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For the weight reduction of suspension coil springs for automobiles, high-strength steel has been developed. But, in general, strength and corrosion fatigue are contradictory. Then the technological basis is to raise durability and sag resistance, and further improvement of the corrosion fatigue strength is important. As a solution for the problem, element design which improved the performance against corrosion and optimum condition of shot peening process were studied. Thus high-strength suspension coil springs have superior corrosion fatigue strength, compared with the spring made of SAE9260 were developed. Moreover, as a new basic evaluation, corrosion fatigue test of springs with an artificial pit was worked out.

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 %.