| 摘要 |
PROBLEM TO BE SOLVED: To homogeneously form a chemical film on the whole face of a high strength hot rolled steel sheet comprising Si, Ti and Nb, to apply it to the conventional production line, and to facilitate operational control and quality control as well. SOLUTION: The hot rolled steel sheet has a composition comprising, by mass, 0.01 to 0.1% C,≤1.5% Si, 0.5 to 3.0% Mn,≤0.07% P,≤0.01% S, 0.015 to 0.1% Al, 0.001 to 0.008% N and either or both of Ti and Nb by 0.02 to 0.4% in total, and satisfying C-(Ti/4)-(Nb/6)>0.002% and A.I. (Aging Index)≥1.5 MPa. In the hot rolling stage, after the completion of finish rolling, the steel sheet is cooled to a coiling temperature at the average cooling rate of≥50°C/s, and is coiled in the temperature range of 400 to 600°C so as to control the A.I. to≥1.5 MPa. COPYRIGHT: (C)2005,JPO&NCIPI
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