[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 hole expansibility and shearing property.
Priority is claimed on Japanese Patent Application No. 2020-143745, 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, hole expansibility 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.
[0004]
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 transformationinduced
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
elongation and hole expansibility 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]
Patent Documents 3 and 4 disclose a high-tension hot -rolled steel sheet
having excellent ductility and hole expansibility and a method for manufacturing the
same. Patent Document 3 discloses a method for manufacturing a high-strength hotrolled
steel sheet having favorable ductility and stretch flangeability, in which a steel
sheet is cooled to a temperature range of 720ac or lower within 1 second after the
- 2 -
finishing of hot rolling, caused to stay in a temperature range of higher than 5ooac to
720°C or lower for a stay time of 1 to 20 seconds, and then coiled in a temperature
range of 35oac to 500°. In addition, Patent Document 4 discloses a high-strength
hot-rolled steel sheet having favorable ductility and stretch flangeability, in which
mainly bainite and an appropriate amount of polygonal ferrite and residual austenite
are contained, and the average grain size of grains that are surrounded by grain
boundaries having a crystal orientation difference of 15° or more is 15 11m or less in a
steel structure excluding the residual austenite.
[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. 2012-251200
[Patent Document 4] Japanese Unexamined Patent Application, First
Publication No. 2015-124410
[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
- 3 -
[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
[Disclosure of the Invention]
[Problems to be Solved by the Invention]
[0009]
Since there are various working methods for vehicle members, the required
formability differs depending on members to which the working methods are applied,
but among these, hole expansibility is placed as important indices for formability. In
addition, vehicle components are formed by press forming, but the press-formed blank
sheet is often manufactured by highly productive shearing working and needs to be
excellent in terms of end surface accuracy after shearing working. Particularly for a
high-strength steel sheet of 980 MPa or more, the load required for a post treatment
such as coining after shearing working is large, and thus it is desired to control the
unevenness of a fractured surface on an end surface after shearing working with
particularly high accuracy.
[0010]
The techniques disclosed in Patent Documents 1 to 4 are all techniques for
improving strength and press formability during hole expansion, but no techniques for
improving shearing property are mentioned, and a post treatment becomes necessary in
a stage of press-forming a component, which makes the production cost presumed to
increase.
[0011]
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
- 4 -
having high strength and excellent hole expansibility and shearing property.
[Means for Solving the Problem]
[0012]
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.
In addition, having excellent shearing property indicates that the linearity of
the boundary between a fractured surface and a sheared surface on an end surface after
shearing working is high. This is because, when the linearity of the boundary
between a fractured surface and a sheared surface on an end surface after shearing
working is high, the end surface accuracy after shearing working can be regarded as
excellent. In addition, the expression of having excellent strength or having high
strength indicates that the tensile strength is 980 MPa or more.
[0013]
(a) In order to obtain excellent tensile (maximum) strength and hole
expansibility, a primary phase structure of the microstructure is preferably full hard.
That is, it is preferable that the fraction of a soft structure such as ferrite or bainite is as
small as possible.

What is claimed is:
CLAIMS
1. A hot-rolled steel sheet comprising, in terms of mass%, as a chemical
composition:
C: 0.040% to 0.250%;
Si: 0.05% to 3.00%;
Mn: 1.00% to 4.00%;
sol. Al: 0.001% to 0.500%;
P: 0.100% or less;
S: 0.0300% or less;
N: 0.1000% or less;
0: 0.0100% orless;
Ti: 0% to 0.300%;
Nb: 0% to 0.300%;
V: 0% to 0.500%;
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;
- 57 -
total,
Sn: 0% to 0.05%; and
a remainder consisting of Fe and impurities,
wherein, a microstructure has,
in terms of area%,
martensite and tempered martensite at more than 92.0% and 100.0% or less in
residual austenite at less than 3.0%, and
ferrite at less than 5.0%,
an E value that indicates periodicity of the microstructure is 11.0 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 les s, 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:
Ti: 0.005% to 0.300%;
Nb: 0.005% to 0.100%;
V: 0.005% to 0.500%;
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%;
- 58 -
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%.