| 摘要 |
<P>PROBLEM TO BE SOLVED: To provide a method for manufacturing a hot-rolled steel sheet excellent in surface quality, and excellent in a ductile crack propagation property. <P>SOLUTION: The method comprises preparing a steel raw material including C: 0.02 to 0.08%, Nb: 0.03 to 0.10%, Ti: 0.005 to 0.05% and a properly adjusted amount of Si, Mn, P, S, Al, N, and then applying a rough rolling process, a finish rolling process and a winding-up process. After the rough rolling process and before the finish rolling process or during the finish rolling process, acceleration quenching is applied which quenches rapidly a surface layer at a cooling speed of 50°C/s or more until it reaches the temperature below the Ar<SB POS="POST">3</SB>transformation point, and then this acceleration quenching is stopped. Thereafter, the finish rolling is applied at a condition that the reduction ratio per one pass is limited to (1.1× uniform elongation)% or below. Thereby, a high tension hot-rolled steel sheet excellent in the surface quality and excellent in toughness, especially the ductile crack propagation property can be manufactured. <P>COPYRIGHT: (C)2013,JPO&INPIT |
