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Image Number 11 for United States Patent #4285741.
Process for producing a high strength, low yield ratio 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. The process comprises cooling the continuously annealed steel sheet under the following conditions:(1) 1.degree. C./second.ltoreq.R.sub.1 .ltoreq.30.degree. C./secondwherein 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./secondwherein 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.
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