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 (TQ) the amount of α’’ significantly decreased, whereas the phase fraction of bainitic ferrite (αB) and RA continuously increased, leading to the formation of fresh martensite (α’final) and the sharp decrease of the RA fraction at TQ exceeding 250°C. Furthermore, with increasing partitioning temperature (TP) a decreasing amount of αB, 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.

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