Manufacturing method of grain-oriented electrical steel sheets

Abstract

Provided is a grain-oriented electrical steel sheet production method, wherein: regarding the particle diameter-number density distribution of precipitates which have equivalent circle diameters D of 50-1,000 nm, a steel sheet after hot-rolled sheet annealing exhibits a mode diameter Dp of 50-350 nm, a mode diameter number density f(Dp) of not less than 1,000,000 particles / g, and a value Wp / Dp, obtained by dividing the mode diameter half-width Wp by the mode diameter Dp, of 0.75-2.25; and in final annealing, during heating, switching to slow heating is performed while the temperature is in the range from 800°C to a final annealing temperature, a switching temperature for the switching to slow heating is more than 880°C but less than 1,010°C, the final annealing temperature is 1,150-1,250°C, the average temperature increase speed for temperatures from 800°C to the switching temperature is not less than 12°C / hour and not more than 50°C / hour, and the average temperature increase speed for temperatures from the switching temperature to the final annealing temperature is not less than 3°C / hour but less than 12°C / hour.

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a grain-oriented electrical steel sheet. This application claims priority based on Japanese Patent Application No. 2024-034330 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 the {110}<001> orientation (Goss orientation) is aggregated. 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, control is performed 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 the inhibitor 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.

[0008] Furthermore, Patent Document 12 shows that, in a decarburized annealed sheet having a specific range of grain sizes, an improvement in magnetic flux density can be expected by regulating the relationship between the amount of nitrogen in the steel sheet after nitriding and the heating rate during the heating process of finish annealing. [Prior art documents] [Patent Documents]

[0009] [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 [Patent Document 12] Japanese Patent No. 4205816 [Overview of the Initiative] [Problems that the invention aims to solve]

[0010] 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.

[0011] 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.

[0012] 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 method for manufacturing 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]

[0013] The gist of this invention is as follows:

[0014] (1) A method for manufacturing grain-oriented electrical steel sheets according to one aspect of the present invention is: It comprises a casting process, a hot rolling process, a hot rolled sheet annealing process, a cold rolling process, a decarburization annealing process, an annealing separator coating process, and a finish annealing process. In the casting process, by mass%, C: 0.0010 to 0.10%, Si: 2.0 to 4.50% , Mn: 0.050 to 0.50% , S: 0 to 0.0350%, Se: 0 to 0.0350%, Total content of S + Se: 0.0030 to 0.0350%, Al: 0.010 to 0.0650%, N: 0.0040 to 0.0120%, Nb: 0 to 0.030%, V: 0 to 0.030%, Mo: 0 to 0.030%, Ta: 0 to 0.030%, W: 0 to 0.030%, Nb+V+Mo+Ta+W total content: 0~0.030%, 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%, ​​​​​​​​​​​​​​Let f(Dp) be the number density of the most mode diameter in units of particles / g. When the full width at half maximum of the most mode diameter is denoted as Wp in units of nm, Dp is 50~350nm, f(Dp) is 1,000,000 particles / g or more. Wp / Dp is 0.75~2.25. Satisfying the conditions, In the cold rolling process, the hot-rolled and annealed steel sheet after the hot-rolled sheet annealing process is cold-rolled to form a cold-rolled steel sheet. In the decarburization annealing process, the cold-rolled steel sheet after the cold-rolling process is decarburized and annealed to obtain a decarburized and annealed steel sheet. In the annealing separation agent application step, the annealing separation agent is applied to the decarburized annealed steel sheet after the decarburization annealing step and dried. In the aforementioned finish annealing process, The decarburized annealed steel sheet to which the annealing separating agent has been applied after the annealing separating agent application step is heated. During the aforementioned heating process, the heating method is switched to slow heating within the temperature range from 800°C to the finish annealing temperature. The switching temperature for slow heating is set to be greater than 880°C and less than 1010°C. The finish annealing temperature is set to 1150°C or higher and 1250°C or lower. The average heating rate from 800°C to the aforementioned switching temperature shall be 12°C / hour or more and 50°C / hour or less, Under the condition that the average heating rate from the aforementioned switching temperature to the aforementioned finish annealing temperature is 3°C / hour or more and less than 12°C / hour, Finish annealing is performed to obtain grain-oriented electrical steel sheets (finish annealed steel sheets). (2) The method for manufacturing grain-oriented electrical steel sheets described in (1) above is: The aforementioned chemical composition may contain at least one element selected from the group consisting of Nb, V, Mo, Ta, and W in a total amount of 0.0030 to 0.030% by mass. (3) The method for manufacturing grain-oriented electrical steel sheets described in (1) or (2) above is: The switching temperature is set to be greater than 950°C and less than 1010°C, and The average heating rate from the switching temperature to the finish annealing temperature may be set to 3°C / hour or more and less than 8°C / hour. (4) The method for manufacturing grain-oriented electrical steel sheets described in (1) or (2) above is: The aforementioned switching temperature is set to be greater than 880°C and less than or equal to 950°C, and, The average heating rate from the switching temperature to the finish annealing temperature may be set to 8°C / hour or more and less than 12°C / hour. (5) The method for manufacturing grain-oriented electrical steel sheets described in (1) or (2) above is: The aforementioned switching temperature is set to be greater than 880°C and less than or equal to 950°C, and, The average heating rate from the switching temperature to the finish annealing temperature may be set to 3°C / hour or more and less than 8°C / hour. [Effects of the Invention]

[0015] According to the above aspects of the present invention, a method for manufacturing grain-oriented electrical steel sheets that can increase magnetic flux density is provided. [Brief explanation of the drawing]

[0016] [Figure 1] This is a schematic diagram of the particle size-number density distribution of precipitates with an equivalent circle diameter D of 50-1000 nm. [Figure 2] This is a flowchart illustrating a method for manufacturing grain-oriented electrical steel sheets according to one embodiment of the present invention. [Modes for carrying out the invention]

[0017] 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.

[0018] 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.

[0019] As mentioned above, there is currently a need to increase the magnetic flux density of grain-oriented electrical steel sheets.

[0020] 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.

[0021] 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 crystal 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 hot-rolled and annealed steel sheet during the manufacturing process of grain-oriented electrical steel sheets, and then implementing a "slow heating switch" by switching the heating rate to a low speed during the heating process of the finish annealing, the secondary recrystallization temperature range is expanded during finish annealing, Goss-oriented grains grow preferentially, and the magnetic flux density of the resulting grain-oriented electrical steel sheet can be increased beyond that of conventional techniques.

[0022] 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.

[0023] 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.

[0024] 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.

[0025] In this embodiment, by controlling the size and distribution of precipitates (inhibitors) contained in the hot-rolled annealed steel sheet to an appropriate range, the temperature range in which secondary recrystallization progresses during the subsequent finish annealing process is expanded, and the selectivity of crystal orientation associated with the progress of secondary recrystallization is enhanced. In addition, in this embodiment, by performing "slow heating switching" during the heating process of finish annealing, the steel sheet is preferably heated slowly in the temperature range in which Goss-oriented grains preferentially grow, thereby enhancing the selectivity of crystal orientation associated with the progress of secondary recrystallization. Specifically, the above effects are obtained by allowing relatively fine inhibitors and relatively coarse inhibitors to coexist in the hot-rolled annealed steel sheet with appropriate size and distribution, and then performing "slow heating switching" to switch the heating rate to a low speed during the heating process of finish annealing.

[0026] The inventors of this invention estimate that the above effect is obtained through the following mechanism.

[0027] First, let's consider the reason why the temperature range for secondary recrystallization expands. 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.

[0028] 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.

[0029] 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.

[0030] 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.

[0031] 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.

[0032] 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.

[0033] 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 recrystallization grain growth rate should be relatively fast relative to the inhibitor decomposition rate.

[0034] However, as mentioned above, when fine and coarse inhibitors coexist, secondary recrystallization begins at a relatively low temperature during the heating process of the finish annealing. In other words, when fine and coarse inhibitors coexist, secondary recrystallization begins at a lower temperature than the conventional temperature range in which it was thought to proceed. Therefore, when fine and coarse inhibitors coexist, simply performing a slow heating process in the temperature range in which it was thought to proceed in the conventional method will not yield the desired results. It is preferable to perform a slow heating process in a temperature range that corresponds to the inhibitor morphology.

[0035] In other words, when fine and coarse inhibitors coexist, secondary recrystallization starts at a relatively low temperature during the heating process of finish annealing and continues up to a relatively high temperature. Therefore, by switching the heating rate to a low speed in the temperature range in which secondary recrystallization proceeds according to the inhibitor morphology, the growth rate of the secondary recrystallized grains becomes favorably faster than the decomposition rate of the inhibitors, and it is thought that Goss-oriented grains will preferentially grow. As a result, it is thought that it will ultimately be possible to increase the magnetic flux density.

[0036] In this embodiment, by comprehensively and inseparably controlling the steel composition, casting conditions, hot rolling conditions, and hot-rolled sheet annealing conditions, relatively fine precipitates and relatively coarse precipitates are made to coexist in an appropriate size and distribution in the hot-rolled annealed steel sheet after the hot-rolled sheet annealing process. Furthermore, in this embodiment, the morphology of the precipitates is favorably controlled by adding auxiliary inhibitor-forming elements. In addition, in this embodiment, "slow heating switching" is performed so that the heating rate is slow in the temperature range corresponding to the above inhibitor morphology.

[0037] In this embodiment, the morphology of the precipitates described above is defined based on hot-rolled and annealed steel sheet (steel sheet immediately before cold rolling).

[0038] The hot-rolled annealed steel sheet controlled by the manufacturing method of grain-oriented electrical steel sheet according to this embodiment (hereinafter also referred to as the hot-rolled annealed steel sheet according to this embodiment) will be described in detail below.

[0039] The hot-rolled and annealed steel sheet according to this embodiment is, by 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%, 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, Regarding the particle size-number density distribution of precipitates with an equivalent circular diameter D of 50-1000 nm, which are the residues obtained by electrolytic extraction of hot-rolled annealed steel sheets, Let Dp be the most frequent diameter in nm. Let f(Dp) be the number density of the most mode, in units of particles / g. When the full width at half maximum of the most mode is denoted as Wp in units of nm, Dp is 50~350nm, f(Dp) is 1,000,000 particles / g or more. Wp / Dp must satisfy the condition 0.75 to 2.25.

[0040] Furthermore, the hot-rolled annealed steel sheet according to this embodiment may contain, in terms of chemical composition, at least one selected from the group consisting of Nb, V, Mo, Ta, and W in a total amount of 0.0030 to 0.030 mass%.

[0041] 1.Chemical composition The chemical composition of the hot-rolled and annealed steel sheet according to this embodiment may be the same as the general chemical composition used for grain-oriented electrical steel sheets.

[0042] It should be noted that the chemical composition of hot-rolled annealed steel sheets, which are intermediate products, is rarely described in publicly available literature on grain-oriented electrical steel sheets. However, since the steel composition hardly changes during the process from slab to before decarburization and annealing, the chemical composition of hot-rolled annealed steel sheets can be considered to be basically the same as the chemical composition of slabs disclosed in publicly available literature.

[0043] The hot-rolled and annealed steel sheet according to this embodiment has a chemical composition that includes basic elements, optional elements as needed, and the remainder being Fe and impurities.

[0044] The hot-rolled and annealed steel sheet according to this embodiment contains, by mass fraction, the following basic elements (major alloying elements): C: 0.0010~0.10%, Si: 2.0~7.0%, Mn: 0.050~1.0%, total S+Se content: 0.0030~0.0350%, Al: 0.010~0.0650%, and N: 0.0040~0.0120%.

[0045] C: 0.0010~0.10% Carbon (C) is an effective element for controlling the primary recrystallization structure during the manufacturing process. However, excessive C content in the final product negatively affects its magnetic properties. Therefore, the C content of hot-rolled annealed steel sheets should be between 0.0010% and 0.10%. The preferred upper limit for C content is 0.0850% and 0.0750%. Note that C is purified in the decarburization annealing and finish annealing processes described later, and after the finish annealing process, it becomes 0.0050% or less. When C is present, considering productivity in industrial production, the C content may be greater than 0% or greater than 0.0010%.

[0046] 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 the sheet more prone to cracking during cold rolling. Therefore, the Si content of hot-rolled and annealed 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%.

[0047] 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 hot-rolled annealed 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%.

[0048] S: 0~0.0350% Se: 0~0.0350% S+Se total content: 0.0030~0.0350% Sulfur (S) and selenium (Se) combine with manganese (Mn) to precipitate as MnS and MnSe, respectively, and function as inhibitors. To favorably control the morphology of these inhibitors (precipitates), the S content of the hot-rolled and annealed steel sheet should be 0 to 0.0350%, the Se content 0 to 0.0350%, and the total S+Se content 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.

[0049] Here, "the total content of S and Se is 0.0030 to 0.0350%" means that the hot-rolled annealed steel sheet may contain only S or Se in its chemical composition, and the content may be between 0.0030 and 0.0350%. Alternatively, it may contain both S and Se, and the total content may be between 0.0030 and 0.0350%.

[0050] 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 hot-rolled and annealed 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 through 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 precipitates 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 stability during secondary recrystallization, the upper limit of the Al content is preferably 0.040%, and more preferably 0.030%.

[0051] N: 0.0040~0.0120% Nitrogen (N) combines with Al to precipitate as AlN or (Al,Si)N, functioning as an inhibitor. The N content of hot-rolled and annealed steel sheets should be between 0.0040% and 0.0120%. In low-temperature slab heating processes, N may be added to the steel during the manufacturing process through nitriding. If the N content exceeds 0.0120%, blistering, a type of defect, is more likely to occur in the steel sheet. The upper limit of the N content is preferably 0.010%, and more preferably 0.0090%. N is purified in the finish annealing process, and after the finish annealing process, the N content becomes 0.0050% or less.

[0052] The hot-rolled and annealed 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%.

[0053] Furthermore, the hot-rolled and annealed 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 Nb, V, Mo, Ta, W, 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%. Moreover, even if these optional elements are included as impurities, the above effects will not be impaired.

[0054] 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 expand the temperature range over which secondary recrystallization progresses. Therefore, the Nb content may be 0-0.030%, the V content may be 0-0.030%, the Mo content may be 0-0.030%, the Ta content may be 0-0.030%, and the W content may be 0-0.030%.

[0055] In this embodiment, Nb, V, Mo, Ta, and / or W may be collectively referred to as "Nb group elements."

[0056] In this embodiment, the hot-rolled and annealed steel sheet preferably contains, as an Nb group element, at least one selected from the group consisting of Nb, V, Mo, Ta, and W in a total amount of 0.0030 to 0.030 mass%.

[0057] When Nb group element precipitates are used as inhibitors, if the total content of Nb group elements in the hot-rolled and annealed 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.

[0058] 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 hot-rolled annealed 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.

[0059] 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.

[0060] 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%.

[0061] In grain-oriented electrical steel sheets, relatively large changes in chemical composition (reduction in element 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 hot-rolled annealed steel sheets. The steel composition hardly changes during the process from slab to before decarburization annealing.

[0062] The chemical composition of the hot-rolled annealed steel sheet according to this embodiment can be measured using general analytical methods for steel. For example, the chemical composition of the hot-rolled annealed steel sheet can be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). Specifically, the chemical composition can be determined by measuring a 35 mm square test piece taken from the hot-rolled annealed steel sheet using ICP-AES under conditions based on a pre-established calibration curve. Note that 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.

[0063] 2. Precipitate Next, we will describe the precipitates contained in the hot-rolled annealed steel sheet according to this embodiment.

[0064] The precipitates (inhibitors) contained in the hot-rolled annealed steel sheet according to this embodiment are not particularly limited in type, as long as the precipitation morphology of the precipitates is controlled. The precipitates may be formed from elements contained in the hot-rolled annealed steel sheet. For example, Mn-based precipitates may be sulfides or selenides, and Al-based precipitates may be nitrides. In addition to these inhibitors, auxiliary inhibitors may include compounds of Nb group elements, compounds of arbitrary elements such as Bi or B, or composite compounds of the above elements.

[0065] While the type (composition) of precipitates may contribute to some extent to the effects obtained in this embodiment, our investigations have shown that the effects obtained in this embodiment are primarily due to the size and distribution of precipitates contained in the hot-rolled annealed steel sheet. Therefore, the size and distribution of precipitates are specified in the hot-rolled annealed steel sheet according to this embodiment.

[0066] The precipitates to be controlled in the hot-rolled annealed steel sheet according to this embodiment are precipitates with an equivalent circular diameter D of 50 to 1000 nm. The "equivalent circular diameter" refers to the diameter of a circle when the area of ​​the precipitate is converted to a circle of the same area. This equivalent circular diameter is equivalent to the equivalent spherical diameter.

[0067] Precipitates with an equivalent circle diameter D smaller than 50 nm in hot-rolled annealed 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 an equivalent circle diameter D smaller than 50 nm in the hot-rolled annealed steel sheet change or disappear in subsequent processes and are less likely to function as inhibitors during finish annealing. Therefore, in the hot-rolled annealed steel sheet according to this embodiment, the size and distribution of precipitates with an equivalent circle diameter D of 50 nm or more are controlled. Furthermore, by considering processes including those after the hot-rolled sheet annealing process, it is expected that precipitates with an equivalent circle diameter D smaller than 50 nm will be able to function as inhibitors in the future.

[0068] Furthermore, precipitates with an excessively large equivalent diameter D may adversely affect the growth of secondary recrystallized grains in the final stage of secondary recrystallization. Also, the formation of precipitates with an excessively large equivalent diameter D may reduce the number of precipitates (number density) contained in the hot-rolled annealed steel sheet. Moreover, precipitates with an excessively large equivalent diameter D are less likely to function as inhibitors. For this reason, it is preferable that the equivalent diameter D of the precipitates be 1000 nm or less on average. In the hot-rolled annealed steel sheet according to this embodiment, the size and distribution of precipitates with an equivalent diameter D of 50 to 1000 nm are controlled to have the effect of expanding the secondary recrystallization temperature range.

[0069] In the hot-rolled annealed steel sheet according to this embodiment, among the precipitates which are the residue obtained by electrolytic extraction of the hot-rolled annealed steel sheet, the particle size-number density distribution of precipitates with an equivalent circular diameter D of 50 to 1000 nm is as follows: Let Dp be the most frequent diameter in nm. Let f(Dp) be the number density of the most mode, in units of particles / g. When the full width at half maximum of the most mode is denoted as Wp in units of nm, Dp is 50~350nm, f(Dp) is 1,000,000 particles / g or more. Wp / Dp is 0.75~2.25. It satisfies the condition.

[0070] Figure 1 shows a schematic diagram of the particle size-number density distribution of precipitates with an equivalent circle diameter D of 50 to 1000 nm. Dp, f(Dp), and Wp are shown as examples in Figure 1.

[0071] When Dp exceeds 350 nm, it is unsuitable because there are fewer fine precipitates to expand the secondary recrystallization temperature range. The upper limit of Dp is preferably 300 nm, and more preferably 250 nm. On the other hand, when Dp is less than 50 nm, as mentioned above, the effect of expanding the secondary recrystallization temperature range is small. The lower limit of Dp is preferably 80 nm, and more preferably 130 nm.

[0072] If f(Dp) is less than 1,000,000 particles / g, there are few precipitates necessary for secondary recrystallization to occur, and the pinning effect cannot be sufficiently obtained, making it unsuitable. On the other hand, there is no particular upper limit to f(Dp), but for example, 50,000,000 particles / g would be appropriate.

[0073] If Wp / Dp is less than 0.75, the ratio of Wp to Dp is small, so the secondary recrystallization temperature range is not sufficiently expanded, which is unsuitable. The lower limit of Wp / Dp is preferably 1.0. On the other hand, if Wp / Dp is greater than 2.25, the ratio of Wp to Dp is too large, so the primary recrystallized grains grow unevenly during normal grain growth, resulting in a mixed grain structure before secondary recrystallization, which is unsuitable. The upper limit of Wp / Dp is preferably 1.75.

[0074] The particle size-number density distribution of precipitates with an equivalent circle diameter D of 50 to 1000 nm can be determined as follows.

[0075] For example, the method described in Japanese Patent No. 6572598 may be used. First, precipitates are electrolytically extracted from the hot-rolled annealed steel sheet. The electrolytic extraction conditions can be, for example, constant current electrolytic extraction (500 mA - 2 hours) using a solution prepared by adding a dispersant such as a surfactant (for example, sodium dodecyl sulfate with a molecular weight of 288.38 g / mol) to an acetylacetone-based electrolyte in advance. Electrolytic extraction should be performed such that the amount of electrolytic material extracted from the hot-rolled annealed steel sheet is 1 g or more.

[0076] The extraction residue (precipitation) should be recovered from the electrolytic extract. The size and distribution of this recovered precipitate should be measured by the Field Flow Fractionation (FFF) method. For the measurement method using the FFF method, please refer to Japanese Patent No. 6572598 mentioned above.

[0077] Note that each parameter can be changed depending on the particle size and type being measured. Here is an example: The FFF instrument should be a Wyatt Eclipse AF4 instrument (Wyatt Technology Europe, Germany). For the dispersion solution of the measurement sample, an aqueous solution of sodium dodecyl sulfate at a concentration of 300 mg / mL should be used. For the cell, an asymmetric diamond-type channel spacer with a channel length of 275 mm and a thickness of 350 μm should be used. As the separation membrane, a regenerated cellulose ultrafiltration membrane with a molecular weight of 30 kDa should be used.

[0078] Before adding the extraction residue (precipitation) recovered from the electrolytic extract, it is necessary to create a calibration curve by correlating particle size with the time until particle detection using standard samples with known particle sizes. The type and number of standard samples should be selected according to the particle size distribution of the extraction residue to be measured, but for example, standard particles of polystyrene latex with particle sizes of 29 to 500 nm can be selected.

[0079] The size of the standard particles must be directly confirmed beforehand using a TEM (Transmission Electron Microscope) or similar device. A minimum of 500 measurements should be performed. The longest side of each standard particle should be measured, and the average value should be calculated. Regarding the types of particle sizes used, for example, six different particle sizes such as 29 nm, 48 nm, 100 nm, 200 nm, 300 nm, and 500 nm can be used.

[0080] The actual separation conditions should be as follows. First, for stabilization before focusing, the eluent flow (channel flow) of the FFF instrument (hereinafter referred to as channel flow) should be 1.0 mL / min and the cross flow 0.5 mL / min for 1 minute. Then, for focusing before sample injection, the focus flow should be 3.0 mL / min for 1 minute. Next, during focusing, the sample should be injected at 0.2 mL / min for 2 minutes. The focusing time after sample injection should be 1 minute. After that, the flow path should be switched, the focus flow should be stopped, and the channel flow should be 1.0 mL / min, and the cross flow should be injected while decreasing the flow rate in direct proportion from 0.5 mL / min to 0.05 mL / min over 35 minutes. A calibration curve should be created by correlating the time from when the injection started until particles were detected with the average value of the particle size of the standard particles that had been measured in advance. The maximum time for particle detection should be 35 minutes, and the injection volume of the liquid in which the sample is dispersed should be 0.1 to 0.4 mL.

[0081] After creating the calibration curve as described above, the extraction residue (precipitate) recovered from the electrolytic extract is then added back into the apparatus. The apparatus settings parameters should be the same as described above.

[0082] In this way, the particle size of nanoparticles contained in the nanoparticle dispersion sample to be measured can be determined.

[0083] Furthermore, the effluent from the FFF instrument (a solution containing precipitates separated by size) can be analyzed for its components using a standard ICP (Inductively Coupled Plasma) mass spectrometer.

[0084] Using particle size distribution data measured by the FFF method, the particle size can be divided into 0.5 nm widths. The number density can then be calculated per unit particle / g from the number of precipitates contained within each particle size width and the amount of electrolysis during extraction. A histogram of particle size and number density can then be created. From this histogram, Dp, f(Dp), and Wp can be determined.

[0085] In this embodiment, the "mode" corresponds to the particle size (particle size category) at which the number density value is largest in the histogram of particle size and number density (particle size-number density distribution of precipitates) described above.

[0086] It is preferable to calculate Dp, f(Dp), Wp, etc., after smoothing the measurement data from the FFF method. For example, the simple moving average method can be used to smooth the measurement data from the FFF method. Furthermore, the value of f(Dp) can be calculated by considering the first three digits of the numerical value as significant.

[0087] 3. Plate thickness The thickness of the hot-rolled annealed steel sheet according to this embodiment is not particularly limited. The hot-rolled annealed steel sheet according to this embodiment is subjected to a subsequent cold-rolling process to be ultimately finished as a grain-oriented electrical steel sheet. Therefore, considering the general manufacturing conditions for grain-oriented electrical steel sheets, the thickness of the hot-rolled annealed steel sheet can be 1.8 to 3.5 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.

[0088] 4. Manufacturing method Next, a method for manufacturing grain-oriented electrical steel sheets according to this embodiment will be described.

[0089] Figure 2 is a flowchart illustrating a method for manufacturing grain-oriented electrical steel sheets according to this embodiment. As shown in Figure 2, the method for manufacturing grain-oriented electrical steel sheets according to this embodiment comprises a casting step, a hot rolling step, a hot-rolled sheet annealing step, a cold rolling step, a decarburization annealing step, an annealing release agent application step, and a finish annealing step. The conditions controlled in these steps will be described in detail later.

[0090] The manufacturing method for grain-oriented electrical steel sheets according to this embodiment comprises a casting step, a hot rolling step, a hot-rolled sheet annealing step, a cold rolling step, a decarburization annealing step, an annealing separation agent application step, and a finish annealing 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%, 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. Regarding the particle size-number density distribution of precipitates with an equivalent circular diameter D of 50-1000 nm in a hot-rolled annealed steel sheet after the hot-rolled sheet annealing process, Let Dp be the most frequent diameter in nm. Let f(Dp) be the number density of the most mode, in units of particles / g. When the full width at half maximum of the most mode is denoted as Wp in units of nm, Dp is 50~350nm, f(Dp) is 1,000,000 particles / g or more. Wp / Dp is 0.75~2.25. Satisfying the conditions, In the cold rolling process, the hot-rolled and annealed steel sheet, after the hot-rolled sheet annealing process, is cold-rolled to form a cold-rolled steel sheet. In the decarburization annealing process, the cold-rolled steel sheet after the cold-rolling process is decarburized and annealed to obtain a decarburized and annealed steel sheet. In the annealing separation agent application process, the annealing separation agent is applied to the decarburized annealed steel sheet after the decarburization annealing process and dried. In the finish annealing process, The decarburized annealed steel sheet, to which the annealing separating agent has been applied after the annealing separating agent application process, is heated. During heating, switch to slow heating within the temperature range from 800°C to the finish annealing temperature. The switching temperature for the above slow heating is set to be between 880°C and 1010°C. The above finish annealing temperature shall be between 1150°C and 1250°C. The average heating rate from 800°C to the switching temperature shall be between 12°C / hour and 50°C / hour, and Under the condition that the average heating rate from the switching temperature to the finish annealing temperature is 3°C / hour or more and less than 12°C / hour, Finish annealing is performed to obtain grain-oriented electrical steel sheets (finish annealed steel sheets).

[0091] Furthermore, in the manufacturing method of grain-oriented electrical steel sheet according to this embodiment, when the total content of the group consisting of Nb, V, Mo, Ta, and W in the chemical composition of the slab is less than 0.0030% by mass or greater than 0.030% by mass, In the hot rolling process, During the slab heating process before rough rolling, the soaking temperature of the slab is set to over 1040°C but less than 1100°C to preferably dissolve a portion of the precipitates contained in the slab (for example, dissolving 14-25% by volume 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 2 hours, and, During rough rolling, the rolling temperature may be set to 940-1030°C and the reduction ratio to 90-95%.

[0092] Furthermore, in the manufacturing method of grain-oriented electrical steel sheet according to this embodiment, when the total content of the group consisting of Nb, V, Mo, Ta, and W in the chemical composition of the slab is 0.0030 to 0.030 mass%, 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 may be set to 940-1070°C and the reduction ratio to 82-95%.

[0093] To control the size and distribution of precipitates in hot-rolled annealed steel sheets, it is necessary to control the steel composition, casting conditions, hot rolling conditions, and hot-rolled sheet annealing conditions. 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. 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.

[0094] 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.

[0095] The important manufacturing conditions for the manufacturing method of grain-oriented electrical steel sheets according to this embodiment are described below. Other manufacturing conditions can be those of conventional known grain-oriented electrical steel sheets.

[0096] (Casting process) In the casting process, the slab is prepared. As mentioned above, the chemical composition hardly changes from the slab to the process before decarburization and annealing, so the chemical composition of the slab is the same as that of the hot-rolled and annealed steel sheet described above.

[0097] 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 requirements for hot-rolled and annealed steel sheets mentioned above.

[0098] 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.

[0099] (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.

[0100] 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.

[0101] During the hot rolling process, when heating the slab after the casting process, the following conditions must be met.

[0102] For example, when the total content of the group consisting of Nb, V, Mo, Ta, and W in the chemical composition of the slab is less than 0.0030% by mass or greater than 0.030% by mass, when heating the slab before rough rolling, the soaking temperature of the slab should be set to more than 1040°C but less than 1100°C to preferably dissolve some of the precipitates contained in the slab (for example, dissolving 14 to 25% by volume of precipitates based on the precipitates contained in the slab at room temperature after the casting process), and the slab should be heated for more than 2 hours to homogenize this dissolved state within the slab.

[0103] On the other hand, when the total content of the group consisting of Nb, V, Mo, Ta, and W in the chemical composition of the slab is 0.0030 to 0.030 mass%, when heating the slab before rough rolling, the soaking temperature of the slab should be set to over 1030°C and 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 the slab should be heated for more than 70 minutes to homogenize this dissolved state within the slab.

[0104] 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).

[0105] Furthermore, Wp, which is the main technical feature of this embodiment, can be increased by controlling the size difference between relatively coarse precipitates that remain precipitated during the slab heating stage (undissolved precipitates) and relatively fine precipitates that do not precipitate during the slab heating stage but precipitate after hot rolling (re-precipitated precipitates).

[0106] 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 hot-rolled sheet annealing process.

[0107] 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.

[0108] 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.

[0109] 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 manufacturing method of grain-oriented electrical steel sheets according to this embodiment, the dissolution state of precipitates is favorably controlled, and slabs in which the dissolution of precipitates is in an equilibrium state are subjected to rough rolling.

[0110] 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.

[0111] 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 grain-oriented electrical steel sheets according to this embodiment, one example is shown of controlling the "solution state of precipitates before rough rolling" by performing soaking at a predetermined temperature for a predetermined time without temporarily increasing the heating temperature during the slab heating process.

[0112] For example, when the total content of the group consisting of Nb, V, Mo, Ta, and W in the chemical composition of the slab is less than 0.0030% by mass or greater than 0.030% by mass, the soaking temperature during slab heating should be greater than 1040°C but less than 1100°C, and the soaking time should be greater than 2 hours. 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 14 to 25% by volume of precipitates).

[0113] When the content of Nb group elements is within the above range, setting the soaking temperature during slab heating to below 1100°C makes it easier to ultimately control the size and distribution of precipitates. For example, if the slab heating temperature is 1100°C or higher, as in the conventional technique, the solutionization of precipitates progresses too much, making it difficult to control the solutionization state favorably (for example, it is difficult to control the precipitate solutionization rate to 25% by volume or less). In this case, the size of the precipitates re-precipitation in subsequent processes becomes larger, making it difficult to have fine inhibitors and coarse inhibitors coexist. As a result, the size difference of precipitates, which is the main technical feature of this embodiment, becomes smaller. Therefore, it is necessary to control the temperature to a lower level than in the conventional technique, as described above.

[0114] Furthermore, if the soaking temperature during slab heating is below 1040°C, the temperature is too low, making it difficult for the precipitates to dissolve and thus difficult to control the dissolution state favorably (for example, it is difficult to control the precipitate dissolution rate to 14% by volume or more). In this case, the amount of precipitates that re-precipitation in subsequent processes will be small, making it difficult to allow fine inhibitors and coarse inhibitors to coexist. As a result, the size difference of the precipitates, which is a key technical feature of this embodiment, becomes smaller.

[0115] In this embodiment, the secondary recrystallization temperature range is expanded by controlling the size difference between the relatively coarse precipitates that remain precipitated during the slab heating stage (undissolved precipitates) and the relatively fine precipitates that precipitate after hot rolling (re-precipitated precipitates) to be large. Therefore, when the content of Nb group elements is within the above range, it is important to set the soaking temperature during slab heating to more than 1040°C but less than 1100°C. This soaking temperature is preferably 1050°C to 1080°C.

[0116] Furthermore, when the Nb group element content is within the above range, a soaking time of 2 hours or less is too short, making it difficult to control the solution state of the precipitate to an equilibrium state. While there is no particular upper limit to the soaking time, considering productivity in industrial production, it may be set to 3 hours.

[0117] On the other hand, when the total content of the group consisting of Nb, V, Mo, Ta, and W in the chemical composition of the slab is 0.0030 to 0.030 mass%, the soaking temperature during slab heating should be greater than 1030°C but less than 1180°C, and the soaking time should be greater than 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).

[0118] When the Nb group element content is within the above range, the slab heating temperature before hot rolling and the rough rolling conditions described later are preferably relaxed. Specifically, even if the slab heating temperature is 1100°C or higher, it is ultimately possible to have both fine and coarse inhibitors coexist. 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 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, since the precipitation nose of Nb group element precipitates (carbonitrides) is at a lower temperature than the precipitation nose of AlN and MnS, the Nb group element precipitates themselves tend to precipitate as even finer precipitates than AlN and the like.

[0119] Therefore, when the content of Nb group elements is within the above range, the upper limit temperature during slab soaking is relaxed to 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, the controllable solubilization state of the precipitates is also relaxed (for example, the upper limit of the precipitate solubilization rate is relaxed to 85% by volume). Even with these relaxed conditions, the effect of the Nb group element precipitates makes it easier to achieve the coexistence of fine and coarse inhibitors.

[0120] Similarly, when the Nb group element content is within the above range, the lower limit temperature for slab soaking is relaxed to over 1030°C. While a lower soaking temperature also suppresses precipitate solution formation, when the Nb group element content is within the above range, the controllable precipitate solution state is also relaxed (for example, the lower limit for precipitate solution rate is relaxed to 12% by volume). Even with these relaxed conditions, it is ultimately possible to achieve coexistence of fine and coarse inhibitors.

[0121] 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.

[0122] 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.

[0123] 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).

[0124] 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.

[0125] 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.

[0126] 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 size and distribution of precipitates contained in the steel sheet after the hot-rolled sheet annealing process, it is important to control the rolling temperature and reduction ratio of the rough rolling after controlling the "solution state of precipitates before rough rolling" described above.

[0127] In the hot rolling process, the following conditions must be met when rough rolling is performed after heating the slab.

[0128] For example, when the total content of the group consisting of Nb, V, Mo, Ta, and W in the chemical composition of the slab is less than 0.0030% by mass or greater than 0.030% by mass, when rough rolling the heated slab, the rolling temperature should be controlled to 940-1030°C and the reduction ratio to 90-95%.

[0129] On the other hand, when the total content of the group consisting of Nb, V, Mo, Ta, and W in the chemical composition of the slab is 0.0030 to 0.030 mass%, when rough rolling the heated slab, the rolling temperature should be controlled to 940 to 1070°C and the reduction ratio to 82 to 95%.

[0130] 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 for 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 and a lower Wp value. On the other hand, there is no particular upper limit to the reduction ratio for rough rolling, but it is reasonable to set it at 95% considering the performance of the rolling mill, etc.

[0131] 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

[0132] Furthermore, if the rolling temperature for rough rolling is higher than the upper limit mentioned above, precipitates of MnS, AlN, and Nb group elements will precipitate 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, and the Wp value decreases. 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.

[0133] 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 that need to be controlled in the hot rolling process, such as the solution state of the precipitates (e.g., the solution rate of the precipitates before rough rolling), the rough rolling temperature, and the rough rolling reduction rate, are relaxed.

[0134] The reason why the control conditions for the solution state of precipitates are relaxed 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, MnS and AlN are re-precipitated more finely due to the precipitates of Nb group elements, so Dp is smaller compared to when Nb group elements are not included. On the other hand, even if Nb group elements are included, the value of Wp does not change significantly. Therefore, when Nb group elements are included, Wp / Dp becomes larger. For these reasons, it is thought that the control conditions for the solution state of precipitates are relaxed.

[0135] 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, resulting in fewer fine precipitates re-precipitation during hot rolling. Consequently, Wp becomes smaller, and 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, resulting in fewer relatively coarse precipitates (undissolved precipitates) in the slab. Consequently, the secondary recrystallization temperature range cannot be sufficiently expanded during finish annealing.

[0136] Furthermore, the reason why the control conditions for the rough rolling reduction rate are relaxed 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 even before rough rolling is greater 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 rate is reduced. For these reasons, it is thought that the control conditions for the rough rolling reduction rate are relaxed.

[0137] When Nb group elements are present, if the reduction ratio during rough rolling is less than 82%, similar to when Nb group elements are not present, the introduction of dislocations due to the rolling process will decrease, and the number of precipitation sites that can be precipitated by the process will decrease. As a result, the particle size of the precipitate will increase, and the Wp value will decrease. When Nb group elements are present, the upper limit of the reduction ratio during rough rolling is preferably 93%.

[0138] Furthermore, the reason why the control conditions for rough rolling temperature are relaxed 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 even before rough rolling is greater 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 these reasons, it is thought that the control conditions for rough rolling temperature are relaxed.

[0139] 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. As a result, the critical precipitation radius of the precipitates re-precipitationd during hot rolling becomes larger. Therefore, the size difference between the relatively coarse precipitates (undissolved precipitates) that have precipitated since the slab heating stage becomes smaller, and the Wp value decreases. When Nb group elements are present, the upper limit of the rough rolling temperature is preferably 1065°C, and more preferably 1040°C.

[0140] Furthermore, although the reason why Nb group elements promote the fine precipitation of precipitates is not clear, it is thought to be as follows.

[0141] 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.

[0142] 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.

[0143] In the hot rolling process, when the slab heating conditions satisfy the above conditions and the rough rolling also satisfies the above conditions, the size and distribution of precipitates are preferably controlled. As a result, after the hot-rolled sheet annealing process, the particle size-number density distribution of precipitates is controlled within the above range.

[0144] 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.

[0145] Furthermore, the conditions for finish rolling in the hot rolling process are not particularly limited; normal hot rolling conditions can be used.

[0146] (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.

[0147] The annealing conditions in the hot-rolled sheet annealing process are not particularly limited, and ordinary hot-rolled sheet annealing conditions may be used. In this embodiment, based on the precipitation morphology of precipitates contained in the hot-rolled steel sheet after the hot-rolling process, the precipitates contained in the hot-rolled annealed steel sheet after the hot-rolled sheet annealing process are controlled to have a particle size-number density distribution within the range described above.

[0148] For example, 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, then a second-stage annealing is performed 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. The steel sheet is preferably held for 20 seconds or more during the second-stage annealing. The cooling rate after the second-stage annealing is preferably an average of 5°C / second or more.

[0149] As described above, in hot-rolled and annealed steel sheets manufactured by comprehensively controlling the above conditions in the casting process, hot-rolling process, and hot-rolled sheet annealing process, the size and distribution of precipitates are preferably controlled, and the particle size-number density distribution of precipitates is controlled within the above range.

[0150] Specifically, in this embodiment, the particle size-number density distribution of precipitates with an equivalent circular diameter D of 50 to 1000 nm in the hot-rolled annealed steel sheet after the hot-rolled sheet annealing process is as follows: Let Dp be the most frequent diameter in nm. Let f(Dp) be the number density of the most mode, in units of particles / g. When the full width at half maximum of the most mode is denoted as Wp in units of nm, Dp is 50~350nm, f(Dp) is 1,000,000 particles / g or more. Wp / Dp is 0.75~2.25. To satisfy these conditions, the casting process, the hot rolling process, and the hot-rolled sheet annealing process are used to comprehensively control the above conditions.

[0151] (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 subjecting it to multiple cold rolling processes (two or more times) via one cold rolling or annealing (intermediate annealing) to a total cold rolling rate of 80-95%, thereby obtaining a cold-rolled steel sheet with a thickness of, for example, 0.10-0.50 mm.

[0152] (Decarburization annealing process) The decarburization annealing process involves performing decarburization annealing (for example, at 700-900°C for 1-3 minutes) on the cold-rolled steel sheet obtained in the cold-rolling process to obtain a decarburized annealed steel sheet in which primary recrystallization has occurred. By performing decarburization annealing on the cold-rolled steel sheet, carbon contained in the cold-rolled steel sheet is removed. Decarburization annealing is preferably performed in a humid atmosphere in order to remove the carbon contained in the cold-rolled steel sheet.

[0153] (nitriding treatment) Nitriding is performed to adjust the strength of the inhibitor during secondary recrystallization. In nitriding, the nitrogen content of the steel sheet can be increased to approximately 40-300 ppm at any time between the start of the decarburization annealing described above and the start of secondary recrystallization during the finish annealing described later. Examples of nitriding treatments include annealing the steel sheet in an atmosphere containing a gas with nitriding ability such as ammonia, or finishing annealing a decarburized annealed steel sheet coated with an annealing separator containing a powder with nitriding ability such as MnN.

[0154] (Annealing and separating agent application process) The annealing separation agent application process involves applying an annealing separation agent to a decarburized annealed steel sheet. Examples of annealing separation agents that can be used include those primarily composed of MgO or alumina.

[0155] After applying the annealing release agent, the decarburized annealed steel sheet is wound into a coil and then finished annealed in the next finish annealing process.

[0156] (Finishing annealing process) The finish annealing process involves applying a finish annealing to a decarburized annealed 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.

[0157] In this embodiment, the hot-rolled and annealed steel sheet is controlled so that fine inhibitors and coarse inhibitors coexist, thereby expanding the secondary recrystallization temperature range during finish annealing, resulting in a level of {110} not previously seen. <001> Preferential growth of azimuthal particles occurs, resulting in a dramatic increase in magnetic flux density.

[0158] In addition, by performing "slow heating switching" so that the heating rate becomes slow in the temperature range corresponding to the inhibitor form described above, the effect of slow heating can be favorably enjoyed. <001> Preferential growth of orientation grains occurs more favorably, and the selectivity of crystal orientation is favorably improved.

[0159] Specifically, in the finish annealing process, The decarburized annealed steel sheet, to which the annealing separating agent has been applied after the annealing separating agent application process, is heated. During heating, switch to slow heating within the temperature range from 800°C to the finish annealing temperature. The switching temperature for slow heating described above is set to be between 880°C and 1010°C. The above finish annealing temperature shall be between 1150°C and 1250°C. The average heating rate from 800°C to the switching temperature shall be between 12°C / hour and 50°C / hour, and Under the condition that the average heating rate from the switching temperature to the finish annealing temperature is 3°C / hour or more and less than 12°C / hour, The grain-oriented electrical steel sheet (finish-annealed steel sheet) can be obtained by performing finish annealing.

[0160] As mentioned above, {110} <001> From the viewpoint of preferential growth of orientation grains, a lower switching temperature and a slower heating rate are preferable. However, from the viewpoint of industrial productivity, a higher switching temperature and a faster heating rate are preferable. Therefore, {110} <001> Depending on the priority of orientation grain growth or industrial productivity, the slow heating switching may be controlled as follows:

[0161] For example, in the finish annealing process, The above switching temperature is set to be greater than 950°C and less than 1010°C, and, The average heating rate from the switching temperature to the finish annealing temperature may be set to 3°C / hour or more and less than 8°C / hour.

[0162] Alternatively, in the finish annealing process, The above switching temperature is set to be greater than 880°C and less than or equal to 950°C, and, The average heating rate from the switching temperature to the finish annealing temperature may be 8°C / hour or more and less than 12°C / hour. Preferably, the switching temperature is between 880°C and 900°C.

[0163] Alternatively, in the finish annealing process, The above switching temperature is set to be greater than 880°C and less than or equal to 950°C, and, The average heating rate from the switching temperature to the finish annealing temperature may be 3°C / hour or more and less than 8°C / hour. Preferably, the switching temperature is between 880°C and 900°C.

[0164] In this embodiment, the average heating rate refers to the value obtained by dividing the temperature range to be heated by the time required for heating.

[0165] The finish annealing time should be between 10 and 40 hours. This annealing (purification annealing) process causes a relatively large change in chemical composition (a decrease in content), and in some cases, the content of certain elements may be reduced to a level undetectable by general analytical methods (less than 1 ppm). In addition, abnormal grain growth of secondary recrystallized grains occurs during the finish annealing process, and after the finish annealing, 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.

[0166] 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. If the above-mentioned hot-rolled annealed steel sheet is used and the above-mentioned finish annealing conditions are met, and the finish annealing conditions disclosed in Patent Documents 9 to 11 are applied, it is even more preferable to expand the secondary recrystallization temperature range.

[0167] The manufacturing method for grain-oriented electrical steel sheets according to this embodiment may include an insulating coating formation step and a magnetic domain control step as needed. Note that the insulating coating formation step and the magnetic domain control step use the {110} crystal orientation. <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.

[0168] (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.

[0169] As described above, in the manufacturing method of grain-oriented electrical 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 each subsequent manufacturing condition. Furthermore, during finish annealing, a "slow heating switch" is performed so that the heating rate is slow in a temperature range corresponding to the inhibitor morphology, {110} <001> The preferential growth of orientation grains is further favorably achieved, the selectivity of crystal orientation is favorably improved, and the magnetic flux density of the grain-oriented electrical steel sheet is improved.

[0170] 6. Grain-oriented electrical steel sheet obtained by the manufacturing method according to this embodiment A brief description of the grain-oriented electrical steel sheet obtained by the manufacturing method according to this embodiment will be provided.

[0171] In this embodiment, the hot-rolled and annealed steel sheet is controlled so that relatively fine precipitates and relatively coarse precipitates coexist in a desirable size and distribution, and slow heating switching is preferred during finish annealing. As a result, the grain-oriented electrical steel sheet obtained by the manufacturing method according to this embodiment exhibits preferential growth of Goss-oriented grains, and the magnetic flux density is preferably increased. Furthermore, since the grain-oriented electrical steel sheet obtained by the manufacturing method according to this embodiment does not suffer deterioration of other properties due to the increased magnetic flux density, it can be used for the same applications as conventional steel sheets.

[0172] The grain-oriented electrical steel sheet obtained by the manufacturing method according to this embodiment contains, in terms of mass fraction, Si (silicon): 2.0 to 7.0% as a basic element (major alloying element).

[0173] 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%.

[0174] 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.

[0175] 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.

[0176] 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.

[0177] The grain-oriented electrical steel sheet obtained by the manufacturing method 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.

[0178] 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.

[0179] 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]

[0180] 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.

[0181] Hot-rolled and annealed steel sheets were manufactured using slabs with the chemical compositions shown in Tables 1 and 2, and subsequently grain-oriented electrical steel sheets were manufactured. The chemical compositions were measured using the method described above. In Tables 1 and 2, "-" indicates that the content was not controlled or measured during manufacturing.

[0182] Furthermore, the hot-rolled annealed steel sheets described above were manufactured based on the manufacturing conditions shown in Tables 3 to 15. 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.

[0183] Although not shown in the table, when the total content of Nb group elements is less than 0.0030 mass% or greater than 0.030 mass%, setting the slab soaking temperature to more than 1040°C but less than 1100°C and the slab soaking time to more than 2 hours will cause 14 to 25 volume% of precipitates to be dissolved, based on the precipitates contained in the slab after the casting process. Furthermore, when the total content of Nb group elements is between 0.0030 and 0.030 mass%, setting the slab soaking temperature to more than 1030°C but less than 1180°C and the slab soaking time to more than 70 minutes will cause 12 to 85 volume% of precipitates to be dissolved, based on the precipitates contained in the slab after the casting process.

[0184] In the hot-rolled sheet annealing process, the hot-rolled steel sheet was annealed after the hot-rolling process. In all examples except No. 192, the hot-rolled steel sheet was heated and recrystallized in a first-stage annealing at a temperature of 1100°C, followed by a second-stage annealing at a lower temperature of 900°C, after which the steel sheet was cooled. The heating rate to the first-stage annealing temperature was set to an average of 5°C / second or more, the holding time during the second-stage annealing was set to 20 seconds or more, and the cooling rate after the second-stage annealing was set to an average of 20°C / second. In example No. 192, the same annealing conditions as above were used, but the second-stage annealing was omitted.

[0185] The precipitation morphology of precipitates was investigated using the manufactured hot-rolled annealed steel sheets based on the method described above. Tables 3 to 15 show the precipitation morphology of precipitates with an equivalent circle diameter D of 50 to 1000 nm. In the tables, Dp represents the mode diameter, f(Dp) represents the number density of the mode diameter, and Wp represents the full width at half maximum of the mode diameter.

[0186] Furthermore, the manufactured hot-rolled and annealed steel sheets were subjected to cold rolling and decarburization annealing. Specifically, a single cold rolling or multiple cold rolling processes with intermediate annealing in between were performed to obtain a cold-rolled steel sheet with a final thickness of 0.22 mm. This cold-rolled steel sheet was subjected to decarburization annealing in an atmosphere of 75% hydrogen:25% nitrogen, held at 850°C for 180 seconds. After decarburization annealing, the steel sheet was subjected to nitriding treatment (nitriding annealing) in a mixed atmosphere of hydrogen-nitrogen-ammonia, held at 750°C for 30 seconds.

[0187] Furthermore, an annealing release agent mainly composed of MgO was applied to the steel plate, and finish annealing was performed. This finish annealing was carried out under the conditions shown in Tables 16 to 28. In the tables, "Heating Rate A" represents the average heating rate from 800°C to the slow heating transition temperature, and "Heating Rate B" represents the average heating rate from the slow heating transition temperature to the finish annealing temperature. In the final stage of finish annealing, the steel plate was held in a hydrogen atmosphere (purification annealing) and allowed to cool naturally.

[0188] 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.

[0189] 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.

[0190] The obtained grain-oriented electrical steel sheets were evaluated for various properties. The evaluation results are shown in Tables 16-28.

[0191] (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.

[0192] 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.

[0193] Of the Nos. 1 to 195, all of the present invention examples showed excellent magnetic flux density as grain-oriented electrical steel sheets. On the other hand, among the Nos. 1 to 195, the comparative examples did not obtain a desirable magnetic flux density as grain-oriented electrical steel sheets.

[0194] [Table 1]

[0195] Table 2

[0196] Table 3

[0197] Table 4

[0198] Table 5

[0199] Table 6

[0200] Table 7

[0201] Table 8

[0202] Table 9

[0203] Table 10

[0204] Table 11

[0205] Table 12

[0206] Table 13

[0207] Table 14

[0208] Table 15

[0209] Table 16

[0210] Table 17

[0211] Table 18

[0212] Table 19

[0213] Table 20

[0214] Table 21

[0215] Table 22

[0216] [Table 23]

[0217] [Table 24]

[0218] [Table 25]

[0219] [Table 26]

[0220] [Table 27]

[0221] [Table 28] [Industrial applicability]

[0222] According to the above aspects of the present invention, it is possible to provide a method for manufacturing grain-oriented electrical steel sheets that can increase magnetic flux density, and therefore has high industrial applicability.

Claims

1. A method for manufacturing grain-oriented electrical steel sheets, wherein the manufacturing method comprises a casting step, a hot rolling step, a hot-rolled sheet annealing step, a cold rolling step, a decarburization annealing step, an annealing release agent application step, and a finish annealing step. In the aforementioned casting process, In mass percent, C: 0.0010 to 0.10%, 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: 0.0040-0.0120%, 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 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%, 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 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. The precipitate, which is the residue obtained by electrolytic extraction of the hot-rolled annealed steel sheet after the hot-rolled sheet annealing process, had its particle size-number density distribution measured based on a calibration curve created from six types of standard particles with particle sizes of 29 nm, 48 nm, 100 nm, 200 nm, 300 nm, and 500 nm. Let Dp be the most frequent diameter in nm. Let the number density of the most mode diameter be f(Dp) in units of particles / g. When the full width at half maximum of the most frequent diameter is denoted as Wp in units of nm, Dp is 50-350 nm, f(Dp) is 1,000,000 particles / g or more. Wp / Dp is 0.75 to 2.

25. Satisfying the conditions, In the cold rolling process, the hot-rolled and annealed steel sheet after the hot-rolled sheet annealing process is cold-rolled to form a cold-rolled steel sheet. In the decarburization annealing process, the cold-rolled steel sheet after the cold-rolling process is decarburized and annealed to obtain a decarburized and annealed steel sheet. In the annealing separation agent application step, the annealing separation agent is applied to the decarburized annealed steel sheet after the decarburization annealing step and dried. In the aforementioned finish annealing process, The decarburized annealed steel sheet to which the annealing separating agent has been applied after the annealing separating agent application step is heated. During the aforementioned heating process, the process is switched to slow heating within a temperature range from 800°C to the finish annealing temperature. The switching temperature to the aforementioned slow heating is set to be greater than 880°C and less than 1010°C. The finish annealing temperature is set to 1150°C or higher and 1250°C or lower. The average heating rate from 800°C to the aforementioned switching temperature shall be 12°C / hour or more and 50°C / hour or less, Under the condition that the average heating rate from the switching temperature to the finish annealing temperature is 3°C / hour or more and less than 12°C / hour, A grain-oriented electrical steel sheet is obtained by performing finish annealing. A method for manufacturing grain-oriented electrical steel sheets, characterized by the following features.

2. The aforementioned chemical composition contains a total of 0.0030 to 0.030% by mass of at least one element selected from the group consisting of Nb, V, Mo, Ta, and W. The method for manufacturing grain-oriented electrical steel sheets according to claim 1.

3. The switching temperature is set to be greater than 950°C and less than 1010°C, and The average heating rate from the switching temperature to the finish annealing temperature is set to be 3°C / hour or more and less than 8°C / hour. A method for manufacturing grain-oriented electrical steel sheets according to claim 1 or 2.

4. The switching temperature is set to be greater than 880°C and less than or equal to 950°C, and The average heating rate from the switching temperature to the finish annealing temperature is set to be 8°C / hour or more and less than 12°C / hour. A method for manufacturing grain-oriented electrical steel sheets according to claim 1 or 2.

5. The switching temperature is set to be greater than 880°C and less than or equal to 950°C, and The average heating rate from the switching temperature to the finish annealing temperature is set to be 3°C / hour or more and less than 8°C / hour. A method for manufacturing grain-oriented electrical steel sheets according to claim 1 or 2.

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