Mixed powder, MgO particles, method for producing grain-oriented electrical steel sheet, method for producing MgO particles, and method for producing mixed powder

Abstract

A mixed powder for an annealing separating agent having MgO as a main ingredient, wherein the average particle diameter of the mixed powder is 0.10 to 8.50 µm, the mixed powder contains B, the content of B in the entire mixed powder is 0.005 mass% or more and less than 0.040 mass%, the proportion of 3-coordinate boron among the B is 5 mass% or more and less than 70 mass%, and the ratio of circumference to thickness of primary particles containing the MgO is 6.0 or more.

Description

Technical Field

[0001] The present invention relates to a method for producing mixed powder, MgO particles, a grain-oriented electrical steel sheet, a method for producing MgO particles, and a method for producing mixed powder. This application claims priority based on Japanese Patent Application No. 2022-137922 filed in Japan on August 31, 2022, and the contents thereof are incorporated herein by reference.

Background Art

[0002] The grain-oriented electrical steel sheet is a soft magnetic material and is mainly used as a core material for transformers. Therefore, the grain-oriented electrical steel sheet is required to have magnetic properties such as high magnetization characteristics and low iron loss. Iron loss is the power loss consumed as heat energy when the core is excited by an alternating magnetic field, and from the perspective of energy saving, it is required that the iron loss be as low as possible.

[0003] In the production of grain-oriented electrical steel sheets, generally, a production method including steps of hot rolling, annealing of hot-rolled sheet, cold rolling, decarburization annealing, and finish annealing is applied to a steel slab adjusted to a predetermined component composition. Among these steps, in the finish annealing step, the steel sheet wound in a coil shape is annealed at a high temperature for a long time to accumulate the crystal orientation into a Goss orientation favorable for magnetic properties (increase the orientation degree). At that time, an annealing release agent is applied to prevent the coil from sticking.

[0004] As the annealing release agent applied to the coil, an annealing release agent mainly composed of magnesium oxide (MgO) is often used. The reason is that when an annealing release agent mainly composed of MgO is used, at the time of finish annealing, silicon dioxide (SiO2) on the surface of the steel sheet reacts with MgO to act on the surface of the steel sheet with tension and form a forsterite (Mg2SiO4)-based film (primary film) that imparts insulation to the steel sheet. That is, by using an annealing release agent mainly composed of MgO, not only can sticking during finish annealing be prevented, but also the magnetic properties of the grain-oriented electrical steel sheet can be improved.

[0005] As mentioned above, the properties of grain-oriented electrical steel sheets can change depending on the annealing separation agent used in their manufacture. Therefore, in recent years, research has been conducted on the trace components contained in magnesium oxide used in annealing separation agents. Furthermore, not only the content of trace components, but also the structure of compounds containing trace elements in the magnesium oxide used in annealing separation agents is being investigated.

[0006] For example, Patent Document 1 discloses an annealing separation agent powder containing 0.04% by mass or more and 0.30% by mass or less of boron, with magnesium oxide as the main component, characterized in that the proportion of three-coordinate boron in the boron is 70% or more and 95% or less.

[0007] Patent Document 2 discloses a method for producing annealing separation agent powder, characterized by calcining a raw material containing either or both magnesium hydroxide and magnesium carbonate, along with boron, and then adjusting the ratio of three-coordinate boron by controlling the humidity of the calcined product, wherein the proportion of three-coordinate boron in the boron contained in the annealing separation agent powder is 70% or more and 95% or less.

[0008] Patent documents 1 and 2 both specify the proportion of three-coordinate boron based on the following findings: 1) the film reaction behavior at high temperatures (above 1100°C) influences the purification of impurities; 2) the three-coordinate boron form influences the film reaction behavior at high temperatures; and 3) the four-coordinate boron form not only does not contribute to the purification of impurities, but also penetrates the steel sheet during high-temperature annealing to form Fe2B, causing deterioration from repeated bending. [Prior art documents] [Patent Documents]

[0009] [Patent Document 1] Japanese Patent Application Publication No. 2017-128772 [Patent Document 2] Japanese Patent Application Publication No. 2020-15982 [Overview of the project] [Problems that the invention aims to solve]

[0010] Patent documents 1 and 2 state that by controlling the amount of boron and the ratio of three-coordinate boron in the annealing separation agent powder, it is possible to solve problems such as poor coating appearance due to insufficient reactivity at high temperatures and insufficient purification of impurities from steel. However, as a result of our investigation, we found that while increasing the boron content in MgO can improve magnetic properties and coating properties (appearance, adhesion), excessively increasing the boron content can lead to too much boron penetration into the steel sheet, causing the development of a non-magnetic layer that inhibits secondary recrystallization, thus degrading magnetic properties. Therefore, the techniques described in patent documents 1 and 2 were not sufficient in improving magnetic properties and coating properties.

[0011] The present invention has been made in view of the above problems, and aims to provide a mixed powder, MgO particles, a method for manufacturing a grain-oriented electrical steel sheet, a method for manufacturing MgO particles, and a method for manufacturing a mixed powder, for producing a grain-oriented electrical steel sheet with good magnetic properties and coating properties, while suppressing the adverse effects of boron application on secondary recrystallization. [Means for solving the problem]

[0012] The inventors investigated a method to obtain sufficient magnetic properties and coating properties due to boron even when the boron content is suppressed. As a result, the inventors found that by suppressing the proportion of three-coordinate boron in small MgO particles, which are highly reactive and react at low temperatures, it is possible to suppress the inclusion of a large amount of boron in the steel sheet and reduce its adverse effect on secondary recrystallization.

[0013] Based on the above findings, the gist of the present invention is as follows: [1] A mixed powder according to one aspect of the present invention is a mixed powder for an annealing separation agent mainly composed of MgO, wherein the average particle size of the mixed powder is 0.10 μm or more and 8.50 μm or less, the mixed powder contains B, the total B content of the mixed powder is 0.005% by mass or more and less than 0.040% by mass, the proportion of 3-coordinate boron in the B is 5% by mass or more and less than 70% by mass, and the ratio of the circumference of the MgO-containing particles to the thickness of the primary particles is 6.0 or more. [2] The mixed powder described in [1] above preferably contains Ca and satisfies the following formula (1). 0.05≦[Ca] / [BO3]<2.00 …(1) formula In formula (1) above, [Ca] is the Ca content (mass%) in the mixed powder, and [BO3] is the content (mass%) of the three-coordinate boron in the mixed powder.

[0014] [3] MgO particles according to another aspect of the present invention have an average particle size of 0.10 μm or more and 8.50 μm or less, a B content of 0.005% by mass or more and less than 0.040% by mass, a proportion of three-coordinate boron of the B of 5% by mass or more and less than 70% by mass, and a ratio of circumference to thickness of primary particles of 6.0 or more. [4] The MgO particles described in [3] above preferably contain Ca and satisfy the following formula (2). 0.05≦[Ca] / [BO3]<2.00 …(2) formula In equation (2) above, [Ca] is the Ca content (mass%) in the MgO particles, and [BO3] is the content (mass%) of the three-coordinate boron in the MgO particles.

[0015] [5] In yet another embodiment of the present invention, a method for manufacturing grain-oriented electrical steel sheets is used, which involves using the mixed powder described in [1] or [2] above or an annealing separation agent containing MgO particles described in [3] or [4] above.

[0016] [6] A further embodiment of the present invention relates to a method for producing MgO particles, comprising: Mg-containing raw material particles containing one or more selected from the group consisting of magnesium hydroxide, basic magnesium carbonate, and magnesium carbonate, wherein the ratio of circumference to thickness of primary particles is 7.0 or more; and B-containing raw material particles containing B, wherein the average particle size is 0.10 μm or more and 8.50 μm or less; and calcining the raw material powder at a temperature of 700°C or more and 1100°C or less. [7] In the method for producing MgO particles described in [6] above, it is preferable that the raw material powder contains more than 0% by mass and 0.02% by mass or less of Ca.

[0017] [8] A further embodiment of the present invention relates to a method for producing MgO particles, comprising calcining raw material particles containing B, one or more selected from the group consisting of magnesium hydroxide, basic magnesium carbonate, and magnesium carbonate, having an average particle size of 0.10 μm or more and 8.50 μm or less, and a ratio of circumference to primary particle thickness of 7.0 or more, at a temperature of 700°C or more and 1100°C or less. [9] In the method for producing MgO particles described in [8] above, it is preferable that the raw material particles contain more than 0% by mass and 0.02% by mass or less of Ca.

[0018]

[10] A further embodiment of the present invention relates to a method for producing a mixed powder, comprising: Mg-containing raw material particles containing one or more selected from the group consisting of magnesium hydroxide, basic magnesium carbonate, and magnesium carbonate, wherein the ratio of circumference to thickness of primary particles is 7.0 or more; and B-containing raw material particles containing B, wherein the average particle size is 0.10 μm or more and 8.50 μm or less; and calcining the raw material powder at a temperature of 700°C or more and 1100°C or less. [Effects of the Invention]

[0019] According to the present invention, it is possible to provide a mixed powder, MgO particles, a method for manufacturing a grain-oriented electrical steel sheet, a method for manufacturing MgO particles, and a method for manufacturing a mixed powder, which suppress the adverse effects of boron application on secondary recrystallization and produce grain-oriented electrical steel sheets with good magnetic properties and coating properties.

Mode for Carrying Out the Invention

[0020] <Mixed powder> The mixed powder according to one embodiment of the present invention is a mixed powder for an annealing separation agent having MgO as a main agent, wherein the average particle size of the mixed powder is 0.10 μm or more and 8.50 μm or less, the mixed powder contains B, the B content contained in the whole of the mixed powder is 0.005% by mass or more and less than 0.040% by mass, the ratio of three-coordinate boron among the B is 5% by mass or more and less than 70% by mass, and the ratio of the perimeter to the thickness of the primary particles of the particles containing MgO is 6.0 or more. This will be described in detail below.

[0021] The mixed powder according to the present embodiment has MgO as a main agent. The mixed powder contains, for example, 50.0% by mass or more of MgO. The ratio of MgO in the mixed powder is preferably 80.0% by mass or more, and more preferably 90.0% by mass or more. The mixed powder contains MgO particles, but the MgO particles contained in the mixed powder are not limited to the MgO particles according to the embodiment of the present invention described later. For example, MgO particles can be used such that the ratio of MgO in the mixed powder is 50% by mass or more.

[0022] The mixed powder according to the present embodiment contains boron (B). The mixed powder contains, for example, B-containing particles containing B. The B-containing particles may contain at least one of pure boron and B compounds. Examples of the B compound include Na2B4O7, borax, calcium borate, magnesium borate, and the like.

[0023] [The average particle size of the mixed powder is 0.10 μm or more and 8.50 μm or less] The average particle size of the mixed powder is 0.10 μm or more and 8.50 μm or less, based on the average circular particle size on a volume basis. If the average particle size of the mixed powder is less than 0.10 μm, the reactivity becomes too high, the reaction with the coating becomes significant at low temperatures, the supply rate of Mg and B increases, the decomposition of precipitates during secondary recrystallization is promoted, and crystal grains other than those with the Goss orientation tend to grow. As a result, the magnetic properties deteriorate. Also, if the average particle size of the mixed powder is less than 0.10 μm, baking becomes a problem, and the appearance of the grain-oriented electrical steel sheet and the adhesion of the coating deteriorate. Therefore, the average particle size of the mixed powder should be 0.10 μm or more. Preferably, the average particle size of the mixed powder is 0.15 μm or more, and more preferably 0.30 μm or more. On the other hand, if the average particle size of the mixed powder exceeds 8.50 μm, it becomes difficult for the steel sheet and the MgO layer to adhere, degassing is promoted, and the heat resistance of the precipitate deteriorates, making it easier for crystal grains in orientations other than the Goss orientation to grow. As a result, the magnetic properties deteriorate. Also, if the average particle size of the mixed powder exceeds 8.50 μm, the film formation becomes insufficient, and the appearance and film adhesion of the grain-oriented electrical steel sheet deteriorate. The average particle size of the mixed powder is preferably 7.50 μm or less, and more preferably 7.00 μm or less.

[0024] The average particle size of the mixed powder is determined by measuring the volume frequency particle size distribution using a laser diffraction particle size distribution analyzer (HORIBA Corporation, LA-920), and the average particle size at the equivalent circle diameter is taken as the average particle size of the mixed powder. The measurement conditions are a refractive index of 1.74 and ultrasonic dispersion treatment in pure water.

[0025] [B content is 0.005% by mass or more and less than 0.040% by mass] The mixed powder contains 0.005% by mass or more and less than 0.040% by mass of boron. If the boron content is less than 0.005% by mass, the film formation will be uneven, and the appearance of the grain-oriented electrical steel sheet and the adhesion of the film to the grain-oriented electrical steel sheet will deteriorate. The boron content is preferably 0.006% by mass or more, and more preferably 0.008% by mass or more. On the other hand, if the B content in the mixed powder is 0.040% by mass or more, degassing from the steel sheet is excessively suppressed, causing the gas pressure at the coating interface to become too high, resulting in degassing accompanied by coating breakdown. This leads to uneven coating formation and deterioration of the appearance of the grain-oriented electrical steel sheet. Therefore, the B content in the mixed powder should be less than 0.040% by mass. Preferably, the B content is 0.035% by mass or less.

[0026] The B content of the mixed powder is determined by quantitative analysis using inductively coupled plasma mass spectrometry (ICP-MS). For quantitative analysis using ICP-MS, the MgO powder is dissolved in a mixed acid of hydrochloric acid and nitric acid. If any residue remains, it is collected and dissolved in an alkaline solution for analysis.

[0027] [The proportion of 3-coordinate boron in B is 5% by mass or more and less than 70% by mass] The boron contained in the mixed powder exists in the forms of three-coordinate boron (BO3) and four-coordinate boron (BO4). Three-coordinate boron is boron that has a three-coordinate structure in which three oxygen atoms are coordinated around the boron element, while four-coordinate boron is boron that has a four-coordinate structure in which four oxygen atoms are coordinated around the boron element. Boron other than three-coordinate boron exists as four-coordinate boron. Investigations into the reactions of 3-coordinate boron and 4-coordinate boron revealed that they have different effects on coating defects and appearance characteristics. Specifically, 3-coordinate boron promotes coating formation more than 4-coordinate boron, while also having a greater impact on precipitate decomposition. Furthermore, it was found that if the amount of 3-coordinate boron becomes too high, the reactions of coating formation and precipitate decomposition occur too concentrated at low temperatures, leading to coating breakdown due to gas release. Therefore, it was found that by staggering the timing of precipitate decomposition, it is possible to form a good coating while preventing coating breakdown. The smaller the particle size, or the higher the 3-coordinate boron content, the more the precipitate decomposition due to coating formation at low temperatures is promoted. The mixed powder according to this embodiment contains 5% by mass or more and less than 70% by mass of three-coordinate boron relative to the B content in the mixed powder. When the proportion of three-coordinate boron in B is 5% by mass or more, the magnetic properties and coating properties can be improved. Therefore, the proportion of three-coordinate boron in B is 5% by mass or more. Preferably, the proportion of three-coordinate boron in B is 8% by mass or more, and more preferably 10% by mass or more. On the other hand, if the proportion of three-coordinate boron in B is 70% by mass or more, the reactivity becomes too high, promoting the decomposition of precipitates and making it easier for crystal grains in orientations other than the Goss orientation to grow. As a result, the magnetic properties deteriorate. Therefore, the proportion of three-coordinate boron in B should be less than 70% by mass. Preferably, the proportion of three-coordinate boron in B is 68% by mass or less, and more preferably 65% ​​by mass or less. Although three-coordinate boron is mainly contained in the B-containing particles as described above, it may also be contained in MgO particles and Al-containing particles.

[0028] The proportion of three-coordinate boron in the mixed powder is determined by the following method: Measurement is performed using NMR (Nuclear Magnetic Resonance). In the obtained spectrum, peaks in the range of 27 ppm or less and 6 ppm or more are considered to be three-coordinate boron, and peaks in the range of less than 6 ppm and -6 ppm or more are considered to be four-coordinate boron. The integral area of ​​the three-coordinate boron peak is divided by the total integral area of ​​the three-coordinate boron peak and the four-coordinate boron peak to determine the proportion of three-coordinate boron in B.

[0029] [The ratio of circumference to thickness of primary particles containing MgO is 6.0 or greater.] The ratio of the circumference to the thickness of the primary particles containing MgO is 6.0 or greater. The MgO particles in this embodiment have a shape such that the ratio of the circumference to the thickness of the primary particles containing MgO is 6.0 or greater. This shape allows the annealing separating agent containing the mixed powder of this embodiment to coat the steel plate surface and, when dried, improves reactivity while coating the steel plate surface, thereby more effectively improving the coating properties due to the application of boron. The ratio of the circumference to the thickness of the primary particles containing MgO may be 8.0 or greater, or 9.1 or greater. For example, the ratio of the circumference to the thickness of the primary particles containing MgO may be 11.5 or less, or 11.2 or less.

[0030] The thickness of primary MgO particles is calculated using the following method. Before mixing, MgO particles are used for thickness calculation. After mixing, elemental analysis of Mg is performed using SEM (Scanning Electron Microscope)-EDS (Energy Dispersive X-ray Spectroscopy), and the primary particles contained in the secondary particles containing the highest amount of Mg, identified from elemental mapping showing the Mg concentration steps, are used for calculation. Before mixing, MgO particles or the mixed powder are observed at 10,000x magnification using SEM. From the obtained secondary electron image, the identified MgO particles that are attached to the surface of the MgO secondary particles, whose contours are determined based on contrast, and which are not hidden by the contours of other particles are used for primary particle thickness calculation. A projection onto a plane is obtained by tracing the outline of the primary particles in the secondary electron image. Tracing is performed using the image analysis software ImageJ (developed by NIH; National Institutes of Health), which determines the contour from the contrast of the secondary electron image. Here, a projection diagram is created for each of the 20 primary particles. In the projection diagram, for each primary particle, a straight line is drawn that passes through the centroid and minimizes the distance between the two points where it intersects the contour line. The median of the distances between two points for each of the 20 primary particles is taken as the thickness of the MgO primary particle. Here, the centroid is determined using the centroid measurement function of the image analysis software ImageJ in an image obtained by tracing the contour with a black line in the secondary electron image, then further filling the area enclosed by the contour with black, and filling the area outside the primary particle with white. The median of the lengths of the contour lines of the 20 primary particles is taken as the perimeter. In this embodiment, the perimeter calculated in this way is 6.0 times or more the thickness.

[0031] [Cl: 0.005 mass% or more and 0.030 mass% or less] Chlorine (Cl) is an element that enhances the reactivity between the SiO2 formed on the surface of the steel sheet after decarburization and the mixed powder. In this embodiment, it is preferable that the mixed powder contains 0.005% by mass or more of Cl, as this further improves the coating properties. More preferably, the Cl content is 0.008% by mass or more. On the other hand, if the Cl content exceeds 0.030 mass%, the strong tendency to sulfurize may cause desulfurization of the steel sheet. If the Cl content is 0.030 mass% or less, desulfurization can be sufficiently suppressed. Therefore, the Cl content is preferably 0.030 mass% or less. More preferably, the Cl content is 0.022 mass% or less.

[0032] [One or more elements selected from the group consisting of Ca, Sr, and Ba, totaling 0.02% by mass or more and 4.00% by mass or less] Calcium (Ca), strontium (Sr), and barium (Ba), like chlorine (Cl), are elements that enhance the reactivity of SiO2 with the mixed powder. Therefore, it is preferable to include at least 0.02% by mass of one or more elements selected from the group consisting of Ca, Sr, and Ba, in total, as this further improves the coating properties. On the other hand, if the total content of one or more elements selected from the group consisting of Ca, Sr, and Ba exceeds 4.00% by mass, the strong tendency to sulfide may cause desulfurization of the steel sheet. If the total content of one or more elements selected from the group consisting of Ca, Sr, and Ba is 4.00% by mass or less, desulfurization can be sufficiently suppressed. Therefore, the total content of one or more elements selected from the group consisting of Ca, Sr, and Ba is preferably 4.00% by mass or less.

[0033] Ca can be included in the mixed powder as a Ca compound. Examples of Ca compounds include calcium sulfate, hemihydrate gypsum, calcined gypsum, and gypsum. Sr can be included in the mixed powder as a Sr compound. Examples of Sr compounds include strontium sulfate. Ba can be included in the mixed powder as a Ba compound. Examples of Ba compounds include barium sulfate. Cl, Sr, and Ba may be included in the mixed powder as particles mainly containing each of them, or they may be included in the aforementioned particles that make up the mixed powder.

[0034] [Ti: 0.25% by mass or more and 5% by mass or less] Titanium (Ti) is an element that helps in film formation by maintaining the oxygen potential of the steel sheet by releasing oxygen at high finishing annealing temperatures. The above effect can be obtained by the mixed powder according to this embodiment containing 0.25% by mass or more of Ti. The Ti content is more preferably 0.5% by mass or more. On the other hand, if the Ti content is 5% by mass or less, the deterioration of magnetic properties due to excessive oxygen release can be suppressed. Therefore, the Ti content is preferably 5% by mass or less. More preferably, the Ti content is 4% by mass or less. Ti can be included in the mixed powder as, for example, TiO2, titanate, TiN, TiB2, or BaTiO3. Ti may be included in the mixed powder as particles mainly containing each compound, or it may be included in the aforementioned particles that make up the mixed powder.

[0035] The content of Cl, Ca, Sr, Ba, and Ti is quantitatively analyzed by ICP-MS using the method described above.

[0036] [impurities] The components in the mixed powder according to this embodiment other than those mentioned above are MgO and impurities. Examples of impurities include Al, Fe, and Si. If the content of each impurity element is 0.04% by mass or less, or the total amount is 0.1% by mass or less, the effect on the magnetic properties or coating properties of the grain-oriented electrical steel sheet is small.

[0037] [0.05≦[Ca] / [BO3]<2.00] The mixed powder according to this embodiment preferably contains Ca and satisfies the following formula (1).

[0038] 0.05≦[Ca] / [BO3]<2.00 …(1) formula In formula (1) above, [Ca] is the Ca content (mass%) in the mixed powder, and [BO3] is the content (mass%) of the three-coordinate boron in the mixed powder.

[0039] When [Ca] / [BO3] satisfies equation (1) above, the amount of three-coordinate boron in SiO2 is maintained by Ca, the precipitate decomposition rate becomes constant, and the magnetic properties are further improved. [Ca] / [BO3] is more preferably 0.10 or higher, and even more preferably 0.15 or higher. Furthermore, [Ca] / [BO3] is more preferably 0.40 or lower, and even more preferably 0.30 or lower.

[0040] The content of three-coordinate boron [BO3] in the mixed powder is determined by multiplying the B content quantified by ICP-MS by the proportion of three-coordinate boron among the B obtained by the method described above.

[0041] <Method for producing mixed powder> The mixed powder according to the embodiment described above is produced by calcining a raw material powder containing one or more selected from the group consisting of magnesium hydroxide (Mg(OH)2), basic magnesium carbonate (mMgCO3·Mg(OH)2·nH2O), and magnesium carbonate (Mg(CO3)), Mg-containing raw material particles having a circumference ratio to primary particle thickness of 7.0 or more, and B-containing raw material particles having B, with an average particle size of 0.10 μm or more and 8.50 μm or less, at a temperature of 700°C or more and 1100°C or less.

[0042] The Mg-containing raw material particles have a circumference-to-thickness ratio of 7.0 or greater for the primary particle thickness. If the circumference-to-thickness ratio of the primary particle is 7.0 or greater, then in the mixed powder after calcination, the circumference-to-thickness ratio of the MgO-containing particles to the primary particle thickness will be 6.0 or greater. The circumference-to-thickness ratio of the Mg-containing raw material particles to the primary particle thickness may be 7.6 or greater, or 9.4 or greater. The circumference-to-thickness ratio of the Mg-containing raw material particles to the primary particle thickness may be, for example, 24.0 or less, or 20.0 or less. The circumference-to-thickness ratio of the Mg-containing raw material particles to the primary particle thickness is calculated using the same method as the calculation method for the circumference-to-thickness ratio of the MgO-containing particles described above.

[0043] B-containing raw material particles include B, BN, B2O3, Na2B4O7, borax, etc. For example, B-containing raw material particles contain 10% or more by mass of B.

[0044] The particle sizes of the Mg-containing raw material particles and B-containing raw material particles constituting the raw material powder may be, for example, the particle size that yields the average particle size of the mixed powder described above, or they may be larger. If the particle size of the raw material powder is larger than the average particle size of the mixed powder, the particle size may be adjusted by known methods such as crushing or classifying the calcined raw material powder. The particle sizes of the Mg-containing raw material particles and B-containing raw material particles may be, for example, 0.05 μm or larger, or 0.08 μm or larger. Alternatively, the particle sizes of the Mg-containing raw material particles and B-containing raw material particles may be, for example, 10.0 μm or smaller, or 8.0 μm or smaller.

[0045] Furthermore, the raw material powder may contain Cl, Ca, Sr, Ba, and Ti as appropriate. If these elements are included in the raw material powder, it is sufficient that one or more of these elements are contained in each particle. For example, Ca is contained in Ca-containing raw material particles that contain Ca compounds. Examples of Ca compounds include calcined gypsum, strontium sulfate, and barium sulfate. Ca-containing raw material particles contain, for example, 10% by mass or more of Ca.

[0046] The B content in the raw material powder is preferably 0.005% by mass or more and 0.040% by mass or less. If the B content of the raw material particles is 0.005% by mass or more and 0.040% by mass or less, the B content of the MgO particles produced can be more reliably set to 0.005% by mass or more and 0.040% by mass or less. The B content is more preferably 0.006% by mass or more, and even more preferably 0.008% by mass or more. Furthermore, the B content is more preferably 0.035% by mass or less, and even more preferably 0.030% by mass or less. However, if the raw material powder contains components that volatilize upon firing, the B content in the raw material powder should be adjusted taking into account the amount of those components. B is mainly contained in the B-containing raw material particles described above, but it may also be contained in other particles constituting the raw material powder, such as Mg-containing raw material particles, Ti-containing particles, etc. The B content in the raw material powder is quantitatively analyzed by ICP-MS using the method described above.

[0047] The raw material particles are fired in an air or nitrogen atmosphere at a temperature between 700°C and 1100°C. Using a firing atmosphere other than air or nitrogen is undesirable because it is economically disadvantageous. Furthermore, if the firing temperature is below 700°C, the firing will be insufficient. Therefore, the firing temperature should be 700°C or higher. Preferably, the firing temperature is 750°C or higher, and more preferably 800°C or higher. On the other hand, by setting the firing temperature to 1100°C or lower, the chemical structure of the fired three-coordinate boron can be stabilized while increasing the proportion of three-coordinate boron in the fired MgO particles without significantly changing the particle size. Therefore, the firing temperature should be 1100°C or lower. Preferably, the firing temperature is 1050°C or lower, and more preferably 1030°C or lower.

[0048] The firing time can be, for example, 5 minutes or more and 120 minutes or less. From the viewpoint of eliminating uneven firing, the firing time is preferably 8 minutes or more, and more preferably 10 minutes or more. On the other hand, from the viewpoint of economy, the firing time is preferably 90 minutes or less, and more preferably 60 minutes or less. We have now explained the method for manufacturing the mixed powder.

[0049] In the above method for producing the mixed powder, on the premise that the particles serving as the Mg source and the particles serving as the B source are different from each other, an embodiment in which the raw material powder contains Mg-containing raw material particles, B-containing raw material particles, and Al-containing raw material particles has been described. However, the present invention is not limited to such an embodiment, and the Mg-containing raw material particles serving as the Mg source may contain B so as to serve as the B source, or may contain other elements. The B-containing raw material particles may contain other elements.

[0050] Further, the mixed powder according to the above-described embodiment may be produced by mixing such that the average particle size is 0.10 μm or more and 8.50 μm or less, the mixed powder contains B, and the proportion of three-coordinate boron in the B is 5% by mass or more and less than 70% by mass.

[0051] <MgO particles> The MgO particles according to an embodiment of the present invention have an average particle size of 0.10 μm or more and 8.50 μm or less, a B content of 0.005% by mass or more and less than <<MASK_E>>0.040% by mass, the proportion of three-coordinate boron in the B is 5% by mass or more and less than 70% by mass, and the ratio of the perimeter to the thickness of the primary particles of the particles containing MgO is 6.0 or more. Since the above characteristics of the MgO particles according to the present embodiment are the same as those of the above-described mixed powder, detailed description thereof will be omitted here. However, the fact that the MgO particles contain the above components means that the above components are contained in the MgO particles constituting the MgO powder, and it means that particles other than the MgO particles do not exist alone.

[0052] <Method for producing MgO particles> The above-described MgO particles are produced by the following production method (I) or (II). (I) A raw material powder containing Mg-containing raw material particles having one or more selected from the group consisting of magnesium hydroxide, basic magnesium carbonate, and magnesium carbonate, with a circumference ratio to primary particle thickness of 7.0 or more, and B-containing particles having B, with an average particle size of 0.10 μm or more and 8.50 μm or less, is calcined at a temperature of 700°C or more and 1100°C or less. (II) Raw material particles containing B, one or more selected from the group consisting of magnesium hydroxide, basic magnesium carbonate, and magnesium carbonate, with an average particle size of 0.10 μm or more and 8.50 μm or less, and a ratio of circumference to primary particle thickness of 7.0 or more, are calcined at a temperature of 700°C or more and 1100°C or less. The following describes each manufacturing method.

[0053] [Manufacturing method (I)] In the manufacturing method of (I), a raw material powder containing Mg-containing raw material particles having one or more selected from the group consisting of magnesium hydroxide, basic magnesium carbonate, and magnesium carbonate, with a circumference ratio to the thickness of the primary particles of 7.0 or more, and B-containing raw material particles having B, and with an average particle size of 0.10 μm to 8.50 μm, is calcined at a temperature of 700°C to 1100°C. If the raw material powder contains components that volatilize upon calcination, the B content is adjusted considering the amount of those components. This manufacturing method is basically the same as the manufacturing method for mixed powder. However, this manufacturing method differs from the manufacturing method for mixed powder described above in that it may involve mixing oxides other than B and Al, which are dissolved as impurities in the MgO particles or are not present in the MgO particles, and then calcining them.

[0054] [Manufacturing method (II)] In the manufacturing method of (II), raw material particles containing B, selected from the group consisting of magnesium hydroxide, basic magnesium carbonate, and magnesium carbonate, having an average particle size of 0.10 μm or more and 8.50 μm or less, and a ratio of circumference to primary particle thickness of 7.0 or more, are calcined at a temperature of 700°C or more and 1100°C or less. If the raw material powder contains components that volatilize upon calcination, the B content is adjusted taking into account the amount of those components.

[0055] Raw material particles containing at least one of magnesium hydroxide, basic magnesium carbonate, and magnesium carbonate contain B. The B content is preferably 0.005% by mass or more and 0.040% by mass or less. If the B content of the raw material particles is 0.005% by mass or more and 0.040% by mass or less, the B content of the manufactured MgO particles can be more reliably set to 0.005% by mass or more and 0.040% by mass or less. The B content is more preferably 0.006% by mass or more, and even more preferably 0.008% by mass or more. Furthermore, the B content is more preferably 0.035% by mass or less, and even more preferably 0.030% by mass or less.

[0056] The content of three-coordinate boron relative to the B content in the raw material particles is preferably 5% by mass or more and 70% by mass or less. If the content of three-coordinate boron relative to the B content is within the above range, the content of three-coordinate boron in the MgO particles after calcination under the calcination conditions described later will be 5% by mass or more and less than 70% by mass.

[0057] The particle sizes of the raw material particles, magnesium hydroxide, basic magnesium carbonate, and magnesium carbonate, may be, for example, a particle size that yields the average particle size of the MgO particles described above, or a larger particle size. If the particle size of the raw material particles is larger than the average particle size of the MgO particles, the MgO particles obtained by calcination using a known method may be crushed or classified. The particle sizes of magnesium hydroxide, basic magnesium carbonate, and magnesium carbonate may be, for example, 0.05 μm or larger, or 0.08 μm or larger. Alternatively, the particle sizes of magnesium hydroxide, basic magnesium carbonate, and magnesium carbonate may be, for example, 10.0 μm or smaller, or 8.0 μm or smaller.

[0058] The raw material particles have a circumference-to-thickness ratio of 7.0 or greater for the primary particles. If the circumference-to-thickness ratio of the primary particles is 7.0 or greater, then in the mixed powder after calcination, the circumference-to-thickness ratio of the raw material particles will be 6.0 or greater for the primary particles. The circumference-to-thickness ratio of the raw material particles may be 7.6 or greater, or 9.4 or greater. The circumference-to-thickness ratio of the raw material particles may be, for example, 20.0 or less, or 15.0 or less. The circumference-to-thickness ratio of the raw material particles is calculated using the same method as the calculation method for the circumference-to-thickness ratio of the primary particles of MgO-containing particles described above.

[0059] The raw material particles are fired in an air or nitrogen atmosphere at a temperature between 700°C and 1100°C. The firing atmosphere, firing temperature, and firing time are the same as those for the raw material particles described above, so a detailed explanation is omitted here.

[0060] <Manufacturing method for grain-oriented electrical steel sheets> A method for manufacturing grain-oriented electrical steel sheets according to one embodiment of the present invention uses the above-mentioned MgO particles or mixed powder as an annealing separating agent. The method for manufacturing grain-oriented electrical steel sheets according to this embodiment includes, for example, a hot rolling step of hot rolling a steel slab to obtain a hot-rolled sheet; a hot-rolled sheet annealing step of annealing the hot-rolled sheet; a cold rolling step of cold rolling the hot-rolled sheet after the hot-rolled sheet annealing step to obtain a cold-rolled sheet; a decarburization annealing step of decarburizing the cold-rolled sheet; and a finish annealing step of applying an annealing separating agent containing the above-mentioned MgO particles or mixed powder to the cold-rolled sheet after the decarburization annealing step, drying it, and then performing finish annealing.

[0061] In this embodiment, the annealing separating agent used to be applied before finish annealing is a slurry made by mixing the mixed powder described above according to this embodiment with water. If the mixed powder does not contain Ti, Ti may be added. By using this annealing separating agent, it is possible to manufacture grain-oriented electrical steel sheets with excellent coating appearance and excellent coating adhesion. In the above manufacturing method, with respect to the chemical composition of the steel slab and the conditions of each process, known manufacturing conditions for grain-oriented electrical steel sheets can be applied, except for the annealing separating agent used.

[0062] When preparing an annealing separation agent by mixing a mixed powder of MgO particles and TiO2 particles with water to form a slurry (an aqueous slurry), it is preferable to mix the TiO2 so that its proportion, when the mass of the mixed powder is considered to be 100%, is between 0.5% and 8.5% in terms of Ti content. [Examples]

[0063] The following are examples of the present invention. The conditions in these examples are merely examples of conditions adopted to confirm the feasibility and effectiveness of the present invention, and the present invention is not limited to the conditions used in the following examples. 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.

[0064] <Example 1> Grain-oriented electrical steel sheets were prepared using MgO particles or a mixed powder of MgO particles and TiO2 particles shown in Table 1A. Specifically, an aqueous slurry of an annealing separator containing the MgO particles or mixed powder from Table 1A was applied to the cold-rolled steel sheet after primary recrystallization annealing. The aqueous slurry was prepared by mixing the MgO powder or mixed powder from Table 1 with water. The solid content (MgO particles or mixed powder) in the aqueous slurry was set to 20% by mass. For all test numbers, the cold-rolled steel sheets coated with the aqueous slurry were subjected to a baking treatment at 300°C for 30 seconds to dry the aqueous slurry and bake in the solid content. After baking, a finish annealing treatment was performed. In the finish annealing treatment, all examples in all test numbers were held at 1200°C for 20 hours. Through the above manufacturing process, a grain-oriented electrical steel sheet with a longitudinal length of 300 mm, a widthwise length of 60 mm, and a thickness of 0.23 mm was produced, comprising a base steel sheet and a glass coating containing a composite oxide such as forsterite (Mg2SiO4). The "BO3 ratio" shown in Table 1A indicates the proportion of three-coordinate boron in B.

[0065] [Table 1A]

[0066] The thickness of primary MgO particles was measured using the following method. A secondary electron image was obtained from the mixed powder using SEM at a magnification of 10,000x, and MgO particles were identified from the obtained secondary electron image using the following method. Specifically, elemental analysis of Mg was performed using SEM-EDS, and the secondary particle containing the most Mg, identified from the elemental mapping showing the Mg concentration steps, was designated as the MgO particle to be calculated. Furthermore, for the identified MgO particles, from the obtained secondary electron image, the particles that were attached to the surface of the MgO secondary particles, whose contours were determined based on contrast, and which were not obscured by the contours of other particles were judged to be primary particles. The outlines of the primary particles in the secondary electron image were traced using ImageJ to obtain projection diagrams onto a plane. For 20 primary particles, projection diagrams were created, and for each primary particle in the created projection diagram, a straight line passing through the centroid was drawn such that the distance between two points intersecting the contour line was the shortest. The median of the two-point distances for the 20 primary particles was taken as the thickness of the primary MgO particle. Furthermore, the median length of the contour lines of the 20 primary particles was used as the circumference. From the calculated thickness and circumference, the ratio of the circumference of the MgO particles to the thickness of the primary particles was calculated. In Table 1A, the circumference / thickness ratio represents the ratio of the circumference of the MgO particles to the thickness of the primary particles.

[0067] The B content and Ca content of MgO particles and mixed powder were measured using inductively coupled plasma mass spectrometry (ICP-MS). Specifically, a solution was used in which the MgO particles or mixed powder were dissolved in a mixed acid of hydrochloric acid and nitric acid. If any residue remained, it was collected and dissolved in an alkaline solution for analysis. The proportion of three-coordinate boron in B was determined by the following method: Measurements were taken using NMR, and in the obtained spectrum, peaks in the range of 27 ppm or less and 6 ppm or more were identified as three-coordinate boron, and peaks in the range of less than 6 ppm and -6 ppm or more were identified as four-coordinate boron. The integral area of ​​the three-coordinate boron peak was divided by the total integral area of ​​the three-coordinate boron peak and the four-coordinate boron peak to determine the proportion of three-coordinate boron in B. The BO3 content [BO3] of MgO particles and mixed powder was determined by multiplying the B content, quantified by ICP-MS, by the proportion of three-coordinate boron in the B.

[0068] A laser diffraction particle size distribution analyzer (HORIBA Corporation, LA-920) was used to measure the volume frequency particle size distribution of MgO particles and mixed powder. The average particle size at the equivalent circle diameter was defined as the average particle size of the MgO particles and mixed powder. The measurement conditions were a refractive index of 1.74 and dispersion treatment using pure ultrasonic waves.

[0069] The obtained grain-oriented electrical steel sheets were evaluated for their magnetic properties (B8), appearance, coating adhesion, and magnetic property degradation amount ΔB8 according to the following procedure.

[0070] [Magnetic properties] The magnetic properties of each grain-oriented electrical steel sheet were evaluated using the following method. Specifically, a sample measuring 300 mm in length in the rolling direction and 60 mm in width was subjected to a magnetic field of 800 A / m, and the magnetic flux density B8 was determined. A B8 of 1.920 T or higher was considered to indicate good magnetic properties.

[0071] [exterior] For samples of each grain-oriented electrical steel sheet, the color tone was evaluated, and then a known insulating coating was formed, followed by an evaluation of coating defects. A sheet was judged to have excellent appearance if the color tone of the primary coating before insulating coating formation was uniform, and there were no coating defects (holes and rusting) after insulating coating formation. Specifically, the evaluation was performed as follows: A: The color tone is uniform before the insulating film is formed, and the maximum area of ​​coating defects after the insulating film is formed is 2 mm². 2 Less than B: The color tone is uniform before the formation of the insulating film, and the maximum area of ​​film defects after the formation of the insulating film is 2-4 mm². 2 That is C: There is color unevenness in the color tone before the insulating coating is formed, or the maximum area of ​​coating defects after the insulating coating is 4 mm². 2 It is super A grade of A or B was considered a passing grade.

[0072] [Coating adhesion] The adhesion of coatings to each grain-oriented electrical steel sheet was evaluated using the following method. Specifically, a sample measuring 50 mm in length and 10 mm in width in the rolling direction was taken from a sample after the insulating coating had been formed. This sample was wrapped around a 20 mm diameter cylinder, bent halfway around, and then unbent. If no coating peeling was observed, the sheet was judged to have excellent adhesion. The evaluation was performed as follows. A: No peeling observed. B: Point-like or island-like delamination of the coating is observed, covering a length less than the entire length of the bent section or the entire width of the sample. C: Coating delamination is observed along the entire length or width of the bent section. The grade was judged as A, meaning it was a passing grade.

[0073] [Magnetic property degradation amount ΔB8] The amount of magnetic property degradation ΔB8 of each grain-oriented electrical steel sheet was evaluated by the following method. Specifically, after evaluating the magnetic flux density B8 using the method described above, the primary coating of the sample was removed with alkali and acid, respectively, and then a magnetic field of 800 A / m was applied again to determine the magnetic flux density B8'. Here, ΔB8 is calculated as ΔB8 = B8' - B8. A magnetic characteristic degradation amount ΔB8 of 0.020T or less was considered to be in good condition. The results are shown in Table 1B.

[0074] [Table 1B]

[0075] In example No. 1, the proportion of three-coordinate boron in B was excessive, and the average particle size of the MgO particles was too small, resulting in an uneven coating and inferior B8, appearance, and coating adhesion of the grain-oriented electrical steel sheet. In example No. 2, the proportion of three-coordinate boron in B was excessive, and the average particle size of MgO particles was extremely large, resulting in a non-uniform coating and inferior B8, appearance, and coating adhesion of the grain-oriented electrical steel sheet. In example No. 6, the low B content resulted in poor film adhesion. In example No. 7, the excessive B content resulted in a lower magnetic property degradation amount ΔB8. In example No. 13, the proportion of three-coordinate boron was excessive, resulting in an inferior appearance. In example No. 14, the proportion of three-coordinate boron was insufficient, resulting in a lower magnetic property degradation amount ΔB8. In example No. 15, the ratio of the circumference to the thickness of the primary MgO particles was insufficient, resulting in poor film adhesion. In examples No. 3-5 and 8-12, grain-oriented electrical steel sheets were obtained with an average particle size of 0.10 μm or more and 8.50 μm or less, a B content of 0.005 mass% or more and less than 0.040 mass%, a proportion of 3-coordinate boron in the B of 5 mass% or more and less than 70 mass%, and a ratio of the circumference of MgO-containing particles to the thickness of the primary particles of 6.0 or more, exhibiting excellent B8, appearance, coating adhesion, and magnetic property degradation ΔB8. In particular, in examples No. 8-11, the [Ca] / [BO3] value was in the range of 0.05 or more and less than 2.00, resulting in a small magnetic property degradation ΔB8 and good magnetic properties.

[0076] <Example 2> Grain-oriented electrical steel sheets were manufactured using the mixed powder shown in Table 2B, which contains MgO particles, B-containing particles, and TiO2 particles in the proportions shown in Table 2A. Specifically, an aqueous slurry of an annealing separator containing the mixed powder from Table 2B was applied to the cold-rolled steel sheet after primary recrystallization annealing. The aqueous slurry was prepared by mixing the mixed powder from Table 2B with water. The solid content (mixed powder) in the aqueous slurry was 20% by mass. In all cases, the cold-rolled steel sheet coated with the aqueous slurry was baked at 300°C for 30 seconds to dry the aqueous slurry and bake the solid content. After baking, a finish annealing treatment was performed. In the finish annealing treatment, in all cases, the sheet was held at 1200°C for 20 hours. Through the above manufacturing process, a grain-oriented electrical steel sheet was produced having a base steel sheet and a glass coating containing a composite oxide such as forsterite (Mg2SiO4), with a longitudinal length of 60 mm, a widthwise length of 300 mm, and a thickness of 0.23 mm.

[0077] [Table 2A]

[0078] [Table 2B]

[0079] The B content, Ca content, Ti content, BO3 content, the proportion of three-coordinate boron in the B (BO3 ratio), the average particle size, and the ratio of the circumference of MgO particles to the thickness of the primary particles were measured in the same manner as in Example 1. Furthermore, the magnetic properties (B8), appearance, coating adhesion, and magnetic property degradation ΔB8 of the obtained grain-oriented electrical steel sheets were evaluated in the same manner as in Example 1. The results are shown in Table 2B.

[0080] In example No. 16, the proportion of three-coordinate boron in B was insufficient, which promoted the formation of Al oxide in the coating. This resulted in a greater degradation of the coating's magnetic properties, leading to a lower magnetic property degradation amount ΔB8. In example No. 17, the proportion of three-coordinate boron in B was excessive, which led to excessive promotion of its formation in the coating, resulting in a greater impact on the degradation of the coating's magnetic properties and a lower magnetic property degradation amount ΔB8. In example No. 18, the amount of B in the mixed powder was excessive, resulting in a lower magnetic property degradation amount ΔB8. In example No. 19, the low B content in the mixed powder resulted in poor film adhesion. In examples No. 20 to 29, the average particle size was between 0.10 μm and 8.50 μm, the mixed powder contained B, the total B content in the mixed powder was between 0.005% by mass and less than 0.040% by mass, the proportion of 3-coordinate boron in the B was between 5% by mass and less than 70% by mass, the ratio of the circumference of the MgO-containing particles to the thickness of the primary particles was 6.0 or more, and grain-oriented electrical steel sheets with excellent B8, appearance, coating adhesion, and magnetic property degradation amount ΔB8 were obtained.

[0081] <Example 3> Grain-oriented electrical steel sheets were manufactured using the mixed powder shown in Table 3B, which contains MgO particles, B-containing particles, Ca-containing particles, and TiO2 particles as shown in Table 3A, under the same conditions as in Example 2. Specifically, an aqueous slurry of an annealing separator containing the mixed powder from Table 3B was applied to the cold-rolled steel sheet after primary recrystallization annealing. The aqueous slurry was prepared by mixing the mixed powder from Table 3B with water. The solid content (mixed powder) in the aqueous slurry was set to 20% by mass. In all examples, the cold-rolled steel sheet coated with the aqueous slurry was baked at 300°C for 30 seconds to dry the aqueous slurry and bake the solid content. After baking, a finish annealing treatment was performed. In the finish annealing treatment, in all examples, the sheet was held at 1200°C for 20 hours. Through the above manufacturing process, a grain-oriented electrical steel sheet was produced, having a base steel sheet and a glass coating containing a composite oxide such as forsterite (Mg2SiO4), with a longitudinal length of 300 mm, a widthwise length of 60 mm, and a thickness of 0.23 mm.

[0082] [Table 3A]

[0083] [Table 3B]

[0084] The B content, Ca content, Ti content, BO3 content, the proportion of three-coordinate boron in the B (BO3 ratio), the average particle size, and the ratio of the circumference of MgO particles to the thickness of the primary particles of the mixed powder were measured using the same method as in Example 1. Furthermore, the magnetic properties (B8), appearance, coating adhesion, and magnetic property degradation ΔB8 of the obtained grain-oriented electrical steel sheets were evaluated using the same procedure as in Example 1. The results are shown in Table 3B.

[0085] In examples No. 30-33, 35, and 36, the average particle size was between 0.10 μm and 8.50 μm, the mixed powder contained boron (B), the total B content in the mixed powder was between 0.005% by mass and less than 0.040% by mass, the proportion of three-coordinate boron in the B was between 5% by mass and less than 70% by mass, and the ratio of the circumference to the thickness of the primary MgO particles was 6.0 or higher. Grain-oriented electrical steel sheets with excellent B8, appearance, coating adhesion, and magnetic property degradation ΔB8 were obtained. In particular, in examples No. 30 and 31, the [Ca] / [BO3] value was in the range of 0.05 to less than 2.00, the magnetic property degradation ΔB8 was small, and good magnetic properties were obtained. In example No. 34, the B content of the mixed powder exceeded 0.040%, and the magnetic property degradation ΔB8 of the grain-oriented electrical steel sheet manufactured using this mixed powder was inferior.

[0086] <Example 4> The raw material particles or powders shown in Table 4A were fired under the conditions shown in Table 4B to produce MgO particles or mixed powders. Using the produced MgO particles or mixed powders, grain-oriented electrical steel sheets were manufactured in the same manner as in Example 1. The B content, Ca content, Ti content, BO3 content, and the proportion of three-coordinate boron in the B (BO3 ratio), average particle size, the ratio of circumference to primary particle thickness for Mg-containing raw material particles, and the ratio of circumference to primary particle thickness for MgO particles in MgO particles or mixed powder were measured in the same manner as in Example 1. Furthermore, the magnetic properties (B8), appearance, coating adhesion, and magnetic property degradation amount ΔB8 of the obtained grain-oriented electrical steel sheets were evaluated in the same manner as in Example 1. The results are shown in Table 4B.

[0087] [Table 4A]

[0088] [Table 4B]

[0089] In examples No. 37 and 39, the B content of the MgO particles was insufficient, resulting in poor film adhesion. In examples No. 38 and 40, the B content of the MgO particles was excessive, resulting in a lower magnetic property degradation amount ΔB8. In examples No. 41-47 and 50-53, a raw material powder containing Mg-containing raw material particles containing one or more selected from the group consisting of magnesium hydroxide, basic magnesium carbonate, and magnesium carbonate, and B-containing particles containing B, with an average particle size of 0.10 μm to 8.50 μm, was calcined at a temperature of 700°C to 1100°C. The MgO particles obtained after calcination had an average particle size of 0.10 μm to 8.50 μm, a B content of 0.005% to less than 0.040% by mass, a proportion of 3-coordinate boron in the B of 5% to less than 70% by mass, and a ratio of circumference to the thickness of the primary particles of the MgO-containing particles of 6.0 or more. As a result, a grain-oriented electrical steel sheet with excellent B8, appearance, film adhesion, and magnetic property degradation amount ΔB8 was obtained. In particular, in cases No. 43-47 and 50-53, the [Ca] / [BO3] value was within the range of 0.05 to less than 2.00, the magnetic property degradation amount ΔB8 was small, and good magnetic properties were obtained. In example No. 48, the calcination temperature was too low, resulting in insufficient calcination of the raw material powder. Consequently, the main component of the mixed powder did not become MgO, and it could not be used as an annealing separating agent. In example No. 49, the firing temperature was too high, causing the average particle size of the mixed powder to fall outside the range, resulting in inferior appearance and coating adhesion of the grain-oriented electrical steel sheet. In example No. 54, the ratio of circumference to primary particle thickness of the Mg-containing raw material particles was less than 7.0, and the ratio of circumference to primary particle thickness of MgO particles or mixed powder MgO particles was below 6.0, resulting in poor coating adhesion.

Claims

1. A mixed powder for annealing separation agents, with MgO as the main component, The average particle size of the mixed powder is 0.10 μm or more and 8.50 μm or less. The mixed powder contains B, and the total B content of the mixed powder is 0.005% by mass or more and less than 0.040% by mass. The proportion of three-coordinate boron in B is 5% by mass or more and less than 70% by mass, The ratio of the circumference of the MgO-containing particles to the thickness of the primary particles is 6.0 or more. Mixed powder.

2. The mixed powder according to claim 1, which contains Ca and satisfies the following formula (1). 0.05≦[Ca] / [BO ３ ]<2.00...Equation (1) In formula (1) above, [Ca] is the Ca content (mass%) in the mixed powder, [BO ３ ] represents the content (mass%) of the three-coordinate boron in the mixed powder.

3. The average particle size is 0.10 μm or more and 8.50 μm or less. The B content is 0.005% by mass or more and less than 0.040% by mass, The proportion of three-coordinate boron in B is 5% by mass or more and less than 70% by mass, MgO particles for annealing separation agent, having a ratio of circumference to thickness of primary particles of 6.0 or more.

4. MgO particles for annealing separation agent according to claim 3, containing Ca and satisfying the following formula (2). 0.05≦[Ca] / [BO ３ ]<2.00...Equation (2) In formula (2) above, [Ca] is the Ca content (mass%) in the MgO particles, [BO ３ ] represents the content (mass%) of the three-coordinate boron in the MgO particles.

5. A method for producing grain-oriented electrical steel sheets, using an annealing separating agent containing the mixed powder described in claim 1 or 2 or the MgO particles described in claim 3 or 4.

6. A method for producing MgO particles for annealing separation agent, wherein the B content is 0.005% by mass or more and less than 0.040% by mass, A method for producing MgO particles for annealing separation agents, comprising: Mg-containing raw material particles containing one or more selected from the group consisting of magnesium hydroxide, basic magnesium carbonate, and magnesium carbonate, wherein the ratio of circumference to thickness of primary particles is 7.0 or more; and B-containing raw material particles containing B, wherein the average particle size is 0.10 μm or more and 8.50 μm or less; and firing the raw material powder at a temperature of 700°C or more and 1100°C or less.

7. The method for producing MgO particles for an annealing separator according to claim 6, wherein the raw material powder contains more than 0% by mass and 0.02% by mass or less of Ca.

8. A method for producing MgO particles for annealing separation agent, wherein the B content is 0.005% by mass or more and less than 0.040% by mass, A method for producing MgO particles for annealing separation agents, comprising calcining raw material particles containing B, one or more selected from the group consisting of magnesium hydroxide, basic magnesium carbonate, and magnesium carbonate, having an average particle size of 0.10 μm or more and 8.50 μm or less, and a ratio of circumference to primary particle thickness of 7.0 or more, at a temperature of 700°C or more and 1100°C or less.

9. The method for producing MgO particles for an annealing separation agent according to claim 8, wherein the raw material particles contain more than 0% by mass and 0.02% by mass or less of Ca.

10. A method for producing a mixed powder for annealing separation agent having a B content of 0.005% by mass or more and less than 0.040% by mass, A method for producing a mixed powder for an annealing separation agent, comprising: Mg-containing raw material particles containing one or more selected from the group consisting of magnesium hydroxide, basic magnesium carbonate, and magnesium carbonate, with a circumference ratio to the thickness of primary particles of 7.0 or more; and B-containing raw material particles containing B, with an average particle size of 0.10 μm or more and 8.50 μm or less; and firing the raw material powder at a temperature of 700°C or more and 1100°C or less.

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