US 12,480,175 B2
Steel sheet, member, and method for producing them
Lingling Yang, Tokyo (JP); Yoshiyasu Kawasaki, Tokyo (JP); Hidekazu Minami, Tokyo (JP); Tatsuya Nakagaito, Tokyo (JP); Shotaro Terashima, Tokyo (JP); Shunsuke Yamamoto, Tokyo (JP); Katsuya Hoshino, Tokyo (JP); and Yuki Takeda, Tokyo (JP)
Assigned to JFE STEEL CORPORATION, Tokyo (JP)
Appl. No. 17/915,696
Filed by JFE STEEL CORPORATION, Tokyo (JP)
PCT Filed Mar. 25, 2021, PCT No. PCT/JP2021/012659
§ 371(c)(1), (2) Date Sep. 29, 2022,
PCT Pub. No. WO2021/200577, PCT Pub. Date Oct. 7, 2021.
Claims priority of application No. 2020-062955 (JP), filed on Mar. 31, 2020; and application No. 2020-122181 (JP), filed on Jul. 16, 2020.
Prior Publication US 2023/0160032 A1, May 25, 2023
Int. Cl. C21D 9/46 (2006.01); B21C 47/02 (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 2/28 (2006.01); C23C 2/40 (2006.01)
CPC C21D 9/46 (2013.01) [B21C 47/02 (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 2/28 (2013.01); C23C 2/29 (2022.08); 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)] 24 Claims
 
1. A steel sheet having a chemical composition comprising, by mass %:
Si: 0.3% to 2.0%;
Mn: 1.0% or more and less than 2.70%;
C: 0.12% to 0.40%;
P: 0.05% or less;
S: 0.02% or less;
Al: 0.01% to 2.0%;
N: 0.01% or less;
optionally at least one selected from the group consisting of Nb: 0.50% or less, Cr: 1.0% or less, Mo: 0.50% or less, B: 0.005% or less, and Ti: 0.05% or less;
optionally at least one selected from the group consisting of Cu, Ni, Sn, As, Sb, Ca, Mg, Pb, Co, Ta, W, REM, Zn, V, Sr, Cs, Hf, and Zr, in a total amount of 0.1% or less;
optionally an equivalent carbon content Ceq in a range of 0.458% or more and less than 0.659%; and
the remainder being Fe and incidental impurities,
wherein the steel sheet has a steel microstructure including a bainitic ferrite area fraction in a range of 10% to 35%, a fresh martensite area fraction in a range of 2% to 15%, a retained austenite area fraction in a range of 5% to 20%, and a ferrite area fraction in a range of 45% to 70%,
a total fraction of the fresh martensite and the retained austenite adjacent to the ferrite is 90% or less of a total area fraction of the fresh martensite and the retained austenite,
crystal grains containing an oxide of at least one of Si and Mn in a region within 4.9 μm in a thickness direction from a surface of the steel sheet have an average grain size in a range of 3 to 10 μm,
the following formula (1) is satisfied:
LSi+LMn≤(TSi+TMn)/4  (1)
where LSi is a lowest Si concentration and LMn is a lowest Mn concentration in the region within 4.9 μm in the thickness direction from the surface of the steel sheet, and TSi is a Si concentration and TMn is a Mn concentration at a quarter thickness position of the steel sheet, and
the steel sheet has a tensile strength (TS) in a range of 780 MPa or more and less than 1180 MPa.