Research On High Temperature Oxidation Behavior And Controlling Technology And Application Of Oxide Scale Of Hot-rolled Non-oriented Silicon Steel

Silicon steel is used to produce cores of electric motors or transformers and it is indispensable in electricity, electronics and military industries. It’s an important magnetically soft alloy, in which non-oriented silicon steel takes up higher proportional of its total production. During hot rolled process of silicon steel oxide scale forms inevitably. On the one hand, because of the existence of Fe2SiO4, compared with plain carbon steel, adhesion of the oxide scale is improved leading to more residual scale on the plate surface. Defects like red scale and press-in scale appeared. Red scale will decrease acid picking efficiency and if distribution of red scale is non-uniform, over-pickling or under-picking will occur which result in bad surface quality. Surface quality will be ruined by press-in scale which would still remain in the subsurface of cold rolled plate and damage its magnetic property. On the other hand, internal oxidation zone will form in the production process of hot rolled silicon steel, which is difficult to be removed by high pressure water descale. The internal oxides will also remain in the subsurface of cold rolled plate and ruin its magnetic property. Thus the major factors to solve the problems mentioned above are to control proportion of red scale, scale/subsurface interface flatness and the thickness of internal oxidation zone.In the present paper, hot-rolled non-oriented silicon steel was used as research object, Silicon content of which is 0.75-2.2wt.%. Its oxidation thermodynamics, kinetics, factors affecting its oxidation behavior and mutual transformation of outer scale and internal scale were studied. Method to control proportion of red scale, flatness of scale/substrate interface and thickness of internal oxidation zone was proposed. Finally improved process of hot-rolled non-oriented silicon steel produced in CSP production line was proposed and its effect was also evaluated. The main work and innovative achievements of the present dissertation are as follows:(1) Oxidation thermodynamics and kinetics analysis of the specimens were studied to provide theoretical basis for scale thickness control technology.From Fe-Si-O phase diagram released by Thermal-calc and Ellingham-Richardson diagram, oxides presented in oxide scale of the specimens can be sorted in descending order according to their formation tendency as:SiO2, Fe2SiO4, FeO, Fe3O4 and Fe2O3. TGA was used to measure oxidation mass gain parabolic rate constant, parabolic rate constant of outer scale, parabolic rate constant of internal oxidation zone at the temperature range of 600～1150℃ under dry air condition, and all the constants increased with temperature. Activation energy of the specimen was also calculated.(2) Melting point of Fe2SiO4 compound was tested. Effect of temperature, Si content and oxidation atmosphere on scale morphology was discussed. Scale morphology included structure, thickness and flatness of scale/substrate interface.Actually Fe2SiO4 formed in the oxide scale was solid solution composed of Fe2SiO4 and Fe3O4 and its melting point was 1140℃which was measured by DSC. Influence of temperature on oxidation behavior of the experimental steels was as follows:thickness of external scale and internal scale increased with temperature increasing. Above melting point of Fe2SiO4, FeO was observed in the oxide scale. Flatness of scale/substrate interface decreased with increasing temperature. When at 800～1000℃ oxide nodules were always observed causing serious damage to the interface. Influence of Si content on oxidation behavior of the experimental steels were as follows:oxidation resistance of the steel was improved by increasing Si content below 1140℃, while above 1140℃ it was decreased. Dimension of internal oxidation participates increased with Si content increasing. Finally at the same temperature decreased Si content would help to improve flatness of scale/substrate interface. Influence of atmosphere on oxidation behavior of the experimental steels was as follows:low oxygen pressure air had no obvious influence on oxidation behavior of the steels. Moisture improved oxidation rate. Structure of the oxide scale formed under wet air condition changed. Voids and cracks were found in it.(3) Mechanism of mutual transformation between external oxidation and internal oxidation was discussed to provide theoretical basis for controlling thickness of internal oxidation zone.Transformation of internal oxidation to external oxidation:when the external scale growing continued corrosion of internal oxidation zone led to the transformation of internal oxidation to external oxidation. By increasing Si content single external scale would form. Another method leading to formation of single external scale was to decrease oxygen content in the atmosphere. Transformation of external oxidation to internal oxidation:transformation of external oxidation to internal oxidation occurred when specimens with oxide scale were held in Ar for a period. It was found that growth of internal oxidation zone was driven by consumption of external scale. It was also found that diffusion coefficient of oxygen in internal oxidation zone was larger than that in iron, which proved that presence of internal oxidation precipitates promoted diffusion of oxygen. And the effect became more apparent with Si content increasing.(4) Proposed controlling technology of hot-rolled non-oriented silicon steel oxide scale was as follows:ensure good operation of rotation descaling. Extreme low oxygen partial pressure should be avoided during reheating stage to prevent rapid conversion of external scale to internal oxidation layer. Temperature of the slab after descaling should be higher than 1000℃, and rolling rate should be raised. For coiling and its cooling stage, on the one hand the coiling temperature should below 700℃to prevent conversion of external scale to internal scale, on the other hand the coil should be coiled tightly to guarantee oxygen-poor atmosphere and the cooling rate should be reduced. According to the above methods the improved process was applied. Statistics of CSP line demonstrated that concession rate due to unsatisfied surface quality concerted with oxide scale of 50WW600 was reduced from 1.57% to 0.89% and that of 50WW1300 was reduced from 3.23% to 1.99% respectively.