Effects of Deoxidation Processes on Inclusions in Environmentally Friendly Free-Cutting Steel

Deoxidation of liquid steel is a key link that affects the quality of environmentally friendly free-cutting steel. The selection and addition sequence of deoxidizer affects the composition, size, distribution, and morphology of non-metallic inclusions, which ultimately affect the machinability of free-cutting steel. The effects of deoxidation processes on inclusions in environmentally friendly free-cutting steel were studied by high-temperature experiments and thermodynamic calculations. The content of total oxygen and inclusion characteristics in steel were analyzed by an oxygen and nitrogen analyzer, a metallographic microscope, a scanning electron microscope, and an energy spectrum analyzer. The results show that the inclusions in the process of an initial Si-Fe alloy addition followed by a manganese addition (H1) are mainly Al2O3-SiO2-MnO composite inclusions in the liquid phase. In addition to the liquid phase Al2O3-SiO2-MnO complex inclusions in the MnO-rich region, there are also some solid phase Al2O3-MnO inclusions with high Al2O3 content in the process of an initial manganese addition followed by a Si-Fe alloy addition (H2). In the two deoxidation experiments, Bi particles mainly exist in the form of adhesion to MnS inclusions. Referring to H2, the average value and median of the aspect ratio is larger and the number of sulfide inclusions with aspect ratio greater than 1.0 increases significantly in H1. In addition, the spheroidization degree of MnS inclusions in the H2 is relatively good.