Spring steel is a kind of steel which is specially used to make springs and elastic elements because of its elasticity in the state of quenching and tempering. The elasticity of steel depends on its elastic deformation ability, that is, within the specified range, the elastic deformation ability makes it bear certain load, and there is no permanent deformation 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 performance (i.e. anti elastic degradation performance, also known as anti relaxation performance), fatigue performance, 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), accurate shape and size.
According to GB / T 13304 “classification of steel”, according to the basic performance and service characteristic I, spring steel belongs to mechanical structure steel; according to the quality grade, it belongs to special quality steel, that is, the steel requiring special strict control of quality and performance in the production process. According to the custom of our country, spring steel belongs to special steel.
（1） Classification by chemical composition
According to GB / T 13304 standard, spring steel is divided into non alloy spring steel (carbon spring steel) and alloy spring steel according to its chemical composition.
1. Carbon spring steel
The carbon content of the spring is generally in the range of 90.0% to 62.0%. According to its manganese content, it can be divided into general manganese content (mass fraction) (0.50% ~ 0.80%), such as 65, 70, 85 and higher manganese content (mass fraction) (0.90 ~ 1.20%), such as 65Mn.
2. Alloy spring steel
Alloy spring steel is a kind of steel which can improve the mechanical properties, hardenability and other properties of the steel by adding one or several alloy elements on the basis of carbon steel, so as to meet the requirements of manufacturing various kinds of spring.
The basic composition series of alloy spring steel include silicon manganese spring steel, silicon chromium spring steel, chromium manganese spring steel, chromium vanadium spring steel, tungsten chromium vanadium spring steel, etc. On the basis of these series, some brands have added molybdenum, vanadium or boron to improve their properties in some aspects.
In addition, some brands are selected from other steel categories, such as high-quality carbon structural steel, carbon tool steel, high-speed tool steel and stainless steel.
（2） Classification according to production and processing methods
1. Hot rolled (forged) steel includes hot rolled round steel, square steel, flat steel and steel plate, and forged round steel and square steel.
2. Cold drawn (rolled) steel includes steel wire, steel strip and cold drawn material (cold drawn round steel).
（3） Classification according to steel delivery status
1. Hot rolled (forged) steel
A. The steel delivered in hot rolled (forged) state is hot formed into spring, and then quenched and tempered.
B. The steel delivered in annealed condition is cold formed into spring, then quenched and tempered.
All of the above shall meet the requirements of the corresponding delivery state hardness.
2. Cold drawn (rolled) steel
A. Steel wire
① Lead bath Austempered cold drawn steel wire (also known as padendre treatment cold drawn steel wire) steel wire made of spring only needs low temperature tempering to eliminate stress.
② After the steel wire is cold drawn to the required size, it is subjected to continuous heating, continuous oil quenching and lead tempering. After the steel wire is made into spring, only the bottom temperature fire is needed to eliminate the stress.
③ Cold drawn steel wire (i.e. steel wire without quenching and tempering treatment)
a. Delivery in cold drawn condition.
b. Delivery in annealed, normalized or tempered condition.
The steel wire delivered in the above two states shall be quenched and tempered after being made into spring.
B. Steel strip
① After being delivered in cold rolling condition, the spring should be tempered at low temperature to relieve stress.
② After being delivered in quenched and tempered condition, the spring shall be tempered at low temperature to remove the stress.
③ After being delivered in annealed condition, the spring shall be quenched and tempered
C. Cold drawn steel
The steel delivered in annealed condition is cold formed into spring, then quenched and tempered.
（4） Other classification methods
In addition to the above, there are other classification methods, such as:
According to the different delivery conditions, it can be divided into delivery by chemical composition (mechanical properties) and delivery by hardenability.
According to the working conditions of spring, it can be divided into static load bearing spring steel, impact load bearing spring steel, high (low) temperature resistant spring steel and corrosion-resistant spring steel.
Spring is used under impact, vibration or long-term cross stress, so spring steel is required to have high tensile strength, elastic limit and high fatigue strength. In the process, the spring steel should have certain hardenability, difficult decarburization and good surface quality, that is, the immature carbon structural steel with carbon content of 0.6% – 0.9%. The alloy spring steels are mainly silicon manganese series steels, which have a little lower carbon content, and their properties are improved mainly by increasing the silicon content WSI; in addition, there are also alloy spring steels of V, W and V. In recent years, combined with the resources of our country, and according to the requirements of new technology for automobile and tractor design, new steel grades with boron, niobium, molybdenum and other elements are developed on the basis of silicon manganese steel, which prolongs the service life of spring and improves the spring quality.