TITLE OF INVENTION
HOT-ROLLED STEEL SHEET AND METHOD FOR PRODUCING THE SAME
TECHNICAL FIELD
[0001]
The present invention relates to a hot-rolled steel sheet and a method for
producing the hot-rolled steel sheet.
BACKGROUND ART
[0002]
In order to compatibly achieve both collision safety and a reduction in weight
of automobiles, the application of high strength steel sheets to automobiles is
progressing. A high strength steel sheet contains a large amount of alloying
elements to increase strength. In particular, a high strength steel sheet having a
tensile strength of 980 MPa or more contains a large amount of Si and Mn.
[0003]
A high strength steel sheet is usually produced by the following method.
First, a slab is hot rolled to produce a hot-rolled steel sheet, which is then coiled in a
coil shape. Next, the hot-rolled steel sheet is pickled, cold-rolled, and annealed.
[0004]
The temperature of the hot-rolled steel sheet when being coiled into a coil
shape (hereunder, referred to as "coiling temperature") may be raised to enhance the
cold workability of the hot-rolled steel sheet. If the coiling temperature is high, an
internal oxidized layer is formed in the vicinity of an outer layer of the hot-rolled
steel sheet. The internal oxidized layer is formed with a thickness of several tens of
pm from the surface of the base metal of the hot-rolled steel sheet toward the center
-;
of the plate thickness. The internal oxidized layer reduces the surface properties,
formability and weldability of the steel sheet after cold rolling (cold-rolled steel
sheet). Therefore, the internal oxidized layer is removed before cold rolling, by
subjecting the hot-rolled steel sheet to a pickling treatment.
2
[0005]
Further, during production of the hot-rolled steel sheet, an oxide film (scale)
is formed on the surface ofthe hot-rolled steel sheet. The scale reduces the surface
properties, formability and weldability of the steel sheet. Therefore, similarly to the
internal oxidized layer, the scale is also removed by subjecting the hot-rolled steel
sheet to a pickling treatment.
[0006]
However, if the internal oxidized layer or the scale is thick, an excessive
workload is applied in the pickling treatment for the hot-rolled steel sheet. In
addition, if the internal oxidized layer or the scale remains, as described above, the
surface properties, formability and weldability of the cold-rolled steel sheet are
reduced. Furthermore, the internal oxidized layer or the scale peel off during
forming of the cold-rolled steel sheet, and cause surface defects such as indentation
defects.
[0007]
The internal oxidized layer is formed as a result of an alloying element in the
base metal being selectively oxidizing. Si and Mn are easily oxidized. Therefore,
an internal oxidized layer is liable to arise in a hot-rolled steel sheet in which the
content of Si and Mn is high. Scale is similarly liable to become thick on a hotrolled
steel sheet having a high Si and Mn content.
[0008]
In addition, as a time period in which the steel sheet has a high temperature
continues, the thicknesses of the internal oxidized layer and scale increase. If the
coiling temperature is raised in order to enhance the cold workability of the hotrolled
steel sheet as described above, an internal oxidized layer is more liable to
arise, and is liable to be thick. This situation similarly applies with respect to scale.
[0009]
Technology for suppressing the formation of the above described internal
oxidized layer and scale is proposed in JP62-13520A (Patent Literature 1), JP2010-
535946A (Patent Literature 2), JP2013-253301A (Patent Literature 3), JP2011-
184741A (Patent Literature 4), JP2011-231391A (Patent Literature 5), JP2012-
036483A (Patent Literature 6), JP2013-216961A (Patent Literature 7), JP2013-
3
103235A (Patent Literature 8), JP2010-503769A (Patent Literature 9), JP2011-
523441A (Patent Literature 10), JP2015-113505A (Patent Literature 11), JP2004-
332099A (Patent Literature 12), JP2013-060657A (Patent Literature 13) and JP2011-
523443A (Patent Literature 14).
[0010]
According to Patent Literature 1, an antioxidant agent is coated onto a steel
sheet surface. It is described in Patent Literature 1 that by this means the formatio!J.
of an internal oxidized layer and scale is suppressed.
[00 11]
According to Patent Literature 2, a hot-rolled steel sheet is coiled at a
comparatively low temperature of 530 to 580°C. It is described in Patent Literature
2 that by this means the formation of an oxidized layer is suppressed.
[0012]
According to Patent Literature 3, a hot-rolled steel sheet after rolling is coiled
at a temperature between 750°C and 600°C to form a coil. After coiling, the coil is
maintained for 10 to 30 minutes, and thereafter the hot-rolled steel sheet is cooled
while dispensing the coil. Subsequently, when the temperature of the hot-rolled
steel sheet reaches 550°C or less, the hot-rolled steel sheet is coiled again to form a
coil. It is described in Patent Literature 3 that in this case the oxidized layer can be
thin.
[0013]
According to Patent Literatures 4 to 6, a heat treatment or a cooling treatment
is performed on a steel sheet after hot rolling or after coiling in an atmosphere in
which the oxygen concentration is reduced. It is described in the aforementioned
Patent Literatures 4 to 6 that the heat treatment or cooling treatment in the
atmosphere in which the oxygen concentration is reduced is effective to reduce scale
and an internal oxidized layer.
[0014]
Acc~rding to Patent Literature 7, descaling is performed prior to coiling on a
hot-rolled steel sheet after hot rolling, to thereby remove oxide scale from the surface
thereof. By removing the oxide scale, an oxygen supply source that is utilized for
formation of an internal oxidized layer during coil cooling decreases. It is
4
described in Patent Literature 7 that, consequently, not only the scale but also the
internal oxidized layer decreases.
[0015]
In Patent Literature 8, a cooling method is proposed for uniformizing an
internal oxidation amount of a hot-rolled steel sheet within a preferable range across
the width direction, in the longitudinal direction thereof.
[0016]
On the other hand, in Patent Literatures 9 to 14, technology that is different
from the technology disclosed in the above described Patent Literatures is proposed.
According to Patent Literature 9, internal oxidation is suppressed by appropriately
controlling the alloy elements of a steel and the conditions for heat treatment of a
hot-rolled steel sheet. Specifically, according to Patent Literature 9, Sb of a content
of 0. 001 to 0.1% is contained in the steel, the steel sheet is reheated to 11 00 to
1250°C and subjected to hot rolling, and coiled at a temperature of 450 to 750°C.
Thereafter, the hot-rolled steel sheet is subjected to pickling and cold rolling, and is
annealed at 700 to 850°C. By this means, formation of an internal oxidized layer is
suppressed.
[0017]
In Patent Literature 10, technology is proposed to appropriately control alloy
elements to suppress formation of an oxide, and thereby improve a plating property.
According to Patent Literature 10, a steel slab is used that contains 0.005 to 0.1% of
Sb, and in which the relation between the contents ofNi, Mn, AI and Ti is adjusted.
The steel slab is subjected to hot working, and hot rolled and coiled at 500 to 700°C.
In addition, the steel slab is subjected to pickling, cold rolling and annealing. By
this means, internal oxidation is suppressed.
[0018]
According to Patent Literature 11, a slab containing 0.02 to 0.10% of Sb is
subjected to hot rolling, pickling, cold rolling, annealing and cooling. In this case,
-; ~.
the finish rolling temperature for the hot rolling is 800 to 1 000°C, and the draft
during the cold rolling is set to 20% or more. In addition, annealing is performed
under conditions of being held for 60 seconds or more in a temperature range of750
to 900°C in an atmosphere in which a dew-point temperature is -35°C or less. After
5
annealing, cooling is performed to 300°C or less at an average cooling rate of
30°C/sec or more, followed by tempering. By this means internal oxidation is
suppressed.
[0019]
Patent Literatures 12 to 14 describe suppressing scale by appropriately
adjusting a content of Si, a heating temperature of a slab, a temperature of finish
rolling and a coiling temperature and the like.
[0020]
However, even when the respective technologies described in Patent
Literatures 1 to 14 are implemented, in some cases a deep internal oxidized layer is
formed or thick scale is formed.
CITATION LIST
PATENT LITERATURE
[0021]
Patent Literature 1: Japanese Patent Application Publication No. 62-13520
Patent Literature 2: National Publication oflnternational Patent Application No.
2010-535946
Patent Literature 3: Japanese Patent Application Publication No. 2013-253301
Patent Literature 4: Japanese Patent Application Publication No. 2011-184741
Patent Literature 5: Japanese Patent Application Publication No. 2011-231391
Patent Literature 6: Japanese Patent Application Publication No. 2012-036483
Patent Literature 7: Japanese Patent Application Publication No. 2013-216961
Patent Literature 8: Japanese Patent Application Publication No. 2013-103235
Patent Literature 9: National Publication oflnternational Patent Application No.
2010-503769
Patent Literature 10: National Publication oflnternational Patent Application No.
2011-523441
·;
Patent Literature 11: Japanese Patent Application Publication No. 2015-113505
Patent Literature 12: Japanese Patent Application Publication No. 2004-332099
Patent Literature 13: Japanese Patent Application Publication No. 2013-060657
6
Patent Literature 14: National Publication oflnternational Patent Application No.
2011-523443
SUMMARY OF INVENTION
TECHNICAL PROBLEM
[0022]
An objective of the present invention is to provide a hot-rolled steel sheet in.
which formation of an internal oxidized layer and scale is suppressed.
SOLUTION TO PROBLEM
[0023]
A hot-rolled steel sheet according to the present embodiment has a chemical
composition consisting of, in mass%, C: 0.07 to 0.30%, Si: more than 1.0 to 2.8%,
Mn: 2.0 to 3.5%, P: 0.030% or less, S: 0.010% or less, AI: 0.01 to less than 1.0%, N:
0.01% or less, 0: 0.01% or less, Sb: 0.03 to 0.30%, Ti: 0 to 0.15%, V: 0 to 0.30%,
Nb: 0 to 0.15%, Cr: 0 to 1.0%, Ni: 0 to 1.0%, Mo: 0 to 1.0%, W: 0 to 1.0%, B: 0 to
0.010%, Cu: 0 to 0.50%, Sn: 0 to 0.30%, Bi: 0 to 0.30%, Se: 0 to 0.30%, Te: 0 to
0.30%, Ge: 0 to 0.30%, As: 0 to 0.30%, Ca: 0 to 0.50%, Mg: 0 to 0.50%, Zr: 0 to
0.50%, Hf: 0 to 0.50% and rare earth metal: 0 to 0.50%, with a balance being Fe and
impurities, and satisfying Formula (1):
Si+Mn 2:: 3.20 (1)
where, a content (mass%) of a corresponding element is substituted for each
symbol of an element in Formula (1 ).
ADVANTAGEOUS EFFECTS OF INVENTION
[0024]
In the hot-rolled steel sheet of the present embodiment, formation of an
internal oxidized layer and scale is suppressed.
BRIEF DESCRIPTION OF DRAWINGS
[0025]
7
[FIG. 1] FIG. 1 is a list view showing SEM images of cross-sections subjected to
nital etching, oxygen mapping images obtained using EPMA at the SEM image
regions, and Sb mapping images with respect to a steel with a high Si and high Mn
content that does not contain Sb (steel not containing Sb), and a steel containing Sb
that is a steel with a high Si and high Mn content that contains 0.1% of Sb.
[FIG. 2] FIG. 2 is a view illustrating the relation between an Sb content (x 1 o-3%) and
the thickness (J..tm) of an internal oxidized layer for a case where the amount of Sb
contained in a steel with a high Si and high Mn content was varied and hot-rolled
steel sheets were produced.
[FIG. 3] FIG. 3 shows SEM images ofthe vicinity of an outer layer in the above
described steel not containing Sb and steel containing Sb.
DESCRIPTION OF EMBODIMENTS
[0026]
The present inventors conducted investigations and studies regarding an
internal oxidized layer and scale with respect to a steel with a high Si and high Mn
content, and obtained the following findings.
[0027]
In an outer layer of a hot-rolled steel sheet after coiling, an internal oxidized
layer is formed within the base metal (ground metal), and scale is formed adjoining
the surface.
[0028]
It is considered that the internal oxidized layer and scale are formed by the
following mechanism. Oxygen ions penetrate int? the hot-rolled steel sheet through
grain boundaries of a near-surface portion of the hot-rolled steel sheet and the surface
of the hot-rolled steel sheet. An internal oxidized layer is formed as a result of the
oxygen ions that penetrated into the inside of the hot-rolled steel sheet oxidizing iron
of the base metal. On the other hand, iron ions in the base metal move to the
surface of the hot-rolled steel sheet through the grain boundaries. Scale is formed
as a result of the Fe that moved to the surface being oxidized.
[0029]
8
It is effective to block the movement path (grain boundaries and surface) of
the oxygen ions and iron ions in order to suppress formation of an internal oxidized
layer and scale. If elements that easily segregate at the grain boundaries and surface
(hereunder, referred to as "segregation elements") are contained in a hot-rolled steel
sheet, the segregation elements segregate at the surface and grain boundaries of the
hot-rolled steel sheet and suppress movement of oxygen ions and iron ions.
Therefore, penetration of oxygen ions into the inside of the hot-rolled steel sheet caq
be suppressed. In addition, movement of iron ions to the hot-rolled steel sheet
surface can be suppressed. As a result, formation of an internal oxidized layer and
scale can be suppressed.
[0030]
The segregation elements are, for example, P, B, and Sb. However, although
P and B segregate at grain boundaries and block the movement path of oxygen ions
and iron ions, they also reduce the mechanical properties of the hot-rolled steel sheet.
[0031]
On the other hand, Sb segregates at the surface of the hot-rolled steel sheet.
Therefore, the present inventors produced a hot-rolled steel sheet from steel with a
high Si and high Mn content, which also contained Sb, and examined the thickness
of scale and an internal oxidized layer.
[0032]
FIG. 1 shows, with respect to a conventional steel with a high Si and high Mn
content that does not contain Sb (hereunder, referred to as "steel not containing Sb")
and a steel containing Sb that is a conventional with a high Si and high Mn content,
which also contains 0.10% ofSb, SEM images at a cross-section in the vicinity ofthe
surface, oxygen mapping images obtained using EPMA at the SEM image regions,
and Sb mapping images. The steel not containing Sb contained, in mass%, C:
0.185%, Si: 1.8%, Mn: 2.6%, P: 0.01 %, S: 0.002%, Al: less than 0.03%, N: 0.003%,
0: 0.0009%, and Ti: 0.005%, with the balance being Fe and impurities. The steel
·~
containing Sb was steel for which 0.10% of Sb was added to the chemical
composition of the steel not containing Sb. For each of these steels, a hot-rolled
steel sheet was formed by hot rolling in a similar manner to the conventional method.
9
The aforementioned microstructure observation and EPMA mapping was performed
with respect to the prepared hot-rolled steel sheets.
[0033]
Referring now to the SEM images in FIG. 1, in the steel not containing Sb,
scale 1 0 was formed on the steel sheet surface, and an internal oxidized layer 20 was
formed in the base metal. On the other hand, in the steel containing Sb, although
the scale 10 was formed, the thickness thereof was thinner than the scale 10 of the
steel not containing Sb. Further, in the steel containing Sb, the internal oxidized
layer 20 was not observed. As a result of performing oxygen mapping using
EPMA, in the steel not containing Sb, oxygen was observed in the scale 10 and the
internal oxidized layer 20 (white region and gray region in the drawing). In
contrast, in the steel containing Sb, oxygen was observed in only the region in which
the scale 10 was formed (white region in the drawing).
[0034]
In addition, Sb mapping was performed using EPMA. As a result, in the
steel containing Sb, a layer 30 containing Sb (white region in the drawing; hereunder
referred to as "Sb concentrated layer") was observed at the interface between the
scale 10 and the base metal.
[0035]
As described above, in a case where Sb is contained in steel with a high Si
and high Mn content, an Sb concentrated layer is formed. It is considered that,
because of the formation of the Sb concentrated layer, the situation is as follows. In
a case where steel with a high Si and high Mn content contains a suitable amount of
Sb, in a hot rolling process, an Sb concentrated layer is formed at the interface
(surface of the hot-rolled steel sheet) between scale and the base metaL The Sb
concentrated layer blocks the penetration of oxygen ions into the base metal.
Consequently, iron in the base metal is not oxidized, and it is difficult for an internal
oxidized layer to form. The Sb concentrated layer also suppresses movement of
·:
iron ions contained in the base metal to the scale. Consequently, growth of the
scale is suppressed, and the thickness of the scale is thin.
[0036]
10
Thus, the Sb concentrated layer functions as a so-called "barrier layer" that
blocks the movement of oxygen ions and iron ions. Therefore, byformation of the
Sb concentrated layer, the penetration of oxygen ions into the base metal from the
scale after coiling of the hot-rolled steel sheet can be suppressed. In addition,
movement of iron ions to the scale from the base metal can be suppressed.
Therefore, the formation of an internal oxidized layer and scale is suppressed.
[0037]
A barrier layer like the Sb concentrated layer is not formed even if P and B
that are elements segregable at grain boundaries are contained in steel with a high Si
and high Mn content. Accordingly, Sb is suitable for suppressing scale and an
internal oxidized layer.
[0038]
FIG. 2 is a view illustrating the relation between the Sb content (x 1 o-3%) and
the thickness (J..Lm) of an internal oxidized layer in a case where the Sb amount
contained in a steel with a high Si and high Mn content is varied and hot-rolled steel
sheets are produced (coiling temperature of750°C). Referring to FIG. 2, it is found
that the thickness of the internal oxidized layer decreases noticeably as the Sb
content increases. Further, in a case where the Sb content is 0.03% or more,
although the thickness of the internal oxidized layer decreases as the Sb content
increases, the margin of the decrease is not as great as when the Sb content is less
than 0.03%. That is, in the relation between the thickness of the internal oxidized
layer and the Sb content, an inflection point exists in the vicinity of an Sb content
equal to 0.03%.
[0039]
In the present embodiment, the Sb concentrated layer also suppresses
movement of carbon contained in the base metal, and not just movement of oxygen
ions and iron ions. Consequently, it is easy to maintain a uniform micro-structure
in the plate thickness direction, and the obtainment of strength in a cold-rolled steel
sheet after ~old rolling and annealing is facilitated.
[0040]
FIG. 3 shows SEM images of the vicinity of an outer layer in the above
described steel not containing Sb and steel containing Sb. Referring to FIG. 3, it is
11
found that in the steel not containing Sb, a decarburization layer 40 is formed in the
outer layer. On the other hand, in the steel containing Sb in which the Sb
concentrated layer is formed at the interface between the base metal and the scale, a
decarburization layer is not formed. Accordingly, the Sb concentrated layer can
also suppress the movement of carbon in the base metal, and not just suppress the
movement of oxygen ions and iron ions.
[0041]
A hot-rolled steel sheet according to the present embodiment that was
completed based on the above described findings has a chemical composition
consisting of, in mass%, C: 0.07 to 0.30%, Si: more than 1.0 to 2.8%, Mn: 2.0 to
3.5%, P: 0.030% or less, S: 0.010% or less, AI: 0.01 to less than 1.0%, N: 0.01% or
less, 0: 0.01% or less, Sb: 0.03 to 0.30%, Ti: 0 to 0.15%, V: 0 to 0.30%, Nb: 0 to
0.15%, Cr: 0 to 1.0%, Ni: 0 to 1.0%, Mo: 0 to 1.0%, W: 0 to 1.0%, B: 0 to 0.010%,
Cu: 0 to 0.50%, Sn: 0 to 0.30%, Bi: 0 to 0.30%, Se: 0 to 0.30%, Te: 0 to 0.30%, Ge:
0 to 0.30%, As: 0 to 0.30%, Ca: 0 to 0.50%, Mg: 0 to 0.50%, Zr: 0 to 0.50%, Hf: 0 to
0.50% and rare earth metal: 0 to 0.50%, with a balance being Fe and impurities, and
satisfying Formula (1):
Si+Mn:?: 3.20 (1)
where, a content (mass%) of a corresponding element is substituted for each
symbol of an element in Formula (1 ).
[0042]
The above described chemical composition may also contain one or more
types of element selected from the group consisting ofTi: 0.005 to 0.15%, V: 0.001
to 0.30% and Nb: 0.005 to 0.15%.
[0043]
The above described chemical composition may also contain one or more
types of element selected from the group consisting of Cr: 0.10 to 1.0%, Ni: 0.10 to
1.0%, Mo: 0.01 to 1.0%, W: 0.01 to 1.0% and B: 0.0001 to 0.010%.
[0044]
The above described chemical composition may also contain Cu: 0.10 to
0.50%.
[0045]
12
The above described chemical composition may also contain one or more
types of element selected from the group consisting of Sn, Bi, Se, Te, Ge and As in a
total amount ofO.OOOI to 0.30%.
[0046]
The above described chemical composition may also contain one or more
types of element selected from the group consisting of Ca, Mg, Zr, Hf and rare earth
metal in a total amount ofO.OOOI to 0.50%.
[0047]
A hot-rolled steel sheet according to the present embodiment includes an Sb
concentrated layer having a thickness of 0.5 J..Lm or more between the surface and
scale.
[0048]
In the micro-structure of the above described hot-rolled steel sheet, a total
area fraction of ferrite and pearlite may be 75% or more, and the tensile strength of
the hot-rolled steel sheet may be 800 MPa or less.
[0049]
In the micro-structure of the above described hot-rolled steel sheet, a total
area fraction of bainite and martensite may be 75% or more, and the tensile strength
of the hot-rolled steel sheet may be 900 MPa or more.
[0050]
In the micro-structure of the above described hot-rolled steel sheet, a total
area fraction of bainite and martensite may be 75% or more, and the tensile strength
of the hot-rolled steel sheet may be 800 MPa or less.
[0051]
Preferably, the thickness of an internal oxidized layer of the hot-rolled steel
sheet is 5 J..Lm or less.
[0052]
Preferably, in the above described hot-rolled steel sheet having a microstructure
in ~hich a total area fraction of ferrite and pearlite is 75% or more and
which has a tensile strength of 800 MPa or less, a scale thickness is 10 J..Lm or less.
[0053]
13
Preferably, in the above described hot-rolled steel sheet having a microstructure
in which a total area fraction of ferrite and pearlite is 75%or more and
which has a tensile strength of 800 MPa or less, a decarburization layer thickness in
an outer layer of the hot-rolled steel sheet is 20 ).lm or less.
[0054]
Preferably, in the above described hot-rolled steel sheet having a microstructure
in which a total area fraction of bainite and martensite is 75% or more and
which has a tensile strength of 900 MPa or more, a scale thickness is 7 ).lm or less.
[0055]
Preferably, in the above described hot-rolled steel sheet having a microstructure
in which a total area fraction of bainite and martensite is 75% or more and
which has a tensile strength of 800 MPa or less, a scale thickness is 7 J.lffi or less.
[0056]
A production method for producing the above described hot-rolled steel sheet
having a micro-structure in which a total area fraction of ferrite and pearlite is 75%
or more and which has a tensile strength of 800 MPa or less includes: a process of
preparing a steel material having the above described chemical composition; a
process of heating the steel material to 1100 to 1350°C and thereafter performing hot
rolling so as to form the steel material into a steel sheet; and a process of coiling the
steel sheet at a temperature of 600 to 750°C, preferably 650 to 750°C, and more
preferably 700 to 750°C.
[0057]
A production method for producing the above described hot-rolled steel sheet
having a micro-structure in which a total area fraction of bainite and martensite is
75% or more and which has a tensile strength of 900 MPa or more includes: a
preparation process of preparing a steel material having the above described
chemical composition; a hot rolling process of heating the steel material to 1100 to
1350°C, thereafter performing hot rolling so as to form the steel material into a steel
sheet, and c~oling the steel sheet to a coiling temperature; and a process of coiling
the steel sheet after cooling, at a temperature of 150 to 600°C, preferably 350 to
500°C, and more preferably 400 to 500°C.
[0058]
14
A production method for producing the above described hot-rolled steel sheet
having a micro-structure in which a total area fraction of bainite and martensite is
75% or more and which has a tensile strength of 800 MPa or less includes: a
preparation process of preparing a steel material having the above described
chemical composition; a hot rolling process of heating the steel material to 1100 to
1350°C, thereafter performing hot rolling so as to form the steel material into a steel
sheet, and cooling the steel sheet to a coiling temperature; a process of coiling the .
steel sheet after cooling, at a temperature of 150 to 600°C, preferably 350 to 500°C,
and more preferably 400 to 500°C; and a process of tempering the steel sheet after
coiling at a temperature of 550°C or more.
[0059]
Hereunder, hot-rolled steel sheets according to the present embodiments are
described in detail.
[0060]
[First Embodiment]
[Chemical Composition]
The chemical composition of the hot-rolled steel sheet according to the
present embodiment contains the following elements. The symbol"%" with respect
to the chemical composition means "percent by mass" unless specified otherwise.
[0061]
C: 0.07 to 0.30%
Carbon (C) forms retained austenite in the hot-rolled steel sheet and enhances
the strength and formability of the steel. Ifthe C content is too low, the
aforementioned effect will not be obtained. On the other hand, if the C content is
too high, the strength of the hot-rolled steel sheet will be too high and a cold rolling
property will be reduced. If the C content is too high, the weldability of the steel
will also decrease. Therefore, the C content is from 0.07 to 0.30%. A preferable
lower limit of the C content is 0.10%, more preferably is 0.12%, and further
·; ~.
preferably is 0.15%. A preferable upper limit of the C content is 0.25%, and more
preferably is 0.22%.
[0062]
Si: more than 1.0 to 2.8%
15
Silicon (Si) suppresses the formation of iron-based carbides and facilitates
formation of retained austenite. The strength and formability of the steel is
improved by formation of retained austenite. If the Si content is too low, the
aforementioned effect will not be obtained. On the other hand, if the Si content is
too high, an internal oxidized layer will grow noticeably and the surface properties of
the hot-rolled steel sheet will decrease. If the Si content is too high, the hot-rolled
steel sheet will also become brittle and the ductility will decrease. Therefore, the ~i
content is from more than 1.0 to 2.8%. A preferable lower limit of the Si content is
1.3%, and more preferably is 1.5%. A preferable upper limit of the Si content is
2.5%, and more preferably is 2.0%.
[0063]
Mn: 2.0 to 3.5%
Manganese (Mn) increases the strength of the steel sheet. If the Mn content
is too low, a large amount of soft micro-structure is formed during cooling after
annealing, and the strength is lowered. On the other hand, if the Mn content is too
high, coarse Mn concentrated parts form at a central portion of the plate thickness
and the steel becomes brittle. Consequently, a slab that is cast is liable to crack. If
the Mn content is too high, the weldability of the steel also decreases. If the Mn
content is too high, the hot-rolled steel sheet will also harden and a cold rolling
property will decrease. Therefore, the Mn content is from 2.0 to 3.5%. A
preferable lower limit of the Mn content is 2.2%, more preferably is 2.3%, and
further preferably is 2.5%. A preferable upper limit of the Mn content is 3.2%, and
more preferably is 3.0%.
[0064]
P: 0.030% or less
Phosphorus (P) segregates at a central portion of the plate thickness of the
steel sheet and embrittles a weld zone. Therefore, the P content is 0.030% or less.
A low P content is preferable. However, making the P content low increases the
·)
production costs. Therefore, when taking the production cost into consideration,
the lower limit of the P content is, for example, 0.0010%.
[0065]
S: 0.010% or less
16
Sulfur (S) reduces the weldability of the steel. S also reduces the
producibility during casting and heat rolling. S also combines with Mn to form
MnS, and reduces the ductility and stretch flangeability of the steel. Therefore, the
content ofS is 0.010% or less. A preferable upper limit ofthe S content is 0.005%,
and more preferably is 0.0025%. A lower limit of the S content is not particularly
limited. However, when taking the production cost into consideration, the lower
limit of the S content is, for example, 0.0001%.
[0066]
AI: 0.01 to less than 1.0%
Aluminum (AI) suppresses formation of iron-based carbides and facilitates
formation of retained austenite. The strength and formability of the steel is
enhanced by the formation of retained austenite. AI also deoxidizes the steel. If
the AI content is too low, the aforementioned effects are not obtained. On the other
hand, if the AI content is too high, the weldability of the steel decreases. Therefore,
the AI content is from 0.01 to less than 1.0%. A preferable lower limit of the AI
content is 0.02%. A preferable upper limit of the AI content is 0.8%, and more
preferably is 0.5%. In the present description, the "AI" content means the content
of "sol. AI" (acid-soluble AI).
[0067]
N: 0.01% or less
Nitrogen (N) forms coarse nitrides and reduces the ductility and stretch
flangeability of the steel. N is also a cause of the occurrence of blowholes during
welding. Therefore, a low N content is preferable. TheN content is 0.01% or
less. A preferable upper limit of theN content is 0.005%. The lower limit of the
N content is not particularly limited. However, when taking the production cost
into consideration, the lower limit of theN content is, for example, 0.0001%.
[0068]
0:0.01% or less
Oxygen (0) forms oxides and reduces the toughness and stretch flangeability
ofthe steel. Therefore, a low 0 content is preferable. The 0 content is 0.01% or
less. A preferable upper limit of the 0 content is 0.008%, and more preferably is
0.006%. The lower limit of the 0 content is not particularly limited. However,
17
when taking the production cost into consideration, a preferable lower limit of the 0
content is, for example, 0.0001%.
[0069]
Sb: 0.03 to 0.30%
Antimony (Sb) is, as described above, an element that easily segregates at the
surface of the steel. Sb forms an Sb concentrated layer in the surface (interface
between scale and base metal) of the hot-rolled steel sheet during hot rolling. The.
Sb concentrated layer suppresses penetration of oxygen ions into the inside of the
hot-rolled steel sheet from grain boundaries that are exposed on the surface of the
hot-rolled steel sheet. The Sb concentrated layer also suppresses movement of iron
ions contained in the base metal to scale. Therefore, formation of an internal
oxidized layer in the hot-rolled steel sheet and the growth of scale are suppressed.
Sb also restricts movement of C to suppress formation of a decarburization layer.
[0070]
If the Sb content is too low, it is difficult to form an Sb concentrated layer and
the above described effects are not obtained. On the other hand, if the Sb content is
too high, the workability of the steel sheet decreases. Further, if the Sb content is
too high, the mechanical properties of the hot-rolled steel sheet also decrease.
Therefore, the Sb content is from 0.03 to 0.30%. A preferable lower limit of the Sb
content is 0.05%, more preferably is 0.07%, further preferably is 0.1 0%, and further
preferably is 0.11 %. A preferable upper limit of the Sb content is 0.25%, and more
preferably is 0.20%.
[0071]
The chemical composition of the above described hot-rolled steel sheet
further satisfies Formula (1):
Si+Mn 2 3.20 (1)
where, a content (mass%) of a corresponding element is substituted for each
symbol of an element in Formula (1 ).
[0072]
If the total content of Si and Mn is less than 3 .20%, retained austenite will not
be stable during annealing performed after cold rolling. In this case, there is a
possibility that the strength or ductility of the steel sheet after annealing will be low.
18
Therefore, the lower limit ofthe total content ofSi and Mn is 3.20%. In this case,
the strength and ductility of the steel sheet will be high even after cold rolling and
annealing. The lower limit of the total content of Si and Mn is preferably 3.50%.
On the other hand, if the total content of Si and Mn is 5.0% or less, a phase
transformation delay during annealing can be suppressed. Therefore, carbon (C)
sufficiently concentrates in untransformed austenite, and the retained austenite is
more stable. Accordingly, an upper limit of the total content ofSi and Mn is
preferably 5.0%, and more preferably is 4.5%.
[0073]
The balance of the chemical composition of the hot-rolled steel sheet ofthe
present embodiment is Fe and impurities. Here, the term "impurities" refers to
elements that, during industrial production of the hot-rolled steel sheet, are mixed in
from ore or scrap used as a raw material, or from the production environment or the
like, and that are allowed within a range that does not adversely affect the hot-rolled
steel sheet according to the present embodiment.
[0074]
[Optional Elements]
The chemical composition of the hot-rolled steel sheet that is described above
may contain optional elements that are described hereunder in addition to the above
described essential elements. The chemical composition need not contain optional
elements.
[0075]
The above described chemical composition may contain one or more types of
element selected from the group consisting of Ti, V and Nb as a substitute for a part
of Fe. Each ofTi, V and Nb is an optional element, and each element increases the
strength of the steel.
[0076]
Ti: 0 to 0.15%
·;
Titanium (Ti) is an optional element, and need not be contained in the steel.
In a case where Ti is contained, Ti forms carbo-nitrides and increases the strength of
the steel. Ti also contributes to fine-grain strengthening of the steel by suppressing
growth of ferrite grains. Ti also contributes to dislocation strengthening ofthe steel
19
through suppression of recrystallization. However, if the Ti content is too high,
carbo-nitrides are excessively formed and the formability of the steel decreases.
Therefore, the Ti content is 0 to 0.15%. A preferable upper limit of the Ti content
is 0.10%, and more preferably is 0.07%. A preferable lower limit ofthe Ti content
is 0.005%, more preferably is 0.010%, and further preferably is 0.015%.
[0077]
V: Oto 0.30%
Vanadium (V) is an optional element, and need not be contained in the steel.
In a case where V is contained, similarly to Ti, V increases the strength of the steel
by precipitation strengthening, fine-grain strengthening and dislocation
strengthening. However, if the V content is too high, carbo-nitrides precipitate
excessively and the formability ofthe steel decreases. Therefore, the V content is
from 0 to 0.30%. A preferable upper limit of the V content is 0.20%, and more
preferably is 0.15%. A preferable lower limit ofthe V content is 0.001%, and more
preferably is 0.005%.
[0078]
Nb: 0 to 0.15%
Niobium (Nb) is an optional element, and need not be contained in the steel.
In a case where Nb is contained, similarly to Ti and V, Nb increases the strength of
the steel by precipitation strengthening, fine-grain strengthening and dislocation
strengthening. However, if the Nb content is too high, carbo-nitrides precipitate
excessively and the formability of the steel decreases. Therefore, the Nb content is
from 0 to 0.15%. A preferable upper limit of the Nb content is 0.10%, and more
preferably is 0.06%. A preferable lower limit of the Nb content is 0.005%, more
preferably is 0.010%, and further preferably is 0.015%.
[0079]
The above described chemical composition may contain one or more types of
element selected from the group consisting of Cr, Ni, Mo, W and B as a substitute for
·,'
a part of Fe. Each ofCr, Ni, Mo, Wand B is an optional element, and each element
increases the strength of the steel.
[0080]
Cr: Oto 1.0%
20
Chromium (Cr) is an optional element, and need not be contained in the steel.
In a case where Cr is contained, Cr suppresses phase transformation at a high
temperature and increases the strength ofthe steel. However, if the Cr content is
too high, the workability of the steel decreases and productivity is reduced.
Therefore, the Cr content is from 0 to 1.0%. A preferable lower limit of the Cr
content is 0.1 0%.
[0081]
Ni: 0 to 1.0%
Nickel (Ni) is an optional element, and need not be contained in the steel. In
a case where Ni is contained, Ni suppresses phase transformation at a high
temperature and increases the strength of the steel. However, ifthe Ni content is
too high, the weldability of the steel decreases. Therefore, the Ni content is from 0
to 1.0%. A preferable lower limit ofthe Ni content is 0.10%.
[0082]
Mo: 0 to 1.0%
Molybdenum (Mo) is an optional element, and need not be contained in the
steel. In a case where Mo is contained, Mo suppresses phase transformation at a
high temperature and increases the strength of the steel. However, ifthe Mo
content is too high, the hot workability of the steel decreases and productivity is
reduced. Therefore, the Mo content is from 0 to 1.0%. A preferable lower limit of
the Mo content is 0.01 %.
[0083]
W: Oto 1.0%
Tungsten (W) is an optional element, and need not be contained in the steel.
In a case where W is contained, W suppresses phase transformation at a high
temperature and increases the strength ofthe steel. However, if theW content is
too high, the hot workability of the steel decreases and productivity is reduced.
Therefore, theW content is from 0 to 1.0%. A preferable lower limit ofthe W
·;
content is 0.01%.
[0084]
B: 0 to 0.010%
21
.;_,
Boron (B) is an optional element, and need not be contained in the steel. In
a case where B is contained, B suppresses phase transformation at a: high temperature
and increases the strength of the steel. However, ifthe B content is too high, the
hot workability of the steel decreases and productivity is reduced. Therefore, the B
content is from 0 to 0.010%. A preferable upper limit ofthe B content is 0.005%,
and more preferably is 0.003%. A preferable lower limit of the B content is
0.0001%, more preferably is 0.0003%, and further preferably is 0.0005%.
[0085]
The above described chemical composition may contain Cu as a substitute for
a part ofF e.
[0086]
Cu: 0 to 0.50%
Copper (Cu) is an optional element, and need not be contained in the steel.
In a case where Cu is contained, Cu precipitates in the steel as fine particles and
increases the strength of the steel. However, the weldability ofthe steel will
decrease if the Cu content is too high. Therefore, the Cu content is from 0 to
0.50%. A preferable lower limit of the Cu content is 0.10%.
[0087]
The above described chemical composition may contain one or more types of
element selected from the group consisting of Sn, Bi, Se, Te, Ge and As as a
substitute for a part ofF e. Each of Sn, Bi, Se, Te, Ge and As is an optional element,
and each element suppresses formation of an internal oxidized layer.
[0088]
Sn: 0 to 0.30%
Bi: 0 to 0.30%
Se: 0 to 0.30%
Te: 0 to 0.30%
Ge: 0 to 0.30%
·,'
As: 0 to 0.30%
Tin (Sn), bismuth (Bi), selenium (Se), tellurium (Te), germanium (Ge) and
arsenic (As) are optional elements, and need not be contained in the steel. If
contained, these elements suppress formation of an internal oxidized layer by
22
suppressing segregation ofMn and Si. However, ifthe content of these elements is
too high, the formability ofthe steel decreases. Therefore, the Sn content is from 0
to 0.30%, the Bi content is from 0 to 0.30%, theSe content is from 0 to 0.30%, the
Te content is from 0 to 0.30%, the Ge content is from 0 to 0.30% and the As content
is from 0 to 0.30%. A preferable upper limit of the Sn content is 0.25%, and more
preferably is 0.20%. A preferable upper limit of the Bi content is 0.25%, and more
preferably is 0.20%. A preferable upper limit of theSe content is 0.25%, and mor~
preferably is 0.20%. A preferable upper limit of the Te content is 0.25%, and more
preferably is 0.20%. A preferable upper limit of the Ge content is 0.25%, and more
preferably is 0.20%. A preferable upper limit of the As content is 0.25%, and more
preferably is 0.20%. A preferable lower limit of the Sn content is 0.0001%. A
preferable lower limit of the Bi content is 0.0001%. A preferable lower limit of the
Se content is 0.0001%. A preferable lower limit of the Te content is 0.0001%. A
preferable lower limit of the Ge content is 0.0001%. A preferable lower limit of the
As content is 0.0001%. Note that, when two or more types of element selected
from the group consisting of Sn, Bi, Se, Te, Ge and As are to be contained in the
steel, it is preferable to make the total content thereof from 0.0001 to 0.30%.
[0089]
The above described chemical composition may contain one or more types of
element selected from the group consisting of Ca, Mg, Zr, Hf and rare earth metal
(REM) as a substitute for a part ofF e. Each of these elements is an optional
element, and each element increases the formability of the steel.
[0090]
Ca: 0 to 0.50%
Mg: 0 to 0.50%
Zr: 0 to 0.50%
Hf: 0 to 0.50%
Rare earth metal (REM): 0 to 0.50%
Calcium (Ca), magnesium (Mg), zirconium (Zr), hafnium (Hf) and rare earth
metal (REM) are each an optional element and need not be contained in the steel. If
contained, these elements enhance the formability of the steel. However, if the
content of these elements is too high, the ductility of the steel decreases. Therefore,
23
the Ca content is from 0 to 0.50%, the Mg content is from 0 to 0.50%, the Zr content
is from 0 to 0.50%, the Hf content is from 0 to 0.50%, and the rare earth metal
(REM) content is from 0 to 0.50%. A preferable lower limit of theCa content is
0.0001%, more preferably is 0.0005%, and further preferably is 0.001%. A
preferable lower limit of the Mg content is 0.0001%, more preferably is 0.0005%,
and further preferably is 0.001%. A preferable lower limit of the Zr content is
0.0001%, more preferably is 0.0005%, and further preferably is 0.001%. A
preferable lower limit of the Hf content is 0.0001%, more preferably is 0.0005%, and
further preferably is 0.001%. A preferable lower limit of the rare earth metal
(REM) content is 0.0001%, more preferably is 0.0005%, and further preferably is
0.001%. Note that, when two or more types of element selected from the group
consisting of Ca, Mg, Zr, Hf and rare earth metal (REM) are to be contained in the
steel, it is preferable to make the total content thereof from 0.0001 to 0.50%.
[0091]
The term "REM" as used in the present description refers to one or more types
of element selected from Sc, Y, and lanthanoids (elements with atomic numbers 57
through 71 from La to Lu). The term "REM content" refers to the total content of
these elements.
[0092]
[Micro-structure]
The micro-structure of the hot-rolled steel sheet of the present embodiment is
not particularly limited. The micro-structure of the hot-rolled steel sheet of the
present embodiment, for example, mainly consists of ferrite and pearlite.
Specifically, in the micro-structure, the combined area fraction of ferrite and pearlite
is 75% or more. In the micro-structure, a region (balance) other than ferrite and
pearlite is one or more types of micro-structure selected from the group consisting of
bainite (including tempered bainite), martensite (including tempered martensite) and
retained austenite.
[0093]
If the total area fraction of ferrite and pearlite in the micro-structure is 75% or
more, the strength of the hot-rolled steel sheet can be suppressed. In this case, the
cold workability is enhanced.
24
[0094]
The area fraction of the respective phases can be determined by the following
methods.
[0095]
[Area fraction of ferrite and pearlite]
The hot-rolled steel sheet is cut along a plane perpendicular to the rolling
direction. The cut surface is mirror polished. Of the entire area of the cut surface.
that was mirror polished, a width-wise central portion of the hot-rolled steel sheet
(range of ±1 0 mm in the width direction from the center in the width direction) that
is a range of±5 mm from a position that is equal to 114 of the plate thickness from
the surface is defined as an observation region. The observation region is corroded
with a nital etching reagent. After corrosion, an arbitrary range of 200 !-liD x 150
!-liD of the observation region is photographed using a scanning electron microscope
(SEM). Ferrite and pearlite are identified using the image of the region that was
photographed (hereunder, referred to as "photographing region"). The total of the
areas of ferrite and pearlite that are identified is determined, and the determined total
area is divided by the sum total of the areas of the entire photographing region to
obtain the total area fraction (%) of ferrite and pearlite. The areas of ferrite and
pearlite are measured using a mesh method or image processing software (product
name: Image Pro).
[0096]
[Area fraction of bainite and martensite]
A method for measuring the area fraction of bainite and martensite is as
follows. A photographing region (200 1-lffi x 150 !-liD) that is the same as in the
above described method for measuring an area fraction of ferrite and pearlite is
photographed using an electron backscattering diffraction method (EBSD method) to
generate a photographic image.
[0097]
A portion excluding pearlite and retained austenite is extracted from the
photographic image by image processing. With respect to a low temperature
transformation phase ofthe remaining region, 15 degrees is defined as a threshold
value of an orientation difference with adjacent grains, and grains are identified.
25
With respect to each identified grain, the average visibility of the Kikuchi Diffraction
pattern in the grains (Grain Average Image Quality: GAIQ) is converted into
numerical values. A histogram of area fractions is created with respect to the GAIQ
values that were converted into numerical values. In a case where the created
histogram has two peaks, a distribution on a side on which the GAIQ is high is taken
as originating from bainite, and a distribution on a side on which the GAIQ is low is
taken as originating from martensite. The total area fraction of grains having a
GAIQ identified as originating from bainite is defined as a bainite area fraction. In
a case where two peaks are overlapping in the histogram, grains that are identified by
the GAIQ up to a boundary at which the distributions overlap are defined as bainite,
and the area fraction of the bainite is detennined.
[0098]
A value(%) obtained by deducting the sum total(%) of the aforementioned
ferrite area fraction, pearlite area fraction and bainite area fraction as well as a
retained austenite area fraction that is described later from 1 00 (%) is defined as the
area fraction of martensite.
[0099]
[Area fraction of retained austenite]
The area fraction of retained austenite is determined by X-ray diffractometry.
Specifically, in a photographing region (200 )lm x 150 )lm) that is the same as in the
above described method for measuring an area fraction of ferrite and pearlite, the
proportion of retained austenite is detennined experimentally by X-ray
diffractometry using the property that the reflection surface intensity differs between
austenite and ferrite. A retained austenite area fraction Vy is determined using the
following formula based on an image obtained by X-ray diffractometry using the Ka.
rayofMo:
Vy = (2/3){ 1 00/(0.7xa.(211)/y(220)+ 1)} + (1/3){ 1 00/(0.78xa.(211)/y(311)+ 1)}
where, a.(211) represents the reflection surface intensity at a (211) surface of
·; ~·.
ferrite, y(220) represents the reflection surface intensity at a (220) surface of
austenite, and y(311) represents the reflection surface intensity at a (311) surface of
austenite.
[0100]
26
>
[Tensile Strength]
A preferable tensile strength of the hot-rolled steel sheet of the present
embodiment is 800 MPa or less, and more preferably is 700 MPa or less. Because
the tensile strength is low, the cold workability is enhanced. Although a lower limit
of the tensile strength is not particularly limited, for example, the lower limit is 400
MPa. The tensile strength can be determined by a tensile testing method for metal
materials in accordance with JIS Z 2241 (2011).
[0101]
[Regarding Sb concentrated layer]
As described above, an Sb concentrated layer is formed at an interface
between a base metal surface and scale of a hot-rolled steel sheet. The existence or
non-existence of the Sb concentrated layer can be observed by electron probe
microanalysis (EPMA). Specifically, the hot-rolled steel sheet is cut along a plane
perpendicular to the rolling direction, and of the entire cut surface, an arbitrary
region of 50 J.!m in the width direction x 45 J.!m in the depth direction of the hotrolled
steel sheet among a width-wise central portion (range of ±1 0 mm in the width
direction from the center in the width direction) that includes the surface is defined
as an observation region. A sample including the observation region is extracted.
Mapping analysis using EPMA is conducted with respect to the observation region.
A portion at which an Sb concentration is 1.5 times or more greater than the region
average is defined as an Sb concentrated layer. In a case where an Sb concentrated
layer is confirmed in 90% or more of the width (50 J.!m) ofthe observation region, it
is determined that an Sb concentrated layer is formed.
[0102]
The thickness of the identified Sb concentrated layer is measured at a pitch of
5 J.!m in the width direction ofthe observation region, and an average value thereof is
defined as the thickness of the Sb concentrated layer. A preferable thickness of the
Sb concentrated layer is 0.5 J.!m or more, more preferably is 1.0 J.!m or more, and
·,'
further preferably is 1.5 J.!m or more.
[0103]
At a high temperature, Sb segregates at grain boundaries and the surface of
the steel, and in particular there is a strong tendency for Sb to segregate and
27
concentrate at the surface. Even in a case where scale covers the base metal, Sb
segregates strongly toward the base metal surface. To sufficiently induce
segregation of Sb and promote the formation of an Sb concentrated layer, it is
preferable to retain the hot-rolled steel sheet in a high temperature region for a long
time period. The Sb concentrated layer is also formed during hot rolling, and is
extended by rolling. Therefore, as described above, the finish rolling temperature is
preferably a high temperature.
[0104]
[Thickness of internal oxidized layer]
In the hot-rolled steel sheet of the present embodiment, because an Sb
concentrated layer is formed, the thickness of an internal oxidized layer is
suppressed. A preferable thickness ofthe internal oxidized layer is 5 J..lm or less.
[0105]
The internal oxidized layer is measured by the following method. A small
piece that includes a part of the surface of the hot-rolled steel sheet is cut out from an
arbitrary position within a width-wise central portion (range of±10 mm in the width
direction from the center in the width direction) of the hot-rolled steel sheet. Ofthe
entire surface of the small piece, a cross-section that is perpendicular to the rolling
direction (hereunder, referred to as "observation face") is mirror polished. The
observation face is subjected to C vapor deposition. After the C vapor deposition, a
portion in the vicinity of the surface ofthe observation face is photographed for
arbitrary visual fields at a magnification of x 1000 using a field-emission scanning
electron microscope (FE-SEM) to obtain images (each visual field is 200 J..lm x 180
J..Lm). The thickness (J..Lm) of the internal oxidized layer is determined based on the
obtained images. Oxides of Si and Mn arise in the base metal in the internal
oxidized layer. Therefore, the scale, the internal oxidized layer and the base metal
can be easily distinguished by means of a backscattered electron image obtained by a
backscattered electron detector that is normally mounted in a common SEM.
[0106]
In the obtained image, a distance from the interface of the scale and base
metal to the lowest edge of the internal oxidized layer is determined at each interval
of 10 J..lm in the rolling direction. This measurement is performed for an arbitrary
28
three visual fields, and the average value of the obtained distances is defined as the
internal oxidized layer thickness (Jlm).
[0107]
[Scale thickness]
In the hot-rolled steel sheet of the present embodiment, because the Sb
concentrated layer is formed, formation of scale is also suppressed. A preferable
thickness of the scale is 1 0 Jlm or less.
[0108]
The scale thickness is measured by the following method. Images are
obtained using an FE-SEM, similarly to when measuring the thickness of the internal
oxidized layer. In the obtained images (it is sufficient to use the same images as
when measuring the internal oxidized layer), the scale is identified, and a distance
between an uppermost edge of the scale and the interface is determined at each
interval of 10 11m in the rolling direction. This measurement is performed for an
arbitrary three visual fields, and the average value of the obtained distances is
defined as the scale thickness (!lm).
[0109]
[Thickness of decarburization layer]
In the hot-rolled steel sheet of the present embodiment, because the Sb
concentrated layer is formed, the thickness of a decarburization phase is also
suppressed. A preferable thickness of the decarburization layer is 20 11m or less.
[0110]
The decarburization layer is measured by the following method. A small
piece that includes a part of the surface of the hot-rolled steel sheet is cut out from an
arbitrary position within a width-wise central portion (range of ±1 0 mm in the width
direction from the center in the width direction) of the hot-rolled steel sheet. Line
analysis of the C-Ka line is performed by means ofEPMA with respect to the
surface of the small piece, and C strengths (line analysis results) in the depth
-; ~·.
direction from the surface of the steel sheet are obtained. Among the obtained line
analysis results, a distance from a position at which the C strength is smallest in the
steel sheet to a depth position at which a difference between the average C strength
29
(C strength ofthe base metal) of the steel sheet and the smallest C strength in the
steel sheet is 98% is defined as the thickness (J..Lm) of the decarburiiation layer.
[0111]
As described above, in the hot-rolled steel sheet ofthe present embodiment,
an Sb concentrated layer suppresses formation of an internal oxidized layer. The Sb
concentrated layer also suppresses formation of scale. Furthermore, the Sb
concentrated layer suppresses formation of a decarburization layer. In addition, in
the hot-rolled steel sheet of the present embodiment, a total area fraction of ferrite
and pearlite in the micro-structure is 75% or more. Therefore, the tensile strength is
suppressed to 800 MPa or less, preferably 700 MPa or less, and the hot-rolled steel
sheet is thus excellent in cold workability.
[0112]
The hot-rolled steel sheet of the present embodiment may also be subjected to
descaling that is described later. In this case, an area fraction of the surface of
island-shaped scale that is caused by fayalite and formed on the surface is lowered.
Consequently, a pickling property is further enhanced.
[0113]
[Production Method]
An example of a method for producing the above described hot-rolled steel
sheet will now be described. The production method includes a preparation
process, a hot rolling process and a coiling process.
[Preparation process]
In the preparation process, a steel material having the above described
chemical composition is prepared. Specifically, molten steel having the above
described chemical composition is produced. A slab as the steel material is
produced using the molten steel. The slab may be produced by a continuous casting
process. Alternatively, an ingot may be produced using the molten steel, and the
ingot may be subjected to blooming to produce the slab.
[0114]
[Hot rolling process]
The prepared steel material (slab) is heated. The heating temperature is from
1100 to 1350°C. Preferably, the heating time period is set to 30 minutes or more.
30
The heated slab is subjected to hot rolling using a roughing mill and a finish rolling
mill and made into a steel sheet. The roughing mill includes a plurality of roll
stands that are arranged in a single row, with each roll stand having a pair of rolls.
The roughing mill may be a reverse-type roughing mill. The finish rolling mill
includes a plurality of roll stands that are arranged in a single row, with each roll
stand having a pair of rolls.
[0115]
[Descaling]
During the hot rolling, descaling may also be performed on the steel sheet that
is being subjected to rolling, by means of one or a plurality of high-pressure water
descaling devices that are installed between the plurality of roll stands (roughing mill
or finish rolling mill). The descaling is preferably performed on the steel sheet
having a temperature of 1050°C or more. In this case, Fe2Si04 (fayalite) arising on
the surface of a steel with a high Si and high Mn content, as in a steel sheet having
the chemical composition of the present embodiment, can be effectively removed.
Iffayalite remains, island-shaped scale will be formed on the surface of the hotrolled
steel sheet. If island-shaped scale remains on the surface of the hot-rolled
steel sheet, it is difficult to remove the scale during pickling. Further, iffayalite
remains, indentation defects arise during cold rolling, and such defects may mar the
external appearance of the cold-rolled steel sheet. Fayalite can be removed by
performing descaling.
[0116]
In a case where one or a plurality of high-pressure water descaling devices are
installed between finishing roll stands during finish rolling, it is preferable that a
steel sheet (rough bar) in a state after rough rolling and before finish rolling is heated
to 1050°C or more by a heating apparatus arranged in the vicinity of the entrance
side of the initial roll stand of the finish rolling mill. The method of heating the
rough bar is not particularly limited. For example, the rough bar is heated by an
-;
induction heating apparatus or a reflow furnace.
[0117]
[Finish rolling temperature FT]
31
In hot rolling, the surface temperature ofthe steel sheet of the exit side ofthe
final stand of the finish rolling miii is defined as a finish rolling temperature FT (0 C).
A preferable finish rolling temperature FT CCC) is the Ar3 transformation temperature
+50°C or more. If the finish roiling temperature FT is less than the Ar3
transformation temperature +50°C, rolling resistance of the steel sheet increases and
productivity decreases. In addition, the steel sheet is roiled in a two-phase region of
ferrite and austenite. In this case, the micro-structure ofthe steel sheet forms a
layered micro-structure and the mechanical properties decrease. Therefore, the
finish rolling temperature FT is the Ar3 transformation temperature +50°C or more.
A preferable finish roiling temperature FT is more than 920°C, and more preferably
is 950°C or more.
[0118]
After finish rolling is completed, the steel sheet is cooled to the coiling
temperature. The cooling method is not particularly limited. The cooling methods
are, for example, water cooling, forced air-cooling and allowing cooling.
[0 119]
[Coiling process]
The hot-rolled steel sheet produced in the hot rolling process is coiled to form
a coil. A surface temperature (hereunder, referred to as "coiling temperature") CT
of the hot-rolled steel sheet when starting coiling of a coil is preferably from 600°C
to 750°C.
[0120]
If the coiling temperature CT is too high, formation of an internal oxidized
layer in the hot-roiled steel sheet is promoted. On the other hand, if the coiling
temperature CT is too low, in steel that contains a large amount of Si, as in the case
of the hot-roiled steel sheet ofthe present embodiment, the strength ofthe hot-rolled
steel sheet wiii be too high and the cold roiling property wiii decrease.
[0121]
-;
If the coiling temperature CT is made a temperature in the range of 600°C to
750°C, an increase in the strength ofthe hot-roiled steel sheet can be suppressed, and
formation of an internal oxidized layer in the steel composition defined in the present
32
embodiment is suppressed. The coiling temperature CT is preferably from 650°C
to 750°C, and more preferably is from 700°C to 750°C.
[0122]
The hot-rolled steel sheet of the present embodiment can be produced by the
above described processes. Note that, the above described production method is
one example of a method for producing a hot-rolled steel sheet in which a total area
fraction of ferrite and pearlite is 75% or more, and a method for producing the hot- .
rolled steel sheet of the present embodiment is not limited thereto.
[0123]
[Second Embodiment]
The micro-structure of the hot-rolled steel sheet may be a micro-structure that
consists mainly of bainite and martensite. Specifically, a combined area fraction of
bainite and martensite may be 75% or more.
[0124]
[~icro-structure]
The chemical composition of a hot-rolled steel sheet according to the present
embodiment (second embodiment) is the same as the chemical composition of the
hot-rolled steel sheet of the first embodiment, and satisfies Formula (1). If the
chemical composition does not satisfy Formula (1), the ductility of the cold-rolled
steel sheet may decrease. If Formula (1) is satisfied, excellent ductility is obtained
in a cold-rolled steel sheet after annealing also.
[0125]
On the other hand, the micro-structure of the hot-rolled steel sheet of the
present embodiment is different from the first embodiment. In the micro-structure
of the hot-rolled steel sheet of the present embodiment, a combined area fraction of
bainite and martensite is 75% or more.
[0126]
A region (balance) other than bainite and martensite is one or more types of
·;
micro-structure selected from the group consisting of ferrite, pearlite and retained
austenite. In the present embodiment, preferably tempering is performed on the
hot-rolled steel sheet after coiling. By this means, the strength of the steel sheet can
be reduced to a certain extent, and the cold workability can be enhanced while
33
maintaining a certain degree of strength. If tempering is performed, the bainite is
mainly tempered bainite, and the martensite is mainly tempered martensite.
Methods for measuring the area fractions of the respective phases in the microstructure
are the same as in the first embodiment.
[0127]
[Tensile strength]
The hot-rolled steel sheet of the present embodiment has the above describeq
chemical composition and micro-structure. In a case where tempering is not
performed after coiling, the tensile strength of the hot-rolled steel sheet of the present
embodiment is 900 MPa or more.
[0128]
On the other hand, if tempering is performed after coiling, the tensile strength
of the hot-rolled steel sheet is 800 MPa or less. In this case the cold workability can
be enhanced, and the load placed on the equipment system during cold rolling can be
reduced. Although the lower limit of the tensile strength is not particularly limited,
for example, the lower limit is 400 MPa. The tensile strength is determined by a
method that is in accordance with JIS Z 2241 (2011).
[0129]
[Production Method]
An example of a method for producing the hot-rolled steel sheet according to
the present embodiment will now be described. The production method includes a
preparation process, a hot rolling process and a coiling process. In comparison to
the production method of the first embodiment, in the present embodiment the
coiling temperature CT in the coiling process differs from the first embodiment.
Preferably, tempering is also performed after the coiling process. The other
processes are the same as in the first embodiment.
[0130]
[Coiling process]
·: ~·.
The steel sheet produced in the hot rolling process is coiled to form a coil. If
the surface temperature (coiling temperature) of the steel sheet when starting coiling
of a coil is too low, the strength of the steel sheet increases and the load placed on a
coiling apparatus becomes large. Therefore, the surface temperature (coiling
34
temperature) CT of the steel sheet when starting coiling is from 150 to 600°C,
preferably from 350 to 500°C, and more preferably from 400°C to 500°C.
[0131]
[Tempering]
Because the coiling temperature CT ofthe hot-rolled steel sheet of the present
embodiment is 600°C or less, preferably 500°C or less, the hardness of the hot-rolled
steel sheet is high. Therefore, tempering may be performed to lower the strength
and enhance the cold rolling property. In the tempering, the steel sheet after coiling
is tempered at a temperature of 550°C or more (Ac1 transformation temperature or
less). If the tempering time period is too short, it is difficult to obtain the above
described effect. On the other hand, if the tempering time period is too long, the
effect saturates. Therefore, a preferable tempering time period is 0.5 to 8 hours in a
temperature range of 550°C or more.
[0132]
The hot-rolled steel sheet of the second embodiment can be produced by the
above described processes.
[0133]
Note that tempering need not be performed. In a case where tempering is
not performed also, in the micro-structure of the hot-rolled steel sheet, a combined
area fraction of bainite and martensite is 75% or more, and the balance is one or
more types of micro-structure selected from the group consisting of ferrite, pearlite
and retained austenite. However, the bainite micro-structure and martensite microstructure
in a case where tempering is not performed are not micro-structures that are
mainly composed of tempered bainite and tempered martensite, but rather are microstructures
mainly composed of bainite and martensite that contain some tempered
bainite and tempered martensite formed during the coiling process.
[0134]
In a case where tempering is not performed, the tensile strength of the hot-
·:
rolled steel sheet is 900 MPa or more. A hot-rolled steel sheet that has not been
subjected to tempering is useful in particular in a case where a high tensile strength is
required as a hot-rolled steel sheet and the like.
[0135]
35
Note that the above described production method is one example of a method
for producing a hot-rolled steel sheet in which the total area fraction of bainite and
martensite is 75% or more, and a method for producing the hot-rolled steel sheet of
the present embodiment is not limited to the above described method.
[0136]
[Other Embodiments]
In the above described first and second embodiments, the micro-structure is
defined. However, the micro-structure of a hot-rolled steel sheet of the present
embodiments is not particularly limited. As long as the above described chemical
composition and Formula (1) are satisfied, an Sb concentrated layer can be formed
and formation of an internal oxidized layer and/or scale can be suppressed while
maintaining the necessary workability and strength.
[0137]
The above described production method is merely one example.
Accordingly, in some cases the hot-rolled steel sheets of the first and second
embodiments can also be produced by other production methods.
[0138]
Note that, in the finish rolling of the hot rolling process, it is preferable to
make the average cooling rate (hereunder, referred to as "ADFT") from the
temperature of the steel sheet at the time when the final descaling is performed until
the temperature of the steel sheet reaches 800°C by cooling after the finish rolling, a
rate of 1 0°C/sec or more. In this case, formation of scale on the hot-rolled steel
sheet surface can be further suppressed.
EXAMPLES
[0139]
Examples of the hot-rolled steel sheet of the present invention will now be
described. The conditions adopted in the Examples are merely one example of
-; ~·.
conditions adopted to confirm the operability and advantageous effects of the present
invention. Therefore, the present invention is not limited to this one example of the
conditions. The present invention can adopt various conditions as long as the
36
objective of the present invention is achieved without departing from the gist ofthe
present invention.
[0140]
[Example 1]
Molten steels having the chemical compositions shown in Table 1 were
produced.
[0141]
37
[Table 1]
TABLE I
Steel
Chemical Composition (Unit Is Mass Percent; Balance Is Fe And Impurities)
Type c Si Mn p s AI N 0 Sb Ti v Nb Cr Ni Mo B Cu
Sn,Bi,Se,
Ca Mg REM Si+Mn
Remarks
Te,Ge,As
A 0.076 1.10 2.39 0.007 0.0011 0.0420 0.0034 0.0014 0.10 - - - - - - - - - - - 3.49 Inventive Example
B 0.120 1.97 2.01 0.011 0.0016 0.0240 0.0022 O.OOII 0.20 - - - - - - - - - - - 3.98 Inventive Example
c 0.182 1.75 2.70 O.O.Q9 ·0.0010 0.0330 0.0034 0.0012 0.05 - - - - - - - - - - - 4.45 Inventive Example
D 0.185 1.80 2.60 0.010 0.0020 0.0300 0.0030 0.0009 0.07 0.005 - - - - - - - - - - 4.40 Inventive Example
E 0.152 1.20 2.II 0.008 0.0021 0.0310 0.0026 0.0019 0.03 0.062 - - 0.12 - - 0.0014 - - - - 3.31 Inventive Example
F 0.152 1.10 2.24 0.007 0.0023 0.0400 0.0016 0.0029 0.03 - - - - 0.24 - - 0.12 - - - 3.34 Inventive Example
G 0.181 1.20 2.41 0.015 0.0034 0.0380 0.0019 0.0015 0.03 - - - - - - - - 0.0006 - - 3.61 Inventive Example
H 0.178 1.20 2.42 0.014 0.0036 0.0410 0.0027 0.0018 0.03 - - - - - - - - - - Ce:0.0013 3.62 Inventive Example
I 0.182 1.10 2.39 0.009 0.0024 0.0190 0.0020 O.OOII 0.03 - - - - - - - - - 0.0013 - 3.49 Inventive Example
. J 0.179 1.10 2.37 0.013 0.0023 0.0680 0.0028 0.0022 0.04 - - - - - - - - - - Nd:0.0007 3.47 Inventive Example
K 0.179 1.10 2.14 0.009 0.0019 0.0350 0.0025 0.0019 0.07 - 0.083 - - - - - - - - - 3.24 Inventive Example
L 0.180 1.10 2.45 0.001 0.0026 0.0980 0.0044 0.0019 0.07 - - - - - - - - - - - 3.55 Inventive Example
M 0.200 1.10 2.30 0.010 0.0010 0.0330 0.0030 0.0015 0.07 - - - - - - - - Sn:0.01 - - - 3.40 Inventive Example
N 0.175 1.75 2.70 0.009 0.0011 0.0330 0.0022 0.0021 0.05 0.005 - - - - - - - Se:0.05 - - - 4.45 Inventive Example
Ge:0.02
0 0.190 1.75 2.80 0.010 0.0010 0.0330 0.0022 0.0014 0.11 0.005 - - - - - - - - - - 4.55 Inventive Example
As:0.03
Bi:0.02
p 0.180 2.20 3.10 0.010 0.0010 0.0330 0.0030 0.0015 0.01 - - - - - - - - - - - 5.30 Comparative Example
Te:0.04
Q 0.150 2.00 2.60 0.010 0.0020 0.0300 0.0030 0.0009 = 4.60 Comparative Example
R 0.429 1.10 2.50 0.007 0.0011 0.0270 0.0022 0.0016 = - - - - - - - - - - - 3.60 Comparative Example
s 0.155 2.88 2.21 0.016 0.0033 0.0240 0.0029 0.0020 = 0.034 - - - - - 0.0009 - - - - 5.09 Comparative Example
T 0.142 1.10 0.70 0.012 0.0038 0.0080 0.0034 0.0026 0.40 - - - - - - - - - - - 180 Comparative Example
u 0.250 1.10 5.61 0.014 0.0030 0.2900 0.0034 0.0019 0.02 - - - - - - - - - - - 6.71 Comparative Example
- '····· (Note) Underlined Numerical Values Indicate That The Values Are Outside The Range Of The Present InventiOn.
38
[0142]
Referring to Table 1, steel types A to 0 are within the range of the chemical
composition of the steel material of the present embodiments. On the other hand,
the chemical compositions of steel types P to U are outside the range of the chemical
composition of the steel material ofthe present embodiments.
[0143]
Steel materials (ingots) were produced by an ingot-making process using the.
above described molten steel. Hot-rolled steel sheets were produced by hot rolling
the steel materials under the hot rolling conditions (heating temperature CCC) and
finish rolling temperature FT (°C)) shown in Table 2 using a hot rolling mill for
testing that was composed of a plurality of hot rolling stands. In addition, a heat
history equivalent to a coil that was coiled at a coiling temperature CT (0 C) shown in
Table 2 was imparted to the respective hot-rolled steel sheets after hot rolling by an
N2.purged annealing furnace.
[0144]
Note that, a reheating furnace simulating a rough bar heater was installed on
the entrance side of the finish rolling mill, and reheating of the hot-rolled steel sheets
was conducted under the conditions shown in Table 2. Further, a high-pressure
water descaling device was disposed between roll stands of the finish rolling mill,
and descaling was performed with respect to steel sheets undergoing finish rolling.
The surface temperatures (descaling temperatures) of the respective steel sheets
immediately before performing descaling were as shown in Table 2.
[0145]
39
[Table 2]
TABLE 2
Test
Number
I
2
3
4
5
6
7
8
9
10
II
12
13
14
15
16
17
18
19
20
21
22
23
24
25
[0146]
Steel
Heating
Type
Temperature
(OC)
A 1280
B 1250
c 1280
c 1250
c 1220
c 1280
D 1250
D 1280
E 1200
F 1200
G 1200
H 1200
I 1250
J 1220
K 1250
L 1220
M 1280
N 1230
0 1280
p 1200
Q 1280
R 1200
s 1200
T 1250
u 1200
Reheating Descaling
FT CT
Temperature Temperature (OC) (OC) Remarks
CC) (OC)
!100 1050 970 655 Inventive Example
!100 1080 930 655 Inventive Example
1090 1050 970 655 Inventive Example
!100 1060 970 700 Inventive Example
!120 1085 970 720 Inventive Example
- 980 850 450 Inventive Example
!170 1100 970 730 Inventive Example
1010 1050 990 500 Inventive Example
1110 1070 930 700 Inventive Example
1140 IIOO 930 655 Inventive Example
1080 1050 930 700 Inventive Example
1090 1080 930 700 Inventive Example
1060 1050 900 655 Inventive Example
1080 1060 930 700 Inventive Example
1100 1080 900 720 Inventive Example
1110 1100 950 730 Inventive Example
!120 1080 950 700 Inventive Example
1080 1050 950 700 Inventive Example
1090 1060 980 700 Inventive Example
1080 1060 980 700 Comparative Example
1060 1060 950 750 Comparative Example
1070 1060 930 400 Comparative Example
- 1000 950 650 Comparative Example
1100 1050 930 680 Comparative Example
1080 1050 900 655 Comparative Example
[Internal oxidized layer thickness and scale thickness measurement test]
40
A small piece was cut out from a width-wise central portion (range of±10
mm in the width direction from the center in the width direction) ofthe hot-rolled
steel sheet of each test number. Of the entire surface of the small piece, a crosssection
(hereunder, referred to as "observation face") perpendicular to the rolling
direction was mirror polished. The observation face was subjected to C (carbon)
vapor deposition. After the C vapor deposition, a portion in the vicinity of the
surface of the observation face was photographed using a field-emission scanning
electron microscope (FE-SEM) and images were obtained. The thickness (f..lm) of
the internal oxidized layer and the scale thickness (f..lm) were determined by the
above described method using the obtained images.
[0147]
[Decarburization layer measurement test]
A small piece was cut out from a width-wise central portion (range of±10
mm in the width direction from the center in the width direction) of the hot-rolled
steel sheet of each test number. Line analysis of C-Ka line was performed by
means ofEPMA with respect to the surface of the small piece. Among the obtained
line analysis results, a distance from a position at which the C strength was smallest
in the steel sheet to a depth position at which a difference between the average C
strength (C strength of the base metal) of the steel sheet and the smallest C strength
in the steel sheet was 98% was defined as the thickness (J..lm) of the decarburization
layer.
[0148]
[Sb concentrated layer measurement test]
The presence or absence of an Sb concentrated layer and the thickness (J..lm)
of an Sb concentrated layer were measured by the measurement method described in
the first embodiment.
[0149]
[Cold rolling property evaluation test]
·; ~·.
No.5 tensile test coupons specified in the JIS Standards were taken at a
width-wise central portion of the respective steel sheets that from a position that was
1/4 of the thickness from the surface. Parallel portions were made parallel in the
rolling direction. A tension test in accordance with JIS Z 2241 (2011) was
41
performed using the respective tensile test specimens at normal temperature (25°C)
in the atmosphere, and a tensile strength (MPa) was obtained.
[0150]
[Pickling property evaluation test]
A test specimen including part of the steel sheet surface was taken from a
width-wise central portion of the steel sheet of each test number. In each test
specimen, a region including part of the steel sheet surface was 50 mm in width x 70
mm in length. A pickling test was performed on each test specimen. In the
pickling test, the test specimen was immersed in an 8% hydrochloric acid aqueous
solution heated to 85°C, and scale on the surface of the test specimen was removed.
A time period (pickling completion time period) taken to remove scale from the
entire surface of the test specimen was measured. The pickling property was
determined as being excellent if the pickling completion time period was within 60
seconds.
[0151]
[Test Results]
The test results are shown in Table 3.
[0152]
[Table 3]
TABLE 3
Microstructure
Test (Area%)
Numb
er Othe F p
r
I
8 I
9 I -
2
8 I
4 6 -
3
4 5
6 4 -
4
4 5
6 4 -
5
4 5 ..
6 4 -
B:75
,
6 0 0 M:18
,
Rg:7
7
5 4
3 7 -
Tensile Decarburizati
Strengt on Layer
Scale
Thickne
hTS Thickness
(MPa) (J.lm)
ss (J.lm)
62I 0.7 5.0
623 0.7 5.0
652 0.7 5.0
6I7 1.1 5.0
618 2.0 5.0
967 0.2 5.0
593 1.5 5.0
42
Internal
Sb
Oxidize Pick! in
Concentrat
d Layer
ed Layer
g
Thickne Remarks Thickness Pro pert
(J.lm) s~ (J.lm)
y
2.3 0.0 0 Inventive Example
2.9 0.0 0 Inventive Example
1.4 0.8 0 Inventive Example
1.4 0.8 0 Inventive Example
1.5 0.8 0 Inventive Example
1.1 0.8 0 Inventive Example
1.9 0.2 0 Inventive Example
:.,...:
8 0 0
B:10
823 0.1 5.0 1.5 0.2 0 Inventive Example
0
9
7 2
650 1.1 7.0 1.1 4.0 o· Inventive Example
8 2 -
10
7 2
577 0.7 9.0 0.8 5.0 0 Inventive Example
5 5 -
11
7 2
529 1.1 9.0 1.1 4.0 0 Inventive Example
1 9 -
12
7 2
523 2.1 6.0 1.1 4.0 0 Inventive Example
1 9 -
13
6 3
590 0.7 6.0 0.9 1.8 0 Inventive Example
9 1 -
14
7 3
562 1.9 6.0 1.2 0.2 0 Inventive Exampl~ 0 0 -
15
7 3
639 2.1 5.0 1.8 0.2 0 Inventive Example
0 0 -
16
7 3
641 1.9 5.0 1.7 0.2 0 Inventive Example
0 0 -
17
6 3
497 1.1 5.0 2.0 0.8 0 Inventive Example
6 4 -
18
4 5
570 2.1 5.0 1.3 0.0 0 Inventive Example
7 3 -
19
4 5
531 1.1 5.0 2.6 0.0 0 Inventive Example
3 7 -
20
7 2
643 1.1 5.0 0.4 20.0
Comparative
5 5 - X Example
21
7 2
596 186.0 75.0 0.0 40.0
Comparative
1 9 - X Example
B:52
,
22 0 0
M:31
904 5.0 100.0 0.0 10.0
Comparative
, X Example
Rg:1
7
23
8 1
643 65.0 100.0 0.0 30.0
Comparative
5 5 - X Example
24
7 2
0.9 3.0 5.3 0.0 0 Comparative
7 3 - - Example
M:92
Comparative
25 0 0 , - 69.0 8.0 0.2 8.0 0
Example
Rg:8
[0153]
In Table 3, the item "micro-structure" shows the micro-structure at a depth
position at 114 of the plate thickness at a width-wise central portion (range of±IO
mm in the width direction from the center in the width direction) of the steel sheet.
In each of the micro-structures shown in Table 3, an area fraction(%) of ferrite in the
respective micro-structures is described in an "F" column, and an area fraction of
pearlite is described in a "P" column. The area fractions of phases other than ferrite
and pearlite are described in an "Other" column. The character "B" denotes the area
fraction(%) of bainite (including tempered bainite), and the character "M" denotes
the area fraction (%) of martensite (including tempered martensite). "Rg" denotes
43
the area fraction (%) of retained austenite. The area fraction of each phase was
measured by the above described measurement method.
[0154]
The item "tensile strength TS" shows the tensile strength TS (MPa) at a
width-wise central portion of the steel sheet. The cold rolling property was
determined as excellent if the tensile strength was 800 MPa or less.
[0155]
An "0" mark in the "pickling property" column in Table 3 indicates that the
pickling completion time period was within 60 seconds. An "x" mark indicates that
the pickling completion time period was more than 60 seconds.
[0156]
Referring to Table 3, the chemical compositions of test numbers 1 to 19 were
appropriate, and also satisfied Formula (1). Therefore, an Sb concentrated layer
was confirmed. The thickness of each of the Sb concentrated layers was 0.5 f.lm or
more. In addition, the scale thickness was 10 f.lm or less, and the thickness of an
internal oxidized layer was 5 f.lm or less. Thus, the internal oxidized layer and scale
could be suppressed. Consequently, the pickling property of the respective
chemical compositions of test numbers 1 to 19 was excellent. Further, the thickness
of the decarburization layer was 20 f.lm or less.
[0157]
In test numbers 1 to 5, test number 7 and test numbers 9 to 19, the production
conditions were suitable for formation of ferrite and pearlite. Therefore, in the
micro-structure of the hot-rolled steel sheet of each of these test numbers, the total
area fraction of ferrite and pearlite was 75% or more. Therefore, the tensile
strength was 800 MPa or less.
[0158]
In test number 6 and test number 8, because the coiling temperature CT was
from 150 to 600°C, the total area fraction of bainite and martensite in the micro-
·;
structure was 75% or more, and the tensile strength was 900 MPa or more.
[0159]
On the other hand, in steel type P used in test number 20, the Sb content was
too low. Therefore, the thickness of the Sb concentrated layer was less than 0.5
44
!liD, and the thickness of the internal oxidized layer was more than 5 !liD.
Consequently, the pickling property was poor.
[0160]
Steel type Q used in test number 21 did not contain Sb. Therefore an Sb
concentrated layer was not confirmed. Consequently, the thickness of an internal
oxidized layer was more than 5 !liD, and the scale thickness was more than I 0 !liD.
Therefore, the pickling property was poor. In addition, the decarburization layer
thickness was more than 20 !lffi·
[OI6I]
In steel type R used in test number 22, the C content was too high. In
addition, Sb was not contained therein. The coiling temperature CT was also too
low. Consequently, the micro-structure mainly consisted of bainite and martensite,
and did not contain ferrite and pearlite. Therefore, the tensile strength was more
than 800 MPa. In addition, because there was no Sb concentrated layer, the
thickness of the internal oxidized layer was more than 5 !lffi, and the scale thickness
was more than I 0 !liD. Therefore, the pickling property was poor.
[OI62]
In steel typeS used in test number 23, the Si content was too high, and Sb was
not contained therein. Consequently, an Sb concentrated layer was not formed, and
the thickness of the internal oxidized layer was more than 5 !liD and the scale
thickness was more than I 0 !liD- Therefore, the pickling property was poor. In
addition, the decarburization layer thickness was more than 20 !liD.
[OI63]
Note that, in test number 23, heating with a rough bar heater was not
performed during hot rolling. Consequently, the descaling temperaturewas less
than I 050°C. As a result, the ratio of island-shaped scale was more than 6%.
[OI64]
In steel type T used in test number 24, the Mn content was too low, the AI
·;
content was too low, and the Sb content was too high. Consequently, embrittlement
of the steel material was severe and the evaluation test was abandoned. In test
number 25, the Mn content was high. Consequently, embrittlement of the steel
material was severe and the evaluation test was abandoned.
45
[0165]
[Example 2]
Molten steels having the chemical compositions shown in Table 4 were
produced.
[0166]
46
[Table 4]
TABLE 4
Steel
Chemical Composition (Unit Is Mass Percent; Balance Is Fe And Impurities)
Type
Remarks
c Si Mn p s AI N 0 Sb Ti v Nb Cr Ni Mo B Cu Sn Bi Te Ca Mg REM Si+Mn
A 0.155 1.25 2.23 0.009 O·.D025 0.0586 0.0042 0.0016 0.05 - - - - - - - - - - - - - - 3.48 Inventive Example
'
B 0.176 1.71 2.49 0.014 0.0014 0.0588 0.0042 0.0023 0.11 - - - - - - - - - - - - - - 4.20 Inventive Example
c 0.178 1.41 2.13 0.013 0.0031 0.0540 0.0039 0.0028 0.15 - - - - - - - - - - - - - - 3.55 Inventive Example
D 0.151 1.19 2.24 0.008 0.0033 0.0608 0.0032 0.0028 0.12 0.062 - - 0.12 - - 0.0014 - - - - - - - 3.42 Inventive Example
E 0.148 1.65 2.73 0.013 0.0022 0.0770 0.0022 0.0013 0.08 - - - - 0.24 - - 0.12 - - - - - - 438 Inventive Example
F 0.169 1.90 2.08 0.013 0.0012 0.0722 0.0020 0.0019 0.22 - - - - - - - - - 0.0008 - 0.0006 - - 3.98 Inventive Example
G 0.186 1.21 2.83 0.009 0.0024 0.0837 0.0043 0.0029 0.08 - - - - - - - - - - 0.05 - 0.0013 - 4.04 Inventive Example
H 0.118 1.67 2.42 0.008 0.0013 0.0790 0.0041 0.0014 0.13 - - - - - - - - - - - - - Nd:0.0007 4.09 Inventive Example
I 0.138 1.32 2.68 0.013 0.0029 0.0742 0.0038 0.0019 0.18 - - - - - 0.14 - - - - - - - - 4.00 Inventive Example
J 0.159 1.79 2.03 0.012 0.0025 0.0905 0.0038 0.0026 0.11 - 0.083 - - - - - - - - - - - - 3.81 Inventive Example
K 0.182 1.74 2.84 0.008 0.0032 0.0859 0.0033 0.0021 :: - 0.053 - - - - - - - - - - - - 4.58 Comparative Example
L 0.113 1.40 2.42 0.008 0.0022 0.0232 0.0030 0.0026 0.41 0.005 - - - - - - - - - - - - - 3.82 Comparative Example
M 0.110 1.86 2.68 0.013 0.0011 0.0974 0.0028 0.0011 0.004 0.005 - - - - - - - - - - - - - 4.54 Comparative Example
N 0.131 1.02 2.05 0.012 0.0034 0.0926 0.0026 0.0016 0.15 - - - - - - - - - - - - - - 3.07 Comparative Example
--
47 .' .t
0 0.172 0.93 2.11 0.008 0.0013 0.0252 0.0021 0.0027 0.03 - - - - - - - - - - - - - - 3.04 Comparative Example
p 0.169 1.88 .L.ll O.ot5 0.0028 0.0205 0.0018 0.0012 0.08 - - - - - - 0.0046 - - - - - - - 3.43 Comparative Example
Q 0.121 2.96 2.21 0.011 0.0014 0.0320 0.0043 0.0023 0.15 - - - - - - - - - - - - - - 5.17 Comparative Example
R 0.123 1.70 3.99 0.008 0.0032 0.0321 0.0041 0.0028 0.09 - - - - - - - - - - - - - - 5.69 Comparative Example
s 0.185 1.75 2.50 0.010 o:oo2o 0.0350 0.0035 0.0016 0.02 - - - - - - - - - - - - - - 4.25 Comparative Example
.......
(Note) Underlined Numerical Values Indicate That The Values Are Outside The Range Of The Present Invention.
48
:~~ ' -:t
[0167]
Using the above described molten steel, steel materials (ingots) were
produced by an ingot-making process. Steel sheets were produced by hot rolling
the steel materials under the hot rolling conditions (heating temperature CCC) and
finish rolling temperature FT CCC)) shown in Table 5 using a hot rolling mill for
testing. In addition, a heat treatment that simulated coiling at a coiling temperature
CT CCC) shown in Table 5 was performed on the respective steel sheets after hot
rolling. Specifically, the steel sheets were stacked and charged into a furnace that
was set to the coiling temperature CT (°C). The inside of the furnace was a
nitrogen atmosphere, and the steel sheet surface was in a state in which the surface
was blocked-off from the atmosphere. That is, the surface state ofthe steel sheet
was equal to the surface state of a coil obtained by actual production. After holding
the steel sheet inside the furnace for 30 minutes at the coiling temperature CT CCC),
the steel sheet was gradually cooled to room temperature at 20°C/hour.
[0168]
[Table 5]
TABLES
Stee
Test I
Numbe Typ
r e
I A
2 B
3 c
4 D
5 E
6 F
7 G
8 H
9 I
10 J
II K
12 L
Heating FT CT Steel
Temperatur (oc (oc Microe
(oC) structur ) ) e
1250 950 750 F+P
1250 950 750 F+P
1250 950 750 F+P
1250 950 750 F+P
1250 950 750 F+P
1250 950 750 F+P
1250 950 750 F+P
1250 950 750 F+P
-; 1250 950 750 F+P
1250 950 750 F+P
1250 950 750 F+P
1250 950 750 F+P
Sb Internal
Concentrate Oxidized TS
d Layer Layer (MPa EL
(%)
Remarks
Thickness Thicknes )
(flm) S(flm)
1.5 I 540 II. Inventive Example Of Present
9 Invention
2.4 0 633 10. Inventive Example Of Present
7 Invention
2.8 0 596 II. Inventive Example Of Present
I Invention
2.5 0 682 10. Inventive Example Of Present
0 Invention
2.2 0 620 II. Inventive Example Of Present
I Invention
3.4 0 585 10. Inventive Example Of Present
I Invention
2.2 0 599 II. Inventive Example Of Present
6 Invention
2.6 0 596 10. Inventive Example Of Present
9 Invention
3.0 0 585 II. Inventive Example Of Present
7 Invention
2.4 0 566 10. Inventive Example Of Present
0 Invention
0 47 648 10. 5 Comparative Example
5.3 0 - - Comparative Example
49
13 M 1180 950 750 F+P 0 34 616 10. Comparative Example 4
14 N 1250 950 750 F+P 2.8 0 534 8.8 Comparative Example
15 0 1250 950 750 F+P 1.1 4 548 8.7 Comparative Example
16 p 1250 950 750 F+P 2.0 0 562 6.7 Comparative Example
17 Q 1250 950 750 F+P 2.7 0 682 7.8 Comparative Example
18 R 1250 950 750 B+M 2.3 0 1317 3.2 Comparative Example
19 s 1200 950 750 F+P 0.3 25 640 10. Comparative Example 2
[0169]
[Ferrite and pearlite area fraction measurement test]
By the same method as in Example 1, the total of the area fractions of ferrite
and pearlite in the steel sheet (hot-rolled steel sheet) after hot rolling was measured.
The results are shown in Table 5. In the "steel micro-structure" column in Table 5,
"F+P" indicates that the total area fraction of ferrite and pearlite in the microstructure
of the hot-rolled steel sheet was 75% or more. In the "steel microstructure"
column in Table 5, "B+M" indicates that the total area fraction of bainite
and martensite in the micro-structure of the hot-rolled steel sheet was 75% or more.
[0170]
[Internal oxidized layer thickness measurement test]
The thickness of an internal oxidized layer in the steel sheet after hot rolling
(hot-rolled steel sheet) of each test number was measured by the same method as in
Example 1. Specifically, a small piece including a part of the surface of the hotrolled
steel sheet was cut out from a width-wise central portion of the hot-rolled steel
sheet. Of the entire surface of the small piece, a cross.:.section (hereunder, refern~d
to as "observation face") perpendicular to the rolling direction was mirror polished.
The observation face was subjected to C (carbon) vapor deposition. After the C
vapor deposition, a portion in the vicinity of the surface of the observation face was
photographed at an observation magnification ofx1000 using a field-emission
scanning eleqtron microscope (FE-SEM) and images were obtained. The thickness
·;
of the internal oxidized layer was measured by the above described method based on
the obtained images. The results are shown in Table 5.
[0171]
50
Note that scale is a layer that is formed when iron ions at the exterior of the
hot-rolled steel sheet are oxidized. On the other hand, an internal oxidized layer is a
layer that contains oxides of Si and Mn and is formed inside the hot-rolled steel
sheet. Therefore, scale, an internal oxidized layer and the base metal can be easily
distinguished using a common SEM.
[0172]
[Tension test]
The tensile strength TS of the hot-rolled steel sheet of each test number was
measured by a method in accordance with JIS Z 2241 (20 11 ). The results are
shown in TableS. In Table 5, the symbol"-" in the "TS (MPa)" column indicates
that cracking occurred at an edge of the hot-rolled steel sheet and measurement was
not possible.
[0173]
[Uniform elongation measurement test]
The hot-rolled steel sheet of each test number was cold-rolled at a draft of
50%. After cold rolling, each steel sheet was subjected to annealing. The
annealing was performed under the following conditions. The steel sheet was
heated to an HC temperature. (Ae3 temperature + 1 0°C) at an average heating rate of
5°C/second, and the steel sheet was subjected to annealing for 90 seconds at this HC
temperature. Thereafter, the steel sheet was gradually cooled at a cooling rate of
2°C/sec to an AC temperature (HC temperature -120°C). In addition, the steel sheet
was rapidly cooled at 80°C/sec from the AC temperature to 420°C. After being
held at 420°C for 300 seconds, the steel sheet was allowed to cool to room
temperature. A tension test was performed by a method in accordance with JIS Z
2241 (20 11) on the steel sheet after annealing. During the tension test, a length that
the test specimen stretched until necking occurred in the test specimen (a section in
which the test specimen exhibited uniform elongation) was measured. The obtained
length was divided by the length of the test specimen and the result was adopted as a
-; ~·.
uniform elongation EL. The results are shown in Table 5. In Table 5, the symbol
"-"in the "EL (%)"column indicates that cracking occurred at an edge of the steel
sheet and measurement was not possible.
[0174]
51
[Test results]
Referring to Table 4 and Table 5, the chemical compositions oftest numbers
1 to 1 0 were appropriate. In addition, the production conditions of test numbers 1
to 10 were appropriate. Therefore, in the micro-structure of the hot-rolled steel
sheets oftest numbers 1 to 10, the total area fraction of ferrite and pearlite was 75%
or more. Further, in the hot-rolled steel sheets of test numbers 1 to 10, an Sb
concentrated layer with a thickness of 0.5 ).!m or more was formed. Furthermore, .
the thickness of an internal oxidized layer was 5 ).!m or less, and thus formation of an
internal oxidized layer was suppressed.
[0175]
In addition, the tensile strength of the hot-rolled steel sheets oftest numbers 1
to 10 was 800 MPa or less, and the workability during cold rolling was excellent.
The uniform elongation ofthe cold-rolled steel sheets oftest numbers 1 to 10 was
10.0% or more, indicating excellent workability after cold rolling also.
[0176]
Steel type K used for test number 11 did not contain Sb. Consequently, in
the hot-rolled steel sheet of test number 11, an Sb concentrated layer was not formed
and an internal oxidized layer had a thick thickness of 47 ).!ill.
[0177]
In steel type L used for test number 12, the Sb content was too high.
Consequently, cracking occurred at an edge of the hot-rolled steel sheet oftest
number 12, and a tension test could not be performed. Therefore, the workability
during cold rolling was poor.
[0178]
In steel type Mused for test number 13, the Sb content was too low.
Consequently, an internal oxidized layer in the hot-rolled steel sheet oftest number
13 had a thick thickness of 34 J..lill.
[0179]
·;
In steel type N used in test number 14, a total content ofSi and Mn was
3.07%, and thus Formula (1) was not satisfied. Consequently, in the cold-rolled
steel sheet oftest number 14, in comparison to test numbers 1 to 10 in which the total
52
area fraction of ferrite and pearlite was 75% or more similarly to test number 14, the
uniform elongation EL was a low value of less than 1 0%.
[0180]
In steel type 0 used in test number 15, the Si content was a low value of
0.93%. In addition, in steel type 0, the total content of Si and Mn was 3.04%, and
thus Formula (1) was not satisfied. Therefore, the uniform elongation ofthe coldrolled
steel sheet oftest number 15 was 8.7%, which was low in comparison to test.
numbers 1 to 10 in which, similarly to test number 15, the total area fraction of
ferrite and pearlite was 75% or more.
[0181]
In steel type P used in test number 16, the Mn content was a low value of
1.55%. Consequently, the uniform elongation of the cold-rolled steel sheet of test
number 16 was 6. 7%, which was low in comparison to test numbers 1 to 10 in which,
similarly to test number 16, the total area fraction of ferrite and pearlite was 75% or
more.
[0182]
In steel type Q used in test number 17, the Si content was a high value of
2.96%. Consequently, the uniform elongation of the cold-rolled steel sheet of test
number 17 was 7.8%, and the workability was poor in comparison to test numbers 1
to 10 in which, similarly to test number 17, the total area fraction of ferrite and
pearlite was 75% or more.
[0183]
In steel type R used in test number 18, the Mn content was a high value of
3.99%. Consequently, the uniform elongation of the cold-rolled steel sheet of test
number 18 was 3.2%, and the workability was poor.
[0184]
In steel typeS used in test number 19, the Sb content was a low value of
0.02%. Consequently, in the hot-rolled steel sheet of test number 19, the thickness
·:
of the Sb concentrated layer was less than 0.5 J..Lm, and the internal oxidized layer had
a thick thickness of 25 J..Lm.
[0185]
[Example 3]
53
Molten steels having the chemical compositions shown in Table 4 were
produced.
[0186]
Using the above described molten steel, steel materials (ingots) were
produced by an ingot-making process. Steel sheets were produced by hot rolling
the steel materials under the hot rolling conditions (heating temperature (0 C) and
finish rolling temperature FT (°C)) shown in Table 6 using a hot rolling mill for
testing. In addition, a heat treatment that simulated coiling at a coiling temperature
CT CCC) shown in Table 6 was performed on the respective steel sheets after hot
rolling. Specifically, the steel sheets were stacked and charged into a furnace that
was set to the coiling temperature CT (°C). The inside of the furnace was a
nitrogen atmosphere, and the steel sheet surface was in a state in which the surface
was blocked-off from the atmosphere. That is, the surface state of the steel sheet
was equal to the surface state of a coil obtained by actual production. After holding
the steel sheet inside the furnace for 30 minutes at the coiling temperature CT (0 C),
the steel sheet was gradually cooled to room temperature at 20°C/hour. In addition,
with respect to the steel sheets oftest numbers other than test numbers 2, 5, 7, 13 and
15, tempering was performed at a tempering temperature (0 C) and for a tempering
time period (hr) as shown in Table 6. In Table 6, the column "tempering time
period (hr)" shows the time period for which the relevant steel sheet was kept at the
tempering temperature shown in Table 6.
[0187]
[Table 6]
54
TABLE 6
Micro-structure (Area Fraction(%))
Sb Internal
Steel
Heating
FT CT
Tempering Tempering Concentrated Oxidized Scale
Test
Temperature Temperature Time Layer Layer Thickness
TS EL
Number Type (OC) (oC) (MPa) (%) Remarks (OC) (OC) Period (hr) F p B M y Thickness Thickness (Jlm)
(J.lm) (Jlm)
.. ·..
I A I250 960' 450 700 0.5 - - 88 I2 - 1.3 0 4.8 62I I2.5 Inventive Example Of Present Invention
2 A I250 980 400 - - - - 68 22 IO 0.8 0 4.8 I095 8.9 Inventive Example Of Present Invention
3 B I250 970 450 650 1.0 - - 86 I4 - 1.9 0 2.7 748 Il.3 Inventive Example Of Present Invention
4 c I250 980 450 600 8.0 - - 85 I5 - 2.5 0 4.8 78I Il.6 Inventive Example Of Present Invention
.5 c I250 950 450 - - - - 86 - I4 2.2 0 4.8 955 6.9 Inventive Example Of Present Invention
6 D I250 980 450 700 0.5 - - 88 I2 - 2.I 0 4.I 6I8 II. I Inventive Example Of Present Invention
7 D 1250 960 450 - - 1 - 88 - 11 1.8 0 4.1 948 12.3 Inventive Example Of Present Invention
8 E I250 930 440 700 0.5 - - 85 I5 - 1.7 0 5.I 610 I2.I Inventive Example Of Present Invention
9 F I250 970 I 50 700 0.5 3 - - 98 2 3.3 0 5.7 628 Il.5 Inventive Example Of Present Invention
10 G I250 955 460 700 0.5 - - 84 I6 - 1.7 0 4.7 654 I3.I Inventive Example Of Present Invention
II H I250 930 460 700 0.5 I - 89 II - 2.2 0 2.6 590 I2.2 Inventive Example Of Present Invention
I2 I I250 950 440 680 0.5 - - 89 II - 2.8 0 3.7 666 I2.5 Inventive Example Of Present Invention
13 I I250 950 450 - - - - 89 - II 2.6 0 3.7 930 7.8 Inventive Example Of Present Invention
I4 J I250 950 460 700 0.5 I - 87 II - 2.0 0 4.I 62I I0.9 Inventive Example Of Present Invention
55
15 A 1250 950 450 - - - - 88 - 12 0.8 0 4.8 1095 9.8 Inventive Example Of Present Invention
16 K 1250 950 450 700 1.0 - - 80 20 - 0.0 30 9.8 608 11.2 Comparative Example
17 L 1250 950 450 650 0.5 - - 91 9 - 5.7 0 5.1 - 12.7 Comparative Example
18 M 1200 950 450 650 1.0 - - 89 II - 0.0 22 12.3 517 10.0 Comparative Example
19 N 1250 95o·· 480 700 0.5 17 - 83 - - 2.5 0 6.9 597 9.2 Comparative Example
20 0 1250 980 450 700 0.5 3 - 86 II - 1.0 0 6.6 635 93 Comparative Example
21 p 1250 950 480 700 0.5 30 - 70 - - 1.7 0 5.7 637 7.5 Comparative Example
22 Q 1250 950 450 700 0.5 - - 88 12 - 2.5 0 4.7 587 7.9 Comparative Example
;23 R 1250 950 450 700 0.5 - - 40 60 - 1.8 0 5.6 597 3.2 Comparative Example
24 s 1200 950 450 650 1.0 - - 85 15 - 0.2 14 8.3 660 11.0 Comparative Example
,_,,,, . '·········· '··· ..
56
[0188]
[Bainite and martensite area fraction measurement test]
The area fraction of bainite and martensite in the hot-rolled steel sheet was
measured by the above described method. The results are shown in Table 6. In
the "micro-structure" columns in Table 6, "F" shows the area fraction of ferrite, "P"
shows the area fraction of pearlite, "B" shows the area fraction of bainite, "M" shows
the area fraction of martensite, and "y" shows the area fraction of austenite.
[0189]
[Internal oxidized layer thickness and scale thickness measurement test]
The internal oxidized layer thickness and scale thickness of the hot-rolled
steel sheet of each test number were measured by the same method as in Example 1.
The results are shown in Table 6.
[0190]
[Sb concentrated layer thickness measurement test]
The presence or absence of an Sb concentrated layer and the thickness (!lm)
of an Sb concentrated layer were measured for the hot-rolled steel sheet of each test
number by the same method as in Example 1. The results are shown in Table 6.
[0191]
[Tension test]
The tensile strength TS (MPa) of each test number was measured by the same
method as in Example 1. The results are shown in Table 6. In Table 6, the symbol
"-"in the "tensile strength" column indicates that cracking occurred at an edge of the
hot-rolled steel sheet and measurement was not possible.
[0192]
[Uniform elongation measurement test]
The uniform elongation EL of each test number was measured by the same
method as in Example 2. The results are shown in Table 6.
[0193]
[Test results]
Referring to Table 4 and Table 6, the chemical compositions of test numbers
1 to 15 were appropriate. In addition, the production conditions of test numbers 1
to 15 were appropriate. Consequently, in the micro-structure ofthe hot-rolled steel
57
sheets oftest numbers 1 to 15, the total area fraction of bainite and martensite was
75% or more. In the hot-rolled steel sheets oftest numbers 1 to 15, an Sb
concentrated layer having a thickness of 0.5 J.!m or more was also confirmed. As a
result, the thickness of the internal oxidized layer was 5 ).!m or less, and formation of
an internal oxidized layer was suppressed. In addition, the scale thickness of the
hot-rolled steel sheets oftest numbers 1 to 15 was 7 J.!m or less, and scale was
suppressed.
[0194]
In test numbers 1, 3, 4, 6, 8 to 12 and 14, tempering was performed.
Consequently, the tensile strength TS was 800 MPa or less and the uniform
elongation EL was 10% or more, and excellent workability was obtained after cold
rolling. On the other hand, in test numbers 2, 5, 7, 13 and 15, tempering was not
performed. Consequently, the tensile strength was 900 MPa or more and excellent
strength was obtained.
[0195]
In contrast, steel type K used in test number 16 did not contain Sb.
Consequently, an Sb concentrated layer was not formed. As a result, the thickness
of an internal oxidized layer was more than 5 ).!m, and the scale thickness was more
than 7 J.!m.
[0196]
In steel type L used in test number 17, the Sb content was 0.41 %, which was
too high. Consequently, in test number 17, cracking occurred at an edge of the hotrolled
steel sheet, and the workability was poor. Therefore a tension test could not
be performed.
[0197]
In steel type Mused in test number 18, the Sb content was 0.004%, which
was too low. Therefore, an Sb concentrated layer was not formed in the hot-rolled
steel sheet oftest number 18. Consequently, the thickness of the internal oxidized
layer was more than 5 J.!m, and the scale thickness was more than 7 ).!ill.
[0198]
58
In steel type N used in test number 19, the total content ofSi and Mn was
3.07%, and thus Formula (1) was not satisfied. Therefore, even though tempering
was performed, the uniform elongation EL was less than 10%.
[0199]
In steel type 0 used in test number 20, the Si content was a low value of
0.93%. In addition, in steel type 0, the total content of Si and Mn was 3.04%, and
thus Formula (1) was not satisfied. Consequently, even though tempering was
performed, the uniform elongation EL was less than 10%.
[0200]
In steel type P used in test number 21, the Mn content was a low value of
1.55%. Consequently, in the micro-structure, the area fraction of ferrite was 30%,
and the combined area fraction of martensite and bainite was less than 75%. As a
result, even though tempering was performed, the uniform elongation EL was less
than 10%.
[0201]
In steel type Q used in test number 22, the Si content was a high value of
2.96%. Consequently, even though tempering was performed, the uniform
elongation EL was less than 10%.
[0202]
In steel type R used in test number 23, the Mn content was a high value of
3.99%. Consequently, even though tempering was performed, the uniform
elongation EL was less than 10%.
[0203]
In steel type S used in test number 24, the Sb content was a low value of
0.02%. Therefore, the thickness of an Sb concentrated layer was less than 0.5 /-lm .
. Consequently, the thickness of an internal oxidized layer was more than 10 /-lm, and
the scale thickness was more than 7 /-lm.
[0204]
-;
Embodiments of the present invention have been described above.
However, the above described embodiments are merely examples for implementing
the present invention. Accordingly, the present invention is not limited to the above
described embodiments, and the above described embodiments can be appropriately
59
modified within a range that does not deviate from the technical scope of the present
invention.

We claim:
1. A hot-rolled steel sheet having a chemical composition consisting of, in
mass%:
C: 0.07 to 0.30%,
Si: more than 1.0 to 2.8%,
Mn: 2.0 to 3.5%,
P: 0.030% or less,
S: 0.010% or less,
AI: 0.01 to less than 1.0%,
N: 0.01% or less,
0: 0.01% or less,
Sb: 0.03 to 0.30%,
Ti: 0 to 0.15%,
V: 0 to 0.30%,
Nb: 0 to 0.15%,
Cr: 0 to 1.0%,
Ni: 0 to 1.0%,
Mo: 0 to 1.0%,
W: 0 to 1.0%,
B: 0 to 0.010%,
Cu: 0 to 0.50%,
Sn: 0 to 0.30%,
Bi: 0 to 0.30%,
Se: 0 to 0.30%,
Te: 0 to 0.30%,
Ge: 0 to 0.30%,
As: 0 to 0.30%,
Ca: 0 to 0.50%,
Mg: 0 to 0.50%,
Zr: 0 to 0.50%,
Hf: 0 to 0.50%, and
61
rare earth metal: 0 to 0.50%,
with a balance being Fe and impurities, and satisfying Formula (1): ·
Si+Mn ~ 3.20 (1)
where, a content (mass%) of a corresponding element is substituted for each
symbol of an element in Formula (1).
2. The hot-rolled steel sheet according to claim 1, containing, in mass%, one or.
more types of element selected from a group consisting of:
Ti: 0.005 to 0.15%,
V: 0.001 to 0.30%, and
Nb: 0.005 to 0.15%.
3. The hot-rolled steel sheet according to claim 1 or claim 2, containing, in
mass%, one or more types of element selected from a group consisting of:
Cr: 0.10 to 1.0%,
Ni: 0.10 to 1.0%,
Mo: 0.01 to 1.0%,
W: 0.01 to 1.0%, and
B: 0.0001 to 0.010%.
4. The hot-rolled steel sheet according to any one of claim 1 to claim 3,
containing, in mass%,
Cu: 0.10 to 0.50%.
5. The hot-rolled steel sheet according to any one of claim 1 to claim 4,
containing, in mass%,
one or more types of element selected from a group consisting of Sn, Bi, Se, Te, Ge
and As in a total amount ofO.OOOl to 0.30%.
6. The hot-rolled steel sheet according to any one of claim 1 to claim 5,
containing, in mass%,
62
one or more types of element selected from a group consisting of Ca, Mg, Zr, Hf and
rare earth metal in a total amount of 0.0001 to 0.50%.
7. The hot-rolled steel sheet according to any one of claim 1 to claim 6,
compnsmg:
an Sb concentrated layer having a thickness of 0.5 J.lm or more between a
surface and scale.
8. The hot-rolled steel sheet according to any one of claim 1 to claim 7, wherein:
in a micro-structure of the hot-rolled steel sheet, a total area fraction of ferrite
and pearlite is 75% or more; and
a tensile strength of the hot-rolled steel sheet is 800 MPa or less.
9. The hot-rolled steel sheet according to any one of claim 1 to claim 7, wherein:
in a micro-structure of the hot-rolled steel sheet, a total area fraction ofbainite
and martensite is 75% or more; and
a tensile strength of the hot-rolled steel sheet is 900 MPa or more.
10. The hot-rolled steel sheet according to any one of claim 1 to claim 7, wherein:
in a micro-structure of the hot-rolled steel sheet, a total area fraction of bainite
and martensite is 75% or more; and
a tensile strength of the hot-rolled steel sheet is 800 MPa or less.
11. The hot-rolled steel sheet according to any one of claim 1 to claim 10,
wherein:
a thickness of an internal oxidized layer of the hot-rolled steel sheet is 5 J.lm
or less.
12. The hot-rolled steel sheet according to claim 8, wherein:
a scale thickness on a surface of the hot-rolled steel sheet is 10 J.lm or less.
13. The hot-rolled steel sheet according to claim 8, wherein:
63
a decarburization layer thickness in an outer layer of the hot-rolled steel sheet
is 20 Jlm or less.
14. The hot-rolled steel sheet according to claim 9 or claim 10, wherein:
a scale thickness on a surface of the hot-rolled steel sheet is 7 Jlm or less.
15. A production method for producing a hot-rolled steel sheet according to claim
8, comprising:
a process of preparing a steel material having a chemical composition
according to claim 8;
a process ofheating the steel material to 1100 to 1350°C, and thereafter
performing hot rolling of the steel material so as to form a steel sheet; and
a process of coiling the steel sheet at a temperature of 600 to 750°C.
16. A production method for producing a hot-rolled steel sheet according to claim
9, comprising:
a preparation process of preparing a steel material having a chemical
composition according to claim 9;
a hot rolling process of heating the steel material to 1100 to 1350°C, and
thereafter performing hot rolling of the steel material so as to form a steel sheet, and
cooling the steel sheet to a coiling temperature; and
a process of coiling the steel sheet after the cooling, at a temperature of 150 to
17. A production method for producing a hot-rolled steel sheet according to claim
10, comprising:
a process of preparing a steel material having a chemical composition
according to claim 1 0;
·: ~.
a process of heating the steel material to 1100 to 1350°C, and thereafter
performing hot rolling of the steel material so as to form a steel sheet;
a process of coiling the steel sheet at a temperature of 150 to 600°C; and
64
a process of tempering the steel sheet after coiling, at a temperature of 550°C
or more.