Research On The Rare Earth Addition Technology And Theory In Refining Process For Special Steel

Rare earth elements using in special steel were mainly restricted by rare earth alloying process. Combining with high alumina special steel09Cr2AlMoRE, the LF refining slag was calculated by the thermodynamic model for the evaluation of mass action concentrations, which was established based on existing measurement data and the ion and molecule coexistence theory, as well as each component in liquid steel. Based on that, the rare earth alloying process in LF refining process was investigated through the slag and steel balance.A thermodynamic model has been developed for determining the surface tension of RE2O3-MgO-SiO2(RE=La, Nd, Sm, Gd and Y) melts based on considering the ionic radii of the components and Butler’s equation. The surface tensions of La2O3, Gd2O3, Nd2O3and Y2O3at1873K were evaluated as686,677,664and541mN/m, respectively. The evaluated results of the surface tension have shown good agreements with literature data. The iso-surface tension lines of La2O3-MgO-SiO2slag melt at1873K have also been calculated.According to the ion and molecule coexistence theory (IMCT) a thermodynamic model for the evaluation of mass action concentrations have been formulated for La2O3-Al2O3-CaF2and CaO-Al2O3-Ce2O3slag systems. The results have shown that the calculated values of NLa2O3were in good agreement with the reported measured values of aLa2O3in the La2O3-Al2O3-CaF2ternary slag at1873K. Similarly, for CaO-Al2O3-Ce2O3ternary slag, the results have shown that the trends of calculated NAl2O3were in good agreement with reported measured trends of aAl2O3and the iso-curves of NCeO3and NCaO were also drawn from the calculated results.During the LF refining process of09Cr2AlMoRE steel, after slagging the oxygen content of the steel was controlled by the Al content. While the Al content in the range of0.075-0.1%the corresponding oxygen content range is3-3.4ppm. In addition, after adding a large number of Al to steel, the oxygen content of the steel was controlled by the Al and Si, because the the refining slag has low SiO2content and the Al content in steel has beyond the optimal control range for oxygen. Meanwhile the Al and Si cntent in molten steel and the SiO2and Al2O3content in slag component have reached approximately balanced state. At last, the LaAlO3phase was found in the slag and the oxygen content in steel was controlled by the RE because of the low RE2O3content in slag after RE treatment.With low oxygen and sulfur conditions, the mechanisms of rare earth element decreasing the harm influence of the residual elements As on09Cr2AlMoRE steel were mainly that the fixation of RE and S on residual elements As has effectively reducing the solution quantity of residual elements As in matrix, which has eliminated the segregation of residual elements As at the grain boundary.