The kinetics of Nb(C,N) precipitation during the isothermal austenite to ferrite transformation in a low-carbon Nb-microalloyed steel

The kinetics of Nb(C,N) precipitation occurring during the isothermal ferritic (α) transformation were quantitatively measured, along with the transformation kinetics at intercritical temperatures ranging from 710 to 790 °C in a Nb-microalloyed steel by means of electrical resistivity and dilatometry. The precipitation occurred most rapidly at 750 °C, which corresponds to a bay temperature on the start curve of the ferritic transformation of the transformation–time–temperature diagram. While interphase precipitation was observed at and above the bay temperature, precipitation in the α matrix was predominantly below the bay temperature. However, precipitation in the untransformed austenite (γ) matrix during the ferritic transformation was also observed, regardless of the intercritical temperatures. It is suggested that the precipitation occurring in the untransformed γ matrix during the ferritic transformation was accelerated owing to carbon enrichment from the α matrix to the γ matrix during the ferritic transformation. The average size of Nb(C,N) particles observed in the α matrix was slightly larger than that of the γ matrix at a given intercritical temperature. This result is proposed to arise primarily from the rapid diffusion of solute Nb atoms in the body-centered cubic α matrix.