The high intensity which is superior in waterproof element embrittlement, weldability and the hole expansion property the black sheet and its production mannered

摘要

PROBLEM TO BE SOLVED: To simultaneously improve hydrogen embrittlement resistance, weldability, hole-expandability, and ductility of a high-strength steel sheet having a tensile strength of 800 MPa or higher.;SOLUTION: This steel sheet comprises, by mass%, specified amounts of C, Si, Mn, P, S, Al and N, specified amounts of one or more elements of Ni, Cu, Cr, Mo and Nb, and the balance being iron with unavoidable impurities; includes, by an area rate, 70% or more one or both of bainite and bainitic in total, and less than 3% retained austenite (V_γ); has a tensile strength (TS) of 800 MPa or higher; and satisfies the expressions (1-1) to (1-3): (3.0Nb+2.5Mo+1/10Si+Mn)-(2C^0.5+2)0 --- (1-1), 0≤0.8×{2Cu+20Mo+3Ni+Cr+V_γ}-{0.1-3.5×10⁷×(TS)^-3.1} --- (1-2), and 0Si+Al+7.67C-1.78 --- (1-3).;COPYRIGHT: (C)2005,JPO&NCIPI