The Temperature Dependence of Superplasticity in Cold‐Rolled Medium Manganese Steel: Achieving 1001% Elongation at 700 °C

The effect of deformation temperature on the microstructural evolution and damage fracture behavior of medium manganese steel is investigated. The results indicate that superplastic deformation accelerates the overall microstructural evolution process of the samples compared to those subjected to isothermal annealing. Grain refinement in the S700 sample occurs due to the combined effects of dynamic recrystallization and austenite reverse transformation during the early stages of superplastic deformation. Grain growth occurs slowly due to the sluggish uphill partition mechanism of Mn and Al atoms and the hindrance of boundary migration mechanisms between dual phase when the true strain ε ≥ 1.25. The total elongation of the S700 sample reaches a peak value of 1001 ± 52%. The ductility of the S800 sample decreases due to the accelerated grain growth, which hinders grain boundary sliding. Due to stress concentration, the damage results indicate that the initial void nucleation sites are located at the polycrystalline boundaries of austenite and ferrite. Multiple voids coalesce into a crack that is parallel to the tensile direction. The high damage tolerance of the S700 sample is an important factor in its excellent superplastic deformation performance.