[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. 2020-143743, 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. Since there are various working
methods for vehicle member s, the required formability differs depending on members
- 1 -
to which the working 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 press-formed 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 the proportion of a sheared surface in an end surface after shearing working
(sheared end surface) (hereinafter, the sheared surface proportion) is not stable (the
amount of change in the sheared surface proportion is large), 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 )lm or less is dispersed in ferrite having an average crystal grain
size of 10 )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.
- 2 -
[0006]
Regarding the technique for improving shearing property, for example, Patent
Document 3 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 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. Hl0-168544
[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
- 3 -
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]
[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
hot-rolled steel sheet and a relationship between a microstructure and mechanical
properties, and completed the present invention. Having excellent shearing property
indicates that the sheared surface proportion in a sheared end surface is stable, that is,
the amount of change in the sheared surface proportion is small. In addition, the
expression of having excellent strength or having high strength indicates that the
- 4 -
tensile strength is 980 MPa or more.
[0012]
(a) In order to obtain an excellent tensile (maximum) strength, it is preferable
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.

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,
- 54 -
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 less than 1.020,
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%.