Computational thermodynamics application on the calcium inclusion treatment of SAE 8620 steel

Abstract

A thermodynamic study was carried out to evaluate the contents of Al, Ca, S and O at which liquid or partially liquid inclusions are formed to optimize the castability of SAE 8620 steel. Samples of steel secondary treatment (Ladle) and continuous-casting billets (Tundish) were obtained from an electric steel mill. Entry data for this study were obtained in heats carried out in the steel plant. Furthermore, analyses were performed with a Scanning Electron Microscope (SEM) and an Energy-Dispersive Spectrometer (EDS), as well as chemical analysis of both steel and total oxygen. In the Ladle sample, after calcium addition, inclusions had concentrations between 30 and 45% CaO and consisted mainly of calcium-aluminates with MgO content of 10%. In the Tundish sample, inclusions contained a high percentage of alumina. Combining the characterization of inclusions with a computational thermodynamics model, the obtained results showed that the ideal range of calcium may vary between 10 and 14 ppm to form liquid inclusions with a minimum CaS content for the O, S and Al levels from SAE 8620 in this process stage. An analysis of Mg influence on the formation of calcium-aluminates was also carried out. For contents of Mg found in aluminium-killed steels treated with Ca, Mg does not significantly influence the formation of calcium-aluminates inclusions. Higher calcium contents can be taken into account, depending on both the process type in steel production and the demanded product properties. The present study shows that the combined application of both inclusions characterization techniques and computational thermodynamics can drastically reduce the empiricism regarding the development of processes for the control of non-metallic inclusions in steels. Thus, this research can contribute to increase the efficiency of manufacturing processes, improve the quality of existent products and make possible the economic development of new ones.