Decarburized annealed steel sheet

Abstract

This decarburization-annealed steel sheet has a chemical composition of at least one element selected from the group consisting of Nb, V, Mo, Ta, and W in total amount in mass% of 0.003-0.030% and has a metal structure containing a Nb-based precipitate that is a precipitate containing at least one element selected from the group consisting of Nb, V, Mo, Ta, and W, and an Al-based precipitate that is a precipitate containing Al; and when the emission intensity of an element for each particle diameter of the Nb-based precipitate and the Al-based precipitate is analyzed using a field-flow fractionation method, the particle diameter DAl at the peak of the emission intensity of Al is 75-250 nm, the particle diameter Dx at the peak of the emission intensity of Nb, V, Mo, Ta, and W is 20-200 nm, and the DAl is greater than the Dx by 10 nm or more, and in the metal structure, the average crystal particle diameter d is 13.0-19.0 μm and σ / d is 0.500 or less where σ is the standard deviation of the crystal particle diameter.

Description

[Technical Field]

[0001] This invention relates to a decarburized annealed steel sheet. This decarburized annealed steel sheet is effective as an intermediate product sheet for grain-oriented electrical steel sheets. This application claims priority based on Japanese Patent Application No. 2024-034139, filed in Japan on March 6, 2024, and the contents of that application are incorporated herein by reference. [Background technology]

[0002] Grain-oriented electrical steel sheets contain approximately 7% by mass or less of Si, {110} <001> It has a secondary recrystallized texture concentrated in a specific orientation (Goss orientation). {110} <001> Orientation refers to the orientation in which the {110} planes of the crystal are aligned parallel to the rolling plane, and the crystal is <001> This means that the axis is positioned parallel to the rolling direction.

[0003] The magnetic properties of grain-oriented electrical steel sheets are {110} <001> It is greatly influenced by the degree of concentration in a particular direction. In particular, the rolling direction of the steel sheet, which is the main magnetization direction when the steel sheet is used, and the crystal, which is the easy magnetization direction. <001> The relationship with direction is considered important. Therefore, in recent practical grain-oriented electrical steel sheets, the crystal structure <001> The angle between the direction of movement and the rolling direction is controlled to fall within a range of approximately 5°.

[0004] Such precise crystal orientation control is achieved by appropriately dispersing fine precipitates called inhibitors in the steel before finish annealing, and by holding the steel sheet at a high temperature during finish annealing. For example, the inhibitors enhance the selective growth of Goss-oriented grains, and as a result, secondary recrystallization proceeds so that Goss-oriented grains preferentially grow during finish annealing. To date, attempts have been made to highly control inhibitors with the aim of precisely controlling crystal orientation.

[0005] For example, Patent Document 1 discloses the use of MnS as an inhibitor and the performance of two cold rolling processes. 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 Publication No. 02-200732 [Patent Document 8] Japanese Patent No. 4962516

Patent Document 9

Patent Document 10

Patent Document 11

Summary of the Invention

Problems to be Solved by the Invention

[0009] In recent years, in the global movement for environmental conservation such as power and energy saving, the demand for higher efficiency of transformers has been increasing. Under such a social environment, there is also a demand for improving the performance of grain-oriented electrical steel sheets used for transformer core materials and the like. In particular, it is required to increase the magnetic flux density of grain-oriented electrical steel sheets.

[0010] As a result of the study by the present inventors, it cannot be said that the conventional inhibitor control techniques disclosed in the above Patent Documents 1 to 8 sufficiently meet the requirements for grain-oriented electrical steel sheets in recent years, and it has been found that further increase in magnetic flux density is necessary.

[0011] Also, in the techniques of Patent Documents 9 to 11, as described above, by precisely controlling the finish annealing, the secondary recrystallization temperature range is expanded, and the preferential growth of Goss-oriented grains closer to the ideal Goss orientation represented by {110}<001> in terms of crystallography is promoted, resulting in improvement of magnetic properties such as magnetic strain of the steel sheet that affects the noise characteristics of the transformer. At the same time, it has been shown that by adding trace amounts of Nb, Ta, Mo, etc., the frequency of sub-grain boundaries increases, the effect of improving magnetic properties increases, and the finish annealing conditions for the manifestation of the effect of improving magnetic properties are relaxed. This suggests that elements such as Nb are effectively functioning as a concrete method to realize the idea of ​​extending 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 these Nb group element precipitates is influencing secondary recrystallization. On the other hand, with these technologies, precipitates of tracely added Nb group elements precipitate at lower temperatures than conventional precipitates such as AlN and MnS. Therefore, conventional hot rolling and hot rolled sheet annealing process conditions cannot adequately control the precipitation state, which can affect the primary recrystallization structure and prevent the magnetic property improvement effect from being fully realized.

[0012] This invention was made in view of the above-mentioned problems. Based on the current demand for increasing the magnetic flux density of grain-oriented electrical steel sheets, this invention aims to provide a decarburized annealed steel sheet, which is an intermediate product sheet of grain-oriented electrical steel sheets, that contributes to improving the magnetic flux density of grain-oriented electrical steel sheets. [Means for solving the problem]

[0013] The inventors of the present invention investigated microstructure control at the intermediate product stage in order to improve the magnetic flux density of grain-oriented electrical steel sheets. As a result, we found that by controlling the grain size and precipitate distribution in the decarburized annealed steel sheet (i.e., the decarburized annealed steel sheet), the magnetic flux density of the final grain-oriented electrical steel sheet can be improved.

[0014] This invention was made in view of the above findings. The gist of this invention is as follows.

[0015] [1] A decarburized annealed steel sheet according to one aspect of the present invention is A decarburized annealed steel sheet, which is an intermediate product sheet of grain-oriented electrical steel sheet,In mass%, the composition is as follows: C: 0.0005~0.010%, Si: 2.0~7.0%, Mn: 0.05~1.00%, S: 0~0.035%, Se: 0~0.035%, total S+Se content: 0.003~0.035%, Al: 0.010~0.065%, N: 0.0040~0.0400%, at least one element selected from the group consisting of Nb, V, Mo, Ta, and W: total 0.003~0.030%, Cu: 0~0.40%, Bi: 0~0.010%, B: 0~0.080%, P: 0~0.500%, Ti: 0~0.015%. The chemical composition consists of Sn: 0-0.100%, Sb: 0-0.100%, Cr: 0-0.300%, Ni: 0-1.000%, the remainder being Fe and impurities, and the metallic structure includes Nb-based precipitates containing at least one element selected from the group consisting of Nb, V, Mo, Ta, and W, and Al-based precipitates containing Al. When the emission intensity of elements for each particle size of the Nb-based precipitates and Al-based precipitates is analyzed using the field flow fractionation method, the emission intensity of Al shows a peak at particle size D. Al However, the particle size D is 75-250 nm and shows peaks in emission intensity for Nb, V, Mo, Ta, and W. x However, it is 20-200 nm, and the aforementioned D Al The above D x It is larger than 10 nm, and in the aforementioned metal structure, the average grain size d is 13.0 to 19.0 μm, and when the standard deviation of the grain size is σ, σ / d is 0.500 or less. [2] The decarburized annealed steel sheet described in [1] above may have an N content of 0.0130 to 0.0400% by mass. [Effects of the Invention]

[0016] According to the above aspects of the present invention, it is possible to provide a decarburized annealed steel sheet, which is an intermediate product sheet of grain-oriented electrical steel sheet, that contributes to improving the magnetic flux density of the grain-oriented electrical steel sheet. [Modes for carrying out the invention]

[0017] A decarburized annealed steel sheet according to one embodiment of the present invention (a decarburized annealed steel sheet according to this embodiment) 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] 1.Chemical composition The chemical composition of the decarburized annealed steel sheet according to this embodiment shall be within the following range. Within the numerical limit ranges described below, ranges indicated by "~" include the values ​​at both ends as the lower and upper limits, respectively. However, numerical values ​​indicated as "greater than" or "less than" are not included in the numerical range. Furthermore, unless otherwise specified, "%" in relation to chemical composition means "mass%".

[0019] The decarburized annealed 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.

[0020] The decarburized annealed steel sheet according to this embodiment contains, by mass%, the following basic elements (major alloying elements): C: 0.0005~0.010%, Si: 2.0~7.0%, Mn: 0.05~1.00%, S: 0~0.035%, Se: 0~0.035%, total S+Se content: 0.003~0.035%, Al: 0.010~0.065%, N: 0.0040~0.0400%, and at least one element selected from the group consisting of Nb, V, Mo, Ta, and W: totaling 0.003~0.030%.

[0021] C: 0.0005~0.010% 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. Therefore, while the C content of the slab is preferably 0.0010 to 0.100%, excessive C content in the final product adversely affects the magnetic properties. Accordingly, the C content of the decarburized annealed steel sheet should be 0.010% or less. The C content is preferably 0.009% or less, and more preferably 0.008% or less. C is purified in the finish annealing process, and after the finish annealing process, it becomes 0.005% or less. In decarburized annealed steel sheets, a lower carbon content is preferable, but considering productivity in industrial production, the carbon content should be 0.0005% or higher. The carbon content may also be 0.0010% or higher.

[0022] Si: 2.0~7.0% 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 2.0%, 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 2.0% 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 of decarburized annealed steel sheets should be 7.0% or less. Preferably, the Si content is 4.5% or less, and more preferably 4.0% or less.

[0023] Mn: 0.05~1.00% Manganese (Mn) is an element that combines with S and Se to precipitate as MnS and MnSe, functioning as an inhibitor. To favorably control the morphology of these inhibitors (precipitates), the Mn content of the decarburized annealed steel sheet is set to 0.05% or higher. If the Mn content falls below 0.05%, the amount of MnS and MnSe that function as inhibitors will be insufficient, thus inhibiting the proper progress of secondary recrystallization. The Mn content is preferably 0.09% or higher. On the other hand, if the Mn content exceeds 1.00%, the amount of MnS and MnSe that function as inhibitors will be excessively precipitated, inhibiting the proper progress of secondary recrystallization. In this embodiment, some of the inhibitory functions 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.

[0024] S: 0~0.035% Se: 0~0.035% S+Se total content: 0.003~0.035% Sulfur (S) and selenium (Se) are elements that combine with manganese (Mn) to precipitate as MnS and MnSe, respectively, and function as inhibitors. To favorably control the morphology of these inhibitors (precipitates), the S content of the decarburized annealed steel sheet should be 0-0.035%, the Se content should be 0-0.035%, and the total S+Se content should be 0.003-0.035%. A total S and Se content of 0.003-0.035% is preferable because it stabilizes secondary recrystallization. On the other hand, in this embodiment, some of the inhibitory functions may be carried out by carbides, nitrides, or carbonitrides of Nb group elements. In this case, the amount of precipitated MnS and MnSe, which are inhibitors, may be controlled to be small. Therefore, the total content of S and Se may be 0.025% or less, or 0.010% or less. As described above, the sulfur (S) and sediment (Se) content in decarburized annealed steel sheets is as shown. However, if S and Se remain in the steel after finish annealing, they can form compounds that degrade iron loss. Therefore, it is preferable to remove S and Se from the steel through purification during finish annealing to reduce their content.

[0025] Here, "the total content of S and Se is 0.003 to 0.035%" means that the decarburized annealed steel sheet may contain only S or Se in its chemical composition, and its content may be between 0.003 and 0.035%. Alternatively, it may contain both S and Se, and their total content may be between 0.003 and 0.035%.

[0026] Al: 0.010~0.065% Aluminum (Al) is an element that combines with nitrogen (N) to precipitate as AlN or (Al,Si)N, functioning as an inhibitor. To favorably control the morphology of these inhibitors (precipitates), the Al content of the decarburized annealed steel sheet is set to 0.010% or higher. When the Al content is 0.010% or higher, AlN and (Al,Si)N precipitate in favorable forms during the nitriding treatment in the low-temperature slab heating process, and secondary recrystallization is particularly stable in the high-temperature range. If the Al content is below 0.010%, the amount of AlN and (Al,Si)N that function as inhibitors precipitated is insufficient, and the proper progress of secondary recrystallization is inhibited. The Al content is preferably 0.020% or higher, and more preferably 0.025% or higher. On the other hand, if the Al content exceeds 0.065%, the amount of AlN and (Al,Si)N, which function as inhibitors, precipitates excessively, inhibiting the proper progress of secondary recrystallization. From the viewpoint of the stability of secondary recrystallization, the Al content is preferably 0.040% or less, and more preferably 0.030% or less.

[0027] N: 0.0040~0.0400% 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 morphology of these inhibitors (precipitates), the N content of decarburized annealed steel sheets is set to 0.0040-0.0400%. If the N content of the slab exceeds 0.0120%, blistering, a type of defect, is more likely to occur in the steel sheet, so it is preferable that the N content of the slab be 0.0120% or less. However, in the low-temperature slab heating process, N is incorporated into the steel by nitriding treatment before secondary recrystallization occurs in the finish annealing process, resulting in an N content of 0.0130-0.0400%. Nitriding is performed during or after the decarburization annealing process. In some cases, MnN or the like may be added to the annealing separating agent and nitrided during the finish annealing process. Therefore, the N content of the decarburized annealed steel sheet before nitriding should be 0.0040 to 0.0400%, assuming that the N content will be increased to 0.0130 to 0.0400% after nitriding if the N content is less than 0.0130%. Preferably, it should be 0.0040 to 0.0120%. On the other hand, the N content of the decarburized annealed steel sheet after nitriding should be 0.0130 to 0.0400%. The N content before nitriding is preferably 0.0100% or less, and more preferably 0.0090% or less. N is purified in the finish annealing process and becomes 0.0050% or less after the finish annealing process.

[0028] At least one element selected from the group consisting of Nb, V, Mo, Ta, and W: totaling 0.003-0.030% Niobium (Nb), vanadium (V), molybdenum (Mo), tantalum (Ta), and tungsten (W) 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, V, Mo, Ta, and W is included. In this embodiment, Nb, V, Mo, Ta, and W may be collectively referred to as "Nb group elements." If the total content of Nb group elements falls below 0.003%, there will be insufficient precipitates of Nb group elements that act as precipitation nuclei, making it difficult for MnS and AlN to become finely granulated. Therefore, the total content of Nb group elements should be 0.003% or higher. Preferably, the total content of Nb group elements should be 0.004% or higher, and more preferably 0.005% or higher. On the other hand, if the total content of Nb group elements exceeds 0.030%, the precipitation temperature range of Nb group element precipitates becomes high, and the Nb group element precipitates tend to be coarse and low in density. In addition, 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. For this reason, the total content of Nb group elements should be 0.030% or less. Preferably, the total content of Nb group elements is 0.020% or less, and more preferably 0.010% or less.

[0029] The reason why precipitates of Nb group elements act as inhibitors and have the effect of favorably expanding the secondary recrystallization temperature range is not clear, but it is thought to be as follows. As will be described later, by appropriately controlling processes such as the hot rolling process and the hot-rolled sheet annealing process, and thereby favorably controlling the primary recrystallized grain structure, it is possible to promote the preferential growth of Goss-oriented grains that occur during the decomposition of precipitates. It is believed that by controlling the precipitation states of AlN, which normally decomposes in a high temperature range of over 1000°C, and Nb group element precipitates, which begin to decompose in a lower temperature range than AlN, the pinning effect of primary recrystallized grains by the Nb group element precipitates and the pinning effect by AlN are separated (more precisely, the combined precipitation state of AlN and Nb group element precipitates is separated into precipitates with a high proportion of Al (high Al / Nb group element ratio in the precipitate) and precipitates with a high proportion of Nb group elements (low Al / Nb group element ratio), and the precipitation state is spatially uniform), the preferential growth of Goss-oriented grains that occurs during the decomposition of Nb group element precipitates and the subsequent preferential growth of Goss-oriented grains during AlN decomposition are made possible in two stages. In particular, carbides, nitrides, or carbonitrides of Nb group elements act as precipitation nuclei for MnS and AlN in the temperature range of 900-1100°C during the cooling process from high temperatures. Therefore, compared to the case where Nb group elements are not present, the presence of Nb group elements results in a greater number of MnS and AlN precipitation sites, making it easier for MnS and AlN to form as precipitates in a spatially uniform manner. Furthermore, in the low temperature range below 900°C, precipitates with a higher proportion of Nb group elements compared to Al precipitate in a spatially uniform and fine manner. By effectively utilizing this phenomenon, the primary recrystallization structure can be controlled to be smaller in diameter and have less variation in grain size than conventional methods. This allows for preferential growth of Goss-oriented grains during the decomposition of precipitates with a high proportion of uniform, fine Nb group elements (low Al / Nb group element ratio). Among these, grains with a crystal orientation closer to the ideal Goss orientation gain a size advantage during the decomposition of AlN (precipitates with a high proportion of Al), and it is thought that Goss-oriented grains closer to the ideal orientation will further undergo preferential growth. In the decarburized annealed steel sheet according to this embodiment, the secondary recrystallization temperature range is expanded by allowing fine inhibitors and coarse inhibitors to coexist. It is believed that precipitates of Nb group elements are particularly effective in expanding the secondary recrystallization temperature range to lower temperatures.

[0030] The decarburized annealed 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 total content of impurities may be, for example, 5.0% or less, preferably 0.5% or less, and more preferably 0.1% or less.

[0031] Furthermore, in the decarburized annealed steel sheet according to this embodiment, in addition to the basic elements and impurities described above, optional elements (arbitrary elements) may be included. For example, instead of a portion of the remaining Fe described above, one or more elements selected from the group consisting of Cu, Bi, B, P, Ti, Sn, Sb, Cr, and Ni may be included as optional elements. These optional elements may be included according to the 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.

[0032] Cu: 0~0.40% Bi: 0~0.010% B: 0~0.080% P: 0-0.50% Ti: 0~0.015% Sn: 0~0.100% Sb: 0~0.100% Cr: 0~0.300% Ni: 0~1.000% Copper (Cu), bismuth (Bi), boron (B), phosphorus (P), titanium (Ti), tin (Sn), antimony (Sb), chromium (Cr), and nickel (Ni) may be included 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%.

[0033] The chemical composition of the decarburized annealed steel sheet according to this embodiment can be measured using general analytical methods for steel. For example, the chemical composition of the decarburized annealed 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 decarburized annealed steel sheet using ICP-AES under conditions based on a pre-established calibration curve. C and S, which are difficult to measure with ICP-AES, can be measured using the combustion-infrared absorption method, and N can be measured using the inert gas fusion-thermal conductivity method.

[0034] 2.Metal structure 2.1 Precipitates Next, the precipitates contained in the decarburized annealed steel sheet according to this embodiment will be described. In the decarburized annealed steel sheet according to this embodiment, its metal structure includes Nb-based precipitates containing at least 1 kind selected from the group consisting of Nb, V, Mo, Ta, and W at 0.003% or more, and Al-based precipitates containing Al. Further, when analyzing the emission intensity of elements for each particle size of Nb-based precipitates and Al-based precipitates using the field flow fractionation (FFF) method, the particle size D at which the emission intensity of Al shows a peak Al is 75 to 250 nm, and the particle size D at which the emission intensity of Nb, V, Mo, Ta, and W shows a peak x is 20 to 200 nm, and the said D Al is 10 nm or more larger than the said D x . By controlling the Nb-based precipitates and Al-based precipitates as described above, the selective growth property of Goss-oriented grains in secondary recrystallization is favorably exhibited, and the selective effect of Goss-oriented grains at the time of decomposition of Al-based precipitates becomes high, whereby the magnetic flux density B8 of the grain-oriented electrical steel sheet is improved. D Al If it is less than 75 nm, the pinning effect of the Al-based precipitates becomes excessively large, so that the preferential growth of the Goss orientation at the time of decomposition of the Nb-based precipitates does not proceed sufficiently. In this case, the size advantage (with respect to matrix grains other than Goss-oriented grains) of Goss-oriented grains having a crystal orientation close to the ideal Goss orientation at the time of decomposition of the Al-based precipitates becomes insufficient, and a sufficient magnetic flux density improvement effect cannot be obtained. D Al If it exceeds 250 nm, the pinning effect of the Al-based precipitates becomes weak, so that the preferential growth of the generated Goss-oriented grains at the time of decomposition of the Nb-based precipitates cannot be stopped. In this case, the second-stage selective preferential growth of Goss-oriented grains at the time of decomposition of the Al-based precipitates disappears, and a sufficient magnetic flux density improvement effect cannot be obtained. D xHowever, below 20 nm, the size and decomposition of Nb-based precipitates during the final annealing process proceed rapidly. In this case, the selective growth of Goss-oriented grains in the first stage is not performed properly, and a sufficient improvement in magnetic flux density cannot be obtained. D x However, beyond 200 nm, the pinning effect of Nb precipitates decreases, resulting in insufficient reduction of the primary recrystallized grain size. In this case, the driving force for grain growth of Nb precipitates during finish annealing is reduced, and the size advantage during the selective growth of Goss-oriented grains in the first stage decreases, making it impossible to obtain a sufficient magnetic flux density improvement effect. Also, D Al is D x If the size is not 10 nm or more larger than that, only one type of precipitate morphology contributes to the pinning effect of the primary recrystallized grains. As a result, the preferential growth of Goss-oriented grains in the second stage, following the preferential growth of Goss-oriented grains at the decomposition temperature of the Nb-based precipitates and subsequent decomposition of the Al-based precipitates, does not occur properly, and a sufficient magnetic flux density improvement effect cannot be obtained.

[0035] D Al and D x This can be obtained by the method described in Patent No. 4572001, as follows. For example, the extraction residue (precipitation) can be recovered from the electrolytic extract, and the size and distribution of this recovered precipitate can be measured using the field flow fractionation (FFF) method. For particle separation using the FFF method, for example, a 0.05 wt% sodium dodecyl sulfate (SDS) solution can be used as the developing solvent and flowed at a rate of 1 mL / min. After separating particles by size in the FFF apparatus and measuring the size and number density, the discharged solution is subjected to component analysis using a standard ICP (inductively coupled plasma) mass spectrometer. A histogram is created from the emission intensity for each particle size measured by the FFF method, with particle size on the x-axis and emission intensity on the y-axis, and the particle size showing the peak emission intensity is determined.

[0036] 2.2 Grain size In the decarburized annealed steel sheet according to this embodiment, the average grain size d in the metal structure is 13.0 to 19.0 μm, and when the standard deviation of the grain size is σ, σ / d is 0.500 or less. Reducing the grain size (primary recrystallization grain size) of the decarburized annealed steel sheet increases the driving force for secondary recrystallization. Therefore, the average grain size d should be 19.0 μm or less. Preferably, the average grain size is 18.0 μm or less. On the other hand, if the average grain size d is less than 13.0 μm, the grain growth driving force at the decomposition temperature of the Nb-based precipitate during finish annealing becomes too large, causing grain growth in grains other than those with the Goss orientation. Therefore, the average grain size should be 13.0 μm or larger. Preferably, the average grain size is 15.0 μm or larger. Furthermore, even if the grain size is small, if there is a large variation in grain size, the selective growth of Goss-oriented grains will be inhibited. For this reason, the ratio σ / d, which is expressed as the average grain size d and the standard deviation σ of the grain size, should be kept below 0.500.

[0037] The average grain size, σ / d, mentioned above can be determined by polishing a cross-section perpendicular to the width direction of the steel sheet (meaning a cross-section of the steel sheet that includes both the vertical direction and the rolling direction), followed by electropolishing, and then measuring the grain size and grain size distribution in the cross-section using the EBSD (Electron Backscatter Diffraction Pattern) method.

[0038] 3. Manufacturing method The decarburized annealed steel sheet according to this embodiment can be manufactured by a manufacturing method that includes the following steps: (I) A casting process in which molten steel having the same chemical composition as the decarburized annealed steel sheet according to the above embodiment is cast to form a slab, (II) A hot rolling process in which the slab is heated to more than 1030°C but less than 1180°C 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, It can be manufactured by a manufacturing method that includes [a specific feature / feature].

[0039] [Casting Process] In the casting process, slabs for hot rolling are prepared. From the slab to the decarburized annealed steel sheet, the chemical composition, except for the carbon content and, if nitriding is performed, the nitrogen content, remains almost unchanged. Therefore, the chemical composition of the slab is set to the chemical composition of the target decarburized annealed steel sheet (the chemical composition of the decarburized annealed steel sheet described above). On the other hand, the carbon content and nitrogen content (mass%) of the slab are as follows.

[0040] An example of a slab manufacturing method is as follows: Molten steel is produced (melted). Slabs are manufactured using this molten steel. For example, slabs may be manufactured by continuous casting. Alternatively, ingots may be manufactured using molten steel, and slabs may be manufactured by bloc rolling of the ingots. The thickness of the slabs is, for example, 150 to 350 mm. Preferably, the thickness of the slabs is 220 to 280 mm.

[0041] C: 0.0010~0.100% Carbon (C) is an effective element for controlling the primary recrystallization structure during the manufacturing process. Therefore, the C content of the slab should be 0.0010% or more. Preferably, it should be 0.010% or more. 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 0.100% or less.

[0042] 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 morphology of these inhibitors (precipitates), the N content of the slab should be 0.0040% or higher. On the other hand, if the nitrogen content of the slab exceeds 0.0120%, blistering, a type of defect in steel plates, is more likely to occur. Therefore, the nitrogen content of the slab should be kept below 0.0120%.

[0043] [Hot rolling process] In the aforementioned hot rolling process, the slab is heated to a temperature between 1030°C and 1180°C. This causes the precipitates contained in the slab at room temperature before heating to be dissolved. If the heating temperature is below 1030°C, sufficient dissolution does not occur. On the other hand, if the heating temperature exceeds 1180°C, the solubilization rate becomes too high. The heating temperature is preferably 1070 to 1130°C. Furthermore, if the soaking time is 40 minutes or less, the time is too short, making it difficult to control the solution state of the precipitate to an equilibrium state. For this reason, it is preferable to set the soaking time to 40 minutes or more. To homogenize the solution state within the slab, it is even preferable to set the soaking time of the slab to more than 70 minutes. The upper limit of the soaking time mentioned above is not particularly limited, but considering the productivity in industrial production, it may be 2 hours. After heating as described above, the slab is hot-rolled (rough rolling and finish rolling) to produce a hot-rolled steel sheet. At this time, the rough rolling temperature, defined as the average value of the start and end temperatures of rough rolling, is set to a temperature range of 940 to 1070°C, and the reduction ratio during rough rolling is set to 82 to 95%. If the rough rolling temperature is higher than 1070°C, MnS precipitation does not occur sufficiently during rough rolling, and many MnS precipitates together with AlN and Nb-based precipitates such as Nb(C,N), which are precipitates of Nb group elements. As a result, the separation of the pinning effect of primary recrystallized grains by Nb group element precipitates and the pinning effect by AlN is not sufficient, and the preferential growth of the Goss orientation during secondary recrystallization is reduced. The rough rolling temperature is preferably 1050°C or lower. Also, if the rough rolling temperature is below 940°C, the slab becomes hard and the rollability decreases. The rough rolling temperature is preferably 960°C or higher, and more preferably 980°C or higher. Furthermore, by setting the roughing reduction ratio within the above range, processing-induced precipitation occurs, making it possible to precipitate a large amount of MnS precipitates. If the roughing reduction ratio is smaller than the lower limit above, many MnS precipitates will be complex precipitates with AlN precipitates and precipitates of Nb group elements such as Nb(C,N) (Nb-based precipitates). As a result, the separation of the pinning effect of primary recrystallized grains by Nb group element precipitates and the pinning effect by AlN will not be sufficient, and the preferential growth of the Goss orientation during secondary recrystallization will be reduced. The roughing reduction ratio should be 95% or less, taking into consideration the performance of the rolling mill, etc. Furthermore, the time between the completion of rough rolling and the start of finish hot rolling should be set to 20 seconds or more. This allows for sufficient precipitation of MnS, effectively reducing the proportion of AlN precipitation and complex precipitation with Nb group elements such as Nb(C,N). In this case, the effect of improving the uniformity of the primary recrystallized grain structure of the fine Nb group element precipitates is obtained, and the effect of promoting the separation of the pinning effect of the primary recrystallized grains by the Nb group element precipitates and the pinning effect by AlN is obtained, thereby improving the preferential growth of the Goss orientation.

[0044] The hot-rolling operating conditions described above primarily control Al-based precipitates. Specifically, in order to ensure Al-based precipitates that contribute to the preferential growth of the second stage during finish annealing, Al-based precipitates that do not precipitate at the same size as Nb-based precipitates, but are larger in size than Nb-based precipitates, are secured during the hot-rolled sheet annealing process. To achieve this, it is important to ensure sufficient AlN precipitates at a temperature higher than the precipitation nose temperature (in the precipitation temperature range) of Nb-based precipitates after hot-rolled sheet annealing. For this purpose, it is effective to allow sufficient MnS to precipitate before finish hot-rolling in the hot-rolling process. Controlling the process in this way facilitates precipitate control based on the hot-rolled sheet annealing process conditions described later. While specific conditions are not limited, completing the rough rolling in a temperature range of 940°C or higher, where AlN precipitation is significant, and ensuring a holding time of 20 seconds or more between the completion of rough rolling in the above temperature range and the start of finish hot rolling (holding time in that temperature range) can significantly increase the difference in size between Nb-based precipitates and Al-based precipitates that precipitate after secondary soaking of the hot-rolled sheet. There is no upper limit to the holding time, but for productivity reasons, the holding time may be 100 seconds or less. The starting temperature for finish rolling is not limited, but it is preferably 930 to 1040°C. Furthermore, it is preferably 960 to 1020°C. The completion temperature for finish rolling is set to 850°C to 950°C, in order to ensure sufficient size of Al precipitates, optimize the size difference between Nb precipitates and Al precipitates, and create a uniform primary recrystallized grain structure. 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 can be wound up at a predetermined temperature. The winding temperature is not limited, but is typically between 300 and 650°C.

[0045] [Hot-rolled sheet annealing process] In the hot-rolled sheet annealing process, the sheet is heated to an annealing temperature of 1040-1120°C (first-stage annealing temperature), held for 0-120 seconds, then cooled to a temperature range of 950-850°C so that the average cooling rate from 1050-900°C is 1-5°C / second, held at that temperature range (second-stage annealing temperature) for 10-150 seconds, and then cooled to a temperature below 500°C so that the average cooling rate from 750-500°C is 5-80°C / second. By setting the first annealing temperature (maximum temperature reached) during hot-rolled sheet annealing within the above range and holding it at that temperature range for 0 to 120 seconds, Al-based precipitates (such as AlN) are preferably dissolved. Furthermore, the dissolved Al-based precipitates tend to re-precipitate in the 1050 to 900°C temperature range during the cooling process. Therefore, controlling the average cooling rate in the 1050 to 900°C temperature range during the cooling process allows for favorable control of Al-based precipitate re-precipitation. For example, at the maximum temperature reached, fine Al-based precipitates formed during hot rolling preferentially dissolve, and if the average cooling rate in the 1050 to 900°C temperature range during the cooling process is within the above range, Al-based precipitates preferentially re-precipitate more coarsely compared to Nb-based precipitates. Since the coarse Al-based precipitates remain dissolved until relatively high temperatures during the heating process of finish annealing, the secondary recrystallization temperature range is preferably extended to higher temperatures. Similarly, when the maximum temperature reached during hot-rolled sheet annealing is within the above range, Nb precipitates are preferably dissolved. The dissolved Nb precipitates tend to reprecipitate in the cooling process at a temperature range of 1000-500°C, below the complete solution temperature determined by the chemical composition of the slab (for example, about 1050°C for a chemical composition containing 0.010% Nb, 0.060% C, and 0.0080% N). Therefore, by controlling the average cooling rate in the temperature range up to 900°C, where Al precipitates coarsely precipitate, Nb precipitates can be precipitated in combination with Al precipitates that remain undissolved at the maximum temperature reached, or they can serve as precipitation nuclei for reprecipitated Al precipitates. Furthermore, by setting the annealing temperature for the second stage to 850-950°C and the holding time to 10-150 seconds, sufficient Al-based precipitates can be precipitated, and the size of the fine Nb-based precipitates that precipitate at temperatures below 850°C during cooling can be well separated from the size of the Al-based precipitates. Furthermore, by controlling the average cooling rate in the 750-500°C temperature range during the cooling process after the second annealing stage, the reprecipitation of Nb-based precipitates with a low Al content can be favorably controlled. For example, if the average cooling rate in the cooling process is within the above range of 750-500°C, fine Nb-based precipitates can be favorably formed. As a result, the uniformity of the primary recrystallized grain structure is improved, and since the fine Nb-based precipitates begin to dissolve from relatively low temperatures during the heating process of the finish annealing, the secondary recrystallization temperature range is favorably expanded to the lower temperature side.

[0046] The average cooling rate between 1050°C and 900°C during the cooling process is calculated as follows: when the first-stage annealing temperature (maximum temperature reached) is 1050°C or lower, the temperature difference from the maximum temperature reached to 900°C is divided by the cooling time from the maximum temperature to 900°C. When the maximum temperature reached is between 1050°C and 1120°C, the average cooling rate is calculated as follows: when the temperature difference from 1050°C to 900°C (150°C) is divided by the cooling time from 1050°C to 900°C. Similarly, the average cooling rate between 750°C and 500°C during the cooling process is calculated as follows: when the temperature difference from 750°C to 500°C (200°C) is divided by the cooling time from 750°C to 500°C.

[0047] [Cold rolling process] In the cold rolling process, the hot-rolled steel sheet after the hot-rolled sheet annealing process is subjected to a single cold rolling (a series of cold rollings without intermediate annealing) or multiple cold rollings (two or more) with annealing (intermediate annealing) in between, 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%.

[0048] 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

[0049] 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, during secondary recrystallization, ideal Goss-oriented grains (ideal {110} <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 have preferentially grown.

[0050] [Decarburization annealing process] The decarburization annealing process is a process in which a cold-rolled steel sheet obtained in the cold-rolling process is subjected to decarburization annealing to obtain a decarburized annealed steel sheet in which primary recrystallization has occurred. In the production of the decarburized annealed steel sheet according to this embodiment, the decarburization annealed steel sheet process includes a heating process to heat up to a soaking temperature, a soaking process to hold at the soaking temperature, and a cooling process to cool after the soaking process. In the heating process, the average heating rate is set to 20 to 300°C / second at 500 to 700°C, and in the soaking process, the soaking temperature is set to 830 to 870°C, with a holding time at the soaking temperature of 60 to 180 seconds. The above conditions are one of the conditions for realizing the features of the intermediate product plate of the present invention. As described above, by controlling the annealing conditions before the decarburization process and controlling the precipitates, and then performing the decarburization annealing process under the above conditions, the primary recrystallized grain size after decarburization annealing can be set to an average grain size d of 13.0 to 19.0 μm, and when the standard deviation of grain size is σ, σ / d can be set to 0.500 or less.

[0051] During the heating process, the average heating rate between 500 and 700°C affects the Nb-based precipitates deposited during the annealing and cooling of the hot-rolled sheet. Since these Nb-based precipitates have a pinning effect on the primary recrystallized grain structure, the average heating rate in this temperature range affects this pinning effect. If the average heating rate between 500 and 700°C is less than 10°C / second, the desired σ / d value cannot be obtained due to changes in the Nb-based precipitates. On the other hand, even if the average heating rate between 500 and 700°C exceeds 300°C / second, the desired σ / d value cannot be obtained. Furthermore, the reason for setting the soaking temperature to 830-870°C and the soaking time to 60-180 seconds is that the 830-870°C temperature range is a temperature range in which Nb-based precipitates and Al-based precipitates are prone to change, and by controlling the soaking time in this temperature range, uniform precipitation of Nb-based carbonitrides and Al-based precipitates can be maintained. If the soaking temperature is below 830°C, the primary recrystallized grain structure becomes mixed due to the influence of fine Al-based precipitates that precipitate below 900°C that did not fully precipitate by 900°C, resulting in a larger σ / d value. In this case, the two-stage selective growth of good Goss orientation grains is suppressed. On the other hand, if the soaking temperature exceeds 870°C, the finely precipitated Nb-based precipitates during hot-rolled sheet annealing become coarser, suppressing the two-stage selective growth of primary recrystallized grain size and good Goss orientation grains. Furthermore, if the soaking time is less than 60 seconds, the decarburization time is short, resulting in insufficient decarburization. On the other hand, if the soaking time exceeds 180 seconds, the primary recrystallized grain size may become coarser, and the changes in precipitates may become too large. While the cooling process is not necessarily limited, if the N content of the cold-rolled steel sheet is low, and the residence time at 750-800°C is less than 30 seconds, the Nb-based precipitates that partially redissolved during soaking may not re-deposit completely, resulting in insufficient pinning effect before secondary recrystallization during finish annealing. Therefore, especially if the nitriding treatment is not yet complete and the N content is low, it is preferable to have a residence time at 750-800°C of 30 seconds or more.

[0052] The decarburized annealed steel sheet according to this embodiment can be obtained by the above manufacturing method. By further applying an annealing separating agent to this decarburized annealed steel sheet, performing finish annealing, and, if necessary, forming an insulating coating and / or controlling the magnetic domains, a grain-oriented electrical steel sheet can be obtained. These steps can be carried out under known conditions.

[0053] [Nitriding process] By applying an annealing release agent to the decarburized annealed steel sheet (cold-rolled steel sheet after the decarburization annealing process) obtained above, and performing finish annealing, a grain-oriented electrical steel sheet can be obtained. After the decarburization annealing process, the nitrogen content of the steel sheet is acceptable if it is in the range of 0.0040 to 0.0400 mass%, but if this decarburized annealed steel sheet is to be used as a grain-oriented electrical steel sheet, the nitrogen content must be 0.0130 mass% (130 ppm) or more before secondary recrystallization begins. If the nitrogen content is less than 0.0130 mass%, the grain structure that was favorably created in the decarburization annealing process cannot be favorably maintained during the finish annealing, and the two-stage preferential growth of Goss-oriented grains due to the decomposition of Nb-based precipitates and the decomposition of Al-based precipitates will not occur properly, making it impossible to obtain a good improvement in magnetic properties. In other words, the decarburized annealed steel sheet according to this embodiment may have an N content of 0.0130 to 0.0400 mass% considering the period after secondary recrystallization. In that case, the manufacturing method of the decarburized annealed steel sheet according to this embodiment may include a nitriding step in which the N content of the cold-rolled steel sheet is set to 0.0130 to 0.0400 mass%. In the manufacturing method of decarburized annealed steel sheet according to this embodiment, the nitriding treatment can be performed by increasing the N content (nitrogen content) of the steel sheet at any timing between the start of the decarburization annealing described above and the start of secondary recrystallization in the finish annealing (for example, when the steel sheet temperature is 800°C or lower). If the N content is excessive, defects may occur in the glass coating; therefore, the N content after the nitriding process may be set to 0.0350% by mass or less. 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.

[0054] The decarburized annealed steel sheet according to this embodiment can then be processed through known steps (for example, the following steps) to produce a grain-oriented electrical steel sheet.

[0055] [Annealing Separating Agent Application Process] The annealing separation agent application process is a process in which an annealing separation agent is applied to the decarburized annealed steel sheet prior to the finish annealing process. As the annealing separation agent, for example, an annealing separation agent mainly composed of MgO or an annealing separation agent mainly composed of alumina (Al2O3) can be used. After applying the annealing release agent, the decarburized annealed steel sheet is wound into a coil.

[0056] [Finishing annealing process] The finish annealing process involves applying an annealing separator to a decarburized annealed steel sheet that has been wound into a coil, and then performing finish annealing to induce secondary recrystallization. In this process, secondary recrystallization is carried out while the growth of primary recrystallized grains is suppressed by an inhibitor, {110} <001> This method prioritizes the growth of azimuthal particles, thereby improving magnetic flux density. When using the decarburized annealed steel sheet according to this embodiment, the temperature range in which the inhibitor decomposition rate is slow during finish annealing is expanded, and the temperature range in which secondary recrystallization progresses, in which the growth rate of secondary recrystallized grains is relatively slow relative to the inhibitor decomposition rate, is expanded to an extent not seen before {100} <011> Preferential growth of orientation grains occurs, resulting in a dramatic improvement in magnetic flux density in the final grain-oriented electrical steel sheet. Additionally, abnormal grain growth of secondary recrystallized grains occurs during finish annealing, and after finish annealing, these secondary recrystallized grains occupy the entire surface of the sheet. The few secondary recrystallized grains cover the entire surface of the steel sheet, and the grain size of each secondary recrystallized grain increases.

[0057] In the finish annealing process, the finish annealing conditions for "expanding the secondary recrystallization temperature range" disclosed in the above-mentioned Patent Documents 9 to 11 may be applied as needed. By using the decarburized annealed steel sheet according to this embodiment and applying the finish annealing conditions disclosed in Patent Documents 9 to 11, the secondary recrystallization temperature range can be further preferably expanded.

[0058] [Insulating film formation process] The insulating coating formation process is a process of forming an insulating coating on grain-oriented electrical steel sheets (finish-annealed steel sheets) after the finish annealing process. An insulating coating mainly composed of phosphate and colloidal silica, or an insulating coating mainly composed of alumina sol and boric acid, can be formed on the steel sheet after finish annealing.

[0059] [Magnetic Domain Control Process] The magnetic domain control process is a process that subdivides the magnetic domains of the grain-oriented electrical steel sheet. This process is carried out at an appropriate time after cold rolling. For example, local minute strains or local grooves can be formed in the grain-oriented electrical steel sheet using known methods such as laser, plasma, mechanical methods, or etching.

[0060] The insulating film formation process and the magnetic domain control process set the crystal orientation to {110} <001> From the perspective of integration, this process is not necessary. However, it is a process that is commonly used in grain-oriented electrical steel sheets to improve practical magnetic properties. [Examples]

[0061] Next, the effects of the present invention will be specifically described in detail with reference to examples. The conditions in the examples are just one example of conditions adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to this one example of conditions. The present invention can adopt various conditions as long as they do not depart from the spirit of the invention and achieve the objectives of the present invention.

[0062] Using slabs with the chemical compositions shown in Tables 1-1 to 1-6, they were heated under the conditions shown in Tables 2-1 to 2-3, held at the heating temperature for 30 to 600 minutes, and then rolled under the conditions shown in Tables 2-1 to 2-3 to obtain hot-rolled steel sheets.

[0063] The obtained hot-rolled steel sheets were subjected to hot-rolled annealing, cold-rolling, and decarburization annealing under the conditions shown in Tables 2-4 to 2-7 to obtain decarburized annealed steel sheets. During hot-rolled sheet annealing, the holding time at the first annealing temperature was 0 to 120 seconds, the holding time at the second annealing temperature was 10 to 150 seconds, and the cooling stop temperature after the second annealing was 100°C or lower.

[0064] The chemical composition of the obtained decarburized annealed steel sheets was measured according to the method described above. The results are shown in Tables 3-1 to 3-6. In Tables 3-1 to 3-6, "-" indicates that no control or manufacturing process was carried out with regard to content, and therefore no content measurement was performed. The total impurity content was 0.1% or less.

[0065] Furthermore, the precipitation morphology and grain structure of the obtained decarburized annealed steel sheets were investigated based on the method described above. Here, d is the average value of the primary recrystallized grain size (average grain size), σ is the standard deviation of the grain size (primary recrystallized grain size), and D x The particle size and D show the peaks of emission intensity for Nb, V, Mo, Ta, and W, derived from FFF analysis. Al This is the particle size at which the emission intensity of Al shows a peak.

[0066] These decarburized annealed steel sheets were subjected to nitriding treatment by holding them at 770°C for 20 seconds in a nitrogen-hydrogen mixed atmosphere containing ammonia. The nitrogen content of the steel sheets after nitriding treatment is shown in Tables 2-4 to 2-7.

[0067] After nitriding, an annealing separation agent mainly composed of MgO was applied to the steel sheet, and finish annealing was performed. In the final stage of finish annealing, the steel sheet was held at 1200°C in a hydrogen atmosphere for 20 hours (purification annealing) and then allowed to cool naturally.

[0068] A coating solution for forming an insulating film, mainly composed of phosphate and colloidal silica and containing chromium, was applied to the primary film (intermediate layer) formed on the surface of the obtained grain-oriented electrical steel sheet (finish annealed steel sheet). The sheet was then heated and held in a hydrogen:nitrogen atmosphere of 75%:25% by volume, and then cooled to form an insulating film.

[0069] The manufactured grain-oriented electrical steel sheet, when viewed from a cross-section parallel to the thickness direction, had an intermediate layer placed in contact with the grain-oriented electrical steel sheet (silicon steel sheet) and an insulating coating placed in contact with this intermediate layer. The intermediate layer was a forsterite coating with an average thickness of 2 μm, and the insulating coating was an insulating coating mainly composed of phosphate and colloidal silica with an average thickness of 1 μm.

[0070] The magnetic properties (magnetic flux density B8(T) in the rolling direction of the steel sheet when excited at 800 A / m) were evaluated for the obtained grain-oriented electrical steel sheets based on the Single Sheet Tester (SST) method specified in JIS C 2556:2015. The results are shown in Tables 4-1 to 4-3. For reference, the iron loss W17 / 50 (W / kg), defined as the power loss per unit weight (1 kg) of the steel sheet, was measured under the condition of an excitation magnetic flux density of 1.7 T.

[0071] We determined that a magnetic flux density B8 of 1.930T or higher indicates superior magnetic properties.

[0072] [Table 1-1]

[0073] [Table 1-2]

[0074] Table 1-3

[0075] Table 1-4

[0076] Table 1-5

[0077] Table 1-6

[0078] Table 2-1

[0079] Table 2-2

[0080] Table 2-3

[0081] Table 2-4

[0082] Table 2-5

[0083] Table 2-6

[0084] Table 2-7

[0085] Table 3-1

[0086] Table 3-2

[0087] Table 3-3

[0088] Table 3-4

[0089] Table 3-5

[0090] Table 3-6

[0091] Table 4-1

[0092] Table 4-2

[0093] Table 4-3

[0094] As can be seen from Tables 1-1 to 4-3, it has the specified chemical composition, D x , D Al , D Al -D x In grain-oriented electrical steel sheets obtained using decarburized annealed steel sheets with average grain size d and σ / d within the range of the present invention, excellent magnetic properties (magnetic flux density B8) are obtained. On the other hand, in terms of chemical composition, D x , D Al , D Al -D x In grain-oriented electrical steel sheets obtained using decarburized annealed steel sheets where at least one of the average grain size d and σ / d is outside the range of the present invention, the magnetic properties are inferior. Regarding No. 52, although it was within the scope of the present invention as a decarburized annealed steel sheet before nitriding treatment, sufficient magnetic properties could not be obtained because the N content after the nitriding treatment process was less than 0.0130 mass%. [Industrial applicability]

[0095] According to the present invention, it is possible to provide a decarburized annealed steel sheet, which is an intermediate product sheet of grain-oriented electrical steel sheet, that contributes to improving the magnetic flux density of grain-oriented electrical steel sheet. Therefore, the present invention has high industrial applicability.

Claims

[Claim 1] An intermediate product sheet of grain-oriented electrical steel, which has been decarburized and annealed, subjected to nitriding, and not subjected to any further nitriding treatment, In mass percent, C: 0.0005-0.010%, Si: 2.0 to 4.5%, Mn: 0.05-0.50%, S: 0 to 0.035%, Se: 0 to 0.035%, S+Se total content: 0.003-0.035%, Al: 0.010-0.065%, N: 0.0130-0.0400%, At least one element selected from the group consisting of Nb, V, Mo, Ta, and W: totaling 0.003 to 0.030%. Cu: 0 to 0.40%, Bi: 0 to 0.010%, B: 0 to 0.080%, P: 0-0.500%, Ti: 0 to 0.015%, Sn: 0-0.100%, Sb: 0 to 0.100%, Cr: 0-0.300%, Ni: 0-1.000%, Remainder: Fe and impurities, It has a chemical composition consisting of, The metal structure includes Nb-based precipitates, which are precipitates containing at least one element selected from the group consisting of Nb, V, Mo, Ta, and W, and Al-based precipitates, which are precipitates containing Al. When the emission intensity of elements in the Nb-based precipitate and the Al-based precipitate was analyzed for each particle size using the field flow fractionation method, the emission intensity of Al showed a peak at particle size D. Ａｌ However, the particle size D is 75–250 nm and shows peaks in emission intensity for Nb, V, Mo, Ta, and W. ｘ However, it is 20 to 200 nm, and the D Ａｌ The above D ｘ It is 10 nm or more larger than that. In the aforementioned metallographic structure, the average grain size d is 13.0 to 19.0 μm, and when the standard deviation of the grain size is σ, σ / d is 0.500 or less. Decarburized annealed steel sheet.

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