Spring steel has hot rolled steel, cold drawn steel, cold rolled steel strip and so on. The smelting of spring steel is very important. It is necessary to ensure excellent metallurgical quality, not only to ensure accurate chemical composition, but also to have high purity. The content of sulfur, phosphorus, oxygen and nitrogen should be low, and the uniformity and stability of steel should be good. The metallurgical quality of spring steel should reach the level of high quality steel and high quality steel.
The surface quality and dimensional accuracy of spring steel have great influence on the performance and life of spring.
Because the surface of the spring steel in the delivery state is no longer processed except the surface shot peening, so it is the working surface of the finished spring, and various defects on the original surface are also left to the spring. Therefore, the surface requirements of spring steel are very high, and the restrictions on decarburization, crack, folding, scab, inclusion and delamination are very strict. All kinds of surface defects will make the spring early damaged and shorten its life. Take the steel wire with circular cross-section as an example, the strength and stiffness of the finished spring are directly proportional to the third and fourth power of the steel wire diameter respectively.
The small change of the steel wire diameter will lead to the huge fluctuation of the spring performance, so the shape and dimension tolerance of the steel should be strictly controlled. In this way, the processing deformation of spring steel (hot rolling, cold rolling, cold drawing, etc.) is very important and must be taken seriously.
Hot rolled spring steel has round steel, square steel, flat steel and so on. Steel surface quality requirements are high, can be delivered as hot rolling or after heat treatment, to ensure that the hardness does not exceed the specified value, easy to spring. The coil spring can be made of hot rolled material in cold state, and the spring with larger section size needs to be heated. After forming, the spring is quenched and tempered at medium temperature. The tempering temperature is about 350-550 ℃ to obtain high yield ratio and good comprehensive mechanical properties.
Due to the large cross-section size of hot-rolled products, the grade with enough hardenability should be adopted to ensure the core hardenability. If there are non martensitic transformation products in the center, such as ferrite and bainite, the properties after tempering will be reduced, especially the fatigue properties.