Investigation on Nonmetallic Inclusions in Ultra-Low-Oxygen Special Steels

Investigation was carried out on inclusions in ultra-low-oxygen special steel by taking systematic samplings not only from secondary refining but also from continuous casting. It was found that RH degassing had a much stronger ability than LF refining in removing inclusions. Total oxygen (TO) can be further reduced from 0.0013 to 0.0015 mass pct to about 0.00047 mass pct without too much difficulty by elongating RH degassing time to 33 minute. Inclusions larger than 5 μm were decreased from 1.49 to 0.08 N/mm² with a removal ratio of about 94 pct. During secondary refining, inclusions experienced continuous evolution from Al₂O₃ to the MgO-Al₂O₃, CaO-MgO-Al₂O₃, and CaO-Al₂O₃ system due to complex reactions among slag, steel, and refractory materials. Because of more efficient removal of solid inclusions, especially those large ones during RH treatment, thorough modification of solid inclusions into liquid CaO-MgO-Al₂O₃ can be achieved. Reoxidation during casting resulted in an obvious rise in Al₂O₃ content in inclusions, which offset the efforts made during secondary refining. Liquid CaO-Al₂O₃ systems after RH were changed to solid CaO-Al₂O₃, MgO-Al₂O₃, or CaO-MgO-Al₂O₃ again. Large inclusions with sizes approaching several hundred micron were frequently observed in casting bloom by a vast area of scanning under an automatic scanning electron microscope despite very low TO of about 0.00059 mass pct. A total of 78 pct of them were cluster-shaped CaO-MgO-Al₂O₃ solid inclusions with sizes as large as 200–330 μm. Most clusters over 100 μm were constituted by smaller particles with similar chemical compositions to other small singular inclusions in steel, which indicated that they were formed due to the aggregations of small inclusions during casting. Moreover, the location of them indicated a close relationship to the solidification microstructure of steel, which has not been covered before. It was impressively found that larger inclusions were mainly in the column grain zone area, while those in the central equiax grain zone of the bloom had smaller sizes.