DUAL PHASE-STRUCTURED HOT ROLLED HIGH-TENSILE STRENGTH STEEL SHEET AND A METHOD OF PRODUCING THE SAME

摘要

A dual phase-structured hot roiled steel sheet having a composition consisting of 0.03-0.15% by weight of C. 0.6-l.8% by weight of MN, 0.04-0.2% by weight of P, not more than 0.10% of A?, not more than 0.008% by weight of S, and the remainder being substantially Fe, and having a microstructure consisting of ferrite and martensite dispersed therein, the area fraction of said ferrite being at least 70% and that of said martensite being at least 5% at the section of the steel sheet, has a high tensile strength and a low yield ratio of not higher than 70%, and has excellent formability. The steel sheet can be produced in a simple manner by cooling directly a hot rolled sheet at an ordinary cooling rate without the use of a particular cooling pattern.