Decarburization-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

Decarburized annealed steel sheet

[0001] The present invention relates to a decarburized annealed steel sheet. This decarburized annealed steel sheet is useful as an intermediate product sheet for grain-oriented electrical steel sheet. This application claims priority based on Japanese Patent Application No. 2024-034139, filed on March 6, 2024, the contents of which are incorporated herein by reference.

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

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

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

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

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

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

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

[0009] In recent years, amid a global trend toward environmental conservation, such as power and energy conservation, there has been an increasing demand for more efficient transformers. In this social environment, there is a demand for improved performance of grain-oriented electrical steel sheets, which are used as iron core materials for transformers. In particular, there is a demand for increased magnetic flux density in grain-oriented electrical steel sheets.

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

[0011] Furthermore, the techniques of Patent Documents 9 to 11, as described above, show that by precisely controlling the final annealing, the secondary recrystallization temperature range is expanded, promoting preferential growth of Goss-oriented grains closer to the ideal Goss orientation represented by the strict {110}<001> crystallographic orientation, and improving the magnetic flux density of the steel sheet while forming subgrain boundaries that result in improved magnetic properties such as steel sheet magnetostriction, which affects the noise characteristics of transformers. At the same time, it has been shown that the addition of trace amounts of Nb, Ta, Mo, etc. increases the frequency of subgrain boundaries, enhancing the effect of improving magnetic properties, and easing the finish annealing conditions under which the magnetic property improvement effect is realized. This is thought to mean that elements such as Nb effectively function as a concrete method for realizing the idea of ​​expanding the secondary recrystallization temperature range and improving the preferential growth of Goss-oriented grains. In particular, since Nb-group elements form carbides, nitrides, and carbonitrides that decompose at lower temperatures than AlN, it is thought that the decomposition of precipitates of these Nb-group elements affects secondary recrystallization. On the other hand, in these techniques, the precipitates of the Nb group elements added in small amounts precipitate at a lower temperature than conventional precipitates such as AlN and MnS, and therefore the precipitation state cannot be appropriately controlled by controlling the conditions in the conventional hot rolling process or hot-rolled sheet annealing process, which affects the primary recrystallized structure and may prevent the magnetic property improvement effect from being effectively achieved.

[0012] The present invention has been made in view of the above-mentioned problems, and in light of the current situation where there is a demand for increasing the magnetic flux density of grain-oriented electrical steel sheets, it is an object of the present invention to provide a decarburization-annealed steel sheet, which is an intermediate product of grain-oriented electrical steel sheets, that contributes to improving the magnetic flux density of grain-oriented electrical steel sheets.

[0013] The present inventors have investigated microstructural control at the intermediate sheet product stage in order to improve the magnetic flux density of grain-oriented electrical steel sheets, and have found that the magnetic flux density of the final grain-oriented electrical steel sheet can be improved by controlling the grain size and precipitate distribution in the steel sheet after decarburization annealing (primary recrystallization annealing), i.e., the decarburization annealed steel sheet.

[0014] The present invention has been made in light of the above findings.

[0015] [1] A decarburization annealed steel sheet according to one embodiment of the present invention contains, in mass%, C: 0.0005 to 0.010%, Si: 2.0 to 7.0%, Mn: 0.05 to 1.00%, S: 0 to 0.035%, Se: 0 to 0.035%, S + Se total content: 0.003 to 0.035%, Al: 0.010 to 0.065%, N: 0.0040 to 0.0400%, at least one selected from the group consisting of Nb, V, Mo, Ta, and W: 0.003 to 0.030% in total, Cu: 0 to 0.40%, Bi: 0 to 0.010%, B: 0 to 0.080%, P: 0 to 0.500%, T The present invention relates to a steel sheet having a chemical composition of Ni: 0 to 0.015%, Sn: 0 to 0.100%, Sb: 0 to 0.100%, Cr: 0 to 0.300%, Ni: 0 to 1.000%, and the balance: Fe and impurities, and the metallographic 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, and when the emission intensity of elements for each particle size of the Nb-based precipitates and the Al-based precipitates is analyzed using a field flow fractionation method, a particle diameter D at which the emission intensity of Al shows a peak is determined. Ａｌ is 75 to 250 nm, and the particle diameter D ｘ is 20 to 200 nm, and Ａｌ is the above D ｘ [2] The decarburization annealed steel sheet according to the above item [1] may have an N content, in mass%, of 0.0130 to 0.0400%.

[0016] According to the above aspect of the present invention, it is possible to provide a decarburization-annealed steel sheet that is an intermediate product of a grain-oriented electrical steel sheet and that contributes to improving the magnetic flux density of the grain-oriented electrical steel sheet.

[0017] A decarburization annealed steel sheet according to one embodiment of the present invention (a decarburization 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 within the scope of the present invention.

[0018] 1. Chemical Composition The chemical composition of the decarburized annealed steel sheet according to this embodiment is within the following range. In the numerical ranges described below, ranges indicated with "to" are included as lower and upper limits. However, values ​​indicated as "greater than" or "less than" are not included in the numerical range. Furthermore, "%" in the chemical composition means "mass %" unless otherwise specified.

[0019] The decarburization annealed steel sheet according to this embodiment contains, as a chemical composition, basic elements, optional elements as needed, and the balance being Fe and impurities.

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

[0021] C: 0.0005 to 0.010% Carbon (C) is an effective element for controlling the primary recrystallization structure during the manufacturing process. In particular, C is effective for controlling the primary recrystallization structure because it forms carbides and carbonitrides with elements such as Nb. Therefore, the C content of the slab is preferably 0.0010 to 0.100%, but excessive C content in the final product adversely affects the magnetic properties. Therefore, the C content of the decarburized annealed steel sheet is set to 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 final annealing process, and after the final annealing process, it is set to 0.005% or less. In decarburized annealed steel sheet, a lower C content is preferable, but considering productivity in industrial production, the C content is set to 0.0005% or more. The C content may be 0.0010% or more.

[0022] Si: 2.0 to 7.0% Silicon (Si) is an element that increases the electrical resistance of grain-oriented electrical steel sheets and reduces iron loss. If the Si content is less than 2.0%, austenite transformation occurs during finish annealing, damaging the crystal orientation of the grain-oriented electrical steel sheet. Therefore, the Si content is set to 2.0% or more. The Si content is preferably 2.5% or more, and more preferably 3.0% or more. On the other hand, if the Si content exceeds 7.0%, cold workability decreases and cracks tend to occur during cold rolling. Therefore, the Si content of decarburized annealed steel sheets is set to 7.0% or less. The Si content is preferably 4.5% or less, and more preferably 4.0% or less.

[0023] Mn: 0.05 to 1.00% Manganese (Mn) is an element that combines with S and Se to precipitate as MnS or MnSe, functioning as an inhibitor. In order to favorably control the morphology of these inhibitors (precipitates), the Mn content of the decarburization-annealed steel sheet is set to 0.05% or more. If the Mn content is below 0.05%, the amount of precipitated MnS and MnSe, which function as inhibitors, is insufficient, thereby inhibiting the appropriate progress of secondary recrystallization. The Mn content is preferably 0.09% or more. On the other hand, if the Mn content exceeds 1.00%, the amount of precipitated MnS and MnSe, which function as inhibitors, is excessive, thereby inhibiting the appropriate progress of secondary recrystallization. In this embodiment, part of the inhibitor function may be performed by a carbide, nitride, or carbonitride of an Nb group element. In this case, the amount of precipitation of inhibitors such as MnS and MnSe may be controlled to be small. Therefore, the Mn content is set to 1.00% or less. The Mn content is preferably 0.50% or less, and more preferably 0.20% or less.

[0024] S: 0 to 0.035% Se: 0 to 0.035% Total content of S+Se: 0.003 to 0.035% Sulfur (S) and selenium (Se) combine with Mn to precipitate as MnS or MnSe, functioning as inhibitors. To favorably control the morphology of these inhibitors (precipitates), the decarburization annealed steel sheet should have an S content of 0 to 0.035%, an Se content of 0 to 0.035%, and a total S+Se content of 0.003 to 0.035%. A total content of S and Se of 0.003 to 0.035% is preferable because it stabilizes secondary recrystallization. On the other hand, in this embodiment, part of the inhibitor function may be performed by carbides, nitrides, carbonitrides, or the like of Nb group elements. In this case, the precipitation amounts of the inhibitors MnS and MnSe may be controlled to be small. Therefore, the total content of S and Se may be set to 0.025% or less, or 0.010% or less. The S content and Se content in decarburized annealed steel sheet are as described above, but if S and Se remain in the steel after finish annealing, they may form compounds that deteriorate iron loss. Therefore, it is preferable to reduce the content of S and Se by purifying them during finish annealing and thereby removing them from the steel.

[0025] Here, "the total content of S and Se is 0.003 to 0.035%" means that the decarburization annealed steel sheet may contain only one of S or Se in its chemical composition, with the content being 0.003 to 0.035%. Alternatively, the decarburization annealed steel sheet may contain both S and Se, with the total content being 0.003 to 0.035%.

[0026] Al: 0.010 to 0.065% Aluminum (Al) combines with 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 decarburized annealed steel sheet is set to 0.010% or more. When the Al content is 0.010% or more, AlN and (Al,Si)N precipitate in a favorable form by nitriding in the low-temperature slab heating process, stabilizing secondary recrystallization, particularly in the high-temperature range. When the Al content is below 0.010%, the amount of precipitated AlN and (Al,Si)N, which function as inhibitors, is insufficient, hindering the proper progress of secondary recrystallization. The Al content is preferably 0.020% or more, and more preferably 0.025% or more. On the other hand, when the Al content exceeds 0.065%, the amount of precipitated AlN and (Al,Si)N, which function as inhibitors, becomes excessive, hindering the proper progress of secondary recrystallization. From the viewpoint of stability of secondary recrystallization, the Al content is preferably 0.040% or less, and more preferably 0.030% or less.

[0027] N: 0.0040 to 0.0400% Nitrogen (N) is an element that combines with Al and precipitates as AlN or (Al,Si)N, functioning as an inhibitor. In order to favorably control the morphology of these inhibitors (precipitates), the N content of decarburized annealed steel sheet is set to 0.0040 to 0.0400%. If the N content of the slab exceeds 0.0120%, blisters, a type of defect, are more likely to occur in the steel sheet. Therefore, the N content of the slab is preferably 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 step during the manufacturing process, so the N content is set to 0.0130 to 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 separator to perform nitriding during finish annealing. Therefore, the N content of the decarburization annealed steel sheet before nitriding should be 0.0040 to 0.0400%, assuming that if the N content is less than 0.0130%, the N content is subsequently adjusted to 0.0130 to 0.0400% by nitriding. Preferably, the N content is 0.0040 to 0.0120%. On the other hand, the N content of the decarburization annealed steel sheet after nitriding is 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 after the finish annealing process, the N content is 0.0050% or less.

[0028] At least one element selected from the group consisting of Nb, V, Mo, Ta, and W: 0.003 to 0.030% in total. Niobium (Nb), vanadium (V), molybdenum (Mo), tantalum (Ta), and tungsten (W) precipitate as carbides, nitrides, or carbonitrides, which act as auxiliary inhibitors, and preferably function as inhibitors. Specifically, they preferably expand the secondary recrystallization progression temperature range. As a result, Goss-oriented grains grow favorably, and the magnetic flux density of the final grain-oriented electrical steel sheet is preferably increased. To achieve this effect, at least one element selected from the group consisting of Nb, V, Mo, Ta, and W is contained. 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 is less than 0.003%, the precipitates of Nb group elements that act as the above-mentioned precipitation nuclei will be insufficient, making it difficult to refine MnS and AlN. Therefore, the total content of Nb group elements is set to 0.003% or more. The total content of Nb group elements is preferably 0.004% or more, and more preferably 0.005% or more. On the other hand, if the total content of Nb group elements exceeds 0.030%, the precipitation temperature range of the Nb group element precipitates will become high, making the Nb group element precipitates coarse and prone to low density. Furthermore, the difference between the precipitation temperature range of the Nb group element precipitates and the precipitation temperature range of MnS and AlN will be large, making it difficult for the Nb group element precipitates to effectively act as precipitation nuclei for refining MnS and AlN. Therefore, the total content of Nb group elements is set to 0.030% or less. The total content of Nb group elements is preferably 0.020% or less, and more preferably 0.010% or less.

[0029] The reason why precipitates of Nb group elements act as inhibitors to preferably expand the temperature range in which secondary recrystallization progresses is not clear, but it is thought to be as follows: As will be described later, by appropriately controlling the hot rolling process and the hot-rolled sheet annealing process, etc., and thereby preferably controlling the primary recrystallized grain structure, it is thought that the preferential growth of Goss-oriented grains that occurs when precipitates are decomposed can be promoted. The precipitation states of AlN, which normally decomposes in a high-temperature range of 1000°C or higher, and the precipitates of Nb-group elements, which begin to decompose in a lower temperature range than AlN, are controlled so that the pinning effect of the Nb-group element precipitates on primary recrystallized grains and the pinning effect of AlN are separated (strictly speaking, so that the composite precipitation state of AlN and precipitates of Nb-group elements is separated into precipitates with as high a proportion of Al as possible (high Al / Nb-group element ratio in the precipitates) and precipitates with a high Nb-group element proportion (low Al / Nb-group element ratio), and a spatially uniform precipitation state is realized). This is thought to enable the preferential growth of Goss-oriented grains to occur in two stages: preferential growth of Goss-oriented grains that occurs during the decomposition of the precipitates of Nb-group elements, and subsequent preferential growth of Goss-oriented grains during the decomposition of AlN. In particular, carbides, nitrides, or carbonitrides of Nb group elements act as precipitation nuclei for MnS and AlN in the temperature range of 900 to 1100° C. during cooling from a high temperature. Therefore, when an Nb group element is contained, compared to when an Nb group element is not contained, the number of precipitation sites for MnS and AlN increases, and as a result, MnS and AlN are more likely to be formed as spatially uniform precipitates, and in the low temperature range of 900° C. or less, precipitates in which the ratio of Nb group elements is higher than that of Al are spatially uniform and finely precipitated. By effectively utilizing this phenomenon, it is possible to control the primary recrystallized structure to be smaller in diameter and with less variation in crystal grain size than conventional structures. As a result, preferential growth of Goss-oriented grains occurs during the decomposition of uniformly fine precipitates with a high proportion of Nb-group elements (a low Al / Nb-group element ratio). Among these, crystal 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 the Goss-oriented grains closer to the ideal Goss orientation will further undergo preferential growth.In the decarburization annealed steel sheet according to this embodiment, the secondary recrystallization progression temperature range is expanded by allowing fine inhibitors and coarse inhibitors to coexist, and it is believed that precipitates of Nb group elements act particularly effectively in expanding the secondary recrystallization progression temperature range to the lower temperature side.

[0030] The decarburization annealed steel sheet according to this embodiment may contain impurities as a chemical composition. "Impurities" refer to elements that are mixed in from raw materials such as ore or scrap, or from the manufacturing environment, during industrial steel production. The 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, the decarburization annealed steel sheet according to this embodiment may contain selective elements (arbitrary elements) in addition to the above-described basic elements and impurities. For example, instead of a portion of the remaining Fe, one or more selective elements selected from the group consisting of Cu, Bi, B, P, Ti, Sn, Sb, Cr, and Ni may be contained. These selective elements may be contained according to the purpose. Therefore, there is no need to set a lower limit for these selective elements, and the lower limit may be 0%. Furthermore, even if these selective elements are contained as impurities, the above-described effects are not impaired.

[0032] Cu: 0 to 0.40%, Bi: 0 to 0.010%, B: 0 to 0.080%, P: 0 to 0.50%, Ti: 0 to 0.015%, Sn: 0 to 0.100%, Sb: 0 to 0.100%, Cr: 0 to 0.300%, Ni: 0 to 1.000%. Copper (Cu), bismuth (Bi), boron (B), phosphorus (P), titanium (Ti), tin (Sn), antimony (Sb), chromium (Cr), and nickel (Ni) may be contained according to known purposes. There is no need to set a lower limit for the content of these optional elements, and it may be 0%.

[0033] The chemical composition of the decarburized annealed steel sheet according to this embodiment may be measured by a general steel analysis method. For example, the chemical composition of the decarburized annealed steel sheet may be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). Specifically, the chemical composition is identified by measuring a 35 mm square test piece taken from the decarburized annealed steel sheet using ICP-AES under conditions based on a pre-created calibration curve. C and S, which are difficult to measure using ICP-AES, may be measured using a combustion-infrared absorption method, and N may be measured using an inert gas fusion-thermal conductivity method.

[0034] 2. Metallographic Structure 2.1 Precipitates Next, the precipitates contained in the decarburization annealed steel sheet according to this embodiment will be described. The decarburization annealed steel sheet according to this embodiment has a metallographic structure that includes Nb-based precipitates containing 0.003% or more of at least one element selected from the group consisting of Nb, V, Mo, Ta, and W, and Al-based precipitates that are precipitates containing Al. Furthermore, when the emission intensity of elements for each particle size of the Nb-based precipitates and the Al-based precipitates was analyzed using a field flow fractionation (FFF) method, the particle diameter D at which the emission intensity of Al peaked was Ａｌ is 75 to 250 nm, and the particle diameter D ｘ is 20 to 200 nm, and Ａｌ is the above D ｘ By controlling the Nb-based precipitates and Al-based precipitates as described above, the selective growth of Goss-oriented grains is favorably exhibited in secondary recrystallization, and the selective effect of Goss-oriented grains during decomposition of Al-based precipitates is enhanced, thereby improving the magnetic flux density B8 of the grain-oriented electrical steel sheet. ＡｌHowever, if the thickness is less than 75 nm, the pinning effect of the Al-based precipitates becomes excessively large, and the preferential growth of the Goss orientation during decomposition of the Nb-based precipitates does not proceed sufficiently. In this case, the size advantage (over matrix grains other than the Goss orientation grains) of the Goss orientation grains, which have a crystal orientation close to the ideal Goss orientation during decomposition of the Al-based precipitates, becomes insufficient, and a sufficient effect of improving the magnetic flux density cannot be obtained. Ａｌ However, if the thickness exceeds 250 nm, the pinning effect of the Al-based precipitates becomes weak, and it becomes impossible to stop the preferential growth of the Goss-oriented grains that are generated when the Nb-based precipitates are decomposed. In this case, the second stage of selective preferential growth of the Goss-oriented grains when the Al-based precipitates are decomposed disappears, and a sufficient effect of improving the magnetic flux density cannot be obtained. ｘ However, if the thickness is less than 20 nm, the change in size and decomposition of Nb-based precipitates during the final annealing process will proceed rapidly. In this case, the first stage of selective preferential growth of Goss-oriented grains will not proceed well, and a sufficient effect of improving magnetic flux density will not be obtained. ｘ However, if the diameter exceeds 200 nm, the pinning effect of the Nb-based precipitates is reduced, and the effect of reducing the diameter of the primary recrystallized grains is insufficient. In this case, the driving force for the grain growth of the Nb-based precipitates during the final annealing is reduced, and the size advantage during the selective growth of the Goss-oriented grains in the first stage is reduced, so that a sufficient effect of improving the magnetic flux density cannot be obtained. Ａｌ D ｘ If the size is not 10 nm or more larger than the above, the form of the precipitates that contribute to the pinning effect of the primary recrystallized grains will be of only one type, and the preferential growth of Goss-oriented grains at the decomposition temperature of the Nb-based precipitates followed by the preferential growth of Goss-oriented grains at the second stage due to the decomposition of Al-based precipitates will not proceed well, and a sufficient effect of improving magnetic flux density will not be obtained.

[0035] D Ａｌ and D ｘcan be determined by the method described in Japanese Patent No. 4572001, as follows. For example, extraction residue (precipitate) is recovered from the electrolytic extraction solution, and the size and distribution of the recovered precipitate are measured by field flow fractionation (FFF). Particle separation conditions using the FFF method may be, for example, using a 0.05 wt% sodium dodecyl sulfate (SDS) solution as the developing solvent and flowing it at a flow rate of 1 mL / min. The solution discharged after size separation and size and number density measurement using the FFF device is subjected to component analysis using a conventional 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 diameter on the horizontal axis and emission intensity on the vertical axis, and the particle diameter showing the peak of the emission intensity is determined.

[0036] 2.2 Grain Size In the decarburization-annealed steel sheet according to this embodiment, the metal structure has an average grain size d of 13.0 to 19.0 μm, and σ / d is 0.500 or less, where σ is the standard deviation of the grain size. A smaller grain size (primary recrystallized grain size) in the decarburization-annealed steel sheet increases the driving force for secondary recrystallization. Therefore, the average grain size d is set to 19.0 μm or less. Preferably, the average grain size is set to 18.0 μm or less. On the other hand, if the average grain size d is less than 13.0 μm, the driving force for grain growth at the decomposition temperature of Nb-based precipitates during finish annealing becomes too large, resulting in the growth of grains other than Goss-oriented grains. Therefore, the average grain size is set to 13.0 μm or more. Preferably, the average grain size is set to 15.0 μm or more. Furthermore, even if the grain size is small, large grain size variation inhibits the selective growth of Goss-oriented grains. Therefore, the value of σ / d, which is expressed by the average grain size d and the standard deviation σ of the grain size, is set to 0.500 or less.

[0037] The average crystal grain size, σ / d, is determined by polishing a cross section perpendicular to the width direction of the steel sheet (meaning a steel sheet cross section including the steel sheet perpendicular direction and the rolling direction) perpendicular to the rolling direction, and then electropolishing the cross section. Then, the crystal grain size and crystal grain size distribution in the cross section are measured by the EBSD (Electron Backscatter Diffraction) method.

[0038] 3. Manufacturing Method The decarburization annealed steel sheet according to the present embodiment can be manufactured by a manufacturing method including the following steps: (I) a casting step of casting molten steel having the same chemical composition as that of the decarburization annealed steel sheet according to the present embodiment to form a slab, (II) a hot rolling step of heating the slab to more than 1030°C and less than 1180°C and hot rolling it to form a hot-rolled steel sheet, (III) a hot-rolled sheet annealing step of heating the hot-rolled steel sheet to a maximum temperature and then cooling it to anneal it, (IV) a cold-rolling step of cold-rolling the hot-rolled steel sheet after the hot-rolled sheet annealing step to form a cold-rolled steel sheet, and (V) a decarburization annealing step of decarburizing the cold-rolled steel sheet.

[0039] [Casting Step] In the casting step, a slab to be subjected to hot rolling is prepared. The chemical composition of the slab remains almost unchanged from the slab to the decarburization-annealed steel sheet, except for the C content and, if nitriding treatment is performed, the N content. Therefore, the chemical composition of the slab is set to the chemical composition of the target decarburization-annealed steel sheet (the chemical composition of the decarburization-annealed steel sheet described above). Meanwhile, the C content and N content (mass%) of the slab are set as follows:

[0040] An example of a method for producing a slab is as follows: Molten steel is produced (smelted). A slab is produced using this molten steel. For example, the slab may be produced by continuous casting. Alternatively, an ingot may be produced using the molten steel, and the ingot may be bloomed to produce a slab. The thickness of the slab is, for example, 150 to 350 mm. The thickness of the slab is preferably 220 to 280 mm.

[0041] C: 0.0010 to 0.100% Carbon (C) is an element effective in controlling the primary recrystallization structure during the manufacturing process. Therefore, the C content of the slab is set to 0.0010% or more, preferably 0.010% or more. On the other hand, an excessive C content in the final product adversely affects the magnetic properties. If the C content of the slab is excessive, it may not be possible to sufficiently reduce the C even with decarburization annealing. Therefore, the C content of the slab is set to 0.100% or less.

[0042] N: 0.0040 to 0.0120% Nitrogen (N) is an element that combines with Al and precipitates as AlN or (Al, Si)N, functioning as an inhibitor. In order to favorably control the form of these inhibitors (precipitates), the N content of the slab is set to 0.0040% or more. On the other hand, if the N content of the slab exceeds 0.0120%, blisters, a type of defect, are more likely to occur in the steel sheet, so the N content of the slab is set to 0.0120% or less.

[0043] [Hot Rolling Process] In the hot rolling process, the slab is heated to a temperature higher than 1030°C and lower than 1180°C. This causes the precipitates contained in the slab at room temperature before heating to be solutionized. A heating temperature of 1030°C or lower does not result in sufficient solutionization. On the other hand, a heating temperature higher than 1180°C results in an excessively high solution rate. The heating temperature is preferably 1070 to 1130°C. Furthermore, a soaking time of 40 minutes or less is too short, making it difficult to control the solution state of the precipitates to an equilibrium state. Therefore, a soaking time of 40 minutes or more is preferable. In order to achieve a homogeneous solution state within the slab, a soaking time of 70 minutes or more is more preferable. The upper limit of the soaking time is not particularly limited, but may be 2 hours in consideration of productivity in industrial production. After the heating, the slab is hot rolled (rough rolling and finish rolling) to produce a hot-rolled steel sheet. In this case, 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 rolling reduction in rough rolling is set to 82 to 95%. If the rolling temperature of rough rolling is higher than 1070 ° C, MnS does not precipitate sufficiently during rough rolling, and there is an increase in the precipitation of AlN and Nb-based precipitates, which are precipitates of Nb group elements such as Nb(C,N). As a result, the pinning effect of the primary recrystallized grains due to the precipitates of Nb group elements and the pinning effect due to AlN are not sufficiently separated, and the preferential growth of the Goss orientation during secondary recrystallization is reduced. The rough rolling temperature is preferably 1050 ° C or less. Furthermore, if the rough rolling temperature is less than 940 ° C, the slab becomes hard and rollability decreases. The rough rolling temperature is preferably 960 ° C or higher, more preferably 980 ° C or higher. Furthermore, by setting the rough rolling reduction within the above range, deformation-induced precipitation occurs, making it possible to mainly precipitate a large amount of MnS precipitates. If the rough rolling reduction is smaller than the above lower limit, the MnS precipitates will often be composite precipitates with AlN precipitates or precipitates of Nb group elements such as Nb(C,N) (Nb-based precipitates). As a result, the pinning effect of the primary recrystallized grains due to the precipitates of Nb group elements and the pinning effect of AlN are not sufficiently separated, and the preferential growth of the Goss orientation during secondary recrystallization is reduced. The rough rolling reduction is set to 95% or less, taking into account the performance of the rolling mill, etc. Furthermore, the time from the completion of rough rolling to the start of finish hot rolling is set to 20 seconds or more.This allows sufficient precipitation of MnS, thereby effectively reducing the rate of precipitation of AlN and of complex precipitation with precipitates of Nb group elements such as Nb(C,N). In this case, the effect of enhancing the regularity of the primary recrystallized grain structure of fine precipitates of Nb group elements and the effect of promoting separation of the pinning effect of the primary recrystallized grains by the precipitates of Nb group elements and the pinning effect by AlN can be obtained, thereby enhancing the preferential growth of the Goss orientation.

[0044] The above-mentioned hot rolling operating conditions mainly control Al-based precipitates. Specifically, in order to ensure Al-based precipitates that contribute to the second-stage preferential growth during finish annealing, Al-based precipitates that do not precipitate with the same size as Nb-based precipitates but are larger in size than Nb-based precipitates are ensured during hot-rolled sheet annealing. For this purpose, it is important to ensure a sufficient amount of AlN that 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, in the hot-rolling process, it is effective to sufficiently precipitate MnS before finish hot rolling. By controlling in this way, precipitate control based on the process conditions of hot-rolled sheet annealing, which will be described later, becomes easy. Although specific conditions are not limited, rough rolling is completed in a temperature range of 940°C or higher where AlN precipitation becomes significant, and the time from the completion of rough rolling completed in the above temperature range to the start of finish hot rolling (holding time in that temperature range) is ensured to be 20 seconds or more, thereby mainly increasing the difference in size between Nb-based precipitates and Al-based precipitates that precipitate after secondary soaking in hot-rolled sheet annealing. The upper limit of the holding time is not limited, but the holding time may be 100 seconds or less from the perspective of productivity. The start temperature of finish rolling is not limited, but is preferably 930 to 1040°C. More preferably, it is 960 to 1020°C. The completion temperature of finish rolling is 850°C or higher and 950°C or lower in order to ensure the size of the Al-based precipitates, to make the difference in size between the Nb-based precipitates and the Al-based precipitates appropriate, and to achieve a granular primary recrystallized grain structure. In the hot rolling process, a hot-rolled steel sheet having a thickness of, for example, 1.8 to 3.5 mm may be produced. After the finish rolling is completed, the hot-rolled steel sheet can be coiled at a predetermined temperature. The coiling temperature is not limited, but is, for example, 300 to 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 to 1120 ° C. (first-stage annealing temperature), held for 0 to 120 seconds, then cooled to a temperature range of 950 to 850 ° C. so that the average cooling rate from 1050 to 900 ° C. is 1 to 5 ° C. / s, and held at that temperature range (second-stage annealing temperature) for 10 to 150 seconds. After holding, the sheet is cooled to a temperature of 500 ° C. or less so that the average cooling rate from 750 to 500 ° C. is 5 to 80 ° C. / s. By setting the first-stage annealing temperature (maximum temperature reached) in the above range and holding at that temperature range for 0 to 120 seconds, Al-based precipitates (such as AlN) are preferably solutionized. Furthermore, the solutionized Al-based precipitates tend to re-precipitate in the temperature range of 1050 to 900 ° C. during the cooling process. Therefore, by controlling the average cooling rate from 1050 to 900°C during the cooling process, the reprecipitation of Al-based precipitates can be favorably controlled. For example, fine Al-based precipitates formed during hot rolling preferentially dissolve at the maximum temperature reached, and when the average cooling rate from 1050 to 900°C during the cooling process is within the above range, Al-based precipitates are favorably reprecipitated coarsely compared to Nb-based precipitates. Since the coarse Al-based precipitates remain dissolved up to relatively high temperatures during the temperature rise process of finish annealing, the secondary recrystallization progression temperature range is favorably expanded to the higher temperature side. Similarly, when the maximum temperature reached during hot-rolled sheet annealing is within the above range, Nb-based precipitates are favorably solutionized. The solutionized Nb-based precipitates tend to reprecipitate in the temperature range of 1000 to 500°C during the cooling process, which is below the complete solution temperature determined by the chemical composition of the slab (for example, approximately 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 coarse Al-based precipitates precipitate, the Nb-based precipitates can be precipitated together with the Al-based precipitates that remain dissolved at the maximum temperature, or can serve as nuclei for the reprecipitated Al-based precipitates. Furthermore, by setting the second-stage annealing temperature to 850 to 950°C and the holding time to 10 to 150 seconds, the Al-based precipitates can be sufficiently precipitated, and the size of the fine Nb-based precipitates that precipitate at 850°C or below during cooling can be well separated from the size of the Al-based precipitates. Furthermore, by controlling the average cooling rate in the temperature range of 750 to 500° C. during the cooling process after the second annealing, it is possible to preferably control the reprecipitation of Nb-based precipitates with a small Al composite ratio.For example, when the average cooling rate during the cooling process is within the above range of 750 to 500°C, fine Nb-based precipitates can be preferably formed. As a result, the uniformity of the primary recrystallized grain structure is improved, and the fine Nb-based precipitates begin to dissolve from a relatively low temperature during the temperature rise process of finish annealing, so the temperature range over which secondary recrystallization progresses is preferably expanded to the lower temperature side.

[0046] The average cooling rate from 1050 to 900 ° C. during the cooling process is the value obtained by dividing the temperature difference from the maximum temperature to 900 ° C. by the cooling time from starting cooling from the maximum temperature to reach 900 ° C. when the first-stage annealing temperature (maximum temperature reached) is 1050 ° C. or less, and when the maximum temperature reached is 1050 to 1120 ° C., it is the value obtained by dividing the temperature difference (150 ° C.) from 1050 ° C. to 900 ° C. by the cooling time from 1050 ° C. to reach 900 ° C. Similarly, the average cooling rate from 750 to 500 ° C. during the cooling process is the value obtained by dividing the temperature difference (200 ° C.) from 750 ° C. to 500 ° C. by the cooling time from 750 ° C. to reach 500 ° C.

[0047] [Cold Rolling Step] In the cold rolling step, the hot rolled steel sheet after the hot rolled sheet annealing step is subjected to a single cold rolling (a series of cold rolling steps without intermediate annealing) or multiple cold rolling steps (two or more) with annealing (intermediate annealing) interposed therebetween to produce a cold rolled steel sheet having a thickness of, for example, 0.10 to 0.50 mm. In the cold rolling step, the reduction ratio of the cold rolling may be controlled to 80 to 95%.

[0048] The cold rolling reduction mentioned above means the cumulative cold rolling reduction in the case where intermediate annealing is not performed, or the cumulative cold rolling reduction after the final intermediate annealing in the case where intermediate annealing is performed. Specifically, the cold rolling reduction is defined as follows: Cold rolling reduction (cumulative reduction) (%) = (1 - "thickness of steel sheet after cold rolling" / "thickness of steel sheet before cold rolling (or after intermediate annealing)") x 100

[0049] When the cold rolling reduction is within the above range, the primary recrystallization texture after decarburization annealing is preferably controlled. Specifically, the primary recrystallization texture becomes a texture that facilitates the preferential growth of ideal Goss-oriented grains (grains with an ideal {110}<001> orientation) during secondary recrystallization (specifically, the primary recrystallization texture has crystal orientations represented by {111}<112> and {411}<148> as its main orientations). As a result, it is preferable that the final product has a secondary recrystallization texture in which ideal Goss-oriented grains grow preferentially.

[0050] [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 to obtain a decarburization annealed steel sheet in which primary recrystallization has occurred. When manufacturing the decarburization annealed steel sheet according to this embodiment, the decarburization annealing steel sheet process includes a heating process in which the steel sheet is heated to a soaking temperature, a soaking process in which the steel sheet is held at the soaking temperature, and a cooling process in which the steel sheet is cooled after the soaking process. In the heating process, the average heating rate from 500 to 700°C is 20 to 300°C / second, and in the soaking process, the soaking temperature is 830 to 870°C, and the holding time at the soaking temperature is 60 to 180 seconds. The above condition is one of the conditions for realizing the features of the intermediate product sheet 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 σ / d of 0.500 or less, where d is the average grain size of 13.0 to 19.0 μm and σ is the standard deviation of the grain size.

[0051] During the heating process, the average heating rate from 500 to 700°C affects the Nb-based precipitates precipitated during the annealing and cooling of the hot-rolled sheet. Because these Nb-based precipitates have a pinning effect on the primary recrystallized grain structure, the average heating rate in this temperature range affects the pinning effect. If the average heating rate from 500 to 700°C is less than 10°C / s, 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 from 500 to 700°C exceeds 300°C / s, the desired σ / d value cannot be obtained. The soaking temperature was set to 830 to 870°C and the soaking time was set to 60 to 180 seconds because the temperature range of 830 to 870°C is a temperature range in which Nb-based precipitates and Al-based precipitates are prone to change, and controlling the soaking time in this temperature range allows for the uniform precipitation of Nb-based carbonitrides and Al-based precipitates to be maintained. If the soaking temperature is less than 830°C, the primary recrystallized grain structure becomes mixed grains due to the influence of fine Al-based precipitates that do not completely precipitate by 900°C but precipitate below 900°C, resulting in a large σ / d value. In this case, the two-stage selective growth of favorable Goss orientation grains is suppressed. On the other hand, if the soaking temperature is more than 870°C, the Nb-based precipitates that are finely precipitated during hot-rolled sheet annealing become coarse, suppressing the coarsening of the primary recrystallized grain size and the two-stage selective growth of favorable Goss orientation grains. Furthermore, if the soaking time is less than 60 seconds, the decarburization time is too short, resulting in poor decarburization. On the other hand, if the soaking time is more than 180 seconds, the primary recrystallized grain size becomes coarse, and the change in precipitates may become too large. Although the cooling process is not necessarily limited, when the N content of the cold-rolled steel sheet is low, if the residence time at 750 to 800°C is less than 30 seconds, the Nb-based precipitates that were partially re-solutioned during soaking will not be able to be completely re-precipitated, and the pinning effect before secondary recrystallization will be insufficient during finish annealing. Therefore, particularly when the nitriding treatment is not completed and the N content is low, it is preferable to set the residence time at 750 to 800°C to 30 seconds or more.

[0052] The decarburization-annealed steel sheet according to this embodiment can be obtained by the above-described manufacturing method. Grain-oriented electrical steel sheet can be obtained by further applying an annealing separator to this decarburization-annealed steel sheet, performing finish annealing, and, if necessary, forming an insulating coating and / or controlling magnetic domains. These steps can be performed under known conditions.

[0053] [Nitriding Process] Grain-oriented electrical steel sheets are obtained by applying an annealing separator to the decarburized annealed steel sheet (cold-rolled steel sheet after the decarburization annealing process) obtained above and then performing finish annealing. After the decarburization annealing process, the N content of the steel sheet may be 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 N content must be 0.0130 mass% (130 ppm) or more before secondary recrystallization begins. If the N content is less than 0.0130 mass%, the grain structure favorably engineered in the decarburization annealing process cannot be favorably maintained during finish annealing, and the two-stage preferential growth of Goss-oriented grains due to the decomposition of Nb-based precipitates and Al-based precipitates is not favorably achieved, resulting in failure to obtain favorable improvements in magnetic properties. That is, the decarburization-annealed steel sheet according to this embodiment may have an N content of 0.0130 to 0.0400 mass% in consideration of the condition after secondary recrystallization. In this case, the method for producing a decarburization-annealed steel sheet according to this embodiment may include a nitriding treatment step in which the N content of the cold-rolled steel sheet is set to 0.0130 to 0.0400 mass%. In the method for producing a decarburization-annealed steel sheet according to this embodiment, the N content (nitrogen amount) of the steel sheet may be increased at any timing between the start of the above-mentioned decarburization annealing and the start of secondary recrystallization in finish annealing (for example, when the steel sheet temperature is 800°C or lower). Since an excessive N content may cause defects in the glass coating, the N content after the nitriding treatment step may be set to 0.0350 mass% or lower. The nitriding method is not limited, and examples thereof include a process of annealing a steel sheet in an atmosphere containing a gas having nitriding ability, such as ammonia, and a process of finish-annealing a decarburized annealed steel sheet coated with an annealing separator containing a powder having nitriding ability, such as MnN.

[0054] The decarburization annealed steel sheet according to this embodiment can then be made into a grain-oriented electrical steel sheet product through known processes (for example, the following processes).

[0055] [Annealing separator application step] The annealing separator application step is a step of applying an annealing separator to a decarburized annealed steel sheet prior to the finish annealing step. Examples of the annealing separator include an annealing separator containing MgO as a main component and an annealing separator containing alumina (Al ２ O ３The decarburized annealed steel sheet after application of the annealing separator is wound into a coil.

[0056] [Finish Annealing Process] The finish annealing process involves finish annealing a decarburized annealed steel sheet coated with an annealing separator and wound into a coil to induce secondary recrystallization. In this process, secondary recrystallization is promoted while the growth of primary recrystallized grains is suppressed by an inhibitor, thereby preferentially growing {110}<001>-oriented grains and improving magnetic flux density. When the decarburized annealed steel sheet according to this embodiment is used, the temperature range in which the inhibitor decomposition rate is slow during finish annealing is expanded, and the secondary recrystallization progression temperature range in which the growth rate of secondary recrystallized grains is relatively slow compared to the inhibitor decomposition rate is expanded. This results in unprecedented preferential growth of {100}<011>-oriented grains, resulting in a dramatic improvement in magnetic flux density in the final grain-oriented electrical steel sheet. Furthermore, abnormal grain growth of secondary recrystallized grains occurs during finish annealing, and the secondary recrystallized grains occupy the entire sheet surface after finish annealing. A small number of secondary recrystallized grains cover the entire surface of the steel sheet, and the grain size of each secondary recrystallized grain becomes large.

[0057] In the finish annealing step, it is possible to apply, as necessary, the finish annealing conditions for "expanding the secondary recrystallization progression temperature range" disclosed in the above Patent Documents 9 to 11. If the decarburization annealed steel sheet according to this embodiment is used and the finish annealing conditions disclosed in Patent Documents 9 to 11 are applied, the secondary recrystallization progression temperature range can be further preferably expanded.

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

[0059] [Magnetic Domain Control Step] The magnetic domain control step is a step of performing a treatment to subdivide the magnetic domains of the grain-oriented electrical steel sheet. This step is appropriately performed at an appropriate timing after cold rolling. For example, localized micro-strains or localized grooves may be formed in the grain-oriented electrical steel sheet by a known method such as laser, plasma, mechanical method, or etching.

[0060] The insulating coating formation process and magnetic domain control process are not necessary from the viewpoint of concentrating the crystal orientation in {110}<001>, but are steps that are adopted in general grain-oriented electrical steel sheets to improve practical magnetic properties.

[0061] Next, the effects of the present invention will be specifically described in detail using examples. The conditions in the examples are examples adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to these examples. Various conditions can be adopted in the present invention as long as they do not deviate from the gist of the present invention and achieve the object of the present invention.

[0062] Slabs having the chemical compositions shown in Tables 1-1 to 1-6 were heated under the conditions shown in Tables 2-1 to 2-3, held at the heating temperature for 30 to 600 minutes, and 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 sheet annealing, cold rolling, and decarburization annealing under the conditions shown in Tables 2-4 to 2-7 to obtain decarburization-annealed steel sheets. In the hot-rolled sheet annealing, the holding time at the first-stage annealing temperature was 0 to 120 seconds, the holding time at the second-stage annealing temperature was 10 to 150 seconds, and the cooling stop temperature after the second-stage annealing was 100°C or lower.

[0064] The chemical compositions of the obtained decarburized annealed steel sheets were measured based on the above-mentioned method. The results are shown in Tables 3-1 to 3-6. In Tables 3-1 to 3-6, "-" indicates that the content was not measured because the control and manufacturing were not carried out with the content in mind. The total impurity content was 0.1% or less.

[0065] The precipitation morphology and grain structure of the obtained decarburized annealed steel sheets were examined based on the above-mentioned method. 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), Dｘ is the particle diameter at which the emission intensity peaks of Nb, V, Mo, Ta, and W are obtained from the FFF analysis, and D Ａｌ is the particle size at which the Al emission intensity peaks.

[0066] These decarburization 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 amounts of the steel sheets after nitriding treatment are shown in Tables 2-4 to 2-7.

[0067] An annealing separator containing MgO as a main component was applied to the steel sheet after the nitriding treatment, and the steel sheet was subjected to finish annealing. In the final step of the finish annealing, the steel sheet was held in a hydrogen atmosphere at 1200°C for 20 hours (purification annealing), and then naturally cooled.

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

[0069] When viewed from a cross section parallel to the thickness direction of the grain-oriented electrical steel sheet, the produced grain-oriented electrical steel sheet had an intermediate layer disposed in contact with the grain-oriented electrical steel sheet (silicon steel sheet) and an insulating coating disposed in contact with the 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) of the obtained grain-oriented electrical steel sheets were evaluated based on the Single Sheet Tester (SST) specified in JIS C 2556:2015. The results are shown in Tables 4-1 to 4-3. For reference, iron loss W17 / 50 (W / kg), defined as the power loss per unit weight (1 kg) of the steel sheet, was also measured under the condition of an excitation magnetic flux density of 1.7 T.

[0071] When the magnetic flux density B8 was 1.930 T or more, it was determined that the magnetic properties were excellent.

[0072]

[0073]

[0074]

[0075]

[0076]

[0077]

[0078]

[0079]

[0080]

[0081]

[0082]

[0083]

[0084]

[0085]

[0086]

[0087]

[0088]

[0089]

[0090]

[0091]

[0092]

[0093]

[0094] As can be seen from Tables 1-1 to 4-3, the compounds have a predetermined chemical composition and D ｘ , D Ａｌ , D Ａｌ -D ｘThe grain-oriented electrical steel sheet obtained using the decarburized annealed steel sheet having the average grain size d and σ / d within the ranges of the present invention exhibits excellent magnetic properties (magnetic flux density B8). ｘ , D Ａｌ , D Ａｌ -D ｘ Grain-oriented electrical steel sheets obtained using decarburization-annealed steel sheets in which at least one of the average grain size d and σ / d is outside the range of the present invention have inferior magnetic properties. No. 52 was within the range of the present invention as a decarburization-annealed steel sheet before nitriding treatment, but the N content after the nitriding treatment was below 0.0130 mass%, so sufficient magnetic properties were not obtained.

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

Claims

1. In mass%, C: 0.0005 to 0.010%, Si: 2.0 to 7.0%, Mn: 0.05 to 1.00%, S: 0 to 0.035%, Se: 0 to 0.035%, total content of S+Se: 0.003 to 0.035%, Al: 0.010 to 0.065%, N: 0.0040 to 0.0400%, at least one element selected from the group consisting of Nb, V, Mo, Ta, and W: 0.003 to 0.030% in total, Cu: 0 to 0.40%, Bi: 0 to 0.010%, B: 0 to 0.080%, P: 0 to 0.500%, Ti: 0 to 0.015%, Sn: 0 to 0.100%, The alloy has a chemical composition consisting of Sb: 0 to 0.100%, Cr: 0 to 0.300%, Ni: 0 to 1.000%, and the balance: Fe and impurities, and the metallographic 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, and when the emission intensity of elements for each particle size of the Nb-based precipitates and the Al-based precipitates is analyzed using a field flow fractionation method, the particle diameter D at which the emission intensity of Al shows a peak is Ａｌ is 75 to 250 nm, and the particle diameter D ｘ is 20 to 200 nm, and Ａｌ is the above D ｘ and in the metal structure, an average grain size d is 13.0 to 19.0 μm, and σ / d is 0.500 or less, where σ is the standard deviation of the grain size.

2. The decarburization annealed steel sheet according to claim 1, wherein the N content is, in mass%, 0.0130 to 0.0400%.