In this study, crack generation and propagation during γ-austenite to α-ferrite phase transformation in Nb-microalloyed steel is observed. The relationship among the cracks, α-ferrite phase, and precipitate is discussed. The effects of the α-ferrite phase and precipitates on the hot ductility are discussed based on the experimental results and thermodynamic calculations. According to the mechanism of synergistic effects, microcracks begin to nucleate at the precipitate/α-ferrite phase interface. This is mainly due to stress concentration near the phase boundary between the soft phase ferrite and the hard phase precipitate. The microcracks grow faster along the γ/α phase boundary than α/α grain boundary. The growth rates are 1.67 and 0.83 µm s−1, for the γ/α and the α/α grain boundaries, respectively.
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