Herein, the precipitation characteristic of sulfide is investigated by confocal laser scanning microscopy for a typical medium‐carbon sulfur‐containing 42CrMo steel under different cooling rates (100, 200, 600 °C min−1) and sulfur contents (0.0018 and 0.0249 wt%). Thermodynamic calculations show that increasing sulfur content can enlarge the liquid–solid‐phase zone and enhance the initial precipitation temperature of MnS in 42CrMo steel. Phase diagram of Mn–Mo–S demonstrates that the sulfides contain Mn, Mo, and S elements can be formed. The experimental results indicate that with an increase of the sulfur content from 0.0018 to 0.0249 wt%, more (Mn, Mo)S sulfides are formed in the steel due to the significant enhancement of supersaturation. Moreover, it is revealed that the average size of (Mn, Mo)S inclusions decreases from 16.11 to 3.55 μm when the cooling rate increases from 100 to 600 °C min−1. The typical of large‐size rodlike (Mn, Mo)S inclusions is observed in a low cooling rates of 100–200 °C min−1. As the cooling rate increases to 600 °C min−1, smaller globular (Mn, Mo)S inclusions with an average size of 3.55 μm are appeared which can act as a core to promote the ferrite formation.

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