A kineticsKinetic model for the six-phase multicomponent among alloy-steelSteel-inclusionInclusion-slag-refractory material was established to predict the variation of the composition of the steelSteel, the slag, and inclusionsInclusion during reaction. Guided by Gibbs free energy analysis, the model includes simple rare earthRare earth compounds (Ce2O3, La2O3, CeS, Ce2S3, LaS, La2S3) and complex rare earthRare earth phases (CeAlO3, Ce2O2S, Ce3S4, LaAlO3, La2O2S, La3S4). The calculation showed that the content of Al2O3 in inclusionsInclusion was over 90%. With the addition of rare earthRare earth metals in the steelSteel, Al2O3 in inclusionInclusion particles was rapidly consumed and initially transformed into Ce2O3 and La2O3. And these oxides subsequently converted into Ce2O2S and La2O2S. At the end of the refining processProcess, inclusionsInclusion consisted predominantly of RE2O2S and RExS phases. The addition of the La–Ce mixed alloy has a negligible effect on slag composition throughout the refining processProcess.