Steel sheet, member, and method for producing them
AssigneeJfe Steel Corporation
InventorsYoshiyasu Kawasaki, Tatsuya Nakagaito, Shunsuke Yamamoto, Katsuya Hoshino
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Abstract
A steel sheet and a member with a TS of 590 MPa or more and a method for producing them are disclosed. A base steel sheet has a specified chemical composition. When nanohardness is measured at 300 points or more in a 50 μm×50 μm region on a sheet surface at a quarter depth position, the ratio of a nanohardness of 7.0 GPa or more is 0.10 or less, the nanohardness of the sheet surface at the quarter depth position has a standard deviation σ of 1.8 GPa or less, and the nanohardness of the sheet surface at a half depth position has a standard deviation σ of 2.2 GPa or less.
Claims16
A steel sheet comprising:
- a base steel sheet with a chemical composition containing, on a mass percent basis,
- C:
- 0.030% or more and 0.500% or less,
- Si:
- 0.01% or more and 3.00% or less,
- Mn:
- 0.30% or more and less than 10.00%,
- P:
- 0.001% or more and 0.100% or less,
- S:
- 0.0200% or less,
- Al:
- 0.005% or more and 2.000% or less, and
- N:
- 0.0100% or less,
- wherein Ceq represented by the following formula (1) satisfies 0.30% or more and 0.85% or less, the remainder being Fe and incidental impurities,
- a surface soft layer with a Vickers hardness of 85% or less with respect to a Vickers hardness at a quarter thickness position of the base steel sheet is formed in a region of 200 μm or less from a surface of the base steel sheet in a thickness direction,
- when nanohardness is measured at 300 points or more in a 50 μm×50 μm region on a sheet surface at a quarter depth position in the thickness direction and at a half depth position in the thickness direction of the surface soft layer from the surface of the base steel sheet,
- a ratio of a number of measurements with a nanohardness of 7.0 GPa or more on the sheet surface at the quarter depth position in the thickness direction of the surface soft layer from the surface of the base steel sheet to a total number of measurements is 0.10 or less,
- the nanohardness of the sheet surface at the quarter depth position in the thickness direction of the surface soft layer from the surface of the base steel sheet has a standard deviation σ of 1.8 GPa or less, and
- the nanohardness of the sheet surface at the half depth position in the thickness direction of the surface soft layer from the surface of the base steel sheet has a standard deviation σ of 2.2 GPa or less,
- Ceq
- =
- [
- %
-
- C
- ]
- +
- [
- %
-
- Mn
- ]
- /
- 6
- +
- [
- %
-
- Si
- ]
- /
- 24
- +
- [
- %
-
- Ni
- ]
- /
- 40
- +
- [
- %
-
- Cr
- ]
- /
- 5
- +
- [
- %
-
- Mo
- ]
- /
- 4
- +
- [
- %
-
- V
- ]
- /
- 14
- formula
-
- (
- 1
- )
- wherein [% C] denotes a C content, [% Mn] denotes a Mn content, [% Si] denotes a Si content, [% Ni] denotes a Ni content, [% Cr] denotes a Cr content, [% Mo] denotes a Mo content, and [% V] denotes a V content, and these are 0 (zero) when not contained.
Dependencies are inferred from the claim text.