Effect of adding nitrogen on microstructure and property of vanadium microalloyed reinforcing bar steel

The mechanical properties of reinforcing bar steel microalloyed with vanadium and nitrogen (known as V-N steel) and its microstructures were studied. The results indicate that by adding nitrogen to the steel, the tensile strength of the steel is increased by 135 MPa and the yield strength is increased by 120 MPa for the same vanadium content of vanadium steel. The saving ratio in vanadium content is higher than 33.3 % for identical or similar yield strength. Nitrogen added to the steels promotes the precipitation of V ( C, N), because the ratio of vanadium content to nitrogen content is near to stoichiometric ratio. The amount of V (C, N) is doubled and fine particles of 1-10 nm are increased by 53 %. At the same time, adding nitrogen to the steel refines ferrite grain size effectively. The practical ferrite average grain size of V-N steel bar is 5.8 mum. So nitrogen in the steel promotes effectively the role of vanadium precipitation strengthening and grain refinement, reaching the target of full utilization of vanadium.