PRODUCTION OF LOW YIELD RATIO 590N/MM2 CLASS HIGH TENSILE STRENGTH STEEL EXCELLENT IN WELDABILITY

摘要

PURPOSE: To produce a high tensile strength steel excellent in weldability and low in a yield ratio by rolling a low carbon-low alloy steel slab substantially contg. no B under specified temp. conditions, subjecting it to hardening and thereafter executing reheating, rehardening and tempering. ;CONSTITUTION: A steel slab having a compsn. contg., by weight, 0.04 to 0.11% C, 0.5% Si, 0.9 to 1.6% Mn, 0.03% P, 0.01% S, 0.01 to 0.05% Nb, 0.01 to 0.06% V, 0.005 to 0.025% Ti, 0.1% Al and 0.006% N, in which Di expressed by the formula is regulated to the range of 1.5 to 3.0, contg. one or more kinds among 0.05 to 0.50% Cr, 0.05 to 0.30% Mo, 0.05 to 2.0% Ni and 0.05 to l.5% Cu and substantially contg. no B is heated to ≥1250°C, is subjected to hot rolling so as to regulate the cumulative draft at ≤900°C to ≥30% and is worked into a thick steel plate or the like. This is hardened from ≥750°C to an ordinary temp., is again reheated to a (γ+α) two-phase region of 720 to 850°C, is hardened and is tempered to the AC₁ transformation point or below, e.g. of 450 to 650°C.;COPYRIGHT: (C)1996,JPO