1200MPa級高強度ばね鋼“ND120S”

抄録

As for automotive suspension coil spring, its weight saving has been achieved by increasing the design stress, since the increasing of design stress requires higher fatigue strength and sag resistance. However, in the case of the conventional spring steel, the high tensile strength over 1900MPa can dramatically reduce the corrosion fatigue strength and delayed fracture strength. In this paper, a newly developed spring steel material, satisfying higher strength and corrosion fatigue life of spring, is proposed, and its application to automotive suspension coil spring under the appropriate spring manufacturing process is introduced. Carbon content of developed steel is less than conventional steels to increase of corrosion fatigue life and fracture toughness. Nickel is added to restrain corrosive pit forming. Boron and niobium are added to improve delayed fracture strength. Experimental results by both the test piece and spring under high strength design, shows that the developed spring material 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 conventional spring steel.