Thermodynamic and experimental study on CO2 injection in RH decarburization process of ultra-low carbon steel

In this work, carbon dioxide (CO2) was investigated as a lifting gas in the Ruhrstahl-Heraeus (RH) decarburization process of ultra-low carbon steel. The main challenge was to determine whether the local carburization reaction leads to the carburization of molten steel with ultra-low carbon content. Thermodynamic analysis of the reaction of the Fe-C-O melt with CO2 demonstrated that compared with traditional Ar injection, the decarburization rate can be improved during the early stage of the decarburization process. This occurs because oxygen that is present not only in molten steel but also produced by CO2 dissociation can react with dissolved carbon to remove the carbon from molten steel. However, it is difficult to determine whether the decarburization rate can be influenced during the later stage by injecting CO2. This is because the carburization reaction at the surface of CO2 bubbles in the up-snorkel and the decarburization reaction at other reaction sites can occur simultaneously. Industrial test results demonstrated that compared with Ar injection, the decarburization rate was not significantly affected by CO2 injection at the early stage, whereas it was reduced during the middle and later stages. Nevertheless, the average carbon content in molten steel after the RH treatment with CO2 injection was only 2.6 ppm higher compared with that in Ar injection. Therefore, Ar can potentially be replaced by CO2 in the RH decarburization process of ultra-low carbon steel based on economic and environmental considerations.