2019 年 68 巻 9 号 p. 718-722
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 cold-rolled, 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 °C, whereas the order of specimens changed to P > B > M above 740 °C. Below 730 °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 °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 °C. Moreover, the austenite growth can progress under local equilibrium in all specimens above 740 °C.