Patent ID: 7918948

Claim:
A method of production of 780 MPa class high strength steel plate superior in low temperature toughness characterized by heating a steel slab of chemical ingredients containing, by mass %, C: 0.06 to 0.15%, Si: 0.05 to 0.35%, Mn: 0.60 to 2.00%, P: 0.015% or less, S: 0.015% or less, Cu: 0.1 to 0.5%, Ni: 0.1 to 1.5%, Cr: 0.05 to 0.8%, Mo: 0.05 to 0.6%, Nb: less than 0.005%, V: 0.005 to 0.060%, Ti: less than 0.003%, Al: 0.02 to 0.10%, B: 0.0005 to 0.003%, and N: 0.002 to 0.006%, having a balance of iron and unavoidable impurities, having a Ceq defined by the equation (1) of 0.41 to 0.61, having a BNP defined by the equation (2) of over 1.5 to less than 4.0, to 1050° C. to 1200° C. in temperature, hot rolling ending at 870° C. to 901° C., waiting for 31 seconds to 90 seconds, then cooling from 840° C. or more in temperature by a 5° C./s or more cooling speed to 200° C., then tempering at 450° C. to 650° C. in temperature for 20 minutes to 60 minutes, Ceq =% C+% Mn/6+(% Cu+% Ni)/15+(% Cr+% Mo+% V)/5 equation (1) BNP =(N−(14/48)Ti)/B equation (2).