A novel process combining thermal deformation and intercritical annealing to enhance mechanical properties and avoid Lüders strain of Fe-0.2C–7Mn TRIP steel

This paper proposed a novel process combining thermal deformation and intercritical annealing (IA) to enhance mechanical properties and avoid Lüders strain of Fe-0.2C–7Mn steel. The experiment results show that the prior microstructure, including prior austenite grain (PAG) size, dislocation density, carbides precipitation and stored deformation energy before IA have great impacts on the nucleation and growth of austenite during IA, and ultimately influence the morphology, grain size, volume fraction and stability of retained austenite (RA) after IA. The low temperature and relatively high strain rate deformation condition sample (referred as 1-LT/RHS) with film-like and block-like mixture morphology structure after IA exhibits the largest product of ultimate tensile strength and total elongation (UTS × TE) value (53 GPa∙%), due to the double-stage discontinuous transformation-induced plasticity (TRIP) effects. Block-like austenite dominates the 1st-stage plastic deformation, while film-like austenite dominates the 2nd-stage. Because the film-like austenite is more stable than the block-like, which is confirmed by the interrupted tensile tests and nanoindentation investigation. Besides, compared to the cold-rolled sample (referred as 3-CR) with single block-like structure after IA, the film-like ferrite in sample 1-LT/RHS initiate dislocation plasticity earlier than the block-like ferrite, resulting in the disappearance of Lüders strain. This is because the different formation mechanisms, leading to the initial dislocation density before deformation in film-like grains much higher than in block-like grains. Moreover, the procedure of this novel process is simple with low industrial cost, which can provide a new guidance for development of lower cost fabricating technology of medium Mn TRIP steel.