Standard and application of spring steel wire

Table 4 GB / t4357-89 carbon spring steel wire
Note: ① grade D steel wire of Φ≤ 4.0mm and grade B and grade C steel wire of Φ≤ 6.0mm shall be wound on the mandrel with the diameter of steel wire for 2 turns, and the surface of the sample after winding shall not produce crack and fracture.
② The D-grade steel wire with a diameter of more than 4.0 mm is wound on the mandrel of twice the diameter of the steel wire for 2 times, and the surface of the sample after winding shall not have cracks and fractures.
③ The steel wire with diameter > 6.00mm shall be subject to bending inspection. The sample shall be bent 900 along the arc of R = 10mm in different directions, and no crack and fracture shall be produced after bending.

Grade B and C steel wires are generally 70 (67a, 72A) or 65Mn (67b), and grade D are produced by t9xta and t8mna (82B).
This standard steel wire is mainly used to make static mechanical spring, its service life vibration frequency is higher than that of non mechanical spring, there are certain requirements for fatigue life, and the torsion performance of finished products is also increased. Therefore, the wire rod for steel wire should be smelted by EAF or EAF + secondary refining method, with P ≤ 0.030%, s ≤ 0.020%, etc. If there is free ferrite in the microstructure of the finished steel wire, the fatigue life of the spring will be reduced. However, 5% – 1.5% of the free ferrite exists in the controlled rolling and controlled cooling coil, so it is not suitable to use wire rod to produce finished steel wire directly. In principle, the steel wire supplied according to this standard should be treated with lead bath before forming to eliminate free ferrite structure, and the microstructure of finished steel wire should be fibrotic sorbite structure.

(3) GB / t4358-1995 carbon spring steel wire for important purposes
The steel wire supplied according to the standard is mainly used to make dynamic spring working 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 spring, group F is suitable for high stress dynamic spring, and group G is suitable for dynamic spring with high fatigue life. The tensile strength, torsion, winding, bending and decarburization are tested for finished steel wire. The mechanical properties of common specifications are shown in Table 5.

Because the steel wire supplied according to this standard is used to make dynamic spring working under medium and high stress state, the finished steel wire must not only maintain high elastic limit and good toughness index, but also consider fatigue limit and fatigue life of spring. Therefore, there are higher requirements for the purity of steel, the content of nonmetallic inclusions and gases, the content of ferrite and the degree of surface decarburization. The wire rod for steel wire must be smelted by electric furnace + secondary refining method. The chemical composition of wire rod should be higher: P ≤ 0.025%, s ≤ 0.020%, Cr ≤ 0.10%, Ni ≤ 0.15% (0.12%), Cu ≤ 0.20%. In order to improve the fatigue life, Mn is often controlled in the high limit in practical production. 70 or 70 Mn (72B) is selected for Group E, t8mna or t9rta is selected for group F, and 65Mn (Mn can be adjusted to 0.9-1.2%) or 67b is selected for group G. The purpose of reducing P and s content in steel, increasing Mn content and adopting secondary refining is to reduce the content of non-metallic inclusions in steel, improve inclusion morphology and gas content, and increase fatigue limit and fatigue life. If there is free ferrite in the microstructure of steel wire, the fatigue life will be greatly reduced. The steel wire supplied according to the standard must be subject to lead quenching before being finished.

Group G steel wire is used to make valve spring working in severe vibration state, which requires very high fatigue life. Therefore, 65Mn wire rod with better toughness is selected. Although the tensile strength is decreased, the fatigue life is guaranteed. The steel wire surface decarburization will form ferrite structure, which will seriously affect the fatigue performance. The standard increases the inspection of decarburization layer for group G steel wire, and stipulates that the total decarburized layer depth shall not be greater than 1.0% d. however, for large size steel wire (Φ > 4.0 mm), the decarburization layer caused by hot-rolled wire rod is difficult to be completely eliminated. The supplementary regulation of the standard is: “with the consent of the demander, the steel with decarburized layer no more than 1.5% d in diameter can be supplied Silk “.
Table 5 GB / t4358-1995 carbon spring steel wire for important use
Note: ① the steel wire with diameter less than 4.0 mm shall be wound on the mandrel equal to the diameter of steel wire, and the steel wire with diameter greater than or equal to 4.0 mm shall be wound for 5 turns on the mandrel with twice the diameter of steel wire, and there shall be no crack or fracture.

② The steel wire with diameter greater than 1.0 mm shall be subjected to bending test. The sample shall be bent 900 along the R arc in different directions, and there shall be no crack or fracture at the bending part, with Φ ≤ 4.0 mm, r = 5 mm, and Φ > 4.0 mm, r = 10 mm.
③ The decarburized layer of group G steel wire is ≤ 1.0d%.
(4) Gjb1497-92 specification for special purpose carbon spring steel wire
In some special occasions, it is required that the spring has the elasticity beyond the conventional requirements. For example, for the convenience of carrying and using, the space occupied by the spring is very small, and the elastic force should be large enough. The steel wire with extra high tensile strength and good toughness must be selected to make the spring. The carbon spring steel wire for special purposes meets this requirement.

The standard stipulates that steel wire shall be supplied in groups a, B and C. Group C is suitable for high stress spring, group B is suitable for high stress spring, group A is suitable for ultra-high stress spring. T9A, T10A and t8mna steel wires are recommended in the standard. In fact, the tensile strength of steel wire increases with the increase of carbon content, while the torsional property (toughness index) decreases with the increase of carbon content. Considering comprehensively, the three wire companies of Dalian special steel group select T9A steel wire added with rare earth to produce group A, B and C steel wire. The tensile strength, torsion, torsional fracture, winding, tensile strength uniformity and size uniformity of finished steel wire are evaluated. 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 by sacrificing part of plasticity and fatigue life. It is only suitable for springs with simple shape and low requirements for fatigue life. This standard should not be popularized.