	US 12,460,285 B2
	Steel sheet
Takafumi Yokoyama, Tokyo (JP); Kazuki Shiokawa, Tokyo (JP); Tatsuya Obuchi, Tokyo (JP); and Kunio Hayashi, Tokyo (JP)
Assigned to NIPPON STEEL CORPORATION, Tokyo (JP)
Appl. No. 17/634,229
Filed by NIPPON STEEL CORPORATION, Tokyo (JP)
PCT Filed Sep. 3, 2020, PCT No. PCT/JP2020/033489 § 371(c)(1), (2) Date Feb. 9, 2022, PCT Pub. No. WO2021/045168, PCT Pub. Date Mar. 11, 2021.
Claims priority of application No. 2019-160683 (JP), filed on Sep. 3, 2019.
Prior Publication US 2022/0275493 A1, Sep. 1, 2022
Int. Cl. C22C 38/60 (2006.01); C21D 8/02 (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 38/42 (2006.01); C22C 38/44 (2006.01); C22C 38/48 (2006.01); C22C 38/50 (2006.01); C22C 38/54 (2006.01); C22C 38/58 (2006.01)

CPC C22C 38/60 (2013.01) [C21D 8/0205 (2013.01); C21D 8/0226 (2013.01); C21D 8/0236 (2013.01); C21D 8/0263 (2013.01); C21D 9/46 (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 38/42 (2013.01); C22C 38/44 (2013.01); C22C 38/48 (2013.01); C22C 38/50 (2013.01); C22C 38/54 (2013.01); C22C 38/58 (2013.01); C21D 2211/001 (2013.01); C21D 2211/005 (2013.01); C21D 2211/008 (2013.01)]

7 Claims

1. A steel sheet comprising a chemical composition that contains, in mass %:

C: 0.18% to 0.35%,

Si: 0.01% to 1.00%,

Mn: 0.10% to 2.40%,

P: 0.050% or less,

S: 0.0050% or less,

Al: 0.001% to 1.00%,

Ti: 0.001% to 0.050%,

B: 0.0005% to 0.0050%,

N: 0.0100% or less,

O: 0.0050% or less,

Cr: 0% to 1.00%,

Mo: 0% to 0.50%,

Cu: 0% to 0.50%,

Ni: 0% to 0.50%,

Co: 0% to 0.50%,

W: 0% to 0.50%,

Sn: 0% to 0.50%,

Sb: 0% to 0.50%,

Nb: 0% to 0.050%,

V: 0% to 0.50%,

Ca: 0% to 0.0100%,

Mg: 0% to 0.0100%,

Ce: 0% to 0.0100%,

Zr: 0% to 0.0100%,

La: 0% to 0.0100%,

Hf: 0% to 0.0100%,

Bi: 0% to 0.0100%,

REM: 0% to 0.0100%, and

a balance consisting of Fe and impurities, and

that satisfies the Formula: 0.001≤Ti−(47.88/14.01)×N≤0.040, wherein each element symbol represents mass % of the corresponding element, and

a steel structure in a cross section in a rolling direction of a steel sheet and in a range of ⅛ thickness to ⅜ thickness centered on ¼ thickness from a surface of the steel sheet,

that contains, in area ratio %,

ferrite: 0 to 5%,

martensite: 90.0 to 99.5%,

a ratio of tempered martensite to total martensite: 80.0 to 100.0%, and

retained austenite: 0.5 to 6.0%,

and

in a measurement region defined by a range of ⅛ thickness to ⅞ thickness from the surface of the steel sheet×25 mm in the cross section in the rolling direction of the steel sheet, a number density of inclusions satisfying a maximum diameter≥3 μm is 40 inclusions/mm²or less,

when the measurement region is divided into 100 sections obtained by dividing the measurement region into 2 sections in a thickness direction and 50 sections in a width direction, and the number density of the inclusions satisfying the maximum diameter≥3 μm in each section is calculated, the number density in a section where the number density of inclusions is in a top 10% is from 2 inclusions/mm²to 80 inclusions/mm²,

the following Formula (A) is satisfied, and

a tensile strength is 1470 MPa or more:

Vγ′/Vγ≥0.1 (A),

wherein Vγ: area ratio of initial retained austenite, Vγ′: area ratio of retained austenite after deep cooling at −196° C.