1
SPECIFICATION
HIGH-STRENGTH STEEL SHEET EXHIBITING EXCELLENT STRETCH-FLANGE
FORMABILITY AND BENDING WORKABILITY, AND METHOD OF PRODUCING
MOLTEN STEEL FOR THE HIGH-STRENGTH STEEL SHEET
Technical Field
[OOO 11
The present invention relates to a high-strength steel sheet suitable for use, for
example, in underbody components of transportation devices, and a method of producing
molten steel for the high-strength steel sheet. In particular, the present invention relates
to a high-strength steel sheet exhibiting excellent stretch-flange formability and bending
workability, and a method of producing molten steel for the high-strength steel sheet.
The present application claims priority based on Japanese Patent Application No.
201 1-038956 filed in Japan on February 24,201 1, Japanese Patent Application No.
201 1-053458 filed in Japan on March 10, 201 1, Japanese Patent Application No.
2012-007784 filed in Japan on January 18,20 12, and Japanese Patent Application No.
2012-007785 filed in Japan on January 18, 2012, the disclosures of which are incorporated
herein by reference in their entirety.
. .
Background Art
[0002]
In recent years, there are growing demands for hot-rolled steel sheets for
automobiles having enhanced strength and reduced weight from the viewpoint of
improvement in safety of automobiles and reduction in fuel consumption, which leads to
environmental conservation. Among the automobile parts, frame-related parts and
arm-related parts, which are called an underbody system, occupy a large portion of the
entire weight of the vehicle. Thus, the entire weight of the vehicle can be reduced by
enhancing the strength of materials used for these parts, and reducing the thickness of
these parts. Further, press forming is widely used for shaping materials into the
underbody system. Thus, in order to prevent these materials from cracking during the
press forming, these materials are required to have a high bending workability. For this
reason, high-strength steel sheets are widely used. In particular, hot-rolled steel sheets
are mainly used because of their price advantages. Yet further, for reinforcing members
or undeffloor members, in particular, for slide rails for seats or other small members
subjected to the bending working, cold-rolled steel sheets or zinc-plated steel sheets are
mainly used to reduce the thickness thereof and reduce the weight thereof through use of
the high-strength steel sheets.
[0003]
Of the steels described above, there are known a low-yield-ratio DP steel sheet
containing a ferrite phase and a martensite phase, and a TRIP steel sheet containing a
ferrite phase and a (retained) austenite phase, as a high-strength steel sheet having
increased strength, improved workability and improved formability. However, although
exhibiting increased strength and excellent workability and ductility, these steel sheets do
not have excellent hole expandability, in other words, stretch-flange formability or
bending workability. Thus, in general, although ductility is slightly inferior,
bainite-based steel sheets are used for structural parts such as underbody components that
are required to have the stretch-flange formability.
[0004]
One of the reasons that a composite-structure steel sheet including the ferrite
phase and the martensite phase (hereinafter, also referred to as "DP steel sheet") has lower
stretch-flange formability is considered to be that, since this steel sheet is a composite
formed by the soft ferrite phase and the hard martensite phase, stress concentrates on a
boundary portion between both phases during the hole-expansion working, and the steel
sheet cannot follow its deformation, whereby this boundary portion is likely to become a
start point of breakage.
[0005]
To solve the problems described above, several steel sheets are proposed on the
basis of the DP steel sheet with the aim of achieving both the mechanical strength property
and the bending workability or hole-expandability (workability). For example, as a
technique for stress relaxation using fine dispersed particles, Patent Document 1 discloses
a composite-structure steel sheet including a ferrite phase and a martensite phase (DP steel
sheet) in which fine Cu precipitates or solid solutions are dispersed. In this technique
disclosed in Patent Document 1, it is found that the bending workability can be
significantly effectively improved without deteriorating the workability, by using Cu
precipitates having a particle size of 2 nm or less and formed by Cu in solid solution or Cu
alone, and on the basis of the findings, a composition ratio of contained components is
defined.
[0006]
As a technique for stress relaxation by reducing the difference in strength in
composite phases, for example, Patent Document 2 discloses a technique relating to a
bainite steel, in which the difference in hardness between ferrite and bainite is reduced by
minimizing C as much as possible to make the bainite structure become the primary phase,
and adjusting the ferrite structure, which has been subjected to solid solution strengthening
or precipitation hardening, so as to have an appropriate volume ratio, and further,
generation of coarsened carbides is eliminated.
[0007]
Patent Document 3 discloses a technique of obtaining a high-strength steel sheet
exhibiting excellent bending workability, by defining the size and the number of
oxide-based inclusions on the assumption that the oxide-based inclusions cause cracking
during the bending working.
[OOOS]
Further, Patent Documents 4 and 5 disclose a technique of obtaining a
high-strength steel sheet exhibiting excellent stretch-flange formability and fatigue
characteristics, by reducing the size of elongated MnS-based inclusions existing in the
steel and deteriorating the fatigue characteristics and the stretch-flange formability (hole
expandability), to be fine spherical inclusions, which are less likely to be a starting point
of the occurrence of cracking, and dispersing the fine spherical inclusions in the steel.
Related Art Documents
Patent Documents
[0009]
Patent Document 1 : Japanese Unexamined Patent Application, First Publication
NO. Hll-199973
Patent Document 2: Japanese Unexamined Patent Application, First Publication
NO. 2001-20033 1
Patent Document 3: Japanese Unexamined Patent Application, First Publication
NO. 2002-363694
Patent Document 4: Japanese Unexamined Patent Application, First Publication
NO. 2008-274336
Patent Document 5: Japanese Unexamined Patent Application, First Publication
NO. 2009-299 136
Disclosure of the Invention
Problems to be Solved by the Invention
[OO lo]
Incidentally, although the steel sheet having fine Cu precipitates or solid
solutions dispersed in the DP steel sheet as disclosed in Patent Document 1 has enhanced
fatigue strength, it is not confirmed whether this steel sheet significantly improves the
stretch-flange formability. Further, the high-strength hot-rolled steel sheet having the
structure of the steel sheet formed mainly by a bainite phase and having a reduced number
of coarsened carbides as disclosed in Patent Document 2 exhibits excellent stretch-flange
formability. However, it cannot be said that the bending workability of this steel sheet is
excellent as compared with the DP steel sheet containing Cu. Additionally, the
occurrence of cracking in the case of severe hole-expanding working cannot be prevented
only by suppressing the generation of the coarsened carbides.
[OO 1 11
Yet further, although the high-strength cold-rolled steel sheet having a reduced
amount of coarsened oxide-based inclusions as disclosed in Patent Document 3 exhibits
excellent bending workability, it is not confirmed whether the fatigue characteristics are
improved and the stretch-flange formability is significantly improved. Additionally, this
steel contains a predetermined amount of Mn and S. According to the present inventors'
findings obtained from experiments, it is considered that containing these elements leads
to generation of coarsened MnS-based inclusions. Thus, as described later, only the
reduction in the amount of coarsened oxide-based inclusions generated is not sufficient to
prevent the occurrence of cracking in the case of the severe hole-expanding working.
[OO 121
Yet further, the high-strength steel sheet having the MnS-based inclusions
dispersed in the steel sheet as fine spherical inclusions as disclosed in Patent Document 4
exhibits excellent stretch-flange formability and fatigue characteristics. However, A1 is
not substantially used in melting and producing a steel, and a desulfurization process is
performed under the condition where relatively high free oxide exists, which makes it
difficult to reduce sulfir to the extremely low sulfur concentration. Besides, the
desulfurization process is performed with Ce, La, or other elements while A1 is not
substantially used, which requires the larger amount of additives to be added
Additionally, the addition efficiency of Ce, La or other elements is low, and hence, the
large amount of additives needs to be added.
[00 131
Yet further, the high-strength steel sheet having MnS-based inclusions dispersed
in the steel sheet as fine spherical inclusions as disclosed in Patent Document 5 is
subjected to deoxidation with A1 during a melting and producing stage in producing the
steel, and further subjected to deoxidation with Ce, La, or the like. Thus, with this steel
sheet, addition efficiency of Ce, La or other elements is high, sulfur can be reduced to the
extremely low sulfur concentration, and excellent stretch-flange formability and fatigue
characteristics can be obtained even with a relatively high S concentration. However, the
large amount of A1203-Ce203-based oxide is generated. This causes clogging of a ladle
nozzle or immersion nozzle during continuous casting processes in a steel-producing stage,
and stops production of steels, which leads to a problem that products cannot be produced
continuously. In the case where Ca is added to eliminate the above-described problem,
there are generated Ca0-A1203-based oxide having a low melting point as illustrated in
FIG 2A and FIG. 6, or coarsened CaS-based inclusions having Fe, Mn or 0 dissolved in
solid solution or having Ca0-A1203 combined therewith as illustrated in FIG. 2B and FIG
7. The oxides or inclusions are elongated as with MnS-based inclusions, deteriorating
the stretch-flange formability. Further, multiply-precipitated MnS-based inclusions also
coarsen, and hence, are likely to be elongated, which leads to a problem that the
stretch-flange formability is more likely to deteriorate. Additionally, in Patent Document
5, Ti is added, and hence, coarsened inclusions precipitate as TiS. CaS or TiS is
heterogeneously nucleated in the complex oxide including Ca0-Al203-based oxide having
the low melting point or Ti oxide. This leads to generation of coarsened Ca0-A1203Ti
oxide or CaSTiS composite oxysulfide. The oxide or oxysulfide forms clusters, and
further coarsens, which largely affects the hole expandability. Further, the oxide or
oxysulfide expands or breaks during rolling, causing a deterioration in the material.
[00 141
According to the study made by the present inventors, the problems that Patent
Documents 1,2, 3,4, and 5 have result mainly from existence of elongated sulfide-based
inclusions formed mainly by MnS in the steel sheet as illustrated in FIG 1B and FIG. 4,
Ca0-A1203-based inclusions having a low melting point as illustrated in FIG. 2A and FIG.
6, and CaS-based inclusions having coarsened and elongated Fe, Mn and 0 dissolved in
solid solution or Ca0-A1203 combined therewith as illustrated in FIG. 2B and FIG 7,
although formation of alumina inclusions that have an effect on the stretch-flange
formability as illustrated in FIG. 1A and FIG 5 is suppressed. In other words, if the steel
sheet receives repetitive deformation, the internal defect occurs in the vicinity of the
elongated and coarsened MnS-based inclusions existing in the surface layer or near the
surface layer, and expands as a crack. This crack leads to the deterioration in the fatigue
characteristics, and is likely to serve as the starting point of the crack during
hole-expanding work or bending work, causing the deterioration in the stretch-flange
formability and bending workability.
[00 151
Next, a detailed description will be made of the existence of the sulfide-based
inclusions formed mainly by MnS as described in Patent Documents 1, 2, 3,4, and 5. As
with C and Si, Mn is an element that effectively strengthens the material. Thus, in
general, the concentration of Mn in the high-strength steel sheet is set higher to secure the
strength of the steel. Further, through normal steel-producing processes, the steel
contains S in the range of 5 ppm to 50 ppm. Thus, casted steels usually contain MnS.
[00 1 61
At the same time, with the increase in soluble Ti, the soluble Ti partially
combines with coarsened TiS or MnS, and (Mn, Ti)S precipitates. When the casted steel
is subjected to hot rolling or cold rolling, the MnS-based inclusions and TiS deform during
the rolling, and become elongated inclusions, causing the deterioration in the fatigue
characteristics and the stretch-flange formability (hole expandability).
[00 171
To deal with this, the invention described in Patent Document 4 disperses the
MnS-based inclusions as fine spherical inclusions in the steel sheet to obtain favorable
stretch-flange formability (hole expandability) and fatigue characteristics. However, this
invention does not substantially perform Al deoxidation, and the steel sheet has high
oxygen potential, which makes a desulfurization reaction less likely to occur. Thus,
extremal values of components or formation of the inclusions are obtained to improve the
material properties in a state where the steel sheet has a relatively high S concentration.
This makes it impossible to remove the sulfur to the extremely low sulfur concentration.
[OO 1 81
Next, a detailed description will be made of the oxygen potential, the sulfur
potential, and components or formation of the inclusions for improving the steel properties.
In general, the acid-soluble A1 is more likely to coarsen because of clustering of oxide in
the acid-soluble Al, which deteriorates the stretch-flange formability, the bending
workability, and the fatigue characteristics. Thus, it is desirable to reduce the
acid-soluble A1 as much as possible. For this reason, a desulfurization process is
performed in a state where the oxygen potential is relatively high, and the concentration of
acid-soluble Al does not exceed 0.0 1 %.
[00 191
The desulfurization reaction is a reducing reaction, and proceeds easily under the
low oxygen potential circumstances. However, the sulfur potential is high in the high
oxygen potential circumstances, and thus, it is extremely difficult to reduce the sulfur to
the extremely low sulfur state. To deal with this, Ce and La are excessively added to
reduce the oxygen potential as much as possible. However, this does not sufficiently
reduce the oxygen potential, and requires high cost. In other words, on the basis of the
concept that the effect of S is removed in the relatively high S concentration, the
stretch-flange formability and the fatigue characteristics are improved by excessively
adding Ce and La to control the component or formation for the inclusions.
[0020]
However, when the component or formation of the inclusions is controlled by
excessively adding Ce and La in order to remove the effect of S in the state where the
concentration of S is relatively high, the degree of removal of the effect of S is limited
because of its relatively high S concentration. For these reasons, there is a demand for
high-strength steel sheets having more favorable stretch-flange formability (hole
expandability) and fatigue characteristics.
[002 11
However, there is no proposal of a high-strength steel sheet exhibiting excellent
stretch-flange formability, bending workability, and fatigue characteristics, and a method
of producing molten steel for the high-strength steel sheet, from the viewpoint of
systematically controlling the operability during a steel-producing process, the oxygen
potential, the sulhr potential, and the components and formation of the inclusions.
[0022]
As with C and Si, Mn is an element that contributes to effectively enhancing the
strength of the materia5 and hence, the concentration of Mn is generally set higher to
obtain the strength of the high-strength steel sheet. Further, the steel sheet contains S of
approximately 50 ppm through normal steel-producing processes. For this reason, a cast
slab usually contains MnS. When the cast slab is subjected to hot rolling and cold rolling,
these MnS-based inclusions elongate, since these MnS-based inclusions are likely to
deform. This causes the deterioration in the bending workability and the stretch-flange
formability (hole expandability). However, conventionally, there is no proposal of a
high-strength steel sheet exhibiting excellent stretch-flange formability and bending
workability, and a method of producing molten steel for the high-strength steel sheet from
the viewpoint of controlling precipitation and deformation of the MnS-based inclusions
described above.
[0023]
In the case where, in Patent Document 5, with the aim of improving the
operability, A1 deoxidation is performed to improve the oxygen potential, the sulfur
potential, and the material properties, Ca needs to be added. This leads to generation of
oxide having a low melting point, deteriorating the material properties. In the molten
steel, Ca exists in the form of liquid or vaporizes, and hence, first forms oxide having the
low melting point. If such oxide in the form of liquid is first generated in the molten
steel, these inclusions in the form of liquid aggregate to form coarsened Ca0-AI2O3-based
oxide having the low melting point, or CaS containing Fe, Mn or 0 in solid solution or
having Ca0-A1203 combined therewith. Thus, even if an attempt is made to control the
formation of inclusions by adding Ce, La or the like thereafter, such control cannot be
achieved.
[0024]
The Ca0-A1203-based oxide having a low melting point, the CaS-based
inclusion containing Fe, Mn or 0 in solid solution or having Ca0-A1203 combined
therewith, and the MnS-based inclusion inevitably formed due to the addition of Mn are
likely to deform when the ingot is subjected to the hot rolling and the cold rolling, and
become elongated Ca0-A1203-based oxide, or coarsened CaS-based inclusion or
MnS-based inclusion, causing the deterioration in the bending workability and the
stretch-flange formability (hole expandability). However, conventionally, there is no
proposal of a high-strength steel sheet exhibiting excellent stretch-flange formability and
bending workability, and a method of producing molten steel for the high-strength steel
sheet, from the view point of controlling the precipitation or deformation of the
Ca0-A1203-based oxide, the coarsened CaS-based inclusion containing coarsened Fe, Mn
or 0 in solid solution or having Ca0-A1203 combined therewith, or the MnS-based
inclusion described above.
[0025]
Further, Ti forms fine TiN or Tic as precipitates, and hence, has an effect of
enhancing the strength of the material. However, Ti also has a problem that Ti is likely to
form coarsened TiS that deforms during rolling as described above.
[0026]
The present invention has been made in view of the problems described above,
and a first object of the present invention is to provide a high-strength steel sheet
exhibiting excellent stretch-flange formability and bending workability and a method of
producing molten steel for the high-strength steel sheet, by applying multiple deoxidation
to molten steel in a steel producing stage to prevent generation of Ca0-A1203-based oxide
and coarsened CaS in an ingot, to make MnS multiple-precipitated fine inclusions in the
oxide or oxysulfide formation, and to make MnS dispersed in the steel sheet as a fine
spherical inclusion, which does not deform during rolling and is less likely to be a starting
point of the occurrence of cracking, thereby improving the stretch-flange formability and
the bending workability.
[0027]
Further, the present invention has been made in view of the problems described
above, and a second object of the present invention is to provide a high-strength steel
sheet exhibiting excellent stretch-flange formability, bending workability, and fatigue
characteristics and a method of producing molten steel for the high-strength steel sheet, by
applying multiple deoxidation to molten steel in a steel-producing stage to prevent
generation of Ca0-A1203-based oxide, and CaS containing coarsened Fe, Mn or 0
dissolved in solid solution or having Ca0-A1203 combined therewith in the ingot, while
controlling generation of coarsened TiS that has an adverse effect on the hole
expandability, thereby improving the stretch-flange formability, the bending workability,
and the fatigue characteristics while obtaining high operability without increasing the cost.
Means for Solving the Problems
[0028]
Main points of the present invention are as follows:
[0029]
(1) A first aspect of the present invention provides a steel sheet including C: 0.03 to
0.25 mass %, Si: 0.1 to 2.0 mass %, Mn: 0.5 to 3.0 mass %, P: not more than 0.05 mass %,
T.0: not more than 0.0050 mass %, S: 0.0001 to 0.01 mass %, N: 0.0005 to 0.01 mass %,
acid-soluble Al: more than 0.01 mass %, Ca: 0.0005 to 0.0050 mass %, and a total of at
least one element of Ce, La, Nd, and Pr: 0.001 to 0.01 mass %, with a balance including
iron and inevitable impurities, in which the steel sheet contains a chemical component on
a basis of mass that satisfies 0.7 < 100 x ([Ce] + [La] + [Nd] + [Pr])/[acid-soluble All 5 70
and 0.2 5 ([Ce] + [La] + [Nd] + [Pr])/[S] 5 10, where [Ce] is an amount of Ce contained,
[La] is an amount of La contained, [Nd] is an amount of Nd contained, [Pr] is an amount
of Pr contained, [acid-soluble All is an amount of acid-soluble A1 contained, and [S] is an
amount of S contained. The steel sheet has a compound inclusion including a first
inclusion phase containing at least one element of Ce, La, Nd, and Pr, containing Ca, and
containing at least one element of 0 and S, and a second inclusion phase having a
component different from that of the first inclusion phase and containing at least one
element of Mn, Si, and Al, the compound inclusion forms a spherical compound inclusion
having an equivalent circle diameter in the range of 0.5 pm to 5 pm, and a ratio of the
number of the spherical compound inclusion relative to number of all inclusions having
the equivalent circle diameter in the range of 0.5 pn to 5 pm is 30% or more.
(2) In the high-strength steel sheet according to (1) above, the spherical inclusion
may be an inclusion having an equivalent circle diameter of 1 pm or more, and the ratio of
the number of elongated inclusions having a major axislminor axis of 3 or less relative to
number of all inclusions having the equivalent circle diameter of 1 pm or more may be
50% or more.
(3) In the high-strength steel sheet according to (1) or (2) above, the spherical
inclusion may contain at least one element of Ce, La, Nd, and Pr, a total of which is in the
range of 0.5 mass % to 95 mass % in an average composition.
(4) In the high-strength steel sheet according to any one of (1) to (3) above, an
average grain diameter of a crystal in a structure of the steel sheet may be 10 pm or less.
(5) The high-strength steel sheet according to any one of (1) to (4) above may
further contain at least one element of Nb: 0.0 1 to 0.10 mass %, and V: 0.0 1 to 0.10
mass %.
(6) The high-strength steel sheet according to any one of (1) to (5) above may
further contain at least one element of: Cu: 0.1 to 2 mass %, Ni: 0.05 to 1 mass %, Cr:
0.01 to 1 mass %, Mo: 0.01 to 0.4 mass %, and B: 0.0003 to 0.005 mass %.
(7) The high-strength steel sheet according to any one of (1) to (6) above may
further contain Zr: 0.00 1 to 0.0 1 mass %.
(8) The high-strength steel sheet according to any one of (1) to (4) above may
further contain at least one element of Nb: 0.0 1 to 0.10 mass %, V: 0.01 to 0.10 mass %,
Cu: 0.1 to 2 mass %, Ni: 0.05 to 1 mass %, Cr: 0.01 to 1 mass %, Mo: 0.01 to 0.4 mass %,
B: 0.0003 to 0.005 mass %, and Zr: 0.001 to 0.01 mass %.
(9) A second aspect of the present invention provides a method of producing molten
steel for the high-strength steel sheet according to any one of (1) to (4) above, having a
refinement process for producing a steel, the refinement process including: a first process
13
of obtaining a first molten steel including applying processing so as to obtain P of not
more than 0.05 mass % and S of not less than 0.0001 mass %, and performing addition or
adjustment such that C is not less than 0.03 mass % and not more than 0.25 mass %, Si is
not less than 0.1 mass % and not more than 2.0 mass %, Mn is not less than 0.5 mass %
and not more than 3.0 mass %, and N is not less than 0.0005 mass % and not more than
0.01 mass %; a second process of obtaining a second molten steel including performing
addition to the first molten steel such that A1 is more than 0.01 mass % in acid-soluble Al,
and T.0 is not more than 0.0050 mass %; a third process of obtaining a third molten steel
including adding at least one element of Ce, La, Nd, and Pr to the second molten steel so
as to satisfy on a basis of mass 0.7 < 100 x ([Ce] + [La] + [Nd] + [Pr])/[acid-soluble All 5
70, 0.2 5 ([Ce] + [La] + [Nd] + [Pr])/[S] 5 10, and 0.00 1 5 [Ce] + [La] + [Nd] + [Pr] I
0.01, where [Ce] is an amount of Ce contained, [La] is an amount of La contained, [Nd] is
an amount of Nd contained, [Pr] is an amount of Pr contained, [acid-soluble All is an
amount of acid-soluble A1 contained, and [S] is an amount of S contained; and a fourth
process of obtaining a fourth molten steel including adding Ca to or performing
adjustment to the third molten steel such that Ca is not less than 0.0005 mass % and not
more than 0.0050 mass %.
(10) In the method of producing molten steel for a high-strength steel sheet according
to (9) above, the third process may include, before the at least one element of Ce, La, Nd,
and Pr is added to the second molten steel, adding at least one element of Nb and V to the
second molten steel such that the second molten steel further contains at least one element
of Nb of not less than 0.01 mass % and not more than 0.10 mass % and V of not less than
0.01 mass % and not more than 0.10 mass %.
(1 1) In the method of producing molten steel for a high-strength steel sheet according
to (9) or (10) above, the third process may include, before the at least one element of Ce,
La, Nd, and Pr is added to the second molten steel, adding at least one element of Cu, Ni,
Cr, Mo, and B to the second molten steel such that the second molten steel further
contains at least one element of Cu of not less than 0.1 mass % and not more than 2
mass %, Ni of not less than 0.05 mass % and not more than 1 mass %, Cr of not less than
0.01 mass % and not more than 1 mass %, Mo of not less than 0.01 mass % and not more
than 0.4 mass %, and B of not less than 0.0003 mass % and not more than 0.005 mass %.
(12) The method of producing molten steel for a high-strength steel sheet according
any one of (9) to (1 1) above, the third process may include, before the at least one element
of Ce, La, Nd, and Pr is added to the second molten steel, adding Zr to the second molten
steel such that the second molten steel further contains Zr of not less than 0.001 mass % to
0.01 mass %.
(1 3) A third aspect of the present invention provides a high-strength steel sheet
including: C: 0.03 to 0.25 mass %, Si: 0.03 to 2.0 mass %, Mn: 0.5 to 3.0 mass %, P: not
more than 0.05 mass %, T.0: not more than 0.0050 mass %, S: 0.0001 to 0.01 mass %,
acid-soluble Ti: 0.008 to 0.20 mass %, N: 0.0005 to 0.01 mass %, acid-soluble Al: more
than 0.01 mass %, Ca: 0.0005 to 0.005 mass %, and a total of at least one element of Ce,
La, Nd, and Pr: 0.001 to 0.01 mass %, with a balance including iron and inevitable
impurities, in which the steel sheet contains a chemical component on a basis of mass that
satisfies 0.7 < 100 x ([Ce] + [La] + [Nd] + [Pr])/[acid-soluble All 5 70, and 0.2 5 ([Ce] +
[La] + [Nd] + [Pr])/[S] 5 10, where [Ce] is an amount of Ce contained, [La] is an amount
of La contained, [Nd] is an amount of Nd contained, [Pr] is an amount of Pr contained,
[acid-soluble All is an amount of acid-soluble Al contained, and [S] is an amount of S
contained. The steel sheet has a compound inclusion including a first inclusion phase
containing at least one element of Ce, La, Nd, and Pr, containing Ca, and containing at
least one element of 0 and S, and a second inclusion phase having a component different
from that of the first inclusion phase and containing at least one element of Mn, Si, Ti, and
Al, the compound inclusion forms a spherical compound inclusion having an equivalent
circle diameter in the range of 0.5 pm to 5 pm, a ratio of the number of the spherical
compound inclusion relative to number of all inclusions having the equivalent circle
diameter in the range of 0.5 pm to 5 pm is 50% or more, and number density of an
inclusion with more than 5 pn is less than 10 pieces/mm2.
(14) In the high-strength steel sheet according to (13) above, the spherical inclusion
may be an inclusion having an equivalent circle diameter of 1 pm or more, and the ratio of
the number of elongated inclusions having a major axislminor axis of 3 or less relative to
number of all inclusions having the equivalent circle diameter of 1 pm or more is 50% or
more.
(15) In the high-strength steel sheet according to (13) or (14) above, the spherical
inclusion may contain at least one element of Ce, La, Nd, and Pr, a total of which is in the
range of 0.5 mass % to 95 mass % in an average composition.
(16) In the high-strength steel sheet according to any one of (13) to (15) above, an
average grain diameter of a crystal in a structure of the steel sheet may be 10 pm or less.
(17) The high-strength steel sheet according to any one of (13) to (16) above may
further contain at least one element of Nb: 0.005 to 0.10 mass %, and V: 0.01 to 0.10 mass
%.
(1 8) The high-strength steel sheet according to any one of (13) to (17) above may
further contain at least one element of: Cu: 0.1 to 2 mass %, Ni: 0.05 to 1 mass %, Cr:
0.01 to 1.0 mass %, Mo: 0.01 to 0.4 mass %, and B: 0.0003 to 0.005 mass %.
(19) The high-strength steel sheet according to any one of (13) to (18) above may
further contain Zr: 0.001 to 0.01 mass %.
(20) The high-strength steel sheet according to any one of (13) to (16) above may
further contain at least one element of Nb: 0.005 to 0.10 mass %, V: 0.01 to 0.10 mass %,
Cu: 0.1 to 2 mass %, Ni: 0.05 to 1 mass %, Cr: 0.01 to 1.0 mass %, Mo: 0.01 to 0.4
mass %, B: 0.0003 to 0.005 mass %, and Zr: 0.001 to 0.01 mass %.
(21) A fourth aspect of the present invention provides a method of producing molten
steel for the high-strength steel sheet according to any one of (1 3) to (1 6) above, having a
refinement process for producing a steel, the refinement process including: a first process
of obtaining a first molten steel including: applying processing so as to obtain P of not
more than 0.05 mass % and S of not less than 0.0001 mass % and not more than 0.01 mass
%, and performing addition or adjustment such that C is not less than 0.03 mass % and not
more than 0.25 mass %, Si is not less than 0.03 mass % and not more than 2.0 mass %,
Mn is not less than 0.5 mass % and not more than 3.0 mass %, and N is not less than
0.0005 mass % and not more than 0.01 mass %; a second process of obtaining a second
molten steel including performing addition to the first molten steel such that A1 is more
than 0.01 mass % in acid-soluble Al, and T.0 is not more than 0.0050 mass %; a third
process of obtaining a third molten steel including adding Ti of not less than 0.008 mass %
and not more than 0.20 mass % in acid-soluble Ti to the second molten steel; a fourth
process of obtaining a fourth molten steel including adding at least one element of Ce, La,
Nd, and Pr to the third molten steel so as to satisfy on a basis of mass 0.7 < 100 x ([Ce] +
[La] + [Nd] + [Pr])/[acid-soluble All 5 70, 0.2 5 ([Ce] + [La] + [Nd] + [Pr])/[S] 5 10, and
0.001 5 [Ce] + [La] + [Nd] + [Pr] 5 0.01, where [Ce] is an amount of Ce contained, [La] is
an amount of La contained, [Nd] is an amount of Nd contained, [Pr] is an amount of Pr
contained, [acid-soluble All is an amount of acid-soluble A1 contained, and [S] is an
amount of S contained; and a fifth process of obtaining a fifth molten steel including
adding Ca to or performing adjustment to the fourth molten steel such that Ca is not less
than 0.0005 mass % and not more than 0.0050 mass %.
(22) In the method of producing molten steel for a high-strength steel sheet according
to (2 1) above, the third process may include, before the at least one element of Ce, La, Nd,
and Pr is added to the second molten steel, adding at least one element of Nb and V to the
second molten steel such that the second molten steel further contains at least one element
of Nb of not less than 0.005 mass % and not more than 0.10 mass %, and V of not less
than 0.01 and not more than 0.10 mass %.
(23) In the method of producing molten steel for a high-strength steel sheet according
to (21) or (22) above, the third process may include, before the at least one element of Ce,
La, Nd, and Pr is added to the second molten steel, adding at least one element of Cu, Ni,
Cr, Mo, and B to the second molten steel such that the second molten steel further contains
at least one element of Cu of not less than 0.1 mass % and not more than 2 mass %, Ni of
not less than 0.05 mass % and not more than 1 mass %, Cr of not less than 0.01 mass %
and not more than 1 mass %, Mo of not less than 0.01 mass % and not more than 0.4
mass %, and B of not less than 0.0003 mass % and not more than 0.005 mass %.
(24) In the method of producing molten steel for a high-strength steel sheet according
to any one of (2 1) to (23) above, the third process may include, before the at least one
element of Ce, La, Nd, and Pr is added to the second molten steel, adding Zr to the second
molten steel such that the second molten steel further contains Zr of not less than 0.001
mass % and not more than 0.01 mass %.
Effects of the Invention
[0030]
According to the high-strength steel sheet exhibiting excellent stretch-flange
formability and bending workability of the first aspect of the present invention, it is
possible to improve the stretch-flange formability and the bending workability, by stably
adjusting components in the molten steel through A1 deoxidation, suppressing generation
of coarsened alumina inclusions, and precipitating fine inclusions multiple-precipitated in
the ingot in the formation of oxide or oxysulfide to disperse the inclusions in the steel
sheet as fine spherical inclusions that do not deform during rolling and are less likely to be
a starting point of the occurrence of cracking, while making the crystal grain diameter fine
in the structure.
[003 11
According to the method of producing molten steel for the high-strength steel
sheet exhibiting excellent stretch-flange formability and bending workability of the second
aspect of the present invention, it is possible to obtain the high-strength hot-rolled steel
sheet exhibiting excellent stretch-flange formability and bending workability, by stably
adjusting components in the molten steel through A1 deoxidation, suppressing generation
of coarsened alumina inclusions, and precipitating fine compound inclusions formed by
oxide or oxysulfide multiple-precipitated in the ingot to disperse the inclusions in the steel
sheet as fine spherical inclusions that do not deform during rolling and are less likely to be
a starting point of the occurrence of cracking, while making the crystal grain diameter fine
in the structure.
[0032]
According to the high-strength steel sheet exhibiting excellent stretch-flange
formability and bending workability of the third aspect of the present invention, it is
possible to improve the stretch-flange formability and the bending workability, by stably
adjusting components in the molten steel through A1 deoxidation, deoxidation with Ce, La,
Nd and Pr, and then Ca deoxidation, suppressing generation of coarsened alumina
inclusions, and generating compound inclusions formed by different fine inclusion phases
in the cast slab to disperse the compound inclusions in the steel sheet as fine spherical
inclusions that do not deform during rolling and are less likely to be a starting point of the
occurrence of cracking, while making the crystal grain diameter fine in the structure.
[0033]
According to the method of producing molten steel for the high-strength steel
sheet exhibiting excellent stretch-flange formability and bending workability of the fourth
aspect of the present invention, it is possible to obtain the high-strength hot-rolled steel
sheet exhibiting excellent stretch-flange formability and bending workability, by stably
adjusting components in the molten steel through deoxidation with Ce, La, Nd and Pr, and
Ca deoxidation thereafter, suppressing generation of coarsened alumina inclusions, and
generating compound inclusions formed by different fine inclusion phases in the case slab
to disperse the inclusions in the steel sheet as fine spherical inclusions that do not deform
during rolling and are less likely to be a starting point of the occurrence of cracking, while
making the crystal grain diameter fine in the structure by adding Ti.
Brief Description of the Drawings
[0034]
FIG. 1A is a diagram for explaining A1203, which is an elongated inclusion
existing in a hot-rolled steel sheet.
FIG. 1B is a diagram for explaining MnS, which is an elongated inclusion
existing in the hot-rolled steel sheet.
FIG. 2A is a diagram for explaining an elongated CaOA1203-based inclusion
existing in the hot-rolled steel sheet.
FIG. 2B is a diagram for explaining an elongated CaS-based inclusion existing in
the hot-rolled steel sheet.
FIG. 3A is a diagram for explaining a compound inclusion relating to a first
embodiment of the present invention, and is a diagram illustrating an example of how a
first inclusion exists.
FIG. 3B is a diagram for explaining a compound inclusion relating to the first
embodiment of the present invention, and is a diagram illustrating an example of how a
second inclusion exists.
FIG. 4 is a diagram illustrating an elongated sulfide-based inclusion formed
mainly by MnS.
FIG. 5 is a diagram illustrating an alumina-based inclusion that has an effect on
stretch-flange formability.
FIG. 6 is a diagram illustrating an elongated Ca0-Al2O3-based oxide having a
lower melting point and having an effect on stretch-flange formability.
FIG. 7 is a diagram illustrating an elongated CaS-based inclusion containing
coarsened Fe, Mn or 0 dissolved in solid solution or combined with Ca0-A1203, and
having an effect on the stretch-flange formability.
FIG. 8A is a diagram illustrating an example of a compound inclusion formed
into a spherical inclusion.
FIG. 8B is a diagram illustrating another example of a compound inclusion
formed into a spherical inclusion.
Embodiments of the Invention
[0035]
[First Embodiment]
The present inventors made a study mainly of a method of improving the
stretch-flange formability and the bending workability by precipitating fine MnS
inclusions in an ingot (cast slab), and dispersing the inclusions in the steel sheet as fine
spherical inclusions that do not deform during rolling and are less likely to be a starting
point of the occurrence of cracking, and of finding additive elements that do not
deteriorate the fatigue characteristics.
As a result, the present inventors found that the hole-expandability or other
properties can be improved in a manner such that: fine and hard Ce oxide, La oxide, Nd
oxide, Pr oxide, cerium oxysulfide, lanthanum oxysulfide, neodymium oxysulfide, andor
praseodymium oxide arelis formed through deoxidation with addition of Ce, La, Nd
andor Pr; a compound inclusion containing an inclusion phase including at least one
element of Ce, La, Nd, and Pr, Ca, and at least one element of 0 and S, and an inclusion
phase further including at least one element of Mn, Si, and Al, the components of these
inclusion phases being different from each other, is further formed through combination
with Ca added; and this compound inclusion is formed into a spherical inclusion having an
equivalent circle diameter in the range of 0.5 pm to 5 pn. With these formations,
precipitated MnS is less likely to deform even during rolling, and hence, the steel sheet
has a significantly reduced number of enlarged and coarsened MnS. Further, MnS-based
inclusion is less likely to be a starting point of the occurrence of cracking or a pathway of
crack propagation even during the repetitive deformation, hole-expanding working or
bending working, so that hole-expandability can be improved.
[0037]
In addition to forming the precipitates into fine oxide and fine MnS-based
inclusions, the present inventors also made a study of sequentially applying multiple
deoxidation with Si, Al, (Ce, La, Nd, Pr), and Ca to reduce sulhr to the low sulfur
concentration so as to reliably fix the residual sulfur to be fine and hard inclusions. As a
result, the present inventors found that, for molten steel subjected first to deoxidation with
Si, second to deoxidation with Al, and then to deoxidation with addition of at least one
element of Ce, La, Nd, and Pr, it is possible to significantly improve the stretch-flange
formability and the bending workability, in a manner such that: by obtaining
predetermined (Ce + La + Nd + Pr)/acid-soluble A1 and (Ce + La + Nd + Pr)/S on the
basis of mass and adding Ca at the end, oxygen potential in the molten steel can be
reduced; under this reduced oxygen potential, sulfur can be reduced to the extremely low
sulfur concentration in a relatively easy manner, and fine MnS-based inclusions can be
obtained; and this makes it possible to reliably fix the residual sulfur to be fine and hard
inclusions.
[003 81
Hereinbelow, a high-strength steel sheet exhibiting excellent stretch-flange
formability and bending workability will be described in detail as a first embodiment
according to the present invention. Below, the unit "mass %" used for compositions will
be expressed simply as "%." Note that the high-strength steel sheet in the present
invention includes a steel sheet subjected to normal hot rolling andlor cold rolling and
used as it is without applying further treatment thereto, and a steel sheet used after
application of surface treatment such as plating and coating.
[0039]
First, experiments concerning the first embodiment according to the present
invention will be described.
[0040]
The present inventors produced a steel ingot by subjecting molten steel
containing C: 0.06%, Si: 1.0%, Mn: 1.4%, P: 0.01% or less, S: 0.005%, and N: 0.003%
with a balance including Fe to deoxidation using various elements. The obtained steel
ingot is hot rolled to form a hot-rolled steel sheet having a thickness of 3 mm. For the
obtained hot-rolled steel sheet, a tensile test, a hole-expanding test, and a bending test
were performed, and examination was made on number density of inclusions, formation
and average composition in the steel sheet.
[004 11
First, in the hot-rolled steel sheet produced by adding Si to the molten steel, and
then subjecting the molten sheet to A1 deoxidation, Alz03-based inclusions precipitated in
the steel ingot as inclusions had a high melting temperature of 2040°C, and remained in an
angulated shape without being elongated during rolling as illustrated in FIG. 1A. Thus,
these inclusions serve as a starting point of cracking of the steel sheet during
hole-expanding work, causing the deterioration in the bending workability and the
stretch-flange formability (hole expandability). The coarsened MnS-based inclusions
precipitated in the steel ingot as inclusions had a low melting point of 16 1 O°C, and were
easily elongated during rolling as illustrated in FIG. 1 B to form elongated MnS-based
inclusions. Further, these inclusions serve as a starting point of cracking of the steel
sheet during hole-expanding work.
[0042]
In the hot-rolled steel sheet produced by adding Ca after the deoxidation with Al,
Ca is melted and aggregates with interfacial energy to be a larger size. Then, Ca
precipitates as coarsened Ca0-A1203-based inclusions or CaS(Fe, Mn, Alz03)-based
inclusions in the ingot. These inclusions have a melting point of approximately 1390°C.
Thus, these inclusions were easily elongated during rolling as illustrated in FIG. 2A and
FIG. 2B to form elongated inclusions having a size in the range of approximately 50 pm to
100 pm, causing the deterioration in the bending workability and the stretch-flange
formability (hole expandability).
[0043]
Further, examination was made on the stretch-flange formability and the bending
workability of a steel sheet produced by adding Si to a molten steel, subjecting the molten
steel to deoxidation with Al, agitating the molten steel for approximately 2 minutes, and
adding at least one element of Ce, La, Nd, and Pr for deoxidation. As a result, with the
steel sheet subjected to the sequential three-step deoxidation with Si, Al, and at least one
element of Ce, La, Nd, and Pr as described above, it is confirmed that the stretch-flange
formability and the bending workability can be further improved. This is because MnS is
precipitated on the fine and hard Ce oxide, La oxide, Nd oxide, Pr oxide, cerium
oxysulfide, lanthanum oxysulfide, neodymium oxysulfide, and/or praseodymium
oxysulfide generated through deoxidation with addition of Ce, La, Nd, and/or Pr, and it is
possible to suppress deformation of the multiple-precipitated oxide or oxysulfide
inclusions during rolling, whereby the number of elongated and coarsened MnS-based
inclusions in the steel sheet can be significantly reduced.
[0044]
It should be noted that the mechanism of making finer the Ce oxide, the La oxide,
the Nd oxide, the Pr oxide, the cerium oxysulfide, the lanthanum oxysulfide, the
neodymium oxysulfide and the praseodymium oxysulfide is that: A1 added later causes
reductive decomposition of the SO2-based inclusions generated first through the Si
deoxidation, thereby forming fine A1203-based inclusions; Ce, La, Nd, andlor Pr is
subjected to reductive decomposition to form fine Ce oxide, La oxide, Nd oxide, Pr oxide,
cerium oxysulfide, lanthanum oxysulfide, neodymium oxysulfide, andlor praseodymium
oxysulfide; and since the interfacial energy between the molten steel and the generated Ce
oxide, La oxide, Nd oxide, Pr oxide, cerium oxysulfide, lanthanum oxysulfide,
neodymium oxysulfide, and praseodymium oxysulfide is low, it is possible to suppress
aggregation of the generated oxides and oxysulfides.
[0045]
The present inventors further produced a steel ingot by then applying A1
deoxidation, applying deoxidation while changing compositions of Ce, La, Nd, and Pr,
and then adding Ca. Thus, the obtained steel ingot was hot rolled to form a hot-rolled
steel sheet having a thickness of 3 mm. For the obtained hot-rolled steel sheet, a
hole-expanding test and a bending test were performed, and examination was made on the
number density of inclusions, formation and average composition in the steel sheet.
[0046]
Through experiments described above, it was found that, by setting a ratio (Ce +
La + Nd + Pr)/acid-soluble A1 in the range of 0.7 to 70 and a ratio of (Ce + La + Nd +
Pr)/S in the range of 0.2 to 10 on the basis of mass, the oxygen potential sharply decreases
in molten steel obtained through multiple deoxidation of adding Si, applying deoxidation
with Al, applying deoxidation with addition of at least one element of Ce, La, Nd, and Pr,
and then adding Ca. In other words, with the effect obtained through the multiple
deoxidation with Al, Si, (Ce, La, Nd, Pr), and Ca, it is possible to obtain the largest
oxygen-potential-reducing effect that conventional deoxidation applications can obtain
with various deoxidation elements. With the effect of multiple deoxidation, it is possible
to extremely lower the A1203 concentration in the generated oxides, and hence, it is
possible to obtain a steel sheet exhibiting excellent stretch-flange formability and bending
workability as with steel sheets produced with little deoxidation with Al.
100471
The reason for this is considered to be as follows:
[0048]
By adding Si, Si02 inclusions are generated, and then, SiO2 inclusions are
reduced to be Si by adding Al. Further, while subjecting SiO2 inclusions to reduction, A1
removes the dissolved oxygen in the molten steel to form A1203-based inclusions. Part
of the A1203-based inclusions rise to the surface and are removed, whereas the rest of the
A1203-based inclusions remain in the molten steel. After this, with the added (Ce, La, Nd,
Pr), the A1203-based inclusions are subjected to reductive decomposition to form fine and
spherical Ce oxide, La oxide, Nd oxide, Pr oxide, and REM oxysulfide such as cerium
oxysulfide, lanthanum oxysulfide, neodymium oxysulfide, and praseodymium oxysulfide.
Then, Ca is added to precipitate A1203, MnS, CaS, (MnCa)S or other precipitations in the
oxides andfor oxysulfides, thereby forming a spherical compound inclusion containing an
Al-O-Ce-La-Nd-Pr-O-S-Ca inclusion phase [for example, A1203(Ce, La, Nd, Pr)202SCa],
a Ca-Mn-S-Ce-La-Nd-Pr-A1-0 inclusion phase [for example, CaMnS(Ce, La, Nd,
Pr)A1203], and a Ce-La-Nd-Pr-O-S-Ca inclusion phase [for example, (Ce, La, Nd,
Pr)202SCa] as illustrated in FIG. 3A, which are inclusion phases in solid solution and
combined with each other to form one inclusion, or a spherical compound inclusion
containing a Ca-Mn-S-Ce-La-Nd-Pr inclusion phase [for example, CaMnS(Ce, La, Nd,
Pr)], a Ce-La-Nd-Pr-O-S-Ca inclusion phase [for example, (Ce, La, Nd, Pr)202SCa], and
a Ce-La-Nd-Pr-O-S-Al-O-Ca inclusion phase [for example, (Ce, La, Nd,
Pr)202SA1203Ca] as illustrated in FIG. 3B, which are combined with each other to form
one inclusion. These compound inclusions are formed mainly by oxysulfide of at least
one element of Ce, La, Nd, and Pr and have a substantially spherical shape. Thus, it is
considered that these compound inclusions are formed such that, during processes in
which added metals such as Ce, La, Nd and Pr are melted and react to form oxysulfide, a
large number of extremely fine cores are formed, and then, are subjected to phase
separation to form the compound inclusions, or a phase having a lower melting point is
partially melted and adhere to a phase having a higher melting point.
[0049]
These fine and spherical compound inclusions have a high melting point of
approximately 2000°C, and do not elongate during hot rolling. This makes these
compound inclusions remain in the fine and spherical formation in the hot-rolled steel
sheet. Thus, by forming the spherical compound inclusion (REM oxysulfide compound
inclusion) having the oxide or oxysulfide formation obtained through the multiple
precipitations as described above, it is possible to eliminate the cause of deteriorating the
bending workability and the stretch-flange formability (hole expandability).
[0050]
With four steps of multiple deoxidation through the addition of Al, Si, (Ce, La,
Nd, Pr), and Ca, it is considered that: although AI203 slightly remains, in most part, there
exist fine and hard oxides or oxysulfides having an equivalent circle diameter in the range
of 0.5 pm to 5 pm and formed by at least one element of Ce, La, Nd, and Pr; in these
oxides or oxysulfides, oxides containing at least one element of Si, Al, and Ca are multiple
precipitated; and, a spherical compound inclusion (REM oxysulfide compound inclusion)
having the oxide or oxysulfide formation in which at least one of MnS, CaS, and (Mn,
Ca)S is multiple precipitated is generated.
[005 11
It should be noted that the fine spherical composite compound cannot be
obtained if Ca is added before the addition of (Ce, La, Nd, Pr).
[0052]
As described above, the present inventors newly found that, by appropriately
performing the deoxidation method using the multiple deoxidation with the addition of Al,
Si, (Ce, La, Nd, Pr), and Ca in the order in which they appear, it is possible to precipitate
the fine and hard spherical compound inclusions (REM oxysulfide compound inclusion)
as described above, and to suppress the deformation of the multiple-precipitated inclusions
even during rolling work. This enables the significant reduction in the number of the
elongated and coarsened MnS-based inclusions in the steel sheet, whereby it is possible to
obtain the effect of improving the bending workability or other properties. Further, with
the multiple deoxidation, the oxygen potential in the molten steel can be reduced, whereby
it is possible to reduce the unevenness in the components.
On the basis of the findings obtained from experiments, the present inventors
examined conditions for chemical components in the steel sheet in the following manner,
and designed the components in the steel sheet.
[0053]
Next, a description will be made of chemical components in the high-strength
steel sheet according to this embodiment exhibiting excellent stretch-flange formability
and bending workability.
[0054]
[C: 0.03% to 0.25%]
C is the most fundamental element that controls the hardenability and the
strength of the steel, and increases the hardness of and the depth of the quench hardening
layer, effectively contributing to improving the fatigue strength. In other words, C is an
essential element for securing the strength of the steel sheet, and C of at least 0.03% is
necessary to obtain the high-strength steel sheet. However, in the case where the amount
of C exceeds 0.25%, the workability and the weldability deteriorate. In order to obtain
the required strength while achieving the workability and the weldability, the
concentration of C is set to be not more than 0.25% in the high-strength steel sheet
according to this embodiment. Thus, the lower limit of C is set to 0.03%, preferably to
0.04%, more preferably to 0.06%. The upper limit of C is set to 0.25%, preferably to
0.20%, more preferably to 0.15%.
[0055]
[Si: 0.1% to 2.0%]
Si is a primary deoxidation element, which increases the number of nucleation
site of austenite during heating in the hardening, suppresses the grain growth in the
austenite, and reduces the grain diameter in the quench hardened layer. Si suppresses the
generation of carbides to prevent the reduction in the strength of the grain boundaries due
to the carbides, and is effective in generating a bainite structure. Thus, Si is an important
element to improve the strength without causing the deterioration in the elongation
property, and improve the hole-expandability with a low yield strength ratio. In order to
reduce the dissolved oxygen concentration in the molten steel, generate the Si02-based
inclusion once, and obtain the minimum value of the final dissolved oxygen through the
multiple deoxidation (this Si02-based inclusion is subjected to reduction with A1 added
later to form the alumina-based inclusion, and then, reduction with Ce, La, Nd, andor Pr
is applied to subject the alumina-based inclusion to reduction), it is necessary to add Si of
0.1% or more. For this reason, in the high-strength steel sheet according to this
embodiment, the lower limit of Si is set to 0.1%. In the case where the concentration of
Si is excessively high, toughness and ductility significantly deteriorate, and the
decarburization of the surface and the damage of the surface increase, resulting in
deteriorated bending workability. Further, in the case where Si is excessively added, Si
has an adverse effect on the weldability and the ductility. For these reasons, in the
high-strength steel sheet according to this embodiment, the upper limit of Si is set to 2.0%.
Accordingly, the lower limit of Si is set to 0.196, preferably to 0.2%, more preferably to
0.5%. The upper limit of Si is set to 2.0%, preferably to 1.8%, more preferably to 1.3%.
[0056]
[Mn: 0.5% to 3.0%]
Mn is an element useful for deoxidation in the steel-producing stage, and is an
element effective in enhancing the strength of the steel sheet as with C and Si. In order
to obtain such an effect, it is necessary to make the steel sheet contain Mn of 0.5% or
more. However, in the case where the amount of Mn contained exceeds 3.0%, Mn
segregates or the solid solution strengthening increases, reducing the ductility. Further,
the weldability and the toughness of the base material also deteriorate. For these reasons,
the upper limit of Mn is set to 3.0%. Thus, the lower limit of Mn is set to 0.5%,
preferably to 0.9%, more preferably to 1%. The upper limit of Mn is set to 3.0%,
preferably to 2.6%, more preferably to 2.3%.
100571
[P: 0.05% or less]
P is an element inevitably contained in the steel, and is effective in that P
hnctions as a substitutional solid-solution strengthening element having a size smaller
than Fe atom. However, in the case where the concentration of P exceeds 0.05%, P
segregates in the grain boundaries of austenite, and the strength of the grain boundaries
deteriorates, reducing the torsion fatigue strength and possibly causing deterioration in the
workability. Thus, the upper limit of P is set to 0.05%, preferably to 0.03%, more
preferably to 0.025%. If the solid solution strengthening is not required, P is not
necessary to be added, and hence, the lower limit value of P includes 0%.
[0058]
[T.O: 0.0050% or less]
T.0 forms oxide as an impurity. In the case where the amount of T.0 is
excessively high, the AlzOs-based inclusion increases, and the oxygen potential in the
steel cannot be made minimized. This leads to the significant deterioration in the
toughness and ductility, and an increase in the surface damage, resulting in the
deterioration in the bending workability. For these reasons, in the high-strength steel
sheet according to this embodiment, the upper limit of T.0 is set to 0.0050%, preferably to
0.0045%, more preferably to 0.0040%.
[0059]
[S: 0.0001% to 0.01%]
S segregates as an impurity, and combines with Mn to form a coarsened and
elongated MnS-based inclusion, which deteriorates the stretch-flange formability. Thus,
it is desirable to reduce the concentration of S as much as possible. By controlling the
formation of the coarsened and elongated MnS-based inclusion in the high-strength steel
sheet according to this embodiment, it is possible to obtain the material more than or
equivalent to the cost without requiring the desulhrization load in the secondary
refinement and without the need of the desulfurization cost, even if the steel sheet contains
a relatively high S concentration of approximately 0.01%. Thus, in the high-strength
steel sheet according to this embodiment, the concentration of S is set in the range of the
extremely low S concentration, which is a concentration obtained on the assumption that
desulfurization is performed in the secondary refinement, to the relatively high S
concentration, that is, the concentration of S is set in the range of 0.0001% to 0.01%.
[0060]
Further, in the high-strength steel sheet according to this embodiment, the
MnS-based inclusion is precipitated and dissolved in solid solution on the compound
inclusion formed by the fine and hard Ce oxide, La oxide, Nd oxide, Pr oxide, cerium
oxysulfide, lanthanum oxysulfide, neodymium oxysulfide, praseodymium oxysulfide, Ca
oxide and the like, and the formation of the MnS-based inclusion is controlled. This
makes the MnS-based inclusion less likely to deform during rolling work, and prevents the
elongation of the inclusion. Thus, the upper limit value of the concentration of S is set
on the basis of the relationship with the total amount of at least one element of Ce, La, Nd,
and Pr as described later. Further, in the case where the concentration of S exceeds
0.01%, the cerium oxysulfide and the lanthanum oxysulfide grow to be over 2 pm in size.
These coarsened oxysulfides make the toughness and the ductility significantly deteriorate,
leading to the increase in the surface damages and deteriorating the bending workability.
For these reasons, in the the high-strength steel sheet according to this embodiment, the
upper limit of S is set to 0.01%, preferably to 0.008%, more preferably to 0.006%.
[006 11
In other words, according to the high-strength steel sheet according to this
embodiment, the formation of MnS is controlled with the inclusions of the Ce oxide, the
La oxide, the cerium oxysulfide, the lanthanum oxysulfide, the neodymium oxysulfide,
and the praseodymium oxysulfide, or the Ca oxide or other elements as described above.
Thus, even if the concentration of S is relatively high but not more than 0.01%, by adding
the corresponding amount of at least one of Ce and La, it is possible to prevent the
occurrence of adverse effects on the material. In other words, even if the concentration
of S is relatively high, by adjusting the amount of Ce or La added so as to correspond to
the amount of S, it is possible to substantially obtain the desulfurization effect, and it is
possible to obtain a material equivalent to the ultra-low sulfur steel. This means that, by
appropriately adjusting the concentration of S in association with the total amount of Ce,
La, Nd and Pr, it is possible to increase the flexibility in the upper limit of the
concentration of S. Thus, the high-strength steel sheet according to this embodiment
does not require desulbrization of the molten steel in the secondary refinement to obtain
the ultra-low sulfur steel, and can omit the desulhrization process. This enables
simplification of the producing processes, and reduction in the cost required for the
accompanying desulhrization process.
[0062]
[N: 0.0005% to 0.01%]
N is captured from air during the steel-melting process, and hence, is an element
that is inevitably contained in the steel. N forms nitrides with A1 or other elements, and
promotes reduction in size of grains in the base material structure. However, in the case
where the amount of N contained exceeds 0.01%, N generates coarsened precipitates, for
example, with Al, deteriorating the stretch-flange formability. For this reason, in the
high-strength steel sheet according to this embodiment, the upper limit of the
concentration of N is set to 0.01%, preferably to 0.005%, more preferably to 0.004%. On
the other hand, the cost required for lowering the N concentration to less than 0.0005% is
high, and hence, the lower limit of the N concentration is set to 0.0005% from the
viewpoint of industrial feasibility.
[0063]
[Acid-soluble Al: over 0.01%]
In general, an oxide of acid-soluble A1 forms a cluster and is likely to coarsen,
which leads to the deterioration in the stretch-flange formability and the bending
workability. Thus, it is desirable to reduce acid-soluble A1 as much as possible.
However, according to the high-strength steel sheet according to this embodiment, a range
of amount of acid-soluble A1 was newly found, which enables obtaining the ultra-low
oxygen potential as described above while preventing clustering and coarsening of
alumina-based inclusion, by employing A1 deoxidation and the deoxidation effect obtained
by sequentially applying multiple deoxidation with Si, Ti, and at least one element of Ce,
La, Nd, and Pr, and adjusting the (Ce, La, Nd, Pr) concentration so as to correspond to the
concentration of acid-soluble Al. In this range, part of the A1203-based inclusions
generated through A1 deoxidation rise to the surface and are removed, whereas the rest of
the A1203-based inclusions remaining in the molten steel are subjected to reductive
decomposition with the Ce and La added later, and the clustered alumina-based oxide is
decomposed to form the fine inclusions.
[0064]
With this finding, according to the high-strength steel sheet according to this
embodiment, it is possible to eliminate the need for setting the limitation that A1 is
substantially not added in order to avoid the coarsened cluster of the alumina-based
inclusion as in the conventional art. In particular, it is possible to increase the flexibility
in the concentration of the acid-soluble Al. By setting the concentration of acid-soluble
A1 to more than 0.01%, it is possible to employ both A1 deoxidation and deoxidation with
addition of Ce and La, thereby eliminating the need for adding deoxidation element of Ce
and La more than necessary as in the conventional art. This makes it possible to solve
the problem of an increase in the oxygen potential in the steel due to deoxidation with Ce
and La. Further, it is possible to obtain the effect of reducing the variation in the
composition of the component elements. The lower limit of acid-soluble A1 is set
preferably to 0.01 3%, more preferably to 0.01 5%.
[0065]
The upper limit value of the acid-soluble A1 concentration can be set on the basis
of 70 2 100 x (Ce + La + Nd + Pr)/acid-soluble A1 > 0.7, which is expressed on the basis
of mass and is a relationship between the acid-soluble A1 and the total amount of at least
one element of Ce, La, Nd, and Pr as described later. However, the upper limit of the
acid-soluble A1 concentration may be set to 1% or less from the viewpoint of the cost
required for adding the alloy of Al, Ce, La, Nd, and Pr.
[0066]
In this specification, the term "acid-soluble A1 concentration" refers to a
measured concentration of A1 dissolved in acid, and this measurement employs a
characteristic in which dissolved A1 is dissolved in acid whereas A1203 is not dissolved in
acid. In this specification, the term "acid" refers, for example, to a mixed acid having
mass ratio of hydrochloric acid: 1, nitric acid: 1, and water: 2. By using such an acid, it
is possible to separate A1 soluble in the acid and A1203 non-soluble to the acid, whereby it
is possible to measure the acid-soluble A1 concentration.
[0067]
[Ca: 0.0005% to 0.0050%]
In the high-strength steel sheet according to this embodiment, Ca is an important
element, which controls the formation of desulfurization such as formation of spherical
sulfides, and also has an effect of causing at least one of MnS, CaS, and (Mn, Ca)S to be
precipitated and dissolved in solid solution in the oxide or oxysulfide obtained through
multiple precipitations to form a compound inclusion, thereby improving the
stretch-flange formability and the bending workability of the steel. In order to obtain
these effects, it is preferable to set the amount of Ca added to 0.0005% or more.
However, even if the amount of Ca contained is excessively high, the effect obtained from
the addition of Ca saturates, and Ca impairs cleanliness of the steel, deteriorating the
ductility of the steel. For these reasons, the upper limit of the amount of Ca is set to
0.0050%. The lower limit of Ca is set to 0.0005%, preferably to 0.0007%, more
preferably to 0.001%, whereas the upper limit of Ca is set to 0.0050%, preferably to
0.0045%, more preferably to 0.0035%.
[0068]
[Total of at least one element of Ce, La, Nd, and Pr: 0.001% to 0.01%]
Ce, La, Nd, and Pr have an effect of: reducing SiO2 generated through Si
deoxidation and A1203 generated sequentially through A1 deoxidation; separating A1203
clusters, which are likely to coarsen; and forming a hard and fine inclusion having a main
phase (target concentration of 50% or more) of Ce oxide (for example, Ce203 and CeOz),
cerium oxysulfide (for example, Ce202S), La oxide (for example, La203 and Lao2),
lanthanum oxysulfide (for example, La202S), Nd oxide (for example, Nd2O3), Pr oxide
(for example, Pr601 ,), Ce oxide-La oxide-Nd oxide-Pr oxide, or cerium
oxysulfide-lanthanum oxysulfide, which are likely to be a precipitation site for the
MnS-based inclusion and are less likely to deform during rolling. Note that it is
preferable to use Ce and La from among Ce, La, Nd and Pr.
[0069]
The above-described inclusion may partially contain MnO, SiO2, or A1203
depending on deoxidation conditions. However, this inclusion sufficiently functions as
the precipitation site for the MnS-based inclusion, and the effect of providing the fine and
hard inclusion is not impaired, provided that this inclusion has the main phase formed by
the oxides described above.
[0070]
Through experiments, it is found that, in order to obtain such an inclusion, it is
necessary to set the total concentration of at least one element of Ce, La, Nd, and Pr to be
not less than 0.001% and not more than 0.0 1 %.
[0071]
In the case where the total concentration of at least one element of Ce, La, Nd,
and Pr is less than 0.001%, Si02 and A1203 inclusions cannot be deoxidized. On the
other hand, in the case where the total amount exceeds 0.01%, at least one of cerium
oxysulfide, lanthanum oxysulfide, neodymium oxysulfide, and praseodymium oxysulfide
is excessively generated, and the generated oxysulfide forms coarsened inclusions,
deteriorating the stretch-flange formability and the bending workability. Note that the
preferable lower limit of the total concentration of at least one element of Ce, La, Nd, and
Pr is set to 0.0013%, and the more preferable lower limit thereof is set to 0.0015%. The
preferable upper limit of the total concentration of at least one element of Ce, La, Nd, and
Pr is set to 0.009%, and the more preferable upper limit is set to 0.008%.
COO721
As conditions for the existence of inclusions having a formation in which MnS is
precipitated in the oxide or oxysulfide formed by at least one element of Ce, La, Nd, and
Pr in the high-strength steel sheet according to this embodiment, the present inventors
focused on the fact that it is possible to determine the degree of improvement of MnS with
the oxide or oxysulfide formed by at least one of Ce, La, Nd, and Pr, by specifying the
degree of improvement using the concentration of S. Then, the present inventors reached
an idea of specifying and simplifLing the degree of improvement using a mass ratio of
chemical components (Ce + La + Nd + Pr)/S in the steel sheet. More specifically, in the
case where this mass ratio is low, the number of the oxide or oxysulfide formed by at least
one element of Ce, La, Nd, and Pr is small, and a large number of MnS is precipitated
alone. In the case where this mass ratio is high, the number of the oxide or oxysulfide
formed by at least one element of Ce, La, Nd, and Pr is higher as compared with that of
MnS, which leads to an increase in the number of inclusions having a formation in which
MnS is precipitated in the oxide or oxysulfide formed by at least one element of Ce, La,
Nd, and Pr. This means that MnS is improved with the oxide or oxysulfide formed by at
least one element of Ce, La, Nd, and Pr. In order to improve the stretch-flange
formability and the bending workability as described above, MnS is caused to precipitated
in the oxide or oxysulfide formed by at least one element of Ce, La, Nd, and Pr, which
leads to prevention of elongated MnS. For these reasons, the above-described mass ratio
can be used as a parameter to determine whether or not these effects can be obtained.
[0073]
In order to determine the chemical component ratio effective in suppressing the
elongation of the MnS-based inclusion, the mass ratio of (Ce + La + Nd + Pr)/S in the
steel sheet was varied to evaluate the formation of the inclusions, the stretch-flange
formability, and the bending workability. As a result, it was found that, by setting the
mass ratio of (Ce + La + Nd + Pr)/S to be in the range of 0.2 to 10, both the stretch-flange
formability and the bending workability significantly improve.
[0074]
In the case where the mass ratio of (Ce + La + Nd + Pr)/S is less than 0.2, the
ratio of the number of the compound inclusions having the formation in which MnS is
precipitated in the oxide or oxysulfide formed by at least one element of Ce, La, Nd, and
Pr is undesirably low. This correspondingly leads to the excessive increase in the ratio of
number of elongated MnS-based inclusions, which are likely to be the starting point of the
occurrence of cracking, deteriorating the stretch-flange formability and the bending
workability.
[0075]
In the case where the mass ratio of (Ce + La + Nd + Pr)/S exceeds 10, the effect
of precipitating MnS in the cerium oxysulfide and lanthanum oxysulfide to improve the
stretch-flange formability and the bending workability saturates, which is not worth the
cost. From these reasons, the mass ratio of (Ce + La + Nd + Pr)/S is set in the range of
0.2 to 10. In the case where the mass ratio of (Ce + La + Nd + Pr)/S is excessively high,
for example, is over 70, the at least one of the cerium oxysulfide, the lanthanum
oxysulfide, the neodymium oxysulfide, and the praseodymium oxysulfide is excessively
generated, and becomes coarsened inclusions, deteriorating the stretch-flange formability
and the bending workability. Thus, the upper limit of the mass ratio of (Ce + La + Nd +
Pr)/S is set to 10.
[0076]
Next, selective elements for the high-strength steel sheet according to this
embodiment will be described. These elements are selective elements, and hence, may
be added or may not be added. Further, it may be possible to add these elements either
alone or in combination of two or more types. In other words, the lower limit of these
selective elements may be set to 0%.
[0077]
For Nb and V
Nb and V form carbides, nitrides, or carbonitrides with C andlor N to facilitate
the reduction in size of grains in the base material structure, and contribute to improving
the toughness.
[0078]
Wb: 0.01% to 0.10%]
In order to obtain composite carbides and composite nitrides described above, it
is preferable to set the concentration of Nb to 0.01% or more, and it is more preferable to
set the concentration of Nb to 0.02% or more. However, in the case where the base
material contains the large amount of Nb in excess of the concentration of 0.10%, the
effect of providing the fine grain in the base material structure saturates, increasing the
producing cost. For these reasons, the upper limit of the concentration of Nb is set to
0.10%, preferably set to 0.09%, more preferably set to 0.08%.
[0079]
[V: 0.01% to 0.10%]
In order to obtain the above-described composite carbides, composite nitrides
and the like, it is preferable to set the concentration of V to 0.01% or more. However,
even if the large amount of V is contained in excess of the concentration of 0.10%, the
effect obtained from V contained saturates, increasing the producing cost. For this
reason, the upper limit of the concentration of V is set to 0.10%.
[OOSO]
For Cu, Ni, Cr, Mo, and B
Cu, Ni, Cr, Mo, and B enhance the strength, and improves the hardenability of
the steel.
[008 11
[Cu: 0.1% to 2%]
Cu contributes to improving the precipitation hardening and the fatigue strength
of ferrite, and may be added depending on applications to fbrther enhance the strength of
the steel sheet. In order to obtain this effect, it is preferable to add Cu of 0.1 % or more.
However, the excessively large amount of Cu contained deteriorates the balance of
strength-ductility. Thus, the upper limit of Cu is set to 2%, preferably to 1.8%, more
preferably to 1.5%.
[0082]
[Ni: 0.05% to 1%]
Ni can be used for solid solution strengthening of ferrite, and may be added
depending on applications to further enhance the strength of the steel sheet. In order to
obtain this effect, it is preferable to add Ni of 0.05% or more. However, the excessively
large amount of Ni contained deteriorates the balance of strength-ductility. Thus, the
upper limit of Ni is set to 1%, preferably to 0.09%, more preferably to 0.08%.
[0083]
[Cr: 0.01% to I%]
Cr may be added depending on applications to further enhance the strength of
the steel sheet. In order to obtain this effect, it is preferable to add Cr of 0.01% or more,
and it is more preferable to add Cr of 0.02% or more. However, the excessively large
amount of Cr contained deteriorates the balance of strength-ductility. Thus, the upper
limit of Cr is set to 1%, preferably to 0.9%, more preferably to 0.8%.
[0084]
[Mo: 0.01% to 0.4%]
Mo may be added depending on applications to further enhance the strength of
the steel sheet. In order to obtain this effect, it is preferable to add Mo of 0.01% or more,
and it is more preferable to add Mo of 0.05% or more. However, the excessively large
amount of Mo contained deteriorates the balance of strength-ductility. Thus, the upper
limit of Mo is set to 0.4%, preferably to 0.3%, more preferably to 0.2%.
[0085]
[B: 0.0003% to 0.005%]
B may be added depending on applications to further enhance the strength of the
grain boundaries to improve the workability. In order to obtain this effect, it is preferable
to add B of 0.0003% or more, and it is more preferable to add B of 0.0005% or more.
However, in the case where the amount of B contained exceeds 0.005%, the effect
obtained from B saturates, and the cleanliness of the steel is impaired, deteriorating the
ductility. Thus, the upper limit of B is set to 0.005%.
[0086]
For Zr
Zr may be added depending on applications to strengthen the grain boundaries
and improve the workability with the control of sulfide formation.
[0087]
[Zr: 0.001% to 0.01%]
In order to obtain the effect of forming spherical sulfides as described above to
improve the toughness of the base material, it is preferable to add Zr of 0.001% or more.
However, the excessively large amount of Zr contained impairs the cleanliness of the steel,
which leads to the deterioration in the ductility. Thus, the upper limit of Zr is set to
0.01%, preferably to 0.009%, more preferably to 0.008%.
[OOSS]
Next, a description will be made of conditions for the existence of inclusions in
the high-strength steel sheet according to this embodiment. In this specification, the term
"steel sheet" means a rolled sheet obtained through hot rolling, or through hot rolling and
cold rolling. Further, the conditions for the existence of inclusions in the high-strength
steel sheet according to this embodiment are set from various viewpoints.
[0089]
In order to obtain the steel sheet exhibiting excellent stretch-flange formability
and bending workability, it is important to minimize the number of elongated and
coarsened MnS-based inclusions in the steel sheet, which are likely to be the starting point
of the occurrence of cracking or the pathway of crack propagation.
[0090]
In this regard, the present inventors found that, as with steel sheets produced
with little deoxidation with Al, it is possible to obtain a steel sheet exhibiting excellent
stretch-flange formability and bending workability, by adding Si to a steel sheet,
subjecting the steel sheet to the deoxidation with Al, then, adding at least one element of
Ce, La, Nd, and Pr, further adding Ca for deoxidation in a manner described above, and
adjusting the ratio (Ce + La + Nd + Pr)/acid-soluble A1 and the ratio of (Ce + La + Nd +
Pr)/S on the basis of mass so as to be those described above, to sharply decrease the
oxygen potential in the molten steel through the multiple deoxidation, subject A1203
generated through A1 deoxidation to reduction, and separate A1203 cluster, which is likely
to coarsen.
[009 11
Further, it was also found that, through deoxidation with addition of Ce, La, Nd,
andlor Pr, and addition of Ca thereafter, although a slight amount of AI2O3 remains, it was
possible to in most parts generate fine and hard Ce oxide, La oxide, Nd oxide, Pr oxide,
cerium oxysulfide, lanthanum oxysulfide, neodymium oxysulfide, praseodymium
oxysulfide, and Ca oxide or Ca oxysulfide, dissolve the generated oxides and oxysulfide in
solid solution, obtain MnS precipitated and dissolved in solid solution, and form a
compound inclusion containing inclusion phases each having a different component.
The obtained compound inclusion is less likely to deform even during rolling work,
whereby the number of the elongated and coarsened MnS can be significantly reduced in
the steel sheet.
[0092]
Further, it was found that, by obtaining, on the basis of mass, the ratio of (Ce +
La + Nd + Pr)/acid-soluble A1 and the ratio of (Ce + La + Nd + Pr)/S as described above,
the number density of fine inclusions having an equivalent circle diameter of 2 pm or less
significantly increases, and the fine inclusions are dispersed in the steel.
[0093]
These fine inclusions are less likely to aggregate, and hence, most of them
remain in the spherical shape or spindle shape. These inclusions have a major axis/minor
axis (hereinafter, also referred to as "elongated ratio") of 3 or less, preferably 2 or less.
In the present invention, these inclusions are referred to as a spherical inclusion.
[0094]
In terms of experiment, the inclusions can be identified easily through
observation using a scanning electron microscope (SEM), and focus was placed on the
number density of inclusions having an equivalent circle diameter of 5 pm or less. Note
that, although the lower limit value for the equivalent circle diameter is not particularly set,
it is preferable to set a target of the observation at the inclusions having approximately 0.5
pm or more, the size of which can be counted and expressed in number. In this
specification, the term "equivalent circle diameter" refers to a value obtained through
(major axis x minor axis)0.5 on the basis of the major axis and the minor axis of the
inclusion with cross-section observation.
[0095]
It is considered that the fine inclusions having a size of 5 pm or less are
dispersed because of the synergistic effect of: the reduced oxygen potential in the molten
steel due to A1 deoxidation; the oxide or oxysulfide formed by at least one element of Ce,
La, Nd, and Pr in which oxide containing at least one element of Si, Al, and Ca is
precipitated and dissolved in solid solution; and the fine compound inclusions formed by
oxide and/or oxysulfide having at least one of MnS, CaS, and (Mn, Ca)S precipitated and
dissolved in solid solution therein.
[0096]
The generated compound inclusions are formed by inclusion phases that have
different components and include an inclusion phase containing at least one element of Ce,
La, Nd, and Pr, further containing Ca, and containing at least one element of 0 and S
(hereinafter, also referred to as a first group of [Ce, La, Nd, Pr]-Ca-[0, S]) and an
inclusion phase further containing at least one element of Mn, Si, and A1 (hereinafter, also
referred to as a second group [Ce, La, Nd, Pr]-Ca-[0, S]-[Mn, Si, All). It is considered
that these compound inclusions form a large number of spherical compound inclusions
having an equivalent circle diameter in the range of 0.5 pn to 5 pn, and these spherical
compound inclusions are less likely to be a starting point of the occurrence of cracking or
pathway of crack propagation, and contribute to relaxation of stress concentration because
of its fine structure, which leads to improvement in the stretch-flange formability and the
bending workability.
[0097]
The present inventors checked whether the elongated and coarsened MnS-based
inclusions, which are likely to be the starting point of the occurrence of cracking or
pathway of crack propagation, are reduced in the steel sheet.
The present inventors experimentally knew that, in the case where the equivalent
circle diameter is less than 1 pm, the elongated MnS does not have any adverse effect in
terms of the starting point of the occurrence of cracking, and does not deteriorate the
stretch-flange formability or bending workability. Further, the inclusions having an
equivalent circle diameter of 1 pm or more can be easily observed with the scanning
electron microscope (SEM) or other devices. For these reasons, by targeting the
observation at the inclusions having the equivalent circle diameter of 1 pm or more in the
steel sheet, their formations and compositions were examined to evaluate the distribution
state of the elongated MnS.
[0099]
It should be noted that, although the upper limit of the equivalent circle diameter
of MnS is not particularly set, MnS having a size of approximately 1 mm may be observed
in practical.
[O 1001
The ratio of the number of the elongated inclusions was measured through
composition analysis on plural pieces (for example, 50 pieces) of inclusions having the
equivalent circle diameter of 1 pn or more and randomly selected using a SEM, and
through measurement of the major axis and the minor axis of the inclusions using a SEM
image. In this specification, the elongated inclusion represents an inclusion having a
major axislminor axis (elongated ratio) of over 3. Further, the ratio of the number of the
elongated inclusions can be obtained by dividing the number of the detected elongated
inclusions by the total number of inclusions analyzed (50 in the case of the
above-described example).
[OlOl]
The reason that the elongated ratio is set to 3 or less is because the inclusions
having the elongated ratio of over 3 in the comparative steel sheet without having the Ce,
La, Nd or Pr added therein were formed mostly by inclusions having, as a core, the oxide
or oxysulfide made of Ce, La, Nd, and Pr through addition of MnS, Ce, La, Nd, or Pr and
having MnS precipitated around the core, the Ca0-A1203-based inclusion having a low
melting point, and the coarsened and elongated CaS. Note that, although the upper limit
of the elongated ratio of MnS is not particularly set, MnS having the elongated ratio of
approximately 50 may be observed in practice.
[O 1 021
As a result, it was found that the stretch-flange formability and the bending
workability were improved in the steel sheet having the controlled formation in which the
ratio of the number of the elongated inclusions having an elongated ratio of 3 or less is
controlled to be 50% or more. More specifically, in the case where the ratio of the
number of the elongated inclusions having the elongated ratio of 3 or less is 50% or more,
there are excessive increases in the ratio of number of MnS, which is likely to be the
starting point of the occurrence of cracking, the ratio of the number of the inclusions
having a core made of oxide or oxysulfide of Ce and La through addition of Ce and La
and having MnS precipitated around the core, the ratio of the number of the
Ca0-AI2O3-based inclusion having the low melting point, and the ratio of the number of
the coarsened and elongated CaS, which leads to the deterioration in the stretch-flange
formability and the bending workability. For these reasons, in the high-strength steel
sheet according to this embodiment, the ratio of the number of the elongated inclusions
having the elongated ratio of 3 or less is set to 50% or more.
[0 1031
The stretch-flange formability and the bending workability become more
favorable with decrease in the number of the elongated MnS-based inclusions. Thus, the
lower limit value of the ratio of the number of the elongated inclusions having the
elongated ratio of over 3 includes 0%. In this specification, the state in which an
inclusion has an equivalent circle diameter of 1 pm or more and the lower limit value of
the ratio of number of an elongated inclusion having the elongated ratio of over 3 is 0%
means that there exists an inclusion having the equivalent circle diameter of 1 pm or more
but there exists no inclusion having the elongated ratio of over 3, or the inclusion is an
elongated inclusion having the elongated ratio of over 3 but the equivalent circle diameters
of all the inclusions are less than 1 pm.
[0 1 041
Further, it was confirmed that the maximum equivalent circle diameter of the
elongated inclusion is smaller as compared with the average grain diameter of crystals in
the structure. This also contributes to a significant improvement in the stretch-flange
formability and the bending workability.
[0 1 051
In the case where a steel sheet has the controlled formation in which the mass
ratio of (Ce + La + Nd + Pr)/S is in the range of 0.2 to 10, and the ratio of the number of
the elongated inclusions having the elongated ratio of 3 or less is 50% or more, the steel
sheet correspondingly has a compound inclusion formed by inclusion phases having
different components and including an inclusion phase (first group of [Ce, La, Nd,
Pr]-Ca-[0, S]) containing at least one element of Ce, La, Nd, and Pr, further containing Ca,
and further containing at least one of 0 and S, and an inclusion phase (second group of
[Ce, La, Nd, Pr]-Ca-[0, S]-[Mn, Si, All) further containing at least one element of Mn, Si,
and Al, and in many cases, this compound inclusion forms a large number of spherical
compound inclusions having an equivalent circle diameter in the range of 0.5 pm to 5 pm.
[O 1061
Further, the spherical compound inclusion having the equivalent circle diameter
in the range of 0.5 pm to 5 pm is a hard inclusion having the high melting point, and is
less likely to deform during rolling. Thus, this spherical compound inclusion remains in
the non-elongated shape in the steel sheet, in other words, is a spherical or spindle-shaped
(also referred to as spherical) inclusion.
[0 1071
In this specification, although not particularly defined, a spherical inclusion
determined to be not elongated represents an inclusion having the elongated ratio of 3 or
less, preferably of 2 or less in the steel sheet. This is because the inclusion in the ingot
stage before rolling was formed by the compound inclusion having a different component
and including an inclusion phase of the first group of [Ce, La, Nd, Pr]-Ca-[0, S], and an
inclusion phase of the second group of [Ce, La, Nd, Pr]-Ca-[0, S]-[Mn, Si, All, was
formed by a spherical compound inclusion having an equivalent circle diameter in the
range of 0.5 pm to 5 pm, and had the elongated ratio of 3 or less. Further, if the spherical
inclusion determined to be not elongated has a completely spherical shape, the elongated
ratio is 1, and hence, the lower limit of the elongated ratio is 1.
[O 1081
The ratio of number of this inclusion was investigated in a similar manner to that
made on the ratio of the number of the elongated inclusions. As a result, it was found
that the stretch-flange formability and the bending workability improve, according to the
steel sheet having a compound inclusion formed by inclusion phases having a different
component and including an inclusion phase of the first group ([Ce, La, Nd, Pr]-Ca-[0,
S]) containing at least one element of Ce, La, Nd, and Pr, further containing Ca, and
further containing at least one element of 0 and S, and an inclusion phase of the second
group ([Ce, La, Nd, Pr]-Ca-[0, S]-[Mn, Si, All) further containing at least one element of
Mn, Si, and Al, in which the steel sheet has a formation controlled such that this
compound inclusion forms a spherical compound inclusion having an equivalent circle
diameter in the range of 0.5 pm to 5 pm, and the ratio of the number of the spherical
compound inclusion relative to the total number of inclusions having an equivalent circle
diameter in the range of 0.5 pm to 5 pm is 30% or more.
[0 1091
In the case where this ratio of number is less than 30%, it is not favorable
because the ratio of the number of the elongated inclusions of MnS correspondingly
excessively increases, deteriorating the the stretch-flange formability and the bending
workability.
[Ol lo]
For these reasons, the ratio of the number of the spherical compound inclusions
having the equivalent circle diameter in the range of 0.5 pm to 5 pm is set to 30% or more.
In this specification, the ratio of number is measured from the SEM image on the basis of
the major axis and the minor axis of 50 pieces of the elongated inclusions randomly
selected using the SEM. Then, the number of the elongated inclusions having the major
axislminor axis (elongated ratio) of 3 or less is divided by the number of all the inclusions
investigated (50 pieces), thereby obtaining the ratio of the number of the elongated
inclusions.
[OI 1 I]
With the increase in the number of spherical compound inclusions having the
equivalent circle diameter in the range of 0.5 pm to 5 pm, the stretch-flange formability
and the bending workability can be more preferably obtained. Thus, the upper limit of
the ratio of number includes 100%.
[OI 121
It should be noted that the spherical compound inclusions having the equivalent
circle diameter in the range of 0.5 pm to 5 pm are less likely to deform even during rolling.
Thus, the equivalent circle diameter is not particularly set, and it may be possible to set the
equivalent circle diameter to 1 pm or more. However, if the inclusions have the
excessively large diameter, the inclusions possibly serve as the starting point of the
occurrence of cracking. Thus, the upper limit of the equivalent circle diameter is set
preferably to above 5 p.m.
[OI 131
On the other hand, these compound inclusions are less likely to deform even
during rolling, and do not serve as the starting point of the occurrence of cracking in the
case where the equivalent circle diameter is less than 0.5 pm. Thus, the lower limit of
the equivalent circle diameter is not particularly set.
[OI 141
Next, the condition for the existence of the compound inclusions in the
high-strength steel sheet according to this embodiment described above is set using
number density of the inclusion per unit volume.
101 151
The distribution of grain diameter of inclusions was obtained through a SEM
evaluation on an electrolyzed surface using a speed method. The SEM evaluation on the
electrolyzed surface using the speed method was performed such that: a surface of a test
piece was polished, and was subjected to electrolyzation using the speed method; and the
surface of the test piece was directly observed with the SEM observation, thereby
evaluating the size or number density of the inclusion. Note that the speed method
represents a method of electrolyzing the surface of the test piece using 10% acetyl
acetone-1% tetramethyl ammonium chloride-methanol, and extracting the inclusion. As
for the amount of electrolysis, electrolyzation was performed until the amount of
electrolysis of the surface of the test piece per lcm2 area reached 1C. The SEM image of
the surface electrolyzed as described above was subjected to image processing, thereby
obtaining a frequency (number of pieces) distribution in terms of equivalent circle
diameter. On the basis of the frequency distribution of the grain diameter, the average
equivalent circle diameter was obtained. Further, the number density of inclusions per
unit volume was calculated by dividing the frequency by the area of the observed view
and the depth obtained from the amount of electrolysis.
[0116]
On the other hand, for the high-strength steel sheet according to this embodiment
described above, the condition for the existence of the spherical compound inclusions
having the equivalent circle diameter in the range of 0.5 pm to 5 pm and formed by
inclusion phases having a different component and including an inclusion phase of the
first group of [Ce, La, Nd, Pr]-Ca-[0, S] and an inclusion phase of the second group of
[Ce, La, Nd, Prl-Ca-[0, S]-[Mn, Si, All is set using the amount of average composition of
Ce, La, Nd or Pr contained in the inclusions.
[0117]
More specifically, as described above, in order to improve the stretch-flange
formability and the bending workability, it is important for the compound inclusions to
exist as the spherical compound inclusions having the equivalent circle diameter in the
range of 0.5 pm to 5 pm and prevent coarsening of the MnS-based inclusions.
These compound inclusions are spherical compound inclusions or
spindle-shaped inclusions having the equivalent circle diameter in the range of 0.5 pm to 5
CLm.
[0119]
Although not particularly set, the spindle-shaped inclusions are inclusions having
an elongated ratio of 3 or less, preferably of 2 or less in the steel sheet. If the inclusions
have a completely spherical shape, the elongated ratio is 1, and hence, the lower limit of
the elongated ratio is 1.
[O 1201
In order to determine a composition effective in suppressing the elongation and
improving the stretch-flange formability and the bending workability, composition
analysis of the compound inclusions was performed.
[0121]
Since the observation becomes easy if the equivalent circle diameter of the
inclusions is 1 pm or more, the target of the observation was set at the inclusion having the
equivalent circle diameter of 1 pm or more for the convenience purpose. However, if the
observation is possible, it may be possible to include the inclusions having the equivalent
circle diameter of less than 1 pm.
[O 1221
Further, since the compound inclusions described above were not elongated, it
was confirmed that all the compound inclusions had the elongated ratio of 3 or less.
Thus, composite analysis was performed for the inclusions having the equivalent circle
diameter of 1 pm or more and the elongated ratio of 3 or less.
[0 1231
As a result, it was found that the inclusions having the equivalent circle diameter
of 1 pm or more and the elongated ratio of 3 or less are formed by compound inclusions
having a formation of components in which there are provided two or more inclusion
phases each having different components and including an inclusion phase of a first group
having a component in which at least one element of Ce, La, Nd, and Pr is contained, Ca
is contained, and at least one element of 0 and S is contained, and an inclusion phase of a
second group having a component in which at least one element of Mn, Si, and Al is
hrther contained, as illustrated in FIG. 3A and FIG. 3B. Further, it was found that the
stretch-flange formability and the bending workability can be improved, by forming the
compound inclusions so as to contain the total amount of at least one element of Ce, La,
Nd, and Pr in the range of 0.5% to 95% in average composition.
[0 1 241
In the case where the average amount of the total of the at least one element of
Ce, La, Nd, and Pr contained is less than 0.5 mass % in the inclusion having the equivalent
circle diameter of 1 pn or more and the elongated ratio of 3 or less, the ratio of the
number of the inclusions having the formation described above largely decreases, while
the ratio of the number of the MnS-based elongated inclusions, which are likely to be the
starting point of the occurrence of cracking, excessively increases correspondingly. Thus,
the stretch-flange formability and the bending workability deteriorate.
[0 1251
On the other hand, in the case where the average amount of the total of the at
least one element of Ce, La, Nd, and Pr contained exceeds 95% in the inclusions having
the equivalent circle diameter of 1 pm or more and the elongated ratio of 3 or less, the
cerium oxysulfide and the lanthanum oxysulfide are largely generated, which leads to
coarsened inclusions having the equivalent circle diameter of approximately 50 pm or
more. Thus, the stretch-flange formability and the bending workability deteriorate.
[0 1261
Next, the structure of the steel sheet will be described.
[0 1271
According to the high-strength steel sheet according to this embodiment, the fine
MnS-based inclusions are precipitated in the ingot, and are dispersed in the steel sheet as
the fine spherical inclusions, which do not deform during rolling and are less likely to be
the starting point of the occurrence of cracking, thereby improving the stretch-flange
formability and the bending workability. Thus, the micro-structure of the steel sheet is
not particularly limited.
[0128]
Although the micro-structure of the steel sheet is not particularly limited, it may
be possible to employ any structure from among a steel sheet having a structure of a phase
formed mainly by bainitic ferrite, a composite-structure steel sheet having a main phase of
a ferrite phase and a second phase of a martensite phase and a bainite phase, and a
composite-structure steel sheet formed by ferrite, retained austenite and a low-temperature
transformation phase (formed by martensite or bainite).
[0 1291
Thus, any of the structures described above are favorable because it is possible to
reduce the crystal grain diameter to 10 pm or less, and the hole-expandability and the
bending workability can be improved. In the case where the average grain diameter
exceeds 10 pm, the degree of improvement in the ductility and the bending workability
reduces. In order to improve the hole-expandability and the bending workability, it is
more preferable to set the crystal grain diameter to 8 pm or less. However, in general, in
the case where excellent stretch-flange formability is required, for example, in the case of
application for underbody components, it is desirable and preferable that the ferrite or
bainite phase be the maximum area-ratio phase, although the ductility is slightly lower.
[0 1 301
Next, producing conditions will be described.
[0131]
According to a method of producing molten steel for the high-strength steel sheet
I according to this embodiment, alloys such as C, Si, and Mn are further added to the
molten steel decarbonized by blowing in a converter or by further using a vacuum
degassing device, and the molten steel is agitated, thereby performing deoxidation and
component adjustment.
As for S, desulfurization may not be performed in the refinement process as
described above, and thus, the desulfurization process can be omitted. However, in the
case where desulfurization of the molten steel is necessary in the secondary refinement to
produce the ultra-low sulfur steel with approximately S 5 20ppm, it may be possible to
perform the component adjustment through desulfurization.
[0133]
It is preferable that, after the elapse of approximately 3 minutes from the
addition of Si described above, A1 be added to perform A1 deoxidation, and then, the
rising time of approximately 3 minutes be set so as to allow A1203 to rise to the surface
and be separated.
[0 1 341
Thereafter, at least one element of Ce, La, Nd, and Pr is added, and components
are adjusted so as to satisfy 70 2 100 x (Ce + La + Nd + Pr)/acid-soluble A1 2 2, and (Ce +
La + Nd + Pr)/S being in the range of 0.2 to 10 on the basis of mass.
[0135]
In the case where a selective element is added, the selective element is added
before the addition of the at least one element of Ce, La, Nd, and Pr, agitation is
sufficiently performed, and the at least one element of Ce, La, Nd, and Pr is added.
Depending on applications, the at least one element of Ce, La, Nd, and Pr may be added
after the component adjustment of the selective element. Then, agitation is sufficiently
performed, and Ca is added. The thus obtained molten steel is subjected to continuous
casting to produce an ingot.
[0136]
The continuous casting not only includes an ordinal slab continuous casting
having a thickness of approximately 250 mm, but also includes a bloom, a billet, and thin
slab continuous casting having a thinner die-thickness than that of ordinal slab
continuous-casting devices, for example, a thickness of 150 mm or less.
[0137]
Hot rolling conditions for producing the high-strength hot-rolled steel sheet will
be described.
[0138]
Since carbonitrides or other inclusions in the steel need to be once dissolved in
solid solution, it is important to set a heating temperature for a slab before hot rolling to
over 1200°C.
[0139]
By making the carbonitrides dissolved in solid solution, it is possible to obtain a
ferrite phase, which is favorable to improve the ductility in the cooling process after the
rolling. On the other hand, in the case where the heating temperature for the slab before
the hot rolling exceeds 1250°C, the surface of the slab is significantly oxidized. In
I particular, wedge-shaped surface defects appear after descaling due to selective oxidation
of the grain boundaries, deteriorating quality of the surface after the rolling. Thus, it is
preferable to set the upper limit of the heating temperature to 1250°C.
[0 1401
After being heated to temperatures in the range described above, the slab is
subjected to the normal hot rolling. In this hot rolling process, the temperature at the
time of completion of the finishing rolling is important to control the structure of the steel
sheet. In the case where the temperature at the time of completion of the finishing rolling
is less than Ar3 point + 30°C, the diameter of the crystal grain in the surface layer portion
is likely to coarsen, which is not favorable in terms of bending workability. On the other
hand, in the case where this temperature exceeds the Ar3 point + 200°C, the diameter of
the austenite grain after the completion of the rolling coarsens, which makes it difficult to
control the structure and the ratio of the phase generated during cooling. Thus, the upper
limit of the temperature is set preferably to the Ar3 point + 200°C.
[0141]
Further, depending on the targeted structure configuration, the condition for the
hot rolling is selected from among a condition in which an average cooling rate for the
steel sheet after the finishing rolling is set in the range of 10°C/sec to 100°C/sec, and the
coiling temperature is set in the range of 450°C to 650°C, and a condition in which the
steel sheet is air cooled at approximately S°C/sec until the temperature reaches 680°C after
the finishing rolling, and is cooled thereafter at the cooling rate of 30°C/sec or more, and
the coiling temperature is set to 400°C or less. By controlling the cooling rate and the
coiling temperature after the rolling, it is possible to obtain a steel sheet having one or
more structures of polygonal ferrite, bainitic ferrite, and a bainite phase, and the
corresponding ratio under the former rolling condition, and a DP steel sheet having a
compound structure including the large amount of polygonal ferrite phase, which are
excellent in ductility, and the martensite phase under the latter rolling condition.
[0 1421
In the case where the average cooling rate described above is less than 10°C/sec,
pearlite, which is not favorable in terms of the stretch-flange formability, is likely to be
generated, which is not preferable. Although setting of the upper limit of the cooling rate
is not necessary from viewpoint of controlling of the structure, the excessively high
cooling rate possibly causes the cooling state of the steel sheet to be nonuniform. Further,
a large amount of cost is required to manufacture equipment that can provide such a high
cooling rate, which leads to increase in prices of the steel sheet. In view of the facts
described above, it is preferable to set the upper limit of the cooling rate to 100°C/sec.
[0 1431
The high-strength cold-rolled steel sheet according to the present invention is
produced by subjecting a steel sheet to hot rolling, coiling, pickling, and skin pass, then
cold rolling the steel sheet, and applying annealing to the steel sheet. In the annealing
processes, batch annealing, continuous annealing or other processes are applied, thereby
obtaining the final cold-rolled steel sheet.
[0 1441
It is needless to say that the high-strength steel sheet according to the present
invention may be used as a steel sheet for electroplating. Application of electroplating
does not change the mechanical properties of the high-strength steel sheet according to the
present invention.
[0 1451
[Second Embodiment]
The present inventors made a study of a method of precipitating fine MnS
inclusion in the cast slab, and dispersing the fine MnS inclusion in the steel sheet as a fine
spherical inclusion that does not deform during rolling and is less likely to be the starting
point of the occurrence of cracking, thereby improving the stretch-flange formability and
the bending workability, and of additional elements that do not deteriorate the fatigue
characteristics.
[0 1461
As a result, it was found that an elongated MnS and coarsened inclusions, which
have an adverse effect on the hole expandability, was significantly reduced in the steel
sheet, and the coarsened inclusions and the MnS-based inclusions are less likely to be the
starting point of the occurrence of cracking or pathway of crack propagation during
repetitive deformation, hole expanding work, and bending work, which leads to an
improvement in the hole-expandability or other properties, by forming a spherical
compound inclusion having an equivalent circle diameter in the range of 0.5 pm to 5 pm
and containing different inclusion phases including a first inclusion phase containing at
least one element of Ce, La, Nd, and Pr, further containing Ca, and further containing at
least one element of 0 and S, and a second inclusion phase further containing at least one
element of Mn, Si, Ti, and Al, as illustrated in FIG 8A and FIG 8B, and controlling the
inclusions such that the ratio of the number of the spherical inclusions is 50% or more, and
number density of inclusions having a size of over 5 pn is less than 10 pieces/mm2.
[0 1471
Further, the present inventors also made a study of sequentially performing
multiple deoxidation with Si, Mn, Al, (Ce, La, Nd, Pr), and Ca to make precipitates fine
oxide or MnS-based inclusions, and remove sulfur to the low sulfur level so as to reliably
fix the residual sulhr to be a fine and hard inclusion. As a result, it was found that, for
molten steel obtained through deoxidation with Si, deoxidation with Ti and Al,
deoxidation with addition of at least one element of Ce, La, Nd, and Pr, and then addition
of Ca, by obtaining predetermined (Ce + La + Nd + Pr)/acid-soluble Al, and (Ce + La +
Nd + Pr)/S on the basis of mass and adding Ca at the end, the oxygen potential in the
molten steel can be reduced, under this reduced oxygen potential, much finer TiS-based
inclusion can be obtained, whereby the residual sulfur can be reliably fixed to be the fine
and hard inclusions. Further it is also found that, with this setting, the stretch-flange
formability and the bending workability significantly improve.
[0148]
It should be noted that, in some observations, TiN is precipitated alone or
multiply precipitated on a compound inclusion containing different inclusion phases
including a first inclusion phase containing at least one element of Ce, La, Nd, and Pr,
further containing Ca, and further containing at least one element of 0 and S, and a second
inclusion phase further containing at least one element of Mn, Si, Ti, and Al. However, it
was confirmed that, since the precipitates were fine precipitates, these precipitates little
affect the stretch-flange formability, the bending workability, and the fatigue
characteristics. Thus, TiN is not considered to be the MnS-based inclusion to which the
high-strength steel sheet according to this embodiment is directed. Further, it was found
that, by adding Ti to increase acid-soluble Ti in the steel, a pinning effect resulting from
solute Ti or carbonitride Ti can be obtained, whereby it is possible to reduce the size of the
crystal grain to the fine crystal grain. Since TiN has little effect on the stretch-flange
formability and the bending workability, TiN is not the target of the MnS-based inclusion.
[0 1491
Next, a detailed description will be made of the high-strength steel sheet
exhibiting excellent stretch-flange formability and bending workability as a second
embodiment of the present invention. Below, the unit "mass %" used for the
composition is expressed simply as "%." Note that the high-strength steel sheet of the
present invention includes a steel sheet subjected to normal hot rolled or cold rolled and
used as it is without applying further treatment thereto, and a steel sheet used after
application of surface treatment such as plating and coating.
[0150]
Next, experiments concerning the second embodiment according to the present
invention will be described.
[0151]
The present inventors produced a steel ingot by subjecting molten steel
containing C: 0.06%, Si: 1.0%, Mn: 1.4%, P: 0.01% or less, S: 0.005%, and N: 0.003%
with a balance including Fe to deoxidation using various elements. The obtained steel
ingot is hot rolled to form a hot-rolled steel sheet with 3 mm. For the obtained hot-rolled
steel sheet, a tensile test, a hole-expanding test, and a bending test were performed, and
examination was made on number density of inclusions, formation and average
composition in the steel sheet.
[0 1521
Further, examination was made on the stretch-flange formability and the bending
workability of a steel sheet produced, by first adding Si to molten steel, subjecting the
molten steel to deoxidation with Al, agitating the molten steel for approximately 2 minutes,
adding Ti, agitating the molten steel for approximately 2 minutes, and adding at least one
element of Ce, La, Nd, and Pr, and deoxidizing with Ca. As a result, with the steel sheet
subjected to the sequential five-step deoxidation with Si, Al, Ti, at least one element of Ce,
La, Nd, and Pr, and Ca as described above, it is confirmed that the stretch-flange
formability and the bending workability can be further improved.
[0153]
It is considered that this is because Al oxide, Ti oxide or AI-Ti compound oxide
generated through deoxidation with Al and Ti and partially containing Mn or Si is changed
through deoxidation with addition of at least one element of Ce, La, Nd, and Pr to form a
(Ce, La, Nd, Pr)-(0) inclusion and a (Mn, Si, Ti, A1)-(Ce, La, Nd, Pr)-(0) inclusion. The
formed inclusions absorb S to form a (Ce, La, Nd, Pr)-(0, S) inclusion and a (Mn, Si, Ti,
A1)-(Ce, La, Nd, Pr)-(0, S). These inclusions are subjected to reduction through
deoxidation with Ca, which causes all the inclusion phases to contain Ca to form a (Ce, La,
Nd, Pr)-(0, S)-(Ca) inclusion phase (hereinafter, also referred to as a first inclusion phase
of [REMI-[Cal-[O,S] or simply as a first inclusion phase) and a (Mn, Si, Ti, A1)-(Ce, La,
Nd, Pr)-(0, S)-(Ca) inclusion phase (hereinafter, also referred to as a second inclusion
phase of [Mn, Si, Ti, All-[REMI-[Cal-[O,S] or simply as a second inclusion phase), so that
these inclusions are combined, or precipitated as an inclusion phase to form the compound
inclusion having different inclusion phases.
[0 1541
FIG. 8A and FIG 8B illustrate examples of the generated compound inclusion.
It should be noted that, in the expression of the (Mn, Si, Ti, A1)-(Ce, La, Nd,
Pr)-(0, S)-(Ca) inclusion phase, the expression (Mn, Si, Ti, Al) represents containing at
least one element of Mn, Si, Ti, and Al, the expression (Ce, La, Nd, Pr) represents
containing at least one element of Ce, La, Nd, and Pr, the expression (0, S) represents
containing at least one element of 0 and S, and the expression (Ca) represents containing
a Ca element.
[0155]
These compound inclusions are subjected to deoxidation with Ca at the last stage,
which has the most strongest deoxidation effect of all the elements in this embodiment,
and contain inclusions having the higher melting point. Thus, these inclusions deform
during rolling with a ratio of the major axis to the minor axis of 3 or less, and are less
likely to deform.
Further, although having a strong deoxidation effect, Ce, La, Nd, Pr and Ca have
favorable wettability with the molten steel, and hence, the generated compound inclusions
are finely dispersed.
In other words, there are formed spherical compound inclusions having an
equivalent circle diameter in the range of 0.5 pm to 5 pm and containing different
inclusion phases including the first inclusion phase of [REMI-[Cal-[O,S] and the second
inclusion phase of [Mn,Si,Ti,Al]-[REMI-[Cal-[O,S].
[0156]
The reason that the above-described inclusion phases are expressed as being
"different inclusion phases" is because they can be separately recognized as inclusion
phases in the compound inclusion through an optical image or electronic image, and are
57
different in concentration through examination on components of the inclusion phases, and
hence, the present inventors considered them as being different inclusion phases. In
other words, in the case where one inclusion phase contains extremely small amount of an
element while the other inclusion phase contains the large amount of the same element,
the one inclusion phase and the other inclusion phase are determined to be different.
[0157]
The present inventors found that the hole-expandability can be improved if the
compound inclusions are spherical inclusions having an equivalent circle diameter in the
range of 0.5 pm to 5 pm, and the ratio of the number of the spherical inclusions is 50% or
more. Note that, although the more favorable effect can be obtained with the increase in
the ratio of the number of the spherical inclusions, the upper limit is considered to be
approximately 98%.
[015S]
The high-strength steel sheet according to this embodiment has a ratio of the
major axis to the minor axis of 3 or less. Further, in the high-strength steel sheet
according to this embodiment, the above-described inclusions are referred to as spherical
inclusions. From the examination made by the present inventors, it was found that
approximately 80% or more of the inclusions having the size in the range of 0.5 pm to 5
pm is formed by the spherical inclusion having the ratio of the major axis to the minor
axis of 3 or less. Note that, in the present case, the number density of the inclusions
having the size in the range of 0.5 pm to 5 pm is approximately several ten pieces per mm2,
in other words, falls within the range of 10 pieces/mm2 to 100 pieces/mm2.
[0159]
Further, the present inventors examined the behavior of TiS generated through
addition of Ti. As a result, the present inventors found that, under the high temperature,
Ti and S are captured on the above-described compound inclusions, and are not
precipitated as the coarsened inclusions of TiS. Further, the present inventors found that,
since TiS precipitated as a fine precipitate in a solid matter slowly disperses, TiS remains
in the solid matter as the fine precipitate.
Through observation, the present inventors found that, according to the steel of
the present embodiment having the compound inclusion containing different inclusion
phases including the first inclusion phase and the second inclusion phase, the size of TiS is
3 pm at the maximum, and inclusions having a size of 3 p or less do not have any
adverse effect on the hole-expandability in the case where the ratio of the number of the
inclusions is 30% or less.
[0161]
Further, TiN particles are generated with addition of Ti. These particles
contribute to achieving a so-called pinning effect of suppressing growth of crystal grains
in the structure of the steel sheet during heating applied before rolling, thereby reducing
the crystal grain diameter of the structure of the steel sheet. This makes the
multiple-precipitated inclusions made of oxide or oxysulfide less likely to be the starting
point of the occurrence of cracking or pathway of crack propagation during repetitive
deformation or hole expanding work. Further, the crystal gain diameter of the structure
of the steel sheet is a fine size, which leads to improvement in the fatigue characteristics as
described above.
[0 1 621
Further, inclusions having a spherical shape, clustering state, or shapes broken
during rolling are partially found as an inclusion having the size of over 5 p. Although
(Ce, La, Nd, Pr) is partially found from among these inclusions, the concentrations are low.
Thus, most of these inclusions are considered to be so-called extrinsic inclusions resulting
from oxide entering the molten steel from slag inclusion or refractory.
[0 1631
The present inventors made a study of how these inclusions having the size of
over 5 p have an effect on the hole expandability. As a result, it is found that, in the
case where the number density is 10 pieces/mm2 or less, these inclusions do not have any
adverse effect on the hole expandability.
[0 1 641
According to the present invention, Ca is added to the molten steel through
blowing after addition of (Ce, La, Nd, Pr). At this time, metal Ca or an alloy containing
metal Ca is used as powder for delivering a so-called flux such as CaO. Thus, it is
considered that the extrinsic inclusions rise to the surface, and this leads to cleanliness of
the molten steel.
[0 1651
The present inventors produced a steel ingot by then performing Al and Ti
deoxidation, performing deoxidation while changing the composition of (Ce, La, Nd, Pr),
and adding Ca. The obtained steel ingot is hot rolled to form a hot-rolled steel sheet
having a thickness of 3 mm. For the obtained hot-rolled steel sheet, a hole-expanding
test, and a bending test were performed, and examination was made on the number density
of inclusions, formation and average composition in the steel sheet.
[0 1661
As a result of the experiments described above, it was found that the oxygen
potential in the molten steel sharply decreases, by obtaining predetermined ratio of (Ce +
La + Nd + Pr)/acid-soluble Al and ratio of (Ce + La + Nd + Pr)/S on the basis of mass in
the steel sheet obtained by adding Si, performing deoxidation with Ti and Al, adding at
least one element of Ce, La, Nd, and Pr, and adding Ca at the end to deoxidize.
[0 1671
In other words, with the effect obtained through multiple deoxidation applied in
the order of Al, Ti, (Ce, La, Nd, Pr), and Ca, it is possible to obtain the largest
oxygen-potential-reducing effect that the former deoxidation applications can obtain with
various deoxidation elements. With the effect of multiple deoxidation, it is possible to
extremely lower the A1203 concentration in the generated oxides, and hence, it is possible
to obtain a steel sheet exhibiting excellent stretch-flange formability and bending
workability as with the steel sheet produced with little deoxidation with Al.
[0 1681
The present inventors found that the predetermined ratio of (Ce + La + Nd +
Pr)/acid-soluble A1 is 70 3 100 x (Ce + La + Nd + Pr)/acid-soluble A1 > 0.2 on the basis of
mass.
[0 1691
Further, the present inventors reached an idea of specification and simplification
using a mass ratio of chemical components (Ce + La + Nd + Pr)/S in the steel sheet.
[0 1701
More specifically, the (Ce + La + Nd + Pr)/S is set so as to be in the range of 0.2
to 10. In the case where 70 3 100 x (Ce + La + Nd + Pr)/acid-soluble A1 > 0.2 is
satisfied and (Ce + La + Nd + Pr)/S is in the range of 0.2 to 10, fine inclusions having an
equivalent circle diameter of 2 pm or less are dispersed as described later.
[0171]
On the other hand, in the case where the value of 100 x (Ce + La + Nd +
Pr)/acid-soluble A1 exceeds 70, the diameter of the inclusions increases. In the case
where the value of 100 x (Ce + La + Nd + Pr)/acid-soluble A1 is less than 0.2, A1203
increases.
[0 1 721
Further, in the case where (Ce + La + Nd + Pr)/S is less than 0.2, large MnS is
precipitated. On the other hand, in the case where (Ce + La + Nd + Pr)/S exceeds 10 and
hrther increases, the effect saturates and the cost for Ce, La, Nd, and Pr increases.
[0 1731
According to the high-strength steel sheet according to this embodiment, the
steel sheet exhibiting excellent stretch-flange formability and bending workability can be
obtained because of the following reasons.
[0 1 741
The present inventors found that the stretch-flange formability (hole
expandability) can be further improved in the case where, in the high-strength steel sheet
according to this embodiment, the ratio of the number of the spherical compound
inclusions having the size of 5 pm or less and the ratio of the major axis to the minor axis
of 3 or less is 50% or more when observation is made of inclusions having the equivalent
circle diameter of 0.5 pm or more. This is because, according to the high-strength steel
sheet according to this embodiment, the compound inclusions having a size of 5 pm or
less are finely dispersed, and are also hard, and hence, deformation of these compound
inclusions can be suppressed during rolling. Further, it is possible to obtain the effect of
improving the bending workability or other properties, by significantly reducing the
number of elongated and coarsened MnS-based inclusions in the steel sheet. Yet further,
with the multiple deoxidation, the oxygen potential in the molten steel can be reduced,
whereby nonuniformity of the components can be reduced.
[0 1751
It should be noted that the fine spherical chemical compound cannot be obtained
by adding Ca before the addition of (Ce, La, Nd, Pr). It is considered that this is because,
in the case where CaS having toughness and ductility is first generated, reduction of CaS
cannot be performed with (Ce, La, Nd, Pr), and CaS remains in the steel.
[0 1 761
On the basis of the findings obtained through the experiments and examination
described above, the present inventors examined conditions of chemical components in
the steel sheet in a manner as described below, and attained the high-strength steel sheet
exhibiting excellent stretch-flange formability and bending workability according to this
embodiment.
[0 1 771
Next, chemical components of the high-strength steel sheet according to this
embodiment will be described.
[0 1781
[C: 0.03% to 0.25%]
C is the most fundamental element that controls the hardenability and the
strength of the steel, and increases the hardness of and the depth of the quench hardened
layer, effectively contributing to improving the fatigue strength. In other words, C is an
essential element for securing the strength of the steel sheet, and C of at least 0.03% is
necessary to obtain the high-strength steel sheet. However, in the case where the amount
of C exceeds 0.25%, the workability and the weldability deteriorate. In order to obtain
the required strength while achieving the workability and the weldability, the
concentration of C is set to be not more than 0.25% in the high-strength steel sheet
according to this embodiment. Thus, the lower limit of C is set to 0.03%, preferably to
0.04%, more preferably to 0.05%. The upper limit of C is set to 0.25%, preferably to
0.20%, more preferably to 0.15%.
[0 1 791
[Si: 0.03% to 2.0%]
Si is a primary deoxidation element, which increases the number of nucleation
site of austenite during heating in the hardening, suppresses the grain growth in the
austenite, and reduces the grain diameter in the quench hardened layer. Si suppresses the
generation of carbides to prevent the reduction in the strength of the grain boundaries due
to the carbides, and is effective in generating a bainite structure. Thus, Si is an important
element to improve the strength without causing the deterioration in the elongation
property, and improve the hole-expandability with a low yield strength ratio. In order to
reduce the dissolved oxygen concentration in the molten steel, generate the SiO2-based
inclusion once, and obtain the minimum value of the final dissolved oxygen through the
multiple deoxidation (this SiO2-based inclusion is subjected to reduction with Al added
later to form the alumina-based inclusion, and then, reduction with Ce, La, Nd, and/or Pr
is applied to subject the alumina-based inclusion to reduction), it is necessary to add Si of
0.03% or more. For this reason, in the high-strength steel sheet according to this
embodiment, the lower limit of Si is set to 0.03%. In the case where the concentration of
Si is excessively high, toughness and ductility significantly deteriorate, and the
decarburization of the surface and the damage of the surface increase, resulting in
deteriorated bending workability. Further, in the case where Si is excessively added, Si
has an adverse effect on the weldability and the ductility. For these reasons, in the
high-strength steel sheet according to this embodiment, the upper limit of Si is set to 2.0%.
Accordingly, the lower limit of Si is set to 0.03%, preferably to 0.05%, more preferably to
0.1%. The upper limit of Si is set to 2.0%, preferably to 1.5%, more preferably to 1.0%.
[0 1 801
[Mn: 0.5% to 3.0%]
Mn is an element useful for deoxidation in the steel-producing stage, and is an
element effective in enhancing the strength of the steel sheet as with C and Si. In order
to obtain such an effect, it is necessary to make the steel sheet contain Mn of 0.5% or
more. However, in the case where the amount of Mn contained exceeds 3.0%, Mn
segregates or the solid solution strengthening increases, reducing the ductility. Further,
the weldability and the toughness of the base material also deteriorate. For these reasons,
the upper limit of Mn is set to 3.0%. Thus, the lower limit of Mn is set to 0.5%,
preferably to 0.7%~m~o re preferably to 1%. The upper limit of Mn is set to 3.0%,
preferably to 2.6%, more preferably to 2.3%.
[0181]
[P: 0.05% or less]
P is effective in that P functions as a substitutional solid-solution strengthening
element having a size smaller than Fe atom. However, in the case where the
concentration of P exceeds 0.05%, P segregates in the grain boundaries of austenite, and
the strength of the grain boundary deteriorates, reducing the torsion fatigue strength and
possibly causing deterioration in the workability. Thus, the upper limit of P is set to
0.05%, preferably to 0.03%, more preferably to 0.025%. If the solid solution
strengthening is not required, P is not necessary to be added, and hence, the lower limit
value of P includes 0%.
[0 1 821
[T.O: 0.0050% or less]
Total oxygen amount (T.0) forms oxide as an impurity. In the case where the
T.0 is excessively high, the Alz03-based inclusions increase, and the oxygen potential in
the steel cannot be made minimized. This leads to the significant deterioration in the
toughness and the ductility and the increase in the surface damage, resulting in the
deterioration in the bending workability. For these reasons, in the high-strength steel
sheet according to this embodiment, the upper limit of T.0 is set to 0.0050%, preferably to
0.0045%, more preferably to 0.0040%.
[O 1 831
[S: 0.0001% to 0.01%]
S segregates as an impurity, and forms a coarsened and elongated MnS-based
inclusion, which deteriorates the stretch-flange formability. Thus, it is desirable to
reduce the concentration of S as much as possible. By controlling the formation of the
coarsened and elongated MnS-based inclusion in the high-strength steel sheet according to
this embodiment, it is possible to obtain the material more than or equivalent to the cost
without causing the desulfurization load in the secondary refinement and without the need
for the desulfurization cost, even if the steel sheet contains a relatively high S
concentration of approximately 0.0 1 %. Thus, in the high-strength steel sheet according
to this embodiment, the concentration of S is set in the range of the extremely low S
concentration, which is a concentration set on the assumption that desulfurization is
performed in the secondary refinement, to the relatively high S concentration, that is, the
concentration of S is set in the range of 0.0001% to 0.01%.
[0 1 841
Further, according to the high-strength steel sheet according to this embodiment,
there is formed the spherical compound inclusion having an equivalent circle diameter in
the range of 0.5 pm to 5 pm and containing different inclusion phases including the first
inclusion phase of [REMI-[Cal-[O,S] and the second inclusion phase of
[Mn,Si,Ti,Al]-[REMI-[Cal-[O,SI.
[0185]
The upper limit value of the concentration of S is set in association with the total
amount of at least one element of Ce, La, Nd, and Pr as described later.
[0 1 861
Further, in the case where the concentration of S exceeds 0.01%, at least one of
the cerium oxysulfide, the lanthanum oxysulfide, the neodymium oxysulfide, and the
praseodymium oxysulfide grows to be over 5 pm in size. These coarsened oxysulfides
make the toughness and the ductility significantly deteriorate, leading to the increase in
the surface damages and deteriorating the bending workability. For these reasons, in the
high-strength steel sheet according to this embodiment, the upper limit of S is set to 0.01%,
preferably to 0.008%, more preferably to 0.006%.
[0187]
In other words, according to the high-strength steel sheet according to this
embodiment, the generation of MnS is suppressed by forming the compound inclusion
containing different inclusion phases including the first inclusion phase of
[REMI-[Cal-[O,S] and the second inclusion phase of [Mn,Si,Ti,Al]-[REMI-[Cal-[Oas, S]
described above. Thus, even if the concentration of S is relatively high but not more than
0.01%, by adding the corresponding amount of at least one element of Ce, La, Nd, and Pr,
it is possible to prevent the occurrence of adverse effect on the material. In other words,
even if the concentration of S is relatively high, by adjusting the amount of at least one
element of Ce, La, Nd, and Pr so as to correspond to the amount of S, it is possible to
substantially obtain the desulfurization effect, and it is possible to obtain a material
equivalent to the ultra-low sulfur steel. This means that, by appropriately adjusting the
concentration of S so as to associated with the total amount of Ce, La, Nd and Pr, it is
possible to increase the flexibility in the upper limit of the concentration of S. Thus, the
high-strength steel sheet according to this embodiment does not require desulfurization of
the molten steel in the secondary refinement to obtain the ultra-low sulfur steel, and can
omit the desulfurization process. This enables simplification of the producing processes,
and reduction in the cost required for the desulfurization process.
[0188]
[Acid-soluble Ti: 0.008% to 0.20%]
Ti is a primary deoxidation element, which forms carbides, nitrides, and
carbonitrides, increases the number of nucleation site of austenite by sufficiently heating
the steel before the hot rolling, and suppresses the grain growth of the austenite. With
these functions, Ti contributes to forming fine grains and enhancing the strength of the
grains, and is effective in dynamic recrystallization during the hot rolling, thereby
significantly improving the stretch-flange formability. To obtain these effects, it is found
through experiments that it is necessary to add the acid-soluble Ti of 0.008% or more.
Thus, in the high-strength steel sheet according to this embodiment, the lower limit of the
acid-soluble Ti is set to 0.008%, preferably to 0.01%, more preferably to 0.015%. Note
that the temperature for the sufficient heating before the hot rolling is required to be set to
a temperature sufficient for dissolving the carbides, nitrides, and carbonitrides generated
during casting in solid solution once, and over 1200°C is necessary. Setting the
temperature to over 1250°C is not preferable from the viewpoint of cost and generation of
scale. Thus, it is preferable to set the temperature to approximately 1250°C. In the case
where the content exceeds 0.2%, the effect of deoxidation saturates, and coarsened
carbides, nitrides, and carbonitrides are formed even if heating is sufficiently applied
before the hot rolling, deteriorating the material. Further, the effect corresponding to the
amount of the element contained cannot be obtained. Thus, in the high-strength steel
sheet according to this embodiment, the upper limit of the concentration of acid-soluble Ti
is set to 0.2%, preferably to 0.18%, more preferably to 0.15%. Note that the term
"acid-soluble Ti concentration" refers to a measured concentration of Ti dissolved in acid,
and this measurement employs a characteristic in which the dissolved Ti is dissolved in
acid whereas Ti oxide is not dissolved in acid. In this specification, the term "acid"
refers, for example, to a mixed acid having mass ratio of hydrochloric acid: 1, nitric acid:
1, and water: 2. By using such an acid, it is possible to separate Ti soluble in the acid and
Ti oxide non-soluble to the acid, whereby it is possible to measure the acid-soluble Ti
concentration.
[0 1891
The present inventors found that it is possible to obtain TiS having a size of 3 pm
or less, by adjusting Ti in the range described above, adjusting (Ce + La + Nd + Pr)/S so
as to be in the range of 0.2 to 10, and adding Ca after the addition of at least one element
of Ce, La, Nd, and Pr.
[0 1901
This is because Ca is contained in all the inclusion phases in the compound
inclusion containing inclusion phases having different components and including the first
inclusion phase of [REMI-[Cal-[O,S] and the second inclusion phase of [Mn, Si, Ti,
All-[REMI-[Cal-[O,S], and hence, Ti and S are more likely to be absorbed by the
compound inclusion. Thus, the TiS inclusion, which is precipitated at a high temperature,
is more likely to be captured by the compound inclusion, and is not precipitated alone.
Further, the TiS inclusion is not competitively precipitated on the compound inclusion.
Thus, only the TiS inclusion is precipitated alone when a temperature is a lower
temperature and a solubility product of Ti and S reaches a precipitation region, and if
precipitated, the TiS inclusion precipitated alone has a size of 3 pn or less.
[0191]
Further, it is considered that, as is the case with the suppression of MnS,
adjustment of (Ce + La + Nd + Pr)/S to be in the range of 0.2 to 10 delays the precipitation
of TiS, and has an effect of reducing the size of precipitated TiS and lowering the ratio of
number of TiS.
[0 1 921
It should be noted that, by adding Ca before addition of at least one element of
Ce, La, Nd, and Pr, it is possible to multiply precipitate MnS, TiS, and (Mn, Ti)S in the
inclusion containing at least one element of Ce, La, Nd, and Pr. However, in this case,
CaS is generated alone. In other words, Ca does not exist in the inclusion containing at
least one element of Ce, La, Nd, and Pr, and hence, unlike the high-strength steel sheet
according to this embodiment, Ti and S are not likely to be absorbed in the compound
inclusion. For these reasons, in the case where Ca is added before the addition of at least
one element of Ce, La, Nd, and Pr, the size of the TiS inclusion may be 3 pn or more, and
the stretch-flange formability becomes worse as compared with that of the high-strength
steel sheet according to this embodiment.
[0 1931
[N: 0.0005% to 0.01%]
N is captured from air during the steel-melting process, and hence, is an element
that is inevitably contained in the steel. N forms nitrides with Al, Ti or other elements,
and promotes reduction in size of grains in the base material structure. However, in the
case where the amount of N contained exceeds 0.01%, N generates coarsened precipitates,
for example, with A1 or Ti, deteriorating the stretch-flange formability. For this reason,
in the high-strength steel sheet according to this embodiment, the upper limit of the
concentration of N is set to 0.01%, preferably to 0.005%, more preferably to 0.004%. On
the other hand, the cost required for lowering the N concentration to less than 0.0005% is
high, and hence, the lower limit of the N concentration is set to 0.0005% from the
viewpoint of industrial feasibility.
[0 1 941
[Acid-soluble Al: over 0.01%]
In general, an oxide of acid-soluble A1 forms a cluster and is likely to coarsen,
which leads to a deterioration in the stretch-flange formability and the bending workability.
Thus, it is desirable to reduce acid-soluble A1 as much as possible. However, according
to the high-strength steel sheet according to this embodiment, a range of amount of
acid-soluble A1 was newly found, which enables obtaining the ultra-low oxygen potential
as described above while preventing clustering and coarsening of alumina-based
inclusions, by employing A1 deoxidation and the deoxidation effect obtained by
sequentially applying multiple deoxidation with Si, Ti, (Ce, La, Nd, and Pr), and Ca, and
adjusting the concentration of at least one element of Ce, La, Nd, and Pr so as to
correspond to the concentration of acid-soluble Al. In this range, part of the A1203-based
inclusions generated through A1 deoxidation rise to the surface and are removed whereas
the rest of the A1203-based inclusions remaining in the molten steel are subjected to
reductive decomposition with at least one element of Ce, La, Nd, and Pr added later, and
the clustered alumina-based oxide is decomposed to form the fine inclusions.
[0 1 951
With this finding, according to the high-strength steel sheet according to this
embodiment, it is possible to eliminate the need for setting the limitation that A1 is
substantially not added in order to avoid the coarsened cluster of the alumina-based
inclusions as in the conventional art. In particular, it is possible to increase the flexibility
in the concentration of the acid-soluble Al. By setting the concentration of acid-soluble
A1 to over 0.01%, preferably to 0.013% or more, more preferably to 0.015% or more, it is
possible to employ the A1 deoxidation, deoxidation with addition of at least one element of
Ce, La, Nd, and Pi-, and Ca deoxidation, thereby eliminating the need for adding the at
least one deoxidation element of Ce, La, Nd, and Pr more than necessary as in the
conventional art. Thus, it is possible to solve the problem of an increase in the oxygen
potential in the steel due to deoxidation with at least one element of Ce, La, Nd, and Pr.
Further, it is possible to obtain the effect of reducing the variation in the composition of
the component elements.
[0 1961
The upper limit value of the concentration of acid-soluble A1 is set in association
with the total amount of at least one element of Ce, La, Nd, and Pr as described later.
[O 1 971
In this specification, the term "acid-soluble A1 concentration" refers to a
measured concentration of Al dissolved in acid, and this measurement employs a
characteristic in which dissolved A1 is dissolved in acid whereas A1203 is not dissolved in
acid. In this specification, the term "acid" refers, for example, to a mixed acid having
mass ratio of hydrochloric acid: 1, nitric acid: 1, and water: 2. By using such an acid, it
is possible to separate A1 soluble in the acid and A1203 non-soluble to the acid, whereby it
is possible to measure the acid-soluble A1 concentration.
[0 1 981
[Ca: 0.0005% to 0.005%]
In the high-strength steel sheet according to this embodiment, Ca is an important
element, which forms the compound inclusion containing different inclusion phases
including the first inclusion phase of [REMI-[Cal-[O,S] and the second inclusion phase of
[Mn,Si,Ti,Al]-[REMI-[Cal-[O,S].
[0 1991
In other words, Ca is added to reduce the inclusions generated through
deoxidation with (Ce, La, Nd, Pr) to make all the inclusion phases contain Ca, thereby
forming the compound inclusion describe above. If Ca is not added, the above-described
compound inclusion is not formed.
[0200]
By forming this compound inclusion, it is possible to improve the stretch-flange
formability and the bending workability of the steel. In order to obtain this effect, it is
preferable to set the amount of Ca added to 0.0005% or more.
[0201]
However, the excessively large amount of Ca added saturates this effect,
impairing the cleanliness of the steel and deteriorating the ductility. Thus, the upper
limit of Ca is set to 0.005%. The lower limit of Ca is set to 0.0005%, preferably to
0.0007%, more preferably to 0.001%. The upper limit of Ca is set to 0.005%, preferably
to 0.0045%, more preferably to 0.0035%.
[0202]
[Total of at least one element of Ce, La, Nd, and Pr: 0.00 1 % to 0.0 1 %]
Ce, La, Nd, and Pr have an effect of reducing Si02 generated through Si
deoxidation and A1203 sequentially generated through A1 deoxidation, and separating
A1203 clusters, which are likely to coarsen. Further, by adding Ca after addition of at
least one element of Ce, La, Nd, and Pr, there is formed the compound inclusion
containing different inclusion phases including the first inclusion phase of
[REMI-[Cal-[O,S] and the second inclusion phase of [Mn,Si,Ti,AI]-[REMI-[Cal-[O,S].
[0203]
The present inventors found experimentally that, in order to obtain such an
inclusion, it is necessary to set the total concentration of at least one element of Ce, La, Nd,
and Pr to be not less than 0.0005% and not more than 0.01%.
[0204]
In the case where the total concentration of at least one element of Ce, La, Nd,
and Pr is less than 0.0005%, the Si02 and A1203 inclusions cannot be reduced. In the
case where the total concentration exceeds 0.01%, the large amount of cerium oxysulfide
and lanthanum oxysulfide is generated, and forms coarsened inclusions, deteriorating the
stretch-flange formability and the bending workability. Note that the lower limit of the
total concentration of at least one element of Ce, La, Nd, and Pr is set preferably to
0.0013%, and more preferably to 0.0015%. The upper limit of the total concentration of
at least one element of Ce, La, Nd, and Pr is set preferably to 0.009%, more preferably to
0.008%.
[0205]
As conditions for the existence of inclusions having a formation in which MnS is
precipitated in the oxide or oxysulfide formed by at least one element of Ce, La, Nd, and
Pr in the high-strength steel sheet according to this embodiment, the present inventors
focused on the fact that it is possible to determine the degree of improvement of MnS with
the oxide or oxysulfide formed by at least one of Ce, La, Nd, and Pr, by specifying the
degree of improvement using the concentration of S. Then, the present inventors reached
an idea of specifying and simplifying the degree of improvement using a mass ratio of
chemical components (Ce + La + Nd + Pr)/S in the steel sheet.
[0206]
More specifically, in the case where this mass ratio is low, the number of the
oxide or oxysulfide formed by at least one element of Ce, La, Nd, and Pr is small, and a
large number of MnS is precipitated alone. As this mass ratio increases, the number of
the inclusions having a formation of the compound inclusion containing different
inclusion phases including the first inclusion phase and the second inclusion phase also
increases as compared with MnS. This means that MnS is improved with the oxide or
oxysulfide formed by at least one element of Ce, La, Nd, and Pr. As described above,
MnS is precipitated in the oxide or oxysulfide formed by at least one element of Ce, La,
Nd, and Pr in order to improve the stretch-flange formability and the bending workability,
which leads to prevention of elongated MnS. For these reasons, the above-described
mass ratio can be used as a parameter to determine whether or not these effects can be
obtained.
In order to determine the chemical component ratio effective in suppressing the
elongation of the MnS-based inclusions, the mass ratio of (Ce + La + Nd + Pr)/S in the
steel sheet was varied to adjust the components in the steel sheet, Ca is then added, and
evaluation was made of the formation of the inclusions, the stretch-flange formability, and
the bending workability. As a result, it was found that, by setting the mass ratio of (Ce +
La + Nd + Pr)/S in the range of 0.2 to 10, both the stretch-flange formability and the
bending workability significantly improve.
[0208] -
In the case where the mass ratio of (Ce + La + Nd + Pr)/S is less than 0.2, the
ratio of the number of the inclusions having the formation of the compound inclusion
containing different inclusion phases including the first inclusion phase of
[REMI-[Cal-[O,S] and the second inclusion phase of [Mn,Si,Ti,Al]-[REMI-[Cal-[Oi,s S]
undesirably low. This correspondingly leads to the excessive increase in the ratio of
number of elongated MnS-based inclusions, which are likely to be the starting point of the
occurrence of cracking, deteriorating the stretch-flange formability and the bending
workability.
[0209]
In the case where the mass ratio of (Ce + La + Nd + Pr)/S exceeds 10, the effect
of generating the compound inclusion containing different inclusion phases including the
first inclusion phase and the second inclusion phase to improve the stretch-flange
formability and the bending workability saturates, which is not worth the cost. From
these reasons, the mass ratio of (Ce + La + Nd + Pr)/S is set in the range of 0.2 to 10. In
the case where the mass ratio of (Ce + La + Nd + Pr)/S is excessively high, for example, is
over 70, the large amount of the cerium oxysulfide and the lanthanum oxysulfide is
generated, and becomes coarsened inclusions, deteriorating the stretch-flange formability
and the bending workability. Thus, the upper limit of the mass ratio of (Ce + La + Nd +
Pr)/S is set to 10.
[02 lo]
It should be noted that, in the high-strength steel sheet according to this
embodiment, the total concentration of the at least one element Ce, La, Nd, and Pr
contained in the compound inclusion containing different inclusion phases including the
first inclusion phase of [REMI-[Cal-[O,S] and the second inclusion phase of
[Mn,Si,Ti,Al]-[REMI-[Cal-[iOs ,inS ]th e range of 0.5% to 95%. In the case where the
total concentration is less than 0.5%, the hard compound inclusion cannot be obtained, and
the ratio of major axidminor axis is 3 or more when subjected to rolling, which adversely
affects the hole-expandability of the steel sheet. On the other hand, in the case where the
total concentration exceeds 95%, the inclusions are more likely to be brittle. Thus, the
inclusions are pulverized and remain in a stranded formation as with the elongated
inclusions, and adversely affect the hole-expandability of the steel sheet.
[0211]
Next, selective elements for the high-strength steel sheet according to this
embodiment will be described. These elements are selective elements, and hence, may
be added or may not be added. Further, it may be possible to add these elements either
alone or in combination of two or more types. In other words, the lower limit of these
selective elements may be set to 0%.
[02 121
For Nb and V
Nb and V form carbides, nitrides, or carbonitrides with C or N to facilitate the
reduction in size of grains in the base material structure, and contribute to improving the
toughness.
[02 131
[Nb: 0.005% to 0.10%]
In order to obtain composite carbides, composite nitrides or other compound
described above, it is preferable to set the concentration of Nb to 0.005% or more, and it is
more preferable to set the concentration of Nb to 0.008% or more. However, in the case
where the base material contains the large amount of Nb in excess of the concentration of
0.10%, the effect of providing the fine grain in the base material structure saturates,
increasing the producing cost. For these reasons, the upper limit of the concentration of
Nb is set to 0.10%, preferably set to 0.09%, more preferably set to 0.08%.
[02 141
[V: 0.01% to 0.10%]
In order to obtain the above-described composite carbides, composite nitrides
and the like, it is preferable to set the concentration of V to 0.01% or more. However,
even if the large amount of V is contained in excess of the concentration of 0.10%, the
effect obtained from V contained saturates, increasing the producing cost. For this
reason, the upper limit of the concentration of V is set to 0.10%.
[02 151
For Cu, Ni, Cr, Mo, and B
Cu, Ni, Cr, Mo, and B enhance the strength, and improve the hardenability of the
steel.
[02 1 61
[Cu: 0.1% to 2%]
Cu contributes to improving the precipitation hardening and the fatigue strength
of ferrite, and may be added depending on applications to further enhance the strength of
the steel sheet. In order to obtain this effect, it is preferable to add Cu of 0.1% or more.
However, the excessively large amount of Cu contained deteriorates the balance of
strength-ductility. Thus, the upper limit of Cu is set to 2%, preferably to 1.8%, more
preferably to 1.5%.
[02 1 71
[Ni: 0.05% to YO]
Ni can be used for solid solution strengthening of ferrite, and may be added
depending on applications to further enhance the strength of the steel sheet. In order to
obtain this effect, it is preferable to add Ni of 0.05% or more. However, the excessively
large amount of Ni contained deteriorates the balance of strength-ductility. Thus, the
upper limit of Ni is set to 1%.
[02 181
[Cr: 0.01% to 1.0%]
Cr may be added depending on applications to further enhance the strength of
the steel sheet. In order to obtain this effect, it is preferable to add Cr of 0.01% or more.
However, the excessively large amount of Cr contained deteriorates the balance of
strength-ductility. Thus, the upper limit of Cr is set to 1.0%.
[02 1 91
[Mo: 0.01% to 0.4%]
Mo may be added depending on applications to further enhance the strength of
the steel sheet. In order to obtain this effect, it is preferable to add Mo of 0.01% or more,
and it is more preferable to add Mo of 0.05% or more. However, the excessively large
amount of Mo contained deteriorates the balance of strength-ductility. Thus, the upper
limit of Mo is set to 0.4%, preferably to 0.3%, more preferably to 0.2%.
[0220]
[B: 0.0003% to 0.005%]
B may be added depending on applications to further enhance the strength of the
grain boundaries, and improve the workability. In order to obtain this effect, it is
preferable to add B of 0.0003% or more, and it is more preferable to add B of 0.0005% or
more. However, in the case where the amount of B contained exceeds 0.005%, the effect
obtained from B saturates, and the cleanliness of the steel is impaired, deteriorating the
ductility. Thus, the upper limit of B is set to 0.005%.
[022 11
For Zr
Zr may be added depending on applications to strengthen the grain boundaries
and improve the workability with the control of sulfide formation.
[0222]
[Zr: 0.001% to 0.01%]
In order to obtain the effect of forming spherical sulfides to improve the
toughness of the base material, it is preferable to add Zr of 0.001% or more. However,
the excessively large amount of Zr contained impairs the cleanliness of the steel, which
leads to a deterioration in the ductility. Thus, the upper limit of Zr is set to 0.01%,
preferably to 0.009%, more preferably to 0.008%.
[0223]
Next, a description will be made of conditions for the existence of inclusions in
the high-strength steel sheet according to this embodiment. In this specification, the term
"steel sheet" means a rolled sheet obtained through hot rolling, or through hot rolling and
cold rolling. Further, the conditions for the existence of inclusions in the high-strength
steel sheet according to this embodiment are set from various viewpoints.
[0224]
In order to obtain the steel sheet exhibiting excellent stretch-flange formability
and bending workability, it is important to minimize the elongated and coarsened
MnS-based inclusions in the steel sheet, which are likely to be the starting point of the
occurrence of cracking or the pathway of crack propagation.
[0225]
In this regard, the present inventors found that, as with steel sheets produced
with little deoxidation with Al, it is possible to obtain a steel sheet exhibiting excellent
stretch-flange formability and bending workability because the oxygen potential in the
molten steel sharply decreases through the multiple deoxidation, A1203 generated through
A1 deoxidation is subjected to reduction, and A1203 cluster, which is likely to coarsen, is
separated, by adding Si to a steel, subjecting the steel to the deoxidation with Al, then,
adding at least one element of Ce, La, Nd, and Pr, further adding Ca for deoxidation in a
manner described above, and obtaining the predetermined ratio (Ce + La + Nd +
Pr)/acid-soluble A1 and ratio of (Ce + La + Nd + Pr)/S on the basis of mass as described
above.
[0226]
Further, it was also found that, with deoxidation through addition of Ce, La, Nd,
andor Pr, and addition of Ca thereafter, the fine and hard compound inclusion containing
different inclusion phases including the first inclusion phase of [REMI-[Cal-[O,S] and the
second inclusion phase of [Mn,Si,Ti,Al]-[REMI-[Cal-[Ois, gSe]n erated in most parts
although a slight amount of A1203 exists, and the precipitated MnS and other inclusions
are less likely to deform even during rolling, whereby the number of the elongated and
coarsened MnS can be significantly reduced in the steel sheet.
[0227]
Further, it was found that, by obtaining the ratio of (Ce + La + Nd +
Pr)/acid-soluble A1 and the ratio of (Ce + La + Nd + Pr)/S on the basis of mass as
described above, the number density of fine inclusions having an equivalent circle
diameter of 2 pm or less significantly increases, and the fine inclusions are dispersed in
the steel.
[0228]
These fine inclusions are less likely to aggregate, and hence, most of them
remain in the spherical shape or spindle shape. These inclusions have a major axislminor
axis (hereinafter, also referred to as "elongated ratio") of 3 or less, preferably 2 or less.
In the present invention, these inclusions are referred to as spherical inclusions.
[0229]
In terms of experiment, the inclusions can be identified easily through
observation using a scanning electron microscope (SEM), and focus was placed on the
number density of inclusions having an equivalent circle diameter of 5 pm or less. Note
that, although the lower limit value for the equivalent circle diameter is not particularly set,
it is preferable to set a target of the observation at the inclusions having approximately 0.5
pm or more, the size of which can be counted and expressed in number. In this
specification, the term "equivalent circle diameter" refers to a value obtained through
(major axis x minor axis)0.5 on the basis of the major axis and the minor axis of the
inclusion with cross-section observation.
[0230]
It is considered that the fine inclusions having a size of 5 pm or less are
dispersed because the oxygen potential in the molten steel is reduced through A1
deoxidation and adjustment of components of at least one element of Ce, La, Nd, and Pr;
the compound inclusions are less likely to aggregate due to the formation of inclusion
phases containing at least one element of Ti, Si, Al, and Ca in the oxide and/or oxysulfide
formed by at least one element of Ce, La, Nd, and Pr and further existence of Ca in each
inclusion phase; and the hardness of the compound inclusions is increased to make the
inclusions fine. It is assumed that, with this formation, the stress concentration occurring
during stretch-flange forming is relaxed, and the hole-expandability sharply improves.
Thus, the compound inclusions are less likely to be the starting point of the occurrence of
cracking or pathway of crack propagation during repetitive deformation and
hole-expanding work, and contributes to relaxing the stress concentration due to the fine
size, which leads to improvement in the stretch-flange formability and the bending
workability.
1023 11
The present inventors checked whether the number of the elongated and
coarsened MnS-based inclusions, which are likely to be the starting point of the
occurrence of cracking or pathway of crack propagation, was reduced in the steel sheet.
[0232]
Through experiments, the present inventors knew that, in the case where the
equivalent circle diameter is less than 1 pm, the elongated MnS does not have any adverse
effect in terms of the starting point of the occurrence of cracking, and does not deteriorate
the stretch-flange formability or bending workability. Further, the inclusions having an
equivalent circle diameter of 1 pm or more can be easily observed with the scanning
electron microscope (SEM) or other devices. For these reasons, by targeting the
observation at the inclusions having the equivalent circle diameter of 0.5 pn or more in
the steel sheet, their formations and compositions were examined to evaluate the
distribution state of the elongated MnS.
[0233]
It should be noted that, although the upper limit of the equivalent circle diameter
of MnS is not particularly set, MnS having a size of approximately 1 mm may be observed
in practical.
[0234]
The ratio of the number of the elongated inclusions was measured through
composition analysis on plural pieces (for example, about 50 pieces) of inclusions having
the equivalent circle diameter of 1 pm or more and randomly selected using a SEM, and
through measurement of the major axis and the minor axis of the inclusions using a SEM
image. In this specification, the elongated inclusion represents an inclusion having a
major axislminor axis (elongated ratio) of over 3. Further, the ratio of the number of the
elongated inclusions can be obtained by dividing the number of the detected elongated
inclusions by the total number of inclusions analyzed (about 50 in the case of the
above-described example). On the other hand, the spherical inclusion represents an
inclusion having the major axislminor axis (elongated ratio) of 3 or less.
[0235]
The reason that the elongated ratio is set to over 3 is because the inclusions
having the elongated ratio of over 3 in the comparative steel sheet without having the Ce,
La, Nd or Pr added therein were formed mostly by MnS. Note that, although the upper
limit of the elongated ratio of MnS is not particularly set, MnS having the elongated ratio
of approximately 50 may be observed in practice as illustrated in FIG 4.
[0236]
As a result, it was found that, with the steel sheet having the controlled formation
in which the ratio of the number of the elongated inclusions having an elongated ratio of 3
or less is controlled to be 50% or more, the stretch-flange formability and the bending
workability improve. More specifically, in the case where the ratio of the number of the
elongated inclusions having the elongated ratio of 3 or less is less than 50%, the ratio of
number of elongated MnS-based inclusions, which are likely to be the starting point of the
occurrence of cracking, excessively increases, and the stretch-flange formability and the
bending workability deteriorate. For these reasons, according to present invention, the
ratio of the number of the elongated inclusions having the elongated ratio of 3 or less is set
to 50% or more.
[0237]
The stretch-flange formability and the bending workability become more
favorable with decrease in the number of the elongated MnS-based inclusions. Thus, the
lower limit value of the ratio of the number of the elongated inclusions having the
elongated ratio of over 3 includes 0%. In this specification, the state in which an
inclusion has an equivalent circle diameter of 1 pm or more and the lower limit value of
the ratio of number of an elongated inclusion having the elongated ratio of over 3 is 0%
means that there exists an inclusion having the equivalent circle diameter of 1 pm or more
but there exists no inclusion having the elongated ratio of over 3, or the inclusion is an
elongated inclusion having the elongated ratio of over 3 but the equivalent circle diameters
of all the inclusions are less than 1 pm.
102381
Further, it was confirmed that the maximum equivalent circle diameter of the
elongated inclusions is smaller as compared with the average grain diameter of crystals in
the structure. This also contributes to the significant improvement in the stretch-flange
formability and the bending workability.
[0239]
In the case where a steel sheet has the controlled formation in which the mass
ratio of (Ce + La + Nd + Pr)/S is in the range of 0.2 to 10, and the ratio of the number of
the elongated inclusions having the elongated ratio of 3 or less is 50% or more, the steel
sheet is correspondingly formed by a spherical compound inclusion having an equivalent
circle diameter in the range of 0.5 pm to 5 pm and containing different inclusion phases
including the first inclusion phase and the second inclusion phase .
[0240]
It should be noted that TiN along with the MnS-based inclusions may be
multiply precipitated on the fine and hard Ce oxide, La oxide, cerium oxysulfide, and
lanthanum oxysulfide. However, as described above, it was confirmed that TiN has little
effect on the stretch-flange formability and the bending workability, and hence, TiN is not
the target of MnS-based inclusion in the high-strength steel sheet according to this
embodiment.
[024 11
Next, the condition for the existence of inclusions in the high-strength steel sheet
according to this embodiment described above is set using number density of the inclusion
per unit volume.
[0242]
The distribution of grain diameters of inclusions was obtained through a SEM
evaluation on an electrolyzed surface using a speed method. The SEM evaluation on the
electrolyzed surface using the speed method was performed such that: a surface of a test
piece was polished, and was subjected to electrolyzation using the speed method; and the
surface of the test piece was directly observed with the SEM observation, thereby
evaluating the size or number density of the inclusion. Note that the speed method
represents a method of electrolyzing the surface of the test piece using 10% acetyl
acetone-1 % tetramethyl ammonium chloride-methanol, and extracting the inclusions. As
for the amount of electrolysis, electrolyzation was performed under the condition that
electric charge of the surface of the test piece per lcm2 area reached 1C (coulomb). The
SEM image of the surface electrolyzed as described above was subjected to image
processing, thereby obtaining a frequency (number of pieces) distribution in terms of
equivalent circle diameter. On the basis of the frequency distribution of the grain
diameter, the average equivalent circle diameter was obtained. Further, the number
density of inclusions per unit volume was calculated by dividing the frequency by the area
of the observed view and the depth obtained from the amount of electrolysis. Further, the
ratio of number was also calculated.
[0243]
In order to determine a composition effective in suppressing the elongation of
MnS-based inclusions, composition analysis was performed on spherical compound
inclusions having an equivalent circle diameter in the range of 0.5 pm to 5 pm and
containing different inclusion phases including the first inclusion phase and the second
inclusion phase.
[0244]
Since the observation becomes easy if the equivalent circle diameter of the
inclusions is 0.5 pm or more, the target of the observation was set at the equivalent circle
diameter of 0.5 pm or more for the convenience purpose. However, if the observation is
possible, it may be possible to include the inclusions having the equivalent circle diameter
of less than 0.5 pm.
lo2451
As a result, it was found that the stretch-flange formability and the bending
workability improve, by forming the inclusions having the equivalent circle diameter of
0.5 pm or more and the elongated ratio of 3 or less so as to contain the total amount of at
least one element of Ce, La, Nd, and Pr in the range of 0.5% to 95% in average
composition.
[0246]
In the case where the average amount of the total of the at least one element of
Ce, La, Nd, and Pr contained is less than 0.5 mass % in the inclusions having the
equivalent circle diameter of 0.5 pm or more and the elongated ratio of 3 or less, the ratio
of the number of the compound inclusions containing different inclusion phases including
the first inclusion phase and the second inclusion phase largely decreases, while the ratio
of the number of the MnS-based elongated inclusions, which are likely to be the starting
point of the occurrence of cracking, excessively increases correspondingly. Thus, the
stretch-flange formability and the bending workability deteriorate.
102471
On the other hand, in the case where the average amount of the total of the at
least one element of Ce, La, Nd, and Pr contained exceeds 95% in the inclusions having
the equivalent circle diameter of 0.5 pm or more and the elongated ratio of 3 or less, at
least one of cerium oxysulfide, lanthanum oxysulfide, neodymium oxysulfide,
praseodymium oxysulfide is largely generated, which leads to coarsened inclusions having
the equivalent circle diameter of approximately 50 pm or more. Thus, the stretch-flange
formability and the bending workability deteriorate.
[0248]
Next, the structure of the steel sheet will be described.
[0249]
According to the high-strength steel sheet according to this embodiment, the fine
MnS-based inclusions are precipitated in the cast slab, and are dispersed in the steel sheet
as the fine spherical inclusions, which do not deform during rolling and are less likely to
be the starting point of the occurrence of cracking, so that the stretch-flange formability
and the bending workability can be improved. Thus, the micro-structure of the steel
sheet is not particularly limited.
[0250]
Although the micro-structure of the steel sheet is not particularly limited, it may
be possible to employ any structure from among a steel sheet having a structure of a phase
formed mainly by bainitic ferrite, a composite-structure steel sheet having a main phase of
a ferrite phase and a second phase of a martensite phase and a bainite phase, and a
composite-structure steel sheet formed by ferrite, retained austenite and a low-temperature
transformation phase (formed by martensite or bainite).
[025 11
Further, by sufficiently applying heat at approximately 1250°C before the hot
rolling, the carbides, the nitrides, and the carbonitrides generated through casting are once
dissolved in solid solution to increase acid-soluble Ti in the steel. Then, with the effect
obtained from solute Ti or carbonitrides of Ti, it is possible to form fine crystal grains, so
that the crystal grain diameter in the steel sheet can be reduced to be 10 pm or less.
[0252]
Thus, any of the structures described above are favorable because it is possible to
reduce the crystal grain diameter to 10 pm or less, and the hole-expandability and the
bending workability can be improved. In the case where the average grain diameter
exceeds 10 pm, the degree of improvement in the ductility and the bending workability
reduces. In order to improve the hole-expandability and the bending workability, it is
more preferable to set the crystal grain diameter to 8 pm or less. However, in general, in
the case where excellent stretch-flange formability is required, for example, in the case of
application for underbody components, it is desirable and preferable that the ferrite or
bainite phase be the maximum area-ratio phase, although the ductility be slightly lower.
[0253]
Next, conditions for producing the steel sheet will be described.
[0254]
According to a method of producing molten steel for the high-strength steel sheet
according to this embodiment, alloys such as C, Si, and Mn are fbrther added to the
molten steel decarbonized by blowing in a converter or by further using a vacuum
degassing device, and the molten steel is agitated, thereby performing deoxidation and
component adjustment.
[0255]
As for S, desulfurization may not be performed in the refinement process as
described above, and thus, the desulfurization process can be omitted. However, in the
case where desulfurization of the molten steel is necessary in the secondary refinement to
produce the ultra-low sulfur steel with approximately S 5 20ppm, it may be possible to
perform desulfurization to adjust the components.
[0256]
It is preferable that, after the elapse of approximately 3 minutes from the
addition of Si described above, A1 be added to perform A1 deoxidation, and then, the rising
time of approximately 3 minutes be set so as to allow A1203 to rise to the surface and be
separated. Ti is added after the A1 deoxidation.
[0257]
Thereafter, at least one element of Ce, La, Nd, and Pr is added, and components
are adjusted so as to satis@ 70 2 100 x (Ce + La + Nd + Pr)/acid-soluble A1 2 2, and (Ce +
La + Nd + Pr)/S being in the range of 0.2 to 10 on the basis of mass.
[0258]
In the case where a selective element is added, the selective element is added
before the addition of the at least one element of Ce, La, Nd, and Pr, agitation is
sufficiently performed, and the at least one element of Ce, La, Nd, and Pr is added.
Depending on application, the at least one element of Ce, La, Nd, and Pr may be added
after components of the selective element are adjusted.
[0259]
Then, agitation is sufficiently performed, and Ca is added. The thus obtained
molten steel is subjected to continuous casting to produce a cast slab.
[0260]
The continuous casting not only includes an ordinal slab continuous casting
having a thickness of approximately 250 mm, but also includes a bloom, a billet, and thin
slab continuous casting having a thinner die-thickness than that of ordinal slab
continuous-casting devices, for example, a thickness of 150 mm or less.
[0261]
Hot rolling conditions for producing the high-strength hot-rolled steel sheet will
be described.
[0262]
Since carbonitrides or other inclusions in the steel need to be once dissolved in
solid solution, it is important to set a heating temperature for a slab before hot rolling to
over 1200°C.
[0263]
By making the carbonitrides dissolved in solid solution, it is possible to obtain a
ferrite phase, which is favorable to improve the ductility in the cooling process after the
rolling. On the other hand, in the case where the heating temperature for the slab before
the hot rolling exceeds 1250°C, the surface of the slab is significantly oxidized. In
particular, wedge-shaped surface defects appear after descaling due to selective oxidation
of the grain boundaries, deteriorating the quality of the surface after the rolling. Thus, it
is preferable to set the upper limit of the heating temperature to 1250°C.
[0264]
After being heated in the temperature range described above, the slab is
subjected to the normal hot rolling. In this hot rolling process, the temperature at the
time of completion of the finishing rolling is important to control the structure of the steel
sheet. In the case where the temperature at the time of completion of the finishing rolling
is less than Ar3 point + 30°C, the diameter of the crystal grain in the surface layer portion
is likely to coarsen, which is not favorable in terms of bending workability. On the other
hand, in the case where this temperature exceeds the Ar3 point + 200°C, the diameter of
the austenite grain after the completion of the rolling coarsens, which makes it difficult to
control the structure and the ratio of the phase generated during cooling. Thus, the upper
limit of the temperature is set preferably to the Ar3 point + 200°C.
[0265]
Further, depending on the targeted structure configuration, a condition for the hot
rolling is selected from among a condition in which an average cooling rate for the steel
sheet after the finishing rolling is set in the range of 10°C/sec to 100°C/sec, and the coiling
temperature is set in the range of 450°C to 650°C, and a condition in which the steel sheet
is air cooled at approximately S°C/sec until the temperature reaches 680°C after the
finishing rolling, and is cooled thereafter at the cooling rate of 30°C/sec or more, and the
coiling temperature is set to 400°C or less. By controlling the cooling rate and the
coiling temperature after the rolling, it is possible to obtain a steel sheet having one or
more structures of polygonal ferrite, bainitic ferrite, and a bainite phase, and the
corresponding ratio under the former rolling condition, and a DP steel sheet having a
compound structure including the large amount of polygonal ferrite phase, which are
excellent in ductility, and the martensite phase under the latter rolling condition.
[0266]
In the case where the average cooling rate described above is less than 10°C/sec,
pearlite, which is not favorable in terms of the stretch-flange formability, is likely to be
generated, which is not preferable. Although setting of the upper limit of the cooling rate
is not necessary from viewpoint of controlling of the structure, the excessively high
cooling rate possibly causes the cooling state of the steel sheet to be nonuniform. Further,
a large amount of cost is required to manufacture the equipment that can provide such a
high cooling rate, which leads to increase in prices of the steel sheet. In view of the facts
described above, it is preferable to set the upper limit of the cooling rate to 100°C/sec.
[0267]
The high-strength cold-rolled steel sheet according to this embodiment is
produced by subjecting a steel sheet to hot rolling, coiling, pickling, and skin pass, then
cold rolling the steel sheet, and applying annealing to the steel sheet. In the annealing
processes, batch annealing, continuous annealing or other processes are applied, thereby
obtaining the final cold-rolled steel sheet.
102681
It is needless to say that the high-strength steel sheet according to this
embodiment may be used as a steel sheet for electroplating. Application of
electroplating does not change the mechanical properties of the high-strength steel sheet
according to this embodiment.
[Examples]
[0269]
[Example 11
Next, Examples according to the present invention along with Comparative
Examples will be described.
[0270]
Molten steels having chemical components shown in Table 1 and Table 2 were
produced through a converter and RH processes. At this time, in the case where the
molten steels were not subjected to a desulfurization process in the secondary refinement,
S was set in the range of 0.003 mass % to 0.01 1 mass %. In the case where the molten
steels were subjected to the desulfurization process, S was set so as to satisfy S 5 20ppm.
[027 11
Si was added to adjust components as shown in Table 1 and Table 2. After
approximately 3 minutes to 5 minutes elapsed from the addition of Si, A1 was added to
perform A1 deoxidation, and then, rising time in the range of approximately 3 minutes to 6
minutes was set so as to allow A1203 to rise to the surface and be separated.
[0272]
Thereafter, depending on charges of experiments, at least one element of Ce, La,
Nd, and Pr was added to adjust components so as to satis@ 70 ? 100 x (Ce + La + Nd +
Pr)/acid-soluble A1 2 2, and (Ce + La + Nd + Pr)/S being in the range of 0.2 to 10 on the
basis of mass.
[0273]
Depending on charges of experiments in which selective elements were added,
the selective elements were added before the addition of at least one element of Ce, La, Nd,
and Pr, agitation was sufficiently performed, and the at least one element of Ce, La, Nd,
and Pr was added. Depending on application, the at least one element of Ce, La, Nd, and
Pr may be added after components of the selective element were adjusted. Then,
agitation was sufficiently performed, and Ca was added. The thus obtained molten steel
was subjected to continuous casting to produce an ingot.
[0274]
For the continuous casting, a normal slab continuous-casting device with a
thickness of approximately 250 mm was used.
[0275]
The ingot subjected to the continuous casting was heated to temperatures in the
range of over 1200°C to 1250°C under hot rolling conditions shown in Table 3.
[0276]
Then, the ingot was subjected to rough rolling, and then to finishing rolling.
Temperatures at the time of completion of the finishing rolling were set to be not less than
Ar3 point + 30°C and not more than Ar3 point + 200°C. In this specification, the Ar3
point was calculated using a normal expression obtained from each of the components.
[0277]
The average cooling rate for the steel sheet after the finishing rolling was set in
the range of 10°C/sec to 100°C/sec. Further, depending on charges of experiments, in
the case where the coiling temperature was set in the range of 450°C to 650°C, the steel
sheet was air cooled at approximately S°C/sec until the temperature reaches 680°C after
the finishing rolling, and was cooled thereafter at a cooling rate of 30°C/sec or more.
With the cooling being applied as described above, it was possible to obtain a
steel sheet having one or more structures of polygonal ferrite, bainitic ferrite, and a bainite
phase.
[0279]
On the other hand, depending on charges of experiments, coiling was performed
at 400°C or less, and it was possible to obtain a DP steel sheet having a compound
structure of a polygonal ferrite phase and a martensite phase.
[0280]
A high-strength cold-rolled steel sheet was obtained, by subjecting the steel sheet
to processes such as hot rolling, coiling, pickling, and skin pass to cold roll the hot-rolled
steel sheet, and applying continuous annealing to form a cold-rolled steel sheet. Further,
to obtain a steel sheet for electroplating, the steel sheet for electroplating was formed in an
electro-plate line or hot-dip zinc plating line.
[028 11
Table 1 and Table 2 show chemical components of the slab.
[0282]
Table 3 shows conditions for hot rolling. Under the conditions, a hot-rolled
plate with a thickness of 3.2 mm was obtained.
[0283]
[Table 11
[0284]
[Table 21
[0285]
[Table 31
[0286]
In Table 1 and Table 2, steel numbers Al, A3, A5, A7, A9, A1 1, A13, A15, A17,
A19, A21, A23, A25, A27, A29, A31, A33, A35, and A37 are configured so as to have
compositions that fall within the range of the high-strength steel sheet according to the
present invention, whereas steel numbers A2, A4, A6, AS, AlO, A12, A14, A16, A1 8,
A20, A22, A24, A26, A28, A30, A32, A34, A36, and A38 are configured as slabs having,
on the basis of mass, the ratio of (Ce + La + Nd + Pr)/acid-soluble Al, the ratio of (Ce +
La + Nd + Pr)/S, and the concentrations of S, T.0, Ca, and Ce + La + Nd + Pr adjusted so
as to fall outside the range of the high-strength steel sheet according to the present
invention.
[02 8 71
It should be noted that, for comparison purposes, in Table 1 and Table 2, steel
number A1 and steel number A2, steel number A3 and steel number A4, steel number A5
and steel number A6, steel number A7 and steel number AS, steel number A9 and steel
number AIO, steel number A1 1 and steel number A12, steel number A13 and steel
number A14, steel number A1 5 and steel number A16, steel number A1 7 and steel
number A1 8, steel number A19 and steel number A20, steel number A21 and steel
number A22, steel number A23 and steel number A24, steel number A25 and steel
number A26, steel number A27 and steel number A28, steel number A29 and steel
number A30, steel number A3 1 and steel number A32, steel number A33 and steel
number A34, steel number A35 and steel number A36, and steel number A37 and steel
number A38 are configured so as to have almost the same composition except that the
compositions such as Ce + La are different.
[0288]
Further, in Table 3, as condition A, a heating temperature was set to 1250°C, a
temperature at the completion of finishing rolling was set to 845"C, a cooling rate after
finishing rolling was set to 75"C/sec, and a coiling temperature was set to 450°C. As
condition B, the heating temperature was set to 1250°C, the temperature at the completion
of finishing rolling was set to 860°C, the steel sheet was air cooled at approximately
S°C/sec until the temperature reaches 680°C afier the finishing rolling, and was cooled
thereafter at a cooling rate of 30°C/sec or more, and the coiling temperature was set to
400°C. As condition C, the heating temperature was set to 1250°C, the temperature at
the completion of finishing rolling was set to 825OC, the cooling rate after the finishing
rolling was set to 45OC/sec, and the coiling temperature was set to 450°C.
[0289]
Condition B was applied to steel number A1 and steel number A2.
Condition B was applied to steel number A3 and steel number A4.
Condition A was applied to steel number A5 and steel number A6.
Condition A was applied to steel number A7 and steel number AS.
Condition A was applied to steel number A9 and steel number A10.
Condition C was applied to steel number A1 l and steel number A12.
Condition B was applied to steel number A13 and steel number A14.
With these applications of conditions, the effects of chemical components can be
compared under the same producing conditions.
[0290]
The thus obtained steel sheets were examined in terms of basic characteristics
including strength (MPa), ductility (%), stretch-flange formability (A%), and limit bending
radius (mm) for bending workability.
[029 11
To obtain existence states of elongated inclusions in the steel sheets, examination
was made on the number density per area of inclusions having a size of 2 pm or less, the
ratio of number of inclusions having an elongated ratio of 3 or less, the number density per
volume, and the average equivalent circle diameter (hereinafter, the average is referred to
as an arithmetic mean) through observation using an optical microscope or observation
using a SEM by targeting the observation at all the inclusions having a size of
approximately 1 pn or more.
[0292]
Further, to obtain existence states of non-elongated inclusions in the steel sheet,
examination was made on the ratio of number of and the number density per volume of a
compound inclusion having a formation having two or more inclusion phases containing
different components and including a first group inclusion phase containing at least one
element of Ce, La, Nd, and Pr, krther containing Ca, and containing at least one of 0 and
S, and a second group inclusion phase further containing at least one element of Mn, Si,
and Al, and the average value of total amount of at least one element of Ce, La, Nd, and Pr
contained in the inclusions having an elongated ratio of 3 or less, by targeting the
observation at all the inclusions having a size of approximately 1 pm or more.
[0293]
It should be noted that the reason that inclusions having a size of approximately
1 pm or more were targeted in the observation is because of easiness of the observation
and also because the inclusions having a size of less than approximately 1 pm do not have
any effect on the deterioration in the stretch-flange formability or bending workability.
[0294]
Table 4 shows results of the examinations for each combination between steel
and rolling condition.
[0295]
[Table 41
[0296]
The strength and the ductility were obtained through a tensile test with Japanese
Industrial Standards (JIS) No.5 test piece taken from the steel sheet in a direction parallel
to the rolling direction. The stretch-flange formability was evaluated such that a punched
hole having a diameter of 10 mm and opened at the center of a steel sheet with 150 mm x
150 mm was pressed and expanded with a conical punch having an angle of 60°, a hole
diameter D (mm) was measured at the time when a through-thickness crack occurred, and
a hole-expanding value h was obtained from h = @ - 10)/10, thereby evaluating the
stretch-flange formability with the hole-expanding value h. The limit bending radius
(mm) used as an index indicating the bending workability was obtained by taking a
bending test piece, and carrying out a V-bending test using a die unit equipped with a die
and a punch. The die used has a recessed portion with a V-shaped cross section and an
angle of aperture of 60". The punch used has an elevated portion that matches the
recessed portion of the die. Various punches were prepared in which bending radii of a
needle portion at a top end portion were varied in 0.5-mm steps, and were subjected to
bending tests to obtain the minimum radius of curvature of the needle portion at the top
end portion of the punch at which a crack occurs at a bent portion of the subjected test
piece. This minimum radius of curvature was evaluated as the limit bending radius.
[0297]
It should be noted that the test piece used was a No. 1 test piece specified in JIS,
which was obtained by equally cutting both sides of a raw sheet (hot rolled sheet) and had
a parallel portion of 25 mm, a radius of curvature R of 100 mm, and a thickness of 3.0
mm.
[0298]
As for inclusions, the major axis and the minor axis of 50 inclusions having an
equivalent circle diameter of 1 pm or more and randomly selected were measured through
SEM observation. Further, with a quantitative analysis function of the SEM,
composition analysis was performed for the randomly selected 50 inclusions having the
equivalent circle diameter of 1 pm or more. These measurement results were used to
obtain the ratio of number of inclusions having an elongated ratio of 3 or less, the average
equivalent circle diameter of the inclusions having the elongated ratio of 3 or less, the
ratio of number of compound inclusions, and the average value of the total of at least one
element of Ce, La, Nd, and Pr in the inclusions having the elongated ratio of 3 or less.
Further, the number density of inclusions per volume was calculated for each formation
with SEM evaluation on electrolyzed surfaces using the speed method.
[0299]
As can be clearly understood from Table 3, with steel numbers Al, A3, A5, A7,
A9, A1 1, A13 and other odd steel numbers to which the method according to the present
invention was applied, it was possible to reduce the number of the elongated MnS-based
inclusions in the steel sheet by generating the compound inclusion specified in the present
invention. In other words, fine spherical compound inclusions having the equivalent
circle diameter in the range of 0.5 pm to 5 pn existed in the steel sheet, and components
of these compound inclusions were formed by inclusion phases containing two or more
inclusion phases having different components and selected from among the first group
inclusion phase of [Ce, La, Nd, Pr]-Ca-[0, S] and the second group inclusion phase of [Ce,
La, Nd, Pr]-Ca-[0, S]-[Mn, Si, All, which are specified in the present invention. Further,
the ratio of the number of the spherical compound inclusions having the equivalent circle
diameter in the range of 0.5 pm to 5 pm relative to the number of all the inclusions having
the equivalent circle diameter in the range of 0.5 pm to 5 pm was 30% or more. The
ratio number of elongated inclusions existing in the steel sheet and having the equivalent
circle diameter of 1 pm or more and the major axislminor axis of 3 or less relative to the
number of all the inclusions having the equivalent circle diameter of 1 pm or more was
50% or more. The average content percentage of the total of at least one element of Ce,
La, Nd, and Pr in the inclusions was in the range of 0.5% to 95%. Note that, in any
structures of the steel sheets, the average crystal grain diameter fell within the range of 1
pm to 8 pm, and were almost equal between the present invention and Comparative
Examples.
[03 001
As a result, the steel sheets of steel numbers Al, A3, A5, A7, A9, A1 1, A13 and
other odd steel numbers, which are steels according to the present invention, exhibited
excellent stretch-flange formability and bending workability as compared with
comparative steels. On the other hand, as for comparative steels (steel numbers A2, A4,
A6, A8, A10, A12, A14 and other even steel numbers), the average crystal grain diameter
exceeded 10 p, there were formed elongated inclusions that little contained Ce, La, Nd,
or Pr and had major axislminor axis of 3 or more, in other words, elongated MnS-based
inclusions, and inclusions distributed in a state different from that specified in the present
invention. As a result, the MnS-based inclusions elongated during working of the steel
sheets served as the starting point of the occurrence of cracking, which led to a
deterioration in the stretch-flange formability and the bending workability.
[0301]
Table 5 and Table 6 show comparison results of the inclusion composition and
the hole-expanding ratio between Example A20 according to the present invention and
Comparative Example A20, the order of addition of Ca and at least one element of Ce, La,
Nd, and Pr being changed between Example A20 and Comparative Example A20. In
Example A20 according to the present invention, Ca was added after addition of Ce from
among Ce, La, Nd, and Pr. In Comparative Example A20, Ce is added after addition of
Ca, and in this case, inclusions had a formation in which MnS and oxide or oxysulfide
formed by Ce were precipitated in CaS. Unlike the inclusions according to the present
invention containing two or more inclusion phases having different components, in this
case, the inclusions had a composition in which the elongation ratio of the inclusions was
high and the hole-expanding ratio reduced as compared with Example according to the
present invention.
[0302]
[Table 51
[0303]
[Table 61
103041
Table 7 and Table 8 show results of the composition of inclusions and the
hole-expanding ratio of Comparative Example A2 1 that did not have Ca added after
addition of two elements of Ce and La in comparison with Example A2 1 according to the
present invention (Ca is added after addition of two elements of Ce and La). In the case
where Ca is not added after addition of two elements of Ce and La, an immersion nozzle
in a continuous casting equipment clogged during casting, not all the molten steel in the
ladle were not able to be completely casted, and casting could not be performed with the
latter ladle, causing production troubles. Although products could be obtained by
applying processes after hot rolling to slabs being processed but not completed, the
inclusions in the products had MnS precipitated in oxide or oxysulfides formed by two
elements of Ce and La, and unlike the inclusions according to the present invention
containing two or more inclusion phases having different components, the inclusions in
the above-described products had a composition in which the elongation ratio of the
inclusions was high and the hole-expanding ratio reduced as compared with Example A2 1
according to the present invention.
[0305]
[Table 71
103061
[Table 81
[0307]
[Example 21
Next, Examples according to the present invention along with Comparative
Examples will be described.
[0308]
Molten steels having chemical components shown in Table 9 and Table 10 were
produced through a converter and RH processes. At this time, in the case where the
molten steels were not subjected to a desulfurization process in the secondary refinement,
S was set in the range of 0.003 mass % to 0.01 1 mass %. In the case where the molten
steels were subjected to the desulfurization process, S was set so as to satis@ S I 20ppm.
[0309]
Si was added to adjust components as shown in Table 9 and Table 10. After
approximately 3 minutes to 5 minutes elapsed from the addition of Si, A1 was added to
perform A1 deoxidation, and then, rising time in the range of approximately 3 minutes to 6
minutes was set so as to allow A1203 to rise to the surface and be separated. Then, Ti was
added.
[03 lo]
Thereafter, depending on charges of experiments, at least one element of Ce, La,
Nd, and Pr was added to adjust components so as to satisfj 70 2 100 x (Ce + La + Nd +
Pr)/acid-soluble A1 2 2, and (Ce + La + Nd + Pr)/S being in the range of 0.2 to 10 on the
basis of mass.
[03 111
Depending on charges of experiments in which selective elements were added,
the selective elements were added before the addition of at least one element of Ce, La, Nd,
and Pr, agitation was sufficiently performed, and the at least one element of Ce, La, Nd,
and Pr was added. Depending on application, the at least one element of Ce, La, Nd, and
Pr may be added after components of the selective element were adjusted.
[03 121
Then, agitation was sufficiently performed, and Ca was added. The thus
obtained molten steel was subjected to continuous casting to produce an ingot. For the
continuous casting, a normal slab continuous-casting device with a thickness of
approximately 250 mm was used. The ingot subjected to the continuous casting was
heated to temperatures in the range of over 1200°C to 1250°C under hot rolling conditions
shown in Table 1 1. Then, the ingot was subjected to rough rolling, and then to finishing
rolling. Temperatures at the time of completion of the finishing rolling were set to be not
less than Ar3 point + 30°C and not more than Ar3 point + 200°C. In this specification,
the Ar3 point was calculated using a normal expression obtained from each of the
components.
[03 131
The average cooling rate for the steel sheet after the finishing rolling was set in
the range of 10°C/sec to 100°C/sec. Further, depending on charges of experiments, in the
case where the coiling temperature was set to temperatures in the range of 450°C to 650°C,
the steel sheet was air cooled at approximately 5"CIsec until the temperature reaches
680°C after the finishing rolling, and was cooled thereafter at a cooling rate of 30°C/sec or
more.
[03 141
With the cooling described above, it was possible to obtain a steel sheet having
one or more structures of polygonal ferrite, bainitic ferrite, and a bainite phase.
[03 151
Depending on charges of experiments, coiling was performed at 400°C or less,
and it was possible to obtain a DP steel sheet having a compound structure of a polygonal
ferrite phase and a martensite phase.
[03 161
A high-strength cold-rolled steel sheet was obtained, by subjecting the steel sheet
to processes such as hot rolling, coiling, pickling, and skin pass to cold roll the hot-rolled
steel sheet, and applying continuous annealing to form a cold-rolled steel sheet. Further,
to obtain a steel sheet for electroplating, the steel sheet for electroplating was formed in an
electro-plate line or hot-dip zinc plating line.
[03 171
Slabs having chemical components shown in Table 9 and Table 10 were
subjected to hot rolling under conditions shown in Table 11 to form a hot-rolled sheet
having a thickness of 3.2 mm.
[03 181
[Table 91
[03 191
[Table 101
[0320]
In Table 9 and Table 10, steel numbers B1, B3, B5, B7, B9, B11, B13, B15, B17,
B 19, B2 1, and B23 are configured so as to have compositions that fall within the range of
the high-strength steel sheet according to the present invention, whereas steel numbers B2,
B4, B6, B 8, B 10, B 12, B 14, B 16, B 1 8, B20, B22, and B24 are configured as slabs having,
on the basis of mass, the ratio of (Ce + La + Nd + Pr)/acid-soluble Al, the ratio of (Ce +
La + Nd + Pr)/S, and the concentrations of S, T.0, Ca, and Ce + La + Nd + Pr adjusted so
as to fall outside the range of the high-strength steel sheet according to the present
invention.
[032 11
It should be noted that, for comparison purposes, in Table 9, steel number B 1 and
steel number B2, steel number B3 and steel number B4, steel number B5 and steel number
B6, steel number B7 and steel number B8, steel number B9 and steel number B10, steel
number B 1 1 and steel number B 12, steel number B 13 and steel number B 14, steel number
B 15 and steel number B 16, steel number B 17 and steel number B 18, steel number B 19
and steel number B20, steel number B2 1 and steel number B22, and steel number B23 and
steel number B24 are configured so as to have almost the same composition except that
the compositions such as Ce + La are different.
[0322]
Further, in Table 10, as condition D, a heating temperature was set to 1250°C, a
temperature at the completion of finishing rolling was set to 845OC, a cooling rate after
finishing rolling was set to 75"C/sec, and a coiling temperature was set to 450°C. As
condition E, the heating temperature was set to 1250°C, the temperature at the completion
of finishing rolling was set to 860°C, the steel sheet was air cooled at approximately
S°C/sec until the temperature reaches 680°C after the finishing rolling, and was cooled
thereafter at a cooling rate of 30°C/sec or more, and the coiling temperature was set to
400°C. As condition F, the heating temperature was set to 1250°C, the temperature at the
completion of finishing rolling was set to 825"C, the cooling rate after the finishing rolling
was set to 45"C/sec, and the coiling temperature was set to 450°C.
[0323]
Condition D was applied to steel number B 1 and steel number B2.
Condition E was applied to steel number B3 and steel number B4,
Condition E was applied to steel number B5 and steel number B6.
Condition F was applied to steel number B7 to steel number B 10.
Condition D was applied to steel number B 11 to steel number B 14.
Condition E was applied to steel number B 15 and steel number B 16.
Condition F was applied to steel number B 17 and steel number B 18.
Condition D was applied to steel number B 19 and steel number B20.
Condition E was applied to steel number B2 1 and steel number B22.
Condition F was applied to steel number B23 and steel number B24.
With these applications of conditions, the effects of chemical components can be
compared under the same producing conditions.
[0324]
[Table 111
[0325]
The thus obtained steel sheets were examined in terms of basic characteristics
including strength (MPa), ductility (%), stretch-flange formability (A%), and limit bending
radius (mm) for bending workability.
[0326]
To obtain existence states of elongated inclusions in the steel sheets, examination
was made on the number density per area of inclusions, and the ratio of number of, the
compositions of, and the equivalent circle diameter of inclusions having an elongated ratio
of 3 or less, through observation using an optical microscope or observation using a SEM,
by targeting the observation at all the inclusions having a size of approximately 0.5 pm or
more.
[0327]
Further, to obtain existence states of non-elongated inclusions in the steel sheet,
examination was made on the ratio of number of spherical compound inclusions
containing different inclusion phases including a first inclusion phase containing at least
one element of Ce, La, Nd, and Pr, hrther containing Ca, and at least one element of 0
and S, and a second inclusion phase further containing at least one element of Mn, Si, Ti,
and Al, the ratio of number of inclusions having the elongated ratio of 3 or less, and the
composition of Ce, La, Nd, and Pr, by targeting the observation at all the inclusions having
a size of approximately 0.5 pm or more. Note that the reason that inclusions having a
size of approximately 0.5 pm or more were targeted in the observation is because of
easiness of the observation and also because the inclusions having a size of less than
approximately 0.5 pm do not have any effect on the deterioration in the stretch-flange
formability or bending workability.
[0328]
Table 12 shows results of the examinations for each combination between steel
and rolling condition.
[0329]
[Table 121
[0330]
The strength and the ductility were obtained through a tensile test with Japanese
Industrial Standards (JIS) No.5 test piece taken from the steel sheet in a direction parallel
to the rolling direction. The stretch-flange formability was evaluated such that a punched
hole having a diameter of 10 mm and opened at the center of a steel sheet with 150 mm x
150 mm was pressed and expanded with a conical punch having an angle of 60°, a hole
diameter D (mm) was measured at the time when a through-thickness crack occurred, and
a hole-expanding value h was obtained from h = (D - 10)/10, thereby evaluating the
stretch-flange formability with the hole-expanding value h. The limit bending radius
(mm) used as an index indicating the bending workability was obtained by taking a
bending test piece, and carrying out a V-bending test using a die unit equipped with a die
and a punch. The die used has a recessed portion with a V shape in cross section and an
angle of aperture of 60". The punch used has an elevated portion that matches the
recessed portion of the die. Various punches were prepared in which bending radii of a
needle portion at a top end portion were varied in 0.5-mm steps, and were subjected to
bending tests to obtain the minimum radius of curvature of the needle portion at the top
end portion of the punch at which a crack occurs at a bent portion of the subjected test
piece. This minimum radius of curvature was evaluated as the limit bending radius.
[033 11
It should be noted that the test piece used was a No. 1 test piece specified in JIS,
which was obtained by equally cutting both sides of a raw sheet (hot rolled sheet) and had
a parallel portion of 25 mm, a radius of curvature R of 100 mm, and a thickness of 3.0
mm.
[0332]
As for inclusions, the major axis and the minor axis of randomly selected 50
inclusions having an equivalent circle diameter of 1 pm or more were measured through
SEM observation. Further, with a quantitative analysis function of the SEM,
composition analysis was performed for the randomly selected 50 inclusions having the
equivalent circle diameter of 1 pm or more. On the basis of the measurement results, the
ratio of number of inclusions having an elongated ratio of 3 or less, the composition
analysis of Ce, La, Nd, and Pr, and the average value of the total of at least one element of
Ce, La, Nd, and Pr in the inclusions were obtained.
[0333]
Although not shown in Table 12, with steel numbers B1, B3, B5, B7, B9, B11,
B 13, B 15, B 17, B 19, B2 1, and B23 to which the method according to the present
invention was applied, it was possible to generate the compound inclusions containing
different inclusion phases including the first inclusion phase of [REMI-[Cal-[O,S] and the
second inclusion phase of [Mn,Si,Ti,Al]-[REMI-[Cal-[O,wSlh,e reby it was possible to
reduce the elongated MnS-based inclusion in the steel sheet.
[0334]
More specifically, although not shown in Table 12, inclusions having the
equivalent circle diameter of 2 pm or less existed in the steel sheet; the ratio of the number
of the spherical compound inclusions formed by inclusion phases including the first
inclusion phase of [REMI-[Cal-[O,S] and the second inclusion phase of [Mn, Si, Ti,
All-[REV-[Cal-[O,S], the components of these inclusion phases being different from
each other, was 50% or more as can be clearly understood from Table 12; the spherical
compound inclusions had the size in the range of 0.5 pm to 5 pm; and the average content
percentage of the total of at least one element of Ce, La, Nd, and Pr in the inclusions
existing in the steel sheet and having elongated ratio of 3 or less was in the range of 0.5%
to 95%. The ratio of the number of the elongated inclusions having the equivalent circle
diameter of 1 pm or more and the elongated ratio of 3 or less was 50% or more. Note
that, in any structures of the steel sheets, the average crystal grain diameter fell within the
range of 2 pm to 10 pm, and were 10 pm or less in the present invention.
As a result, the steel sheets numbered B 1, B3, B5, B7, B9, B11, B 13, B 15, B17,
B 19, B21, and B23 exhibited excellent stretch-flange formability and bending workability
as compared with comparative steels.
[03 361
On the other hand, as for comparative steels (B2, B4, B6, B8, BlO, B12, B14,
B 16, B 18, B20, B22, and B24), although the average crystal grain diameters of all the
comparative steels were 10 pm or less, the ratio of the number of the small spherical
compound inclusions having the size in the range of 0.5 pm to 5 pm and containing
different inclusion phases including the first inclusion phase and the second inclusion
phase was apparently low, and the distribution state of the compound inclusions was
different from that specified in the present invention. Thus, the MnS-based inclusions
elongated during processes applied to the steel sheet served as the starting point of the
occurrence of cracking, deteriorating the stretch-flange formability and the bending
workability.
[03 3 71
Table 13 and Table 14 show an example of comparison between a case of the
present invention where Ca is added after addition of La (see steel number B25 according
to the present invention) and a case where La is added after addition of Ca (see steel
number B26 of Comparative Example). In the case where Ca was added after addition of
La, the ratio of the number of the spherical inclusions having the size of 5 pm or less
increased, the density of inclusions having the size of over 5 pm reduced, and the
hole-expandability improved.
[0338]
[Table 131
[0339]
[Table 141
[0340]
Table 15 and Table 16 show examples of a case of the present invention where
Ca was added after addition of Ce (see steel number B27) and a case where Ca was not
added (steel number B28 of Comparative Example). In the case where Ca was added
after addition of Ce, it is confirmed that the ratio of number of spherical inclusions having
the size of 5 pm or less increased, and the hole-expandability improved.
[0341]
[Table 151
[0342]
[Table 161
[0343]
It should be noted that, in the case of steel number B28 in Table 15 and Table 16,
the immersion nozzle clogged in the middle of the continuous casting process, not all the
molten steel in the ladle was able to be completely casted, and casting could not be
performed with the latter ladle, causing the production troubles. Further, processes of the
hot rolling or later were applied to slabs being processed but not completed, so that
products could be obtained.
Industrial Applicability
[0344]
According to the present invention, it is possible to obtain a high-strength steel
sheet exhibiting improved and excellent stretch-flange formability and bending
workability, and a method of producing molten steel for the high-strength steel sheet.

Table 2 I

Table 4

Table 1 1
Condition
D
E
F
Heating temperature
PC)
1250
1250
1250
Temperature at completion of
finishing rolling ("C)
845
860
825
Cooling rate after finishing rolling
("Clsec)
75
30
45
Coiling temperature
PC)
450
400
450
Table 12
number
cluster shape ratio of 3 or less equivalent circle ( P I
diameter of 0.5 to
Condition Strength
(MPa)
Elongation
(%I
Ratio of number of
compound
inclusion of Ce, La,
Nd, Pr, Si, AI, Ca,
Mn, Ca, 0, and S
having equivalent
Number density of
compound
oxysulfide having
over 5 pm and
having spherical or
Ratio of number of
inclusion having
equivalent circle
diameter of I pm or
more and elongated
Average
concentration of
total of at least one
element of ~ eLa,,
Nd, and Pr in
inclusion having
Average grain
diameter of crystal
in metal structure
Hole
expanding
value h
Limit bending
radius (mm)

CLAIMS
1. A high-strength steel sheet comprising:
C: 0.03 to 0.25 mass %,
Si: 0.1 to 2.0 mass %,
Mn: 0.5 to 3.0 mass %,
P: not more than 0.05 mass %,
T.0: not more than 0.0050 mass %,
S: 0.0001 to 0.0 1 mass %,
N: 0.0005 to 0.01 mass %,
acid-soluble Al: more than 0.01 mass %,
Ca: 0.0005 to 0.0050 mass %, and
a total of at least one element of Ce, La, Nd, and Pr: 0.001 to 0.01 mass %,
with a balance including iron and inevitable impurities, wherein:
the steel sheet contains a chemical component on a basis of mass that satisfies
0.7 < 100 x ([Ce] + [La] + [Nd] + [Pr])/[acid-soluble All 5 70, and
0.2 5 ([Ce] + [La] + [Nd] + [Pr])/[S] 5 10,
where [Ce] is an amount of Ce contained, [La] is an amount of La contained,
[Nd] is an amount of Nd contained, [Pr] is an amount of Pr contained, [acid-soluble All is
an amount of acid-soluble A1 contained, and [S] is an amount of S contained;
the steel sheet has a compound inclusion including a first inclusion phase
containing at least one element of Ce, La, Nd, and Pr, containing Ca, and containing at
least one element of 0 and S, and a second inclusion phase having a component different
from that of the first inclusion phase and containing at least one element of Mn, Si, and Al;
the compound inclusion forms a spherical compound inclusion having an
equivalent circle diameter in a range of 0.5 pm to 5 pm; and
a ratio of number of the spherical compound inclusion relative to number of all
inclusions having the equivalent circle diameter in the range of 0.5 ym to 5 pn is 30% or
more.
2. The high-strength steel sheet according to Claim 1, wherein
the spherical inclusion is an inclusion having an equivalent circle diameter of 1
pm or more, and
a ratio of number of elongated inclusions having a major axislminor axis of 3 or
less relative to number of all inclusions having the equivalent circle diameter of 1 pm or
more is 50% or more.
3. The high-strength steel sheet according to Claim 1, wherein
the spherical inclusion contains at least one element of Ce, La, Nd, and Pr, a total
of which is in a range of 0.5 mass % to 95 mass % in an average composition.
4. The high-strength steel sheet according to Claim 1, wherein
an average grain diameter of a crystal in a structure of the steel sheet is 10 pm or
less.
5. The high-strength steel sheet according to any one of Claims 1 to 4, further
containing at least one element of Nb: 0.01 to 0.10 mass %, and V: 0.01 to 0.10 mass %.
6. The high-strength steel sheet according to any one of Claims 1 to 4, fbrther
containing at least one element of:
Cu: 0.1 to 2 mass %,
Ni: 0.05 to 1 mass %,
Cr: 0.01 to 1 mass %,
Mo: 0.01 to 0.4 mass %, and
B: 0.0003 to 0.005 mass %.
7. The high-strength steel sheet according to any one of Claims 1 to 4, further
containing Zr: 0.001 to 0.01 mass %.
8. The high-strength steel sheet according to any one of Claims 1 to 4, further
containing at least one element of
Nb: 0.01 to 0.10 mass %,
V: 0.0 1 to 0.10 mass %,
Cu: 0.1 to 2 mass %,
Ni: 0.05 to 1 mass %,
Cr: 0.01 to 1 mass %,
Mo: 0.01 to 0.4 mass %,
B: 0.0003 to 0.005 mass %, and
Zr: 0.001 to 0.01 mass %.
9. A method of producing molten steel for the high-strength steel sheet according to
any one of Claims 1 to 4, the method having a refinement process for producing a steel,
the refinement process including:
a first process of obtaining a first molten steel including
applying processing so as to obtain P of not more than 0.05 mass % and
S of not less than 0.0001 mass %, and
performing addition or adjustment such that C is not less than 0.03 mass
% and not more than 0.25 mass %, Si is not less than 0.1 mass % and not more than 2.0
mass %, Mn is not less than 0.5 mass % and not more than 3.0 mass %, and N is not less
than 0.0005 mass % and not more than 0.01 mass %;
a second process of obtaining a second molten steel including
performing addition to the first molten steel such that A1 is more than
0.01 mass % in acid-soluble Al, and T.0 is not more than 0.0050 mass %;
a third process of obtaining a third molten steel including
adding at least one element of Ce, La, Nd, and Pr to the second molten
steel so as to satisfj on a basis of mass
0.7 < 100 x ([Ce] + [La] + md] + [Pr])/[acid-soluble All 570,
0.2 5 ([Ce] + [La] + [Nd] + [Pr])/[S] 5 10, and
0.001 5 [Ce] + [La] + wd] + [Pr] 5 0.01,
where [Ce] is an amount of Ce contained, [La] is an amount of La
contained, [Nd] is an amount of Nd contained, [Pr] is an amount of Pr contained,
[acid-soluble All is an amount of acid-soluble A1 contained, and [S] is an amount of S
contained; and
a fourth process of obtaining a fourth molten steel including
adding Ca to or performing adjustment to the third molten steel such that
Ca is not less than 0.0005 mass % and not more than 0.0050 mass %.
10. The method of producing molten steel for a high-strength steel sheet according to
Claim 9, wherein
the third process includes, before the at least one element of Ce, La, Nd, and Pr is
added to the second molten steel, adding at least one element of Nb and V to the second
molten steel such that the second molten steel further contains at least one element of Nb
of not less than 0.01 mass % and not more than 0.10 mass % and V of not less than 0.01
mass % and not more than 0.10 mass %.
11. The method of producing molten steel for a high-strength steel sheet according to
Claim 9 or 10, wherein
the third process includes, before the at least one element of Ce, La, Nd, and Pr is
added to the second molten steel, adding at least one element of Cu, Ni, Cr, Mo, and B to
the second molten steel such that the second molten steel further contains at least one
element of Cu of not less than 0.1 mass % and not more than 2 mass %, Ni of not less than
0.05 mass % and not more than 1 mass %, Cr of not less than 0.01 mass % and not more
than 1 mass %, Mo of not less than 0.01 mass % and not more than 0.4 mass %, and B of
not less than 0.0003 mass % and not more than 0.005 mass %.
12. The method of producing molten steel for a high-strength steel sheet according to
Claim 9 or 10, wherein
the third process includes, before the at least one element of Ce, La, Nd, and Pr is
added to the second molten steel, adding Zr to the second molten steel such that the
second molten steel further contains Zr of not less than 0.001 mass % to 0.01 mass %.
13. A high-strength steel sheet comprising:
C: 0.03 to 0.25 mass %,
Si: 0.03 to 2.0 mass %,
Mn: 0.5 to 3.0 mass %,
P: not more than 0.05 mass %,
T.0: not more than 0.0050 mass %,
S: 0.0001 to 0.01 mass %,
acid-soluble Ti: 0.008 to 0.20 mass %,
N: 0.0005 to 0.01 mass %,
acid-soluble Al: more than 0.01 mass %,
Ca: 0.0005 to 0.005 mass %, and
a total of at least one element of Ce, La, Nd, and Pr: 0.001 to 0.01 mass %,
with a balance including iron and inevitable impurities, wherein:
the steel sheet contains a chemical component on a basis of mass that satisfies
0.7 < 100 x ([Ce] + [La] + [Nd] + [Pr])/[acid-soluble All 170, and
0.2 5 ([Ce] + [La] + md] + [Pr])/[S] 5 10,
where [Ce] is an amount of Ce contained, [La] is an amount of La contained,
[Nd] is an amount of Nd contained, [Pr] is an amount of Pr contained, [acid-soluble All is
an amount of acid-soluble Al contained, and [S] is an amount of S contained;
the steel sheet has a compound inclusion including a first inclusion phase
containing at least one element of Ce, La, Nd, and Pr, containing Ca, and containing at
least one element of 0 and S, and a second inclusion phase having a component different
from that of the first inclusion phase and containing at least one element of Mn, Si, Ti, and
Al;
the compound inclusion forms a spherical compound inclusion having an
equivalent circle diameter in a range of 0.5 pm to 5 pm;
a ratio of number of the spherical compound inclusion relative to number of all
inclusions having the equivalent circle diameter in the range of 0.5 pm to 5 pm is 50% or
more; and
number density of an inclusion with more than 5 pm is less than 10 pieces/mm2.
14. The high-strength steel sheet according to Claim 13, wherein
the spherical inclusion is an inclusion having an equivalent circle diameter of 1
pm or more, and
a ratio of number of elongated inclusions having a major axislminor axis of 3 or
less relative to number of all inclusions having the equivalent circle diameter of 1 pm or
more is 50% or more.
15. The high-strength steel sheet according to Claim 13, wherein
the spherical inclusion contains at least one element of Ce, La, Nd, and Pr, a total
of which is in a range of 0.5 mass % to 95 mass % in an average composition.
16. The high-strength steel sheet according to Claim 13, wherein
an average grain diameter of a crystal in a structure of the steel sheet is 10 pm or
less.
17. The high-strength steel sheet according to any one of Claims 13 to 16, further
containing at least one element of:
Nb: 0.005 to 0.10 mass %, and
V: 0.0 1 to 0.10 mass %.
18. The high-strength steel sheet according to any one of Claims 13 to 16, further
containing at least one element of:
Cu: 0.1 to 2 mass %,
Ni: 0.05 to 1 mass %,
Cr: 0.01 to 1.0 mass %,
Mo: 0.01 to 0.4 mass %, and
B: 0.0003 to 0.005 mass %.
19. The high-strength steel sheet according to any one of Claims 13 to 16, hrther
containing Zr: 0.001 to 0.01 mass %.
20. The high-strength steel sheet according to any one of Claims 13 to 16, further
containing at least one element of:
Nb: 0.005 to 0.10 mass %,
V: 0.0 1 to 0.10 mass %,
Cu: 0.1 to 2 mass %,
Ni: 0.05 to 1 mass %,
Cr: 0.01 to 1.0 mass %,
Mo: 0.01 to 0.4 mass %,
B: 0.0003 to 0.005 mass %, and
Zr: 0.001 to 0.01 mass %.
2 1. A method of producing molten steel for the high-strength steel sheet according to
any one of Claims 13 to 16, having a refinement process for producing a steel, the
refinement process including:
a first process of obtaining a first molten steel including:
applying processing so as to obtain P of not more than 0.05 mass % and
S of not less than 0.0001 mass % and not more than 0.01 mass %, and
performing addition or adjustment such that C is not less than 0.03 mass
% and not more than 0.25 mass %, Si is not less than 0.03 mass % and not more than 2.0
mass %, Mn is not less than 0.5 mass % and not more than 3.0 mass %, and N is not less
than 0.0005 mass % and not more than 0.01 mass %;
a second process of obtaining a second molten steel including
performing addition to the first molten steel such that A1 is more than
0.01 mass % in acid-soluble Al, and T.0 is not more than 0.0050 mass %;
a third process of obtaining a third molten steel including
adding Ti of not less than 0.008 mass % and not more than 0.20 mass %
in acid-soluble Ti to the second molten steel;
a fourth process of obtaining a fourth molten steel including
adding at least one element of Ce, La, Nd, and Pr to the third molten
steel so as to satisfy on a basis of mass
0.7 < 100 x ([Ce] + [La] + [Nd] + [Pr])/[acid-soluble All 570,
0.2 5 ([Ce] + [La] + [Nd] + [Pr])/[S] 5 1 0, and
0.001 5 [Ce] + [La] + [Nd] + [Pr] 5 0.01,
where [Ce] is an amount of Ce contained, [La] is an amount of La
contained, v d ] is an amount of Nd contained, [Pr] is an amount of Pr contained,
[acid-soluble All is an amount of acid-soluble A1 contained, and [S] is an amount of S
contained; and
a fifth process of obtaining a fifth molten steel including
adding Ca to or performing adjustment to the fourth molten steel such
that Ca is not less than 0.0005 mass % and not more than 0.0050 mass %.
22. The method of producing molten steel for a high-strength steel sheet according to
Claim 21, wherein
the third process further includes, before the at least one element of Ce, La, Nd,
and Pr is added to the second molten steel, adding at least one element of Nb and V to the
second molten steel such that the second molten steel further contains at least one element
of Nb of not less than 0.005 mass % and not more than 0.10 mass %, and V of not less
than 0.0 1 and not more than 0.10 mass %.
23. The method of producing molten steel for a high-strength steel sheet according to
Claim 21 or 22, wherein
the third process hrther includes, before the at least one element of Ce, La, Nd,
and Pr is added to the second molten steel, adding at least one element of Cu, Ni, Cr, Mo,
and B to the second molten steel such that the second molten steel further contains at least
one element of Cu of not less than 0.1 mass % and not more than 2 mass %, Mi 9f not less
than 0.05 mass % and not more than 1 mass %, Cr of not less than 0.01 mass % and not
more than 1 mass %, Mo of not less than 0.01 mass % and not more than 0.4 mass %, and
B of not less than 0.0003 mass % and not more than 0.005 mass %.
24. The method of producing molten steel for a high-strength steel sheet according to
Claim 2 1 or 22, wherein
the third process further includes, before the at least one element of Ce, La, Nd,
and Pr is added to the second molten steel, adding Zr to the second molten steel such that
the second molten steel further contains Zr of not less than 0.001 mass % and not more
than 0.0 1 mass %.