METHOD FOR MANUFACTURING VERY HIGH STRENGTH, COLD-ROLLED, DUAL PHASE STEEL SHEETS, AND SHEETS THUS PRODUCED

Cold-rolled and annealed steel sheet comprises (in wt.%): carbon (0.055-0.095); manganese (2-2.6); silicon (0.005-0.35); sulfur (= 0.005); phosphorus (= 0.05); aluminum (0.1-0.3); molybdenum (0.05-0.25); chromium (0.2-0.5); nickel (= 0.1); niobium (0.01-0.04); titanium (0.01-0.05); boron (0.0005-0.0025); and nitrogen (0.002-0.007), where the rest of the composition is iron and impurities. An independent claim is included for a process for preparation of the steel sheet comprising pouring the steel in the form of semi-product, carrying the semi-product at a temperature of 1150-1250[deg] C, hot rolling of the semi-finished product with a temperature of end-rolling of greater than argon (Ar3) to obtain a hot rolled product, coiling the hot-rolled at a temperature of 500-570[deg] C, cleaning the hot rolled product, making cold rolling with a reduction rate of 30-80% to obtain cold rolled product, heating the cold rolled product having a speed of 1-5[deg] C/s and annealing temperature including Ac1+40[deg] C-Ac3-30[deg] C , (where Ac1 is initial temperature of allotropic transformation, and Ac3 is final temperature of allotropic transformation) for 30-300 seconds to obtain a heated and annealed product with a structure comprising austenite and cooling the product to a temperature below the initial temperature of formation of martensite (M s) temperature with sufficient speed so that austenite is completely transformed to martensite.