| Soft-killed process is adopted after tapping for some ultra-low carbon steel, oxygen activity in steel was very so high that effective desulfurization cannot be performed in LF furnace, even that resulfurization would be happened. So desulfurization was carrried out after RH deoxidation alloying, but left time for desulfurization was short and difficult to remove sulfur from steel in soft killed process.In the present study, based on the severe problem that resulfurization during RH melting, short time for RH desulfurization in soft-killed process and corrosion of RH submerged tube after RH desulfurizer input for the melting of ultra-low carbon electric steel, high efficient desulfurizer and control range of every component in top slag were obtained through thermodynamic calculation and experimental study in laboratory, improvement measures for the corrosion of RH submerged tube were also put forward, the research results were gotten as following:The effects of components on Ls were calculated using FACTSage thermodynamic software and the results show that:①overhigh content of (FeO+MnO) was the main reason for resulfurization, Ls would be increased and oxygen activity in steel decreased with the decrease of FeOcontent. For the avoiding of resulfurization, (FeO+MnO) content in top slag should be controlled below 15% and after RH desulfurization should be controlled below 10% at least;②the increase of (%CaO)/(%Al2O3) increases the sulfur distribution. The component of CaO in slag can increase sulphide capacity of top-slag, and decrease FeO activity, and (%CaO)/(%Al2O3) should be controlled 2.54;③no substantial effect of MgO on the sulphide distribtution when it increases from 7 to 13 mass%, and a certain amount of MgO in slag helps to alleviate corrosion and should be controlled around 9%;④SiO2 can decrease the sulphide capacity and increase the activity of FeO in slag so SiO2 concentration in top slag should be controlld strictly.The effect of the ratio of CaO/Al2O3 and the content CaF2 on desulfurizing ability of desulfurizer based on CaO-Al2O3-CaF2 slag system was analyzed through desulfurization experiment performed in MoSi2 furnace. The results show that:①the sulfur distribution and desulfurizing ability of desulfurizer is found to increase with the increase of (%CaO)/(%Al2O3);②the addition of CaF2 can effectively increase the desulfurizing ability of desulfurizer but when CaF2 content is in the range 15%20%, sulfur distribution goes near to steadiness;③Considering that large amount of Al2O3 produced by deoxidizing may reduced the ratio of CaO/Al2O3 of desulfurizer, the optimum desulfurizer composition was obtained as follows: %CaO/%Al2O3=2.5-4;SiO2<5%;F-<8%,the melting point should be controlled lower than 1350℃.As for the influence of various components in desulfurizer containing BaO and Na2O on Ls based on CaO-SiO2-Al2O3-MgO-BaO-Na2O-CaF2 slag system, the quadratic regression orthogonal method was used for the experimental study and the results show that:①with the increase of the ratio of CaO/Al2O3, Ls increases at first, then decreases and the least Ls was obtained when CaO/Al2O3 was 1.9;②with the increase of the content of MgO, Ls increases at first, then decreases and the largest Ls was obtained when MgO content was 6%;③Ls decreased with the increase of BaO content and goes near to steadiness when BaO content was larger than 10%;④Ls decreased with the increase of CaF2 content;⑤the melting point of slag can be effectively decreased with the addition of BaO,Na2O and CaF2;⑥no addition of BaO and Na2O in desulfurizer was decided to be used in industrial test, and desulfurizer composition was obtained as follows: %CaO/%Al2O3=2.5-4;SiO2<5%;F-<8%,the melting point should be controlled lower than 1350℃.The reaction between RH desulfurizer and high alumina refractory castable for RH submerged tube was thermodynamic analyzed using FACTSage software and the impact on corrosion of CaF2 and MgO in desulfurier and castable was also analyzed. The verification test was also carried out, the results indicate that:①CaO·6Al2O(3CA6) and CaO·2Al2O(3CA2)are the main phase produced by the reaction between castable and by RH desulfurizer, and the reactants with low melting point dissolved into slag;②CaF2 reacts with CaO and Al2O3 to produce 11CaO·7Al2O3·CaF2 which may quicken the penetration and sharpen the erosion;③the saturation solubility of Al2O3 in RH desulfurizer is lower after addition of 5% MgO, Meanwhile, compacted MgO·Al2O3(MA) spinel with high melting point can be produced, which can effectively reduce corrosion and extend RH submerged tube utility longevity;④thermodynamic calculation was demonstrated by experiments and the results indicate that after addition of 5% MgO in desulfurizer, corrosion of desulfurizer to corundum crucible was reduced.The influence factors of desulfurization process kinetics in RH process were analyzed in this paper and a model for RH desulfurization using the method of top addition was also established, the results show that the main influence factor of RH desulfurization is the reaction volumetric coefficient, increasing Ar gas flow rate, improving vaccum degree and sulfuide capacity of slag can effectively increase the rate of RH desulfurization.Industrial test shows that after the content of (FeO+MnO) was controlled below 15% and 10% after RH through the slag property changing treatment, oxygen activity in steel decreased and Ls increased obviously, resulfurization can be avoidable. Combined with the process of slag modification and application of RH desulphurizer, the efficient rate of RH desulfurization can be close to 30%, sulphur content in products is lower than 0.005%, and meets the needs of steel grade.
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