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

摘要

A method for producing a cold-rolled steel strip with a yield ratio Re/Rm of at least 0.7, the cold-rolled steel product including iron, unavoidable impurities and (in wt. %) C: 0.05-0.20%, Si: 0.25-1.00%, Mn: 1.0-3.0%, Al: 0.02-1.5%, Cr: 0.1-1.5%, N: <0.Q2%, P: <0.03%, S: <0.05% and optionally one or more of Ti, Mo, Nb, V, and B, Ti: up to 0.15%, Mo: <2%, Nb: <0.1%, V: <0.12%, and B: 0.0005-0.003%. The cold-rolled flat steel product undergoes heat treatment for 4.5-24 hours at a temperature of 150-400° C. Also, a cold rolled flat steel product discussed above having a structure including at least two phases, selected from (in vol. %) at least 10% tempered martensite, <10% bainite, <10% residual austenite and remainder ferrite, a yield ratio of at least 0.7, a tensile strength of ≧750 MPa and a hole expansion of at least 18%.