Development of Pb-free Free-cutting Microalloyed Steel with High Toughness for Crankshafts of Small Multipurpose Engines

A ferrite-pearlite type microalloyed steel has been developed for use in the crankshafts of small multipurpose engines. Although the thermal refining process after hot forging has been eliminated, this steel possesses equivalent strength and toughness to thermally refined steel, and can also be induction hardened. The design of the alloy sought to ensure toughness by increasing the ratio of ferrite in the structure of the steel and refining the structure through the use of intragranular ferrite and, at the same time, to ensure strength by strengthening of solid solution and precipitation strengthening and increasing hardenability while still employing a carbon content within the range enabling induction hardening. In addition, conditions for the temperature of heating during hot forging and for cooling after hot forging were established to control coarsening of the structure and optimize the characteristics of the alloy. At the same time, the design ensured that while no lead was added, a level of machinability equivalent to that conventional free-cutting steel with added lead was achieved. This has reduced the cost of crankshaft manufacture by eliminating the thermal refining process and done away with the necessity of adding lead, an environmentally harmful substance.