| 摘要 |
PROBLEM TO BE SOLVED: To provide a method for improving an extensive flange characteristic in a DP steel sheet excellent in fatigue-resistance characteristic. SOLUTION: The steel sheet has a composition containing 0.03-0.2% C, 0.5-3% Mn, 0.01-0.05% Al, and limited to≤2.5% Si,≤0.02% P,≤0.01 S,≤0.01% N and the balance Fe with inevitable impurities. After reheating the steel sheet at≥Ac<SB>3</SB>point to≤1,300°C, the steel sheet is hot-rolled at Ar<SB>3</SB>point to (Ar<SB>3</SB>point+100)°C as the finish temperature, and the rolled steel sheet is cooled to 600 to 800°C at the average cooling speed of <14°C/s, and further, cooled to the room temperature to≤450°C at the average cooling speed of≥14°C/s. Further, a bending-processing is performed by 30-180°contacting angle with roll in which the ratio of the obtained steel sheet thickness t[mm] and the roll radius r[mm] fill 15≤r/t≤50, at least in one time, and successively, a heat treatment is performed at 440 to 550°C for 1 to 10s. COPYRIGHT: (C)2008,JPO&INPIT
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