Patent ID: 8110292

Claim:
High strength steel plate with excellent low temperature toughness, having a chemical composition containing, by mass %, C: 0.01 to 0.08%, Si: 0.01 to 0.50%, Mn: 0.5 to 2.0%, S: 0.0001 to 0.005%, Ti: 0.003 to 0.030%, Mo: 0.05 to 1.00%, B: 0.0003 to 0.010%, and 0 : 0.0001 to 0.008%, limiting P: 0.050% or less and Al: 0.020% or less, and having a balance of iron and unavoidable impurities, having a Ceq, calculated by the following (formula 1), of 0.30 to 0.53, having a Pcm, found the following (formula 2), of 0.10 to 0.20, and having a metal structure with an area percentage of polygonal ferrite of 20 to 90% and a balance of a hard phase comprised of one or both of bainite and martensite: Ceq =C+Mn/6+(Ni+Cu)/15+(Cr+Mo+V)/5 . . . (formula 1) Pcm =C+Si/30+(Mn+Cu+Cr)/20+Ni/60+Mo/15+V/10+5B . . . (formula 2) where, C, Si, Mn, Ni, Cu, Cr, Mo, V, and B are contents of the individual elements (mass %).