Study on flow and mixing characteristics of molten steel in RH and RH-KTB refining processes

Abstract

The flow and mixing characteristics of molten steel during the vacuum circulation refining, including RH(Ruhrstahl-Heraeus) and RH-KTB(Ruhrstahl-Heraeus-Kawasaki top blowing) processes, were investigated on a 1/5 linear scale water model of a 90 t multifunction RH degasser. The circulation rate was directly and more accurately determined, using a new method by which the more reliable results can be obtained. The fluid flow pattern and flow field in the ladle were demonstrated, observed and analyzed. The mixing time of liquid in the ladle was measured using electrical conductivity method. The residence time distribution in the RH model was obtained by tracer response technique. The influence of the main technological and geometric factors, including the gas top blowing (KTB) operation, was examined. The results indicated that the circulation rate of molten steel in the RH degasser can be fairly precisely calculated by the formula: Q lp=0.0333Q g 0.26 D u 0.69 D d 0.80 (t/min), where Q g - the lifting gas flow rate (NL/min); D u and D d - the inner diameters of the up and down-snorkels (cm), respectively. The maximum value of circulation rate of molten steel in the case of the 30 cm diameters either of the up-and down-snorkels for the RH degasser (the “saturated” rate) is approximately 31 t/min. The corresponding gas flow rate is 900 NL/min. Blowing gas into the vacuum chamber through the top lance like KTB operation does not markedly influence the circulatory flow and mixing characteristics of the RH process under the conditions of the present work. There exist a major loop and a large number of small vortices and eddies in the ladle during the RH refining process. A liquid-liquid two-phase flow is formed between the descending stream from the down-snorkel and the liquid around the stream. All of these flow situation and pattern will strongly influence and determine the mixing and mass transfer in the ladle during the refining. The correlation between the mixing time and the stirring energy density is τ m∞e −0.50 for the RH degasser. The mixing time rapidly shortens with an increase in the lifting gas flowrate. At a same gas flow rate, the mixing times with the up-and down-snorkel diameters either of 6 and 7 cm are essentially same. The 30 cm diameters either of the up-and down-snorkels for the RH degasser would be reasonable. The concentration-time curve showed that three circulation cycles are at least needed for complete mixing of the liquid steel in the RH degasser.