Effects of grain boundary characters for secondary recrystallisation in grain oriented silicon steel

The grain growth due to the orientation selectivity during the secondary recrystallisation has been intensively investigated for grain oriented silicon steels, however there still remain several points where common agreement is not obtained yet. Therefore the present paper reviews the arguments and examines the effect of grain boundary characters on this type of abnormal grain growth. So far, we have reported an important role of coherent boundaries, namely, the elucidated selective growth infers significant correlation of Sigma 9 type CSL orientation relationship between the growing grains and the expended grains in HI-B (c) material, and that of Sigma 5 type in CGO material. During the grain boundary migration, second phase fine particles spreading in these materials provide constant Zener drug effect, therefore the atomic coherency of Sigma 9 grain boundaries was implied to receive weaker pinning force than those of other general boundaries. Regarding the low mobility of coherent grain boundaries, the preferential growth of {110} < 001 > orientation in CGO material which is mostly surrounded by low angle grain boundaries suggests that the strict distinction between the intrinsic mobility effects for the migration and the tension effect which provides extrinsic driving force to interpret the "orientation pinning effect". Among the models suggested for the origin of the abnormal grain growth, a surface tracking computer simulation of grain growth was made as a trial and it was concluded that the weak pinning is by itself able to generate abnormal grain growth accompanied by the ascertained orientation selection. On the other hand, as the orientation relationships between the adjacent grains are not simply related to the specific characters of grain boundaries, the atomic structures are considered to play an important role for the actual occurrence of the orientation relationships. In the presentation, we will review our recent HRTEM characterization performed for secondary recrystallisation phenomena from this point of view.