Magnesium oxide for annealing separator and grain-oriented electrical steel

Abstract

The purpose of this invention is to provide a magnesium oxide annealing separator for obtaining oriented electrical steel with excellent coating properties. This invention provides a magnesium oxide annealing separator with a CAA40% of 50 seconds to 170 seconds, and a ratio (T2 / T1) of 3.0 or higher between the time (T2) to reach a reaction rate of 20 mol% and the time (T1) to reach a reaction rate of 10 mol% in a constant pH method.

Description

Technical Field

[0001] This invention relates to magnesium oxide (MgO) for annealing separating agents and oriented electrical steel. Background Technology

[0002] Oriented electrical steel used in transformers or generators is typically produced by hot-rolling silicon steel containing approximately 3% silicon (Si), followed by cold rolling to the final sheet thickness, and then decarburization annealing and finishing annealing. In the decarburization annealing (single recrystallization annealing), a SiO2 coating is formed on the steel sheet surface. A slurry containing magnesium oxide (an annealing separating agent) is then applied to the surface, dried, rolled into a coil, and finished annealed. The SiO2 reacts with MgO to form a forsterite (Mg2SiO4) coating on the steel sheet surface. This forsterite coating imparts surface tension to the steel sheet, reduces iron loss, improves magnetic properties, and also provides insulation.

[0003] The magnesium olivine coating determines the product's appearance, electrical insulation properties, and consequently, its market value. The coating formation process influences the decomposition behavior of inhibitors on the steel plate surface, thus affecting secondary recrystallization. Consequently, the quality of the coating affects the product's magnetic properties. Furthermore, the coating's appearance determines the final appearance of the oriented electrical steel product. Therefore, the coating's appearance also significantly impacts product value and yield; uneven coating reduces the manufacturing yield. Thus, improving the properties of this coating plays a crucial role in the manufacturing technology of oriented electrical steel.

[0004] In the past, studies have been conducted on magnesium oxide as an annealing separator in order to improve the properties of oriented electrical steel, and various efforts have been made to improve its quality.

[0005] For example, the forsterite coating forms during the final annealing of the finished product, so adjusting the activity of the coating formation reaction at this time is quite important. Therefore, the activity of magnesium oxide used as an annealing separator was studied. Previously, citric acid activity (CAA) was commonly used as one of the indicators of magnesium oxide activity. CAA is expressed as the time taken for the citric acid solution to become neutral after adding the final reaction equivalent of magnesium oxide to a 0.4N aqueous solution of citric acid at a specified temperature (e.g., 303K) mixed with phenolphthalein as an indicator and stirring. Empirically, CAA can serve as an evaluation index for magnesium oxide used as an annealing separator for oriented electrical steel. On the other hand, CAA ultimately approximates the reactivity of MgO with the secondary oxide scale on the surface of oriented electrical steel; it is a single-point evaluation at a certain final reaction rate and does not reflect the breadth of the activity distribution. Therefore, it is well known that even two magnesium oxides used as annealing separators with CAA values ​​close to 40% may have different coating-forming abilities.

[0006] Therefore, a magnesium oxide annealing separator with a wide range of CAA activity distribution has been proposed. For example, Patent Documents 1 and 2 disclose technical solutions for magnesium oxide annealing separators in which the activity, particle size, or specific surface area of ​​CAA 40% and CAA 80% are respectively limited to specified values. In addition, Patent Document 3 discloses a technical solution for an annealing separator for oriented electrical steel in which the CAA 70% and the ratio of CAA 70% to CAA 40%, particle size, and specific surface area are respectively limited to specified values.

[0007] Existing technical documents

[0008] Patent documents

[0009] Patent Document 1: Japanese Patent Application Publication No. 06-033138

[0010] Patent Document 2: Japanese Patent Application Publication No. 11-158558

[0011] Patent Document 3: Japanese Patent Application Publication No. 11-269555 Summary of the Invention

[0012] The problem the invention aims to solve

[0013] However, the aforementioned magnesium oxide used as a conventional annealing separator cannot completely prevent poor coating of oriented electrical steel, nor can it reliably produce oriented electrical steel with excellent coating properties, thus lacking reliability. In other words, a magnesium oxide annealing separator with sufficient performance has not yet been discovered.

[0014] Therefore, the object of the present invention is to provide magnesium oxide for use as an annealing separator for obtaining oriented electrical steel with excellent coating properties. Specifically, the object of the present invention is to provide magnesium oxide for use as an annealing separator that can form a magnesium olivine coating on the surface of a steel sheet with excellent coating appearance and adhesion. Furthermore, the object of the present invention is to provide a method for manufacturing oriented electrical steel using the above-mentioned magnesium oxide for use as an annealing separator.

[0015] Solution for solving the problem

[0016] To solve the aforementioned technical problems, the inventors decided to conduct in-depth research on the so-called highly active fraction of magnesium oxide powder used as an annealing separator, which has not received much attention until now. In particular, they conducted a detailed study using the constant pH method, which can evaluate the highly active fraction of magnesium oxide more rigorously than CAA. The results showed that even among magnesium oxides with the same CAA 40% value, which is an indicator of the so-called medium-activity region, magnesium oxide with suppressed initial reactivity is more suitable for obtaining oriented electrical steel with excellent coating appearance and coating adhesion, thus completing the present invention.

[0017] That is, the main purpose of the present invention is to provide a magnesium oxide annealing separator with a CAA40% of 50 seconds or more and 170 seconds or less, and a ratio (T2 / T1) of 3.0 or more for the time (T2) to reach a reaction rate of 20 mol% to the time (T1) to reach a reaction rate of 10 mol%.

[0018] In addition, the present invention also includes magnesium oxide for annealing separation agent with a CAA40% of 50 seconds or more and 170 seconds or less, and a ratio (T2 / T1) of 3.0 or more and less than 11.0 of the time (T2) to reach a reaction rate of 20 mol% to reach a reaction rate of 10 mol% in the constant pH method.

[0019] Furthermore, the magnesium oxide used as the annealing separator in this invention preferably has a BET specific surface area of ​​12.0 m². 2 / g or more and 30.0m 2 / g or less, Cl content below 500ppm, and cumulative 50% particle size (D) on a volume basis. 50 The thickness is between 0.5 μm and 7.0 μm. This is achieved by adjusting the BET specific surface area, Cl content, and D... 50 Within the specified range, magnesium oxide used as an annealing separator for magnesium olivine coatings can be obtained more reliably, exhibiting a coating appearance and excellent adhesion on the surface of steel plates.

[0020] Furthermore, the magnesium oxide used as an annealing separator of the present invention preferably has a total content of Zn, Zr, Ni, Co, and Mn of 200 ppm or more and 10,000 ppm or less, more preferably a Zn content of 200 ppm or more and 10,000 ppm or less. Additionally, the Zn content is more preferably 200 ppm or more and 9,000 ppm or less. By containing the above elements within the specified range, the initial reactivity of magnesium oxide can be suppressed. Thus, magnesium oxide used as an annealing separator with a magnesium olivine coating that forms a film on the surface of a steel sheet and exhibits excellent adhesion can be obtained.

[0021] Furthermore, the main point of this invention is an annealing separator comprising magnesium oxide as described above for use in annealing separators. By using the annealing separator of this invention, it is possible to manufacture oriented electrical steel with excellent magnetic and insulating properties.

[0022] Furthermore, the main objective of this invention is a method for manufacturing oriented electrical steel, comprising: a step of forming a SiO2 coating on the surface of a steel sheet; and a step of applying the aforementioned annealing separating agent to the surface of the SiO2 coating and annealing thereon, thereby forming a magnesium olivine coating on the surface of the steel sheet. The manufacturing method of this invention enables the production of oriented electrical steel with excellent magnetic and insulating properties.

[0023] The effects of the invention

[0024] According to the present invention, magnesium oxide for use as an annealing release agent for obtaining oriented electrical steel with excellent magnetic and insulating properties can be provided. Specifically, according to the present invention, magnesium oxide for use as an annealing release agent can be provided, which forms a magnesium olivine coating on the surface of a steel sheet with excellent coating appearance and coating adhesion. Detailed Implementation

[0025] In the annealing separating agent of the present invention, the CAA40% of magnesium oxide is 50 seconds or more and 170 seconds or less, and the ratio (T2 / T1) of the time (T2) to reach a reaction rate of 20 mol% to reach a reaction rate of 10 mol% in the constant pH method is 3.0 or more. Furthermore, the ratio (T2 / T1) is preferably 3.0 or more and less than 11.0.

[0026] The initial reactivity of magnesium oxide used as an annealing separator in this invention was determined by the following constant pH method. In conventional CAA-based determinations, it is difficult to accurately measure regions of high activity. In this invention, the constant pH method allows for more rigorous determination and control of the initial reactivity of magnesium oxide.

[0027] The constant pH method is a method for maintaining the pH at a specified value during the reaction of an alkali (magnesium oxide in this invention) and an acid (citric acid in this invention), and recording / measuring the amount of acid or alkali solution added (consumed), the time elapsed, etc. It can be implemented using a commercially available conventional pH titration apparatus. The determination based on the constant pH method of this invention is performed as follows: First, 100 mL of deionized water is added to a 200 mL beaker and placed in a constant temperature bath at 285 K. A pH electrode is set up, and the mixture is stirred at 800 rpm using a magnetic stirrer. Simultaneously, 400 mg of weighed magnesium oxide powder is added, and 0.4 N·citric acid aqueous solution is immediately added dropwise. After the addition begins, the pH is maintained at 10, and the amount of citric acid aqueous solution consumed and the elapsed time are measured. The time (T1) to reach a reaction rate of 10 mol% and the time (T2) to reach a reaction rate of 20 mol% are calculated. Here, the time (T1) until the reaction rate reaches 10 mol% refers to the time required for the addition of an aqueous citric acid solution to react with magnesium oxide equivalent to 10 mol% of the magnesium oxide supplied for the determination, and the time (T2) until the reaction rate reaches 20 mol% refers to the time required for the addition of an aqueous citric acid solution to react with magnesium oxide equivalent to 20 mol% of the magnesium oxide supplied for the determination.

[0028] In the constant pH method for annealing separators using magnesium oxide of the present invention, the ratio (T2 / T1) of the time (T2) to reach a reaction rate of 20 mol% to the time (T1) to reach a reaction rate of 10 mol% is 3.0 or more. Preferably, it is 3.3 or more, more preferably 3.5 or more. Furthermore, the upper limit of the ratio (T2 / T1) is, for example, less than 11.0, preferably less than 10.5, more preferably less than 10.0. The range of the ratio (T2 / T1) is, for example, 3.0 or more and less than 11.0, preferably 3.3 or more and less than 10.5, more preferably 3.5 or more and less than 10.0.

[0029] A larger T2 / T1 ratio indicates a longer time from when the reaction rate reaches 10 mol% to when it reaches 20 mol%, which means that the initial reactivity of magnesium oxide is suppressed.

[0030] Magnesium oxide with a (T2 / T1) ratio of 3.0 or higher has its initial reactivity sufficiently suppressed, resulting in a forsterite coating with excellent appearance and adhesion on the steel plate surface. Magnesium oxide with a (T2 / T1) ratio less than 3.0 has its initial reactivity not sufficiently suppressed, preventing the formation of a forsterite coating with excellent adhesion on the steel plate surface. On the other hand, magnesium oxide with a (T2 / T1) ratio of 11.0 or higher has its initial reactivity excessively suppressed, preventing the formation of a forsterite coating with excellent adhesion on the steel plate surface.

[0031] In the magnesium oxide used as an annealing separator of the present invention, the (T2 / T1) ratio can be adjusted by various methods, such as adjusting the secondary particle size of the precursor magnesium hydroxide, adjusting the calcination conditions of the precursor magnesium hydroxide, adding metal elements, or mixing multiple magnesium oxide powders, but not limited to these methods. For example, the ratio can be adjusted by containing Zn, Zr, Ni, Co, and Mn as metal elements within a specified range in the magnesium oxide used as an annealing separator. Typically, this can be adjusted by increasing the value of (T2 / T1) by adding a certain amount of Zn, Zr, Ni, Co, and Mn. Preferably, by using Zn, the (T2 / T1) ratio can be stably adjusted; typically, this can be adjusted by increasing the value of (T2 / T1) by adding a certain amount of Zn.

[0032] Furthermore, the total content of Zn, Zr, Ni, Co, and Mn is preferably 200 ppm to 10000 ppm. Preferred contents include, for example, 250 ppm to 9500 ppm, more preferably 300 ppm to 9000 ppm. Additionally, 250 ppm to 8000 ppm is preferred, more preferably 300 ppm to 6000 ppm. If the total content is less than 200 ppm, the initial reactivity cannot be sufficiently reduced, and magnesium oxide with a (T2 / T1) value of 3.0 or higher cannot be obtained. If the total content exceeds 10000 ppm, the impact on the sintering properties, coating formation, and even coating characteristics of magnesium oxide becomes greater, becoming a cause of poor coating. The contents of Zn, Zr, Ni, Co, and Mn can be controlled by known methods, for example, by controlling the amount of trace elements described later. The aforementioned Zn, Zr, Ni, Co, and Mn can be added to the magnesium oxide precursor for annealing separation agents in the form of their oxides, hydroxides, chlorides, sulfides, carbonates, sulfates, etc. In particular, when adding Zn, zinc chloride and / or zinc oxide are preferred. It should be noted that unless otherwise specified, ppm in the specification refers to mass ppm.

[0033] Furthermore, the value of T1 is preferably 850 seconds or more, more preferably 870 seconds or more, and even more preferably 890 seconds or more. Additionally, it is preferably 5000 seconds or less, more preferably 4800 seconds or less, and even more preferably 4500 seconds or less. As a range, it is preferably 850 to 5000 seconds, more preferably 870 to 4800 seconds, and even more preferably 890 to 4500 seconds. If the value of T1 is less than 850 seconds, the initial reactivity is too high, and a forsterite coating with excellent appearance and adhesion cannot be formed on the surface of the steel plate. On the other hand, if the value of T1 is greater than 5000 seconds, the initial reactivity is too low, and a forsterite coating with excellent appearance and adhesion cannot be formed on the surface of the steel plate.

[0034] CAA (Carbon Adsorption) is a method for measuring the reactivity of magnesium oxide particles, including primary particles, by empirically simulating the solid-solid reaction between silica and magnesium oxide on the surface of actual electrical steel based on a solid-liquid phase reaction. As mentioned above, CAA40% is used as an indicator of the intermediate active region. If the CAA40% of magnesium oxide is greater than 170 seconds, the reactivity of the magnesium oxide particles is poor, the formation rate of the forsterite coating is slow, and therefore a sufficient coating cannot be formed, resulting in poor iron loss and magnetic flux density characteristics of the oriented electrical steel. On the other hand, if the CAA40% of magnesium oxide is less than 50 seconds, the reactivity of the magnesium oxide particles becomes too fast, and a uniform forsterite coating cannot be formed, resulting in poor appearance and adhesion of the coating on the oriented electrical steel. In other words, if CAA40% is less than 50 seconds, the hydration amount becomes too high, and if it exceeds 170 seconds, the reactivity is too low; in either case, good coating characteristics cannot be obtained. Therefore, in this invention, CAA40% is 50 seconds or more and 170 seconds or less, preferably in the range of 50 to 150 seconds, and more preferably in the range of 60 to 130 seconds.

[0035] The preferred BET specific surface area of ​​the magnesium oxide in this invention is 12.0 m². 2 / g or more and 30.0m 2 / g or less, more preferably 12.0m 2 / g or more and 23.0m 2 The BET specific surface area of ​​magnesium oxide is less than 12.0 m² / g. 2 / At g, the primary particle size of magnesium oxide becomes coarser, the reactivity of magnesium oxide particles decreases, the formation rate of forsterite coating declines, and due to the coarse particle size, residues are more likely to remain after acid removal. Magnesium oxide has a BET specific surface area greater than 30.0 m². 2 When the particle size of magnesium oxide decreases to / g, the reactivity of magnesium oxide particles becomes too fast, making it difficult to form a uniform magnesium olivine coating.

[0036] Cumulative 50% particle size on a volume basis (D) 50 Preferably, the particle size is 0.5 μm or larger and 7.0 μm or smaller. If the cumulative 50% particle size (D) on a volume basis... 50 If the particle size is less than 0.5 μm, the activity is high, and it is generally difficult to suppress the initial reactivity of the powder. If the cumulative 50% particle size (D) on a volume basis... 50 If the primary particle size exceeds 7.0 μm, the magnesium oxide particles become coarse, their reactivity decreases, and therefore the formation rate of the forsterite coating slows down, making it difficult to form a sufficient coating. 50 More preferably, the micrometer is 0.7 μm or more and 6.0 μm or less, and even more preferably, it is 1.0 μm or more and 5.0 μm or less.

[0037] In addition to the zinc (Zn), zirconium (Zr), nickel (Ni), cobalt (Co), and manganese (Mn) mentioned above, the magnesium oxide of the present invention may also contain trace amounts of calcium (Ca), silicon (Si), aluminum (Al), iron (Fe), phosphorus (P), boron (B), sulfur (S), fluorine (F), and chlorine (Cl).

[0038] When the magnesium oxide of the present invention contains calcium (Ca), the calcium content, converted to CaO, is preferably 0.2 to 2.0% by mass. When the magnesium oxide of the present invention contains silicon (Si), the silicon content is preferably 0.05 to 0.5% by mass. When the magnesium oxide of the present invention contains aluminum (Al), the aluminum content is preferably 0.01 to 0.5% by mass. When the magnesium oxide of the present invention contains iron (Fe), the iron content is preferably 0.01 to 0.5% by mass. When the magnesium oxide of the present invention contains phosphorus (P), the phosphorus content, converted to P₂O₃, is preferably 0.01 to 0.15% by mass. When the magnesium oxide of the present invention contains boron (B), the boron content is preferably 0.04 to 0.15% by mass. When the magnesium oxide of the present invention contains sulfur (S), the sulfur content, converted to SO₃, is preferably 0.01 to 1.5% by mass. When the magnesium oxide of the present invention contains fluorine (F), the fluorine content is preferably 0.05% by mass or less. When the magnesium oxide of the present invention contains chlorine (Cl), the chlorine content is preferably 500 ppm or less, more preferably 400 ppm or less, and even more preferably 300 ppm or less.

[0039] In this invention, magnesium oxide can be manufactured using known methods. For example, magnesium chloride is used as a raw material, and calcium hydroxide is added to its aqueous solution in a slurry state and reacted to form magnesium hydroxide. The magnesium hydroxide can then be filtered, washed with water, dried, and calcined in a furnace to form magnesium oxide, which is then pulverized to the desired particle size for further processing.

[0040] Alternatively, alkaline compounds with hydroxyl groups, such as sodium hydroxide and potassium hydroxide, can be used instead of calcium hydroxide. Magnesium oxide can also be produced via the Aman process, which involves introducing an aqueous solution containing magnesium chloride, such as seawater, irrigation water (wheat slurry), or brine, into a reactor and directly generating magnesium oxide and hydrochloric acid at 1773–2273 K. This magnesium oxide is then pulverized to the desired particle size.

[0041] Alternatively, magnesium oxide obtained from roasted magnesite can be hydrated, the resulting magnesium hydroxide can be roasted, and then pulverized to the desired particle size to produce magnesium oxide.

[0042] The amount of trace elements in MgO can be controlled using known methods. For example, to control the amount of trace elements in MgO, the amount can be controlled during the crude product manufacturing process or before final calcination, ensuring that the amount of trace elements in MgO remains within a specified range. Control during the crude product manufacturing process can be achieved, for example, by analyzing the amount of trace elements contained in the raw materials, and based on the results, by adding or removing the trace elements as a controlled substance using a specified amount, either wet or dry. Adding trace elements can be performed, for example, by mixing the added elements and drying them. Furthermore, removing trace elements can be performed, for example, by wet physical washing of excess substances or chemical separation. Chemical separation can be performed, for example, by forming soluble hydrates, dissolving, filtering, and washing to separate them; or by forming insoluble compounds, precipitating them, and separating them by adsorbing the precipitates. The amount of trace elements in the crude product before final calcination can be controlled, for example, by combining and mixing crude products with different compositions, adjusting the excess or deficiency of trace elements to bring the trace elements within a specified range, and then finally calcining them. Alternatively, to control the amount of trace elements, crude product MgO can be produced in any case, and after analyzing the obtained MgO, the above steps can be repeated based on the results related to the amount of trace elements. Another method is to control the amount of trace elements in MgO by mixing the target trace elements into the calcined MgO.

[0043] D of magnesium oxide 50 The BET specific surface area and CAA40% can be adjusted using known methods, for example, by adjusting the reaction temperature and alkali source concentration in the magnesium hydroxide manufacturing process, controlling the primary and secondary particle sizes of magnesium hydroxide, and thus adjusting the D of magnesium oxide. 50 , BET specific surface area and CAA 40%. Additionally, the D of magnesium oxide can be adjusted by controlling the calcination temperature and time of magnesium hydroxide with controlled particle size. 50 BET specific surface area and CAA 40%. Additionally, as D... 50 The adjustment methods for BET specific surface area and CAA 40% can also be achieved by measuring the D after pulverization. 50 The BET specific surface area and CAA 40% can be adjusted through multiple calcinations. Alternatively, the calcined magnesium oxide can be pulverized using various pulverizers such as jaw crushers, rotary crushers, cone crushers, impact crushers, roller crushers, choppers, pulverizers, ring mills, roller mills, spray mills, hammer mills, pin pulverizers, rotary mills, vibratory mills, planetary mills, and ball mills to adjust the D of the pulverized magnesium oxide. 50BET specific surface area and CAA 40%. Additionally, as D... 50 The adjustment methods for BET specific surface area and CAA 40% can also be achieved by measuring the D after pulverization. 50 The BET specific surface area and CAA 40% can be adjusted by multiple grinding processes. Alternatively, a pulverizer with a built-in classifier can be used to adjust the D of magnesium oxide. 50 BET specific surface area and CAA 40%. Alternatively, D can be adjusted by combining and mixing various magnesium oxide powders. 50 BET specific surface area and CAA 40%.

[0044] The oriented electrical steel of the present invention can be manufactured, for example, by the following method. The oriented electrical steel can be manufactured as follows: a silicon steel slab containing 2.5–4.5% Si is hot-rolled, pickled, and then cold-rolled or cold-rolled with two intermediate annealing processes to adjust to a specified thickness. Next, the cold-rolled coil is subjected to recrystallization annealing, which also serves as decarburization, at 923–1173 K, during which an oxide coating mainly composed of silicon dioxide (SiO2) is formed on the steel plate surface. An annealing separating agent containing magnesium oxide, the annealing separating agent of the present invention, is uniformly dispersed in water to obtain an aqueous slurry. This slurry is continuously coated onto the steel plate using roller coating or spraying and dried at approximately 573 K. The steel plate obtained in this manner is then subjected to a final annealing at 1473 K for 20 hours, for example, to form a magnesium olivine coating (Mg2SiO4) on the steel plate surface. Magnesia olivine coating is an insulating coating that can impart tension to the surface of steel plates, improving the iron loss value of oriented electrical steel.

[0045] Example

[0046] The present invention will be described in detail through the following embodiments, but these embodiments do not limit the present invention in any way.

[0047] <Determination Methods / Test Methods>

[0048] (1) Methods for determining the content of metallic elements

[0049] After the sample was completely dissolved in acid, it was diluted with ultrapure water, and the content of metal elements in the sample was determined using an ICP emission spectrometer (PS3520VDD, manufactured by Hitachi High-Tech Science Co., Ltd.).

[0050] (2) Method for determining the content of chlorine (Cl)

[0051] The sample was dissolved in nitric acid, diluted with ultrapure water, and its mass was measured using a spectrophotometer (UV-2550, manufactured by Shimadzu Corporation). The concentration of chlorine (Cl) in the sample was then calculated.

[0052] (3) Method for determining BET specific surface area

[0053] The BET specific surface area was determined using a specific surface area measuring device (trade name: Macsorb, manufactured by Mountech Co., Ltd.) via the gas adsorption method (BET method).

[0054] (4) Cumulative 50% particle size on a volume basis (D) 50 Determination method of )

[0055] The sample was dispersed in methanol, and the cumulative 50% particle size (D) of the sample on a volume basis was measured using a laser diffraction scattering particle size distribution analyzer (MT3300EX-II, manufactured by LEEDS & NORTHRUP). 50 At this point, use 40W ultrasonic waves to disperse the sound for 180 seconds.

[0056] (5) Determination method of CAA 40%

[0057] Add 100 mL of 0.4 N citric acid solution and 2 mL of 1% phenolphthalein solution as an indicator to a 200 mL beaker. Adjust the liquid temperature to 303 K and stir with a magnetic stirrer at 700 rpm. At the same time, add 40% of the final reaction equivalent of magnesium oxide (2.0 g) to the citric acid solution. Measure the time until the final reaction, that is, the time until the citric acid is consumed and the solution becomes neutral.

[0058] (6) Initial reactivity evaluation (pH-stat method)

[0059] Initial reactivity was evaluated using a commercially available pH titration apparatus (Automatic Titration Apparatus: AUT-701, manufactured by Toa DKK Corporation) via the constant pH method. Specifically, first, 100 mL of deionized water was added to a 200 mL beaker and placed in a thermostat maintained at 285 K. A pH electrode was set, and the mixture was stirred at 800 rpm using a magnetic stirrer. Simultaneously, 400 mg of the weighed sample powder was added, and the addition of 0.4 N citric acid aqueous solution was immediately initiated. After the addition began, the pH was maintained at 10, and the time (in seconds) required to consume the citric acid aqueous solution to achieve a reaction rate of 10 mol% and 20 mol% was measured. Under these conditions, the amount of 0.4 N citric acid aqueous solution required to achieve a reaction rate of 10 mol% relative to 400 mg (approximately 0.1 mol) of sample powder was 5 mL, and the amount required to achieve a reaction rate of 20 mol% was 10 mL.

[0060] (7) Fabrication of steel plates for testing

[0061] The test steel used was a steel plate made of silicon steel slab for oriented electrical steel, which was hot-rolled and cold-rolled to a final thickness of 0.28 mm using known methods, and then decarburized and annealed in a humid atmosphere of 25% nitrogen and 75% hydrogen. The composition of the steel plate before decarburization annealing, in mass percent, was C: 0.01%, Si: 3.29%, Mn: 0.09%, Al: 0.03%, S: 0.07%, N: 0.0053%, with the balance being unavoidable impurities and Fe. Magnesium oxide was coated onto this steel plate to study the coating characteristics of the forsterite coating.

[0062] Specifically, the magnesium oxide of the present invention or the magnesium oxide of the comparative example is prepared into a slurry, with a weight of 14 g / m³ after drying. 2 The coating is applied to the steel plate in a specific manner, and after drying, the final product is annealed at 1473K for 20.0 hours. After the final product annealing is completed, it is cooled, the steel plate is washed with water, pickled with hydrochloric acid solution, washed with water again, and dried to form a magnesium olivine coating on the steel plate.

[0063] (8) Evaluation of the appearance of the forsterite coating

[0064] The appearance of the forsterite coating is judged based on its appearance after cleaning. Specifically, a uniform and relatively thick gray forsterite coating is rated ◎, a uniform but relatively thin coating is rated ○, and an uneven and thin coating without any exposed parts of the base steel plate, or an uneven and very thin coating with obvious exposed parts of the base steel plate, is rated ×.

[0065] (9) Evaluation of the adhesion of the forsterite coating

[0066] The adhesion of the forsterite coating is judged based on its coating condition. That is, a coating that is uniformly formed and has no peeling areas, or a coating that is slightly uneven but has no peeling areas, is rated as ○; a coating that is uneven and has pinhole-like peeling areas, or a coating that is uneven and has definite peeling areas, is rated as ×.

[0067] <Example 1>

[0068] Calcium hydroxide slurry was added to a brine containing 2.0 mol / L magnesium ions to obtain a magnesium hydroxide concentration of 1.2 mol / L after the reaction, resulting in a mixed solution. The mixture was stirred at 600 rpm and reacted at 323 K for 7.0 hours. The solution was then filtered, washed with water, and dried to obtain magnesium hydroxide. Zinc chloride (manufactured by Kanto Chemical, premium grade) was mixed with this magnesium hydroxide to achieve a Zn content of 720 ppm in the calcined magnesium oxide. The mixture was then calcined in a rotary kiln at 1173 K for 0.5 hours and pulverized to obtain the magnesium oxide powder of Example 1. It should be noted that the calcination was carried out under conditions where the CAA40% of the magnesium oxide was in the range of 70–90 seconds.

[0069] <Example 2>

[0070] Magnesium oxide powder was obtained in the same manner as in Example 1, except that zinc chloride (premium grade) was mixed with calcined magnesium oxide in a manner that the Zn content was 2250 ppm.

[0071] <Example 3>

[0072] Magnesium oxide powder was obtained in the same manner as in Example 1, except that zinc chloride (premium grade) was mixed with calcined magnesium oxide in a manner that the Zn content was 4300 ppm.

[0073] <Comparative Example 1>

[0074] Magnesium oxide powder was obtained in the same manner as in Example 1, except that it was not mixed with zinc chloride (premium grade).

[0075] For the magnesium oxide powders obtained in Examples 1-3 and Comparative Example 1, the composition and other parameters were determined as described above, and the oriented electrical steels obtained using these magnesium oxide powders were evaluated. The results are shown in Table 1. It should be noted that for metallic elements not shown in the table, the content is at a typical impurity level.

[0076] [Table 1]

[0077]

[0078] As shown in Table 1, the forsterite coatings formed using magnesium oxide (Examples 1-3) with a (T2 / T1) value of 3.0 or higher determined by the constant pH method exhibit excellent (a) appearance and (b) adhesion. On the other hand, the forsterite coatings formed using magnesium oxide (Comparative Example 1) with a (T2 / T1) value less than 3.0 determined by the constant pH method exhibit poor (a) appearance and (b) adhesion.

[0079] <Example 4>

[0080] Calcium hydroxide slurry was added to a brine containing 2.0 mol / L magnesium ions to bring the magnesium hydroxide concentration after the reaction to 1.2 mol / L, resulting in a mixed solution. Zinc chloride (manufactured by Kanto Chemical, premium grade reagent) was then mixed into this solution to achieve a Zn concentration of 8800 ppm in the calcined magnesium oxide. The mixture was stirred at 600 rpm and reacted at 323 K for 7.0 hours. The mixture was then filtered, washed with water, and dried using a filter press to obtain magnesium hydroxide. This magnesium hydroxide was calcined in a rotary kiln at 1173 K for 0.5 hours and then pulverized to obtain the magnesium oxide powder of Example 4. It should be noted that the calcination was carried out under conditions where the CAA40% of magnesium oxide was in the range of 70–95 seconds.

[0081] <Example 5>

[0082] Except for replacing zinc chloride, zinc oxide (Wako Pure Chemicals, Special Grade Reagent) was mixed with calcined magnesium oxide in a manner that resulted in a Zn content of 5250 ppm, magnesium oxide powder was obtained in the same manner as in Example 4.

[0083] <Example 6>

[0084] Except for the absence of zinc chloride, magnesium oxide powder was obtained in the same manner as in Example 4, wherein zinc oxide (Wako Pure Chemicals, premium reagent) was mixed in such a manner that the Zn content was 5150 ppm, thereby obtaining the target magnesium oxide powder.

[0085] <Comparative Example 2>

[0086] Magnesium oxide powder was obtained in the same manner as in Example 4, except that zinc chloride (premium grade) was mixed with calcined magnesium oxide in a manner that the Zn content was 10300 ppm.

[0087] For the magnesium oxide powders obtained in Examples 4-6 and Comparative Example 2, the composition and other parameters were determined as described above, and the oriented electrical steels obtained using these magnesium oxide powders were evaluated. The results are shown in Table 2. It should be noted that for metallic elements not shown in the table, the content is at a typical impurity level.

[0088] [Table 2]

[0089]

[0090] As shown in Tables 1 and 2, the forsterite coatings formed using magnesium oxide (Examples 1-6) with a (T2 / T1) value of 3.0 or higher and less than 11.0 determined by the constant pH method exhibit excellent (a) appearance and (b) adhesion. On the other hand, the forsterite coatings formed using magnesium oxide with a (T2 / T1) value less than 3.0 (Comparative Example 1) and magnesium oxide with a (T2 / T1) value of 11.0 or higher (Comparative Example 2) exhibit poor (a) appearance and (b) adhesion.

[0091] In summary, the magnesium oxide used as an annealing separator according to the present invention can obviously produce oriented electrical steel with an excellent forsterite coating.

[0092] Industrial availability

[0093] According to the present invention, magnesium oxide for use as an annealing separator that can provide excellent coating properties for oriented electrical steel can be provided.

Claims

1. A magnesium oxide annealing separator, wherein the CAA40% is 50 seconds or more and 170 seconds or less, and the ratio of the time T2 to reach a reaction rate of 20 mol% to the time T1 to reach a reaction rate of 10 mol% in the constant pH method is 3.0 or more and less than 11.

0.

2. The magnesium oxide used as an annealing separator according to claim 1, has a BET specific surface area of ​​12.0 m². 2 / g or more and 30.0m 2 / g or less, Cl content is less than 500ppm, and the cumulative 50% particle size on a volume basis is D 50 It is between 0.5μm and 7.0μm.

3. The magnesium oxide for annealing separation agent according to claim 1 or 2, wherein, The total content of Zn, Zr, Ni, Co and Mn is above 200 ppm and below 10,000 ppm.

4. The magnesium oxide for annealing separation agent according to claim 1 or 2, wherein, The Zn content is above 200 ppm and below 10,000 ppm.

5. An annealing separator comprising magnesium oxide for annealing separators according to any one of claims 1 to 4.

6. A method for manufacturing oriented electrical steel, comprising the following steps: The process of forming a SiO2 coating on the surface of a steel plate; and, The process of applying the annealing separating agent as described in claim 5 to the surface of the SiO2 coating and annealing it to form a magnesium olivine coating on the surface of the steel plate.

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