	US 12,331,378 B2
	Steel sheet and method for producing same
Takahiko Kohtake, Tokyo (JP); Koutarou Hayashi, Tokyo (JP); and Yoshihiro Suwa, Tokyo (JP)
Assigned to NIPPON STEEL CORPORATION, Tokyo (JP)
Appl. No. 17/911,974
Filed by NIPPON STEEL CORPORATION, Tokyo (JP)
PCT Filed Mar. 17, 2021, PCT No. PCT/JP2021/010853 § 371(c)(1), (2) Date Sep. 15, 2022, PCT Pub. No. WO2021/200164, PCT Pub. Date Oct. 7, 2021.
Claims priority of application No. 2020-067685 (JP), filed on Apr. 3, 2020.
Prior Publication US 2023/0021370 A1, Jan. 26, 2023
Int. Cl. C21D 9/46 (2006.01); C21D 8/02 (2006.01); C22C 38/00 (2006.01); C22C 38/02 (2006.01); C22C 38/04 (2006.01); C22C 38/06 (2006.01); C23C 2/06 (2006.01); C23C 2/40 (2006.01)

CPC C21D 9/46 (2013.01) [C21D 8/0226 (2013.01); C21D 8/0236 (2013.01); C21D 8/0273 (2013.01); C22C 38/001 (2013.01); C22C 38/002 (2013.01); C22C 38/02 (2013.01); C22C 38/04 (2013.01); C22C 38/06 (2013.01); C23C 2/06 (2013.01); C23C 2/40 (2013.01); C21D 2211/001 (2013.01); C21D 2211/002 (2013.01); C21D 2211/005 (2013.01); C21D 2211/008 (2013.01)]

17 Claims

1. A steel sheet, having a chemical composition consisting of, in mass %,

C: more than 0.18% and less than 0.30%,

Si: 0.01% or more and less than 2.00%,

Mn: more than 2.50% and 4.00% or less,

sol. Al: 0.001% or more and less than 3.00%,

P: 0.100% or less,

S: 0.010% or less,

N: less than 0.050%,

O: less than 0.020%,

Cr: 0% or more and less than 2.00%,

Mo: 0 to 2.00%,

W: 0 to 2.00%,

Cu: 0 to 2.00%,

Ni: 0 to 2.00%,

Ti: 0 to 0.300%,

Nb: 0 to 0.300%,

V: 0 to 0.300%,

B: 0 to 0.010%,

Ca: 0 to 0.010%,

Mg: 0 to 0.010%,

Zr: 0 to 0.010%,

REM: 0 to 0.010%,

Sb: 0 to 0.050%,

Sn: 0 to 0.050%,

Bi: 0 to 0.050%, and

the balance: Fe and impurities,

wherein:

in a cross section parallel to a rolling direction and a sheet thickness direction of the steel sheet, a steel micro-structure at a position at a depth of ¼ of a sheet thickness from a surface is, in area %:

tempered martensite: 25 to 90%,

ferrite: 5% or less,

retained austenite: 10 to 50%, and

bainite: 5% or less;

at a position at a depth of ¼ of the sheet thickness from a surface of a cross section parallel to the rolling direction and the sheet thickness direction of the steel sheet, a proportion of a total area of retained austenite grains having an area of 1 μm²or more and having a grain shape circularity of 0.1 or more is less than 50% with respect to an entire area of the retained austenite; and

an Mn concentration in the steel micro-structure at a position at a depth of ¼ of the sheet thickness from the surface satisfies formula (i) below:

C_Mnγ/C_Mnα≥1.2 (i)

where, meaning of each symbol in formula (i) above is as follows:

C_Mnγ: average Mn concentration (mass %) in retained austenite

C_Mnα: average Mn concentration (mass %) in ferrite and tempered martensite.