In general, road wheels for vehicles such as automobiles are manufactured by forming and welding a steel plate material. The steel plate therefore needs a forming property, a burning property, and a weldability.
A steel plate material of a strength 38-60 kg/mm2 grade has been used as a material of a conventional road wheel. The steel plate material of a strength 38-60 kg/mm2 grade is manufactured into road wheels by a press forming operation. The steel plate material then undergoes low temperature heat treatment to obtain a precipitating hardening effect and soft-nitriding treatment to improve the strength, respectively.
However, there is a need in the automobile industry to reduce weight of components. If a road wheel is made of a material having a low strength, and the vehicle including the road wheel get a shock while traveling such as might occur by passing over a small object or a large pot-hole, the road wheel is easily distorted. Thereafter, airs in tires leaks quickly and often so fast the depletion can be termed a blowout, and reliable operation of the wheel is lost. This is a significant safety concern.
The strength of the material of a conventional road wheel is low so that, in order to manufacture a road wheel having a sufficient strength, the width of the material should be thick. The road wheel accordingly becomes heavy.
Wheels are conventionally manufactured by a process that includes press forming, or press molding, a metal plate. When performing a press forming of a road wheel rim using the conventional road wheel material having a low strength, with the resultant high thickness, a crack is easily generated around a cooling hole and a center hole. This reduces the percentage of manufactured tires that pass inspection and, if the flaw is not noticed, this reduces the life-span of a road wheel due to fatigue fracture propagation during the use of the road wheel.
However, when a conventional high strength material, that is, of a strength higher than 38-60 kg/mm2 grade, is used for the road wheel, the press forming property is not adequate and a spring back occurs. Also, a defect such as cracking or breaking during the press forming is still generated so that it is difficult to manufacture the road wheel.
A road wheel composition, and a method for manufacturing a road wheel using the same, which is capable of improving the strength of a finally completed road wheel while paying attention about the press forming property to form a lightweight and a durable road wheel have been greatly demanded.