Jaromir Heger

The Numerical And Experimental Investigation Of A Concasting Process

Solidification and cooling of a continuously cast steel slab and simultaneously heating of a mould is from the viewpoint of thermokinetics, a very complicated problem of non-stationary heat and mass transfer. This process is described by the Fourier-Kirchhoff equation, in a mould by the Fourier equation. The solving of such a problem is impossible without numerical models of the temperature field, not only of the concasting itself, while it is being processed through the caster, but of the mould as well. A three-dimensional (3D) numerical model of the temperature field of a solidifying concasting has been used. This model is able to simulate the temperature field of a concasting machine (caster) as a whole, or any of its parts. It is also able to solve current thermokinetic problems both globally and in detail. Simultaneously, together with the numerical computation, the experimental research and measuring have to take place. The experimental investigation was focused mainly on the temperature in the tundish, the temperatures of the walls of the mould and the surface temperatures within tertiary cooling zone (measured by means of thermocouples), on the surface temperatures of the slab under the mould (measured by means of pyrometers) and the metallurgical length of the concast slab using radio-isotope methods. The cooling intensity of individual cooling jets had to be conducted on the experimental laboratory device. Each jet had been measured separately on the hot plate-model, on which the hot surface of the slab, which is cooled by a moving jet, can be modeled. The temperatures measured beneath the surface of the modeling plate by means of thermocouples are converted to cooling intensities using an inverse task, which, in turn, are converted to the courses of the heat transfer coefficients using an expanded numerical model. This laboratory facilities is also capable of measuring the effect of radiation, which is dependent not only on the surface temperature but also on the actual quality of the surface. Experimental research and measuring have to take place not only to confront it with the numerical model, but also to make it more accurate throughout the course of the process. The similarity of the results attained from the computed and from the experimentally measured temperature field of the steel slab is very satisfactory.

Industrial Application Of Two Numerical Models In Concasting Technology

This paper deals with the causes of a transversal crack in a steel slab with a 1300x145 mm cross-section by means of two numerical models. Samples were taken from and around the crack in order to analyze the concentration, as well as the chemical heterogeneity y of the constituent elements and impurities. Simultaneously, the concentration of elements at the surface of the crack was measured after the crack was opened. The heterogeneity of elements was analyzed by the JEOL JXA 8600/KEVEX device. The measurement results were processed using mathematical statistics. The chemical heterogeneity of elements in the steel matrix around the crack, and at the crack surface, had been evaluated with the help of heterogeneity y parameters, i.e. the arithmetic mean of concentration, the standard deviation of concentration and the index of heterogeneity of the analyzed elements. The results proved that there was an internal crack initiating immediately below the solidus temperature and consecutively propagating.

A Numerical Model of the Crystallization of Pure Aluminium

The model was originally designed to confirm and enhance the capabilities of experimental research in the crystallization of pure aluminium (99.99% Al), specifically to determine the zone of the occurrence of columnar and equiaxed crystals and the positions of the transition interface. The character of primary crystallization was investigated on a simple cylindrical sample, crystallizing in a cast-iron mould pre-heated to various temperatures. The experimental research comprised the measurement of temperatures using thermocouples, the evaluation of the experimentally acquired temperature gradients G, and the shift rate of the phase transition interface R. The numerical model had been developed to expand the limited experimental capabilities of the evaluation of G and R to every point of the longitudinal section, based on the investigation of the 3D transient temperature field within the system comprising the casting, the mould and ambient. This enabled the prediction of the character of the crystallization in greater detail.Copyright

Two Numerical Models for Optimization of the Foundry Technology of the Ceramics EUCOR

Corundo-baddeleyit material — EUCOR — is a heat- and wear-resistant material even at extreme temperatures. This article introduces a numerical model of solidification and cooling of this material in a non-metallic mould. The model is capable of determining the total solidification time of the casting and also the place of the casting which solidifies last. Furthermore, it is possible to calculate the temperature gradient in any point and time, and also determine the local solidification time and the solidification interval of any point. The local solidification time is one of the input parameters for the cooperating model of chemical heterogeneity. This second model and its application on samples of EUCOR prove that the applied method of measurement of chemical heterogeneity provides detailed quantitative information on the material structure and makes it possible to analyse the solidification process. The analysis of this process entails statistical processing of the results of the measurements of the heterogeneity of the components of EUCOR and performs correlation of individual components during solidification. The crystallisation process seems to be very complicated, where the macro- and microscopic segregations differ significantly. The verification of both numerical models was conducted on a real cast 350 × 200 × 400 mm block.Copyright

Two Numerical Models for Prediction of an Industrial Concasting Process

This paper introduces the application of two three-dimensional (3D) numerical models of the temperature field of a caster. The first model simulates the temperature field of a caster—either as a whole, or any of its parts. Experimental research and data acquisition take place simultaneously with the numerical computation in order to enhance the numerical model and to perfect it in the course of the process. In order to apply the second original numerical model—a model of dendritic segregation of elements—it is necessary to analyze the heterogeneity of samples of the constituent elements and impurities in characteristic places of the solidifying slab. The samples are taken from places, which provide information on the distribution of elements under both standard and extreme conditions for solidification, where the mean solidification (crystallization) rate is known for points between the solidus and liquidus curves. Using this method, it is possible to forecast the occurrence of the critical points of a slab from the viewpoint of its susceptibility to crack and fissure. Verification of the technological impact of optimization, resulting from both models, is conducted on a real industrial caster.Copyright

Calculation of the Temperature Field of the Solidifying Ceramic Material EUCOR

EUCOR, a corundo-badelleyit material, which is not only resistant to wear but also to extremely high temperatures, is seldom discussed in literature. The solidification and cooling of this ceramic material in a non-metallic mould is a very complicated problem of heat and mass transfer with a phase and structure change. Investigation of the temperature field, which can be described by the three-dimensional (3D) Fourier equation, is not possible without the employing of a numerical model of the temperature field of the entire system—comprising the casting, the mould and the surroundings. The temperature field had been investigated on a 350×200×400 mm block casting—the so-called “stone”—with a riser of 400 mm, and using a numerical model with graphical input and output. The computation included the automatic generation of the mesh, and the successive display of the temperature field using iso-zones and iso-lines. The thermophysical properties of the cast, as well as the mould materials, were gathered, and the initial derivation of the boundary conditions was conducted on all boundaries of the system. The initial measurements were conducted using thermocouples in a limited number of points. The paper provides results of the initial computation of the temperature field, which prove that the transfer of heat is solvable, and also, using the numerical model, it is possible to optimise the technology of production of this ceramic material, which enhances its utilisation. The results are complemented with an approximated measurement of the chemical heterogeneity of EUCOR.Copyright

The Influence of Thermophysical Properties on a Numerical Model of Solidification

Solidification and cooling of a (con)casting, with the simultaneous heating of the mold, is a case of transient spatial heat and mass transfer. This paper introduces an original and universal numerical model of solidification, cooling and heating, of a one-to-three-dimensional stationary and transient temperature field in a system comprising the casting, the mold and its surroundings. This model simulates both traditional as well as non-traditional technologies of casting conducted in foundries, metallurgical plants, forging operations, heat-treatment processes, etc. The casting process is influenced not only by the thermophysical properties (i.e. heat conductivity, the specific heat capacity and density in the solid and liquid states) of the metallic and non-metallic materials, but also by the precision with which the numerical simulation is conducted. Determining these properties is often more demanding than the actual calculation of the temperature field of the solidifying object. Since the influence of individual properties should be neither under- nor over-estimated, it is necessary to investigate them via a parametric study. It is also necessary to determine the order of these properties in terms of their importance.© 2002 ASME

Numerical Models of Crystallization and Its Direction for Metal and Ceramic Materials in Technical Application

Structure of metallic and also majority of ceramic alloys is one of the factors, which significantly influence their physical and mechanical properties. Formation of structure is strongly affected by production technology, casting and solidification of these alloys. Solidification is a critical factor in the materials industry, e.g. (Chvorinov, 1954). Solute segregation either on the macroor micro-scale is sometimes the cause of unacceptable products due to poor mechanical properties of the resulting non-equilibrium phases. In the areas of more important solute segregation there occurs weakening of bonds between atoms and mechanical properties of material degrade. Heterogeneity of distribution of components is a function of solubility in solid and liquid phases. During solidification a solute can concentrate in inter-dendritic areas above the value of its maximum solubility in solid phase. Solute diffusion in solid phase is a limiting factor for this process, since diffusion coefficient in solid phase is lower by three up to five orders than in the melt (Smrha, 1983). When analysing solidification of these alloys so far no unified theoretical model was created, which would describe this complex heterogeneous process as a whole. During the last fifty years many approaches with more or less limiting assumptions were developed. Solidification models and simulations have been carried out for both macroscopic and microscopic scales. The most elaborate numerical models can predict micro-segregation with comparatively high precision. The main limiting factor of all existing mathematical micro-segregation models consists in lack of available thermodynamic and kinetic data, especially for systems of higher orders. There is also little experimental data to check the models (Kraft & Chang, 1997).

Pilot Simulation Of The Temperature Field Of AContinuous Casting

Solidification and cooling of a continuously cast steel billet is a very complicated problem of transient heat and mass transfer. The solving of such a problem is impossible without a numerical model of the temperature field, not only of the concasting itself while it is being processed through the caster but of the mold as well. This process is described by the Fourier-Kirchhoff equation. An original 3D numerical off-line model of the temperature field of a caster has been developed. It has graphical input and output automatic generation of the net and plotting of temperature fields in the form of color iso-therms and iso-zones, and temperature-time curves for any point of the system being investigated. This numerical model is capable of simulating the temperature field of a caster as a whole, or any of its parts. Experimental research and data acquisition have to be conducted simultaneously with the numerical computation—not only to confront it with the actual numerical model, but also to make it more accurate throughout the process. After computation, it is possible to obtain the temperatures at each node of the network, and at each time of the process. The utilization of the numerical model of solidification and cooling of a concasting plays an indispensable role in practice. The potential change of technology—on the basis of computation—is constantly guided by the effort to optimize, i.e. to maximize the quality of the process.

Prediction Of Cracks Of The ContinuouslyCast Carbon-steel Slab

This paper deals with surface morphology, the mechanism of origination and causes of cross cracking of a concast low alloy manganese steel slab. The cross cracking was identified in a steel slab with a sectional size of 145x1300 mm and the length of the asymmetrical cracking was approximately 600 mm. The light microscopy and the scanning electron microscopy have been applied for determination of the metallographic structure of steel and for the study of micro-morphology and the trajectory of cracking. The chemical microheterogeneity of the steel matrix and the surface of cracking have been estimated by means of an X-ray micro-analyser JEOL JXA 8600/KEVEX. The analyses of Al, Si, P, Ti, Cr, Mn and Fe on the metallograpic samples of the matrix of steel, of the neighbourhood of cracking and of the surface of cracking have been realized. It has been found that the cross cracking is characterized by high macro-heterogeneity of manganese, carbon and sulphur. The causes of cross cracking have been explained by means of a thermokinetic calculation of a slab transient temperature field and by mean of an application of the theory of physical similarity and dimensionless criteria. It has been confirmed that two solidification cones are formed asymmetrically in the course of slab solidification and it is probable that the asymmetrically passing crack initiated on one of these two apexes. Computer Methods and Experimental Measurements for Surface Effects and Contact Mechanics VII 111