Manufacturing method of hot-rolled steel sheet having excellent surface quality and ductile crack propagation characteristics

摘要

PROBLEM TO BE SOLVED: To provide a method for producing a high-tensile-strength hot-rolled steel sheet excellent in surface quality and ductile crack propagation characteristics.;SOLUTION: The method includes subjecting a steel raw material having a composition containing 0.02-0.08% C, 0.03-0.10% Nb, 0.005-0.05% Ti and Si, Mn, P, S, Al, and N in suitably adjusted amounts to a rough rolling step, a finish rolling step, and a winding step. In these steps, accelerated cooling including rapidly cooling the surface at a cooling rate of 50°C/s or higher until it reaches a temperature not higher than the Ar₃ transformation temperature is performed between the rough rolling step and the finish rolling step and/or during the finish rolling step, and the accelerated cooling is stopped to recuperate the cooled steel to a surface temperature not lower than the Ac₃ transformation temperature at which reverse transformation is completed, then the steel raw material is subjected to the finish rolling step. Thus, the method can prevent the formation of defects on the surface and is possible to produce a high-tensile-strength hot-rolled steel sheet excellent in surface quality, excellent in toughness, and excellent in, especially ductile crack propagation characteristics.;COPYRIGHT: (C)2009,JPO&INPIT