PROCESS FOR PRODUCING THICK HIGH-STRENGTH STEEL PLATE EXCELLENT IN BRITTLE FRACTURE ARRESTABILITY AND TOUGHNESS OF ZONE AFFECTED BY HEAT IN LARGE-HEAT-INPUT WELDING AND THICK HIGH-STRENGTH STEEL PLATE EXCELLENT IN BRITTLE FRACTURE ARRESTABILITY AND TOUGHNESS OF ZONE AFFECTED BY HEAT IN LARGE-HEAT-INPUT WELDING

This method for producing a thicker high-strength steel plate that includes: cooling a continuously cast slab containing, in terms of mass %, C: 0.05 to 0.12%, Si: not more than 0.3%, Mn: 1 to 2%, P: not more than 0.015%, S: not more than 0.005%, B: 0.0003 to 0.003%, V: 0.01 to 0.15%, Al: 0.001 to 0.1%, Ti: 0.005 to 0.02%, N: 0.002 to 0.01%, and O: not more than 0.004%, with the remainder being iron and unavoidable impurities, to a temperature of Ar 3 -200°C or lower, and subsequently reheating the slab to 950 to 1,100°C; subjecting the continuously cast slab to rough rolling at 900°C or higher with a cumulative reduction ratio of at least 30%; subsequently performing finish rolling at 700°C or higher with a cumulative reduction ratio of at least 50% under conditions that both of the finish rolling start temperature and the finish rolling completion temperature are not higher than the temperature represented by the formula: {-0.5 × (slab heating temperature (°C)) + 1.325} (°C), thereby forming a rolled plate; and then cooling the rolled plate to 500°C or lower by accelerated cooling to obtain a steel plate. In the continuously cast slab, a calculated value of an amount of B {effective B amount: Bef(%)} which is solid-solubilized into an austenite base material prior to transformation is not more than 0%, and a carbon equivalent Ceq satisfies a range from 0.32 to 0.42%.