Grain-oriented electrical steel sheet and method for forming insulating coating

Abstract

This grain-oriented electromagnetic steel sheet includes a parent material steel sheet and an insulating coating film that is formed on the surface of the parent material steel sheet. The insulating coating film includes an intermediate layer that includes a crystalline metal phosphate and is formed on the parent material steel sheet side and a tension coating film layer that is formed on the surface side of the insulating coating film. The porosity of the intermediate layer is less than 40%.

Description

[Technical Field]

[0001] The present invention relates to a grain-oriented electrical steel sheet and a method for forming an insulating coating. This application claims priority based on Japanese Patent Application No. 2023-064837, filed on April 12, 2023, the contents of which are incorporated herein by reference. [Background technology]

[0002] Grain-oriented electrical steel sheets are primarily used in transformers. Transformers are continuously excited over a long period of time, from installation to disposal, and continue to generate energy loss. Therefore, the energy loss during magnetization with AC, i.e., core loss, is a major indicator that determines the performance of a transformer.

[0003] To reduce the iron loss of grain-oriented electrical steel sheets, (a) {110} <001> Many technologies have been developed to date, from the perspective of (a) increasing the concentration in the orientation (Goss orientation), (b) increasing the content of solid solution elements such as Si to increase the electrical resistance of the steel sheet, or (c) reducing the thickness of the electrical steel sheet.

[0004] In addition, applying tension to steel sheets is effective in reducing iron loss. Forming a coating made of a material with a smaller thermal expansion coefficient than the steel sheet at high temperatures on the steel sheet surface is an effective means of reducing iron loss. Forsterite-based coatings (inorganic coatings) with excellent coating adhesion are produced by the reaction of oxides on the steel sheet surface with annealing separators during the finish annealing process of electrical steel sheets. This coating can apply tension to steel sheets.

[0005] Furthermore, for example, the method disclosed in Patent Document 1, in which an insulating coating is formed by baking a coating liquid mainly composed of colloidal silica and phosphate onto the surface of a steel sheet, is an effective method for reducing iron loss because it is highly effective in applying tension to the steel sheet. Therefore, a common method for producing grain-oriented electrical steel sheets is to leave the forsterite-based coating formed in the final annealing process and then apply an insulating coating mainly composed of phosphate on top of it.

[0006] However, in recent years, there has been an increasing demand for smaller and higher-performance transformers. To achieve this, grain-oriented electrical steel sheets are required to have excellent high-field iron loss characteristics, i.e., good iron loss even at high magnetic flux densities. At the same time, it has become clear that forsterite-based coatings hinder domain wall movement, adversely affecting iron loss. In grain-oriented electrical steel sheets, magnetic domains change due to domain wall movement under an AC magnetic field. Smooth and rapid domain wall movement is effective in reducing iron loss. However, forsterite-based coatings are nonmagnetic and have an uneven structure at the steel sheet / coating interface. This uneven structure is thought to hinder domain wall movement and adversely affect iron loss. Therefore, as a means of improving high magnetic field iron loss, methods of removing inorganic coatings by mechanical means such as polishing or chemical means such as pickling are being researched. Other research being conducted includes techniques for manufacturing grain-oriented electrical steel sheets that do not have inorganic coatings by preventing the formation of inorganic coatings during high-temperature finish annealing, and techniques for making the steel sheet surface mirror-finished (in other words, techniques for magnetically smoothing the steel sheet surface).

[0007] As a technique for preventing the formation of inorganic coatings, for example, Patent Document 2 discloses a technique in which, after normal finish annealing, the steel sheet is pickled to remove surface deposits, and then chemically or electrolytically polished to a mirror finish. It has been found that by forming a tensioned insulating coating on the surface of an inorganic-coating-free grain-oriented electrical steel sheet obtained by such a known method, an even more excellent iron loss improvement effect can be obtained. Furthermore, in addition to improving iron loss, tensioned insulating coatings can also impart various properties such as corrosion resistance, heat resistance, and slip resistance.

[0008] However, inorganic coatings not only provide insulation but also function as an intermediate layer that ensures adhesion when forming a tension coating (tension-applying insulating coating). In other words, inorganic coatings are formed so that they penetrate deeply into the steel sheet, providing excellent adhesion to the metal steel sheet. Therefore, when a tension-applying coating (tension coating) containing colloidal silica or phosphate as its main component is formed on the surface of an inorganic coating, the coating provides excellent adhesion. On the other hand, since it is generally difficult for metals and oxides to bond together, it has been difficult to ensure sufficient adhesion between the tension coating and the steel sheet surface in the absence of an inorganic coating. Therefore, when forming a tension coating on a grain-oriented electrical steel sheet that does not have an inorganic coating, it is being considered to provide a layer that takes the role of the intermediate layer of the inorganic coating.

[0009] For example, Patent Document 3 discloses a technique in which grain-oriented electrical steel sheet having no inorganic coating is annealed in a weakly reducing atmosphere to selectively thermally oxidize the silicon inevitably contained in the silicon steel sheet, thereby forming an SiO2 layer on the steel sheet surface, and then a tension-applying insulating coating is formed. Also, Patent Document 4 discloses a technique in which grain-oriented electrical steel sheet having no inorganic coating is anodically treated in a silicate aqueous solution to form an SiO2 layer on the steel sheet surface, and then a tension-applying insulating coating is formed.

[0010] However, the technology disclosed in Patent Document 3 requires the preparation of annealing equipment capable of controlling the atmosphere in order to perform annealing in a weakly reducing atmosphere, which results in a problem of processing costs. Also, in the technology disclosed in Patent Document 4, in order to perform anodic electrolysis in a silicate aqueous solution to obtain an SiO layer on the steel sheet surface that maintains sufficient adhesion with the tension-applying insulating coating, it is necessary to prepare new electrolysis equipment, which results in a problem of processing costs.

[0011] In contrast, Patent Document 5 discloses a grain-oriented electrical steel sheet having a base steel sheet and an insulating coating formed on the surface of the base steel sheet, the insulating coating being formed on the base steel sheet side, an intermediate layer containing a crystalline metal phosphate, and a tensile coating layer formed on the surface side of the insulating coating. In this grain-oriented electrical steel sheet, the intermediate layer can be formed by chemical conversion treatment.

[0012] Furthermore, Patent Document 6 relates to a method for manufacturing a grain-oriented silicon steel sheet, and describes a method for applying a secondary recrystallized grain-oriented silicon steel sheet with 0.1 g / m per side of the steel sheet prior to forming a tension-applying insulating film. 2 More than 10g / m 2 The following method for forming a coating mainly composed of zinc phosphate is disclosed. [Prior art documents] [Patent documents]

[0013] [Patent Document 1] Japanese Patent Publication No. 48-039338 [Patent Document 2] Japanese Patent Publication No. 49-96920 [Patent Document 3] Japanese Patent Publication No. 6-184762 [Patent Document 4] Japanese Patent Application Publication No. 11-209891 [Patent Document 5] International Publication No. 2022 / 215710 [Patent Document 6] Japanese Patent Application Publication No. 2005-139481 Summary of the Invention [Problem to be solved by the invention]

[0014] However, as a result of the inventors' investigation of the conventional techniques described in Patent Documents 5 and 6, they found that improving adhesion by using a coating (intermediate layer) mainly composed of metal phosphate in grain-oriented electrical steel sheets obtained by conventional methods can result in degraded magnetic properties. Further investigation into this point revealed that the cause of the degradation of magnetic properties is that the metal phosphate crystals precipitated during the chemical conversion treatment used to form the intermediate layer become coarse, resulting in the formation of many voids in the intermediate layer after the tension coating is applied and baked. It was found that the formation of many voids in the intermediate layer can result in a decrease in the space factor when an actual transformer is manufactured, resulting in a decrease in magnetic flux density per unit volume and increased (deteriorated) transformer iron loss.

[0015] Therefore, an object of the present invention is to provide a grain-oriented electrical steel sheet and an insulating coating forming method that have excellent adhesion and magnetic properties of a tensile coating and do not reduce the space factor of a transformer (core). [Means for solving the problem]

[0016] The inventors have discovered that when forming a layer containing a metal phosphate as an intermediate layer to improve adhesion between the base steel sheet and the tensile coating layer, adding a predetermined amount of a specific oxidizing agent to the chemical conversion treatment solution can reduce the void ratio in the resulting intermediate layer, resulting in a grain-oriented electrical steel sheet that has excellent adhesion and magnetic properties of the tensile coating and does not reduce the space factor of the transformer (core).

[0017] The present invention has been made in view of the above findings. The gist of one aspect of the present invention is as follows.

[0018] [1] A grain-oriented electrical steel sheet according to one aspect of the present invention comprises a base steel sheet and an insulating coating formed on the surface of the base steel sheet; and The insulating coating is an intermediate layer formed on the base steel sheet side and containing a crystalline metal phosphate; a tensile coating layer formed on the surface side of the insulating coating, The porosity within the intermediate layer is less than 40%. [2] In the grain-oriented electrical steel sheet according to the above [1], the crystalline metal phosphate in the intermediate layer may contain one or more of manganese phosphate, iron manganese phosphate, zinc phosphate, and zinc calcium phosphate. [3] In the grain-oriented electrical steel sheet according to the above [1] or [2], the crystalline metal phosphate in the intermediate layer may be in the form of a plate, a particle, or a column. [4] In the grain-oriented electrical steel sheet according to the above [1] or [2], the crystalline metal phosphate in the intermediate layer may have an average crystal grain size of 0.1 to 10.0 μm. [5] A method for forming an insulating coating according to one aspect of the present invention is a method for forming the insulating coating provided on the grain-oriented electrical steel sheet according to [1] above, comprising the steps of: a finish annealing process in which an annealing separator containing 10 to 100 mass% of Al2O3 is applied to the steel sheet, dried, and then finish annealed; an annealing separator removing step of removing excess annealing separator from the steel sheet after the finish annealing step; a pickling step in which the steel sheet after the annealing separator removing step is pickled with 0.1 to 5.0 mass % of one inorganic acid selected from sulfuric acid, chloric acid, nitric acid, and phosphoric acid for 1 to 20 seconds; an immersion step of immersing the steel sheet after the pickling step in a treatment solution containing a metal phosphate and an oxidizing agent for 2 to 60 seconds; a drying step of removing the steel sheet after the immersion step from the treatment solution, removing excess treatment solution, and then drying the steel sheet; a tensile coating layer forming step of applying a coating liquid containing a metal phosphate and colloidal silica, and having a total concentration of the metal phosphate and colloidal silica of 10 to 40 mass% in terms of solid content, to the steel sheet after the drying step, drying the coating liquid, and then maintaining the steel sheet at a sheet temperature of 750 to 950°C for 10 to 120 seconds; Equipped with In the treatment solution, the blending amount of the metal phosphate is 0.1 to 30.0 g / L, the blending amount of the oxidizing agent is 0.1 to 20.0 g / L, and the metal ion concentration is 0.5 to 10.0 g / L; The oxidizing agent is one or more of nitrates, nitrites, chlorates, chlorites, bromates, perborates, and aqueous hydrogen peroxide. [6] In the method for forming an insulating coating described in [5] above, in the immersion step, the steel sheet after the pickling step may be immersed in the treatment liquid for 2 to 60 seconds. [Effects of the Invention]

[0019] According to one aspect of the present invention, it is possible to provide a grain-oriented electrical steel sheet and an insulating coating forming method that have excellent adhesion and magnetic properties of a tensile coating and do not reduce the space factor of a transformer (core). [Brief explanation of the drawings]

[0020] [Figure 1] 1 is an example of a cross-sectional view of a grain-oriented electrical steel sheet according to an embodiment of the present invention. [Figure 2] FIG. 2 is a schematic diagram for explaining a method for determining the porosity according to the present embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0021] A grain-oriented electrical steel sheet according to one embodiment of the present invention (grain-oriented electrical steel sheet according to the present embodiment) and a method for manufacturing the grain-oriented electrical steel sheet according to the present embodiment, including a method for forming an insulating coating provided on the grain-oriented electrical steel sheet according to the present embodiment, will be described. First, the grain-oriented electrical steel sheet according to this embodiment will be described.

[0022] As shown in FIG. 1, a grain-oriented electrical steel sheet 100 according to this embodiment has a base steel sheet 1 and an insulating coating 2 formed on the surface of the base steel sheet 1.

[0023] The grain-oriented electrical steel sheet 100 according to this embodiment does not substantially have a forsterite-based coating on the surface of the base steel sheet 1. That is, in this embodiment, a forsterite-based coating is not intentionally formed on the surface of the base steel sheet 1, but the coating amount of the forsterite-based coating is 1 g / m 2If the amount is less than 0 to 1 g / m, its presence is acceptable (in this case, it is present in a part between the base steel sheet 1 and the insulating coating 2). That is, in the grain-oriented electrical steel sheet 100 according to this embodiment, the surface of the base steel sheet 1 is 2 The forsterite-based coating may be formed of the above.

[0024] The insulating coating 2 has a tensile coating layer 22 formed on the surface side of the insulating coating 2 (i.e., the surface side of the grain-oriented electrical steel sheet 100), and an intermediate layer 21 formed on the base steel sheet 1 side and containing a crystalline metal phosphate. Each component of the grain-oriented electrical steel sheet 100 will be described below.

[0025] <Base material steel plate> (chemical composition) The grain-oriented electrical steel sheet 100 according to this embodiment is significantly characterized by the structure of the insulating coating 2 formed on the surface of the base steel sheet 1, and the base steel sheet 1 included in the grain-oriented electrical steel sheet 100 is not limited in terms of its chemical composition. However, in order to obtain the properties generally required of grain-oriented electrical steel sheets, it is preferable that the chemical components contain the following: In this embodiment, % relating to the chemical components is % by mass unless otherwise specified.

[0026] C: 0.010% or less C (carbon) is an element effective for controlling the structure of steel sheets in the manufacturing process up to the completion of the decarburization annealing step. However, if the C content exceeds 0.010%, the magnetic properties of the finished grain-oriented electrical steel sheet will deteriorate. Therefore, in the base steel sheet of the grain-oriented electrical steel sheet according to this embodiment, the C content is preferably 0.010% or less. The C content is more preferably 0.005% or less. The lower the C content, the better; however, even if the C content is reduced to less than 0.0001%, the effect of structural control will saturate and the manufacturing cost will simply increase. Therefore, the C content may be 0.0001% or more.

[0027] Si: 2.50 to 4.00% Silicon (Si) is an element that increases the electrical resistance of grain-oriented electrical steel sheets and improves their core loss characteristics. If the Si content is less than 2.50%, a sufficient eddy current loss reduction effect cannot be obtained. Therefore, the Si content is preferably 2.50% or more. The Si content is more preferably 2.70% or more, and even more preferably 3.00% or more. On the other hand, if the Si content exceeds 4.00%, the grain-oriented electrical steel sheet becomes embrittled and the threading property deteriorates significantly. Furthermore, the workability of the grain-oriented electrical steel sheet deteriorates, and the steel sheet may break during rolling. Therefore, the Si content is preferably 4.00% or less. The Si content is more preferably 3.80% or less, and even more preferably 3.70% or less.

[0028] Mn: 0.01 to 0.50% Mn (manganese) is an element that combines with S to form MnS during the manufacturing process. This precipitate functions as an inhibitor (a suppressor of normal grain growth) and induces secondary recrystallization in steel. Mn also improves the hot workability of steel. If the Mn content is less than 0.01%, the above-mentioned effects cannot be fully obtained. Therefore, the Mn content is preferably 0.01% or more. The Mn content is more preferably 0.02% or more. On the other hand, if the Mn content exceeds 0.50%, secondary recrystallization does not occur, and the magnetic properties of the steel deteriorate. Therefore, in the base steel sheet of the grain-oriented electrical steel sheet according to this embodiment, the Mn content is preferably 0.50% or less. The Mn content is more preferably 0.20% or less, and even more preferably 0.10% or less.

[0029] N: 0.010% or less N (nitrogen) is an element that bonds with Al during the manufacturing process to form AlN, which functions as an inhibitor. However, if the N content exceeds 0.010%, an excessive amount of inhibitor remains in the grain-oriented electrical steel sheet, resulting in a deterioration in magnetic properties. Therefore, in the base steel sheet of the grain-oriented electrical steel sheet according to this embodiment, the N content is preferably 0.010% or less. The N content is more preferably 0.008% or less. On the other hand, although the lower limit of the N content is not particularly specified, reducing it to less than 0.001% would only increase the manufacturing cost, and therefore the N content may be set to 0.001% or more.

[0030] sol.Al: 0.020% or less Sol-Al (acid-soluble aluminum) is an element that bonds with N to form AlN, which functions as an inhibitor, during the manufacturing process of grain-oriented electrical steel sheet. However, if the sol-Al content of the base steel sheet exceeds 0.020%, excessive inhibitors remain in the base steel sheet, resulting in reduced magnetic properties. Therefore, in the base steel sheet of the grain-oriented electrical steel sheet according to this embodiment, the sol-Al content is preferably 0.020% or less. The sol-Al content is more preferably 0.010% or less, and even more preferably less than 0.001%. There is no particular restriction on the lower limit of the sol-Al content, but reducing it to less than 0.0001% simply increases manufacturing costs. Therefore, the sol-Al content may be 0.0001% or more.

[0031] S: 0.010% or less S (sulfur) is an element that combines with Mn during the manufacturing process to form MnS, which functions as an inhibitor. However, if the S content exceeds 0.010%, the magnetic properties will be reduced due to the remaining inhibitor. Therefore, in the base steel sheet of the grain-oriented electrical steel sheet according to this embodiment, the S content is preferably 0.010% or less. It is more preferable that the S content in the grain-oriented electrical steel sheet is as low as possible, for example, less than 0.001%. However, reducing the S content in the grain-oriented electrical steel sheet to less than 0.0001% will only increase the manufacturing cost. Therefore, the S content in the grain-oriented electrical steel sheet may be 0.0001% or more.

[0032] Remainder: Fe and impurities The chemical composition of the base steel sheet of the grain-oriented electrical steel sheet according to this embodiment may contain the above-mentioned elements, with the remainder being Fe and impurities. However, for the purpose of improving magnetic properties, etc., Sn, Cu, Se, and Sb may also be contained in the ranges shown below. Furthermore, even if other elements are contained, for example, one or more of W, Nb, Ti, Ni, Co, V, Cr, and Mo in a total amount of 1.0% or less, this does not impair the effects of the grain-oriented electrical steel sheet according to this embodiment. Here, impurities refer to elements that are mixed in from raw materials such as ore or scrap, or the manufacturing environment, when the base steel sheet is industrially manufactured, and are permissible to be contained in amounts that do not adversely affect the function of the grain-oriented electrical steel sheet according to this embodiment.

[0033] Sn: 0 to 0.50% Sn (tin) is an element that contributes to improving magnetic properties by controlling the primary crystal structure. To obtain the effect of improving magnetic properties, the Sn content is preferably 0.01% or more. The Sn content is more preferably 0.02% or more, and even more preferably 0.03% or more. On the other hand, if the Sn content exceeds 0.50%, secondary recrystallization becomes unstable and magnetic properties deteriorate. Therefore, the Sn content is preferably 0.50% or less. The Sn content is more preferably 0.30% or less, and even more preferably 0.10% or less.

[0034] Cu: 0 to 0.50% Copper (Cu) is an element that contributes to increasing the Goss orientation occupancy rate in the secondary recrystallized structure. To achieve the above effect, the Cu content is preferably 0.01% or more. The Cu content is more preferably 0.02% or more, and even more preferably 0.03% or more. On the other hand, if the Cu content exceeds 0.50%, the steel sheet becomes embrittled during hot rolling. Therefore, in the base steel sheet of the grain-oriented electrical steel sheet according to this embodiment, the Cu content is preferably 0.50% or less. The Cu content is more preferably 0.30% or less, and even more preferably 0.10% or less.

[0035] Se: 0 to 0.020% Se (selenium) is an element that has the effect of improving magnetic properties. When Se is contained, the Se content is preferably 0.001% or more in order to effectively exhibit the effect of improving magnetic properties. The Se content is more preferably 0.003% or more, and even more preferably 0.006% or more. On the other hand, if the Se content exceeds 0.020%, the adhesion of the coating deteriorates. Therefore, the Se content is preferably 0.020% or less. The Se content is more preferably 0.015% or less, and even more preferably 0.010% or less.

[0036] Sb: 0 to 0.50% Sb (antimony) is an element that has the effect of improving magnetic properties. When Sb is contained, the Sb content is preferably 0.005% or more in order to effectively exhibit the effect of improving magnetic properties. The Sb content is more preferably 0.01% or more, and even more preferably 0.02% or more. On the other hand, if the Sb content exceeds 0.50%, the adhesion of the coating significantly deteriorates. Therefore, the Sb content is preferably 0.50% or less. The Sb content is more preferably 0.30% or less, and even more preferably 0.10% or less.

[0037] As described above, the chemical composition of the base steel sheet of the grain-oriented electrical steel sheet in this embodiment is, for example, one that contains the above-mentioned elements with the balance being Fe and impurities.

[0038] The chemical composition of the base steel sheet of the grain-oriented electrical steel sheet according to this embodiment can be measured using a known ICP atomic emission spectroscopy. However, if an insulating coating is formed on the surface, it must be removed before measurement. The removal can be achieved by immersing the sample in a highly concentrated alkaline solution (e.g., a 30% sodium hydroxide solution heated to 85°C) for 20 minutes or more. Peeling can be determined visually. For small samples, removal can also be achieved by surface grinding.

[0039] <Insulating coating> In the grain-oriented electrical steel sheet 100 according to this embodiment, an insulating coating 2 is formed on the surface of a base steel sheet 1 . The insulating coating 2 has a structure in which an intermediate layer 21 and a tensile coating layer 22 are laminated in this order from the base steel sheet 1 side.

[0040] (middle class) As described above, grain-oriented electrical steel sheets generally have a forsterite-based coating formed in the final annealing process and an insulating coating (tensile insulating coating) formed thereon. However, in recent years, it has become clear that this forsterite-based coating hinders the movement of domain walls and adversely affects iron loss. Therefore, grain-oriented electrical steel sheets without a forsterite-based coating are being studied to further improve magnetic properties. However, without a forsterite-based coating, it is difficult to ensure sufficient adhesion between the tensile insulating coating and the surface of the base steel sheet.

[0041] In the grain-oriented electrical steel sheet 100 according to this embodiment, an intermediate layer 21 containing a crystalline metal phosphate is formed between the base steel sheet 1 and the tensile coating layer 22, thereby improving the adhesion between the base steel sheet 1 and the tensile coating layer 22 via the intermediate layer 21. This is because when the intermediate layer 21 contains a crystalline metal phosphate, the tensile coating formed thereon (which becomes the tensile coating layer 22 after formation) also contains a metal phosphate, resulting in high affinity and excellent adhesion between the intermediate layer and the tensile coating layer. Furthermore, when the intermediate layer 1 is formed by immersion in a treatment solution containing a metal phosphate, as described below, it can be formed on the surface of the base steel sheet 1 by utilizing a chemical reaction, and the adhesion between the intermediate layer 21 and the base steel sheet 1 can also be ensured.

[0042] If the intermediate layer 21 does not contain a crystalline metal phosphate, the above-mentioned effect cannot be obtained. The proportion of the crystalline metal phosphate in the intermediate layer 21 is preferably 80 mass % or more, and may be 100 mass %. In terms of adhesion, the metal phosphate is preferably one of manganese phosphate, iron manganese phosphate, zinc phosphate, and zinc calcium phosphate. The intermediate layer 21 may contain oxides and elements such as Fe and Si diffused from the base steel sheet 1 as the remainder of the metal phosphate.

[0043] The inventors of the present invention have investigated the adhesion and magnetic properties of intermediate layers 21 using metal phosphates and found that using an intermediate layer mainly made of metal phosphates to improve adhesion between the base material and the insulating coating can sometimes result in a deterioration of the magnetic properties of grain-oriented electrical steel sheets. Further investigation into this point revealed that the deterioration of magnetic properties is caused by the coarsening of the metal phosphate crystals precipitated by the chemical conversion treatment used to form the intermediate layer, resulting in the formation of many voids in the intermediate layer after the application and baking of the tension coating. It was found that if many voids are formed in the intermediate layer, the space factor will decrease when an actual transformer is manufactured, resulting in a decrease in magnetic flux density per unit volume and increased (deteriorated) transformer iron loss.

[0044] Therefore, the grain-oriented electrical steel sheet according to this embodiment is manufactured by adding a predetermined amount of a specific oxidizing agent to the treatment solution used to form the intermediate layer, in order to suppress coarsening of the crystals of the crystalline metal phosphate in the intermediate layer 21. As a result, the intermediate layer 21 is able to suppress coarsening of the crystals of the crystalline metal phosphate and has a reduced porosity.

[0045] The porosity of the intermediate layer 21 is less than 40%. If the porosity in the intermediate layer 21 increases, the adhesion between the tensile coating layer formed on the intermediate layer 21 and the intermediate layer 21 (adhesion to the base steel sheet 1 via the intermediate layer 21) decreases. Furthermore, if the porosity in the intermediate layer 21 increases, the space factor decreases, which may result in a decrease in magnetic flux density per unit volume and increase transformer iron loss. Furthermore, if the porosity exceeds 40%, the space factor decreases and the effect of the tensile coating decreases, resulting in poor iron loss. Therefore, the lower the porosity of the intermediate layer 21, the better, and the porosity may even be 0%. However, since it is practically difficult to achieve a porosity of 0% in the intermediate layer 21, the porosity may be greater than 0%.

[0046] Here, the mechanism by which voids are generated in the intermediate layer 21 will be described below. When the chemical conversion treatment solution that forms the intermediate layer 21 comes into contact with the steel sheet surface, iron ions dissolve from the steel sheet surface and hydrogen gas is generated. This hydrogen gas is in a very fine gaseous state. Therefore, the generated hydrogen gas adheres to the steel sheet surface and prevents the chemical conversion treatment solution from coming into contact with the steel sheet, forming metal phosphate crystals that grow in a specific direction, or is incorporated into the formed metal phosphate crystals, inhibiting the densification of the void layer, resulting in the formation of an intermediate layer with large surface irregularities. When a treatment solution for a tensile coating layer is applied and baked on an intermediate layer with large surface irregularities, it is presumed that the hydrogen gas remaining in the intermediate layer and the tensile coating treatment solution that has penetrated into the intermediate layer expand during baking, resulting in the formation of voids in the intermediate layer and the tensile coating.

[0047] In the intermediate layer 21 having the reduced porosity as described above, it can be said that the coarsening of the crystals of the crystalline metal phosphate is naturally suppressed. For example, the average crystal grain size of the crystalline metal phosphate in the intermediate layer 21 is 10.0 μm or less. To further suppress a decrease in the space factor, the average crystal grain size of the crystalline metal phosphate is preferably 8.0 μm or less. There is no particular lower limit to the average crystal grain size of the crystalline metal phosphate, but it may be 0.1 μm or more.

[0048] The porosity of the intermediate layer 21 is determined by the following method. FIG. 2 is an SEM image taken along the thickness direction of the intermediate layer 21 according to this embodiment. First, as shown in Figure 2, observation is performed using an SEM at 5000x magnification to obtain an observation image including the interface between the base steel sheet 1 and the intermediate layer 21, and the interface between the intermediate layer 21 and the tensile coating layer 22. Then, the interface path of the interface between the intermediate layer 21 and the tensile coating layer 22 in the observation image (i.e., the curved path tracing the actual interface) is determined as the interface length L. Next, the sum of the widths W of the voids observed in the observation image is determined, and the ratio (W / L) of the widths W of the voids to the interface length L is calculated. This ratio (W / L) of the widths W of the voids to the interface length L is defined as the porosity of the intermediate layer 21.

[0049] The interface length L can be easily determined by using an application system such as "Luzex AP" manufactured by Nireco Corporation on a cross-sectional image taken by an electron microscope.

[0050] As described above, the "porosity" in this embodiment is the ratio of the gap width W to the interface length L. According to the inventors' investigations, it is believed that the adhesion between the tensile coating layer 22 and the intermediate layer 21 (adhesion to the base steel sheet 1 via the intermediate layer 21) and the tensile performance of the tensile coating layer 22 are correlated with the "width" of the gaps in the intermediate layer 21. The reason for this is presumed to be that adhesion and transmission of tension to the steel sheet are lost in the gap areas. Therefore, in this embodiment, the "width" of these gaps is reduced, and the ratio of voids in the intermediate layer 21 is reduced, thereby ensuring good adhesion and high tension.

[0051] The thickness of the intermediate layer 21 is preferably 1.0 to 9.0 μm. If the average thickness of the intermediate layer 21 is less than 1.0 μm, the effect of improving the adhesion between the base steel sheet 1 and the insulating coating 2 via the intermediate layer 21 may not be sufficiently obtained. On the other hand, if the average thickness of the intermediate layer 21 exceeds 9.0 μm, the magnetic properties may deteriorate.

[0052] The thickness of the intermediate layer 21 can be determined by the following method. The thickness of the intermediate layer 21 can be determined by measurement using a scanning electron microscope (SEM) and an energy dispersive elemental analyzer. That is, a sample consisting of the base steel sheet 1 and the insulating coating layer 2 is cut, and the polished cross section is observed with a scanning electron microscope at 5000x magnification to measure the thickness of the insulating coating layer 2. At this time, the energy dispersive elemental analyzer is used to determine the thickness of the intermediate layer 21 by calculating the thickness of the insulating coating layer 2, assuming that the portion of the insulating coating layer 2 that contains Si is the tensile coating layer 22 and the portion that does not contain Si is the intermediate layer 21. Measurements are taken at five or more locations, and the average is taken as the thickness of the intermediate layer 21.

[0053] The crystalline metal phosphate contained in the intermediate layer 21 preferably has a plate-like, particulate, or columnar shape. When the crystalline metal phosphate has a plate-like, particulate, or columnar shape, the bonding state between the crystals is good, making it possible to form a dense intermediate layer 21. As a result, adhesion is improved and a decrease in the space factor can be suppressed. It is not necessary for all of the crystalline metal phosphates contained in the intermediate layer 21 to have the same shape; they may have any of plate-like, particulate, or columnar shapes. In other words, plate-like, particulate, and columnar crystalline metal phosphates may be mixed. The shape of the crystalline metal phosphate can be tailored by controlling the crystal growth rate. Specifically, methods include adjusting the concentration of each ion in the treatment solution, the type of oxidizing agent, the temperature, the stirring force, etc., and by appropriately adjusting and combining these conditions, it is possible to control the crystal morphology.

[0054] Here, with regard to the shape of the crystalline metal phosphate, "plate-like" refers to a flat shape in which two of the three dimensions (height, width, and depth) are five or more times longer than the other dimension. "Particulate" refers to a nearly isotropic shape in which the three dimensions (width and depth) are approximately equal. "Columnar" refers to a shape in which one of the three dimensions (height, width, and depth) is five or more times longer than the other two dimensions, and the other two dimensions are approximately equal in length.

[0055] The intermediate layer 21 is formed at a different time from the tensile coating layer 22 formed thereon, but both the intermediate layer 21 and the tensile coating layer 22 function as the insulating coating 2 .

[0056] The mass proportion of the crystalline metal phosphate and the type of the metal phosphate in the intermediate layer 21 can be determined by measuring a cross section along the thickness direction of the intermediate layer 21 using a scanning electron microscope (SEM) and an energy dispersive elemental analyzer. Whether the metal phosphate in the intermediate layer 21 is a crystalline metal phosphate can be determined by X-ray crystal structure analysis.

[0057] The base steel sheet 1 and the insulating coating 2 can be distinguished by the presence or absence of phosphorus. Of the insulating coating 2, the intermediate layer 21 and the tensile coating layer 22 can be distinguished by the presence or absence of silicon.

[0058] (Tension coating layer) In the grain-oriented electrical steel sheet 100 according to this embodiment, a tensile coating is formed on the surface of the intermediate layer 21 , so that a tensile coating layer 22 is provided on the surface side of the insulating coating 2 . The tensile coating layer 22 is not particularly limited as long as it is used as an insulating coating for grain-oriented electrical steel sheets, but it preferably contains a metal phosphate from the viewpoint of adhesion to the intermediate layer 21 (adhesion to the base steel sheet 1 via the intermediate layer 21). In particular, it is preferable that the tensile coating layer 22 has a composition containing aluminum phosphate and silica as its main components.

[0059] The tensile coating layer 22 preferably contains metal phosphate and silica (derived from colloidal silica in the coating liquid) so that the silica content is 20.0% by mass or more. On the other hand, if the silica content of the tensile coating layer 22 exceeds 60.0% by mass, it may cause powdering, so it is preferably 60.0% by mass or less. It also preferably contains a total of 70.0% by mass or more of metal phosphate and silica. The remainder other than the metal phosphate and silica may contain ceramic particles such as alumina and silicon nitride. As the metal phosphate, aluminum phosphate is preferred from the viewpoint of heat resistance.

[0060] Although there are no limitations on the thickness of the tensile coating layer 22, the average thickness of the insulating coating 2 (intermediate layer 21 + tensile coating layer 22) is preferably 2.0 to 20.0 μm, assuming that the average thickness of the intermediate layer 21 is within the above range. If the average thickness of the insulating coating 2 is less than 2.0 μm, sufficient coating tension cannot be obtained. Furthermore, excessive leaching of phosphoric acid may occur. This may cause stickiness and reduced corrosion resistance, and may even lead to coating peeling. Furthermore, if the thickness of the insulating coating 2 exceeds 20.0 μm, the space factor may decrease, resulting in deterioration of magnetic properties, or cracks may occur, resulting in reduced adhesion and reduced corrosion resistance.

[0061] In the tensile coating layer 22, the mass proportion of the metal phosphate and the type of the metal phosphate can be determined in a cross section along the thickness direction in the same manner as in the intermediate layer 21. As mentioned above, the tension coating layer 22 and the intermediate layer 21 can be distinguished by the silica content.

[0062] The thickness of the tensile coating layer 22 can be determined in the same manner as the intermediate layer 21. The sum of the thickness of the tensile coating layer 22 and the thickness of the intermediate layer 21 is the thickness of the insulating coating 2.

[0063] <Method of manufacturing grain-oriented electrical steel sheets> The grain-oriented electrical steel sheet according to this embodiment can be suitably manufactured by a manufacturing method that satisfies the manufacturing conditions described below. However, it goes without saying that the grain-oriented electrical steel sheet according to this embodiment is not particularly limited to a manufacturing method. In other words, a grain-oriented electrical steel sheet having the above-described configuration is considered to be the grain-oriented electrical steel sheet according to this embodiment, regardless of its manufacturing conditions.

[0064] The grain-oriented electrical steel sheet according to this embodiment can be manufactured by a manufacturing method including the following steps. (I) a hot rolling step in which a steel slab having a predetermined chemical composition is hot rolled to obtain a hot-rolled sheet; (II) a hot-rolled sheet annealing step of annealing the hot-rolled sheet; (III) a cold rolling step in which the hot-rolled sheet after the hot-rolled sheet annealing step is cold-rolled to obtain a steel sheet (cold-rolled sheet); (IV) a decarburization annealing step of performing decarburization annealing on the steel sheet after the cold rolling step; (V) a finish annealing step of applying an annealing separator containing 10 to 100 mass% of Al2O3 to a steel sheet, drying the steel sheet, and then finish annealing the steel sheet; (VII) an annealing separator removing step of removing excess annealing separator from the steel sheet after the finish annealing step; (VIII) a pickling step in which the steel sheet after the annealing separator removing step is pickled with 0.1 to 5.0 mass % of one inorganic acid selected from sulfuric acid, chloric acid, nitric acid, and phosphoric acid for 1 to 20 seconds; (IX) an immersion step of immersing the steel sheet after the pickling step in a treatment solution containing a metal phosphate and an oxidizing agent for 2 to 60 seconds; (X) a drying step of removing the steel sheet after the immersion step from the treatment solution, removing excess treatment solution, and then drying the steel sheet; (XI) A tensile coating layer forming process, which comprises applying a coating liquid containing a metal phosphate and colloidal silica, and having a total concentration of the metal phosphate and colloidal silica of 10.0 to 40.0 mass % in terms of solid content, to the steel sheet after the drying process, drying the coating liquid, and then maintaining the steel sheet at a temperature of 750 to 950°C for 10 to 120 seconds.

[0065] In the treatment solution used in the immersion step, the blending amount of metal phosphate is 0.1 to 30.0 g / L, the blending amount of oxidizing agent is 0.1 to 20.0 g / L, the metal ion concentration is 0.5 to 10.0 g / L, and the oxidizing agent is two or more of nitrate, nitrite, chlorate, chlorite, bromate, perborate, and hydrogen peroxide solution.

[0066] Furthermore, in the immersion step, it is preferable that the treatment liquid satisfies the following conditions. (A) Metal ion concentration: 0.5 to 10.0 g / L, (B) Phosphate ion concentration: 0.1 to 30 g / L, (C) Oxidant concentration is 0.1 to 20 g / L.

[0067] Furthermore, the method for producing a grain-oriented electrical steel sheet according to this embodiment further includes the steps of: (XII) a nitriding treatment step of nitriding the steel sheet between the decarburization annealing step and the finish annealing step; (XIII) a magnetic domain refining step for controlling the magnetic domains of the steel sheet after the tension coating layer forming step; may include either or both of the following:

[0068] Of these, the manufacturing of the grain-oriented electrical steel sheet according to this embodiment is characterized by the steps (V) finish annealing step to (XI) tension coating layer forming step (sometimes collectively referred to as the insulating coating forming step), which are mainly related to the formation of the insulating coating, and known conditions can be used for other steps or conditions not described. These steps will be described below.

[0069] [Hot rolling process] In the hot rolling process, a steel billet such as a slab having a predetermined chemical composition is heated and then hot rolled to obtain a hot-rolled sheet. The heating temperature of the steel billet is preferably within the range of 1100 to 1450°C, and more preferably 1300 to 1400°C.

[0070] The chemical composition of the slab may be changed depending on the chemical composition of the grain-oriented electrical steel sheet that is ultimately desired to be obtained, but an example of such a chemical composition may include, in mass %, C: 0.01 to 0.20%, Si: 2.50 to 4.00%, sol. Al: 0.01 to 0.040%, Mn: 0.01 to 0.50%, N: 0.020% or less, S: 0.005 to 0.040%, Cu: 0 to 0.50%, Sn: 0 to 0.50%, Se: 0 to 0.020%, Sb: 0 to 0.50%, and the balance being Fe and impurities.

[0071] The hot rolling conditions are not particularly limited and may be appropriately set based on the desired properties. The thickness of the hot rolled sheet is preferably within the range of 2.0 mm to 3.0 mm, for example.

[0072] [Hot-rolled sheet annealing process] The hot-rolled sheet annealing process is a process of annealing the hot-rolled sheet manufactured through the hot rolling process. By performing such an annealing treatment, recrystallization occurs in the steel sheet structure, and good magnetic properties can be achieved, which is preferable.

[0073] When hot-rolled sheet annealing is performed, the hot-rolled sheet produced through a hot rolling process may be annealed according to a known method. The means for heating the hot-rolled sheet during annealing is not particularly limited, and known heating methods can be used. The annealing conditions are also not particularly limited. For example, the hot-rolled sheet may be annealed in a temperature range of 900 to 1200°C for 10 seconds to 5 minutes.

[0074] [Cold rolling process] In the cold rolling process, the hot-rolled sheet after the hot-rolled sheet annealing process is subjected to cold rolling to obtain a steel sheet (cold-rolled sheet). The cold rolling may be a single cold rolling (a series of cold rolling without annealing in between), or may be multiple cold rolling passes with intermediate annealing between them, with the cold rolling being interrupted and at least one or two or more intermediate annealing passes being performed before the final pass of the cold rolling process.

[0075] When intermediate annealing is performed, it is preferable to hold the steel sheet at a temperature of 1000 to 1200° C. for 5 to 180 seconds. The annealing atmosphere is not particularly limited. In consideration of production costs, it is preferable to perform intermediate annealing three times or less. Furthermore, before the cold rolling step, the surface of the hot-rolled sheet may be subjected to pickling.

[0076] In the cold rolling step according to the present embodiment, the hot-rolled sheet after the hot-rolled sheet annealing step may be cold-rolled to obtain a steel sheet according to a known method. For example, the final rolling reduction may be in the range of 80 to 95%. If the final rolling reduction is 80% or more, the {110} <001> This is preferable because it is possible to obtain Goss nuclei with a high degree of orientation accumulation in the rolling direction. On the other hand, if the final rolling reduction exceeds 95%, it is not preferable because the secondary recrystallization is likely to become unstable in the subsequent finish annealing step. The final rolling reduction is the cumulative rolling reduction of cold rolling, and in the case where intermediate annealing is performed, it is the cumulative rolling reduction of cold rolling after final intermediate annealing.

[0077] [Decarburization annealing process] In the decarburization annealing step, the obtained steel sheet is subjected to decarburization annealing. In the decarburization annealing, the conditions for the decarburization annealing are not limited as long as the steel sheet undergoes primary recrystallization and C, which adversely affects magnetic properties, can be removed from the steel sheet. For example, the oxidation degree (PH2O / PH2) in the annealing atmosphere (furnace atmosphere) can be set to 0.3 to 0.6, and the annealing temperature can be set to 800 to 900°C, and the steel sheet can be held for 10 to 600 seconds.

[0078] [Nitriding process] Nitriding treatment may be carried out between the decarburization annealing step and the finish annealing step described below. In the nitriding process, for example, the steel sheet after the decarburization annealing process is nitrided by maintaining it at approximately 700 to 850°C in a nitriding atmosphere (an atmosphere containing hydrogen, nitrogen, and ammonia or other nitriding gases). When AlN is used as an inhibitor, it is preferable that the N content of the steel sheet after the nitriding process be 40 ppm or more by the nitriding process. On the other hand, if the N content of the steel sheet after the nitriding process exceeds 1000 ppm, excess AlN will remain in the steel sheet even after the completion of secondary recrystallization in the finish annealing. Such AlN can cause iron loss degradation. For this reason, it is preferable that the N content of the steel sheet after the nitriding process be 1000 ppm or less.

[0079] [Finishing annealing process] In the final annealing step, an annealing separator containing 10 to 100 mass% of Al2O3 is applied to the steel sheet that has been subjected to the decarburization annealing step or further nitriding treatment (nitriding treatment step), dried, and then final annealing is performed.

[0080] In conventional methods for manufacturing grain-oriented electrical steel sheets, a forsterite-based coating is formed on the surface of the steel sheet (cold-rolled sheet) by applying an annealing separator mainly composed of MgO and then performing finish annealing. In contrast, in the method for manufacturing a grain-oriented electrical steel sheet according to this embodiment, an annealing separator containing Al2O3 is used so that a forsterite-based film is hardly formed.

[0081] On the other hand, the proportion of Al2O3 may be 100% by mass, but from the viewpoint of preventing Al2O3 from seizing onto the steel sheet surface, in the method for producing grain-oriented electrical steel sheet according to this embodiment, it is preferable that the annealing separator contains MgO. MgO may be 0%, but to obtain the above effects, the proportion of MgO is preferably 5% by mass or more. If MgO is contained, the proportion of MgO is 90% by mass or less to ensure 10% by mass or more of Al2O3. The proportion of MgO is preferably 50% by mass or less.

[0082] Furthermore, in the method for producing a grain-oriented electrical steel sheet according to this embodiment, the annealing separator may further contain chloride. When the annealing separator contains chloride, the effect of making it more difficult for a forsterite-based coating to form is obtained. The chloride content is not particularly limited and may be 0%, but in order to obtain the above effect, a content of 0.5 to 10 mass% is preferable. Effective chlorides include, for example, bismuth chloride, calcium chloride, cobalt chloride, iron chloride, and nickel chloride.

[0083] The finish annealing conditions are not limited, but for example, conditions of holding at a temperature of 1150 to 1250° C. for 10 to 60 hours can be adopted.

[0084] [Annealing separator removal process] In the annealing separator removal step, excess annealing separator is removed from the steel sheet after the finish annealing step. For example, excess annealing separator can be removed by washing with water.

[0085] [Pickling process] In the pickling process, the steel sheet after the annealing separator removal process is pickled with 0.1 to 10.0 mass % of one inorganic acid selected from sulfuric acid, chloric acid, nitric acid, and phosphoric acid at a solution temperature of 30 to 85°C for 1 to 20 seconds. By pickling under these conditions, it is possible to sufficiently remove the forsterite-based coating and, if any, to remove MgO.

[0086] [Soaking process] [Drying process] In the immersion step, the steel sheet after the pickling step is immersed in a treatment solution containing a metal phosphate and an oxidizing agent for 2 to 60 seconds, and in the drying step, the steel sheet after the immersion step is pulled out of the treatment solution, excess treatment solution is removed, and then the steel sheet is dried, thereby forming an intermediate layer on the surface of the base steel sheet.

[0087] In the immersion step, the blending amount of the metal phosphate is 0.1 to 30.0 g / L, the blending amount of the oxidizing agent is 0.1 to 20 g / L, and the oxidizing agent is two or more of nitrate, nitrite, chlorate, chlorite, bromate, perborate, and hydrogen peroxide solution.

[0088] <Metal phosphate blend amount: 0.1 to 30.0 g / L> The blending amount of the metal phosphate in the treatment solution is 0.1 to 30.0 g / L. If the blending amount of the metal phosphate is less than 0.1 g / L, the deposition of the metal phosphate will be relatively slow, resulting in increased elution of iron ions from the steel sheet, which may increase the porosity of the resulting intermediate layer 21 and result in deterioration of the magnetic properties and space factor. Therefore, the blending amount of the metal phosphate is preferably 1.0 g / L or more, and more preferably 3.0 g / L or more. On the other hand, if the blending amount of the metal phosphate exceeds 30.0 g / L, the pH of the treatment solution increases and the deposition rate of the metal phosphate decreases, which may cause precipitation in the treatment solution or prevent the intermediate layer from being formed. Furthermore, if the blending amount of the metal phosphate exceeds 30.0 g / L, the porosity of the resulting intermediate layer 21 may increase, which may result in deterioration of the magnetic properties and space factor. Therefore, the blending amount of the metal phosphate is preferably 25.0 g / L or less, and more preferably, the phosphate ion concentration is 20.0 g / L or less.

[0089] <Type of oxidizing agent> The oxidizing agent added to the treatment liquid is one or more of nitrite, chlorate, chlorite, bromate, perborate, and hydrogen peroxide. In this embodiment, in the drying step described below, hydrogen gas generated from the applied treatment liquid is oxidized to suppress the generation of voids and reduce the void size. In other words, from the perspective of reducing porosity, it is preferable to use an oxidizing agent that can quickly oxidize hydrogen gas. Specifically, examples of the oxidizing agent include nitrate, nitrite, chlorite, bromate, perborate, and hydrogen peroxide. Among these, bromate is particularly preferable.

[0090] In the present embodiment, the treatment liquid in the immersion step is preferably adjusted to satisfy the following conditions (A) to (C).

[0091] (A) Metal ion concentration: 0.5 to 10 g / L (B) Phosphate ion concentration: 1 to 30 g / L The treatment solution contains a metal phosphate and an oxidizing agent. Examples of the metal phosphate include zinc phosphate, manganese phosphate, zinc calcium phosphate, and iron manganese phosphate, with zinc phosphate being preferred. The metal ion concentration and phosphate ion concentration in the treatment solution are 0.5 to 10 g / L and 0.1 to 30 g / L, respectively. If the metal ion concentration is less than 0.5 g / L, the precipitation of metal phosphate salts will be relatively slow, resulting in increased elution of iron ions from the steel sheet, which may increase the porosity of the resulting intermediate layer 21 and result in deterioration of magnetic properties and space factor. If the phosphate ion concentration is less than 1 g / L, the precipitation rate of phosphate salts will also be relatively slow, which may result in etching of the steel sheet surface. Therefore, the metal ion concentration is preferably 0.5 g / L or more, and the phosphate ion concentration is preferably 0.1 g / L or more. On the other hand, if the metal ion concentration exceeds 10 g / L, the pH of the treatment solution may increase and the deposition rate of the metal phosphate may decrease. Furthermore, if the phosphate ion concentration exceeds 30 g / L, the porosity of the resulting intermediate layer 21 may increase, resulting in deterioration of the magnetic properties and space factor. Therefore, the metal ion concentration is preferably 10 g / L or less, and the phosphate ion concentration is preferably 30 g / L or less.

[0092] (C) Oxidant (additive) concentration: 0.1 to 20 g / L If the oxidizing agent concentration is less than 0.1 g / L, the ability to oxidize hydrogen gas is low, which may increase the porosity. On the other hand, if the oxidizing agent concentration is excessively high, the steel sheet surface may be oxidized, inhibiting the precipitation of metal phosphate, reducing the density of the intermediate layer 21 and increasing the porosity. Therefore, the oxidizing agent concentration is set to 20 g / L or less.

[0093] The immersion time in the treatment solution is preferably 2 to 60 seconds. If the immersion time is less than 2 seconds, the intermediate layer may not be formed sufficiently, resulting in poor adhesion. On the other hand, if the immersion time is more than 60 seconds, the intermediate layer may have regions where the crystalline metal phosphate is excessively precipitated, resulting in poor space factor.

[0094] The temperature of the treatment solution may be 20 to 85° C. If the solution temperature is below 20° C., the intermediate layer may not be sufficiently formed, resulting in poor adhesion. On the other hand, if the solution temperature is above 85° C., the intermediate layer may have regions where the crystalline metal phosphate is excessively precipitated, resulting in poor space factor.

[0095] In the drying step, if the drying temperature is high, voids may be generated, increasing the porosity of the intermediate layer 21 and potentially reducing adhesion, so the drying temperature is preferably 300°C or lower, more preferably 200°C or lower. The drying temperature is preferably 100°C or higher.

[0096] [Tension coating layer formation process] In the tensile coating layer formation process, a coating liquid containing metal phosphate and colloidal silica and having a concentration of 10 to 40 mass % is applied to the steel sheet after the drying process, dried, and then heated and held at a sheet temperature of 700 to 950°C for 10 to 50 seconds to form a tensile coating layer on the surface of the intermediate layer.

[0097] If the sheet temperature during holding is less than 700°C, the tension will be low and the magnetic properties will be poor. Therefore, it is preferable that the sheet temperature be 700°C or higher. On the other hand, if the sheet temperature is above 950°C, the rigidity of the steel sheet will decrease and it will be prone to deformation. In this case, strain may be introduced into the steel sheet due to transportation, etc., resulting in poor magnetic properties. Therefore, it is preferable that the sheet temperature be 950°C or lower.

[0098] Furthermore, if the retention time is less than 10 seconds, the dissolution property will be poor. Therefore, the retention time is set to 10 seconds or more. On the other hand, if the retention time is more than 50 seconds, the adhesion of the tensile coating layer may be poor. Therefore, the retention time is preferably 50 seconds or less.

[0099] The coating liquid (insulating coating solution) is adjusted so that it contains a total of 10 to 40 mass % of metal phosphate and colloidal silica in terms of solid content. If the total concentration of the metal phosphate and colloidal silica is less than 10% by mass, the applied treatment liquid may easily run, which may cause uneven application.If the total concentration of the metal phosphate and colloidal silica is more than 40% by mass, the viscosity may become too high, which may cause uneven patterns or application.

[0100] As the metal phosphate, for example, one or a mixture of two or more selected from aluminum phosphate, zinc phosphate, magnesium phosphate, nickel phosphate, copper phosphate, lithium phosphate, cobalt phosphate, etc. can be used. Among these, aluminum phosphate is preferred.

[0101] The coating liquid may contain additional elements such as vanadium, tungsten, molybdenum, zirconium, etc. When these elements are contained, they can be added to the coating liquid as, for example, an oxygen acid.

[0102] Colloidal silica can be of either type S or type C. Type S colloidal silica refers to an alkaline silica solution, while type C refers to a silica particle surface that has been aluminum-treated, resulting in an alkaline to neutral silica solution. Type S colloidal silica is widely used and relatively inexpensive, but care must be taken as it may aggregate and precipitate when mixed with an acidic metal phosphate solution. Type C colloidal silica is stable even when mixed with a metal phosphate solution and does not precipitate, but it is relatively expensive due to the large processing steps required. It is best to use the appropriate type depending on the stability of the coating solution being prepared.

[0103] [Magnetic domain refinement process] The method for producing a grain-oriented electrical steel sheet according to this embodiment may further include a magnetic domain refining step of refining magnetic domains on the steel sheet after the tensile coating layer forming step. By performing the magnetic domain refining treatment, it is possible to further reduce the iron loss of the grain-oriented electrical steel sheet.

[0104] Methods of magnetic domain subdivision include a method of narrowing the width of 180° magnetic domains (subdividing 180° magnetic domains) by forming linear or point-like grooves extending in a direction intersecting the rolling direction at predetermined intervals along the rolling direction, and a method of narrowing the width of 180° magnetic domains (subdividing 180° magnetic domains) by forming linear or point-like stress distortion portions or grooves extending in a direction intersecting the rolling direction at predetermined intervals along the rolling direction.

[0105] When forming stress-strained portions, laser beam irradiation, electron beam irradiation, etc. can be applied. When forming grooves, mechanical groove formation methods using gears, etc., chemical groove formation methods in which grooves are formed by electrolytic etching, and thermal groove formation methods using laser irradiation can be applied. If the insulating coating is damaged by the formation of stress-strained portions or grooves, and the insulating properties and other characteristics are deteriorated, the insulating coating may be formed again to repair the damage. [Example]

[0106] A slab containing, by mass%, C: 0.08%, Si: 3.31%, sol. Al: 0.028%, N: 0.008%, Mn: 0.07%, S: less than 0.0005%, and the balance being Fe and impurities was cast. The slab was heated to 1350°C and then hot rolled to form a hot-rolled sheet having a thickness of 2.2 mm. This hot-rolled sheet was annealed by holding it at 1100°C for 10 seconds (hot-rolled sheet annealing). Thereafter, the hot-rolled sheet was subjected to cold rolling to obtain a cold-rolled sheet having a thickness of 0.22 mm. This cold-rolled sheet was subjected to decarburization annealing by holding it at 830°C for 90 seconds.

[0107] After decarburization annealing, an annealing separator containing 95% MgO and Al2O3 and 5% BiCl3 (bismuth chloride) was applied, dried, and then final annealing was performed at 1200°C for 20 hours. After the finish annealing, the steel sheet was washed with water to remove excess annealing separator, but no forsterite-based film was formed on the surface of the steel sheet. This steel sheet was subjected to light pickling in 3 mass % sulfuric acid at a liquid temperature of 85°C for 10 seconds.

[0108] After light pickling, an intermediate layer was formed using a treatment solution containing a mixture of metal phosphate and an additive (oxidizing agent) shown in Table 1. The drying temperature was as shown in Table 1. The properties of the obtained intermediate layer are as shown in Table 2.

[0109] Then, an insulating coating treatment solution containing metal phosphate and colloidal silica as its main components, as shown in Table 2, was applied, and the steel sheet was dried at 850°C for 20 seconds to form a tensile coating layer on the surface of the steel sheet. The thickness of the insulating coating (intermediate layer and tensile coating layer) was as shown in Table 2. The tensile coating layer was essentially composed of metal phosphate and silica.

[0110] The obtained steel sheet (grain-oriented electrical steel sheet) was subjected to magnetic domain refinement treatment by irradiating it with a laser beam under conditions of UA (irradiation energy density) of 2.0 J and irradiation intervals of 5.0 mm pitch. The iron loss W17 / 50 (iron loss at 50 Hz at 1.7 T) of the steel sheet after magnetic domain refinement treatment was measured using a single sheet magnetic property measurement method (Single Sheet Tester: SST) in accordance with JIS C2556 (2015). If the iron loss W17 / 50 was 0.65 W / kg or less, it was determined that good magnetic properties were ensured. The space factor was measured as follows.

[0111] [Occupancy rate] The space factor was measured in accordance with JIS C 2550-5 (2020). Thirty test pieces, each 30 mm wide and 320 mm long, were used. After measuring the total mass of the sample, the distance between the upper and lower backing plates sandwiching the laminate was measured and calculated under a pressure of 1 MPa. If the space factor is 96.0% or higher, it is determined that a high space factor is ensured.

[0112] The coating adhesion, coating tension, corrosion resistance, and elution resistance of the steel sheets after the magnetic domain refining treatment were evaluated by the following methods. The porosity of the intermediate layer was also determined by the following method. The results are shown in Tables 2 and 3.

[0113] [Porosity] First, a scanning electron microscope was used to observe the interface between the base steel sheet and the intermediate layer, and the interface between the intermediate layer and the tensile coating layer at 5000x magnification to obtain an observation image. The interface path between the intermediate layer and the tensile coating layer in the observation image was then calculated as the interface length L. Next, the sum of the widths W of the voids observed in the observation image was calculated, and the ratio of the void width W to the interface length L (W / L) was calculated as the porosity. The interface length L was determined using an application system such as "Luzex AP" manufactured by Nireco Corporation on the cross-sectional images obtained by the electron microscope.

[0114] [Coating adhesion] The adhesion of the coating was evaluated by taking a sample 30 mm wide and 300 mm long from the steel plate, annealing this sample for stress relief at 800°C for 2 hours in a nitrogen stream, then winding it around a 10 mm diameter cylinder and unwinding it, and then performing a bending adhesion test.The degree of peeling of the coating (area ratio) was measured. The evaluation criteria were as follows, and a rating of ⊚ or ◯ was judged to indicate excellent coating adhesion. ◎: Peeling area rate 0-0.5%. ○: Peeled area rate over 0.5% and 5.0% or less. △: Peeled area rate over 5.0%.

[0115] [Coating tension] The coating tension was calculated by back-calculating from the state of curvature when one side of the insulating coating was peeled off. If the obtained coating tension was 4.0 MPa or more, it was determined that the coating had sufficient tension.

[0116] [Corrosion resistance] Corrosion resistance was evaluated by subjecting the sample to a 5% NaCl aqueous solution that had fallen naturally onto the sample for 7 hours in a 35°C atmosphere in accordance with the JIS salt spray test (JIS Z2371:2015). Thereafter, the rusted area was evaluated on a 10-point scale. The evaluation criteria are as follows: Regarding corrosion resistance, a rating of 5 or more was determined to be excellent in corrosion resistance. 10: No rust occurred 9: Very little rust (area rate 0.10% or less) 8: Area ratio of rusted surface = over 0.10% and 0.25% or less 7: Area ratio of rusted surface = over 0.25% and 0.50% or less 6: Area ratio of rusted surface = over 0.50% and 1.0% or less 5: Area ratio of rusted surface = over 1.0% to 2.5% 4: Area ratio of rusted surface = over 2.5% and 5.0% or less 3: Area ratio of rusted surface = over 5.0% and 10% or less 2: Area ratio of rusted surface = over 10% and less than 25% 1: Area ratio of rusted surface = 25% to 50%

[0117] [Elution resistance] The resistance to elution was evaluated based on whether or not the elution of phosphoric acid from the sample could be inhibited. The amount of elution was measured by boiling the sample in boiling pure water for 10 minutes, measuring the amount of phosphoric acid eluted in the pure water, and dividing the amount of phosphoric acid by the area of ​​the insulating coating of the boiled grain-oriented electrical steel sheet.The amount of phosphoric acid eluted in the pure water was measured by cooling the pure water (solution) into which the phosphoric acid had eluted, and then diluting the cooled solution with pure water to measure the phosphoric acid concentration of the sample using ICP-AES. Elution amount: 40mg / m 2 If the resistance is less than this, the resin is deemed to have excellent resistance to elution.

[0118] [Table 1]

[0119] [Table 2]

[0120] [Table 3]

[0121] As can be seen from Tables 1 to 3, the examples of the present invention are extremely excellent in various properties, including coating adhesion, and have improved iron loss and space factor. On the other hand, the comparative examples were inferior in one or more of the coating adhesion, magnetic properties, corrosion resistance, elution resistance, and space factor of the transformer (core). [Explanation of symbols]

[0122] 100 grain-oriented electrical steel sheet 1 Base steel plate 2. Insulation coating 21 Middle Class 22 Tension coating layer [Industrial Applicability]

[0123] According to the above aspect of the present invention, a grain-oriented electrical steel sheet can be obtained that has excellent adhesion of a tensile coating and magnetic properties, and that does not reduce the space factor of a transformer (core). Therefore, the grain-oriented electrical steel sheet obtained can be suitably used as an iron core material for transformers, and has high industrial applicability.

Claims

1. A base steel plate; an insulating coating formed on the surface of the base steel sheet; and The insulating coating is an intermediate layer formed on the base steel sheet side and containing a crystalline metal phosphate; a tensile coating layer formed on the surface side of the insulating coating, A grain-oriented electrical steel sheet, characterized in that the porosity of the intermediate layer is less than 40%.

2. 2. The grain-oriented electrical steel sheet according to claim 1, wherein the crystalline metal phosphate in the intermediate layer comprises at least one of manganese phosphate, iron manganese phosphate, zinc phosphate, and zinc calcium phosphate.

3. 3. The grain-oriented electrical steel sheet according to claim 1, wherein the crystalline metal phosphate in the intermediate layer is in the form of a plate, a particle, or a column.

4. 3. The grain-oriented electrical steel sheet according to claim 1, wherein the crystalline metal phosphate in the intermediate layer has an average crystal grain size of 0.1 to 10.0 μm.

5. A method for forming the insulating coating provided on the grain-oriented electrical steel sheet according to claim 1, comprising: Steel plate, Al ２ O ３ a finish annealing process in which an annealing separator containing 10 to 100 mass% of the above is applied, dried, and then finish annealed; an annealing separator removing step of removing excess annealing separator from the steel sheet after the finish annealing step; a pickling step in which the steel sheet after the annealing separator removing step is pickled with 0.1 to 5.0 mass% of one inorganic acid selected from sulfuric acid, chloric acid, nitric acid, and phosphoric acid for 1 to 20 seconds; an immersion step of immersing the steel sheet after the pickling step in a treatment solution containing a metal phosphate and an oxidizing agent; a drying step of removing the steel sheet after the immersion step from the treatment solution, removing excess treatment solution, and then drying the steel sheet; a tensile coating layer forming step of applying a coating liquid containing a metal phosphate and colloidal silica, and having a total concentration of the metal phosphate and colloidal silica of 10 to 40 mass % in terms of solid content, to the steel sheet after the drying step, drying the coating liquid, and then maintaining the steel sheet at a sheet temperature of 750 to 950°C for 10 to 120 seconds; Equipped with In the treatment solution, the blending amount of the metal phosphate is 0.1 to 30.0 g / L, the blending amount of the oxidizing agent is 0.1 to 20.0 g / L, and the metal ion concentration is 0.5 to 10.0 g / L, A method for forming an insulating coating, wherein the oxidizing agent is one or more of nitrates, nitrites, chlorates, chlorites, bromates, perborates, and aqueous hydrogen peroxide.

6. 6. The method for forming an insulating coating according to claim 5, wherein in the immersion step, the steel sheet after the pickling step is immersed in the treatment solution for 2 to 60 seconds.

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