<p>In the continuous casting of medium-carbon steel, highly crystalline mold fluxes are employed to suppress surface longitudinal cracks; however, the lubrication performance of such mold fluxes is generally poor. One hypothesis is that the composition segregation of the remaining liquid slag after crystallization in the mold fluxes leads to the deterioration of lubrication performance, but no direct experimental data currently confirm this hypothesis. Thus, remaining liquid slag in the glassy state after crystallization was obtained by rapidly cooling samples. The composition of original slag, crystals, and remaining liquid slag was analyzed using an electron probe microanalysis system. The performance changes caused by the compositional changes in the remaining liquid slag at 200&#xa0;μm away from the crystals were calculated using FactSage. The results indicated that the crystalline phase of all mold fluxes is cuspidine (3CaO·2SiO₂·CaF₂). After the mold fluxes precipitate crystals, the content of non-cuspidine components in the remaining slag increases. As for the cuspidine component, the CaO content has significantly decreased. Overall, the content of SiO₂ in the remaining slag decreases, while the content of F increases. The experimental results confirmed that there is obvious segregation in the remaining liquid slag after the mold flux selectively crystallizes. Compared with the original slag, the liquidus temperature of the remaining liquid slag is significantly reduced, and the maximum crystallization ratio of cuspidine also decreases. The poor lubricity of the mold fluxes for medium-carbon steel is its nature, rather than caused by segregation.</p>