With the development of plastic compression springs application technology, more requirements are put forward for plastic compression springs materials. Mainly in terms of improving fatigue life and anti-relaxation performance under high stress; secondly, according to different uses, it is required to have corrosion resistance, non-magnetic, electrical conductivity, wear resistance, heat resistance, etc. To this end, in addition to the development of new varieties of plastic compression springs materials, beneficial results have been achieved in terms of strict control of chemical composition, reduction of non-metallic inclusions, and improvement of surface quality and dimensional accuracy.
(1) Development of plastic compression springs steel production technology To improve the quality of plastic compression springs steel, industrially developed China has generally adopted out-of-furnace refining technology, continuous casting technology, new rolling, and online automatic detection and control equipment.
To ensure the chemical composition of the steel and reduce the content of gases and various non-metallic inclusions, a large-capacity electric furnace or converter is used for smelting, and a ladle is used outside the furnace for refining to reduce the oxygen content (mass fraction) to (0.0021~0.0010)%. Produce ultra-pure steel, which greatly improves the design and working stress of plastic compression springs.
The continuous casting production process has been adopted in the production of plastic compression springs steel. Continuous casting can reduce the segregation of steel by electromagnetic stirring, low-temperature casting, and other technologies, reduce secondary oxidation, improve surface decarburization, and make the structure and performance stable and uniform.
The split-type full continuous rolling mill can improve the dimensional accuracy and surface quality, and at the same time make the steel microstructure uniform along the length. To ensure the surface quality of the product during the rolling process, online automatic detection and control are adopted. To be suitable for the production of variable cross-section plastic compression springs flat steel, new austenite rolling process has been developed, that is, the steel is heated to the austenite zone and then quenched to the metastable austenite zone for plastic processing and quenching. This process can increase the strength of steel without reducing its plasticity. Besides, the performance of plastic compression springs steel can be improved by online heat treatment and surface hardening treatment after rolling.
(2) The development of alloy steel The main function of alloying elements is to improve mechanical properties, improve process performance, and give certain special properties. SiCr steel has been applied to the valve plastic compression springs and suspension plastic compression springs. Si is an anti-stress relaxation alloy element. Adding V and Mo to SiCr steel forms SiCrV and SiCrMo steel, which can improve fatigue life and anti-relaxation properties. At the same time, SiCr drew wire has better relaxation resistance when working at high temperatures than piano wire and carbon plastic compression springs wire for important purposes. With the high-speed miniaturization of engines, Ti alloys with good anti-flutter performance, lightweight, and small elastic modulus have been widely used, and their strength can reach 2000MPa.