Effect of N on Mirostructures and Mechanical Properties of Vanadium Containing Q345 Steel

The test steels in this study had base chemistry of Q345 steel with 0.08%V and various nitrogen contents of 0%, 0.022%,0.034% and 0.042%.Thermo-Calc software was used for the thermodynamic calculation.When steel is free of nitrogen,the calculated V(C,N) precipitation temperature in austeniteis 933℃.When the nitrogen content in the steel is 420×10^-6.the precipitation temperature of V(C,N) is 1 340℃.TEM observation indicates that increasing nitrogen content can result in a large number of V(C,N) which plays the role of precipitation strengthening and refining grain.The mechanical properties of the steel increase significantly with increasing nitrogen content.The steel free of nitrogen has yield strength of 486 MPa,and tensile strength of 686 MPa.When the nitrogen content is 340×10^-6,the yield strength of the test steel is 610 MPa,and the tensile strength is 732 MPa.Fracture observation show that toughness and ductility of Q345 steel has been enhanced with increase of nitrogen content.