Grain-oriented electrical steel sheet and method for manufacturing same

Abstract

This grain-oriented electrical steel sheet has a base steel sheet, an optional glass coating film that is formed on the surface of the base steel sheet, and an optional insulating coating film that is formed on the surface of the base steel sheet or on the surface of the glass coating film. The base steel sheet has a chemical composition that contains, in terms of mass%, 0.0005-0.0050% of C, 1.0-7.0% of Si, 0.0030-0.0300% in total of one or more elements selected from among Nb group elements that are Nb, Ta, V, and Mo, 0.0001-0.0300% of acid-insoluble Al, 0.0001-0.0200% of Mg, 0.0001-0.0200% of O, and 0.0001-0.0100% of N. In a region of the base steel sheet ranging from a position of t / 20 from the surface to a position of (19t) / 20 from the surface, the number ratio of the precipitates that are in the form of composite precipitates and have a region in which the Nb group element is detected as a first peak by EDS analysis is 50% or more with respect to all the precipitates that contain the Nb group element.

Description

Grain-oriented electrical steel sheet and its manufacturing method

[0001] The present invention relates to a grain-oriented electrical steel sheet and a method for manufacturing the same. This application claims priority based on Japanese Patent Application No. 2024-034142, filed on March 6, 2024, the contents of which are incorporated herein by reference.

[0002] Grain-oriented electrical steel sheets contain approximately 7 mass% or less of Si and have a secondary recrystallization texture concentrated in the {110}<001> orientation (Goss orientation). The {110}<001> orientation means that the {110} plane of the crystal is aligned parallel to the rolling surface and the <001> axis of the crystal is aligned parallel to the rolling direction.

[0003] The magnetic properties of grain-oriented electrical steel sheets are significantly affected by the degree of concentration of the {110}<001> orientation. In particular, the relationship between the rolling direction of the steel sheet, which is the primary magnetization direction during use, and the crystallographic <001> direction, which is the easy magnetization direction, is considered to be important. Therefore, in recent years, the angle between the crystallographic <001> direction and the rolling direction has been controlled to within a range of approximately 5° in grain-oriented electrical steel sheets.

[0004] Such precise crystal orientation control is achieved by dispersing fine precipitates called inhibitors in the steel before final annealing and then maintaining the steel sheet at a high temperature during the final annealing. For example, the inhibitors enhance the selective growth of Goss-oriented grains, resulting in secondary recrystallization during the final annealing so that Goss-oriented grains grow preferentially. To date, attempts have been made to precisely control the inhibitors in order to precisely control the crystal orientation.

[0005] For example, Patent Document 1 discloses using MnS as an inhibitor and performing two cold rolling passes. Patent Documents 2 and 3 disclose controlling MnS+AlN and MnS (and / or MnSe)+Sb as inhibitors, respectively. Patent Document 4 discloses a technique for preferably controlling inhibitors in order to lower the slab heating temperature for the purpose of reducing production costs.

[0006] Patent Document 5 discloses controlling the primary recrystallized grain size and its dispersion related to inhibitors. Patent Documents 6 to 8 disclose adding Nb, V, etc. to grain-oriented electrical steel sheets.

[0007] Furthermore, Patent Documents 9 to 11 disclose techniques for improving magnetostriction by precisely controlling the atmosphere and residence time during finish annealing to form sub-boundaries within secondary recrystallized grains. These techniques demonstrate the technical idea of ​​expanding the temperature range in which secondary recrystallization progresses in order to form sub-boundaries, and also show that an improvement in magnetic flux density can be expected.

[0008] Japanese Patent Publication No. 30-3651 Japanese Patent Publication No. 40-15644 Japanese Patent Publication No. 51-13469 Japanese Patent Publication No. 62-40315 Japanese Patent Publication No. 2008-261022 Japanese Patent Publication No. 52-024116 Japanese Patent Publication No. 02-200732 Japanese Patent No. 4962516 International Publication No. 2020 / 027215 International Publication No. 2020 / 027218 International Publication No. 2020 / 027219

[0009] In recent years, amid a global trend toward power and energy conservation and other efforts to protect the global environment, there has been an increasing demand for more efficient transformers. In this social environment, there is also a demand for improved performance of grain-oriented electrical steel sheets, which are used as iron core materials in transformers. In particular, there is a demand for increasing the magnetic flux density of grain-oriented electrical steel sheets, thereby improving their iron loss characteristics (reducing iron loss).

[0010] As a result of investigations by the present inventors, it has been found that the conventional inhibitor control techniques disclosed in the above Patent Documents 1 to 8 do not fully satisfy the requirements for grain-oriented electrical steel sheets, and that further increases in magnetic flux density are necessary.

[0011] Furthermore, the techniques of Patent Documents 9 to 11, as described above, show that by precisely controlling the final annealing, the secondary recrystallization temperature range is expanded, the preferential growth of Goss-oriented grains closer to the ideal Goss orientation represented by the strict crystallographic {110}<001> orientation is promoted, and the magnetic flux density of the steel sheet is improved while forming subgrain boundaries that improve magnetic properties such as magnetostriction of the steel sheet, which affects the noise characteristics of transformers. At the same time, it has been shown that the addition of trace amounts of Nb, Ta, Mo, etc. (hereinafter sometimes referred to as "Nb-group elements") increases the frequency of subgrain boundaries, thereby enhancing the effect of improving magnetic properties, and easing the finish annealing conditions under which the effect of improving magnetic properties is realized. This is thought to mean that elements such as Nb function effectively as a specific method for expanding the secondary recrystallization temperature range and improving the preferential growth of Goss-oriented grains. In particular, since Nb-group elements form carbides, nitrides, and carbonitrides that decompose at lower temperatures than AlN, it is thought that the decomposition of precipitates of these Nb-group elements affects secondary recrystallization. On the other hand, in these techniques, the precipitates of Nb group elements added in small amounts tend to precipitate as carbonitrides in the steel. Therefore, if a large amount of Nb remains after finish annealing, even if the C content in the steel is reduced to 0.0050% or less, there is a concern that fine carbides will precipitate in the steel sheet when used as a transformer core, causing a phenomenon known as magnetic aging that deteriorates the iron loss characteristics.

[0012] In view of the above, an object of the present invention is to provide a grain-oriented electrical steel sheet in which magnetic aging is suppressed and which has excellent core loss characteristics, and a method for manufacturing the same.

[0013] As described above, Nb group elements contribute to improving magnetic properties and easing finish annealing conditions, but there have been concerns that they may cause magnetic aging. As a result of studies by the present inventors, it has been found that in grain-oriented electrical steel sheets containing Nb group elements, magnetic aging can be suppressed by precipitating Nb-based precipitates (precipitates of Nb group elements) not alone but in combination with fine nitrides, oxides, or sulfides.

[0014] The present invention has been made in light of the above findings. [1] A grain-oriented electrical steel sheet according to one aspect of the present invention comprises: a base steel sheet; optionally, a glass coating formed on a surface of the base steel sheet; and optionally, an insulating coating formed on the surface of the base steel sheet or on the surface of the glass coating, wherein the base steel sheet contains, in mass %, C: 0.0005 to 0.0050%, Si: 1.0 to 7.0%, one or more Nb group elements of Nb, Ta, V, and Mo: 0.0030 to 0.0300% in total, acid-insoluble Al: 0.0001 to 0.0300%, Mg: 0.0001 to 0.0200%, O: 0.0001 to 0.0200%, N: 0.0001 to 0.0100%, Mn: 0 to 1.00%, and a chemical composition consisting of S: 0 to 0.0150%, Se: 0 to 0.0150%, Cu: 0 to 0.40%, Bi: 0 to 0.010%, B: 0 to 0.080%, P: 0 to 0.50%, Sn: 0 to 0.10%, Sb: 0 to 0.10%, Cr: 0 to 0.30%, Ni: 0 to 1.00%, Ti: 0 to 0.0050%, Ca: 0 to 0.0050%, and the balance: Fe and impurities, wherein, when the thickness of the base steel plate is t, precipitates observed by a TEM replica method in a region from a position t / 20 to a position (19t) / 20 in the thickness direction of the base steel plate The proportion of precipitates that have a region in which an Nb group element is detected as a first peak by EDS analysis and are composite precipitates to all precipitates containing the Nb group element is 50% or more.[2] A method for producing a grain-oriented electrical steel sheet according to another embodiment of the present invention comprises, in mass %, C: 0.0010 to 0.1000%, Si: 1.0 to 7.0%, at least one selected from the group consisting of Nb, Ta, V, and Mo: 0.0030 to 0.0300% in total, Mn: 0.05 to 1.00%, S: 0 to 0.0350%, Se: 0 to 0.0350%, acid-soluble Al: 0.0100 to 0.0650%, Mg: 0.0001 to 0.0100%, O: 0.0001 to 0.0200%, N: 0.0040 to 0.0120%, Cu: 0 to 0.40%, Bi: 0 to 0.010%, B: 0 to 0.080%, a casting step of casting molten steel having a chemical composition consisting of P: 0 to 0.50%, Sn: 0 to 0.10%, Sb: 0 to 0.10%, Cr: 0 to 0.30%, Ni: 0 to 1.00%, Ti: 0 to 0.0050%, Ca: 0 to 0.0050%, and the balance: Fe and impurities to form a slab; a hot rolling step of heating the slab and hot rolling it to form a hot rolled steel sheet; a hot rolled sheet annealing step of annealing the hot rolled steel sheet; a cold rolling step of cold rolling the hot rolled steel sheet after the hot rolled sheet annealing step to form a cold rolled steel sheet; a decarburization annealing step of decarburization annealing the cold rolled steel sheet; and a finish annealing step of applying an annealing separator to the cold rolled steel sheet after the decarburization annealing step, and then performing finish annealing. and a stress relief annealing process in which the cold-rolled steel sheet after the finish annealing process is annealed, wherein in the decarburization annealing process, the C content of the cold-rolled steel sheet is set to 0.0050% or less, and in the finish annealing process, the residence time at 1210°C or higher is set to 2 hours or less and the residence time at 1180°C or higher is set to 5 to 30 hours, and the residence time at 950 to 800°C during cooling is set to 5 hours or more, and in the stress relief annealing process, the residence time at 860°C or higher is set to 30 seconds or less and the residence time at 750°C or higher is set to 50 seconds or more.

[0015] According to the above aspects of the present invention, it is possible to provide a grain-oriented electrical steel sheet in which magnetic aging is suppressed and which has excellent core loss characteristics, and a method for manufacturing the same.

[0016] FIG. 1 is a photograph of a composite precipitate of NbC observed by a TEM replica method, and a diagram showing the results of EDS analysis of two areas (cross intersections) where the precipitate is present.

[0017] A grain-oriented electrical steel sheet according to one embodiment of the present invention (grain-oriented electrical steel sheet according to this embodiment) and a method for manufacturing the same will be described below. However, the present invention is not limited to the configuration disclosed in this embodiment, and various modifications are possible within the scope of the present invention.

[0018] A grain-oriented electrical steel sheet according to one embodiment of the present invention (grain-oriented electrical steel sheet according to the present embodiment) has a base steel sheet having a predetermined chemical composition. The grain-oriented electrical steel sheet according to the present embodiment may also have a base steel sheet and a glass coating formed on the surface of the base steel sheet. The grain-oriented electrical steel sheet according to the present embodiment may also have a base steel sheet, a glass coating formed on the surface of the base steel sheet, and an insulating coating formed on the surface of the glass coating. The grain-oriented electrical steel sheet according to the present embodiment may also have a base steel sheet and an insulating coating formed on the surface of the base steel sheet. Furthermore, when the thickness of the base steel plate is t, in a region from the surface to t / 20 to (19t) / 20 in the plate thickness direction of the base steel plate, in precipitates observed by TEM replica method, the precipitates have a region where an Nb group element is detected as a first peak by EDS analysis and are precipitated in a composite manner, and the number ratio of the precipitates to all precipitates containing Nb group elements is 50% or more (i.e., the ratio of Nb-based precipitates that are precipitated in a composite manner is 50 to 100%). Each of these will be explained below.

[0019] [Base steel sheet] <Chemical composition> The chemical composition of the base steel sheet of the grain-oriented electrical steel sheet according to this embodiment (which can be said to be the chemical composition of the grain-oriented electrical steel sheet when the glass coating and insulating coating are not included) is within the following range. In the numerical ranges indicated below with "to" between, the values ​​at both ends are included as the lower and upper limits of the range. Numerical values ​​indicated as "greater than" or "less than" are not included in the numerical range. Furthermore, "%" regarding the chemical composition means "mass %" unless otherwise specified.

[0020] The base steel sheet of the grain-oriented electrical steel sheet according to this embodiment contains, as a chemical composition, basic elements, optional elements as needed, and the balance being Fe and impurities.

[0021] The base steel sheet of the grain-oriented electrical steel sheet according to this embodiment contains, as basic elements (main alloying elements), in mass %, C: 0.0005 to 0.0050%, Si: 1.0 to 7.0%, acid-insoluble Al: 0.0001 to 0.0300%, Mg: 0.0001 to 0.0200%, one or more of the Nb group elements Nb, Ta, V, and Mo: 0.0030 to 0.0300% in total, O: 0.0001 to 0.0200%, and N: 0.0001 to 0.0100%.

[0022] C: 0.0005 to 0.0050% Carbon (C) is an effective element for controlling the primary recrystallization structure during the manufacturing process. In particular, C is effective for controlling the primary recrystallization structure because it forms carbides and carbonitrides with elements such as Nb. Generally, C is purified during the final annealing process, resulting in a reduced C content. However, if purification is insufficient, excessive C content in the final product (grain-oriented electrical steel sheet) can degrade magnetic properties. Furthermore, C combines with Si, Ti, etc., and precipitates as fine carbides or carbonitrides, causing magnetic aging. A C content exceeding 0.0050% significantly deteriorates magnetic properties, so the C content is set to 0.0050% or less. The C content is preferably 0.0030% or less, and more preferably 0.0020% or less. While a lower C content is preferable, it is difficult to achieve a C content of less than 0.0005% given the productivity of industrial production. Therefore, the C content is set to 0.0005% or more.

[0023] Si: 1.0 to 7.0% Silicon (Si) is an element that increases the electrical resistance of grain-oriented electrical steel sheets and reduces iron loss. If the Si content is less than 1.0%, austenite transformation occurs during finish annealing, damaging the crystal orientation of the grain-oriented electrical steel sheets. Therefore, the Si content is set to 1.0% or more. The Si content is preferably 1.5% or more, and more preferably 3.0% or more. On the other hand, if the Si content exceeds 7.0%, cold workability decreases, making cracks more likely to occur during cold rolling. Therefore, the Si content of grain-oriented electrical steel sheets is set to 7.0% or less. The Si content is preferably 5.5% or less, and more preferably 4.0% or less.

[0024] Acid-insoluble Al (Insol. Al): 0.0001 to 0.0300% Acid-soluble Al is an important element because it causes the preferential growth of Goss-oriented grains, which occurs during decomposition, to occur in two stages due to two types of precipitates, Nb-based precipitates and Al-based precipitates, thereby improving the magnetic flux density of the steel sheet. Most of the acid-soluble Al contained in the slab becomes acid-insoluble Al after final annealing and remains as oxide in the glass coating, but some remains in the steel, and Al ２ O ３ This Al forms complex oxides with Mg, such as Al and spinel (including inclusions formed during steelmaking). ２ O ３ Complex oxides with Mg, such as magnesium oxide and spinel, effectively function as precipitation sites for Nb-based precipitates and suppress the occurrence of magnetic aging due to Nb-based precipitates. To achieve the above effect, the acid-insoluble Al content is set to 0.0001% or more. On the other hand, if the acid-insoluble Al content exceeds 0.0300%, the amount of Al-containing oxides in the steel becomes too large, deteriorating the iron loss of the material. Therefore, the acid-insoluble Al content is set to 0.0300% or less. It is preferably 0.0200% or less, and more preferably 0.0100% or less.

[0025] Mg: 0.0001 to 0.0200% Magnesium (Mg) is an element that forms composite oxides such as spinel with Al, effectively functions as a precipitation site for Nb-based precipitates, and has the effect of suppressing the occurrence of magnetic aging due to Nb-based precipitates. To achieve this effect, the Mg content is 0.0001% or more. The Mg content is preferably 0.0002% or more. On the other hand, if the Mg content exceeds 0.0200%, the amount of Mg-containing oxides in the steel becomes too large, resulting in deterioration of the iron loss of the material. Therefore, the Mg content is 0.0200% or less. Preferably, it is 0.0100% or less. Mg is not added as a slab steel component, but may be mixed in from furnace materials during steelmaking, or part of MgO, the main component of the annealing separator during finish annealing, may diffuse into the steel. Even in these cases, the Mg content may be within the above-mentioned range.

[0026] One or more of Nb, Ta, V, and Mo: 0.0030 to 0.0300% in total. Niobium (Nb), tantalum (Ta), vanadium (V), and molybdenum (Mo) precipitate as carbides, nitrides, or carbonitrides, which act as auxiliary inhibitors, and preferably function as inhibitors. Specifically, they preferably expand the secondary recrystallization progression temperature range. As a result, Goss-oriented grains grow favorably, and the magnetic flux density of the final grain-oriented electrical steel sheet is preferably increased. To achieve this effect, at least one element selected from the group consisting of Nb, Ta, V, and Mo is contained in an amount of 0.0030% or more. In this embodiment, Nb, Ta, V, and Mo may be collectively referred to as "Nb group elements." Preferably, the content of the Nb group elements is greater than 0.0050%, and in terms of the magnetic enhancement effect, the total content of Nb and Ta is more preferably greater than 0.0050%. On the other hand, if the total content of Nb group elements exceeds 0.0300%, the precipitation temperature range of the Nb group element precipitates becomes high, and the Nb group element precipitates tend to become coarse and low-density. Furthermore, the difference between the precipitation temperature range of the Nb group element precipitates and the precipitation temperature range of MnS and AlN becomes large, making it difficult for the Nb group element precipitates to effectively function as precipitation nuclei for refining MnS and AlN. Furthermore, carbonitrides of Nb group elements (Nb-based carbonitrides) in grain-oriented electrical steel sheets deteriorate the magnetic properties. Therefore, the total content of Nb group elements is set to 0.0300% or less. The total content of Nb group elements is preferably 0.0200% or less, and more preferably 0.0100% or less.

[0027] O: 0.0001 to 0.0200% O is an element that forms fine oxides that precipitate together with Nb-based precipitates. To achieve this effect, the O content is set to 0.0001% or more. On the other hand, if the O content exceeds 0.0200%, the deterioration of iron loss becomes significant. The O content is preferably 0.0150% or less, and more preferably 0.0100% or less.

[0028] N: 0.0001 to 0.0100% N is an element that forms fine nitrides that precipitate together with Nb-based precipitates. To achieve this effect, the N content is set to 0.0001% or more. On the other hand, if the N content exceeds 0.0100%, the deterioration of iron loss becomes significant. The N content is preferably 0.0050% or less, and more preferably 0.0030% or less.

[0029] The base steel sheet of the grain-oriented electrical steel sheet according to this embodiment may contain impurities as a chemical composition. "Impurities" refer to elements that are mixed in from raw materials such as ore or scrap, or from the manufacturing environment, during industrial steel production. The upper limit of the total impurity content may be, for example, 5.0%.

[0030] Furthermore, the base steel sheet of the grain-oriented electrical steel sheet according to this embodiment may contain optional elements in addition to the above-described basic elements and impurities. For example, instead of a portion of the remaining Fe, one or more of Mn, S, Se, Cu, Bi, B, P, Sn, Sb, Cr, Ni, Ti, and Ca may be contained as optional elements. These optional elements may be contained according to the purpose. Therefore, there is no need to set a lower limit for these optional elements, and the lower limit may be 0%. Furthermore, even if these optional elements are contained as impurities, the above-described effects are not impaired.

[0031] Mn: 0 to 1.00% Manganese (Mn) is an element that combines with S and Se to precipitate as MnS or MnSe, functioning as an inhibitor. If the Mn content exceeds 1.00%, the amount of MnS or MnSe that functions as an inhibitor will be excessive, hindering the appropriate progress of secondary recrystallization. In this embodiment, part of the inhibitor function may be performed by carbides, nitrides, or carbonitrides of Nb group elements. In this case, the amount of MnS or MnSe that functions as an inhibitor may be controlled to be small. Therefore, the Mn content is set to 1.00% or less. The Mn content is preferably 0.50% or less, and more preferably 0.20% or less. On the other hand, the lower limit of the Mn content of the base steel sheet is not particularly limited and may be 0%, but since it has been confirmed that MnS or MnSe may be formed to have the effect of acting as an inhibitor during secondary recrystallization, or may be used to control the precipitation of AlN, and that MnS or Mn silicate may function as a precipitation site for Nb-based precipitates to prevent magnetic aging, the Mn content may be 0.001% or more. The Mn content is preferably 0.02% or more.

[0032] S: 0-0.0150% Se: 0-0.0150% Cu: 0-0.40% Bi: 0-0.010% B: 0-0.080% P: 0-0.50% Sn: 0-0.10% Sb: 0-0.10% Cr: 0-0.30% Ni: 0-1.00% Sulfur (S), selenium (Se), copper (Cu), bismuth (Bi), boron (B), phosphorus (P), tin (Sn), antimony (Sb), chromium (Cr), and nickel (Ni) may be contained within the above ranges according to known purposes. There is no need to set a lower limit for the content of these optional elements, and 0% may be sufficient. When Mn is contained in steel, the remaining S and Se form sulfides and selenides and function as precipitation sites for Nb-based precipitates. Therefore, the total content of S and Se is preferably 0.0001% or more. On the other hand, since a large amount of sulfides and selenides deteriorates magnetic properties, the S content and Se content are preferably 0.0150% or less and 0.0050% or less, respectively. Copper (Cu) affects precipitates and primary recrystallization texture, and has the effect of improving secondary recrystallization. Taking into account contamination from scrap, etc., the Cu content may be 0.01% or more. To improve secondary recrystallization, it is preferably 0.05% or more, more preferably 0.10% or more. On the other hand, from the perspective of improving magnetic properties and avoiding cost increases due to element addition, the Cu content is preferably 0.40% or less. Bismuth (Bi) is an element that has the effect of improving secondary recrystallization. Therefore, it may be added. To achieve the above effects, the Bi content is preferably 0.001% or more. On the other hand, if the Bi content exceeds 0.010%, it may be difficult to form a good glass coating on the finished sheet. Therefore, the Bi content is preferably 0.010% or less. Boron (B) is an element that precipitates or segregates and has the effect of improving secondary recrystallization. Therefore, it may be contained. To achieve the above effects, the B content is preferably 0.001% or more. On the other hand, if the B content exceeds 0.080%, it may be difficult to form a good glass coating on the finished sheet. Therefore, the B content is preferably 0.080% or less. Phosphorus (P), tin (Sn), and antimony (Sb) are elements that, as segregating elements, have the effect of stably realizing secondary recrystallization and are effective in forming a glass coating.Therefore, it may be contained. To obtain the above effects, the P content is preferably 0.005% or more, more preferably 0.01% or more. The Sn content is preferably 0.01% or more. The Sb content is preferably 0.01% or more. On the other hand, if the P content exceeds 0.50%, the Sn content exceeds 0.10%, or the Sb content exceeds 0.10%, it may be difficult to form a glass coating. Therefore, it is preferable that the P content be 0.50% or less, the Sn content be 0.10% or less, and the Sb content be 0.10% or less. The P content is more preferably 0.30% or less, even more preferably 0.10% or less, and even more preferably 0.05% or less. The Sn content is more preferably 0.09% or less. The Sb content is more preferably 0.05% or less. Chromium (Cr) is an element that affects oxidation during decarburization annealing and is effective in forming a glass coating. Therefore, it may be contained. To obtain the above effects, the Cr content is preferably 0.02% or more. On the other hand, if the Cr content exceeds 0.30%, it may be difficult to form a glass coating. Therefore, the Cr content is preferably 0.30% or less. The Cu content is more preferably 0.20% or less. Nickel (Ni) is an element that contributes to the uniform dispersion of precipitates and has the effect of improving secondary recrystallization. Therefore, it may be contained. To obtain the above effects, the Ni content is preferably 0.005% or more, more preferably 0.01% or more. On the other hand, if the Ni content exceeds 1.00%, it may be difficult to decarburize and form a glass coating. Therefore, the Ni content is set to 1.00% or less. From the viewpoint of cost, the Ni content is more preferably 0.30% or less.

[0033] Ti: 0 to 0.0050% Titanium (Ti) exists in the base steel sheet of grain-oriented electrical steel sheet as precipitates such as TiN, and reduces the magnetic properties of the steel sheet. Therefore, the Ti content is set to 0.0050% or less. The Ti content is preferably 0.0025% or less. On the other hand, Ti forms precipitates such as TiN that effectively function as precipitation sites for Nb-based precipitates, similar to Al-Mg-based oxides. Therefore, to obtain the above effect, the Ti content may be set to 0.0002% or more. Ti is present in the ferrosilicon raw material or in the annealing separator containing TiO. ２ When the element is contained as a sintered material, it may be mixed into the steel sheet even if it is not intentionally added. Even in this case, it is acceptable as long as the content is within the above range.

[0034] Ca: 0 to 0.0050% Calcium (Ca) is an element that deteriorates magnetic properties. If the Ca content exceeds 0.0050%, the deterioration of magnetic properties becomes significant, so the Ca content is set to 0.0050% or less. On the other hand, Ca-based precipitates such as CaO effectively function as precipitation sites for Nb-based precipitates. Therefore, the Ca content may be set to 0.0001% or more. Even if not intentionally added, Ca may be contained even after final annealing due to fluorite during steelmaking decarburization or Ca contained as an impurity in MgO, an annealing separator. Even in this case, the Ca content should be within the above range.

[0035] The chemical composition of the base steel sheet of the grain-oriented electrical steel sheet according to this embodiment may be measured by a general steel analysis method. For example, the chemical composition of the base steel sheet of the grain-oriented electrical steel sheet may be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). Specifically, the chemical composition is identified by measuring a 35 mm square test piece taken from the grain-oriented electrical steel sheet using ICP-AES under conditions based on a pre-created calibration curve. C and S may be measured using a combustion-infrared absorption method, and N may be measured using an inert gas fusion-thermal conductivity method. If a glass coating and an insulating coating are formed on the surface of the steel sheet, these may be removed before measuring the components. Specifically, the grain-oriented electrical steel sheet having an insulating coating is immersed in a solution of 30 to 50 mass% NaOH and H ２ The insulating coating can be removed by immersing the grain-oriented electrical steel sheet in an aqueous sodium hydroxide solution at 80 to 90°C containing 50 to 70% by mass of sodium hydroxide for 7 to 10 minutes. The grain-oriented electrical steel sheet from which the insulating coating has been removed can be rinsed with water and then dried with a hot air blower for just under 1 minute. Furthermore, if a glass coating is formed underneath the insulating coating, the glass coating can be removed by immersing the grain-oriented electrical steel sheet with the glass coating in an aqueous hydrochloric acid solution at 80 to 90°C containing 30 to 40% by mass of HCl for 1 to 10 minutes. After immersion, the base steel sheet can be rinsed with water and then dried with a hot air blower for just under 1 minute. The above steps allow the base steel sheet to be removed from the grain-oriented electrical steel sheet on which the glass coating and / or insulating coating has been formed.

[0036] <Precipitates> As mentioned above, grain-oriented electrical steel sheets containing Nb group elements have sometimes exhibited magnetic aging, which deteriorates the iron loss characteristics of the grain-oriented electrical steel sheet, which is the material for the iron core, when used in transformers. The first suspected cause of this is the Nb group elements that remain in the steel even after final annealing. Nb group elements that are also present in the steel are likely to form carbonitrides, so it is thought that magnetic aging may occur even with a C content that would normally not cause deterioration in iron loss characteristics. The inventors have investigated the effects of magnetic aging on the insulation coating, known as a secondary coating, mainly composed of colloidal silica and aluminum phosphate, and the forsterite (Mg ２ SiO ４ When the content of Nb-group elements in a steel sheet (base steel sheet) from which a glass coating called a primary coating mainly composed of Nb-group elements was removed was analyzed, it was found that when the content of Nb-group elements was 0.0030% or more, magnetic aging occurred when the remaining C content was 0.0005% or more. Although magnetic aging can be suppressed by reducing the C content, as described above, it is not easy to make the C content less than 0.0005% in consideration of industrial production. Therefore, it was found that in practical steel sheets, when the content of Nb-group elements is 0.0030% or more, the problem of magnetic aging is an unavoidable issue.

[0037] However, as a result of further investigations, the present inventors have found that even within the above-mentioned composition range, magnetic aging is suppressed when Nb-based precipitates are not precipitated alone but are precipitated in a composite with spinel-based oxides (Al—Mg—O), MnS, TiN, CaO, etc. Specifically, when the thickness of the base steel sheet is t, the precipitates observed by TEM replica method in the region from the surface at position t / 20 to position (19t) / 20 in the thickness direction of the base steel sheet have a region in which an Nb-group element is detected as a first peak by EDS analysis, and the number ratio of the precipitates that are composite precipitates to all precipitates containing Nb-group elements is 50% or more (i.e., the proportion of Nb-based precipitates that are composite precipitates is 50 to 100%). It is presumed that the reason why magnetic aging is suppressed by the composite precipitation is that the composite precipitation of Nb group elements causes the precipitates to become coarse and suppresses the precipitation as fine carbides during use of the transformer. The reason why the target precipitates are in the region from the surface to the position of t / 20 to the position of (19t) / 20 is that the state of the precipitates may be different in the extreme surface layers (the range from the front and back surfaces to less than t / 20), and the effect on the properties of the steel sheet is small.

[0038] The composite form of the precipitates is not limited, but examples include a form in which spinel-based oxides (Al—Mg—O), MnS, TiN, and CaO exist in the center like a precipitation nucleus, surrounded by Nb-based precipitates, or a form in which Nb-based precipitates are superimposed on the spinel-based oxides (Al—Mg—O), MnS, TiN, and CaO when the size of the spinel-based oxides (Al—Mg—O), MnS, TiN, and CaO is large, i.e., 0.5 μm or larger. For example, in the composite precipitate shown in FIG. 1 , as can be seen from the EDS analysis results for measurement point 139 on the right, the detection intensities of Al, Mg, and O are high in the center, indicating the presence of spinel-based oxides (Al—Mg—O) like a precipitation nucleus. Furthermore, as can be seen from the EDS analysis results for measurement point 142 on the left, the spinel-based oxides (Al—Mg—O) are surrounded by Nb-based precipitates. The average size of the composite precipitates having a region where the Nb group elements have a first peak is preferably 10 nm or more, more preferably 100 nm or more, because if the average size is too small, the number of precipitates increases, deteriorating the iron loss characteristics of the steel sheet itself. For the same amount of precipitates, the larger the average size, the better the magnetic properties. However, to achieve this, a sufficient holding time is required in a predetermined temperature range to precipitate coarse Nb-based precipitates such as NbC, which increases industrial costs. Therefore, the average size of the composite precipitates is preferably 1000 nm or less, more preferably 500 nm or less.

[0039] The proportion of precipitates that have a region in which an Nb-group element is detected as a first peak by EDS analysis and that are composite precipitates, i.e., the proportion of composite precipitates of Nb-based precipitates, is determined by the following method: A sample for TEM observation prepared by the carbon extraction replica method is observed at positions t / 20 to (19t) / 20 (e.g., position t / 2), and a region showing the presence of integrated precipitates (where peaks of elements other than the sample substrate are detected by EDS measurement) surrounded by a precipitate-free space (a region in which only the peak of C, an element of the sample substrate, and the material components of the grid (e.g., Cu or Ni) are observed by EDS measurement and where the peak intensity is low (a region that can usually be determined by a person performing TEM observation without EDS analysis)) is found. EDS measurement is performed on areas where a contrast in shade that is thought to be due to the components is present, and the EDS measurement energy profiles indicating the presence of elements are different between the two areas. It is determined that composite precipitates are present. For example, Nb-based precipitates are determined to be composite precipitates when, among the different contrasts within the precipitates, one peak indicates the presence of an Nb-group element and the other peak is extremely weak, or when the peak of the Nb-group element in one peak is not clearly weaker than the other peak, but the peaks of elements other than the Nb-group element (e.g., Al, Mg, Si, Ca, Mn, Ti, S, N, and O) are stronger. Here, the carbon extraction replica method is a method for preparing a TEM observation sample by extracting fine precipitates and inclusions present in steel from the base steel sheet (parent phase). The preparation procedure involves polishing the steel to be observed and then corroding (etching) the parent steel. Carbon is then vapor-deposited on the corroded surface, followed by immersion in a stripping solution that elutes only the parent phase without dissolving the precipitates. The released replica film is then washed and placed on a TEM observation grid, completing the TEM observation sample.For example, a sample cut into a size of 20 mm x 20 mm x plate thickness is polished with sandpaper (starting from #80 and gradually increasing the number to #1500) so that the surface in the plate thickness direction becomes the observation surface, and then buffed with 6 μm and then 1 μm in order. The obtained sample is then subjected to preliminary etching to remove surface dirt (the amount of coulombs is determined by the sample surface area (cm)) using a SPEED etching device (for example, Fujiwara Seisakusho: FV-138) with an electrolyte (89 mass% methanol, 10 mass% acetylacetone, 1 mass% tetramethylammonium chloride) at a step potential of -200 mV. ２ ) / 100[C]), and then the main etching (the amount of coulombs is determined by the sample surface area (cm ２) / 10 [C]). Carbon deposition of approximately 30 nm is performed on the electropolished surface thus obtained (for example, using a JFE-400 manufactured by JEOL). Then, methyl acetate and the sample are placed in a petri dish and immersed for approximately 2 hours under fluorescent light. Once the replica film floats up, the film is immersed in a new methyl acetate solution for approximately 30 minutes and washed. The film is then scooped up with a Cu mesh for observation and allowed to air dry for approximately 30 seconds to obtain a sample for observation. Furthermore, replica samples for TEM observation are prepared on a surface perpendicular to the direction normal to the steel plate surface (ND direction) at positions t / 20 to (19t) / 20, for example, at a position 1 / 2 of the plate thickness from the surface of the steel plate (t / 2 position). However, if there is a possibility that the precipitation state of the precipitates differs between the t / 2 position and near the surface of the steel plate, for example, the t / 20 layer, the plate thickness to be observed may be increased and the plate thickness averaged to calculate the proportion of composite precipitation. Specifically, samples for TEM observation can be prepared by polishing the specimen in increments of 1 / 20 of the plate thickness, and the composite precipitation ratio can be calculated as the average value of a total of 19 positions, from t / 20 to (19t) / 20. Furthermore, during observation, precipitates are observed spatially evenly across 100 or more fields of view from the TEM observation grid (usually 3 mm diameter) at a magnification of 10,000 times, and the average size and number density of the precipitates can be calculated. Additionally, 100 or more precipitates are randomly selected, and EDS analysis is performed at a magnification of 50,000 times on at least two areas within each precipitate, namely, areas with high and low contrast, to determine whether Nb-group elements are detected and whether the precipitate is a composite precipitate. This allows the proportion of composite precipitates among Nb-based precipitates to be quantified in increments of at least 1%.

[0040] During the above observation, the long and short sides of the precipitates are measured, and the size of each precipitate is determined as the average value. The average size can be calculated based on the measurements of at least 50 precipitates.

[0041] [Glass Coating] In the grain-oriented electrical steel sheet according to this embodiment, a glass coating may be formed on the surface of the base steel sheet. The glass coating is an inorganic coating containing magnesium silicate as a main component. The glass coating is formed during finish annealing by a reaction between an annealing separator containing magnesia (MgO) applied to the surface of the base steel sheet and the components of the surface of the base steel sheet, and has a composition derived from the components of the annealing separator and the base steel sheet (more specifically, Mg ２ SiO ４ When an annealing separator containing alumina as the main component is used during the final annealing, the glass film may not be formed.

[0042] [Insulating Coating] In the grain-oriented electrical steel sheet according to this embodiment, an insulating coating may be formed on the surface of the base steel sheet or the surface of the glass coating. The insulating coating imparts electrical insulation to the grain-oriented electrical steel sheet, thereby reducing eddy current loss and improving the iron loss characteristics of the grain-oriented electrical steel sheet. The insulating coating has the function of applying tension to the grain-oriented electrical steel sheet. Applying tension to the grain-oriented electrical steel sheet facilitates domain wall motion in the grain-oriented electrical steel sheet, thereby improving the iron loss characteristics of the grain-oriented electrical steel sheet. Furthermore, the insulating coating provides various properties such as corrosion resistance, heat resistance, and slip resistance in addition to the electrical insulation described above. In the grain-oriented electrical steel sheet according to this embodiment, the insulating coating may be a known coating formed, for example, by applying a coating liquid containing phosphate and colloidal silica as main components to the surface of the glass coating (forsterite coating) and baking it.

[0043] [Manufacturing Method] The grain-oriented electrical steel sheet according to this embodiment can achieve the effects described above as long as it has the characteristics described above, regardless of the manufacturing method, but is preferably manufactured stably by a manufacturing method including the following steps: (I) a casting step of casting molten steel having a predetermined chemical composition to form a slab, (II) a hot rolling step of heating the slab and hot-rolling it to form a hot-rolled steel sheet, (III) a hot-rolled sheet annealing step of heating the hot-rolled steel sheet to a maximum temperature and then cooling it to anneal it, (IV) a cold-rolling step of cold-rolling the hot-rolled steel sheet after the hot-rolled sheet annealing step to form a cold-rolled steel sheet, (V) a decarburization annealing step of decarburization annealing the cold-rolled steel sheet, (VI) a finish annealing step of applying an annealing separator to the cold-rolled steel sheet after the decarburization annealing step and then performing finish annealing, and (VII) a stress relief annealing step of annealing the cold-rolled steel sheet after the finish annealing step. Preferred conditions for each step will be explained below.

[0044] "Casting Process" In the casting process, a slab is prepared. An example of a method for producing a slab is as follows: Molten steel is produced (smelted). A slab is produced using the molten steel. The slab may be produced by continuous casting. The molten steel may be used to produce an ingot, which is then bloomed to produce a slab. The thickness of the slab is, for example, 150 to 350 mm. The thickness of the slab is preferably 220 to 280 mm. A so-called thin slab having a thickness of 10 to 70 mm may be used as the slab. When a thin slab is used, rough rolling before finish rolling can be omitted in the hot rolling process.

[0045] The chemical composition of the slab is determined taking into consideration changes in the chemical composition (such as C and inhibitor components) that occur during processing so as to obtain the final chemical composition of the grain-oriented electrical steel sheet. For example, the slab may contain the following elements, with the balance being Fe and impurities:

[0046] C: 0.0010 to 0.1000% Carbon (C) is an element that is effective in controlling the primary recrystallization structure during the manufacturing process. Therefore, the C content of the slab is set to 0.0010% or more. On the other hand, an excessive C content in the final product has a negative effect on the magnetic properties. If the C content of the slab is excessive, it may not be possible to sufficiently reduce the C even with decarburization annealing. Therefore, the C content of the slab is set to 0.1000% or less.

[0047] Si: 1.0 to 7.0% Silicon (Si) increases the electrical resistance of grain-oriented electrical steel sheets and reduces iron loss. If the Si content is less than 1.0%, austenite transformation occurs during finish annealing, damaging the crystal orientation of the grain-oriented electrical steel sheets. Therefore, the Si content is 1.0% or more. The Si content is preferably 2.5% or more, and more preferably 3.0% or more. On the other hand, if the Si content exceeds 7.0%, cold workability decreases, making cracks more likely to occur during cold rolling. Therefore, the Si content is 7.0% or less. The Si content is preferably 4.5% or less, and more preferably 4.0% or less.

[0048] At least one element selected from the group consisting of Nb, V, Mo, and Ta: total 0.0030 to 0.0300%. Nb-based carbonitrides have the effect of improving the selective growth of Goss-oriented grains during secondary recrystallization. Therefore, 0.0030% or more is contained in the slab. On the other hand, if the total content of Nb-group elements exceeds 0.0300%, the decomposition temperature of precipitates of Nb-group elements during secondary recrystallization becomes high, and the difference with the decomposition temperature of AlN becomes small. This results in the loss of the effect of expanding the secondary recrystallization temperature range and the loss of the effect of improving magnetic flux density. Therefore, the total content of Nb-group elements is set to 0.0300% or less. The total content of Nb-group elements is preferably 0.0200% or less, and more preferably 0.0100% or less.

[0049] Mn: 0.05 to 1.00% Manganese (Mn) combines with S or Se to produce MnS or MnSe, which functions as an inhibitor. A Mn content within the range of 0.05 to 1.00% is preferred because it stabilizes secondary recrystallization. In this embodiment, part of the inhibitor function can be performed by nitrides of Nb group elements. In this case, the strength of MnS or MnSe as a general inhibitor is controlled to be weak. For this reason, the Mn content is preferably 0.50% or less, and more preferably 0.20% or less.

[0050] S: 0 to 0.0350% Se: 0 to 0.0350% Sulfur (S) and selenium (Se) combine with Mn to form MnS or MnSe, which function as inhibitors. The S content may be 0 to 0.0350%, and the Se content may be 0 to 0.0350%. When at least one of S and Se is contained, a total S and Se content of 0.0030 to 0.0350% is preferred because secondary recrystallization is stabilized. In this embodiment, part of the inhibitor function can be performed by nitrides of Nb group elements. In this case, the strength of MnS or MnSe, which is a typical inhibitor, is controlled to be weak. For this reason, the preferred upper limit of the total S and Se content is 0.0250%, and more preferably 0.0100%. If S and Se remain after final annealing, they form compounds that deteriorate iron loss. Therefore, it is preferable to reduce S and Se as much as possible by purification during final annealing.

[0051] Here, "the total content of S and Se is 0.0030 to 0.0350%" may mean that the chemical composition of the slab contains only one of S or Se, and the total content of either S or Se is 0.0030 to 0.0350%, or that the slab contains both S and Se, and the total content of S and Se is 0.0030 to 0.0350%.

[0052] Acid-soluble Al (sol. Al): 0.0100 to 0.0650% Aluminum (Al) combines with N to precipitate as (Al,Si)N, functioning as an inhibitor. When the acid-soluble Al content is within the range of 0.0100 to 0.0650%, AlN, which acts as an inhibitor and is formed by nitriding (described later), expands the secondary recrystallization temperature range, stabilizing secondary recrystallization particularly in the high-temperature range, which is preferable. The acid-soluble Al content is preferably 0.0200% or more, more preferably 0.0250% or more. From the viewpoint of secondary recrystallization stability, the Al content is preferably 0.0400% or less, more preferably 0.0350% or less.

[0053] N: 0.0040 to 0.0120% Nitrogen (N) is an element that combines with Al and precipitates as AlN or (Al,Si)N, functioning as an inhibitor. In order to favorably control the form of these inhibitors (precipitates), the N content is set to 0.0040% or more. On the other hand, if the N content exceeds 0.0120%, blisters, a type of defect, are more likely to occur in the steel sheet, so the N content is set to 0.0120% or less.

[0054] O: 0.0001 to 0.0200% O is an element that forms fine oxides that precipitate together with Nb-based precipitates. To achieve this effect, the O content is set to 0.0001% or more. Since the O content depends on the ore grade, deoxidation time, etc., in reality, in consideration of costs, it is preferably 0.0010% or more, and more preferably 0.0020% or more. On the other hand, if the O content exceeds 0.0200%, the deterioration of iron loss becomes significant. Therefore, the O content is set to 0.0200% or less. The O content is preferably 0.0150% or less, and more preferably 0.0100% or less.

[0055] Mg: 0.0001 to 0.0100% Mg is an element that forms Al-Mg oxides such as spinel, and forms precipitates that are used as precipitation sites for Nb-based precipitates. Therefore, the Mg content is set to 0.0001% or more. On the other hand, if the Mg content exceeds 0.0100%, the amount of Mg-containing oxides in the steel becomes too large, deteriorating the iron loss of the material. Therefore, the Mg content is set to 0.0100% or less. The Mg content is preferably 0.0050% or less.

[0056] Cu: 0-0.40%, Bi: 0-0.010%, B: 0-0.080%, P: 0-0.50%, Sn: 0-0.10%, Sb: 0-0.10%, Cr: 0-0.30%, Ni: 0-1.00%, Ti: 0-0.0050%, Ca: 0-0.0050%. The slabs used to manufacture the grain-oriented electrical steel sheet according to this embodiment may contain optional elements in addition to the above-described basic elements and impurities. For example, instead of a portion of the remaining Fe, one or more of Cu, Bi, B, P, Sn, Sb, Cr, Ni, Ti, and Ca may be contained as optional elements. These optional elements may be added according to their purpose, and their content may be the same as the content in the grain-oriented electrical steel sheet that is ultimately desired. There is no need to set a lower limit for these optional elements; the lower limit may be 0%. Furthermore, even if these selected elements are contained as impurities, the above effects are not impaired.

[0057] [Hot Rolling Process] In the hot rolling process, the slab is heated and hot rolled (rough rolling and finish rolling) to produce a hot-rolled steel sheet. The conditions are not particularly limited. In the hot rolling process, a hot-rolled steel sheet having a thickness of, for example, 1.8 to 3.5 mm may be produced. After finish rolling, the hot-rolled steel sheet may be coiled at a predetermined temperature. The slab heating temperature is not limited, but when using an innate inhibitor that completely dissolves precipitates used as inhibitors during hot rolling and forms secondary inhibitors used for secondary recrystallization during hot rolling and hot-rolled sheet annealing, a temperature of 1280 to 1400°C is preferred. If the temperature is too high, the crystal grains become coarse, causing secondary recrystallization defects known as linear fine grains. If the temperature is too low, complete solution of the precipitates on the skid is insufficient, resulting in variations in magnetic properties due to non-uniform precipitation of the precipitates. Furthermore, when nitriding the steel sheet in a subsequent process and using precipitates formed during finish annealing, so-called acquired inhibitors, as secondary inhibitors during secondary recrystallization, the slab heating temperature is preferably 1050 to 1250°C. If the slab heating temperature exceeds 1250°C, the primary recrystallization structure will become mixed due to the uneven precipitation state of fine precipitates reprecipitated after re-solution and coarse precipitates remaining in the solution, preventing good secondary recrystallization. Furthermore, the soaking time at the slab heating temperature is preferably 30 minutes or more, more preferably 60 minutes or more, in order to reduce temperature unevenness within the slab. On the other hand, a long soaking time reduces productivity, so the soaking time is preferably 600 minutes or less, more preferably 300 minutes or less.

[0058] [Hot-rolled sheet annealing process] The hot-rolled sheet annealing process is a process in which the hot-rolled steel sheet obtained in the hot-rolling process is annealed under predetermined temperature conditions and then cooled to obtain an annealed steel sheet before cold rolling. The conditions may be any known method (e.g., annealing temperature of 750 to 1200°C, holding for 30 seconds to 10 minutes, and then cooling), as long as they are determined to provide good magnetic properties. The maximum temperature is adjusted to 1000 to 1150°C, and the soaking time is adjusted. Then, if necessary, soaking is performed at an intermediate temperature (e.g., 1000 to 800°C) called a secondary soaking temperature during cooling. Adjusting the subsequent cooling rate, etc., can favorably induce secondary recrystallization, resulting in good magnetic properties. This is preferable.

[0059] [Cold Rolling Step] In the cold rolling step, the hot-rolled steel sheet after the hot-rolled sheet annealing step is subjected to a single cold rolling or multiple (two or more) cold rolling steps via annealing (intermediate annealing) to form a cold-rolled steel sheet having a thickness of, for example, 0.10 to 0.50 mm. In the cold rolling step, the reduction ratio of the cold rolling may be controlled to 80 to 95%.

[0060] The cold rolling reduction mentioned above means the cumulative cold rolling reduction in the case where intermediate annealing is not performed, or the cumulative cold rolling reduction after the final intermediate annealing in the case where intermediate annealing is performed. Specifically, the cold rolling reduction is defined as follows: Cold rolling reduction (cumulative reduction) (%) = (1 - "thickness of steel sheet after cold rolling" / "thickness of steel sheet before cold rolling (or after intermediate annealing)") x 100

[0061] When the cold rolling reduction is within the above range, the primary recrystallization texture after decarburization annealing is preferably controlled. Specifically, the primary recrystallization texture becomes a texture that facilitates the preferential growth of ideal Goss-oriented grains (grains with an ideal {110}<001> orientation) during secondary recrystallization (specifically, the primary recrystallization texture has crystal orientations represented by {111}<112> and {411}<148> as its main orientations). As a result, it is preferable that the final product has a secondary recrystallization texture in which ideal Goss-oriented grains grow preferentially.

[0062] "Decarburization Annealing Step" The decarburization annealing step is a step in which the cold-rolled steel sheet obtained in the cold rolling step is subjected to decarburization annealing (for example, at 700 to 900°C for 60 to 180 seconds) to obtain a cold-rolled steel sheet (decarburization-annealed steel sheet) in which primary recrystallization has occurred. By subjecting the cold-rolled steel sheet to decarburization annealing, C contained in the cold-rolled steel sheet is removed. Decarburization annealing is preferably performed in a humid atmosphere to remove "C" contained in the cold-rolled steel sheet. A high content of Nb-group elements increases the amount of C remaining in the decarburization-annealed steel sheet, which can deteriorate the magnetic properties of the finished steel sheet. Therefore, in the manufacturing method of grain-oriented electrical steel sheet according to this embodiment, the effects of the present invention are achieved by performing the decarburization annealing step so that the C content after the decarburization annealing step is 0.0050% or less. The C content after decarburization annealing can be controlled by adjusting the dew point, humid atmosphere (oxidation degree), decarburization annealing temperature, decarburization annealing time, etc. during decarburization annealing. The C content after the decarburization annealing step can be measured by using a combustion-infrared absorption method on the steel sheet after the decarburization annealing step.

[0063] "Nitriding Step" In the manufacturing method of the grain-oriented electrical steel sheet according to this embodiment, a nitriding treatment may be further performed to adjust the strength of the inhibitor in secondary recrystallization. The nitriding treatment may be performed by increasing the nitrogen content of the steel sheet to approximately 0.004 to 0.040 mass% at any timing between the start of the decarburization annealing and the start of secondary recrystallization in the finish annealing. Preferably, the nitrogen content of the steel sheet after the nitriding treatment is 0.015 to 0.040 mass%. The nitriding method is not limited, but examples include annealing the steel sheet in an atmosphere containing a nitriding gas such as ammonia, and finish-annealing a decarburization-annealed steel sheet coated with an annealing separator containing a nitriding powder such as MnN. The N content after the nitriding treatment can be measured using an inert gas fusion-thermal conductivity method for the steel sheet after the nitriding treatment.

[0064] "Finish Annealing Step" "Stress Relief Annealing" In the manufacturing method of grain-oriented electrical steel sheet according to this embodiment, in the finish annealing step and stress relief annealing step, Al-Mg-based oxides, TiN, CaO, MnS, MnSe, and the like are used as precipitation nuclei to cause complex precipitation of Nb-based precipitates. Specifically, in the finish annealing step (purification annealing), an annealing separator is applied to the cold-rolled steel sheet after the decarburization annealing step or after further performing the nitriding treatment step, and then the steel is annealed. During annealing, the residence time at 1210°C or higher is set to 2 hours or less, the residence time at 1180°C or higher is set to 5 to 30 hours, and the residence time at 950 to 800°C during cooling is set to 5 hours or more. Furthermore, in the subsequent stress relief annealing step, the residence time at 860°C or higher is set to 30 seconds or less (including 0 seconds), and the residence time at 750°C or higher is set to 50 seconds or more. In the final annealing process, if the residence time at 1210°C or higher exceeds 2 hours, the solution of spinel, TiN, and MnS, which are precipitation sites for Nb-based precipitates such as NbC, progresses, and the number of precipitation sites for Nb-based precipitates within the crystal grains decreases. In this case, fine re-precipitates preferentially precipitate at secondary recrystallization grain boundaries, promoting heterogeneity of residual precipitates in the steel and significantly deteriorating magnetic properties. The residence time at 1180°C or higher is set to 5 to 30 hours to ensure the purification time, suppress heterogeneity due to ripening of the precipitation state of spinel, TiN, and MnS, and optimize the total number and spatial uniformity of precipitation sites for Nb-based precipitates. Furthermore, if the residence time at 950 to 800°C during cooling is less than 5 hours, precipitates that serve as precipitation sites for Nb-based precipitates may not be completely reprecipitated, or Nb-based precipitates may not be sufficiently coarsely reprecipitated at the precipitation sites, resulting in magnetic aging, in which the dissolved Nb precipitates finely with residual C during use of the transformer. The residence time is preferably 10 hours or more. There is no upper limit to the residence time, but a longer residence time reduces productivity, so it may be 50 hours or less. Furthermore, if the residence time at 860°C or higher exceeds 30 seconds in the stress relief annealing process, re-solution of Nb-based precipitates progresses, resulting in insufficient composite precipitation during stress relief annealing. From the perspective of uniformly and compositely precipitating Nb-based precipitates, the residence time at 860°C or higher may be greater than 0 seconds (e.g., 5 seconds or more).Furthermore, if the residence time at 750°C or higher is less than 50 seconds, the Nb-based precipitates that have re-solutioned during stress relief annealing will not be completely reprecipitated, resulting in magnetic aging. There is no upper limit to the residence time at 750°C or higher, but a longer time will reduce productivity, so it may be set to 300 seconds or less. Regarding the annealing separator, a known annealing separator may be applied by a known method. Strain relief annealing is preferably performed simultaneously with the baking annealing of the insulating coating. To control the oxidation state of the steel sheet surface, it is preferable to perform the treatment in a mixed atmosphere of hydrogen and nitrogen with a hydrogen content of less than 5% by volume, rather than in a mixed atmosphere of nitrogen containing 5% or more by volume of hydrogen. To ensure the adhesion of the insulating coating on grain-oriented electrical steel sheets with smoothed interfaces, known as mirror-finished GO, externally oxidized SiO is applied to the steel sheet surface. ２ When the dew point is controlled in a mixed atmosphere of 5% by volume or more of hydrogen and nitrogen in order to form an oxide film, SiO ２ It is necessary to control the heat cycle not only in the annealing for forming the oxide film but also in the annealing for baking the insulating coating.

[0065] The above steps produce a grain-oriented electrical steel sheet, but the method for producing a grain-oriented electrical steel sheet according to this embodiment may further include an insulating coating formation step of forming an insulating coating on the surface of the steel sheet (grain-oriented electrical steel sheet) after the stress relief annealing step, and a magnetic domain refinement step of refining magnetic domains in the grain-oriented electrical steel sheet with the insulating coating formed thereon. These steps further improve the magnetic properties.

[0066] "Insulating Coating Forming Step" An insulating coating may be formed on the surface of the grain-oriented electrical steel sheet. There are no limitations on the insulating coating to be formed, and any known coating may be used. In addition, known methods may be used for forming the insulating coating.

[0067] "Magnetic Domain Refinement Process" The magnetic domain refinement process may be performed by a known method. For example, there is a method in which linear or dot-like grooves extending in a direction intersecting the rolling direction are formed at predetermined intervals along the rolling direction to narrow the width of the 180° magnetic domains (to refine the 180° magnetic domains). Alternatively, if performed after the insulating coating formation process, there is a method in which linear or dot-like stress-strain portions or grooves extending in a direction intersecting the rolling direction are formed at predetermined intervals along the rolling direction to narrow the width of the 180° magnetic domains (to refine the 180° magnetic domains). Laser beam irradiation, electron beam irradiation, and the like can be used to form stress-strain portions. Furthermore, grooves can be formed by mechanical groove formation using gears or the like, chemical groove formation using electrolytic etching, thermal groove formation using laser irradiation, and the like. If the formation of stress-strain portions or grooves damages the insulating coating, resulting in deterioration of its insulating properties, the damage can be repaired by forming a new insulating coating.

[0068] Molten steel having the chemical composition shown in Tables 1-1 to 1-4 was cast to form a slab. The obtained slab was heated under the conditions shown in Tables 2-1 and 2-2, and hot-rolled to form a hot-rolled steel sheet. The obtained hot-rolled steel sheet was annealed by heating to 1100°C, soaking at this temperature for 10 seconds, and then furnace-cooling to the secondary soaking temperature of 900°C over 100 seconds. Thereafter, cooling was performed at an average cooling rate of 20°C / second between the secondary soaking temperature and 500°C. The average cooling rate between the secondary soaking temperature and 500°C is the temperature difference from the secondary soaking temperature to 500°C divided by the cooling time from the secondary soaking temperature to reach 500°C. The hot-rolled steel sheet after hot-rolled sheet annealing was cold-rolled under the conditions shown in Tables 2-3 and 2-4 to obtain a cold-rolled steel sheet. The obtained cold-rolled steel sheets were subjected to decarburization annealing in a nitrogen-hydrogen mixed atmosphere of 75% hydrogen by volume and 25% nitrogen by volume, with the dew point, soaking temperature, and soaking time conditions shown in Tables 2-3 and 2-4, and the C content was varied. The C content after the decarburization annealing process was equivalent to the chemical composition of the steel sheets shown in Tables 3-1 to 3-4. Thereafter, with the exception of some examples, a nitriding treatment was performed to increase the nitrogen content of the steel sheets. A known annealing separator mainly composed of MgO was then applied, and finish annealing was performed under the conditions shown in Tables 2-3 and 2-4, followed by stress relief annealing. Strain relief annealing was performed in a hydrogen-nitrogen atmosphere containing 3% hydrogen by volume.

[0069] The chemical compositions of the steel sheets (cold-rolled steel sheets) after stress relief annealing were investigated, and the results are shown in Tables 3-1 to 3-4.

[0070] In addition, an insulating coating mainly composed of colloidal silica and aluminum phosphate was formed on the steel sheet after stress relief annealing.

[0071] In the obtained steel sheet, the number ratio of the precipitates having a region where the Nb group element is detected as a first peak and which are composite precipitates at a position t / 2 from the surface in the sheet thickness direction of the base steel sheet, relative to all precipitates containing the Nb group element, and the average diameter thereof were determined by the method described above. The measurement was performed in the same manner as described above. A JEM-2100 transmission electron microscope manufactured by JEOL Ltd. was used, and an EX-24065JGP was used for EDS measurement.

[0072] In addition, using steel sheets sampled from a portion adjacent to the region where the precipitates were observed, the magnetic flux density B8 and iron loss W17 / 50 were evaluated by measurement using the Epstein test method described in JIS C2550-1: 2011. If the magnetic flux density B8 was 1.920 T or more, it was determined that a sufficient magnetic flux density was obtained.

[0073] Furthermore, to evaluate the iron loss degradation of the steel sheet due to magnetic aging, the same sample was annealed (aged) at 150°C for 200 hours after the magnetic measurement, and then the iron loss was evaluated in the same manner. The change in the iron loss value before and after aging was evaluated, and the difference ΔW17 / 50 was taken as the iron loss degradation due to magnetic aging. If the iron loss W17 / 50 after aging was 0.870 W / kg or less and the iron loss degradation due to magnetic aging ΔW17 / 50 was 0.020 W / kg or less, it was determined that the iron loss degradation due to magnetic aging was small and that sufficient iron loss characteristics were obtained even after magnetic aging.

[0074]

[0075]

[0076]

[0077]

[0078]

[0079]

[0080]

[0081]

[0082]

[0083]

[0084]

[0085]

[0086]

[0087]

[0088] As can be seen from the results in Tables 1-1 to 4-2, the examples of the present invention, which have a predetermined chemical composition, a region in which an Nb-group element is detected as a first peak, and in which the proportion of composite precipitates to all precipitates containing Nb-group elements is 50% or more, have high magnetic flux density, small iron loss deterioration due to magnetic aging, and low iron loss even after magnetic aging. On the other hand, the comparative examples, which have a chemical composition outside the range of the present invention and / or a small proportion of composite precipitates of Nb-based precipitates, do not satisfy the targets for one or more of the magnetic flux density, iron loss after aging, and iron loss deterioration due to magnetic aging.

[0089] According to the present invention, it is possible to provide a grain-oriented electrical steel sheet in which magnetic aging is suppressed and which has excellent core loss characteristics, and a method for manufacturing the same. Therefore, the present invention has high industrial applicability.

Claims

1. A steel sheet comprising a base steel sheet, optionally a glass coating formed on the surface of the base steel sheet, and optionally an insulating coating formed on the surface of the base steel sheet or on the surface of the glass coating, wherein the base steel sheet contains, in mass %, C: 0.0005 to 0.0050%, Si: 1.0 to 7.0%, one or more of Nb group elements, i.e., Nb, Ta, V, and Mo: 0.0030 to 0.0300% in total, acid-insoluble Al: 0.0001 to 0.0300%, Mg: 0.0001 to 0.0200%, O: 0.0001 to 0.0200%, N: 0.0001 to 0.0100%, Mn: 0 to 1.00%, S: 0 to 0.0150%, and a chemical composition consisting of Se: 0 to 0.0150%, Cu: 0 to 0.40%, Bi: 0 to 0.010%, B: 0 to 0.080%, P: 0 to 0.50%, Sn: 0 to 0.10%, Sb: 0 to 0.10%, Cr: 0 to 0.30%, Ni: 0 to 1.00%, Ti: 0 to 0.0050%, Ca: 0 to 0.0050%, and the balance: Fe and impurities, wherein, when the thickness of the base steel plate is t, precipitates observed by a TEM replica method in a region from a position t / 20 to a position (19t) / 20 in the thickness direction of the base steel plate A grain-oriented electrical steel sheet, characterized in that the proportion of precipitates that have a region in which an Nb group element is detected as a first peak by EDS analysis and that are composite precipitates to all precipitates containing the Nb group element is 50% or more.

2. In mass%, C: 0.0010 to 0.1000%, Si: 1.0 to 7.0%, at least one selected from the group consisting of Nb, Ta, V, and Mo: 0.0030 to 0.0300% in total, Mn: 0.05 to 1.00%, S: 0 to 0.0350%, Se: 0 to 0.0350%, acid-soluble Al: 0.0100 to 0.0650%, Mg: 0.0001 to 0.0100%, O: 0.0001 to 0.0200%, N: 0.0040 to 0.0120%, Cu: 0 to 0.40%, Bi: 0 to 0.010%, B: 0 to 0.080%, P: 0 to 0.50%, a casting step of casting molten steel having a chemical composition consisting of Sn: 0 to 0.10%, Sb: 0 to 0.10%, Cr: 0 to 0.30%, Ni: 0 to 1.00%, Ti: 0 to 0.0050%, Ca: 0 to 0.0050%, and the balance: Fe and impurities to form a slab; a hot rolling step of heating the slab and hot rolling it to form a hot-rolled steel sheet; a hot-rolled sheet annealing step of annealing the hot-rolled steel sheet; a cold-rolling step of cold-rolling the hot-rolled steel sheet after the hot-rolled sheet annealing step to form a cold-rolled steel sheet; a decarburization annealing step of decarburization annealing the cold-rolled steel sheet; and a finish annealing step of applying an annealing separator to the cold-rolled steel sheet after the decarburization annealing step, and then performing finish annealing. a stress relief annealing step of annealing the cold-rolled steel sheet after the finish annealing step, wherein in the decarburization annealing step, the C content of the cold-rolled steel sheet is set to 0.0050% or less, in the finish annealing step, a residence time at 1210°C or higher is set to 2 hours or less and a residence time at 1180°C or higher is set to 5 to 30 hours, and a residence time at 950 to 800°C during cooling is set to 5 hours or more, and in the stress relief annealing step, a residence time at 860°C or higher is set to 30 seconds or less and a residence time at 750°C or higher is set to 50 seconds or more.