Effects of B2O3 on Viscosity, Structure, and Crystallization of Mold Fluxes for Casting Rare Earth Alloyed Steels

Amounts of rare earth oxides were transferred into mold fluxes for the continuous casting of rare earth alloyed heavy railway steels. B2O3 was added to improve the viscosity properties and the crystallization behavior of this traditional mold flux. Thus, in this paper, effects of B2O3 on viscosity, structure, and crystallization of CeO2-bearing mold fluxes for casting rare earth alloyed heavy railway steels were investigated by rotating cylinder method, Raman spectroscopy, and magic angle spinning nuclear magnetic resonance (MAS NMR). Experimental results revealed that with the addition of B2O3, viscosity values of mold fluxes at some certain temperatures decreased significantly. By analyzing the structure of glassy mold fluxes, it can be observed that B2O3 behaved as a network-forming oxide within the studied system. However, the introduction of the weak bond energy of B-O and the transition from four-fold coordination boron to three-fold coordination boron with loose structural units mainly reduced the viscosity. Moreover, results of X-ray diffraction (XRD) and differential scanning calorimetry (DSC) confirmed that B2O3 decreased the crystallization of the crystal phase Na4Ca4(Si6O18) and the solidus temperature of mold fluxes. Thus, these results obtained can provide guidelines for designing new mold fluxes for casting rare earth alloyed steels.