In the case of Quenching & Partitioning (Q&P) steels, striving for large retained austenite (RA) fractions, the exact selection of the process temperatures is required. In this context, the quenching temperature (TQ) is known as a crucial parameter for setting the appropriate microstructure. In recent years, high research effort has therefore been put in the development of models that forecast the microstructural evolution during Q&P processing. In particular, the constrained carbon equilibrium (CCE) methodology is extensively widespread, although it represents a simplified state and therefore exhibits large deviations compared to experimentally obtained results. Hence, the aim of this work was to develop a new model that considers the experimentally observed competing reactions occurring during Q&P processing, including the formation of bainitic ferrite (αB) and incomplete C-partitioning into the remaining austenite (γremain). Moreover, the mechanical stabilization of γremain due to the occurrence of compressive stresses was also taken into account. Furthermore, a relation between C-enrichment in γremain and RA stability against strain-induced martensitic transformation (SIMT) could be established. The findings were summarized in a model, which allows for the accurate prediction of the phase fractions and the mechanical stability of RA as a function of the applied TQ in case of Q&P steels with an example for lean medium Mn compositions. The basic principles of this model, taking into account the competing reactions and mechanical stabilization of RA, can be however applied to all RA containing advanced high strength steels (AHSS), in which RA is primarily stabilized by C-enrichment during heat-treating.