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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.