The quenching and partitioning (Q&P) process of lean medium Mn steels is a novel approach for producing ultra-high strength and good formable steels. First, the steel is fully austenitized, followed by quenching to a specific quenching temperature (TQ) in order to adjust an appropriate amount of initial martensite (α’initial). Subsequently, the steel is reheated to a partitioning temperature (TP) in order to ensure C-partitioning from α’initial to remaining austenite (γremain) and thus retained austenite (RA) stabilization. After isothermal holding, the steel is quenched to room temperature (RT), in order to achieve a martensitic-austenitic microstructure, where the meta-stable RA undergoes the strain-induced martensitic transformation by the so-called transformation induced plasticity (TRIP) effect. This paper systematically investigates the influence of the Q&P process on the isothermal bainitic transformation (IBT) kinetics in a 0.2C-4.5Mn-1.3Al lean medium Mn steel by means of dilatometry. Therefore, the Q&P annealing approach was precisely compared to the TRIP-aided bainitic ferrite (TBF) process, where the samples were directly quenched to the temperature of the IBT after full austenitization. The results indicated an accelerated IBT for the Q&P samples, caused by the formation of α’initial during quenching below the martensite start (MS) temperature. Furthermore, a significant influence of the annealing parameters, such as TQ and TP, was observed with regard to the transformation behavior. For further characterization, light optical microscopy (LOM) and scanning electron microscopy (SEM) were applied, showing a microstructure consisting of a martensitic-bainitic matrix with finely distributed RA islands. Saturation magnetization method (SMM) was used to determine the amount of RA, which was primarily depending on TQ. Furthermore, the hardness according to Vickers revealed a remarkable impact of the annealing parameters, such as TQ and TP, on the predicted mechanical properties.

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Herein, the investigation of the tensile behavior of lean medium Mn Quenching and Partitioning (Q&P) steels containing 0.2 wt% C, 4.5 wt% Mn, and additions of 1.5 wt% Si or 1.3 wt% Al is concentrated upon. By the variation of the quenching temperature (TQ), different volume fractions of primary martensite (α′prim) are adjusted, influencing the subsequent microstructural evolution during Q&P processing. Using scanning electron microscopy (SEM), the final microstructure consisting of tempered martensite (α″), retained austenite (RA), partially bainitic ferrite (αB), and final martensite (α′final) is characterized. Furthermore, interrupted tensile tests at gradually increased strains are conducted to investigate the stability of RA against strain‐induced martensite transformation (SIMT) and overall tensile behavior of lean medium Mn Q&P steels. The investigations manifest the formation of larger amounts of αB and consequently lower RA contents, when Si is substituted by Al. As an aftermath, the tendency to form α′final is significantly lower, compared with the Si‐alloyed composition, reflected in the overall stress–strain behavior. Especially in case of the Si‐alloyed samples containing RA fractions exceeding 15 vol%, an over‐accelerated SIMT is observed, inducing failures occurring prior to necking, whereas for the Al‐alloyed samples, a wider process window is obtained.

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