HIGH-DUCTILITY STEEL SHEET EXCELLENT IN PRESS FORMABILITY AND STRAIN AGE HARDENABILITY, AND METHOD FOR MANUFACTURING THE SAME

摘要

A steel sheet composition contains appropriate amounts of C, Si, Mn, P, S, Al and N and 0.5 to 3.0% Cu. A composite structure of the steel sheet has a ferrite phase or a ferrite phase and a tempered martensite phase as a primary phase, and a secondary phase containing retained austenite in a volume ratio of not less than 1%. In place of the Cu, at least one of Mo, Cr, and W may be contained in a total amount of not more than 2.0%. This composition is useful in production of a high-ductility hot-rolled steel sheet, a high-ductility cold-rolled steel sheet and a high--ductility hot-dip galvanized steel sheet having excellent press formability and excellent stain age hardenability as represented by a ATS of not less than 80 MPa, in which the tensile strength increases remarkably through a heat treatment at a relatively low temperature after press forming.