Evolution from free-cutting to super free-cutting steels: A comprehensive review

ABSTRACT Free-cutting steels are widely used for high-productivity machining of precision components. This review summarizes the historical development, machinability-enhancing mechanisms, and recent strategies for developing lead-free or low-lead free-cutting steels, tracing the evolution from resulfurized MnS-containing steels to Pb-containing, Te-modified, Bi-containing, and multi-component systems, alongside the parallel development of free-cutting stainless steels. The principal machinability-enhancing effects include inclusion-induced stress concentration and chip segmentation, interfacial lubrication and transfer-layer formation, liquid-metal-assisted embrittlement, tool-protective oxide film formation, and solid-solution-induced reduction of matrix ductility. The occurrence states and functional roles of major free-cutting elements and phases—including MnS, Pb, Te, Bi, Ca, Ti, Sn, Se, and P—are systematically examined, together with their trade-offs against in-service corrosion resistance, toughness, and fatigue life. Recent research has increasingly focused on Pb-free or low-Pb alloy design, multi-component inclusion engineering, application-specific grades, and three-dimensional inclusion characterization combined with data-driven approaches. Future progress hinges on establishing quantitative composition–solidification–inclusion–machinability relationships across well-defined cutting regimes.