The present invention relates to a hot-rolled steel sheet.
Priority is claimed on Japanese Patent Application No. 2022-208520, filed on
December 26, 2022, 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. In order to meet this demand, several techniques
have been conventionally proposed. Since there are various working methods for
vehicle members, the required formability differs depending on members to which the
working methods are applied. Among these, limit fracture sheet thickness reduction
ratio and hole expansibility is placed as important indices for formability.
- 1 -
[0004]
The limit fracture sheet thickness reduction ratio is a value calculated from a
sheet thickness of a tensile test piece before fracture and a minimum value of a sheet
thickness of the tensile test piece after fracture. When the limit fracture sheet
thickness reduction ratio is high, it is preferable since early fracture become to be hardly
occurred when tensile strain is applied during press forming.
[0005]
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.
[0006]
For example, when a linearity of a boundary between a fractured surface and a
sheared surface at the sheared end surface is low, the accuracy of the sheared end
surface significantly deteriorates.
[0007]
Furthermore, for steel sheets applied to vehicle members, further improved
corrosion resistance is required.
[0008]
For example, Patent Document 1 discloses a hot-rolled steel sheet that can be
used as a raw material for cold-rolled steel sheet with excellent surface properties after
press forming, in which the degrees of Mn segregation and P segregation in the center
part of sheet thickness are controlled.
PRIOR ART DOCUMENT
Patent Document
- 2 -
[0009]
Patent Document 1: W02020/044445
Non-Patent Document
[0010]
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
SUMMARY OF INVENTION
Technical Problem
[0011]
However, in Patent Document 1, a limit fracture sheet thickness reduction
ratio, shearing property, and corrosion resistance of the hot-rolled steel sheet are not
considered.
[0012]
The present invention has been made in view of the circumstances described
above, and an object of the present invention is to provide a hot-rolled steel sheet
having high strength and limit fracture sheet thickness reduction ratio, excellent hole
expansibility, shearing property and corrosion resistance.
Solution to Problem
[0013]
The gist of the present invention is as follows.
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( 1) A hot-rolled steel sheet according to one aspect of the present invention
comprising, in terms of mass%, as a chemical composition,
C: 0.040% to 0.250%,
Si: 0.40% to 1.00%,
Mn: 1.00% to 4.00%,
sol. Al: 0.100% to 0.500%,
Cr: 0.50% to 2.00%,
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 0.100%,
V: 0% to 0.500%,
Cu: 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 0.0200%,
As: 0% to 0.100%,
Zr: 0% to 1.00%,
Co: 0% to 1.00%,
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Zn: 0% to 1.00%,
W: 0% to 1.00%,
Sn: 0% to 0.05%,
a remainder comprising Fe and impurities, and
the following formula (A) is satisfied,
in a microstructure at a position of 114 from a surface in a sheet thickness
direction,
total,
in terms of area%,
martensite and tempered martensite is more than 92.0% and 100.0% or less in
residual austenite is less than 3.0%,
ferrite is less than 5.0%, and
an Entropy value indicated by the following formula (1) is 11.0 or more, an
Inverse difference normalized value indicated by the following formula (2) is less than
1.020, and a Cluster Shade value indicated by the following formula (3) is -8.0x105 to
8.0xl05, which are obtained by analyzing SEM images of the microstructure with a gray
level co-occurrence matrices method,
a standard deviation of Mn concentrations is 0.60 mass% or less,
at the surface, a number density of Si-Al-Cr oxides having a circle equivalent
radius of 0.500 )lm or more is 2.0x1 03 pieces/cm2 or less, and a number density of SiAl-
Cr oxides having a circle equivalent radius of 0.005 )lm to 0.050 )lm is l.Ox1 05
pieces/cm2 or more,
Zr+Co+Zn+W:::; 1.00% ... (A),
here, each element symbol in the formula (A) indicates the content of the
element in terms of mass %, and 0 is substituted when the element is not contained,
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P(i,j) in the following formulas (1) to (5) is a gray level co-occurrence matrix,
Lin the following formula (2) is possible Quantization levels of grayscale of the SEM
images, i and j in the following formulas (2) and (3) are natural numbers from 1 to the
L, Jlx and Jly in the following formula (3) are indicated by the in the following formulas
(4) and (5).
[Formula 1]
• • • ( 1 )
[Formula 2]
I I P(i,J)
Inverse difference normalized = . .
1
. .
1
I J z- J
1 +-'--'L
• • • ( 2)
[Formula 3]
• • • ( 3 )
[Formula 4]
• • • ( 4 )
[Formula 5]
• • • ( 5 )
(2) The hot-rolled steel sheet according to (1 ), in which the chemical
composition may comprise, in terms of mass%, one or two or more selected from the
group consisting of
Ti: 0.001% to 0.300%,
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Nb: 0.001% to 0.100%,
V: 0.001% to 0.500%,
Cu: 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%,
Bi: 0.0005% to 0.0200%,
As: 0.001% to 0.100%,
Zr: 0.01% to 1.00%,
Co: 0.01% to 1.00%,
Zn: 0.01% to 1.00%,
W: 0.01% to 1.00%, and
Sn: 0.01% to 0.05%.
Advantageous Effects of Invention
[0014]
According to the above aspect according to the present invention, it is possible
to obtain a hot-rolled steel sheet having high strength and limit fracture sheet thickness
reduction ratio, excellent hole expansibility, shearing property, and corrosion 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.
A hot-rolled steel sheet comprising, in terms of mass%, as a chemical
composition:
C: 0.040% to 0.250%,
Si: 0.40% to 1.00%,
Mn: 1.00% to 4.00%,
sol. Al: 0.100% to 0.500%,
Cr: 0.50% to 2.00%,
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 0.100%,
V: 0% to 0.500%,
Cu: 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 0.0200%;
As: 0% to 0.100%;
Zr: 0% to 1.00%;
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Co: 0% to 1.00%;
Zn: 0% to 1.00%;
W: 0% to 1.00%;
Sn: 0% to 0.05%;
a remainder comprising Fe and impurities, and
the following formula (A) is satisfied,
wherein, in a microstructure at a position of 114 from a surface in a sheet
thickness direction,
total,
in terms of area%,
martensite and tempered martensite is more than 92.0% and 100.0% or less in
residual austenite is less than 3.0%,
ferrite is less than 5.0%, and
an Entropy value indicated by the following formula (1) is 11.0 or more, an
Inverse difference normalized value indicated by the following formula (2) is less than
1.020, and a Cluster Shade value indicated by the following formula (3) is -8.0xl05 to
8.0xl05
, which are obtained by analyzing SEM images of the microstructure with a gray
level co-occurrence matrices method,
a standard deviation of Mn concentrations is 0.60 mass% or less,
at the surface, a number density of Si-Al-Cr oxides having a circle equivalent
radius of 0.500 J.lm or more is 2.0xl 03 pieces/cm2 or less, and a number density of SiAl-
Cr oxides having a circle equivalent radius of 0.005 J.lm to 0.050 J.lm is l.Oxl05
pieces/cm2 or more,
Zr+Co+Zn+W::::; 1.00% . .. (A),
here, each element symbol in the formula (A) indicates the content of the
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element in terms of mass %, and 0 is substituted when the element is not contained,
P(i,j) in the following formulas (1) to (5) is a gray level co-occurrence matrix,
Lin the following formula (2) is possible Quantization levels of grayscale of the SEM
images, i and j in the following formulas (2) and (3) are natural numbers from 1 to the
L, J.lx and J.ly in the following formula (3) are indicated by the in the following formulas
(4) and (5).
[Formula 1]
• • • ( 1 )
[Formula 2]
P(i,J)
Inverse difference normalized = "" . "" .
1
. .
1
LJILJ) l-j
1 + .!.--.!..
• • • ( 2 )
L
[Formula 3]
• • • ( 3)
[Formula 4]
• • • ( 4 )
[Formula 5]
• • • ( 5 )
[Claim 2] The hot-rolled steel sheet according to claim 1, wherein the chemical
composition comprises, in terms of mass%, one or two or more selected from the group