Mechanism of Grain Growth During Annealing of Si-Al Electrical Steel Strips Deformed in Tension
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Mechanism of Grain Growth During Annealing of Si-Al Electrical Steel Strips Deformed in Tension J. Salinas B.1, A. Salinas R.2 Centro de Investigación y Estúdios Avanzados del IPN. P. O. Box 663, Saltillo Coahuila, México 25000. E-mails: [email protected], [email protected] ABSTRACT The mechanism of recrystallization as a result of annealing during 600-7200 seconds at 700 °C of a Si-Al, low C electrical steel strip is investigated in samples deformed in tension. The evolution of grain size during annealing is evaluated by optical microscopy and electron backscatter diffraction in the scanning electron microscope. It is found that grain growth starts after an incubation time of 600 s with no apparent evidence of primary recrystallization. After that, the grain size-time relationship exhibits two different stages. Initially, the grain size increases linearly with time up to about 3600 s. During this time, some selected grains grow until they consume the deformed microstructure. In the second stage, the rate of growth decreases significantly and a final grain size of about 150 m is reached after 7200 seconds of annealing. Grain orientation spread maps obtained from EBSD data of deformed and partially recrystallized samples during the stage of linear growth reveals that the growing grains exhibit lower misorientation and therefore smaller stored energy than the non-recrystallized matrix grains. Analysis of image quality maps reveal that the IQ values for {100} orientations are higher than those observed for {111} orientations thus suggesting that the {100} orientations grow at the expense of {111} orientations by a mechanism of strain-induced boundary migration. INTRODUCTION Non-oriented electrical steels are used in motors and transformer cores due their low energy losses [1]. Their magnetic properties, such as magnetization curves, permeability, coercive force and specific magnetic energy losses are all related to their microstructure [2, 3] which must be strictly controlled during fabrication. Electrical steel strips are processed like most cold rolled steels: hot rolling, cold rolling, box annealing and temper rolling [4]. In the case of semi-processed grain-non oriented electrical steel strips, the final processing stage at the steelmaking plant is a small deformation by rolling known as “skin-pass” or “temper rolling”. The final magnetic properties are then developed during an annealing treatment conducted after punching the laminations at the motor/transformer fabrication plant. The temper rolling strain is of paramount importance in manufacturing this type of steel strips because the final grain size and, therefore, the resulting magnetic properties, depend on it. In the present study the mechanisms of recrystallization and grain growth during annealing of samples obtained from a low-C, Si-Al electrical steel strip deformed to small tensile strains is investigated using orientation imaging microscopy. Image quality and intragranular misorientation maps are analyzed to qualitatively estim
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