Dislocation and twinning mechanisms for dynamic recrystallization of as-cast Mn18Cr18N steel

Hot deformation behavior and microstructure evolution of as-cast Mn18Cr18N austenitic stainless steel were studied by uniaxial compression experiments at temperatures from 1123 K to 1473 K under strain rates of 0.001-1 s−1 up to the true strain of 0.69. The true stress-strain curves, characterized by hardening and subsequent softening, varied with temperatures and strain rates. By multiple linear regressions of the flow stress-strain data, the hyperbolic sine constitutive equation for the steel was developed and the hot deformation activation energy is calculated to be 672 KJ mol−1, which implies that the recrystallization is sluggish during hot deformation. Furthermore, two dynamic recrystallization mechanisms, dislocation and twinning, were demonstrated. At higher temperatures and lower strain rates, the dynamic recrystallization was controlled by dislocation slip and climbing. The fully recrystallized grains, composed of sub-grains and various dislocation configurations, can be obtained at 1473 K and 0.001 s−1 up to the true strain of 0.69. However, the dynamic recrystallization was controlled by twinning at higher strain rates and lower temperatures. Most of recrystallized grains are composed of twins, micro-twins and dislocation cells in twins. It suggests that twinning plays an important role during nucleation and progress of DXR by inducing separation of bulged grain boundaries and accelerating the expansion of migratory boundaries. The uniformly refined recrystallized grains with a large number of Σ3 twins were obtained at 1373 K and 1 s−1 up to the true strain of 0.69.