Effect of Gadolinium on Inclusions in a High-Sulfur Free-Cutting Steel and Converting Two-Dimensional to Three-Dimensional Morphology of Inclusions Through Matlab Programming

The influence of the various rare earth element gadolinium contents on inclusions, microstructure, and hardness of free-cutting Y15Mn steel was investigated. And the Matlab programming was explored to provide a more accurate method for the statistics of MnS inclusions in two-dimensional state. Scanning electron microscopy (SEM) was employed to observe the type, morphology, and distribution of inclusions in samples. Results revealed that with the increase of T.Gd content, the transformation sequence of inclusions followed a specific order: MnS → Al–Gd–O → Gd–O–S → (Gd–O–S)–(Gd–S) → Gd–S and the proportion of MnS distributed along the grain boundary gradually decreased in the direction from the center. Notably, the type of inclusions can be affected by the feeding sequence of alloying elements during the smelting process. The Al–Gd–O inclusion was the product of the full reaction of rare earth Gd with Al₂O₃ crucible, which was only observed in the experiment, where rare earth Gd was preferentially added. And the holding time after adding rare earth Gd in the smelting process was positively correlated with the size of inclusions. The morphology of inclusions was changed from dendritic to short rod-like shape, eventually forming spindle-like rare earth inclusions as the increase of at the T.Gd content was higher than 462 ppm, inclusions were evenly distributed in the sample. By comparing the two-dimensional morphology of MnS inclusions in the steel and the three-dimensional morphology after electrolysis, it was found that the method of counting MnS inclusions through the cross section of the sample was not accurate. The MnS distributed along the grain boundary was well identified by Matlab programming so that inclusion statistics were closer to the real state of the three-dimensional situation. As the T.Gd content increased from 0 ppm to 462 ppm, the hardness of the steel increased from 44.9 to 49.9 HRA.