Dieter Janke

Oxygen probes based on calcia-doped hafnia or calcium zirconate for use in metallic melts

Up to the present, fully or partially stabilized zirconia has been used as a solid electrolyte material in probes for the determination of oxygen in metallic melts. In the present study, the ionic conduction behavior of HfO2 (CaO) solid solutions and the compound calcium zirconate CaZrO2 have been investigated. Both polarization experiments and EMF measurements on oxygen concentration cells point out that these two highly refractory oxide materials are also most suitable solid electrolytes. Their use is particularly recommended for oxygen probe measurements in deoxidized steel melts where extremely high chemical stability and low partial electronic conductivity of the solid electrolyte is required. In the paper, properties such as crystal structure, free energy of formation, thermal expansivity, ionic and total electrical conductivity are summarized and compared for fully and partially stabilized ZrO2, calcium zirconate CaZrO3, HfO2 (CaO), and ThO2 (Y2O3) solid solutions.

Numerical simulation of fluid flow and disperse phase behaviour in continuous casting tundishes

A new Euler-Lagrange model of steel melt flows including dispersed secondary phases in continuous-casting tundishes is presented. The system of fundamental equations for the flow field is closed by different turbulence models. The calculation of tundish flows and a comparison with experimental results show that the flow fields of the steel melt and the behaviour of the dispersed secondary phases in the melt are described adequately by the model. However, the quality of the results for the dispersed phase depends essentially on the choice of turbulence model. Criteria for deciding which turbulence model is best suited for a specific flow situation are suggested. These criteria are deduced from investigations of different tundish flows.

Recent development of solid ionic sensors to control iron and steel bath composition

Abstract A series of attempts were made to measure the activities of other components than oxygen in molten iron and steel using oxygen concentration cells in conjunction with appropriate auxiliary electrode. A survey of such solid state sensors will be provided.

Formation of oxide inclusions during deoxidation and solidification of high purity steels

Abstract The deoxidation of low alloy steels has been theoretically and experimentally investigated. A calculation method for the description of oxide precipitation in liquid and solidifying steel is presented. Electromotive force sensors were used for continuous control of oxygen activity after aluminium deoxidation in liquid and solidifying steel under laboratory conditions. Examination of the deoxidation products was conducted by means of optical and scanning electron microscopy techniques.

Secondary Materials in Steel Production and Recycling

With regard to the utilisation of the by-products of the steelmaking process (slag, dust, sludge, mill scale) the German steel industry has been in a leading position for many years. Slags, especially those from the blast furnace process, are utilised as materials for the construction industry (road building, cement production, fertilisers). Dusts, sludges and mill scale are either re-used directly in the steelmaking process or treated in special equipment to recover valuable metals (iron, zinc). Steel is undoubtedly one indispensable material of our modern technologydriven society. At present, steel encompasses a class of over 2500 different steel grades, hence there is a wide variety of properties leading to an even wider range of uses. There are countless possibilities of combinations with regard to micro and macro structures, alloying elements, heat and mechanical treatment procedures. Steels represent materials with a high potential for innovation, although they have been known to mankind for several thousand years. Another peculiarity of steels is their high rate of recycling, some aspects of which will be discussed in this paper. There exist two different technological routes for steel production. One is based primarily on the reduction and smelting of iron ore, this is the so called blast furnace – oxygen converter route (BF-BOF). The other one is based on steel scrap smelting and employs electric arc furnaces (EAF). Steel scrap is used as a source of iron in both processes but the scrap proportions in the charge are quite different. While in electric steelmaking the iron-bearing charge consists only of scrap, the oxygen converter uses up to 25% scrap, the rest being hot metal from the blast furnace. It is evident that electric steelmaking plays a more important role in the recycling of steel than the BF-BOF process. That is why this paper concentrates on the material cycle steel-scrap-steel and discusses the complex relationships between

A new immersion sensor for the rapid electrochemical determination of dissolved oxygen in metallic melts

Abstract A newly developed solid electrolyte “needle sensor” is described bearing 100–200μm thick coatings of a metal-metal-oxide reference and a ZrO 2 - or ThO 2 -based electrolyte material. The sensor represents a suitable and low-cost device for the rapid measurement of oxygen activities in liquid metals, especially in steel melts.

Modelling of temperature and stress profiles in oxygen sensors upon immersion into steel baths

Abstract Using a previously established non-stationary thermal model it is possible to calculate the distribution of temperature and thermal stresses in emf sensors based on stabilised ZrO2 which are frequently applied for the measurement of oxygen activity in steel melts. The model may provide a basis for optimised construction of these sensors. The preheating behaviour is in the first step characterised for the sensor itself and in the second step for the complete one-reading measuring device.

Electrochemical Sensing of Carbon, Oxygen and Silicon in Iron Melts

Activities of carbon and oxygen in Fe-O-C-Xn melts (Xn = Cr, Mn, Si) and of silicon in Fe-C-Si melts were determined in-situ by emf measurements. Electrochemical probes were developed and tested, based on ZrO2 and ThO2 solid electrolytes for C and O sensing, and based on molten silicate electrolytes for Si sensing. The probes can be immediately immersed into the metal bath at a response time of 20 to 30 s and 30 to 60 s, respectively. Accurate C and O activities were obtained from the emf measurements with ZrO2 and ThO2 electrolyte probes up to carbon saturation when a controlled pco is maintained. A reproducible emf vs silicon activity relationship resulted from measurements with a silicate electrolyte probe in carbon-saturated Fe-C-Si melts up to 1.8 wt% Si. The present results are based on laboratory-scale experiments. Further efforts are thought to be promising to develop these probes to an industrial standard.

Nachweis der Ionenleitfähigkeit

Fur den Nachweis der Ionenleitfahigkeit fester Oxide und Mischoxide bei hohen Temperaturen eignen sich zwei Mesverfahren: a) Die Messung der elektromotorischen Krafte an einer Sauerstoffkonzentrationskette; b) die Messung der elektrischen Leitfahigkeit in Abhangigkeit vom Sauerstoffpartialdruck der Atmosphare.

Zielsetzung der Arbeit

Die vorherrschende Ionenleitfahigkeit der untersuchten ZrO2-Y2O3-, ZrO2-CaO- und ThO2-Y2O3-Festelektrolyte ist bei mittleren Anwendungstemperaturen zwischen 800 und 1200oC in weiten Grenzen des Sauerstoffdrucks sowohl durch EMK-Messungen als auch durch Aufnahme von Leitfahigkeit -Sauerstoffdruck-Isothermen nachgewiesen.