Nucleation and growth of selective oxide particles on ferritic steel

In the continuous annealing process, steel sheets are annealed at 800 °C in an atmosphere of nitrogen and hydrogen (5 vol.%) containing low partial water pressure (20–50 Pa). Under these conditions, the most oxidizable alloying elements in the steel segregate towards the surface where they form oxide particles. The nucleation and growth of those oxides were examined. Oxide nucleation mainly occurs between 650 and 750 °C. During their growth, the oxides take the form of a spherical cap and are composed of MnO, Mn2SiO4 (or MnSiO3), MnAl2O4, SiO2, Al2O3 and B2O3. Particle nucleation and growth are favored on grain boundaries.