Patent ID: 8394209

Claim:
A high-strength steel sheet, having a composition comprising: C at 0.10 to 0.16% (herein, the term “%” means “mass %”, the same is true hereinbelow), Si at 0.05 to 0.50%, Mn at 1.3 to 1.9%, Al at 0.01 to 0.05%, Ti at 0.005 to 0.025%, Nb at 0.005 to 0.025%, V at 0.005 to 0.06%, Cr at 0.05 to 0.25%, N at 0.0030 to 0.01%, and Mo at 0.00 to 0.02%, with a balance consisting of iron and inevitable impurities, wherein a DE value defined by the following Equation (1) is 0.0340% or more, where DE value=[Ti]+[Nb]+0.3[V]+0.0075[Cr] (1) and [Ti], [Nb], [V], and [Cr] represent contents (mass %) of Ti, Nb, V, and Cr, respectively; wherein V and Cr are made to remain as respective carbides in the steel sheet by the combined content of Nb; wherein a carbon equivalent Ceq defined by the following Equation (2) is 0.45% or less, where Ceq =[C]+[Mn]/6+([Cr]+[Mo]+[V])/5+([Cu]+[Ni])/15 (2) and [C], [Mn], [Cr], [Mo], [V], [Cu], and [Ni] represent contents (mass %) of C, Mn, Cr, Mo, V, Cu, and Ni, respectively; wherein the steel sheet has a thickness in a range of from 30 to 70 mm; wherein the steel sheet is obtained by a process comprising rolling an ingot having the composition of the steel sheet wherein the rolling is carried out at a reduction rate of 10% or more in a non-recrystalline temperature region; and wherein the steel sheet has a tensile strength in a range of from 591 to 658 MPa after the steel sheet is annealed at 615° C. for 23 hours.