Steel sheet, member, and method for producing them

Patent metadata

Field	Value
Publication number	US-12590350-B2
Application number	US-202218847395-A
Country	US
Kind code	B2
Filing date	Nov 30, 2022
Priority date	Mar 31, 2022
Publication date	Mar 31, 2026
Grant date	Mar 31, 2026

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.

First claim

The invention claimed is:

1 . 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.

2 . The steel sheet according to claim 1 , wherein the chemical composition further contains, on a mass percent basis, at least one element selected from
Nb: 0.200% or less, Ti: 0.200% or less, V: 0.200% or less, B: 0.0100% or less, Cr: 1.000% or less, Ni: 1.000% or less, Mo: 1.000% or less, Sb: 0.200% or less, Sn: 0.200% or less, Cu: 1.000% or less, Ta: 0.100% or less, W: 0.500% or less, Mg: 0.0200% or less, Zn: 0.0200% or less, Co: 0.0200% or less, Zr: 0.1000% or less, Ca: 0.0200% or less, Se: 0.0200% or less, Te: 0.0200% or less, Ge: 0.0200% or less, As: 0.0500% or less, Sr: 0.0200% or less, Cs: 0.0200% or less, Hf: 0.0200% or less, Pb: 0.0200% or less, Bi: 0.0200% or less, and REM: 0.0200% or less.

3 . The steel sheet according to claim 1 , comprising one or two or more selected from the following (1) to (4):
(1) wherein ferrite and bainite present in the surface soft layer have a total area fraction of 20% or more, and
an area fraction of martensite present in the surface soft layer divided by a total area fraction of martensite and tempered martensite present in the surface soft layer is 0.45 or less,
(2) comprising a metal coated layer as a first coated layer on one or both surfaces of the base steel sheet, (3) comprising a metal coated layer as a second coated layer on an outermost surface layer of one or both surfaces of the steel sheet, and (4) wherein a test specimen after 90-degree V-bending in which R/t obtained by dividing a critical bending radius R by a sheet thickness t is 4.5 or more and 5.0 or less has a surface roughness depth of 20.0 μm or less.

4 . The steel sheet according to claim 2 , comprising one or two or more selected from the following (1) to (4):
(1) wherein ferrite and bainite present in the surface soft layer have a total area fraction of 20% or more, and
an area fraction of martensite present in the surface soft layer divided by a total area fraction of martensite and tempered martensite present in the surface soft layer is 0.45 or less,
(2) comprising a metal coated layer as a first coated layer on one or both surfaces of the base steel sheet, (3) comprising a metal coated layer as a second coated layer on an outermost surface layer of one or both surfaces of the steel sheet, and (4) wherein a test specimen after 90-degree V-bending in which R/t obtained by dividing a critical be

Assignees

• Jfe Steel Corp

Inventors

• Kawasaki Yoshiyasu
• Nakagaito Tatsuya
• Yamamoto Shunsuke
• Hoshino Katsuya

Classifications

• C21D8/02

  during manufacturing of plates or strips (C21D8/12 takes precedence) · CPC title

• C22C38/60

  containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur · CPC title

• C22C38/58

  with more than 1.5% by weight of manganese · CPC title

• C22C38/54

  with boron · CPC title

• C22C38/52

  with cobalt · CPC title