Precipitation kinetics of Nb carbonitride in austenite and acicular ferrite and its effect on hardness of high-Nb steel

The dissolved Nb contents in four high-Nb steels were quantitatively measured after hot deformation at 925 °C and reheating at 600 °C. The Nb carbonitride precipitation kinetics in austenite and in acicular ferrite were studied, and the effects of Nb carbonitride on the precipitation hardness were discussed. The results show that the nucleation rate of Nb precipitation in austenite is strongly depended on the dissolved Nb content in matrix. The parameter n of the JMAK equation is increased with the increase of the dissolved Nb content in matrix. The n value of the factor of nucleation rate of Nb carbonitride precipitated in acicular ferrite is related to the dissolved Nb and C contents in matrix. Nb carbonitride precipitated in austenite decreases the hardness of the final products by accelerating the phase transformation. By contrast, Nb carbonitride precipitated in acicular ferrite exhibits high precipitation strengthening potency.