Grain-oriented electrical steel sheet

Abstract

In this grain-oriented electrical steel sheet, an insulating film does not contain a chromium compound, the film tension of the insulating film is 4.9 MPa or greater, and in a spectrum obtained by measuring the insulating film using a fluorescent X-ray analysis method, when the potassium intensity is defined as P (K) and the sodium intensity is defined as P (Na), the ratio P (K) / P (Na) between P (K) and P (Na) is greater than 13.0 and less than or equal to 100.

Description

grain-oriented electrical steel sheet

[0001] The present invention relates to a grain-oriented electrical steel sheet coated with a chromate-free insulating coating. In particular, the present invention relates to a grain-oriented electrical steel sheet that exhibits little decrease in coating tension of the insulating coating even after long-term storage, despite not containing chromate. This application claims priority based on Japanese Patent Application No. 2024-070781, filed on April 24, 2024, the contents of which are incorporated herein by reference.

[0002] Grain-oriented electrical steel sheets are steel sheets that are primarily used as iron cores for transformers, etc. Such grain-oriented electrical steel sheets typically have two surface coating layers: a forsterite layer (also referred to as a primary coating or forsterite coating) formed during high-temperature finish annealing, and a phosphate coating (also referred to as a secondary coating or insulating coating) that is formed by applying a treatment solution containing phosphate as a main component and then baking it during heat flattening of the steel sheet.

[0003] Phosphate coatings are required to provide grain-oriented electrical steel sheets with electrical insulation and reduce eddy current loss, thereby improving core loss. Phosphate coatings are also required to have various other properties in addition to insulation, such as corrosion resistance, heat resistance, slipperiness, and adhesion. This is necessary to facilitate various manufacturing processes when grain-oriented electrical steel sheets are processed into iron cores for transformers and other devices. For example, if the heat resistance, slipperiness, and adhesion of a phosphate coating are poor, the phosphate coating may peel off during stress relief annealing in core manufacturing, preventing the phosphate coating from demonstrating its inherent insulating properties or hindering smooth lamination of steel sheets, resulting in poor workability.

[0004] Furthermore, one important characteristic of the insulating coating of grain-oriented electrical steel sheets is the ability to apply tension to the steel sheets. Applying tension to steel sheets can improve the iron loss of grain-oriented electrical steel sheets by facilitating domain wall motion. Applying tension 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, specific techniques such as those 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 treatment solution containing a specific composition of aluminum phosphate, chromate, and colloidal silica as main components to a forsterite coating formed on the surface of a steel sheet after finish annealing, followed by baking the solution. The technique disclosed in Patent Document 1 enables the formation of an insulating coating with high tensile strength on the surface of the steel sheet, thereby reducing the iron loss and magnetostriction of the grain-oriented electrical steel sheet.

[0007] Furthermore, Patent Document 2 discloses a method in which a treatment solution containing ultrafine colloidal silica particles with a particle size of 8 μm or less, primary phosphate, and chromate in specific proportions is applied to a steel sheet and then baked. The technology disclosed in Patent Document 2 makes it possible to maintain the high tensile strength of the insulating coating and further improve the smoothness of the insulating coating.

[0008] Furthermore, Patent Document 3 discloses a technology for forming a high-tensile insulating coating on the surface of a grain-oriented electrical steel sheet by depositing a specific amount of an insulating coating whose main components are phosphate, chromate, and colloidal silica having a glass transition point of 950°C to 1200°C.

[0009] The techniques disclosed in Patent Documents 1 to 3 above made it possible to form insulating coatings with significantly superior coating properties and improved coating tension. However, all of the techniques disclosed in Patent Documents 1 to 3 contain chromate, a chromium compound, in the insulating coating. In recent years, with increasing attention being paid to environmental issues, there has been a social demand to prohibit or restrict the use of compounds such as lead, chromium, and cadmium.

[0010] Therefore, research has been conducted into technologies that can form good insulating coatings without containing the above-mentioned chromium compounds.However, with insulating coatings that do not contain chromium compounds, there has been an issue of insufficient tension being applied to the steel sheet.

[0011] As a method for solving the above problem, for example, Patent Document 4 discloses a method for treating colloidal silica with SiO ２The present invention discloses a method for treating an insulating coating on a grain-oriented electrical steel sheet, which involves baking a treatment solution at 300°C or higher containing 20 parts by weight of ammonium nitrate, 10 to 120 parts by weight of aluminum phosphate, 2 to 10 parts by weight of boric acid, and 4 to 40 parts by weight in total of one or more sulfates of Mg, Al, Fe, Co, Ni, and Zn.

[0012] Furthermore, Patent Document 5 discloses a technology relating to a coating agent for forming a film, which 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] Patent Document 6 discloses a surface treatment agent for grain-oriented electrical steel sheet containing primary phosphates of Al, Mg, and Ca and colloidal silica, which also contains one or more organic acid salts of Ca, Mn, Fe, Mg, Zn, Co, Ni, Cu, B, and Al. Patent Document 6 also lists formate, acetate, oxalate, tartrate, lactate, citrate, succinate, and salicylate as examples of organic acid salts.

[0014] Furthermore, Patent Document 7 discloses a technology in which, in an insulating coating treatment agent for grain-oriented electrical steel sheets containing a phosphate and colloidal silica, the metal components in the phosphate are a combination of specific proportions of divalent metal elements, trivalent metal elements, and metal elements having a valence of tetravalent or higher.

[0015] Furthermore, Patent Document 8 discloses a grain-oriented electrical steel sheet comprising a steel sheet and an insulating coating containing a first metal phosphate which is a metal phosphate of one or more metals selected from Al, Fe, Mg, Mn, Ni, and Zn, a second phosphate which is a metal 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, which contains aluminum cations, manganese cations, dihydrogen phosphate, hydrogen phosphate and / or phosphate anions, colloidal silicon dioxide, and optionally iron cations.

[0017] In addition, Patent Document 10 discloses a method for forming a surface coating by applying a phosphorus salt-based top coating containing colloidal silica containing 0.01 to 1.0 mass % of Na as an impurity, and then applying a potassium compound in an amount of 10 to 60 mg / m in terms of potassium. ２ The method for producing a grain-oriented electrical steel sheet is disclosed, in which the coating is applied in a range of 100 to 150° C., dried, and then heat-treated at 500 to 850° C. for 1 minute to 5 hours.

[0018] Japanese Patent Application Publication No. 48-39338 Japanese Patent Application Publication No. 61-41778 Japanese Patent Application Publication No. 11-071683 Japanese Patent Application Publication No. 54-143737 Japanese Patent Application Publication No. 7-278828 Japanese Patent Application Publication No. 2000-178760 Japanese Patent Application Publication No. 2010-13692 International Publication No. 2017 / 057513 Japanese Patent Application Publication No. 2022-519691 Japanese Patent Application Publication No. 2015-147988

[0019] These proposed technologies have improved various properties of insulating coatings, such as the ability to apply tension to steel sheets. However, previous research by the present inventors has revealed that, in the case of insulating coatings that do not contain chromium compounds, moisture absorption into the insulating coating gradually progresses with long-term storage, resulting in a gradual decrease in coating tension. After manufacturing, electrical steel sheets are often loaded onto ships in coil form and transported for long periods of time in high-temperature, high-humidity environments. Therefore, if moisture absorption progresses during long-term transportation, the coating tension may decrease, potentially preventing the expected iron loss value from being achieved. None of the above technologies has yet achieved the same level of coating tension after long-term storage as conventional coatings containing chromic acid, leaving room for improvement.

[0020] As described above, the insulating coating of grain-oriented electrical steel sheets must have electrical insulation properties and be capable of applying a large tension to the surface of the steel sheet. In addition, the insulating coating of grain-oriented electrical steel sheets must be resistant to changes in coating tension even when stored for long periods under high-temperature and high-humidity conditions.

[0021] One aspect of the present invention has been made to solve the above-mentioned problems, and aims to provide a grain-oriented electrical steel sheet in which the insulating coating does not contain chromate, and which exhibits the same or better performance as conventional coatings in terms of tensioning the base steel sheet, and in which the coating tension decreases little even when stored for a long period of time in a high-temperature, high-humidity atmosphere.

[0022] The gist of the present invention is as follows.

[0023] (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 does not contain a chromium compound, the insulating coating has a coating tension of 4.9 MPa or more, and wherein, when the potassium intensity of a spectrum obtained by measuring the insulating coating by X-ray fluorescence analysis is defined as P(K) and the sodium intensity of the spectrum is defined as P(Na), the ratio of P(K) to P(Na), P(K) / P(Na), is greater than 13.0 and not more than 100. (2) In the grain-oriented electrical steel sheet according to (1) above, when the chromium intensity of the spectrum is defined as P(Cr) and the silicon intensity of the spectrum is defined as P(Si), the ratio of P(Cr) to P(Si), P(Cr) / P(Si), may be not more than 0.01. (3) In the grain-oriented electrical steel sheet according to (1) or (2), the base steel sheet contains, as a chemical composition, 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, Se: 0.010% or less, Cr: 0 to 0.30%, Cu: 0 to 0.40%, P: 0 to 0.50%, Ni: 0 to 1.00%, Sn: 0 to 0.30%, Sb: 0 to 0.30%, B: 0 to 0.0100%, Mo: 0 to 0.1%, and Bi: 0 to 0.01%; The balance may consist of Fe and impurities.

[0024] According to the above aspect of the present invention, it is possible to reliably obtain grain-oriented electrical steel sheets in which the insulating coating does not contain chromate and the ability to apply tension to the base steel sheet is equal to or better than conventional methods, and in which the coating tension decreases little even when the steel sheet is stored for a long period of time in a high-temperature, high-humidity atmosphere.

[0025] FIG. 1 is a diagram showing the relationship between the amount of potassium in the insulating coating treatment solution and the decrease in coating tension before and after maintaining constant temperature and humidity. ０ , Q １ , Q ２ , Q ３ The four types of Q ｎ 1 is a diagram showing the relationship between the P(K) / P(Na) value of the insulating coating and the decrease in coating tension before and after being kept at constant temperature and humidity.

[0026] Preferred embodiments of the present invention will be described in detail below. However, the present invention is not limited to the configurations disclosed in the present embodiments, and various modifications are possible within the scope of the present invention. Furthermore, the numerical ranges shown in the present embodiments include the lower and upper limits. Numerical values ​​indicated as "greater than" or "less than" do not include the numerical range. "%" regarding the content of each element means "mass %" unless otherwise specified.

[0027] The results of the preliminary experiments that led to the production of the grain-oriented electrical steel sheet according to this embodiment will be described below.

[0028] <Experiment> A finish-annealed grain-oriented electrical steel sheet having a thickness of 0.23 mm, manufactured by a known method, was sheared to a width of 60 mm and a length of 300 mm, and the annealing separator adhering to the surface was removed by washing with water to prepare a base steel sheet.

[0029] Next, 0 to 10 ml of a 30 mass % aqueous potassium hydroxide solution was added to 220 g of a stock solution containing 40 g of aluminum phosphate monobasic, 36 g of colloidal silica, and the remainder being water, to prepare 13 types of insulation coating treatment solutions, as shown in Table 1. Note that because sodium-stabilized colloidal silica was used, the amount of Na contained in the insulation coating treatment solution was 0.12 mass %.

[0030]

[0031] Then, the insulating coating treatment solutions were applied to the base steel sheets prepared above so that the coating adhesion amount after baking was 4.5 g / m per side. ２The coating was then applied to both sides using a roll coater so that the coating was as follows: Then, the coating was baked at a temperature of 850° C. for 30 seconds.

[0032] A steel plate was used as a test specimen, which was prepared by applying an insulating coating treatment liquid to both sides of a base steel plate and baking the solution. The test specimen was then subjected to constant temperature and humidity storage (temperature: 50°C, humidity: 90%, 1 week). By performing constant temperature and humidity storage, it is possible to simulate long-term storage in a high-temperature, high-humidity atmosphere. For example, the constant temperature and humidity storage can be performed by placing the steel plate in a constant temperature and humidity chamber (temperature: 50°C, humidity: 90%) for 1 week.

[0033] The coating tension was also measured before and after being kept at constant temperature and humidity. The coating tension refers to the tension applied to the steel sheet by the coating. The coating tension was measured by the following method. The steel sheet was sheared to a length of 300 mm and a width of 30 mm, and one side of the steel sheet was protected with tape. The steel sheet was then immersed in a 20% aqueous sodium hydroxide solution at 80°C to peel off the insulating coating from the side of the steel sheet that was not protected with tape. Using this steel sheet from which the insulating coating had been peeled off, the radius of curvature of the curve of the steel sheet caused by the peeling of the insulating coating was measured, and the coating tension was calculated using Stoney's equation: σ = Ed / {3 × (1 - v) × R}. Here, σ is the coating 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: 1 / m). Since the Young's modulus and Poisson's ratio of the steel sheet are values ​​that vary depending on the type of steel, values ​​appropriate for the steel sheet used may be substituted as appropriate. In this experiment, a steel sheet with a thickness of 0.23 mm, a Young's modulus of 115 GPa, and a Poisson's ratio of 0.38 was used.

[0034] The above evaluation results were summarized in FIG. 1, which shows the relationship between the amount of potassium in the insulating coating solution and the decrease in coating tension before and after maintaining constant temperature and humidity.

[0035] As shown in Figure 1, increasing the amount of potassium in the insulating coating solution reduces the decrease in coating tension. That is, increasing the amount of potassium in the insulating coating solution reduces the decrease in coating tension even after long-term storage in a high-temperature, high-humidity atmosphere.

[0036] Regarding the above experimental results, the details of the mechanism by which the decrease in coating tension before and after constant temperature and humidity maintenance decreases as the amount of potassium in the insulating coating treatment solution increases are not clear at present, but the inventors believe as follows.

[0037] The decrease in coating tension that occurs when the coating is kept at constant temperature and humidity is thought to be due to a hydrolysis reaction in which water molecules act on the P-O-P bonds of the phosphate that forms the insulating coating, cleaving the P-O-P bonds. Therefore, it is important to replace the P (phosphorus)-O (oxygen)-P (phosphorus) bonds with other bonds to create P-O-M (M is a metal element).

[0038] The structure of phosphate is PO, where four oxygen atoms are coordinated around the phosphorus atom. ４ The tetrahedron is used as a unit, and the oxygen atom is replaced by another PO ４ The oxygen atoms that connect the tetrahedra are called bridging oxygens, and the number of bridging oxygen atoms, n, is used as an index to determine the Q ｎ As shown in Figure 2, ０ , Q １ , Q ２ , Q ３ The four types of Q ｎ They can be classified into structures.

[0039] As the number n of bridging oxygen atoms decreases, the polarity around the phosphorus atom decreases, and the interaction with highly polarizable water molecules weakens, making the bond less likely to be broken.

[0040] Here, regarding the polarity around the phosphorus atom, the smaller the force of M (M is a metal element) shown in P-O-M to attract oxygen, the smaller the polarity around the phosphorus atom. This force of M (metal element) to attract oxygen is expressed as field strength. Field strength is the value obtained by dividing the valence (Z) of the metal ion by the square of the ionic radius (r) (Z / r ２ ) and Z / r of potassium ２ The value is very small, 0.4 (for example, Kobe Steel Pantec Technical Report, Vol. 35, No. 1, 27).

[0041] As the amount of potassium in the insulating coating treatment solution increases, the P-O-P bonds in the insulating coating are replaced by P-O-M bonds, and potassium acts as the M in P-O-M, further reducing the polarity around the phosphorus atom. As a result, it is thought that the hydrolysis reaction of the insulating coating is suppressed even in a high-temperature, high-humidity atmosphere, thereby reducing the decrease in coating tension.

[0042] The grain-oriented electrical steel sheet according to this embodiment will be described in detail below.

[0043] The grain-oriented electrical steel sheet according to this embodiment comprises a base steel sheet and an insulating coating, wherein the insulating coating does not contain a chromium compound, the insulating coating has a coating tension of 4.9 MPa or more, and the ratio of P(K) to P(Na), P(K) / P(Na), is greater than 13.0 and not greater than 100, where P(K) is the potassium intensity and P(Na) is the sodium intensity of a spectrum obtained by measuring the insulating coating by X-ray fluorescence analysis.

[0044] <Base Steel Sheet for Grain-Oriented Electrical Steel Sheet> First, the base steel sheet for the grain-oriented electrical steel sheet according to this embodiment will be described. The base steel sheet for the grain-oriented electrical steel sheet is not particularly limited, but the chemical composition 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 balance being Fe and impurities. Hereinafter, the "%" related to the chemical composition refers to the mass % relative to the total mass of the base steel sheet.

[0045] 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 final annealing. If the C concentration in the final product exceeds 0.010%, C precipitates during aging, deteriorating hysteresis loss. Therefore, the C concentration is set to 0.010% or less. The C concentration is preferably 0.007% or less, more preferably 0.005% or less. The lower limit of the C concentration includes 0%, but the C concentration may exceed 0%. However, reducing the C concentration to less than 0.0001% significantly increases manufacturing costs, so 0.0001% is the practical lower limit for practical steel sheets. In grain-oriented electrical steel sheets, the C concentration is typically reduced to approximately 0.001% or less by decarburization annealing.

[0046] 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 of the steel structure occurs during finish annealing, damaging the crystal orientation of the steel sheet. Therefore, the Si concentration is set to 2.00% or more. The Si concentration is preferably 2.50% or more, and more preferably 3.00% or more. 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 set to 4.00% or less. The Si concentration is preferably 3.50% or less.

[0047] Mn: 0.05 to 1.00% Mn (manganese) is an element that prevents cracking during hot rolling and combines with S and / or Se to produce MnS and MnSe, which function as inhibitors. If the Mn concentration is less than 0.05%, the effect of adding Mn is not fully realized, so the Mn concentration is set to 0.05% or more. The Mn concentration is preferably 0.07% or more, more preferably 0.09% or more. On the other hand, if the Mn concentration exceeds 1.00%, the precipitation and dispersion of MnS and MnSe becomes non-uniform, the desired secondary recrystallization structure cannot be obtained, and the magnetic flux density decreases. Therefore, the Mn concentration is set to 1.00% or less. The Mn concentration is preferably 0.80% or less, more preferably 0.60% or less.

[0048] Al: 0.010 to 0.065% Al (aluminum) 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 fully realized, and secondary recrystallization does not proceed sufficiently. Therefore, the Al concentration is set to 0.010% or more. The Al concentration is preferably 0.015% or more, more preferably 0.020% or more. On the other hand, if the Al concentration exceeds 0.065%, the precipitation dispersion of the inhibitor becomes nonuniform, the desired secondary recrystallization structure cannot be obtained, and the magnetic flux density decreases. Therefore, the Al concentration is set to 0.065% or less. The Al concentration is preferably 0.050% or less, more preferably 0.040% or less.

[0049] N: 0.004% or less N (nitrogen) is an element that combines with aluminum to form AlN and other elements that function as inhibitors. However, if the N concentration in the final product exceeds 0.004%, the N in the steel sheet precipitates as AlN, deteriorating hysteresis loss. Therefore, the N concentration is set to 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, so 0.0001% is the practical lower limit for practical steel sheets. In grain-oriented electrical steel sheets, the N concentration is usually reduced to approximately 0.001% or less by finish annealing.

[0050] S: 0.010% or less S (sulfur) is an element that combines with Mn to form MnS, which functions as an inhibitor. However, if the S concentration in the final product exceeds 0.010%, the S in the steel sheet precipitates as MnS, deteriorating hysteresis loss. Therefore, the S concentration is set to 0.010% or less. The lower limit of the S concentration includes 0%, but reducing the S concentration to less than 0.0001% significantly increases production costs, so 0.0001% is the substantial 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.

[0051] In the present embodiment, the base steel sheet may contain impurities. Note that the term "impurities" refers to substances that are mixed in from raw materials such as ore or scrap, or from the manufacturing environment, when steel is industrially manufactured.

[0052] Furthermore, in this embodiment, the base steel sheet may contain optional elements in addition to the above-described elements and impurities. For example, instead of a portion of the remaining Fe, at least one of Se, Cr, Cu, P, Ni, Sn, Sb, B, Mo, or Bi may be contained as an optional element. These optional elements may be contained according to their intended purpose. Therefore, there is no need to set a lower limit for these optional elements, and the lower limit may be 0%. Furthermore, even if these optional elements are contained as impurities, the effects obtained by the grain-oriented electrical steel sheet according to this embodiment are not impaired.

[0053] For example, in this embodiment, the base steel sheet may contain one or more of the following selective elements (optionally added elements): 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 impairing the magnetic properties.

[0054] Se: 0 to 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%, Se in the steel sheet precipitates as MnSe, deteriorating hysteresis loss. Therefore, the Se concentration is set to 0.010% or less. The lower limit of the Se concentration may be 0%, or even 0.0001%. In grain-oriented electrical steel sheets, the Se concentration is usually reduced to about 0.005% or less by finish annealing.

[0055] Cr: 0 to 0.30% Cr (chromium) is an element that improves the oxide layer during decarburization annealing and is effective in forming a glass film. Therefore, Cr may be added to the base steel sheet in a range of 0.30% or less. If the Cr concentration exceeds 0.30%, decarburization properties are significantly impaired, so the upper limit of the Cr concentration is preferably 0.30%.

[0056] Cu: 0 to 0.40% Cu (copper) is an element that is effective in 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 becomes a cause of surface defects called "copper scuffs" during hot rolling, so the upper limit of the Cu concentration is preferably 0.40%.

[0057] P: 0 to 0.50% P (phosphorus) is an element that is effective in increasing the resistivity of the base steel sheet and reducing iron loss. If the P concentration exceeds 0.50%, problems arise with rollability, so the upper limit of the P concentration is preferably 0.50%.

[0058] Ni: 0 to 1.00% Ni (nickel) is an element effective in increasing the resistivity of the base steel sheet and reducing iron loss. Ni is also an element effective in controlling the steel structure of the hot-rolled sheet and improving the magnetic properties. However, if the Ni concentration exceeds 1.00%, secondary recrystallization becomes unstable, so the upper limit of the Ni concentration is preferably 1.00%.

[0059] Sn: 0 to 0.30% Sb: 0 to 0.30% Sn (tin) and Sb (antimony) are well-known grain boundary segregation elements. In this embodiment, since the base steel sheet contains Al, depending on the finish annealing conditions, Al may be oxidized by moisture released from the annealing separator, resulting in fluctuations in inhibitor strength at the coil position. As a result, magnetic properties may vary at the coil position. One solution to this problem is to prevent Al oxidation by adding these grain boundary segregation elements. 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 concentrations of these elements exceed 0.30%, Si is less likely to be oxidized during decarburization annealing, resulting in insufficient glass film formation and significantly impairing decarburization annealing performance. For this reason, the upper limit of the concentrations of these elements is preferably 0.30%.

[0060] B: 0 to 0.0100% Boron (B) is an element that combines with N in the base steel sheet and precipitates 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, to fully exert 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 precipitated BN becomes non-uniformly dispersed, the desired secondary recrystallization structure cannot be obtained, and the magnetic flux density decreases. Therefore, 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.

[0061] Mo: 0 to 0.1% Mo (molybdenum) is an element effective in improving surface properties during hot rolling. However, if the Mo concentration exceeds 0.1%, the effect of adding Mo becomes saturated, so the upper limit of the Mo concentration is preferably 0.1%.

[0062] Bi: 0 to 0.01% Bi (bismuth) has the effect of stabilizing precipitates such as sulfides and strengthening its inhibitory function. However, if the Bi concentration exceeds 0.01%, Bi has an adverse effect on the formation of the glass coating, so the upper limit of the Bi concentration is preferably 0.01%.

[0063] The above-mentioned chemical composition may be measured by a general analysis method for steel. For example, the chemical composition may be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). Al may be measured as total aluminum in accordance with JIS G1257-10-1:2013. C and S may be measured using a combustion-infrared absorption method, N may be measured using an inert gas fusion-thermal conductivity method, and, if necessary, O may be measured using an inert gas fusion-non-dispersive infrared absorption method.

[0064] The above chemical composition is that of the base steel sheet. If the grain-oriented electrical steel sheet to be measured has an insulating coating on its surface, remove the insulating coating by the following method before measuring the chemical composition.

[0065] For example, the insulating coating can be removed by immersing the grain-oriented electrical steel sheet having the coating in a high-temperature alkaline solution. ２ O: The insulating coating can be removed from the grain-oriented electrical steel sheet by immersing the sheet in a 50 to 70 mass % aqueous sodium hydroxide solution at 80 to 90°C for 5 to 10 minutes, followed by rinsing with water and drying. The immersion time in the aqueous sodium hydroxide solution can be changed depending on the thickness of the insulating coating.

[0066] <Insulating Coating of Grain-Oriented Electrical Steel Sheet> Next, the insulating coating of the grain-orientated electrical steel sheet according to this embodiment will be described. The insulating coating of the grain-orientated electrical steel sheet does not contain a chromium compound, the coating tension of the insulating coating is 4.9 MPa or more, and the ratio of P(K) to P(Na), P(K) / P(Na), where P(K) is the potassium intensity and P(Na) is the sodium intensity of a spectrum obtained by measuring the insulating coating by X-ray fluorescence analysis, is greater than 13.0 and less than or equal to 100. Note that wavelength-dispersive X-ray fluorescence analysis is used, and the measurement atmosphere is vacuum. The measurement conditions are as follows: an X-ray source is Rh, the X-ray tube voltage is 40 kV, and the tube current is 75 mA. LiF may also be used as the analyzing crystal.

[0067] P(K) / P(Na) ratio: more than 13.0 and not more than 100 The inventors of the present invention conducted a detailed study of insulating coatings and found that the decrease in coating tension before and after holding at constant temperature and humidity decreases as the ratio of potassium intensity to sodium intensity in the fluorescent X-ray spectrum of the insulating coating, i.e., the value of P(K) / P(Na), increases, as shown in Figure 3. In particular, they found that when the value of P(K) / P(Na) is in the range of more than 13.0 and not more than 100, the decrease in coating tension before and after holding at constant temperature and humidity can be suppressed to an acceptable level.

[0068] If the value of P(K) / P(Na) is 13.0 or less, potassium will not be able to sufficiently suppress the hydrolysis reaction of the phosphate in the insulating coating. On the other hand, if the value of P(K) / P(Na) exceeds 100, potassium will excessively cut the network structure of the phosphate in the insulating coating, resulting in an excessively low coating tension immediately after baking the insulating coating (before maintaining the temperature and humidity constant).

[0069] The value of P(K) / P(Na) is preferably 30 or more, and more preferably 35 or more. The value of P(K) / P(Na) is preferably 70 or less, and more preferably 45 or less.

[0070] The above-mentioned P(K) / P(Na) value can be determined by subjecting the surface of the insulating coating to X-ray fluorescence analysis in accordance with JIS K0119:2008, measuring the potassium intensity (K Kα 0.3742 nm X-ray fluorescence intensity cps) and the sodium intensity (Na Kα 1.191 nm X-ray fluorescence intensity cps) of the X-ray fluorescence spectrum, and dividing the potassium intensity by the sodium intensity. Since an intensity ratio is to be determined, the same X-ray irradiation conditions can be used when measuring the potassium intensity and the sodium intensity. For example, the X-ray fluorescence analysis can be performed using a wavelength-dispersive optical system under the following conditions: an Rh X-ray source, a tube voltage of 40 kV, a tube current of 75 mA, and a LiF analyzing crystal; other well-known conditions can be applied. Furthermore, the measurement conditions are not limited to those described above, and any X-ray irradiation conditions that have sufficient energy to generate fluorescent X-rays of potassium and sodium and that provide sufficient fluorescent X-ray intensity can be used. The above-mentioned P(K) / P(Na) can also be determined by X-ray fluorescence analysis using an energy-dispersive optical system. During the measurement, it is preferable to perform X-ray fluorescence analysis on the insulating coating in at least 10 or more different regions that are sufficiently separated from one another, and to obtain an average value from the results of these multiple measurements.

[0071] Furthermore, in the insulating coating of the grain-oriented electrical steel sheet according to this embodiment, when the chromium intensity and silicon intensity of the spectrum obtained by subjecting the insulating coating to X-ray fluorescence analysis under the above conditions are defined as P(Cr) and P(Si), respectively, the ratio of P(Cr) to P(Si), P(Cr) / P(Si), may be 0.01 or less.

[0072] The insulating coating of the grain-oriented electrical steel sheet according to this embodiment does not contain chromium compounds. Therefore, the ratio of the chromium intensity to the silicon intensity in the fluorescent X-ray spectrum of the insulating coating, i.e., the value of P(Cr) / P(Si), may be 0.01 or less. The lower limit of the value of P(Cr) / P(Si) is not particularly limited and may be 0. However, the value of P(Cr) / P(Si) may not be 0 due to Cr contained in the base steel sheet, etc. Therefore, the lower limit of the value of P(Cr) / P(Si) may be greater than 0.

[0073] Furthermore, in the insulating coating of the grain-oriented electrical steel sheet according to this embodiment, when the phosphorus intensity and silicon intensity of the spectrum obtained by subjecting the insulating coating to X-ray fluorescence analysis under the above conditions are defined as P(P) and P(Si), respectively, the ratio P(P) / P(Si) of P(P) to P(Si) may be 0.8 or more and 1.0 or less.

[0074] The insulating coating of the grain-oriented electrical steel sheet according to this embodiment is formed by baking an insulating coating treatment solution containing mainly colloidal silica and phosphate. The phosphate functions as a binder for the colloidal silica, forming the coating. It is preferable that the phosphate and colloidal silica are present in an appropriate ratio when the coating is formed. Therefore, the value of P(P) / P(Si) may be 0.8 or more and 1.0 or less.

[0075] When the value of P(P) / P(Si) is 0.8 or more, a coating is preferably formed, and when the value of P(P) / P(Si) is 1.0 or less, sufficient coating tension is preferably obtained.

[0076] The above-mentioned values ​​of P(Cr) / P(Si) and P(P) / P(Si) can be determined by subjecting the surface of the insulating coating to X-ray fluorescence analysis in accordance with JIS K0119:2008, measuring the chromium intensity (Cr X-ray fluorescence intensity at Kα 0.2291 nm, cps), phosphorus intensity (P X-ray fluorescence intensity at Kα 0.6155 nm, cps), and silicon intensity (Si X-ray fluorescence intensity at Kα 0.7126 nm, cps) in the X-ray fluorescence spectrum, and calculating the respective ratios. The measurement conditions for the X-ray fluorescence analysis can be the same as those described above.

[0077] In the grain-oriented electrical steel sheet according to this embodiment, the coating tension of the insulating coating may be 4.5 MPa or more, but is preferably 4.9 MPa or more. Specifically, the coating tension is preferably 4.9 MPa or more, regardless of whether the coating tension is before or after constant temperature and humidity maintenance. If the coating tension does not satisfy this value, the iron loss improvement effect may be insufficient, and good magnetic properties may not be obtained. The coating tension of the insulating coating is preferably 5.0 MPa or more, more preferably 6.0 MPa or more, even more preferably 6.5 MPa or more, and even more preferably 7.0 MPa or more. On the other hand, although there is no upper limit on the coating tension, the coating tension should be 10 MPa or less, since the iron loss improvement effect saturates.

[0078] Furthermore, in the grain-oriented electrical steel sheet according to this embodiment, it is more preferable that the coating tension before being maintained at a constant temperature and humidity is 4.9 MPa or more. Since the coating tension is likely to decrease due to the maintenance of a constant temperature and humidity, it is preferable that the coating tension before being maintained at a constant temperature and humidity is a high value. The coating tension before being maintained at a constant temperature and humidity is preferably 5.0 MPa or more, more preferably 6.0 MPa or more, even more preferably 6.5 MPa or more, even more preferably 7.0 MPa or more, and even more preferably 7.5 MPa or more.

[0079] Furthermore, in the grain-oriented electrical steel sheet according to this embodiment, it is further preferable that the coating tension after being kept at a constant temperature and humidity is 4.9 MPa or more. Since the coating tension is likely to decrease due to being kept at a constant temperature and humidity, it is preferable that the coating tension after being kept at a constant temperature and humidity is a high value. The coating tension after being kept at a constant temperature and humidity is preferably 5.0 MPa or more, more preferably 6.0 MPa or more, even more preferably 6.5 MPa or more, and even more preferably 7.0 MPa or more.

[0080] The coating tension of the insulating coating can be measured in the same manner as described above. Specifically, a steel sheet is sheared to a length of 300 mm and a width of 30 mm, and one side of the steel sheet is protected with tape. The steel sheet is then immersed in a 20% aqueous sodium hydroxide solution at 80°C to remove the insulating coating from the side of the steel sheet not protected with tape. Using this steel sheet from which the insulating coating has been removed, the radius of curvature of the steel sheet caused by the removal of the insulating coating can be measured, and the coating tension can be calculated using Stoney's equation: σ = Ed / {3 × (1 − v) × R}. Here, σ is the coating 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), v is the Poisson's ratio of the steel sheet (unit: -), and R is the radius of curvature of the steel sheet (unit: 1 / m). Strictly speaking, the Young's modulus and Poisson's ratio of a steel plate are values ​​that vary depending on the type of steel, so values ​​appropriate for the steel plate used may be substituted as appropriate.

[0081] In this embodiment, the chemical composition of the insulating coating is not particularly limited, but may satisfy, for example, the following as main contained elements: P: 5 to 30 atomic %, Si: 5 to 30 atomic %, O: 30 to 80 atomic %, Al: 0.1 to 10 atomic %, Cr: less than 1 atomic %, Fe: less than 25 atomic %, Mg: 0 to 10 atomic %, Mn: 0 to 10 atomic %, Ni: 0 to 10 atomic %, Zn: 0 to 10 atomic %, V: 0 to 10 atomic %, W: 0 to 10 atomic %, Zr: 0 to 10 atomic %, Co: 0 to 10 atomic %, and Mo: 0 to 10 atomic %.

[0082] The insulating coating of the grain-oriented electrical steel sheet according to this embodiment does not contain chromium compounds. For example, in this embodiment, when the Cr concentration in the insulating coating is less than 1 atomic %, the insulating coating is determined to contain no chromium compounds. The Cr concentration is preferably 0.8 atomic % or less, and more preferably 0.5 atomic % or less.

[0083] The chemical composition of the insulating coating can be determined by, for example, analyzing the composition of the cut surface using SEM-EDS (Scanning Electron Microscope-Energy Dispersive X-ray Spectroscopy) or TEM-EDS (Transmission Electron Microscope-Energy Dispersive X-ray Spectroscopy). During measurement, the composition analysis can be performed by irradiating the insulating coating on the cut surface with an electron beam. Furthermore, the composition analysis can be performed under well-known conditions.

[0084] In addition, in the grain-oriented electrical steel sheet according to this embodiment, the coating weight of the insulating coating is not particularly limited, but is preferably 2.0 to 7.0 g / m per side. ２ The coating weight of the insulating coating is 2.0 g / m ２ In the above cases, high tension can be preferably imparted to the grain-oriented electrical steel sheet, and the grain-oriented electrical steel sheet can also be preferably provided with insulating properties and corrosion resistance. ２ In the following cases, the decrease in the space factor of the grain-oriented electrical steel sheet can be suppressed and the transformer characteristics can be preferably improved. The coating weight of the insulating coating is more preferably 3.0 g / m ２ More preferably, 4.0 g / m ２ The coating weight of the insulating coating is more preferably 6.0 g / m ２ More preferably 5.0 g / m or less ２ The following is the result.

[0085] The coating weight of the insulating coating can be determined from the change in mass before and after removing the insulating coating. The insulating coating can be removed by the method described above.

[0086] Furthermore, the insulating coating of the grain-oriented electrical steel sheet according to this embodiment means an insulating coating that minimizes insufficient baking or cracking due to poor baking conditions. If the insulating coating suffers from insufficient baking or excessive cracking due to poor baking conditions, the insulating coating may not satisfy the electrical insulation, tensioning, corrosion resistance, heat resistance, slip properties, adhesion, and other properties required of the insulating coating.

[0087] <Method for manufacturing grain-oriented electrical steel sheet> Next, a method for manufacturing the grain-oriented electrical steel sheet according to this embodiment will be described. Note that the method for manufacturing the grain-oriented electrical steel sheet according to this embodiment is not limited to the method described below. The manufacturing method described below is one example for manufacturing the grain-oriented electrical steel sheet according to this embodiment.

[0088] Molten steel having a predetermined chemical composition is cast by a conventional method to produce a silicon steel slab. The chemical composition of the silicon steel slab is not limited to a specific composition as long as it can provide the magnetic and mechanical properties required for grain-oriented electrical steel sheet, but an example of the chemical composition of a silicon steel slab is as follows. For example, the silicon steel slab contains, in mass %, 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%, and S: 0.010% or less.

[0089] C: 0.085% or less Carbon (C) is an element effective in controlling the primary recrystallization structure, but has a negative effect on magnetic properties, so it is removed by decarburization annealing before final annealing. If the C concentration exceeds 0.085%, the decarburization annealing time increases and productivity decreases, so the C concentration is set to 0.085% or less. The C concentration is preferably 0.070% or less, more preferably 0.050% or less. The lower limit of the C concentration is not particularly limited, and may be 0% or more. Considering productivity in industrial production and the magnetic properties of the product, 0.0001% is the substantial lower limit of the C concentration. In grain-oriented electrical steel sheets, the C concentration is typically reduced to approximately 0.001% or less by decarburization annealing.

[0090] Si: 2.00 to 4.00% Si (silicon) is an element that increases the electrical resistance of steel sheet and improves iron loss characteristics. If the Si concentration is less than 2.00%, γ transformation occurs during finish annealing, damaging the crystal orientation of the steel sheet, so the Si concentration is set to 2.00% or more. The Si concentration is preferably 2.50% or more, and more preferably 3.00% or more. On the other hand, if the Si concentration exceeds 4.00%, workability decreases and cracks occur during rolling, so the Si concentration is set to 4.00% or less. The Si concentration is preferably 3.50% or less.

[0091] Mn: 0.05 to 1.00% Mn (manganese) is an element that prevents cracking during hot rolling and also combines with S and / or Se to form MnS and MnSe, which function as inhibitors. If the Mn concentration is less than 0.05%, the additive effect is not fully realized, so the Mn concentration is set to 0.05% or more. The Mn concentration is preferably 0.07% or more, more preferably 0.09% or more. On the other hand, if the Mn concentration exceeds 1.00%, the precipitation and dispersion of MnS and MnSe becomes non-uniform, the desired secondary recrystallization structure cannot be obtained, and the magnetic flux density decreases. Therefore, the Mn concentration is set to 1.00% or less. The Mn concentration is preferably 0.80% or less, more preferably 0.06% or less.

[0092] Al: 0.010 to 0.065% Al (aluminum) 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 is not fully realized and secondary recrystallization does not proceed sufficiently, so the Al concentration is set to 0.010% or more. The Al concentration is preferably 0.015% or more, more preferably 0.020% or more. 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 recrystallization structure cannot be obtained, and the magnetic flux density decreases. Therefore, the Al concentration is set to 0.065% or less. The Al concentration is preferably 0.050% or less, more preferably 0.040% or less.

[0093] N: 0.004 to 0.012% N (nitrogen) is an element that combines with Al to form AlN and other compounds that function as inhibitors. However, it is also an element that forms blisters (voids) in the steel sheet during cold rolling. If the N concentration is less than 0.004%, the formation of AlN is insufficient, so the N concentration is set to 0.004% or more. The N concentration is preferably 0.006% or more, more preferably 0.007% or more. On the other hand, if the N concentration exceeds 0.012%, there is a concern that blisters (voids) may form in the steel sheet during cold rolling, so the N concentration is set to 0.012% or less. The N concentration is preferably 0.010% or less, more preferably 0.009% or less.

[0094] 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 precipitation and dispersion of MnS becomes non-uniform after purification, the desired secondary recrystallization structure cannot be obtained, the magnetic flux density decreases, and the hysteresis loss deteriorates. Alternatively, MnS remains after purification, which deteriorates the hysteresis loss. Although there is no particular lower limit, the S concentration may be 0%, and is preferably 0.003% or more. The S concentration is more preferably 0.007% or more.

[0095] In this embodiment, the silicon steel slab may contain impurities. The term "impurities" refers to substances that are mixed in from raw materials such as ore or scrap, or from the manufacturing environment, during industrial steel production.

[0096] In this embodiment, the silicon steel slab may contain selective elements in addition to the above-described elements and impurities. For example, instead of a portion of the remaining Fe, at least one of Se, Cr, Cu, P, Ni, Sn, Sb, B, Mo, or Bi may be contained as a selective element. These selective elements may be contained according to the purpose. Therefore, there is no need to set a lower limit for these selective elements, and the lower limit may be 0%. Furthermore, even if these selective elements are contained as impurities, the above-described effects are not impaired.

[0097] For example, in this embodiment, the silicon steel slab may contain one or more of 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, within a range that does not impair the magnetic properties of the grain-oriented electrical steel sheet and can enhance other properties.

[0098] In the hot rolling process, a slab having the above chemical composition is hot-rolled to obtain a hot-rolled sheet. The hot-rolling conditions are not particularly limited, and ordinary conditions can be used. The hot-rolled sheet obtained by the hot-rolling process is wound into a coil.

[0099] Before the slab is subjected to hot rolling, the slab may be heated to a temperature of more than 1300°C in order to sufficiently dissolve the inhibitor components MnS and AlN. From the viewpoint of productivity and manufacturing costs, the slab may also be heated to about 1250°C on the premise that the inhibitors will be strengthened in a subsequent nitriding treatment.

[0100] In the hot-rolled sheet annealing process, the coiled hot-rolled sheet is recoiled into a strip-shaped hot-rolled sheet, and then the strip-shaped hot-rolled sheet 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 conventional conditions can be used.

[0101] In the cold rolling process, the annealed hot-rolled sheet is subjected to one or more cold rolling processes to obtain a cold-rolled sheet having a final thickness. In this cold rolling process, the annealed hot-rolled sheet may be subjected to two or more cold rolling processes with intermediate annealing between them to obtain a cold-rolled sheet. Annealing performed before the finish (final) cold rolling homogenizes the crystalline structure. The cold rolling conditions are not particularly limited, and ordinary conditions can be used.

[0102] In the decarburization annealing process, a decarburization annealed sheet is obtained by subjecting the cold-rolled sheet to decarburization annealing. In this decarburization annealing process, the cold-rolled sheet is heat-treated in wet hydrogen to reduce the C content in the cold-rolled sheet to an amount that will not cause deterioration due to magnetic aging in the product steel sheet, and to induce primary recrystallization in the cold-rolled sheet, preparing for the subsequent secondary recrystallization. The decarburization annealing conditions are not particularly limited, and ordinary conditions can be used. The surface of the decarburization annealed sheet obtained by this decarburization annealing process has SiO ２ When a cold-rolled steel sheet is manufactured from a slab heated to about 1250°C, the decarburization-annealed steel sheet is annealed in an ammonia atmosphere after decarburization annealing, thereby generating AlN, which functions as an inhibitor, in the decarburization-annealed steel sheet.

[0103] In the method for producing a grain-oriented electrical steel sheet 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 may be a grain-oriented electrical steel sheet without a forsterite coating.

[0104] In the case of grain-oriented electrical steel sheets having a normal forsterite coating, an annealing separator containing MgO as the main component is applied in the annealing separator application process, which is the process following the decarburization annealing process, in order to prevent seizure during the finish annealing process. The amount of annealing separator applied is 6.0 to 14.0 g / m per side of the decarburization annealed sheet. ２ is.

[0105] In the case of grain-oriented electrical steel sheets that do not have a forsterite coating, alumina (Al ２ O ３ The decarburized annealed sheet coated with the annealing separator is then wound into a coil after the annealing separator has dried.

[0106] In the finish annealing step, the coil-shaped decarburized annealed sheet coated with the annealing separator is subjected to finish annealing to obtain a base steel sheet for the final product (grain-oriented electrical steel sheet). In this finish annealing step, secondary recrystallization occurs in the decarburized annealed sheet by performing finish annealing at a temperature of 1100°C or higher. Note that, in order to reduce hysteresis loss in the final product, the finish annealed sheet after completion of secondary recrystallization may be subjected to purification annealing so that the precipitates used as inhibitors are rendered harmless.

[0107] An insulating coating is formed on the surface of the steel sheet after secondary recrystallization. The method for forming this insulating coating includes a mixing step of an insulating coating treatment liquid, a coating step of applying the insulating coating treatment liquid to the surface of the steel sheet, and a baking step of baking the insulating coating treatment liquid. The insulating coating is formed through these steps.

[0108] In the mixing step, metal phosphate, colloidal silica, and the like are mixed, as will be described in detail later in the section <Manufacturing Method of Insulation Coating Solution for Grain-Oriented Electrical Steel Sheets>. At this time, sodium and potassium are included in the insulation coating solution. For example, sodium may be derived from sodium or sodium borate contained in colloidal silica, and potassium may be derived from potassium hydroxide, potassium chloride, or dipotassium hydrogen phosphate.

[0109] In the method for producing grain-oriented electrical steel sheet according to this embodiment, it is necessary to control the stirring time of the insulating coating treatment solution prepared as described above in the mixing step. Specifically, the stirring time of the insulating coating treatment solution in the mixing step is set to 60 minutes or more.

[0110] The stirring time of the insulating coating treatment solution described above is longer than the stirring time conventionally considered. Conventional insulating coating treatment solutions that do not contain both sodium and potassium have a stirring time of approximately 30 minutes. For example, because a short stirring time is preferable industrially, the prior art has set the stirring time to approximately 30 minutes. In contrast, in this embodiment, the stirring time of the insulating coating treatment solution in the mixing step is set to 60 minutes or more. The present inventors have found that when the insulating coating treatment solution contains potassium and sodium, the grain-oriented electrical steel sheet according to this embodiment can be obtained by stirring the insulating coating treatment solution for 60 minutes or more. The upper limit of this stirring time is not particularly limited and may be, for example, 150 minutes.

[0111] In addition, in the manufacturing method of grain-oriented electrical steel sheet according to this embodiment, it is necessary to control the holding time of the insulating coating treatment solution after stirring in the mixing step. Specifically, the holding time of the insulating coating treatment solution after stirring is set to within 300 minutes before application to the steel sheet.

[0112] The above-described holding time of the insulating coating treatment solution after stirring is shorter than the holding time conventionally considered. Conventional insulating coating treatment solutions that do not contain both sodium and potassium have been considered to be sufficient for a holding time of within one week after stirring. For example, because it is industrially preferable to prepare the insulating coating treatment solution in advance, conventional techniques have sometimes held the insulating coating treatment solution for approximately one week after stirring. In contrast, in this embodiment, the holding time from the end of stirring the insulating coating treatment solution until application to the steel sheet is set to within 300 minutes. The present inventors have found that when the insulating coating treatment solution contains potassium and sodium, the grain-oriented electrical steel sheet according to this embodiment can be obtained by holding the insulating coating treatment solution after stirring for within 300 minutes. Note that the lower limit of this holding time is not particularly limited and may be, for example, 0 minutes.

[0113] In the coating process, the insulating coating solution prepared in the mixing process is applied to the steel sheet within 300 minutes after mixing. After finish annealing or purification annealing, the steel sheet is washed with water to remove excess annealing separator, and then subjected to pickling in a sulfuric acid bath or the like, and then water rinsing. This cleans and activates the surface of the steel sheet, and then in the coating process, the insulating coating solution is applied to the steel sheet.

[0114] There are no limitations on the method for applying the insulating coating solution to the steel sheet, but it is usually applied using 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 below, thereby forming an insulating coating on the surface.

[0115] In the baking process, the grain-oriented electrical steel sheet coated with the insulating coating treatment liquid is heated to a baking soaking temperature, held at the baking soaking temperature, and then cooled.

[0116] The baking soaking temperature (°C) refers to the sheet temperature (maximum sheet temperature) reached in the baking process, and must be 800°C or higher and 1000°C or lower. If the baking soaking temperature is lower than 800°C, the coating formation reaction of the insulating coating will not proceed sufficiently, resulting in not only a poor appearance of the coating but also in the possibility that sufficient tension cannot be imparted to the steel sheet. On the other hand, if the baking soaking temperature exceeds 1000°C, cracks may occur in the insulating coating, reducing the coating tension and insulating properties, and scratches may also occur on the steel sheet. The baking soaking temperature is more preferably 850°C or higher and 950°C or lower.

[0117] The soaking time (seconds) indicates the time required to hold the insulating coating 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 insulating coating may not be baked properly, resulting in a decrease in coating tension. A soaking time of 20 seconds or more is desirable. On the other hand, the soaking time is set to 60 seconds or less. If the soaking time exceeds 60 seconds, excessive crystallization of the insulating coating may occur, causing cracks and a decrease in coating tension. A soaking time of 45 seconds or less is more preferable, as this provides sufficient coating properties.

[0118] As mentioned above, there are no particular limitations on the type of base steel sheet to be treated with the insulating coating. The grain-oriented electrical steel sheet according to this embodiment is primarily characterized by the configuration 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 apply large tension to the surface of the steel sheet, good adhesion and corrosion resistance, and excellent long-term stability despite not containing chromate, can be achieved regardless of the type of base steel sheet.

[0119] Preferably, the above-described insulating coating treatment may be applied to grain-oriented electrical steel sheets manufactured using the technology disclosed in, for example, Japanese Patent Laid-Open Publication No. 7-268567. In this case, the effect of further reducing iron loss can be obtained. Specifically, by applying the above-described insulating coating treatment to grain-oriented electrical steel sheets containing, by mass%, at least 0.005% or less of C and 2.5 to 7.0% of Si, and optionally containing other alloying elements (e.g., Mn: 0 to 1.0%, Al: 0 to 0.03%, N: 0.01% or less, P: 0.01% or less, and S: 0.01% or less) within ranges that do not impair the properties, with the balance being Fe and impurities, an average grain size of 1 to 10 mm, and an average angle between the (110)

[001] crystal orientation and the rolling direction of 8° or less, the effect of further reducing iron loss can be obtained.

[0120] <Insulating Coating Solution for Grain-Oriented Electrical Steel Sheet> Next, the insulating coating solution (hereinafter simply referred to as "insulating coating solution") used for the grain-orientated electrical steel sheet according to this embodiment will be described.

[0121] The insulating coating treatment solution contains colloidal silica and a metal phosphate salt of one or more metals selected from Al, Fe, Mg, Mn, Ni, Zn, Co, Mo, V, W, and Zr, but does not contain chromate. The colloidal silica used in the insulating coating treatment solution typically contains 0.010 to 1.0 mass% Na to enhance dispersion stability. Therefore, the amount of Na in the insulating coating can be controlled by appropriately selecting the colloidal silica. Alternatively, the amount of Na in the insulating coating can be controlled by adding a sodium compound to the insulating coating treatment solution.

[0122] The metal phosphate is preferably one or more phosphates selected from Al, Mg, Ni, V, and W. This is because when these phosphates are selected, a flat and uniform appearance can be obtained under a wide range of baking conditions.

[0123] Furthermore, potassium must be added to improve the hydrolysis resistance of the insulating coating. The amount of potassium contained in the insulating coating treatment solution is preferably 0.10 mol / kg or more, more preferably 0.13 mol / kg or more. On the other hand, the upper limit is not particularly limited, but is, for example, 0.30 mol / kg or less, preferably 0.20 mol / kg or less. If the potassium amount is less than 0.10 mol / kg, the coating tension may decrease when the insulating coating is stored for a long period of time under a high-temperature and high-humidity atmosphere. On the other hand, if the potassium amount exceeds 0.30 mol / kg, the potassium may excessively cut the phosphate network structure of the insulating coating, resulting in an excessively low coating tension immediately after baking the insulating coating (before maintaining the insulating coating at constant temperature and humidity).

[0124] In this embodiment, the amount of potassium in the insulation coating treatment solution can be analyzed as follows: The amount of potassium contained in the insulation coating treatment solution is measured by atomic absorption spectrometry and converted into the amount of substance. The amount of potassium can then be calculated by dividing the amount by the weight of the insulation coating treatment solution.

[0125] <Method for producing insulating coating solution for grain-oriented electrical steel sheet> Next, a method for producing the insulating coating solution used for the grain-oriented electrical steel sheet according to this embodiment (hereinafter simply referred to as the "method for producing the insulating coating solution") and the reasons for the limitations thereon will be described.

[0126] The method for producing the insulating coating solution includes a mixing step of metal phosphate, colloidal silica, sodium, and potassium. The sodium may be derived from the sodium contained in the colloidal silica. The method for adding potassium is not particularly limited as long as a predetermined amount can be added to the insulating coating solution. An example is as follows:

[0127] The phosphate is based on a metal phosphate of one or more metals selected from Al, Fe, Mg, Mn, Ni, Zn, Co, Mo, V, W, and Zr, and the insulating coating treatment solution may further contain potassium in the form of potassium hydroxide, potassium chloride, or dipotassium hydrogen phosphate.

[0128] The size of the colloidal silica (silica particles) used in this embodiment is not particularly limited, but the average particle size (average primary particle size) is preferably 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 may reduce the stability of the insulating coating treatment solution, or the insulating coating may become porous with large gaps, resulting in a decrease in the coating tension of the insulating coating. On the other hand, if the average particle size of the colloidal silica exceeds 35 nm, the reactivity of the colloidal silica may decrease, which may result in insufficient mixing of the phosphate binder with the colloidal silica, or cracks may occur in the insulating coating, resulting in a decrease in the coating tension.

[0129] Furthermore, since the smaller the particle size of the colloidal silica, the denser the coating film formed and the higher the coating tension, it is more preferable that the upper limit of the average particle size of the colloidal silica is 31 nm, 22 nm, 18 nm, or 12 nm. Furthermore, it is more preferable that the surface of the colloidal silica is chemically treated with aluminum. The average particle size (average primary particle size) of the colloidal silica can be determined, for example, by conversion from the specific surface area measured by the BET adsorption method (in accordance with JIS Z 8830:2013).

[0130] In the method for producing an insulating coating solution, the ratio of metal phosphate to colloidal silica is not particularly limited. As long as the insulating coating solution contains sodium and potassium, and the potassium content is 0.10 mol / kg or more and 0.30 mol / kg or less, the insulating coating of grain-oriented electrical steel sheet produced using this insulating coating solution will exhibit excellent properties. Preferred values ​​are shown below.

[0131] For example, the insulating coating treatment solution may be prepared by mixing 100 parts by mass (solids content) of metal phosphate, 30 to 130 parts by mass (solids content) of colloidal silica (sodium-stabilized), and potassium hydroxide or dipotassium hydrogen phosphate as potassium. The insulating coating treatment solution thus prepared may contain potassium in an amount of 0.10 mol / kg or more and 0.30 mol / kg or less.

[0132] The colloidal silica content in the insulating coating treatment solution is preferably 25 to 55% by mass, calculated as solids, relative to the total mass of the insulating coating treatment solution. A colloidal silica content of less than 25% by mass is undesirable because the insulating coating may not have sufficient film tension. A colloidal silica content of more than 55% by mass is undesirable because it may reduce the adhesion of the insulating coating or result in excessive sodium content in the insulating coating, resulting in a significant decrease in film tension. The colloidal silica content in the insulating coating treatment solution is more preferably 27% by mass or more, more preferably 30% by mass or more, 32% by mass or more, or even 35% by mass or more, calculated as solids, relative to the total mass of the insulating coating treatment solution. The colloidal silica content in the insulating coating treatment solution is more preferably 45% by mass or less, and even more preferably 40% by mass or less.

[0133] In the mixing step, various oxides such as titanium oxide and molybdenum oxide, boric acid, sodium borate, pigments, and inorganic compounds such as barium titanate may be further mixed into the insulating coating treatment solution.

[0134] As described above, the stirring time of the insulating coating treatment liquid in the mixing step is set to 60 minutes or more, and the holding time from the end of stirring of the insulating coating treatment liquid until application to the steel sheet is set to 300 minutes or less.

[0135] Next, the effects of one embodiment of the present invention will be explained in more detail using examples, but the conditions in the examples are merely examples adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to these examples. Various conditions can be adopted in the present invention as long as they do not deviate from the gist of the present invention and the object of the present invention is achieved.

[0136] A slab (slab satisfying the chemical composition of the silicon steel slab described above) whose chemical composition was adjusted so that the final product base steel sheet had the chemical composition shown in Table 2 was heated to 1150°C and subjected to hot rolling to obtain a hot-rolled steel sheet with a thickness of 2.6 mm. This hot-rolled steel sheet was subjected to hot-rolled sheet annealing as necessary, and then subjected to one cold rolling or multiple cold rollings 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 to nitriding annealing in which the sheet was held in an ammonia-containing atmosphere during cooling. Note that well-known conditions were applied in the processes from slab heating to nitriding annealing.

[0137] The decarburization-annealed steel sheets after the above-described decarburization annealing and nitriding annealing were coated with an annealing separator mainly composed of MgO and dried. The decarburization-annealed steel sheets coated with the annealing separator were subjected to finish annealing at 1200°C for 20 hours.

[0138] After that, excess annealing separator was removed by washing with water using a scrubber. The insulating coating treatment solution was mixed under the conditions shown in Tables 3, 5, and 6. The insulating coating treatment solution was then applied and baked under the conditions shown in Table 4 to form an insulating coating on the base steel sheet. The sodium content in the insulating coating treatment solution was adjusted by adjusting the sodium contained in the colloidal silica to enhance dispersion stability, and the potassium content was adjusted by adding alkali metal salts such as a 30% potassium hydroxide aqueous solution, a 5% potassium chloride aqueous solution, or a 5% potassium dihydrogen phosphate aqueous solution. In addition, insulating coating treatment solution B5 contained 0.5 mass% titanium oxide, and insulating coating treatment solution B13 contained 0.5 mass% boric acid. In Table 3, the contents of the metal phosphate salt, colloidal silica, sodium content, alkali metal salt, titanium oxide, and boric acid are expressed in mass% converted to solid content. In Tables 5 and 6, the "mixing step" refers to the mixing step of the insulating coating treatment solution.

[0139] For the obtained grain-oriented electrical steel sheets Nos. D1 to D33, d1 to d15, and E1 to E6, the chemical composition of the base steel sheet, the chemical composition of the insulating coating, the coating weight of the insulating coating, the fluorescent X-ray intensity ratio of the insulating coating, and the like were measured according to the methods described above.

[0140] Furthermore, the coating tension of the obtained grain-oriented electrical steel sheets Nos. D1 to D33, d1 to d15, and E1 to E6 was evaluated based on the above-mentioned method.

[0141] The coating tension was measured before and after the following constant temperature and humidity holding, and the decrease in coating tension was calculated. The constant temperature and humidity holding was performed by placing the grain-oriented electrical steel sheet with the insulating coating in a constant temperature and humidity chamber (temperature: 50°C, humidity: 90%) for one week. A coating was judged to be acceptable if the coating tension was 4.9 MPa or more both before and after the constant temperature and humidity holding, and if the decrease in coating tension after the constant temperature and humidity holding was 50% or less compared to the coating tension before the constant temperature and humidity holding.

[0142] The resulting grain-oriented electrical steel sheets were evaluated for the fluorescent X-ray intensity ratio of the insulating coating, the coating tension of the insulating coating, and the decrease in strength before and after being kept at constant temperature and humidity. The results are shown in Tables 5 and 6.

[0143] Although not shown in the table, the chemical composition of the insulating coating of Examples D1 to D33 and E1 to E6 of the present invention satisfied the following main elements: P: 5 to 30 atomic %, Si: 5 to 30 atomic %, O: 30 to 80 atomic %, Al: 0.1 to 10 atomic %, Cr: less than 1 atomic %, Fe: less than 25 atomic %, Mg: 0 to 10 atomic %, Mn: 0 to 10 atomic %, Ni: 0 to 10 atomic %, Zn: 0 to 10 atomic %, V: 0 to 10 atomic %, W: 0 to 10 atomic %, Zr: 0 to 10 atomic %, Co: 0 to 10 atomic %, and Mo: 0 to 10 atomic %. The coating weight of the insulating coating was 2.0 to 7.0 g / m per side. ２ was satisfied.

[0144] As can be seen from Tables 2 to 6, the product characteristics of invention examples D1 to D33 and E1 to E6 satisfied the range of the present invention, and were excellent in terms of the decrease in film tension before and after constant temperature and humidity maintenance and the decrease in film tension after constant temperature and humidity maintenance. In contrast, the product characteristics of comparative examples d1 to d15 did not satisfy the range of the present invention, and were inferior in at least one of the decrease in film tension before and after constant temperature and humidity maintenance and the decrease in film tension after constant temperature and humidity maintenance. Note that comparative example d1 contains chromate (chromium compound), and is therefore outside the range of the present invention.

[0145]

[0146]

[0147]

[0148]

[0149]

[0150] According to the above aspect of the present invention, it is possible to reliably obtain a grain-oriented electrical steel sheet in which the insulating coating does not contain chromate, and the ability to apply tension to the base steel sheet is equal to or better than conventional methods, and in which the coating tension decreases little even when the steel sheet is stored for a long period of time in a high-temperature, high-humidity atmosphere, thereby providing high industrial applicability.

Claims

1. A grain-oriented electrical steel sheet comprising a base steel sheet and an insulating coating, wherein the insulating coating does not contain chromium compounds, the insulating coating has a coating tension of 4.9 MPa or more, and the ratio of P(K) to P(Na), P(K) / P(Na), is greater than 13.0 and less than or equal to 100, where P(K) is the potassium intensity and P(Na) is the sodium intensity of a spectrum obtained by measuring the insulating coating using X-ray fluorescence analysis.

2. The grain-oriented electrical steel sheet according to claim 1, wherein, when the chromium intensity of the spectrum is defined as P(Cr) and the silicon intensity is defined as P(Si), the ratio of P(Cr) to P(Si), P(Cr) / P(Si), is 0.01 or less.

3. The grain-oriented electrical steel sheet according to claim 1, characterized in that the base steel sheet contains, as a chemical composition, 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, Se: 0.010% or less, Cr: 0 to 0.30%, Cu: 0 to 0.40%, P: 0 to 0.50%, Ni: 0 to 1.00%, Sn: 0 to 0.30%, Sb: 0 to 0.30%, B: 0 to 0.0100%, Mo: 0 to 0.1%, Bi: 0 to 0.01%, with the balance consisting of Fe and impurities.

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