Seppo Louhenkilpi

Real-time simulation of heat transfer in continuous casting

A real-time heat-transfer model for continuous slab casting is presented. The model calculates the strand temperatures and the solid shell thickness profile along the machine as a function of the actual casting variables, strand geometry, and steel grade. The special requirements con-cerning the real-time use of the model and, in general, the accuracy of the model are also studied and discussed. The model has been tested by carrying out industrial trials. Some examples of the differences between the calculated and measured surface temperatures are presented. A spe-cial procedure to determine the boundary conditions for the secondary cooling zones from tem-perature measurements is also described.

Calculation of thermophysical properties of carbon and low alloyed steels for modeling of solidification processes

Algorithms and a computer software package for calculating thermophysical material properties of carbon and low-alloyed steels, associated with the simulation of solidification processes, have been developed. The earlier studies on kinetic phase transformation modeling are applied and are the base of the present work. The calculation algorithms are based on thermodynamic theory connected to thermodynamic assessment data, as well as on regression formulas of experimental data, and they take into account the temperature, the cooling rate, and the steel composition. The calculation algorithms and some results of calculations are presented in this article.

IDS: Thermodynamic-kinetic-empirical tool for modelling of solidification, microstructure and material properties

IDS (InterDendritic Solidification) is a thermodynamic-kinetic-empirical tool for simulation of solidification phenomena of steels including phase transformations from melt down to room temperature. In addition, important thermophysical material properties (enthalpy, thermal conductivity, density, etc.) are calculated. The model has been developed in the Laboratory of Metallurgy, Helsinki University of Technology, Finland, since 1984. IDS includes two main modules, the IDS module and the ADC (Austenite DeComposition) module. IDS module simulates the solidification phenomena from liquid down to 1000^oC and ADC the austenite decomposition down to room temperature. Both modules have their own recommended composition ranges. The IDS module is based on the so-called sharp interface concept. The ADC is mainly statistical based on empirical CCT (Continuous Cooling Transformation) diagrams. IDS tool is also coupled with the thermodynamic programmers library, called ChemApp, developed by a German company, GTT-Technologies. This coupled package is used to simulate among other things multiphase inclusions during solidification. The present paper summarises the features of the IDS tool including the coupling with the ChemApp library.

Numerical simulation of dehydrogenation of liquid steel in the vacuum tank degasser

Vacuum tank degassers are often utilized to remove hydrogen from liquid steel. A new comprehensive numerical model, which has been developed to simulate hydrogen removal in the vacuum degassers, is presented in this paper. The degassing model consists of two sub-models, which calculate the gas-steel flow field and the species transport of hydrogen. An extended k–ε turbulence model is adopted to consider the effect of gas injection on the turbulent properties and an interfacial area concentration model is introduced to compute the interfacial area density between liquid steel and the bubbles. The fluid dynamic sub-model is validated with a physical gas stirred tank, which is believed to have similar flow phenomena as the studied vacuum degasser based on the modified Froude number. Two fundamental expressions for mass transfer coefficient, which have been paid little attention by the researchers concentrating on vacuum degassing, are evaluated with a simulation case corresponding to practical operation. The effect of vacuum pressure on the dehydrogenation process is investigated and, moreover, the integrated model is verified with industrial measurements. The predicted final hydrogen contents in liquid steel show good agreement with the measured ones. The model and the main results are presented.

Numerical study on the removal of hydrogen and nitrogen from the melt of medium carbon steel in vacuum tank degasser

The steelmaking field has been seeing an increased demand of reducing hydrogen and nitrogen in liquid steel before casting. This is often accomplished by vacuum treatment. This paper focuses on developing a numerical model to investigate the removal of hydrogen and nitrogen from the melt of medium carbon steel in a commercial vacuum tank degasser. An activity coefficient model and the eddy-cell expression are implemented in the ANSYS FLUENT code to compute the activities of related elements and mass transfer coefficients of hydrogen and nitrogen in liquid steel. Several cases are simulated to assess the effect of gas flow rate and initial nitrogen content in liquid steel on degassing process and the calculated results are compared with industrial measured data.

Thermodynamic Evaluation of Inclusions Formation and Behaviour in Steels during Casting and Solidification

For demanding applications high steel cleanliness and strictly controlled inclusions are required. Primary inclusions are formed during steel treatments in the ladle. Most of these are removed to the ladle slag or on the lining. However, the rest of the inclusions still remain through the successive process stages, and additionally new inclusions are formed during casting and solidification due to decreasing thermodynamic solubility of oxygen in the steel at lower temperatures, reactions with surrounding slag, refractory materials and eventual contact with air. Inclusions formation and transformation are simulated by thermodynamic calculations in the steel/inclusions/slag system taking into account the solidification phenomena. In this paper inclusions in Si/Mn-deoxidised steel and Al-deoxidised Ca-treated steels are contributed. Calculations are compared with experimental results from steel plants.

Electron Probe Microanalysis of Ni Silicides Using Ni-L X-Ray Lines

We report electron probe microanalysis measurements on nickel silicides, Ni5Si2, Ni2Si, Ni3Si2, and NiSi, which were done in order to investigate anomalies that affect the analysis of such materials by using the Ni L3-M4,5 line (Lα). Possible sources of systematic discrepancies between experimental data and theoretical predictions of Ni L3-M4,5 k-ratios are examined, and special attention is paid to dependence of the Ni L3-M4,5 k-ratios on mass-attenuation coefficients and partial fluorescence yields. Self-absorption X-ray spectra and empirical mass-attenuation coefficients were obtained for the considered materials from X-ray emission spectra and relative X-ray intensity measurements, respectively. It is shown that calculated k-ratios with empirical mass attenuation coefficients and modified partial fluorescence yields give better agreement with experimental data, except at very low accelerating voltages. Alternatively, satisfactory agreement is also achieved by using the Ni L3-M1 line (Lℓ) instead of the Ni L3-M4,5 line.

Continuous Casting of Steel

Continuous casting process has grown into the biggest casting method for steel, exceeding the conventional ingot casting route in the mid-1980s. Nowadays, the continuous casting ratio has reached the level of 95%. Continuous casting offers not only a high level of productivity and yield but also improved quality. The research and development work in the continuous casting field is continuing intensively because the requirements for steel quality from customers become all the time stricter and the energy efficiency, productivity, and ecological aspects are of increasing importance. One aim of the development has been to construct lower and simpler machines with smaller need for space, low investment costs, and high flexibility in production and maintenance. Today, we have fairly good knowledge of the complex phenomena taking place in continuous casting. Computational simulation and modeling of different phenomena in casting have greatly helped to solve practical problems in industrial casters and to improve process practices and control. Altogether, we still need deeper understanding of the complex solidification phenomena and transformations of microstructure in continuous casting in order to rise to the increasing requirements. This chapter attempts to overview the continuous casting method including machines, solidification phenomena, defects formation, and modeling aspects.

Characterizing the Inner Structure of Continuously Cast Sections by Using a Heat Transfer Model

The inner structure of the continuously cast semis has a great importance from the point of view of further processing and application. The main reason for it is the very direct effect of the inner structure’s features (i.e. porosity, macrosegregations, geom etry of primary dendrites) on the technological characteristics features of the semis during fur ther processing (i.e. crack sensitivity, formability, etc.). The paper deals with the possible ways of mac rostructure determination on the basis of the results of mathematical modeling of continuous casting process. We paid special attention to the geometry of primary dendrites and to the columnar-e quiaxed transition as a function of heat parameters of the casting process. Introduction In the continuous casting shops an obvious way to set up the production is t o increase the casting rate. In general, in order to realize the higher rate the enhancement of secondary cooling intensity is required. In turn the cooling circumstances have a direc t eff t on the quality parameters of the cast product, i.e. on the surface and subsurface crack susc eptibility, on the geometrical parameters of the primary structure, and on the position of the colum nar to equiaxed transition (CET). The connection between the cooling intensity and the surface and subsurfac e c ack susceptibility has a great importance in the case of mircoall oyed grades, because microalloying elements change the ductility of the steel as a function of temperature in a v ry drastic manner. The quality of the dendritic structure affects the microsegrega tion besides the mechanical properties at elevated temperature. From both points of view, the f iner primary and secondary dendritic arm spacing is beneficial. The third factor mentioned above is the solidification pattern (columna r or equiaxed), which has its effect mainly on the macrosegregation, i.e. on the distribut ion of the impurity enriched melt. In the continuous casting process the appearance of macrosegregation is inevitable, and in some cases it can result in a very serious centerline segregation. I f mainly columnar dendritic growth takes places, the solute enriched liquid flows towards the centerline , which increases the macrosegregation. The so called bridge formation enhances the m acrosegregation, because the columnar dendrites growing from both sides separate the melt, and re sult in locally a mini-ingot behaviour. In the other case, if equiaxed dendrites are growing, the enr iched melt can remain on the surface of individual dendrites, and can percolate through grain boundaries, a nd less solute enriched liquid will reach the centerline. So the macrosegregation can be de creas d in two manners, decreasing the concentration differences during solidification (i.e . lowering the amount of impurities), or controlling the distribution of the enriched melt. All kinds of effects which enhance the formation of finer dendritic structure are beneficial from t he point of view of macrosegregation. From this aspect the increasing of secondary cooling intensity can be detrimental because of the worsening the conditions of equiaxed growth. Materials Science Forum Online: 2003-01-15 ISSN: 1662-9752, Vols. 414-415, pp 461-470 doi:10.4028/www.scientific.net/MSF.414-415.461

The effect of Fluid Flow on Heat Transfer and Shell Growth in Continuous Casting of Copper

Molten metal is cooled in a continuous casting mould forming initially a thin shell that grows thicker. The main phenomena in the mould are: fluid flow, heat transfer and solidification. A lot of mathematical models have been developed to simulate these phenomenons in continuous casting machines but most of the models developed are not calculating the fluid flow at all. In these models, it is assumed that the strand (solid and liquid) is withdrawn through the machine with a constant velocity field (= casting speed) and the convective heat transfer generated by the fluid flow is taken into account by using an effective thermal conductivity method. Also at the Helsinki University of Technology, these kinds of heat transfer models have been developed (TEMPSIMU for steels and CTEMP3D for coppers). The flow in the mould is three-dimensional and turbulent. Coupled models calculate the fluid flow, heat transfer and solidification simultaneously. The fluid flow is affected by many things: inlet flow rate, design of the inlet nozzle (SEN), immersion depth of the SEN, movement of the solid shell, natural convection, solidification shrinkage, etc. and the fully coupled, turbulent fluid flow and heat transfer models are generally subjected to convergence difficulties and they need a lot of computing time. Due to these reasons, these kinds of models are not so much used in industry so far. In the present study, a commercial FLOW-3D package is used to make coupled simulations of heat transfer, turbulent fluid flow and solidification in a copper continuous casting machine. The effect of thermophysical material data are also studied and presented. The material data are calculated by a model developed at the Helsinki University of Technology, called CASBOA.