Study On Preparation Of High Silicon Steel By Molten Salt Pulse Electrodeposition

With the purpose of achieving excellent soft magnetic property 6.5 wt%Si high silicon steel, this dissertation studied every link in the preparation of high silicon steel by pulse electrodeposition in molten salts on the detailed analysis and synthesize of present research condition in the silicon steel at home and abroad.Si diffusion layer on grain oriented 3.53 wt% Si steel substrate was produced by pulse electrodeposition in KCl-NaCl-NaF-SiO₂. The Si concentration gradient of the siliconized layer was flat after diffusion annealing and high silicon steel was obtained. Symmetry simplex was applied to design the formula of molten salts. A regression equation was found to be a better fit for the relationship between thickness of the siliconized layer and salts components.The crystallized temperature, phase structure, density, conductivity and surface tension of KCl-NaCl-NaF-SiO₂ molten salts were studied using differential thermal analysis (DTA), X ray diffraction (XRD), Archimedes method, four parallel connecting electrodes method and tension hookup method.The effects of average current density, duty cycle, frequency, deposition temperature and deposition time on deposition rate, composition distribution, microstructure characteristics and phase structure were systematically studied by weight increase method, glow discharge spectrometry(GDS), optical microscope, scanning electron microscope(SEM), atomic force microscope(AFM) and XRD. The diffusion rule of Si in substrate during process of electrodeposition was studied on the basis of the Flick's second law. The possible phases in the siliconized layer were calculated using thermodynamic theory. The component and relative content of phase in different amount of Si in Fe-Si alloys were calculated using equilibrium constant. Furthermore, its correlation was verified by layer-by-layer analysis of the sample by X ray diffraction (XRD). The distribution curves of Si concentration through the cross section of the layers after electrodeposition were measured by GDS. The electrodeposition technology of achieving average content of Si of 6.5 wt% in the silicon steel after diffusion annealing was determined by integral quantity using ORIGIN software. The required time to homogenize was by diffusion law. The texture evolution of substrate, deposited sample and diffused sample was analysed by orientation distribution function (ODF) and orientation distribution intensities.The results showed that the optimal formulation of the salts was xNaCl: xKCl: xNaF: xSiO₂=1:1:3:0.3(mole ratio). Using this salt formulation,a maximal siliconized layer could be achieved on low silicon steel sheet. NaF was the most significant factor for the thickness. The thickness increased gradually with the increase of NaF content. When NaF was 0.4-0.6 mole fraction, the thickness increased remarkably. The liquidus temperature of xNaCl: xKCl: xNaF: xSiO₂=1:1:3:0.3(mole ratio) was 783.0℃. There were NaCl, KCl, NaF and Na2SiF6 phases, and new Na2SiF6 phase was formed in the molten salts. When the temperature was 800℃, the density of the molten salts was 1.8252 g·cm^-3, conductivity of 0.0485 S.cm-1 and surface tension of 0.1718 N/m. It is found that the temperature had great influence on the diffusion of Si in the substrate. Diffusion coefficient values of Si in the substrate at 750, 800 and 850℃were 0.053439μm2/s, 0.14378μm²/s and 0.904757μm2/s, respectively. The diffusion activation energy Q was about 269.32KJ/mol, which the diffusion mechanism of Si in substrate was controlled by vacancy diffusion. The phase structure of the siliconized layer from surface to the substrate was Fe₃Si→Fe₃Si +α-Fe(Si)→α-Fe(Si). The sample produced by electrodeposition temperature of 800℃, the period of 1000μs, duty cycle of 20%, an electrodeposition time of 60 min and current density of 120 mA/cm2 followed by diffusion annealing to obtain high silicon steel. Diffusion annealing was carried out under argon gas protection, the temperature was kept at 1050°C, and the annealing time was 40 min.The textures of substrate, deposited sample and diffused sample were analysed by orientation distribution function (ODF) and orientation distribution intensities. The results showed that favorable Gauss texture {110} <001> and cube texture {100} <100> were formed and developed during process of diffused annealing, disadvantageous texture {111} <110> and {111} <112> were weakened, presenting that the magnetic properties were improved. The core loss P5/1K of the high silicon steel with 300μm thick is 9.735 W/kg and is lower than that of 300μm thick common grain-oriented silicon steel. The magnetic induction of the high silicon steel is 1.916 T and is higher than that of 300μm thick common grain-oriented silicon steel.