The effect of double intercritical annealing on micromechanical behavior of a medium Mn transformation induced plasticity steel investigated by in situ high energy X-ray diffraction

Abstract

The micromechanical behavior of a medium Mn transformation induced plasticity (TRIP) steel processed by intercritical annealing (IA) and double intercritical annealing (DIA) has been investigated by using in situ high energy X-ray diffraction during uniaxial tensile deformation. The results show that the ultimate tensile strength (UTS) and total elongation are 777.1 MPa and 39.6 % for IA and 953.6 MPa and 59.1 % for DIA, and that the product of UTS and total elongation is 30.8 GPa·% for IA and 56.4 GPa·% for DIA, respectively. Discontinuous phase transformation from austenite to martensite leads to Portevin-Le Châtelier (PLC) band propagation for both IA and DIA. The pronounced TRIP effect takes place in DIA step by step during deformation and contributes to higher strain hardening, which results in almost doubling the product of UTS and total elongation. The stepwise phase transformation highly corresponds to the lattice strain evolution of ferrite and austenite, indicating that load partitioning occurs during the deformation. The change of grain structure generated by different heat-treatments correlate with the load partitioning, thereby inducing large discrepancy in micromechanical behavior. The present investigation provides a deeper fundamental understanding of the effect of double inter-critical annealing on micromechanical behavior and effective engineering strategies for improving the product of UTS and total elongation for the medium Mn TRIP steels.