| 摘要 |
A high-strength cold-rolled steel sheet with dual-phase structure, recently developed by the Japanese steel producers have a low yield ratio of approximately 0.6 or lower, is free of the yield-point elongation and is of excellent press-formability. One of the production methods of this steel is a continuous annealing at a temperature range of two-phase structure of ferrite ( alpha )+austenite ( gamma ). In order to improve the formability, the artificial-aging hardenability after forming, yield ratio and ductility over those of the known dual-phase steel, the present invention provides a method for producing a dual-phase steel, which comprise a step of cooling from the annealing temperature to a temperature not higher than 200 DEG C. at an average cooling rate (R1) in the range of 1 DEG C./second>/=R1</=30 DEG C./second over the primary cooling stage from the annealing temperature down to an intermediate temperature (T) in the range of 420 DEG C.</=T</=700 DEG C., and at an average cooling rate (R2) in the range of 100 DEG C./second</=R2</=300 DEG C./second over the secondary cooling stage from the intermediate temperature (T) down to the temperature not higher than 200 DEG C. |
