Study on the Effect of Long Holding Time on Improving the Microstructure and Mechanical Properties of Nb Microalloyed Steels

The Nb content and holding time are crucial for studying the correlation between the microstructure, precipitation behavior, and mechanical properties of rebar. This paper studied the effect of holding at 650 °C for longer periods of time (500, 650, 800 and 950 s) on the microstructure transformation, precipitation behavior, and mechanical properties of Nb microalloyed steels. The microstructure, precipitation behavior, and mechanical properties of the experimental rebar were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), a universal tensile testing machine, and an HV-100 microhardness tester. The results showed that when the holding time was increased from 500 to 950 s, the distribution of ferrite and pearlite in the experimental Nb-containing rebar was more uniform. Moreover, the ferrite grain size was refined to 8.24 μm, the pearlite lamellar spacing was refined to 131.19 nm, the pearlite ratio reached 30.1%, the composite precipitations were finer, the precipitates changed from (V, Ti) C to (V, Ti, Nb) C, and the size of precipitates was reduced to 20-30 nm and distributed more uniformly on the ferrite or the grain boundaries. The microhardness increased to 245.89 HV, the yield strength increased to 632.47 MPa, the tensile strength increased to 766.96 MPa.