	US 12,258,646 B2
	High-strength steel sheet and method for producing same
Hidekazu Minami, Tokyo (JP); Yuji Tanaka, Tokyo (JP); and Takeshi Yokota, Tokyo (JP)
Assigned to JFE STEEL CORPORATION, Tokyo (JP)
Appl. No. 17/593,062
Filed by JFE STEEL CORPORATION, Tokyo (JP)
PCT Filed Feb. 21, 2020, PCT No. PCT/JP2020/007200 § 371(c)(1), (2) Date Sep. 8, 2021, PCT Pub. No. WO2020/184154, PCT Pub. Date Sep. 17, 2020.
Claims priority of application No. 2019-044066 (JP), filed on Mar. 11, 2019.
Prior Publication US 2022/0195552 A1, Jun. 23, 2022
This patent is subject to a terminal disclaimer.
Int. Cl. C21D 9/46 (2006.01); B32B 15/01 (2006.01); C21D 6/00 (2006.01); C21D 8/02 (2006.01); C22C 38/00 (2006.01); C22C 38/02 (2006.01); C22C 38/06 (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 38/38 (2006.01); C22C 38/40 (2006.01); C23C 2/02 (2006.01); C23C 2/28 (2006.01)

CPC C21D 9/46 (2013.01) [B32B 15/013 (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 38/02 (2013.01); C22C 38/06 (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 38/38 (2013.01); C22C 38/40 (2013.01); C23C 2/02 (2013.01); C23C 2/0224 (2022.08); C23C 2/024 (2022.08); C23C 2/28 (2013.01); C21D 2211/001 (2013.01); C21D 2211/005 (2013.01)]

17 Claims

1. A high-strength steel sheet comprising:

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: 1.50% or more and 2.45% 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,

N: 0.0100% or less,

Mo: 0.500% or less,

Cr: 0.300% or less,

Ca: 0.0200% or less, and

Sb: 0.200% or less,

with a balance being Fe and inevitable impurities, wherein Mn_eqcalculated according to the following formula (1) is 2.40% or more and 3.40% or less,

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

where [% Si], [% Mn], [% P], [% Mo], and [% Cr] denotes content in mass % of Si, Mn, P, Mo, and Cr in steel, respectively, and is 0 in the case where each of Si, Mn, P, Mo, and Cr is not contained;

a steel microstructure in which an area ratio of ferrite is 15% or more and 55% or less, an area ratio of hard phase which is any of bainitic ferrite, bainite, tempered martensite, and quenched martensite is 40% or more and 85% or less, a volume fraction of retained austenite is 4% or more and 20% or less, a carbon concentration in the retained austenite is 0.55% or more and 1.10% or less, an amount of diffusible hydrogen in the steel sheet is 0.80 mass ppm or less, a surface layer softening thickness is 5 μm or more and 150 μm or less, and a corresponding grain boundary frequency in a surface layer of the steel sheet after a high-temperature tensile test is 0.45 or less, where the high-temperature tensile test is performed such that after heating a test piece to 900° C. at 100° C./s, the test piece immediately gas-cooled at 40° C./s and after reaching 700° C., a tensile test immediately conducted at a crosshead rate of 50 mm/s until the test piece fractured; and

a tensile strength of 980 MPa or more.