Influence of Hot Deformation on MnS Inclusions and the Composite Inclusions in 20MnCr5 Gear Steel Bars

The control of morphologies of MnS inclusions is crucial for the development of high‐quality gear steels. In this study, single‐pass compression tests are conducted on 20MnCr5 gear steel bars using a hot deformation machine. The effects of different hot deformation conditions on the morphology of MnS and its composite inclusions are investigated. The results show that after hot deformation with different deformation amounts, the long strip MnS can be broken into several small segments, while the irregular MnS can be broken into a large block and a small block of MnS. The sulfides with oxides as the core may deform into nearly spindle shape or conical shape. The composite inclusions may also separate into small pieces of MnS, oxide, and MnS+oxide after crushing. When the deformation temperature is 950 °C. and the deformation amount is 30%, the inclusions undergo fragmentation, with the smallest particle size and the lowest aspect ratio. As the cooling rate is increased, the size and aspect ratio of inclusions decrease, and the quantity of inclusions increases. Therefore, the inclusions have the best morphologies after hot rolling when the deformation amount is 30% and the deformation temperature is 950 °C.