Reducing yield ratio while enhancing strength in 500 MPa grade wind power steel via intercritical annealing

This study achieved a favourable combination of high strength and a low yield ratio on a novel wind power steel subjected to a single-step intercritical annealing (IA) followed by water cooling strategy. The microstructure, mechanical properties and strain-hardening behavior of the steel were investigated before and after IA. Results showed that the experimental steel was transformed from a bainitic structure to a ferrite-martensite dual-phase (DP) steel. As the annealing temperature increased to 800 °C, the tensile strength increased significantly from 732 MPa to 877 MPa, while the yield ratio decreased effectively from 0.85 to 0.52. The former is attributed to the high volume fraction of martensite, while the latter is primarily due to the chain-like networked martensite distribution which enhances ductility and strain-hardening capacity. Finally, strain-hardening characteristics were analyzed using the Hollomon and modified Crussard-Jaoul (MC-J) models. The results indicate that the MC-J model demonstrated a superior fit for the ferrite-martensite DP steel. In conclusion, single-step IA is proven to be an effective heat treatment for optimizing the mechanical properties of wind power steels. • A desirable combination of high strength and low yield ratio was achieved in intercritically annealed 500 MPa grade wind power steel. • The microstructures and its relation to strengthening mechanisms were revealed. • Strain-hardening characteristics were well-analyzed by the Hollomon and modified Crussard-Jaoul (MC-J) models.