In the continuous casting of ultra-low carbon steels, recarburization from the mold powder negatively affects steel quality. Therefore, to develop mold powders without free carbon, different carbides or nitrides have been proposed and evaluated in laboratory and field studies. Among these, SiC and Si ₃N ₄ were selected for the present study. Additionally, thermodynamic calculations were performed to quantitatively describe their effect on the melting behavior of the mold powder. To verify the results under high heating rates, mixtures of raw material components were placed in a steel crucible with a lid, inserted into a furnace preheated to different temperatures, and investigated mineralogically. The results agreed with those of the thermodynamic calculations. Moreover, the results suggested that Si ₃N ₄ and SiC were suitable raw material components and alternatives to carbon in the mold powder. They prevented solid–solid reactions between raw material components to form new phases and were still found at the temperature of 1200 °C. Consequently, the CaO/SiO ₂ ratio was higher before oxidation and affected the liquid-phase formation. The addition of antioxidants reduced the liquefaction-controlling effect of SiC. Additionally, the study shows that the sample preparation impacts the phase formation: granules facilitate phase formation due to a higher contact of reactants, further they show earlier melting close to their surface as sodium content is increased there by the spray drying procedure.