JP7200687B2 - Grain-oriented electrical steel sheet and manufacturing method thereof - Google Patents

Description

TECHNICAL FIELD The present invention relates to a grain-oriented electrical steel sheet and a method for manufacturing the same.

A grain-oriented electrical steel sheet is used as a core material for transformers and the like. A grain-oriented electrical steel sheet is required to have magnetic properties such as high magnetic flux density and low iron loss.

In order to ensure magnetic properties, the crystal orientation of a grain-oriented electrical steel sheet is controlled, for example, in an orientation (Goss orientation) in which {110} planes are aligned parallel to the steel sheet surface and <100> axes are aligned in the rolling direction. A secondary recrystallization process using AlN, MnS, etc. as inhibitors is widely used to increase the accumulation of Goss orientations.

Generally, a film is formed on the surface of a grain-oriented electrical steel sheet for the purpose of reducing iron loss. This film reduces iron loss as a steel sheet veneer by applying tension to the grain-oriented electrical steel sheet. This film further reduces iron loss as an iron core by ensuring electrical insulation between steel sheets when grain-oriented electrical steel sheets are laminated and used.

As the film-formed grain-oriented electrical steel sheet, a finish annealing coating, which is an oxide film containing Mg, is formed on the surface of the mother steel sheet, and an insulating coating is further formed on the surface of the finishing annealing coating. There is something That is, in this case, the coating on the mother steel sheet includes the finish annealing coating and the insulating coating. Each of the finish tempered pure coating and the insulating coating has both the function of ensuring insulation and the function of applying tension to the mother steel sheet.

The finish annealing film, which is an oxide film containing Mg, is formed by separating an annealing separator containing magnesia (MgO) as a main component and a mother steel sheet at a temperature of 600 to 1200 ° C. It is formed by reaction during heat treatment that is applied for 30 hours or more.

The insulating coating is formed by applying a coating solution containing, for example, phosphoric acid or a phosphate, colloidal silica, and chromic anhydride or a chromate to the mother steel sheet after final annealing, and heating at 300 to 950° C. for 10 seconds or more. It is formed by baking and drying.

In order for the coatings to exhibit the functions of insulating properties and imparting tension to the mother steel sheet, high adhesion between these coatings (finish annealing coating and insulation coating) and the mother steel sheet is required.

Conventionally, the above-mentioned adhesion has been ensured mainly by the anchoring effect of unevenness at the interface between the mother steel sheet and the finish baked pure coating. However, the unevenness of the interface hinders the movement of the domain wall when the grain-oriented electrical steel sheet is magnetized, and is a factor that hinders the reduction of iron loss.

Therefore, in order to further reduce the iron loss, there is a technique for ensuring the adhesion of the insulating film in a state where the above-mentioned interface is smoothed without the presence of the finish annealing film, which is an oxide film containing Mg. , Japanese Patent Application Laid-Open No. 49-096920 (Patent Document 1) and International Publication No. 2002/088403 (Patent Document 2).

In the method for producing a grain-oriented electrical steel sheet disclosed in Patent Document 1, the finish annealing film is removed by pickling or the like, and the surface of the mother steel sheet is smoothed by chemical polishing or electropolishing. In the method for producing a grain-oriented electrical steel sheet disclosed in Patent Document 2, an annealing separating agent containing alumina (Al ₂ O ₃ ) is used during finish annealing to suppress the formation of the finish annealing film itself, so that the surface of the mother steel sheet is smoothed. Smooth. However, in the manufacturing methods of Patent Documents 1 and 2, when an insulating film is formed in contact with a smooth mother steel plate surface (directly on the mother steel plate surface), it is difficult for the insulating film to adhere to the mother steel plate surface. (Sufficient adhesion cannot be obtained).

In order to deal with such problems, a technique for forming an intermediate layer (base film) between the mother steel plate and the insulating film in order to improve the adhesion of the film to the smoothed surface of the mother steel plate is disclosed, for example, in Japanese Unexamined Patent Application Publication No. 05- 279747 (Patent Document 3), JP-A-06-184762 (Patent Document 4), JP-A-09-078252 (Patent Document 5), and JP-A-07-278833 (Patent Document 6) Proposed.

Patent Document 3 discloses a method of applying an aqueous solution of a phosphate or an alkali metal silicate to the surface of a mother steel plate to form an intermediate layer on the surface of the mother steel plate. Further, in Patent Documents 4 to 6, a mother steel plate is subjected to heat treatment for several tens of seconds to several minutes in which the temperature and atmosphere are appropriately controlled, so that an externally oxidized silicon oxide film is formed as an intermediate layer. A method of forming is disclosed.

The intermediate layer proposed in Patent Documents 3 to 6 is effective to some extent in improving the adhesion of the insulating coating and reducing iron loss by smoothing the interface between the mother steel plate and the coating by suppressing the formation of unevenness. demonstrated. However, there is a demand for further improvement in adhesion.
Therefore, JP-A-2002-322566 (Patent Document 7), JP-A-2002-363763 (Patent Document 8), JP-A-2003-313644 (Patent Document 9), JP-A-2003-171773 (Patent Document Document 10), Japanese Patent Application Laid-Open No. 2002-348643 (Patent Document 11), and Japanese Patent Application Laid-Open No. 2004-342679 (Patent Document 12) have proposed new techniques.

Patent Document 7 discloses a technique for forming an outer oxide film mainly composed of silicon oxide and a granular outer oxide. Further, Patent Document 8 discloses a technique for controlling cavities in an external oxidation-type oxide film mainly composed of silicon oxide.

In Patent Documents 9 to 11, metallic iron and metal-based oxides (eg, Si—Mn—Cr oxide, Si—Mn—Ca—Ti oxide, Fe oxide, etc. are added to an external oxide film mainly composed of silicon oxide. ) is disclosed to modify the outer oxide film.

Patent Document 12 discloses a grain-oriented electrical steel sheet having a multi-layer intermediate layer including an oxide film mainly composed of silicon oxide generated by an oxidation reaction and a coating layer mainly composed of silicon oxide formed by coating and baking. ing.

As described above, by using an external oxide film mainly composed of silicon oxide as an intermediate layer, even if the surface of the mother steel sheet is smoothed, the adhesion of the insulation film to the mother steel sheet is ensured through the intermediate layer, and , grain-oriented electrical steel sheets with excellent magnetic properties have been proposed.

By the way, when a grain-oriented electrical steel sheet is used as a wound core or an EI core as an iron core of a transformer, after being processed into a desired shape, moisture in the air or moisture in the oil in which the iron core is immersed is exposed to water. come into contact with Therefore, water resistance is required. However, in the above-mentioned patent documents relating to the grain-oriented electrical steel sheet having an intermediate layer mainly composed of silicon oxide, there is no description of peeling of the insulating coating due to water, and no document discussing the water resistance of the insulating coating as a problem. As a result of studies by the present inventors, when a three-layer structure of "mother steel plate/intermediate layer/insulating coating" is used as in Patent Documents 7 to 12, a general grain-oriented electrical steel sheet that is widely put into practical use " It was found that the water resistance of the insulation film is sometimes lower than in the case of a three-layer structure of "base steel plate/oxide film containing Mg/insulation film".

JP-A-49-096920 WO2002/088403 JP-A-05-279747 JP-A-06-184762 JP-A-09-078252 JP-A-07-278833 JP 2002-322566 A JP-A-2002-363763 JP 2003-313644 A Japanese Patent Application Laid-Open No. 2003-171773 JP-A-2002-348643 Japanese Patent Application Laid-Open No. 2004-342679

As described above, the film structure of a general grain-oriented electrical steel sheet, which is currently in wide practical use, is the three-layer structure shown in FIG. The structure is the basic structure. The insulating coating is generally a ceramic coating formed by mixing amorphous phosphate and colloidal silica.

On the other hand, in order to reduce iron loss and ensure adhesion between the mother steel sheet and the insulating coating, the coating structure of the grain-oriented electrical steel sheet, in which the interface between the mother steel sheet and the coating is macroscopically uniform and smooth, is simple. In this case, it is preferable to use a three-layer structure of "mother steel plate 1/intermediate layer 2B/insulating coating 3" as shown in FIG. 2 as a basic structure.

However, in the case of the grain-oriented electrical steel sheet having the structure shown in FIG. 2, the water resistance of the insulation film may be lower than that of the grain-oriented electrical steel sheet having the finish annealing film, which is the oxide film containing Mg shown in FIG. . The decrease in water resistance becomes more pronounced as the film (intermediate layer and insulating film) becomes thinner.

The present invention has been made in view of the above problems. An object of the present invention is to provide a grain-oriented electrical steel sheet having an intermediate layer mainly composed of silicon oxide and having excellent adhesion and water resistance of the insulating coating.

The gist of the present invention is as follows.
(1) A steel sheet comprising a mother steel sheet, an intermediate layer mainly composed of silicon oxide and formed in direct contact with the surface of the mother steel sheet, and an insulating coating formed on the surface of the intermediate layer. A grain-oriented electrical steel sheet, wherein the insulating coating contains voids, and the void ratio in the insulating coating is 5 to 30% in terms of area ratio.
(2) The grain-oriented electrical steel sheet according to (1), wherein the average diameter of the voids is 1/4 or less and 1/20 or more of the thickness of the insulating coating.
(3) The grain-oriented electrical steel sheet according to (1) or (2), wherein the insulating coating comprises a compound containing P, O and Si and Cr, and the average Cr concentration in the insulating coating is , 0.1 atomic % or more, a grain-oriented electrical steel sheet.
(4) The grain-oriented electrical steel sheet according to any one of (1) to (3), wherein the distance from the average center of gravity of the voids to the interface between the insulating coating and the intermediate layer is A grain-oriented electrical steel sheet having a film thickness of 4/5 or less of the insulation coating.
(5) The grain-oriented electrical steel sheet according to any one of (1) to (4), wherein the Cr concentration on the surface of the insulation coating is less than 80% of the average Cr concentration of the entire insulation coating. A grain-oriented electrical steel sheet having a total area ratio of Cr-deficient regions of 50% or less.
(6) A preparation step of preparing a slab, a hot rolling step of heating the slab at 1280 ° C. or less and then performing hot rolling to manufacture a hot-rolled steel sheet for a grain-oriented electrical steel sheet, A hot-rolled sheet annealing step of performing hot-rolled sheet annealing on a hot-rolled steel sheet for steel plate to produce an annealed steel sheet, and a cold-rolling step of performing cold rolling on the annealed steel sheet to produce a cold-rolled steel sheet. A rolling step, a decarburization annealing step of decarburizing and annealing the cold-rolled steel sheet to produce a mother steel sheet, an annealing separator applying step of applying an annealing separator to the mother steel sheet, and the annealing separator. A finish annealing step of performing finish annealing on the coated mother steel sheet to manufacture the mother steel sheet on which the finish annealing film is formed, and, if necessary, removing the finish annealing film and removing the finish annealing film. A mother steel plate surface smoothing step for making a smooth surface having a surface roughness Ra of 1.0 μm or less, and a heat treatment is performed on the mother steel plate after the finish annealing step or the mother steel plate smoothing step, so that the mother steel plate is smoothed. an intermediate layer forming step of forming an intermediate layer mainly composed of silicon oxide in contact with a surface; and an insulating coating forming step of forming an insulating coating on the surface of the intermediate layer. a coating step of coating a coating solution containing phosphate, colloidal silica, and chromic anhydride or chromate on the surface of the intermediate layer formed on the surface of the mother steel plate; and coating the coating solution. and a baking step of heating the mother steel plate and performing baking treatment at 600 to 1150° C. for 5 to 300 seconds to form the insulating film. A method for producing a grain-oriented electrical steel sheet, wherein the average heating rate in a temperature range of 600°C is 10 to 200°C/sec.

The grain-oriented electrical steel sheet of the present invention has an intermediate layer mainly composed of silicon oxide on the surface of the mother steel sheet, and the insulation coating has cavities formed therein, so that the adhesion of the insulation coating is excellent. Further, in the grain-oriented electrical steel sheet of the present invention, when the insulating film contains Cr, it is possible to improve adhesion and suppress deterioration of water resistance.

It is a schematic diagram which shows the example of sectional drawing of the conventional grain-oriented electrical steel plate. FIG. 3 is a schematic diagram showing an example of a cross-sectional view of a grain-oriented electrical steel sheet having an intermediate layer 2B mainly made of silicon oxide. BRIEF DESCRIPTION OF THE DRAWINGS It is a schematic diagram which shows the example of sectional drawing of the grain-oriented electrical steel plate which concerns on this embodiment.

The present inventors have studied a technique for solving the above-described problems.

The present inventors first studied a method for improving the adhesion of the insulation coating to the mother steel plate via the intermediate layer (hereinafter sometimes simply referred to as the adhesion of the insulation coating to the mother steel plate). As a result of investigation, it was thought that the adhesion of the insulating coating to the mother steel plate would be enhanced if a plurality of voids (gaps) existed in the insulating coating. The reason for this is as follows. If voids exist in the insulating film, the voids act as stress relaxation sites when stress such as bending is applied. If the stress is relieved, the stress acting on the interface between the mother steel plate and the intermediate layer and the adhesion interface between the intermediate layer and the insulating coating is reduced. As a result, it is considered that the adhesion of the insulating film to the mother steel plate is enhanced.

The present inventors have further proposed a grain-oriented electrical steel sheet in which an intermediate layer mainly composed of silicon oxide is formed in contact with the surface of the mother steel sheet, and an insulating film is formed in contact with the surface of the intermediate layer. , we investigated the cause of the deterioration of the water resistance of the insulating film.

First, the inventors have developed a grain-oriented electrical steel sheet in which an intermediate layer mainly composed of silicon oxide is formed in contact with the surface of the mother steel sheet, and an insulating film is formed in contact with the surface of the intermediate layer. , found that the water resistance of the insulating coating is lowered when the insulating coating contains Cr. Therefore, the inventors further investigated the cause of the decrease in the water resistance of the insulating coating when the insulating coating contains Cr.

The decrease in the water resistance of the insulating coating containing Cr becomes more pronounced as the intermediate layer becomes thinner. Therefore, the present inventors considered that the decrease in the water resistance of the insulating coating is related to the effect associated with mass transfer between the mother steel sheet and the insulating coating. As a result of further studies, the inventors of the present invention have considered that the mechanism by which the water resistance of the insulating coating is lowered in grain-oriented electrical steel sheets having an insulating coating and an intermediate layer mainly composed of silicon oxide is as follows. .

That is, when the insulation coating is baked, Fe in the mother steel sheet penetrates into the insulation coating from the mother steel sheet through the intermediate layer due to thermal diffusion, and Fe diffuses into the insulation coating. When the Fe concentration in the insulating film is low, a considerable amount of Cr can be solid-solved in the amorphous phosphate that is the matrix of the insulating film. However, when the Fe concentration in the insulating coating increases, Fe combines with Cr in the insulating coating to form a crystalline phosphide of Fe and Cr ((Fe, Cr) ₂ P ₂ O ₇ ). If crystalline phosphides of Fe and Cr are formed, a Cr-depleted region (hereinafter referred to as a Cr-depleted region) is formed in the matrix of the insulating film. As the proportion of the Cr-deficient region in the insulating coating increases, the water resistance of the insulating coating decreases.

Based on the mechanism of the deterioration of the water resistance of the insulating coating described above, the inventors of the present invention have found that, in order to suppress the deterioration of the water resistance of the insulating coating containing Cr, the insulating coating should be removed from the mother steel sheet during baking. It was thought that it would be effective to suppress the penetration of Fe into the .

Based on such a way of thinking, the present inventors proceeded with further studies. As a result, during baking, voids (bulk defects) are intentionally introduced into the insulating film in a temperature range of 550°C or less, which is the diffusion limit temperature of Fe, so that the diffusion path of Fe is physically blocked by the voids. It was found that Fe can be suppressed from entering the surface side of the insulating film beyond the voids.

A grain-oriented electrical steel sheet completed on the basis of the above findings includes a mother steel sheet, an intermediate layer formed on the surface of the mother steel sheet in direct contact with the surface of the mother steel sheet, and an intermediate layer mainly composed of silicon oxide, and a layer formed on the surface of the intermediate layer. and an insulating coating. The intermediate layer is formed in direct contact with the surface of the mother steel sheet, and substantially no final annealing film, which is an oxide film containing Mg, exists on the surface. Moreover, the void ratio in the insulating film is 5 to 30%.

When the insulating film is composed of a compound containing P, O and Si and Cr, and the average Cr concentration in the insulating film is 0.1 atomic % or more, the insulating film is also excellent in water resistance, which is preferable.
More preferably, on the surface of the insulation coating, the total area ratio of the Cr-deficient region having a Cr concentration of less than 80% of the average Cr concentration of the entire insulation coating to the entire coating is 50% or less.

In addition, such a grain-oriented electrical steel sheet is
(S0) a preparation step of preparing a slab;
(S1) a hot rolling step of heating the slab at 1280° C. or lower and then performing hot rolling to manufacture a hot-rolled steel sheet for grain-oriented electrical steel sheets;
(S2) a hot-rolled sheet annealing step of performing hot-rolled sheet annealing on the hot-rolled steel sheet for grain-oriented electrical steel sheet to produce an annealed steel sheet;
(S3) a cold-rolling step of cold-rolling the annealed steel sheet to produce a cold-rolled steel sheet;
(S4) a decarburization annealing step of performing decarburization annealing on the cold-rolled steel sheet to produce a mother steel sheet;
(S5) an annealing separator application step of applying an annealing separator to the mother steel plate;
(S6) a finish annealing step of performing finish annealing on the mother steel sheet after application of the annealing separator to manufacture the mother steel sheet on which the finish annealing film is formed;
(S7) if necessary, a step of smoothing the surface of the mother steel sheet to remove the finish annealing film to make the surface roughness of the mother steel sheet Ra 1.0 μm or less;
(S8) An intermediate layer forming step of heat-treating the mother steel plate after the finish annealing step or the mother steel plate smoothing step to form an intermediate layer mainly composed of silicon oxide in contact with the surface of the mother steel plate. When,
(S9) an insulating coating forming step of forming an insulating coating on the surface of the intermediate layer;
obtained by a manufacturing method comprising

A grain-oriented electrical steel sheet according to an embodiment of the present invention (a grain-oriented electrical steel sheet according to the present embodiment) and a method for manufacturing the same will be described below in detail with reference to the drawings.

[Grain-oriented electrical steel sheet]
FIG. 3 is a cross-sectional view near the surface of the grain-oriented electrical steel sheet according to the present embodiment. Referring to FIG. 3 , the grain-oriented electrical steel sheet according to the present embodiment includes a mother steel sheet 1 and a coating 20 formed in contact with the surface of the mother steel sheet 1 . Coating 20 includes intermediate layer 2B formed in direct contact with the surface of mother steel plate 1, and insulating coating 30 formed in contact with the surface of intermediate layer 2B.

[Mother steel plate]
The grain-oriented electrical steel sheet according to the embodiment of the present invention is particularly characterized by the structure of the insulation coating 30 .
The chemical composition and structure of the mother steel sheet 1 are not directly related to the configuration of the insulation coating 30 . Therefore, the chemical composition and structure of the mother steel sheet 1 of the grain-oriented electrical steel sheet according to this embodiment are not particularly limited. For example, the mother steel sheet 1 may be a mother steel sheet used for general grain-oriented electrical steel sheets. An example of the mother steel sheet 1 included in the grain-oriented electrical steel sheet according to the present embodiment will be described below.

[Chemical Composition of Mother Steel Plate 1]
As the chemical composition of the mother steel sheet 1 described above, the chemical composition of a mother steel sheet in a general grain-oriented electrical steel sheet can be used. The chemical composition of mother steel sheet 1 contains, for example, the following elements. Unless otherwise specified, "%" used for the content of each element in the chemical composition of the mother steel sheet 1 means % by mass.

The mother steel sheet 1 of the oriented electrical steel sheet according to the present embodiment contains, for example, Si: 0.50 to 7.00%, C: 0.005% or less, and N: 0.0050% or less, and the balance is It consists of Fe and impurities. Hereinafter, reasons for limitation of a representative example of the chemical composition of the mother steel sheet 1 of the grain-oriented electrical steel sheet according to the present embodiment will be described.

Si: 0.50-7.00%
Silicon (Si) increases the electrical resistance of the grain-oriented electrical steel sheet and reduces iron loss. If the Si content is less than 0.50%, this effect cannot be sufficiently obtained. Therefore, the Si content is preferably 0.50% or more. The Si content is more preferably 1.50% or more, still more preferably 2.50% or more.
On the other hand, when the Si content exceeds 7.00%, the saturation magnetic flux density of the mother steel plate 1 decreases. Therefore, iron loss deteriorates. Therefore, preferably the Si content is 7.00% or less. The Si content is more preferably 5.50% or less, still more preferably 4.50% or less.

C: 0.005% or less Carbon (C) forms a compound in the mother steel sheet 1 and deteriorates core loss. Therefore, the C content is preferably 0.005% or less. The C content is preferably as low as possible. The C content is more preferably 0.004% or less, still more preferably 0.003% or less.
On the other hand, the C content is preferably as low as possible, so it may be 0%, but C may be contained as an impurity in steel. Therefore, the C content may exceed 0%.

N: 0.0050% or less Nitrogen (N) forms a compound in the mother steel sheet 1 and deteriorates iron loss. Therefore, the N content is preferably 0.0050% or less. N content is preferably as low as possible. The N content is more preferably 0.0040% or less, still more preferably 0.0030% or less.
On the other hand, since the N content is preferably as low as possible, it may be 0%, but N may be contained as an impurity in the steel. Therefore, the N content may exceed 0%.

The remainder of the chemical composition of the mother steel plate 1 consists of Fe and impurities. The "impurities" referred to here are inevitably mixed from components contained in raw materials or components mixed in the manufacturing process when the mother steel plate 1 is industrially manufactured, and are grain-oriented electrical steel sheets according to the present embodiment. means an element that does not substantially affect the effect obtained by

[Arbitrary element]
The chemical composition of the mother steel plate 1 may contain one or more arbitrary elements in the following range instead of part of Fe for the purpose of improving magnetic properties and solving manufacturing problems. . Examples of optional elements contained in place of part of Fe include the following elements. Since these elements do not have to be contained, the lower limit is 0%. On the other hand, if the content of these elements is too high, precipitates are formed and the iron loss is deteriorated, ferrite transformation is suppressed, the GOSS orientation cannot be obtained sufficiently, and the saturation magnetic flux density is lowered. As a result, iron loss deteriorates. Therefore, even when it is contained, it is preferable to set it within the following range.
Al: 0.065% or less,
Mn: 1.00% or less,
S and Se: 0.001% or less in total,
Bi: 0.010% or less,
B: 0.0080% or less,
Ti: 0.015% or less,
Nb: 0.020% or less,
V: 0.015% or less,
Sn: 0.50% or less,
Sb: 0.50% or less,
Cr: 0.30% or less,
Cu: 0.40% or less,
P: 0.50% or less,
Ni: 1.00% or less, and Mo: 0.10% or less.

The chemical composition of the mother steel sheet of the grain-oriented electrical steel sheet according to the present embodiment described above is obtained by using a slab having the chemical composition described later.

[Surface roughness of mother steel plate]
The roughness of the surface of mother steel plate 1 is not particularly limited. However, from the viewpoint of preventing the formation of unevenness at the interface between the film 20 and the mother steel plate 1 and preventing the iron loss reducing action from being hindered, for example, Ra (arithmetic mean roughness) should be 1.0 μm or less. preferable. A more preferable upper limit of the arithmetic mean roughness Ra of the surface of the mother steel plate 1 is 0.7 μm, and a further preferable upper limit is 0.5 μm.

The arithmetic mean roughness Ra of the surface of the mother steel plate 1 is measured by the following method.
A sample is taken with a cross-section perpendicular to the rolling direction of the grain-oriented electrical steel sheet as an observation surface. The roughness of the surface of the mother steel plate on the obtained observation surface is measured by the following method. When the coating 20 such as the finish annealing coating or the intermediate layer 2B is formed on the surface of the mother steel sheet, the coating 20 is not formed at the interface between the mother steel sheet 1 and the intermediate layer 2B on the observation surface (cross section). When the base steel plate surface is exposed, the positional coordinates in the plate thickness direction of the base steel plate surface on the observation surface (cross section) are measured with an accuracy of 0.01 μm or more, and arithmetic is performed in accordance with JIS B 0601 (2001). Calculate the average roughness Ra.
The measurement is performed in a continuous range of 2 mm (total of 20000 points) at a pitch of 0.1 μm in the direction parallel to the surface of the mother steel sheet, and the arithmetic mean roughness Ra is obtained with the reference length of 2 mm. Arithmetic mean roughness Ra is determined by the above method at at least five arbitrary points on the mother steel sheet surface, and the average value of the Ra values obtained at each point is defined as the arithmetic mean roughness Ra of the mother steel sheet surface. This observation can be performed with a scanning electron microscope (SEM), and it is practical to apply image processing to measure the positional coordinates.

[Regarding the final annealing film that is an oxide film containing Mg]
On the surface of the mother steel sheet 1, substantially no final annealing film (glass film), which is an oxide film containing Mg, exists. Here, the “glass film” is an oxide film containing Mg formed on the surface of the mother steel sheet 1 by the reaction of the annealing separator MgO and the mother steel sheet 1 by performing the final annealing. means. A "glass film" is an oxide film having a Mg content in the film in the range of 15 to 55 atomic %.

The "finish annealing film" includes not only products (for example, inorganic minerals such as forsterite and oxides containing Al) produced by the reaction between the annealing separator and the mother steel sheet 1, but also unreacted annealing A separating agent may be included.

“There is substantially no finish annealing film” on the surface of the mother steel sheet 1 means that the formation of the finish annealing film is intentionally suppressed in the finish annealing process, and as a result, the outermost surface of the mother steel sheet 1 is finished. The finish annealing film may be substantially absent, or the finish annealing film may be left on the surface of the mother steel plate 1 as a result of removing substantially all of the finish annealing film from the surface of the mother steel plate 1 after the finish annealing process. It may be substantially non-existent. Furthermore, in the step of smoothing the surface of the mother steel sheet described later, after leaving the finish annealing film on the surface of the mother steel plate 1 after the finish annealing, substantially all of the finish annealing film disappeared in the steps after the intermediate layer forming step. As a result, the finish annealing film may not substantially exist on the surface of the mother steel sheet 1 .

[Intermediate layer 2B]
The intermediate layer 2B is formed in contact with the surface of the mother steel sheet 1 on which the finish annealing film is substantially absent. The intermediate layer 2B is an external oxide film mainly composed of silicon oxide. Here, "mainly composed of silicon oxide" means that the composition is composed of Fe content of less than 80 atomic %, P content of less than 5 atomic %, and Si content of 20 atomic % or more. , O content of 50 atomic % or more and Mg content of 10 atomic % or less.

Intermediate layer 2B is a layer arranged between mother steel plate 1 and insulating coating 30, and has a function of adhering mother steel plate 1 and insulating coating 30 together.

The silicon oxide that forms the main component of the intermediate layer 2B is preferably SiOx (x=1.0 to 2.0), more preferably SiOx (x=1.5 to 2.0). This is because silicon oxide is more stable. Silica (SiO ₂ ) can be formed by performing sufficient heat treatment to form silicon oxide on the surface of mother steel plate 1 .

For example, hydrogen: 20 to 50% by volume, balance: nitrogen and impurities, dew point: -20 to 2 ° C., under conditions such as holding for 10 to 600 seconds in a temperature range of 600 to 1150 ° C. When heat treatment is applied to form the intermediate layer 2B, the silicon oxide remains amorphous. The intermediate layer 2B containing such amorphous silicon oxide has a high strength to withstand thermal stress, and has a relatively small elastic modulus, so that the intermediate layer 2B is made of a dense material that can easily relax the thermal stress. .

The mother steel sheet 1 of the grain-oriented electrical steel sheet contains Si at a high concentration (for example, Si: 0.80% by mass or more and 7.00% by mass or less). A strong chemical affinity develops between them, and the intermediate layer 2B and the mother steel plate 1 are firmly adhered to each other.

The thickness of the intermediate layer 2B is not particularly limited. If the thickness is 2 nm or more, the adhesion of the insulating coating 30 to the mother steel sheet 1 is more effectively enhanced, so the thickness of the intermediate layer 2B is preferably 2 nm or more, more preferably 5 nm or more. On the other hand, when the thickness of the intermediate layer 2B is 400 nm or less, defects such as voids 33 and cracks in the intermediate layer 2B are effectively suppressed. Therefore, the thickness of the intermediate layer 2B is preferably 400 nm or less, more preferably 300 nm or less. If the thickness of the intermediate layer 2B is reduced within the range in which film adhesion can be ensured, the formation time can be shortened, contributing to high productivity and suppressing a decrease in the lamination factor when used as an iron core. The thickness of 2B is preferably 100 nm or less, more preferably 50 nm or less.

A method for measuring the thickness of the intermediate layer 2B is as follows.
The cross section of the intermediate layer 2B is observed and measured with a TEM (transmission electron microscope) with an electron beam diameter of 10 nm. Specifically, for example, for TEM observation, a sample is cut so as to have an observation cross section parallel to the plate thickness direction, and the width of the observation cross section of the sample in the direction parallel to the surface of the mother steel plate 1 is 10 μm or more. , and from the measurement area including the intermediate layer 2B, the mother steel plate 1 described later, and the insulating coating 30 described later, 5 or more measurement positions separated from each other by 2 μm or more in the width direction are selected, and the intermediate layer 2B thickness is measured by TEM. Let the average of the measured values be the thickness of the intermediate layer 2B. When measuring the thickness of the intermediate layer 2B at each measurement position in the measurement area with a TEM, the layer existing between the mother steel plate 1 and the insulation coating 30, which will be described later, is measured as the intermediate layer 2B.

[Insulating film 30]
Insulating coating 30 is formed on the surface of intermediate layer 2B. The insulating coating 30 applies tension to the mother steel sheet 1 to reduce iron loss as a steel sheet veneer. The insulating coating 30 further ensures electrical insulation between steel sheets when the grain-oriented electrical steel sheets are laminated and used.

The insulating coating 30 may have a general composition. The insulating coating 30 may be mainly composed of phosphate or aluminum borate, for example. For example, the coating may be a mixture of phosphate and colloidal silica and may consist of a compound containing P, O and Si.

The insulation coating 30 may further contain various elements and compounds to improve various properties.
The insulating coating 30 may not contain Cr. However, when the insulation coating 30 contains Cr, if the insulation coating has cavities as will be described later, the adhesion between the mother steel plate 1 and the insulation coating 30 as well as the water resistance are enhanced.

When the insulation coating 30 contains Cr, the average Cr concentration in the insulation coating 30 is preferably 0.1 atomic % or more. A preferable upper limit of the average Cr concentration in the insulating coating 30 is 5.0 atomic %. If the Cr concentration is less than 0.1 atomic percent, the above effect cannot be sufficiently obtained.

When the insulating film 30 contains Cr, it is not particularly limited as long as the average Cr concentration of the whole is 0.1 atomic % or more, but it may contain chromic anhydride or chromate. Furthermore, the insulating coating 30 may further contain various elements and compounds to improve various properties.

The method for measuring the average Cr concentration is as follows. The entire insulating coating 30 is observed with a TEM, and EDS (energy dispersive X-ray analysis) plane analysis is performed on the Cr concentration distribution so that the entire thickness of the insulating coating 30 in the thickness direction is included. The average value of the Cr concentration at each position of the insulating coating 30 obtained by surface analysis is defined as the average Cr concentration (atomic %). Plane analysis is performed by 20 µm or more in a direction parallel to the steel plate surface.

Voids 33 in insulating coating 30 enhance the adhesion of insulating coating 30 to mother steel plate 1 . The reason for this is as follows.
If voids exist in the insulating film, the voids act as stress relaxation sites when stress such as bending is applied. If the stress is relieved, the stress acting on the adhesion interface between the intermediate layer 2B and the mother steel sheet 1 is reduced. As a result, the adhesion of the insulating coating to the mother steel plate 1 is enhanced.

If the void ratio in insulating coating 30 is less than 5%, the adhesion of insulating coating 30 is not sufficiently improved. Therefore, the void ratio in the insulating coating 30 is 5% or more. A preferable lower limit of the void ratio in the insulating coating 30 is 7%, more preferably 10%.
On the other hand, if the void ratio of the insulating coating 30 exceeds 30%, the insulating properties deteriorate. Therefore, the void ratio in the insulating coating 30 is 30% or less. A preferable upper limit of the void ratio in the insulating coating 30 is 25%, more preferably 20%.

From the viewpoint of efficient stress relaxation, the average diameter of voids is preferably 1/4 or less and 1/20 or more, more preferably 1/5 or less and 1/20 or more, of the thickness of the insulating coating. preferable. That is, it is preferable that R/t is 0.05 to 0.25, where R is the average diameter of the voids and t is the film thickness of the insulating coating.

When the insulation coating contains Cr, the voids 33 in the insulation coating 30 are formed by Fe from the mother steel plate 1 through the intermediate layer 2B and into the insulation coating 30 due to thermal diffusion during the baking step in the insulation coating formation step described later. Physically block the intrusion. As a result, the Fe concentration in the insulating film 30 can be lowered, and the formation of crystalline phosphide by combining Fe with Cr in the insulating film 30 can be suppressed. As a result, the insulating coating 30 can be prevented from forming a Cr-deficient region, and the insulating coating 30 can maintain high water resistance.
If the porosity of the insulating coating 30 is less than 5%, the effect of preventing Fe penetration is small.

Suppression of thermal diffusion of Fe by the voids 33 described above is effective for Fe on the side of the mother steel sheet 1 with respect to the voids 33 . Therefore, from the viewpoint of suppressing the formation of Cr-depleted regions, the distance from the average center of gravity of each void to the interface between the intermediate layer and the insulating coating is preferably within 4/5 of the thickness of the insulating coating. It is more preferably within 2/3, still more preferably within 1/2.

The porosity, the average diameter of the voids, and the average center-of-gravity position of the insulating coating 30, which rises due to the formation of voids, can be determined by the following measurement methods.
A section perpendicular to the rolling direction of the steel sheet is observed by SEM for 1 mm or more in a direction parallel to the surface of the steel sheet, and the area ratio of voids is calculated by image analysis. The average diameter is obtained by averaging circle-equivalent diameters calculated from the area of each void in the observation region. The average center-of-gravity position is determined by image analysis.
Specifically, a backscattered electron image taken at a magnification of 10,000 times is converted into a 256-gradation monochrome image. Of the 256 gradations, 50% of the gradations from the black side are converted into a binary image as a threshold value, and the black region is defined as an insulating film (including voids). Also, among the 256 gradations, 20% of the gradations from the black side are converted into a binary image as a threshold value, and the black regions are defined as voids.
The void ratio in the insulating film is calculated by the following formula from the area of each region obtained from the two binarized images.
(Void ratio) = (area of voids) / (area of insulating coating (including voids)) x 100 (%)
Also, the average diameter of the voids is the average circle equivalent diameter of the voids obtained by particle analysis with respect to the binarized image.
Furthermore, the average center-of-gravity position of the void is the average distance h _ave between the mother steel plate surface and all pixels inside the void, which is obtained by the following equation, where h is the distance from the mother steel plate surface to the image pixels inside the void.

Here, in the above formula, n is the total number of pixels within the void, hi is the distance from the _i -th pixel within the void to the surface of the mother steel plate.

[Total Area Ratio of Cr Depleted Region in Insulating Film 30]
In the insulating coating 30, a region where the Cr concentration is less than 80% of the average Cr concentration of the entire insulating coating 30 is defined as a "Cr-deficient region". In the insulating coating 30 in the cross section perpendicular to the surface of the mother steel sheet 1, the void ratio of the insulating coating 30 is 5 to 30%, and the distance from the average center of gravity of the voids to the interface between the intermediate layer and the insulating coating is the distance of the insulating coating. Within 4/5 of the thickness, the area ratio of the Cr deficient region in the insulating film 30 can be controlled to 50% or less.

The total area ratio of the Cr-deficient region is calculated by analyzing 20 μm or more of the surface of the steel sheet in a direction parallel to the surface of the steel sheet. Measurements are made using a TEM. The entire insulating film 30 is observed with a TEM. EDS (energy dispersive X-ray spectroscopy) surface analysis is performed on the Cr concentration distribution on the observed surface. If the Cr concentration is less than 80% of the average concentration of the entire insulation film 30 in terms of atomic concentration, it is identified as a Cr-deficient region.
The average Cr concentration of the entire insulating coating 30 is the average value of all surface analysis results excluding cavities of the coating. The ratio (%) of the total area of the identified Cr-deficient region to the total area of the entire insulating coating 30 in the viewing field is defined as the total area ratio (%) of the Cr-deficient region to the entire coating area of the viewing field. Define.

The thickness of the insulating coating 30 is not particularly limited. The thickness of the insulating coating 30 is preferably 0.1 μm or more, more preferably 0.5 μm or more. In this case, tension can be effectively applied to the mother steel sheet 1, and excellent water resistance is exhibited.
On the other hand, the thickness of the insulating coating 30 is preferably 10 μm or less, more preferably 5 μm or less. In this case, it is possible to suppress a decrease in the space factor due to an increase in the thickness of the insulating film, and it is possible to suppress deterioration of iron loss.

The thickness of the insulating coating 30 can be measured by observing the cross section of the coating 20 with SEM or TEM.

The insulating coating 30 may optionally be subjected to a magnetic domain refining treatment to form local micro-strain regions or grooves by laser, plasma, mechanical methods, etching, or other techniques.

[Manufacturing method of grain-oriented electrical steel sheet]
The grain-oriented electrical steel sheet according to the present embodiment described above can be manufactured, for example, by the following manufacturing method. An example of the method for manufacturing the grain-oriented electrical steel sheet of the present embodiment will be described in detail below.

A grain-oriented electrical steel sheet according to the present embodiment is obtained by a manufacturing method including the following steps S0 to S9.
S0: Preparatory step S1: Hot rolling step S2: Hot rolled sheet annealing step S3: Cold rolling step S4: Decarburization annealing step S5: Annealing separator application step S6: Finish annealing step S7: Mother (if necessary) Steel plate surface smoothing step S8: Intermediate layer forming step S9: Insulating film forming step Each step will be described below.

[S0: Preparatory step]
In the preparation step, a slab (steel material) to be used for manufacturing the grain-oriented electrical steel sheet is prepared. As for the chemical composition of the slab, the chemical composition of the mother steel sheet in a general grain-oriented electrical steel sheet can be used. An example of a slab manufacturing method is as follows.
Manufacture (smelt) molten steel. Molten steel is used to produce slabs. A slab may be produced by a continuous casting method. An ingot may be produced using molten steel, and the ingot may be bloomed to produce a slab. Other methods may be used to produce slabs. The thickness of the slab is not particularly limited. The thickness of the slab is, for example, 150-350 mm. The thickness of the slab is preferably 220-280 mm. A so-called thin slab having a thickness of 10 to 70 mm may be used as the slab. When a thin slab is used, rough rolling before finish rolling in the next process can be omitted in the hot rolling process.

[Slab chemical composition]
The chemical composition of the slab contains, for example, the following elements. Percentages relating to chemical compositions are mass percentages.

Si: 0.80-7.00%
Silicon (Si) increases the electrical resistance of the grain-oriented electrical steel sheet and reduces iron loss. If the Si content is less than 0.80%, γ-transformation occurs during finish annealing, and the crystal orientation of the grain-oriented electrical steel sheet is damaged. Therefore, the Si content is preferably 0.80% or more. The Si content is more preferably 2.00% or more, still more preferably 2.50% or more.
On the other hand, if the Si content exceeds 7.00%, the cold workability deteriorates and cracks are likely to occur during cold rolling. Therefore, preferably the Si content is 7.00% or less. The Si content is more preferably 4.50% or less, still more preferably 4.00% or less.

C: 0.085% or less,
Carbon (C) is inevitably contained. C is an element effective in controlling the primary recrystallized structure, but it adversely affects the magnetic properties. Therefore, the C content is preferably 0.085% or less. The C content is preferably as low as possible.
However, considering productivity in industrial production, the lower limit of the C content is preferably 0.020%, more preferably 0.050%. C is purified in the decarburization annealing process and the finish annealing process, which will be described later, and becomes 0.005% or less in the mother steel sheet after the finish annealing process.

Acid-soluble Al: 0.010-0.065%
Acid-soluble aluminum (Al) combines with N to precipitate as (Al, Si)N and functions as an inhibitor. Secondary recrystallization is stable when the content of acid-soluble Al is in the range of 0.010 to 0.065%. Therefore, the content of acid-soluble Al is preferably 0.010 to 0.065%. The acid-soluble Al content is more preferably 0.015% or more, still more preferably 0.020% or more. From the viewpoint of secondary recrystallization stability, the acid-soluble Al content is more preferably 0.045% or less, and still more preferably 0.035% or less.
If acid-soluble Al remains after finish annealing, it forms a compound and deteriorates iron loss. Therefore, it is preferable to reduce acid-soluble Al as much as possible in the mother steel sheet of the grain-oriented electrical steel sheet by purification during finish annealing.

N: 0.0040 to 0.0120%
Nitrogen (N) binds to Al and functions as an inhibitor. If the N content is less than 0.0040%, a sufficient amount of inhibitor is not produced. Therefore, the N content is preferably 0.0040% or more. The N content is more preferably 0.0050% or more, still more preferably 0.0060% or more.
On the other hand, if the N content exceeds 0.0120%, blisters, which are a type of defect, tend to occur in the steel sheet. Therefore, the N content is preferably 0.0120% or less. The N content is more preferably 0.0110% or less, still more preferably 0.01000% or less.
N is purified in the finish annealing process, and becomes 0.0050% or less in the mother steel sheet after the finish annealing process.

Mn: 0.05-1.00%
Manganese (Mn) combines with S or Se to produce MnS or MnSe and functions as an inhibitor. Secondary recrystallization is stable when the Mn content is in the range of 0.05 to 1.00%. Therefore, the Mn content is preferably 0.05-1.00%. The Mn content is more preferably 0.06% or more, still more preferably 0.08% or more. The Mn content is more preferably 0.50% or less, still more preferably 0.20% or less.

S and Se: 0.003 to 0.015% in total
Sulfur (S) and selenium (Se) combine with Mn to produce MnS or MnSe and act as inhibitors. If the total content of S and Se is 0.003 to 0.015%, secondary recrystallization is stabilized. Therefore, the total content of S and Se is preferably 0.003 to 0.015%.
If S and Se remain after finish annealing, they form compounds and deteriorate iron loss. Therefore, it is preferable to reduce the S content and the Se content in the mother steel sheet after the finish annealing as much as possible by purification during the finish annealing.

Here, "the total content of S and Se is 0.003 to 0.015%" means that the chemical composition of the slab contains only either S or Se, and either S or Se The content of one may be 0.003 to 0.015% in total, or the slab contains both S and Se, and the total content of S and Se is 0.003 to 0.015% may be

The remainder of the slab chemical composition consists of Fe and impurities. The “impurities” referred to here are mixed from components contained in raw materials or components mixed in the manufacturing process when the mother steel sheet 1 is industrially manufactured, and are obtained by the grain-oriented electrical steel sheet according to the present embodiment. It means an element that does not substantially affect the effect.

[Arbitrary element]
The chemical composition of the slab may contain one or more arbitrary elements in the following range in place of a part of Fe, considering the effect on the magnetic properties and the enhancement of the inhibitor function due to the formation of the compound. good. Examples of optional elements contained in place of part of Fe include the following elements.
Bi: 0.010% or less,
B: 0.080% or less,
Ti: 0.015% or less,
Nb: 0.20% or less,
V: 0.15% or less,
Sn: 0.10% or less,
Sb: 0.10% or less,
Cr: 0.30% or less,
Cu: 0.40% or less,
P: 0.50% or less,
Ni: 1.00% or less, and Mo: 0.10% or less.

[S1: Hot rolling step]
In the hot-rolling step, the prepared slab is hot-rolled using a hot rolling mill to produce a hot-rolled steel sheet (hot-rolled steel sheet for grain-oriented electrical steel sheet).
Specifically, first, the slab is heated. For example, the slab is loaded into a known heating furnace or a known soaking furnace and heated. A preferred heating temperature for the slab is 1280° C. or less. By setting the slab heating temperature to 1280°C or less, for example, various problems (necessity of a dedicated heating furnace, large amount of molten scale, etc.) when heated at a temperature higher than 1280°C can be avoided. be able to.

The lower limit of the slab heating temperature is not particularly limited. If the heating temperature is too low, hot rolling becomes difficult and productivity may decrease. Therefore, the heating temperature should be set in the range of 1280° C. or less in consideration of productivity. A preferable lower limit of the slab heating temperature is 1100°C. A preferred upper limit for the heating temperature of the slab is 1250°C.

It is also possible to omit the slab heating process itself and start hot rolling after casting before the temperature of the slab drops.

Next, the heated slab is hot-rolled using a hot rolling mill to produce a hot-rolled steel sheet. A hot rolling mill, for example, comprises a roughing mill and a finishing mill arranged downstream of the roughing mill. The roughing mill comprises a row of roughing stands. Each roughing stand includes a plurality of rolls arranged one above the other. The finishing mill likewise comprises a row of finishing stands. Each finishing stand includes a plurality of rolls arranged one above the other. After the heated steel material is rolled by a rough rolling mill, it is further rolled by a finishing rolling mill to produce a hot-rolled steel sheet.

The thickness of the hot-rolled steel sheet manufactured by hot rolling is not particularly limited. The thickness of the hot-rolled steel sheet is, for example, 3.5 mm or less.
The finishing temperature in the hot rolling process (the temperature of the steel sheet at the delivery side of the finishing rolling stand where the steel sheet is finally rolled down in the finishing mill) is, for example, 900 to 1000.degree. A hot-rolled steel sheet is manufactured by the hot rolling process described above.

[S2: Hot-rolled sheet annealing step]
In the hot-rolled sheet annealing step, hot-rolled sheet annealing is performed on the hot-rolled steel sheet for grain-oriented electrical steel sheets manufactured in the hot-rolling process to manufacture an annealed steel sheet.

As for the conditions for hot-rolled sheet annealing, for example, the annealing temperature in hot-rolled sheet annealing (furnace temperature in a hot-rolled sheet annealing furnace) is 750 to 1200.degree. The holding time at the annealing temperature is, for example, 30-600 seconds.

[S3: Cold rolling step]
In the cold-rolling process, the annealed steel sheet after the hot-rolled sheet annealing process is cold-rolled to produce a cold-rolled steel sheet. Cold rolling is performed using a cold rolling mill. A cold rolling mill comprises a plurality of cold rolling stands arranged in a row. Each cold rolling stand includes multiple cold rolling rolls.

In the cold rolling process, cold rolling may be performed only once or may be performed multiple times. When cold rolling is performed multiple times, intermediate annealing is performed for the purpose of softening after cold rolling, and then cold rolling is performed again. A known method is used for the intermediate annealing conditions.

When performing a plurality of cold rolling steps without performing an intermediate annealing step, it may be difficult to obtain uniform properties in the produced grain-oriented electrical steel sheet. On the other hand, when the cold rolling process is performed a plurality of times and the intermediate annealing process is performed between each cold rolling process, the produced grain-oriented electrical steel sheet may have a low magnetic flux density. Therefore, the number of cold rolling steps and the presence or absence of an intermediate annealing step are determined according to the properties and production costs required for the grain-oriented electrical steel sheet to be finally produced.

In the cold rolling performed once or multiple times, the cumulative cold rolling reduction (cumulative rolling reduction) is preferably 80% or more, more preferably 90% or more. A preferred upper limit for the cumulative cold rolling reduction is 95%. Here, the cumulative cold rolling rate (%) is defined as follows.
Cold rolling rate (%) = (1-thickness of cold-rolled steel sheet after last cold rolling/thickness of annealed steel sheet before starting first cold-rolling) x 100

Before cold rolling the annealed steel sheet, the annealed steel sheet may be pickled. The manufactured cold-rolled steel sheet is wound into a coil. The thickness of the cold-rolled steel sheet is not particularly limited, but is preferably 0.35 mm or less, more preferably 0.30 mm or less, in order to further reduce iron loss.

[S4: Decarburization annealing step]
In the decarburization annealing step, the cold-rolled steel sheet produced in the cold rolling step is subjected to decarburization annealing and primary recrystallization. Decarburization annealing is performed, for example, by the following method.
A cold-rolled steel sheet is charged into a heat treatment furnace. The temperature of the heat treatment furnace (decarburization annealing temperature) is, for example, 800 to 950° C., and the atmosphere of the heat treatment furnace is a moist atmosphere containing hydrogen and nitrogen. By performing decarburization annealing, primary recrystallization occurs and carbon in the steel sheet is removed from the steel sheet. As described above, an example of the decarburization annealing temperature in the decarburization annealing step is 800 to 950° C., and an example of the holding time at the decarburization annealing temperature is 15 to 150 seconds.

[S5: Annealing separator application step]
In the annealing separator application step, an annealing separator is applied to the surface of the steel sheet. The annealing separator is not particularly limited. For example, an annealing separator containing alumina (Al ₂ O ₃ ) as a main component, an annealing separator containing magnesia (MgO) as a main component, or an annealing separator containing both of them as a main component may be used.

As the annealing separator, an annealing separator containing alumina as a main component is preferable. This is because the formation of unevenness at the interface between the finish annealing film and the mother steel sheet 1 can be suppressed. As the annealing separator containing alumina as a main component, an annealing separator containing both alumina and magnesia as main components is preferable. This is because Al contained in the steel sheet can be incorporated into the finish annealing film to purify the steel sheet, so that internal oxidation of Al contained in the steel sheet and an increase in iron loss can be suppressed.

In addition, the annealing separator containing alumina and magnesia as main components preferably has a mass ratio of magnesia in the main component of 10 to 60%, more preferably 20 to 50%, particularly 20 to 40%. preferable. This is because if the mass ratio of magnesia in the main components is less than 20% (the mass ratio of alumina exceeds 80%), the Al contained in the steel sheet cannot be incorporated into the finish annealing film to purify the steel sheet. On the other hand, if the mass ratio of magnesia is high, the magnesia tends to react with the mother steel sheet 1 during the finish annealing, causing unevenness at the interface between the finish annealing film and the steel sheet, thereby deteriorating iron loss.

The mother steel sheet 1 (decarburized annealed steel sheet) after the annealing separating agent application step is wound into a coil shape and supplied to the next finish annealing step.

[S6: Finish annealing step]
In the finish annealing process, the mother steel sheet 1 after the annealing separating agent application process is subjected to finish annealing. This causes secondary recrystallization in the mother steel plate 1 . Further, when finish annealing is performed, the annealing separator reacts with the mother steel sheet 1 to form a finish annealing film on the surface of the mother steel sheet. The finish annealing coating contains a product generated by the reaction between the annealing separator and the steel sheet, and may contain unreacted annealing separator. As a result, the mother steel sheet on which the finish annealing film is formed is manufactured by the finish annealing step.

For example, when an Al-containing annealing separator, such as an alumina-based annealing separator, is applied, the annealing separator reacts with the mother steel plate 1 to produce an Al-containing oxide as the main component. A finish baked pure coating is formed. Further, even when the annealing separator not containing Al is applied, the annealing separator and Al contained in the mother steel sheet 1 react with each other to form a finish annealed coating mainly composed of oxides containing Al on the surface of the steel sheet. is formed. Further, when an annealing separating agent containing magnesia as a main component is applied, the annealing separating agent reacts with the mother steel sheet 1 to form a final annealing film mainly containing forsterite (Mg ₂ SiO ₄ ). . Furthermore, when an annealing separator containing Al or Mg is applied, the annealing separator does not react completely, and a finish annealing film containing unreacted annealing separator is formed. There is

The finish annealing conditions are not particularly limited, and for example, it is performed by heating at a temperature within the range of 1100 to 1300° C. for 20 to 24 hours.

Further, when an annealing separator not containing Al is applied, the annealing separator and Al contained in the mother steel plate 1 are reacted to finish annealing mainly composed of oxides containing Al on the surface of the steel plate. Also in the case of forming a film, the finish annealing conditions do not have to be special conditions, and general conditions may be used.

Finish annealing also serves as refinement annealing. Purification annealing removes the inhibitor components such as Al, N, Mn, S and Se from the steel.

[S7: mother steel plate surface smoothing step]
The mother steel plate surface smoothing step is performed as necessary. That is, the mother steel plate surface smoothing step may not be performed. When the mother steel sheet surface smoothing step is performed, in the mother steel sheet surface smoothing step, the surface of the mother steel sheet after finish annealing is optionally smoothed so as to avoid interference with the iron loss reducing action of the insulating coating 30. Adjust to a smooth surface (smooth). Specifically, it is preferable to adjust the arithmetic mean roughness Ra of the surface of the mother steel sheet to, for example, 1.0 μm or less. This is because it is possible to effectively avoid interference with the action of reducing iron loss. It is more preferable to adjust the arithmetic mean roughness Ra of the surface of the mother steel sheet after smoothing to 0.7 μm or less, more preferably 0.5 μm or less.

The necessity of performing the step of smoothing the surface of the mother steel sheet depends on whether unevenness is formed at the interface between the finish annealing coating and the mother steel sheet 1 or when unevenness is not formed at the interface between the finish annealing coating and the mother steel sheet 1. broadly classified. Each case will be described below.

“When irregularities are formed at the interface between the finish annealing film and the mother steel sheet 1” means that the finish annealing film is formed on the mother steel sheet 1 like a conventional grain-oriented electrical steel sheet in which the finish annealing film mainly composed of forsterite is formed. It means a case where a so-called "root" is formed at the interface with the steel plate 1 to a deep position inside the mother steel plate, and the effect of the insulation coating 30 to reduce iron loss is hindered. Specifically, it means that the arithmetic mean roughness Ra of the surface of the mother steel plate after finish annealing exceeds, for example, 1.0 μm.
On the other hand, "the case where unevenness is not formed at the interface between the finish-annealed coating and the mother steel sheet 1" means the case where the interface between the finish-annealed coating and the mother steel sheet 1 does not have unevenness. Specifically, it means that Ra (arithmetic mean roughness) of the mother steel plate surface after finish annealing is, for example, 1.0 μm or less.

(1) When unevenness is formed at the interface between the finish-annealed coating and the mother steel sheet 1 When unevenness is formed at the interface between the finish-annealed coating and the mother steel sheet 1, the insulating coating 30 acts to reduce iron loss. A step of smoothing the surface of the mother steel sheet is carried out to avoid interference by unevenness of the interface. Specifically, the finish annealing film is completely removed from the surface of the steel sheet after finish annealing, and the surface of the mother steel sheet after finish annealing is smoothed to adjust the surface to a smooth surface.

As a method of removing all of the finish annealing coating, a method of carefully removing it by means of pickling, grinding, etc., and exposing the mother steel sheet 1 is used. As a method for smoothing the surface of the mother steel sheet after finish annealing, a method of smoothing the surface of the mother steel sheet by chemical polishing or electropolishing is used.

(2) When unevenness is not formed at the interface between the finish annealing film and the mother steel sheet 1 If unevenness is not formed at the interface between the finish annealing film and the mother steel sheet 1, the mother steel sheet surface smoothing step need not be performed. .

[S8: Intermediate layer forming step]
In the intermediate layer forming step, the mother steel plate 1 is heat-treated after the finish annealing step, or after the mother steel plate surface smoothing step when unevenness is formed at the interface between the finish annealing film and the mother steel plate 1. to form an intermediate layer 2B mainly composed of silicon oxide in contact with the surface of the mother steel plate 1. As shown in FIG.

The conditions for the heat treatment are not particularly limited, but when forming the intermediate layer 2B with a thickness of 2 to 400 nm, it is preferable to hold the temperature in the temperature range of 300 to 1150° C. for 5 to 120 seconds. More preferably, the temperature range is maintained for 10 to 60 seconds.

Furthermore, from the viewpoint of preventing the inside of the mother steel sheet 1 from being oxidized, it is preferable that the atmosphere during the temperature rise and the temperature maintenance during annealing be a reducing atmosphere, and more preferably a nitrogen atmosphere mixed with hydrogen. . As the nitrogen atmosphere mixed with hydrogen, for example, an atmosphere consisting of hydrogen: 5 to 50% by volume and the remainder: nitrogen and impurities, and having a dew point: -20 to 2°C can be mentioned. Among them, an atmosphere containing 10 to 35% by volume of hydrogen, the remainder consisting of nitrogen and impurities, and having a dew point of -10 to 0°C is preferable.

In the intermediate layer forming step, hydrogen: 20 to 50% by volume, the balance: nitrogen and impurities, dew point: in an atmosphere of -20 to 2 ° C. in a temperature range of 600 to 1150 ° C. Hold for 10 to 60 seconds to form a steel plate. It is preferable to heat-treat the

[S9: Insulating film forming step]
In the insulating film forming step, an insulating film 30 made of a compound containing P, O and Si is formed on the surface of the intermediate layer. The insulating coating 30 may contain Cr. The insulating film forming step includes a coating step of applying a coating solution for the insulating film 30 onto the surface of the intermediate layer 2B, and a step of heating the mother steel plate 1 coated with the coating solution and performing a baking treatment to form the insulating film 30. and a baking step to form the The coating process and the baking process will be described in detail below.

[Coating process]
In the application step, a coating solution containing phosphate, colloidal silica, and optionally chromic anhydride or chromate is applied onto the surface of intermediate layer 2B on mother steel plate 1 . Chromic anhydride or chromate may not be contained. Phosphates include, for example, phosphates of Ca, Al, Mg, Sr, and the like. Colloidal silica is particulate silicon dioxide (SiO ₂ ) with a diameter of about 1 nm to 1 μm. Colloidal silica is not particularly limited, and its particle size can be used as appropriate. Examples of chromates include chromates of Na, K, Ca, Sr, and the like. In addition, various elements and compounds may be further added to the coating solution to improve various properties. The coating method is also not particularly limited, and a known method is sufficient.

[Baking process]
In the baking step, the mother steel plate 1 coated with the coating solution is heated and baked to form the insulating coating 30 . At the same time, voids 33 are formed in the insulating coating 30 by this baking process. The formed voids suppress penetration of Fe into the insulating coating 30 due to thermal diffusion.

In the baking process, the mother steel sheet 1 coated with the coating solution is heated in a heat treatment furnace. Then, the baking process is performed in a temperature range (baking temperature) of 600 to 1150°C. The holding time at the baking temperature is 5 to 300 seconds.

At the time of heating before reaching the baking temperature, the average heating rate in the temperature range of 100° C. to 600° C. of the mother steel plate 1 is set to 10 to 200° C./sec. The insulating film 30 emits contained moisture and oxygen as gas in a temperature range between 100.degree. C. and 600.degree. By controlling the heating rate in the temperature range between 100° C. and 600° C., it is believed that the amount of voids 33 caused by degassing (the void ratio of the insulating coating) and the size of the voids 33 (average diameter) can be controlled. .
If the average heating rate in the surface temperature range of 100° C. to 600° C. of the mother steel sheet 1 is less than 10° C./sec, the voids 33 are not sufficiently formed in the insulating coating 30 because the degassing rate is slow. As a result, the void ratio in the insulating coating 30 becomes less than 5%, and the adhesion of the insulating coating is lowered. On the other hand, if the average heating rate exceeds 200.degree. Therefore, the void ratio in the insulating coating 30 exceeds 30%. In this case, the insulating properties of the steel plate are degraded. Therefore, the average heating rate in the surface temperature range of 100° C. to 600° C. of mother steel plate 1 is set to 10 to 200° C./sec.

Further, when the insulation coating 30 contains Cr, the voids 33 are formed in the surface temperature range of the mother steel plate 1 from 100° C. to 600° C., and at the same time, the subsequent insulation coating baking temperature is set at 800° C. or more and less than 900° C. By controlling, the position of voids can be controlled to the vicinity of the interface between the intermediate layer and the insulating coating, and the diffusion of Fe into the insulating coating 30 can be effectively suppressed. Therefore, water resistance can also be improved. The reason for this is as follows.
The effect of suppressing Fe diffusion by the voids is effective for Fe on the mother steel sheet 1 side (the interface side between the intermediate layer 2B and the insulating coating 30) rather than the voids. Therefore, the closer the voids exist to the interface side at the baking temperature at which the diffusion speed is the fastest, the more the diffusion of Fe is suppressed, and the effect of suppressing the formation of the Cr-depleted region is high. The reason why the position of the void 33 changes due to the above control is not clear. It is believed that the reaction rate between phosphate and colloidal silica during film formation influences the formation of void nuclei in appropriate regions near the interface between the intermediate layer and the insulating film. Furthermore, the degassing process of water vapor, etc. remaining in the voids also affects the baking temperature, and the growth of voids becomes predominant at positions near the interface between the intermediate layer and the insulating film. As a result, the void distribution is in a favorable state, and it is thought that the diffusion of Fe and the resulting expansion of the Cr depleted layer can be efficiently suppressed.

Other conditions in the baking process may be well-known conditions. The atmosphere for the baking treatment may be, for example, an atmosphere having a gas oxidation degree (PH ₂ O/PH ₂ ): 0.001 to 0.1, or may be another atmosphere.

In the insulating film forming step, it is preferable to cool the steel sheet in an atmosphere in which the degree of oxidation of the gas is kept low so that the insulating film 30 and the intermediate layer 2B do not change after baking. As cooling conditions, general conditions may be used, but for example, hydrogen: 75% by volume and the remainder: nitrogen and impurities, dew point: 5 to 10° C., and degree of gas oxidation (PH ₂ O/PH ₂ ). : Cool in an atmosphere of less than 0.01.

[Other manufacturing processes]
The method for manufacturing a grain-oriented electrical steel sheet according to the present embodiment may further include steps generally performed in a method for manufacturing a grain-oriented electrical steel sheet. Preferably, the method for manufacturing a grain-oriented electrical steel sheet according to the present embodiment includes a nitriding treatment step for increasing the N content in the steel sheet after the decarburization annealing step and before the finish annealing step. It may contain further. This is because the magnetic flux density can be stably improved even if the temperature gradient given to the steel sheet at the boundary portion between the primary recrystallized region and the secondary recrystallized region is low. Any general treatment may be used as the nitriding treatment. For example, annealing in an atmosphere containing a gas with nitriding ability such as ammonia, finish annealing of a mother steel sheet (decarburized annealing steel sheet) coated with an annealing separator containing powder with nitriding ability such as MnN, etc. is mentioned.

The invention is not limited to the embodiments described above. The above-described embodiment is an example, and any device that has substantially the same configuration as the technical idea described in the claims of the present invention and produces similar effects is the present invention. It is included in the technical scope of the invention.

EXAMPLES Hereinafter, the present invention will be specifically described by presenting examples. In the following, the conditions in the examples are examples of conditions adopted for confirming the feasibility and effects of the present invention. The present invention is not limited to this one conditional example. Various conditions can be adopted in the present invention as long as the object of the present invention is achieved without departing from the gist of the present invention.

<Example 1>
In % by mass, Si: 3.30%, C: 0.050%, acid-soluble Al: 0.030%, N: 0.0080%, and Mn: 0.10%, S and Se: 0.1% in total. 005%, with the balance being Fe and impurities.
The slab was soaked at 1150° C. for 60 minutes.
The slab after heating was subjected to hot rolling to produce a hot-rolled steel sheet having a thickness of 2.6 mm.
Annealed steel sheets were manufactured by performing hot-rolled sheet annealing on the manufactured hot-rolled steel sheets. The conditions for hot-rolled sheet annealing were to keep the temperature at 900° C. for 120 seconds.
The produced annealed steel sheets were cold-rolled to produce cold-rolled steel sheets having a thickness of 0.3 mm.

Decarburization annealing was applied to the produced cold-rolled steel sheets. The decarburization annealing was carried out in an atmosphere containing 75% by volume of hydrogen and the balance of nitrogen and impurities under conditions of holding at 850° C. for 90 seconds.

An annealing separator was applied to the surface of the obtained mother steel sheet. For test numbers 1 to 8, the annealing separator was alumina (Al ₂ O ₃ ) and magnesia (MgO) at a mass ratio of 60:40. Further, for test numbers 9 to 16, an annealed separator composed only of magnesia (MgO) was used.

The steel plate to which the annealing separator was applied was subjected to finish annealing to obtain a mother steel plate. The finish annealing was carried out under the conditions of heating to 1200° C. at a rate of temperature increase of 15° C./hour in a hydrogen-nitrogen mixed atmosphere and then holding at 1200° C. for 20 hours in a hydrogen atmosphere. After that, it was naturally cooled to obtain a steel sheet in which secondary recrystallization and purification were completed.
The chemical composition of the steel sheet after finish annealing is all in mass %, Si: 3.30%, C: 0.0020% or less, acid-soluble Al: 0.0030% or less, N: 0.0020% or less, and It contained Mn: 0.10%, S and Se: 0.0005% or less in total, and the balance consisted of Fe and impurities.

After the finish annealing, the base steel sheet was subjected to heat treatment, and for test numbers 9 to 16, after smoothing the surface of the base material, an intermediate layer was formed. The conditions of the intermediate layer forming process were as follows. Hydrogen: 35% by volume, balance: nitrogen and unavoidable impurities, dew point: -2°C atmosphere, heated to 800°C and held for 30 seconds. After that, it was naturally cooled. In any example, the composition of the intermediate layer is Fe content less than 80 atomic %, P content less than 5 atomic %, Si content 20 atomic % or more, O content 50 atomic % or more, Mg content satisfies 10 atomic % or less.

An insulating coating was formed on the mother steel plate on which the intermediate layer was formed. When forming the insulating coating, first, a coating solution was applied to the surface of the intermediate layer. The composition of the coating solution was aluminum phosphate: 60% and colloidal silica: 40%. The steel plate coated with the coating solution was heated to 850° C. in an atmosphere containing 75% by volume of hydrogen and the balance of nitrogen and impurities. At that time, the average heating rate in the temperature range between 100° C. and 600° C. of the surface temperature of the steel sheet was as described in Table 1, “heating rate in baking step”. The heated steel plate was held for 30 seconds to bake the insulating coating. After baking, the steel plate was cooled to room temperature.

On the surfaces of the steel sheets of Test Nos. 1 to 16, insulating coatings 30 containing colloidal silica 32 and containing phosphate as a main component were formed. The thickness of the insulating coating was 1.5 to 2.0 μm. A grain-oriented electrical steel sheet was produced through the above steps.
For the obtained grain-oriented electrical steel sheets, the void ratio in the insulating coating was measured. Also, the adhesion and insulating properties of the film were evaluated.

[Measurement of void content in insulating film]
The void ratio in the insulating film was measured.
Specifically, a test piece was taken from the grain-oriented electrical steel sheet of each test number so that a cross section perpendicular to the rolling direction of the grain-oriented electrical steel sheet was obtained. The cross section of the test piece was observed using an SEM at a magnification of 10,000 times and a visual field area of 8 μm×6 μm. The observation area was 1 mm in the direction parallel to the surface of the grain-oriented electrical steel sheet. The area ratio of cavities was calculated by image analysis.
More specifically, a backscattered electron image taken at a magnification of 10,000 times is converted into a monochrome image of 256 gradations, and of the 256 gradations, 50% of the gradation from the black side is used as a threshold for conversion into a binary image. , the black area was defined as the insulating film (including voids). Also, among the 256 gradations, 20% of the gradations from the black side were converted into a binary image as a threshold value, and the black regions were defined as voids.
Then, the void ratio in the insulating film was calculated by the following formula from the area of each region obtained from the two binarized images.
(Void ratio) = (area of voids) / (area of insulating coating (including voids)) x 100 (%)
Table 1 shows the results.
Also, the average diameter of voids was obtained by averaging circle-equivalent diameters obtained by particle analysis for the binarized image using the same image.

[Adhesion test]
The adhesion test was performed according to the flex resistance test of JIS K 5600-5-1 (1999). From the grain-oriented electrical steel sheets of Test Nos. 1 to 16, test pieces of 80 mm in the rolling direction and 40 mm in the direction perpendicular to the rolling were taken. The sampled test piece was wound around a round bar with a diameter of 16 mm. In the adhesion test, 180° bending was performed using a type 1 testing apparatus described in the bending resistance test of JIS K 5600-5-1 (1999). The total area of the portions where the insulating coating 30 was peeled off was measured for the bent test piece. The ratio of the total area of the portion where the insulating coating 30 was peeled to the total surface area of the test piece on the coating side was defined as the φ16 mm bending coating peeling rate (%). Table 1 shows the results.
If the φ16 mm bending film peeling rate (%) was 50% or less, it was determined that the adhesion was excellent.

[Insulation test]
The insulation test was performed according to JIS C 2550-4 (2011). From the grain-oriented electrical steel sheets of test numbers 1 to 14, test pieces of 30 mm in the direction perpendicular to the rolling direction and 280 mm in the rolling direction were taken. Interlayer resistance (Ω·cm ² ) described in JIS C 2550-4 was measured for the sampled test piece. The voltage was 0.5 V and the pressure was 2 N/mm ² . An interlayer resistance value was calculated from the total current value flowing through the ten contactor electrodes. The average value of the measured inter-layer resistance values was obtained and used as the inter-layer resistance. Table 1 shows the results. If the interlayer resistance was 100 Ω·cm ² or more, it was determined that the insulation was excellent.

With reference to Table 1, in Test Nos. 3 to 6 and Test Nos. 11 to 14, the φ16 mm bending film peeling rate was 50% or less, the interlayer resistance was 100 Ω·cm ² or more, and adhesion and insulation were obtained. Excellent in nature.

On the other hand, in Test Nos. 1, 2, 9 and 10, the heating rate in the baking process was less than 10° C./sec, and the void ratio in the insulating coating was less than 5%. As a result, the φ16 mm bending film peeling rate exceeded 50%. That is, the adhesiveness was low.

In test numbers 7, 8, 15 and 16, the heating rate in the baking process exceeded 200°C/sec, and the void ratio in the insulating coating exceeded 30%. As a result, the interlayer resistance was less than 100 Ωcm ² . That is, the insulating property was low.

<Example 2>
In % by mass, Si: 3.30%, C: 0.050%, acid-soluble Al: 0.030%, N: 0.0080%, and Mn: 0.10%, S and Se: 0.1% in total. 005%, with the balance being Fe and impurities.
The slab was soaked at 1150° C. for 60 minutes.
The slab after heating was subjected to hot rolling to produce a hot-rolled steel sheet having a thickness of 2.6 mm.
Annealed steel sheets were manufactured by performing hot-rolled sheet annealing on the manufactured hot-rolled steel sheets. The conditions for hot-rolled sheet annealing were to keep the temperature at 900° C. for 120 seconds.
The produced annealed steel sheets were cold-rolled to produce cold-rolled steel sheets having a thickness of 0.3 mm.

Decarburization annealing was applied to the produced cold-rolled steel sheets. The decarburization annealing was carried out in an atmosphere containing 75% by volume of hydrogen and the balance of nitrogen and impurities under conditions of holding at 850° C. for 90 seconds.

An annealing separator was applied to the surface of the obtained mother steel sheet. The annealing separator was alumina (Al ₂ O ₃ ) and magnesia (MgO) at a mass ratio of 60:40. The steel plate to which the annealing separator was applied was subjected to finish annealing to obtain a mother steel plate. The finish annealing was carried out under the conditions of heating to 1200° C. at a rate of temperature increase of 15° C./hour in a hydrogen-nitrogen mixed atmosphere and then holding at 1200° C. for 20 hours in a hydrogen atmosphere. After that, it was naturally cooled to obtain a steel sheet in which secondary recrystallization and purification were completed.
In addition, the chemical composition of the steel sheet after finish annealing is, in mass%, Si: 3.30%, C: 0.0020% or less, acid-soluble Al: 0.0030% or less, N: 0.0020% or less. , and Mn: 0.10%, S and Se: 0.0005% or less in total, and the balance consisted of Fe and impurities.
An intermediate layer was formed by heat-treating the mother steel sheet after finish annealing. The conditions of the intermediate layer forming process were as follows. Hydrogen: 35% by volume, balance: nitrogen and unavoidable impurities, dew point: -2°C atmosphere, heated to 800°C and held for 30 seconds. After that, it was naturally cooled. In any example, the composition of the intermediate layer is Fe content less than 80 atomic %, P content less than 5 atomic %, Si content 20 atomic % or more, O content 50 atomic % or more, Mg content satisfies 10 atomic % or less.

An insulating coating was formed on the mother steel plate on which the intermediate layer was formed. First, a coating solution was applied to the surface of the intermediate layer. The composition of the coating solution was aluminum phosphate: 50%, colloidal silica: 50%, and chromic anhydride: 10%.
The steel plate coated with the coating solution was heated to the temperature shown in the "baking temperature" column in Table 2 in an atmosphere containing 75% by volume hydrogen and the balance nitrogen and impurities. In addition, the average heating rate in the temperature range between 100° C. and 600° C. of the surface temperature of the steel sheet was as shown in Table 2, “heating rate in baking step”. The heated steel plate was held for 30 seconds to bake the insulating coating.

After baking, the steel plate was cooled to room temperature. An insulation film was formed from a compound containing P, O and Si and Cr. The thickness of the insulating coating was 1.5 to 2.0 μm. A grain-oriented electrical steel sheet was produced through the above steps.
For the obtained grain-oriented electrical steel sheets, the void ratio in the insulating coating, the average diameter of voids, the position of the center of gravity, the Cr concentration in the insulating coating, and the total area ratio of Cr-deficient regions were measured. Also, the adhesion, insulation and water resistance of the film were evaluated.

[Measurement of void content in insulating film]
A test piece was taken from the grain-oriented electrical steel sheet of each test number so that a cross section perpendicular to the rolling direction of the grain-oriented electrical steel sheet was obtained. The cross section of the test piece was observed using an SEM at a magnification of 10,000 times and a visual field area of 8 μm×6 μm. The observation area was 1 mm in the direction parallel to the surface of the grain-oriented electrical steel sheet. The area ratio of voids was calculated by image analysis in the same manner as in Example 1.
Table 2 shows the results.

[Measuring the average diameter of voids in the insulating film and the position of the center of gravity]
Using the image used to measure the void ratio, the average diameter of the voids was obtained by averaging the circle-equivalent diameters calculated from the areas of the voids in the observation region in the same manner as in Example 1.
Further, the average center of gravity of the void was determined as the average distance h _ave between the surface of the mother steel sheet and all the pixels in the void, where h is the distance from the mother steel sheet surface to the image pixels in the void. .

Here, in the above formula, n is the total number of pixels within the void, hi is the distance from the _i -th pixel within the void to the surface of the mother steel plate.
Table 2 shows the results. A to D in the "average center of gravity position of voids" column in Table 2 indicate the following.
A: The distance from the average center of gravity of the voids to the interface between the intermediate layer and the insulating coating is more than 4/5 of the thickness of the insulating coating. B: The distance from the average center of gravity of the voids to the interface between the intermediate layer and the insulating coating is , 4/5 or less, more than 2/3 of the insulating film thickness C: The distance from the average center of gravity of the void to the interface between the intermediate layer and the insulating film is 2/3 or less, more than 1/2 of the insulating film thickness D: The distance from the average center of gravity of voids to the interface between the intermediate layer and the insulation film is 1/2 or less of the thickness of the insulation film.

[Measurement of Cr concentration in insulating film]
The Cr concentration in the insulating film was calculated by analyzing 10 μm in a direction parallel to the grain-oriented electrical steel sheet surface of each test number. Observing the entire insulating film with a TEM, EDS (energy dispersive X-ray analysis) surface analysis is performed on the Cr concentration distribution so that the entire thickness of the insulating film is included, and the insulation obtained by the surface analysis The average Cr concentration at each position of the film was defined as the average Cr concentration (atomic %). The measurement conditions were a magnification of 1000 times and a total visual field area of the coating portion of 20 μm ² .

[Measurement of total area ratio of Cr-deficient region in insulating film]
The total area ratio of the Cr-deficient region of the insulating film was measured. As a result of measuring the Cr concentration in the insulating coating as described above, it was determined that the Cr-deficient region was found when the Cr concentration was less than 80% of the average concentration of the entire insulating coating in terms of atomic concentration. The average Cr concentration of the entire insulating coating was the average value of all surface analysis results excluding cavities in the coating. The ratio (%) of the total area of the specified Cr-deficient region to the total area of the entire insulating coating in the viewing plane was defined as the area ratio (%) of the Cr-deficient region.

[Adhesion test]
The adhesion test was performed according to the flex resistance test of JIS K 5600-5-1 (1999). From the grain-oriented electrical steel sheets of test numbers 17 to 30, test pieces of 80 mm in the rolling direction and 40 mm in the direction perpendicular to the rolling were taken. The sampled test piece was wound around a round bar with a diameter of 16 mm. In the adhesion test, 180° bending was performed using a type 1 testing apparatus described in the bending resistance test of JIS K 5600-5-1 (1999). The total area of the portions where the insulating coating 30 was peeled off was measured for the bent test piece. The ratio of the total area of the portion where the insulating coating 30 was peeled to the total surface area of the test piece on the coating side was defined as the φ16 mm bending coating peeling rate (%). Table 2 shows the results.
If the φ16 mm bending film peeling rate (%) was 50% or less, it was determined that the adhesion was excellent.

[Insulation test]
The insulation test was performed according to JIS C 2550-4 (2011). From grain-oriented electrical steel sheets of test numbers 17 to 30, test pieces of 30 mm in the direction perpendicular to the rolling direction and 280 mm in the rolling direction were taken. Interlayer resistance (Ω·cm ² ) described in JIS C 2550-4 was measured for the sampled test piece. The voltage was 0.5 V and the pressure was 2 N/mm ² . An interlayer resistance value was calculated from the total current value flowing through the ten contactor electrodes. The average value of the measured inter-layer resistance values was obtained and used as the inter-layer resistance. Table 2 shows the results. If the interlayer resistance was 100 Ω·cm ² or more, it was determined that the insulation was excellent.

[Water resistance test]
The water resistance of the insulating coating was evaluated using the test piece after the above adhesion test. The bent portion of the test piece after the adhesion test was immersed in water while being bent. After 1 minute had passed, the test piece was pulled up, and the total area of the portion where the insulating film was peeled off was measured. The ratio of the total area of the portion where the insulating film was peeled off to the entire surface area of the test piece on the film side was taken as the film peeling rate (%) after immersion in water. Table 2 shows the results.
The water resistance was judged to be excellent when the difference between the film peeling rate after φ16 mm bending and the film peeling rate after immersion in water was 20% or less.

With reference to Table 2, in test numbers 17 to 22 and 25 to 28, the φ16 mm bending film peeling rate was 50% or less, the interlayer resistance was 100 Ω·cm ² or more, and the adhesion and insulation properties were excellent.
In test numbers 18 to 20 and 25 to 28, the film peeling rate after immersion in water was (φ 16 mm bending film peeling rate + 20)% or less, and the water resistance was also excellent.

On the other hand, in Test Nos. 23 and 24, the heating rate in the baking process was less than 10° C./sec, and the void ratio in the insulating coating was less than 5%. As a result, the φ16 mm bending film peeling rate exceeded 50%. That is, the adhesiveness was low. Moreover, it was inferior also in water resistance.

In test numbers 29 and 30, the heating rate in the baking process exceeded 200° C./sec, and the void ratio in the insulating coating exceeded 30%. As a result, the interlayer resistance was less than 100 Ωcm ² . That is, the insulating property was low.

REFERENCE SIGNS LIST 1 mother steel plate 2A final annealing film which is an oxide film containing Mg 2B intermediate layer 3, 30 insulating film 31 matrix 33 void

Claims (6)

a mother steel plate;
an intermediate layer formed in direct contact with the surface of the mother steel sheet and mainly composed of silicon oxide;
and an insulating film formed on the surface of the intermediate layer,
A grain-oriented electrical steel sheet, wherein the insulating coating contains voids, and the void ratio in the insulating coating is 5 to 30% in terms of area ratio. 2. The grain-oriented electrical steel sheet according to claim 1, wherein the average diameter of the voids is 1/4 or less and 1/20 or more of the thickness of the insulating coating.
Oriented electrical steel sheet. The grain-oriented electrical steel sheet according to claim 1 or 2,
The insulating film is composed of a compound containing P, O and Si and Cr, and the average Cr concentration in the insulating film is 0.1 atomic % or more.
Oriented electrical steel sheet. The grain-oriented electrical steel sheet according to any one of claims 1 to 3,
The distance from the average center of gravity of the voids to the interface between the insulating coating and the intermediate layer is 4/5 or less of the thickness of the insulating coating.
Oriented electrical steel sheet. The grain-oriented electrical steel sheet according to any one of claims 1 to 4,
On the surface of the insulating coating,
The total area ratio of the Cr-deficient region having a Cr concentration of less than 80% of the average Cr concentration of the entire insulating film is 50% or less.
Oriented electrical steel sheet. a preparation step of preparing a slab;
a hot rolling step of heating the slab at 1280° C. or less and then performing hot rolling to manufacture a hot rolled steel sheet for a grain-oriented electrical steel sheet;
A hot-rolled sheet annealing step of performing hot-rolled sheet annealing on the hot-rolled steel sheet for grain-oriented electrical steel sheet to produce an annealed steel sheet;
A cold-rolling step of cold-rolling the annealed steel sheet to produce a cold-rolled steel sheet;
a decarburization annealing step of performing decarburization annealing on the cold-rolled steel sheet to produce a mother steel sheet;
An annealing separator application step of applying an annealing separator to the mother steel plate;
A finish annealing step of performing finish annealing on the mother steel plate after the annealing separator application step to manufacture the mother steel plate on which the finish annealing film is formed;
A mother steel sheet surface smoothing step of removing the finish annealing film as necessary and smoothing the surface roughness of the mother steel sheet to 1.0 μm or less in Ra;
an intermediate layer forming step of heat-treating the mother steel plate after the finish annealing step or the mother steel plate smoothing step to form an intermediate layer mainly composed of silicon oxide in contact with the surface of the mother steel plate;
an insulating film forming step of forming an insulating film on the surface of the intermediate layer;
with
The insulating film forming step includes:
a coating step of coating a coating solution containing phosphate, colloidal silica, and chromic anhydride or chromate on the surface of the intermediate layer formed on the surface of the mother steel plate; a baking step of heating the mother steel plate and performing baking treatment at 600 to 1150° C. for 5 to 300 seconds to form the insulating film;
including
In the baking step, when heating the mother steel sheet, the average heating rate in the temperature range of 100 to 600 ° C. is set to 10 to 200 ° C./sec.
A method for producing a grain-oriented electrical steel sheet.

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