Jean Lehmann

Kinetic model of non-metallic inclusions' precipitation during steel solidification

A numerical simulation which describes precipitation of non-metallic inclusions (liquid or solid oxides, nitrides) during steel solidification is proposed. Its aim is to predict the compositions and size distributions of these precipitates. The model takes into account microsegregation and includes homogeneous nucleation in the interdendritic liquid, and a mixed-controlled growth stage combining transport of solute elements in the boundary layer around precipitates and interfacial kinetics. We have observed that the calculated compositions of oxide inclusions differ from those of inclusions precipitating at equilibrium and are close to industrial observations on semi-killed steels.

Crystallization kinetics of Al2O3-CaO-SiO2 based oxide inclusions

Abstract 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.

Kinetics of precipitation of non-metallic inclusions during solidification of steel

A numerical simulation which describes precipitation of non-metallic inclusions during steel solidification is proposed. The model includes nucleation laws in the homogeneous phase, growth controlled by diffusion of solute elements and kinetics of interfacial reaction, as well as microsegregation. The calculated results, i.e., the total amount, size and number of inclusions, were compared to experimental observations obtained in the laboratory on TiN precipitation by quenching during unidirectional solidification. Sulphur content of the steel was observed to influence the kinetics of precipitation.

THERMODYNAMIC MODELS FOR MONITORING DEOXIDATION TREATMENTS AND CONTROLLING INCLUSIONS IN STEELS

A multiphase equilibrium calculation, based on IRSID’s statistical thermodynamics slag model, is used to predict the composition of endogeneous oxide and sulphide inclusions precipitated as a result of the deoxidation treatment, during metal solidification, or due to late reoxidations. This evaluation shows that the composition of inclusions is strongly affected by the content of trace elements (Ca, Mg and Al) in the steel. On the basis of these models, the monitoring of inclusion control by calcium treatment (Al-killed steels), or slag treatment (semi-killed steels) will be discussed.