HIGH-STRENGTH COLD-ROLLED STEEL SHEET AND PROCESS FOR MANUFACTURING SAME

摘要

The present invention addresses the problem of providing a high-strength cold-rolled steel sheet which has a tensile strength (TS) of 1180MPa or more and exhibits improved elongation, stretch flangeability and bendability by adjusting the metal structure of a cold-rolled steel sheet which does not contain any expensive alloying element. In order to solve the problem, this high-strength cold-rolled steel sheet has a specific composition and a structure which comprises, in volume fraction, 40 to 60% of ferrite, 10 to 30% of bainite, 20 to 40% of tempered martensite, and 5 to 20% of retained austenite and in which tempered martensite phases having major-axis lengths of 5mum or less account for 80 to 100% of the total volume fraction of the tempered martensite.