The effect of a subsequent galvannealing (GA) treatment in the temperature range of 480–560 °C after a quenching and partitioning (Q&P) process was investigated via two medium-Mn steels 0.2C–4Mn-1.5Si and 0.2C–4Mn-1.5Al, focusing on the microstructural evolution and mechanical behavior. Thereby, the influence of a replacement of the alloy element Si by Al on the microstructure, especially on the retained austenite (RA), was analyzed via scanning electron microscopy, electron backscatter diffraction, and X-ray diffraction. Heat treatment schedule and alloy steel turned out to be the main influencing factors on the amount of RA and its morphology. By traditional Q&P, Si-steel was more efficient in stabilizing high amounts of lath like and globular RA, compared to Al. However, an additional GA-treatment led to a drastic decomposition of RA into pearlite and fresh martensite with increasing temperature for the Si-steel. In contrast, the GA-treatment showed no significant reduction at the RA content in the Al-alloyed steel. The different decomposition behavior of RA during GA was also reflected in the mechanical behavior measured by means of tensile testing. Hereby, it could be shown that Al, compared to Si ensured high elongations, which are reached due to a sufficient amount of stable RA (good TRIP-behavior) even after high temperature galvannealing.