2. Development of spring materials
With the development of spring application technology, more requirements are put forward for spring materials. It is mainly to improve the fatigue life and anti relaxation performance under high stress; secondly, it is required to have corrosion resistance, non-magnetic, electrical conductivity, wear resistance, heat resistance and other aspects according to different uses. Therefore, in addition to the development of new spring materials, we have also achieved beneficial results in strictly controlling chemical composition, reducing non-metallic inclusions, and improving surface quality and dimensional accuracy.
(1) In order to improve the quality of spring steel, secondary refining technology, continuous casting process, new rolling and on-line automatic detection and control equipment have been commonly used in developed countries.
In order to ensure the chemical composition of the steel and reduce the content of gas and various nonmetallic inclusions, a large capacity electric furnace or converter is used for smelting, and the ladle refining is used outside the furnace to reduce the oxygen content (mass fraction) to (0.0021 ~ 0.0010)%, so as to produce ultra pure steel, which greatly improves the design and working stress of spring.
Continuous casting process has been widely used in spring steel production. Continuous casting can reduce steel segregation, reduce secondary oxidation and improve surface decarburization by electromagnetic stirring, low temperature casting and other technologies, so that the microstructure and properties are stable and uniform.
The dimension accuracy, surface quality and microstructure uniformity along the length of the steel can be improved by adopting split type full continuous rolling mill. In order to ensure the surface quality of products in the rolling process, online automatic detection and control are adopted. In order to adapt to the production of variable cross-section spring flat steel, a new austenitic rolling forming process was developed, that is, the steel was heated to the austenite region, then quenched to the metastable austenite region for plastic processing and quenching treatment. This process can improve the strength of the steel without reducing the plasticity. In addition, the properties of spring steel can be improved by on-line heat treatment and surface hardening treatment after rolling.
(2) The development of alloy steel, the main role of alloy elements is to improve mechanical properties, improve process properties and give some special properties. SiCr steel has been widely used in valve spring and suspension spring. Si is the best alloy element to resist stress relaxation. Adding V and Mo to SiCr steel can improve fatigue life and relaxation resistance. At the same time, the relaxation resistance of SiCr drawing steel wire is better than that of piano steel wire and carbon spring steel wire for important purposes at high temperature. With the rapid miniaturization of engine, Ti alloy with good flutter resistance, 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 38simnb is a new type of high performance spring steel independently researched and developed in China. Based on this, 38SiMnVBE has more advantages, such as high strength and toughness, high hardenability, high applicability and high performance ratio. After ultra-fine grain controlled rolling, the tensile strength = (2030 ~ 2140) MPa, yield strength = (900 ~ 2010) MPa, elongation = (12 ~ 15)%, surface shrinkage = (48 ~ 55)%. It provides a high performance material for a few variable section panel spring.
(4) The development of stainless steel in China is a big country producing stainless steel. With the development of stainless steel production, many kinds of stainless steel have been developed naturally. At present, more than 50 kinds of stainless steel have been developed, which basically meet the needs of domestic production development. This paper briefly introduces some new varieties developed at present.
1) The initial formation of austenitic stainless steel system. Low carbon austenitic stainless steels 0Cr18Ni9 and 00cr17ni2mo2 were developed to eliminate the corrosion fatigue of stainless steel grain boundaries caused by carbon. In order to improve its special properties, Cu, Ti, Nb, Mn, Cr, Si and n elements can be added.
2) The development of nitrogen containing stainless steel. The substitution of nitrogen for carbon in stainless steel has been achieved. N and C have many common characteristics in austenitic stainless steel. The effect of N on stabilizing austenite is greater than that of Ni, which is equivalent to that of C. The combination of N and Mn 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 of C and Cr. 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) Development of super ferritic stainless steel. Ferritic stainless steel has good corrosion resistance and oxidation resistance, and its stress corrosion resistance is better than that of austenitic stainless steel. The price is cheaper than austenitic stainless steel. However, it has the disadvantages of poor weldability and brittleness, which limits the production and application. The weldability and brittleness of ferritic steel can be improved by reducing the content of carbon and nitrogen, adding stabilizing elements such as Ti, Nb, Zr, Ta, and adding toughening elements such as Cu, AI and V in weld metal.
4) The development of super austenitic steel. Super austenitic steel refers to the austenitic steel with Cr, Mo and N contents significantly higher than that of conventional stainless steel. Among them, the famous one is the steel containing 6% Mo (245s Mo). This kind of steel has very good local corrosion resistance. It has good pitting corrosion resistance (PI 40) and good stress corrosion resistance under the conditions of seawater, aeration, crevice and low velocity erosion. It is a substitute for Ni based alloy and titanium alloy.
5) The development of super martensitic stainless steel. The traditional martensitic stainless steels 2Cr13, 3Cr13, 4Cr13 and 1Cr17Ni2 are lack of enough ductility and are very sensitive to stress during cold upsetting, so it is difficult to cold work. In addition, the weldability of steel is relatively poor, so its application range 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 and increasing the content of Ni. This kind of steel has high tensile strength,