Abstract: There is provided an ultrahigh strength steel sheet containing 0.10 to 0.40 mass % of C, 0.01 to 3.5 mass % of Cr, at least one selected from the group consisting of 0.10 to 2.0 mass % of Mo, 0.20 to 1.5 mass % of W, 0.002 to 1.0 mass % of V, 0.002 to 1.0 mass % of Ti and 0.005 to 1.0 mass % of Nb, 0.02 mass % or less of P and 0.01 mass % or less of S as impurities and the balance being Fe and unavoidable impurities based on the total mass of the steel sheet and having a base structure of lower bainite, a prior austenite grain size of 30 ?m or smaller and a tensile strength of 980 MPa or higher. There is also provided an automotive strength part using the ultrahigh strength steel sheet.

Abstract: There is provided a high-tensile steel sheet containing 0.15 to 0.25 mass % of C, 0.1 to 2.0 mass % of Si, 0.10 to 1.0 mass % of Mn, 0.5 to 3.5 mass % of Cr, 0.02 mass % or less of P, 0.01 mass % or less of S and the balance being Fe and unavoidable impurities based on the total mass of the steel sheet and satisfying a content ratio of Mn/(Mn+Cr)<0.50.