JP7795149B2 - Grain-oriented electrical steel sheet and method for forming insulating coating - Google Patents

Description

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. 2023-064836, filed on April 12, 2023, the contents of which are incorporated herein by reference.

Grain-oriented electrical steel sheets are primarily used in transformers. Transformers are continuously excited over long periods of time, from installation to disposal, and continue to generate energy loss. Therefore, the energy loss when magnetized with alternating current, i.e., iron loss, is a key indicator that determines the performance of a transformer.

To reduce the iron loss of grain-oriented electrical steel sheets, many technologies have been developed to date, including (a) increasing the concentration 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.

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

For example, the method disclosed in Patent Document 1, in which a coating liquid primarily composed of colloidal silica and phosphate is baked onto the surface of a steel sheet to form an insulating coating, is an effective method for reducing iron loss because it effectively applies tension to the steel sheet. Therefore, the typical method for manufacturing grain-oriented electrical steel sheets is to leave the forsterite-based coating formed in the final annealing process and then apply an insulating coating primarily composed of phosphate on top of it.

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 themselves nonmagnetic and have an uneven structure at the interface between the steel sheet and the coating. This uneven structure is thought to hinder domain wall movement and adversely affect iron loss.
Therefore, as means for improving high magnetic field iron loss, research has been conducted on a variety of techniques, including methods for removing the forsterite-based coating (inorganic coating) by mechanical means such as polishing or by chemical means such as pickling, techniques for producing grain-oriented electrical steel sheets that do not have a forsterite-based coating by preventing the formation of a forsterite-based coating 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).

As a technique for preventing the formation of forsterite-based 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 even more excellent iron loss improvement effects can be achieved by forming a tensioned insulating coating on the surface of grain-oriented electrical steel sheet that does not have a forsterite-based coating and is obtained using such a known method. Furthermore, in addition to improving iron loss, tensioned insulating coatings can also impart various other properties, such as corrosion resistance, heat resistance, and slip resistance.

However, in addition to exhibiting insulating properties, forsterite-based coatings also function as intermediate layers that ensure adhesion when forming tension coatings (tension-applying insulating coatings). That is, because forsterite-based coatings are formed in a state where they penetrate deeply into the steel sheet, they have excellent adhesion to the metal steel sheet. Therefore, when a tension-applying coating (tension coating) containing colloidal silica, phosphate, or the like as a main component is formed on the surface of a forsterite-based coating, the coating has excellent adhesion. However, because bonding between metals and oxides is generally difficult, it has been difficult to ensure sufficient adhesion between a tension coating and the steel sheet surface in the absence of a forsterite-based coating.
Therefore, when forming a tension coating on a grain-oriented electrical steel sheet that does not have a forsterite-based coating, it is being considered to provide a layer that takes the role of the intermediate layer of the forsterite-based coating.

For example, Patent Document 3 discloses a technique in which a grain-oriented electrical steel sheet not having a forsterite-based coating (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 a grain-oriented electrical steel sheet not having a forsterite-based coating (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.

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, new electrolytic processing equipment must be prepared in order to perform anodic electrolytic processing in a silicate aqueous solution to obtain an _SiO2 layer on the steel sheet surface that maintains sufficient adhesion with the tension-applying insulating coating, which results in a problem of processing costs.

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.

Japanese Patent Publication No. 48-039338 Japanese Patent Publication No. 49-96920 Japanese Patent Publication No. 6-184762 Japanese Patent Application Publication No. 11-209891 International Publication No. 2022/215709

The technology of Patent Document 5 provides an intermediate layer made of a crystalline metal phosphate between the base steel sheet and the tension coating, thereby improving the coating adhesion, tension, and magnetic properties. Furthermore, since the intermediate layer can be formed by chemical conversion treatment, no special equipment is required. Therefore, this is a useful technology.
However, as a result of investigations by the present inventors, it was found that improving the adhesion of the grain-oriented electrical steel sheet of Patent Document 5 may result in a deterioration in the magnetic properties of the transformer. Further investigation into this point revealed that the cause of the deterioration in magnetic properties is that the crystals of the metal phosphate precipitated by the chemical conversion treatment become coarse, which reduces the space factor when an actual transformer is manufactured.

Therefore, the objective of this invention is to provide a grain-oriented electrical steel sheet in which a layer containing metal phosphate is formed on the surface of a forsterite-based coating steel sheet by chemical conversion treatment, which has excellent adhesion and magnetic properties of the tensile coating and does not reduce the space factor of the transformer (core). However, this is premised on not reducing the basic properties required of the coating, such as corrosion resistance and resistance to phosphate leaching.

The inventors have discovered that when forming a layer containing metal phosphate as an intermediate layer to improve adhesion between the base steel sheet and the tensile coating layer, the coarsening of metal phosphate crystals can be suppressed by adding a substance to the chemical conversion treatment solution that suppresses phosphate crystallization.

The present invention has been made in light of the above findings.
[1] A grain-oriented electrical steel sheet according to one embodiment of the present invention is a grain-oriented electrical steel sheet that does not have a forsterite-based coating , and that comprises a base steel sheet and an insulating coating formed on the surface of the base steel sheet, wherein the insulating coating comprises an intermediate layer formed on the base steel sheet side and containing a crystalline metal phosphate and one or more of amorphous silica, an inorganic filler, and a metal oxide , and a tensile coating layer formed on the surface side of the insulating coating and containing a metal phosphate and silica , wherein the intermediate layer has a Si content of less than 10 mass %, the tensile coating layer has a Si content of 10 mass % or more, and the amorphous silica, the inorganic filler, and the metal oxide or more have an average particle size of 10 to 500 nm.
[2] In the grain-oriented electrical steel sheet according to [1], the inorganic filler may contain one or more of alumina, BN, AlN, and kaolin.
[3] In the grain-oriented electrical steel sheet according to [1] or [2], the metal oxide may be one or more of titanium oxide, zinc oxide, and calcium oxide.
[4] In the grain-oriented electrical steel sheet according to any one of [1] to [3], the crystalline metal phosphate may have an average crystal grain size of 1.0 to 12.0 μm.
[5] A method for forming an insulating coating according to another aspect of the present invention is a method for forming the insulating coating provided on the grain-oriented electrical steel sheet according to [1], comprising the steps of: a finish annealing step of applying an annealing separator containing 10 to 100 mass% of Al ₂ O ₃ to a steel sheet, drying the steel sheet, and then finish annealing the steel sheet; an annealing separator removal step of removing excess annealing separator from the steel sheet after the finish annealing step; a light pickling step of pickling the steel sheet after the annealing separator removal step for 1 to 20 seconds at a solution temperature of 30 to 85°C with a pickling solution containing 0.10 to 10.0 mass% of one inorganic acid selected from sulfuric acid, chloric acid, nitric acid, and phosphoric acid; and a light pickling step of pickling the steel sheet after the light pickling step for 1 to 20 seconds at a solution temperature of 30 to 85°C with 0.3 to 10.0 mass% of a metal phosphate and 0.01 to 10.0 g/L of an average particle size of 1000 μm or more. the steel sheet after the immersion step is removed from the treatment solution, and excess treatment solution is removed, followed by drying; and a tensile coating layer forming step is performed in which a coating solution containing a metal phosphate and colloidal silica, the metal phosphate and the colloidal silica having a total concentration of 10 to 40 mass %, is applied to the steel sheet after the drying step, dried, and then heated and maintained at a sheet temperature of 700 to 950°C for 10 to 50 seconds.

According to the above aspect of the present invention, it is possible to provide a grain-oriented electrical steel sheet that has excellent adhesion and magnetic properties of the tension coating and does not reduce the space factor of the transformer (core).

1 is an example of a cross-sectional view of a grain-oriented electrical steel sheet according to an embodiment of the present invention.

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.

1 , the 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. In the grain-oriented electrical steel sheet 100 according to this embodiment, a forsterite-based coating is not intentionally formed on the surface of the base steel sheet 1, and in many cases a forsterite-based coating is not present, but its presence is permissible as long as the coating mass is 1.0 g/ ^m2 or less (in which case, it is present between the base steel sheet 1 and the insulating coating 2).
The insulating coating 2 also 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.
The intermediate layer 21 also contains a crystalline metal phosphate and one or more of amorphous silica having an average particle size of 10 to 500 nm, an inorganic filler, and a metal oxide.
Each of these will be explained below.

<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 following chemical components are included. In this embodiment, percentages relating to chemical components are mass % unless otherwise specified.

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 process. However, if the C content exceeds 0.010%, the magnetic properties of the finished grain-oriented electrical steel sheet 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. Although a lower C content is preferable, reducing the C content to less than 0.0001% saturates the effect of structural control and simply increases manufacturing costs. Therefore, the C content may be 0.0001% or more.

Si: 2.50-4.00%
Silicon (Si) is an element that increases the electrical resistance of grain-oriented electrical steel sheets and improves their iron 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.

Mn: 0.01-0.50%
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 the steel. Mn also improves the hot workability of the 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.

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 base steel sheet of 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, the lower limit of the N content is not particularly specified, but reducing it to less than 0.001% would only increase the manufacturing cost, so the N content may be 0.001% or more.

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 sheets. 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% only increases manufacturing costs. Therefore, the sol. Al content may be 0.0001% or more.

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. The S content in the grain-oriented electrical steel sheet is preferably as low as possible. For example, less than 0.001%. However, reducing the S content in the base steel sheet of the grain-oriented electrical steel sheet to less than 0.0001% will only increase the manufacturing cost. Therefore, the S content in the base steel sheet of the grain-oriented electrical steel sheet may be 0.0001% or more.

The balance: 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 balance 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 than these, such as one or more of W, Nb, Ti, Ni, Co, V, Cr, and Mo, are contained 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.

Sn: 0-0.50%
Sn (tin) is an element that contributes to improving magnetic properties through controlling the primary recrystallization 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.

Cu: 0-0.50%
Cu (copper) 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.

Se: 0-0.020%
Se (selenium) is an element that has a magnetic property improving effect. When Se is contained, the Se content is preferably 0.001% or more in order to effectively exhibit the magnetic property improving effect. 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, more preferably 0.015% or less, and even more preferably 0.010% or less.

Sb: 0-0.50%
Sb (antimony) is an element that has a magnetic property improving effect. When Sb is contained, the Sb content is preferably 0.005% or more in order to effectively exhibit the magnetic property improving effect. 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, more preferably 0.30% or less, and even more preferably 0.10% or less.

As described above, in this embodiment, the chemical composition of the base steel sheet of the directional electrical steel sheet is, for example, one that contains the above-mentioned elements, with the remainder consisting of Fe and impurities.

The chemical composition of the base steel sheet of the grain-oriented electrical steel sheet according to this embodiment can be measured using the known ICP atomic emission spectroscopy method. However, if an insulating coating is formed on the surface, it must be removed before measurement. The removal method involves 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.

<Insulating coating>
The grain-oriented electrical steel sheet 100 according to this embodiment has an insulating coating 2 formed on the surface of a base steel sheet 1 .
The insulating coating 2 is made up of an intermediate layer 21 and a tensile coating layer 22 in this order from the base steel sheet 1 side.

(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 forsterite are being studied to further improve magnetic properties. However, if there is no forsterite-based coating, it is difficult to ensure sufficient adhesion between the tensile coating and the surface of the base steel sheet.

In the grain-oriented electrical steel sheet 100 of this embodiment, an intermediate layer 21 containing a crystalline metal phosphate is formed between the base steel sheet 1 and the tensile coating, thereby improving the adhesion between the base steel sheet 1 and the tensile coating layer 22 via the intermediate layer 21.
When the intermediate layer 21 contains crystalline metal phosphate, the tensile coating formed thereon (which becomes the tensile coating layer 22 after formation) also contains metal phosphate, resulting in high affinity and excellent adhesion between the intermediate layer and the tensile coating layer. Furthermore, when the intermediate layer is formed by immersion in a treatment solution containing metal phosphate, as described below, it can be formed on the surface of the base steel sheet 1 by utilizing a chemical reaction, and adhesion between the intermediate layer 21 and the base steel sheet 1 can also be ensured.
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 is preferably 80 mass % or more, preferably 90 mass % or more, and may be 99 mass % or more. In terms of adhesion, the metal phosphate is preferably one or more of zinc phosphate, manganese phosphate, iron phosphate, and zinc calcium phosphate.
The intermediate layer may contain oxides and elements such as Fe and Si diffused from the base steel sheet as the remainder of the metal phosphate.

However, if the crystals of the crystalline metal phosphate in the intermediate layer become coarse, when an actual transformer is manufactured, the space factor decreases, which reduces the magnetic flux density per unit volume and increases the transformer iron loss.
Therefore, in the grain-oriented electrical steel sheet according to this embodiment, in order to suppress the crystallization of the crystalline metal phosphate and thereby suppress the coarsening of the crystals, one or more of colloidal silica, inorganic filler, and metal oxide having an average particle size of 10 to 500 nm are added as additives to the treatment solution used to form the intermediate layer. As a result, the intermediate layer is formed containing the crystalline metal phosphate and one or more of amorphous silica, inorganic filler, and metal oxide, with the amorphous silica, inorganic filler, and metal oxide having an average particle size of 10 to 500 nm. In such an intermediate layer, the average crystal grain size of the crystalline metal phosphate is, for example, 1.0 to 12.0 μm.
At least one of colloidal silica, inorganic filler, and metal oxide having an average particle size of 10 to 500 nm is preferably dispersed uniformly in the intermediate layer without localization, for example, in the form of aggregates of several μm or less even if secondary aggregation occurs.

Among the amorphous silica, inorganic fillers, and metal oxides that are added as additives and remain in the intermediate layer, amorphous silica is preferred in terms of ease of availability. Amorphous silica differs in both form and effect from crystalline silica that is produced by thermal oxidation annealing or the like.
The inorganic filler preferably contains one or more of alumina, BN, AlN, and kaolin in an amount of 90% by mass or more (with a purity of 90% by mass or more). BN preferably has a hexagonal crystal structure in consideration of ease of dispersion.
In addition, the metal oxide is preferably one or more of titanium oxide, zinc oxide, and calcium oxide from the viewpoint of the stability of the treatment liquid.
Carbonates can also be used as additives, but this is not preferred because carbonates do not remain in the intermediate layer under normal baking conditions and special baking conditions are required to obtain an intermediate layer containing carbonates.
The content of one or more of amorphous silica, inorganic filler, and metal oxide is preferably 0.01 to 1.00% by mass. If it is less than 0.01% by mass, the effect of suppressing crystal coarsening of the metal phosphate may be inferior, and if it exceeds 1.00% by mass, there is a concern that the adhesion of the intermediate layer may be inferior.

The intermediate layer 21 is formed at a different time from the tensile coating formed on top of it, but both the intermediate layer 21 and the tensile coating layer 22 function as the insulating coating 2.

The thickness of the intermediate layer 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 and the insulating coating via the intermediate layer may not be sufficient. On the other hand, if the average thickness of the intermediate layer exceeds 9.0 μm, the magnetic properties may deteriorate.

The mass fraction and type of the crystalline metal phosphate in the intermediate layer can be determined by measuring a cross section of the intermediate layer in the thickness direction using a scanning electron microscope 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.
The base steel plate and the insulating coating can be distinguished by the P (phosphorus) concentration (if the P content is 1.0 mass% or more, it is an insulating coating, and if it is less than 1.0 mass%, it is determined to be a steel plate). Of the insulating coating 2, the intermediate layer 21 and the tensile coating layer 22 can be distinguished by the difference in Si concentration (if the Si content is 10 mass% or more, it is determined to be a tensile coating layer, and if it is less than 10 mass%, it is determined to be an intermediate layer).

The average crystal grain size of the crystalline metal phosphate can be determined by the following method.
The steel sheet is cut into pieces of several mm square that are easy to observe, and then subjected to ion milling (CP processing) to remove microscopic shape defects such as sagging and cracks. Then, a cross section parallel to the rolling direction and thickness direction of the steel sheet, and a cross section perpendicular to the rolling direction and parallel to the thickness direction of the steel sheet are observed using a scanning electron microscope. The crystal morphology of the metal phosphate observed in the cross section is observed, and the average long and short diameters of each crystal are measured for at least five crystals in each cross section. The measured values are taken as the particle size. The magnification of the electron microscope during observation is 1000x.

The contents and average particle diameters of amorphous silica, inorganic filler, and metal oxide can be determined by the following method.
The content is measured by cutting the steel sheet into pieces of several mm square that are easy to observe, and then performing ion milling (CP processing) to remove microscopic shape defects such as sagging and cracks. Then, cross sections parallel to the rolling direction and thickness direction of the steel sheet, and cross sections perpendicular to the rolling direction and parallel to the thickness direction of the steel sheet, are observed with a scanning electron microscope at a magnification of 5000x. The content is measured by analyzing the intermediate layer portion at five or more cross sections on each of the cross sections using an energy dispersive elemental analyzer.
Regarding the average particle diameter, the presence of amorphous silica, inorganic filler, and metal oxide is confirmed by elemental analysis at 10 or more locations in the intermediate layer using a transmission electron microscope on a cross-sectional sample that has also been subjected to ion milling processing, and the particle diameter is then calculated as the average of the major axis and minor axis of the particles observed at a magnification of 20,000 times.

The thickness of the intermediate layer can be determined by the following method.
The cross section of the sample is observed with a scanning electron microscope, and the average total thickness of the intermediate layer and insulating coating can be measured by measuring the thickness at five or more points. The intermediate layer and the insulating coating can be distinguished by the difference in silicon (Si) concentration derived from silica. Therefore, the thickness of the intermediate layer can be calculated by subtracting the thickness of the insulating coating from the total average thickness at each measurement point.

(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 from the viewpoint of adhesion to the intermediate layer 21 (adhesion to the base steel sheet 1 via the intermediate layer 21), it is preferable that the composition be mainly composed of metal phosphate and silica. It is more preferable that the composition be essentially composed of metal phosphate and silica.
The tensile coating layer 22 preferably contains a 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. Furthermore, it preferably contains a total of 70% by mass or more of the metal phosphate and silica. The total of the metal phosphate and silica may be 100% by mass. The remainder other than the metal phosphate and silica may include ceramic particles such as alumina or silicon nitride. Aluminum phosphate is preferred as the metal phosphate in terms of heat resistance.
Although the thickness of the tensile coating layer 22 is not limited, the average thickness of the insulating coating 2 (intermediate layer 21 + tensile coating layer 22) is preferably 1.0 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 1.0 μm, sufficient coating tension cannot be obtained. Furthermore, the amount of phosphate elution increases. This can 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 decreases, deteriorating magnetic properties, or cracks can occur, resulting in reduced adhesion and reduced corrosion resistance.

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 in the thickness direction in the same manner as in the intermediate layer.
As mentioned above, the tension coating layer and the intermediate layer can be distinguished by the Si content.

The thickness of the tensile coating layer can be determined in the same manner as the intermediate layer. The sum of the thickness of the tensile coating layer and the thickness of the intermediate layer is the thickness of the insulating coating.

<Manufacturing method>
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.

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 of hot rolling a steel billet such as a slab having a predetermined chemical composition 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 Al ₂ O ₃ to the steel sheet, drying the steel sheet, and then performing finish annealing;
(VI) an annealing separator removing step of removing excess annealing separator from the steel sheet after the finish annealing step;
(VII) a light pickling step in which the steel sheet after the annealing separator removing step is pickled at a solution temperature of 30 to 85°C with a pickling solution of 0.10 to 10.0 mass% of one inorganic acid selected from sulfuric acid, chloric acid, nitric acid, and phosphoric acid for 1 to 20 seconds;
(VIII) an immersion step of immersing the steel sheet after the light pickling step in a treatment solution having a solution temperature of 30 to 85°C for 5 to 150 seconds, the treatment solution containing 0.3 to 10.0 mass% of a metal phosphate and 0.01 to 10.0 g/L of one or more of colloidal silica, an inorganic filler, and a metal oxide, each having an average particle size of 10 to 500 nm;
(IX) 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;
(X) A tensile coating layer forming step of applying a coating liquid containing a metal phosphate and colloidal silica, the total concentration of the metal phosphate and the colloidal silica being 10 to 40 mass %, to the steel sheet after the drying step, drying the coating liquid, and then heating the steel sheet to maintain a sheet temperature of 700 to 950°C for 10 to 50 seconds.
Furthermore, the method for producing a grain-oriented electrical steel sheet according to this embodiment further includes the steps of:
(XI) a nitriding treatment step of performing a nitriding treatment on the steel sheet between the decarburization annealing step and the finish annealing step;
(XII) A magnetic domain refining step for controlling the magnetic domains of the steel sheet after the tensile coating layer forming step, or both of these steps may be included.
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 (X) tension coating layer forming step (these steps may be collectively referred to as the method for forming an insulating coating), which are mainly related to the formation of an insulating coating, and known conditions may be used for the other steps or conditions not described.
These steps will be described below.

[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.
The chemical composition of the slab may be changed depending on the chemical composition of the base steel sheet of the grain-oriented electrical steel sheet that is ultimately 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.
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 a range of 2.0 to 3.0 mm, for example.

[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.
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.

[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.
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 manufacturing 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.

In the cold rolling process according to this embodiment, the hot-rolled sheet after the hot-rolled sheet annealing process is cold-rolled according to a known method to obtain a steel sheet. For example, the final reduction can be in the range of 80 to 95%. A final reduction of 80% or more is preferable because it is possible to obtain Goss nuclei with a high concentration of {110}<001> orientation in the rolling direction. On the other hand, a final reduction of more than 95% is not preferable because it increases the possibility of unstable secondary recrystallization in the subsequent finish annealing process.
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.

[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 (PH ₂ O/PH ₂ ) in the annealing atmosphere (furnace atmosphere) is set to 0.3 to 0.6, and the annealing temperature is set to 800 to 900°C, and the steel sheet is held for 10 to 600 seconds.

[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 maintained at approximately 700 to 850°C in a nitriding atmosphere (an atmosphere containing hydrogen, nitrogen, and ammonia or other nitriding gases) to perform nitriding. When AlN is used as an inhibitor, it is preferable to set the N content of the steel sheet after the nitriding process to 40 ppm or more by nitriding. On the other hand, if the N content of the steel sheet after the nitriding process exceeds 1000 ppm, excess AlN remains in the steel sheet even after secondary recrystallization is completed in finish annealing. Such AlN causes iron loss degradation. For this reason, it is preferable to set the N content of the steel sheet after the nitriding process to 1000 ppm or less.

[Finishing annealing process]
In the final annealing step, an annealing separator containing 10 to 100 mass% of Al ₂ O ₃ is applied to the steel sheet after the decarburization annealing step or after further nitriding (nitriding step), dried, and then final annealing is performed.
In conventional methods for producing grain-oriented electrical steel sheets, an annealing separator mainly containing MgO is applied and then finish annealing is performed to form a forsterite-based coating on the surface of the steel sheet (cold-rolled sheet). In contrast, in the method for producing grain-oriented electrical steel sheets according to the present embodiment, an annealing separator containing _Al2O3 is used so that _a forsterite-based coating is hardly formed.
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 _a grain-oriented electrical steel sheet according to this embodiment, it is preferable that the annealing separator contains MgO. While the MgO content may be 0%, to obtain the above effect, the proportion of MgO is preferably 5% by mass or more. When _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. It is sufficient that the total of _Al2O3 and MgO exceeds 50% by mass in terms of solid content relative to the annealing separator.
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 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.
The conditions for the finish annealing are not limited, but for example, conditions in which the steel is held at a temperature of 1150 to 1250° C. for 10 to 60 hours can be adopted.

[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.

[Light pickling process]
In the light pickling step, the steel sheet after the annealing separator removal step is pickled with 0.1 to 10.0 mass % of one inorganic acid selected from sulfuric acid, chloric acid, nitric acid, and phosphoric acid for 1 to 20 seconds at a solution temperature of 30 to 85° C. The inorganic acid is preferably one selected from sulfuric acid, nitric acid, and phosphoric acid.
This has the effect of densifying the crystalline metal phosphate.
If the conditions for the mild pickling are not appropriate, the adhesion of the tension coating layer may be poor and the resistance to elution may be poor.

[Soaking process]
[Drying process]
In the immersion step, the steel sheet after the light pickling step is immersed in the treatment solution for 5 to 150 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. As a result, an intermediate layer is formed on the surface of the base steel sheet.
In the immersion step, the treatment solution is adjusted to have a liquid temperature of 30 to 85°C and to contain 0.3 to 10 mass% of a metal phosphate, and 0.01 to 10.0 g/L of one or more of colloidal silica, inorganic filler, and metal oxide, each having an average particle size of 10 to 500 nm.
By including colloidal silica (which becomes amorphous silica in the intermediate layer), inorganic filler, and one or more metal oxides (sometimes referred to as additives), crystallization of the metal phosphate that forms the intermediate layer is suppressed, and as a result, the average crystal grain size of the crystalline metal phosphate in the intermediate layer is reduced.
However, if the content of the additive in the treatment solution is less than 0.01 g/L, sufficient effects cannot be obtained, whereas if it exceeds 10.0 g/L, the treatment solution becomes unstable.
If the average particle size of the additive is less than 10 nm, aggregation occurs, making the treatment liquid unstable, whereas if the average particle size is more than 500 nm, the particles precipitate, making the treatment liquid less dispersible.

If the temperature of the treatment solution is less than 30°C or the treatment time is less than 5 seconds, the adhesion will be poor. On the other hand, if the temperature of the treatment solution is more than 85°C or the treatment time is more than 150 seconds, the average crystal grain size of the crystalline metal phosphate will become too large.
On the other hand, if the metal phosphate content of the treatment solution exceeds 10 mass %, the average crystal grain size of the metal phosphate may become coarse, which may cause a decrease in adhesion. The metal phosphate contained in the treatment solution may be one or more of zinc phosphate, manganese phosphate, and zinc calcium phosphate.
On the other hand, if the metal phosphate content of the treatment solution is less than 0.3 mass %, the formation of the intermediate layer will be slow and the cost will be high industrially. To achieve a uniform thickness of the intermediate layer, the metal phosphate content is preferably 1.0 mass % or more.
Furthermore, if the drying temperature is too high, voids may occur, resulting in poor 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.

[Tension film layer formation process]
In the tensile coating layer forming process, a coating liquid containing metal phosphate and colloidal silica, with a total concentration of the metal phosphate and colloidal silica 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, thereby forming a tensile coating layer on the surface of the intermediate layer.
If the sheet temperature during holding is less than 700°C, the tension will be low and the magnetic properties will be inferior. Therefore, it is preferable that the sheet temperature be 700°C or higher. On the other hand, if the sheet temperature is higher than 950°C, the rigidity of the steel sheet will decrease and it will be more likely to deform. In this case, strain may be introduced into the steel sheet due to transportation, etc., resulting in inferior magnetic properties. Therefore, it is preferable that the sheet temperature be 950°C or lower.
Furthermore, if the holding time is less than 10 seconds, the elution resistance will be poor. Therefore, the holding time is set to 10 seconds or more. On the other hand, if the holding time is more than 50 seconds, the adhesion of the tensile coating layer will be poor. Therefore, the holding time is preferably 50 seconds or less.
The coating liquid (insulating coating solution) contains 10 to 40 mass % of metal phosphate and colloidal silica.
If the total concentration of the metal phosphate and colloidal silica is less than 10% by mass, the applied treatment liquid will tend to flow, causing unevenness in the amount of coating, whereas if it exceeds 40% by mass, the viscosity will be too high, causing uneven patterns and coating.
The metal phosphate may be 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. Aluminum phosphate is preferred in terms of the stability of the treatment solution.

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.
Colloidal silica can be of type S or type C. Type S colloidal silica refers to colloidal silica in which the silica solution is alkaline, while type C colloidal silica refers to silica in which the silica particle surface is aluminum-treated and the silica solution is alkaline to neutral. Type S colloidal silica is widely used and relatively inexpensive, but caution is required as there is a risk of aggregation and precipitation when mixed with an acidic metal phosphate solution. Type C colloidal silica is stable even when mixed with a metal phosphate solution and there is no risk of precipitation, but it is relatively expensive due to the large number of processing steps required. It is preferable to use the appropriate type depending on the stability of the coating liquid to be prepared.

[Magnetic domain refining process]
The method for manufacturing 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 magnetic domain refining treatment, it is possible to further reduce the iron loss of grain-oriented electrical steel sheets.
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-strain portions or grooves extending in a direction intersecting the rolling direction at predetermined intervals along the rolling direction.
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 using 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.

A slab containing, in mass %, 0.08% C, 3.31% Si, 0.028% sol. Al, 0.008% N, 0.07% Mn, less than 0.0005% S, with 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 (annealing of hot-rolled sheet).
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 under conditions of holding at 830°C for 90 seconds.
After decarburization annealing, an annealing separator containing 45 mass% MgO, 50 mass _{% Al2O3} , and 5 mass% _BiCl3 (bismuth chloride) was applied, dried, and then finish annealed at 1200°C for 20 hours.
After the finish annealing, the steel sheet was washed with water to remove excess annealing separator, and it was found that no forsterite-based coating was formed on the surface of the steel sheet.
This steel sheet was subjected to light pickling under the conditions shown in Table 2-1.
After light pickling, an intermediate layer was formed using a treatment solution containing a mixture of phosphate and additives shown in Table 1. The drying temperature was 200°C. The obtained intermediate layer had the properties shown in Table 2-2. The proportion of crystalline metal phosphate in the intermediate layer was 80 mass% or more.

Thereafter, an insulating coating treatment solution containing metal phosphate and colloidal silica as main ingredients shown in Table 2-3 was applied, and after application, 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 2-3. The tensile coating layer consisted essentially of metal phosphate and silica.

The obtained steel sheet (grain-oriented electrical steel sheet) was subjected to a magnetic domain refining treatment 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 50 Hz at 1.7 T) of the steel sheet after the magnetic domain refining treatment was measured by a single sheet magnetic property measurement method (Single Sheet Tester: SST) in accordance with JIS C2556 (2015).
The space factor was measured as follows.

[Occupancy factor]
The space factor was measured according to 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 space factor was calculated by measuring the distance between the upper and lower backing plates sandwiching the laminate under a pressure of 1 MPa.
If the space factor was 96.0% or more, it was determined that a high space factor was ensured.

In addition, the coating adhesion, coating tension, corrosion resistance, and elution resistance of the steel sheets after magnetic domain refinement treatment were evaluated using the following methods. The results are shown in Table 3.

[Coating adhesion]
The adhesion of the coating was evaluated by taking a sample having a width of 30 mm and a length of 300 mm from the steel sheet, subjecting this sample to stress relief annealing at 800°C for 2 hours in a nitrogen gas flow, and then winding it around a 10 mmφ cylinder and unwinding it, followed by a bending adhesion test.
The evaluation criteria were as follows, and a rating of A or B was determined to indicate excellent coating adhesion.
A: Peeling area ratio 0 to 0.5%
B: Peeling area rate more than 0.5%, 5.0% or less C: Peeling area rate more than 5.0%, 20% or less D: Peeling area rate more than 20%, 50% or less E: Peeling area rate more than 50%

[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.

[Corrosion resistance]
Corrosion resistance was evaluated by subjecting the sample to a 5% NaCl aqueous solution that was allowed to fall 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 scale of 1 to 10. The evaluation criteria were as follows: A score of 5 or more (5 to 10) was considered to be excellent in corrosion resistance.
10: No rust occurred 9: Very little rust occurred (area ratio = 0.10% or less)
8: Area ratio where rust has occurred = more than 0.10% but not more than 0.25% 7: Area ratio where rust has occurred = more than 0.25% but not more than 0.50% 6: Area ratio where rust has occurred = more than 0.50% but not more than 1.0% 5: Area ratio where rust has occurred = more than 1.0% but not more than 2.5% 4: Area ratio where rust has occurred = more than 2.5% but not more than 5.0% 3: Area ratio where rust has occurred = more than 5.0% but not more than 10% 2: Area ratio where rust has occurred = more than 10% but not more than 25% 1: Area ratio where rust has occurred = more than 25% but not more than 50%

[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 calculated by cooling the pure water (solution) into which the phosphoric acid had eluted, diluting the cooled solution with pure water, and measuring the phosphoric acid concentration of the sample using ICP-AES.
If the amount of elution was less than 40 mg/ ^m2 , the elution resistance was deemed to be excellent.

As can be seen from Tables 1 to 3, the examples of the present invention are extremely excellent in the main properties of the coating, including adhesion, and the iron loss and space factor are improved.
On the other hand, in the comparative examples, the insulating coating did not have a desirable configuration, and one or more of the adhesion of the tensile coating, magnetic properties, corrosion resistance, resistance to phosphoric acid elution, and space factor of the transformer (core) were inferior.

The present invention provides a grain-oriented electrical steel sheet that has excellent adhesion and magnetic properties to the tension coating and does not reduce the space factor of the transformer (core). Therefore, it has high industrial applicability.

100 Grain-oriented electrical steel sheet 1 Base steel sheet 2 Insulation coating 21 Intermediate layer 22 Tensile coating layer

Claims (5)

A grain-oriented electrical steel sheet that does not have a forsterite-based coating,
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 and one or more of amorphous silica, an inorganic filler, and a metal oxide;
a tensile coating layer formed on a surface side of the insulating coating , the tensile coating layer containing a metal phosphate and silica ,
The intermediate layer has a Si content of less than 10% by mass, and the tension coating layer has a Si content of 10% by mass or more.
the average particle size of one or more of the amorphous silica, the inorganic filler, and the metal oxide is 10 to 500 nm;
A directional electrical steel sheet characterized by: The grain-oriented electrical steel sheet according to claim 1, characterized in that the inorganic filler contains one or more of alumina, BN, AlN, and kaolin. The metal oxide is one or more of titanium oxide, zinc oxide, and calcium oxide.
The grain-oriented electrical steel sheet according to claim 1 or 2, The average crystal particle size of the crystalline metal phosphate is 1.0 to 12.0 μm.
The grain-oriented electrical steel sheet according to claim 1 or 2, A method for forming the insulating coating provided on the grain-oriented electrical steel sheet according to claim 1, comprising:
a finish annealing process in which an annealing separator containing 10 to 100 mass% of Al ₂ O ₃ is applied to a 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 light pickling step in which the steel sheet after the annealing separator removal step is pickled at a solution temperature of 30 to 85°C with a pickling solution containing 0.10 to 10.0 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 light pickling step in a treatment solution having a solution temperature of 30 to 85°C for 5 to 150 seconds, the treatment solution containing 0.3 to 10.0 mass% of a metal phosphate and 0.01 to 10.0 g/L of one or more of colloidal silica, inorganic filler, and metal oxide having an average particle size of 10 to 500 nm;
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, the total concentration of the metal phosphate and the colloidal silica being 10 to 40 mass %, to the steel sheet after the drying step, drying the coating liquid, and then heating the steel sheet to maintain a sheet temperature of 700 to 950°C for 10 to 50 seconds;
A method for forming an insulating coating, comprising: