Wire rod for car suspension spring has strict requirements for steel purity, dimensional accuracy and surface quality. The practice shows that the diameter accuracy of wire rod is of great significance to reduce the wear of wire drawing die, reduce energy consumption, reduce wire breakage and ensure the stability of drawing. The diameter tolerance of wire rod for suspension spring shall be controlled within ± 0.2mm, and the ovality shall not be greater than 60% of the diameter tolerance. Cracks, folds, scratches and other defects on the wire surface seriously affect the quality of the steel wire and reduce the fatigue life of the suspension spring. The wire surface shall be smooth without cracks, folds, scars, ears, etc. The size of local bumps, pits, pitted surfaces and other defects shall not be greater than 0.10mm. The mechanical properties of wire rod are good and the fluctuation is small. The fluctuation of tensile strength of wire rod shall be ≤ 40MPa, the fluctuation of area shrinkage shall be ≤ 15%, and the volume fraction of sorbite shall be ≥ 85%. Free ferrite has a great impact on fatigue life. The volume fraction shall be controlled within 1.5%, the grain size shall be above grade 8, and there shall be no martensite.
As the source of fatigue crack, inclusion will lead to the propagation of fatigue crack and eventually lead to spring fracture. The spring failure caused by inclusion has become one of the main reasons for spring damage. Especially for the spring used under high-pressure stress, the fatigue limit is no longer linear with the hardness. When the material hardness HV value exceeds 400, the micro inclusions will become the source of fatigue cracks and reduce the fatigue limit of the material. However, the automobile suspension spring is usually used when the HV value is 430 ~ 535, which shall not be greater than 15 μ M.
Surface decarburization will reduce the surface hardness and easily form the source of surface fatigue cracks in machining, which has an important impact on the fatigue life of suspension spring. The total decarburization layer on the wire surface shall not be greater than 1% of the nominal diameter of the wire to avoid full decarburization.
The fluctuation of carbon content will cause the fluctuation of mechanical properties: carbon content is positively correlated with tensile strength and negatively correlated with area shrinkage. Reducing carbon content can reduce decarburization and improve formability and elastic resistance. The fluctuation range of carbon mass fraction shall be less than 0.03%. Silicon element has high solid solution strengthening effect, which is beneficial to improve the anti ballistic property. Too high silicon content is easy to form silicate inclusions and decarburize. Manganese can enhance the strength and hardenability of the spring, but also increase the overheating sensitivity and tempering brittleness of the steel. For example, in high-quality 55sicr, the mass fraction of silicon is generally 1.33% ~ 1.45%, and the mass fraction of manganese is 0.67% ~ 0.69%. The content of sulfur and phosphorus shall be strictly controlled. For high-quality suspension spring products, the mass fraction of sulfur and phosphorus shall be less than 0.01% and 0.015% respectively, and the mass fraction of oxygen shall be ≤ 15 × 10. Mass fraction of nitrogen ≤ 35 × 10。
Through the understanding of current products, we can also know that spring manufacturers have strict requirements in the process of manufacturing various springs, such as bending springs, so they can not meet the demand for springs of the product.