Method of producing dual phase structure cold rolled steel sheet

A method of producing a dual phase structure cold rolled steel sheet having tensile strength 35 to 50 kg/mm.sup.2, yield ratio less than 60% and high elongation comprising steps of hot rolling and cold rolling by conventional process a steel containing 0.01 to 0.05% C, less than 0.2% Si, 1.7 to 2.5% Mn, 0.01 to 0.10% Al, and at least one of 0.005 to 0.050% rare earth metal, 0.01 to 0.1% Zr, 0.001 to 0.02% Ca, less than 1.0% Cr, less than 0.5% Ni, less than 0.5% Mo, and 0.0005 to 0.0050% B with the balance being Fe and unavoidable impurities, holding the produced steel sheet for 20 seconds to 20 minutes at a temperature 720.degree. to 850.degree. C., and cooling the steel sheet at a cooling rate between 3.degree. and 30.degree. C. per second and also above a value (.degree.C. per second) as shown by following formulae:

• 12.times.[Mn(%)].sup.2 -62.times.[Mn(%)]+81.