	US 12,297,515 B2
	Steel sheet, member, and method for producing them
Yoshiyasu Kawasaki, Tokyo (JP); Lingling Yang, Tokyo (JP); Shotaro Terashima, Tokyo (JP); Tatsuya Nakagaito, Tokyo (JP); Shunsuke Yamamoto, Tokyo (JP); Katsuya Hoshino, Tokyo (JP); and Yuki Takeda, Tokyo (JP)
Assigned to JFE STEEL CORPORATION, Tokyo (JP)
Appl. No. 17/915,020
Filed by JFE STEEL CORPORATION, Tokyo (JP)
PCT Filed Mar. 25, 2021, PCT No. PCT/JP2021/012662 § 371(c)(1), (2) Date Sep. 27, 2022, PCT Pub. No. WO2021/200580, PCT Pub. Date Oct. 7, 2021.
Claims priority of application No. JP2020-062804 (JP), filed on Mar. 31, 2020; and application No. JP2020-122184 (JP), filed on Jul. 16, 2020.
Prior Publication US 2023/0151451 A1, May 18, 2023
Int. Cl. C21D 8/02 (2006.01); B32B 15/01 (2006.01); C21D 6/00 (2006.01); C21D 9/46 (2006.01); C22C 38/00 (2006.01); C22C 38/02 (2006.01); C22C 38/04 (2006.01); C22C 38/06 (2006.01); C22C 38/08 (2006.01); C22C 38/10 (2006.01); C22C 38/12 (2006.01); C22C 38/14 (2006.01); C22C 38/16 (2006.01); C22C 38/38 (2006.01); C22C 38/60 (2006.01); C23C 2/28 (2006.01)

CPC C21D 9/46 (2013.01) [B32B 15/013 (2013.01); C21D 6/001 (2013.01); C21D 6/002 (2013.01); C21D 6/005 (2013.01); C21D 6/007 (2013.01); C21D 6/008 (2013.01); C21D 8/0205 (2013.01); C21D 8/0226 (2013.01); C21D 8/0236 (2013.01); C21D 8/0263 (2013.01); C22C 38/001 (2013.01); C22C 38/002 (2013.01); C22C 38/005 (2013.01); C22C 38/008 (2013.01); C22C 38/02 (2013.01); C22C 38/04 (2013.01); C22C 38/06 (2013.01); C22C 38/08 (2013.01); C22C 38/10 (2013.01); C22C 38/12 (2013.01); C22C 38/14 (2013.01); C22C 38/16 (2013.01); C22C 38/38 (2013.01); C22C 38/60 (2013.01); C23C 2/28 (2013.01); C23C 2/29 (2022.08); C21D 2211/001 (2013.01); C21D 2211/005 (2013.01)]

24 Claims

1. A steel sheet having a chemical composition comprising, by mass %:

Si: 0.20% to 2.00%;

Mn: 2.70% to 4.00%;

C: 0.120% to 0.400%;

P: 0.001% to 0.100%;

S: 0.0200% or less;

Al: 0.010% to 2.000%;

N: 0.0100% or less;

optionally at least one selected from the group consisting of:

Sb: 0.200% or less, and

Sn: 0.200% or less

optionally at least one selected from the group consisting of:

Ti: 0.200% or less,

Nb: 0.200% or less,

V: 0.100% or less,

B: 0.0100% or less,

Cu: 1.000% or less,

Cr: 1.000% or less,

Ni: 1.000% or less,

Mo: 0.500% or less,

Ta: 0.100% or less,

W: 0.500% or less,

Mg: 0.0200% or less,

Zn: 0.020% or less,

Co: 0.020% or less,

Zr: 0.020% or less,

Ca: 0.0200% or less,

Ce: 0.0200% or less,

Se: 0.0200% or less,

Te: 0.0200% or less,

Ge: 0.0200% or less,

As: 0.0200% 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 (except Ce): 0.0200% or less;

optionally an equivalent carbon content Ceq of 0.697% or more;

the remainder being Fe and incidental impurities,

wherein:

the steel has a microstructure including a ferrite area fraction in a range of 5% to 30%, a bainitic ferrite area fraction in a range of 5% to 35%, a tempered martensite area fraction in a range of 20% to 50%, and a retained austenite volume fraction in a range of 5% to 35%,

in a region within 4.9 μm in a thickness direction from a surface of the steel sheet, a region with a Si concentration not more than one-third of the Si concentration in the chemical composition of the steel sheet and with a Mn concentration not more than one-third of the Mn concentration in the chemical composition of the steel sheet has a thickness of 1.0 μm or more,

the lowest Si concentration L_Siand the lowest Mn concentration L_Mnin the region within 4.9 μm in the thickness direction from the surface of the steel sheet and a Si concentration T_Siand a Mn concentration T_Mnat a quarter thickness position of the steel sheet satisfy the following formula (1):

L_Si+L_Mn≤(T_Si+T_Mn)/4 (1),

the steel sheet has a tensile strength of 1180 MPa or more, and

optionally, an amount of diffusible hydrogen in the steel sheet is 0.50 ppm or less by mass.