With high ductility, production mannered null mass of the ultrahigh strength cold rolling steel sheet which possesses the tensile strength above 780MPa which is superior in

摘要

PROBLEM TO BE SOLVED: To provide an ultrahigh strength cold rolled steel sheet having high ductility, excellent chemical conversion treatability and a 780 to 1,180 MPa class TS and also to provide its manufacturing method.;SOLUTION: The ultrahigh strength cold rolled steel sheet having high ductility, excellent chemical conversion treatability and a tensile strength of ≥780 MPa has a composition consisting of, by mass, 0.05 to 0.2% C, 0.5 to 2.0% Si, 1.5 to 3.5% Mn, 0.001 to 0.05% P, 0.0001 to 0.005% S, 0.005 to 0.08% Al, 0.001 to 0.01% N and the balance Fe with inevitable impurities. The steel sheet has a composite structure consisting of a ferrite phase and a tempered martensite phase, and the volume fraction of the tempered martensite phase is 20 to 70%, and further, a ratio between Si concentration Si(0) at the surface of the steel sheet and Si concentration Si(0.1) at a depth of 0.1 mm from the surface of the steel sheet, [Si(0)/Si(0.1)], ranges from 1.0 to 1.6 and the standard deviation of the ratio is ≤0.50.;COPYRIGHT: (C)2007,JPO&INPIT