Aplicação da termodinâmica computacional para o desenvolvimento de pós fluxantes sem flúor para aços peritéticos.

Abstract

During continuous casting, the steel starts to solidify forming a strand shell from contact with the copper mold. Once the mold flux is released on the liquid steel in the mold and melts on the steel surface, the slag infiltrates between the mold and the strand shell and is exposed to different cooling conditions, which can favor or prevent its crystallization. In mold fluxes, fluorides can reduce the melting temperature and slag viscosity, in addition to being important for the precipitation of the phase called cuspidine (Ca4Si2O7F2) which is vital for controlling the heat transfer between the liquid steel and the mold. However, fluorides will react with SiO2 to form a low melting point compound (SiF2) that will evaporate into the atmosphere during steel casting and will also form the HF gas compound which is one of the major villains of the process by be terribly toxic. In addition to the environmental concern, the fluorides generated by these reactions will also cause corrosion problems in the continuous casting equipment. Therefore, there is an urgent need to develop fluorine-free slags. Considering the development of fluorine-free mold fluxes for the casting of peritetic steel plates, control of crystallization is essential. The main problem is to effectively control the heat transfer between the steel shell and the mold. The objective of the present work is to analyze the effects of the components of a new fluorine-free flowing powder for the continuous casting of peritetic steel plates, replacing the CaO-SiO2-CaF2 system by the CaOSiO2- TiO2 system, considering the relationship between temperature, composition and precipitation of crystals capable of imparting properties similar to the properties of fluorinebased mold fluxes. With the aid of computational thermodynamics the present work started from the basic system CaO – SiO2 – Al2O3 at a temperature of 1200 ° C, it was noticed that the addition of Na2O, MgO and B2O3 is very effective in decreasing the liquidus temperature of the flux slag. Subsequently, the relationship of these components was evaluated with the addition of 5 wt% of TiO2 in the precipitation of the perovskite phase (CaTiO3), which acts in a similar way to cuspidine in the control of crystallization, allowing the new system to replace the old one. Finally, the proportion of the different crystals precipitated during cooling from 1200 ° C to 900 ° C was analyzed, which allowed us to perceive that the new mold flux presents excellent perspectives as a substitute for the traditional fluorine based flux powders.