Development of non-reactive F-free mold fluxes for high aluminum steels: non-isothermal crystallization kinetics for devitrification

Abstract

It is necessary to study crystallization from glass (devitrification) for non-reactive F-free CaO-Al2O3 based mold fluxes; this is especially important for the development of mold fluxes for continuous casting of high aluminum steels. To the knowledge of the authors, there is no report in the literature regarding devitrification of F-free mold fluxes based on the CaO-Al2O3-B2O3-Na2O-Li2O system. Therefore, crystallization kinetics for particular compositions in this system, with different w(CaO)/w(Al2O3) ratios, was investigated by Differential Scanning Calorimeter, Field-Emission Environmental Scanning Electron Microscopy / Energy Dispersive Spectroscopy, and X-ray Diffraction techniques. The first crystal, which precipitates during heating from glass, is Ca12Al14O33, followed by CaO. For the first crystal, which precipitates as plate-like (2-dimensional), it was found that, when using the Matusita–Sakka model, agreement between the calculated Avrami parameters and the micrographs obtained from electron microscope was reached. In the same way, agreement was found for the second event—CaO precipitation—which grows 2-dimensionally or 3-dimensionally, depending on the w(CaO)/w(Al2O3) ratio. The most important event (in terms of energy liberated and amount of crystals) is Ca12Al14O33 precipitation. For this event, the effective activation energy for crystallization, EG, decreases with the increase of w(CaO)/w(Al2O3) ratio. The activation energy for crystallization reflects the energy barrier for crystallization. Thus, it can be concluded that mold fluxes crystallization during heating is enhanced when increasing the w(CaO)/w(Al2O3) ratio, for constant contents of B2O3, Na2O, and Li2O.