Juergen Schneider

Thermomechanical processing of high Si-steel (up to 6.3% Si)

Good properties for magnetostriction and hysteresis losses are obtained in steel with 6.5% Si, making high-Si steels interesting in electrical applications. A research program is described to evaluate the possibility to produce high-silicon steel up to 6.5% Si using an appropriate hot rolling program followed by cold rolling. A final thickness between 0.50 and 0.40 mm should be obtained. Thermomechanical processing of high silicon steel (up to 6.3% Si) by hot rolling appears to be possible whenever special conditions of temperature and rolling are maintained. The texture and microstructure of the materials are described. Magnetic properties of some annealed cold rolled samples were also measured.

Evolution and Interaction of the Microstructure and Texture at the Different Processing Steps for Ferritic Nonoriented Electrical Steels

The fabrication process of nonoriented electrical steels comprises casting, hot rolling, cold rolling, and final annealing. The development of new technologies for the fabrication of hot band offers new possibilities. In this paper, we will shortly describe the similarities and differences with respect to the evolution of microstructure (grain structure) and texture along the conventional processing route and thin strip casting. We will point out the most relevant features at the different processing steps, which are important for the optimum texture and microstructure of the finally processed material. Thereby, we will regard ferritic FeSi steels, where no homogenization of the microstructure appears due to the austenite-ferrite phase transformation.

Hot Rolling of FeSi Steels – Effects by Hot Rolling in the Two Phase Region

Finish rolling in the two-phase region may offer new possibilities for improving the properties of the FeSi steels with phase transformation. Therefore a deeper understanding of the effects by hot rolling in the two phase region or mixed rolling: multistep hot rolling in the two phase region and in the ferrite region on the evolution of the microstructure is desirable. In this paper we will present the results of our experimental studies on the effect of hot rolling in the intercritical state on the hardening and softening in the ferrite state. It will be demonstrated that depending on the process conditions at hot rolling the austenite ferrite transformation affects the stress strain behaviour in the ferrite state at multistep hot rolling remarkable.

Influence of cubic texture intensity of hot rolled ferritic non-oriented electrical steels on the microstructure and texture in the final processed material

The magnetic properties of non-oriented electrical steels are determined by the microstructure and texture of the material. Besides optimum grain size (microstructure) for low values of specific magnetic losses, a high intensity of θ-fibre texture and low intensity of γ-fibre and α-fibre texture is desirable. Each of the processing steps influences the intensity of the θ-fibre in the final processed material. In this paper the interplay of the various processing steps on the intensity of the θ-fibre is regarded for ferritic Iron-Silicon steels with 2.4 wt.% Si and 3.0 wt.% Si.

Effect of Thermal Treatment after Hot Rolling on the Microstructure of Fe-2.4wt.%Si

The grain structure and intensities of the relevant magnetic texture of the hot band affect remarkable the resulting microstructure and texture in the cold rolled and annealed nonoriented electrical steel grades based on ferritic Fe-Si alloys. The coiling or annealing after finishing the hot rolling process is an important factor, which gives rise to different grain structure across the thickness. We describe in this paper the effect of annealing in the temperature range of 750 °C up to 850°C by a confocal laser scanning microscopy technology.

Microstructure Characterization by EBSD of Hot Rolled High-Silicon Steel

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.

Effects by the microstructure after hot and cold rolling on the texture and grain size after final annealing of ferritic non-oriented FeSi electrical steel

The magnetic properties of fully processed non-oriented FeSi electrical steel are characterized by their magnetization behavior and specific magnetic losses. The magnetic properties are determined by the texture and microstructure. Less gamma fiber intensity and a high intensity of preferable texture components, especially cube fiber texture, are desirable to obtain an excellent magnetizing behavior. Furthermore, large grain sizes are necessary to reach low values of the specific magnetic losses. The fabrication route of the fully processed non-oriented electrical steels comprises a heavy cold rolling of the hot rolled material before final annealing. To fulfill the requirements on large grain size for low loss materials, grain growth, which appears after complete recrystallization, plays an important role. In this paper we will analyze the influence of different microstructures of the hot strip and the resulting microstructure after cold rolling on the appearance of recrystallization and grain growth aft...

Frequency Dependence of Magnetization Behavior for FeSi Materials With Different Thickness

Electrical steels with lower thickness become more and more important to realize electrical machines with high nominal frequency due to high number of the poles and to provide higher power at a given volume of the electrical machine. For application at higher frequencies the dynamic magnetization processes have to be analyzed. In the paper the experimental data for frequency dependence of the magnetization curves, the coercive force, the permeability, and of the magnetic losses for nonoriented electrical steels with thicknesses in the range of 0.10 mm up to 0.50 mm will be reported. For thin electrical steels a different character of the dynamic magnetization behavior at increasing frequency of the applied external field compared to thicker materials is observed. This is attributed to differences in the number of moving walls.

Characterisation by EBSD of Cold Rolled High-Silicon Steel

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.

Crystallographic texture evolution during processing of hot dipped and diffusion annealed high silicon electrical steel

The texture of high Si and Al electrical steel produced by hot dipping and diffusion annealing can be significantly modified by the thermomechanical processing. The diffusion of Si and Al from the coating affects the final texture depending on the Si content of the substrate. In low Si substrates a phase transformation gamma-alpha is possible and two different textures are observed. Crystallographic texture is an important characteristic of electrical steel. For the production route described here, a modification of the thermomechanical processing has an important influence on texture development. Further investigation shows how far can be gone in order to reduce the magnetic losses through an advanced texture control.