Using Sodium Polyacrylate Aqueous Solution to Replace Lead Bath for Patenting High-Strength Steel Rope Wire of Automobile Industry

In order to eventually eliminate the lead toxicity in the steel wire making industry and to protect our environment from pollution, a polymer quenchant, aqueous sodium polyacrylate (SPA) solution, was selected to replace the lead bath and used as a substitution quenchant in patenting of high strength and precise steel rope wire for a car window auto lifter. A series of different concentrations of aqueous SPA solution were made up, cooling curves of these solutions at different temperatures were measured, and the cooling characteristics of the quenchant were evaluated according to the requirements for high strength steel wire patenting treatment. The cooling mechanism of patenting treatment and the cooling behavior of steel wires in the different concentrations of SPA solution bath at different temperatures were systematically explored and analyzed. Then, the cold-drawn KSC72A (0.72 % carbon) steel wires were subjected to patenting treatment in the optimized concentration of the SPA solution bath. The effect of the staying time in the water bath treatment on the microstructure and mechanical properties of the steel wire was investigated by means of a scanning electron microscope, a transmission electronic microscope, and material testing machines. The results show that when the SPA solution concentration is between 5–20 %, both the cooling rate at the high temperature range and the vapor blanket breaking the temperature decrease with the bath temperature increasing. When the temperature of the SPA aqueous solution bath is between 70°C∼95°C, the concentration has a barely significant effect on the cooling rate at a higher temperature range, which is needed for the sorbite transformation; however, the higher bath temperatures can make the vapor blanket stage (A-stage) maintain at 400°C. The patenting test results show that, within the designed length of the patenting bed, both the sorbite colony size and the lamella space decrease with the extension of processing time in the solution bath, and the mechanical properties (tensile strength, percentage of elongation, and reduction of area) of the steel wires do increase as the sorbite colony size and the lamella space decrease. When the KSC72A (0.72 % carbon) steel rope wires with diameters of 0.8 mm are subjected to the patenting treatment in the SPA solution bath at 90°C with the concentration of 10 %, the satisfactory metallurgical results and mechanical properties can be obtained by using a wire feed speed of 50 cm/min within the 50-cm length of the solution bath. The sorbite lamellae space and the sorbite colony size of the steel rope wires are about 82.76 nm and 2.86 μm, respectively. The tensile strength, elongation percentage, area reduction, and micro-hardness reach 1,266 MPa, 6.4 %, 54.5 %, and 340 HV, correspondingly. This excellent combination of the mechanical properties completely meets the technical requirements of the high strength steel rope wire. Hence, the SPA aqueous solution bath can replace the traditional lead bath to be used for the patenting process of high strength steel rope wires.