Lieven Pandelaers

Effect of High Cooling Rates on the Mineralogy and Hydraulic Properties of Stainless Steel Slags

This article investigates the effect of chemical composition and cooling rate during solidification on the mineralogy and hydraulic properties of synthetic stainless steel slags. Three synthetic slags, covering the range of typical chemical composition in industrial practice, were subjected to high cooling rates, by melt spinning granulation or quenching in water, and to low cooling rates, by cooling inside the furnace. Both methods of rapid cooling led to volumetrically stable slags unlike the slow cooling which resulted in a powder-like material. Stabilized slags consisted predominantly of lamellar β-dicalcium silicate (β-C2S) and Mg, Ca-silicates (merwinite and bredigite); the latter form the matrix at low basicity and are segregated along the C2S grain boundaries at high basicities. Slowly cooled slags consist of the γ-C2S polymorph instead of the β-C2S and of less Mg, Ca-silicates. Isothermal conduction calorimetry and thermogravimetric analysis indicate the occurrence of hydration reactions in the stabilized slags after mixing with water, while calcium silicate hydrates (C-S-H) of typical acicular morphology are identified by SEM. The present results demonstrate that the application of high cooling rates can result in a stable, environmental-friendly, hydraulic binder from stainless steel slags, rich in β-C2S, without the necessity of introducing any additions to arrest the β polymorph.

Theoretical evaluation of influence of convective heat transfer and original sample size on shell melting time during Ti dissolution in secondary steelmaking

Abstract The dissolution of Ti additions in liquid steel during secondary steelmaking occurs in a two step process. In the first step, a steel shell solidifies around the initial cold addition, whereas in the second step, after this shell has remelted, the Ti dissolves directly in the steel bath. The initial presence of this steel shell modifies the position of dissolution and influences the local concentration and thus the inclusion formation. Further complications arise from the fact that part of the Ti will dissolve while enclosed by the steel shell, altering the alloy composition first released in the ladle and effectively shortening the subsequent free dissolution period. The duration of the steel shell period and the fraction of predissolved Ti have been investigated using a conservative one-dimensional sharp interface model solving the coupled heat and mass transfer in a cylindrical shell/addition composite. The influence of the convection conditions and the original Ti radius was evaluated in a parametric study. A pronounced effect of the convective heat transfer on the shell melting time was found. It is thus concluded that the dissolution behaviour is strongly dependent on the local flow conditions, which is determined by factors such as stirring conditions and addition characteristics.

Interfacial Reactions during the Dissolution of Titanium in Liquid Iron

The interfacial reactions between a solidified Fe shell and Ti were investigated within the framework of steel alloying. Ti cylinders were immersed into liquid Fe for various durations and subsequently water-quenched. An Fe shell solidifies around the Ti. At the interface between Fe and Ti a reaction zone is formed. Initially it consists of a liquid eutectic layer, though in later stages all stable phases at elevated temperature can be found in the quenched microstructure. The larger part of this reaction zone is fluid at high temperature and both Ti and Fe dissolve into it. Moreover, intermetallic compound formation and mixing of Fe and Ti generate extra heat, shortening the time required for shell melt-back. The reaction zone reaches thicknesses up to 40 % of the initial sample’s radius and when the shell has completely remolten, a discontinuity in the thickness-time profile is expected. The exact morphology of the reaction zone at high temperature remains to be determined: presence of a solid layer of Fe2Ti may impede mixing in the initial stages.

Stabilization of Free Lime in BOF Slag by Melting and Solidification in Air

The present study reports on the mineralogy of BOF slag after melting and solidification under Ar and air atmosphere. Results indicate that free lime can be effectively stabilized in air without additional stabilizers like SiO2. The stabilization mechanism was analyzed, and the role of the oxygen partial pressure was explored. Wustite is believed to be oxidized to hematite under high oxygen partial pressure and then stabilizes the free lime by forming brownmillerite.

Effect of Al2O3 and SiO2 Addition on the Viscosity of BOF Slag

The effect of solid phases and SiO2 and Al2O3 additions on the viscosity of BOF slag was measured with a rotational viscometer in the temperature range of 1500°C-1700°C. Various viscosity models for completely liquid slag were evaluated and parameters in the Einstein-Roscoe equation were optimized to estimate the influence of the solid phases on the BOF slag viscosity.

Experimental Evaluation of the Dissolution Rates of Ti and FeTi70 in Liquid Fe

During secondary steelmaking, improving alloy yield and engineering inclusion content require understanding and quantification of the alloy distribution in the melt. When additions are dropped in the melt, a steel shell solidifies around them. After this shell has melted, the alloy is spread in the melt. The influence of process parameters on the duration of the shell period for Ti and FeTi70 additions has been experimentally evaluated. For Ti, the melt temperature and the initial addition size were varied and for FeTi70, only the melt temperature was varied. By continuously measuring the apparent weight of submerged samples with a load cell, the shell period and the amount of molten alloy within the shell were determined. The shell period increases at lower superheats and for larger sample sizes. For a certain size of Ti additions, the molten content within the shell increases with increasing shell period. The importance of this period, relative to the total dissolution time, increases at lower superheats. All investigated FeTi70 samples may melt completely within the shell. While the shell period lasts longer for FeTi70 than for the corresponding Ti samples, this fast internal melting yields a net reduction in total dissolution time.

Viscosity of Heterogeneous Silicate Melts: A Review

The viscosity of solid phase containing heterogeneous silicate melts is discussed in this review. The crucial parameters characterizing silicate melt structure are first presented, i.e., volume fraction of solid particles, particle shape and size. The measurement and calculation methods of these parameters are summarized and the influence of the parameters on viscosity is discussed. Next, a theoretical description of rheological behavior in heterogeneous silicate melts is given, focusing on non-Newtonian characteristics, e.g., shear-dependence, time-dependence, and yield stress. The viscosity models which were proposed in the last half century are reviewed and critically assessed. This paper concludes with opportunities and suggestions for future work.