Process for producing cold rolled steel strip having excellent mechanical strength and useful for motor vehicles

A process for producing a high strength cold rolled steel strip useful for motor vehicles, comprising the steps of:

• preparing a steel slab comprising
• 0.008 to 0.020% by weight of carbon,
• 0.01 to 0.45% by weight of manganese,
• 0.05 to 0.10% by weight of phosphorus,
• 0.005 to 0.050% by weight of acid-soluble aluminum,
• and the balance consisting of iron and unavoidable impurities in which nitrogen is limited to a content of 40 ppm or less;
• heating said steel slab to a temperature of 1200.degree. C. or less;
• hot rolling said heated steel slab at a temperature not lower than the Ar.sub.3 point of said steel;
• descaling said resultant hot rolled steel strip;
• cold rolling said descaled steel strip at a rolling reduction of 65% or more;
• continuously annealing said cold rolled steel strip by heating it to a temperature of from 700.degree. to 900.degree. C., by soaking the steel strip for 20 seconds to 3 minutes and by cooling the steel strip at a cooling rate of from 5.degree. C./sec to 30.degree. C./sec in the temperature range of 650.degree. C. or more and then, at a cooling rate of more than 30.degree. C./sec in the temperature range of below 650.degree. C.;
• overaging said annealed steel strip at a temperature of from 320.degree. to 450.degree. C. for 1 to 10 minutes;
• cooling said overaged steel strip to the ambient temperature, and;
• temper-rolling said cooled steel strip at the ambient temperature.