MANUFACTURE OF STEEL MATERIAL EXCELLENT IN RESISTANCE TO STRESS CORROSION CRACKING AND HAVING LOW YIELD RATIO IN SULFIDE-RICH CIRCUMSTANCES

摘要

PURPOSE:To obtain a steel material excellent stress corrosion cracking resistance in sulfide-richcircumstances and having low yield ratio, by heating a steel slab having a specific composition containing Nb up to a temp. at which Nb is allowed to enter into solid solution in or after hot rolling and then applying natural cooling. CONSTITUTION:A steel slab consisting of 0.02-0.20% C, 0.01-0.50% Si, 0.2-2.5% Mn, =0.025% P, =0.02% S, =0.1% Al, 0.01-0.10% Nb, and the balance essentially Fe is used as a stock, or, if necessary, trace amounts of one or more elements among Cu, Ce, Ca, and N or further one or more elements among Ti, B, Cr, Mo, and Ni are incorporated to the above steel. The above steels are subjected to heating up to a temp. where Nb is allowed to enter into solid solution in the course of hot rolling and then natural cooling after hot rolling, or, after rolling, the above steels are immediately heated up to the temp. where Nb enters into solid solution and then cooled naturally. By this method, the steel material having superior stress corrosion cracking resistance even in a sulfide-corrosion atmosphere and also having low yielding ratio can be obtained.