Grain-oriented electrical steel sheet and method for manufacturing the 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

Technical Field

[0001] The present invention relates to a grain-oriented electrical steel sheet and a method for producing the same. This application claims priority based on Japanese Patent Application No. 2024-034142 filed in Japan on March 6, 2024, and incorporates the content herein by reference.

Background Art

[0002] The grain-oriented electrical steel sheet contains about 7 mass% or less of Si and has a secondary recrystallized texture in which grains are oriented in the {110}<001> direction (Goss direction). The {110}<001> direction means that the {110} plane of the crystal is arranged parallel to the rolling plane, and the <001> axis of the crystal is arranged parallel to the rolling direction.

[0003] The magnetic properties of the grain-oriented electrical steel sheet are greatly affected by the degree of orientation in the {110}<001> direction. In particular, the relationship between the rolling direction of the steel sheet, which becomes the main magnetization direction during use of the steel sheet, and the <001> direction of the crystal, which is the easy magnetization direction, is considered important. Therefore, in recent practical grain-oriented electrical steel sheets, control is performed so that the angle formed by the <001> direction of the crystal and the rolling direction is within a range of about 5°.

[0004] Such precise crystal orientation control is performed by appropriately dispersing fine precipitates called inhibitors in the steel before finish annealing and holding the steel sheet at a high temperature during finish annealing. For example, the inhibitor enhances the selective growth property of Goss-oriented grains, and as a result, secondary recrystallization proceeds so that Goss-oriented grains preferentially grow during finish annealing. So far, attempts have been made to highly control inhibitors for the purpose of precisely controlling crystal orientation. <00者0020> For example, Patent Document 1 discloses using MnS as an inhibitor and performing cold rolling twice. 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 controlling inhibitors to lower the slab heating temperature in order to reduce manufacturing costs.

[0006] Patent Document 5 discloses a method for controlling the primary recrystallized particle size and its dispersion related to inhibitors. Patent documents 6 to 8 disclose the addition of Nb, V, etc., to grain-oriented electrical steel sheets.

[0007] Furthermore, Patent Documents 9 to 11 describe techniques for improving magnetostriction by precisely controlling the atmosphere and residence time during finish annealing to form subgrain boundaries within the secondary recrystallized grains. These techniques demonstrate the technical concept of expanding the temperature range in which secondary recrystallization proceeds in order to form subgrain boundaries, and simultaneously show that an improvement in magnetic flux density can also be expected. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Japan Special Publication No. 30-3651 [Patent Document 2] Japan Special Publication No. 40-15644 [Patent Document 3] Japan Special Publication No. 51-13469 [Patent Document 4] Japanese Patent Publication No. 62-40315 [Patent Document 5] Japanese Patent Application Publication No. 2008-261022 [Patent Document 6] Japanese Patent Publication No. 52-024116 [Patent Document 7] Japanese Patent Application Publication No. 02-200732 [Patent Document 8] Japanese Patent No. 4962516 [Patent Document 9] International Publication No. 2020 / 027215 [Patent Document 10] International Publication No. 2020 / 027218 [Patent Document 11] International Publication No. 2020 / 027219 [Overview of the project] [Problems that the invention aims to solve]

[0009] In recent years, amidst global efforts to conserve electricity and energy and protect the global environment, the demand for higher efficiency in transformers has been increasing. In this social environment, there is a growing need to improve the performance of grain-oriented electrical steel sheets used in transformer core materials. In particular, there is a demand to increase the magnetic flux density of grain-oriented electrical steel sheets, thereby improving their iron loss characteristics (reducing iron loss).

[0010] As a result of our investigation, we found that the conventional inhibitor control technologies disclosed in the above-mentioned Patent Documents 1 to 8 do not adequately meet the requirements for grain-oriented electrical steel sheets, and that further increases in magnetic flux density are necessary.

[0011] Furthermore, in the technologies described in Patent Documents 9-11, as mentioned above, the secondary recrystallization temperature range is extended by precisely controlling the finish annealing, thereby achieving crystallographically precise {110} <001> It has been shown that preferential growth of Goss-oriented grains closer to the ideal Goss orientation, as represented by , is promoted, and that the magnetic flux density of the steel sheet is improved while forming subgrain boundaries that bring about improvements in magnetic properties such as magnetostriction of the steel sheet, which affects the noise characteristics of the transformer. 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, increases the effect of improving magnetic properties, and relaxes the finish annealing conditions under which the effect of improving magnetic properties is exhibited. This suggests that elements such as Nb are effectively functioning as a concrete method to extend the secondary recrystallization temperature range and improve 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 these Nb group element precipitates is influencing secondary recrystallization. On the other hand, in these technologies, since the precipitates of the trace-added Nb group elements tend to precipitate as carbonitrides in steel, when the amount of remaining Nb is large after finish annealing, even when the C content in the steel is reduced to 0.0050% or less, there is a concern that a phenomenon called magnetic aging occurs, in which fine carbides precipitate in the steel sheet when used as a transformer core, deteriorating the iron loss characteristics.

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

Means for Solving the Problems

[0013] As described above, although the Nb group elements contribute to the improvement of magnetic properties and the relaxation of finish annealing conditions, there has been a concern that they may cause magnetic aging. As a result of investigations by the present inventors, it has been found that in a grain-oriented electrical steel sheet containing Nb group elements, magnetic aging can be suppressed by causing Nb-based precipitates (precipitates of Nb group elements) to precipitate in combination with fine nitrides, oxides, or sulfides rather than alone.

[0014] The present invention has been made in view of the above findings. The gist of the present invention is as follows. [1] The grain-oriented electrical steel sheet according to one aspect of the present invention has a base steel sheet, [[ID=二十一]]optionally, a glass film formed on the surface of the base steel sheet, optionally, an insulating film formed on the surface of the base steel sheet or the surface of the glass film, and the base steel sheet contains, in mass%, C: 0.0005 to 0.0050%, [[ID=3']]]Si: 1.0 to 7.0%, one or more of the Nb group elements of Nb, Ta, V, and Mo: a total of 0.0030 to 0.0300%, acid-insoluble Al: 0.0001 to 0.0300%, Mg: 0.0001 to 0.0200%, O: 0.0001 to 0.0200%. N: 0.0001~0.0100%, Mn: 0~1.00%, 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%, Ti: 0~0.0050%, Ca: 0~0.0050%, Remainder: Fe and impurities, It has a chemical composition consisting of, When the thickness of the base steel sheet is denoted as t, in the region of the base steel sheet in the thickness direction from the surface at a position t / 20 to a position (19t) / 20, in the precipitate observed by the TEM replica method, EDS analysis reveals that the precipitates containing Nb group elements have a region where they are detected as the primary peak, and the proportion of these precipitates relative to the total precipitates containing Nb group elements is 50% or more. [2] A method for manufacturing grain-oriented electrical steel sheets according to another aspect of the present invention is: A method for manufacturing grain-oriented electrical steel sheets as described in [1], In mass%, C: 0.0010~0.1000%, Si: 1.0~7.0%, At least one element selected from the group consisting of Nb, Ta, V, and Mo: totaling 0.0030 to 0.0300%. Mn: 0.05~1.00%, S: 0~0.0350%, Se: 0~0.0350%, Acid soluble Al: 0.0100~0.0650%, Mg: 0.0001~0.0100%, O: 0.0001~0.0200%, N: 0.0040~0.0120%, 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%, A casting process in which molten steel having a chemical composition consisting of the remainder being Fe and impurities is cast into a slab, The aforementioned slab is heated and hot-rolled to form a hot-rolled steel sheet in a hot-rolling process, A hot-rolled steel sheet annealing step, The hot-rolled steel sheet after the hot-rolled sheet annealing process is cold-rolled to produce a cold-rolled steel sheet in a cold-rolling process, A decarburization annealing step for decarburizing the cold-rolled steel sheet, A finish annealing step is performed in which an annealing release agent is applied to the cold-rolled steel sheet after the decarburization annealing step, and then finish annealing is carried out. A distortion-relieving annealing step is performed on the cold-rolled steel sheet after the finish annealing step, Equipped with, In the decarburization annealing process, the carbon content of the cold-rolled steel sheet is set to 0.0050% or less. In the aforementioned finish annealing process, The maximum stay time at temperatures above 1210°C is 2 hours or less, and the maximum stay time at temperatures above 1180°C is 5 to 30 hours. During cooling, the stay time at 950-800°C should be 5 hours or more. In the aforementioned strain-relieving annealing process, The duration of stay at temperatures above 860°C must be 30 seconds or less, and the duration of stay at temperatures above 750°C must be 50 seconds or more. [Effects of the Invention]

[0015] According to the above aspects of the present invention, it is possible to provide grain-oriented electrical steel sheets and a method for manufacturing the same, which have suppressed magnetic aging and excellent iron loss characteristics. [Brief explanation of the drawing]

[0016] [Figure 1] This figure shows a photograph of a precipitate containing NbC, observed by TEM replicating, and the results of EDS analysis at two locations (the intersections of the cross) where the precipitate is located. [Modes for carrying out the invention]

[0017] A grain-oriented electrical steel sheet (grain-oriented electrical steel sheet according to this embodiment) and a method for manufacturing the same according to one embodiment of the present invention will be described. However, the present invention is not limited to the configuration disclosed in this embodiment, and various modifications are possible without departing from the spirit of the invention.

[0018] A grain-oriented electrical steel sheet according to one embodiment of the present invention (grain-oriented electrical steel sheet according to this embodiment) has a base steel sheet having a predetermined chemical composition. Furthermore, the grain-oriented electrical steel sheet according to this embodiment may have a base steel sheet and a glass coating formed on the surface of the base steel sheet. Furthermore, the grain-oriented electrical steel sheet according to this embodiment may 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. Furthermore, the grain-oriented electrical steel sheet according to this embodiment may 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 sheet is denoted as t, in the region from the position t / 20 to (19t) / 20 from the surface in the thickness direction of the base steel sheet, the precipitates observed by TEM replica method have a region in which Nb group elements are detected as the first peak by EDS analysis and are complex precipitates, and the number ratio of these precipitates to the total precipitates containing Nb group elements is 50% or more (i.e., the proportion of complex precipitates of Nb-based materials is 50-100%). I will explain each of them.

[0019] [Base material steel plate] <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 if it does not include the glass coating or insulating coating) shall be within the following range. In the numerical limit range indicated by "~" below, the values ​​at both ends are included in the range as the lower and upper limits. Numerical values ​​indicated as "greater than" or "less than" are not included in the numerical range. In addition, "%" in relation to chemical composition means "mass%" unless otherwise specified.

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

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

[0022] C: 0.0005~0.0050% Carbon (C) is an effective element for controlling the primary recrystallization structure during the manufacturing process. In particular, C forms carbides and carbonitrides with elements such as Nb, making it effective for controlling the primary recrystallization structure. Generally, C is purified during the finish annealing process, and its content decreases. However, if the purification is insufficient, or if the C content of the final product (grain-oriented electrical steel sheet) is excessive, the magnetic properties will deteriorate. Furthermore, by bonding with Si, Ti, etc., and precipitating as fine carbides or carbonitrides, it can cause magnetic aging. If the C content exceeds 0.0050%, the deterioration of magnetic properties becomes significant, so the C content should be 0.0050% or less. Preferably, the C content is 0.0030% or less, and more preferably 0.0020% or less. While a lower carbon content is preferable, it is difficult to achieve a carbon content below 0.0005% considering productivity in industrial production. Therefore, the carbon content should be 0.0005% or higher.

[0023] Si: 1.0~7.0% 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, and the crystal orientation of the grain-oriented electrical steel sheet is impaired. Therefore, the Si content should be 1.0% or more. Preferably, the Si content is 1.5% or more, and more preferably 3.0% or more. On the other hand, if the Si content exceeds 7.0%, the cold workability decreases, and cracks are more likely to occur during cold rolling. Therefore, the Si content of grain-oriented electrical steel sheets should be 7.0% or less. Preferably, the Si content is 5.5% or less, and more preferably 4.0% or less.

[0024] Acid-insoluble Al (insol.Al): 0.0001~0.0300% Acid-soluble aluminum is an important element because it causes the preferential growth of Goss-oriented grains during decomposition in two stages, through the formation of two types of precipitates: Nb-based precipitates and Al-based precipitates, thereby improving the magnetic flux density of the steel sheet. After finish annealing, most of the acid-soluble aluminum contained in the slab becomes acid-insoluble aluminum and remains as an oxide in the glass coating, but some remains in the steel and forms composite oxides with Mg such as Al2O3 and spinel (including inclusions formed during steelmaking). These composite oxides with Mg such as Al2O3 and spinel effectively function as precipitation sites for Nb-based precipitates, suppressing the occurrence of magnetic aging by Nb-based precipitates. To obtain the above effect, the acid-insoluble aluminum content should be 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 excessive, leading to deterioration of the iron loss of the material. Therefore, the acid-insoluble Al content should be 0.0300% or less. Preferably, it should be 0.0200% or less, and more preferably 0.0100% or less.

[0025] Mg: 0.0001~0.0200% Magnesium (Mg), together with Al, forms complex oxides such as spinel, effectively functioning as a precipitation site for Nb-based precipitates and having the effect of suppressing the occurrence of magnetic aging by Nb-based precipitates. To obtain the above effect, the Mg content should be 0.0001% or more. Preferably, the Mg content is 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 excessive, and the iron loss of the material deteriorates. Therefore, the Mg content should be 0.0200% or less. Preferably, it should be 0.0100% or less. While magnesium (Mg) is not added as a component of the slab steel, it may be introduced from furnace materials during steelmaking, or some of the MgO, the main component of the annealing separator during finish annealing, may diffuse into the steel. However, even in these cases, the content must remain within the above-mentioned range.

[0026] One or more of the following: Nb, Ta, V, and Mo: Total of 0.0030-0.0300% Niobium (Nb), tantalum (Ta), vanadium (V), and molybdenum (Mo) precipitate as auxiliary inhibitors such as carbides, nitrides, and carbonitrides, and function favorably as inhibitors. Specifically, they favorably expand the temperature range over which secondary recrystallization progresses. As a result, Goss-oriented grains grow favorably, and the magnetic flux density of the resulting grain-oriented electrical steel sheet is favorably increased. To obtain this effect, at least one element selected from the group consisting of Nb, Ta, V, and Mo is included 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 Nb group elements is greater than 0.0050%, and more preferably, the total content of Nb and Ta is greater than 0.0050% due to the magnetic enhancement effect. On the other hand, if the total content of Nb group elements exceeds 0.0300%, the precipitation temperature range for Nb group element precipitates becomes high, and the Nb group element precipitates tend to be coarse and low in density. Furthermore, the difference between the precipitation temperature range of Nb group element precipitates and the precipitation temperature range of MnS and AlN becomes large, making it difficult for Nb group element precipitates to effectively act as precipitation nuclei for refining MnS and AlN. In addition, carbonitrides of Nb group elements (Nb-based carbonitrides) in grain-oriented electrical steel sheets degrade the magnetic properties. For this reason, the total content of Nb group elements should be 0.0300% or less. Preferably, the total content of Nb group elements is 0.0200% or less, and more preferably 0.0100% or less.

[0027] O: 0.0001~0.0200% O is an element that forms fine oxides in combination with Nb-based precipitates. To achieve this effect, the O content should be 0.0001% or higher. 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: 0.0001~0.0100% N is an element that forms fine nitrides in combination with Nb-based precipitates. To achieve this effect, the N content should be 0.0001% or higher. 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 in its chemical composition. "Impurities" refer to elements that are introduced from the ore or scrap used as raw materials, or from the manufacturing environment, etc., during the industrial production of steel. The upper limit of the total impurity content may be, for example, 5.0%.

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

[0031] Mn: 0~1.00% Manganese (Mn) is an element that combines with S and Se to precipitate as MnS and MnSe, functioning as an inhibitor. If the Mn content exceeds 1.00%, the amount of MnS and MnSe that function as inhibitors will be excessive, and the proper progress of secondary recrystallization will be inhibited. In this embodiment, some of the inhibitory function may be carried out by carbides, nitrides, or carbonitrides of Nb group elements. In this case, the amount of MnS and MnSe that precipitate as inhibitors may be controlled to be smaller. Therefore, the Mn content should be 1.00% or less. Preferably, the Mn content is 0.50% or less, and more preferably 0.20% or less. On the other hand, there is no particular limit to the Mn content of the base steel sheet, and it may be 0%. However, since Mn can form MnS or MnSe and act as inhibitors during secondary recrystallization, or be used to control the precipitation of AlN, and it has also been confirmed that MnS or Mn silicate can function as precipitation sites for Nb-based precipitates to prevent magnetic aging, the Mn content may be 0.001% or more. Preferably, the Mn content is 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 included within the above ranges depending on the known purpose. There is no need to set a lower limit for the content of these selected elements, and it may be 0%. When Mn is included in the steel, the remaining S and Se form sulfides and selenides, which function as precipitation sites for Nb-based precipitates. Therefore, it is preferable that the total content of S and Se be 0.0001% or more. On the other hand, if there are too many sulfides and selenides, the magnetic properties deteriorate. Therefore, it is preferable that the S content and Se content be 0.0150% or less and 0.0050% or less, respectively. Copper (Cu) affects the precipitate and primary recrystallization texture, and has the effect of improving secondary recrystallization. The Cu content may be 0.01% or more, taking into account contamination from scrap, etc., and preferably 0.05% or more, more preferably 0.10% or more, in order to improve secondary recrystallization. On the other hand, from the viewpoint of improving magnetic properties and the cost increase due to elemental addition, the Cu content is preferably 0.40% or less. Bismuth (Bi) is an element that improves secondary recrystallization. Therefore, it may be included. To obtain the above effect, the Bi content is preferably 0.001% or more. On the other hand, if the Bi content exceeds 0.010%, it may become difficult to form a good glass coating on the product plate. Therefore, the Bi content is preferably 0.010% or less. Boron (B) is an element that precipitates and segregates, improving secondary recrystallization. Therefore, it may be included. To obtain the above effect, the B content is preferably 0.001% or more. On the other hand, if the B content exceeds 0.080%, it may become difficult to form a good glass coating on the product plate. Therefore, the B content is preferably 0.080% or less. Phosphorus (P), tin (Sn), and antimony (Sb) are segregation elements that have the effect of stably inducing secondary recrystallization and forming glass coatings. Therefore, they may be included. 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 become 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 also 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 included. To obtain the above effect, the Cr content is preferably 0.02% or more. On the other hand, if the Cr content exceeds 0.30%, glass coating formation may become difficult. 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 included. To obtain the above effect, 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%, decarburization becomes difficult, and glass film formation may become difficult. Therefore, the Ni content should be 1.00% or less. From a cost perspective, the Ni content is more preferably 0.30% or less.

[0033] Ti: 0~0.0050% Titanium (Ti) exists in the base steel sheet of grain-oriented electrical steel sheets as precipitates such as TiN, and degrades the magnetic properties of the steel sheet. Therefore, the Ti content should be 0.0050% or less. Preferably, the Ti content is 0.0025% or less. On the other hand, Ti, like Al-Mg oxides, forms precipitates such as TiN that effectively function as precipitation sites for Nb-based precipitates. Therefore, to obtain the above effect, the Ti content may be set to 0.0002% or higher. Ti may be present in the steel sheet even without intentional addition, either as a component of the ferrosilicon raw material or as TiO2 in the annealing separation agent. In this case, however, the content must be within the above-mentioned range.

[0034] Ca: 0~0.0050% Calcium (Ca) is an element that causes deterioration of magnetic properties. Since the deterioration of magnetic properties becomes significant when the Ca content exceeds 0.0050%, the Ca content should be kept below 0.0050%. On the other hand, calcium-based precipitates such as CaO function effectively as precipitation sites for nb-based precipitates. Therefore, the calcium content may be set to 0.0001% or higher. Ca may be present even after finish annealing, without intentional addition, from fluorite during decarburization in steelmaking, or as an impurity in MgO, which is used as an annealing separating agent. In this case, however, the content is acceptable as long as it is within the above-mentioned range.

[0035] The chemical composition of the base steel sheet of the grain-oriented electrical steel sheet according to this embodiment can be measured by general analytical methods for steel. For example, the chemical composition of the base steel sheet of the grain-oriented electrical steel sheet can be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). Specifically, the chemical composition can be determined 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-established calibration curve. C and S can be measured using the combustion-infrared absorption method, and N can be measured using the inert gas fusion-thermal conductivity method. If a glass coating or insulating coating is formed on the surface of the steel plate, these should be removed before measuring the components. Specifically, the insulating coating on grain-oriented electrical steel sheets can be removed by immersing them in an aqueous sodium hydroxide solution containing 30-50% by mass of NaOH and 50-70% by mass of H2O at 80-90°C for 7-10 minutes. After the insulating coating is removed, the grain-oriented electrical steel sheets can be washed with water and then dried with a hot air blower for slightly less than one minute. Furthermore, if a glass coating is formed beneath the insulating coating, the glass coating can be removed from the grain-oriented electrical steel sheet having the glass coating by immersing it in a hydrochloric acid aqueous solution containing 30-40% by mass of HCl at 80-90°C for 1-10 minutes. After immersion, the base steel sheet should be washed with water and then dried with a hot air blower for slightly less than one minute. Through the above process, the base steel sheet can be extracted from the grain-oriented electrical steel sheet on which the glass coating and / or insulating coating has been formed.

[0036] <Precipitate> As mentioned above, in grain-oriented electrical steel sheets containing Nb group elements, there were cases where a magnetic aging phenomenon occurred, causing the iron loss characteristics of the grain-oriented electrical steel sheet, which is the core material, to deteriorate when used as a transformer. The primary suspected cause of this is the Nb group elements that remain in the steel even after finish annealing. Since the Nb group elements present in steel readily form carbonitrides, it is thought that magnetic aging may occur even at carbon content levels that would not normally cause deterioration of iron loss characteristics. The inventors analyzed the Nb group element content in the steel sheet (base steel sheet) obtained by removing an insulating coating called a secondary coating mainly composed of colloidal silica and aluminum phosphate, and a glass coating called a primary coating mainly composed of forsterite (Mg2SiO4) formed during finish annealing, from a product sheet of grain-oriented electrical steel sheet that undergoes magnetic aging. They found that magnetic aging occurs when the Nb group element content is 0.0030% or more and when the C content remains at 0.0005% or more. Although magnetic aging can be suppressed by reducing the C content, as mentioned above, it is not easy to reduce the C content to less than 0.0005% when considering industrial production. Therefore, it was found that in practical steel sheets, the problem of magnetic aging is unavoidable when the Nb group element content is 0.0030% or more.

[0037] However, as a result of further investigation by the inventors, they found that even within the above component range, magnetic aging is suppressed when Nb-based precipitates do not precipitate alone but are precipitated in combination with spinel-based oxides (Al-Mg-O), MnS, TiN, CaO, etc. Specifically, when the thickness of the base steel sheet is t, we found that magnetic aging is greatly suppressed when, in the region from the surface at a position t / 20 to a position (19t) / 20 in the thickness direction of the base steel sheet, the precipitates observed by the TEM replica method have a region in which Nb group elements are detected as the first peak by EDS analysis and are complex precipitates, and the number ratio of these precipitates to the total precipitates containing Nb group elements is 50% or more (i.e., the proportion of complex precipitates containing Nb-based elements is 50-100%). It is presumed that magnetic aging is suppressed by composite deposition because, due to the composite deposition of Nb group elements, the precipitates become coarser, and the deposition of fine carbides during transformer use is suppressed. The reason for focusing on precipitates in the region from t / 20 to (19t) / 20 from the surface is that the state of precipitates may differ in the very surface layer (the range less than t / 20 from both the front and back surfaces), and the impact on the properties of the steel sheet is small.

[0038] While the composite morphology of the precipitates is not limited, examples include a morphology in which spinel oxides (Al-Mg-O), MnS, TiN, or CaO exist at the center like a precipitation nucleus, surrounded by Nb-based precipitates, or, if the size of the spinel oxides (Al-Mg-O), MnS, TiN, or CaO is large (0.5 μm or more), a morphology in which Nb-based precipitates precipitate on top of them in a composite manner. For example, in the composite precipitate shown in Figure 1, as can be seen from the EDS analysis results for measurement point 139 on the right, the detection intensity of Al, Mg, and O is high at the center, indicating the presence of a spinel-based oxide (Al-Mg-O) like a precipitation nucleus. Also, as can be seen from the EDS analysis results for measurement point 142 on the left, Nb-based precipitates surround it. The average size of composite precipitates having a region where Nb group elements are the first peak is preferably 10 nm or larger, more preferably 100 nm or larger, because if it is too small, the number of precipitates increases and the iron loss properties of the steel sheet itself deteriorate. For the same amount of precipitate, the magnetic properties improve as the average size increases, but this requires a sufficient holding time in a predetermined temperature range to coarsely precipitate Nb-based precipitates such as NbC, which becomes industrially costly. Therefore, the average size of composite precipitates is preferably 1000 nm or less, and more preferably 500 nm or less.

[0039] The proportion of precipitates that have a region in which Nb group elements are detected as the primary peak by EDS analysis and that are complex precipitates, i.e., the proportion of complex precipitates of Nb-based elements, is determined by the following method. A TEM observation sample prepared by the carbon extraction replica method is observed at positions t / 20 to (19t) / 20 (for example, at position t / 2). Within a region indicating the presence of a unified precipitate surrounded by spaces without precipitates (where only the peak of C, an element of the sample substrate, and the material components of the grid (such as Cu or Ni) are observed by EDS measurement, and elemental peaks other than those of the sample substrate are detected by EDS measurement), a contrast in density thought to be due to the components is found. EDS measurements are then performed at locations with different density levels, and if the energy profiles of the EDS measurements indicating the presence of elements differ between one of the areas with different density levels and the other, it is determined that the precipitate is a composite precipitate. To give an example, if, within the different contrasts within the precipitate, a peak indicating the presence of Nb group elements is observed in one area, while the peak indicating the presence of Nb group elements is extremely weak in the other area, or if, compared to one area, the peak of Nb group elements in the other area does not become significantly smaller, but the peaks of other elements (such as Al, Mg, Si, Ca, Mn, Ti, S, N, and O) become stronger, then it can be determined that Nb-based precipitates are present in a complex precipitate. Here, the carbon extraction replica method is a method for preparing TEM observation samples obtained by extracting fine precipitates and inclusions present in steel from the base steel sheet. The procedure for this method involves polishing the steel material to be observed, then etching the base steel material. After that, carbon is deposited onto the etched surface, and the sample is immersed in a stripping solution that dissolves only the base material without dissolving the precipitates. The stripped and floating replica film is washed, collected on a TEM observation grid, and the TEM observation sample is completed. For example, a sample cut to a size of 20mm x 20mm x plate thickness is polished with sandpaper (starting from grit #80 and gradually increasing the grit to #1500) so that the surface in the thickness direction becomes the observation surface. Then, the sample obtained by buffing from 6μm to 1μm is subjected to a SPEED etching apparatus (e.g., Fujiwara Seisakusho: FV-138) with an electrolyte (89% methanol by mass, 10% acetylacetone by mass, 1% tetramethylammonium chloride by mass) to a step potential of -200mV, and preliminary etching is performed to remove surface contaminants (Coulomb amount is the sample surface area (cm²)). 2 ) / 100[C]) and then the main etching (the amount of Coulomb is the sample surface area (cm²) 2 ) / 10[C]) is performed. Carbon deposition of about 30 nm is carried out on the electropolished surface obtained in this way (for example, using JEOL's JFE-400). Then, methyl acetate and the sample are placed in a petri dish and immersed for about 2 hours under fluorescent light. Once the replica film floats to the surface, the film is washed by immersing it in a fresh solution of methyl acetate for about 30 minutes, and then the film is scooped up with an observation Cu mesh and allowed to air dry for about 30 seconds to obtain an observation sample. Furthermore, replica samples for TEM observation are prepared on a plane perpendicular to the direction perpendicular to the plate surface (ND direction) of the steel plate, at positions from t / 20 to (19t) / 20, for example, at a position half the thickness of the steel plate from the surface (t / 2 position). However, if the precipitation state of precipitates may differ between the t / 2 position and near the surface of the steel plate, for example, the t / 20 layer, the proportion of composite precipitation may be calculated by increasing the plate thickness observed as needed and taking the average of the plate thicknesses. Specifically, samples for TEM observation may be prepared by polishing the plate to a thickness of 1 / 20 of the plate thickness, and the proportion of composite precipitation may be calculated as the average value of a total of 19 positions from t / 20 to (19t) / 20. Furthermore, during observation, precipitates can be observed spatially evenly across more than 100 fields of view from a TEM observation grid (usually 3 mm in diameter) at a magnification of 10,000x, allowing for the calculation of the average size and number density of the precipitates. In addition, more than 100 precipitates are randomly selected, and EDS analysis is performed at a magnification of 50,000x on at least two areas within each precipitate—one with high contrast and one with low contrast—to determine whether Nb group elements are detected and whether the precipitate is a composite precipitate. This allows for the quantification of the proportion of composite precipitates among Nb-based precipitates to at least 1% increments.

[0040] Furthermore, during the observation described above, the long and short sides of the precipitates are measured, and the size of each precipitate is determined by taking the average of these measurements. Based on the measurements of at least 50 precipitates, the average size can be calculated.

[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 mainly composed of magnesium silicate. The glass coating is formed during finish annealing by a reaction between the annealing release agent containing magnesia (MgO) applied to the surface of the base steel sheet and the components of the base steel sheet surface, and has a composition derived from the annealing release agent and the components of the base steel sheet (more specifically, a composition mainly composed of Mg2SiO4). If an annealing release agent primarily composed of alumina is used during the final annealing process, a glass coating 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 on the surface of the glass coating. The insulating coating improves the iron loss characteristics of grain-oriented electrical steel sheets by reducing eddy current losses through the imparting of electrical insulation properties to the sheet. The insulating coating also has the function of imparting tension to the grain-oriented electrical steel sheet. By imparting tension to the grain-oriented electrical steel sheet and facilitating magnetic domain wall movement within the sheet, the iron loss characteristics of the grain-oriented electrical steel sheet can be improved. Furthermore, in addition to the electrical insulation properties mentioned above, insulating coatings can provide various other characteristics such as corrosion resistance, heat resistance, and slipperiness. In the grain-oriented electrical steel sheet according to this embodiment, the insulating coating may be a known coating formed by, for example, applying a coating solution mainly composed of phosphate and colloidal silica to the surface of a glass coating (forsterite coating) and baking it.

[0043] [Manufacturing method] The grain-oriented electrical steel sheet according to this embodiment can achieve the above-described effects regardless of the manufacturing method, but a manufacturing method including the following steps is preferable because it can be manufactured stably. (I) A casting process in which molten steel having a predetermined chemical composition is cast to form a slab, (II) A hot rolling process in which the slab is heated and hot-rolled to make a hot-rolled steel sheet, (III) A hot-rolled steel sheet annealing step, in which the hot-rolled steel sheet is heated to the highest temperature it can reach and then cooled to anneal it, (IV) A cold rolling step in which the hot-rolled steel sheet after the hot-rolled sheet annealing step is cold-rolled to obtain a cold-rolled steel sheet, (V) A decarburization annealing step in which the cold-rolled steel sheet is decarburized, (VI) A finish annealing step in which an annealing release agent is applied to the cold-rolled steel sheet after the decarburization annealing step, and then finish annealing is performed. (VII) A distortion-relieving annealing step in which the cold-rolled steel sheet is annealed after the finish annealing step. The following describes the preferred conditions for each process.

[0044] "Casting process" In the casting process, slabs are prepared. An example of a slab manufacturing method is as follows: Molten steel is produced (melted). Slabs are manufactured using the molten steel. Slabs may also be manufactured by continuous casting. Alternatively, ingots may be manufactured using the molten steel, and slabs may be manufactured by bloc rolling of the ingots. The thickness of the slab is, for example, 150 to 350 mm. Preferably, the thickness of the slab is 220 to 280 mm. As slabs, so-called thin slabs with a thickness of 10 to 70 mm may be used. When using thin slabs, rough rolling before finish rolling can be omitted in the hot rolling process.

[0045] The chemical composition of the slab is determined by considering changes in the chemical composition (such as carbon and inhibitor components) during intermediate processes, so that the final chemical composition of the grain-oriented electrical steel sheet is obtained. For example, it contains the following elements, with the remainder being Fe and impurities.

[0046] C: 0.0010~0.1000% Carbon (C) is an effective element for controlling the primary recrystallization structure during the manufacturing process. Therefore, the carbon content of the slab should be 0.0010% or higher. On the other hand, excessive carbon content in the final product negatively affects its magnetic properties. If the carbon content of the slab is excessive, it may not be possible to sufficiently reduce the carbon content even after decarburization annealing. Therefore, the carbon content of the slab should be 0.1000% or less.

[0047] Si: 1.0~7.0% Silicon (Si) increases the electrical resistance of grain-oriented electrical steel sheets, thereby reducing 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 sheet. Therefore, the Si content is 1.0% or more. Preferably, the Si content is 2.5% or more, and more preferably 3.0% or more. On the other hand, if the Si content exceeds 7.0%, the cold workability decreases, and cracks are more likely to occur during cold rolling. Therefore, the Si content is 7.0% or less. Preferably, the Si content is 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 0.0030-0.0300% Nb-based carbonitrides have the effect of improving the selective growth of Goss-oriented grains during secondary recrystallization. Therefore, they should be included in the slab at a concentration of 0.0030% or more. On the other hand, if the total content of Nb group elements exceeds 0.0300%, the decomposition temperature of the Nb group element precipitates during secondary recrystallization becomes higher, and the difference with the decomposition temperature of AlN becomes smaller, causing the expansion effect of the secondary recrystallization temperature range to disappear and the improvement effect of magnetic flux density to be lost. For this reason, the total content of Nb group elements should be 0.0300% or less. Preferably, the total content of Nb group elements is 0.0200% or less, and more preferably 0.0100% or less.

[0049] Mn: 0.05~1.00% Manganese (Mn) combines with S or Se to form MnS or MnSe, which functions as an inhibitor. A Mn content in the range of 0.05 to 1.00% is preferable because it stabilizes secondary recrystallization. In this embodiment, it is possible to have a part of the inhibitory function taken over by nitrides of Nb group elements. In this case, the strength of MnS or MnSe, which acts 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: 0~0.0350% Se: 0~0.0350% Sulfur (S) and selenium (Se) combine with manganese (Mn) to form MnS or MnSe, which function as inhibitors. The S content should be 0 to 0.0350%, and the Se content should be 0 to 0.0350%. When at least one of S and Se is included, a total S and Se content of 0.0030 to 0.0350% is preferable because it stabilizes secondary recrystallization. In this embodiment, it is possible for nitrides of Nb group elements to perform part of the inhibitor function. In this case, the strength of MnS or MnSe, which acts as a general inhibitor, is controlled to be weak. Therefore, the preferred upper limit for the total S and Se content is 0.0250%, and more preferably 0.0100%. If S and Se remain after finish annealing, they form compounds and degrade iron loss. Therefore, it is preferable to reduce S and Se as much as possible by purification during finish annealing.

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

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

[0053] N: 0.0040~0.0120% Nitrogen (N) is an element that combines with Al to precipitate as AlN or (Al,Si)N, functioning as an inhibitor. To favorably control the form of these inhibitors (precipitates), the N content should be 0.0040% or higher. On the other hand, if the N content exceeds 0.0120%, blistering, a type of defect, is more likely to occur in the steel sheet, so the N content should be kept below 0.0120%.

[0054] O: 0.0001~0.0200% O is an element that forms fine oxides that precipitate in combination with Nb-based precipitates. To obtain this effect, the O content should be 0.0001% or more. Since the O content depends on the ore grade and deoxidation time, considering the cost, it is practically preferable to have 0.0010% or more, and even more preferable to have 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 should be 0.0200% or less. Preferably, the O content is 0.0150% or less, and more preferably 0.0100% or less.

[0055] Mg: 0.0001~0.0100% Mg is an element that forms Al-Mg oxides such as spinel, and precipitates that are used as precipitation sites for Nb-based precipitates. Therefore, the Mg content should be 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 high, and the iron loss of the material deteriorates. Therefore, the Mg content should be 0.0100% or less. Preferably, the Mg content is 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% In the slab used to manufacture the grain-oriented electrical steel sheet according to this embodiment, in addition to the basic elements and impurities described above, optional elements may be included. For example, instead of a portion of the remaining Fe described above, one or more of the following optional elements may be included: Cu, Bi, B, P, Sn, Sb, Cr, Ni, Ti, and Ca. These optional elements may be included according to their purpose, and their content should be the same as that of the grain-oriented electrical steel sheet to be ultimately obtained. There is no need to limit the lower limit of these optional elements, and the lower limit may be 0%. Furthermore, even if these optional elements are included as impurities, the above effects will not be impaired.

[0057] [Hot rolling process] In the aforementioned hot rolling process, the slab is heated and hot-rolled (rough rolling and finish rolling) to produce a hot-rolled steel sheet. The conditions for this process are not particularly limited. In the hot rolling process, for example, a hot-rolled steel sheet with a thickness of 1.8 to 3.5 mm can be used. After the finish rolling is complete, the hot-rolled steel sheet should be wound up at a predetermined temperature. While the slab heating temperature is not limited, it is preferable to use a temperature of 1280-1400°C when using innate inhibitors that completely dissolve precipitates used as inhibitors during hot rolling and form secondary inhibitors used for secondary recrystallization during hot rolling or hot-rolled sheet annealing. 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, the precipitates on the skid will not be completely dissolved, resulting in fluctuations in magnetic properties due to the non-uniformity of the precipitate precipitation state. Furthermore, when the steel sheet is nitrided in a subsequent process and precipitates formed during finish annealing, so-called acquired inhibitors, are used as secondary inhibitors during secondary recrystallization, it is preferable to set the slab heating temperature to 1050-1250°C. If the slab heating temperature exceeds 1250°C, a mixture of the primary recrystallized structure occurs due to the uneven precipitation state of fine precipitates re-precipitation after re-solution treatment and coarse precipitates that remain undissolved, preventing proper secondary recrystallization. Furthermore, the soaking time at the slab heating temperature is preferably 30 minutes or more, and more preferably 60 minutes or more, in order to minimize temperature unevenness within the slab. On the other hand, productivity decreases if the soaking time is long, so the soaking time is preferably 600 minutes or less, and 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 at this time may be those of known methods (for example, setting the annealing temperature to 750 to 1200°C, holding it for 30 seconds to 10 minutes, and then cooling), and should be determined in such a way that good magnetic properties are obtained. It is preferable to adjust the maximum temperature reached to 1000 to 1150°C and the soaking time, and then, if necessary, soak the sheet at an intermediate temperature called the secondary soaking temperature (for example, 1000 to 800°C) during the cooling process, and then adjust the subsequent cooling rate, etc., in order to suit secondary recrystallization and obtain good magnetic properties.

[0059] [Cold rolling process] In the cold rolling process, the hot-rolled steel sheet, after the hot-rolled sheet annealing process, is subjected to one cold rolling or multiple cold rollings (two or more times) via annealing (intermediate annealing) to produce a cold-rolled steel sheet with a thickness of, for example, 0.10 to 0.50 mm. In the cold rolling process, the reduction ratio should be controlled to 80-95%.

[0060] The cold rolling reduction ratio described above refers to the cumulative cold rolling reduction ratio when no intermediate annealing is performed, or, if intermediate annealing is performed, the cumulative cold rolling reduction ratio after the final intermediate annealing. Specifically, the cold rolling reduction ratio is defined as follows: Cold rolling reduction ratio (cumulative reduction ratio) (%) = (1 - "thickness of steel sheet after cold rolling" / "thickness of steel sheet before cold rolling (or after intermediate annealing)") × 100

[0061] When the reduction ratio of cold rolling is within the above range, the primary recrystallized texture after decarburization annealing is preferably controlled. Specifically, the primary recrystallized texture is such that the grains have an ideal Goss orientation (ideal {110}) during secondary recrystallization. <001> The texture becomes such that orientation grains (which have an orientation) preferentially grow (specifically, in the primary recrystallized texture, {111} <112> ya{411} <148> (The crystal orientation represented by becomes the dominant orientation.) As a result, it is preferable because it is easier to obtain a secondary recrystallized texture in the final product in which grains with the ideal Goss orientation preferentially grow.

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

[0063] "Nitriding process" In the manufacturing method of grain-oriented electrical steel sheets according to this embodiment, nitriding treatment may be performed in order to adjust the strength of the inhibitor in secondary recrystallization. In the nitriding treatment, the nitrogen content of the steel sheet can be increased to about 0.004 to 0.040 mass% at any timing between the start of the decarburization annealing described above and the start of secondary recrystallization in finish annealing. Preferably, the nitrogen content of the steel sheet after nitriding treatment is 0.015 to 0.040 mass%. The nitriding treatment is not limited to any particular method, but examples include annealing a steel sheet in an atmosphere containing a gas with nitriding ability such as ammonia, or finishing annealing a decarburized annealed steel sheet coated with an annealing separator containing a powder with nitriding ability such as MnN. The nitrogen content after nitriding can be measured using the inert gas fusion-thermal conductivity method on a nitrided steel sheet.

[0064] "Finishing annealing process" "Distortion-removing annealing" In the manufacturing method of grain-oriented electrical steel sheets according to this embodiment, in the finish annealing step and the strain-relieving annealing step, Nb-based precipitates are complexly precipitated using Al-Mg oxides, TiN, CaO, MnS, MnSe, etc. as precipitation nuclei. Specifically, in the finish annealing process (purification annealing), an annealing release agent is applied to the cold-rolled steel sheet after the decarburization annealing process, or after the nitriding process, and then annealed. During annealing, the stay time at 1210°C or higher is 2 hours or less, the stay time at 1180°C or higher is 5 to 30 hours, and the stay time at 950 to 800°C during cooling is 5 hours or more. Furthermore, in the subsequent distortion-relieving annealing process, the stay time at 860°C or higher is set to 30 seconds or less (including 0 seconds), and the stay time at 750°C or higher is set to 50 seconds or more. In the finish annealing process, if the stay time exceeds 1210°C for more than 2 hours, the solution treatment of spinel, TiN, and MnS, which are precipitation sites for Nb-based precipitates such as NbC, progresses, and the number of Nb-based precipitate precipitation sites within the crystal grains decreases. In this case, finely re-precipitation preferentially precipitates at the secondary recrystallization grain boundaries, which exacerbates the non-uniformity of residual precipitates in the steel, leading to a greater deterioration of magnetic properties. Furthermore, the reason for setting the residence time at 1180°C or higher to 5 hours or more and 30 hours or less is to ensure sufficient purification time, suppress heterogeneity due to lithoning of the precipitation state of spinel, TiN, and MnS, and to optimize the total amount and spatial uniformity of the precipitation sites of Nb-based precipitates. Furthermore, if the residence time at 950-800°C during cooling is less than 5 hours, the precipitates that serve as precipitation sites for Nb-based precipitates may not re-precipitation completely, or the Nb-based precipitates may not re-precipitation sufficiently coarsely at the precipitation sites, leading to magnetic aging where the dissolved Nb precipitates finely with residual C during transformer use. The residence time is preferably 10 hours or more. There is no upper limit to the residence time, but productivity decreases if it is too long, so it may be limited to 50 hours or less. Furthermore, in the stress relief annealing process, if the residence time at 860°C or higher exceeds 30 seconds, the re-solution of Nb-based precipitates progresses, resulting in insufficient composite precipitation during stress relief annealing. From the viewpoint of uniform and composite precipitation of Nb-based precipitates, the residence time at 860°C or higher may be greater than 0 seconds (for example, 5 seconds or more). Furthermore, if the residence time above 750°C is less than 50 seconds, the Nb-based precipitates that redissolve during strain-relieving annealing may not re-deposit completely, leading to magnetic aging. While there is no upper limit on the residence time above 750°C, longer periods will reduce productivity, so it is advisable to limit it to 300 seconds or less. As for the annealing separating agent, a known annealing separating agent can be applied using a known method. Distortion-relieving annealing is preferably performed simultaneously with the baking annealing of the insulating coating, and in order to control the oxidation state of the steel sheet surface, it is preferable to use a mixed atmosphere of hydrogen and nitrogen with a hydrogen content of less than 5 volume%, rather than a mixed atmosphere of nitrogen containing 5 volume% or more of hydrogen. In order to ensure the adhesion of the insulating coating on a grain-oriented electrical steel sheet called mirror-finish GO, which has a smoothed interface, if the atmospheric dew point is deliberately controlled by using a mixed atmosphere of hydrogen and nitrogen at a concentration of 5% or more by volume to form an externally oxidized SiO2 oxide film on the steel sheet surface, it is necessary to control the heat cycle not only for annealing to form the SiO2 oxide film, but also for annealing to bake the insulating coating.

[0065] The above process yields a grain-oriented electrical steel sheet. However, the manufacturing method for grain-oriented electrical steel sheets 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 strain-relieving annealing process, and a magnetic domain subdivision step of subdividing the magnetic domains of the grain-oriented electrical steel sheet on which the insulating coating has been formed. These steps further improve the magnetic properties.

[0066] "Insulating film formation process" An insulating coating may be formed on the surface of the grain-oriented electrical steel sheet described above. The insulating coating to be formed is not limited and may be any known coating. Furthermore, a known method can be applied to form the coating.

[0067] "Magnetic domain refining process" A known method may be used for the magnetic domain subdivision process. For example, one method involves narrowing the width of the 180° magnetic domain (subdividing the 180° magnetic domain) by forming linear or dot-shaped grooves extending in a direction intersecting the rolling direction at predetermined intervals along the rolling direction. Another method, when performed after the insulating film formation process, involves narrowing the width of the 180° magnetic domain (subdividing the 180° magnetic domain) by forming linear or dot-shaped stress-strained areas or grooves extending in a direction intersecting the rolling direction at predetermined intervals along the rolling direction. When forming stress-strained areas, laser beam irradiation and electron beam irradiation can be applied. Furthermore, when forming grooves, mechanical groove formation methods using gears, chemical groove formation methods using electrolytic etching, and thermal groove formation methods using laser irradiation can be applied. If the insulating coating is damaged due to the formation of stress-strained areas or grooves, and its properties such as insulation deteriorate, the insulating coating may be reapplied to repair the damage. [Examples]

[0068] Molten steel having the chemical compositions listed in Tables 1-1 to 1-4 was cast to form slabs. The obtained slabs were heated and hot-rolled according to the conditions in Tables 2-1 and 2-2 to produce hot-rolled steel sheets. The obtained hot-rolled steel sheet was subjected to hot-rolled sheet annealing. After heating to 1100°C, it was soaked at this temperature for 10 seconds, then furnace-cooled to the secondary soaking temperature of 900°C over 100 seconds. Subsequently, it was cooled 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 value obtained by dividing the temperature difference from the secondary soaking temperature to 500°C by the cooling time required to reach 500°C from the secondary soaking temperature. After annealing the hot-rolled steel sheet, it was cold-rolled under the conditions shown in Tables 2-3 and 2-4 to obtain cold-rolled steel sheets. The obtained cold-rolled steel sheets were subjected to decarburization annealing in a nitrogen-hydrogen mixed atmosphere of 75% hydrogen and 25% nitrogen by volume, with the dew point, soaking temperature, and soaking time set to the conditions shown in Tables 2-3 and 2-4, thereby changing the carbon content. The carbon content after the decarburization annealing process was equivalent to the chemical composition of the steel sheets shown in Tables 3-1 to 3-4. Subsequently, with a few exceptions, nitriding treatment was performed to increase the nitrogen content of the steel sheets. Subsequently, a known annealing separation agent mainly composed of MgO was applied, and finish annealing was performed under the conditions shown in Tables 2-3 and 2-4, followed by stress relief annealing. Stress relief annealing was performed in a hydrogen-nitrogen atmosphere with 3 volume% hydrogen introduced.

[0069] The chemical composition of steel sheets (cold-rolled steel sheets) after distortion-relieving annealing was investigated. The results are shown in Tables 3-1 to 3-4.

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

[0071] In the obtained steel sheet, the number ratio of precipitates containing Nb group elements to the total number of precipitates containing Nb group elements, and their average diameter, were determined by the method described above, which have a region in which Nb group elements are detected as the first peak and are complex precipitates at a position t / 2 from the surface in the thickness direction of the base steel sheet. The measurements were performed according to the procedure described above. A JEM-2100 transmission electron microscope manufactured by JEOL Ltd. was used, and the EX-24065JGP was used for EDS measurements.

[0072] Furthermore, the magnetic flux density B8 and iron loss W17 / 50 were evaluated using steel plates sampled from areas adjacent to the region where precipitates were observed, by measuring them according to the Epstein test method described in JIS C2550-1:2011. A magnetic flux density B8 of 1.920T or higher was considered sufficient.

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

[0074] [Table 1-1]

[0075] [Table 1-2]

[0076] [Table 1-3]

[0077] [Table 1-4]

[0078] [Table 2-1]

[0079] [Table 2-2]

[0080] Table 2-3

[0081] Table 2-4

[0082] Table 3-1

[0083] Table 3-2

[0084] Table 3-3

[0085] Table 3-4

[0086] Table 4-1

[0087] Table 4-2

[0088] As can be seen from the results in Tables 1-1 to 4-2, the present invention example, in which precipitates having a predetermined chemical composition, having a region in which Nb group elements are detected as the first peak, and having a number ratio of 50% or more of composite precipitates to the total precipitates containing Nb group elements, exhibits high magnetic flux density, small iron loss degradation due to magnetic aging, and low iron loss even after magnetic aging. On the other hand, in comparative examples where the chemical composition is outside the scope of the present invention and / or the proportion of Nb-based precipitates is small, one or more of the magnetic flux density, iron loss after aging, and the rate of iron loss degradation due to magnetic aging do not satisfy the target. [Industrial applicability]

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

Claims

1. Base material steel plate and Optionally, a glass coating formed on the surface of the base steel plate, Optionally, an insulating coating formed on the surface of the base steel plate or on the surface of the glass coating, It has, The aforementioned base steel plate is, by mass%, C: 0.0005-0.0050%, Si: 1.0 to 4.0%, One or more elements from the Nb group, including Nb, Ta, V, and Mo: total of 0.0030 to 0.0300%. Acid-insoluble Al: 0.0001 to 0.0300%, Mg: 0.0001-0.0200%, O: 0.0001 to 0.0200%, N: 0.0001 to 0.0100%, Mn: 0 to 0.50%, 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: 0-0.50%, 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: 0-0.0050%, Remainder: Fe and impurities, It has a chemical composition consisting of, When the thickness of the base steel sheet is denoted as t, in the region of the base steel sheet in the thickness direction from the surface at a position t / 20 to a position (19t) / 20, in the precipitate observed by the TEM replica method, EDS analysis reveals that the precipitates containing Nb group elements have a region where Nb group elements are detected as the primary peak and are complex precipitates, and the proportion of these precipitates to the total precipitates containing Nb group elements is 50% or more. A grain-oriented electrical steel sheet characterized by the following features.

2. A method for manufacturing grain-oriented electrical steel sheets according to claim 1, In mass percent, C: 0.0010-0.1000%, Si: 1.0 to 4.0%, At least one selected from the group consisting of Nb, Ta, V, and Mo: totaling 0.0030 to 0.0300%. Mn: 0.05-0.50%, 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: 0.0040-0.0120%, Cu: 0 to 0.40%, Bi: 0 to 0.010%, B: 0 to 0.080%, P: 0-0.50%, 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: 0-0.0050%, The casting process involves casting molten steel having a chemical composition consisting of the remainder: Fe and impurities, into a slab. The aforementioned slab is heated and hot-rolled to form a hot-rolled steel sheet in a hot-rolling process, A hot-rolled steel sheet annealing step, The hot-rolled steel sheet after the hot-rolled sheet annealing process is cold-rolled to produce a cold-rolled steel sheet in a cold-rolling process, A decarburization annealing step for decarburizing the cold-rolled steel sheet, A finish annealing step is performed in which an annealing release agent is applied to the cold-rolled steel sheet after the decarburization annealing step, and then finish annealing is carried out. A distortion-relieving annealing step is performed on the cold-rolled steel sheet after the finish annealing step, Equipped with, In the decarburization annealing process, the carbon content of the cold-rolled steel sheet is set to 0.0050% or less. In the aforementioned finish annealing process, The maximum stay time at 1210°C or higher is 2 hours or less, and the maximum stay time at 1180°C or higher is 5 to 30 hours. During cooling, the stay time at 950-800°C should be 5 hours or more. In the aforementioned strain-relieving annealing process, The duration of stay at temperatures above 860°C must be 30 seconds or less, and the duration of stay at temperatures above 750°C must be 50 seconds or more. A method for manufacturing grain-oriented electrical steel sheets, characterized by the following features.

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