| 摘要 |
PROBLEM TO BE SOLVED: To provide a high strength steel sheet which is used for automotive room structural parts and hardly causes stretch-flange rupture at press working and can suppress the deformation of the room at collision and also to provide its manufacturing method. SOLUTION: The steel sheet has a composition consisting of, by weight, 0.05-0.17% C, 0.5-1.5% Si, 0.8-2.4% Mn, <=0.02% P, <=0.004% S, 0.01-0.1% Al, <=0.006% N, 0.0005-0.004% Ca and the balance Fe with inevitable impurities and has a structure containing >=30% ferrite by volume fraction, >=10% bainite by volume fraction and <6%, in total, of martensite and austenite by volume fraction. In the steel sheet: average grain size of ferrite is <=22μm; the volume fraction of carbides of >=0.1μm circle-equivalent radius is <=0.1%;, stretch-flange formability is >=80% by bore expanding ratio; upper yield stress at 500/s strain rate is >=1.5 times the quasi-static tensile strength; and strength is >=490 MPa.
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