Abstract
In order to save Mo in Cr-Mo steels for automobile components, medium carbon-boron steels were investigated. Boron is a popular alloying element for structural steels, however, to date boron steels have not been widely used because of their unstable hardenability and poor machinability. Therefore, in this study, the optimum content of boron was reexamined, and also the appropriate addition of Ti as a stabilizer of boron was investigated from the view point of hardenability. Furthermore the upper limit of Mn content was studied to keep good machinability. The new steel grades, established on the basis of the above fundamental research, have been applied to vital components of passenger cars, and exhibited the same performance as that of Cr-Mo steel.
