The Siliconizing Behavior Of Cold-rolled Non-Oriented Silicon Steel

High silicon steels are the excelent soft magnetic material due to its superior comprehensive soft magnetic properties, such as high permeability, very low core loss and nearly zero magnetostriction, so it can be widely used in power and electric industries. However, it is extremely difficult to produce high silicon steel sheet by conventional rolling techniques at room temperature. Up to know, many methods were proposed for the synthesizing of high silicon steel, however only the chemical vapor deposition (CVD) method can be used for a small-scale production in Japan, the key problems, that restrains the large-scale application of the CVD method, included the follows:serious corrosion of equipment and sheet surface owing to high percentage of halide and high 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 FeCl2gas.In this work, nanostructure was synthesized in the surface layer of non-oriented silicon steel be means of asymmetric rolling. 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 can be significantly reduced. Base on previous work,0.5%and2.5%non-oriented silicon steel were selected to be rolled to different thickness by using the asymmetric rolling. Solid siliconizing treatments were performed at different temperatures for different durations in order to obtain compound layer for the cold rolled samples of different thick. Diffusion annealing was adopted at different temperatures for different durations in order to promote the Si diffusion from the compound layer to the internal matrix. The cross-sectional microstructure, Si variation along the depth and phases in the top surface layer of the samples after the cold rolling, siliconizing and diffusion annealing were examined by using OM, XRD, SEM and EDS, and the effects of rolling reduction, siliconizing and diffusion annealing parameters on the Si diffusion behaviors were analyzed. Main results are summarized as follows:1. By using solid siliconizing technology, compound layer with the thickness of more than several tens micrometers can be obtained for the cold rolled samples after annealing in Si+0.5wt.%halide powder at550～T4℃for20～30min. Our research work shows that temperature and halide percentage of the siliconizing treatment can be reduced significantly by introducing the asymmetric rolling into the cold rolling process of0.5%and2.5%non-oriented silicon steel.2. The Si diffusion process can be promoted by the enhanced siliconizing temperature and duration, however can not be affected by rolling reduction.3. The enhancements of temperature and duration in the siliconizing process and the Si content in original sheet in some ranges are helpful for the increment of the density of the compound layer and the reduction of the surface roughness.4. Diffusion annealing can promoted the Si diffusion from surface compound layer to internal matrix for the sample after the cold rolling and siliconizing, then the interface between compound layer and matrix disappears, and the compound layer becomes more dense due to the reduction of holes.5. During diffusion annealing process, the Si tends to change from gradient distribution to uniform one throughout the whole sheet with the increment of treatment temperature and duration.6. The phases in the top surface layer of the sample after the siliconizing and diffusion annealings are Fe3Si and FeSi.