Process for producing cold-rolled high strength steel sheet having

Abstract

A process for producing a cold-rolled high strength steel sheet having an excellent formability and conversion-treatability, comprising the steps of: heating to a temperature of 1150.degree. C. or lower, a steel slab comprising; 0.005 wt % or less of C, 0.5-1.5 wt % of Si, 0.1-0.5 wt % of Mn, 0.003-0.05 wt % and at least 3.4.times.N wt % of Ti, 0.003-0.05 wt % and at least 7.8.times.C wt % of Nb, 0.01-0.1 wt % of Al, 0.05-0.15 wt % of P, 0.010 wt % or less of S, 0.005 wt % or less of N, 0.0001-0.0050 wt % of B, the balance consisting of Fe and unavoidable impurities; hot-rolling the steel slab with a finishing rolling temperature of the A.sub.r3 point of the steel or higher to obtain a hot-rolled steel sheet; cooling the hot-rolled steel sheet at a cooling rate of 10.degree. C./sec or more in the stage between said finishing rolling and the following coiling step, with the provision that said cooling is started not later than 2 sec after the hot-rolling is finished and is continued for a duration of at least 3 sec; coiling the steel sheet at a temperature of 650.degree. C. or higher; cold-rolling the steel sheet at a usual rolling draft; and continuous-annealing the cold-rolled steel sheet at a heat-cycle of holding for 1 sec or longer at a temperature of 930.degree. C. or lower to complete the recrystallization of the cold-rolled steel sheet.