Mathematical Modeling Of Non-equilibrium Decarburization Process During The RH Refining Of Molten Steel

The developing history and progresses obtained in the RH refining technology molten steel have been reviewed, and its metallurgical functions were briefly described, the available achievements and present situations for study on physical and mathematical modeling of this refining process have been analyzed and summarized. Also, the investigations of the gas bubble diameter in the liquid in the up-snorkel and the decarburization mechanism during the RH refining process of molten steel have been discussed.The basic fundamentals of non-equilibrium thermodynamics have been introduced and described. Taking the vacuum circulation (RH) refining of clean steel (ultra-low-carbon and ultra-low-sulphur steel) as an example, the non-linear and non-equilibrium features of metallurgical processes have been illustrated. The similarities and differences between metallurgical reaction engineering and non-equilibrium thermodynamics have been analyzed. The necessity and feasibility investigating and dealing with practical metallurgical processes from the viewpoints, fundamentals and methods of non-equilibrium thermodynamics with metallurgical reaction engineering have been discussed. It is pointed out that to get a clear and true understanding of the natures and internal patterns of practical metallurgical processes and really and quantitatively describe the processes, their features of non-linear and non-equilibrium must be fully be taken into account. Non-equilibrium thermodynamics should and can play its proper role in the metallurgical area, and the studies on non-equilibrium thermodynamics of metallurgical processes and its applications should be enhanced and accelerated to develop and carry out. The corresponding constitutive relations between the thermodynamic fluxes and forces, interacting and phenomenological coefficients of a few of special cases located in the linear region of non-equilibrium thermodynamics, have been analyzed. Simultaneously, some available studies in the literature have been briefly introduced and summarized. Proceeded from themodynamic stability, the studies on application of non-linear and non-equilibrium thermodynamics theory to the two metallurgical processes of electrolysis of aluminum and decarburization of molten steel have also been reviewed.The flow and mixing characteristics of molten steel during the vacuum circulation refining were investigated on a 1:5 linear scale water model unit of a 90 t RH degasser, respectively with the diameters of the ports of 1.2 and 0.8 mm. The flow patterns for the two situations were essentially same, and there were no obvious changes. The relation of the circulation flow rate withsome main process parameters at din = 1.2 mm can be expressed as Q₁ ∝ Q_g^0.26D_u^0.69D_d^0.80 , The circulation flow rate slightly increases with an increase of the port diameter and the corresponding relation can be described by Q₁ = 2.40Q_g^0.23D_u^0.72D_d^0.88d_in^0.13 . Relevantly, themixing time little decreases with an increase of the port diameter. The relations of between mixing time and stirring energy density are rm oc eos and T oc e049, respectively at dm = 0.8 mm anddm - 1.2 mm.Based on the two-fluid (Eulerian-Eulerian) model for a gas-liquid two-phase flow and the modified k-s model for turbulent flow, a three-dimensional mathematical model for the flow of the molten steel in the whole degasser during the RH refining process of molten steel has been proposed and developed with considering the physical characteristics of the process, particularly the behaviors of gas-liquid two-phase flow in the up-snorkel and the momentum exchange between the two phases. The related parameters of the model have been determined, and the lifting gas properties at the inlet section in the up-snorkel has been calculated with considering the gas stream in the gas blowing pipe being a heating and friction flow. In addition, the lifting gas temperature reached in the molten steel of the up-snorkel has been determined from the estimated results for the heat transfer between the gas jets and the liquid steel. The fluid flow fields and the gas holdups of liquid phases and others respectively in a 901 RH degasser and its water model unit with a 1/5 linear scale have been computed using this model. The results showed that the flow pattern of molten steel in the whole RH degasser could be well modeled by the model. The liquid can be fully mixed during the refining process except the area close to the free surface of liquid and zone between the two snorkels in the ladle, but there is a boundary layer between the descending liquid stream from the down-snorkel and its surrounding liquid, which is a typical liquid-liquid two phase flow, and the molten steel in the ladle is not in a perfect mixing state. The lifting gas blown is rising mostly near the up-snorkel wall, which is more obvious under the conditions of a practical RH degasser, and the flow pattern of the bubbles and liquid in the up-snorkel is closer to an annular flow. The calculated circulation rates for the model unit at differ-ent lifting gas rates are in good agreement with the determined values.Considering the characteristics of the non-equilibrium decarburization process during the RH refining of molten steel, a new mathematical model has been proposed and developed on the base of the metallurgical reaction engineering and non-equilibrium thermodynamics theories and the two-fluid model for gas-liquid two-phase flow. The details have been presented, including the establishment of the control equations, the determination of the boundary conditions and the source items and the related parameters. The decarburization processes in a 90 t RH degasser under the RH and RH-KTB operating conditions have been computed using this model. The results showed that the carbon and oxygen contents in the molten steel during the refining process could well precisely be modeled using this model. The flow characteristics governed the distributions of the carbon and oxygen concentrations in the molten steel. When the initial carbon concentration is higher than 400x10"4 mass%, the top oxygen blowing can not only supply the oxygen needed for the decarburization process, and accelerate the decarburization...