Reaction mechanism between porous ferrosilicon nitride ceramic and molten steel

A novel porous ferrosilicon nitride (Fe3Si-Si3N4) ceramic sliding gate was prepared and applied in a ladle in a steel mill; it was characterised by X-ray diffraction analysis, scanning electron microscopy, and energy-dispersive spectroscopy. The results showed that after use, the sliding gate consisted of a reaction layer (0-1.5 mm), a transition layer (1.5-2.2 mm), a gradient layer (2.2-20 mm), and an unaltered layer. In the reaction layer, Si3N4 is decomposed, and the structure is damaged; in the transition layer, Si3N4 is stable, and the structure shows no obvious change; in the gradient layer, Si3N4 is oxidised slightly. The reaction mechanism is as follows. The structure of the original porous ferrosilicon nitride resembles a sea urchin, and β-Si3N4 columns are bundled together by Fe3Si. During operation, Fe3Si reacts with molten steel to form an Fe/FeSi melt, which attacks the structure, whereas Si3N4 decomposes to the product N2(g) [Si3N4(s)+Fe(l)→FeSi(l)+N2(g)]. N2(g) forms a high-pressure gas cushion that blocks the molten steel, protecting the inner part of the sliding gate, so Si3N4 remains stable in the transition layer. Further inward, Si3N4 is oxidised slightly by trace [O] from the molten steel to generate silicon oxynitride. The reaction model was established.