Ultra-purification Process Investigation And Practice Of30cr2Ni4MoV Steel For Low Pressure Rotor

The lower-pressure turbine-rotor is the core part of generator set, and the purity of material is a key factor affect the property and service life of turbine-rotor. In this paper, the influence of EAF, refining and casting process to [P],[S],[O],[H] and content of inclusions were studied. The manufacturing process was made according to the data of successful production trial. By studying the chemical composition of EAF samples, refining samples, slag, tundish samples and phase analysis of refining slag, we got following conclusions.(1) After strengthen the dephosphorization, the content of impurity elements met the requirement of ultra-low30Cr2Ni4MoV lower-pressure turbine-rotor steel. The content of [P] was less than0.002%that met the requirement of ultra-low phosphor steel,[S] was less than0.002%, met the requirement of ultra-low sulfur steel.[H] was less than1.5ppm.[O] was less than20ppm during tapping and less than30ppm in the tundish, and residual metal elements also can be controlled at low levels.(2) The main presence form of CaO-Al2O3in high aluminum slag that used in refining furnace was12CaO·7Al2O3determined by the method of XRD analysis.12CaO·7Al2O3has the lowest melting point, thus the high aluminum also has good dynamic conditions which is beneficial for the removal of inclusions in the steel.(3) The capacity of desulfuration, dephosphorization and deoxidation in refining furance met the requirement of ultra-pure steel. The average content of [S],[O] and [H] in5test was less than0.002%,20ppm and0.4ppm, respectively. The requirement of ultra-pure30Cr2Ni4MoV steel can be met.(4) The refining effect of100t tundish was better than60t tundish, the removing efficiency of100t tundish reached90%, but the removing efficiency of60t tundish was39～73%according to the height of liquid steel.(5) The analysis of the samples of100t tundish big and small head end showed that the average values of [O] of molten steel at big and small head end were27and36ppm, respectively. It indicated that tundish retaining wall can be very effective to stop inclusions on the side of small head end, prevent the inclusion into the ingot and promote the floating inclusions to be removed and thereby significantly improves the purity of the molten steel coming into ingot mold.(6) The content of [O] in steel can be reduced by using long nozzle, the consumption of [Al] and other elements reduced72.5%, and the decrease of N content3～5×10-6.