Crystallization kinetics of Al2O3–CaO–SiO2 based oxide inclusions

The crystallization of oxide inclusions present in steel affects their deformation during hot rolling, and as a consequence, the quality of the steel product. To define optimal compositions of these inclusions, the crystallization kinetics were experimentally determined on laboratory prepared mixtures Al2O3–CaO–SiO2–X (with X=MgO,Na2O,CaF2,TiO2,ZrO2). The samples were quenched after an isothermal treatment and then measured by X-ray diffraction and optical and scanning electron microscopy. The corresponding time–temperature-transformation (TTT) curves are drawn. The calculated average ratio Tnose/Tliquidus is 0.86±0.06. The effect of TiO2 and ZrO2 as nucleating agents was tested in Al2O3–CaO–SiO2–Na2O–CaF2 liquids. Such additions were found to increase both the incubation time and the growth rate, which is diffusion controlled. A Johnson–Mehl–Avrami approach has been applied to samples which precipitate only one crystalline phase with a single-nosed TTT curve but is inappropriate for compositions in which several crystalline phases precipitate or when the TTT curve shape is not single-nosed. The main factors affecting the glass forming ability are the viscosities and the liquidus temperatures of the liquids.