Mould powders are additives in the continuous casting process. They are added to the meniscus within the mould. Due to the high temperatures they melt and infiltrate the gap between the mould and the strand to lubricate the strand and control the heat flow. One of the most essential parameters in the continuous casting process to provide casting conditions stability and product surface quality is the melting behaviour of the mould powder. Operating experience has shown that the knowledge of only the chemical composition is not always sufficient for the mould powder selection. It is likely that beside the chemical composition the phase composition and the phase distribution are of influence on the mould powder performance. Therefore the phase composition of fourteen commercial mould powders was investigated by reflected light and scanning electron microscopy as well as X-ray diffraction. Several methods have been applied to investigate the melting behaviour of these powders. After annealing at different temperatures specimens have been investigated by X-ray diffraction, reflected light microscopy, scanning electron microscopy and X-ray microanalysis. Furthermore simultaneous thermal analysis and hot stage microscopy have been applied. The CaO/SiO2 ratio of the specimens was in the range of 0.6-1.3 and the carbon content between 2.2% and 15.2%. Although the mineralogical composition of the mould powders as received differs, there are still similarities concerning their components. The analysis of the green mould powders shows that the main components are silicates, sodium and calcium carbonates, fluorite (CaF2) and carbon carriers With respect to the melting behaviour the mould powders show some similarities. In all cases the carbonates start to dissociate below 500 °C and finish not later than 790 °C. As a consequence Na2O diffuses into the surrounding silicates and lowers their melting range. The formation of the first liquid phases depends on the used silicates. Especially the presence of glass scrap leads to the formation of liquid phases at low temperatures and enhances the melting of mould powders. Between 750°C and 900°C sodium-lime-silicates recrystallize out of the sodium saturated liquid. Due to the dissolution of alumina and silicates containing Al2O3 > 18 mass% sodium-aluminium-silicates are formed between 900 °C and 1000 °C. Cuspidine is the most important phase formed within fluorine containing silica based mould powders because it is the last melting phase. At temperatures around 600 °C it is formed due to diffusion of fluorine into sodium silicates. But the main quantity crystallizes out of the liquid phase between 690 °C and 1073 °C when a considerable amount of liquid is present. The formation of a homogeneous liquid phase takes place between 1020 °C and 1260 °C and depends on the CaO/SiO2 ratio of the mould powders. By contrast a not otherwise specified phase containing approximately 17 mol% Al was formed in the only calcium alumina based mould powder investigated above 900 °C due to its high alumina content of 25,3 mass%. Here cuspidine was no longer the main crystalline phase.
|Translated title of the contribution
|Phase Composition of Mould Powders and its Alteration with Increasing Temperature
|Published - 2013
Bibliographical noteembargoed until null
- mould powders
- continuous casting
- melting behaviour