Patent ID: 8043447

Claim:
A method of manufacturing a high tensile strength thick steel plate having high weldability and low-temperature toughness of a welded joint, with a tensile strength of 780 MPa or more and a yield stress of 685 MPa or more, the steel plate having a thickness of 12-40 mm, the method comprising: heating to 950-1100° C. a steel slab or a cast slab having a component composition which includes, in mass %, 0.030-0.055% of C, 3.0-3.5% of Mn, and 0.002-0.10% of Al, and restricts P to 0.01% or less, S to 0.0010% or less, N to 0.0060% or less, Mo to 0.03% or less, Si to 0.09% or less, V to 0.01% or less, Ti to 0.003% or less, B to 0.0003% or less, Nb to 0.003% or less, and Ni to 0.03% or less, wherein a Pcm value representing a weld cracking parameter is within a range of 0.20-0.24% and a DI value representing a hardenability index is within a range of 1.00-2.60, and comprises a balance of Fe with inevitable impurities; performing a first rolling with a cumulative draft of 70-90% at a temperature of 850° C. or more, thereafter performing a second rolling with a cumulative draft of 10-40% at a temperature in a range of 780-830° C.; and subsequently starting accelerated cooling at a cooling rate of 8-80° C./sec from 700° C. or higher, and stopping the accelerated cooling at a temperature between room temperature and 350° C., wherein Pcm=[C]+[Si]/30+[Mn]/20+[Cu]/20+[Ni]/60+[Cr]/20+[Mo]/15+[V]/10+5[B], DI=0.367([C] 1/2 )(1+0.7[Si])(1+3.33[Mn])(1+0.35[Cu])(1+0.36[Ni])(1+2.16[Cr])(1+3.0[Mo])(1+1.75[V])(1+1.77[Al]), where [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V], [Al], and [B] are the amounts, expressed in mass %, of C, Si, Mn, Cu, Ni, Cr, Mo, V, Al and B respectively.