A phenomenon of strain induced bainitic transformation and its effect on strength enhancement in a lightweight transformation-induced-plasticity steel

Microstructure evolution, transformation behavior and mechanical property are examined for a Fe-0.35C-1.1Mn-4.1Al-0.38Si (in wt%) ferrite-based lightweight transformation induced plasticity (TRIP) steel under tensile deformation at room temperature (25 degrees C) to 450 degrees C. The influence of deformation temperature and strain on mechanical stability of retained austenite leads to the occurrence of various transformations in the TRIP steel. From room temperature towards 150 degrees C, strain induced martensitic transformation (SIMT) takes place resulting in the formation of twin-type martensite, and it is progressively suppressed with increasing deformation temperature. A phenomenon of strain induced bainitic transformation (SIBT) of retained austenite is observed at higher deformation temperatures, and it is enhanced with increasing deformation strain and temperature ranging from 150 degrees C to 300 degrees C. During the SIBT, the formation of carbide-free bainite with high dislocation density is primarily attributed to the increased carbon diffusivity. The occurrence of the SIBT reveals the significant importance of the diffusion-associated mechanism for the bainitic transformation. Above 300 degrees C, the activation of dislocation movement in retained austenite gradually replaces the SIBT with increasing deformation temperature. The highest tensile strength obtained at 300 degrees C indicates the strength enhancement due to the SIBT.