The Surface Nanocrystallization And Siliconizing Behavior Of3%Non-grain Oriented Silicon Steel

It is known that high silicon steels can be widely used in power and electric industries due to its superior comprehensive soft magnetic properties, such as high permeability, very low core loss and nearly zero magnetostriction compared with conventional3%silicon steel. Unfortunately, it is extremely difficult to produce high silicon steel sheet by conventional rolling techniques due to its poor ductility. Up to know, more synthesizing methods were proposed for the high silicon steels, however only the chemical vapor deposition (CVD) method among them can be used for a small-scale production in Japan, the key problems, that restrain the large-scale application of the CVD method, include serious corrosion of equipment and sheet surface induced by high siliconizing temperature and high percentage of halide, complex warm-rolling process (necessary for the reduction of roughness of sheet surface induced by Cl-corrosion), iron loss of the sheet surface and environment pollution of FeCl2gas.Surface nanocrystallization (SNC) technique can introduce novel properties of the nanocrystalline materials for the surface of the metallic materials by generation of a nanostructured surface layer. The large volume fraction of grain boundaries, dislocations and vacancies in the surface layer can provide channels for atom diffusion, meanwhile the large amount of active atoms existing in the top surface layer can promote chemical reaction, so it is reasonably expected to significantly reduce the siliconizing temperature and percentage of halide by introducing the SNC into the production of high silicon steel. In this work, a new method was proposed for the SNC of3%non-grain oriented silicon steel by means of cross-shear rolling (CSR), of which the rolling parameters were designed according to the basic principle of SNC, then a hybrid technique of CSR and low-temperature siliconizing (LTS, i.e. CSR/LTS technique) was developed for the fabrication of high silicon steel. This CSR/LTS technique can effectively solve the problems existing in the CVD process, therefore it is expected to achieve practical application.In order to develop the CSR/LTS technique, systematical research activities were carried out as follows:first,3%non-grain oriented silicon steel was selected to be treated by using surface mechanical attrition treatment (SMAT), SMAT+CSR and CSR, and gradient nano-micro-structured surface layers were obtained for the treated samples, the structural evolution along the depth of the treated samples and influence factors were investigated. Second, solid siliconizing treatments were performed at550-700℃for1-8h in Si+1-5wt.%halide, and microstructural evolutions were characterized in order to clarify the influences of siliconizing treatment parameters (temperature, duration and halide percentage) and gradient structure of matrix (grain size of top surface layer and thickness of gradient structure) on thickness of siliconizing layer and phase composition, finally diffusion coefficients and activation energies of Si in gradient structure of3%non-grain oriented silicon steel were calculated according to the diffusion theory for the SMAT, SMAT+CSR and CSR samples, respectively. Third, diffusion annealing at1000and1100℃for15min was performed for typical CSR/LTS samples, and the structural evolution of siliconizing layer was examined. The main results are summarized as follows:1. Deformation-induced SNC of3%non-grain oriented silicon steel:1) Gradient nano-micro-structures form in the surface layers of3%non-grain oriented silicon steel after the SMAT, SMAT+CSR and CSR. The grain sizes of top surface layer are10,10～20and20～50nm, and the thicknesses of gradient nano-micro-structured surface layers are120,80and80μm, respectively. Our research work shows that the SNC can be realized for iron and steel during the rolling process by using the CSR.2) The SNC of the SMAT, SMAT+CSR and CSR samples is dominated by dislocation activities, the action directions of external forces do not change the SNC process.3) The surface hardness values are increased by86%,54%and48%respectively after the SMAT, SMAT+CSR and CSR. With the increment of the depth, the hardness reduces gradually and tends to achieve constant value as the matrix. The increment of the hardness can be attributed to the grain refinement and work hardening.2. Diffusion and thermodynamics of Si in gradient nano-micro-structured surface layers:1) By using solid siliconizing technology, siliconizing layer of about several tens micrometers can be obtained for the SMAT, SMAT+CSR and CSR samples after annealing at550～700℃for1-8h in Si+1-3wt.%halide. Our research work shows that temperature and halide percentage of the siliconizing treatment can be reduced significantly by introducing the SNC into the rolling process of high silicon steel.2) The thickness of siliconizing layer increases with elevated temperature, especially when annealing duration and halide percentage is increased properly.3) The increment of annealing duration cannot change the thickness of siliconizing layer at lower temperature (≤600℃), but can significantly increase the thickness of siliconizing layer at higher temperature (≥650℃).4) Properly increment of halide percentage at higher temperature for a short duration is helpful for the increasing of the thickness of siliconizing layer, but this relationship becomes weaken at lower temperature for longer duration.5) The thickness of siliconizing layer increases with the increment of gradient structure layer, and keeps unchanged while the grain sizes of top surface layer vary within the nano-scale.6) The phase in siliconizing layer of the SMAT, SMAT+CSR and CSR samples after annealing at550～600℃is Fe3Si, and partial or entire phase transformation from Fe3Si to FeSi occurs with the increments of siliconizing temperature, duration and halide percentage.7) The diffusion coefficients of Si in the SMAT, SMAT+CSR and CSR samples at550℃are3.70x10-10、3.45x10-10and3.02x10-10cm2/s respectively, and at650℃are5.81x10"9、3.01×10-9and2.53x10-9cm2/s respectively.8) The activation energies of Si diffusion in the SMAT, SMAT+CSR and CSR samples are141,155and171kJ/mol, respectively. 3. Microstructural evolution of the CSR/LTS samples after the high temperature diffusion annealing1) During high temperature diffusion annealing process, Si diffuses from surface to internal matrix, the interface between siliconizing layer and matrix disappears, and the siliconizing layer becomes more dense due to the reduction of the pores.2) During high temperature diffusion annealing process, the Si tends to change from gradient distribution to uniform one throughout the whole sheet.