Method of manufacturing laminated ring and heat treatment apparatus for use in such method

A method of manufacturing a laminated ring for use as a belt in transmitting power in a continuously variable transmission, comprising the steps of:

• severing a cylindrical drum, which comprises a sheet of maraging steel with welded opposite ends, into a plurality of rings of predetermined width, the maraging steel being a low-carbon steel containing 17 to 19% of Ni, and Co, Mo, and Ti;
• correcting circumferential lengths of the rings;
• aging the rings by placing the rings into an aging chamber, heating the aging chamber to a predetermined aging temperature, and keeping the rings at said predetermined aging temperature for a predetermined period of time;
• thereafter, nitriding said rings by transferring the rings from said aging chamber into a nitriding chamber which is disposed independently of:
• said aging chamber and has been heated to a predetermined nitriding temperature, while maintaining said rings at said aging temperature, keeping said rings at said nitriding temperature in an atmosphere containing at least an ammonia gas for a predetermined period for time, and cooling said rings;
• and
• stacking the rings into a laminated ring,
• wherein said rings are aged in a temperature range of from 450° C. to 500° C. for a period of time ranging from 2 to 3 hours, and nitrided in, a temperature range of from 450° C. to 500° C. at the nitriding temperature which is the same as or higher than said aging temperature for a period of time ranging from 30 to 120 minutes, so that errors of circumferential lengths of the nitrided rings with respect to the corrected circumferential lengths of the rings fall within a predetermined error range.