Cold rolled dual-phase structure steel sheet having an excellent deep drawability and a method of manufacturing the same

摘要

A cold rolled dual-phase structure steel sheet having an excellent deep drawability is disclosed, which consists of ferrite phase and low temperature transformation product phase and comprises 0.001-0.008 wt % of C, not more than 1.0 wt % of Si, 0.05-1.8 wt % of Mn, not more than 0.15 wt % of P, 0.01-0.10 wt % of Al, 0.002-0.050 wt % of Nb and 0.0005-0.0050 wt % of B provided that the value of Nb (%)+10B (%) is in a range of 0.010-0.080%, and, if necessary, 0.05-1.00 wt % of Cr and the balance being substantially Fe with inevitable impurities. This steel sheet is manufactured by hot and cold rolling a steel slab with the above chemical composition and continuously annealing the resulting steel sheet in such a manner than the steel sheet is heated and soaked at a temperature from alpha -> gamma transformation point to 1,000 DEG C. and then cooled at an average cooling rate of not less than 0.5 DEG C./sec but less than 20 DEG C./sec in a temperature range of from the soaking temperature to 750 DEG C., and subsequently at an average cooling rate of not less than 20 DEG C./sec in a temperature range of from 750 DEG C. to not more than 300 DEG C.