METHOD FOR PRODUCING HOT-ROLLED STEEL SHEET WHICH IS EXCELLENT IN DUCTILE CRACKING-ARRESTING CHARACTERISTIC AND SOUR-RESISTANCE

摘要

PROBLEM TO BE SOLVED: To provide a method for producing a high tension hot-rolled steel sheet excellent in a surface quality, a ductile cracking arresting characteristic and a sour-resistance.;SOLUTION: A steel blank having the composition containing 0.02-0.08% C, 0.03-0.10% Nb, 0.005-0.05% Ti, ≤0.003% Ca and the suitable contents of Si, Mn, P, S, Al, N, O, and wherein the specific relation of Ca, O, S is satisfied, is subjected to a rough-rolling process, a finish-rolling process and a winding process in this order. An accelerating cooling is applied before the finish-rolling process after the rough-rolling process and/or during the finish-rolling process, in which the surface layer part is rapidly cooled till reaching the temperature of Ar₃ transformation point or lower at ≥50°C/s cooling speed, and thereafter, this accelerating cooling is stopped. After heat-recovering the temperature of the surface layer part to the temperature of ≥Ac₃ transformation point completing the reverse-transformation, the finish-rolling is applied. In this way, the high tension hot-rolled steel sheet having excellent surface quality in prevention of the surface defect, and excellent toughness, especially excellent ductile cracking-arresting characteristic and further, excellent sour-resistance, can be obtained.;COPYRIGHT: (C)2009,JPO&INPIT