- - METHOD FOR PRODUCING A COLD-ROLLED FLAT STEEL PRODUCT WITH HIGH YIELD STRENGTH AND FLAT COLD-ROLLED STEEL PRODUCT

摘要

The invention provides a method for producing a cold-rolled steel strip having a yield ratio (Re / Rm) of at least 0.7, wherein the cold-rolled steel product comprises, in addition to iron and unavoidable impurities, C: 0.1 to 1.5% of Cr, less than 0.02% of N, less than 0.03% of P, less than 0.05% of S, 0.05 to 1.00% of Si, 1.0 to 3.0% Mo, Nb, V, and B "under the conditions of Ti: not more than 0.15%, Mo: less than 2%, Nb: less than 0.1%, V: less than 0.12% And a steel containing one or more elements from the group. According to the present invention, the provided cold-rolled plate-shaped steel products are further heat treated and annealed during annealing for 4.5 to 24 hours at an annealing temperature of 150 ^° C to 400 ^° C. Wonderfully, as a result of such a long-time annealing performed at a relatively low temperature, a remarkable increase in the yield strength Re and an improvement in hole expandability occur. The present invention comprises a steel comprising the above-mentioned steel and comprising at least two phases, comprising at least 10% by volume of tempered martensite, less than 10% by volume of bainite, less than 10% by volume of retained austenite, Wherein the steel has a yield ratio (Re / Rm) of at least 0.7, a tensile strength (Rm) in excess of 750 MPa and a hole expandability of at least 18% (LA).