Effect of La₂O₃ on the Viscosity, Crystallization, and Structure of Calcium-silicate-based Mold Flux for Continuous Casting La-bearing FeCrAl Alloy

Abstract

The influence of La₂O₃ on the properties and structure of calcium-silicate-based mold flux for continuous casting La-bearing FeCrAl alloy was studied through employing rotating viscometer, SEM-EDS, XRD, and Raman spectroscopy. The results showed that the viscosity of mold fluxes decreased with the increase of La₂O₃ content from 0 mass% to 15 mass%. The apparent activation energy for viscous flow decreased from 108.56 ± 1.96 kJ/mol to 87.29 ± 7.29 kJ/mol with increasing La₂O₃. Deconvolution Raman analysis showed that with increasing La₂O₃, the mole fraction of Q³ units decreased, while that of Q⁰, Q¹, and Q² units increased. Furthermore, the values of NBO/Si increased from 1.27 to 1.83 with the increase of La₂O₃, which indicated that the degree of polymerization of melt structure was reduced and lead to the decrease of viscosity. During the cooling process, cuspidine (Ca₄F₂Si₂O₇) was the main crystalline phase in calcium-silicate-based mold fluxes. Nevertheless, when La₂O₃ was excessively added, a new phase of CaLa₂(SiO₄)₂ was formed owing to the charge balance of Ca²⁺ and La³⁺ on the simple structural units Q⁰ ([SiO₄]⁴⁻). Therefore, with increasing La₂O₃ can increase the break temperature and accelerate the formation of crystalline phases Ca₄F₂Si₂O₇ and CaLa₂(SiO₄)₂ at high temperature.