Phase relation between mold flux composition and cuspidine was researched fundamentally. Solidification temperatures of the mold flux were measured at various compositions, and crystal compositions in their solidified structure were identified by X-ray diffraction analysis. Based on these results, the phase relation between the mold flux composition and cupsidine was considered. In the relation between basicity (T.CaO/SiO₂) and the solidification temperature, there was a peak of the solidification temperature and the basicity at the peak varied according to the contents of Na₂O and F. Taking the affinity of F to Na rather than Ca into consideration, NaF was considered to exist as a component of the molten flux. Treating the mold flux composition as CaO-SiO₂-CaF₂-NaF system, it became possible to estimate the phase relation between the mold flux composition and crystal composition such as cuspidine, CaF₂ and NaCa₂FSiO₄ accurately.
According to this study, conventional compositions of the mold flux were estimated to be out of the primary crystallization field of cuspidine. On the other hand, the composition of the mold flux, that enabled the high speed casting of hypoperitectic steel slabs at 5.0m/min without longitudinal surface cracking, was estimated to be in the field and close to be cuspidine.