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For the purpose of weight reduction and improvement in fuel economy of vehicles, we developed and put in practical use the high strength bolt (following 13 T) which is excellent in delayed fracture resistance in all the environment of the bolt currently used for the automobiles, and which guarantees 1300 MPa or more of tensile strength. The points of development are in the optimization of chemical composition design and heat treatment conditions which noted the following three points. 1) Toughness of grain-boundary be a fracture course 2) Restraint of hydrogen invation into bolt 3) Diffusion restraint of invaded hydrogen into bolt. In the delayed fracture tests which simulated much corrosion environments like the salt splaying in axle parts and acid corrosion environment like the blowby gas inside engine, high reliability of the developed 13T bolt was confirmed.

Gas carburizing is a surface hardening method that is used for automobile steel parts such as gears. The conventional gas carburizing(at 1223K) takes longer time. Shortening the carburizing time will contribute greatly to the energy saving and CO2 reduction of parts production. The new carburizing process at higher temperature,1273K, and vacuum is made without adding reheating and quenching process. That process can shorten treatment time to one-fourth of conventional process. The new alloy steel for high-temperature vacuum carburizing is developed in order to prevent the grain coarsening. This paper describes the outline of this alloy steel.

We have developed a new torsional damper spring which lowers the torsional rigidity of the clutch disc while retaining its conventional size. The following two items have been adopted in the newly developed spring: 1) A new steel wire which suppresses any core-softening of the element wire through nitriding. 2) A dual-stage shot peening method which uses harder steel shots (rather than conventional shots) in order to obtain an optimal residual stress profile. As a result of evaluating the fatigue characteristics of this spring, it was discovered that its fatigue strength is approximately 35% higher than that of the conventional spring. A clutch disc using this spring was able to absorb rattling noises which conventional clutch discs could not.

This newly developed helical spring can be used at a stress level up to 1300 MPa. The material is composed of Fe-C-Si-Mn-Ni-Cr-Mo-V alloy. Its strength-toughness balance was greatly superior to that of other spring steels. To improve the fatigue strength at a higher stress level, decarburization at the surface upon austenitizing was severely controlled, applying induction heating. Then, a special shot peening process, introduced for the first time, was applied to obtain a surface residual stress at the surface of over 1000 MPa. The spring was first applied to a 1992 TOYOTA model car. Plans are to increase the use since the spring material achieves a weight reduction of at least 30 % and, possibly, 35 to 40 %.