Magnesium oxide for annealing separator, method for producing same, and method for producing grain-oriented electromagnetic steel sheet using same

Abstract

Provided is magnesium oxide for an annealing separator with which it is possible to form a smooth forsterite film on the surface of a grain-oriented electromagnetic steel sheet. Magnesium oxide for an annealing separator according to the present invention is capable of forming a smooth forsterite coating on the surface of a grain-oriented electrical steel sheet. The magnesium oxide contains chlorine. The average chlorine content of the magnesium oxide is 20-300 ppm. The variation coefficient of the chlorine concentration in the magnesium oxide is 0.25 or less.

Description

Magnesium oxide for annealing separator, its manufacturing method, and manufacturing method of grain-oriented electrical steel sheet using the same

[0001] The present invention relates to magnesium oxide for use in an annealing separator, a method for producing the same, and a method for producing a grain-oriented electrical steel sheet using the same.

[0002] Annealing separators intended to improve the smoothness of the forsterite film in grain-oriented electrical steel sheets are known. For example, Patent Document 1 discloses an annealing separator for grain-oriented electrical steel sheets. The annealing separator contains Cl: 0.01 to 0.05 mass%, B: 0.05 to 0.15 mass%, CaO: 0.1 to 2 mass%, and P. ２ O ３ : Contains 0.03 to 1.0 mass% and is mainly composed of magnesia. The magnesia has a citric acid activity of 30 to 120 seconds at 40% CAA and a specific surface area of ​​8 to 50 m2 by the BET method. ２ / g, a hydration amount by ignition loss of 0.5 to 5.2 mass%, and a content of particles of 45 μm or more of particle size of 0.1 mass% or less. Furthermore, the annealing separator contains 0.05 mass% or more and 20 mass% or less of a water-insoluble compound having a particle size of 45 μm or more and 150 μm or less. Patent Document 1 states that by using the annealing separator, a uniform and smooth forsterite coating can be easily formed.

[0003] International Publication No. WO2013 / 051270

[0004] In Patent Document 1, the content of trace elements in the magnesia, i.e., magnesium oxide, of the annealing separator is specified in order to form a smooth forsterite coating. However, Patent Document 1 does not describe variations in the concentration of trace elements in magnesium oxide. Therefore, there is room for further improvement in the technology for forming a smooth forsterite coating.

[0005] An object of the present invention is to provide magnesium oxide for an annealing separator capable of forming a smooth forsterite film on the surface of grain-oriented electrical steel sheet, a method for producing the same, and a method for producing grain-oriented electrical steel sheet using the same.

[0006] The present invention includes the following disclosures.

[0007] (First Disclosure) The first disclosure is magnesium oxide for use as an annealing separator. The magnesium oxide contains chlorine. The average content of chlorine in the magnesium oxide is 20 to 300 ppm. The coefficient of variation of the chlorine concentration in the magnesium oxide is 0.25 or less.

[0008] (Second Disclosure) In the second disclosure, in the first disclosure, the average crystallite size of the magnesium oxide is 25 to 60 nm.

[0009] (Third Disclosure) In a third disclosure, in the first or second disclosure, the magnesium oxide further contains at least one element selected from Al, Mn, Fe, and Cu.

[0010] (Fourth Disclosure) The fourth disclosure is a method for producing magnesium oxide for an annealing separator. The method for producing the magnesium oxide includes a reaction step and a formation step. In the reaction step, a chlorine-containing magnesium hydroxide raw material and an alkali raw material are continuously supplied to a reaction vessel, and the magnesium hydroxide raw material and the alkali raw material are reacted in a turbulent state. An upper layer slurry of magnesium hydroxide formed by the reaction is then continuously removed from the reaction vessel. In the formation step, the removed magnesium hydroxide is fired to form magnesium oxide. The magnesium oxide contains chlorine. The average chlorine content in the magnesium oxide is 20 to 300 ppm. The coefficient of variation of the chlorine concentration in the magnesium oxide is 0.25 or less.

[0011] (Fifth Disclosure) The fifth disclosure relates to a method for manufacturing a grain-oriented electrical steel sheet. The method for manufacturing the grain-oriented electrical steel sheet includes an application step of applying a slurry containing magnesium oxide for an annealing separator to a steel sheet that has been decarburization-annealed, and an annealing step of annealing the steel sheet to which the slurry has been applied. The magnesium oxide contains chlorine. The average chlorine content in the magnesium oxide is 20 to 300 ppm. The coefficient of variation of the chlorine concentration in the magnesium oxide is 0.25 or less.

[0012] According to the present invention, it is possible to provide magnesium oxide for an annealing separator that can form a smooth forsterite coating on the surface of grain-oriented electrical steel sheet, a method for producing the same, and a method for producing grain-oriented electrical steel sheet using the same.

[0013] Hereinafter, preferred embodiments of the magnesium oxide for an annealing separator of the present invention, the method for producing the same, and the method for producing a grain-oriented electrical steel sheet using the same will be described.

[0014] [Magnesium oxide for annealing separator] The magnesium oxide for annealing separator of the present invention is a powder containing magnesium oxide as a main component, but may contain other trace elements. However, "containing" a trace element means that the trace element is contained inside and / or outside the magnesium oxide particles. The magnesium oxide content of the annealing separator is at least 95% by mass or more, preferably 98% by mass or more. Therefore, the trace element content of the annealing separator is less than 5% by mass, preferably less than 2% by mass. Hereinafter, magnesium oxide for annealing separator will also be simply referred to as magnesium oxide.

[0015] Magnesium oxide contains chlorine (Cl) as a trace element. When a forsterite film is formed by the reaction of magnesium oxide for the annealing separator with silicon dioxide on the steel sheet surface, chlorine is added to the forsterite phase (Mg ２ SiO ４ This has the effect of lowering the melting point of the forsterite phase, thereby improving the flowability of the forsterite phase and improving the smoothness of the overall appearance of the forsterite coating that is formed.

[0016] If the chlorine content in magnesium oxide is too low, the effect of lowering the melting point of the forsterite phase is less likely to be achieved. On the other hand, if the chlorine content is too high, the effect of lowering the melting point is likely to be reduced. Therefore, the chlorine content in magnesium oxide is preferably 15 to 400 ppm. When magnesium oxide contains 20 ppm or more of chlorine, the effect of lowering the melting point of the forsterite phase is more likely to be achieved. This improves the flowability of the forsterite phase, allowing for a smooth foresterite coating. When magnesium oxide contains 400 ppm or less of chlorine, the reduction in the effect of lowering the melting point of the forsterite phase due to excessive chlorine can be suppressed. This improves the flowability of the forsterite phase, allowing for a smooth foresterite coating. Furthermore, the distribution of chlorine is less likely to be uneven, preventing the occurrence of unevenness in the foresterite coating. The lower limit of the chlorine content is preferably 20 ppm. The upper limit of the chlorine content is preferably 300 ppm.

[0017] As described above, the effect of lowering the melting point of the forsterite phase can vary depending on the chlorine content in magnesium oxide. That is, the smoothness of the forsterite coating can vary depending on the chlorine content. Therefore, it is preferable that not only the chlorine content is within the above-mentioned predetermined range, but also the chlorine distribution is uniform. In other words, it is preferable that not only the chlorine content is within the above-mentioned predetermined range, but also the chlorine concentration variation is low. Therefore, in magnesium oxide for annealing separators, the coefficient of variation, which indicates the degree of variation in chlorine concentration, is preferably less than 0.30. The coefficient of variation is more preferably 0.25 or less. However, the coefficient of variation corresponds to the variation in chlorine concentration contained in individual magnesium oxide particles among multiple magnesium oxide particles in magnesium oxide. The method for determining the coefficient of variation will be described later.

[0018] The magnesium oxide for the annealing separator has at least a chlorine content within a predetermined range, and the variation in the chlorine concentration is suppressed. Therefore, by uniformly applying the magnesium oxide for the annealing separator to a steel sheet, it is possible to uniformly distribute chlorine on the steel sheet at a content within the predetermined range. This results in a uniform decrease in melting point throughout the forsterite phase, improved flowability, and improved overall smoothness of the forsterite coating. In other words, the magnesium oxide for the annealing separator of the present invention can form a smooth forsterite coating on the surface of grain-oriented electrical steel sheet.

[0019] The average crystallite size of magnesium oxide particles is not particularly limited as long as it can adjust the reactivity of magnesium oxide. The average crystallite size is preferably 25 to 60 nm. If the average crystallite size is 25 nm or less, the reactivity is excessively high, which can lead to hydration during preparation of the coating slurry or localized forsterite formation. If the average crystallite size is 60 nm or more, the reactivity of magnesium oxide decreases, making it difficult to form forsterite. The method for determining the average crystallite size will be described later.

[0020] The shape of the magnesium oxide particles is not particularly limited as long as the trace element content can be within a predetermined range and the variation in the concentration of the trace elements between particles can be reduced. Examples of the planar shape of the particles include polygonal, rectangular, polygonal, elliptical, circular, irregular, and combinations thereof. The particle shape can be confirmed from the image obtained by photographing the particles at a magnification of 20,000 times using a scanning electron microscope (SEM).

[0021] The particle size distribution of magnesium oxide is not particularly limited as long as it can reduce the variation in the content and concentration of trace elements between particles. D10 is preferably 0.5 to 2.0 μm. D50 is preferably 1.5 to 4.0 μm. D90 is preferably 6.0 to 14 μm. The volume mean diameter MV is, for example, 2.0 to 7.0 μm. In the particle size distribution of magnesium oxide, the particle diameters are distributed in a range that is relatively small but not too small, so that the dispersibility can be improved while suppressing the aggregation of particles in the slurry. The method for determining the particle size distribution will be described later.

[0022] Magnesium oxide may contain various trace elements known to contribute to facilitating coating formation, improving coating properties, and / or improving magnetic properties of grain-oriented electrical steel sheets, such as at least one element selected from aluminum (Al), boron (B), sodium (Na), copper (Cu), phosphorus (P), iron (Fe), manganese (Mn), titanium (Ti), and calcium (Ca), or compounds thereof.

[0023] [Method for producing magnesium oxide for annealing separator] There are no particular limitations on the method for producing magnesium oxide as long as it is possible to produce magnesium oxide having the above-mentioned configuration, particularly magnesium oxide with little variation in the concentrations of trace elements. Examples of the production method include the following three methods.

[0024] In the first method, an aqueous solution of magnesium hydroxide raw material containing trace elements and an aqueous solution of alkali raw material are continuously supplied to a reaction vessel while reacting under a turbulent flow condition, and an upper layer slurry of magnesium hydroxide formed by the reaction is continuously removed from the reaction vessel. The removed magnesium hydroxide is then calcined to obtain magnesium oxide. However, the reaction may also be allowed to proceed while the aqueous solution of trace element raw material, the aqueous solution of magnesium hydroxide raw material, and the aqueous solution of alkali raw material are each continuously supplied to the reaction vessel.

[0025] In the second method, magnesium hydroxide is synthesized by a batch reaction under turbulent conditions while continuously supplying an aqueous solution of an alkali raw material to an aqueous solution of a magnesium hydroxide raw material containing trace elements in a reaction vessel. The synthesized magnesium hydroxide is then calcined to obtain magnesium oxide. However, the reaction may also be carried out while continuously supplying an aqueous solution of a trace element raw material and an aqueous solution of an alkali raw material to the aqueous solution of the magnesium hydroxide raw material in the reaction vessel.

[0026] In the third method, magnesium oxide is obtained by mixing powders of trace element raw materials with magnesium hydroxide powder and calcining the mixture. However, the magnesium oxide powder can be obtained, for example, by a method in which the trace element raw materials are not added when producing magnesium hydroxide in the first or second method.

[0027] Conversely, when the magnesium hydroxide raw material contains an excess of a trace element, a chelating agent suitable for the trace element may be added to the magnesium hydroxide raw material.

[0028] However, examples of magnesium hydroxide raw materials include water-soluble magnesium salts or hydrates thereof, and specifically, magnesium chloride hexahydrate, magnesium chloride dihydrate, and anhydrous magnesium chloride are suitable. In addition, seawater, brine, and bittern may also be used as magnesium hydroxide raw materials.

[0029] Examples of the source of the target trace element, i.e., the target trace element source, include the trace element itself and a compound of the trace element. When the trace element is chlorine, examples of the chlorine compound that is the chlorine source include magnesium chloride, sodium chloride, aluminum chloride, calcium chloride, potassium chloride, ammonium chloride, and hydrochloric acid.

[0030] Examples of alkali raw materials include calcium hydroxide, sodium hydroxide, and potassium hydroxide. When magnesium hydroxide is calcined, the calcination atmosphere can be air or nitrogen.

[0031] In the first method, for example, a target trace element source and a magnesium hydroxide source are first added to deionized water to form an aqueous solution containing the target trace element source and the magnesium hydroxide source, i.e., a magnesium hydroxide source aqueous solution. Meanwhile, an alkali source is added to deionized water to form an aqueous solution containing the alkali source, i.e., an alkali source aqueous solution. Next, the magnesium hydroxide source aqueous solution and the alkali source aqueous solution are continuously poured into a reaction vessel at predetermined flow rates. Then, in the reaction vessel, the magnesium hydroxide source to which the target trace element source has been added and the alkali source are reacted under turbulent conditions caused by stirring, while the upper layer of magnesium hydroxide slurry formed by the reaction is continuously removed from the reaction vessel (reaction step). The magnesium hydroxide source aqueous solution and the alkali source aqueous solution are poured into the reaction vessel at flow rates such that the ratio of Mg to OH is approximately 1:2. "Approximately" refers to a flow rate error within a range of ±50%. During this process, the reaction vessel is maintained at a predetermined pressure and temperature, as necessary. In this way, the magnesium hydroxide raw material to which the target trace element raw material has been added and the alkali raw material are continuously reacted in a turbulent state to synthesize the target trace element-added magnesium hydroxide. If necessary, washing with water, filtration, and drying are performed. The trace element-added magnesium hydroxide is then calcined at a predetermined temperature to obtain the trace element-added magnesium oxide (forming step).

[0032] In the second method, for example, first, a target trace element source and a magnesium hydroxide source are added to deionized water to form an aqueous solution containing the target trace element source and the magnesium hydroxide source, i.e., a magnesium hydroxide source aqueous solution. Meanwhile, an alkali source is added to deionized water to form an aqueous solution containing the alkali source, i.e., an alkali source aqueous solution. Next, the alkali source aqueous solution is poured into the magnesium hydroxide source aqueous solution in a reaction vessel at a predetermined flow rate, and the magnesium hydroxide source with the target trace element source added and the alkali source are reacted in the reaction vessel under turbulent conditions due to stirring (reaction step). If necessary, the reaction vessel is maintained at a predetermined pressure and temperature. In this way, magnesium hydroxide with the target trace element source added is synthesized by gradually adding the alkali source to the magnesium hydroxide source under turbulent conditions. If necessary, washing with water, filtration, and drying are performed. The magnesium hydroxide with the trace elements added is then calcined at a predetermined temperature to obtain magnesium oxide with the trace elements added (formation step).

[0033] In the third method, for example, a magnesium hydroxide raw material is first added to deionized water to form an aqueous solution containing the magnesium hydroxide raw material, i.e., a magnesium hydroxide raw material aqueous solution (without trace elements). Meanwhile, an alkali raw material is added to deionized water to form an aqueous solution containing the alkali raw material, i.e., an alkali raw material aqueous solution. Next, the alkali raw material aqueous solution is poured into the magnesium hydroxide raw material aqueous solution (without trace elements) in a reaction tank at a predetermined flow rate, and the magnesium hydroxide raw material and the alkali raw material are reacted in the reaction tank. During this process, the reaction tank is maintained at a predetermined pressure and temperature while stirring as necessary. By reacting the magnesium hydroxide raw material and the alkali raw material in this manner, magnesium hydroxide free of trace elements is synthesized (reaction step). A powder of the target trace element raw material is mixed with this magnesium hydroxide powder free of trace elements. The magnesium hydroxide powder mixed with the powder of the target trace element raw material is then fired at a predetermined temperature to obtain magnesium oxide with added trace elements (formation step).

[0034] Another example of the manufacturing method is a method using magnesium oxide obtained by calcining the mineral magnesite. In this manufacturing method, first, magnesium oxide obtained from the mineral magnesite is hydrated to obtain magnesium hydroxide powder, and then the magnesium hydroxide powder is used to obtain magnesium oxide by carrying out the third method described above.

[0035] The terms used in this specification regarding the calcination of magnesium hydroxide samples have the following meanings. "Heating time" refers to the time it takes to heat from room temperature to reach the desired maximum temperature when calcining a sample. "Holding temperature" refers to the desired maximum temperature when calcining a sample. It is also called the calcination temperature. "Holding time" refers to the time it takes to maintain the holding temperature when calcining a sample. "Cooling time" refers to the time it takes to cool from the holding temperature to room temperature after the holding time has elapsed when calcining a sample. Note that cooling includes active cooling using a cooling means as well as gradual cooling such as by leaving it to cool.

[0036] <Caking Conditions> Magnesium oxide can be controlled by adjusting the final calcination conditions when obtaining magnesium oxide and the trace elements contained in the precursor to be subjected to the final calcination. The calcination conditions include the temperature rise time, the holding temperature, the holding time, and the temperature fall time.

[0037] As conditions for obtaining magnesium oxide, for example, the temperature rise time is preferably 0.5 to 3 hours, and more preferably 1 to 2 hours. The holding temperature is preferably 600°C to 1300°C, and more preferably 700°C to 1200°C. The holding time is preferably 0.1 to 15 hours, and more preferably 0.2 to 13 hours. The temperature drop time is preferably 0.1 to 6 hours, and more preferably 0.2 to 5 hours.

[0038] If the temperature-raising time, holding time, and temperature-lowering time are shorter than the above ranges, the calcination of magnesium hydroxide may not be completed or the reactivity of magnesium oxide may be too high during the formation of the forsterite coating, making it difficult to form uniform forsterite.On the other hand, if the temperature-raising time, holding time, and temperature-lowering time are longer than the above ranges, the reactivity of magnesium oxide may be too low, making it difficult to form forsterite.

[0039] The boron contained in magnesium oxide has the effect of lowering the melting point of the material to be sintered when the holding temperature during sintering is approximately 1200 to 1300°C. The boron content in magnesium oxide is preferably 0.05 to 0.15 mass%. When magnesium oxide having a boron content within this range is used as an annealing separator, the characteristics of the forsterite coating can be improved, and the magnetic properties and insulating properties of the grain-oriented electrical steel sheet can be improved.

[0040] The sodium content in magnesium oxide is preferably 0.1 to 200 ppm. When magnesium oxide having a sodium content in this range is used as an annealing separator, the properties of the forsterite coating can be improved, and the magnetic properties and insulating properties of the grain-oriented electrical steel sheet can be improved.

[0041] <Firing atmosphere> The atmosphere during firing may be either nitrogen or air. It is sufficient that heat is uniformly applied to the material to be fired. The material to be fired may be uniformly stirred during firing. An example of an apparatus for performing such firing is a rotary kiln.

[0042] <Control of the Content of Trace Elements in Magnesium Oxide> The content of trace elements in magnesium oxide can be controlled as follows. First, the content of trace elements contained in raw materials for producing magnesium oxide is measured. Then, based on the results, trace elements are added to or removed from the raw materials or intermediate products so that the content of trace elements contained in magnesium oxide becomes the desired content. Examples of raw materials include magnesium raw materials, the mineral magnesite, and alkalis reacted with the magnesium raw materials. Examples of intermediate products include magnesium hydroxide.

[0043] The method of adding trace elements is not particularly limited. For example, a method of mixing a compound containing the trace element to be restricted with a raw material or an intermediate product can be used. The mixing method may be a wet method or a dry method. The intermediate product includes the above-mentioned precursor.

[0044] The method for removing trace elements is not particularly limited. For example, the method may be a method for washing raw materials or intermediate products. A specific example of washing is washing with water.

[0045] It is also possible to obtain magnesium oxide with a desired content of trace elements by mixing intermediate products of different compositions, adjusting the excess or deficiency of trace elements, and then performing final calcination, or by mixing magnesium oxides of different compositions after final calcination, adjusting the excess or deficiency of trace elements, and obtaining magnesium oxide with a desired content of trace elements.

[0046] In the manufacturing process of magnesium oxide, various additives known to improve film properties and magnetic properties may be effectively added, such as copper (Cu), phosphorus (P), manganese (Mn), titanium (Ti), calcium (Ca), and compounds thereof.

[0047] <Control of Magnesium Oxide Particle Size Distribution (D10, D50, D90, MV)> The particle size distribution of magnesium oxide can be controlled by the following methods. One method is to adjust at least one of the reaction temperature, reaction rate, and stirring conditions when synthesizing magnesium hydroxide by reacting a magnesium raw material with an alkali raw material. Another method is to pulverize the precursor before final calcination. Another method is to control the calcination conditions of magnesium hydroxide. Another method is to re-calcinate or pulverize the magnesium oxide after final calcination.

[0048] Under the above conditions, magnesium oxide for use as an annealing separator is produced. Since this magnesium oxide has the above-mentioned predetermined composition, it is possible to suppress variations in the contents of trace elements in the magnesium oxide.

[0049] [Method for manufacturing grain-oriented electrical steel sheet using magnesium oxide for annealing separator] Next, a method for manufacturing grain-oriented electrical steel sheet using the above-mentioned magnesium oxide for annealing separator will be described. The manufacturing method includes an application step of applying the above-mentioned magnesium oxide-containing slurry to a steel sheet that has been decarburized and annealed, and a high-temperature annealing step of annealing the steel sheet coated with magnesium oxide.

[0050] <Coating Step> The magnesium oxide is uniformly dispersed in a liquid, such as water, to form a slurry containing magnesium oxide. The liquid is, for example, water. The slurry is formed at a low temperature, such as 5°C, to prevent hydration of the magnesium oxide.

[0051] The magnesium oxide concentration is, for example, 5 to 30 mass%. The lower limit of the magnesium oxide concentration is preferably 7 mass% from the viewpoint of facilitating uniform application of the slurry to the steel sheet. The upper limit of the annealing separator concentration is preferably 25 mass% from the viewpoint of achieving a viscosity that makes the slurry easy to apply.

[0052] The viscosity of the slurry at 5°C is, for example, 2.2 to 5.2 mPa·s. The lower limit of the viscosity of the slurry at 5°C is preferably 2.6 mPa·s from the viewpoint of ensuring a sufficient amount of coating. The upper limit of the viscosity of the slurry at 5°C is preferably 4.6 mPa·s from the viewpoint of making the slurry easy to coat.

[0053] The slurry is continuously applied to the decarburization-annealed steel sheet using a roll coating device or a spray device. However, the slurry is applied at a low temperature, such as 5°C, to prevent hydration of the magnesium oxide. Since magnesium oxide having the above-mentioned configuration is used, the slurry containing the magnesium oxide is uniformly applied to the steel sheet, thereby suppressing variations in the content of trace elements in the applied slurry. The applied slurry is then dried, for example, at a temperature of about 300 to 500°C.

[0054] <High-Temperature Annealing Step> The steel sheet to which the above-described slurry has been applied, and thus coated with magnesium oxide, is annealed. Annealing conditions include, for example, 1000 to 1200°C and 10 to 20 hours. As a result, a forsterite coating is formed on the surface of the steel sheet, and then, by performing known predetermined treatments as necessary, a grain-oriented electrical steel sheet using the above-described annealing separator is formed.

[0055] The method for producing a grain-oriented electrical steel sheet of the present invention has the above-mentioned specific configuration, and therefore by uniformly applying magnesium oxide to the steel sheet, it is possible to form a forsterite coating on the steel sheet with reduced variation in the content of trace elements, thereby obtaining a grain-oriented electrical steel sheet with improved magnetic properties.

[0056] The magnesium oxide for an annealing separator of the present invention, its manufacturing method, and its manufacturing method for a grain-oriented electrical steel sheet are not limited to the above-described embodiments or the examples described below, and can be appropriately combined, substituted, or modified within a scope that does not deviate from the object and spirit of the present invention.

[0057] <Measurement and test methods> Various measurement and test methods are as follows.

[0058] 1. Trace Element Concentration, Its Average Value, Average Content, Standard Deviation, and Coefficient of Variation The trace element concentration, its average value, standard deviation, and coefficient of variation were determined by the following method. (1) Magnesium oxide (powder) to be measured was prepared. (2) The magnesium oxide was formed into a 4 mm x 4 mm pellet to serve as a sample. (3) The sample was placed in a PHI ADEPT-1010 D-SIMS (Dynamic Secondary Ion Mass Spectrometry) manufactured by ULVAC-PHI, Inc. The trace element concentrations were measured at 250 measurement points in the depth direction, from the surface of the sample to a depth of approximately 5 μm, with a primary acceleration voltage of 5.0 kV. (4) The average value (μ) and standard deviation (σ) of the trace element concentrations were determined based on the measured trace element concentrations at the 250 points. (5) The coefficient of variation was calculated by dividing the standard deviation (σ) by the mean value (μ). (6) The average content was calculated by dividing the mean value (μ) by the molecular weight of chlorine, Avogadro's number, and the density of magnesium oxide. The trace element concentrations at each D-SIMS measurement point do not necessarily represent the concentrations of trace elements in each particle. Measurement points are located every 5 μm / 250 = 0.02 μm, and the measurement intervals are generally smaller than the particle diameter of the magnesium oxide. Therefore, the trace element concentrations at each measurement point can approximate the concentrations of trace elements in more detail than those of each particle.

[0059] 2. Average Crystallite Size The average crystallite size was determined by the following method. (1) Magnesium oxide (powder) to be measured was prepared. (2) The magnesium oxide was placed in a powder X-ray diffractometer (Malvern Panalytical Empyrean X-ray diffractometer) and subjected to powder X-ray diffraction measurement to obtain diffraction data. The measurement conditions were: X-ray output: 45 kV, 40 mA, scanning speed: 40 degrees / min, step width: 0.02 degrees, and X-ray: CuKα radiation. (3) Based on the diffraction data, the constituent components were identified, and the average crystallite size was determined from the half-width of the peak assigned to the (200) plane of magnesium oxide. The integrated powder X-ray analysis software HighScore Plus was used as the XRD analysis software. The average crystallite size was determined by the following Scherrer equation using the diffraction peak assigned to the (200) plane of magnesium oxide. L(200) = (K λ) / (β cos θ) (L: average crystallite size, λ: 1.542 Å (CuKα), θ: Bragg diffraction angle, β: half-width, K: 0.94) The diffraction peak attributable to the (200) plane of magnesium oxide is observed in the vicinity of 2θ = 42.80° to 42.90°.

[0060] 3. Particle size distribution The particle size distribution was measured using a particle size distribution measuring device MT3300EXII (manufactured by Microtrac Bell Co., Ltd.). First, the particle size distribution measuring device was filled with ethanol, and the solvent was circulated. An appropriate amount of the sample to be measured was added thereto, and it was confirmed that it was within the appropriate range. Thereafter, the solvent containing the sample was circulated for 1 minute, and then the particle size distribution was measured. The measurement time was 30 seconds.

[0061] 4. Coating Properties 3.5 g of the magnesium oxide to be evaluated was suspended in 30 ml of water at 5°C to obtain a slurry. The obtained slurry was applied to a steel plate measuring 150 mm in length, 80 mm in width, and 0.5 mm in thickness, and the plate was passed through a rubber roll to homogenize it, followed by baking to bake the magnesium oxide. The coating properties of the magnesium oxide film on the steel plate, i.e., gloss, adhesion, and smoothness, were evaluated visually.

[0062] The present invention will be further described below with reference to examples and comparative examples, although the present invention is not limited to these examples and comparative examples.

[0063] (1) Regarding the Samples The samples of Examples 1 to 3 and Comparative Examples 1 and 2 were produced as follows.

[0064] Example 1: 386 g of magnesium chloride hexahydrate (Wako Pure Chemical Industries, Ltd.) was dissolved in 1.9 L of deionized water to obtain a 1 mol / L magnesium aqueous solution. At atmospheric pressure and 25°C, 1.9 L of this aqueous solution and 1.7 L of a 2 mol / L sodium hydroxide aqueous solution were each poured into a 220 mL overflow vessel and allowed to react continuously to obtain a magnesium hydroxide slurry. During the reaction, a 2.5 cm diameter screw propeller was used to stir the mixture at a rotation speed of 450 rpm. The aqueous solution was then heated to 45°C and heat-treated for 5.0 hours with stirring at 400 rpm. The treated magnesium hydroxide slurry was then filtered to obtain a cake. The solids of the resulting cake were washed twice with 25 times the weight of pure water and dried at 105°C for 12 hours to obtain magnesium hydroxide. The magnesium hydroxide was then calcined at 900°C for 2.0 hours to obtain magnesium oxide. The particle size distribution of the magnesium oxide obtained was measured, and it was found that D10 was 1.7 μm, D50 was 3.8 μm, and D90 was 13 μm.

[0065] Example 2: 386 g of magnesium chloride hexahydrate (Wako Pure Chemical Industries, Ltd.) was dissolved in 1.9 L of deionized water to obtain a 1 mol / L magnesium aqueous solution. At atmospheric pressure and 25°C, 1.7 L of a 2 mol / L sodium hydroxide aqueous solution was poured into 1.9 L of this aqueous solution at a rate of 170 mL / min to obtain a magnesium hydroxide slurry. During the reaction, a 2.5 cm diameter screw propeller was used to stir the solution at a rotation speed of 500 rpm. The aqueous solution was then heated to 40°C, and heat-treated for 5.0 hours while stirring at 400 rpm. The treated magnesium hydroxide slurry was then filtered to obtain a cake. The solids of the resulting cake were washed twice with 30 times the weight of pure water, and then dried at 105°C for 12 hours to obtain magnesium hydroxide. The magnesium hydroxide was then calcined at 900°C for 5 hours to obtain magnesium oxide. The particle size distribution of the obtained magnesium oxide was measured, and it was found that D10 was 1.5 μm, D50 was 3.2 μm, and D90 was 12 μm.

[0066] Example 3: 386 g of magnesium chloride hexahydrate (Wako Pure Chemical Industries, Ltd.) was dissolved in 1.9 L of deionized water to obtain a 1 mol / L magnesium aqueous solution. At atmospheric pressure and 25°C, 1.9 L of this aqueous solution and 1.7 L of a 2 mol / L sodium hydroxide aqueous solution were each poured into a 220 mL overflow vessel and allowed to react continuously to obtain a magnesium hydroxide slurry. During the reaction, a 2.5 cm diameter screw propeller was used to stir the mixture at a rotation speed of 400 rpm. The aqueous solution was then heated to 40°C and heat-treated for 4.0 hours with stirring at 500 rpm. The treated magnesium hydroxide slurry was then filtered to obtain a cake. The solids of the resulting cake were washed twice with 25 times the weight of pure water and dried at 105°C for 12 hours to obtain magnesium hydroxide. The magnesium hydroxide was then calcined at 700°C for 1 hour to obtain magnesium oxide. The particle size distribution of the obtained magnesium oxide was measured, and it was found that D10 was 1.0 μm, D50 was 2.1 μm, and D90 was 8.0 μm.

[0067] Comparative Example 1: 386 g of magnesium chloride hexahydrate (Wako Pure Chemical Industries, Ltd.) was dissolved in 1.9 L of deionized water to obtain a 1 mol / L magnesium aqueous solution. At atmospheric pressure and 25°C, 1.9 L of this aqueous solution and 1.7 L of a 2 mol / L sodium hydroxide aqueous solution were poured into a 220 mL overflow container and allowed to react continuously to obtain a magnesium hydroxide slurry. During the reaction, a 2.5 cm diameter screw propeller was used to stir the mixture at a rotation speed of 400 rpm. The aqueous solution was then heated to 40°C and heat-treated for 5.0 hours under stirring at 350 rpm. The treated magnesium hydroxide slurry was then filtered to obtain a cake. The solids of the resulting cake were washed twice with 25 times the weight of pure water, and then dried at 105°C for 12 hours to obtain magnesium hydroxide. 110 g of this magnesium hydroxide was mixed with 0.05 g of sodium chloride (Wako Pure Chemical Industries, Ltd.) and calcined at 800° C. for 1.5 hours to obtain magnesium oxide. The particle size distribution of the obtained magnesium oxide was measured, and D10 was 2.0 μm, D50 was 3.6 μm, and D90 was 13 μm.

[0068] Comparative Example 2: 386 g of magnesium chloride hexahydrate (Wako Pure Chemical Industries, Ltd.) was dissolved in 1.9 L of deionized water to obtain a 1 mol / L magnesium aqueous solution. At atmospheric pressure and 25°C, 1.9 L of this aqueous solution and 1.7 L of a 2 mol / L sodium hydroxide aqueous solution were poured into a 220 mL overflow container and reacted continuously to obtain a magnesium hydroxide slurry. During the reaction, a 2.5 cm diameter screw propeller was used for stirring at a rotation speed of 300 rpm. Next, the water bath temperature was set to 40°C, and heat treatment was performed for 5.5 hours under stirring at 300 rpm. The treated magnesium hydroxide slurry was then filtered to obtain a cake. The solids of the obtained cake were washed twice with 30 times the weight of pure water and dried at 105°C for 12 hours to obtain magnesium hydroxide. This magnesium hydroxide was then calcined at 1000°C for 3.0 hours to obtain magnesium oxide. The particle size distribution of the obtained magnesium oxide was measured, and it was found that D10 was 1.8 μm, D50 was 4.8 μm, and D90 was 15 μm.

[0069] Comparative Example 3: 386 g of magnesium chloride hexahydrate (Wako Pure Chemical Industries, Ltd.) was dissolved in 1.9 L of deionized water to obtain a 1 mol / L magnesium aqueous solution. At atmospheric pressure and 25°C, 1.7 L of a 2 mol / L sodium hydroxide aqueous solution was poured into 1.9 L of this aqueous solution at a rate of 170 mL / min to obtain a magnesium hydroxide slurry. During the reaction, a 2.5 cm diameter screw propeller was used to stir the solution at a rotation speed of 400 rpm. Next, the temperature of the aqueous solution was set to 45°C, and heat treatment was performed for 4.0 hours under stirring conditions of 400 rpm. The treated magnesium hydroxide slurry was then filtered to obtain a cake. The solids of the obtained cake were washed twice with pure water in an amount 50 times the weight of the cake, and then dried at 105°C for 12 hours to obtain magnesium hydroxide. This magnesium hydroxide was then calcined at 800°C for 1 hour to obtain magnesium oxide. The particle size distribution of the obtained magnesium oxide was measured, and it was found that D10 was 1.7 μm, D50 was 3.4 μm, and D90 was 11 μm.

[0070] The manufacturing conditions for Examples 1 to 3 and Comparative Examples 1 to 3 are summarized in Table 1.

[0071] (2) Evaluation Items The magnesium oxides of Examples 1 to 3 and Comparative Examples 1 to 3 were evaluated for the average crystallite size, the average chlorine content, the coefficient of variation of the chlorine concentration, and the coating properties of the magnesium oxide film, i.e., gloss, adhesion, and smoothness.

[0072] (3) Regarding the evaluation results (a) Example 1 In the completed magnesium oxide, the average chlorine content in the magnesium oxide measured by D-SIMS was 216 ppm. The coefficient of variation of the chlorine concentration at that time was 0.09. The average size of the magnesium oxide crystallites measured by X-ray diffraction was 45 nm. Regarding the coating properties, with regard to gloss, the coating was slightly non-uniform but glossy. With regard to adhesion, the coating was uniform and no peeled areas were present. With regard to smoothness, the coating surface was very smooth. Overall, the condition was very good.

[0073] (b) Example 2 The average chlorine content in the finished magnesium oxide measured by D-SIMS was 23 ppm. The coefficient of variation of the chlorine concentration at that time was 0.22. The average size of the magnesium oxide crystallites measured by X-ray diffraction was 60 nm. Regarding the coating properties, with regard to gloss, the coating was slightly uneven but in a glossy state. With regard to adhesion, the coating was slightly uneven but in a state where no peeled areas were present. With regard to smoothness, the coating surface was in very good smoothness. Overall, the condition was good.

[0074] (c) Example 3 The average chlorine content in the finished magnesium oxide measured by D-SIMS was 300 ppm. The coefficient of variation of the chlorine concentration at that time was 0.15. The average size of the magnesium oxide crystallites measured by X-ray diffraction was 25 nm. Regarding the coating properties, with regard to gloss, the coating was slightly non-uniform but glossy. With regard to adhesion, the coating was uniform and no peeled areas were present. With regard to smoothness, the coating surface was smooth and in good condition. Overall, the condition was good.

[0075] (f) Comparative Example 1 The average chlorine content in the finished magnesium oxide measured by D-SIMS was 1957 ppm. The coefficient of variation of the chlorine concentration at that time was 0.14. The average size of the magnesium oxide crystallites measured by X-ray diffraction was 40 nm. With regard to the coating properties, with regard to gloss, the coating was non-uniform, with the underlying steel sheet being partially exposed. With regard to adhesion, the coating was non-uniform, with pinhole-like peeled areas present. With regard to smoothness, the smoothness of the coating surface was poor. Overall, the condition was very poor.

[0076] (g) Comparative Example 2 The average chlorine content in the finished magnesium oxide measured by D-SIMS was 20 ppm. The coefficient of variation of the chlorine concentration at that time was 0.38. The average size of the magnesium oxide crystallites measured by X-ray diffraction was 60 nm. With regard to the coating properties, with regard to gloss, the coating was non-uniform and lacked luster. With regard to adhesion, the coating was non-uniform and there were obvious peeled areas. With regard to smoothness, the coating surface was in a good state of smoothness. Overall, the condition was poor.

[0077] (g) Comparative Example 3 The average chlorine content in the finished magnesium oxide measured by D-SIMS was 12 ppm. The coefficient of variation of the chlorine concentration at that time was 0.24. The average size of the magnesium oxide crystallites measured by X-ray diffraction was 40 nm. With regard to the coating properties, with regard to gloss, the coating was non-uniform and lacked gloss. With regard to adhesion, the coating was non-uniform and had pinhole-like peeled areas. With regard to smoothness, the coating surface was in a good state of smoothness. Overall, the condition was poor.

[0078] The evaluation results of Examples 1 to 3 and Comparative Examples 1 to 3 are summarized in Table 2. The meanings of the indications for gloss, adhesion, and smoothness in the table are as follows: Gloss ◎: The coating is uniform and very glossy 〇: The coating is slightly uneven, but glossy △: The coating is uneven and matte ×: The coating is uneven, with some of the underlying steel sheet exposed Adhesion ◎: The coating is uniform and there are no peeled areas ○: The coating is slightly uneven, but there are no peeled areas △: The coating is uneven, with pinhole-like peeled areas ×: The coating is uneven, with obvious peeled areas Smoothness ◎: The coating surface is very smooth ○: The coating surface is good smoothness △: The coating surface is poorly smooth ×: The coating surface is very poorly smooth

[0079] As can be seen from the above data, the magnesium oxide for an annealing separator of the present invention has at least a chlorine content within a predetermined range, and the chlorine concentration variation is suppressed. Therefore, by uniformly applying the magnesium oxide for an annealing separator to a steel sheet, it is possible to uniformly distribute chlorine on the steel sheet at a content within the predetermined range. This results in a uniform decrease in melting point throughout the forsterite phase, improved flowability, and improved overall smoothness of the forsterite coating. In other words, the magnesium oxide for an annealing separator of the present invention can form a smooth forsterite coating on the surface of a grain-oriented electrical steel sheet. Therefore, the magnetic properties of grain-oriented electrical steel sheets manufactured using the magnesium oxide for an annealing separator can be improved.

Claims

1. Magnesium oxide for use as an annealing separator, having an average chlorine content of 20 to 300 ppm, and a coefficient of variation of the chlorine concentration of 0.25 or less.

2. The magnesium oxide for use in an annealing separator according to claim 1, wherein the average crystallite size is 25 to 60 nm.

3. The magnesium oxide for an annealing separator according to claim 1 or 2, further comprising at least one element selected from the group consisting of Al, Mn, Fe and Cu.

4. A method for producing magnesium oxide for an annealing separator, comprising: a reaction step of continuously supplying a chlorine-containing magnesium hydroxide raw material and an alkali raw material to a reaction vessel, reacting the magnesium hydroxide raw material with the alkali raw material in a turbulent state, and continuously removing an upper layer slurry of magnesium hydroxide formed by the reaction from the reaction vessel; and a formation step of calcining the removed magnesium hydroxide to form magnesium oxide, wherein the average content of chlorine in the magnesium oxide is 20 to 300 ppm, and the coefficient of variation of the chlorine concentration in the magnesium oxide is 0.25 or less.

5. A method for producing grain-oriented electrical steel sheet, comprising: an application step of applying a slurry containing magnesium oxide for an annealing separator to a steel sheet that has been decarburized and annealed; and an annealing step of annealing the steel sheet to which the slurry has been applied, wherein the average chlorine content in the magnesium oxide is 20 to 300 ppm, and the coefficient of variation of the chlorine concentration in the magnesium oxide is 0.25 or less.

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