The present invention relates to a hot-rolled steel sheet.
Priority is claimed on Japanese Patent Application No. 2020-026996, filed in
Japan on February 20, 2020, the content of which is incorporated herein by reference.
[Related Art]
[0002]
In recent years, from the viewpoint of regulations for greenhouse gas emission
in association with global warming countermeasures, there has been a demand for
additional improvement in the fuel efficiency of vehicles. In addition, in order to
reduce the weights of vehicle bodies and secure collision safety, the application of high
strength steel sheets to components for a vehicle is becoming increasingly widespread.
However, for steel sheets that are used for components for a vehicle, not only
strength but also a variety of workability that is required at the time of forming
components such as press workability or weldability are required. Specifically, from
the viewpoint of press workability or formability, bending workability and elongation
are often required for steel sheets. However, since the formability of steel sheets tends
to deteriorate with the high-strengthening of the materials, it is difficult to achieve both
a high strength and favorable formability.
Therefore, for the application of high strength steel sheets to components for a
vehicle, it has become an important problem to realize excellent bending workability
and elongation together with a high strength of a tensile strength of 980 MPa or more.
[0003]
- 1 -
It is reported in Non-Patent Document 1 that bending workability is improved
by controlling the structure to a single structure of ferrite, bainite, martensite, or the like
by microstructure control.
[0004]
Patent Document 1 discloses a method for realizing a tensile strength of 590
MPa or more and 750 MPa or less and excellent bending workability by controlling a
steel sheet containing, by mass%, 0.010% to 0.055% of C, 0.2% or less of Si, 0.7% or
less of Mn, 0.025% or less of P, 0.02% or less of S, 0.01% or less of N, 0.1% or less of
Al, and 0.06% to 0.095% of Ti to a structure including 95% or more of ferrite by an area
ratio and controlling the diameters of carbide particles containing Ti in ferrite crystal
grains and a structure in which only TiS having an average diameter of 0.5 J.lm or less is
dispersed and precipitated as a sulfide containing Ti.
[0005]
Patent Document 2 discloses a method for improving bending workability
while maintaining a tensile strength of 780 MPa or more by, for a steel sheet containing,
by mass%, 0.05% to 0.15% of C, 0.2% to 1.2% of Si, 1.0% to 2.0% of Mn, 0.04% or
less of P, 0.0030% or less of S, 0.005% to 0.10% of Al, 0.01% or less ofN, and 0.03%
to 0.13% of Ti, controlling the structure inside the steel sheet to a bainite single phase or
a structure including bainite in a fraction of more than 95% and setting, in the structure
of the steel sheet surface layer area, bainite to a fraction of less than 80% and ferrite that
is rich in workability to a fraction of 10% or more.
[0006]
Furthermore, Patent Document 3 discloses that a high- strength hot-rolled steel
sheet having a high strength of a yield strength of 960 MPa or more and excellent
bending workability and being excellent in terms of low temperature toughness can be
- 2 -
obtained by making the high-strength hot-rolled steel sheet contain, by mass%, 0.08%
to 0.25% of C, 0.01% to 1.0% of Si, 0.8% to 1.5% ofMn, 0.025% orless of P, 0.005%
or less of S, 0.005% to 0.10% of Al, 0.001% to 0.05% ofNb, 0.001% to 0.05% ofTi,
0.1% to 1.0% of Mo, and 0.1% to 1.0% of Cr and controlling the structure to a structure
in which a tempered martensite is a primary phase with a volume percentage of 90% or
more, the average grain size of prior austenite grains is 20 11m or less in a cross section
parallel to a rolling direction, the average grain size of prior austenite grains is 15 11m or
less in a cross section orthogonal to the rolling direction, and the anisotropy of the prior
y grains is reduced.
[0007]
Patent Document 4 discloses that a hot-rolled steel sheet having excellent local
deformability and a small anisotropy in bending workability can be obtained by
controlling the pole density of each orientation of a specific crystal orientation group at
the center portion in the sheet thickness, which is a range of 5/8 to 3/8 of the sheet
thickness from the steel sheet surface and setting rC, which is the Lankford value in a
direction perpendicular to a rolling direction, to 0.70 or more and 1.10 or less and r30,
which is the Lankford value in a direction at an angle of 30° with respect to the rolling
direction, to 0. 70 or more and 1.10 or less.
[Prior Art Document]
[Patent Document]
[0008]
[Patent Document 1] Japanese Unexamined Patent Application, First
Publication No. 2013-133499
[Patent Document 2] Japanese Unexamined Patent Application, First
Publication No. 2012-62558
- 3 -
[Patent Document 3] Japanese Unexamined Patent Application, First
Publication No. 2012-77336
[Patent Document 4] PCT International Publication No. W020121121219
[Non-Patent Document]
[0009]
[Non-Patent Document 1] Takahashi et al., Nippon Steel Technical Report,
"Development of High Strength Steels for Automobiles", No. 378, p. 2 top. 6, (2003).
[Disclosure of the Invention]
[Problems to be Solved by the Invention]
[0010]
As described above, in recent years, there has been a demand for increasing the
strengths of steel sheets and then further improving the bending workability and the
elongation. However, it cannot be said that both the strength and the bending
workability and the elongation are sufficiently satisfied with the above-described
techniques of Patent Documents 1 to 4.
[0011]
An object to be solved by the present invention is to provide a high-strength
hot-rolled steel sheet being excellent in terms of the bending workability and the
elongation and having a tensile strength of 980 MPa or more.
The above-described bending workability is an index indicating the
unlikelihood of cracks being initiated in a processed portion during bending or an index
indicating the unlikelihood of the cracks growing. However, in the present invention,
unlike the related art, the bending workability relates to cracks (inside bend cracks)
being initiated from the inside of the bending portion during bending.
[Means for Solving the Problem]
- 4 -
[001 2]
The present inventors studied the above-described object. As a result, it was
found that, when the microstructure includes, by volume percentage, 70% or more of
martensite, tempered martensite and bainite in total and 5 to 20% of residual austenite,
it is possible to manufacture a steel sheet having a tensile strength of 980 MPa or more
while securing workability.
[0013]
In addition, the present inventors intensively investigated the bending
workability of high strength steel sheets. As a result, it was clarified that, as the
strengths of the steel sheets increase, it becomes easier for cracks to be initiated during
bending. In addition, regarding cracking during the bending of a steel sheet, in the
related art, it is usual that cracks are initiated from the surface of the steel sheet or the
vicinity of the end surface on the outside bend, but it was found that, in association with
the high-strengthening of steel sheet, there is a case where minute cracks are initiated in
the inside bend. From findings in the related art, no method for suppressing such
minute cracks that are initiated in the inside bend (hereinafter, referred to as inside bend
cracks) has been found.
From research by the present inventors, it was found that the inside bend cracks
are likely to be initiated in steel sheets having a tensile strength of 780 MPa class or
higher, becomes significant in steel sheets having a tensile strength of 980 MPa class or
higher, and becomes a more significant problem in steel sheets having a tensile strength
of 1180 MPa class or higher.
[0014]
The present inventors presumed that the mechanism of the init iation of the
above-described inside bend cracks is attributed to the unevenness of deformation and
- 5 -
examined a method for suppressing the inside bend cracks with attention paid to texture
and uniformity of hardness.
As a result, it was found that, when the texture is relatively random,
deformation resistance is also uniform, and thus deformation is likely to occur
uniformly; however, when a specific texture develops, unevenness of deformation is
caused between crystals having an orientation in which the deformation resistance is
large and crystals having the other orientations, which promotes generation of a shear
deformation band. Conversely, when crystals having an orientation in which the
deformation resistance is large are reduced, deformation occurs uniformly and the shear
deformation band is less likely to be generated. That is, the present inventors found
that the inside bend cracks can be suppressed by controlling the texture particularly in
the surface layer region in the sheet thickness direction where cracks are initiated.
[0015]
The present invention has been made based on the above-described findings,
and the gist of the present invention is as follows.
(1) A hot-rolled steel sheet according to one aspect of the present invention
containing, as a chemical composition, by mass%, C: 0.02% to 0.30%, Si: 0.01% to
2.50%, Mn: 1.00% to 3.00%, P: 0.100% or less, S: 0.0001% to 0.0100%, Al: 0.005% to
1.000%, N: 0.010% or less, Ti: 0% to 0.20%, Nb: 0% to 0.20%, V: 0% to 0.200%, Ni:
0% to 2.00%, Cu: 0% to 2.00%, Cr: 0% to 2.00%, Mo: 0% to 2.00%, W: 0% to 0.100%,
B: 0% to 0.0100%, REM: 0% to 0.0300%, Ca: 0% to 0.0300%, Mg: 0% to 0.0300%,
and a remainder of Fe and impurities, in which the chemical composition satisfies Si +
Al 2: 1.00%, a microstructure includes, by volume percentage, 70% or more of
martensite, tempered martensite, and bainite in total and 5% to 20% of residual
austenite, in a surface layer region that is a range from a surface to a position at 1/10 of
- 6 -
a sheet thickness, a sum of an average pole density of an orientation group consisting of
{211 }<111> to { 111 }<112> and a pole density of a crystal orientation of { 110}<001> is
6.0 or less, a concentration of solid solution carbon in the residual austenite is 0.5
mass% or more, and a tensile strength is 980 MPa or more.
(2) The hot-rolled steel sheet according to (1) may contain, as the chemical
composition, by mass%, one or more selected from Ti: 0.001% to 0.20%, Nb: 0.001%
to 0.20%, V: 0.001% to 0.200%, Ni: 0.01% to 2.00%, Cu: 0.01% to 2.00%, Cr: 0.01%
to 2.00%, Mo: 0.01% to 2.00%, W: 0.005% to 0.100%, B: 0.0005% to 0.0100%, REM:
0.0003% to 0.0300%, Ca: 0.0003% to 0.0300%, and Mg: 0.0003% to 0.0300%.
(3) The hot-rolled steel sheet according to (1) or (2) may further include a hotdip
galvanized layer on the surface.
(4) The hot-rolled steel sheet according to (3), in which the hot-dip galvanized
layer may be a hot-dip galvannealed layer.
[Effects of the Invention]
[0016]
According to the above-described aspect of the present invention, it is possible
to obtain a hot-rolled steel sheet that has a tensile strength of 980 MPa or more, is
capable of suppressing the initiation of inside bend cracks, and is excellent in terms of
bending workability and elongation.
[Brief Description of the Drawings]
[0017]
FIG. 1 is a diagram showing crystallite orientation distribution functions
(ODF) in a cross section with 2 = 45° and an orientation group consisting of
{211 }<111> to { 111 }<112> and a { 110}<001> orientation.
[Embodiments ofthe Invention]
- 7 -
[0018]
Hereinafter, a hot-rolled steel sheet according to an embodiment of the present
invention (the steel sheet according to the present embodiment) will be described.
[0019]
1. Microstructure

First, the reasons for limiting the microstructure will be described.
In the steel sheet according to the present embodiment, the primary phase of
the microstructure is 70% or more of one or more selected from martensite, tempered
martensite and bainite by volume percentage. The microstructure further includes 5%
to 20% of residual austenite.
The steel sheet according to the present embodiment includes one or more
selected from martensite, tempered martensite, and bainite, which are low temperature
transformation-forming primary phases, in order to satisfy both a tensile strength (TS)
of 980 MPa or more and bending workability. When an attempt is made to increase
the strength in a structural configuration with a low total volume percentage of
martensite, tempered martensite, and/or bainite, unevenness of deformation is caused
between the above-described full hard structure and soft structures other than the abovedescribed
structure, and the bending workability deteriorates. When the total volume
percentage of martensite, tempered martensite and/or bainite is less than 70%, a
sufficient strength cannot be obtained or sufficient bending workability cannot be
obtained.
In addition, the steel sheet according to the present embodiment includes 5% or
more of residual austenite by volume percentage in order to obtain excellent elongation.
- 8 -
When the volume percentage of residual austenite is less than 5%, sufficient elongation
cannot be obtained. On the other hand, when a manufacturing condition where more
than 20% of residual austenite is left is selected, other desired structures or strengths
cannot be obtained. Therefore, the practical upper limit of the volume percentage of
residual austenite is 20%.
The remainder other than the above-described structures may be one or more of
ferrite and pearlite.
[0020]
In the present embodiment, regarding the volume percentages of pearlite,
bainite, tempered martensite, and ferrite, a sample is collected such that a cross section
in the sheet thickness direction parallel to a rolling direction of the hot-rolled steel sheet
serves as an observed section, the observed section is polished and Nital-etched, a range
of 118 to 3/8 of the sheet thickness ( 118 thickness to 3/8 thickness) from the surface in
which a position of a 114 depth ( 114 thickness) of the sheet thickness from the surface is
centered is observed using a field emission scanning electron microscope (FE-SEM) at
a magnification of 5000 times, the area ratio of each structure is measured, and the area
ratio is regarded as the volume percentage. At that time, the area ratios are measured
at 10 visual fields, and the average value thereof is regarded as the volume percentage.
[0021]
Each structure has the following characteristics. Therefore, in the
measurement of the area ratio, each structure is identified based on the following
characteristics, and the area ratio is obtained.
Ferrite is equiaxed grains containing no iron-based carbides, and pearlite is a
layered structure of ferrite and cementite.
Bainite includes upper bainite and lower bainite, and the upper bainite is a
- 9 -
aggregation of lath-shaped crystal grains and an aggregate of laths containing a carbide
between the laths. The lower bainite is a aggregation of lath-shaped crystal grains and
contains an iron-based carbide having a major axis of 5 nm or more therein, and the
carbide belongs to a single variant, that is, a group of iron-based carbides elongated in
the same direction. Here, the group of iron-based carbides elongated in the same
direction means that the difference in the elongation direction of the iron-based carbide
group is within 5°.
Tempered martensite is a aggregation of lath-shaped crystal grains and contains
an iron-based carbide with a major axis of 5 nm or more therein, and the carbide
belongs to a plurality of variants, that is, a group of iron-based carbides elongated in
two or more directions. Usually, tempered martensite refers to structures containing an
iron-based carbide such as cementite in many cases; however, in the present
embodiment, martensite including a fine precipitate containing Ti is also defined as
tempered martensite.
[0022]
Martensite (fresh martensite) and residual austenite are not sufficiently
corroded by Nital etching and thus can be clearly distinguished from the abovedescribed
structures (ferrite, pearlite, bainite, and tempered martensite) in the
observation with the FE-SEM. Therefore, the volume percentage of martensite can be
obtained as a difference between the volume percentage obtained as the area ratio of a
non-corroded region that is observed with the FE-SEM and the volume percentage of
residual austenite measured with X-rays described below.
[0023]
The volume percentage of the residual austenite is obtained by an X-ray
diffraction method. Specifically, in the cross section in the sheet thickness direction
- 10 -
parallel to the rolling direction at the 1/4 depth position of the sheet thickness of the
steel sheet, the integrated intensities of a total of six peaks of a( 11 0), a(200), a(211 ),
y(111), y(200), and y(220) are obtained using Co-Ka rays, and the volume percentage of
the residual austenite is obtained by calculation using an intensity averaging method.
However, in the steel sheet according to the present embodiment, only the total
volume percentage of martensite, tempered martensite, and bainite needs to be
specified, and it is not essential to distinguish these structures.
[0024]

When the concentration of solid solution carbon in residual austenite is set to
0.5 mass% or more, residual austenite is appropriately stabilized, a large amount of
transformation-induced plastic property (TRIP) is likely to be generated in a high strain
region in the late deformation stage, and the elongation and bending workability of the
steel sheet improve. Therefore, the concentration of solid solution carbon in residual
austenite is set to 0.5 mass% or more. The concentration of solid solution carbon in
residual austenite is preferably 0.7 mass% or more.
When the concentration of solid solution carbon in residual austenite to 2.0
mass% or less, it is possible to suppress excessive stabilization of residual austenite and
to more reliably develop transformation-induced plastic property (TRIP). Therefore,
the concentration of solid solution carbon in residual austenite is preferably set to 2.0
mass% or less.
[0025]
The concentration of solid solution carbon in residual austenite is obtained by
X-ray diffraction. Specifically, X-ray analysis with Cu-Ka rays is performed on the
- 11 -
metallographic structure at a depth of 114 of the sheet thickness from the surface of the
steel sheet in the cross section in the sheet thickness direction parallel to the rolling
direction at the central position in the sheet width direction, the lattice constant a (unit:
angstrom) is obtained from the reflection angles of the (200) plane, (220) plane, and
(3 11) plane of residual austenite, and the concentration of solid solution carbon (Cy) in
residual austenite is calculated according to the following Equation (A).
Cy = (a - 3.572)/0.033 · · · (A)
[0026]
to
{ 111 }<112> and pole density of crystal orientation of { 110}<001> being 6.0 or less>
The present inventors intensively investigated the bending workability of high
strength steel sheets. As a result, it was found that minute cracks may be initiated in
inside bends in association with the high-strengthening of the steel sheets. As a result
of additional studies, the mechanism of such inside bend cr acks is presumed as follows.
At the time of bending, compressive stress is generated in the inside bend. In
the beginning, the working proceeds while the entire inside bend is uniformly distorted;
however, as the working amount increases, deformation becomes too significant to be
carried by uniform deformation alone, and microscopic unevenness of deformation is
caused (the generation of a shear deformation band). As this shear deformation band
further grows, cracks are initiated along the shear band from the surface of the inside
bend and propagates. It is presumed that the reason for the inside bend cracks to be
more likely to be initiated in association with high- strengthening is that deterioration of
work hardening capability in association with high-strengthening makes it difficult for
uniform deformation to proceed and makes it easy for unevenness of deformation to be
- 12 -
caused, which generates a shear deformation band at an early stage of the working (or
under moderate working conditions).
When the steel sheet is bent and deformed, the strain increases toward the
surface with the center of the sheet thickness as the boundary, and the strain becomes
maximum at the outermost surface. Therefore, cracks of inside bend cracks are
initiated on the surface of the steel sheet. Since the structure of the surface layer
region that is a range from the surface of the steel sheet to 1110 of the sheet thickness in
the sheet thickness direction contributes to such initiation of cracks, the structure of the
surface layer region is controlled.
[0027]
The present inventors paid attention to the texture in order to suppress the
unevenness of deformation that acts as the cause for the inside bend cracks during
bending.
Specifically, when the steel sheet is distorted, the responsiveness of a slip
system against deformation in each crystal orientation differs (Schmid factor). This is
considered to be because deformation resistance differs in each crystal orientation.
That is, when the texture is relatively random, the deformation resistance is also
uniform, and thus deformation is likely to occur uniformly; however, when a specific
texture develops, unevenness of deformation is caused between cr ystals having an
orientation in which the deformation resistance is large and crystals having the other
orientations and a shear deformation band is likely to be generated. Conversely, it is
considered that, when crystals having an orientation in which the deformation resistance
is large are reduced, deformation occurs uniformly, and the shear deformation band is
less likely to be generated.
[0028]
- 13 -
In the steel sheet according to the present embodiment, based on the abovedescribed
idea, in a surface layer region that is a range from the surface of the steel
sheet to a position at 1110 of the sheet thickness, the sum of the average pole density of
an orientation group consisting of { 211 }<111> t o { 111 }< 112> and the pole density of a
crystal orientation of { 110}<001> is set to 6.0 or less. This makes it possible to
suppress the inside bend cracks.
In the case of a steel sheet where development of texture differs on the front
and back surfaces, if the texture that is specified in the present embodiment is satisfied
even in a range from the surface on one side to the position at 1/10 of the sheet
thickness alone, it is possible to obtain the inside bend crack suppression effect in
bending where the surface becomes the inside bend.
[0029]
The orientation group consisting of {211 }<111> to { 111 }<112> and the crystal
orientation of { 110}<001> are orientations that easily develop in the surface layer
regions of high-strength hot-rolled steel sheets produced by a common method. In
addition, these orientations are orientation groups where deformation resistance is
particularly large in inside bends during bending, and thus a shear deformation band is
likely to be generated due to a difference in deformation resistance from other
orientation groups. Therefore, the inside bend cracks can be suppressed by reducing
the pole densities of these orientation groups. Here, when only any one of the average
pole density of the orientation group consisting of {211 }<111> to { 111 }<112> and the
pole density of the crystal orientation of { 110}<001> is reduced, the effect of the
present embodiment cannot be obtained, and it is important to reduce the sum thereof.
[0030]
When the sum of the average pole density of the orientation group consisting
- 14 -
of {211 }<111> to { 111 }<112> and the pole density of the crystal orientation of
{ 110 }<001> is more than 6.0 in the surface layer region that is a range from the surface
of the steel sheet to 1110 of the sheet thickness, the shear deformation band is
significantly likely to be generated, which acts as a cause for the initiation of inside
bend cracks. In this case, R/t, which is the average value of the minimum bend radii in
an L axis and in a C axis/the sheet thickness, exceeds 1.5. Therefore, the sum thereof
is set to 6.0 or less. From this viewpoint, the sum of the average pole density of the
orientation group consisting of {211 } to { 111 }<112> and the pole density of the
crystal orientation of { 110 }<001> is preferably 5.0 or less and more preferably 4.0 or
less.
The sum of the average pole density of the orientation group consisting of
{211 } to { 111 }<112> and the pole density of the crystal orientation of
{ 110 }<001> is preferably as small as possible, but it is difficult to set the sum to less
than 0.5 in high- strength hot-rolled steel sheets of 980 MPa or more, and thus the
practical lower limit is 0.5.
[0031]
The pole density can be measured by an electron backscatter diffraction pattern
(EBSP) method. In a sample to be subjected to analysis by the EBSP method, a cut
surface parallel to the rolling direction and perpendicular to the sheet surface is
mechanically polished, and strain is removed by chemical polishing, electrolytic
polishing, or the like after the mechanical polishing. Using thi s sample, the
measurement intervals are set to 4.0 1-!m in the range from the surface of the steel sheet
to the position at 1110 of the sheet thickness, and the analysis by the EBSP method is
performed such that the measurement area becomes 150000 !-!m2 or larger.
[0032]
- 15 -
FIG. 1 shows crystallite orientation distribution functions (ODF) at a ~2 = 45o
cross section, the orientation group consisting of { 211 }<111> to { 111 }<112>, and the
orientation { 110 }<001>. The orientation group consisting of {211 }<111> to
{ 111 }<112> refers to a range where the texture analysis is Bunge-expressed and the
crystallite orientation distribution functions (ODF) at the ~2 = 45° cross section are ~ 1 =
85° to goo, = 30° to 60°, and ~2 = 45o. The average pole density of this orientation
group is calculated in the above-described range shown in FIG. 1. Strictly speaking,
the {211 }<111> to { 111 }<112> orientation group is a range of ~1 =goo, = 30° to
60°, and ~2 = 45° on ODE However, since there is a measurement error arising from
test piece working or sample setting, in the steel sheet according to the present
embodiment, the average pole density is calculated in a range of ~1 = 85° to goo, =
30° to 60°, and ~2 = 45°. In the following average pole density analyses as well,
angular ranges from which the average value is taken are determined in the same
manner in consideration of a measurement error arising from test piece working or
sample setting.
Similarly, the pole density of the crystal orientation of { 110}<001> refers to a
range where the crystallite orientation distribution functions (ODF) at the ~2 = 4SO cross
section are ~1 = 85° to goo, = 85° to goo, and ~2 = 45o. The pole density of this
crystal orientation is calculated in the above-described range shown in FIG. 1.
[0033]
Here, for the crystal orientation of the rolled sheet, a lattice plane parallel to the
sheet surface is normally expressed by (hkl) or {hkl}, and an orientation parallel to the
rolling direction is expressed by [uvw] or . {hkl} and are general terms
for equivalent lattice planes and directions, and (uvw) and [hkl] refer to individual
lattice planes and directions. That is, in the steel sheet according to the present
- 16 -
embodiment, the bee structure is covered, and thus, for example, (110), (-110), (1-10), (-
1-10), (101), (-101), (10-1), (-10-1), (011), (0-11), (01 -1), and (0-1-1) are equivalent
lattice planes and cannot be distinguished. In this case, these lattice planes are
collectively referred to as { 110}.
[0034]
2. Chemical composition
Hereinafter, the chemical composition of the steel sheet according to the
present embodiment will be described in detail.
Numerical limitation ranges described below using "to" include values at both
ends of the ranges as the lower limit value and the upper limit value. However,
numerical values expressed with 'more than' or 'less than' are not included in numerical
ranges. "%" relating to the amount of each element indicates "mass%" unless
otherwise described.

1. A hot-rolled steel sheet comprising, as a chemical composition, by mass%:
C: 0.02% to 0.30%;
Si: 0.01% to 2.50%;
Mn: 1.00% to 3.00%;
P: 0.100% or less;
S: 0.0001% to 0.0100%;
Al: 0.005% to 1.000%;
N: 0.010% or less;
Ti: 0% to 0.20%;
Nb: 0% to 0.20%;
V: 0% to 0.200%;
Ni: 0% to 2.00%;
Cu: 0% to 2.00%;
Cr: 0% to 2.00%;
Mo: 0% to 2.00%;
W: 0% to 0.100%;
B: 0% to 0.0100%;
REM: 0% to 0.0300%;
Ca: 0% to 0.0300%;
Mg: 0% to 0.0300%; and
a remainder of Fe and impurities,
wherein the chemical composition satisfies Si + Al 2: 1.00%,
a microstructure includes, by volume percentage, 70% or more of martensite,
tempered martensite, and bainite in total and 5% to 20% of residual austenite,
- 52 -
in a surface layer region that is a range from a surface to a position at 1110 of a
sheet thickness, a sum of an average pole density of an orientation group consisting of
{ 211 }< 111 > to { 111 }<112> and a pole density of a crystal orientation of { 110 }<001 > is
6.0 or less,
a concentration of solid solution carbon in the residual austenite is 0.5 mass%
or more, and
a tensile strength is 980 MPa or more.
2. The hot-rolled steel sheet according to Claim 1, comprising, as the chemical
composition, by mass%, one or more selected from:
Ti: 0.001% to 0.20%;
Nb: 0.001% to 0.20%;
V: 0.001% to 0.200%;
Ni: 0.01% to 2.00%;
Cu: 0.01% to 2.00%;
Cr: 0.01% to 2.00%;
Mo: 0.01% to 2.00%;
W: 0.005% to 0.100%;
B: 0.0005% to 0.0100%;
REM: 0.0003% to 0.0300%;
Ca: 0.0003% to 0.0300%; and
Mg: 0.0003% to 0.0300%.
3. The hot-rolled steel sheet according to Claim 1 or 2, further comprising:
a hot-dip galvanized layer on the surface.
4. The hot-rolled steel sheet according to Claim 3,
wherein the hot-dip galvanized layer is a hot-dip galvannealed layer.