Mihály Réger

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

Macrosegregation in CC Slabs

In continuously cast semi-finished products, fluid flow results in macrosegregation patterns. There are two reasons for fluid flow during solidification: density differences in the liquid phase and outer forces or constraints, which mix, suck or squeeze the liquid inside. The calculation method described in this paper can be used to analyze the effects of fluid flow caused by outer constraints and to estimate the expected macrosegregation level.

The Effect of Alloying Elements on the Stability of Centerline Segregation

The paper deals with the stability of centerline inhomogenity of continuously cast slabs and hot rolled products. The centerline segregation is a disadvantageous failure of slabs which can affect the quality properties of the final products. During hot rolling of slabs the centerline segregation pattern will become thin and stretch and it can also be detected in the middle part of heavy plates and coils. It is a common experience that the centerline segregation of heavy plates can not be easily decreased by post heat treatment. The pattern of the centerline segregation was modeled physically by preparing a sandwich structure of steel plates with different levels of carbon and alloying elements. Homogenization experiments were performed and the samples were examined metallographically. Diffusional calculations proved the governing role of carbon activity which is influenced by the distribution of alloying elements.

Macrostructure Prediction of CC Slabs

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 this is the very direct effect of the inner structure’s features (ie. porosity, macrosegregations, geometry of primary dendrites) on the technological characteristic features of the semis during further processing (ie. crack sensitivity, formability, etc.). The paper deals with the possible ways of macrostructure determination on the basis of the results of mathematical modeling of continuous casting process. We pay a special attention to the columnar-equiaxed transition as a function of heat parameters of the casting process and to the macrosegregation formation caused by the motion of solute enriched interdendritic liquid in the mushy zone.

Investigation of Local Corrosion Degradation Developed on a Pipeline System in Service Period

Corrosion degradation was observed in a nuclear power plant spent fuel cooling system. A systematic and comprehensive investigation program was developed which was negatively influenced by the limit of sampling (contaminated material). Corrosion tests, mechanical and microstructural investigations were carried out and also microbiological effect was examined. Major contributors to the degradation were identified.

Mathematical model for the optimal distribution of district heat sources

Different heat sources are weighed by their significance and then, using linear programming algorithm known as knapsack problem, assigned to cover the heat demand of a settlement using district heating.

Determining the priority of heat sources used in district heating

In this paper the measure of priority attributed to the different heat sources produced locally and used in the energy supply of a given settlement using district heating will be shown. The rate of district heating is introduced and its critical value that shows whether the settlement is able to self-supplement its district heat demand or not is determined. Two algorithms - depending on the legal environment - are established for the optimal use of local heat sources, one that deals with cogeneration and one that does not.

Optimizing Steel-Making Secondary Metallurgy Slag at ISD DUNAFERR Zrt

ISD DUNAFERR Co. Ltd., formerly named as Dunai Vasmű (Danube Ironworks), has a history of over half a century. Currently it is the only factory in Hungary that is operated by integrated steel-making technology and has a hot metal production capacity of 1.7 million tons (Figure 1). The metallurgical combine includes coke production, hot metal production, steel-making, hot and cold rolling, profiling and manufacturing of galvanized products too. Steel is produced by 2 BOFs with the capacity of 135 tons each.

Effect of Mushy Zone Permeability on the Formation of Centerline Segregation in Slab Casting

A complex mathematical model characterizing the centerline segregation level in the mid region of continuously cast slabs was developed. The basic heat transfer and solidification model connected to the semi-empirical liquid feeding model (LMI - Liquid Motion Intensity model) gives the possibility to estimate the centerline segregation parameters of slab cast under industrial circumstances. Solid shell deformation changes the volume of the space available for the liquid inside the slab and hereby also changes the conditions of liquid supply.

Effect of Supporting Roll Setting on the Inner Structure of Cc Slabs

Mathematical model was developed to estimate the flow rate and direction and of the expected porosity level in the centre part of a slab. Calculations show that centreline segregation is basically affected, at a given composition and cooling technology by the setting, deformation and eccentricity of the supporting rolls. Bulging of the strand between the supporting rolls can also play role.