Ludger Lahn

Effects of topology on abnormal grain growth in silicon steel

This work addresses the role of grain topology on abnormal grain growth in silicon steel. The question was investigated whether the abnormal grain growth of Goss grains during secondary recrystallization can be interpreted in terms of an initial size advantage that these grains inherit from rolling and primary recrystallization. For this purpose the correlation between crystallographic orientation, size and number of next neighbors of large grains in the subsurface layer of a primary recrystallized silicon steel sheet was investigated. It was found that most of the large grains have an orientation on the h-fiber ( 001 axis parallel to the rolling direction) but are not particularly close to the Goss orientation. Also, no tendency of grains to be larger the closer they are to the Goss orientation was visible. Rather it was found that the scatter of the angular deviation to the Goss orientation is similar over a large range of grain sizes and this was found to be true too if the number of next neighbors of a grain rather than its grain size was checked. One single grain, however, was found that was close to the Goss orientation and had a high number of next neighbors and might therefore act as a nucleus for secondary recrystallization. Nevertheless, grains with a similarly high number of neighbors and a large deviation to the Goss orientation were found, too. Thus, a topological reason for the Goss texture evolution could not yet be proved. However, it might be that the extreme rareness of Goss nuclei (1 out of 10 6 grains) has prevented, up to now, from observing a true nucleus.

Investigation of the primary recrystallization microstructure of cold rolled and annealed Fe 3% Si single crystals with Goss orientation

A silicon steel single crystal with {110}<001> Goss orientation was cold rolled up to 89 % thickness reduction and subsequently annealed. The evolution of the macroscopic cold rolling texture was investigated by x-ray diffraction. Local orientation relationships and the microstructure around and within Goss grains of deformed and annealed samples were analysed using the electron backscatter diffraction (EBSD) technique. During cold rolling a texture consisting of two strong {111}<112> components and a minor {110}<001> Goss component develops. After primary recrystallisation the texture is characterized by a strong Goss component. Goss-oriented grains that remain after high deformation are considered to be the origin for the primary recrystallisation texture.

Microstructure–magnetic property relations in grain-oriented electrical steels: quantitative analysis of the sharpness of the Goss orientation

We have investigated microstructure–magnetic property relations in several high-permeability grain-oriented steels by large-area electron backscatter diffraction (EBSD) mapping. The evaluation of the Goss sharpness determined from orientation distribution functions provides a more quantitative and detailed texture analysis than conventional pole figure estimates. Accordingly, it results in a more quantitative treatment of texture–magnetic property relations. The analysis of crystal-orientation distribution by a statistical binning method provides further insights into the occurrence of secondary texture components. Specifically, we have found the formation of two weak secondary components rotated about 4°–6° and 8°–10° from the texture center, respectively. These texture components have a strong influence on the magnetic polarization but a small influence on the core loss. We explain this effect in terms of the magneto-crystalline anisotropy energy of a cubic crystal and magnetic domain–microstructure feature interactions.

Survival of Goss Grains during Cold Rolling of a Silicon Steel Single Crystal

A silicon steel single crystal with initial Goss orientation, i.e. the {110}<001> orientation, was cold rolled up to 89 % thickness reduction. Most of the crystal volume rotates into the two symmetrical equivalent {111}<112> orientations. However, a weak Goss component is still present after high strain, although the Goss orientation is mechanically instable under plane strain loading. Two types of Goss-oriented crystal volumes are found in the highly deformed material. We suggest that their origin is different. The Goss-oriented regions that are observed within shear bands form during the cold rolling process. In contrast, those Goss-oriented crystal volumes that are found inside of microbands survive the cold rolling.