Cold rolled steel flat product and method for its production

Cold reduced flat steel product comprises perlite and bainite structure comprising 20-40 vol.% of martensite, 2-15 vol.% of retained austenite and residual ferrite. The flat steel product is made of a steel consisting of 0.12-0.19 wt.% of carbon, 1.5-2.5 wt.% of manganese, 0.60-1 wt.% of silicon, = 0.1 wt. of aluminum, 0.2-0.6 wt.% of chromium, 0.05-0.15 wt.% of titanium, and remaining iron and unavoidable impurities, and has an elongation at rupture (A80) of 15%, a tensile strength of at least 880 MPa, a yield strength of at least 550 MPa, and hole expansion ratio of more than 6%. An independent claim is included for producing the cold-rolled flat steel product comprising Casting a steel melt containing of carbon, manganese, silicon, aluminum, chromium, titanium and remaining iron and unavoidable impurities to form an intermediate in which there is a slab or thin slab, heating the intermediate at an austenitizing temperature of 1100-1300[deg] C, hot rolling the heated intermediate to form a hot strip at a temperature of 850-960[deg] C, cooling the hot strip at a coiling temperature of 500-650[deg] C, rolling the cooled strip using a roller, optionally pickling the hot-rolled strip, cold-rolling the hot-rolled strip into a cold-rolled flat steel product having cold rolling ratio of at least 30%, continuously annealing the cold-rolled steel flat product at a temperature of 750-900[deg] C for 80-300 seconds, and then cooling in two stages, aging the flat steel product at an aging temperature of 100-400[deg] C for 210-710 seconds, cooling the flat product to room temperature, temper-rolling the flat steel product with a roll of 0.2-2%, and optional coating the flat steel product with a metallic protective layer, where the step of cooling the cold-rolled steel flat product includes in the first stage, cooling the product with a cooling rate of 8-100[deg] C/sec to a temperature of 450-550[deg] C and in the second stage, cooling from the intermediate temperature at a cooling rate of 2-100[deg] K/second to a temperature of 350-450[deg] C.