Oil tempered spring steel wire is a high-strength spring steel wire produced by cold drawing medium-carbon or alloy steel wire rod to the target size, followed by continuous heat treatment consisting of heating for austenitizing, oil quenching, and medium-temperature tempering. Its microstructure is uniform tempered martensite, with tensile strength reaching 1800–2200 MPa and a yield ratio of approximately 0.85–0.9, offering significantly better fatigue performance than ordinary cold-drawn hard wire. It is mainly used in high-stress, high-frequency springs such as automotive suspension springs and valve springs, and is classified into three fatigue grades: FD, TD, and VD.