Method for producing a dual-phase steel sheet having excellent formability, high artificial-aging hardenability after forming, high strength, low yield ratio, and high ductility

A method for producing a dual-phase steel sheet mainly composed of a ferrite phase and at least one rapidly-cooled transformed phase selected from the group consisting of a martensite phase, a bainite phase and a retained austenite phase, and having a tensile strength not lower than 40 kg/mm.sup.2, excellent formability and high artificial-aging hardenability after forming, comprising the steps of:

• hot rolling a steel containing from 0.01 to 0.12% carbon and from 0.7 to 1.7% manganese, followed by coiling;
• continuously annealing the steel sheet, which has undergone the hot rolling, at an annealing temperature in the range of from 730.degree. to 900.degree. C., and;
• cooling in two successive stages from the annealing temperature to a temperature lower than 200.degree. C. at an average primary cooling rate (R.sub.1) in the range of 1.degree. C./second<R.sub.1 <30.degree. C./second over a primary cooling stage from said annealing temperature down to an intermediate temperature (T) in the range of 420<T<700.degree. C., and at an average secondary cooling rate (R.sub.2) in the range of 100.degree. C./second<R.sub.2 <300.degree. C./second over a secondary cooling stage from the intermediate temperature (T) down to a temperature not higher than 200.degree. C., said secondary average cooling rate (R.sub.2) being high enough within its said range so as to provide the cooled steel sheet with said excellent formability and high artificial-hardenability after forming.