Crack Evolution in an Automotive Lever Steel: Interaction Between Casting-Origin Lamination and Process-Induced Surface Cracks

The present study investigated the origin and evolution of cracks observed in an automotive lever steel (S45CS1, JASO M106/JIS) processed through quenching and tempering, cold drawing, thread rolling, induction hardening, and fine grinding. Chemical analysis showed Mn levels of 0.93–0.95 wt.% and S levels of 0.043–0.051 wt.%. In the present steel, sulfur was intentionally added to improve machinability, and the observed sulfur level is therefore considered part of the designed material concept rather than incidental contamination. In samples containing internal lamination, the defects were identified as layered discontinuities formed from mushy-zone porosity during continuous casting and subsequently flattened during rolling and drawing. Scanning electron microscopy revealed layered internal structures and MnS particles within the lamination, supporting a casting-origin porosity-based defect rather than a simple machining crack. During induction hardening, surface quenching cracks propagated inward and selectively opened the pre-existing lamination layers, resulting in repeated deflection, parallel propagation along the lamination, and zigzag crack paths. The results demonstrate that two distinct crack mechanisms coexisted in the investigated component: lamination-assisted crack evolution in MnS-bearing material and mechanically induced surface cracking independent of internal casting defects.