GRAIN-ORIENTED ELECTRICAL STEEL SHEET AND METHOD FOR MANUFACTURING SAME

Description

TECHNICAL FIELD

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

Priority is claimed on Japanese Patent Application No. 2022-182571, filed Nov. 15, 2022, the content of which is incorporated herein by reference.

BACKGROUND ART

In general, a grain-oriented electrical steel sheet has a three-layer structure of a base steel sheet, a primary coating (sometimes referred to as a glass coating) formed on a surface of the base steel sheet, and a tension-applying insulation secondary coating formed on a surface of the primary coating. A grain-oriented electrical steel sheet is a soft magnetism material and is mainly used as an iron core material of a transformer. Thus, the grain-oriented electrical steel sheet is required to have magnetic characteristics such as high magnetization characteristics and a low iron loss.

The iron loss is a power loss due to consumption as thermal energy that occurs when the iron core is excited by an AC magnetic field, and the iron loss is required to be as low as possible from the viewpoint of energy saving. The level of iron loss is affected by a magnetic susceptibility, a sheet thickness, a coating tension, an impurity amount, an electric resistivity, a crystal grain size, a magnetic domain size, and the like. The iron loss of the grain-oriented electrical steel sheet is the sum of an eddy-current loss depending on the specific resistance, the sheet thickness, the size of the magnetic domain, and the like and a hysteresis loss depending on crystal orientation, the smoothness of the surface, and the like. Regarding the sheet thickness, for example, thinning the base steel sheet (for example, less than 0.22 mm) is effective in reducing the iron loss by reducing the eddy-current loss. However, the thinning changes the behavior of the secondary recrystallization itself, and there is a concern that the development degree of the Goss orientation is deteriorated.

The development of the Goss orientation occurs through a phenomenon (secondary recrystallization) through advanced control of the texture and the precipitate (inhibitor). The texture is preferably in a state in which there are many Goss orientations (nuclei of secondary recrystallization) and there are many orientations (corresponding orientations) on which the Goss orientation encroaches. On the other hand, the inhibitor is preferably in a state in which the pinning force of the inhibitor gradually decreases (the heat resistance of the inhibitor is high) during the secondary recrystallization annealing. In other words, a rapid decrease in the pinning force of the inhibitor during the secondary recrystallization annealing is not preferable from the viewpoint of promoting growth of the Goss orientation.

Here, in order to obtain a thin material, it is necessary to increase the rolling reduction in cold rolling (cold rolling ratio). When rolling is performed under a condition of a high cold rolling ratio, the Goss orientation, which is a nucleus of secondary recrystallization, decreases, and secondary recrystallization becomes unstable.

Also, in a thin material, the cause is not clear, but the heat resistance of the inhibitor decreases, and it is not possible to secure a time for preferential growth of the Goss orientation, and secondary recrystallization becomes unstable.

That is, in a thin material, there is a problem that secondary recrystallization becomes unstable from the viewpoint of both texture and inhibitor.

Under such circumstances, various studies for stabilizing secondary recrystallization of a thin material have been heretofore made.

For example, Patent Document 1 and Patent Document 2 disclose techniques for avoiding texture deterioration by optimally controlling a cold rolling ratio by preliminary cold rolling.

In addition, for example, Patent Document 3 discloses an inhibitor control technique via primary coating control. In Patent Document 3, by applying MgO containing 0.15 to 2.0% of Cl and/or SO₃ as an annealing separator, a formation reaction of glass coating is significantly enhanced, and primary coating is controlled to a form preferable for magnetism.

CITATION LIST

Patent Document

• Patent Document 1: Japanese Unexamined Patent Application, First Publication No. S59-126722
• Patent Document 2: Japanese Patent No. 2562254
• Patent Document 3: Japanese Patent No. 3021241

SUMMARY OF INVENTION

Technical Problem

In the case of the methods of Patent Documents 1 and 2, since the number of steps of the cold rolling step increases, a manufacturing problem occurs.

Also, according to the method of Patent Document 3, inhibitor control is possible, but texture control is not considered. Therefore, for example, when the present technique is applied to a thin electrical steel sheet with a sheet thickness of a base steel sheet of less than 0.22 mm, there is a high possibility that a high quality secondary recrystallization orientation cannot be obtained.

As described above, there has not been disclosed a method for stabilizing secondary recrystallization of a thin material to obtain a grain-oriented electrical steel sheet having good magnetic characteristics without reducing productivity.

In recent years, in addition to magnetic characteristics such as high magnetization characteristics and low iron loss, a grain-oriented electrical steel sheet is also required to reduce noise generated when the steel sheet is applied to a transformer or the like.

From the above, an object of the present disclosure is to provide a grain-oriented electrical steel sheet having good magnetic characteristics and noise characteristics without reducing productivity, and a method for manufacturing the same.

Solution to Problem

The present inventors have conducted studies on a thin material from the viewpoint of controlling both the texture and the precipitate (inhibitor) in order to achieve both good magnetic characteristics due to the development of the Goss orientation through secondary recrystallization and good noise characteristics.

As a result, in some of the thin materials, it was confirmed that the magnetic characteristics were very excellent as compared with the magnetic characteristics of the conventional grain-oriented electrical steel sheet without causing destabilization of secondary recrystallization, and it was also confirmed that the magnetic characteristics were compatible with good noise characteristics. As a result of more detailed investigation, it has been found that, among average dispersion angles from the ideal Goss orientation in the secondary recrystallization structure, when each of an average dispersion angle β(°) from the ideal Goss orientation having an orthogonal-to-rolling direction TD as the rotation axis, an average dispersion angle α (°) from the ideal Goss orientation having a rolled surface normal direction ND as the rotation axis, and an average dispersion angle γ (°) from the ideal Goss orientation having a rolling direction RD as the rotation axis satisfies the order shown in the following formula (1), secondary recrystallization is stabilized even in the thin material, and it is possible to achieve both good magnetic characteristics and good noise characteristics.

Hitherto, the average dispersion angle β(°) and the average dispersion angle α (°) have been known as factors affecting magnetic characteristics. However, in the conventional technique, there has been a limit to sufficiently reducing the average dispersion angles β and α, and further reduction of these average dispersion angles has been desired. Furthermore, it has been sometimes difficult to achieve both magnetic characteristics and noise characteristics at a high level only by reducing the average dispersion angles β and α. Therefore, as a result of intensive studies by the present inventors, it has been found that the balance among the average dispersion angles β, α, and γ has a strong influence in addition to further reduction of the average dispersion angle β in order to achieve both the magnetic characteristics and the noise characteristics of the thin material, and also, a new means capable of further reducing the average dispersion angle β has been found.

Specifically, the present inventors have found that by appropriately controlling both the texture and the inhibitor, the average dispersion angle β can be greatly reduced, and the balance among the average dispersion angles β, α, and γ can be achieved. More specifically, the present inventors have found that a dense oxide film capable of sufficiently securing the Goss orientation and stably exhibiting the function of the inhibitor during the secondary recrystallization annealing can be formed by increasing the temperature rising rate in a predetermined temperature range in the temperature-raising step during decarburization annealing.

Hereinafter, the results of studies by the present inventors will be described in detail.

First, from the viewpoint of inhibitor control, the present inventors have investigated the cause of deterioration of the effect of secondary recrystallization stabilization by an inhibitor in a thin material. As a result, it has been found that in the thin material, the decomposition rate of the inhibitor increases due to a relative increase in the surface area percentage of the steel material itself, and thus secondary recrystallization becomes unstable. In addition, the present inventors have found that the trigger for the decomposition of the inhibitor is an interaction between Mg₂SiO₄ as a primary coating (glass coating) and AlN as an inhibitor. Then, based on the obtained novel findings, the present inventors have found that deterioration of the secondary recrystallization stabilization effect by the inhibitor can be suppressed by decreasing the formation rate of the primary coating (that is, increasing the formation temperature of the primary coating).

Moreover, as a result of further studies by the present inventors, it has been found that an increase in the formation temperature of the primary coating can be achieved by thickening and densifying SiO₂ formed on a surface of a cold-rolled steel sheet during the decarburization annealing (in particular, during the temperature-raising step). The mechanism by which the formation temperature of the primary coating at the time of secondary recrystallization can be increased by thickening and densifying SiO₂ is unknown, but it is considered that the thickened and densified SiO₂ reduces the mobility of Mg ions in SiO₂.

Further, the present inventors have found that the Goss orientation is increased by increasing the temperature rising rate in a predetermined temperature range during the decarburization annealing from the viewpoint of texture. Furthermore, it has been found that by inclusion of P (phosphorus) in the steel, a favorable texture for secondary recrystallization during cold rolling can be obtained.

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

[1] A grain-oriented electrical steel sheet according to an embodiment of the present invention includes:

• • a base steel sheet having a chemical composition containing, by mass %:
  • C: 0 to 0.010%,
  • Si: 3.00 to 4.00%,
  • Sol.Al: 0 to 0.010%,
  • Mn: 0.01 to 0.50%,
  • N: 0.010% or less,
  • S+Se: 0.0100% or less,
  • P: 0.005 to 0.100%,
  • Sn: 0 to 0.50%,
  • Cu: 0 to 0.50%,
  • Cr: 0 to 0.50%,
  • Sb: 0 to 0.20%,
  • Mo: 0 to 0.10%,
  • Ni: 0 to 0.20%,
  • Nb: 0 to 0.0200%,
  • B: 0 to 0.0200%,
  • Ti: 0 to 0.0200%, and
  • Bi: 0 to 0.0200%,
  • with the remainder being Fe and impurities;
  • a glass coating provided on a surface of the base steel sheet; and
  • a tension-applying insulation coating provided on a surface of the glass coating;
  • in which
  • the base steel sheet has a texture oriented in the Goss orientation, and
  • in the base steel sheet,
  • when an average dispersion angle from an ideal Goss orientation with a rolled surface normal direction ND as the rotation axis is defined as α (°),
  • an average dispersion angle from the ideal Goss orientation with an orthogonal-to-rolling direction TD as the rotation axis is defined as β(°), and
  • an average dispersion angle from the ideal Goss orientation with a rolling direction RD as the rotation axis is defined as γ (°), the α, the β, and the γ satisfy the following formula (1).

0.1 ≤ ❘ "\[LeftBracketingBar]" β ❘ "\[RightBracketingBar]" < 2.5 < ❘ "\[LeftBracketingBar]" α ❘ "\[RightBracketingBar]" < 4. ≤ ❘ "\[LeftBracketingBar]" γ ❘ "\[RightBracketingBar]" ≤ 10. ( 1 )

[2] In the grain-oriented electrical steel sheet according to [1], the β may satisfy the following formula (2).

0. ≤ ❘ "\[LeftBracketingBar]" β ❘ "\[RightBracketingBar]" ≤ 2. 0 ( 2 )

[3] In the grain-oriented electrical steel sheet according to [1] or [2], the base steel sheet may have a sheet thickness of 0.16 mm or more and less than 0.20 mm.

[4] A method for manufacturing a grain-oriented electrical steel sheet according to an embodiment of the present invention includes:

• • a heating step of heating a steel piece containing:
  • in % by mass,
  • C: 0.020 to 0.150%,
  • Si: 3.00 to 4.00%,
  • Sol.Al: 0.010 to 0.050%,
  • Mn: 0.01 to 0.50%,
  • N: 0.001 to 0.020%,
  • S+Se: 0.001 to 0.040%,
  • P: 0.005 to 0.100%,
  • Sn: 0 to 0.50%,
  • Cu: 0 to 0.50%,
  • Cr: 0 to 0.50%,
  • Sb: 0 to 0.20%,
  • Mo: 0 to 0.10%,
  • Ni: 0 to 0.20%,
  • Nb: 0 to 0.0200%,
  • B: 0 to 0.0200%,
  • Ti: 0 to 0.0200%, and
  • Bi: 0 to 0.0200%
  • with the remainder being Fe and impurities;
  • a hot rolling step of hot rolling the steel piece after the heating step to obtain a hot-rolled steel sheet;
  • a hot-band annealing step of annealing the hot-rolled steel sheet to obtain a hot-rolled annealed steel sheet;
  • a cold rolling step of cold rolling the hot-rolled annealed steel sheet to obtain a cold-rolled steel sheet;
  • a decarburization annealing step of subjecting the cold-rolled steel sheet to decarburization annealing to obtain a decarburization-annealed steel sheet;
  • a final annealing step of applying an annealing separator to the decarburization-annealed steel sheet and then performing final annealing to obtain a final-annealed steel sheet; and
  • an insulating coating forming step of forming an insulating coating on a surface of the final-annealed steel sheet,
  • in which
    • the decarburization annealing step includes a temperature-raising step and a soaking step, and
    • in the temperature-raising step,
      • an average temperature rising rate in a temperature range of 550 to 800° C. is set to 400° C./sec or more and 3000° C./sec or less,
      • maximum heating temperature T1 (° C.) is set to 850° C. or higher and 950° C. or lower,
      • an average temperature rising rate in a temperature range of 800 to the maximum heating temperature T1 (° C.) is set to 100° C./sec or more and 1500° C./sec or less, and
      • a dew point of an atmosphere in a temperature range of 800 to the maximum heating temperature T1 (° C.) is set to 0° C. or lower.

[5] In the method for manufacturing a grain-oriented electrical steel sheet according to [4], the annealing separator may contain MgO as a main component, and the annealing separator may contain one or more selected from Ti, Sb, Sr, and Cl in an amount of 0.10 to 10.00% with respect to the weight of MgO.

Advantageous Effects of Invention

According to the above aspect of the present disclosure, it is possible to provide a grain-oriented electrical steel sheet having good magnetic characteristics and noise characteristics without reducing productivity, and a method for manufacturing the same.

BRIEF DESCRIPTION OF DRAWING

FIG. 1A A diagram schematically illustrating a structure of a grain-oriented electrical steel sheet according to the present embodiment.

FIG. 1B A diagram schematically illustrating a structure of a grain-oriented electrical steel sheet according to the present embodiment.

FIG. 2 A schematic diagram illustrating a deviation angle α, a deviation angle β, and a deviation angle γ.

FIG. 3 A flowchart illustrating an example of a flow of a method for manufacturing a grain-oriented electrical steel sheet according to the present embodiment.

DESCRIPTION OF EMBODIMENTS

Hereinafter, preferred embodiments of the present disclosure will be described in detail with reference to the drawings. In the present specification and the drawings, constituent elements having substantially the same functional configuration are denoted by the same reference numbers, and redundant description is omitted.

The grain-oriented electrical steel sheet according to an embodiment of the present disclosure (the grain-oriented electrical steel sheet according to the present embodiment) includes a base steel sheet having a predetermined chemical composition described later, a glass coating formed on the base steel sheet, and a tension-applying insulation coating formed on the glass coating. Also, the base steel sheet has a texture oriented in the Goss orientation. Moreover, in the base steel sheet, when an average dispersion angle from an ideal Goss orientation with a rolled surface normal direction ND as the rotation axis is defined as α (°), an average dispersion angle from the ideal Goss orientation with an orthogonal-to-rolling direction TD as the rotation axis is defined as β(°), and an average dispersion angle from the ideal Goss orientation with a rolling direction RD as the rotation axis is defined as γ(°), the α, the β, and the γ satisfy the following formula (1).

0.1 ≤ ❘ "\[LeftBracketingBar]" β ❘ "\[RightBracketingBar]" < 2.5 < ❘ "\[LeftBracketingBar]" α ❘ "\[RightBracketingBar]" < 4. ≤ ❘ "\[LeftBracketingBar]" γ ❘ "\[RightBracketingBar]" ≤ 10. ( 1 )

Hereinafter, the grain-oriented electrical steel sheet according to the present embodiment will be described.

[Grain-Oriented Electrical Steel Sheet]

FIGS. 1A and 1B are diagrams schematically illustrating a structure of the grain-oriented electrical steel sheet according to the present embodiment.

As illustrated in FIG. 1A, a grain-oriented electrical steel sheet 10 according to the present embodiment includes a base steel sheet 11, a primary coating (glass coating) 13 formed on a surface of the base steel sheet 11, and a secondary coating (tension-applying insulation coating) 15 which is an example of an insulating coating formed on a surface of the glass coating 13. The glass coating 13 and the tension-applying insulation coating 15 may be formed on at least one surface of the base steel sheet 11 but are usually formed on both surfaces of the base steel sheet 11 as schematically illustrated in FIG. 1B.

Hereinafter, a characteristic configuration of the grain-oriented electrical steel sheet 10 according to the present embodiment will be mainly described. In the following description, detailed descriptions of known configurations and some configurations that can be implemented by those skilled in the art may be omitted.

(Base Steel Sheet 11)

The base steel sheet 11 exhibits excellent noise characteristics and magnetic characteristics by being manufactured from a steel piece containing a chemical composition as described in detail below. The chemical composition of the base steel sheet 11 will be described in detail below.

(Glass Coating 13)

The glass coating 13 is an inorganic coating which is disposed on the surface of the base steel sheet 11 and contains magnesium silicate as a main component. The glass coating 13 is formed by a reaction between an annealing separator containing magnesia (MgO) applied to the surface of the base steel sheet and the component of the surface of the base steel sheet 11 in final annealing. That is, the glass coating 13 has a composition derived from the components of the annealing separator and the base steel sheet (more specifically, a composition containing Mg₂SiO₄ as a main component).

(Tension-Applying Insulation Coating 15)

The tension-applying insulation coating 15 is disposed on the surface of the glass coating 13. By imparting electrical insulation properties to the grain-oriented electrical steel sheet 10 by the tension-applying insulation coating 15, it is possible to reduce eddy-current loss and improve iron loss of the grain-oriented electrical steel sheet 10. Also, the tension-applying insulation coating 15 realizes various properties such as corrosion resistance, heat resistance, and slippage in addition to the electrical insulation properties as described above.

Further, the tension-applying insulation coating 15 has a function of applying tension to the grain-oriented electrical steel sheet 10. By applying tension to the grain-oriented electrical steel sheet 10 by the tension-applying insulation coating 15 to facilitate the movement of domain walls in the grain-oriented electrical steel sheet 10, the iron loss of the grain-oriented electrical steel sheet 10 can be improved.

The surface of the tension-applying insulation coating 15 may be subjected to magnetic domain refining treatment using a continuous-wave laser beam or an electron beam. In this case, linear thermal strain forming a predetermined angle φ with the sheet width direction, which is a direction orthogonal to the rolling direction, is periodically formed at predetermined intervals along the rolling direction. Accordingly, in the grain-oriented electrical steel sheet according to the present embodiment, the magnetic characteristics can be further improved.

The tension-applying insulation coating 15 is formed, for example, by applying a coating liquid containing a metal phosphate and silica as main components onto the surface of the glass coating 13 and baking the coating liquid.

(Sheet Thickness)

The sheet thickness of the base steel sheet of the grain-oriented electrical steel sheet 10 according to the present embodiment (the sheet thickness t in FIGS. 1A and 1B) is not particularly limited, and may be, for example, 0.16 mm or more and 0.30 mm or less. Also, in the present embodiment, the smaller the sheet thickness of the cold-rolled sheet (the base steel sheet 11) after the cold rolling, the more sufficiently the effect of reducing iron loss can be obtained. Therefore, the sheet thickness t of the base steel sheet of the grain-oriented electrical steel sheet 10 is preferably 0.24 mm or less, and more preferably 0.23 mm or less. In addition, for example, in the case of a thin material in which the sheet thickness of the cold-rolled sheet is less than 0.22 mm (that is, a thin material), the above effect becomes remarkable, and the iron loss is further excellent. Therefore, the sheet thickness t of the base steel sheet of the grain-oriented electrical steel sheet 10 is more preferably, for example, less than 0.20 mm from the viewpoint of magnetic characteristics. The lower limit of the sheet thickness t may be 0.16 mm or more, or may be 0.17 mm or more.

(Chemical Composition)

Next, the base steel sheet 11 of the grain-oriented electrical steel sheet 10 according to the present embodiment and the chemical composition of the steel piece used in manufacturing the grain-oriented electrical steel sheet 10 will be described in detail. Hereinafter, unless specified otherwise, “%” represents “mass %”.

When a steel piece (for example, a slab) having the following chemical composition becomes a grain-oriented electrical steel sheet through the manufacturing steps as described in detail below, the same contents as those of the steel piece are maintained for components other than carbon (C), acid-soluble aluminum (sol.Al), nitrogen (N), sulfur (S), and bismuth (Bi) of the base steel sheet 11. That is, the contents of components other than carbon (C), acid-soluble aluminum (sol.Al), nitrogen (N), and sulfur (S) of the base steel sheet 11 are substantially the same as those of the steel piece and the product sheet. On the other hand, the contents of carbon (C), acid-soluble aluminum (sol.Al), nitrogen (N), sulfur (S), and bismuth (Bi) of the base steel sheet 11 change as compared with the composition of the steel piece through the manufacturing steps as described in detail below. In addition, since silicon (Si), manganese (Mn), chromium (Cr), boron (B), and titanium (Ti) of the base steel sheet 11 are absorbed into the glass coating in the middle of manufacturing, the contents thereof may be slightly reduced as compared with the composition of the steel piece.

C: 0.020 to 0.150%

C (Carbon) is an element having an effect of increasing the magnetic flux density. On the other hand, when the C content of the steel piece is less than 0.020%, the effect of improving the magnetic flux density cannot be obtained. Thus, the C content of the steel piece is 0.020% or more. The C content is preferably 0.040% or more, and more preferably 0.050% or more.

On the other hand, when the C content of the steel piece exceeds 0.150%, the steel undergoes phase transformation in the secondary recrystallization annealing (that is, final annealing), secondary recrystallization does not sufficiently proceed, and a favorable magnetic flux density and iron loss characteristics cannot be obtained. Thus, the C content of the steel piece is 0.150% or less. The lower the C content, the better for iron loss reduction. From the viewpoint of iron loss reduction, the C content is preferably 0.120% or less, and more preferably 0.100% or less.

The C content in the steel piece as described above becomes 0.010% (100 ppm) or less in the base steel sheet 11 by forming the grain-oriented electrical steel sheet 10 according to the present embodiment through the steps as described in detail below. The C content in the base steel sheet 11 may be 0%. However, it is difficult to set the C content to 0% in a practical steel sheet in terms of manufacturing, and thus the C content may exceed 0%.

Si: 3.00 to 4.00%

Si (silicon) is an extremely effective element for increasing electric resistance (specific resistance) of steel to reduce eddy-current loss constituting a part of iron loss. When the Si content of the steel piece is less than 3.00%, the steel undergoes phase transformation in the secondary recrystallization annealing, secondary recrystallization does not sufficiently proceed, and a favorable magnetic flux density and iron loss characteristics cannot be obtained. Thus, the Si content of the steel piece is 3.00% or more. The Si content of the steel piece is preferably 3.10% or more, and more preferably 3.20% or more.

On the other hand, when the Si content of the steel piece exceeds 4.00%, the steel sheet is embrittled, and passability of the sheet in the manufacturing steps is remarkably deteriorated. Thus, the Si content of the steel piece is 4.00% or less. The Si content of the steel piece is preferably 3.80% or less, and more preferably 3.50% or less.

The Si content in the steel piece as described above may be reduced by forming the grain-oriented electrical steel sheet 10 according to the present embodiment through the steps as described in detail below. For example, when Si is consumed as a glass coating, the Si content in the base steel sheet 11 may be slightly reduced. However, the reduced amount is within a range that does not impair the operation and effects of the present embodiment, and the effects of the present embodiment can be obtained by setting the Si content of the steel piece within the above range. In consideration of the reduced amount of the Si content, the Si content of the base steel sheet 11 may be 2.80 to 3.80%.

Sol.Al: 0.010 to 0.050%

Sol.Al (acid-soluble aluminum) is a constituent element of a main inhibitor among compounds called inhibitors that influence secondary recrystallization in the grain-oriented electrical steel sheet, and is an essential element from the viewpoint of development of secondary recrystallization in the base steel sheet according to the present embodiment. When the sol.Al content of the steel piece is less than 0.010%, AlN functioning as an inhibitor is not sufficiently generated, and secondary recrystallization becomes insufficient, and iron loss characteristics are not improved. Thus, the content of sol.Al in the steel piece is 0.010% or more. The content of sol.Al is preferably 0.020% or more.

On the other hand, when the content of sol.Al exceeds 0.050%, embrittlement of the steel sheet becomes remarkable. Thus, the content of sol.Al of the steel piece is 0.050% or less. The content of sol.Al is preferably 0.040% or less, and more preferably 0.030% or less.

The sol.Al content in the steel piece as described above becomes 0.010% (100 ppm) or less in the base steel sheet 11 by forming the grain-oriented electrical steel sheet 10 according to the present embodiment through the steps as described in detail below. The lower limit of the sol.Al content in the base steel sheet 11 is not particularly limited, and may be 0%.

Mn: 0.01 to 0.50%

Mn (manganese) is an important element that forms MnS, which is one of the major inhibitors. When the Mn content of the steel piece is less than 0.01%, the absolute amount of MnS required to cause secondary recrystallization is insufficient. Thus, the Mn content of the steel piece is 0.01% or more. The Mn content is preferably 0.03% or more, and more preferably 0.06% or more.

On the other hand, when the Mn content of the steel piece exceeds 0.50%, the steel undergoes phase transformation in the secondary recrystallization annealing, secondary recrystallization does not sufficiently proceed, and a favorable magnetic flux density and iron loss characteristics cannot be obtained. Thus, the Mn content of the steel piece is 0.50% or less. The Mn content is preferably 0.20% or less, and more preferably 0.10% or less.

The Mn content in the steel piece as described above may be reduced by forming the grain-oriented electrical steel sheet 10 according to the present embodiment through the steps as described in detail below. For example, when Mn is absorbed into the glass coating, the Mn content in the base steel sheet 11 may be slightly reduced. However, the reduced amount is within a range that does not impair the operation and effects of the present embodiment, and the effects of the present embodiment can be obtained by setting the Mn content of the steel piece within the above range. In consideration of the reduced amount of the Mn content, the Mn content of the base steel sheet 11 may be 0 to 0.40%.

N: 0.001 to 0.020%

N (nitrogen) is an element that reacts with the acid-soluble Al to form AlN that functions as an inhibitor. In order to bond to Al to form AlN functioning as an inhibitor, the N content of the steel piece is 0.001% or more. The N content is preferably 0.004% or more, and more preferably 0.006% or more.

On the other hand, when the N content of the steel piece exceeds 0.020%, blisters (pores) are generated in the steel sheet during cold rolling, the strength increases, and passability of the sheet during manufacturing deteriorates. Thus, the N content of the steel piece is 0.020% or less. The N content is preferably 0.015% or less, and more preferably 0.010% or less.

The N content in the steel piece as described above becomes 0.010% (100 ppm) or less in the base steel sheet 11 by forming the grain-oriented electrical steel sheet 10 according to the present embodiment through the steps as described in detail below. The lower limit of the N content in the base steel sheet 11 is not particularly limited, and may be 0%.

S+Se: 0.0010 to 0.0400%

Sulfur(S) and Selenium (Se) are important elements that react with the Mn to form an inhibitor MnS or MnSe. When the total of the S content and the Se content of the steel piece is less than 0.0010%, a sufficient inhibitor effect cannot be obtained. Thus, the total of the S content and the Se content in the steel piece is 0.0010% or more. The total of the S content and the Se content is preferably 0.0100% or more, and more preferably 0.0150% or more.

On the other hand, when the total of the S content and the Se content of the steel piece exceeds 0.0400%, hot embrittlement is caused, and hot rolling is significantly difficult. Thus, the total of the S content and the Se content of the steel piece is 0.0400% or less. The total of the S content and the Se content is preferably 0.0300% or less.

The S content in the steel piece as described above may be reduced by forming the grain-oriented electrical steel sheet 10 according to the present embodiment through the steps as described in detail below, and in this case, the S content in the base steel sheet 11 may be 0.0100% (100 ppm) or less. The lower limit of the total of the S content and the Se content in the base steel sheet 11 is not particularly limited and may be 0.0005%. The lower limit of the total of the S content and the Se content in the base steel sheet 11 may include 0%. It should be noted that identification of S or Se of less than 0.0005% requires careful attention. When the S content or the Se content is below the lower detection limit of the analyzer, the total of the S content and the Se content may be regarded as 0%. In a practical steel sheet, the lower limit of the total of the substantial S content and Se content is 0.0005%.

P: 0.005 to 0.100%

P (phosphorus) is an element that contributes to obtaining a favorable texture for secondary recrystallization and has an effect of improving magnetic characteristics. Thus, the P content of the steel piece is 0.005% or more. The P content is preferably 0.010% or more.

On the other hand, P is an element that lowers the workability in rolling. When the P content exceeds 0.100%, rolling workability may be deteriorated, leading to fracture of the steel sheet during manufacturing. Thus, the P content is 0.100% or less. The P content is preferably 0.070% or less, and more preferably 0.030% or less.

Remainder: Fe and impurities

The chemical composition of the steel piece and the base steel sheet 11 according to the present embodiment basically contains the above-described elements (basic elements) with the remainder being Fe and impurities. However, for the purpose of improving magnetic characteristics and the like, one or more (optional elements) selected from the group consisting of Sn, Cu, Cr, Sb, Mo, Ni, Nb, B, Ti, and Bi may be further contained in the following ranges. Sn, Cu, Cr, Sb, Mo, Ni, Nb, B, Ti, and Bi are optional elements in the steel piece and the base steel sheet 11 according to the present embodiment, and thus the lower limit of the content thereof is 0%.

Sn: 0 to 0.50%,

Sn (tin) is an element having an effect of improving magnetic characteristics. Thus, Sn may be contained. When Sn is contained, the content of Sn is preferably 0.01% or more in order to favorably exhibit the effect of improving magnetic characteristics. The Sn content is more preferably 0.03% or more in consideration of both magnetic characteristics and coating adhesion.

On the other hand, when the Sn content exceeds 0.50%, adhesion of the glass coating is significantly deteriorated. Therefore, when contained, the Sn content is 0.50% or less. The Sn content is preferably 0.40% or less, and more preferably 0.30% or less.

Cu: 0 to 0.50%

Cu (copper) is an element that contributes to an increase in the occupancy rate of the Goss orientation in the secondary recrystallization structure and contributes to an improvement in the glass coating adhesion. In the case of obtaining the above effect, the Cu content is preferably 0.01% or more. The Cu content is more preferably 0.05% or more.

On the other hand, when the Cu content exceeds 0.50%, the steel sheet is embrittled during hot rolling. Thus, when contained, the Cu content of the steel piece is 0.50% or less. The Cu content is preferably 0.40% or less and more preferably 0.30% or less.

Cr: 0 to 0.50%

Cr (chromium) is an element that contributes to an increase in the occupancy rate of the Goss orientation in the secondary recrystallization structure to improve the magnetic characteristics, and contributes to an improvement in the glass coating adhesion, similarly to Sn and Cu. In order to obtain the above effect, the Cr content is preferably set to 0.01% or more. The Cr content is more preferably 0.03% or more.

On the other hand, when the Cr content exceeds 0.50%, Cr oxide is formed, and the magnetic characteristics are deteriorated. Thus, when contained, the Cr content is 0.50% or less. The Cr content is preferably 0.40% or less, and more preferably 0.30% or less.

Sb: 0 to 0.20%

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

On the other hand, when the Sb content exceeds 0.20%, the glass coating is significantly deteriorated. Therefore, when contained, the upper limit of the Sb content is to 0.20%. The Sb content is preferably 0.15% or less and 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 contained. When Mo is contained, the Mo content is preferably 0.01% or more in order to favorably exhibit the effect of improving magnetic characteristics.

On the other hand, when the Mo content is more than 0.10%, the cold rolling characteristics may be deteriorated, leading to fracture. Therefore, when contained, the Mo content is 0.10% or less. The Mo content is preferably 0.05% or less, and more preferably 0.03% or less.

Ni: 0 to 0.20%

Ni (nickel) is an effective element that affects crystal orientation rotation occurring during cold rolling and obtains a favorable texture for secondary recrystallization. Ni (nickel) is also an element effective for increasing the specific resistance of the steel sheet and reducing the iron loss. Thus, Ni may be contained. When Ni is contained, the Ni content is preferably 0.01% or more in order to obtain these effects.

On the other hand, when the Ni content exceeds 0.20%, secondary recrystallization may become unstable. Thus, when contained, the Ni content is 0.20% or less. The Ni content is preferably 0.15% or less, and more preferably 0.10% or less.

Nb: 0 to 0.0200%

Nb (niobium) is an element effective for enhancing the function of the inhibitor to stably obtain secondary recrystallization. Thus, Nb may be contained. When Nb is contained, the Nb content is preferably 0.0005% or more in order to stably obtain secondary recrystallization. The Nb content is more preferably 0.0010% or more.

On the other hand, when the Nb content exceeds 0.0200%, secondary recrystallization may become unstable. Thus, when contained, the Nb content is 0.0200% or less. The Nb content is preferably 0.0100% or less, and more preferably 0.0050% or less.

B: 0 to 0.0200%

B (boron) is an element effective for enhancing the function of the inhibitor to stably obtain secondary recrystallization. Thus, B may be contained. When B is contained, the B content is preferably 0.0005% or more in order to stably obtain secondary recrystallization. The B content is more preferably 0.0010% or more.

On the other hand, when the B content exceeds 0.0200%, secondary recrystallization may become unstable. Thus, when contained, the B content is 0.0200% or less. The B content is preferably 0.0100% or less, and more preferably 0.0050% or less.

Ti: 0 to 0.0200%

Ti (titanium) is an element that increases the development degree of the Goss orientation and improves magnetism. Although the cause is not clear, T may bond to N to form TiN, and function as an inhibitor. When Ti is contained, the Ti content is 0.0005% or more, and preferably 0.0010% or more.

On the other hand, when the content of Ti exceeds 0.0200%, TiN is excessively precipitated, and the iron loss is deteriorated. Thus, the Ti content is 0.0200% or less. The Ti content is preferably 0.0100% or less, and more preferably 0.0050% or less.

Bi: 0 to 0.0200%

Bi (bismuth) is an element that increases the development degree of the Goss orientation and improves magnetism. When Bi is contained, the Bi content is 0.0010% or more, and preferably 0.0020% or more.

On the other hand, when Bi exceeds 0.0200%, the coating adhesion is deteriorated. Thus, the Bi content is 0.0200% or less. The Bi content is preferably 0.0100% or less.

The Bi content in the steel piece as described above becomes 0.0100% (100 ppm) or less in the base steel sheet 11 by forming the grain-oriented electrical steel sheet 10 according to the present embodiment through the steps as described in detail below. The lower limit of the Bi content in the base steel sheet 11 is not particularly limited and may be 0%. The lower limit of the Bi content may exceed 0%.

Here, the respective contents of Cr, B, and Ti in the steel piece as described above may be reduced by forming the grain-oriented electrical steel sheet 10 according to the present embodiment through the steps as described in detail below. For example, when Cr is absorbed into the glass coating or an inclusion such as BN or TiN is formed in the glass coating, the content of each of these elements in the base steel sheet 11 may be slightly reduced. However, any of the reduced amounts is within a range that does not affect the operation and effects of the present embodiment.

Also, in order to obtain the total amount of chemical composition in the base steel sheet 11 from the grain-oriented electrical steel sheet 10, measurement may be performed using inductively coupled plasma-atomic emission spectrometry (ICP-AES).

Specifically, first, the grain-oriented electrical steel sheet 10 is subjected to a cleaning treatment with an alkaline solution to remove the tension-applying insulation coating 15, and further, the glass coating 13 is removed by pickling, and then measurement is performed using ICP-AES. At this time, C and S may be measured by a combustion-infrared absorption method, N may be measured by an inert gas fusion-thermal conductivity method, and O may be measured by an inert gas fusion-non-dispersive infrared absorption method.

As a method for removing the tension-applying insulation coating, a method of immersing the grain-oriented electrical steel sheet including the coating in a high-temperature alkaline solution may be applied. Specifically, the grain-oriented electrical steel sheet including the coating is immersed in a sodium hydroxide aqueous solution (NaOH: 30 to 50 mass %+H₂O: 50 to 70 mass %) at 80 to 90° C. for 5 minutes to 10 minutes, then washed with water, and dried. Thereby, the tension-applying insulation coating can be removed from the grain-oriented electrical steel sheet. The time of immersion in the sodium hydroxide aqueous solution may be appropriately changed according to the thickness of the tension-applying insulation coating.

In addition, as a method for removing the glass coating, the grain-oriented electrical steel sheet from which the tension-applying insulation coating has been removed is immersed in hydrochloric acid (concentration: 30 to 40%) at 80 to 90° C. for 1 minute to 5 minutes, then washed with water, and dried. Thereby, the glass coating can be removed from the grain-oriented electrical steel sheet.

As described above, it is preferable to selectively remove using an alkaline solution for removing the tension-applying insulation coating and hydrochloric acid for removing the glass coating. By removing the tension-applying insulation coating and the glass coating, the base steel sheet can be exposed, and the chemical composition of the base steel sheet can be measured.

In addition, the steel component of the slab (steel piece) may be subjected to composition analysis by collecting a sample from molten steel before casting or subjected to composition analysis by removing a surface oxide film or the like from the slab after casting.

The remainder of the chemical composition of the steel piece and the base steel sheet 11 according to the present embodiment other than the above-described elements (basic element, optional element) is basically Fe and impurities. Here, the “impurity” is present in the steel piece and the base steel sheet 11 regardless of the intention of addition. That is, the impurity is an element that is a contaminant derived from ore or scrap as a raw material, a manufacturing environment, or the like when the base steel sheet is industrially manufactured and means an element that is allowed to be contained in a content that does not adversely affect the effect of the grain-oriented electrical steel sheet according to the present embodiment.

(Texture)

The base steel sheet 11 of the present embodiment has a secondary recrystallization texture developed in a {110}<001> orientation (Goss orientation). Moreover, in the base steel sheet 11, when an average dispersion angle from an ideal Goss orientation with a rolled surface normal direction ND as the rotation axis is defined as a) (°, an average dispersion angle from the ideal Goss orientation with an orthogonal-to-rolling direction TD as the rotation axis is defined as β(°), and an average dispersion angle from the ideal Goss orientation with a rolling direction RD as the rotation axis is defined as γ (°), the α, the β, and the γ satisfy the following formula (1). The β preferably satisfies the following formula (2).

0.1 ≤ ❘ "\[LeftBracketingBar]" β ❘ "\[RightBracketingBar]" < 2.5 < ❘ "\[LeftBracketingBar]" α ❘ "\[RightBracketingBar]" < 4. ≤ ❘ "\[LeftBracketingBar]" γ ❘ "\[RightBracketingBar]" ≤ 10. ( 1 ) 0. ≤ ❘ "\[LeftBracketingBar]" β ❘ "\[RightBracketingBar]" ≤ 2. 0 ( 2 )

In the present embodiment, two {110}<001> orientations of the “actual crystal {110}<001> orientation” and the “ideal {110}<001> orientation” are distinguished. This is because, in the present embodiment, it is necessary to distinguishably handle the {110}<001> orientation when displaying the crystal orientation of the practical steel sheet and the {110}<001> orientation as an academic crystal orientation.

In general, in the measurement of the crystal orientation of a recrystallized practical steel sheet, the crystal orientation is defined without strictly distinguishing an angle difference of about ±2.5°. In the case of a conventional grain-oriented electrical steel sheet, an angular range of about ±3.0° centered on a geometrically strict {110}<001> orientation is defined as a “{110}<001> orientation”. However, in the present embodiment, it is also necessary to clearly distinguish an angle difference of +3.0° or less.

Therefore, in the present embodiment, in the case of meaning the orientation of the grain-oriented electrical steel sheet in a practical sense, it is simply described as a “{110}<001> orientation (Goss orientation)” as conventional. On the other hand, in the case of meaning the {110}<001> orientation as a geometrically strict crystal orientation, it is described as an “ideal {110}<001> orientation (ideal Goss orientation)” in order to avoid confusion with the {110}<001> orientation used in conventional known documents and the like.

In the present embodiment, the deviation between the actual crystal orientation of the grain-oriented electrical steel sheet and the ideal Goss orientation is defined using the following three angles α, β, and γ (unit: °).

Average dispersion angle (deviation angle) α: a deviation angle around the rolled surface normal direction (ND)

Average dispersion angle (deviation angle) β: a deviation angle around the orthogonal-to-rolling direction (TD)

Average dispersion angle (deviation angle) γ: a deviation angle around the rolling direction (RD)

FIG. 2 shows a schematic diagram of the deviation angle α, the deviation angle β, and the deviation angle γ.

As illustrated in FIG. 2, the deviation angle α is an angle formed by the <001> direction of crystal projected on the rolled surface and the rolling direction RD when viewed from the rolled surface normal direction ND. The deviation angle β is an angle formed by the <001> direction of crystal projected on L cross section (cross section whose normal direction is the orthogonal-to-rolling direction TD) and the rolling direction RD when viewed from the orthogonal-to-rolling direction TD (sheet width direction). The deviation angle γ is an angle formed by the <110> direction of crystal projected on C-section (cross section whose normal direction is the rolling direction RD) and the rolled surface normal direction ND when viewed from the rolling direction RD.

It is known that the deviation angles α and β among the deviation angles α, β, and γ affect magnetic characteristics. Among them, the deviation angle β affects magnetostriction. The magnetostriction is a phenomenon in which a shape of magnetic body changes by application of magnetic field. In a grain-oriented electrical steel sheet used for a transformer or the like, since the magnetostriction causes noise, the magnetostriction is required to be small. Generally, in order to reduce the magnetostriction, the crystal orientation is controlled so that the deviation angle β becomes small (specifically, so that the maximum value and the average value of the absolute value |β| of the deviation angle β become small). However, in the conventional grain-oriented electrical steel sheet, there has been a limit to reduction of the deviation angle β, and further reduction has been desired.

Therefore, as a result of studies by the present inventors, it has been found that by stably securing the effect of secondary recrystallization stabilization by the inhibitor and increasing the temperature rising rate of the temperature-raising step among the decarburization annealing conditions, the deviation angle β can be greatly reduced as compared with the conventional case, and the Goss orientation can be sufficiently secured.

In the present embodiment, the secondary recrystallization texture of the base steel sheet satisfies the above formula (1), and preferably satisfies the above formula (2).

When the deviation angle γ exceeds 10.0° in the formula (1), the magnetic characteristics may be deteriorated. In addition, the deviation angles α and β have a greater influence on the magnetic characteristics than the deviation angle γ. Therefore, the deviation angles α and β are made smaller than the deviation angle γ. However, since the deviation angle γ affects noise characteristics, it is not preferable to make the deviation angle γ excessively large. Therefore, in the formula (1), the deviation angle γ is 10.0° or less. Further, when the deviation angle γ is controlled to be less than 4.0°, the deviation angle β may exceed 2.5°. When the deviation angle β exceeds 2.5°, the noise characteristics may be deteriorated. Therefore, the lower limit of the deviation angle γ is 4.0° or more. Furthermore, by controlling the deviation angles α and β to be less than 4.0°, the magnetic flux density with respect to the rolling direction is improved, and good magnetic characteristics are obtained. From the viewpoint of improving magnetic characteristics, the deviation angles α and β are preferably less than 3.5°, and more preferably less than 3.0°. However, when the deviation angle α is controlled to 2.5° or less, the deviation angle β becomes 2.5° or more. Although the cause of this is unknown, it is presumed that there is a trade-off relationship between the primary recrystallization texture capable of reducing the deviation angle α and the primary recrystallization texture capable of reducing the deviation angle β. When the deviation angle β is 2.5° or more, the noise characteristics may be deteriorated. Therefore, the lower limit of the deviation angle α exceeds 2.5°, and the upper limit of the deviation angle β is less than 2.5°. From the above, from the viewpoint of achieving both the magnetic characteristics and the noise characteristics, it is effective that the deviation angles α, β, and γ satisfy the following relationship in the present embodiment.

0.1 ≤ ❘ "\[LeftBracketingBar]" β ❘ "\[RightBracketingBar]" < 2.5 < ❘ "\[LeftBracketingBar]" α ❘ "\[RightBracketingBar]" < 4. ≤ ❘ "\[LeftBracketingBar]" γ ❘ "\[RightBracketingBar]" ≤ 10. ( 1 )

Also, in the above formula (2), when the deviation angle β exceeds 2.0°, the noise characteristics may be deteriorated. Therefore, the deviation angle β is preferably 2.0° or less, and more preferably 1.8° or less. Since it is more preferable as the deviation angle β is smaller, the lower limit thereof is not limited. The deviation angle β may be 0°.

The crystal orientation of the grain-oriented electrical steel sheet can be obtained experimentally using, for example, a Laue diffractometer (RIGAKU RASCO-L II V).

For example, a grain-oriented electrical steel sheet of 60 mm in the width direction×300 mm in the length direction is irradiated with X-rays at intervals of 5 mm in the length direction and 5 mm in the width direction to obtain Laue diffraction spots. Then, by fitting the obtained Laue diffraction spots with analysis software on a PC, Euler angles φ1, Φ, and φ2 are obtained. For example, since the Euler angles of the Goss orientation are given by φ1=0°, Φ=45°, φ2=0° or φ1=90°, Φ=90°, and φ2=45° in Bunge notation, the deviation angles α, β, and γ are obtained by comparing the orientation angles obtained in the experiment with the angles of the Goss orientation.

[Method for Manufacturing Grain-Oriented Electrical Steel Sheet]

Next, a grain-oriented electrical steel sheet according to an embodiment of the present invention and a method for manufacturing the same (a method for manufacturing the grain-oriented electrical steel sheet according to the present embodiment) will be described with reference to FIG. 3. FIG. 3 is a flowchart illustrating an example of a flow of a method for manufacturing a grain-oriented electrical steel sheet according to the present embodiment.

A method for manufacturing a grain-oriented electrical steel sheet according to the present embodiment includes:

• • (I) a heating step of heating a steel piece having a predetermined chemical composition;
  • (II) a hot rolling step (step S101) of hot rolling the steel piece after the heating step to obtain a hot-rolled steel sheet;
  • (III) a hot-band annealing step (step S103) of annealing the hot-rolled steel sheet to obtain a hot-rolled annealed steel sheet;
  • (IV) a cold rolling step (step S105) of subjecting the hot-rolled annealed steel sheet to cold rolling to obtain a cold-rolled steel sheet;
  • (V) a decarburization annealing step (step S107) of subjecting the cold-rolled steel sheet to decarburization annealing to obtain a decarburization-annealed steel sheet;
  • (VI) a final annealing step (step S109) of applying an annealing separator to the decarburization-annealed steel sheet and then performing final annealing to obtain a final-annealed steel sheet; and
  • (VII) an insulating coating forming step (step S111) of forming an insulating coating on a surface of the final-annealed steel sheet.

Hereinafter, preferred conditions for each step will be described. Known conditions can be applied to conditions that are not described.

(Heating Step)

In the heating step, a steel piece such as a slab having the above chemical composition is heated prior to hot rolling. The heating temperature of the steel piece is not particularly limited, but is preferably in the range of 1100 to 1450° C. The heating temperature is more preferably 1300 to 1400° C.

(Hot Rolling Step: S101)

In the hot rolling step (step S101), the steel piece after the heating step is hot rolled to obtain a hot-rolled steel sheet. The hot rolling conditions are not particularly limited, and is appropriately set on the basis of required characteristics. The thickness of the hot-rolled steel sheet processed by hot rolling is preferably, for example, in a range of 2.0 mm or more and 3.0 mm or less.

(Hot-Band Annealing Step: S103)

The hot-band annealing step is a step of annealing the hot-rolled steel sheet manufactured through the hot rolling step to obtain a hot-rolled annealed steel sheet. By performing such an annealing treatment, recrystallization occurs in the metallographic structure, and favorable magnetic characteristics can be realized.

In the hot-band annealing step of the present embodiment, the hot-rolled steel sheet manufactured through the hot rolling step may be annealed according to a known method. Methods for heating the hot-rolled steel sheet at the time of annealing is not particularly limited, and a known heating method can be adopted. The annealing conditions are also not particularly limited, but for example, the hot-rolled steel sheet can be annealed in a temperature range of 900 to 1200° C. for 10 seconds to 5 minutes.

(Cold Rolling Step: S105)

In the cold rolling step (step S105), the hot-rolled annealed steel sheet is subjected to cold rolling including at least one or more passes to obtain a cold-rolled steel sheet. The cold rolling may not include one or more intermediate annealing between each rolling pass. A plurality of times of cold rolling including an intermediate annealing therebetween may be performed by interrupting the cold rolling and performing at least one or more intermediate annealing before the final pass of the cold rolling step.

When the intermediate annealing is performed, it is preferable to hold the intermediate annealing 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.

Also, in the cold rolling step, the surface of the hot-rolled steel sheet may be subjected to pickling under known conditions before rolling.

The cold rolling conditions are not limited, but for example, the final rolling reduction can be within a range of 80% or more and 95% or less. When the final rolling reduction is less than 80%, there is a high possibility that a Goss nucleus in which the {110}<001> orientation has a high development degree in a rolling direction cannot be obtained, which is not preferable.

On the other hand, when the final rolling reduction exceeds 95%, there is a high possibility that secondary recrystallization is unstable in the subsequent final annealing step, which is not preferable. By setting the final rolling reduction within the above range, 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.

Here, the sheet thickness of the cold-rolled steel sheet subjected to cold rolling (the sheet thickness after the cold rolling) is usually different from the sheet thickness of the grain-oriented electrical steel sheet to be finally manufactured (the product sheet thickness including the thickness of the tension-applying insulation coating). The product sheet thickness of the grain-oriented electrical steel sheet is as described above.

(Decarburization Annealing Step: S107)

The decarburization annealing step (step S107) is a step of subjecting a cold-rolled steel sheet to decarburization annealing to obtain a decarburization-annealed steel sheet, and in the present embodiment, it is an important step for appropriately controlling both the texture and the inhibitor and reducing the average dispersion angle β.

In decarburization annealing step, the cold-rolled steel sheet is primarily recrystallized, and C (carbon) that adversely affects magnetic characteristics is removed from the steel sheet. Also, in the decarburization annealing step, in order to promote stable secondary recrystallization in the subsequent final annealing step, the Goss nuclei are increased, and SiO₂ formed by annealing is thickened and densified. In the method for manufacturing a grain-oriented electrical steel sheet according to the present embodiment, the secondary recrystallization structure can be precisely controlled in the subsequent final annealing step by performing the annealing treatment according to a predetermined heat treatment condition in the decarburization annealing step.

The decarburization annealing step according to the present embodiment includes two steps of a temperature-raising step (step S131) and a soaking step (step S133) in order to obtain a desired secondary recrystallization structure.

The temperature-raising step (step S131) is a step of raising the temperature of the cold-rolled steel sheet obtained in the cold rolling step from room temperature to a temperature T1 (° C.) in a range of 850° C. or higher and 950° C. or lower (maximum heating temperature) at a predetermined temperature rising rate. Also, the soaking step (step S131) is a step of annealing the cold-rolled steel sheet heated at a predetermined temperature rising rate by cooling the cold-rolled steel sheet to a predetermined temperature and holding the cold-rolled steel sheet in a predetermined temperature range for a predetermined time.

<Temperature-Raising Step>

The temperature-raising step according to the present embodiment is an important step for precisely controlling the texture of the secondary recrystallized grains.

In a thin material, the rolling reduction in cold rolling (cold rolling ratio) is increased in order to reduce the sheet thickness, but as the cold rolling ratio increases, the Goss orientation decreases and secondary recrystallization becomes unstable. On the other hand, in the present embodiment, in the temperature-raising step of the decarburization annealing step, the average temperature rising rate in the temperature range of 550 to 800° C. is set to 400° C./sec or more to increase the Goss orientation. In addition, the temperature range of 550 to 800° C. affects the behavior of recrystallization, that is, transition. In the temperature-raising step, recrystallization is completed at the maximum attainment temperature through recovery of the structure. This recovery and recrystallization most affect the final annealing step (secondary recrystallization annealing step) which is a subsequent step. In the present embodiment, by setting the average temperature rising rate in the temperature range of 550 to 800° C. to 400° C./sec or more, it is possible to increase the nucleus of the Goss orientation having geometrically strict {110}<001>. On the other hand, when the average temperature rising rate in this temperature range is less than 400° C./sec, the nucleus of the Goss orientation having geometrically strict {110}<001> cannot be sufficiently increased.

The upper limit of the average temperature rising rate in the temperature range of 550 to 800° C. is not necessarily limited from the viewpoint of characteristics, but it is not desirable to excessively increase the average temperature rising rate from the viewpoint of a load on equipment and devices to be used. Therefore, the average temperature rising rate in the temperature range of 550 to 800° C. is set to 3000° C./sec or less.

As described above, the temperature range for controlling the average temperature rising rate is set to 550 to 800° C. because this temperature range is an important temperature range for enrichment and recovery of the Goss orientation. This temperature range is a range found by the present inventors by investigating the influence of the staying time at each temperature on the existence frequency and recovery of the Goss orientation.

That is, as a result of investigating the influence of the staying time at each temperature on the existence frequency and recovery of the Goss orientation, the present inventors have found that the Goss orientation frequency in the primary recrystallization texture decreases due to staying in the temperature range of 550 to 800° C. In other words, by increasing the temperature rising rate in the temperature range of 550 to 800° C., the existence frequency of the Goss orientation can be increased. If the existence frequency of the Goss orientation can be increased, the coarsening probability of the Goss orientation can be increased in the final annealing step, which leads to an increase in magnetic characteristics.

Furthermore, it has been found that, in the temperature-raising step of the present embodiment, SiO₂ formed on the surface layer of the steel sheet can be thickened and densified by setting the average temperature rising rate in the temperature range of 800 to 850° C. to 100° C./sec or more.

Hereinafter, the relationship between the formation form of SiO₂ and the secondary recrystallization stabilization will be described.

In the thin material, since the percentage of the surface area is large, the decomposition rate of the inhibitor is high, and secondary recrystallization becomes unstable. The trigger for the decomposition of the inhibitor is an interaction between Mg₂SiO₄ as a primary coating and AlN. In order to suppress the decomposition of the inhibitor, it is effective to reduce the formation rate of the primary coating, that is, to increase the formation temperature of the primary coating. Also, the formation rate of the primary coating is affected by the abundance of SiO₂ in the decarburization-annealed steel sheet.

In addition, as a result of more detailed studies by the present inventors, it has been found that the increase in the formation temperature of the primary coating can be achieved by thickening and densifying SiO₂ formed in the temperature-raising step during the decarburization annealing step. The mechanism by which the formation temperature of the primary coating can be increased by thickening and densifying SiO₂ is unknown, but it is considered that the thickened and densified SiO₂ reduces the mobility of Mg ions in SiO₂.

The form of SiO₂ formed in the temperature-raising step is roughly divided into an “externally oxidized film” formed on the sheet surface and an “internally oxidized film” formed on the surface layer of the steel sheet, and the “internally oxidized film” is further divided into a “spherical oxide” and a “lamellar oxide film”. The present inventors have particularly focused on the “externally oxidized film” among them, and have found that by forming the externally oxidized film thick and dense in the temperature-raising step, the formation rate of the primary coating can be reduced (that is, the formation temperature of the primary coating can be increased) in the subsequent final annealing step, and the stabilization (reduction in the decomposition rate) of the inhibitor can be realized. Although the mechanism by which the decomposition of the inhibitor is suppressed and the stabilization can be achieved is unknown, it is considered that the thick and densely formed externally oxidized film of SiO₂ suppresses the entry of Mg into the steel sheet, and the formation of the primary coating (glass coating) is delayed. It is considered that this is because the reaction between Mg₂SiO₄ and AlN (that is, the decomposition reaction of AlN) is also delayed as the formation of Mg₂SiO₄ itself is delayed. The increase in the temperature of the decomposition reaction of AlN leads to the securing of the grain growth rate of the Goss orientation, and thus is a preferable direction as the magnetic characteristics.

Also, in the temperature-raising step of the present embodiment, the temperature of the cold-rolled steel sheet is raised to the temperature T1 (° C.) in the range of 850° C. or higher and 950° C. or lower (maximum heating temperature). The maximum heating temperature T1 is an effective element for suppressing internal oxidation in the subsequent soaking step. In order to sufficiently externally oxidize the cold-rolled steel sheet and sufficiently cover the sheet surface with an externally oxidized film in the temperature-raising step, the maximum heating temperature T1 is set to 850° C. or higher. This makes it possible to suppress the internal oxidation (to suppress the formation of the spherical oxide and the lamellar oxide film) in the soaking step. The maximum heating temperature T1 is preferably 870° C. or higher, and more preferably 900° C. or higher. On the other hand, since excessively increasing the maximum heating temperature T1 causes an excessive burden on equipment, the maximum heating temperature T1 is set to 950° C. or lower.

In the temperature-raising step, by setting the average temperature rising rate in the temperature range of 800 to the maximum heating temperature T1 (° C.) to 100° C./sec or more, the externally oxidized film of SiO₂ formed on the sheet surface can be formed thick and dense as described above. On the other hand, when the average temperature rising rate in the temperature range of 800 to the maximum heating temperature T1 (° C.) is less than 100° C./sec, the thickness of the externally oxidized film of SiO₂ becomes insufficient, and an oxide film other than SiO₂ (for example, Fe₂SiO₄ or the like) is allowed. The oxide film other than SiO₂ may promote decomposition of the inhibitor during the final annealing step. Although the cause of this is not clear, an oxide film such as Fe₂SiO₄ may have higher affinity with AlN than SiO₂. Therefore, the average temperature rising rate in the temperature range of 800 to the maximum heating temperature T1 (° C.) is set to 100° C./sec or more, preferably 200° C./sec or more, and more preferably 400° C./sec or more. In order to further suppress the formation of the oxide film other than SiO₂ (in particular, Fe₂SiO₄) and further promote the formation of the externally oxidized film of SiO₂, the control range of the average temperature rising rate in the temperature-raising step is preferably set to 550 to the maximum heating temperature T1 (° C.). That is, the average temperature rising rate in the temperature range of 550 to the maximum heating temperature T1 (° C.) is preferably set to 100° C./sec or more. From the viewpoint of suppressing the formation of Fe₂SiO₄, a preferable temperature range to be controlled is 600 to the maximum heating temperature T1 (° C.), and more preferably 650 to the maximum heating temperature T1 (° C.).

On the other hand, the upper limit of the average temperature rising rate in the temperature range of 800 to the maximum heating temperature T1 (° C.) is not necessarily limited from the viewpoint of characteristics, but since a special device is required to obtain an average temperature rising rate exceeding 1500° C./sec, the average temperature rising rate is set to 1500° C./sec or less.

Also, in the temperature-raising step of the present embodiment, the dew point of the atmosphere during the temperature rising in the temperature range from 800° C. to the maximum heating temperature T1 (° C.) is set to 0° C. or less. The dew point in the temperature-raising step also affects the formation of the oxide film other than SiO₂. In order to promote the formation of SiO₂ and suppress the formation of the oxide film other than SiO₂, it is effective to lower the degree of oxidation of the atmosphere in the temperature-raising step. Therefore, the dew point of the atmosphere in the temperature range of 800 to the maximum heating temperature T1 (° C.) is set to 0° C. or less, preferably −5° C. or less, and more preferably −10° C. or less. In the temperature-raising step, setting the dew point of the atmosphere in the temperature range of room temperature or higher and lower than 800° C. to 0° C. or lower does not impair the effect of the present invention.

<Soaking Step>

After the temperature-raising step, a soaking step is performed.

When carbon remains in the steel sheet, iron loss characteristics deteriorate over time (known as magnetic aging). Therefore, the atmosphere at the time of decarburization annealing is usually set to a relatively high oxygen potential in order to reduce the amount of carbon. However, in the annealing step, when the oxygen potential of the atmosphere is high, the amount of SiO₂ formation increases, and the primary coating formation rate increases.

Therefore, in the soaking step, the annealing atmosphere is performed at a low oxygen potential. Thereby, the formation rate of the primary coating is reduced.

Specifically, in the soaking step, the cold-rolled steel sheet is preferably held in an atmosphere having a temperature of 780 to 860° C. and an oxygen potential (PH₂O/PH₂) of 0.20 or more and 0.60 or less for 100 seconds to 300 seconds. In the present embodiment, since heating is performed beyond the holding temperature in the soaking process in the temperature-raising step, cooling is performed up to a predetermined soaking holding temperature by air cooling, natural cooling, or the like at the time of transition to the soaking step, but the cooling does not impair the effect shown in the present embodiment.

When the holding temperature in the soaking step is lower than 780° C., decarburization failure occurs due to restriction of diffusion rate control. When decarburization failure occurs, carbon remains in the steel sheet, which causes iron loss deterioration. Alternatively, since a phase change occurs, secondary recrystallization itself cannot occur.

On the other hand, when the holding temperature exceeds 860° C., decarburization failure also occurs due to the restriction of interface rate control. This is because a covering oxide film that is harmful to decarburization is formed.

Also, when the oxygen potential in the soaking process is less than 0.20, decarburization failure occurs. Decarburization is a chemical reaction between carbon in a steel sheet and oxygen in an annealing atmosphere. The low oxygen potential is synonymous with the low oxygen partial pressure and means a situation in which a decarburization reaction is unlikely to occur.

On the other hand, when the oxygen potential in the soaking step process exceeds 0.60, decarburization failure also occurs. This is because a covering oxide film that inhibits decarburization is formed.

In addition, when the oxygen potential is not controlled, the atmospheric dew point temperature may be set to 30 to 80° C. as a mixed gas of nitrogen and hydrogen or a nitrogen gas.

(Nitriding Treatment Step)

In the method for manufacturing a grain-oriented electrical steel sheet according to the present embodiment, a nitriding treatment may be performed between the decarburization annealing step and a final annealing step described later.

In the nitriding treatment step, for example, the cold-rolled steel sheet after the decarburization annealing step is maintained at about 700 to 850° C. in a nitriding treatment atmosphere (an atmosphere containing a gas having nitriding ability, such as hydrogen, nitrogen, or ammonia). Here, it is preferable to subject the steel sheet to a nitriding treatment such that the N content of the cold-rolled steel sheet is 40 to 1000 ppm on a mass basis. When the N content of the cold-rolled steel sheet after the nitriding treatment is less than 40 ppm, AlN may not be sufficiently precipitated in the cold-rolled steel sheet, and AlN may not function as an inhibitor. Therefore, when AlN is utilized as an inhibitor, the N content of the cold-rolled steel sheet is preferably set to 40 ppm or more.

On the other hand, when the N content of the cold-rolled steel sheet exceeds 1000 ppm, AlN is excessively present in the steel sheet even after completion of secondary recrystallization in the final annealing. Such AlN causes iron loss deterioration. Therefore, the N content of the steel sheet is preferably set to 1000 ppm or less.

(Final Annealing Step (Secondary Recrystallization Annealing Step): S109)

In the final annealing step (step S109), an annealing separator is applied to the decarburization-annealed steel sheet (after the decarburization annealing step or after the nitriding treatment step), and then final annealing is performed to obtain a final-annealed steel sheet.

The final annealing conditions are not limited, but the final annealing may be performed, for example, under conditions in which the temperature is raised to 1150 to 1250° C. in an atmosphere gas containing hydrogen and nitrogen, and annealing (holding) is performed in the temperature range for 10 to 60 hours.

The final annealing is generally performed for a long time in a state where the steel sheet is wound in a coil shape. Thus, prior to the final annealing, an annealing separator is applied to the cold-rolled steel sheet and dried for the purpose of preventing seizure inside and outside the winding of the coil. As the annealing separator to be applied, an annealing separator containing MgO as a main component is used. By using an annealing separator containing MgO as a main component, a glass coating can be formed on the surface of the base steel sheet. When MgO is not the main component, no primary coating (glass coating) is formed. This is because the primary coating is a Mg₂SiO₄ or a MgAl₂O₄ compound, and Mg, which is necessary for the formation reaction, is not supplied.

In the method for manufacturing a grain-oriented electrical steel sheet according to the present embodiment, the annealing separator applied to the decarburization-annealed steel sheet preferably contains at least one selected from Ti, Sb, Sr, and Cl in a total amount of 0.10 to 10.00% with respect to the weight of MgO.

When the annealing separator containing MgO as a main component contains one or more selected from Ti, Sb, Sr, and Cl, the reactivity between the primary coating and

AlN is suppressed, and the magnetic characteristics can be improved. Although the mechanism is unknown, it is considered that the segregation of these elements at the interface between the primary coating and the base steel sheet suppresses the reaction between the primary coating and AlN. The total amount of Ti, Sb, Sr, and Cl is preferably 0.10 to 10.00% by wt %. When the total amount of Ti, Sb, Sr, and Cl is less than 0.10% with respect to the weight of MgO, the effect of improving the magnetic characteristics may not be sufficiently obtained. On the other hand, when the total amount of Ti, Sb, Sr, and Cl exceeds 10.00%, a sufficient primary coating amount is not formed, a coating tension effective for improving magnetic characteristics cannot be secured, and the iron loss may be poor.

Incidentally, the existence form (that is, the addition means) of Ti, Sb, Sr, and Cl in the annealing separator may exist as a compound or may exist alone. For example, when Ti is contained in the annealing separator, Ti may be contained as simple Ti or may be contained as a Ti oxide (for example, TiO₂).

(Insulating Coating Forming Step: S111)

In the insulating coating forming step (step S111), an insulating coating (a tension-applying insulation coating) is formed on the surface (one surface or both surfaces) of the final-annealed steel sheet. The conditions for forming the insulating coating are not particularly limited. A treatment liquid may be applied and dried by a known method using a known insulating coating treatment liquid. When the insulating coating is formed on the sheet surface, the magnetic characteristics of the grain-oriented electrical steel sheet can be further improved.

The surface of the steel sheet on which the insulating coating 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 a pickling 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 coating formed on the surface of the steel sheet is not particularly limited as long as it is used as an insulating coating for a grain-oriented electrical steel sheet, and a known insulating coating can be used. Examples of such an insulating coating include a coating containing a phosphate and colloidal silica as main components. In addition, a composite insulating coating mainly containing an inorganic substance and further containing an organic substance can be exemplified. Here, the composite insulating coating is, for example, an insulating coating 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 coating using a metal phosphate, a Zr or Ti coupling agent, or a carbonate or ammonium salt thereof as a starting material may be used.

(Magnetic Domain Refinement Step)

The method for manufacturing a grain-oriented electrical steel sheet according to the present embodiment may include a magnetic domain refinement step after the insulating coating forming step.

In the magnetic domain refinement step, the surface of the insulating coating is irradiated with energy rays such as laser beam and electron beam at predetermined intervals in the rolling direction, thereby introducing a plurality of linear strains (thermal strains caused by rapid heating by energy ray irradiation and subsequent rapid cooling). The interval at which the plurality of linear strains are formed (that is, the interval between adjacent strains) is preferably 3.0 to 9.0 mm in the rolling direction. The energy ray include a laser beam and an electron beam. The laser beam may be a continuous wave laser or a pulsed laser. Examples of the laser beam type include a fiber laser, a YAG laser, and a CO₂ laser. The electron beam may be a continuous beam or an intermittent beam.

EXAMPLES

As shown in Table 1, slabs having different chemical compositions for each steel number (Nos. a to 1) were prepared. Each slab was then used to manufacture grain-oriented electrical steel sheets (Test Nos. 1 to 19).

Specifically, the slab was heated to a temperature of 1100 to 1380° C. and then hot rolled to prepare a hot-rolled steel sheet with a sheet thickness of 2.3 mm.

Next, the obtained hot-rolled steel sheet was subjected to hot-band annealing. Specifically, the hot-rolled steel sheet was annealed under the conditions of an annealing temperature of 1100° C. and a holding time of 20 seconds.

Next, the hot-rolled steel sheet after the hot-rolled annealing (the hot-band annealed steel sheet) was subjected to surface scale removal by pickling or the like, and then cold rolling including a plurality of passes without intermediate annealing was performed to prepare a cold-rolled steel sheet with a sheet thickness of 0.16 to 0.23 mm.

The obtained cold-rolled steel sheet was subjected to decarburization annealing under the conditions shown in Table 3. The retention time in the soaking process was set to 120 seconds in the soaking process.

Test Nos. 1, 2, 9, 10, 14, and 15 were further subjected to nitriding treatment to increase the nitrogen amount to 200 ppm.

Next, a final annealing step was performed on the decarburization-annealed steel sheet. Specifically, on the surface of the cold-rolled steel sheet, an annealing separator mainly composed of MgO and containing elements shown in Table 3 was applied to the sheet surface by water slurry application. The components contained in the annealing separator shown in Table 3 indicate the weight fraction of the element alone.

Then, the decarburization-annealed steel sheet to which the annealing separator was applied was held at 1200° C. for 20 hours to prepare a steel sheet having a primary coating (a glass coating) on the base steel sheet (the final-annealed steel sheet).

Next, an insulating coating was formed on the steel sheet. Specifically, an insulating coating forming liquid mainly composed of colloidal silica and phosphate was applied to the surface of the steel sheet (more specifically, the surface of the glass coating as the primary coating) and heat-treated (baked). As a result, a grain-oriented electrical steel sheet including a base steel sheet, a glass coating formed on the base steel sheet, and an insulating coating formed on the glass coating was obtained. The chemical composition of the base steel sheet of the obtained grain-oriented electrical steel sheet is shown in Table 2. The notation “-” in the chemical composition of Table 2 means that a corresponding element content is 0% in the significant digits (numerical value up to the minimum digit) defined in the embodiment.

TABLE 1
	Chemical composition of slab (mass%, remainder: Fe and impurities)

No.	C	Si	Mn	S + Se	sol. Al	N	P	Cu	Sn	Cr	B	Ti	Nb	Ni	Sb	Bi	Mo
a	0.031	3.05	0.01	0.0050	0.038	0.003	0.085	0.45	—	—	—	—	—	—	—	—	—
b	0.132	3.85	0.40	0.0301	0.014	0.016	0.055	—	—	—	—	0.0078	—	—	—	—	0.09
c	0.055	3.30	0.05	0.0155	0.023	0.005	0.040	0.20	—	—	—	—	0.0051	—	0.01	0.0063	—
d	0.063	3.40	0.20	0.0220	0.032	0.015	0.043	0.15	—	—	0.0051	—	—	0.05	—	—	—
e	0.079	3.45	0.08	0.0231	0.026	0.008	0.015	0.10	0.10	0.05	—	0.0052	—	0.05	—	—	0.02
f	0.085	3.50	0.15	0.0205	0.034	0.009	0.023	0.10	0.20	0.30	0.0033	0.0040		—	0.03	—	—
g	0.045	3.45	0.11	0.0154	0.022	0.007	0.010	—	—	—	—	—	—	—	—	—	—
h	0.050	3.33	0.15	0.0168	0.029	0.008	0.011	—	—	—	—	—	—	—	—	—	—
i	0.061	3.25	0.09	0.0152	0.028	0.007	0.012	—	—	—	—	—	—	—	—	—	—
j	0.072	3.42	0.07	0.0255	0.024	0.009	0.015	—	—	—	—	—	—	—	—	—	—
k	0.084	3.39	0.08	0.0241	0.026	0.008	0.013	—	—	—	—	—	—	—	—	—	—
l	0.086	3.38	0.10	0.0223	0.025	0.007	0.025	—	—	—	—	—	—	—	—	—	—

TABLE 2
	Chemical composition of base steel sheet (mass%, remainder: Fe and impurities)

No.	C	Si	Mn	S + Se	sol. Al	N	P	Cu	Sn	Cr	B	Ti	Nb	Ni	Sb	Bi	Mo
a	-	3.00	0.01	—	—	0.001	0.085	0.45	—	—	—	—	—	—	—	—	—
b	-	3.74	0.38	0.0009	—	0.001	0.055	—	—	—	—	0.0075	—	—	—	—	0.09
c	-	3.25	0.04	0.0010	—	0.001	0.040	0.20	—	—	—	—	0.0051	—	0.01	—	—
d	-	3.35	0.19	0.0013	—	—	0.043	0.15	—	—	0.0049	—	—	0.05	—	—	—
e		3.31	0.08	0.0008	—	—	0.015	0.10	0.10	0.03	—	0.0049	—	0.05	—	—	0.02
f	-	3.39	0.14	—	—	—	0.023	0.10	0.20	0.25	0.0030	0.0038		—	0.03	—	—
g	-	3.39	0.09	—	—	—	0.010	—	—	—	—	—	—	—	—	—	—
h	-	3.25	0.12	—	—	—	0.011	—	—	—	—	—	—	—	—	—	—
i	-	3.05	0.08	—	—	—	0.012	—	—	—	—	—	—	—	—	—	—
j	-	3.25	0.06	0.0010	—	—	0.015	—	—	—	—	—	—	—	—	—	—
k	-	3.25	0.07	0.0009	—	—	0.013	—	—	—	—	—	—	—	—	—	—
l	-	3.19	0.08	0.0011	—	—	0.025	—	—	—	—	—	—	—	—	—	—

TABLE 3
			Decarburization annealing

					Average
					temperature
					rising	Dew
					rate (800°	point	Annealing separator

			Average		C. to	(800°					Ti +
			temperature	Maximum	maximum	C. to					Sr +
	Com-	Sheet	rising rate	heating	heating	maximum					C1 +
	po-	thick-	(550 to	temperature	temperature	heating	Ti	Sr	Cl	B	B
Test	sition	ness	800° C.)	T1	T1)	temperature	(wt	(wt	(wt	(wt	(wt
No.	No.	(mm)	(° C./sec)	(° C.)	(° C./sec)	T1) (° C.)	%)	%)	%)	%)	%)
1	a	0.23	400	850	200	0	—	—	—	—	0.00
2	b	0.23	400	850	200	0	—	—	—	—	0.00
3	c	0.22	700	880	500	−15	—	—	—	—	0.00
4	d	0.22	700	880	500	−15	—	—	—	—	0.00
5	e	0.19	1000	920	800	−20	3.50	—	—	—	3.50
6	f	0.19	1000	920	800	−20	3.50	1.65	—	—	5.15
7	c	0.19	2000	950	1100	−30	3.00	1.43	0.10	—	4.53
8	d	0.19	2000	950	1100	−30	3.00	1.43	0.10	0.10	4.63
9	a	0.22	1000	950	500	5	2.50	—	—	=	2.50
10	b	0.22	200	900	400	0	—	1.90	—	—	1.90
11	c	0.22	400	830	400	0	—	—	0.05	—	0.05
12	d	0.22	200	830	600	0	—	—	—	0.05	0.05
13	e	0.22	400	950	50	0	—	—	—	—	0.00
14	g	0.18	1200	850	700	−15	3.00	1.43	0.08	0.07	4.58
15	h	0.18	1200	850	700	−15	3.00	1.43	0.08	0.07	4.58
16	i	0.18	1200	880	1000	−30	3.00	—	0.08	0.07	3.15
17	j	0.17	1000	880	1000	−30	4.20	—	0.03	0.03	4.26
18	k	0.17	1000	920	1400	−30	4.20	—	0.03	0.03	4.26
19	l	0.16	1000	920	1400	−30	4.20	—	0.03	0.03	4.26
20	a	0.23	400	850	200	0	—	—	—	0.20	0.20
21	b	0.23	400	850	200	0	0.07	—	0.15	0.35	0.57
22	c	0.22	400	850	200	0	—	0.05	0.15	0.35	0.55
23	d	0.22	400	850	200	0	0.05	0.05	0.15	0.05	0.30
24	a	0.22	400	850	200	0	9.50	—	0.75	—	10.25
25	b	0.22	400	850	200	0	9.50	—	0.75	—	10.25

TABLE 4
		Characteristics

	Average	Mag-
	dispersion	netic	Iron
	angle	flux	loss

Test

B

a

Y

density

(W/

Noise

No.	(°)	(T)	kg)	(dBA)	Remark

1	2.3	3.4	4.3	1.91	0.77	59.5	Invention
							Example
2	2.4	3.1	4.5	1.91	0.76	58.4	Invention
							Example
3	1.8	3.3	4.5	1.92	0.72	57.2	Invention
							Example
4	1.9	3.5	4.3	1.92	0.71	56.9	Invention
							Example
5	1.4	2.8	4.6	1.95	0.68	54.5	Invention
							Example
6	1.3	2.8	4.9	1.96	0.67	54.3	Invention
							Example
7	1.5	3.0	4.8	1.96	0.67	54.8	Invention
							Example
8	1.3	2.8	4.7	1.93	0.66	54.4	Invention
							Example
9	2.1	3.3	11.2	1.90	0.74	62.3	Compar-
							ative
							Example
10	2.8	3.8	3.5	1.89	0.86	61.4	Compar-
							ative
							Example
11	1.8	4.3	4.8	1.89	0.87	57.8	Compar-
							ative
							Example
12	3.2	2.3	4.2	1.92	0.76	63.0	Compar-
							ative
							Example
13	3.0	3.5	4.5	1.89	0.87	64.2	Compar-
							ative
							Example
14	1.5	2.7	4.5	1.95	0.66	53.2	Invention
							Example
15	1.3	2.8	4.5	1.95	0.65	53.4	Invention
							Example
16	1.5	2.8	4.6	1.96	0.65	52.9	Invention
							Example
17	1.2	2.6	4.3	1.94	0.62	52.5	Invention
							Example
18	1.2	2.7	4.5	1.96	0.61	53.1	Invention
							Example
19	1.1	2.6	5.1	1.97	0.60	51.5	Invention
							Example
20	1.7	3.7	4.7	1.91	0.74	57.3	Invention
							Example
21	1.7	3.4	4.8	1.91	0.75	57.4	Invention
							Example
22	1.9	3.6	4.8	1.91	0.74	56.8	Invention
							Example
23	1.6	3.6	4.6	1.91	0.74	55.9	Invention
							Example
24	2.4	3.6	4.9	1.92	0.78	58.8	Invention
							Example
25	2.4	3.7	5.1	1.92	0.76	59.2	Invention
							Example

Magnetic characteristics (iron loss and magnetic flux density) of the obtained grain-oriented electrical steel sheet were evaluated in the following manner. The results are shown in Table 4.

[Iron Loss]

The obtained steel sheet sample was cut into a size of 60 mm parallel to the sheet width direction and 300 mm parallel to the rolling direction, and these samples were held at 800±50° C. for 2 hours to perform straightening annealing. Thereafter, magnetic domain refinement was performed by laser irradiation.

The sample after laser irradiation was subjected to magnetism evaluation by a single sheet tester (SST), and an iron loss (W_17/50) was measured. In addition, a magnetic field of 800 A/m was applied to the sample, and a magnetic flux density B₈ (T) was also measured.

The iron loss (W_17/50) was evaluated according to the following criteria. Evaluations A to C were determined to have excellent iron loss characteristics (low iron loss), and evaluation D was determined to have poor iron loss characteristics.

• • Less than 0.70: evaluation A
  • 0.70 or more and less than 0.75: evaluation B
  • 0.75 or more and less than 0.80: evaluation C
  • 0.80 or more: evaluation D

With respect to the magnetic flux density (Bs), a magnetic flux density of 1.88 T or more was defined as acceptable, and a magnetic flux density of less than 1.88 T was defined as unacceptable.

[Noise Characteristics]

The magnetostriction of a sample having a width of 60 mm×a length of 300 mm subjected to the magnetic domain control was measured by an AC magnetostriction measurement method using a magnetostriction measurement device. The magnetostriction measurement device includes a laser Doppler vibrometer, an exciting coil, an excitation power supply, a magnetic flux detection coil, an amplifier, and an oscilloscope.

Specifically, an AC magnetic field was applied to the sample so that the maximum magnetic flux density was 1.7 T in the rolling direction. A change in the length of the sample due to extension and contraction of the magnetic domain was measured with a laser Doppler vibrometer to obtain a magnetostrictive signal. The obtained magnetostrictive signal was subjected to Fourier analysis to determine the amplitude Cn of each frequency component fn (n is a natural number of 1 or more) of the magnetostrictive signal. Using an A correction coefficient an of each frequency component fn, a magnetostriction velocity level LVA (dB) expressed by the following equation was determined.

LVA = 2 ⁢ 0 × Log ⁡ ( √ ( ∑ ( ρ ⁢ c × 2 ⁢ π × fn × α ⁢ n × Cn / √ 2 ) 2 ) / Pe ⁢ 0 )

Here, ρc is a specific acoustic resistance, and ρc=400. Pe0 is a minimum audible sound pressure, and Pe0=2×10⁻⁵ (Pa) was used. As the A correction coefficient an, values shown in Table 2 of JIS C 1509-1 (2005) were used.

Based on the obtained magnetostriction velocity level (LVA), the noise characteristics were evaluated according to the following criteria.

When the magnetostriction velocity level was 60.0 dBA or less, it was determined that “the noise characteristics are excellent” (evaluation C). When it was 57.5 dBA or less, it was determined to be further excellent (evaluation B), and when it was 55.0 dBA or less, it was determined to be particularly excellent (evaluation A). When the magnetostriction velocity level exceeded 60.0 dBA, it was determined that “the noise characteristics were insufficient (evaluation D)”.

Samples having a magnetic flux density (B₈) of 1.88 T or more and the iron loss and noise characteristics rated evaluation C or higher were defined as acceptable.

As can be seen from Tables 1 to 4, in the steel sheet in which the deviation angles α, β, and γ satisfy the above formula (1), both good magnetic characteristics and noise characteristics could be achieved.

REFERENCE SIGNS LIST

10 . . . . Grain-oriented electrical steel sheet, 11 . . . . Base steel sheet, 13 . . . . Primary coating (glass coating), 15 . . . . Secondary coating (tension-applying insulation coating)

INDUSTRIAL APPLICABILITY

According to the above aspect of the present disclosure, it is possible to provide a grain-oriented electrical steel sheet having good magnetic characteristics and noise characteristics without reducing productivity, and a method for manufacturing the same, and thus industrial applicability is high.