Are superior in processability 400 - 800N/mm2 class high intensity hot rolling steel sheet and its production manner

摘要

PROBLEM TO BE SOLVED: To obtain a high strength hot rolled steel plate excellent in workability by producing a steel, containing specific elements by the amounts in weight percentages within specific ranges, by a specific method. ;SOLUTION: A steel, having a composition consisting of, by weight, 0.05-0.2% C, 0.01-0.5% Mn, 0.01-0.5% Si, 0.005-0.1% Al, ≤0.05% P, ≤0.01% S, ≤0.007% N, 0.05-0.3% Ti, and the balance iron with inevitable impurities, is heated at 1200-1350°C, roughed, and hot-rolled at ≥Ar₃ finishing temp. Successively, the resultant steel plate is cooled from the finishing temp. down to 600-750°C at a cooling rate in the range of ±5°C/S with respect to the cooling rate determined from equation CR=-200×(0.5×C%+2.5×Ti%)+TS/3-85 [where TS means the desired tensile strength (N/mm²)]. Then coiling is performed at a temp. between room temp. and 750°C. By this method, the (400 to 800)N/mm² class high strength hot rolled steel plate excellent in workability, in which the ratio of C precipitating as cementite to the total C content, represented by M=(C% as cementite)/(total C%), is regulated to ≤0.03, can be produced.;COPYRIGHT: (C)1997,JPO