Effect of heating temperature on the microstructure and mechanical properties of 700 MPa anti-seismic steel bar

Producing steel bars with higher strength grades is an effective measure to ensure building safety, but achieving a coordinated improvement in the strength and toughness of steel bars is also a challenge. This study produced a low-carbon equivalent Nb-V-Ti-N multi-element coupled steel bar by combining microalloying and rolling parameter control methods. The results indicate that the microstructure of the steel bar is primarily composed of ferrite and pearlite. Additionally, high-temperature treatment promotes pearlite phase transformation in the Nb-V-Ti-N multi-element coupled strengthened steel. As the temperature of heating process increases, the volume fraction, colony size and interlamellar spacing of pearlite all increase, while the proportionality of low-angle grain boundaries, fracture elongation, and shear lip width on the fracture surface decrease. The strength-ductility balance of the steel bar is primarily attained through phase transformation strengthening, supplemented by grain refinement and precipitation strengthening. When the heating temperature is elevated to 1275 °C, the yield strength of the steel bar is 733 MPa, the elongation at break is 15.4%, and the strength-yield ratio is 1.28. This study presents a temperature control strategy aimed at optimizing the performance of high-strength anti-seismic steel bars with a strength grade exceeding 700 MPa.