HOT-ROLLED STEEL SHEET EXCELLENT IN SURFACE QUALITY AND DUCTILE CRACK PROPAGATION CHARACTERISTIC AND PRODUCTION METHOD THEREFOR

摘要

PROBLEM TO BE SOLVED: To provide a production method for a hot-rolled steel sheet excellent in the surface quality and the ductile crack propagation characteristic.;SOLUTION: A steel blank having the composition of 0.02-0.08% C, 0.03-0.10% Nb, 0.005-0.05% Ti and Si, Mn, P, S, Al and N adjusted to suitable contents is subjected to a rough rolling process, a finish rolling process and a coiling process in order. After the rough rolling process and before the finish rolling process and/or during the finish rolling process, an accelerated cooling is applied to a the surface layer part to a temperature of 930°C or below and above the Ar₃ transformation point at ≥50°C/s cooling rate or the accelerated cooling is applied to the surface layer part to a temperature of the Ar₃ transformation point or below at ≥50°C/s cooling rate, then this accelerated cooling is stopped, and thereafter the finish rolling is applied. Then, when the accelerated cooling of rapidly cooling the surface layer part to a temperature of Ar₃ transformation point or below has been performed, the rolling reduction ratio for 1 pass in the finish rolling is desirable to be limited to ≤(1.1×uniform extension)%. In this way, the high tension hot rolled steel sheet excellent in the surface quality, ductility and especially the ductile crack propagation characteristic can be produced.;COPYRIGHT: (C)2009,JPO&INPIT