Secondary recrystallization mechanism of grain oriented electrical steels

Key words:  grain oriented electrical steel  orientation evolution  Goss grains  abnormal grain growth

Abstract:

In response to the various doubts currently existing in theory, the process of orientation evolution in the production of grain oriented electrical steel has been comprehensively reviewed. Ultimately, the mechanism of secondary recrystallization was fully elucidated. Only a small fraction of those Goss grains that formed in the surface layer of the hot band during hot rolling under large pass reductions can survive the heavy cold rolling and are further enhanced through subsequent recrystallization annealing. The nucleation process based on polygonization results in recrystallized Goss grains that do not have the size advantage over non-Goss grains. The molar volume expansion effect that occurs during the precipitation of inhibitors such as MnS and AlN in ferrite forces the precipitated particles to maintain a dispersed distribution and hinders the growth behavior of all grains. However, the molar volume expansion effect of the steel sheet near the free surface area will be significantly weakened, resulting in an obvious decrease in particle density due to the opportunity for coarsening of the inhibitor particles in the area. As a result, the surface grains exhibit a growth advantage over the internal grains. Especially at high temperatures, the strong elastic anisotropy of ferrite leads to a higher density of inhibitor particles in the surface Goss grains than in non-Goss grains. Therefore, when the surface grains begin to grow, the speed of grain boundary migration towards Goss grains is significantly lower than that towards non-Goss grains. As a result, while the grain size inside the steel remains basically unchanged, some Goss grains gradually become the largest among all surface grains. As the temperature of the secondary recrystallization annealing increases and the inhibitor particles gradually dissolve back into the ferrite, the surface Goss grains, which are already the largest in size, engulf all grains and complete the secondary recrystallization process characterized by abnormal growth. It is the molar volume effect of inhibitors and the strong elastic anisotropy of ferrites that lead to the abnormal growth of Goss grains originating from the surface layer of hot band during secondary recrystallization.