To provide a method for manufacturing structural parts for an automobile made from steel, which have fatigue strength equivalent to that of a nitrocarburized product or an induction-hardened product, even when machined while skipping a heat treatment step after a hot forging step.
This manufacturing method comprises the steps of: hot-forging a steel material into a formed workpiece; cooling it to room temperature into a hot-forged workpiece having a micro metal structure in which an austenite structure occupies 50% or more by an area rate; machining the hot-forged workpiece; and then, cold-working only a portion in which the fatigue strength is desired to be improved to impart a strain with a true strain of 0.10 or more to the portion. The manufacturing method further comprises subjecting the cold-worked workpiece to tempering treatment at 400 to 650°C, or sub-zero treatment at minus 196°C or higher but lower than 0°C, as needed.
NAITO KENICHIRO
MIZUNO YOSHIKI
MATSUDA HIDEKI
KOBAYASHI SEIJI
HONDA MOTOR CO LTD
Yasuhiko