The wire used for car suspension springs has strict requirements for the purity, dimensional accuracy, and surface quality of steel. Practice has proven that the accuracy of wire diameter is of great significance in reducing drawing die wear, reducing energy consumption, reducing wire breakage, and ensuring stable drawing. The diameter tolerance of the suspension spring wire should be controlled within ± 0.2mm, and the ovality should not exceed 60% of the diameter tolerance. The defects such as cracks, folds, and scratches on the surface of the wire seriously affect the quality of the steel wire and reduce the fatigue life of the suspension spring. The surface of the wire should be smooth, without cracks, folds, scars, ears, etc. The size of local defects such as protrusions, pits, and pits shall not exceed 0.10mm. The mechanical properties of the wire are good and the fluctuation is small. The fluctuation of tensile strength of wire rod is ≤ 40MPa, the fluctuation of cross-sectional shrinkage is ≤ 15%, the volume fraction of martensite is ≥ 85%, and the free state ferrite has a significant impact on fatigue life. The volume fraction should be controlled within 1.5%, and the grain size should be above grade 8, without martensite.

Inclusions, as the source of fatigue cracks, can cause the propagation of fatigue cracks and ultimately lead to spring fracture. The spring failure caused by them has become one of the main reasons for spring damage. Especially for springs used under high-pressure stress conditions, the relationship between fatigue limit and hardness is no longer linear. When the material hardness HV value exceeds 400, micro inclusions will become the source of fatigue cracks, causing a decrease in the material's fatigue limit. However, automotive suspension springs are usually used when the HV value is 430-535, requiring no brittle inclusions greater than 15 μ m.

Surface decarburization can cause a decrease in surface hardness, leading to the formation of surface fatigue crack sources during processing, which has a significant impact on the fatigue life of suspension springs. Require that the total decarburization layer on the surface of the wire should not exceed 1% of the nominal diameter of the wire to avoid complete decarburization.

The fluctuation of carbon content can cause fluctuations in mechanical properties: carbon content is positively correlated with tensile strength and negatively correlated with cross-sectional shrinkage. Reducing carbon content can reduce decarburization, improve formability and resilience. The fluctuation range of carbon mass fraction should be less than 0.03%. Silicon element has a high solid solution strengthening effect, which is beneficial for improving the resistance to elastic deformation. Excessive silicon content can easily form silicate inclusions and lead to decarburization tendency. Manganese can enhance the strength and hardenability of springs, but it also increases the overheating sensitivity and tempering brittleness of steel. In high-quality 55SiCr, the mass fraction of silicon is generally between 1.33% and 1.45%, and the mass fraction of manganese is between 0.67% and 0.69%. The sulfur and phosphorus content must be strictly controlled. High quality suspension spring products require sulfur and phosphorus mass fractions to be less than 0.01% and 0.015%, respectively, oxygen mass fraction to be ≤ 15 × 10, and nitrogen mass fraction to be ≤ 35 × 10.

Through the understanding of the current product, it can also be known that spring manufacturers have strict requirements in the process of manufacturing various springs, such as bent springs, which means that they cannot meet the spring requirements of the product.