Abstract
As the diameters of suspension springs become smaller due to reduction of the weight of motor cars and increase of the stress levels of the coil springs, cold formed springs have come to draw attention. In contrast to the hot formed springs, however, there are few materials appropriate for cold formed springs.
The only steel available so far is SAE9254, but its application to a spring in a higher stress level is substantially impossible for lack of sag resistance. SAE9254 has a long history as an oil tempered wire for springs. And of some elements to improve the sag resistance, we selected vanadium, for vanadium is known to have no direct effect on heat treatment of the steel. The addition of vanadium into SAE9254 was found to enhance both sag resistance and corrosion fatigue resistance of coil springs for automotive suspension.
The most prominent effect of vanadium is seen in the refinement of austemite grains upon heat treating. The grain size diminishes from No. 9.5 to No. 12. This refinement of the grains is supposed to be responsible for the improvement of sag resistance by 25% as well as corrosion fatigue resistance.
