Development of spring materials

With the development of spring application technology, more requirements are put forward for 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 and so on. Therefore, in addition to developing new varieties of spring materials, beneficial results have been achieved from strictly controlling chemical composition, reducing non-metallic inclusions, improving surface quality and dimensional accuracy.

(1) The development of spring steel production process In order to improve the quality of spring steel, industrial developed countries have generally adopted out-of-furnace refining technology, continuous casting process, new rolling and online automatic detection and control equipment.

In order to ensure the chemical composition of the steel, reduce the content of gases and various non-metallic additions, the use of large-capacity electric furnace or converter smelting, the use of outside the furnace ladle refining, so that the oxygen content (mass fraction) is reduced to (0.0021~0.0010) %, the production of ultra-pure steel, which greatly improves 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, reduce secondary oxidation, improve surface decarburization, and make the structure and performance stable and uniform by electromagnetic stirring, low temperature casting and other technologies.

The use of separate continuous rolling mill can improve the dimensional accuracy, surface quality, and microstructure of steel along the length. In order to ensure the surface quality of the product in the rolling process, on-line automatic detection and control are adopted. In order to adapt to the production of spring flat steel with variable section, a new process of austenite rolling was developed, that is, the steel was first heated to the austenite zone and then cooled to the metastable austenite zone for plastic processing and quenching. This process allows the steel to increase strength without reducing plasticity. In addition, the properties of spring steel can be improved by online heat treatment and surface hardening after rolling.

(2) The development of alloy steel The main role of alloying elements is to improve mechanical properties, improve process properties and give some special properties. Valve springs and suspension springs have been widely used SiCr steel. Si is the best alloying element to resist stress relaxation. Adding V and Mo to SiCr steel to form SiCrV and SiCrMo steel can improve the fatigue life and anti-relaxation properties. At the same time, SiCr drawing steel wire has better anti-relaxation performance when working at high temperature than piano steel wire and carbon spring steel wire for important purposes. With the high-speed miniaturization of engines, Ti alloy with good anti-flutter performance, light weight and small elastic modulus has been widely used, and its strength can reach 2000 MPa.

(3) The development of low carbon austenitic steel Low carbon austenitic steel 38SiMnB is a new type of high-performance spring steel independently developed in China, on this basis, 38SiMnVBE developed more superiority, with high strength toughness, high hardenability, high application and high performance ratio. After ultra-fine grain control rolling, the tensile strength is (2030~2140) MPa, yield strength is (900~2010) MPa, elongation is (12~15) %, surface shrinkage is (48~55) %. The high performance material is provided for the variable section plate spring with few pieces.

(4) The development of stainless steel China is a big country in the production of stainless steel, with the development of stainless steel production, naturally also developed a lot of varieties, has reached more than 50 kinds, basically meet the needs of domestic production and development, the current development of some new varieties for a brief description.

1) The initial formation of austenitic stainless steel system. In order to eliminate the grain boundary corrosion fatigue of stainless steel caused by carbon elements, low carbon austenitic stainless steel 0Cr18Ni9 and 00Cr17Ni2Mo2 were developed. In order to improve its special performance, elements such as Cu, Ti, Nb, Mn, Cr, Si and N can be added.

2) The development of nitrogenous stainless steel. Results have been obtained by substituting nitrogen for carbon in stainless steel. N and C have many common properties in austenitic stainless steel. The effect of N stable austenite is larger than Ni and comparable to C. N and Mn binding energy can replace the more expensive Ni.

N is also one of the most effective solid solution strengthening elements in austenite. The affinity between N and Cr is smaller than that between C and Cr, and the precipitation of Cr2N is rarely seen in austenitic steel. Therefore, N can improve the strength of stainless steel without reducing the corrosion resistance.

3) The development of super-strong ferritic stainless steel. Ferritic stainless steel has good corrosion and oxidation resistance, and its stress corrosion resistance is better than that of austenitic stainless steel. It's cheaper than austenitic stainless steel. But there are poor weldability, brittle tendency is relatively large shortcomings, production and use is limited. The weldability and brittleness of ferritic steel can be improved by reducing the content of carbon and nitrogen in steel, adding stabilizing elements such as Ti, Nb, Zr and Ta, and adding weld metal toughening elements such as Cu, AI and V.

4) The development of super austenitic steel. Super austenitic steel refers to austenitic steel with significantly higher Cr, Mo and N content than conventional stainless steel. One of the more famous is steel containing 6% Mo (245S Mo). This kind of steel has very good local corrosion resistance, in seawater, aerated, there are gaps, low speed erosion conditions, has good pitting resistance (PI 40) and better stress corrosion resistance, is a substitute material for Ni-based alloys and titanium alloys.

5) The development of super martensitic stainless steel. Traditional martensitic stainless steels 2Cr13, 3Cr13, 4Cr13 and 1Cr17Ni2 lack sufficient ductility and are very sensitive to stress during cold top forging deformation, making it difficult to form in cold working. In addition, the weldability of the steel is relatively poor, and the scope of use is limited. In order to overcome the above shortcomings of martensitic steel, an effective way has been found recently, which is to develop a new series of alloy steel - super martensitic steel by reducing the content of C and Ti in steel and increasing the content of Ni. This kind of steel has high tensile strength, good ductility, and improved welding performance, so super martensitic steel is also known as soft martensitic steel or weldable martensitic steel.

(5) The development of spring steel wire After more than 100 years of development, the process technology has experienced from lead quenching to oil quenching, and now it has developed to induction heating quenching. Coupled with the continuous innovation and improvement of process technology and equipment, the quality of varieties is constantly updated. Recently developed valve spring steel wire induction heating quenching and tempering process, the test proves that due to the induction heating time is short, the quenching structure is small, the surface of the steel wire almost no decarburization layer, so its plasticity, toughness, relaxation resistance, fracture toughness, delayed fracture resistance, fatigue life and so on than the oil quenched tempering steel wire has been greatly improved.

Another kind of steel wire with ultra-fine grain deformable heat treatment has been applied in the field. Ultra-fine grain deformable heat treatment is a composite strengthening and toughening process which combines ultra-fine microstructure and deformable heat treatment. It can not only improve the mechanical properties of steel wire, but also improve the surface quality of steel wire. The surface quality of materials has great influence on the fatigue properties. In order to ensure surface quality, the peeling process is used for materials with special requirements, and the surface layer is removed by 0.1mm. Eddy current testing is used for defects with a depth of 0.5mm. For the uneven surface of the drawing process, electrolytic grinding can be used to reduce the surface roughness to = (6.5~3.4) m.

(6) The development of stainless steel wire in recent years, foreign stainless spring steel wire production has developed rapidly. The varieties with increasing domestic demand are mainly 1Cr18Ni9 and 0Cr17Ni7AI.

The advanced steel wire production process is characterized by the first peeling of the wire rod to remove the defects caused by hot processing on the surface, in addition to pickling after the first solution treatment, the entire cold processing process is to maintain a bright surface.

With the development of the process, the production process of non-embroidery steel wire has been further simplified, and the quality control of some original metal products industry has been simplified, and it has been converted into the requirements for the quality of wire rods. After the rough drawing machine, wipe with a cleaning ball and clean in water to remove the surface coating and residual lubricating film. Before bright heat treatment, it is equipped with electrolytic pickling, alkali neutralization, water washing and drying devices to completely remove the oil on the surface of the steel wire and improve the surface quality.

(7) Development of shape memory alloys At present, one-way shape memory alloys with promising applications in springs have the best performance of 50Ti and 50Ni. A spring made of shape memory alloy that can be stretched by temperature. It is mainly used in the control system of constant temperature, constant load and constant deformation. Because the actuator is driven by the spring expansion, the working stress of the spring changes greatly.

(8) The application of ceramics ceramics have high elastic modulus and low fracture strength, and are suitable for places with little deformation. At present, there are heat-resistant, wear-resistant and good insulation ceramics under development, and the application of superplastic zinc alloy (SPZ) has high strength at room temperature. In addition, there are high-strength silicon nitride, which can withstand high temperatures, up to 1000℃. However, ceramic springs are not suitable for operation under impact loads.

Glass fiber reinforced plastic (GFRP) plate springs have been widely used in the United Kingdom, the United States and Japan, in addition to transverse suspension, can also be used for special light vehicles, such as the longitudinal suspension of racing cars. At present, carbon fiber reinforced plastic (GFRP) suspension spring has been successfully developed, which is 20% lighter than metal plate spring.