Insulating coating composition for grain-oriented electrical steel sheet, method for producing same, grain-oriented electrical steel sheet having an insulating coating formed on its surface using the same, and method for producing same

Abstract

Provided are an insulating coating composition for grain-oriented electrical steel sheets that has coating strength and insulating properties, a method for producing the same, and grain-oriented electrical steel sheets using the same. [Solution] The insulating coating composition for grain-oriented electrical steel sheet of the present invention contains 100 parts by weight of metal phosphate, 10 to 250 parts by weight of colloidal silica, 0.5 to 5 parts by weight of cobalt hydroxide, and 20 to 170 parts by weight of powder. The production method includes the steps of simultaneously adding the metal oxide, cobalt hydroxide, and powder to phosphoric acid (H3PO4) and heating the mixture to produce a first composition containing 0.5 to 5 parts by weight of cobalt hydroxide and 20 to 170 parts by weight of powder, per 100 parts by weight of the metal phosphate, and mixing 10 to 250 parts by weight of colloidal silica with the first composition.

Description

[Technical Field]

[0001] The present invention relates to an insulating coating composition for grain-oriented electrical steel sheet, a manufacturing method thereof, a grain-oriented electrical steel sheet having an insulating coating formed on its surface using the same, and a manufacturing method thereof. More specifically, the present invention relates to an insulating coating composition for grain-oriented electrical steel sheet having excellent coating strength and insulating properties, as well as excellent gloss and a beautiful hue, a manufacturing method thereof, a grain-oriented electrical steel sheet having an insulating coating formed on its surface using the same, and a manufacturing method thereof. [Background technology]

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

[0003] It is known that reducing the iron loss of grain-oriented electrical steel sheets and improving their insulation properties can further improve their magnetic properties. Currently, commercial grain-oriented electrical steel sheets aim to reduce iron loss by applying tensile stress to the steel sheet by utilizing the difference in thermal expansion coefficients between the steel sheet and the insulating coating (secondary coating) formed on the forsterite-based base coating (primary coating). In this regard, active research is being conducted into forming a high-tensile insulating coating on the surface as one way to reduce iron loss in grain-oriented electrical steel sheets.

[0004] In particular, methods of forming primary coatings and insulating coatings have been proposed to improve surface properties or impart surface tension. A known primary coating is a forsterite (2MgO·SiO2) layer formed during high-temperature annealing, which is formed by a reaction between silicon oxide (SiO2) formed on the surface of electrical steel sheet during the primary recrystallization annealing process and magnesium oxide (MgO), which is used as an annealing separator. This primary coating formed during high-temperature annealing must have a uniform color without any defects in appearance. Functionally, it prevents fusion between sheets in the coil state and imparts tensile stress to the sheet due to the difference in thermal expansion coefficients between the sheet and the primary coating, thereby improving the core loss of the sheet.

[0005] This primary coating must have a uniform color without defects on the material surface. However, since commercial products are manufactured in large coils, it is very difficult to maintain uniform coating properties throughout the entire length of the coil. The main reason for this is the annealing separator, which contains water.

[0006] That is, during the process in which magnesium oxide slurry is applied to a steel sheet, wound into a coil, and subjected to a final high-temperature annealing process, the amount of heat transferred from the annealing furnace to the coil varies, which causes variations in the amount of hydrated water discharged depending on the position and location of the coil. As a result, oxidation defects, known as bare spots, which are a type of defect caused by the rapid discharge of hydrated water from the coil, mainly occur in the outermost winding. In particular, in the case of a secondary coating formed using a conventional phosphate-based tension coating agent, defects in the primary coating are exposed due to insufficient shielding properties due to its high transparency. Therefore, in mass production, exposed defective areas are discarded before shipping, which increases the unit price of the product.

[0007] The coils, which are coated with the secondary tension coating, are supplied in hoop form after being slit to the appropriate size, and are then manufactured into stacked core transformers or wound core transformers depending on the application. In addition, inter-plate insulation is extremely important for improving efficiency when manufacturing transformers, and generally, increasing the thickness of the coating improves insulation, but reduces no-load loss, which may ultimately result in a decrease in transformer efficiency.

[0008] To solve these problems and improve the film tension and insulation, insoluble materials such as powders are typically added to the coating material, but the addition of insoluble materials leads to technical problems such as reduced roughness and gloss, and the generation of voids in the film after film formation.

[0009] Therefore, there is a need for a colored secondary coating product that improves oxidation defects that occur in the primary coating and has excellent insulation properties even when the coating is thin. However, grain-oriented electrical steel sheet coating agents generally do not meet these requirements, and commercialization technology is currently required.

[0010] Meanwhile, in response to the recent tightening of environmental regulations, active development of coating agents that do not contain chromium oxide has been promoted for electrical steel sheets, but in the case of coating agents for directional electrical steel sheets, colloidal silica having functional groups is introduced to compensate for the weakening of corrosion resistance and adhesion due to the absence of chromium oxide. However, directional coating agents based on phosphate or colloidal silica still have limitations in overcoming the weakening of corrosion resistance and adhesion due to the absence of chromium oxide.

[0011] In the case of chromium-free tension coating agents for grain-oriented electrical steel sheets, two methods have been proposed: one is to introduce colloidal silica modified with Fe, Al, Ga, Ti, etc., as disclosed in Japanese Patent Laid-Open No. 2007-23329; and the other is to introduce oxides of Fe, Co, Cu, etc., to improve corrosion resistance and coating tension, as disclosed in Korean Patent Publication No. 10-2008-0025733. However, the former method requires a fairly complicated process of modifying colloidal silica by reacting it with Fe, Al, etc., which has many disadvantages in terms of production costs and is not effective enough, making it difficult to apply in industry. On the other hand, the latter method is easier to use than the former, but the introduced oxides merely prevent the generation of free phosphoric acid when the coating agent dries, thereby incidentally improving coating density and coating tension. Therefore, it has limitations in meeting the requirements for high-grade grain-oriented electrical steel sheets, which generally require high corrosion resistance and coating tension. Therefore, the current situation is that no commercial technology has been proposed for a directional non-chromium coating agent that is satisfactory in all physical properties. Summary of the Invention [Problem to be solved by the invention]

[0012] An object of the present invention is to provide an insulating coating composition for grain-oriented electrical steel sheet that has excellent coating strength and insulating properties, as well as excellent gloss and a beautiful hue, a method for producing the same, and grain-oriented electrical steel sheet using the same. [Means for solving the problem]

[0013] According to one finding of the present invention, there is provided an insulating coating composition for grain-oriented electrical steel sheet, which comprises 100 parts by weight of metal phosphate, 10 to 250 parts by weight of colloidal silica, 0.5 to 5 parts by weight of cobalt hydroxide, and 20 to 170 parts by weight of powder.

[0014] According to another finding of the present invention, there is provided a method for producing an insulating coating composition for grain-oriented electrical steel sheet, the method comprising the steps of: simultaneously adding a metal oxide, cobalt hydroxide, and powder to phosphoric acid (H3PO4) and then heating the mixture to produce a first composition containing 0.5 to 5 parts by weight of cobalt hydroxide and 20 to 170 parts by weight of powder per 100 parts by weight of metal phosphate; and mixing 10 to 250 parts by weight of colloidal silica with the first composition.

[0015] According to another finding of the present invention, there is provided a grain-oriented electrical steel sheet having an insulating coating formed on its surface using the insulating coating composition of the present invention, and a method for producing the same. [Effects of the Invention]

[0016] The present invention can not only improve the coating tension and insulation properties of grain-oriented electrical steel sheets, but also effectively provide grain-oriented electrical steel sheets having excellent magnetic properties.

[0017] The present invention can impart a beautiful roughness to the insulating coating of a grain-oriented electrical steel sheet, thereby imparting excellent luster and a beautiful color hue. [Brief explanation of the drawings]

[0018] [Figure 1] This is a photograph taken for evaluating the corrosion resistance of a conventional grain-oriented electrical steel sheet. [Figure 2]1 is a photograph taken for evaluating the corrosion resistance of an electrical steel sheet according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0019] While the present invention may be embodied in various different forms, it is not intended to be limited to the embodiments set forth herein, and the scope of the present invention is not limited to the embodiments set forth herein.

[0020] Generally, the main components of a chromium-based insulating coating composition are colloidal silica, which imparts coating tension, a coating agent, a metal phosphate, which imparts adhesion to the steel sheet interface, chromium oxide, and trace amounts of other additives to reinforce functionality. However, if chromium oxide is removed from this composition, the drying speed decreases, which in turn reduces the insulating properties and the ability of the coating to impart tension.

[0021] In chromium-based insulating coating compositions, hexavalent chromium (Cr 6+ ) reacts with the water present in the coating agent to form the H2CrO4 compound, which then reacts with the Fe present in the steel sheet to produce FeO. This creates a state in which FeO and metal phosphate can react. This action is very useful in improving the adhesion between the steel sheet and the coating agent, which in turn helps to improve the film tension. In addition, hexavalent chromium (Cr 6+ ) also reacts with the generated FeO to form trivalent chromium (Cr 3+ ), which can be reduced to trivalent chromium (Cr 3+ The condensation polymerization of (C) can increase the density of the coating, which has a great effect on improving the coating tension.

[0022] In an insulating coating forming composition containing chromium (Cr), chromium oxide not only suppresses the generation of free phosphate that causes poor adhesion after the coating is dried, but also improves the reactivity of phosphate even at low temperatures, as shown in the following formula (1):

[0023] CrO3+2H3PO4→Cr(PO4)2+6H2O ···(1)

[0024] Therefore, it is necessary to solve the problem of the deterioration of corrosion resistance and adhesion of insulating coating-forming compositions that do not contain chromium oxide.

[0025] The present invention solves the problems of moisture absorption and reduced corrosion resistance that can occur with chromium-free insulating coating compositions that contain phosphate by incorporating chromium hydroxide and nano- or micro-sized fine carbon black and clay. The present invention provides a chromium-free insulating coating composition for grain-oriented electrical steel sheets, which is based on metal phosphate with added cobalt hydroxide and colloidal silica and contains nano- or micro-sized fine carbon black and clay powder, thereby achieving beautiful colored surface characteristics and excellent insulation even after being processed to a thinner thickness than normal coating thickness; a method for manufacturing the same; and a grain-oriented electrical steel sheet having an insulating coating formed on its surface using the same and a method for manufacturing the same.

[0026] After surface coating using an insulating coating composition for grain-oriented electrical steel sheets containing a large amount of phosphate, moisture absorption, dust generation, and a decrease in corrosion resistance occur over time due to free phosphate. Therefore, in order to reduce surface defects caused by free phosphate, it goes without saying that pure phosphate and metal oxide must be prepared in an appropriate molar ratio, and the proportion of phosphate in the insulating coating composition is very important.

[0027] As explained above, in the present invention, it was confirmed that free phosphoric acid can be suppressed by adding 0.5 to 5 parts by weight of cobalt hydroxide on a solids basis to 100 parts by weight of metal phosphate, and that a dense coating can be formed by adding 20 to 170 parts by weight of powder. Ultimately, it was revealed that cobalt hydroxide and powder take over the function of suppressing free phosphoric acid generation due to the reaction between chromium oxide and phosphoric acid.

[0028] An insulating coating composition for grain-oriented electrical steel sheet according to one embodiment of the present invention may contain 100 parts by weight of metal phosphate, 10 to 250 parts by weight of colloidal silica, 0.5 to 5 parts by weight of cobalt hydroxide, and 20 to 170 parts by weight of powder.

[0029] Colloidal silica increases the strength and hardness of the coating itself through an intramolecular network reaction when the coating dries after coating, thereby applying tension to the material. Various colloidal silicas can be used without limitation as the colloidal silica, and for example, commercially available colloidal silica can be used. More specifically, basic colloidal silica can be used.

[0030] Generally, insulating coating compositions for grain-oriented electrical steel sheets contain both chromium oxide to reinforce corrosion resistance after coating formation and solid silica and the like to adjust the roughness and coatability of the coating.

[0031] The insulating coating composition for grain-oriented electrical steel sheets may contain, on a solids basis, 10 to 250 parts by weight of colloidal silica per 100 parts by weight of metal phosphate. If the colloidal silica content is less than 10 parts by weight, the tensile strength-improving effect of the colloidal silica may not be fully achieved, while if the colloidal silica content is more than 250 parts by weight, the amount of metal phosphate is relatively reduced, resulting in poor adhesion of the insulating coating. More specifically, the insulating coating composition for grain-oriented electrical steel sheets may contain, on a solids basis, 100 to 200 parts by weight, or 125 to 175 parts by weight, of colloidal silica per 100 parts by weight of metal phosphate.

[0032] During the drying process of the coating, colloidal silica undergoes a condensation reaction due to a chain reaction of silica as shown in formula (2) below, forming a network structure such as -(HO-Si-O-Si)-n.

[0033] -(HO-Si-OH-)n+-(HO-Si-OH-)n=-(HO-Si-O-Si-)n+H2O···(2)

[0034] However, insulating coating-forming compositions for electrical steel sheets that contain colloidal silica can form an excessively uniform network structure, which can limit the density of the coating. Therefore, there is a limit to the adhesion and corrosion resistance between the electrical steel sheet material and the insulating coating-forming composition, and metal phosphate and chromium oxide can be added to reinforce this.

[0035] Colloidal silica can have a particle size of 7 to 40 nm. If the particle size is less than 7 nm, compatibility with other components in the coating agent decreases, which can lead to an increase in viscosity of the coating agent. If the particle size is more than 40 nm, the coating may become less dense, which can reduce corrosion resistance.

[0036] The solid fraction of colloidal silica can be 25 to 55 wt%. If the solid fraction is less than 25 wt%, the insulating properties may be reduced, and if the solid fraction is more than 55 wt%, the compatibility of the coating agent after preparation may be reduced. More specifically, the solid fraction of colloidal silica can be 45 to 50 wt%.

[0037] Colloidal silica Na + The content can be 0.1 to 1.0% by weight. + If the content is less than 0.1 wt%, the density of the coating may decrease. + If the content exceeds 1.0 wt %, the increase in cations in the insulating coating composition may hinder the compatibility between the components. + The content can be 0.3 to 0.7% by weight.

[0038] The pH of the colloidal silica may be 9.5 to 10.5. If the pH is less than 9.5 or more than 10.5, the pH difference between the components other than the colloidal silica in the insulating coating film-forming composition becomes too great, which may result in phase separation. More specifically, the pH may be 9.5 to 10.0.

[0039] The viscosity of colloidal silica can be 3.5 to 6.5 cp. If the viscosity is less than 3.5 cp, problems with the application of the coating agent can occur, while if it exceeds 6.5 cp, the viscosity can increase over time, causing problems with aging. More specifically, the viscosity of colloidal silica can be 4 to 6 cp. The viscosity of colloidal silica can be measured at 20°C using a Brookfield viscometer based on a solids content of 30% by weight of colloidal silica.

[0040] The specific gravity of the colloidal silica may be 1.1 to 1.3. If the specific gravity is less than 1.1, it becomes difficult to control the amount of the insulating coating-forming composition applied, and if the specific gravity exceeds 1.3, sedimentation may occur after the insulating coating-forming composition is prepared. More specifically, the specific gravity of the colloidal silica may be 1.15 to 1.25.

[0041] Metal phosphates act as binders in insulating coating compositions. Metal phosphates are essentially manufactured by adding metal oxide to pure phosphoric acid (H3PO4) and reacting it at 80°C or higher. To improve the adhesion of the metal phosphate, boric acid is added during the reaction and maintained for 3 hours or more, which induces a condensation reaction between magnesium metal phosphate and boric acid. The manufactured metal phosphate is highly acidic.

[0042] The metal phosphate may contain 50 to 70% by weight of solids. If the solids content is less than 50% by weight, the amount of free phosphoric acid in the metal phosphate increases, which may cause surface moisture absorption after production of the metal phosphate. If the solids content exceeds 70% by weight, the solids content may be excessive relative to the pure phosphoric acid, which may cause reaction failure and precipitation.

[0043] The metal phosphates and metal oxides can include a variety of metals without limitation.

[0044] Specifically, the metal oxide may include one or more of magnesium oxide (MgO) and aluminum oxide (Al2O3). More specifically, the metal oxide may include magnesium oxide (MgO) and aluminum oxide (Al2O3).

[0045] Specifically, the metal phosphate may include one or more of monomagnesium phosphate (Mg(H2PO4)2) and monoaluminum phosphate (Al(H2PO4)3). More specifically, the metal phosphate may include monomagnesium phosphate (Mg(H2PO4)2) and monoaluminum phosphate (Al(H2PO4)3).

[0046] In this case, the metal phosphate may contain 10 to 40 parts by weight of aluminum monophosphate and 60 to 90 parts by weight of magnesium monophosphate, based on the total weight of 100 parts by weight of solids. If the metal phosphate contains less than 10 parts by weight of aluminum monophosphate, the tensile strength improvement effect of the addition of aluminum monophosphate is insufficient. If the metal phosphate contains more than 40 parts by weight of aluminum monophosphate, the aluminum component may increase silica crystallization, potentially causing cracks in the insulating coating. Specifically, the metal phosphate may contain 15 to 35 parts by weight of aluminum monophosphate and 65 to 85 parts by weight of magnesium monophosphate, based on the total weight of 100 parts by weight of solids, and more specifically, 20 to 30 parts by weight of aluminum monophosphate and 70 to 80 parts by weight of magnesium phosphate.

[0047] On the other hand, in the case of a coating agent mainly composed of colloidal silica and metal phosphate, when chromium oxide is removed, problems of surface adhesion or powder precipitation due to free phosphoric acid occur, as mentioned above. Therefore, in the present invention, in order to solve these problems and find a substance that can replace the reaction between chromium oxide and free phosphoric acid as shown in formula (1), a wide range of metal oxides or hydroxides were used and their effects were verified. The materials used include cobalt phosphate hydrate, nickel oxide, strontium peroxide, iron oxide, copper oxide, manganese oxide, cobalt hydroxide, strontium hydroxide, iron citrate hydrate, nickel hydroxide, ammonium ferric citrate, germanium dioxide, niobium oxide, molybdenum oxide, barium oxide, lanthanum oxide, tantalum oxide, and yttrium oxide, as well as cobalt hydroxide. It was confirmed that chromium hydroxide (H2O3) was used instead of chromium oxide and had the best effect of improving the deposition of free phosphoric acid and the density of the coating.

[0048] The insulating coating composition for grain-oriented electrical steel sheet of the present invention may contain 0.5 to 5 parts by weight, particularly 2 parts by weight, of cobalt hydroxide in terms of solid weight ratio per 100 parts by weight of metal phosphate. If the insulating coating composition for grain-oriented electrical steel sheet contains less than 0.5 part by weight or more than 5 parts by weight of cobalt hydroxide, there is a problem in that the physical properties of the grain-oriented electrical steel sheet cannot be improved.

[0049] The powder can improve the density, shielding properties, and tension of the coating. Here, "improving density" means increasing the density of the coating to improve corrosion resistance, "improving shielding properties" means decreasing transparency to improve color and shielding performance, and "improving coating tension" means decreasing the thermal expansion coefficient of the coating formed using the powder's low thermal expansion coefficient to impart tensile stress to the material.

[0050] The insulating coating composition for grain-oriented electrical steel sheets may contain 20 to 170 parts by weight of powder (solids content) per 100 parts by weight of metal phosphate. When the insulating coating composition for grain-oriented electrical steel sheets contains less than 20 parts by weight of powder per 100 parts by weight of metal phosphate, there is an advantage in that almost no voids are generated in the formed coating. However, the coating color becomes lighter and more transparent, which may significantly reduce the shielding effect of the primary coating. Furthermore, the tension applied to the substrate by the coating is minimal, and the effect of coating tension on improving iron loss cannot be expected. When the insulating coating composition contains more than 170 parts by weight of powder per 100 parts by weight of metal phosphate, the solid fraction in the insulating coating composition becomes high, which may cause aggregation and sedimentation between powder particles. Furthermore, voids may form at the powder interfaces after the coating is formed, significantly reducing the coating tension and insulating properties. Specifically, the insulating coating composition for grain-oriented electrical steel sheet may contain 30 to 80 parts by weight, more specifically 40 to 60 parts by weight of powder based on the solid content.

[0051] The powder may include clay and carbide, specifically, the clay may include one or more components selected from the group consisting of 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.

[0052] The clay may contain 15 to 45 wt% Al, 40 to 70 wt% Si, 0.1 to 5 wt% Mg, and 1 to 10 wt% Fe.

[0053] If the clay contains less than 15% Al by weight, the coating modulus is low and no coating tension effect is observed; if it contains more than 45% by weight, electrical conductivity is high and the coating insulation properties are reduced. If the clay contains less than 40% Si by weight, the coating tension improvement effect is not observed due to the difference in thermal expansion coefficients. If it contains less than 70% by weight, the coating hardness is high and the material workability is significantly reduced. If the clay contains less than 0.1% Mg by weight, the affinity between the water and powder in the coating solution is reduced, reducing the uniform dispersion of the powder in the coating solution. If it contains more than 5% by weight, the coating modulus is reduced and no coating tension improvement effect is observed. Furthermore, if the clay contains less than 1% Fe by weight, the heat resistance properties after coating formation are reduced. If it contains more than 10% Fe by weight, the specific gravity of the powder is increased, accelerating powder sedimentation and reducing the corrosion resistance of the coating.

[0054] The clay may be present in an amount of 5 to 95% by weight based on the total weight of the clay and carbide. If the clay content is less than 5% by weight, the transparency of the coating increases, significantly reducing the shielding effect of the primary coating. If the clay content exceeds 95% by weight, the hardness of the coating may increase excessively, potentially reducing the processability of the product. Specifically, the clay content may be present in an amount of 7 to 93% by weight based on the total weight of the clay and carbide.

[0055] The average particle size of the clay and carbide may be 101 nm to 106 nm. If the average particle size of the clay and carbide is less than 101 nm, it is difficult to uniformly disperse them in the solution due to the electrostatic attraction of the particles themselves, and if it exceeds 106 nm, rapid sedimentation occurs in the solution, making it difficult to achieve appropriate performance.

[0056] In addition to the above-mentioned components, the insulating coating composition for grain-oriented electrical steel sheets may further contain a solvent.

[0057] A method for manufacturing an insulating coating composition for grain-oriented electrical steel sheet according to an embodiment of the present invention may include the steps of simultaneously adding a metal oxide, cobalt hydroxide, and powder to phosphoric acid (H3PO4) and heating the mixture to prepare a first composition containing 0.5 to 5 parts by weight of cobalt hydroxide and 20 to 170 parts by weight of powder, based on 100 parts by weight of metal phosphate, and mixing 10 to 250 parts by weight of colloidal silica with the first composition.

[0058] Each stage will be explained in detail below.

[0059] If the reaction between phosphoric acid and a metal oxide is carried out first and then cobalt hydroxide and powder are added, the viscosity of the metal phosphate increases rapidly after the metal phosphate is produced. Therefore, even if cobalt hydroxide and powder are introduced and mixed, the cobalt hydroxide does not dissolve in the phosphate, and the powder does not disperse uniformly in the metal phosphate, resulting in a phenomenon in which particles aggregate with each other.

[0060] In one embodiment of the present invention, a metal oxide, cobalt hydroxide, and powder are simultaneously added to phosphoric acid to allow the reaction between the phosphoric acid and the metal oxide to proceed. Because the cobalt hydroxide and powder are added when the reaction is not yet underway and the phosphoric acid has a low viscosity, a highly uniform phosphate / cobalt / powder mixture is formed due to the flow induced by stirring. This gradually progresses to a highly viscous metal phosphate / cobalt / powder mixture, thereby improving the dispersibility of the powder in the coating composition and significantly improving the barrier properties of the coating. Hereinafter, this phosphate / cobalt / powder mixture is referred to as the first composition.

[0061] 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 produce a first composition containing 0.5 to 5 parts by weight of cobalt hydroxide and 20 to 170 parts by weight of powder per 100 parts by weight of metal phosphate.

[0062] In the step of preparing the first composition, the heating temperature may be 80° C. or higher. If the heating temperature is lower than 80° C., the reaction between the metal oxide and cobalt hydroxide and phosphoric acid is not smooth, making it difficult to form the metal phosphate. In this situation, even if stirring is carried out, the powder may not form a uniform mixed phase in the metal phosphate, and the particles may aggregate.

[0063] The metal oxides, metal phosphates, powders, oxides and carbon-based compounds contained in the powders, and their contents have been explained in relation to the colored coating composition for forming an insulating coating on an electrical steel sheet, and therefore, redundant explanations will be omitted.

[0064] In one embodiment of the present invention, 10 to 250 parts by weight of colloidal silica may be mixed into the first composition.

[0065] The colloidal silica and chromium oxide and their contents have been explained in relation to the insulating coating composition for grain-oriented electrical steel sheets, and therefore, redundant explanations will be omitted.

[0066] Colloidal silica becomes basic and metal phosphate becomes acidic, and when they are mixed together, a temporary gelling phenomenon occurs due to the significant pH difference between the two components, which prevents the formation of a dense coating.

[0067] In one embodiment of the present invention, cobalt hydroxide is used as a buffering agent to serve to neutralize the colloidal silica.

[0068] An electrical steel sheet according to one embodiment of the present invention includes an electrical steel sheet substrate and an insulating coating disposed on one or both sides of the electrical steel sheet substrate, and the insulating coating may have a surface roughness (Ra) of 0.5 μm or less and a coating transparency of 20% or less.

[0069] The electrical steel sheet substrate may be a non-oriented electrical steel sheet without a separate insulating coating, a regular grain-oriented electrical steel sheet with a primary coating, or a glass-less grain-oriented electrical steel sheet without a primary coating. In one embodiment of the present invention, the effects are achieved by the composition of the insulating coating and the characteristics of the insulating coating, regardless of the composition of the electrical steel sheet substrate. The composition of the grain-oriented electrical steel sheet substrate will be described below.

[0070] The grain-oriented electrical steel sheet substrate contains 2.0 to 7.0 wt% silicon (Si), 0.020 to 0.040 wt% aluminum (Al), 0.01 to 0.20 wt% manganese (Mn), 0.01 to 0.15 wt% phosphorus (P), 0.01 wt% or less (excluding 0%) carbon (C), 0.005 to 0.05 wt% N, and 0.01 to 0.15 wt% antimony (Sb), tin (Sn), or a combination thereof, with the balance being Fe and other unavoidable impurities. The description of each component of the grain-oriented electrical steel sheet substrate is the same as that generally known, and therefore a detailed description will be omitted.

[0071] A metal oxide layer (base coating layer, primary coating) formed by reaction between the annealing separator and the oxide layer of the steel sheet during the secondary recrystallization process may exist between the grain-oriented electrical steel sheet substrate and the insulating coating. An example of a metal oxide layer is a forsterite layer. It is also possible to suppress the formation of the metal oxide layer or remove the metal oxide layer during the manufacturing process of the grain-oriented electrical steel sheet so that the grain-oriented electrical steel sheet substrate and the insulating coating come into contact with each other.

[0072] If the surface roughness of the insulating coating is too high, the space factor increases during lamination of materials for transformer manufacturing, which can lead to problems such as reduced transformer efficiency. The insulating coating formed from the insulating coating composition for grain-oriented electrical steel sheet of the present invention can have excellent surface roughness. For example, the insulating coating can have a surface roughness (Ra) of 0.5 μm or less.

[0073] Furthermore, the insulating coating may have a coating transparency of 20% or less. Coating 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) on the test piece, or the reciprocal of (1 / Rabs). That is, coating transparency 1 / Rabs = Ir / Ii. If the coating has high transmittance and almost all of the incident light is reflected, the coating transparency approaches 100%. On the other hand, if the coating has low transmittance and almost all of the incident light is absorbed or scattered by the coating, the coating transparency approaches 0%. When the coating transparency is 20% or less, the coating is opaque and almost all of the light incident from the outside is absorbed or scattered by the coating, preventing the original color of the test piece from being discerned, resulting in excellent shielding effect. Conversely, when the coating transparency exceeds 20%, the coating exhibits poor shielding properties and allows the original color of the test piece to be discerned, failing to achieve the objectives of the present invention. Specifically, the coating transparency can be 19% or less, and more specifically 10% or less.

[0074] In the manufacturing process of the insulating coating composition for grain-oriented electrical steel sheet of the present invention, the above-mentioned coating transparency value can be obtained by adjusting the mixing order and mixing amounts of clay and carbide.

[0075] The insulating coating may contain, by weight, 6.0 to 45% P, 11 to 80% Si, 0.5 to 7% Cr, 3 to 30% Ti, 1.5 to 25% N, 1.5 to 25% C, and the remainder being oxygen and unavoidable impurities.

[0076] The insulating coating may contain 6.0 to 45% by weight of phosphorus (P). Phosphorus (P) may originate from metal phosphates, carbides, and nitrides in the insulating coating composition, or may diffuse from the electrical steel sheet substrate. If the phosphorus (P) content is less than 6.0% by weight, the coating may have poor adhesion, while if it is more than 45% by weight, the coating may become sticky after formation. More specifically, the insulating coating may contain 10 to 30% by weight of phosphorus (P).

[0077] The insulating coating may contain 11 to 60% by weight of silicon (Si). Silicon (Si) may originate from colloidal silica, carbides, and nitrides in the insulating coating composition, or may diffuse from the electrical steel sheet substrate. If the silicon (Si) content is less than 11% by weight, the coating tension effect of silicon may be reduced, while if it is more than 60% by weight, the coating may be too hard and its workability may be reduced. More specifically, the insulating coating may contain 20 to 40% by weight of silicon (Si).

[0078] The insulating coating may contain 0.5 to 7 wt. % cobalt (Co). Cobalt (Co) may originate from chromium oxide in the insulating coating composition or may diffuse from the electrical steel sheet substrate. If the cobalt (Co) content is less than 0.5 wt. %, corrosion resistance may be reduced, while if it exceeds 7 wt. %, the viscosity of the coating agent may increase, causing problems with application. More specifically, the insulating coating may contain 1 to 6 wt. % chromium (Cr).

[0079] The insulating coating may contain 1.5 to 25% by weight of carbon (C). The carbon (C) may originate from carbides and clay in the insulating coating composition or may diffuse from the electrical steel sheet substrate during the insulating coating formation process. If the carbon content is less than 1.5% by weight, the coating may become too transparent, resulting in poor shielding properties. If the carbon content is more than 25% by weight, ash may form on the surface after the coating is formed. More specifically, the insulating coating may contain 5 to 15% by weight of carbon (C).

[0080] The insulating coating may contain the remainder oxygen (O). In addition to the above-mentioned Si, P, Cr, and Fe, other elements may be included, in which case they are included instead of oxygen (O). Specifically, the insulating coating may contain 35 to 75 wt % oxygen.

[0081] The insulating coating may further contain one or more of Al: 1.5 to 12 wt % and Mg: 4.5 to 34 wt %.

[0082] The aforementioned Al, Mg, etc. can be derived from metal phosphates added to the insulating coating composition, and B can be derived from boron added to the insulating coating composition. Adding additional Al and Mg within the aforementioned ranges can further improve adhesion.

[0083] A method for manufacturing an electrical steel sheet according to an embodiment of the present invention includes the steps of applying an insulating coating composition to one or both surfaces of an electrical steel sheet substrate, and drying the steel sheet coated with the insulating coating composition for grain-oriented electrical steel sheet to form an insulating coating.

[0084] Prior to the application step, the insulating coating composition for grain-oriented electrical steel sheets can be stored at a temperature of 10 to 30°C. If stored at a temperature below 10°C, the viscosity increases, making it difficult to control a uniform amount of coating, while if stored at a temperature above 30°C, the gelation of the insulating coating composition for grain-oriented electrical steel sheets can be accelerated, potentially resulting in a deterioration in surface quality. More specifically, the insulating coating composition for grain-oriented electrical steel sheets can be stored at a temperature of 15 to 25°C.

[0085] In the coating step, the insulating coating composition for grain-oriented electrical steel sheet is applied in an amount of 0.5 to 6.0 g / m 2 It can be applied in the range of 0.5g / m 2 If applied at less than 6.0 g / m, the tension provided by the insulating coating will be weak, and 2 If the amount is exceeded, the insulating coating becomes too thick, which may result in poor adhesion to the steel sheet and poor space factor in the electrical steel sheet product. More specifically, the insulating coating composition for grain-oriented electrical steel sheet is applied in an amount of 1.0 to 5.0 g / m 2 It can be applied in the range of

[0086] In the step of forming the insulating coating, drying can be performed at 550 to 900°C for 10 to 50 seconds. If the drying temperature and time are outside the above ranges, the coating may not be dried properly, resulting in a decrease in adhesion, or the coating may be overdried, resulting in oxidation and discoloration. [Example]

[0087] Example Hereinafter, embodiments of the present invention will be described in detail, but these are presented as examples and are not intended to limit the present invention, which is defined by the scope of the claims that follow.

[0088] A grain-oriented electrical steel sheet (300 x 60 mm) containing 3.1% Si by weight and having a primary coating that had been finish-annealed to a thickness of 0.23 mm was prepared as a substrate.

[0089] An insulating coating composition for grain-oriented electrical steel sheets was prepared containing the ingredients listed in Table 1 below and 100 parts by weight of water. The manufacturing method involved adding powder to metal phosphates containing (A) or not containing (B, C) cobalt hydroxide, heating the mixture to produce a first composition, and then adding colloidal silica to the first composition and mixing it. This was then compared with a conventional tension coating agent (D).

[0090] The powder used contained montmorillonite as the clay and carbon black as the carbide.

[0091] The insulating coating composition for grain-oriented electrical steel sheets was applied to the substrate at a rate of 4 g / m 2 After application, the coating was dried at 850°C for 30 seconds. Corrosion resistance was evaluated by immersing the test piece in a 5% NaCl solution at 35°C for 8 hours to see if rust occurred on the test piece. In this test, a rusted area of ​​5% or less was evaluated as excellent, 20% or less as good, 20-50% as slightly poor, and 50% or more as poor.

[0092] The coating properties of the test pieces were evaluated using the evaluation methods described above, and the results are shown in Table 1.

[0093] [Table 1]

[0094] *The uniformity of the coating solution was evaluated based on the dispersion of the powder in the solution immediately after application, and the composition of the insulating coating formed on the evaluation test piece was summarized in Table 2 below.

[0095] [Table 2]

[0096] [Table 3]

[0097] Corrosion resistance was recorded by evaluating the area of ​​rust that developed on the test piece after immersing it in a 5% by weight NaCl solution at 35°C for 8 hours. The coating tension was evaluated by removing the insulating coating formed on one of both surfaces, applying tensile stress to the insulating coating formed on the remaining surface, and bending the piece in one direction, measuring the force required to change the degree of bending.

[0098] The insulation properties were measured by measuring the stored current value when a current of 1.0 A was passed through the capacitor at an input voltage of 0.5 V under a pressure of 300 PSI.

[0099] Coating transparency was measured by reflectometry.

[0100] As can be seen from Tables 1 to 3, when the composition of the insulating coating is 15 to 45 wt %, Si 40 to 70 wt %, Mg 0.1 to 5 wt %, and Fe 1 to 10 wt %, and cobalt hydroxide is included, it can be confirmed that the insulation properties, coating shielding properties, corrosion resistance, coating tension, and coating tension after SRA are all improved.

[0101] In particular, in Example 3, a powder containing 35 g of clay and 15 g of carbide was added. In this case, the coating tension and coating shielding factor before and after stress-relieving annealing (SRA) were confirmed to be significantly superior compared to not only the comparative examples but also other examples. The coating after SRA also showed no voids, which is expected to improve workability during transformer manufacturing (FIG. 2). Furthermore, referring to Tables 1 and 3, it can be seen that the coating tensions of Comparative Examples 7 and 8 were higher than those of Comparative Examples 5 and 6. In other words, it can be seen that the coating tension increased as the ratio of clay to carbide in the powder increased.

[0102] Referring to Table 3, it can be seen that in the case of Comparative Example 9, which was produced using the conventional phosphate-based insulating coating composition manufacturing method D, the coating tension before and after stress relief annealing (SRA) was inferior to that of the inventive examples. This was confirmed to be because the phosphate-based coating agent had a high surface roughness, which caused the coating volume to shrink due to silica crystallization during heat treatment.

[0103] Referring to FIGS. 1 and 2, when a secondary coating was formed on a test specimen having a primary coating containing surface defects using the colored coating composition for forming an insulating coating on an electrical steel sheet prepared in Example 3, it was confirmed that the resulting secondary coating had a very uniform and elegant color and had an excellent function of concealing the surface defects of the primary coating.

[0104] The present invention is not limited to the above-described embodiments, and can be manufactured in various different forms, and a person skilled in the art will understand that the present invention can be embodied in other specific forms without changing the technical idea or essential features of the present invention. Therefore, the above-described embodiments should be understood to be illustrative in all respects and not limiting.

Claims

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

2. The metal phosphate is phosphoric acid (H ３ P.O. ４ 2. The insulating coating composition for grain-oriented electrical steel sheet according to claim 1, which is produced by adding a metal oxide to a mixture of the above-mentioned components and heating the mixture.

3. The metal oxides are magnesium oxide (MgO) and aluminum oxide (Al ２ O ３ 3. The insulating coating composition for grain-oriented electrical steel sheet according to claim 2, comprising at least one of:

4. The insulating coating composition for grain-oriented electrical steel sheet according to claim 1 , wherein the powder contains clay and carbide.

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

6. 5. The insulating coating composition for grain-oriented electrical steel sheet according to claim 4, wherein the clay is contained in an amount of 5 to 95% by weight based on the total weight of the powder.

7. 2. The insulating coating composition for grain-oriented electrical steel sheet according to claim 1, wherein the metal phosphate comprises, based on a solid content, 10 to 40 parts by weight of aluminum monophosphate and 60 to 90 parts by weight of magnesium monophosphate, relative to a total of 100 parts by weight.

8. Phosphate (H ３ P.O. ４ ) and then heating the mixture to prepare a first composition containing 0.5 to 5 parts by weight of cobalt hydroxide and 20 to 170 parts by weight of powder per 100 parts by weight of metal phosphate; A method for producing an insulating coating composition for grain-oriented electrical steel sheets, comprising the step of mixing 10 to 250 parts by weight of colloidal silica with a first composition.

9. The method for producing an insulating coating composition for grain-oriented electrical steel sheet according to claim 8, wherein the heating temperature is 80°C or higher.

10. Electrical steel sheet substrate and an insulating coating located on one or both surfaces of the electrical steel sheet substrate; The electrical steel sheet is characterized in that the insulating coating has a surface roughness (Ra) of 0.5 μm or less and a coating transparency of 20% or less.

11. Applying the insulating coating composition for grain-oriented electrical steel sheet according to claim 1 to one or both surfaces of an electrical steel sheet substrate; and The method for manufacturing an electrical steel sheet, comprising the step of drying the steel sheet coated with the insulating coating composition for grain-oriented electrical steel sheet to form an insulating coating.

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

13. In the coating step, the insulating coating composition for grain-oriented electrical steel sheets is applied in an amount of 0.5 to 6.0 g / m ２ The method for producing an electrical steel sheet according to claim 11, wherein the coating is performed in a range of

14. The method for manufacturing an electrical steel sheet according to claim 11, wherein in the step of forming the insulating coating, drying is performed at 550 to 900°C for 10 to 50 seconds.

Images