Qian Xu, Dong-ping Zhan, Wei-li Xu, Fu-hua Fan, Hong-tao Li, Huai-zhu Li, Shang-kun Wang
By increasing the nitrogen content in vanadium-containing steel, the mechanical properties of the steel can be effectively improved. In order to better utilize the strengthening effect of N in vanadium-containing steel and reduce the use of ferrosilicon, which will reduce production costs and provide a theoretical basis for adjusting the composition of steel bars, we designed three different vanadium-containing steel samples with varying N and Si contents. Through mechanical property testing, metallographic microscopy, scanning electron microscopy, transmission electron microscopy, and other test methods, the evolution of the structure and mechanical properties of HRB400e steel under different N and Si contents were studied. The results showed that increasing the nitrogen content in steel can improve the yield strength of threaded steel. Before nitrogen addition, the yield strength of S1 steel exceeds the national standard at 29 MPa. After increasing the N content by 0.0056%, the yield strength of the steel exceeds the national standard of 64.4 MPa. Thermodynamic calculations revealed the presence of VN, V(C, N), Si3N4, Fe3C, and M7C3 phases in the steel, and VN, Fe3C, and M7C3 phases were found in the test steel through the transmission electron microscope and high resolution transmission electron microscope. Increasing the N content can also increase the precipitation temperature of the precipitates, allowing them to better anchor the austenite grain boundaries during hot rolling and prevent the growth of austenite grains. This conclusion was also confirmed through high-temperature coagulation experiments. After refinement of the austenite grain size, the final ferrite and pearlite structure is further refined, and their distribution becomes more uniform.
