High-strength steel plate and producing method therefor

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 Ac3 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+log2m using an average number m of crystal grains per 1 mm2 in a cross section of a sample piece, and
• 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.