Standard, application scope and process characteristics of spring steel wire
The standard of spring steel wire in China is formulated according to ISO and JIS standards. The current national and industrial recommended standards include 6 carbon spring steel wire standards and 4 alloy spring steel wire standards. The standard details are shown in Table 2.
Table 2 standard details of spring steel wire
2.1 standard, application scope and process characteristics of carbon spring steel wire
The current national and industrial recommended standards for carbon spring steel wire are divided into two types: one is cold deformation strengthening steel wire, also known as cold drawn spring steel wire. The cold drawn carbon spring steel wire is first quenched with lead to obtain sorbite structure, and then the surface is phosphated. The steel wire is drawn to the finished product size with a large reduction rate. The steel wire structure is fibrous, with high tensile strength and elastic limit, and good bending and torsion properties. Cold drawn spring steel wire has high dimensional accuracy, smooth surface, no oxidation and decarburization defects, stable fatigue life, and is the most widely used spring steel wire.
Another type of carbon spring steel wire is martensitic strengthening steel wire, also known as oil quenching and tempering steel wire. Carbon steel wire can obtain good comprehensive mechanical properties by quenching and tempering. When the steel wire specification is small (φ ≤ 2.0 mm), the strength indexes of oil quenched and tempered steel wire are lower than those of cold drawn steel wire after sorbite treatment. When the size of steel wire is large (φ ≥ 6.0 mm), sorbitic steel wire can not obtain the required strength index by using large reduction ratio. However, oil quenched and tempered steel wire can obtain higher performance than cold drawn steel wire as long as it is fully quenched. Under the same tensile strength, martensite reinforced steel wire has higher elastic limit than cold deformation strengthened steel wire. The microstructure of cold drawn steel wire is fibrous with obvious anisotropy. The microstructure of oil quenched and tempered steel wire is homogeneous tempered martensite, which is almost isotropic. At the same time, the relaxation resistance of oil quenched and tempered steel wire is better than that of cold drawn steel wire, and the service temperature (150-190 ℃) is also higher than that of cold drawn steel wire (≤ 120 ℃). In recent years, medium and large size oil quenched and tempered steel wire will replace cold drawn steel wire. The application scope and process characteristics of various kinds of carbon spring steel wire are introduced according to the standard.
1) Yb / t5220-93 carbon spring steel wire for non mechanical spring
The standard is applicable to carbon spring steel wire for non mechanical spring such as sofa cushion spring, cushion, back tension spring, clip spring and so on. The steel wire is divided into A1, A2, A3 according to different tensile strength requirements A9 nine groups, each group of steel wire regardless of size according to a strength range, tensile strength deviation ≤ 200MPa. Groups A1, A2 and A3 are used to make lower stress springs. A3, A4 and A5 are used to make general stress springs. A7, a8 and A9 are used to make high stress springs. Mattress spring is generally selected A3 and A4 groups.
From the analysis of service state, the standard steel wire basically belongs to static spring, and the finished steel wire only tests three properties of tensile strength, winding and single bending. See Table 3 for details.
Table 3 Yb / t5220-93 carbon spring steel wire for non mechanical spring
Note: ① the steel wire with diameter ≤ 4.0 mm is wound on the 2D mandrel for two times without crack or fracture.
② The steel wire with a diameter of more than 4.0 mm is subjected to bending test. The sample is bent for 90o along the arc of R = 10mm, and there is no crack or fracture at the bend.
In general, 45-70 steel is selected for A1-A3 group, 65Mn or 70 steel is selected for a3-a6 group, and 70 or t8mna (82B) is selected for a7-a9 group steel wire. As the steel wire supplied according to the standard is basically used to make static spring, the requirement of fatigue life is relatively loose, and the steel wire can be directly drawn with controlled rolling and controlled cooling coil. Normalizing can be used instead of lead bath in heat treatment of steel wire before forming. At the same time, converter killed steel is allowed to be used as raw material.
(2) GB / t4357-89 carbon spring steel wire
This standard is a general standard for cold drawn carbon spring steel wire, which is mainly used to make static spring working under various stress conditions. According to the working stress state of the spring, the steel wire can be supplied in three levels: Grade B for low stress spring, grade C for medium stress spring and grade D for high stress spring. The tensile strength, torsion, winding and bending properties of finished steel wire are evaluated. The mechanical properties of common specifications are shown in Table 4.