US 12,077,831 B2
Steel sheet, member, and methods for producing them
Shimpei Yoshioka, Tokyo (JP); Yoshihiko Ono, Tokyo (JP); Yuma Honda, Tokyo (JP); and Nobuyuki Nakamura, Tokyo (JP)
Assigned to JFE STEEL CORPORATION, Tokyo (JP)
Appl. No. 17/415,532
Filed by JFE STEEL CORPORATION, Tokyo (JP)
PCT Filed Oct. 25, 2019, PCT No. PCT/JP2019/041817
§ 371(c)(1), (2) Date Jun. 17, 2021,
PCT Pub. No. WO2020/129402, PCT Pub. Date Jun. 25, 2020.
Claims priority of application No. 2018-238963 (JP), filed on Dec. 21, 2018.
Prior Publication US 2022/0056549 A1, Feb. 24, 2022
This patent is subject to a terminal disclaimer.
Int. Cl. C21D 9/46 (2006.01); B22D 11/18 (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); C22C 38/12 (2006.01); C22C 38/14 (2006.01); C23C 2/06 (2006.01)
CPC C21D 9/46 (2013.01) [B22D 11/182 (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); C22C 38/12 (2013.01); C22C 38/14 (2013.01); C23C 2/06 (2013.01); C21D 2211/001 (2013.01); C21D 2211/002 (2013.01); C21D 2211/005 (2013.01); C21D 2211/008 (2013.01)] 16 Claims
OG exemplary drawing
 
1. A steel sheet having a chemical composition comprising, by mass %:
C: 0.13% or more and 0.40% or less;
Si: 1.5% or less;
Mn: 1.7% or less;
P: 0.010% or less;
S: 0.0020% or less;
sol. Al: 0.20% or less;
N: less than 0.0055%,%;
O: 0.0025% or less;
Nb: 0.002% or more and 0.035% or less;
Ti: 0.002% or more and 0.10% or less;
B: 0.0002% or more and 0.0035% or less; and
the balance being Fe and incidental impurities,
wherein the steel sheet has a microstructure including martensite and bainite, a total area fraction of the martensite and the bainite being in a range of 95% or more and 100% or less, the balance being one or more selected from ferrite and retained austenite, and
a total of (i) a density of inclusion particles having a long-axis length in a range of 20 μm or more and 80 μm or less and a minimum interparticle distance of more than 10 μm and (ii) a density of inclusion particle clusters each having a long-axis cluster length in a range of 20 μm or more and 80 μm or less and each including two or more inclusion particles having a long-axis length of 0.3 μm or more and a minimum interparticle distance of 10 μm or less is 5 pieces/mm2 or less,
a local P concentration in a region extending from a position 1/4 of a thickness of the steel sheet in a thickness direction from a surface of the steel sheet to a position 3/4 of the thickness of the steel sheet in the thickness direction from the surface of the steel sheet is 0.060% or less by mass, and a degree of Mn segregation in the region is 1.50 or less,
the steel sheet has a tensile strength of 1,320 MPa or more, and
formulae (1) and (2) are satisfied:
[% Ti]+[% Nb]>0.007  (1)
[% Ti]×[% Nb]2≤7.5×10−6  (2)
where, in each of formulae (1) and (2), [% Nb] and [% Ti] are a Nb content (%) and a Ti content (%), respectively, of the steel sheet.