Research In Hot Working Of Microstructure Evolution Law And Technology For6.5%Si Steels

6.5%Si steel is mainly applied to manufacture high speed and high frequency motor and the iron core of high-frequency transformer. It is the excellent soft magnetic materials with lower core loss, higher permeability and almost zero magnetostriction especially at high frequency. These properties make it has extensive application prospect. The research of6.5%Si steel has become a hot topic in the field of materials.6.5%Si steel is too brittle to be produced by conventional rolling methods because high silicon content. Comparing other methods, conventional rolling still has incomparable advantage, such as lower cost, higher efficiency, lower energy consumption, lower pollution. It has been the dream of the metallurgy and materials researcher to produce6.5%Si steel with this method. Therefore, conventional rolling process is worthy researching and making a breakthrough.So this paper used the single-pass compression test to study6.5%Si steel thermal processing, analysised the influence of different deformation conditions to the flow stress and the microstructure evolution, calculated the mathematical model of the flow stress and established processing maps with different strains. The main conclusions in this paper can be drawn as follows:(1) The effect of hot working processing parameters on flow stress of6.5%Si steel had been studied through the single-pass compression test. During the process of hot deformation, the flow stress of6.5%Si steel increased with gradually increasing strain rate and. decreasing deformation temperature. The effect of strain on flow stress had a complex change. The different deformation conditions leaded to different degree of dynamic recovery (DRV).(2) According to experimental data, a mathematical model of flow stress for6.5%Si steel had been established, which had a good fitting accuracy.(3) When the deformation temperature and strain rate of equiaxed6.5%Si steel were1000℃and1s-1,the number of subgrain in deformed grains and the degree of dynamic recovery was increasing with the increase of strain.(4) At the same strain rate,with the temperature increasing, the degree of dynamic recovery and softing was increasing. The influence of temperature on the microstructure evolution was not different when the structures of6.5%Si steel were equiaxed crystal and columnar crystal(5) For equiaxed crystal or columnar crystal samples, the influence of strain rate on microstructure evolution was similar, With strain rate decreasing, the number and size of subgrain were increasing,the degree of dynamic recovery became intense.(6) The EBSD result showed that a large number of small-angle grain boundaries obviously outlined the subgrain. As the strain and deformation temperature increased while strain rate reduced, subgrain generated from the trigeminal boundaries, and the number of subgrain gradually increased and extended to the internal of deformed grain. However, because of the lack of large angle grain boundary and recrystallized grains, the main softening mechanism in heat deformation was dynamic recovery.(7) Based on the Z-H constitutive theory and the Arrhenius equation, a high temperature consistive equation included the effect of strain had been established. Comparing data between calculation and experiment,it had a good fitting accuracy which satisfied the engineering application.(8) The basic theory of dynamic material model (DMM) had been studied. The hot processing maps for6.5%Si steel with different strains had been established by the data from single-pass compression. To optimize hot working processing, the distribution of safe regions and instable regions of processing maps had been analyzed combined with the evolution of microstructure. And the optimal processing area was discovered:the best hot working temperatures range from950℃～1000℃and the strain rates range fromls-1～4s-1.