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Abstract

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: (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; (2) preparing steel ingots by continuous casting the melting steel; (3) hot rolling the steel ingots to hot-rolled bands; (4) coiling the hot-rolled bands at temperature ranging from 560.degree. C. to 720.degree. C.; (5) after cold rolling, soaking the steel sheets at temperature ranging from 780.degree. C. to 900.degree. C. for less than five minutes to proceed intercritical (ferrite plus austenite) annealing treatment; (6) gradually cooling the steel sheets in the air to temperature ranging from 650.degree. C. to 750.degree. C.; and (7) cooling the steel sheets to temperature ranging from 200.degree. C. to 400.degree. C. by roller-quenching at the cooling rate ranging from 50.degree. C./sec to 400.degree. C./sec to proceed overageing treatment for a time duration ranging from 1 minute to 6 minutes.

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