The austenite formation under isochronal conditions in Nb microalloyed low carbon steel was studied by means of dilatometric analysis and the data was adjusted to the Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation, for different heating rates and for three initial microstructures. It was shown that the kinetics of austenitization of a pearlite+ferrite structure is faster than that of martensite (tempered martensite) at a heating rate of 0.1ºC/s. For heating rates higher than 0.1ºC/s, the kinetics of austenitization of a martensite structure is faster than of pearlite+ferrite one. The K parameter of the JMAK equation increases with the heating rate for the three previous microstructures and it is greater for the initial microstructure composed of ferrite+pearlite. At lower heating rates, the formation of austenite in this steel is controlled by carbon diffusion, independently of the initial microstructure. At higher heating rates, the formation of austenite from an initial microstructure composed of pearlite and ferrite is controlled by interface-controlled transformation.
austenite formation; kinetics; low carbon steel
