Bar and Wire Steels for Gears and Valves of Automobiles--Eco-friendly Free Cutting Steel without Lead Addition

摘要

Free cutting steel consists of two types, one for machine structural use and the other based on low carbon steel for hard cutting, such as SAE 12L14. Conventionally, Pb has been added to free cutting steels to improve their machinability, but Pb in steel is thought to have a harmful effect on the human body. Therefore, as alternatives to Pb addition in free cutting steels, JFE Steel and NKK BARS & SHAPES studied graphite precipitation in the steel and control of the sulft. de morphology by the chemical composition and developed new Pb-free free cutting steels based on these respective concepts. In the graphite-type free cutting steel for machine structural use, JFE Steel realized machinability equal to that of the Pb-added steel by controlling the state of existence of C in the steel, and achieved an excellent balance of cold workability and high fatigue strength after quenching and tempering. As a substitute for the low carbon Pb-added free cutting steel (SAE 12L14), NKK BARS & SHAPES developed a steel with improved machinability by crystallizing large sulfide inclusions, which is realized by increasing S addition while also adding Cr on the basis of the estimation from the calculated phase diagrams.