Generally, pickling and phosphating are used to remove iron oxide scale and form a phosphating film; a few methods also employ mechanical treatment. The goal is to meet the requirements of the cold drawing process and obtain a smooth surface. For spring steel wire requiring high fatigue life, such as valve spring steel wire, the wire rod should be peeled to reduce surface defects. If the steel mill can perform grinding on the billet, it will also help reduce defects.
The drawing process of the finished product has a significant impact on its performance. Generally, a relatively large total area reduction rate of around 90% (see area reduction rate) and a smaller pass area reduction rate (approximately 10%-20%) are used to ensure product toughness. For high-strength spring steel wire, the exit temperature of the wire in each pass should be controlled below 150℃ during drawing to prevent torsional cracks caused by strain aging, a major defect leading to wire scrap. Therefore, good lubrication and sufficient cooling are essential during drawing. Using a smaller pass area reduction rate and drawing speed helps reduce the temperature rise of the wire.
Carbon steel spring wire commonly uses lead bath quenching, which produces a very fine pearlitic structure (sorbite), beneficial for improving deep drawing performance and spring properties. Fluidized bed quenching, an alternative to lead bath quenching, has not yet been widely adopted and is only used for some small-diameter wires. Alloy steel wire generally undergoes annealing heat treatment to adapt the microstructure to drawing deformation. Stainless steel wire undergoes solution treatment, also aimed at improving the microstructure to meet drawing requirements. Oil quenching is used on finished steel wires, employing induction heating or gas/oil-fired furnaces to heat the wire to its austenitizing temperature, hold it for a period of time, and then perform a medium-temperature tempering after quenching. Although nominally called oil quenching, it is basically done using water quenching or water quenching with added polymer materials.
