Formation of Low-Melting-Point Inclusions in Al-Deoxidized Steel Refined by High-Basicity Calcium Aluminate Slag in ZrO₂ Crucible Experiments

ZrO₂ crucible experiments were carried out in the laboratory to study the formation of low-melting-point inclusions in steel, during which aluminum deoxidization and a high-basicity calcium aluminate refining slag (CaO/SiO₂: 6 to 8, Al₂O₃ 40 to 45 pct) were used. Four experiments were done with different slag/steel reaction times (30, 60, 90, and 180 minutes). It was found that inclusions were mainly composed of CaO-Al₂O₃-ZrO₂ with very limited SiO₂, in spherical morphology and with sizes mainly less than 5 μm. They can be classified into two types according to the ZrO₂ content. The first type contained much lower ZrO₂, whereas a much higher level of ZrO₂ was detected in the other type. An evolution of inclusions with the reaction time was observed and studied. The obtained results indicated that chemical compositions of inclusions were widely scattered in the CaO-Al₂O₃-ZrO₂ phase diagram after 30 min reaction. However, the composition of inclusions became much more uniform and concentrated in low-melting-temperature regions at 60, 90, and 180 minutes, which would be favorable to prevent nozzle clogging and fatigue problems. Compared with the authors’ previous results obtained in MgO crucible experiments, it was found that low-melting-temperature inclusions can be targeted in shorter time in ZrO₂ crucible experiments, without any degradation of cleanliness.