The current national and industry recommended standards for carbon spring steel wire are divided into two types:
 
Cold drawn spring steel wire
One type is cold deformation strengthened steel wire, also known as cold drawn spring steel wire. Cold drawn carbon spring steel wire is first subjected to lead quenching treatment to obtain a martensite structure, and then the surface is phosphated and drawn to the finished size with a large reduction rate. The steel wire structure is fibrous, with high tensile strength and elastic limit, and good bending and torsional properties. Cold drawn spring steel wire has high dimensional accuracy, smooth surface, no oxidation or decarburization defects, and relatively stable fatigue life, making it the most widely used spring steel wire.
 
Oil quenched and tempered steel wire
Another type of carbon spring steel wire is martensitic strengthened steel wire, also known as oil quenched and tempered steel wire. Carbon steel wire can obtain good comprehensive mechanical properties through quenching and tempering treatment. When the steel wire specification is small (φ ≤ 2.0mm), the various strength indicators of oil quenched and tempered steel wire are lower than those of cold drawn steel wire after austenitization treatment. When the steel wire specification is large (φ ≥ 6.0mm), it is impossible to achieve the required strength index with a large reduction rate for the steel wire with martensitic structure, while oil quenched and tempered steel wire can achieve higher performance than cold drawn steel wire as long as it is completely quenched. Under the same tensile strength conditions, martensitic reinforced steel wire has a higher elastic limit than cold deformation reinforced steel wire. The metallographic structure of cold drawn steel wire is fibrous with obvious anisotropy, while the metallographic structure of oil quenched and tempered steel wire is uniform tempered martensite, almost isotropic. At the same time, the anti relaxation performance of oil quenched and tempered steel wire is better than that of cold drawn steel wire, and the operating temperature (150-190 ℃) is also higher than that of cold drawn steel wire (≤ 120 ℃). In recent years, there has been a trend for medium and large-sized oil quenched and tempered steel wires to replace cold drawn steel wires.
 
Application scope and process characteristics
Below, we will introduce the application scope and process characteristics of various types of carbon spring steel wires according to the standards.
(1) YB/T5220-93 Carbon Spring Steel Wire for Non Mechanical Springs
This standard applies to carbon spring steel wire for non mechanical springs such as sofa cushion springs, seat cushions, backrest tension springs, snap springs, clip springs, etc. According to different tensile strength requirements, steel wires are divided into nine groups: A1, A2, A3... A9. Each group of steel wires is supplied within a strength range regardless of specifications, with a tensile strength deviation of ≤ 200Mpa. A1, A2, and A3 groups are used to manufacture lower stress springs. A3, A4, and A5 are used to manufacture general stress springs. A7, A8, and A9 are used to manufacture high stress springs. Mattress springs are generally selected from A3 and A4 groups.
From the analysis of usage status, this standard steel wire basically belongs to static springs, and the finished steel wire is only assessed for tensile strength, winding, and single bending performance, as shown in Table 3. Table 3 YB/T5220-93 Carbon Spring Steel Wire for Non Mechanical Springs
Explanation:
① The steel wire with a diameter of no more than 4.0mm is wound twice on a 2D core rod without cracks or breaks.
② The steel wire with a diameter greater than 4.0mm shall undergo a bending test, and the sample shall be bent 90 degrees in different directions along an R=10mm arc. There shall be no cracks or fractures at the bending point.
A1-A3 groups of steel wires generally use 45-70 steel, A3-A6 groups use 65Mn or 70 steel, and A7-A9 groups of steel wires use 70 or T8MnA (82B) for production. Due to the fact that the steel wire supplied according to this standard is mainly used for making static springs, the requirements for fatigue life are relatively relaxed. The steel wire can be directly drawn into finished products using controlled rolling and cooling coils. Pre heat treatment of steel wire can also be replaced by normalizing instead of lead bath treatment. Simultaneously, it is allowed to use converter killed steel as raw material.
(2) GB/T4357-89 "Carbon Spring Steel Wire"
This standard is a universal standard for cold drawn carbon spring steel wire, mainly used to produce static springs that work under various stress states. According to the working stress state of the spring, the steel wire can be supplied in three levels: level B is used for low stress springs, level C is used for medium stress springs, and level D is used for high stress springs. The finished steel wire is evaluated for its tensile strength, torsion, winding, and bending properties. The mechanical properties of common specifications are shown in Table 4. Table 4 GB/T4357-89 Carbon Spring Steel Wire
Explanation:
① D-grade steel wire with a diameter of no more than 4.0mm and B-grade and C-grade steel wire with a diameter of no more than 6.0mm shall be wound twice on a core rod equal to the diameter of the steel wire. The surface of the wound sample shall not produce cracks or fractures.
② D-grade steel wire with a diameter greater than 4.0mm shall be wound twice on a core rod with a diameter twice that of the steel wire. The surface of the wound sample shall not produce cracks or fractures.
③ Steel wires with a diameter greater than 6.00mm should undergo bending inspection. The sample should be bent 900 degrees in different directions along an R=10mm arc, and no cracks or fractures should occur after bending.
B-grade and C-grade steel wires are generally produced using 70 (67A, 72A) or 65Mn (67B), while D-grade steel wires are produced using T9XtA and T8MnA (82B).
This standard steel wire is mainly used to make static mechanical springs, and its service life vibration frequency is higher than that of non mechanical springs, with certain requirements for fatigue life. The finished product also increases the assessment of torsional performance. Therefore, for steel wire coils, electric furnace or electric furnace+external refining method should be used for smelting, with P ≤ 0.030%, S ≤ 0.020%, etc. in the coils. If there is free ferrite in the microstructure of finished steel wire, it will reduce the fatigue life of the spring. Generally, 5% to 1.5% of free ferrite is present in controlled rolling and controlled cooling coils, and it is not advisable to directly produce finished steel wire using coils. The steel wire supplied according to this standard should be treated with a lead bath before production to eliminate free ferrite structure. The microstructure of the finished steel wire should be fibrous martensite structure.
(3) GB/T4358-1995 "Carbon Spring Steel Wire for Important Applications"
The steel wire supplied according to this standard is mainly used to make dynamic springs that work under various stress states. According to the working stress state of the spring, the steel wire is supplied in three groups: Group E is suitable for medium stress dynamic springs, Group F is suitable for high stress dynamic springs, and Group G is suitable for high fatigue life dynamic springs. The finished steel wire is evaluated based on five performance indicators: tensile strength, torsion, winding, bending, and decarburization. The mechanical properties of common specifications are shown in Table 5. Table 5 GB/T4358-1995 Carbon Spring Steel Wire for Important Applications
Explanation:
① Steel wires with a diameter less than 4.0mm shall be wound 5 times on a core rod equal to the diameter of the wire, and steel wires with a diameter greater than or equal to 4.0mm shall be wound 5 times on a core rod twice the diameter of the wire, without cracks or breaks.
② The steel wire with a diameter greater than 1.0mm shall undergo a bending test, and the sample shall be bent in different directions along the R arc for 900 degrees. There shall be no cracks or breaks at the bending point, Φ≤4.0mm，R=5mm；Φ＞4.0mm，R=10mm。
③ The decarburization layer of G group steel wire is ≤ 1.0d%.
Due to the fact that the steel wire supplied according to this standard is used to produce dynamic springs working under medium and high stress conditions, the finished steel wire must not only maintain high elastic limits and good toughness indicators, but also consider fatigue limits and the fatigue life of the spring. Therefore, there are higher requirements for the purity, non-metallic inclusion content, gas content, ferrite content, and surface decarburization degree of steel. The wire rod used for steel wire must be smelted using an electric furnace+external refining method, with higher requirements for the chemical composition of the rod: P ≤ 0.025%, S ≤ 0.020%, Cr ≤ 0.10%, Ni ≤ 0.15% (0.12%), Cu ≤ 0.20%. In actual production, Mn is often controlled at a high limit to improve fatigue life. Group E uses 70 or 70Mn (72B), Group F uses T8MnA or T9RtA, and Group G uses 65Mn (Mn can be adjusted to 0.9-1.2%) or 67B. The purpose of reducing the content of P and S in steel, increasing the content of Mn, and adopting external refining is to reduce the content of non-metallic inclusions in steel, improve the morphology of inclusions, reduce gas content, and increase fatigue limit and fatigue life. If the microstructure of steel wire contains free ferrite, it will significantly reduce the fatigue life. Steel wire supplied according to this standard must undergo lead quenching treatment before production.
G group steel wire is used to make valve springs that work under severe vibration conditions, with extremely high fatigue life requirements. Therefore, 65Mn wire rods with better toughness are selected. Although the tensile strength has decreased, the fatigue life is more guaranteed. The decarburization on the surface of steel wire and the formation of ferrite structure seriously affect the fatigue performance. The standard adds decarburization layer inspection to G group steel wire, and stipulates that the total decarburization layer depth shall not exceed 1.0% d. However, for larger specification steel wire (Φ>4.0mm), the decarburization layer caused by hot-rolled wire rod is difficult to completely eliminate due to the reduction rate limitation. The standard supplement stipulates: "With the consent of the demand side, steel wire with decarburization layer not exceeding 1.5% diameter can be supplied.
(4) GJB1497-92 "Specification for Special Purpose Carbon Spring Steel Wire"
In certain specific situations, springs need to have elasticity beyond conventional requirements, such as firearm springs. In order to facilitate portability and use, springs occupy a small space and must have sufficient elasticity. Steel wires with high tensile strength and good toughness must be selected to make springs. The "Special Purpose Carbon Spring Steel Wire" meets this requirement standard.
This standard stipulates that steel wires are supplied in three groups: Group A, Group B, and Group C. Group C is suitable for high stress springs, Group B is suitable for high stress springs, and Group A is suitable for ultra-high stress springs. The standard recommends using T9A, T10A, and T8MnA to manufacture steel wire. In fact, the tensile strength of steel wire increases with the increase of carbon content, while the torsional performance (toughness index) decreases with the increase of carbon content. After comprehensive consideration, Liaoning Special Steel Group's three major steel wire companies in Dalian choose T9A steel wire with rare earth elements to produce Group A, B, and C steel wires. The finished steel wire is evaluated based on six performance indicators: tensile strength, torsion, torsion fracture, winding, tensile strength uniformity, and dimensional uniformity. The mechanical properties of common specifications are shown in Table 6.
It should be pointed out that the ultra-high tensile strength of steel wire is obtained at the expense of some plasticity and fatigue life, and is only suitable for making springs with simple shapes and low requirements for fatigue life. This standard is not suitable for promotion and use. Table 6 GJB 1497-92 Specification for Special Purpose Carbon Spring Steel Wire
Explanation:
① The difference in tensile strength between the two ends of each coil of steel wire shall not exceed 100Mpa.
② When twisting the steel wire, there shall be no visible cracks or delamination within the specified number of twists.
③ The steel wire shall undergo winding test, and shall not break or rupture after being wrapped around the core rod for 1-5 turns. The diameter of the core rod is equal to the diameter of the steel wire.
(5) YB/T5103-93 Oil quenched tempered carbon spring steel wire
Oil quenched and tempered steel wire is first produced to the finished size, and then subjected to oil quenched and tempered treatment to temper the martensitic structure before delivery. Organizational uniformity is an important indicator that determines the performance of oil quenched and tempered steel wire. Due to the limited hardenability of carbon spring steel wire, the core of steel wire with too large specifications cannot be completely transformed into martensite after oil quenching and tempering. Therefore, the standard stipulates that the supply diameter of oil quenched and tempered carbon spring steel wire should be less than 12.0mm.
Compared with cold drawn carbon spring steel wire, oil quenched and tempered steel wire with a diameter of ≤ 2.0mm has lower tensile strength and elastic limit than cold drawn steel wire. However, it is not possible to achieve high tensile strength by using a large reduction rate drawing method for large-sized steel wire (Φ ≥ 6.0mm) after Soxhlet treatment, while oil quenched and tempered steel wire can achieve higher tensile strength than cold drawn steel wire as long as it is completely quenched. Even under the same tensile strength conditions, the elastic limit of oil quenched and tempered steel wire is higher than that of cold drawn steel wire.
The anisotropy of cold drawn spring steel wire is obvious, while oil quenched and tempered steel wire is almost identical. The fatigue life and stress relaxation resistance of oil quenched and tempered steel wire are much better than those of cold drawn steel wire, with a higher working temperature (150-170 ℃) and better creep resistance. In addition, the oil quenched and tempered steel wire has good straightness, and after opening the coil, the steel wire bounces straight with almost no bending. The forming performance is better when winding the spring. Therefore, in industrialized countries, large-sized oil quenched and tempered carbon spring steel wire has almost replaced cold drawn carbon spring steel wire.
(6) YB/T510-293 "Valve Oil Quenching and Tempering Carbon Spring Steel Wire"
The relationship between YB/T5102-93 and YB/T5103-93 is equivalent to the relationship between GB/T4358-1995 and GB/T4357-89. The former is used to make dynamic springs, while the latter is a universal standard mainly used to make static springs. Due to the stricter requirements for microstructure uniformity in dynamic oil quenching and tempering springs, carbon steel has limited hardenability, and YB/T5102 specifies smaller supply specifications (diameter ≤ 6.0mm).
The scope of use and quality control requirements for valve oil quenched and tempered carbon steel wire are basically equivalent to GB/T4358-1995 Group G, and will not be repeated here.