1) high strength: in order to improve the spring's ability to resist fatigue failure and relaxation, the spring material should have a high yield strength and elastic limit, and in particular, a high ratio of strength to strength.In general, the elastic limit of the material is directly proportional to the yield strength, so the spring design and the manufacturer always want the material to have a high yield strength.The tensile strength and yield strength of the spring material are close, for example, about 90% of the carbon steel wire of cold drawn is about: since the tensile strength is easier to be measured than the yield strength, the tensile strength is provided in the delivery of the material, so the tensile strength is generally used as the basis in design and manufacture.However, the material's tensile strength is not the higher the better, the higher the strength will reduce the material's plasticity and toughness, and increase brittle tendency.The tensile strength of the material is related to its chemical composition, metallographic structure, heat treatment, degree of cold processing (drawing or rolling) and other factors such as strengthening process.There is also a certain relationship between tensile strength and fatigue strength. When the material is below 1600MPa, its fatigue strength increases with the increase of tensile strength.
2) good plasticity and toughness: during the spring manufacturing process, the material has to undergo processing deformation of different degrees, so a certain plasticity is required.For example, the hook ring and torsion arm of the spring with complex shape, when the radius of curvature is very small, the spring material shall not have defects such as cracks and broken in the process of winding or stamping forming.At the same time, when the spring bears the impact load or variable load, the material should have good toughness, which will be of great benefit to the life of the spring.
3) excellent surface state and the fatigue performance: the biggest spring under the surface of the work stress, and fatigue failure often starts from the surface of steel wire, for use in the important occasion of the spring, such as valve spring, valve spring and the suspension spring are required to have millions of times, even tens of millions of times more long cycle life, it's on the material fatigue performance of the high demands are put forward.There are many factors that affect the fatigue properties of materials, such as the chemical composition of materials, hardness, purity of steel, surface quality and metallographic structure, especially the surface quality of materials.The surface defects of materials, such as cracks, folds, scales, rust, pits, scratches and indentations, can easily cause stress concentration in the working process of the spring.The stress concentration is often the source of fatigue failure.The fatigue source is also easy to occur at the site of surface decarburization, so strict control of decarburization depth is also a very important quality indicator.In order to improve the surface quality of the spring material, the material surface can be polished or polished. Before steel wire drawing, a layer of material epidermis can be removed by peeling technology. In this way, most surface defects can be removed.Spring heat treatment can be used to control the atmosphere or vacuum heat treatment to prevent surface decarburization and oxidation.
4) strict dimensional accuracy: many springs have higher requirements on load accuracy. For example, the load deviation of valve springs should not be greater than 5%-6% of the specified load. Take pull and pressure springs with round steel wire as an example.It can be seen that strict dimensional accuracy is also very important to ensure the quality of the spring.
5) good uniformity: the uniformity requirements for materials refer to the uniformity and stability requirements for the chemical composition, mechanical properties, dimensional deviation and other indicators of materials.If all aspects of the material performance is inconsistent, it will bring great difficulties to the spring production, resulting in the product geometric size, hardness, load and other parameters of dispersion, serious non-uniformity and even waste.
In order to meet the above performance requirements, spring steel must have excellent metallurgical quality, including strictly controlled chemical composition, high purity, low impurity content, low non-metallic inclusion content, and control its shape, particle size and distribution.In addition, homogeneity and stability of steel are required.Spring steel shall also have good surface quality (including surface decarburization) and high precision shape and size.
At present, spring steel is mainly produced by electric furnace or converter, which is characterized by high power and capacity, and is used as spring material for general machinery.For materials with high performance springs, ladle refining, vacuum degassing and other outside refining technologies are widely used, such as ladle blowing argon, ladle refining furnace (LF), vacuum treatment (RH), etc., to strictly control chemical composition and reduce the content of gas and various non-metallic inclusions.In addition, non-metallic inclusion control technology is adopted to control the shape, particle size and distribution of non-metallic inclusions, such as feeding human calcium wire and spraying calcium powder to make non-metallic inclusions contain CaO and control their particle size.In the process of steelmaking, the composition of non-metallic inclusions is changed by adjusting the deoxidizer and furnace slag to make it easy to plastic deformation during hot rolling.By using RH degassing technology, the oxygen content can be greatly reduced and the fatigue life can be significantly improved.
Continuous casting has been widely used in spring steel production.Compared with mold casting, continuous casting can reduce the segregation of steel and improve the uniformity of steel by electromagnetic stirring and low temperature casting.Reduce secondary oxidation and improve surface decarburization of steel;Make the structure and properties of steel stable and uniform;Increase yield and productivity;In combination with outside refining technology to reduce oxygen content and effectively control the chemical composition of steel.
The longitudinal continuous rolling mill is widely used in foreign countries for high speed, no twist, no tension rolling under the condition of increasing the dimensional deviation of spring steel, section shape, surface quality (including the depth of decarburization) and the microstructure uniformity along the length of steel.Short stress line or prestress mill is used to ensure steel size accuracy.