Thermomechanical Processing for Recovery of Desired $$ {\left\langle {001} \right\rangle } $$ Fiber Texture in Electric
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IRON-SILICON steel is known for its excellent soft magnetic properties achieved by a high solute level of silicon. High silicon content contributes to high electrical resistivity and low magnetic losses,[1] which makes this alloy extremely attractive for electrical devices such as transformers, motors, and generators. The development of grain-oriented iron-silicon steel has been a great scientific and technological achievement in controlling the texture and improving the magnetic properties of soft magnetic alloys. For transformer sheet application, the Goss f011gh100i texture[2] provides superior magnetic properties, because the h100i direction is parallel to the rolling direction (RD) and transformers are designed in such a way that the magnetic field travels in a closed loop along the RD, i.e., h100i direction, which is the easy direction of magnetization.[3,4] However, in AC motors and generators, where the magnetic field constantly rotates in a plane,[5] the Goss texture is not adequate. The texture that should give the best magnetic properties is k fiber, where the h001i crystal direction is normal to the sheet, while the other two directions, h100i and h010i, are distributed uniformly in the plane of sheet (Figure 1). Goss textured sheet has been the industry standard for many years, while sheets with the k fiber have never been produced commercially in the bcc iron-silicon. DEJAN STOJAKOVIC, Student, and ROGER D. DOHERTY and SURYA R. KALIDINDI, Professors, are with the Department of Materials Science and Engineering, Drexel University, Philadelphia, PA 19104. FERNANDO J.G. LANDGRAF, Professor, is with the Department of Metallurgical and Materials Engineering, University of Sao Paulo, Sao Paulo, 05508-900, Brazil. Contact e-mail: [email protected] Manuscript submitted November 30, 2007. Article published online April 26, 2008 1738—VOLUME 39A, JULY 2008
The k fiber can be produced by directional solidification (DS),[6] where during solidification the direction of dendritic growth in cubic metals (fcc as well as bcc) is h100i and the desired texture is conventionally produced in regions of columnar grain growth.[7] Very low magnetic losses were reported in the directionally solidified electrical steel with a columnar grain structure near the ideal k fiber texture.[8] Unfortunately, in order to obtain sheets with optimal thickness and grain size for electric motors, additional heavy rolling and recrystallization are required. After heavy rolling and recrystallization, the preferred k fiber from the as-cast state was largely destroyed and the magnetic properties deteriorated.[9] Following the discussion between two of the co-authors in 2002 on the loss of the preferred k fiber texture, an additional processing step was proposed to try to restore the previously lost k fiber texture. The additional step was a light (10 pct) rolling reduction followed by recrystallization of the previously heavily rolled and recrystallized DS steel. The suggestion was successful: the additional step recovered much of the lost k fiber texture.
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