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

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

Technical Field

[0001] The present invention relates to a decarburized and nitrided steel sheet for a grain-oriented electrical steel sheet. This application claims priority based on Japanese Patent Application No. 2024-034214 filed in Japan on March 6, 2024, and incorporates its content herein.

Background Art

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

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

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

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

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

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

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

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

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

[0011] One aspect of the present invention has been made in view of the above-mentioned problems. One aspect of the present invention aims to provide a decarburized nitrided steel sheet for grain-oriented electrical steel sheets that can increase the magnetic flux density, given the current demand for increasing the magnetic flux density of grain-oriented electrical steel sheets. [Means for solving the problem]

[0012] The gist of this invention is as follows:

[0013] (1) A decarburized nitrided steel sheet for grain-oriented electrical steel sheets according to one aspect of the present invention is In mass%, C: 0.0005~0.010%, Si:2.0~ 4.50% , Mn:0.050~ 0.50% , S: 0~0.0350%, Se: 0~0.0350%, S+Se total content: 0.0030~0.0350%, 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%, 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%, containing, with the balance consisting of Fe and impurities, Among the precipitates collected by the extraction replica method from the decarburized and nitrided steel sheet, for precipitates with a major axis D of 50 to 150 nm, When the Al content is denoted as R(Al) in atomic %, and the total content of Nb + V + Mo + Ta + W is denoted as R(Nb) in atomic %, the precipitates with a value of R(Nb)÷R(Al) of 0.1 or more are 35% or more and 95% or less in terms of the number ratio.

Advantages of the Invention

[0014] According to the above aspect of the present invention, a decarburized and nitrided steel sheet for a grain-oriented electromagnetic steel sheet capable of increasing the magnetic flux density is provided.

Brief Description of the Drawings

[0015] [Figure 1] It is a flowchart of a method for manufacturing a decarburized and nitrided steel sheet for a grain-oriented electromagnetic steel sheet according to an embodiment of the present invention.

Embodiments for Carrying Out the 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 without departing from the spirit of the invention. Furthermore, the numerical limits described below include both lower and upper limits. Numerical values ​​indicated as "greater than" or "less than" are not included in the numerical range. Also, "%" in relation to chemical composition means "mass%" unless otherwise specified.

[0017] Furthermore, in the following description, regarding the precipitates in the steel that are characteristic of this embodiment, the term "inhibitor" will be used mainly in descriptions related to the secondary recrystallization mechanism, and the term "precipitate" will be used mainly in descriptions related to the compound phase observed in the microstructure. However, in this embodiment, the terms "inhibitor" and "precipitate" are not used with the intention of strictly distinguishing between them.

[0018] As mentioned above, there is currently a need to increase the magnetic flux density of grain-oriented electrical steel sheets.

[0019] Therefore, the inventors focused on the technical concept of "expanding the secondary recrystallization temperature range" disclosed in the above-mentioned Patent Documents 9 to 11. These Patent Documents 9 to 11 primarily utilize this "expanding the secondary recrystallization temperature range" technique for the formation of subgrain boundaries within secondary recrystallized grains and the associated noise reduction. The inventors considered that further improvement in magnetic flux density would be possible if this "expanding the secondary recrystallization temperature range" technique were optimized to enhance the selectivity of crystal orientation.

[0020] Specifically, we investigated how to effectively expand the secondary recrystallization temperature range by appropriately controlling the inhibitor morphology in the steel, and how to preferentially grow grains with a desirable crystal orientation during the secondary recrystallization process within the expanded temperature range. As a result, we found that by optimally controlling the morphology of precipitates contained in the decarburized and nitrided steel sheet during the manufacturing process of grain-oriented electrical steel sheets, the secondary recrystallization temperature range can be expanded during finish annealing, Goss-oriented grains can grow preferentially, and the magnetic flux density of the final grain-oriented electrical steel sheet can be increased beyond that of conventional techniques.

[0021] Generally, inhibitors are fine precipitates in steel with a diameter of approximately 1000 nm or less. These inhibitors have a grain boundary pinning effect, suppressing grain growth. When the temperature exceeds approximately 1000°C during finish annealing, these inhibitors dissolve into the αFe phase, the matrix phase, and the grain boundary pinning effect weakens. As a result, abnormal grain growth occurs, a phenomenon known as secondary recrystallization.

[0022] For example, sulfides and selenides are used as Mn-based precipitates, and nitrides are used as Al-based precipitates, serving as major inhibitors. Mn-based inhibitors and Al-based inhibitors (Al-based inhibitors controlled before cold rolling) are mainly used in manufacturing methods where the slab heating temperature before hot rolling is 1300°C or higher (hereinafter sometimes 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 where the slab heating temperature before hot rolling is 1280°C or lower, and nitriding is performed between cold rolling and finish annealing (hereinafter sometimes 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., are sometimes used as auxiliary inhibitors.

[0023] Traditionally, in order to form inhibitors with appropriate functions within steel, the steel composition and manufacturing conditions have been controlled during the production of grain-oriented electrical steel sheets. In particular, the steel composition, hot rolling conditions, and decarburization annealing conditions have been recognized as manufacturing conditions that greatly influence the morphology of the inhibitors, and these conditions have been precisely controlled.

[0024] In this embodiment, by appropriately controlling the precipitates (inhibitors) contained in the decarburized nitrided steel sheet, the secondary recrystallization temperature range is expanded during the subsequent finish annealing process, and the selectivity of the crystal orientation as secondary recrystallization progresses is enhanced. Specifically, the above effects are obtained by appropriately controlling the type and number ratio of fine precipitates with a major axis D of 50 to 150 nm, among the relatively fine and relatively coarse inhibitors contained in the decarburized nitrided steel sheet.

[0025] The inventors of this invention estimate that the above effect is obtained through the following mechanism.

[0026] First, let's consider the reason why the temperature range for secondary recrystallization is expanded. As mentioned above, secondary recrystallization occurs because the pinning effect of the grain boundaries weakens as the inhibitor dissolves. During finish annealing, it is thought that fine inhibitors dissolve and disappear earlier than coarse inhibitors. Therefore, when fine and coarse inhibitors coexist, it is thought that the fine inhibitors preferentially disappear early in the heating process of finish annealing. In particular, when at least one selected from Nb, V, Mo, Ta, and W is added, it becomes possible to favorably control the decomposition of fine inhibitors at lower temperatures than conventional inhibitors such as AlN.

[0027] As fine inhibitors dissolve, coarse inhibitors may grow, similar to Ostwald growth. However, the increase in pinning force due to the growth of coarse inhibitors is thought to have a smaller impact than the decrease in pinning force due to the disappearance of fine inhibitors. Therefore, if fine and coarse inhibitors coexist, and the fine inhibitors dissolve earlier than the coarse inhibitors, secondary recrystallization is thought to begin at a relatively low temperature during the heating process of the finish annealing.

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

[0029] In other words, when fine inhibitors and coarse inhibitors coexist, secondary recrystallization starts at a relatively low temperature during the heating process of the finish annealing and continues up to a relatively high temperature, thus expanding the temperature range in which secondary recrystallization can proceed.

[0030] Next, let's consider the reason why the selectivity of crystal orientation improves. As mentioned above, secondary recrystallization proceeds with the preferential growth of Goss-oriented grains. This preferential growth of Goss-oriented grains is thought to be due to the special characteristics of the grain boundaries (grain boundary properties) and the special characteristics of the grain size (size advantage) of Goss-oriented grains.

[0031] However, the driving force for the preferential growth of Goss-oriented grains is not very strong. Therefore, if grain boundary movement occurs relatively easily even in grains other than Goss-oriented grains during secondary recrystallization, for example, if the inhibitor decomposition is rapid and the grain growth pinning effect is weak, resulting in a relatively high grain growth rate (a relatively high driving force for grain growth), then grains other than Goss-oriented grains will also grow easily. In this case, the preferential growth of Goss-oriented grains is inhibited.

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

[0033] When it is difficult to industrially extend the finish annealing time (when it is difficult to change the heating rate if the maximum temperature reached is the same), even if the heating rate is constant, if the temperature range in which secondary recrystallization proceeds can be expanded by slowing down the decomposition rate of the inhibitor, it is possible to extend the time during which secondary recrystallization proceeds, make the growth rate of secondary recrystallized grains relatively fast, and improve the preferential growth of secondary recrystallized grains without reducing productivity. For example, considering that the entire surface of the grain-oriented electrical steel sheet will eventually be covered with secondary recrystallized grains, it can be understood that extending the time during which secondary recrystallization proceeds leads to an acceleration of the growth rate of secondary recrystallized grains relative to the decomposition rate of the inhibitor.

[0034] In other words, when fine and coarse inhibitors coexist, the temperature range in which the inhibitor decomposition rate is slow expands, and the temperature range in which secondary recrystallization progresses, where the growth rate of secondary recrystallized grains is relatively fast relative to the inhibitor decomposition rate, expands. As a result, it is thought that Goss-oriented grains are more likely to grow preferentially. Consequently, it is thought that it becomes possible to ultimately increase the magnetic flux density.

[0035] In this embodiment, the type and proportion of fine precipitates with a major axis D of 50 to 150 nm, among the relatively fine and relatively coarse inhibitors contained in the decarburized steel sheet after the decarburized nitriding process, are appropriately controlled by comprehensively and inseparably controlling the steel composition, casting conditions, hot rolling conditions, hot roll annealing conditions, cold rolling conditions, and decarburized nitriding conditions. Furthermore, in this embodiment, the morphology of the precipitates is preferably controlled by adding auxiliary inhibitor-forming elements.

[0036] In this embodiment, the morphology of the precipitate described above is defined based on the decarburized and nitrided steel sheet.

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

[0038] The decarburized and nitrided steel sheet according to this embodiment is, by mass%, C: 0.0005~0.010%, Si: 2.0~7.0%, Mn: 0.050~1.0%, S: 0~0.0350%, Se: 0~0.0350%, S+Se total content: 0.0030~0.0350%, Al: 0.010~0.0650%, N: 0.010~0.040%, Nb: 0~0.030%, V: 0~0.030%, Mo: 0~0.030%, Ta: 0~0.030%, W: 0~0.030%, Nb+V+Mo+Ta+W total content: 0.0030~0.030%, Cu: 0~0.40%, Bi: 0~0.010%, B: 0~0.080%, P: 0-0.50% Ti: 0~0.0150%, Sn: 0~0.10%, Sb: 0~0.10%, Cr: 0~0.30%, Ni: 0~1.0%, It contains and has a chemical composition consisting of Fe and impurities as the remainder, Among the precipitates collected from decarburized and nitrided steel sheets by the extraction replica method, those with a major axis D of 50-150 nm, The Al content is expressed in atomic percent as R(Al). When the total content of Nb+V+Mo+Ta+W is expressed in atomic percent as R(Nb), Precipitates with a R(Nb)÷R(Al) value of 0.1 or greater account for 35% to 95% of the total number of precipitates.

[0039] 1.Chemical composition The chemical composition of the decarburized and nitrided steel sheet according to this embodiment may be the same as the general chemical composition used for grain-oriented electrical steel sheets.

[0040] It should be noted that the chemical composition of decarburized and nitrided steel sheets, which are intermediate products, is rarely described in publicly available literature on grain-oriented electrical steel sheets.

[0041] The decarburized and nitrided steel sheet according to this embodiment has a chemical composition that includes basic elements, optional elements as needed, and the remainder being Fe and impurities.

[0042] The decarburized and nitrided steel sheet according to this embodiment contains, by mass fraction, the following basic elements (major alloying elements): C: 0.0005~0.010%, Si: 2.0~7.0%, Mn: 0.050~1.0%, total S+Se content: 0.0030~0.0350%, Al: 0.010~0.0650%, N: 0.0040~0.040%, and total Nb+V+Mo+Ta+W content: 0.0030~0.030%.

[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 forms carbides and carbonitrides with elements such as Nb, making it effective in controlling the primary recrystallization structure. Therefore, the C content of the slab is preferably 0.0010 to 0.10%. However, excessive C content in the final product adversely affects the magnetic properties. Therefore, the C content of decarburized steel sheets should be 0.0005 to 0.010%. The preferred upper limit for C content is 0.009% and 0.008%. Note that C is purified in the finish annealing process described later, and after the finish annealing process, it becomes 0.0050% or less. When C is included, considering the productivity in industrial production, the C content may be greater than 0%, 0.0005%, or 0.0010% or more.

[0044] Si: 2.0~7.0% Silicon (Si) increases the electrical resistance of grain-oriented electrical steel sheets, thereby reducing iron loss. If the Si content is less than 2.0%, austenite transformation occurs during finish annealing, damaging the crystal orientation of the grain-oriented electrical steel sheet. On the other hand, if the Si content exceeds 7.0%, cold workability decreases, making it more prone to cracking during cold rolling. Therefore, the Si content of decarburized nitrided steel sheets should be between 2.0% and 7.0%. The preferred lower limit of the Si content is 2.50%, and more preferably 3.0%. The preferred upper limit of the Si content is 4.50%, and more preferably 4.0%.

[0045] Mn: 0.050~1.0% Manganese (Mn) combines with S and Se to precipitate as MnS and MnSe, functioning as inhibitors. To favorably control the morphology of these inhibitors (precipitates), the Mn content of the decarburized and nitrided steel sheet should be between 0.050% and 1.0%. If the Mn content is below 0.050%, the amount of MnS and MnSe that function as inhibitors will be insufficient, thus inhibiting the proper progression of secondary recrystallization. Conversely, if the Mn content exceeds 1.0%, the amount of MnS and MnSe that function as inhibitors will be excessive, also inhibiting the proper progression of secondary recrystallization. In this embodiment, some of the inhibitory functions may be carried out by carbides, nitrides, or carbonitrides of Nb group elements. In this case, the amount of MnS and MnSe that precipitate as inhibitors may be controlled to be lower. Therefore, the upper limit of the Mn content is preferably 0.50%, and more preferably 0.20%.

[0046] S: 0~0.0350% Se: 0~0.0350% S+Se total content: 0.0030~0.0350% Sulfur (S) and selenium (Se) combine with Mn to precipitate as MnS and MnSe, respectively, and function as inhibitors. To favorably control the morphology of these inhibitors (precipitates), the S content of the decarburized and nitrided steel sheet should be 0 to 0.0350%, the Se content should be 0 to 0.0350%, and the total S+Se content should be 0.0030 to 0.0350%. A total S and Se content of 0.0030 to 0.0350% is preferable because it stabilizes secondary recrystallization. In this embodiment, some of the inhibitor functions may be carried out by carbides, nitrides, or carbonitrides of Nb group elements. In this case, the amount of MnS and MnSe precipitated as inhibitors may be controlled to be smaller. Therefore, the upper limit of the total S and Se content is preferably 0.0250%, and more preferably 0.010%. Furthermore, if S and Se remain in the steel after finish annealing, they can form compounds that degrade iron loss. Therefore, it is preferable to remove S and Se from the steel through purification during finish annealing to reduce their content.

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

[0048] Al: 0.010~0.0650% Aluminum (Al) combines with nitrogen (N) to precipitate as AlN or (Al,Si)N, functioning as inhibitors. To favorably control the morphology of these inhibitors (precipitates), the Al content of the decarburized nitrided steel sheet should be between 0.010% and 0.0650%. If the Al content is 0.010% or higher, AlN and (Al,Si)N precipitate in favorable forms during the nitriding treatment in the low-temperature slab heating process, and secondary recrystallization is particularly stable in the high-temperature range. If the Al content is below 0.010%, the amount of AlN and (Al,Si)N that function as inhibitors precipitated is insufficient, hindering the proper progress of secondary recrystallization. If the Al content exceeds 0.0650%, the amount of AlN and (Al,Si)N that function as inhibitors precipitates is excessive, also hindering the proper progress of secondary recrystallization. The lower limit of the Al content is preferably 0.020%, and more preferably 0.0250%. From the viewpoint of the stability of secondary recrystallization, the upper limit of the Al content is preferably 0.040%, and more preferably 0.030%.

[0049] N: 0.010~0.040% Nitrogen (N) combines with Al to precipitate as AlN or (Al,Si)N, functioning as an inhibitor. The N content of the decarburized and nitrided steel sheet should be between 0.010% and 0.040%. In this embodiment, the nitrogen content of the steel sheet is increased by nitriding treatment in the decarburized and nitrided step. If the N content exceeds 0.040%, blistering, a type of defect, is more likely to occur in the steel sheet. The upper limit of the N content is preferably 0.035%, and more preferably 0.0250%. The N is purified in the finish annealing step, and after the finish annealing step, it becomes 0.0050% or less.

[0050] Nb+V+Mo+Ta+W total content: 0.0030~0.030% Nb: 0~0.030% V: 0~0.030% Mo: 0~0.030% Ta: 0~0.030% W: 0~0.030% Niobium (Nb), vanadium (V), molybdenum (Mo), tantalum (Ta), and tungsten (W) precipitate as auxiliary inhibitors such as carbides, nitrides, and carbonitrides, and function favorably as inhibitors. Specifically, they favorably extend the temperature range over which secondary recrystallization can proceed. Therefore, the Nb content is 0-0.030%, the V content is 0-0.030%, the Mo content is 0-0.030%, the Ta content is 0-0.030%, the W content is 0-0.030%, and the total Nb+V+Mo+Ta+W content is 0.0030-0.030%. The lower limit of the Nb, V, Mo, Ta, and / or W content is preferably 0.0040%, and more preferably 0.0050%. Furthermore, the upper limit of the content of Nb, V, Mo, Ta, and / or W is preferably 0.020%, and 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 decarburized and nitrided steel sheet according to this embodiment contains, as Nb group elements, one or more elements selected from the Nb group elements consisting of Nb, V, Mo, Ta, and W, in a total mass of 0.0030 to 0.030%.

[0053] When Nb group element precipitates are used as inhibitors, if the total content of Nb group elements in the decarburized nitrided steel sheet is 0.030% or less (preferably 0.0030% to 0.030%), the morphology of the Nb group element precipitates is preferably controlled, and the secondary recrystallization temperature range is preferably expanded. As a result, Goss-oriented grains grow preferably, and the magnetic flux density of the resulting grain-oriented electrical steel sheet is preferably increased.

[0054] The reason why precipitates of Nb group elements function favorably as inhibitors is not clear, but it is thought to be as follows: Carbides, nitrides, or carbonitrides of Nb group elements are thought to precipitate non-equilibrium during the cooling process from high temperatures and act as precipitation nuclei for MnS and AlN that precipitate thereafter. Therefore, compared to cases where Nb group elements are not present, when Nb group elements are present, there are many more precipitation sites for MnS and AlN, and as a result, MnS and AlN are more likely to form as fine precipitates. In the decarburized and nitrided steel sheet according to this embodiment, the secondary recrystallization temperature range is expanded by allowing fine and coarse inhibitors to coexist, and precipitates of Nb group elements are thought to be particularly effective in expanding the secondary recrystallization temperature range to the lower temperature side.

[0055] The total content of Nb group elements is preferably 0.0040% or more, and more preferably 0.0050% or more. Furthermore, the total content of Nb group elements is preferably 0.020% or less, and more preferably 0.010% or less. If the total content of Nb group elements falls below 0.0030%, there will be insufficient precipitates of Nb group elements that act as precipitation nuclei, making it difficult for MnS and AlN to become finely ground. On the other hand, if the total content of Nb group elements exceeds 0.030%, the precipitation temperature range of the Nb group element precipitates will be high, and the precipitates of Nb group elements will tend to be coarse and low in density. In addition, the difference between the precipitation temperature range of Nb group element precipitates and the precipitation temperature range of MnS and AlN will be large, making it difficult for the Nb group element precipitates to effectively act as precipitation nuclei for the refinement of MnS and AlN.

[0056] Here, "total content of Nb group elements is 0.0030 to 0.030%" means that the decarburized and nitrided steel sheet contains at least one element selected from the group consisting of Nb, V, Mo, Ta, and W as part of its chemical composition, and its content may be 0.0030 to 0.030%. Alternatively, it means that the decarburized and nitrided steel sheet contains at least two or more elements selected from the group consisting of Nb, V, Mo, Ta, and W, and its total content may be 0.0030 to 0.030%.

[0057] The decarburized and nitrided steel sheet according to this embodiment may contain impurities in its chemical composition. "Impurities" refer to elements that are introduced from raw materials such as ore and scrap, or from the manufacturing environment, during the industrial production of steel. The upper limit of the total impurity content may be, for example, 5%.

[0058] Furthermore, the decarburized and nitrided steel sheet according to this embodiment may contain optional elements in addition to the basic elements and impurities described above. For example, instead of a portion of the remaining Fe described above, optional elements such as Cu, Bi, B, P, Ti, Sn, Sb, Cr, and Ni may be included. These optional elements may be included according to their purpose. Therefore, there is no need to limit the lower limit of these optional elements, and the lower limit may be 0%. Moreover, even if these optional elements are included as impurities, the above effects will not be impaired.

[0059] Cu: 0~0.40% Bi: 0~0.010% B: 0~0.080% P: 0-0.50% Ti: 0~0.0150% Sn: 0~0.10% Sb: 0~0.10% Cr: 0~0.30% Ni: 0~1.0% Copper (Cu), bismuth (Bi), boron (B), phosphorus (P), titanium (Ti), tin (Sn), antimony (Sb), chromium (Cr), and nickel (Ni) may be included according to known purposes. There is no need to set a lower limit for the content of these selected elements, and the lower limit may be 0%.

[0060] In grain-oriented electrical steel sheets, relatively large changes in chemical composition (reduction in content) occur through decarburization annealing and purification annealing during secondary recrystallization. For some elements, purification annealing can reduce the content to levels undetectable by general analytical methods (less than 1 ppm). However, the above chemical composition refers to that of decarburized and nitrided steel sheets.

[0061] The chemical composition of the decarburized and nitrided steel sheet according to this embodiment can be measured using general analytical methods for steel. For example, the chemical composition of the decarburized and nitrided steel sheet can be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). Specifically, the chemical composition can be determined by measuring a 35 mm square test piece taken from the decarburized and nitrided steel sheet using ICP-AES under conditions based on a pre-established calibration curve. C and S can be measured using the combustion-infrared absorption method, and N can be measured using the inert gas fusion-thermal conductivity method.

[0062] 2. Precipitate Next, we will describe the precipitates contained in the decarburized nitrided steel sheet according to this embodiment.

[0063] The precipitates (inhibitors) contained in the decarburized and nitrided steel sheet according to this embodiment may be precipitates formed from elements contained in the decarburized and nitrided steel sheet. For example, Mn-based precipitates (Mn-containing precipitates) may be sulfides or selenides, Al-based precipitates (Al-containing precipitates) may be nitrides, and Nb-group element-containing precipitates may be carbides, nitrides, or carbonitrides. In addition to these inhibitors, compounds of arbitrary elements such as Bi and B, or composite compounds with the above elements may also be included.

[0064] The decarburized and nitrided steel sheet according to this embodiment contains relatively fine precipitates and relatively coarse precipitates, with a major axis D of 50 to 1000 nm. Among these precipitates, the precipitates to be controlled in this embodiment are those with a major axis D of 50 to 150 nm. The "major axis" refers to the longest line segment among the line segments connecting non-adjacent vertices of the cross-sectional contour of the precipitate on the observation surface.

[0065] Precipitates with a major axis D smaller than 50 nm in decarburized and nitrided steel sheets currently have little effect in expanding the secondary recrystallization temperature range. The reason for this is not clear, but it is thought that precipitates with a major axis D smaller than 50 nm in the decarburized and nitrided steel sheet change or disappear in subsequent processes and are less likely to function as inhibitors during finish annealing. Therefore, in the decarburized and nitrided steel sheet according to this embodiment, precipitates with a major axis D of 50 nm or more are controlled. Furthermore, by considering processes after the decarburized and nitrided process, it is expected that precipitates with a major axis D smaller than 50 nm will be able to function as inhibitors in the future.

[0066] Furthermore, precipitates with excessively large major axis D may adversely affect the growth of secondary recrystallized grains in the final stage of secondary recrystallization. Also, the formation of precipitates with excessively large major axis D may reduce the number of precipitates (number density) contained in the decarburized nitrided steel sheet. In addition, precipitates with excessively large major axis D are less likely to function as inhibitors. For this reason, it is preferable that the major axis D of the precipitates be 1000 nm or less on average. In the decarburized nitrided steel sheet according to this embodiment, precipitates with a major axis D of 50 to 150 nm are controlled to be particularly effective in expanding the secondary recrystallization temperature range to the lower temperature side.

[0067] In the decarburized nitrided steel sheet according to this embodiment, among the precipitates collected from the decarburized nitrided steel sheet by the extraction replica method, precipitates with a major axis D of 50 to 150 nm are selected. The Al content is expressed in atomic percent as R(Al). When the total content of Nb+V+Mo+Ta+W (content of Nb group elements) is expressed as R(Nb) in atomic percent, Precipitates with a R(Nb)÷R(Al) value of 0.1 or greater account for 35% to 95% of the total number of precipitates.

[0068] When steel contains Al and Nb group elements as part of its composition, and Al-containing precipitates and Nb group element-containing precipitates coexist in the steel, precipitates with a major axis D of 50 to 150 nm are often composite precipitates of Al-containing precipitates and Nb group element-containing precipitates. Furthermore, precipitates with a major axis D of 50 to 150 nm have the effect of extending the secondary recrystallization temperature range to the lower temperature side, and precipitates with a high Nb group element content within the composite precipitate exhibit this effect particularly favorably. Specifically, precipitates with an R(Nb)÷R(Al) value of 0.1 or higher favorably exhibit the above effect.

[0069] Therefore, among precipitates with a major axis D of 50 to 150 nm, precipitates with a value of R(Nb)÷R(Al) of 0.1 or more should account for 35% or more in number proportion. To obtain the above effect favorably, the above number proportion is preferably 38% or more, and more preferably 40% or more. On the other hand, the upper limit of the above number proportion can be set to 95% in order to obtain a favorable magnetic flux density.

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

[0071] For example, precipitates can be collected from decarburized and nitrided steel sheets using the extraction replica method. The extraction replica method involves polishing and etching the steel sheet to expose the precipitates on the surface, depositing a replica film onto the steel sheet surface to incorporate the precipitates into the replica film, and then peeling the replica film off the steel sheet to collect only the precipitates. These precipitates can then be observed and analyzed for composition using TEM-EDS (Transmission Electron Microscope-Energy Dispersive X-ray Spectroscopy).

[0072] Specifically, the major axis D of precipitates collected from decarburized and nitrided steel sheets by extraction replica method can be confirmed by TEM, and the Al content and Nb group element content of precipitates with a major axis D of 50-150 nm can be analyzed by EDS.

[0073] For example, for a TEM observation sample prepared from a decarburized and nitrided steel sheet using the replica method, select 30 or more precipitates with a major axis D of 50-150 nm from among the precipitates in any carbon mesh of the observation sample. Then, select precipitates in the same way at two other locations in the carbon mesh of the observation sample, so that a total of 100 precipitates are selected from the three locations. Perform compositional analysis using EDS so that the electron beam is irradiated to the central part of the precipitate (the intersection of the major and minor axes). The Al content obtained from the above analysis can be denoted as R(Al) in atomic percent, and the Nb group element content can be denoted as R(Nb) in atomic percent.

[0074] 3. Plate thickness The thickness of the decarburized and nitrided steel sheet according to this embodiment is not particularly limited. The decarburized and nitrided steel sheet according to this embodiment is subjected to subsequent processes, namely the annealing separating agent coating process and the finish annealing process, to finally be finished into a grain-oriented electrical steel sheet. Therefore, considering the general manufacturing conditions for grain-oriented electrical steel sheets, the thickness of the decarburized and nitrided steel sheet can be 0.10 to 0.50 mm. However, it is not limited to this thickness, and any known thickness or a thickness that is used in practical applications may be adopted.

[0075] 4. Manufacturing method Next, a method for manufacturing a decarburized nitrided steel sheet for grain-oriented electrical steel sheets according to one embodiment of the present invention will be described. Note that the method for manufacturing the decarburized nitrided 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 decarburized nitrided steel sheet according to this embodiment.

[0076] Figure 1 is a flowchart illustrating the manufacturing process of a decarburized and nitrided steel sheet according to this embodiment. Figure 1 also shows the manufacturing process of a grain-oriented electrical steel sheet using this decarburized and nitrided steel sheet. As shown in Figure 1, the manufacturing method of the decarburized and nitrided steel sheet according to this embodiment comprises a casting process, a hot rolling process, a hot-rolled sheet annealing process, a cold rolling process, and a decarburized and nitrided process. The conditions controlled in these processes will be described in detail later.

[0077] Furthermore, the processes following the annealing separation agent application process shown in Figure 1, namely the annealing separation agent application process and the finish annealing process, are manufacturing processes for grain-oriented electrical steel sheets (finish annealed steel sheets). The effect of the decarburized and nitrided steel sheet according to this embodiment can be confirmed in the final product, the grain-oriented electrical steel sheet, so the conditions to be controlled in these processes will also be described later.

[0078] The method for manufacturing a decarburized and nitrided steel sheet according to this embodiment comprises a casting step, a hot rolling step, a hot rolled sheet annealing step, a cold rolling step, and a decarburized and nitrided step. In the casting process, In mass%, C: 0.0010~0.10%, Si: 2.0~7.0%, Mn: 0.050~1.0%, S: 0~0.0350%, Se: 0~0.0350%, S+Se total content: 0.0030~0.0350%, Al: 0.010~0.0650%, N: 0.0040~0.0120%, Nb: 0~0.030%, V: 0~0.030%, Mo: 0~0.030%, Ta: 0~0.030%, W: 0~0.030%, Nb+V+Mo+Ta+W total content: 0.0030~0.030%, Cu: 0~0.40%, Bi: 0~0.010%, B: 0~0.080%, P: 0-0.50% Ti: 0~0.0150%, Sn: 0~0.10%, Sb: 0~0.10%, Cr: 0~0.30%, Ni: 0~1.0%, Molten steel having a chemical composition containing and the remainder consisting of Fe and impurities is cast and formed into a slab. In the hot rolling process, the slab after the casting process is heated, roughly rolled, and then finish-rolled to form a hot-rolled steel sheet. In the hot-rolled sheet annealing process, the hot-rolled steel sheet after the hot-rolling process is annealed 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 to form a cold-rolled steel sheet with a cumulative reduction ratio of 80-95%. In the decarburization and nitriding process, the cold-rolled steel sheet after the cold-rolling process is subjected to decarburization annealing by holding it in a humid atmosphere at 700-900°C for 1-3 minutes, and then subjected to a nitriding treatment to increase the nitrogen content of the steel sheet by 40-300 ppm to obtain a decarburization and nitrided steel sheet.

[0079] Furthermore, in the method for manufacturing decarburized nitrided steel sheets according to this embodiment, In the hot rolling process, During the slab heating process before rough rolling, the soaking temperature of the slab is set to over 1030°C but less than 1180°C to preferably dissolve a portion of the precipitates contained in the slab (for example, dissolving 12 to 85 volume percent of precipitates based on the precipitates contained in the slab after the casting process), and in order to homogenize this dissolved state within the slab, the soaking time of the slab is set to over 70 minutes, and, During rough rolling, the rolling temperature is set to 940-1070°C, and the reduction ratio is set to 82-95%.

[0080] To control the size and distribution of precipitates in decarburized and nitrided steel sheets, it is necessary to control the steel composition, casting conditions, hot rolling conditions, hot-rolled sheet annealing conditions, cold rolling conditions, and decarburized and nitrided conditions, respectively. In particular, it is important to control the steel composition, slab heating conditions (solution state of precipitates before rough rolling), rough rolling temperature, and rough rolling reduction ratio, and then to control the hot-rolled sheet annealing conditions, cold rolling conditions, and decarburized and nitrided conditions. Furthermore, to control the "solution state of precipitates before rough rolling" mentioned above, it is important to control the steel composition and slab heating conditions, respectively.

[0081] Furthermore, the slab heating described above does not require temporarily increasing the heating temperature during the heating process; instead, it is sufficient to perform soaking at a predetermined temperature for a predetermined time. In this case, the soaking temperature of the slab refers to the surface temperature of the slab, and the soaking time refers to the holding time after the surface temperature of the slab reaches the soaking temperature. For example, although it is also affected by the steel composition and heating rate, if the surface temperature of the slab reaches the soaking temperature during the heating process, the solution state of precipitates on the surface of the slab is favorably controlled. Also, if the surface temperature of the slab is held for the soaking time after reaching the soaking temperature, the solution state of precipitates is favorably controlled all the way to the center of the slab.

[0082] The important manufacturing conditions for the decarburized and nitrided steel sheet manufacturing method according to this embodiment are described below. Other manufacturing conditions can be those of conventional known grain-oriented electrical steel sheets.

[0083] (Casting process) In the casting process, a slab is prepared. Since the chemical composition other than C (carbon) and N (nitrogen) hardly changes from the slab to the decarburized nitriding process, the chemical composition of the slab should be the same as the target decarburized nitrided steel sheet (the chemical composition of the decarburized nitrided steel sheet described above), excluding C (carbon) and N (nitrogen). The C content of the slab should be 0.0010 to 0.10%, and the N content of the slab should be 0.0040 to 0.0120%.

[0084] Furthermore, the chemical composition of the slab affects the "solution state of precipitates before rough rolling" mentioned above. As will be explained in more detail later, the chemical composition of the slab needs to be controlled in combination with other manufacturing conditions that affect the "solution state of precipitates before rough rolling," in addition to satisfying the chemical composition described above.

[0085] An example of a slab manufacturing method is as follows: Molten steel is produced (melted). A slab is manufactured using this molten steel. For example, the slab may be manufactured by continuous casting. Alternatively, an ingot may be manufactured using molten steel, and the ingot may be rolled into a slab. The thickness of the slab is, for example, 150 to 350 mm. Preferably, the thickness of the slab is 220 to 280 mm. A so-called thin slab with a thickness of 10 to 70 mm may be used as the slab.

[0086] (Hot rolling process) The hot rolling process involves heating a slab to a predetermined temperature and performing hot rolling (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, rough rolling is performed, and then finish rolling is performed to produce a hot-rolled steel sheet with a predetermined thickness of 1.8 to 3.5 mm. After the finish rolling is completed, the hot-rolled steel sheet is wound up at a predetermined temperature.

[0088] During the hot rolling process, when heating the slab after the casting process, the following conditions must be met.

[0089] When heating the slab before rough rolling, the soaking temperature of the slab should be set to over 1030°C but less than 1180°C to preferably dissolve some of the precipitates contained in the slab (for example, dissolving 12 to 85 volume percent of precipitates based on the precipitates contained in the slab at room temperature after the casting process), and in order to homogenize this dissolved state within the slab, the slab should be heated for more than 70 minutes.

[0090] In the pre-rough rolling stage, it is desirable to dissolve some of the precipitates contained in the slab in order to ultimately achieve a desirable balance between the amount of relatively coarse precipitates that remain dissolved after the slab heating stage (undissolved precipitates) and the amount of relatively fine precipitates that do not precipitate during the slab heating stage but precipitate after hot rolling (re-precipitation precipitates).

[0091] The "solution state of precipitates before rough rolling" mentioned above refers to the "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 thickness direction of the sheet. If a slab in this non-equilibrium state is subjected to rough rolling, it becomes difficult to control the size and distribution of precipitates contained in the steel sheet after the decarburization and nitriding process.

[0092] For example, in order to bring the solution state of the precipitates closer to equilibrium, it is preferable to keep the value obtained by subtracting the slab center temperature from the slab surface temperature within the range of greater than -10°C and less than 50°C during slab heating and extraction. In particular, if the above temperature difference is less than -10°C, the steel plate surface becomes less expandable, and the occurrence of defects becomes significant. Also, if the above temperature difference is greater than 50°C, the solution state of the precipitates becomes non-uniform in the thickness direction of the plate, making it difficult to control the size of the precipitates.

[0093] Although different from the slab heating method in 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 keep the difference between the surface temperature at the time of maximum temperature attainment and the surface temperature at the time of slab extraction from the heating furnace to 80°C or less. In this case, it is preferable to hold the slab in the lower temperature region of the slab heating furnace for at least 20 minutes after it has cooled from the maximum temperature attainment, so that the difference between the surface temperature and the core 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 core temperature of the slab should be between 0 and 30°C.

[0094] In conventional techniques that perform slab heating at temperatures below 1280°C, known as low-temperature slab heating processes, there was no technical concept of dissolving only a specific proportion of precipitates contained in the slab, nor was there any knowledge that it was necessary to bring this dissolution of precipitates closer to an equilibrium state. In the method for manufacturing decarburized nitrided steel sheets according to this embodiment, the dissolution state of the precipitates is favorably controlled, and a slab in which the dissolution of precipitates is in an equilibrium state is subjected to rough rolling.

[0095] The "solution state of precipitates before rough rolling" described above is a characteristic that is influenced by both 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 of the above manufacturing conditions in a complex and inseparable manner, taking into account the influence of each manufacturing condition on the "solution state of precipitates." For example, a person skilled in the art can perform material control including precipitation behavior, so if they understand that each of the above conditions affects the "solution state," they can control the "solution state" by combining the above conditions.

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

[0097] In the hot rolling process, when heating the slab before rough rolling, the soaking temperature should be set to over 1030°C but less than 1180°C, and the soaking time should be set to over 70 minutes. In this case, it is preferable to dissolve a portion of the precipitates contained in the slab (for example, based on the precipitates contained in the slab at room temperature after the casting process, it is preferable to dissolve 12 to 85 volume percent of the precipitates).

[0098] When the Nb group element content is within the above range, even if the slab heating temperature is 1100°C or higher, it is possible to ultimately achieve the coexistence of fine and coarse inhibitors. For example, if the slab heating temperature is high and the solution treatment of AlN and MnS is promoted during the slab heating stage, these AlN and MnS particles tend to reprecipitate coarsely in subsequent processes. However, when the Nb group element content is within the above range, the Nb group element precipitates act as precipitation nuclei for MnS and AlN, reducing the size of the reprecipitated AlN and MnS. Furthermore, because the precipitation nose of Nb group element precipitates (carbonitrides) is at a lower temperature than the precipitation noses of AlN and MnS, the Nb group element precipitates themselves tend to precipitate as even finer precipitates than AlN and other elements.

[0099] Therefore, when the content of Nb group elements is within the above range, the upper limit temperature during slab soaking should be less than 1180°C. While higher soaking temperatures promote the solubilization of precipitates, when the content of Nb group elements is within the above range, it is easier to favorably solubilize a portion of the precipitates contained in the slab (for example, the upper limit of the precipitate solubilization rate should be 85% by volume). When these conditions are met, the effect of the Nb group element precipitates described above makes it easier to achieve the coexistence of fine and coarse inhibitors.

[0100] Similarly, when the Nb group element content is within the above range, the lower limit temperature for slab soaking should be above 1030°C. While lower soaking temperatures suppress the solution of precipitates, when the Nb group element content is within the above range, it is easier to favorably solubilize some of the precipitates contained in the slab (for example, the lower limit for the precipitate solution rate should be 12% by volume). When these conditions are met, it becomes possible to ultimately have both fine and coarse inhibitors coexist.

[0101] The mechanism by which the above effects are obtained is thought to be related to the fact that precipitates of Nb group elements (carbonitrides) precipitate more easily than MnS and AlN (in particular, MnS is difficult to precipitate without support such as dislocation proliferation due to rolling, and when it does precipitate, it becomes larger in size), and that precipitates of Nb group elements function as precipitation nuclei in the precipitation of MnS and AlN, suppressing the coarsening of re-precipitated AlN and MnS.

[0102] Furthermore, when the Nb group element content is within the above range, a soaking time of 70 minutes or less is too short, making it difficult to control the solution state of the precipitate to an equilibrium state. While the upper limit of the soaking time is not particularly limited, it may be set to 2 hours considering 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 amount of relatively coarse precipitates that remain precipitated during the slab heating stage (undissolved precipitates) and the amount of relatively fine precipitates that precipitate after hot rolling (re-precipitation precipitates).

[0104] The soaking temperature of the slab mentioned above represents the surface temperature of the slab, and the soaking time of the slab represents the holding time after the surface temperature of the slab reaches the soaking temperature. For example, although it is also affected by the steel composition and heating rate, if the surface temperature of the slab reaches the soaking temperature mentioned above during the heating of the slab, the solution state of precipitates on the surface of the slab will be favorably controlled. Furthermore, if the surface temperature of the slab is held for the soaking time mentioned above after reaching the soaking temperature, the solution state of precipitates will be favorably controlled all the way to the center of the slab.

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

[0106] In the hot rolling process, hot rolling is carried out following the slab heating described above. 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 decarburizing and nitriding process, it is important to control the "solution state of precipitates before rough rolling" described above, and then control each condition from hot rolling onward.

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

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

[0109] By setting the reduction ratio within the above range, processing-induced precipitation occurs, making it possible to precipitate fine and abundant precipitates. If the reduction ratio in rough rolling is smaller than the lower limit above, the introduction of dislocations due to the rolling process will decrease, and the number of precipitation sites that can undergo processing-induced precipitation will decrease, resulting in larger particle sizes for the precipitates. On the other hand, there is no particular upper limit to the reduction ratio in rough rolling, but it is reasonable to set it at 95% considering the performance of the rolling mill, etc.

[0110] The reduction ratio for rough rolling mentioned above refers to the cumulative reduction ratio during rough rolling. Specifically, the reduction ratio for rough rolling is defined as follows: Rough rolling reduction ratio (cumulative reduction ratio) (%) = (1 - "steel plate thickness after rough rolling" / "steel plate thickness before rough rolling") × 100

[0111] Furthermore, if the rolling temperature for rough rolling is higher than the upper limit mentioned above, processing-induced precipitation of precipitates such as MnS, AlN, and Nb group elements will occur at a higher temperature than or near the precipitation nose, resulting in a larger critical radius for precipitation of precipitates that re-precipitation during hot rolling. Consequently, the size difference between these precipitates and the relatively coarse precipitates (undissolved precipitates) that have precipitated since the slab heating stage becomes smaller. On the other hand, there is no particular lower limit to the rolling temperature for rough rolling, but since the slab hardens and its rollability decreases at lower temperatures, it is advisable to roll at, for example, 940°C or higher. The rough rolling temperature is defined as the average value of the start and end temperatures of rough rolling.

[0112] Furthermore, when the chemical composition preferably contains Nb group elements, during rough rolling, precipitates of Nb group elements (especially carbides and nitrides) precipitate in addition to MnS and AlN. These Nb group element precipitates act as precipitation nuclei for MnS and AlN that precipitate later, resulting in finer reprecipitation of MnS and AlN. Therefore, when the chemical composition preferably contains Nb group elements, the 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 rate should be controlled as described above.

[0113] When Nb group elements are suitably included, the reason why the conditions of the hot rolling process should be controlled as described above is as follows: When Nb group elements are included, MnS and AlN re-precipitation occurs more finely due to the precipitates of Nb group elements, so the particle size of the re-precipitationd precipitates becomes smaller compared to when Nb group elements are not included. Therefore, it becomes easier to favorably balance the amount of precipitates between the relatively coarse precipitates that remain precipitated during the slab heating stage (undissolved precipitates) and the precipitates that do not precipitate during the slab heating stage but precipitate relatively finely after hot rolling (re-precipitationd precipitates). For this reason, it is considered that the conditions of the hot rolling process should be controlled as described above.

[0114] For example, if the solution state of precipitates is not favorably controlled when Nb group elements are present (for example, if the "solution rate of precipitates before rough rolling" is lower than 12 volume%), the precipitates will not be sufficiently dissolved at the time of slab heating, similar to the case where Nb group elements are not present. As a result, the fine precipitates that re-precipitate during hot rolling will be reduced. Therefore, the secondary recrystallization temperature range cannot be sufficiently expanded during finish annealing. Also, if the solution state of precipitates is not favorably controlled when Nb group elements are present (for example, if the "solution rate of precipitates before rough rolling" is higher than 85 volume%), most of the precipitates will be dissolved at the time of slab heating, similar to the case where Nb group elements are not present. As a result, the relatively coarse precipitates (undissolved precipitates) in the slab will be reduced. Therefore, the secondary recrystallization 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 Nb group elements are suitably included, compared to when Nb group elements are not suitably included, is thought to be as follows: When Nb group elements are included, precipitates of Nb group elements tend to precipitate finely in the steel, so the number of fine precipitates contained in the steel is greater even before rough rolling compared to when Nb group elements are not included. Therefore, when Nb group elements are included, there are more precipitation sites for precipitates, and processing-induced precipitation is more likely 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 Nb group elements are present, if the reduction ratio during rough rolling is less than 82%, the introduction of dislocations due to the rolling process will be reduced, similar to when Nb group elements are not present, resulting in fewer precipitation sites that can be precipitated by the process, and thus the particle size of the precipitate will increase. When Nb group elements are present, the upper limit of the reduction ratio during rough rolling is preferably 93%.

[0117] Furthermore, the reason why the rough rolling temperature should be controlled as described above when Nb group elements are suitably included, compared to when Nb group elements are not suitably included, is thought to be as follows: When Nb group elements are included, as mentioned above, the number of fine precipitates contained in the steel increases even before rough rolling, compared to when Nb group elements are not included. Therefore, when Nb group elements are included, there are more precipitation sites for precipitates, and the precipitates that re-precipitation during hot rolling tend to be finer. For this reason, it is thought that the rough rolling temperature should be controlled as described above.

[0118] When Nb group elements are present, if the rough rolling temperature is higher than 1070°C, all precipitates, including MnS, AlN, and Nb group elements, will undergo processing-induced precipitation at a temperature higher than the precipitation nose of the precipitates. This increases the critical precipitation radius of the precipitates that re-precipitation during hot rolling. As a result, the size difference between the relatively coarse precipitates (undissolved precipitates) that have precipitated since the slab heating stage becomes smaller. When Nb group elements are present, the upper limit of the rough rolling temperature is preferably 1065°C, and more preferably 1040°C.

[0119] Furthermore, although the reason why Nb group elements promote the fine precipitation of precipitates is not clear, it 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 temperature ranges where all MnS and AlN would 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 temperature ranges where Nb group elements precipitate. Therefore, when Nb group elements precipitate during rough rolling, these precipitates are thought to act as precipitation nuclei for MnS and AlN that precipitate later, causing fine precipitates of MnS and AlN. Specifically, compared to the case where Nb group element precipitates are not present, the presence of Nb group element precipitates many more precipitation sites for MnS and AlN, resulting in finer precipitates of MnS and AlN.

[0121] Furthermore, when precipitates of Nb group elements that acted as precipitation nuclei for MnS and AlN are covered with MnS or AlN, the further growth of those Nb group element precipitates is suppressed. In this case, the Nb group elements that would have been consumed in the growth of the precipitates are thought to precipitate as new, fine precipitates. These new, fine precipitates of Nb group elements are thought to act as new precipitation nuclei for MnS and AlN, contributing to the further fine precipitation of MnS and AlN. Thus, precipitates of Nb group elements are thought to synergistically contribute to the fine precipitation of MnS and AlN.

[0122] When the above-mentioned conditions in the hot rolling process are satisfied, and the conditions after the hot rolling process are also satisfied, precipitates are preferably controlled. As a result, after the decarburization and nitriding process, precipitates with a major axis D of 50 to 150 nm are controlled as described above.

[0123] Furthermore, the slab soaking temperature during slab heating before rough rolling, and the rolling temperature during rough rolling, as described above, are temperatures that are controlled with a specific purpose. These temperatures are not due to the natural temperature drop that occurs when the slab is removed from the slab heating furnace and subjected to rough rolling. For example, in general operation, the slab soaking temperature and rough rolling temperature are not controlled with a specific purpose. Normally, if the slab soaking temperature is high, the rough rolling temperature will also be high, and if the slab soaking temperature is low, the rough rolling temperature will also be low. On the other hand, in this embodiment, the slab soaking temperature and rough rolling temperature described above are controlled with a specific purpose. For example, even if the slab soaking temperature is high within the above range, the rough rolling temperature is controlled to remain within the above range, and similarly, even if the slab soaking temperature is low within the above range, the rough rolling temperature is controlled to remain within the above range.

[0124] Furthermore, the conditions for finish rolling in the hot rolling process are not particularly limited; normal hot rolling conditions can be used.

[0125] (Hot-rolled sheet annealing process) The hot-rolled sheet annealing process is a process in which hot-rolled steel sheets are annealed after the hot-rolling process to obtain hot-rolled annealed steel sheets. 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 favorably adjust the morphology of precipitates in the steel.

[0126] In this embodiment, the hot-rolled steel sheet after the hot-rolling process is heated and recrystallized in a first-stage annealing at a temperature range of 1000 to 1150°C, followed by a second-stage annealing at a lower temperature range of 800 to 1000°C, and then 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 reach the first-stage annealing temperature is preferably an average of 5°C / second or more. In particular, the heating rate from 200 to 900°C to reach the first-stage annealing temperature is preferably an average of 100°C / second or less. In the second-stage annealing, it is preferable to hold the steel sheet for 20 seconds or more. The cooling rate after the second-stage annealing is preferably an average of 5°C / second or more. In particular, the cooling rate from 750 to 500°C after the second-stage annealing is preferably an average of 25 to 80°C / second.

[0127] In the hot-rolled sheet annealing process, additional precipitates with a major axis D of 50-150 nm may precipitate. For example, if the heating rate to the first-stage annealing temperature of 200-900°C is less than 5°C / second, the amount of AlN dissolved in the first-stage annealing tends to increase, and when Al precipitates later as precipitates with a major axis D of 50-150 nm, it tends to precipitate together with Nb group elements. 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 more may decrease. Also, for example, if the heating rate to the first-stage annealing temperature of 200-900°C is greater than 100°C / second, precipitates containing Nb group elements do not dissolve sufficiently in the first-stage annealing, and when precipitates later precipitate with a major axis D of 50-150 nm, Nb group elements are less likely to be included. As a result, the R(Nb) value of the precipitate decreases, and the proportion of precipitates where the R(Nb)÷R(Al) value is 0.1 or greater may decrease.

[0128] (Cold rolling process) The cold rolling process involves taking the hot-rolled and annealed steel sheet obtained in the hot-rolled sheet annealing process and cold-rolling it multiple times (two or more times) via annealing (intermediate annealing) to obtain a cold-rolled steel sheet with a thickness of, for example, 0.10 to 0.50 mm.

[0129] In the cold rolling process, when rolling the hot-rolled and annealed steel sheet after the hot-rolled sheet annealing process, the cold rolling reduction ratio (cumulative reduction ratio) should be controlled to 80-95%.

[0130] The cold rolling reduction ratio (cumulative reduction ratio) mentioned above refers to either the cumulative cold rolling ratio without intermediate annealing, or the cumulative cold rolling ratio after intermediate annealing. Specifically, the cold rolling reduction ratio (cumulative reduction ratio) is defined as follows. Cold rolling reduction ratio (cumulative reduction ratio) (%) = (1 - "steel sheet thickness after cold rolling" / "steel sheet thickness before cold rolling (or after intermediate annealing)") × 100

[0131] When the reduction ratio of cold rolling is within the above range, the primary recrystallized texture after decarburization annealing is preferably controlled. Specifically, the primary recrystallized texture is such that, during secondary recrystallization, ideal Goss-oriented grains (ideal {110} <001> The texture becomes such that orientation grains (which have an orientation) preferentially grow (specifically, in the primary recrystallized texture, {111} <112> ya{411} <148> (The crystal orientation represented by becomes the dominant orientation.) As a result, it is preferable because it is easier to obtain a secondary recrystallized texture in the final product in which ideal Goss orientation grains have preferentially grown.

[0132] (Decarburizing and nitriding process) The decarburizing and nitriding process involves performing decarburizing annealing (for example, at 700-900°C for 1-3 minutes) on the cold-rolled steel sheet obtained in the cold-rolling process, and then performing a nitriding treatment (for example, an increase of 40-300 ppm) to increase the nitrogen content of the steel sheet, thereby obtaining a decarburizing and nitrided steel sheet that has undergone primary recrystallization.

[0133] (Decarburization annealing) Decarburization annealing is performed to remove carbon contained in cold-rolled steel sheets. In decarburization annealing, it is preferable to heat the steel sheet evenly in a humid atmosphere at 700-900°C for 1-3 minutes.

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

[0135] As described above, the method for manufacturing a decarburized nitrided steel sheet according to this embodiment comprises a casting step, a hot rolling step, a hot-rolled sheet annealing step, a cold rolling step, and a decarburized nitride step. In the decarburized nitrided steel sheet manufactured by comprehensively controlling the above conditions in each step, precipitates with a major axis D of 50 to 150 nm are preferably controlled. As a result, the secondary recrystallization 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 decarburized nitrided steel sheets according to this embodiment, the amount of relatively coarse precipitates (undissolved precipitates) that remain precipitated during the slab heating stage is controlled mainly by the slab soaking temperature and slab soaking time during slab heating before rough rolling, and the amount of relatively fine precipitates (re-precipitated precipitates) is controlled by the subsequent manufacturing conditions, thereby controlling the characteristics of the decarburized nitrided steel sheet within the above range. As a result, the secondary recrystallization temperature range is expanded during finish annealing, the selective growth of Goss-oriented grains is improved, and the magnetic flux density of the grain-oriented electrical steel sheet is improved.

[0137] 5. How to use decarburized nitrided steel sheets The effects of the decarburized and nitrided steel sheet according to this embodiment can be confirmed in the final product, the grain-oriented electrical steel sheet. Therefore, from the perspective of how to use the decarburized and nitrided steel sheet according to this embodiment, the manufacturing process of the grain-oriented electrical steel sheet following the decarburized and nitrided process will be described.

[0138] The manufacturing method for grain-oriented electrical steel sheets includes an annealing and separating agent application step and a finish annealing step. Additionally, an insulating film formation step and a magnetic domain control step may be included as needed. These steps can utilize known general process conditions. The manufacturing method will be described below as an example.

[0139] (Annealing and separating agent application process) The annealing separation agent application process involves applying an annealing separation agent to a decarburized and nitrided steel sheet. Examples of annealing separation agents that can be used include those primarily composed of MgO or alumina.

[0140] After the annealing release agent is applied, the decarburized and nitrided steel sheet is wound into a coil and then finished annealed in the next finish annealing process.

[0141] (Finishing annealing process) The finish annealing process involves applying a finish annealing to a decarburized and nitrided steel sheet coated with an annealing separating agent to induce secondary recrystallization. This process allows secondary recrystallization to proceed while the growth of primary recrystallized grains is suppressed by an inhibitor, thus {110} <001> This method prioritizes the growth of azimuthal particles, thereby improving magnetic flux density.

[0142] When using the decarburized nitrided steel sheet according to this embodiment, the secondary recrystallization temperature range is expanded during finish annealing, resulting in a level of {100} not previously seen. <011> Preferential growth of oriented grains occurs, resulting in a dramatic increase in magnetic flux density. Additionally, abnormal grain growth of secondary recrystallized grains occurs during finish annealing, and after finish annealing, these secondary recrystallized grains occupy the entire surface of the plate. The few secondary recrystallized grains cover the entire surface of the steel plate, and the grain size of each secondary recrystallized grain increases.

[0143] Furthermore, in the finish annealing process, the finish annealing conditions for "expanding the secondary recrystallization temperature range" disclosed in the above-mentioned Patent Documents 9 to 11 may be applied as needed. By using the decarburized nitrided steel sheet according to this embodiment and applying the finish annealing conditions disclosed in Patent Documents 9 to 11, the secondary recrystallization temperature range can be further preferably expanded.

[0144] The following insulating film formation and magnetic domain control steps use the crystal orientation {110}. <001> From the perspective of integration, this process is not necessary. However, it is a process that is commonly used in grain-oriented electrical steel sheets to improve practical magnetic properties. (Insulating film formation process) The insulating coating formation process is a process of forming an insulating coating on grain-oriented electrical steel sheets (finish-annealed steel sheets) after the finish annealing process. An insulating coating mainly composed of phosphate and colloidal silica, or an insulating coating mainly composed of alumina sol and boric acid, can be formed on the steel sheet after finish annealing.

[0145] (Magnetic domain control process) The magnetic domain control process is a process that subdivides the magnetic domains of the grain-oriented electrical steel sheet. This process is carried out at an appropriate time after cold rolling. For example, local minute strains or local grooves can be formed in the grain-oriented electrical steel sheet using known methods such as laser, plasma, mechanical methods, or etching.

[0146] 6. Grain-oriented electrical steel sheet obtained using the decarburized and nitrided steel sheet according to this embodiment A brief description of grain-oriented electrical steel sheets manufactured using decarburized and nitrided steel sheets according to this embodiment will be provided.

[0147] In the decarburized and nitrided steel sheet according to this embodiment, relatively fine and relatively coarse precipitates coexist in a favorable size and distribution. Therefore, in the grain-oriented electrical steel sheet obtained using the decarburized and nitrided steel sheet according to this embodiment, Goss-oriented grains preferentially grow, and the magnetic flux density is favorably increased. Furthermore, since the grain-oriented electrical steel sheet manufactured using the decarburized and nitrided steel sheet according to this embodiment does not suffer deterioration of other properties due to the increased magnetic flux density, it can be used in the same applications as conventional materials.

[0148] The grain-oriented electrical steel sheet manufactured using the decarburized and nitrided steel sheet according to this embodiment contains, by mass fraction, Si (silicon): 2.0 to 7.0% as a basic element (major alloying element).

[0149] Furthermore, impurities may be present. "Impurities" refer to elements introduced during the industrial production of steel, either from the raw materials (ore or scrap) or from the manufacturing environment. The total impurity content may be capped at, for example, 5%.

[0150] Furthermore, in addition to the basic elements and impurities mentioned above, optional elements may also be included. For example, instead of a portion of the remaining Fe mentioned above, optional elements such as Nb, V, Mo, Ta, W, C, Mn, S, Se, Al, N, Cu, Bi, B, P, Ti, Sn, Sb, Cr, Ni, etc., may be included. These optional elements may be included according to their purpose. Therefore, there is no need to limit the lower limit of these optional elements, and the lower limit may be 0%. Also, these optional elements may be included as impurities.

[0151] In grain-oriented electrical steel sheets, relatively large changes in chemical composition (reduction in content) occur through decarburization annealing and purification annealing during secondary recrystallization. Depending on the element, the content may be reduced to a level undetectable by general analytical methods (less than 1 ppm) through purification annealing. Generally, the chemical composition of the final product differs from that of the starting material slab. However, the arbitrary elements mentioned above are elements that remained in the final product from those contained in the slab, and the content of each element will not exceed the aforementioned content range for the slab, but will be within the content range corresponding to the content in the slab and subsequent manufacturing processes.

[0152] The above chemical composition is that of grain-oriented electrical steel sheet. If the grain-oriented electrical steel sheet used as a measurement sample has an insulating coating or the like on its surface, the coating or the like should be removed by a known method before measuring the chemical composition.

[0153] The grain-oriented electrical steel sheet manufactured using the decarburized and nitrided steel sheet according to this embodiment may have 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.

[0154] For example, the above-mentioned intermediate layer may be a layer mainly composed of oxides, a layer mainly composed of carbides, a layer mainly composed of nitrides, a layer mainly composed of borides, a layer mainly composed of silicides, a layer mainly composed of phosphides, a layer mainly composed of sulfides, or a layer mainly composed of intermetallic compounds. These intermediate layers are formed primarily to ensure adhesion between the silicon steel sheet and the insulating coating, and may be known to be formed by heat treatment in an atmosphere with controlled oxidation-reduction properties, chemical vapor deposition (CVD), physical vapor deposition (PVD), etc.

[0155] Furthermore, typical examples of the above-mentioned insulating coatings include those mainly composed of phosphate and colloidal silica with an average thickness of 0.1 to 10 μm, and those mainly composed of alumina sol and boric acid with an average thickness of 0.5 to 8 μm. [Examples]

[0156] Next, the effects of the present invention will be specifically described in detail with reference to examples. The conditions in the examples are just one example of conditions adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to this one example of conditions. The present invention can adopt various conditions as long as they do not depart from the spirit of the invention and achieve the objectives of the present invention.

[0157] Decarburized and nitrided steel sheets were manufactured using slabs with the chemical compositions shown in Tables 1 and 2. The chemical composition of the manufactured decarburized and nitrided steel sheets was equivalent to that of the slabs shown in Tables 1 and 2, except for C (carbon) and N (nitrogen). The C and N content of the manufactured decarburized and nitrided steel sheets is shown in Tables 15 to 20. These chemical compositions were measured based on the method described above. In Tables 1 and 2, "-" indicates that the content was not controlled or measured during manufacturing.

[0158] Furthermore, the decarburized and nitrided steel sheets described above were manufactured based on 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. In the tables, the soaking temperature represents the surface temperature of the slab after heating, and the soaking time represents the slab heating time from the time 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%, setting the slab soaking temperature to over 1030°C and less than 1180°C, and the slab soaking time to over 70 minutes, results in 12 to 85 volume percent of precipitates being dissolved, based on the precipitates contained in the slab after the casting process.

[0160] In the hot-rolled sheet annealing process, the hot-rolled steel sheet was annealed after the hot-rolling process. In all examples except No. 90, the hot-rolled steel sheet was annealed under the annealing conditions shown in Tables 9 to 14. In the case of example No. 90, a second-stage annealing was not performed.

[0161] Furthermore, in the decarburization and nitriding process, decarburization 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 manufactured decarburized and nitrided steel sheets, the Al content (atomic %) R(Al) and Nb group element content (atomic %) R(Nb) of precipitates with a major axis D of 50 to 150 nm were investigated based on the method described above. In Tables 15 to 20, the "number percentage" represents the number percentage of precipitates with a major axis D of 50 to 150 nm where the value of R(Nb)÷R(Al) is 0.1 or greater.

[0163] Using the manufactured decarburized and nitrided steel sheets, an annealing separation agent mainly composed of MgO was applied to the steel sheets, and finish annealing was performed. In the final stage of finish annealing, the steel sheets were held at 1200°C in a hydrogen atmosphere for 20 hours (purification annealing) and then allowed to cool naturally.

[0164] A coating solution for forming an insulating film, mainly composed of phosphate and colloidal silica and containing chromium, was applied to the primary film (intermediate layer) formed on the surface of the manufactured grain-oriented electrical steel sheet (finish annealed steel sheet). The sheet was then heated and held in an atmosphere of hydrogen:nitrogen at 75% by volume:25% by volume, and then cooled to form an insulating film.

[0165] The manufactured grain-oriented electrical steel sheet, when viewed from a cross-section parallel to the thickness direction, had an intermediate layer placed in contact with the grain-oriented electrical steel sheet (silicon steel sheet) and an insulating coating placed in contact with this intermediate layer. The intermediate layer was a forsterite coating with an average thickness of 2 μm, and the insulating coating was an insulating coating mainly composed of phosphate and colloidal silica with an average thickness of 1 μm.

[0166] The obtained grain-oriented electrical steel sheets were evaluated for various properties. 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) method specified in JIS C 2556:2015.

[0168] As a measure of magnetic properties, the magnetic flux density B8(T) in the rolling direction of the steel sheet was measured when excited at 800 A / m. A magnetic flux density B8 of 1.930 T or higher was considered acceptable. For reference, iron loss W, defined as power loss per unit weight (1 kg) of the steel sheet, was also measured under the conditions of AC frequency: 50 Hz and excitation magnetic flux density: 1.7 T. 17 / 50 (W / kg) was measured.

[0169] Of Nos. 1 to 93, the examples of the present invention showed favorable control of precipitates contained in the decarburized and nitrided steel sheets, and all exhibited excellent magnetic flux density as grain-oriented electrical steel sheets. On the other hand, of Nos. 1 to 93, the comparative examples did not show favorable control of precipitates contained in the decarburized and nitrided steel sheets, and could not obtain a favorable magnetic flux density as grain-oriented electrical steel sheets.

[0170] Table 1

[0171] Table 2

[0172] Table 3

[0173] Table 4

[0174] Table 5

[0175] Table 6

[0176] Table 7

[0177] Table 8

[0178] Table 9

[0179] Table 10

[0180] Table 11

[0181] [Table 12]

[0182] [Table 13]

[0183] [Table 14]

[0184] [Table 15]

[0185] [Table 16]

[0186] [Table 17]

[0187] [Table 18]

[0188] [Table 19]

[0189] [Table 20] [Industrial applicability]

[0190] According to the above aspects of the present invention, it is possible to provide a decarburized nitrided steel sheet for grain-oriented electrical steel sheets that can increase magnetic flux density, thus having high industrial applicability.

Claims

[Claim 1] In decarburized nitrided steel sheets for grain-oriented electrical steel sheets, The decarburized and nitrided steel sheet is, by mass%, C: 0.0005-0.010%, Si: 2.0 to 4.50%, Mn: 0.050 to 0.50%, S: 0-0.0350%, Se: 0 to 0.0350%, S+Se total content: 0.0030-0.0350%, Al: 0.010-0.0650%, N: 0.010-0.040%, Nb: 0 to 0.030%, V: 0 to 0.030%, Mo: 0 to 0.030%, Ta: 0 to 0.030%, W: 0-0.030%, 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: 0-0.50%, Ti: 0 to 0.0150%, Sn: 0 to 0.10%, Sb: 0 to 0.10%, Cr: 0 to 0.30%, Ni: 0-1.0%, It contains and has a chemical composition consisting of Fe and impurities as the remainder, Of the precipitates collected from the aforementioned decarburized and nitrided steel sheet by the extraction replica method, those with a major axis D of 50 to 150 nm, The Al content is expressed in atomic percent as R(Al). When the total content of Nb + V + Mo + Ta + W is expressed as R(Nb) in atomic percent, Precipitates with a R(Nb)÷R(Al) value of 0.1 or greater account for 35% to 95% of the total number of precipitates. A decarburized and nitrided steel sheet for grain-oriented electrical steel sheets, characterized by the following features.

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