Effect of different isothermal times on the microstructure and mechanical properties of high-strength rebar

Abstract High-strength rebar plays a supporting role in large engineering structures due to its excellent performance. In this study, the effect of different isothermal time treatments (30, 60, 100, and 200 s) at 650°C on the microstructure transformation and mechanical properties of rebars was investigated. The hot-rolling process was simulated by Gleeble-3800 thermal simulator. The microstructure, precipitates, and mechanical properties of high-strength rebar were characterized by scanning electron microscopy, transmission electron microscopy (TEM), and a universal tensile test machine. Results show that when the isothermal time increased from 30 to 200 s, the ferrite grain size decreased from 10.632 to 8.326 μm, and the pearlite lamellar spacing was refined from 0.230 to 0.142 μm. The TEM confirmed that when the isothermal time increased from 30 to 200 s, the nanoscale (Nb, V, and Ti) C precipitates were uniformly distributed in the ferrite matrix and grain boundary, and the size of precipitates decreased from 34.014 to 29.916 nm; thus, the tensile strength increased from 752.477 to 780.713 MPa, and the yield strength increased from 574.714 to 621.434 MPa.