Abstract
Characteristics of superplasticity in a duplex stainless steel produced by a direct strip casting (DSC) technique have been studied. The thickness of a strip is 1.2mm. The effect of temperature, initial strain rate and cold rolling is investigated. The results are compared with the same type of steel manufactured by the conventional process. An as-cast strip shows superplasticity at 1273K. The superplasticity is enhanced by cold reduction through recrystallization and sigma phase precipitation during heating. An as-cast strip followed by 50% cold reduction without annealing is superplastic at 1173K or higher temperatures. The obtained maximum elongation of cold rolled strip is 1029% when strained for a rate of 8.33×10-4s-1 at 1273K. The mechanism of superplasticity in these materials is discussed. The precipitation of fine austenite phases occurs during heating, and the straining at a high temperature causes grain rotation so that the microstructure is equiaxed. Rapid cooling by a pair of drums makes it possible to reduce compositional segregation. The effect of cold reduction is remarkable by acceleration of precipitation of both sigma phase and austenite phase to supply large grain or inter phase boundary area. The superplastic ductility by the sigma phase is enhanced at a relatively higher strain rate range than by austenite phase. This may be explained by that sigma phase precipitation occurs faster than recrystallization of austenite phases at heating and straining stage.
