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Microstructure evolution in deformed and recrystallized electrical steel
Dissertation   Open access

Microstructure evolution in deformed and recrystallized electrical steel

Dejan Stojakovic
Doctor of Philosophy (Ph.D.), Drexel University
Mar 2008
DOI:
https://doi.org/10.17918/etd-2728
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Abstract

Steel, Electrical Materials--Texture Materials Science
A processing route has been developed for recovering the desired lambda fiber in ironsilicon electrical steel needed for superior magnetic properties in electric-motor application. The lambda fiber texture is available in directionally solidified iron-silicon steel with the <001> columnar grains but was lost after heavy rolling and recrystallization required for motor laminations. Two steps of light rolling each followed by recrystallization were found to largely restore the desired fiber texture. This strengthening of the <001> fiber texture had been predicted on the basis of the strain induced boundary migration mechanism during recrystallization of lightly rolled steel from existing grains of near the ideal orientation, due to postulated low stored energies. Taylor and finite element models supported the idea of the low stored energy of the lambda fiber grains. A novel methodology has been developed for converting the nanoindentation loaddisplacement data into indentation stress-strain curves and extracting the elastic and postelastic behavior. Extracted variations of effective indentation modulus with orientation were in excellent agreement with previously developed model. Furthermore, an intrinsic orientation dependence of indentation yield strength was extracted in a strain-free material. Developed nanoindentation methodology was successfully used for characterization of microstructure evolution in terms of stored energy variation with orientation during plane strain compression. Variations in stored energy at the grain-scale level were extracted from an increment in indentation yield due to increase in dislocation density. It was found that nanoindentation yield strength is about 2 times the yield strength of homogeneous compression. Moreover, higher indentation yield strength was observed in regions that have rotated during deformation to non-lambda orientations with higher Taylor factors. Experimental results have supported idea of correlation between the Taylor factor and stored energy that was used in multistage processing for successful recovery of lambda texture. Hypothesis for observed much higher strain hardening in nanoindentation than in homogeneous plane strain compression is that the rate of generation of new dislocations is dependent on the dislocation density alone while the rate of annihilation of dislocation is strongly dependent on both dislocation density and the type of dislocations being generated which can be influenced by deformation mode.

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