Insulating film composition for grain-oriented electrical steel sheet, method for producing said composition, grain-oriented electrical steel sheet having insulating film formed on surface using said composition, and method for producing said grain-oriented electrical steel sheet

Abstract

The insulating film composition for a grain-oriented electrical steel sheet according to the present invention comprises 10-250 parts by weight of colloidal silica, 0.5-5 parts by weight of cobalt hydroxide, and 20-170 parts by weight of a powder with respect to 100 parts by weight of a metal phosphate. The method for preparing an insulating film composition for a grain-oriented electrical steel sheet according to the present invention comprises the steps of: simultaneously adding a metal oxide, cobalt hydroxide, and a powder to phosphoric acid (H3PO4) and then heating to prepare a first composition containing 0.5-5 parts by weight of cobalt hydroxide and 20-170 parts by weight of a powder with respect to 100 parts by weight of a metal phosphate; and mixing 10-250 parts by weight of colloidal silica in the first composition. According to the insulating film composition for a grain-oriented electrical steel sheet and the method for manufacturing the same, a grain-oriented electrical steel sheet having improved film tension and insulating characteristics can be obtained.

Description

Technical Field

[0001] The present invention relates to an insulating film composition for grain-oriented electrical steel sheets, a preparation method of the composition, a grain-oriented electrical steel sheet having an insulating film formed on the surface using the composition, and a manufacturing method thereof. More specifically, the present invention relates to an insulating film composition for grain-oriented electrical steel sheets having excellent film strength and insulating properties and having excellent gloss and gorgeous colors, a preparation method of the composition, a grain-oriented electrical steel sheet having an insulating film formed on the surface using the composition, and a manufacturing method thereof. Background Art

[0002] Grain-oriented electrical steel sheets are steel sheets that generally contain a large amount of silicon components and exhibit excellent magnetic properties in the rolling direction.

[0003] It is known that when improving the insulation by reducing the iron loss of grain-oriented electrical steel sheets, the magnetic properties can be further improved. For currently commercialized grain-oriented electrical steel sheets, by utilizing the difference in thermal expansion coefficients between the steel sheet and the insulating film (secondary film) formed on the forsterite-based film (primary film), tensile stress is applied to the steel sheet to achieve the effect of reducing iron loss. For this, as one of the methods for reducing the iron loss of grain-oriented electrical steel sheets, a method of forming an insulating film with high tension on the surface is being actively studied.

[0004] In particular, for improving the surface physical properties or imparting surface tension, a method of forming a primary film and an insulating film has been proposed. As the primary film, a forsterite (2MgO·SiO2) layer formed by the reaction of silicon oxide (SiO2) generated on the surface of the electrical steel sheet material during the primary recrystallization annealing process and magnesium oxide (MgO) used as an annealing separator is known. Thus, the primary film formed during high-temperature annealing should have a uniform color without appearance defects, and functionally can prevent fusion between plates in the coiled state, and due to the difference in thermal expansion coefficients between the material and the primary film, tensile stress is imparted to the material, so that the effect of improving the iron loss of the material can be brought about.

[0005] Such a primary film should basically have a uniform color without defects on the material surface. However, for commercial products, since they are in the form of large-scale coils, it is difficult to maintain uniform film properties throughout the entire length of the coil. The biggest reason for this is the annealing separator containing water.

[0006] That is, a magnesia slurry is coated on a steel sheet and rolled into a coiled sheet state. During the process of the final high-temperature annealing process, due to the different amounts of heat transferred from the annealing furnace to the coil, the amount of discharged hydrated moisture also varies depending on the position and part of the coil. Therefore, oxidation defects called bare spots occur in the outermost coil part, which is one of the defects caused by the too-fast discharge of hydrated moisture in the coiled sheet. In particular, in the case of a secondary film formed by a conventional phosphate-based tension coating agent, the high transparency results in insufficient shielding property, directly exposing the primary coating defects. Therefore, in the mass production process, the defective parts are cut off and shipped, resulting in an increase in the unit price of the product.

[0007] In this way, a coiled sheet with a tension coating as a secondary film is formed, slit into appropriate sizes, and supplied in a hoop form. Depending on the use, it is made into a stacked core type transformer and a wound core type transformer. And when making a transformer, in order to improve efficiency, the inter-sheet insulation is very important. Usually, when increasing the coating thickness, the insulation is improved, but the no-load loss is reduced, and finally the transformer efficiency will instead decrease.

[0008] To solve this problem and improve the film tension and insulation, insoluble substances such as powders are introduced into the usual coating agent. However, when introducing insoluble substances, the roughness and glossiness are reduced, and technical problems such as pores generated in the film after film formation occur.

[0009] Therefore, there is a demand to develop a colored secondary film product that can improve the oxidation defects generated in the primary film and still has excellent insulation even at a thin film coating thickness. However, in the case of a conventional grain-oriented electrical steel sheet coating agent, this condition cannot be met, so there is a current need for relevant commercial technologies.

[0010] In addition, due to the strengthening of environmental regulations in recent years, the development of coating agents that exclude chromium oxide in electrical steel sheets is actively underway. In the case of a grain-oriented electrical steel sheet coating agent, colloidal silica with functional groups is introduced to reinforce the weakening of corrosion resistance and adhesion due to the absence of chromium oxide. However, the grain-oriented coating agent mainly based on phosphate or colloidal silica still has limitations in overcoming the weakening of corrosion resistance and adhesion due to the absence of chromium oxide.

[0011] In the case of a non-chromium-based tension coating agent for grain-oriented electrical steel sheets, the following methods have been proposed. For example, the method of colloidal silica modified with Fe, Al, Ga, Ti, etc. disclosed in Japanese Patent Laid-Open No. 2007-23329, and the method of introducing oxides such as Fe, Co, Cu, etc. to improve corrosion resistance and film tension as disclosed in Korean Patent Publication No. 10-2008-0025733. In the former case, the process of modifying colloidal silica by reacting it with Fe, Al, etc. is quite complex, and there are also many disadvantages in terms of manufacturing cost, and its effect is also insufficient. Therefore, it is difficult to be applied industrially. In the latter case, although it can be used more simply compared to the former, the introduced oxides have a mechanism of incidentally improving film densification or film tension by simply preventing the effect of free phosphoric acid generated during the drying of the coating agent. Thus, there are limitations in meeting the level of high-grade grain-oriented electrical steel sheets that require overall high corrosion resistance and film tension. Therefore, a commercialization technology of a non-chromium-based coating agent for orientation that can meet all physical properties has not been proposed yet. Summary of the Invention

[0012] (I) Technical Problems to be Solved

[0013] An object of the present invention is to provide an insulating film composition for grain-oriented electrical steel sheets having excellent film strength and insulating properties and having excellent gloss and gorgeous colors, a preparation method of the composition, and a grain-oriented electrical steel sheet using the composition.

[0014] (II) Technical Solutions

[0015] According to one aspect of the present invention, there is provided an insulating film composition for grain-oriented electrical steel sheets, which comprises 10-250 parts by weight of colloidal silica, 0.5-5 parts by weight of cobalt hydroxide, and 20-170 parts by weight of powder based on 100 parts by weight of metal phosphate.

[0016] According to another aspect of the present invention, there is provided a preparation method of an insulating film composition for grain-oriented electrical steel sheets, the preparation method comprising the following steps: heating after simultaneously adding a metal oxide, cobalt hydroxide, and powder in phosphoric acid (H3PO4) to prepare a first composition, the first composition comprising 0.5-5 parts by weight of cobalt hydroxide and 20-170 parts by weight of powder based on 100 parts by weight of metal phosphate; and mixing 10-250 parts by weight of colloidal silica in the first composition.

[0017] According to another aspect of the present invention, there is provided a grain-oriented electrical steel sheet having an insulating film formed on its surface using the insulating film composition of the present invention and a manufacturing method thereof.

[0018] (III) Advantageous Effects

[0019] The present invention can effectively provide an oriented electrical steel sheet that can improve the film tension and insulation properties of the oriented electrical steel sheet and has excellent magnetic properties.

[0020] By imparting delicate roughness to the insulating film of the oriented electrical steel sheet, the present invention can impart excellent gloss and gorgeous colors. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 It is a photograph taken for evaluating the corrosion resistance of an existing oriented electrical steel sheet.

[0022] Figure 2 It is a photograph taken for evaluating the corrosion resistance of the electrical steel sheet of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0023] Hereinafter, embodiments of the present invention will be described in detail so that those of ordinary skill in the art to which the present invention pertains can easily implement the present invention. However, the present invention can be implemented in various different forms and is not limited to the embodiments described herein.

[0024] Generally, the main components of a chromium-based insulating film composition are composed of colloidal silica that imparts film tension, a coating agent, metal phosphate that imparts adhesion to the steel sheet interface, chromium oxide, and trace additives for reinforcing other functions, etc. However, if chromium oxide is excluded from this composition, the drying rate decreases, and thus the insulation and the ability to impart tension by coating will be reduced.

[0025] In the chromium-based insulating film composition, when drying the film, hexavalent chromium (Cr 6+ ) reacts with the water present in the coating agent to become an H2CrO4 compound, and H2CrO4 reacts with Fe present in the steel sheet to generate FeO. Therefore, it becomes a state in which FeO and metal phosphate can react. This action greatly helps to improve the adhesion between the steel sheet and the coating agent, and ultimately helps to improve the film tension. In addition, hexavalent chromium (Cr 6+ ) reacts with the generated FeO and is reduced to trivalent chromium (Cr 3+ ), and the film densification can be improved by the polycondensation reaction of the reduced trivalent chromium (Cr 3+ ), which has a great influence on the improvement of the film tension.

[0026] In an insulating film-forming composition containing chromium (Cr), as shown in the following formula (1), chromium oxide can inhibit the generation of free phosphoric acid that causes poor adhesion after film drying, and at the same time improve the reactivity of phosphate at low temperatures.

[0027] CrO3 + 2H3PO4 → Cr(PO4)2 + 6H2O (1)

[0028] Therefore, it is necessary to solve the reduction in corrosion resistance and adhesion of the insulating film-forming composition that excludes chromium oxide.

[0029] In the present invention, by introducing chromium hydroxide, particulate carbon black and clay in nano or micron units, the problem of reduced hygroscopicity and corrosion resistance that may occur in a non-chromium-based insulating film-forming composition containing phosphate is solved. The present invention provides a non-chromium-based insulating film composition for grain-oriented electrical steel sheets, a method for preparing the composition, a grain-oriented electrical steel sheet having an insulating film formed on its surface using the composition, and a method for manufacturing the same. The non-chromium-based insulating film composition for grain-oriented electrical steel sheets is based on metal phosphate added with cobalt hydroxide and colloidal silica, and has colored and excellent surface characteristics and excellent insulation even when the coating thickness is thinner than usual by including particulate carbon black (Carbon Black) and clay (Clay) powder in nano or micron units.

[0030] After surface coating with an insulating film composition for grain-oriented electrical steel sheets containing a large amount of phosphate, over time, free phosphoric acid causes a reduction in hygroscopicity, chalking, and corrosion resistance. Therefore, in order to reduce surface defects caused by free phosphoric acid, it is necessary to prepare pure phosphate and metal oxide in an appropriate molar ratio, and the proportion of phosphate in the insulating film composition is very important.

[0031] In the present invention, as described above, 0.5 - 5 parts by weight of cobalt hydroxide is added based on 100 parts by weight of metal phosphate on a solid basis to suppress free phosphoric acid, and it is confirmed that 20 - 170 parts by weight of powder is added to form a dense film. Finally, it is found that cobalt hydroxide and powder replace the function of suppressing free phosphoric acid by the reaction of chromium oxide and phosphoric acid.

[0032] The insulating coating film composition for grain-oriented electrical steel sheets according to an embodiment of the present invention may contain 10 - 250 parts by weight of colloidal silica, 0.5 - 5 parts by weight of cobalt hydroxide, and 20 - 170 parts by weight of powder based on 100 parts by weight of metal phosphate.

[0033] Colloidal silica improves the strength and hardness of the film itself through an intramolecular network reaction when the film dries after coating, and thus plays a role in applying tension within the film-forming material. As the colloidal silica, various colloidal silicas can be used without limitation. For example, commercially available colloidal silica can be used. More specifically, basic colloidal silica can be used.

[0034] Generally, the insulating film composition for grain-oriented electrical steel sheets contains chromium oxide for reinforcing the corrosion resistance after film formation and solid silica for adjusting the roughness and coatability of the film, etc.

[0035] Based on solids, the insulating film composition for grain-oriented electrical steel sheets may contain 10 to 250 parts by weight of colloidal silica relative to 100 parts by weight of metal phosphate. When the content of colloidal silica is less than 10 parts by weight, there is a problem that the effect of increasing the tension brought by colloidal silica cannot be fully obtained. When the content of colloidal silica exceeds 250 parts by weight, the amount of metal phosphate relatively decreases, and there is a problem that the adhesion of the insulating film deteriorates. More specifically, based on solids, the insulating film composition for grain-oriented electrical steel sheets may contain 100 to 200 parts by weight of colloidal silica relative to 100 parts by weight of metal phosphate, or may contain 125 to 175 parts by weight of colloidal silica.

[0036] During the drying process of the film, colloidal silica undergoes a condensation reaction through the silica chain reaction shown in the following formula (2) to form a network structure such as -(HO-Si-O-Si)-n.

[0037] -(HO-Si-OH-)n + -(HO-Si-OH-)n = -(HO-Si-O-Si-)n + H2O (2)

[0038] However, the insulating film forming composition for electrical steel sheets containing colloidal silica may form an overly uniform network structure. Therefore, there may be limitations in terms of the denseness of the film. Therefore, there are limitations in imparting adhesion and corrosion resistance between the electrical steel sheet material and the insulating film forming composition. To reinforce it, metal phosphate and chromium oxide may be included.

[0039] The particle size of colloidal silica can be 7 to 40 nm. When the particle size is less than 7 nm, the compatibility with other components in the coating agent decreases, and there is a problem of increasing the viscosity of the coating agent. When the particle size exceeds 40 nm, the denseness of the film decreases, which may reduce the corrosion resistance.

[0040] The solid fraction of colloidal silica can be 25 to 55 wt%. When the solid fraction is less than 25 wt%, there may be a problem of reduced insulation. When the solid fraction exceeds 55 wt%, there may be a problem of reduced compatibility after preparing the coating agent. More specifically, the solid fraction of colloidal silica can be 45 to 50 wt%.

[0041] The Na + content of colloidal silica can be 0.1 to 1.0 wt%. When the Na + content is less than 0.1 wt%, there may be a problem of reduced denseness of the film. When the Na + content exceeds 1.0 wt%, the cations in the insulating film forming composition increase, and there may be a problem of hindering the compatibility between components. More specifically, Na+ The content can be 0.3 - 0.7% by weight.

[0042] The pH of the colloidal silica can be 9.5 to 10.5. When the pH is below 9.5 or exceeds 10.5, the pH difference of the components other than the colloidal silica in the insulating film forming composition is extremely large, and a phase separation phenomenon may occur. More specifically, the pH can be 9.5 to 10.0.

[0043] The viscosity of the colloidal silica can be 3.5 - 6.5 cp. When the viscosity is less than 3.5 cp, problems may occur in the coating property of the coating agent. When the viscosity exceeds 6.5 cp, thickening occurs during long-term use, and thus aging problems may occur. More specifically, the viscosity of the colloidal silica can be 4 - 6 cp. The viscosity of the colloidal silica can be measured with a Brookfield viscometer at a temperature of 20°C based on 30% by weight of the solids of the colloidal silica.

[0044] The specific gravity of the colloidal silica can be 1.1 to 1.3. When the specific gravity is less than 1.1, it may be difficult to control the coating amount of the insulating film forming composition. When the specific gravity exceeds 1.3, sedimentation problems may occur after preparing the insulating film forming composition. More specifically, the specific gravity of the colloidal silica can be 1.15 to 1.25.

[0045] The metal phosphate acts as a binder in the insulating film forming composition. The preparation of the metal phosphate must undergo a process of adding metal oxide to pure phosphoric acid (H3PO4) and reacting at 80°C or higher. In order to improve the adhesion of the metal phosphate, boric acid is further added during the reaction process and maintained for more than 3 hours, thereby inducing the condensation reaction of magnesium metal phosphate and boric acid. The prepared metal phosphate is strongly acidic.

[0046] The solids of the metal phosphate can be 50 - 70% by weight. When the solids are less than 50% by weight, the free phosphoric acid in the metal phosphate may increase, and surface moisture absorption may occur after preparing the metal phosphate. When the solids exceed 70% by weight, due to the excess of solids relative to pure phosphoric acid, poor reaction and precipitation may occur.

[0047] The metal phosphate and the metal oxide can contain various metals without limitation.

[0048] Specifically, the metal oxide can contain one or more of magnesium oxide (MgO) and aluminum oxide (Al2O3). More specifically, the metal oxide can contain magnesium oxide (MgO) and aluminum oxide (Al2O3).

[0049] In addition, specifically, the metal phosphate may contain one or more of magnesium dihydrogen phosphate (Mg(H2PO4)2) and aluminum dihydrogen phosphate (Al(H2PO4)3). More specifically, the metal phosphate may contain magnesium dihydrogen phosphate (Mg(H2PO4)2) and aluminum dihydrogen phosphate (Al(H2PO4)3).

[0050] In this case, based on the solid content, relative to a total of 100 parts by weight, the metal phosphate may contain 10 - 40 parts by weight of aluminum dihydrogen phosphate and 60 - 90 parts by weight of magnesium dihydrogen phosphate. When the metal phosphate contains less than 10 parts by weight of aluminum dihydrogen phosphate, the effect of increasing the tension brought about by the addition of aluminum dihydrogen phosphate may be insufficient, and when it contains more than 40 parts by weight of aluminum dihydrogen phosphate, the aluminum component may increase the crystallization of silica and cracks may occur in the insulating film. Specifically, based on the solid content, relative to a total of 100 parts by weight, the metal phosphate may contain 15 - 35 parts by weight of aluminum dihydrogen phosphate and 65 - 85 parts by weight of magnesium dihydrogen phosphate. More specifically, it may contain 20 - 30 parts by weight of aluminum dihydrogen phosphate and 70 - 80 parts by weight of magnesium dihydrogen phosphate.

[0051] In addition, in the case of a coating agent containing colloidal silica and metal phosphate as main components, when chromium oxide is excluded, as described above, problems such as surface adhesion caused by free phosphoric acid or precipitation in the form of powder will occur. Therefore, in the present invention, in order to solve such problems and to find a substance to replace the reaction between chromium oxide and free phosphoric acid as shown in formula (1), a wide range of metal oxides or hydroxides were applied and their effects were verified. The substances used include Cobalt phosphate hydrate, Nickel oxide, strontium peroxide, Iron oxide, Copper oxide, Manganese oxide, Cobalt hydroxide, Strontium hydroxide, Iron citratehydrate, Nickel hydroxide, Ammonium Ferric citrate, Germanium dioxide, Niobium oxide, Molybdenum oxide, Barium oxide, Lanthanum oxide, Tantalum oxide, Yttriumoxide, etc. It was confirmed that cobalt hydoroxide has the best effect in replacing chromium oxide in terms of improving the precipitation of free phosphoric acid and the denseness of the film.

[0052] The insulating film composition for grain-oriented electrical steel sheets of the present invention may contain 0.5 - 5 parts by weight of cobalt hydroxide based on the solid weight ratio with respect to 100 parts by weight of the metal phosphate. In particular, it may contain 2 parts by weight of cobalt hydroxide. When the insulating film composition for grain-oriented electrical steel sheets contains less than 0.5 part by weight or more than 5 parts by weight of cobalt hydroxide, the grain-oriented electrical steel sheets have problems in that their physical properties cannot be improved.

[0053] The powder can improve the denseness, shielding property and film tension of the film. Among them, improving the denseness means improving the corrosion resistance by increasing the denseness of the film, improving the shielding property means reducing the transparency to improve the color and shielding property, and improving the film tension means using the low coefficient of thermal expansion of the powder to reduce the coefficient of thermal expansion of the formed film and impart tensile stress to the material.

[0054] Based on the solid content, relative to 100 parts by weight of metal phosphate, the insulating film composition for grain-oriented electrical steel sheets may contain 20 - 170 parts by weight of powder. When the insulating film composition for grain-oriented electrical steel sheets contains less than 20 parts by weight of powder relative to 100 parts by weight of metal phosphate, it has the advantage that almost no pores are generated in the film after film formation. However, the film color is lighter and the transparency becomes higher, so the effect of shielding the primary film may be significantly reduced. In addition, the tension applied to the material through the film is tiny, so the effect of improving iron loss brought by the film tension cannot be expected. When the insulating film forming composition contains more than 170 parts by weight of powder relative to 100 parts by weight of metal phosphate, the solid fraction in the insulating film forming composition increases, and aggregation and sedimentation phenomena may occur between the powders, and pores are formed at the powder interface after film formation, thus significantly reducing the film tension and insulation. Therefore, specifically, based on the solid content, the insulating film composition for grain-oriented electrical steel sheets may contain 30 - 80 parts by weight of powder, and more specifically, it may contain 40 - 60 parts by weight of powder.

[0055] The powder may contain clay and carbide. Specifically, the clay may contain one or more components selected from Al, Si, Mg, Na, Ca, K, and Fe, and the carbide may be carbon (C). More specifically, the clay may be montmorillonite, and the carbide may be carbon black.

[0056] The clay may contain 15 - 45 wt% of Al, 40 - 70 wt% of Si, 0.1 - 5 wt% of Mg, and 1 - 10 wt% of Fe.

[0057] When the clay contains less than 15 wt% of Al, the film elastic modulus decreases, and the film tension effect may not be achieved. When the clay contains more than 45 wt% of Al, the conductivity increases and the film insulation decreases. When the clay contains less than 40 wt% of Si, the effect of increasing the film tension cannot be achieved due to the difference in thermal expansion coefficient. When the clay contains less than 70 wt% of Si, the film hardness increases and the workability of the material is significantly reduced. When the clay contains less than 0.1 wt% of Mg, the affinity between water and powder in the coating solution decreases, resulting in a decrease in the uniform dispersion of the powder in the coating solution. When the clay contains more than 5 wt% of Mg, the film elastic modulus decreases and the effect of increasing the film tension cannot be achieved. In addition, when the clay contains less than 1 wt% of Fe, the heat resistance characteristics after film formation decrease. When the clay contains more than 10 wt% of Fe, the powder specific gravity is higher, accelerating the sedimentation of the powder and reducing the corrosion resistance of the film.

[0058] Based on the total mass of the clay and the carbide, the clay can be 5 wt% to 95 wt%. When the proportion of the clay is less than 5 wt%, the transparency of the film increases, so the primary film shielding effect may be significantly reduced. When the proportion of the clay exceeds 95 wt%, the hardness of the film increases excessively, and the processability of the product may be reduced. Specifically, based on the total mass of the clay and the carbide, the proportion of the clay can be 7 wt% to 93 wt%.

[0059] The average particle size of the clay and the carbide can be 101 nm to 106 nm. When the average particle size of the clay and the carbide is less than 101 nm, it is difficult to be uniformly dispersed in the solution due to the electrostatic attraction of the particles themselves. When the average particle size of the clay and the carbide exceeds 106 nm, sedimentation occurs rapidly in the solution, so there is a problem that it is difficult to exhibit appropriate performance.

[0060] In addition to the above components, the insulating film composition for grain-oriented electrical steel sheets may further contain a solvent.

[0061] The method for preparing an insulating film composition for grain-oriented electrical steel sheets according to an embodiment of the present invention may include the following steps: heating while simultaneously adding a metal oxide, cobalt hydroxide, and powder in phosphoric acid (H3PO4) to prepare a first composition, wherein the first composition contains 0.5 - 5 parts by weight of cobalt hydroxide and 20 - 170 parts by weight of powder relative to 100 parts by weight of metal phosphate; and mixing 10 - 250 parts by weight of colloidal silica in the first composition.

[0062] Hereinafter, each step will be specifically described.

[0063] When the reaction between phosphoric acid and the metal oxide is carried out first and then cobalt hydroxide and powder are added, the viscosity of the metal phosphate increases sharply after the metal phosphate is prepared. Therefore, even if cobalt hydroxide and powder are introduced therein to induce mixing, cobalt hydroxide cannot be dissolved in the phosphate, and the powder cannot be uniformly dispersed in the metal phosphate, and a phenomenon of aggregation between the particles will occur.

[0064] In an embodiment of the present invention, the reaction between phosphoric acid and the metal oxide is carried out by simultaneously adding the metal oxide, cobalt hydroxide, and powder in phosphoric acid, and cobalt hydroxide and powder are added in a state where the reaction has not been carried out and the viscosity of the phosphoric acid is low. Therefore, with the flow induced by stirring, a very uniform phosphate / cobalt / powder mixed phase is formed, and it gradually becomes a highly viscous metal phosphate / cobalt / powder uniform mixed phase. Therefore, the dispersibility of the powder in the coating composition can be improved, and thus the shielding property of the film can be significantly improved. Hereinafter, the phosphate / cobalt / powder uniform mixed phase is referred to as the first composition.

[0065] In one embodiment of the present invention, a metal oxide, cobalt hydroxide, and powder are simultaneously added to phosphoric acid (H3PO4) and then heated to prepare a first composition. The first composition contains 0.5 - 5 parts by weight of cobalt hydroxide and 20 - 170 parts by weight of powder with respect to 100 parts by weight of metal phosphate.

[0066] In the step of preparing the first composition, the heating temperature can be 80 °C or higher. When the heating temperature is lower than 80 °C, the reaction of the metal oxide and cobalt hydroxide with phosphoric acid does not proceed smoothly, and it is difficult to form metal phosphate. In this case, even with stirring, the powder in the metal phosphate cannot form a uniform mixed phase, and there may be a phenomenon of aggregation between particles.

[0067] The descriptions of the metal oxide, metal phosphate, powder, the oxides and carbon-based compounds contained in the powder, and the descriptions of their contents have been given in the above-mentioned colored coating composition for forming an insulating film of an electrical steel sheet, so the repeated descriptions will be omitted.

[0068] In one embodiment of the present invention, 10 - 250 parts by weight of colloidal silica can be mixed in the first composition.

[0069] The descriptions of colloidal silica and chromium oxide and the descriptions of their contents have been given in the above-mentioned insulating film composition for grain-oriented electrical steel sheets, so the repeated descriptions will be omitted.

[0070] Colloidal silica is alkaline and metal phosphate is acidic. When they are mixed, a significant pH difference between the two components causes a temporary gelation phenomenon, which hinders the formation of a dense film.

[0071] In one embodiment of the present invention, cobalt hydroxide is used as a buffer to neutralize colloidal silica.

[0072] An electrical steel sheet according to one embodiment of the present invention may include: an electrical steel sheet substrate; and an insulating film located on one or both surfaces of the electrical steel sheet substrate. The surface roughness (Ra) of the insulating film can be 0.5 μm or less, and the film transparency can be 20% or less.

[0073] The electrical steel sheet substrate can be a non-oriented electrical steel sheet without a separately formed insulating film, a normal grain-oriented electrical steel sheet with a primary film, or a grain-oriented electrical steel sheet in a glass-free form without a primary film. In one embodiment of the present invention, the effects are exerted through the insulating film composition and the characteristics of the insulating film regardless of the composition of the electrical steel sheet substrate. A supplementary description of the composition of the grain-oriented electrical steel sheet substrate is as follows.

[0074] The substrate of the grain-oriented electrical steel sheet may contain: silicon (Si): 2.0 - 7.0 wt%, aluminum (Al): 0.020 - 0.040 wt%, manganese (Mn): 0.01 - 0.20 wt%, phosphorus (P): 0.01 - 0.15 wt%, carbon (C): less than 0.01 wt% (except 0%), N: 0.005 - 0.05 wt%, antimony (Sb), tin (Sn) or a combination thereof: 0.01 - 0.15 wt%, and the balance of Fe and other inevitable impurities. The description of each component of the substrate of the grain-oriented electrical steel sheet is the same as that of the generally known content, so the detailed description is omitted.

[0075] A metal oxide layer (base coating, primary film) formed by reacting with an annealing separating agent and the oxide layer of the steel sheet during the secondary recrystallization process may exist between the substrate of the grain-oriented electrical steel sheet and the insulating film. As an example of the metal oxide layer, there is a forsterite layer. During the manufacturing process of the grain-oriented electrical steel sheet, by suppressing the formation of the metal oxide layer or removing the metal oxide layer, the substrate of the grain-oriented electrical steel sheet and the insulating film can be in contact.

[0076] If the surface roughness of the insulating film is too high, the space factor increases when laminating materials for manufacturing transformers, and problems such as a decrease in transformer efficiency may occur. The insulating film formed from the insulating film composition for grain-oriented electrical steel sheets of the present invention can have excellent surface roughness. For example, the surface roughness (Ra) of the insulating film can be 0.5 μm or less.

[0077] In addition, the film transparency of the insulating film can be 20% or less. The film transparency is measured by reflectometry and is defined as the absolute intensity ratio (Ir / Ii) of the reflected light intensity (Ir) to the incident light intensity (Ii) of the specimen or the reciprocal of (1 / Rabs). That is, the film transparency is 1 / Rabs = Ir / Ii. When the transmittance of the film is high and most of the incident light is reflected, the film transparency approaches 100%. On the other hand, when the transmittance of the film is low and most of the incident light is absorbed or scattered by the film, the film transparency approaches 0%. When the film transparency is 20% or less, the film is opaque and most of the light source incident from the outside is absorbed or scattered by the film, so that the color of the specimen under the film cannot be distinguished, and thus it has an excellent shielding effect. On the other hand, when the film transparency is greater than 20%, the shielding property becomes poor and the original color of the specimen under the film can be distinguished, so the object of the present invention cannot be achieved. Specifically, the film transparency can be 19% or less, and more specifically, it can be 10% or less.

[0078] In the preparation step of the insulating film composition for grain-oriented electrical steel sheets of the present invention, by adjusting the mixing order and mixing amount of the clay and the carbide, the above film transparency value can be obtained.

[0079] By weight percentage, the insulating film may contain: P: 6.0 - 45%, Si: 11 - 80%, Cr: 0.5 - 7%, Ti: 3 - 30%, N: 1.5 - 25%, C: 1.5 - 25%, and the balance of oxygen and inevitable impurities.

[0080] The insulating film may contain 6.0 - 45 wt% of phosphorus (P). The phosphorus (P) may be derived from metal phosphates, carbides, and nitrides in the insulating film composition, or may diffuse from the electrical steel sheet substrate. When the content of phosphorus (P) is less than 6.0 wt%, there may be a problem of reduced adhesion of the coating agent, and when the content of phosphorus (P) exceeds 45 wt%, the film may become sticky after film formation. More specifically, the insulating film may contain 10 - 30 wt% of phosphorus (P).

[0081] The insulating film may contain 11 - 60 wt% of silicon (Si). The silicon (Si) may be derived from colloidal silica, carbides, and nitrides in the insulating film composition, or may diffuse from the electrical steel sheet substrate. When the content of silicon (Si) is less than 11 wt%, the film tension effect brought by silicon may be reduced, and when the content of silicon (Si) exceeds 60 wt%, the hardness of the film is too high, which may reduce the processability. More specifically, the insulating film may contain 20 - 40 wt% of silicon (Si).

[0082] The insulating film may contain 0.5 - 7 wt% of cobalt (Co). The cobalt (Co) may be derived from chromium oxide in the insulating film composition, or may diffuse from the electrical steel sheet substrate. When the content of cobalt (Co) is less than 0.5 wt%, there may be a problem of reduced corrosion resistance, and when the content of cobalt (Co) exceeds 7 wt%, the viscosity of the coating agent increases, and there may be problems with coatability. More specifically, the insulating film may contain 1 - 6 wt% of chromium (Cr).

[0083] The insulating film may contain 1.5 - 25 wt% of carbon (C). The carbon (C) may be derived from carbides and clay in the insulating film composition, or may diffuse from the electrical steel sheet substrate during the formation of the insulating film. When the content of carbon is less than 1.5 wt%, the transparency of the film increases, and there may be a problem of reduced shielding property, and when the content of carbon exceeds 25 wt%, there may be a problem of ash formation on the surface after film formation. More specifically, the insulating film may contain 5 - 15 wt% of carbon (C).

[0084] The insulating film may contain the balance of oxygen (O). Elements other than the above Si, P, Cr, and Fe may be further contained, and in this case, they may be contained in place of oxygen (O). Specifically, the insulating film may contain 35 - 75 wt% of oxygen.

[0085] The insulating film may further contain one or more of Al: 1.5 - 12% by weight or Mg: 4.5 - 34% by weight.

[0086] The above-mentioned Al, Mg, etc. may be derived from metal phosphates added to the insulating film composition, and B may be derived from boron added to the insulating film composition. When Al and Mg within the above ranges are further added, the adhesiveness can be further improved.

[0087] A method for manufacturing an electrical steel sheet according to an embodiment of the present invention includes the following steps: coating an insulating film composition on one or both sides of an electrical steel sheet substrate; and drying the steel sheet coated with the insulating film composition for the oriented electrical steel sheet to form an insulating film.

[0088] Before the coating step, the insulating film composition for the oriented electrical steel sheet may be stored at a temperature of 10 - 30°C. When stored at a temperature below 10°C, the viscosity increases, and it may be difficult to manage a uniform coating amount. When stored at a temperature exceeding 30°C, the gelation phenomenon of the insulating film composition for the oriented electrical steel sheet may be accelerated, thereby possibly reducing the surface quality. More specifically, the insulating film composition for the oriented electrical steel sheet may be stored at 15 - 25°C.

[0089] In the coating step, the insulating film composition for the oriented electrical steel sheet may be coated in the range of 0.5 - 6.0 g / m 2 When coating is performed at less than 0.5 g / m 2 , the tension imparted by the insulating film may be weakened. When coating is performed at more than 6.0 g / m 2 , the insulating film becomes too thick, and the adhesiveness to the steel sheet and the duty factor of the electrical steel sheet product may deteriorate. More specifically, the insulating film composition for the oriented electrical steel sheet may be coated in the range of 1.0 - 5.0 g / m 2 .

[0090] In the step of forming the insulating film, drying may be performed at 550 - 900°C for 10 - 50 seconds. When drying is performed outside the above temperature and time ranges, the phenomenon of reduced adhesiveness may occur due to the film not being dried, or the problem of oxidation discoloration caused by over-drying of the film may occur. Detailed Description

[0091] Examples

[0092] Hereinafter, specific examples of the present invention will be described in detail. However, these are merely examples, and the present invention is not limited thereto. The present invention is defined by the scope of the claims.

[0093] As a base material, an oriented electrical steel sheet (300×60 mm) is prepared. The oriented electrical steel sheet contains 3.1% Si by weight and has a primary film that is finally annealed at a plate thickness of 0.23 mm.

[0094] Prepare an insulating film composition for an oriented electrical steel sheet containing the components listed in Table 1 below and 100 parts by weight of water. The preparation method is as follows: After adding powder to cobalt hydroxide (A) or metal phosphates (B, C) without cobalt hydroxide, heat to prepare a first composition, then add colloidal silica to the first composition and mix to prepare, and then compare with a normal tension coating agent (D).

[0095] The powder used is a powder containing montmorillonite as clay and carbon black as carbide.

[0096] The prepared insulating film composition for an oriented electrical steel sheet is coated on the base material at 4 g / m 2 and then dried at 850 °C for 30 seconds. The corrosion resistance is evaluated as follows: Evaluate whether the test piece rusts during an 8-hour period in a 5%, 35 °C, NaCl solution. In this test, when the rusted area is 5% or less, it is marked as excellent; when it is 20% or less, it is marked as good; when it is 20 - 50%, it is marked as slightly poor; when it is 50% or more, it is marked as poor.

[0097] Evaluate the film physical properties of the test piece by the above evaluation method and show the results in Table 1.

[0098] [Table 1]

[0099]

[0100] : For the coating solution uniformity, judge whether the dispersion state of the powder in the solution is good / bad after the solution, and sort out the composition of the insulating film formed on the test piece and show it in Table 2 below.

[0101] [Table 2]

[0102]

[0103] [Table 3]

[0104]

[0105] For the corrosion resistance, evaluate and record the rusted area of the test piece in a 5 wt%, 35 °C, NaCl solution during an 8-hour period. For the film tension, remove the insulating film formed on one of the two sides, and use the tensile stress applied by the insulating film formed on the remaining surface to bend it in one direction, and measure the force required for the bending degree to evaluate the film tension caused by the insulating film.

[0106] The insulation property is measured by the received current value when a voltage of 0.5 V and a current of 1.0 A are applied under a pressure of 300 PSI (pounds per square inch).

[0107] The film transparency is measured by the reflected light measurement method.

[0108] As can be confirmed from Tables 1 to 3, when the composition of the insulating film is 15 - 45 wt%, Si is 40 - 70 wt%, Mg is 0.1 - 5 wt%, Fe is 1 - 10 wt%, and cobalt hydroxide is included, it can be confirmed that the insulation property, film shielding property, corrosion resistance, film tension, and the film tension after SRA are improved as a whole.

[0109] In particular, in the case of Invention Example 3, a powder containing 35 g of clay and 15 g of carbide is added. In this case, it can be confirmed that the film tension and film shielding rate before / after Stress Relieving Annealing (SRA) are very excellent not only compared to the Comparative Example but also compared to other Embodiments. No pores are generated in the film after SRA, so it is expected to improve the processability when manufacturing a transformer ( Figure 2 ). In addition, referring to Tables 1 and 3, compared with Comparative Example 5 and Comparative Example 6, it can be confirmed that the film tension of Comparative Example 7 and Comparative Example 8 is high. That is, as the ratio of clay to carbide in the powder increases, the film tension increases.

[0110] Referring to Table 3, in the case of Comparative Example 9 prepared by the existing preparation method D of the phosphate - based insulating film composition, it can be confirmed that the coating tension before / after Stress Relieving Annealing (SRA) is worse than that of the Invention Example of the present invention. It is confirmed that this is because the surface roughness is rough in the case of the phosphate - based coating agent, and the shrinkage of the film volume occurs due to the crystallization of silica during heat treatment.

[0111] Referring to Figure 1 and Figure 2 , it can be confirmed that when forming a secondary film on a test piece with a primary film including surface defects using the colored coating composition for forming an insulating film of the electrical steel sheet prepared in Invention Example 3, the color of the generated secondary film is very uniform and gorgeous, and the function of shielding the surface defects of the primary film is very excellent.

[0112] The present invention is not limited to the above - mentioned embodiments and can be manufactured in various forms different from each other. Those of ordinary skill in the technical field to which the present invention pertains can understand that the present invention can be implemented in other specific ways without changing the technical idea and essential features of the present invention. Therefore, it should be understood that the above - described embodiments are exemplary in all aspects and not restrictive.

Claims

1. An insulating film composition for grain-oriented electrical steel sheets, wherein, The composition comprises 10 - 250 parts by weight of colloidal silica, 0.5 - 5 parts by weight of cobalt hydroxide, and 20 - 170 parts by weight of powder relative to 100 parts by weight of metal phosphate.

2. The insulating film composition for grain-oriented electrical steel sheets according to claim 1, wherein, The metal phosphate is prepared by heating after adding metal oxide into phosphoric acid (H3PO4).

3. The insulating film composition for oriented electrical steel sheets according to claim 2, wherein, The metal oxide includes one or more of magnesium oxide (MgO) and aluminum oxide (Al2O3).

4. The insulating film composition for grain-oriented electrical steel sheets according to claim 1, wherein, The powder includes clay and carbide.

5. The insulating film composition for oriented electrical steel sheets according to claim 4, wherein, The clay contains one or more components selected from Al, Si, Mg, Na, Ca, K, and Fe, and the carbide is carbon (C).

6. The insulating film composition for grain-oriented electrical steel sheets according to claim 4, wherein, The clay accounts for 5 wt% to 95 wt% of the total weight of the powder.

7. The insulating film composition for grain-oriented electrical steel sheets according to claim 1, wherein, Based on solids, relative to a total of 100 parts by weight, the metal phosphate comprises 10 - 40 parts by weight of aluminum dihydrogen phosphate and 60 - 90 parts by weight of magnesium dihydrogen phosphate.

8. A method for preparing an insulating film composition for grain-oriented electrical steel sheet, which comprises the following steps: Simultaneously adding metal oxide, cobalt hydroxide, and powder into phosphoric acid (H3PO4) and then heating to prepare a first composition, which comprises 0.5 - 5 parts by weight of cobalt hydroxide and 20 - 170 parts by weight of powder relative to 100 parts by weight of metal phosphate; and Mixing 10 - 250 parts by weight of colloidal silica into the first composition.

9. The method for preparing an insulating film composition for an oriented electrical steel sheet according to claim 8, wherein, The heating temperature is 80 °C or higher.

10. An electrical steel sheet, comprising: an electrical steel sheet Substrate; And an insulating film located on one or both surfaces of the electrical steel sheet substrate, The surface roughness (Ra) of the insulating film is 0.5 μm or less, and the film transparency is 20% or less.

11. A method for manufacturing an electrical steel sheet, which comprises the following steps: Coating the insulating film composition for grain-oriented electrical steel sheet prepared by the method according to claim 1 on one or both surfaces of the electrical steel sheet substrate; And Drying the steel sheet coated with the insulating film composition for grain-oriented electrical steel sheet to form an insulating film.

12. The manufacturing method of the electrical steel sheet according to claim 11, wherein, Before the coating step, it further includes the step of storing the insulating film composition for grain-oriented electrical steel sheet at a temperature of 10 - 30 °C.

13. The manufacturing method of the electrical steel sheet according to claim 11, wherein, In the coating step, the insulating film composition for the grain-oriented electrical steel sheet is coated in the range of 0.5 - 6.0 g / m 2 .

14. The method for manufacturing an electrical steel sheet according to claim 11, wherein, In the step of forming the insulating film, drying is carried out at 550 - 900 °C for 10 - 50 seconds.

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