What is the status and development direction of Spring steel in China

Spring steel is mainly used in the automotive, engine manufacturing and railway industries. At present, the problems of Chinese spring steel products are that there are excess middle and low-end products, and there are no high-end and special varieties. China spring steel has a big gap with foreign countries in terms of purity, fatigue resistance, surface quality and quality stability. It meets the requirements of spring steel performance for high-end passenger car suspension springs, valve springs, railways and special springs for heavy-duty trucks. China's high-end and deep-processed spring steel still relies on imports. The imported varieties are mainly spring steel for cars, spring round bars for railways, and spring steel wires for oil pump valves.

Although reducing the oxygen and inclusion content in steel is a way to obtain pure steel, it is difficult to obtain zero-inclusion spring steel. For this reason, researchers have proposed oxide metallurgy technology, which is an effective fine grain. The method is the most effective way to achieve double the strength and toughness of steel materials. It utilizes finely dispersed high-melting non-metallic inclusions in steel, mainly oxides, sulfides, and nitrides, as the core of the nucleation of intragranular ferrite, thereby refining the grains. Systematic research on Ti and Zr oxide systems has been carried out at home and abroad, and it is considered that titanium-containing oxides are the most ideal. The oxidized material point of titanium in the austenite grains becomes an effective nucleation site for acicular ferrite, which promotes the formation of intragranular ferrite. However, due to the limitations of steel composition, the promotion of titanium oxide metallurgy has been limited. In recent years, research on rare earth elements has been carried out, and the effects of rare earth oxides on the properties of steels can be studied by utilizing the strong deoxidation and desulfurization ability of rare earth elements and the high melting point of the products.

In the automotive industry, the requirements for the strength of the suspension spring are getting higher and higher, and the design stress is increased to 1100-1200 MPa. For this reason, Japan has developed a steel that adds alloys to improve strength and improve corrosion fatigue strength. China's spring steel can not meet the performance requirements of high-end passenger car suspension spring steel. Spring steel with suspension spring products of 1200MPa and above is all dependent on imports. However, in recent years, in order to avoid resource risks, reduce costs, and achieve a global supply of raw materials, it is strongly required to use standard steel (SAE9254) to maintain high strength, and strongly demand to improve the toughness of steel, so more and more shot peening is used. The treatment replaces the surface hardening heat treatment with high processing cost. Shot peening treatment exerts compressive residual stress on the surface, which can improve fatigue resistance and reduce the influence of surface defects. Therefore, it has been regarded as an indispensable technology for surface treatment in recent years. With the development of surface strengthening technology, the design stress of the suspension spring has reached 1200MPa. It is expected that the requirements for strength, toughness, corrosion resistance and durability of high-strength suspension spring steel will become higher and higher in the future. In the future, with the lightweight of automobiles, the development of high-strength, excellent anti-ballistic performance and anti-fatigue performance of spring steel for automotive suspension is an inevitable trend to improve China's high-end equipment parts and components and effectively replace imports.

Among all spring products, valve springs are the most demanding material, especially high stress and profiled valve springs are almost demanding on materials. At present, the production of special spring steel for foreign valve springs is mainly concentrated in Japan, South Korea and Sweden. The production companies include Japan Suzuki, Sanxing, Sumitomo, Kobelco Steel, Korea KisWire, Sweden Garphyttan, etc., which almost monopolize all of China's profiled sections and heights. Stress valve spring steel market. Since 2000, with the development of new engines, the requirements for the engine's rotational speed, weight reduction, and compactness have become higher and higher. Therefore, Japan has adopted OT steel wires of 2100-2200 MPa. In this case, not only the alloy composition needs to be adjusted, but also the existing manufacturing process is improved, and low-temperature dispersion hardening becomes an indispensable process. However, the shape of the spring after the low-temperature dispersion hardening changes, and in order to improve the control precision of the shape and the size, the technique of controlling the shape change in the entire manufacturing process has begun to attract attention.

In the future, in order to meet the development needs of localization of high-end spring basic components, high-performance spring steel products should be continuously developed. On the one hand, it is to develop in high strength, and it is required to simultaneously improve fatigue life and anti-relaxation performance under high stress; It is developed in a functional direction and is required to have corrosion resistance, non-magnetic properties, electrical conductivity, wear resistance, heat resistance, etc. depending on the application.