[Technical Field of the Invention]
[0001]
The present invention relates to a hot-rolled steel sheet. Specifically, the
present invention relates to a hot-rolled steel sheet that is formed into various shapes
by press working or the like to be used, and particularly relates to a hot-rolled steel
sheet that has high strength and has excellent shearing workability.
Priority is claimed on Japanese Patent Application No. 2020-143746, filed on
August 27, 2020, the content of which is incorporated herein by reference.
[Background Art]
[0002]
In recent years, from the viewpoint of protecting the global environment,
efforts have been made to reduce the amount of carbon dioxide gas emitted in many
fields. Vehicle manufacturers are also actively developing techniques for reducing
the weight of vehicle bodies for the purpose of reducing fuel consumption. However,
it is not easy to reduce the weight of vehicle bodies since the emphasis is placed on
improvement in collision resistance to secure the safety of the occupants.
[0003]
In order to achieve both vehicle body weight reduction and collision
resistance, an investigation has been conducted to make a member thin by using a
high- strength steel sheet. Therefore, there is a strong demand for a steel sheet having
both high strength and excellent formability, and several techniques have been
conventionally proposed to meet this demand. Vehicle members are formed by press
forming, and the press-formed blank sheet is often manufactured by highly productive
- 1 -
shearing working. Since the clearance in blanking is not always constant, it is
preferable that the end surface accuracy after shearing working is stable even with
various clearances. For example, it is preferable that the proportion of a shear droop
in the sheared end surface after shearing working is stable regardless of the clearance.
[0004]
Regarding the shearing workability, for example, Patent Document 1 discloses
a technique for controlling burr height after punching by controlling a ratio dsldb of the
ferrite grain size ds of the surface layer to the ferrite crystal grain db of an inside to 0.95
or less.
Patent Document 2 discloses a technique for improving separations or burrs
on an end surface of a sheet by reducing a P content.
[Prior Art Document]
[Patent Document]
[0005]
[Patent Document 1] Japanese Unexamined Patent Application, First
Publication No. H10-168544
[Patent Document 2] Japanese Unexamined Patent Application, First
Publication No. 2005-298924
[Non-Patent Document]
[0006]
[Non-Patent Document 1] J. Webel, J. Gola, D. Britz, F. Mucklich, Materials
Characterization 144 (2018) 584-596
[Non-Patent Document 2] D. L. Naik, H. U. Sajid, R. Kiran, Metals 2019, 9,
546
[Non-Patent Document 3] K. Zuiderveld, Contrast Limited Adaptive
- 2 -
Histogram Equalization, Chapter VIII. 5, Graphics Gems IV. P. S. Heckbert (Eds.),
Cambridge, MA, Academic Press, 1994, pp. 474-485
[Disclosure of the Invention]
[Problems to be Solved by the Invention]
[0007]
However, in Patent Document 1, the target is IF steel, and it may be difficult
to apply the target to members having a high strength of 780 MPa or more. In Patent
Document 2, a strength of 780 MPa or more is obtained, but the stability of a shear
droop on a sheared end surface after shearing working is not investigated.
[0008]
The present invention has been made in view of the above problems of the
related art, and an object of the present invention is to provide a hot-rolled steel sheet
having high strength and excellent shearing workability.
[Means for Solving the Problem]
[0009]
In view of the above problems, the present inventors obtained the following
findings (a) to (f) as a result of intensive studies on the chemical composition of the
hot-rolled steel sheet and a relationship between a microstructure and mechanical
properties, and completed the present invention. Having excellent shearing
workability indicates that the proportion of a shear droop in the sheared end surface
after shearing working is stable regardless of the clearance during shearing working.
In addition, the expression of having excellent strength or having high strength
indicates that the tensile strength is 780 MPa or more.
[0010]
(a) In order to obtain an excellent tensile (maximum) strength, it is preferable
- 3 -
to utilize a full hard structure. That is, it is preferable to contain martensite or bainite
in the microstructure.
[0011]
(b) In order to stabilize the proportion of a shear droop in a sheared end
surface, it is important that Mn segregation does not occur much, the microstructural
morphology is periodic, and the microstructure is made to be non-uniform (poorly
uniform).
[0012]
(c) Specifically, it is effective to control the standard deviation of the Mn
concentration to a certain value or less and to control the periodicity of the
microstructure and the uniformity of the microstructure for stabilizing the proportion
of a shear droop in the sheared end surface.
[0013]
(d) In order to control the standard deviation of the Mn concentration to a
certain value or less, a slab heating step and a subsequent hot rolling step are important.
For example, it is effective that the steel sheet is held in a temperature range of 700°C
to 850°C for 900 seconds or longer, further heated, and held in a temperature range of
1100oc or higher for 6000 seconds or longer and that hot rolling is performed such that
a total of 90% or more of the sheet thickness reduces in a temperature range of 850°C
to ll00°C.
[0014]
(e) In order to enhance the periodicity of the microstructure, it is important to
control the recrystallization behavior of austenite during hot rolling. For example, it
is effective to control the rolling reduction and rolling temperature of the final stage of
hot rolling to within a predetermined range, set stress that is loaded to the steel sheet
- 4 -
after rolling one stage before the final stage of hot rolling and before the final stage
rolling to 170 kPa or more, and set stress that is loaded to the steel sheet after the final
stage of hot rolling and until the steel sheet is cooled to 75oac to 200 kPa or more.
Such hot rolling conditions make it possible to produce fine and flat austenite grains
and make it possible to enhance the periodicity of the microstructure as a result in
combination with conditions for subsequent cooling.
[0015]
(f) In order to reduce the uniformity of the microstructure, it is effective to
promote the precipitation of an iron carbide by cooling the steel sheet to a temperature
range of 400°C or higher and lower than 6ooac and then coiling the steel sheet.
[0016]
The gist of the present invention made based on the above findings is as
follows.
(1) A hot-rolled steel sheet according to one aspect of the present invention
containing, in terms of mass%, as a chemical composition,
C: 0.050% to 0.250%,
Si: 0.05% to 3.00%,
Mn: 1.00% to 4.00%,
one or two or more of Ti, Nb, and V: 0.060% to 0.500% in total,
sol. Al: 0.001% to 2.000%,
P: 0.100% or less,
S: 0.0300% or less,
N: 0.1000% or less,
0: 0.0100% orless,
Cu: 0% to 2.00%,
- 5 -
Cr: 0% to 2.00%,
Mo: 0% to 1.00%,
Ni: 0% to 2.00%,
B: 0% to 0.0100%,
Ca: 0% to 0.0200%,
Mg: 0% to 0.0200%,
REM: 0% to 0.1000%,
Bi: 0% to 0.020%,
one or two or more of Zr, Co, Zn, and W: 0% to 1.00% in total,
Sn: 0% to 0.05%, and
a remainder consisting of Fe and impurities,
in which, in a microstructure,
in terms of area%, residual austenite is less than 3.0%, ferrite is less than
15.0%, and pearlite is less than 5.0%,
an E value that indicates periodicity of the microstructure is less than 10.7,
and an I value that indicates uniformity of the microstructure is less than 1.020,
a standard deviation of a Mn concentration is 0.60 mass% or less, and
a tensile strength is 780 MPa or more.
(2) The hot-rolled steel sheet according to (1), in which ds/dq, which is a ratio
of an average crystal grain size ds of a surface layer to an average crystal grain size dq
at a 1/4 depth position of a sheet thickness from a surface, may be 0.95 or less.
(3) The hot-rolled steel sheet according to (1) or (2) may further contain, in
terms of mass%, one or two or more selected from the group consisting of, as the
chemical composition
Cu: 0.01% to 2.00%,
- 6 -
Cr: 0.01% to 2.00%,
Mo: 0.01% to 1.00%,
Ni: 0.02% to 2.00%,
B: 0.0001% to 0.0100%,
Ca: 0.0005% to 0.0200%,
Mg: 0.0005% to 0.0200%,
REM: 0.0005% to 0.1000%, and
Bi: 0.0005% to 0.020%.
[Effects ofthe Invention]
[0017]
According to the above aspect according to the present invention, it is
possible to obtain a hot-rolled steel sheet having excellent strength and shearing
workability. In addition, according to the preferable aspect according to the present
invention, it is possible to obtain a hot-rolled steel sheet which has the above various
properties and, furthermore, suppresses the occurrence of inside bend cracking, that is,
has excellent inside bend cracking resistance.
The hot-rolled steel sheet according to the above aspect of the present
invention is suitable as an industrial material used for vehicle members, mechanical
structural members, and building members.
[Brief Description of the Drawings]
[0018]
FIG. 1 is a view for describing a method for measuring the proportion of a
shear droop in a sheared end surface after shearing working.
[Embodiments ofthe Invention]
[0019]
- 7 -
The chemical composition and microstructure of a hot-rolled steel sheet
according to the present embodiment (hereinafter, sometimes simply referred to as the
steel sheet) will be more specifically described below. However, the present
invention is not limited only to a configuration disclosed in the present embodiment,
and various modifications can be made without departing from the scope of the gist of
the present invention.
The numerical limit range described below with "to" in between includes the
lower limit and the upper limit. Regarding the numerical value indicated by "less
than" or "more than", the value does not fall within the numerical range. In the
following description, % regarding the chemical composition of the steel sheet is
mass% unless particularly otherwise specified.
[0020]
1. Chemical Composition
The hot-rolled steel sheet according to the present embodiment includes, in
terms of mass%, C: 0.050% to 0.250%, Si: 0.05% to 3.00%, Mn: 1.00% to 4.00%, one
or two or more of Ti, Nb, and V: 0.060% to 0.500% in total, sol. Al: 0.001% to 2.000%,
P: 0.100% or less, S: 0.0300% or less, N: 0.1000% orless, 0: 0.0100% or less, and a
remainder consisting of Fe and impurities. Each element will be described in detail
below.
[0021]
(1-1) C: 0.050% to 0.250%
C increases the fraction of a hard phase and increases the strength of ferrite by
bonding to a precipitation hardening element such as Ti, Nb, or V. When the C
content is less than 0.050%, it is difficult to obtain a desired strength. Furthermore,
the stability of a shear droop on the sheared end surface deteriorates. Therefore, the
- 8 -
C content is set to 0.050% or more. The C content is preferably 0.060% or more,
more preferably 0.070% or more, and still more preferably 0.080% or more.
On the other hand, when the C content is more than 0.250%, the weldability
of the hot-rolled steel sheet deteriorates. Therefore, the C content is set to 0.250% or
less. The C content is preferably 0.150% or less.
[0022]
(1-2) Si: 0.05% to 3.00%
Si has an action of increasing the strength of the hot-rolled steel sheet by solid
solution strengthening. In addition, Si has an action of making steel sound by
deoxidation (suppressing the occurrence of a defect such as a blowhole in steel).
When the Si content is less than 0.05%, an effect by the action cannot be obtained.
Therefore, the Si content is set to 0.05% or more. The Si content is preferably 0.50%
or more and more preferably 0.80% or more.
However, when the Si content is more than 3.00%, the surface properties, the
chemical convertibility, the ductility and the weldability of the hot-rolled steel sheet
are significantly deteriorated, and the A3 transformation point is significantly increased.
Therefore, it becomes difficult to perform hot rolling in a stable manner. Furthermore,
the stability of a shear droop on the sheared end surface deteriorates. Therefore, the
Si content is set to 3.00% or les s. The Si content is preferably 2.70% or less and
more preferably 2.50% or less.
[0023]
(1-3) Mn: 1.00% to 4.00%
Mn has an action of suppressing ferritic transformation to achieve the highstrengthening
of the hot-rolled steel sheet. When the Mn content is less than 1.00%, a
tensile strength of 780 MPa or more cannot be obtained. Furthermore, the stability of
- 9 -
a shear droop on the sheared end surface deteriorates. Therefore, the Mn content is
set to 1.00% or more. The Mn content is preferably 1.30% or more and more
preferably 1.50% or more.
On the other hand, when the Mn content is more than 4.00%, cracking occurs
in the vicinity of the sheet thickness center due to center segregation of Mn, and the
sheared end surface properties after shearing working deteriorate. Therefore, the Mn
content is set to 4.00% or less. The Mn content is preferably 3.70% or less and more
preferably 3.50% or less.
[0024]
(1-4) One or two or more of Ti, Nb, and V: 0.060% to 0.500% in total
Ti, Nb, and V are elements that are finely precipitated in steel as a carbide and
a nitride and improve the strength of steel by precipitation hardening. In addition, Ti,
Nb, and V are elements that fix C by forming the above carbide and suppress the
formation of cementite that is harmful to shearing workability. When the total
amount of Ti, Nb, and Vis less than 0.060%, these effects cannot be obtained.
Therefore, the total amount ofTi, Nb, and Vis set to 0.060% or more. Not all ofTi,
Nb, and V need to be contained, and any one thereof may be contained in a quantity of
0.060% or more. In a case where two or more of Ti, Nb, and V are contained, the
total content thereof may be 0.060% or more. The total amount of Ti, Nb, and V is
preferably 0.080% or more and more preferably 0.100% or more.
On the other hand, when the total amount of Ti, Nb, and V exceeds 0.500%,
the workability of the hot-rolled steel sheet deteriorates. Therefore, the total amount
of Ti, Nb, and V is set to 0.500% or less. The total amount of Ti, Nb, and V is
preferably 0.300% or less, more preferably 0.250% or less, and still more preferably
0.200% or less.
- 10 -
[0025]
(1-5) soL Al: 0.001% to 2.000%
Similar to Si, Al has an action of deoxidizing steel to make steel sound.
When the soL Al content is less than 0.001%, an effect by the action cannot be
obtained. Therefore, the soL Al content is set to 0.001% or more. The soL Al
content is preferably 0.010% or more.
On the other hand, when the soL Al content is more than 2.000%, the above
effects are saturated, which is not economically preferable, and thus the sol. Al content
is set to 2.000% or less. The soL Al content is preferably 1.500% or less, more
preferably 1.300% or less, and still more preferably 1.000% or less.
The sol. Al means acid-soluble Al and refers to solid solution Al present in
steel in a solid solution state.
[0026]
( 1-6) P: 0.100% or less
Pis an element that is generally contained as an impurity, and has an action of
increasing the strength of the hot-rolled steel sheet by solid solution strengthening.
Therefore, P may be positively contained, but Pis an element that is easily segregated,
and, when the P content exceeds 0.100%, the deterioration of ductility attributed to
boundary segregation becomes significant. Therefore, the P content is set to 0.100%
or less. The P content is preferably 0.030% or less. The lower limit of the P content
does not need to be particularly specified, but is preferably set to 0.001% from the
viewpoint of the refining cost.
[0027]
(1-7) S: 0.0300% or less
S is an element that is contained as an impurity and forms a sulfide-based
- 11 -
inclusion in steel to degrade the ductility of the hot -rolled steel sheet. When the S
content is more than 0.0300%, the ductility of the hot-rolled steel sheet significantly
deteriorates. Therefore, the S content is set to 0.0300% or less. The S content is
preferably 0.0050% or less. The lower limit of the S content does not need to be
particularly specified, but is preferably set to 0.0001% from the viewpoint of the
refining cost.
[0028]
(1-8) N: 0.1000% orless
N is an element that is contained in steel as an impurity and has an action of
degrading the ductility of the hot-rolled steel sheet. When the N content is more than
0.1000%, the ductility of the hot-rolled steel sheet significantly deteriorates.
Therefore, theN content is set to 0.1000% or less. TheN content is preferably
0.0800% or less, more preferably 0.0700% or less, and still more preferably 0.0100%
or less. Although the lower limit of theN content does not need to be particularly
specified, in a case where one or two or more of Ti, Nb, and V are contained to further
refine the microstructure, theN content is preferably set to 0.0010% or more and more
preferably set to 0.0020% or more to promote the precipitation of a carbonitride.
[0029]
(1-9) 0: 0.0100% or less
When a large amount of 0 is contained in steel, 0 forms a coarse oxide that
becomes the starting point of fracture and causes brittle fracture and hydrogen-induced
cracks. Therefore, the 0 content is set to 0.0100% or less. The 0 content is
preferably 0.0080% or less and more preferably 0.0050% or less. The 0 content may
be set to 0.0005% or more or 0.0010% or more to disperse a large number of fine
oxides when molten steel is deoxidized.
- 12 -
[0030]
The remainder of the chemical composition of the hot-rolled steel sheet
according to the present embodiment may be Fe and an impurity. In the present
embodiment, the impurities mean substances that are incorporated from ore as a raw
material, a scrap, manufacturing environment, or the like and/or substances that are
permitted to an extent that the hot-rolled steel sheet according to the present
embodiment is not adversely affected.
[0031]
Instead of a part of Fe, the hot-rolled steel sheet according to the present
embodiment may contain Cu, Cr, Mo, Ni, B, Ca, Mg, REM, Bi, Zr, Co, Zn, W, and Sn
as optional elements. In a case where the above optional elements are not contained,
the lower limit of the content thereof is 0%. Hereinafter, the above optional elements
will be described in detail.
[0032]
(1-10) Cu: 0.01% to 2.00%, Cr: 0.01% to 2.00%, Mo: 0.01% to 1.00%, Ni:
0.02% to 2.00%, and B: 0.0001% to 0.0100%
All of Cu, Cr, Mo, Ni, and B have an action of enhancing the hardenability of
the hot-rolled steel sheet. In addition, Cu and Mohave an action of being precipitated
as a carbide in steel to increase the strength of the hot-rolled steel sheet. Furthermore,
in a case where Cu is contained, Ni has an action of effectively suppressing the grain
boundary cracking of a slab caused by Cu. Therefore, one or two or more of these
elements may be contained.
[0033]
Cu has an action of enhancing the hardenability of the hot-rolled steel sheet
and an action of being precipitated as a carbide in steel at a low temperature to increase
- 13 -
the strength of the hot-rolled steel sheet. In order to more reliably obtain the effect by
the action, the Cu content is preferably set to 0.01% or more and more preferably set to
0.05% or more. However, when the Cu content is more than 2.00%, grain boundary
cracking may occur in the slab in some cases. Therefore, the Cu content is set to
2.00% or less. The Cu content is preferably 1.50% or less and more preferably 1.00%
or less.
[0034]
As described above, Cr has an action of enhancing the hardenability of the
hot-rolled steel sheet. In order to more reliably obtain the effect by the action, the Cr
content is preferably set to 0.01% or more and more preferably set to 0.05% or more.
However, when the Cr content is more than 2.00%, the chemical convertibility of the
hot-rolled steel sheet significantly deteriorates. Therefore, the Cr content is set to
2.00% or less.
[0035]
As described above, Mo has an action of enhancing the hardenability of the
hot-rolled steel sheet and an action of being precipitated as a carbide in steel to
increase the strength of the hot-rolled steel sheet. In order to more reliably obtain the
effect by the action, the Mo content is preferably set to 0.01% or more and more
preferably set to 0.02% or more. However, even when the Mo content is set to more
than 1.00%, the effect by the action is saturated, which is not economically preferable.
Therefore, the Mo content is set to 1.00% or less. The Mo content is preferably
0.50% or less and more preferably 0.20% or less.
[0036]
As described above, Ni has an action of enhancing the hardenability of the
hot-rolled steel sheet. In addition, in a case where Cu is contained, Ni has an action
- 14 -
of effectively suppressing the grain boundary cracking of the slab caused by Cu. In
order to more reliably obtain the effect by the action, the Ni content is preferably set to
0.02% or more. Since Ni is an expensive element, it is not economically preferable to
contain a large amount of Ni. Therefore, the Ni content is set to 2.00% or less.
[0037]
As described above, B has an action of enhancing the hardenability of the hotrolled
steel sheet. In order to more reliably obtain the effect by this action, the B
content is preferably set to 0.0001% or more and more preferably set to 0.0002% or
more. However, when the B content is more than 0.0100%, the formability of the
hot-rolled steel sheet significantly deteriorates, and thus the B content is set to
0.0100% or less. The B content is preferably 0.0050% or less.
[0038]
(1-11) Ca: 0.0005% to 0.0200%, Mg: 0.0005% to 0.0200%, REM: 0.0005%
to 0.1000%, and Bi: 0.0005% to 0.020%
All of Ca, Mg, and REM have an action of enhancing the ductility of the hotrolled
steel sheet by adjusting the shape of inclusions in steel to a preferable shape.
In addition, Bi has an action of enhancing the ductility of the hot-rolled steel sheet by
refining the solidification structure. Therefore, one or two or more of these elements
may be contained. In order to more reliably obtain the effect by the action, it is
preferable that the amount of any one or more of Ca, Mg, REM, and Bi is set to
0.0005% or more. However, when theCa content or Mg content is more than
0.0200% or when the REM content is more than 0.1000%, an inclusion is excessively
formed in steel, and thus the ductility of the hot-rolled steel sheet may be conversely
degraded in some cases. In addition, even when the Bi content is set to more than
0.020%, the above effect by the action is saturated, which is not economically
- 15 -
preferable. Therefore, the Ca content and the Mg content are each set to 0.0200% or
less, the REM content is set to 0.1000% or less, and the Bi content is set to 0.020% or
less. The Bi content is preferably 0.010% or less.
Here, REM refers to a total of 17 elements consisting of Sc, Y, and
lanthanoids, and the REM content refers to the total amount of these elements. In the
case of the lanthanoids, the lanthanoids are industrially added in the form of misch
metal.
[0039]
(1-12) One or two or more of Zr, Co, Zn, or W: 0% to 1.00% in total and Sn:
0% to 0.05%
Regarding Zr, Co, Zn, and W, the present inventors have confirmed that, even
when a total of 1.00% or less of these elements are contained, the effect of the hotrolled
steel sheet according to the present embodiment is not impaired. Therefore,
one or two or more of Zr, Co, Zn, or W may be contained in a total of 1.00% or less.
In addition, the present inventors have confirmed that, even when a small
amount of Sn is contained, the effect of the hot-rolled steel sheet according to the
present embodiment is not impaired. However, when a large amount of Sn is
contained, a defect may be generated during hot rolling, and thus the Sn content is set
to 0.05% or less.
[0040]
The chemical composition of the above hot-rolled steel sheet may be
measured by a general analytical method. For example, inductively coupled plasmaatomic
emission spectrometry (ICP-AES) may be used for measurement. sol. Al may
be measured by the ICP-AES u sing a filtrate after a sample is decomposed with an acid
by heating. C and S may be measured by using a combustion-infrared absorption
- 16 -
method, N may be measured by using the inert gas melting-thermal conductivity
method, and 0 may be measured using an inert gas melting-non-dispersive infrared
absorption method.
[0041]
2. Microstructure of Hot-Rolled Steel Sheet
Next, the microstructure of the hot-rolled steel sheet according to the present
embodiment will be described.
In the microstructure of the hot-rolled steel sheet according to the present
embodiment, in terms of area%, residual austenite is less than 3.0%, ferrite is less than
15.0%, and pearlite is less than 5.0%, theE value that indicates the periodicity of the
microstructure is less than 10.7, the I value that indicates the uniformity of the
microstructure is less than 1.020, and the standard deviation of the Mn concentration is
0.60 mass % or less. Therefore, the hot-rolled steel sheet according to the present
embodiment can obtain a high strength and excellent shearing workability. In the
present embodiment, the microstructural fractions, theE value, the I value, and the
standard deviation of the Mn concentration in the microstructure at a 1/4 depth position
of the sheet thickness from the surface in a sheet thickness cross section parallel to the
rolling direction are specified. The reason therefor is that the microstructure at this
position indicates a typical microstructure of the steel sheet.
[0042]
(2-1) Area Fraction of Residual Austenite: Less than 3.0%
In the present invention, when the area fraction of residual austenite is too
large, the proportion of a shear droop in the sheared end surface may become unstable.
It is presumed that residual austenite improves the work hardening capability of the
hot-rolled steel sheet by transformation-induced plasticity (TRIP) and thus the
- 17 -
proportion of a shear droop in the sheared end surface becomes unstable. When the
area fraction of the residual austenite is 3.0% or more, the shearing workability of the
hot-rolled steel sheet deteriorates. Therefore, the area fraction of the residual
austenite is set to less than 3.0%. The area fraction of the residual austenite is
preferably less than 1.5% and more preferably less than 1.0%. Since residual
austenite is preferably as little as possible, the area fraction of the residual austenite
may be 0%.
[0043]
As the measurement method of the area fraction of the residual austenite,
methods by X-ray diffraction, electron back scatter diffraction image (EBSP, electron
back scattering diffraction pattern) analysis, and magnetic measurement and the like
may be used and the measured values may differ depending on the measurement
method. In the present embodiment, the area fraction of the residual austenite is
measured by X-ray diffraction.
In the measurement of the area fraction of the residual austenite by X-ray
diffraction in the present embodiment, the integrated intensities of a total of 6 peaks of
a(llO), a(200), a(211), y(lll), y(200), and y(220) are obtained at a 1/4 depth position
of the sheet thickness (a region between a depth of 1/8 of the sheet thickness from the
surface and a depth of 3/8 of the sheet thickness from the surface) of the hot-rolled
steel sheet using Co-Ka rays, and the area fraction of the residual austenite is obtained
by calculation using the strength averaging method.
[0044]
(2-2) Area Fraction of Ferrite: Less than 15.0%
Ferrite is a structure formed when fcc transforms into bee at a relatively high
temperature. Since ferrite has a high work hardening capability, when the area
- 18 -
fraction of the ferrite is too large, the proportion of a shear droop in the sheared end
surface becomes unstable. Therefore, the area fraction of the ferrite is set to less than
15.0%. The area fraction of the ferrite is preferably 12.0% or less, more preferably
10.0% or less, and still more preferably 8.0% or less. The area fraction of the ferrite
is preferably as small as possible, and the lower limit of the area fraction of the ferrite
may be 3.0%, 2.0%, or 0 %.
[0045]
(2-3) Area Fraction of Pearlite: Less than 5.0%
Pearlite is a lamellar microstructure in which cementite is precipitated in
layers between ferrite. In addition, pearlite is a soft microstructure compared with
bainite and martensite. When the area fraction of the pearlite is 5.0% or more, carbon
is consumed by cementite that is contained in pearlite, and the strengths of martensite
and bainite, which are the remainder in microstructure, decrease, and a tensile strength
of 780 MPa or more cannot be obtained. Therefore, the area fraction of the pearlite is
set to less than 5.0%. The area fraction of the pearlite is preferably 3.0% or less. In
order to improve the stretch flangeability of the hot-rolled steel sheet, the area fraction
of the pearlite is preferably reduced as much as possible, and the lower limit of the area
fraction of the pearlite is preferably 2.0%, more preferably 1.0%, and still more
preferably 0%.
[0046]
The hot-rolled steel sheet according to the present embodiment contains a full
hard structure consisting of one or two or more of bainite, martensite, and tempered
martensite as the remainder in microstructure other than residual austenite, ferrite, and
pearlite.
[0047]
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Measurement of the area fractions of the microstructure is conducted by the
following method. A sheet thickness cross section parallel to the rolling direction is
mirror-finished and, furthermore, polished at room temperature with colloidal silica not
containing an alkaline solution for 8 minutes, thereby removing strain introduced into
the surface layer of a sample. In a random position of the sample cross section in a
longitudinal direction, a region with a length of 50 11m and at a 114 depth position of
the sheet thickness from the surface (a region between a 1/8 depth of the sheet
thickness from the surface and a 3/8 depth of the sheet thickness from the surface) is
measured at a measurement interval of 0.1 11m by electron backscatter diffraction to
obtain crystal orientation information. For the measurement, an EBSD device
configured of a thermal field emission scanning electron microscope (JSM-7001F
manufactured by JEOL) and an EBSD detector (DVC5 type detector manufactured by
TSL) is used. At this time, the degree of vacuum inside the EBSD device is set to 9.6
x 10·5 Pa or less, the acceleration voltage is set to 15 kV, the irradiation current level is
set to 13, and the electron beam irradiation level is set to 62. Furthermor e, a reflected
electron image is photographed at the same visual field. First, cr ystal grains where
ferrite and cementite are precipitated in layers are specified from the reflected electron
image, and the area fraction of the crystal grains is calculated, thereby obtaining the
area fraction of pearlite. After that, for crystal grains except the crystal grains
determined as pearlite, from the obtained crystal orientation information, regions
where the grain average misorientation value is 1.0° or less are determined as ferrite
using a "Grain Average Misorientation" function installed in software "OIM Analysis
(registered trademark)" included in the EBSD analyzer. The area fraction of the
region determined as the ferrite is obtained, thereby obtaining the area fraction of the
ferrite.
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[0048]
Subsequently, under a condition of defining a 5° grain boundary in the
residual region (a region where the grain average misorientation value is more than
1.0°) as a crystal grain boundary, when the maximum value of "Grain Average IQ" of a
ferrite region is indicated by Ia, a region with more than Ia/2 is extracted as bainite,
and a region with Ia/2 or less is extracted as "pearlite, martensite, and tempered
martensite". The area fraction of the bainite is obtained by calculating the area ratio
of the extracted bainite. In addition, the total of the area ratios of the martensite and
the tempered martensite is obtained by calculating the area fractions of the extracted
"pearlite, martensite, and tempered martensite" and subtracting the area fraction of the
pearlite obtained by the above EBSD analysis.

What is claimed is:
CLAIMS
1. A hot-rolled steel sheet comprising, in terms of mass%, as a chemical
composition:
C: 0.050% to 0.250%;
Si: 0.05% to 3.00%;
Mn: 1.00% to 4.00%;
one or two or more of Ti, Nb, and V: 0.060% to 0.500% in total;
sol. Al: 0.001% to 2.000%;
P: 0.100% or less;
S: 0.0300% or less;
N: 0.1000% or less;
0: 0.0100% orless;
Cu: 0% to 2.00%;
Cr: 0% to 2.00%;
Mo: 0% to 1.00%;
Ni: 0% to 2.00%;
B: 0% to 0.0100%;
Ca: 0% to 0.0200%;
Mg: 0% to 0.0200%;
REM: 0% to 0.1000%;
Bi: 0% to 0.020%;
one or two or more of Zr, Co, Zn, and W: 0% to 1.00% in total;
Sn: 0% to 0.05%; and
a remainder consisting of Fe and impurities,
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wherein, a microstructure has,
in terms of area%, residual austenite at less than 3.0%, ferrite at less than
15.0%, and pearlite at less than 5.0%,
an E value that indicates periodicity of the microstructure is less than 10.7,
and an I value that indicates uniformity of the microstructure is less than 1.020,
a standard deviation of a Mn concentration is 0.60 mass% or less, and
a tensile strength is 780 MPa or more.
2. The hot-rolled steel sheet according to claim 1,
wherein ds/dq, which is a ratio of an average crystal grain size ds of a surface
layer to an average crystal grain size dq at a 114 depth position of a sheet thickness
from a surface, is 0.95 or less.
3. The hot-rolled steel sheet according to claim 1 or 2, further comprising, in
terms of mass%, one or two or more selected from the group consisting of, as the
chemical composition:
Cu: 0.01% to 2.00%;
Cr: 0.01% to 2.00%;
Mo: 0.01% to 1.00%;
Ni: 0.02% to 2.00%;
B: 0.0001% to 0.0100%;
Ca: 0.0005% to 0.0200%;
Mg: 0.0005% to 0.0200%;
REM: 0.0005% to 0.1000%; and
Bi: 0.0005% to 0.020%.