| 摘要 |
PROBLEM TO BE SOLVED: To provide an ultrahigh strength steel sheet combining excellent formability and delayed fracture resistance, and to provide a strength part for an automobile utilizing the same. SOLUTION: An ultrahigh strength steel sheet comprising molybdenum, tungsten, vanadium, titanium, niobium or the like, wherein the base structure thereof is annealed martensite, the grain diameter of prior austenite is 30μm or less and the tensile strength thereof is 980 MPa or greater. The steel sheet comprises 0.10 to 0.40% C, 0.01 to 3.5% Cr, 0.10 to 2.0% Mo, 0.20 to 1.5% W, 0.002 to 1.0% V, 0.002 to 1.0% Ti, 0.005 to 1.0% Nb or the like, and comprises 0.02% or less P and 0.01% or less S and the balance of Fe and unavoidable impurities, wherein the average grain diameter of prior austenite is 3 to 10μm. Further, there is provided a strength part for an automobile utilizing the ultrahigh strength steel sheet. COPYRIGHT: (C)2008,JPO&INPIT
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