In order to improve steel quality, it is necessary to increase the cleanliness of the liquid steel; that is, to reduce the number and size of the inclusions in the liquid steel, as well as to control their chemical composition. For this purpose, processes (with different operating costs) are used, such as the bubbling of inert gas in the steel ladle and its treatment in the vacuum degasser RH. This article deals with inclusion removal through an argon purging process and RH treatment, and provides a comparison of their effectiveness using industrial data and a mathematical model. The inclusion count shows a strong exponential decay for RH treated heats with chemical heating, as expected. The same behavior is not seen for RH treated heats without chemical heating as well as an argon purging process. It is suggested that in the later cases, there could exist competition between inclusion removal and inclusion generation from sources, such as refractory-slag-metal interaction and open eye atmosphere metal interaction. It has been concluded that the RH and the argon purging processes are both able to reduce the amount of inclusions. However, the bubbling process would require a much smaller gas flow rate to avoid open eye formation.

The developed model is an association of thermodynamic calculations for dissolution of alloys, slag formers and the deoxidation reaction in the molten steel with two artificial neural network (ANN) models trained with industrial data, to predict the molten steel temperature drop from the blowing end of the BOF until the first measurement at secondary metallurgy. To calculate the associated energy for deoxidation, an experiment was designed to set up the parameters for oxygen partitioning among deoxidants, with timed aluminum addition during teeming being the main parameter. The temperature control in the teeming stage presented a standard deviation for the error of prediction of 5.46 o C, for transportation from the rinsing station to the secondary metallurgy of 2.79 o C. The association of all calculations presented an error standard deviation of 7.49 o C. The operational validation presented superior accuracy compared with the current method for controlling the temperature, resulting in a reduction in the aluminum consumption for heating at secondary metallurgy with a potential economy of U$ 4.07 million per year for a steel shop producing 5 million tons of steel yearly. The artificial neural network model confirmed its capacity for modeling a complex multivariable process and the separation of thermodynamic calculation provides a better adaptability to different steel grades with different teeming strategies.

In the steel industry, lime is used as a desulfurizing agent and fluorspar as flux in the hot metal desulfurization process. However there are environmental concerns regarding the use of fluorspar, even when its harmful effects are not fully known. In order to reduce such risks and also to anticipate possible future restrictions, which could undermine the sustainability of steel production, it is proposed to replace the fluorspar for alternative materials in the composition of the desulfurizing lime based mixture. For this purpose new mixtures using aluminum dross, ilmenite, sodalite and ulexite were tested, comparing them with a reference mixture containing fluorspar. The impact of these mixtures on the refractories commonly used in hot metal pretreatment was also checked, since this is an important cost parameter for the steel industries. The laboratory tests showed ulexite as a feasible substitute since the lime (84.03%)-ulexite (10.97%)-graphite (5%) mixture presented the highest desulfurization ratio and low refractory wear (less than the standard fluorspar mixture). The sodalite mixture was the worst by comparison. The ilmenite and aluminum dross mixtures showed similar behavior when compared to the fluorspar mixture.

Resumo Gás inerte é usualmente injetado no refratário da válvula superior durante o lingotamento continuo visando minimizar a sucção de ar para o sistema e consequente oxidação do aço, deposição de inclusões e obstrução do sistema de refratários. Nesse contexto, esse estudo pretende investigar a formação e o comportamento das bolhas de gás no interior de um sistema com fluxo líquido, simulando o sistema de refratários, utilizando-se para isto da injeção via placas refratárias, com características distintas, produzidas a partir de receitas industriais da empresa RHI Magnesita. Esta investigação foi conduzida na região do refratário e na região da válvula submersa via modelo físico, em acrílico, e o efeito do refratário e das vazões de gás e liquido foram discutidos.As filmagens foram realizadas com câmera de alta velocidade à 3680 frames por segundo (fps), tendo sido implantada uma sistemática para medição do Diâmetro Médio (Dm) das bolhas e sua validação estatística.Os experimentos confirmaram o efeito do Refratário, do fluxo e da vazão de gás no diâmetro médio das bolhas para fluxo bifásico em canal.

Synthetic slag has been added during BOF tapping in order to modify the physicochemical properties of resulting slag with the aim of assessing the influence on inclusion density, composition and size distribution. Viscosities and activities of slag oxides were calculated by FactSage 7.2, using the FToxid database and Viscosity module. All heats were treated under vacuum in an RH degasser for 15 minutes. A comparative analysis of top slags with two levels of FeO+MnO (> 10% and ≤ 10%) was carried out. The higher level of FeO+MnO led to almost thrice more inclusion than the lower level. Top slags after treatment with synthetic slag brought the best result of inclusions features thanks to their higher CaO activity, lower viscosity and lower activity of Al 2 O 3 . These heats presented smaller inclusions and lower initial inclusion density at ladle arrival at RH. Based on these evidences, top slags with synthetic slags added during the tapping and FeO+MnO content ≤ 10% conferred the best condition in steel cleanliness.