Study On Crystallization Behaviors And Heat Transfer Of High Al Steel Mould Fluxes

The high aluminium steels, such as alumina-containing TRIP steels, non-magnetic steel, electrical steel, and so on, generally contain more than 0.5% Al, whose chemical property is so lively that can react easily with SiO₂ in mould fluxes during casting. The oxidation-reduction reaction which result in the decrease in (SiO₂) and increase in (Al₂O₃) contents, as well as the rapid increase in the basicity [w(CaO)/w(SiO₂)] of the mould fluxes usually occurred within the formerly 10 minutes. The physicochemical properties, for example the viscosity, melting point and and heat flux, could become unstable accordingly. So the mould fluxes of high aluminium steels can not match excellently with the continuous casting process up to now.In order to ensure the appropriate physicochemical properties of degeneration fluxes, mould fluxes with low basicity are chosen localised and generalised to cast high aluminium steels. However, some surface longitudinal cracks are frequently found on the strands occurring before basicity was modified because these types of fluxes are too glassy to enable heat transfer control. The ways of modelling studying on heat flux, slag film thickness and microstructure of fluxes with the increasing w(Al₂O₃)/w(SiO₂) are prior research, including Single Hot Thermocouple (SHTT), heat flux simulation and Scanning Electron Microscopy. The results show that both Li2O and B2O3 can stable heat flux. The heat flux, melting point and viscosity can remain stable during and after the oxidation-reduction reaction by adding both Li2O and B2O3. Therefore, the mould fluxes with low basicity were optimized commendably.The critical SiO₂ content was acquired by thermodynamic calculation and liquid-liquid phase reaction carrying in a vacuum induction furnace to avoid these problems due to the oxidation-reduction reaction. The effect of Li2O,B2O3,Na2O,F-,MgO on the crystallisation and heat transfer of non-reactive mold fluxes was then investigated via the single hot thermocouple technique, heat flux simulation, and Scanning Electron Microscopy. With the increase in Li2O and F- content of CaO-Al₂O₃ slag system the incubation time became shorter and the critical cooling rate became greater. However, the increasing Li2O (0-2%), Na2O, B2O3, MgO and BaO (replace CaO) can prolong the incubation time and reduce the critical cooling rate. In the CaO-Al₂O₃ slag system, both maximum and average heat flux can become greater and the slag film thickness can become thinner for the increasing Na2O, B2O3, MgO, F-, BaO (replace CaO) and SrO. But when the Li2O content increased from 2% to 5%, both maximum and average and slag film thickness heat flux minished.The industrial casting experiment of 20Mn23AlV nonmagnetic steel was carried using the optimized mold fluxes with low basicity and the new designed non-reactive mold fluxes. The slab quality corresponding to the low basicity slag was eligible ,and the qualified rate of rolled products reached 100 percent. The designed non-reactive mold fluxes whose melting point is comparatively high can not form a normal melting slag pool during the casting. But the application effect of non-reactive mold fluxes also integrally good except few slag inclusion. So the intention to optimize or design the mould fluxes of high aluminium steels has been obtained. So the next research direction was advised to improve the melting property of the non-reactive mold fluxes.