Effect of Solution and Precipitation of Nb on Microstructure and Strengthening Mechanism of High-Strength Anti-seismic Rebar

The microstructure, precipitates, and mechanical properties of high-strength anti-seismic rebar with varying Nb contents of 0.02, 0.03, and 0.064 wt.% were investigated by using physical and chemical phase analysis, extraction replica, and dissolution, among others. The strengthening mechanisms of the tested rebars were quantitatively calculated. When compared to 20 Nb steel, 30 Nb steel exhibits increased yield and tensile strength, particularly a notable increase in tensile strength, resulting in a significant rise in the Rm/Rel ratio. 64 Nb steel contains a higher concentration of solute Nb compared to 30 Nb steel. However, the difference in the volume fraction of ferrite is not significant. Additionally, the effect of solute Nb on dragging C atoms reaches a peak influence on the transformation amount of the austenite–ferrite transformation. Furthermore, the reheating process introduces more undissolved and solute Nb into the austenite of 64Nb steel, leading to finer ferrite grains and promoting the formation of bainite. Consequently, the yield strength increases. Although the precipitation of 64Nb steel increases, the precipitation strengthening increment is only 7 MPa higher than that of 30 Nb steel, thus resulting in an insignificant strengthening effect. Additionally, the presence of coarse precipitates causes the tensile strength of 64Nb steel to be lower than that of 30 Nb steel.