Electrical insulating coating for electrical steel

Abstract

The invention relates to ferrous metallurgy, in particular to an electrical insulating coating for electrical steel, which can be used for any known purpose. The proposed electrical insulating coating composition for electrical steel comprises the following components, wt.%: AlPO4 and Mg3(PO4)2 20-40%, nanodispersed SiO2 2-15%, K3O4V, VOHPO4, MnO(OH) 0.3-8%, TiO2 0.01- 2.5%, and balance is water (to 100%). This composition is proposed for use as a coating for electrical steel. The proposed invention provides a uniform electrical insulating coating with a matting effect, high resistance coefficient, high corrosion resistance and moisture resistance, excellent marketable appearance, and high adhesion to metal.

Description

[0001] ELECTRICAL INSULATING COATING FOR ELECTRICAL STEEL

[0002] SPECIFICATION

[0003] Field of the Invention

[0004] The invention relates to ferrous metallurgy, in particular to an electrical insulating coating for electrical steel, which can be used for any known purpose. The electrical insulating coating is applied to various electrically conductive metal surfaces (windings of electrical machines, electrical equipment, etc.). Preferably, the electrical insulating coating according to the invention is applied to grain-oriented electrical steel used for the manufacture of magnetic cores of power and distribution transformers.

[0005] Background Art

[0006] An electrical insulating coating is a coating with an increased resistance coefficient, namely at least 1.0 Ohm-cm2.

[0007] The main purpose of the electrical insulating coating for use with grain-oriented electrical steel (GOES) is to create an insulation layer between the plates of the magnetic cores of transformers. To ensure good quality of electrical products, the coating should have high technical performance: adhesion to metal, corrosion resistance, and dielectric (electrical insulating) properties.

[0008] The electrical insulating coating in the process flow of electrical steel production is formed in two stages. Initially, the high-temperature annealing process forms a primer layer with a forsterite-like composition. Then, on the thermoflattening line, a solution of electrical insulating coating based on orthophosphoric acid, silica sol and modifying additives based on metal oxides is applied to the surface of the steel strip with a primer layer, followed by heat treatment at a temperature of 800-850°C. The heat treatment converts the components of the solution of electrical insulating coating and the primer layer into a composite with the properties determined by the physical features of the primer layer and by the electrical insulating coating composition solution.

[0009] Nowadays, most electrical steel manufacturers in the world use an electrical insulating coating composition based on orthophosphoric acid and silica sol, comprising chromium (Cr) compounds as modifying additives (see US3985583, US3562011, US2753282). The technical effect of the use of modifying additives based on Cr compounds in the electrical insulating coating composition is high corrosion resistance and moisture resistance of the phosphate coating, which is especially important during transportation and further processing of electrical steel in high humidity conditions.

[0010] The negative effect of using Cr compounds as modifying additives in the magnetical coating (MC) composition is caused by the risks related to use and storage of the solution due to the toxicity of these components, degradation of the coating adhesion to the finished GOES metal due to the high chemical activity of the solution, deterioration of the marketable appearance of the finished GOES due to strong oxidizing agents in the composition in the absence of a matting effect (diversity of the primerlayer is emphasized).

[0011] The goal of most studies aimed at improving the electrical insulating coating compositions is to eliminate the use of Cr compounds as modifying additives and to provide a coating with the required level of corrosion and moisture resistance, dielectric properties, and adhesion to metal.

[0012] In recent years, due to the high level of competition in the global electrical steel market, the requirements for the commercial type of products have increased significantly. The variety of coatings and the presence of cosmetic defects that do not have a negative impact on the technical characteristics of electrical steel often limit the available markets and reduce the product margins.

[0013] There are numerous options of similar compositions close to those discussed above, which comprise phosphates and silica sol and use vanadium(V) compounds (US20140245926 Al, EP2180082 Bl), boron (B) compounds (US6461741 Bl), titanium (Ti) phosphates (EP3135793 Al, EP3101157 Al), and zirconium (Zr) compounds (RU2706082) as modifying additives. However, the use of these materials, while solving the problem of toxicity of the solution, does not allow providing a coating with the required level of moisture resistance (especially under conditions of long-term transportation of finished products in containers by sea), adhesion of the electrical insulating coating to a metal and marketable appearance of the electrical insulating coating.

[0014] RU2675887 published on 25.12.2018 discloses a method for manufacturing textured sheet electrical steel with an insulating coating, wherein a working solution comprising at least one chemical element selected from the group comprising phosphate of at least one chemical element selected from the group consisting of Mg, Ca, Ba, Sr, Zn, Al, and Mn, colloidal silica, and a compound of M, where M is a chemical element selected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, Mo, and W. The abov-mentioned patent shows the results of experimental examples 1 (examples No. 24, 31, 34), 2 (examples No. 31, 32), and 3 (examples No. 31, 32), wherein 50 / 70 parts by weight of colloidal silica and 25 / 100 / 120 parts by weight of titania was added per 100 parts by weight of magnesium and aluminum phosphates.

[0015] The disadvantage of this insulating coating is that a low TiCh content (below 10 parts by weight) in the composition results in a high content of coating defects and low adhesion of the coating to metal.

[0016] RU2758423 published on 28.10.2021 discloses a liquid for producing a chromium-free insulating coating, comprising at least one phosphoric acid salt selected from the phosphate salts of any one of Mg, Ca, Ba, Sr, Zn, Al, and Mn, colloidal silica, which content, based on SiCh solid matter, is 50 to 120 parts by weight per 100 parts by weight of at least one salt of phosphoric acid, and particles of a compound comprising a metal element, which content, based on the element, is 5 to 60 parts by weight per 100 parts by weight of at least one phosphate salt. Preferably, the particles comprise metal element compound and belong to at least one type of particles selected from any one of TiCh, ZrCh, HfCh, MgO, ZnO, Nb2Os, V2O5, TiN, and ZrN.

[0017] A disadvantage of this insulatingon coating is also that a low TiCh content in the composition (less than 10 parts by weight) causes a high content of coating defects and low adhesion of the coating to a metal.

[0018] RU2688982 published 23.05.2019 discloses an insulating coating comprising a first metal phosphate, which is a phosphate of one or more metals selected from Al, Fe, Mg, Mn, Ni, and Zn, a second metal phosphate, which is a phosphate of one or more metals selected from Co, Mo, V, W, and Zr, and colloidal silica. The insulating coating may comprise at least one compound selected from the group consisting of boric acid, sodium borate, titania, molybdenum oxide, pigment, and barium titanate. The content of colloidal silica in the insulating coating is preferably 25-55 wt.% as the solid content in the total weight of the insulating coating.

[0019] A further disadvantage of this insulating coating is that a low TiCh content in the composition (less than 10 parts by weight) causes a high content of coating defects and low adhesion of the coating to a metal.

[0020] RU2765555 published on 01.02.2022 is chosen as the closest prior art, it discloses an electrical insulating coating composition based on aluminum and magnesium phosphates and silica sol for grain-oriented electrical steel, comprising zirconium silicate ZrSiCU and potassium orthovanadate K3O4V as modifying additives, vanadyl hydrogen phosphate VOHPO4, and manganese metahydroxide MnO(OH) in the following ratio of components, wt.%:

[0021] Al and Mg phosphates 20-40, silica sol 20-45 zirconium silicate ZrSiC as modifying additive 0.01-2, potassium orthovanadate K3O4V as modifying additive 0.1-3, vanadyl hydrogen phosphate VOHPO4 as modifying additive 0.1-3, manganese metahydroxide as modifying additive MnO(OH) 0.1-2, and water q.s. to 100.

[0022] The disadvantages of this insulating coating comprise the need to use hard-to-get zirconium silicate, as well as insufficiently high appearance characteristics due to the less pronounced matting properties.

[0023] Summary

[0024] The problem of the present invention and the aimed technical effect thereof is to provide a uniform electrical insulating coating with a matting effect, high resistance coefficient, high corrosion resistance and moisture resistance, excellent marketable appearance, and high adhesion to metal.

[0025] The solution to the problem and the technical effect is achieved through the use of a composition (formulation) of an electrical insulating coating for electrical steel, comprising the following components, wt.%:

[0026] AIPO4 and Mg3(PO4)2 20-40%, nanodispersed SiCh 2-15%,

[0027] K3O4V, VOHPO4, MnO(OH) 0.3-8%,

[0028] TiO20.01-2.5%, and water q.s. to 100%.

[0029] The boundary conditions for the content of components are determined on the basis of laboratory and industrial experiments.

[0030] In the course of studies, during drafting of this specification, it was established that providing a uniform electrical insulating coating with a matting effect, high resistance coefficient, high corrosion resistance and moisture resistance, excellent marketable appearance, and high adhesion to metal can be achieved only due to the synergistic effect of all components included into the composition, and each of the components allows minimizing the negative consequences of reduced content of the other component.

[0031] The above content range of aluminum and magnesium phosphates AIPO4 and Mg3(PO4)2 is due to the fact that a decrease in this content below 20 wt.% leads to a lower corrosion resistance, resistance coefficient of the electrical insulating coating, and adhesion of the coating to metal, and an increase in this content above 40 wt.% leads to a lower moisture resistance of the coating.

[0032] The above content range of nanodispersed SiCh is due to the fact that a decrease in this content below 2 wt.% leads to a lower corrosion resistance, resistance coefficient of the electrical insulating coating, and adhesion of the coating to metal, and an increase in this content above 15 wt.% leads to a lower adhesion of the coating to metal.

[0033] Nanodispersed silica consists of SiCh particles, where all three dimensions range from 1 to 10,000 nm.

[0034] The above content range of potassium orthovanadate K3O4V, vanadyl hydrogen phosphate VOHPO4, and manganese metahydroxide MnO(OH) is due to the fact that a decrease in this content below 0.3 wt.% leads to a lower corrosion resistance and moisture resistance, and an increase in this content above 8 wt.% leads to the formation of a sediment in solution and deterioration of the coating appearance.

[0035] The lower limit of the content of the additive based on titania TiCh is due to the following reason: a decrease in this content below 0.01 wt.% leads to the absence of a significant effect from the use of the additive to increase the resistance coefficient of the electrical insulating coating and improve the marketable appearance of the electrical insulating coating of electrical steel.

[0036] The upper limit of the titanium oxide TiCh additive content is due to the following reasons. An increase in the titanium oxide TiCh content over 3 wt.% leads to technical difficulties in the preparation, transportation, and storage of the electrical insulating coating solution due to sedimentation of additive particles; moreover, it is not economically reasoned due to no significant increase in the resistance coefficient of the electrical insulating coating and improvement in the marketable appearance of the electrical insulating coating when using the specified additive in an amount exceeding 2.5 wt.%.

[0037] In apreferred embodiment, the TiCh content is 0.5-2 wt. %, more preferably 0.5-1.9 wt. %.

[0038] In a preferred embodiment, the weight ratio of AIPO4 : Mgs(PO4)2 is (3-5) : 1, preferably (3.5-4.5) : 1, more preferably 4 : 1.

[0039] Despite the fact that the above components, at any ratio, make it possible to achieve the stated technical effect, the above ratio allows achieving the highest characteristics of the marketable appearance of the coating.

[0040] In a preferred embodiment, the weight ratio of K3O4V : VOHPO4 : MnO(OH) is (3.5-4.5) : (3.5-4.5) : (2.5-3.5), preferably 4 : 4 : 3 . Despite the fact that the above components, at any ratio, make it possible to achieve the stated technical effect, the above ratio allows achieving the highest characteristics of the marketable appearance of the coating.

[0041] To the fullest extent, all the advantages of the resulting electrical insulating coating composition can be used when using it as a coating for electrical steel.

[0042] The proposed coating can be used for any type of steel that, for one or another reason, needs to be provided with an electrical insulating coating, for example, for various electrically conductive metal surfaces (steel pipes, windings of electrical machines, electrical equipment, etc.).

[0043] To the fullest extent, all the advantages of the resulting electrical insulating coating composition can be used when using it as a coating for electrical steel, which, in particular, can be used for the manufacture of magnetic cores of power and distribution transformers.

[0044] Implementation of the Invention

[0045] Below are embodiments of the invention, which do not exclude other options within the claims, but confirm the effective use of the electrical insulating coating with the proposed composition.

[0046] Example. A series of melts were provided in 150-ton converters (composition, wt.%: 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 at continuous steel casting plants (CSCP) into slabs, which were heated in heating furnaces to 1240-1260°C and then rolled on a continuous wide-band hot rolling mill into strips 2.5 mm thick. Hot rolled strips were etched. The etched strips were twice cold rolled (on a 1300 mill to a thickness of 0.70 mm and a reversing mill to a thickness of 0.27 mm. A heat-resistant coating was applied to the cold-rolled strips after the second cold rolling. Then the strips with the applied heat-resistant coating were subjected to high-temperature annealing to provide secondary recrystallization. After high-temperature annealing in the line of the electrical insulating coating unit, an electrical insulation coating with the proposed composition was applied to the strips and subjected to flattening annealing. After the final treatment, the adhesion, resistance coefficient of the electrical insulating coating, corrosion and moisture resistance of the coating, and quality (marketable appearance) of the electrical insulating coating on the finished steel were assessed.

[0047] Table 1 shows the options of the produced coating compositions (formulations).

[0048] Table 2 shows the results of assessing the quality parameters of the produced coatings.

[0049] Table 1. Options for manufactured coatings

[0050]

[0051] Table 2. Results of assessing the quality parameters of produced coatings

[0052]

[0053] 1Note. Adhesion assessed on the inner side of a strip as required by GB / T 2522 * Assessment according to the results of three test methods (+ test passed, - test failed).

[0054] 1. Testing for the presence of corrosion spots after keeping hermetically sealed packed samples of grain-oriented electrical steel (GOES) moistened with distilled water for 24 hours in a drying cabinet at 80°C.

[0055] 2. Testing samples in a salt fog chamber at 50°C for 24 hours.

[0056] 3. Testing coils of packaged finished metal in a unit simulating the process of long-term transportation in containers (periodic exposure (heating) to live steam followed by natural cooling, test frequency 7-10 days, change of heating / cooling mode every 12 hours).

[0057] The surface quality assessment after each test was carried out according to the following criteria.

[0058] High degree of corrosion resistance: no changes in the appearance of the coating on the samples (indicated as “+” in Table 2)

[0059] Satisfactory degree of corrosion resistance: external changes (turbidity, etc.) are allowed without visible corrosion spots (indicated as “+-” in Table 2)

[0060] Unsatisfactory: changes in the appearance of the coating on the samples as rainbow color (temper color), red spots and obvious corrosion spots (indicated as “+-” in Table 2).

[0061] ** Assessment of the moisture resistance of the coating using the method, which consists in determining the concentration of phosphoric acid (based on phosphorus, mg / 1) in an aqueous solution. Free orthophosphoric acid appears in solution as a result of boiling transformer steel samples in distilled water. The determination of phosphates is carried out photometrically, using the property of phosphoric acid to form colored phosphorus -molybdenum complexes. During the experiment, transformer steel plates were boiled in distilled water for 60 minutes. Next, the phosphate content in this solution was determined.

[0062] *** Measurements of current and calculation of the resistance coefficient of the electrical insulating coating. The current is measured using a ten-pin Franklin unit in accordance with IEC 60404-11 (IEC 60404-11:2021) or GOST 12119.8 (GOST 12119.8-98). To measure the resistance coefficient of an electrical insulating coating using the Franklin method, two unannealed samples are taken from the beginning and end of the roll. The sample size is 50 mm over the entire width of the strip. On two samples (one each from the beginning and end of the roll), five measurements are taken from the side of the marking (front side), on two other samples (from the beginning and end of the roll), five measurements are taken from the side opposite the marking (back side). The resistance coefficient is calculated using the formula:

[0063] R = 6.45-(l / Iav -1) [Ohm-on2], where R is the calculated resistance coefficient; Imean is an arithmetic mean for the results of 20 measurements of current (A).

[0064] As follows from the Tables, the use of our proposed composition allows solving the problems existing in the prior art and providing a uniform electrical insulating coating with a matting effect, which has a high resistance coefficient, high corrosion resistance and moisture resistance, excellent marketable appearance and high adhesion to metal.

[0065] The described embodiments are provided for illustrative purposes only. It will be obvious to a person skilled in the art that other embodiments are possible without changing the essence of the invention.

Claims

CLAIMS1. A composition of an electrical insulating coating for electrical steel, comprising the following components, wt.%: AIPO4 and Mg3(PO4)220-40% nanodispersed SiO22-15%K3O4V, VOHPO4, MnO(OH) 0.3-8%TiO20.01-2.5% water balance.

2. The composition according to claim 1, characterized in that the TiO2content is 0.5-2 wt.%, preferably 0.5-1.9 wt. %.

3. The composition according to claim 1, characterized in that the weight ratio of AIPO4 : Mg3(PO4)2is (3-5) : 1, preferably (3.5-4.5) : 1, more preferably 4 : 1.

4. The composition according to claim 1, characterized in that the weight ratio of K3O4V : VOHPO4 : MnO(OH) is (3.5-4.5) : (3.5-4.5) : (2.5-3. 5), preferably 4:4:3.

5. Use of the composition according to claim 1 as a coating for electrical steel.

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