Abstract: The causes of hard spot defects in cutting process of 20CrMnTiH gear steel Φ120 mm product were systematically analyzed by metallographic microscopy, scanning electron microscopy and energy dispersive spectrometer. The results show that the main cause of hard spot defects in gear steel sawing process is the aggregation of large inclusions of 30 ~ 80μm in gear steel round bar. The main inclusions are Al₂O₃ inclusions and TiN inclusions. By controlling the dissolved oxygen content of 120 t converter tapping steel less than 200 X 10 ~6, adjusting the parameters of calcium and aluminium (0.015% -0.025% Al,0.001 5% ~0.0020%Ca) at refining end point,reducing nitrogen absorption during continuous casting, decreasing casting liquid temperature from 1535°C to 1525 °C and optimizing secondary cooling water to accelerate the solidification of steel, the hard point defect in 20CrMnTiH gear steel produced by converter is eliminated.

Key words: 20 CrMnTiH, Hard Spot, BOF, Al₂O₃ , +TiN Inclusions, Parameters of Calcium and Aluminum ,