Microstructure–Mechanical Property Relationship and Austenite Stability in Transformation-Induced Plasticity Steels: Effects of Quenching and Partitioning Processing and Quenching and Tempering Treatments

We elucidate here the underlying reason for the differences in mechanical properties of cold-rolled lightweight medium manganese TRIP steel subjected to two unique heat treatments with the objective to optimize the mechanical properties. Furthermore, the relationship between the mechanical properties of the material and the stability of austenite was explored. After quenching and tempering (Q and T) treatment, tensile strength and total elongation of the experimental steel were 942 ± 11-1380 ± 22 MPa and 12.5 ± 1.5-52.5 ± 2.1%, respectively. While, the tensile strength and total elongation of experimental steel after quenching and partitioning (Q and P) treatment were in the range of 890 ± 10-1470 ± 24 MPa and 5 ± 0.8-47.4 ± 1.7%, respectively. The primary reason for the difference in mechanical properties is that the volume fraction of austenite in Q and T steel was higher, and the degree of TRIP effect was significantly greater. Interestingly, when the volume fraction of austenite was similar, the elongation of steel after Q and P treatment was greater. Short-time heat treatment results in non-uniform distribution of Mn in austenite grain. The Mn content at the austenite grain boundary regions is higher than that inside the grain, which leads to low stability of austenite. Austenite transforms in the short time during deformation, which cannot provide strong TRIP effect, resulting in low elongation. However, the diffusion time of Mn in the steel heat treated by Q and P is relatively long, the diffusion distribution of Mn in austenite is more uniform, and the stability of austenite is enhanced, which can delay the emergence of TRIP effect during deformation, and provides strong work hardening ability and increases elongation.