Work hardening behavior of ultrafine-grained Mn transformation-induced plasticity steel

Ultrafine grain refinement by intercritical annealing at 680 °C and 640 °C was investigated in a Fe-0.05%C-6.15%Mn-1.4%Si multiphase TRIP steel. A large volume fraction of retained austenite was obtained at room temperature in both cases. Whereas a pronounced localization of the deformation during tensile testing appeared in the steel annealed at 640 °C, strain localization occurred only in the initial deformation stages in the steel annealed at 680 °C. The retained austenite was transformed to strain-induced martensite during tensile testing in the sample annealed at 680 °C. However, no martensitic transformation was observed in the sample annealed at 640 °C. The activation volume showed a sharp decrease during the tensile test and saturated to the same value in both cases. Two different dislocation structures were observed in the ferrite grains of the samples annealed intercritically at 680 °C after tensile deformation, but only the dislocation-free structure of ferrite was observed in the sample annealed at 640 °C.