US 11,634,802 B2Granted

Cold-rolled steel sheet

AssigneeNippon Steel Corporation

InventorsHiroshi Kaido, Mai Nagano, Koutarou Hayashi

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Abstract

The cold-rolled steel sheet having a high bake hardening amount and excellent bendability after bake hardening according to the present invention has a predetermined composition, and contains 20% or more and 70% or less of ferrite and 30% or more of tempered martensite in terms of area ratio, in which a sum of ferrite and tempered martensite is 90% or more, and in a case where a microstructure image of 30 μm×30 μm obtained by photographing a structure at a magnification of 2,000-fold is disposed in an xy coordinate system having a sheet thickness direction as an x-axis and a rolling direction as a y-axis, the microstructure image is divided into 1024 pieces in an x-axis direction and 1024 pieces in a y-axis direction to form 1024×1024 divided regions, and a two-dimensional image is created by performing double gradation by assuming a value of “1” in each of the divided regions in one case where the structure is ferrite and assuming a value of “0” in the other cases, a heterogeneity α when two-dimensional discrete Fourier transform is performed on the two-dimensional image is 1.20 or less.

A cold-rolled steel sheet comprising, by mass %:

C:
0.05% to 0.30%;
Si:
0.200% to 2.000%;
Mn:
2.00% to 4.00%;
P:
0.100% or less;
S:
0.010% or less;
Al:
0.001% to 2.000%;
N:
0.010% or less;
Ti:
0% to 0.100%;
Nb:
0% to 0.100%;
V:
0% to 0.100%;
Cu:
0% to 1.000%;
Ni:
0% to 1.000%;
Mo:
0% to 1.000%;
Cr:
0% to 1.000%;
W:
0% to 0.005%;
Ca:
0% to 0.005%;
Mg:
0% to 0.005%;
REM:
0% to 0.010%;
B:
0% to 0.0030%;
and
a remainder comprising Fe and impurities,
wherein the cold-rolled steel sheet contains 20% or more and 70% or less of ferrite and 30% or more of tempered martensite in terms of area ratio,
a sum of ferrite and tempered martensite is 90% or more, and in a case where in a sheet thickness cross section perpendicular to a sheet width direction of the steel sheet at a position from 1/8 to 7/8 of a sheet width of the cold-rolled steel sheet, a microstructure image of 30 μm×30 μm obtained by photographing a structure at a position from 1/4 to 3/8 of a sheet thickness from a surface of the cold-rolled steel sheet at a magnification of 2,000-fold is disposed in an xy coordinate system having a sheet thickness direction as an x-axis and a rolling direction as a y-axis, the microstructure image is divided into 1024 pieces in an x-axis direction and 1024 pieces in a y-axis direction to form 1024×1024 divided regions, and a two-dimensional image is created by performing double gradation by assuming a value of “1” in each of the divided regions in one case where the structure is ferrite and assuming a value of “0” in the other cases, a heterogeneity a defined by Formula (1) for the two-dimensional image is 1.20 or less,
[
Formula
⁢
⁢
1
]
α
=
S
u
S
v
(
1
)
in Formula (1), Su is defined by Formula (2), and Sv is defined by Formula (3),
[
Formula
⁢
⁢
2
]
S
u
=
∑
u
=
1
1023
⁢

F
⁡
(
u
,
0
)

(
2
)
S
v
=
∑
u
=
1
1023
⁢

F
⁡
(
0
,
v
)

(
3
)
in Formula (2) and Formula (3), F(u,v) is defined by Formula (4), and
[
Formula
⁢
⁢
3
]
⁢
F
⁡
(
u
,
v
)
=
∑
1023
x
=
0
⁢
∑
1023
y
=
0
⁢
f
⁡
(
x
,
y
)
⁢
e
-
2
⁢
π
⁢
⁢
i
⁡
(
ux
1
⁢
0
⁢
2
⁢
4
+
v
⁢
y
1
⁢
0
⁢
2
⁢
4
)
(
4
)
in Formula (4), f(x,y) represents a gradation of coordinates (x,y) of the two-dimensional image.

173 more elements

Abstract

Claims2