Grain-oriented electrical steel sheet and method for producing insulating coating film treatment liquid for grain-oriented electrical steel sheet

Abstract

This grain-oriented electrical steel sheet is provided with a base steel sheet and an insulating coating film, wherein the insulating coating film contains, in terms of mass%, 10%-20% of P, 18%-25% of Si, 1.1%-2.7% of V, and 2%-5% of Al as a chemical composition, and Cr is limited to less than 0.1%.

Description

Method for manufacturing grain-oriented electrical steel sheets and insulating coating solutions for grain-oriented electrical steel sheets

[0001] The present invention relates to a grain-oriented electrical steel sheet coated with an insulating film that does not contain chromate, and a method for producing an insulating film treatment solution. This application claims priority based on Japanese Patent Application No. 2024-227448, filed in Japan on December 24, 2024, the contents of which are incorporated herein by reference.

[0002] Grain-oriented electrical steel sheets are steel sheets primarily used as cores for transformers and other electrical equipment. Typically, such grain-oriented electrical steel sheets have two surface coatings: a forsterite layer (also called a primary coating or forsterite coating) formed during high-temperature finish annealing, and a phosphate coating (also called a secondary coating or insulating coating) formed by baking during heat flattening of the steel sheet after applying a treatment solution mainly composed of phosphates.

[0003] Phosphate coatings are necessary to impart electrical insulation to grain-oriented electrical steel sheets, reduce eddy current losses, and improve iron loss. In addition to insulation, phosphate coatings are also required to have various other properties such as corrosion resistance, heat resistance, slipperiness, and adhesion. This is to facilitate various manufacturing processes when processing grain-oriented electrical steel sheets to form the cores of transformers and other devices. For example, if the heat resistance, slipperiness, and adhesion of the phosphate coating are poor, the coating may peel off during the stress-relieving annealing process in core manufacturing, preventing the phosphate coating from exhibiting its intended insulation properties, or making it difficult to laminate the steel sheets smoothly, resulting in poor workability.

[0004] Furthermore, an important characteristic of the insulating coating on grain-oriented electrical steel sheets is that it imparts tension to the steel sheet. By applying tension to the steel sheet, the iron loss of the grain-oriented electrical steel sheet can be improved by facilitating magnetic domain wall movement. Applying tension to the steel sheet can also reduce magnetostriction (one of the main causes of noise in transformers).

[0005] In order to improve the various properties of grain-oriented electrical steel sheets as described above, the technologies disclosed in the following Patent Documents 1 to 10 have been researched and developed.

[0006] For example, Patent Document 1 discloses a method of applying an insulating coating solution mainly composed of aluminum phosphate, chromate, and colloidal silica of a specific composition onto a forsterite coating formed on the surface of a steel sheet after finish annealing, and then baking it. According to the technology disclosed in Patent Document 1, an insulating coating with high tension can be formed on the surface of a steel sheet, thereby reducing iron loss and magnetostriction of grain-oriented electrical steel sheets.

[0007] Furthermore, Patent Document 2 discloses a method of applying a treatment solution containing ultrafine colloidal silica particles with a particle size of 8 μm or less, a primary phosphate, and a chromate in specific proportions to a steel plate, and then baking it. According to the technology disclosed in Patent Document 2, it is possible to maintain the high tensile strength of the insulating film and further improve the slipperiness of the insulating film.

[0008] Furthermore, Patent Document 3 discloses a technique for forming a high-tensile insulating film on the surface of a grain-oriented electrical steel sheet by depositing a specific amount of an insulating film mainly composed of phosphate, chromate, and colloidal silica having a glass transition temperature of 950°C to 1200°C.

[0009] According to the technologies disclosed in the above-mentioned Patent Documents 1 to 3, it was possible to form insulating coatings with significantly superior coating properties and improved coating tension. However, the technologies disclosed in Patent Documents 1 to 3 all contain chromate, a chromium compound, in the insulating coating. In recent years, with environmental issues coming into focus, there has been a social demand to prohibit or restrict the use of compounds such as lead, chromium, and cadmium.

[0010] Therefore, technologies are being investigated that can form a good insulating coating without containing the above-mentioned chromium compounds. However, in the case of insulating coatings that do not contain chromium compounds, insufficient tension on the steel plate has been a problem.

[0011] As a method to solve the above-mentioned problems, for example, Patent Document 4 describes colloidal silica as SiO ２A method for treating grain-oriented electrical steel sheets with an insulating coating is disclosed, which involves baking a treatment solution containing 20 parts by weight of aluminum phosphate, 10 to 120 parts by weight of aluminum phosphate, 2 to 10 parts by weight of boric acid, and a total of 4 to 40 parts by weight of one or more sulfates selected from Mg, Al, Fe, Co, Ni, and Zn at 300°C or higher.

[0012] Furthermore, Patent Document 5 discloses a technology relating to a coating agent for forming a film that contains a mixture of boric acid and alumina sol and an organic solvent that is compatible with water, and has a tension-imparting effect on grain-oriented electrical steel sheets.

[0013] Furthermore, Patent Document 6 discloses a surface treatment agent for grain-oriented electrical steel sheets containing monophosphates of Al, Mg, and Ca, and colloidal silica, which includes one or more organic acid salts of Ca, Mn, Fe, Mg, Zn, Co, Ni, Cu, B, and Al. Patent Document 6 also provides examples of organic acid salts such as formate, acetate, oxalate, tartrate, lactate, citrate, succinate, and salicylate.

[0014] Furthermore, Patent Document 7 discloses a technique for an insulating coating agent for grain-oriented electrical steel sheets containing phosphate and colloidal silica, wherein the metal component in the phosphate is a specific combination of divalent metal elements, trivalent metal elements, and metal elements with a valency of 4 or higher.

[0015] Furthermore, Patent Document 8 discloses a grain-oriented electrical steel sheet comprising an insulating coating and a steel sheet, the insulating coating containing a first metal phosphate salt which is a phosphate of one or more metals selected from Al, Fe, Mg, Mn, Ni, and Zn, a second metal phosphate salt which is a phosphate of one or more metals selected from Co, Mo, V, W, and Zr, and colloidal silica.

[0016] Furthermore, Patent Document 9 discloses an aqueous composition for coating grain-oriented electrical steel, comprising an aluminum cation, a manganese cation, dihydrogen phosphate, hydrogen phosphate and / or anion phosphate, colloidal silicon dioxide, and optionally an iron cation.

[0017] Further, Patent Document 10 discloses a chromium-free insulating coating treatment liquid for a grain-oriented electromagnetic steel sheet that contains one or more selected from phosphates of Mg, Ca, Ba, Sr, Zn, Al, and Mn, and in the selected phosphate, based on PO ４ and with respect to 1 mol of this PO ４ , colloidal silica is blended in an amount of 0.5 to 10 mol in terms of SiO ２ conversion and a water-soluble vanadium compound is blended in an amount of 0.1 to 2.0 mol in terms of V.

[0018] Japanese Patent Application Laid-Open No. 48-39338, Japanese Patent Application Laid-Open No. 61-41778, Japanese Patent Application Laid-Open No. 11-071683, Japanese Patent Application Laid-Open No. 54-143737, Japanese Patent Application Laid-Open No. 7-278828, Japanese Patent Application Laid-Open No. 200-178760, Japanese Patent Application Laid-Open No. 2010-13692, International Publication No. 2017 / 057513, Japanese Patent Application Laid-Open No. 2022-519691, Japanese Patent Application Laid-Open No. 2009-041074

[0019] By these proposed technologies, various properties of the insulating coating have been improved. However, according to the inventors' previous research, in the case of an insulating coating that does not contain a chromium compound, with long-term storage, moisture absorption into the insulating coating gradually progresses, the elution amount of phosphorus from the insulating coating significantly increases, and the film tension gradually decreases. Electromagnetic steel sheets are loaded onto ships in the form of coils after production and may be transported for a long time in a high-temperature and high-humidity atmosphere. Therefore, when moisture absorption of the insulating coating progresses during long-term transportation, there is a possibility of problems such as stickiness between coils and in some cases, the coils not being able to unwind, and also the film tension decreases, and there is a possibility that the assumed iron loss value cannot be obtained. Even when using any of the above technologies, regarding the phosphorus elution amount and film tension after long-term storage, it has not reached the same level as that of the conventional coating containing chromic acid, and there is still room for improvement.

[0020] In addition, according to the previous research of the present inventors, in the case of an insulating film treatment liquid that does not contain a chromium compound and contains a large amount of vanadium, the properties of the treatment liquid are likely to change with time. For example, if the insulating film treatment liquid is stored for several days after mixing, the viscosity of the treatment liquid increases significantly. As a result, it has been found that the surface properties of the insulating film after baking are significantly deteriorated. When the surface properties of the insulating film deteriorate, voids are likely to occur when electromagnetic steel sheets are laminated, and the stacking ratio of the grain-oriented electromagnetic steel sheet may decrease, resulting in deterioration of the transformer characteristics.

[0021] As described above, the insulating film of the grain-oriented electromagnetic steel sheet must have electrical insulation and be capable of applying a large tension to the surface of the steel sheet. In addition, for the insulating film of the grain-oriented electromagnetic steel sheet, it is also required that the amount of phosphorus elution does not easily increase and the film tension does not easily change even when stored for a long time under high-temperature and high-humidity conditions. In addition, it is also required that the insulating film treatment liquid does not easily change with time and the insulating film after baking has excellent surface properties.

[0022] One aspect of the present invention has been made to solve the above problems. One aspect of the present invention is based on the premise that the insulating film does not contain chromate and has moisture resistance and tension application to the base steel sheet equal to or better than the conventional level. Furthermore, even when stored in a high-temperature and high-humidity atmosphere for a long time, it aims to provide a grain-oriented electromagnetic steel sheet with a small amount of phosphorus elution from the insulating film, a small decrease in film tension, and excellent surface properties. In addition, the present invention aims to provide a method for manufacturing an insulating film treatment liquid for such a grain-oriented electromagnetic steel sheet.

[0023] The gist of the present invention is as follows.

[0024] (1) A grain-oriented electrical steel sheet according to one aspect of the present invention comprises a base steel sheet and an insulating coating, wherein the insulating coating is limited to containing, in mass%, P: 10-20%, Si: 18-25%, V: 1.1-2.7%, Al: 2-5%, and Cr: less than 0.1%. (2) A method for producing an insulating coating treatment liquid for a grain-oriented electrical steel sheet according to one aspect of the present invention, wherein when an insulating coating treatment liquid is obtained by mixing a metal phosphate salt and colloidal silica without adding chromate, the insulating coating treatment liquid contains PO in the metal phosphate salt ４ Based on this, 1.0 mol PO ４ In contrast, SiO ２ It contains, in terms of equivalent weight, 1.0 mol to 2.0 mol of colloidal silica and, in terms of metallic weight, 0.050 mol to less than 0.12 mol of vanadium.

[0025] According to the above embodiment of the present invention, assuming that the insulating coating does not contain chromate and that its moisture resistance and ability to impart tension to the base steel sheet are equivalent to or better than conventional methods, it is possible to stably obtain grain-oriented electrical steel sheets that exhibit low phosphorus leaching from the insulating coating, minimal decrease in coating tension, and excellent surface properties, even when stored for a long period in a high-temperature, high-humidity atmosphere. In addition, a method for producing such an insulating coating treatment liquid for grain-oriented electrical steel sheets can be provided.

[0026] This figure shows the relationship between the amount of vanadium contained in the insulating coating and the coating tension before constant temperature and humidity maintenance. This figure also shows the relationship between the amount of vanadium contained in the insulating coating and the decrease in coating tension before and after constant temperature and humidity maintenance.

[0027] Preferred embodiments of the present invention are described in detail below. However, the present invention is not limited to the configuration disclosed in these embodiments, and various modifications are possible without departing from the spirit of the invention. In addition, the numerical limit ranges shown in these embodiments include both lower and upper limits. Numerical values ​​indicated as "greater than" or "less than" are not included in the numerical range. Unless otherwise specified, "%" for the content of each element means "mass%".

[0028] The grain-oriented electrical steel sheet according to this embodiment has a small amount of phosphorus eluted from the insulating film, a small decrease in the film tension of the insulating film, and excellent surface properties even when stored for a long time without containing chromate. Hereinafter, the results of preliminary experiments for obtaining the grain-oriented electrical steel sheet according to this embodiment will be described.

[0029] <Experiment> A finished annealed grain-oriented electrical steel sheet with a thickness of 0.23 mm manufactured by a well-known method was sheared into a width of 60 mm and a length of 300 mm, and the annealing release agent (the annealing release agent applied to the surface of the base steel sheet before finish annealing) adhering to the surface was removed by washing with water, and the resulting sheet was prepared as the base steel sheet.

[0030] Next, 100 g of an aqueous solution of a metal phosphate (solid content fraction: 40% by mass), 120 g of colloidal silica (solid content fraction: 30% by mass), and, if necessary, other additives (such as vanadium compounds) were mixed to obtain an insulating film treatment liquid. Note that, other than the solid content, the above was water and did not contain chromate. Aluminum phosphate was used as the metal phosphate. As shown in Table 1, 10 types of insulating film treatment liquids with varying amounts of aluminum and vanadium were prepared. Note that the mol ratio of PO ４ in the metal phosphate to the total amount of metal (aluminum and vanadium) was adjusted so that the mol ratio was 2.8 mol: 1 mol.

[0031]

[0032] Regarding the insulating film treatment liquids shown in Table 1, when the content of each inclusion contained in the treatment liquid was converted to the number of moles based on 1 mol of PO ４ in the metal phosphate, it was as shown in Table 2.

[0033]

[0034] Then, after storing the above insulating film treatment liquid at room temperature for 3 days, the base steel sheet prepared earlier was coated on both sides with a roll coater so that the film adhesion amount after baking was 4.5 g / m ２ per side. Thereafter, baking was performed at a temperature of 850°C for 30 seconds.

[0035] The surface properties of the insulating coating after baking were investigated. Surface properties were evaluated by the packing factor. The method for measuring the packing factor will be described in detail later, but when the surface properties of the insulating coating deteriorate, the variation in the thickness of the insulating coating increases, and the packing factor of the grain-oriented electrical steel sheet decreases. If this packing factor is 95.0% or higher, it can be judged that the surface properties are excellent.

[0036] Furthermore, the chemical composition of the insulating coating after baking was quantitatively analyzed. The chemical composition analysis method will be described in detail later, but the chemical composition of the insulating coating (unit: mass%) was derived from the mass of the insulating coating calculated from the mass difference before and after dissolution of the insulating coating, and from the quantitative analysis results of each element contained in the insulating coating dissolution solution.

[0037] As described above, a steel plate was coated with an insulating coating solution on both sides of the base steel plate and baked. This steel plate was then used as a test specimen and subjected to constant temperature and humidity maintenance (temperature 50°C, humidity 90%, 1 week). By maintaining constant temperature and humidity, it is possible to simulate long-term storage in a high-temperature, high-humidity atmosphere. For example, to maintain constant temperature and humidity, the steel plate can be placed in a constant temperature and humidity chamber (temperature 50°C, humidity 90%) for one week.

[0038] The film tension was measured before and after constant temperature and humidity maintenance. Film tension refers to the tension exerted by the film on the steel plate. The method for measuring film tension will be described in detail later, but the film tension was derived by measuring the radius of curvature of the bending of the steel plate caused by peeling off the insulating film on one side. In this experiment, a steel plate with a thickness of 0.23 mm, a Young's modulus of 115 GPa, and a Poisson's ratio of 0.38 was used. If the film tension before constant temperature and humidity maintenance is 8.0 MPa or higher, and the decrease in film tension after constant temperature and humidity maintenance is 60% or less, the film tension can be judged to be sufficient.

[0039] Figure 1 shows the relationship between the amount of vanadium contained in the insulating film and the film tension before constant temperature and humidity maintenance, based on the evaluation results described above.

[0040] As shown in Figure 1, the film tension increased as the amount of vanadium in the insulating film increased. For example, when the amount of vanadium in the insulating film was 1.1 mass% or more, the film tension remained almost constant at 8.0 MPa or higher. Although not shown in the figure, when the amount of vanadium contained in the insulating film exceeded 2.7 mass%, the packing ratio as a grain-oriented electrical steel sheet was not satisfied.

[0041] Next, Figure 2 shows the results of a study summarizing the relationship between the amount of vanadium contained in the insulating coating and the decrease in coating tension before and after constant temperature and humidity maintenance.

[0042] As shown in Figure 2, the decrease in film tension decreased as the amount of vanadium in the insulating coating increased. For example, when the amount of vanadium in the insulating coating was 1.1 mass% or more, the decrease in film tension was 60% or less. Although not shown in the figure, as described above, when the amount of vanadium contained in the insulating coating exceeded 2.7 mass%, the packing ratio as a grain-oriented electrical steel sheet was not satisfied.

[0043] Regarding the above experimental results, the mechanism by which the insulating coating solution containing a predetermined amount of vanadium increases the film tension before constant temperature and humidity maintenance, and reduces the decrease in film tension before and after constant temperature and humidity maintenance, is not yet clear. However, the inventors believe the following.

[0044] The properties of phosphates can be altered by mixing network-forming oxides, modified oxides, and intermediate oxides in various proportions. Vanadium is known to be a network-forming oxide with high single-bond strength, and it is thought that the inclusion of vanadium strengthens the bonds between phosphates, resulting in increased film tension.

[0045] Furthermore, the P (phosphorus)-O (oxygen)-P (phosphorus) bond in the phosphate that forms the insulating film is susceptible to hydrolysis when water molecules act on it. Therefore, by including V (vanadium), the P-O-P bond changes to a P-O-V bond, resulting in a phosphate structure that is highly resistant to hydrolysis, and thus reducing the decrease in film tension before and after constant temperature and humidity maintenance.

[0046] Furthermore, as mentioned above, if the amount of vanadium contained in the insulating coating solution is excessive, the insulating coating solution is prone to deterioration over time, and the surface properties of the insulating coating after baking tend to deteriorate. The inventors have found the optimal amount of vanadium for the insulating coating that can achieve both good coating properties and good surface properties.

[0047] The grain-oriented electrical steel sheet according to this embodiment will be described in detail below.

[0048] The grain-oriented electrical steel sheet according to this embodiment comprises a base steel sheet and an insulating coating.

[0049] <Base Steel Sheet for Grain-Grain Electrical Steel Sheet> First, the base steel sheet for the grain-grain electrical steel sheet according to this embodiment will be described. The chemical composition of the base steel sheet for grain-grain electrical steel sheet is not limited to a specific composition as long as the magnetic and mechanical properties required for grain-grain electrical steel sheet can be obtained, but an example of the chemical composition of the base steel sheet is as follows. For example, the base steel sheet for grain-grain electrical steel sheet may contain, in mass%, C: 0.010% or less, Si: 2.00 to 4.00%, Mn: 0.05 to 1.00%, Al: 0.010 to 0.065%, N: 0.004% or less, S: 0.010% or less, with the remainder being Fe and impurities.

[0050] Hereafter, the percentages related to the chemical composition shall refer to the mass percentage relative to the total mass of the base steel sheet.

[0051] C: 0.010% or less. Carbon (C) is an element effective in controlling the primary recrystallization structure, but it adversely affects magnetic properties, so it is removed by decarburization annealing before finish annealing. If the C concentration in the final product exceeds 0.010%, C precipitates due to aging, and the hysteresis loss deteriorates, so the C concentration is preferably 0.010% or less. The C concentration is preferably 0.007% or less, and more preferably 0.005% or less. The lower limit of the C concentration includes 0%, but the C concentration may exceed 0%. However, if the C concentration is reduced to less than 0.0001%, the manufacturing cost increases significantly, so 0.0001% is the practical lower limit for practical steel sheets. In grain-oriented electrical steel sheets, the C concentration is usually reduced to about 0.001% or less by decarburization annealing.

[0052] Si: 2.00 to 4.00% Si (silicon) is an element that increases the electrical resistance of steel sheets and improves iron loss characteristics. If the Si concentration is less than 2.00%, a γ transformation of the steel structure occurs during finish annealing, and the crystal orientation of the steel sheet is impaired, so the Si concentration is preferably 2.00% or higher. The Si concentration is preferably 2.50% or higher, and more preferably 3.00% or higher. On the other hand, if the Si concentration exceeds 4.00%, the workability of the grain-oriented electrical steel sheet decreases and cracks occur during rolling, so the Si concentration is preferably 4.00% or lower. The Si concentration is preferably 3.50% or lower.

[0053] Mn: 0.05-1.00% Manganese (Mn) is an element that prevents cracking during hot rolling and, by bonding with S and / or Se, produces MnS and MnSe, which function as inhibitors. If the Mn concentration is less than 0.05%, the effect of adding Mn is not fully expressed, so the Mn concentration is preferably 0.05% or higher. The Mn concentration is preferably 0.07% or higher, more preferably 0.09% or higher. On the other hand, if the Mn concentration exceeds 1.00%, the precipitation dispersion of MnS and MnSe becomes non-uniform, the desired secondary recrystallized structure cannot be obtained, and the magnetic flux density decreases, so the Mn concentration is preferably 1.00% or lower. The Mn concentration is preferably 0.80% or lower, more preferably 0.60% or lower.

[0054] Al: 0.010-0.065% Aluminum (Al) is an element that combines with N to form (Al,Si)N or AlN, which functions as an inhibitor. If the Al concentration is less than 0.010%, the effect of adding Al is not sufficiently expressed, and secondary recrystallization does not proceed sufficiently, so the Al concentration is preferably 0.010% or higher. The Al concentration is preferably 0.015% or higher, more preferably 0.020% or higher. On the other hand, if the Al concentration exceeds 0.065%, the precipitation dispersion of the inhibitor becomes non-uniform, the desired secondary recrystallized structure cannot be obtained, and the magnetic flux density decreases, so the Al concentration is preferably 0.065% or lower. The Al concentration is preferably 0.050% or lower, more preferably 0.040% or lower.

[0055] N: 0.004% or less. Nitrogen (N) is an element that combines with Al to form AlN and other inhibitors. However, if the N concentration in the final product exceeds 0.004%, the N in the steel sheet will precipitate as AlN, degrading the hysteresis loss. Therefore, the N concentration is preferably 0.004% or less. The lower limit of the N concentration includes 0%, but reducing the N concentration to less than 0.0001% significantly increases manufacturing costs. Therefore, 0.0001% is the practical lower limit for practical steel sheets. In grain-oriented electrical steel sheets, the N concentration is usually reduced to about 0.001% or less by finish annealing.

[0056] S: 0.010% or less. S (sulfur) is an element that combines with Mn to produce MnS, which functions as an inhibitor. However, if the S concentration in the final product exceeds 0.010%, the S in the steel sheet will precipitate as MnS, degrading the hysteresis loss, so the S concentration is preferably 0.010% or less. The lower limit of the S concentration includes 0%, but if the S concentration is reduced to less than 0.0001%, the manufacturing cost will increase significantly, so 0.0001% is the practical lower limit for practical steel sheets. In grain-oriented electrical steel sheets, the S concentration is usually reduced to about 0.005% or less by finish annealing.

[0057] In this embodiment, the base steel sheet may contain impurities. "Impurities" refer to substances that are introduced during the industrial production of steel from raw materials such as ore and scrap, or from the manufacturing environment.

[0058] Furthermore, in this embodiment, the base steel sheet may contain optional elements in addition to the elements and impurities mentioned above. For example, instead of a portion of the remaining Fe mentioned above, at least one of Se, Cr, Cu, P, Ni, Sn, Sb, B, Mo, or Bi may be included as an optional element. These optional elements may be included according to their purpose. Therefore, there is no need to limit the lower limit of these optional elements, and the lower limit may be 0%.

[0059] For example, in this embodiment, the base steel sheet may be enriched with one or more of the following elements as optional additives: Se: 0.010% or less, Cr: 0.30% or less, Cu: 0.40% or less, P: 0.50% or less, Ni: 1.00% or less, Sn: 0.30% or less, Sb: 0.30% or less, B: 0.0100% or less, Mo: 0.1% or less, and Bi: 0.01% or less, in order to enhance other properties without hindering the magnetic properties of the base steel sheet.

[0060] Se: 0-0.010% Se (selenium) is an element that combines with Mn to produce MnSe, which functions as an inhibitor. However, if the Se concentration in the final product exceeds 0.010%, the Se in the steel sheet will precipitate as MnSe, degrading the hysteresis loss, so the Se concentration is preferably 0.010% or less. The lower limit of the Se concentration can be 0%, or it can be 0.0001%. In grain-oriented electrical steel sheets, the Se concentration is usually reduced to about 0.005% or less by finish annealing.

[0061] Cr: 0-0.30% Cr (chromium) is an element that improves the oxide layer in decarburization annealing. For this reason, Cr may be added to the base steel sheet in a range of 0.30% or less. If the Cr concentration exceeds 0.30%, the decarburization effect is significantly inhibited, so it is preferable that the upper limit of the Cr concentration is 0.30%.

[0062] Cu: 0-0.40% Cu (copper) is an effective element for increasing the resistivity of the base steel sheet and reducing iron loss. If the Cu concentration exceeds 0.40%, the iron loss reduction effect saturates and it can cause surface defects called "copper chips" during hot rolling, so it is preferable that the upper limit of the Cu concentration is 0.40%.

[0063] P: 0-0.50% Phosphorus (P) is an effective element for increasing the resistivity of the base steel sheet and reducing iron loss. If the P concentration exceeds 0.50%, problems arise with rolling, so it is preferable that the upper limit of the P concentration be 0.50%.

[0064] Ni: 0-1.00% Nickel (Ni) is an effective element for increasing the resistivity of the base steel sheet and reducing iron loss. Ni is also an effective element for controlling the steel structure of hot-rolled sheets and improving their magnetic properties. However, secondary recrystallization becomes unstable when the Ni concentration exceeds 1.00%, so it is preferable that the upper limit of the Ni concentration be 1.00%.

[0065] Sn: 0-0.30% Sb: 0-0.30% Sn (tin) and Sb (antimony) are well-known grain boundary segregation elements. If the base steel sheet contains Al, depending on the finish annealing conditions, Al may be oxidized by moisture released from the annealing separating agent, causing the inhibitor strength to fluctuate at the coil position. As a result, the magnetic properties may fluctuate at the coil position. One countermeasure is to prevent the oxidation of Al by adding these grain boundary segregation elements, and for this purpose, Sn and Sb may be added to the base steel sheet at concentrations of 0.30% or less. On the other hand, if the concentration of these elements exceeds 0.30%, Si is less likely to be oxidized during decarburization annealing, resulting in insufficient glass film formation and significantly hindering decarburization annealing. For this reason, it is preferable that the upper limit of the concentration of these elements is 0.30%.

[0066] B: 0 to 0.0100% Boron (B) is an element that combines with N in the base steel sheet and precipitates in a complex with MnS to form BN, which functions as an inhibitor. The lower limit of the B concentration is not particularly limited and may be 0%, as described above. However, in order to fully exhibit the effect of adding B, the lower limit of the B concentration is preferably 0.0005%. The B concentration is preferably 0.0010% or more, more preferably 0.0015% or more. On the other hand, if the B concentration exceeds 0.0100%, the deposition dispersion of BN becomes non-uniform, the desired secondary recrystallized structure cannot be obtained, and the magnetic flux density decreases. For this reason, the B concentration is preferably 0.0100% or less. The B concentration is preferably 0.0080% or less, more preferably 0.0060% or less, and more preferably 0.0040% or less.

[0067] Mo: 0-0.1% Mo (molybdenum) is an effective element for improving surface properties during hot rolling. However, if the Mo concentration exceeds 0.1%, the effect of adding Mo saturates, so it is preferable that the upper limit of the Mo concentration be 0.1%.

[0068] Bi: 0-0.01% Bi (bismuth) has the effect of stabilizing precipitates such as sulfides and enhancing their function as inhibitors. However, if the Bi concentration exceeds 0.01%, Bi adversely affects glass film formation, so it is preferable that the upper limit of the Bi concentration be 0.01%.

[0069] The chemical composition of the base steel sheet described above can be measured using general analytical methods for steel. For example, the chemical composition of the base steel sheet can be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). Al can be measured as total aluminum in accordance with JIS G1257-10-1:2013. C and S can be measured using the combustion-infrared absorption method, and N can be measured using the inert gas fusion-thermal conductivity method.

[0070] The above chemical composition refers to the components of the base steel sheet. If the grain-oriented electrical steel sheet used as the measurement sample has an insulating coating on its surface, this insulating coating should be removed using the method described below before measuring the chemical composition of the base steel sheet.

[0071] For example, as a method for removing the insulating coating, the grain-oriented electrical steel sheet having the coating can be immersed in a high-temperature alkaline solution. Specifically, NaOH: 10-50% by mass + H ２ The insulating coating can be removed from grain-oriented electrical steel sheets by immersing them in a 50-90% by mass sodium hydroxide aqueous solution at 80-90°C for 5-30 minutes, followed by rinsing with water and drying. The immersion time in the sodium hydroxide aqueous solution should be adjusted according to the thickness of the insulating coating.

[0072] <Insulating coating for grain-oriented electrical steel sheet> Next, the insulating coating for grain-oriented electrical steel sheet according to this embodiment will be described. The insulating coating for grain-oriented electrical steel sheet may be limited to containing, in mass%, P: 10-20%, Si: 18-25%, V: 1.1-2.7%, Al: 2-5%, and Cr: less than 0.1%.

[0073] Hereafter, percentages related to chemical composition shall refer to mass percentages relative to the total mass of the insulating coating.

[0074] P: 10-20% P (phosphorus) is an element derived from phosphates. The insulating coating on the grain-oriented electrical steel sheet according to this embodiment is mainly formed by baking a treatment solution which is a mixture of metal phosphate and colloidal silica. Therefore, the P concentration should be 10% or more. Preferably, the P concentration should be 12% or more, and more preferably 13% or more. Also, the P concentration should be 20% or less. Preferably, the P concentration should be 18% or less, and more preferably 15% or less.

[0075] Si: 18-25% Si (silicon) is an element derived from colloidal silica. The insulating coating of the grain-oriented electrical steel sheet according to this embodiment is mainly formed by baking a processing solution which is a mixture of metal phosphate salt and colloidal silica. Therefore, the Si concentration should be 18% or more. Preferably, the Si concentration should be 20% or more, and more preferably 21% or more. Also, the Si concentration should be 25% or less. Preferably, the Si concentration should be 24% or less, and more preferably 23% or less.

[0076] V: 1.1-2.7% V (vanadium) is an important element for controlling the amount of phosphorus leaching and the film tension after long-term storage of the insulating coating. Therefore, the V concentration should be 1.1% or higher. Preferably, the V concentration should be 1.3% or higher, and more preferably 1.4% or higher.

[0077] On the other hand, V is also an element that easily alters the properties of insulating coating solutions over time. For example, insulating coating solutions that do not contain chromium compounds and contain a large amount of vanadium will see a significant increase in viscosity after being stored for just a few days after mixing. When the viscosity of the solution increases, uneven application of the insulating coating solution is more likely to occur, resulting in a significant decrease in the surface properties of the insulating coating after baking. Industrially, insulating coating solutions are not used immediately after mixing, but may be stored for several days within the process. However, insulating coating solutions that do not contain chromium compounds and contain a large amount of vanadium are prone to changes over time, degrading the surface properties of the insulating coating after baking. This is thought to be because vanadium excessively breaks the phosphate network structure of the insulating coating, causing a loss of continuity in the insulating coating and making it easier for unevenness to occur in appearance. For this reason, the V concentration should be 2.7% or less. Preferably, the V concentration is 2.2% or less, and more preferably 2.0% or less.

[0078] In particular, when the V concentration is 2.2% or less, the surface properties of the insulating film after baking are significantly improved. For example, the packing ratio as a grain-oriented electrical steel sheet is significantly improved. As shown in the examples described later, when the V concentration is greater than 2.2%, the packing ratio is less than 97%, but when the V concentration is 2.2% or less, the packing ratio is 97% or more. This result indicates that the surface properties of the insulating film after baking are significantly improved.

[0079] Al: 2-5% Al (aluminum) is an element included in the insulating film when an insulating film is formed using Al phosphate. In this case, the Al concentration should be 2% or more. Preferably, the Al concentration should be 2.5% or more, and more preferably 3.0% or more. Also, the Al concentration should be 5% or less. Preferably, the Al concentration should be 4.5% or less, and more preferably 4.0% or less.

[0080] Cr: Less than 0.1% Cr (chromium), when added to the processing solution as a chromium compound, is an element that significantly improves the coating properties of the insulating film. However, as mentioned above, in recent years there has been a social demand to prohibit or restrict the use of chromium compounds. The insulating film of the grain-oriented electrical steel sheet according to this embodiment is an insulating film formed without containing chromium compounds. When the Cr concentration is less than 0.1%, it can be considered an insulating film that does not contain chromium compounds. Therefore, the Cr concentration is set to less than 0.1%. The Cr concentration is preferably 0.05% or less, and more preferably 0.01% or less. The lower limit of the Cr concentration is not particularly limited, and the lower limit may be 0%.

[0081] Furthermore, in this embodiment, the insulating coating may contain optional elements in addition to the elements described above. For example, at least one of Mg, Mn, Ni, Zn, W, Zr, Co, and Mo may be included as the optional elements. These optional elements may be included according to the purpose. Therefore, there is no need to limit the lower limit of these optional elements, and the lower limit may be 0%.

[0082] For example, in this embodiment, the insulating coating may contain one or more of the following elements as optional additives, in mass%, with the aim of not hindering its coating properties while enhancing other properties: Mg: 0-14%, Mn: 0-26%, Ni: 0-28%, Zn: 0-30%, W: 0-54%, Zr: 0-37%, Co: 0-28%, Mo: 0-38%.

[0083] Furthermore, in this embodiment, the insulating coating may contain the above elements, with the remainder consisting of Fe (iron), O (oxygen), and impurities. Fe is an element that originates from diffusion from the base steel sheet during the insulating coating baking process, and it is industrially difficult to specify the Fe concentration. O is an element that originates from the oxidation reaction during the insulating coating baking process, and it is industrially difficult to specify the O concentration. Impurities refer to elements that are introduced from raw materials or the manufacturing environment during the industrial production of the insulating coating. For example, the impurities are not particularly limited, but should be 1% or less.

[0084] The chemical composition of the insulating film can be measured, for example, using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry).

[0085] For example, the measurement can be performed as follows: A grain-oriented electrical steel sheet with an insulating coating is sheared into a 3 cm square (3 mm x 3 mm x thickness) and the mass before the insulating coating is removed (weighing I) is measured. The sheared steel sheet is placed in a fluororesin beaker to which 20 ml of a 20% NaOH aqueous solution has been added, and the insulating coating is removed by gently heating it on a hot plate set to 250°C for 45 minutes. The steel sheet from which the insulating coating has been removed is removed, washed with water, dried, and the mass after the insulating coating has been removed (weighing II) is measured. The mass of the removed insulating coating can be determined by subtracting weighing II from weighing I. 30 ml of hydrochloric acid (1+1) is added to the 20% NaOH aqueous solution containing the removed insulating coating to completely dissolve the insulating coating. Then, water is added to quantify the total volume to 100 ml, a portion of which is taken and the emission intensity is measured by ICP-AES, and the mass of each element contained is derived in mg. The concentration (in mass %) of each element contained in the insulating coating can be determined by dividing the mass (mg) of each element by the mass (mg) of the removed insulating coating.

[0086] Furthermore, it is preferable to optimize the immersion conditions in the sodium hydroxide aqueous solution according to the thickness of the insulating coating. For example, when dissolving the insulating coating in the sodium hydroxide aqueous solution using the above method, it is preferable to determine immersion conditions in which only the insulating coating of the grain-oriented electrical steel sheet is dissolved and the base steel sheet is not affected, while observing the dissolution state of the insulating coating by looking at the cut surface parallel to the thickness direction of the sheet, and then dissolving only the insulating coating of the grain-oriented electrical steel sheet based on those conditions.

[0087] Furthermore, in the grain-oriented electrical steel sheet according to this embodiment, the amount of insulating coating applied is not particularly limited, but is 2.0 to 7.0 g / m² per side. ２ That would be fine. The amount of insulating coating applied is 2.0 g / m². ２In the above case, high tensile strength can be preferably imparted to the grain-oriented electrical steel sheet, and the insulation and corrosion resistance of the grain-oriented electrical steel sheet can also be preferably obtained. Furthermore, the amount of insulating coating attached is 7.0 g / m². ２ In the following cases, the decrease in the packing factor of the grain-oriented electrical steel sheet can be suppressed, thereby favorably improving the transformer characteristics. The amount of insulating coating applied is more preferably 3.0 g / m². ２ More preferably 4.0 g / m ２ That concludes the explanation. The amount of insulating coating applied is more preferably 6.0 g / m². ２ More preferably, 5.0 g / m ２ The following applies:

[0088] The amount of insulating coating attached can be determined from the change in mass before and after the removal of the insulating coating. The removal of the insulating coating can be carried out using the method described above.

[0089] Furthermore, the insulating coating of the grain-oriented electrical steel sheet according to this embodiment means an insulating coating that minimizes insufficient baking and crack formation caused by poor baking conditions of the insulating coating. If there is an excessive amount of insufficient baking and crack formation caused by poor baking conditions in the insulating coating, the insulating coating may not satisfy the requirements for electrical insulation, tension, corrosion resistance, heat resistance, slipperiness, adhesion, etc.

[0090] Furthermore, the insulating coating of the grain-oriented electrical steel sheet according to this embodiment can be evaluated by its surface properties using its packing factor. When the surface properties of the insulating coating deteriorate, the variation in the thickness of the insulating coating increases, and the packing factor of the grain-oriented electrical steel sheet decreases. It is preferable that this packing factor be 95.0% or higher. The packing factor can be measured in accordance with JIS C2550-5:2020.

[0091] Furthermore, the insulating coating of the grain-oriented electrical steel sheet according to this embodiment has a phosphorus elution amount of 50 mg / m², both before and after constant temperature and humidity maintenance. ２The following is preferable. The amount of phosphorus leached may be measured by the following method. Constant temperature and humidity maintenance should be performed at a temperature of 50°C, a humidity of 90%, and for one week. The phosphorus leaching test should be performed by immersing three 40 mm x 60 mm test pieces in distilled water at 100°C for 20 minutes and boiling them to leach phosphorus from the surface of the coating, and then quantitatively analyzing the phosphorus. The quantitative analysis of phosphorus should be performed in accordance with JIS K 0102:2019 Industrial wastewater testing method 46.1.1, and PO ４ (Unit: mg / m ２ ) should be quantified as follows.

[0092] Furthermore, it is preferable that the insulating coating of the grain-oriented electrical steel sheet according to this embodiment has a coating tension of 8.0 MPa or more before constant temperature and humidity holding. It is also preferable that the decrease in coating tension before and after constant temperature and humidity holding is 60% or less. The coating tension can be measured by the following method. Constant temperature and humidity holding can be performed under conditions of 50°C, 90% humidity, and 1 week. To measure the coating tension, the steel sheet is sheared to a length of 300 mm x width of 30 mm, one side of the steel sheet is protected with tape, and then the steel sheet is immersed in a 20% sodium hydroxide aqueous solution at 80°C to peel off the insulating coating from the side of the steel sheet that is not protected with tape. Using the steel sheet from which the insulating coating has been peeled off one side, the radius of curvature of the bending of the steel sheet caused by the peeling of the insulating coating can be measured, and the coating tension can be derived using Stoney's formula: σ = Ed / {3 × (1 - ν) × R}. Here, σ is the film tension (unit: Pa), E is the Young's modulus of the steel sheet (unit: Pa), d is the thickness of the steel sheet (unit: m), ν is the Poisson's ratio of the steel sheet (unit: -), and R is the radius of curvature of the steel sheet (unit: m). Note that the Young's modulus and Poisson's ratio of the steel sheet are values ​​that vary depending on the type of steel, so you should substitute the appropriate values ​​according to the steel sheet used.

[0093] <Method for Manufacturing Grain-Grain Electrical Steel Sheets> Next, a method for manufacturing grain-grain electrical steel sheets according to this embodiment will be described. Note that the method for manufacturing grain-grain electrical steel sheets according to this embodiment is not limited to the method described below. The manufacturing method described below is one example for manufacturing grain-grain electrical steel sheets according to this embodiment.

[0094] A silicon steel slab is formed by casting molten steel having a predetermined chemical composition using conventional methods. The chemical composition of the silicon steel slab is not limited to a specific composition as long as the magnetic and mechanical properties required for grain-oriented electrical steel sheets can be obtained. However, an example of the chemical composition of a silicon steel slab is as follows: For example, the silicon steel slab may contain, in mass percent, C: 0.085% or less, Si: 2.00 to 4.00%, Mn: 0.05 to 1.00%, Al: 0.010 to 0.065%, N: 0.004 to 0.012%, S: 0.010% or less, with the remainder being Fe and impurities.

[0095] C: 0.085% or less. Carbon (C) is an element effective in controlling the primary recrystallization structure, but it adversely affects magnetic properties, so it is removed by decarburization annealing before finish annealing. If the C concentration exceeds 0.085%, the decarburization annealing time becomes longer and productivity decreases, so the C concentration is preferably 0.085% or less. The C concentration is preferably 0.070% or less, and more preferably 0.050% or less. The lower limit of the C concentration is not particularly limited and may be 0%, or it may be greater than 0%. When considering productivity in industrial production and the magnetic properties of the product, 0.0001% is the practical lower limit of the C concentration. In grain-oriented electrical steel sheets, the C concentration is usually reduced to about 0.001% or less by decarburization annealing.

[0096] Si: 2.00 to 4.00% Si (silicon) is an element that increases the electrical resistance of steel sheets and improves iron loss characteristics. If the Si concentration is less than 2.00%, γ transformation occurs during finish annealing, and the crystal orientation of the steel sheet is impaired, so the Si concentration is preferably 2.00% or higher. The Si concentration is preferably 2.50% or higher, and more preferably 3.00% or higher. On the other hand, if the Si concentration exceeds 4.00%, workability decreases and cracks occur during rolling, so the Si concentration is preferably 4.00% or lower. The Si concentration is preferably 3.50% or lower.

[0097] Mn: 0.05-1.00% Manganese (Mn) is an element that prevents cracking during hot rolling and forms MnS or MnSe, which function as inhibitors by bonding with S and / or Se. If the Mn concentration is less than 0.05%, the additive effect will not be sufficiently expressed, so the Mn concentration is preferably 0.05% or higher. The Mn concentration is preferably 0.07% or higher, more preferably 0.09% or higher. On the other hand, if the Mn concentration exceeds 1.00%, the precipitation dispersion of MnS or MnSe becomes non-uniform, the desired secondary recrystallized structure cannot be obtained, and the magnetic flux density decreases, so the Mn concentration is preferably 1.00% or lower. The Mn concentration is preferably 0.80% or lower, more preferably 0.06% or lower.

[0098] Al: 0.010-0.065% Aluminum (Al) is an element that combines with N to form (Al,Si)N or AlN, which functions as an inhibitor. If the Al concentration is less than 0.010%, the additive effect will not be sufficiently expressed and secondary recrystallization will not proceed sufficiently, so the Al concentration is preferably 0.010% or higher. The Al concentration is preferably 0.015% or higher, more preferably 0.020% or higher. On the other hand, if the Al concentration exceeds 0.065%, the precipitation dispersion of (Al,Si)N and the like becomes non-uniform, the desired secondary recrystallized structure cannot be obtained, and the magnetic flux density decreases, so the Al concentration is preferably 0.065% or lower. The Al concentration is preferably 0.050% or lower, more preferably 0.040% or lower.

[0099] N: 0.004-0.012% Nitrogen (N) is an element that combines with Al to form AlN and other inhibitors, but it is also an element that forms blisters (vacancies) in the steel sheet during cold rolling. If the N concentration is less than 0.004%, the formation of AlN will be insufficient, so the N concentration is preferably 0.004% or higher. The N concentration is preferably 0.006% or higher, more preferably 0.007% or higher. On the other hand, if the N concentration exceeds 0.012%, there is a concern that blisters (vacancies) will be generated in the steel sheet during cold rolling, so the N concentration is preferably 0.012% or lower. The N concentration is preferably 0.010% or lower, more preferably 0.009% or lower.

[0100] S: 0.010% or less. S (sulfur) is an element that combines with Mn to form MnS, which functions as an inhibitor. If the S concentration exceeds 0.010%, the deposition and dispersion of MnS after purification will be non-uniform, the desired secondary recrystallized structure may not be obtained, the magnetic flux density may decrease, and the hysteresis loss may deteriorate, or MnS may remain after purification, resulting in a deterioration of the hysteresis loss. There is no particular lower limit, but the S concentration may be 0%, and preferably 0.003% or more. The S concentration is more preferably 0.007% or more.

[0101] In this embodiment, the silicon steel slab may contain impurities. "Impurities" refer to substances that are introduced during the industrial production of steel, either from the raw materials such as ore or scrap, or from the manufacturing environment.

[0102] Furthermore, in this embodiment, the silicon steel slab may contain optional elements in addition to the elements and impurities described above. For example, instead of a portion of the remaining Fe described above, at least one of Se, Cr, Cu, P, Ni, Sn, Sb, B, Mo, or Bi may be included as an optional element. These optional elements may be included according to their purpose. Therefore, there is no need to limit the lower limit of these optional elements, and the lower limit may be 0%. Moreover, even if these optional elements are included as impurities, the above effects will not be impaired.

[0103] For example, in this embodiment, the silicon steel slab may contain one or more of the following in amounts that do not hinder the magnetic properties of the grain-oriented electrical steel sheet and can enhance other properties: Se: 0.010% or less, Cr: 0.30% or less, Cu: 0.40% or less, P: 0.50% or less, Ni: 1.00% or less, Sn: 0.30% or less, Sb: 0.30% or less, B: 0.0100% or less, Mo: 0.1% or less, and Bi: 0.01% or less.

[0104] In the hot rolling process, a hot-rolled sheet is obtained by hot-rolling a slab having the above chemical composition. The hot-rolling conditions are not particularly limited, and ordinary conditions can be used. The hot-rolled sheet obtained in the hot-rolling process is wound into a coil.

[0105] Before subjecting the slab to hot rolling, it may be heated to a temperature exceeding 1300°C to sufficiently dissolve the inhibitor components such as MnS and AlN. Alternatively, from the viewpoint of productivity and manufacturing cost, the slab may be heated to around 1250°C, assuming that the inhibitors will be strengthened in a subsequent nitriding treatment.

[0106] In the hot-rolled sheet annealing process, a coiled hot-rolled sheet is rewound into a strip, and then the strip is subjected to hot-rolled sheet annealing to obtain an annealed hot-rolled sheet. The hot-rolled sheet annealing conditions are not particularly limited, and ordinary conditions can be used.

[0107] In the cold rolling process, a cold-rolled sheet with a final thickness is obtained by cold-rolling the annealed hot-rolled sheet once or more times. In this cold rolling process, the cold-rolled sheet may also be obtained by cold-rolling the annealed hot-rolled sheet twice or more times, with an intermediate annealing in between. The annealing performed before the finish (final) cold rolling is for homogenization of the crystalline structure. The cold rolling conditions and intermediate annealing conditions are not particularly limited, and normal conditions can be used.

[0108] In the decarburization annealing process, a decarburized annealed sheet is obtained by decarburizing the cold-rolled sheet. In this decarburization annealing process, the cold-rolled sheet is heat-treated in humid hydrogen to reduce the amount of carbon in the cold-rolled sheet to a level that does not deteriorate due to magnetic aging as a product steel sheet, and to induce primary recrystallization in the cold-rolled sheet, preparing it for the subsequent secondary recrystallization. The decarburization annealing conditions are not particularly limited, and ordinary conditions can be used. On the surface of the decarburized annealed sheet obtained by such a decarburization annealing process, SiO ２ An oxide film is formed. Furthermore, when cold-rolled sheets are manufactured from slabs heated to approximately 1250°C, after decarburization annealing, the decarburized annealed sheets are annealed in an ammonia atmosphere to generate AlN, which functions as an inhibitor, in the decarburized annealed sheets.

[0109] In the manufacturing method of grain-oriented electrical steel sheets according to this embodiment, the steel sheet on which the insulating coating is formed may be a grain-oriented electrical steel sheet having a normal forsterite coating, or it may be a grain-oriented electrical steel sheet without a forsterite coating.

[0110] In the case of grain-oriented electrical steel sheets with a typical forsterite coating, an annealing separator, mainly composed of MgO, is applied in the annealing separator application step, which is the next step after the decarburization annealing process, in order to prevent seizing during the finish annealing process. The amount of annealing separator applied is, for example, 6.0 to 14.0 g / m² per side of the decarburized annealed sheet. ２ That is the case.

[0111] In the case of grain-oriented electrical steel sheets without a forsterite coating, alumina (Al) is used in the annealing separation agent application process. ２ O ３ An annealing separation agent, mainly composed of ), is applied. The decarburized annealed plate, to which the annealing separation agent has been applied, is wound into a coil after the annealing separation agent has dried.

[0112] In the finish annealing process, a coil-shaped decarburized annealed sheet coated with an annealing separating agent is subjected to finish annealing to obtain the base steel sheet for the final product (grain-oriented electrical steel sheet). In this finish annealing process, the decarburized annealed sheet undergoes secondary recrystallization by performing finish annealing at a temperature of 1100°C or higher. In order to reduce hysteresis loss in the final product, the finish annealed sheet may be subjected to purification annealing after secondary recrystallization is complete so that the precipitates used as inhibitors are rendered harmless.

[0113] An insulating film is formed on the surface of a steel sheet that has undergone secondary recrystallization. The method for forming this insulating film includes a mixing step of an insulating film treatment solution, a coating step of applying this insulating film treatment solution to the surface of the steel sheet, and a baking step of curing the insulating film treatment solution. The insulating film is formed by these steps.

[0114] In the mixing step of the insulating coating solution, when the insulating coating solution is obtained by mixing metal phosphate salt and colloidal silica without adding chromate, the PO in the metal phosphate salt ４ Based on this, 1.0 mol PO ４ In contrast, SiO ２ It should contain, in terms of conversion, 1.0 mol to 2.0 mol of colloidal silica and, in terms of metallic equivalent, 0.050 mol to less than 0.12 mol of vanadium.

[0115] In the coating process, the insulating coating solution prepared in the mixing process is applied to the steel sheet. After finish annealing or purification annealing, the steel sheet undergoes a process to remove excess annealing separating agent by rinsing with water, followed by pickling in a sulfuric acid bath or the like, and then rinsing with water. This cleans and activates the surface of the steel sheet, after which the insulating coating solution is applied to the steel sheet in the coating process.

[0116] There are no restrictions on the method of applying the insulating coating solution to the steel sheet, but it is usually applied by a roll coater. The grain-oriented electrical steel sheet to which the insulating coating solution has been applied is subjected to a baking process under the conditions described later, thereby forming an insulating coating on the surface.

[0117] In the baking process, the grain-oriented electrical steel sheet coated with an insulating coating solution is heated to the baking soaking temperature, held at that temperature, and then cooled.

[0118] The baking soaking temperature (°C) refers to the plate temperature reached during the baking process (maximum plate temperature), and it must be between 800°C and 1000°C. If the baking soaking temperature is below 800°C, the insulating film will not undergo sufficient film formation reaction, resulting not only in a poor appearance of the film but also in an inability to impart sufficient tension to the steel plate. On the other hand, if the baking soaking temperature exceeds 1000°C, cracks may form in the insulating film, leading to a decrease in film tension, reduced insulation properties, and even damage to the steel plate. More preferably, the baking soaking temperature is between 850°C and 950°C.

[0119] The soaking time (seconds) indicates the holding time at the baking soaking temperature. A soaking time of 10 seconds or more is required. If the soaking time is less than 10 seconds, the baking of the insulating film may be insufficient, and the film tension may decrease. Preferably, it should be 20 seconds or more. On the other hand, the soaking time should be 60 seconds or less. If the soaking time exceeds 60 seconds, excessive crystallization of the insulating film may occur, causing cracks and a decrease in film tension. A soaking time of 45 seconds or less is more preferable, as it allows for the acquisition of necessary and sufficient film properties.

[0120] As mentioned above, the type of base steel sheet to which the insulating coating is applied is not particularly limited. The main feature of the grain-oriented electrical steel sheet according to this embodiment lies in the structure of the insulating coating, and the effects of the insulating coating of the grain-oriented electrical steel sheet according to this embodiment, namely the ability to impart large tension to the surface of the steel sheet, good adhesion and corrosion resistance, and excellent long-term stability despite not containing chromate, are exhibited regardless of the type of base steel sheet.

[0121] Preferably, the above-described insulating coating treatment can be applied to grain-oriented electrical steel sheets, such as those manufactured using the technology disclosed in Patent Document No. 7-268567. In this case, an effect of further reducing iron loss can be obtained. Specifically, by applying the above-described insulating coating treatment to steel sheets having an average grain size of 1 to 10 mm and an average angle of 8° or less between the (110)

[001] crystal orientation and the rolling direction, an effect of further reducing iron loss can be obtained.

[0122] <Insulating coating treatment liquid for grain-oriented electrical steel sheets> Next, we will describe the insulating coating treatment liquid (hereinafter also simply referred to as "insulating coating treatment liquid") used for grain-oriented electrical steel sheets according to this embodiment.

[0123] In this embodiment, the insulating coating solution does not contain chromium compounds and mainly contains metal phosphate salts and colloidal silica. This insulating coating solution contains PO in the metal phosphate salt ４ Based on this, 1.0 mol PO ４ In contrast, SiO ２ It should contain 1.0 mol to 2.0 mol of colloidal silica (calculated equivalently) and 0.050 mol to less than 0.12 mol of vanadium (calculated equivalently). This insulating coating solution contains 1.0 mol of PO ４ In contrast, chromium should be limited to less than 0.01 mol in metallic terms.

[0124] Specifically, the insulating coating solution is a mixture of a metal phosphate salt containing one or more metals selected from Al, Fe, Mg, Mn, Ni, Zn, Co, Mo, V, W, and Zr, and colloidal silica without the addition of a chromium compound, and the PO in the metal phosphate salt ４Based on this, this PO ４ : For 1.0 mol, add colloidal silica to SiO ２ It should contain 1.0 mol or more and 2.0 mol or less in terms of conversion, and vanadium in terms of 0.050 mol or more and less than 0.12 mol in terms of metallic equivalent. In this insulating coating solution, 1.0 mol of PO ４ In contrast, chromium should be limited to less than 0.01 mol in metallic terms.

[0125] <Method for manufacturing insulating coating solution for grain-oriented electrical steel sheets> Next, the method for manufacturing the insulating coating solution used in grain-oriented electrical steel sheets according to this embodiment (hereinafter simply referred to as "method for manufacturing insulating coating solution") and the reasons for its limitation will be described.

[0126] In this embodiment, the method for producing the insulating coating solution includes a mixing step. In this mixing step, without adding chromate, metal phosphate, colloidal silica, and other additives as needed are mixed to obtain the insulating coating solution. This insulating coating solution contains PO in the metal phosphate ４ Based on this, 1.0 mol PO ４ In contrast, SiO ２ It should contain 1.0 mol to 2.0 mol of colloidal silica (calculated equivalently) and 0.050 mol to less than 0.12 mol of vanadium (calculated equivalently). This insulating coating solution contains 1.0 mol of PO ４ In contrast, chromium should be limited to less than 0.01 mol in metallic terms.

[0127] The metal phosphate salt is preferably a phosphate containing one or more metals selected from Al, Fe, Mg, Mn, Ni, Zn, Co, Mo, V, W, and Zr. For example, aluminum phosphate may be used. When these metal phosphate salts are selected, a flat and uniform appearance is easily obtained under a wide range of baking conditions. That is, the above insulating coating treatment liquid is PO in aluminum phosphate ４ Based on this, 1.0 mol PO ４ In contrast, SiO ２ It should contain colloidal silica in an amount of 1.0 mol to 2.0 mol (calculated equivalent) and vanadium in an amount of 0.050 mol to less than 0.12 mol (calculated equivalent) equivalent.

[0128] Colloidal silica is PO in the above phosphate ４ : SiO per 1.0 mol ２ The amount to be added should be between 1.0 mol and 2.0 mol in conversion. During baking, the colloidal silica reacts with the above-mentioned phosphate to form an insulating film, and this insulating film imparts tension to the base steel sheet.

[0129] The size of the colloidal silica (silica particles) used in this embodiment is not particularly limited, but it is preferable that the average particle size (average primary particle diameter) is 4 to 35 nm. If the average particle size of the colloidal silica is less than 4 nm, the colloidal silica tends to aggregate, which can lead to poor stability of the insulating coating solution, or the insulating coating may become a porous coating with large gaps, reducing the coating tension, which is undesirable. On the other hand, if the average particle size of the colloidal silica is greater than 35 nm, the reactivity of the colloidal silica becomes poor, which can lead to insufficient mixing of the phosphate binder and the colloidal silica, or cracks may occur in the insulating coating, reducing the coating tension, which is undesirable.

[0130] Furthermore, the smaller the particle size of the colloidal silica, the denser the coating that can be formed, and the higher the coating tension can be. Therefore, it is even more preferable that the upper limit of the average particle size of the colloidal silica be 31 nm, 22 nm, 18 nm, or 12 nm. Moreover, it is even more preferable that the surface of the colloidal silica be chemically treated with aluminum. The average particle size (average primary particle diameter) of the colloidal silica can be determined, for example, by conversion from the specific surface area measured by the BET adsorption method (according to JIS Z 8830:2013).

[0131] Furthermore, the colloidal silica in the insulating coating solution is phosphate PO ４It is preferable that the amount of colloidal silica in the insulating coating solution is between 1.0 mol and 2 mol per 1.0 mol. If the amount of colloidal silica in the insulating coating solution is less than 1.0 mol under the above conditions, the film tension of the insulating coating may not be sufficient, which is undesirable. On the other hand, if the amount of colloidal silica in the insulating coating solution exceeds 2.0 mol under the above conditions, there will be insufficient phosphate, which functions as a binder, resulting in insufficient film formation and low film tension. The amount of colloidal silica blended is preferably 1.3 mol or more, and more preferably 1.5 mol or more, under the above conditions. Furthermore, the amount of colloidal silica blended is preferably 1.8 mol or less, and more preferably 1.7 mol or less, under the above conditions.

[0132] In this embodiment, the insulating coating solution mainly consists of a metal phosphate salt and colloidal silica. Therefore, the insulating coating solution preferably contains more than 50% by mass of metal phosphate salt and colloidal silica in total, and more preferably 80% by mass or more, in terms of solid content.

[0133] Vanadium is present in the PO in the above phosphate ４ For every 1.0 mol, the amount of vanadium added should be between 0.050 mol and less than 0.12 mol in terms of metal equivalent. When vanadium is added at a concentration of 0.050 mol or more under the above conditions, even if the insulating film is stored for a long period of time in a high-temperature, high-humidity atmosphere, the amount of phosphorus leached from the insulating film and the decrease in film tension can be reduced. Therefore, the amount of vanadium added should be such that it is equivalent to the amount of PO in the phosphate. ４ For every 1.0 mol, the amount of metal equivalent to 0.050 mol or more is sufficient. Under the above conditions, the amount of vanadium is preferably 0.055 mol or more, more preferably 0.060 mol or more, and even more preferably 0.065 mol or more.

[0134] On the other hand, when vanadium is added in an amount of 0.12 mol or more under the above conditions, the viscosity of the treatment solution changes significantly over time. For example, an insulating coating treatment solution that does not contain chromium compounds and contains a large amount of vanadium will have a significantly increased viscosity after being stored for just a few days after mixing. When the viscosity of the treatment solution increases, uneven application of the insulating coating treatment solution is likely to occur, and as a result, the surface properties of the insulating coating after baking are significantly reduced. Industrially, insulating coating treatment solutions are not used immediately after mixing, but may be stored for several days within the process. However, insulating coating treatment solutions that do not contain chromium compounds and contain a large amount of vanadium are prone to changes over time and degrade the surface properties of the insulating coating after baking. Therefore, the amount of vanadium added should be adjusted according to the PO in the phosphate. ４ For every 1.0 mol, the amount should be less than 0.12 mol in terms of metal equivalent. Under the above conditions, the amount of vanadium is preferably less than 0.10 mol, more preferably less than 0.094 mol, and even more preferably less than 0.090 mol.

[0135] In particular, when the vanadium content is less than 0.094 mol, the surface properties of the insulating film after baking are significantly improved. For example, the packing ratio as a grain-oriented electrical steel sheet is significantly improved. As shown in the examples described later, when the vanadium content is 0.094 mol or more, the packing ratio is less than 97%, but when the vanadium content is less than 0.094 mol, the packing ratio is 97% or more. This result indicates that the surface properties of the insulating film after baking are significantly improved.

[0136] Vanadium may be mixed into the treatment solution as a metal phosphate salt, or as a water-soluble vanadium compound. Examples of water-soluble vanadium compounds include vanadium sulfate, vanadium chloride, sodium vanadate, and ammonium vanadate, or their hydrates may be used.

[0137] In the above mixing step, various oxides such as titanium dioxide and molybdenum oxide, boric acid, sodium borate, pigments, and inorganic compounds such as barium titanate may be further mixed with the insulating coating solution.

[0138] Next, the effects of one aspect of the present invention will be described in more detail with reference to examples. However, the conditions in the examples are merely examples of conditions adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to these examples of conditions. The present invention can adopt various conditions as long as they do not depart from the spirit of the invention and achieve the objectives of the present invention.

[0139] As the final product, a slab (slab satisfying the chemical composition of the silicon steel slab described above) with a chemical composition adjusted to match the composition shown in Table 3 was heated to 1150°C and subjected to hot rolling to obtain a hot-rolled steel sheet with a thickness of 2.6 mm. After hot-rolled steel sheet annealing as necessary, it was subjected to one cold-rolling or multiple cold-rolling with intermediate annealing in between to obtain a cold-rolled steel sheet with a final thickness of 0.23 mm. This cold-rolled steel sheet was subjected to decarburization annealing, and then nitriding annealing was performed while holding it in an ammonia-containing atmosphere during the cooling process. In the process from slab heating to nitriding annealing, well-known conditions were applied.

[0140] The decarburized annealed plates, after the decarburized annealing and nitriding annealing described above, were coated with an annealing separator mainly composed of MgO and dried. The decarburized annealed plates coated with the annealing separator were then subjected to finish annealing at 1200°C for 20 hours.

[0141] Subsequently, excess annealing separating agent was removed by washing with water using a scrubber. Then, the insulating coating solution was mixed under the conditions shown in Tables 4 to 6, and the insulating coating solution was applied and baked under the conditions shown in Tables 4 to 6 to form an insulating coating on the base steel plate. The insulating coating solution was prepared by mixing metal phosphate salt and colloidal silica. The amounts of V and Al in the insulating coating solution were adjusted by adding vanadium sulfate and aluminum phosphate. After storing this insulating coating solution at room temperature for 3 days, the amount of coating adhesion after baking was 4.5 g / m² per side. ２ The coating was applied to both sides of the steel plate and then baked on.

[0142] For the obtained grain-oriented electrical steel sheets No. 1 to 51, the chemical composition of the base steel sheet and the chemical composition of the insulating coating were measured based on the above method. For example, the chemical composition of the base steel sheet and the chemical composition of the insulating coating can be measured using ICP-AES. Al can be measured as total aluminum in accordance with JIS G1257-10-1:2013. C and S can be measured using the combustion-infrared absorption method, and N can be measured using the inert gas fusion-thermal conductivity method.

[0143] Furthermore, for the obtained grain-oriented electrical steel sheets No. 1 to 51, the packing factor, phosphorus elution amount after constant temperature and humidity holding, and coating tension before and after constant temperature and humidity holding were evaluated based on the above method. For example, the packing factor can be measured in accordance with JIS C2550-5:2020. The amount of phosphorus elution can be quantitatively analyzed by immersing three 40 mm x 60 mm test pieces in distilled water at 100°C for 20 minutes and boiling them to elute phosphorus from the coating surface. The coating tension can be determined by shearing the steel sheet to a length of 300 mm x width of 30 mm, protecting one side of the steel sheet with tape, and then immersing the steel sheet in a 20% sodium hydroxide aqueous solution at 80°C to peel off the insulating coating on the unprotected side of the steel sheet. Using a steel sheet with the insulating coating peeled off from one side, the radius of curvature of the bending of the steel sheet caused by the peeling of the insulating coating can be measured, and the coating tension can be derived using Stoney's formula: σ = Ed / {3 × (1 - ν) × R}. The steel sheet had a thickness of 0.23 mm, a Young's modulus of 115 GPa, and a Poisson's ratio of 0.38. The grain-oriented electrical steel sheet was held under constant temperature and humidity conditions: holding temperature: 50°C, holding humidity: 90%, and holding time: 1 week.

[0144] The surface properties include a packing density of 95.0% or higher, and a phosphorus elution amount of 50 mg / m² after constant temperature and humidity maintenance. ２ The following criteria were used to determine whether a product was acceptable: the film tension before constant temperature and humidity maintenance was 8.0 MPa or higher, and the decrease in film tension before and after constant temperature and humidity maintenance was 60% or less. The manufacturing and evaluation results are shown in Tables 7 to 9.

[0145] As can be seen from Tables 3 to 9, among grain-oriented electrical steel sheets No. 1 to 51, the example of the present invention had an insulating coating that did not contain chromate, and even when stored for a long period in a high-temperature, high-humidity atmosphere, the amount of phosphorus leaching from the insulating coating was small, the coating tension was high before constant temperature and humidity maintenance, the decrease in coating tension before and after constant temperature and humidity maintenance was small, and the surface properties were also excellent. In contrast, among grain-oriented electrical steel sheets No. 1 to 51, the comparative examples were inferior in at least one of the following: packing density, amount of phosphorus leaching after constant temperature and humidity maintenance, coating tension, or decrease in coating tension. Note that comparative examples No. 28 and 29 contain chromate (chromium compounds) and are therefore outside the scope of the present invention.

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[0153] According to the above embodiment of the present invention, assuming that the insulating coating does not contain chromate and that its moisture resistance and ability to impart tension to the base steel sheet are equivalent to or better than conventional methods, it is possible to stably obtain grain-oriented electrical steel sheets that exhibit low phosphorus leaching from the insulating coating, minimal decrease in coating tension, and excellent surface properties even when stored for a long period in a high-temperature, high-humidity atmosphere. In addition, a method for producing such an insulating coating solution for grain-oriented electrical steel sheets can be provided. Therefore, it has high potential for industrial application.

Claims

1. A grain-oriented electrical steel sheet comprising a base steel sheet and an insulating coating, wherein the insulating coating is limited to having a chemical composition of P: 10-20% by mass%, Si: 18-25%, V: 1.1-2.7%, Al: 2-5%, and Cr: less than 0.1%.

2. When obtaining an insulating coating solution by mixing a metal phosphate salt and colloidal silica without adding chromate, the insulating coating solution contains PO in the metal phosphate salt. ４ Based on this, 1.0 mol PO ４ In contrast, SiO ２ A method for producing an insulating coating solution for grain-oriented electrical steel sheets, characterized by containing 1.0 mol or more and 2.0 mol or less of colloidal silica, and 0.050 mol or more and less than 0.12 mol of vanadium, in metallic terms.

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