Characterizing Grain-Oriented Silicon Steel Sheet Using Automated High-Resolution Laue X-ray Diffraction
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Grain-oriented (GO) silicon steel, also denoted as grain-oriented electrical steel, is arguably the most highly refined of all steel products. It is employed in the cores of power transformers where high magnetic permeability and low power losses are essential for long-term efficiency and minimum environmental loading. This material containing ~3.25 pct Si consists of thin sheets, typically 0.2 to 0.3 mm in thickness, having a glassy insulating coating that is applied in a final stage of the process. Its processing route, which is complex, results in a remarkably sharp so-called Goss texture, (110)[001],[1] and large grains with average sizes from about 5 to 30 mm penetrating the full thickness of the sheet. Perfection of the texture, and in particular the alignment of the directions with the sheet RD, is essential for optimum soft magnetic quality, since these are the directions of easy magnetization. However,
PETER LYNCH and MATTHEW BARNETT are with the Institute for Frontier Materials, Deakin University, Geelong VIC 3217, Australia. ANDREW STEVENSON is with CSIRO Manufacturing, Private Bag 10, Clayton South, VIC 3169, Australia. BEVIS HUTCHINSON is with SwereaKIMAB, Box 7047, SE-16407 Kista, Sweden. Contact e-mail: [email protected] Manuscript submitted February 22, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
quantification of the textures is difficult as the huge grain size precludes use of standard X-ray goniometers. One alternative is to employ neutron diffraction,[2] which can penetrate several centimeters, giving a volumetric measure of the texture, although this is not feasible for routine purposes. Grain by grain assessment of the texture is possible, but traditional Laue diffraction is tedious for the large numbers of individual measurements that are required for a quantitative texture description. An alternative method using electron backscattered diffraction (EBSD) for individual grain orientations[3] is also time consuming and necessitates careful alignment of stacked samples since the total texture spread to be measured is only a few degrees. Precipitates and dislocations also impinge on the magnetic quality. These can be seen using transmission electron microscopy and electron channelling contrast microscopy[4] but are not convenient for characterizing such extremely coarse grain structures. Sub-boundaries have been reported to be present in the large grains during secondary recrystallization at high-temperature anneal using synchrotron X-ray topography[5] and by high-resolution synchrotron X-ray Laue scanning with a fine beam size of 4 lm.[6] According to some views, these boundaries play a special role in the growth process and selection of the specific Goss orientation.[7,8] However, synchrotron-based instruments do not lend themselves to routine quality examinations of commercial products. There is, accordingly, a need for a method that can scan both long-range and local orientation spreads with good accuracy and precision under normal laboratory conditions. An instrument with these capabilit
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