Spring steel is the main material specially used to manufacture all kinds of spring and elastic components or structural parts with similar performance requirements. 1. Performance requirements: ① high elastic limit σ E and yield strength ratio σ s / σ B, to ensure excellent elastic properties, that is to absorb a lot of elastic energy without plastic deformation; ② For high fatigue limit, fatigue is one of the most important failure forms of spring. Besides the composition and structure of steel, the fatigue performance is mainly affected by the metallurgical quality of steel (such as non-metallic inclusions) and the surface quality of spring (such as decarburization); ③ sufficient plasticity and toughness to prevent impact fracture; ④ Other properties, such as good heat treatment and plastic processing performance, heat resistance or corrosion resistance requirements under special conditions.
The content and function of carbon and alloy elements in steel are as follows: (1) in general, medium and high carbon, carbon spring steel WC = 0.6% ~ 0.9%, alloy spring steel WC = 0.45% ~ 0.70%. Tempered troostite structure is obtained after quenching and tempering at medium temperature, which can meet the performance requirements of spring. (2) The alloy spring steel for general use is generally low alloy steel. The main additive elements are Si, Mn, Cr, etc. their main functions are to improve the hardenability, solid solution strengthening matrix and improving tempering stability; the auxiliary elements are mo, W, V and other strong carbide forming elements, which mainly prevent the decarburization defects caused by Si and overheat defects caused by Mn, and improve the tempering stability and heat resistance.
3. Common spring steels are listed in table 12-10 for the grades, performance characteristics and main uses of spring steels commonly used in China. Their composition, heat treatment process and mechanical properties can refer to relevant national standards (such as GB 1222-1984). Table 12-10 performance characteristics and application types of main spring steel grades and performance characteristics of main spring steels for main uses carbon spring steels with common Mn content of 65 have high hardness, strength and yield strength ratio, but poor hardenability, poor heat resistance, low ability to bear dynamic load and fatigue load, and low price. It is mainly used for small springs with working temperature less than 12mm The alloy spring steel Si Mn 55si2mn with small cross section less than 15mm has high strength, good elasticity and good tempering stability, but it is easy to decarburize and graphitize. The hardenability of steel containing B is improved obviously. The main spring steel is widely used. It can be used to manufacture all kinds of important spring with medium section less than 25 mm, such as automobile, tractor leaf spring, coil spring, etc. 60si2mn55si2mnb55simnvbcr series 50CrVA has excellent hardenability, high tempering stability, low decarburization and graphitization tendency; It has good comprehensive mechanical properties and certain corrosion resistance. The springs containing V, Mo, W and other elements have certain high-temperature resistance. Because they are all high-quality steel, their fatigue properties are further improved. They are used to manufacture heavy-duty and large-scale springs (50-60mm) with large load, such as engine valve spring, conventional weapon spring, crusher spring; Heat resistant spring, such as boiler safety valve spring, spray nozzle spring, cylinder expansion ring, etc.,
60crmna60crmnba60crmnmoa60si2cra60si2crva1) carbon spring steel (i.e. non alloy spring steel) has low price but poor hardenability, which is suitable for non important spring with small section size, among which 65 and 65Mn are most commonly used. 2) The alloy spring steel can be divided into two categories according to the main alloying elements: Si Mn spring steel and Cr spring steel. The former has higher hardenability than carbon steel, and its price is not very expensive, so it is widely used in all kinds of springs with cross-section size less than 25 mm, and 60Si2Mn is the typical representative; the latter has good hardenability, high comprehensive mechanical properties, and is not easy to decarbonize on the surface of spring, but its price is relatively high. It is generally used for important springs with large section size, and 50CrVA is its typical representative.
4. Heat treatment characteristics the heat treatment of spring steel depends on the processing and forming method of spring, which can be generally divided into hot forming spring and cold forming spring: 1) hot forming spring adopts hot forming (such as hot rolling and hot coil) for all kinds of large and complex springs with section size > 10 mm, such as leaf spring and coil spring of automobile, tractor and train. The simple processing route is as follows: cutting flat steel or round steel → heating bending or winding → quenching and tempering at medium temperature → surface shot peening treatment. The structure of the used state is tempered troostite. The surface fatigue property of the spring can be improved significantly. 2) Cold forming (such as cold rolling and cold rolling) can be used for various small springs with cross-section size less than 10 mm, such as coil spring, spring and spring sheet in instrument. This kind of spring should be cold drawn (cold rolled), Quenched and tempered at medium temperature or lead bath austempering before forming, and then cold drawn (rolled) strengthened. In this process, the metal will be further strengthened, but large internal stress and brittleness will be produced. Therefore, low temperature stress relief annealing (generally 200-400 ℃) should be carried out afterwards.