GRAIN-ORIENTED ELECTRICAL STEEL SHEET

Description

TECHNICAL FIELD

The present invention relates to a grain-oriented electrical steel sheet.

Priority is claimed on Japanese Patent Application No. 2022-151341, filed Sep. 22, 2022, the content of which is incorporated herein by reference.

BACKGROUND ART

In recent years, reduction in noise and vibration has been increasingly required for electromagnetic application equipment such as a transformer, and grain-oriented electrical steel sheets used for a transformer core have been required to be a material suitable for low noise and low vibration as well as low iron loss. It is presumed that one of the factors of noise and vibration of transformers due to material is magnetostriction of a grain-oriented electrical steel sheet. Herein, the magnetostriction is a vibration of a grain-oriented electrical steel sheet that is observed in the rolling direction when the grain-oriented electrical steel sheet is excited by alternating current and the outer shape of the grain-oriented electrical steel sheet is slightly changed due to strength change in the magnetization. Although the magnitude of the magnetostriction is as very small as 10⁻⁶ order, the magnetostriction generates vibration of the core, and the vibration propagates to the external structure such as the tank of the transformer to generate noise.

The magnetostriction characteristics vary depending on various factors such as the structure and state of the grain-oriented electrical steel sheet, specifically, the development degree of crystal orientation, tension applied to the steel sheet by an insulating film, and strain inherent in steel. When the magnetostriction characteristics change, the noise level changes, and noise can be reduced in some cases.

As a grain-oriented electrical steel sheet that can reduce noise from an electric member when used as a material of the electric member, for example, Patent Document 1 discloses a grain-oriented electrical steel sheet in which the average value DLmax of the length of the grain in the rolling direction is 12 mm or more, the film tension is 1 MPa or less, and the sheet thickness is 0.35 mm or less, and when the length of the grain in the rolling direction is divided into four equal regions and a distance of within 2 mm from the grain boundary present in the two external regions to the inner side of the grain is defined as near-grain boundary region, the absolute value of a diving angle in crystal orientation is defined as β, and satisfies (the area of the near-grain boundary region in which β is 4.0° or less)/(the total area of the near-grain boundary region)≥0.50.

In addition, as a grain-oriented electrical steel sheet excellent in magnetostriction characteristics, Patent Document 2 discloses a grain-oriented electrical steel sheet including, as a composition, 3.0 to 7.0 mass % of Si, 0.04 to 0.15 mass % of Mn, 0.01 to 0.10 mass % of Sb, 0.01 to 0.20 mass % of Sn, and a balance including Fe and unavoidable impurities, and having a tension-applying insulating film formed such that the total including the tensile stress applied by a forsterite film is 10 MPa or more, in which an average azimuth difference angle δ is 6° or less in a crystal orientation with a rolling direction as a rotation axis in Goss orientation {110}<001> grains, and a magnetostriction λ_p-p is 1.7×10⁻⁶ or less when a compressive stress of 3.92 MPa is applied in the rolling direction and magnetization is performed at 50 Hz and 1.7 T.

However, in the above techniques, improvement in iron loss characteristics (reduction in iron loss) is not sufficient. Although it has been known that iron loss is effectively lowered by irradiating the surface of a grain-oriented electrical steel sheet with a laser or the like for magnetic domain control, the methods disclosed in Patent Documents 1 and 2 cannot be applied to a laser irradiation material.

As a technique for reducing iron loss and magnetostriction by laser beam irradiation, Patent Document 3 discloses a manufacturing method of a grain-oriented electrical steel sheet including: selecting a grain-oriented electrical steel sheet in which the magnetic flux density B8 generated under a magnetizing force of 800 A/m is 1.92 T or more, and, assuming that the sheet thickness direction component of an angular deviation between the rolling direction and an easy magnetization axis (100)<001> is defined as β angle and that the ratio of the total area of grains simultaneously including a region having an absolute value of the β angle of 0.5° or less and a region having an absolute value of the β angle of 2° to 6° in the grain with respect to the total area of the steel sheet is defined as Rs, Rs is 30% to 100%; and irradiating the surface thereof with a laser beam generated from a fiber laser having a fiber core diameter of 5 μm to 400 μm periodically and in a direction substantially perpendicular to the rolling direction of the grain-oriented electrical steel sheet so that the rolling direction width of the surface irradiation mark of the laser beam or the rolling direction width of the closure domain formed in the laser irradiation part is 10 μm to 200 μm to further reduce iron loss compared to before laser beam irradiation. Patent Document 3 shows that, according to the above method, it is possible to provide a grain-oriented electrical steel sheet having an extremely low iron loss and magnetostriction particularly in a grain-oriented electrical steel sheet having a high magnetic flux density.

However, according to the study of the present inventors, it has been found that transformer noise is effectively reduced when the 200 Hz component in magnetostriction waveform is reduced. In the technique in Patent Document 3, the 200 Hz component in magnetostriction waveform has not been studied, and the characteristics may be insufficient.

In addition, Patent Document 4 discloses a novel and improved manufacturing method for a grain-oriented electrical steel sheet that can manufacture a grain-oriented electrical steel sheet with lower iron loss when the temperature is rapidly increased at a higher temperature rise rate than before in the primary recrystallization annealing, and a grain-oriented electrical steel sheet manufactured by the manufacturing method. Patent Document 4 discloses that the above technique can reduce noise while improving the magnetic characteristics of a transformer.

However, Patent Document 4 does not consider the 200 Hz component in magnetostriction waveform, either. Patent Document 4 describes it is assumed that transformer noise is increased by a magnetic domain control process.

CITATION LIST

Patent Documents

Patent Document 1

• • Japanese Patent No. 6606991

Patent Document 2

• • Japanese Patent No. 5896112

Patent Document 3

• • Japanese Patent No. 4616623

Patent Document 4

• • WO 2019/181952

SUMMARY OF INVENTION

Technical Problem

As described above, conventionally, in a grain-oriented electrical steel sheet on the premise of implementation of magnetic domain control with laser irradiation for the reduction of iron loss, a grain-oriented electrical steel sheet that has a reduced 200 Hz component in magnetostriction waveform, which is correlated with transformer noise, has not been disclosed.

Therefore, an object of the present invention is to provide a grain-oriented electrical steel sheet with low iron loss and low noise, which is suitable for application to a transformer or the like.

Solution to Problem

The present inventors have studied the reduction of transformer noise. As a result, it has been found that transformer noise is strongly correlated with the 200 Hz component in magnetostriction waveform, and transformer noise can be reduced by reducing the 200 Hz component in magnetostriction waveform.

In addition, when laser irradiation is performed on the surface of a grain-oriented electrical steel sheet in order to reduce the iron loss, magnetostriction generally increases. That is, the reduction in iron loss and the reduction in magnetostriction are in a trade-off relationship. Therefore, conventionally, when magnetostriction is reduced, there is no choice but to adjust irradiation conditions such as laser intensity and select conditions where the iron loss reduction effect is sacrificed to some extent and magnetostriction is reduced.

However, it is difficult to further reduce iron loss and magnetostriction at the same time only by adjusting laser irradiation conditions.

Therefore, the present inventors have studied a method of controlling the microstructure and the like of a grain-oriented electrical steel sheet to reduce magnetostriction (particularly, the 200 Hz component in magnetostriction waveform) without impairing iron loss.

As a result, the present inventors have found that magnetostriction can be reduced without impairing iron loss when the crystal grain size (in particular, the distance between the grain boundaries in the rolling direction) is controlled. In addition, it has been found that the crystal grain size is effectively controlled by performing a special treatment in coiling to change the curvature of the steel sheet.

The present invention has been made in view of the above findings. The gist of the present invention is as follows.

[1] In an aspect of the present invention, a grain-oriented electrical steel sheet includes: a base steel sheet; a forsterite film formed on a surface of the base steel sheet; and an insulating film formed on a surface of the forsterite film, wherein the base steel sheet includes, as a chemical composition, in terms of mass %, 0.80 to 7.00% of Si, in the surface of the base steel sheet, grains where the distance between grain boundaries of a grain is 3.0 mm or more and 13.0 mm or less in a rolling direction have an area ratio of 70% or more, a magnetic flux density B8 generated under a magnetizing force of 800 A/m is 1.88 T or more, when a sheet thickness of the base steel sheet is defined as t in a unit mm, an iron loss W17/50 is 13.1×t²−4.3×t+1.2 or less in a unit W/kg under a frequency of 50 Hz and a maximum magnetic flux density of 1.7 T, and a LvA200 Hz, which is a 200 Hz component of magnetostriction waveform is 60 to 78 dBA.

[2] In the grain-oriented electrical steel sheet according to [1], optionally, in the surface of the base steel sheet, a plurality of linear strains extending in a direction intersecting a rolling direction is formed, and the plurality of linear strains has an interval of 3 to 10 mm in the rolling direction.

[3] In the grain-oriented electrical steel sheet according to [1] or [2], optionally, the base steel sheet has: a closure domain with a strip shape in an area percentage of 10% or less; and a main domain with a stripe shape having a width of 1.2 mm or less, each of which is observed in the surface.

[4] In the grain-oriented electrical steel sheet according to [1] or [2], optionally, when the magnetic flux density is 1.92 T or more and the sheet thickness is 0.18 to 0.23 mm, the W17/50 is 0.74 W/kg or less.

[5] In the grain-oriented electrical steel sheet according to [3], optionally, when the magnetic flux density is 1.92 T or more and the sheet thickness is 0.18 to 0.23 mm, the W17/50 is 0.74 W/kg or less.

Advantageous Effects of Invention

According to the aspect of the present invention, it is possible to provide a grain-oriented electrical steel sheet having low iron loss and low noise (excellent in iron loss characteristics and noise characteristics).

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an example of an observed magnetic domain pattern.

FIG. 2 is a diagram for illustrating a closure domain (lancet).

FIG. 3 is a diagram illustrating an example of an arrangement of spacers placed between steel sheets in a coiling step.

FIG. 4 is a diagram illustrating an example of an embodiment in which an annealing separator is applied in a coiling step.

DESCRIPTION OF EMBODIMENTS

The grain-oriented electrical steel sheet according to an embodiment of the present invention (the grain-oriented electrical steel sheet according to the embodiment) includes: a base steel sheet; a forsterite film formed on a surface of the base steel sheet; and an insulating film formed on a surface of the forsterite film.

In the grain-oriented electrical steel sheet according to the embodiment, the base steel sheet includes, as a chemical composition, in terms of mass %, 0.80 to 7.00% of Si, in the surface of the base steel sheet, grains where the distance between grain boundaries of a grain is 3.0 mm or more and 13.0 mm or less in a rolling direction have an area ratio of 70% or more, a magnetic flux density B8 generated under a magnetizing force of 800 A/m is 1.88 T or more, when a sheet thickness is defined as t in a unit mm, an iron loss W17/50 is 13.1×t²−4.3×t+1.2 or less in a unit W/kg under a frequency of 50 Hz and a maximum magnetic flux density of 1.7 T, and a LvA200 Hz, which is a 200 Hz component of magnetostriction waveform is 60 to 78 dBA.

Each will be described below.

<Base Steel Sheet>

(Chemical Composition)

Si: 0.80 to 7.00%

Si (silicon) is an element that increases the electric resistance of the grain-oriented electrical steel sheet and improves iron loss characteristics. When the Si content is less than 0.80%, a sufficient eddy-current loss reducing effect cannot be obtained. Therefore, the Si content is 0.80% or more. The Si content is preferably 1.00% or more, and more preferably 1.20% or more.

On the other hand, when the Si content exceeds 7.00%, the steel sheet becomes brittle, and the steel sheet may be broken during rolling. In addition, the workability of the grain-oriented electrical steel sheet is deteriorated. Therefore, the Si content is 7.00% or less. The Si content is preferably 6.80% or less, more preferably 6.70% or less, and more preferably 4.00% or less.

The content of other elements is not particularly limited as long as the base steel sheet of the grain-oriented electrical steel sheet according to the embodiment includes, as a chemical composition, in terms of mass %, 0.80 to 7.00% of Si.

However, in accordance with the characteristics required for the grain-oriented electrical steel sheet, the following elements may be included in the range shown below in addition to Si as the components (elements) constituting the chemical composition. In the embodiment, % relating to the content of each element is mass % unless otherwise specified.

C: 0.070% or Less

C (carbon) is an element effective in controlling the microstructure of the steel sheet in steps up to completion of the decarburization annealing step in the manufacturing process. However, when the C content exceeds 0.070%, the magnetic characteristics of the grain-oriented electrical steel sheet, which is a product sheet, are deteriorated. Therefore, in the base steel sheet of the grain-oriented electrical steel sheet according to the embodiment, the C content is preferably 0.070% or less. The C content is more preferably 0.050% or less, still more preferably 0.020% or less. The C content is preferably as low as possible. Even when the C content is reduced to less than 0.0001%, the microstructure control effect is saturated, and manufacturing cost is merely increased. Therefore, the C content may be 0.0001% or more.

Mn: 0.01 to 0.50%

Mn (manganese) is an element that is bonded to S in the manufacturing process to form MnS. This precipitate functions as an inhibitor (inhibitor for normal grain growth) and causes secondary recrystallization in steel. Mn is an element that also enhances hot workability of steel. When the Mn content is less than 0.01%, the above effect cannot be sufficiently obtained. Therefore, the Mn content is preferably 0.01% or more. The Mn content is more preferably 0.02% or more.

On the other hand, when the Mn content exceeds 0.50%, secondary recrystallization does not occur, and magnetic characteristics of steel are deteriorated. Therefore, in the base steel sheet of the grain-oriented electrical steel sheet according to the embodiment, the Mn content is preferably 0.50% or less. The Mn content is more preferably 0.20% or less, and still more preferably 0.10% or less.

N: 0.0100% or Less

N (nitrogen) is an element that is bonded to Al in the manufacturing process to form AlN that functions as an inhibitor. When the N content is more than 0.0100%, the inhibitor excessively remains in the grain-oriented electrical steel sheet, and the magnetic characteristics are deteriorated. Therefore, in the base steel sheet of the grain-oriented electrical steel sheet according to the embodiment, the N content is preferably 0.0100% or less. The N content is more preferably 0.0080% or less.

On the other hand, the lower limit of the N content is not particularly limited. Even when the N content is reduced to less than 0.0010%, manufacturing cost is merely increased. Therefore, the N content may be 0.0010% or more.

Sol. Al: 0.030% or Less

Sol. Al (acid-soluble aluminum) is an element that is bonded to N in the manufacturing process of the grain-oriented electrical steel sheet to form AlN that functions as an inhibitor. However, when the Sol. Al content in the base steel sheet exceeds 0.030%, the inhibitor excessively remains in the base steel sheet and the magnetic characteristics are deteriorated. Therefore, in the base steel sheet of the grain-oriented electrical steel sheet according to the embodiment, the Sol. Al content is preferably 0.030% or less. The Sol. Al content is more preferably 0.020% or less, still more preferably 0.015% or less. The lower limit of the Sol. Al content is not particularly limited. Even when the content is reduced to less than 0.0001%, manufacturing cost is merely increased. Therefore, the Sol. Al content may be 0.0001% or more.

S: 0.010% or Less

S (sulfur) is an element that is bonded to Mn in the manufacturing process to form MnS that functions as an inhibitor. However, when the S content exceeds 0.010%, the remaining inhibitor deteriorates magnetic characteristics. Therefore, in the base steel sheet of the grain-oriented electrical steel sheet according to the embodiment, the S content is preferably 0.010% or less. The S content is more preferably as low as possible in the grain-oriented electrical steel sheet. For example, the S content is less than 0.001%. However, even when the S content is reduced to less than 0.0001% in the grain-oriented electrical steel sheet, manufacturing cost is merely increased. Therefore, the S content may be 0.0001% or more in the grain-oriented electrical steel sheet.

Balance: Fe and Impurities

In the grain-oriented electrical steel sheet according to the embodiment, the chemical composition of the base steel sheet contains, for example, the above-described elements, and the balance may be Fe and impurities. However, for the purpose of improving magnetic characteristics and the like, P, Cr, Sn, Cu, Se, Sb, and Mo may be further included in the following ranges (these elements are not necessarily included and the lower limit thereof are 0%). In addition, even when other elements, for example, at least any one of W, Nb, Ti, Ni, Bi, Co, and V, are included in a total amount of 1.0% or less, the effect of the grain-oriented electrical steel sheet according to the embodiment is not impaired.

Here, the impurities are contaminated from ore or scrap as a raw material, or from a manufacturing environment or the like when the base steel sheet is industrially manufactured. The impurities mean elements allowed to be included in such a content that the operation of the grain-oriented electrical steel sheet according to the embodiment is not adversely affected.

P: 0 to 0.030%

P (phosphorus) is an element that lowers the workability in rolling. When the P content is 0.030% or less, it is possible to suppress excessive reduction in rolling workability and to suppress fracture during manufacturing. From such a viewpoint, the P content is preferably 0.030% or less. The P content is more preferably 0.020% or less, and further preferably 0.010% or less.

The lower limit of the P content is not limited, and the P content may be 0%. In a practical steel sheet, the lower limit of the P content is substantially 0.0001%. In addition, P is also an element having an effect of improving the texture and improving the magnetic characteristics. In order to obtain this effect, the P content may be 0.001% or more or 0.005% or more.

Cr: 0 to 0.50%

Cr (chromium) is an element that contributes to an increase in Goss orientation occupancy in the secondary recrystallization structure and improvement of magnetic characteristics. In order to obtain the above effect, the Cr content is preferably 0.01% or more, more preferably 0.05% or more, and still more preferably 0.10% or more.

On the other hand, when the Cr content exceeds 0.50%, Cr oxide is formed, and the magnetic characteristics are deteriorated. Therefore, the Cr content is preferably 0.50% or less. The Cr content is more preferably 0.30% or less, and still more preferably 0.15% or less.

Sn: 0 to 0.50%

Sn (tin) is an element that contributes to improvement in magnetic characteristics through primary recrystallization structure control. In order to obtain an effect of improving magnetic characteristics, the Sn content is preferably 0.01% or more. The Sn content is more preferably 0.02% or more, and still more preferably 0.03% or more.

On the other hand, when the Sn content exceeds 0.50%, secondary recrystallization is unstable, and magnetic characteristics are deteriorated. Therefore, the Sn content is preferably 0.50% or less. The Sn content is more preferably 0.30% or less, and still more preferably 0.10% or less.

Cu: 0 to 0.50%

Cu (copper) is an element that contributes to an increase in Goss orientation occupancy in the secondary recrystallization structure. Cu is an optional element in the base steel sheet of the grain-oriented electrical steel sheet according to the embodiment. Therefore, the lower limit of the content is 0%. In order to obtain the above effect, the Cu content is preferably 0.01% or more. The Cu content is more preferably 0.02% or more, and still more preferably 0.03% or more.

On the other hand, when the Cu content exceeds 0.50%, the steel sheet is embrittled during hot rolling. Therefore, in the base steel sheet of the grain-oriented electrical steel sheet according to the embodiment, the Cu content is preferably 0.50% or less. The Cu content is more preferably 0.30% or less, and still more preferably 0.10% or less.

Se: 0 to 0.020%

Se (selenium) is an element having an effect of improving magnetic characteristics. Thus, Se may be included. When Se is contained, the content is preferably 0.001% or more such that Se favorably exhibits the effect of improving magnetic characteristics. The Se content is more preferably 0.003% or more, and still more preferably 0.006% or more.

On the other hand, when the Se content exceeds 0.020%, the adhesion of the forsterite film is deteriorated. Therefore, the Se content is preferably 0.020% or less. The Se content is more preferably 0.015% or less, and still more preferably 0.010% or less.

Sb: 0 to 0.50%

Sb (antimony) is an element having an effect of improving magnetic characteristics. Thus, Sb may be included. When Sb is contained, the content is preferably 0.005% or more such that Sb favorably exhibits the effect of improving magnetic characteristics. The Sb content is more preferably 0.01% or more, and still more preferably 0.02% or more.

On the other hand, when the Sb content exceeds 0.50%, the adhesion of the forsterite film is remarkably deteriorated. Therefore, the Sb content is preferably 0.50% or less. The Sb content is more preferably 0.30% or less, and still more preferably 0.10% or less.

Mo: 0 to 0.10%

Mo (molybdenum) is an element having an effect of improving magnetic characteristics. Thus, Mo may be included. When Mo is contained, the Mo content is preferably 0.01% or more in order to favorably exhibit the effect of improving magnetic characteristics. The Mo content is more preferably 0.02% or more, and still more preferably 0.03% or more.

On the other hand, when the Mo content is more than 0.10%, the cold rolling characteristics may be deteriorated, leading to breakage. Therefore, the Mo content is preferably 0.10% or less. The Mo content is more preferably 0.08% or less, and still more preferably 0.05% or less.

As described above, in the grain-oriented electrical steel sheet according to the embodiment, for example, the chemical composition of the base steel sheet includes C, Si, Mn, N, Sol. Al, S, and a balance of Fe and impurities, or the chemical composition of the base steel sheet includes elements C, Si, Mn, N, Sol. Al, and S, and further includes one or more of P, Cr, Sn, Cu, Se, Sb, Mo, W, Nb, Ti, Ni, Bi, Co, and V, and a balance of Fe and impurities. The total content of the elements W to V may be 0.05% or less.

In the grain-oriented electrical steel sheet according to the embodiment, the Si content in the chemical composition of the base steel sheet is determined by a method specified in JIS G 1212 (1997) (Methods for determination of silicon content). Specifically, when the above-described swarf is dissolved in an acid, silicon oxide is separated as a precipitate, and thus the precipitate (silicon oxide) is filtered with filter paper, and the mass is measured to determine the Si content.

For others, the chemical composition of the base steel sheet may be determined by a well-known component analysis method. Specifically, swarf is generated from the base steel sheet using a drill, the swarf is recovered, and the recovered swarf is dissolved in an acid to obtain a solution. The solution is subjected to ICP-AES to perform elemental analysis of the chemical composition.

However, elements that are difficult to measure by ICP-AES, for example, the C content and the S content, are determined by a well-known high frequency combustion method (combustion-infrared absorption method). Specifically, optionally, the above-described solution is combusted by high-frequency heating in an oxygen stream, and generated carbon dioxide and sulfur dioxide are detected to determine the C content and the S content. The N content may be determined using a well-known inert gas melting-thermal conductivity method.

When a forsterite film and/or an insulating film are formed on the surface of the grain-oriented electrical steel sheet according to the embodiment, the chemical composition of the base steel sheet is measured after these films are removed.

The insulating film can be removed by immersing the grain-oriented electrical steel sheet in a sodium hydroxide aqueous solution containing 30 to 50 mass % of NaOH and 50 to 70 mass % of H₂O at 80 to 90° C. for 7 to 10 minutes.

The grain-oriented electrical steel sheet from which the insulating film has been removed is washed with water, and after water washing, dried with a warm air blower for slightly less than 1 minute. The dried grain-oriented electrical steel sheet (grain-oriented electrical steel sheet not provided with an insulating film) is immersed in a hydrochloric acid aqueous solution containing 30 to 40 mass % of HCl at 80 to 90° C. for 1 to 10 minutes. Thereby, the forsterite film can be removed.

After immersion, the base steel sheet is washed with water, and after water washing, dried with a warm air blower for slightly less than 1 minute. Thereby, the base steel sheet can be isolated from the grain-oriented electrical steel sheet having a forsterite film and an insulating film.

(Microstructure)

In the grain-oriented electrical steel sheet according to the embodiment, the microstructure is controlled in its grain size in the rolling direction. Specifically, in the surface of the base steel sheet, grains where the distance between grain boundaries of a grain is 3.0 mm or more and 13.0 mm or less in a rolling direction have an area ratio of 70% or more.

A grain boundary crossing the rolling direction changes the magnetic domain pattern. In particular, as the distance between the grain boundaries decreases, the 200 Hz component (sometimes referred to as LvA200 Hz) in magnetostriction waveform decreases. This is presumably because, when the distance between the grain boundaries is decreased, the width of the main domain is narrowed, so that an effect of suppressing the closure domain (lancet) in a grain is exhibited.

When grains where the distance between grain boundaries (grain boundary interval) of a grain is 3.0 mm or more and 13.0 mm or less in a rolling direction have an area ratio of less than 70%, the effect of reducing LvA200 Hz cannot be sufficiently obtained. The area ratio of grains having a grain boundary interval of 3.0 mm or more and 13.0 mm or less is preferably 75% or more, more preferably 80% or more, and still more preferably 90% or more. The upper limit of the area ratio is not limited and may be 100%.

The grain for which the area ratio is controlled is set to grains (regions) in which the distance between the grain boundaries is 3.0 mm or more and 13.0 mm or less, because, in grains in which the grain boundary interval is more than 13.0 mm, the domain wall interval is not sufficiently narrowed, and the effect of reducing LvA200 Hz is small. On the other hand, the grain boundary interval is preferably short from the viewpoint of reducing LvA200 Hz. However, a grain having a grain boundary interval of less than 3.0 mm may hinder the movement of a domain wall, thereby deteriorating the iron loss characteristics.

FIG. 1 illustrates an example of the magnetic domain pattern observed in the grain-oriented electrical steel sheet according to the embodiment.

In order to obtain the above-described microstructure, there is a method of irradiating a laser to the rolling direction of the base steel sheet such that the scanning direction of the laser crosses the rolling direction, in which irradiation is preferably performed such that fine grains are not generated on the irradiation marks. When fine grains are not generated, it is possible to suppress the generation of grains where the grain has a distance between grain boundaries of less than 3.0 mm in a rolling direction in the surface of the base steel sheet.

The average grain size in the rolling direction is preferably 3.0 to 20.0 mm.

When the average grain size is small, it is difficult that grains where the distance between grain boundaries (grain boundary interval) of a grain is 3.0 mm or more and 13.0 mm or less in a rolling direction have an area ratio of 70% or more, and it is concerned that the movement of a domain wall is hindered and thereby the iron loss characteristics are deteriorated. On the other hand, when the average grain size exceeds 20.0 mm, it is difficult that the area ratio of grains having a size of 3.0 mm or more and 13.0 mm or less is 70% or more, and it is concerned that noise is increased.

The area ratio of grains in each of which the distance between grain boundaries of a grain is 3.0 mm or more and 13.0 mm or less in a rolling direction can be determined by the following method.

First, the distance between grain boundaries in the rolling direction is measured by observing magnetic domains in the surface of the grain-oriented electrical steel sheet using an element for magnetic domain observation. Specifically, the surface of the grain-oriented electrical steel sheet is observed by using a reflection electron microscope or an MO sensor using the Faraday effect, an image is obtained, the grain boundary is determined from the image according to the direction and interval of domain walls and contrast discontinuity, and the distance between the grain boundaries in the rolling direction is measured. Thus, the distance is determined.

The area ratio of the grains having a predetermined grain boundary interval is determined as follows: in the surface, a region of 100 mm in the width direction and 500 mm in the rolling direction is set as an observation range; and in the observation range, the area of the grains in each of which the distance between grain boundaries is 3.0 mm or more and 13.0 mm or less in the rolling direction is divided by the area of the measurement region (for percentage, ×100). At that time, the area of the objective grains (the grains in each of which the distance between grain boundaries is 3.0 mm or more and 13.0 mm or less in the rolling direction) may be measured as follows: a scale along the rolling direction is used to specify the grains out of 3.0 mm or more and 13.0 mm or less; the area thereof is measured; and the measured area is subtracted from the area of the measurement region.

In the above measurement, the average grain size in the rolling direction can also be measured as follows: a 500 mm straight line is drawn from one end to the other end in the rolling direction in the observation range; and the length of the straight line is divided by the number of grain boundaries intersecting the straight line.

The area ratio is measured in the surface of the base steel sheet. According to the above method, the measurement can be performed even when the forsterite film and the insulating film are formed on the base steel sheet. Therefore, it is not necessary to remove the forsterite film and the insulating film for measurement, but measurement may be performed after removal thereof.

As illustrated in FIG. 2, in the magnetic domain observed in the surface of the base steel sheet of the grain-oriented electrical steel sheet according to the embodiment, a closure domain (lancet) 100 with a strip shape is present within the main domain. In the grain-oriented electrical steel sheet according to the embodiment, the area percentage of the closure domain (lancet) 100 is preferably 10% or less of the whole. When the area percentage of the closure domain 100 exceeds 10%, it is difficult to obtain a predetermined magnetic flux density and iron loss. The lower limit of the area ratio is not limited and may be 0%.

In the grain-oriented electrical steel sheet according to the embodiment, the main domain, which is observed in the surface and seen as being separated in a stripe shape, preferably has a width of 1.2 mm or less. When the width increases beyond this, the lancet 100 is likely to occur, and it is difficult to obtain a predetermined magnetic flux density and iron loss. More preferably, the width of the main domain is 1.0 mm or less, and still more preferably 0.9 mm or less. The width of the main domain is affected by the average grain size in the rolling direction, the magnitude of strain caused by the difference in laser irradiation conditions or curvature in a coil state, and the like.

The area percentage of the closure domain and the width of the main domain are measured with a measuring instrument such as CMOS-Mag View using a magneto-optical effect in a range of 100 mm in the width direction×500 mm in the rolling direction in the surface of the base steel sheet. According to this method, magnetic domain observation is also possible in an electrical steel sheet in which films are formed. Thereafter, based on the obtained image, the area of the lancet in the steel sheet surface is measured, the abundance ratio thereof to the whole is evaluated, or the width of the main domain is measured. However, the observation is performed in a demagnetized state.

(Magnetic Characteristics)

The grain-oriented electrical steel sheet according to the embodiment has been subjected to magnetic domain control by laser irradiation and is excellent in magnetic characteristics. Specifically, the magnetic flux density B8 generated under a magnetizing force of 800 A/m is 1.88 T or more; and when the sheet thickness of the base steel sheet is defined as t in a unit mm, the iron loss W17/50 is 13.1×t²−4.3×t+1.2 or less in a unit W/kg under a frequency of 50 Hz and a maximum magnetic flux density of 1.7 T. When B8 is less than 1.88 T or W17/50 is more than 13.1×t²−4.3×t+1.2 W/kg, the magnetic characteristics is not sufficient.

Although B8 is preferably high, on the premise that the area of the above-described grains is increased, the upper limit is substantially about 1.95 T, and thus B8 may be 1.95 T or less.

The sheet thickness t is a nominal thickness, and when t=0.23 mm, W17/50 is 0.90 W/kg or less.

The above B8 and W17/50 are achieved when fine regions such as a texture or a magnetic domain are controlled by controlling the manufacturing method including laser irradiation. However, there is a plurality of factors that change magnetic characteristics, and measurement thereof is not easy. Therefore, in the grain-oriented electrical steel sheet according to the embodiment, the grain-oriented electrical steel sheet is defined by the values of B8 and W17/50.

As the magnetic characteristics, B8 is preferably 1.92 T or more. In addition, the iron loss W17/50 is preferably 0.74 W/kg or less in a range where the sheet thickness (t) is 0.18 to 0.23 mm.

B8 and W17/50 are measured by a single sheet magnetic characteristics measurement method (Single Sheet Tester: SST) in accordance with JIS C 2556:2015 using a test piece with a size of 100 mm in the width direction and 500 mm in the rolling direction that has been cut from the grain-oriented electrical steel sheet.

(Noise Characteristics)

In the grain-oriented electrical steel sheet according to the embodiment, the 200 Hz component LvA200 Hz in magnetostriction waveform is 60 to 78 dBA. As a result, in a use for a material of a transformer, noise from the transformer can be reduced. When LvA200 Hz exceeds 78 dBA, the noise reduction effect is small. On the other hand, when predetermined magnetic characteristics are secured, it is not easy that LvA200 Hz is less than 60 dBA, and LvA200 Hz is 60 dBA or more.

LvA200 Hz is determined by measuring the elongation and shrink of the steel sheet with a laser Doppler vibration measuring instrument in accordance with IEC standard IEC 606404-17 ED1.

In the measurement, the elongation and shrink of the steel sheet are generated by applying a magnetic field of 50 Hz from the outside. After the elongation and shrink of the length against time is obtained as magnetostriction waveform, the waveform is subjected to frequency analysis, and separated into 100 Hz and 200 Hz components for evaluation.

As a result of magnetic domain control, the grain-oriented electrical steel sheet according to the embodiment has B8 and W17/50 described above, and LvA200 Hz is 60 to 78 dBA. That is, the grain-oriented electrical steel sheet according to the embodiment is a magnetic domain-controlled material.

The method of magnetic domain control is not limited. For example, magnetic domain control is preferably performed by laser irradiation under conditions described later. In this case, in the surface of the base steel sheet, a plurality of linear strains extending in a direction intersecting the rolling direction is formed, such that the plurality of linear strains has an interval of 3 to 10 mm in the rolling direction. Therefore, the plurality of linear strains preferably has an interval of 3 to 10 mm in the rolling direction. The interval between the linear strains in the rolling direction is the distance from the center of a linear strain in the width direction to the center of the adjacent linear strain.

In addition, the width of the linear strain in the rolling direction is preferably 250 μm or less to contribute to improvement in iron loss characteristics. Linear includes a straight line that is a continuous line, and a dotted line that is an intermittent line.

The location of the linear strains can be analyzed using a residual strain measurement technique by an X-ray diffraction method (for example, K. Iwata, et. al, j. Appl. Phys. 117.17 A910 (2015)). When an energy ray irradiation mark can be confirmed on the surface of the steel sheet, the irradiation mark may be determined as a strain.

For the observed linear strains, a predetermined distance L, where L>5 mm, is provided in the rolling direction, and the number n of the strains present therein may be counted. The interval between the linear strains in the rolling direction is defined as L/n. The width of the linear strain in the rolling direction may be an average value of the measured n widths.

(Sheet Thickness)

In the grain-oriented electrical steel sheet according to the embodiment, the sheet thickness of the base steel sheet is not limited but is preferably 0.17 to 0.30 mm in consideration of application to a core of a transformer requiring low noise as well as low iron loss.

As the sheet thickness becomes thinner, an effect of reducing eddy-current loss can be obtained and a preferable iron loss can be obtained. Therefore, the upper limit of the sheet thickness of the base steel sheet is preferably 0.30 mm. On the other hand, special equipment is required to manufacture a base steel sheet having a sheet thickness of less than 0.17 mm, which is not preferable in terms of production such as an increase in manufacturing cost. Therefore, the lower limit of the sheet thickness is industrially preferably 0.17 mm. The sheet thickness is preferably 0.18 to 0.23 mm.

<Forsterite Film>

In the grain-oriented electrical steel sheet according to the embodiment, a forsterite film (may be also referred to as glass film) is formed on the surface of the base steel sheet. The forsterite film may be any known film. Generally, the forsterite film is an inorganic film containing magnesium silicate as a main component.

The forsterite film is formed in final annealing when an annealing separator including magnesia (MgO) that is applied on the surface of the base steel sheet reacts with the components in the surface of the base steel sheet. The forsterite film has a composition derived from the annealing separator and the components of the base steel sheet and has a microstructure including a Mg₂SiO₄ phase as a main phase (50 area % or more) and an MgAl₂O₄ phase. In addition to these phases, precipitates may be included in an amount of about 1% or less.

<Insulating Film>

In the grain-oriented electrical steel sheet according to the embodiment, an insulating film (tension-applying insulating film) is formed on the surface of the forsterite film. The insulating film may be any known film used in the art.

The insulating film imparts electrical insulation properties to the grain-oriented electrical steel sheet to reduce eddy-current loss and improve the iron loss characteristics (reduce iron loss) of the grain-oriented electrical steel sheet. Further, according to the insulating film, various properties such as corrosion resistance, heat resistance, and slippage can be obtained in addition to the electrical insulation properties as described above. Further, the insulating film has a function of applying tension to the grain-oriented electrical steel sheet. When tension is applied to the grain-oriented electrical steel sheet to facilitate the movement of domain walls in the grain-oriented electrical steel sheet, the iron loss of the grain-oriented electrical steel sheet can be reduced (the iron loss characteristics can be improved). The insulating film is formed, for example, by applying a coating liquid containing a metal phosphate and silica as main components onto the surface of the forsterite film and baking the coating liquid.

<Manufacturing Method>

The grain-oriented electrical steel sheet according to the embodiment can be manufactured by a manufacturing method including the following steps.

• • (i) Hot rolling step of hot-rolling a steel piece having a predetermined chemical composition to obtain a hot rolled sheet (hot-rolled steel sheet)
  • (ii) Cold rolling step of cold-rolling the hot rolled sheet to obtain a cold rolled sheet (cold-rolled steel sheet)
  • (iii) Decarburization annealing step of performing decarburization annealing to the cold rolled sheet
  • (iv) Final annealing step of applying an annealing separator including MgO onto the cold rolled sheet after the decarburization annealing step, then coiling the cold rolled sheet in a coil shape, and performing final annealing
  • (v) Insulating film-forming step of forming an insulating film on the surface of the cold rolled sheet after the final annealing to obtain a grain-oriented electrical steel sheet
  • (vi) Laser irradiation step of performing laser irradiation to the grain-oriented electrical steel sheet having the insulating film

The manufacturing method of a grain-oriented electrical steel sheet according to the embodiment may further include the following steps.

• • (I) Hot rolled sheet annealing step of annealing the hot rolled sheet
  • (II) Nitriding treatment step of increasing the nitrogen amount in the cold rolled sheet

The manufacturing method of a grain-oriented electrical steel sheet according to the embodiment is characterized by the above-described (iv) final annealing step and (vi) laser irradiation step. Preferred conditions in these steps will be described below. In other steps, known manufacturing conditions for a grain-oriented electrical steel sheet can be applied.

Each step will be described.

(Hot Rolling Step)

In the hot rolling step, a steel piece such as a slab is heated and then hot-rolled to obtain a hot rolled sheet. The heating temperature of the steel piece is not particularly limited, but is preferably in the range of 1100 to 1450° C.

The hot rolling conditions are not particularly limited and is appropriately set on the basis of required characteristics. The sheet thickness of the hot rolled sheet obtained by hot rolling is preferably, for example, within a range of 2.0 to 3.0 mm.

The chemical composition of the steel piece may be in a preferable range to obtain the chemical composition of the base steel sheet described above in consideration of the manufacturing steps after the hot rolling step (in consideration of the change in the chemical composition in each step).

(Hot Rolled Sheet Annealing Step)

The hot rolled sheet annealing step is a step of annealing a hot rolled sheet manufactured through the hot rolling step. By performing such an annealing treatment, recrystallization occurs in the metallographic structure, and favorable magnetic characteristics can be realized. Therefore, the hot rolled sheet annealing step may be performed.

When hot rolled sheet annealing is performed, the hot rolled sheet manufactured through the hot rolling step is annealed according to a known method. The annealing conditions are not particularly limited. For example, the hot rolled sheet can be annealed in a temperature range of 900 to 1200° C. for 10 seconds to 5 minutes. The means to heat the hot rolled sheet for annealing is not particularly limited, and a known heating method can be adopted. Two-stage annealing, in which the annealing temperature is changed in the middle, may be used.

(Cold Rolling Step)

In the cold rolling step, the hot rolled sheet after the hot rolled sheet annealing step is subjected to cold rolling including several passes to obtain a cold rolled sheet. The cold rolling may be performed one time (continuously performed without intervening intermediate annealing(s)). Alternatively, before the final pass in the cold rolling step, intermediate annealing may be performed at least one time by interrupting cold rolling, that is, cold rolling may be performed several times with intervening intermediate annealing(s).

When intermediate annealing is performed, it is preferable to hold the hot rolled sheet at a temperature of 1000 to 1200° C. for 5 to 180 seconds. The annealing atmosphere is not particularly limited. The number of times of intermediate annealing is preferably three or less in consideration of manufacturing cost.

In the cold rolling step, the hot rolled sheet may be cold-rolled according to a known method to obtain a cold rolled sheet. For example, the final rolling reduction can be in a range of 80 to 95%. When the final rolling reduction is 80 to 95%, it is possible to obtain Goss nuclei in which the {110} <001> orientation has a high development degree in the rolling direction and to suppress destabilization of secondary recrystallization.

The final rolling reduction is a cumulative rolling reduction of cold rolling, and when intermediate annealing is performed, the final rolling reduction is a cumulative rolling reduction of cold rolling after the final intermediate annealing.

Before the cold rolling step, the surface of the hot rolled sheet may be subjected to pickling under known conditions.

(Decarburization Annealing Step)

In the decarburization annealing step, the cold rolled sheet is subjected to decarburization annealing. In decarburization annealing, the cold rolled sheet is primarily recrystallized, and C, which adversely affects magnetic characteristics, is removed from the steel sheet. In the decarburization annealing step, the number of Goss nuclei is increased in order to obtain fine secondary recrystallized grains in the final annealing described later. Considering that the grain boundary itself has a function as a magnetic pole (leakage magnetic flux-generating site), the miniaturization of the secondary recrystallized grains increases the static magnetic energy of the entire system. That is, the driving force for magnetic domain refinement becomes high.

The conditions for decarburization annealing may be in a known range. Examples thereof include conditions in which the cold rolled sheet is held at an annealing temperature of 750 to 900° C. for 10 to 600 seconds in a wet hydrogen and nitrogen atmosphere. From the viewpoint of controlling the average grain size in consideration of the balance between iron loss and noise, the decarburization annealing temperature is optimally about 835 to 845° C.

(Nitriding Treatment Step)

In the nitriding treatment step, the nitrogen amount in the steel sheet is increased. The nitriding treatment step is performed at any one or more timings of: during the decarburization annealing step; between the decarburization annealing step and the final annealing step; and during the heating process in the final annealing step and by the start of secondary recrystallization.

Examples of the method of increasing the nitrogen amount of the steel sheet include: a method of controlling the nitrogen amount in the steel sheet by performing annealing in an atmosphere containing a gas having nitriding ability; and when the nitriding treatment step is performed during the heating process in the final annealing step, a method of adding a powder having nitriding ability such as MnN to the annealing separator.

(Final Annealing Step)

In the final annealing step, a predetermined annealing separator is applied to one surface or both surfaces of the cold rolled sheet, which is obtained in the decarburization annealing step or further subjected to nitriding treatment, then the cold rolled sheet is coiled into a coil shape, and final annealing is performed.

In manufacturing the grain-oriented electrical steel sheet according to the embodiment, the steel sheet (steel strip) is provided with a certain or more curvature during coiling into a coil. As a result, the distance between the grain boundaries in the rolling direction is controlled, and the area ratio of grains in each of which the distance between the grain boundaries is 3.0 to 13.0 mm is increased.

The method of providing curvature includes the following two methods.

• • (a) During coiling, a spacer is arranged between the steel sheets.
  • (b) The application amount of the annealing separator is periodically changed.

Hereinafter, (a) and (b) will be described.

Even when neither (a) nor (b) is performed, the steel sheet is provided with curvature when forming a coil. A larger curvature is provided to the inner side of the coil as compared with the outer side. However, although the curvature is relatively large on the inner side of a coil, conventional coiling cannot provide the steel sheet with curvature enough to obtain the grain-oriented electrical steel sheet according to the embodiment.

(a) A Spacer is Arranged Between the Steel Sheets

In this method, as illustrated in FIG. 3, a steel sheet 1 is coiled into a coil shape while ceramic round bars, as spacers 2, are periodically inserted perpendicularly to the coiling direction and the curvature of the steel sheet is periodically changed. The steel sheet meanders such that the spacers 2 adjacent in the rolling direction of the steel sheet are alternately in contact with the front surface, the back surface, the front surface, the back surface (continues). Thereby, curvature is formed.

When the curvature is small, the area ratio of the grains in each of which the distance between the grain boundaries is 3.0 to 13.0 mm cannot be sufficiently increased. On the other hand, when the curvature is too large, a preferable crystal orientation cannot be obtained, and magnetic characteristics are deteriorated. In order to obtain a predetermined curvature, the diameter (φ) of the spacer 2 (ceramic round bar) is 3 to 20 mm, and the interval (L1 in FIG. 3) between the spacers 2 is 15 to 100 mm in the rolling direction of the steel sheet 1.

(b) the Application Amount of the Annealing Separator is Periodically Changed

In this method, as illustrated in FIG. 4, the application amount (thickness) of the annealing separator 3 is changed so that the steel sheet becomes wavy along the rolling direction.

In order to provide a certain curvature, the annealing separator 3 is applied (disposed) so that the wave height (h in FIG. 4) changes within 3 to 20 mm in a cycle of 15 to 100 mm (L2 in FIG. 4) in the rolling direction of the steel sheet 1.

The annealing separator is applied to the cold rolled sheet for the purpose of preventing seizure between the inside and the outside of the winding of the coil and forming a forsterite film.

In the manufacturing method of a grain-oriented electrical steel sheet according to the embodiment, an annealing separator containing MgO as a main component (for example, containing 80 mass % or more thereof) is used as the applied annealing separator. When an annealing separator containing MgO as a main component is used, a forsterite film can be formed on the surface of the base steel sheet. When the main component does not include MgO, the primary film (forsterite film) is not formed. This is because the forsterite film is Mg₂SiO₄ or an MgAl₂O₄ compound, and Mg, which is necessary for the formation reaction, is not supplied.

The annealing separator may further include TiO₂. When TiO₂ is included, an effect of suppressing defective formation of the glass film can be obtained. The content of TiO₂ is, for example, 0 to 10 mass %.

The annealing separator can be applied to the steel sheet after mixed with water to form a slurry.

After being coiled into a coil shape, the cold rolled sheet is subjected to final annealing. For example, in an atmosphere gas containing hydrogen and nitrogen, the temperature is raised to 1150 to 1250° C. and held in the temperature range for 10 to 60 hours.

(Insulating Film-Forming Step)

In the insulating film-forming step, an insulating film (tension-applying insulating film) is formed on one surface or both surfaces of the cold rolled sheet after final annealing. The conditions for forming the insulating film are not particularly limited. The treatment liquid may be applied and dried by a known method using a known insulating film treatment liquid. When the insulating film is formed on the surface of the steel sheet, the magnetic characteristics of the grain-oriented electrical steel sheet can be further improved.

The surface of the steel sheet on which the insulating film is formed may be: a surface that has been subjected to an arbitrary pretreatment such as a degreasing treatment with alkali or the like or an acid washing treatment with hydrochloric acid, sulfuric acid, phosphoric acid, or the like, before the treatment liquid is applied; or may be a surface as it is after final annealing, having not been subjected to such a pretreatment.

The insulating film formed on the surface of the steel sheet is not particularly limited as long as it is used as an insulating film for a grain-oriented electrical steel sheet, and a known insulating film can be used. Examples of such an insulating film include a film containing a phosphate and colloidal silica as main components. In addition, a composite insulating film mainly containing an inorganic substance and further containing an organic substance can be exemplified. Here, the composite insulating film is, for example, an insulating film mainly made of at least one of a chromate metal salt, a phosphate metal salt, colloidal silica, and an inorganic substance such as a Zr compound and a Ti compound and having a fine organic resin particle dispersed therein. In particular, from the viewpoint of reducing the environmental load in manufacturing, which has been increasingly required in recent years, an insulating film using a metal phosphate, a Zr or Ti coupling agent, or a carbonate or ammonium salt thereof as a starting material may be used.

(Laser Irradiation Step)

In the laser irradiation step, the grain-oriented electrical steel sheet on which the insulating film is formed is irradiated with a laser beam to perform magnetic domain control. This makes it possible to improve magnetic characteristics.

As the laser irradiation conditions, for example, the laser input energy Ua is 1.0 to 4.0 mJ/mm², and the laser power density Ip is 500 to 4000 W/mm². Further, the laser beam is irradiated such that the laser beam extends in a direction intersecting the rolling direction (for example, a direction of 60 to 120° against the rolling direction) (preferably from one end to the other end in the width direction of the steel sheet) and is irradiated several times such that the interval PL in the rolling direction is 3 to 10 mm, and the irradiation directions are substantially parallel to each other (preferably over the entire length of the steel sheet).

When Ua is less than 1.0 mJ/mm² or Ip is less than 500 W/mm², a sufficient magnetic domain refinement effect cannot be obtained. On the other hand, when Ua exceeds 4.0 mJ/mm² or Ip exceeds 4000 W/mm², noise characteristics are deteriorated. This is presumably because many closure domains are generated. Ip is preferably 2000 W/mm² or less.

EXAMPLES

Hereinafter, the grain-oriented electrical steel sheet according to the present invention will be specifically described with reference to Examples. The following Examples are merely examples, and the grain-oriented electrical steel sheet according to the present invention is not limited to the following Examples.

Example 1

A steel slab including 0.055% of C, 0.86 to 3.15% of Si, 0.14% of Mn, 0.007% of S, 0.027% of Sol. Al, 0.12% of Cr, 0.0075% of N, and a balance of Fe and impurities was heated to 1150° C., and then hot-rolled to obtain a hot rolled sheet having a sheet thickness of 2.3 mm.

Subsequently, the hot rolled sheet was subjected to hot rolled sheet annealing. In the hot rolled sheet annealing, the hot rolled sheet was heated to 1120° C. and held for 180 seconds, then cooled to 900° C., and held at this temperature for 120 seconds. Thereafter, the hot rolled sheet was rapidly cooled using hot water at 100° C.

Thereafter, pickling was performed, and then cold rolling was performed to obtain a cold rolled sheet having a sheet thickness of 0.23 mm.

Subsequently, decarburization annealing in which the cold rolled sheet (steel sheet) was heated to the temperature described in Table 1 in an atmosphere of wet hydrogen and nitrogen and held for 150 seconds was performed.

After the decarburization annealing, nitriding treatment in which the cold rolled sheet was heated to 750° C. in an atmosphere in which an atmosphere made of 25% of N₂ and 75% of H₂ is added with NH₃ and held at this temperature for 30 seconds to perform was performed for increasing the N content in the steel sheet to 180 ppm.

After the nitriding treatment, final annealing in which a known annealing separator containing MgO and TiO₂ as main components and containing Na, B, Cl, and the like was applied, and held at 1200° C. for 20 hours was performed. At this time, the final annealing was performed after spacers were arranged at regular intervals between the steel sheets so that the steel sheets having a wavy shape was coiled into a coil shape. The diameter and arrangement interval of the spacers (ceramic rods) were as shown in Table 1. After the final annealing, a forsterite film was formed on the surface.

After the final annealing, the surface of the steel sheet (steel sheet in which the forsterite film was formed on the surface of the base steel sheet) was applied with a coating solution mainly containing chromic anhydride and aluminum phosphate, followed by baking and annealing. Thus, an insulating film was formed.

Thereafter, the surface of the steel sheet on which the insulating film was formed was irradiated with a laser beam to perform magnetic domain refinement (magnetic domain control). At that time, the laser input energy and the laser power density were as shown in Table 1. The scanning direction of the laser beam (the extending direction of the irradiation mark) was the 90° direction against the rolling direction. The interval of the irradiation positions of the adjacent laser beams in the rolling direction (becomes the interval of the plurality of linear strains in the rolling direction) was 4 mm.

For the steel sheet after magnetic domain control (grain-oriented electrical steel sheet), the chemical composition, the percentage of grains in each of which the distance between the grain boundaries was 3.0 to 13.0 mm in the rolling direction, the area ratio of the lancet, the width of the main domain having a stripe shape, the magnetic characteristics (B8 and W17/50), and the value of LvA200 Hz were determined by the above-described methods. At this time, magnetic domain observation was performed in a demagnetized state. In addition, the average grain size in the rolling direction in the surface and the width of each of the linear strains formed at 4 mm intervals in the rolling direction over the entire region of the steel sheet in the rolling direction (average of the plurality of linear strains) were also determined.

Table 1 shows the results.

The content of each element other than Si is not shown in the table, and the C content was 0.014 to 0.055%, the S content was 0.001 to 0.007%, the Sol. Al content was 0.011 to 0.027%, and the N content was 0.0049 to 0.0075%. The Mn content and the Cr content did not change significantly compared to the stage of slab.

TABLE 1
								Area
								percentage of
								grains having
				Laser	Si	Average	Width	distance
			Laser	input	content of	grain size	of	between grain
	Diameter	Interval of	power	energy	base steel	in rolling	linear	boundaries of
	of spacer	spacer	density Ip	Ua	sheet	direction	strain	3.0 to 13.0
No.	(mm)	(mm)	(W/mm2)	(mJ/mm2)	(mass %)	(mm)	(μm)	mm (%)
11	15	90	500	4.0	3.15	5.1	220	79
12	15	90	1100	3.1	3.05	2.8	132	50
13	15	90	1500	4.0	3.06	20.2	225	60
14	15	90	540	3.6	3.12	10.9	153	83
15	15	90	520	3.9	3.10	9.5	197	91
16	15	90	2000	4.1	3.14	10.8	251	82
17	15	90	1900	4.4	3.09	9.4	263	92
18	15	90	3900	4.0	0.86	3.1	248	90
19	2	200	1500	4.0	3.08	21.2	225	60
20	15	90	200	3.8	3.10	20.2	168	60

		Area	Width of	Decarburization
		ratio of	main	annealing		LvA	W17/
		lancet	domain	temperature	B8	200 Hz	50
	No.	(%)	(mm)	(° C.)	(T)	(dB)	(W/kg)	Note
	11	8	1.0	840	1.92	60	0.77	Present invention
								example
	12	20	0.9	830	1.87	80	0.95	Comparative
								example
	13	10	0.8	850	1.89	82	0.92	Comparative
								example
	14	5	1.0	840	1.91	67	0.73	Present invention
								example
	15	3	1.0	840	1.93	65	0.72	Present invention
								example
	16	1	0.7	840	1.92	81	0.93	Comparative
								example
	17	0	0.6	840	1.93	83	0.91	Comparative
								example
	18	9	0.5	840	1.88	78	0.90	Present invention
								example
	19	0	0.8	845	1.95	80	0.72	Comparative
								example
	20	9	1.5	845	1.89	68	0.95	Comparative
								example

As can be seen from Table 1, in the present invention examples, grains having a distance between grain boundaries of 3.0 mm or more and 13.0 mm or less in a rolling direction had an area ratio of 70% or more, the magnetic flux density B8 generated under a magnetizing force of 800 A/m was 1.88 T or more, W17/50 was 13.1×t²−4.3×t+1.2 or less in a unit W/kg, and LvA200 Hz was 60 to 78 dBA. Fine grains were not generated on the irradiation marks.

On the other hand, in the comparative examples, one or more of the above are out of the scope of the present invention, and iron loss and noise could not be sufficiently reduced at the same time.

For example, in No. 12, although the decarburization annealing temperature was slightly low and Ip was 1100 W/mm², the average grain size was as small as 2.8 mm. Accordingly, the area percentage of grains where the distance between grain boundaries of a grain was 3.0 mm or more and 13.0 mm or less in a rolling direction was small. In addition, B8 was low, iron loss was high, and LvA200 Hz was high.

In No. 13, the decarburization annealing temperature was slightly high and the average grain size was as large as 20.2 mm. Accordingly, the area percentage of grains where the distance between grain boundaries of a grain was 3.0 mm or more and 13.0 mm or less in a rolling direction was small. As a result, LvA200 Hz was high.

In No. 16, the laser input energy Ua was as relatively high as 4.1 mJ/mm², and as a result, LvA200 Hz was high.

In No. 17, the laser input energy Ua was as relatively high as 4.4 mJ/mm², and as a result, LvA200 Hz was high.

In No. 19, since the diameter of the spacer and the interval between the spacers were not in the preferable ranges, the average grain size was as large as 21.2 mm. Accordingly, the area percentage of grains where the distance between grain boundaries of a grain was 3.0 mm or more and 13.0 mm or less in a rolling direction was small. As a result, LvA200 Hz was high.

In No. 20, the laser power density Ip was as low as 200 W/m². Accordingly, the area percentage of grains where the distance between grain boundaries of a grain is 3.0 mm or more and 13.0 mm or less in a rolling direction was small. As a result, the iron loss was high.

Example 2

A steel slab including 0.070% of C, 3.08 to 3.24% of Si, 0.09% of Mn, 0.006% of S, 0.026% of Sol. Al, 0.11% of Cr, 0.0076% of N, and a balance of Fe and impurities was heated to 1140° C., and then hot-rolled to obtain a hot rolled sheet having a sheet thickness of 2.4 mm.

Subsequently, the hot rolled sheet was subjected to hot rolled sheet annealing. In the hot rolled sheet annealing, the hot rolled sheet was heated to 1120° C. and held for 180 seconds, then cooled to 900° C., and held at this temperature for 120 seconds. Thereafter, the hot rolled sheet was rapidly cooled using hot water at 100° C.

Thereafter, pickling was performed, and then cold rolling was performed to obtain a cold rolled sheet having a sheet thickness of 0.23 mm.

Subsequently, the cold rolled sheet (steel sheet) was heated to the temperature described in Table 2 in an atmosphere of wet hydrogen and nitrogen and held for 150 seconds for decarburization annealing.

After the decarburization annealing, a known annealing separator containing MgO and TiO₂ as main components and containing Na, B, Cl, and the like was applied, and held at 1200° C. for 20 hours for final annealing. At this time, as illustrated in FIG. 4, the final annealing was performed after the annealing separator was distributed between the steel sheets so that the thickness thereof changes at a constant interval, and the steel sheet was coiled into a coil shape. The annealing separator was applied by changing the thickness so that the steel sheet had a wavy shape in which the wave height changes in the wave period (rolling direction) shown in Table 2. After the final annealing, a forsterite film was formed on the surface.

After the final annealing, the surface of the steel sheet (steel sheet in which the forsterite film was formed on the surface of the base steel sheet) was applied with a coating solution mainly containing chromic anhydride and aluminum phosphate, followed by baking and annealing. Thus, an insulating film was formed.

Thereafter, the surface of the steel sheet on which the insulating film was formed was irradiated with a laser beam to perform magnetic domain refinement (magnetic domain control). At that time, the laser input energy and the laser power density were as shown in Table 2. The scanning direction of the laser beam (the extending direction of the irradiation mark) was the 90° direction against the rolling direction. The interval of the irradiation positions of the adjacent laser beams in the rolling direction was 4 mm.

TABLE 2
					Si			Area percentage of
			Laser	Laser	content	Average		grains having
			power	input	of base	grain size	Width of	distance between
	Wave	Wave	density	energy	steel	in rolling	linear	grain boundaries of
	period	height	Ip	Ua	sheet	direction	strain	3.0 to 13.0 mm
No.	(mm)	(mm)	(W/mm2)	(mJ/mm2)	(mass %)	(mm)	(μm)	(%)
21	90	10	500	3.9	3.24	6.3	231	72
22	90	10	560	3.2	3.23	11.8	135	85
23	90	10	530	3.8	3.19	8.4	188	94
24	90	10	1700	3.3	3.14	2.7	166	49
25	90	10	2000	2.5	3.14	21.1	120	58
26	90	10	2100	4.2	3.13	10.6	241	83
27	90	10	1800	4.6	3.08	9.7	266	91
28	200	2	1500	4.0	3.11	21.4	214	60

		Area		Decarburization
		ratio of	Width of	annealing		LvA	W17/
		lancet	main domain	temperature	B8	200 Hz	50
	No.	(%)	(mm)	(° C.)	(T)	(dB)	(W/kg)	Note
	21	0	1.0	840	1.93	60	0.74	Present invention
								example
	22	2	0.9	840	1.92	66	0.72	Present invention
								example
	23	0	1.0	840	1.94	64	0.71	Present invention
								example
	24	20	1.3	830	1.86	81	0.96	Comparative
								example
	25	18	1.5	850	1.89	84	0.93	Comparative
								example
	26	0	1.5	840	1.93	80	0.98	Comparative
								example
	27	0	1.4	840	1.93	84	0.92	Comparative
								example
	28	0	1.4	845	1.95	80	0.73	Comparative
								example

For the steel sheet after magnetic domain control (grain-oriented electrical steel sheet), the chemical composition, the percentage of grains in each of which the distance between the grain boundaries was 3.0 to 13.0 mm in the rolling direction, the area ratio of the lancet, the width of the main domain having a stripe shape, the magnetic characteristics (B8 and W17/50), and the value of LvA200 Hz were determined by the above-described methods. Magnetic domain observation was performed in a demagnetized state. In addition, the average grain size in the rolling direction in the surface and the width of each of the plurality of linear strains formed at 4 mm intervals in the rolling direction (average of the plurality of linear strains) were also determined.

Table 2 shows the results.

The content of each element other than Si is not shown in the table, and the C content was 0.016 to 0.070%, the S content was 0.001 to 0.006%, the Sol. Al content was 0.010 to 0.026%, and the N content was 0.0051 to 0.0076%. The Mn content and the Cr content did not change significantly compared to the stage of slab.

As can be seen from Table 2, in the present invention examples, in the surface of the base steel sheet, grains where the distance between grain boundaries of a grain is 3.0 mm or more and 13.0 mm or less in a rolling direction have an area ratio of 70% or more, the magnetic flux density B8 generated under a magnetizing force of 800 A/m is 1.88 T or more, W17/50 is 13.1×t²−4.3×t+1.2 or less in a unit W/kg, and LvA200 Hz is 60 to 78 dBA. Here, laser irradiation was performed so that the laser irradiation crosses the rolling direction of the steel sheet, which did not generate fine grains on the irradiation marks.

On the other hand, in the comparative examples, one or more of the above are out of the scope of the present invention, and iron loss and noise cannot be sufficiently reduced at the same time.

In No. 24, the decarburization annealing temperature was slightly low. Therefore, although the laser power density and the like fell within the preferable range, the area percentage of grains where the distance between grain boundaries of a grain is 3.0 mm or more and 13.0 mm or less in a rolling direction was small. In addition, B8 was low, LvA200 Hz was high, and W17/50 was high.

In No. 25, the decarburization annealing temperature was slightly high, and the average grain size was large. Accordingly, the area percentage of grains in each of which the distance between the grain boundaries of a grain was 3.0 to 13.0 mm in the rolling direction was small. As a result, LvA200 Hz was high. In addition, W17/50 was high.

In No. 26 and No. 27, the laser input energy was large, and as a result, LvA200 Hz was high. In addition, W17/50 was high.

In No. 28, the wave period was large, and the average grain size was large. Accordingly, the area percentage of grains where the distance between grain boundaries of a grain is 3.0 mm or more and 13.0 mm or less in a rolling direction was small. As a result, LvA200 Hz was high.

REFERENCE SIGNS LIST

• • 100 Lancet (Closure domain)
  • 1 Steel sheet
  • 2 Spacer
  • 3 Annealing separator
  • L1 Interval between spacers
  • L2 Cycle
  • h Wave height