Electrical insulating coating for grain-oriented electrical steel sheets

Abstract

The present invention relates to the composition of an electrical insulating coating for grain-oriented electrical steel sheets based on phosphates of aluminum and magnesium and on a silica sol. The claimed composition has the following proportions of components: 20-40% by weight of Al and Mg phosphates, 20-45% by weight of silica sol, and 0.01-2% by weight of modifying additives in the form of zirconium silicate ZrSiO4, 0.1-3% by weight of potassium orthovanadate K3VO4, 0.1-3% by weight of vanadyl hydrogen phosphate VOHPO4 and 0.1-2% by weight of manganese oxide hydroxide MnO(OH), and up to 100% by weight of water. As a result, an electrically insulating coating is obtained which is free of chromium compounds (CrIII and CrVI) and which exhibits high corrosion and moisture resistance, excellent adhesion to metals, good appearance and a high coefficient of electrical resistance.

Description

[Technical Field]

[0001] This invention relates to ferrous metallurgy, and in particular to electrical insulating coatings on grain oriented electrical steel sheets used in the manufacture of magnetic cores for power and distribution transformers. [Background technology]

[0002] The main purpose of electrical insulating coatings on grain-oriented electrical steel (GOES) is to form an insulating layer between the plates of a transformer core. To ensure the excellent quality of electrical products, the coating must have high technical properties, namely, strong adhesion to the metal, corrosion resistance, and dielectric (electrical insulating) properties. In the process flow for manufacturing grain-oriented electrical steel sheets, the electrical insulating coating is formed in two stages and is a composite material. First, a primer layer with a forsterite-like composition is formed through a high-temperature annealing process. Next, a magnetic coating (MC) solution based on orthophosphoric acid, silica sol, and metal oxide-based modifying additives is applied to the surface of the primer layer in a thermal flattening line, followed by heat treatment at a temperature of 800-850°C. During heat treatment, the components of the MC solution and the components of the primer layer form a complex, the properties of which are determined by the physical properties of the primer layer and the composition of the MC solution. Currently, the majority of grain-oriented electrical steel manufacturers worldwide use MC formulations based on orthophosphoric acid and silica sol, containing CrVI compounds or combinations of CrVI and CrIII in various proportions as modifying additives (U.S. Pat. No. 3,985,583 (1); U.S. Pat. No. 3,562,011 (2); U.S. Pat. No. 2,753,282 (3)). The technical effect of using CrVI and / or CrIII-based modifying additives in electrical insulating coating compositions is high corrosion and moisture resistance of the phosphate coating, which is particularly important in the transportation and further processing of electrical steel in humid conditions. The adverse effects of using CrVI and CrIII as modifying additives in MC are due to the following: -Risks associated with the use and storage of the solution due to the toxicity of the above ingredients; -Poor adhesion of the coating to the metal of the finished GOES due to the high chemical activity of the solution; - Deterioration of the marketable appearance of the finished GOES due to the presence of strong oxidizing agents in the composition and lack of a matte effect (the color change of the primer layer is accentuated). The goal of most efforts aimed at improving electrical insulating coating compositions is to eliminate the use of toxic CrVI and CrIII as modifying additives and to obtain coatings with the required levels of adhesion to metal, moisture resistance, and matte properties that improve the marketable appearance of steel. An important factor in evaluating the results of efforts to improve electrical insulating coating compositions is the cost of production consideration. There are many similar compositions close to those of (1-3), which are based on the use of phosphates, silica sols, and modifying additives such as vanadium (V) compounds (US Patent Application Publication No. 20140245926 (A1) (4) and EP Patent Application Publication No. 2180082 (B1) (5)), boron (B) compounds (US Patent Application Publication No. 6,461,741 (B1) (7)), titanium (Ti) compounds (EP Patent Application Publication No. 3135793 (A1) (9) and EP Patent Application Publication No. 3101157 (A1) (10)), and zirconium (Zr) compounds (Russian Patent Application Publication No. 2706082 (11)). However, although the use of these materials solves the problem of the toxicity of the solutions, it does not make it possible to obtain a coating having the required level of moisture resistance (especially under the conditions of long-term transport of the finished product in containers by sea), adhesion to the metal, and a commercially acceptable appearance. Summary of the Invention

[0003] The authors of the present invention, using a composition based on Russian Patent Application No. 2706082 (11) as the closest prior art, continued to search for solutions in this field and proposed the following solution: to obtain a chromate-free (environmentally friendly) coating with the required level of adhesion, moisture resistance and a commercially acceptable appearance, zirconium silicate ZrSiO4 modifier was used together with potassium orthovanadate K3VO4, vanadyl hydrogen phosphate VOHPO4, manganese oxide hydroxide MnO(OH) in the following proportions (by weight): TIFF0007778820000001.tif43156 Add to the solution composition.

[0004] The boundary conditions for the content of the zirconium silicate-based modifying additive were determined based on laboratory and industrial experiments. The lower limit of the content of the zirconium silicate-based modifying additive is due to the following reason: if the content falls below 0.01% by mass, the use of the modifying additive will no longer have a significant effect on obtaining the required technical and commercial properties of grain-oriented electrical steel sheets (commercially acceptable appearance, adhesion, resistance coefficient of the electrical insulating coating, and corrosion resistance). The upper limit for the content of modifying additives based on zirconium silicate is due to the following reasons: - when the content of zirconium silicate modifying additive increases above 2% by weight, precipitation of modifying additive particles causes technical difficulties during the preparation, transportation and storage of the MC solution; and Increasing the content of zirconium silicate modifying additive beyond 2% by weight is economically unreasonable, since there is no substantial improvement in technical and commercial properties when using more than 2% by weight of modifying additive.

[0005] The boundary conditions for the content of manganese oxide hydroxide (MnO(OH)) modifying additive were determined based on laboratory and industrial experiments. The lower limit of the manganese oxide hydroxide (MnO(OH)) modifying additive content is due to the following reason: if the content falls below 0.01 mass%, the use of the modifying additive will no longer have an effective effect on obtaining the required technical and commercial properties of grain-oriented electrical steel sheets (commercially acceptable appearance, adhesion, resistance coefficient of electrical insulating coatings, and corrosion resistance). The upper limit for manganese oxide hydroxide (MnO(OH)) modifying additive content is due to the following reasons: - Increasing the content of manganese oxide hydroxide (MnO(OH)) modifying additive beyond 2% by weight is economically unreasonable, since there is no substantial improvement in technical properties when using an amount of modifying additive greater than 2% by weight; - In laboratory and industrial tests, negative trends were observed in the product properties of the appearance of the finished product when using modifying additives in amounts greater than 2% by weight.

[0006] The boundary conditions for the content of modifying additives based on vanadium compounds (vanadyl hydrogen phosphate VOHPO4 and potassium orthovanadate K3VO4) were determined on the basis of laboratory and industrial experiments. The lower limits of the contents of the vanadyl hydrogen phosphate (VOHPO4) and potassium orthovanadate (K3VO4) modifying additives are set for the following reason: if the content of each compound falls below 0.01 mass %, the use of the modifying additives will no longer have an effective effect in obtaining the required technical and commercial properties of grain-oriented electrical steel sheet (commercially valuable appearance and corrosion resistance). The upper limit of the content of modifying additives based on vanadium compounds (vanadyl hydrogen phosphate VOHPO4 and potassium orthovanadate K3VO4) is due to the following reasons: - Increasing the content of modifying additives based on vanadium compounds (vanadyl hydrogen phosphate VOHPO4 and potassium orthovanadate K3VO4) beyond 3% by weight for each compound is not practical, since there is no substantial improvement in the technical properties (commercially acceptable appearance and corrosion resistance) and it is economically unreasonable.

[0007] A unique feature of the proposed composition when compared with the closest prior art (11) is the balance of the level of "unbound" (free) acids, which ensures high corrosion and moisture resistance of the electrical insulating coating on the finished grain-oriented electrical steel sheet. The free acid appears at a specific pH value and its presence can be explained by the following reaction scheme for the hydrolysis of magnesium and aluminum phosphates: Mg(H2PO4)2+2H2O=Mg(OH)2+2H3PO4 Al(H2PO4)3+3H2O=Al(OH)3+3H3PO4 The presence of modifying additives based on vanadium IV compounds (vanadyl hydrogen phosphate VOHPO4) and vanadium IV compounds (potassium orthovanadate K3VO4) in the proposed compositions makes it possible to prevent the appearance of "free" phosphate ions in the solution, since when excess orthophosphate anions appear, the orthovanadate converts into vanadyl cations, which bind to these anions and prevent the formation of free orthophosphate. When the pH drops and excess phosphate needs to be combined, the reaction is as follows: VO4 3- +2H + =H2VO4 - H2VO4 - +4H + +1e - =VO2 + +3H2O Thus, the vanadyl cation binds excess orthophosphate to vanadyl hydrogen phosphate. As the pH value increases, reactions occur which have the effect of maintaining the acidity in the desired pH range and preventing loss of stability of the composition. VO2 + +H2O-1e - =VO2 + +H + VO2 + +2OH - =H2VO4 - In this way, excess hydroxide ions are bound and the pH value is prevented from rising. As a result, the use of a compound containing orthovanadate ions in combination with a compound containing vanadyl cations in a solution provides the MC solution with the property of maintaining compositional stability within a desired pH range. The presence of modifying additives based on zirconium silicate ZrSiO4 and manganese oxide hydroxide MnO(OH) in the proposed composition makes it possible to obtain a ready-made electrical insulating coating with high commercial properties on the surface of grain-oriented electrical steel sheets by obtaining a uniform, monochromatic coating with a matte effect.

[0008] The analysis of science, technology and patent literature shows that the unique features of the claimed method are inconsistent with those of known technical solutions, and based on this, it is concluded that the claimed technical solution meets the criteria for an inventive step. The use of the present invention makes it possible to obtain a GOES having an electrical insulating coating produced without the use of environmentally harmful modifying additives (based on CrIII and CrVI), while at the same time obtaining a coating on the finished grain-oriented electrical steel sheet with the required technical and commercial properties in terms of adhesion of the electrical insulating coating, appearance, coefficient of electrical insulating coating of the finished GOES having the required level of corrosion resistance and moisture resistance, which are superior to similar products.

[0009] Below are presented embodiments of the present invention that confirm the effectiveness of the use of electrical insulating coatings with the proposed compositions, without excluding other variations within the scope of the claims. Example: A series of meltings was carried out in a 150-ton converter (contents, by mass: 3.10-3.14% Si, 0.032-0.034% C, 0.003-0.004% S, 0.50-0.51% Cu, 0.015-0.017% Al, 0.010-0.011% N). The melts were cast into slabs in a continuous steel casting plant, which were then heated to a temperature of 1240-1260°C in a heating furnace and subsequently rolled to a 2.5 mm thick strip in a continuous wide hot rolling mill. The hot-rolled strips were pickled. The pickled strips were double cold-rolled (1300 mm to a thickness of 0.70 mm, then reversed to a thickness of 0.27 mm). A heat-resistant coating was applied to the cold-rolled strips after the second cold rolling. The steel strip coated with the heat-resistant coating was then subjected to high-temperature annealing for secondary recrystallization. After high-temperature annealing in the electrical insulation coating line, an electrical insulation coating of the proposed composition was applied to the steel strip, and the steel strip was subjected to flattening annealing. After final processing, a series of measurements were carried out to determine the adhesion, the resistance coefficient of the electrical insulation coating, the corrosion resistance, the moisture resistance of the coating, and the quality and marketable appearance of the electrical insulation coating on the finished steel.

[0010] Table 1 shows the results of evaluation of adhesion, resistance coefficient of the electrical insulating coating, corrosion resistance, coating quality, and marketable appearance for grain-oriented electrical steel sheets manufactured according to the known composition (closest prior art (11)) and the claimed composition. [Table 1-1] [Table 1-2] [Table 1-3]

[0011] * Evaluation based on the results of three test methods (+pass, -fail): 1. Examination of sealed GOES specimens moistened with distilled water for the presence of corrosion spots after 24 hours exposure in a drying oven at 80°C. 2. Test the specimens in a salt spray chamber at 50°C for 24 hours. 3. Testing of packaged finished metal coils in a simulator that models the process of long-term transportation in containers (cyclic exposure to live steam followed by natural cooling, test intervals of 7-10 days, alternating between heating and cooling modes every 12 hours). After each test, a surface quality evaluation was performed according to the following criteria: High corrosion resistance - no change in the appearance of the coating on the specimen (shown as "+" in the table) Acceptable corrosion resistance - A change in appearance (opacity, etc.) without visible corrosion is permitted (shown as "+-" in the table) Fail - Changes in the coating appearance of the sample, such as iridescence (oxidation), red spots, and obvious corrosion (shown as "-" in the table)

[0012] ** The moisture resistance of coatings was evaluated using the following method: This method consists of determining the concentration of phosphoric acid (equivalent to mg / l of phosphorus) in an aqueous solution. Free orthophosphate appears in solution as a result of boiling grain-oriented steel samples in distilled water. The determination of phosphate is carried out photometrically, taking advantage of the property of phosphoric acid to form colored phosphorus-molybdenum complexes. During the experiment, grain-oriented steel samples were boiled in distilled water for 60 minutes. The phosphate content of the solution was then determined. *** Measurement of current and calculation of the resistance coefficient of the electrical insulating coating. The current was measured in a 10-contact Franklin apparatus according to IEC 60404-11 or GOST 12119.8. To measure the resistance coefficient of the electrical insulating coating using the Franklin method, two unannealed samples were taken from the beginning and end of the coil. The sample size is 50 mm across the full width of the steel strip. Five measurements are carried out on two samples (one for the head and one for the tail of the coil) on the opposite side (underside) of the marking. The resistance coefficient is calculated using the following formula: R=6.45-(I / Iav-1),[Ohm×cm 2 ], where R is the calculated resistance coefficient; I meanis the arithmetic mean of the results of 20 current measurements (A).

[0013] Judging from the data (Table 1), the use of the claimed composition for electrical insulating coatings makes it possible to obtain ready-made metals with high-quality electrical insulating coatings, in comparison with the prior art using ZrSiO4-based modifying additives, as well as with compositions using other modifying additives (4, 5, 8, 9, 10), which provide high consumer properties with a higher adhesion rate (upgrading from adhesion class A, B, C to O) in terms of defect and appearance levels, the required level of resistance coefficient of the electrical insulating coating, a high level of corrosion and moisture resistance, without using environmentally harmful materials in the composition.

[0014] References 1. United States Patent 3,985,583, 12.10.1976 2. United States Patent 3,562,011, 09.02.1971 3. United States Patent 2,753,282, 03.07.1956 4. US 20140245926 A1, 04.09.2014 5. EP 2 180082 B1, 02.04.2014 6. US 2009 / 0208764 A1, 20.08.2009 7. US 2011 / 0067786 A1, 24.03.2011 8. US 6,461,741 B1, 08.10.2002 9. EP 3 135 793 A1, 01.03.2017 10. EP 3 101 157 A1, 07.12.2016 11. RU 2706082, 17.01.2019 Another aspect of the present invention may be as follows. [1] An electrical insulating coating composition for grain-oriented electrical steel sheets based on aluminum phosphate, magnesium phosphate, and silica sol, containing zirconium silicate ZrSiO as a modifying additive. 4 , potassium orthovanadate K 3 VO 4 , Vanadyl hydrogen phosphate VOHPO 4 , manganese oxide hydroxide MnO(OH) in the following component ratio (mass%): TIFF0007778820000005.tif43156 1. An electrically insulating coating composition comprising:

Claims

[Claim 1] An electrical insulating coating composition for grain-oriented electrical steel sheets based on aluminum and magnesium phosphates and silica sol, containing zirconium silicate ZrSiO as a modifying additive. 4 , potassium orthovanadate K 3 VO 4 , vanadyl hydrogen phosphate VOHPO 4 , manganese oxide hydroxide MnO(OH) in the following component ratio (mass%): and the concentration of SiO 2 in said silica sol is 10% to 30%.

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