With the development of hydraulic valve spring application technology, more requirements are put forward for hydraulic valve spring 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. For this reason, in addition to the development of new varieties of special-shaped spring 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 special-shaped spring steel production technology In order to improve the quality of special-shaped spring steel, industrially developed China has generally adopted out-of-furnace refining technology, continuous casting technology, new rolling and online automatic detection and control equipment.
In order 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 an external ladle is used for refining to reduce the oxygen content (mass fraction) to (0.0021~0.0010)%. The ultra-pure steel is produced, which greatly improves the design and working stress of the special-shaped spring.
The continuous casting production process has been adopted in the production of special-shaped spring 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. In the rolling process, in order to ensure the surface quality of the product, online automatic detection and control are adopted. In order to be suitable for the production of variable cross-section special-shaped spring flat steel, a new austenite rolling process has been developed, that is, the steel is first heated to the austenite zone and then quenched to the metastable austenite zone for plastic processing and quenching treatment. This process can increase the strength of the steel without reducing its plasticity. In addition, the performance of special-shaped spring steel can be improved through 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 valve special-shaped springs and suspension special-shaped springs. Si is an anti-stress relaxation alloy element. Adding V and Mo to SiCr steel forms SiCrV and SiCrMo steels, which can improve fatigue life and anti-relaxation properties. At the same time, SiCr drawn wire has better relaxation resistance when working at high temperatures than piano wire and carbon special-shaped spring wire for important purposes. With the high-speed and miniaturization of engines, Ti alloys with good anti-flutter performance, light weight, and small elastic modulus have been widely used, and their strength can reach 2000MPa.