Crystallization behavior and structure analysis for molten CaO-SiO2-B2O3 based fluorine-free mold fluxes

An investigation had been conducted to study the effect of CaO/SiO2 mass ratio and B2O3 on the crystallization behavior and the melt structure of fluorine-free mold fluxes. The results showed that the increase of CaO/SiO2 ratio tends to promote the crystallization behavior of the fluorine-free mold flux system, due to the depolymerization of the original complex borosilicate structure by the released free oxygen (O2-) ions with the added CaO. Small amount of B2O3 would initially inhibit the mold flux crystallization, then the further addition would promote crystallization. The melt structure analysis results suggested that with the addition of B2O3 content, the existing silicate structure would connect with each other and also link with the dissociative [BO3]-trihedral units to form borosilicate chain. Simultaneously, O-2(-) could destroy the original diborate structure and form two simple borate structures. Subsequently, the simple borate structure would connect with the dissociative [BO3]-trihedral and the formed borosilicate chain to form a more complex borosilicate structure. Thus, the degree of polymerization of whole molten slag was getting improved, but the overall effect on the mold flux crystallization was depended on the balance between the molten slag structure and viscosity, in which 6 mass% was the critical point in the current mold flux system.