Method of producing high tensile-high toughness steel

A method for manufacturing high tensile-high toughness steel plate comprising the steps of:

• preparing a molten steel alloy consisting, by weight, of
• 0. 03 to 0.20% C,
• 0.01 to 0.70% Si,
• 0.50 to 1.80% Mn,
• one or two selected from the group consisting of 0.005 to 0.05% Ti and 0.005 to 0.05% Zr,
• 0.005 to 0.10% Nb,
• not greater than 0.025% P,
• not greater than 0.015% S,
• not greater than 0.080% Al,
• not greater than 0.0030% N, and
• the balance Fe and impurities incidentally mixed in the normal steel manufacturing process and having a value not smaller than 0.60 of D.sub.I * defined by formula:
• ##EQU3## preparing a steel slab or ingot by casting the said molten steel alloy, rolling said slab or ingot with an accumulative rolling reduction of at least 30% in a temperature range between (Ar.sub.3 +150.degree. C.) and Ar.sub.3 during a cooling after casting, or in another cooling after reheating a cold steel slab or ingot in a temperature range between 1000.degree. C. and 1300.degree. C., quenching the rolled steel alloy from a temperature not less than (Ar.sub.3 -30.degree. C.) within a period of time in which neither recovering nor recrystallization substantially occur, and tempering at a temperature of not higher than Ac.sub.1.