Dissolution of Al 2 O 3 -based inclusions are paramount during production of special steel and slag engineering is key to enhance this phenomenon. This work evaluates the influence of thermodynamic driving force (ΔC) and effective viscosity (η e ) on steel cleanliness of three distinct steel grades and slag composition (Steel/Slag A, Steel/Slag B and Steel/Slag C). Initial and final steel and slag samples were withdrawal on an electric steelmaking facility. The first samples after addition of aluminum as deoxidizer and the second after vacuum degassing stage. X-ray fluorescence and optical spectrometry obtained initial and final samples chemical composition. An ASPEX Explorer acquired inclusions data and FactSage v.7.2 performed thermodynamic calculations. Analysis of steel Al and O content highlighted an inefficient dissolution of Al 2 O 3 -based inclusions for Steel/Slag A, deteriorating steel cleanliness. Furthermore, this phenomenon is supported by average inclusions composition on initial and final samples plotted in pseudo-ternary diagram. Steel/Slag B and C provided improved results regarding the dissolution of Al 2 O 3 -based inclusions with a final inclusion density below 0.50 mm -2 . These two slags composition achieved values of ΔC above 25 and η e close to 0.10 Pa•s. In addition, their combined effect (ΔC/η e ) presented values above 250 and had the highest linear fit among these analyses. These properties are influenced by slags chemical composition and can be improved controlling parameters as binary basicity and CaO/Al 2 O 3 ratio. Slags B and C were selected to define an optimal range for these parameters, with binary basicity between 3.00-3.50 and CaO/Al 2 O 3 ratio in a range from 2.50-3.50.
In order to investigate the formation of inclusions by reoxidation in the tundish, an MgO-CaO gunning mass (GM1), an MgO gunning mass (GM2), a calcium aluminate tundish slag (TS), and a rice husk ash (RHA) silica based are evaluated. The interaction between the steel and these materials is analyzed under laboratory conditions. The experimental results for the chemical composition of steel and the types of inclusions formed satisfactory reach the thermodynamic equilibrium calculated by FactSage. GM1 presents better results for inclusion density and average size of inclusions compared to GM2. The experiment with TS presents a good steel cleanliness despite the formation of numerous alumina inclusions, caused by the reaction between the slag and the alumina crucible. The addition of RHA increases the inclusion density and average size of inclusions, and also the formation of CaO-Al 2 O 3 -SiO 2 oxides inclusions is favored. RHA has harmful effects concerning reoxidation events, deteriorating the good cleanliness obtained with the TS. The experiments with GM1 and TS are less reoxidative and presents a better steel cleanliness. Thus, the usage of this gunning mass and a calcium aluminate slag without RHA is preferable in order to diminish reoxidation during the tundish steelmaking.
Al 2 O 3 (alumina) and MgO•Al 2 O 3 (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 FactSage TM 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 SiO 2 contents in the slag, an increase of binary basicity (%CaO/%SiO 2). 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).
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