Effect of cooling rate and Nb synergistic strengthening on microstructure and mechanical properties of high-strength rebar

Rebar is an extremely important building material. The cooling rate and the presence of niobium (Nb) element are key factors influencing the overall performance of rebars. In this work, the high-strength rebar's microstructure, precipitated phase, and mechanical properties were characterized using scanning electron microscopy, transmission electron microscopy, HVS-1000 hardness tester, and MTS810 universal tensile testing machine. The results showed that a shift in cooling rate from 0.3 to 3 degrees Cs(-1) resulted in noticeable changes in the microstructures of rebars, particularly between Nb-free and Nb-containing variants. In the case of Nb-containing rebars, there was an increase of 8.26% in the proportion of pearlite, along with a decrease of 10.63 mu m in the average grain size of ferrite. Furthermore, the lamellar spacing of pearlite experienced a decrease of 0.0495 mu m, the proportion of low-angle grain boundaries saw an increment of 4.13%, and the size of the precipitated phase (Nb, Ti, V) C reduced by 18.9 nm. These changes collectively led to a significant increase in hardness (98.56 HV), yield strength (179.02 MPa), and ultimate strength (199.43 MPa). The resultant fracture morphology manifested as a dimple pattern.