Key Takeaways & Executive Findings
- •• In-situ observation reveals that Al2O3 inclusions undergo instantaneous morphological transition to spherical calcium aluminates during calcium treatment, with overall regularity increasing over time. • An empirical formula linking inclusion composition (CaO/Al2O3 ratio) to overall regularity enables real-time prediction of inclusion modification state. • A CaO/Al2O3 mass ratio of 0.451 corresponds to ideal spherical calcium aluminate inclusions with overall regularity of 1, providing a quantitative target for process control. • Smaller Al2O3 particle sizes accelerate the transformation to spherical calcium aluminates, highlighting the importance of inclusion size control in steel refining.
Abstract
The instantaneous morphological transition of triangular Al2O3 particles with various sizes in the molten Ca-treated steel was observed using a confocal scanning laser microscope at the steelmaking temperature. The composition of inclusions at different times was analyzed using scanning electron microscopy–energy dispersive spectroscopy. The shape evolution of particles was characterized by the shape parameter of overall regularity. It was found that the overall regularity of particles gradually increased with time during the calcium treatment. The geometry of particles tended to be more rounded and regular as the overall regularity increased during the modification process. An empirical formula was proposed to predict the composition of inclusion particles based on their overall regularity during the calcium treatment. When the CaO/Al2O3 mass ratio in the particle increased to 0.451, the particle was considered an ideal spherical calcium aluminate inclusion with the overall regularity of 1. Smaller particle sizes promoted the transformation of Al2O3 inclusions to spherical calcium aluminates during the calcium treatment.
1. Introduction
The control of inclusions in the steelmaking process was of great importance to improve the steel cleanliness [1–4]. The mechanical performance of steel products was influenced by inclusion characteristics [5–7]. Irregular and hard alumina inclusions in the Al-killed steel can lead to the stress concentration and lower the plasticity, toughness, and fatigue resistance of steel products [3,8–11]. The quality of steels was related to the number, size, morphology, and distribution of inclusions in steels [12–16]. In Al-killed steels, calcium treatment was widely used to transform solid alumina inclusions into liquid calcium aluminates to avoid the nozzle clogging during the continuous casting process [17–18]. Liquid calcium aluminates were hardly removed and remained in steels since the contact angle between calcium aluminates and the molten steel was less than 90° [19]. Solid alumina inclusions with irregular shape reduced the quality of steel, while liquid calcium aluminates inclusions can hardly be effectively removed in steels. Thus, it is necessary to study the evolution mechanism of the morphology and composition of alumina inclusions during the calcium treatment.
Yue et al. [20] studied the fractal dimension of inclusions with different compositions, which was closely related to the composition and melting temperature of inclusions. The theory of fractal dimension was widely used to quantify morphology characteristics of cluster inclusions in steels [21–22]. Parameters of aspect ratio (AR), sphericity (S), and convexity (C) were commonly used to characterize the shape of the single particle [23–25]. It should be emphasized that values of AR, S, and C are quantified in the range from 0 to 1. As the shape of the particle changed from irregular to spherical, parameter values of AR S, and C showed an increasing trend. It is necessary to determine the quantitative relation between the shape value of AR, S, and C and the composition of inclusions. Yang and Luo [26] proposed a collective description of overall regularity (OR) to characterize the irregularity of the particle shape, and the OR was defined as the average of AR, S, and C. The particle exhibited more regular spherical shapes as the OR value increased. Normally, inclusion characters were mainly analyzed after the solidification of steel samples. In addition, the average AR of inclusions in steel was frequently used to characterize the shape of inclusions. However, the AR used to quantitatively characterize the shape of inclusions was too simple.
The morphology of inclusions was correlated with their chemical composition during the calcium treatment. Calcium aluminates with low melting temperatures were liquid and spherical at the steelmaking temperature [27]. The morphology evolution of inclusions at the steelmaking temperature was observed in this study.
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Guojun Chen, Hejun Zhang, Ying Ren, Lifeng Zhang (2025). Instantaneous transition of composition and morphology of inclusions with an initial Al2O3 composition in the molten steel during calcium treatment. Int. Journal of Minerals, Metallurgy and Materials (矿物冶金与材料学报). https://doi.org/10.1007/s12613-025-3101-2
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Frequently Asked Questions
What is the main objective of this study?
The study aims to observe the instantaneous morphological transition of Al2O3 inclusions during calcium treatment in molten steel and to establish a quantitative relationship between inclusion composition and shape parameter (overall regularity).
How was the morphology of inclusions characterized?
The morphology was characterized using the shape parameter of overall regularity (OR), which is the average of aspect ratio, sphericity, and convexity, all ranging from 0 to 1.
What is the significance of the CaO/Al2O3 mass ratio of 0.451?
When the CaO/Al2O3 mass ratio reaches 0.451, the inclusion is considered an ideal spherical calcium aluminate with an overall regularity of 1, indicating complete modification.
How does particle size affect the transformation of Al2O3 inclusions?
Smaller particle sizes promote the transformation of Al2O3 inclusions to spherical calcium aluminates during calcium treatment, as they modify more rapidly.
What practical applications does this research have?
The empirical formula proposed can be used to predict inclusion composition in real-time based on shape, aiding in process control to improve steel cleanliness and prevent nozzle clogging.
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