EP4653555A1

EP4653555A1 - Method for producing grain-oriented electrical steel sheet

Info

Publication number: EP4653555A1
Application number: EP24744693.3A
Authority: EP
Inventors: Ryo Matsubara; Yoshiyuki Ushigami
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 2023-01-18
Filing date: 2024-01-18
Publication date: 2025-11-26
Also published as: CN120322568A; KR20250122492A; JPWO2024154772A1; WO2024154772A1

Abstract

The method of manufacturing a grain-oriented electrical steel sheet includes: a hot rolling step; a hot-rolled sheet annealing step performed as necessary; a cold rolling step; a decarburization annealing step; an annealing separator applying step; a nitriding treatment step; and a final annealing step, wherein the final annealing step has a temperature raising process and a soaking process, and a temperature gradient of 0.5 °C/cm or more is generated in a boundary region between a primary recrystallization region and a secondary recrystallization region at least in one period from the start of secondary recrystallization to completion of the secondary recrystallization in the temperature raising process; and the nitriding treatment step is performed by annealing in an atmosphere containing a gas having nitriding ability at least in one stage of: during the decarburization annealing step; between the decarburization annealing step and the final annealing step; and during the temperature raising process in the final annealing step and before the start of the secondary recrystallization, so that the nitrogen amount in the steel sheet is 210 ppm or more based on mass after the nitriding treatment step.

Description

TECHNICAL FIELD

The present invention relates to a method of manufacturing a grain-oriented electrical steel sheet.
Priority is claimed on Japanese Patent Application No. 2023-005564, filed January 18, 2023 , the content of which is incorporated herein by reference.

BACKGROUND ART

The grain-oriented electrical steel sheet (also referred to as grain-oriented silicon steel sheet) is a soft magnetic material, and is mainly used as an iron core material of a transformer. Therefore, the grain-oriented electrical steel sheet is required to have a small energy loss (low iron loss). For example, the magnetic flux density: B8 (magnetic flux density in a magnetic field of 800 A/m) is the most dominant factor of iron loss characteristics. It is known that the higher the value of the magnetic flux density: B8, the lower the iron loss, and the better the iron loss characteristics. In addition, the iron core can be downsized as the value of the magnetic flux density: B8 becomes higher, which is advantageous in terms of the device configuration of the transformer and also advantageous in terms of the manufacturing cost of the transformer. In order to increase the value of the magnetic flux density: B8, it is important to highly align the crystal orientation. The crystal orientation control is achieved using a catastrophic grain growth phenomenon called secondary recrystallization.
Hitherto, many inventions relating to a grain-oriented electrical steel sheet having a high magnetic flux density have been proposed. However, in industrially produced grain-oriented silicon steel sheets investigated so far, the magnetic flux density (B8) thereof is smaller than the theoretical upper limit of silicon steel, and there is still room for significant improvement.
In order to improve the magnetic flux density, for example, Patent Document 1 discloses a method of manufacturing a grain-oriented electrical steel sheet having a high magnetic flux density in which a silicon steel slab containing 0.015% or less of C, 4% or less of Si, 0.012% or less of S, 0.020 to 0.065% of acid-soluble Al, and 0.0030 to 0.0095% of T.N is heated at 1270°C or lower and then hot-worked into a hot rolled sheet, the hot rolled sheet is coiled at 700 to 950°C and then cold-rolled at a rolling reduction of 65% or more, the steel sheet is annealed for primary recrystallization for a short time, and then the steel sheet is subjected to final annealing at high temperature including treatment of growing secondary recrystallization grains while applying a temperature gradient of 2 °C/cm or more to the steel sheet in the boundary portion between the primary recrystallization region and the secondary recrystallization region.
In addition, Patent Document 2 discloses a method of manufacturing a grain-oriented silicon steel sheet having an ultra-low iron loss including achieving crystal orientation control through secondary recrystallization and a smooth steel sheet surface to manufacture a grain-oriented electrical steel sheet having an ultra-low iron loss at low cost also for a material having a thin sheet thickness (for example, 0.13 mm), which has been difficult to manufacture conventionally. Patent Document 2 discloses that it is necessary to raise the temperature to 1000 to 1100°C at a temperature rising rate of 50 °C/hr or more in the final annealing in order to secure a temperature gradient of at least 2 °C/cm.
Further, for example, Patent Document 3 discloses a method of manufacturing a grain-oriented silicon steel sheet (band) having a high magnetic flux density in which secondary recrystallization proceeds while the steel sheet (band) is applied with a temperature gradient in the boundary region between the primary recrystallization region and the secondary recrystallization region during the manufacturing process of a grain-oriented silicon steel sheet (band).
In addition, for example, Patent Documents 4 and 5 disclose equipment and methods for imparting a temperature gradient to a coiled steel sheet.

Citation List

Patent Document

Patent Document 1: Japanese Examined Patent Application Publication No. S59-41488
Patent Document 2: Japanese Unexamined Patent Application, First Publication No. H05-311238
Patent Document 3: Japanese Examined Patent Application Publication No. S58-50295
Patent Document 4: Japanese Unexamined Patent Application, First Publication No. S57-164935
Patent Document 5: Japanese Unexamined Patent Application, First Publication No. S58-1019

SUMMARY OF INVENTION

Technical Problem

As described above, in the techniques described in Patent Documents 1 and 2, it is necessary that a temperature gradient of 2 °C/cm or more be applied during final annealing. It is thought that the equipment and methods disclosed in Patent Documents 4 and 5 or the like are adopted to apply a temperature gradient. However, using the equipment and methods disclosed therein, it is difficult to control the temperature gradient to be constant throughout an industrial scale coil in the longitudinal direction and in the width direction, and to control the temperature gradient to be high throughout the same, and there are regions with a low-temperature gradient of about 0.5 °C/cm (small temperature gradient) in some parts. In such a low-temperature gradient region, it is difficult to sufficiently improve the magnetic flux density.
The present inventors calculated the temperature gradient of each portion of a coil by simulation using a known heat transfer calculation software Fluent, (registered trademark), developed by ANSYS, and found that even when a temperature gradient of 2 °C/cm or more is applied, a region having a low-temperature gradient of about 0.5 °C/cm (small temperature gradient) exists in some parts of the coil. It was found that when a coil is applied with a temperature gradient, particularly to a large temperature gradient, the temperature gradient tends to be smaller inside the coil than outside the coil. That is, for example, even when the temperature gradient is 2 °C/cm or more outside the coil, there is a low-temperature gradient of less than 2 °C/cm in some regions inside the coil or the like. In addition, when a temperature gradient is applied in the width direction of a coil, the temperature gradient tends to be smaller on the low-temperature end than on the high-temperature end. For example, even when the temperature gradient is 2 °C/cm or more on the high-temperature end of the coil, there may be a low-temperature gradient of less than 2 °C/cm in some parts on the low-temperature end or the like. Therefore, it is difficult to apply a temperature gradient of 2 °C/cm or more throughout a coil.
For this reason, in order to obtain a sufficient effect of improving the magnetic flux density throughout a coil, a method for obtaining the effect of improving the magnetic flux density even under a smaller temperature gradient has been desired.
Patent Document 3 discloses that an effect of improving B8 characteristics is recognized by imparting a temperature gradient of 0.5 °C/cm. However, Patent Document 3 indicates that a remarkable effect is obtained at 2 °C/cm or more. In fact, the B8 value of a grain-oriented electrical steel sheet having a Si content of 2.95% is about 1.92 T under a temperature gradient of 0.5 °C/cm, which means that a certain effect of improving the magnetic flux density is obtained, but it cannot be said to be sufficient for the recent advanced requirements.
The present invention has been made in view of the above problem. An object of the present invention is to provide a method of manufacturing a grain-oriented electrical steel sheet in which final annealing is performed while applying a temperature gradient in the boundary region between the primary recrystallization region and the secondary recrystallization region to manufacture a grain-oriented electrical steel sheet having a high magnetic flux density, the method being a method for manufacturing a grain-oriented electrical steel sheet stably having a high magnetic flux density throughout a coil by obtaining a sufficient effect of improving magnetic flux density even under a small temperature gradient.

Solution to Problem

On the premise of a method of performing final annealing while applying a temperature gradient in the boundary region between the primary recrystallization region and the secondary recrystallization region, the present inventors examined a method for obtaining a sufficient effect of improving the magnetic flux density even under a relatively small temperature gradient (even when the lower limit of the temperature gradient is small in a case where a large temperature gradient part and a small temperature gradient part are generated).
As a result, it was found that by controlling a nitrogen amount of a steel sheet to be 210 ppm or more before secondary recrystallization a sufficient effect of improving the magnetic flux density is obtained even under a relatively small temperature gradient.
The present invention has been made in view of the above findings. The gist of the present invention is as follows.

[1] In an embodiment of the present invention, a method of manufacturing a grain-oriented electrical steel sheet includes: a hot rolling step of heating a silicon steel material containing, in terms of mass%, 0.80 to 7.00% of Si to a temperature of 1280°C or lower and hot rolling the silicon steel material, to obtain a hot rolled sheet; a hot-rolled sheet annealing step of annealing the hot rolled sheet as necessary; a cold rolling step of cold-rolling the hot rolled sheet after the hot rolling step or after the hot-rolled sheet annealing step to obtain a steel sheet having a final sheet thickness; a decarburization annealing step of decarburization-annealing the steel sheet after the cold rolling step; an annealing separator applying step of applying an annealing separator on the steel sheet after the decarburization annealing step and coiling the steel sheet into a coil shape; a nitriding treatment step of increasing a nitrogen amount in the steel sheet; and a final annealing step of final-annealing the steel sheet coiled into a coil shape, wherein the final annealing step has a temperature raising process and a soaking process, and a temperature gradient of 0.5 °C/cm or more is generated in a boundary region between a primary recrystallization region and a secondary recrystallization region at least in one period from a start of secondary recrystallization to completion of the secondary recrystallization in the temperature raising process; and the nitriding treatment step is performed by annealing in an atmosphere containing a gas having nitriding ability at least in one stage of: during the decarburization annealing step; between the decarburization annealing step and the final annealing step; and during the temperature raising process in the final annealing step and before the start of the secondary recrystallization so that the nitrogen amount in the steel sheet is 210 ppm or more based on mass after the nitriding treatment step.
[2] In the method of manufacturing a grain-oriented electrical steel sheet according to [1], in the temperature raising process in the final annealing step, an atmosphere in a furnace from the start of the secondary recrystallization to the completion of the secondary recrystallization may include N₂ gas in a proportion of 20% or more in terms of mass%.
[3] In the method of manufacturing a grain-oriented electrical steel sheet according to [1] or [2], the nitrogen amount in the steel sheet may be 210 ppm or more and 350 ppm or less based on mass after the nitriding treatment step.
[4] In the method of manufacturing a grain-oriented electrical steel sheet according to [1] or [2], a formula (1) below may be satisfied when [N] is defined as the nitrogen amount in the steel sheet after the nitriding treatment step in terms of ppm based on mass; [N₂] is defined as the proportion of N₂ gas in terms of mass% in an atmosphere in a furnace in the final annealing step; and [T] is defined as the temperature gradient in the final annealing step in terms of a unit of °C/cm. $[T] \geq 1 / (0.0026 \times [N]) - 0.005 \times [N_{2}] - 0.5$
[5] In the method of manufacturing a grain-oriented electrical steel sheet according to [3], a formula (1) below may be satisfied when [N] is defined as the nitrogen amount in the steel sheet after the nitriding treatment step in terms of ppm based on mass; [N₂] is defined as the proportion of N₂ gas in terms of mass% in an atmosphere in a furnace in the final annealing step; and [T] is defined as the temperature gradient in the final annealing step in terms of a unit of °C/cm. $[T] \geq 1 / (0.0026 \times [N]) - 0.005 \times [N_{2}] - 0.5$
[6] In the method of manufacturing a grain-oriented electrical steel sheet according to [1] or [2], the silicon steel material may include, as a chemical composition, in terms of mass%, 0.80 to 7.00% of Si, 0.085% or less of C, 0.010 to 0.065% of acid-soluble Al, 0.004 to 0.012% of N, 0 to 1.00% of Mn, 0 to 0.30% of Cr, 0 to 0.4% of Cu, 0 to 0.5% of P, 0 to 1.00% of Ni, 0 to 0.015% in total of S and Se, and a balance of Fe and impurities.
[7] In the method of manufacturing a grain-oriented electrical steel sheet according to [3], the silicon steel material may include, as a chemical composition, in terms of mass%, 0.80 to 7.00% of Si, 0.085% or less of C, 0.010 to 0.065% of acid-soluble Al, 0.004 to 0.012% of N, 0 to 1.00% of Mn, 0 to 0.30% of Cr, 0 to 0.4% of Cu, 0 to 0.5% of P, 0 to 1.00% of Ni, 0 to 0.015% in total of S and Se, and a balance of Fe and impurities.
[8] In the method of manufacturing a grain-oriented electrical steel sheet according to [4], the silicon steel material may include, as a chemical composition, in terms of mass%, 0.80 to 7.00% of Si, 0.085% or less of C, 0.010 to 0.065% of acid-soluble Al, 0.004 to 0.012% of N, 0 to 1.00% of Mn, 0 to 0.30% of Cr, 0 to 0.4% of Cu, 0 to 0.5% of P, 0 to 1.00% of Ni, 0 to 0.015% in total of S and Se, and a balance of Fe and impurities.
[9] In the method of manufacturing a grain-oriented electrical steel sheet according to [5], the silicon steel material may include, as a chemical composition, in terms of mass%, 0.80 to 7.00% of Si, 0.085% or less of C, 0.010 to 0.065% of acid-soluble Al, 0.004 to 0.012% of N, 0 to 1.00% of Mn, 0 to 0.30% of Cr, 0 to 0.4% of Cu, 0 to 0.5% of P, 0 to 1.00% of Ni, 0 to 0.015% in total of S and Se, and a balance of Fe and impurities.

Advantageous Effects of Invention

According to the embodiment of the present invention, it is possible to provide a method of manufacturing a grain-oriented electrical steel sheet in which final annealing is performed while applying a temperature gradient in the boundary region between the primary recrystallization region and the secondary recrystallization region to manufacture a grain-oriented electrical steel sheet having a high magnetic flux density, the method being a method of manufacturing a grain-oriented electrical steel sheet capable of obtaining a sufficient effect of improving magnetic flux density even under a small temperature gradient.

DESCRIPTION OF EMBODIMENTS

Hereinafter, a method of manufacturing a grain-oriented electrical steel sheet according to an embodiment of the present invention (a method of manufacturing a grain-oriented electrical steel sheet according to the embodiment) will be described.
The method of manufacturing a grain-oriented electrical steel sheet according to the embodiment includes:

(i) a hot rolling step of heating a silicon steel material having a predetermined chemical composition to a temperature of 1280°C or lower and hot rolling the silicon steel material, to obtain a hot rolled sheet;
(ii) a hot-rolled sheet annealing step of annealing the hot rolled sheet as necessary;
(iii) a cold rolling step of cold-rolling the hot rolled sheet after the hot rolling step or after the hot-rolled sheet annealing step to obtain a steel sheet having a final sheet thickness;
(iv) a decarburization annealing step of decarburization-annealing the steel sheet after the cold rolling step;
(v) an annealing separator applying step of applying an annealing separator on the steel sheet after the decarburization annealing step and coiling the steel sheet into a coil shape;
(vi) a nitriding treatment step of increasing a nitrogen amount in the steel sheet; and
(vii) a final annealing step of final-annealing the steel sheet coiled into a coil shape.

Hereinafter, the conditions of each step will be described.

[Hot Rolling Step]

In the hot rolling step, a silicon steel material such as a slab having a chemical composition described later is heated to a temperature of 1280°C or lower and hot rolling, to obtain a hot rolled sheet (hot-rolled steel sheet).
In a method of manufacturing a grain-oriented electrical steel sheet, generally, a method is industrially performed in which a fine precipitate called inhibitor completely becomes a solid solution when the silicon steel material is heated before hot rolling, and then the inhibitor is finely precipitated during the hot rolling and the subsequent annealing steps. In this method, it is necessary to heat the silicon steel material at a high temperature of 1350°C or higher in order to make a complete solid solution of the precipitate. However, this temperature is higher than the slab heating temperature of ordinary steel by about 200°C, and there is a problem that a dedicated heating furnace for this purpose is required and the amount of molten scale is large.
Therefore, in the method of manufacturing a grain-oriented electrical steel sheet according to the embodiment, the heating temperature is 1280°C or lower to avoid the above-described problem due to high-temperature heating. The lower limit of the heating temperature is preferably 700°C or higher in order to prevent cracking due to hot rolling.
The hot rolling conditions other than the heating temperature are not limited, and may be in a known range according to required characteristics and the like.
In addition, the silicon steel material to be subjected to hot rolling is obtained by smelting steel in a converter, an electric furnace, or the like, subjecting the molten steel to a vacuum degassing treatment as necessary, and then subjecting the molten steel to continuous casting or blooming after an ingot is made.
The silicon steel material contains 0.80 to 7.00% of Si in terms of mass%. Preferably, the silicon steel material includes, as the chemical composition, in terms of mass%, 0.80 to 7.00% of Si, 0.085% or less of C, 0.010 to 0.065% of acid-soluble Al, 0.004 to 0.012% of N, 0 to 1.00% of Mn, 0 to 0.30% of Cr, 0 to 0.4% of Cu, 0 to 0.5% of P, 0 to 1.00% of Ni, 0 to 0.015% in total of S and Se, and a balance of Fe and impurities.
The reason for the content of each element will be described. Hereinafter, % regarding the content is mass%.

(Si: 0.80 to 7.00%)

When the Si content is less than 0.80%, γ transformation occurs during the final annealing, and the crystal orientation of the steel sheet is impaired. In addition, when Si is contained, electrical resistance is increased and iron loss characteristics are improved. Therefore, in the silicon steel material, the Si content is 0.80% or more. The Si content is preferably 1.50% or more, more preferably 2.00% or more, and still more preferably 2.50% or more.
On the other hand, when the Si content is more than 7.00%, it is extremely difficult to perform cold rolling, and there is a possibility of cracking during rolling. Therefore, the Si content is 7.00% or less. As a range suitable for industrial production, the Si content may be 4.80% or less, or may be 4.00% or less.

(C: 0.085% or less)

C is an effective element for controlling the primary recrystallization structure, but has an adverse effect on the magnetic characteristics. Therefore, it is necessary to perform decarburization before the final annealing. When the C content is more than 0.085% in the silicon steel material, the decarburization annealing time becomes long, and industrial productivity is impaired. Therefore, the C content is preferably 0.085% or less. The lower limit of the C content is not particularly limited, but is preferably 0.020% or more, and more preferably 0.050% or more in consideration of industrial productivity and magnetic properties of products.

(Acid-soluble Al: 0.010 to 0.065%)

Acid-soluble Al (sol.Al) is an element that binds to N and functions as an inhibitor as AlN or (Al, Si)N. As a range in which the secondary recrystallization is stabilized, the acid-soluble Al content is preferably 0.010 to 0.065%. The acid-soluble Al content may be 0.040% or less, and further, may be 0.030% or less.

(N: 0.004 to 0.012%)

N is an element that binds to Al and functions as an inhibitor. When the N content is less than 0.004%, a sufficient amount of inhibitor cannot be obtained. Therefore, the N content is preferably 0.004% or more. The N content is more preferably 0.006% or more, still more preferably 0.007% or more.
On the other hand, when the N content is more than 0.012%, pores called blisters may be generated in the steel sheet during cold rolling. Therefore, the N content is preferably 0.012% or less.
The silicon steel material may contain, as the chemical composition, the above elements, and the balance may be Fe and impurities. On the other hand, in order to improve various properties, the following elements may be further contained.
The following elements are also allowed to be contained as impurities as long as they fall within the range described later.

(Mn: 0 to 1.00%)

Mn is an element having an effect of increasing specific resistance and reducing iron loss. Thus, Mn may be included.
Mn is an effective element for preventing the occurrence of cracking in hot rolling caused by S or Se. In order to prevent the occurrence of cracking, the Mn content is preferably in a range satisfying Mn/(S + Se) ≥ 4 in relation to the total amount of S and Se.
On the other hand, when the Mn content exceeds 1.00%, the magnetic flux density of the grain-oriented electrical steel sheet decreases. Therefore, the Mn content is preferably 1.00% or less.

(Cr: 0 to 0.30%)

Cr is an element that brings the composition and amount of the oxide layer of the decarburization annealing into a preferable state and promotes the formation of a glass coating. Thus, Cr may be included.
On the other hand, when the Cr content is more than 0.30%, decarburization is inhibited. Therefore, the Cr content is preferably 0.30% or less.

(Cu: 0 to 0.4%)

Cu is an effective element for increasing specific resistance and reducing iron loss. Thus, Cu may be included.
On the other hand, when the Cu content is more than 0.4%, the iron loss reduction effect is saturated, and a surface defect called "copper scab" is caused during hot rolling. Therefore, the Cu content is preferably 0.4% or less.

(P: 0 to 0.5%)

P is an effective element for increasing specific resistance and reducing iron loss. Thus, P may be included.
On the other hand, when the P content is more than 0.5%, the rollability is deteriorated. Therefore, the P content is preferably 0.5% or less.

(Ni: 0 to 1.00%)

Ni is an effective element for increasing specific resistance and reducing iron loss. In addition, Ni is an effective element for controlling the metallographic structure of the hot rolled sheet to improve magnetic characteristics. Thus, Ni may be included.
On the other hand, when the Ni content exceeds 1.00%, secondary recrystallization becomes unstable. Therefore, the Ni content is preferably 1.00% or less.

(S and Se: 0 to 0.015% in total)

S and Se are elements that adversely affect magnetic characteristics. Therefore, the content of these elements is preferably 0.015% or less in total. The content of these elements is preferably as small as possible, and may be 0%.
In addition to the above-described elements, for example, B, O, Mg, Ca, Ti, Mo, V, Nb, Sn, Sb, and Bi may be each contained as impurities in an amount of 0.10% or less. Impurity refers to an element that is mixed from a raw material or in a manufacturing process and does not clearly affect the characteristics of the grain-oriented electrical steel sheet.
The chemical composition of the silicon steel material may be measured by a known method. For example, the chemical composition may be measured using inductively coupled plasma-atomic emission spectrometry (ICP-AES). Note that Al may be measured by ICP-AES using, as acid-soluble Al, a filtrate after a sample is thermally decomposed with an acid. Si may be measured by the silicon dioxide weight method, C and S may be measured by the combustion-infrared absorption method, and N may be measured by the inert gas fusion-thermal conductivity method. O may be measured using the inert gas fusion-non-dispersive infrared absorption method.
The above chemical composition refers to the components of the silicon steel sheet as a base metal. When the grain-oriented electrical steel sheet as a measurement sample has a glass coating, an insulating coating, or the like on the surface, the chemical composition is measured after these coatings are removed by a known method.

[Hot-Rolled sheet Annealing Step]

After the hot rolling step, the hot rolled sheet obtained by hot rolling may be subjected to annealing (hot rolled sheet annealing) in order to enhance magnetic characteristics. The annealing condition is not limited, but may be, for example, a condition of holding at 900 to 1200°C for 30 seconds to 30 minutes. The annealing temperature may be 950 to 1050°C.

[Cold Rolling Step]

In the cold rolling step, the hot rolled sheet after the hot rolling step or after the hot-rolled sheet annealing step is subjected to cold rolling to obtain a steel sheet (cold rolled sheet) having a sheet thickness equal to the final sheet thickness (the sheet thickness of the completed grain-oriented electrical steel sheet (when a glass coating or an insulating coating is formed on the surface thereof, the sheet thickness of the base steel sheet excluding the coatings)). The cold rolling may be performed one time (in series without intervening intermediate annealing) or two or more times with intervening annealing (intermediate annealing).
For the cold rolling, in order to develop a preferable primary recrystallization orientation such as {411} during the decarburization annealing, the final rolling reduction is preferably 80% or more. 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.

[Decarburization Annealing Step]

In the decarburization annealing step, the steel sheet after the cold rolling step is subjected to decarburization annealing.
In the decarburization annealing, decarburization annealing conditions are not limited as long as the steel sheet can be primarily-recrystallized and C, which adversely affects magnetic characteristics, can be removed from the steel sheet. For example, the steel sheet is held at an annealing temperature of 770 to 950°C for 10 to 600 seconds with a degree of oxidation (PH₂O/PH₂) of 0.15 to 1.00 in an annealing atmosphere (furnace atmosphere).

[Annealing Separator Applying Step]

In the annealing separator applying step, the steel sheet after the decarburization annealing step is applied with an annealing separator, and then coiled into a coil shape.
The annealing separator to be applied may be a known one, but an annealing separator containing magnesia as a main component is preferable. By applying an annealing separator containing magnesia as a main component and performing the subsequent final annealing, a glass coating (forsterite coating) is formed on the surface of the steel sheet.

[Nitriding Treatment Step]

In the nitriding treatment step, the nitrogen amount in the steel sheet is increased. The nitriding treatment step is performed at least in one stage of: during the decarburization annealing step; between the decarburization annealing step and the final annealing step; and after the start of the final annealing step and before the start of the secondary recrystallization in the temperature raising process in the final annealing step. Between the decarburization annealing step and the final annealing step means between the completion of the decarburization annealing step and the start of the final annealing step. The nitriding treatment step is preferably performed after the completion of the decarburization annealing step and before the start of the annealing separator applying step.
However, regardless of which stage the nitriding treatment step is performed in, the nitrogen amount in the steel sheet is required to be 210 ppm (0.0210 mass%) or more based on mass after the final nitriding treatment step. When the nitrogen amount in the steel sheet before the start of secondary recrystallization is increased, the amount of the inhibitor is increased, and the inhibitor becomes thermally stable. As a result, a sufficient magnetic flux density improving effect can be obtained even under a relatively small temperature gradient. The nitrogen amount may be 250 ppm or more, and further, may be 300 ppm or more. On the other hand, when the nitrogen amount is more than 350 ppm, the effect of improving the magnetic flux density may be saturated, and there is also a possibility that purification after secondary recrystallization will be disadvantageous. Therefore, the nitrogen amount is preferably 350 ppm or less.
Conventionally, when final annealing is performed by applying a temperature gradient, the nitrogen amount in the steel sheet is usually 200 ppm or less. For example, JP S59-215419 A discloses that when secondary recrystallization annealing is performed while the boundary region between the primary recrystallization region and the secondary recrystallization region is applied with a temperature gradient in the final annealing, the nitrogen content in the steel sheet is 130 to 200 ppm at the start of secondary recrystallization. JP S59-215419 A describes that the magnetic flux density improving effect is saturated at a nitrogen content of 180 to 200 ppm.
On the other hand, the present inventors have found that when the nitrogen amount is increased to 210 ppm or more, the lower limit of the temperature gradient at which high magnetic flux density (for example, B8 is stably 1.940 T or more) can be achieved is expanded compared with the conventional case (high B8 is stably obtained even when the temperature gradient is about 0.5 °C/cm).
The nitrogen amount after the nitriding treatment step against the intended temperature gradient is preferably in a range satisfying a formula (1) below when [N] is defined as the nitrogen amount in the steel sheet after the final nitriding treatment step in terms of ppm based on mass; [N₂] is defined as the proportion of N₂ gas in terms of mass% in the atmosphere in the temperature raising process in the final annealing step described later; and [T] is defined as the temperature gradient in the temperature raising process in the final annealing step in terms of a unit of °C/cm.
In this case, a higher magnetic flux density is obtained. For example, when the nitrogen amount and the proportion of N₂ gas in terms of mass% in the atmosphere are determined so that the minimum temperature gradient throughout the coil calculated by simulation or the like satisfies the formula (1), a higher magnetic flux density can be stably obtained throughout the coil.
A higher magnetic flux density can be obtained when the formula (1) is satisfied presumably because: when the nitrogen amount in the steel sheet before the start of secondary recrystallization is increased, the amount of the inhibitor is increased and the inhibitor becomes thermally stable; and when the proportion of N₂ gas in terms of mass% in the atmosphere is increased, the removal of the inhibitor can be suppressed, and the preferential growth of { 110} <001> oriented grains can be further enhanced during secondary recrystallization. $[T] \geq 1 / (0.0026 \times [N]) - 0.005 \times [N_{2}] - 0.5$
In addition, examples of the method for increasing the nitrogen amount in the steel sheet include a method of annealing the steel sheet in an atmosphere containing a gas having nitriding ability to control the nitrogen amount in the steel sheet.
In addition to the above, the nitrogen amount in the steel sheet may be increased by adding a powder having nitriding ability such as MnN to an annealing separator in the temperature raising process in the final annealing step.
The nitrogen amount in the steel sheet after the nitriding treatment can be measured by a known method using, for example, an oxygen-nitrogen-hydrogen analyzer (EMGA-930) manufactured by HORIBA, Ltd. or an apparatus equivalent thereto. As the known method, a general analysis method such as the inert gas fusion-thermal conductivity method can be used. A sample having an arbitrary size may be collected from the steel sheet after the nitriding treatment step in the manufacturing process, and measured using these devices and methods.

[Final Annealing Step]

In the final annealing step, the steel sheet coiled into a coil shape is subjected to final annealing.
The final annealing step includes: a temperature raising process of heating to a final annealing temperature for secondary recrystallization; and a soaking process of holding the steel sheet at the final annealing temperature.

In the final annealing step, under the state where the nitrogen amount in the steel sheet is controlled to be 210 ppm or more based on mass in the nitriding treatment step, a temperature gradient of 0.5 °C/cm or more is generated in a boundary region between a primary recrystallization region and a secondary recrystallization region at least in one period from the start of secondary recrystallization to completion of the secondary recrystallization in the temperature raising process, and { 110} <001> oriented grains are preferentially grown through secondary recrystallization. Even if a temperature gradient is applied at a time other than the above time, for example, before the final annealing, a similar effect cannot be obtained.
The temperature rising rate is not limited as long as the temperature gradient is satisfied, and may be 50 °C/h or less.
In the final annealing, secondary recrystallized grains are generated in a portion heated to the secondary recrystallization temperature or higher. When the steel sheet is heated in a state where a temperature gradient exists in a certain direction, secondary recrystallization proceeds from the region where the temperature is equal to or higher than the secondary recrystallization temperature; and between the above region and the region where the secondary recrystallization temperature has not been reached and the primary recrystallization structure still remains, a region where primary recrystallization grains and secondary recrystallization grains are mixed in the sheet thickness direction (boundary region) is generated along the isotherm. As the steel sheet is heated and the temperature increases, this boundary region moves along the temperature gradient to the region where the primary recrystallization structure remains, so that the region that has become the secondary recrystallization structure expands, and the entire surface of the steel sheet is finally covered with the secondary recrystallization grains. Throughout this process, the temperature of the boundary region is kept relatively constant. Regarding the direction of the temperature gradient, in the final annealing, a coil-shaped grain-oriented electrical steel sheet is usually disposed in a columnar shape inside a furnace. Therefore, the temperature gradient is preferably applied in the width direction of the steel sheet. When a temperature gradient is applied in the width direction, the temperature gradient is formed in one direction in the entire width direction of the steel sheet (so that one end serves as the high-temperature end, and the other end serves as the low-temperature end).
As described above, it is not easy to apply a temperature gradient of 2.0 °C/cm or more throughout a coil, and problems of productivity, variations in characteristics in the steel sheet, and the like may arise. However, in the method of manufacturing a grain-oriented electrical steel sheet according to the embodiment, the nitrogen amount in the steel sheet is 210 ppm or more at the start of secondary recrystallization. Therefore, the amount of the inhibitor increases and the inhibitor becomes thermally stable, so that a sufficient magnetic flux density improving effect can be obtained even under a relatively small temperature gradient. When the temperature gradient varies, the lower limit of the temperature gradient at which a sufficient magnetic flux density improving effect can be obtained can be reduced. However, when the temperature gradient is less than 0.5 °C/cm, the effect of improving the magnetic flux density cannot be sufficiently obtained. Therefore, the temperature gradient is 0.5 °C/cm or more. When the temperature gradient varies in each portion of the coil or the steel sheet, the minimum temperature gradient throughout the coil or the steel sheet is 0.5 °C/cm or more. It is not necessary to limit the upper limit of the temperature gradient. However, even if the temperature gradient exceeds 10.0 °C/cm, the effect is saturated and the equipment load increases. Therefore, the temperature gradient throughout the coil may be 10.0 °C/cm or less. In the present application, since a sufficient magnetic flux density improving effect can be obtained even under a relatively small temperature gradient, the temperature gradient throughout the coil may be 5.0 °C/cm or less, or may be 2.0 °C/cm or less. Particularly when a relatively uniform temperature gradient is applied, the temperature gradient may be 1.5 °C/cm or less, or may be 1.0 °C/cm or less. When the temperature gradient varies in each portion of the coil or the steel sheet, the minimum temperature gradient throughout the coil or the steel sheet may be 5.0 °C/cm or less, or may be 2.0 °C/cm or less, and further may be 1.5 °C/cm or less, or may be 1.0 °C/cm or less.
The temperature gradient is preferably set in a range satisfying a formula (1) below when [N] is defined as the nitrogen amount in the steel sheet after the nitriding treatment step in terms of ppm based on mass; [N₂] is defined as the proportion of N₂ gas in terms of mass% in the atmosphere in the temperature raising process in the final annealing step described later; and [T] is defined as the temperature gradient in the temperature raising process in the final annealing step in terms of a unit of °C/cm.
In this case, a higher magnetic flux density is obtained. $[T] \geq 1 / (0.0026 \times [N]) - 0.005 \times [N_{2}] - 0.5$
In addition, the atmosphere in a furnace from the start of the secondary recrystallization to the completion of the secondary recrystallization preferably includes N₂ gas in a proportion of 20% or more in terms of mass%. When the proportion of N₂ gas (nitrogen gas) in the atmosphere is 20% or more in terms of mass%, the removal of the inhibitor can be suppressed, and thus a more preferable effect is obtained. In the temperature raising process in the final annealing, the proportion of N₂ gas (nitrogen gas) in the atmosphere in a furnace is preferably 25% or more. The proportion of N₂ gas (nitrogen gas) may be 100%, but when the proportion is 100%, coating defects are likely to occur. Therefore, the proportion of N₂ gas (nitrogen gas) is preferably less than 100%. The proportion is more preferably 75% or less, still more preferably 50% or less.
When the final annealing is performed while applying a temperature gradient, secondary recrystallization starts from the high-temperature side. Therefore, from the start to the completion of the secondary recrystallization means from the start of the secondary recrystallization in a partial region of the steel sheet on the high-temperature side of the temperature gradient to the completion of the secondary recrystallization throughout the steel sheet.
Regarding the application of a temperature gradient, although the temperature at the position of the boundary region is not constant, depending on the type of the steel sheet and the annealing conditions, the temperature in the boundary region can be known when a preliminary experiment or the like is performed and the temperature at which secondary recrystallization occurs is confirmed under the assumed type of the steel sheet and annealing conditions. Therefore, by applying a temperature gradient in a position where the temperature is around the temperature of the boundary region examined in this way, a temperature gradient can be applied in the boundary region between the primary recrystallization region and the secondary recrystallization region. For example, when the steel sheet contains MnS and AlN as an inhibitor and has a Si content of about 3 mass%, the temperature of the boundary region is about 950 to 1100°C.
When the boundary region is not clear, a temperature gradient may be applied to a wider range or the entire coil (steel sheet).
In addition, the effect can be obtained by applying a temperature gradient to the boundary region at least in one period from the generation to the growth of secondary recrystallization grains. However, in order to obtain a sufficient effect, the temperature gradient is preferably applied to the boundary region after secondary recrystallization starts and until the entire surface of the steel sheet is covered with secondary recrystallization grains (until the completion of the secondary recrystallization). That is, the temperature gradient may be generated from the beginning to the end of the temperature raising process of the final annealing (until reaching the soaking temperature).
The temperature gradient can be applied by increasing the temperature such that the furnace has a temperature difference therein, or by heating and/or cooling the coil ends to increase the temperature such that the coil has a temperature difference therein in the final annealing furnace. Regarding the magnitude of the temperature gradient, for example, when the temperature gradient is applied in the width direction of the coil, sensors such as thermocouples are arranged at constant intervals (interval at which temperature gradient can be measured, for example, 100 mm interval) in the width direction to measure the temperature history, so that the temperature gradient of each portion in the steel sheet is calculated. In addition, the minimum value of the temperature gradient throughout (in the entire region of) the coil can be determined by calculating the temperature gradient of each portion.
In addition, the temperature gradient varies depending on the size of the furnace, the temperature difference in the furnace, the size and weight of the coil, and the like. In this case, optionally, physical property values such as thermal diffusivity are calculated by using the results of the temperature history at a plurality of portions of the coil actually measured as described above, and then, for example, the furnace wall temperature is provided as a boundary condition, and the temperature gradient of each portion of the coil is calculated by simulation using a known heat transfer calculation software, Fluent (registered trademark), developed by ANSYS or the like. In the simulation, various conditions are set so that the temperature gradient of each portion of the coil (for example, a range of arbitrary 100 mm intervals in the width direction of the coil) is calculated in consideration of the variation of the temperature gradient. By calculating the temperature gradient of each portion, the minimum value of the temperature gradient throughout the coil can be determined.
When there is a difference in the temperature gradient in the radial direction of the coil, sensors such as thermocouples are arranged at constant intervals in the width direction (interval at which the difference in temperature gradient can be measured, for example, 100 mm interval) at a plurality of locations in the radial direction of the coil, and the temperature history in the width direction of each location is measured, whereby the difference in temperature gradient in the radial direction can be calculated. The minimum value of the temperature gradient throughout the coil can be determined from the temperature gradient calculated at each measurement point in the radial direction of the coil. For example, when a temperature gradient is applied in the width direction of the coil, the temperature history in the radial direction of the coil may be measured at three points or more in total, such that each of the steel sheet located on the outermost side of the coil, the steel sheet located in the radial intermediate portion of the coil, and the steel sheet located on the innermost side of the coil has one or more measurement points in the longitudinal direction of the coil. Similarly, simulation makes it possible to calculate the temperature gradient in the width direction at a plurality of locations in the coil radial direction (for example, each position at intervals of 100 mm in width in the coil radial direction, or each position of the outermost side, the intermediate portion, and the innermost side in the coil radial direction).
When the temperature gradient varies in the coil, the temperature gradient tends to be relatively small on the side of the low-temperature end of the coil, and tends to be relatively small at the position on the radially innermost side of the coil. Therefore, the temperature gradient calculated by measurement or simulation at the position that is on the side of the low-temperature end and the radially innermost side of the coil may be determined as the minimum temperature gradient throughout the coil.

In the soaking process, impurities that adversely affect magnetic properties, such as N, S, and Se, are purified. Therefore, the final annealing temperature (soaking temperature) is preferably 1150 to 1250°C. The annealing time (soaking time) is preferably 10 to 30 hours after the low-temperature side of the temperature gradient of the coil reaches the soaking temperature.

[Insulating Coating-Forming Step]

The method of manufacturing a grain-oriented electrical steel sheet according to the embodiment may further include an insulating coating-forming step of forming an insulating coating on the surface of the steel sheet. The insulating coating to be formed is not limited, and may be a known coating.

[Magnetic Domain Refinement Step]

The method of manufacturing a grain-oriented electrical steel sheet according to the embodiment may further include a magnetic domain refinement step of performing magnetic domain refinement on the steel sheet.
By performing the magnetic domain refinement treatment, iron loss of the grain-oriented electrical steel sheet can be further reduced.
The method of the magnetic domain refinement treatment is not limited. There is a method for narrowing the width of a 180° magnetic domain (performing refinement of a 180° magnetic domain) by forming linear or dotted groove parts extending in a direction intersecting a rolling direction at predetermined intervals in the rolling direction, and a method for narrowing the width of a 180° magnetic domain (performing refinement of a 180° magnetic domain) by forming linear or dotted stress-strain parts or groove parts extending in a direction intersecting a rolling direction at predetermined intervals in the rolling direction.
In a case where a stress-strain part is formed, laser beam irradiation, electron beam irradiation, and the like can be applied. In a case where a groove part is formed, a mechanical groove-forming method using a gear or the like, a chemical groove-forming method by electrolytic etching, a thermal groove-forming method by laser irradiation, and the like can be applied.
In a case where the insulating coating is damaged due to formation of a stress-strain part or a groove part and characteristics such as insulation properties are deteriorated, the insulating coating may be formed again to repair the damage.

Examples

By casting, a silicon steel material containing, in terms of mass%, 3.30% of Si, 0.052% of C, 0.027% of acid-soluble Al, 0.008% of N, and a balance of Fe and impurities is obtained.
The silicon steel material is heated to 1150°C and then hot-rolled to prepare a hot rolled sheet having a sheet thickness of 2.3 mm.
This hot rolled sheet is subjected to annealing (hot rolled sheet annealing) in which the hot rolled sheet is heated to 1100°C, then cooled to 900°C, and held for 30 seconds.
The hot rolled sheet after the hot rolled sheet annealing is cold-rolled to a thickness of 0.22 mm to obtain a steel sheet (cold rolled sheet). A sample steel sheet having a length of 200 mm in the rolling direction and a length of 600 mm in the width direction is cut out from the steel sheet.
Thereafter, the sample steel sheet is subjected to decarburization annealing in which the temperature is raised to 840°C and held for 120 seconds in a wet atmosphere gas containing 75% of hydrogen and 25% of nitrogen to generate primary recrystallization. In addition, nitriding treatment is performed at least in one timing of: during the temperature raising and soaking process in the decarburization annealing step; between the decarburization annealing step and the final annealing step; and during the temperature raising process in the final annealing step and before the start of the secondary recrystallization, and the nitrogen amount after the final nitriding treatment step is controlled to be 160 to 380 ppm. In the table, for example, "Between decarburization annealing step and final annealing step" means that nitriding treatment is performed one time "between the completion of the decarburization annealing step and the start of the final annealing step", and "Between decarburization annealing step and final annealing step; and during temperature raising process in final annealing step and before start of secondary recrystallization" means that nitriding treatment is performed two times, "between the completion of the decarburization annealing step and the start of the final annealing step" and "from the start of the final annealing step to the start of the secondary recrystallization in the temperature raising process in the final annealing step".
After the decarburization annealing, the steel sheet is applied with an annealing separator containing MgO as a main component, and then subjected to final annealing such that the steel sheet end has a high temperature, and a temperature gradient of 0 to 5.0 °C/cm is applied to the entire region in the direction perpendicular to the rolling direction (width direction) to generate secondary recrystallization. The average temperature rising rate from the start of the secondary recrystallization to the completion of the secondary recrystallization in the boundary region is 10 °C/hr, the final annealing temperature is 1200°C, and the soaking time is 30 hours.
The temperature gradient is uniformly applied throughout the sample steel sheet. The temperature gradient is applied by increasing the temperature such that the furnace has a temperature difference therein. The magnitude of the temperature gradient is controlled by raising the temperature while measuring the temperature at intervals of 100 mm in the width direction of the steel sheet.
A sample of 60 mm in the width direction and 200 mm in the rolling direction is collected from the obtained steel sheet, and magnetic properties of this sample is measured by the SST method (see JISC2556:2015 Annex JA) to measure the magnetic flux density B8 in the rolling direction.

Tables 1A and 1B show the results.

[Table 1A]

Sample No.	Nitriding treatment step		Final annealing step		Magnetic flux density B8 (T)	Note
	Nitriding treatment step		Temperature raising process
	Performing timing	Nitrogen amount after nitriding treatment (ppm)	Temperature gradient (°C/cm)	Proportion of N₂ gas in atmosphere (mass%)
1	Between decarburization annealing step and final annealing step	210	1.5	10	1.961	Inventive Example
2	During decarburization annealing step	210	1.5	10	1.961	Inventive Example
3	Between decarburization annealing step and final annealing step; and during temperature raising process in final annealing step and before start of secondary recrystallization	210	1.5	10	1.961	Inventive Example
4	During decarburization annealing step; and during temperature raising process in final annealing step and before start of secondary recrystallization	210	1.5	10	1.961	Inventive Example
5	Between decarburization annealing step and final annealing step; during decarburization annealing step; and during temperature raising process in final annealing step and before start of secondary recrystallization	210	1.5	10	1.961	Inventive Example
6	Between decarburization annealing step and final annealing step; and during decarburization annealing step	210	1.5	10	1.961	Inventive Example
7	During decarburization annealing step	210	1.5	20	1.962	Inventive Example
8	Between decarburization annealing step and final annealing step; and during temperature raising process in final annealing step and before start of secondary recrystallization	210	1.5	20	1.962	Inventive Example
9	During decarburization annealing step; and during temperature raising process in final annealing step and before start of secondary recrystallization	210	1.5	20	1.962	Inventive Example
10	Between decarburization annealing step and final annealing step; during decarburization annealing step; and during temperature raising process in final annealing step and before start of secondary recrystallization	210	1.5	20	1.962	Inventive Example
11	Between decarburization annealing step and final annealing step; and during decarburization annealing step	210	1.5	20	1.962	Inventive Example

[Table 1B]

Sample No.	Nitriding treatment step		Final annealing step		Magnetic flux density B8 (T)	Note
	Nitriding treatment step		Temperature raising process
	Performing timing	Nitrogen amount after nitriding treatment (ppm)	Temperature gradient (°C/cm)	Proportion of N₂ gas in atmosphere (mass%)
12	Between decarburization annealing step and final annealing step	160	0.0	10	1.910	Comparative Example
13	Between decarburization annealing step and final annealing step	160	0.5	10	1.922	Comparative Example
14	Between decarburization annealing step and final annealing step	160	1.0	10	1.932	Comparative Example
15	Between decarburization annealing step and final annealing step	160	1.5	10	1.939	Comparative Example
16	During decarburization annealing step	160	1.5	10	1.939	Comparative Example
17	Between decarburization annealing step and final annealing step; and during temperature raising process in final annealing step and before start of secondary recrystallization	160	1.5	10	1.939	Comparative Example
18	During decarburization annealing step; and during temperature raising process in final annealing step and before start of secondary recrystallization	160	1.5	10	1.939	Comparative Example
19	Between decarburization annealing step and final annealing step; during decarburization annealing step; and during temperature raising process in final annealing step and before start of secondary recrystallization	160	1.5	10	1.939	Comparative Example
20	Between decarburization annealing step and final annealing step; and during decarburization annealing step	160	1.5	10	1.939	Comparative Example
21	Between decarburization annealing step and final annealing step	210	0.0	10	1.920	Comparative Example
22	Between decarburization annealing step and final annealing step	210	0.5	10	1.944	Inventive Example
23	Between decarburization annealing step and final annealing step	210	1.0	10	1.954	Inventive Example
24	Between decarburization annealing step and final annealing step	210	1.5	10	1.960	Inventive Example
25	Between decarburization annealing step and final annealing step	210	2.0	10	1.964	Inventive Example
26	Between decarburization annealing step and final annealing step	210	5.0	10	1.967	Inventive Example

[Table 1C]

Sample No.	Nitriding treatment step		Final annealing step		Magnetic flux density B8 (T)	Note
	Nitriding treatment step		Temperature raising process
	Performing timing	Nitrogen amount after nitriding treatment (ppm)	Temperature gradient (°C/cm)	Proportion of N₂ gas in atmosphere (mass%)
27	Between decarburization annealing step and final annealing step	270	0.9	10	1.961	Inventive Example
28	During decarburization annealing step	270	0.9	10	1.961	Inventive Example
29	Between decarburization annealing step and final annealing step; and during temperature raising process in final annealing step and before start of secondary recrystallization	270	0.9	10	1.961	Inventive Example
30	During decarburization annealing step; and during temperature raising process in final annealing step and before start of secondary recrystallization	270	0.9	10	1.961	Inventive Example
31	Between decarburization annealing step and final annealing step; during decarburization annealing step; and during temperature raising process in final annealing step and before start of secondary recrystallization	270	0.9	10	1.961	Inventive Example
32	Between decarburization annealing step and final annealing step; and during decarburization annealing step	270	0.9	10	1.961	Inventive Example
33	Between decarburization annealing step and final annealing step	300	0.0	10	1.925	Comparative Example
34	Between decarburization annealing step and final annealing step	300	0.5	10	1.955	Inventive Example
35	Between decarburization annealing step and final annealing step	300	1.0	10	1.962	Inventive Example
36	Between decarburization annealing step and final annealing step	300	1.5	10	1.967	Inventive Example
37	Between decarburization annealing step and final annealing step	300	0.5	20	1.956	Inventive Example
38	Between decarburization annealing step and final annealing step	300	1.0	20	1.962	Inventive Example
39	Between decarburization annealing step and final annealing step	300	1.5	20	1.963	Inventive Example
40	Between decarburization annealing step and final annealing step	350	0.0	10	1.925	Comparative Example

[Table 1D]

Sample No.	Nitriding treatment step		Final annealing step		Magnetic flux density B8 (T)	Note
	Nitriding treatment step		Temperature raising process
	Performing timing	Nitrogen amount after nitriding treatment (ppm)	Temperature gradient (°C/cm)	Proportion of N₂ gas in atmosphere (mass%)
41	Between decarburization annealing step and final annealing step	350	0.5	10	1.958	Inventive Example
42	Between decarburization annealing step and final annealing step	350	1.0	10	1.964	Inventive Example
43	Between decarburization annealing step and final annealing step	350	1.5	10	1.967	Inventive Example
44	Between decarburization annealing step and final annealing step	350	0.5	20	1.962	Inventive Example
45	Between decarburization annealing step and final annealing step	350	1.0	20	1.965	Inventive Example
46	Between decarburization annealing step and final annealing step	350	1.5	20	1.968	Inventive Example
47	Between decarburization annealing step and final annealing step	380	0.0	10	1.925	Comparative Example
48	Between decarburization annealing step and final annealing step	380	0.5	10	1.960	Inventive Example
49	Between decarburization annealing step and final annealing step	380	1.0	10	1.965	Inventive Example
50	Between decarburization annealing step and final annealing step	380	1.5	10	1.967	Inventive Example
51	During decarburization annealing step	380	1.5	10	1.967	Inventive Example
52	Between decarburization annealing step and final annealing step; and during temperature raising process in final annealing step and before start of secondary recrystallization	380	1.5	10	1.967	Inventive Example
53	During decarburization annealing step; and during temperature raising process in final annealing step and before start of secondary recrystallization	380	1.5	10	1.967	Inventive Example
54	Between decarburization annealing step and final annealing step; during decarburization annealing step; and during temperature raising process in final annealing step and before start of secondary recrystallization	380	1.5	10	1.967	Inventive Example

[Table 1E]

Sample No.	Nitriding treatment step		Final annealing step		Magnetic flux density B8 (T)	Note
	Nitriding treatment step		Temperature raising process
	Performing timing	Nitrogen amount after nitriding treatment (ppm)	Temperature gradient (°C/cm)	Proportion of N₂ gas in atmosphere (mass%)
55	Between decarburization annealing step and final annealing step; and during decarburization annealing step	380	1.5	10	1.967	Inventive Example
56	Between decarburization annealing step and final annealing step; and during decarburization annealing step	210	0.0	0	1.923	Comparative Example
57	Between decarburization annealing step and final annealing step	210	0.5	0	1.944	Inventive Example
58	Between decarburization annealing step and final annealing step	210	1.0	0	1.954	Inventive Example
59	Between decarburization annealing step and final annealing step	210	1.5	0	1.960	Inventive Example
60	Between decarburization annealing step and final annealing step	210	0.0	20	1.925	Comparative Example
61	Between decarburization annealing step and final annealing step	210	0.5	20	1.949	Inventive Example
62	Between decarburization annealing step and final annealing step	210	1.0	20	1.957	Inventive Example
63	Between decarburization annealing step and final annealing step	210	1.5	20	1.962	Inventive Example
64	Between decarburization annealing step and final annealing step	210	0.0	50	1.926	Comparative Example
65	Between decarburization annealing step and final annealing step	210	0.5	50	1.951	Inventive Example
66	Between decarburization annealing step and final annealing step	210	1.0	50	1.959	Inventive Example
67	Between decarburization annealing step and final annealing step	210	1.5	50	1.963	Inventive Example
68	Between decarburization annealing step and final annealing step	210	0.0	80	1.927	Comparative Example
69	Between decarburization annealing step and final annealing step	210	0.5	80	1.954	Inventive Example

[Table 1F]

Sample No.	Nitriding treatment step		Final annealing step		Magnetic flux density B8 (T)	Note
	Nitriding treatment step		Temperature raising process
	Performing timing	Nitrogen amount after nitriding treatment (ppm)	Temperature gradient (°C/cm)	Proportion of N₂ gas in atmosphere (mass%)
70	Between decarburization annealing step and final annealing step	210	1.0	80	1.960	Inventive Example
71	Between decarburization annealing step and final annealing step	210	1.5	80	1.964	Inventive Example
72	Between decarburization annealing step and final annealing step	210	0.0	100	1.927	Comparative Example
73	Between decarburization annealing step and final annealing step	210	0.5	100	1.955	Inventive Example
74	Between decarburization annealing step and final annealing step	210	1.0	100	1.960	Inventive Example
75	Between decarburization annealing step and final annealing step	210	1.5	100	1.964	Inventive Example
76	Between decarburization annealing step and final annealing step	210	0.4	30	1.938	Comparative Example
77	During decarburization annealing step	210	0.4	30	1.938	Comparative Example
78	Between decarburization annealing step and final annealing step; and during temperature raising process in final annealing step and before start of secondary recrystallization	210	0.4	30	1.938	Comparative Example
79	During decarburization annealing step; and during temperature raising process in final annealing step and before start of secondary recrystallization	210	0.4	30	1.938	Comparative Example
80	Between decarburization annealing step and final annealing step; during decarburization annealing step; and during temperature raising process in final annealing step and before start of secondary recrystallization	210	0.4	30	1.938	Comparative Example
81	Between decarburization annealing step and final annealing step; and during decarburization annealing step	210	0.4	30	1.938	Comparative Example

As can be seen from Tables 1A to 1F, when the nitrogen amount after the final nitriding treatment is 210 ppm or more, a high magnetic flux density (magnetic flux density B8 is 1.940 T or more) can be achieved under a temperature gradient of 0.5 °C/cm or more, regardless of the timing and the number of times of the nitriding treatment. Furthermore, under the condition that the formula (1) is satisfied, a higher magnetic flux density (magnetic flux density B8 is 1.960 T or more) can be achieved.

A silicon steel material (slab) having a chemical composition shown in Table 2 (unit: mass%, balance: Fe and impurities) is obtained by casting.
The silicon steel material is heated to 1100 to 1200°C and then hot-rolled to prepare a hot rolled sheet having a sheet thickness of 2.3 mm.
This hot rolled sheet is subjected to annealing (hot rolled sheet annealing) in which the hot rolled sheet is heated to 1100°C, then cooled to 900°C, and held for 30 seconds.
The hot rolled sheet after the hot rolled sheet annealing is cold-rolled to a thickness of 0.22 mm to obtain a steel sheet (cold rolled sheet). A sample steel sheet having a length of 200 mm in the rolling direction and a length of 600 mm in the width direction is cut out from the steel sheet.
Thereafter, the sample steel sheet is subjected to decarburization annealing in a wet atmosphere gas containing 75% of hydrogen and 25% of nitrogen to generate primary recrystallization. Nitriding treatment is performed between the decarburization annealing step and the final annealing step, and the nitrogen amount is controlled to be 210 ppm. The nitriding treatment is performed one time between the completion of the decarburization annealing step and the start of the final annealing step.
After the decarburization annealing, the steel sheet is applied with an annealing separator containing MgO as a main component, and then subjected to final annealing such that the steel sheet end has a high temperature, and a temperature gradient of 0.5 °C/cm is applied to the entire region in the direction perpendicular to the rolling direction (width direction) to generate secondary recrystallization. The final annealing temperature is 1150 to 1250°C, and the soaking time is 10 to 30 hours. In the temperature raising process in the final annealing step, the atmosphere in a furnace from the start of the secondary recrystallization to the completion of the secondary recrystallization includes N₂ gas in a proportion of 30% in terms of mass%.
The temperature gradient is uniformly applied throughout the sample steel sheet. The temperature gradient is applied by increasing the temperature such that the furnace has a temperature difference therein. The magnitude of the temperature gradient is controlled by raising the temperature while measuring the temperature at intervals of 100 mm in the width direction of the steel sheet.
A sample of 60 mm in the width direction and 200 mm in the rolling direction is collected from the obtained steel sheet, and magnetic properties of this sample is measured by the SST method (see JISC2556:2015 Annex JA) to measure the magnetic flux density B8 in the rolling direction.

Table 2 shows the results.

[Table 2]

Sample No	Chemical composition (mass%) balance : Fe and impurities										Magnetic flux density B8 (T)	Note
Sample No	Si	C	sol. Al	N	Mn	S + Se	Cr	Cu	P	Ni	Magnetic flux density B8 (T)	Note
82	3.30	0.052	0.027	0.008	-	-	-	-	-	-	1.944	Inventive Example
83	2.70	0.050	0.027	0.008	-	-	-	-	-	-	1.950	Inventive Example
84	4.00	0.062	0.027	0.008	-	-	-	-	-	-	1.940	Inventive Example
85	4.80	0.083	0.027	0.007	-	-	-	-	-	-	1.940	Inventive Example
86	3.30	0.085	0.027	0.008	-	-	-	-	-	-	1.944	Inventive Example
87	3.10	0.035	0.027	0.008	-	-	-	-	-	-	1.945	Inventive Example
88	3.30	0.058	0.015	0.004	-	-	-	-	-	-	1.940	Inventive Example
89	3.30	0.052	0.020	0.006	-	-	-	-	-	-	1.940	Inventive Example
90	3.30	0.052	0.040	0.012	-	-	-	-	-	-	1.942	Inventive Example
91	3.20	0.052	0.027	0.008	0.14	-	-	-	-	-	1.952	Inventive Example
92	3.20	0.052	0.029	0.009	0.45	-	-	-	-	-	1.946	Inventive Example
93	3.00	0.052	0.030	0.009	0.75	-	-	-	-	-	1.941	Inventive Example
94	3.20	0.052	0.027	0.008	0.14	0.007	-	-	-	-	1.950	Inventive Example
95	3.00	0.052	0.030	0.009	0.75	0.015	-	-	-	-	1.940	Inventive Example
96	3.30	0.052	0.027	0.008	-	-	0.15	-	-	-	1.944	Inventive Example
97	3.30	0.035	0.027	0.008	-	-	0.25	-	-	-	1.942	Inventive Example
98	3.30	0.060	0.029	0.009	-	-	-	0.2	-	-	1.945	Inventive Example
99	3.10	0.060	0.029	0.009	-	-	-	0.4	-	-	1.943	Inventive Example
100	3.20	0.052	0.027	0.008	-	-	-	-	0.2	-	1.943	Inventive Example
101	3.20	0.052	0.027	0.008	-	-	-	-	0.4	-	1.941	Inventive Example
102	3.20	0.052	0.027	0.008	-	-	-	-	-	0.20	1.944	Inventive Example
103	3.00	0.052	0.030	0.009	-	-	-	-	-	0.75	1.940	Inventive Example

As can be seen from Table 2, an excellent magnetic flux density B8 is obtained in all of the chemical compositions.

INDUSTRIAL APPLICABILITY

According to the present invention, it is possible to provide a method of manufacturing a grain-oriented electrical steel sheet in which final annealing is performed while applying a temperature gradient in the boundary region between the primary recrystallization region and the secondary recrystallization region to manufacture a grain-oriented electrical steel sheet having a high magnetic flux density, the method being a method of manufacturing a grain-oriented electrical steel sheet capable of obtaining a sufficient magnetic flux density improving effect even under a small temperature gradient. Therefore, the present invention has high industrial applicability.

Claims

A method of manufacturing a grain-oriented electrical steel sheet, the method comprising:
a hot rolling step of heating a silicon steel material containing, in terms of mass%, 0.80 to 7.00% of Si to a temperature of 1280°C or lower, and hot rolling the silicon steel material, to obtain a hot rolled sheet;

a hot-rolled sheet annealing step of annealing the hot rolled sheet as necessary;

a cold rolling step of cold-rolling the hot rolled sheet after the hot rolling step or after the hot-rolled sheet annealing step to obtain a steel sheet having a final sheet thickness;

a decarburization annealing step of decarburization-annealing the steel sheet after the cold rolling step;

an annealing separator applying step of applying an annealing separator on the steel sheet after the decarburization annealing step, and coiling the steel sheet into a coil shape;

a nitriding treatment step of increasing a nitrogen amount in the steel sheet; and

a final annealing step of final-annealing the steel sheet coiled into a coil shape,

wherein the final annealing step has a temperature raising process and a soaking process, and a temperature gradient of 0.5 °C/cm or more is generated in a boundary region between a primary recrystallization region and a secondary recrystallization region at least in one period from a start of secondary recrystallization to completion of the secondary recrystallization in the temperature raising process; and

the nitriding treatment step is performed by annealing in an atmosphere containing a gas having nitriding ability at least in one stage of: during the decarburization annealing step; between the decarburization annealing step and the final annealing step; and during the temperature raising process in the final annealing step and before the start of the secondary recrystallization, so that the nitrogen amount in the steel sheet is 210 ppm or more based on mass after the nitriding treatment step.
The method of manufacturing a grain-oriented electrical steel sheet according to claim 1, wherein
in the temperature raising process in the final annealing step, an atmosphere in a furnace from the start of the secondary recrystallization to the completion of the secondary recrystallization includes N₂ gas in a proportion of 20% or more in terms of mass%.
The method of manufacturing a grain-oriented electrical steel sheet according to claim 1 or 2, wherein
the nitrogen amount in the steel sheet is 210 ppm or more and 350 ppm or less based on mass after the nitriding treatment step.
The method of manufacturing a grain-oriented electrical steel sheet according to claim 1 or 2, wherein
the formula (1) below is satisfied when [N] is defined as the nitrogen amount in the steel sheet after the nitriding treatment step in terms of ppm based on mass; [N₂] is defined as the proportion of N₂ gas in terms of mass% in an atmosphere in a furnace in the final annealing step; and [T] is defined as the temperature gradient in the final annealing step in terms of a unit of °C/cm, $[T] \geq 1 / (0.0026 \times [N]) - 0.005 \times [N_{2}] - 0.5$
The method of manufacturing a grain-oriented electrical steel sheet according to claim 3, wherein
the formula (1) below is satisfied when [N] is defined as the nitrogen amount in the steel sheet after the nitriding treatment step in terms of ppm based on mass; [N₂] is defined as the proportion of N₂ gas in terms of mass% in an atmosphere in a furnace in the final annealing step; and [T] is defined as the temperature gradient in the final annealing step in terms of a unit of °C/cm,. $[T] \geq 1 / (0.0026 \times [N]) - 0.005 \times [N_{2}] - 0.5$
The method of manufacturing a grain-oriented electrical steel sheet according to claim 1 or 2, wherein
the silicon steel material includes, as a chemical composition, in terms of mass%, 0.80 to 7.00% of Si, 0.085% or less of C, 0.010 to 0.065% of acid-soluble Al, 0.004 to 0.012% of N, 0 to 1.00% of Mn, 0 to 0.30% of Cr, 0 to 0.4% of Cu, 0 to 0.5% of P, 0 to 1.00% of Ni, 0 to 0.015% in total of S and Se, and a balance of Fe and impurities.
The method of manufacturing a grain-oriented electrical steel sheet according to claim 3, wherein
the silicon steel material includes, as a chemical composition, in terms of mass%, 0.80 to 7.00% of Si, 0.085% or less of C, 0.010 to 0.065% of acid-soluble Al, 0.004 to 0.012% of N, 0 to 1.00% of Mn, 0 to 0.30% of Cr, 0 to 0.4% of Cu, 0 to 0.5% of P, 0 to 1.00% of Ni, 0 to 0.015% in total of S and Se, and a balance of Fe and impurities.
The method of manufacturing a grain-oriented electrical steel sheet according to claim 4, wherein
the silicon steel material includes, as a chemical composition, in terms of mass%, 0.80 to 7.00% of Si, 0.085% or less of C, 0.010 to 0.065% of acid-soluble Al, 0.004 to 0.012% of N, 0 to 1.00% of Mn, 0 to 0.30% of Cr, 0 to 0.4% of Cu, 0 to 0.5% of P, 0 to 1.00% of Ni, 0 to 0.015% in total of S and Se, and a balance of Fe and impurities.
The method of manufacturing a grain-oriented electrical steel sheet according to claim 5, wherein
the silicon steel material includes, as a chemical composition, in terms of mass%, 0.80 to 7.00% of Si, 0.085% or less of C, 0.010 to 0.065% of acid-soluble Al, 0.004 to 0.012% of N, 0 to 1.00% of Mn, 0 to 0.30% of Cr, 0 to 0.4% of Cu, 0 to 0.5% of P, 0 to 1.00% of Ni, 0 to 0.015% in total of S and Se, and a balance of Fe and impurities.