Study On Preparation Of High Silicon Steel Foil By EB-PVD And Its Post-Treatment Process

High Si steel has excellent magnetic properties, such as low core loss in medium and high frequency, high permeability, low coercive force and almost zero magnetostriction. Also, the core loss is lower when the thickness is smaller. It is especially suitable for application in the high frequency and high speed electric machine, medium and high frequency transformer and reactor. But its poor ductility hinders its preparation and application. Therefore, preparation of high Si steel foil becomes a critical issue. Now, the preparation method mainly includes rapidly solidified method, special rolling method and CVD, etc. Except CVD process, fabrication and commercial scale production of high Si steel foil can not be developed due to various problems. And the CVD method has its own shortcoming, such as high repair rate and environmental pollution due to the application of a corrosive and poisonous gas SiCl4.Electron beam vapor deposition has a lot of merits, such as high deposition rate, pollution free and being easy to produce foil with from several tens to several hundreds microns and it is expected to be a good method for the preparation. Therefore, we did many researches on the preparation by EB-PVD and got some exciting results.The thickness and the density of the finally obtained high Si steel foil are 64μm and 7.5g/cm3, respectively. The coercive force is 1.89Oe and saturation magnetization is 1.88T. The minimum core loss P1/10k is 8.2W/kg, 45.6% of the loss of the 0.1mm thick grain oriented steel foil and the minimum core loss P0.5/20k is 5.2W/kg, 37.1% of the core loss of the 0.1mm thick grain oriented steel foil.Experimental results and theoretical analysis showed that high Si steel foil can't be prepared by one source method using Fe-6.5 wt. % Si source since the Si atom oxidized actively in the vacuum chamber. While high Si steel foil can be obtained by two source method. The silicon content of steel foil decreased when the target-substrate distance was lower and high Si steel foil can be manufactured when the target-substrate distance was smaller than 415 millimeters in some conditions. The density of the high Si steel foil was 7.26 g/cm3, 97 percent of theoretical values, the electrical resistivity was 90μ?·cm, larger than theoretical one 82μ?·cm. And the saturation magnetization was 1.73T, smaller than theoretical value 1.88T. This indicated that that the electrical and magnetic properties were comparatively good and still required to being enhanced by increase of the densification.XRD,EDS,SEM and M?ssbauer spectroscopy were used to characterized and analyzed the high Si steel foil. It was found that: The as-deposited high Si steel foil from static substrate had different phase composition and microstructures on the side close to the substrate, middle place and the site far from the substrate. The high Si steel foil was composed of A2 phase with no texture on the side close to the substrate while the steel foil in the middle and on the side far form the substrate consisted of an ordering phase B2 with strong (100) texture. It was composed of small equiaxed crystal and columnar crystallite, the diameter of which was larger when it is more far from the side close to the substrate. The width of columnar grain size was 25~50μm on the side far from the substrate. Similarly, the high Si steel foil from a rotated substrate had different microstructures and phase compositions. It was composed of an ordering phase DO3 with no texture while it consisted an ordering phase B2 with weak (110) preferential orientation. It was composed of equiaxed crystal-columnar grain-equiaxed crystal-columnar grain. The width of the columnar grain grew larger when it was closer to the side far from the substrate and the width was 2~3μm on the side far from the substrate.In the evaporation-deposition process, the growth of high Si steel foil obeys the mechanism"random nucleation-preferential growth". In other words, the evaporated atoms nucleate randomly at the beginning on the substrate. As a result, the formed grains are small equiaxed crystals. With the continual deposition, some grains with a special orientation are reserved due to being easy to grow. As a consequence, the width of columnar crystal grew larger and the texture degree enhanced.Experimental Research showed that high temperature rapid annealing can not improve the density and decreased a little on the contrary. Stick-like pores almost perpendicular to the surface of the Si steel foil were formed between the columnar grains and other stick-like pores parallel to the surface of the foil appeared in the interface between equiaxed grains and columnar grains. On the contrary, the hot pressure process can improve the densification degree of high Si steel foil. The density was improved from 7.2 g/cm3 to 7.5 g/cm3 and the saturation magnetization was enhanced from 1.74T to 1.88T as the pressure increased form 0MPa to 60MPa. In addition, vacuum annealing can improve soft magnetic properties markedly. Coercive force and remnant magnetism decreased from 6.87Oe and 593G to the minimum 1.89Oe and 132G, respectively.