Superplasticity of as rolled duplex stainless steel.

Superplasticity of a microduplex stainless steel with a relatively small amount of elongated austenite phases is studied. Specimens used for this study were prepared by different manufacturing histories: one was an as hot-rolled strip worked by a planetary mill which can provide a very high rolling speed (up to 170s-1) followed by water cooling, the other was an as cold-rolled strip. High temperature tensile tests and microstructural observations were conducted.In spite of the characteristic microstructure that was not considered as suitable for superplasticity, under a certain condition both the as hot-rolled and as cold-rolled plates showed superplastic behavior. Observed total elongation was over 500% in the strain rate range between 1.67×10-3 and 8.33×10-3s-1. Elongation above 3000% was obtained under a specific condition. The m-values in the superplastic state were larger than 0.3. In the relation between total elongation and initial strain rate, at least two ductility peaks were shown. This seemed to be related to the superplastic deformation mechanism of the steel. From the microstructural observation and the estimation of activation energy for superplastic flow, it was considered that sigma phase transformation contributed to the grain refinement which was accelerated by accumulated strain induced by water cooling or cold working.