Grain-oriented electrical steel sheet, iron core for transformer, transformer, and method for reducing noise of transformer

Abstract

Provided is a grain-oriented electrical steel sheet which can be processed into a core for a transformer and which can exhibit low noise characteristics under actual operation conditions. The grain-oriented electrical steel sheet has an insulating coating film, the insulating coating film contains Si, P and O and at least one element selected from the group consisting of Mg, Ca, Ba, Sr, Zn, Al, Mn and Co, the crystallinity is 20% or more, and the minimum tension imparted to the steel sheet by the insulating coating film at 100-200 DEG C is 10 MPa or more. Preferably, the static friction coefficient of the insulating coating film is 0.21-0.50. It is preferable that the insulating coating film does not contain Cr.

Description

[0001] This application is a divisional application of an application with an application date of November 17, 2017, an application number of 201780080754.4, and an invention title of "Directional Electromagnetic Steel Sheet, Core of Transformer, Transformer, and Method for Reducing Noise of Transformer". Technical Field

[0002] The present invention relates to a directional electromagnetic steel sheet, a core of a transformer, a transformer, and a method for reducing noise of a transformer, and particularly relates to a directional electromagnetic steel sheet having excellent low-noise properties. Background Art

[0003] Generally, in a directional electromagnetic steel sheet, a film is provided on the steel sheet surface to impart insulation, workability, rust prevention properties, etc. The film usually consists of a base film mainly composed of forsterite formed during final annealing and a topcoat film of a phosphate system formed thereon.

[0004] Since the above film is formed at a high temperature and has a low thermal expansion rate, when it cools down to room temperature, tension is imparted to the steel sheet due to the difference in thermal expansion rates between the steel sheet and the film. As a result, iron loss and magnetostriction are reduced. In particular, when magnetostriction is reduced, the magnetostriction amplitude of the core becomes smaller, and the noise of the transformer can be suppressed to a lower level. In recent years, the demand for low-noise transformers has been continuously increasing. Therefore, it is desired to impart as high a tension as possible to the steel sheet.

[0005] In order to meet such an urgent expectation of high tension imparting, various films have been proposed. For example, a film mainly composed of magnesium phosphate, colloidal silica, and chromic anhydride is proposed in Patent Document 1, and a film mainly composed of aluminum phosphate, colloidal silica, and chromic anhydride is proposed in Patent Document 2.

[0006] However, it cannot be said that the tensile stress generated by the phosphate-based glass coatings described in Patent Document 1 or Patent Document 2 is sufficient, and further improvement is desired.

[0007] On the other hand, Patent Document 3 discloses a directional electromagnetic steel sheet that generates a high tensile stress and reduces iron loss by forming a coating containing elements of P, Si, Cr, and O and at least one element selected from Mg, Al, Ni, Co, Mn, Zn, Fe, Ca, and Ba and having a crystal phase of 5 mass% or more of phosphate.

[0008] In addition, Patent Document 4 discloses a high-tensile insulating film formed on the surface mainly composed of a metal phosphate and colloidal silica, with the crystallinity of the metal phosphate being 60% or less and not containing chromium. Patent Document 5 discloses a high-tensile insulating film formed mainly of a phosphate and colloidal silica and uniformly dispersed throughout the surface with crystalline magnesium phosphate and not containing chromium.

[0009] Crystallizing a part of the glassy film of the phosphate does contribute to improving the adhesion and the tension applied to the steel plate. However, it is known that any of the techniques in Patent Documents 3, 4, and 5 has a problem of relatively high noise generated by the transformer when actually manufacturing a transformer.

[0010] Prior Art Documents

[0011] Patent Documents

[0012] Patent Document 1: Japanese Patent Application Laid-Open No. 50-79442

[0013] Patent Document 2: Japanese Patent Application Laid-Open No. 48-39338

[0014] Patent Document 3: Japanese Patent Re-Publication No. 2013 / 099455

[0015] Patent Document 4: Japanese Patent Application Laid-Open No. 2007-217758

[0016] Patent Document 5: Japanese Patent Re-Publication No. 2007 / 136115 Summary of the Invention

[0017] An object of the present invention is to solve the above problems and provide a grain-oriented electrical steel sheet that can be processed into a core of a transformer and can exhibit low-noise characteristics under actual operating conditions. Another object is to provide a core of a transformer, a transformer using the above grain-oriented electrical steel sheet, and a method for reducing the noise of the transformer.

[0018] The inventors of the present invention conducted intensive studies and obtained the following insights.

[0019] Different coatings were applied to the same grain-oriented electrical steel sheet, and the differences between the steel sheet with low transformer noise (i.e., low noise) and the steel sheet with high transformer noise were investigated in depth. As a result, it was found that in the steel sheet with high transformer noise, the tension applied to the steel sheet by the film is greatly reduced under the conditions of actual transformer operation, that is, at a temperature of about 100°C to 200°C.

[0020] Based on this result, it is considered that the cause of noise generation is that the tension applied to the steel sheet is greatly reduced at a temperature of about 100°C to 200°C. Moreover, it can be seen that from the perspective of low noise, what is important is not the tension applied to the steel sheet at room temperature that has been measured and evaluated so far, but the tension applied to the steel sheet under the conditions of actual transformer operation, that is, at a temperature of about 100°C to 200°C. Further research was carried out, and the following insights were also obtained: by making the insulating film contain a crystalline phase, the tension applied to the steel sheet becomes higher due to crystallization.

[0021] The present invention is based on such insights, and the gist is as follows.

[0022] [1] A grain-oriented electrical steel sheet having an insulating film, the insulating film containing at least one selected from Mg, Ca, Ba, Sr, Zn, Al, Mn, Co and Si, P, O, having a crystallinity of 20% or more, and the minimum tension applied to the steel sheet by the insulating film at 100°C to 200°C being 10 MPa or more.

[0023] [2] The grain-oriented electrical steel sheet according to the above [1], wherein the static friction coefficient of the insulating film is 0.21 to 0.50.

[0024] [3] The grain-oriented electrical steel sheet according to the above [1] or [2], wherein the insulating film does not contain Cr.

[0025] [4] The grain-oriented electrical steel sheet according to any one of the above [1] to [3], wherein the average film thickness of the insulating film is 4.5 μm or less.

[0026] [5] An iron core of a transformer made of the grain-oriented electrical steel sheet according to any one of the above [1] to [4].

[0027] [6] A transformer having the iron core of the transformer according to the above [5].

[0028] [7] A method for reducing the noise of a transformer, which is a method for reducing the noise of a transformer,

[0029] using the grain-oriented electrical steel sheet according to any one of the above [1] to [4] as the grain-oriented electrical steel sheet constituting the iron core of the transformer.

[0030] According to the present invention, a grain-oriented electrical steel sheet with excellent low-noise performance is obtained. It can reduce the noise of a transformer and is useful as a material for a low-noise transformer. The iron core of the transformer using the grain-oriented electrical steel sheet of the present invention and the transformer have excellent low-noise performance. Detailed embodiments

[0031] A detailed description will be given below. It should be noted that the unit of the content of the component composition, i.e., “%”, represents “mass %” unless otherwise specified.

[0032] The insulating film formed on the surface of the directional electromagnetic steel sheet of the present invention contains at least one selected from Mg, Ca, Ba, Sr, Zn, Al, Mn, Co and Si, P, O, the crystallinity is 20% or more, and the minimum imparted tension of the insulating film at 100°C to 200°C to the steel sheet is 10 MPa or more.

[0033] It should be noted that in the present invention, the insulating film refers to a phosphate-based tension insulating film (top coat film).

[0034] It is considered that the cause of the noise of the transformer is mainly attributed to the magnetostriction of the iron core. Magnetostriction refers to the phenomenon of expansion and contraction when iron is magnetized, and it is known that the magnetostriction becomes larger when compressive stress is applied to iron. The iron core of the transformer is formed by laminating steel sheets, and the iron core of a large transformer uses dozens of tons of steel sheets. Therefore, compressive stress acts on the steel sheet due to its own weight. Therefore, as long as tension is applied to the steel sheet in advance, the influence of the compressive stress can be eliminated. Thus, by applying as high a tension as possible to the steel sheet, an increase in magnetostriction can be prevented, and the noise of the transformer can be reduced.

[0035] From the above considerations, in the present invention, as the tension imparted to the steel sheet, the minimum imparted tension of the insulating film at 100°C to 200°C to the steel sheet is 10 MPa or more. Assuming that the transformer is actually in operation, the minimum imparted tension of the insulating film at 100°C to 200°C to the steel sheet is evaluated, whereby the low-noise characteristics can be improved. Considering from the aspect of deviating too much from the temperature during actual operation and improving the low-noise characteristics, the evaluation at a temperature less than 100°C or higher than 200°C is not suitable. In addition, the minimum imparted tension to the steel sheet is 10 MPa or more. When the tension of the insulating film is less than 10 MPa, the improvement of the compressive stress characteristics of magnetostriction is insufficient and the noise becomes larger. It is preferably 12 MPa or more. There is no particular limitation on the upper limit. Since increasing the tension beyond the required level will lead to an increase in cost, it is preferably 30 MPa or less from the economic point of view.

[0036] It should be noted that the minimum imparted tension of the insulating film at 100°C to 200°C to the steel sheet is measured as follows.

[0037] The tension imparted to the steel sheet is the tension in the rolling direction, and is calculated by the following formula (1) from the warpage amount of the steel sheet after peeling off the insulating film on one side of the steel sheet using an alkali, an acid, etc.

[0038] Tension imparted to the steel sheet [MPa] = Young's modulus of the steel sheet [GPa] × plate thickness [mm] × warpage amount [mm] ÷ (warpage measurement length [mm])2 × 103... Formula (1)

[0039] Among them, the Young's modulus of the steel plate is 132 GPa.

[0040] Then, the measurement sample is heated from 100°C to 200°C at a rate of 20°C / hr, and the tension applied to the steel plate calculated using the value of the warpage amount when the warpage amount is the smallest is taken as the minimum tension applied to the steel plate by the insulating film from 100°C to 200°C.

[0041] In the present invention, the fact that the minimum tension applied to the steel plate by the insulating film from 100°C to 200°C is 10 MPa or more means that the tension applied to the steel plate by the insulating film in the temperature range of 100°C to 200°C is 10 MPa or more.

[0042] The insulating film targeted in the present invention contains at least one selected from Mg, Ca, Ba, Sr, Zn, Al, Mn, Co and Si, P, O. In addition, the insulating film of the present invention may not contain Cr, but from the viewpoint of environmental load, it is preferably not contain Cr.

[0043] P is necessary to form a network structure of P-O-P in the form of phosphate to ensure the adhesion between the insulating film matrix (substrate films such as metal matrix, forsterite film and other ceramic films) and the insulating film.

[0044] Si forms a network structure of Si-O-Si in the form of silicate, which helps to improve the tension imparting property due to the moisture absorption resistance, heat resistance and thermal expansion coefficient of the insulating film.

[0045] In order to ensure the stability of the network structures of P-O-P and Si-O-Si with each other, it is necessary to contain at least one metal element selected from Mg, Ca, Ba, Sr, Zn, Al, Mn, Co.

[0046] In addition, the insulating film of the present invention may have metal elements other than the above. Examples of the above metal elements include Li, Zr, Na, K, Hf, Ti, W.

[0047] It should be noted that whether the insulating film contains the above elements can be determined by, for example, fluorescent X-ray analysis, GD-OES (glow discharge optical emission spectroscopy).

[0048] The insulating film of the present invention can be obtained by, for example, applying a treatment liquid mixed with at least one of phosphates selected from Mg, Ca, Ba, Sr, Zn, Al, Mn, and Co, colloidal silica, and any additives that can be used, for example, to the surface of a directional electromagnetic steel sheet, and then sintering the treated liquid to obtain an insulating film formed by the above-mentioned composition. In order to improve the compatibility and dispersibility in the treatment liquid, the silica surface of the colloidal silica can be surface treated with Al or the like, and a dispersant such as aluminate can be appropriately added to the colloidal solution. In addition, as a type of phosphate, dihydrogen phosphate (heavy phosphate, heavy phosphate) is easily available and can be preferably used.

[0049] The optional additives are not particularly limited, and examples thereof include Li 2 O, NaOH, K 2 SO 4 、TiOSO 4 ·nH 2 O, ZrO 2 , HfO 2 、Na 2 WO 4 etc., preferably Li 2 O, ZrO 2 .

[0050] In addition, the content ratio of phosphate to colloidal silica in the treatment liquid is preferably 50 to 150 parts by mass of colloidal silica, more preferably 50 to 120 parts by mass, relative to 100 parts by mass of phosphate, calculated on a solid basis. In addition, when any additive is used, the content of the additive is preferably 1.0 to 15 parts by mass, more preferably 2.0 to 10 parts by mass, relative to 100 parts by mass of phosphate, calculated on a solid basis.

[0051] The insulating film has a crystallinity of 20% or more.

[0052] Generally, a glassy insulating film mainly composed of phosphate is applied to a grain-oriented electrical steel sheet. The insulating film is formed at a high temperature of 800°C to 1000°C. By making the thermal expansion coefficient of the insulating film smaller than that of the steel sheet, tensile stress can be applied to the steel sheet after the insulating film is sintered. The insulating film is usually glassy, ​​but by dispersing a crystalline phase with a low thermal expansion coefficient in the glass, lower thermal expansion can be achieved.

[0053] Based on the above viewpoints, in the present invention, the insulating film contains a crystal phase with a crystallinity of 20% or more to improve the tension applied to the steel sheet. In order to sufficiently reduce the thermal expansion coefficient of the insulating film, the crystallinity needs to be 20% or more. The upper limit of the crystallinity can be 100%, that is, it can be all crystal phases. However, from the viewpoint of corrosion resistance, etc., it is preferably 80% or less. More preferably, it is 60% or less.

[0054] It should be noted that the crystallinity refers to the proportion of the crystalline phase in the insulating film, and the crystallinity can be obtained by the following methods, namely, the method of X-ray diffraction; or the method of slightly etching the insulating film with an acid, a base, warm water, etc., and observing the unevenness of the surface by utilizing the difference in the etching rates of the glass phase and the crystalline phase to obtain the area ratio, etc. From the viewpoint of easy measurement, the latter method is preferred.

[0055] The desired crystallinity can be obtained by controlling the heating rate before reaching the sintering temperature, the sintering temperature, the sintering time, etc. during the sintering treatment.

[0056] The simplest method for precipitating a low thermal expansion crystalline phase in a phosphate-based glassy insulating film is the method disclosed in Patent Documents 3 and 4 for crystallization by heat treatment, etc. In this method, mainly pyrophosphates (Mg 2 P 2 O 7 , Ni 2 P 2 O 7 , etc.) are precipitated. For these pyrophosphates, for example, the average thermal expansion coefficient of Mg 2 P 2 O 7 shows a very small value of 43×10 －7 (°C －1 ) from 25°C to 1000°C, and thus greatly helps to reduce the thermal expansion coefficient of the insulating film. However, Mg 2 P 2 O 7 contracts due to structural phase dislocation at a temperature of about room temperature to 70°C, and thus the average thermal expansion coefficient from 100°C to 1000°C becomes as large as 70×10 －7 (°C －1 ). Due to the influence of this contraction, the tension applied to the steel plate near 100°C is greatly reduced.

[0057] The iron core of the transformer is immersed in insulating oil, and during operation, the insulating oil is heated to a temperature of about 150°C due to energy losses such as iron loss and copper loss. Therefore, what helps with the noise in the actual use state is the compressive stress characteristic of magnetostriction at a temperature of 100°C to 200°C. Even for the conventional insulating film with only a glass phase, the tension decreases slightly due to the temperature rise compared to the tension at room temperature, and the degree is approximately (sintering temperature - iron core temperature) ÷ (sintering temperature - room temperature). Even assuming that the sintering temperature is 800°C, it is (800 - 150) / (800 - 25) = 0.84, and the decrease is approximately 16%.

[0058] This phenomenon is common in pyrophosphates. However, the temperature at which structural phase dislocation occurs varies depending on the type of pyrophosphate. Therefore, it is preferable to use pyrophosphates (e.g., Zr 2 P 2 O 7 , (MgCo) 2 P 2 O 7 , Co 2 P 2 O 7 ) in which the temperature of precipitating structural phase dislocation is 200 °C or higher.

[0059] In addition, for the purpose of avoiding structural phase dislocation itself, as the crystallized phase to be formed, it is further preferable to precipitate other low thermal expansion crystallized phases that are not pyrophosphates. For example, cordierite, β-spodumene, quartz, zircon, zirconium phosphate-based, and tungsten phosphate-based crystallized phases can be cited.

[0060] The static friction coefficient of the insulating film is preferably 0.21 to 0.50, and more preferably 0.25 to 0.50. The iron core of the transformer is made by laminating oriented electromagnetic steel sheets. The higher the static friction coefficient between the steel sheets, the more integrated the laminate deforms, so the rigidity of the iron core is improved and the noise can be further suppressed to a lower level. Therefore, it is preferably 0.21 or more, and more preferably 0.25 or more. On the other hand, in the iron core assembly operation, it is necessary to slide the steel sheets to adjust the shape, and the workability is poor in steel sheets with little sliding. Therefore, it is preferably 0.50 or less.

[0061] As a method for adjusting the static friction coefficient, for example, a method of promoting the surface smoothing of the glassy film by increasing the sintering temperature or extending the time, reducing the roughness, and increasing the contact area between the steel sheets to increase the static friction coefficient can be cited.

[0062] The static friction coefficient can be measured by the method of the following examples.

[0063] From the viewpoint of environmental load, it is preferable that the insulating film does not contain Cr. In the present invention, even without containing Cr, the effects of the present invention are achieved. Problems such as insufficient tension imparting, deterioration of moisture absorption resistance, and fusion during stress relief annealing do not occur.

[0064] The average film thickness of the insulating film is preferably 4.5 μm or less, and more preferably 3.0 μm or less. If the average film thickness of the insulating film is too thick, the duty factor of the steel sheet decreases, the effective excitation magnetic flux density becomes high, and the magnetostrictive vibration becomes large. Therefore, the average film thickness of the insulating film is preferably 4.5 μm or less, and more preferably 3.0 μm or less.

[0065] The oriented electrical steel sheet with the insulating film of the present invention usually forms a ceramic film mainly composed of forsterite on the surface in advance before forming the insulating film, but other ceramic films such as metal nitrides (e.g., TiN, Si 3 N 4 etc.) can also be applied to its surface, and the film of the present invention can also be directly applied to the metal substrate.

[0066] An example of the method for forming the insulating film in the present invention will be described. For the oriented electrical steel sheet after final annealing, after washing with water to remove the excess annealing separating agent, stress relief annealing is carried out as required, and pickling treatment, water washing treatment, etc. are carried out. Then, the insulating film treatment liquid is coated on the surface of the steel sheet, sintered and dried to form an insulating film on the surface of the steel sheet. As the oriented electrical steel sheet after final annealing, both the steel sheet with a forsterite film and the steel sheet without a forsterite film can be used. The insulating film treatment liquid only needs to make the insulating film contain at least one selected from Mg, Ca, Ba, Sr, Zn, Al, Mn, Co and Si, P, O. For the sintering conditions and drying conditions, in order to make the crystallinity 20% or more, the sintering temperature is preferably the crystallization temperature + 10°C or more to 1100°C or less, and more preferably 1000°C or less. The sintering time is preferably 10 seconds to 90 seconds. Of course, for crystallization, it is necessary to be above the crystallization temperature obtained by TG-DTA (Thermo Gravimetry-Differential Thermal Analysis), and in order to make the crystallinity 20% or more, it is preferably sintered at the crystallization temperature + 10°C or more. In addition, considering the through-plate property of the thin steel sheet, it is preferably 1100°C or less, and more preferably 1000°C or less. For crystallization, the sintering time is preferably maintained for 10 seconds or more, and from the economic point of view, it is preferably 90 seconds or less.

[0067] Example 1

[0068] An oriented electrical steel sheet after final annealing with a thickness of 0.23 mm manufactured by a known method was cut into a size of 300 mm in the rolling direction × 100 mm in the direction perpendicular to the rolling, and after washing with water to remove the unreacted annealing separating agent (annealing separating agent mainly composed of MgO), stress relief annealing (800°C, 2 hours, N 2 atmosphere) was carried out. A forsterite film was formed on the surface of the above steel sheet after stress relief annealing. Then, it was slightly pickled with 5% by mass phosphoric acid. For the obtained oriented electrical steel sheet, the treatment liquids (phosphate, colloidal silica, optional additives) shown in Table 1 were coated on both sides of the steel sheet in such a way that the unit area weight after sintering treatment was 8 g / m 2 respectively, and then sintering treatment was carried out under the various conditions shown in Table 1. Nitrogen was used as the atmosphere during the sintering treatment.

[0069] The phosphate is separately the aqueous solution of dihydrogen phosphate, and the amount thereof is expressed as the amount obtained by converting based on the solid component.

[0070] The colloidal silica used is AT-30 manufactured by ADEKA Corporation, and the amount thereof is expressed by conversion based on the solid component in the form of SiO 2 .

[0071] Average film thickness

[0072] The average film thickness of the insulating film is calculated as the average film thickness of one side by observing the cross section using SEM.

[0073] Identification of crystal phase

[0074] The identification of the crystal phase is carried out by X-ray diffraction method.

[0075] Crystallinity

[0076] For the measurement of crystallinity, the surface of the insulating film of the sample is polished into a mirror surface using diamond paste, immersed in ion-exchanged water at 100 °C for 30 minutes, and then the surface is observed by SEM. The dissolved part is taken as the glass phase (AG), and the undissolved part is taken as the crystal phase (AC). The area thereof is measured, and the calculation is carried out by crystallinity R = AC / (AC + AG) × 100.

[0077] The minimum imparted tension of the insulating film at 100 °C to 200 °C to the steel plate

[0078] The imparted tension to the steel plate is the tension in the rolling direction, and is calculated using the following formula (1) from the warping amount of the steel plate after peeling off the insulating film on one side using alkali, acid, etc.

[0079] Imparted tension to the steel plate [MPa] = Young's modulus of the steel plate [GPa] × plate thickness [mm] × warping amount [mm] ÷ (warping measurement length [mm]) 2 ×10 3 …… Formula (1)

[0080] Among them, the Young's modulus of the steel plate is 132 GPa.

[0081] For the measurement of the warping amount between 100 °C and 200 °C, the sample is heated from 100 °C to 200 °C at a rate of 20 °C / hr, and the value at the minimum warping amount (i.e., the minimum imparted tension between 100 °C and 200 °C) is used.

[0082] Coefficient of static friction

[0083] The coefficient of static friction is measured using the static friction measuring instrument TYPE10 manufactured by Shinto Kagaku Co., Ltd.

[0084] Transformer noise (low-noise characteristic)

[0085] Regarding the noise of the transformer, a transformer with a capacity of 100 kVA is manufactured, and the noise is measured and evaluated at a position 1 m away from the transformer body.

[0086]

[0087] Based on the above results, in the examples of the present invention, the transformer noise can be made 40 dBA or less.

[0088] Example 2

[0089] The directionally electromagnetic steel sheet after final annealing with a plate thickness of 0.27 mm manufactured by a known method is cut into a size of 300 mm in the rolling direction × 100 mm in the direction perpendicular to the rolling. After washing with water to remove the unreacted annealing release agent (annealing release agent mainly composed of MgO), stress relief annealing (800 °C, 2 hours, N 2 atmosphere) is carried out. A forsterite film is formed on the surface of the above steel sheet after stress relief annealing. Then, it is slightly pickled with 5% by mass phosphoric acid. For the directionally electromagnetic steel sheet obtained in this way, the treatment liquids (phosphate, colloidal silica, optional CrO 3 and additives) shown in Table 2 are coated on both sides in such a way that the unit area weight after sintering treatment is 12 g / m 2 respectively, and then sintering treatment is carried out under various conditions shown in Table 2. Nitrogen is used as the atmosphere during sintering treatment.

[0090] Each phosphate uses an aqueous solution of dihydrogen phosphate, and the amount thereof represents the amount obtained by converting the solid components.

[0091] The colloidal silica uses ST-C manufactured by Nissan Chemical Industries, Ltd., and the amount thereof is expressed by converting the solid components in the form of SiO 2

[0092] Average film thickness

[0093] Regarding the average film thickness of the insulating film, the average film thickness of one side is calculated by observing the cross section using SEM.

[0094] Identification of crystal phase

[0095] The identification of the crystal phase uses X-ray diffraction method.

[0096] Crystallinity

[0097] ​For the measurement of crystallinity, the surface of the insulating film of the specimen was polished into a mirror surface using diamond paste, immersed in ion-exchanged water at 100 °C for 30 minutes, and then the surface was observed by SEM. The dissolved part was regarded as the glass phase (AG), and the undissolved part was regarded as the crystalline phase (AC). The areas were measured, and the crystallinity R = AC / (AC + AG)×100 was calculated.

[0098] The minimum tension applied to the steel plate by the above insulating film at 100 °C to 200 °C

[0099] The tension applied to the steel plate is the tension in the rolling direction, and is calculated using the following formula (1) from the warpage amount of the steel plate after peeling off the insulating film on one side using alkalis, acids, etc.

[0100] Tension applied to the steel plate [MPa] = Young's modulus of the steel plate [GPa] × plate thickness [mm] × warpage amount [mm] ÷ (warpage measurement length [mm]) 2 ×10 3 ……Formula (1)

[0101] Among them, the Young's modulus of the steel plate is 132 GPa.

[0102] For the measurement of the warpage amount between 100 °C and 200 °C, the sample was heated from 100 °C to 200 °C at a rate of 20 °C / hr, and the value at the minimum warpage amount (i.e., the minimum tension applied between 100 °C and 200 °C) was used.

[0103] Coefficient of static friction

[0104] The coefficient of static friction was measured using a static friction measuring instrument TYPE10 manufactured by Shinto Kagaku Co., Ltd.

[0105] Transformer noise

[0106] Regarding the noise of the transformer, a transformer with a capacity of 100 kVA was manufactured, and the noise was measured and evaluated at a position 1 m away from the transformer body.

[0107]

[0108] According to Table 2, it can be seen that whether the insulating film treatment liquid contains Cr or not, the noise of the transformer can be 40 dBA or less when the crystallinity of the insulating film is 20% or more and the minimum tension applied to the steel plate between 100 °C and 200 °C is 10 MPa or more.

[0109] Example 3

[0110] The influence of the average film thickness of the insulating film on the noise of the transformer was investigated. The average film thickness of the insulating film was changed by using the treatment liquids of No. 1, No. 2, and No. 3 shown in Example 2, Table 2 and setting the coating amount as shown in Table 3. As the test steel plate for forming the insulating film, a steel plate obtained by the following method was used: a directionally electromagnetic steel plate with a plate thickness of 0.20 mm after final annealing manufactured by a known method was cut into a size of 300 mm in the rolling direction × 100 mm in the direction perpendicular to rolling, and after removing the unreacted annealing release agent (annealing release agent mainly composed of MgO), stress relief annealing (800 °C, 2 hours, N 2 atmosphere) was carried out, and the steel plate with a forsterite film formed on the surface was slightly pickled with 5% by mass phosphoric acid to obtain the steel plate.

[0111] The average film thickness, identification of crystal phase, crystallinity, the minimum tension imparted to the steel plate by the above film at 100 °C to 200 °C, static friction coefficient, and transformer noise were measured by the same method as in Example 2.

[0112]

[0113] According to Table 3, it can be seen that regardless of whether the insulating film treatment liquid contains Cr or not, when the crystallinity of the insulating film is 20% or more and the minimum tension imparted to the steel plate at 100 °C to 200 °C is 10 MPa or more, the noise of the transformer can be 40 dBA or less.

Claims

1. A directional electromagnetic steel sheet having an insulating film, the insulating film containing at least one selected from Mg, Ca, Ba, Sr, Zn, Al, Mn, Co and Si, P, O, and having a crystallinity of 20% or more, and the minimum imparted tension of the insulating film to the steel sheet at 100°C to 200°C being 12.0 MPa or more, The insulating film contains Zr 2 P 2 O 7 , (MgCo) 2 P 2 O 7 , Co 2 P 2 O 7 , any one of cordierite, β-spodumene, quartz, zircon, a crystal phase of a zirconium phosphate system, and a crystal phase of a tungsten phosphate system.

2. The directional electromagnetic steel sheet according to claim 1, wherein, the static friction coefficient of the insulating film is 0.21 to 0.

50.

3. The directional electromagnetic steel sheet according to claim 1 or 2, wherein, the insulating film does not contain Cr.

4. The directional electromagnetic steel sheet according to any one of claims 1 to 3, wherein, the average film thickness of the insulating film is 4.5 μm or less.

5. An iron core of a transformer, which is made of the directional electromagnetic steel sheet according to any one of claims 1 to 4.

6. A transformer having the iron core of the transformer according to claim 5.

7. A method for reducing the noise of a transformer, which is a method for reducing the noise of a transformer, using the directional electromagnetic steel sheet according to any one of claims 1 to 4 as the directional electromagnetic steel sheet constituting the iron core of the transformer.

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