Patent ID: 8216400

Claim:
A high-strength steel plate consisting of the following composition: by mass %, 0.18 to 0.23% of C; 0.1 to 0.5% of Si; 1.0 to 2.0% of Mn; 0.020% or less of P; 0.010% or less of S; 0.5 to 3.0% of Ni; 0.003 to 0.10% of Nb; 0.05 to 0.15% of Al; 0.0003 to 0.0030% of B; 0.006% or less of N; and optionally further containing one or more kinds selected from the group consisting of: by mass %, 0.05 to 0.5% of Cu; 0.05 to 1.5% of Cr; 0.03 to 0.5% of Mo; and 0.01 to 0.10% of V; 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 0.36% or less, wherein [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 A c3 transformation point is equal to or less than 830° C., a percentage value of a martensite structure is equal to or greater than 90%, prior austenite grains are polygonal grains which have uniform size, a yield strength is equal to or greater than 1300 MPa, 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 of the prior austenite grains 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, the prior austenite grain size number Nγ of the high-strength steel plate is in a range of 8.0 to 11.0, if the tensile strength is less than 1550 MPa, the prior austenite grain size number Nγ and the tensile strength satisfy the formulae Nγ≧([TS]−1400)×0.004+8.0 and Nγ≦11.0, 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 formulae Nγ≧([TS]−1550)×0.008+8.6 and Nγ11.0, where [TS] (MPa) is the tensile strength, and an average value of an absorbed energy of the high-strength steel plate at −20° C. is equal to or greater than 27 J per 10 mm regarding a width of a Charpy specimen.