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 with a yield ratio Re/Rm of at least 0.7, the cold-rolled steel product consisting of a steel comprising iron, unavoidable production 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: less than 0.02%, P: less than 0.03%, S: less than 0.05% and optionally one or more elements from the group "Ti, Mo, Nb, V, B" subject to Ti: being up to 0.15%, Mo: less than 2%, Nb: less than 0.1%, V: less than 0.12%, and B: 0.0005 - 0.003%. According to the invention the cold-rolled flat steel product provided undergoes additional heat treatment, during which it is annealed for an annealing time of 4.5 - 24 hours at an annealing temperature of up to 150 - 400°C. It has surprisingly transpired that as a result of such long-time annealing carried out at comparatively low temperatures a dramatic increase in the yield strength Re and an improvement in the hole expansion occurs. The invention also provides a cold rolled flat steel product consisting of the steel mentioned before and having a structure comprising at least two phases, which (in vol.-%) contains at least 10% tempered martensite, less than 10 % bainite, less than 10 % residual austenite and as the remainder ferrite, the steel having a yield ratio Re/Rm of at least 0.7, a tensile strength R of more than 750 MPa and a hole expansion LA of at least 18%.