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.
Highlights
CONSIDERING the development of non-reactiveF-free mold fluxes for the continuous casting of high aluminum steels, it is imperative to understand their crystallization behavior in the mold, since control of horizontal heat transfer is related to crystallization tendency
Mold fluxes play a crucial role in the continuous casting of defect-free steel products, providing appropriate heat transfer control, lubrication, thermal insulation, inclusion absorption, and oxidation prevention.[1]
It is well accepted that horizontal heat transfer rate can be controlled when controlling mold slag crystallization.[2,3]
Summary
During continuous casting of high aluminum steels using traditional CaO-SiO2-CaF2 based mold fluxes, operational issues and quality problems arise due to the steel/ slag interfacial reaction: Al + (SiO2) = Si + (Al2O3). This reaction is fast and leads to increase of Al2O3 content in the mold slag, causing problems such as massive formation of slag rim, poor lubrication, and uneven horizontal heat transfer. This work concluded that some recipes in the CaO-Al2O3-B2O3-Na2O-Li2O system are potential substitutes for traditional mold powders Due to their non-reactive behavior (no SiO2), these new fluorine-free mold fluxes could be applied for casting of high Al steels. The present paper reports an investigation using DSC with multiple heating rates, considering non-isothermal crystallization kinetics for glassy fluorine-free mold fluxes in the CaO-Al2O3-B2O3-Na2O-Li2O system. Crystallization mechanisms and activation energies for devitrification were determined for three mold flux samples, with the ratios w(CaO)/w(Al2O3) 0.9, 1.0, and 1.1
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
