Thermodynamic Analysis on the Formation and Chemical Development of Nonmetallic Inclusions in Ladle Treatment of Tool Steels

摘要

A thermodynamic analysis was carried out to reveal the mechanisms of the formation and chemical development of nonmetallic inclusions in ladle treatment of tool steels. In parallel, an industrial investigation was conducted at Uddeholm Tooling AB, Hagfors, Sweden. The inclusions found in the steel samples taken at different stages of ladle treatment were analyzed and identified under a scanning electron microscope with an EDS x-ray analyzer. Six types of inclusions were found. The types of inclusions were found to vary with the stage of ladle treatment. Thermodynamic analysis could successfully explain the formation and the chemical development of the inclusions throughout the process. Three types of inclusions were found in the liquid steel before deoxidation, namely, type A (liquid inclusions with high SiO_2 concentration), type B (spinel) and type C (a combination of A and B). Thermodynamic analysis indicated that these types of inclusions could be generated by the reaction between EAF slag and the ladle glaze during the filling of the ladle. The addition of aluminum would form the alumina inclusions (type E), which agglomerated and floated up quickly. The spinel inclusions would not be stable at low oxygen potentials. The inclusions of this type would react with the liquid metal, forming inclusions of type F (spinel in the center surrounded by an oxide solution containing Al_2O_3, CaO and MgO). Furthermore, reaction between the liquid metal and inclusions of type F would result in the inclusions of type G, an oxide solution consisting mostly of Al_2O_3 and CaO with small amounts of MgO and SiO_2.