Development of New High Strength Spring Steel and Its Application to Automotive Coil Spring

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 σ_B=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. The chemical elements of developed material were designed under the following philosophies, where Si stays at 1.8% for sag-resistance, C is reduced to 0.4% for the corrosion fatigue life, Ni is added by 0.5% for the control of forming corrosive pit, and minute B and Nb is added for the strength of austenitic grain boundary and fine grain size respectively. Experimental results by both test piece and spring under high strength design, shows that the developed spring materials is effective for improving the corrosive fatigue life and delayed fracture strength, and can achieve the 20% weight-saving under 1200MPa design stress, comparing with the conventional spring steel.