PRODUCTION OF HIGH TENSILE STRENGTH STEEL MATERIAL EXCELLENT IN TOUGHNESS AT LOW TEMPERATURE

摘要

PROBLEM TO BE SOLVED: To produce a high tensile strength steel material excellent in toughness at low temp. by successively applying heating, roughing, cooling, and finish rolling under respectively specified conditions to a slab of a steel in which composition and oxide grain content are respectively specified. ;SOLUTION: A slab of a steel, having a composition which consists of, excluding oxide grains, 0.01-0.20%, by weight, C, 0.03-1.0% Si, 0.30-2.0% Mn, 0.001-0.1% Al, 0.001-0.01% N, and the balance Fe with inevitable impurities and in which the contents of P and S as impurities are limited to ≤0.015% and ≤0.010%, respectively, and further containing oxide grains of 0.1-1μm grain size by 0.02-0.5 pieces per μm² of area of base material, is heated to a temp. between the Ac₃ transformation point and 1250°C. After roughing in the austenitic region at 10-70% cumulative draft, cooling is performed at a rate of (0.1 to 10)°C/sec until finish rolling is started. Then, from the state of 50-90% ferrite proportion, finish rolling of 30-90% cumulative draft is finished at 650-750°C. By this method, the high tensile strength steel material, having ≤ about 3μm average α-grain size and also having ultra-fine grained α-structure, can be obtained.;COPYRIGHT: (C)1997,JPO