Decarburized annealed steel sheet for grain-oriented electrical steel sheet

Abstract

Provided is a decarburized annealed steel sheet for a grain-oriented electrical steel sheet wherein, with regard to the particle diameter-detected intensity distribution of precipitates for which the equivalent circle diameter D is 50-1000 nm, when the mode diameter of an Al-based precipitate in units of nm is represented as DpAl and the mode diameter of a Nb-based precipitate in units of nm is represented as DpNb, DpNb is 45-250 nm and DpAl-DpNb is 10-50 nm, and the average particle diameter of the decarburized annealed steel sheet is 13-19 μm.

Description

Decarburized annealed steel sheets for grain-oriented electrical steel sheets

[0001] The present invention relates to a decarburization annealed steel sheet for grain-oriented electrical steel sheet. This application claims priority to Japanese Patent Application No. 2024-034154, filed on March 6, 2024, the contents of which are incorporated herein by reference.

[0002] Grain-oriented electrical steel sheets contain 7% by 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 oriented parallel to the rolling surface and the <001> axis of the crystal is oriented 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 by holding 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 proceeding so that Goss-oriented grains grow preferentially during the final annealing. 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 for grain-oriented electrical steel sheets, and that further increases in magnetic flux density are necessary.

[0011] One aspect of 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, an object of one aspect of the present invention is to provide a decarburization-annealed steel sheet for grain-oriented electrical steel sheets that can increase the magnetic flux density.

[0012] The gist of the present invention is as follows.

[0013] (1) A decarburization annealed steel sheet for grain-oriented electrical steel sheet according to one embodiment of the present invention has, in mass%, C: 0 to 0.10%, Si: 2.0 to 7.0%, Mn: 0.050 to 1.0%, S: 0 to 0.0350%, Se: 0 to 0.0350%, S+Se total content: 0.0030 to 0.0350%, Al: 0.010 to 0.0650%, N: 0.0040 to 0.0120%, Nb: 0 to 0.030%, V: 0 to 0.030%, Mo: 0 to 0.030%, Ta: 0 to 0.030%, W: 0 to 0.030%, Nb+V+Mo+Ta+W total content: 0.0030 to 0.030%, The decarburization annealed steel sheet has a chemical composition containing Cu: 0 to 0.40%, Bi: 0 to 0.010%, B: 0 to 0.080%, P: 0 to 0.50%, Ti: 0 to 0.0150%, Sn: 0 to 0.10%, Sb: 0 to 0.10%, Cr: 0 to 0.30%, Ni: 0 to 1.0%, with the balance being Fe and impurities, and the particle size-detection intensity distribution of precipitates that are residues obtained by electrolytic extraction from the decarburization annealed steel sheet and have a circle equivalent diameter D of 50 to 1000 nm is Ａｌ The most frequent diameter of Nb-based precipitates is Dp in units of nm. Ｎｂ When this is done, Dp Ｎｂ is 45 to 250 nm, Dp Ａｌ -Dp Ｎｂ is 10 to 50 nm, and the average grain size of the decarburized annealed steel sheet is 13 to 19 μm.

[0014] According to the above aspect of the present invention, there is provided a decarburization annealed steel sheet for use in a grain-oriented electrical steel sheet that is capable of increasing magnetic flux density.

[0015] 1 is a schematic diagram of particle size-detection intensity distribution of precipitates having a circle-equivalent diameter D of 50 to 1000 nm, and FIG. 2 is a flow chart of a method for producing a decarburization annealed steel sheet for a grain-oriented electrical steel sheet according to one embodiment of the present invention.

[0016] A preferred embodiment of the present invention will be described in detail. 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. Furthermore, the numerical ranges described below include the lower and upper limits. Numerical values ​​indicated as "greater than" or "less than" are not included in the numerical range. Furthermore, "%" in relation to chemical composition means "mass %" unless otherwise specified.

[0017] In the following description, the term "inhibitor" is mainly used for the precipitates in the steel that are a feature of this embodiment in the description related to the secondary recrystallization mechanism, and the term "precipitates" is mainly used for the description related to the compound phases observed in the structure. However, in this embodiment, the terms "inhibitor" and "precipitates" are not used with the intention of strictly distinguishing them.

[0018] As described above, there is currently a demand for increasing the magnetic flux density of grain-oriented electrical steel sheets.

[0019] Therefore, the present inventors focused on the technical idea of ​​"expanding the secondary recrystallization progression temperature range" disclosed in the above-mentioned Patent Documents 9 to 11. In these Patent Documents 9 to 11, the technology of "expanding the secondary recrystallization progression temperature range" is utilized mainly for the formation of subgrain boundaries within secondary recrystallized grains and the associated noise reduction. The present inventors believed that if this technology of "expanding the secondary recrystallization progression temperature range" could be optimized to increase the selectivity of crystal orientation, it would be possible to further improve magnetic flux density.

[0020] Specifically, they investigated how to more effectively expand the temperature range in which secondary recrystallization proceeds by appropriately controlling the morphology of inhibitors in the steel, and how to preferentially grow crystal grains with a preferred crystal orientation during the secondary recrystallization process in the expanded temperature range. As a result, they found that if the morphology of precipitates contained in decarburized annealed steel sheets is optimally controlled in the process of manufacturing grain-oriented electrical steel sheets, the temperature range in which secondary recrystallization proceeds can be expanded during finish annealing, and Goss-oriented grains can preferentially grow, thereby enabling the magnetic flux density of the resulting grain-oriented electrical steel sheets to be increased above that of conventional techniques.

[0021] Generally, inhibitors are fine precipitates with a diameter of about 1000 nm or less contained in steel. These inhibitors have a pinning effect on the grain boundaries and suppress the growth of grains. When the temperature reaches about 1000°C or higher during the final annealing, these inhibitors dissolve into the α-Fe phase, which is the parent phase, and the pinning effect on the grain boundaries weakens. As a result, abnormal grain growth, known as secondary recrystallization, occurs.

[0022] For example, sulfides and selenides as Mn-based precipitates and nitrides as Al-based precipitates are used as main inhibitors. Mn-based inhibitors and Al-based inhibitors (Al-based inhibitors controlled before cold rolling) are mainly used in manufacturing methods in which the slab heating temperature before hot rolling is 1300°C or higher (hereinafter, this may be referred to as the "high-temperature slab heating process"). Al-based inhibitors (Al-based inhibitors controlled after cold rolling) are mainly used in manufacturing methods in which the slab heating temperature before hot rolling is 1280°C or lower and nitriding treatment is performed after cold rolling and before finish annealing (hereinafter, this may be referred to as the "low-temperature slab heating process"). In addition to the above inhibitors, carbides and nitrides of Nb, V, Mo, Ta, W, etc. may be used as auxiliary inhibitors.

[0023] Conventionally, in order to form inhibitors with appropriate functions in steel, when grain-oriented electrical steel sheets are manufactured, the steel composition and manufacturing conditions have been controlled. In particular, the steel composition, hot rolling conditions, and decarburization annealing conditions have been recognized as manufacturing conditions that have a significant impact on the morphology of the inhibitor, and these conditions have been precisely controlled.

[0024] In this embodiment, by utilizing Nb group elements and controlling the size and distribution of precipitates (inhibitors) contained in the decarburization-annealed steel sheet within appropriate ranges, the secondary recrystallization progression temperature range is expanded during the subsequent finish annealing process, and the selectivity of crystal orientation accompanying the progression of secondary recrystallization is improved. Specifically, the above effects are achieved by utilizing precipitates of Nb group elements as relatively fine inhibitors and mainly using AlN as relatively coarse inhibitors, and by allowing the inhibitors to coexist in the decarburization-annealed steel sheet with appropriate sizes and distributions.

[0025] The present inventors presume that the above-mentioned effects are obtained as follows.

[0026] First, we speculate on the reason why the secondary recrystallization progression temperature range expands. As described above, secondary recrystallization occurs due to the weakening of the pinning effect of the grain boundaries as the inhibitor dissolves. During final annealing, fine inhibitors are thought to dissolve and disappear earlier than coarse inhibitors. Therefore, when fine inhibitors and coarse inhibitors coexist, it is thought that the fine inhibitors disappear preferentially early in the temperature rise process of final annealing. In particular, when at least one element selected from Nb, V, Mo, Ta, and W (Nb group element) is added, it becomes possible to preferably control the fine inhibitors that decompose at a lower temperature than conventional inhibitors such as AlN.

[0027] As fine inhibitors dissolve, coarse inhibitors may grow, similar to Ostwald ripening. However, the increase in pinning force that accompanies the growth of coarse inhibitors is thought to have a smaller effect than the decrease in pinning force that accompanies the disappearance of fine inhibitors. Therefore, if fine inhibitors and coarse inhibitors coexist and the fine inhibitors dissolve earlier than the coarse inhibitors, secondary recrystallization is thought to start at a relatively low temperature during the temperature rise process of final annealing.

[0028] In addition, it is considered that the coarse inhibitors remain dissolved in a non-equilibrium state until a relatively high temperature is reached during the temperature rise process of the finish annealing, and that their pinning effect is maintained up to a high temperature. Therefore, it is considered that when fine inhibitors and coarse inhibitors coexist and the coarse inhibitors remain up to a high temperature, the pinning effect is maintained up to a high temperature, and the secondary recrystallization continues up to a relatively high temperature.

[0029] That is, when fine inhibitors and coarse inhibitors coexist, secondary recrystallization starts from a relatively low temperature during the temperature rise process of finish annealing and continues up to a relatively high temperature, which is thought to expand the temperature range in which secondary recrystallization progresses.

[0030] Next, we speculate on the reason for the improved selectivity of crystal orientation. As mentioned above, secondary recrystallization proceeds by preferentially growing Goss-oriented grains. This preferential growth of Goss-oriented grains is thought to be due to the unique grain boundary characteristics and unique crystal grain size (size advantage) of Goss-oriented grains.

[0031] However, the driving force for the preferential growth of Goss-oriented grains is not so strong. Therefore, when the grain growth rate is relatively high (when the driving force for grain growth is relatively high) due to the rapid decomposition of the inhibitor and the weakening of the pinning effect of grain growth, grains other than Goss-oriented grains also grow easily during secondary recrystallization. In this case, the preferential growth of Goss-oriented grains is inhibited.

[0032] Therefore, to preferentially grow Goss-oriented grains, the decomposition rate of the inhibitor should be as slow as possible, the grain growth rate during secondary recrystallization should be relatively high relative to the decomposition rate of the inhibitor, and secondary recrystallization should be maintained for a long period of time. For example, the rate of temperature rise in the temperature range where the inhibitor strength weakens (the temperature range where the inhibitor dissolves) should be slowed down, the dissolution rate of the inhibitor should be slowed down, and the growth rate of the secondary recrystallized grains should be relatively high relative to the decomposition rate of the inhibitor. However, this method inevitably results in a long total finish annealing time, resulting in a decrease in productivity.

[0033] In cases where it is difficult to extend the finish annealing time industrially (in cases where it is difficult to change the heating rate if the maximum temperature is the same), even if the heating rate is constant, if the temperature range in which secondary recrystallization proceeds can be expanded by slowing the decomposition rate of the inhibitor, the time during which secondary recrystallization proceeds can be extended without reducing productivity, the growth rate of secondary recrystallized grains can be made relatively high, and the preferential growth of secondary recrystallized grains can be enhanced. For example, considering that the entire surface of a grain-oriented electrical steel sheet is ultimately occupied by secondary recrystallized grains, it can be understood that extending the time during which secondary recrystallization proceeds leads to an increase in the growth rate of secondary recrystallized grains relative to the decomposition rate of the inhibitor.

[0034] That is, when fine inhibitors and coarse inhibitors coexist, the temperature range where the decomposition rate of the inhibitors is slow expands, and the temperature range where secondary recrystallization progresses, where the growth rate of secondary recrystallization grains is relatively high relative to the decomposition rate of the inhibitors, expands, which is thought to facilitate preferential growth of Goss-oriented grains, ultimately making it possible to increase the magnetic flux density.

[0035] In this embodiment, by controlling the steel composition, casting conditions, hot rolling conditions, hot-rolled sheet annealing conditions, cold rolling conditions, and decarburization annealing conditions in a composite and inseparable manner, relatively fine precipitates and relatively coarse precipitates are allowed to coexist with appropriate sizes and distributions in the decarburization annealed steel sheet after the decarburization annealing step. In particular, in this embodiment, the morphology of the precipitates is preferably controlled by adding a supplementary inhibitor-forming element (Nb group element).

[0036] In this embodiment, the morphology of the above precipitates is defined based on the decarburized annealed steel sheet.

[0037] The decarburization annealed steel sheet for grain-oriented electrical steel sheet according to this embodiment will be described in detail below.

[0038] The decarburization annealed steel sheet according to this embodiment has, in mass%, C: 0 to 0.10%, Si: 2.0 to 7.0%, Mn: 0.050 to 1.0%, S: 0 to 0.0350%, Se: 0 to 0.0350%, S+Se total content: 0.0030 to 0.0350%, Al: 0.010 to 0.0650%, N: 0.0040 to 0.0120%, Nb: 0 to 0.030%, V: 0 to 0.030%, Mo: 0 to 0.030%, Ta: 0 to 0.030%, W: 0 to 0.030%, Nb+V+Mo+Ta+W total content: 0.0030 to 0.030%, Cu: 0 to 0.40%, The steel sheet has a chemical composition containing Bi: 0 to 0.010%, B: 0 to 0.080%, P: 0 to 0.50%, Ti: 0 to 0.0150%, Sn: 0 to 0.10%, Sb: 0 to 0.10%, Cr: 0 to 0.30%, Ni: 0 to 1.0%, with the balance being Fe and impurities, and the particle size-detection intensity distribution of precipitates having a circle equivalent diameter D of 50 to 1000 nm among precipitates that are residues obtained by electrolytic extraction of a decarburized annealed steel sheet is Ａｌ The most frequent diameter of Nb-based precipitates is Dp in units of nm. Ｎｂ When this is done, Dp Ｎｂ is 45 to 250 nm, Dp Ａｌ -Dp Ｎｂ is 10 to 50 nm, and the average grain size of the decarburized annealed steel sheet is 13 to 19 μm.

[0039] 1. Chemical Composition The chemical composition of the decarburized annealed steel sheet according to this embodiment may be a general chemical composition used in grain-oriented electrical steel sheets.

[0040] It should be noted that it is rare for publicly known literature on grain-oriented electrical steel sheets to describe the chemical composition of decarburized annealed steel sheets, which are intermediate products.

[0041] 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.

[0042] The decarburization annealed steel sheet according to this embodiment contains, as basic elements (main alloying elements), the following mass fractions: Si: 2.0 to 7.0%, Mn: 0.050 to 1.0%, a total content of S+Se: 0.0030 to 0.0350%, Al: 0.010 to 0.0650%, N: 0.0040 to 0.0120%, and a total content of Nb+V+Mo+Ta+W: 0.0030 to 0.030%.

[0043] Si: 2.0 to 7.0% Silicon (Si) increases the electrical resistance of grain-oriented electrical steel sheets and reduces iron loss. If the Si content is less than 2.0%, austenite transformation occurs during finish annealing, damaging the crystal orientation of the grain-oriented electrical steel sheet. On the other hand, if the Si content exceeds 7.0%, cold workability decreases, making cracks more likely to occur during cold rolling. Therefore, the Si content of decarburized annealed steel sheets should be 2.0 to 7.0%. The preferred lower limit of the Si content is 2.50%, more preferably 3.0%. The preferred upper limit of the Si content is 4.50%, more preferably 4.0%.

[0044] Mn: 0.050 to 1.0% Manganese (Mn) combines with S and Se to precipitate as MnS or MnSe, functioning as an inhibitor. To favorably control the morphology of these inhibitors (precipitates), the Mn content of the decarburization-annealed steel sheet should be 0.050 to 1.0%. If the Mn content is below 0.050%, the amount of precipitated MnS and MnSe, which function as inhibitors, is insufficient, thereby inhibiting the appropriate progress of secondary recrystallization. Furthermore, if the Mn content exceeds 1.0%, the amount of precipitated MnS and MnSe, which function as inhibitors, becomes excessive, thereby inhibiting the appropriate progress of secondary recrystallization. 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 amount of precipitated MnS and MnSe, which act as inhibitors, may be controlled to be small. Therefore, the upper limit of the Mn content is preferably 0.50%, and more preferably 0.20%.

[0045] S: 0 to 0.0350% Se: 0 to 0.0350% Total content of S+Se: 0.0030 to 0.0350% 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 0.0350%, an Se content of 0 to 0.0350%, and a total content of S+Se of 0.0030 to 0.0350%. A total content of S and Se of 0.0030 to 0.0350% is preferable because it stabilizes secondary recrystallization. In this embodiment, part of the inhibitor function may be performed by carbides, nitrides, or carbonitrides of Nb group elements. In this case, the amount of precipitation of inhibitors MnS and MnSe may be controlled to be small. Therefore, the upper limit of the total content of S and Se is preferably 0.0250%, more preferably 0.010%. If S and Se remain in the steel after final annealing, they may form compounds that deteriorate the iron loss. Therefore, it is preferable to reduce the content of S and Se by purifying them during final annealing to remove them from the steel.

[0046] Here, "the total content of S and Se is 0.0030 to 0.0350%" means that the decarburization annealed steel sheet may contain only one of S or Se in its chemical composition, with the content being 0.0030 to 0.0350%. Alternatively, the decarburization annealed steel sheet may contain both S and Se, with the total content being 0.0030 to 0.0350%.

[0047] Al: 0.010 to 0.0650% 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 the decarburization annealed steel sheet should be 0.010 to 0.0650%. If the Al content is 0.010% or more, AlN or (Al,Si)N precipitates in a favorable form through nitriding in the low-temperature slab heating process, stabilizing secondary recrystallization, particularly in the high-temperature range. If the Al content is below 0.010%, the amount of AlN or (Al,Si)N precipitated, which functions as an inhibitor, is insufficient, hindering the proper progress of secondary recrystallization. Furthermore, if the Al content exceeds 0.0650%, the amount of AlN or (Al,Si)N precipitated, which functions as an inhibitor, becomes excessive, hindering the proper progress of secondary recrystallization. The lower limit of the Al content is preferably 0.020%, more preferably 0.0250%. From the viewpoint of the stability of secondary recrystallization, the upper limit of the Al content is preferably 0.040%, more preferably 0.030%.

[0048] N: 0.0040 to 0.0120% Nitrogen (N) combines with Al and precipitates as AlN or (Al, Si)N, functioning as an inhibitor. The N content of decarburized annealed steel sheet may be 0.0040 to 0.0120%. Note that in the low-temperature slab heating process, N may be added to the steel by nitriding during the manufacturing process. If the N content exceeds 0.0120%, blisters, a type of defect, are more likely to occur in the steel sheet. The upper limit of the N content is preferably 0.010%, more preferably 0.0090%. N is purified in the finish annealing process, and after the finish annealing process, the N content is 0.0050% or less.

[0049] Total content of Nb + V + Mo + Ta + W: 0.0030 to 0.030% Nb: 0 to 0.030% V: 0 to 0.030% Mo: 0 to 0.030% Ta: 0 to 0.030% W: 0 to 0.030% 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 temperature range in which secondary recrystallization progresses. Therefore, the Nb content is set to 0 to 0.030%, the V content to 0 to 0.030%, the Mo content to 0 to 0.030%, the Ta content to 0 to 0.030%, the W content to 0 to 0.030%, and the total content of Nb + V + Mo + Ta + W to 0.0030 to 0.030%. The lower limit of the content of Nb, V, Mo, Ta, and / or W is preferably 0.0040%, more preferably 0.0050%. The upper limit of the content of Nb, V, Mo, Ta, and / or W is preferably 0.020%, more preferably 0.010%.

[0050] In this embodiment, Nb, V, Mo, Ta, and W may be collectively referred to as "Nb group elements."

[0051] The decarburization annealed steel sheet according to this embodiment contains, as the Nb group element, one or more elements selected from the Nb group elements consisting of Nb, V, Mo, Ta, and W in a total amount of 0.0030 to 0.030 mass%.

[0052] When the precipitates of Nb group elements are utilized as inhibitors, when the total content of the Nb group elements in the decarburization annealed steel sheet is 0.030% or less (preferably 0.0030% or more and 0.030% or less), the morphology of the precipitates of Nb group elements is favorably controlled, the secondary recrystallization proceeding temperature range is favorably expanded, and as a result, Goss-oriented grains grow favorably, and the magnetic flux density of the finally obtained grain-oriented electrical steel sheet is favorably increased.

[0053] The reason why precipitates of Nb group elements preferably function as inhibitors is unclear, but is thought to be as follows. Carbides, nitrides, or carbonitrides of Nb group elements are thought to precipitate non-equilibrium during cooling from high temperatures and act as precipitation nuclei for the subsequent precipitation of MnS and AlN. Therefore, compared with a steel sheet not containing an Nb group element, when an Nb group element is contained, the number of precipitation sites for MnS and AlN increases, and as a result, MnS and AlN are thought to be more likely to form as fine precipitates. In the decarburization-annealed steel sheet according to this embodiment, the coexistence of fine inhibitors and coarse inhibitors expands the secondary recrystallization progression temperature range, and the precipitates of Nb group elements are thought to be particularly effective in expanding the secondary recrystallization progression temperature range to the lower temperature side. In this embodiment, it is important to control the manufacturing conditions so as to increase the difference between the mode diameter of Nb-based precipitates and the mode diameter of Al-based precipitates.

[0054] The total content of Nb group elements is preferably 0.0040% or more, more preferably 0.0050% or more. Furthermore, the total content of Nb group elements is preferably 0.020% or less, more preferably 0.010% or less. If the total content of Nb group elements is less than 0.0030%, the precipitates of Nb group elements that act as the above-mentioned precipitation nuclei are insufficient, making it difficult to refine MnS and AlN. 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 becomes 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 becomes large, making it difficult for the Nb group element precipitates to effectively act as precipitation nuclei for refining MnS and AlN.

[0055] Here, "the total content of Nb group elements is 0.0030 to 0.030%" means that the decarburization annealed steel sheet may contain, as a chemical composition, at least one element selected from the group consisting of Nb, V, Mo, Ta, and W, with the content being 0.0030 to 0.030%. Alternatively, the decarburization annealed steel sheet may contain at least two elements selected from the group consisting of Nb, V, Mo, Ta, and W, with the total content being 0.0030 to 0.030%.

[0056] The decarburization annealed steel sheet according to this embodiment may contain impurities as a chemical composition. The term "impurities" refers to elements that are mixed in from raw materials such as ore or scrap, or from the manufacturing environment, during industrial steel production. The upper limit of the total impurity content may be, for example, 5%.

[0057] Furthermore, the decarburization annealed steel sheet according to this embodiment may contain optional elements in addition to the above-described basic elements and impurities. For example, instead of a portion of the Fe balance, optional elements such as C, Cu, Bi, B, P, Ti, Sn, Sb, Cr, and Ni may be contained. These optional elements may be contained according to their intended purpose. Therefore, there is no need to set a lower limit for these optional elements, and the lower limit may be 0%. Furthermore, even if these optional elements are contained as impurities, the above-described effects are not impaired.

[0058] C: 0 to 0.10% 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 0.10%. However, excessive C content in the final product adversely affects the magnetic properties. Therefore, the C content of the decarburized annealed steel sheet should be 0 to 0.10%. The preferred upper limit of the C content is 0.0850% or 0.0750%. Note that C is purified in the finish annealing process described below, and after the finish annealing process, the C content is 0.0050% or less. When C is included, the C content may be greater than 0% or greater than 0.0010%, taking into account productivity in industrial production.

[0059] Cu: 0 to 0.40%, Bi: 0 to 0.010%, B: 0 to 0.080%, P: 0 to 0.50%, Ti: 0 to 0.0150%, Sn: 0 to 0.10%, Sb: 0 to 0.10%, Cr: 0 to 0.30%, Ni: 0 to 1.0%. 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 the lower limit may be 0%.

[0060] In grain-oriented electrical steel sheets, decarburization annealing and purification annealing during secondary recrystallization cause relatively large changes in the chemical composition (reduction in content). Depending on the element, purification annealing can reduce the content to a level that cannot be detected by general analytical methods (1 ppm or less). However, the above chemical composition is the chemical composition of decarburization-annealed steel sheets.

[0061] 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 determined by measuring a 35 mm square test piece taken from the decarburized annealed steel sheet using ICP-AES under conditions based on a previously prepared calibration curve. C and S may be measured using a combustion-infrared absorption method, and N may be measured using an inert gas fusion-thermal conductivity method.

[0062] 2. Precipitates Next, the precipitates contained in the decarburized annealed steel sheet according to this embodiment will be described.

[0063] The precipitates (inhibitors) contained in the decarburization-annealed steel sheet according to this embodiment may be precipitates formed from elements contained in the decarburization-annealed steel sheet. For example, sulfides and selenides may be used as Mn-based precipitates (Mn-containing precipitates), nitrides may be used as Al-based precipitates (Al-containing precipitates), and carbides, nitrides, and carbonitrides may be used as Nb-based precipitates (Nb-group element-containing precipitates). In addition to these inhibitors, auxiliary inhibitors may be used, such as compounds of Nb-group elements excluding Nb, compounds of optional elements such as Bi and B, and complex compounds with the above elements.

[0064] The precipitates to be controlled in the decarburization annealed steel sheet according to this embodiment are precipitates having an equivalent circle diameter D of 50 to 1000 nm. Note that the "equivalent circle diameter" refers to the diameter of a circle when the area of ​​a precipitate is converted into a circle having the same area. This equivalent circle diameter is equivalent to the equivalent sphere diameter.

[0065] Precipitates contained in decarburized annealed steel sheets and having an equivalent circle diameter D of less than 50 nm currently have little effect in expanding the secondary recrystallization progression temperature range. Although the reason for this is not clear, it is thought that precipitates having an equivalent circle diameter D of less than 50 nm at the time of decarburized annealed steel sheets change or disappear in subsequent processes and are therefore less likely to function as inhibitors during finish annealing.

[0066] Specifically, precipitates having an equivalent circle diameter D of less than 50 nm tend to reduce the size of precipitates such as AlN precipitated in subsequent processes, and therefore are less likely to contribute to increasing the difference in decomposition temperature between precipitates that decompose at relatively low temperatures and precipitates that decompose at relatively high temperatures during finish annealing. In the decarburization-annealed steel sheet according to this embodiment, it is important to ensure a difference between the most frequent diameter of Nb-based precipitates and the most frequent diameter of Al-based precipitates. Therefore, in the decarburization-annealed steel sheet according to this embodiment, the size and distribution of precipitates having an equivalent circle diameter D of 50 nm or more are controlled. It is expected that, by considering processes subsequent to the decarburization-annealing process, precipitates having an equivalent circle diameter D of less than 50 nm will function as inhibitors.

[0067] Furthermore, precipitates with an excessively large equivalent circle diameter D may adversely affect the growth of secondary recrystallized grains in the final stage of secondary recrystallization. Furthermore, the formation of precipitates with an excessively large equivalent circle diameter D may reduce the number of precipitates (number density) contained in the decarburization-annealed steel sheet. Furthermore, precipitates with an excessively large equivalent circle diameter D are unlikely to function as inhibitors. Therefore, it is preferable that the average equivalent circle diameter D of the precipitates is 1000 nm or less. In the decarburization-annealed steel sheet according to this embodiment, the size and distribution of precipitates with an equivalent circle diameter D of 50 to 1000 nm are controlled as precipitates that have the effect of expanding the secondary recrystallization progression temperature range.

[0068] In the decarburization annealed steel sheet according to this embodiment, among the precipitates that are residues of electrolytic extraction from the decarburization annealed steel sheet, the particle size-detection intensity distribution of precipitates having a circle equivalent diameter D of 50 to 1000 nm is as follows: Ａｌ The most frequent diameter of Nb-based precipitates is Dp in units of nm. Ｎｂ When this is done, Dp Ｎｂ is 45 to 250 nm, Dp Ａｌ -Dp Ｎｂ is 10 to 50 nm, and

[0069] FIG. 1 shows a schematic diagram of the particle size-detection intensity distribution of precipitates having a circle-equivalent diameter D of 50 to 1000 nm. Ａｌ and Dp Ｎｂ Here is an example:

[0070] Dp Ｎｂ If Dp exceeds 250 nm, the number of fine precipitates for expanding the secondary recrystallization temperature range becomes small, which is inappropriate. Ｎｂ The upper limit of Dp is preferably 200 nm, and more preferably 100 nm. Ｎｂ If Dp is less than 45 nm, the effect of widening the temperature range in which secondary recrystallization progresses is small, as described above. Ｎｂ The lower limit is preferably 50 nm, and more preferably 65 nm.

[0071] Dp Ａｌ -Dp ＮｂIf the value of Dp is less than 10 nm, the balance between the relatively fine inhibitors and the relatively coarse inhibitors is not favorable, and the secondary recrystallization temperature range is not sufficiently expanded, which is inappropriate. Ａｌ -Dp Ｎｂ The lower limit of Dp is preferably 15 nm, and more preferably 20 nm. Ａｌ -Dp Ｎｂ If the value of Dp exceeds 50 nm, the coarse inhibitors are too large, so that the primary recrystallized grains grow unevenly during normal grain growth, and the grain structure before secondary recrystallization becomes a mixed grain structure, which is unsuitable. Ａｌ -Dp Ｎｂ The upper limit is preferably 45 nm, and more preferably 40 nm.

[0072] The particle size-detection intensity distribution of precipitates having a circle-equivalent diameter D of 50 to 1000 nm can be determined as follows.

[0073] For example, the method described in Japanese Patent No. 6,572,598 may be used. First, a precipitate is electrolytically extracted from a decarburized annealed steel sheet. The electrolytic extraction may be performed under constant current electrolytic extraction (500 mA, 2 hours) using an acetylacetone-based electrolyte to which a dispersant such as a surfactant (e.g., sodium dodecyl sulfate with a molecular weight of 288.38 g / mol) has been added in advance. The electrolytic extraction may be performed by electrolysis so that the amount of electrolysis of the decarburized annealed steel sheet is 1 g or more.

[0074] The extraction residue (precipitate) is recovered from the electrolytic extraction solution. The size and distribution of the recovered precipitates are measured by FFF-ICP-MS (Field Flow Fractionation-Inductively Coupled Plasma-Mass Spectrometry). Regarding the particle separation conditions using the FFF method, refer to the above-mentioned Japanese Patent No. 6,572,598.

[0075] Each parameter may be changed depending on the particle size and type to be measured. An example is shown below. A Wyatt Eclipse AF4 device (Wyatt Technology Europe, Germany) may be used as the FFF device. A sodium dodecyl sulfate aqueous solution at a concentration of 300 mg / mL may be used as the dispersion solution for the measurement sample. A cell may have a channel length of 275 mm and an asymmetric diamond-shaped channel spacer with a thickness of 350 μm. A regenerated cellulose ultrafiltration membrane with a molecular weight of 30 kDa may be used as the separation membrane.

[0076] Before adding the extraction residue (precipitate) recovered from the electrolytic extraction solution, it is necessary to create a calibration curve by using standard samples with known particle sizes to correlate the particle size with the time it takes for the particles to be detected.The type and number of standard samples can be selected according to the particle size distribution of the extraction residue to be measured, and for example, polystyrene latex standard particles with a particle size of 29 to 500 nm can be selected.

[0077] The size of the standard particles needs to be directly confirmed in advance using a TEM (Transmission Electron Microscope) or the like. The number of measurements should be 500 or more. The long sides of each standard particle are measured, and the average value is calculated. Furthermore, six particle diameters, for example, 29 nm, 48 nm, 100 nm, 200 nm, 300 nm, and 500 nm, may be used for the standard particles.

[0078] The actual separation conditions are as follows: First, for stabilization before focusing, the eluent effluent (hereinafter referred to as channel flow) of the FFF device is set to 1.0 mL / min, the cross flow to 0.5 mL / min, and the time is 1 minute. Then, for focusing before sample injection, the focus flow is set to 3.0 mL / min and the time is 1 minute. Next, for focusing, the sample is injected at 0.2 mL / min for 2 minutes. The focusing time after sample injection is 1 minute. Then, the flow path is switched, the focus flow is stopped, and the channel flow is set to 1.0 mL / min, and the cross flow is decreased from 0.5 mL / min to 0.05 mL / min over 35 minutes while decreasing the flow rate in direct proportion. A calibration curve can be created by correlating the time from the start of the liquid delivery until particle detection with the average particle size of the standard particles measured in advance. The maximum time for particle detection is 35 minutes, and the injection volume of the sample dispersion liquid is set to 0.1 to 0.4 mL.

[0079] After preparing the calibration curve as described above, the extraction residue (precipitate) recovered from the electrolytic extraction solution is again charged into the apparatus. The apparatus setting parameters can be the same as those described above.

[0080] In this way, the particle size of the nanoparticles contained in the nanoparticle dispersion sample to be measured can be measured.

[0081] Furthermore, the effluent from the FFF device (a solution containing precipitates separated by size) may be subjected to component analysis using inductively coupled plasma mass spectrometry (ICP-Mass).

[0082] The particle size distribution data calculated from the measurement results of the FFF-ICP-MS method can be used to create the distribution of particle size and ICP-MS detection intensity (Al detection intensity and Nb group element detection intensity). Ａｌ and Dp Ｎｂ All we need to do is find the answer.

[0083] In this embodiment, the Nb-based precipitates refer to precipitates containing at least one element selected from the Nb group elements Nb, V, Mo, Ta, and W. Ｎｂmeans the most frequent diameter of the Nb-based precipitates. In this embodiment, the "most frequent diameter" corresponds to the particle diameter at which the detected intensity value is the largest in the particle diameter-detected intensity distribution described above.

[0084] In addition, the above Dp Ａｌ , Dp Ｎｂ It is preferable to perform the calculations such as the above after smoothing the measurement data obtained by the FFF-ICP-MS method. The method for smoothing the measurement data obtained by the FFF-ICP-MS method may be, for example, a simple moving average method.

[0085] 3. Average Grain Size Next, the average grain size of the decarburized annealed steel sheet according to this embodiment will be described.

[0086] The average grain size of the decarburized annealed steel sheet according to this embodiment is 13 to 19 μm. In the decarburized annealed steel sheet according to this embodiment, relatively fine precipitates and relatively coarse precipitates coexist with appropriate sizes and distributions, so the average grain size of the decarburized annealed steel sheet (steel sheet after primary recrystallization) is preferably small.

[0087] The average grain size of the decarburized annealed steel sheet is preferably 18 μm or less, and more preferably 17 μm. The lower limit of the average grain size is not particularly limited. For example, the average grain size of the decarburized annealed steel sheet may be 13 μm or more.

[0088] The average grain size of the decarburized annealed steel sheet according to this embodiment may be determined based on the intercept method of JIS G0551 (2013). For example, an L-section (a cross section normal to the direction perpendicular to the rolling direction) of the decarburized annealed steel sheet is photographed using an optical microscope at a magnification of 200 times, and the grain size of the cross-sectional structure is measured along the sheet thickness direction based on the intercept method. This measurement is carried out at least five times at different measurement locations to determine the average grain size.

[0089] 4. Sheet Thickness The sheet thickness of the decarburization-annealed steel sheet according to this embodiment is not particularly limited. The decarburization-annealed steel sheet according to this embodiment is subjected to subsequent processes, an annealing separator application process and a finish annealing process, and is finally finished into a grain-oriented electrical steel sheet. Therefore, taking into account the manufacturing conditions of general grain-oriented electrical steel sheets, the sheet thickness of the decarburization-annealed steel sheet may be 0.10 to 0.50 mm. However, the sheet thickness is not limited to this, and any known sheet thickness or a sheet thickness used in practice may be adopted.

[0090] 5. Manufacturing Method Next, a method for manufacturing a decarburization-annealed steel sheet for grain-oriented electrical steel sheet according to one embodiment of the present invention will be described. Note that the method for manufacturing the decarburization-annealed steel sheet according to this embodiment is not limited to the method described below. The manufacturing method described below is one example for manufacturing the decarburization-annealed steel sheet according to this embodiment.

[0091] Fig. 2 is a flow chart illustrating the manufacturing process of a decarburized annealed steel sheet according to this embodiment. Fig. 2 also shows the manufacturing process of a grain-oriented electrical steel sheet using this decarburized annealed steel sheet. As shown in Fig. 2, the manufacturing method of a decarburized annealed steel sheet according to this embodiment includes a casting process, a hot rolling process, a hot-rolled sheet annealing process, a cold rolling process, and a decarburization annealing process. The conditions controlled in these processes will be described in detail below.

[0092] 2 , i.e., the annealing separator application step and the finish annealing step, are manufacturing steps for a grain-oriented electrical steel sheet (finish annealed steel sheet). The effects of the decarburization annealed steel sheet according to this embodiment can be confirmed in the grain-oriented electrical steel sheet, which is the final product, and the conditions for controlling these steps will also be described later.

[0093] The method for producing a decarburization annealed steel sheet according to this embodiment includes a casting step, a hot rolling step, a hot-rolled sheet annealing step, a cold rolling step, and a decarburization annealing step, and in the casting step, the steel sheet contains, in mass %, C: 0.0010 to 0.10%, Si: 2.0 to 7.0%, Mn: 0.050 to 1.0%, S: 0 to 0.0350%, Se: 0 to 0.0350%, S+Se total content: 0.0030 to 0.0350%, Al: 0.010 to 0.0650%, N: 0.0040 to 0.0120%, Nb: 0 to 0.030%, V: 0 to 0.030%, Mo: 0 to 0.030%, Ta: 0 to 0.030%, W: 0 to 0.030%, a hot rolling process for forming the slab from molten steel having a chemical composition containing Nb+V+Mo+Ta+W total content: 0.0030 to 0.030%, Cu: 0 to 0.40%, Bi: 0 to 0.010%, B: 0 to 0.080%, P: 0 to 0.50%, Ti: 0 to 0.0150%, Sn: 0 to 0.10%, Sb: 0 to 0.10%, Cr: 0 to 0.30%, Ni: 0 to 1.0%, with the balance being Fe and impurities; a hot rolling process for heating the slab after the casting process, rough rolling the slab, and finish rolling the slab to form a hot rolled steel sheet; a hot rolled sheet annealing process for annealing the hot rolled steel sheet after the hot rolling process to obtain a hot rolled annealed steel sheet; In the cold rolling step, the hot-rolled and annealed steel sheet after the hot-rolled sheet annealing step is rolled to form a cold-rolled steel sheet, and in the decarburization annealing step, the cold-rolled steel sheet after the cold rolling step is decarburized and annealed to obtain a decarburization annealed steel sheet.

[0094] In the method for manufacturing a decarburization annealed steel sheet according to this embodiment, in the hot rolling step, when heating the slab before rough rolling, the slab soaking temperature may be more than 1030°C and less than 1180°C, and the slab soaking time may be more than 70 minutes; when rough rolling, the rolling temperature may be 940 to 1070°C, and the reduction may be 82 to 95%; in the hot-rolled sheet annealing step, the maximum temperature may be 1000 to 1140°C, and during the cooling process, the average cooling rate from 1050 to 900°C may be 1 to 5°C / second, and the average cooling rate from 900 to 500°C may be 10 to 50°C / second; in the cold rolling step, the reduction may be 80 to 95%; and in the decarburization annealing step, during the heating process, the average heating rate from 200 to 750°C may be 15 to 2000°C / second, and the maximum temperature may be 800 to 900°C.

[0095] In order to control the size and distribution of precipitates contained in decarburization annealed steel sheet, it is necessary to control each of the steel composition, casting conditions, hot rolling conditions, hot-rolled sheet annealing conditions, cold rolling conditions, and decarburization annealing conditions. In particular, it is important to control the steel composition, slab heating conditions, rough rolling temperature, and rough rolling reduction, and then control the cooling process after hot-rolled sheet annealing, the cold rolling reduction, and the temperature rise process during decarburization annealing.

[0096] The slab heating can be performed by soaking the slab at a predetermined temperature for a predetermined time without temporarily increasing the heating temperature during the heating process. In this case, the soaking temperature of the slab refers to the surface temperature of the slab, and the soaking time of the slab refers to the time the surface temperature of the slab is maintained at the soaking temperature. Although this is affected by, for example, the steel composition and the heating rate, if the surface temperature of the slab is maintained at the soaking temperature for the soaking time, the precipitation state of the precipitates can be uniformly controlled from the surface to the center of the slab.

[0097] Important manufacturing conditions for the method for manufacturing a decarburized annealed steel sheet according to this embodiment will be described below. Other manufacturing conditions may be the same as those for conventionally known grain-oriented electrical steel sheets.

[0098] (Casting Process) In the casting process, a slab is prepared. Since the chemical composition of the slab, other than C (carbon), remains almost unchanged during the process from slab to decarburization annealing, the chemical composition of the slab may be the chemical composition of the target decarburization annealed steel sheet (the chemical composition of the decarburization annealed steel sheet described above) except for C (carbon). The C content of the slab may be 0.0010 to 0.10%.

[0099] 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 the slab. The thickness of the slab is, for example, 150 to 350 mm. The thickness of the slab is preferably 220 to 280 mm. A so-called thin slab having a thickness of 10 to 70 mm may also be used as the slab.

[0100] (Hot Rolling Step) The hot rolling step is a step in which a slab is heated to a predetermined temperature and hot rolled (rough rolling and finish rolling) to obtain a hot-rolled steel sheet.

[0101] For example, in the hot rolling process, the slab after the casting process is heated, subjected to rough rolling, and then subjected to finish rolling to form a hot-rolled steel sheet having a predetermined thickness of 1.8 to 3.5 mm. After the finish rolling is completed, the hot-rolled steel sheet may be coiled at a predetermined temperature.

[0102] In the hot rolling process, when the slab after the casting process is heated, the following conditions should be satisfied.

[0103] During the slab heating before rough rolling, the slab may be heated to a soaking temperature of more than 1030°C and less than 1180°C for a soaking time of more than 70 minutes. At this time, it is preferable that a portion of the precipitates contained in the slab is preferably solutionized (for example, it is preferable that 12 to 85% by volume of the precipitates are solutionized, based on the precipitates contained in the slab at room temperature after the casting step).

[0104] By preferably bringing some of the precipitates contained in the slab into solution before rough rolling, it becomes easier to achieve a favorable final balance between the amounts of relatively coarse precipitates (residual precipitates) that remain precipitated during the slab heating stage and relatively fine precipitates (re-precipitated precipitates) that do not precipitate during the slab heating stage but precipitate after hot rolling.

[0105] If an Nb-group element is not contained, precipitates such as AlN and MnS that are solutionized during slab heating tend to be non-uniform when they precipitate in a subsequent process. Therefore, in this embodiment, an Nb-group element is contained in the slab. The Nb-group element assists uniform reprecipitation of AlN, MnS, and the like, while contributing to the distribution of relatively fine inhibitors and relatively coarse inhibitors in a preferred form, thereby favorably enhancing the preferential growth of secondary recrystallization.

[0106] Furthermore, as described above, in this embodiment, the soaking time for the slab is set to more than 70 minutes. In conventional manufacturing methods, the soaking time for heating the slab was at most about 60 minutes from a cost perspective. For example, in the case of conventional manufacturing methods in which the slab does not contain 0.0030% or more of an Nb-group element, even if the soaking time for the slab is extended, there is no significant change in the state of reprecipitation of solution-treated AlN, MnS, and the like in subsequent processes, and the soaking time for the slab does not have a significant effect on the magnetic properties of the final product.

[0107] However, when the slab contains 0.0030% or more of an Nb-group element, if the soaking time of the slab exceeds 70 minutes, the Nb-based precipitates contained in the slab will be in a complete solution state or a state close to that state during the slab heating. In this case, precipitates such as AlN and MnS will be reprecipitated in a preferable state in a subsequent process, and as a result, secondary recrystallization will preferably occur.

[0108] Therefore, in this embodiment, the slab contains an Nb group element, and in the hot rolling process, when the slab is heated before rough rolling, the soaking temperature during slab heating is set to more than 1030°C and less than 1180°C, and the soaking time is set to more than 70 minutes. In this case, it is easy to preferably bring some of the precipitates contained in the slab into solution (for example, it is easy to bring 12 to 85 volume % of the precipitates into solution, based on the precipitates contained in the slab at room temperature after the casting process).

[0109] When the content of the Nb-group elements is within the above range, even if the slab heating temperature is 1100°C or higher, it is possible to ultimately allow fine inhibitors and coarse inhibitors to coexist. For example, if the slab heating temperature is high and solution formation of AlN, MnS, etc. is promoted during the slab heating stage, these AlN and MnS are likely to re-precipitate as coarse particles in subsequent processes. However, when the content of the Nb-group elements is within the above range, the precipitates of the Nb-group elements act as precipitation nuclei for MnS and AlN, reducing the size of the re-precipitated AlN and MnS. Furthermore, since the precipitation nose of the Nb-group element precipitates (carbonitrides) is located on the lower temperature side than the precipitation noses of AlN and MnS, the precipitates of the Nb-group elements themselves are likely to precipitate as finer precipitates than AlN, etc.

[0110] Therefore, when the content of the Nb group elements is within the above range, the upper limit temperature during slab soaking may be less than 1180° C. Note that, as the soaking temperature increases, the solution formation of precipitates is also promoted, and when the content of the Nb group elements satisfies the above range, the effect of the precipitates of the Nb group elements described above ultimately makes it easier for fine inhibitors and coarse inhibitors to coexist.

[0111] Similarly, when the content of the Nb group element is within the above range, the lower limit temperature during slab soaking may be more than 1,030° C. Note that, as the soaking temperature decreases, solution formation of precipitates is also suppressed, but when the content of the Nb group element satisfies the above range, it becomes possible to finally allow fine inhibitors and coarse inhibitors to coexist.

[0112] The mechanism by which the above-mentioned effects are obtained is thought to be related to the fact that precipitates (carbonitrides) of Nb group elements are more likely to precipitate than MnS or AlN (MnS in particular is difficult to precipitate without support such as dislocation multiplication due to rolling, and when it does precipitate, its size increases), and that precipitates of Nb group elements function as precipitation nuclei in the precipitation of MnS and AlN, thereby suppressing the coarsening of the re-precipitated AlN and MnS.

[0113] Furthermore, when the content of the Nb group element is within the above range, if the soaking time is 70 minutes or less, the time is too short and it is difficult to control the solution state of the precipitates to an equilibrium state. The upper limit of the soaking time is not particularly limited, but may be 2 hours in consideration of productivity in industrial production.

[0114] Controlling the "solution state of precipitates before rough rolling" to the above conditions makes it easier to achieve a favorable final balance between the amounts of relatively coarse precipitates (residual precipitates) that remain precipitated at the slab heating stage and relatively fine precipitates (re-precipitated precipitates) that precipitate after hot rolling.

[0115] The soaking temperature of the slab refers to the surface temperature of the slab, and the soaking time of the slab refers to the time for which the surface temperature of the slab is maintained after reaching the soaking temperature. Although this is affected by, for example, the steel composition and the heating rate, if the surface temperature of the slab is maintained for the soaking time after reaching the soaking temperature, the precipitation state of the precipitates from the surface to the center of the slab can be controlled uniformly.

[0116] In the hot rolling process, hot rolling is performed following the above-mentioned slab heating. Generally, hot rolling is divided into rough rolling and finish rolling. In this embodiment, in order to control the size and distribution of precipitates contained in the steel sheet after the decarburization annealing process, it is important to control the above-mentioned "solution state of precipitates before rough rolling" and then control each condition after hot rolling.

[0117] In the hot rolling process, when rough rolling is performed after heating the slab, the following conditions should be satisfied.

[0118] When the heated slab is rough-rolled, the rolling temperature is controlled to 940 to 1070°C and the rolling reduction is controlled to 82 to 95%. The rough-rolling temperature is defined as the average value of the start temperature and end temperature of the rough rolling.

[0119] By setting the reduction rate within the above range, deformation-induced precipitation occurs, making it possible to precipitate fine and large amounts of precipitates. If the rough rolling reduction rate is smaller than the above lower limit, the introduction of dislocations by rolling processing is reduced, and the number of precipitation sites available for deformation-induced precipitation is reduced, resulting in a larger particle size of the precipitates. On the other hand, the upper limit of the rough rolling reduction rate is not particularly limited, but may be set to 95% taking into account the performance of the rolling mill, etc.

[0120] The rough rolling reduction mentioned above means the cumulative reduction in rough rolling. Specifically, the rough rolling reduction is defined as follows: rough rolling reduction (cumulative reduction) (%) = (1 - "steel sheet thickness after rough rolling" / "steel sheet thickness before rough rolling") x 100

[0121] Furthermore, if the rolling temperature of the rough rolling is higher than the above upper limit, the deformation-induced precipitation of precipitates such as MnS, AlN, and Nb group element precipitates occurs on the higher temperature side or near the nose of the precipitation, so the precipitation critical radius of the precipitates that reprecipitate during hot rolling becomes larger. Therefore, the size difference with the relatively coarse precipitates (residual precipitates) that have been precipitated since the slab heating stage becomes smaller. On the other hand, the lower limit of the rolling temperature of the rough rolling is not particularly limited, but since the slab becomes hard and the rollability decreases at low temperatures, it is sufficient to roll at, for example, 940 ° C or higher.

[0122] When the Nb group elements are suitably contained in the chemical composition, in addition to MnS and AlN, precipitates of the Nb group elements (particularly carbides and nitrides) are precipitated during rough rolling. These precipitates of the Nb group elements act as precipitation nuclei for the MnS and AlN that subsequently precipitate, resulting in finer re-precipitation of MnS and AlN. Therefore, when the Nb group elements are suitably contained in the chemical composition, it is sufficient to control the various control conditions, such as the rough rolling temperature and the rough rolling reduction, as described above.

[0123] When an Nb group element is suitably contained, the reason why the conditions of the hot rolling process should be controlled as described above is thought to be as follows. When an Nb group element is contained, MnS and AlN are reprecipitated more finely due to the precipitates of the Nb group element, and the particle size of the reprecipitated precipitates is smaller than when an Nb group element is not contained. Therefore, it is easier to achieve a favorable balance between the amounts of relatively coarse precipitates (residual precipitates) that remain precipitated at the slab heating stage and relatively fine precipitates (reprecipitated precipitates) that are not precipitated at the slab heating stage but precipitate relatively finely after hot rolling. Therefore, it is thought that the conditions of the hot rolling process should be controlled as described above.

[0124] For example, when an Nb group element is contained, if the solution state of the precipitates is not favorably controlled (e.g., if the "solution rate of precipitates before rough rolling" is lower than 12% by volume), the precipitates are unlikely to be fully dissolved at the time of slab heating, as in the case where an Nb group element is not contained, and the number of fine precipitates that reprecipitate during hot rolling is likely to be small. Therefore, it is difficult to sufficiently expand the secondary recrystallization progression temperature range during finish annealing. Furthermore, when an Nb group element is contained, if the solution state of the precipitates is not favorably controlled (e.g., if the "solution rate of precipitates before rough rolling" is higher than 85% by volume), most of the precipitates are likely to be fully dissolved at the time of slab heating, as in the case where an Nb group element is not contained, and the number of relatively coarse precipitates (residual precipitates) in the slab is likely to be small. Therefore, it is difficult to sufficiently expand the secondary recrystallization progression temperature range during finish annealing.

[0125] Furthermore, the reason why the rough rolling reduction ratio should be controlled as described above when the Nb group elements are suitably contained, compared to when the Nb group elements are not suitably contained, is thought to be as follows: When the Nb group elements are contained, precipitates of the Nb group elements are likely to precipitate finely in the steel, so the number of fine precipitates contained in the steel even before rough rolling is greater, compared to when the Nb group elements are not contained. Therefore, when the Nb group elements are contained, the number of precipitation sites for precipitates increases, and deformation-induced precipitation is likely to occur even when the reduction ratio is reduced. For this reason, it is thought that the rough rolling reduction ratio should be controlled as described above.

[0126] When an Nb group element is contained, if the rough rolling reduction is less than 82%, the introduction of dislocations by rolling is reduced, and the number of precipitation sites available for deformation-induced precipitation is reduced, as in the case where an Nb group element is not contained, resulting in a larger particle size of the precipitates. When an Nb group element is contained, the upper limit of the rough rolling reduction is preferably 93%.

[0127] Furthermore, the reason why the rough rolling temperature should be controlled as described above when the Nb group elements are suitably contained, compared to when the Nb group elements are not suitably contained, is thought to be as follows. When the Nb group elements are contained, as described above, the number of fine precipitates contained in the steel even before rough rolling is greater, compared to when the Nb group elements are not contained. Therefore, when the Nb group elements are contained, the number of precipitation sites for precipitates increases, and the precipitates that re-precipitate during hot rolling tend to be finer. Therefore, it is thought that the rough rolling temperature should be controlled as described above.

[0128] When Nb group elements are contained, if the rolling temperature of rough rolling is higher than 1070 ° C, all precipitates of MnS, AlN, Nb group elements, etc. will precipitate at a temperature higher than the precipitation nose of the precipitate, so the precipitation critical radius of the precipitates that reprecipitate during hot rolling will be large. Therefore, the size difference with the relatively coarse precipitates (residual precipitates) that have precipitated since the slab heating stage will be small. When Nb group elements are contained, the upper limit of the rolling temperature of rough rolling is preferably 1065 ° C, more preferably 1040 ° C.

[0129] The reason why Nb group elements promote the fine precipitation of precipitates is not clear, but is thought to be as follows.

[0130] During rough rolling, the steel sheet temperature drops rapidly over time. Therefore, the rough rolling process is considered to be in a non-equilibrium state. Even in a temperature range where all MnS and AlN precipitate in an equilibrium state, dissolved MnS and AlN may exist in a non-equilibrium state. For example, because the rough rolling process is in a non-equilibrium state, dissolved MnS and AlN are thought to exist even in a temperature range where precipitates of Nb group elements precipitate. Therefore, when precipitates of Nb group elements precipitate during rough rolling, these precipitates of Nb group elements are thought to act as precipitation nuclei for the subsequently precipitated MnS and AlN, causing fine precipitation of MnS and AlN. Specifically, compared to when precipitates of Nb group elements are not present, when precipitates of Nb group elements are present, there are more precipitation sites for MnS and AlN, resulting in fine precipitation of MnS and AlN.

[0131] Furthermore, when precipitates of Nb group elements that acted as precipitation nuclei for MnS or AlN are covered with MnS or AlN, further growth of the precipitates of Nb group elements is suppressed. In this case, it is believed that the Nb group elements that would have been consumed for the growth of the precipitates precipitate finely as new precipitates. It is believed that these new fine precipitates of Nb group elements act as new precipitation nuclei for MnS or AlN, contributing to further fine precipitation of MnS or AlN. In this way, it is believed that the precipitates of Nb group elements synergistically contribute to the fine precipitation of MnS or AlN.

[0132] When the above-mentioned conditions in the hot rolling step are satisfied and the conditions in the subsequent steps are also satisfied, the size and distribution of the precipitates are preferably controlled, and as a result, the particle size-detection intensity distribution of the precipitates after the decarburization annealing step is controlled within the above-mentioned range.

[0133] The slab soaking temperature during slab heating before rough rolling and the rolling temperature during rough rolling are temperatures that are controlled with a purpose. These temperatures are not caused by a natural temperature drop that occurs when the slab is removed from the slab heating furnace and subjected to rough rolling. For example, in general operations, the slab soaking temperature and rough rolling temperature are not controlled with a purpose. Typically, if the slab soaking temperature is high, the rough rolling temperature also increases accordingly, and if the slab soaking temperature is low, the rough rolling temperature also decreases accordingly. On the other hand, in this embodiment, the slab soaking temperature and the rough rolling temperature are controlled with a purpose. For example, even if the slab soaking temperature is high within the above range, the rough rolling temperature is controlled to be within the above range. Similarly, even if the slab soaking temperature is low within the above range, the rough rolling temperature is controlled to be within the above range.

[0134] The conditions for finish rolling in the hot rolling step are not particularly limited, and ordinary hot rolling conditions may be employed.

[0135] (Hot-rolled sheet annealing process) The hot-rolled sheet annealing process is a process in which the hot-rolled steel sheet after the hot-rolling process is annealed to obtain a hot-rolled annealed steel sheet. The hot-rolled sheet annealing is generally performed to control the steel sheet structure such as the recrystallization rate, residual strain, and grain size, and to preferably adjust the morphology of precipitates in the steel, by annealing the hot-rolled steel sheet after the hot-rolling process.

[0136] In the hot-rolled sheet annealing step, when annealing the hot-rolled steel sheet after hot rolling, the following conditions may be satisfied.

[0137] During annealing of the hot-rolled sheet, the maximum temperature reached is set to 1000 to 1140°C, and during the cooling process, the average cooling rate from 1050 to 900°C is controlled to 1 to 5°C / sec, and the average cooling rate from 900 to 500°C is controlled to 10 to 50°C / sec.

[0138] When the maximum temperature reached during hot-rolled sheet annealing is within the above range, Al-based precipitates (such as AlN) are preferably solutionized. Furthermore, the solutionized Al-based precipitates tend to reprecipitate 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 preferably controlled. For example, when the average cooling rate from 1050 to 900°C during the cooling process is within the above range, Al-based precipitates are preferably reprecipitated coarsely compared to Nb-based precipitates. Since the coarse Al-based precipitates remain in solution in a non-equilibrium state until a relatively high temperature during the temperature-raising process of finish annealing, the secondary recrystallization progression temperature range is preferably expanded toward the higher temperature side.

[0139] Similarly, when the maximum temperature reached during hot-rolled sheet annealing is within the above range, Nb-based precipitates are preferably put into solution. The solution-formed Nb-based precipitates tend to re-precipitate in the temperature range of 900 to 500°C during the cooling process. Therefore, by controlling the average cooling rate from 900 to 500°C during the cooling process, the re-precipitation of Nb-based precipitates can be preferably controlled. For example, when the average cooling rate during the cooling process is 900 to 500°C within the above range, fine Nb-based precipitates can be preferably formed. Since fine Nb-based precipitates begin to dissolve at a relatively low temperature during the temperature rise process of finish annealing, the secondary recrystallization progression temperature range is preferably expanded to the lower temperature side.

[0140] 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 maximum temperature is 1000 to 1050 ° C., and 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. when the maximum temperature is 1050 to 1140 ° C. Similarly, the average cooling rate from 900 to 500 ° C. during the cooling process is the value obtained by dividing the temperature difference (400 ° C.) from 900 ° C. to 500 ° C. by the cooling time from 900 ° C. to reach 500 ° C.

[0141] (Cold Rolling Step) The cold rolling step is a step of obtaining a cold rolled steel sheet having a thickness of, for example, 0.10 to 0.50 mm by cold rolling the hot rolled annealed steel sheet obtained in the hot rolled sheet annealing step once, or by cold rolling the hot rolled annealed steel sheet multiple times (two or more times) via annealing (intermediate annealing).

[0142] In the cold rolling step, when the hot-rolled and annealed steel sheet after the hot-rolled sheet annealing step is rolled, the reduction ratio of the cold rolling may be controlled to 80 to 95%.

[0143] The cold rolling reduction mentioned above means the cumulative cold rolling reduction without intermediate annealing, or the cumulative cold rolling reduction after intermediate annealing. Specifically, the cold rolling reduction is defined as follows: Cold rolling reduction (cumulative reduction) (%) = (1 - "steel sheet thickness after cold rolling" / "steel sheet thickness before cold rolling (or after intermediate annealing)") x 100

[0144] 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 having the 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.

[0145] (Decarburization annealing step) The decarburization annealing step is a step of performing decarburization annealing on the cold-rolled steel sheet obtained in the cold rolling step to obtain a decarburization annealed steel sheet in which primary recrystallization has occurred. By performing decarburization annealing on the cold-rolled steel sheet, C contained in the cold-rolled steel sheet is removed. The decarburization annealing is preferably performed in a humid atmosphere in order to remove "C" contained in the cold-rolled steel sheet.

[0146] In the decarburization annealing step, when annealing the cold-rolled steel sheet after the cold rolling step, the following conditions may be satisfied.

[0147] During decarburization annealing, the average temperature rise rate from 200 to 750°C during the temperature rise process may be controlled to 15 to 2000°C / sec, and the maximum temperature may be controlled to 800 to 900°C.

[0148] When the average heating rate during the temperature rise process of decarburization annealing 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 preferential growth of ideal Goss oriented grains during secondary recrystallization. As a result, the final product has a magnetic flux density B ８ In addition, when the maximum temperature reached in decarburization annealing is within the above range, the primary recrystallized grain size can be controlled within a suitable range. As a result, after the Nb-based precipitates are decomposed on the low-temperature side during the temperature rise process in finish annealing, secondary recrystallized grains having the Goss orientation can grow sufficiently, which is preferable.

[0149] The average temperature rise rate from 200 to 750°C during the temperature rise process is the temperature difference (550°C) from 200°C to 750°C divided by the temperature rise time from 200°C to 750°C.

[0150] As described above, the method for producing a decarburization-annealed steel sheet according to this embodiment includes a casting process, a hot-rolling process, a hot-rolled sheet annealing process, a cold-rolling process, and a decarburization-annealing process. The decarburization-annealed steel sheet produced by controlling the above conditions in each process in a composite manner has favorably controlled precipitate size and distribution, and the precipitate particle size-detection intensity distribution is controlled within the above range. As a result, the secondary recrystallization progression temperature range is expanded during finish annealing, the selective growth of Goss-oriented grains is enhanced, and the magnetic flux density of the grain-oriented electrical steel sheet is improved.

[0151] For example, as described above, in the manufacturing method of the decarburized annealed steel sheet according to this embodiment, the amount of relatively coarse precipitates (residual precipitates) remaining after slab heating is controlled mainly by the slab soaking temperature and slab soaking time during slab heating before rough rolling, and the amount of relatively fine precipitates (reprecipitated precipitates) is controlled by the subsequent manufacturing conditions, thereby controlling the characteristics of the decarburized annealed steel sheet within the above ranges. As a result, the secondary recrystallization progression temperature range is expanded during finish annealing, the selective growth of Goss-oriented grains is enhanced, and the magnetic flux density of the grain-oriented electrical steel sheet is improved.

[0152] 6. Method of Using Decarburization Annealed Steel Sheet The effects of the decarburization annealed steel sheet according to this embodiment can be confirmed in the final product, that is, grain-oriented electrical steel sheet. Therefore, from the perspective of a method of using the decarburization annealed steel sheet according to this embodiment, the manufacturing process of the grain-oriented electrical steel sheet subsequent to the decarburization annealing step will be described.

[0153] The manufacturing method of grain-oriented electrical steel sheet includes an annealing separator application step and a finish annealing step. If necessary, the manufacturing method may also include a nitriding treatment, an insulating coating formation step, and a magnetic domain control step. These steps may be performed under well-known general process conditions. Below, a manufacturing method using nitriding treatment as a low-temperature slab heating process will be described as an example.

[0154] (Nitriding Treatment) Nitriding treatment is carried out to adjust the strength of the inhibitor in secondary recrystallization. In the nitriding treatment, the nitrogen content of the steel sheet may be increased to about 40 to 300 ppm at any timing between the start of the above-mentioned decarburization annealing and the start of secondary recrystallization in the finish annealing described below. Examples of nitriding treatment include a treatment in which a steel sheet is annealed in an atmosphere containing a gas with nitriding ability such as ammonia, and a treatment in which a decarburization-annealed steel sheet coated with an annealing separator containing a powder with nitriding ability such as MnN is finish-annealed.

[0155] (Annealing separator application step) The annealing separator application step is a step of applying an annealing separator to the decarburized annealed steel sheet. As the annealing separator, for example, an annealing separator containing MgO as a main component or an annealing separator containing alumina as a main component can be used.

[0156] The decarburized annealed steel sheet after application of the annealing separator is wound into a coil and then finish-annealed in the subsequent finish-annealing step.

[0157] (Finish annealing step) The finish annealing step is a step in which the decarburized annealed steel sheet coated with the annealing separator is subjected to finish annealing to cause secondary recrystallization. In this step, the growth of primary recrystallized grains is suppressed by an inhibitor, and secondary recrystallization is allowed to proceed, thereby preferentially growing {110}<001> oriented grains and improving magnetic flux density.

[0158] When the decarburized annealed steel sheet according to this embodiment is used, the secondary recrystallization progression temperature range is expanded during final annealing, and preferential growth of {100}<011> oriented grains occurs to an extent not previously observed, resulting in a dramatic improvement in magnetic flux density. Furthermore, abnormal grain growth of secondary recrystallized grains occurs during final annealing, and the secondary recrystallized grains occupy the entire sheet surface after final annealing. The few secondary recrystallized grains cover the entire steel sheet surface, and the grain size of each secondary recrystallized grain becomes larger.

[0159] Furthermore, 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 decarburized 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.

[0160] The following insulating coating formation process and magnetic domain control process are not necessary from the viewpoint of concentrating the crystal orientation in {110}<001>. However, they are processes adopted in general grain-oriented electrical steel sheets to improve practical magnetic properties. (Insulating Coating Formation Process) The insulating coating formation process is a process for forming an insulating coating on the grain-oriented electrical steel sheet (finish-annealed steel sheet) 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 may be formed on the steel sheet after the finish-annealing process.

[0161] (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.

[0162] 7. Grain-oriented electrical steel sheet obtained using the decarburization annealed steel sheet according to this embodiment A brief description will be given of the grain-oriented electrical steel sheet produced using the decarburization annealed steel sheet according to this embodiment.

[0163] In the decarburization-annealed steel sheet according to the present embodiment, relatively fine precipitates and relatively coarse precipitates coexist in a preferred size and distribution, and therefore, in the grain-oriented electrical steel sheet obtained using the decarburization-annealed steel sheet according to the present embodiment, Goss-oriented grains grow preferentially, and magnetic flux density is preferably increased. Furthermore, since the increased magnetic flux density does not cause deterioration in other properties of the grain-oriented electrical steel sheet produced using the decarburization-annealed steel sheet according to the present embodiment, it can be used in the same applications as conventional ones.

[0164] The grain-oriented electrical steel sheet manufactured using the decarburization annealed steel sheet according to this embodiment contains, as a base element (main alloying element), 2.0 to 7.0% by mass of Si (silicon).

[0165] Furthermore, impurities may be contained. Here, "impurities" refer to elements that are mixed in from raw materials such as ore or scrap, or from the manufacturing environment, during industrial production of steel. The upper limit of the total content of impurities may be, for example, 5%.

[0166] Furthermore, in addition to the above-described basic elements and impurities, selective elements may be contained. For example, instead of a portion of the remaining Fe, Nb, V, Mo, Ta, W, C, Mn, S, Se, Al, N, Cu, Bi, B, P, Ti, Sn, Sb, Cr, Ni, etc. may be contained as selective elements. 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, these selective elements may be contained as impurities.

[0167] In grain-oriented electrical steel sheets, relatively large changes in chemical composition (reduction in content) occur as a result of decarburization annealing and purification annealing during secondary recrystallization. Purification annealing can reduce the content of some elements to a level that cannot be detected by common analytical methods (1 ppm or less). Generally, the chemical composition of the final product differs from that of the starting slab. However, the above-mentioned optional elements are elements contained in the slab that remain in the final product. The content of each element does not exceed the content range described above for the slab, but rather falls within a content range that corresponds to the content in the slab and the subsequent manufacturing process.

[0168] The above chemical composition is that of the grain-oriented electrical steel sheet. If the grain-oriented electrical steel sheet to be measured has an insulating coating or the like on its surface, the coating or the like is removed by a known method before measuring the chemical composition.

[0169] The grain-oriented electrical steel sheet manufactured using the decarburization annealed steel sheet according to this embodiment may have an intermediate layer disposed on and in contact with the grain-oriented electrical steel sheet (silicon steel sheet), and an insulating coating disposed on and in contact with the intermediate layer.

[0170] For example, the intermediate layer may be a layer mainly made of an oxide, a layer mainly made of a carbide, a layer mainly made of a nitride, a layer mainly made of a boride, a layer mainly made of a silicide, a layer mainly made of a phosphide, a layer mainly made of a sulfide, a layer mainly made of an intermetallic compound, etc. These intermediate layers are formed mainly to ensure adhesion between the silicon steel sheet and the insulating coating, and may be any known layer formed by heat treatment in an atmosphere with controlled oxidation-reduction, chemical vapor deposition (CVD), physical vapor deposition (PVD), etc.

[0171] Representative examples of the insulating coating include an insulating coating mainly composed of phosphate and colloidal silica and having an average thickness of 0.1 to 10 μm, and an insulating coating mainly composed of alumina sol and boric acid and having an average thickness of 0.5 to 8 μm.

[0172] Next, the effects of the present invention will be specifically described in detail using examples. The conditions in the examples are 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 deviate from the gist of the present invention and the object of the present invention is achieved.

[0173] Decarburized annealed steel sheets were manufactured using slabs having the chemical compositions shown in Tables 1 and 2. The chemical compositions of the manufactured decarburized annealed steel sheets were equivalent to the chemical compositions of the slabs shown in Tables 1 and 2, except for C (carbon). The C contents of the manufactured decarburized annealed steel sheets are shown in Tables 17 to 23. These chemical compositions were measured based on the above-mentioned method. In Tables 1 and 2, "-" indicates that the content was not measured and that control and manufacturing were not carried out with the content in mind.

[0174] The decarburization annealed steel sheets were manufactured under the manufacturing conditions shown in Tables 3 to 16. Slab heating was performed by soaking at a predetermined temperature for a predetermined time without temporarily increasing the heating temperature during the slab heating process. The soaking temperature shown in the tables indicates the surface temperature of the slab after heating, and the soaking time shown in the tables indicates the slab heating time from when the slab surface temperature reached the soaking temperature.

[0175] Although not shown in the table, when the total content of Nb group elements is 0.0030 to 0.030 mass%, by setting the soaking temperature of the slab to more than 1030°C and less than 1180°C and the soaking time of the slab to more than 70 minutes, 12 to 85 volume % of the precipitates are solutionized, based on the precipitates contained in the slab after the casting process.

[0176] The average grain size and the grain size-detection intensity distribution of precipitates were measured using the manufactured decarburized annealed steel sheets based on the above-mentioned method. The measurement results are shown in Tables 17 to 23. In the tables, Dp Ａｌ represents the most frequent diameter of Al-based precipitates, and Dp Ｎｂ represents the most frequent diameter of Nb-based precipitates.

[0177] The produced decarburized annealed steel sheets were then subjected to nitriding treatment (nitriding annealing) in a mixed atmosphere of hydrogen, nitrogen and ammonia.

[0178] Furthermore, an annealing separator containing MgO as a main component was applied to the steel sheet, 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.

[0179] 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 manufactured grain-oriented electrical steel sheet (finish-annealed steel sheet), and the resulting solution was heated in an atmosphere of hydrogen:nitrogen 75% by volume:25% by volume, maintained at this temperature, and then cooled to form an insulating coating.

[0180] When viewed from a cross section parallel to the thickness direction of the grain-oriented electrical steel sheet, the produced 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 with an average thickness of 1 μm, mainly composed of phosphate and colloidal silica.

[0181] The obtained grain-oriented electrical steel sheets were evaluated for various properties. The evaluation results are shown in Tables 17 to 23.

[0182] (1) Magnetic Properties of Grain-Oriented Electrical Steel Sheets The magnetic properties of the grain-oriented electrical steel sheets were measured based on the Single Sheet Tester (SST) specified in JIS C 2556:2015.

[0183] As a magnetic property, the magnetic flux density B in the rolling direction of the steel sheet when excited at 800 A / m ８ (T) was measured. ８ For reference, the iron loss W, defined as the power loss per unit weight (1 kg) of the steel sheet under the conditions of AC frequency: 50 Hz, excitation magnetic flux density: 1.7 T, was １７／５０ (W / kg) was measured.

[0184] Among Nos. 1 to 111, the inventive examples had favorably controlled average grain size of the decarburized annealed steel sheet and particle size-detection intensity distribution of the precipitates contained in the decarburized annealed steel sheet, and all of them exhibited excellent magnetic flux density as grain-oriented electrical steel sheets. On the other hand, among Nos. 1 to 111, the comparative examples had not favorably controlled average grain size of the decarburized annealed steel sheet or particle size-detection intensity distribution of the precipitates contained in the decarburized annealed steel sheet, and did not obtain favorable magnetic flux density as grain-oriented electrical steel sheets.

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[0208] According to the above-described aspects of the present invention, it is possible to provide a decarburization annealed steel sheet for use in a grain-oriented electrical steel sheet that can increase magnetic flux density, and therefore the present invention has high industrial applicability.

Claims

1. A decarburization annealed steel sheet for grain-oriented electrical steel sheet, the decarburization annealed steel sheet having, in mass%, C: 0 to 0.10%, Si: 2.0 to 7.0%, Mn: 0.050 to 1.0%, S: 0 to 0.0350%, Se: 0 to 0.0350%, S+Se total content: 0.0030 to 0.0350%, Al: 0.010 to 0.0650%, N: 0.0040 to 0.0120%, Nb: 0 to 0.030%, V: 0 to 0.030%, Mo: 0 to 0.030%, Ta: 0 to 0.030%, W: 0 to 0.030%, Nb+V+Mo+Ta+W total content: 0.0030 to 0.030%, The decarburization annealed steel sheet has a chemical composition containing Cu: 0 to 0.40%, Bi: 0 to 0.010%, B: 0 to 0.080%, P: 0 to 0.50%, Ti: 0 to 0.0150%, Sn: 0 to 0.10%, Sb: 0 to 0.10%, Cr: 0 to 0.30%, Ni: 0 to 1.0%, with the balance being Fe and impurities, and the particle size-detection intensity distribution of precipitates that are residues obtained by electrolytic extraction from the decarburization annealed steel sheet and have a circle equivalent diameter D of 50 to 1000 nm is Ａｌ The most frequent diameter of Nb-based precipitates is Dp in units of nm. Ｎｂ When this is done, Dp Ｎｂ is 45 to 250 nm, Dp Ａｌ -Dp Ｎｂ and the average grain size of the decarburization-annealed steel sheet is 13 to 19 μm.

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