Improving cleanliness of 30Cr2Ni4MoV low-pressure rotor steel by CaO–SiO2–MgO–Al2O3 slag refining

The optimal composition of a CaO–SiO2–Al2O3–MgO slag in the ladle furnace refining process was investigated to precisely control the contents of [O] and [Si] and improve the cleanliness of 30Cr2Ni4MoV steel. The iso-[O] lines and iso-[Si] lines of the equilibrium between the CaO–SiO2–Al2O3–MgO refining slag and 30Cr2Ni4MoV steel at 1873 K were calculated by the thermodynamic software FactSage 7.3, and the activities of SiO2, Al2O3 and CaO in the refining slag were discussed to achieve the optimal composition range of the refining slag. Finally, combined with high-temperature “slag–steel” equilibrium experiments, the effects of different refining slags on the oxygen contents, chemical compositions, quantities and sizes of inclusions in steels were studied, and then the thermodynamic formation mechanism of MgAl2O4 inclusions in 30Cr2Ni4MoV steel was discussed. The results showed that the contents of dissolved [O] and [Si] in steel can be controlled below 10 × 10–6 and 0.05%, respectively; when the slag basicity is above 7, the CaO/Al2O3 ratio is above 1, and the mass fraction of SiO2 in the slag does not exceed 7%. The chemical composition of the slag has a great influence on the removal and composition of inclusions. The assessed stability phase diagrams of MgO, Al2O3 and MgO·Al2O3 inclusion formation in the Fe–Al–Mg–O system calculated by FactSage 7.3 show good agreement with the experimental results.