| The high silicon steels with the silicon content of about 6.5% are energy-saving and environment-protected soft magnetic material with low-energy as well as low noise due to the excellent magnetic properties such as high magnetic permeability, low core loss, and low magnetostriction. However, the silicon steels with 6.5% silicon are too brittle to be rolled thus cannot be fabricated with the traditional rolling process. Up to now, only the Japanese have achieved industrialized production by CVD method with high cost. During recent decades, powder metallurgy, with low cost, has become another approach for the fabrication of the high silicon steels.In this paper, powder rolling and bulk rolling were applied with different raw powder materials to explore the process conditions of high silicon steels fabrication by powder metallurgy. The factors affecting the rolling process and magnetic properties of silicon steels were investigated. The suitable fabrication process as well as silicon steels with good properties were obtained.At the beginning, the simulation and experiment was carried out based on the finite element method, with iron as the objection. The loose structure was more likely to appear due to different load-carrying conditions between the inner and the out surface during powder rolling. The strip quality was greatly influenced by the roll gap and the powder flow ability since the gap was the main factor contributing to the strip density and dense strip could be attained with high flow ability powder. When another poor flow ability silicon phase existed in the raw material, the content of the strip could drift due to easy loss of such silicon phase.Pure iron is difficult to densify during sintering below 1400℃. The diffusion reaction between iron and silicon is closely related to the powder granularity and temperature, thus affecting the rolling ability as well as the final properties. With the proper silicon powder granularity, the green strip, having a good deformation ability and no elemental Si after the first heat treatment, could transform to high density silicon steels after the second rolling and heat treatment.The reaction between Si and Fe could be stimulated by small introduction of Sn thus being achieved in low temperature and short time, which also improved the sintering of Fe with a large crystal grain. The atmosphere with H2 deteriorated the sintering of silicon steel and resulted in the oxidation of Si while Ar atmosphere is better.The bulk material, with the blend of ultra-fine Si and Fe powder, had the deformation ability after 500-1000℃SPS treatment, which can transform to strip with the thickness of 0.3-0.4mm by rolling. They were severely oxidized after high temperature heat treatment with lower density and magnetic properties compared to materials by powder rolling.The silicon steel ingots can be fabricated by SPS technique with Fe and FeSi as the raw materials. The powder granularity as well as SPS temperature and pressure can affect the diffusion reaction between Fe and FeSi particles, hence influencing the microstructure of the final products. The bulk material with coarse powder had good ductility and performed ductile failure. With the decreasing granularity and the increasing temperature, the ductility degraded and brittle failure gradually took place. The proper processing condition for dense and ductile silicon steels should be:the coarse FeSi particles with the sintering temperature below 1100℃and the pressure 50MPa.The high silicon steels can be produced by roll-thinning and annealing with the highly dense silicon steel bulk as the raw materials. The saturated magnetization was 17740Gs and coercive force was 3.67Oe. The saturated magnetization is equivalent to that silicon steel blended by Fe and Si, but the coercive force reduced 9 times. |
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