Decarburized and nitrided steel sheet for grain-oriented electromagnetic steel sheet

Abstract

In this decarburized and nitrided steel sheet for a grain-oriented electrical steel sheet, as regards precipitates having a major axis D of 50-150 nm, the percentage of precipitates having an R(Nb) ÷ R(Al) value of 0.1 or more is 35-95% when the Al content is taken to be R(Al) in at% and the total content of Nb + V + Mo + Ta + W is taken to be R(Nb) in at%.

Description

Decarburized and nitrided steel sheets for grain-oriented electrical steel sheets

[0001] The present invention relates to a decarbonitrided steel sheet for grain-oriented electrical steel sheet. This application claims priority to Japanese Patent Application No. 2024-034214, 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 decarbonitrided 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 decarbonitrided steel sheet for grain-oriented electrical steel sheet according to one aspect of the present invention has, in mass%, C: 0.0005 to 0.010%, Si: 2.0 to 7.0%, Mn: 0.050 to 1.0%, S: 0 to 0.0350%, Se: 0 to 0.0350%, total content of S+Se: 0.0030 to 0.0350%, Al: 0.010 to 0.0650%, N: 0.010 to 0.040%, 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%, total content of Nb+V+Mo+Ta+W: 0.0030 to 0.030%, The decarbonitrided 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 among precipitates collected from the decarbonitrided steel sheet by an extraction replica method, for precipitates having a major axis D of 50 to 150 nm, when the Al content is R(Al) in atomic % and the total content of Nb+V+Mo+Ta+W is R(Nb) in atomic %, the proportion of precipitates such that the value of R(Nb) / R(Al) is 0.1 or more is 35% to 95% by number.

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

[0015] 1 is a flowchart of a method for manufacturing a decarbonitrided steel sheet for a grain-oriented electrical steel sheet according to an 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 decarbonitrided 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 appropriately controlling the precipitates (inhibitors) contained in the decarbonitrided steel sheet, 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 appropriately controlling the type and number ratio of fine precipitates having a major axis D of 50 to 150 nm among the relatively fine inhibitors and relatively coarse inhibitors contained in the decarbonitrided steel sheet.

[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, the fine inhibitors are thought to 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 is added, it becomes possible to favorably control the fine inhibitors that decompose at lower temperatures 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, the steel composition, casting conditions, hot rolling conditions, hot-rolled sheet annealing conditions, cold rolling conditions, and decarbonitriding conditions are controlled in a composite and inseparable manner, thereby appropriately controlling the type and number ratio of fine precipitates having a major axis D of 50 to 150 nm among the relatively fine inhibitors and relatively coarse inhibitors contained in the decarbonitrided steel sheet after the decarbonitriding step. Also, in this embodiment, the morphology of the precipitates is preferably controlled by adding a supplementary inhibitor-forming element.

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

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

[0038] The decarbonitrided steel sheet according to this embodiment has, in mass %, C: 0.0005 to 0.010%, 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: 0.010 to 0.040%, 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 among precipitates collected from a decarbonitrided steel sheet by an extraction replica method, for precipitates having a major axis D of 50 to 150 nm, when the Al content is R(Al) in atomic % and the total content of Nb+V+Mo+Ta+W is R(Nb) in atomic %, the proportion of precipitates such that the value of R(Nb) / R(Al) is 0.1 or more is 35% to 95% by number.

[0039] 1. Chemical Composition The chemical composition of the decarburized-nitrided 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 decarbonitrided steel sheets, which are intermediate products.

[0041] The decarbonitrided 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 decarbonitrided steel sheet according to this embodiment contains, as basic elements (main alloying elements), by mass fraction, C: 0.0005 to 0.010%, 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 0.040%, and a total content of Nb+V+Mo+Ta+W: 0.0030 to 0.030%.

[0043] C: 0.0005-0.010% Carbon (C) is an effective element for controlling the primary recrystallization structure during the manufacturing process. In particular, C is effective for controlling the primary recrystallization structure because it forms carbides and carbonitrides with elements such as Nb. Therefore, the C content of the slab is preferably 0.0010-0.10%. However, excessive C content in the final product adversely affects the magnetic properties. Therefore, the C content of the decarbonitrided steel sheet should be 0.0005-0.010%. The preferred upper limit of the C content is 0.009% or 0.008%. 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, considering productivity in industrial production, the C content may be greater than 0%, 0.0005%, or 0.0010% or more.

[0044] 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 and cracks tend to occur during cold rolling. Therefore, the Si content of decarbonitrided 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%.

[0045] 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 decarbonitrided 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%.

[0046] 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 form of these inhibitors (precipitates), the decarbonitrided steel sheet should have an S content of 0 to 0.0350%, an Se content of 0 to 0.0350%, and an S+Se total content 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 a carbide, nitride, or carbonitride of an Nb group element. 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.

[0047] Here, "the total content of S and Se is 0.0030 to 0.0350%" means that the decarbonitrided steel sheet may contain only one of S or Se in its chemical composition, with its content being 0.0030 to 0.0350%. Alternatively, the decarbonitrided steel sheet may contain both S and Se, with their total content being 0.0030 to 0.0350%.

[0048] 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 decarbonitrided 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 during nitriding in a 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. On the other hand, 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%.

[0049] N: 0.010 to 0.040% Nitrogen (N) combines with Al and precipitates as AlN or (Al, Si)N, functioning as an inhibitor. The N content of the decarbonitrided steel sheet may be 0.010 to 0.040%. In the process of this embodiment, the nitrogen content of the steel sheet is increased by nitriding treatment in the decarbonitrided steel sheet. If the N content exceeds 0.040%, blisters, a type of defect, are more likely to occur in the steel sheet. The upper limit of the N content is preferably 0.035%, more preferably 0.0250%. N is purified in the final annealing process, and the N content is 0.0050% or less after the final annealing process.

[0050] 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%.

[0051] In this embodiment, Nb, V, Mo, Ta, and W may be collectively referred to as "Nb group elements."

[0052] The decarbonitrided 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 %.

[0053] When the precipitates of Nb group elements are utilized as inhibitors, when the total content of Nb group elements in the decarbonitrided 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.

[0054] The reason why precipitates of Nb group elements preferably function as inhibitors is not clear, 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 is thought to increase, and as a result, MnS and AlN are thought to be more likely to form as fine precipitates. In the decarbonitrided steel sheet according to this embodiment, the coexistence of fine inhibitors and coarse inhibitors expands the secondary recrystallization progression temperature range, and it is thought that precipitates of Nb group elements are particularly effective in expanding the secondary recrystallization progression temperature range toward the lower temperature side.

[0055] 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.

[0056] Here, "the total content of Nb group elements is 0.0030 to 0.030%" means that the decarbonitrided 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, it means that the decarbonitrided 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%.

[0057] The decarbonitrided 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%.

[0058] Furthermore, the decarbonitrided steel sheet according to this embodiment may contain optional elements in addition to the basic elements and impurities described above. For example, Cu, Bi, B, P, Ti, Sn, Sb, Cr, Ni, etc. may be contained as optional elements in place of a portion of the remaining Fe described above. These optional elements may be contained according to the purpose. Therefore, there is no need to set a lower limit for these optional elements, and the lower limit may be 0%. Furthermore, even if these optional elements are contained as impurities, the above-mentioned effects are not impaired.

[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 decarburized, nitrided steel sheets.

[0061] The chemical composition of the decarbonitrided steel sheet according to this embodiment may be measured by a general steel analysis method. For example, the chemical composition of the decarbonitrided 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 decarbonitrided steel sheet using ICP-AES under conditions based on a pre-created calibration curve. C and S may be measured using a combustion-infrared absorption method, and N may be measured using an inert gas fusion-thermal conductivity method.

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

[0063] The precipitates (inhibitors) contained in the decarbonitrided steel sheet according to this embodiment may be precipitates formed from elements contained in the decarbonitrided 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-group element-containing precipitates. In addition to these inhibitors, compounds of optional elements such as Bi and B, and complex compounds with the above elements may also be contained.

[0064] The decarbonitrided steel sheet according to this embodiment includes relatively fine precipitates and relatively coarse precipitates, each having a major diameter D of 50 to 1000 nm. Of these precipitates, the precipitates to be controlled in this embodiment are those having a major diameter D of 50 to 150 nm. Note that the "major diameter" refers to the longest line segment among the line segments connecting non-adjacent vertices of the cross-sectional profile of the precipitate on the observation surface.

[0065] Precipitates contained in decarbonitrided steel sheets with a major 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 with a major diameter D of less than 50 nm at the time of decarbonitrided steel sheets change or disappear in subsequent processes and are less likely to function as inhibitors during finish annealing. Therefore, in the decarbonitrided steel sheets according to this embodiment, precipitates with a major diameter D of 50 nm or more are controlled. It is expected that in the future, precipitates with a major diameter D of less than 50 nm will function as inhibitors by considering processes including those after the decarbonitrided steel sheet.

[0066] Furthermore, precipitates with an excessively large major 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 major diameter D may reduce the number of precipitates (number density) contained in the decarbonitrided steel sheet. Furthermore, precipitates with an excessively large major diameter D are unlikely to function as inhibitors. Therefore, the major diameter D of the precipitates is preferably 1000 nm or less on average. In the decarbonitrided steel sheet according to this embodiment, precipitates with a major diameter D of 50 to 150 nm are controlled as precipitates that are particularly effective in expanding the secondary recrystallization progression temperature range toward the lower temperature side.

[0067] In the decarbonitrided steel sheet according to this embodiment, among precipitates collected from the decarbonitrided steel sheet by an extraction replica method, for precipitates having a major diameter D of 50 to 150 nm, when the Al content is R(Al) in atomic % and the total content of Nb+V+Mo+Ta+W (Nb group element content) is R(Nb) in atomic %, the proportion of precipitates such that the value of R(Nb) / R(Al) is 0.1 or more is 35% or more and 95% or less by number.

[0068] When a steel composition contains Al and an Nb group element and Al-containing precipitates and Nb group element-containing precipitates coexist in the steel, precipitates having a major diameter D of 50 to 150 nm are often composite precipitates of Al-containing precipitates and Nb group element-containing precipitates. Furthermore, precipitates having a major diameter D of 50 to 150 nm have the effect of expanding the secondary recrystallization progression temperature range toward the lower temperature side, and particularly, precipitates having a high Nb group element content within the composite precipitates preferably exhibit this effect. Specifically, precipitates for which the value of R(Nb)÷R(Al) is 0.1 or greater preferably exhibit this effect.

[0069] Therefore, among the precipitates having a major diameter D of 50 to 150 nm, the number percentage of precipitates having a value of R(Nb)÷R(Al) of 0.1 or more is set to 35% or more. To preferably obtain the above effect, the number percentage is preferably 38% or more, and more preferably 40% or more. On the other hand, the upper limit of the number percentage may be set to 95% in order to obtain a preferable magnetic flux density.

[0070] The Al content, Nb group element content, and number of precipitates having a major diameter D of 50 to 150 nm can be determined as follows.

[0071] For example, precipitates can be collected from decarbonitrided steel sheets by extraction replica method. The extraction replica method is a method in which the steel sheet is polished and etched to expose the precipitates on the surface, a replica film is vapor-deposited on the steel sheet surface to capture the precipitates in the replica film, and the replica film is then peeled off from the steel sheet to collect only the precipitates. These precipitates can be observed and their composition analyzed using TEM-EDS (Transmission Electron Microscope-Energy Dispersive X-ray Spectroscopy).

[0072] Specifically, the major diameter D of precipitates sampled from the decarbonitrided steel sheet by the extraction replica method is confirmed by TEM, and the Al content and Nb group element content of precipitates having a major diameter D of 50 to 150 nm are subjected to composition analysis by EDS.

[0073] For example, for a TEM observation sample prepared by a replica method from a decarbonitrided steel sheet, 30 or more precipitates having a major axis D of 50 to 150 nm are selected from among the precipitates in an arbitrary carbon mesh of the observation sample, and precipitates are similarly selected at two other locations on the carbon mesh of the observation sample by changing the location, so that a total of 100 precipitates are selected from three locations. A composition analysis is then performed using EDS such that the central portion (the intersection of the major axis and minor axis) of the precipitate is irradiated with an electron beam, and the Al content obtained by the above analysis is designated R(Al) in atomic %, and the Nb group element content is designated R(Nb) in atomic %.

[0074] 3. Sheet Thickness The sheet thickness of the decarbonitrided steel sheet according to this embodiment is not particularly limited. The decarbonitrided 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 decarbonitrided 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.

[0075] 4. Manufacturing Method Next, a method for manufacturing a decarbonitrided 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 decarbonitrided 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 decarbonitrided steel sheet according to this embodiment.

[0076] Fig. 1 is a flow chart illustrating the manufacturing process of a decarbonitrided steel sheet according to this embodiment. Fig. 1 also shows the manufacturing process of a grain-oriented electrical steel sheet using this decarbonitrided steel sheet. As shown in Fig. 1, the manufacturing method of a decarbonitrided 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 decarbonitrided steel sheet. The conditions controlled in these processes will be described in detail below.

[0077] 1 , 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 decarbonitrided 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.

[0078] The method for producing a decarbonitrided 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 decarbonitrided steel sheet, 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 process, the hot-rolled and annealed steel sheet after the hot-rolled sheet annealing process is rolled at a cumulative reduction rate of 80 to 95% to form a cold-rolled steel sheet, and in the decarbonitriding process, the cold-rolled steel sheet after the cold rolling process is subjected to decarbonization annealing by holding it in a moist atmosphere at 700 to 900°C for 1 to 3 minutes, and then subjected to nitriding treatment to increase the nitrogen content of the steel sheet by 40 to 300 ppm, thereby obtaining a decarbonitrided steel sheet.

[0079] In the manufacturing method of a decarbonitrided steel sheet according to this embodiment, in the hot rolling step, when heating the slab before rough rolling, the soaking temperature of the slab is set to more than 1030°C and less than 1180°C, thereby preferably bringing some of the precipitates contained in the slab into solution (for example, 12 to 85% by volume of the precipitates based on the precipitates contained in the slab after the casting step are brought into solution), and in order to make this solution state uniform within the slab, the soaking time of the slab is set to more than 70 minutes, and when rough rolling, the rolling temperature is set to 940 to 1070°C and the reduction is set to 82 to 95%.

[0080] In order to control the size and distribution of precipitates contained in decarbonitrided 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 decarbonitriding conditions, and it is particularly important to control each of the steel composition, slab heating conditions (solution state of precipitates before rough rolling), rough rolling temperature, and rough rolling reduction, and then control the hot-rolled sheet annealing conditions, cold rolling conditions, and decarbonitriding conditions. Furthermore, in order to control the above-mentioned "solution state of precipitates before rough rolling," it is important to control each of the steel composition and slab heating conditions.

[0081] The slab heating may be performed by soaking at a predetermined temperature for a predetermined time without temporarily increasing the heating temperature during the slab 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 holding time after the surface temperature of the slab reaches the soaking temperature. Although this is affected by, for example, the steel composition and the heating rate, if the surface temperature of the slab reaches the soaking temperature during heating, the solution state of the precipitates on the surface of the slab is preferably controlled. Furthermore, if the surface temperature of the slab reaches the soaking temperature and is held for the soaking time, the solution state of the precipitates is preferably controlled all the way to the center of the slab.

[0082] Important manufacturing conditions for the method for manufacturing a decarbonitrided 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.

[0083] (Casting Process) In the casting process, a slab is prepared. Since the chemical composition of the slab remains almost unchanged from the slab to the decarbonitriding process except for C (carbon) and N (nitrogen), the chemical composition of the slab may be the chemical composition of the target decarbonitriding steel sheet (the chemical composition of the decarbonitriding steel sheet described above) except for C (carbon) and N (nitrogen). The C content of the slab may be 0.0010 to 0.10%, and the N content of the slab may be 0.0040 to 0.0120%.

[0084] The chemical composition of the slab affects the "solution state of precipitates before rough rolling" as described above. As will be described in detail later, the chemical composition of the slab needs to satisfy the above-mentioned chemical composition and also needs to be controlled in combination with other manufacturing conditions that affect the "solution state of precipitates before rough rolling."

[0085] 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.

[0086] (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.

[0087] 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.

[0088] In the hot rolling process, when the slab after the casting process is heated, the following conditions should be satisfied.

[0089] When heating the slab before rough rolling, the soaking temperature of the slab is set to more than 1030°C and less than 1180°C, so that a portion of the precipitates contained in the slab are preferably solutionized (for example, 12 to 85 volume% of the precipitates are solutionized based on the precipitates contained in the slab at room temperature after the casting process), and in order to make this solution state uniform within the slab, the slab is heated so that the soaking time of the slab is more than 70 minutes.

[0090] Preferably bringing some of the precipitates contained in the slab into solution before rough rolling is necessary 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.

[0091] The above "solution state of precipitates before rough rolling" means "solution state of precipitates before rough rolling" in an equilibrium state, not a non-equilibrium state. In a non-equilibrium state, for example, the solution state of precipitates becomes non-uniform near the surface and near the center in the sheet thickness direction. If a slab in this non-equilibrium state is subjected to rough rolling, it ultimately becomes difficult to control the size and distribution of precipitates contained in the steel sheet after the decarbonitriding process.

[0092] For example, in order to bring the solution state of the precipitates closer to an equilibrium state, it is preferable to set the value obtained by subtracting the temperature at the center of the slab from the surface temperature of the slab within a range of more than -10°C and less than 50°C during slab heating and extraction. In particular, if the temperature difference is -10°C or less, the steel sheet surface becomes difficult to elongate, resulting in significant occurrence of defects. Furthermore, if the temperature difference is 50°C or more, the solution state of the precipitates becomes non-uniform in the sheet thickness direction, making it difficult to control the size of the precipitates.

[0093] Although different from the slab heating method of this embodiment, the heating temperature may be temporarily increased during the slab heating process to shorten the soaking time. In this case, it is effective to set the difference between the surface temperature at the maximum temperature reached and the surface temperature at the time of heating and extraction of the slab to 80°C or less. In this case, after the temperature is reduced from the maximum temperature reached, it is preferable to hold the slab in a low-temperature region of the slab heating furnace for at least 20 minutes or more, so that the difference between the surface temperature and the center temperature at the time of extraction from the slab heating furnace is less than 50°C. More preferably, the difference between the surface temperature and the center temperature of the slab should be 0 to 30°C.

[0094] In the conventional technology known as the low-temperature slab heating process, in which slab heating is performed at a temperature of 1280°C or less, there was no technical idea of ​​solutionizing only a specific proportion of the precipitates contained in the slab, nor any knowledge that the solution of these precipitates needs to approach an equilibrium state. In the manufacturing method of a decarbonitrided steel sheet according to this embodiment, the solution state of the precipitates is preferably controlled, and a slab in which the solution of the precipitates is in an equilibrium state is subjected to rough rolling.

[0095] The "solution state of precipitates before rough rolling" is a characteristic that is affected by the steel composition and the hot rolling conditions (slab heating conditions). To control this "solution state of precipitates before rough rolling," it is necessary to control each manufacturing condition in a composite and inseparable manner, taking into consideration the influence of the manufacturing conditions on the "solution state of precipitates." For example, a person skilled in the art can control the material properties, including precipitation behavior, and can control the "solution state" by combining the above conditions, as long as he or she understands that each of the above conditions affects the "solution state."

[0096] For example, as described above, the "solution state of the precipitates before rough rolling" may be controlled by temporarily increasing the heating temperature during the slab heating process and maintaining the temperature for a certain period of time after cooling. However, in the manufacturing method of a decarbonitrided steel sheet according to this embodiment, as an example, a method is shown in which the "solution state of the precipitates before rough rolling" is controlled by soaking at a predetermined temperature for a predetermined period of time without temporarily increasing the heating temperature during the slab heating process.

[0097] In the hot rolling process, when the slab is heated before rough rolling, the soaking temperature during slab heating may be set to more than 1030°C and less than 1180°C, and the soaking time may be set to more than 70 minutes. In this case, a portion of the precipitates contained in the slab is easily and preferably solutionized (for example, 12 to 85% by volume of the precipitates are easily solutionized, based on the precipitates contained in the slab at room temperature after the casting process).

[0098] 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.

[0099] 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 of precipitates is also promoted, and when the content of the Nb group elements is within the above range, it is easy to preferably bring some of the precipitates contained in the slab into solution (for example, the upper limit of the solution rate of the precipitates may be 85% by volume). When these conditions are satisfied, the effect of the precipitates of the Nb group elements described above ultimately makes it easy to make fine inhibitors and coarse inhibitors coexist.

[0100] Similarly, when the content of the Nb group element is within the above range, the lower limit temperature during slab soaking may be greater than 1,030°C. Note that, as the soaking temperature decreases, the solution of precipitates is also suppressed, but when the content of the Nb group element is within the above range, it is easy to preferably bring some of the precipitates contained in the slab into solution (for example, the lower limit of the solution rate of precipitates may be 12% by volume). When these conditions are satisfied, it is ultimately possible to allow fine inhibitors and coarse inhibitors to coexist.

[0101] 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.

[0102] 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.

[0103] Controlling the "solution state of precipitates before rough rolling" to the above conditions is necessary to ultimately achieve a favorable 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 precipitate after hot rolling.

[0104] 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. For example, although it is affected by the steel composition and the heating rate, if the surface temperature of the slab reaches the soaking temperature during heating, the solution state of the precipitates on the surface of the slab is preferably controlled. Furthermore, if the surface temperature of the slab is maintained for the soaking time after reaching the soaking temperature, the solution state of the precipitates is preferably controlled all the way to the center of the slab.

[0105] The specific value of the solution ratio is not particularly limited. As described above, by controlling the steel composition and the slab heating conditions, the "solution state of precipitates before rough rolling" can be preferably controlled. However, if necessary, the specific value of the solution ratio may be determined using integrated thermodynamic calculation software. For example, "Thermo-Calc" is known as a commonly available integrated thermodynamic calculation software. In this embodiment, the solution ratio was calculated from the chemical composition and temperature of the slab using "Thermo-Calc" (2019a ver.) and used as a reference.

[0106] 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 precipitates contained in the steel sheet after the decarbonitriding process, it is important to control the above-mentioned "solution state of the precipitates before rough rolling" and then control each condition after hot rolling.

[0107] In the hot rolling process, when rough rolling is performed after heating the slab, the following conditions should be satisfied.

[0108] When the heated slab is subjected to rough rolling, the rolling temperature may be controlled to 940 to 1070° C. and the rolling reduction to 82 to 95%.

[0109] 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.

[0110] 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

[0111] Furthermore, if the rolling temperature of rough rolling is higher than the above upper limit, deformation-induced precipitation occurs on the higher temperature side or near the nose of precipitates of MnS, AlN, Nb group elements, etc., 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 precipitated since the slab heating stage becomes smaller. On the other hand, the lower limit of the rolling temperature of 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. The rough rolling temperature is defined as the average value of the start temperature and end temperature of rough rolling.

[0112] In addition, when Nb group elements are suitably contained in the chemical composition, in addition to MnS and AlN, precipitates of Nb group elements (particularly carbides and nitrides) are precipitated during rough rolling. These precipitates of Nb group elements act as precipitation nuclei for the subsequently precipitated MnS and AlN, resulting in finer re-precipitation of MnS and AlN. Therefore, when Nb group elements are suitably contained in the chemical composition, it is sufficient to control the various control conditions, such as the solution state of the precipitates (for example, the solution rate of the precipitates before rough rolling), the rough rolling temperature, and the rough rolling reduction, as described above.

[0113] 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.

[0114] For example, if the solution state of the precipitates is not favorably controlled when an Nb group element is contained (e.g., if the "solution rate of precipitates before rough rolling" is lower than 12% by volume), the precipitates are not sufficiently dissolved at the time of slab heating, just as in the case where an Nb group element is not contained, and the number of fine precipitates that reprecipitate during hot rolling is reduced. As a result, the secondary recrystallization progression temperature range cannot be sufficiently expanded during finish annealing. Furthermore, if the solution state of the precipitates is not favorably controlled when an Nb group element is contained (e.g., if the "solution rate of precipitates before rough rolling" is higher than 85% by volume), just as in the case where an Nb group element is not contained, most of the precipitates are dissolved at the time of slab heating, and the number of relatively coarse precipitates (residual precipitates) in the slab is reduced. As a result, the secondary recrystallization progression temperature range cannot be sufficiently expanded during finish annealing.

[0115] 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.

[0116] 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%.

[0117] 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.

[0118] 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.

[0119] The reason why Nb group elements promote the fine precipitation of precipitates is not clear, but is thought to be as follows.

[0120] 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.

[0121] 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.

[0122] When the above-mentioned conditions in the hot rolling step are satisfied and the conditions in the steps after the hot rolling step are also satisfied, precipitates are preferably controlled, and as a result, precipitates having a major diameter D of 50 to 150 nm are controlled as described above after the decarbonitriding step.

[0123] 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.

[0124] The conditions for finish rolling in the hot rolling step are not particularly limited, and ordinary hot rolling conditions may be employed.

[0125] (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.

[0126] In this embodiment, the hot-rolled steel sheet after the hot rolling process is heated and subjected to first-stage annealing for recrystallization in a temperature range of 1000 to 1150°C, followed by second-stage annealing in a lower temperature range of 800 to 1000°C, after which the steel sheet is cooled. The first-stage annealing temperature is preferably 1020 to 1130°C. The second-stage annealing temperature is preferably 800 to 950°C. The heating rate to the first-stage annealing temperature is preferably 5°C / sec or more on average. In particular, the heating rate from 200 to 900°C during heating to the first-stage annealing temperature is preferably 100°C / sec or less on average. In addition, it is preferable that the steel sheet be held for 20 seconds or more during second-stage annealing. The cooling rate after second-stage annealing is preferably 5°C / sec or more on average. In particular, the cooling rate from 750 to 500°C during cooling after second-stage annealing is preferably 25 to 80°C / sec on average.

[0127] In the hot-rolled sheet annealing process, precipitates with a major diameter D of 50 to 150 nm may be additionally precipitated. For example, if the heating rate from 200 to 900 °C to the first-stage annealing temperature is less than 5 °C / s, the amount of AlN that goes into solution in the first-stage annealing is likely to increase, and Al is likely to precipitate simultaneously with Nb-group elements when precipitates with a major diameter D of 50 to 150 nm are subsequently precipitated. As a result, the R(Al) value of the precipitates increases, and the proportion of precipitates with an R(Nb) / R(Al) value of 0.1 or greater may decrease. Furthermore, for example, if the heating rate from 200 to 900 °C to the first-stage annealing temperature is greater than 100 °C / s, precipitates containing Nb-group elements may not fully dissolve during the first-stage annealing, and Nb-group elements may not be readily incorporated when precipitates with a major diameter D of 50 to 150 nm are subsequently precipitated. As a result, the R(Nb) value of the precipitates becomes smaller, and the proportion of the number of precipitates in which the value of R(Nb) / R(Al) is 0.1 or more may decrease.

[0128] (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).

[0129] In the cold rolling step, when the hot-rolled and annealed steel sheet after the hot-rolled sheet annealing step is rolled, the reduction rate (cumulative reduction rate) of the cold rolling may be controlled to 80 to 95%.

[0130] The cold rolling reduction (cumulative 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 (cumulative 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

[0131] 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.

[0132] (Decarbonitriding Step) The decarbonitriding step is a step in which the cold-rolled steel sheet obtained in the cold rolling step is subjected to decarbonization annealing (for example, at 700 to 900°C for 1 to 3 minutes) and then subjected to nitriding treatment to increase the nitrogen content of the steel sheet (for example, an increase of 40 to 300 ppm), thereby obtaining a decarbonitrided steel sheet in which primary recrystallization has occurred.

[0133] (Decarburization Annealing) Decarburization annealing is carried out to remove C contained in the cold-rolled steel sheet. In decarburization annealing, the steel sheet is preferably soaked in a moist atmosphere at 700 to 900°C for 1 to 3 minutes.

[0134] (Nitriding Treatment) Nitriding treatment is carried out to adjust the strength of the inhibitor in secondary recrystallization. In nitriding treatment, the nitrogen content of the steel sheet may be increased to about 40 to 300 ppm. For example, nitriding treatment may be performed by annealing the steel sheet in an atmosphere containing a nitriding gas such as ammonia.

[0135] As described above, the method for producing a decarbonitrided 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 decarbonitriding step. A decarbonitrided steel sheet produced by comprehensively controlling the above conditions in each step preferably controls precipitates having a major diameter D of 50 to 150 nm. As a result, the secondary recrystallization progression temperature range is preferably 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.

[0136] For example, as described above, in the manufacturing method of a decarbonitrided steel sheet according to this embodiment, the amount of relatively coarse precipitates (residual precipitates) that remain precipitated at the slab heating stage is controlled by mainly varying 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 various subsequent manufacturing conditions, thereby controlling the characteristics of the decarbonitrided 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.

[0137] 5. Method of Using the Decarbonitrided Steel Sheet The effects of the decarbonitrided steel sheet according to this embodiment can be confirmed in the final product, which is a grain-oriented electrical steel sheet. Therefore, from the perspective of a method of using the decarbonitrided steel sheet according to this embodiment, the manufacturing process of the grain-oriented electrical steel sheet subsequent to the decarbonitrided steel sheet process will be described.

[0138] The method for producing grain-oriented electrical steel sheet includes an annealing separator application step and a finish annealing step. If necessary, the method may also include an insulating coating formation step and a magnetic domain control step. These steps may be carried out under well-known general process conditions. The following describes the production method as an example.

[0139] (Annealing separator application step) The annealing separator application step is a step of applying an annealing separator to the decarbonitrided 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.

[0140] The decarbonitrided steel sheet after being coated with the annealing separator is wound into a coil and then finish-annealed in the subsequent finish-annealing step.

[0141] (Finish annealing step) The finish annealing step is a step in which the decarbonitrided 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.

[0142] When the decarbonitrided steel sheet according to this embodiment is used, the secondary recrystallization progression temperature range is expanded during finish 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 finish annealing, and the secondary recrystallized grains occupy the entire sheet surface after finish annealing. The few secondary recrystallized grains cover the entire steel sheet surface, and the grain size of each secondary recrystallized grain becomes larger.

[0143] 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 decarbonitrided steel sheet according to the present 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.

[0144] 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.

[0145] (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.

[0146] 6. Grain-oriented electrical steel sheet obtained using the decarbonitrided steel sheet according to this embodiment A brief description will be given of the grain-oriented electrical steel sheet produced using the decarbonitrided steel sheet according to this embodiment.

[0147] In the decarbonitrided 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 decarbonitrided 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 result in deterioration of other properties, the grain-oriented electrical steel sheet produced using the decarbonitrided steel sheet according to the present embodiment can be used in the same applications as conventional ones.

[0148] The grain-oriented electrical steel sheet manufactured using the decarbonitrided steel sheet according to this embodiment contains, as a base element (main alloying element), 2.0 to 7.0% by mass of Si (silicon).

[0149] 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, when steel is industrially produced. The upper limit of the total content of impurities may be, for example, 5%.

[0150] 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.

[0151] 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.

[0152] 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.

[0153] The grain-oriented electrical steel sheet manufactured using the decarbonitrided 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.

[0154] 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.

[0155] 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.

[0156] 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.

[0157] Decarbonitrided steel sheets were manufactured using slabs having the chemical compositions shown in Tables 1 and 2. The chemical compositions of the manufactured decarbonitrided steel sheets were equivalent to the chemical compositions of the slabs shown in Tables 1 and 2, except for C (carbon) and N (nitrogen). The C and N contents of the manufactured decarbonitrided steel sheets are shown in Tables 15 to 20. These chemical compositions were measured based on the above-mentioned method. In Tables 1 and 2, "-" indicates that the contents were not measured, and that control and manufacturing were not carried out with the contents in mind.

[0158] The decarbonitrided steel sheets were manufactured under the manufacturing conditions shown in Tables 3 to 14. 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.

[0159] 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.

[0160] In the hot-rolled sheet annealing step, the hot-rolled steel sheet after the hot rolling step was annealed. In Examples other than No. 90, the hot-rolled steel sheet after the hot rolling step was annealed under the annealing conditions shown in Tables 9 to 14. In Example No. 90, second-stage annealing was not performed.

[0161] In the decarbonitriding step, decarbonization annealing was performed in a humid atmosphere under the conditions shown in Tables 9 to 14, and nitriding treatment was performed in a hydrogen-nitrogen-ammonia mixed atmosphere under the conditions shown in Tables 9 to 14.

[0162] Using the produced decarbonitrided steel sheets, the Al content (atomic %): R(Al) and the Nb group element content (atomic %): R(Nb) of precipitates having a major diameter D of 50 to 150 nm were examined based on the above-mentioned method. Tables 15 to 20 show, as "number ratio," the number ratio of precipitates having a major diameter D of 50 to 150 nm, where the value of R(Nb) / R(Al) is 0.1 or more.

[0163] The decarbonitrided steel sheets were then coated with an annealing separator mainly composed of MgO and subjected to finish annealing. In the final step of the finish annealing, the steel sheets were held in a hydrogen atmosphere at 1200°C for 20 hours (purification annealing) and then naturally cooled.

[0164] 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.

[0165] 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.

[0166] The grain-oriented electrical steel sheets thus obtained were evaluated for various properties, and the evaluation results are shown in Tables 15 to 20.

[0167] (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.

[0168] 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.

[0169] Among Nos. 1 to 93, the inventive examples had favorable control of precipitates contained in the decarbonitrided steel sheets, and all exhibited excellent magnetic flux density as grain-oriented electrical steel sheets. On the other hand, among Nos. 1 to 93, the comparative examples had poor control of precipitates contained in the decarbonitrided steel sheets, and did not achieve favorable magnetic flux density as grain-oriented electrical steel sheets.

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[0190] According to the above-described aspects of the present invention, it is possible to provide a decarbonitrided steel sheet for grain-oriented electrical steel sheet that can increase magnetic flux density, and therefore the present invention has high industrial applicability.

Claims

1. A decarbonitrided steel sheet for grain-oriented electrical steel sheet, comprising, in mass%, C: 0.0005 to 0.010%, Si: 2.0 to 7.0%, Mn: 0.050 to 1.0%, S: 0 to 0.0350%, Se: 0 to 0.0350%, total content of S+Se: 0.0030 to 0.0350%, Al: 0.010 to 0.0650%, N: 0.010 to 0.040%, 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 decarbonitrided steel sheet 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%, and the balance being Fe and impurities, wherein, among precipitates collected from the decarbonitrided steel sheet by an extraction replica method, precipitates having a major axis D of 50 to 150 nm, when the Al content is R(Al) in atomic %, and the Nb+V+Mo+Ta+W total content is R(Nb) in atomic %, A decarbonitrided steel sheet for grain-oriented electrical steel sheet, characterized in that the number of precipitates having a value of R(Nb)÷R(Al) of 0.1 or more is 35% or more and 95% or less.