A Strategy for Sulfur Reduction and Machinability Improvement in Al‐Killed Gear Steel Through B‐Microalloying

To improve the machinability of sulfur‐containing gear steel while reducing sulfur use, Al‐killed 20MnCr5 steels with different sulfur levels and B / S ratios were designed as model materials. BN microsegregation model, Thermo‐Calc calculations, SEM‐EDS, automated inclusion analysis, nanoindentation, and turning tests were combined to clarify the effects of B microalloying on inclusion evolution and machinability. BN was found to precipitate mainly in the mushy zone and solid‐state region during the late stage of solidification. With increasing B / S ratio, the dominant inclusions changed from MnS‐based inclusions to B ‐bearing complex inclusions. In high‐sulfur steels, BN preferentially nucleated on MnS, refining coarse MnS and forming MnS‐BN‐based inclusions. In low‐sulfur steels, BN tended to nucleate on Al ₂ O ₃ , producing fine and dispersed Al ₂ O ₃ –BN and Al ₂ O ₃ –MnS–BN inclusions. Thermodynamic calculations showed that increasing B content promoted BN precipitation, whereas the maximum equilibrium precipitation amount of MnS was governed primarily by the S content, with preferred points at B / S ≈ 0.23 and 2.16 for high‐ and low‐sulfur steels, respectively. B addition improved chip breakability and surface finish. The low‐sulfur steel with B / S = 2.16 showed the best machinability, achieving 39.06% C‐shaped chips and R _a  = 2.28 μm. Thus, appropriate B microalloying can partially replace S to achieve sulfur reduction and high machinability in gear steels.