Magnesium oxide for annealing separator, annealing separator, and method for manufacturing grain-oriented electrical steel sheet

Abstract

An object of the present invention is to provide magnesium oxide for an annealing separator that can form a forsterite coating on the surface of a steel sheet, the coating having excellent appearance and adhesion. The present invention provides a method for producing a crystalline silicon substrate having a BET specific surface area S measured by a water vapor adsorption method. H2O and the BET specific surface area S measured by nitrogen adsorption method N2 Relative to S H2O / S N2 The magnesium oxide for an annealing separator has a β-value of 0.40 or more and 1.30 or less.

Description

[Technical Field]

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

[0002] Grain-oriented electrical steel sheets used in transformers and generators are generally manufactured by hot-rolling silicon steel containing approximately 3% silicon (Si), followed by cold rolling to the final thickness, followed by decarburization annealing and finish annealing. In the decarburization annealing (primary recrystallization annealing), a SiO2 coating is formed on the steel sheet surface. A slurry containing magnesium oxide as an annealing separator is applied to the surface, dried, and the steel is wound into a coil. This finish annealing causes the SiO2 and MgO to react, forming a forsterite (Mg2SiO4) coating on the steel sheet surface. This forsterite coating applies tension to the steel sheet surface, reduces core loss, improves magnetic properties, and also provides insulation to the steel sheet.

[0003] This forsterite coating determines the product's appearance and electrical insulation, and ultimately its market value. The coating formation process affects the inhibitor decomposition behavior in the steel sheet surface and, ultimately, secondary recrystallization. As a result, the quality of the coating affects the product's magnetic properties. Furthermore, the appearance of the coating determines the final appearance of the grain-oriented electrical steel sheet. Therefore, the appearance of the coating affects product value and has a significant impact on product yield; an uneven coating reduces product yield. Therefore, improving the properties of this coating plays an important role in the manufacturing technology of grain-oriented electrical steel sheets.

[0004] Previously, research into magnesium oxide for use as an annealing separator has been conducted to improve the properties of grain-oriented electrical steel sheets, and various innovations have been made to improve quality, including various studies on the powder properties of magnesium oxide for use as an annealing separator, such as the concentrations of impurities such as B, CaO, SO3, and Cl, citric acid activity (CAA), BET specific surface area, and particle size distribution. However, because magnesium oxide functions in a very complex way in the manufacturing process of grain-oriented electrical steel sheets, it has not yet been fully elucidated how changes in each of the magnesium oxide powder properties affect the product properties of grain-oriented electrical steel sheets.

[0005] Here, magnesium oxide for use as an annealing separator is suspended in water to form a slurry, which is then applied to a steel sheet. However, when the magnesium oxide is slurried, it hydrates over time and changes to magnesium hydroxide, causing an increase in the viscosity of the slurry. If the change to magnesium hydroxide is significant and the increase in the viscosity of the slurry is too great, the application to the steel sheet becomes non-uniform, causing problems in the formation of a forsterite film. Therefore, it is considered important to stabilize the annealing separator slurry in order to form a good forsterite film, and the following studies have been conducted to date.

[0006] Patent Document 1 describes a method for preparing an annealing separator slurry containing magnesia as a main component to be applied to grain-oriented electrical steel sheets, in which the addition of a carboxylic acid or a magnesium salt of a carboxylic acid can suppress an increase in viscosity after slurrying. Patent Document 2 describes that for an annealing separator containing magnesium oxide as a main component, the slurry is subjected to a pulverization treatment using a bead mill or the like after and / or during dispersion in water to activate the particle surfaces, thereby stabilizing the particles. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2007-246973 [Patent Document 2] Japanese Patent Application Publication No. 6-212249 Summary of the Invention [Problem to be solved by the invention]

[0008] Although the annealing separator slurry can be stabilized by the above-mentioned methods, the methods of Patent Documents 1 and 2 require the addition of an acid or a pulverization treatment, which may change the properties of magnesium oxide and may adversely affect the formation of a forsterite coating during finish annealing.

[0009] As described above, the stability of the annealing separator slurry is important, but on the other hand, the dispersibility of magnesium oxide when suspending magnesium oxide in water is also important for imparting an appropriate viscosity to the annealing separator slurry and improving its coatability and adhesion.

[0010] Therefore, an object of the present invention is to discover a new magnesium oxide for an annealing separator that has excellent dispersibility during slurrying and from which a stable annealing separator slurry can be obtained without the addition of acid or pulverization, thereby providing magnesium oxide for an annealing separator for obtaining grain-oriented electrical steel sheets with excellent coating properties. That is, an object of the present invention is to provide magnesium oxide for an annealing separator that can form a forsterite coating on the surface of a steel sheet with excellent coating appearance and adhesion. Another object of the present invention is to propose an annealing separator and a method for producing grain-oriented electrical steel sheets using the above-mentioned magnesium oxide for an annealing separator. [Means for solving the problem]

[0011] In order to solve the above problems, the present inventors have conducted research focusing on the surface state of magnesium oxide for use as an annealing separator, and have found that the BET specific surface area S H2O and the BET specific surface area S measured by nitrogen adsorption method N2 Relative to S H2O / S N2The inventors have found that by controlling the content of magnesium oxide in the annealing separator to fall within a predetermined range, it is possible to obtain magnesium oxide for an annealing separator that has excellent dispersibility when made into a slurry and from which a stable annealing separator slurry can be obtained without the need for adding an acid or performing a pulverization treatment during the slurry preparation, and that this magnesium oxide for an annealing separator is capable of forming a good forsterite coating and is therefore suitable for obtaining an excellent grain-oriented electrical steel sheet, thereby completing the present invention.

[0012] [1] BET specific surface area S measured by water vapor adsorption method H2O and the BET specific surface area S measured by nitrogen adsorption method N2 Relative to S H2O / S N2 Magnesium oxide for use as an annealing separator, wherein the value of β-aminobenzoic acid is 0.40 or more and 1.30 or less.

[0013] [2] BET specific surface area S measured by water vapor adsorption method H2O is 10.0m 2 / g or more 25.0m 2 / g or less of magnesium oxide for annealing separator [1].

[0014] [3] CAA40% is 50 seconds or more and 170 seconds or less, the CaO content is 0.2 to 1.0 mass%, the Cl content is 500 ppm or less, the B content is 400 to 1500 ppm, the SO3 content is 0.01 to 1.00 mass%, and the volume-based cumulative 50% particle diameter (D 50 ) is 0.5 μm or more and 7.0 μm or less.

[0015] [4] An annealing separator containing the magnesium oxide for an annealing separator according to any one of [1] to [3].

[0016] [5] A method for producing grain-oriented electrical steel sheet, comprising: a step of forming an SiO2 coating on the surface of a steel sheet; and a step of applying the annealing separator of [4] to the surface of the SiO2 coating and annealing the steel sheet to form a forsterite coating on the surface of the steel sheet. [Effects of the Invention]

[0017] According to the present invention, it is possible to provide magnesium oxide for an annealing separator that can improve the dispersibility of magnesium oxide for an annealing separator when it is made into a slurry and stabilize the annealing separator slurry after the slurrying, thereby enabling the formation of a forsterite coating on the surface of a steel sheet that has excellent appearance and adhesion to the coating. DETAILED DESCRIPTION OF THE INVENTION

[0018] The magnesium oxide of the present invention has a BET specific surface area S H2O and the BET specific surface area S measured by nitrogen adsorption method N2 Relative to S H2O / S N2 In the present invention, S H2O / S N2 is preferably 0.50 or more and 1.20 or less, and more preferably 0.60 or more and 1.10 or less.

[0019] The present inventors have investigated the BET specific surface area S of magnesium oxide for use as an annealing separator, which is measured by a water vapor adsorption method. H2O (m 2 / g) and the BET specific surface area S measured by nitrogen adsorption method N2 (m 2 / g) H2O / S N2 It has been found that by adjusting the ratio of the annealing separator to a range of 0.40 to 1.30, the dispersibility can be improved and the annealing separator slurry can be stabilized, and by using this magnesium oxide for the annealing separator, an excellent forsterite coating can be formed on the surface of the steel sheet. Although the detailed mechanism is not clear, it is presumed that this is due to the following reasons.

[0020] Generally, nitrogen gas is adsorbed to the entire surface of a particle, whereas water vapor tends to be selectively adsorbed to highly hydrophilic surfaces. Therefore, the BET specific surface area S measured by the water vapor adsorption method H2O and the BET specific surface area S measured by nitrogen adsorption method N2Relative to S H2O / S N2 It is thought that S can be an index showing the degree of hydrophilicity of the particle surface. In other words, a large value indicates high hydrophilicity of the particle surface, and a small value indicates low hydrophilicity of the particle surface. H2O / S N2 If S is larger than 1.30, the particles tend to aggregate when suspended in water, resulting in poor dispersibility. In addition, hydration tends to proceed, making it impossible to stabilize the slurry state. This will also have an adverse effect on film formation. H2O / S N2 If S is smaller than 0.40, problems such as insufficient dispersibility of magnesium oxide in water and tendency of magnesium oxide to settle in water occur. H2O / S N2 By controlling the content of magnesium oxide in the annealing separator to fall within the above range, the dispersibility of magnesium oxide in water can be improved and the stability of the slurry can be improved when the magnesium oxide is suspended in water to form a slurry. As a result, the use of the magnesium oxide for an annealing separator of the present invention makes it possible to form a good coating when applied to a steel sheet, and an excellent grain-oriented electrical steel sheet can be produced.

[0021] In the present invention, the BET specific surface area S of magnesium oxide measured by the water vapor adsorption method H2O For example, 10.0m 2 / g or more 25.0m 2 / g or less, preferably 10.0m 2 / g or more 22.0m 2 / g or less, and more preferably 10.0m 2 / g or more 20.0m 2 / g or less. The BET specific surface area S measured by the water vapor adsorption method H2O When the content of the polymer falls within the above range, a good coating film can be formed.

[0022] In the present invention, the BET specific surface area S of magnesium oxide measured by the nitrogen adsorption method N2 For example, 8.0m 2 / g or more 25.0m2 The BET specific surface area S measured by the nitrogen adsorption method is preferably 1 / g or less. N2 When the content of the polymer falls within the above range, a good coating film can be formed.

[0023] In the present invention, the volume-based cumulative 50% particle diameter (D 50 ) is, for example, 0.5 μm or more and 7.0 μm or less, preferably 0.7 μm or more and 6.0 μm or less, and more preferably 1.0 μm or more and 5.0 μm or less. D 50 If the particle size is smaller than 0.5 μm, the activity is high and the particles tend to aggregate, making handling difficult and making it difficult to form a good coating. 50 If the particle size exceeds 7.0 μm, the primary particle size of the magnesium oxide becomes coarse and the reactivity of the magnesium oxide particles deteriorates, so that the rate of forsterite film formation slows down and it becomes difficult to form a sufficient film.

[0024] In the present invention, the proportion of particles with a particle diameter of 1 μm or less in the volume-based particle size distribution of magnesium oxide is preferably, for example, 20% or less. A proportion of particles with a particle diameter of 1 μm or less of 20% is advantageous for forming a good coating. That is, a high proportion of particles with a particle diameter of 1 μm or less tends to promote coating formation, but if the proportion of particles with a particle diameter of 1 μm or less is more than 20%, the reaction rate becomes too fast and it becomes difficult to form a good coating.

[0025] In the present invention, the CAA 40% of magnesium oxide is, for example, 50 seconds or more and 170 seconds or less, preferably 50 seconds or more and 150 seconds or less, and more preferably 60 seconds or more and 130 seconds or less. The CAA empirically simulates the reactivity of the solid-solid reaction between silicon dioxide and magnesium oxide that occurs on the surface of an actual electrical steel sheet through a solid-liquid phase reaction, and measures the reactivity of magnesium oxide particles, including primary particles. If the CAA 40% of magnesium oxide is longer than 170 seconds, the reactivity of the magnesium oxide particles is poor, slowing the rate of forsterite film formation, resulting in insufficient film formation and tending to deteriorate the core loss and magnetic flux density characteristics of the grain-oriented electrical steel sheet. On the other hand, if the CAA 40% of magnesium oxide is shorter than 50 seconds, the reactivity of the magnesium oxide particles becomes too fast, preventing the formation of a uniform forsterite film and tending to deteriorate the appearance and adhesion of the film on the grain-oriented electrical steel sheet.

[0026] In the magnesium oxide for an annealing separator of the present invention, the contents of, for example, calcium (Ca), chlorine (Cl), boron (B), and sulfur (S) are preferably adjusted to fall within specific ranges.

[0027] Calcium (Ca) is an element that affects the formation and morphology of a forsterite coating. In the present invention, the calcium (Ca) content of magnesium oxide, calculated as CaO, is, for example, 0.2 to 1.0 mass%, preferably 0.2 to 0.8 mass%, and more preferably 0.2 to 0.5 mass%. If the Ca content is less than 0.2 mass%, the coating tends to peel off, and if it is more than 0.5 mass%, a sufficient coating is not formed, and in either case, good coating properties cannot be obtained.

[0028] Chlorine (Cl) is an element that promotes the formation of a forsterite film and has the effect of lowering the film formation temperature. In the present invention, the chlorine (Cl) content of magnesium oxide is, for example, 500 ppm or less, preferably 400 ppm or less, and more preferably 300 ppm or less. If the chlorine (Cl) content is more than 500 ppm, an excessive film is formed, which can cause point defects and the like, and good film properties cannot be obtained, and the effect of lowering the film formation temperature is not fully exerted. In addition, ppm in the specification means ppm by mass unless otherwise specified.

[0029] Boron (B) is an element that promotes the formation of a forsterite film. In the present invention, the boron (B) content of magnesium oxide is, for example, 400 to 1500 ppm, preferably 400 to 1200 ppm, and more preferably 400 to 1000 ppm. If the content is less than 400 ppm, the forsterite film is not sufficiently formed, while if the content is more than 1500 ppm, the film is excessively formed, causing point defects and the like, and in either case, good film properties cannot be obtained.

[0030] Sulfur (S) is an element that affects the formation and morphology of a forsterite coating. The sulfur (S) content of the magnesium oxide of the present invention, calculated as SO3, is, for example, 0.01 to 1.00 mass%, preferably 0.01 to 0.75 mass%, and more preferably 0.01 to 0.50 mass%. If the content is less than 0.01 mass%, the coating is likely to peel off, and if the content is more than 1.00 mass%, a sufficient coating is not formed, and in either case, good coating properties cannot be obtained.

[0031] The magnesium oxide for an annealing separator of the present invention may contain trace amounts of, in addition to the above, silicon (Si), aluminum (Al), iron (Fe), phosphorus (P), and the like.

[0032] When the magnesium oxide of the present invention contains silicon (Si), the silicon content is preferably, for example, 0.05 to 0.5 mass %.

[0033] When the magnesium oxide of the present invention contains aluminum (Al), the aluminum content is preferably, for example, 0.01 to 0.5 mass %.

[0034] When the magnesium oxide of the present invention contains iron (Fe), the iron content is preferably, for example, 0.01 to 0.5 mass %.

[0035] When the magnesium oxide of the present invention contains phosphorus (P), the phosphorus content is preferably, for example, 0.01 to 0.15 mass % in terms of P2O3.

[0036] The magnesium oxide for an annealing separator of the present invention has excellent dispersibility when prepared as a slurry. Here, the dispersibility of magnesium oxide is related to the initial viscosity when prepared as a slurry. The initial viscosity of the magnesium oxide for an annealing separator of the present invention (see the Examples for a measurement method) is, for example, in the range of 10 to 40 cP, which can provide the annealing separator slurry with appropriate spreadability and adhesion. The viscosity of the annealing separator slurry generally affects the formation of a forsterite film. A low viscosity reduces the spreadability and adhesion to the steel sheet surface, causing uneven application and adversely affecting film formation. On the other hand, an excessively high viscosity can cause problems such as the slurry clogging in the piping, making uniform application difficult and resulting in an uneven appearance of the film. In addition, if grooves are present on the steel sheet surface, the slurry does not penetrate into the grooves, resulting in insufficient formation of a forsterite film.

[0037] The magnesium oxide for an annealing separator of the present invention has excellent stability after being made into a slurry. The magnesium oxide for an annealing separator of the present invention exhibits high slurry stability, with the absolute value of the viscosity change (see Examples for the measurement method) after 4 hours after being made into a slurry being, for example, 10 cP or less. An increase in the viscosity of the slurry is undesirable because it tends to make the application to the steel sheet non-uniform. Furthermore, the viscosity change after being made into a slurry is usually thought to be due to the conversion of magnesium oxide to magnesium hydroxide due to hydration, but the hydration of magnesium oxide is generally known to have an adverse effect on film formation and is therefore undesirable.

[0038] In the present invention, known methods can be used to produce magnesium oxide. For example, magnesium chloride is used as a raw material, and calcium hydroxide is added in a slurry state to an aqueous solution of magnesium chloride to cause a reaction, forming magnesium hydroxide. Next, this magnesium hydroxide is filtered, washed with water, dried, and then fired in a heating furnace to form magnesium oxide, which can then be pulverized to a desired particle size.

[0039] Instead of calcium hydroxide, alkaline compounds having a hydroxyl group, such as sodium hydroxide or potassium hydroxide, can also be used.Also, magnesium oxide can be produced by introducing an aqueous solution containing magnesium chloride, such as seawater, flooding water, or bittern, into a reactor and directly producing magnesium oxide and hydrochloric acid at 1500 to 2000°C using the Aman process, which produces magnesium oxide, which is then pulverized to a desired particle size.

[0040] Furthermore, magnesium oxide can also be produced by hydrating magnesium oxide obtained by calcining the mineral magnesite, calcining the resulting magnesium hydroxide, and pulverizing it to a desired particle size.

[0041] The amounts of Ca, Cl, B, S, and other trace elements (e.g., Si, Al, Fe, P) in MgO can be controlled by known methods. For example, the amount of trace elements in MgO can be controlled during the crude product production process or by controlling the amount of trace elements in the resulting crude product before final firing so that the amount of trace elements in MgO is within a predetermined range. Control during the crude product production process can be achieved, for example, by analyzing the amount of trace elements such as Ca, Cl, B, and S contained in the raw materials and, based on the results, adding or removing the target trace elements by a wet or dry method to achieve a predetermined amount. Trace elements can be added, for example, by mixing the elements to be added and drying the mixture. Trace elements can be removed, for example, by physically washing away excess elements by a wet method or by chemically separating them. Chemical separation can be achieved, for example, by forming a soluble hydrate, dissolving, filtering, and washing the resulting compound for separation, or by forming an insoluble compound, precipitating the compound, and then adsorbing and separating the precipitate. The amount of trace elements such as Ca, Cl, B, and S in the crude product before final calcination can be controlled, for example, by combining and mixing crude products with different compositions to adjust the amount of trace elements so that the trace elements fall within a predetermined range, and then calcining the resulting mixture. Furthermore, to control the amount of trace elements, in either case, crude MgO is produced, the resulting MgO is analyzed, and the above procedures can be repeated or combined depending on the individual results regarding the amount of trace elements.

[0042] BET specific surface area S of magnesium oxide measured by water vapor adsorption method H2O and the BET specific surface area S measured by nitrogen adsorption method N2 Relative to S H2O / S N2 can be adjusted by known methods, for example, the following methods.

[0043] For example, by controlling the baking temperature, baking time, temperature rise time, and temperature fall time when baking magnesium hydroxide, H2O and SH2O / S N2 For example, the firing temperature is preferably 700°C to 1100°C, the firing time is preferably 0.1 to 5.0 hours, the temperature rise time is preferably 0.1 to 5.0 hours, and the temperature fall time is preferably 0.1 to 5.0 hours. For example, under these conditions, by increasing the firing temperature and lengthening the firing time, S H2O and S H2O / S N2 By lowering the firing temperature and shortening the firing time, S H2O and S H2O / S N2 However, the specific conditions are adjusted appropriately according to the purpose.

[0044] More specifically, when firing in a rotary kiln or muffle furnace, for example, S is produced by the following conditions and methods. H2O and S H2O / S N2 In rotary kiln firing, the S can be adjusted by controlling the rate at which raw materials are fed, the rotation speed of the furnace, and the atmosphere inside the furnace (air volume, oxygen concentration, etc.). H2O / S N2 In muffle furnace firing, S can be adjusted by controlling the amount of raw material charged and the stirring conditions of the fired material during firing. H2O and S H2O / S N2 can be adjusted.

[0045] In addition, by controlling the surrounding environment (temperature, humidity) when crushing the calcined magnesium oxide, S H2O and S H2O / S N2For example, the ambient temperature during pulverization is preferably 40°C or less, and the relative humidity is preferably 60% or less. For pulverization of magnesium oxide, pulverizers such as jaw crushers, gyratory crushers, cone crushers, impact crushers, roll crushers, cutter mills, stamp mills, ring mills, roller mills, jet mills, hammer mills, pin mills, rotary mills, vibration mills, planetary mills, and ball mills can be used. Furthermore, by performing heat treatment after pulverization, S H2O and S H2O / S N2 However, the heat treatment is preferably carried out at a temperature of 100°C to 300°C so that the particles do not aggregate or sinter due to the heat treatment.

[0046] Others, S H2O and S H2O / S N2 By combining multiple magnesium oxides with different properties and mixing them, S H2O and S H2O / S N2 can be adjusted.

[0047] Magnesium oxide particle size (D 50 etc.), CAA 40% and BET specific surface area S by nitrogen adsorption method N2 can be adjusted by known methods, for example, by the following method: By adjusting the reaction temperature and the concentration of the alkali source in the magnesium hydroxide production process, the primary particle size and secondary particle size of the magnesium hydroxide can be controlled, and the particle size of magnesium oxide, CAA 40% and S N2 In addition, by controlling the calcination temperature and time of magnesium hydroxide with controlled particle size, the particle size of magnesium oxide, CAA40% and S can be adjusted. N2 It is also possible to adjust the particle size, CAA40% and S N2 The adjustment method is particle size after grinding, CAA 40% and S N2Furthermore, the particle size, CAA40% and S of the magnesium oxide after crushing can also be adjusted by crushing the calcined magnesium oxide using a crusher such as a jaw crusher, a gyratory crusher, a cone crusher, an impact crusher, a roll crusher, a cutter mill, a stamp mill, a ring mill, a roller mill, a jet mill, a hammer mill, a pin mill, a rotary mill, a vibration mill, a planetary mill or a ball mill. N2 It is also possible to adjust the particle size, CAA40% and S N2 The adjustment method is particle size after grinding, CAA 40% and S N2 It is also possible to adjust the particle size of magnesium oxide, CAA40% and S by measuring the particle size and grinding several times. N2 It is possible to adjust the particle size, CAA40% and S by combining and mixing multiple magnesium oxide powders. N2 can be adjusted.

[0048] The annealing separator of the present invention is characterized by containing the magnesium oxide for an annealing separator of the present invention, and other than that, there are no particular limitations. For example, it may optionally contain known additives for annealing separators.

[0049] The grain-oriented electrical steel sheet of the present invention can be produced, for example, by the following method. To produce the grain-oriented electrical steel sheet, a silicon steel slab containing 2.5 to 4.5% Si is hot-rolled, pickled, and then cold-rolled or cold-rolled twice with intermediate annealing to adjust the sheet thickness to a desired value. The cold-rolled coil is then subjected to recrystallization annealing, which also serves as decarburization, in a wet hydrogen atmosphere at 650 to 900°C, during which an oxide film primarily composed of silica (SiO2) is formed on the steel sheet surface. The annealing separator containing magnesium oxide for use in the annealing separator of the present invention is uniformly dispersed in water to obtain an aqueous slurry. This aqueous slurry is continuously applied to a steel sheet using roll coating or spraying, and then dried at approximately 300°C. The steel sheet coil thus treated is then subjected to final annealing, for example, at 1200°C for 20 hours to form a forsterite film (Mg2SiO4) on the steel sheet surface. The forsterite coating is an insulating coating and also imparts tension to the steel sheet surface, thereby improving the iron loss value of the grain-oriented electrical steel sheet. [Example]

[0050] The present invention will be described in detail with reference to the following examples, but these examples are not intended to limit the present invention in any way.

[0051] <Measurement and evaluation methods> (1) BET specific surface area S by water vapor adsorption method H2O Measurement method The measurement sample was heated and degassed at 105°C for 1 hour under a 10 Pa atmosphere, and then the BET specific surface area was measured by gas adsorption using a specific surface area and pore size distribution analyzer (Microtrac-Bell BELSORP MAXII). H2O was used as the adsorption gas, and measurements were performed at an adsorption temperature of 25°C over a relative pressure range of 0 to 0.30. The results were analyzed by the BET method to determine the BET specific surface area, S H2O was calculated.

[0052] (2) BET specific surface area S by nitrogen adsorption method N2 Measurement method The measurement sample was heated and degassed at 150°C for 15 minutes in a nitrogen atmosphere, and then the BET specific surface area was measured by gas adsorption using a specific surface area measuring device (Macsorb, manufactured by Mountech). N2 was used as the adsorption gas, and measurements were performed at an adsorption temperature of -196°C and in a relative pressure range of 0 to 0.30. The results were analyzed by the BET method to determine the BET specific surface area S N2 was calculated.

[0053] (3) Volume-based cumulative 50% particle size (D 50 ) and measurement method for the proportion of particles with a particle diameter of 1 μm or less in the volume-based particle size distribution 0.1 g of the measurement sample and 40 mL of methanol were placed in a 100 mL beaker and dispersed for 180 seconds using ultrasonic waves at an output of 120 W. The sample was then placed in a laser diffraction scattering particle size distribution analyzer (MT3300EX-II manufactured by LEEDS & NORTHRUP) to measure the volumetric particle size distribution. From the measured particle size distribution, the cumulative 50% particle size (D 50 ) and the proportion of particles with a particle size of 1 μm or less were calculated.

[0054] (4) CAA40% measurement method 100 mL of 0.4 N citric acid solution and an appropriate amount (2 mL) of 1% phenolphthalein solution as an indicator were placed in a 200 mL beaker, the liquid temperature was adjusted to 30°C, and while stirring at 700 rpm using a magnetic stirrer, 40% of the final reaction equivalent of magnesium oxide (2.0 g) was added to the citric acid solution, and the time until the final reaction, i.e., the time until the citric acid was consumed and the solution became neutral, was measured.

[0055] (5) Measurement method for calcium (Ca) and sulfur (S) content The measurement sample was press-molded using an aluminum ring (φ35 mm) at a total pressure of 30 MPa, and the content of oxides (CaO, SO3) of each element in the sample was measured using a fluorescent X-ray analyzer (Rigaku Simultix15 model).

[0056] (6) Measurement method for chlorine (Cl) content The measurement sample was dissolved in acid, then diluted with ultrapure water, and the mass was measured using a spectrophotometer (Shimadzu UV-2550) to calculate the chlorine (Cl) concentration in the sample.

[0057] (7) Measurement method for boron (B) content The measurement sample was dissolved in acid, then diluted with ultrapure water, and the boron (B) content in the sample was measured using an ICP optical emission spectrometer (PS3520VDD, manufactured by Hitachi High-Tech Science).

[0058] (8) Evaluation of the stability of the annealing separator slurry A 1000 mL beaker containing 800 mL of 10°C ion-exchanged water was charged with 89 g of the sample and stirred for 15 minutes at 2000 rpm using a stirrer (Shinto Scientific Co., Ltd., Three-One Motor BL3000). The slurry was prepared by stirring at 2000 rpm for 15 minutes. The viscosity of the slurry was measured immediately after stirring stopped using a Brookfield viscometer (Toki Sangyo Co., Ltd., VISCOMETER Model BL). This was designated the initial viscosity. The slurry was then stirred at 500 rpm while maintaining the liquid temperature at 10°C. After 4 hours, the viscosity was measured again, and the change in viscosity (cP) from the initial viscosity to the viscosity after 4 hours was calculated. The change in viscosity of the slurry is generally believed to be due to the hydration of magnesium oxide to magnesium hydroxide. This viscosity change can be used as an indicator of the stability of the magnesium oxide in the slurry to evaluate the stability of the annealing separator slurry.

[0059] (9) Evaluation of the appearance and adhesion of the forsterite coating The test specimens were silicon steel slabs for grain-oriented electrical steel sheets, which were hot-rolled and cold-rolled by known methods to a final thickness of 0.28 mm. These steel sheets were then decarburized and annealed in a humid atmosphere of 25% nitrogen and 75% hydrogen. The composition of the steel sheets before decarburization and annealing was, in mass %, 0.01% C, 3.29% Si, 0.09% Mn, 0.03% Al, 0.07% S, and 0.0053% N, with the remainder consisting of unavoidable impurities and Fe. Magnesium oxide was applied to the steel sheets, and the coating properties of the forsterite coating were investigated.

[0060] Specifically, the magnesium oxide of the present invention or the magnesium oxide of the comparative example was made into a slurry, and the weight after drying was 14 g / m 2 After drying, the steel sheet was subjected to final annealing at 1200°C for 20.0 hours. After the final annealing was completed, the steel sheet was cooled, rinsed with water, pickled with an aqueous hydrochloric acid solution, rinsed with water again, and dried to form a forsterite coating on the steel sheet.

[0061] The appearance of the forsterite coating was judged from the appearance of the coating after cleaning: ◯ indicates that the gray forsterite coating was uniformly thick, or that the coating was uniform but somewhat thin, and × indicates that the coating was uneven and thin but with no areas where the underlying steel sheet was exposed, or that the coating was uneven and very thin with areas where the underlying steel sheet was clearly exposed.

[0062] The adhesion of the forsterite coating was judged from the state of the coating after cleaning: a rating of ∘ indicates that the coating was uniformly formed with no peeled areas, or that the coating was slightly uneven but with no peeled areas, and an rating of × indicates that the coating was uneven with pinhole-like peeled areas, or that the coating was uneven with clear peeled areas.

[0063] Example 1 Boric acid was added to an aqueous magnesium chloride solution containing magnesium ions at a concentration of 2.0 mol / L, followed by the addition of calcium hydroxide slurry to a molar ratio of MgCl / Ca(OH) of 1.1. The mixture was stirred and reacted at 50°C for 6.0 hours to obtain magnesium hydroxide slurry. The magnesium hydroxide slurry was filtered, washed with 25 times the amount of pure water relative to the solid matter, and dried to obtain magnesium hydroxide. The magnesium hydroxide was then calcined in a rotary kiln under the following conditions: a heating time of 1.5 hours, a calcination temperature of 900°C, a calcination time of 1.0 hour, and a cooling time of 0.5 hours. After calcination, the product was pulverized in an atmosphere at 25°C and a relative humidity of 50%, yielding the magnesium oxide powder of Example 1.

[0064] <Example 2> Magnesium oxide powder of Example 2 was obtained in the same manner as in Example 1, except that after pulverization, heat treatment was carried out at 180°C for 1 hour.

[0065] Example 3 The magnesium oxide powder of Example 3 was obtained in the same manner as in Example 1, except that the temperature rise time during firing was 0.5 hours, the temperature fall time was 1.0 hour, and the grinding was carried out in an atmosphere at a temperature of 25°C and a relative humidity of 55%.

[0066] Example 4 Boric acid and magnesium sulfate were added to an aqueous magnesium chloride solution containing magnesium ions at a concentration of 2.0 mol / L, followed by the addition of calcium hydroxide slurry at a molar ratio of MgCl / Ca(OH) of 1.1. The mixture was stirred at 50°C for 6.0 hours to produce a magnesium hydroxide slurry. The magnesium hydroxide slurry was filtered, washed with 50 times the amount of pure water relative to the solids, and dried to produce magnesium hydroxide. The magnesium hydroxide was then calcined in a rotary kiln under the following conditions: a heating time of 1.0 hour, a calcination temperature of 1,050°C, a calcination time of 1.0 hour, and a cooling time of 1.0 hour. After calcination, the magnesium oxide powder of Example 4 was obtained by pulverizing the calcined magnesium hydroxide at 25°C and a relative humidity of 50%.

[0067] <Example 5> The magnesium oxide powder of Example 5 was obtained in the same manner as in Example 4, except that the amounts of boric acid and magnesium sulfate added were changed, the firing temperature was set to 1000°C, and the pulverization was carried out in an atmosphere at a temperature of 25°C and a relative humidity of 60%.

[0068] <Comparative Example 1> Boric acid and magnesium sulfate were added to an aqueous magnesium chloride solution containing magnesium ions at a concentration of 2.0 mol / L, followed by the addition of calcium hydroxide slurry so that the molar ratio of MgCl2 / Ca(OH)2 was 1.1. The mixture was allowed to react at 50°C for 6.0 hours to obtain magnesium hydroxide slurry. This magnesium hydroxide slurry was filtered, washed with 50 times the amount of pure water relative to the solid matter, and dried to obtain magnesium hydroxide. This magnesium hydroxide was fired in a rotary kiln under the following conditions: a heating time of 2.0 hours, a firing temperature of 1100°C, a firing time of 1.0 hour, and a cooling time of 1.0 hour. After firing, the magnesium oxide powder of Comparative Example 1 was obtained by pulverizing the fired magnesium hydroxide in an atmosphere at 25°C and a relative humidity of 70% or higher.

[0069] The above measurements and evaluations were carried out on the obtained magnesium oxide powders of Examples 1 to 5 and Comparative Example 1. The results are shown in Table 1. The proportion of particles with a particle diameter of 1 μm or less in the volume-based particle size distribution was 20% or less in all cases.

[0070] [Table 1]

[0071] As is clear from Table 1, S H2O / S N2 All of the forsterite coatings formed using the magnesium oxides of Examples 1 to 5, in which S was in the range of 0.40 or more and 1.30 or less, were excellent in appearance and adhesion of the coating. H2O / S N2 The forsterite coating formed using the magnesium oxide of Comparative Example 1, which was outside the range of 0.40 to 1.30, was poor in both appearance and adhesion of the coating. H2O / S N2 It was suggested that by adjusting the ratio of SiO2 to SiO2 in the range of 0.40 or more and 1.30 or less, it is possible to obtain magnesium oxide for use in an annealing separator that can form a forsterite coating on the steel sheet surface that has excellent appearance and adhesion to the coating.

[0072] Looking at the viscosity of the magnesium oxide slurries, the magnesium oxides of Examples 1 to 5 all had sufficiently high initial viscosities when suspended in water to form slurries. The initial viscosity immediately after suspension can be said to be an index of dispersibility, suggesting that the magnesium oxides of Examples 1 to 5 had excellent dispersibility. Furthermore, it was thought that such initial viscosities of the slurries imparted appropriate application properties and adhesive properties to the slurries. On the other hand, the magnesium oxide of Comparative Example 1 had a low initial viscosity when suspended in water to form a slurry, suggesting poor dispersibility. Furthermore, it was thought that such initial viscosities of the slurries were undesirable because they reduced application properties and adhesive properties.

[0073] Furthermore, the magnesium oxides of Examples 1 to 5 showed little change in viscosity after being suspended in water to form slurries, indicating that these magnesium oxide slurries had excellent stability. Here, the change in viscosity that occurred after sufficient suspension in water is thought to be caused by the magnesium oxide hydrating and changing into magnesium hydroxide. Therefore, it was suggested that the magnesium oxide slurries of Examples 1 to 5 were less prone to hydration, which is advantageous for forming an excellent coating. On the other hand, the magnesium oxide of Comparative Example 1 showed a large change in viscosity after being formed into a slurry, indicating that the slurry had poor stability.

[0074] From the above, S H2O / S N2 It has been found that magnesium oxide having a value of 0.40 or more and 1.30 or less has excellent dispersibility when made into a slurry, and the magnesium oxide slurry is stable. Therefore, it has been found that the magnesium oxide for use as an annealing separator is advantageous for forming a forsterite coating on the surface of a steel sheet that has excellent appearance and adhesion to the coating. [Industrial Applicability]

[0075] According to the present invention, it is possible to provide magnesium oxide for an annealing separator that can form a forsterite coating on the surface of a steel sheet, the forsterite coating having excellent appearance and adhesion.

Claims

1. BET specific surface area S measured by water vapor adsorption method Ｈ２Ｏ and the BET specific surface area S measured by the nitrogen adsorption method Ｎ２ Ratio to S Ｈ２Ｏ / S Ｎ２ is 0.40 or more and 1.30 or less, a BET specific surface area S H2O measured by a water vapor adsorption method of 10.0 m 2 / g or more and 25.0 m 2 / g or less; Magnesium oxide for an annealing separator, wherein a ratio of particles having a particle diameter of 1 μm or less in a volume-based particle size distribution is 20% or less, The initial viscosity of the slurry is in the range of 10 to 40 cP, and the absolute value of the viscosity change after 4 hours from the slurry formation is 10 cP or less. Magnesium oxide for annealing separator.

2. The CAA 40% is 50 seconds or more and 170 seconds or less, the CaO content is 0.2 to 1.0 mass%, the Cl content is 500 ppm or less, the B content is 400 to 1500 ppm, and the SO ３ The content of the powder is 0.01 to 1.00 mass %, and the volume-based cumulative 50% particle diameter (D ５０ 2. The magnesium oxide for an annealing separator according to claim 1, wherein the average particle size is 0.5 μm or more and 7.0 μm or less.

3. An annealing separator comprising the magnesium oxide for use in an annealing separator according to claim 1 or 2.

4. SiO on the steel plate surface ２ forming a coating; The annealing separator according to claim 3 is ２ a process of forming a forsterite coating on the surface of the coating and annealing the steel sheet; A method for producing a grain-oriented electrical steel sheet, comprising:

Images