WO2024111637A1

WO2024111637A1 - Grain-oriented electrical steel sheet and manufacturing method therefor

Info

Publication number: WO2024111637A1
Application number: PCT/JP2023/042039
Authority: WO
Inventors: 克高橋; 雅人安田; 秀行濱村; 直樹和田; 将嵩岩城; 尚茂木
Original assignee: 日本製鉄株式会社
Priority date: 2022-11-22
Filing date: 2023-11-22
Publication date: 2024-05-30

Abstract

This grain-oriented electrical steel sheet has a silicon steel sheet, an oxide layer that is formed on a surface of the silicon steel sheet and comprises an oxide or oxides of one or more of Mg, Al, and Si, and an insulating film layer that is formed on the surface of the oxide layer. In the silicon steel sheet, in a range that is 10 μm in the sheet thickness direction from the interface between the silicon steel sheet and the oxide layer, instances of the oxide or oxides of one or more of Mg, Al, and Si with an equivalent circle diameter of 0.1-3.0 μm are present at a density of 0.010-0.200 instances/μm2. On the surface side of the silicon steel sheet, there are flat crystal grains that have an average thickness of 0.5-5.0 μm in the direction normal to the surface, have an aspect ratio of 1.5 or greater, said aspect ratio being the ratio of the grain width in a direction parallel to the surface to the average thickness, and have crystal orientations deviating by 10° or more from the Goss orientation. In a cross-section in the sheet thickness direction, the length of the grain boundaries of the flat crystal grains accounts for 70% or more of the length of the interface between the silicon steel sheet and the oxide layer.

Description

Grain-oriented electrical steel sheet and its manufacturing method

The present invention relates to a grain-oriented electrical steel sheet and a manufacturing method thereof.
This application claims priority based on Japanese Patent Application No. 2022-186165, filed on November 22, 2022, the contents of which are incorporated herein by reference.

Grain-oriented electrical steel sheet is a soft magnetic material that is primarily used as a transformer core material. Grain-oriented electrical steel sheet is a steel sheet that contains, for example, 2.00-6.00% Si, and the crystal orientation of the product is highly concentrated in the {110}<001> orientation. Its magnetic properties require high magnetic flux density, as represented by the B8 value, and low iron loss, as represented by W17/50. In particular, there has been a growing demand in recent years for reduced power loss in transformers from the perspective of energy conservation, and there is an increasing demand for reduced iron loss in grain-oriented electrical steel sheet.

In response to this demand, a so-called magnetic domain refining technique has been developed to reduce the magnetic domain width in the steel sheet as a means of reducing the iron loss of grain-oriented electrical steel sheet. Hereinafter, this technique, i.e., the technique of refining magnetic domains, will be referred to as "magnetic domain control technique," and the effect of magnetic domain control technique will also be referred to as "magnetic domain control effect."
For example, Patent Document 1 discloses a method of irradiating the surface of a grain-oriented electrical steel sheet after finish annealing with a laser beam to subdivide magnetic domains (reduce the magnetic domain width) and reduce eddy current loss, thereby reducing iron loss. However, the reduction in iron loss by this method utilizes the magnetic domain subdivision phenomenon caused mainly by thermal distortion introduced into the steel sheet by laser irradiation, and the method cannot be used for wound core applications that require stress relief annealing after forming the transformer core.

Wound cores, which are often used primarily in small and medium-sized transformers, are often manufactured using a core manufacturing method that involves mechanical bending. In this manufacturing method, in order to eliminate the increase in iron loss caused by processing distortion introduced into the steel sheet by bending, stress relief annealing (for example, at 800°C for 2-4 hours) is generally performed after the core shape is machined. Although such stress relief annealing reduces or eliminates the distortion introduced into the core by the machining process, the thermal distortion introduced for magnetic domain refinement disappears in steel sheets that have been subjected to magnetic domain control by the aforementioned laser irradiation. For this reason, grain-oriented electrical steel sheets that have been subjected to magnetic domain refinement by the introduction of thermal distortion, as typified by laser irradiation, are generally considered not applicable to wound cores.

A widely known magnetic domain control technology that does not lose its magnetic domain control effect even when the above-mentioned strain relief annealing is performed is the "groove-introducing magnetic domain control technology" that forms linear grooves periodically in a direction intersecting the rolling direction. Such groove-introducing magnetic domain control technologies include groove formation technology by machining, groove formation technology by etching, and groove formation technology by laser irradiation. For example, Patent Document 2 discloses a groove formation technology by laser irradiation. However, these groove formation methods alone are not sufficient to meet the increasing demand for iron loss reduction in recent years.

As another technique for reducing iron loss, Patent Document 3 discloses a technique in which sharp and fine irregularities are formed on the surface of a steel sheet before decarburization annealing to activate the surface, and an oxide layer rich in silica is formed after decarburization annealing.
Furthermore, Patent Document 4 discloses a technology in which, in order to improve film properties and magnetic properties, an annealed film made of oxides mainly containing Mg, Si, and Al is provided on the surface, and the proportion of crystal orientation grains having a deviation angle of 10 degrees or less from the Goss orientation is set to 50% or less in the crystal orientation distribution of steel sheet crystal grains in a steel sheet portion within 3 μm of the boundary between the film and the steel sheet, or in a mixed region of the film and steel sheet crystal grains.

However, these methods are not sufficient to meet the increasing demand for reducing iron loss in recent years.

Japanese Patent Publication No. 56-51522 Japanese Patent Publication No. 2005-59014 Japanese Patent Publication No. 62-151522 Japanese Patent Publication No. 2003-27194

As described above, studies have been conducted on improving magnetic flux density and achieving a corresponding reduction in iron loss, but these efforts have not been sufficient to meet the increasingly high demands of recent years. In particular, in grain-oriented electrical steel sheets (GO) suitable for use in wound cores that are manufactured by performing stress relief annealing during core processing, it has not been possible to achieve a sufficient improvement in magnetic flux density and a corresponding reduction in iron loss.
Therefore, an object of the present invention is to provide a grain-oriented electrical steel sheet having excellent magnetic properties (high magnetic flux density and low core loss commensurate with the magnetic flux density) and a manufacturing method thereof. Preferably, the object of the present invention is to provide a grain-oriented electrical steel sheet that is manufactured without non-heat-resistant magnetic domain control (the aforementioned method of controlling magnetic domains by introducing thermal strain into the steel sheet through laser irradiation of the steel sheet surface) and that has excellent magnetic properties (high magnetic flux density and low core loss commensurate with the magnetic flux density) and a manufacturing method thereof, assuming application to iron cores that are subjected to stress relief annealing, such as wound cores.

The inventors have investigated how to improve the magnetic properties of grain-oriented electrical steel sheets suitable for use in wound cores, i.e., how to increase magnetic flux density and reduce iron loss. As a result, they have found that by having one or more oxides of Mg, Al, and Si present at a predetermined density near the surface of the silicon steel sheet (base steel sheet) that the grain-oriented electrical steel sheet comprises, and by forming flat crystal grains on the surface side of the silicon steel sheet whose crystal orientation deviates by 10° or more from the Goss orientation ({110}<001> orientation), it is possible to energetically control the 180° magnetic domain width to a small state, and as a result, it is possible to reduce eddy current loss and iron loss.

The present inventors also investigated the influence of production conditions, and as a result, obtained the following findings.
That is, the Goss orientation, which develops high magnetic properties in grain-oriented electrical steel sheets, is highly accumulated by the abnormal grain growth phenomenon called "secondary recrystallization" that utilizes the pinning effect of the precipitates, which are called inhibitors, and are precipitated at the grain boundaries in the final annealing process of the manufacturing process. After the accumulation of the Goss orientation in the steel sheet is completed, that is, after the steel sheet surface is almost completely covered with Goss orientation grains, the inhibitor that has completed its role is decomposed and oxidized by the temperature rise in the latter half of the final annealing process and removed from the steel sheet. That is, it is not preferable that the decomposition and oxidation of the inhibitor occur before the Goss orientation is sufficiently accumulated in the steel sheet. Furthermore, by suppressing the decomposition and oxidation of the inhibitor to a higher temperature, it is possible to accumulate the Goss orientation to a higher degree, that is, to accumulate crystals closer to the ideal Goss orientation. Therefore, a method is used to increase the heat resistance of the precipitates that act as inhibitors.
The present inventors have found that, as a method for enhancing the heat resistance of the inhibitor, it is effective to make oxides capable of suppressing the decomposition and oxidation of the inhibitor present on the surface of the steel sheet during the decarburization annealing process normally performed in the manufacture of grain-oriented electrical steel sheets, which can be performed during the subsequent finish annealing. Furthermore, they have found that by making oxides capable of suppressing the decomposition and oxidation of the inhibitor present on the surface of the steel sheet by utilizing the decarburization annealing process before the finish annealing, it is possible to generate flat crystal grains whose crystal orientation is deviated from the Goss orientation by 10° or more near the interface between the oxides on the surface of the steel sheet and the steel sheet, and that these flat crystal grains contribute to improving the magnetic properties.
The inventors also discovered that in order to generate more preferable flat crystal grains for improving magnetic properties, it is effective to form oxide particles more densely, thickly and uniformly on the surface side of the cold-rolled sheet that becomes the base steel sheet in the decarburization annealing process, and that in order to form oxide particles more densely, thickly and uniformly, it is effective to grind the cold-rolled sheet under specified conditions before the decarburization annealing process in order to remove reaction products with the surface of the steel sheet that inhibit uniform oxidation of the steel sheet surface during decarburization annealing.

In addition, they discovered that by combining the above steel sheet with groove-type magnetic domain control technology under certain conditions, they could further reduce iron loss.

The present invention has been made in view of the above findings.
[1] A grain-oriented electrical steel sheet according to one embodiment of the present invention comprises a silicon steel sheet, an oxide layer formed on the surface of the silicon steel sheet and made of one or more oxides of Mg, Al, and Si, and an insulating coating layer formed on the surface of the oxide layer, wherein the silicon steel sheet has an oxide of one or more oxides of Mg, Al, and Si with a circle equivalent diameter of 0.1 to 3.0 μm within a range of 10 μm from an interface between the silicon steel sheet and the oxide layer in the sheet thickness direction, and the oxide of one or more oxides of Mg, Al, and Si is present at a density of 0.010 to 0.200 particles/μm. and a density of ^2.2 , and flat crystal grains are present on a surface side of the silicon steel sheet, the flat crystal grains having an average thickness in a direction perpendicular to the surface of 0.5 to 5.0 μm, an aspect ratio which is a ratio of a grain width in a direction parallel to the surface to the average thickness of 1.5 or more, and a crystal orientation that deviates from the Goss orientation by 10° or more, and in the cross section in the sheet thickness direction, a length of the grain boundary of the flat crystal grains accounts for 70% or more of the length of the interface between the silicon steel sheet and the oxide layer.
[2] In the grain-oriented electrical steel sheet according to [1], the average thickness of the flat crystal grains may be more than 2.0 μm and not more than 5.0 μm.
[3] In the grain-oriented electrical steel sheet according to [1] or [2], the coverage of the oxide layer on the surfaces of the flat crystal grains constituting the interfaces may be 50% or more.
[4] In the grain-oriented electrical steel sheet according to [1] or [2], the silicon steel sheet may have a plurality of grooves having a depth of 10 to 30 μm and extending in a direction at an angle of 80 to 100° to the rolling direction, and the distance between adjacent grooves in the rolling direction may be 1.0 to 20.0 mm.
[5] The grain-oriented electrical steel sheet according to [3] may have a plurality of grooves in the silicon steel sheet, each groove having a depth of 10 to 30 μm and extending in a direction at an angle of 80 to 100° to the rolling direction, and the distance between adjacent grooves in the rolling direction may be 1.0 to 20.0 mm.
[6] The grain-oriented electrical steel sheet according to [4] may have flat grains on the surface side of the grooves of the silicon steel sheet, the flat grains having an average diameter of 0.5 to 5.0 μm in a direction perpendicular to the surface of the grooves, an aspect ratio which is a ratio of a grain width in a direction parallel to the surface to the average diameter of 2.0 or more, and a crystal orientation which deviates from the Goss orientation by 10° or more, and in a cross section in the sheet thickness direction perpendicular to the extension direction of the grooves, the length of the grain boundaries of the flat grains in the grooves accounts for 70% or more of the length of the inner surface of the grooves.
[7] The grain-oriented electrical steel sheet according to [5] may have flat grains on the surface side of the groove of the silicon steel sheet, the flat grains having an average diameter of 0.5 to 5.0 μm in a direction perpendicular to the surface of the groove, an aspect ratio which is a ratio of a grain width in a direction parallel to the surface to the average diameter of 2.0 or more, and a crystal orientation which deviates from the Goss orientation by 10° or more, and in a cross section in the sheet thickness direction perpendicular to the extension direction of the groove, the length of the grain boundary of the flat grains in the groove accounts for 70% or more of the length of the inner surface of the groove.
[8] In the grain-oriented electrical steel sheet according to [6], the average of the average diameter of the flat crystal grains in the grooves may be more than 2.0 μm and not more than 5.0 μm.
[9] In the grain-oriented electrical steel sheet according to [7], the average of the average diameter of the flat crystal grains in the grooves may be more than 2.0 μm and not more than 5.0 μm.
[10] A method for producing a grain-oriented electrical steel sheet according to another aspect of the present invention includes a hot rolling process of heating and hot rolling a slab to obtain a hot-rolled sheet, a hot-rolled sheet annealing process of annealing the hot-rolled sheet after the hot rolling process, a pickling process of pickling the hot-rolled sheet after the hot-rolled sheet annealing process, a cold rolling process of cold-rolling the hot-rolled sheet after the pickling process to obtain a cold-rolled sheet, a grinding process of grinding a surface of the cold-rolled sheet after the cold rolling process, a contacting process of contacting the cold-rolled sheet after the grinding process with an aqueous liquid having a pH of 4.0 to 10.0, a decarburization annealing process of performing decarburization annealing on the cold-rolled sheet after the contacting process, and a sintering process of the cold-rolled sheet after the decarburization annealing process. The method includes a finish annealing process in which a annealing separator is applied to the surface of the cold-rolled sheet which becomes a base steel sheet, and the surface is finish annealed to form an oxide layer made of one or more oxides of Mg, Al, and Si; and an insulating coating forming process in which an insulating coating layer is formed on the surface of the oxide layer after the finish annealing process. In the grinding process, grinding is performed with a reduction amount of 1.0 to 5.0 mm and a grinding speed of 500 mpm or more using abrasive grains having a Knoop hardness of 1000 or more and a maximum particle size of more than 50 μm and not more than 500 μm, or abrasive paper, roll, or brush to which the abrasive grains are fixed, and the grinding amount of the cold-rolled sheet is 0.10 to 10.0 g/m ² on at least one surface.
[11] The method for producing a grain-oriented electrical steel sheet according to [10] may further include, prior to the grinding step, a groove forming step of forming, in the cold-rolled sheet, a plurality of grooves having a depth of 10 to 30 μm, extending in a direction forming an angle of 80 to 100° with respect to the rolling direction, at intervals of 1.0 to 20 mm in the rolling direction.
[12] In the method for manufacturing a grain-oriented electrical steel sheet according to [11], in the groove forming step, the groove may be formed by irradiating a laser onto a surface of the cold-rolled sheet to melt a part of the steel sheet surface and removing the molten material from the surface.

The above aspect of the present invention makes it possible to provide a grain-oriented electrical steel sheet with excellent magnetic properties and a method for manufacturing the same.

1 is a schematic diagram of a cross section of a grain-oriented electrical steel sheet according to an embodiment of the present invention. FIG. FIG. 2 is a schematic diagram of a cross section of the grain-oriented electrical steel sheet according to the present embodiment when a groove is formed. FIG. 2 is a diagram for explaining a method for measuring the average thickness and aspect ratio of crystal grains. FIG. 2 is a diagram illustrating a method for measuring the coverage of an oxide layer on a flat crystal grain.

The following describes a grain-oriented electromagnetic steel sheet according to one embodiment of the present invention (grain-oriented electromagnetic steel sheet according to this embodiment) and its manufacturing method.

<Grain-oriented electrical steel sheet>
As shown in FIG. 1 , the grain-oriented electrical steel sheet 1 according to this embodiment has a silicon steel sheet 11 (hereinafter sometimes referred to as a base steel sheet or simply as a steel sheet), an oxide layer 21 formed on the surface of the silicon steel sheet 11 and made of one or more oxides of Mg, Al, and Si, and an insulating coating layer 31 formed on the surface of the oxide layer 21.
The oxide layer 21 and the insulating coating layer 31 may be formed on only one side of the steel sheet, but it is preferable from the viewpoint of insulation properties and the like that they be formed on both sides.
Each of these will be explained below.

[Silicon steel sheet]
(One or more oxides of Mg, Al, and Si, each having a circle equivalent diameter of 0.1 to 3.0 μm, are present at a density of 0.010 to 0.200 pieces/ ^μm2 within a range of 10 μm in the plate thickness direction from the interface between the silicon steel plate and the oxide layer.)
In grain-oriented electrical steel sheets, by suppressing the decomposition and oxidation of inhibitors (precipitates present at grain boundaries, such as AlN) during final annealing and allowing them to exist up to high temperatures, it becomes possible to accumulate a high degree of Goss orientation during secondary recrystallization, in other words, to accumulate crystals closer to the ideal Goss orientation, thereby improving magnetic flux density and reducing iron loss.
The size of the precipitates that become inhibitors is very small, ranging from several tens of nm to about 100 nm in circle equivalent diameter. There is also a size distribution. When there is a size distribution, the decomposition and oxidation of small-sized inhibitors is completed at low temperatures, and the inhibitor effect is lost. In that case, secondary recrystallization in the Goss orientation closer to the ideal Goss orientation becomes difficult, and it is difficult to improve the magnetic flux density. On the other hand, the above problem can be solved by controlling the size distribution of the inhibitors to a constant value (so that the size difference is small), but this is extremely difficult industrially.
On the other hand, if the inhibitor can be made to exist up to high temperatures by suppressing decomposition and oxidation by some method even in a state where the size distribution of the inhibitor occurs, secondary recrystallization of crystal grains closer to the ideal Goss orientation can be caused. In addition, a method of using a highly heat-resistant inhibitor can be used to suppress the decomposition and oxidation of the inhibitor. On the other hand, as a method of achieving this without changing the components of the inhibitor, it is known that oxide particles of Si (hereinafter sometimes referred to as Si-based pre-oxides) formed on the steel sheet surface or in the surface layer of the steel sheet (in the steel) in the decarburization annealing process contribute. Although the mechanism is speculation, it is thought that the oxidation of the inhibitor occurs when a small amount of oxygen contained in the finish annealing atmosphere oxidizes AlN and the like on the steel sheet surface, and that the above-mentioned Si-based pre-oxides prevent and reduce the oxidation.
However, the Si-based pre-oxides tend to be formed unevenly at various locations on the surface of the silicon steel sheet, and if the formation is uneven, the inhibitor's effect of suppressing decomposition and oxidation varies depending on the location on the steel sheet surface, making it difficult to obtain the intended effect.

The present inventors have investigated the cause of the non-uniform formation of the oxide layer at each site on the surface after finish annealing, and have found that Fe-based oxides and reaction products between the surface metal of the steel sheet and oiliness agents or extreme pressure additives contained in the rolling oil used during cold rolling are present non-uniformly on the surface of the silicon steel sheet (cold-rolled sheet) before decarburization annealing, and that these Fe-based oxides and reaction products prevent the Si-based pre-oxide from being densely and uniformly formed in a certain thickness region from the surface during decarburization annealing.
Since it is difficult to make the Fe-based oxide film and reaction products uniform during cold rolling, the present inventors have conducted research into neutralizing the factors inhibiting the formation of these Si-based pre-oxides. As a result, as described below, the inventors have found that by uniformly grinding the surface (at least one side) of the cold-rolled sheet before the decarburization annealing process using abrasive grains or abrasive paper, roll, or brush with abrasive grains fixed thereon to expose a clean metal surface, and then immediately contacting the surface with an aqueous liquid, it is possible to remove the Fe-based oxides and reaction products that are factors inhibiting the formation of Si-based pre-oxides from the surface of the steel sheet, and it is possible to form the Si-based pre-oxides at a predetermined number density in a region of a certain thickness from the surface of the steel sheet after the decarburization annealing process.
Based on these findings, in the grain-oriented electrical steel sheet according to the present embodiment, as shown in Fig. 1, within a range of 10 µm in the sheet thickness direction from the interface between the steel sheet (silicon steel sheet) 11 and the oxide layer 21, oxides 101 (oxide particles) which are oxides of one or more of Mg, Al, and Si and have a circle equivalent diameter of 0.1 to 3.0 µm, and which are oxides formed by oxidation of an inhibitor or a solid-phase reaction with an annealing separator in a process such as finish annealing, are present at a density of 0.010 to 0.200 pieces/µm ^2. The oxides 101 may be one or more oxides of Mg, Al, and Si (including composite oxides), but when the manufacturing conditions described later are assumed, they are often oxides containing Mg, Al, and Si, such as spinel (MgAl ₂ O ₄ ), alumina (Al ₂ O ₃ ), and mullite (2SiO ₂ ·3Al ₂ O ₃ ).
If the number density of the oxides 101 is too small, the adhesion of the oxide layer 21 to the steel sheet is poor, and the formation of the flat crystal grains 102 described below becomes non-uniform. On the other hand, if the number density of the oxides 101 is too large, the area occupied by the metal portion of the steel sheet 11 is reduced, and the magnetic flux density is reduced. In addition, the proportion of the flat crystal grains 102 is relatively small, making it difficult to obtain the effect of reducing iron loss.
The oxide 101 is uniformly formed in a predetermined region, which reduces the variation from location to location in the effect of suppressing the decomposition and oxidation of the inhibitor during finish annealing, thereby improving the magnetic flux density in the grain-oriented electrical steel sheet 1. In addition, by appropriately forming the flat crystal grains 102, the 180° magnetic domain width becomes smaller, and an iron loss reduction effect commensurate with the magnetic flux density can be obtained.
Considering the formation process, the oxide 101 is often present in flat crystal grains 102, which will be described later.

(On the surface side of the silicon steel sheet, there are flat crystal grains with an average thickness in the direction perpendicular to the surface of 0.5 to 5.0 μm, an aspect ratio, which is the ratio of the grain width in the direction parallel to the surface to the average thickness, of 1.5 or more, and a deviation of the crystal orientation from the Goss orientation of 10° or more.)
(In the cross section in the plate thickness direction, the length of the grain boundary of the flat crystal grains accounts for 70% or more of the length of the interface between the silicon steel plate and the oxide layer)
As described above, in the grain-oriented electrical steel sheet according to this embodiment, the decarburization annealing process is mainly used to uniformly form Si-based pre-oxides in the surface layer (within 10 μm from the surface) of the silicon steel sheet (base steel sheet), thereby suppressing the decomposition and oxidation of the inhibitor during the final annealing and allowing it to exist up to high temperatures. In this case, it is possible to highly accumulate the Goss orientation, that is, to accumulate crystals closer to the ideal Goss orientation, resulting in improved magnetic flux density. In other words, it is possible to reduce iron loss.
On the other hand, inducing secondary recrystallization at a higher temperature means that only crystal grains closer to the ideal Goss orientation undergo secondary recrystallization. In this case, the number of Goss-oriented grains undergoing secondary recrystallization decreases, and the number of Goss-oriented grains per unit area of the steel sheet decreases. In other words, the crystal grain size per Goss-oriented grain becomes larger.
The iron loss required for grain-oriented electrical steel sheets is divided into hysteresis loss and eddy current loss. Hysteresis loss is reduced by improving the magnetic flux density. On the other hand, there are two types of eddy current loss: classical eddy current loss, which is reduced by reducing the sheet thickness and increasing the resistivity of the steel sheet, and abnormal eddy current loss, which is reduced by reducing the magnetic domain width formed in the Goss-oriented grains. Since the reduction in sheet thickness and the increase in the resistivity of the steel sheet in the reduction of classical eddy current loss often affect productivity, it is important to reduce the abnormal eddy current loss, i.e., to reduce the magnetic domain width. The magnetic domain width is generally correlated with the crystal grain size of the Goss orientation. In general, the magnetic domain width of the so-called 180° magnetic domain generated in the grain-oriented electrical steel sheet is also reduced by reducing the grain size.
That is, although the magnetic flux density is improved by controlling the above oxides, there is a concern that the abnormal eddy current loss increases due to the coarsening of the crystal grain size, and therefore the iron loss reduction effect commensurate with the improvement in magnetic flux density cannot be obtained.

Therefore, the inventors have studied a method of reducing iron loss commensurate with the improvement of magnetic flux density, that is, a method of reducing the magnetic domain width to solve the coarsening of the grain size that occurs secondarily after increasing the frequency of the ideal Goss orientation crystal grains. As a result, it was found that, as described above, secondary recrystallization is caused at a higher temperature, and only crystal grains closer to the ideal Goss orientation undergo secondary recrystallization, and even when the grain size is large, the surface of the steel sheet is made to have flat crystal grains (flat crystal grains) with a deviation angle of 10° or more from the Goss orientation, so that the 180° magnetic domain width can be energetically controlled to a small state, and an increase in eddy current loss can be suppressed.
Specifically, as shown in FIG. 1, it has been found that eddy current loss is reduced when flat crystal grains 102 are present on the surface side of the base steel sheet 11 (flat crystal grains 102 are present as grains constituting the outermost layer of the silicon steel sheet 11) having an average thickness in a direction perpendicular to the surface of 0.5 to 5.0 μm, an aspect ratio which is the ratio of the grain width in a direction parallel to the surface to the average thickness of 1.5 or more, and a crystal orientation that deviates (deviation angle) from the Goss orientation of 10° or more.
Crystal grains with an average thickness of less than 0.5 μm, an aspect ratio of less than 1.5, or a deviation from the Goss orientation of less than 10° cannot sufficiently reduce the magnetic domain width, and therefore cannot sufficiently reduce core loss.
On the other hand, since the crystal grains are deviated from the Goss orientation, if the average thickness of the crystal grains exceeds 5.0 μm, the overall magnetic properties deteriorate, that is, the magnetic flux density decreases and the core loss increases.
The average thickness of each flat crystal grain is preferably more than 2.0 μm and not more than 5.0 μm in order to fully obtain the effect of reducing the magnetic domain width.

In order to fully obtain the above-mentioned magnetic domain refinement effect, the length of the grain boundary of the flat crystal grains accounts for 70% or more of the length of the interface between the base steel sheet and the oxide layer in the cross section in the sheet thickness direction.
If the ratio of flat crystal grains constituting the interface is small, the effect of reducing the magnetic domain width is insufficient, and therefore a sufficient effect of reducing iron loss cannot be obtained.

In the manufacturing method of grain-oriented electrical steel sheet, during finish annealing, tiny Goss orientation grains on the inside of the steel sheet in the sheet thickness direction grow while encroaching on surrounding crystal grains having orientations other than the Goss orientation, and as a result, the proportion of Goss orientation grains (crystal grains in which the longitudinal direction of the silicon steel sheet is the <100> direction and the surface direction is the <110> direction) increases from the inside of the sheet thickness direction to the sheet thickness surface, and further in the rolling direction and width direction.
In the grain-oriented electrical steel sheet according to this embodiment, as described above, oxides are present discretely (at a predetermined number density) in the surface layer portion of the silicon steel sheet. As a result, when the Goss-oriented grains present inside the sheet thickness grow, tiny flat crystal grains remain in the surface layer portion of the steel sheet without being encroached upon by the Goss-oriented grains, resulting in the formation of "flat crystal grains" that are confirmed as flat grains.

The average thickness, aspect ratio, and deviation of crystal orientation of the crystal grains present on the surface side can be measured by the following method.
A sample of, for example, about 20 mm square is cut out from the steel sheet so that a surface parallel to the rolling direction (RD direction) is obtained as a cross section, and the cross section is polished to a mirror surface. In addition, since it is difficult to measure the crystal orientation when the steel sheet is distorted by polishing, a polishing material such as colloidal silica is used in the final polishing process to prepare a polished sample so that no distortion is introduced. The polished sample is used to observe the cross-sectional shape with an FE-SEM, and then the crystal orientation is measured by EBSD measurement. For the FE-SEM, "SU-70" (manufactured by Hitachi High-Tech Corporation) is used as an example, and for the EBSD measurement, "Digiview" (manufactured by TSL Solutions) is used as an example. Specific examples of the method include the following. The cross section is observed with an FE-SEM at a magnification of 500 times, and an electron microscope image is obtained. The interface between the insulating coating layer and the oxide layer and the interface between the oxide layer and the steel sheet, which will be described later, are identified from the difference in electron density in the electron microscope image. When identifying the above-mentioned interface, if the FE-SEM is equipped with an elemental analyzer (EDS), it is possible to identify the interface more precisely from differences in elemental species, such as P, B, O, and Fe, contained in the insulating coating layer, the oxide layer, and the silicon steel plate.
Next, the crystal orientation of the steel sheet is measured by EBSD on the cross section of the same field of view. Specifically, in a 500x field of view where it is assumed that 100 or more flat grains are included, the crystal orientation is measured at measurement point pitches of 0.25 μm in an area with a cross-sectional length of 200 μm in the rolling direction and 70 μm in the sheet thickness direction. The boundary where the crystal orientation difference is 15° or more is defined as a grain boundary, and the area surrounded by this grain boundary is defined as a grain. If the number of grains in the field of view is less than 100, measurements are performed on additional fields of view.
Regarding these crystal grains, the average thickness of the crystal grains is determined by the methods shown in a) to d) in FIG.
a) Draw an imaginary line (1) in the thickness direction (normal direction) of the steel plate to determine both ends of the grain.
b) Draw imaginary lines (2) in the thickness direction at 2.5% of the distance L between both ends of the crystal grain (the lines between them indicate 95% of the grain width).
c) Between the imaginary lines drawn in b) above (95% width portion of the crystal grain), draw an average line (3) on the envelope of the interface between the crystal grain and the oxide layer and on the lower side of the crystal grain (the grain boundary on the opposite side to the oxide layer).
d) The distance between the two average lines drawn in c) above is calculated as the thickness t(4) (average of both ends, the center, and the midpoint between both ends and the center, a total of five points).
The range between both ends of the grain drawn in the above step a) is regarded as the width of this crystal grain, and the aspect ratio is calculated.

The crystal orientation of the Fe ferrite phase is measured for all of the above crystal grains with an average thickness of 0.5 to 5.0 μm and an aspect ratio of 1.5 or more. The measured crystal orientation is then used to obtain a crystal orientation map called an IPF map, which shows the crystal orientation relative to the rolling direction (RD direction) and the normal direction of the steel sheet surface (ND direction). The average orientation difference between each crystal grain and the Goss orientation is calculated, and this is taken as the deviation from the Goss orientation. If the deviation from the Goss orientation is 10° or more, the grain is considered to be a flattened grain.

The average (simple average) of the average thickness of the flat crystal grains is obtained by dividing the sum of the average thicknesses of each flat crystal grain obtained above by the number of flat crystal grains.

Since the flat crystal grains are flat in the rolling direction (longitudinal direction) and width direction, any method may be used to observe the cross section in the thickness direction, but the method of obtaining a cross section of the above-mentioned steel plate parallel to the rolling direction (RD direction) and obtaining a crystal orientation map by EBSD to confirm the presence of "flat crystal grains" is preferable because it has high accuracy. Another method of simply confirming the presence of "flat crystal grains" is to polish a surface parallel to the rolling direction (RD direction) to obtain a smooth cross section, and then to confirm by a method such as the so-called nital method (nitric acid ethanol method, described in JIS-G-0553 (2019)), which reveals the crystal grain boundaries. However, this method does not identify the crystal orientation, and it is necessary to measure the crystal orientation separately using EBSD, etc., so in this embodiment, the method of using the above-mentioned FE-SEM and EBSD in combination is adopted.

The ratio of the length of the grain boundary of the flat crystal grains to the length of the interface between the base steel sheet and the oxide layer can be determined by the following method.
For example, in a field of view observed at a magnification of 500 times, SEM observation and EBSD measurement are performed on an area of 200 μm in cross-sectional length in the rolling direction of the interface between the silicon steel sheet and the oxide layer. This is performed at five locations, that is, for an interface length of 1000 μm. The proportion (percentage) of the grain boundaries of flat crystal grains with an average thickness of 0.5 to 5.0 μm, an aspect ratio of 1.5 or more, and an orientation difference from the Goss orientation of 10° or more is measured in the length (1000 μm) of the interface between the silicon steel sheet and the oxide layer. Identification of the interfaces of the insulating coating layer, the oxide layer, and the silicon steel sheet, and of the flat crystal grains, etc. can be performed in the same manner as above.
In the measurement, the length of the measurement range B where an oxide layer is formed on the surface of the silicon steel sheet is defined as B' (if the oxide layer is formed over the entire measurement range, B = B'), and the lengths of the parts where flat crystal grains are formed on the outermost layer of the silicon steel sheet and the interface between the silicon steel sheet and the oxide layer are the grain boundaries of the flat crystal grains are defined as b1, b2...bi (i = 3 in the figure), and the total length of b1 to bi (Σbi) is divided by the length where an oxide layer is formed on the surface of the silicon steel sheet by B'(Σbi/B') to measure the ratio of the length of the grain boundaries of the flat crystal grains to the length of the interface between the base steel sheet and the oxide layer.

(groove)
By forming linear grooves periodically in a direction intersecting the rolling direction, magnetic domain control can be performed. In the grain-oriented electrical steel sheet according to this embodiment, in order to obtain this effect, it is preferable that grooves G are formed on the surface of the base steel sheet 11 as shown in FIG. 2. Specifically, it is preferable that the silicon steel sheet (base steel sheet) 11 has a plurality of grooves G with a depth (sheet thickness direction) of 10 to 30 μm and extending in a direction at an angle of 80 to 100° to the rolling direction, and the distance between adjacent grooves G in the rolling direction is 1.0 to 20.0 mm. The distance between adjacent grooves G in the rolling direction is more preferably 2.0 to 10.0 mm.
If the size and spacing of the grooves are not within the above ranges, sufficient effect cannot be obtained. The spacing of the grooves is the distance from the center of the width of one groove to the center of the width of the adjacent groove.
The shape of the groove is not limited, and for example, the cross section is substantially rectangular or substantially triangular. The cross section may also be a bow shape constituting a part of a circle. The width of the groove is preferably about 0.5 to 3.0 times the depth of the groove. If the width of the groove is less than 0.5 times the depth of the groove, a sufficient magnetic domain control effect cannot be obtained, and the groove itself is difficult to form. On the other hand, if the width of the groove is more than 3.0 times the depth of the groove, the occupancy rate of the groove on the steel sheet surface increases, resulting in a decrease in magnetic flux density, while the magnetic domain control effect is saturated, so that the iron loss reduction effect cannot be obtained, and may even lead to an increase in iron loss.

The above-mentioned effect of reducing abnormal eddy current loss due to flat crystal grains is also effective for magnetic domain control material formed by grooves.
That is, when the inner surface (bottom surface, side surface) of the groove is the surface of the base steel sheet, as shown in FIG. 2, there are flat crystal grains (flat crystal grains in the groove) G102 on the surface side of the groove of the base steel sheet, which have an average diameter in the direction perpendicular to the surface of 0.5 to 5.0 μm, an aspect ratio which is the ratio of the grain width in the direction parallel to the surface to the average diameter of 2.0 or more, and a deviation of the crystal orientation from the Goss orientation of 10° or more (the flat crystal grains in the groove G102 exist as grains constituting the outermost layer of the groove of the silicon steel sheet), and in a cross section in the plate thickness direction perpendicular to the extension direction, if the length of the grain boundary of the flat crystal grains in the groove G102 occupies 70% or more of the length of the inner surface of the groove G, in addition to the magnetic domain refinement effect due to the groove formation, the eddy current loss reduction effect due to the flat crystal grains in the groove G102 can be obtained, which is more preferable.
The average diameter of the flat crystal grains G102 in the grooves is more preferably more than 2.0 μm and not more than 5.0 μm.

The presence or absence of flat crystal grains in grooves, as well as their average diameter, aspect ratio, and deviation of crystal orientation from the Goss orientation, can be determined in the same manner as for the flat crystal grains on the surface of the base steel sheet described above.
However, when the grooves on the surface of the steel sheet are not straight but curved, for example, a cross section perpendicular to the tangent of the curve in the observation target portion is revealed. In this case, the deviation angle of the cross section with respect to the rolling direction (RD direction) is measured and corrected when measuring the crystal orientation of the flattened crystal grains.
In addition, the length of the grain boundaries of the flat crystal grains in the groove relative to the length of the inner surface of the groove can be determined by performing EBSD measurement of a cross section in the plate thickness direction perpendicular to the extension direction of the groove in the same manner as the measurement of the flat crystal grains on the surface side of the silicon steel plate.

(Chemical Composition)
The chemical composition of the silicon steel sheet is not limited as long as it is the same as the base steel sheet of a known grain-oriented electrical steel sheet. For example, the composition may be within the range described below.
The chemical composition of the silicon steel sheet contains, in mass%, 2.00 to 6.00% Si, in order to control the crystal orientation to a Goss texture concentrated in the {110}<001> orientation and ensure good magnetic properties.
The other elements are not particularly limited, and known elements may be contained in known ranges in place of Fe, with the balance being Fe and impurities.
Representative content ranges (mass %) of representative elements other than Si are as follows:
C: 0 to 0.0050%,
Mn: 0 to 1.0%,
S: 0 to 0.0150%,
Se: 0 to 0.0150%,
Al: 0 to 0.0650%,
N: 0 to 0.0050%,
Cu: 0 to 0.40%,
Bi: 0 to 0.010%,
B: 0 to 0.080%,
P: 0 to 0.50%,
Ti: 0 to 0.0150%,
Sn: 0 to 0.10%,
Sb: 0 to 0.10%,
Cr: 0 to 0.30%,
Ni: 0 to 1.0%,
Nb: 0 to 0.030%,
V: 0 to 0.030%,
Mo: 0 to 0.030%,
Ta: 0 to 0.030%,
W: 0 to 0.030%,
These selective elements may be contained according to the purpose, so there is no need to limit the lower limit, and they may not be contained substantially. Furthermore, even if these selective elements are contained as impurities, the effects of the present invention are not impaired. Impurities refer to elements that are unintentionally contained, and refer to elements that are mixed in from raw materials such as ores and scraps, or the manufacturing environment, when industrially manufacturing the base steel sheet.

The chemical composition of the silicon steel sheet is determined by the following method.
A solution is obtained by acid decomposing a silicon steel sheet with hydrochloric acid or the like. Then, a calibration curve is obtained by ICP (inductively coupled plasma) analysis of each element solution whose concentration is already known. The solution obtained is then analyzed, and the elements contained therein are quantified and determined.
When an oxide layer and/or an insulating coating layer is formed on the surface of a silicon steel sheet (when a grain-oriented electrical steel sheet includes a silicon steel sheet, an oxide layer, and an insulating coating layer), the oxide layer and the insulating coating layer can be removed before measurement.
Specifically, when an insulating coating layer is formed, the insulating coating layer is removed by immersing the grain-oriented electrical steel sheet having the insulating coating layer for 7 to 10 minutes in an aqueous sodium hydroxide solution at 80 to 90°C containing 30 to 50 mass% NaOH and 50 to 70 mass% H ₂ O. The grain-oriented electrical steel sheet from which the insulating coating layer has been removed is washed with water, and then dried with a hot air blower for just under 1 minute.
When an oxide layer is formed, the oxide layer is removed by immersing the grain-oriented electrical steel sheet having the oxide layer for 1 to 10 minutes in an aqueous hydrochloric acid solution containing 10 mass% HCl at 80 to 90° C. After immersion, the base steel sheet is washed with water and then dried with a hot air blower for just under 1 minute.
Through the above steps, a silicon steel sheet, which is the base steel sheet, can be taken out from the grain-oriented electrical steel sheet on which the oxide layer and/or insulating coating layer is formed.

(Thickness)
The thickness of the silicon steel sheet of the grain-oriented electrical steel sheet according to this embodiment is not limited, but is preferably 0.15 to 0.35 mm. If the thickness exceeds 0.35 mm, the classical eddy current loss described above increases, resulting in large iron loss. On the other hand, if the thickness is less than 0.15 mm, the rolling efficiency decreases, which is disadvantageous in terms of productivity and cost.

[Oxide layer]
In the grain-oriented electrical steel sheet according to this embodiment, an oxide layer made of one or more oxides of Mg, Al, and Si is formed on the surface of the base steel sheet.
This oxide layer is formed by a solid-phase reaction between Mg and/or Al contained in the annealing separator and the Si-based pre-oxide formed on the steel sheet surface during the final annealing. For example, when an annealing separator containing MgO is used, a forsterite (Mg ₂ SiO ₄ ) film is mainly formed as the oxide layer. Furthermore, AlN contained in the steel as an inhibitor is oxidized by oxygen in the annealing atmosphere on the surface of the silicon steel sheet in the latter half of the final annealing. As a result, spinel (MgAl ₂ O ₄ ), alumina (Al ₂ O ₃ ), or mullite (2SiO ₂ .3Al ₂ O ₃ ) is formed on the surface of the silicon steel sheet. However, when an annealing separator containing MgO as the main component is used, it is mostly formed as spinel (MgAl ₂ O ₄ ).
The oxide layer covers the surfaces of the flat crystal grains, thereby improving the adhesion to the insulating coating layer applied thereon. To obtain a sufficient effect, it is preferable that the coverage of the oxide layer on the flat crystal grains is 50% or more.

The coverage can be determined by the following method.
That is, the presence of flat crystal grains is identified by EBSD in the manner described above. Then, attention is paid to FE-SEM images of each flat crystal grain or elemental analysis images obtained by performing elemental analysis using EDS or the like based on the FE-SEM images. The length of the flat crystal grain where one or more oxide layers of Mg, Al, and Si exist between the insulating coating layer and the flat crystal grain or in the projected portion from the surface side of the flat crystal grain toward the inside of the steel sheet is measured. The percentage of the length where the oxide layer exists per 1000 μm of the interface length between the oxide layer or insulating coating layer and the flat crystal grain is calculated.
For example, in the state shown in FIG. 4, the coverage rate (%) can be calculated by (A1+A2+A3)/(a1+a2+a3)×100.

[Insulating coating layer]
In the grain-oriented electrical steel sheet according to the present embodiment, an insulating coating layer is formed on the surface (as an upper layer) of the oxide layer. This insulating coating layer is essential when the grain-oriented electrical steel sheet is used as a transformer. When the grain-oriented electrical steel sheet is used as a transformer, it is laminated and used. When the laminated steel sheets (silicon steel sheets) are short-circuited, eddy currents are generated in the transformer core, which causes an increase in core core loss. Therefore, an insulating coating layer is formed on the surface of the steel sheet to impart electrical insulation, thereby reducing the core core loss of the transformer. In addition, by applying tension to the steel sheet in the insulating coating of the grain-oriented electrical steel sheet, the magnetic domain width can be reduced, and abnormal eddy current loss and therefore iron loss can be reduced.
In addition to the electrical insulation properties described above, the insulating coating of grain-oriented electrical steel sheets is also required to have various properties necessary for producing iron cores, such as corrosion resistance, heat resistance, and slipperiness. In consideration of these needs, for example, a coating type whose main components are phosphate and colloidal silica is used for the insulating coating. In addition, for the purpose of imparting greater tension to the steel sheet, a coating whose main component is aluminum borate or a coating made of aluminum borate and silica may be used. Either coating may be a known coating formed by applying a coating liquid in which the components contained therein are dissolved or dispersed to the surface of an oxide layer and baking it.

<Production Method>
The grain-oriented electrical steel sheet according to this embodiment can obtain its effects as long as it has the above-mentioned characteristics regardless of the manufacturing method, but a manufacturing method including the following steps is preferable because it can be stably manufactured.
(I) a hot rolling step of heating and hot rolling the slab into a hot-rolled sheet;
(II) a hot-rolled sheet annealing step of annealing the hot-rolled sheet after the hot rolling step;
(III) a pickling process for pickling the hot-rolled sheet after the hot-rolled sheet annealing process;
(IV) a cold rolling step of cold rolling the hot-rolled sheet after the pickling step to obtain a cold-rolled sheet;
(V) a grinding step of grinding the surface of the cold-rolled sheet after the cold rolling step;
(VI) a contacting step of contacting the cold-rolled sheet after the grinding step with an aqueous liquid having a pH of 4.0 to 10.0;
(VII) a decarburization annealing step of performing decarburization annealing on the cold-rolled sheet after the contact step;
(VIII) a finish annealing step in which an annealing separator is applied to the cold-rolled sheet after the decarburization annealing step, and finish annealing is performed to form an oxide layer composed of one or more oxides of Mg, Al, and Si on the surface of the cold-rolled sheet which becomes a base steel sheet;
(IX) An insulating coating forming step of forming an insulating coating layer on the surface of the oxide layer after the final annealing step.
The method for producing a grain-oriented electrical steel sheet according to this embodiment may further include any one or more of the following steps.
(X) A groove forming step, which is performed before the grinding step, further comprises forming a plurality of grooves having a depth of 10 to 30 μm in the cold-rolled sheet, the grooves extending in a direction at an angle of 80 to 100° with respect to the rolling direction, the grooves being spaced apart from each other in the rolling direction by 1.0 to 20.0 mm;
(XI) A nitriding process for increasing the nitrogen content of the cold-rolled sheet.

Among these, the manufacturing method of the grain-oriented electrical steel sheet according to the present embodiment is characterized by the grinding step, the contacting step, and the groove forming step, while the hot rolling step, the hot-rolled sheet annealing step, the cold rolling step, the decarburization annealing step, the nitriding treatment step, the finish annealing step, and the insulating coating forming step can be performed under known conditions.
Preferred conditions are described below. Even if conditions are not described, the reaction can be carried out under known conditions.

[Hot rolling process]
In the hot rolling process, a slab having a predetermined chemical composition (a chemical composition corresponding to the chemical composition of the silicon steel sheet of the grain-oriented electrical steel sheet according to this embodiment) is heated and hot rolled to form a hot-rolled sheet.
The slab heating temperature is, for example, 1000 to 1400°C.

The chemical composition of the slab to be subjected to hot rolling may be determined according to the desired chemical composition of the grain-oriented electrical steel sheet, taking into consideration changes in the chemical composition in each process.
When obtaining the chemical composition of the silicon steel sheet of the grain-oriented electrical steel sheet according to the preferred embodiment described above, the hot rolling stage may contain, by mass%, C: 0.040 to 0.100%, Si: 2.00 to 6.00%, and in addition, Al, Mn, Se, S, B, N, etc. are contained in predetermined ranges as inhibitors to obtain AlN, MnS, MnSe, BN, and further elements such as Cu, Sn, Cr, Ni, Mo, Nb, Bi, Sb, P, Ti, V, Ta, W, etc. as necessary.

The method for obtaining the slab is not limited. For example, molten steel having a predetermined chemical composition may be melted and the molten steel may be used to produce the slab. The slab may be produced by a continuous casting method, or the molten steel may be used to produce an ingot and the ingot may be bloomed to produce the slab. Alternatively, the slab may be produced by other methods.
The thickness of the slab is not particularly limited, but is, for example, 150 to 350 mm. The thickness of the slab is preferably 220 to 280 mm. As the slab, a so-called thin slab having a thickness of 10 to 70 mm may be used.
A so-called hot-rolled sheet (hot-rolled steel sheet) is obtained by hot rolling. The thickness (finished thickness) of the hot-rolled sheet is not particularly limited. However, the hot-rolled sheet is annealed, pickled, and then cold-rolled. It is known that the so-called cold rolling reduction rate affects the magnetic properties of the grain-oriented electrical steel sheet, and the thickness of the hot-rolled sheet is selected taking into account the required cold rolling reduction rate for the final thickness. For example, when the final thickness is 0.20 to 0.30 mm, the finished thickness of the hot-rolled sheet is preferably in the range of 2.0 to 4.0 mm.

[Hot-rolled sheet annealing process]
In the hot-rolled sheet annealing process, the hot-rolled sheet after the hot rolling process is annealed. By carrying out such annealing treatment, recrystallization occurs in the steel sheet structure, making it possible to realize good magnetic properties.
In the hot-rolled sheet annealing process of this embodiment, the hot-rolled sheet manufactured through the hot rolling process may be annealed according to a known method. The means for heating the hot-rolled sheet during annealing is not particularly limited, and a known heating method can be adopted. For example, so-called continuous annealing may be used, or the hot-rolled sheet may be coiled and subjected to batch annealing. The annealing conditions are also not particularly limited, but for example, the hot-rolled sheet may be annealed for 10 seconds to 5 minutes in a temperature range of 900 to 1200 ° C. The atmosphere is not particularly limited, but it is preferable to suppress oxidation of the steel sheet, and it is preferable to perform the annealing in a non-oxidizing atmosphere such as nitrogen, argon, or hydrogen.

[Pickling process]
In the pickling process, scale (oxides) formed on the surface of the steel sheet during hot rolling and hot-rolled sheet annealing is removed. In the pickling process of this embodiment, a known method is used. As the pickling solution, known acids such as hydrochloric acid, sulfuric acid, and nitric acid are used. In addition, known pickling inhibitors, pickling accelerators, and the like may be added to the pickling solution as necessary. Furthermore, before the steel sheet is brought into contact with the pickling solution, it is also possible to perform physical treatment such as shot blasting on the steel sheet before pickling in order to penetrate the pickling solution into the interface between the scale and the steel sheet and improve the pickling efficiency.

[Cold rolling process]
In the cold rolling process, the steel sheet after pickling is cold rolled to obtain a cold rolled sheet. The cold rolling may be a single cold rolling (a series of cold rolling without intermediate annealing) or may be multiple cold rollings with intermediate annealing by interrupting the cold rolling and performing at least one or more intermediate annealings before the final pass of the cold rolling process.
The cold rolling conditions may be in accordance with known methods. The cold rolling reduction of grain-oriented electrical steel sheet has a large effect on its magnetic properties. In particular, the final reduction has a large effect, and the final reduction can be set to 80 to 95%. The final reduction is the cumulative reduction of cold rolling, and in the case where intermediate annealing is performed, it is the cumulative reduction of cold rolling after final intermediate annealing.
When intermediate annealing is performed, for example, the steel sheet is held at a temperature of 800 to 1200°C for 5 to 180 seconds. The annealing atmosphere is not particularly limited, but it is preferable to perform the annealing in a non-oxidizing atmosphere such as nitrogen, argon, or hydrogen in order to prevent oxidation of the steel sheet. The annealing method may be either so-called continuous annealing or batch annealing in a coil shape, or other methods may be used. The number of times intermediate annealing is preferably three or less, taking into account the manufacturing cost.

[Groove forming process]
In the groove forming step, grooves are formed in the cold-rolled sheet before the grinding step, extending in a direction at an angle of 80 to 100° with respect to the rolling direction, and having a depth of 10 to 30 μm. A plurality of grooves are formed so that the intervals in the rolling direction are 1.0 to 20.0 mm. The intervals in the rolling direction are more preferably 2.0 to 10.0 mm.
By forming the above-mentioned grooves on the surface of the cold-rolled sheet (base steel sheet), the magnetic domains are subdivided due to the effect of the grooves when the Goss orientation is recrystallized, improving the magnetic properties. Specifically, abnormal eddy current loss is reduced, and iron loss is reduced. If the direction, interval, shape, etc. of the grooves are outside the above ranges, sufficient effects cannot be obtained.
The method for forming the grooves is not particularly limited, and known methods such as those shown below can be used. For example, physical contact methods (methods of scratching the surface of the steel sheet with a blade or the like, or methods of roll transfer or pressing using a die with a blade, etc.), non-physical contact methods (methods of locally melting a part of the steel sheet surface with a laser, electron beam, plasma, etc. and removing the molten material outside the system, etc.), and chemical methods (methods of masking the steel sheet surface with a resin or the like, removing a part of the mask according to the shape of the groove to be formed, contacting the part where the mask has been removed with an acid or the like, and etching the steel sheet to dissolve and form a groove) can be mentioned.
Among these, the above-mentioned methods not relying on physical contact are superior to the methods relying on physical contact and the chemical methods in the following respects.
In physical contact methods, blades and dies are brought into contact with the steel sheet, which causes distortion in the steel sheet and leads to deterioration of the magnetic properties. In addition, grooves are introduced into the steel sheet at a pitch of 1.0 to 20.0 mm in the rolling direction, so the blades and dies are subject to significant wear when grooves are applied to a steel sheet coil that is several thousand meters long. This necessitates frequent replacement, which has the disadvantage of poor productivity.
As for the chemical method, as mentioned above, it is necessary to carry out multiple steps such as masking the steel sheet surface with a resin, removing a part of the mask, and then etching, which is a problem mainly in terms of productivity. Furthermore, a strong acidic solution with a pH of about 1, such as hydrochloric acid, is often used for etching, and the costs of removing Fe dissolved in the strong acidic solution and treating the strong acidic solution as waste liquid are high.
As a method that does not rely on physical contact, one method is to irradiate the surface of a cold-rolled sheet with a laser, melt a part of the steel sheet surface, and remove the molten material from the surface to form grooves. This method has the great advantage that it uses a high-energy source with high linearity such as a laser, which allows for high-level control of the irradiation position on the steel sheet surface and allows grooves to be formed accurately at a specified location. In addition, the molten material generated from the steel sheet during irradiation can be removed outside the system by installing a suction duct in the laser irradiation section, and this does not affect the laser irradiation control. In addition, it is preferable to remove the molten material without allowing it to adhere to the steel sheet surface, but even if the molten material does adhere to the steel sheet surface, it can be removed from the steel sheet surface in the grinding process before the decarburization annealing process, and the steel sheet surface can be kept clean.
When irradiating the laser, for example, a high-power laser generally used for industrial purposes, such as a fiber laser, a YAG laser, a semiconductor laser, or a _CO2 laser, can be used. The output form may be a pulsed laser or a continuous wave laser. In order to form a groove of a predetermined shape, it is preferable to set the laser output to 200 to 3000 W, the laser light focusing spot diameter in the rolling direction (diameter including 86% of the laser output) to 10 to 1000 μm, the focusing spot diameter in the sheet width direction to 10 to 1000 μm, and the laser scanning speed to 5 to 100 m/s. In addition, as a method for removing the molten material from the surface, there is a method of blowing an assist gas, for example, air, _CO2 , argon, etc., is blown onto the irradiated part at the same time as the laser irradiation, while providing a suction part in the vicinity thereof, thereby reducing the reattachment of the molten material to the steel sheet surface.
The groove forming step is not essential and can be omitted.

[Grinding process]
In the grinding process, the surface of the cold-rolled sheet after the cold rolling process (the cold-rolled sheet after the groove forming process when the groove forming process is performed) is ground. At that time, grinding is performed using abrasive grains having a Knoop hardness of 1000 or more and a maximum particle size of more than 50 μm and 500 μm or less, or abrasive paper, rolls, or brushes to which the abrasive grains are fixed. When grinding a coiled cold-rolled sheet, it is preferable in terms of productivity and quality to continuously grind using a sheet passing line. In that case, it is common to use a brush in which the abrasive grains are mainly fixed. Of course, it is also possible to use a plate-shaped cold-rolled sheet instead of a coil, in which case grinding can also be performed using abrasive paper or the like.
As described above, by allowing the inhibitors (precipitates present at grain boundaries, such as AlN) to exist at as high a temperature as possible during the final annealing, only crystal grains having a crystal orientation closer to the ideal Goss orientation are allowed to grow, thereby improving the magnetic flux density.
However, although the size of the inhibitor is very small, ranging from several tens of nm to about 100 nm, there is a size distribution. When there is a size distribution, the small size inhibitor starts to decompose at a low temperature. In that case, secondary recrystallization of only crystal grains close to the Goss orientation (ideal Goss orientation) becomes difficult, and it is difficult to improve the magnetic flux density. On the other hand, it is extremely difficult industrially to control the size of the inhibitor to a constant preferred size (so that the size difference is small).
On the other hand, if the decomposition and oxidation of the inhibitor can be suppressed so that the inhibitor can exist up to high temperatures, secondary recrystallization can occur only in crystal grains closer to the Goss orientation. It is also known that the aforementioned Si-based pre-oxides formed in the base steel sheet (the cold-rolled sheet to be used) in the decarburization annealing process contribute to the suppression of the decomposition and oxidation of the inhibitor.
However, these Si-based pre-oxides are easily affected by the process before the decarburization annealing process, and the formation state of these pre-oxides tends to be non-uniform at each part of the steel sheet surface. If the formation state is non-uniform, the inhibitor's effect of suppressing decomposition and oxidation varies depending on the location on the steel sheet surface, and the desired effect cannot be obtained.

Therefore, in the manufacturing method of the grain-oriented electrical steel sheet according to this embodiment, in order to make the state of formation of the oxide layer after finish annealing as uniform as possible over a certain thickness region on the surface of the steel sheet, Fe-based oxides that have been formed non-uniformly on the steel sheet surface during cold rolling, etc., and that inhibit the uniform formation of such oxide layers, as well as reaction products with the steel sheet surface such as oiliness agents or extreme pressure additives, are removed from the steel sheet surface before decarburization annealing in addition to grinding the steel sheet surface.
Specifically, at least one surface of the steel sheet is ground with abrasive grains having a Knoop hardness of 1000 or more and a maximum particle size exceeding 50 μm and not exceeding 500 μm, or with abrasive paper, roll or brush to which the abrasive grains are fixed, thereby removing the Fe-based oxide film and reaction products from the surface of the steel sheet.
If the Knoop hardness is less than 1000, the hardness of the abrasive grains is insufficient for the steel plate, making grinding difficult. Or the grinding efficiency is reduced. Also, if the maximum particle size of the abrasive grains is 50 μm or less, the particle size of the abrasive grains becomes relatively small compared to the roughness of the steel plate surface, making grinding difficult. Or the grinding efficiency is reduced. On the other hand, if the maximum particle size exceeds 500 μm, the particle size of the abrasive grains becomes too large relative to the roughness of the steel plate surface, making surface scratches more noticeable during grinding and reducing the quality of the product's appearance. Although there is no upper limit for the Knoop hardness, hard abrasive grains tend to become brittle, and continuous use of abrasive paper, rolls, brushes, etc. containing abrasive grains tends to cause problems such as grinding defects, so that the Knoop hardness is preferably 8000 or less, more preferably 5000 or less. The abrasive grains are mainly made of alumina (Knoop hardness: about 2000), silicon carbide (Knoop hardness: about 2500), boron nitride (Knoop hardness: about 5000), diamond (Knoop hardness: about 7000), or the like.
Specifically, the process of grinding a cold-rolled sheet will be described using an example of a brush roll containing abrasive grains. The brush roll is a metal roll with a resin lining on the surface, and the abrasive grains are embedded in fibers made of acrylic resin or the like, which are then planted in the form of hairs on the resin layer surface of the roll. To explain the application to a continuous sheet passing line as an example, the sheet passing speed of the steel sheet when grinding the steel sheet with the brush roll is in the range of about 20 to 200 mpm (meter per minute), and the steel sheet is ground by contacting the brush roll, which rotates in the direction opposite to the sheet passing direction, with the steel sheet while moving the steel sheet at the position where the steel sheet and the brush roll contact each other. When grinding the steel sheet with the brush roll, the steel sheet is sandwiched between the brush roll and the idle roll, and the brush roll is pressed down against the sheet passing line (pass line) to the idle roll side to perform grinding. The amount of reduction at this time is 1.0 to 5.0 mm. If the amount of reduction is small, the amount of grinding is small. On the other hand, if the reduction amount is increased to increase the amount of grinding, the brush roll and the steel sheet pass in opposite directions, and the friction between the steel sheet and the brush roll makes it easy for the steel sheet to pass smoothly and move in an inching motion, which is called "chattering". "Chattering" is an extremely undesirable phenomenon that should be avoided because it causes uneven grinding of the steel sheet surface. Brush rolls with a diameter of about 200 to 500 mm are usually used. If the diameter is too small, the brush and abrasive grains wear out quickly, and if the diameter is too large, the metal roll becomes too large and the equipment becomes large. As mentioned above, the brush is rotated in the direction opposite to the passing direction of the steel sheet to perform grinding. The passing speed of the steel sheet is in the range of 20 to 200 mpm as mentioned above, and in this case, the grinding speed (corresponding to the rotation speed in the case of a brush roll) is set to 500 mpm or more. If the grinding speed (rotation speed in the case of a brush roll) is low, the amount of grinding becomes insufficient, and the Si-based preoxides are not sufficiently formed, resulting in insufficient formation of flat crystal grains.
On the other hand, in the case of a brush roll, if the rotation speed exceeds 2000 mpm, the frictional force between the brush roll and the steel plate becomes too large, causing not only the above-mentioned "chatter" but also an overload on the motor driving the brush roll. Therefore, the rotation speed of the brush roll is preferably 2000 mpm or less.
In addition, in order to sufficiently remove the Fe-based oxide film and reactants unevenly formed on the surface of the cold-rolled sheet, the grinding amount is 0.10 g/ ^m2 or more on at least one surface. On the other hand, if the grinding amount exceeds 10.0 g/ ^m2 , while the Fe-based oxide film and reactants are sufficiently removed from the steel sheet surface, the service life of the abrasive grains is shortened and sludge generation due to grinding becomes significant, and the processing of the sludge is time-consuming, which causes defects on the steel sheet surface due to pressing, etc. Therefore, the grinding amount is 10.0 g/ ^m2 or less.
The amount of grinding can be confirmed from the weight difference of the steel sheet before and after grinding. The amount of grinding is the amount of grinding per side. When grinding is performed on both sides, the amount of grinding per both sides is obtained, and for convenience, this value is halved, and from the viewpoint of removing the front and rear Fe-based oxide films and reaction products from the entire surface of the steel sheet, the preferable range of the amount of grinding is 0.30 g/ ^m2 or more and 3.0 g/ ^m2 or less.

As described below, when grooves are formed on cold-rolled sheet prior to the grinding process, the optimum groove depth is 10 to 30 μm, and the steel sheet surface inside the groove is also ground, so the effect of grinding is also effective on the inner surface of the groove formed on the surface of the cold-rolled sheet. Therefore, flat crystal grains (flat crystal grains inside the groove) are also formed on the surface side inside the groove of the base steel sheet that has undergone decarburization annealing and finish annealing.

[Contacting step]
In the contact step, after the grinding step and before the decarburization annealing step, the surface of the cold-rolled sheet is brought into contact with an aqueous liquid having a pH of 4.0 to 10.0. This removes the abrasive grains attached to the surface of the steel sheet during grinding and the steel sludge generated during grinding. The aqueous liquid may be ion-exchanged water, or may contain minerals such as Ca and Mg, or may contain carbonic acid or silicic acid as a counter ion. In addition, an acid selected from sulfuric acid, nitric acid, phosphoric acid, carbonic acid, carboxylic acid, phosphonic acid, etc. may be added at about 0.01 wt %, and the pH may be adjusted with an alkali metal or alkaline earth metal. In particular, carboxylic acid and phosphonic acid are highly effective in removing abrasive grains and sludge from the steel sheet. In the case of ion-exchanged water, the electrical conductivity is preferably 0.1 to 10 μS/cm from the viewpoint of preventing dissolution.
If the pH is less than 4.0, the steel sheet surface is etched by the acidic aqueous solution, causing corrosion of the steel sheet. If the pH is more than 10.0, the alkaline aqueous solution acts to promote oxidation of the metal surface after grinding, reducing the effect of removing the Fe-based oxides that were unevenly formed on the steel sheet surface in the grinding process. In this case, the initial intended effect of uniformly forming an oxide layer and oxide particles after finish annealing cannot be sufficiently obtained.
For the above purpose, the contact time is preferably 0.1 to 60 seconds, more preferably 1 to 60 seconds, and even more preferably 5 to 60 seconds. The flow rate of the aqueous liquid is preferably 1 to 100 L/min.
Furthermore, by carrying out the contact step, abrasive grains and sludge can be removed from the surface of the steel sheet, and factors that inhibit the uniform formation of an oxide layer and oxide particles after the finish annealing can be avoided.
The surface of the cold-rolled sheet may be brought into contact with the aqueous liquid during the grinding step, but the above-mentioned effect cannot be obtained unless the contacting step is carried out after the grinding step.

[Decarburization annealing process]
In the decarburization annealing process, the cold-rolled sheet after the grinding process is subjected to decarburization annealing, which removes (decarburizes) C, which adversely affects magnetic properties, from the steel sheet and causes primary recrystallization of the cold-rolled sheet.
The decarburization annealing conditions are not limited, but the annealing is performed in a nitrogen/hydrogen mixed atmosphere for decarburization, in which the oxygen potential is increased by humidification. In addition, since it is necessary to form a primary recrystallized structure, the humidification temperature (dew point) is determined from the viewpoints of the annealing temperature required for recrystallization and the oxygen potential capable of decarburization at the annealing temperature.
The annealing temperature is about 700 to 900° C., and since annealing is generally performed in a continuous annealing process, soaking is performed for about 60 seconds. As described above, since annealing is performed in a humid atmosphere with a high oxygen potential for decarburization, it is known that the so-called Si contained in the steel forms a layered oxide on the steel sheet surface and oxide particles inside the steel sheet (hereinafter, as above, these will be referred to as Si-based pre-oxides).

[Nitriding process]
In the nitriding process, the nitrogen content of the steel sheet is increased to increase the amount of nitrides, thereby promoting secondary recrystallization of crystal grains closer to the Goss orientation in the finish annealing process. In the nitriding process, the nitrogen content of the steel sheet after the nitriding process is preferably 0.015 to 0.050 mass%. The method of the nitriding process is not limited, and any known method may be used.
The nitriding step is not essential and may be omitted. If nitriding is performed, it is preferable to perform it between the decarburization annealing step and the finish annealing step.

[Finish annealing process]
In the final annealing process, an annealing separator is applied to the cold-rolled sheet after the decarburization annealing process (or after the nitriding process if nitriding has been performed), and the cold-rolled sheet is then finish-annealed to form an oxide layer made of one or more oxides of Mg, Al, and Si on the surface of the cold-rolled sheet which becomes the base steel sheet (silicon steel sheet).
Since the annealing time is long, the steel sheet is usually wound into a coil and then batch annealed. Since the temperature of the steel sheet rises to about 1200°C, an annealing separator is applied to prevent the coiled steel sheet from seizing. MgO is generally used as the annealing separator. By applying such an annealing separator and then performing the finish annealing, Mg contained in the annealing separator reacts with the Si-based pre-oxide formed on the steel sheet surface in the decarburization annealing process in a solid phase, and an oxide layer consisting of one or more oxides of Mg and Si is formed on the surface of the cold-rolled sheet. For example, when an annealing separator containing MgO is used, a layer of forsterite (Mg ₂ SiO ₄ ) coating is mainly formed as the oxide layer. Furthermore, AlN contained in steel as an inhibitor is oxidized by oxygen in the annealing atmosphere on the surface of the steel sheet in the latter half of the finish annealing, and at that time, it forms spinel (MgAl ₂ O ₄ ), alumina (Al ₂ O ₃ ), or mullite (2SiO ₂ .3Al ₂ O ₃ ).When an annealing separator consisting essentially of MgO is used, it forms almost entirely as spinel (MgAl ₂ O ₄ ).
In the final annealing process, the steel sheet is heated to cause secondary recrystallization of the primary recrystallized grains obtained in the decarburization annealing process, thereby obtaining crystal grains having the Goss orientation. By holding the steel sheet at an annealing temperature close to 1200°C for a predetermined time, precipitates in the steel, such as nitrides (e.g., AlN) and sulfides (e.g., MnS), which have completed their role as inhibitors, are removed (purified) so as not to adversely affect the magnetic properties.
In the method for producing a grain-oriented electrical steel sheet according to the present embodiment, the size of the inhibitor is controlled to be larger and more uniform than usual in the cold-rolled sheet to be subjected to finish annealing, so that secondary recrystallization occurs only in grains close to the Goss orientation (grains having an orientation close to the Goss orientation).
The conditions for the finish annealing are not limited, but for example, the temperature is raised from room temperature at a rate of 10 to 100°C/h, and in the temperature range of 900 to 1000°C, where secondary recrystallization in the Goss orientation generally occurs, the temperature is raised at a rate of 5 to 20°C/h to promote preferential growth (secondary recrystallization) in the Goss orientation, and then, as described above, the inhibitor that has completed its role is purified at around 1200°C (for example, 1150 to 1250°C).Then, the coil is slowly cooled in a non-oxidizing atmosphere such as hydrogen or nitrogen, and then removed from the furnace.

[Insulating film forming process]
In the insulating coating forming step, an insulating coating layer is formed on the surface of the oxide layer after the final annealing step.
For example, the insulating coating layer can be formed by applying a coating solution containing phosphoric acid or a phosphate, colloidal silica, and chromic anhydride or a chromate to a cold-rolled sheet (base steel sheet + oxide layer) after finish annealing, and baking and drying at 300 to 950 ° C for 10 seconds or more. The atmosphere during baking is not particularly limited, but it is preferable to suppress oxidation of the steel sheet, and it is preferable to perform the baking in a non-oxidizing atmosphere such as nitrogen, argon, or hydrogen. It is also possible to use a coating solution mainly composed of boric acid and alumina sol instead of the above-mentioned phosphate, or a coating solution mainly composed of boric acid and aluminosilicate (kaolin mineral, etc.), etc., as the coating type, and form an insulating coating mainly composed of aluminum borate. The application of aluminum borate can impart a large tension to the steel sheet, thereby reducing iron loss.
This process also plays a role in flattening the coiled steel sheet produced by the batch annealing in the above-mentioned final annealing, by continuous annealing. That is, the insulating coating is baked and the coiled steel sheet is subjected to continuous annealing at about 800°C while applying a certain tension to obtain a flat steel sheet. For this reason, it is sometimes called the flattening annealing process.

Through these processes, a grain-oriented electrical steel sheet can be obtained that comprises a silicon steel sheet (base steel sheet), an oxide layer, and an insulating coating layer.

Example 1
Molten steel containing 3.25 mass% Si, 0.13 mass% Mn, 0.006 mass% S, 0.050 mass% C, 0.025 mass% acid-soluble Al, and 0.007 mass% N was continuously cast to obtain a slab with a thickness of 300 mm.
The slab was heated at 1150° C. for 60 minutes in an electric furnace adjusted to a nitrogen atmosphere, and then roughly hot rolled to obtain a steel plate having a thickness of 40 mm. The slab was then finish rolled to obtain a hot rolled plate having a thickness of 2.3 mm.
Thereafter, the hot-rolled sheet was annealed in a continuous annealing furnace adjusted to a nitrogen atmosphere by heating at 1100° C. for 60 seconds and then cooling.
The obtained steel sheet (hot-rolled sheet) was pickled with 10% hydrochloric acid to remove scale from the steel sheet.
Thereafter, cold rolling was carried out to obtain a cold-rolled sheet having a thickness of 0.22 mm.
The surface of the obtained cold-rolled sheet was ground using various brushes containing abrasive grains as shown in Table 1 while flowing ion-exchanged water having a pH of 2.5 to 12.0. After grinding, the surface was brought into contact with ion-exchanged water having a pH of 2.5 to 12.0. However, as shown in Table 1, for comparison, some steel sheets were not ground, and were not brought into contact with ion-exchanged water after grinding. During the contact, the contact time was 5 seconds, and the flow rate of the aqueous liquid was 10 L/min.

For the steel sheets that had been ground and contacted with an aqueous liquid (the cold-rolled sheets after cold rolling if neither was performed, or the cold-rolled sheets after the grinding step if no contact with an aqueous liquid was performed), samples of 1000 mm x 1000 mm were taken and visually observed to evaluate the appearance.
The criteria for judgment were as follows:
5: Very beautiful 4: Beautiful 3: Some streaks present 2: Unevenness of attached matter present 1: Streaks present over the entire surface When the appearance was rated as 1, it was determined that the general requirements for appearance were not met, and further evaluation was not carried out.

In addition, decarburization annealing was performed on the steel sheet that had been ground and contacted with an aqueous liquid (the cold-rolled sheet after cold rolling when neither was performed, or the cold-rolled sheet after the grinding process when no contact with an aqueous liquid was performed). The annealing atmosphere was a 50% nitrogen + 50% hydrogen atmosphere, and the oxygen potential (P _H2O /P _H2 ) was 0.33. The oxygen potential was adjusted by humidifying the atmosphere before introducing it into the furnace. In this atmosphere, decarburization annealing was performed by soaking at 850°C for 60 seconds.
Thereafter, the material was subjected to nitriding treatment by soaking in a nitrogen-hydrogen-ammonia atmosphere at 750° C. for 30 seconds, with the ammonia concentration adjusted so that the nitrogen content after the nitriding treatment was 0.020 mass %.
Thereafter, an aqueous slurry of an annealing separator mainly composed of MgO was prepared, and the annealing separator was applied to both sides of the steel sheet so that the post-dry adhesion amount per side was 6 g/ ^m2 , and then dried. The composition of the annealing separator was 100 parts by mass of MgO, 5 parts by weight of _TiO2 , and 0.020% by mass of _FeCl2 as Cl.
Then, as a finish annealing, the sample was placed in a batch annealing furnace and heated at an average heating rate of 20°C/h in an atmosphere of 50% nitrogen and 50% hydrogen. After heating up to 1200°C, the atmosphere was switched to 100% hydrogen and soaked for 20 hours, and then cooled to room temperature.
After the completion of the final annealing, the steel sheet was taken out of the furnace and the annealing separator was removed by washing with water. At this time, a glass coating made of forsterite and an oxide layer made of granular spinel (MgAl ₂ O ₄ ), alumina (Al ₂ O ₃ ) and/or mullite were formed on the surface of the steel sheet (silicon steel sheet).
A chemical solution containing insulating coating components consisting of aluminum phosphate, colloidal silica, and chromic anhydride was applied to this steel sheet (a steel sheet having a glass coating, which is an oxide layer, formed on the surface of a silicon steel sheet, which is a base steel sheet), and the sheet was baked by heating to 800°C in a nitrogen atmosphere and holding for 30 seconds to form an insulating coating layer. The amount of the insulating coating layer attached was 4.8 g/ ^m2 per side.

In addition, the oxides and flat crystal grains in the range of 10 μm from the interface with the oxide layer in the sheet thickness direction of the obtained silicon steel sheet (having the silicon steel sheet, the glass coating (oxide layer), and the insulating coating layer) were evaluated by the above-mentioned method. The results are shown in Table 2.
In this example, as shown in the table, one or more oxides of Mg, Al, and Si having a circle equivalent diameter of 0.1 to 3.0 μm within a range of 10 μm from the interface with the oxide layer in the plate thickness direction were spinel (MgAl ₂ O ₄ ), alumina (Al ₂ O ₃ ), and mullite (2SiO _2.3Al ₂ O ₃ ), that is, oxides containing Mg, Al, and Si.

From the obtained grain-oriented electrical steel sheet (having a silicon steel sheet, a glass coating (oxide layer), and an insulating coating layer), 36 samples each measuring 30 mm in the sheet width direction and 280 mm in the rolling direction were taken by shearing, and these samples were subjected to stress relief annealing by holding them at 800°C for 2 hours in a nitrogen atmosphere.
Thereafter, the magnetic flux density when excited with a magnetizing force of 800 A/m (hereinafter referred to as B8) and the iron loss when excited with an excitation frequency of 50 Hz and a magnetic flux density of 1.7 T (hereinafter referred to as W17/50) were measured using the magnetic property measurement method specified by the Epstein method described in JIS-C-2550-1:2011.
If B8 was 1.90 T or more and W17/50 was 0.85 W/kg or less, it was determined that the material had excellent magnetic properties. The results are shown in Table 2.

In addition, a sample of 300 mm in the rolling direction × 300 mm in the width direction was taken from the obtained grain-oriented electrical steel sheet, and this sample was wound around a SUS304 round bar with a diameter of 20 mm (φ20 mm). After unwinding, the insulating coating in the recess on the inside where it was wound was observed to evaluate the adhesion of the insulating coating.
The criteria for judgment were as follows:
G (GOOD): no coating peeling off P (POOR): partial coating peeling off B (BAD): full coating peeling off The results are shown in Table 2.

From the results shown in Tables 1 and 2, in the examples where the surface of the steel sheet was ground and brought into contact with an aqueous liquid under the conditions of the present invention, one or more oxides of Mg, Al, and Si having a circle equivalent diameter of 0.1 to 3.0 μm were present at a density of 0.010 to 0.200 pieces/ ^μm2 within a range of 10 μm in the sheet thickness direction from the interface between the silicon steel sheet and the oxide layer, and the length of the grain boundary of the flat crystal grains accounted for 70% or more of the length of the interface between the silicon steel sheet and the oxide layer in the cross section in the sheet thickness direction. Furthermore, as a result, these examples had excellent magnetic properties.
On the other hand, in the cases where the contact with the prescribed aqueous liquid was not performed or the grinding conditions were not favorable, the appearance required in general was not satisfied, or the oxide in the surface layer was not sufficiently formed or the flat crystal grains were not sufficiently formed, and as a result, the magnetic properties were inferior (some of the appearance defects were not evaluated).

Example 2
The same molten steel and slab as those used in Example 1 were used, and hot rolling, hot-rolled sheet annealing, pickling, and cold rolling were carried out in the same manner as in Example 1 to obtain a cold-rolled sheet having a thickness of 0.22 mm.
One side of the obtained cold-rolled sheet was irradiated with a laser using a commercially available fiber laser under the conditions of the laser output, focused spot diameter (sheet width direction TD and rolling direction RD), and scanning speed shown in the table. Argon was sprayed as an assist gas simultaneously with the laser irradiation so that the molten material generated from the steel sheet during the laser irradiation would not reattach to the steel sheet, and a suction duct was installed at a position opposite to the assist gas outlet to collect dust caused by the molten material generated by the laser irradiation. This laser irradiation formed linear grooves on the surface, with a cross-sectional projection shape of approximately a triangle and a width and depth shown in the table. The grooves extended in the direction shown in the table, and each groove was formed parallel to the rolling direction and periodically at intervals shown in the table in the rolling direction.
Next, the surface of the cold-rolled sheet on which the grooves were formed was ground while flowing ion-exchanged water having a pH of 4.0 to 6.0, and the surface was ground under the conditions shown in Table 2. After grinding was completed, the steel sheet was brought into contact with ion-exchanged water having a pH of 4.0 to 6.0. The contact time was 5 seconds, and the flow rate of the aqueous liquid was 10 L/min.

Thereafter, under the same conditions as in Example 1, decarburization annealing, nitriding treatment, application of an annealing separator containing MgO as a main component, and finish annealing were performed.
After the final annealing, the annealing separator was removed by rinsing with water, and as a result, a glass film (oxide layer) was formed on the surface of the steel sheet.
A chemical solution containing insulating coating components consisting of aluminum phosphate, colloidal silica, and chromic anhydride was applied to the silicon steel plate having the glass coating, and the insulating coating was baked by heating to 800°C in a nitrogen atmosphere and holding for 30 seconds. At this time, the amount of the insulating coating layer attached was 5.0 g/ ^m2 per side.

In the obtained silicon steel plate (having a silicon steel plate, a glass coating (oxide layer), and an insulating coating layer), the oxides in a range of 10 μm from the interface with the oxide layer in the plate thickness direction and the flat crystal grains were evaluated in the same manner as in Example 1. In this example, the flat crystal grains in the grooves were also evaluated.

From the obtained grain-oriented electrical steel sheet (having a silicon steel sheet, a glass coating (oxide layer), and an insulating coating layer), 36 samples each measuring 30 mm in the sheet width direction and 280 mm in the rolling direction were taken by shearing, and these samples were subjected to stress relief annealing at 800°C for 2 hours in a nitrogen atmosphere.
Thereafter, the magnetic flux density when excited with a magnetizing force of 800 A/m (hereinafter referred to as B8) and the iron loss when excited with an excitation frequency of 50 Hz and a magnetic flux density of 1.7 T (hereinafter referred to as W17/50) were measured using the magnetic property measurement method specified by the Epstein method described in JIS-C-2550-1:2011.

In addition, a sample of 300 mm in the rolling direction × 300 mm in the width direction was taken from the obtained grain-oriented electrical steel sheet, and this sample was wound around a SUS304 round bar with a diameter of 20 mm (φ20 mm). After unwinding, the insulating coating in the recess on the inside where it was wound was observed to evaluate the adhesion of the insulating coating.
The criteria for judgment were as follows:
G (GOOD): No coating peeling P (POOR): Partial coating peeling B (BAD): Full coating peeling

As can be seen from Tables 3 to 5, sufficient adhesion of the coating was obtained in all cases, and low iron loss of 0.85 W/kg or less was achieved. However, in the example where the grooves were formed under favorable conditions, the iron loss was even lower, at 0.76 W/kg or less.

The present invention provides a grain-oriented electrical steel sheet with excellent magnetic properties and a manufacturing method thereof. Therefore, it has high industrial applicability.

REFERENCE SIGNS LIST 1 Grain-oriented electrical steel sheet 11 Silicon steel sheet 21 Oxide layer 31 Insulation coating layer 101 Oxide (oxide particles)
102 Flat crystal grain G Groove t Thickness G102 Flat crystal grain in groove

Claims

A silicon steel sheet;
an oxide layer formed on a surface of the silicon steel sheet and made of one or more oxides of Mg, Al, and Si;
an insulating coating layer formed on a surface of the oxide layer;
having
One or more oxides of Mg, Al, and Si having a circle equivalent diameter of 0.1 to 3.0 μm are present at a density of 0.010 to 0.200 pieces/ μm2 within a range of 10 μm in the sheet thickness direction from the interface between the silicon steel sheet and the oxide layer of the silicon steel sheet,
The silicon steel sheet has flat crystal grains on a surface side thereof, the flat crystal grains having an average thickness in a direction perpendicular to the surface of 0.5 to 5.0 μm, an aspect ratio, which is the ratio of the grain width in a direction parallel to the surface to the average thickness, of 1.5 or more, and a crystal orientation deviation from the Goss orientation of 10° or more;
In the cross section in the plate thickness direction, the length of the grain boundary of the flat crystal grains accounts for 70% or more of the length of the interface between the silicon steel plate and the oxide layer.
The grain-oriented electrical steel sheet is characterized in that
The average thickness of the flat crystal grains is more than 2.0 μm and 5.0 μm or less.
The grain-oriented electrical steel sheet according to claim 1 .
The coverage of the oxide layer on the surface of the flat crystal grain constituting the interface is 50% or more.
The grain-oriented electrical steel sheet according to claim 1 or 2.
The silicon steel plate has a plurality of grooves having a depth of 10 to 30 μm and extending in a direction of 80 to 100° with respect to the rolling direction,
The interval between adjacent grooves in the rolling direction is 1.0 to 20.0 mm;
The grain-oriented electrical steel sheet according to claim 1 or 2.
The silicon steel plate has a plurality of grooves having a depth of 10 to 30 μm and extending in a direction of 80 to 100° with respect to the rolling direction,
The interval between adjacent grooves in the rolling direction is 1.0 to 20.0 mm;
The grain-oriented electrical steel sheet according to claim 3 .
On the surface side of the groove of the silicon steel plate, there are present groove flattened crystal grains having an average diameter of 0.5 to 5.0 μm in a direction perpendicular to the surface of the groove, an aspect ratio which is a ratio of a grain width in a direction parallel to the surface to the average diameter of 2.0 or more, and a crystal orientation deviation from the Goss orientation of 10° or more;
In a cross section in the plate thickness direction perpendicular to the extension direction of the groove, the length of the grain boundary of the flat crystal grain in the groove is 70% or more of the length of the inner surface of the groove.
The grain-oriented electrical steel sheet according to claim 4 .
On the surface side of the groove of the silicon steel plate, there are present groove flattened crystal grains having an average diameter of 0.5 to 5.0 μm in a direction perpendicular to the surface of the groove, an aspect ratio which is a ratio of a grain width in a direction parallel to the surface to the average diameter of 2.0 or more, and a crystal orientation deviation from the Goss orientation of 10° or more;
In a cross section in the plate thickness direction perpendicular to the extension direction of the groove, the length of the grain boundary of the flat crystal grain in the groove is 70% or more of the length of the inner surface of the groove.
The grain-oriented electrical steel sheet according to claim 5 .
The average diameter of the flat crystal grains in the groove is more than 2.0 μm and 5.0 μm or less.
The grain-oriented electrical steel sheet according to claim 6 .
The average diameter of the flat crystal grains in the groove is more than 2.0 μm and 5.0 μm or less.
The grain-oriented electrical steel sheet according to claim 7 .
a hot rolling step of heating and hot rolling the slab into a hot rolled sheet;
A hot-rolled sheet annealing process for annealing the hot-rolled sheet after the hot rolling process;
A pickling process of pickling the hot-rolled sheet after the hot-rolled sheet annealing process;
A cold rolling process in which the hot-rolled sheet after the pickling process is cold-rolled to obtain a cold-rolled sheet;
A grinding step of grinding the surface of the cold-rolled sheet after the cold rolling step;
A contacting step of contacting the cold-rolled sheet after the grinding step with an aqueous liquid having a pH of 4.0 to 10.0;
a decarburization annealing step of performing decarburization annealing on the cold-rolled sheet after the contact step;
A finish annealing process in which an annealing separator is applied to the cold-rolled sheet after the decarburization annealing process, and the cold-rolled sheet is finish annealed to form an oxide layer composed of one or more oxides of Mg, Al, and Si on the surface of the cold-rolled sheet which becomes a base steel sheet;
an insulating coating forming step of forming an insulating coating layer on a surface of the oxide layer after the final annealing step;
Equipped with
In the grinding step, grinding is performed using abrasive grains having a Knoop hardness of 1000 or more and a maximum particle size of more than 50 μm and not more than 500 μm, or abrasive paper, roll or brush to which the abrasive grains are fixed, at a rolling reduction of 1.0 to 5.0 mm and a grinding speed of 500 mpm or more, and the grinding amount of the cold-rolled sheet is 0.10 to 10.0 g / m 2 on at least one surface.
A method for producing a grain-oriented electrical steel sheet.
Prior to the grinding step, the method further includes forming a groove in the cold-rolled sheet, the groove having a depth of 10 to 30 μm and extending in a direction at an angle of 80 to 100° with respect to the rolling direction, the groove being spaced apart from each other by 1.0 to 20 mm in the rolling direction.
The method for producing a grain-oriented electrical steel sheet according to claim 10 .
In the groove forming step, a laser is irradiated onto the surface of the cold-rolled sheet to melt a part of the steel sheet surface and remove the molten material from the surface to form the groove.
The method for producing a grain-oriented electrical steel sheet according to claim 11.