	US 12,221,682 B2
	Ultrahigh-strength steel having excellent cold workability and SSC resistance, and manufacturing method therefor
Dae-Woo Kim, Pohang-si (KR); and Young-Jin Jung, Pohang-si (KR)
Assigned to POSCO CO., LTD, Pohang-si (KR)
Appl. No. 17/294,572
Filed by POSCO, Pohang-si (KR)
PCT Filed Nov. 29, 2019, PCT No. PCT/KR2019/016706 § 371(c)(1), (2) Date May 17, 2021, PCT Pub. No. WO2020/111863, PCT Pub. Date Jun. 4, 2020.
Claims priority of application No. 10-2018-0153164 (KR), filed on Nov. 30, 2018.
Prior Publication US 2022/0002851 A1, Jan. 6, 2022
Int. Cl. C22C 38/04 (2006.01); C21D 1/19 (2006.01); C21D 1/26 (2006.01); C21D 6/00 (2006.01); C21D 8/02 (2006.01); C21D 9/46 (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 38/58 (2006.01)

CPC C22C 38/58 (2013.01) [C21D 1/19 (2013.01); C21D 1/26 (2013.01); C21D 6/004 (2013.01); C21D 6/005 (2013.01); C21D 6/008 (2013.01); C21D 8/0226 (2013.01); C21D 8/0263 (2013.01); C21D 9/46 (2013.01); C22C 38/02 (2013.01); C22C 38/04 (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); C21D 2211/001 (2013.01); C21D 2211/002 (2013.01); C21D 2211/005 (2013.01); C21D 2211/008 (2013.01)]

5 Claims

1. A steel comprising, by wt %, more than 0.08% and 0.2% or less of carbon (C), 0.05 to 0.5% of silicon (Si), 0.5 to 2% of manganese (Mn), 0.005 to 0.1% of aluminum (Al), 0.01% or less of phosphorus (P), 0.0015% or less of sulfur(S), 0.001 to 0.03% of niobium (Nb), 0.001 to 0.03% of vanadium (V), 0.001 to 0.03% of titanium (Ti), 0.01 to 1% of chromium (Cr), 0.01 to 0.15% of molybdenum (Mo), 0.01 to 0.5% of copper (Cu), 0.05 to 4% of nickel (Ni), and 0.0005 to 0.004% of calcium (Ca), with a balance of Fe and other inevitable impurities,

wherein a microstructure of a surface layer portion, which is a region from a surface of the steel to 10% of a total thickness of the steel, comprises 90 area % or more of polygonal ferrite, a microstructure of a region excluding the surface layer portion comprises 90 area % or more of tempered martensite or 90 area % or more of a mixed structure of tempered martensite and tempered bainite, and

the surface layer portion has a dislocation density of 3×10¹⁴/m²or less.