ULTRAHIGH STRENGTH STEEL SHEET AND STRENGTH PART FOR AUTOMOBILE UTILIZING THE SAME

摘要

An ultrahigh strength steel sheet comprising, based on the mass of steel sheet, at least one member selected from the group consisting of 0.10 to 0.40 mass% C, 0.01 to 3.5 mass% Cr, 0.10 to 2.0 mass% Mo, 0.20 to 1.5 mass% W, 0.002 to 1.0 mass% V, 0.002 to 1.0 mass% Ti and 0.005 to 1.0 mass% Nb, impurities of 0.02 mass% or less P and 0.01 mass% or less S and the balance of Fe and unavoidable impurities, wherein the base structure thereof is a lower bainite, or annealed lower bainite, or annealed martensite, and wherein the grain diameter of prior austenite is 30 µm or less and the tensile strength thereof is 980 MPa or greater. Further, there is provided a strength part for automobile utilizing the ultrahigh strength steel sheet.