JP7328777B2 - Magnesium oxide for annealing separator containing magnesium titanate, method for producing the same, annealing separator, and grain-oriented electrical steel sheet - Google Patents

Description

TECHNICAL FIELD The present invention relates to magnesium oxide for an annealing separator containing magnesium titanate, a method for producing the same, an annealing separator, and a grain-oriented electrical steel sheet.

Grain-oriented electrical steel sheets used in transformers and generators are generally manufactured by hot rolling silicon steel containing about 3% silicon (Si), then cold rolling to final thickness, and then decarburizing annealing. , finish annealed and manufactured. Here, in the decarburization annealing (primary recrystallization annealing), a silicon dioxide (SiO ₂ ) film is formed on the surface of the steel sheet. Then, on the surface, a slurry mainly composed of magnesium oxide (MgO) for an annealing separator and additives such as titanium oxide is added as an annealing separator is applied as an annealing separator, dried, wound into a coil shape, and then finish annealed. do. As a result, SiO ₂ and MgO on the surface of the steel sheet react with each other to form a forsterite (Mg ₂ SiO ₄ ) film on the surface of the steel sheet. This forsterite coating adds tension to the surface of the steel sheet, reduces core loss, improves magnetic properties, and also plays a role of imparting insulation to the steel sheet.

In order to improve the properties of grain-oriented electrical steel sheets, research has been conducted on titanium oxide to be added to the annealing separator.

Patent Document 1 attempts to control the particle size of titanium oxide as an additive in order to uniformize the glass coating of the grain-oriented electrical steel sheet.

Furthermore, studies have been made focusing on the activity due to the reaction rate between magnesium oxide particles and acid, that is, citric acid activity (CAA: Citric Acid Activity). CAA is prepared by mixing an indicator phenolphthalein in a 0.4 N citric acid aqueous solution at a predetermined temperature (e.g. 303K), adding a final reaction equivalent of magnesium oxide and stirring, until the citric acid aqueous solution becomes neutral. It is expressed as the time until It is empirically known that CAA can be an evaluation index for magnesium oxide used as an annealing separator for grain-oriented electrical steel sheets.

As an invention related to the distribution of CAA in the reaction equivalent of magnesium oxide, Patent Document 2 discloses that in each case of a final reaction rate of 20%, 40%, 60% and 70%, the activity is adjusted to control CAA within a narrow range. An invention of magnesium oxide for an annealing separator is disclosed. Further, Patent Documents 3 and 4 disclose inventions of magnesium oxide for annealing separators in which the activity, particle size or specific surface area of CAA40% and CAA80% are respectively limited to predetermined values. Furthermore, Patent Document 5 discloses an invention of an annealing separator for grain-oriented electrical steel sheets in which 70% CAA, the ratio of 70% CAA to 40% CAA, the particle size, the specific surface area, etc. are each limited to predetermined values. there is All of these inventions control hydration and reactivity of magnesium oxide particles.

JP-A-61-79781 JP-A-55-58331 JP-A-6-33138 JP-A-11-158558 JP-A-11-269555

The magnetic properties, insulating properties, and market value of grain-oriented electrical steel sheets are greatly influenced by the performance of the forsterite coating, specifically, whether defects are formed due to uneven coating and agglomerated particles. . In other words, the properties and value of the grain-oriented electrical steel sheet are highly dependent on the performance of the annealing separator magnesium oxide for forming the forsterite coating.

However, when titanium oxide is added as an additive to conventional magnesium oxide for annealing separators, defects and coating irregularities due to reaggregation of titanium oxide occur, and defects in the coating of grain-oriented electrical steel sheets are completely prevented. However, it has not been able to prevent it, and it lacks reliability because a certain effect cannot be obtained.

As described above, in Patent Literature 1, reaggregation of titanium oxide is suppressed by adding titanium oxide with good dispersibility to magnesium oxide for an annealing separator. However, even with the method described in this document, the uniformity of the titanium oxide is not sufficient, and it has not been possible to prevent defects due to uneven coating of the forsterite coating and agglomerated particles.

Thus, conventionally, many attempts have been made to improve the quality of the forsterite coating by controlling the dispersibility of titanium oxide as an additive added to magnesium oxide for annealing separators. However, in order to fully satisfy the requirements imposed on the quality of the forsterite coating (coating without coating unevenness and defects due to agglomerated particles), there is a demand for further improvement in the quality of magnesium oxide for annealing separators. .

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide magnesium oxide for annealing separators for obtaining grain-oriented electrical steel sheets having excellent magnetic properties and insulating properties. Specifically, it is an object of the present invention to provide a magnesium oxide for an annealing separating agent that can form a highly uniform forsterite coating free from coating irregularities and defects due to agglomerated particles.

As a result of intensive research, the present inventors added a titanium compound to a magnesium oxide precursor and fired it to convert a part of the magnesium oxide into magnesium titanate during firing, resulting in an oxide in which the titanium compound was uniformly dispersed. It is possible to obtain and use magnesium to form a highly uniform forsterite coating free from coating unevenness and defects due to agglomerated particles, and to obtain a grain-oriented electrical steel sheet with excellent magnetic properties and insulating properties. This finding led to the present invention.

The present invention is a magnesium oxide for an annealing separator containing 0.1 to 20.0% by mass of magnesium titanate. According to the magnesium oxide for annealing separator of the present invention, it is possible to form an excellent forsterite coating on the surface of a steel sheet, free from coating unevenness and defects due to agglomerated particles.

Moreover, this invention is an annealing separator containing the above-mentioned magnesium oxide for annealing separators. By using the annealing separator of the present invention, a grain-oriented electrical steel sheet having excellent magnetic properties and insulating properties can be produced.

Further, the present invention is a grain-oriented electrical steel sheet having a coating formed on the surface thereof using the above-described magnesium oxide for annealing separation agent. A grain-oriented electrical steel sheet obtained by forming a film using the magnesium oxide for annealing separator of the present invention is a grain-oriented electrical steel sheet having excellent magnetic properties and insulating properties.

Further, the present invention provides an annealing separator comprising a step of firing a magnesium oxide precursor mixed with a titanium compound so that the content of magnesium titanate in the obtained magnesium oxide is 0.1 to 20.0% by mass. It is a method for producing magnesium oxide for use. According to the production method of the present invention, it is possible to produce magnesium oxide for annealing separators, which can provide grain-oriented electrical steel sheets with excellent magnetic properties and insulating properties.

ADVANTAGE OF THE INVENTION According to this invention, the magnesium oxide for annealing separators for obtaining the grain-oriented electrical steel sheet excellent in a magnetic property and an insulation property can be provided. Specifically, according to the present invention, it is possible to provide magnesium oxide for an annealing separator that is capable of forming an excellent forsterite coating on the surface of a steel sheet without coating unevenness or defects due to agglomerated particles. .

The magnesium oxide for annealing separator of the present invention contains 0.1 to 20.0% by mass of magnesium titanate. That is, the magnesium oxide for an annealing separator of the present invention is characterized by containing a predetermined amount of a titanium compound as magnesium titanate. As a result, the titanium compound is considered to be contained in magnesium oxide in a state of being sufficiently uniformly dispersed, and a forsterite coating free from coating unevenness and defects due to agglomerated particles can be formed. Here, if the content of magnesium titanate is too small, the forsterite film is not sufficiently formed and unevenness of the film occurs, which is not preferable. The content of magnesium oxide for annealing separators of the present invention is 0.1 to 20.0% by mass, more preferably 1.0 to 15.0% by mass, and particularly preferably 5.0 to 12.0% by mass. %.

The magnesium oxide for annealing separator of the present invention preferably contains 0.04 to 0.15% by mass of boron and has a chlorine content of 0.10% by mass or less. When the content of boron and chlorine is within the predetermined range, it is possible to more reliably obtain the magnesium oxide for the annealing separator for obtaining the grain-oriented electrical steel sheet having excellent magnetic properties and insulating properties.

The magnesium oxide for annealing separator of the present invention preferably has a BET specific surface area of 12.0×10 ³ to 25.0×10 ³ m ² ·kg ⁻¹ . Here, the BET specific surface area is a specific surface area measured by a gas adsorption method (BET method).

In the measurement of the BET specific surface area by the BET method, since even fine pores in the aggregated particles can be measured, the specific surface area (BET specific surface area) including the surface area of the primary particles constituting the aggregated particles can be measured.

In the magnesium oxide of the present invention, when the BET specific surface area is less than 12.0×10 ³ m ² ·kg ⁻¹ , the primary particle size of magnesium oxide becomes coarse and the reactivity of the magnesium oxide particles becomes poor. An uneven stellite coating occurs. Also, since the particles are coarse, a residue remains when removed with an acid, resulting in defects due to agglomerated particles.

In the magnesium oxide of the present invention, when the BET specific surface area is larger than 25.0×10 ³ m ² ·kg ⁻¹ , the primary particle size of magnesium oxide becomes small, the reactivity of magnesium oxide particles becomes too fast, and uniform Forsterite film cannot be formed. Therefore, unevenness occurs in the forsterite coating of the grain-oriented electrical steel sheet.

The BET specific surface area of the magnesium oxide of the present invention is preferably 12×10 ³ to 23×10 ³ m ² ·kg ⁻¹ , more preferably 14×10 ³ to 20×10 ³ m ² ·kg ⁻¹ . be.

The magnesium oxide of the present invention preferably has a citric acid activity (CAA) of 50 to 200 seconds, more preferably 60 to 170 seconds. Here, the citric acid activity (CAA) is the time until the final reaction when 40% of the final reaction equivalent of magnesium oxide is added to a 0.4 N citric acid aqueous solution at a temperature of 303 K and stirred. It means the time until the citric acid is consumed and the solution becomes neutral.

CAA empirically simulates the reactivity of the solid-solid reaction between silicon dioxide (SiO ₂ ) and magnesium oxide (MgO) that occurs on the surface of an actual electrical steel sheet through a solid-liquid reaction. It is possible to measure the reactivity of magnesium oxide particles, including primary particles.

If the CAA of magnesium oxide is greater than 200 seconds, the primary particle size of magnesium oxide becomes coarse and the reactivity of the magnesium oxide particles becomes poor, resulting in a decrease in the forsterite film formation rate. Also, since the particles are coarse, a residue remains when removed with an acid, resulting in defects due to agglomerated particles.

If the CAA of magnesium oxide is less than 50 seconds, the primary particle size of magnesium oxide becomes small and the reactivity of magnesium oxide particles becomes too fast. As a result, a uniform forsterite coating cannot be formed, and unevenness of the forsterite coating occurs.

If the magnesium oxide for annealing separator of the present invention is used, a grain-oriented electrical steel sheet having excellent insulating properties and magnetic properties can be produced.

The magnesium oxide for annealing separator of the present invention is a step of firing a magnesium oxide precursor mixed with a titanium compound so that the content of magnesium titanate in the obtained magnesium oxide is 0.1 to 20.0% by mass. It can be produced by a production method (manufacturing method of magnesium oxide for annealing separator of the present invention) containing. In this manner, part of the magnesium oxide is changed to magnesium titanate during firing, and magnesium oxide in which the titanium compound is sufficiently uniformly dispersed can be obtained.

In the present invention, the magnesium oxide precursor is not particularly limited as long as it can be calcined to obtain magnesium oxide. Examples thereof include magnesium hydroxide, magnesium carbonate, magnesium nitrate, and the like, preferably magnesium hydroxide. be. Specifically, for example, synthetic magnesium chloride is used as a raw material, and calcium hydroxide is added in a slurry state to this aqueous solution and reacted to form magnesium hydroxide as a magnesium oxide precursor. At this time, instead of calcium hydroxide, an alkaline compound having a hydroxyl group such as sodium hydroxide or potassium hydroxide can also be used. Furthermore, magnesium hydroxide as a magnesium oxide precursor can also be obtained by hydrating magnesium oxide obtained by firing the mineral magnesite. In addition, magnesium chloride-containing aqueous solutions such as seawater, sprinkling water, bittern, etc., or magnesium hydroxide, magnesium carbonate, magnesium nitrate, etc. obtained therefrom can also be used as magnesium oxide precursors.

In the present invention, the titanium compound is not particularly limited as long as it can be mixed with a magnesium oxide precursor and fired to form magnesium titanate. Examples include titanium oxide, titanium chloride, and titanium hydroxide. , titanium nitride, titanium bromide, titanium fluoride, magnesium titanate, etc., preferably titanium oxide. Moreover, in the present invention, the titanium compound is preferably a titanium compound having an average particle size of 0.1 to 10.0 μm. By using a titanium compound having such an average particle size, it is possible to obtain magnesium oxide in which the titanium compound is more uniformly dispersed. From this point of view, the average particle size of the titanium compound is more preferably 0.2 to 5.0 μm, particularly preferably 0.3 to 4.0 μm.

In the present invention, the amount of the titanium compound to be mixed with the magnesium oxide precursor is such that the content of magnesium titanate in the obtained magnesium oxide is 0.1 to 20.0% by mass, preferably 1 0 to 15.0% by mass, and 5.0 to 12.0% by mass.

Desired magnesium oxide can be obtained by mixing the above-described magnesium oxide precursor and titanium compound and firing the mixture. Specifically, when the magnesium oxide precursor is magnesium hydroxide, for example, it is mixed with a titanium compound as a slurry, filtered, washed with water, dried, and then fired in a heating furnace. Also, when the magnesium oxide precursor is a magnesium chloride-containing aqueous solution such as seawater, sprinkling water, bittern, etc., for example, these can be introduced into the reactor together with the titanium compound to directly produce magnesium oxide and HCl at 1773-2273K. (Aman process). Magnesium oxide obtained by firing is optionally pulverized to a desired particle size, and for example, the BET specific surface area, CAA, etc. are also adjusted as described later.

In the present invention, the firing conditions are not particularly limited, and general firing conditions for obtaining magnesium oxide for an annealing separator by firing a magnesium oxide precursor are applied. The firing temperature is, for example, preferably 1073-1473K, more preferably 1123-1373K. The firing time is preferably 0.1 to 10.0 hours, more preferably 0.3 to 3.0 hours. Here, it is considered that all of the titanium compound mixed with the magnesium oxide precursor becomes magnesium titanate under the firing conditions for obtaining the magnesium oxide for the annealing separating agent as described above.

In the present invention, the BET specific surface area and CAA of magnesium oxide for annealing separators can be adjusted, for example, by the following methods. That is, by adjusting the reaction temperature and the concentration of the alkali source during the production process of magnesium hydroxide, the primary particle size and secondary particle size of magnesium hydroxide are controlled, and the BET specific surface area and CAA of magnesium oxide are adjusted. be able to. The BET specific surface area and CAA of magnesium oxide can also be adjusted by controlling the firing temperature and time of magnesium hydroxide whose particle size is controlled. Moreover, as a method for adjusting the BET specific surface area and CAA, the BET specific surface area and CAA after pulverization can be measured, and the particles can be calcined a plurality of times. Furthermore, calcined magnesium oxide is processed into jaw crushers, gyratory crushers, cone crushers, impact crushers, roll crushers, cutter mills, stamp mills, ring mills, roller mills, jet mills, super micron mills, cyclone mills, hammer mills, and pin mills. The BET specific surface area and CAA of magnesium oxide can also be adjusted by grinding using a grinder such as a rotary mill, vibration mill, planetary mill, or ball mill. Moreover, as a method for adjusting the BET specific surface area and CAA, the BET specific surface area and CAA after pulverization can be measured and pulverized multiple times.

In the present invention, the amount of trace elements such as boron and chlorine contained in the magnesium oxide for annealing separator can be controlled by a known method. As a method for controlling the amount of trace contents in magnesium oxide, for example, during the production process of the crude product or the obtained crude product so that the amount of trace contents in magnesium oxide is within a predetermined range This can be done by controlling the trace content of the product prior to final firing. The control in the manufacturing process of the crude product is, for example, analyzing the amount of trace inclusions contained in the raw material, and based on the results, wet or dry so that the trace content to be controlled is a predetermined amount. It can be controlled by adding at low temperature or removing by wet process. Addition of a trace content can be carried out by, for example, mixing the elements to be added and drying. Further, the removal of trace inclusions can be carried out by, for example, wet-type physical washing of excess inclusions or chemical separation. Chemical separation is, for example, by forming a soluble hydrate, dissolving, filtering, washing and separating, or forming an insoluble compound, precipitating, and adsorbing and separating the precipitate. It can be done by Control of the amount of trace elements in the crude product prior to the final calcination can be achieved, for example, by combining and mixing crude products having different compositions, thereby increasing the amount of trace elements so that the trace elements are within a predetermined range. The deficit can be adjusted and controlled by the final firing. In addition, in order to control the amount of trace elements, in each case after producing a crude magnesium oxide and analyzing the resulting magnesium oxide, depending on the individual results regarding the amount of trace elements, The above steps can be repeated and combined.

The grain-oriented electrical steel sheet of the present invention is a grain-oriented electrical steel sheet having a coating formed on the surface using the magnesium oxide for annealing separator of the present invention, and can be produced, for example, by the following method. Steel sheets for grain-oriented electrical steel sheets are produced by hot-rolling a silicon steel slab containing 2.5 to 4.5% Si, pickling it, and then cold-rolling it. It can be manufactured by rolling to adjust the plate thickness to a predetermined value. Next, a coil obtained by cold rolling this steel sheet is subjected to recrystallization annealing in a wet hydrogen atmosphere at 923 to 1173K, which also serves as decarburization. At this time, an oxide film containing silica (SiO ₂ ) as a main component is formed on the surface of the steel sheet. Next, an annealing separator containing magnesium oxide for an 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 plate having an oxide film formed on its surface by roll coating or spraying, and dried at about 573K. The steel sheet thus treated is subjected to final finish annealing at 1473 K for 20.0 hours, for example, to form a forsterite coating (Mg ₂ SiO ₄ coating) on the surface of the steel sheet. The forsterite coating is an insulating coating and can improve the iron loss value of the grain-oriented electrical steel sheet by applying tension to the surface of the steel sheet.

The present invention will be explained in detail by the following examples, which are not intended to limit the invention in any way.

(1) Method for measuring content of boron (B) After completely dissolving the measurement sample, the content of boron (B) was measured using an ICP measuring device (SPS1700HVR, manufactured by Seiko Instruments Inc.).

(2) Method for measuring the content of chlorine (Cl) A measurement sample is pressure-molded at a total pressure of 215 kN using an aluminum ring of 35 mmφ, and a fluorescence X-ray analyzer (Simultix 12 type, manufactured by Rigaku Co., Ltd.). , the content of chlorine (Cl) in the sample was measured.

(3) Method for measuring CAA Put 1×10 ⁻⁴ m ³ of 0.4 N citric acid solution and an appropriate amount of 2×10 ⁻⁴ m ³ of phenolphthalein solution as an indicator into a beaker, adjust the liquid temperature to 303 K, and use a magnet. While stirring at 700 rpm using a stirrer, 40% final reaction equivalent of magnesium oxide was dosed into the citric acid solution and the time to final reaction, i.e., until the citric acid was consumed and the solution was neutral. time was measured.
(4) Measurement method of BET specific surface area The BET specific surface area was measured by a gas adsorption method (BET method) using nitrogen gas using a specific surface area measuring device (Macsorb, manufactured by Mounttech Co., Ltd.).

(5) Evaluation method for coating unevenness of forsterite coating and defects due to agglomerated particles Test sample As a test steel, a silicon steel slab for grain-oriented electrical steel sheets was hot-rolled and cold-rolled by a known method. A steel sheet having a final thickness of 0.35×10 ⁻³ m and further decarburized and annealed in a wet atmosphere of 25% nitrogen and 75% hydrogen was used. The composition of the steel sheet before decarburization annealing is, in mass %, C: 0.01%, Si: 3.29%, Mn: 0.09%, Al: 0.03%, S: 0.07%, N : 0.0053%, the balance being unavoidable impurities and Fe. After coating the magnesium oxide to be evaluated on this steel sheet, it was annealed to form a forsterite coating on the steel sheet, and the properties of this coating were evaluated. Specifically, the magnesium oxide of the present invention or the magnesium oxide of the comparative example is made into a slurry, applied to a steel plate so that the weight after drying is 14 g m ⁻² , and after drying, at 1473 K for 20.0 hours. was subjected to final finish annealing. After the final annealing was completed, the steel sheet was cooled, washed with water, acid washed with an aqueous solution of hydrochloric acid, washed with water again, and dried. Coating unevenness and defects due to agglomerated particles were judged from the appearance of the coating after washing. That is, the coating unevenness of the gray forsterite coating is ◎ when it is formed thick and there is no unevenness, ○ when the coating is formed slightly thin and there is little unevenness, △ when the coating is thin and has a little unevenness, and the coating is thick. A very thin sample with a large amount of unevenness was evaluated as x. Defects caused by agglomerated particles: ◎ indicates that agglomerated particles are completely removed and there are no defects; O indicates that some agglomerated particles remain and some defects remain; was rated as Δ, and those in which aggregates clearly remained and many defects remained were rated as x.

<Example 1>
Magnesium chloride (reagent special grade) was dissolved in pure water to prepare a magnesium chloride aqueous solution of 0.5×10 ³ mol·m ⁻³ . Next, calcium hydroxide (reagent special grade) was added to pure water to prepare a calcium hydroxide dispersion of 0.5×10 ³ mol·m ⁻³ . These magnesium chloride aqueous solution and calcium hydroxide dispersion were mixed at a molar ratio of MgCl ₂ /Ca(OH) ₂ =1.1 so as to obtain a mixture of 1×10 ⁻³ m ³ . After that, an aqueous solution of boric acid adjusted to 0.3×10 ³ mol·m ⁻³ with pure water is added to the mixed liquid so that the B content in the finally obtained magnesium oxide is about 550 ppm. Using a single-spring stirring blade, the mixture was stirred at 300 rpm and reacted at 363 K for 6.0 hours to obtain a magnesium hydroxide slurry. After that, titanium oxide having an average particle size of 1.5 μm was dispersed in pure water, and titanium in the finally obtained magnesium oxide was added so as to be 5.0% by mass in terms of titanium oxide, followed by magnesium hydroxide slurry. was filtered, washed with pure water of 100 times the weight of the obtained magnesium hydroxide, and dried at 378 K for 12.0 hours to obtain magnesium hydroxide powder. The obtained magnesium hydroxide powder was calcined at 1173K for 1.0 hour using an electric furnace. Thus, a magnesium oxide powder containing the added titanium oxide as magnesium titanate was obtained. At this time, the content of magnesium titanate in the obtained magnesium oxide powder was 7.28% by mass.

<Example 2>
Magnesium chloride (reagent special grade) was dissolved in pure water to prepare a magnesium chloride aqueous solution of 0.5×10 ³ mol·m ⁻³ . Next, calcium hydroxide (reagent special grade) was added to pure water to prepare a calcium hydroxide dispersion of 0.5×10 ³ mol·m ⁻³ . These magnesium chloride aqueous solution and calcium hydroxide dispersion were mixed at a molar ratio of MgCl ₂ /Ca(OH) ₂ =1.1 so as to obtain a mixture of 1×10 ⁻³ m ³ . After that, an aqueous solution of boric acid adjusted to 0.3×10 ³ mol·m ⁻³ with pure water is added to the mixed liquid so that the B content in the finally obtained magnesium oxide is about 550 ppm. A stirring blade of a leaf spring was used to react at 303 K for 6.0 hours while stirring at 300 rpm to obtain a magnesium hydroxide slurry. After that, titanium oxide having an average particle size of 1.5 μm is dispersed in pure water, and titanium in the finally obtained magnesium oxide is added so as to be 6.0% by mass in terms of titanium oxide. The slurry was filtered, washed with 100 times as much pure water as the weight of magnesium hydroxide obtained, and dried at 378 K for 12.0 hours to obtain magnesium hydroxide powder. The obtained magnesium hydroxide powder was calcined at 1223K for 0.5 hours using an electric furnace. Thus, a magnesium oxide powder containing the added titanium oxide as magnesium titanate was obtained. At this time, the content of magnesium titanate in the obtained magnesium oxide powder was 9.04% by mass.

<Comparative Example 1>
Magnesium chloride (reagent special grade) was dissolved in pure water to prepare a magnesium chloride aqueous solution of 0.5×10 ³ mol·m ⁻³ . Next, calcium hydroxide (reagent special grade) was added to pure water to prepare a calcium hydroxide dispersion of 0.5×10 ³ mol·m ⁻³ . These magnesium chloride aqueous solution and calcium hydroxide dispersion were mixed at a molar ratio of MgCl ₂ /Ca(OH) ₂ =1.1 so as to obtain a mixture of 1×10 ⁻³ m ³ . After that, an aqueous solution of boric acid adjusted to 0.3×10 ³ mol·m ⁻³ with pure water is added to the mixed liquid so that the B content in the finally obtained magnesium oxide is about 550 ppm. While stirring at 600 rpm using a leaf-spring stirring blade, reaction was carried out at 363 K for 5.5 hours to obtain a magnesium hydroxide slurry. After that, the magnesium hydroxide slurry was filtered, washed with pure water of 100 times the weight of the obtained magnesium hydroxide, and dried at 378 K for 12.0 hours to obtain magnesium hydroxide powder. The obtained magnesium hydroxide powder was calcined at 1173K for 1.0 hour using an electric furnace. Thus, magnesium oxide was obtained. After that, titanium oxide having an average particle size of 1.5 μm was added and mixed so that the finally obtained magnesium oxide contained 5.0% by mass of titanium in terms of titanium oxide. Thus, a magnesium oxide powder containing the added titanium oxide as it was was obtained.

<Results>
A forsterite coating was formed on the steel sheet surface using the obtained magnesium oxide powder according to the evaluation method described above, and defects due to forsterite coating unevenness and agglomerated particles were evaluated. Table 1 shows the composition, CAA and BET specific surface areas, and evaluation results for each magnesium oxide powder.

As shown in Table 1, the forsterite coating formed using magnesium oxide containing a predetermined amount of titanium compound as magnesium titanate (Examples 1 and 2) had no defects due to coating unevenness and agglomerated particles. It was an excellent forsterite coating with a small amount. Therefore, it has been clarified that by using the magnesium oxide of the present invention, a grain-oriented electrical steel sheet having excellent magnetic properties and insulating properties can be produced. On the other hand, the forsterite coating formed by using magnesium oxide (Comparative Example 1) containing titanium oxide as it is instead of magnesium titanate as a titanium compound has a little unevenness in the coating and many defects due to agglomerated particles. Therefore, it was clear that the desired grain-oriented electrical steel sheet could not be obtained.

ADVANTAGE OF THE INVENTION According to this invention, the magnesium oxide for annealing separators for obtaining the grain-oriented electrical steel sheet excellent in a magnetic property and an insulation property can be provided.

Claims (6)

It contains 5.0 to 20.0% by mass of magnesium titanate, has a BET specific surface area of 12.0×10 ³ to 25.0×10 ³ m ² ·kg ⁻¹ , and has a citric acid activity of 50 to 200. Magnesium oxide for an annealing separator containing 0.04 to 0.15% by mass of boron and having a chlorine content of 0.10% by mass or less . An annealing separator containing magnesium oxide for an annealing separator according to claim 1 . A grain-oriented electrical steel sheet having a coating formed on the surface thereof using the magnesium oxide for annealing separator according to claim 1 . A method for producing magnesium oxide for an annealing separator, comprising:
A production method comprising a step of firing a magnesium oxide precursor mixed with a titanium compound such that the content of magnesium titanate in the obtained magnesium oxide is 0.1 to 20.0% by mass. 5. The production method according to claim 4 , wherein the titanium compound is titanium oxide. The production method according to claim 4 or 5 , wherein the titanium compound has an average particle size of 0.1 to 10.0 µm.