Spring steel mainly has good elasticity, and because it works under dynamic load environmental conditions, the most important material for manufacturing springs should have high yield strength; it should not cause plastic deformation when subjected to heavy loads; should be High fatigue strength, long service life under repeated loads; and sufficient toughness and plasticity to prevent sudden brittle fracture under impact force.
As economic and environmental pressures continue to grow, the automotive industry continues to improve to reduce weight. For example, the weight of the coil spring has been reduced from 4kg in 1975 to 2kg today, a half reduction. This is mainly due to improved design and increased hardness (the current hardness is 52～56HRC). The higher strength can not only reduce weight but also improve the resistance to bending deformation, but also provide the possibility for the new design of the suspension system. However, higher hardness means increased sensitivity to surface defects during fatigue and corrosion fatigue. Indeed, the fracture of high-strength suspension springs is mainly caused by corrosion pits. Therefore, in order to ensure reliability, in addition to improving strength, it should also have good plasticity (impact toughness) and corrosion resistance.
Studies have found that reducing the carbon content can improve corrosion resistance, impact toughness and plasticity. However, in order to obtain the same strength when the carbon content is reduced, the tempering temperature must be lowered, and the carbon content should be controlled as far as possible to 0.45 ~ 0.55wt%; silicon can improve the yield strength and toughness, but in the process of heat treatment Silicon is also easy to cause decarburization. Therefore, the content of silicon should be controlled at 1.6～2.1wt%; the addition of Cr, Ni, Cu can be adjusted to control the morphology of corrosion pits to improve corrosion resistance and corrosion fatigue resistance, and obtain the required hardenability And keep enough low cost to ensure the economy of new steel grades; the microalloying elements Ti, V, Nb can make the steel have finely dispersed carbonitride precipitates. These precipitates, especially those containing V, are beneficial Obtain higher hardness and yield strength. Nb, Ti and a small amount of V help control the grain size during the austenitization process and obtain a fine structure, thereby improving toughness, corrosion resistance and bending resistance.
The biggest challenge in designing high-strength steel for spiral springs is to ensure that they have high hardness, while having good plasticity and corrosion fatigue resistance and reasonable cost. Kobe Steel and ASCOM Metal jointly developed a new type of high-strength spring steel (HRC>55). The design idea of this kind of steel is: adding controllable alloying elements and microalloying elements to improve the fatigue, corrosion fatigue and bending deformation resistance of steel. Combining the effects of these elements on hardness, corrosion resistance, corrosion pit morphology, hydrogen embrittlement resistance, toughness and cost, the content of alloying elements can be precisely controlled.
Compared with SiCr steel, this type of steel has excellent hydrogen embrittlement resistance, toughness and corrosion resistance, and can meet the needs of higher quality springs that can improve corrosion fatigue resistance under high stress (1300MPa). The difference in the chemical composition of these steels allows users to choose the best composition combination by adjusting cost, corrosion fatigue and toughness according to their needs.
Spring steel definition
Spring steel refers to steel specially used for manufacturing springs and elastic elements due to its elasticity in the quenched and tempered state. The elasticity of steel depends on its elastic deformation ability, that is, within the specified range, the elastic deformation ability makes it bear a certain load, and no permanent deformation occurs after the load is removed.
Spring steel should have excellent comprehensive properties, such as mechanical properties (especially elastic limit, strength limit, yield ratio), elastic reduction properties (that is, elastic reduction resistance, also called relaxation resistance), fatigue properties, hardenability , Physical and chemical properties (heat resistance, low temperature resistance, oxidation resistance, corrosion resistance, etc.). In order to meet the above performance requirements, spring steel has excellent metallurgical quality (high purity and uniformity), good surface quality (strict control of surface defects and decarburization), precise shape and size.
According to the GB/T 13304 “Steel Classification” standard, according to basic performance and service characteristics 1, spring steel belongs to mechanical structural steel; according to the quality grade, it belongs to special quality steel, that is, the steel that requires strict quality and performance control during the production process . According to our country’s custom, spring steel belongs to special steel. When making spring steel, the technical requirements are relatively high, and the technical excellence directly determines the quality.