Papers by Author: Zsolt Csepeli

Abstract: 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.

Abstract: Along the metallurgical length at continuous casting, the temperature within the strand ranges from above melt temperature (>1500) to about 900 oC. In this range, mechanical properties drastically drop at above the zero deformation temperature (ZDT) and the zero strength temperature (ZST), therefore crack susceptibility increases. These temperatures depend on (local) chemical and microstructural properties. We investigated the change in ZDT and ZST for ST52-3 steel grade by taking samples from the as-cast strand from positions of the columnar grains and from the centreline segregated zone. We found more than 100 oC drop in ZDT due to segregations in the centreline compared to the columnar zone within the strand.

Abstract: In spite of speedy development of technical tools for examination, the importance of macro structural investigation has not decreased; on the contrary, these methods are in the state of their revival. One of evident reasons for it is the fact, that the results of macro structural examinations and the conclusions drawn from them are in the closest relation with the parameters of technology, reflect their changes, and the deviations from the specified values. It is expedient from time to time to check the methods proved earlier suitable for macro structural investigation, possibly to develop and implement new methods. We have to take in account that the methods now widely applied were developed when the technology, composition, type and quantity of pollutants were still basically different from those of the present. We mention as an example that in the fundamental work of George F. Vander Voort published in 1984 the examinations of macrostructure of continuously cast semi products occupied hardly one to two pages, while nowadays they produce about 90% of the annual 1.1 billion tonne steel quantity in continuous casting plants, thus the decisive proportion of continuously cast semi products examination can hardly be questioned. Taking in consideration the above points of view, in the frame of a consortium work we examined the macrostructure of samples taken from bloom slabs cast on vertical continuous casting machine from nine consciously chosen charges, with methods known from the literature or personal information.

Abstract: The final microstructure of DP and TRIP assisted steels can evolve after hot working (hot rolling) or during post heat treatment process. In the formation of the final structure a number of different technological parameters have important role, e.g. finishing temperature of rolling, cooling rates, temperature of intercritical annealing, etc. As a result of the individual factors and their combinations a lot of production technology routes are feasible. The effect of the different combinations of these technological parameters on the microstructure can be mapped by a special Jominy end-quench test (so called intercritical Jominy end-quench test) described in this paper. Unlike the traditional Jominy test, in this case there is a partial austenizing between A1 and A3 temperatures which results in a given amount of ferrite in the microstructure before quenching. The amount of ferrite depends on the temperature. In some cases the quenching process was interrupted for a given period of time in order to model the cooling process on the run-out table. During cooling each point of the Jominy specimen has a different cooling rate, so the effect of cooling rate on the microstructure can be evaluated along the length of the specimen.

Abstract: The macrosegregation is one of the most recurrent phenomena in continuous steel casting of slabs. It often impedes the further processing and has detrimental effect on the quality of rolled strip. The macrosegregation can be explained by the interpretation of unit steps of solidification in continuous casting slabs as well as on the basis of the interaction of the inclusions at the interface of solid and liquid phases. Development of solid shell and interaction between growing dendrites and MnS segregation has been presented. Only the pushing effect could be the real cause of MnS segregation among the three possible interactive mechanisms. The sulphur print could give misleading information on sulphur segregation.

Abstract: The different solidification processes occuring in steels can be well studied by using the microstructure investigation. The microstructure can be observed after the sample preparation by using a special type of Oberhoffer etching. The changes in the cooling rate can be described by determining the secondary dendrite arm spacing. A relative cooling rate can be calculated by applying the relationship between the secondary dendrite arm spacing and the cooling rate which characterises the changes of cooling conditions in the ingot during the solidification..

Abstract: 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.

Abstract: The continuous development of steel products requires the detailed investigation of relationships between the production technology, microstructure, and mechanical properties. This paper describes the detailed characterisation of normalising rolled and of thermomechanically rolled steel wide strip microstructures and presents the relationship between their microstructures and their mechanical properties. The morphology of the microstructures were characterised by using an image analyser. The most important measured stereological parameters were the area, perimeter, length and breadth of the ferrite and pearlite cross-sections.