Two stage mass transfer kinetics and control of the deep deoxidation window with low splashing during vacuum final carbon deoxidation of molten steel

Vacuum final carbon deoxidation is regarded as an important route for producing ultra-low oxygen clean steel because its deoxidation product is removed in the form of CO gas and, in principle, does not introduce solid oxide inclusions. However, the process still involves unresolved issues related to deep deoxidation capability, the deoxidation rate at the later stage, and splashing risk under high oxygen conditions. In this study, the effects of ambient pressure, initial oxygen content, and carbon content in molten steel on vacuum final carbon deoxidation behavior were systematically investigated in a 2 kg vacuum induction furnace. A pressure constraint model for CO bubble nucleation was established, and the deoxidation process was divided into an initial exponential decay stage and a later linear stage for quantitative kinetic characterization. The results show that ambient pressure significantly affects the deep deoxidation capability at the later stage. Under pressures of <30 Pa and 5000 Pa, the initial deoxidation rates are similar, whereas both the initial and later stage deoxidation processes are markedly suppressed at 50000 Pa, with the later stage linear deoxidation rate decreasing to only -0.49×10 -6 /min. The initial oxygen content mainly affects the reaction intensity and splashing risk in the early stage. Under high oxygen conditions, the carbon oxygen reaction is violent and prone to cause splashing, whereas initiating vacuum final carbon deoxidation at a medium to low oxygen level is more favorable for stable operation with low splashing. The carbon content in molten steel has a significant influence on the final deoxidation limit, but within the investigated carbon content range and operating conditions, its effect on the macroscopic deoxidation rate is limited, suggesting that the deoxidation rate under the present experimental conditions is mainly controlled by mass transfer through the liquid phase boundary layer. Under the present experimental conditions, especially at an ambient pressure below 30 Pa and within the investigated carbon content range, the final oxygen content of molten steel was reduced to 3×10 -6 to 6×10 -6 , with an inclusion number density of about 0.6 mm -2 . The observed inclusion characteristics suggest that the residual inclusions were more likely associated with refractory or slag related sources rather than conventional alloy deoxidation products.