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[Document Type] SPECIFICATION
[Title of Invention] HOT-ROLLED STEEL SHEET AND PRODUCTION
METHOD THEREOF
[Technical Field]
[0001]
The present invention relates to a hot-rolled steel sheet which is excellent in
local deformability such as bendability, stretch flangeability, burring workability, and
hole expansibility, and azimuthal dependence of formability is small, and which is
excellent in ductility, and a production method thereof Particularly, the present
invention relates to a steel sheet using a Transformation Induced Plasticity (TRIP)
phenomenon.
Priority is claimed on Japanese Patent Application No. 2011-070725, filed on
March 28, 2011, and the content of which is incorporated herein by reference.
[Background Art]
[0002]
So as to suppress carbon dioxide emissions from vehicles, weight saving of a
vehicle body using a high-strength steel sheet has been in progress. In addition, in
order to secure safety of passengers, besides a soft steel sheet, a high-strength steel
sheet is frequently used for the vehicle body. Furthermore, for the weight saving of
the vehicle body to progress in the fiiture, it is necessary to increase a strength of the
high-strength steel sheet in use fiirther than that of the related art. Accordingly, for
example, so as to use the high-strength steel sheet for underbody components, it is
necessary to improve local deformability for a burring process.
[0003]
However, generally, when the strength of a steel sheet is increasing.
- 1
formability decreases. Therefore, uniform elongation, that is important for drawing
or stretching, decreases. In contrast, Non-Patent Document 1 discloses a method of
securing the uniform elongation by making austenite remain in a steel sheet.
[0004]
Furthermore, Non-Patent Document 1 also discloses a method of controlling a
metallographic structure of the steel sheet to improve local ductility that is necessary
for a bending, a hole expanding process, or a burring process. In addition, Non-
Patent Document 2 discloses that reduction of a difference in hardness between
microstructures by controlling inclusions so as to control the microstructures into a
single structure is effective for bendability or the hole expanding process.
[0005]
For coexistence between ductility and strength, Non-Patent Document 3
discloses a technology of obtaining an appropriate fraction of ferrite and bainite. In
the technology, a metallographic structure control is performed by a cooling control
after hot rolling, precipitates and a transformation structure are to be controlled.
However, all of the methods are improving methods of local deformability depending
on the structure control (a categorical microstructure control), and thus local
deformability is greatly affected by a base structure.
[0006]
On the other hand, Non-Patent Document 4 discloses a technology of
improving a material quality of a hot-rolled steel sheet by increasing a rolling
reduction amount in a continuous hot rolling process. This technology is a so-called
grain refinement technology. In Non-Patent Document 4, large rolling reduction is
performed at a very low temperature in an austenite region to transform nonrecrystallized
austenite into ferrite. According to this, the grains of ferrite that is a
main phase of a product is refined, and thus strength and toughness are increased.
However, in the production method disclosed in Non-Patent Document 4, an
improvement of local deformability and ductility is not considered.
[0007]
As described above, for improving local deformability of the high-strength
steel sheet, the structure control including inclusions is mainly performed .
[0008]
In jaddition, in order to use the high-strength steel sheet as members for
vehicles, the balance of strength and ductility is needed . For this requirement,
hitherto, a TRIP steel sheet, in which transformation induced plasticity of retained
austenite is used, is suggested (for example, refer to Patent Document 1 and Patent
Document 2).
[0009]
However, the TRIP steel has characteristics in which strength and ductility are
excellent, but generally, local deformability such as hole expansibility is low.
Therefore, it is necessary for local deformability such as hole expansibility to be
improved so as to use the TRIP steel, for example, as a high-strength steel sheet of
underbody components.
[Citation List]
[Patent Literature]
[0010]
[Patent Document 1] Japanese Unexamined Patent Application, First
Publication No. S61-217529
[Patent Document 2] Japanese Unexamined Patent Application, First
Publication No. H5-59429
- 3 -
[Non-Patent Literature]
[0011]
[Non-Patent Document 1] Takahashi et al, Nippon Steel Technical Report
(2003) No. 378, R 7
[Non-Patent Document 2] Kato et al., Iron-Making research (1984) vol. 312,
P41
[Non-Patent Document 3] K. Sugimoto et al., ISIJ International (2000) Vol.
40, p. 920
[Non-Patent Document 4] NFG product introduction of NAKAYAMA
STEEL WORKS, LTD.; http://www.nakayama-steel.co.jp/menu/product/nfg.html
[Summary of Invention]
[Problem to be Solved by the Invention]
[0012]
The present invention is an object to provide a high-strength hot-rolled steel
sheet of TRIP steel, which is excellent in local deformability, in which azimuthal
dependence of formability is small, and which is excellent in ductility in TRIP steel,
and a production method thereof In addition, the present invention is an object to
provide a production method of a high-strength hot-rolled steel sheet in which
anisotropy of the hot-rolled steel sheet is improved by controlling a texture through hot
rolling.
[Means for Solving the Problems]
[0013]
The present inventors have found that in the TRIP steel, when a pole density
of a predetermined crystal orientation is appropriately controlled, local deformability is
improved. In addition, the present inventors have succeeded in producing a steel
sheet which is excellent in local deformability and other mechanical properties by
optimizing chemical components and production conditions of the TRIP steel so as to
control a microstructure of the steel sheet.
The essence of the present invention is as follows.
[0014]
(1) According to an aspect of the present invention, there is provided a hotrolled
steel sheet being a steel sheet having a chemical composition, by mass%, of C:
0.02% to 0.5%, Si: 0.001% to 4.0%, Mn: 0.001% to 4.0%, Al: 0.001% to 4.0%, P:
0.15% or less, S: 0.03% or less, N: 0.01% or less, O: 0.01% or less, and the balance
consisting of Fe and unavoidable impurities, in which a sum of a content of the Si and
a content of the Al is 1.0% to 4.0% in the chemical composition of the steel sheet, an
average pole density of an orientation group from {100}<011> to {223}<110>, which
is apole density expressed by an arithmetic average of pole densities of respective
crystal orientations {100}<011>, {116)<110>, {114}<110>, {112}<110>, and
{223}<110>, is 1.0 to 6.5, and apole density of a crystal orientation {332}<113> is 1.0
to 5.0 in a sheet-thickness central portion within a range of 5/8 to 3/8 of a sheet
thickness, a microstructure of the steel sheet includes a plurality of grains, the
microstructure of the steel sheet includes, by an area ratio, 2% to 30% of retained
austenite, 20%o to 50% of ferrite, 10% to 60% of bainite, 20% or less of pearlite, and
20%) or less of martensite, and rC that is a Lankford value in a direction orthogonal to a
rolling direction is 0.70 to 1.10, and r30 that is a Lankford value in a direction forming
an angle of 30° with the rolling direction is 0.70 to 1.10.
[0015]
(2) In the hot-rolled steel sheet according to (1), in which the chemical
composition of the steel sheet may further includes, by mass%, one or more selected
from the group consisting of Ti: 0.001% to 0.2%, Nb: 0.001% to 0.2%, V: 0.001% to
1.0%, W: 0.001% to 1.0%, Cu: 0.001% to 2.0%, B: 0.0001% to 0.005%, Mo: 0.001%
to 1.0%, Cr: 0.001% to 2.0%, As: 0.0001% to 0.50%, Mg: 0.0001% to 0.010%, REM:
0.0001% to 0.1%, Ca: 0.0001% to 0.010%), Ni: 0.001% to 2.0%), Co: 0.0001%) to 1.0%),
Sn: 0.0001% to 0.2%, and Zr: 0.0001% to 0.2%.
[0016]
(3) In the hot-rolled steel sheet according to (1) or (2), a volume average
diameter of the grains may be 1 ^m to 4 i^m.
[0017]
(4) In the hot-rolled steel sheet according to (1) or (2), the average pole
density of the orientation group from {100}<011> to {223}<110> may be 1.0 to 5.0,
and the pole density of the crystal orientation {332}<113> may be 1.0 to 4.0.
[0018]
(5) In the hot-rolled steel sheet according to any one of (1) to (4), among the
plurality of grains, an area ratio of grains which exceed 20 |j,m may be limited to 10%i
or less.
[0019]
(6) In the hot-rolled steel sheet according to any one of (1) to (5), with regard
to at least 100 grains of the retained austenite and the martensite, a standard deviation
of a distance LMA between the grains closest to each other may be 5 \xm or less.
[0020]
(7) According to the hot-rolled steel sheet related to the aspect of the present
invention, there is provided a production method of the hot-rolled steel sheet, the
production method may have: a first hot-rolling process of performing a hot-rolling
with respect to a steel, so as to set an average austenite grain size of the steel to 200
|am or less, the first hot-rolling process includes, in which a pass is performed, at least
one or more times, with a rolling reduction ratio of 40%) or more, in a temperature
range of 1,000°C to 1,200°C, the steel includes a chemical composition which includes,
by mass%, C: 0.02% to 0.5%, Si: 0.001% to 4.0%, Mn: 0.001% to 4.0%, Al: 0.001% to
4.0%, P: 0.15% or less, S: 0.03% or less, N: 0.01% or less, O: 0.01% or less, and the
balance consisting of Fe and unavoidable impurities, and a sum of a content of the Si
and a content of the Al is 1.0% to 4.0%, a second hot rolling process of performing the
hot-rolling with respect to the steel, the second hot-rolling process includes, in which
large-rolling-reduction passes with a rolling reduction ratio of 30% or more in a
temperature range of Tl + 30°C to Tl + 200°C when a temperature calculated by the
following Expression 1 is set to T1°C, an accumulative rolling reduction ratio in a
temperature range of Tl + 30°C to Tl + 200°C is 50%) or more, an accumulative
rolling reduction ratio in a temperature range, that is higher than or equal to Ar3°C and
lower than Tl + 30°C, is limited to 30% or less, and a rolling terminal temperature is
Ar3°C or higher; a primary cooling process of performing a cooling with respect to the
steel, in which a standby time t (second), which is set as a time from a completion of
the final pass among the large-rolling-reduction passes to a cooling start, satisfies the
following Expression 2; a secondary cooling process of performing a cooling with
respect to the steel, in which the steel is cooled to a temperature T3 within a range of
630°C to 800°C at an average cooling rate of 10°C/s to 100°C/s; a retention process of
performing a retaining, in which the steel is retained within the temperature range of
630°C to 800°C for 1 second to 20 seconds, or a slow cooling process of a slow
cooling with respect to the steel, in which the steel is slowly cooled from the
temperature T3 to a temperature range within lower than T3 and higher than or equal to
550°C at an average cooling rate of 20°C/s or less; a winding process of performing a
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winding of the steel in a temperature range of 350°C to 500°C; and an air cooling
process of performing a cooling of the steel with air, in which the steel, which is
retained at a temperature range of 350°C to 500°C for 30 minutes to 300 minutes, is
then cooled by the air. Herein,
Tl = 850 + 10 X ([C] + [N]) X [Mn] (Expression 1)
Here, [C], [N], and [Mn] represent mass percentages of the content of C, the
content of N, and the content of Mn in the steel, respectively.
t < 2.5 X tl (Expression 2)
Here, tl is expressed by the following Expression 3.
tl = 0.001 X ((Tf-Tl) X Pl/100)^ - 0.109 x ((Tf- Tl) x Pl/100) + 3.1
(Expression 3)
Here, Tf represents a Celsius temperature of the steel at the time of completion
of the final pass, and PI represents a percentage of the rolling reduction ratio during
the final pass.
[0021]
(8) In the production method of the hot-rolled steel sheet according to (7), the
production method may have, in which the steel may include the chemical composition
which further may include, by mass%, one or more selected from the group consisting
of Ti: 0.001% to 0.2%, Nb: 0.001% to 0.2%, V: 0.001% to 1.0%, W: 0.001% to 1.0%,
Cu: 0.001% to 2.0%, B: 0.0001% to 0.005%, Mo: 0.001% to 1.0%, Cr: 0.001% to
2.0%, As: 0.0001% to 0.50%, Mg: 0.0001% to 0.010%, REM: 0.0001% to 0.1%, Ca:
0.0001% to 0.010%, Ni: 0.001% to 2.0%, Co: 0.0001% to 1.0%, Sn: 0.0001% to 0.2%,
and Zr: 0.0001% to 0.2%, and in which a temperature calculated by the following
Expression 4 in place of the temperature calculated by the Expression 1 may be set as
Tl.
- 8 -
Tl = 850 + 10 X ([C] + [N]) X [Mn] + 350 x [Nb] + 250 x [Ti] + 40 x [B] +
10 X [Cr] + 100 x[Mo] + 100 x [V] (Expression 4)
Here, [C], [N], [Mn], [Nb], [Ti], [B], [Cr], [Mo], and [V] represent mass
percentages of the content of C, the content of N, the content of Mn, the content of Nb,
the content of Ti, the content of B, the content of Cr, the content of Mo, and the content
of V in the steel, respectively.
[0022]
(9) In the production method of the hot-rolled steel sheet according to (7) or
(8), in which the standby time t (second) may further satisfy the following Expression
5 using tl.
t < tl (Expression 5)
[0023]
(10) In the production method of the hot-rolled steel sheet according to (7) or
(8), in which the standby time t (second) may ftirther satisfy the following Expression
6 using tl.
tl < t < tl X 2.5 (Expression 6)
[0024]
(11) In the production method of the hot-rolled steel sheet according to any
one of (7) to (10), in the primary cooling process, the average cooling rate may be
50°C/s or more, a cooling temperature variation that is a difference between a steel
temperature at the start time of cooling and a steel temperature at the finish time of
cooling may be 40°C to 140°C, and the steel temperature at the finish time of the
cooling may be Tl + 100°C or lower.
[0025]
(12) In the production method of the hot-rolled steel sheet according to any
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one of (7) to (11), wherein the final pass of rolling within a temperature range of Tl +
30°C to Tl + 200°C may be the large-rolling-reduction pass.
[0026]
(13) In the production method of the hot-rolled steel sheet according to any
one of (7) to (12), wherein in the temperature range control, a temperature variation
rate may be -40°C/h to 40°C/h.
[0027]
(14) In the production method of the hot-rolled steel sheet according to any
one of (7) to (13), wherein the primary cooling process may be performed between
rolling stands.
[Effects of the Invention]
[0028]
According to the aspects of the present invention, it is possible to provide a
high-strength hot-rolled steel sheet which is excellent in local deformability such as
bendability, stretch flangeability, burring workability, and hole expansibility, and in
which azimuthal dependence of formability is small, and which is excellent in ductility,
and a production method thereof When the steel sheet is used, particularly, weight
saving of vehicles and collision safety of vehicles may be compatible with each other,
and thus industrial contribution is significant.
[Brief Description of the Drawings]
[0029]
FIG. 1 is a diagram illustrating a relationship between an average pole density
of an orientation group from {100}<011> to {223}<110> and d/RmC (sheet thickness
d/minimum bend radius RmC).
FIG. 2 is a diagram illustrating a relationship between a pole density of an
- 10 -
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orientation {332}<113> and d/RmC.
FIG. 3 is a diagram illustrating a relationship between an r value (rC) in a
direction orthogonal to a rolling direction and d/RmC.
FIG. 4 is a diagram illustrating a relationship between an r value (r30) in a
direction forming an angle of 30° with the rolling direction and d/RmC.
FIG. 5 is a diagram illustrating a relationship between the number of rolling
times of 40% or more in rough rolling and an austenite grain size of the rough rolling.
FIG. 6 is a flowchart illustrating the outline of a production method of the hotrolled
steel sheet related to an embodiment of the present invention.
[Description of Embodiments]
[0030]
As described above, according to the finding in the related art, hole
expansibility, bendability, and the like are improved by an inclusion control,
precipitates refinement, homogenization of a microstructure, a single phase structure
control, and a reduction in hardness difference between microstructures, and the like.
However, with only these technologies, there is no choice but to limit a main structure
configuration. Furthermore, for high strength, when representative elements such as
Nb and Ti that is largely contribute to an increase in strength are added, anisotropy
largely increases. Therefore, there is a problem in that another formability factor may
be sacrificed, or a direction of punching blanks before formation is limited.
Therefore, a usage of the steel sheet is limited.
[0031]
In the TRIP steel sheet which is one of technologies to increase ductility,
during an annealing process, by means of concentration of C in austenite, and thus an
amount of retained austenite or the amount of C in the retained austenite increases.
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Accordingly, tensile strength is improved.
[0032]
Therefore, concerning with the TRIP steel sheet, the present inventors have
made an examination and a research about grain refinement of a microstructure and a
texture control in a hot rolling process so as to improve bending workability and hole
expansibility. As a result, they have made clear that local deformability of the TRIP
steel sheet may be sufficiently improved, by controlling the pole density of crystal
orientation, to be described later. In addition, the present inventors have made clear
that particularly local deformability of the TRIP steel sheet is dramatically improved,
in a case where rC that is a Lankford value (r value) in a direction orthogonal to a
rolling direction, and r30 that is a Lankford value (r value) in a direction forming an
angle of 30° with the rolling direction are in balance with each other.
[0033]
Hereinafter, the hot-rolled steel sheet related to an embodiment of the present
invention will be described in detail.
First, the pole density of the crystal orientation of the hot-rolled steel sheet
will be described.
[0034]
Pole Density (Dl and D2) of Crystal Orientation:
In the hot-rolled steel sheet related to the embodiment, as pole densities of
two kinds of crystal orientations, with respect to a sheet-thickness cross-section, which
is parallel with a rolling direction, at a sheet-thickness central portion within a range of
5/8 to 3/8 of the sheet thickness (that is a range distant from a surface of the steel sheet
by a distance of a range of 5/8 to 3/8 of the sheet thickness in a sheet-thickness
direction (depth direction) of the steel sheet), an average pole density Dl of an
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orientation group from {100}<011> to {223}<110> (hereinafter, may be abbreviated
as an average pole density), and a pole density D2 of a crystal orientation {332}<113>
are controlled.
In the embodiment, the average pole density is a characteristic (an orientation
integration degree, a development degree of a texture) of a particularly important
texture (a crystal orientation of a grain in a microstructure). In addition, the average
pole density is a pole density expressed by an arithmetic average of pole densities of
respective crystal orientations {100}<011>, {116}<110>, {114}<110>, {112}<110>,
and {223}<110>.
With respect to a cross-section at a sheet-thickness central portion within a
range of 5/8 to 3/8 of a sheet thickness, Electron Back Scattering Diffraction (EBSD)
or X-ray diffraction is performed to obtain intensity ratios of electron diffraction
intensity or X-ray diffraction intensity of respective orientations for a random sample,
and the average pole density of an orientation group from {100}<011> to {223}<110>
may be obtained from the respective intensity ratios.
When the average pole density of the orientation group from {100}<011> to
{223}<110> is 6.5 or less, d/RmC (an index obtained by dividing a sheet-thickness d
by minimum bend radius RmC (C-direction bending)), which is necessary for
processing of underbody components or skeleton components, may satisfy 1.5 or more.
This condition is one condition for satisfying the following two conditions, particularly,
one is between tensile strength TS and hole expansion ratio A., and another is between
tensile strength TS and elongation EL, which are necessary for underbody members,
that is, TSxX > 30,000 and TSxEL > 14,000. Furthermore, when the average pole
density is 5.0 or less, a ratio (Rm45/RmC) of minimum bend radius Rm45 of 45°-
direction bending to minimum bend radius RmC of C-direction bending, which is an
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^^k
index of azimuthal dependence (isotropy) of formability, decreases, and thus high local
deformability that does not depend on a bending direction may be secured.
Accordingly, the average pole density may be 6.5 or less, and preferably 5.0 or less.
In a case where further excellent hole expansibility or further smaller bending limit
characteristic is necessary, the average pole density is more preferably less than 4.0,
and still more preferably less than 3.0.
When the average pole density of the orientation group from {100}<011> to
{223}<110> exceeds 6.5, anisotropy of mechanical properties of the steel sheet
significantly increases. As a result, local deformability only in a specific direction is
improved, but local deformability in other directions different from the direction
significantly deteriorates. Therefore, in this case, the steel sheet may not satisfy
d/RmC > 1.5 as shown in FIG. 1.
[0035]
On the other hand, when the average pole density is less than 1.0, there is a
concern that local deformability deteriorates. Therefore, it is preferable that the
average pole density is 1.0 or more.
[0036]
Furthermore, from the same reason, the pole density of the crystal orientation
{332}<113> at the sheet-thickness central portion within a range of 5/8 to 3/8 of the
sheet thickness is set to 5.0 or less. This condition is one condition in which the steel
sheet safisfies d/RmC >1.5. Particularly, the condition is one condition for safisfying
the following two conditions between tensile strength TS and hole expansion ratio X,
and tensile strength TS and elongation EL, which are necessary for underbody
members, that is, both TSxX > 30,000 and TSxEL > 14,000.
Furthermore, when the pole density is 4.0 or less, TSX?L or d/RmC may be
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further increased. Accordingly, it is preferable that the pole density is 4.0 or less, and
more preferably 3.0 or less. When the pole density exceeds 5.0, anisotropy of
mechanical properties of the steel sheet significantly increases. As a result, local
deformability only in a specific direction is improved, but local deformability in other
directions different from the direction significantly deteriorates. Therefore, in this
case, the steel sheet may not reliably satisfy d/RmC > 1.5 as shovm in FIG. 2.
On the other hand, when the pole density is less than 1.0, there is a concern
that local deformability deteriorates. Therefore, it is preferable that the pole density
of the crystal orientation {332}<113> is 1.0 or more.
[0037]
The pole density has the same meaning as an X-ray random intensity ratio.
The X-ray random intensity ratio is a numerical value obtained by dividing diffraction
intensity of a sample material by diffraction intensity of a standard sample not having
integration in a specific orientation. The diffraction intensity (X-ray or electron) of
the standard sample, and the diffraction intensity of the sample material may be
obtained by measurement using an X-ray diffraction method and the like under the
same conditions. The pole density may be measured using X-ray diffraction. Electron
Back Scattering Diffraction (EBSD), or electron channeling. For example, the pole
density of the orientation group from {100}<011> to {223}<110> may be obtained as
follows. The pole densities of respective orientations {100}<011>, {116}<110>,
{114}<110>, {112}<110>, and {223}<110> are obtained from a three-dimensional
texture (ODF) calculated by a series-expanding method using a plurality of pole
figures among the pole figures {110}, {100}, {211}, and {310} measured by the
methods, and these pole densities are arithmetically averaged to obtain the pole density
of the orientation group {100}<011>to {223}<110>.
15 -
[0038]
With respect to the sample that is provided for the X-ray diffiaction, the
EBSD, and the electron channeling, the thickness of the steel sheet may be reduced by
mechanical polishing or the like to a predetermined sheet thickness. Next, at the
same time to may remove a strain by chemical polishing, electrolysis polishing, or the
like, the sample may be adjusted in order for an appropriate surface including a range
of 5/8 to 3/8 of the sheet thickness to be a measurement surface. And the pole density
may be measured according to the above-described methods. With regard to a sheet
width direction, it is preferable that the sample is collected in the vicinity of 1/4 or 3/4
of a sheet-thickness position (a position is distant from an end surface of the steel sheet
by a distance that is 1/4 of a sheet-width of the steel sheet).
[0039]
With regard to not only the sheet-thickness central portion but also as many as
possible sheet-thickness positions, when the steel sheet satisfies the above-described
pole density, local deformability is further improved. However, the orientation
integration of the above-described sheet-thickness central portion is the strongest, and
an effect on the anisotropy of the steel sheet is large, and thus the material quality of
the sheet-thickness central portion is generally representative of material properties of
the entirety of the steel sheet. Accordingly, the average pole density of the orientation
group from {100}<011>to{223} < 110> and the pole density of the crystal orientation
{332} in a range of 5/8 to 3/8 of the sheet thickness central portion are specified.
[0040]
Here, {hkl} represents that when the sample is collected by the abovedescribed
method, a normal direction of a sheet surface is parallel with , and a
rolling direction is parallel with . In addition, with regard to a crystal
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orientation, an orientation that is commonly orthogonal to the sheet surface is
expressed by (hkl) or {hkl}, and an orientation that is parallel with the rolling direction
is expressed by [uvw] or . {hkl} collectively represents equivalent
planes, and (hkl)[uvw] represents individual crystal plane. That is, in the
embodiment, since a body centered cubic structure (bcc structure) is a target, for
example, respective planes (111), (-111), (1-11), (11-1), (-1-11), (-11-1), (1-1-1), and (-
1-1-1) are equivalent, and thus are not distinguishable. In this case, these orientations
are collectively called a plane {111}. The ODF expression is also used for orientation
expression of other crystal structures having a low symmetric property, and thus in the
ODF expression, an individual orientation is generally expressed by (hkl)[uvw].
However, in the embodiment, {hkl} and (hkl)[uvw] have the same meaning.
[0041]
r Value (rC) in Direction Orthogonal to Rolling Direction:
The r value (Lankford value) of the steel sheet is important in the embodiment.
That is, as a result of the intensive investigation by the present inventors, as shown in
FIG. 3, the present inventors have found that when the respective pole densities, which
are described above, are set within the above-described ranges, at the same time, and
rC is set to 0.70 or more, good hole expansibility and good bendability may be
obtained. Accordingly, rC may be 0.70 or more.
The upper limit of rC may be 1.10 or less to obtain fiirther excellent hole
expansibility and bendability.
[0042]
r Value (r30) in Direction Having Angle of 30° with Rolling Direction:
The r value (Lankford value) of the steel sheet is important in the present
invention. That is, as a result of the intensive investigation by the present inventors.
17
as shown in FIG. 4, the present inventors have found that when the respective pole
densities, which are described above, are set within the above-described ranges, at the
same time, and r30 is set to 1.10 or less, good hole expansibility and good bendability
may be obtained. Accordingly, r30 may be 1.10 or less.
The lower limit of r30 may be 1.10 to obtain fiirther excellent hole
expansibility and bendability.
[0043]
The above-described r value is evaluated by a tensile test using a tensile test
specimen of JIS No. 5. In consideration of a common high-strength steel sheet, the r
value may be evaluated within a range in which tensile strain is within a range of 5%
to 15% and within a range in which corresponds to uniform elongation.
[0044]
However, generally, it is known that the texture and the r value have a
correlation with each other, but in the hot-rolled steel sheet related to the embodiment,
as already mentioned, the limitation for the pole density of the crystal orientation and
the limitation for the r value are different from each other. Therefore, when both of
the limitations are satisfied concurrently, good local deformability may be obtained.
[0045]
Next, a microstructure of the hot-rolled steel sheet related to the embodiment
will be described.
A basic microstructure of the hot-rolled steel sheet related to the embodiment
includes ferrite, bainite, and retained austenite. In the embodiment, in addition to the
basic components of the microstructure (in place of a part of ferrite, bainite, and
retained austenite), one or more kinds of pearlite and martensite (including tempered
martensite) may be included in the microstructure as a selective component of the
18
microstructure as necessary or in an unavoidable manner. In addition, in the
embodiment, an individual microstructure is evaluated by an area ratio.
[0046]
Ferrite and bainite concentrate C in the retained austenite, and thus ferrite and
bainite are necessary for improvement of ductility by the TRIP effect. Furthermore,
ferrite and bainite also contribute to improvement of hole expansibility. The fraction
of ferrite and the fraction of bainite may be allowed to vary depending on a strength
level that is an aim of development, when ferrite is set to from 20% to 50% and bainite
is set to from 10% to 60%, excellent ductility and excellent hole expansibility are
capable of being obtained. Accordingly, ferrite is set to from 20% to 50%, and
bainite is set to from 10% to 60%.
[0047]
The retained austenite is a structure that increases ductility, particularly,
uniform elongation by transformation induced plasticity, and it is necessary for the
retained austenite to be 2% or more in terms of an area ratio. In addition, the retained
austenite is transformed to martensite by processing, and also contributes to
improvement of strength. The higher the area ratio of the retained austenite is, the
more preferable. However, it is necessary to increase the content of C and Si so as to
secure retained austenite exceeding 30% in terms of an area ratio, and in this case,
weldability or surface qualities deteriorate. Therefore, the upper limit of the area
ratio of the retained austenite is set to 30% or less. In addition, in a case where it is
necessary to fiarther increase the uniform elongation, it is preferable that the retained
austenite is 3% or more, more preferably 5% or more, and still more preferably 8% or
more.
[0048]
19 -
In addition, the microstructure may contain each of pearlite and martensite
(including tempered martensite) in a fraction of 20%. When the amount of pearlite
and martensite increases, workability and local deformability of the steel sheet
decrease, or a utilization rate of C, that generates retained austenite, decreases.
Therefore, in the microstructure, pearlite is limited to 20% or less, and martensite is
limited to 20% or less.
[0049]
Here, the area ratio of austenite may be determined from diffraction intensity
that may be obtained by performing X-ray diffraction with respect to a plane, which is
parallel with a sheet surface, in the vicinity of 1/4 sheet-thickness position.
In addition, the area ratio of ferrite, pearlite, bainite, and martensite may be
determined from an image that may be obtained by observing 1/8 to 3/8 of sheetthickness
range (that is, a sheet-thickness range in which 1/4 sheet-thickness position
becomes the center) using a Field Emission Scanning Electron Microscope (FE-SEM).
In the FE-SEM observation, a sample is collected in such a manner that a sheetthickness
cross-section which is parallel with the rolling direction of the steel sheet
becomes an observation surface, and polishing and Nital etching are performed with
respect to the observation surface.
In addition, with regard to the sheet-thickness direction, in the vicinity of the
surface of the steel sheet and in the vicinity of the center of the steel sheet, the
microstructure (component) of the steel sheet may be largely different from other
portions due to decarburization and Mn segregation. Therefore, in the embodiment,
the observation of the microstructure is performed at the 1/4 of sheet-thickness position,
which is the reference.
[0050]
- 20 -
#
Furthermore, in a case of further improving the elongation, the size of the
grain in the microstructure, particularly, a volume average diameter may be made fine.
Furthermore, by making refinement of the volume average diameter, and thus fatigue
properties (fatigue limit ratio) that are necessary for steel sheets for vehicles are
improved.
The number of coarse grains has a high influence rate on the elongation
compared to fine grains, and thus the elongation has a close correlation with a volumeaverage
diameter calculated as a weighted average of the volume compared to a
number-average diameter. Therefore, in a case of obtaining the above-described
effect, the volume-average diameter may be fi^om 1 |j.m to 15 pirn, preferably fi-om 1
[im to 9.5 |xm, and more preferably from 1 |im to 4 [im.
[0051]
In addition, when the volume-average diameter decreases, local strain
concentration that occurs in a micrometer order is suppressed, and thus strain during
local deformation may be dispersed. Accordingly, it is considered that elongation,
particularly, uniform elongation is improved. In addition, when the volume-average
diameter decreases, a grain boundary, which is serving as a barrier of dislocation
motion, may be appropriately controlled. In addition, the grain boundary acts on
repetitive plastic deformation (fatigue phenomenon) that occurs due to the dislocation
motion, and thus fatigue properties are improved.
[0052]
In addition, the diameter of an individual grain (grain unit) may be determined
as follows.
Pearlite is specified by structure observation using an optical microscope. In
addition, the grain units of ferrite, austenite, bainite, and martensite are specified by
- 21
#
EBSD. When a crystal structure of a region which is determined by the EBSD is a
face-centered cubic structure (fee structure), this region is determined as austenite. In
addition, when a crystal structure of a region which is determined by the EBSD is a
body-centered cubic structure (bcc structure), this region is determined as any one of
ferrite, bainite, and martensite. Ferrite, bainite, and martensite may be distinguished
using a Kernel Average Misorientation (KAM) method that is equipped to EBSP-OIM
(registered trademark. Electron Back Scatter Diffraction Pattern-Orientation Image
Microscopy). In the KAM method, a difference in orientation between respective
pixels is averaged in a first approximation (total seven pixels) in which an arbitrary
regular hexagonal pixel (central pixel) among measurement data and six pixels that are
adjacent to the pixel are used, in a second approximation (total 19 pixels) in which 12
pixels positioned further outside the six pixels are also used, or in a third
approximation (total 37 pixels) in which 18 pixels positioned further outside the 12
pixels are also used. Then, an average value that is obtained is determined as a value
of the central pixel, and this operation is performed with respect to the entirety of the
pixels. When the calculation according to the KAM method is performed without
exceeding a grain boundary, a map, which is expressing an intragranular orientation
variation, may be created. This map shows a strain distribution based on the
intragranular local orientation variation.
[0053]
In the embodiment, the orientation difference between adjacent pixels is
calculated by the third approximation in the EBSP-OIM (registered trademark). The
grain size of ferrite, bainite, martensite, and austenite may be obtained as follows.
For example, the above-described orientation measurement is performed at a
measurement step of 0.5 ^m below with a magnification of 1,500 times, a position at
22
V
which the orientation difference between measurement points, which are adjacent to
each other, exceeds 15° is determined as a grain boundary (this grain boundary,
necessarily, may not be a general grain boundary), and an equivalent circle diameter is
calculated to obtain the grain size. In a case where pearlite is contained in the
microstructure, with respect to an image obtained by an optical microscope, the
pearlite grain size may be calculated by applying an image processing method such as
binarization processing and an intercept method.
[0054]
In the grain (grain unit) defined as described above, in a case where an
equivalent circle radius (a half value of the equivalent circle diameter) is set to r, the
volume of an individual grain may be obtained by 4x7rxr''/3, and a volume average
diameter may be obtained by a weighted average of the volume.
In addition, an area ratio of a following coarse grain may be obtained by
dividing the area ratio of the coarse grain, which is obtained by the method, by an area
of an object to be measured.
Furthermore, the following distance LMA may be determined using a boundary
between austenite and a grain other than austenite and a boundary between martensite
and a grain other than martensite which are obtained by the above method (only, FESEM
with which EBSD is possible).
[0055]
Furthermore, in a case of fiirther improving bendability, with respect to total
components of the microstructure, a ratio of an area (area ratio of a coarse grain) that is
occupied by a grain (coarse grain) having a grain size, which is exceeding 20 |j,m per a
unit area, may be limited to 10% or less. When a grain having a large grain size
increases, tensile strength decreases, and thus local deformability also decreases.
23
Therefore, it is preferable to make the grain as fine as possible. Furthermore, when
all grains are uniformly and equivalently received a strain, bendability is improved.
Accordingly, local strain of the grain may be suppressed by limiting the amount of
coarse grains.
[0056]
In addition, to further improve local deformability such as bendability, stretch
flangeability, burring workability, and hole expansibility, it is preferable that a hard
structure such as retained austenite and martensite is dispersed. Therefore, among
grains of retained austenite and martensite, the standard deviation of a distance LMA
[[itn] between closest crystal grains (retained austenite or martensite) with each other
may be set to 5 (am. In this case, with respect to at least 100 grains of retained
austenite and martensite, the standard deviation of the distance LMA may be obtained
by measuring the distance LMA-
[0057]
Next, the reason why the chemical components (chemical elements) of the
hot-rolled steel sheet related to the embodiment are limited will be described. Here,
"%" in the content of respective chemical components represents "by mass%".
[0058]
C: 0.02% to 0.5%
C is necessary to secure high strength and retained austenite. It is necessary
for the content of C to be 0.02% or more so as to obtain a sufficient amount of retained
austenite. On the other hand, when the steel sheet excessively contains C, weldability
deteriorates, and thus the upper limit of the content of C is set to 0.5% or less. In a
case of further improving strength and elongation, it is preferable that the content of C
is 0.05% or more, more preferably 0.06% or more, and still more preferably 0.08% or
24
^g
more. In addition, in a case of further improving weldability, it is preferable that the
content of C is 0.45% or less, and more preferably 0.40% or less.
[0059]
Si: 0.001% to 4.0%
Si is a deoxidizer, and it is preferable that a steel contains 0.001%) or more of
Si. In addition. Si stabilizes ferrite during a temperature control after hot rolling, and
suppresses cementite precipitation after winding (during bainitic transformation).
Accordingly, Si increases the concentration of C in austenite, and contributes to
securement of retained austenite. The more the content of Si is, the further the effect
increases. However, when Si is excessively added to the steel, surface qualities,
paintability, weldability, and the like deteriorate. Therefore, the upper limit of the
content of Si is set to 4.0% or less. In a case that an effect of obtaining stable retained
austenite is sufficiently exhibited by Si, it is preferable that the content of Si is 0.02%
or more, more preferably 0.20% or more, and still more preferably 0.50% or more. In
addition, in a case of further securing the surface qualities, paintability, weldability,
and the like, it is preferable that the content of Si is 3.5%) or less, and more preferably
3.0% or less.
[0060]
Mn: 0.001% to 4.0%
Mn is an element that stabilizes austenite, and increase hardenability. It is
necessary for steel to contain 0.001% or more of Mn so as to secure sufficient
hardenability. On the other hand, when Mn is excessively added in the steel, ductility
deteriorates, and thus the upper limit of the content of Mn is set to 4.0%). To secure
further higher hardenability, it is preferable that the content of Mn is 0.1% or more,
more preferably 0.5% or more, and still more preferably 1.0% or more. In addition.
- 25 -
9
in a case of securing further higher ductility, it is preferable that the content of Mn is
3.8% or less, and more preferably 3.5% or less.
[0061]
P: 0.15% or less
P is an impurity, and when P is excessively contained in steel, ductility or
weldability deteriorates. Therefore, the upper limit of the content of P is set to 0.15%
or less. In addition, P operates as a solid-solution hardening element, but P is
unavoidably contained in steel. Accordingly, it is not necessary to particularly limit
the lower limit of the content of P, and the lower limit is 0%. In addition, when
considering recent general refining (including secondary refining), the lower limit of
the content of P may be 0.001%. In a case of further increasing ductility and
weldability, it is preferable that the content of P is 0.12%) or less, and more preferably
0.10% or less.
[0062]
S: 0.03% or less
S is an impurity, and when S is excessively contained in steel, MnS that
elongates by hot rolling is generated. Therefore, formability such as ductility and
hole expansibility deteriorates. Therefore, the upper limit of the content of S is set to
0.03%. In addition, since S is unavoidably contained in steel, it is not necessary to
particularly limit the lower limit of the content of S, and the lower limit is 0%. In
addition, when considering recent general refining (including secondary refining), the
lower limit of the content of S may be 0.0005%. In a case of further increasing
ductility and hole expansibility, it is preferable that the content of S is 0.020% or less,
and more preferably 0.015% or less.
[0063]
26
§
0:0.01% or less
O (oxygen) is an impurity, and when the content of O exceeds 0.01%,
ductility deteriorates. Therefore, the upper limit of the content of O is set to 0.01%.
In addition, since O is unavoidably contained in steel, it is not necessary to particularly
limit the lower limit of the content of O, and the lower limit is 0%. In addition, when
considering recent general refining (including secondary refining), the lower limit of
the content of O may be 0.0005%.
[0064]
Al: 0.001% to 4.0%
Al is a deoxidizer, and when considering recent general refining (including
secondary refining), it is preferable that 0.001% or more of Al is contained in steel.
In addition, Al stabilizes ferrite during a temperature control after hot rolling, and
suppresses cementite precipitation during bainitic transformation. Accordingly, Al
increases the concentration of C in austenite, and contributes to securement of retained
austenite. When the content of Al is increasing, the effect further increases.
However, when Al is excessively added to steel, surface qualities, paintability, and
weldability deteriorate. Therefore, the upper limit of the content of Al is set to 2.0%.
In a case that an effect of obtaining stable retained austenite to be sufficiently exhibited
by Al, it is preferable that the content of Al is 0.005% or more, and more preferably
0.01% or more. In addition, in a case where it is necessary to further secure the
surface qualities, paintability, weldability, and the like, it is preferable that the content
of Al is 3.5% or less, and more preferably 3.0% or less.
[0065]
N: 0.01% or less
N is an impurity, and when the content of N exceeds 0.01%), ductility
27
§
deteriorate. Therefore, the upper limit of the content of N is set to 0.01% or less. In
addition, since N is unavoidably contained in steel, it is not necessary to particularly
limit the lower limit of the content of N, and the lower limit is 0%. In addition, when
considering recent general refining (including secondary refining), the lower limit of
the content of N may be 0.0005%. In a case of further increasing ductility, it is
preferable that the content of N is 0.005% or less.
[0066]
Si+Al: 1.0% to 4.0%
These elements are deoxidizers as described above. In addition, both Si and
Al stabilize ferrite during a temperature control after hot rolling, and suppress
cementite precipitation after winding (during bainitic transformation). Accordingly,
these elements increase the concentration of C in austenite, and contribute to
securement of retained austenite. As a result, it is preferable that the sum of the
content of Si and the content of Al is 1.0% or more. However, when these elements
are excessively added to steel, surface qualities, paintability, weldability, and the like
deteriorate, and thus the sum of the content of Si and the content of Al is set to 4.0% or
less. In a case of further securing surface qualities, paintability, weldability, and the
like, it is preferable that the sum is 3.5% or less, and more preferably 3.0% or less.
[0067]
The above-described chemical elements are basic components (basic
elements) of steel in the embodiment, and the chemical composition in which the basic
elements are controlled (contained or limited), and in which the balance including of
Fe and unavoidable impurities is a basic composition of the embodiment. However,
in the embodiment, in addition to the basic components (in place of a part of Fe of the
balance), the following chemical elements (selective elements) may be further
28
#
contained in steel as necessary. In addition, even when the selective elements are
unavoidably contained (for example, in an amount less than the lower limits of the
amounts of the respective selective elements) in steel, the effect in the embodiment
does not deteriorate.
That is, the hot-rolled steel sheet related to the embodiment may contain one
or more kinds among Ti, Nb, B, Mg, REM, Ca, Mo, Cr, V, W, Ni, Cu, Co, Sn, Zr, and
As as a selective element to improve local deformability, for example, by inclusions
control or precipitates refinement.
[0068]
Furthermore, in a case of obtaining strength by precipitation strengthening,
fine carbo-nitrides may be allowed to be generated. It is effective to add Ti, Nb, V, W,
and Cu so as to obtain the precipitation strengthening. In addition, the steel sheet
may contain one or more kinds of these as necessary.
[0069]
To obtain the effect by addition of Ti, Nb, V, W, and Cu, the content of Ti is
preferably 0.001% or more, the content of Nb is preferably 0.001%) or more, the
content of V is preferably 0.001%) or more, the content of W is preferably 0.001% or
more, and the content of Cu is preferably 0.001%) or more. However, even when the
chemical elements are excessively added to steel, an increase in strength is saturated,
and in addition to this, recrystallization after hot rolling is suppressed, and thus, it is
difficult to control crystal orientation. Therefore, the content of Ti is limited to 0.2%
or less, the content of Nb is limited to 0.2% or less, the content of V is limited to 1.0%
or less, the content of W is limited to 1.0% or less, and the content of Cu is limited to
2.0% or less. In addition, in consideration of alloy cost reduction, it is not necessary
to purposely add these chemical elements to steel, and all of the lower limits of the
29 -
^^
content of Ti, the content of Nb, the content of V, the content of W, and the content of
Cu are 0%.
[0070]
In a case of increasing hardenability of a structure and performing a secondary
phase control to secure strength, it is effective to add one or more kinds among B, Mo,
Cr, and As according to necessity. To obtain the effect, the content of B is preferably
0.0001% or more, the content of Mo and the content of Cr are preferably 0.001% or
more, and the content of As is preferably 0.0001% or more. However, when these
chemical elements are excessively added, workability conversely deteriorates, and thus
the upper limit of the content of B is limited to 0.005%, and the upper limit of the
content of Mo is limited to 1.0%, and the upper limit of the content of Cr is limited to
2.0%, and the upper limit of the content of As is limited to 0.50%. In addition, for
cost reduction of alloy, it is not necessary to purposely add these chemical elements to
steel, and all of the lower limits of the content of B, the content of Mo, the content of
Cr, and the content of As are 0%.
[0071]
Mg, REM (Rare Earth Metal), and Ca are important selective elements to
improve local deformability of the steel sheet by controlling inclusions into a harmless
type. Accordingly, one or more kinds among Mg, REM, and Ca may be added to
steel as necessary. In this case, all of the lower limits of the respective chemical
elements are preferably 0.0001% or less. On the other hand, when these chemical
elements are excessively added to steel, cleanliness deteriorates. Therefore, with
regard to the upper limits of the contents of the respective chemical elements, the
content of Mg is limited to 0.010% or less, the content of REM is limited to 0.1% or
less, and the content of Ca is limited to 0.010%) or less. In addition, for cost reduction
- 30
#
of alloy, it is not necessary to purposely add these chemical elements to the steel, and
all of the lower limits of the content of Mg, the content of REM, and the content of Ca
are 0%.
[0072]
Ni, Co, Sn, and Zr are selective elements to increase strength, and one or more
kinds of these chemical elements may be added to steel as necessary. In this case, the
content of Ni is preferably 0.001%) or more, the content of Co is preferably 0.0001% or
more, the content of Sn is preferably 0.0001% or more, and the content of Zr is
preferably 0.0001% or more. However, when these chemical elements are
excessively added to steel, formability is lost. Therefore, with regard to the upper
limits of the respective chemical elements, the content of Ni is limited to 2.0% or less,
the content of Co is limited to 1.0% or less, the content of Sn is limited to 0.2% or less,
and the content of Zr is limited to 0.2% or less. In addition, for cost reduction of
alloy, it is not necessary to purposely add these chemical elements to steel, and all of
the lower limits of the content of Ni, the content of Co, the content of Sn, and the
content of Zr are 0%.
[0073]
As described above, the hot-rolled steel sheet related to the embodiment has a
chemical composition containing the above-described basic elements, the balance
including of Fe and unavoidable impurities, or a chemical composition containing
the above-described basic elements and at least one of the above-described selective
elements, the balance including Fe and unavoidable impurities.
[0074]
In addition, the hot-rolled steel sheet related to the embodiment may be
subjected to a surface treatment. For example, the hot-rolled steel sheet may include
31
various kinds of film (film or coating) by applying surface treatments such as electro
coating, hot-dip coating, deposition coating, an alloying treatment after coating,
organic film formation, film laminating, a treatment using organic salts/inorganic salts,
and a non-chromium treatment (non-chromate treatment). As an example of these
films, the hot-rolled steel sheet may include a hot-dip galvanized layer or a
galvanneald layer on a surface thereof Even when the hot-rolled steel sheet includes
the film, local deformability may be sufficiently maintained.
[0075]
In addition, in this embodiment, the sheet thickness of the hot-rolled steel
sheet is not particularly limited, but for example, the sheet thickness may be from 1.5
mm to 10 mm, or from 2.0 mm to 10 mm. In addition, the strength of the hot-rolled
steel sheet is also not particularly limited, and for example, the tensile strength may be
from 440 MPato 1,500 MPa.
[0076]
The hot-rolled steel sheet related to the embodiment is applicable to overall
uses of a high-strength steel sheet, and local deformability such as bending workability
and hole expansibility of the high-strength steel sheet is significantly improved.
[0077]
In addition, a direction, which is performed of bending processing, to the hotrolled
steel sheet is different depending on components to be processed, and the
direction is not particularly limited. In the hot-rolled steel sheet related to the
embodiment, the same properties may be obtained in all bending directions, and thus
the hot-rolled steel sheet is applicable to composite forming including processing
modes such as bending, stretching, and drawing.
[0078]
32
#
Next, a production method of the hot-rolled steel sheet related to an
embodiment of the present invention will be described.
To realize excellent local deformability, it is important to form a texture (nondeveloped
texture) which has a pole density of less anisotropy, and to appropriately
control rC and r30. Details of production conditions to control respective pole density,
rC, and r30 with respect to the hot-rolled steel sheet that is produced will be described
below.
[0079]
A production method preceding hot rolling is not particularly limited. For
example, various kinds of secondary refining may be performed subsequently to
smelting and refining using a blast furnace, an electric furnace, a converter, or the like
to melt steel which is satisfying the above-described chemical composition, whereby
steel (molten steel) may be obtained. Then, to obtain a steel ingot or slab from the
steel, for example, the steel may be casted by casting methods such as a common
continuous casting method, an ingot method, and a thin slab casting method. In the
case of the continuous casting, the steel may be hot-rolled after cooling the steel once
to a low temperature (for example, room temperature), and reheating the steel.
Alternatively, steel (casted slab) immediately after being casted may be continuously
hot-rolled. In addition, as a raw material of steel (molten steel), scrap may be used.
[0080]
To obtain a high-strength hot-rolled steel sheet that is excellent in local
deformability, it is preferable to satisfy the following conditions.
It is preferable that the austenite grain size before finish rolling is small so as
to increase local deformability. In addition, it has been proven that when an average
austenite grain size before finish rolling is 200 ^im or less, this is effective to obtain
33
#
sufficient local deformability.
As shown in FIG. 5, to obtain an average austenite grain size of 200 |am or
less before finish rolling, steel may be rolled one or more times with a rolling reduction
ratio of 40% or more by rough rolling (first hot rolling) within a temperature range of
from 1,000°C to 1,200°C (preferably, 1,150°C or lower).
[0081]
As the rolling reduction ratio and the number of rolling reduction times
increase, a fiirther fine austenite grain may be obtained. For example, in the rough
rolling, it is preferable to control the average austenite grain size to 100 |j,m or less.
To perform a grain size control, rolling in which a rolling reduction ratio of one pass is
40% or more may be performed two or more times (two or more passes). However,
with regard to the rough rolling, when the rolling reduction ratio of one pass is limited
to 70% or less, or the number of rolling reduction times (the number of passes) is
limited to 10 times or less, a concern about a decrease of temperature or excessive
generation of scales are capable of being reduced. Accordingly, in the rough rolling,
the rolling reduction ratio of one pass may be 70% or less, and the number of rolling
reduction times (the number of passes) may be 10 times or less.
As described above, when the austenite grain size before finish rolling is
made small, recrystallization of austenite in the subsequent finish rolling is promoted,
and thus the reduction of the austenite grain size is effective to improve local
deformability.
[0082]
The effect is assumed to be because an austenite grain boundary after the
rough rolling (that is, before the finish rolling) functions as one of recrystallization
nuclei during finish rolling.
- 34
f6
So as to confirm the austenite grain size after the rough rolling, it is preferable
to quickly cool the steel (steel sheet) before entering the finish rolling at a cooling rate
as high as possible. For example, the steel sheet is cooled at an average cooling rate
of 10°C/s or higher. Furthermore, a cross-section of a sheet piece collected from the
steel sheet obtained after cooling is etched to make an austenite grain boundary in a
microstructure emerge to the fi-ont, and then measurement using an optical microscope
is performed. At this time, with respect to 20 viewing fields or more at a
magnification of 50 times, the austenite grain size is measured by image analysis or an
intercept method, and respective austenite grain sizes are averaged to obtain an average
austenite grain size.
[0083]
After the rough rolling, finish rolling may be continuously performed after
jointing a sheet bar. At this time, a rough bar may be wound at once into a coil, and
may be stored in a cover having a heat retention fiinction as necessary, and jointing
may be performed after rewinding the coil again.
[0084]
In addition, as one condition for controlling the average pole density of the
orientafion group from {100}<011> to {223}<110> and the pole density of the crystal
orientation {332}<113> at the sheet thickness central portion within a range of the 5/8
to 3/8 of sheet thickness within the above-described pole density ranges, a rolling is
controlled in the finish rolling (second hot rolling) after the rough rolling with a
temperature Tl (°C), which may be determined as shown in the following Expression 7
by a chemical composition (by mass%) of steel, set as a reference.
Tl = 850 + 10 X ([C] + [N]) X [Mn] + 350 x [Nb] + 250 x [Ti] + 40 x [B] +
10 x [Cr] + 100 x[Mo] + 100 x [V] (Expression 7)
35 -
m
In addition, in Expression 1, [C], [N], [Mn], [Nb], [Ti], [B], [Cr], [Mo], and
[V] represent mass percentages of the content of C, the content of N, the content of Mn,
the content of Nb, the content of Ti, the content of B, the content of Cr, the content of
Mo, and the content of V in the steel, respectively. In addition, calculation is
performed while setting the content of chemical elements (chemical components) not
contained in Expression 7 to 0%. Therefore, in the basic composition that contains
only the above-described basic components, the following Expression 8 may be used
instead of Expression 7.
Tl = 850 + 10 X ([C] + [N]) X [Mn] (Expression 8)
In addition, when steel contains selective elements, it is necessary for a
temperature calculated by Expression 7 instead of the temperature calculated by
Expression 8 to be set as Tl (°C).
In the finish rolling, the temperature Tl (°C) that may be obtained by
Expression 7 or Expression 8 is set as a reference, a large rolling reduction ratio is
secured in a temperature range of Tl + 30°C to Tl + 200°C (preferably, a temperature
range of Tl + 50°C to Tl + 100°C), and the rolling reduction ratio is limited to a small
range (including 0%) in a temperature range that is higher than or equal to Ar3°C and
lower than Tl + 30°C. When the finish rolling is performed in addition to the rough
rolling, local deformability of a final product may be raised.
When the large rolling reduction ratio is secured in a temperature range of Tl
+ 30°C to Tl + 200°C, and the rolling reduction ratio is limited in a temperature range
that is higher than or equal to Ar3°C and lower than Tl + 30°C, the average pole
density of the orientation group from {100}<0U> to {223}<110> and the pole density
of the crystal orientation {332}<113> at the sheet thickness central portion within a
range of the 5/8 to 3/8 of the sheet thickness are sufficiently controlled. Accordingly,
36
#
local deformability of the final product is dramatically improved. The temperature Tl
itself is empirically obtained. The present inventors have empirically found the
following fact through experiment. That is, a temperature range in which
recrystallization in an austenite range of each steel is promoted may be determined
with the temperature Tl set as a reference. So as to obtain further satisfactory local
deformability, it is important to accumulate a large amount of strains by rolling
reduction, and thus an accumulative rolling reduction ratio within a temperature range
of Tl + 30°C to Tl + 200°C is 50% or more. In addition, from the viewpoint of
promoting recrystallization by strain accumulation, it is preferable that the
accumulative rolling reduction ratio is 70% or more. In addition, when the upper
limit of the accumulative rolling reduction ratio is limited, the rolling temperature may
be further sufficiently secured, and thus a rolling load may be further suppressed.
Accordingly, the accumulative rolling reduction ratio may be 90% or less.
[0085]
Furthermore, so as to increase the homogeneity of the hot-rolled sheet, and to
raise the elongation and local ductility of a final product to the limit, the finish rolling
is controlled to include a large-rolling-reduction pass having a rolling reduction ratio of
30% or more in a temperature range of Tl + 30°C to Tl + 200°C. In this manner, in
the finish rolling, in a temperature range of Tl + 30°C to Tl + 200°C, at least one time
of rolling reduction having a rolling reduction ratio of 30%o or more is performed.
Particularly, when considering the cooling control, to be described later, the rolling
reduction ratio of the final pass in the temperature range is 30% or more. That is, it is
preferable that the final pass is the large-rolling-reduction pass. In a case where
further higher workability is required, the rolling reduction ratios of final two passes
may be set to 30% or more, respectively. In a case of further raising homogeneity of
- 37
#
a hot-rolled sheet, the rolling reduction ratio of the large-rolling-reduction pass (one
pass) may be 40% or more. In addition, in a case of obtaining a further satisfactory
shape of a steel sheet, the rolling reduction ratio of the large-rolling-reduction pass
(one pass) may be 70% or less.
[0086]
In addition, in a temperature range of Tl + 30°C to Tl + 200°C, when
temperature rising of a steel sheet between respective rolling passes is suppressed (for
example, 18°C or lower), further uniform recrystallized austenite may be obtained.
[0087]
Furthermore, uniform recrystallization is promoted by release of accumulated
strains. Accordingly, after rolling reduction in a temperature range of Tl + 30°C to
Tl + 200°C is terminated, an amount of processing in a temperature range that is
higher than or equal to Ar3°C and lower than Tl + 30°C (preferably, T1°C to lower
than Tl + 30°C) is suppressed to be as small as possible. Accordingly, the
accumulative rolling reduction ratio in a temperature range that is higher than or equal
to Ar3°C and lower than Tl + 30°C is limited to 30% or less. In a case of securing
excellent sheet shape in this temperature range, the accumulative rolling reduction ratio
of 10%> or more is preferable. However, in a case where high value is set on local
deformability, it is preferable that the accumulative rolling reduction ratio is 10%) or
less, and more preferably 0%. That is, in a temperature range that is higher than or
equal to Ar3°C and lower than Tl + 30°C, it is not necessary to perform the rolling
reduction, and even when the rolling reduction is performed, the accumulative rolling
reduction ratio is set to 30%) or less.
In addition, when the rolling reduction ratio in a temperature range that is
higher than or equal to Ar3°C and lower than Tl + 30°C is large, recrystallized
38
#
austenite grain is expanded, and thus local deformability deteriorates.
That is, with regard to production conditions related to the embodiment, when
austenite is uniformly and finely recrystallized in the finish rolling, the texture and r
value of a hot-rolled product are controlled. Accordingly, local deformability such as
hole expansibility and bendability may be improved.
[0088]
When rolling is performed in a temperature range lower than Ar3°C, or the
accumulative rolling reduction ratio in a temperature range that is higher than or equal
to Ar3°C and lower than Tl + 30°C is too large, the texture of austenite develops. As
a result, a hot-rolled steel sheet that may be ultimately obtained that does not satisfy at
least one of a condition in which the average pole density of the orientation group from
{100}<0I1> to {223}<110> in the sheet thickness central portion is 1.0 to 6.5, and a
condition in which the pole density of the crystal orientation {332}<113> is 1.0 to 5.0.
On the other hand, in the finish rolling, when rolling is performed in a temperature
range higher than Tl + 200°C, or the accumulative rolling reduction ratio is too small,
coarse grains or mixed grains may be included in the microstructure, or the
microstructure may be constituted by mixed grains. In addition, in this case, an area
ratio of grains exceeding 20 \xm. or a volume average diameter increases.
Here, the rolling reduction ratio may be obtained by actual results or
calculation in measurement of a rolling load or a sheet thickness, and the like. In
addition, a rolling temperature (for example, each of the temperature ranges above)
may be obtained by actual measurement using a thermometer between stands, by
calculation through a calculation simulation in consideration of processing heat
generation due to a line speed, a rolling reduction ratio, or the like, or by performing
both of them (actual measurement and calculation). In addition, in the above
39 -
^
description, the rolling reduction ratio in one pass represents a percentage of a rolling
reduction amount in one pass to an inlet sheet thickness before passing through a
rolling stand (a difference between the inlet sheet thickness before passing through the
rolling stand and an outlet sheet thickness after passing the rolling stand). When an
inlet sheet thickness before first pass in rolling in each of the temperature ranges above
is set as a reference, the accumulative rolling reduction ratio represents a percentage of
an accumulative rolling reduction amount to the reference (a difference between the
inlet sheet thickness before first pass in the rolling in each of the temperature ranges
above and the outlet sheet thickness after a final pass in rolling in each of the
temperature ranges). Furthermore, Ara temperature is obtained by the following
Expression 9.
Ara = 879.4 - 516.1 x [C] - 65.7 x [Mn] + 38.0 x [Si] + 274.7 x [P]
(Expression 9)
[0089]
With regard to the hot rolling (finish rolling) that is performed as described
above, when the hot rolling is terminated at a temperature lower than A13 (°C), steel is
rolled at two-phase region (two-phase temperature region) including austenite and
ferrite, and thus integration of the crystal orientation to the orientation group from
{100}<011> to {223}<110> becomes strong. As a result, local deformability
significantly deteriorates. Here, when the rolling termination temperature of the
finish rolling is T1°C or higher, an amount of strain in a temperature range of T1°C or
lower may be reduced, and thus anisotropy may be fiirther reduced. As a result, local
deformability may be further increased. Accordingly, the rolling termination
temperature of the finish rolling may be T1°C or higher.
[0090]
40
#
In addition, cooling (primary cooling) after final large-rolling-reduction pass
(rolling reduction at a rolling stand) of the rolling in a temperature range of Tl + 30°C
to Tl + 200°C has a large effect on a grain size of a final product. In addition, due to
the cooling, an equiaxed (uniform-size) grain is obtained, and thus the microstructure
may be controlled to have less coarse grain.
Steel is cooled after a rolling stand corresponding to the final pass among the
large-rolling-reduction passes in such a manner that a standby time t (second), which is
taken before primary cooling initiation after completion of the final pass among the
large-rolling-reduction passes (as described above, the large-rolling-reduction passes
represent rolling reduction (pass) having a rolling reduction ratio of 30% or more in the
temperature range of Tl + 30°C to Tl + 200°C) in the hot rolling, satisfies Expression
10. Here, tl in Expression 10 may be obtained by the following Expression 11. In
Expression 11, Tf represents a temperature (°C) of a steel sheet at the time of
completion of the final pass of the large-rolling-reduction passes, and PI represents a
rolling reduction ratio (%) in the final pass among the large-rolling reduction passes.
Here, when considering runability (for example, shape correction or controllability of
secondary cooling), the primary cooling may be performed between rolling stands.
When the standby time t exceeds the right-side value (2.5 x tl) of Expression
10, recrystallization is almost completed, on the other hand, grains significantly are
grown, and thus a grain size increases. Therefore, the r value and elongation decrease.
Accordingly, the standby time t is set to 2.5 x tl seconds or less.
t < 2.5 X tl (Expression 10)
tl = 0.001 X ((Tf-Tl) X Pl/100)^ - 0.109 x ((Tf- Tl) x Pl/100) + 3.1
(Expression 11)
[0091]
- 41
#
When the standby time t is fiirther limited to be less than tl seconds, the
growth of the grain may be largely suppressed. In this case, a volume-average
diameter of a final product may be controlled to 4 |im or less. As a result, even when
recrystallization does not sufficiently progress, the elongation of the steel sheet may be
sufficiently improved, and at the same time, fatigue properties may be improved.
[0092]
On the other hand, when the standby time t is further limited to tl seconds to
2.5 X tl seconds, the volume-average diameter increases (for example, exceeding 4
(am) compared to a case in which the standby time tl is less than tl. However,
recrystallization sufficiently progresses, and thus the crystal orientation becomes
random. Accordingly, the elongation of the steel sheet may be sufficiently improved,
and at the same time, isotropy may be largely improved.
[0093]
In addition, the above-described primary cooling may be performed between
rolling stands or after the final rolling stand. That is, after performing the primary
cooling, rolling having a low rolling reduction ratio (for example, 30% or less (or less
than 30%)) may be performed in a temperature range of Ar3°C or higher (for example,
from Ar3 (°C) to Tl + 30 (or Tf) (°C)).
[0094]
It is preferable that a cooling temperature variation that is a difference
between a steel sheet temperature (steel temperature) at the time of cooling initiation
and a steel sheet temperature (steel temperature) at the time of cooling termination in
the primary cooling is 40°C to 140°C. In addition, it is preferable that the steel sheet
temperature T2 at the time of cooling completion of the primary cooling is Tl + 100°C
or lower. When the cooling temperature variation is 40°C or higher, grain growth of
42
#
the recrystallized austenite grain may be further suppressed. When the cooling
temperature variation is 140°C or lower, recrystallization may further sufficiently
progress, and thus the pole density may be fiirther improved. In addition, when the
cooling temperature variation is limited to I40°C or lower, the temperature of the steel
sheet may be controlled in a relatively easy manner, and variant selection (avoiding of
variant limitation) may be controlled in a relatively effective manner, and thus
development of a texture may be further suppressed. Accordingly, in this case,
isotropy may be further raised, and thus orientation dependence of workability may be
further decreased. Furthermore, when the steel sheet temperature T2 at the time of
cooling termination of the primary cooling is Tl + 100°C or lower, a fiirther sufficient
cooling effect may be obtained. Due to the cooling effect, grain growth may be
suppressed, and thus an increase of austenite grain size may be further suppressed.
In addition, it is preferable that an average cooling rate in the primary cooling
is 50°C/s or more. When the average cooling rate in the primary cooling is 50°C/s or
more, grain growth of recrystallized austenite grain may be further suppressed. On
the other hand, it is not necessary to particularly set the upper limit of the average
cooling rate, but the average cooling rate may be 200°C/s or less from the viewpoint of
a sheet shape.
[0095]
After the finish rolling, steel is cooled at an average cooling rate of from
10°C/s to 100°C/s to a temperature T3 within a range of 630°C to 800°C that is in the
vicinity of a nose of a pro-eutectoid ferrite range (secondary cooling). Then, the steel
is retained for 1 to 20 seconds in a temperature range of 630°C to 800°C, or slowly
cooled to a temperature within a range that is higher than or equal to 550°C and lower
than the temperature T3 from the temperature T3 at an average cooling rate of 20°C/s.
43
^B
A sufficient amount of ferrite may be easily obtained by the temperature control. In
addition, a grain may be refined by the cooling in 630°C to 800°C at an average
cooling rate of 10°C/s. In a case of a substantial isothermal retention treatment, when
a retention time exceeds 20 seconds, a fraction of ferrite becomes too high, and thus
strength decreases. On the other hand, when the retention time is less than one
second, an amount of generation of ferrite becomes deficient. In addition, when a
temperature at which the slow cooling is stopped is lower than 550°C or a cooling
stopping temperature before the retention or slow cooling is lower than 630°C, there is
a possibility in that pearlite transformation may occur. Therefore, the temperature at
which the slow cooling is stopped is set to 550°C or higher, and the cooling stopping
temperature before the retention or slow cooling is set to 630°C or higher.
[0096]
Furthermore, the steel is cooled to a temperature within a range of 350°C to
500°C and is wound. After performing a temperature range control of retaining the
wound coil (steel) within a range of 350°C to 500°C for 30 minutes to 300 minutes, the
resultant coil is cooled with air. When the winding temperature is higher than 500°C,
bainitic transformation excessively progresses. In addition, when the winding
temperature is lower than 350°C, the bainitic transformation is excessively suppressed,
and thus stabilization of retained austenite by C-concentration is not sufficient.
Furthermore, in this case, martensitic transformation occurs during air cooling, and
thus it is not possible to obtain a sufficient amount of retained austenite. In addition,
when the retention time at 350°C to 500°C is less than 30 minutes, the progress of
bainitic transformation is not sufficient, and the fraction of retained austenite decreases.
On the other hand, the retention time exceeds 300 minutes, cementite precipitates or
precipitated cementite grows, and thus the target fraction of retained austenite may not
- 44
^B:
be obtained. Furthermore, when a temperature variation rate of the coil in the
temperature range control is -40°C/h to 40°C/h, a temperature variation in the coil
gradually occurs. Accordingly, material qualities in the coil may be controlled to be
further homogeneous.
According to the above-described production method, a hot-rolled steel sheet
having excellent local deformability may be obtained.
[0097]
In addition, with respect to the hot-rolled steel sheet that is obtained, skin pass
rolling may be performed as necessary. According to the skin pass rolling, a stretcher
strain that occurs during machining may be prevented, a shape of a steel sheet may be
corrected.
[0098]
In addition, the hot-rolled steel sheet that is obtained may be subjected to a
surface treatment. For example, surface treatments such as electro coating, hot-dip
coating, deposition coating, an alloying treatment after coating, organic film formation,
film laminating, a treatment using organic salts/inorganic salts, and a non-chromium
treatment may be applied to the hot-rolled steel sheet that is obtained. As an example
of these treatments, a hot-dip galvanized layer or a galvanneald layer may be formed
on a surface the hot-rolled steel sheet. Even when the surface treatments are
performed, local deformability may be sufficiently maintained.
[0099]
For reference, FIG. 6 shows a flowchart illustrating the outline of a production
method of the hot-rolled steel sheet related to the embodiment.
Examples
[0100]
45
#
The technical content of the present invention will be described with reference
to examples of the present invention.
Results of examination performed using steel a to steel t having a chemical
composition (the balance includes Fe and unavoidable impurities) shown in Tables 1
and 2 will be described. The steel was melted and casted. Then, the steel was
heated to a temperature range of 900°C to 1,300°C by reheating the steel as is, or
reheating steel that was cooled at once to room temperature. Then, hot rolling (an
austenite range that is a temperature range of Ara or higher) and a temperature control
(cooling or retention) was performed under production conditions shown in Tables 3 to
6, whereby a hot-rolled steel sheet having a thickness of from 2 mm to 5 mm was
obtained.
[0101]
[Table 1]
[0102]
[Table 2]
[0103]
Tables 7 to 9 show characteristics and mechanical properties of a
microstructure (including a texture). In addition, in Tables 7 to 9, y, F, B, M, and P
represent area ratios of retained austenite, ferrite, bainite, martensite, and pearlite,
respectively. In addition, fjo, dv, and OMA represent a percentage of an area ratio of a
grain (coarse grain) exceeding 20 jim, a volume-average diameter of a grain, and
standard deviation of the above-described distance LMA, respectively.
As an index of local deformability, hole expanding ratio X and the limit bend
radius (d/RmC) according to 90° V-bending of the final product were used. In
addition, a tensile test (measurement of TS and EL), a bending test, and a hole
46
expanding test were performed according to JIS Z 2241, JIS Z 2248 (V-block 90°
bending test), and Japan Iron and Steel Federation Standard JFS TlOOl, respectively.
In addition, with respect to the sheet thickness central portion within a region of 5/8 to
3/8 of a sheet thickness cross-section which is parallel with a rolling direction at a 1/4
position in a sheet width direction, a pole density was measured at a pitch of 0.5 )j,m
using the above-described EBSD. In addition, r values (rC, r30) of respective
directions were measured by the above-described method according to JIS Z 2254
(2008) (ISOl 0113 (2006)).
In addition, an underline in Tables 1 to 8 indicates conditions not satisfying
the conditions of the present invention. In addition, in Production No. 38, since
rolling within a temperature range of Tl + 30°C to Tl + 200°C does not include a pass
of 30% or more, as a value of PI, a rolling reduction ratio of the final pass in the
rolling within a temperature range of Tl + 30°C to Tl + 200°C was used.
[0104]
[Table 3]
[0105]
[Table 4]
[0106]
[Table 5]
[0107]
[Table 6]
[0108]
[Table 7]
[0109]
[Table 8]
47
^^&
[0110]
[Table 9]
[0111]
Production Nos. 1 to 14, and 25 to 34 satisfy the conditions of the present
invention, and thus d/RmC, TSxX, and TSxEL of the steel sheets that were obtained in
the production numbers were excellent. In addition, when the production conditions
were optimized, d/RmC, TSxX, and TSxEL were further improved.
On the other hand, in Production Nos. 15 to 24, and 35, the conditions of the
present invention were not satisfied, and thus at least one of d/RmC, TSxA,, and TSxEL
of the steel sheet that was obtained was not sufficient.
[0112]
Hereinbefore, the preferred examples of the present invention have been
described, but the present invention is not limited to the examples. Addition,
omission, substitution, and other modifications of configuration may be made within a
range not departing from the essence of the present invention. The present invention
is not limited by the above-described description, and is limited only by the attached
claims.
Industrial Applicability
[0113]
With regard to TRIP steel, a hot-rolled steel sheet which is excellent in local
deformability, in which orientation dependence of formability is small, and which is
excellent in ductility, and a production method thereof are provided.
48 -
v ^
TABLE 1
STEEL
No.
a
b
c
d
e
f
g
h
i
J
k
1
m
n
0
P
q
r
s
t
CHEMICAL COMPONENT / BY MASS %
C
0.15
0.15
0.2
0.44
0.81
0.22
0.28
0.34
0.24
0.001
0.025
0.073
0.095
0.11
0.13
0.19
0.075
0.061
0.15
0.03
Si
1.5
1.58
2.1
0.5
1.55
1.5
1.5
0.15
0.94
2.5
1.3
1.5
1.4
0.4
0.002
2.1
2.8
3.9
1.8
0.003
Mn
2.5
3.47
2.15
1.8
3
2.5
0.05
2.9
3.5
3.6
2.5
2.3
1.9
2.4
1.6
1.8
0.8
0.7
0.005
1.3
P
0.11
0.09
0.08
0.01
0.07
0.09
0.04
0.018
02
0.13
0.11
0.08
0.02
0.06
0.08
0.04
0.13
0.12
0.06
0.08
S
0.005
0.004
0.01
0.009
0.012
0.011
0.017
0.022
0.04
0.006
0.004
0.003
0.008
0.01
0.006
0.007
0.013
0.012
0.011
0.027
Al
0.01
0.8
0.01
1.5
1.2
3.8
2.1
3.2
1.5
1
0.3
0.1
0.01
1.5
2.1
0.01
0.01
0.01
0.01
3.9
Si+AI
1.51
2.38
2.11
2
2.75
5J
3.6
3.35
2.44
3.5
1.6
1.6
1.41
1.9
2.102
2.11
2.81
3.91
1.81
3.903
N
0.003
0.0025
0.001
0.004
0.006
0.0055
0.0013
0.0089
0.01
0.05
0.004
0.003
0.008
0.009
0.01
0.003
0.002
0.009
0.002
0.01
0
0.005
0.001
0.003
0.002
0.004
0.009
0.008
0.004
0.0036
0.002
0.005
0.006
0.008
0.009
0.01
0.007
0.002
0.003
0.004
0.008
T1
/ °0
854
870
892
888
874
887
883
899
879
888
851
875
852
903
882
882
917
864
860
880
Ar3
/°C
725
659
737
556
342
683
800
524
616
773
782
770
764
697
729
754
930
983
887
801
HHiMBiJjJJJiuimy)
TABLE 2
0
STEEL
No.
a
b
c
d
e
f
g
h
i
J
k
1
m
n
0
P
q
r
s
t
CHEMICAL COMPONENT / BY MASS %
Ti
0
0.06
0
0
0
0
0.09
0
0.04
0
0
0
0
0.16
0
0.1
0.2
0.001
0.03
0
Nb
0
0
0
0
0
0.09
0.03
0
0.03
0
0
0
0
0
0
0
0.03
0
0
0
V
0
0
0
0.3
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0.2
w
• 0
0.03
0.04
0
0.06
0.1
0
0
0.02
0
0
0
0
0
0.03
0
0
0.09
0
0
Cu
0
0
0
0
0
0.002
0
0
0.002
0
0
0
0
0
0.014
0
0.03
0.025
0
0
B
0
0
0
0
0
0
0
0.0022
0
0
0
0
0
0
0
0.002
0
0
0
0
Mo
0
0
0.38
0
0
0
0
0
0
0.36
0
0.23
0
0.1
0.3
0.03
0
0
0.01
0.03
Cr
0
0
0
0
0
0
0
^9
0
0
0
0
0
0
0
0
0.6
1.3
0.1
0.6
As
0
0
0
0
0
0
0.3
0
0
0
0
0
0
0
0
0
0
0
0.03
0
Mg
0
0
0
0
0
0.008
0
0
0
0
0
0
0.003
0
0
0
0
0
0
0
REM
0
0
0
0.O3
0
0
0
0
0
0
0
0
0
0.09
0
0
0
0
0.04
0
Ca
0
0
0
0
0
0
0.009
0
0
0
0
0.004
0
0
0
0
0
0
0
0
Ni
0
0
0
0
0.005
0
0
0
0
0
0
0
0
0
0.6
0
0
0
1.9
0
Co
0.05
0
0
0
0
0
0
0
0
0
0
0
0
0.03
0
0
0
0
0
0
Sn
0
0.1
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0.15
0
0
0
Zr
0
0
0
0.015
0
0
0
0
0
0
0
0
0
0.15
0
0
0
0
0
0
TABLE 3
PRODUCTION
No.
1
2
3
4
5
6
7
8
g
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
STEEL
No.
a
a
b
b
c
c
d
d
a
a
b
c
c
d
a
b
c
d
S
J
e
f
h
i
k
1
m
n
0
P
q
r
s
t
a
c
c
c
c
J
T1
/°C
854
854
870
870
892
892
888
888
854
854
870
892
892
888
854
870
892
888
883
888
874
887
899
879
851
875
852
903
882
882
917
864
860
880
854
892
892
892
892
888
NUMBER OF
TIMES OF
ROLUNG
REDUCTION OF
40% OR MORE
AT 1,000°C
TO 1.200°C/-
2
2
2
2
2
1
2
2
2
0
1
ROLUNG
REDUCTION
RATIO OF 40%
OR MORE AT
1,000°CTO
1,200°G/%
50
45
40
50
45
45
40
45
50
40
45
45
40
50
50
45
45
40
50
40
45
45
40
40
45
45
45
AUSTENITE
GRAIN SIZE
/urn
120
130
140
130
130
130
160
150
120
70
70
70
95
90
120
70
70
95
350
T20
CRACKED DURING HOT ROLLING
CRACKED DURING HOT ROLUNG
CRACKED DURING HOT ROLLING
CRACKED DURING HOT ROLLING
1
2
1
1
2
1
2
1
1
1
1
2
2
2
2
1
50
45
50
50
45
50
50
50
50
50
45
40
40
40
40
50
45
45
45
40
40
40
40
105
90
130
130
80
105
65
130
160
180
130
95
95
95
95
120
TABLE 4
•z. o
o
Q.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
ACCUMULATIVE
ROLUNG
REDUCTION
RATIO AT
T1 + 30°C TO
T1 + 200°C/%
73.0
67.5
59.1
62.1
58.6
79.0
61.5
64.3
60.8
71.3
67.0
62.8
55.8
58.7
40.5
51.0
50.3
70.6
48.7
74.8
ROLUNG
REDUCTION
RATIO BEFORE
ONE PASS OF
FINAL PASS AT
T1 + 30°C TO
T1 + 200°C/%
55
50
35
45
40
65
45
49
44
59
45
38
33
41
15
30
29
58
21
58
ROLUNG
REDUCTION
RATIO OF FINAL
PASS AT
T1 + 30°C TO
T1 + 200°C/%
40
35
37
31
31
40
30
30
30
30
40
40
34
30
30
30
30
30
35
40
NUMBER OF TIMES
OF ROLUNG
REDUCTION OF
30% OR MORE
AT T1 + 30°C
TO T1 + 200°C/-
2
2
2
2
2
2
2
2
2
2
2
2
2
2
MAXIMUM
TEMPERATURE
RISING DURING
ROLUNG
REDUCTION
AT T1 + 30°C
TO T1 + 200°C/°C
15
5
15
18
13
14
16
17
18
18
13
15
20
20
12
20
15
12
30
40
CRACKED DURING HOT ROLUNG
CRACKED DURING HOT ROLUNG
CRACKED DURING HOT ROLUNG
CRACKED DURING HOT ROLUNG
73.0
69.8
68.5
70.8
76.0
75.5
54.5
64.3
72.0
69.0
45.6
45.7
55.8
57.0
55.8
74.8
55
45
55
55
60
65
30
49
60
55
15
33
33
20
33
58
40
45
30
35
40
30
35
30
30
31
36
19
34
20
34
40
2
2
2
2
2
2
2
2
2
2
1
1
2
0
2
1
20
15
10
15
15
20
16
24
23
16
12
20
20
20
20
40
#
TABLE 5
2
O
o
Cd
0.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
ACCUMULATIVE
ROLUNG REDUCTION
RATIO AT A
TEMPERATURE EQUAL
TO OR HIGHER THAN
Ar3°C AND LOWER
THAN T1 + 30°C/%
10
0
25
5
15
0
25
5
15
0
0
10
25
15
45
21
29
29
35
35
Tf
935
892
945
920
955
934
970
960
921
990
1012
985
965
993
880
930
1075
890
910
860
PI
/%
40
35
37
31
31
40
30
30
30
30
40
40
34
30
30
30
30
30
35
40
t1
/s
0.62
1.83
0.85
1.65
1.35
1.55
1.02
1.21
1.31
0.32
0.14
0.43
1.01
0.66
2.31
1.46
0.13
3.03
2.16
4.45
2.5 x t l
/s
1.55
4.57
2.11
4.13
3.38
3.88
2.56
3.03
3.28
0.79
0.34
1.07
2.53
1.65
5.78
3.66
0.32
7.59
5.40
11.12
t
/s
0.8
2
1
2.3
2
2.2
0.9
1
2
0.7
0.3
0.9
1.2
0.8
5
5
0.1
15
0.5
9
t / t l
/ -
1.29
1.10
1.18
1.39
1.48
1.42
0.88
0.83
1.52
2.21
2.22
2.10
1.19
1.21
2.16
3 ^
0.77
4J4
0.23
2.02
COOUNG
RATE OF
PRIMARY
COOUNG
/°G/s
50
60
60
70
80
70
70
70
70
70
60
60
50
50
30
40
60
80
60
60
VARIATION IN
COOLING
TEMPERATURE
AT PRIMARY
COOUNG/°C
50
60
90
80
80
70
90
60
80
40
60
60
60
60
60
90
50
10
30
60
CRACKED DURING HOT ROLUNG
CRACKED DURING HOT ROLUNG
CRACKED DURING HOT ROLUNG
CRACKED DURING HOT ROLUNG
10
16
20
16
14
13
11
12
16
18
45
25
45
25
25
0
880
910
900
890
920
940
945
885
870
915
880
965
965
965
965
860
40
45
30
35
40
30
35
30
30
31
36
33
34
20
34
40
1.97
1.63
1.74
3.62
1.67
1.51
2.13
2.45
2.78
2.04
2.17
1.05
1.01
1.72
1.01
4.45
4.93
4.08
4.34
9.04
4.19
3.77
5.32
6.13
6.96
5.09
5.42
2.64
2.53
4.30
2.53
11.12
1.5
2.5
0.8
2.2
0.6
1.2
1.6
1.8
0.2
0.6
5
1.2
1.2
2
1.2
9
0.76
1.53
0.46
0.61
0.36
0.80
0.75
0.73
0.07
0.30
2.31
1.14
1,19
1.16
1.19
2.02
50
60
60
60
60
80
60
60
60
70
30
50
50
50
50
60
90
80
100
160
50
60
90
100
80
75
60
60
60
60
60
60
#
TABLE 6
o
h
o
Q.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
COOUNG
RATE
FROM
800°C
TO T3°C
/°C/s
40
50
50
60
50
40
80
70
70
60
60
50
50
40
40
60
5
70
40
50
RETENTION
TIME AT
630°C TO
800°C/s
12
15
14
16
18
11
10
16
8
-
-
-
3
5
5
7
30
10
-
60
T3
/°C
680
680
695
695
670
670
665
665
675
675
660
685
685
675
680
665
655
695
670
680
COOUNG RATE
FROM T3°C TO A
TEMPERATURE
EQUAL TO OR
HIGHER THAN
550°C AND LOWER
THAN T3°C/°C/s
-
-
-
-
-
-
-
-
-
15
18
10
-
-
-
-
-
-
25
-
WINDING
TEMPERATURE
/"C
420
450
468
472
438
447
459
387
459
364
483
415
456
369
550
405
150
395
450
259
RETENTION
TIME
AFTER
WINDING
/min.
290
300
250
290
280
240
100
150
90
50
260
290
300
300
300
250
400
150
10
40
TEMPERATURE
VARIATION
RATE DURING
RETENTION
/°C/h
-20
-10
10
30
40
-20
-40
30
20
-10
40
20
30
-10
-2
-10
40
20
10
50
CRACKED DURING HOT ROLUNG
CRACKED DURING HOT ROLUNG
CRACKED DURING HOT ROLUNG
CRACKED DURING HOT ROLUNG
50
50
40
40
60
90
40
20
40
40
40
50
50
50
50
50
10
8
9
-
-
4
15
10
13
-
5
3
3
3
-
10
680
670
675
680
655
695
675
680
665
680
680
685
685
685
685
685
-
-
-
10
15
-
-
-
-
8
-
-
-
-
30
-
380
375
400
425
410
375
360
480
490
425
550
456
456
456
456
380
100
120
160
120
150
200
300
240
250
100
300
300
300
300
300
100
10
15
-15
-20
17
25
-10
-5
25
35
-2
30
30
30
30
10
TABLE 7
PRODUCTION
No.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
STEEL
No.
a
a
b
b
c
c
d
d
a
a
b
c
c
d
a
b
c
d
g
J
e
f
h
i
k
1
m
n
0
P
q
r
s
t
a
c
c
c
c
J
D1
/ -
2.6
2.2
2.9
2.7
3.5
3
3.9
2.9
3.5
2.1
2.9
3
3.8
3.4
M
5.2
5.8
6.4
M
LI
D2
2.2
2.1
2.8
2.7
3.2
2.8
3.5
2.7
2.9
2
2.6
2.9
3.8
3.1
M
4.3
4.5
4.9
M
M
rC
/ -
0.87
0.9
0.79
0.85
0.72
0.73
0.7
0.9
0.75
0.95
0.72
0.85
0.75
0.75
0.67
0.64
0.71
0.68
0.65
0.65
r30
/ -
1.04
0.96
1.05
1.02
1.1
1.1
1.08
1.06
1.05
1.02
1.06
0.95
0.98
1.05
1.24
1.15
1.08
1.18
1.22
1.15
f20
/%
8
6
8
9
8
7
8
8
9
8
9
7
6
8
12
15
14
15
75
14
dv
7
5
9
5
5
8
4
3
9
8
7
6
5
6
7
16
12
10
12
7
C M A
4.8
4.7
4.2
4.6
3.9
3.8
4.6
4.7
4.8
5
4.8
3.6
3.2
4.5
5.1
4.6
8.3
7.6
8.7
5.8
CRACKED DURING HOT ROLUNG
CRACKED DURING HOT ROLLING
CRACKED DURING HOT ROLLING
CRACKED DURING HOT ROLLING
5.8
3.8
5.9
5.5
5.5
5.7
5.5
5.1
5.5
5.5
6 ^
M
6J
6.4
3.9
4.8
4.8
3.7
5
4.8
4.5
4.5
4.2
4.7
4.7
4.8
M
5J.
5
4.5
3.8
3.8
0.87
0.78
0.9
0.75
0.7
0.75
0.75
0.88
0.78
0.89
0.64
0.68
0.64
0.65
0.74
0.88
1.05
1.1
1.1
1.05
0.99
1.02
0.99
1.02
1.05
1.09
1.25
1.12
1.08
1.25
1.01
1.03
8
10
5
4
6
8
4
3
9
5
10
9
12
15
6
9
2
5
3
2
2
4
2
2
3
3
8
7
8
8
5
8
2.5
5.5
3.5
2.9
4.9
5.6
4.5
1.8
5.5
4.9
4.8
3.2
3.2
3.2
3.2
2.5
^p
TABLE 8
PRODUCTION
No.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
r
/%
13.5
10.5
8.5
10.6
7.8
16.8
14.8
14.2
13
7.8
10
10.5
10.6
12.6
4.8
3.9
4.7
12.5
3.6
0^
F
/%
26
35
42
38
42
39
22
26
25
47
39
42
38
21
20.8
35
52
23
22
62
B
/%
45
48
39
40
36
41
45
42
40
29
39
39
42
38
20
36
0
43
5
8
P
/%
3
0
0
0
10
0
7
5
4
2
0
0
0
10
4
8
0
3
25
0
M
/%
12.5
6.5
10.5
11.4
4.2
3.2
11.2
12.8
18
14.2
12
8.5
9.4
18.4
0.4
17.1
43.3
18.5
44.4
29.5
CRACKED DURING HOT ROLUNG
CRACKED DURING HOT ROLUNG
CRACKED DURING HOT ROLUNG
CRACKED DURING HOT ROLUNG
2.5
3.5
3.7
8.9
12.4
5.8
4.5
3.8
14.5
7.8
3
10.5
10.8
10.5
M
0.8
45
29
34
26
27
24
36
36
25
44
20.8
40
42
42
20
48
39
45
45
46
55
55
45
54
51
35
71.8
43
40
39
38
45
2
6
7
0
0
0
0
4
4
10
4
0
0
0
32
1
11.5
16.5
10.3
19.1
5.6
15.2
14.5
2.2
5.5
3.2
0.4
6.5
7.2
8.5
8.2
5.2
fY
#
TABLE 9
PRODUCTION
No.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
TS
/MPa
1026
985
859
1055
850
1148
1450
1426
760
735
890
788
850
1056
805
840
780
1609
948
489
EL
/%
20
30
35
18
25
18
13
14
28
20
16
19
19
18
15
7
16
8
12
32
A
/%
70
80
85
75
80
40
30
35
60
90
100
80
70
40
35
25
25
10
30
60
d/RmC
/ -
3.3
3.2
3.2
2.3
2.7
2.3
2.1
3.3
3.2
3.2
2.7
2.3
1.5
1.6
1.1
1.2
1.2
1.2
1.1
1.2
TSx A
/MPa%
71820
78800
73015
79125
68000
45920
43500
49910
45600
66150
89000
63040
59500
42240
28175
21000
19500
16090
28440
29340
TSX EL
/MPa%
20520
29550
30065
18990
21250
20664
18850
19964
21280
14700
14240
14972
16150
19008
12075
5880
12480
12872
11376
15648
CRACKED DURING HOT ROLUNG
CRACKED DURING HOT ROLUNG
CRACKED DURING HOT ROLUNG
CRACKED DURING HOT ROLUNG
457
594
764
793
864
1126
945
602
1015
530
750
790
820
830
870
440
39
31
27
25
23
20
25
32
24
34
11
18
20
22
10
30
80
78
45
60
55
35
32
75
31
95
21
35
30
35
12
60
2.5
2.1
3.1
2.7
1.9
1.7
1.8
1.7
1.5
1.6
1.3
1.3
1.3
1.3
1.1
2.1
36560
46332
34380
47580
47520
39410
30240
45150
31465
50350
15750
27650
24600
29050
10440
26400
17823
18414
20628
19825
19872
22520
23625
19264
24360
18020
8250
14220
16400
18260
8700
13200
REMARKS
EXAMPLE
EXAMPLE
EXAMPLE
EXAMPLE
EXAMPLE
EXAMPLE
EXAMPLE
EXAMPLE
EXAMPLE
EXAMPLE
EXAMPLE
EXAMPLE
EXAMPLE
EXAMPLE
COMPARATIVE
EXAMPLE
COMPARATIVE
EXAMPLE
COMPARATIVE
EXAMPLE
COMPARATIVE
EXAMPLE
COMPARATIVE
EXAMPLE
COMPARATIVE
EXAMPLE
COMPARATIVE
EXAMPLE
COMPARATIVE
EXAMPLE
COMPARATIVE
EXAMPLE
COMPARATIVE
EXAMPLE
EXAMPLE
EXAMPLE
EXAMPLE
EXAMPLE
EXAMPLE
EXAMPLE
EXAMPLE
EXAMPLE
EXAMPLE
EXAMPLE
COMPARATIVE
EXAMPLE
COMPARATIVE
EXAMPLE
COMPARATIVE
EXAMPLE
COMPARATIVE
EXAMPLE
COMPARATIVE
EXAMPLE
COMPARATIVE
EXAMPLE
f ^

#
[Document Type] CLAIMS
[Claim 1]
A hot-rolled steel sheet being a steel sheet having a chemical composition, by
mass%, of
C: 0.02% to 0.5%,
Si: 0.001% to 4.0%,
Mn: 0.001% to 4.0%,
Al: 0.001% to 4.0%,
P: 0.15% or less,
S: 0.03% or less,
N: 0.01% or less,
0:0.01% or less,
and the balance consisting of Fe and unavoidable impurities,
wherein a sum of a content of the Si and a content of the Al is 1.0% to 4.0% in
the chemical composition of the steel sheet,
an average pole density of an orientation group from {100}<011> to
{223}<110>, which is a pole density expressed by an arithmetic average of pole
densities of respective crystal orientations {100}<011>, {116}<110>, {114}<110>,
{112}<110>, and {223}<110>, is 1.0 to 6.5, and a pole density of a crystal orientation
{332}<113> is 1.0 to 5.0 in a sheet thickness central portion within a range of 5/8 to
3/8 of a sheet thickness,
a microstmcture of the steel sheet includes a plurality of grains,
the microstructure of the steel sheet includes, by an area ratio, 2% to 30% of
retained austenite, 20% to 50% of ferrite, \0% to 60% of bainite, 20% or less of
pearlite, and 20% or less of martensite, and
A9' -
^p
rC that is a Lankford value in a direction orthogonal to a rolling direction is
0.70 to 1.10, and r30 that is a Lankford value in a direction forming an angle of 30°
with the rolling direction is 0.70 to 1.10.
[Claim 2]
The hot-rolled steel sheet according to Claim 1,
wherein the chemical composition of the steel sheet further includes, by
mass%, one or more selected from the group consisting of
Ti: 0.001% to 0.2%,
Nb: 0.001% to 0.2%,
V: 0.001% to 1.0%,
W: 0.001% to 1.0%,
Cu: 0.001% to 2.0%,
B: 0.0001% to 0.005%,
Mo: 0.001% to 1.0%,
Cr: 0.001% to 2.0%,
As: 0.0001% to 0.50%,
Mg: 0.0001% to 0.010%,
REM: 0.0001% to 0.1%,
Ca: 0.0001% to 0.010%,
Ni: 0.001% to 2.0%,
Co: 0.0001% to 1.0%,
Sn: 0.0001% to 0.2%, and
Zr: 0.0001% to 0.2%.
[Claim 3]
The hot-rolled steel sheet according to Claim 1 or 2,
wherein a volume average diameter of the grains is 1 jxm to 4 |j,m.
[Claim 4]
The hot-rolled steel sheet according to Claim 1 or 2,
wherein the average pole density of the orientation group from {100}<011>to
{223}<110> is 1.0 to 5.0, and the pole density of the crystal orientation {332}<113> is
1.0 to 4.0.
[Claim 5]
The hot-rolled steel sheet according to Claim 1 or 2,
wherein among the plurality of grains, an area ratio of grains which exceed 20
lim is limited to 10% or less.
[Claim 6]
The hot-rolled steel sheet according to Claim 1 or 2,
wherein with regard to at least 100 grains of the retained austenite and the
martensite, a standard deviation of a distance LMA between the grains closest to each
other is 5 |am or less.
[Claim 7]
A production method of the hot-rolled steel sheet, the production method
comprising:
a first hot-rolling process of performing a hot rolling with respect to a steel, so
as to set an average austenite grain size of the steel to 200 |j,m or less, the first hotrolling
process includes,
wherein a pass is performed, at least one or more times, with a rolling
reduction ratio of 40% or more, in a temperature range of 1,000°C to 1,200°C,
the steel comprises a chemical composition which includes, by mass%,
C: 0.02% to 0.5%,
6b
#
Si: 0.001% to 4.0%,
Mn: 0.001% to 4.0%,
Al: 0.001% to 4.0%,
P: 0.15% or less,
S: 0.03% or less,
N: 0.01% or less,
O: 0.01% or less, and
the balance consisting of Fe and unavoidable impurities, and
wherein a sum of a content of the Si and a content of the Al is 1.0% to 4.0%;
a second hot-rolling process of performing the hot rolling with respect to the
steel, the second hot-rolling process includes,
wherein large-rolling-reduction passes with a rolling reduction ratio of 30% or
more in a temperature range of Tl + 30°C to Tl + 200°C when a temperature
calculated by a following Expression 1 is set to T1°C,
an accumulative rolling reduction ratio in the temperature range of Tl + 30°C
to Tl + 200°C is 50% or more,
an accumulative rolling reduction ratio in a temperature range, that is higher
than or equal to Ar3°C and lower than Tl + 30°C, is limited to 30% or less, and
a rolling terminal temperature is Ar3°C or higher;
a primary cooling process of performing a cooling with respect to the steel,
wherein a standby time t (second), which is set as a time fi"om a completion of
a final pass among the large-rolling-reduction passes to a cooling start, satisfies
Expression 2;
a cooling process of performing a cooling with respect to the steel,
wherein the steel is cooled to a temperature T3 within a range of 630°C to
#
800°C at an average cooling rate of 10°C/s to 100°C/s;
a retention process of performing a retaining, wherein the steel is retained
within the temperature range of 630°C to 800°C for 1 second to 20 seconds, or a slow
cooling process of a slow cooling with respect to the steel, wherein the steel is slowly
cooled from the temperature T3 to a temperature range within lower than T3 and
higher than or equal to 550°C at an average cooling rate of 20°C/s or less;
a winding process of performing a winding of the steel in a temperature range
of350°Cto500°C;and
an air cooling process of performing a cooling of the steel with air,
wherein the steel, which is retained at the temperature range of 350°C to
500°C for 30 minutes to 300 minutes, is then cooled by the air,
herein,
Tl = 850 + 10 X ([C] + [N]) X [Mn] (Expression 1)
here, [C], [N], and [Mn] represent mass percentages of the content of C, the
content of N, and the content of Mn in the steel, respectively,
t < 2.5 X tl (Expression 2)
here, tl is expressed by the following Expression 3,
tl = 0.001 X ((Tf-Tl) X Pl/100)^ - 0.109 x ((Tf- Tl) x Pl/100) + 3.1
(Expression 3)
here, Tf represents a celsius temperature of the steel at the time of completion
of the final pass, and PI represents a percentage of the rolling reduction ratio during
the final pass.
[Claim 8]
The production method of the hot-rolled steel sheet according to Claim 7, the
production method comprising
9-
61
#
wherein the steel comprises the chemical composition which further includes,
by mass%, one or more selected from the group consisting of
Ti: 0.001% to 0.2%,
Nb: 0.001% to 0.2%,
V: 0.001% to 1.0%,
W: 0.001% to 1.0%,
Cu: 0.001% to 2.0%,
B: 0.0001% to 0.005%,
Mo: 0.001% to 1.0%,
Cr: 0.001% to 2.0%,
As: 0.0001% to 0.50%,
Mg: 0.0001% to 0.010%,
REM: 0.0001% to 0.1%,
Ca: 0.0001% to 0.010%,
Ni: 0.001% to 2.0%,
Co: 0.0001% to 1.0%,
Sn: 0.0001% to 0.2%,
Zr: 0.0001% to 0.2% of Zr, and
wherein a temperature calculated by the following Expression 4 in place of
the temperature calculated by the Expression 1 is set as the TI.
TI = 850 + 10 X ([C] + [N]) X [Mn] + 350 x [Nb] + 250 x [Ti] + 40 x [B] +
10 X [Cr] + 100 x[Mo] + 100 x [V] (Expression 4)
here, [C], [N], [Mn], [Nb], [Ti], [B], [Cr], [Mo], and [V] represent mass
percentages of the content of C, the content of N, the content of Mn, the content of Nb,
the content of Ti, the content of B, the content of Cr, the content of Mo, and the content
#
of V in the steel, respectively.
[Claim 9]
The production method of the hot-rolled steel sheet according to Claim 7 or 8,
wherein the standby time t (second) further satisfies the following Expression
5 using tl.
t < tl (Expression 5)
[Claim 10]
The production method of the hot-rolled steel sheet according to Claim 7 or 8,
wherein the standby time t (second) further satisfies the following Expression
6 using tl.
tl < t < tl X 2.5 (Expression 6)
[Claim 11]
The production method of the hot-rolled steel sheet according to Claim 7 or 8,
wherein in the primary cooling process, the average cooling rate is 50°C/s or
more, a cooling temperature variation that is a difference between a steel temperature
at the start time of a cooling and a steel temperature at the finish time of the cooling is
40°C to 140°C, and the steel temperature at the finish time of the cooling is Tl +
100°C or lower
[Claim 12]
The production method of the hot-rolled steel sheet according to Claim 7 or 8,
wherein a final pass of rolling within the temperature range of Tl + 30°C to
Tl + 200°C is the large-rolling-reduction pass.
[Claim 13]
The production method of the hot-rolled steel sheet according to Claim 7 or 8,
wherein in the temperature range control, a temperature variation rate is -
«
40°C/h to 40°C/h.
[Claim 14]
The production method of the hot-rolled steel sheet according to Claim 7 or 8,
wherein the primary cooling process is performed between rolling stands.