Development of Microalloyed Steel for Fracture Split Connecting Rod

In Europe and the U.S., fracture split connecting rods are used in many types of current engines. This process can eliminate the machining of crankshaft end and eliminate the dowel pin for positioning. The most important key for fracture split connecting rods is a reduction in the plastic deformation during the fracture splitting process. For this reason, sinter-forged materials and pearlitic steels (C70S6) are used for fracture split connecting rods because of their low ductility. Such types of steel, however, are inferior to the hot forged microalloyed steels typically used as connecting rod material in Japan in terms of buckling strength and machinability although they are easier to fracture split. On the other hand, the conventional microalloyed steels used for connecting rods in Japan are not suitable for fracture splitting. The reason is that these steels have too much ductility and associated plastic deformation for fracture splitting. The extensive plastic deformation does not permit accurate rejoining during the manufacture of connecting rods. Based on a fundamental study about the influence of chemical composition on the ductility of microalloyed steels, the authors have developed a new type of steel composed of 0.4% carbon, 0.6% silicon, 0.8% manganese, 0.1% vanadium and 0.1% phosphorus. The development of this steel shows minimal deformation when processed by the fracture splitting method. The deformation is as minimal as that of the pearlitic steel (C70S6), which is widely used for connecting rod production by the fracture splitting method. Furthermore, this newly developed steel shows strength and machinability which is equivalent, if not superior, to conventional microalloyed steel. The developed steel has already been adopted by Nissan Motor Co. for its mainstay model engines.