With the development of spring application technology, more requirements for spring materials are put forward. The main reason is to improve the fatigue life and anti relaxation performance under high stress; secondly, according to different uses, it is required for corrosion resistance, non magnetic, electrical conductivity, wear resistance, heat resistance and so on. In addition to the development of new varieties, the spring materials have achieved beneficial results in the aspects of strict control of chemical composition, reducing non-metallic inclusions, improving the surface quality and size accuracy.
(1) development of the production process of spring steel
In order to improve the quality of spring steel, the industrial developed countries have generally adopted the refining technology, continuous casting technology, new rolling and on-line automatic testing and control equipment.
In order to ensure the chemical composition of steel, reduce the content of gas and various non metal addition, the large capacity electric furnace or converter smelting, using the ladle refining to reduce oxygen content (mass fraction) to (0.0021 to 0.0010)%, produce super pure steel, thus greatly improve the design and working stress of the spring.
Continuous casting process has been widely used in spring steel production. Continuous casting can reduce the segregation of steel by electromagnetic stirring and low temperature casting technology, reduce the two oxidation, improve the decarburization of the surface, and make the structure and properties stable and even.
The split continuous rolling mill can improve the dimensional accuracy and surface quality, and also make the steel uniformly distributed along the length. In order to ensure the surface quality of products in the rolling process, on-line automatic detection and control are adopted. In order to fit the variable cross section spring flat steel production, a new austenite rolling process was developed, that is, the steel was first heated to the austenite region and then to the metastable austenite area to be plastic and quenched. This process enables the steel to improve its strength without lowering its plasticity. In addition, the properties of spring steel can be improved by on-line heat treatment and surface hardening treatment after rolling.
(2) development of alloy steel
The main role of alloy elements is to improve mechanical properties, improve process performance and give some special properties. Valve spring and suspension spring have been widely used in SiCr steel. Si is the best alloy element to resist stress relaxation. Adding V and Mo into SiCr steel to form SiCrV and SiCrMo steel can improve fatigue life and anti slack performance. At the same time, SiCr drawing steel wire has good anti slack performance when working at high temperature, better than Qin steel wire and important carbon spring steel wire. With the high speed and miniaturization of engine, Ti alloy with good flutter resistance, light weight and small elastic modulus has been widely applied, and its intensity can reach 2000MPa.
(3) development of low carbon austenite steel
Low carbon austenite steel 38SiMnB is a new type of high performance spring steel developed by our country. On this basis, the developed 38SiMnVBE has more advantages, such as high strength, toughness, high hardenability, high application and high performance ratio. After the controlled rolling of superfine grain, the tensile strength = (2030~2140) MPa, yield strength = (900~2010) MPa, elongation = (12~15)%, and surface shrinkage = (48~55)%. It provides high performance materials for less chip section spring.