Industrial applications for disordered materials: the case of mold fluxes for steel production

Steel production is made through continuos casting, with the need to reduce the frictional force between molten steel (which is solidifying on the outer shell) and the crystallisator. Mold fluxes are especially made for this purpose: they are synthetic oxy-fluoride glass-ceramic systems used also to control the amount of heat transfer rate. Fluorine in mold fluxes is used as an additive to control melting temperature, to form the main crystalline phase (cuspidine), and to reduce viscosity by depolymerization of silicate network. Despite its criticality, fluorine has been limited due to some environmental and industrial issues: evaporation and formation of HF, erosion/corrosion of facilities such as nozzle and caster frame, cost for pretreatment before discharging the waste-water, etc. In order to reduce these detrimental effects, B2O3 has been regarded as the alternative to CaF2 in conventional mold flux systems. It is well known by previous studies that the role of B2O3 is similar to that of CaF2 as a flux to reduce melting temperature. However, the contribution of B2O3 in terms of viscosity and crystallization kinetics is quite controversial. To elucidate uneven viscosity behaviour, importance of chemical structure identification has come to the fore. Shin et al. proved that variation of viscosity depends on the form of borate; [BO3]-triangular or [BO4]-tetrahedral structure. Measured viscosity showed trend that increases at first with increasing B2O3 contents, however, it turned into decreasing tendency with further B2O3 additions. At the early stage, most of borates existed in the form of [BO4]-tetrahedral structure, which could combine successfully with silicate anions, thus viscosity increases. When further amounts of B2O3 are added, [BO3]-triangular structures are predominantly formed due to limited alkali oxide. By degree of polymerization analysis, it is verified that [BO3]-triangular structures could simplify and even attack silicate network.