Effect of V and N on Microstructures and Properties of Grade-70 Tire Cord Steel during Cold Drawing

Commercial grade-70 tire cord steel was microalloyed with V and N to explore the possibility of substituting V for C and the effect of V and N on cementite dissolution during cold drawing process. Tensile strength, elongation and torsion laps of cold-drawn wires were determined. Microstructures of secondary phases were studied by transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS). Saturation magnetization was measured by using a vibrating sample magnetometer (VSM) to evaluate cementite dissolution during cold drawing process. Experimental results showed that with the increase of V content, tensile strength increased, elongation slightly decreased, and the torsion laps of the cold-drawn wires decreased. Results also showed that with the increase of N content, a peak (increasing first and then decreasing) appeared in the tensile strength graph, and an increase of elongation and a decrease of torsion laps of the cold-drawn wires occured. N is soluble in ferrite when its content is low. The increase in the content of N to 82 × 10−6 will result in the precipitation of the V-containing secondary phases. Also, cementite tended to be stable during cold drawing process with the increase of V and N contents. Grade-80 tire cord steel can be replaced by grade-70 tire cord steel microalloyed with 0.086 mass% V.