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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.

Warm Shot Peening is a shot peening process within warm temperature range. However, the effect of warm shot peening to fatigue strength was not certain and no substantial study has been made. The requirements of coil spring with higher fatigue strength and sag resistance have been increasing to obtain the mass saving of vehicle. While several new spring materials have been developed in recent years, the essence of those developments were to make the strength(hardness) of spring higher, so-called high strengthened spring. As for high strengthened spring, the shot peening process becomes further essential to relieve the increased notch sensitivity due to higher strength. However, for the shot peening process to high strengthened spring, the shot with higher hardness is also required. This can decrease the life of both shot and shot peening equipments.