Magnesium oxide powder, method for producing magnesium oxide powder, method for producing magnesium oxide particles, and method for producing grain-oriented electromagnetic steel sheet

Abstract

This magnesium oxide (MgO) powder is for an annealing separator, and contains B at 0.030 mass% or more but less than 0.250 mass%. When the particles constituting the MgO powder are classified by a classifier with a classification point of D50, and of the total particles, the MgO particles having an average particle diameter of less than 0.8 μm are classified as small-diameter particles and the MgO particles having an average particle diameter of 0.8 μm or more are classified as large-diameter particles, it is found that: Bs, which is the B content included in the small-diameter particles, is 0.0020 or more but less than 0.0800 mass%; Bl, which is the B content included in the large-diameter particles, is 0.0400 mass% or more but less than 0.3000 mass%; the proportion of the small-diameter particles to the total particles is 5 to 80 mass%; and Bl / Bs, which is the ratio of Bl to Bs, is 1.05 to 30.0.

Description

MgO powder, manufacturing method of MgO powder, manufacturing method of MgO particles, manufacturing method of grain-oriented electrical steel sheet

[0001] The present invention relates to an MgO powder, a method for producing an MgO powder, a method for producing MgO particles, and a method for producing a grain-oriented electrical steel sheet.

[0002] Grain-oriented electrical steel sheets are soft magnetic materials primarily used as transformer core materials. Therefore, grain-oriented electrical steel sheets are required to have magnetic properties, such as 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. From the perspective of energy conservation, iron loss should be as low as possible. The production of grain-oriented electrical steel sheets generally involves a manufacturing method that includes hot rolling, hot-rolled sheet annealing, cold rolling, decarburization annealing, and finish annealing of a steel slab adjusted to a predetermined composition. Among these processes, the finish annealing process involves annealing a coiled steel sheet at a high temperature for a long period of time to concentrate the crystal orientation into the GOSS orientation, which is favorable for magnetic properties (enhancing the degree of orientation concentration). During this process, an annealing separator is applied to prevent coil seizure.

[0003] As the annealing separator applied to the coil, an annealing separator mainly composed of magnesium oxide (MgO) is often used. The reason for this is that when an annealing separator mainly composed of MgO is used, silicon dioxide (SiO ２ ) reacts with MgO to form forsterite (Mg ２ SiO ４ In other words, by using an annealing separator containing MgO as a main component, it is possible to prevent seizure during finish annealing and also to improve the magnetic properties of the grain-oriented electrical steel sheet.

[0004] As described above, in the production of grain-oriented electrical steel sheets, the properties of the grain-oriented electrical steel sheets can change depending on the annealing separator. Therefore, in recent years, research has been conducted on the trace elements contained in annealing separators. Furthermore, not only the contents of the trace elements but also the structures of compounds containing the trace element elements in magnesium oxide for annealing separators have been investigated.

[0005] For example, Patent Document 1 discloses a powder for an annealing separator containing 0.04 mass% to 0.30 mass% boron and composed primarily of magnesium oxide, wherein the proportion of tricoordinated boron in the boron is 80% to 95%. Patent Document 2 also 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 proportion of tricoordinated boron by adjusting the humidity of the calcined product, wherein the proportion of tricoordinated boron in the boron contained in the powder for an annealing separator is 70% to 95%. In both Patent Documents 1 and 2, it is stated that 1) the behavior of the coating reaction at high temperatures (1100°C or higher) affects the purification of impurities, 2) the behavior of the coating reaction at high temperatures is affected by boron in a three-coordinated state, and 3) boron in a four-coordinated state not only does not contribute to the purification of impurities but also penetrates into the steel sheet during high-temperature annealing and becomes Fe. ２ The ratio of tricoordinated boron is specified based on the knowledge that tricoordinated boron forms B and causes deterioration due to repeated bending.

[0006] Japanese Patent No. 6613919 Japanese Patent Application Publication No. 2020-15982

[0007] As described in 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 and poor purification of impurities from steel, which are caused by insufficient reactivity at high temperatures. However, as a result of studies by the present inventors, these techniques sometimes fail to achieve both an improvement in the secondary recrystallized structure by utilizing the effect of tricoordinated boron in suppressing precipitate decomposition and a sufficient increase in coating tension, which affects the improvement of magnetic properties. Therefore, the techniques described in Patent Documents 1 and 2 cannot be said to be sufficient in improving magnetic properties.

[0008] The present invention has been made in view of the above-mentioned problems. 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.

[0009] The present inventors have investigated the improvement of coating properties of grain-oriented electrical steel sheets by controlling the annealing separator. As a result, they found that adding boron (B) to the MgO powder contained in the annealing separator can improve coating properties and magnetic properties, but that adding more than a certain amount of boron increases the amount of boron penetrating into the steel sheet, thereby inhibiting secondary recrystallization and purification. Therefore, in order to improve coating properties and magnetic properties, they investigated a method for achieving sufficient coating properties and magnetic properties due to boron even with a reduced B content. As a result, they found that ensuring a certain amount of MgO particles with small particle sizes is effective among the MgO particles constituting the MgO powder. They also found that controlling the proportion of tricoordinated boron among the boron (B) contained in the MgO powder and / or including one or more of Cl, Ca, Sr, and Ba in the MgO powder further improves properties.

[0010] The present invention was made based on the above findings. The gist of the present invention is as follows. [1] 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 when MgO particles having an average particle size of less than 0.8 μm, obtained by classifying all particles constituting the MgO powder using a classifier at a classification point D50, are defined as small-diameter particles and MgO particles having an average particle size of 0.8 μm or more 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, and the ratio of Bl to Bs, Bl / Bs, is 1.05 to 30.0. [2] In the MgO powder according to [1], the proportion of tricoordinated boron in the B contained in the small particles may be 40% by mass or more and 70% by mass or less, and the proportion of tricoordinated boron in the B contained in the large particles may be 80% by mass or more and 95% by mass or less. [3] In the MgO powder according to [1], the ratio of the amount of tricoordinated boron in the large particles to the amount of tricoordinated boron in the small particles may be 2.00 to 20.00. [4] In the MgO powder according to [2], the ratio of the amount of tricoordinated boron in the large particles to the amount of tricoordinated boron in the small particles may be 2.00 to 20.00. [5] In the MgO powder according to any one of [1] to [4], the MgO powder may further contain 0.005% by mass or more and 0.080% by mass or less of Cl. [6] 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% by mass to 4.00% by mass. [7] 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% by mass to 4.00% by mass.[8] 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 μm 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 μm or more, so that the ratio of the B content of the MgO particles having an average particle size of 0.8 μm or more to the B content of the MgO particles having an average particle size of less than 0.8 μm is 1.05 or more, and the mass ratio of the MgO particles having an average particle size of less than 0.8 μm to the MgO powder after mixing is 0.05 or more and 0.80 or less. [9] Another embodiment of the present invention provides a method for producing MgO particles having an average particle size of less than 0.8 μm, 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 μm, is heated and fired in air or nitrogen.

[10] Another aspect of the present invention provides a method for producing MgO particles having an average particle size of less than 0.8 μm, as described in [8], comprising adding boron or a boron compound to a raw material powder having an average particle size of less than 0.8 μm and consisting of one or more selected from magnesium hydroxide, basic magnesium carbonate, and magnesium carbonate, so that the mass ratio of B to the MgO equivalent of the raw material powder is 0.0020 to 0.0800, and then heating and firing the resulting mixture in air or nitrogen.

[11] In the method for producing MgO particles described in [9] or

[10] , the firing temperature may be 700 to 1100° C.

[12] 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, wherein the annealing separator is a mixture of the MgO powder and TiO according to any one of [1] to [4]. ２

[13] 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, wherein the annealing separator is a mixture of the MgO powder and TiO according to [5]. ２

[14] 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, wherein the annealing separator is a mixture of the MgO powder and TiO according to [6]. ２

[15] 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, wherein the annealing separator is a mixture of the MgO powder and TiO according to [7]. ２ The annealing separator is prepared by mixing the above with water to form a slurry.

[0011] According to the above aspects of the present invention, it is possible to provide an MgO powder for use in an annealing separator that is applied to a steel sheet before finish annealing in the manufacturing process of a grain-oriented electrical steel sheet, and by using this MgO powder, it is possible to obtain excellent magnetic properties, appearance, and coating tension in the grain-oriented electrical steel sheet after finish annealing. Furthermore, the present invention can provide a method for manufacturing a grain-oriented electrical steel sheet using this MgO powder, a method for manufacturing this MgO powder, and a method for manufacturing MgO particles contained in this MgO powder.

[0012] An MgO powder according to one embodiment of the present invention (MgO powder according to the present embodiment) is an MgO powder for use as an annealing separator. The MgO powder according to the present embodiment has the following characteristics: the MgO powder contains 0.030% by mass or more and less than 0.250% by mass of B; the B content Bs of the small diameter particles is 0.0020% by mass or more and less than 0.0800% by mass; the B content Bl of 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 the particles is 5 to 80% by mass; and the ratio of Bl to Bs, Bl / Bs, is 1.05 to 30.0. In this embodiment, of all particles constituting the MgO powder, the small-diameter particles are particles with an average particle size of less than 0.8 μm obtained using an air classifier with a classification point D50 of 0.8 μm, and the large-diameter particles are particles with an average particle size of 0.8 μm or more obtained using an air classifier with a classification point D50 of 0.8 μm. However, classification is performed only once. The reasons for each limitation are explained below.

[0013] [MgO Powder Contains 0.030% by Mass or More and Less than 0.250% by Mass of B] By including boron (B) in the MgO powder, the coating properties and magnetic properties of a grain-oriented electrical steel sheet obtained by applying an annealing separator containing this MgO powder and then performing finish annealing can be improved. If the B content in MgO is less than 0.030% by mass, the above effects cannot be fully achieved. Therefore, the B content in the MgO powder (total) is set to 0.030% by mass or more. On the other hand, if the B content in MgO is too high, excessive boron will penetrate into the steel sheet, thereby inhibiting secondary recrystallization and purification. Therefore, the B content in the MgO powder (total) is set to less than 0.250% by mass. Here, "B is contained in the MgO powder" refers to B being contained within the MgO particles that constitute the MgO powder, and does not refer to the presence of boron or boron compound particles alone.

[0014] The B content in the MgO powder is determined by quantitative analysis of the MgO powder using inductively coupled plasma mass spectrometry (ICP-MS). For quantitative analysis by ICP-MS, the MgO powder is dissolved 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 further analysis.

[0015] [Proportion of small-diameter particles relative to total particles is 5 to 80% by mass] Among MgO particles, small-diameter particles are highly reactive and react with the coating at low temperatures. Therefore, to obtain excellent properties of the grain-oriented electrical steel sheet, the proportion of small-diameter particles relative to total particles is set to 5% by mass or more, preferably 15% by mass or more. On the other hand, if the proportion of small-diameter particles exceeds 80% by mass, excessive sintering during finish annealing may occur, adversely affecting the shape of the steel sheet, or the steel sheets may stick together, making it difficult to remove after finish annealing. Therefore, the proportion of small-diameter particles is set to 80% by mass or less.

[0016] The proportion of small diameter particles in all particles is determined as the ratio of the weight of powder classified as small diameter particles having an average particle size of less than 0.8 μm after classification with a classifier to the weight before classification.

[0017] [B content in small diameter particles is 0.0020% by mass or more and less than 0.0800% by mass] [B content in large diameter particles is 0.0400% by mass or more and less than 0.3000% by mass] In the MgO powder according to this embodiment, when all particles constituting the MgO powder are classified using a classifier at a classification point D50, MgO particles having an average particle size of less than 0.8 μm are defined as small diameter particles, and MgO particles having an average particle size of 0.8 μm or more are defined as large diameter particles. The B content (Bs) in the small diameter particles is 0.0020% by mass or more and less than 0.0800% by mass, and the B content (B1) in the large diameter particles is 0.0400% by mass or more and less than 0.3000% by mass. The B contained in the small diameter particles and the B contained in the large diameter particles have different operating 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 a coating, is effective even if it is relatively low. Specifically, the effect is obtained at 0.0020% by mass or more. On the other hand, if the B content of small-diameter particles is high, the coating may develop excessively at low temperatures, inhibiting the decomposition of precipitates and excessively increasing the secondary recrystallization temperature. Therefore, Bs is set to less than 0.0800% by mass. In contrast, 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% by mass or more and less than 0.3000% by mass.

[0018] [Bl / Bs is 1.05 or more and 30.0 or less] If the ratio (Bl / Bs) of the B content (Bl) of large-diameter particles (MgO particles having an average particle size of 0.8 μm or more) to the B content (Bs) of small-diameter particles (MgO particles having an average particle size of less than 0.8 μm) is less than 1.05, a high pinning force is maintained at low temperatures, but precipitate decomposition occurs rapidly at high temperatures, resulting in a rapid decrease in pinning force. In this case, the time required for secondary recrystallization to achieve ideal orientation cannot be ensured, and grains also grow in orientations deviating from those with good magnetic properties, resulting in inferior magnetic properties of the grain-oriented electrical steel sheet. On the other hand, if (Bl / Bs) exceeds 30.0, precipitate decomposition at high temperatures becomes too slow, making secondary recrystallization less likely to occur. In this case, fine grains may form in areas where secondary recrystallization did not occur. Therefore, (Bl / Bs) is set to 1.05 or more and 30.0 or less.

[0019] The B content of the small and large particles is determined by quantitatively analyzing the small (<0.8 μm) and large (≧0.8 μm) particles obtained by classifying the particles using an airflow classifier with a classification point D50 of 0.8 μm using ICP-MS. The 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.

[0020] [Preferably, 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 is mainly tricoordinated boron (BO ３ ) or tetracoordinate boron (BO ４) state. Among these, 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. Therefore, it is preferable that the proportion of tricoordinated boron among the B contained in the small-diameter particles is 40 mass% or more, and that the proportion of tricoordinated boron among the B contained in the large-diameter particles is 80 mass% or more. On the other hand, highly reactive MgO quickly supplies Mg. Mg forms an extremely stable composite oxide with Al, causing decomposition of precipitates containing Al. In other words, 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. Small-diameter particles, which react at a lower temperature, have a higher BO content than large-diameter particles. ３ Specifically, from the viewpoint of suppressing the decomposition of AlN, it is preferable that the proportion of tricoordinated boron in the B contained in the small diameter particles is 70 mass % or less, and the proportion of tricoordinated boron in the B contained in the large diameter particles is 95 mass % or less.

[0021] [Preferably, 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 is 2.00 or more and 20.00 or less] Among MgO particles, small-diameter particles have high reactivity and undergo reaction at low temperatures. Therefore, by increasing the proportion of tricoordinated boron, particularly in small-diameter particles, the effect of suppressing precipitate decomposition is significant. Therefore, in order to maintain the effect of suppressing precipitate decomposition by boron from low to high temperatures, 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 (BO ３ It is preferable that the ratio of the amount of tricoordinated boron to the amount of tricoordinated boron (sometimes referred to as the ratio of the amount of tricoordinated boron to the amount of tricoordinated boron) is 2.00 or more. On the other hand, if the ratio of the amount of tricoordinated boron exceeds 20.00, the effect of suppressing precipitate decomposition at high temperatures decreases. Therefore, it is preferable that the ratio of the amount of tricoordinated boron to the amount of tricoordinated boron is 20.00 or less.

[0022] 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 spectra, the amount in the range of 27 to 6 ppm is defined as tricoordinated boron, and the amount in the range of less than 6 to -6 ppm is defined as 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. Furthermore, 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 and the proportion of tricoordinated boron in the small-diameter particles and the mass ratio of the small-diameter particles to the large-diameter particles.

[0023] [Preferably, the MgO powder further contains Cl in an amount of 0.005 mass % or more and 0.080 mass % or less, and / or one or more elements selected from the group consisting of Ca, Sr, and Ba in an amount of 0.02 mass % or more and 4.00 mass % or less in total] Cl (chlorine) or alkaline earth metal is a metal such as SiO ２ It is an element that enhances reactivity with Cr. 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). On the other hand, if the Cl content exceeds 0.080 mass% or the total content of one or more elements selected from the group consisting of Ca, Sr, and Ba exceeds 4.00 mass%, this is not preferable because the strong sulfidation tendency causes desulfurization of the steel sheet.

[0024] In the MgO powder according to this embodiment, Cl, Ca, Sr, and Ba are contained in the MgO particles, and their contents 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 after dissolution, it is recovered and dissolved in an alkaline solution for further analysis.

[0025] <Method for producing MgO powder> The MgO powder according to this embodiment can be produced by the following method. However, the effect can be obtained regardless of the production method as long as the above-mentioned characteristics are present, and therefore the production method is not limited to the following. [Example of production method] MgO particles having a B content of 0.0020% by mass or more but less than 0.0800% by mass and an average particle size of less than 0.8 μm are mixed with MgO particles having a B content of 0.0400% by mass or more but less than 0.3000% by mass and an average particle size of 0.8 μm or more, so that the ratio of the B content of the MgO particles having an average particle size of 0.8 μm or more to the B content of the MgO particles having an average particle size of less than 0.8 μm 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 μm to the mass of the total MgO powder after mixing is 0.05 or more but 0.80 or less.

[0026] From the viewpoint of preventing seizure, MgO particles having an average particle size of less than 0.8 μm are preferably particles having an average particle size of 0.3 μm or more. Furthermore, from the viewpoint of ensuring the shape of the steel sheet, particles having an average particle size of 0.8 μm or more are preferably particles having an average particle size of 4.0 μm or less. To achieve the above-mentioned predetermined content ratio or mass ratio during mixing, for example, the content can be controlled by methods such as removing trace elements by ion exchange of the raw materials before firing and adding trace elements during MgO firing, and the particle size can be controlled by methods such as classification as needed, and the mass ratio can be controlled by the mixing ratio.

[0027] [Method for Producing MgO Particles with an Average Particle Size Less than 0.8 μm] MgO particles with an average particle size less than 0.8 μm can be produced by either method (I) or (II) below. (I) 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 to 0.0800 (the B content is 0.0020 to 0.0800 mass% when the MgO equivalent of the raw material powder is taken as 100%), and the raw material powder has an average particle size less than 0.8 μm, is heated and fired in air or nitrogen. Here, the MgO equivalent is the weight of MgO in the weight of the raw material powder, and is calculated as (MgO equivalent) = (weight of raw material powder) × (molecular weight of MgO) / (stoichiometric molecular weight of raw material powder). (II) 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 μm is added with boron or a boron compound 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 heated and fired in air or nitrogen. That is, 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 μm and consisting of one or more selected from magnesium hydroxide, basic magnesium carbonate, and magnesium carbonate is fired in air or nitrogen while controlling the sum of the B content contained in the raw material powder (e.g., as an impurity) and the B content added as needed to the raw material powder so that the mass ratio to the MgO equivalent of the raw material powder is 0.0020 to 0.0800, and then MgO particles are produced. During firing, the firing temperature is set to 700 to 1100°C, and the fired BO ３This is preferable because it stabilizes the chemical structure of the MgO particles while allowing firing without significantly changing the particle size, thereby increasing the proportion of tricoordinated boron in the MgO particles. In terms of maintaining the shape of the powder, the firing temperature is more preferably 700 to 1000°C. Furthermore, the firing time is preferably 5 to 10 minutes. It is not preferable to perform the firing in an atmosphere other than air or nitrogen, as this is economically disadvantageous. Examples of boron compounds to be added include boron, Na, ２ B ４ O ５ (OH) ４ ・8H ２ O, magnesium borate, etc.

[0028] MgO particles having an average particle size of 0.8 μm or more can be produced, for example, by classifying a raw material powder, adding boron or a boron compound to adjust the B content as necessary, and then controlling the firing conditions.

[0029] [Method for manufacturing grain-oriented electrical steel sheet] A grain-oriented electrical steel sheet can be manufactured by a manufacturing method including the following steps: 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. In this embodiment, the annealing separator to be applied before finish annealing is a mixture of the MgO powder and TiO according to the embodiment described above. ２ and water to form a slurry. By using this annealing separator, it is possible to produce a grain-oriented electrical steel sheet that has excellent magnetic properties, excellent coating appearance, and excellent coating tension. In the above production method, with the exception of the annealing separator used, known production conditions for grain-oriented electrical steel sheets can be applied to the chemical composition of the steel slab and the conditions for each step.

[0030] The annealing separator is composed of MgO powder and TiO ２When the MgO powder for the annealing separator is mixed with water to form a slurry (to form an aqueous slurry), the amount of TiO ２ It is preferable to mix the above at a mass ratio of 0.5 to 8.5%.

[0031] Example 1 MgO particles with an average particle size of less than 0.8 μm and MgO particles with an average particle size of 0.8 μm or more, each having a different B content, were mixed to obtain MgO powders for annealing separators shown in Tables 1A and 1B. This MgO powder was mixed with TiO in an amount of 5.0% by mass, assuming the mass of the MgO powder to be 100%. ２ and then water was added to obtain 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 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. Through the above manufacturing process, the base steel sheet and forsterite (Mg ２ SiO ４ Grain-oriented electrical steel sheets having a primary coating containing complex oxides such as Cr, Ni, and Cu, were manufactured.

[0032] The obtained grain-oriented electrical steel sheets were evaluated for magnetic properties, appearance, and coating tension in the following manner.

[0033] [Magnetic Properties] The 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. 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. A magnetic flux density of 1.93 T or higher was determined to have excellent magnetic properties.

[0034] [Appearance] A sample measuring 50 mm in length in the rolling direction and 60 mm in width was taken from each grain-oriented electrical steel sheet. The color tone of the sample was evaluated, and then a known insulating coating was formed on the surface, and coating defects were evaluated. If the color tone of the primary coating of each grain-oriented electrical steel sheet before the insulating coating was formed was uniform and there were no coating defects (holes and rust) after the insulating coating was formed, the appearance was judged to be excellent. Specifically, the evaluation was made as follows: Ex (Excellent): The color tone before the insulating coating was formed was uniform and the maximum area of ​​coating defects after the insulating coating was formed was 2 mm ２ G (GOOD): The color tone before the insulating coating is uniform, and the maximum area of ​​the coating defect after the insulating coating is formed is 2 to 4 mm ２ P (Poor): The color tone before the insulating coating is uneven, or the maximum area of ​​the coating defect after the insulating coating is formed is 4 mm ２ It's super.

[0035] [Coating Tension] The coating tension of each grain-oriented electrical steel sheet No. 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 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, and for example, the method disclosed in the "Post-Evaluation Report on the Development of Innovative Magnetic Materials for Reducing Power Losses in Transformers" (February 2006) by the Research and Evaluation Committee of the New Energy and Industrial Technology Development Organization (NEDO) can be used. ２ If the value was equal to or greater than this, it was determined that the coating tension was excellent.

[0036]

[0037]

[0038] As can be seen from Tables 1A and 1B, the MgO powders described in Nos. 1 to 6, 13 to 21, 25, and 26 all had B contents within the range of the present invention for the MgO powder, MgO particles with an average particle size of less than 0.8 μm, and MgO particles with an average particle size of 0.8 μm or more, and the B content of the B / Bs ratio and the proportion of small-diameter particles to all particles were also within the range of the present invention. Therefore, grain-oriented electrical steel sheets produced by applying an annealing separator using these MgO powders had a B content of 350 gf / mm ２ The MgO powders described in Nos. 7 and 24 had excellent coating tension, excellent appearance with few appearance defects, and excellent magnetic properties. In contrast, the B content of the MgO powders described in Nos. 7 and 24 was outside the range, and the resulting grain-oriented electrical steel sheets had poor coating tension. The MgO powder described in No. 8 had a B content of MgO particles of 0.8 μm or more outside the range, and the resulting grain-oriented electrical steel sheets had poor coating tension. The MgO powders described in Nos. 9 and 10 had a B content of MgO particles less than 0.8 μm outside the range, and the magnetic properties and appearance were poor. The MgO powders described in Nos. 11 and 12 had a B content within the range for the entire powder, but a Bs / Bl ratio outside the range, and the magnetic properties and coating tension were poor. In the MgO powder described in No. 22, the MgO particles having an average particle size of less than 0.8 μm had a B content outside the range, and the resulting grain-oriented electrical steel sheet had poor coating tension. In the MgO powder described in No. 23, the MgO particles having an average particle size of less than 0.8 μm had a B content outside the range, and the resulting grain-oriented electrical steel sheet had poor appearance.

[0039] 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 μm. These MgO particles were then mixed with MgO particles with an average particle size of 0.8 μm or more (B content 0.0660 to 0.0770 mass%, proportion of tricoordinated boron 82 mass%) as shown in Table 3 to prepare the MgO powders shown in Tables 4A, 4B, and 4C. The MgO powder was mixed with 5.0% TiO when the mass of the MgO powder was taken as 100%. ２Then, water was added to prepare an aqueous slurry of the annealing separator. This annealing separator was applied to the surface of 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), and then baked at 300°C for 30 seconds to dry the aqueous slurry. After baking, the steel sheet was subjected to final annealing at 1200°C for 20 hours. Through the above manufacturing process, the base steel sheet and forsterite (Mg ２ SiO ４ Grain-oriented electrical steel sheets having a primary coating containing complex oxides such as Cr, Ni, and Cu, were manufactured.

[0040] The magnetic properties, appearance, and coating tension of the obtained grain-oriented electrical steel sheets were evaluated in the same manner as in Example 1. The results are shown in Table 4C.

[0041]

[0042]

[0043]

[0044]

[0045]

[0046]

[0047] As can be seen from Tables 2A to 4C, Nos. 101 to 111 and 118 to 127 had the B content of the MgO powder, small diameter particles, and large diameter particles, Bl / Bs, and the ratio of small diameter particles to all particles that were within the ranges of the present invention. Therefore, the grain-oriented electrical steel sheets manufactured by applying an annealing separator using this MgO powder had a tensile strength of 350 gf / mm ２ These samples had excellent coating tension, 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 ratios outside the ranges of the invention, resulting in inferior magnetic properties and appearance. Furthermore, Nos. 128 and 129 had B contents in large-diameter particles outside the range of the invention, resulting in inferior appearance. Nos. 130 and 131 had ratios of small-diameter particles outside the range of the invention, resulting in inferior appearance.

[0048] According to the present invention, it is possible to provide an MgO powder that, when used, can provide grain-oriented electrical steel sheets after finish annealing with excellent magnetic properties, appearance, and coating tension. Furthermore, according to the present invention, it is possible to provide 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.

Claims

1. 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, wherein, of all particles constituting the MgO powder, MgO particles having an average particle size of less than 0.8 μm obtained by classifying the MgO powder using a classifier at a classification point of D50 are defined as small diameter particles and MgO particles having an average particle size of 0.8 μm 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, and the ratio of Bl to Bs, Bl / Bs, is 1.05 to 30.

0.

2. The MgO powder according to claim 1, characterized in that 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.

3. The MgO powder according to claim 1, wherein 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 is 2.00 or more and 20.00 or less.

4. The MgO powder according to claim 2, wherein 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 is 2.00 or more and 20.00 or less.

5. The MgO powder according to any one of claims 1 to 4, characterized in that the MgO powder further contains Cl in an amount of 0.005 mass % or more and 0.080 mass % or less.

6. The MgO powder according to any one of claims 1 to 4, further comprising 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%.

7. The MgO powder according to claim 5, further comprising at least one element selected from the group consisting of Ca, Sr, and Ba in a total amount of 0.02 mass % to 4.00 mass %.

8. A method for producing MgO powder as set forth in claim 1, comprising 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 μm 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 μm or more, so that the ratio of the B content of the MgO particles having an average particle size of 0.8 μm or more to the B content of the MgO particles having an average particle size of less than 0.8 μm is 1.05 or more, and the mass ratio of the MgO particles having an average particle size of less than 0.8 μm to the MgO powder after mixing is 0.05 or more and 0.80 or less.

9. A method for producing MgO particles having an average particle size of less than 0.8 μm as set forth in claim 8, characterized in that the raw material powder is made of one or more materials selected from magnesium hydroxide, basic magnesium carbonate, and magnesium carbonate, the mass ratio of B to MgO equivalent of the raw material powder being 0.0020 or more and 0.0800 or less, and the raw material powder having an average particle size of less than 0.8 μm, and is heated and fired in air or nitrogen.

10. A method for producing MgO particles having an average particle size of less than 0.8 μm as set forth in claim 8, characterized in that boron or a boron compound is added to a raw material powder consisting of one or more materials selected from magnesium hydroxide, basic magnesium carbonate, and magnesium carbonate and having an average particle size of less than 0.8 μm, 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 the mixture is heated and fired in air or nitrogen.

11. The method for producing MgO particles according to claim 9 or 10, wherein the firing temperature is set to 700 to 1100°C.

12. A method for producing a steel slab, comprising: 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, wherein the annealing separator is a mixture of MgO powder and TiO according to any one of claims 1 to 4. ２ and water to form a slurry of an annealing separator.

13. A method for producing a steel slab, comprising: 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, wherein the annealing separator is a mixture of MgO powder and TiO according to claim 5. ２ and water to form a slurry of an annealing separator.

14. A method for producing a steel slab, comprising: 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, wherein the annealing separator is a mixture of MgO powder and TiO according to claim 6. ２ and water to form a slurry of an annealing separator.

15. A method for producing a steel slab, comprising: 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, wherein the annealing separator is a mixture of MgO powder and TiO according to claim 7. ２ and water to form a slurry of an annealing separator.

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