1
DESCRIPTION
HOT-ROLLED STEEL SHEET AND METHOD FOR PRODUCING SAME
Technical Field
[OOO 11
The present invention relates to a high-strength hot-rolled steel sheet which is
excellent in uniform deformability contributing to stretchability, dfawability, or the like
and is excellent in local deformability contributing to bendability, stretch flangeability,
burring formability, or the like, and relates to a method for producing the same.
Particularly, the present invention relates to a steel sheet including a Dual Phase @P)
structure.
Priority is claimed on Japanese Patent Application No. 201 1-1 17432, filed on
May 25,20 11, and the content of which is incorporated herein by reference.
Background of Invention
[0002]
In order to suppress emission of carbon dioxide gas from a vehicle, a weight
reduction of an automobile body has been attempted by utilization of a high-strength
steel sheet. Moreover, from a viewpoint of ensuring safety of a passenger, the
utilization of the high-strength steel sheet for the automobile body has been attempted in
addition to a mild steel sheet. However, in order to further improve the weight
reduction of the automobile body in future, a usable strength level of the high-strength
steel sheet should be increased as compared with that of conventional one. Moreover,
in order to utilize the high-strength steel sheet for suspension parts or-_the like of the
2 -
automobile body, the local deformability contributing to the burring formability or the
like should also be improved in addition to the uniform deformability.
[0003]
However, in general, when the strength of steel sheet is increased, the
formability (deformability) is decreased. For example, Non-Patent Document 1
discloses that uniform elongation which is important for drawing or stretching is
decreased by strengthening the steel sheet.
[0004]
Contrary, Non-Patent Document 2 discloses a method which secures the uniform
elongation by compositing metallographic structure of the steel sheet even when the
strength is the same.
[0005]
In addition, Non-Patent Document 3 discloses a metallographic structure control
method which improves local ductility representing the bendability, hole expansibility, or
the burring formability by controlling inclusions, controlling the microstructure to single
phase, and decreasing hardness difference between microstructures. In the Non-Patent
Document 3, the microstructure of the steel sheet is controlled to the single phase by
microstructure control, and thus, the local defoimability contributing to the hole
expansibility or the like is improved. However, in order to control the microstructure to
the single phase, a heat treatment from an austenite single phase is a basis producing
method as described in Non-Patent Document 4.
[0006]
In addition, the Non-Patent Document 4 discloses a technique which satisfies
both the strength and the ductility of the steel sheet by controlling a cooling after a
hot-rolling in order to control the metallographic structure, specifically, in order to obtain
3
intended morphologies of precipitates and transformation structures and to obtain an
appropriate fiaction of ferrite and bainite. However, all techniques as described above
are the improvement methods for the local deformability which rely on the
microstructure control, and are largely influenced by a microstructure formation of a
5 base.
[0007]
Also, a method, which improves material properties of the steel sheet by
increasing reduction at a continuous hot-rolling in order to refine grains, is known as a -
related art. For example, Non-Patent Document 5 discloses a technique which improves
10 the strength and toughness of the steel sheet by conducting a large reduction rolling in a
comparatively lower temperature range within an austenite range in order to refine the
grains of ferrite which is a primary phase of a product by transforming non-recrystallized
austenite into the ferrite. However, in Non-Patent Document 5, a method for improving
the local deformability to be solved by the present invention is not considered at all.
15
Related Art Documents
Non-Patent Documents
[OOOS]
[Non-Patent Document 11 Kishida: Nippon Steel Technical Report No.371
20 (2999), p. 13.
won-Patent Document 21 0. Matsumura et al: Trans. ISIJ vo1.27 (1987),
p.570.
won-Patent Document 31 Katoh et al: Steel-manufacturing studies vo1.3 12
(1984), p.41.
25 won-Patent Document 41 K. Sugimoto et al: vol. 40 (2000), p.920.
4
mon-Patent Document 51 NFG product introduction of NAKAYAMA STEEL
WORKS, LTD.
Summary of Invention
Technical Problem
[0009]
As described above, it is the fact that the technique, which simultaneously
satisfies the high-strength and both properties of the uniform deformability and the local
deformability, is not found. For example, in order to improve the local deformability of
the high-strength steel sheet, it is necessary to conduct the microstructure control
including the inclusions. However, since the improvement relies on the microstructure
control, it is necessary to control the fraction or the morphology of the microstructure
such as the precipitates, the ferrite, or the bainite, and therefore the metallographic
structure of the base is limited. Since the metallographic structure of the base is
restricted, it is difficult not only to improve the local deformability but also to
simultaneously improve the strength and the local deformability.
[OO lo]
An object of the present invention is to provide a hot-rolled steel sheet which
has the high-strength, the excellent uniform deformability, the excellent local
deformability, and small orientation dependence (anisotropy) of formability by
controlling texture and by controlling the size or the morphology of the grains in addition
to the metallographic structure of the base, and is to provide a method for producing the
same. Herein, in the present invention, the strength mainly represents tensile strength,
and the high-strength indicates the strength of 440 MPa or more in the tensile strength.
In addition, in the present invention, satisfaction of the high-strength, the excellent
5
uniform deformability, and the excellent local deformability indicates a case of
simultaneously satisfying all conditions of TS > 440 (unit: MPa), TS x u-EL > 7000
(unit: ma.%), TS x h 2 30000 (unit: m a % ) , and d / RmC 2 1 (no unit) by using
characteristic values of the tensile strength (TS), the uniform elongation (u-EL), hole
5 expansion ratio (A), and d / RmC which is a ratio of thickness d to minimum radius RmC
of bending to a C-direction.
Solution to Problem
[OO 111
10 In the related arts, as described above, the improvement in the local
deformability contributing to the hole expansibility, the bendability, or the like has been
attempted by controlling the inclusions, by refining the precipitates, by homogenizing the
microstructure, by controlling the microstructure to the single phase, by decreasing the
hardness difference between the microstructures, or the like. However, only by the
15 above-described techniques, main constituent of the microstructure must be restricted.
In addition, when an element largely contributing to an increase in the strength, such as
representatively Nb or Ti, is added for high-strengthening, the anisotropy may be
significantly increased. Accordingly, other factors for the formability must be
abandoned or directions to take a blank before forming must be limited, and as a result,
20 the application is restricted. On the other hand, the uniform deformability can be
improved by dispersing hard phases such as martensite in the metallographic structure.
[OO 121
In order to obtain the high-strength and to improve both the uniform
deformability contributing to the stretchability or the like and the local deformability
25 contributing to the hole expansibility, the bendability, or the like, the inventors have
6
newly focused influences of the texture of the steel sheet in addition to the control of the
fraction or the morphology of the metallographic structures of the steel sheet, and have
investigated and researched the operation and the effect thereof in detail. As a result,
the inventors have found that, by controlling a chemical'composition, the metallographic
5 structure, and the texture represented by pole densities of each orientation of a specific
crystal orientation group of the steel sheet, the high-strength is obtained, the local
deformability is remarkably improved due to a balance of Lankford-v.a . lues (r values) in a
rolling direction, in a direction (C-direction) making an angle of 90" with the rolling
direction, in a direction making an angle of 30" with the rolling direction, or in a
10 direction making an angle of 60" with the rolling direction, and the uniform
deformability is also secured due to the dispersion of the hard phases such as the
martensite.
[00 131
An aspect of the present invention employs the following.
15 (1) A hot-rolled steel sheet according to an aspect of the present invention
includes, as a chemical composition, by mass%, C: 0.0 1% to 0.4%, Si: 0.00 1% to 2.5%,
Mn: 0.001% to 4.0%, Al: 0.001% to 2.0%, P: limited to 0.15% or less, S: limited to
0.03% or less, N: limited to 0.01% or less, 0: limited to 0.01% or less, and a balance
consisting of Fe and unavoidable impurities, wherein: an average pole density of an
20 orientation group of {100}<011> to (22314 lo>, which is a pole density represented by
an arithmetic average of pole densities of each crystal orientation { 100}<0 1 1>,
{116}<110>, {114}<110>, {112}<110>, and {223}<110>, is 1.0 to 5.0 and apole
density of a crystal orientation {332}<113> is 1.0 to 4.0 in a thickness central portion
which is a thickness range of 518 to 318 based on a surface of the steel sheet; the steel
25 sheet includes, as a metallographic structure; plural grains, and includes, by area%, a
7
ferrite and a bainite of 30% to 99% in total and a martensite of 1% to 70%; and when an
area fraction of the martensite is defined as fM in unit of area%, an average size of the
martensite is defined as dia in unit of pm, an average distance between the martensite is
defined as dis in unit of pm, and a tensile strength of the steel sheet is defined as TS in
5 unit of MPa, a following Expression 1 and a following Expression 2 are satisfied.
dia 5 13 pm . . . (Expression 1)
TS / fM x dis / dia > 500 . . . (Expression 2)
(2) The hot-rolled steel sheet according to (1) may further includes, as the
chemical composition, by mass %, at least one selected from the group consisting of Mo:
10 0.001% to 1.0%, Cr: 0.001% to 2.0'74 Ni: 0.001% to 2.0%, Cu: 0.001% to 2.0%, B:
0.0001% to 0.005%, Nb: 0.001% to 0.2%, Ti: 0.001% to 0.2%, V: 0.001% to 1.0%, W:
0.001% to 1.0%, Ca: 0.0001% to 0.01%, Mg: 0.0001% to 0.01%, Zr: 0.0001% to 0.2%,
Rare Earth Metal: 0.0001% to 0.1%, As: 0.0001% to 0.5%, Co: 0.0001% to 1.0%, Sn:
0.0001% to 0.2%, Pb: 0.0001% to 0.2%, Y 0.0001% to 0.2%, and Hf: 0.0001% to 0.2%.
15 (3) In the hot-rolled steel sheet according to (1) or (2), a volume average
diameter of the grains may be 5 pm to 30 pm.
(4) In the hot-rolled steel sheet according to (1) or (2), the average pole density
of the orientation group of (1 00)<0 11s to (2231-4 10> may be 1.0 to 4.0, and the pole
density of the crystal orientation (33214 13> may be 1.0 to 3.0.
20 (5) In the hot-rolled steel sheet according to any one of (1) to (4), when a
major axis of the martensite is defined as La, and a minor axis of the martensite is
defined as Lb, an area fraction of the martensite satisfying a following Expression 3 may
be 50% to 100% as compared with the area fraction fM of the martensite.
La / Lb I 5.0 . . . (Expression 3)
8
(6) In the hot-rolled steel sheet according to any one of (1) to (9, the steel
sheet may include, as the metallographic structure, by area%, the ferrite of 30% to 99%.
(7) In the hot-rolled steel sheet according to any one of (1) to (6), the steel
sheet may include, as the metallographic structure, by area%, the bainite of 5% to 80%.
(8) In the hot-rolled steel sheet according to any one of (1) to (7), the steel
sheet may include a tempered martensite in the martensite.
(9) In the hot-rolled steel sheet according to any one of (1) to (8), an area
fraction of coarse grain having grain size of more than 35 pm may be 0% to 10% among
the grains in the metallographic structure of the steel sheet.
10 (1 0) In the hot-rolled steel sheet according to any one of (1) to (9), a hardness
H of the ferrite may satisfy a following Expression 4.
H<200 + 30 x [Si] +21 x [Mn] +270 x [PI + 78 x w]'" + 108 x
[ ~ i ] ".~. (.E xpression 4)
(1 1) In the hot-rolled steel sheet according to any one of (I) to (lo), when a
15 hardness of the ferrite or the bainite which is a primary phase is measured at 100 points
or more, a value dividing a standard deviation of the hardness by an average of the
hardness may be 0.2 or less.
(12) A method for producing a hot-rolled steel sheet according to an aspect of
the present invention includes: first-hot-rolling a steel in a temperature range of 1000°C
20 to 1200°C under conditions such that at least one pass whose reduction is 40% or more is
included so as to control an average grain size of an austenite in the steel to 200 pm or
less, wherein the steel includes, as a chemical composition, by mass%, C: 0.01% to 0.496,
Si: 0.001% to 2.5%, Mn: 0.001% to 4.0%, Al: 0.001% to 2.0%, P: limited to 0.15% or
less, S: limited to 0.03% or less, N: limited to 0.01% or less, 0: limited to 0.01% or less,
and a balance consisting of Fe and unavoidable impurities; second-hot-rolling the steel
under conditions such that, when a temperature calculated by a following Expression 5 is
defined as T1 in unit of "C and a ferritic transformation temperature calculated by a
following Expression 6 is defined as AT3 in unit of "C, a large reduction pass whose
5 reduction is 30% or more in a temperature range of T1 + 30°C to T1 + 200°C is included,
a cumulative reduction in the temperature range of Tl + 30°C to T1 + 200°C is 50% or
more, a cumulative reduction in a temperature range o f k 3 to lower than T1 + 30°C is
limited to 30% or less, and a rolling finish temperature is AT3 or higher; first-cooling the
steel under conditions such that, when a waiting time from a finish of a final pass in the
10 large reduction pass to a cooling start is defined as t in unit of second, the waiting time t
satisfies a following Expression 7, an average cooling rate is 50 "Clsecond or faster, a
cooling temperature change which is a difference between a steel temperature at the
cooling start and a steel temperature at a cooling finish is 40°C to 140°C, and the steel
temperature at the cooling finish is T1 + 100°C or lower; second-cooling the steel to a
15 temperature range of 600°C to 800°C under an average cooling rate of 15 "Clsecond to
300 "Clsecond after finishing the second-hot-rolling; holding the steel in the temperature
range of 600°C to 800°C for 1 second to 15 seconds; third-cooling the steel to a
temperature range of a room temperature to 350°C under an average cooling rate of 50
"Clsecond to 300 "Clsecond after finishing the holding; coiling the steel in the
20 temperature range of the room temperature to 350°C.
TI = 850 + 10 x ([C] + [N]) x [Mn] . . . (Expression 5)
here, [C], [N], and [Mn] represent mass percentages of C, N, and Mn
respectively.
AT3 = 879.4 - 516.1 x [C] - 65.7 x [Mn] + 38.0 x [Si] + 274.7 x [P] ...
(Expression 6)
here, in Expression 6, [C], [Mn], [Si] and [PI represent mass percentages of C,
Mn, Si, and P respectively.
t I 2.5 x tl.. . (Expression 7)
5 here, tl is represented by a following Expression 8.
tl =0.001 x ((Tf-T1) x P1 / 100)~- 0.109 x ((Tf-TI) x P1 / 100)+3.1 ...
(Expression 8)
here, Tf represents a celsius temperature of the steel at the finish of the final pass,
and P1 represents a percentage of a reduction at the final pass.
10 (13) In the method for producing the hot-rolled steel sheet according to (12),
the steel may further includes, as the chemical composition, by mass%, at least one
selected from the group consisting of Mo: 0.001% to 1.0%, Cr: 0.001% to 2.0%, Ni:
0.001% to 2.0%, Cu: 0.001% to 2.0%, B: 0.0001% to 0.005%, Nb: 0.001% to 0.2%, Ti:
0.001% to 0.2%, V: 0.001% to 1.0%, W: 0.001% to 1.0%, Ca: 0.0001% to 0.01%, Mg:
15 0.0001% to 0.01%, Zr: 0.0001% to 0.2%, Rare Earth Metal: 0.0001% to 0.1%, As:
0.0001% to 0.5%, Co: 0.0001% to 1.0%, Sn: 0.0001% to 0.2%, Pb: 0.0001% to 0.2%, Y
0.0001% to 0.2%, and Hf 0.0001% to 0.2%, wherein a temperature calculated by a
following Expression 9 may be substituted for the temperature calculated by the
Expression 5 as TI.
20 T1=850+10x([C]+~)x[Mn]+350x[Nb]+250x[Ti]+40x[B]+10x
[Cr] + 100 x [Mo] + 100 x [V] . . . (Expression 9)
here, CCI, N, [Mnl, [Nbl, [Ti], P I , [Crl, [Moly and [Vl represent mass
percentages of C, N, Mn, Nb, Ti, B, Cry Mo, and V respectively.
(14) In the method for producing the hot-rolled steel sheet according to (12) or
25 (13), the waiting time t may further satisfy a following Expression 10.
11
0 I t < tl . . . (Expression 10)
(15) In the method for producing the hot-rolled steel sheet according to (12) or
(1 3), the waiting time t may firther satisfy a following Expression 11.
tl I t 5 tl x 2.5.. . (Expression 11)
(16) In the method for producing the hot-rolled steel sheet according to any
one of (12) to (15), in the first-hot-rolling, at least two times of rollings whose reduction
is 40% or more may be conducted, and the average grain size of the austenite may be
controlled to 100 km or less.
(17) In the method for producing the hot-rolled steel sheet according to any
10 one of (12) to (16), the second-cooling may start within 3 seconds after finishing the
second-hot-rolling.
(1 8) In the method for producing the hot-rolled steel sheet according to any
one of (12) to (17), in the second-hot-rolling, a temperature rise of the steel between
passes may be 18°C or lower.
(19) In the method for producing the hot-rolled steel sheet according to any
one of (12) to (18), a final pass of rollings in the temperature range of T1 + 30°C to T1 +
200°C may be the large reduction pass.
(20) In the method for producing the hot-rolled steel sheet according to any
one of (12) to (19), in the holding, the steel may be held in a temperature range of 600°C
20 to 680°C for 3 seconds to 15 seconds.
(2 1) In the method for producing the hot-rolled steel sheet according to any
one of (12) to (20), the first-cooling may be conducted at an interval between rolling
stands.
12
Advantageous Effects of Invention
[00 141
According to the above aspects of the present invention, it is possible to obtain a
hot-rolled steel sheet which has the high-strength, the excellent uniform deformability,
5 the excellent local deformability, and the small anisotropy even when the element such as
Nb or Ti is added.
Brief Description of Drawings
[00 151
10 FIG. 1 shows a relationship between an average pole density Dl of an
orientation group of _/100}<011> to {223)<110> and d / RmC (thickness d I minimum
bend radius RrnC).
FIG. 2 shows a relationship between a pole density D2 of a crystal orientation
{332}<113> and d I RmC.
15
Detailed Description of Preferred Embodiments
[00 161
Hereinafter, a hot-rolled steel sheet according to an embodiment of the present
invention will be described in detail. First, a pole density of a crystal orientation of the
20 hot-rolled steel sheet will be described.
[00 171
Average Pole Density Dl of Crystal Orientation: 1.0 to 5.0
Pole Density D2 of Crystal Orientation: 1.0 to 4.0
In the hot-rolled steel sheet according to the embodiment, as the pole densities
25 of two kinds of the crystal orientations, the average pole density Dl of an orientation
13
group of { 100)<011> to (22314 10> (hereinafter, referred to as "average pole density")
and the pole density D2 of a crystal orientation {332)<113> in a thickness central portion,
which is a thickness range of 518 to 318 (a range which is 518 to 318 of the thickness
distant from a surface of the steel sheet along a normal direction (a depth direction) of the
5 steel sheet), are controlled in reference to a thickness-cross-section (a normal vector
thereof corresponds to the normal direction) which is parallel to a rolling direction.
[00 181
In the embodiment, the average pole density Dl is an especially-important
characteristic (orientation integration and development degree of texture) of the texture
10 (crystal orientation of grains in metallographic structure). Herein, the average pole
density Dl is the pole density which is represented by an arithmetic average of pole
densities of each crystal orientation {100)<011>, (1 1614 lo>, (1 1414 lo>,
{112)<110>, and {223)<110>.
[00 191
15 A intensity ratio of electron difiaction intensity or X-ray diffraction intensity of
each orientation to that of a random sample is obtained by conducting Electron Back
Scattering Diffraction (EBSD) or X-ray diffraction on the above cross-section in the
thickness central portion which is the thickness range of 518 to 318, and the average pole
density Dl of the orientation group of {100)<011> to (2231-4 10> can be obtained from
20 each intensity ratio.
[0020]
When the average pole density Dl of the orientation group of { 100}<011> to
{223)<110> is 5.0 or less, it is satisfied that d I RrnC (a parameter in which the thickness
d is divided by a minimum bend radius RmC (C-direction bending)) is 1.0 or more,
25 which is minimally-required for working suspension parts or frame parts. Particularly,
14
the condition is a requirement in order that tensile strength TS, hole expansion ratio h,
and total elongation EL preferably satisfy TS x h 2 30000 and TS x EL 2 14000 which
are two conditions required for the suspension parts of the automobile body.
[0021]
In addition, when the average pole density Dl is 4.0 or less, a ratio (Rm45 /
RmC) of a minimum bend radius Rm45 of 45"-direction bending to the minimum bend
radius RmC of the C-direction bending is decreased, in which the ratio is a parameter of
orientation dependence (isotropy) of formability, and the excellent local deformability
which is independent of the bending direction can be secured. As described above, the
10 average pole density Dl may be 5.0 or less, and may be preferably 4.0 or less. In a case
where the further excellent hole expansibility or small critical bending properties are
needed, the average pole density Dl may be more preferably less than 3.5, and may be
furthermore preferably less than 3.0.
[0022]
15 When the average pole density Dl of the orientation group of { 100)<011> to
(22314 10> is more than 5.0, the anisotropy of mechanical properties of the steel sheet
is significantly increased. As a result, although the local deformability in only a
-
specific direction is improved, the local deformability in a direction different from the
specific direction is significantly decreased. Therefore, in the case, the steel sheet
20 cannot satisfjr d 1 RmC 2 1 .O.
[0023]
On the other hand, when the average pole density Dl is less than 1.0, the local
deformability may be decreased. Accordingly, preferably, the average pole density Dl
may be 1.0 or more.
2 5 [0024]
15
In addition, from the similar reasons, the pole density D2 of the crystal
orientation {332)<113> in the thickness central portion which is the thickness range of
518 to 318 may be 4.0 or less. The condition is a requirement in order that the steel sheet
satisfies d / RmC 2 1.0, and particularly, that the tensile strength TS, the hole expansion
5 ratio h, and the total elongation EL preferably satisfy TS x h 2 30000 and TS x EL 2
14000 which are two conditions required for the suspension parts.
100251
Moreover, when the pole density D2 is 3.0 or less, TS x h or d 1 RmC can be
further improved. The pole density D2 may be preferably 2.5 or less, and may be more
10 preferably 2.0 or less. When the pole density D2 is more than 4.0, the anisotropy of the
mechanical properties of the steel sheet is significantly increased. As a result, although
the local deformability in only a specific direction is improved, the local deformability in
a direction different from the specific direction is significantly decreased. Therefore, in
the case, the steel sheet cannot sufficiently satisfy d 1 RmC 2 1 .O.
On the other hand, when the average pole density D2 is less than 1.0, the local
deformability may be decreased. Accordingly, preferably, the pole density D2 of the
crystal orientation {332}<113> may be 1.0 or more.
The pole density is synonymous with an X-ray random intensity ratio. The
X-ray random intensity ratio can be obtained as follows. Diffraction intensity (X-ray or
electron) of a standard sample which does not have a texture to a specific orientation and
diffraction intensity of a test material are measured by the X-ray diffraction method in the
same conditions. The X-ray random intensity ratio is obtained by dividing the
25 diffraction intensity of the test material by the diffraction intensity of the standard sample.
16
The pole density can be measured by using the X-ray diffraction, the Electron Back
Scattering Diffraction (EBSD), or Electron Channeling Pattern (ECP). For example, the
average pole density Dl of the orientation group of {100}<0 11> to (2231-4 10> can be
obtained as follows. The pole densities of each orientation (1 00)<1 lo>, (1 161-4 lo>,
(1 14)<1 lo>, (1 1214 lo>, and (22314 10> are obtained fiom a three-dimensional
texture (ODF: Orientation Distribution Functions) which is calculated by a series
expanding method using plural pole figures in pole figures of { 11 0}, { 100}, (2 1 1 }, and
(3 10) measured by the above methods. The average pole density Dl is obtained by
calculating an arithmetic average of the pole densities.
[0028]
With respect to samples which are supplied for the X-ray diffraction, the EBSD,
and the ECP, the thickness of the steel sheet may be reduced to a predetermined thickness
by mechanical polishing or the like, strain may be removed by chemical polishing,
electrolytic polishing, or the like, the samples may be adjusted so that an appropriate
surface including the thickness range of 518 to 318 is a measurement surface, and then the
pole densities may be measured by the above methods. With respect to a transverse
direction, it is preferable that the samples are collected in the vicinity of 114 or 314
position of the thickness (a position which is at 114 of a steel sheet width distant from a
side edge the steel sheet).
[0029]
When the above pole densities are satisfied in many other thickness portions of
the steel sheet in addition to the thickness central portion, the local deformability is
further improved. However, since the texture in the thickness central portion
significantly influences the anisotropy of the steel sheet, the material properties of the
thickness central portion approximately represent the material properties of the entirety
17
of the steel sheet. Accordingly, the average pole density Dl of the orientation group of
{100)<011> to (2231-4 lo> and the pole density D2 of the crystal orientation
{332)<113> in the thickness central portion of 518 to 318 are prescribed.
[0030]
Herein, {hkl) indicates that the normal direction of the sheet surface is
parallel to and the rolling direction is parallel to when the sample is
collected by the above-described method. In addition, generally, in the orientation of
the crystal, an orientation perpendicular to the sheet surface is represented by (hkl) or
{hkl) and an orientation parallel to the rolling direction is represented by [uvw] or
10 . {hkl) indicates collectively equivalent planes, and (hkl)[uvw] indicates
each crystal plane. Specifically, since the embodiment targets a body centered cubic
(bcc) structure, for example, (1 1 I), (-1 1 I), (1-1 I), (1 1 -11, (-1-1 I), (-11-I), (1-1-I), and
(-1-1-1) planes are equivalent and cannot be classified. In the case, the orientation is
collectively called as (1 11). Since the ODF expression is also used for orientation
15 expressions of other crystal structures having low symmetry, generally, each orientation
is represented by (hkl)[uvw] in the ODF expression. However, in the embodiment,
{hkl) and (hkl)[uvw] are synonymous.
[003 11
Next, a metallographic structure of the hot-rolled steel sheet according to the
20 embodiment will be described.
[0032]
A metallographic structure of the hot-rolled steel sheet according to the
embodiment is fundamentally to be a Dual Phase @P) structure which includes plural
grains, includes ferrite andlor bainite as a primary phase, and includes martensite as a
25 secondary phase. The strength and the uniform deformability can be increased by
or the bainite which is the primary phase and has the excellent deformability. The
improvement in the uniform deformability is derived from an increase in work hardening
rate by finely dispersing the martensite which is the hard phase in the metallographic
I 5 structure. Moreover, herein, the ferrite or the bainite includes polygonal ferrite and
I bainitic ferrite.
[0033]
The hot-rolled steel sheet according to the embodiment includes residual
austenite, pearlite, cementite, plural inclusions, or the like as the microstructure in
10 addition to the ferrite, the bainite, and the martensite. It is preferable that the
microstructures other than the ferrite, the bainite, and the martensite are limited to, by
area %, 0% to 10%. Moreover, when the austenite is retained in the microstructure,
secondary work embrittlement or delayed fracture properties deteriorates. Accordingly,
except for the residual austenite of approximately 5% in area fraction which unavoidably
15 exists, it is preferable that the residual austenite is not substantially included.
[0034]
Area fraction of Ferrite and Bainite which are Primary Phase: 30% to less than
99%
The ferrite and the bainite which are the primary phase are comparatively soft,
20 and have the excellent deformability. When the area fraction of the ferrite and the
bainite is 30% or more in total, both properties of the uniform deformability and the local
deformability of the hot-rolled steel sheet according to the embodiment are satisfied.
More preferably, the ferrite and the bainite may be, by area%, 50% or more in total. On
the other hand, when the area fraction of the ferrite and the bainite is 99% or more in
25 total, the strength and the uniform deformability of the steel sheet are decreased. -
Preferably, the area fraction of the ferrite which is the primary phase may be
30% to 99%. By controlling the area fraction of the ferrite which is comparatively
excellent in the deformability to 30% to 99%, it is possible to preferably increase the
5 ductility (deformability) in a balance between the strength and the ductility
(deformability) of the steel sheet. Particularly, the ferrite contributes to the
improvement in the uniform deformability.
[0036]
Alternatively, the area fraction of the bainite which is the primary phase may be
10 5% to 80%. By controlling the area fraction of the bainite which is comparatively
excellent in the strength to 5% to SO%, it is possible to preferably increase the strength in
a balance between the strength and the ductility (deformability) of the steel sheet. By
increasing the area fraction of the bainite which is harder phase than the ferrite, the
strength of the steel sheet is improved. In addition, the bainite, which has small
15 hardness difference from the martensite as compared with the ferrite, suppresses
initiation of voids at an interface between the soft phase and the hard phase, and
improves the hole expansibility.
[0037]
Area fraction fM of Martensite: 1 % to 70%
20 By dispersing the martensite, which is the secondary phase and is the hard phase,
in the metallographic structure, it is possible to improve the strength and the uniform
deformability. When the area fraction of the martensite is less than I%, the dispersion
of the hard phase is insufficient, the work hardening rate is decreased, and the uniform
deformability is decreased. Preferably, the area fraction of the martensite may be 3% or
25 more. On the other hand, when the area fraction of the martensite is more than 70%, the
20
area fraction of the hard phase is excessive, and the deformability of the steel sheet is
significantly decreased. In accordance with the balance between the strength and the
deformability, the area fraction of the martensite may be 50% or less. Preferably, the
area fraction of the martensite may be 30% or less. More preferably, the area fraction of
5 the martensite may be 20% or less.
[003 81
Average Grain Size dia of Martensite: 13 pm or less
When the average size of the martensite is more than 13 pm, the uniform
deformability of the steel sheet may be decreased, and the local deformability may be
10 decreased. It is considered that the uniform elongation is decreased due to the fact that
contribution to the work hardening is decreased when the average size of the martensite
is coarse, and that the local deformability is decreased due to the fact that the voids easily
initiates in the vicinity of the coarse martensite. Preferably, the average size of the
martensite may be less than 10 pm. More preferably, the average size of the martensite
15 may be 7 pm or less.
Relationship of TS 1 fM x dis 1 dia: 500 or more
Moreover, as a result of the investigation in detail by the inventors, it is found
that, when the tensile strength is defined as TS (tensile strength) in unit of MPa, the area
20 fraction of the martensite is defined as fM (fraction of Martensite) in unit of %, an
average distance between the martensite grains is defined as dis (distance) in unit of pm,
and the average grain size of the martensite is defmed as dia (diameter) in unit of pm, the
uniform deformability of the steel sheet is improved in a case that a relationship among
the TS, the fM, the dis, and the dia satisfies a following Expression 1.
2 1
TS / fM x dis / dia 2 500 . . . (Expression 1)
[0040]
When the relationship of TS I fM x dis I dia is less than 500, the uniform
deformability of the steel sheet may be significantly decreased. A physical meaning of
5 the Expression 1 has not been clear. However, it is considered that the work hardening
more effectively occurs as the average distance dis between the martensite grains is
decreased and as the average grain size dia of the martensite is increased. Moreover,
the relationship of TS 1 fM x dis / dia does not have particularly an upper limit.
However, from an industrial standpoint, since the relationship of TS / fM x dis 1 dia
10 barely exceeds 10000, the upper limit may be 10000 or less.
[0041]
Fraction of Martensite having 5.0 or less in Ratio of Major Axis to Minor Axis:
50% or more
In addition, when a major axis of a martensite grain is defined as La in unit of
15 pm and a minor axis of a martensite grain is defined as Lb in unit of pm, the local
deformability may be preferably improved in a case that an area fraction of the
martensite grain satisfying a following Expression 2 is 50% to 100% as compared with
the area fraction fM of the martensite.
La I Lb I 5.0 . . . (Expression 2)
[0042]
The detail reasons why the effect is obtained has not been clear. However, it is
considered that the local deformability is improved due to the fact that the shape of the
martensite varies from an acicular shape to a spherical shape and that excessive stress
concentration to the ferrite or the bainite near the martensite is relieved. Preferably, the
22
area fraction of the martensite grain having Lakb of 3.0 or less may be 50% or more as
compared with the fM. More preferably, the area fraction of the martensite grain having
L a b of 2.0 or less may be 50% or more as compared with the fM. Moreover, when
the fraction of equiaxial martensite is less than 50% as compared with the £Myth e local
5 deformability may deteriorate. Moreover, a lower limit of the Expression 2 may be 1 .O.
[0043]
Moreover, all or part of the martensite may be a tempered martensite. When
the martensite is the tempered martensite, although the strength of the steel sheet is
decreased, the hole expansibility of the steel sheet is improved by a decrease in the
10 hardness difference between the primary phase and the secondary phase. In accordance
with the balance between the required strength and the required deformability, the area
fraction of the tempered martensite may be controlled as compared with the area fraction
fM of the martensite.
[0044]
15 The metallographic structure such as the ferrite, the bainite, or the martensite as
described above can be observed by a Field Emission Scanning Electron Microscope
(FE-SEM) in a thickness range of 118 to 318 (a thickness range in which 114 position of
the thickness is the center). The above characteristic values can be determined from
micrographs which are obtained by the observation. In addition, the characteristic
20 values can be also determined by the EBSD as described below. For the observation of
the FE-SEM, samples are collected so that an observed section is the
thickness-cross-section (the normal vector thereof corresponds to the normal direction)
which is parallel to the rolling direction of the steel sheet, and the observed section is
polished and nital-etched. Moreover, in the thickness direction, the metallographic
25 structure (constituent) of the steel sheet may be significantly different between the
23
vicinity of the surface of the steel sheet and the vicinity of the center of the steel sheet
because of decarburization and Mn segregation. Accordingly, in the embodiment, the
metallographic structure based on 114 position of the thickness is observed.
[0045]
Volume Average Diameter of Grains: 5 pm to 30 pm
Moreover, in order to hrther improve the deformability, size of the grains in the
metallographic structure, particularly, the volume average diameter may be refined.
Moreover, fatigue properties (fatigue limit ratio) required for an automobile steel sheet or
the like are also improved by refining the volume average diameter. Since the number
10 of coarse grains significantly influences the deformability as compared with the number
of fine grains, the deformability significantly correlates with the volume average
diameter calculated by the weighted average of the volume as compared with a number
average diameter. Accordingly, in order to obtain the above effects, the volume average
diameter may be 5 pm to 30 pm, may be more preferably 5 pm to 20 pm, and may be
15 furthermore preferably 5 pm to 10 pm.
[0046]
Moreover, it is considered that, when the volume average diameter is decreased,
local strain concentration occurred in micro-order is suppressed, the strain can be
dispersed during local deformation, and the elongation, particularly, the uniform
20 elongation is improved. In addition, when the volume average diameter is decreased, a
grain boundary which acts as a barrier of dislocation motion may be appropriately
controlled, the grain boundary may affect repetitive plastic deformation (fatigue
phenomenon) derived from the dislocation motion, and thus, the fatigue properties may
be improved.
. .
[0047]
24
Moreover, as described below, the diameter of each grain (grain unit) can be
determined. The pearlite is identified through a metallographic observation by an
optical microscope. In addition, the grain units of the ferrite, the austenite, the bainite,
and the martensite are identified by the EBSD. If crystal structure of an area measured
5 by the EBSD is a face centered cubic structure (fcc structure), the area is regarded as the
austenite. Moreover, if crystal structure of an area measured by the EBSD is the body
centered cubic structure (bcc structure), the, area is regarded as the any one of the ferrite,
the bainite, and the martensite. The ferrite, the bainite, and the martensite can be
identified by using a Kernel Average Misorientation (KAM) method which is added in an
10 Electron Back Scatter Diffraction Pattern-Orientation Image Microscopy (EBSP-OM,
Registered Trademark). In the KAM method, with respect to a first approximation
(total 7 pixels) using a regular hexagonal pixel (central pixel) in measurement data and 6
pixels adjacent to the central pixel, a second approximation (total 19 pixels) using 12
pixels further outside the above 6 pixels, or a third approximation (total 37 pixels) using
15 18 pixels further outside the above 12 pixels, an misorientation between each pixel is
averaged, the obtained average is regarded as the value of the central pixel, and the above
operation is performed on all pixels. The calculation by the KAM method is performed
so as not to exceed the grain boundary, and a map representing intragranular crystal
rotation can be obtained. The map shows strain distribution based on the intragranular
20 local crystal rotation.
[0048]
In the embodiment, the misorientation between adjacent pixels is calculated by
using the third approximation in the EBSP-OM (registered trademark). For example,
the above-described orientation measurement is conducted by a measurement step of 0.5
25 - ' pm or less at a magnification of 1500-fold, a position in which the misorientation
25
between the adjacent measurement points is more than 15" is regarded as a grain border
(the grain border is not always a general grain boundary), the circle equivalent diameter
is calculated, and thus, the grain sizes of the ferrite, the bainite, the martensite, and the
austenite are obtained. When the pearlite is included in the metallographic structure,
5 the grain size of the pearlite can be calculated by applying an image processing method
such as binarization processing or an intercept method to the micrograph obtained by the
optical microscope.
COO491
In the grain (grain unit) defined as described above, when a circle equivalent
10 radius (a half value of the circle equivalent diameter) is defined as r, the volume of each
grain is obtained by 4 x n x r3 / 3, and the volume average diameter can be obtained by
the weighted average of the volume. In addition, an area fraction of coarse grains
described below can be obtained by dividing area fraction of the coarse grains obtained
using the method by measured area. Moreover, except for the volume average diameter,
15 the circle equivalent diameter or the grain size obtained by the binarization processing,
the intercept method, or the like is used, for example, as the average grain size dia of the
martensite.
[0050]
The average distance dis between the martensite grains may be determined by
20 using the border between the martensite grain and the grain other than the martensite
obtained by the EBSD method (however, FE-SEM in which the EBSD can be conducted)
in addition to the FE-SEM observation method.
[005 11
Area fraction of Coarse Grains having Grain Size of more than 35 pm: 0% to
25 10%
26
In addition, in order to further improve the local deformability, with respect to
all constituents of the metallographic structure, the area fraction (the area fraction of the
coarse grains) which is occupied by grains (coarse grains) having the grain size of more
than 35 pm occupy per unit area may be limited to be 0% to 10%. When the grains
5 having a large size are increased, the tensile strength may be decreased, and the local
deformability may be also decreased. Accordingly, it is preferable to refine the grains.
Moreover, since the local deformability is improved by straining all grains uniformly and
equivalently, the local strain of the grains may be suppressed by limiting the fraction of
the coarse grains.
10 [0052]
Standard Deviation of Average Distance dis between Martensite Grains: 5 pm or
less
Moreover, in order to further improve the local deformability such as the
bendability, the stretch flangeability, the burring formability, or the hole expansibility, it
15 is preferable that the martensite which is the hard phase is dispersed in the
metallographic structure. Therefore, it is preferable that the standard deviation of the
average distance dis between the martensite grains is 0 pm to 5 pm. In the case, the
average distance dis and the standard deviation thereof may be obtained by measuring
the distance between the martensite grains at 100 points or more.
[0053]
Hardness H of Ferrite: it is preferable to satisfy a following Expression 3
The ferrite which is the primary phase and the soft phase contributes to the
improvement in the deformability of the steel sheet. Accordingly, it is preferable that
the average hardness H of the ferrite satisfies the following Expression 3. When a - - -
25 ferrite which is harder than the following Expression 3 is contained, the improvement
27
effects of the deformability of the steel sheet may not be obtained. Moreover, the
average hardness H of the ferrite is obtained by measuring the hardness of the ferrite at
100 points or more under a load of 1 mN in a nano-indenter.
H<200+30x[Si]+21x ~ ] + 2 7 0 x ~ ] + 7 [8Nxb1 1"+108x
5 [ ~ i ] ".~ .(.E xpression 3)
Here, [Si], [Mn], [PI, [Nb], and [Ti] represent mass percentages of Si, Mn, P, Nb,
and Ti respectively.
[0054]
Standard Deviation /Average of Hardness of Ferrite or Bainite: 0.2 or less
As a result of investigation which is focused on the homogeneity of the ferrite or
bainite which is the primary phase by the inventors, it is found that, when the
homogeneity of the primary phase is high in the microstructure, the balance between the
uniform deformability and the local deformability may be preferably improved.
Specifically, when a value, in which the standard deviation of the hardness of the ferrite
15 is divided by the average of the hardness of the ferrite, is 0.2 or less, the effects may be
preferably obtained. Moreover, when a value, in which the standard deviation of the
hardness of the bainite is divided by the average of the hardness of the bainite, is 0.2 or
less, the effects may be preferably obtained. The homogeneity can be obtained by
measuring the hardness of the ferrite or the bainite which is the primary phase at 100
20 points or more under the load of 1 mN in the nano-indenter and by using the obtained
average and the obtained standard deviation. Specifically, the homogeneity increases
with a decrease in the value of the standard deviation of the hardness / the average of the
hardness, and the effects may be obtained when the value is 0.2 or less. In the
nano-indenter (for example, UMIS-2000 manufactured by CSIRO corporation), by using
25 a smaller indenter than the grain size, the hardness of a single grain which does not
28
include the grain boundary can be measured.
[0055]
Next, a chemical composition of the hot-rolled steel sheet according to the
embodiment will be described.
[0056]
Hereinafter, description will be given of the base elements of the hot rolled steel
sheet according to the embodiment and of the limitation range and reasons for the
limitation. Moreover, the % in the description represents mass%.
[0057]
C: 0.01% to 0.4%
C (carbon) is an element which increases the strength of the steel sheet, and is an
essential element to obtain the area fraction of the martensite. A lower limit of C
content is to be 0.01% in order to obtain the martensite of 1% or more, by area%. On
the other hand, when the C content is more than 0.40%, the deformability of the steel
15 sheet is decreased, and weldability of the steel sheet also deteriorates. Preferably, the C
content may be 0.30% or less.
[0058]
. .
Si: 0.001% to 2.5%
Si (silicon) is a deoxidizing element of the steel and is an element which is
20 effective in an increase in the mechanical strength of the steel sheet. Moreover, Si is an
element which stabilizes the ferrite during the temperature control after the hot-rolling
and suppresses cementite precipitation during the bainitic transformation. However,
when Si content is more than 2.5%, the deformability of the steel sheet is decreased, and
surface dents tend to be made on the steel sheet. On the other hand, when the Si content
25 is less than 0.001%, it is difficult to obtain the effects.
[0059]
Mn: 0.001% to 4.0%
Mn (manganese) is an element which is effective in an increase in the
mechanical strength of the steel sheet. However, when Mn content is more than 4.0%,
5 the deformability of the steel sheet is decreased. Preferably, the Mn content may be
3.5% or less. More preferably, the Mn content may be 3.0% or less. On the other
hand, when the Mn content is less than 0.001%, it is difficult to obtain the effects. In
addition, Mn is also an element which suppresses cracks during the hot-rolling by fixing
S (sulfur) in the steel. When elements such as Ti which suppresses occurrence of cracks
10 due to S during the hot-rolling are not sufficiently added except for Mn, it is preferable
that the Mn content and the S content satisfy Mn / S 2 20 by mass%.
[0060]
Al: 0.001% to 2.0%
A1 (aluminum) is a deoxidizing element of the steel. Moreover, A1 is an
15 element which stabilizes the ferrite during the temperature control after the hot-rolling
and suppresses the cementite precipitation during the bainitic transformation. In order
to obtain the effects, A1 content is to be 0.001% or more. However, when the A1 content
is more than 2.0%, the weldability deteriorates. In addition, although it is difficult to
quantitatively show the effects, A1 is an element which significantly increases a
20 temperature & at which transformation starts from y (austenite) to a (ferrite) at the
cooling of the steel. Accordingly, Ar3 of the steel may be controlled by the A1 content.
[0061]
The hot-rolled steel sheet according to the embodiment includes unavoidable
impurities in addition to the above described base elements. Here, the unavoidable
25 impurities indicate elements such as P, S, N, 0, Cd, Zn, or Sb which are unavoidably
30
mixed from auxiliary raw materials such as scrap or fiom production processes. In the
elements, P, S, N, and 0 are limited to the following in order to preferably obtain the
effects. It is preferable that the unavoidable impurities other than P, S, N, and 0 are
individually limited to 0.02% or less. Moreover, even when the impurities of 0.02% or
less are included, the effects are not affected. The limitation range of the impurities
includes 0%, however, it is industrially difficult to be stably 0%. Here, the described %
is mass%.
[0062]
P: 0.15% or less
P (phosphorus) is an impurity, and an element which contributes to crack during
the hot-rolling or the cold-rolling when the content in the steel is excessive. In addition,
P is an element which deteriorates the ductility or the weldability of the steel sheet.
Accordingly, the P content is limited to 0.15% or less. Preferably, the P content may be
limited to 0.05% or less. Moreover, since P acts as a solid solution strengthening
element and is unavoidably included in the steel, it is not particularly necessary to
prescribe a lower limit of the P content. The lower limit of the P content may be 0%.
Moreover, considering current general refining (includes secondary refining), the lower
limit of the P content may be 0.0005%.
[0063]
S: 0.03% or less
S (sulfur) is an impurity, and an element which deteriorates the deformability of
the steel sheet by forming MnS stretched by the hot-rolling when the content in the steel
is excessive. Accordingly, the S content is limited to 0.03% or less. Moreover, since S
is unavoidably included in the steel, it is not particularly necessary to prescribe a lower
limit of the S content. The lower limit of the S content may be 0%. Moreover,
3 1
considering the current general refining (includes the secondary refining), the lower limit
of the P content may be 0.0005%.
[0064]
N: 0:O 1 % or less
N (nitrogen) is an impurity, and an element which deteriorates the deformability
of the steel sheet. Accordingly, the N content is limited to 0.01% or less. Moreover,
since N is unavoidably included in the steel, it is not particularly necessary to prescribe a
lower limit of the N content. The lower limit of the N content may be 0%. Moreover,
considering the current general refming (includes the secondary refining), the lower limit
10 of the N content may be 0.0005%.
100651
0: 0.01% or less
0 (oxygen) is an impurity, and an element which deteriorates the deformability
of the steel sheet. Accordingly, the 0 content is limited to 0.01% or less. Moreover,
15 since 0 is unavoidably included in the steel, it is not particularly necessary to prescribe a
lower limit of the 0 content. The lower limit of the 0 content may be 0%. Moreover,
considering the current general refining (includes the secondary refining), the lower limit
of the 0 content may be 0.0005%.
[0066]
2 0 The above chemical elements are base components (base elements) of the steel
in the embodiment, and the chemical composition, in which the base elements are
controlled (included or limited) and the balance consists of Fe and unavoidable
impurities, is a base composition of the embodiment. However, in addition to the base
elements (instead of a part of Fe which is the balance), in the embodiment, the following
25 chemical elements (optional elements) may be additionally included in the steel as
3 2
necessary. Moreover, even when the optional elements are unavoidably included in the
steel (for example, amount less than a lower limit of each optional element), the effects in
the embodiment are not decreased.
[0067]
Specifically, the hot-rolled steel sheet according to the embodiment may hrther
include, as a optional element, at least one selected fkom a group consisting of Mo, Cr, Ni,
Cu, B, Nb, Ti, V, W, Ca, Mg, Zr, REM, As, Co, Sn, Pb, Y, and Hf in addition to the base
elements and the impurity elements. Hereinafter, numerical limitation ranges and the
limitation reasons of the optional elements will be described. Here, the described % is
[0068]
Ti: 0.001% to 0.2%
Nb: 0.001% to 0.2%
15 Ti (titanium), Nb (niobium), and B (boron) are the optional elements which form
fine carbon-nitrides by fixing the carbon and the nitrogen in the steel, and which have the
effects such as precipitation strengthening, microstructure control , or grain refinement
strengthening for the steel. Accordingly, as necessary, at least one of Ti, Nb, and B may
be added to the steel. In order to obtain the effects, preferably, Ti content may be
20 0.001% or more, Nb content may be 0.00 1% or more, and B content may be 0.0001% or
more. However, when the optional elements are excessively added to the steel, the
effects may be saturated, the control of the crystal orientation may be difficult because of
suppression of recrystallization after the hot-rolling, and the workability (deformability)
of the steel sheet may deteriorate. Accordingly, preferably, the Ti content may be 0.2%
25 or less, the Nb content may be 0.2% or less, and the B content may be 0.005% or less.
33
Moreover, even when the optional elements having the amount less than the lower limit
are included in the steel, the effects in the embodiment are not decreased. Moreover,
since it is not necessary to add the optional elements to the steel intentionally in order to
reduce costs of alloy, lower limits of amounts of the optional elements may be 0%.
5 [0069]
Mg: 0.0001% to 0.01%
REM: 0.0001% to 0.1%
Ca: 0.0001% to 0.01%
Ma (magnesium), REM (Rare Earth Metal), and Ca (calcium) are the optional
10 elements which are important to control inclusions to be harmless shapes and to improve
the local deformability of the steel sheet. Accordingly, as necessary, at least one of Mg,
REM, and Ca may be added to the steel. In order to obtain the effects, preferably, Mg
content may be 0.0001% or more, REM content may be 0.0001% or more, and Ca
content may be 0.0001% or more. On the other hand, when the optional elements are
15 excessively added to the steel, inclusions having stretched shapes may be formed, and the
deformability of the steel sheet may be decreased. Accordingly, preferably, the Mg
content may be 0.01% or less, the REM content may be 0.1% or less, and the Ca content
may be 0.01% or less. Moreover, even when the optional elements having the amount
less than the lower limit are included in the steel, the effects in the embodiment are not
20 decreased. Moreover, since it is not necessary to add the optional elements to the steel
intentionally in order to reduce costs of alloy, lower limits of amounts of the optional
elements may be 0%.
[0070]
In addition, here, the REM represents collectively a total of 16 elements which
25 are 15 elements from lanthanum with atomic number 57 to lutetium with atomic number
3 4
71 in addition to scandium with atomic number 21. In general, REM is supplied in the
state of misch metal which is a mixture of the elements, and is added to the steel.
[007 11
Mo: 0.001% to 1.0%
5 Cr: 0.001% to 2.0%
Ni: 0.001% to 2.0%
W: 0.001% to 1.0%
Zr: 0.0001% to 0.2%
As: 0.0001% to 0.5%
10 Mo (molybdenum), Cr (chromium), Ni (nickel), W (tungsten), Zr (zirconium),
and As (arsenic) are the optional elements which increase the mechanical strength of the
steel sheet. Accordingly, as necessary, at least one of Mo, Cr, Ni, W, Zr, and As may be
-added to the steel. In order to obtain the effects, preferably, Mo content may be 0.001%
or more, Cr content may be 0.001% or more, Ni content may be 0.001% or more, W
15 content may be 0.001% or more, Zr content may be 0.0001% or more, and As content
may be 0.0001% or more. However, when the optional elements are excessively added
to the steel, the deformability of the steel sheet may be decreased. Accordingly,
preferably, the Mo content may be 1.0% or less, the Cr content may be 2.0% or less, the
Ni content may be 2.0% or less, the W content may be 1.0% or less, the Zr content may
20 be 0.2% or less, and the As content may be 0.5% or less. Moreover, even when the
optional elements having the amount less than the lower limit are included in the steel,
the effects in the embodiment are not decreased. Moreover, since it is not necessary to
add the optional elements to the steel intentionally in order to reduce costs of alloy, lower
limits of amounts of the optional elements may be 0%.
2 5 100721 . .
v: 0.001% 1.0%
Cu: 0.001% to 2.0%
V (vanadium) and Cu (copper) are the optional elements which is similar to Nb,
Ti, or the like and which have the effect of the precipitation strengthening. In addition,
5 a decrease in the local deformability due to addition of V and Cu is small as compared
with that of addition of Nb, Ti, or the like. Accordingly, in order to obtain the
high-strength and to further increase the local deformability such as the hole
expansibility or the bendability, V and Cu are more effective optional elements than Nb,
Ti, or the like. Therefore, as necessary, at least one of V and Cu may be added to the
10 steel. In order to obtain the effects, preferably, V content may be 0.001% or less and Cu
content may be 0.001% or less. However, the optional elements are excessively added
to the steel, the deformability of the steel sheet may be decreased. Accordingly,
preferably, the V content may be 1.0% or less and the Cu content may be 2.0% or less.
Moreover, even when the optional elements having the amount less than the lower limit
15 are included in the steel, the effects in the embodiment are not decreased. In addition,
since it is not necessary to add the optional elements to the steel intentionally in order to
reduce costs of alloy, lower limits of amounts of the optional elements may be 0%.
[0073]
Co: 0.0001% to 1.0%
20 Although it is difficult to quantitatively show the effects, Co (cobalt) is the
optional element which significantly increases the temperature at which the
transformation starts from y (austenite) to a (ferrite) at the cooling of the steel.
Accordingly, AT3 of the steel may be controlled by the Co content. In addition, Co is the
optional element which improves the strength of the steel sheet. In order to obtain the
25 effect, preferably, the Co content may be 0.000 1 % or more. However, when Co is
36
excessively added to the steel, the weldability of the steel sheet may deteriorate, and the
deformability of the steel sheet may be decreased. Accordingly, preferably, the Co
content may be 1.0% or less. Moreover, even when the optional element having the
amount less than the lower limit are included in the steel, the effects in the embodiment
5 are not decreased. In addition, since it is not necessary to add the optional element to
the steel intentionally in order to reduce costs of alloy, a lower limit of an amount of the
optional element may be 0%.
[0074]
Sn: 0.0001% to 0.2%
Pb: 0.0001% to 0.2%
Sn (tin) and Pb (lead) are the optional elements which are effective in an
improvement of coating wettability and coating adhesion. Accordingly, as necessary, at
least one of Sn and Pb may be added to the steel. In order to obtain the effects,
preferably, Sn content may be 0.0001% or more and Pb content may be 0.0001% or more.
15 However, when the optional elements are excessively added to the steel, the cracks may
occur during the hot working due to high-temperature embrittlement, and surface dents
tend to be made on the steel sheet. Accordingly, preferably, the Sn content may be
0.2% or less and the Pb content may be 0.2% or less. Moreover, even when the optional
elements having the amount less than the lower limit are included in the steel, the effects
20 in the embodiment are not decreased. In addition, since it is not necessary to add the
optional elements to the steel intentionally in order to reduce costs of alloy, lower limits
of amounts of the optional elements may be 0%.
[0075]
Y 0.0001% to 0.2%
Hf: 0.0001% to 0.2%
3 7
Y (yttrium) and Hf (hafnium) are the optional elements which are effective in an
improvement of corrosion resistance of the steel sheet. Accordingly, as necessary, at
least one of Y and Hf may be added to the steel. In order to obtain the effect, preferably,
Y content may be 0.0001% or more and Hf content may be 0.0001% or more. However,
5 when the optional elements are excessively added to the steel, the local deformability
such as the hole expansibility may be decreased. Accordingly, preferably, the Y content
may be 0.20% or less and the Hf content may be 0.20% or less. Moreover, Y has the
effect which forms oxides in the steel and which adsorbs hydrogen in the steel.
Accordingly, diffusible hydrogen in the steel is decreased, and an improvement in
10 hydrogen embrittlement resistance properties in the steel sheet can be expected. The
effect can be also obtained within the above-described range of the Y content.
Moreover, even when the optional elements having the amount less than the lower limit
are included in the steel, the effects in the embodiment are not decreased. In addition,
since it is not necessary to add the optional elements to the steel intentionally in order to
15 reduce costs of alloy, lower limits of amounts of the optional elements may be 0%.
COO761
As described above, the hot-rolled steel sheet according to the embodiment has
the chemical composition which includes the above-described base elements and the
balance consisting of Fe and unavoidable impurities, or has the chemical composition
20 which includes the above-described base elements, at least one selected fiom the group
consisting of the above-described optional elements, and the balance consisting of Fe and
unavoidable impurities.
COO771
Moreover, surface treatment may be conducted on the hot-rolled steel sheet
25 according to the embodiment. For example, the surface treatment such as electro
38
coating, hot dip coating, evaporation coating, alloying treatment after coating, organic
film formation, film laminating, organic salt and inorganic salt treatment, or non-chrome
treatment (non-chromate treatment) may be applied, and thus, the hot-rolled steel sheet
may include various kinds of the film (film or coating). For example, a galvanized
5 layer or a galvannealed layer may be arranged on the surface of the hot-rolled steel sheet.
Even if the hot-rolled steel sheet includes the above-described coating, the steel sheet can
obtain the high-strength and can sufficiently secure the uniform deformability and the
local deformability.
[0078]
Moreover, in the embodiment, a thickness of the hot-rolled steel sheet is not
particularly limited. However, for example, the thickness may be 1.5 mm to 10 rnrn,
and may be 2.0 mm to 10 mm. Moreover, the strength of the hot-rolled steel sheet is
not particularly limited, and for example, the tensile strength may be 440 MPa to 1500
MPa.
15 [0079]
The hot-rolled steel sheet according to the embodiment can be applied to general
use for the high-strength steel sheet, and has the excellent uniform deformability and the
remarkably improved local deformability such as the bending workability or the hole
expansibility of the high-strength steel sheet.
20 [OOSO]
In addition, since the directions in which the bending for the hot-rolled steel
sheet is conducted differ in the parts which are bent, the direction is not particularly
limited. In the hot-rolled steel sheet according to the embodiment, the similar
properties can be obtained in any bending direction, and the hot-rolled steel sheet can be
25 subjected to the composite forming including working modes such as bending, stretching,
or drawing.
[008 11
Next, a method for producing the hot-rolled steel sheet according to an
embodiment of the present invention will be described. In order to produce the
5 hot-rolled steel sheet which has the high-strength, the excellent uniform deformability,
and the excellent local deformability, it is important to control the chemical composition
of the steel, the metallographic structure, and the texture which is represented by the pole
densities of each orientation of a specific crystal orientation group. The details will be
described below.
10 [0082]
The production process prior to the hot-rolling is not particularly limited. For
example, the steel (molten steel) may be obtained by conducting a smelting and a
refining using a blast furnace, an electric furnace, a converter, or the like, and
subsequently, by conducting various kinds of secondary refining, in order to melt the
15 steel satisfying the chemical composition. Thereafter, in order to obtain a steel piece or
a slab from the steel, for example, the steel can be cast by a casting process such as a
continuous casting process, an ingot making process, or a thin slab casting process in
general. In the case of the continuous casting, the steel may be subjected to the
hot-rolling after the steel is cooled once to a lower temperature (for example, room
20 temperature) and is reheated, or the steel (cast slab) may be continuously subjected to the
hot-rolling just after the steel is cast. In addition, scrap may be used for a raw material
of the steel (molten steel).
[0083]
In order to obtain the high-strength steel sheet which has the high-strength, the
25 excellent uniform deformability, and the excellent local deformability, the following
40
conditions may be satisfied. Moreover, hereinafter, the "steel" and the "steel sheet" are
synonymous.
[0084]
First-Hot-Rolling Process
In the first-hot-rolling process, using the molten and cast steel piece, a rolling
pass whose reduction is 40% or more is conducted at least once in a temperature range of
1000°C to 1200°C (preferably, 1150°C or lower). By conducting the first-hot-rolling
under the conditions, the average grain size of the austenite of the steel sheet after the
first-hot-rolling process is controlled to 200 pm or less, which contributes to the .-
10 improvement in the uniform deformability and the local deformability of the finally
obtained hot-rolled steel sheet.
[0085]
The austenite grains are refined with an increase in the reduction and an increase
in the frequency of the rolling. For example, in the first-hot-rolling process, by
15 conducting at least two times (two passes) of the rolling whose reduction is 40% or more
per one pass, the average grain size of the austenite may be preferably controlled to 100
pm or less. In addition, in the first-hot-rolling, by limiting the reduction to 70% or less
per one pass, or by limiting the frequency of the rolling (the number of times of passes)
to 10 times or less, a temperature fall of the steel sheet or excessive formation of scales
20 may can be decreased. Accordingly, in the rough rolling, the reduction per one pass
may be 70% or less, and the frequency of the rolling (the number of times of passes) may
be 10 times or less.
[0086]
As described above, by refining the austenite grains after the first-hot-rolling
- -
25 process, it is preferable that the austenite grains can be further refined by the post
4 1
processes, and the ferrite, the bainite, and the martensite transformed from the austenite
at the post processes may be finely and uniformly dispersed. As a result, the anisotropy
and the local deformability of the steel sheet are improved due to the fact that the texture
is controlled, and the uniform deformability and the local deformability (particularly,
5 uniform deformability) of the steel sheet are improved due to the fact that the
metallographic structure is refined. Moreover, it seems that the grain boundary of the
austenite refined by the first-hot-rolling process acts as one of recrystallization nuclei
during a second-hot-rolling process which is the post process.
[0087]
In order to inspect the average grain size of the austenite after the
first-hot-rolling process, it is preferable that the steel sheet after the first-hot-rolling
process is rapidly cooled at a cooling rate as fast as possible. For example, the steel
sheet is cooled under the average cooling rate of 10 "Clsecond or faster. Subsequently,
the cross-section of the sheet piece which is taken fiom the steel sheet obtained by the
15 cooling is etched in order to make the austenite grain boundary visible, and the austenite
grain boundary in the microstructure is observed by an optical microscope. At the time,
visual fields of 20 or more are observed at a magnification of 50-fold or more, the grain
size of the austenite is measured by the image analysis or the intercept method, and the
average grain size of the austenite is obtained by averaging the austenite grain sizes
20 measured at each of the visual fields.
[OOSS]
After the first-hot-rolling process, sheet bars may be joined, and the
second-hot-rolling process which is the post process may be continuously conducted.
At the time, the sheet bars may be joined after a rough bar is temporarily coiled in a coil
25 shape, stored in a cover having a heater as necessary, and recoiled again.
[0089]
Second-Hot-Rolling Process
In the second-hot-rolling process, when a temperature calculated by a following
Expression 4 is defined as T1 in unit of "C, the steel sheet after the first-hot-rolling
5 process is subjected to a rolling under conditions such that, a large reduction pass whose
reduction is 30% or more in a temperature range of Tl + 30°C to T1 + 200°C is included,
a cumulative reduction in the temperature range of Tl + 30°C to T1 + 200°C is 50%, a
cumulative reduction in a temperature range of Ar3"C to lower than T1 + 30°C is limited
to 30% or less, and a rolling finish temperature is Ar3"C or higher.
10 [0090]
As one of the conditions in order to control the average pole density Dl of the
orientation group of (100)<0 11> to (22314 10> and the pole density D2 of the crystal
orientation (33214 13> in the thickness central portion which is the thickness range of
518 to 318 to the above-described ranges, in the second-hot-rolling process, the rolling is
15 controlled based on the temperature TI (unit: "C) which is determined by the following
Expression 4 using the chemical composition (unit: mass%) of the steel.
T1=850+10x([C]+~)x[Mn]+350x[Nb]+250x[Ti]+40x[B]+10x
[Cr] + 100 x [Mo] + 100 x [V] . . . (Expression 4)
In Expression 4, [C], M, [Mn], [Nb], [Ti], [B], [Cr], [Mo], and [V] represent
20 mass percentages of C, N, Mn, Nb, Ti, B, Cr, Mo, and V respectively.
[0091]
The amount of the chemical element, which is included in Expression 4 but is
not included in the steel, is regarded as 0% for the calculation. Accordingly, in the case
of the chemical composition in which the steel includes only the base elements, a
following Expression 5 may be used instead of the Expression 4.
T1 = 850 + 10 x ([C] + m) x [Mn] . . . (Expression 5)
In addition, in the chemical composition in which the steel includes the optional
elements, the temperature calculated by Expression 4 may be used for TI (unit: "C),
5 instead of the temperature calculated by Expression 5.
[0092]
In the second-hot-rolling process, on the basis of the temperature TI (unit: "C)
obtained by the Expression 4 or 5, the large reduction is included in the temperature
range of Tl + 30°C to T1 + 200°C (preferably, in a temperature range of Tl + 50°C to T1
10 + 100°C), and the reduction is limited to a small range (includes 0%) in the temperature
range of Ar30C to lower than T1 + 30°C. By conducting the second-hot-rolling process
in addition to the first-hot-rolling process, the uniform deformability and the local
deformability of the steel sheet is preferably improved. Particularly, by including the
large reduction in the temperature range of Tl + 30°C to T1 + 200°C and by limiting the
15 reduction in the temperature range of Ar3"C to lower than TI + 30°C, the average pole
density Dl of the orientation group of (1 00)<0 11> to (22314 10> and the pole density
D2 of the crystal orientation (3321-4 13> in the thickness central portion which is the
thickness range of 518 to 318 are sufficiently controlled, and as a result, the anisotropy
and the local defonnability of the steel sheet are remarkably improved.
20 [0093]
The temperature T1 itself is empirically obtained. It is empirically found by
the inventors through experiments that the temperature range in which the
recrystallization in the austenite range of each steels is promoted can be determined
based on the temperature T1. In order to obtain the excellent uniform defonnability and
44
the excellent local deformability, it is important to accumulate a large amount of the
strain by the rolling and to obtain the fine recrystallized grains. Accordingly, the rolling
having plural passes is conducted in the temperature range of Tl + 30°C to T1 + 200°C,
and the cumulative reduction is to be 50% or more. Moreover, in order to hrther
5 promote the recrystallization by the strain accumulation, it is preferable that the
cumulative reduction is 70% or more. Moreover, by limiting an upper limit of the
cumulative reduction, a rolling temperature can be sufficiently held, and a rolling load
can be further suppressed. Accordingly, the cumulative reduction may be 90% or less.
[0094]
10 When the rolling having the plural passes is conducted in the temperature range
of Tl + 30°C to T1 + 200°C, the strain is accumulated by the rolling, and the
recrystallization of the austenite is occurred at an interval between the rolling passes by a
driving force derived from the accumulated strain. Specifically, by conducting the
rolling having the plural passes in the temperature range of Tl + 30°C to T1 + 200°C, the
15 recrystallization is repeatedly occurred every pass. Accordingly, it is possible to obtain
the recrystallized austenite structure which is uniform, fine, and equiaxial. In the
temperature range, dynamic recrystallization is not occurred during the rolling, the strain
is accumulated in the crystal, and static recrystallization is occurred at the interval
between the rolling passes by the driving force derived from the accumulated strain. In
20 general, in dynamic-recrystallized structure, the strain which introduced during the
working is accumulated in the crystal thereof, and a recrystallized area and a
non-crystallized area are locally mixed. Accordingly, the texture is comparatively
developed, and thus, the anisotropy appears. Moreover, the metallographic structures
may be a duplex grain structure. In the method for producing the hot-rolled steel sheet
25 according to the embodiment, the austenite is recrystallized by the static recrystallization.
Accordingly, it is possible to obtain the recrystallized austenite structure which is
uniform, fine, and equiaxial, and in which the development of the texture is suppressed.
In order to increase the homogeneity, and to preferably increase the uniform
5 deformability and the local deformability of the steel sheet, the second-hot-rolling is
controlled so as to include at least one large reduction pass whose reduction per one pass
is 30% or more in the temperature range of Tl + 30°C to T1 + 200°C. In the
second-hot-rolling, in the temperature range of Tl + 30°C to T1 + 200°C, the rolling
whose reduction per one pass is 30% or more is conducted at least once. Particularly,
10 considering a cooling process as described below, the reduction of a final pass in the
temperature range may be preferably 25% or more, and may be more preferably 30% or
more. Specifically, it is preferable that the final pass in the temperature range is the
large reduction pass (the rolling pass with the reduction of 30% or more). In a case that
the further excellent deformability is required in the steel sheet, it is hrther preferable
15 that all reduction of first half passes are less than 30% and the reductions of the final two
passes are individually 30% or more. In order to more preferably increase the
homogeneity of the steel sheet, a large reduction pass whose reduction per one pass is
40% or more may be conducted. Moreover, in order to obtain a more excellent shape of
the steel sheet, a large reduction pass whose reduction per one pass is 70% or less may be
20 conducted.
[0096]
Moreover, in the rolling in the temperature range of Tl + 30°C to T1 + 200°C,
by suppressing a temperature rise of the steel sheet between passes of the rolling to 1 X°C
or lower, it is possible to preferably obtain the recrystallized austenite which is more
25 uniform.
In order to suppress the development of the texture and to keep the equiaxial
recrystallized structure, after the rolling in the temperature range of Tl + 30°C to T1 +
200°C, an amount of working in the temperature range of Ar3"C to lower than T1 + 30°C
5 (preferably, T1 to lower than T1 + 30°C) is suppressed as small as possible.
Accordingly, the cumulative reduction in the temperature range of Ar3"C to lower than
T1 + 30°C is limited to 30% or less. In the temperature range, it is preferable that the
cumulative reduction is 10% or more in order to obtain the excellent shape of the steel
sheet, and it is preferable that the cumulative reduction is 10% or less in order to further
10 improve the anisotropy and the local deformability. In the case, the cumulative
reduction may be more preferably 0%. Specifically, in the temperature range of Ar3"C
to lower than TI + 30°C, the rolling may not be conducted, and the cumulative reduction
is to be 30% or less even when the rolling is conducted.
[0098]
15 When the cumulative reduction in the temperature range ofAr3"C to lower than
T1 + 30°C is large, the shape of the austenite grain recrystallized in the temperature
range of Tl + 30°C to T1 + 200°C is not to be equiaxial due to the fact that the grain is
stretched by the rolling, and the texture is developed again due to the fact that the strain
is accumulated by the rolling. Specifically, as the production conditions according to
20 the embodiment, the rolling is controlled at both of the temperature range of Tl + 30°C
to T1 + 200°C and the temperature range of Ar3"C to lower than TI + 30°C in the
second-hot-rolling process. As a result, the austenite is recrystallized so as to be
uniform, fine, and equiaxial, the texture, the metallographic structure, and the anisotropy
of the steel sheet are controlled, and therefore, the uniform deformability and the local
47
deformability can be improved. In addition, the austenite is recrystallized so as to be
uniform, fine, and equiaxial, and therefore, the ratio of major axis to minor axis of the
martensite, the average size of the martensite, the average distance between the
martensite, and the like of the finally obtained hot-rolled steel sheet can be controlled.
[0099]
In the second-hot-rolling process, when the rolling is conducted in the
temperature range lower than Ar3"C or the cumulative reduction in the temperature range
of Ar3"C to lower than T1 + 30°C is excessive large, the texture of the austenite is
developed. As a result, the finally obtained hot-rolled steel sheet does not satisfy at
10 least one of the condition in which the average pole density Dl of the orientation group
of {100)

to {223)<110> is 1.0 to 5.0 and the condition in which the pole density
D2 of the crystal orientation (3321-4 13> is 1.0 to 4.0 in the thickness central portion.
On the other hand, in the second-hot-rolling process, when the rolling is conducted in the
temperature range higher than T1 + 200°C or the cumulative reduction in the temperature
15 range of Tl + 30°C to T1 + 200°C is excessive small, the recrystallization is not
uniformly and finely occurred, coarse grains or mixed grains may be included in the
metallographic structure, and the metallographic structure may be the duplex grain
structure. Accordingly, the area fraction or the volume average diameter of the grains
which is more than 35 pm is increased.
[O 1 001
Moreover, when the second-hot-rolling is finished at a temperature lower than
Ar3 (unit: "C), the steel is rolled in a temperature range of the rolling finish temperature
to lower than AT3 (unit: OC) which is a range where two phases of the austenite and the
ferrite exist (two-phase temperature range). Accordingly, the texture of the steel sheet is
4 8
developed, and the anisotropy and the local deformability of the steel sheet significantly
deteriorate. Here, when the rolling finish temperature of the second-hot-rolling is T1 or
more, the anisotropy may be further decreased by decreasing an amount of the strain in
the temperature range lower than T1, and as a result, the local deformability may be
5 further increased. Therefore, the rolling finish temperature of the second-hot-rolling
may be T1 or more.
[OlOl]
Here, the reduction can be obtained by measurements or calculations from a
rolling force, a thickness, or the like. Moreover, the rolling temperature (for example,
10 the above each temperature range) can be obtained by measurements using a
thermometer between stands, by calculations using a simulation in consideration of
deformation heating, line speed, the reduction, or the like, or by both (measurements and
calculations). Moreover, the above reduction per one pass is a percentage of a reduced
thickness per one pass (a difference between an inlet thickness before passing a rolling
15 stand and an outlet thickness after passing the rolling stand) to the inlet thickness before
passing the rolling stand. The cumulative reduction is a percentage of a cumulatively
reduced thickness (a difference between an inlet thickness before a first pass in the
rolling in each temperature range and an outlet thickness after a final pass in the rolling
in each temperature range) to the reference which is the inlet thickness before the first
20 pass in the rolling in each temperature range. Ar3, which is a ferritic transformation
temperature from the austenite during the cooling, is obtained by a following Expression
6 in unit of "C. Moreover, although it is dificult to quantitatively show the effects as
described above, A1 and Co also influence Ar3.
Ar3 = 879.4 - 516.1 x [C] - 65.7 x [Mn] +38.0 x [Si] +274.7 x [PI...
25 (Expression 6)
49
In the Expression 6, [C], [Mn], [Si] and p] represent mass percentages of C, Mn,
Si and P respectively.
[O 1 021
First-Cooling Process
In the first-cooling process, after a final pass among the large reduction passes
whose reduction per one pass is 30% or more in the temperature range of Tl + 30°C to
T1 + 200°C is finished, when a waiting time from the finish of the final pass to a start of
the cooling is defined as t in unit of second, the steel sheet is subjected to the cooling so
that the waiting time t satisfies a following Expression 7. Here, tl in the Expression 7
10 can be obtained from a following Expression 8. In the Expression 8, Tf represents a
temperature (unit: "C) of the steel sheet at the finish of the final pass among the large
reduction passes, and PI represents a reduction (unit: %) at the final pass among the large
reduction passes.
T I 2.5 x tl . . . (Expression 7)
tl = 0.001 x ((Tf-Tl) x Pl / 100)~- 0.109 x ((Tf -TI) x P1 / 100) + 3.1 ...
(Expression 8)
[0 1031
The first-cooling after the final large reduction pass significantly influences the
grain size of the finally obtained hot-rolled steel sheet. Moreover, by the first-cooling,
20 the austenite can be controlled to be a metallographic structure in which the grains are
equiaxial and the coarse grains rarely are included (namely, uniform sizes).
Accordingly, the finally obtained hot-rolled steel sheet has the metallographic structure in
which the grains are equiaxial and the coarse grains rarely are included (namely, uniform
sizes), and the ratio of the major axis to the minor axis of the martensite, the average size
5 0
of the martensite, the average distance between the martensite, and the like may be
preferably controlled.
[0 1041
The right side value (2.5 x tl) of the Expression 7 represents a time at which the
5 recrystallization of the austenite is substantially finished. When the waiting time t is
more than the right side value (2.5 x tl) of the Expression 7, the recrystallized grains are
significantly grown, and the grain size is increased. Accordingly, the strength, the
uniform deformability, the local deformability, the fatigue properties, or the like of the
steel sheet are decreased. Therefore, the waiting time t is to be 2.5 x tl seconds or less.
10 In a case where runnability (for example, shape straightening or controllability of a
second-cooling) is considered, the first-cooling may be conducted between rolling stands.
Moreover, a lower limit of the waiting time t is to be 0 seconds or more.
[0 1051
Moreover, when the waiting time t is limited to 0 second to shorter than tl
15 seconds so that 0 5 t < tl is satisfied, it may be possible to significantly suppress the
grain growth. In the case, the volume average diameter of the finally obtained
hot-rolled steel sheet may be controlled to 30 pm or less. As a result, even if the
recrystallization of the austenite does not sufficiently progress, the properties of the steel
sheet, particularly, the uniform deformability, the fatigue properties, or the like may be
20 preferably improved.
[0 1061
Moreover, when the waiting time t is limited to tl seconds to 2.5 x tl seconds so
that tl 5 t 5 2.5 x tl is satisfied, it may be possible to suppress the development of the
texture. In the case, although the volume average diameter may be increased because
5 1
the waiting time t is prolonged as compared with the case where the waiting time t is
shorter than t 1 seconds, the crystal orientation may be randomized because the
recrystallization of the austenite sufficiently progresses. As a result, the anisotropy, the
local deformability, and the like of the steel sheet may be preferably improved.
[0 1071
Moreover, the above-described first-cooling may be conducted at an interval
between the rolling stands in the temperature range of Tl + 30°C to T1 + 200°C, or may
be conducted after a final rolling stand in the temperature range. Specifically, as long as
the waiting time t satisfies the condition, a rolling whose reduction per one pass is 30%
10 or less may be further conducted in the temperature range of Tl + 30°C to T1 + 200°C
and between the finish of the final pass among the large reduction passes and the start of
the first-cooling. Moreover, after the first-cooling is conducted, as long as the reduction
per one pass is 30% or less, the rolling may be further conducted in the temperature
range of Tl + 30°C to T1 + 200°C. Similarly, after the first-cooling is conducted, as
15 long as the cumulative reduction is 30% or less, the rolling may be further conducted in
the temperature range ofAr3"C to T1 + 30°C (or Ar3"C to Tf "C). As described above,
as long as the waiting time t after the large reduction pass satisfies the condition, in order
to control the metallographic structure of the finally obtained hot-rolled steel sheet, the
above-described first-cooling may be conducted either at the interval between the rolling
20 stands or after the rolling stand.
[0108]
In the first-cooling, it is preferable that a cooling temperature change which is a
difference between a steel sheet temperature (steel temperature) at the cooling start and a
steel sheet temperature (steel temperature) at the cooling finish is 40°C to 140°C. When
52
the cooling temperature change is 40°C or higher, the growth of the recrystallized
austenite grains may be hrther suppressed. When the cooling temperature change is
140°C or lower, the recrystallization may more sufficiently progress, and the pole density
may be preferably improved. Moreover, by limiting the cooling temperature change to
5 140°C or lower, in addition to the comparatively easy control of the temperature of the
steel sheet, variant selection (variant limitation) may be more effectively controlled, and
the development of the recrystallized texture may be preferably controlled. Accordingly,
in the case, the isotropy may be further increased, and the orientation dependence of the
formability may be further decreased. When the cooling temperature change is higher
10 than 140°C, the progress of the recrystallization may be insuficient, the intended texture
may not be obtained, the ferrite may not be easily obtained, and the hardness of the
obtained ferrite is increased. Accordingly, the uniform deformability and the local
deformability of the steel sheet may be decreased.
[0 1091
15 Moreover, it is preferable that the steel sheet temperature T2 at the first-cooling
finish is T1 + 100°C or lower. When the steel sheet temperature T2 at the first-cooling
finish is T1 + 100°C or lower, more sufficient cooling effects are obtained. By the
cooling effects, the grain growth may be suppressed, and the growth of the austenite
grains may be further suppressed.
Moreover, it is preferable that an average cooling rate in the first-cooling is 50
"Clsecond or faster. When the average cooling rate in the first-cooling is 50 "Clsecond
or faster, the growth of the recrystallized austenite grains may be further suppressed.
On the other hand, it is not particularly necessary to prescribe an upper limit of the
5 3
average cooling rate. However, from a viewpoint of the sheet shape, the average
cooling rate may be 200 "Clsecond or slower.
[Olll]
Second-Cooling Process
In the second-cooling process, the steel sheet after the second-hot-rolling and
after the first-cooling process may be preferably cooled to a temperature range of 600°C
to 800°C under an average cooling rate of 15 "Clsecond to 300 "Clsecond. When a
temperature (unit: "C) of the steel sheet becomes AT3 or lower by cooling the steel sheet
during the second-cooling process, the martensite starts to be transformed to the ferrite.
10 When the average cooling rate is 15 "Clsecond or faster, grain coarsening of the austenite
may be preferably suppressed. It is not particularly necessary to prescribe an upper
limit of the average cooling rate. However, fiom a viewpoint of the sheet shape, the
average cooling rate may be 300 "Clsecond or slower. In addition, it is preferable to
start the second-cooling within 3 seconds after finishing the second-hot-rolling or after
15 the first-cooling process. When the second-cooling start exceeds 3 seconds, coarsening
of the austenite may occur.
[0112]
Holding Process
In the holding process, the steel sheet after the second-cooling process is held in
20 the temperature range of 600°C to 800°C for 1 second to 15 seconds. By holding in the
temperature range, the transformation fiom the austenite to the ferrite progresses, and
therefore, the area fraction of the ferrite can be increased. It is preferable that the steel
is held in a temperature range of 600°C to 680°C. By conducting the ferritic
transformation in the above comparatively lower temperature range, the ferrite structure
5 4
may be controlled to be fine and uniform. Accordingly, the bainite and the martensite
which are formed in the post process may be controlled to be fine and uniform in the
metallographic structure. In addition, in order to accelerate the ferritic transformation, a
holding time is to be 1 second or longer. However, when the holding time is longer than
5 15 seconds, the ferrite grains may be coarsened, and the cementite may precipitate. In a
case where the steel is held in the comparatively lower temperature range of 600°C to
680°C, it is preferable that the holding time is 3 seconds to 15 seconds.
[0113]
Third-Cooling Process
10 In the third-cooling process, the steel sheet after the holding process is cooled to
a temperature range of a room temperature to 350°C under an average cooling rate of 50
"Clsecond to 300 "Clsecond. During the third-cooling process, the austenite which is
not transformed to the ferrite even after the holding process is transformed to the bainite
and the martensite. When the third-cooling process is stopped at a temperature higher
15 than 350°C, the bainitic transformation excessively progresses due to the excessive high
temperature, and the martensite of 1% or more in unit of area% cannot be finally
obtained. Moreover, it is not particularly necessary to prescribe a lower limit of the
cooling stop temperature of the third-cooling process. However, in a case where water
cooling is conducted, the lower limit may be the room temperature. In addition, when
20 the average cooling rate is slower than 50 "Clsecond, the pearlitic transformation may
occur during the cooling. Moreover, it is not particularly necessary to prescribe an
upper limit of the average cooling rate in the third-cooling process. However, from an
industrial standpoint, the upper limit may be 300°C. By decreasing the average cooling
rate within the above-described range of the average cooling rate, the area fraction of the
5 5
bainite may be increased. On the other hand, by increasing the average cooling rate
within the above-described range of the average cooling rate, the area fraction of the
martensite may be increased. In addition, the grain sizes of the bainite and the
martensite are also refined.
[0114]
In accordance with properties required for the hot-rolled steel sheet, the area
fractions of the ferrite and the bainite which are the primary phase may be controlled, and
the area fraction of the martensite which is the second phase may be controlled. As
described above, the ferrite can be mainly controlled in the holding process, and the
10 bainite and the martensite can be mainly controlled in the third-cooling process. In
addition, the grain sizes or the morphologies of the ferrite and the bainite which are the
primary phase and of the martensite which is the secondary phase significantly depend
on the grain size or the morphology of the austenite which is the microstructure before
the transformation. Moreover, the grain sizes or the morphologies also depend on the
15 holding process and the third-cooling process. Accordingly, for example, the value of
TS 1 fM x dis 1 dia, which is the relationship of the area fraction fM of the martensite, the
average size dia of the martensite, the average distance dis between the martensite, and
the tensile strength TS of the steel sheet, may be satisfied by multiply controlling the
above-described production processes.
[0115]
Coiling Process
In the coiling process, the steel sheet after the third-cooling starts to be coiled at
a temperature of the room temperature to 350°C which is the cooling stop temperature of
the third-cooling, and the steel sheet is air-cooled. As described above, the hot-rolled
25 steel sheet according to the embodiment can be produced.
5 6
[0116]
Moreover, as necessary, the obtained hot-rolled steel sheet may be subjected to a
skin pass rolling. By the skinpass rolling, it may be possible to suppress a stretcher
strain which is formed during working of the steel sheet, or to straighten the shape of the
5 steel sheet.
[0117]
Moreover, the obtained hot-rolled steel sheet may be subjected to a surface
treatment. For example, the surface treatment such as the electro coating, the hot dip
coating, the evaporation coating, the alloying treatment after the coating, the organic film
10 formation, the film laminating, the organic salt and inorganic salt treatment, or the
non-chromate treatment may be applied to the obtained hot-rolled steel sheet. For
example, a galvanized layer or a galvannealed layer may be arranged on the surface of
the hot-rolled steel sheet. Even if the surface treatment is conducted, the uniform
deformability and the local deformability are sufficiently maintained.
15 [0118]
Moreover, as necessary, a tempering treatment or an ageing treatment may be
conducted as a reheating treatment. By the treatment, Nb, Ti, Zr, V, W, Mo, or the like
which is solid-soluted in the steel may be precipitated as carbides, and the martensite
may be softened as the tempered martensite. As a result, the hardness difference
20 between the ferrite and the bainite which are the primary phase and the martensite which
is the secondary phase is decreased, and the local deformability such as the hole
expansibility or the bendability is improved. The effects of the reheating treatment may
be also obtained by heating for the hot dip coating, the alloying treatment, or the like.
25 Example
[0119]
Hereinafter, the technical features of the aspect of the present invention will be
described in detail with reference to the following examples. However, the condition in
the examples is an example condition employed to confirm the operability and the effects
5 of the present invention, and therefore, the present invention is not limited to the example
condition. The present invention can employ various conditions as long as the
conditions do not depart fiom the scope of the present invention and can achieve the
object of the present invention.
[O 1201
Steels S1 to S98 including chemical compositions (the balance consists of Fe
and unavoidable impurities) shown in Tables 1 to 6 were examined, and the results are
described. After the steels were melt and cast, or after the steels were cooled once to
the room temperature, the steels were reheated to the temperature range of 900°C to
1300°C. Thereafter, the hot-rolling and the temperature control (cooling, holding, or
15 the like) were conducted under production conditions shown in Tables 7 to 14, and
hot-rolled steel sheets having the thicknesses of 2 to 5 rnm were obtained.
[0121]
In Tables 15 to 22, the characteristics such as the metallographic structure, the
texture, or the mechanical properties are shown. Moreover, in Tables, the average pole
20 density of the orientation group of {100}<011> to {223)<110> is shown as Dl and the
pole density of the crystal orientation {332}<113> is shown as D2. In addition, the area
fractions of the ferrite, the bainite, the martensite, the pearlite, and the residual austenite
are shown as F, B, fM, P, and y respectively. Moreover, the average size of the
martensite is shown as dia, and the average distance between the martensite is shown as
25 dis. Moreover, in Tables, the standard deviation ratio of hardness represents a value
58
dividing the standard deviation of the hardness by the average of the hardness with
respect to the phase having higher area fraction among the ferrite and the bainite.
As a parameter of the local deformability, the hole expansion ratio h and the
5 critical bend radius (d 1 RmC) by 90" V-shape bending of the final product were used.
The bending test was conducted to C-direction bending. Moreover, the tensile test
(measure~nenot f TS, u-EL and EL), the bending test, and the hole expansion test were
respectively conducted based on JIS Z 2241, JIS Z 2248 (V block 90" bending test) and
Japan Iron and Steel Federation Standard JFS TI00 1. Moreover, by using the
10 above-described EBSD, the pole densities were measured by a measurement step of 0.5
pm in the thickness central portion which was the range of 518 to 318 of the
thickness-cross-section (the normal vector thereof corresponded to the normal direction)
which was pa~alletlo the rolling direction at 114 position of the transverse direction.
Moreover, the r values (Lankford-values) of each direction were measured based on JIS
15 Z 2254 (2008) (IS0 10 113 (2006)). Moreover, the underlined value in the Tables
indicates out of the range of the present invention, and the blank column indicates that no
alloying element was intentionally added.
[Oi 231
Preduction Nos. PI, P2, P7, P10, Pll, P13, P14, P16 to P19, P21, P23 to P27,
20 P29 to P31, P33, P34, P36 to P41, P48 to P77, and P141 to PI80 are the examples which
satisfy the conditions of the present invention. In the examples, since all conditions of
TS 2 440 (unit: MPa), TS x u -EL 2 7000 (unit: MPa.%), TS x h 2 30000 (unit: MPa-%),
and d I RmC 2 1 (no unit) were simultaneously satisfied, it can be said that the hot-rolled
steel sheets have the high-strength, the excellent uniform deformability, and the excellent
local deformability.
[0 1241
On the other hand, P3 to P6, P8, P9, P12, P15, P20, P22, P28, P32, P35, P42 to
P47, and P78 to PI40 are the comparative examples which do not satisfy the conditions
of the present invention. In the comparative examples, at least one condition of TS 2
440 (unit: MPa), TS x u - EL 2 7000 (unit: MPa.%), TS x h 2 30000 (unit: ma.%), and
d / RmC 2 1 (no unit) was not satisfied.
[0125]
In regard to the examples and the comparative examples, the relationship
between Dl and d 1 RmC is shown in FIG. 1, and the relationship between D2 and d 1
RmC is shown in FIG. 2. As shown in FIG. 1 and FIG. 2, when Dl is 5.0 or less and
when D2 is 4.0 or less, d 1 RmC 2 1 is satisfied.
[0126]
[Table 11
[0127]
[Table 21
[0128]
[Table 31
101291
[Table 41
[0 1301
[Table 51
[0131]
[Table 61
[0 1321
[Table 71
[0133]
[Table 81
[0 1341
[Table 91
[0135]
[Table 101
[0136]
[Table 111
[0137]
[Table 121
[0138]
[Table 131
[0139]
[Table 141
[0 1 401
[Table 151
[0141]
[Table 161
[0 1421
[Table 171
[0 1431
[Table 181
[0 1441
[Table 191
[O 1451
[Table 201
C0146l
[Table 2 11
[Table 221
. . . . - . . ---
Industrial Applicability
[0148]
10 According to the above aspects of the present invention, it is possible to obtain
the hot-rolled stee! sheet which simultaneously has the high-strength, the excellent
uniform deformability, and the excellent local deformability. Accordingly, the present
inventioh has significant industrial applicability.

TABLE 3 -- 1 STEEL I CHEMICAL COMPOS I T ION/mass%
TABLE 4
--
, S38 85 1 764 235 -@ARATIVE w'!
s39 85 1 76 1 234 CoHPARATlVE WYL
540 952 762 234 COWARATIVE WLE
S41 87 1 765 232 COMPARATIVE EXAMPLE.
S42 851 766 234 COMPARATIVE WLE,
S43 ------- 851 767 232 mAUH(PEL
S44 851 762 233 COMPARATIVE WLE;
S45 I f 921 764 269 MMlPARATlVE WPLE
, S46 I 901 758 282 COMPARATIVE WLE~
547 1.010 I 952 762 235 COMPARATIVE WLE
s48 1.01 0 851 763 234 WAAATIVE WLE
549 0.01 10 1 851 765 234 COMPARATIVE WLE~
S50 0.01 10 1 851 764 235 COPARATIVE MWLE
S51 0.2010 I 85 1 768 235 COMPARATIVE WLE
S52 o.1010 I 1 ! 85 1 762 235 COMPARATIVE ME
553 0.5010 1 I 851 760 233 COMPARATIVE UlAMPLE
s54 1 .oioo 851 842 234 COMPARATIVE WLE
S55 0.2010 851 765 232 COMPARATIVE WLE
S56 ,. -. Oa10 851 764 232 COMPARATIVE WLE
S57 I
S58 1
S59 1
, S60 1
S61 '
S62
, S63
, S64
S65
I
0.201 0
S66 1 851
0.2010
760
851
851
851
851
85 1
854
85 1
851
85 1
234
766
762
7 62
765
760
764
767
759
761
EXAMPLE
234
235
234
234
232
233
233
233
233
COMPARATIVE WLE
CMllPARATlVE WPLE
EXAMPLE
EXAMPLE
EXAMPLE
EXAMPLE .
EXAMPLE
EXAMPLE ,
EXAMPLE

grjs 69
TABLE 7-2
TABLE 8- 1
lo/%- +'
TABLE 8-2
9 ?2
TABLE 9-1
12/38 =) 3
TABLE 9-2 /'
STEEL
F I RST-COOL I NG
pRoDuGTIoN
Ib
ROUlffi IN RANGE OF Ar3
TO lOWER MAN T1+30"c
tl
/s
S51
552
S53
OmMTAT1TIEFM(P ElRlA TURE
X:
PI33 I 0
PI34 ) 0
PI% 1 0
TEIPERATURE
AT bUlK
FINISH
PC
2.5 x tl
/s
935 1 0.99
935 1 03s
935 / 0.99
t
/s
2.47 1 0.90
2.47 1 0.90
247 1 0.90
t/tl
/-
0.61
0.91
0.91
AVERAGE
MY18
RATE
I0C/second
CWllNG
TEMPERATR
CHAR
PC
--
, 113
113
113
90 / 842
90 1 842
90 1 842

TABLE 10-2
STEEL
--
S96 PI78 0 935 0.99 2.47 0.90 0.91 1 11 3 90 842
S97 PI79 0 935 0.99 2.47 0.90 0.91 1 113 90 842
$96 PI80 0 935 0.M 2.47 0.90 0.91 / 113 90 842
PRoDuGT1oN
lo
ROLLING IN RANGE OF br
TO LNR lHM T1+30"c F I RST-COOL I NG
COOlfi
TEMPERATURE
CHME
PC
TWERATURE
AT MILIN6
FINISH
PC
t
/s
2.5 x tl
/s
tl
/s
IMtATNE
1%
t/tl
/-
!!;:!/
TEMPERATUR
PC
AVERAGE
C(E(I$
RATE
/%/second
15/38 Q6
TART ,E 1 1
TABLE 12

2 0 l a 9 8
TABLE 15-2
I SIZE OF METALLOGRAPH I C
STRUCTURE I
TABLE 16-1
I I TEXTURE AREA FRACT I ON OF METALLOGRAPH 1 C STRUCTURE
2 2 w %
TABLE 16-2

24&/
TABLE 17-2

T--A -B -L E 18-2
PRODUCTION
No.
S I ZE OF METALLOGRAPH I C
STRUCTURE
AREA FRACT ! OI
MERE La/Lb
55.0 1s
QTISFIED
1%
di s
/ P m
VOLUME
AVERAGE
DIAMETER
Iflm
d i a
/ P m
2 7 / 9 9
TABLE 19-1
PRODKTION
No.
REMARKS
LANKFORD-VLAUE
r60
/-
r30
/-
rL
/-
rC
/-

29/W 4.
TABLE 19-3
1 ~ 4 51C racks occur dur ing Hot rol l inglCOMPARATlVE EXAMPLEI
TABLE 20-1
3 1q 3~/
TABLE 20-2
3 2 ? . 43
TABLE 20-3
3 3 p &j Y
TABLE 21-1
/
3 4 / w 7~
TABLE 2 1-2

3 6 r n qe
TABLE 22-1
6
TABLE 22-2
38/3&Fq
TABLE 22-3

1. A steel sheet which is a hot-rolled steel sheet, the steel sheet comprising, as a
chemical composition, by mass%,
C: 0.01% to 0.496,
Si: 0.001% to 2.5%,
Mn: 0.001% to 4.0%,
Al: 0.001% to 2.0%,
P: limited to 0.15% or less,
S: limited to 0.03% or less,
N: limited to 0.01% or less,
0: limited to 0.0 1 % or less, and
a balance consisting of Fe and unavoidable impurities,
wherein: an average pole density of an orientation group of (1 00)<0 11> to
15 (22314 lo>, which is a pole density represented by an arithmetic average of pole
densities of each crystal orientation (1 00)<01 1>, (1 16}<1 lo>, (1 1414 lo>,
(1 1214 lo>, and (2231-4 lo>, is 1 .O to 5.0 and a pole density of a crystal orientation
(33214 13> is 1 .O to 4.0 in a thickness central portion which is a thickness range of 518
to 318 based on a surface of the steel sheet;
20 the steel sheet includes, as a metallographic structure, plural grains, and includes,
by area%, a ferrite and a bainite of 30% to 99% in total and a martensite of 1% to 70%;
and
when an area fraction of the martensite is defined as fM in unit of area%, an
average size of the martensite is defined as dia in unit of pm, an average distance
25 between the martensite is defined as dis in unit of pm, and a tensile strength of the steel
- = f ?
-. F-"
**, , ex- " *:
* F
I) /6+ 1'01
sheet is defined as TS in unit of MPa, a following Expressi
Expression 2 are satisfied,
dia I 13 pm . . . (Expression l),
TS / fM x dis / dia 2 500 . . . (Expression 2).
5
2. The hot-rolled steel sheet according to claim 1, further comprising, as the
chemical composition, by mass %, at least one selected fiom the group consisting of
Mo: 0.001% to 1.0%,
Cr: 0.001% to 2.0%,
10 Ni: 0.001% to 2.0%,
Cu: 0.001% to 2.0%,
B: 0.0001% to 0.005%,
Nb: 0.001% to 0.2%,
Ti: 0.001% to 0.2%,
15 V: 0.001% to 1.0%,
W: 0.001% to 1.0%,
Ca: 0.0001% to 0.01%,
Mg: 0.0001% to 0.01%,
Zr: 0.0001% to 0.2%,
2 0 Rare Earth Metal: 0.0001% to 0.1%,
As: 0.0001% to 0.5%,
Co: 0.0001% to 1.0%,
Sn: 0.0001% to 0.2%,
Pb: 0.0001% to 0.2%,
2 5 Y 0.0001% to 0.2%, and
3. The hot-rolled steel sheet ,according to claim 1 or 2,
wherein a volume average diameter of the grains is 5 pm to 30 pm.
4. The hot-rolled steel sheet according to claim 1 or 2,
wherein the average pole density of the orientation group of (100)<011> to
(22314 10> is 1.0 to 4.0, and the pole density of the crystal orientation (332)<113> is
1 .O to 3.0.
5. The hot-rolled steel sheet according to claim 1 or 2,
wherein, when a major axis of the martensite is defined as La, and a minor axis
of the martensite is defined as Lb, an area fraction of the martensite satisfying a
following Expression 3 is 50% to 100% as compared with the area fraction fM of the
15 martensite,
La / Lb < 5.0 . . . (Expression 3).
6. The hot-rolled steel sheet according to claim 1 or 2,
wherein the steel sheet includes, as the metallographic structure, by area%, the
20 ferrite of 30% to 99%.
7. The hot-rolled steel sheet according to claim 1 or 2,
whereili the steel sheet includes, as the metallographic structure, by area%, the
bainite of 5% to 80%.
,r : t p C. c- . ', . -'-C . CY 4
' ,-.- 4
r a , t - ' : ' . \ i 6 ~yo3" '? I \ :i s.1 - 9 'r: : ti r
8. The hot-rolled steel sheet according to claim 1 or 2, - ' Ncl 1'
wherein the steel sheet includes a tempered martensite in the martensite. ?-
9. The hot-rolled steel sheet according to claim 1 or 2,
wherein an area fraction of coarse grain having grain size of more than 35 pm is
0% to 10% among the grains in the metallographic structure of the steel sheet.
10. The hot-rolled steel sheet according to claim 1 or 2,
wherein a hardness H of the ferrite satisfies a following Expression 4,
H < 2 0 0 + 3 0 x [ S i ] + 2 1 x ~ ] + 2 7 0 x ~ ] + 7 8 x [ ~ b ] ' ~ + 1 0 8 x
[~i]'". . .(Expression 4).
11. The hot-rolled steel sheet according to claim 1 or 2,
wherein, when a hardness of the ferrite or the bainite which is a primary phase is
15 measured at 100 points or more, a value dividing a standard deviation of the hardness by
an average of the hardness is 0.2 or less.
12. A method for producing a hot-rolled steel sheet, comprising:
first-hot-rolling a steel in a temperature range of 1000°C to 1200°C under
20 conditions such that at least one pass whose reduction is 40% or more is included so as to
control an average grain size of an austenite in the steel to 200 pm or less, wherein the
steel includes, as a chemical composition, by mass%,
C: 0.01% to 0.4%,
Si: 0.001% to 2.5%,
Mn: 0.001% to 4.0%,
Al: 0.001% to 2.0%,
P: limited to 0.15% or less,
S: limited to 0.03% or less,
N: limited to 0.01% or less,
0: limited to 0.01% or less, and
a balance consisting of Fe and -unavoidable impurities;
second-hot-rolling the steel under conditions such that, when a temperature
calculated by a following Expression 5 is defined as T1 in unit of "C and a ferritic
transformation temperature calculated by a following Expression 6 is defined as Ar3 in
10 unit of "C, a large reduction pass whose reduction is 30% or more in a temperature range
of Tl + 30°C to T1 + 200°C is included, a cumulative reduction in the temperature range
of Tl + 30°C to T1 + 200°C is 50% or more, a cumulative reduction in a temperature
range of& to lower than T1 + 30°C is limited to 30% or less, and a rolling finish
temperature is AT3 or higher;
15 first-cooling the steel under conditions such that, when a waiting time from a
finish of a final pass in the large reduction pass to a cooling start is defined as t in unit of
second, the waiting time t satisfies a following Expression 7, an average cooling rate is
50 "Clsecond or faster, a cooling temperature change which is a difference between a
steel temperature at the cooling start and a steel temperature at a cooling finish is 40°C to
20 140°C, and the steel temperature at the cooling finish is T1 + 100°C or lower;
second-cooling the steel to a temperature range of 600°C to 800°C under an
average cooling rate of 15 "Clsecond to 300 "Clsecond after finishing the
second-hot-rolling;
holding the steel in the temperature range of 600°C to 800°C for 1 second to 15
seconds;
third-cooling the steel to a temperature range of a room temperature to 350°C
under an average cooling rate of 50 "Clsecond to 300 "Clsecond after finishing the
holding;
5 coiling the steel in the temperature range of the room temperature to 350°C,
T1 = 850 + 10 x ([C] + M) x [Mh] . . . (Expression 5),
here, [C], N, and [Mn] represent mass percentages of C, N, and Mn
respectively,
Ar3 = 879.4 - 516.1 x [C] - 65.7 x [Mn] + 38.0 x [Si] +274.7 x [PI ...
10 (Expression 6),
here, in Expression 6, [C], [Mn], [Si] and [PI represent mass percentages of C,
Mh, Si, and P respectively,
t I 2.5 x t l . . . (Expression 7),
here, tl is represented by a following Expression 8,
15 tl = 0.001 x ((Tf-Tl) x Pl 1100)~- 0 .109 x ((Tf- T1) x P1 1100) + 3.1 ...
(Expression 8),
here, Tf represents a celsius temperature of the steel at the finish of the final pass,
and PI represents a percentage of a reduction at the final pass.
20 13. The method for producing the hot-rolled steel sheet according to claim 12,
wherein the steel further includes, as the chemical composition, by mass%, at
least one selected from the group consisting of
Mo: 0.001% to 1.0%,
Cr: 0.001% to 2.0%,
Ni: 0.001% to 2.0%, . -
Cu: 0.001% to 2.0%,
B: 0.0001% to 0.005%,
Nb: 0.001% to 0.2%,
5 Ti: 0.001% to 0.2%,
V: 0.001% to 1.0%,
W: 0.001% to 1.0%,
Ca: 0.0001% to 0.01%,
Mg: 0.0001% to 0.01%,
10 Zr: 0.0001% to 0.2%,
Rare Earth Metal: 0.0001% to 0.1%,
As: 0.0001% to 0.5%,
Co: 0.0001% to 1.0%,
Sn: 0.0001% to 0.2%,
15 Pb: 0.0001% to 0.2%,
Y 0.0001% to 0.2%, and
Hf: 0.0001% to 0.2%,
wherein a temperature calculated by a following Expression 9 is substituted for
the temperature calculated by the Expression 5 as TI,
20 T1= 850 + 10 x ([C] + M) x [Mn] + 350 x [Nb] + 250 x [Ti] + 40 x [B] + 10 x
[Cr] + 100 x [Mo] + 100 x [V]. . . (Expression 9),
here, [CIY [Nl, [MnI, [Nbl, [Ti], P I , [Crl, WoI, and [V1 represent mass
percentages of C, N, Mn, Nb, Ti, By Cry Mo, and V respectively.
25 14. The method for producing the hot-rolled steel sheet according to claim 12 or 13,
. *
r t .
wherein the waiting time t further satisfies a following Expression 10, ,,,,! 7 ~ ~ 2
0 l t < t 1.. . (Expression 10).
15. The method for producing the hot-rolled steel sheet according to claim 12 or 13;
wherein the waiting time t further satisfies a following Expression 11,
tl l t l tl x 2.5.. . (Expression 11).
16. The method for producing the hot-rolled steel sheet according to claim 12 or 13,
wherein, in the first-hot-rolling, at least two times of rollings whose reduction is
10 40% or more are conducted, and the average grain size of the austenite is controlled to
100 pm or less.
17. The method for producing the hot-rolled steel sheet according to claim 12 or 13,
wherein the second-cooling starts within 3 seconds after finishing the
15 second-hot-rolling.
18. The method for producing the hot-rolled steel sheet according to claim 12 or 13,
wherein, in the second-hot-rolling, a temperature rise of the steel between passes
is 18°C or lower.
20
19. The method for producing the hot-rolled steel sheet according to claim 12 or 13,
wherein a final pass of rollings in the temperature range of T1 + 30°C to TI +
200°C is the large reduction pass.
20. The method for producing the hot-rolled steel sheet according to claim 12 or 13,
wherein, in the holding, the steel is held in a temperature range of 600°C to
680°C for 3 seconds to 15 seconds.
5 21. The method for producing the hot-rolled steel sheet according to claim 12 or 13, .
wherein the first-cooling is conducted at an interval between rolling stands.
Dated this 2211.1 1201 3
(NEHA SRTVASTAVA)
OF REMFRY & SAGAR
ATTORNEY FOR THE APPLICANTS