Bainitic transformation during the two-step Q&P process in a lean medium Mn steel containing silicon

Lean medium Mn Quenching and Partitioning (Q&P) steels with Mn contents up to 5 wt-% have recently gained a lot of interest due to their promising combination of strength and ductility. This steel group is characterized by a microstructure consisting of a carbon-depleted tempered martensitic (α'') matrix and a considerable amount of retained austenite (RA) stabilized by both, C and Mn, in order to ensure a sufficient strain-induced austenite to martensite transformation (TRIP-effect). Furthermore, the hard α'' matrix contributes to the excellent performance of these steels in bending, sheet cutting and hole expansion operations. In this contribution, the impact of the Q&P process on the transsformation behavior of a 0.2C-4.5Mn-1.5Si lean medium Mn steel is presented and discussed in detail. This includes dilatometric experiments examining the influence of the Q&P process parameters on the phase transformations, light optical and scanning electron microstructural (LOM, SEM) investigations, as well as hardness measurements according to Vickers. Moreover, the Q&P annealing approach was compared to the TRIP-assisted bainitic ferrite (TBF) process, in order to evaluate the influence of primary martensite (α'_initial) being present in the Q&P samples at the onset of isothermal bainitic transformation (IBT) on the transformation kinetics. With increasing quench temperature (T_Q) the amount of α'' significantly decreased, whereas the phase fraction of bainitic ferrite (α_B) and RA continuously increased, leading to the formation of fresh martensite (α'_final) and the sharp decrease of the RA fraction at T_Q exceeding 250^°C. Furthermore, with increasing partitioning temperature (T_P) a decreasing amount of α_B, associated with an increasing amount of α'_final, could be examined. The comparison of the Q&P and TBF process in terms of transformation behavior manifested a pronounced influence of the presence of α'_initial, since the IBT was intensively accelerated in the Q&P samples.