Change of Microduplex Structure and Its Refinement by Hot Compressive Deformation in (α+γ) Two-Phase Stainless Steel

The change of microstructures in Fe-24%Cr-7%Ni (ferrite (α)+austenite (γ)) two-phase stainless steel during hot compressive deformation at the corresponding temperature of (α+γ) two phases was investigated. Two kinds of specinnens with different microstructures are prepared for the hot compressive deformation. One has a microduplex structure which consists of fine α subgrains and γ grains. The other has an α single phase structure which was deformed to the reduction of 80% in thickness by cold rolling and compression. In the former case, high angle α grain boundaries appear in the early stages of compressive deformation at 1173 K under a strain rate of 1.1×10−3 s−1, due to local recrystallization of α subgrains. Misorientation angles among the α grains increase with increasing compressive deformation. Such a microduplex structure with high angle grain boundaries is presumed to be responsible for superplastic deformation at the high temperature. In the latter case, the recovery in the α matrix and the precipitation of the γ phase occur simultaneously, during compressive deformation at 1173 K under a strain rate of 2.2×10−2 s−1. Since the specimen deforms heterogeneously, in the areas where large strain is stored the recovery occurs rapidly and the ultra-fine α subgrains are formed during hot compressive deformation. Then, at those sub-boundaries, γ particles predominantly nucleate. Consequently, an ultra-fine microduplex structure with average grain size of 0.5 μm is formed.