Microstructure and Microtexture Development in Grain Oriented Electrical Steel

Abstract

The first Si-Fe electrical steel was produced in 1905, and the grain-oriented steel was discovered in 1930 after Goss demonstrated how optimal combinations of heat treatment and cold rolling could produce a texture giving Si-Fe strip good magnetic properties when magnetised along its rolling direction. This technology has reduced the power loss in transformers greatly and remains the basis of the manufacturing process today. Since then, many postulations reported on the mechanism on abnormal grain growth (AGG) which is the key for Si-Fe superior magnetic properties. However, none have provided a concrete understanding of this phenomenon. Identifying and classifying the driving force behind Goss abnormal grain growth is of industrial and academic importance to further optimise the manufacturing process and reduce losses. In the current investigation, the deviation from easy magnetisation direction <001> was studied to find a correlation between crystallographic orientation and magnetic domain structure. Both deviation angles α: the angle between <001> and in-plane rolling direction (RD), and β: the angle between <001> and out-plane rolling direction were calculated using electron backscatter diffraction (EBSD) raw data. Further, EBSD combined with forescatter detector (FSD) is used to reveal the magnetic domain configuration within individual oriented grains. The magnetic domain patterns were directly imaged and correlated to the crystal orientation and α and β deviation angles. It was demonstrated that the size of the deviated orientation grains from ideal (110) <001> Goss orientation is a critical microtexture parameter for the optimisation of magnetic property. It is concluded that the magnetic domain patterns and α and β angle of deviations are strongly correlated to the magnetic losses in GOES (grain oriented electrical steel).Furthermore, the effect of grain boundaries, grain size, heating rate and dislocation density on Goss abnormal grain growth was investigated using EBSD. It was found that in the early stages of secondary recrystallisation random grains grow and abnormal growth of Goss achieved in low heating rate. The advantage of Goss abnormal grain growth in secondary recrystallisation is lost while annealing at a high heating rate, and random orientation can grow abnormally. Also, statistical analysis of grain boundaries, including CSL (coincident site lattice), shows no distinct behaviour and high angle grain boundaries and CSL are not exclusive to Goss oriented grains. In addition, GND (geometrically necessary dislocation) and Taylor Factor showed to be randomly distributed around Goss grains, and the hypothesis of Goss grains grow by consuming high GND and Taylor Factor grains cannot be the reason for Goss abnormal grain growth. Neutron diffraction experiment was conducted at Rutherford Appleton Laboratory, ISIS facility at Oxford, UK using GEM beamline. It was demonstrated that Si atom positions in the solid solution disorder α-Fe cubic unit cell that cause lattice distortions and BCC symmetry reduction is the most influential factor in early stages of Goss AGG than what was previously thought to be dislocation related stored energy, grain boundary characteristics and grain size/orientation advantages. Finally, heat flux, heat flow direction, and strain effect on Goss abnormal grain growth investigated. It was found that heat flow direction greatly impacts the rate of abnormal grain growth of Goss. Also, strain areas can disrupt Goss AGG and promotes randomly oriented grains to grow abnormally.