Crystallization kinetics of Al2O3-CaO-SiO2 based oxide inclusions

The crystallization of oxide inclusions present in steel affects their defo rmation juring hot rolling? and as a consequence, the quality of the steel product. To define optimal compositions of these inclusions, the crystalliz ation kinetics were experimentally determined on laboratory prepared mixtur es Al2O3-CaO-SiO2-X (with X = MgO, Na2O, CaF2, TiO2, ZrO2). The samples wer e quenched after an isothermal treatment and then measured by X-ray diffrac tion and optical and scanning electron microscopy. The corresponding time-t emperature-transformation (TTT) curves are drawn. The calculated average ra tio T-nose/T-liquidus is 0.86 +/- 0.06. The effect of TiO2 and ZrO2 as nucl eating agents was tested in Al2O3-CaO-SiO2-Na2O-CaF2 liquids. Such addition s were found to increase both the incubation time and the growth rate, whic h is diffusion controlled. A Johnson-Mehl-Avrami approach has been applied to samples which precipitate only one crystalline phase with a single-nosed ITT curve but is inappropriate for compositions in which several crystalli ne phases precipitate or when the TIT curve shape is not single-nosed. The main factors affecting the glass forming ability are the viscosities and th e liquidus temperatures of the liquids. (C) 2001 Elsevier Science B.V. All rights reserved.