Study On The Process And Principle Of Powder Rolling Technology For Preparation Of High Silicon Steel Sheets

The high silicon steels have a potential application in magnetic devices such astransformers, high-speed motors and power generators at high frequency due to theirhigh permeability, near zero magnetostriction and very low core loss compared withconventional silicon steels. The weight and size, as well as the noise, of magneticdevices can be drastically reduced by using high silicon steels. However with the Sicontent increases the steels become brittle and make the rolling procedure difficult,which restricts the application.For overcoming the brittle of high silicon steels, some mechanisms by controllingover the process of powder rolling were investigated in the dissertation for thepurpose of finding a simple and industrial solution.In the present dissertation, the preparation of Fe-Si composite powders and theeffect of powder's characteristic on the powder rolling were studied. The ultrafine Sipowders were prepared by ball milling and crushing. The composite powders weresynthesized with graded Fe and Si powders. The Si powders are bonded togetherwith Fe powders in different forms. The coarse Si powders distribute among the Fepowders. The Si powders with the particle size of around 10μm are embedded ingrooves of Fe powders. The fine Si powders are adsorbed on the surface of Fepowders. The homogeneity of composite powders was evaluated by measuring thestandard deviation of silicon content. The results show that the powders achieve thebest homogeneity after mixing for 2.5 to 3 hours. When the composite powders areprepared with the biggish silicon powders, the two kinds of powders separate intolayers because of low compatibility. The composite powders prepared with differentpowder components were characterized by using powder comprehensivecharacteristic tester. The fluidity of composite powders decreases with decreasing thepowder's size. The apparent density and tap density of the composite powders areimproved with the Horsfield graded powders according to the maximum packingdensity theory.The simulated experiments for powder rolling had been done based the theory of powder rolling. The results show the compact coefficient increases with increasingthe pressure. Under the same pressure the compact coefficient is independent of thethickness of the specimen. After powders are pressed under the pressure of 350MPa,the relative density of the specimen is 80% and the compact coefficient is 1.8. Thenumerical simulation for the process of powder rolling based on the finite elementmethod (FEM) was implemented. The results show that there is slip phenomenon inthe powder rolling process andα(P_max) does not equal toα_neutral. The velocity of thepowder at center is 3～12% smaller than the velocity of the powder close to the roll.The effect of roll gap on the thickness and density of the green strip during powderrolling was studied. With the increasing of roll gap the thickness of the green stripincreases and the density decreases. The powder rolling process can be divided intothree stages: initial instable stage, stable stage and past instable stage. The criticalfeed height ensuring uniform thickness and density of the strip had been determined60mm by the experiment.Based on the Miedema's semi-empirical theory, the formation heats of all kinds ofphases in Fe-Si binary alloy were calculated. The sintering process was analyzed bythe calculation results. The activity of the component and interdiffusion coefficientwere calculated using thermodynamic data. The changes of density and phases aswell as effect of particle size on sintering of green strips sintered at the temperatureranging from 700℃to 1200℃respectively were investigated through XRD andEPMA. The quantity of the solid solubility of Si in sintered Fe-Si alloys wasevaluated by DSC technique. According to the diffusion coefficients and growthkinetic model of compound layers, the mechanism of reaction diffusion duringsintering was discussed. At the initial stage, the rich-Fe area forms in the vicinity ofthe Fe/Si interface, and then the Fe₃Si layer grows by the reaction of Fe and Si atoms.When the Fe₃Si layer grows up to the stated thickness, the FeSi layer forms in thevicinity of the Fe₃Si/Si interface.Following this principle derived from experiment and analysis of powder rollingand sintering, the best technological parameters for preparation of high silicon steelssheets were determined, which are primary sintering at 1000℃for 3h, secondarysintering at 1200℃for 3h and stress relieving annealing at 800℃for 3h. The changes of density, phase composition and mechanical property during the processwere analyzed. The reaction between Fe and Si powders occurres as follows duringthe primary sintering: Fe + Si→Fe(Si) + Fe₃Si. The secondary sintering makes thephase of strips change into the single Fe(Si) and the whole strips homogeneous. Thecore loss of specimens is relative low in high frequencies of 1kHz and above. Thedependence of core losses on the peak inductions and magnetizing frequencies in thehigh Si steel obtained through powder rolling processing are studied and comparedwith the results of Fe-6.5wt.%Si samples manufactured by a CVD processing. Thereason of the difference of the percentage of three components out of the core lossesbetween two kinds of high silicon steel is the difference of microstructures includinginclusions, homogeneousness and grain size.