HIGH-STRENGTH COLD-ROLLED STEEL SHEET SUPERIOR IN BENDING WORKABILITY AND DELAYED FRACTURE RESISTANCE, AND MANUFACTURING METHOD THEREFOR

摘要

PROBLEM TO BE SOLVED: To provide a cold-rolled steel sheet which has a tensile strength of 1,470 MPa or more and is superior in bending workability and delayed fracture resistance.;SOLUTION: This steel sheet comprises 0.15-0.20% C, 1.0-2.0% Si, 1.5-2.5% Mn, 0.020% or less P, 0.005% or less S, 0.01-0.05% Al, 0.005% or less N, 0.1% or less Ti, 0.1% or less Nb, 5-30 ppm B, and the balance Fe with unavoidable impurities; and has a metal structure in which the volume ratio of a tempered martensite phase is 97% or more, and the volume ratio of a retained austenite phase is less than 3%.;COPYRIGHT: (C)2010,JPO&INPIT