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For the requirements of high power output, lightweight and improved fuel consumption of motorcycles to respond to global environmental needs, high fatigue strength to cope with high stress is becoming more important than ever for valve springs. To satisfy such needs, a new alloy steel that softens less in tempering (1),(2) and nitriding that increases surface hardness of valve springs has been developed using oil-tempered silicone-chrome steel wire as the base material. Also, with regards to shot-peening to create compression residual stress on the surface, studies are being performed for multi-stage and high-hardness peening. The research reported in this paper is aimed at an increase of internal hardness after nitriding while maintaining the cold-coiling-ability by adding elements that reduce softening from tempering while reducing non-dissolved carbide in the material.

In addition to the requirements of high power output and compactness, further reduction of weight is being required for motorcycle engines from the standpoint of fuel economy and reduction of CO2 emissions. For this purpose, it is important to reduce crankshaft weight, which is the heaviest rotating part in the engine. The crankshaft has to be strong enough to bear loads, as the demands of weight reduction are increasing. Yet, productivity has to be considered at the same time even when increasing crankshaft strength. In this report of crankshaft material studies that feature high fatigue strength, machinability and distortion correct-ability, attention is given to the fact that the amount of vanadium, which is known as an element that enhances the strength with its precipitation, accelerates deposition, dissolved in the steel depends on the heating temperature.

Evolution continues for small, light weight and high output engines for motorcycles. As a result, high strength steel is a necessity for internal engine parts, including mission shafts. The transmission shafts must have sufficient torsional and bending strengths to withstand the higher torque, along with intermittent sharp torque inputs, and higher bending force. From a production point of view, using a coil fed cold forge process results in superior productivity. This productivity cannot decrease by changing to a high strength material. Also, there needs to be a balance of high strength versus post carburizing straightening. A new class of steel was developed by adding boron and adjusting Cr content of a conventional case hardening steel to produce a flat quench curve (Jominy curve). A flat Jominy curve is desired to increase core hardness and reduce variability.

There are strong demands for reduced production costs of ordinary bolts, of which a large number are used throughout automobiles. In addition, there are continued demands for higher performance and lower weight in automobiles. For this reason, there is an increasing trend to develop steel for high strength bolts or to adopt the plastic region tightening method. At present, the principal materials used in high strength bolts of class 10.9 are medium carbon alloy steel. When a low carbon boron steel bolt is used as a class 10.9 bolt under high stress, delayed fracture may occur, so that these cannot always be used for the body and chassis applications. The authors have developed a new low carbon boron steel with increased delayed fracture strength on the same order as that of JIS-SCM435 (equivalent to SAE4135) medium carbon alloy steel. Attention was focused principally on decreasing the amounts of phosphorus and sulfur in the steel.

In recent years, efforts for development of high-performance and compact automobile engine are being made more actively than ever before. The connecting rod bolt is one of those parts which are very much required to be compact and light, since its size and weight affect the performance of the engine. However, if it is used under high stress, delayed fracture can be caused. Therefore, it is a common practice to keep the stress below 1177MPa. The developed alloy (HB149) is a precipitation hardening alloy steel of low carbon martensite, composed mainly of 0.35C-1.2Cr-1.0Mo-0.3V. With lowered intergranular inclusions by suppressing impurity elements such as phosphorus and sulfur, and with reduced grain boundary embrittlement by tempering it at a high temperature, this steel is excellent in resistance to delayed fracture. Using this steel, we developed a 1372MPa class (Class 14.9*) high strength connecting rod bolt.

A β-titanium alloy wire with a tensile strength of 210 kgf/mm2 (2060 MPa) was made for automotive valve springs. The fatigue of the β-titanium valve springs were heavily affected with the existence of the oxide layer on the surface. The repeated contact of coils to the ends also gives rise to heavy abrasion during the service, indicating the necessity of some measures to prevent it. Through this investigation, technological basis for β-Ti valve springs was established.