Mold Fluxes For Continuous Casting Research Articles

Development of Non-reactive F-Free Mold Fluxes for High Aluminum Steels: Non-isothermal Crystallization Kinetics for Devitrification

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.

Viscous characteristics and modelling of CaO–Al2O3-based mould flux with B2O3 as a substitute for CaF2

ABSTRACTFollowing a series of laboratory studies on the development of fluoride-free or low-fluoride CaO–Al2O3-based mould flux for continuous casting of high-aluminium steel, the viscosity properties of CaO–Al2O3-based mould flux with B2O3 as a substitute for CaF2 were investigated using the rotating cylinder method. It was found that B2O3 behaved in a similar manner to CaF2 in changing the viscosity of the flux system. From calculations of the viscous activation energies, it was found that similar changes in the concentrations of CaF2 and B2O3 resulted in almost the same changes in activation energy. A viscosity prediction model was developed using the optical basicity concept as an indicator of melt behaviour. Predicted viscosity values showed good agreement with measurements reported in the literature.

Stability of Fluorine-Free CaO-SiO2-Al2O3-B2O3-Na2O Mold Fluxes

B2O3 and Na2O are key components of fluorine-free mold fluxes for continuous casting, but both are highly volatile, which affects the flux stability. This paper investigated the evaporation of the SiO2-CaO-Al2O3-B2O3-Na2O fluxes (Na2O: 6 to 10 wt pct, CaO/SiO2 ratio: 0.8 to 1.3) in the temperatures ranging from 1573 K to 1673 K (1300 °C to 1400 °C) using thermogravimetric analysis. The weight loss as a result of the flux evaporation increased with the increasing temperature for all fluxes. The rate of evaporation was found to be very small for the Na2O-free flux but significantly increased with the addition of Na2O. The high evaporation rate of fluxes in the presence of B2O3 and Na2O was attributed to the formation of highly volatile NaBO2. Changing the ratio of CaO/SiO2, however, did not affect the rate of evaporation. Kinetic analysis of the evaporation processes demonstrated that external mass transfer contributed to the rate of evaporation.

Dissolution rate of Al2O3 into molten CaO–Al2O3–CaF2 flux

Dissolution of Al2O3 into molten CaO–Al2O3–CaF2, a base system of mould flux for continuous casting of high Al steel, has been investigated by employing a rotating cylinder method. The dissolution rate of an alumina rod into molten CaO–Al2O3–CaF2 flux increased with increase in rotating speed and temperature. The apparent activation energy for mass transport of flux C was calculated to be 255·6 kJ mol−1. The rate controlling step during the dissolution process of the alumina rod into molten CaO–Al2O3–CaF2 flux was found to be the diffusion of the solute in the flux boundary layer. The dissolution rate of alumina into molten CaO–Al2O3–CaF2 flux increased with increasing CaO/Al2O3, and it may be caused by the increase in thermodynamic driving force and the decrease in the viscosity of the flux. When the Al2O3 rod was immersed into molten flux, an intermediate compound of CaO.2Al2O3 formed firstly and then dissolved into molten flux.

Effects of Na 2CO 3 on properties of low fluoride content mould flux for thin slab continuous casting

To reduce fluorine pollution, partial fluoride could be replaced by Na 2C0 3 in mould flux for continuous casting. The properties including melting temperature, viscosity fluidity, surface tension and A1 20 3 inclusion adsorption of low fluoride content mould flux with addition of Na 2C0 3 for thin slab casting were studied. The results showed that with the increase in Na 2C0 3 content, melting temperature, viscosity and viscous activation energy of mould flux decreased obviously, fluidity increased, surface tension changed little, and A1 20 3 inclusion adsorption of mould flux increased. The Na 2C0 3 could play effective role when its content was less than 9%. The results would play a guiding role in designing of low fluoride content mould flux for thin slab continuous casting.