Papers by Author: Ibolya Kardos

Abstract: The aim of our investigation was to study the effect of deformation on the start temperature of the austenite-ferrite transformation. The deformation was carried out during cooling before the beginning of the transformation. The transformation start temperature was determined by dilatometric measurement with a Gleeble 3800 thermomechanical simulator. A novel method was developed to study the deformation induced ferrite transformation (DIFT) effect and successful experiments were carried out on S460MC grade steel specimens to determine the effect of the deformation stored energy on the transformation temperature of the austenite. Evaluating the dilatograms a strong relationship was observed between the temperature of the deformation and the austenite transformation start and finish temperatures. Lower deformation temperature resulted finer microstructure after austenite transformation.

Abstract: A lot of phases can be present in the structure of cast iron depending on the quality and the heat treatment processes. By using the color etching, the different phases – e.g. the carbides – can exactly be identified and their quantity can be measured. The carbide of iron (cementite) or other types of carbides existing in the structure have a significant influence on the properties of cast product therefore strict prescriptions regulate their quantity mainly in case of making cast products for the motor-car industry. In the course of our experiments performed by using the color etching, we proved that carbides can be detected and their quantity can be determined by using the digital image processing method by means of the different reagents. Other different characteristics of the structure can become visible by means of the reagents used during the color etching. During the investigations, it was experienced that some of the ferrite grains are etched in different color. This is probably connected to the orientation of grains as the thickness of interference-film is different on the anode phases having different orientation and therefore they color in different tones.

Abstract: In the course of our experiments we have developed an etching process by means of which the austenite grains formed subsequent to roughing can be made visible in the case of Grade X80Mo0 and X80Mo2 microalloyed steels. The definition of the size and elongation of the grains enables the production process to be optimized and the microstructure features associated with the desired mechanical properties attained thereby.

Abstract: 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 the slabs the centerline segregation pattern will become thin and stretched 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.

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: 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.

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: Color etching is a widely used technique for visualizing different phases in metallic materials. Its advantage to the traditional etching techniques is that it gives additional information within one phase, namely, the color shade of a given phase can change in a certain range. This paper demonstrates that, due to the physics of the color etching, the shade of a phase also depends on the crystallographic orientation of the investigated grain. As a test material, spheroidal graphite cast iron was used, and individual grain orientation was identified by automated electron back scattering diffraction (EBSD). Results showed that there is a strong correlation between grain orientation and the shades obtained by color etching.

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.