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Application of microalloyed steel to automobile parts is becoming increasingly common in Japan. However, fatigue properties of actual automotive forged parts with slight notches on their surface have not been fully clarified. In this work, the fatigue properties of microalloyed steel were studied using test specimens and also actual automotive parts. The results indicated that microalloyed steel with an optimal microstructure showed higher notch fatigue resistance than quenched-tempered steel. The improvement of material technology and the application of microalloyed steel have not only served to bring product costs down, but have paved the way for part weight reductions. Lightweight connecting rods for the newly developed Nissan engines have been produced, contributing to improved engine performance.

We have developed a lead-free, free-machining 0.1 - 0.8% C steel for machine structural use, which demonstrates machinability almost equivalent to lead-containing free-machining steels. This free-machining steel is characterized by controlling the sulfide shape by Ca-Al deoxidation, and, unlike conventional Ca-deoxidized steels (1) and Al-deoxidized CaS free-machining steels (2), its machinability is improved with the aid of manganese-calcium sulphides which precipitate wrapping calcium-aluminate oxides. In order that these duplex inclusions generate, it is essential that the activity balance between [O] and [S] be controlled to 15 ≦ aS/aO ≦ 100. The free-machining steel which satisfies this activity balance has an outstanding machinability and fatigue strength, which are equivalent to lead containing steels. It has been under consideration to apply this novel free-machining steel to automotive parts such as crank shafts and connecting rods.

The cold forging process is one of the most popular in the manufacture the automotive parts such as gears and shafts, cold forging saves material and machining costs by near-net shape the principle of forming. However, abnormal austenite grain growth sometimes occurs when the cold forged parts are heated for surface carburizing without a prior normalizing process. The size of the coarse grains can be large, sometimes ASTM Grain Size Number -2 to -4. The abnormal grain growth may cause post-carburizing distortion and is harmful to both fracture toughness and fatigue strength of the parts [1]. The purpose of our research was to develope new steels which would keep the fine grains during the carburizing treatment without normalizing. First, we studied the influence of elements on the grain growth property of case hardening steels and Naiobum (Nb) was selected as the element to control the grain growth. Secondly, we developed an ultra fine grain steel containing a small amount of Nb.

For the purpose of saving natural resources and energy, the requirements of vehicle weight-saving have been increasing continuously. As for Automotive Suspension Coil Spring, its weight-saving has been achieved by increasing the design stress. Since the increase of design stress requires higher fatigue life and sag resistance, the strength of spring is usually increased. However, in case of the conventional spring steel, the high strength over σB=1900MPa can dramatically reduce the corrosion fatigue life of spring, to decrease the reliability of spring at the actual usage. In this paper, newly developed spring steel material, satisfying higher strength and corrosion fatigue life simultaneously, is proposed, and its application of Automotive Suspension Coil Spring under the appropriate spring manufacturing processes in introduced.