A high-strength case-hardening steel maintaining good cold forgeability has been developed for use in the manufacture of transmission shafts. The cold forgeability of the steel enables workability to be maintained when the steel is used in the form of wire in a multiple-stage former, as has now become standard. The developed material is also able to match the strength increase that was previously achieved using a special heat treatment (cold quenching) by using a standard carburization quenching (hot quenching). Higher strength represents an impediment to the straightening process that follows carburization, and it was therefore necessary to achieve a balance between high strength and straightenability. In order to do so, flattening of the hardenability curve (Jominy curve) was focused on. Boron was added to a conventional JIS SCr alloy steel and Cr content was adjusted to develop a new steel. During the development process, the mechanism by which the addition of boron causes a decline in the hardenability of the carburized case was clarified, and hardenability was enhanced by the addition of Cr, enabling the achievement of the development goals.
Article of Honda R&D Technical Review Vol.20 No.1 (134 - 140)