Effect of Magnesium Addition on Acicular Ferrite Nucleation in FH420 Steels

The present work investigates the effect of inclusions on acicular ferrite (AF) nucleation in FH420 steels. Six heats with Mg contents of 0, 5, 21, 37, 56, and 80 ppm are prepared. The ferrite transformation is directly monitored in situ using high‐temperature confocal laser scanning microscopy. The composition and size of inclusions and their AF‐nucleating behavior are characterized by scanning electron microscopy coupled with energy‐dispersive spectroscopy, while electron backscatter diffraction is employed to verify inclusion‐induced intragranular AF and its crystallographic features. Mg addition transforms the dominant oxides toward MgO‐rich inclusions and produces a nonmonotonic AF response: the AF fraction increases from 49.5% to 60.9% and peaks at 61.9% at 21 ppm Mg, but decreases to 44.4–32.6% at 37–80 ppm. Based on the experimental results, relationships among the cosine of the equivalent contact angle parameter at the inclusion–ferrite interface, the inclusion diameter, and the normalized energy barrier are established, which are consistent with the predictions of classical nucleation theory.