To provide a crankshaft which can exhibit both excellent machinability and high fatigue strength though it has a soft-nitrided surface.
The crankshaft is composed of a steel with a nitrided or soft-nitrided surface. The steel comprises, as alloying elements, 0.07 to 0.12 mass% C, 0.05 to 0.25 mass% Si, 0.1 to 0.5 mass% Mn, 0.8 to 1.5 mass% Cu, 2.4 to 4.5 mass% Ni, 0.8 to 1.5 mass% Al, 0.5 to 1.5 mass% Ti, with the balance being Fe and unavoidable impurities. When the core part of the steel not affected by the nitriding is subjected to a solution treatment at 1,200°C for 1 hour and then cooled at a cooling rate of 0.3 to 1.5°C/sec in a temperature range of 900 to 300°C, the steel exhibits a bainite content of 80% or higher and a Vickers hardness of 180 to 280 Hv.
MATSUDA HIDEKI
KOBAYASHI SEIJI
HIRAOKA YASUSHI
TAKADA KATSUNORI
KUREBAYASHI YUTAKA
DAIDO STEEL CO LTD