Papers by Author: Juergen Schneider

Abstract: The magnetic properties in electrical steels are strongly dependent on the crystallographic texture as well as other microstructural features such as grain size. Both, texture and grain size, are determined by the thermo-mechanical history of the material. This work regards a set of different thermo-mechanical paths applied on two types of non-oriented electrical steels containing 2.4% and 3.0%Si, respectively. The evolution of grain size, microstructure morphology and texture throughout processing were studied in detail by optical microscopy, X-Ray diffraction and Electron BackScatter Diffraction (EBSD). The impact of the texture on the magnetic properties was evaluated. This was done by the calculation of the magnetic anisotropy energy and the A parameter, i.e. a parameter defined in scientific literature that describes the magnetic "quality" of the texture, which can be correlated with the magnetic properties of the materials. Finally, the influence of further laser cutting on the crystallographic texture will be examined as well.

Abstract: Steel containing a high Si-content is mainly used as electrical steel in flux carrying electrical machines. These materials are divided in the categories: grain oriented and non oriented electrical steels, mainly used in transformers and electrical motors, respectively. Their industrial production is normally limited to silicon contents lower than 3.5 m.-%, due to the generation of brittle ordered structures if the Si content is increased beyond this value. The paper reports on microstructure and texture evolution during processing by rolling of electrical steel in the high Si-range. The materials studied are two industrial electrical steels with a silicon content of 2.4 and 3.2 m.-%, their situation was as-received after hot rolling and industrial annealing. The different processing parameters, as rolling temperatures and cooling conditions have a strong influence on the final microstructures and textures. The importance of hot rolling and intermediate annealing processes is enhanced since above 2 m.-% Si these steels do not experience the usual α-γ-α phase transformation, because they present a bcc crystal structure over the entire solidus domain. Consequently, their microstructures and textures are strongly inherited from the earlier processing steps into the final product. The as-received materials were cold rolled with a nominal reduction of 75%. Their microstructures and textures were analysed by EBSD. The results obtained were compared with those of the industrial hot band. The textures were studied by the interpretation of the most important crystallographic fibre textures, extracted from the ODF’s of φ2 = 45o section of the Euler space. Special attention was given to the evolution of the most important magnetic textural components. Although in terms of grain shape, IQ, texture and normalised thickness position or ‘s’-parameter the microstructures obtained before and after cold rolling are totally different, the overall crystallographic textures seem not to differ very much.

Abstract: Exposure of metals and alloys to high temperatures leads to the formation of oxide scales, with a large impact on surface quality. The most important features of the oxide layer are its thickness, composition, structure, adherence and coherence. Temperature, time, gas atmosphere and chemical composition determine the growth of oxide layers. In this paper, the characteristics of the high temperature oxidation properties of Fe-Si alloys are discussed in terms of oxide growth mechanism, kinetics and phase morphology. The oxidation kinetics of different Fe-Si alloy steels in air, its scale structure and composition were investigated over the temperature range 900-1250°C. Oxidation experiments were performed in air, to analyse the oxidation process. Experiments were carried out in an electric furnace at temperatures ranging from 900 to 1250°C, for times between 16 and 7200s. Thus treated specimens were characterised by metallography and their scale thickness was measured by optical microscopy. Scale morphology was studied and scale composition confirmed by EDS (Energy Dispersive Spectroscopy) and EBSD (Electron Backscattered Diffraction) analysis. Results show that high temperature oxidation of Si- alloys presents the classic three layered oxide scale. On the grain boundaries in the scale, iron-silicate was found. Observations show a Sienrichment at the scale –metal interface. This enrichment is present in the form a mixed wustite-iron silicate (fayalite) phase, FeO-Fe2SiO4. A very rapid build-up of oxide occurs when a liquid phase, due to the wustite-fayalite eutectic, is present in the surface of the steel.

Abstract: An oxide scale layer always forms at the strip surface during the hot rolling process. As a consequence, de-scaling and pickling operations must be performed prior or after hot rolling. Many surface defects caused by hot rolling are related to oxidation in the reheating furnace. One of these is the melting of eutectic FeO/Fe2SiO4 during reheating over 1170°C giving as a result red scale defects in Si-added steel. On the other hand, steel strip surface oxidation during hot rolling causes an industrial and environmental problem: secondary oxide is removed after roughing, but tertiary oxide scales already start to form before entering the finishing stands. Their properties affect the final steel surface quality and its response to further processing. Furthermore, the addition of alloying elements has an important impact on scale properties. In particular the alloying of silicon effects the region between scale and substrate. It causes peculiar surface properties inherited from its specific oxidation characteristics. Conventional oxidation experiments in air of silicon steels are a valuable tool to study the influence of Si on steel oxidation. After oxidation in air in the temperature range of 900-1250°C it has been observed that Si enhance markedly scale adhesion, especially above 1177°C (the eutectic temperature of FeO-Fe2SiO4 ) and also at lower temperatures. Special attention has been paid on the investigation of the effects of alloying Si on the high-temperature oxidation of steel, for a better understanding of the behaviour of modern steels during hot rolling.

Abstract: Non oriented electrical steels are soft magnetic materials used in the core of electrical motors. No preferential anisotropy of the electrical texture in the rolling plane is desired. Nowadays these special steels are mainly alloyed with Si, Al and some additives to improve the magnetic properties and to reach a good of formability. For (Si, Al)-concentrations higher than 2 wt.% the α- γ-α phase transformation is suppressed, resulting in a bcc crystalline structure from liquidus to room temperature. These electrical steels, which will be discussed in the paper, exhibit the lowest values of the magnetic losses. Hot rolling of FeSi electrical steels has been found to be one of the fundamental steps in producing these materials with optimum properties. The resulting properties, as well known, are determined by the type of magnetic textures and the structural inhomogeneities. Electron Backscattered Diffraction (EBSD) is a reliable tool for microstructural and texture characterization of different materials. Two compositions of electrical steel are studied by optical microscopy and EBSD, with special attention paid to characterize the grain morphology and its texture through thickness.