Abstract
Many different bolts are employed in automobiles for different purposes and uses, and their strength generally ranges from 700 N/(mm)^2. Automobiles face the issue of making improvements in fuel economy as an environmental measure, and there is consequently a requirement to lighten component parts. The creation of higher-strength bolts is an important factor in achieving lighter weight. Increasing the tensile strength, however, can also bring about an increased incidence of delayed fracture, and the conventional solution used to require the application of special steels such as expensive maraging steel. The present development addressed this issue by focusing on high carbon steel rod, which had been considered less susceptible to delayed fracture, although heading was also considered to be difficult. Heading techniques were therefore devised that made it possible not only to form bolts from this material, but also to provide satisfactory strength. The result was successful development of 1600 N/(mm)^2 class high strength bolts.
