Formation and removal mechanism of SiO2-MnO-Al2O3 inclusions in Al-killed 42CrMo4 wind turbine bearing steel forgings

SiO2-MnO-Al2O3 (SMA) inclusions were identified in the defective areas during the ultrasonic inspection of Al-killed 42CrMo4 wind turbine bearing steel forgings. To explore their formation mechanism, we systematically sampled 42CrMo4 steel through industrial experiments, analysing SMA inclusions' morphology and composition at various stages. Thermodynamic calculations were also employed to assess the origin and formation of these inclusions. It was observed that these inclusions primarily formed during electric arc furnace tapping, where the mass ratio of SiO2 to MnO was close to 1, while the Al2O3 content varied. Despite undergoing the ladle furnace and vacuum degassing (VD) refining processes, these inclusions persisted in the molten steel. Following the conclusion of electric furnace smelting, the dissolved oxygen content in the molten steel could reach 193 ppm. During the tapping process, high oxidising steel comes into direct contact with silicon-manganese alloy. During this phase, the uneven distribution of Al content around the alloy results in the formation of SMA inclusions with varying Al2O3 content. Dynamic calculations suggest that in subsequent smelting processes, SMA inclusions with high Al2O3 content and large size are relatively easier to eliminate, while those with low Al2O3 content and small size are more likely to persist in the molten steel. During solidification, residual SMA inclusions agglomerate, forming larger sizes, consequently resulting in the ultrasonic inspection failure of 42CrMo4 steel forgings. After the process improvement, the SMA inclusions are greatly reduced and the effect is obvious.