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This newly developed helical spring can be used at a stress level up to 1300 MPa. The material is composed of Fe-C-Si-Mn-Ni-Cr-Mo-V alloy. Its strength-toughness balance was greatly superior to that of other spring steels. To improve the fatigue strength at a higher stress level, decarburization at the surface upon austenitizing was severely controlled, applying induction heating. Then, a special shot peening process, introduced for the first time, was applied to obtain a surface residual stress at the surface of over 1000 MPa. The spring was first applied to a 1992 TOYOTA model car. Plans are to increase the use since the spring material achieves a weight reduction of at least 30 % and, possibly, 35 to 40 %.

We developed new microalloyed steels, containing about 0.05% sulfur, which have excellent as hot-forged toughness even when forged at the temperatures of about 1300°C(2375°F). We also estimated the various properties of the new microalloy steel in the as hot-forged condition, comparing them to quench and tempered SAE1055 steel used in the front axle of a small truck. The results showed the new steel has improved yield strength, fatigue strength, absorbed impact energy and machinability over the SAE1055 steel.