  Hot rolled commercial metastable austenitic steel 321 with strongly elongated
thin delta-ferrite particles in its microstructure was the object of
investigations. Ultrafine-grained (UFG) microstructure in steel 321 was formed
by Equal Channel Angular Pressing (ECAP) at 150 oC and 450 oC. When heating the
UFG steel specimens, the nucleation of sigma-phase particles blocking the grain
boundary migration was observed. The maximum elongation to failure (~250%) was
achieved at the deformation temperature 750 oC. The process of superplastic
deformation of the UFG steel 321 is controlled by simultaneous grain boundary
sliding and power-law creep. The contribution of each process depends on the
grain growth rate in the superplasticity regime as well as on defect
accumulation on the grain boundaries. The fracture of the UFG steel 321
specimens has a cavitational character - an intensive formation of large
elongated pores at the non-metallic particles as well as of the submicron pores
at the sigma-phase particles in the course of superplastic deformation were
observed.
