Estudo da cinética de cristalização em pó fluxante F-free à base de CaO-SiO2-TiO2 utilizando os modelos isoconversionais: Friedman, FWO e KAS

Abstract

During the steelmaking process, many defects such as poor quality of the finished produce and breakouts happen due to the high or insufficient heat flux between the mold and melt steel. Mold fluxes are necessary during the continuous casting of steel, because it improve the quality of the finished produce, in which the fluorine have a importante role in a heat flux control because it cristalization control of the mold fluxes. Due to environmental damage caused by fluorine, demands for developing a fluorine-free mold fluxes for continuous casting of steel are increasing. Therefore improve the fluorine-free mold fluxes is essencial, in which is necessary to replace it with oxides, which must maintain the technological parameters, related to viscosity, melting characteristics and behavior crystallization. Previously, the crystals precipitation sequence for the mold flux (CaO–SiO2–Na2O–Al2O3–TiO2–B2O3–MgO) with basicity 1 during cooling was defined, combining Differential Scanning Calorimetry (DSC) and a thermal cycle together with X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM) previously performed by SHU et al., 2020. For this slag system, it is interesting to know the effect of Na2O concentration regarding crystallization kinetics, especially for peritectic steel continuous casting, because peritectic steel show more surface defects. A modified Avrami model was used to calculate the parameter n values. It was found that the first crystal which precipitates (Ca2SiO4) have a three-dimensional growth, because its n values are near 2,5. This agrees with the SEM micrographs, where dendritic structure is observed. The Friedman, Flynn–Wall–Ozawa (FWO) and Kissinger–Akahira–Sunose (KAS) differential isoconversional methods were applied for determining the effective activation energy for non-isothermal crystallization, because they gives relevant information without necessary know the form of the kinetic equation. In this work, the Friedman differential isoconversional method have more relative crystallinity variation sensitivity than FWO and KAS. The results of effective activation energy for non-isothermal crystallization for all of differential isoconversional methods used show that crystallization becomes slower with increasing temperature. That is to say the crystallization of Ca2SiO4 during cooling follows anti-Arrhenius behavior.