Process for producing high-strength, low yield ratio and high ductility dual-phase structure steel sheets

In a process for producing a dual-phase structure steel sheet comprising hot rolling a steel containing 0.03 to 0.12% C, not more than 0.8% Si, and 0.8 to 1.7% Mn with the balance being iron and unavoidable impurities, and continuously annealing the hot rolled steel sheet in a range from 730.degree. to 900.degree. C., improvements comprising cooling the continuously annealed steel sheet under the following conditions:

• (1) 1.degree. C./second.ltoreq.R.sub.1 .ltoreq.30.degree. C./second
• wherein R.sub.1 represents an average cooling rate from the continuous annealing temperature down to an intermediate temperature T.degree.C. in the cooling process
• (2) 4.degree. C./second.ltoreq.R.sub.2 .ltoreq.100.degree. C./second
• wherein R.sub.2 represents an average cooling rate from T.degree.C. to a temperature not higher than 200.degree. C.
• (3) R.sub.1 <R.sub.2
• and
• (4) 420.degree. C..ltoreq.T.ltoreq.700.degree. C.
• to obtain high strength, low yield ratio of not over 0.6 and high ductility dual-phase structure steel sheet having a structure composed mainly of a ferrite phase and a rapidly cooled transformation phase and having excellent formability with a tensile strength of 40 kg/mm.sup.2 or higher.