[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 ductility and shearing property.
Priority is claimed on Japanese Patent Application No. 2020-143742, 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 resi stance,
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. Since there are various working methods for vehicle
members, the required formability differs depending on members to which the working
- 1 -
methods are applied, but among these, ductility is placed as important indices for
formability. In addition, vehicle members are formed by press forming, and the pressformed
blank sheet is often manufactured by highly productive shearing working. A
blank sheet manufactured by shearing working needs to be excellent in terms of the end
surface accuracy after shearing working. For example, when a secondary sheared
surface consisting of a sheared surface, a fractured surface, and a sheared surface is
generated in the appearance of the end surface after shearing working (sheared end
surface), the accuracy of the sheared end surface significantly deteriorates.
[0004]
Regarding the technique for improving ductility, for example, Patent Document
1 discloses a high-strength steel sheet for a vehicle having excellent collision resistant
safety and formability, in which residual austenite having an average crystal grain size
of 5 J.lm or less is dispersed in ferrite having an average crystal grain size of 10 J.lm or
less. In the steel sheet containing residual austenite in the microstructure, while the
austenite is transformed into martensite during working and large elongation is
exhibited due to transformation-induced plasticity, the formation of full hard martensite
impairs hole expansibility. Patent Document 1 discloses that not only ductility but also
hole expansibility are improved by refining the ferrite and the residual austenite.
[0005]
Patent Document 2 discloses a high-strength steel sheet having excellent
ductility and stretch flangeability and having a tensile strength of 980 MPa or more, in
which a second phase consisting of residual austenite and/or martensite is finely
dispersed in crystal grains.
[0006]
Regarding the technique for improving shearing property, for example, Patent
- 2 -
Document 3 discloses a technique for controlling burr height after punching by
controlling a ratio ds/db 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 4 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]
[0007]
[Patent Document 1] Japanese Unexamined Patent Application, First
Publication No. Hll-61326
[Patent Document 2] Japanese Unexamined Patent Application, First
Publication No. 2005-179703
[Patent Document 3] Japanese Unexamined Patent Application, First
Publication No. H 10-168544
[Patent Document 4] Japanese Unexamined Patent Application, First
Publication No. 2005-298924
[Non-Patent Document]
[0008]
[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
Histogram Equalization, Chapter VIII. 5, Graphics Gems IV. P. S. Heckbert (Eds.),
Cambridge, MA, Academic Press, 1994, pp. 474-485
- 3 -
[Disclosure of the Invention]
[Problems to be Solved by the Invention]
[0009]
The techniques disclosed in Patent Documents 1 to 4 are all techniques for
improving either ductility or an end surface property after shearing working.
However, Patent Documents 1 to 3 do not refer to a technique for achieving both of the
properties. Patent Document 4 refers to achievement of both shearing property and
press formability. However, since the strength of a steel sheet disclosed in Patent
Document 4 is less than 850 MPa, it may be difficult to apply the steel sheet to a
member having a high strength of 980 MPa or more.
[0010]
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 ductility and shearing property.
[Means for Solving the Problem]
[0011]
In view of the above problems, the present inventors obtained the following
findings (a) to (i) as a result of intensive studies on the chemical composition of the hotrolled
steel sheet and a relationship between a microstructure and mechanical properties,
and completed the present invention. In addition, the expression of having excellent
shearing property indicates that the generation of a secondary sheared surface is
suppressed during shearing working. In addition, the expression of having excellent
strength or having high strength indicates that the tensile strength is 980 MPa or more.
[0012]
(a) In order to obtain an excellent tensile (maximum) strength, it is preferable
- 4 -
to utilize a full hard structure. That is, it is preferable to contain martensite or bainite
in the microstructure.
[0013]
(b) However, since a full hard structure is a structure with poor ductility,
excellent ductility cannot be secured simply by forming a microstructure mainly
composed of these.
[0014]
(c) In order to make a high-strength hot rolled steel sheet also having excellent
ductility, it is effective to add an appropriate amount of highly ductile ferrite.
[0015]
(d) Since ferrite is generally soft, it is necessary to utilize Ti, Nb, V, or the like
as a precipitation hardening element in order to obtain a desired strength. Therefore, it
is effective to perform intermediate air cooling in the hot rolling process to obtain an
appropriate amount of precipitation-hardened ferrite.
[0016]
(e) In order to suppress the generation of a secondary sheared surface, it is
important to form a fractured surface after a sheared surface is sufficiently formed. In
order for that, it is necessary to suppress the early occurrence of cracking from the
cutting edge of the tool during shearing working. In order for that, it is important that
Mn segregation does not occur much, the microstructural morphology is not periodic,
but random, and the microstructure is highly uniform.
[0017]
(f) Specifically, it is effective to control the standard deviation of the Mn
concentration to a certain value or less and to control the periodicity and uniformity of
the microstructure for suppressing the secondary sheared surface.
- 5 -
[0018]
(g) 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
1100°C.
[0019]
(h) In order to reduce the periodicity of the microstructural morphology, 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 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 800°C to less than 200
kPa. Such hot rolling conditions make it possible to produce fine and equiaxed
recrystallized austenite grains and make it possible to reduce the periodicity of the
microstructural morphology in combination with conditions for subsequent cooling.
[0020]
(i) In order to enhance the uniformity of the microstructure, it is effective to
suppress the precipitation of an iron carbide by cooling the steel sheet to 350°C or lower
after a desired amount of ferrite is obtained by performing intermediate air cooling.
[0021]
The gist of the present invention made based on the above findings is as
- 6 -
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%,
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,
- 7 -
in terms of area%, residual austenite is less than 3.0%, ferrite is 15.0% or more
and less than 60.0%, and pearlite is less than 5.0%,
an E value that indicates periodicity of the microstructure is 10.7 or more, and
an I value that indicates uniformity of the microstructure is 1.020 or more,
a standard deviation of a Mn concentration is 0.60 mass% or les s, and
a tensile strength is 980 MPa or more.
(2) The hot-rolled steel sheet according to (1), in which an average crystal
grain size of a surface layer may be less than 3.0 11m.
(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%,
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]
[0022]
According to the above aspect according to the present invention, it is possible
to obtain a hot-rolled steel sheet having excellent strength, ductility, and shearing
property. In addition, according to the preferable aspect according to the present
- 8 -
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]
[0023]
FIG. 1 is an example of a sheared end surface of a hot-rolled steel sheet
according to a present invention example.
FIG. 2 is an example of a sheared end surface of a hot-rolled steel sheet
according to a comparative example.
[Embodiments ofthe Invention]
[0024]
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.
- 9 -
[0025]
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% orless, and a
remainder consisting of Fe and impurities. Each element will be described in detail
below.
[0026]
(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. Therefore, the 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 ductility of the
hot-rolled steel sheet deteriorates due to a decrease in the fraction of ferrite. Therefore,
the C content is set to 0.250% or less. The C content is preferably 0.150% or less.
[0027]
(1-2) Si: 0.05% to 3.00%
Si has an action of improving the ductility of the hot -rolled steel sheet by
promoting the formation of ferrite and has an action of increasing the strength of the
hot-rolled steel sheet by the solid solution strengthening of ferrite. 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
- 10 -
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,
chemical convertibility, furthermore, ductility, and weldability of the steel sheet
significantly deteriorate, and the A3 transformation point significantly increases.
Therefore, it becomes difficult to perform hot rolling in a stable manner. Therefore,
the Si content is set to 3.00% or less. The Si content is preferably 2.70% or less and
more preferably 2.50% or less.
[0028]
(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 980 MPa or more cannot be obtained. 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%, due to the
segregation of Mn, the form of the hard phase becomes a periodic band shape, and it
becomes difficult to obtain a desired shearing property. 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.
[0029]
(1-4) One or two or more ofTi, 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. When the total
amount of Ti, Nb, and Vis less than 0.060%, these effects cannot be obtained.
- 11 -
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, and the total
amount thereof may be 0.060% or more. The total amount of Ti, Nb, and Vis
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 deteriorates. Therefore, the total amount of Ti, Nb, and V is set to 0.500%
or less. The total amount ofTi, Nb, and Vis preferably 0.300% or less, more
preferably 0.250% or less, and still more preferably 0.200% or less.
[0030]
(1-5) sol. Al: 0.001% to 2.000%
Similar to Si, Al has an action of making steel sound by deoxidizing the steel
and has an action of enhancing the ductility of the hot-rolled steel sheet by promoting
the formation of ferrite. 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.
[0031]
(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.
- 12 -
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 limited 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.
[0032]
(1-7) S: 0.0300% or less
S is an element that is contained as an impurity and forms a sulfide-based
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 limited 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.
[0033]
(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, the N content is preferably set to 0.0010% or more and more
- 13 -
preferably set to 0.0020% or more to promote the precipitation of a carbonitride.
[0034]
(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.
[0035]
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.
[0036]
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.
[0037]
(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%
- 14 -
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.
[0038]
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
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.
[0039]
As described above, Cr has an action of enhancing the hardenability of the hotrolled
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.
[0040]
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
- 15 -
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.
[0041]
As described above, Ni has an action of enhancing the hardenability of the hotrolled
steel sheet. In addition, in a case where Cu is contained, Ni has an action 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.
[0042]
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 hotrolled
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.
[0043]
(1-ll) 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
- 16 -
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 any one or more of Ca, Mg, REM, and Bi are set to 0.0005% or more.
However, when the Ca 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 preferable. Therefore, theCa content
and the Mg content are 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
[0044]
(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 hot-rolled
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
- 17 -
contained, a defect may be generated during hot rolling, and thus the Sn content is set to
0.05% or less.
[0045]
The chemical composition of the above hot-rolled steel sheet may be measured
by a general analytical method. For example, inductively coupled plasma-atomic
emission spectrometry (ICP-AES) may be used for measurement. soL Al may be
measured by the ICP-AES using a filtrate after a sample is decomposed with an acid by
heating. C and S may be measured by using a combustion-infrared absorption 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.
[0046]
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 15.0% or
more and less than 60.0%, and pearlite is less than 5.0%, theE value that indicates the
periodicity of the microstructure is 10.7 or more, the I value that indicates the
uniformity of the microstructure is 1.020 or more, 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 ductility and shearing
property. 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
depth of 1/4 of the sheet thickness from the surface and the center position in the sheet
width direction in a cross section parallel to the rolling direction are specified. The
- 18 -
reason therefor is that the microstructure at this position indicates a typical
microstructure of the steel sheet.
[0047]
(2-1) Area Fraction of Residual Austenite: Less than 3.0%
Residual austenite is a microstructure that is present as a face-centered cubic
lattice even at room temperature. Residual austenite has an action of enhancing the
ductility of the hot-rolled steel sheet by transformation-induced plasticity (TRIP). On
the other hand, residual austenite transforms into high-carbon martensite during
shearing working, which inhibits the stable occurrence of cracking and causes the
formation of a secondary sheared surface. When the area fraction of the residual
austenite is 3.0% or more, the action is actualized, and the shearing property of the hotrolled
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%.
[0048]
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, first, 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 in the cross
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section parallel to the rolling direction at a depth of 114 of the sheet thickness (a region
between a depth of 1/8 of the sheet thickness from the surface to a depth of 3/8 of the
sheet thickness from the surface) and the center position in the sheet width direction 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.
[0049]
(2-2) Area Fraction of Ferrite: 15.0% or more and less than 60.0%
Ferrite is a structure formed when fcc transforms into bee at a relatively high
temperature. Ferrite has a high work hardening rate and thus has an action of
enhancing the strength- ductility balance of the hot -rolled steel sheet. In order to
obtain the above action, the area fraction of the ferrite is set to 15.0% or more. The
area fraction of the ferrite is preferably 20.0% or more, more preferably 25.0% or more,
and still more preferably 30.0% or more.
Since ferrite has a low strength, when the area fraction is excessive, a desired
tensile strength cannot be obtained. Therefore, the area fraction of the ferrite is set to
less than 60.0%. The area fraction of the ferrite is preferably 50.0% or less and more
preferably 45.0% or less.
[0050]
(2-3) Area Fraction of Pearlite: Less than 5.0%
Pearlite is a lamellar microstructure in which cementite is precipitated in layers
between ferrite and is a soft microstructure as 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 980 MPa or more cannot
be obtained. Therefore, the area fraction of the pearlite is set to less than 5.0%. The
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area fraction of the pearlite is preferably 3.0% or less. In order to improve the stretch
flangeability of the steel sheet, the area fraction of the pearlite is preferably reduced as
much as possible, and the area fraction of the pearlite is more preferably 0%.

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% or less;
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, in a microstructure,
in terms of area%, residual austenite is less than 3.0%, ferrite is 15.0% or more
and less than 60.0%, and pearlite is less than 5.0%,
an E value that indicates periodicity of the microstructure is 10.7 or more, and
an I value that indicates uniformity of the microstructure is 1.020 or more,
a standard deviation of a Mn concentration is 0.60 mass% or less, and
a tensile strength is 980 MPa or more.
2. The hot-rolled steel sheet according to claim 1,
wherein an average crystal grain size of a surface layer is less than 3.0 11m.
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%.