Mold slag crawling down the outside of the submerged entry nozzle (SEN) may occur during continuous casting of steel affecting the SEN port geometry and steel flow patterns in the mold, and leading to defect formation in the final product due to slag accumulations shearing off from the SEN body. There are two possible mechanisms contributing to slag crawling, (a) refractory wetting together with chemical reactions at the refractory – mold slag – steel interface, and (b) liquid steel flow induced drag forces, displacing slag down the SEN. In this work, we applied thermodynamic modeling to study the extent of chemical reactions at the refractory – slag – steel interface using FactSage 7.2 thermochemical software. The modeling was validated by industrial and experimental observations of slag crawling. A finite number of reaction zones was identified at the interface, and it was assumed chemical reactions reach equilibrium in the designated effective reaction zone volumes. The refractory – slag carbothermic reaction, slag – steel exchange reaction, gas back-infiltration into the refractory body, and deposit formation on the SEN, observed in steel continuous casting, were investigated in detail. Thermodynamic insight into interfacial chemical reactions form the basis for successful development of the process kinetic model.