	US 12,435,384 B2
	High-strength steel sheet and method for manufacturing same
Hidekazu Minami, Tokyo (JP); Takeshi Yokota, Tokyo (JP); Yoshiyasu Kawasaki, Tokyo (JP); and Shotaro Terashima, Tokyo (JP)
Assigned to JFE STEEL CORPORATION, Tokyo (JP)
Appl. No. 17/630,504
Filed by JFE STEEL CORPORATION, Tokyo (JP)
PCT Filed Jun. 19, 2020, PCT No. PCT/JP2020/024251 § 371(c)(1), (2) Date Jan. 27, 2022, PCT Pub. No. WO2021/019947, PCT Pub. Date Feb. 4, 2021.
Claims priority of application No. 2019-140307 (JP), filed on Jul. 30, 2019.
Prior Publication US 2022/0251676 A1, Aug. 11, 2022
This patent is subject to a terminal disclaimer.
Int. Cl. C22C 38/00 (2006.01); B32B 15/01 (2006.01); C21D 1/22 (2006.01); C21D 6/00 (2006.01); C21D 8/02 (2006.01); C21D 9/46 (2006.01); C22C 38/04 (2006.01); C22C 38/20 (2006.01); C22C 38/22 (2006.01); C22C 38/24 (2006.01); C22C 38/26 (2006.01); C22C 38/28 (2006.01); C22C 38/30 (2006.01); C22C 38/32 (2006.01); C22C 38/44 (2006.01); C22C 38/58 (2006.01)

CPC C21D 9/46 (2013.01) [B32B 15/013 (2013.01); C21D 1/22 (2013.01); C21D 6/002 (2013.01); C21D 6/004 (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); C22C 38/001 (2013.01); C22C 38/002 (2013.01); C22C 38/005 (2013.01); C22C 38/008 (2013.01); C22C 38/04 (2013.01); C22C 38/20 (2013.01); C22C 38/22 (2013.01); C22C 38/24 (2013.01); C22C 38/26 (2013.01); C22C 38/28 (2013.01); C22C 38/30 (2013.01); C22C 38/32 (2013.01); C22C 38/44 (2013.01); C22C 38/58 (2013.01); C21D 2211/001 (2013.01); C21D 2211/002 (2013.01); C21D 2211/005 (2013.01); C21D 2211/008 (2013.01)]

8 Claims

1. A high-strength steel sheet having a tensile strength of 1180 MPa or more, and a yield ratio YR is 65% or more and 70% or less, which comprises

a chemical composition containing, in mass %,

C: 0.120% or more and 0.250% or less,

Si: 0.80% or more and 2.00% or less,

Mn: more than 2.45% and 4.00% or less,

P: 0.001% or more and 0.100% or less,

S: 0.0200% or less,

Al: 0.010% or more and 1.000% or less, and

N: 0.0100% or less, with

Mn_eqobtained with the following expression (1) satisfying a relationship of 3.00% or more and 4.20% or less, and

the balance consisting of Fe and inevitable impurities, wherein

Mn_eq=0.26×[% Si]+[% Mn]+3.5×[% P]+2.68×[% Mo]+1.29×[% Cr] (1)

where the [% X] in the expression (1) represents a content of element X in mass % in the steel, and the [% X] is 0 when the element X is not contained, and

a steel microstructure wherein

an area ratio of ferrite is 25% or less,

a total area ratio of tempered martensite and bainite is 65% or more and 96% or less,

an area ratio of quenched martensite is 15% or less,

a volume fraction of retained austenite is 4% or more and 20% or less,

an amount of diffusible hydrogen in the steel sheet is 0.60 mass ppm or less,

a thickness of softened surface layer is 5 μm or more and 150 μm or less,

should a high-temperature tensile test be performed on the steel sheet, a frequency of coincidence boundary in a surface layer of the steel sheet after the high-temperature tensile test is 0.45 or less, and

the yield ratio YR is expressed by “yield stress YS/tensile strength TS” of the steel sheet.