Patent ID: 9255305
Filing Date: 2016-02-09
CPC Classification: C21D,C22C

Claim Text:
1. A high-strength steel sheet having superior toughness at extreme low temperatures, comprising, in weight percentage, 0.02 to 0.06% of C, 0.1 to 0.35% of Si, 1.0 to 1.6% of Mn, 0.02% or less (but not 0%) of Al, 0.7 to 2.0% of Ni, 0.4 to 0.9% of Cu, 0.003 to 0.015% of Ti, 0.003 to 0.02% of Nb, 0.01% or less of P, 0.005% or less of S, the remainder being Fe and unavoidable impurities, wherein the high-strength steel sheet satisfies the condition of [Mn]+5.4[Si]+26[Al]+32.8[Nb]<4.3 where [Mn], [Si], [Al], and [Nb] indicate contents of Mn, Si, Al, and Nb in weight percentage, respectively, wherein microstructure of the steel sheet comprises, in area percentage, 99% or more of acicular ferrite and 1% or less austenite/martensite (M&A).