Classifications

• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C22/00—Chemical surface treatment of metallic material by reaction of the surface with a reactive liquid, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals

Definitions

• the present invention relates to MgO powder, a method for producing MgO powder, a method for producing MgO particles, and a method for producing grain-oriented electrical steel sheet.
• Grain-oriented electrical steel sheets are soft magnetic materials that are primarily used as iron core materials in transformers. Therefore, grain-oriented electrical steel sheets are required to have magnetic properties, including high magnetization and low iron loss. Iron loss is the power loss consumed as thermal energy when an iron core is excited by an AC magnetic field, and from the perspective of energy conservation, iron loss should be as low as possible.
• a production method is generally applied to a steel slab adjusted to a predetermined chemical composition, the production method including the steps of hot rolling, hot-rolled sheet annealing, cold rolling, decarburization annealing, and finish annealing.
• the coiled steel sheet is annealed at high temperature for a long time to concentrate the crystal orientation into the GOSS orientation, which is good for magnetic properties (to increase the degree of orientation concentration).
• an annealing separator is applied to prevent the coil from seizing.
• an annealing separator containing magnesium oxide (MgO) as its main component is often used.
• MgO magnesium oxide
• silicon dioxide (SiO 2 ) on the surface of the steel sheet reacts with MgO during finish annealing, forming a forsterite (Mg 2 SiO 4 )-based coating (primary coating) on the surface of the steel sheet, which applies tension to the steel sheet surface and also serves to impart insulation properties to the steel sheet.
• SiO 2 silicon dioxide
• SiO 4 forsterite
• using an annealing separator containing MgO as its main component not only prevents seizure during finish annealing, but also improves the magnetic properties of the grain-oriented electrical steel sheet.
• Patent Document 1 discloses a powder for an annealing separator containing 0.04 mass % or more and 0.30 mass % or less of boron and containing magnesium oxide as a main component, in which the proportion of tricoordinate boron in the boron is 80% or more and 95% or less.
• Patent Document 2 discloses a method for producing a powder for an annealing separator, which comprises calcining a raw material containing either or both of magnesium hydroxide and magnesium carbonate and boron, and then adjusting the ratio of tricoordinated boron by controlling the humidity of the calcined product, in which the ratio of tricoordinated boron in the boron contained in the powder for an annealing separator is 70% or more and 95% or less.
• Patent Documents 1 and 2 controlling the amount of boron and the ratio of tricoordinated boron in powders for annealing separators can solve problems such as poor coating appearance caused by insufficient reactivity at high temperatures and insufficient purification of impurities from steel.
• these technologies sometimes fail to improve the secondary recrystallized structure by utilizing the effect of tricoordinated boron in suppressing precipitate decomposition, while also sufficiently increasing the coating tension, which affects the improvement of magnetic properties. Therefore, the technologies described in Patent Documents 1 and 2 cannot be said to be sufficient in improving magnetic properties.
• An object of the present invention is to provide an MgO powder for use in an annealing separator applied to steel sheet before finish annealing, which MgO powder provides excellent magnetic properties, appearance, and coating tension in grain-oriented electrical steel sheet after finish annealing.
• Another object of the present invention is to provide a method for producing grain-oriented electrical steel sheet using this MgO powder for annealing separator, a method for producing this MgO powder, and a method for producing MgO particles contained in this MgO powder.
• the present inventors have investigated the improvement of the coating properties of grain-oriented electrical steel sheets by controlling the annealing separator. As a result, they have found that the coating properties and magnetic properties can be improved by adding boron (B) to the MgO powder contained in the annealing separator, but that if the amount of boron contained exceeds a certain level, the amount of boron that penetrates into the steel sheet increases, which actually inhibits secondary recrystallization and purification. Therefore, in order to improve the coating properties and magnetic properties, the present inventors have investigated a method for obtaining sufficient coating properties and magnetic properties due to boron even when the content of B is reduced.
• B boron
• the present inventors have found that it is effective to ensure that a certain amount of MgO particles having small particle sizes are included in the MgO particles that make up the MgO powder. They also found that the properties can be further improved by controlling the proportion of tricoordinated boron among the boron (B) contained in the MgO powder and/or by making the MgO powder contain one or more of Cl, Ca, Sr, and Ba.
• An MgO powder according to one aspect of the present invention is an MgO powder for an annealing separator, the MgO powder containing 0.030% by mass or more and less than 0.250% by mass of B, and of all particles constituting the MgO powder, MgO particles having an average particle size of less than 0.8 ⁇ m obtained by classifying the MgO powder with a classifier at a classification point D50 are defined as small diameter particles and MgO particles having an average particle size of 0.8 ⁇ m or more are defined as large diameter particles.
• the B content Bs contained in the small diameter particles is 0.0020% by mass or more and less than 0.0800% by mass
• the B content Bl contained in the large diameter particles is 0.0400% by mass or more and less than 0.3000% by mass
• the proportion of the small diameter particles to all particles is 5 to 80% by mass
• the ratio Bl/Bs of Bl to Bs is 1.05 to 30.0.
• the proportion of tricoordinated boron in the B contained in the small diameter particles may be 40% by mass or more and 70% by mass or less
• the proportion of tricoordinated boron in the B contained in the large diameter particles may be 80% by mass or more and 95% by mass or less.
• the ratio of the amount of tricoordinated boron contained in the large diameter particles to the amount of tricoordinated boron contained in the small diameter particles may be 2.00 or more and 20.00 or less.
• the ratio of the amount of tricoordinated boron contained in the large diameter particles to the amount of tricoordinated boron contained in the small diameter particles may be 2.00 or more and 20.00 or less.
• the MgO powder according to any one of [1] to [4] may further contain Cl in an amount of 0.005 mass % or more and 0.080 mass % or less.
• the MgO powder according to any one of [1] to [4] may further contain one or more elements selected from the group consisting of Ca, Sr, and Ba in a total amount of 0.02 mass% to 4.00 mass%.
• the MgO powder according to [5] may further contain one or more elements selected from the group consisting of Ca, Sr, and Ba in a total amount of 0.02 mass% or more and 4.00 mass% or less.
• Another aspect of the present invention relates to a method for producing MgO powder, which is the method for producing MgO powder according to [1], and which comprises mixing MgO particles having a B content of 0.0020% by mass or more and 0.0800% by mass or less and an average particle size of less than 0.8 ⁇ m with MgO particles having a B content of 0.0400% by mass or more and 0.3000% by mass or less and an average particle size of 0.8 ⁇ m or more, so that the ratio of the B content of the MgO particles having an average particle size of 0.8 ⁇ m or more to the B content of the MgO particles having an average particle size of less than 0.8 ⁇ m is 1.05 or more, and the mass ratio of the MgO particles having an average particle size of less than 0.8 ⁇ m to the MgO powder after mixing is 0.05 or more and 0.80 or less.
• Another embodiment of the present invention provides a method for producing MgO particles having an average particle size of less than 0.8 ⁇ m, as described in [8], in which a raw material powder made of one or more materials selected from magnesium hydroxide, basic magnesium carbonate, and magnesium carbonate, in which the mass ratio of B to the MgO equivalent of the raw material powder is 0.0020 or more and 0.0800 or less, and the raw material powder has an average particle size of less than 0.8 ⁇ m, is heated and fired in air or nitrogen.
• Another aspect of the present invention relates to a method for producing MgO particles, which is a method for producing the MgO particles having an average particle size of less than 0.8 ⁇ m as described in [8], and which comprises adding boron or a boron compound to a raw material powder consisting of one or more types selected from magnesium hydroxide, basic magnesium carbonate, and magnesium carbonate and having an average particle size of less than 0.8 ⁇ m so that the mass ratio of B to the MgO equivalent of the raw material powder is 0.0020 or more and 0.0800 or less, and then heating and firing the powder in air or nitrogen.
• the firing temperature may be 700 to 1100°C.
• a method for producing a grain-oriented electrical steel sheet includes a hot rolling step of hot-rolling a steel slab to obtain a hot-rolled sheet, a hot-rolled sheet annealing step of annealing the hot-rolled sheet, a cold rolling step of cold-rolling the hot-rolled sheet after the hot-rolled sheet annealing step to obtain a cold-rolled sheet, a decarburization annealing step of decarburization annealing the cold-rolled sheet, and a finish annealing step of applying an annealing separator to the cold-rolled sheet after the decarburization annealing step, drying the cold-rolled sheet, and then finish annealing the cold-rolled sheet.
• a method for producing a grain-oriented electrical steel sheet according to another aspect of the present invention includes a hot rolling step of hot-rolling a steel slab to obtain a hot-rolled sheet, a hot-rolled sheet annealing step of annealing the hot-rolled sheet, a cold rolling step of cold-rolling the hot-rolled sheet after the hot-rolled sheet annealing step to obtain a cold-rolled sheet, a decarburization annealing step of decarburization annealing the cold-rolled sheet, and a finish annealing step of applying an annealing separator to the cold-rolled sheet after the decarburization annealing step, drying the cold-rolled sheet, and then finish annealing the cold-rolled sheet.
• a method for producing a grain-oriented electrical steel sheet according to another aspect of the present invention includes a hot rolling step of hot-rolling a steel slab to obtain a hot-rolled sheet, a hot-rolled sheet annealing step of annealing the hot-rolled sheet, a cold rolling step of cold-rolling the hot-rolled sheet after the hot-rolled sheet annealing step to obtain a cold-rolled sheet, a decarburization annealing step of decarburization annealing the cold-rolled sheet, and a finish annealing step of applying an annealing separator to the cold-rolled sheet after the decarburization annealing step, drying the cold-rolled sheet, and then finish annealing the cold-rolled sheet.
• a method for producing a grain-oriented electrical steel sheet according to another aspect of the present invention includes a hot rolling step of hot-rolling a steel slab to obtain a hot-rolled sheet, a hot-rolled sheet annealing step of annealing the hot-rolled sheet, a cold rolling step of cold-rolling the hot-rolled sheet after the hot-rolled sheet annealing step to obtain a cold-rolled sheet, a decarburization annealing step of decarburization annealing the cold-rolled sheet, and a finish annealing step of applying an annealing separator to the cold-rolled sheet after the decarburization annealing step, drying the cold-rolled sheet, and then finish annealing the cold-rolled sheet.
• the annealing separator the annealing separator prepared by mixing the MgO powder, TiO2
• an MgO powder used in an annealing separator applied to steel sheet before final annealing in the manufacturing process of grain-oriented electrical steel sheet which MgO powder provides excellent magnetic properties, appearance, and coating tension in the grain-oriented electrical steel sheet after final annealing.
• the present invention also provides a method for manufacturing grain-oriented electrical steel sheet using this MgO powder, a method for manufacturing this MgO powder, and a method for manufacturing the MgO particles contained in this MgO powder.
• the small diameter particles are particles with an average particle size of less than 0.8 ⁇ m obtained using an air classifier with a classification point D50 of 0.8 ⁇ m
• the large diameter particles are particles with an average particle size of 0.8 ⁇ m or more obtained using an air classifier with a classification point D50 of 0.8 ⁇ m.
• classification is performed only once. The reasons for each limitation will be explained below.
• MgO powder contains 0.030 mass % or more and less than 0.250 mass % of B]
• B boron
• the B content in the MgO powder (total) is set to less than 0.250 mass%.
• B is contained in the MgO powder means that B is contained within the MgO particles that make up the MgO powder, and does not mean that particles of boron or a boron compound are present alone.
• the B content in MgO powder is determined by quantitative analysis of the MgO powder using inductively coupled plasma mass spectrometry (ICP-MS). Quantitative analysis using ICP-MS is performed by dissolving the MgO powder in a mixed acid of hydrochloric acid and nitric acid. If any residue remains after dissolution, it is recovered and dissolved in an alkaline solution for analysis.
• ICP-MS inductively coupled plasma mass spectrometry
• the proportion of small-diameter particles in all particles is 5 to 80% by mass
• small-diameter particles are highly reactive and react with the coating at low temperatures. Therefore, in order to obtain excellent properties of the grain-oriented electrical steel sheet, the proportion of small-diameter particles to all particles is set to 5% by mass or more, preferably 15% by mass or more.
• the proportion of small diameter particles exceeds 80 mass%, sintering during final annealing proceeds excessively, adversely affecting the shape of the steel sheet, or the steel sheets stick together, making it difficult to remove them after final annealing. Therefore, the proportion of small diameter particles is set to 80 mass% or less.
• the proportion of small particles among all particles is calculated as the ratio of the weight of the powder classified as small particles with an average particle size of less than 0.8 ⁇ m after classification using a classifier to the weight before classification.
• MgO particles having an average particle size of less than 0.8 ⁇ m obtained by classifying using a classifier with a classification point of D50, are defined as small-diameter particles, and MgO particles having an average particle size of 0.8 ⁇ m or more are defined as large-diameter particles.
• the B content (Bs) contained in the small-diameter particles is 0.0020% by mass or more and less than 0.0800% by mass, and the B content (Bl) contained in the large-diameter particles is 0.0400% by mass or more and less than 0.3000% by mass.
• the B contained in small-diameter particles and the B contained in large-diameter particles have different active temperatures.
• the B contained in highly active small-diameter particles is more effective at lower temperatures, while the B contained in large-diameter particles allows for the supply of B at higher temperatures and for a longer period of time.
• the B content (Bs) of small-diameter particles, which function before the formation of the coating begins, is effective even if it is relatively small.
• the effect can be obtained at 0.0020% by mass or more.
• the coating may develop excessively at low temperatures, inhibiting the decomposition of precipitates and causing the secondary recrystallization temperature to rise excessively, so Bs is set to less than 0.0800% by mass.
• large diameter particles require a higher concentration of MgO than small diameter particles, and secondary recrystallization is stable when the B content (B1) is 0.0400 mass % or more and less than 0.3000 mass %.
• (Bl/Bs) is set to 1.05 or more and 30.0 or less.
• the B content of the small diameter particles and the large diameter particles is determined by quantitatively analyzing, by ICP-MS, the small diameter particles ( ⁇ 0.8 ⁇ m) and the large diameter particles ( ⁇ 0.8 ⁇ m) obtained by classifying the particles using an air flow classifier with a classification point D50 of 0.8 ⁇ m.
• Quantitative analysis by ICP-MS is carried out by dissolving the MgO powder in a mixed acid of hydrochloric acid and nitric acid. If any residue remains after dissolution, it is recovered and dissolved in an alkaline solution for analysis.
• the proportion of tricoordinated boron in the B contained in the small diameter particles is 40% by mass or more and 70% by mass or less, and the proportion of tricoordinated boron in the B contained in the large diameter particles is 80% by mass or more and 95% by mass or less]
• the boron (B) contained in MgO exists mainly in the form of tricoordinated boron ( BO3 ) or tetracoordinated boron ( BO4 ).
• tricoordinated boron has better reactivity with the coating than tetracoordinated boron. In other words, the higher the proportion of tricoordinated boron, the greater the effect can be obtained with a smaller B content.
• the proportion of tricoordinated boron in the B contained in the small diameter particles is 40 mass % or more, and the proportion of tricoordinated boron in the B contained in the large diameter particles is 80 mass % or more.
• highly reactive MgO rapidly supplies Mg.
• Mg forms an extremely stable composite oxide with Al, causing decomposition of Al-containing precipitates. That is, if the proportion of tricoordinated boron is too high to increase the reactivity with the coating, not only the supply of B but also the supply of Mg will be rapid, which may result in the decomposition of the precipitates too quickly. Therefore, it is preferable to keep the proportion of tricoordinated boron below a certain level.
• the optimal proportion of BO3 is lower than for large-diameter particles.
• the proportion of tricoordinated boron in the B contained in the small-diameter particles be 70 mass% or less, and the proportion of tricoordinated boron in the B contained in the large-diameter particles be 95 mass% or less.
• the ratio of the amount of tricoordinated boron contained in the large diameter grains to the amount of tricoordinated boron contained in the small diameter grains is 2.00 or more and 20.00 or less.
• Small-diameter particles of MgO have high reactivity and undergo reaction at low temperatures. Therefore, increasing the proportion of tricoordinated boron, particularly in small-diameter particles, significantly suppresses the decomposition of precipitates.
• the ratio of the amount of tricoordinated boron contained in the large-diameter particles to the amount of tricoordinated boron contained in the small-diameter particles (sometimes referred to as the BO3 ratio) be 2.00 or more.
• the ratio of the amount of tricoordinated boron exceeds 20.00, the effect of suppressing the decomposition of precipitates at high temperatures decreases, so it is preferable to set the ratio of the amount of tricoordinated boron to 20.00 or less.
• the proportion of tricoordinated boron in the B contained in the small-diameter particles and the large-diameter particles is determined by the following method.
• the small-diameter particles and the large-diameter particles are each measured by NMR (Nuclear Magnetic Resonance), and from the obtained spectrum, the area in the range of 27 to 6 ppm is considered to be tricoordinated boron, and the area in the range of less than 6 to -6 ppm is considered to be tetracoordinated boron.
• the abundance ratio of tricoordinated boron to tetracoordinated boron is determined from the ratio obtained by dividing the integrated area of the former by the total integrated area of the former and the latter.
• the ratio of the amount of tricoordinated boron contained in the large-diameter particles to the amount of tricoordinated boron contained in the small-diameter particles can be calculated from the B content, the proportion of tricoordinated boron in the small-diameter particles and the large-diameter particles, and the mass ratio of the small-diameter particles to the large-diameter particles.
• the MgO powder further contains 0.005% by mass or more and 0.080% by mass or less of Cl, and/or 0.02% by mass or more and 4.00% by mass or less of one or more elements selected from the group consisting of Ca, Sr, and Ba.
• Cl (chlorine) or alkaline earth metals are elements that increase the reactivity with SiO 2. Therefore, it is preferable that the MgO powder according to this embodiment contains 0.005 mass% or more of Cl and/or 0.02 mass% or more in total of one or more elements selected from the group consisting of Ca, Sr, and Ba, since this further improves the coating properties (appearance, coating tension).
• Cl, Ca, Sr, and Ba are contained in the MgO particles, and their content can be measured by quantitatively analyzing the powder using inductively coupled plasma mass spectrometry (ICP-MS). Quantitative analysis using ICP-MS is performed by dissolving the MgO powder in a mixed acid of hydrochloric acid and nitric acid. If any residue remains undissolved, it is recovered and dissolved in an alkaline solution for analysis.
• ICP-MS inductively coupled plasma mass spectrometry
• the MgO powder according to this embodiment can be manufactured by the following method.
• the manufacturing method is not limited to the following, since the effect can be obtained regardless of the manufacturing method as long as the MgO powder has the above characteristics.
• MgO particles having a B content of 0.0020% by mass or more and less than 0.0800% by mass and an average particle size of less than 0.8 ⁇ m are mixed with MgO particles having a B content of 0.0400% by mass or more and less than 0.3000% by mass and an average particle size of 0.8 ⁇ m or more so that the ratio of the B content of the MgO particles having an average particle size of 0.8 ⁇ m or more to the B content of the MgO particles having an average particle size of less than 0.8 ⁇ m is 1.05 or more (preferably 30.0 or less), and the ratio of the mass of the MgO particles having an average particle size of less than 0.8 ⁇ m to the mass of the entire MgO powder after mixing is 0.05 or more and 0.80 or less.
• MgO particles having an average particle size of less than 0.8 ⁇ m are preferably particles having an average particle size of 0.3 ⁇ m or more. Furthermore, from the viewpoint of ensuring the shape of the steel sheet, it is preferable that the particles having an average particle size of 0.8 ⁇ m or more are particles having an average particle size of 4.0 ⁇ m or less.
• examples include controlling the content by methods such as removing trace elements by ion exchange of the raw materials before firing and adding trace elements during MgO firing, and controlling the particle size by methods such as classification as needed, and controlling the mass ratio by the mixing rate.
• MgO particles having an average particle size of less than 0.8 ⁇ m can be produced by either of the following methods (I) and (II).
• a raw material powder consisting of one or more materials selected from magnesium hydroxide, basic magnesium carbonate, and magnesium carbonate, in which the mass ratio of B to the MgO equivalent of the raw material powder is 0.0020 or more and 0.0800 or less (the B content is 0.0020% by mass or more and 0.0800% by mass or less when the MgO equivalent of the raw material powder is taken as 100%), and the average particle size is less than 0.8 ⁇ m, is heated and fired in air or nitrogen.
• the MgO equivalent is the weight of MgO in the weight of the raw material powder
• (MgO equivalent) (weight of raw material powder) x (molecular weight of MgO) / (stoichiometric molecular weight of raw material powder).
• (II) Boron or a boron compound is added to a raw material powder consisting of one or more selected from magnesium hydroxide, basic magnesium carbonate, and magnesium carbonate and having an average particle size of less than 0.8 ⁇ m, so that the mass ratio of B to the MgO equivalent of the raw material powder is 0.0020 to 0.0800 (so that the B content is 0.0020 to 0.0800 mass% when the MgO equivalent of the raw material powder is taken as 100%), and then the mixture is heated and fired in air or nitrogen.
• a raw material powder having an average particle size of less than 0.8 ⁇ m and consisting of one or more types selected from magnesium hydroxide, basic magnesium carbonate, and magnesium carbonate is calcined in air or nitrogen to form MgO particles, while controlling the total content of B contained in the raw material powder (for example, as an impurity) and the content of B added as needed so that the mass ratio relative to the MgO equivalent of the raw material powder is 0.0020 or more and 0.0800 or less.
• a firing temperature of 700 to 1100°C is preferred because it stabilizes the chemical structure of the fired BO3 while allowing firing without significantly changing the particle size, thereby increasing the proportion of tricoordinated boron in the MgO particles.
• the firing temperature is more preferably 700 to 1000°C. Furthermore, the firing time is preferably 5 to 10 minutes. It is not preferable that the firing atmosphere is not air or nitrogen, since this is economically disadvantageous.
• the boron compound to be added include boron, Na 2 B 4 O 5 (OH) 4.8H 2 O, and magnesium borate.
• MgO particles with an average particle size of 0.8 ⁇ m or more can be produced, for example, by classifying the raw material powder, adding boron or a boron compound as needed to adjust the B content, and then controlling the firing conditions.
• the annealing separator to be applied before finish annealing is an annealing separator prepared by mixing the MgO powder according to the embodiment described above, TiO 2 , and water to form a slurry.
• the chemical composition of the steel slab and the conditions of each step can be those of known grain-oriented electrical steel sheets, except for the annealing separator used.
• the annealing separator is prepared by mixing MgO powder, TiO2 , and water to form a slurry (to prepare an aqueous slurry), it is preferable to mix the MgO powder for the annealing separator so that the mass ratio of TiO2 is 0.5 to 8.5% when the mass of the MgO powder for the annealing separator is taken as 100%.
• Example 1 MgO particles with an average particle size of less than 0.8 ⁇ m and MgO particles with an average particle size of 0.8 ⁇ m or more and differing in B content were mixed to obtain the MgO powder for the annealing separator shown in Tables 1A and 1B.
• TiO2 was added in an amount of 5.0% by mass, based on the mass of the MgO powder being 100%, and water was further added to prepare an aqueous slurry of the annealing separator.
• This aqueous slurry of the annealing separator was applied to a cold-rolled sheet (a known steel sheet used as a raw material for grain-oriented electrical steel sheets) after decarburization annealing (after primary recrystallization annealing).
• the cold-rolled sheet with the annealing separator applied to its surface was baked at 300°C for 30 seconds to dry the aqueous slurry. After baking, the steel sheet was subjected to a final annealing treatment at 1200°C for 20 hours.
• a grain-oriented electrical steel sheet having a base steel sheet and a primary coating containing a composite oxide such as forsterite (Mg 2 SiO 4 ) was manufactured.
• the magnetic properties, appearance, and coating tension of the resulting grain-oriented electrical steel sheets were evaluated as follows.
• Magnetic properties were determined by the following method. A sample measuring 300 mm in length in the rolling direction and 60 mm in width was taken from each grain-oriented electrical steel sheet No. A magnetic field of 800 A/m was applied to the sample in accordance with the SST method described in Appendix JA of JIS C2556:2015, and the magnetic flux density B8 was determined. When the magnetic flux density was 1.93 T or more, it was determined that the magnetic properties were excellent.
• G The color tone before the insulating coating is formed is uniform, and the maximum area of coating defects after the insulating coating is formed is 2 to 4 mm2 .
• P The color tone before the insulating coating was formed was uneven, or the maximum area of the coating defects after the insulating coating was formed was more than 4 mm2 .
• the coating tension of each grain-oriented electrical steel sheet was evaluated by the following method. Specifically, a sample measuring 300 mm in length in the rolling direction and 60 mm in width was taken from the grain-oriented electrical steel sheet after finish annealing. The primary coating was removed from only one side of the sample by pickling, and the coating tension was then determined from the radius of curvature of the curved steel sheet.
• the coating tension can be determined from the radius of curvature by any known method, such as the method disclosed in "Ex-post evaluation report on the development of innovative magnetic materials for reducing power loss in transformers" (February 2006) by the Research and Evaluation Committee of the New Energy and Industrial Technology Development Organization (NEDO). If the coating tension was 350 gf/mm 2 or more, it was determined that the coating tension was excellent.
• MgO powders described in No. 8 had B contents outside the range, and the resulting grain-oriented electrical steel sheets had poor coating tension.
• the B content of MgO particles of 0.8 ⁇ m or more was outside the range, and the resulting grain-oriented electrical steel sheet had poor coating tension.
• the MgO powders described in Nos. 9 and 10 had B contents outside the range in MgO particles of less than 0.8 ⁇ m, resulting in inferior magnetic properties and appearance.
• the MgO powders described in Nos. 11 and 12 had a B content within the range for the entire powder, but the Bs/Bl ratio was outside the range, resulting in inferior magnetic properties and coating tension.
• the B content of the MgO particles having an average particle size of less than 0.8 ⁇ m was outside the range, and the resulting grain-oriented electrical steel sheet had poor coating tension.
• the B content of the MgO particles having an average particle size of less than 0.8 ⁇ m was outside the range, and the appearance of the obtained grain-oriented electrical steel sheet was inferior.
• Example 2 Magnesium hydroxide, basic magnesium carbonate, and magnesium carbonate were mixed in the proportions shown in Tables 2A and 2B based on the MgO equivalent, and boron or a boron compound was added as necessary. The mixture was then fired at the temperatures shown in Tables 2A and 2B to obtain MgO particles with an average particle size of less than 0.8 ⁇ m. These MgO particles were further mixed with MgO particles having an average particle size of 0.8 ⁇ m or more (B content: 0.0660 to 0.0770 mass %, proportion of tricoordinated boron: 82 mass %) as shown in Table 3 to prepare MgO powders shown in Tables 4A, 4B, and 4C.
• the B contents of the MgO powder, small-diameter particles, and large-diameter particles, Bl/Bs, and the ratio of small-diameter particles to all particles were within the ranges of the present invention for Nos. 101 to 111 and 118 to 127. Therefore, the grain-oriented electrical steel sheets produced by applying an annealing separator using this MgO powder had an excellent coating tension of 350 gf/ mm2 or more, an excellent appearance with few appearance defects, and excellent magnetic properties. In contrast, Nos. 112 to 117 had B contents in small diameter particles and Bl/Bs outside the range of the present invention, and were inferior in magnetic properties and appearance. In addition, Nos.
• the present invention provides an MgO powder that, when used, can provide grain-oriented electrical steel sheets with excellent magnetic properties, appearance, and coating tension after finish annealing.
• the present invention also provides a method for producing grain-oriented electrical steel sheets using this MgO powder, a method for producing this MgO powder, and a method for producing MgO particles contained in this MgO powder. Therefore, the present invention has high industrial applicability.

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