SPECIFICATION
TITLE OF INVENTION
HIGH-STRENGTH STEEL SHEET AND METHOD FOR PRODUCING SAME
5 Field of the Invention
[0001]
The present invention relates to a high-strength steel sheet which can be
preferably mainly pressed and used in the underbody parts of automobiles and the like
and structural materials, and is excellent in terms of hole expansion and ductility, and a
10 method of producing the same.
Priority is claimed on Japanese Patent Application No. 2010-108431, filed May
10, 2010, and Japanese Patent Application No. 2010-133709, filed June 11, 2010, the
contents of which are incorporated herein by reference.
15 Description of Related Art
[00021
A steel sheet used for the structure of an automobile body needs to have
favorable formability and strength. Asa high strength steel sheet having both
formability and high strength, a steel sheet composed of ferrite and martensite, a steel
20 sheet composed of ferrite and bainite, a steel sheet including retained austenite in the
microstructure, and the like are known.
[0003]
The above complex microstructure steel sheets are disclosed in, for example,
Patent Citations Ito 3. However, there is a demand for a complex microstructure steel
25 sheet having more favorable hole expansion than in the conventional technique in order
2
to meet demands for an additional decrease in the weight of modem automobiles and the
capability of parts to have more complicated shapes.
[0004]
A complex microstructure steel sheet including martensite dispersed in a ferrite
5 matrix has a low yield ratio, a high tensile strength, and an excellent elongation.
However, in the complex microstructure steel sheet, stress concentrates on the interfaces
between ferrite and martensite, cracks easily occur at the interfaces, and thus the complex
microstructure steel sheet has the disadvantage of poor hole expansion.
[0005]
10 In contrast to the above, Patent Citation 4 discloses a high-strength hot-rolled
steel sheet having excellent hole expansion that are required for the recent wheel and
underbody member materials. In Patent Citation 4, the amount of C in the steel sheet is
decreased as much as possible so that a solid solution-hardened or precipitation-hardened
ferrite is included in the steel sheet which includes bainite as a major part of the
15 microstructure at an appropriate volume fraction, the difference in hardness between the
ferrite and the bainite decreases, and generation of coarse carbides is prevented.
[0006]
In addition, Patent Citations 5 and 6 disclose methods in which MnS-based
coarse inclusions present in slabs are dispersed and precipitated in a steel sheet as fine
20 spherical inclusions which include MnS so as to provide ahigh-strength steel sheet that is
excellent in terms of hole expansion without deteriorating fatigue characteristics. In
Patent Citation 5, deoxidation is carried out by adding Ce and La without substantially
adding A], and fine MnS is precipitated on fine and hard Cc oxides, La oxides, cerium
oxysulfides, and lanthanum oxysulfides, all of which are generated by the deoxidation.
25 In this technique, MnS does not elongate during rolling, and therefore the MrS does not
3
easily serve as a starting point of cracking or crack propagation path, and the hole
expansion can be improved.
Patent Citation
5 [0007]
[Patent Citation 1] Japanese Unexamined Patent Application, First Publication
No. F16-128688
[Patent Citation 2] Japanese Unexamined Patent Application, First Publication
No. 2000-319756
10 [Patent Citation 3] Japanese Unexamined Patent Application, First Publication
No. 2005120436
[Patent Citation 4] Japanese Unexamined Patent Application, First Publication
No. 2001-200331
[Patent Citation 5] Japanese Unexamined Patent Application, First Publication
15 No. 2007-146290
[Patent Citation 6] Japanese Unexamined Patent Application, First Publication
No. 2008274336
SUMMARY OF THE INVENTION
20 Problems to be Solved by the Invention
[0008]
The high-strength hot-rolled steel sheet as disclosed in Patent Citation 4, in
which a major part of the microstructure is bainite , and generation of coarse carbides is
suppressed, exhibits excellent hole expansion, but the ductility is poor compared to a
25 steel sheet mainly including ferrite and martensite. In addition, while generation of
4
coarse carbides is suppressed, it is still difficult to prevent occurrence of cracks in a case
in which a strict hole expanding is carried out.
[0009]
According to studies by the inventors, it was found that the above disadvantages
5 result from elongated sulfide-based inclusions mainly including MnS in the steel sheet.
When the steel sheet is repeatedly deformed, internal defects are caused in the vicinity of
elongated coarse MnS-based inclusions that are present in and in the vicinity of the
surface layer of the steel sheet, the internal defects propagate as cracks, and the fatigue
characteristics deteriorate. In addition, the elongated coarse MnS-based inclusions
10 become liable to serve as starting points of cracking during a hole expanding.
[0010]
Therefore, it is desirable to make MnS-based inclusions in steel into a fine
spherical shape while preventing the MnS-based inclusions from being elongated as
much as possible.
15 [0011]
However, since Mn is an element that increases the strength of materials
together with C or Si, in a high strength steel sheet, it is common to set the concentration
of Mn to a high percentage in order to secure the strength. Furthermore, when a heavy
treatment for desulfurization is not carried out in a secondary refining, 50 ppm or more of
20 S is included in steel. Therefore, generally, MnS is present in slabs.
[0012]
In addition, when the concentration of soluble Ti is increased in order to
improve stretch flangeability, the soluble Ti partially bonds with coarse TiS and MnS so
as to precipitate (Mn, Ti)S.
25 Since 1VhrS-based inclusions (hereinafter, three inclusions of MnS, TiS, and (Mn,,
5
Ti)S will be referred to as "MnS-based inclusions " for convenience) are liable to deform
when steels are hot=rolled or cold-rolled , the MnS-based inclusions are elongated, which
causes degradation of hole expansion.
[0013]
5 In contrast to Patent Citation 4, in Patent Citations 5 and 6, since fine
MnS-based inclusions are precipitated in slabs, and the MnS-based inclusions are
dispersed in the steel sheet as fine spherical inclusions that do not easily serve as starting
points of cracking while not deforming during rolling, it is possible to manufacture a
hot-rolled steel sheet that is excellent in terms of hole expansion.
10 [0014]
However, in Patent Citation 5, since the steel sheet has a microstructure mainly
including bainite, sufficient ductility cannot be expected compared to a steel sheet having
microstructures mainly including ferrite and martensite . In addition, in a steel sheet
having microstructures mainly including ferrite and martensite , which are significantly
15 different in hardness , hole expansion are not significantly improved even when
MnS-based inclusions are finely precipitated using the techniques of Patent Citations 5
and 6.
[0015]
The present invention has been made to solve the problems of the conventional
20 techniques, and provides a complex microstructure type high-strength steel sheet that is
excellent in terms of hole expansion and ductility, and a method of manufacturing the
same.
Methods for Solving the Problem
25 [0016]
6
Hole expansion are a characteristic that is dependent on the uniformity of the
microstructure, and, in a multiphase steel sheet mainly including ferrite and martensite
having a large difference in hardness in the microstructure, stress concentrates in the
interfaces between the ferrite and the martensite, and cracks are liable to occur at the
5 interfaces. Additionally, the hole expansion are significantly deteriorated by
sulfide-based inclusions in which MnS and the like are elongated.
[0017]
As a result of thorough studies, the inventors found that, when chemical
components and manufacturing conditions are adjusted so as to prevent the hardness of a
10 martensite phase (martensite) in a multiphase steel sheet mainly including ferrite and
martensite from excessively increasing, and MnS-based inclusions are finely precipitated
through deoxidation by addition of Cc and La, hole expansion can be significantly
improved even in a steel sheet having a microstructure in which ferrite and martensite are
mainly included, and completed the present invention.
15 [0018]
Meanwhile, an example in which TiN is precipitated on fine and hard Cc oxides,
La oxides, cerium oxysulfides, and lanthanum oxysulfides together with MnScbased
inclusions was also observed, but it was confirmed that such an example has little
influence on hole expansion and ductility.
20 Therefore, in the present invention, TiN will not be taken into account as a
partner of MnSnbased inclusions.
[0019]
The purports of the present invention are as follows:
[0020]
25 (1) A high strength steel sheet according to an aspect of the present invention
7
includes, by mass%, C: 0.03% to 0.30%, Si: 0.08% to 2.1%, Mn: 0.5% to 4.0%, P: 0.05%
or less, S: 0.0001% to 0.1%, N: 0.01% or less, acid-soluble Al: more than 0.004% and
less than or equal to 2.0%, acid. soluble Ti: 0.0001% to 0.20%, at least one selected from
Cc and La: 0.001% to 0.04% in total, and a balance of iron and inevitable impurities, in
5 which [Ce], [La], [acid-soluble Al], and [S] satisfy 0.02< _ ([Ce] + [La]) / [acid-soluble
Al] < 0.25, and 0.4 <_ ([Ce] + [La]) / [S] <_ 50 in a case in which the mass percentages of
Ce, La, acid-soluble Al, and S are defined to be [Ce], [La], [acid-soluble Al], and [S],
respectively, and the microstructure of the high-strength steel sheet includes 1 % to 50%
of martensite in terms of an area ratio.
10 [0021]
(2) The high-strength steel sheet according to the above (1) may further include,
by mass%, at least one selected from a group consisting of Mo: 0.001% to 1.0%, Cr:
0.001% to 2.0%, Ni: 0.001% to 2.0%, Cu: 0.001% to 2.0%, B: 0.0001% to 0.005%, Nb:
0.001% to 0.2%, V: 0.001%to 1.0%, W: 0,001% to 1,0%, Ca: 0.0001% to 0.01%, Mg:
15 0.0001% to 0.01%, Zr: 0.0001% to 0.2%, at least one selected from Se and lanthanoids
ofPr through Lu: 0.0001%to 0.1%, As: 0.0001% to 0.5%, Co: 0.0001% to 1.0%, Sn:
0.0001% to 0.2%, Pb: 0.0001% to 0.2%, Y: 0.0001% to 0.2%, and Hf: 0.0001% to 0.2%.
[0022]
(3) In the high-strength steel sheet according to the above (1) or (2), the amount
20 of the acid-soluble Ti may be more than or equal to 0.0001% and less than 0.008%.
[0023]
(4) In the high-strength steel sheet according to the above (1) or (2), the amount
of the acid-soluble Ti may be 0.008% to 0.20%.
[0024]
25 (5) In the high-strength steel sheet according to the above (1) or (2), [Ce], [La],
8
[acid-soluble Al], and [S] may satisfy 0.02 < ([Ce] + [La]) / [acid-soluble All < 0.15.
[0025]
(6) In the high-strength steel sheet according to the above (1) or (2), [Ce], [La],
[acid-soluble Al], and [S] may satisfy 0.02< ([Ce] + [La]) / [acid-soluble Al] < 0.10.
5 [0026]
(7) In the high-strength steel sheet according to the above (1) or (2), the amount
of acid-soluble Al may be more than 0.0 1% and less than or equal to 2.0%.
[0027]
(8) In the high-strength steel sheet according to the above (1) or (2), the number
10 density of inclusions having an equivalent circle diameter of 0.5 μm to 2 μm in the
microstructure may be 15 inclusions/mm2 or more.
[0028]
(9) In the high-strength steel sheet according to the above (1) or (2), of
inclusions having an equivalent circle diameter of 1.0 μm or more in the microstructure,
15 the number percentage of elongated inclusions having an aspect ratio of 5 or more
obtained by dividing the long diameter by the short diameter may be 20% or less.
[0029]
(10) In the high-strength steel sheet according to the above (1) or (2), of
inclusions having an equivalent circle diameter of 1.0 t m or more in the microstructure,
20 the number percentage of inclusions having at least one of MnS, TiS, and (Mn, Ti)S
precipitated to an oxide or oxysulfide composed of at least one of Cc and La, and at least
one of 0 and S, or an oxide or oxysulfide composed of at least one of Cc and La, at least
one of Si and Ti, and at least one of 0 and S may be 10% or more.
[0030]
9
(11) In the high-strength steel sheet according to the above (1) or (2), the volume
number density of elongated inclusions having an equivalent circle diameter of I μm or
more, and an aspect ratio of 5 or more obtained by dividing the long diameter by the
short diameter may be 1.0 x 104 inclusions/mm3 or less in the steel structure.
5 [0031]
(12) In the high-strength steel sheet according to the above (1) or (2), in the
microstructure, the volume number density of inclusions having at least one of MnS, TiS,
and (Mn, Ti)S precipitated to an oxide or oxysulfide composed of at least one of Cc and
La, and at least one of 0 and S, or an oxide or oxysulfide composed of at least one of Cc
10 and La, at least one of Si and Ti, and at least one of 0 and S may be 1.0 x 103
inclusions/mm3 or more.
[0032]
(13) In the high-strength steel sheet according to the above (1) or (2), elongated
inclusions having an equivalent circle diameter of 1 μm or more, and an aspect ratio of 5
15 or more obtained by dividing the long diameter by the short diameter may be present in
the microstructure, and the average equivalent circle diameter of the elongated inclusions
maybe 10 μm or less.
[0033]
(14) In the high-strength steel sheet according to the above (1) or (2), inclusions
20 having at least one of MnS, TiS, and (Mn, Ti)S precipitated to an oxide or oxysulfde
composed of at least one of Cc and La, and at least one of 0 and S, or an oxide or
oxysulfide composed of at least one of Cc and La, at least one of Si and Ti, and at least
one of 0 and S may be present in the microstructure, and the inclusions may include a
total of 0.5 mass% to 95 mass% of at least one of Ce and La in terms of an average
10
composition.
[0034]
(15) In the highstrength steel sheet according to the above (1) or (2), the
average grain size in the microstructure may be 10 μm or less.
5 [0035]
(16) In the high-strength steel sheet according to the above (1) or (2), the
maximum hardness of martensite included in the microstructure may be 600 Hv or less.
[0036]
(17) In the high-strength steel sheet according to the above (1) or (2), the sheet
10 thickness maybe 0.5 mm to 20 mm.
[0037]
(18) The high-strength steel sheet according to the above (1) or (2) may further
have a galvanized layer or a galvannealed layer on at least one surface.
[0038]
15 (19) A method of manufacturing a high-strength steel sheet according to the
aspect of the present invention includes a first process in which molten steel having the
chemical components according to the above (1) or (2) is subjected to continuous casting
so as to be processed into a slab; a second process in which hot rolling is carried out on
the slab in a finishing temperature of 850°C to 970°C, and a steel sheet is manufactured;
20 and a third process in which the steel sheet is cooled to a cooling control temperature of
650°C or lower at an average cooling rate of 10 °C/second to 100 °C/second, and then
coiled at a coiling temperature of more than and equal to 300°C and less than 650°C.
[0039]
(20) In the method of manufacturing the high-strength steel sheet according to
11
the above (19), in the third process, the cooling control temperature may be 450°C or
lower, the coiling temperature may be 300°C to 450°C, and a hot-rolled steel sheet may
be manufactured.
[0040]
5 (21) The method of manufacturing the high-strength steel sheet according to the
above (19) may further include, after the third process, a fourth process in which the steel
sheet is pickled, and cold rolling is carried out on the steel sheet at a reduction in
thickness of 40% or more; a fifth process in which the steel sheet is annealed at a
maximum temperature of 750°C to 900°C; a sixth process in which the steel sheet is
10 cooled to 450°C or lower at an average cooling rate of 0.1 °C/second to 200 °C/second;
and a seventh process in which the steel sheet is held in a temperature range of 300°C to
450°C for I second to 1000 seconds so as to manufacture a cold-rolled steel sheet.
[0041]
(22) In the method of manufacturing the high-strength steel sheet according to
15 the above (20) or (21), galvanizing or galvannealing may be carried out on at least one
surface of the hot-rolled steel sheet or the cold-rolled steel sheet.
[0042]
(23) In the method of manufacturing the high-strength steel sheet according to
the above (19), the slab may be reheated to 1100°C or higher after the first process and
20 before the second process.
Effects of the Invention
[0043]
According to the present invention, it is possible to stably adjust the chemical
12
composition of molten steel, suppress generation of coarse alumina inclusions, and
precipitate sulfides in a slab through fine MnS-based inclusions by controlling Al
deoxidation and deoxidation by addition of Cc and La. Since the fine MnS-based
inclusions are dispersed in the steel sheet as fine spherical inclusions, do not deform
5 during rolling, and do not easily serve as starting points of cracking, it is possible to
obtain a high-strength steel sheet that is excellent in terms of hole expansion and
ductility.
[0044]
Since the high-strength steel sheet according to the above (1) is a multiphase
10 steel sheet mainly including ferrite and martensite, the ductility is excellent. In addition,
in the high-strength steel sheet according to the above (16), since the hardness of the
martensite phase is controlled, it is possible to further enhance the effect of improving
hole expansion by controlling the morphology of inclusions. Furthermore, in the
method of manufacturing the high-strength steel sheet according to the above (19), it is
15 possible to manufacture a multiphase steel sheet mainly including ferrite and martensite,
in which fine MnS-based inclusions are dispersed, that is, a high-strength steel sheet that
is excellent in terms of hole expansion and ductility.
BRIEF DESCRIPTION OF THE DRAWINGS
20 [0045]
FIG. 1 is a view showing a. relationship between the maximum hardness and the
hole expansion of a martensite phase.
FIG 2 is a flowchart showing a method of manufacturing a high-strength steel
sheet according to an embodiment of the present invention.
25
13
DETAILED DESCRIPTION OF THE INVENTION
[0046]
Hereinafter, the high-strength steel sheet of the present invention will be
described in detail. Hereinafter, mass% in chemical components (chemical
5 compositions) will be denoted simply by %.
[0047]
Firstly, experiments that have been made until completion of the present
invention will be described.
[0048]
10 Deoxidation by various amounts (chemical components in molten steel) of Cc
and La were carried out together with Al deoxidation so as to manufacture slabs. The
slabs were hot--rolled so as to manufacture 3 mm hot-rotted steel sheets. Furthermore,
the hot-rolled steel sheets were pickled, then cold-rolled at a reduction in thickness of
50%, and annealed under a variety of annealing conditions so as to manufacture
15 cold-rolled steel sheets. The inventors provided the cold-rolled steel sheets for hole
expansion tests and tension tests, and investigated the number densities, morphologies,
and average chemical compositions of inclusions in the steel sheets.
[0049]
As a result of the above tests, it was found that, in molten steel obtained by
20 adding Si, then adding A], then adding one or both of Cc and La, and thereby carrying
out deoxidation, in a case in which ([Ce] + [La]) / [acid-soluble Al] and ([Ce] + [La]) /
[S] are in predetermined ranges, the oxygen potential in the molten steel abruptly
decreases, the concentration of A1203 being generated decreases, and a steel sheet that is
excellent in terms of hole expansion can be obtained, Here, [Ce], [La], [acid-soluble
25 Al], and [S] represent by mass% of Ce, La, acid-soluble Al, and S that are included in
14
steel, respectively (hereinafter, the same expression as this description will be used).
[0050]
The amount of increase in the hole expansion ratio of a cold-rolled steel sheet to
which one or both of Cc and La were added with respect to the hole expansion ratio of a
5 cold-rolled steel sheet to which neither Cc nor La were added was varied by the hardness
of a martensite phase in the steel sheet, and the amount of increase increased as the
hardness decreased.
[0051]
It could be confirmed that, when the maximum hardness of the martensite phase
10 was 6001Tv or less, the hole expansion were improved more clearly by adding one or
both of Cc and La. The maximum hardness of the martensite phase refers to the
maximum value of micro Vickers hardness obtained by randomly pressing an indenter
with a load of 10 gf on a hard phase (other than a ferrite phase) 50 times.
[0052]
15 The cold-rolled steel sheet to which neither Ce nor La were added (the steel
sheet used to compare the hole expansion ratios) was annealed under the same conditions
so as to have the same tensile strength as the cold-rolled steel sheet to which one or both
of Cc and La were added. In this case, it was confirmed that uniform elongation of the
cold-rolled steel sheet to which neither Cc nor La were added and uniform elongation of
20 the cold-rolled steel sheet to which one or both of Cc and La were added were the same,
and deterioration of the ductility due to the addition of Cc and La was not observed.
[0053]
Meanwhile, in arnicrostructure that is substantially composed of bainite, the
hole expansion were significantly improved by addition of Cc and La, but the ductility
25 was small compared, to the steel sheet mainly including ferrite and martensite.
15
[0054]
Reasons why the hole expansion were improved by addition of Cc and La are
considered to be as follows:
[0055]
5 It is considered that, when Si is added to molten steel in manufacturing a slab,
SiO2 inclusions are formed, but the SiO2 inclusions are reduced to Si by later addition of
Al. Al reduces SiO2 inclusions, and deoxidizes dissolved oxygen in the molten steel so
as to form A12O3-based inclusions. Some ofA12O3-based inclusions are removed
though floatation, and the rest of the A12O3-based inclusions remain in the molten steel.
10 [0056]
After that, when Cc and La are added to the molten steel, a little amount of
A12O3 remains, but the A]2O3=based inclusions in the molten steel are reduced and
decomposed, and fine and hard Cc oxides, La oxides, cerium oxysulfides, and lanthanum
oxysulfides are formed by deoxidation using Cc and La.
15 [0057]
When Al deoxidation is appropriately carried out based on the above
deoxidation, similarly to a case in which Al deoxidation is rarely carried out, it is
possible to precipitate MnS on the fine and hard Cc oxides, La oxides, and cerium
oxysulfides which are formed by deoxidation by addition of Cc and La. As a result, it is
20 possible to suppress deformation of the precipitated MnS during rolling, and therefore
elongated coarse MnS in the steel sheet can be significantly reduced, and the hole
expansion can be improved. Additionally, since it is also possible to further lower the
oxygen potential of the molten steel by Al deoxidation, fluctuation in the chemical
composition can be reduced.
25 [0058]
16
Reasons why the degree of the hole expansion improved is varied by the
hardness of the martensite phase in steel sheets having the same tensile strength and
uniform elongation are considered to be as follows.
[0059]
5 Hole expansion are significantly affected by the local ductility of a steel, and the
most dominant factor in relation to hole expansion is considered to be the difference in
hardness between microstructures (herein, between the martensite phase and the ferrite
phase). Other powerful dominant factors in relation to hole expansion include the
presence of nonmetallic inclusions, such as MnS, and many publications report that voids
10 are formed from the inclusions as the starting points, grow, and link together such that the
steel breaks.
[0060]
Therefore, if the hardness of the martensite phase is excessively high, there are
cases in which, even when the morphology of inclusions are controlled by addition of Cc
15 and La, and occurrence of voids due to the inclusions is suppressed, stress concentrates at
the interfaces between ferrite and martensite, voids are formed due to the difference in
strength between the microstructures, and thereby the steel may break.
[0061]
The inventors newly found that, if the cooling conditions after hot rolling in the
20 case of a hot-rolled steel sheet and the annealing conditions in the case of a cold-rolled
steel sheet are appropriately controlled, and the hardness of the martensite phase is
reduced, it is possible to further enhance the effect of suppressing occurrence of voids by
controlling the morphology of the inclusions. In addition, the inventors found that a
steel sheet that is excellent in terms of ductility and hole expansion can be obtained by
25 securing a predetermined amount or more of martensite in a microstructure mainly
17
including ferrite and martensite, and controlling the morphology of inclusions by adding
Cc and La.
[0062]
Meanwhile, it is possible to add Ti to the molten steel after Al is added and
5 before Cc and La are added. At this point in time, since oxygen in the molten steel is
already deoxidized by Al, the amount of oxygen to be deoxidized by Ti is small. After
that, due to Cc and La that have been added to the molten steel, A12O3-based inclusions
are reduced and decomposed, and fine Cc oxides, La oxides, cerium oxysulfides, and
lanthanum oxysulfides are formed.
l0 [0063]
As described above, it is considered that, when complex deoxidation is carried
out by adding At, Si, Ti, Ce, and La, a small amount ofA12O3 remains, but fine and hard
Cc oxides, La oxides, cerium oxysulfides, lanthanum oxysulfides, and Ti oxides are
mainly formed.
15 [0064]
During the complex deoxidation by addition of Al, Si, Ti, Cc, and La, if the At
deoxidation is appropriately carried out based on the deoxidation as described above,
similarly to a case in which Al deoxidation is rarely carried out, it is possible to
precipitate MnS, TiS, or (Mn, Ti)S on fine and hard oxides, such as Cc oxides, La oxides,
20 and Ti oxides, or fine and hard oxysulfides, such as cerium oxysulfides and lanthanum
oxysulfides. Asa result, in a case in which a predetermined amount or more of Ti is
added to the molten steel, the kinds of chemical elements included in inclusions slightly
vary, but a mechanism that suppresses elongation of MnS-based inclusions was the same
as in a case in which Ti is rarely added.
25 [0065]
18
Based on the finding obtained from experimental studies, the inventors studied
the chemical compositions, microstructures, and manufacturing conditions of steel sheets
as described below. Firstly, a high-strength steel sheet according to an embodiment of
the present invention will be described.
5 [0066]
Hereinafter, reasons why the chemical compositions are limited in the
high-strength steel sheet according to the embodiment of the present invention will be
described.
[0067]
10 C is the most fundamental element that controls the hardenability and strength of
steel, which increases the hardness and thickness of a layer hardened by quenching so as
to improve the fatigue strength. That is, C is an essential element for securing the
strength of a steel sheet. In order to form retained austenite and low. temperature
transformation phases that are necessary to obtain a desired high-strength steel sheet, the
15 concentration of C needs to be 0.03% or more. When the concentration of C exceeds
0.30%, formability and weldability deteriorate. Therefore, in order to achieve a
necessary strength and secure formability and weldability, the concentration of C needs
to be 0.30% or less. When the balance between strength and formability is taken into
account, the concentration of C is preferably 0.05% to 0.20%, and more preferably
20 0.10% to 0.15%.
[0068]
Si is one major deoxidizing element. In addition, Si increases the number of
nucleation sites of austenite during heating for quenching, and suppresses the grain
growth of austenite so as to refine the grain size in a layer hardened by quenching. In
25 addition, Si suppresses formation of carbides, and suppresses degradation of grain
19
boundary strength due to carbides. Furthermore, Si is also effective for forming bainite,
and plays a critical role from the viewpoint of securing the overall strength.
[0069]
In order to develop the above effects, it is necessary to add 0.08% or more of Si
5 to steel. When the concentration of Si is too high, even in a case in which Al
deoxidation is sufficiently carried out, the concentration of SiO2 in inclusions increases,
and coarse inclusions become liable to be formed. In addition, in this case, toughness,
ductility, and weldability deteriorate, and surface decarburization and surface flaws
increase so as to deteriorate fatigue characteristics. Therefore, the upper limit of the
10 concentration of Si needs to be 2.1%. When the balance between strength and other
mechanical properties is taken into account, the concentration of Si is preferably 0.10%
to 1.5%, and more preferably 0.12% to 1.0%.
[0070]
Mn is a useful element for deoxidation in a steelmaking step, and an effective
15 element for increasing the strength of the steel sheet together with C and Si. In order to
obtain the above effect, the concentration of Mn needs to be 0.5% or more. When more
than 4.0% of Mn is included in steel, ductility degrades due to segregation of Mn and
enhancement of solid solution strengthening. In addition, since weldability and the
toughness of a base metal deteriorate, the upper limit of the concentration of Mn is 4.0%.
20 When the balance between strength and other mechanical properties is taken into account,
the concentration of Mn is preferably 1.0% to 3.0%, and more preferably 1.2% to 2.5%.
[0071]
P is useful in a case in which P is used as an element for substitutional solid
solution strengthening which is smaller than an Fe atom. When the concentration of P
25 in steel exceeds 0.05%, there are cases in which P segregates at the grain boundaries of
20
austenite, the grain boundary strength degrades, and formability may deteriorate.
Therefore, the upper limit of the concentration of P is 0.05%. When solid solution
strengthening is not required, it is not necessary to add P to steel, and therefore the lower
limit of the concentration of P includes 0%. Meanwhile, for example, the lower limit of
5 the concentration of P may be 0.0001% in consideration of the concentration of P
included as an impurity.
[0072]
N is an element that is inevitably incorporated into steel since nitrogen in the air
is trapped into molten steel during treating molten steel. N has an action of forming
10 nitrides with chemical elements, such as Al and Ti, so as to promote refining of the
microstructure in the base metal. However, when the concentration of N exceeds 0.01%,
N forms coarse precipitates with chemical elements, such as Al and Ti, and hole
expansion deteriorate. Therefore, the upper limit of the concentration of Nis 0.01%.
On the other hand, when the concentration of N is reduced to less than 0.0005%, the cost
15 increases, and therefore the lower limit of the concentration of N may be 0.0005% from
the viewpoint of industrial feasibility.
[0073]
S is included in the steel sheet as an impurity, and liable to segregate in steel.
Since S forms elongated coarse MnS-based inclusions so as to deteriorate hole expansion,
20 the concentration is preferably extremely low. In the conventional techniques, it was
necessary to significantly decrease the concentration of S in order to secure hole
expansion.
[0074]
However, when an attempt is made to decrease the concentration of S to less
25 than 0.0001 °/0, the desulfurization load during secondary refining increases, and the
21
desulfurization cost increases excessively. In a case in which desulfurization during
secondary refining is assumed, when the desulfurization cost in accordance with the
quality of the steel sheet is taken into consideration, the lower limit of the concentration
of S is 0.0001%. Meanwhile, in a case in which the costs for secondary refining are
5 further suppressed, and the effect of addition of Cc and La are more effectively used, the
concentration of S is preferably more than 0.0004%, more preferably 0.0005% or more,
and most preferably 0.0010% or more.
[0075]
In addition, in the present embodiment, MnS-based inclusions are precipitated
10 on fine and hard inclusions, such as Cc oxides, La oxides, cerium oxysul fides, and
lanthanum oxysulfides, so as to control the morphology of MnS-based inclusions.
Therefore, inclusions do not easily deform during rolling, and elongation of the
inclusions is prevented. Therefore, the upper limit of the concentration of S is specified
by the relationship between the concentration of S and the total amount of one or both of
15 Cc and La as described below. For example, the upper limit of the concentration of S is
0.1%.
[0076]
In the embodiment, since the morphology of MnS-based inclusions are
controlled by inclusions, such as Cc oxides, La oxides, cerium oxysulfides, and
20 lanthanum oxysulfides, even when the concentration of S is high, it is possible to prevent
S from adversely affecting the qualities of the steel sheet by adding one or both of Cc and
La at an amount that corresponds to the concentration of S. That is, even when the
concentration of S increases to a certain extent, a substantial desulfurization effect can be
obtained by adding one or both of Cc and La to steel at an amount that corresponds to the
25 concentration of S. and steel having the same qualities as extremely low sulfur steel can
22
be obtained.
[0077]
In other words, since the concentration of S is appropriately adjusted in
accordance with the total amount of Ce and La, the flexibility is large for the upper limit
5 of the concentration of S. Asa result, in the embodiment, it is not necessary to carry
out desulfurization of the molten steel during the secondary refining in order to obtain
extremely low sulfur steel, and it is possible to skip the secondary refining. Therefore,
it is possible to simplify the manufacturing processes of the steel sheet and, accordingly,
reduce the costs for the desulfirrization.
10 [0078]
Generally, since oxides of Al are liable to form clusters so as to be coarse and
deteriorate hole expansion, it is preferable to suppress acid-soluble Al in the molten steel
as much as possible. However, the inventors newly found areas in which alumina-based
oxides are prevented from forming clusters so as to be coarse by controlling the
15 concentrations of Cc and La in the molten steel in accordance with the concentration of
the acid-soluble Al while Al deoxidation is carried out. In the areas, of A12O3-based
inclusions formed by the Al deoxidation, some of the A12O3-based inclusions are
removed through floatation, and the rest of the A12O3-based inclusions in the molten steel
are reduced and decomposed by Cc and La that are to be added afterwards, thereby
20 forming fine inclusions.
[0079]
Therefore, in the embodiment, it is substantially unnecessary to add Al to steel,
and, particularly, the flexibility is large for the concentration of the acid-soluble Al. For
example, the concentration of the acid-soluble At may be more than 0.004% in
25 consideration of the relationship between the concentration of the acid-soluble Al and the
23
total amount of one or both of Cc and La, which will be described below.
[0080]
In addition, in order to jointly use Al deoxidation and deoxidation by the
addition of Cc and La, the concentration of the acid-soluble Al may be more than 0.010%.
5 In this case, unlike the conventional techniques, it becomes unnecessary to increase the
amounts of Cc and La in order to secure the total amount of deoxidizing elements, the
oxygen potential in steel can be further lowered, and variation in the amount of each
chemical element in the chemical composition can be suppressed. Meanwhile, in a case
in which the effect of jointly using Al deoxidation and deoxidation by the addition of Cc
10 and La is further enhanced, the concentration of the acid-soluble Al is preferably more
than 0.020%, and more preferably more than 0.040%.
[0081]
The upper limit of the concentration of the acid-soluble Al is specified by the
relationship between the acid-soluble Al and the total amount of one or both of Cc and La
15 as described below. For example, the concentration of the acid-soluble Al may be 2.0%
or less in consideration of the above relationship.
[0082]
Here, the concentration of the acid-soluble Al is determined by measuring the
concentration ofAl that dissolves in an acid. For analysis of the acid-soluble Al, the
20 fact that dissolved Al (or solute Al in a solid solution) dissolves in an acid, but A12O3
does not dissolve in an acid is used. Here, examples of the acid include a mixed acid in
which chloric acid, nitric acid, and water are mixed at a ratio (mass ratio) of 1:1:2.
Using such an acid, Al that is soluble in the acid and A1203 that is insoluble in the acid
are separated, and the concentration of the acid-soluble Al can be measured. Meanwhile,
25 the acid-insoluble Al (A1203 that is insoluble in the acid) is determined as an inevitable
24
impurity.
[0083]
Ti is a major deoxidizing element, and increases the number of the nucleation
sites of austenite when carbides, nitrides, and carbonitrides are formed, and the slabs are
5 sufficiently heated before hot rolling. As a result, since the grain growth of austenite is
suppressed, Ti contributes to refining of crystal grains and an increase in the strength of
the steel sheet, promotes dynamic recrystallization during hot rolling, and significantly
improves hole expansion.
[0084]
10 Therefore, in a case in which the above effect is sufficiently enhanced, 0.008%
or more of the acid-soluble Ti may be added to steel. In a case in which the above
effect does not need to be sufficiently secured, and a case in which the slabs cannot be
sufficiently heated, the concentration of the acid-soluble Ti may be less than 0.008%.
Examples of imaginable situations in which the slabs cannot be sufficiently heated
15 include a case in which the operation rate of the hot rolling is high and a case in which
sufficient heating capacity is not provided in the hot rolling. Meanwhile, the lower limit
of the concentration of the acid-soluble Ti in steel is not particularly limited, but may be,
for example, 0.0001% since Ti is inevitably included in steel.
[0085]
20 In addition, when the concentration of the acid-soluble Ti exceeds 0.2%, the
deoxidation effect of Ti is saturated, coarse carbides, nitrides, and carbonitrides are
formed by heating of the slabs before hot rolling, and the qualities of the steel sheet
deteriorate. In this case, an effect in accordance with the addition of Ti cannot be
obtained. Therefore, in the embodiment, the upper limit of the concentration of the
25 acid-soluble Ti is 0.2%.
25
[0086]
Therefore, the concentration of the acid-soluble Ti needs to be 0.0001% to 0.2%.
In addition, in a case in which the effect of the carbides, nitrides, and carbonitrides of Ti
is sufficiently secured, the concentration of the acid-soluble Ti is preferably 0.008% to
5 0.2%. In this case, in order to more reliably prevent the carbides, nitrides, and
carbonitrides of Ti from coarsening, the concentration of the acid-soluble Ti may be
0.15% or less. On the other hand, in a case in which the effect of the carbides, nitrides,
and carbonitrides of Ti and the deoxidation effect of Ti are not sufficiently secured, the
concentration of the acid-soluble Ti is preferably more than or equal to 0.0001% and less
10 than 0.008%.
[0087]
When the slab is heated at a sufficient heating temperature before hot rolling,
carbides, nitrides, and carbonitrides formed during casting can be made to temporarily
dissolve so as to form solid solutions. Therefore, in order to obtain an effect in
15 accordance with addition of Ti, the heating temperature before hot rolling is preferably
higher than 1200°C. In this case, since fine carbides, nitrides, and carbonitrides
precipitates again from solute Ti, it is possible to refine the crystal grains of the steel
sheet and increase the strength of the steel sheet. On the other hand, the heating
temperature before hot rolling exceeding 1250°C is not preferred from the viewpoint of
20 costs and scale forming. Therefore, the heating temperature before hot rolling is
preferably 1250°C or lower.
[0088]
The concentration of the acid-soluble Ti is determined by measuring the
concentration of Ti dissolved in an acid. For analysis of the acid-soluble Ti, the fact that
25 dissolved Ti (or solute Ti in a solid solution) dissolves in an acid, but Ti oxides do not
26
dissolve in an acid is used. Here, examples of the acid include a mixed acid in which
chloric acid, nitric acid, and water are mixed at a ratio (mass ratio) of 1:1:2. Using such
an acid, Ti that is soluble in the acid and Ti oxides that are insoluble in the acid are
separated, and the concentration of the acid-soluble Ti can be measured. Meanwhile,
5 the acid-insoluble Ti (Ti oxides that are insoluble in the acid) is determined as an
inevitable impurity.
[0089]
Cc and La are liable to reduce A12O3 formed by Al deoxidation and SiO2 formed
by Si deoxidation, and serve as precipitation sites of MnS-based inclusions.
10 Furthermore, Ce and La form inclusions (hard inclusions) including Cc oxides (for
example, Ce203 and CeO2), cerium oxysulfides (for example, Ce2O2S), La oxides (for
example, La203 and LaO2), lanthanum oxysulfides (for example, La2O2S), Cc oxide-La
oxide, or cerium oxysulfide-lanthanum oxysulfide which are hard and fine, and do not
easily deform during rolling, as a main compound (for example, the total amount of the
15 compounds is 50% or more).
[0090]
There are cases in which the hard inclusions include MnO, SiO2, TiO2, Ti2O3, or
A12O3 due to deoxidation conditions. However, when the main compound is the Cc
oxides, cerium oxysulfides, La oxides, lanthanum oxysulfides, Cc oxide-La oxide, or
20 cerium oxysulfi.de-lanthanum oxysulfide, the hard inclusions sufficiently serve as the
precipitation sites of MnS-based inclusions while maintaining the size and hardness
thereof.
[0091]
The inventors experimentally found that the total concentration of one or both of
25 Cc and La needs to be 0.001 % to 0.04% in order to obtain the above inclusions.
27
[0092]
When the total concentration of one or both of Ce and La is less than 0.001%,
A12O3 inclusions and SiO2 inclusions cannot be reduced. In addition, when the total
concentration of one or both of Cc and La exceeds 0 . 04%, large amounts of cerium
5 oxysulfides and lanthanum oxysulfides are formed, and the oxysulfides coarsen such that
hole expansion deteriorate . Therefore, the total of at least one selected from Cc and La
is preferably 0.001% to 0.04 %. In order to more reliably reduce A12O3 inclusions and
SiO2 inclusions , the total concentration of one or both of Cc and La is most preferably
0.015% or more.
10 [0093]
hi addition, the inventors paid attention to the fact that the amount of MnS
reformed by oxides or oxysulfides that are composed of one or both of Cc and La
(hereinafter sometimes also referred to as "hard compounds ") is expressed using the
concentrations of Ce, La, and S, and obtained an idea that the concentration of S and the
15 total concentration of Cc and La in steel are controlled using ([Cc] + [La]) / [S].
[0094]
Specifically, when ([Cc] + [La]) / [S] is small, the amount of the hard
compounds is small , and a large amount of MnS alone precipitates . When ([Cc] + [La])
/ [S] increases , the amount of the hard compounds becomes larger than that of MnS, and
20 inclusions having a morphology in which MnS precipitates on the hard compounds
increase. That is, MnS is reformed by the hard compounds. As a result, hole
expansion are improved , and MnS is prevented from elongating.
[0095]
That is, it is possible to use ([Cc ] + [La]) / [S] as a parameter that controls the
25 morphology of MnS=based inclusions. Therefore, the inventors varied ([Cc] + [La]) /
28
[S] of the steel sheet, and evaluated the morphology of inclusions and hole expansion in
order to clarify the composition ratio that is effective for suppressing the elongation of
MnS-based inclusions. As a result, it was found that, when ([Ce] + [La]) / [S] is 0.4 to
50, hole expansion are drastically improved.
5 [0096]
When ([Ce] + [La]) / [S] is less than 0.4, the number percentage of inclusions
having a morphology in which MnS precipitates on the hard compounds significantly
decreases, and the number percentage of MnS-based elongated inclusions that are liable
to serve as starting points of cracking increases such that hole expansion degrade.
10 [0097]
When ([Ce] + [La]) / [S] exceeds 50, large amounts of formed cerium
oxysulfides and lanthanum oxysulfides form coarse inclusions, and therefore hole
expansion deteriorate. For example, when ([Ce] +. [La]) / [S] exceeds 70, cerium
oxysulfides and lanthanum sulfides form coarse inclusions having an equivalent circle
15 diameter of 50 μin or more.
[0098]
In addition, when ([Ce] + [La]) / [S] exceeds 50, the effect of controlling the
morphology of MnS-based inclusions is saturated, and thereby the effect which is
appropriate for the costs cannot be obtained. From the above results, ([Ce] + [La]) / [S]
20 needs to be 0.4 to 50. When the degree of controlling the morphology of MnS-based
inclusions and the costs are taken into account, ([Ce] + [La]) / [S] is preferably 0.7 to 30,
and more preferably 1.0 to 10. Furthermore, in a case in which the morphology of
MnS-based inclusion are most efficiently controlled while the chemical components in
molten steel is adjusted, ([Ce] + [La]) / [S] is most preferably 1.1 or more.
25 [0099]
29
In addition, the inventors paid attention to the total concentration of one or both
of Cc and La with respect to the concentration of the acid-soluble A] in the steel sheet of
the embodiment, which is obtained from molten steel that has undergone deoxidation by
Si, deoxidation by Al, and deoxidation by one or both of Cc and La, and obtained an idea
5 of using ([Cc] + [La]) / [acid-soluble Al] as a parameter that appropriately controls the
oxygen potential in the molten steel.
[0100]
The inventors experimentally found that, in a case in which ([Cc] + [La]) /
[acid-soluble Al] is 0.02 or more in the molten steel that has undergone deoxidation by Si,
10 deoxidation by Al, and then deoxidation by at least one of Cc and La, it is possible to
obtain a steel sheet that is excellent in terms of hole expansion. In this case, the oxygen
potential in the molten steel abruptly decreases, and, consequently, the concentration of
A12O3 formed decreases. Therefore, even in a case in which deoxidation by Al is
actively carried out, similarly to a case in which deoxidation by Al is rarely carried out, a
15 steel sheet that is excellent in terms of hole expansion could be obtained. In addition, in
a case in which ([Cc] + [La]) / [acid-soluble Al] is less than 0.25, the costs for Cc or La
decreases, and transfer of oxygen between chemical elements in the molten steel can also
be efficiently controlled based on the affinity of each chemical element to oxygen.
Meanwhile, in the embodiment, it is not necessary to actively carry out deoxidation by A],
20 and simply necessary to control the total concentration of at least one of Ca and La and
the concentration of the acid-soluble Al so that ([Cc] + [La]) / [acid-soluble Al] satisfies
more than or equal to 0.02 and less than 0.25.
[0101]
It was confirmed that, in a case in which ([Cc] 1- [La]) / [acid-soluible Al] is less
25 than 0.02, the amount of Al added to at least one of Ca and La becomes too large even
30
when one or both of Cc and La are added to steel, and therefore coarse alumina clusters
that deteriorate hole expansion are formed. In addition, in a case in which ([Cc] + [La])
/ [acid-soluble Al] is 0.25 or more, there are cases in which the morphology of inclusions
are not sufficiently controlled. For example, cerium oxysulfides and lanthanum
5 oxysulfides form coarse inclusions, and sufficient deoxidation is not carried out in the
molten steel. Therefore, ([Cc] + [La]) / [acid-soluble Al] needs to be more than or equal
to 0.02 and less than 0.25. In addition, in order to further reduce the cost, and
appropriately control the oxygen transfer between chemical elements in the molten steel,
([Cc] + [La]) / [acid-soluble Al] is preferably less than 0. 15, and more preferably less
10 than 0.10. As such, even when desulfurization through the secondary refining is not
carried out, a steel sheet that is excellent in terms of ductility and hole expansion can be
obtained by controlling ([Cc] + [La]) / [S] and ([Cc] + [La]) / [acid-soluble Al].
[0102]
Hereinafter, in the embodiment, reasons why the amount of each optional
15 element in the chemical composition is limited will be described. The chemical
elements are optional elements, and can be arbitrarily (optionally) added to steel.
Therefore, the chemical elements may not be added to steel, and at least one selected
from a group consisting of the chemical elements may be added to steel. Meanwhile,
since there are cases in which the chemical elements are inevitably included in steel, the
20 lower limit of the concentration of the chemical elements is a threshold value that
determines inevitable impurities.
[0103]
Nb, W, and V form carbides, nitrides, and carbonitrides with C or N, promotes
refining of the microstructure in a base metal, and improves toughness.
25 [0104]
31
In order to obtain complex carbides, complex nitrides, and the like, 0.01% or
more of Nb may be added to steel. However, even when a large amount of Nb is added
so that the concentration of Nb exceeds 0.20%, the effect of refining the microstructure
in the base metal is saturated, and the manufacturing cost increases. Therefore, the
5 upper limit of the concentration of Nb is 0.20%. Ina case in which the cost of Nb is
reduced, the concentration of Nb may be controlled to 0.10% or less. Meanwhile, the
lower limit of the concentration of Nb is 0.001%.
In order to obtain the complex carbides, complex nitrides, and the like, W may
be added to steel. However, even when a large amount of W is added so that the
10 concentration of W exceeds 1.0%, the effect of refining the microstructure in the base
metal is saturated, and the manufacturing cost increases. Therefore, the upper limit of
the concentration of W is 1.0%. Meanwhile, the lower limit of the concentration of W
is 0.001%.
[0105]
15 In order to obtain complex carbides, complex nitrides, and the like, 0.01% or
more of V may be added to steel. However, even when a large amount of V is added so
that the concentration of V exceeds 1.0%, the effect of refining the microstructure in the
base metal is saturated, and the manufacturing cost increases. Therefore, the upper limit
of the concentration of V is 1.0%. Ina case in which the cost of V is reduced, the
20 concentration of V may be controlled to be 0.05% or less. Meanwhile, the lower limit
of the concentration of V is 0.001%.
[0106]
Cr, Me, and B are chemical elements that improve the hardenability of steel.
[010'7]
25 Cr can be included in steel according to necessity in order to further secure the
32
strength of the steel sheet. For example, in order to obtain the effect, 0.01% or more of
Cr may be added to steel. When a large amount of Cr is included in steel, the balance
between strength and ductility deteriorate. Therefore, the upper limit of the
concentration of Cr is 2.0%. In a case in which the cost of Cr is reduced, the
5 concentration of Cr maybe controlled to be 0.6% or less. In addition, the lower limit of
the concentration of Cr is 0.001%.
[0108]
Mo can be included in steel according to necessity in order to further secure the
strength of the steel sheet. For example, in order to obtain the effect, 0.01% or more of
10 Mo may be added to steel. When a large amount of Mo is included in steel, it becomes
difficult to suppress formation of pro-eutectic ferrite, and therefore the balance between
strength and ductility deteriorate. Therefore, the upper limit of the concentration of Mo
is 1.0%. Ina case in which the costs of Mo are reduced, the concentration of Mo may
be controlled to be 0.4% or less. In addition, the lower limit of the concentration of Mo
15 is 0.001%.
[0109]
B can be included in steel according to necessity in order to further strengthen
grain boundaries and improve formability. For example, in order to obtain the effect,
0.0003% or more of B may be added to steel. Even when a large amount of B is
20 included in steel, the effect is saturated, the cleanliness of steel is impaired, and the
ductility deteriorates. Therefore, the Lipper limit of the concentration of B is 0.005%.
In a case in which the cost of B is reduced, the concentration or B may be controlled to
be 0.003% or less. In addition, the lower limit of the concentration of B is 0.0001%.
[0110]
25 In order to strengthen grain boundaries and improve formability by controlling
33
the morphology of sulfides, Ca, Mg, Zr, Sc, lanthanoids of Pr through Lu (Pr, Nd, Pm,
Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, and Yb) can be included in steel according to necessity.
[0111]
Ca controls the morphology of sulfides by the spheroidizing of sulfides or the
5 like, so as to strengthen grain boundaries and improve the formability of the steel sheet.
For example, in order to obtain the effect, the concentration of Ca may be 0.0001% or
more. Even when a large amount or Ca is included in steel, the effect is saturated, the
cleanliness of steel is impaired, and the ductility deteriorates. Therefore, the upper limit
of the concentration of Ca is 0.01%. In a case in which the cost of Ca is reduced, the
10 concentration of Ca maybe controlled to be 0.004% or less. In addition, the lower limit
of the concentration of Ca is 0.0001%.
Similarly, since Mg has almost the same effects as Ca, the concentration of Mg
is from 0.0001% to 0.01%.
[0112]
15 In order to spheroidize sulfides so as to improve the toughness of the base metal,
0.001% or more of Zr may be added to steel. When a large amount of Zr is included in
steel, the cleanliness of steel is impaired, and the ductility deteriorates. Therefore, the
upper limit of the concentration of Zr is 0.2%. In a case in which the cost of Zr is
reduced, the concentration of Zr may be controlled to be 0.01% or less. In addition, the
20 lower limit of the concentration of Zr is 0.0001%.
Similarly, in a case in which the morphology (shapes) of sulfides is controlled,
the total concentration of at least one selected from Sc, and lanthanoids of Pr through Lu
may be from 0.0001 %a to 0.1 %.
[0113]
25 In the embodiment, 0.001% to 2.0% of Cu and 0.001% to 2.0% of Ni can be
34
included in steel according to necessity. The chemical elements improve hardenability
so as to enhance the strength of steel. Meanwhile, in a case in which quenching is
efficiently carried out using the chemical elements, the concentration of Cu maybe
0.04% to 2.0%, and the concentration of Ni may be 0.02% to 1.0%.
5 Furthermore, in a case in which scraps or the like are used as some of starting
materials, there are cases in which As, Co, Sn, Ph, Y, and I-If are inevitably incorporated.
In order to prevent the chemical elements from adversely affecting the mechanical
properties (for example, hole expansion) of the steel sheet, the concentration of each
chemical elements is limited as below. The upper limit of the concentration ofAs is
10 0.5%. The upper limit of the concentration of Co is 4.0%. In addition, the upper
limits of the concentrations of Sn, Pb, Y. and Hf are all 0.2%. Meanwhile, the lower
limits of the chemical elements are all 0.0001%.
In the embodiment, the optional elements as described above can be optionally
included in steel.
15 [0114]
Next, the microstructure of the high-strength steel sheet according to the
embodiment will be described.
[0115]
Hole expansion are significantly affected by the local ductility of a steel, and the
20 most dominant factor in relation to hole expansion is the difference in hardness between
microstructures. Another powerful dominant factor in relation to hole expansion is the
presence of nonmetallic inclusions, such as MnS. Generally, voids are caused from the
inclusions as the starting point, grow and link together such that the steel breaks.
[0116]
25 That is, when the hardness of the martensite phase is too large compared to the
35
hardness of other microstructures (for example, the ferrite phase), there are cases in
which, even when the morphology of inclusions are controlled by adding Cc and La, and
occurrence of voids due to the inclusions is suppressed, stress concentrates at the
interfaces between ferrite and martensite, voids are caused due to the difference in
5 strength between the microstructures, and the steel may break.
[0117]
When the cooling conditions after hot rolling in the case of a hot-rolled steel
sheet, and the annealing conditions in the case of a cold-rolled steel sheet are
appropriately controlled, and the hardness of the martensite phase is reduced, the effect
10 of suppressing occurrence of voids by controlling the morphology of inclusions can be
further enhanced. In this case, the effect of controlling the morphology of inclusions by
Cc and La that are included in steel sheet is significantly exhibited as described above.
FIG. 1 schematically shows a relationship between the maximum hardness (Vickers
hardness) of martensite and hole expansion ratios (hole expansion) A. As shown in FIG,
15 1, in a case in which the hardness of the martensite phase is suppressed to a certain value
so that the morphology of inclusions are controlled using at least one of Cc and La, hole
expansion can be significantly improved compared to a case in which the morphology of
inclusions are not controlled. In addition, in a microstructure substantially composed of
bainite, the degree of the hole expansion improved by the addition of Cc and La is large,
20 but the ductility is poor compared to a steel sheet mainly including ferrite and martensite.
[0118]
In the embodiment, a steel sheet that is excellent in terms of both hole expansion
and ductility is provided. Therefore, the major microstructure is ferrite and martensite,
and the microstructure includes 1% to 50% of the martensite phase in terms of the area
25 ratio, optionally includes bainite and retained austenite, and has a remainder composed of
36
a ferrite phase. In this case, in order to obtain uniform deformability, for example,
bainite and retained austenite are controlled to 10% or less each. When the area ratio of
the martensite phase is less than 1%, the work-hardenability is weak. In order to further
enhance the work-hardenability, the area ratio of the martensite phase is preferably 3% or
5 more, and more preferably 5% or more. On the other hand, when the area ratio of the
martensite phase exceeds 50%, the uniform deformability of the steel sheet decreases
significantly. In order to obtain a large uniform deformability, the area ratio of the
martensite phase is preferably 30% or less, and more preferably 20% or less.
Meanwhile, some or all of the martensite phase may be tempered martensite. The ratio
10 of the martensite phase is determined by the area ratio of the martensite phase in a
microstructure photograph obtained using an optical microscope. Herein, the inclusions
as described below are included in the microstructures (the martensite phase, the ferrite
phase, the bainite, and the retained austenite).
[0119]
15 Since the hardness of the ferrite phase and the martensite phase included in steel
varies with the chemical composition and manufacturing conditions (for example, the
amount of strains caused during rolling or cooling rate) of steel, the hardness is not
particularly limited. When it is taken into account that the hardness of the martensite
phase is high compared to those of other microstructures, the maximum hardness of the
20 martensite phase included in steel is preferably 600 Hv or less. The maximum hardness
of the martensite phase is the maximum value of micro-Vickers hardness obtained by
randomly pressing an indenter with a load of 10 gf on a hard phase (other than the ferrite
phase) 50 times.
[0120]
25 Next, the presence conditions of inclusions in the high-strength steel sheet of the
37
embodiment will be described. Here, the steel sheet refers to a. rolled sheet obtained
after hot rolling or cold rolling.
[0121]
In the embodiment, the presence conditions of inclusions in the steel sheet can
5 be optionally specified from a -variety of viewpoints.
[0122]
In the first feature in relation to inclusions, the number density of inclusions that
are present in the steel sheet and have an equivalent circle diameter of 0.5 μm to 2 μm is
15 inclusions/mm2 or more.
10 [0123]
In order to obtain a steel sheet that is excellent in terms of ductility and hole
expansion, it is important to reduce as much as possible elongated coarse MnS-based
inclusions that easily act as starting points of cracking or crack propagation paths.
[0124]
15 As described above, the inventors found that, in a case in which ([Cc] + [La]) /
[acid-soluble At] and ([Cc] + [La]) / [S] are in the above ranges, since the oxygen
potential in the molten steel abruptly lowers due to the complex deoxidations, and the
concentration of A1203 in inclusions decreases, a steel sheet that is deoxidized by Si, then,
deoxidized by Al, and then deoxidized by at least one of Cc and La is excellent in terms
20 of ductility and hole expansion, similarly to a steel sheet manufactured with little
deoxidation by Al.
[0125]
In addition, the inventor also found that, since MnS precipitates on fine and hard
Cc oxides, La oxides, cerium oxysulfides, and lanthanum oxysulfides that are formed due
25 to deoxidation by the addition of Cc and La, and the precipitated MnS does not easily
38
deform during rolling, the elongated coarse MnS is significantly reduced in the steel
sheet.
[0126]
That is, it was found that, in a case in which ([Cc] + [La]) / [acid soluble Al] and
S ([Cc] + [La]) / [Si are in the above ranges, the number density of fine inclusions having
an equivalent circle diameter of 2 Fun or less abruptly increases, and the fine inclusions
are dispersed in steel.
[0127]
Since the fine inclusions do not easily aggregate, most of the inclusions have a
10 spherical or spindle shape. In addition, since inclusions having MnS precipitated on Cc
oxides, La oxides, cerium oxysulfides, and lanthanum oxysulfides have a high melting
point and do not easily deform, the inclusions maintain an almost spherical shape even
during hot rolling. As a result, the long diameter/ short diameter (hereinafter sometimes
referred to as the "elongation ratio") of most of the inclusions is generally 3 or less.
15 [0128]
Since the likelihood of the inclusions to serve as starting points of fracture
significantly varies with the shapes of the inclusions, the elongation ratio of the
inclusions is preferably 2 or less.
[01.29]
20 Experimentally, attention was paid to the number density of inclusions having an
equivalent circle diameter of 0.5 μm to 2 Irm so that the inclusions can be easily
identified through observation using a scanning electron microscope (SEM) or the like.
With regard to the lower limit of the equivalent circle diameter, inclusions that are large
enough to be sufficiently counted are used. That is, the number of inclusions was
25 counted with respect to inclusions of 0.5 μm or more. The equivalent circle diameter is
39
obtained by measuring the long diameter and the short diameter of an inclusion observed
on a cross-section, and computing (long diameter x short diameter)°'.
[0130]
While the detailed mechanism is not clear, it is considered that fine inclusions of
5 2 pn or less are dispersed in the microstructure at 15 inclusions/mm2 or more due to a
synergetic effect of the lowering of the oxygen potential in the molten steel by Al
deoxidation and the refining of MnS-based inclusions. It is assumed that, due to the
above, stress concentration caused during forming of hole expanding or the like is
alleviated, and an effect of abruptly improving hole expansion is exhibited. Asa result,
10 it is considered that, during repetitive deformation or hole expanding, MnS-based
inclusions are fine, and therefore MnS-based inclusions do not easily act as starting
points of cracking or crack propagation paths, alleviate stress concentration, and it p ove
formability, such as hole expansion . As such, with regard to the morphology of the
inclusions, the number density of inclusions that are present in the steel sheet and have an
15 equivalent circle diameter of 0.5 pin to 2 μm is preferably 15 inclusions/mm2 or more.
[0131]
In the second feature in relation to inclusions, of inclusions that are present in
the steel sheet and have an equivalent circle diameter of 1μm or more, the number
percentage of elongated inclusions having an aspect ratio (elongation ratio) of 5 or more
20 obtained by dividing the long diameter by the short diameter is 20% or less.
[0132]
The inventors investigated whether or not elongated coarse MnS-based
inclusions that easily act as starting points of cracking or crack propagation paths are
reduced.
25 [0 3]
40
The inventor experimentally found that, when the equivalent circular diameters
of the inclusions are less than 1 μm, even in a case in which MnS is elongated, the
inclusions do not act as starting points of cracking, and ductility and hole expansion are
not deteriorated. In addition, since inclusions having an equivalent circle diameter of 1
5 μm or more can be easily observed using a scanning electron microscope (SEM) or the
like, the morphology and chemical compositions of inclusions having an equivalent
circle diameter of 1 μm or more in the steel sheet were investigated, and the distribution
of the elongated MnS was evaluated. The upper limit of the equivalent circle diameter
of MnS is not particularly specified; however, for example, there are cases in which MnS
10 of approximately 1 mm is observed in the steel sheet.
[0134]
The number percentage of elongated inclusions is obtained in the following
manner. Here, the elongated inclusion refers to an inclusion having a long diameter/
short diameter (elongation ratio) of 5 or more.
15 [0135]
The chemical compositions of a plurality (for example, a predetermined number
of 50 or more) of inclusions having an equivalent circle diameter of 1 μm or more which
are randomly selected using a SEM are analyzed, and the long diameter and short
diameter of the inclusions are measured from a SEM image (secondary electron image).
20 The number percentage of the elongated inclusions can be obtained by dividing the
number of the detected elongated inclusions by the number of all inclusions investigated
(in the above example, a predetermined number of 50 or more).
[0136]
A reason why the elongated inclusions are defined as inclusions having an
25 elongation ratio of 5 or more is that most of inclusions having an elongation ratio of 5 or
41
more in the steel sheet to which Ce and La are not added are MnS. The upper limit of
the elongation ratio of MnS is not particularly specified; however, for example, there are
cases in which MnS having an elongation ratio of approximately 50 is observed in the
steel sheet.
5 [0137]
As a result of evaluation by the inventors, it was found that, in the steel sheets
for which the number percentage of the elongated inclusions having an elongation ratio
of 5 or more with respect to inclusions having an equivalent circle diameter of I .xm or
more is controlled to be 20% or less, the hole expansion are improved . When the
10 number percentage of the elongated inclusions exceeds 20 %, since a number of
MnS-based elongated inclusions that easily act as starting points of cracking are present,
the hole expansion degrades . In addition, as the grain sizes of the elongated inclusions
increase, that is, as the equivalent circle diameters increase, stress concentration occurs
more easily during forming and deformation, and therefore the elongated inclusions
15 easily act as starting points of cracking or crack propagation paths, and the hole
expansion abruptly deteriorates.
[0138]
Therefore, in the embodiment, the number percentage of the elongated
inclusions is preferably 20% or less. Since the hole expansion become better as the
20 elongated MnS-based inclusions become smaller, the lower limit of the number
percentage of the elongated inclusions include 0%-
[01391
In a case in which inclusions having an equivalent circle diameter of I μm or
more are included, and elongated inclusions having an elongation ratio of 5 or more are
25 not present in the inclusions, or in a case in which the equivalent circle diameters of
42
inclusions are all less than I μm, the number percentage of elongated inclusions having
an elongation ratio of 5 or more in inclusions having an equivalent circle diameter of 1
μm or more is determined to be 0%.
[0140]
5 It is confirmed that the maximum equivalent circle diameters of elongated
inclusions are also small compared to the average grain size of crystals (metallic crystals)
in the microstructure, and the reduction of the maximum equivalent circle diameters of
the elongated inclusions are also considered to be a factor that can drastically improve
the hole expansion.
10 [0141]
In the third feature in relation to inclusions, of inclusions having an equivalent
circle diameter of 1.0 μm or more in the steel sheet, the number percentage of inclusions
having at least one of MnS, TiS, and (Mn, Ti)S precipitated on an oxide or oxysulfide
composed of at least one of Cc and La, and at least one of 0 and S, or an oxide or
15 oxysulfide composed of at least one of Cc and La, at least one of Si and Ti, and at least
one of 0 and S is 10% or more.
[0142]
For example, in a steel sheet having ([Cc] + [La]) / [S] of 0.4 to 50, MnS-based
inclusions precipitate on an oxide or oxysulfide including one or both of Cc and La, or an
20 oxide or oxysulfide including one or both of Ce and La, and one or both of Si and Ti (the
above hard compounds). Meanwhile, in a steel sheet in which the acid-soluble Ti is less
than 0.008%, there are many cases in which oxides or oxysulfides including one or both
of Si and Ti are not formed.
[0143]
25 The morphology of the inclusions is not particularly specified as long as
43
MnS-based inclusions precipitate on the hard compounds, and there are many cases in
which MnS-based inclusions precipitate around the hard compounds as nuclei.
[0144]
Also, there are cases in which TiN precipitates together with MnS-based
5 inclusions on the fine and hard Cc oxides, La oxides, cerium oxysulfides, and lanthanum
oxysulfides. However, since TiN has little influence on ductility and hole expansion as
described above, TiN itself is not included in MnS-based inclusions.
[0145]
Since inclusions having MnS-based inclusions precipitated on the hard
10 compounds in the steel sheet do not easily deform during rolling, the inclusions have a
shape that is not elongated, that is, a spherical or spindle shape.
[0146]
Here, inclusions that are determined to be not elongated (spherical inclusions)
are not particularly specified; however, for example, the inclusions are an inclusion
15 having an elongation ratio of 3 or less, and preferably an inclusion having an elongation
ratio of 2 or less. This is because the elongation ratio of an inclusion having MnS-based
inclusions precipitated on the hard compounds in a slab before rolling is 3 or less. In
addition, when the spherical inclusion is a perfectly spherical body, the elongation ratio is
1, and therefore the lower limit of the elongation ratio is 1.
20 [0147]
The inventors investigated the number percentage of the inclusions (spherical
inclusions) by the same method as the method of measuring the number percentage of the
elongated inclusions. That is, the chemical compositions of a plurality (for example, a
predetermined number of 50 or more) of inclusions having an equivalent circle diameter
25 of 1.0 μm or more which are randomly selected using a SEM are analyzed, and the long
44
diameter and short diameter of the inclusions are measured from a SEM image
(secondary electron image). The number percentage of the spherical inclusions can be
obtained by dividing the number of the spherical inclusions having a detected elongation
ratio of 3 or less by the number of all inclusions investigated (in the above example, a
5 predetermined number of 50 or more). Asa result, in the steel sheet for which the
number percentage of inclusions having MnS-based inclusions precipitated on the hard
compounds (spherical inclusions) is controlled to be 10% or more, the hole expansion are
improved.
[0148]
10 When the number percentage of the inclusions having MnS-based inclusions
precipitated on the hard compounds is less than 10%, the number percentage of
MnSnbased elongated inclusions increases, and the hole expansion degrades. Therefore,
in the embodiment, of the inclusions having an equivalent circle diameter of 1.0 pm or
more, the number percentage of inclusions having MnS-based inclusions precipitated on
15 the hard compounds is 10% or more.
[0149]
Since the hole expansion are improved by precipitating a number of MnS-based
inclusion on the hard compounds, the upper limit value of the number percentage of
inclusions having MnS-based inclusions precipitated on the hard compounds includes
20 100%.
[0150]
Meanwhile, since the inclusions having MnS-based inclusions precipitated on
the hard compounds do not easily deform during rolling, the equivalent circle diameter is
not particularly specified, and hole expansion are not adversely affected even when the
25 equivalent circle diameter is 1 pin or more. However, when the equivalent circle
45
diameter is too large, there is a possibility for inclusions to act as starting points of
cracking, and therefore the upper limit of the equivalent circle diameter is preferably
approximately 50 pm.
[0151]
5 Additionally, in a case in which the equivalent circle diameters of inclusions are
less than 1 μm, since the inclusions do not easily act as starting points of cracking, the
lower limit of the equivalent circle diameter is not specified.
[0152]
In the fourth feature in relation to inclusions, of inclusions that are present in the
10 steel sheet and have an equivalent circle diameter of 1 μm or more, the volume number
density of elongated inclusions having an aspect ratio of 5 or more obtained by dividing
the long diameter by the short diameter (elongation ratio) is 1.0 x IW inclusions/mm3 or
less.
[0153]
15 The grain size distribution of inclus ons s obtained through, for example, SEM
observation of electrolyzed surfaces according to the SPEED method (Selective
Potentiostatic Etching by Electrolytic Dissolution method). In the SEM observation of
an electrolyzed surface by the SPEED method, a surface of a test specimen obtained from
a steel sheet is polished, then, electrolyzed by the SPEED method, and the sample
20 surface is directly observed using a SEM, whereby the sizes and number density of
inclusions are evaluated.
[0154]
The SPEED method is a method in which a metal matrix on the sample surface
is electrolyzed using a solution of 10% acetyl acetone, l°/0 tetramethyl ammonium
25 chloride, and methanol, and inclusions are shown. The electrolysis is performed, for
46
example, in I coulomb per an area of the sample surface of 1 cm2. A SEM image on the
electrolyzed sample surface is processed by an image-processing, and the equivalent
circle diameter and frequency (number) distribution of inclusions are obtained. The
frequency distribution is divided by the depth of electrolysis so as to compute the number
5 density of inclusions per volume.
[0155]
The inventors evaluated the volume number density of elongated inclusions
having an equivalent circle diameter of 1 μm or more and an elongation ratio of 5 or
more as inclusions that act as starting points of cracking and deteriorate hole expansion.
10 As a result, it was found that, when the volume number density of the elongated
inclusion is 1.0 x 104 inclusionshnm3 or less, hole expansion improves.
[0156]
When the volume number density of the elongated inclusions exceeds 1.0 x 104
inclusions/mm3, the number density of MnS-based elongated inclusions that easily act as
15 starting points of cracking increases, and hole expansion degrade. Therefore, the
volume number density of elongated inclusions having an equivalent circle diameter of 1
lam or more and an elongation ratio of 5 or more is limited to 1.0 x 104 inclusions/mm3 or
less. Since hole expansion improve as elongated MnS=based inclusions decrease, the
lower limit value of the volume number density of the elongated inclusions includes 0%.
20 [0157]
Meanwhile, similarly to the second feature in relation to inclusions, it is found
that, in a case in which inclusions having an equivalent circle diameter of 1 pm or more
and an elongation ratio of 5 or more are not present, or a case in which the equivalent
circle diameters of inclusions are all less than 1 μm, of inclusions having an equivalent
47
circle diameter of 1 lrm or more, the volume number density of elongated inclusion
having an elongation ratio of 5 or more is 0%.
[0158]
In the fifth feature in relation to inclusions, of inclusions having an equivalent
5 circle diameter of I μm or more in the steel sheet, the volume number density of
inclusions having at least one of MnS, TiS, and (Mn, Ti)S precipitated on an oxide or
oxysulfide (hard compound) composed of at least one of Cc and La, and at least one of 0
and S, or an oxide or oxysulfide composed of at least one of Cc and La, at least one of Si
and Ti, and at least one of 0 and S is 1.0 x 103 inclusions/mm3 or more.
10 [0159]
Investigation by the inventors showed that unelongated MnS-based inclusions
had MnS-based inclusions precipitated on the hard compounds and had an almost
spherical or spindle shape.
[0160]
15 The morphology of the inclusions are not particularly specified as long as
MnS-based inclusions are precipitated on the hard compounds, but there are many cases
in which MnS-based inclusions precipitate around the hard compounds as nuclei.
[0161]
The spherical inclusion is defined in the same manner as in the third feature in
20 relation to inclusions, and, the volume number density of the spherical inclusions is
measured using the same SPEED method as in the fourth feature in relation to inclusions.
[0162]
As a result of investigation by the inventor on the volume number density of the
spherical inclusions, it was found that in steel sheets for which the volume number
25 density of inclusions having MnS-based inclusions precipitated around the hard
48
compounds as nuclei (spherical inclusions) is controlled to be 1.0 x 103 inclusions/mm3
or more, hole expansion improves.
[0163]
When the volume number density of inclusions having MnS-based inclusions
5 precipitated on the hard compounds becomes less than 1.0 x 103 inclusions /rnm3, the
number percentage of MnS-based elongated inclusions increases, and hole expansion
degrades . Therefore, the volume number density of inclusion having MnS-based
inclusion precipitated on the hard compounds is 1.0 x 103 inclusions /mm3 or more.
Since hole expansion are improved by precipitating a number of MnS-based inclusions
10 using the hard compounds as nuclei, the upper limit of the volume number density is not
specified.
[0164]
The equivalent circle diameters of inclusions having MnS-based inclusions
precipitated on the hard compounds are not particularly specified. However, when the
15 equivalent circle diameter is too large, there is a possibility for inclusions to act as
starting points of cracking, and therefore the upper limit of the equivalent circle diameter
is preferably approximately 50 μm.
[0165]
Additionally, in a case in which the equivalent circle diameters of inclusions are
20 less than I μm, no problem occurs, and therefore the lower limit of the equivalent circle
diameter is not specified.
[0166]
In the sixth feature in relation to inclusions , of inclusions that are present in the
steel sheet and have an equivalent circle diameter of I pm or more, the average
49
equivalent circle diameter of inclusions having an aspect ratio of 5 or more obtained by
dividing the long diameter by the short diameter (elongation ratio) is 10 μm or less.
[0167]
The inventors evaluated the average equivalent circle diameter of elongated
5 inclusions having an equivalent circle diameter of 1 μm or more and a elongation ratio of
5 or more as inclusions that act as starting points of cracking and deteriorate hole
expansion. Asa result, it was found that, when the average equivalent circle diameter
of the elongated inclusions is 10 lrm or less, hole expansion improves . This is assumed
to be because, as the amount of Mn or S in the molten steel increases, the number of
10 MnS-based inclusions being formed increases, and the sizes of MnS-based inclusions
being formed also increase.
[0168]
As a result, attention was paid to a phenomenon in which the average equivalent
circle diameter of the elongated inclusions increases as the number percentage of the
15 elongated inclusions increases, and the average equivalent circle diameter of the
elongated inclusions was specified as a parameter.
[0169]
When the average equivalent circle diameter of the elongated inclusions exceeds
10 μm, the number percentage of coarse MnS-based inclusions that easily act as starting
20 points of cracking increases. As a result, hole expansion degrades, and therefore the
morphology of inclusions is controlled so that the average equivalent circle diameter of
the elongated inclusions having equivalent circle diameter of I μm or more and an
elongation ratio of 5 or more becomes 10 [Lm or less.
[0170]
50
Since the average equivalent circle diameter of the elongated inclusions is
obtained by measuring the equivalent circle diameters of inclusions that are present in the
steel sheet and have an equivalent circle diameter of I pin or more using a SEM, and
dividing the total of equivalent circle diameters of a plurality (for example, a
5 predetermined number of 50 or more) of inclusions by the number of the plurality of
inclusions, the lower limit of the average equivalent circle diameter is I μm.
[0171]
In the seventh feature in relation to inclusion, inclusions having at least one of
MnS, TiS, and (Mn, Ti)S precipitated on an oxide or oxysulfide composed of at least one
10 of Cc and La, and at least one of 0 and S, or an oxide or oxysulfide composed of at least
one of Cc and La, at least one of Si and Ti, and at least one of O and S are present in the
steel sheet, and the inclusions include a total of 0.5 mass% to 95 mass% of at least one of
Cc and La in terms of an average chemical composition.
[0172]
15 As described above, in order to improve hole expansion, it is important to
precipitate MnS-based inclusions on the hard compounds and prevent elongation of
MnS-based inclusions. With regard to the morphology of the inclusions, MnS-based
inclusions may be precipitated on hard inclusions, and, generally, MnSabased inclusions
precipitate around hard inclusions as nuclei.
20 [0173]
The inventors analyzed the chemical compositions of inclusions having
MnS-based inclusions precipitated on the hard inclusions through SEM and energy
dispersive X-ray spectroscopy (EDX) in order to clarify the chemical compositions of
inclusions, which are effective for suppressing elongation of MnS-based inclusions.
25 When the equivalent circle diameters of the inclusions are 1 μrn or more, since inclusions
51
are easily observed, the composition analysis was carried out on inclusions having an
equivalent circle diameter of 1 tun or more. In addition, since inclusions having
MnS-based inclusions precipitated on hard inclusions are not elongated as described
above, the elongation ratios are all 3 or less. Therefore, the composition analysis was
5 carried out on spherical inclusions having an equivalent circle diameter of 1 tun or more
and an elongation ratio of 3 or less, which are defined in the third feature in terms of
inclusions.
[0174]
As a result, it was found that, when the spherical inclusions include a total of
10 0.5% to 95% of one or both of Cc and La in terms of an average chemical composition,
hole expansion improves.
[0175]
When the average amount of the sum of one or both of Cc and La in the
spherical inclusions is less than 0.5 mass%, the number percentage of inclusions having
15 MnS-based inclusions precipitated on the hard compounds significantly decreases, and
therefore the number percentage of MnS-based elongated inclusions that easily act as
starting points of cracking increases, and hole expansion and fatigue characteristics
degrade. Meanwhile, the larger the average amount of the sum of one or both of Ce and
La, the more preferable. For example, the upper limit of the average amount may be
20 95% or 50% according to the amount of MLnS-based inclusions.
[0176]
When the average amount of the sum of one or both of Cc and La in the
spherical inclusions exceeds 95%, large amounts of cerium oxysulfides and lanthanum
oxysulfides form coarse inclusions having an equivalent circle diameter of 50μm or
25 more, hole expansion and fatigue characteristics deteriorate.
52
[0177]
Meanwhile, the high-strength steel sheet of the embodiment may be a
cold-rolled steel sheet or a hot-rolled steel sheet. In addition, the high-strength steel
sheet of the embodiment may be a coated steel sheet having a coating, such as a
5 galvanized layer or a galvannealed layer, on at least one surface thereof.
[0178]
Next, the manufacturing conditions of the high-strength steel sheet according to
an embodiment of the present invention will be described. Meanwhile, the chemical
composition of the molten steel is the same as the chemical composition of the
10 high-strength steel sheet of the above embodiment.
[0179]
in the present invention, an alloy of C, Si, Mn, and the like is added to molten
steel that has been blown and decarburized in a converter, and stirred so as to carry out
deoxidization and adjust the chemical components. Meanwhile, according to necessity,
15 deoxidization can be carried out using a vacuum degassing apparatus.
[0180]
Meanwhile, with regard to S, since desulfurization need not be carried out in the
refining process as described above, a desulfurization process can be skipped. However;
in a case in which desulfurization of the molten steel is required in secondary refining in
20 order to melt extremely low sulfur steel having a concentration of S of 20 ppm or less,
the amount of the chemical components may be controlled by carrying out
desulfurization.
[0484]
Deoxidation and composition control are carried out in the following manner.
25 [0182]
53
After Si (for example, Si or a compound including Si) is added to the molten
steel, and approximately three minutes pass, At (for example, Al or a compound
including Al) is added to the molten steel, and deoxidization is carried out. Afloatation
time of approximately 3 minutes is preferably secured in order to make oxygen and Al
5 combine together so as to float A12O3. After that, in a case in which addition of Ti (for
example, Ti or a compound including Ti) is required, Ti is added to the molten steel. In
this case, a floatation time of approximately 2 to 3 minutes is preferably secured in order
to make oxygen and Ti combine together so as to float TiO2 and Ti2O3.
[0183]
10 After that, the chemical composition are controlled by adding one or both of Cc
and La to the molten steel so as to satisfy 0.02 <_ ([Ce] + [La]) / [acid-soluble Al] < 0.25,
and 0.45 ([Ce] + [La]) / [S] < 50.
[0184]
In a case in which optional elements are added, addition of the optional elements
15 is completed before one or both of Cc and La are added to the molten steel. In this case,
the molten steel is sufficiently stirred so as to adjust the amounts of the optional elements,
and then one or both of Cc and La are added to the molten steel. The molten steel
manufactured in the above manner is subjected to continuous casting so as to
manufacture slabs.
20 [0185]
With regard to the continuous casting, the embodiment can be sufficiently
applied not only to ordinary slab continuous casting in which approximately 250
mm4hick slabs are manufactured but also to, for example, thin slab continuous casting in
which 150 mm or less thick slabs are manufactured.
25 [0186]
54
In the embodiment, the high-strength hot-rolled steel sheet can be manufactured
in the following manner.
[0187]
The obtained slab is reheated to 1100°C or higher, and preferably 1150°C or
5 higher according to necessity. Particularly, in a case in which it is necessary to
sufficiently control the morphology (for example, fine precipitation) of carbides and
nitrides, it is necessary to temporarily form solid solutions by dissolving carbides and
nitrides in steel, and therefore the heating temperature of the slab before hot rolling
preferably exceeds 1200°C. A ferrite phase whose ductility is improved in a cooling
10 process after rolling can be obtained by forming solid solutions by dissolving carbides
and nitrides in steel.
[0188]
When the heating temperature of the slab before hot rolling exceeds 1250°C,
there are cases in which the surfaces of the slab are significantly oxidized. Particularly,
15 there are cases in which wedge-shaped surface defects caused by selective oxidation of
grain boundaries are liable to remain after descaling, and the qualities of the surfaces
after rolling are impaired. Therefore, the upper limit of the heating temperature is
preferably 1250°C. Meanwhile, the heating temperature is preferably as low as possible
in terms of costs.
20 [0189]
Next, hot rolling is carried out at a finishing temperature of 850°C to 970°C on
the slab so as to manufacture a steel sheet. When the finishing temperature is lower
than 850°C, the rolling is carried out in a two-phase region, and therefore ductility
degrades. When the finishing temperature exceeds 970°C, austenite grain sizes become
55
coarse, the ratio of the ferrite phase decreases, and ductility degrades.
[0190]
After the hot rolling, the steel sheet is cooled to a temperature range of 450°C or
lower (cooling control temperature) at an average cooling rate of 10 °C/second to 100
5 °C/second, the steel sheet is coiled in a temperature of NOT to 450°C (coiling
temperature). A hot-rolled steel sheet is manufactured as a final product in the above
manner. In a case in which the cooling control temperature after hot rolling is higher
than 450°C, a ratio of desired martensite phase cannot be obtained, and therefore the
upper limit of the coiling temperature is 450°C. Meanwhile, in a case in which the
10 martensite phase is secured more flexibly, the upper limits of the cooling control
temperature and the coiling temperature are preferably 440°C. When the coiling
temperature is 300°C or lower, the hardness of the martensite phase excessively increases,
and therefore the lower limit of the coiling temperature is 300°C.
[0191]
15 In addition, when the cooling rate is less than 10 °C/second, pearlite is liable to
be formed, and, when the cooling rate exceeds 100 °C/second, it is difficult to control the
coiling temperature.
[0192]
When a hot-rolled steel sheet is manufactured by controlling the hot rolling
20 conditions and the cooling conditions after hot rolling in the above manner, a
high-strength steel sheet that is excellent in terms of hole expansion and ductility, and
mainly includes ferrite and martensite can be manufactured.
[0193]
In addition, in the embodiment, the high-strength cold-rolled steel sheet can be
56
manufactured in the following manner.
[0194]
After the casting, the slab having the above chemical composition is reheated to
1100°C or higher according to necessity. Meanwhile, reasons why the temperature of
5 the slab before the hot rolling is controlled are the same as in a case in which the above
high-strength hot-rolled steel sheet is manufactured.
[0195]
Next, hot rolling is carried out at afnishing temperature of 850°C to 970°C on
the slab so as to manufacture a steel sheet. Furthermore, the steel sheet is cooled to a
10 temperature range of 300°C to 650°C (cooling control temperature) at an average cooling
rate of 10 °C/second to 100 °C/second. After that, the steel sheet is coiled at a
temperature of 300°C to 650°C (coiling temperature) so as to manufacture a hot-rolled
steel sheet as an intermediate material.
[0196]
15 When the cooling control temperature and the coiling temperature are higher
than 650°C, lamellar pearlite is liable to be formed, and the lamellar pearlite cannot be
sufficiently melt through annealing, and therefore hole expansion degrades. In addition,
when the coiling temperature is lower than 300°C, the hardness of the martensite phase
excessively increases, and therefore it is difficult to efficiently coil the steel sheet.
20 Meanwhile, reasons why the cooling rate and the finishing temperature of the hot rolling
are limited are the same as in a case in which the above high-strength hot-rolled steel
sheet is manufactured.
[0197]
The hot-rolled steel sheet (steel sheet) manufactured in the above manner is
57
pickled, then, subjected to cold rolling at a reduction in thickness of 40% or more, and
annealed at a maximum temperature of 750°C to 900°C. After that, the steel sheet is
cooled to 450°C or lower at an average cooling rate of 0.1 °C/second to 200 °C/second,
and, subsequently, held for 1 second to 1000 seconds in a temperature range of 300°C to
5 450°C. Ahigh-strength cold-rolled steel sheet that is excellent in terms of elongation
and hole expansion can be manufactured as a final product in the above manner.
[0198]
In manufacturing the cold-rolled steel sheet, when the reduction in thickness is
less than 40%, it is not possible to sufficiently refine crystal grains after the annealing.
10 [0199]
In a case in which the maximum temperature of the annealing is lower than
750°C, the amount of austenite obtained through the annealing is small, and therefore it
is not possible to form a desired amount of martensite in the steel sheet. When the
annealing temperature increases, the grain sizes of the aisl:enite becomes coarse, ductility
15 degrades, and manufacturing cost increases, and therefore the upper limit of the
maximum temperature of the annealing is 900°C.
[0200]
The cooling after the annealing is important to promote transformation from
austenite to ferrite and martensite. When the cooling rate is less than 0.1 °C/second,
20 since pearlite is formed such that hole expansion and strength degrade, the lower limit of
the cooling rate is 0.1 °C/second. In a case in which the cooling rate exceeds 200
°C/second, it is not possible to sufficiently proceed with ferrite transformation, and
ductility degrades, and therefore the upper limit of the cooling rate is 200 °C/second.
[0201]
59
The cooling temperature during the cooling after the annealing is 450°C or lower.
When the cooling temperature exceeds 450°C, it is difficult to form martensite. Next,
the cooled steel sheet is held in a temperature range of 300°C to 450°C for 1 second to
1000 seconds.
5 [0202]
A reason why the lower limit of the cooling temperature cannot be provided is
that martensite transformation can be promoted by once cooling the steel sheet to a
temperature lower than the holding temperature. Meanwhile, even when the cooling
temperature is 300°C or lower, as long as the steel sheet is held in a temperature higher
10 than the cooling temperature, the martens ite is tempered, and it is possible to reduce the
difference in hardness between the martensite and the ferrite.
[0203]
When the holding temperature is lower than 300°C, the hardness of the
martensite phase excessively increases. In addition, when the holding time is less than
15 1 second, thermal shrinkage-induced residual strains remain, and elongation degrades.
When the holding time exceeds 1.000 seconds, more bainite and the like are formed than
is necessary, and a desired amount of martensite cannot be formed.
[0204]
As described above, when a hot-rolled steel sheet is manufactured by controlling
20 the hot rolling conditions and the cooling conditions after the hot rolling, and a
cold-rolled steel sheet is manufactured from the hot-rolled steel sheet by controlling the
cold rolling conditions, the annealing conditions, the cooling conditions, and the holding
conditions, it is possible to manufacture a high-strength cold-rolled steel sheet that is
excellent in terms of hole expansion and ductility, and mainly includes ferrite and
59
martensite.
[0205]
Therefore, in the embodiment, molten steel is processed into a slab, hot rolling is
carried out on the slab at a finishing temperature of 850°C to 9%0°C so as to manufacture
S a steel sheet. After that, the steel sheet is cooled to a cooling control temperature of
650°C or lower at an average cooling rate of 10 °C/second to 100 °C/second, and then
coiled at a coiling temperature of 300°C to 650°C. Here, in a case in which a hot-rolled
steel sheet is manufactured, the cooling control temperature is 450°C or lower, and the
coiling temperature is 300°C to 450°C. In addition, when a cold-rolled steel sheet is
10 manufactured, the coiled steel sheet is pickled, cold rolling is carried out on the steel
sheet at a reduction in thickness of 40% or more, the cold-rolled steel sheet is annealed at
a maximum temperature of750°C to 900°C, cooled to 450°C or lower at an average
cooling rate of 0.1 °C/second to 200 °C/second, and held in a. temperature range of 300°C
to 450°C for 1 second to 1000 seconds.
15 Meanwhile, a flowchart of the method of manufacturing the high-strength steel
sheet of the embodiment is shown in FIG. 2 for easy of understanding. Meanwhile, the
broken lines in the flowchart indicate processes or manufacturing conditions that are
selected according to necessity.
[0206]
20 Furthermore, coating may be appropriately carried out on at least one surface of
the hot-rolled steel sheet and the cold-rolled steel sheet. For example, zinc-based
coating such as coating using galvanizing and galvannealing can be formed as a coating.
The zinc-based coating can also be formed through electroplating or hot dipping. The
galvannealing coating can be obtained by, for example, alloying a zinc coating
60
(galvanizing coating) that is fonned through electroplating or hot dipping in a
predetermined temperature (for example, a temperature of 450°C to 600°C, and a time of
10 seconds to 90 seconds). A galvanizing steel sheet and a galvannealed steel sheet can
be manufactured as final products in the above manner.
5 [0207]
Additionally, a variety of organic films and coatings can be formed on the
hot-rolled steel sheet, the cold-rolled steel sheet, the galvanized steel sheet, and the
galvannealed steel sheet..
[Examples]
10 [0208]
Hereinafter, examples of the present invention will be described.
[0209]
Steels that had been prepared and incited in a converter and had the chemical
components as shown in Tables 1 to 3 were cast so as to produce slabs. The steels
15 having each chemical component were heated to a temperature of 1150°C or higher in a
heating furnace, subjected to hot rolling at a finishing temperature of 850°C to 920°C,
cooled at an average cooling rate of 30 °C/second, and coiled in a coiling temperature of
100°C to 600°C, thereby producing 2.8 mm to 3.2 mm-thick hot-rolled steel sheets.
The manufacturing conditions and mechanical properties of the hot-rolled steel sheets are
20 shown in Tables 4 to 6, and the microstructures of the hot-rolled steel sheets are shown in
Tables 7 to 9.
[0210]
[Table 1]
- CheodosI components 6nass%1 ([Ce]T[La])
([CcHTA)
Steel
No. C Si Mn P S N
Acid-soluble Aeid-soluble
Ti
Cr Nb 4 M. & B Ca Cu N Ce Ia
/[Acid-soluble
All IS]
Al 0.067 0.48 19 0.015 0.0049 0.0033 0.024 - - - - - - - - - 0.0040 - 0.17 0.8
A2 0.135 0.52 2.1 0.015 0.0030 0.0044 0.042 - - - - - - 0.001 - - - 0.0050 0.12 1.7
A3 0.068 0.60 3.5 0.030 00049 0.0037 0.034 - - - - - - - - - - 0.0040 0.12 0.8
A4 0.157 0.41 2.4 0.016 0.0029 00044 0.036 - - - - - - - 0.1 - - 0.0050 0.14 1J
AS 0.153 1.15 22 0.008 0.0029 0.0006 0.030 0.046 0.5 0.01 - - - 0.001 - - - 0.0050 0.17 1.7
A6 0.135 058 24 0.012 0.0038 0.0026 0.041 - 0.4 - - 0.10 0005 - - - 0.05 0.0060 0.15 1.6
a1 0.070 052 L9 0.015 0.0048 0.0036 0.026 - - - - - - - - 0.0010 0.04 0.2
e2 0.155 1.02 2.1 0.008 0.0031 0.0045 0.031 0.006 0.001 - - - - - c
u3 0.072 0.62 2.4 0.020 0.0052 0.0035 0.034 - - - - - - - - - ' =
a4 0 081 -0.71 -T---0 Oli 0.OOti 0.0037 0.024 - - - - - - - - - 0.0400 00400 332 53 3
ee 0.083 0.62 2.3 0.016 0.0013 0.0026 0.003 0.045 - 0.03 - 0.15 - 0.002 - - - 00300 0.0450 1.53 5>2
a6 01167 050 2.0 0 015 00039 0.0034 0.035 0250 - 0.15 - - - - - - 00010 0°0010 0.08 LO
c7 0.153 0.98 0.008 0.0029 0.0046 0.030 0.006 5.3 0.01 - - 0.001 - - 0.0050 - 0.17 1.7
indicates that the corresponding chemical element is not added.
'The under es is this Table indicate that the corresponding amount does not satisfy the conditions of the chemical components according t0 the present inveutioit
62
[0211]
[Table 2]
Steel Chemical components (mass%) ([Ce]±[La]) C +
No. C Si Mn p S N
Acid-soluble
At
Acid-soluble
Ti
Cr Alb V Me Zr B Ca Cu Ni Cc La
/[Acid-soluble
Al]
e a
([
/[S]
B 1 0,07 042 1.9 0.011 0.0015 0.0034 0.025 0.02 - - - - - - 0.004 - 0,16 2.67
32 0.07 0.50 2.1 0.015 0.0030 0.0036 0.034 005 - - - 0.006 0.18 2.00
B3 0.14 0,49 2.0 0.015 0.0031 0,0047 0.041 0.02 - - - 0.001 - - 0005 0.12 1.61
B4 0.07 0.58 2.5 0.020 0.0051 0.0034 0.035 0.10 - - - - - - - 0,004 0.11 0,79
B5 0.08 0.59 23 0.015 0.0049 0.0033 0,042 0.02 - - - - - - - 0.05 0.002 0.002 010 0.82
B6 0.15 0.49 2,6 0.009 0.0010 0.0045 0.036 0.01 - - - - - - - - 0.001 0.003 0,11 4.03
B7 0.16 2.07 2,0 0.010 0.0024 0.IXL2 0.031 0.02 - 0.02 - - - 0.001 - - - 0.002 0.004 0.20 2.46
BS 0.15 1.03 20 0.008 0.0030 0.0006 0.031 0.01 0.5 0,01 - - - 0.001 - 0.005 0.16 1.67
B9 0.15 0.61 28 0.012 0.0042 0.0023 0.042 0.01 0.4 - 010 - - 0.05 - 0.006 0.14 144
b1 0,07 0.49 20 0.016 0.0029 0.0034 0,036 0.05 - - - - - - - 0.001 0.02 028
b2 0.13 0.52 2.2 0.015 0.0030 0.0046 0.040 0.02 - - - 0.001 - - 0.00 0.00
b3 0.08 0.59 2.3 0.015 0.0048 0.0037 0.039 0.02 - - - - - - 0.05 - - 0.00 0.00
b4 0.08 0.59 21 0.015 0.0029 0.0024 0.030 0.05 - 0.03 - 0.15 - 0.002 - - - 0.001 - 0.03 0.34
b5 O.15 197 1.9 0.010 0.0026 0.0018 0.030 0.02 - 0.02 - 0.001 - - - - 0.00 0.00
b6 0.14 0.59 2.7 0.011 0.0038 0.0027 0.041 0.01 0.4 - - 0.10 0.005 - 0.05 - 0.00 0.00
b7 0.07 0.38 1.S 0.015 0.0015 0.0037 0.025 0.02 - - - - 0040 0.040 3.19 53.33
b8 0.16 049 2.5 OA09 0,0010 0.0046 0.035 0 01 - - - - - - - - 0.035 0.030 1.88 64.01
b9 0.35 062 3.6 0.012 0.0039 00025 0.041 04 010 0.005 0.05 0.002 0.002 0.09 0.90
b10 0.07 OSO 2.0 0.015 00029 0.0034 0.035 025 - 0.15 - - - - - - 0.002 0.001 0.08 1.04
b11 j 0.15 0.98 22 0.003 0.0029 0.0006 0030 0.02 53 0.01 - - - 0.001 - - 0.005 0.000 0.17 1.74
`-" indicates that the corresponding chemical element is not added.
"The underlines in this Table indicate that the corresponding amount does not satisfy the conditions of the chemical components according to the present invention.
64
[0212]
[Table 3]
Chemical components ( mesa%) (CCo]+[Lal)
([Ce]+(Ia])
Steel Acid-soluble Acid-soluble
Nb V M Z 3 M W Ni Others C. ha
/jAeld-soluble
1151
Na C Si Mn Is S N Cr o r g A[]
M Ti
Cl 0040 042 L8 0.015 00029 00027 0.043 0050 0.01 - 0.05 - - - 14 - - 0,0010 0.0020 0.07 1.04
- - - - 0.0013 00024 0.09 1.48
C3 0 110 0 92 22 0015 0. 0025 0.0035 0,039 0.002 - - - -
C3
.
0 165
.
1.45 2 5 0.008 0.0029 0.0025 0.040 0.004 0.01 - - - 0001 0.004 - - 0.0025 - 006 0.87
C4
.
0.130 1.00 2.2 0010 0.0002 0.0036 0030 0.010 - 0.03 005 - - - - - - - 0.0015 10010 0.08 12.50
As -0 02,
C5 0.060 070 20 0,010 0.0072 0.0035 0.039 0.040 - - - - - - - - - Co:02, 00010 00020 008 0.42
S.:0,002
Sn:O04
0 0030 - 0.08 079
C6 0.161 1.20 2.8 0010 0.0038 00035 0.040 0.004 - 0.02 - - - - - pb005
.
-
Dy:0.003,
0 0015 0 0020 009 1.00
C7 0.110 1.10 2.1 0012 0.0035 00034 0.038 000? - - - - - - - - Nd0003
.
Y:0.002, - 0=022 007 550
C8 0.080 0.87 1.5 0009 0 0114 00033 0.032
C9 0 080 0.60 10 0.017 0.0105 00035 0.103 0.020 - 0.03 - - - - - - - 0.0100 0.0120 0.12 114
CIO
.
1190 1 70 2.5 0010 00420 00040 0.105 0.006 0.6 - - - - - - - - 0.000 0.0210 030 0.50
e1 0 250
,
0 61 4.2 0.012 0,0039 0.0025 0.041 - 0.4 - - 0.10 0.005 - - 0.05 - 0.0015 0.01_0 0.09 0.90
. .
- - - - 00090 O1110 0.01 250
0. 110 0,05 22 0,010 0.0040 0.0036 1.900 0.010 - - - -
- - - - 00020 1025 0.08 1,29
070 195 18 0012 0.0035 0.0025 1032 - - - - - -
- itdi,acs that the corneapondt8 chemical eleanent is not added.
"'The underlines in 5ris Table indicate that the eortespond htn amount does not satisfy the conditions ofthe chemical components din- to the present Ineevfion.
66
[0213]
[Table 4]
Hot-rolled conditions Mechanical properties
Steel
sheet
No.
Steel
No,
Heating
temperature
°C
Finishing
temperature
C
Coiling
temperature
C
Tensile
strength
,1,5
MPa
Elongation
El
%
Hole
expansion
TS*Elx,
Al-hl At 1180 900 350 572 30.2 94 1. 6E+06
A1-h2 Al 1160 890 180 645 28.2 51 9.3E+05
A2-h1 Al 1180 900 360 745 23.4 76 1.3E+06
A2-h2 A2 1170 880 110 802 20.8 38 6.3E+05
A3-hl A3 1200 890 380 720 24.5 79 T4E+06
A3-h2 A3 1170 900 100 813 21,2 33 5.7E+05
A4-hl AT 1150 880 330 932 17.3 67 1. I G+06
A4-h2 A4 1180 870 180 1023 16,1 31 5. 11 i+05
A5-111 AS 1190 880 400 1072 14,6 62 9.6E+05
AS-h2 Al 1170 900 150 1196 15.6 21 3 9E+05
A6-hl A6 1200 890 330 1068 15.3 65 1.IE+06
Anh2 A6 1180 900 130 1236 14,2 23 4.0E+05
al-hl at 1180 900 360 569 30.1 65 1.1E+06
a2-hl a2 1200 990 410 1098 14,8 42 6.8E+05
a3-hl a3 1160 870 400 725 24,2 54 9.5E+05
a4-hl a4 1190 860 340 562 31.2 62 1,1E+06
a5-ht a5 1210 900 370 932 18,2 45 7.6E+05
a6-hl a6 1250 910 320 921 8,8 45 3,6E+05
a7-hl a7 1200
_
880 350 1320 7.2 55 5.2E+05
The underlines ur the Table indicate that the corresponding coil
does not satisfy the manufacturing conditions according to the present invention.
[0214]
5 [Table 5]
67
Hot rolled conditions Mechanical properties
Steel
sheet
No.
Steel
No,
Healing
temperature
°C.
Finishing
temperature
C
Coiling
temperature
°C
Tensile
strength
TS
MPa
Elongation
El
Hole
expansion
SxElx7.
81-hl BI 1250 900 360 575 30.8 95 1.7E+06
Bi-h2 B1 1250 890 180 643 28.2 55 1.0E+06
112-hl B2 1250 900 360 531 32,3 108 1.9E+06
B2-h2 B2 1250 880 110 646 26.8 51 8.8E+05
B3-hl B3 1250 880 330 760 22.6 78 1.3E+06
133-h2 B3 1250 870 180 837 19.0 42 6,7E+05
B4-hl B4 1250 900 390 777 22.7 77 1.4E+06
B4-h2 B4 1250 880 150 835 19.9 40 6.7E+05
85-hl B5 1250 900 310 783 21.4 73 1,2E+06
B5-h2 B5 1250 910 100 845 18.2 42 6.5E+05
136-hl B6 1250 890 330 964 15.7 57 8.7E+05
B6-h2 B6 1250 900 180 1086 15.1 35 5.7E+95
B7-hl B7 1250 880 350 1075 14.1 52 7.9E+05
B7-h2 B7 1250 890 150 1199 12.8 38 5.8E+05
88-hl B8 1210 870 370 1062 14.0 59 8.8E+05
B8-h2 B8 1200 880 180 1250 14.2 35 6,2E+05
B9-hl B9 1210 900 390 1156 14,1 48 7,8E+05
89-h2 B9 1210 880 160 1235 12.9 32 5.1E+05
bl-h1 bl 1250 890 360 533 33.1 76 1.33E-06
b2-hl b2 1250 870 330 754 22,7 55 9.33E+05
b3-hl b3 1250 910 310 777 21.3 51 8.43E+05
b4-hI b4 1250 900 360 950 16,8 41 6.58E+05
b5-hl b5 1250 880 350 1070 15.2 36 5.92E+05
b6-hl b6 1210 870 370 1053 13.8 41 6.00E+05
b7-hl b7 1250 900 360 574 31,3 67 1.20E+06
b8-hl b8 1250 890 330 954 15.7 40 5,97E+05
b9-hl b9 1250 880 350 1170 8.1 05E+05
bl0-hl b10 1250 880 320 905 91
L
72+05
bll -hi bit 1250 890 350 1313 7,1 5O L4.65^E+05
The underlines in the Table indicate that the corresponding cell
does not satisfy the manufacturing conditions according to the present invention.
[0215)
[Table 6]
68
Hot-rolled conditions Mechanical properties
Steel
sheet
No.
Steel
No.
Heating
temperature
aC
Finishing
temperature
oc
Coiling
temperature
oC
Tensile
strength
„rS
MPa
Elongation
El
%
4expansion
1x2v
Cl-hi Cl 1250 920 380 552 31.2 +06
Cl- h2 Cl 1250 910 150 623 29.2 1-06
C2-hl C2 1200 890 350 983 16.6 +06
C2-h2 C2 1200 900 180 1092 15.3 39 6.5E+05
0-hl C3 1250 950 400 1176 14.9 65 1.1E+06
C3-h2 C3 1250 940 180 1265 13.8 31 5.4E+05
C4-hl C4 1250 910 450 892 19.2 81 1.4E+06
C4-h2 C4 1250 930 160 102,4 17.6 38 6.8E+05
C5-hl C5 1200 880 350 621 27.8 121 2.1E+06
C5-h2 C5 1200 880 180 644 28.4 58 1.1E+06
C6-hl C6 1250 880 380 1206 14.2 68 1.2E+06
C6-h2 C6 1250 890 150 1289 12.4 28 4.5E+05
C7-hl C7 1200 900 400 945 18.6 76 1.3E-1-06
C7-h2 C7 1200 920 180 1056 16.2 36 6.2E+05
C8-hl C8 1250 880 330 561 32.6 119 2.2E+06
C8-h2 C8 1250 890 160 603 30.1 67 1.2E+06
C9-hl C9 1250 930 300 702 26,8 102 1.9E+06
C9-h2 C9 1250 930 150 791 24.1 42 8.0E+05
C10-hl CIO 1200 880 320 1191 16.3 78 1.5E+06
C10-h2 C10 1200 880 130 1253 13.4 21 3.5E+05
cl-hi cl 1150 880 380 1074 9.3 35 3,3E+05
c2-hl c2 1200 900 350 989 1s2 32
_
5.8E+05
c3-hl c3 1200 920 380 773 9.3 31 3,3E+05
* The underlies in the Table indicate that the corresponding cell
does not satisfy the manufacturing conditions according to the present invention
[0216]
[Table 7]
Fine inclusions Elongated ni clusions Inclusions including sulfides Martensite phase
Steel Number
Volume
Average
equivalent Number
Volume
Average
concentration Ratio Maximum
sheet
No.
number
percentage
mnnber
d i -
circle percentage
number
densn3'
oC hardness
No. density ens R
diameter % s
^Fe)+(La] °
/o HV
inclusions/mm^
3
inclusions/mm urclusions/mm
nm
AI hl At 42 0 0 6 83 2 5x10' 34 10.2 487
Al-h2 Al 41 1 0 6 82 28x10° 38 10.4 643
A2-hl A2 42 0 0 5 84 1 2.4x10° 40 18.2 516
A2-h2 A2 41 1 0 6 83 2.8x10° 28 16.4 665
A3-hl A3 38 1 0 9 81 2.3x10" 31 16.2 534
A3-h2 A3 42 0 0 9 90 2.5x10° 33 16.4 681
A4-hl A4 36 1 _ 0 88 2.1x10° 28 26.1 549
A4-h2 A4 37 1 0 7 92 2.8x10° 40 25.3 668
A5-hl AS 37 0 0 5 88 2.1x10° 37 33.2 546
A5-h2 AS 35 1 0 8 91 2.9x10° 28 34.6 630
A6-hl A6 45 0 0 7 82 2,7x10° 25 35.6 511
A6-h2 A6 46 1 0 7 83 25x10' 31 34.6 657
al-hl al 7 45 2.7x10° 29 3 5,0x702 2 10.1 484
a2-hl a2 9 32 2.8x10° 17 0 0 0 32.8 590
a3-hl a3 6 41 3.0x104 26 1 0 0 16,5 553
a4-hl a4 52 0 0 7 89 2.140° 42 10.7 447
a5-hl a5 49 1 0 5 92 3.0x10° 53 29.8 510
a6-hl a6 63 0 0 9 84 2.0x10° 18 31.5 479
a7-hl a7 33 0 0 7 88 25x10° 32 56.2 475
Fine inclusions Elongated inclusions Inclusions including sulfides Martensite phase
Steel Area Volume
Average Volme eerage
Steel Number equivalent Number
number
conc entration Ratio Maximum
sheet
No.
nunber
percentage i circle percentage densit
of hardness
No. density dens ty
diameter %
y
[Ce]+{La] % HV
Inclusions/arm2 llKlnsions/nine
3
Fun inclnslons/nntt Ox,
B1-hl Bl 42 0 0 6 82 2.1x10 34 9.4 514
B14i2 31 41 1 0 7 81 2.8x10' 36 9.5 685
32-h1 32 . 38 0 0 4 83 2.3x10' 41) 9.3 420
B2 h2 32 37 0 0 8 81 2.6x10' 42 10.3 657
133-hl 33 37 0 0 6 81 2.5x10° 28 19.1 532
B3-b2 B3 40 1 0 6 80 2.6x10° 30 17.6 664
34-hl 34 39 0 0 7 83 2,4x10 41 19.4 539
B4-h2 B4 38 0 0 8 84 2.5x10° 39 19.8 613
135-111 B5 41 1 0 5 81 2.6x10' 35 20.9 518
B5-h2 35 42 0 0 6 82 2.4x10' 37 187 656
36-h1 36 41 1 0 6 81 2.4x10' 43 28.2 556
B6-h2 B6 39 0 0 5 80 21x10' 42 29.7 627
37-1i B7 39 0 0 6 81 25x10' 31 39,9 491
B7-h2 37 41 0 0 7 80 2.0x10' 33 36.8 607
38-h1 B8 38 0 0 9 80 2.1x10` 42 43.2 450
38-12 B8 42 1 0 7 80 2.5x10° 43 37.4 624
B94u1 B9 39 0 0 6 83 23x10' 40 40.8 523
39-h2 39 36 0 0 5 81 2.8x10' 39 37.4 618
bl1il bl 5 45 2.1x10` 29 0 5.0x102 1 9.2 423
b2-hl b2 7 32 2.8x100 17 0 0 0 18.9 537
b3-hl b3 6 41 3.0x10' 26 0 0 0 20.4 528
b4-hl b4 4 40 23xl0' 25 0 5,0.102 1 29.3 531
b5-hl b5 6 42 2,7x10' 24 0 0 0 38.9 502
b6-hl b6 4 40 3.0x10' 23 0 0 0 42,2 458
b7-hl b7 55 0 6 89 21x10' 68 9.0 534
b8-hl b8 63 2 0 5 91 3.0x10' 72 27.5 568
b9-hl b9 34 0 0 6 81 1.8x10' 21 59.9 397 1I
b10-hl b10 41 0 0 9 84 2.0x10' 18 2°2 494
bll-hl b11 33 0 0 7 89 25x10' 32 49.8 515
0
Fine inclusions Elongated inclusions inclusions including sulfides Martensite phase
Steel Area Volume
Average Volume
Average
Steel Number equivalent Number ber
concentration Ratio Maximum
sheet
No.
number
d it
percentage
number
density
circle percentage
num
density
of hardness
No ens y
% diameter
me
% s [Ce]+[La]
°
/° 1-NV
inclusions/=2 inclusions/mm3 Pm inclusions/mm %
Cl-hl Cl 42 1 0 7 82 2.6x10° 32 2.5 580
Ci-h2 Cl 45 0 0 8 80 2.0xl0' 34 3 725
C2-hl C2 38 0 0 5 83 2.1x10° 33 29 515
C2-h2 C2 36 1 0 6 85 2.2x10° 35 32 656
C3-hl C3 42 0 0 7 80 2.0x10° 36 42 536
C3-h2 C3 48 0 0 7 84 2.3x10' 39 39 672
C4-h1 C4 25 1 0 6 81 2.5x104 35 28 492
C4-h2 C4 27 1 0 7 80 2.2x104 37 26 648
C5-hl C5 41 1 0 81 2.0x104 34 5.5 550
C5-h2 CS 44 1 0 8 80 2.5x104 31 6 783
C6-hl C6 40 0 0 8 81 2.1x104 32 45 563
C6-h2 J C6 46 0 0 7 83 2.7x104 i 36 42 692
C7-h1 C7 27 0 0 6 80 22x104 34 25 521
C7-h2 C7 28 0 0 6 82 23x104 36 27 666
C8-h1 C8 21 0 0 6 88 1.6x104 42 6 569
C8-h2 C8 22 0 0 7 85 1.7x104 39 7 702
C9-h1 C9 76 1 0 8 81 43x104 25 13 563
C9-h2 C9 82 1 0 7 76 3.9x104 27 14 673
CIO-hl CIO 103 1 1.0x103 8 79 6.2x104 24 42 562
C10-h2 CIO Ill 1 1.0x103 9 83 5.8x104 25 41 715
cl-hl cl 25 2 1.0xlo' 7 85 13x104 18 48 556
c2-hl c2 10 5 3.0x103 9 8 S.Oxlor 5 25 582
c3-hl c3 26 1 10x103 8 82 15x10 4 21 26 571
72
compositions were cast, heated to 1150°C or higher, subjected to hot rolling in a finishing
temperature of 850°C to 910°C, cooled at an average cooling rate of 30 °C/second, and
coiled at a coiling temperature of 450°C to 610°C, thereby producing 2.8 mm to 3.2
mm-thick hot-rolled steel sheets. After that, the hot-rolled steel sheets were pickled,
5 and then subjected to cold rolling , annealing, and holding under the conditions as shown
in Tables 10 to 12, thereby producing cold rolled steel sheets. The manufacturing
conditions and mechanical properties of the cold rolled steel sheets are shown in Tables
10 to 12 and the microstructures of the cold-rolled steel sheets are shown in Tables 13 to
15. The sheet thicknesses of the cold-rolled steel sheets were 0.5 mm to 2.4 mm.
10 [0220]
[Table 10]
Hot-rolled conditions Cold-rolled conditions Mechanical properties
Steel
sheet
No.
Steel
No.
Fleatmg
temperature
°C
Fntior t
temperature
C
Coiling
temperature
°C
Reduction
%
Annealing
temperature
C
Average
coaling
rate
Holding
temperature
Holdnig
time
s
Sheet
thickness
mm
Tensile
strength
TS
MPa
Elongation
El
%
Hole
expansion
%
S281x).
Al-cl Al 1180 900 600 55 790 14 350 330 0.8 562 322 102 1.8E+06
Al-c2 Al 1160 890 580 55 790 18 250 330 0.8 662 28.2 48 9.0E+05
A2-cl A2 1180 900 590 55 810 14 380 300 1.6 722 24.3 75 1.3E+06
A2-c2 A2 1170 880 610 55 810 17 280 300 1.6 791 20.3 38 6,IE+05
A3-cl A3 1200 890 500 60 810 15 340 320 1.2 720 22.0 81 1.3E+06
A3-c2 A3 1170 900 510 60 810 19 )40 320 1.2 813
-
19.8
-----------------------
35 5.6E+05
A4-c1 A4 1150 880 450 60 830 16 350 320 1.4 945 17.5 71 1.2E+06
A4-c2 A4 1180 870 480 60 830 19 260 320 1.4 1032 13.9 28 4.6E+05
A5-cl AS 1190 880 560 50 800 14 380 350 L2 1082 13.2 59 8.4E+05
A5-c2 AS 1170 900 570 50 800 17 280 350 12 1203 13.9 19 3.2E+05
A6-cl A6 1200 890 540 60 810 15 350 350 0.6 1072 15.8 67 1 1E+06
A6-c2 A6 1180 900 520 60 810 18 250 350 0.6 1251 13.1 17 2.8E+05
al-cl al 1180 900 590 55 810 15 350 330 2.1 572 30.1 59 1.OE+06
a2-cl a2 1200 890 550 50 800 14 380 350 0.9 1075 13.1 43 6.1E+05
a3-cl a3 1160 870 490 60 810 15 340 320 1.5 725 23.8 51 8.8E+05
a4-cl a4 1190 850 400 55 810 13 400 300 2.3 557 33.2 67 1.2E+06
aS-cl a5 1210 900 520 60 850 15 340 300 1.6 941 18.9. 44 - 7.8E+05
a6-cl a6 1250 910 570 55 800 14 380 300 1.4 932 82 47 3.6E+05
a7-cl a7 1200 880 570 50 800 14 380 350 0.7 1280 73 51 4.8E+05
"The underlin the Table indicate that the corresponding cell does not satisfy the manufact g conditions according to the present ention
Hot-rolled conditions Cold-rolled conditions Mechanical properties
Steel
sheet
No.
Steel
No.
Edaating
tempcraACe
Firnslwrg
temperature
Coiling
temperature
°C
Reduction
%
Annealing
temperanne
oC
Average
cooling
rate
C
Holding
temperance
eC
Holding
time
e
Sheet
thickness
mm
Tensile
strength
TS
MPa
Elongation
El
E
Hole
expansion
7.
%
S-EM,
BI-c1 B1 1250 900 500 60 810 15 350 330 2.1 547 31 5 105 1.8E+06
Bl-c2 BI 1250 900 510 60 810 18 250 330 2.1 623 28.8 61 LIE+06
B2 -el B2 1250 910 580 55 870 16 380 300 0.7 535 33.1 115 2013 06
132-c2 32 1250 910 600 55 870 19 280 300 0-7 658 27.8 55 1.0E+06
B3-.l B3 1250 880 490 60 790 15 330 360 1.0 785 22.1 78 1413-506
133-c2 B3 1250 890 480 60 790 17 260 360 L0 804 206 63 1.0E-5-06
134-cl 134 1250 900 560 55 800 13 400 360 0.8 759 22.9 85 1.5E+06
134-c2 134 1250 900 550 55 800 20 200 360 0-8 853 19.8 58 9.8E+05
135-el B5 1250 910 590 50 810 16 320 330 1 0.5 763 23.4 8B 1.63+06
E5-c2 Be 1250 900 600 50 810 18 250 330 0-5 841 19.2 56 9.0E-105
36-e1 B6 1250 900 580 60 850 15 380 330 1.2 916 18.2 75 1.3'13+06
136c2 B6 1250 900 570 60 850 20 250 330 12 1074 15.0 36 582805
137-c1 B7 1250 900 580 60 800 14 380 250 1.2 1086 14.3 58 9.0E+05
137c2 B7 1250 900 590 60 800 18 250 250 1.2 1254 12.8 28 456-105
138-c1 B8 1220 900 610 55 820 17 310 300 1.4 1036 14.7 63 9.6E+05
138-c2 B8 1210 890 590 55 820 18 280 300 1.4 1251 145 31 5.61305
139-cl 139 1210 890 550 50 820 15 350 330 2.0 1132 14.4 54 8-813+05
139c2 B9 1220 880 540 50 820 19 250 330 2.0 1207 13.9 29 4913+05
bl-c1 Ill 1250 900 590 55 870 16 380 300 1.8 524 324 79 1,3E+06
b2-el b2 1250 900 500 60 790 15 330 360 0.7 779 22.5 54 9.5E+05
b3-cl 125 1250 890 600 50 810 16 320 330 0.9 771 22.2 61 1.0E+06
b4-el It, 1250 900 540 60 810 15 350 360 Lo 939 17.1 48 7 713+05
135-c1 b5 1250 890 600 60 800 14 380 250 1-4 1109 14,6 37 6.0E+05
b6-c1 b6 1200 910 600 55 820 17 310 300 0.6 1045 14.2 41 6.1E+05
b7c1 b7 1250 900 510 60 810 15 350 330 L2 554 31.1 72 1213+06
b8-c1 68 1250 890 580 60 850 15 380 360 1.6 914 173 48 7.6E+05
b9-c I 69 1250 900 500 55 810 15 350 350 0.6 1216 8.0 38 3713+05
b10-ei blO 1250 910 550 55 850 16 350 300 1.1 887 8.8 48 3713+05
bl I-el b11 1250 880 570 50 880 l9 310 350 12 1347 7.0 31 2.9E-105
x Ilse underlines in the Table indicate that the eorrespondiog cell does not cana l the manufaeltuing conditions according to Ote present invention.
Hot-rolled conditions Cold-rolled conditions Mechanical properties
Steel
sheet
No
Steel
No.
Heating
temperature
°C
Finishing
temperature
°C
Coiling
temperature
°C
Reduction
%
Annealing
temperature
°C
Average
cooling
rate
oC
Holding
temperature
oC
Holding
.
time
s
Sheet
t ickness
m`n
Tensile
strength
TS
Mr.
Elongation
El
Hole
expansion
SxEIX%
Cl-hl Cl 1250 900 500 45 810 15 350 330 1.6 572 31.1 129 2.3E+06
01-h2 C1 1250 900 510 45 810 18 250 330 1.6 621 32.3 58 120406
C2-hl C2 1200 880 550 60 850 14 300 300 1.3 983 16.8 76 1.3E+06
C2-h2 C2 1200 880 550 60 850 17 220 300 1.3 1092 15.6 39 6.6E+05
C3-hl C3 1250 900 550 55 820 18 320 300 1-4 1176 15.5 65 1.20+06
C3-h2 C3 1250 900 550 55 820 22 200 300 1.4 1265 14.5 31 57E-+05
04-hl C4 1250 900 500 55 860 15 380 400 I 1.7 892 19.2 81 1.40+06
04-h2 C4 1250 900 500 55 860 18 250 400 1.7 1024 17.6 38 6.8E+05
05-hl C5 1200 880 600 80 820 16 350 600 08 601 30.1 121 3.2E+06
C5-h2 C5 1200 880 610 80 820 21 180 600 0.8 647 30.3 63 1.2E+06
06-hl C6 1250 900 350 70 850 16 350 300 1.2 1201 14.2 67 1.1E+06
C6-12 06 1250 900 360 70 850 21 250 300 L2 1296 14,1 6.011405
07-111 C7 1200 910 430 60 820 17 400 600 1.0 946 19.1 82 1.50+06
C7-- 07 1200 910 450 60 820 23 250 600 1.0 1079 175 41 7.7E+05
C8-hl 08 1250 880 550 65 860 7 300 500 2.2 543 342 112 2.10406
08-9.2 C8 1250 890 530 65 860 9 220 500 22 631 296 65 1.2E+06
C9-hl C9 1250 930 500 55 810 35 350 660 2.4 697 23.6 102 1.70+06
09-112 C9 1250 930 510 55 810 41 250 660 2.4 746 225 48 8.1E+05
C10-h1 010 1200 880 550 60 820 53 400 320 1.6 1201 15.8 74 1.4E+06
010-h2 010 1200 880 550 60 820 62 220 320 1.6 1291 141 23 420+05
al-hl ci 1150 880 600 60 810 7 350 300 1.3 1281 8.5 31 3-411+05
c2-hl o2 1200 880 600 60 820 15 330 300 OS 989 17.6 32 5.6E+05
c3-hl c3 1200 930 550 55 810 8 330 450 1.9 811 9.5 33 5.6E+05
The underlines in die Table indicatethat the corresponding cell does not satisfy the manutaetimng conditions according to me present invention.
76
[0223]
[Table 13]
Fine inclusions Elongated inclusions Inclusions including sulfides Martensite phase
Steel Area Volume
Average Volume Average
sheet
Steel
number
Number number
equivalent Number
number
number
concentration Ratio Maximum
No.
No.
density
percentage density
circle percentage density
of hardness
o 0 diameter %
3
[Ce]+[La; HV
inclusions/mm2 inclusions/mms μm inclusions/mm %
Al-cl Al 45 0 0 6 81 2.2x104 37 11.2 431
Al-c2 Al 38 1 0 6 89 2.5xl04 41 9.8 717
A2-ci A2 37 0 0 5 79 2.3x]04 43 17.3 528
A2-c2 A2 38 1 0 6 88 3.0xl04 29 166.8 636
A3-cl A3 39 1 0 8 78 2.5x104 32 19.5 476
A3-c2 A3 38 0 0 8 87 2.2404 33 17.9 632
A4-cl A4 34 1 0 5 91 2.3x10 27 302 514
A4-c2 A4 35 1 0 7 85 2.5x104 39 28.3 614
AS-cl AS 36 0 0 4 93 2.2x104 40 37.8 514
A5-c2 AS 33 1 0 7 95 2.7x104 29 34.4 638
A6-c1 A6 41 0 0 6 88 3.0x104 23 36.6 522
A6-c2 A6 45 1 0 6 76 2.6x104 34 35.2 657
al-el al 7 46 2.5x104 26 3 S.Oxioc 2 11.5 442
a2-cI a2 8 32 2.4x104 16 0 0 0 37.4 514
a3-cl a3 5 41 3.1x101 I 24 1 0 0 19 492
a4-cl a4 49 0 0 6 97 23x104 41 103 449
a5-01 a5 50 1 0 1 5 96 3.1x104 49 28.2 541
a6-cl a6 57 0 0 8 78 2.1x104 16 31.2 491
a7-ci a7 32 0 0 7 92 2.8x10 32 J 50A 501
Fine inclusions Elongated inclusions Inclusions including sulfides Martensite phase
Steel Area Volume
Average Volume
Average
sheet
Steel
munber
Ntunber number
egnivalent Numuber number
ooncentranon Ratio Maximum
No
No.
density
percentage density
circle percentage density
of hardness
, %
'
diameter a/ 3 LCej+[La] - AV
inclusions /nun'' iucInsious/mm inclusions/mm %
BI-cl BI 38 1 0 8 82 2.1x10° 48 9.2 467
B1-c2 BI 39 0 0 7 73 29x10° 48 9.1 659
B2-cl B2 40 0 0 7 98 2.2x10° 49 89 447
B2-c2 B2 35 1 0 6 78 2.1x108 37 9.9 696
B3-cl E3 36 0 0 6 85 2.6x10° 38 18.7 574
B3-c2 B3 38 0 0 5 83 2.0x10° 35 17.9 618
B4-cl B4 42 1 0 6 90 2.9x10° 50 20.0 511
B4-c2 B4 35 1 0 5 85 2.8x10° 50 IS 6 661
B5-cl B5 41 0 0 5 81 23x10" 48 20.9 498
B5-c2 B5 42 0 0 5 74 23x10' 47 18.1 662
B6-cI B6 41 9 0 6 83 2.3x104 37 29.5 502
B6-c2 B6 40 0 0 5 85 2.7x10° 48 28.4 647
B7-cl B7 39 0 0 6 74 ? 8x10° 42 39.2 501
B7-c2 B7 41 1 0 6 80 2.140' 44 36.8 634
B8-cl B8 38 0 0 5 81 2.6x1& 50 43.8 431
B8-c2 B8 45 0 0 8 86 2.3x10° 49 35.8 646
B9-cl B9 42 0 0 7 76 2.6x10° 37 41.5 504
E9-c2 B9 36 0 0 7 81 28x10' 46 36.0 616
bl-ct bl 4 43 2.5x10° 27 0 5.0xl0c I 8.7 428
b2-cl b2 6 35 2.3x101 21 0 0 0 18.5 570
b3-c1 b3 6 42 31x10' 24 0 0 0 21.2 499
b4-cl b4 3 45 2.8x10' 25 0 5. 0x102 1 29.0 527
b5-cl b5 6 40 2.1x10' 21 0 0 0 39.3 514
b6-cl b6 3 39 2.3x10 20 0 0 0 42.7 445
b7-cl b7 50 1 0 5 92 2.2x10' 72 9.1 488
b8-c1 b8 62 2 0 6 92 2.1x10' 65 29.4 501
b9-cl b9 31 0 0 7 89 2.0x10' 35 57.5 424
b10-cl b10 38 0 I 0 8 82 2.5x10' 27 28.0 499
bli-cl bit 36 0 0 6 84 26x10" 37 S1a 523
00
Fine inclusions Elongated niclusions Inclusions including sulfides Martensite phase
Steel Area Volume
Average Volume
Average
sheet
Steel
number
Number number
equivalent Number number
concentration Ratio Maximum
No.
No.
density
percentage densry
circle percentage
density
of hardness
% diameter % [Ce]+[L,a] - HV
inclusions/min inclusions/mm inclusions/mm
um %
Cl-hl Cl 41 0 0 8 79 2.0x104 42 2.5 550
CS-h2 Cl 46 1 0 8 82 2 4x104 38 2 753
C2=n] C2 37 1 0 5 84 2.3x104 36 29 515
C2-h2 C2 36 1 0 6 81 2.5xl04 39 32 656
C3-h1 C3 43 0 0 6 83 2.64104 42 42 536
C3-h2 C3 46 0 0 5 I 81 2.1x104 39 39 672
C4-hl C4 27 1 0 7 85 22x10a 34 28 492
C4-h2 C4 29 1 0 8 82 22x104 38 26 648
CS-hi CS 41 0 0 6 81 2.3x104 29 6 543
C5-h2 C5 44 0 0 6 80 21 x104 31 5.5 753
C6-hi C6 44 0 0 7 82 2.1x104 45 45 556
C6-h2 C6 45 0 0 8 80 2.3x104 43 43 692
C7-hl C7 29 0 0 6 84 25x104 32 29 512
C7-h2 C7 31 0 0 7 83 2.7x104 35 31 656
C8-hl C8 23 1 0 7 87 17x104 41 8 545
C8-h2 C8 21 1 0 8 86 1.8x104 39 9 682
C9-hl C9 78 1 0 8 82 4.2x104 28 18 584
C9-h2 C9 81 1 0 7 77 3.6x104 26 15 642
C10-hl CIO 102 0 1.0x103 9 78 6.4x104 22 42 578
C10-h2 ClO 113 1 1.0x103 9 82 5.9x104 23 41 725
cl-hl c1 26 2 0 8 82 1.3x104 22 43 556
c2-hl c2 11 1 3.0x103 9 7 3.0x103 5 25 582
e3-hl c3 27 1 1.0x103 7 85 1.9x104 25 26 554
80
With regard to the elongated inclusions in the steel sheets, the presence of coarse
inclusions was confirmed using an optical microscope, and the area number density of
inclusions having an equivalent circle diameter of 2 μm or less with respect to inclusions
having an equivalent circle diameter of 0.5 μm or more was investigated through
5 observation using a SEM. Even for inclusions having an elongation ratio of 5 or more,
the number percentage, the volume number density, and the average equivalent circle
diameter were investigated.
[0227]
Furthermore, with regard to unelongated inclusions in the steel sheet, the
10 number percentage and volume number density of inclusions having MnS precipitated on
oxides or oxysul fides (hard compounds) including at least one of Cc and La with respect
to inclusions having an equivalent circle diameter of I μm or more, and the average value
of the total amount of one or both of Cc and La that are included in the inclusions were
investigated.
15 [0228]
The investigation results of inclusions in the hot-rolled steel sheets are shown in
Tables 7 to 9, and the investigation results of inclusions in the cold-rolled steel sheets are
shown in Tables 13 to 15. Meanwhile, in Tables 7 to 9 and Tables 13 to 15, fine
inclusions refer to inclusions having an equivalent circle diameter of 0.5 μm to 2 μm,
20 elongated inclusions refer to inclusions having an equivalent circle diameter of 1 μm or
more and an elongation ratio of 5 or more, and inclusions including sulfides refer to
inclusions that have MnS-based inclusions precipitated on oxides or oxysulfides
including at least one of Cc and La and have an equivalent circle diameter of I μm or
more.
81
[0229]
Firstly, the test results of manufacturing of hot-rolled steel sheets will be
described with reference to Tables 1 to 9.
[0230]
5 In Steel sheet Nos. b9hl and c3-hl in which Steel Nos. b9 and c3 are used, the
concentration of C exceeds 0.3%. In Steel sheet No. cl-hl in which Steel No. cl is
used, the concentration of Mn exceeds 4.0%. In Steel sheet Nos. a6-hl and b10-hl in
which Steel Nos. a6 and b10 are used, the concentration of the acid-soluble Ti exceeds
0.20%. Asa result, in Steel sheet Nos. b9-hl, c3-hl, cl-hl, a6-hl, and b10-hl,
10 elongation and hole expansion were significantly small.
[0231]
In addition, in Steel sheet No. c2-hl in which Steel No. c2 was used, the
concentration of Si exceeded 2.1%, and ([Cc] + [La]) / [acid-soluble Al] was less than
0.02, and therefore hole expansion were small.
15 [0232]
In Steel sheet Nos. a7-hl and bll-hl in which Steel Nos. a7 and bl I were used,
the concentration of Cr exceeded 2.0%, and therefore elongation was significantly small.
[0233]
In Steel sheet Nos. al-ht to a5-hl and hl-hl to b8-hl in which Steel Nos. at to
20 a5 andbI to b8 were used, ([Cc] + [La]) / [S] was less than 0.4, or exceeded 50,
Therefore, in the steel sheets, the morphologies of inclusions were not sufficiently
controlled, and elongation and hole expansion degraded compared to steel sheets having
the same chemical composition except for Cc and La.
[0234]
25 in Steel sheet Nos. Al-h2 to A6-h2, BI-h2 to B9-h2, and CI-h2 to C10-h2 in
82
which Steel sheet Nos. Al to A6, BIto B9, and CIto CIO were used, the coiling
temperature was lower than 300°C. Therefore, in the above steel sheet Nos., the
difference in hardness between martensite and ferrite lowered, and hole expansion
degraded compared to Steel sheet Nos, Al-h1 toA6-h1, BI-hl to B9-hl, and C1-hl to
5 CIO-hl having the same chemical composition.
[0235]
In Steel sheet Nos. Al-hl to A6-hl, B1-hl to B9-h1, and Cl-hl to C10-hl in
which Steel sheet Nos. Al to A6, BI to B9, and C1 to C 10 were used, the morphologies
of inclusions were sufficiently controlled, and therefore elongation and hole expansion
10 were sufficient.
[0236]
Next, the test results of manufacturing of cold-rolled steel sheets will be
described with reference to Tables 1 to 3 and 10 to 15.
[0237]
15 Similarly to the test results of manufacturing of hot-rolled steel sheets, in Steel
sheet Nos. a6-cl, a7-cl, b9-cl to bl l-cl, cl-cl to c3-cl in which Steel Nos. a6, a7, b9 to
bl1, and cl to c3 were used, elongation or hole expansion were significantly small.
[0238]
In addition, in Steel sheetNos. al-cl to a5-eI and bl-cl to b8-cl in which Steel
20 Nos. at to a5 and bl to b8 were used, ([Ce] + [La]) / [S] was less than 0.4 or exceeded 50.
Therefore, in the steel sheets, the morphologies of inclusions were not sufficiently
controlled, and elongation and hole expansion degraded compared to steel sheets having
the same chemical composition except for Cc and La.
[0239]
25 In Steel sheet Nos. A] -c2 to A6-c2, B1-c2 to B9-c2, and C1-c2 to C10-c2 in
83
which Steel sheet Nos. Al to A6, BIto B9, and CIto C10 were used, the coiling
temperature was lower than 300°C. Therefore, in the above steel sheet Nos., the
difference in hardness between martensite and ferrite lowered, and hole expansion
degraded compared to Steel sheet Nos. Al-cl to A6-cl, Bl-cl to B9-cl, and CI-c1 to
5 C10-c1 having the same chemical composition.
[0240]
In Steel sheet Nos. Al-cl to A6-cl, B1-cl to B9-cl, and Cl-cl to C10-cl in
which Steel sheet Nos. Al to A6, BI to B9, and Cl to C10 were used, the morphologies
of inclusions were sufficiently controlled, and therefore elongation and hole expansion
10 were sufficient.
Industrial Applicability
[0241]
According to the present invention, since it is possible to obtain a high-strength
15 steel sheet that can be preferably mainly pressed and used for underbody parts of
automobiles and the like and structural materials, and is excellent in terms of hole
expansion and ductility, the present invention significantly contributes to steel industry,
and has a large industrial availability.
20
84

What is claimed is:
1. A high-strength steel sheet comprising, by mass%:
C: 0.03% to 0.30%;
5 Si: 0.08%to2.1%;
Mn: 0.5% to 4.0%;
P: 0.05% or less;
S: 0.0001%to 0.1%;
N: 0.01% or less;
10 acid-soluble Al: more than 0.004% and less than or equal to 2.0%;
acid-soluble Ti: 0.0001% to 0.20%;
at least one selected from Cc and La: 0.001% to 0.04% in total; and
a balance of iron and inevitable impurities,
wherein [Ce], [La], [acid-soluble Al], and [S] satisfy 0.02<_ ([Ce] + [La]) /
15 [acid-soluble Al] < 0.25, and 0.4 <_ ([Ce] + [La]) / [S] s 50 in a case in which mass
percentages of Ce, La, acid-soluble Al, and S are defined to be [Ce], [La], [acid-soluble
Al], and [S], respectively, and
a microstructure thereof includes 1% to 50% of martensite in terms of an area
ratio.
20
2. The high-strength steel sheet according to claim 1 further comprising, by mass%, at
least one selected from a group consisting of
Me: 0.001% to 1.0%,
Cr: 0.001% to 2.0%,
25 Ni: 0.00 1% to 2.0%,
85
Cu: 0.001% to 2.0%,
B: 0.0001% to 0.005%
Nb: 0.001%to 0.2%
V: 0.001% to 1.0%,
5 W: 0.001% to 1.0%,
Ca: 0.0001% to 0.01%,
Mg: 0.0001% to 0.01%,
Zr: 0.0001% to 0.2%,
at least one selected from Se and lanthanoids of Pr through Lu: 0.0001% to 0.1%
10 in total,
As: 0.0001 % to 0.5%,
Co: 0.0001% to 1.0%,
Sn: 0.0001% to 0.2%,
Pb: 0.0001% to 0.2%,
15 Y: 0.0001% to 0.2%, and
Hf: 0.0001% to 0.2%.
3. The high-strength steel sheet according to claim 1 or 2,
wherein an amount of the acid-soluble Ti
20 less than 0.008%.
s more than or equal to 0.0001% and
The high-strength steel sheet according to claim I or 2,
wherein an amount of the acid-soluble T s 0.008% to 0.20%.
25 5. The high-strength steel sheet according to claim 1 or 2,
86
wherein [Cc], [La], [acid-soluble Al], and [S] satisfy 0.02 <_ ([Cc] + [La]) /
[acid-soluble Al] < 0.15.
6. The high-strength steel sheet according to claim 1 or 2,
5 wherein [Cc], [La], [acid-soluble Al], and [S] satisfy 0.02 <_ ([Cc] + [La]) /
[acid-soluble Al] < 0.10.
7. The high-strength steel sheet according to claim I or 2,
wherein an amount of the acid-soluble Al is more than 0.01% and less than or
10 equal to 2.0%.
8. The high-strength steel sheet according to claim 1 or 2,
wherein a. number density of inclusions having an equivalent circle diameter of
0.5 μm to 2 lrm in the microstructure is 15 inclusions/mm2 or more.
15
9. The high-strength steel sheet according to claim 1 or 2,
wherein, of inclusions having an equivalent circle diameter of 1.0 μm or more in
the microstructure, a number percentage of elongated inclusions having an aspect ratio of
5 or more obtained by dividing a long diameter by a short diameter is 20% or less.
20
10. The high-strength steel sheet according to claim I or 2,
wherein, of inclusions having an equivalent circle diameter of 1.0 μm or more in
the microstructure, a number percentage of inclusions having at least one of MnS, TiS,
and (Mn, Ti)S precipitated to an oxide or oxysulfide composed of at least one of Cc and
87
La, and at least one of O and S, or an oxide or oxysulfide composed of at least one of Cc
and La, at least one of Si and Ti, and at least one of 0 and S is 10% or more.
11. The high-strength steel sheet according to claim 1 or 2,
5 wherein a volume number density of elongated inclusions having an equivalent
circle diameter of 1 μn1 or more, and an aspect ratio of 5 or more obtained by dividing a
long diameter by a short diameter is 1.0 x 104 inclusions/mm3 or less in the
microstructure.
10 12. The high-strength steel sheet according to claim 1 or 2,
wherein, in the microstructure, a volume number density of inclusions having at
least one of MnS, TiS, and (Mn, Ti)S precipitated in an oxide or oxysulfide composed of
at least one of Cc and La, and at least one of 0 and S, or an oxide or oxysulfide
composed of at least one of Cc and La, at least one of Si and Ti, and at least one of 0 and
15 S is 1.0 x 103 inclusions/mm3 or more.
13. The high-strength steel sheet according to claim 1 or 2,
wherein elongated inclusions having an equivalent circle diameter of 1 μm or
more, and an aspect ratio of 5 or more obtained by dividing a long diameter by a short
20 diameter are present in the microstructure, and an average equivalent circle diameter of
the elongated inclusions is 10 μm or less.
14. The high-strength steel sheet according to claim I or 2,
wherein inclusions having at least one of MnS, TiS, and (Mn, Ti)S precipitated
88
to an oxide or oxysulfide composed of at least one of Ce and La, and at least one of 0
and S, or an oxide or oxysulfide composed of at least one of Cc and La, at least one of Si
and Ti, and at least one of 0 and S are present in the microstructure, and the inclusions
include a total of 0.5 mass% to 95 nlass% of at least one of Cc and La in terms of an
5 average composition.
15. The high-strength steel sheet according to claim 1 or 2,
wherein an average grain size in the microstructure is 10 μm or less.
10 16. The high-strength steel sheet according to claim 1 or 2,
wherein a maximum hardness of martensite included in the microstructure is
600 Hv or less.
17. The high-strength steel sheet according to claim 1 or 2,
15 wherein a sheet thickness thereof is 0.5 mm to 20 mm.
18. The high-strength steel sheet according to claim 1 or 2, further comprising a
galvanized layer or a galvannealed layer on at least one surface.
20 19. A method of manufacturing a high-strength steel sheet, the method comprising:
a first process in which a molten steel having the chemical components
according to claim 1 or 2 is subjected to a continuous casting so as to be processed into a
slab;
a second process in which a hot rolling is carried out on the slab in a finishing
25 temperature of 850°C to 970°C, and a steel sheet is manufactured; and
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a third process in which the steel sheet is cooled to a cooling control temperature
of 650°C or lower at an average cooling rate of 10 to 100 °C/second, and then coiled at a
coiling temperature of 300°C to 650°C.
5 20. The method of manufacturing the high-strength steel sheet according to claim 19,
wherein, in the third process, the cooling control temperature is 450°C or lower,
the coiling temperature is 300°C to 450°C, and a hot-rolled steel sheet is manufactured.
21. The method of manufacturing the high-strength steel sheet according to claim 19,
10 the method further comprising after the third process:
a fourth process in which the steel sheet is pickled, and cold rolling is carried out
on the steel sheet at a reduction in thickness of 40% or more;
a fifth process in which the steel sheet is annealed at a maximum temperature of
750°C to 900°C;
15 a sixth process in which the steel sheet is cooled to 450°C or lower at an average
cooling rate of 0.1 °C/second to 200 °C/second; and
a seventh process in which the steel sheet is held in a temperature range of
300°C to 450°C for 1 second to 1000 seconds so as to manufacture a cold-rolled steel
sheet.
20
22. The method of manufacturing the high-strength steel sheet according to claim 20 or
21,
wherein galvanizing or galvannealed is carried out on at least one surface of the
hot- rolled steel sheet or the cold=rolled steel sheet.
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23. The method of manufacturing the high-strength steel sheet according to claim 19,
wherein the slab is reheated to 1100°C or higher after the first process and
before the second process.