US 12,305,266 B2
High-strength steel plate having excellent formability, toughness and weldability, and production method of same
Hiroyuki Kawata, Tokyo (JP); Eisaku Sakurada, Tokyo (JP); Kohichi Sano, Tokyo (JP); and Takafumi Yokoyama, Tokyo (JP)
Assigned to NIPPON STEEL CORPORATION, Tokyo (JP)
Appl. No. 17/312,853
Filed by NIPPON STEEL CORPORATION, Tokyo (JP)
PCT Filed Dec. 11, 2018, PCT No. PCT/JP2018/045547
§ 371(c)(1), (2) Date Jun. 10, 2021,
PCT Pub. No. WO2020/121417, PCT Pub. Date Jun. 18, 2020.
Prior Publication US 2021/0340653 A1, Nov. 4, 2021
Int. Cl. C21D 9/46 (2006.01); C21D 8/02 (2006.01); C22C 38/00 (2006.01); C22C 38/02 (2006.01); C22C 38/06 (2006.01); C22C 38/42 (2006.01); C22C 38/44 (2006.01); C22C 38/46 (2006.01); C22C 38/48 (2006.01); C22C 38/50 (2006.01); C22C 38/54 (2006.01); C22C 38/58 (2006.01); C23C 2/02 (2006.01); C23C 2/06 (2006.01); C23C 2/40 (2006.01)
CPC C22C 38/58 (2013.01) [C21D 8/0205 (2013.01); C21D 8/0226 (2013.01); C21D 9/46 (2013.01); C22C 38/001 (2013.01); C22C 38/002 (2013.01); C22C 38/005 (2013.01); C22C 38/02 (2013.01); C22C 38/06 (2013.01); C22C 38/42 (2013.01); C22C 38/44 (2013.01); C22C 38/46 (2013.01); C22C 38/48 (2013.01); C22C 38/50 (2013.01); C22C 38/54 (2013.01); C23C 2/02 (2013.01); C23C 2/0224 (2022.08); C23C 2/06 (2013.01); C23C 2/40 (2013.01); C21D 2211/001 (2013.01); C21D 2211/005 (2013.01); C21D 2211/008 (2013.01)] 29 Claims
 
1. A steel sheet comprising a chemical composition comprising: by mass %,
C in a range from 0.05 to 0.30%;
Si of 2.50% or less;
Mn in a range from 0.50 to 3.50%;
P of 0.100% or less;
S of 0.0100% or less;
Al in a range from 0.001 to 2.000%;
N of 0.0150% or less;
O of 0.0050% or less; and
the balance consisting of Fe and inevitable impurities,
the steel sheet comprising a microstructure in a region from ⅛t to ⅜ t from a steel sheet surface, the microstructure comprising: by volume %,
acicular ferrite of 20% or more;
martensite of 10% or more;
polygonal ferrite of 20% or less;
residual austenite of 2.0% or less; and
5% or less of a structure other than a structure comprising bainite and bainitic ferrite in addition to the above whole structure, wherein
the martensite satisfies a formula (A) below,

OG Complex Work Unit Math
where: t represents sheet thickness, di represents a circle-equivalent diameter in μm of the i-th largest island-shaped martensite in the microstructure in the region of ⅛ t to ⅜ t, and ai represents an aspect ratio of the i-th largest island-shaped martensite in the microstructure in the region of ⅛ t to ⅜ t.