Hot-rolled annealed steel sheet for grain-oriented electrical steel sheet

Abstract

This hot-rolled annealed steel sheet for a grain-oriented electrical steel sheet has, regarding the particle diameter-number density distribution of precipitates having an equivalent circle diameter D of 50-1000 nm, a mode diameter Dp of 100-300 nm, a number density f(Dp) of the mode diameter of 1,000,000 / g or more, and a value Wp / Dp obtained by dividing the half-value width Wp of the mode diameter by the mode diameter Dp of 1.0-2.0, and the average particle diameter of the hot-rolled annealed steel sheet is 20.0-21.5 μm.

Description

Hot-rolled and annealed steel sheets for grain-oriented electrical steel sheets

[0001] The present invention relates to a hot-rolled and annealed steel sheet for grain-oriented electrical steel sheet. This application claims priority to Japanese Patent Application No. 2024-034126, filed on March 6, 2024, the contents of which are incorporated herein by reference.

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

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

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

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

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

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

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

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

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

[0011] One aspect of the present invention has been made in view of the above-mentioned problems, and in light of the current situation where there is a demand for increasing the magnetic flux density of grain-oriented electrical steel sheets, an object of one aspect of the present invention is to provide a hot-rolled annealed steel sheet for grain-oriented electrical steel sheets that can increase the magnetic flux density.

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

[0013] (1) A hot-rolled annealed steel sheet for grain-oriented electrical steel sheet according to one aspect of the present invention has, in mass%, C: 0.0010 to 0.10%, Si: 2.0 to 7.0%, Mn: 0.050 to 1.0%, S: 0 to 0.0350%, Se: 0 to 0.0350%, 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%, The hot-rolled annealed steel sheet has a chemical composition containing Nb+V+Mo+Ta+W total content: 0.0030 to 0.030%, Cu: 0 to 0.40%, Bi: 0 to 0.010%, B: 0 to 0.080%, P: 0 to 0.50%, Ti: 0 to 0.0150%, Sn: 0 to 0.10%, Sb: 0 to 0.10%, Cr: 0 to 0.30%, Ni: 0 to 1.0%, with the balance being Fe and impurities, and the particle size-number density distribution of precipitates having a circle equivalent diameter D of 50 to 1000 nm among precipitates that are residues obtained by electrolytic extraction from the hot-rolled annealed steel sheet is as follows: the most frequent diameter is Dp in the unit of nm, and the number density of the most frequent diameter is f(Dp) in the unit of particles / g, When the half width of the mode diameter is Wp in the unit of nm, Dp is 100 to 300 nm, f(Dp) is 1,000,000 particles / g or more, and Wp / Dp is 1.0 to 2.0, and the average grain size of the hot-rolled annealed steel sheet is 20.0 to 21.5 μm.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

[0036] In this embodiment, the morphology of the above-mentioned precipitates is defined based on a hot-rolled annealed steel sheet (a steel sheet immediately before cold rolling).

[0037] Hereinafter, the hot-rolled annealed steel sheet for grain-oriented electrical steel sheet according to this embodiment will be described in detail.

[0038] The hot rolled annealed steel sheet according to this embodiment has, in mass%, C: 0.0010 to 0.10%, Si: 2.0 to 7.0%, Mn: 0.050 to 1.0%, S: 0 to 0.0350%, Se: 0 to 0.0350%, S+Se total content: 0.0030 to 0.0350%, Al: 0.010 to 0.0650%, N: 0.0040 to 0.0120%, Nb: 0 to 0.030%, V: 0 to 0.030%, Mo: 0 to 0.030%, Ta: 0 to 0.030%, W: 0 to 0.030%, Nb+V+Mo+Ta+W total content: 0.0030 to 0.030%, A steel sheet having a chemical composition containing Cu: 0 to 0.40%, Bi: 0 to 0.010%, B: 0 to 0.080%, P: 0 to 0.50%, Ti: 0 to 0.0150%, Sn: 0 to 0.10%, Sb: 0 to 0.10%, Cr: 0 to 0.30%, Ni: 0 to 1.0%, with the balance being Fe and impurities, wherein, among precipitates that are residues obtained by electrolytic extraction from a hot-rolled annealed steel sheet, the particle size-number density distribution of precipitates having an equivalent circle diameter D of 50 to 1000 nm is such that, when the most frequent diameter is Dp in the unit of nm, the number density of the most frequent diameter is f(Dp) in the unit of particles / g, and the half width of the most frequent diameter is Wp in the unit of nm, Dp is 100 to 300 nm, f(Dp) is 1,000,000 particles / g or more, Wp / Dp is 1.0 to 2.0, and the average grain size of the hot-rolled annealed steel sheet is 20.0 to 21.5 μm.

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

[0040] It should be noted that although the chemical composition of hot-rolled and annealed steel sheets, which are intermediate products, is rarely described in publicly known literature on grain-oriented electrical steel sheets, the chemical composition of hot-rolled and annealed steel sheets can be considered to be basically the same as the chemical composition of slabs disclosed in publicly known literature, since the steel composition hardly changes during the process from slab to decarburization annealing.

[0041] The hot-rolled and annealed steel sheet according to this embodiment contains, as a chemical composition, basic elements, optional elements as needed, and the balance being Fe and impurities.

[0042] The hot-rolled annealed steel sheet according to this embodiment contains, as basic elements (major alloying elements), the following mass fractions: C: 0.0010 to 0.10%, Si: 2.0 to 7.0%, Mn: 0.050 to 1.0%, S+Se total content: 0.0030 to 0.0350%, Al: 0.010 to 0.0650%, N: 0.0040 to 0.0120%, and Nb+V+Mo+Ta+W total content: 0.0030 to 0.030%.

[0043] C: 0.0010 to 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 adversely affects the magnetic properties. Therefore, the C content of the hot-rolled annealed steel sheet may be 0.0010 to 0.10%. The preferred upper limit of the C content is 0.0850%, or 0.0750%. Note that C is purified in the decarburization annealing process and the finish annealing process described below, and after the finish annealing process, the C content is 0.0050% or less. When C is contained, taking into consideration productivity in industrial production, the C content may be more than 0% or may be 0.0010% or more.

[0044] Si: 2.0 to 7.0% Silicon (Si) increases the electrical resistance of grain-oriented electrical steel sheets and reduces iron loss. If the Si content is less than 2.0%, austenite transformation occurs during finish annealing, damaging the crystal orientation of the grain-oriented electrical steel sheet. On the other hand, if the Si content exceeds 7.0%, cold workability decreases, making cracks more likely to occur during cold rolling. Therefore, the Si content of hot-rolled annealed steel sheets should be 2.0 to 7.0%. A preferred lower limit of the Si content is 2.50%, more preferably 3.0%. A preferred upper limit of the Si content is 4.50%, more preferably 4.0%.

[0045] Mn: 0.050 to 1.0% Manganese (Mn) combines with S and Se to precipitate as MnS or MnSe, functioning as an inhibitor. To favorably control the morphology of these inhibitors (precipitates), the Mn content of the hot-rolled annealed steel sheet should be 0.050 to 1.0%. If the Mn content is below 0.050%, the amount of precipitated MnS and MnSe, which function as inhibitors, is insufficient, thereby inhibiting the appropriate progress of secondary recrystallization. Furthermore, if the Mn content exceeds 1.0%, the amount of precipitated MnS and MnSe, which function as inhibitors, is excessive, thereby inhibiting the appropriate progress of secondary recrystallization. In this embodiment, part of the inhibitor function may be performed by carbides, nitrides, carbonitrides, or the like of Nb group elements. In this case, the amount of precipitated MnS and MnSe, which act as inhibitors, may be controlled to be small. Therefore, the upper limit of the Mn content is preferably 0.50%, and more preferably 0.20%.

[0046] S: 0 to 0.0350% Se: 0 to 0.0350% Total content of S + Se: 0.0030 to 0.0350% Sulfur (S) and selenium (Se) combine with Mn to precipitate as MnS or MnSe, functioning as inhibitors. To favorably control the form of these inhibitors (precipitates), the S content of the hot-rolled annealed steel sheet should be 0 to 0.0350%, the Se content should be 0 to 0.0350%, and the total content of S + Se should be 0.0030 to 0.0350%. A total content of S and Se of 0.0030 to 0.0350% is preferable because secondary recrystallization is stabilized. In this embodiment, part of the inhibitor function may be performed by carbides, nitrides, carbonitrides, or the like of Nb group elements. In this case, the amount of precipitation of inhibitors MnS and MnSe may be controlled to be small. Therefore, the upper limit of the total content of S and Se is preferably 0.0250%, more preferably 0.010%. If S and Se remain in the steel after final annealing, they may form compounds that deteriorate the iron loss. Therefore, it is preferable to reduce the content of S and Se by purifying them during final annealing to remove them from the steel.

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

[0048] Al: 0.010 to 0.0650% Aluminum (Al) combines with N to precipitate as AlN or (Al,Si)N, functioning as an inhibitor. To favorably control the morphology of these inhibitors (precipitates), the Al content of the hot-rolled annealed steel sheet should be 0.010 to 0.0650%. If the Al content is 0.010% or more, AlN or (Al,Si)N precipitates in a favorable form by nitriding in the low-temperature slab heating process, stabilizing secondary recrystallization, particularly in the high-temperature range. If the Al content is below 0.010%, the amount of AlN or (Al,Si)N precipitated, which functions as an inhibitor, is insufficient, thereby inhibiting the proper progress of secondary recrystallization. On the other hand, if the Al content exceeds 0.0650%, the amount of AlN or (Al,Si)N precipitated, which functions as an inhibitor, becomes excessive, thereby inhibiting the proper progress of secondary recrystallization. The lower limit of the Al content is preferably 0.020%, more preferably 0.0250%. From the viewpoint of the stability of secondary recrystallization, the upper limit of the Al content is preferably 0.040%, more preferably 0.030%.

[0049] N: 0.0040 to 0.0120% Nitrogen (N) combines with Al and precipitates as AlN or (Al, Si)N, functioning as an inhibitor. The N content of hot-rolled annealed steel sheet may be 0.0040 to 0.0120%. Note that in the low-temperature slab heating process, N may be added to the steel by nitriding during the manufacturing process. If the N content exceeds 0.0120%, blisters, a type of defect, are more likely to occur in the steel sheet. The upper limit of the N content is preferably 0.010%, more preferably 0.0090%. N is purified in the finish annealing process, and after the finish annealing process, the N content is 0.0050% or less.

[0050] Total content of Nb + V + Mo + Ta + W: 0.0030 to 0.030% Nb: 0 to 0.030% V: 0 to 0.030% Mo: 0 to 0.030% Ta: 0 to 0.030% W: 0 to 0.030% Niobium (Nb), vanadium (V), molybdenum (Mo), tantalum (Ta), and tungsten (W) precipitate as carbides, nitrides, or carbonitrides, which act as auxiliary inhibitors, and preferably function as inhibitors. Specifically, they preferably expand the temperature range in which secondary recrystallization progresses. Therefore, the Nb content is set to 0 to 0.030%, the V content to 0 to 0.030%, the Mo content to 0 to 0.030%, the Ta content to 0 to 0.030%, the W content to 0 to 0.030%, and the total content of Nb + V + Mo + Ta + W to 0.0030 to 0.030%. The lower limit of the content of Nb, V, Mo, Ta, and / or W is preferably 0.0040%, more preferably 0.0050%. The upper limit of the content of Nb, V, Mo, Ta, and / or W is preferably 0.020%, more preferably 0.010%.

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

[0052] The hot-rolled annealed steel sheet according to this embodiment contains, as the Nb group element, one or more elements selected from the Nb group elements consisting of Nb, V, Mo, Ta, and W in a total amount of 0.0030 to 0.030 mass%.

[0053] When the precipitates of Nb group elements are utilized as inhibitors, when the total content of Nb group elements in the hot-rolled annealed steel sheet is 0.030% or less (preferably 0.0030% or more and 0.030% or less), the morphology of the precipitates of Nb group elements is favorably controlled, the secondary recrystallization proceeding temperature range is favorably expanded, and as a result, Goss-oriented grains grow favorably, and the magnetic flux density of the finally obtained grain-oriented electrical steel sheet is favorably increased.

[0054] The reason why precipitates of Nb group elements preferably function as inhibitors is not clear, but is thought to be as follows. It is thought that carbides, nitrides, or carbonitrides of Nb group elements precipitate non-equilibrium during cooling from high temperatures and act as precipitation nuclei for the subsequent precipitation of MnS and AlN. Therefore, when an Nb group element is contained, the number of precipitation sites for MnS and AlN increases compared to when an Nb group element is not contained, and as a result, it is thought that MnS and AlN are more likely to form as fine precipitates. In the hot-rolled annealed steel sheet according to this embodiment, the coexistence of fine inhibitors and coarse inhibitors expands the secondary recrystallization progression temperature range, and it is thought that precipitates of Nb group elements particularly act effectively to expand the secondary recrystallization progression temperature range toward the lower temperature side.

[0055] The total content of Nb group elements is preferably 0.0040% or more, more preferably 0.0050% or more. Furthermore, the total content of Nb group elements is preferably 0.020% or less, more preferably 0.010% or less. If the total content of Nb group elements is less than 0.0030%, the precipitates of Nb group elements that act as the above-mentioned precipitation nuclei are insufficient, making it difficult to refine MnS and AlN. On the other hand, if the total content of Nb group elements exceeds 0.030%, the precipitation temperature range of the Nb group element precipitates becomes high, making the Nb group element precipitates coarse and prone to low density. Furthermore, the difference between the precipitation temperature range of the Nb group element precipitates and the precipitation temperature range of MnS and AlN becomes large, making it difficult for the Nb group element precipitates to effectively act as precipitation nuclei for refining MnS and AlN.

[0056] Here, "the total content of Nb group elements is 0.0030 to 0.030%" means that the hot-rolled annealed steel sheet may contain, as a chemical composition, at least one element selected from the group consisting of Nb, V, Mo, Ta, and W, with the content being 0.0030 to 0.030%. Alternatively, it means that the hot-rolled annealed steel sheet may contain at least two elements selected from the group consisting of Nb, V, Mo, Ta, and W, with the total content being 0.0030 to 0.030%.

[0057] The hot-rolled annealed steel sheet according to this embodiment may contain impurities as a chemical composition. The term "impurities" refers to elements that are mixed in from raw materials such as ore or scrap, or from the manufacturing environment, during industrial steel production. The upper limit of the total impurity content may be, for example, 5%.

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

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

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

[0061] The chemical composition of the hot-rolled annealed steel sheet according to this embodiment may be measured by a general steel analysis method. For example, the chemical composition of the hot-rolled annealed steel sheet may be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). Specifically, the chemical composition is determined by measuring a 35 mm square test piece taken from the hot-rolled annealed steel sheet using ICP-AES under conditions based on a previously prepared calibration curve. C and S may be measured using a combustion-infrared absorption method, and N may be measured using an inert gas fusion-thermal conductivity method.

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

[0063] The type of precipitates (inhibitors) contained in the hot-rolled annealed steel sheet according to this embodiment is not particularly limited as long as the precipitation form of the precipitates is controlled. The precipitates may be precipitates formed from elements contained in the hot-rolled annealed steel sheet. For example, sulfides and selenides may be used as Mn-based precipitates (Mn-containing precipitates), nitrides may be used as Al-based precipitates (Al-containing precipitates), and carbides, nitrides, and carbonitrides may be used as Nb-group element-containing precipitates. In addition to these inhibitors, compounds of optional elements such as Bi and B, and complex compounds with the above elements may also be included.

[0064] Although the type (composition) of precipitates may contribute to some extent to the above-described effects obtained in this embodiment, the inventors have found that the above-described effects obtained in this embodiment are mainly due to the size and distribution of the precipitates contained in the hot-rolled and annealed steel sheet. Therefore, the size and distribution of the precipitates are specified in the hot-rolled and annealed steel sheet according to this embodiment.

[0065] The precipitates to be controlled in the hot-rolled annealed steel sheet according to this embodiment are precipitates having an equivalent circle diameter D of 50 to 1000 nm. The "equivalent circle diameter" means the diameter of a circle when the area of ​​a precipitate is converted into a circle having the same area. This equivalent circle diameter is equivalent to the equivalent sphere diameter.

[0066] Precipitates contained in hot-rolled annealed steel sheets with an equivalent circle diameter D of less than 50 nm currently have little effect in expanding the secondary recrystallization progression temperature range. The reason for this is unclear, but it is thought that precipitates with an equivalent circle diameter D of less than 50 nm at the time of hot-rolled annealed steel sheets change or disappear in subsequent processes, making them less likely to function as inhibitors during finish annealing. Therefore, in the hot-rolled annealed steel sheets according to this embodiment, the size and distribution of precipitates with an equivalent circle diameter D of 50 nm or more are controlled. It is expected that, by considering processes including those after the hot-rolled sheet annealing process, precipitates with an equivalent circle diameter D of less than 50 nm will function as inhibitors.

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

[0068] In the hot-rolled annealed steel sheet according to this embodiment, the particle size-number density distribution of precipitates having an equivalent circle diameter D of 50 to 1000 nm among the precipitates that are residues after electrolytic extraction of the hot-rolled annealed steel sheet satisfies the following, where Dp is the most frequent diameter in the unit of nm, f(Dp) is the number density of the most frequent diameter in the unit of particles / g, and Wp is the half-width of the most frequent diameter in the unit of nm: Dp is 100 to 300 nm, f(Dp) is 1000000 particles / g or more, and Wp / Dp is 1.0 to 2.0.

[0069] 1 shows a schematic diagram of particle size-number density distribution of precipitates having a circle-equivalent diameter D of 50 to 1000 nm, in which Dp, f(Dp), and Wp are exemplified.

[0070] If Dp exceeds 300 nm, the amount of fine precipitates required to expand the secondary recrystallization temperature range will be reduced, which is inappropriate. The upper limit of Dp is preferably 275 nm, more preferably 250 nm. On the other hand, if Dp is 100 nm or more, the secondary recrystallization proceeding temperature range is preferably expanded. The lower limit of Dp is preferably 125 nm, more preferably 150 nm.

[0071] When f(Dp) is 1,000,000 particles / g or more, the precipitates necessary for the occurrence of secondary recrystallization are sufficiently precipitated, and the pinning effect is preferably obtained. On the other hand, the upper limit of f(Dp) is not particularly limited, but may be set to, for example, 50,000,000 particles / g.

[0072] When Wp / Dp is 1.0 or more, the ratio of Wp to Dp becomes a preferable value, and the secondary recrystallization temperature range is preferably expanded. The lower limit of Wp / Dp is preferably 1.2. On the other hand, when Wp / Dp is 2.0 or less, the ratio of Wp to Dp becomes a preferable value, and primary recrystallized grains grow uniformly during normal grain growth, resulting in a preferable grain structure before secondary recrystallization. The upper limit of Wp / Dp is preferably 1.75.

[0073] The particle size-number density distribution of precipitates having a circle-equivalent diameter D of 50 to 1000 nm can be determined as follows.

[0074] For example, this method may be performed using the method described in Japanese Patent No. 6,572,598. First, a precipitate is electrolytically extracted from a hot-rolled annealed steel sheet. The electrolytic extraction may be performed under conditions such as constant current electrolytic extraction (500 mA, 2 hours) using an acetylacetone-based electrolyte to which a dispersant such as a surfactant (e.g., sodium dodecyl sulfate with a molecular weight of 288.38 g / mol) has been added in advance. The electrolytic extraction may be performed by electrolysis such that the amount of electrolysis of the hot-rolled annealed steel sheet is 1 g or more.

[0075] The extraction residue (precipitate) is recovered from the electrolytic extraction solution, and the size and distribution of the recovered precipitates are measured by the Field Flow Fractionation (FFF) method. For the measurement method using the FFF method, see Japanese Patent No. 6,572,598 mentioned above.

[0076] Each parameter may be changed depending on the particle size and type to be measured. An example is shown below. A Wyatt Eclipse AF4 device (Wyatt Technology Europe, Germany) may be used as the FFF device. A sodium dodecyl sulfate aqueous solution at a concentration of 300 mg / mL may be used as the dispersion solution for the measurement sample. A cell may have a channel length of 275 mm and an asymmetric diamond-shaped channel spacer with a thickness of 350 μm. A regenerated cellulose ultrafiltration membrane with a molecular weight of 30 kDa may be used as the separation membrane.

[0077] Before adding the extraction residue (precipitate) recovered from the electrolytic extraction solution, it is necessary to create a calibration curve by using standard samples with known particle sizes to correlate the particle size with the time it takes for the particles to be detected.The type and number of standard samples can be selected according to the particle size distribution of the extraction residue to be measured, and for example, polystyrene latex standard particles with a particle size of 29 to 500 nm can be selected.

[0078] The size of the standard particles needs to be directly confirmed in advance using a TEM (Transmission Electron Microscope) or the like. The number of measurements should be 500 or more. The long sides of each standard particle are measured, and the average value is calculated. Furthermore, six particle diameters, for example, 29 nm, 48 nm, 100 nm, 200 nm, 300 nm, and 500 nm, may be used for the standard particles.

[0079] The actual separation conditions are as follows: First, for stabilization before focusing, the eluent effluent (hereinafter referred to as channel flow) of the FFF device is set to 1.0 mL / min, the cross flow to 0.5 mL / min, and the time is 1 minute. Then, for focusing before sample injection, the focus flow is set to 3.0 mL / min and the time is 1 minute. Next, for focusing, the sample is injected at 0.2 mL / min for 2 minutes. The focusing time after sample injection is 1 minute. Then, the flow path is switched, the focus flow is stopped, and the channel flow is set to 1.0 mL / min, and the cross flow is decreased from 0.5 mL / min to 0.05 mL / min over 35 minutes while decreasing the flow rate in direct proportion. A calibration curve can be created by correlating the time from the start of the liquid delivery until particle detection with the average particle size of the standard particles measured in advance. The maximum time for particle detection is 35 minutes, and the injection volume of the sample dispersion liquid is set to 0.1 to 0.4 mL.

[0080] After preparing the calibration curve as described above, the extraction residue (precipitate) recovered from the electrolytic extraction solution is again charged into the apparatus. The apparatus setting parameters can be the same as those described above.

[0081] In this way, the particle size of the nanoparticles contained in the nanoparticle dispersion sample to be measured can be measured.

[0082] Furthermore, the effluent from the FFF device (a solution containing precipitates separated by size) can be subjected to component analysis using a conventional ICP (Inductively Coupled Plasma) mass spectrometer.

[0083] Using particle size distribution data measured by the FFF method, the particle size is divided into 0.5 nm intervals, and the number density is calculated in units / g from the number of precipitates within this particle size interval and the amount of electrolysis in the extraction electrolysis, and a histogram of particle size and number density is then created. Dp, f(Dp), and Wp can then be determined from this histogram.

[0084] In this embodiment, the "mode diameter" corresponds to the particle diameter (particle diameter category) at which the number density value is greatest in the above-mentioned histogram of particle diameters and number densities (particle diameter-number density distribution of precipitates).

[0085] It is preferable to calculate the above Dp, f(Dp), Wp, etc. after smoothing the measurement data obtained by the FFF method. The method for smoothing the measurement data obtained by the FFF method may be, for example, a simple moving average method. The value of f(Dp) may be calculated by regarding the top three digits as significant.

[0086] 3. Average Grain Size Next, the average grain size of the hot-rolled and annealed steel sheet according to this embodiment will be described.

[0087] The average grain size of the hot-rolled annealed steel sheet according to this embodiment is 20.0 to 21.5 μm. In the hot-rolled annealed steel sheet according to this embodiment, relatively fine precipitates and relatively coarse precipitates coexist with appropriate sizes and distributions, and the average grain size of the hot-rolled annealed steel sheet (steel sheet after hot-rolled sheet annealing) is an appropriate grain size, which is achieved by lowering the first-stage annealing temperature and also lowering the second-stage annealing temperature during hot-rolled sheet annealing.

[0088] The average grain size of the hot-rolled annealed steel sheet is preferably 21.5 μm or less, and more preferably 21.0 μm. The lower limit of the average grain size is not particularly limited in terms of the effect of widening the precipitate size distribution. However, in order to reduce the average grain size, it is better to use a higher second-stage annealing temperature, but in that case, fine precipitates will be scarce, and the size distribution of the precipitates will not be sufficiently widened. For this reason, for example, the average grain size of the hot-rolled annealed steel sheet may be 20.0 μm or more.

[0089] The average grain size of the hot-rolled annealed steel sheet according to this embodiment may be determined based on the cutting method of JIS G0551: 2013. For example, an L-section (a cross section normal to the direction perpendicular to the rolling direction) of the hot-rolled annealed steel sheet is photographed using an optical microscope at a magnification of 200 times, and the grain size of the cross-sectional structure is measured along the sheet thickness direction based on the above-mentioned cutting method. This measurement is carried out at least five times at different measurement locations to determine the average grain size.

[0090] 3. Sheet Thickness The sheet 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 and is finally finished into a grain-oriented electrical steel sheet. Therefore, taking into account the manufacturing conditions of general grain-oriented electrical steel sheets, the sheet thickness of the hot-rolled annealed steel sheet may be 1.8 to 3.5 mm. However, the sheet thickness is not limited to this, and any known sheet thickness or a sheet thickness that is practically used may be adopted.

[0091] 4. Manufacturing Method Next, a method for manufacturing a hot-rolled and annealed steel sheet for grain-oriented electrical steel sheet according to one embodiment of the present invention will be described. Note that the method for manufacturing the hot-rolled and annealed steel sheet according to this embodiment is not limited to the method described below. The manufacturing method described below is one example for manufacturing the hot-rolled and annealed steel sheet according to this embodiment.

[0092] Fig. 2 is a flow chart illustrating the manufacturing process of a hot-rolled annealed steel sheet according to this embodiment. Fig. 2 also shows the manufacturing process of a grain-oriented electrical steel sheet using this hot-rolled annealed steel sheet. As shown in Fig. 2, the manufacturing method of a hot-rolled annealed steel sheet according to this embodiment includes a casting process, a hot-rolling process, and a hot-rolled sheet annealing process. The conditions controlled in these processes will be described in detail below.

[0093] 2 , i.e., the cold rolling step, decarburization annealing step, annealing separator application step, and finish annealing step, are manufacturing steps for a grain-oriented electrical steel sheet (finish annealed steel sheet). The effects of the hot-rolled annealed steel sheet according to this embodiment can be confirmed in the grain-oriented electrical steel sheet, which is the final product, and the conditions for controlling these steps will also be described later.

[0094] The method for producing a hot-rolled and annealed steel sheet according to this embodiment includes a casting step, a hot rolling step, and a hot-rolled sheet annealing step, and in the casting step, the steel sheet contains, in mass %, C: 0.0010 to 0.10%, Si: 2.0 to 7.0%, Mn: 0.050 to 1.0%, S: 0 to 0.0350%, Se: 0 to 0.0350%, S+Se total content: 0.0030 to 0.0350%, Al: 0.010 to 0.0650%, N: 0.0040 to 0.0120%, Nb: 0 to 0.030%, V: 0 to 0.030%, Mo: 0 to 0.030%, Ta: 0 to 0.030%, W: 0 to 0.030%, a hot rolling process in which molten steel having a chemical composition containing Nb+V+Mo+Ta+W total content: 0.0030 to 0.030%, Cu: 0 to 0.40%, Bi: 0 to 0.010%, B: 0 to 0.080%, P: 0 to 0.50%, Ti: 0 to 0.0150%, Sn: 0 to 0.10%, Sb: 0 to 0.10%, Cr: 0 to 0.30%, Ni: 0 to 1.0%, with the balance being Fe and impurities, is cast and formed into a slab; and in a hot rolling process, the slab after the casting process is heated, rough 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 heated and subjected to first-stage annealing in a temperature range of 1040 to 1080°C, and second-stage annealing in a lower temperature range of 810 to 880°C, and then cooled at an average cooling rate of 5 to 80°C / second to obtain a hot-rolled annealed steel sheet.

[0095] In the manufacturing method of the hot-rolled annealed steel sheet according to this embodiment, in the hot rolling step, when heating the slab before rough rolling, the soaking temperature of the slab is set to more than 1030°C and less than 1180°C, thereby preferably solutionizing a portion of the precipitates contained in the slab (for example, 12 to 85% by volume of the precipitates based on the precipitates contained in the slab after the casting step), and in order to make this solution state uniform within the slab, the soaking time of the slab is set to more than 70 minutes, and when rough rolling, the rolling temperature is set to 940 to 1070°C and the reduction ratio is set to 82 to 95%.

[0096] In order to control the size and distribution of precipitates contained 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, respectively. In particular, it is important to control the steel composition, slab heating conditions (solution state of precipitates before rough rolling), rough rolling temperature, rough rolling reduction, and hot-rolled sheet annealing conditions, respectively. In addition, in order to control the above-mentioned "solution state of precipitates before rough rolling," it is important to control the steel composition and slab heating conditions, respectively.

[0097] The slab heating may be performed by soaking at a predetermined temperature for a predetermined time without temporarily increasing the heating temperature during the slab heating process. In this case, the soaking temperature of the slab refers to the surface temperature of the slab, and the soaking time of the slab refers to the holding time after the surface temperature of the slab reaches the soaking temperature. Although this is affected by, for example, the steel composition and the heating rate, if the surface temperature of the slab reaches the soaking temperature during heating, the solution state of the precipitates on the surface of the slab is preferably controlled. Furthermore, if the surface temperature of the slab reaches the soaking temperature and is held for the soaking time, the solution state of the precipitates is preferably controlled all the way to the center of the slab.

[0098] Important manufacturing conditions for the method for manufacturing a hot-rolled annealed steel sheet according to this embodiment will be described below. Other manufacturing conditions may be the same as those for conventionally known grain-oriented electrical steel sheets.

[0099] (Casting step) In the casting step, a slab is prepared. As described above, the chemical composition of the slab hardly changes during the steps from slab production to decarburization annealing, and therefore the chemical composition of the slab is set to the chemical composition of the target hot-rolled annealed steel sheet (the chemical composition of the hot-rolled annealed steel sheet described above).

[0100] The chemical composition of the slab affects the "solution state of precipitates before rough rolling" as described above. As will be described in detail later, the chemical composition of the slab needs to satisfy the chemical composition of the hot-rolled annealed steel sheet as described above, and also needs to be controlled in combination with other manufacturing conditions that affect the "solution state of precipitates before rough rolling".

[0101] An example of a method for producing a slab is as follows: Molten steel is produced (smelted). A slab is produced using this molten steel. For example, the slab may be produced by continuous casting. Alternatively, an ingot may be produced using the molten steel, and the ingot may be bloomed to produce the slab. The thickness of the slab is, for example, 150 to 350 mm. The thickness of the slab is preferably 220 to 280 mm. A so-called thin slab having a thickness of 10 to 70 mm may also be used as the slab.

[0102] (Hot Rolling Step) The hot rolling step is a step in which a slab is heated to a predetermined temperature and hot rolled (rough rolling and finish rolling) to obtain a hot-rolled steel sheet.

[0103] For example, in the hot rolling process, the slab after the casting process is heated, subjected to rough rolling, and then subjected to finish rolling to form a hot-rolled steel sheet having a predetermined thickness of 1.8 to 3.5 mm. After the finish rolling is completed, the hot-rolled steel sheet may be coiled at a predetermined temperature.

[0104] In the hot rolling process, when the slab after the casting process is heated, the following conditions should be satisfied.

[0105] When heating the slab before rough rolling, the soaking temperature of the slab is set to more than 1030°C and less than 1180°C, so that a portion of the precipitates contained in the slab are preferably solutionized (for example, 12 to 85 volume% of the precipitates are solutionized based on the precipitates contained in the slab at room temperature after the casting process), and in order to make this solution state uniform within the slab, the slab is heated so that the soaking time of the slab is more than 70 minutes.

[0106] Preferably bringing some of the precipitates contained in the slab into solution before rough rolling is necessary to achieve a favorable final balance between the amounts of relatively coarse precipitates (residual precipitates) that remain precipitated during the slab heating stage and relatively fine precipitates (re-precipitated precipitates) that do not precipitate during the slab heating stage but precipitate after hot rolling.

[0107] Furthermore, the value of Wp, which is a main technical feature of this embodiment, can be increased by controlling the size difference between the relatively coarse precipitates (residual precipitates) that remain precipitated at the slab heating stage and the relatively fine precipitates (re-precipitated precipitates) that do not precipitate at the slab heating stage but precipitate after hot rolling.

[0108] The above "solution state of precipitates before rough rolling" means "solution state of precipitates before rough rolling" in an equilibrium state, not a non-equilibrium state. In a non-equilibrium state, for example, the solution state of precipitates becomes non-uniform near the surface and near the center in the thickness direction. 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.

[0109] For example, in order to bring the solution state of the precipitates closer to an equilibrium state, it is preferable to set the value obtained by subtracting the temperature at the center of the slab from the surface temperature of the slab within a range of more than -10°C and less than 50°C during slab heating and extraction. In particular, if the temperature difference is -10°C or less, the steel sheet surface becomes difficult to elongate, resulting in significant occurrence of defects. Furthermore, if the temperature difference is 50°C or more, the solution state of the precipitates becomes non-uniform in the sheet thickness direction, making it difficult to control the size of the precipitates.

[0110] Although different from the slab heating method of this embodiment, the heating temperature may be temporarily increased during the slab heating process to shorten the soaking time. In this case, it is effective to set the difference between the surface temperature at the maximum temperature reached and the surface temperature at the time of heating and extraction of the slab to 80°C or less. In this case, after the temperature is reduced from the maximum temperature reached, it is preferable to hold the slab in a low-temperature region of the slab heating furnace for at least 20 minutes or more, so that the difference between the surface temperature and the center temperature at the time of extraction from the slab heating furnace is less than 50°C. More preferably, the difference between the surface temperature and the center temperature of the slab should be 0 to 30°C.

[0111] In the conventional technology of performing slab heating at a temperature of 1280°C or less, which is called a low-temperature slab heating process, there was no technical idea of ​​solutionizing only a specific proportion of the precipitates contained in the slab, nor any knowledge that it was necessary to bring the solution of these precipitates close to an equilibrium state. In the manufacturing method of a hot-rolled annealed steel sheet according to this embodiment, the solution state of the precipitates is preferably controlled, and a slab in which the solution of the precipitates is in an equilibrium state is subjected to rough rolling.

[0112] The "solution state of precipitates before rough rolling" is a characteristic that is affected by the steel composition and the hot rolling conditions (slab heating conditions). To control this "solution state of precipitates before rough rolling," it is necessary to control each manufacturing condition in a composite and inseparable manner, taking into consideration the influence of the manufacturing conditions on the "solution state of precipitates." For example, a person skilled in the art can control the material properties, including precipitation behavior, and can control the "solution state" by combining the above conditions, as long as he or she understands that each of the above conditions affects the "solution state."

[0113] For example, as described above, the "solution state of the precipitates before rough rolling" may be controlled by temporarily increasing the heating temperature during the slab heating process and maintaining the temperature for a certain period of time after cooling. However, in the manufacturing method of the hot-rolled annealed steel sheet according to this embodiment, as an example, a method of controlling the "solution state of the precipitates before rough rolling" is shown, in which the heating temperature is not temporarily increased during the slab heating process, but rather the slab is soaked at a predetermined temperature for a predetermined period of time.

[0114] In the hot rolling process, when the slab is heated before rough rolling, the soaking temperature during slab heating may be set to more than 1030°C and less than 1180°C, and the soaking time may be set to more than 70 minutes. In this case, a portion of the precipitates contained in the slab is easily and preferably solutionized (for example, 12 to 85% by volume of the precipitates are easily solutionized, based on the precipitates contained in the slab at room temperature after the casting process).

[0115] When the content of the Nb-group elements is within the above range, even if the slab heating temperature is 1100°C or higher, it is possible to ultimately allow fine inhibitors and coarse inhibitors to coexist. For example, if the slab heating temperature is high and solution formation of AlN, MnS, etc. is promoted during the slab heating stage, these AlN and MnS are likely to re-precipitate as coarse particles in subsequent processes. However, when the content of the Nb-group elements is within the above range, the precipitates of the Nb-group elements act as precipitation nuclei for MnS and AlN, reducing the size of the re-precipitated AlN and MnS. Furthermore, since the precipitation nose of the Nb-group element precipitates (carbonitrides) is located on the lower temperature side than the precipitation noses of AlN and MnS, the precipitates of the Nb-group elements themselves are likely to precipitate as finer precipitates than AlN, etc.

[0116] Therefore, when the content of the Nb group elements is within the above range, the upper limit temperature during slab soaking may be less than 1180°C. Note that, as the soaking temperature increases, the solution of precipitates is also promoted, and when the content of the Nb group elements is within the above range, it is easy to preferably bring some of the precipitates contained in the slab into solution (for example, the upper limit of the solution rate of the precipitates may be 85% by volume). When these conditions are satisfied, the effect of the precipitates of the Nb group elements described above ultimately makes it easy to make fine inhibitors and coarse inhibitors coexist.

[0117] Similarly, when the content of the Nb group element is within the above range, the lower limit temperature during slab soaking may be greater than 1,030°C. Note that, as the soaking temperature decreases, the solution of precipitates is also suppressed, but when the content of the Nb group element is within the above range, it is easy to preferably bring some of the precipitates contained in the slab into solution (for example, the lower limit of the solution rate of precipitates may be 12% by volume). When these conditions are satisfied, it is ultimately possible to allow fine inhibitors and coarse inhibitors to coexist.

[0118] The mechanism by which the above-mentioned effects are obtained is thought to be related to the fact that precipitates (carbonitrides) of Nb group elements are more likely to precipitate than MnS or AlN (MnS in particular is difficult to precipitate without support such as dislocation multiplication due to rolling, and when it does precipitate, its size increases), and that precipitates of Nb group elements function as precipitation nuclei in the precipitation of MnS and AlN, thereby suppressing the coarsening of the re-precipitated AlN and MnS.

[0119] Furthermore, when the content of the Nb group element is within the above range, if the soaking time is 70 minutes or less, the time is too short and it is difficult to control the solution state of the precipitates to an equilibrium state. The upper limit of the soaking time is not particularly limited, but may be 2 hours in consideration of productivity in industrial production.

[0120] The soaking time during slab heating may be relaxed by changing the hot-rolled sheet annealing conditions described later. For example, by favorably changing the hot-rolled sheet annealing conditions, it may be possible to appropriately control precipitates even with a soaking time of 70 minutes or less. For example, even if the soaking time is 60 minutes or more and 70 minutes or less, desired properties can be obtained by controlling other conditions and hot-rolled sheet annealing conditions within appropriate ranges (for example, magnetic flux density B ８ (The value may be 1.935T or more.)

[0121] Controlling the "solution state of precipitates before rough rolling" to the above conditions is necessary to ultimately achieve a favorable balance between the amounts of relatively coarse precipitates (residual precipitates) that remain precipitated during the slab heating stage and relatively fine precipitates (re-precipitated precipitates) that precipitate after hot rolling.

[0122] The soaking temperature of the slab refers to the surface temperature of the slab, and the soaking time of the slab refers to the time for which the surface temperature of the slab is maintained after reaching the soaking temperature. For example, although it is affected by the steel composition and the heating rate, if the surface temperature of the slab reaches the soaking temperature during heating, the solution state of the precipitates on the surface of the slab is preferably controlled. Furthermore, if the surface temperature of the slab is maintained for the soaking time after reaching the soaking temperature, the solution state of the precipitates is preferably controlled all the way to the center of the slab.

[0123] The specific value of the solution ratio is not particularly limited. As described above, by controlling the steel composition and the slab heating conditions, the "solution state of precipitates before rough rolling" can be preferably controlled. However, if necessary, the specific value of the solution ratio may be determined using integrated thermodynamic calculation software. For example, "Thermo-Calc" is known as a commonly available integrated thermodynamic calculation software. In this embodiment, the solution ratio was calculated from the chemical composition and temperature of the slab using "Thermo-Calc" (2019a ver.) and used as a reference.

[0124] In the hot rolling process, hot rolling is performed following the above-mentioned slab heating. Generally, hot rolling is divided into rough rolling and finish rolling. In this embodiment, in order to control the size and distribution of precipitates contained in the steel sheet after the hot-rolled sheet annealing process, it is important to control the rolling temperature and reduction rate of the rough rolling after controlling the above-mentioned "solution state of precipitates before rough rolling".

[0125] In the hot rolling process, when rough rolling is performed after heating the slab, the following conditions should be satisfied.

[0126] When the heated slab is subjected to rough rolling, the rolling temperature may be controlled to 940 to 1070° C. and the rolling reduction to 82 to 95%.

[0127] By setting the reduction rate within the above range, deformation-induced precipitation occurs, making it possible to precipitate fine and large amounts of precipitates. If the rough rolling reduction rate is smaller than the above lower limit, the introduction of dislocations by rolling processing is reduced, and the number of precipitation sites available for deformation-induced precipitation is reduced, resulting in larger particle sizes of precipitates and a smaller Wp value. On the other hand, the upper limit of the rough rolling reduction rate is not particularly limited, but may be set to 95% taking into account the performance of the rolling mill, etc.

[0128] The rough rolling reduction mentioned above means the cumulative reduction in rough rolling. Specifically, the rough rolling reduction is defined as follows: rough rolling reduction (cumulative reduction) (%) = (1 - "steel sheet thickness after rough rolling" / "steel sheet thickness before rough rolling") x 100

[0129] Furthermore, if the rolling temperature of rough rolling is higher than the above upper limit, deformation-induced precipitation occurs on the higher temperature side or near the nose of precipitates such as MnS, AlN, and Nb group element precipitates, so the precipitation critical radius of the precipitates that reprecipitate during hot rolling becomes larger. Therefore, the size difference with the relatively coarse precipitates (residual precipitates) that have precipitated since the slab heating stage becomes smaller, and the value of Wp becomes smaller. On the other hand, the lower limit of the rolling temperature of rough rolling is not particularly limited, but since the slab becomes hard and the rollability decreases at low temperatures, it is sufficient to roll at, for example, 940 ° C or higher. The rough rolling temperature is defined as the average value of the start temperature and end temperature of rough rolling.

[0130] In addition, when Nb group elements are suitably contained in the chemical composition, in addition to MnS and AlN, precipitates of Nb group elements (particularly carbides and nitrides) are precipitated during rough rolling. These precipitates of Nb group elements act as precipitation nuclei for the subsequently precipitated MnS and AlN, resulting in finer re-precipitation of MnS and AlN. Therefore, when Nb group elements are suitably contained in the chemical composition, it is sufficient to control the various control conditions, such as the solution state of the precipitates (for example, the solution rate of the precipitates before rough rolling), the rough rolling temperature, and the rough rolling reduction, as described above.

[0131] When an Nb group element is suitably contained, the reason why the conditions of the hot rolling step can be controlled as described above is believed to be as follows. When an Nb group element is contained, MnS and AlN are reprecipitated more finely due to the precipitates of the Nb group element, so Dp becomes smaller compared to when an Nb group element is not contained. On the other hand, even if an Nb group element is contained, the value of Wp does not change significantly. Therefore, when an Nb group element is contained, Wp / Dp becomes larger compared to when an Nb group element is not contained. Therefore, in order to suitably control the values ​​of Dp and Wp / Dp, it is believed that the conditions of the hot rolling step can be controlled as described above.

[0132] For example, if the solution state of the precipitates is not favorably controlled when an Nb group element is contained (e.g., if the "solution rate of precipitates before rough rolling" is lower than 12% by volume), the precipitates are not sufficiently dissolved at the time of slab heating, as in the case where an Nb group element is not contained, and the number of fine precipitates that reprecipitate during hot rolling is reduced. As a result, Wp is reduced, and the secondary recrystallization progression temperature range cannot be sufficiently expanded during finish annealing. Furthermore, if the solution state of the precipitates is not favorably controlled when an Nb group element is contained (e.g., if the "solution rate of precipitates before rough rolling" is higher than 85% by volume), as in the case where an Nb group element is not contained, most of the precipitates are dissolved at the time of slab heating, and the number of relatively coarse precipitates (residual precipitates) in the slab is reduced. As a result, the secondary recrystallization progression temperature range cannot be sufficiently expanded during finish annealing.

[0133] Furthermore, the reason why the rough rolling reduction ratio should be controlled as described above when the Nb group elements are suitably contained, compared to when the Nb group elements are not suitably contained, is thought to be as follows: When the Nb group elements are contained, precipitates of the Nb group elements are likely to precipitate finely in the steel, so the number of fine precipitates contained in the steel even before rough rolling is greater, compared to when the Nb group elements are not contained. Therefore, when the Nb group elements are contained, the number of precipitation sites for precipitates increases, and deformation-induced precipitation is likely to occur even when the reduction ratio is reduced. For this reason, it is thought that the rough rolling reduction ratio should be controlled as described above.

[0134] When an Nb group element is contained, if the rough rolling reduction is less than 82%, as in the case where an Nb group element is not contained, the introduction of dislocations by rolling is reduced, and the number of precipitation sites available for deformation-induced precipitation is reduced, resulting in an increase in the particle size of the precipitates and a decrease in the value of Wp. When an Nb group element is contained, the upper limit of the rough rolling reduction is preferably 93%.

[0135] Furthermore, the reason why the rough rolling temperature should be controlled as described above when the Nb group elements are suitably contained, compared to when the Nb group elements are not suitably contained, is thought to be as follows. When the Nb group elements are contained, as described above, the number of fine precipitates contained in the steel even before rough rolling is greater, compared to when the Nb group elements are not contained. Therefore, when the Nb group elements are contained, the number of precipitation sites for precipitates increases, and the precipitates that re-precipitate during hot rolling tend to be finer. Therefore, it is thought that the rough rolling temperature should be controlled as described above.

[0136] When Nb group elements are contained, if the rolling temperature of rough rolling is higher than 1070 ° C, all precipitates of MnS, AlN, and Nb group elements will precipitate at a temperature higher than the precipitation nose of the precipitate, and the precipitation critical radius of the precipitates that reprecipitate during hot rolling will increase. Therefore, the size difference with the relatively coarse precipitates (residual precipitates) that have precipitated since the slab heating stage will decrease, and the value of Wp will decrease. When Nb group elements are contained, the upper limit of the rolling temperature of rough rolling is preferably 1065 ° C, more preferably 1040 ° C.

[0137] The reason why Nb group elements promote the fine precipitation of precipitates is not clear, but is thought to be as follows.

[0138] During rough rolling, the steel sheet temperature drops rapidly over time. Therefore, the rough rolling process is considered to be in a non-equilibrium state. Even in a temperature range where all MnS and AlN precipitate in an equilibrium state, dissolved MnS and AlN may exist in a non-equilibrium state. For example, because the rough rolling process is in a non-equilibrium state, dissolved MnS and AlN are thought to exist even in a temperature range where precipitates of Nb group elements precipitate. Therefore, when precipitates of Nb group elements precipitate during rough rolling, these precipitates of Nb group elements are thought to act as precipitation nuclei for the subsequently precipitated MnS and AlN, causing fine precipitation of MnS and AlN. Specifically, compared to when precipitates of Nb group elements are not present, when precipitates of Nb group elements are present, there are more precipitation sites for MnS and AlN, resulting in fine precipitation of MnS and AlN.

[0139] Furthermore, when precipitates of Nb group elements that acted as precipitation nuclei for MnS or AlN are covered with MnS or AlN, further growth of the precipitates of Nb group elements is suppressed. In this case, it is believed that the Nb group elements that would have been consumed for the growth of the precipitates precipitate finely as new precipitates. It is believed that these new fine precipitates of Nb group elements act as new precipitation nuclei for MnS or AlN, contributing to further fine precipitation of MnS or AlN. In this way, it is believed that the precipitates of Nb group elements synergistically contribute to the fine precipitation of MnS or AlN.

[0140] When the slab heating conditions and rough rolling conditions in the hot rolling process satisfy the above conditions, the size and distribution of the precipitates are preferably controlled, and as a result, the particle size-number density distribution of the precipitates is controlled within the above range after the hot-rolled sheet annealing process.

[0141] The slab soaking temperature during slab heating before rough rolling and the rolling temperature during rough rolling are temperatures that are controlled with a purpose. These temperatures are not caused by a natural temperature drop that occurs when the slab is removed from the slab heating furnace and subjected to rough rolling. For example, in general operations, the slab soaking temperature and rough rolling temperature are not controlled with a purpose. Typically, if the slab soaking temperature is high, the rough rolling temperature also increases accordingly, and if the slab soaking temperature is low, the rough rolling temperature also decreases accordingly. On the other hand, in this embodiment, the slab soaking temperature and the rough rolling temperature are controlled with a purpose. For example, even if the slab soaking temperature is high within the above range, the rough rolling temperature is controlled to be within the above range. Similarly, even if the slab soaking temperature is low within the above range, the rough rolling temperature is controlled to be within the above range.

[0142] The conditions for finish rolling in the hot rolling step are not particularly limited, and ordinary hot rolling conditions may be employed.

[0143] (Hot-rolled sheet annealing process) The hot-rolled sheet annealing process is a process in which the hot-rolled steel sheet after the hot-rolling process is annealed to obtain a hot-rolled annealed steel sheet. The hot-rolled sheet annealing is generally performed to control the steel sheet structure such as the recrystallization rate, residual strain, and grain size, and to preferably adjust the morphology of precipitates in the steel, by annealing the hot-rolled steel sheet after the hot-rolling process.

[0144] The annealing conditions in the hot-rolled sheet annealing step may be the annealing conditions for the hot-rolled sheet described later. In this embodiment, the precipitates contained in the hot-rolled annealed steel sheet after the hot-rolled sheet annealing step are controlled to have a particle size-number density distribution within the above-mentioned range.

[0145] For example, in this embodiment, the hot-rolled steel sheet after the hot rolling process is heated and subjected to first-stage annealing for recrystallization in a temperature range of 1040 to 1080°C, followed by second-stage annealing in a lower temperature range of 810 to 880°C, after which the steel sheet is cooled at an average cooling rate of 5 to 80°C / s. The first-stage annealing temperature is preferably 1040 to 1060°C. The second-stage annealing temperature is preferably 830 to 870°C. The heating rate to the first-stage annealing temperature is preferably 5°C / s or more on average. In the second-stage annealing, the steel sheet is preferably held for 20 seconds or more. The average cooling rate after the second-stage annealing is preferably 10°C / s or more, and more preferably 20°C / s or more. There is no particular upper limit to the average cooling rate, but to prevent breakage during cold rolling, the average cooling rate is preferably 50°C / s or less, and more preferably less than 40°C / s. The above-mentioned average cooling rate means a value obtained by dividing the temperature range from the second-stage annealing temperature to 500°C by the time required for cooling.

[0146] By setting the first-stage annealing temperature to 1040 to 1080°C, the number of precipitates mainly due to Nb group elements that precipitate during cooling after second-stage annealing can be preferably increased, thereby preferably increasing Wp. For example, annealing at a temperature higher than 1080°C tends to increase the amount of AlN that goes into solution in the first-stage annealing temperature range, and tends to increase the number of fine AlN that precipitate during second-stage annealing. In this case, Nb group elements tend to attach to the fine AlN and precipitate during cooling after second-stage annealing, which may result in a small Wp. Furthermore, for example, annealing at a temperature lower than 1040°C tends to make it difficult for precipitates due to Nb group elements to go into solution sufficiently, and tends to reduce the number of precipitates due to Nb group elements that precipitate during cooling after second-stage annealing, which may result in a small Wp.

[0147] By setting the second-stage annealing temperature to 810 to 880°C, precipitates of Nb group elements that precipitate during subsequent cooling can be preferably finely precipitated, and Wp can be preferably increased. For example, when annealing is performed at a temperature higher than 880°C, the subsequent cooling proceeds from a high temperature range, so that the Nb group elements that precipitate during cooling tend to precipitate large, and Wp may be reduced. Furthermore, for example, when annealing is performed at a temperature lower than 840°C, the number of relatively large Nb group element precipitates that precipitate during second-stage annealing increases, and the number of fine Nb group element precipitates that precipitate during subsequent cooling tends to decrease, and Wp may be reduced.

[0148] Furthermore, by controlling the annealing conditions in the hot-rolled sheet annealing step as described above, the average grain size of the hot-rolled and annealed steel sheet (steel sheet after hot-rolled sheet annealing) is preferably controlled to 20.0 to 21.5 μm.

[0149] As described above, the method for producing a hot-rolled annealed steel sheet according to this embodiment includes a casting process, a hot-rolling process, and a hot-rolled sheet annealing process. The hot-rolled annealed steel sheet produced by controlling the above conditions in each process in a composite manner has favorably controlled precipitate size and distribution, and the particle size-number density distribution of the precipitates is controlled within the above range. As a result, the secondary recrystallization progression temperature range is expanded during finish annealing, the selective growth of Goss-oriented grains is enhanced, and the magnetic flux density of the grain-oriented electrical steel sheet is improved.

[0150] For example, as described above, in the manufacturing method of the hot-rolled annealed steel sheet according to this embodiment, the amount of relatively coarse precipitates (residual precipitates) remaining after slab heating is controlled mainly by the slab soaking temperature and slab soaking time during slab heating before rough rolling, and the amount of relatively fine precipitates (re-precipitated precipitates) is controlled by the subsequent manufacturing conditions, thereby controlling the characteristics of the hot-rolled annealed steel sheet within the above ranges. As a result, the secondary recrystallization progression temperature range is expanded during finish annealing, the selective growth of Goss-oriented grains is enhanced, and the magnetic flux density of the grain-oriented electrical steel sheet is improved.

[0151] 5. Method of Using Hot-Rolled Annealed Steel Sheet The effects of the hot-rolled annealed steel sheet according to this embodiment can be confirmed in the final product, that is, grain-oriented electrical steel sheet. Therefore, from the perspective of a method of using the hot-rolled annealed steel sheet according to this embodiment, the manufacturing process of the grain-oriented electrical steel sheet subsequent to the hot-rolled sheet annealing process will be described.

[0152] The manufacturing method of grain-oriented electrical steel sheet includes a cold rolling process, a decarburization annealing process, an annealing separator application process, and a finish annealing process. If necessary, the manufacturing method may also include an insulating coating formation process and a magnetic domain control process. These processes may employ well-known general process conditions. Below, a manufacturing method using nitriding treatment as a low-temperature slab heating process will be described as an example.

[0153] (Cold Rolling Step) The cold rolling step is a step of cold rolling the hot-rolled annealed sheet obtained in the hot-rolled sheet annealing step once, or cold rolling the hot-rolled annealed sheet multiple times (two or more times) via annealing (intermediate annealing) (for example, a total cold rolling rate of 80 to 95%) to obtain a cold-rolled steel sheet having a thickness of, for example, 0.10 to 0.50 mm.

[0154] (Decarburization annealing step) The decarburization annealing step is a step in which the cold-rolled steel sheet obtained in the cold rolling step is subjected to decarburization annealing (for example, at 700 to 900°C for 1 to 3 minutes) to obtain a decarburization annealed steel sheet in which primary recrystallization has occurred. By subjecting the cold-rolled steel sheet to decarburization annealing, C contained in the cold-rolled steel sheet is removed. The decarburization annealing is preferably performed in a humid atmosphere in order to remove "C" contained in the cold-rolled steel sheet.

[0155] (Nitriding Treatment) Nitriding treatment is carried out to adjust the strength of the inhibitor in secondary recrystallization. In the nitriding treatment, the nitrogen content of the steel sheet may be increased to about 40 to 300 ppm at any timing between the start of the above-mentioned decarburization annealing and the start of secondary recrystallization in the finish annealing described below. Examples of nitriding treatment include a treatment in which a steel sheet is annealed in an atmosphere containing a gas with nitriding ability such as ammonia, and a treatment in which a decarburization-annealed steel sheet coated with an annealing separator containing a powder with nitriding ability such as MnN is finish-annealed.

[0156] (Annealing separator application step) The annealing separator application step is a step of applying an annealing separator to the decarburized annealed steel sheet. As the annealing separator, for example, an annealing separator containing MgO as a main component or an annealing separator containing alumina as a main component can be used.

[0157] The decarburized annealed steel sheet after application of the annealing separator is wound into a coil and then finish-annealed in the subsequent finish-annealing step.

[0158] (Finish annealing step) The finish annealing step is a step in which the decarburized annealed steel sheet coated with the annealing separator is subjected to finish annealing to cause secondary recrystallization. In this step, the growth of primary recrystallized grains is suppressed by an inhibitor, and secondary recrystallization is allowed to proceed, thereby preferentially growing {110}<001> oriented grains and improving magnetic flux density.

[0159] When the hot-rolled annealed steel sheet according to this embodiment is used, the secondary recrystallization progression temperature range is expanded during finish annealing, and preferential growth of {100}<011> orientation grains occurs to an extent not previously observed, resulting in a dramatic improvement in magnetic flux density. Furthermore, abnormal grain growth of secondary recrystallized grains occurs during finish annealing, and the secondary recrystallized grains occupy the entire sheet surface after finish annealing. The few secondary recrystallized grains cover the entire steel sheet surface, and the grain size of each secondary recrystallized grain becomes larger.

[0160] Furthermore, in the finish annealing step, it is possible to apply, as necessary, the finish annealing conditions for "expanding the secondary recrystallization progression temperature range" disclosed in the above Patent Documents 9 to 11. If the hot-rolled annealed steel sheet according to this embodiment is used and the finish annealing conditions disclosed in Patent Documents 9 to 11 are applied, the secondary recrystallization progression temperature range can be further preferably expanded.

[0161] The following insulating coating formation process and magnetic domain control process are not necessary from the viewpoint of concentrating the crystal orientation in {110}<001>. However, they are processes adopted in general grain-oriented electrical steel sheets to improve practical magnetic properties. (Insulating Coating Formation Process) The insulating coating formation process is a process for forming an insulating coating on the grain-oriented electrical steel sheet (finish-annealed steel sheet) after the finish-annealing process. An insulating coating mainly composed of phosphate and colloidal silica, or an insulating coating mainly composed of alumina sol and boric acid may be formed on the steel sheet after the finish-annealing process.

[0162] (Magnetic Domain Control Step) The magnetic domain control step is a step of performing a treatment to subdivide the magnetic domains of the grain-oriented electrical steel sheet. This step is appropriately performed at an appropriate timing after cold rolling. For example, localized micro-strains or localized grooves may be formed in the grain-oriented electrical steel sheet by a known method such as laser, plasma, mechanical method, or etching.

[0163] 6. Grain-oriented electrical steel sheet obtained using the hot-rolled annealed steel sheet according to this embodiment A brief description will be given of the grain-oriented electrical steel sheet produced using the hot-rolled annealed steel sheet according to this embodiment.

[0164] In the hot-rolled annealed steel sheet according to the present embodiment, relatively fine precipitates and relatively coarse precipitates coexist with a preferred size and distribution, so that in the grain-oriented electrical steel sheet obtained using the hot-rolled annealed steel sheet according to the present embodiment, Goss-oriented grains grow preferentially, and magnetic flux density is preferably increased. Furthermore, since the increased magnetic flux density does not cause deterioration of other properties of the grain-oriented electrical steel sheet produced using the hot-rolled annealed steel sheet according to the present embodiment, it can be used in the same applications as conventional ones.

[0165] The grain-oriented electrical steel sheet manufactured using the hot-rolled annealed steel sheet according to this embodiment contains, as a base element (main alloying element), 2.0 to 7.0% by mass of Si (silicon).

[0166] Furthermore, impurities may be contained. Here, "impurities" refer to elements that are mixed in from raw materials such as ore or scrap, or from the manufacturing environment, during industrial production of steel. The upper limit of the total content of impurities may be, for example, 5%.

[0167] Furthermore, in addition to the above-described basic elements and impurities, selective elements may be contained. For example, instead of a portion of the remaining Fe, Nb, V, Mo, Ta, W, C, Mn, S, Se, Al, N, Cu, Bi, B, P, Ti, Sn, Sb, Cr, Ni, etc. may be contained as selective elements. These selective elements may be contained according to the purpose. Therefore, there is no need to set a lower limit for these selective elements, and the lower limit may be 0%. Furthermore, these selective elements may be contained as impurities.

[0168] In grain-oriented electrical steel sheets, relatively large changes in chemical composition (reduction in content) occur as a result of decarburization annealing and purification annealing during secondary recrystallization. Purification annealing can reduce the content of some elements to a level that cannot be detected by common analytical methods (1 ppm or less). Generally, the chemical composition of the final product differs from that of the starting slab. However, the above-mentioned optional elements are elements contained in the slab that remain in the final product. The content of each element does not exceed the content range described above for the slab, but rather falls within a content range that corresponds to the content in the slab and the subsequent manufacturing process.

[0169] The above chemical composition is that of the grain-oriented electrical steel sheet. If the grain-oriented electrical steel sheet to be measured has an insulating coating or the like on its surface, the coating or the like is removed by a known method before measuring the chemical composition.

[0170] The grain-oriented electrical steel sheet manufactured using the hot-rolled annealed steel sheet according to this embodiment may have an intermediate layer disposed on and in contact with the grain-oriented electrical steel sheet (silicon steel sheet), and an insulating coating disposed on and in contact with the intermediate layer.

[0171] For example, the intermediate layer may be a layer mainly made of an oxide, a layer mainly made of a carbide, a layer mainly made of a nitride, a layer mainly made of a boride, a layer mainly made of a silicide, a layer mainly made of a phosphide, a layer mainly made of a sulfide, a layer mainly made of an intermetallic compound, etc. These intermediate layers are formed mainly to ensure adhesion between the silicon steel sheet and the insulating coating, and may be any known layer formed by heat treatment in an atmosphere with controlled oxidation-reduction, chemical vapor deposition (CVD), physical vapor deposition (PVD), etc.

[0172] Representative examples of the insulating coating include an insulating coating mainly composed of phosphate and colloidal silica and having an average thickness of 0.1 to 10 μm, and an insulating coating mainly composed of alumina sol and boric acid and having an average thickness of 0.5 to 8 μm.

[0173] Next, the effects of the present invention will be specifically described in detail using examples. The conditions in the examples are one example of conditions adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to this one example of conditions. The present invention can adopt various conditions as long as they do not deviate from the gist of the present invention and the object of the present invention is achieved.

[0174] Hot-rolled and annealed steel sheets were produced using slabs having the chemical compositions shown in Tables 1 and 2. The chemical compositions of the produced hot-rolled and annealed steel sheets were equivalent to the chemical compositions of the slabs shown in Tables 1 and 2. These chemical compositions were measured based on the above-mentioned method. In Tables 1 and 2, "-" indicates that control and production were not carried out with the content in mind, and that the content was not measured.

[0175] The above hot-rolled and annealed steel sheets were manufactured under the manufacturing conditions shown in Tables 3 to 9. Slab heating was performed by soaking at a predetermined temperature for a predetermined time without temporarily increasing the heating temperature during the slab heating process. The soaking temperature shown in the tables indicates the surface temperature of the slab after heating, and the soaking time shown in the tables indicates the slab heating time from when the slab surface temperature reached the soaking temperature.

[0176] Although not shown in the table, when the total content of Nb group elements is 0.0030 to 0.030 mass%, by setting the soaking temperature of the slab to more than 1030°C and less than 1180°C and the soaking time of the slab to more than 70 minutes, 12 to 85 volume % of the precipitates are solutionized, based on the precipitates contained in the slab after the casting process.

[0177] In the hot-rolled sheet annealing step, the hot-rolled steel sheet after the hot rolling step was annealed. In Examples other than No. 100, the hot-rolled steel sheet after the hot rolling step was annealed under the annealing conditions shown in Tables 3 to 9. In this case, the heating rate to the first-stage annealing temperature was set to an average of 5°C / second or more, and the holding time in the second-stage annealing was set to 20 seconds or more. In Example No. 100, second-stage annealing was not performed.

[0178] The average grain size and precipitation morphology of the produced hot-rolled and annealed steel sheets were examined based on the above-mentioned methods. The precipitation morphology of precipitates having a circle-equivalent diameter D of 50 to 1000 nm is shown in Tables 10 to 16. In the tables, Dp represents the mode diameter, f(Dp) represents the number density of the mode diameter, and Wp represents the half-width of the mode diameter.

[0179] The hot-rolled and annealed steel sheets were then subjected to cold rolling and decarburization annealing under known conditions. The cold rolling was performed at a reduction rate of 90.7% to a sheet thickness of 0.26 mm. The decarburization annealing temperature was in the range of 830 to 860°C for 90 seconds, and the steel sheets after decarburization annealing were then subjected to nitriding treatment (nitriding annealing) in a hydrogen-nitrogen-ammonia mixed atmosphere to adjust the nitrogen content of the steel sheets to 0.020 to 0.023 mass% (200 to 230 ppm).

[0180] Furthermore, an annealing separator containing MgO as a main component was applied to the steel sheet, and the steel sheet was subjected to finish annealing. In the final step of the finish annealing, the steel sheet was held in a hydrogen atmosphere at 1200°C for 20 hours (purification annealing), and then naturally cooled.

[0181] A coating solution for forming an insulating coating, which was mainly composed of phosphate and colloidal silica and also contained chromium, was applied to the primary coating (intermediate layer) formed on the surface of the manufactured grain-oriented electrical steel sheet (finish-annealed steel sheet), and the resulting solution was heated in an atmosphere of hydrogen:nitrogen 75% by volume:25% by volume, maintained at this temperature, and then cooled to form an insulating coating.

[0182] When viewed from a cross section parallel to the thickness direction of the grain-oriented electrical steel sheet, the produced sheet had an intermediate layer disposed in contact with the grain-oriented electrical steel sheet (silicon steel sheet) and an insulating coating disposed in contact with the intermediate layer. The intermediate layer was a forsterite coating with an average thickness of 2 μm, and the insulating coating was an insulating coating with an average thickness of 1 μm, mainly composed of phosphate and colloidal silica.

[0183] The grain-oriented electrical steel sheets thus obtained were evaluated for various properties, and the evaluation results are shown in Tables 10 to 16.

[0184] (1) Magnetic Properties of Grain-Oriented Electrical Steel Sheets The magnetic properties of the grain-oriented electrical steel sheets were measured based on the Single Sheet Tester (SST) specified in JIS C 2556:2015.

[0185] As a magnetic property, the magnetic flux density B in the rolling direction of the steel sheet when excited at 800 A / m ８ (T) was measured. ８ For reference, the iron loss W, defined as the power loss per unit weight (1 kg) of the steel sheet under the conditions of AC frequency: 50 Hz, excitation magnetic flux density: 1.7 T, was １７／５０ (W / kg) was measured.

[0186] Among Nos. 1 to 103, the particle size-number density distribution of the precipitates contained in the hot-rolled annealed steel sheet was preferably controlled in the inventive examples, and all of them exhibited excellent magnetic flux density as grain-oriented electrical steel sheets. On the other hand, among Nos. 1 to 103, the particle size-number density distribution of the precipitates contained in the comparative examples was not preferably controlled, and they did not obtain magnetic flux density preferable as grain-oriented electrical steel sheets.

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[0203] According to the above aspects of the present invention, it is possible to provide a hot-rolled annealed steel sheet for grain-oriented electrical steel sheet that can increase magnetic flux density, and a method for manufacturing the same, and therefore the present invention has high industrial applicability.

Claims

1. A hot-rolled and annealed steel sheet for grain-oriented electrical steel sheet, comprising, in mass%, C: 0.0010 to 0.10%, Si: 2.0 to 7.0%, Mn: 0.050 to 1.0%, S: 0 to 0.0350%, Se: 0 to 0.0350%, 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%, The hot-rolled annealed steel sheet has a chemical composition containing Nb+V+Mo+Ta+W total content: 0.0030 to 0.030%, Cu: 0 to 0.40%, Bi: 0 to 0.010%, B: 0 to 0.080%, P: 0 to 0.50%, Ti: 0 to 0.0150%, Sn: 0 to 0.10%, Sb: 0 to 0.10%, Cr: 0 to 0.30%, Ni: 0 to 1.0%, with the balance being Fe and impurities, and the particle size-number density distribution of precipitates having a circle equivalent diameter D of 50 to 1000 nm among precipitates that are residues obtained by electrolytic extraction from the hot-rolled annealed steel sheet is as follows: the most frequent diameter is Dp in the unit of nm, and the number density of the most frequent diameter is f(Dp) in the unit of particles / g, a hot-rolled annealed steel sheet for grain-oriented electrical steel sheet, characterized in that, when Wp is the half-width of the mode diameter in the unit of nm, Dp is 100 to 300 nm, f(Dp) is 1,000,000 particles / g or more, and Wp / Dp is 1.0 to 2.0; and the hot-rolled annealed steel sheet has an average grain size of 20.0 to 21.5 μm.

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