Achieving low-temperature superplasticity in a cold-rolled medium Mn steel with an equilibrium ultrafine equiaxed dual-phase microstructure

Abstract

The present study reports an equilibrium ultrafine equiaxed dual-phase microstructure to regulate and achieved the low deformation temperature and high hot ductility balance in a novel 3.6 Al medium Mn steel. The cold-rolled specimens were subjected to high-temperature tensile tests and microstructure characterization with deformation temperature as a variable. The results showed that during the hot tensile process, the initial ultrafine martensite grains were transformed into ferrite + austenite equiaxed dual-phase grains, accompanied by dynamic recovery and recrystallization. At the deformation conditions of 625°C and 5 × 10-4 s-1, the accumulation of dislocations is obvious and the grain refinement is caused significantly. The reason for its failure is the development of the neck. The maximum elongation of 880% was obtained at 675°C and 5 × 10-4 s-1 due to the dynamic balance between ‘the grain refinement and strain hardening caused by dislocation creep and accumulation’ and ‘the grain coarsening and softening caused by dynamic recovery and recrystallization’. When the deformation temperature continues to increase to 775 °C, the elongation sharply decreases up to 491%. The stress concentration at the phase boundary induces cavity behavior, which is the main reason for the premature failure at 775 °C.