Effect of Initial Microstructures on Austenite Formation Behavior during Intercritical Annealing in Low-Carbon Steel

The effect of initial microstructures of low-carbon steel on austenite formation behavior during intercritical annealing was investigated. Three types of hot-rolled sheet specimens with different microstructures were used; specimen P consisting of ferrite and pearlite, specimen B consisting of bainitic structures, and specimen M consisting of fully martensitic structures. After the hot rolling, these specimens were coldrolled, and subsequently heated to target temperature, and then water-quenched to room temperature. The martensite and/or bainite fraction corresponds to the fraction of austenite during intercritical annealing since the austenite transforms into martensite and/or bainite during the cooling process. The austenite fraction in specimen M was larger than that in specimens P and B below 730 degrees C, whereas the order of specimens changed to P > B > M above 740 degrees C. Below 730 degrees C, austenite connected along the rolling direction was observed in specimens P and B, while the distribution of austenite in specimen M was uniform. In contrast, austenite was connected and elongated along the rolling direction in all the specimens above 740 degrees C. The nucleation and growth of austenite can proceed under local equilibrium in specimens P and B, whereas that can proceed under paraequilibrium in specimen M below 730 degrees C. Moreover, the austenite growth can progress under local equilibrium in all specimens above 740 degrees C.