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

Abstract

To provide a magnesium oxide for an annealing separator that allows a slurry of the magnesium oxide for an annealing separator to be evenly applied onto a steel sheet.SOLUTION: The magnesium oxide for an annealing separator comprises reticular particles having multiple depressions on the surfaces, and plate-shaped particles. A proportion of the number of plate-shaped particles and the number of reticular particles is 1:99 to 20:80.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] A method for producing grain-oriented electrical steel sheet is known, in which steel slag or molten steel is subjected to a hot rolling process, a preliminary annealing process, a cold rolling process, a decarburization annealing process, and a high-temperature annealing process. During the high-temperature annealing process, an annealing separator primarily composed of magnesium oxide is applied to the surface of the decarburization-annealed steel sheet. This results in the formation of a forsterite coating on the surface of the steel sheet. The coating properties of the forsterite coating affect the magnetic properties of the steel sheet. Therefore, efforts have been made to improve the coating properties of the forsterite coating. For example, Patent Document 1 discloses a magnesia-based annealing separator used to form a forsterite coating in the final high-temperature annealing process of grain-oriented silicon steel sheet. This annealing separator is primarily composed of magnesia. The magnesia is obtained by blending magnesium hydroxide with Cu or a Cu compound in an amount of 0.1 to 10% by weight, calculated as the metal element, relative to the calcined MgO, and then calcining the mixture. According to Patent Document 1, by incorporating Cu into the annealing separator, the coating properties of the forsterite coating, namely, the uniformity of the appearance, adhesion, and space factor, are improved. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 4-56781 Summary of the Invention [Problem to be solved by the invention]

[0004] According to Patent Document 1, the coating properties of a forsterite coating are improved when the annealing separator contains copper (Cu) in the range of 0.1 to 10 wt %. However, because the annealing separator contains a large amount of copper, there is a possibility that a large amount of copper will diffuse into the steel sheet during the high-temperature annealing process. In this case, the copper content in the steel sheet will be too high, which may adversely affect the magnetic properties of the steel sheet.

[0005] In order to obtain a forsterite coating having good coating properties regardless of the amount of copper, it is important to uniformly apply the annealing separator slurry onto the steel sheet.

[0006] An object of the present invention is to provide magnesium oxide for an annealing separator that enables a magnesium oxide slurry for an annealing separator to be uniformly applied onto a steel sheet, and a method for producing a grain-oriented electrical steel sheet using the same. [Means for solving the problem]

[0007] The present invention includes the following disclosures.

[0008] (First Disclosure) The first disclosure relates to magnesium oxide for use in an annealing separator, which is mainly composed of magnesium oxide. The magnesium oxide for use in an annealing separator includes reticulate particles having a plurality of depressions on the surface thereof and plate-like particles. The ratio of the number of the plate-like particles to the number of the reticulate particles is 1:99 to 20:80.

[0009] (Second Disclosure) In a second disclosure, in the first disclosure, the mesh particles have a particle diameter of 0.05 to 5 μm, and the size of each of the plurality of recesses in the mesh particles is 0.005 to 0.2 μm.

[0010] (Third Disclosure) In a third disclosure, in the first or second disclosure, the particle diameter of the plate-like particles is 0.05 to 5 μm.

[0011] (Fourth Disclosure) In a fourth disclosure, in any one of the first to third disclosures, the magnesium oxide contains copper, and the copper content is 3 to 200 ppm.

[0012] (Fifth Disclosure) In the fifth disclosure, in any one of the first to fourth disclosures, the magnesium oxide has a particle size distribution in which D10 is 0.65 to 1.7 μm. In the particle size distribution, D50 is 1.5 to 3.6 μm. In the particle size distribution, D90 is 2.0 to 14 μm. In the particle size distribution, the volume average diameter is 1.55 to 6.4 μm.

[0013] (Sixth Disclosure) In a sixth disclosure, in any one of the first to fifth disclosures, the magnesium oxide further includes aggregated particles that are larger than the plate-like particles and the reticulated particles.

[0014] (7th Disclosure) In a seventh disclosure, in any one of the first to sixth disclosures, when a slurry containing 5 to 30 mass % of magnesium oxide for the annealing separator is formed using water as a solvent, the viscosity of the slurry at 5°C is 2.2 to 5.2 mPa·s.

[0015] (Eighth Disclosure) The eighth disclosure is a method for manufacturing a grain-oriented electrical steel sheet. The method for manufacturing the grain-oriented electrical steel sheet includes an application step and a high-temperature annealing step. The application step is a step of applying a slurry containing magnesium oxide for the annealing separator according to any one of the first to seventh disclosures to a steel sheet that has been decarburized and annealed. The high-temperature annealing step is a step of annealing the steel sheet to which the slurry has been applied. [Effects of the Invention]

[0016] According to the present invention, it is possible to provide magnesium oxide for an annealing separator that enables a magnesium oxide slurry for an annealing separator to be uniformly applied onto a steel sheet, and a method for producing a grain-oriented electrical steel sheet using the same. [Brief explanation of the drawings]

[0017] [Figure 1] FIG. 2 is a schematic diagram showing an example of a reticulated particle and a plate-like particle in magnesium oxide for an annealing separator according to an embodiment. [Figure 2] FIG. 2 is an SEM photograph showing examples of reticulated particles and plate-like particles in the magnesium oxide of Example 1. [Figure 3] FIG. 3 is an SEM photograph showing an example of aggregated particles in the magnesium oxide of Example 1 to which aggregated particles were added. [Figure 4] FIG. 4 is an SEM photograph showing an example of particles in the magnesium oxide of Comparative Example 1. [Figure 5] FIG. 5 is a graph showing an example of the pore distribution in the magnesium oxide of Example 1. [Figure 6] FIG. 6 is a graph showing an example of the pore distribution in the magnesium oxide of Example 2. [Figure 7] FIG. 7 is a graph showing an example of the pore distribution in the magnesium oxide of Example 3. [Figure 8] FIG. 8 is a graph showing an example of the pore distribution in the magnesium oxide of Comparative Example 1. [Figure 9] FIG. 9 is a graph showing an example of the pore distribution in the magnesium oxide of Comparative Example 3. [Figure 10] FIG. 10 is a graph showing an example of the pore distribution in the magnesium oxide of Comparative Example 4. [Figure 11] FIG. 11 is an optical observation image showing an example of the coating test results for the magnesium oxide of Example 1. [Figure 12] FIG. 12 is a 3D observation image showing an example of the coating test results for the magnesium oxide of Example 1. [Figure 13]FIG. 13 is a 3D observation image showing an example of the coating test results for magnesium oxide in Example 2. [Figure 14] FIG. 14 is a 3D observation image showing an example of the coating test results for magnesium oxide in Example 3. [Figure 15] FIG. 15 is an optical observation image showing an example of the coating test results for the magnesium oxide of Comparative Example 1. [Figure 16] FIG. 16 is a 3D observation image showing an example of the coating test results for magnesium oxide of Comparative Example 1. [Figure 17] FIG. 17 is a 3D observation image showing an example of the coating test results for magnesium oxide of Comparative Example 3. [Figure 18] FIG. 18 is a 3D observation image showing an example of the coating test results for magnesium oxide of Comparative Example 4. DETAILED DESCRIPTION OF THE INVENTION

[0018] Hereinafter, preferred embodiments of the magnesium oxide for an annealing separator of the present invention and the method for producing a grain-oriented electrical steel sheet using the same will be described.

[0019] [Magnesium oxide for annealing separator] The magnesium oxide for an annealing separator of the present invention is a powder containing magnesium oxide as a main component. Hereinafter, the magnesium oxide for an annealing separator will also be simply referred to as magnesium oxide. Magnesium oxide includes reticulate particles and plate-like particles. Reticulate particles refer to particles that have a plurality of depressions on the surface of magnesium oxide particles constituting magnesium oxide. In other words, reticulate particles are reticulate particles having a skeletal portion forming a net portion and depression portions forming a mesh portion. However, the skeletal portion may be flat and plate-like. Part or all of the skeletal portion may be bent or distorted in any direction. Part or all of the depressions of the reticulate particles may include through-holes that penetrate the magnesium oxide particles (skeletal portion).

[0020] 1A and 1B are schematic diagrams showing examples of reticular particles and plate-like particles in magnesium oxide according to an embodiment, in which Fig. 1(a) shows reticular particle 1, and Fig. 1(b) shows plate-like particle 2.

[0021] As shown in FIG. 1( a), the reticulated particle 1 is a plate-like particle having a plurality of depressions 12 on its surface 11. The planar shape of the reticulated particle 1 is not particularly limited. Examples of the planar shape of the reticulated particle 1 include thin plate-like, polygonal, rectangular, polygonal, elliptical, circular, irregular, and combinations thereof. The planar shape of the depressions 12 is not particularly limited. Examples of the planar shape of the depressions 12 include polygonal, rectangular, polygonal, elliptical, circular, irregular, and combinations thereof. The arrangement of the depressions 12 on the surface 11 of the reticulated particle 1 is not particularly limited. Examples of the arrangement of the depressions 12 include a staggered arrangement, a matrix arrangement, a random arrangement, and combinations thereof. The reticulated particle may have a skeleton that is flat and plate-like as shown in FIG. 1( a), or may have a shape that is bent or distorted in any direction (not shown).

[0022] As shown in FIG. 1(b), the plate-like particle 2 is a particle that does not have any noticeable depressions on the surface 21 of the plate-like particle. In other words, the surface 21 of the plate-like particle 2 is a particle that has a flat surface. However, the plate-like particle 2 may have a small number of shallow depressions, although not as many as the depressions 12 seen in the reticulated particle 1. The planar shape of the plate-like particle 2 is not particularly limited. Examples of the planar shape of the plate-like particle 2 include thin plate-like, polygonal, rectangular, polygonal, elliptical, circular, irregular, and combinations thereof.

[0023] In magnesium oxide, the magnesium oxide particles, which are the main component, contain reticular particles and plate-like particles. Therefore, when magnesium oxide is mixed with a liquid such as water, the magnesium oxide blends with the water and is less likely to aggregate in the liquid, mainly due to the presence of the reticular particles. Furthermore, the magnesium oxide particles are easily dispersed in the solution, spaced apart from each other, mainly due to the presence of the plate-like particles. As a result, a slurry in which the magnesium oxide is dispersed generally uniformly in the liquid can be obtained. Then, by applying this slurry to a steel sheet, a coating film in which the magnesium oxide is dispersed generally uniformly can be formed on the steel sheet. In other words, the magnesium oxide of this embodiment makes it easy to sufficiently disperse magnesium oxide in the liquid, making it possible to apply the magnesium oxide as uniformly as possible to the steel sheet.

[0024] The particle size of the reticulate particles is not particularly limited as long as it is large enough to form depressions. Examples of particle sizes of the reticulate particles include 0.05 to 5 μm. The lower limit of the particle size of the reticulate particles is preferably 0.1 μm from the viewpoint of making the particles less likely to aggregate in the slurry. The upper limit of the particle size of the reticulate particles is preferably 4 μm from the viewpoint of uniformly dispersing the particles in the slurry. The particle size of the reticulate particles can be measured using a 20,000x scanning electron microscope (SEM) photograph, as described below.

[0025] The pore size of the depressions of the reticulated particles is not particularly limited as long as the depressions have a desired shape. Examples of the pore size of the depressions include 0.005 to 0.2 μm. The lower limit of the pore size of the depressions is preferably 0.01 μm, from the viewpoint of making it easier for the particles to retain moisture in the slurry. The upper limit of the pore size of the depressions is preferably 0.15 μm, from the viewpoint of the size at which multiple depressions can be formed within the particles. The pore size of the depressions can be measured using a 20,000x scanning electron microscope (SEM) photograph, as described below.

[0026] The particle size of the plate-like particles is not particularly limited as long as it is approximately the same as that of the net-like particles. Examples of the particle size of the plate-like particles include 0.05 to 5 μm. The lower limit of the particle size of the plate-like particles is preferably 0.1 μm from the viewpoint of stably dispersing the particles in the slurry. The upper limit of the particle size of the plate-like particles is preferably 4 μm from the viewpoint of uniformly dispersing the particles in the slurry. The particle size of the plate-like particles can be measured using a scanning electron microscope (SEM) photograph at 20,000 magnifications, as described below.

[0027] However, it is not necessary for all of the magnesium oxide to be composed of reticulate particles and plate-like particles. The total proportion of reticulate particles and plate-like particles to the total magnesium oxide is preferably 70% by weight or more. This is from the perspective of the effects achieved by magnesium oxide having reticulate particles and plate-like particles. This proportion is more preferably 90% by weight or more. This proportion is even more preferably 95% by weight or more. As described below, this proportion can be determined, for example, from a scanning electron microscope (SEM) photograph taken at a magnification of 20,000 times or more.

[0028] The relationship between the number of reticulate particles and the number of plate-like particles contained in magnesium oxide is such that the number of reticulate particles is preferably greater than the number of plate-like particles. The greater number of reticulate particles facilitates the reticulate particles to retain an appropriate amount of moisture in the slurry, and the plate-like particles facilitate particle dispersibility. This synergistic effect allows for the production of a coating film with higher uniformity and fewer lumps. The number of particles can be compared, for example, using scanning electron microscope (SEM) photographs taken at a magnification of 20,000 times or more, as described below.

[0029] The relationship between the number of reticulate particles and the number of plate-like particles contained in magnesium oxide is, for example, 1:99 to 20:80 (where the total ratio is 100). The ratio of the number of plate-like particles to the number of reticulate particles is preferably 5:95 to 15:85. By ensuring that the ratio of the number of reticulate particles to the number of plate-like particles in magnesium oxide falls within the above range, a slurry viscosity suitable for application can be more easily obtained when magnesium oxide is suspended in a liquid such as water. Furthermore, the reticulate particles in magnesium oxide retain an appropriate amount of moisture, and the plate-like particles contribute to the dispersibility of the particles, ensuring application properties such as higher uniformity and fewer lumps.

[0030] Alternatively, the relationship between the reticulate particles and the plate-like particles contained in the magnesium oxide may be, for example, 1:99 to 20:80 (weight of the plate-like particles):(weight of the reticulate particles) (where the total ratio is 100). The ratio of the number of reticulate particles to the number of plate-like particles in the magnesium oxide is preferably 5:95 to 15:85. By ensuring that the ratio of the number of reticulate particles to the number of plate-like particles in the magnesium oxide falls within the above range, a slurry viscosity suitable for application can be more easily obtained when the magnesium oxide is suspended in a liquid such as water. Furthermore, in the magnesium oxide, the reticulate particles retain an appropriate amount of moisture, and the plate-like particles contribute to the dispersibility of the particles, ensuring application properties such as higher uniformity and fewer lumps.

[0031] In addition to the reticulate particles and plate-like particles, magnesium oxide may further include aggregated particles larger than the plate-like particles and reticulate particles. The aggregated particles are not two-dimensional like plates, but are three-dimensional, with the longest diameter (dimension) of the particle being within four times the shortest diameter (dimension). Such aggregated particles can function as spacers to prevent seizure when the decarburization-annealed steel sheet is rolled and annealed at high temperatures. The aggregated particles have multiple surfaces. Each of the multiple surfaces may be flat or may be convex or concave. Each of the multiple surfaces of the aggregated particles may have the same shape, or some or all of the surfaces may have different shapes. The aggregated particles may be formed by bonding several aggregated particles having different shapes. The planar shape of the aggregated particles is not particularly limited. Examples of the planar shape of the aggregated particles include polygonal, rectangular, polygonal, elliptical, circular, irregular, and combinations thereof.

[0032] The particle size of the aggregated particles is not particularly limited as long as it is larger than the particle size of the plate-like particles and the net-like particles. The particle size of the aggregated particles is, for example, larger than 5 μm and not larger than 100 μm. The lower limit of the particle size of the aggregated particles is preferably 10 μm from the viewpoint of functioning as a spacer to prevent seizure during high-temperature annealing. The upper limit of the particle size of the aggregated particles is preferably 50 μm from the viewpoint of uniformly dispersing the particles in the slurry. The particle size of the aggregated particles can be measured, for example, using an SEM photograph taken at a magnification of 500 times or more, as described below. The ratio of the aggregated particles to the total magnesium oxide is, for example, 1 to 10 wt %. This is from the viewpoint of functioning as a spacer.

[0033] The particle size distribution of magnesium oxide is not particularly limited as long as the magnesium oxide contains reticular particles and plate-like particles, and preferably also contains aggregated particles. D10 is preferably 0.65 to 1.7 μm. D50 is preferably 1.5 to 3.6 μm. D90 is preferably 2.0 to 14 μm. The volume mean diameter MV is, for example, 1.55 to 6.4 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 particle aggregation in the slurry can be suppressed and dispersibility can be improved.

[0034] Incidentally, "mainly composed of magnesium oxide" means that the content of magnesium oxide in the annealing separator is 50% by mass or more. From the viewpoint of functioning well as an annealing separator, the content of magnesium oxide is preferably 90% by mass or more. The content of magnesium oxide is more preferably 95% by mass or more. In this embodiment, the content of magnesium oxide in the annealing separator is 98% by mass or more.

[0035] Magnesium oxide may contain other substances besides magnesium oxide. Examples of such other substances include copper (Cu). The inclusion of copper can improve the coating properties of the forsterite coating and the magnetic properties of the steel sheet. The lower limit of the copper content relative to magnesium oxide, from the viewpoint of improving properties, is, for example, 3 ppm, preferably 30 ppm, and more preferably 50 ppm. The upper limit of the copper content relative to magnesium oxide, from the viewpoint of suppressing excessive diffusion of copper into the steel sheet, is, for example, 200 ppm.

[0036] Magnesium oxide may contain various trace elements known to promote film formation, improve film properties, and / or improve magnetic properties of grain-oriented electrical steel sheets. Examples of such trace elements include chlorine (Cl), boron (B), sodium (Na), phosphorus (P), aluminum (Al), titanium (Ti), manganese (Mn), calcium (Ca), and compounds thereof. For example, chlorine is an element that promotes the formation of a forsterite film. Boron is an element that promotes the formation of a forsterite film. Sodium is an element that adjusts the rate of forsterite film formation. Phosphorus is an element that promotes the formation of a forsterite film. Aluminum is an element that promotes the formation of a forsterite film.

[0037] When a slurry containing 5 to 30 mass % magnesium oxide is formed using water as a solvent, the viscosity of the slurry at 5°C is preferably 2.2 to 5.2 mPa·s. A more preferable lower limit is 3.5 mPa·s. When the viscosity is not too low, a stable coating film can be maintained when the magnesium oxide-containing slurry is applied to a steel sheet. On the other hand, when the viscosity is not too high, the slurry can be easily applied to the entire surface of the steel sheet. The reason why the viscosity of the slurry is specified as 5°C is as follows: When magnesium oxide is dispersed in water to form a slurry, if the slurry is formed at room temperature, the magnesium oxide will hydrate. Applying a slurry containing such magnesium oxide hydrate to the surface of a steel sheet may adversely affect the properties of the steel sheet. Therefore, to suppress the hydration of magnesium oxide, it is preferable to form the slurry at a low temperature, such as 5°C, and apply it to the surface of the steel sheet. Therefore, the viscosity of the slurry, which is easy to apply, is specified as 5°C.

[0038] [Method of manufacturing magnesium oxide for annealing separator] The method for producing magnesium oxide is not particularly limited as long as it can produce magnesium oxide having the above-described structure. For example, a method for producing magnesium oxide includes reacting a magnesium raw material with an alkaline raw material to synthesize magnesium hydroxide, and then calcining the magnesium hydroxide to obtain magnesium oxide. Examples of the magnesium raw material include water-soluble magnesium salts or hydrates thereof. Suitable magnesium raw materials include magnesium chloride hexahydrate, magnesium chloride dihydrate, and anhydrous magnesium chloride. Seawater, flooding water, and bittern may also be used as the magnesium raw material. When calcining magnesium hydroxide, the calcination atmosphere may be, for example, air or nitrogen. Examples of alkaline raw materials include calcium hydroxide, sodium hydroxide, and potassium hydroxide.

[0039] Another example of the production method is a method using magnesium oxide obtained by calcining the mineral magnesite, in which magnesium oxide obtained from the mineral magnesite is hydrated to obtain magnesium hydroxide, which is then calcined to obtain magnesium oxide.

[0040] 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.

[0041] <Firing conditions> The shape of particles contained in magnesium oxide, such as net-like particles, plate-like particles, and aggregated particles, 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, holding temperature, holding time, and temperature fall time.

[0042] <Plate-shaped particles> Conditions for obtaining magnesium oxide containing plate-like particles include, for example, a temperature rise time of preferably 0.5 to 2.5 hours, more preferably 1.0 to 2.0 hours. The holding temperature is preferably 400 to 700°C or 1000 to 1300°C, more preferably 450 to 600°C or 1050 to 1200°C. When the holding temperature is 400 to 700°C, the shape of the magnesium hydroxide is easily maintained, making it easier to obtain plate-like particles. On the other hand, when the holding temperature is 1000 to 1300°C, sintering of the magnesium oxide proceeds appropriately, making it easier to obtain plate-like particles. The holding time is preferably 0.1 to 15.0 hours, more preferably 0.2 to 13.0 hours. The temperature drop time is preferably 0.1 to 6.0 hours, more preferably 0.2 to 5.0 hours. The temperature rise time, holding time, and temperature drop time are common to the two holding temperature ranges.

[0043] If the holding temperature is lower than the above range, magnesium hydroxide may remain. If the holding temperature is outside the above range, i.e., 700°C to 1000°C, reticulated particles are likely to be formed, making it difficult to form plate-like particles. If the holding temperature is higher than the above range, sintering will proceed too much, making it easy to form agglomerates, making it difficult to form plate-like particles.

[0044] <Reticulated particles> Conditions for obtaining magnesium oxide containing reticulated particles include, for example, a temperature-raising time of preferably 0.5 to 2.0 hours, more preferably 1.0 to 1.5 hours, a holding temperature of preferably 700°C to 1000°C, more preferably 750°C to 950°C, a holding time of preferably 0.1 to 24 hours, more preferably 0.2 to 1.0 hour, and a temperature-lowering time of preferably 0.1 to 1.0 hour, more preferably 0.2 to 0.8 hour.

[0045] If the temperature rise time, holding time, and temperature fall time are shorter than the above ranges, it becomes difficult to form network particles, and uneven firing is likely to occur. If the holding temperature is lower than the above range, it becomes difficult to form network particles. On the other hand, if the temperature rise time, holding time, and temperature fall time are longer than the above ranges, it becomes easy to form plate-like particles, making it difficult to obtain network particles. If the holding temperature is higher than the above range, it becomes difficult to form network particles. Furthermore, production efficiency decreases.

[0046] <Agglomerated particles> The temperature-raising time is preferably 0.5 to 2.5 hours, and more preferably 1.0 to 2.0 hours. The holding temperature is preferably 1100°C to 1300°C, and more preferably 1200°C to 1300°C. The holding time is preferably 0.1 to 15.0 hours, and more preferably 0.2 to 13.0 hours. The temperature-lowering time is preferably 0.1 to 6.0 hours, and more preferably 0.2 to 5.0 hours.

[0047] If the temperature rise time, holding time, and temperature fall time are shorter than the above ranges, plate-like particles are more likely to be formed, and aggregated particles are less likely to be formed. If the holding temperature is lower than the above ranges, the particle size will not be large enough. On the other hand, if the temperature rise time, holding time, and temperature fall time are longer than the above ranges, the particle size will be too large. If the holding temperature is higher than the above ranges, the particle size will be too large.

[0048] <Effects of trace elements during firing> When forming reticulate particles and plate-like particles, the melting point of the material to be fired varies depending on the trace elements contained in the material, so a scanning electron microscope (SEM) is used to confirm whether the desired reticulate particles and / or plate-like particles have been formed.

[0049] For example, the chlorine (Cl) content has the effect of lowering the melting point of the material to be fired when the firing temperature is maintained at approximately 700 to 900°C, and mainly affects the formation of reticulated particles. The Cl content in magnesium oxide obtained after firing is preferably 0.001 to 0.1 mass%.

[0050] The boron (B) content 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, and mainly affects the formation of plate-like particles and agglomerated particles. The B content contained in the magnesium oxide obtained after sintering is preferably 0.03 to 0.15 mass%. When magnesium oxide having a boron content within this numerical range is used as an annealing separator, the magnetic properties and insulating properties of grain-oriented electrical steel sheet can be improved.

[0051] <Firing atmosphere> The atmosphere during firing may be either nitrogen or air. It is sufficient that heat is evenly applied to the material being fired. The material being fired may be uniformly stirred during firing. An example of an apparatus for performing such firing is a rotary kiln.

[0052] <Controlling the ratio of plate-like particles to net-like particles> The ratio of plate-like particles to net-like particles can be controlled by mixing magnesium oxides having different ratios of plate-like particles to net-like particles after final calcination. The ratio may also be controlled by the final calcination conditions for obtaining magnesium oxide, such as the temperature rise time, holding temperature, holding time, and temperature fall time, or by adjusting the trace elements contained in the precursor (intermediate product) to be subjected to final calcination.

[0053] <Controlling the ratio of plate-like particles, net-like particles, and aggregate particles> The ratio of plate-like particles to net-like particles to aggregated particles can be controlled by mixing magnesium oxides having different ratios of plate-like particles to net-like particles to aggregated particles after final calcination. The ratio may also be controlled by setting the final calcination conditions for obtaining magnesium oxide, such as the temperature rise time, holding temperature, holding time, and temperature fall time, or by adjusting the trace elements contained in the precursor (intermediate product) to be subjected to final calcination.

[0054] <Control of trace element content in magnesium oxide> The content of trace elements in magnesium oxide is controlled as follows. First, the content of trace elements contained in raw materials used to produce 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.

[0055] The method of adding the trace element is not particularly limited. For example, the method may be a method of mixing a compound containing the trace element to be restricted with a raw material or an intermediate product. The mixing method may be a wet method or a dry method. The intermediate product includes the above-mentioned precursor.

[0056] 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.

[0057] 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.

[0058] For example, a method of adding copper (Cu) to magnesium oxide includes mixing copper or a copper compound with an intermediate product, adjusting the copper content, and then performing final firing. Alternatively, a method includes mixing multiple magnesium oxides after final firing to adjust the copper content. This allows magnesium oxide to have a desired copper content. A preferred method includes mixing a predetermined amount of copper or a copper compound with a magnesium hydroxide slurry, which is an intermediate product, before final firing. This facilitates uniform dispersion of copper in the resulting magnesium oxide. A preferred copper compound is, for example, copper oxide (CuO).

[0059] One method for adding boron (B) to magnesium oxide is to mix a boron compound with an intermediate product, adjust the boron content, and then perform final calcination. Another method involves mixing multiple magnesium oxides after final calcination to adjust the boron content. This allows magnesium oxide to have a desired boron content. Examples of boron compounds include boric acid, alkali metal borates, ammonium borates, alkali metal metaborates, and boron dioxide.

[0060] In the manufacturing process of magnesium oxide, various additives known to improve film properties and magnetic properties may be effectively added in addition to copper (Cu), such as titanium (Ti), manganese (Mn), aluminum (Al), calcium (Ca), and their compounds.

[0061] <Control of magnesium oxide particle size distribution (D10, D50, D90, MV)> The particle size distribution of magnesium oxide can be controlled by the following method. 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 for magnesium hydroxide. Another method is to re-calcinate or pulverize the magnesium oxide after final calcination.

[0062] Under the above conditions, magnesium oxide for use as an annealing separator is produced. Since this magnesium oxide has the above-mentioned specific composition, it becomes possible to uniformly apply the magnesium oxide for use as an annealing separator onto a steel sheet.

[0063] [Method of manufacturing grain-oriented electrical steel sheets using magnesium oxide as an annealing separator] Next, a method for producing a grain-oriented electrical steel sheet using the above-mentioned magnesium oxide for use as an annealing separator will be described. The production method includes a coating step of coating a decarburization-annealed steel sheet with the above-mentioned magnesium oxide-containing slurry, and a high-temperature annealing step of annealing the magnesium oxide-coated steel sheet.

[0064] <Coating process> The magnesium oxide is uniformly dispersed in a liquid, such as water, to form a slurry containing magnesium oxide. The liquid is prepared at a low temperature, such as 5°C, to prevent hydration of the magnesium oxide.

[0065] As described above, the concentration of magnesium oxide 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.

[0066] As described above, 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 lower limit is more preferably 3.5 mPa·s. 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.

[0067] 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 structure is used, the slurry containing the magnesium oxide can be applied uniformly to the steel sheet. Thereafter, the applied slurry is dried at a temperature of, for example, about 300 to 500°C.

[0068] <High-temperature annealing process> The steel sheet coated with the above-described slurry and thus coated with magnesium oxide is then annealed. Annealing conditions include, for example, 1000 to 1200°C and 10 to 20 hours. As a result, a forsterite film is formed on the surface of the steel sheet, and then, by performing a known predetermined treatment as necessary, a grain-oriented electrical steel sheet using the above-described annealing separator is formed.

[0069] The method for producing a grain-oriented electrical steel sheet of the present invention uses magnesium oxide having the above-mentioned specific composition and capable of being uniformly applied to the steel sheet. Therefore, a forsterite coating with good coating properties can be formed on the steel sheet. As a result, a grain-oriented electrical steel sheet with improved magnetic properties can be obtained.

[0070] The magnesium oxide for an annealing separator of the present invention and the method for producing a grain-oriented electrical steel sheet using the same are not limited to the above-described embodiments or the examples described below, and can be appropriately combined, substituted, or modified within the scope of the object and intent of the present invention.

[0071] <Measurement and test methods> The various measurement and test methods are as follows:

[0072] 1. Particle size of net-like particles, plate-like particles, and aggregate particles, pore size of depressions in net-like particles, total ratio of net-like particles and plate-like particles, and ratio of net-like particles to plate-like particles The particle sizes of the reticulated particles and plate-like particles, as well as the pore size of the depressions in the reticulated particles, were confirmed using images obtained by photographing the particles at a magnification of 20,000 times using a scanning electron microscope (SEM). The particle size of the aggregated particles was confirmed using images obtained by photographing the particles at a magnification of 500 times using a scanning electron microscope (SEM). However, when measuring particle size or pore size using an image, the diameter of the smallest circle surrounding the particle or depression to be measured was used as the particle size or pore size. The range of particle size or pore size was determined, for example, by selecting 10 random particles or depressions and measuring the range from the minimum value to the maximum value. The average particle size or pore size was determined, for example, by selecting 10 random particles or depressions and measuring the average of the 10 particle size or pore size values ​​as the final average particle size or pore size.

[0073] The ratio of the total weight of the reticulate particles and plate-like particles to the total weight of magnesium oxide was confirmed by photographing the sample at a magnification of 20,000 times using a scanning electron microscope (SEM). Specifically, the ratio of the total area of ​​the reticulate particles and plate-like particles in the image to the area of ​​the image was approximated to the ratio of the total weight of the reticulate particles and plate-like particles to the total weight of magnesium oxide. The weight ratio of the reticulate particles to the plate-like particles was confirmed by photographing the sample at a magnification of 20,000 times using a scanning electron microscope (SEM). Specifically, the ratio of the total area of ​​the reticulate particles to the total area of ​​the plate-like particles in the image was approximated to the weight ratio of the reticulate particles to the plate-like particles.

[0074] 2.Particle size distribution The particle size distribution was measured using a particle size distribution analyzer MT3300EXII (manufactured by Microtrac Bell Corporation). First, the particle size distribution analyzer was filled with ion-exchanged water, and the ion-exchanged water was circulated. Next, an appropriate amount of sample was added, and after confirming that it was within the appropriate range, the water was circulated for 1 minute and then measured. The measurement time was 30 seconds.

[0075] 3. Relationship between the number of plate-like particles and the number of net-like particles The relationship between the number of plate-like particles and the number of net-like particles was determined by taking a photograph using a scanning electron microscope (SEM) at a magnification of, for example, 20,000 times or more, and visually observing the resulting image. One hundred particles were randomly selected from the image, and the number of plate-like particles and the number of net-like particles were counted to calculate the ratio of the number of plate-like particles to the number of net-like particles.

[0076] 4. BET specific surface area, pore distribution, and total pore volume Approximately 0.5 g of sample was weighed into a cell, and as a pretreatment, it was vacuum degassed and heated at 105°C for 1 hour. The pretreated cell was then measured for specific surface area, pore size distribution, and total pore volume by nitrogen adsorption using a BELSORP MAX pore size distribution analyzer (Microtrac-Bell Corporation). The specific surface area was analyzed using the BET multipoint method. The pore size distribution was analyzed using the BJH method using desorption isotherms. The total pore volume was determined using the single-point method.

[0077] 5. Trace element content 0.5 g of sample was dissolved in 5 ml of 30% HNO3 solution, and then the volume was adjusted to 100 ml with ultrapure water. This was used as the test solution and measured using the calibration curve method with an emission spectrometer SPS3520-DD (Hitachi High-Tech Science Corporation).

[0078] 6. Slurry viscosity 30 ml of ion-exchanged water was placed in a beaker equipped with a stirrer and the temperature was adjusted to 5°C. All subsequent procedures were carried out at a temperature of 5°C. 3.5 g of sample was weighed using an electronic top-dish balance and poured into the beaker. After pouring, the sample was left to stand for 30 seconds, and then stirred and mixed with a stirrer for 2 minutes to prepare the test liquid. 7 ml of the test liquid was placed in the chamber of a temperature-adjusted viscometer LVDV2T (manufactured by Brookfield) and measured. The spindle was SC4-18(18) and the rotation speed was 200 rpm.

[0079] 7. Slurry application test 3.5 g of magnesium oxide 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 after homogenization through a rubber roll, it was baked at 500°C for 20 seconds to bake in the MgO. Optical and 3D images of the obtained steel plate were taken using a one-shot 3D shape measuring instrument, VR-6000 (manufactured by Keyence Corporation). [Example]

[0080] 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.

[0081] (1) About the sample [Example 1] A 1.7 mol / L magnesium chloride aqueous solution and a 2.2 mol / L calcium hydroxide aqueous solution were mixed and reacted at 130°C for 4 hours to obtain a magnesium hydroxide slurry. Copper oxide (CuO) was added to the obtained slurry so that the copper (Cu) content in the final magnesium oxide was 0.012 mass%. Furthermore, 0.3 × 10 pure water was added to the obtained slurry so that the boron (B) content in the final magnesium oxide was 0.09 mass%. 3 mol m -3 After the addition, the slurry was filtered and washed with purified water in an amount 20 times the mass of the solid content to obtain a magnesium hydroxide cake. The obtained cake was dried at 180°C for 40 minutes to obtain magnesium hydroxide.

[0082] The obtained magnesium hydroxide was calcined in an air atmosphere using a rotary kiln under the following conditions: heating time: 1.25 hours, holding temperature: 900°C, holding time: 0.25 hours, and cooling time: 0.25 hours, to obtain a calcined product. The calcined product was pulverized using a jet mill to obtain magnesium oxide A. When 100 random particles of the obtained magnesium oxide A were observed using an SEM photograph at 20,000x magnification, reticulated particles were confirmed, but plate-like particles were not.

[0083] Separately prepared magnesium oxide with a D50 of 3.1 μm was rehydrated at 90°C for 4 hours, dehydrated, and dried to obtain magnesium hydroxide powder. The obtained magnesium hydroxide powder was fired in an air atmosphere using a tunnel kiln under the following conditions: a heating time of 2 hours, a holding temperature of 1100°C, a holding time of 12 hours, and a cooling time of 4 hours to obtain a fired product. The fired product was pulverized using a jet mill to obtain magnesium oxide B. When 100 random particles of the obtained magnesium oxide B were observed using SEM photographs at 20,000x magnification, no reticulated particles were observed, but plate-like particles were confirmed. Furthermore, when the obtained magnesium oxide B was observed using SEM photographs at 500x magnification, aggregated particles were confirmed.

[0084] Magnesium oxide B and magnesium oxide A were mixed in a weight ratio of 10:90 to prepare the magnesium oxide of Example 1. In the magnesium oxide of Example 1, the ratio of the number of plate-like particles to the number of reticulate particles was 10:90. Therefore, the ratio of the number of plate-like particles to the number of reticulate particles was within the range of 1:99 to 20:80.

[0085] FIG. 2 is an SEM photograph showing examples of reticulate particles and plate-like particles in the magnesium oxide of Example 1. The magnification is 20,000 times. FIG. 2(a) shows an example of a typical reticulate particle 1. FIG. 2(b) shows an example of a typical plate-like particle 2. As shown in FIG. 2(a), reticulate particle 1 had multiple depressions on the surface of the plate-like particle. Its planar shape was roughly hexagonal. Some reticulate particles could also be seen around it. On the other hand, as shown in FIG. 2(b), plate-like particle 2 had some irregularities on the surface of the plate-like particle, but no noticeable depressions. Its planar shape was a deformed hexagon. A few reticulate particles could also be seen around it.

[0086] Figure 3 is an SEM photograph showing an example of aggregated particles in the magnesium oxide of Example 1 to which aggregated particles were added. The magnification was 500x. As shown in the figure, aggregated particle 3 had multiple surfaces. Small mesh-like particles and plate-like particles were attached to the surface of aggregated particle 3.

[0087] [Example 2] 1.6 mol / L bittern and 12 mol / L aqueous sodium hydroxide solution were mixed and reacted at 130°C for 2 hours to obtain magnesium hydroxide slurry. The obtained slurry was filtered and washed with soft water in an amount 20 times the mass of the solid content to obtain a magnesium hydroxide cake. The obtained cake was dried at 180°C for 40 minutes to obtain magnesium hydroxide.

[0088] The obtained magnesium hydroxide was calcined in an air atmosphere using a rotary kiln under the following conditions: heating time: 0.5 hour, holding temperature: 550°C, holding time: 1.0 hour, and cooling time: 0.1 hour, to obtain a calcined product. The calcined product was pulverized using an impact crusher to obtain magnesium oxide C. When 100 random particles of the obtained magnesium oxide C were observed using SEM photographs at 20,000x magnification, no reticulated particles were observed, but plate-like particles were confirmed. Furthermore, when the obtained magnesium oxide C was observed using SEM photographs at 500x magnification, no aggregated particles were confirmed.

[0089] 1.6 mol / L bittern and 12 mol / L aqueous sodium hydroxide solution were mixed and reacted at 130°C for 2 hours to obtain a magnesium hydroxide slurry. The obtained slurry was filtered and washed with soft water in an amount 20 times the mass of the solid content to obtain a magnesium hydroxide cake. The obtained cake was dried at 150°C for 40 minutes, and the obtained dried product was pulverized using an impact pulverizer to obtain magnesium hydroxide.

[0090] The obtained magnesium hydroxide was fired in an air atmosphere using a tunnel kiln under conditions of a heating time of 0.5 hours, a holding temperature of 850°C, a holding time of 24 hours, and a cooling time of 0.1 hours, to obtain magnesium oxide D. When 100 random particles of the obtained magnesium oxide D were observed using an SEM photograph at 20,000x magnification, net-like particles were confirmed, but no plate-like particles were found.

[0091] Magnesium oxide C and magnesium oxide D were mixed at a weight ratio of 5:95 to prepare magnesium oxide of Example 2. In the magnesium oxide of Example 2, the ratio of the number of plate-like particles to the number of reticulate particles was 5:95. Therefore, the ratio of the number of plate-like particles to the number of reticulate particles was within the range of 1:99 to 20:80.

[0092] [Example 3] Magnesium oxide C and magnesium oxide D were mixed at a weight ratio of 15:85 to prepare magnesium oxide of Example 3. In the magnesium oxide of Example 3, the ratio of the number of plate-like particles to the number of reticulate particles was 15:85. Therefore, the ratio of the number of plate-like particles to the number of reticulate particles was within the range of 1:99 to 20:80.

[0093] [Comparative Example 1] First, magnesium oxide was prepared, containing, in oxide equivalents, 92.9 wt% MgO, 2.39 wt% SiO, 2.36 wt% CaO, 0.19 wt% AlO, and 0.55 wt% FeO. This magnesium oxide was introduced into an aqueous solution of ammonium nitrate (NHNO / MgO molar ratio = 2) in a reactor equipped with a stirrer, a cooler, a thermometer, and a gas inlet, heated to 100°C. The aqueous solution was then stirred to dissolve and react the magnesium oxide, producing a magnesium nitrate solution and releasing ammonia gas. The produced ammonia gas was added to water, recovered, and concentrated to 10.0 mol / L to serve as an alkali source. The produced magnesium nitrate solution was filtered, recovered, and adjusted to 3.5 mol / L to serve as a magnesium source.

[0094] The purified magnesium nitrate solution, ammonia water, and water were mixed at 25°C in a ratio of 1.46 mol Mg(NO3)2 (417.14 ml): 2.92 mol NH3 HO (292.0 ml): 16.14 mol HO (290.86 ml), causing the magnesium nitrate and ammonia to react, yielding a magnesium hydroxide slurry.

[0095] The obtained magnesium hydroxide slurry was heated at 130°C for 4 hours to obtain a magnesium hydroxide slurry. An appropriate amount of an aqueous boric acid solution was added to the obtained magnesium hydroxide slurry, filtered, and washed with pure water in an amount 20 times the mass of the solid content to obtain a magnesium hydroxide cake.

[0096] The resulting cake was dried at 180°C for 40 minutes to obtain magnesium hydroxide. The resulting magnesium hydroxide was calcined in an air atmosphere using a rotary kiln under conditions of a heating time of 1.5 hours, a holding temperature of 900°C, a holding time of 0.5 hours, and a cooling time of 0.5 hours to obtain a calcined product. The resulting calcined product was pulverized using a jet mill to obtain magnesium oxide of Comparative Example 1. When 100 random particles of the magnesium oxide of Comparative Example 1 were observed using an SEM photograph at 20,000x magnification, the ratio of the number of plate-like particles to the number of net-like particles was 0:100. Therefore, the ratio of the number of plate-like particles to the number of net-like particles was outside the range of 1:99 to 20:80.

[0097] Figure 4 is an SEM photograph showing an example of particles in the magnesium oxide of Comparative Example 1. The magnification was 20,000 times. As shown in the figure, the skeleton portion was bent or distorted in arbitrary directions, and relatively small reticular particles (or aggregates thereof) were present compared to the case of Figure 2(a), but particles resembling plate-like particles were hardly present. Furthermore, although not shown, no aggregated particles were confirmed in the SEM photograph at a magnification of 500 times.

[0098] Comparative Example 2 A 1.7 mol / L aqueous magnesium chloride solution and a 12 mol / L aqueous sodium hydroxide solution were mixed and reacted at 130°C for 4 hours to obtain a magnesium hydroxide slurry. The obtained slurry was filtered and washed with pure water in an amount 20 times the mass of the solid content to obtain a magnesium hydroxide cake. The obtained cake was dried at 180°C for 40 minutes to obtain magnesium hydroxide.

[0099] The obtained magnesium hydroxide was calcined in an air atmosphere using a rotary kiln under the following conditions: a heating time of 1.5 hours, a holding temperature of 950°C, a holding time of 0.5 hours, and a cooling time of 0.5 hours, to obtain a calcined product. The obtained calcined product was pulverized using a jet mill to obtain the magnesium oxide of Comparative Example 2. When 100 random particles of the obtained magnesium oxide of Comparative Example 2 were observed using an SEM photograph at 20,000x magnification, the ratio of the number of plate-like particles to the number of net-like particles was 0:100. Therefore, the ratio of the number of plate-like particles to the number of net-like particles was outside the range of 1:99 to 20:80.

[0100] Comparative Example 3 Magnesium oxide consisting only of magnesium oxide C was designated as Comparative Example 3. In the magnesium oxide of Comparative Example 2, the ratio of the number of plate-like particles to the number of reticulate particles was 100:0. Therefore, the ratio of the number of plate-like particles to the number of reticulate particles was outside the range of 1:99 to 20:80.

[0101] Comparative Example 4 Magnesium oxide C and magnesium oxide D were mixed in a weight ratio of 30:70 to prepare magnesium oxide of Comparative Example 4. In the magnesium oxide of Comparative Example 4, the ratio of the number of plate-like particles to the number of reticulate particles was 30:70. Therefore, the ratio of the number of plate-like particles to the number of reticulate particles was outside the range of 1:99 to 20:80.

[0102] (2) Evaluation items The magnesium oxide of Example 1 and the magnesium oxides of Comparative Examples 1 and 2 were evaluated for particle size, pore size of depressions, particle size distribution, slurry viscosity, trace element content, pore distribution, pore volume, BET specific surface area, and slurry application test. For Examples 2, 3, Comparative Examples 3, and 4, the particle size, pore size of the depressions, particle size distribution, trace element content, pore distribution, pore volume, BET specific surface area, and slurry application test were evaluated.

[0103] (3) Evaluation results (a) Particle size and pore size In the magnesium oxides of Examples 1, 2, and 3, the particle size of the reticulate particles was generally in the range of 0.05 to 5 μm. In the magnesium oxides of Examples 1, 2, and 3, the pore size of the depressions in the reticulate particles was generally in the range of 0.005 to 0.2 μm. In the magnesium oxides of Examples 1, 2, and 3, the particle size of the plate-like particles was generally in the range of 0.05 to 5 μm, which was similar to that of the reticulate particles. In the magnesium oxide of Example 1, the particle size of the aggregated particles was generally in the range of 5 to 100 μm.

[0104] On the other hand, in the magnesium oxides of Comparative Examples 1 and 2, reticulate particles were formed but plate-like particles were not formed. (b) Particle size distribution In the magnesium oxide of Example 1, D10=1.24 μm, D50=2.55 μm, D90=9.97 μm, and the volume mean diameter MV was 4.57 μm. In the magnesium oxide of Example 2, D10=0.77 μm, D50=1.53 μm, D90=2.54 μm, and the volume mean diameter MV was 1.62 μm. In the magnesium oxide of Example 3, D10=0.72 μm, D50=1.54 μm, D90=2.58 μm, and the volume mean diameter MV was 1.62 μm. That is, in the magnesium oxides of Examples 1, 2, and 3, D10 was in the range of 0.65 to 1.7 μm, D50 was in the range of 1.5 to 3.6 μm, D90 was in the range of 2.0 to 14 μm, and MV was in the range of 1.55 to 6.4 μm. That is, the particle size distribution of the magnesium oxides of Examples 1, 2, and 3 was a relatively small particle size.

[0105] On the other hand, the magnesium oxide of Comparative Example 1 had D10=1.29 μm, D50=6.15 μm, D90=40.01 μm, and a volume mean diameter MV of 17.17 μm. That is, the magnesium oxide of Comparative Example 1 had D10 within the range of 0.65 to 1.7 μm, but D50 outside the range of 1.5 to 3.6 μm, D90 outside the range of 2.0 to 14 μm, and MV outside the range of 1.55 to 6.4 μm. The magnesium oxide of Comparative Example 2 had D10=1.02 μm, D50=3.86 μm, and D90=70.52 μm, and the volume mean diameter MV was 25.53 μm. That is, the magnesium oxide of Comparative Example 2 had D10 within the range of 0.65 to 1.7 μm, but D50 outside the range of 1.5 to 3.6 μm, D90 outside the range of 2.0 to 14 μm, and MV outside the range of 1.55 to 6.4 μm. The magnesium oxide of Comparative Example 3 had D10=1.14 μm, D50=4.00 μm, and D90=6.75 μm, and the volume mean diameter MV was 4.12 μm. That is, the magnesium oxide of Comparative Example 3 had D10 within the range of 0.65 to 1.7 μm, D50 outside the range of 1.5 to 3.6 μm, D90 within the range of 2.0 to 14 μm, and MV within the range of 1.55 to 6.4 μm. That is, the particle size distribution of the magnesium oxides of Comparative Examples 1, 2 and 3 was relatively large. The magnesium oxide of Comparative Example 4 had D10=0.52 μm, D50=1.43 μm, and D90=2.57 μm, and the volume mean diameter MV was 1.50 μm. That is, the magnesium oxide of Comparative Example 4 had D10 outside the range of 0.65 to 1.7 μm, D50 outside the range of 1.5 to 3.6 μm, D90 within the range of 2.0 to 14 μm, and MV outside the range of 1.55 to 6.4 μm. That is, the particle size distribution of the magnesium oxide of Comparative Example 4 was a relatively small particle size.

[0106] (c) Slurry viscosity For the magnesium oxide of Example 1, the viscosity of the slurry at 5° C. was 3.69 mPa·s. Therefore, the viscosity of the slurry at 5° C. was within the range of 2.2 to 5.2 mPa·s, which is easy to apply.

[0107] On the other hand, the magnesium oxides of Comparative Examples 1 and 2 had slurries with viscosities of 3.39 mPa·s and 3.70 mPa·s at 5°C, respectively. Therefore, the slurries had viscosities of 2.2 to 5.2 mPa·s at 5°C, which was within the range of easy application.

[0108] (d) Trace element content In the magnesium oxide of Example 1, the typical trace elements were boron (B) 795 ppm, aluminum (Al) 205 ppm, copper (Cu) 109 ppm, iron (Fe) 266 ppm, manganese (Mn) 91 ppm, and titanium (Ti) 12 ppm.

[0109] In the magnesium oxide of Example 2, the typical trace elements were boron (B) at 236 ppm, aluminum (Al) at 51 ppm, copper (Cu) at 4 ppm, iron (Fe) at 2 ppm, manganese (Mn) at less than 1 ppm, and titanium (Ti) at less than 1 ppm. In the magnesium oxide of Example 3, the typical trace elements were boron (B) at 244 ppm, aluminum (Al) at 261 ppm, copper (Cu) at 4 ppm, iron (Fe) at 8 ppm, manganese (Mn) at less than 1 ppm, and titanium (Ti) at less than 1 ppm.

[0110] On the other hand, the magnesium oxide of Comparative Example 1 contained 852 ppm of boron (B), 22 ppm of aluminum (Al), 1 ppm of copper (Cu), 135 ppm of iron (Fe), 8 ppm of manganese (Mn), and 2 ppm of titanium (Ti) as typical trace elements. The magnesium oxide of Comparative Example 2 contained 200 ppm of boron (B), less than 1 ppm of aluminum (Al), 2 ppm of copper (Cu), 0 ppm of iron (Fe), less than 1 ppm of manganese (Mn), and less than 1 ppm of titanium (Ti) as typical trace elements.

[0111] In the magnesium oxide of Comparative Example 3, the typical trace elements were boron (B) at 144 ppm, aluminum (Al) at 22 ppm, copper (Cu) at 4 ppm, iron (Fe) at less than 1 ppm, manganese (Mn) at less than 1 ppm, and titanium (Ti) at less than 1 ppm. In the magnesium oxide of Comparative Example 4, the typical trace elements were boron (B) at 240 ppm, aluminum (Al) at 331 ppm, copper (Cu) at 4 ppm, iron (Fe) at 11 ppm, manganese (Mn) at less than 1 ppm, and titanium (Ti) at less than 1 ppm.

[0112] The results of (b) to (d) above are summarized in Table 1. [Table 1]

[0113] (e) Pore size distribution, pore volume, and BET specific surface area FIG. 5 is a graph showing an example of the pore distribution in the magnesium oxide of Example 1. The vertical axis represents the differential pore volume (cm 3 ·g -1 nm -1 ), and the horizontal axis is the pore diameter (nm). In the magnesium oxide of Example 1, the total pore volume was 0.276 cm 3 / g, and the BET specific surface area is 16.84 m 2 / g. Therefore, the total pore volume and BET specific surface area were relatively large. In addition, peaks in the pore volume were present around pore diameters of 2 to 6 nm and 20 to 70 nm.

[0114] FIG. 6 is a graph showing an example of the pore distribution in the magnesium oxide of Example 2. The vertical axis represents the differential pore volume (cm 3 ·g -1 nm -1 ), and the horizontal axis is the pore diameter (nm). For the magnesium oxide of Example 2, the total pore volume was 0.290 cm 3 / g, and the BET specific surface area is 20.28 m 2 / g. Therefore, the total pore volume and BET specific surface area were relatively large. In addition, peaks in the pore volume were present in the vicinity of pore diameters of 2 to 6 nm and 30 to 70 nm.

[0115] Fig. 7 is a graph showing an example of the pore distribution in the magnesium oxide of Example 3. The vertical axis represents the differential pore volume (cm 3 ·g -1 nm -1 ), and the horizontal axis is the pore diameter (nm). For the magnesium oxide of Example 3, the total pore volume was 0.259 cm 3 / g, and the BET specific surface area is 32.83 m 2 / g. Therefore, the total pore volume and BET specific surface area were relatively small. In addition, a pore volume peak was present in the vicinity of pore diameters of 3 to 6 nm. A pore volume peak was present in the vicinity of pore diameters of 30 to 70 nm, but it was small.

[0116] Fig. 8 is a graph showing an example of the pore distribution in the magnesium oxide of Comparative Example 1. The vertical axis represents the differential pore volume (cm 3 ·g -1 nm -1 ), and the horizontal axis is the pore diameter (nm). In the magnesium oxide of Comparative Example 1, the total pore volume was 0.201 cm 3 / g, and the BET specific surface area is 15.34 m 2 / g. Therefore, the total pore volume and BET specific surface area were relatively small. In addition, a pore volume peak was present in the vicinity of pore diameters of 2 to 6 nm. However, no pore volume peak was present in the vicinity of pore diameters of 20 to 70 nm. Although a pore volume peak was present in the vicinity of pore diameters of 50 to 80 nm, it was small.

[0117] 9 is a graph showing an example of the pore distribution in the magnesium oxide of Comparative Example 3. The vertical axis represents the differential pore volume (cm 3 ·g -1 nm -1 ), and the horizontal axis is the pore diameter (nm). In the magnesium oxide of Comparative Example 3, the total pore volume was 0.281 cm 3 / g, and the BET specific surface area is 121.67 m 2 / g. Therefore, the total pore volume and BET specific surface area were relatively small. In addition, a peak in the pore volume was observed in the vicinity of pore diameters of 3 to 6 nm.

[0118] Fig. 10 is a graph showing an example of the pore distribution in the magnesium oxide of Comparative Example 4. The vertical axis represents the differential pore volume (cm 3 ·g -1 nm -1 ), and the horizontal axis is the pore diameter (nm). In the magnesium oxide of Comparative Example 4, the total pore volume was 0.282 cm 3 / g, and the BET specific surface area is 46.43 m 2 / g. Therefore, the total pore volume and BET specific surface area were relatively small. In addition, a peak in the pore volume was observed in the vicinity of pore diameters of 3 to 6 nm.

[0119] (f) Optical observation images and 3D observation images based on the coating test FIG. 11 is an optical observation image showing an example of the results of a coating test on the magnesium oxide of Example 1. The magnification was 40x. FIG. 12 is a 3D observation image showing an example of the results of a coating test on the magnesium oxide of Example 1. The magnification was 120x. As can be seen, it was found that the coating film formed with the slurry containing the magnesium oxide of Example 1 had few spots or lumps.

[0120] Fig. 13 is a 3D observation image showing an example of the coating test results for the magnesium oxide of Example 2. The magnification was 120x. As can be seen from the image, it was found that the coating film formed with the slurry containing the magnesium oxide of Example 2 had few spots or lumps.

[0121] 14 is a 3D observation image showing an example of the coating test results for the magnesium oxide of Example 3. The magnification was 120x. As can be seen from the image, it was found that the coating film formed with the slurry containing the magnesium oxide of Example 3 had few spots or lumps.

[0122] Meanwhile, Fig. 15 is an optical observation image showing an example of the coating test results for the magnesium oxide of Comparative Example 1. The magnification was 40x. Fig. 16 is a 3D observation image showing an example of the coating test results for the magnesium oxide of Comparative Example 1. The magnification was 120x. As can be seen from the image, it was found that the coating film formed with the slurry containing the magnesium oxide of Comparative Example 1 had many spots and lumps.

[0123] 17 is a 3D observation image showing an example of the coating test results for the magnesium oxide of Comparative Example 3. The magnification was 120 times. As can be seen from the image, it was found that the coating film formed with the slurry containing the magnesium oxide of Comparative Example 3 had many spots and lumps.

[0124] 18 is a 3D observation image showing an example of the coating test results for the magnesium oxide of Comparative Example 4. The magnification was 120 times. As can be seen from the image, it was found that the coating film formed with the slurry containing the magnesium oxide of Comparative Example 4 had many spots and lumps.

[0125] As can be seen from the above data, the magnesium oxide for an annealing separator of the present invention has the above-mentioned specific composition, and therefore, it is possible to uniformly apply the magnesium oxide for an annealing separator to a steel sheet. As a result, by using the magnesium oxide for an annealing separator, a forsterite coating with good coating properties can be obtained. Therefore, the magnetic properties of grain-oriented electrical steel sheets manufactured using the magnesium oxide for an annealing separator can be improved. [Explanation of symbols]

[0126] 1. Reticulated particles 2 Plate-like particles 3. Agglomerated particles 11 Surface 12 depression 21 Surface

Claims

[Claim 1] Magnesium oxide for an annealing separator, The magnesium oxide is a reticulated particle having a plurality of depressions on its surface; Plate-like particles; Including, the ratio of the number of the plate-like particles to the number of the net-like particles is 1:99 to 20:80; Magnesium oxide for annealing separator.

Images