Crystallization control for fluorine-free mold fluxes: effect of Na2O content on non-isothermal melt crystallization kinetics

Abstract

There are increasing demands for developing fluorine-free mold fluxes for continuous casting of steel. When removing fluorine from mold flux composition, it is necessary to replace it with oxides, which must maintain the technological parameters, related to viscosity, melting characteristics, and crystallization behavior. For industrial developments in the CaO–SiO2–Na2O–Al2O3–TiO2–B2O3–MgO (with basicity = 1, Al2O3 = 7%, TiO2 = 5%, B2O3 = 3%, MgO = 2%) slag system, it is necessary to know the effect of Na2O concentration regarding crystallization kinetics. This is especially important for fluorine-free mold fluxes for peritectic steel slab casting. In this work, the crystals´ precipitation sequence for this system during cooling was determined, combining Differential Scanning Calorimetry (DSC), X-Ray Diffraction (XRD), and Scanning Electron Microscopy (SEM). The Friedman differential isoconversional method was applied for determining the effective activation energy for non-isothermal crystallization, since it gives relevant information without knowing the form of the kinetic equation. A modified Avrami model was used to calculate the n values; it was found that they are near 2.5, for all analyzed samples, which means that it is related to the crystallization mode diffusion controlled, with constant nucleation rate and three-dimensional growth. This agrees with the SEM micrographs, where dendritic structure is observed for all crystalline samples. Additionally, structural information got from Raman spectroscopy, for the samples in vitreous state, was used to interpret crystallization tendency, i.e., the fact that crystallization was enhanced by increasing Na2O content, due to slag depolymerization. Moreover, computational thermodynamics was used to analyze mold fluxes crystallization behavior.