Morphological Differences of MnS Inclusions in Medium‐Carbon Steel with Different Manganese and Sulfur Contents

The morphology of MnS inclusion is one of the key elements influencing the machinability of steel. Herein, it is shown in the thermodynamic analysis results that the interaction coefficient causes a 1.2% deviation for the precipitation of MnS in low‐sulfur‐content steel (S1), while the deviation is 14% in high‐sulfur steel (S2), and the content of C and Si is the main element causing the deviation. The microstructure of medium‐carbon steel is mainly pearlite and ferrite. The increase of manganese and sulfur content leads to the decrease of ferrite content and increase of pearlite content. Almost all MnS inclusions in S1 are polyhedral, while S2 contains spherelike, cluster, and polyhedral MnS inclusions. Manganese and sulfur mainly affect the nucleation rate of MnS inclusion, where the number density of MnS in S2 is higher than that in S1, which increases the probability of viscous sintering of MnS inclusions in S2, and form spherelike‐ and rodlike‐polycrystalline MnS particles. Polyhedron inclusions are mostly octahedron, and it is the final equilibrium morphology of sulfide in S1 and S2.