Mold powders used for the continuous casting of steels contain different carbon carriers controlling the melting behavior. During melting carbon particles may accumulate at the liquid slag pool surface resulting in steel recarburization, which significantly changes the product quality. Thus, to reduce or prevent this reaction, mold powders with considerably reduced carbon content, or even without free carbon, are required. To replace carbon with another melting-controlling component, the effect on the melting behavior of mold powders under near-process conditions, such as high heating rates, should be investigated. Consequently, a different procedure was developed to evaluate the effect of various carbon contents on the melting behavior. In this study, a granulated ultra-low carbon (ULC) mold powder was selected, and different graphite contents (0%, 1%, 2%, and 5%) were added. Furthermore, a sample obtained by granule disintegration and 5% graphite addition was produced and compared with a powdery sample. Subsequently, the steel crucibles were covered to reduce oxygen supply, filled with the samples, inserted into a preheated furnace (700–1300 °C), held at the selected temperatures for 10 min, and quenched to room temperature. The samples were mineralogically investigated, and the experimental method was applicable for characterizing the melting behavior of mold powders. At lower temperatures, carbon reduces the reactions between raw material components owing to reduced particle contact and prevents the formation of new solid (for example, cuspidine) and liquid phases. With increasing carbon content, the reactions shifted to higher temperatures, further delaying liquid phase formation. This effect was more evident for the powdery mold powder than for the granulated mold powders. At elevated temperatures, the powdery sample contained a coherent liquid phase, whereas the granules melted independently. This study provides a foundation for alternative melting-controlled additives for mold powders. It demonstrates that carbon substitutes should delay the reaction between raw material components and be stable until high temperatures.