[Alcoa China] Reoxidation is a common phenomenon during the steelmaking process, particularly in the tundish. When severe reoxidation occurs, it introduces unwanted impurities into the molten steel, which can significantly reduce its cleanliness and negatively impact the mechanical properties and overall quality of the final product. This study aimed to investigate the adverse effects of reoxidation in the tundish on non-metallic inclusions in molten steel, with a specific focus on calcium-treated aluminum-killed steels and their impact on end products. Inclusions at different stages—RH refining, tundish, slab, and steel plate—were analyzed in API-X70 pipeline steel. To ensure more accurate and comprehensive results, an automated scanning electron microscope (APEXPSEMEXPLORER) was used to examine a larger area of the samples. The findings revealed that reoxidation in the tundish caused well-formed liquid calcium aluminate inclusions to transform into solid Alâ‚‚O₃-rich inclusions. These inclusions are more prone to nozzle clogging compared to pure Alâ‚‚O₃. The inclusion density increased from 2.8 μmâ»Â² after RH refining, rose to 13.5 μmâ»Â² in the tundish, and then decreased to 6.0 μmâ»Â² at the slab and steel plate stages. This indicates that reoxidation in the tundish reduces the effectiveness of inclusion removal during calcium treatment and RH refining. Furthermore, the proportion of liquid inclusions was found to correlate directly with the average CaO/Alâ‚‚O₃ ratio in the inclusions. Large aggregates and hairline inclusions were observed at 1/2 and 1/4 of the thickness of slabs and steel plates. The ridged inclusions found in the steel plate are believed to be remnants of these aggregates formed in the slab during rolling, which can cause failure in ultrasonic testing. Understanding these mechanisms is crucial for improving steel cleanliness and ensuring the reliability of final products.
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