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Development of surface insulation coatings for oriented silicon steel

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  The preparation of the coating is the last step in the production of silicon steel. The type, process and quality of the surface coating will have a great influence on the insulation, adhesion, punching and corrosion resistance of electrical steel, which will affect the electrical steel. Its own ratio of total loss and other magnetic properties, even the key to the safe operation of the entire transformer.
  As an important magnetic material, oriented silicon steel has excellent magnetic properties and low iron loss, and is widely used as a core material for power transformers.
  The untreated electrical steel sheet will corrode and rust, which not only affects the performance of the silicon steel material itself, but also threatens the safety of the grid operation. Usually, the surface of the silicon steel sheet is coated with a very thin (0.5-5 μm) insulating coating, so that the silicon steel sheet has a high interlayer resistivity, which minimizes the power loss between the silicon steel sheets. At the same time, the silicon steel sheet is protected from corrosion by various corrosive media during storage, transportation and use to prevent rust.
  The magnesium silicate underlayer formed by silicon steel after high temperature annealing has certain insulation and corrosion resistance, and at the same time, tensile stress is generated in the steel, and the interlayer resistance is greater than 4 Ω·cm?/piece, which can meet the needs of the coil core distribution transformer. However, for large and medium-sized transformers with laminated cores, the resistance is not large enough, and an insulating coating must be applied to the surface. A schematic cross-sectional view of the surface of the oriented silicon steel product is shown in FIG.
  Figure 1 is a schematic cross-sectional view of the oriented silicon steel product
  The preparation of the coating is the last step in the production of silicon steel. The type, process and quality of the surface coating will have a great influence on the insulation, adhesion, punching and corrosion resistance of electrical steel, which will affect the electrical steel. The ratio of total loss and other magnetic properties, or even the key to the safe operation of the entire transformer, is urgently needed to develop an insulating coating material with excellent performance, which not only has good electrical insulation, coating adhesion, but also Good punching and workability, weldability and corrosion resistance, can adapt to high temperature annealing at 700-800 °C, and has high stacking factor and uniform surface color. At home and abroad, the coating process is continuously researched and improved to further improve the performance of oriented silicon steel products.
  1. Overview of research on foreign oriented silicon steel coating
  Oriented silicon steel surface insulation coatings used at home and abroad mainly include organic coatings, semi-inorganic coatings and inorganic coatings. Organic coating is a relatively traditional domestic insulating coating, mainly composed of acrylic resin, vinyl acetate resin, modified alkyd tree and epoxy resin. It is commonly used in the manufacture of high-grade silicon steel for large motors. The organic coating has good film-forming properties, good insulation and impact resistance, high shear filming and interlayer resistance, but low hardness, heat shrinkage deformation, heat resistance and weldability, and heat. It is easy to creep afterwards. It can be seen from Fig. 2 that as the content of the organic resin in the coating increases, the punching property is improved, but the weldability is lowered. However, in some special occasions, the coating needs to be heat treated at 600 ° C and then used as an insulating coating. The resin in the organic coating has been degraded and volatilized at 600 ° C, losing the insulating ability, and volatilizing harmful gases, causing environmental pollution. Therefore, the stress relief annealing treatment cannot be performed. At present, organic coatings have gradually been eliminated.
  Figure 2 Relationship between organic resin content and number of punches and welding speed
  In order to improve the adverse effect of secondary grain size on iron loss, Nippon Steel proposed a stress coating of ultrafine particle colloid SiO2 mixed with phosphate solution. The stress coating produced various isotropic tensile stresses on the surface of the steel plate to make iron loss P. And the magnetostriction is reduced, and the adverse effects of external stress on silicon steel can be offset. Kawasaki uses a stress coating of magnesium silicate (Mg(H2PO4)2) added to the gelatinous SiO2 and Cr03 to improve the moisture absorption resistance of the coating and also improve the appearance. However, the chromium compound contains hexavalent Cr, the paint pollutes the environment, and the waste liquid is troublesome. Nippon Steel has proposed to use a solid solution type composite metal hydroxide with an average particle size of less than 1 μm or 2-3 μm plus borate or acetate. , silicate, etc., good adhesion, corrosion resistance and sinter resistance, but not hygroscopic and heat resistance. The addition of a fine particle colloidal oxide reacts with free phosphoric acid to form a more stable phosphoric acid compound, thereby improving moisture absorption resistance and heat resistance, and also having better lubricity. In order to prevent the aggregation of the colloidal oxide of the particles, the organic acid salt is added, but the coating liquid is unstable, and an inorganic compound such as ultrafine particles Fe(OH)2 of about 10 nm is added to adjust the structure of the colloidal oxide to make the phosphate solution more stable. .
  The semi-inorganic coating is an insulating coating formed by mixing a phosphate and chromate inorganic solution with a latex resin solution. An organic resin powder having a particle diameter of 2 to 50 μm is added to the latex resin, and then mixed with the inorganic solution and coated on the surface of the silicon steel for baking. The inorganic component of the conventional semi-inorganic coating is chromate, which has good corrosion resistance. The organic resin powder is usually one or several kinds of polyethylene, polypropylene, polyamine, benzodiaminotriazine, polypropylene resin or polystyrene copolymer or mixture, among which polyphenylene The copolymer of the copolymer has the best adhesive strength.
  Nippon Steel first proposed a tension coating in 1973. Its main component is ultrafine colloidal SiO2 (silica sol) formulated with a phosphate coating solution. The tensile stress generated by the coating can refine the magnetic domain and effectively reduce iron loss and magnetostriction. Related studies have shown that iron loss P1.7 can be reduced by up to 0.1 W/kg by adjusting the coating formulation and using a suitable drying and sintering process.
  One of the most effective ways to reduce the iron loss of oriented silicon steel is to develop new coating technologies. The results show that the magnesium oxide coating has an important influence on the magnetic properties and surface properties of oriented silicon steel. In addition, Japan has adopted chemical vapor deposition to coat thin films to form ceramic insulating films instead of forsterite films, and to reduce iron loss by laser treatment and changing the inorganic acid species of the insulating film coating liquid. As a result, it has been found that by improving the composition of the insulating film coating liquid and improving the viscosity stability and the film tension, an oriented silicon steel sheet having high magnetic induction and low iron loss can be obtained.
  At present, foreign countries have made some research progress in the field of environmental protection coating new technology research, but have not achieved satisfactory results in ensuring the comprehensive performance of insulating coatings.
  2. Research status of surface coating of oriented silicon steel in China
  In the 1940s, hot-rolled silicon steel was coated with insulating paint on the surface. In the 1980s, this method was still used in China, and even the glass-coated cold-rolled silicon steel was coated with insulating varnish. In the early days of China, pure organic coatings were mainly studied. In view of the shortcomings of low hardness and poor heat resistance, domestic silicon steel enterprises are currently used less.
  Later, companies such as the United States and Japan successively developed phosphate surface coatings with better insulation properties. However, chromate is generally added to previous coatings to improve the corrosion resistance and interlayer resistance of the coating. For a long time, almost all insulating coating materials containing hexavalent chromium have been used in the production of cold-rolled silicon steel in China. There is no clear standard for the environmental protection of insulating coatings, and water-based chromium-free silicon steel coatings, usually water-based solvents. It is an environmentally friendly coating, but this inorganic coating is subject to various conditions in practical applications. At present, semi-inorganic coatings are still dominant in China. The semi-inorganic insulating coatings used by domestic steel companies are mainly shown in Table 1.
  Table 1 Coating code and classification of major domestic electrical steel production enterprises
  In the production process of oriented silicon steel, in order to prevent the steel plate from being connected during the annealing process, the surface of the steel plate needs to be coated with magnesium oxide (Mg0) before the high temperature annealing, and during the annealing process, Mg0 reacts with the oxide formed on the surface layer of the steel plate to form a magnesium silicate underlayer. Then, a phosphate tension coating is applied to the outer surface to form a two-layer structure of a magnesium silicate underlayer and a phosphate coating. Due to the different coefficients of thermal expansion of the two-layer structure coating, the phosphate coating exerts a tensile effect on the substrate after the high-temperature cooling, that is, the tension coating. The thermal expansion coefficient of the magnesium silicate underlayer is between the steel plate and the phosphate coating, which is beneficial to the adhesion of the phosphate coating and the exertion of the tension. Hu Shoutian et al. chemically removed the surface phosphate coating and the magnesium silicate underlayer of oriented silicon steel to study the coating structure and its effect on core loss. The results are shown in Table 2.
  Table 2 Iron loss of oriented silicon steel sheet with different coating structure
  The results in Table 2 demonstrate that the phosphate coating can reduce the loss of oriented silicon steel by about 9%. The embedded structure of the magnesium silicate underlayer makes the coating have good adhesion, but it has an adverse effect on the core loss of the oriented silicon steel. Based on the synergy of the two layers, the eddy current loss during the use of the core can be effectively reduced, and the surface tension provided can also reduce the noise and loss due to magnetostriction and domain wall movement during the use of the core.
  3. Development direction of silicon steel surface insulation coating
  The core loss, magnetostrictive properties and surface quality of oriented silicon steel are closely related to the surface insulating coating. The tensile stress generated by the coating on the silicon steel sheet can offset the compressive stress generated by the material in the transformer assembly, thereby reducing the noise of the transformer. Due to the coating, the stress sensitivity of the steel sheet is reduced during magnetostriction, and the transformer noise is also reduced. At the same time, the oriented silicon steel sheet insulation coating has the effect of reducing the total loss, and the iron loss can be reduced by 9%-14%. The research and application of high performance silicon steel surface insulation coatings have an important impact on improving the overall performance of oriented silicon steel.
  Hexavalent chromium can cause serious damage during the production and use of paints and in the discharge of waste liquids. Although semi-inorganic coatings have improved some of the disadvantages of inorganic coatings and organic coatings, they are currently the most widely used, but some of the volatile organic solvents contained therein are also toxic. Minimize or even eliminate volatile organic solvents. The development of inorganic coatings with excellent performance is a research focus of insulating coatings. Therefore, chromium-free environmentally-friendly inorganic oriented silicon steel insulation coating is the key development direction of oriented silicon steel insulation coating.
  4 Conclusion
  1. Surface insulation coating can reduce iron loss, weaken the sensitivity of magnetostriction to stress, and reduce transformer noise. It is an important process in the production process of oriented silicon steel.
  2. Oriented silicon steel surface insulation coatings used at home and abroad mainly include organic coatings, semi-inorganic coatings and inorganic coatings. Organic coatings have been phased out and inorganic coatings will gradually replace semi-inorganic coatings.
  3. In order to meet environmental protection requirements, research and development of chromium-free environmentally friendly inorganic coatings containing no chromium and no volatile organic components is the development direction of silicon steel surface insulation coating.
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