Effect of ladle age on formation of nonmetallic inclusions in ladle treatment

An experimental study was carried out at Uddeholm Tooling, Hagfors, Sweden to examine the impact of ladle glaze on the formation of non-metallic inclusions during the ladle treatment process. Steel samples were taken at various stages of the process and from ladles of different ages. Inclusion numbers were counted under an optical microscope. It was found that the total number of inclusions increased with the ladle age before deoxidation and at the end of the ladle treatment. The increase was substantial after the ladle had been used more than 18 times. Inclusions having the smallest sizes were found to make a large contribution to this increase. This observation was further confirmed by the difference between total oxygen content and dissolved oxygen content in the steel samples, which also showed an increase with ladle age. Analysis by SEM-EDX revealed two types of inclusions before casting, namely, inclusions consisting only of an oxide solution having a composition very close to 3CaO.Al2O3, and inclusions consisting of the same oxide solution as well as MgO phase. This finding was in accordance with the reported result that both 3CaO.Al2O3 and MgO were present in the slag infiltrated layer of the ladle glaze. It was concluded that ladle glaze is the foremost source of non-metallic inclusions in tool steel during ladle treatment.