Effect of tempering and partitioning (T&P) treatment on microstructure and mechanical properties of a low-carbon low-alloy quenched and dynamically partitioned (Q-DP) steel

We performed a direct quenching and dynamical partitioning (Q-DP) process, following intercritical annealing, to a low-carbon low-alloy steel. Performing an additional tempering treatment on the Q-DP steel, we investigated the carbon partitioning behavior upon tempering. We emphatically analyzed the effect of tempering and partitioning (T&P) treatment on the microstructure evolution and mechanical properties of the Q-DP steel, especially on carbon partition, retained austenite stabilization, TRIP effect and work hardening. Results show that the synergetic partition of carbon and Mn from ferrite to austenite during intercritical annealing and the dynamical partition of carbon upon quenching jointly achieve the stabilization of retained austenite in the Q-DP sample. The actual volume fraction of retained austenite is larger than the theoretical calculating value, which is attributed to the dynamical partition of carbon. The T&P treatment at 300 °C for 15 min does not result in obvious decomposition of retained austenite, but leads to apparent carbon partition from martensite to retained austenite. The carbon partition upon tempering enhances the mechanical stability of retained austenite, thus weakening the TRIP effect during tensile deformation and promoting the TRIP effect to occur at higher true strain level. The much higher work hardening rate at the late deformation stage near necking, for the quenched tempered and partitioned (Q-T&P) sample, leads to a higher uniform elongation, compared with the Q-DP sample. The existence of carbon-enriched stable retained austenite after necking happens finally enhances the post necking elongation of the steel.