STEEL SHEET FOR PRESS FORMING WITH EXCELLENT DUCTILITY AFTER FORMING, ITS FORMING METHOD AND AUTOMOTIVE PARTS USING THE STEEL SHEET FOR PRESS FORMING

摘要

PROBLEM TO BE SOLVED: To provide a steel sheet for press forming having a high strength of 780 to 1,180 N/mm² class and excellent ductility after forming and its forming method and also to provide automotive parts using the steel sheet for press forming.;SOLUTION: A steel sheet which has a composition consisting of, by mass, 0.1 to 0.4% C, 0.001 to 3.0% Si, 1.5 to 4.0% Mn, ≤0.1% P, ≤0.05% S, 0.005 to 0.1% Al, ≤0.01% N and the balance Fe with inevitable impurities and also has a microstructure composed of ferrite and pearlite or of ferrite, cementite and pearlite is heated at 1 to 100°C/sec heating rate, held in a temperature region of 700 to 850°C for 10 to 6,000 sec and then press formed in a temperature region of 550 to 700°C. The resultant formed steel sheet has a structure consisting of, by area ratio after cooling, 40 to 90% ferrite as a main phase, 10 to 60% martensite as a second phase and the balance bainite.;COPYRIGHT: (C)2007,JPO&INPIT