Effect of vanadium and nitrogen contents on microstructure and mechanical properties of 650–700 MPa high-strength rebar steel

Abstract

The present work investigated the effects of vanadium contents (V, 0.06 wt%–0.22 wt%), nitrogen contents (N, 0.009 wt%–0.043 wt%), and V/N ratio (2.9–23.3) on microstructure, mechanical properties, and underlying mechanisms of 650–700 MPa rebar steel. The variation of the mechanical properties before and after artificial aging at 300°C for 1 h was also examined. The results demonstrated that the microstructure of the rebar steels consists of polygonal ferrite and pearlite (P). An increase in V content led to higher ferrite fraction and strength for the rebar steel, along with a decrease in grain size. Similarly, an increase in N content resulted in higher ferrite fraction accompanied by a decrease in grain size. The hardening process experienced by the ferrite phase led to an increase in nano-hardness and Young's modulus, thereby enhancing the strength of the experimental steels. However, when the N content reached 0.043 wt%, a reduction in ferrite fraction along with a decrease in strength resulting from an increase in grain size, leading to softer ferrite, decreased nano-hardness, Young's modulus, and ductility. The artificial aging results indicated that when the V/N ratio does not exceed 5.5, a large amount of free N will exist in the steel, while excessive V atoms dissolved in matrix when the V/N ratio equal to 23.3. Then, there will be an increase in strength with less obvious reduction in plasticity but worsened toughness. The optimal V/N ratio compatible with the present thermo-mechanically controlled process was 9.2, yielding comprehensive mechanical properties that far exceed the requirements of 700 MPa rebar steel. Subsequently, industrial trial was conducted with a V and N addition of 0.15 wt% and 0.020 wt%, respectively. The results showed that V(C,N) precipitates were uniformly dispersed and fine, located within matrix, near dislocation lines, and at grain boundaries. The yield strength (R_eL), tensile strength (R_m), elongation (A), and total elongation at maximum force (A_gt) were 657 MPa, 900 MPa, 19.5%, and 10.5%, respectively, indicating that with lower V and appropriate N contents, the steel fully meets the performance requirements for 650 MPa grade rebar steel, while also reducing cost.