Influence of Q&P-Parameters and Al-Content on the Microstructural Evolution of Lean-Medium-Mn-Steels

This report investigates the impact of different heat treatment parameters and varying Al-contents on the microstructure of Quenching & Partitioning (Q&P) steels. Therefore, three lean-medium-Mn-steels with Al-contents between 0.3 and 0.9 wt-% underwent heat treatments according to Quenching & Partitioning regimes. For comparison, the steels were subjected to a transformation induced plasticity (TRIP) aided-bainitic-ferrite (TBF) process. In both cases, the samples were fully austenitized at 900 °C for 120 s, using dilatometry. For the Quenching & Partitioning process, the quenching temperature (T_Q) ranged from 210 °C to 330 °C, while the transformation induced plasticity (TRIP) aided-bainitic-ferrite samples were cooled to 360 °C. Afterwards, the specimen were re-heated to the partitioning temperature (T_P) of 400 °C and isothermally held for partitioning times (t_p) of 40, 120 and 200 s. Subsequently, these steels were analyzed with regard to their phase fractions and hardness. The results indicated that in the Quenching & Partitioning process, the microstructure was primarily influenced by the partitioning temperature (T_Q), while partitioning times (t_p) played a minor role due to the time-independent martensitic transformation during quenching. In general, rising quenching temperature (T_Q) led to an increase in retained austenite (RA) fraction. In the transformation induced plasticity (TRIP) aided-bainitic-ferrite (TBF) process, a substantial influence of partitioning times (t_p) was found, which can be explained by the kinetics of the isothermal bainitic transformation. Regardless of the heat treatment concept, an increasing Al-content contributed to elevated retained austenite contents.