Method for forming grain-oriented electrical steel sheets and insulating coatings

Abstract

This grain-oriented electrical steel sheet has a base steel sheet and an insulating coating film that is formed on the surface of the base steel sheet. The insulating coating film has: an intermediate layer that is formed on the base steel sheet side and contains a crystalline metal phosphate salt; and a tension coating layer that is formed on the surface side of the insulating coating film. In a cross-section of the intermediate layer along the thickness direction, when the interface between the base steel sheet and the intermediate layer is observed at a magnification of 5,000 times with a scanning electronic microscope, the ratio of the interface length L to the observation image width W is 100.0% to 120.0%.

Description

Technical Field

[0001] The present invention relates to a grain-oriented electrical steel sheet and a method for forming an insulating film. This application claims priority based on Japanese Patent Application No. 2023-064826 filed in Japan on April 12, 2023, and incorporates its content herein.

Background Art

[0002] Grain-oriented electrical steel sheets are mainly used in transformers. Transformers are continuously excited over a long period from installation to disposal and continue to generate energy losses. Therefore, the energy loss during magnetization with alternating current, that is, iron loss, becomes a major index determining the performance of transformers.

[0003] In order to reduce the iron loss of grain-oriented electrical steel sheets, many techniques have been developed so far from the viewpoints of (a) enhancing the aggregation in the {110}<001> 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] Also, applying tension to the steel sheet is effective in reducing iron loss. Forming a film of a material with a smaller coefficient of thermal expansion than the steel sheet on the steel sheet surface at a high temperature is an effective means for reducing iron loss. In the finish annealing process of the electrical steel sheet, a forsterite-based film (inorganic film) with excellent film adhesion, which is generated by the reaction of the oxide on the steel sheet surface and the annealing separation agent, is a film that can apply tension to the steel sheet.

[0005] Also, for example, the method of forming an insulating film by baking a coating liquid mainly composed of colloidal silica and phosphate on the steel sheet surface, as disclosed in Patent Document 1, is an effective method for reducing iron loss because the effect of applying tension to the steel sheet is large. Therefore, leaving the forsterite-based film generated in the finish annealing process and applying an insulating coating mainly composed of phosphate thereon has become a general method for manufacturing grain-oriented electrical steel sheets.

[0006] However, in recent years, there has been a growing demand for smaller and more powerful transformers. To achieve this miniaturization, grain-oriented electrical steel sheets are required to exhibit excellent high-field iron loss performance, meaning that iron loss is low even at high magnetic flux densities. At the same time, it has recently become clear that forsterite coatings hinder the movement of magnetic domain walls and adversely affect iron loss. In grain-oriented electrical steel sheets, magnetic domains change under alternating magnetic fields as magnetic domain walls move. Smooth and rapid movement of these magnetic domain walls is effective in reducing iron loss, but forsterite coatings are nonmagnetic themselves and have an uneven structure at the steel sheet / coating interface. This uneven structure is thought to hinder the movement of magnetic domain walls and thus adversely affect iron loss. Therefore, as a means of improving high-field iron loss, methods for removing inorganic coatings using mechanical means such as polishing, or chemical means such as pickling, are being researched. In addition, technologies for manufacturing grain-oriented electrical steel sheets without inorganic coatings by preventing the formation of inorganic coatings during high-temperature finish annealing, and technologies for making the steel sheet surface mirror-like (in other words, technologies for magnetically smoothing the steel sheet surface) are also being researched.

[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, surface deposits are removed by pickling, and then the steel sheet surface is polished to a mirror finish by chemical polishing or electrolytic polishing. It has been found that by forming a tension-imparting insulating coating on the surface of a grain-oriented electrical steel sheet that does not have an inorganic coating, obtained by such a known method, an even better iron loss improvement effect can be obtained. In addition, the tension-imparting insulating coating can impart various properties other than iron loss improvement, such as corrosion resistance, heat resistance, and slipperiness.

[0008] However, inorganic coatings not only exhibit insulating properties but also act as an intermediate layer to ensure adhesion when forming tension coatings (tension-imparting insulating coatings). In other words, because inorganic coatings are formed deeply embedded in the steel sheet, they exhibit excellent adhesion to the metallic steel sheet. Therefore, when a tension-imparting coating (tension coating) mainly composed of colloidal silica or phosphate is formed on the surface of an inorganic coating, the coating adhesion is excellent. On the other hand, since bonding between metals and oxides is generally difficult, it was difficult to ensure sufficient adhesion between the tension coating and the steel plate surface when an inorganic coating was not present. Therefore, when forming a tension coating on grain-oriented electrical steel sheets that do not have an inorganic coating, it is being considered to provide a layer that replaces the role of the inorganic coating as an intermediate layer.

[0009] For example, Patent Document 3 discloses a technique for forming an SiO2 layer on the surface of a grain-oriented electrical steel sheet without an inorganic coating by annealing it in a weakly reducing atmosphere and selectively thermally oxidizing the silicon that is inevitably contained in the silicon steel sheet, and then forming a tension-imparting insulating coating. Patent Document 4 also discloses a technique for forming an SiO2 layer on the surface of a grain-oriented electrical steel sheet without an inorganic coating by anodic electrolytic treatment in a silicate aqueous solution, and then forming a tension-imparting insulating coating.

[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 presents a problem in terms of processing costs. Furthermore, in the technology disclosed in Patent Document 4, in order to obtain an SiO2 layer on the steel sheet surface that maintains sufficient adhesion with the tension-imparting insulating coating by performing anodic electrolysis in an aqueous silicate solution, it is necessary to prepare new electrolytic treatment equipment, which presents a problem in terms of processing costs.

[0011] In contrast, Patent Document 5 discloses a grain-oriented electrical steel sheet comprising a base steel sheet and an insulating coating formed on the surface of the base steel sheet, wherein the insulating coating is formed on the base steel sheet side and comprises an intermediate layer containing a crystalline metal phosphate salt and a tension 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 unidirectional silicon steel sheet, and describes a method for forming a tension-impregnating insulating film on a unidirectional silicon steel sheet that has undergone secondary recrystallization, by applying 0.1 g / m² per side of the steel sheet. 2 More than 10g / m 2 The following method for forming a film 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 [Overview of the project] [Problems that the invention aims to solve]

[0014] However, as a result of the inventors' examination of the conventional techniques described in Patent Documents 5 and 6, it has been found that in the case of a grain-oriented electromagnetic steel sheet obtained by a conventional method, when the adhesion is improved by a film (intermediate layer) mainly composed of a metal phosphate, the magnetic properties may deteriorate. As a result of further examination on this point, it has been found that the cause of the deterioration of the magnetic properties is that when the intermediate layer is formed, that is, during the chemical conversion treatment, the surface of the base material is etched simultaneously with the precipitation of phosphate crystals, and an uneven structure is formed at the interface between the base material and the intermediate layer. As a result, the flow of magnetic flux in the base material is suppressed.

[0015] Therefore, an object of the present invention is to provide a grain-oriented electromagnetic steel sheet excellent in the adhesion and magnetic properties of a tension film and not reducing the occupation ratio of a transformer (core), and a method for forming an insulating film.

Means for Solving the Problems

[0016] When the inventors provide a layer containing a metal phosphate as an intermediate layer for enhancing the adhesion between the base steel sheet and the tension film layer, by adjusting the ratios of metal ions, phosphate ions, and nitrate ions in the chemical conversion treatment liquid within a specific range, the interface between the intermediate layer and the base material can be smoothed. As a result, a grain-oriented electromagnetic steel sheet excellent in the adhesion and magnetic properties of the tension film and not reducing the occupation ratio of a transformer (core) can be obtained.

[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] The grain-oriented electromagnetic steel sheet according to one aspect of the present invention includes a base steel sheet and an insulating film formed on the surface of the base steel sheet, and has the insulating film includes an intermediate layer formed on the base steel sheet side and containing a crystalline metal phosphate, and a tension film layer formed on the surface side of the insulating film. When observing the interface between the base steel plate and the intermediate layer at 5000 times magnification with a scanning electron microscope in a cross-section along the thickness direction of the intermediate layer, the ratio of the interface length L to the observed image width W is 100.0 to 120.0%. [2] In the grain-oriented electrical steel sheet according to [1] above, the tension coating layer may contain a metal phosphate and silica. [3] The method for forming an insulating coating according to one aspect of the present invention is a method for forming the insulating coating provided in the grain-oriented electrical steel sheet according to [1] above, A finishing annealing step of applying an annealing separator containing 10 to 100% by mass of Al2O3 to the steel sheet, drying it, and then performing finishing annealing; An annealing separator removing step of removing the excess annealing separator from the steel sheet after the finishing annealing step; An acid pickling step of pickling the steel sheet after the annealing separator removing step with one kind of inorganic acid selected from sulfuric acid, chloric acid, nitric acid, and phosphoric acid at a concentration of 0.1 to 5% by mass for 1 to 20 seconds; An immersion step of immersing the steel sheet after the acid pickling step in a treatment liquid containing a metal phosphate, an oxidizing agent, and iron ions; A drying step of pulling up the steel sheet from the treatment liquid after the immersion step, removing the excess treatment liquid, and then drying it; A tension coating layer forming step of applying a coating liquid containing a metal phosphate and colloidal silica to the steel sheet after the drying step, drying it, and then holding it at a plate temperature of 700 to 950 °C for 10 to 120 seconds; comprising the treatment liquid is Metal ion concentration: 1.0 to 10.0 g / L, Phosphate ion concentration: 2.0 to 25.0 g / L, Nitrate ion concentration is 2.0 to 40.0 g / L, Iron ion concentration is 1.0 to 20.0 g / L, and The ratio of the phosphate ion concentration to the metal ion concentration is 1.5 to 5.0, The ratio of the nitrate ion concentration to the phosphate ion concentration is 0.5 to 10.0. [4] In the method for forming an insulating coating described in [3] above, the steel sheet after the pickling step may be immersed in the treatment liquid at a liquid temperature of 20 to 85°C for 2 to 60 seconds. [5] In the method for forming an insulating film described in [3] above, the nitrate ion concentration may be 2.0 to 25.0 g / L. [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 that have excellent adhesion and magnetic properties of the tension coating and do not reduce the packing factor of the transformer (core). [Brief explanation of the drawing]

[0020] [Figure 1] This is an example of a cross-sectional view of a grain-oriented electrical steel sheet according to this embodiment. [Figure 2] This is a schematic diagram illustrating how to determine the surface roughness index according to this embodiment. [Modes for carrying out the invention]

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

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

[0023] In the grain-oriented electrical steel sheet 100 according to this embodiment, the surface of the base steel sheet 1 does not substantially have a forsterite-based coating. In other words, in this embodiment, a forsterite-based coating is not intentionally formed on the surface of the base steel sheet 1, but the amount of forsterite-based coating is 1 g / m². 2The presence of such particles is permissible if it is within the following limits (in which case, it exists in a portion 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 contains 0 to 1 g / m² 2 It may have a forsterite-based coating.

[0024] The insulating coating 2 comprises a tension 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 salt.

[0025] In the intermediate layer 21, when the interface between the base steel plate and the intermediate layer is observed at 5000x magnification using a scanning electron microscope in a cross-section along the thickness direction of the intermediate layer, the ratio of the interface length L to the observed image width W (roughness index) is 100-120%. The following describes each component of the grain-oriented electrical steel sheet 100.

[0026] <Base material steel plate> (chemical composition) The grain-oriented electrical steel sheet 100 according to this embodiment is 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 of 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 for grain-oriented electrical steel sheets, it is preferable that the chemical components include the following. In this embodiment, the percentages for chemical components are mass percentages unless otherwise specified.

[0027] C: 0.010% or less Carbon (C) is an effective element for controlling the microstructure of steel sheets during the manufacturing process up to the completion of the decarburization annealing process. However, if the C content exceeds 0.010%, the magnetic properties of the grain-oriented electrical steel sheet, which is the finished product, 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. More preferably, the C content is 0.005% or less. While a lower C content is preferable, reducing the C content to less than 0.0001% will saturate the effect of microstructure control, only increasing manufacturing costs. Therefore, the C content may be 0.0001% or more.

[0028] Si: 2.50~4.00% Silicon (Si) is an element that increases the electrical resistance of grain-oriented electrical steel sheets and improves iron loss characteristics. If the Si content is less than 2.50%, a sufficient eddy current loss reduction effect cannot be obtained. Therefore, it is preferable that the Si content be 2.50% or more. More preferably, the Si content is 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 brittle, and its treadability deteriorates significantly. In addition, the workability of the grain-oriented electrical steel sheet decreases, and the sheet may break during rolling. For this reason, it is preferable to keep the Si content at 4.00% or less. More preferably, the Si content is 3.80% or less, and even more preferably 3.70% or less.

[0029] Mn: 0.01~0.50% Manganese (Mn) is an element that combines with sulfur (S) during the manufacturing process to form MnS. This precipitate functions as an inhibitor (an agent that suppresses normal grain growth) and causes secondary recrystallization in steel. Furthermore, Mn is an element that also improves the hot workability of steel. If the Mn content is less than 0.01%, the above effects cannot be fully obtained. Therefore, it is preferable that the Mn content be 0.01% or more. More preferably, the Mn content is 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. More preferably, the Mn content is 0.20% or less, and even more preferably 0.10% or less.

[0030] N: 0.010% or less Nitrogen (N) is an element that combines with Al during the manufacturing process to form AlN, which functions as an inhibitor. However, if the N content exceeds 0.010%, an excess of inhibitor remains in the grain-oriented electrical steel sheet, leading to a decrease 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. More preferably, the N content is 0.008% or less. On the other hand, there is no specific lower limit for the N content, but reducing it to less than 0.001% would only increase manufacturing costs. Therefore, the N content may be 0.001% or higher.

[0031] sol.Al: 0.020% or less sol.Al (acid-soluble aluminum) is an element that, during the manufacturing process of grain-oriented electrical steel sheets, combines with N to form AlN, which functions as an inhibitor. However, if the sol.Al content of the base steel sheet exceeds 0.020%, an excess of inhibitor remains in the base steel sheet, reducing its 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. More preferably, the sol.Al content is 0.010% or less, and even more preferably less than 0.001%. There is no particular lower limit for the sol.Al content, but reducing it to less than 0.0001% would only increase manufacturing costs. Therefore, the sol.Al content may be 0.0001% or more.

[0032] 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 deteriorate due to the remaining inhibitor. Therefore, in the base steel sheet of the grain-oriented electrical steel sheet according to this embodiment, it is preferable that the S content be 0.010% or less. It is even preferable that the S content in the grain-oriented electrical steel sheet be 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% would only increase manufacturing costs. Therefore, the S content in the grain-oriented electrical steel sheet may be 0.0001% or more.

[0033] Remainder: Fe and impurities The chemical composition of the base steel sheet of the grain-oriented electrical steel sheet according to this embodiment contains the elements described above, with the remainder being Fe and impurities. However, for the purpose of improving magnetic properties, etc., Sn, Cu, Se, and Sb may be further included in the ranges shown below. Furthermore, even if one or more of the following elements, such as W, Nb, Ti, Ni, Co, V, Cr, and Mo, are included in a total amount of 1.0% or less, it will not impair the effects of the grain-oriented electrical steel sheet according to this embodiment. Here, "impurities" refers to elements that are mixed in during the industrial manufacturing of the base steel sheet from raw materials such as ore, scrap, or the manufacturing environment, and which are permitted to be present in amounts that do not adversely affect the function of the grain-oriented electrical steel sheet according to this embodiment.

[0034] Sn: 0~0.50% Tin (Sn) is an element that contributes to improving magnetic properties through the control of the primary crystal structure. To obtain the effect of improving magnetic properties, it is preferable to have a Sn content of 0.01% or more. More preferably, the Sn content is 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 the magnetic properties deteriorate. Therefore, it is preferable to keep the Sn content at 0.50% or less. More preferably, the Sn content is 0.30% or less, and even more preferably 0.10% or less.

[0035] Cu: 0~0.50% Copper (Cu) is an element that contributes to increasing the Goss orientation occupancy in the secondary recrystallized structure. To obtain the above effect, it is preferable to have a Cu content of 0.01% or more. More preferably, the Cu content is 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 brittle during hot rolling. Therefore, in the base steel sheet of the grain-oriented electrical steel sheet according to this embodiment, it is preferable to have a Cu content of 0.50% or less. The Cu content is more preferably 0.30% or less, and even more preferably 0.10% or less.

[0036] Se: 0~0.020% Se (selenium) is an element that has the effect of improving magnetic properties. When Se is included, it is preferable to have a Se content of 0.001% or more in order to exhibit a good 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, it is preferable to keep the Se content at 0.020% or less. More preferably, the Se content is 0.015% or less, and even more preferably 0.010% or less.

[0037] Sb: 0~0.50% Antimony (Sb) is an element that improves magnetic properties. When Sb is included, it is preferable to have an Sb content of 0.005% or more in order to exhibit a good magnetic property improvement effect. More preferably, the Sb content is 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 deteriorates significantly. Therefore, it is preferable to keep the Sb content at 0.50% or less. More preferably, the Sb content is 0.30% or less, and even more preferably 0.10% or less.

[0038] As described above, in this embodiment, the chemical composition of the base steel sheet of the grain-oriented electrical steel sheet is exemplified as containing the above-mentioned elements, with the remainder being Fe and impurities.

[0039] 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 emission spectrometry method. However, if an insulating film is formed on the surface, it must be removed before measurement. The film can be removed by immersing the sheet in a high-concentration alkaline solution (for example, a 30% sodium hydroxide solution heated to 85°C) for 20 minutes or more. Whether the film has been removed can be determined visually. For small samples, the film may be removed by surface grinding.

[0040] <Insulating coating> In this embodiment, the grain-oriented electrical steel sheet 100 has an insulating coating 2 formed on the surface of the base steel sheet 1. Furthermore, this insulating coating 2 has a structure in which an intermediate layer 21 and a tension coating layer 22 are laminated in order from the base steel plate 1 side.

[0041] (Middle class) As mentioned above, grain-oriented electrical steel sheets generally have a forsterite-based coating formed in the finish annealing process and an insulating coating (tension insulating coating) formed on top of it. However, in recent years, it has become clear that this forsterite-based coating hinders the movement of magnetic domain walls and adversely affects iron loss, so grain-oriented electrical steel sheets without a forsterite-based coating are being investigated in order to further improve magnetic properties. However, in the absence of a forsterite-based coating, it is difficult to ensure sufficient adhesion between the tension insulating coating and the surface of the base steel sheet.

[0042] In the grain-oriented electrical steel sheet 100 according to this embodiment, an intermediate layer 21 containing a crystalline metal phosphate salt is formed between the base steel sheet 1 and the tension coating layer 2, thereby improving the adhesion between the base steel sheet 1 and the tension coating layer 22 via the intermediate layer 21. This is because when the intermediate layer 21 contains a crystalline metal phosphate salt, the tension coating formed on it (which becomes the tension coating layer 22 after formation) also contains a metal phosphate salt, resulting in high affinity and excellent adhesion between the intermediate layer and the tension coating layer. Furthermore, when the intermediate layer 1 is formed by immersion in a treatment solution containing a metal phosphate salt, as described later, it can be formed on the surface of the base steel sheet 1 using a chemical reaction, and the adhesion between the intermediate layer 21 and the base steel sheet 1 can also be ensured.

[0043] If the intermediate layer 21 does not contain a crystalline metal phosphate salt, the above effects cannot be obtained. The proportion of crystalline metal phosphate salt in the intermediate layer 21 is preferably 80% by mass or more, and may be 100% by mass. As the metal phosphate salt, it is preferable to use one of zinc phosphate, manganese phosphate, zinc calcium phosphate, or iron manganese phosphate in terms of adhesion. The intermediate layer 21 may contain oxides or elements such as Fe and Si diffused from the base steel sheet 1 as the remainder of the metal phosphate salt.

[0044] Here, the inventors investigated the adhesion and magnetic properties of an intermediate layer 21 using a metal phosphate salt. They found that when attempting to improve the adhesion between the base material and the insulating film using an intermediate layer mainly composed of a metal phosphate salt, the magnetic properties of the grain-oriented electrical steel sheet may deteriorate. Further investigation revealed that the reason for the deterioration of magnetic properties is that during the formation of the intermediate layer, i.e., during the chemical conversion treatment, the surface of the base steel sheet is etched at the same time as the deposition of phosphate crystals, forming an uneven structure at the interface between the base steel sheet and the intermediate layer. As a result, the flow of magnetic flux through the base steel sheet is obstructed.

[0045] Furthermore, the inventors' investigations revealed that the surface roughness index also affects the packing density. The surface roughness index increases because the chemical treatment solution etches the surface of the steel plate during the formation of the intermediate layer, but at the same time, the fine irregularities on the surface side of the intermediate layer 21 also increase. As a result, the fine irregularities of the tension coating layer 22 formed above the intermediate layer 21 also increase, and consequently the packing density decreases.

[0046] Therefore, in the grain-oriented electrical steel sheet according to this embodiment, the degree of unevenness (roughness) at the interface between the base steel sheet 1 and the intermediate layer 21 is reduced, thereby ensuring good adhesion and space factor while suppressing deterioration of magnetic properties. Specifically, in the grain-oriented electrical steel sheet according to this embodiment, when the interface between the base steel sheet 1 and the intermediate layer 21 is observed at 5000x magnification using a scanning electron microscope (SEM) in a cross-section along the thickness direction of the intermediate layer 21, the ratio of the interface length L to the observed image width W (roughness index), L / W, is 100.0 to 120.0%.

[0047] Figure 2 is a schematic diagram illustrating how to determine the roughness index L / W, and is a schematic cross-sectional view of the intermediate layer 21 along the thickness direction. First, as shown in Figure 2, the cross-section is observed at 5000x magnification using a scanning electron microscope (SEM) to obtain an observation image that includes the interface between the base steel sheet 1 and the intermediate layer 21. Then, the distance between two points connected by a straight line at both ends of the interface between the base steel sheet 1 and the intermediate layer 21 in the observation image is defined as the observation image width W, and the interface length L is defined as the actual interface path (i.e., a curved path tracing the actual interface) at both ends of the interface, and the roughness index L / W (%) is calculated. This roughness index L / W is an indicator of the degree of roughness of the interface, and a smaller roughness index L / W indicates a smaller degree of roughness and superior interface smoothing. Note that the degree of roughness observed changes depending on the observation magnification of the SEM. Therefore, the observation magnification of the SEM is set to a range suitable for observing the interface, and in this embodiment, it is set to 5000x. Furthermore, when selecting the cross-section from which to obtain the observation image for measuring the roughness index L / W of the grain-oriented electrical steel sheet, the cross-section should be a flat portion of the grain-oriented electrical steel sheet surface that is free from surface defects or micro-processing. In addition, when calculating the roughness index L / W, the average value of the values ​​measured from three observation images should be used.

[0048] The observed image width W and interface length L can be easily determined from cross-sectional images obtained with an electron microscope using an application system such as "LuzeX AP" manufactured by Nireco Corporation.

[0049] In this embodiment, the surface roughness index L / W is 100.0 to 120.0%. If the surface roughness index L / W exceeds 120.0%, the degree of roughness at the interface increases, which may degrade the magnetic properties. Therefore, the surface roughness index is 120.0% or less. Preferably it is less than 120.0%, more preferably 118.0% or less, even more preferably 115.0% or less, and even more preferably 110.0% or less. The lower limit of the surface roughness index L / W is 100.0%. A surface roughness index L / W of 100.0% means that the observed image width W and the interface length L are the same, that is, the degree of roughness is zero and the interface is smooth.

[0050] 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 film 2 via the intermediate layer 21 may not be sufficiently obtained. On the other hand, if the average thickness of the intermediate layer 21 is greater than 9.0 μm, the magnetic properties may deteriorate.

[0051] 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 measuring it using a scanning electron microscope (SEM) and an energy-dispersive elemental analyzer. Specifically, a sample consisting of a base steel plate 1 and an insulating coating layer 2 is cut, and the polished cross-section is observed with a 5000x scanning electron microscope to measure the thickness of the insulating coating layer 2. At this time, using an energy-dispersive elemental analyzer, the portion of the insulating coating layer 2 containing Si is considered the tension coating layer 22, and the portion not containing Si is considered the intermediate layer 21. By calculating this, the thickness of the intermediate layer 21 can be determined. Measurements are taken at five or more locations, and the average is used as the thickness of the intermediate layer 21.

[0052] The intermediate layer 21 is formed at a different time than the tension coating layer 22 formed on top of it, but both the intermediate layer 21 and the tension coating layer 22 function as insulating coatings 2.

[0053] The mass ratio and type of crystalline 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 crystallography.

[0054] Furthermore, the base steel plate 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 tension coating layer 22 can be distinguished by the presence or absence of silicon.

[0055] (Tension coating layer) In the grain-oriented electrical steel sheet 100 according to this embodiment, a tension coating is formed on the surface of the intermediate layer 21, thereby providing a tension coating layer 22 on the surface side of the insulating coating 2. The tension 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 is preferable that it contains a metal phosphate salt 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 tension coating layer 22 has a composition mainly composed of aluminum phosphate and silica.

[0056] The tension coating layer 22 preferably contains metal phosphate and silica (derived from colloidal silica in the coating solution) such that the silica content is 20.0% by mass or more. On the other hand, if the silica content of the tension coating layer 22 exceeds 60.0% by mass, it may cause powdering, so it is preferable to keep it at 60.0% by mass or less. It is also preferable that the total amount of metal phosphate and silica is 70.0% by mass or more. The remainder other than metal phosphate and silica may include ceramic fine particles such as alumina or silicon nitride. As the metal phosphate, aluminum phosphate is preferred from the viewpoint of heat resistance.

[0057] The thickness of the tension coating layer 22 is not limited, but the average thickness of the insulating coating 2 (intermediate layer 21 + tension coating layer 22) is preferably 2.0 to 20.0 μm, given 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. In addition, there will be a large amount of phosphoric acid leaching. In this case, it may cause stickiness and reduced corrosion resistance, and may also cause the coating to peel off. Furthermore, if the thickness of the insulating coating 2 exceeds 20.0 μm, the packing factor will decrease, the magnetic properties will deteriorate, and adhesion will decrease due to cracks, etc., and corrosion resistance will decrease.

[0058] In the tension coating layer 22, the mass ratio and type of metal phosphate salt can be determined in the same manner as for the intermediate layer 21, using a cross-section along the thickness direction. As mentioned above, the tension coating layer 22 and the intermediate layer 21 can be distinguished by their different silica content.

[0059] The thickness of the tension coating layer 22 can be determined in the same manner as the intermediate layer 21. The sum of the thickness of the tension coating layer 22 and the thickness of the intermediate layer 21 is the thickness of the insulating coating 2.

[0060] <Manufacturing method for 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, naturally, the grain-oriented electrical steel sheet according to this embodiment is not particularly limited to the manufacturing method. That is, a grain-oriented electrical steel sheet having the above-described configuration is considered to be a grain-oriented electrical steel sheet according to this embodiment, regardless of its manufacturing conditions.

[0061] The grain-oriented electrical steel sheet according to this embodiment can be manufactured by a manufacturing method that includes the following steps. (I) A hot rolling process to obtain a hot-rolled sheet by hot-rolling a steel billet having a predetermined chemical composition, (II) A hot-rolled sheet annealing step in which the hot-rolled sheet is annealed, (III) A cold rolling step in which the hot-rolled sheet after the hot-rolled sheet annealing step is subjected to cold rolling to obtain a steel sheet (cold-rolled sheet), (IV) A decarburization annealing step in which decarburization annealing is performed on the steel sheet after the cold rolling step, (V) A finish annealing step in which an annealing release agent containing 10 to 100% by mass of Al2O3 is applied to the steel plate, dried, and then finish annealing is performed. (VII) An annealing separating agent removal step in which excess annealing separating agent is removed from the steel sheet after the finish annealing step, (VIII) A pickling step in which the steel sheet after the annealing separating agent removal step is pickled with one inorganic acid selected from sulfuric acid, chloric acid, nitric acid, and phosphoric acid in an amount of 0.1 to 5% by mass for a period of 1 to 20 seconds, (IX) an immersion step in which the steel sheet after the pickling step is immersed for 5 to 150 seconds in a treatment solution containing a metal phosphate salt, nitric acid, and iron ions at a liquid temperature of 20 to 85°C; (X) a drying step in which the steel sheet after the immersion step is removed from the treatment solution, excess treatment solution is removed, and then the sheet is dried. (XI) A tension coating layer formation step, wherein a coating liquid containing a metal phosphate salt and colloidal silica, wherein the total concentration of the metal phosphate salt and colloidal silica is 10.0 to 40.0% by mass in terms of solid content, is applied to the steel sheet after the drying step, and the sheet is dried, and then the sheet temperature is maintained at 700 to 950°C for 10 to 120 seconds.

[0062] In the manufacturing method of grain-oriented electrical steel sheets according to this embodiment, the treatment liquid satisfies the following conditions during the immersion step. (A) Metal ion concentration: 1.0~10.0 g / L (B) Phosphate ion concentration: 2.0~25.0 g / L, (C) Nitrate ion concentration is 2.0 to 40.0 g / L, (D) Iron ion concentration is 1.0 to 20.0 g / L, (E) The ratio of phosphate ion concentration to metal ion concentration is 1.5 to 5.0. (F) The ratio of nitrate ion concentration to phosphate ion concentration is 0.5 to 10.0.

[0063] Furthermore, the manufacturing method of grain-oriented electrical steel sheets according to this embodiment further includes: (XII) Between the decarburization annealing step and the finish annealing step, a nitriding step is performed on the steel sheet, (XIII) After the tension coating layer formation step, a magnetic domain subdivision step is performed to control the magnetic domains of the steel sheet, It may include either or both of the following: Of these, the characteristic features of the manufacturing of grain-oriented electrical steel sheets according to this embodiment are the processes (V) finish annealing process to (XI) tension coating layer formation process (these may be collectively referred to as the insulating coating formation process), which are mainly related to the formation of an insulating coating, and other processes or conditions not described can be those of known origin. The following explains these processes.

[0064] [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 in the range of 1100 to 1450°C. More preferably, the heating temperature is 1300 to 1400°C.

[0065] The chemical composition of the steel billet can be changed according to the chemical composition of the grain-oriented electrical steel sheet that is ultimately to be obtained. For example, a chemical composition can be exemplified by containing, in mass%, C: 0.01~0.20%, Si: 2.50~4.00%, sol.Al: 0.01~0.040%, Mn: 0.01~0.50%, N: 0.020% or less, S: 0.005~0.040%, Cu: 0~0.50%, Sn: 0~0.50%, Se: 0~0.020%, Sb: 0~0.50%, with the remainder being Fe and impurities.

[0066] The hot rolling conditions are not particularly limited and should be set appropriately based on the desired characteristics. The thickness of the hot-rolled sheet is preferably in the range of 2.0 mm to 3.0 mm.

[0067] [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. This annealing treatment is preferable because it induces recrystallization in the steel sheet structure, making it possible to achieve good magnetic properties.

[0068] When performing hot-rolled sheet annealing, the hot-rolled sheet manufactured through the hot-rolling process should be annealed according to known methods. The means of heating the hot-rolled sheet during annealing are not particularly limited, and known heating methods can be used. Similarly, the annealing conditions are not particularly limited. For example, the hot-rolled sheet can be annealed at a temperature range of 900 to 1200°C for 10 seconds to 5 minutes.

[0069] [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). Cold rolling may be performed in a single cold rolling cycle (a series of cold rolling cycles that do not include annealing in between), or multiple cold rolling cycles may be performed with intermediate annealing in between, by interrupting the cold rolling before the final pass of the cold rolling process and performing at least one or more intermediate annealing cycles.

[0070] When performing intermediate annealing, it is preferable to hold the material at a temperature of 1000-1200°C for 5-180 seconds. The annealing atmosphere is not particularly limited. Considering the manufacturing cost, it is preferable to perform intermediate annealing no more than three times. Furthermore, the surface of the hot-rolled sheet may be pickled before the cold-rolling process.

[0071] In the cold rolling process according to this embodiment, the hot-rolled sheet after the hot-rolled sheet annealing process is cold-rolled to form a steel sheet according to a known method. For example, the final reduction ratio can be in the range of 80 to 95%. If the final reduction ratio is 80% or more, {110} <001> This is preferable because it allows for the acquisition of Goss nuclei with a high degree of concentration in the rolling direction. On the other hand, if the final reduction ratio exceeds 95%, it is undesirable because there is a high possibility that secondary recrystallization will become unstable in the subsequent finish annealing process. The final reduction ratio is the cumulative reduction ratio of cold rolling, and if intermediate annealing is performed, it is the cumulative reduction ratio of cold rolling after the final intermediate annealing.

[0072] [Decarburization annealing process] In the decarburization annealing process, the obtained steel sheet is subjected to decarburization annealing. In decarburization annealing, the conditions are not limited as long as the steel sheet undergoes primary recrystallization and carbon, which adversely affects magnetic properties, is removed from the steel sheet. For example, it is exemplified to set the degree of oxidation (PH2O / PH2) in the annealing atmosphere (furnace atmosphere) to 0.3 to 0.6 and hold the annealing temperature at 800 to 900°C for 10 to 600 seconds.

[0073] [Nitriding process] Nitriding may be performed between the decarburization annealing process and the finish annealing process described later. In the nitriding process, for example, the steel sheet after the decarburization annealing process is subjected to nitriding by maintaining it at approximately 700-850°C in a nitriding atmosphere (an atmosphere containing gases with nitriding ability such as hydrogen, nitrogen, and ammonia). When using AlN as an inhibitor, it is preferable to have a nitrogen content of 40 ppm or more in the steel sheet after the nitriding process. On the other hand, if the nitrogen content of the steel sheet after the nitriding process exceeds 1000 ppm, excess AlN will remain in the steel sheet even after secondary recrystallization is completed in finish annealing. Such AlN can cause iron loss degradation. Therefore, it is preferable to have a nitrogen content of 1000 ppm or less in the steel sheet after the nitriding process.

[0074] [Finishing annealing process] In the finish annealing process, an annealing release agent containing 10-100% by mass of Al2O3 is applied to the steel sheet after the decarburization annealing process, or after further nitriding treatment (after the nitriding treatment process), and after drying, finish annealing is performed.

[0075] In conventional methods for manufacturing grain-oriented electrical steel sheets, a forsterite-based coating was formed on the surface of the steel sheet (cold-rolled sheet) by applying an annealing separating agent mainly composed of MgO and performing finish annealing. In contrast, in the manufacturing method of grain-oriented electrical steel sheets according to this embodiment, an annealing separating agent containing Al2O3 is used so that almost no forsterite-based coating is formed.

[0076] On the other hand, while the proportion of Al2O3 may be 100% by mass, in order to prevent Al2O3 from burning onto the surface of the steel sheet, it is preferable that the annealing separating agent in the manufacturing method of grain-oriented electrical steel sheet according to this embodiment contains MgO. While the amount of MgO may be 0%, in order to obtain the above effect, it is preferable that the proportion of MgO be 5% by mass or more. When MgO is included, the proportion of MgO should be 90% by mass or less in order to ensure that 10% by mass or more of Al2O3 is secured. Preferably, the proportion of MgO is 50% by mass or less.

[0077] Furthermore, in the method for manufacturing grain-oriented electrical steel sheets according to this embodiment, the annealing separating agent may further contain chlorides. Including chlorides in the annealing separating agent has the effect of making it more difficult for forsterite-based coatings to form. The chloride content is not particularly limited and may be 0%, but 0.5 to 10% by mass is preferred to obtain the above effect. Examples of effective chlorides include bismuth chloride, calcium chloride, cobalt chloride, iron chloride, nickel chloride, etc.

[0078] The finish annealing conditions are not limited, but for example, conditions such as holding at a temperature of 1150-1250°C for 10-60 hours can be used.

[0079] [Annealing Separating Agent Removal Process] In the annealing release agent removal process, excess annealing release agent is removed from the steel sheet after the finish annealing process. For example, excess annealing release agent can be removed by washing with water.

[0080] [Pickling process] In the pickling process, the steel sheet, after the annealing separation agent removal process, is pickled with 0.1 to 10% by mass of one inorganic acid selected from sulfuric acid, phosphoric acid, nitric acid, and chloric acid, heated to 30 to 85°C, for 1 to 20 seconds. By pickling under these conditions, the forsterite-based coating can be sufficiently removed, and if any remains, MgO can be removed. However, in the pickling process of this embodiment, it is difficult to form so-called pickling pits.

[0081] [Soaking process] [Drying process] In the immersion process, the steel sheet after the pickling process is immersed in a treatment solution containing metal phosphate and nitric acid for 5 to 150 seconds. In the drying process, the steel sheet after the immersion process is removed from the treatment solution, excess solution is removed, and then it is dried. This forms an intermediate layer on the surface of the base steel sheet.

[0082] During the immersion process, the treatment solution is adjusted to satisfy the following conditions (A) to (F).

[0083] (A) Metal ion concentration: 1.0~10.0 g / L (B) Phosphate ion concentration: 2.0~25.0 g / L The treatment solution contains a metal phosphate salt and nitric acid as an oxidizing agent. Examples of metal phosphates include zinc phosphate, manganese phosphate, zinc calcium phosphate, and iron manganese phosphate, but zinc phosphate is preferred from the viewpoint of controlling the surface roughness index. The metal ion concentration and phosphate ion concentration in the processing solution should be 1.0 to 10.0 g / L and 2.0 to 25.0 g / L, respectively. If the metal ion concentration is less than 1.0 g / L, the phosphate deposition rate will be relatively slow, leading to etching of the steel plate surface. This increases the degree of surface roughness at the interface, increases the surface roughness index L / W, and may degrade the magnetic properties. Similarly, if the phosphate ion concentration is less than 2.0 g / L, the degree of surface roughness at the interface will increase, the surface roughness index L / W will increase, and may degrade the magnetic properties. Therefore, it is preferable that the metal ion concentration and phosphate ion concentration be 1.0 g / L or higher and 2.0 g / L or higher, respectively. On the other hand, if the metal ion concentration exceeds 10.0 g / L, the pH of the treatment solution increases, and the deposition rate of metal phosphate salts decreases, increasing the degree of surface roughness at the interface, which increases the surface roughness index L / W and may degrade the magnetic properties. Similarly, if the phosphate ion concentration exceeds 25.0 g / L, the degree of surface roughness at the interface increases, which increases the surface roughness index L / W and may degrade the magnetic properties. Therefore, it is preferable that the metal ion concentration and phosphate ion concentration be 10.0 g / L or less and 25.0 g / L or less, respectively.

[0084] (C) Nitrate ion concentration: 2.0~40.0 g / L As described above, nitric acid is added as an oxidizing agent to the processing solution according to this embodiment. By using nitric acid as an oxidizing agent, the generation of hydrogen gas can be prevented, and a dense intermediate layer can be efficiently formed. However, if the amount of nitric acid added to the processing solution is excessively large, the surface of the steel plate will be excessively oxidized, the phosphate deposition rate will decrease, and as a result, the degree of surface roughness at the interface will increase, the roughness index L / W will increase, and the magnetic properties may deteriorate. For this reason, the nitrate ion concentration is preferably 40.0 g / L or less. More preferably 25.0 g / L or less. On the other hand, if the amount of nitric acid added to the processing solution is excessively small, the generation of hydrogen gas cannot be suppressed, and areas with insufficient phosphate deposition will be formed in some parts, and as a result, the degree of surface roughness at the interface will increase, the roughness index L / W will increase, and the magnetic properties may deteriorate. For this reason, the acid ion concentration is preferably 2.0 g / L or more.

[0085] (D) Iron ion concentration: 1.0~20.0 g / L It is presumed that the appropriate presence of iron ions in the treatment solution suppresses the etching action of the metal phosphate treatment solution and reduces the rate at which the steel sheet surface is dissolved. To obtain this effect, it is desirable to have a concentration of 1.0 g / L or more. On the other hand, if there is an excess of iron ions in the treatment solution, iron will precipitate in the resulting intermediate layer, reducing adhesion, so it is preferable to have a concentration of 20.0 g / L or less.

[0086] (E) Ratio of phosphate ion concentration to metal ion concentration: 1.5~5.0 As stated above, from the viewpoint of optimizing the surface roughness index, it is effective to keep the phosphate concentration and metal ion concentration within the above range, but it is also preferable to adjust the ratio of phosphate ion concentration to metal ion concentration. Specifically, the ratio of phosphate ion concentration to metal ion concentration (phosphate ion concentration / metal ion concentration) should be between 1.5 and 5.0. If the ratio of phosphate ion concentration to metal ion concentration is too high, etching may proceed, causing the steel plate to become excessively rough and potentially resulting in a lower packing efficiency. Therefore, it is preferable to keep the ratio of phosphate ion concentration to metal ion concentration at 5.0 or less. On the other hand, if the ratio of phosphate ion concentration to metal ion concentration is too low, the pH may rise, preventing the precipitation of metal phosphate salts, or causing precipitation to take a very long time. Therefore, it is preferable to keep the ratio of phosphate ion concentration to metal ion concentration at 1.5 or higher.

[0087] (F) Ratio of nitrate ion concentration to phosphate ion concentration: 0.5~10.0 Furthermore, from the viewpoint of deposition rate, the ratio of nitrate ion concentration to phosphate ion concentration is also adjusted. Specifically, the ratio of nitrate ion concentration to phosphate ion concentration (nitrate ion concentration / phosphate ion concentration) should be between 0.5 and 10.0. If the ratio of nitrate ion concentration to phosphate ion concentration is too high, there is a risk of excessive etching of the steel plate. Therefore, it is preferable to keep the ratio of nitrate ion concentration to phosphate ion concentration at 10.0 or less. On the other hand, if the ratio of nitrate ion concentration to phosphate ion concentration is too low, the deposition of metal phosphate salts may be suppressed, and it may take a long time to form the intermediate layer. Therefore, it is preferable to keep the ratio of nitric acid concentration to phosphate ion concentration at 0.5 or higher.

[0088] The temperature of the treatment solution should preferably be between 20 and 85°C, and the immersion time between 5 and 150 seconds. If the temperature is below 20°C or the treatment time is less than 5 seconds, the formation of the intermediate layer may be insufficient, potentially resulting in poor adhesion. On the other hand, if the temperature is above 85°C or the treatment time is above 150 seconds, areas where crystalline metal phosphate salts precipitate excessively may occur in the intermediate layer, potentially resulting in poor packing density.

[0089] In the drying process, if the drying temperature is too high, voids may form and adhesion may be poor, so it is preferable to keep the drying temperature below 300°C. More preferably below 200°C. It is preferable to keep the drying temperature above 100°C.

[0090] [Tension coating layer formation process] In the tension coating layer formation process, a coating solution containing a metal phosphate salt and colloidal silica, with a concentration of 10-40% by mass, is applied to the steel sheet after the drying process. After drying, the sheet is heated and held at a temperature of 700-950°C for 10-50 seconds to form a tension coating layer on the surface of the intermediate layer.

[0091] If the plate temperature during holding is below 700°C, the tension will be low and the magnetic properties will be inferior. Therefore, it is preferable to keep the plate temperature at 700°C or higher. On the other hand, if the plate temperature exceeds 950°C, the rigidity of the steel plate decreases and it becomes more susceptible to deformation. In this case, the steel plate may become distorted due to transport, etc., and the magnetic properties may be inferior. Therefore, it is preferable to keep the plate temperature at 950°C or lower.

[0092] Furthermore, if the holding time is less than 10 seconds, the elution properties will be inferior. Therefore, the holding time should be 10 seconds or more. On the other hand, if the holding time exceeds 50 seconds, the adhesion of the tension coating layer may be inferior. Therefore, a holding time of 50 seconds or less is preferable.

[0093] The coating solution (insulating film solution) is prepared so that it contains a total of 10-40% by mass of metal phosphate salts and colloidal silica, based on solid content. If the combined concentration of metal phosphate and colloidal silica is less than 10% by mass, the applied treatment solution may flow easily, potentially resulting in uneven application. Conversely, if the combined concentration of metal phosphate and colloidal silica exceeds 40% by mass, the viscosity may be too high, potentially causing uneven application or patterns.

[0094] As the metal phosphate salt, one or more selected from aluminum phosphate, zinc phosphate, magnesium phosphate, nickel phosphate, copper phosphate, lithium phosphate, cobalt phosphate, etc., or a mixture of two or more can be used. Aluminum phosphate is particularly preferred.

[0095] The coating solution may also contain additional elements such as vanadium, tungsten, molybdenum, and zirconium. If these elements are included, they can be added to the coating solution, for example, as an oxygen acid.

[0096] Colloidal silica can be of either S-type or C-type. S-type colloidal silica refers to silica solutions that are alkaline, while C-type colloidal silica refers to silica particles treated with aluminum, resulting in a silica solution that is alkaline to neutral. S-type colloidal silica is widely used and relatively inexpensive, but caution is needed as it may aggregate and precipitate when mixed with acidic metal phosphate solutions. C-type colloidal silica is stable even when mixed with metal phosphate solutions and does not precipitate, but it is relatively more expensive due to the increased processing steps. It is preferable to choose the type of colloidal silica based on the stability of the coating solution being prepared.

[0097] [Magnetic domain refining process] The manufacturing method for grain-oriented electrical steel sheets according to this embodiment may further include a magnetic domain subdivision step in which magnetic domains are subdivided in the steel sheet after the tension coating layer formation step. By performing magnetic domain subdivision processing, iron loss in grain-oriented electrical steel sheets can be further reduced. Methods for refining magnetic domains include a method of narrowing the width of 180° magnetic domains (refining 180° magnetic domains) by forming linear or point-shaped 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 (refining 180° magnetic domains) by forming linear or point-shaped stress-strain areas or grooves extending in a direction intersecting the rolling direction at predetermined intervals along the rolling direction. When forming stress-strained areas, laser beam irradiation and electron beam irradiation can be applied. Furthermore, when forming grooves, mechanical groove formation methods using gears, chemical groove formation methods using electrolytic etching, and thermal groove formation methods using laser irradiation can be applied. If the insulating coating is damaged due to the formation of stress-strained areas or grooves, and its properties such as insulation deteriorate, the insulating coating may be reapplied to repair the damage. [Examples]

[0098] A slab was cast containing, by mass%, C: 0.08%, Si: 3.31%, sol.Al: 0.028%, N: 0.008%, Mn: 0.07%, and S: less than 0.0005%, with the remainder being Fe and impurities. The slab was heated to 1350°C, then hot-rolled to produce a hot-rolled sheet with 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). Subsequently, the hot-rolled sheet was cold-rolled to produce a cold-rolled sheet with a thickness of 0.22 mm. This cold-rolled sheet was subjected to decarburization annealing under the condition of being held at 830°C for 90 seconds.

[0099] After decarburization annealing, an annealing separation agent containing 95% MgO and Al2O3 and 5% Bismuth chloride (BiCl3) was applied, dried, and then a finish annealing was performed at 1200°C for 20 hours. After the final annealing, the steel sheet was rinsed with water to remove excess annealing separator, and no forsterite-based coating was formed on the surface. This steel plate was lightly pickled with 3% by mass sulfuric acid at 80°C for 10 seconds.

[0100] After light pickling, an intermediate layer was formed using the treatment solution shown in Table 1. Steel wool was used as the source of iron ions, and the treatment solution was adjusted to the concentrations shown in Table 1. The immersion conditions were as shown in Table 1. The resulting intermediate layer was as shown in Table 1.

[0101] Subsequently, an insulating coating solution mainly composed of metal phosphate salts and colloidal silica, as shown in Table 2, was applied, and the sheet was dried at 850°C for 20 seconds to form a tension coating layer on the steel sheet surface. Note that the "molar ratio of metal elements" in Table 2 indicates the relative abundance of metal elements when two or more metal elements are present in a metal phosphate salt. The thicknesses of the insulating coatings (intermediate layer and tension coating layer) were as shown in Table 2. The tension coating layer consisted substantially of metal phosphate and silica.

[0102] The obtained steel sheet (grain-oriented electrical steel sheet) was subjected to magnetic domain refinement by irradiating it with a laser beam under conditions of a UA (irradiation energy density) of 2.0 J and an irradiation interval of 5.0 mm pitch. The iron loss W17 / 50 (iron loss at 50Hz at 1.7T) of the steel plate after magnetic domain subdivision treatment was measured using the Single Sheet Tester (SST) method in accordance with JIS C2556 (2015). If the iron loss W17 / 50 was 0.68 W / kg or less, it was determined that good magnetic properties were ensured. Furthermore, the space utilization rate was measured according to the following procedure.

[0103] [Occupancy rate] The packing factor was measured according to the method compliant with JIS C 2550-5 (2020). Thirty test specimens, each 30 mm wide and 320 mm long, were used. After measuring the total mass of the sample, the packing factor was calculated by measuring the distance between the upper and lower backing plates sandwiching the laminate while under a pressure of 1 MPa. If the occupancy rate is 96.0% or higher, it is judged that a high occupancy rate has been secured.

[0104] Furthermore, the film adhesion, film tension, corrosion resistance, and elution resistance of the steel plates after magnetic domain refinement treatment were evaluated using the following methods. The interface roughness index was also determined using the following method. The results are shown in Tables 2 and 3.

[0105] [Interface roughness index] First, a scanning electron microscope was used to observe the region including the interface between the base steel sheet and the intermediate layer at 5000x magnification, and an observation image was obtained. Then, the distance between two points connected by a straight line at both ends of the interface between the base steel sheet and the intermediate layer in the observation image was defined as the observation image width W, and this observation image width W was determined. Furthermore, the actual interface path (i.e., the curved path tracing the actual interface) was set as the interface length L, and the interface length L was determined, and the surface roughness index L / W was calculated. The observation image width W and interface length L were determined using the application system "LuzeX AP" manufactured by Nireco Corporation on the cross-sectional image obtained by the electron microscope.

[0106] [Coating adhesion] The adhesion of the coating was evaluated by taking a sample measuring 30 mm in width and 300 mm in length from a steel plate, performing stress-relieving annealing at 800°C for 2 hours in a nitrogen atmosphere, then wrapping it around a 10 mm diameter cylinder and unwrapping it, followed by a bending adhesion test, and measuring the degree of coating peeling (area ratio). The evaluation criteria were as follows, and a rating of ◎ or ○ indicated excellent film adhesion. ◎: Peeling area ratio 0-0.5%. ○: Peeling area ratio greater than 0.5% and 5.0% or less. △: Peeling area ratio exceeds 5.0%.

[0107] [Coating tension] The coating tension was calculated by working backward from the curvature of the insulating coating after peeling off one side. A coating tension of 4.0 MPa or higher was considered sufficient.

[0108] [Corrosion resistance] Corrosion resistance was tested according to the JIS salt spray test (JIS Z2371:2015), by allowing a 5% NaCl aqueous solution to naturally fall onto the sample in a 35°C atmosphere for 7 hours. Subsequently, the rusted area was evaluated on a 10-point scale. The evaluation criteria were as follows: Regarding corrosion resistance, a score of 5 or higher was considered to indicate excellent corrosion resistance. 10: No rust occurred. 9: Very little rust occurs (area ratio of 0.10% or less) 8. Percentage of rusted area = over 0.10% and under 0.25% 7. Percentage of rusted area = over 0.25% and under 0.50% 6. Percentage of rusted area = over 0.50% and under 1.0% 5. Percentage of rusted area = over 1.0% and 2.5% or less 4. Percentage of rusted area = Over 2.5% to 5.0% or less 3: Area percentage where rust occurred = more than 5.0% but less than or equal to 10% 2: Percentage of rusted area = over 10% and 25% or less 1: Area percentage where rust has occurred = more than 25% but less than or equal to 50%

[0109] [Elution resistance] Dissolution resistance was evaluated by whether the reagent could suppress the elution of phosphate from the sample. The amount of eluted phosphoric acid was measured by boiling the sample in boiling pure water for 10 minutes, measuring the amount of phosphoric acid dissolved in the pure water, and dividing the amount of phosphoric acid by the area of ​​the insulating coating on the boiled grain-oriented electrical steel sheet. The amount of phosphoric acid dissolved in the pure water was calculated by cooling the pure water (solution) in which the phosphoric acid had dissolved, and then measuring the phosphoric acid concentration of the sample obtained by diluting the cooled solution with pure water using ICP-AES. The amount of elution is 40 mg / m². 2 If the value is less than this, it is considered to have excellent elution resistance.

[0110] [Table 1]

[0111] [Table 2]

[0112] [Table 3]

[0113] As can be seen from Tables 1 to 3, the present invention demonstrates extremely excellent properties, including film adhesion, and improves iron loss and space factor. On the other hand, the comparative example was inferior in one or more of the following aspects: coating adhesion, magnetic properties, corrosion resistance, elution resistance, and transformer (core) packing ratio. [Explanation of symbols]

[0114] 100 grain-oriented electrical steel sheet 1 Base steel plate 2. Insulating coating 21 Middle Class 22 Tension coating layer [Industrial applicability]

[0115] According to the above embodiment of the present invention, a grain-oriented electrical steel sheet is obtained that exhibits excellent adhesion and magnetic properties of the tension coating and does not reduce the packing factor of the transformer (core). Therefore, the obtained grain-oriented electrical steel sheet can be suitably applied as a core material for transformers, and thus has high industrial applicability.

Claims

1. Base material steel plate and An insulating coating formed on the surface of the base steel plate, It has, The insulating coating, An intermediate layer formed on the base steel sheet side, containing a crystalline metal phosphate salt, The insulating film has a tension coating layer formed on the surface side, A grain-oriented electrical steel sheet characterized in that, in a cross-section along the thickness direction of the intermediate layer, when the interface between the base steel sheet and the intermediate layer is observed at 5000x magnification using a scanning electron microscope, the ratio of the interface length L to the observed image width W is 100.0 to 120.0%.

2. The grain-oriented electrical steel sheet according to claim 1, characterized in that the tension coating layer contains a metal phosphate salt and silica.

3. The grain-oriented electrical steel sheet according to claim 1 or 2, characterized in that the thickness of the intermediate layer is 1.0 to 9.0 μm.

4. A method for forming the insulating coating on a grain-oriented electrical steel sheet according to claim 1, On the steel plate, Al ２ O ３ A finish annealing step involves applying an annealing release agent containing 10 to 100% by mass of [the substance], drying it, and then performing finish annealing. An annealing separating agent removal step is performed on the steel plate after the finish annealing step to remove any excess annealing separating agent, The steel plate after the annealing separating agent removal step is subjected to an acid pickling step in which it is pickled with 0.1 to 5% by mass of one inorganic acid selected from sulfuric acid, chloric acid, nitric acid, and phosphoric acid for a period of 1 to 20 seconds. The process involves immersing the steel sheet after the pickling process in a treatment solution containing a metal phosphate salt, an oxidizing agent, and iron ions, A drying step is performed in which the steel plate after the immersion step is removed from the processing liquid, the excess processing liquid is removed, and then the plate is dried. A tensile coating layer formation step is performed, wherein a coating solution containing a metal phosphate salt and colloidal silica, with a total concentration of the metal phosphate salt and colloidal silica being 10 to 40% by mass in terms of solid content, is applied to the steel sheet after the drying step, and the sheet is dried, and then the sheet temperature is maintained at 700 to 950°C for 10 to 120 seconds. Equipped with, The aforementioned processing liquid Metal ion concentration: 1.0–10.0 g / L Phosphate ion concentration: 2.0–25.0 g / L Nitrate ion concentration is 2.0 to 40.0 g / L. Iron ion concentration is 1.0 to 20.0 g / L. And, The ratio of phosphate ion concentration to metal ion concentration is 1.5 to 5.

0. A method for forming an insulating coating, characterized in that the ratio of nitrate ion concentration to phosphate ion concentration is 0.5 to 10.

0.

5. The method for forming an insulating coating according to claim 4, characterized in that, in the immersion step, the steel plate after the pickling step is immersed in the treatment liquid having a liquid temperature of 20 to 85°C for 2 to 60 seconds.

6. The method for forming an insulating film according to claim 4, characterized in that the nitrate ion concentration is 2.0 to 25.0 g / L.

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