Interpretation and Classification of Non-Metallic Inclusions

Abstract

Abstract The non-metallic inclusions in steel are one of the most important parameters of metallurgical quality. Despite the extensive progress in thermodynamic description of non-metallic inclusions formation in the liquid and solidifying steel, a complete simulation of their evolution during secondary metallurgy, casting and solidification is impossible. The processes for non-metallic inclusion formation, growth and elimination, taking into account macro- and micro-segregation evolution, are too complicated. Therefore, a statistically reliable estimation of the amount, morphology, size distribution and chemical composition of non-metallic inclusions obtained by modern experimental approaches could be an important basis for understanding steelmaking technology. Automated feature SEM/EDS analysis of the chemical composition, size, and volume fraction of more than 600 non-metallic inclusions in rail steel was conducted. Analysis of these non-metallic inclusions' database by developed software found the following clusters of inclusions: (1) oxides, Al-Ca-Si-Mn-S-O; (2) predominantly sulfides, Mn-S-Al-Ti-O; (3) oxides and sulfides, Mn-S-O; and (4) complex non-metallic inclusions, Ti-Mn-O(N). All found compositions of non-metallic inclusions were placed on the ternary diagram Mn-Al-S and they lined up from the Al corner to the MnS point on the Mn-S axis. The chemical compositions of the non-metallic inclusions evolved from Al-Ca-Si-Mn-S-O to Mn-S-Al-Ti-O and, finally, to the Mn-S-O+Ti-Mn-O(N) system. Interpretation of the chemical composition of non-metallic inclusions by thermodynamic modeling revealed the nature of each cluster with correspondence to the secondary metallurgy stage where they were formed. This information could be useful for improvement of steelmaking technology. Accumulation of cluster analysis results for non-metallic inclusions in different steels could be a basis for the development of a universal classification of non-metallic inclusions produced by modern secondary metallurgy technologies.