Thermodynamic Model for Carbonitride Precipitation in Low Carbon Steel with Nb-Ti-V and Verification

Carbonitride precipitates influence the microstructure and properties of steel by grain refinement and precipitation strengthening.Based on a two-sub lattice model（metal atom sub lattice and interstitial atom sub lattice） of regular solution,a thermodynamic model for the precipitates of carbonitride,aluminium nitride and manganese sulfide was established and then used to calculate the starting-temperature of precipitates,the equilibrium molar fraction and the evolution of precipitation.The precipitation pattern was verified by Thermo-Calc software,transmission electron microscope（TEM） and energy dispersive spectrometer（EDS）.The calculation results show that starting-temperature of precipitation of MnS and AlN are 1 200 ℃ and 675 ℃,their molar fraction are 1.05×10^-4 and 1.21×10^-4.The evolution order of precipitation particles is(Nb_0.29Ti_0.68V_0.03)(C_0.01N_0.99),(Nb_xTi_vV_1-x-v)(C_yN_1-y) and(Nb_0.29Ti_0.45V_0.26)(C_0.87N_0.13).By comparison,the simulation results of the model are in good agreement with the calculation by the Thermo-Calc software and the TEM observation results.