HIGH TENSILE STRENGTH STEEL HAVING EXCELLENT WELD CRACKING SENSITIVITY AND LOW YIELD RATIO, AND ITS PRODUCTION

摘要

PROBLEM TO BE SOLVED: To provide a method of production of a high tensile strength steel of ≤100 mm plate thickness, excellent in characteristics of weld cracking resistance and low yield ratio, at a low cost. ;SOLUTION: A steel stock, having a composition which consists of, by weight, 0.06-0.11% C, 0.01-0.4% Si, 0.5-1.6% Mn, 0.1% Cu, 0.1% Ni, 0.1% Cr, 0.1-1% Mo, 0.01-0.1% V, 0.005% Nb, 0.005% Ti, 0.0003% B, and the balance iron with inevitable impurities and in which the value of P cm, represented by P cm=C%+Si%/30+Mn%/20+Cu%/20+Ni%/30+Cr%/20+Mo%/15+V %/10+5B%, is regulated to ≤0.22%, is heated to 1000 to 1250°C and hot-rolled. Subsequently, the resultant hot rolled steel stock is directly quenched from a temp. not lower than the Ar₃ transformation point, reheated to a dual-phase region temp. between the Ac₁ and the Ac₃ point, quenched, and further subjected to tempering treatment at a temp. not higher than the Ac₁ point.;COPYRIGHT: (C)1999,JPO