Metallurgical Behaviors In Ladle Furnace With Gas Injection Through Hollow Graphite Electrode

Ladle furnace (is called LF) is one of the important methods for clean steel refining, and it is used widely in the domestic and foreign iron and steel enterprise. With the development of the clean steel smelting technology, during the refining of low-carbon and/or low nitrogen clean steel in LF, the main disadvantages of it are carbon and nitrogen pickup, lower heating rate and longer refining time. This causes the request of the high efficiency LF (fast, high grade, high cleanness) to be getting more and more urgent. In recent years, some improvements regarding above problems have been made through optimization of the LF power supply and argon stirring system, application of the premelted slag, the foaming slag technology and so on. However, these problems of LF still have not been solved.Based on the wide investigation of the domestic and foreign literatures, the metallurgical behaviors during LF refining with gas injection through hollow electrode were systematically studied. Applying the plasma local equilibrium thermodynamics and metallurgical thermodynamics theories, a calculation model of argon-hydrogen plasma component concentration was established. Using this model, the particle densities of the argon-hydrogen system were calculated. The decarburization and denitrogenation mechanisms during Ar-H2 and Ar-CH4 injection in LF with hollow graphite electrode were studied.Based on above theoretical research, the gas injection refining experiments using hollow graphite electrode were carried out in 100 kg LF in laboratory and 14 t LF at one steel corporation. The effects of the injection of gases such as Ar-H2, Ar-CH4, Ar-CO2, Ar-liquid gas and Ar-natural gas on the carbon, nitrogen and hydrogen contents, temperature of the molten steel, consumption of the graphite electrode were investigated. A prediction model of molten steel temperature used the method of back-propagation (BP) artificial neural network and a consumption model of the hollow graphite electrode were developed. Through the above research, the main conclusions are obtained as follows:(1) In the temperature range of the plasma system consisting of Ar-H2 or Ar-CH4 injected through hollow electrode in LF, the particle density of the hydrogen atom in the electric arc plasma is the biggest. The partial pressures of the products reacted by hydrogen atom with carbon or nitrogen are far greater than that of the hydrogen molecular. The hydrogen atom is the main reactants of decarburization and denitrogenation. The methane and ammonia are the main reaction products of the hydrogen atom decarburization and denitrogenation.(2) The carbon pickup values are reduced during Ar-H2, Ar-CH4 or Ar-CO2 gas mixtures injection into LF with hollow graphite electrode. When direct-current power supply, the maximum carbon pickup ratios in the cases of the injection of Ar-H2, Ar-CH4 or Ar-CO2 are 66.7%,31.48% or 74.2% than that in the case of the solid graphite electrode respectively.(3) The denitrogenation of the molten steel takes place during Ar-H2, Ar-CH4 or Ar-liquid gas injection into LF with hollow graphite electrode. The lowest denitrogenation rates of them are 0.30×10-6/min, 0.233×10-6/min and 0.18×10-6/min, respectively.(4) All the maximum heating rates increased obviously during Ar-H2, Ar-CH4, Ar-CO2 and Ar-liquid gas injection into LF through hollow graphite electrode, which are 6.92,5.35,3.05 and 12.17 times than that of the solid graphite electrode respectively when direct-current power supply.(5) The temperature prediction model of the LF with hollow electrode injection gas was established based on BP neural network. Prediction results show that the error absolute values of 30.8% heats are less than 10 K, and 76.9% are less than 20 K. The everage error absolute value is 16.9 K. The least error absolute value is 3 K.(6) When direct-current power supply, the consumption ratios of the graphite electrode during the injection of Ar-H2, Ar-CH4 or Ar-CO2 are less than that of the solid graphite electrode, and it is decreased by 2.1%,32.4% and 34.7% respectively.(7) The errors between the prediction values calculated by the consumption model of the hollow graphite electrode and the actual value measured by the experiements are small. The error absolute values of 39.4% heats are smaller than 0.05 kg. The everage error absolute value is 0.101 kg.(8) Industrial test results show that, during Ar-liquid gas injection into LF with the hollow graphite electrode, there are decarburization and denitrogenation for all the heats. The highest decarburization rate is 4×10-6/min, the highest decarburization ratio achieves 13.25%. The average denitrogenation rate is 1.12×10-6/min, the average denitrogenation ratio is 18.57% Compared with solid electrode refining, the average heating rate enhances 26.18%.