Carbon spring steel wire is used for non-mechanical springs such as sofa cushion springs, seat cushions, backrest springs, retaining springs, and clip springs. The standard classifies the steel wire into nine groups (A1, A2, A3…A9) according to different tensile strength requirements. Each group of wire is supplied within a specific strength range, regardless of size, with a tensile strength deviation ≤200 MPa. Groups A1, A2, and A3 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 higher stress springs. Mattress springs generally use groups A3 and A4.
Steel wire in groups A1-A3 is generally made of 45-70 steel; groups A3-A6 use 65Mn or 70 steel; and groups A7-A9 use 70 or T8MnA (82B). Since the steel wire supplied according to this standard is primarily used for making static springs, the requirements for fatigue life are relatively lenient, and the wire can be directly drawn from controlled-rolling and controlled-cooling coils. Normalizing can be used instead of lead bath treatment for the pre-forming heat treatment of steel wire. Converter-killed steel is also permitted as raw material.
