CONTINUOUSLY CAST SLAB FOR HIGH STRENGTH STEEL EXCELLENT IN TOUGHNESS AT LOW TEMPERATURE, ITS PRODUCTION, AND HIGH STRENGTH STEEL EXCELLENT IN TOUGHNESS AT LOW TEMPERATURE

摘要

PROBLEM TO BE SOLVED: To inhibit the coarsening of austenite on reheating of a cast slab and to obtain a high strength hot rolled steel plate having fine-grained structure and excellent toughness at low temperature by specifying the area ratio of transgranular transformed ferrite in martensitic and bainitic structures of a cast slab. ;SOLUTION: A steel composition, consisting of, by mass ratio, 0.03-0.10% C, 0.6% Si, 1.2 2.5% Mn, 0.005% P, 0.003% S, 0.1-1.0% Ni, 0.15-0.60% Mo, 0.005-0.10% Nb, 0.001-0.006% N, 0.005-0.006% Ti, and the balance essentially Fe and containing, if necessary, one or ≥2 kinds among 0.10% Cr, 1.0% Cu, 0.10% V, 0.0005-0.0020% B, 0.006% Ca, 0.02% REM, 0.006% Mg, and 0.10% Zn, is provided. Simultaneously, a cast slab, in which the area ratio of transgranular transformed ferrite in martensitic and bainitic structures is regulated to ≥10%, is provided. Further, this cast slab is formed into steel plate by controlled rolling.;COPYRIGHT: (C)1999,JPO