2022 年 62 巻 1 号 p. 227-236
The austenite formation kinetics of a low–carbon steel under two initial conditions—annealed and cold–rolled (0.08C–1.22Mn–0.73Si) were determined by dilatometric analysis during continuous heating. In both conditions, the austenite formation occurred in two stages. The critical transformation temperatures are a function of the heating rate but not of the initial microstructure (annealed or cold–rolled) that is, for a given heating rate (0.06, 0.36, or 0.66 K s−1), the critical transformation temperatures are similar for both conditions, despite in the cold–rolled condition, the cementite is spheroidized prior the austenite formation. The volume fraction of austenite was fitted based on the Johnson–Mehl–Avrami–Kolmogorov model to calculate the kinetics parameters k and n. The parameter k is proportional to the heating rate, and n changes between stages and heating rates for both conditions suggesting variations in the nucleation mode. The austenite–formation rate was calculated as a function of the microstructural evolution and transformation time. The austenite formation rate in the first stage is lower in the cold–rolled steel, during a specific range of temperatures, than in annealed steel which presented a formation rate maximum at a peak temperature. In the second stage, the behavior was similar for both conditions with a peak at the rate maximum.
