Grain Refining Behavior by Dynamic Recrystallization of As-Forged Austenitic Stainless Steel with High N

Flow behavior and microstructure evolution of as-forged austenitic stainless steel with high N were investigated by isothermal compression tests at 950-1200 °C with strain rate of 0.001-1 s⁻¹ and strain of 30-50% using Gleeble-3800 thermal mechanical simulation machine. The hyperbolic sine constitutive equation was established during isothermal compression with different parameters. The activation energy and stress exponent were 550 KJ mol⁻¹ and 4.715, which is lower than the value of as-casted austenitic stainless steel with high N and higher than other austenitic stainless steel such as 304 and 316. It indicates that the dynamic softening behavior is related to the original structure and material composition. The dynamic softening behavior and grain refining progress were studied employing microstructure analysis techniques. It was found by comparing with the as-casted microstructure, the dynamic recrystallization behavior for as-forged sample was more sensitive to strain rate and less affected by temperature, strain and original structure. This should mainly depend on the stacking fault energy related to the material composition. When strain rate is less than 0.1 s⁻¹, the dynamic softening is characterized by serration, bulging and strain-induced separation of original grain boundary. While strain rate is 1 s⁻¹, the nucleation and progress of dynamic recrystallization grain were related to formation and interaction of Σ3 twin boundaries. With the increase of strain, the recrystallization fraction increases linearly with the number of twin boundaries. The recrystallization fraction increased to 86.2% and the grain size was refined to 16.6μm depending on twinning mechanism by compressing to 50% at 1100 °C with strain rate of 1s^-1.