Characteristics, formation mechanisms and removal of large-sized CaO-MgO-Al ₂ O ₃ inclusions in an aluminum-deoxidized and sulfur-containing XC45 steel

XC45 steel, an aluminum-deoxidized and sulfur-containing steel, is widely used for mechanical components, whose fatigue performance and service life are strongly affected by CaO-MgO-Al 2 O 3 (CMA) inclusions with large size and low interfacial tension, and commonly identified as D- or DS-type inclusions. To investigate their formation mechanism and control, systematic industrial sampling was conducted during LF refining, VD treatment, and tundish stages, together with inclusion characterization by SEM-EDS and thermodynamic analysis. The results showed that the composition of CMA inclusions in the billet was fixed at tundish stage, while their size was significantly influenced by the CaO content in inclusions. The changes of MgO content in inclusions affected their aggregation and melting/wetting behavior, thereby altering inclusion growth and removal trend. Based on these findings, a control strategy was proposed by maintaining total calcium at 0.0003–0.0005 wt.%, promoting the formation of removable MgO·Al 2 O 3 and CaO·2Al 2 O 3 inclusions while suppressing CaS precipitation. This ensures the liquid phase fraction of all inclusions exceeding 20%, thereby alleviating nozzle clogging during continuous casting. Industrial trials confirmed a significant reduction in inclusion amounts larger than 5 μm in the billets. This study provides an effective guideline for improving steel cleanliness and product quality of aluminum-deoxidized and sulfur-containing steels.