Numerical Simulation of Dehydrogenation of Liquid Steel in the Vacuum Tank Degasser
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is well known that in the production of special steels, the control of hydrogen in steel is of special importance since hydrogen can lead to many problems such as the formation of flakes, the occurrence of breakouts, and hydrogen embrittlement. The content of hydrogen in liquid steel can sometimes be required to be 1 ppm or lower and if the level of hydrogen in solid steel after casting is still too high, additional heat treatment in the hydrogen removal furnace is needed, which can take several days. The hydrogen pickup and removal in liquid steel are both complicated and the net hydrogen level in steel depends on the extent of pickup and removal during various stages of the steelmaking process. Hydrogen sources are refractory materials, lime, alloying elements, scrap, atmosphere, etc. Moisture from additions for deoxidation and alloying have also been found to increase the hydrogen content in liquid steel.[1] Because the solubility of hydrogen in liquid steel is considerably higher than in solid steel, it is a prerequisite to remove hydrogen from liquid steel before casting. In steelmaking industries, the removal of hydrogen in liquid steel is usually accomplished within the vacuum tank degasser in which intensive gas stirring and vacuum treatment are SHAN YU, Doctoral Candidate, and SEPPO LOUHENKILPI, Professor, are with the Laboratory of Metallurgy, Department of Materials Science and Engineering, Aalto University, PO Box 16200, Vuorimiehentie 2, Espoo 00076, Aalto, Finland. Contact e-mail: shan.yu@aalto.fi Manuscript submitted August 17, 2012. Article published online December 12, 2012. METALLURGICAL AND MATERIALS TRANSACTIONS B
utilized to provide favorable conditions for dehydrogenation. The main parameters that influence the hydrogen removal in the tank degasser are vacuum pressure, treatment time, liquid steel composition, flow rates of stirring gas, and circulation flow in the liquid bath. It has been reported that the efficient removal can be obtained at the bath surface and areas close to the gas plume, where hydrogen can be picked up by the argon bubbles.[2] In general, the vacuum degassing equipment will probably not change drastically and it is impossible to observe the degassers and take steel samples under vacuum and high temperature. Therefore, the main phenomena in the vacuum degasser, such as fluid flow and mass transfer, have been studied experimentally and numerically for decades.[3–10] Compared to physical modeling, the numerical approach has been receiving more attention nowadays mostly due to its incomparable advantages: The numerical approach only has a little difficulty in representing the processes with high temperature and large scale dimensions. In addition, there is no inaccessible location in a computational domain and no disturbance caused by a probe, which commonly occur in a physical model.[11] In principle, the multiphase flow (e.g., gas–liquid flow in the vacuum degasser) can be modeled with the Euler–Euler (two-fluid) method or by using the Lagrangian particle tracking technique. The former
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