The Surface Nanocrystallization And Heat Stability Of Non-grain Oriented Silicon Steel Processed By Cross Shear Rolling

Surface nanocrystallization (SNC) is one of the synthesizing techniques for the nanocrystalline materials that might achieve practical application. However, the treatment methods, proposed up to now, cannot provide flexible selection for the application investigations due to the disadvantages as complex operation, longer processing duration, significant increment of surface roughness and, especially, they are only restrained for the treatment of bulk metals. Rolling is a suitable process for the production of large scale sheet, among the modern rolling techniques, the cross shear rolling (CSR) can introduce severe plastic deformation into the surface layer of rolled sheet, then the grains can be refined into sub-micro scale. So, it is reasonably deduced that the grains might be further refined into nano-scale were the rolling parameters optimized according to the surface nanocrystalli-zation (SNC) principle. On the other hand, excellent properties of the metallic materials with a nanostructured surface layer are usually related to its heat stability, however, little investigations were focused on this topic up to now.In this paper, a 3% non-grain oriented silicon steel was selected to be rolled by conventional rolling and cross shear rolling with the rolling parameters (include mismatch speed ratio, reduction and pass) varying in certain ranges, and then typical rolled samples were selected to be annealed in vacuum and Si+halide powder, respectively. Different experimental techniques, including optical microscope, X-ray diffraction, scanning electron microscopy, transmission electron microscopy and hardness measurement, were adopted for the microstructural characterizations, in order to clarify microstructural evolutions in the rolling and the annealing processes, as well as the effects of treatment parameters. The main results are summarized as follows:1) Dislocation cells and nanocrystallines with random orientations form in the surface layers of the non-grain oriented silicon steel sheets after the conventional rolling for 92% reduction and 20 passes and the cross shear rolling for 92% reduction, 20?40 passes with the mismatch speed ratios of?1.18and-1.31, respectively. This work indicates that the surface nanocrystallization can be realized for large-dimensional non-grain oriented silicon steel sheets during the rolling process by using the cross shear rolling.2) The surface nanocrystallization can be related to the shear stress acting on the sur-face of the CSRed sheet, the increments of the mismatch speed ratio, reduction and pass can increase the shear stress and provide repeated loads necessary for the SNC respectively, then the gram refinement process can be accelerated.3) After the rolling, the structure and hardness show gradient variations from the top-surface layer to the center part of the rolled sheet, and the enhancements of mismatch speed ratio, reduction and pass are helpful for the strengthening of the top-surface layer, however they cannot obviously strengthen the center part.4) While annealed in vacuum, hysteresis effect can be found for the recrystallization of the rolled sheet in the heating procedure, however this effect is just valid in certain range above the recrystallization temperature. The size of recrystallized grains is mainly dominated by annealing duration, and the difference between the microstructures of the sheets rolled with different parameters tends to disappear with the increments of annealing temperature and duration.5) While annealed in Si+halide powder, Si diffusions in gradient nano-micro-structures of the CSRed sheet occur, that together with the reduction of grain size induced by the cross shear rolling can significantly delay the occurrence of recovery and recrystallization. Besides, it is found that smaller grain size in the top surface layer of the CSRed sheet is helpful for the increment of the silicon content in compound layer.