Plunger member used for belt type continuously variable transmission

A manufacturing method of a plunger member used for a belt type continuously variable transmission, wherein

• the plunger member is fixed to a shaft so as to face a movable-side pulley half body configuring a pulley together with a fixed-side pulley half body in the belt type continuously variable transmission to define an oil chamber formed by a cylinder member into a pulley oil chamber and a canceller oil chamber,
• the plunger member comprising:
• an expanding flange portion having a large diameter, the expanding flange portion formed on one end side by press molding a blank material, the expanding flange portion slidably abutting on the cylinder member;
• a sleeve portion having a small diameter, the sleeve portion formed on the other end side, the sleeve portion fit and fixed to the shaft;
• and
• one or more step-like formed portions having diameters stepwisely smaller from the expanding flange portion, the one or more step-like formed portions continuing to the sleeve portion, and
• the plunger member is configured by cold press molding the blank material according to deep-drawing molding, and closed forging, compression molding or a combined molding thereof, wherein during the cold press molding, a thickness of a bent corner portion, which at least makes the sleeve portion and the step-like formed portions continuous to each other, is configured to be increased by 30% or more relative to a thickness of the blank material, and then a hardened surface layer is formed on both of entire front and back sides of the plunger member by performing a soft nitriding treatment.