Effect of Vanadium on the Microstructure and Mechanical Properties of 2100 MPa Ultra-High Strength High Plasticity Spring Steel Processed by a Novel Online Rapid-Induction Heat Treatment

Advanced automotive industries generate large demand for the next generation of high strength and high toughness spring steels. Vanadium-containing 55SiCrV spring steels subjected to rapid-induction heating treatment can fulfil such requirements. However, the effect of vanadium microalloying under online rapid-induction heat treatments is rarely reported. A comparative study of the microstructure and tensile properties of 55SiCr and 55SiCrV spring steel wires subjected to a novel online rapid induction heat treatment has been demonstrated herein. It is found that the tensile strength of the 55SiCr spring wire decreases with the decrease in the wire speed in online rapid-induction heating, and the plasticity increases. Whereas, the tensile strength of the 55SiCrV steel wire increases with the decrease in the wire speed with the retained high plasticity, which is attributed to the strengthening effect of the dislocations. Through the optimized rapid-induction heating/cooling thermal cycles and intermediate-temperature tempering treatment, the tensile strength of the 55SiCrV steel wire approaches 2106 MPa with total elongation of 9.7%. Compared with the 55SiCr spring steel, the addition of V in 55SiCrV spring steel changes the strengthening and toughening mechanisms via the grain refinement and enhancement in the hardenability and tempering resistance. The finely dispersed V-containing secondary phases are rarely found in the matrix, which indicates that the precipitation effect stemming from the addition of V is not the dominant strengthening factor in the online rapid-induction heat process. The proposed novel online rapid-induction heat treatment provides a promising pathway for the mechanical property improvement of the spring steel.

Automated summary

This study investigates the effect of vanadium microalloying under online rapid-induction heat treatments on the microstructure and tensile properties of 55SiCr and 55SiCrV spring steel wires. It is found that the tensile strength of 55SiCr spring wire decreases with the decrease in wire speed during rapid-induction heating, while the tensile strength of 55SiCrV steel wire increases and retains high plasticity. By optimizing the thermal cycles and tempering treatment, the tensile strength of 55SiCrV steel wire reaches 2106 MPa with a total elongation of 9.7%. Compared to 55SiCr spring steel, the addition of vanadium in 55SiCrV spring steel changes the strengthening and toughening mechanisms through grain refinement and enhancement in hardenability and tempering resistance.