PRODUCTION OF HIGH STRENGTH STEEL EXCELLENT IN DELAYED FRACTURE CHARACTERISTIC

摘要

PURPOSE:To obtain a steel plate having high strength and excellent in delayed fracture characteristics by hot-rolling a steel with a specific composition under specific conditions to elongate austenite grains in the surface layer part of plate thickness and then quenching the above plate without delay. CONSTITUTION:A steel slab has a composition containing, by weight ratio, 0.15-0.40% C, 0.05-0.5% Si, 0.3-1.5% Mn, 0.1-1.0% Cr, 0.03-2.0% Mo, 0.0050-0.050% Ti, 0.0005-0.0050% B, and 0.01-0.10% solAl, also containing, as impurities, =0.010% P, =0.010% S, =0.0060% N, and =0.0030% O, and having the balance Fe with inevitable impurities. The above steel slab is heated up to 1,050-1,250 deg.C and subjected to rolling at a draft at least of 30-70% at 800-950 deg.C, by which the structure in the surface layer part of plate thickness is formed into austenite grains. After rolling, the above plate is subjected to quenching without delay to undergo direct hardening, by which the structure is formed into martensitic structure. By this method, effective grain diameter can be reduced, and superior delayed fracture characteristics as well as high toughness can be obtained.