Abstract Al2O3 (alumina) and MgO·Al2O3 (spinel) inclusions cause valve obstruction (clogging) in continuous casting and can deteriorate the quality of the final product. In this context, industrial heats of the bearing steel SAE 52100 was examined. Samples were collected in the final steps of the steelmaking process, both after vacuum treatment and during continuous casting. A scanning electron microscope (SEM) equipped with a energy-dispersive spectrometer (EDS) and automated particle characterization analysis was used to characterize the inclusions present in the steel samples. Thermodynamic calculations were performed with the commercial software FactSageTM 7.2. Based on thermodynamic predictions, parameters such as solid fraction, liquid fraction, MgO saturation point of the slags, content of dissolved elements in steel (Al, Mg, Ca,...) and the construction of a phase stability diagram were determined. The results in this study showed a tendency for increase in MgO content in the inclusions with the decrease of %FeO and SiO2 contents in the slag, an increase of binary basicity (%CaO/%SiO2). It is verified that the MgO contents in the slag were close to the saturation, increasing the probability for the formation of inclusions rich in MgO and/or spinel. On the other hand, stability diagrams confirm the formation of spinel inclusions for each of the heats analyzed. During the final step of the steelmaking process, there is a tendency for re-oxidation, which is verified by an increase in the density of inclusions (or total oxygen TO values).

Secondary refining slag samples with different chemical compositions (45-54 % CaO, 7-13 % Al2O3, 20-29 % SiO2, 9-16 % MgO, 0-5 % CaF2) were investigated to verify the influence of their effective viscosity on inclusion cleanliness of DIN 38MnS6 in a steelmaking plant. The steel samples were collected during the production process for analysis of inclusions. Using the commercial software FactSage 6.4, thermodynamic calculations were performed to determine the effective viscosity, solid fraction, liquid fraction and MgO saturation point of these slags at 1560ºC. The results showed that all the slags were saturated in MgO, revealing a better protection of the ladle refractory. The addition of 2 to 5 % of CaF2 reduced the effective viscosity values for the analyzed slags from 0.45 Pa∙s to 0.10 Pa∙s , in comparison to the slags without the addition of CaF2, with an effective viscosity of 0.40 Pa∙s, 0.27 Pa∙s and 0.22 Pa∙s, decreasing the level of non-metallic inclusions for some of the analysed heats. However, it was detected during the manufacturing process that high slag fluidity and re-oxidation events continue to be a challenge associated with reducing the level of non-metallic inclusions.

The process of vacuum degassing occurs during the secondary refining of special steels. Its main function is to remove undesirable gases. However, during this process, flotation phenomenon and inclusions absorption are reported. The aim of the present work was to study the slag viscosity and vacuum degassing (tank type) capacity in steel cleanliness from an industry perspective. To achieve this objective, slag and steel samples were taken before and after the vacuum stage. The results in steel cleanliness were related to a kinetic stirring parameter (βs) of the vacuum station and to the effect of slag viscosity. The removal of inclusions during the vacuum stage reached 64, 75 and 78% in the diameter ranges of 2.5-5, 5-15 and ≥ 15 µm, respectively. After the degassing process, the composition of non-metallic inclusions seemed to approach the slags' chemical compositions. The stirring process in the vacuum degassing station promotes a significant decrease in the inclusion densities for the 2.5-15 µm diameter range and also in the sulfur content in liquid steel. Regarding the effective viscosities of slags, it was concluded that lower values (0.20 Pa.s) increased slag capacity in inclusion removal, whereas higher values (> 0.40 Pa.s) were detrimental to steel cleanliness.

A eficiência do processo de fabricação do aço durante a etapa de refino secundário na panela está relacionada aos fenômenos de mistura. Em função disto, é de muito interesse o conhecimento do estado de mistura que ocorre durante o tratamento do aço líquido na panela tendo em vista a otimização do processo. Assim sendo, o presente estudo tem como objetivo determinar, através de um modelo físico de uma panela industrial em escala reduzida (1:6), o efeito de variáveis operacionais como: vazão de gás, posição de injeção de gás, adição de traçador e posições de medida, sobre os tempos de mistura na homogeneização do sistema. A análise dos resultados permite concluir que o tempo de mistura tende a diminuir com um aumento de vazão de gás e que, para injeções de gás a 1/2 raio, encontram-se tempos de mistura superiores do que a injeção a 1/5 raio. Esses resultados encontram-se em concordância com os resultados de outros pesquisadores, no que se refere ao efeito de parede e à escala do modelo.

Production efficiency during the secondary refining stage in the ladle is related to the mixture's phenomenas. Because of this, it is very important to understand the mixture's state as it occurs during the liquid steel treatment in the ladle, aiming for process optimization. So, the present study wants to determine, through a physical model in reduced scale (1:6), the effect of operational variables such as: gas injection, gas injection position, tracer addition and measurement position, and the mixing times in the homogenization system. The results make it possible to conclude that the mixing time tends to decrease with gas injection increase and that the mixing time at 1/2 radios is greater than the one at 1/5 radios. These results are in accordance with other researchers' papers referring to the wall effect and the model scale.