Evolution of precipitates in Nb-Ti binary low-carbon microalloyed steels

A thermodynamic model of binary low-carbon microalloyed steel with 0.023% Nb and 0.012% Ti additions was established to calculate the equilibrium molar fraction and chemical driving force of carbonitride precipitation as well as the molar fraction of each element in austenite at temperatures from 1 073 to 1 523 K. The evolution of precipitation in the microalloyed steel was studied, and the precipitation pattern was verified by transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS). The calculating results show that the chemical formula of precipitation particles is (Nb_0.15Ti_0.85) (C_0.16N_0.84) at 1523 K, Ti-rich precipitates gradually transit to Nb-Ti uniform precipitates. The evolution order of precipitation particles is (Nb_0.15Ti_0.85) (C_0.16N_0.84), (Nb_xTi_1-x) (C_yN_1-y) and (Nb_0.5 Ti_0.5) (C_0.56 N_0.44), which accord with the experimental results. The Ti/Nb mass ratio decreases gradually with decreasing temperature, and TiC is more insoluble than NbC. The critical core size and the relative nucleation rate were computed under two nucleation conditions, and the temperatures at which the size of the second phase is the smallest, i. e. the nucleation rate is maximum are 1 198 K and 1 123 K for homogeneous nucleation and dislocation nucleation, respectively.