Development of niobium-vanadium microalloyed 9.8 grade high-strength, high-toughness non-quenched and tempered cold heading steel wire rod

In response to the demand for high strength and toughness 9.8 grade non-quenched and tempered cold heading steel wire rod for the production of large deformation automotive fastener products and reduced mold loss, based on the chemical composition of the original MFT8 non-quenched and tempered cold heading steel wire rod, the niobium-vanadium composite microalloying was used, and the C, Si, Mn and other elements were fine-tuned. By controlling w(V)/w(N), the strength and toughness of the wire rod was ensured, and by controlling w(Al)/w (N), the strain aging performance of the wire rod after cold working plastic deformation was ensured. Combined with the Bauschinger effect produced by cold drawing of the wire rod, the strength and plastic toughness of the final products were ensured. Low temperature rolling, controlled rolling and controlled cooling process were adopted to give full play to the precipitation strengthening effects of dispersed fine carbonitrides. While improving the strength of the wire rod, the plastic toughness was not reduced, and the high strength and toughness of non-quenched and tempered cold heading steel wire rod with excellent cold working properties was obtained. This wire rod could be directly drawn and cold heading without undergoing spheroidizing annealing + quenching and tempering treatment.It could be used to produce 9.8 grade high-strength fasteners with large deformation such as long rod flange bolts.