Many different types of bolts are employed in automotive applications for a variety of different purposes and uses, and their strength generally ranges from 700 N/mm2 to 1 200 N/mm2. The present increasing focus on increasing fuel economy to prevent global warming has led to the requirement to increase the strength of bolts in order to lower weight. However, increasing the tensile strength of conventional bolts increases the incidence of delayed fractures, and is therefore not suitable for mass-market vehicles. This project studied the use of high carbon steel rods, which are less susceptible to delayed fracture than conventional bolt materials, as a solution to this issue. The difficulty of heading when employing high carbon steel rods was solved by improving the material through the addition of the optimum amount of Cr, utilizing a new forming method in which the spindle is hardened during heading, and adopting a hexalobular shape with excellent formability. The application of these techniques made it possible to form bolts from high carbon steel rods, and has enabled the development of a 1 600 N/mm2 class high-strength bolt. The new material and techniques have been used in the manufacture of the conrod bolts used in 2003 models, and have enabled the size of the bolts to be reduced from conventionally used M12 to M9, and the weight of individual bolt to be reduced by 80g.
Article of Honda R&D Technical Review Vol.15 No.2 (183 - 188)