Bake-hardening cold-rolled steel sheet having dual-phase structure and process for manufacturing it

摘要

A bake hardening cold-rolled steel sheet which has good bake hardenability, good dent resistance, and low yield ratio which contributes to the shape-fixability of the steel sheet. The steel is suitable for the outer panel of an automobile. The process for manufacturing the steel sheets includes the following steps:P P(1) preparing a melting steel which contains 0.02 to 0.06% carbon by weight, 0.60% to 1.40% manganese by weight, 0.5% silicon by weight at most, 0.1% phosphorus by weight at most, 0.1% aluminum by weight at most, 0.01% nitrogen by weight at most, 0.1% titanium by weight at most, and 50 ppm of boron at most;PP(2) preparing steel ingots by continuous casting the melting steel;PP(3) hot rolling the steel ingots to hot-rolled bands;PP(4) coiling the hot-rolled bands at temperature ranging from 560° C. to 720° C.; PP(5) after cold rolling, soaking the steel sheets at temperature ranging from 780° C. to 900° C. for less than five minutes to proceed intercritical (ferrite plus austenite) annealing treatment;PP(6) gradually cooling the steel sheets in the air to temperature ranging from 650° C. to 750° C.; and P P(7) cooling the steel sheets to temperature ranging from 200° C. to 400° C. by roller-quenching at the cooling rate ranging from 50° C./sec to 400° C./sec to proceed overageing treatment for a time duration ranging from 1 minute to 6 minutes.