The Siliconizing Behavior Of Non-grain Oriented Silicon Steel Subjected To Cross Shear Rolling

High silicon steels exhibits superior comprehensive soft magnetic properties, such as high permeability, very low core loss and nearly zero magnetostriction, so it is expected to be used in power and electric industries. However, its large-scale production by using conventional rolling techniques is impossible due to its brittleness at room temperature originating from the increment of silicon. Up to know, several methods were proposed for the synthesizing of high silicon steel, however only the chemical vapor deposition (CVD) method was used for a small-scale production in Japan, the key problems, that restrains the large-scale application of the CVD method, were summarized as follows:severe corrosion of equipment and sheet surface induced by high percentage of halide at higher siliconizing temperature, complex warm-rolling process (necessary for the reduction of roughness of sheet surface induced by high percentage of halide), iron loss of the sheet surface and environment pollution of FeCl2 gas.In this work,3% non-grain oriented silicon steel was selected to be rolled by using cross-shear rolling (CSR), in order to synthesis nanocrystallines in the surface layer, then the large volume fraction of grain boundaries, dislocations and vacancies in the surface layer can provide channels for silicon atom diffusion, and the large amount of active atoms existing in the top surface layer can promote chemical reaction, therefore the siliconizing temperature and the percentage of halide were expected to be significantly reduced. To optimize the siliconizing technique and obtain dense compound layer, systematical research activities were carried out as follows:the CSR with different mismatch speed ratios (MSR) was adopted to roll 3% non-grain oriented silicon steel in order to obtain nanocrystallines with different sizes. Solid siliconizing treatments were performed in Si+0～3Wt.% halide powder at T1～T4℃ for 10-30min in the following processes for the CSRed sample:heating+keeping at selected temperature+cooling in furnace、heating+keeping at selected temperature+cooling in air and directly keeping at selected temperature+cooling in air. The phases of the top surface layer, the cross-sectional microstructure and the silicon variation along the depth of the CSRed sample after the siliconizing treatment, as well as the surface microstructure of the CSRed sample were examined by using XRD、SEM、EDS and TEM, in order to clarify the influences of the siliconizing treatment parameters (temperature, duration and halide percentage) on the diffusion behavior of the silicon atoms. Main results are summarized as follows:1. After the CSR for 20 passes and 92% reduction, nanocrystallines with nearly random orientations form in the top-surface layer of 3% non-grain oriented silicon steel, the grain sizes are in the range of 10-30nm and 30～50nm for the sheets rolled with the MSR=-1.31 and～1.18 respectively.2. By using solid siliconizing technology, compound layer with the thickness of more than several tens micrometers forms for the CSRed samples after annealing in Si+0.5-3Wt.% halide powder at T1～T4℃ for 10-30min. Our research work shows that temperature and halide percentage of the siliconizing treatment can be reduced significantly by introducing the CSR into the rolling process of 3% non-grain oriented silicon steel.3. The enhancements of siliconizing temperature, duration and halide percentage are helpful for the diffusion process, until the silicon element approaches 15wt.% for the whole sheet.4. The phases in the compound layer of the CSRed samples are detected to be Fe3Si and FeSi, the FeSi phase increases with the annealing temperature, and remains unchanged with the variations of the annealing duration and halide percentage.5. The enhancements of siliconizing temperature, annealing duration and halide percentage in some ranges are helpful for the increment of the density of the compound layer and the reduction of the surface roughness.6. The siliconizing process is not dominated by the grain sizes of the nanocrystallines in the top surface layer of the CSRed sample.