The Diffusion Behavior Of Si In 3% Cold-rolled Non-grain Oriented Silicon Steel

High silicon steel (also known as electrical steel) generally refers to the iron-silicon alloy containing 6.5wt.% silicon, it is one of the most important soft magnetic materials used in the electric power and telecommunications industry for the manufacture of motors, generators, relays, transformers and other electrical instrumentation. Although high silicon steel exhibits superior comprehensive soft magnetic properties, such as high permeability, very low core loss and nearly zero magnetostriction, its large-scale production by using conventional rolling techniques is impossible due to its brittleness at room temperature originating from the increment of silicon, so it difficult to obtain a satisfactory industrial products. 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 originate from the decomposition of 5 ?35V.% SiCl4 at ?1200?, high temperature and halide content may cause equipment and sheet metal surface corrosion seriously (then subsequent complex warm rolling process is necessary for the reduction of surface roughness), iron loss and FeCl2 gas pollution of the environment.In our previous work, we propose a new method (i.e. asymmetric rolling and solid siliconizing) for the production of high silicon steel, it includes the following key operations, first to synthesize nanoscrystallines in the top surface layer of sheet by using asymmetric rolling; second to obtain a compound layer through solid siliconizing; third to drive the Si in the compound layer to diffuse toward the internal matrix be means of diffusion annealing. In order to minimize the negative effects of the high concentrations of Cl-, the expensive SiCl4 is replaced by a mixture of cheap silicon powder (used as silicon infiltration sources) and an extremely small amount of halide (as a catalyst), the high activity of the nanocrystallines in the top surface layer of the cold rolled sheet can compensate for the low activity of silicon powder. In this work, hot-rolled sheet of 3% non-grain oriented silicon steel was selected to be rolled to different thickness by using asymmetric rolling. Solid siliconizing treatments were performed for different parameters (include temperature, durations and the composition of the mixed powder) to obtain compound layer for the cold rolled samples of different thick. Diffusion annealing was adopted for different parameters (include temperature and duration) 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, compounder layer can be obtained for cold-rolled 3% non-oriented silicon. The thickness and the density of the compound layer can be enhanced by the increment of the siliconizing temperature and duration, and not be affected by rolling reduction.2. In the siliconizing process with fixed temperature and duration, the compound layer forms when the content of the halide in the mixed powder is large than 0.25wt.%, the thickness of the compound layer increases with the increment of the content of the halide, and remains unchanged when the content of the halide is large than 0.5wt.%.3. In the solid siliconizing process in the mixture of Si+halide powder, dense compound layer can be obtained when the Si and halide powder is mixed uniformly, otherwise holes and cracks may form in the compound layer.4. The thickness of the compound layer reduces with the increment of reuse times of the Si powder, and the SiO2 being added to the Si+halide powder as the filler can reduce the thickness of the compound layer and the combination of the compound layer/matrix.5. The Si element in the compound layer of the sample after the cold rolling and siliconizing diffuses to the matrix in the diffusion annealing process, tends to distribute uniformly throughout the whole sheet with the increment of annealing temperature and duration.6. The hole inside the compound layer reduced and the compound layer becomes denser due to the diffusion annealing.7. The phases in the top surface layer of 3% non-grain oriented silicon steel after the siliconizing and diffusion annealings are Fe3Si and FeSi.