In situ observation of instantaneous evolution of CaO-Al2O3-MgO-SiO2 slag-entrained inclusions in Al-killed steel

To clarify the compositional difference between the slag and slag-entrained inclusions in steels, it is of great significance to study the evolution mechanism of slag-entrained inclusions in the molten steel. The evolution mechanism of various slag-entrained inclusions was little studied through laboratory experiments. In the current study, the instantaneous evolution of CaO-Al2O3-MgO-SiO2 slag-entrained inclusions in steels was studied through a series of in situ observation experiments. Large CaO-Al2O3-MgO-SiO2 inclusions were designed according to the composition of various industrial slags, including refining slag, tundish flux, and mold flux. To simulate the reaction process of slag-entrained inclusions in the molten steel, an inclusion particle was placed on the steel surface and maintained for a constant time after steel melting using confocal laser scanning microscopy. The results showed that the CaO content declined in slag-entrained inclusions from refining slag and tundish flux. Contents of CaO and SiO2 in the slag-entrained inclusion from mold flux gradually decreased simultaneously over time. Additionally, the impact of dissolved aluminum content on the composition evolution of CaO-Al2O3 based slag-entrained inclusions was also discussed. Then, thermodynamic and kinetic analyses were employed to explore the reaction mechanism and transfer behavior during the composition evolution of CaO-Al2O3 based slag-entrained inclusions. Thermodynamic analyses revealed that higher dissolved aluminum content promoted the desulfurization reaction that consumed CaO to form CaS in inclusions. Kinetic analyses indicated that the reaction of dissolved aluminum reducing CaO was dominantly controlled by the internal diffusion of CaO and Al2O3 in inclusions. Based on the diffusion-controlled kinetic model, the mass transfer coefficient for CaO and Al2O3 diffused in CaO-Al2O3 based inclusions was determined from experimental data, with the value calculated as 3.23×10-6 m·s-1. Importantly, the current experimental method can be widely used to simulate the evolution behavior of inclusions in the molten steel from refining slag, tundish slag, and mold flux.