Study of Inclusions in High Sulfur, Al-Killed Ca-Treated Steel via Experiments and Thermodynamic Calculations

Abstract

In the production of special steels, inclusions have a significant influence on final product quality. Moreover, there is a growing need for constant improvement of mechanical and machinability properties, such as in the production of complex pieces for the automotive applications. The main objective of this study was to conduct a thermodynamic study aimed at preventing clogging problems during continuous casting (CC) and the formation of undesirable inclusions. Alumina (Al2O3), spinels, and calcium sulfide (CaS) are potential sources of defects during rolling and steel forging. The specific goals of this study were: (1) To determine the phases and compounds formed in the inclusions as a function of composition and casting temperature adopted for high sulfur Al-killed Ca-treated steel, and (2) To establish chemical composition conditions resulting in the formation of less harmful inclusions for steel castability. Based on the literature and previous studies by the authors, calculations were carried out using FactSage commercial software and related databases. The calculations were performed using the steel temperature and chemical composition in the CC tundish. The results of these calculations were used to estimate the composition of steel and non-metallic inclusions (oxides and sulfides) at the given steel temperature. Based on these calculations, it was predicted a calcium content range where inclusions contained a high proportion of liquid phase (i.e., “liquid window”). The results of thermodynamic simulation were compared to industrial outcomes in terms of inclusion composition and casting index. The thermodynamic predictions were validated and confirmed by industrial results.