Chromium-free electrically insulating coating for improving the magnetic properties of electrical steel

Abstract

The present application relates to ferrous metallurgy, in particular to an electrically insulating coating for electrical steel, which can be used for any known use. A composition of an electrically insulating coating for electrical steel is proposed, which comprises the following components (wt%): AlPO4 and Mg3(PO4)2 20-40%, nano-dispersed SiO2 2-15%, K3VO4, VOHPO4, MnO(OH) 0.3-8%, TiO2 0.01-2.5%, ZrO2, SnO2 and H3BO3 0.01-5%, and the rest is water (up to 100%). The use of the composition as an electrical steel coating is also proposed. The present application can prepare a uniform electrically insulating coating, which has extinction effect, high insulating coating resistivity, corrosion resistance and moisture resistance, excellent commercial appearance, and high adhesion of the coating to the metal, high elastic tensile stress value, thereby obtaining advantages in the level of magnetic properties.

Description

Technical Field

[0001] This invention relates to ferrous metallurgy, and more particularly to an electrically insulating coating for electrical steel, which can be used for any known purpose. The electrically insulating coating according to the invention is applied to grain-oriented electrical steel (GOES) used to manufacture the cores of power transformers and distribution transformers. Background Technology

[0002] The primary purpose of electrical insulating coatings used on grain-oriented electrical steel is to form an insulating layer between the laminations of transformer cores. To ensure high-quality electrical products, the coating (or coating layer) should possess high technical characteristics, including adhesion to metals, corrosion resistance, and dielectric (electrical insulation) properties.

[0003] Electrically insulating coatings are formed in two stages during the production cycle of grain-oriented electrical steel. First, a base coat with an olivine-like composition is formed during high-temperature annealing. Then, on a hot-straightening line, a solution of the electrically insulating coating based on orthophosphoric acid, silica sol, and metal oxide-modified additives is applied to the surface of the steel strip with the base coat, followed by heat treatment at 800-850°C. During heat treatment, the composition of the electrically insulating coating solution forms a composite material with the base coat, the properties of which are determined by the physical properties of the base coat and the composition of the electrically insulating coating solution.

[0004] Currently, most electrical steel manufacturers worldwide use electrical insulating coating compositions based on orthophosphoric acid and silica sol, which contain chromium (Cr) compounds as modifying additives (see US 3985583, US 3562011, and US2753282). The technical benefit of using Cr-based modifying additives in electrical insulating coatings lies in the high corrosion resistance and moisture resistance of the phosphate coating, which is particularly important for transporting and subsequently processing electrical steel under high humidity conditions.

[0005] The use of Cr compounds as modifying additives in magnetically active coatings (MAC) has negative effects because: these components are environmentally unsafe and toxic, posing risks to the application and storage of the coating solution; the high chemical activity of the solution leads to decreased adhesion between the coating and the finished GOES metal; and the presence of strong oxidants in the composition and the lack of matting effect (manifested as uneven undercoat hues) results in a poor appearance of the finished GOES.

[0006] Most studies aimed at improving the formulation of electrical insulating coatings aim to eliminate the use of Cr(VI) compounds as modifying additives and obtain coatings with the desired levels of corrosion resistance, moisture resistance, dielectric properties, and metal adhesion.

[0007] However, in recent years, due to intense competition in the global electrical steel market, the requirements for the marketable appearance and environmental friendliness of coatings have significantly increased, as have the requirements for the magnetic properties of electrical steel. To meet current and future customer demands for magnetic properties while maintaining a high level of marketable appearance and environmental friendliness in finished products, electrical steel manufacturers are exploring various ways to improve coating processes and formulations. It is known in the art that the domain structure of steel can be further improved by adjusting the coating composition to generate elastic tensile stress in the metal matrix. Therefore, a promising approach is to introduce components into the coating that increase the difference in the linear coefficient of thermal expansion (CLTE) between the metal and the coating, thereby increasing the elastic tensile stress generated by the coating in the metal. Another important objective in this research area is to develop a method for determining the coefficient of thermal expansion of materials based on orthophosphoric acid, silica sol, and modified additives used as coatings for electrical steel.

[0008] If manufacturers fail to meet future requirements for magnetic properties, are environmentally unfriendly, have poor adhesion between coatings and metals, or have appearance defects (which, while not affecting the technical characteristics of electrical steel, will reduce its product appearance), they will face the risk of limited market acceptability and a significant decline in product profit margins.

[0009] Several similar formulation variations exist, based on compositions containing phosphates, silica sols, and vanadium (V) compounds (US 20140245926 A1 and EP2180082 B1), boron (B) compounds (US6461741 B1), titanium (Ti) phosphates (EP 3135793 A1 and EP 3101157 A1), zirconium (Zr) compounds (RU2706082), (K, V, Mn, Zr) compounds (RU 2765555 C1), and (K, V, Mn, Ti) compounds (RU2803614 C1) as modifying additives. However, while using these materials solves the problem of solution environmental friendliness, it is impossible to produce a coating that simultaneously meets all the requirements: high elastic tensile stress, good moisture resistance (especially under conditions of long-term shipping in ocean containers), good adhesion of the electrically insulating coating to the metal, and excellent product appearance.

[0010] RU 2675887, published on December 25, 2018, discloses a method for producing textured sheet electrical steel with an insulating coating. The method involves applying a working solution to the steel, the working solution containing at least one chemical element selected from the group consisting of: phosphates of at least one chemical element selected from Mg, Ca, Ba, Sr, Zn, Al, and Mn; colloidal silica; and compound M, wherein M is a chemical element selected from Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Mo, and W. The patent provides results for Experimental Examples 1 (Examples 24, 31, 34), 2 (Examples 31, 32), and 3 (Examples 31 and 32), wherein 50 / 70 parts by weight of colloidal silica and 25 / 100 / 120 parts by weight of TiO2 are added per 100 parts by weight of magnesium phosphate and aluminum phosphate.

[0011] The disadvantages of this insulating coating are that when the TiO2 content in the composition is reduced (less than 10 parts by weight), more coating application defects, poor adhesion between the coating and the metal, and insufficient corrosion resistance and moisture resistance are observed.

[0012] RU 2758423, published on October 28, 2021, discloses a composition for producing a chromium-free insulating coating, comprising: at least one phosphate selected from Mg, Ca, Ba, Sr, Zn, Al, and Mn; colloidal silica, calculated as SiO2 and on a solid basis, in an amount of 50 to 120 parts by weight per 100 parts by weight of the at least one phosphate; and particles containing a metal element, calculated as the metal element, in an amount of 5 to 60 parts by weight per 100 parts by weight of the at least one phosphate. Preferably, the particles containing the metal element are at least one selected from TiO2, ZrO2, HfO2, MgO, ZnO, Nb2O5, V2O5, TiN, and ZrN.

[0013] The disadvantages of this insulating coating are: when the TiO2 content in the composition is reduced (less than 10 parts by weight), significant coating application defects and low adhesion between the coating and the metal are observed, as well as insufficient corrosion resistance and moisture resistance.

[0014] RU 2688982, published on May 23, 2019, discloses an insulating coating comprising a first metal phosphate selected from one or more metals chosen from Al, Fe, Mg, Mn, Ni, and Zn; a second metal phosphate selected from one or more metals chosen from Co, Mo, V, W, and Zr; and colloidal silica. The insulating coating may include at least one compound selected from boric acid, sodium borate, titanium dioxide, molybdenum oxide, pigments, and barium titanate. The content of colloidal silica in the insulating coating is preferably 25-55% by weight of solids based on the total weight of the insulating coating.

[0015] The disadvantages of this insulating coating also include: when the TiO2 content in the composition is reduced (less than 10% by weight), significant coating application defects and low adhesion between the coating and the metal are observed, as well as insufficient corrosion resistance and moisture resistance of the coating.

[0016] The closest prior art to this invention is RU 2803614 C1, published on September 18, 2023, which discloses an electrically insulating coating composition based on aluminum phosphate, magnesium phosphate, and silica sol for electrical grain-oriented steel. This coating contains titanium dioxide (TiO2), potassium orthovanadate (K3VO4), vanadium hydrogen phosphate (VOHPO4), and manganese metahydroxide (MnO(OH)) as modifying additives, with the following component ratios (wt%): AlPO4 and Mg3(PO4)2 20-40% Nano-dispersed SiO2 2-15% K3VO4, VOHPO4, MnO(OH) 0.3-8% TiO2 0.01-2.5% Water balance.

[0017] The drawback of this insulating coating is that, due to the absence of components that increase the difference in the coefficient of linear thermal expansion (CLTE) between the steel and the coating in the finished metal, the level of elastic tensile stress generated by the coating in the metal is insufficient. This does not contribute to the efforts of electrical steel manufacturers to achieve advanced magnetic performance requirements. Summary of the Invention

[0018] The main objective of this invention and the technical achievement it aims to achieve is to produce an electrically insulating coating with a matte surface, high elastic tensile stress, high resistivity, high corrosion and moisture resistance, excellent product appearance, and good adhesion between the coating and metal. Another objective is to develop a mechanism for predicting changes in the magnetic properties of electrical steel by introducing components that affect the level of elastic tensile stress into the coating formulation, thereby accelerating the development and implementation of new coating formulations.

[0019] The above-mentioned main objectives and technical achievements were achieved by using an electrical insulating coating composition for electrical steel, which comprises the following components in weight percent: AlPO4 and Mg3(PO4)2 20-40% Nano-dispersed SiO2 2-15% K3VO4, VOHPO4, MnO(OH) 0.3-8% TiO2 0.01-5% ZrO2, SnO2, H3BO3 0.01-5% Water balance.

[0020] The boundary conditions for component content were determined based on laboratory and industrial experiments.

[0021] In the research conducted to prepare this specification, it has been confirmed that only through the synergistic effect of all components contained in the composition can a uniform electrically insulating coating with matte finish, high elastic tensile stress, high insulating coating resistivity, high corrosion and moisture resistance, excellent product appearance, and good adhesion to metals be obtained. Each component minimizes the negative consequences of reducing the content of another component.

[0022] According to RU 2803614 C1: - The above ranges for aluminum phosphate and magnesium phosphate (AlPO4 and Mg3(PO4)2) are due to the fact that: a content of less than 20% by weight will lead to a decrease in corrosion resistance, resistivity of the electrical insulating coating and adhesion between the coating and the metal, while a content of more than 40% by weight will lead to a decrease in the moisture resistance of the coating. - The above range of nano-dispersed SiO2 content is due to the fact that: a content of less than 2% by weight will lead to a decrease in corrosion resistance, resistivity of the electrical insulating coating, and adhesion between the coating and the metal, while a content of more than 15% by weight will lead to a decrease in adhesion between the coating and the metal. - Nanodispersed SiO2 contains SiO2 particles with sizes ranging from 1 to 10,000 nm in all three dimensions; - The above ranges for the contents of potassium vanadate K3VO4, vanadium hydroxide phosphate VOHPO4, and manganese hydroxide MnO(OH) are because: contents below 0.3% by weight will lead to a decrease in corrosion resistance and moisture resistance, while contents above 8% by weight will lead to the formation of precipitates in the solution and a deterioration in the appearance of the coating. - The lower limit for TiO2-based additives is due to the fact that a total content of less than 0.01% by weight results in no significant effect of using the additive in improving the resistivity of electrical insulation coatings and improving the appearance of electrical insulation coatings on electrical steel. The upper limit for TiO2 content is due to the fact that a total TiO2 content exceeding 3% by weight leads to technical difficulties in the preparation, transportation, and storage of the electrical insulating coating solution due to additive particle sedimentation. When the TiO2 content exceeds 2.5% by weight, there is no fundamental increase in the resistivity of the electrical insulating coating or improvement in the commercial appearance of the electrical insulating coating.

[0023] During the research, the authors of this invention determined the lower limits of the content of ZrO2, SnO2 and H3BO3 additives because: a total content of less than 0.01% by weight resulted in no significant effect of using the additives in achieving high levels of elastic tensile stress, improving the resistivity of electrical insulation coatings, and improving the commercial appearance of electrical steel electrical insulation coatings.

[0024] During the research, the authors of this invention determined the upper limit of the content of ZrO2, SnO2, and H3BO3 additives. This is because an increase in the total content of ZrO2, SnO2, and H3BO3 additives exceeding 5% by weight would lead to technical difficulties in the preparation, transportation, and storage of the electrical insulating coating solution due to additive particle sedimentation. Furthermore, it is uneconomical because the increased profit margin resulting from improved magnetic properties ensured by reducing the linear thermal expansion coefficient through the introduction of these components is offset by a deterioration in the product's appearance. Therefore, the authors propose the following technical solution: A chromium-free electrical insulating coating composition for improving the magnetic properties of electrical steel, comprising the following components by weight percent: AlPO4 and Mg3(PO4)2 20-40% Nano-dispersed SiO2 2-15% K3VO4, VOHPO4, MnO(OH) 0.3-8% TiO2 0.01-2.5% ZrO2, SnO2, H3BO3 0.01-5% Water balance.

[0025] The content of ZrO2, SnO2, and H3BO3 additives is 0.01-5% by weight, preferably 1.75-3.0% by weight, more preferably 2.2-2.6% by weight. In a preferred embodiment, the weight ratio of ZrO2, SnO2, and H3BO3 additives is (1-3):1:(1-3), preferably (1.5-2.5):1:(1.5-2.5), more preferably 2:1:2. The weight ratio of AlPO4 to Mg3(PO4)2 is 4:1. The weight ratio of K3VO4, VOHPO4, and MnO(OH) is 4:4:3.

[0026] Although the aforementioned components can achieve the technical results in any proportion, the above proportions enable the highest level of elastic tensile stress generated by the coating to be achieved by introducing an increase in the difference in the coefficient of linear thermal expansion (TCLE) between the steel and the coating on the finished metal.

[0027] When the resulting insulating coating composition is used as an electrical steel coating, all its advantages can be fully realized.

[0028] The proposed coating can be used on any steel type that requires an insulating coating for some reason, such as various conductive metal surfaces (motor windings, electrical equipment, etc.).

[0029] When the resulting insulating coating composition is used as a coating for electrical steel (which is particularly useful for manufacturing the cores of power transformers and distribution transformers), all its advantages are most effectively realized. Detailed Implementation

[0030] The following provides embodiments of the invention that support the effectiveness of insulating coating compositions. These embodiments do not preclude other embodiments within the scope of the claims.

[0031] Example A series of smelting processes were carried out in a 150-ton converter (composition, by weight %): 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, and cast into slabs in a continuous casting machine (CSCU). The slabs were heated to 1240–1260°C in a reheat furnace and then rolled into 2.5 mm thick strip in a continuous wide strip hot rolling mill. The hot-rolled strip was pickled. The pickled strip was then cold-rolled twice (to a thickness of 0.70 mm on a 1300 mill and to a thickness of 0.27 mm on a reversible mill). After the second cold rolling, a heat-resistant coating was applied to the cold-rolled strip. The strip with the heat-resistant coating is then subjected to high-temperature annealing to achieve secondary recrystallization. After high-temperature annealing on the electrical insulation coating production line, the strip is coated with the proposed electrical insulation coating composition, followed by leveling annealing. After final treatment, the adhesion and resistivity of the electrical insulation coating are measured, and the corrosion resistance and moisture resistance of the coating, as well as the quality of the electrical insulation coating on the finished steel (product appearance), are evaluated.

[0032] Table 1 lists the formulation (composition) of the prepared coatings.

[0033] Table 2 lists the evaluation results of the quality parameters of the prepared coatings.

[0034] Table 1. Examples of the prepared coatings Table 2. Evaluation results of the quality parameters of the prepared coating 1 Note: Adhesion assessment shall be performed on the inside of the strip in accordance with GB / T 2522.

[0035] * Measure the current and calculate the resistivity of the electrical insulation coating. Current measurements are performed using a ten-pin Franklin tester, according to IEC 60404-11 (IEC 60404-11:2021) or GOST 12119.8 (GOST 12119.8-98). To measure the resistivity of the electrical insulation coating using the Franklin method, two unannealed samples are taken from each end of the strip roll. The sample size is 50 mm along the full width of the strip. Five measurements are performed on the marked side (front) of each of the two samples (one from the beginning and one from the end of the strip roll). Five measurements are performed on the opposite side (back) of each of the other two samples (one from the beginning and one from the end of the strip roll). The resistivity is calculated using the following formula: R = 6.45 - (1 / I av - 1), [Ω·cm 2 ] In the formula: R is the calculated resistivity; I av The arithmetic mean of 20 current measurements is expressed in amperes (A).

[0036] **A method for evaluating the moisture resistance of coatings involves determining the concentration of phosphoric acid (mg / L as phosphorus) in an aqueous solution. Free orthophosphoric acid is formed in the solution by boiling a transformer steel sample in distilled water. The phosphate content is determined spectrophotometrically, utilizing the ability of phosphoric acid to form colored phosphomolybdate complexes. In the experiment, the transformer steel plate is boiled in distilled water for 60 minutes, and then the phosphate content in the solution is determined.**

[0037] ***The elastic tensile stress δ generated in the metal by the electrical insulating coating was estimated using a specialized technique (see LS Karenina, "The Effect of Deformation Fractionality during Cold Rollingon Properties" / LS Karenina, MPPuzanov, OV Pervushina, MA Pankratov, and A. Yu. Tarasova / / Steel, Vol. 10, 2021, pp. 13-18).

[0038] ****Measurements were performed according to GOST 12119.0-98 (specific magnetic loss was given using an Epstein apparatus on an electrical grain-oriented steel sample with a thickness of 0.27 mm).

[0039] *****The samples used for CLTE measurements were prepared from raster samples, and the preparation process included the following main steps: drying, grinding, pressing, firing, and sample preparation. CLTE measurements of the resulting samples were performed according to GOST 10978-2014 using a DIL 402C dilatometer.

[0040] The data in Tables 1 and 2 show that the composition proposed by the inventors solves the problems existing in the prior art and provides a uniform electrical insulating coating with matting effect, high corrosion and moisture resistance, excellent product appearance, high adhesion to metals, high tensile stress value, and improved magnetic properties compared to the prototype.

[0041] The exemplary embodiments described above are given for illustrative purposes only. Those skilled in the art will understand that other embodiments are possible without departing from the spirit of the invention.

Claims

1. A chromium-free electrical insulating coating composition for improving the magnetic properties of electrical steel, comprising the following components in weight percent: AlPO4, Mg3(PO4)2 20-40%, Nano-dispersed SiO2 2-15%, K3VO4, VOHPO4, MnO(OH) 0.3-8%, TiO2 0.01-2.5%, ZrO2, SnO2, H3BO3 0.01-5%, Water balance.

2. The composition according to claim 1, characterized in that, The content of ZrO2, SnO2 and H3BO3 additives is 0.01-5% by weight, preferably 1.75-3.0% by weight, more preferably 2.2-2.6% by weight, wherein the weight ratio of ZrO2, SnO2 and H3BO3 additives is (1-3):1:(1-3), preferably (1.5-2.5):1:(1.5-2.5), more preferably 2:1:

2.

3. Use of the composition according to claim 1 as a grain-oriented electrical steel coating.

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