Patent ID: 8500924

Claim:
A high-strength steel plate comprising the following composition: 0.18 to 0.23 mass % of C; 0.1 to 0.5 mass % of Si; 1.0 to 2.0 mass % of Mn; 0.020 mass % or less of P; 0.010 mass % or less of S; greater than 0.5 mass % and equal to or less than 3.0 mass % of Cu; 0.25 to 2.0 mass % of Ni; 0.003 to 0.10 mass % of Nb; 0.05 to 0.15 mass % of Al; 0.0003 to 0.0030 mass % of B; 0.006 mass % or less of N; and a balance composed of Fe and inevitable impurities, wherein a weld crack sensitivity index Pcm is calculated by Pcm=[C]+[Si]/30 +[Mn]/20 +[Cu]/20 +[Ni]/60 +[Cr]/20 +[Mo]/15 +[V]/10 +5 [B], and is 0.39 mass % or less, where [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V], and [B] are the concentrations (mass %) of C, Si, Mn, Cu, Ni, Cr, Mo, V, and B, respectively, an Ac 3 transformation point is equal to or less than 850° C., a percentage value of a martensite structure is equal to or greater than 90%, a yield strength is equal to or greater than 1300 MPa, and a tensile strength is equal to or greater than 1400 MPa and equal to or less than 1650 MPa, a prior austenite grain size number Nγ is calculated by Nγ=−3+log 2 m using an average number m of crystal grains per 1 mm 2 in a cross section of a sample piece, if the tensile strength is less than 1550 MPa, the prior austenite grain size number Nγ and the tensile strength satisfy the formula Nγ≧([TS]−1400 )×0.006+7.0 , and if the tensile strength is equal to or greater than 1550 MPa, the prior austenite grain size number Nγ and the tensile strength satisfy the formula Nγ([TS]-1550 )×0.01+7.9, where [TS] (MPa) is the tensile strength, and wherein the prior austenite grain size number Nγ is 11 or less.