Method for manufacturing grain-oriented electrical steel sheet

Abstract

The present invention provides a method for manufacturing a grain-oriented electrical steel sheet that exhibits superior magnetic properties compared to conventional methods by actively utilizing inhibitors while highly controlling the microstructure of the primary recrystallized sheet. A method for manufacturing a grain-oriented electrical steel sheet comprises heating a steel billet to a temperature exceeding the γ phase precipitation temperature and below 1380°C, and performing a true strain ε introduction process at a temperature above (the temperature at which the γ phase fraction is maximum - 20°C) including two or more passes. t The hot rolled sheet is subjected to rough rolling with a rolling temperature of 0.50 or more, and finish rolling is performed with the rolling end temperature set to 900°C or more to produce a hot rolled sheet. The hot rolled sheet is cooled at a cooling rate of 70°C / s or more within 2 seconds after the finish rolling for more than 1 second, and is coiled at a coiling temperature of 600°C or less. The hot rolled sheet is annealed at a soaking temperature of 1000°C to (1150-2.5Y)°C when the recrystallization rate of the center layer of the plate thickness after the coiling is set to Y(%), and then cold rolling, primary recrystallization annealing and secondary recrystallization annealing are performed.

Description

Technical Field

[0001] The present invention relates to a method for producing a grain-oriented electrical steel sheet. Background Art

[0002] Grain-oriented electromagnetic steel sheets are mainly used as materials for the iron core inside transformers. In order to improve the energy efficiency of transformers, it is required that the iron loss of oriented electromagnetic steel sheets be low. As methods for reducing the iron loss of oriented electromagnetic steel sheets, in addition to methods such as increasing the resistivity of the steel sheets, increasing the film tension, and thinning, methods such as surface processing of the steel sheets and sharpening the crystal orientation to the {110}<001> orientation of the grains (hereinafter referred to as Gaussian orientation) can also be cited. As an indicator of magnetic properties, the iron loss W per 1 kg of steel sheet when magnetized to 1.7 T in an AC magnetic field with an excitation frequency of 50 Hz is mainly used. 17 / 50 In particular, as an indicator of the sharpening of the crystal orientation toward the {110}<001> orientation of the grains (hereinafter referred to as the Gossian orientation), the magnetic flux density B8 at a magnetic field strength of 800 A / m is primarily used. To increase the integration of the Gossian orientation, it is important to: add a difference in grain boundary activity in a manner that preferentially grows only sharp Gossian-oriented grains, that is, to form the aggregate structure of the primary recrystallized plate into a predetermined structure, and to use precipitates called inhibitors to suppress the growth of recrystallized grains other than the Gossian orientation. Technologies utilizing such inhibitors include, for example, a method utilizing AlN and MnS disclosed in Patent Document 1, and a method utilizing MnS and MnSe disclosed in Patent Document 2. These methods have both been put to practical use industrially.

[0003] These inhibitors are preferably uniformly and finely dispersed in the steel. Therefore, in methods utilizing inhibitors, the billet is typically heated to a high temperature of 1300°C or higher before hot rolling to dissolve the inhibitor components and finely precipitate them in subsequent steps. For example, in Patent Document 3, Al is added to steel, and after hot rolling, the hot-rolled sheet is annealed at 750-1200°C, followed by rapid cooling. This precipitates fine AlN, resulting in an extremely high magnetic flux density.

[0004] On the other hand, research is also underway into methods for producing grain-oriented electrical steel sheets that do not rely on inhibitors (inhibitor-free methods). This method, which does not rely on inhibitors, is characterized by utilizing higher-purity steel and controlling the crystal aggregate structure to achieve secondary recrystallization. This method eliminates the need for high-temperature heating of the billet to dissolve the inhibitor components, making it possible to produce grain-oriented electrical steel sheets at low cost. For example, Patent Document 3 demonstrates that the presence of a large number of {554}<225>-oriented grains and {411]<148>-oriented grains in the primary recrystallization structure increases the concentration of Goss-oriented grains after secondary recrystallization, resulting in a higher magnetic flux density.

[0005] Prior art literature

[0006] Patent Literature

[0007] Patent Document 1: Japanese Patent Publication No. 40-15644

[0008] Patent Document 2: Japanese Patent Publication No. 51-13469

[0009] Patent Document 3: Japanese Patent Application Laid-Open No. 2001-60505 Summary of the Invention

[0010] In order to increase the magnetic flux density of grain-oriented electrical steel sheets, it is considered necessary to highly control the inhibitors and the microstructure of the primary recrystallized sheet. However, in order to actively utilize the inhibitors and finely disperse them in the steel, the microstructure before cold rolling is usually refined, making it difficult to control the primary recrystallized microstructure. In the existing manufacturing process of grain-oriented electrical steel sheets, fine inhibitors are formed during the hot-rolled sheet annealing, and these inhibitors significantly hinder the grain growth of recrystallized grains in the subsequent intermediate annealing process. In addition, the larger the grain size before cold rolling, the more frequently Goss-oriented grains are generated in the subsequent primary recrystallization process. Therefore, when the grain size is refined by intermediate annealing, the formation of Goss-oriented grains is extremely unfavorable.

[0011] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a method for producing a grain-oriented electrical steel sheet that exhibits superior magnetic properties compared to conventional methods while actively utilizing inhibitors and highly controlling the microstructure of the primary recrystallized sheet.

[0012] The inventors have conducted repeated and in-depth research to solve the above-mentioned problems. As a result, the inventors have found that in order to form a preferred aggregate structure for obtaining good magnetic properties in a primary recrystallized plate, it is important not only to coarsen the grain size before cold rolling, but also to increase the frequency of existence of grains with less strain before cold rolling. In addition, it was found that in order to increase the frequency of existence of grains with less strain before cold rolling, hard rolling and the number of passes within the temperature range where the γ phase fraction is maximum in the rough rolling conditions of hot rolling are important. Furthermore, it was found that by changing the annealing temperature of the hot rolled plate according to the existence ratio of grains with less strain in the hot rolled plate, and then introducing skin pass rolling, it is possible to actively utilize inhibitors while also producing a good primary recrystallized aggregate structure. As a result, an extremely high magnetic flux density can be obtained after the secondary recrystallization annealing, thereby developing the present invention.

[0013] The present invention is based on the above findings. That is, the gist of the present invention is as follows.

[0014] [1] A method for manufacturing a grain-oriented electrical steel sheet,

[0015] A steel slab having the following composition, wherein the steel slab comprises 0.005-0.085 mass % C, 2.00-4.50 mass % Si, 0.03-1.00 mass % Mn, 0.008 mass % or more and less than 0.030 mass % sol. Al, and 0.004-0.009 mass % N, and further comprises at least one of 0.0005-0.02 mass % S and 0.0005-0.02 mass % Se, with the remainder being Fe and unavoidable impurities, is heated to a temperature exceeding a γ phase precipitation temperature and not exceeding 1380° C.

[0016] Next, the steel slab is subjected to a true strain ε of plate thickness at a temperature not lower than (the temperature at which the γ phase fraction is maximum - 20°C) including two or more passes. t The rough rolling of the rolling with a temperature of 0.50 or above is made into rough rolled plate.

[0017] Next, the rough-rolled sheet is subjected to finish rolling at a rolling end temperature of 900° C. or higher to obtain a hot-rolled sheet.

[0018] Next, within 2 seconds after the completion of the finish rolling, the hot rolled sheet is cooled at a cooling rate of 70°C / s or higher for 1 second or longer.

[0019] The cooled hot rolled sheet is coiled at a coiling temperature below 600°C.

[0020] Next, the coiled hot-rolled sheet is subjected to hot-rolled sheet annealing at a soaking temperature of 1000° C. to (1150-2.5Y)° C. for 60 seconds or more, when the recrystallization rate of the center layer of the sheet thickness is set to Y (%), to produce a hot-rolled annealed sheet.

[0021] Next, the hot-rolled annealed sheet is subjected to cold rolling at a rolling reduction of 88% to 91% to obtain a cold-rolled sheet having a final thickness.

[0022] Then, the cold rolled sheet is subjected to primary recrystallization annealing to prepare a primary recrystallization annealed sheet.

[0023] Next, the primary recrystallization annealed sheet is subjected to secondary recrystallization annealing to obtain a grain-oriented electrical steel sheet.

[0024] Here, the true strain ε of the plate thickness is t Calculated by the following formula (1).

[0025] ε t = -ln(plate thickness after rolling / plate thickness before rolling)…(1)

[0026] [2] The method for producing a grain-oriented electrical steel sheet according to [1] above, wherein the composition further contains one or two selected from the group consisting of Sb: 0.005 to 0.500 mass % and Sn: 0.005 to 0.500 mass %.

[0027] [3] The method for producing a grain-oriented electrical steel sheet according to [1] or [2], wherein the composition further contains Ni: 0.01-1.50 mass%, Cr: 0.005-0.50 mass%, Cu: 0.03-0.50 mass%, P: 0.005-0.500 mass%, As: 0.0005-0.050 mass%, Bi: 0.005-0.50 0 mass%, Mo: 0.005-0.100 mass%, B: 0.0002-0.0025 mass%, Te: 0.0005-0.0100 mass%, Zr: 0.001-0.010 mass%, Nb: 0.001-0.010 mass%, V: 0.001-0.010 mass% and Ta: 0.001-0.010 mass%.

[0028] [4] The method for producing a grain-oriented electrical steel sheet according to any one of [1] to [3] above, wherein the rough rolling includes one or more passes of rolling from (temperature at which the γ phase fraction is maximum - 20°C) to (temperature at which the γ phase fraction is maximum + 50°C).

[0029] [5] The method for producing a grain-oriented electrical steel sheet according to any one of [1] to [4], wherein the rough rolling is performed four or more times in total.

[0030] [6] A method for manufacturing a grain-oriented electrical steel sheet according to any one of [1] to [5] above, wherein, for the hot-rolled sheet after soaking, a first average cooling rate v1 from the soaking temperature to 800°C is set to less than 40°C / s, and a second average cooling rate v2 from 800°C to 650°C is set to be greater than v1 and cooling is performed.

[0031] [7] The method for producing a grain-oriented electrical steel sheet according to any one of [1] to [6], wherein the recrystallization rate Y is 18% or more.

[0032] [8] The method for manufacturing a grain-oriented electrical steel sheet according to any one of [1] to [7] above, wherein the recrystallization rate Y is 20% or more, and temper rolling with an elongation of 0.05% or more is performed after the finish rolling and before the hot-rolled sheet is annealed.

[0033] [9] The method for producing a grain-oriented electrical steel sheet according to any one of [1] to [8], wherein the magnetic flux density B8 in the rolling direction of the grain-oriented electrical steel sheet is 1.940 T or more.

[0034] According to the present invention, it is possible to provide a method for producing a grain-oriented electrical steel sheet that exhibits superior magnetic properties compared to conventional methods while actively utilizing inhibitors and highly controlling the microstructure of the primary recrystallized sheet. DETAILED DESCRIPTION

[0035] First, the experiments that led to the development of the present invention will be described. The inventors carefully observed the crystal structure of hot-rolled sheets to verify whether coarsening the grain size before cold rolling is effective in forming a structure that is optimal for improving magnetic properties in primary recrystallized grains of grain-oriented electrical steel sheets.

[0036] Experiment 1

[0037] A steel raw material (C: 0.060 mass%, Si: 3.40 mass%, Mn: 0.06 mass%, sol.Al: 0.014 mass%, N: 0.007 mass%, S: 0.020 mass%, Sb: 0.035 mass%), the remainder of which is composed of Fe and inevitable impurities, is melted to form a steel billet, which is then heated to 1310°C. Next, the steel billet is subjected to a true strain ε at 1200°C. t 0.6 one-pass rolling, plate thickness true strain ε at 1150℃ t 0.4 in one-pass rolling and true strain ε of plate thickness at 1100℃ t The rough-rolled sheet was then subjected to a single pass of 0.4°C to produce a rough-rolled sheet. The rough-rolled sheet was then subjected to finish rolling at a finishing temperature of 1050°C to produce a hot-rolled sheet with a thickness of 2.2 mm. One second after the finish rolling, the sheet was cooled at a cooling rate of 80°C / s for 5 seconds, and then coiled at a coiling temperature of 520°C. The hot-rolled sheet was then soaked at 1100°C for 90 seconds, cooled to 600-450°C for 2 minutes, and then water-cooled to 100°C for hot-rolled annealing to produce a hot-rolled annealed sheet. The hot-annealed sheet was then cold-rolled at a reduction of 90% to produce a cold-rolled sheet with a final thickness of 0.22 mm. The cold-rolled sheet was then subjected to a primary recrystallization annealing process using a known method to produce a primary recrystallization annealed sheet. The primary recrystallization annealed sheet was then subjected to a secondary recrystallization annealing process to produce a grain-oriented electrical steel sheet.

[0038] The microstructure of the vertical cross section (L cross section) of the hot rolled sheet after coiling, which is parallel to the rolling direction, was observed, and many grains elongated (extended) in the rolling direction were observed. It is believed that the grains elongated in the rolling direction are caused by residual strain. Here, the grains elongated in the rolling direction refer to grains whose ratio of the grain size in the rolling direction to the grain size in the plate thickness direction is 2.0 or more. The recrystallization rate Y of the plate thickness center layer described later is 5%. In addition, the microstructure of the L cross section of the hot rolled annealed sheet was observed, and many grains elongated in the rolling direction were observed. The magnetic flux density B8 of the grain-oriented electrical steel sheet after secondary recrystallization annealing was evaluated by the Epstein test described later, and the result was 1.930T. It should be noted that B8 refers to the magnetic flux density of the sample when the sample is excited with a magnetizing force of 800A / m in the rolling direction.

[0039] Next, a steel composition having the same chemical composition as above was prepared into a steel slab in the same manner as above. The steel slab was heated to 1310°C. Then, the steel slab was subjected to true strain ε at 1220°C. t 0.5% of the plate thickness true strain ε in one-pass rolling at 1180°C t True strain ε of plate thickness at 1140℃ and 1-pass rolling of 0.4 t The rough rolled sheet is made by a single pass of 0.5°C. Next, the rough rolled sheet is subjected to finish rolling with the finish rolling end temperature set at 1050°C to produce a hot rolled sheet with a thickness of 2.2 mm. Next, the hot rolled sheet is cooled at a cooling rate of 80°C / s for 5 seconds 1 second after the finish rolling, and then coiled at a coiling temperature of 520°C. Next, the hot rolled sheet is subjected to hot rolled sheet annealing at 1100°C for 60 seconds to produce a hot rolled annealed sheet. Next, the hot rolled annealed sheet is subjected to a single cold rolling to produce a cold rolled sheet with a final thickness of 0.22 mm. Then, the cold rolled sheet is subjected to a primary recrystallization annealing by exactly the same method as above to produce a primary recrystallization annealed sheet, and then the primary recrystallization annealed sheet is subjected to a secondary recrystallization annealing to produce a grain oriented electromagnetic steel sheet.

[0040] Observation of the L-section microstructure of the hot-rolled sheet after coiling revealed, as described above, numerous grains elongated in the rolling direction. However, the recrystallization rate Y, described below, was higher than above, at 20%. Furthermore, observation of the L-section microstructure of the hot-rolled annealed sheet revealed a lower proportion of grains elongated in the rolling direction than in the above examples. The magnetic flux density B8 of the grain-oriented electrical steel sheet after secondary recrystallization annealing was evaluated using the Epstein test and found to be 1.941 T.

[0041] Based on the above results, the present inventors discovered that the rough rolling process of hot rolling has a significant impact on the microstructure of the hot-rolled sheet. Furthermore, the present inventors discovered that by appropriately controlling the microstructure of the hot-rolled sheet, the magnetic flux density of the grain-oriented electrical steel sheet after secondary recrystallization annealing is increased. The method that actively utilizes inhibitors reduces recrystallization during hot rolling due to the high billet heating temperature and large grain size after heating. Therefore, the present inventors concluded that the method that actively utilizes inhibitors is the only one that effectively controls the microstructure of the hot-rolled sheet by optimizing the rough rolling conditions, leading to the discovery of the present invention.

[0042] Furthermore, the present inventors have determined that if the microstructure of a hot-rolled sheet can be appropriately controlled, it will be possible to redefine an appropriate hot-rolled sheet annealing temperature in a method that actively utilizes inhibitors.

[0043] Based on the above, the present inventors further conducted the following experiments.

[0044] Experiment 2

[0045] A steel raw material (C: 0.065 mass%, Si: 3.40 mass%, Mn: 0.060 mass%, sol.Al: 0.017 mass%, N: 0.007 mass%, Se: 0.006 mass%, Sb: 0.035 mass%), the remainder of which is composed of Fe and inevitable impurities, is melted to form a steel billet, which is then heated to 1330°C and subjected to true strain ε at 1200°C. t 0.6 one-pass rolling, plate thickness true strain ε at 1150℃ t 0.5% of the plate thickness true strain ε in one-pass rolling at 1100°C tThe rough-rolled sheet was then subjected to a rough rolling process consisting of one rolling pass at a thickness of 0.4°C to produce a rough-rolled sheet. Next, the rough-rolled sheet was subjected to finish rolling at a finishing temperature of 1060°C to produce a hot-rolled sheet with a thickness of 2.1 mm. Next, one second after the finish rolling, the sheet was cooled at a cooling rate of 80°C / s for 5 seconds, and then coiled at a coiling temperature of 520°C. The hot-rolled sheet obtained in this manner will be hereinafter referred to as Hot-Rolled Sheet A. Separately, a steel slab having the same composition as above was subjected to rough rolling, consisting of one rolling pass at 1220°C with a true strain of 0.6°C, one rolling pass at 1180°C with a true strain of 0.3°C, and one rolling pass at 1100°C with a true strain of 0.4°C to produce a rough-rolled sheet. Next, the rough-rolled sheet was subjected to finish rolling at a finishing temperature of 1060°C to produce a hot-rolled sheet with a thickness of 2.1 mm. Next, 1 second after the finish rolling, cooling was performed at a cooling rate of 80°C / s for 5 seconds, and then coiling was performed at a coiling temperature of 520°C. The hot-rolled sheet obtained in this manner is hereinafter referred to as hot-rolled sheet B. Hot-rolled sheets A and B were annealed at four conditions: 1030°C for 90 seconds, 1070°C for 90 seconds, 1100°C for 90 seconds, and 1130°C for 90 seconds, respectively, to produce hot-rolled annealed sheets. Next, the hot-rolled annealed sheets were cold rolled at a rolling ratio of 90% to produce cold-rolled sheets with a final thickness of 0.22 mm. The cold-rolled sheets were then subjected to primary recrystallization annealing by a known method to produce primary recrystallization annealed sheets. The primary recrystallization annealed sheets were then subjected to secondary recrystallization annealing to produce grain-oriented electrical steel sheets. Table 1 shows the magnetic flux density B8 of grain-oriented electrical steel sheets using hot-rolled sheets A and B. In the experiment using hot-rolled sheet A, the hot-rolled sheet annealing temperature at which the magnetic flux density of the grain-oriented electrical steel sheet reached the maximum was 1100°C, whereas in the experiment using hot-rolled sheet B, the hot-rolled sheet annealing temperature at which the magnetic flux density of the grain-oriented electrical steel sheet reached the maximum was 1130°C.

[0046] [Table 1]

[0047]

[0048] Based on the above results, the present inventors believe that a higher magnetic flux density can be obtained by appropriately determining the hot-rolled sheet annealing according to the microstructure of the hot-rolled sheet.

[0049] Next, the present inventors conducted the following experiments to investigate in more detail the effect of rough rolling on the recrystallization rate Y of a hot-rolled sheet.

[0050] Experiment 3

[0051] A steel raw material (C: 0.060 mass%, Si: 3.40 mass%, Mn: 0.060 mass%, sol. Al: 0.017 mass%, N: 0.008 mass%, Se: 0.006 mass%, Cu: 0.03%, As: 0.005 mass%, Sb: 0.02 mass%), with the remainder consisting of Fe and inevitable impurities, was melted to form a slab. The slab was then heated to 1330°C. The slab was then rough-rolled under various rolling conditions to produce a rough-rolled plate. The rough-rolled plate was then finish-rolled at a finishing temperature of 1040-1100°C to produce a hot-rolled plate with a thickness of 2.2 mm. One second after the finish rolling, the plate was cooled at a cooling rate of 80°C / s for 5 seconds, and then coiled at a coiling temperature of 500-550°C. The microstructure of the L-section of the hot-rolled sheet after coiling was observed to evaluate the recrystallization rate Y. The method for evaluating the recrystallization rate Y will be described later.

[0052] The results are shown in Table 2.

[0053]

[0054] Based on the present results, the present inventors have inferred the following tendencies (i) to (iii).

[0055] (i) If the plate thickness true strain ε is introduced at a temperature above (temperature with the maximum γ phase fraction - 20°C) t By applying rough rolling with a rolling angle of 0.50 or more to the slab, a high recrystallization rate Y of 15% or more can be obtained in the hot rolled sheet. Here, it is known from a preliminary equilibrium calculation that the temperature at which the γ phase fraction is maximum in this experiment is 1150°C.

[0056] (ii) Rough rolling in hot rolling includes at least one rolling pass at a temperature of (temperature at which the γ phase fraction is maximum - 20°C) or higher and (temperature at which the γ phase fraction is maximum + 50°C) or lower, and a higher recrystallization rate Y (18% or higher in the above results) can be obtained.

[0057] (iii) When the number of rough rolling passes is 4 or more in total, a higher recrystallization rate Y can be obtained (20% or more in the above results).

[0058] Next, the present inventors conducted an experiment in which the soaking temperature of the subsequent hot-rolled sheet annealing was changed by several levels for each hot-rolled sheet having a different recrystallization rate Y.

[0059] Experiment 4

[0060] First, the 2.2 mm thick hot-rolled sheet produced in Experiment 3 was used as the test material. The hot-rolled sheet was annealed under conditions where the soaking temperature was varied several times. The soaking time was 100 seconds. After soaking, the sheet was cooled to 600-450°C for 2 minutes, and then water-cooled to 100°C to obtain a hot-rolled annealed sheet. After the hot-rolled annealing, the hot-rolled annealed sheet was cold-rolled at a rolling ratio of 90% to obtain a cold-rolled sheet with a final thickness of 0.22 mm. The cold-rolled sheet was then subjected to a primary recrystallization annealing process using a known method to obtain a primary recrystallization annealed sheet. The primary recrystallization annealed sheet was then subjected to a secondary recrystallization annealing process to obtain a grain-oriented electrical steel sheet. The magnetic flux density B8 of the resulting grain-oriented electrical steel sheet was evaluated using the Epstein test described below. Table 3 shows the soaking temperature for the hot-rolled sheet annealing and the magnetic flux density B8 of the resulting grain-oriented electrical steel sheet. The relationship between the recrystallization rate Y of each hot-rolled plate and the soaking temperature of the hot-rolled annealed plate that gives the maximum magnetic flux density B8 was investigated. The results showed that a high magnetic flux density can be obtained when the soaking temperature of the hot-rolled plate annealing is approximately (1150-2.5Y)℃.

[0061]

[0062] The following describes embodiments of the present invention. It should be noted that the present invention is not limited to the following embodiments. First, the appropriate range of chemical compositions for the steel slabs used as raw materials for the grain-oriented electrical steel sheets of the present invention and the reasons for these ranges will be described. It should be noted that in the following description, numerical ranges indicated by "to" include the numerical values ​​before and after "to" as the lower and upper limits.

[0063] C: 0.005-0.085 mass%

[0064] If C is less than 0.005% by mass, the grain boundary strengthening effect of C is lost, cracks are generated in the blank, and manufacturing is hindered. In addition, the uneven deformation that is preferred for improving magnetic properties due to strain aging during rolling is suppressed. On the other hand, if the C amount exceeds 0.085% by mass, it is difficult to reduce the C amount to less than 0.005% by mass, which does not cause magnetic aging, during the primary recrystallization annealing. Therefore, C is in the range of 0.005 to 0.085% by mass. The C amount is preferably 0.010% by mass or more, more preferably 0.030% by mass or more. In addition, the C amount is preferably 0.080% by mass or less, more preferably 0.070% by mass or less.

[0065] Si: 2.00-4.50 mass%

[0066] Si is an important element for increasing the resistivity of steel sheets and reducing iron loss. Adding less than 2.00% by mass of Si does not fully demonstrate these effects. On the other hand, if the Si content exceeds 4.50% by mass, the brittleness of the steel sheet increases, making rolling difficult. Therefore, the Si content is in the range of 2.00 to 4.50% by mass. The Si content is preferably 2.50% by mass or more, more preferably 3.0% by mass or more. In addition, the Si content is preferably 4.50% by mass or less, more preferably 4.0% by mass or less.

[0067] Mn: 0.03-1.00 mass%

[0068] Mn is an element necessary for improving the hot workability of steel. A Mn content of less than 0.03 mass% is insufficient to achieve the aforementioned effects. On the other hand, if the Mn content exceeds 1.00 mass%, the magnetic flux density of the finished sheet decreases. Therefore, the Mn content is within the range of 0.03 to 1.00 mass%. The Mn content is preferably 0.05 mass% or greater, more preferably 0.06 mass% or greater. The Mn content is preferably 0.20 mass% or less, more preferably 0.15 mass% or less.

[0069] Acid-soluble Al (sol.Al): 0.008 mass% or more and less than 0.030 mass%

[0070] Al is an important element that acts as an inhibitor and causes the Gossian oriented grains to undergo secondary recrystallization. In order to achieve this effect, 0.008% by mass or more is required. On the other hand, if added in excess, not only will grain growth be excessively inhibited, the Gossian oriented grains will not undergo secondary recrystallization, but a dense oxide film will form on the surface, which sometimes makes it difficult to control the amount of nitriding during nitriding or hinders decarburization. Therefore, sol.Al is suppressed to less than 0.030% by mass. The Al content is preferably 0.010% by mass or more, more preferably 0.013% by mass or more. The Al content is preferably 0.022% by mass or less, more preferably 0.020% by mass or less.

[0071] N: 0.004-0.009 mass%

[0072] Like Al, N is an important element that acts as an inhibitor and causes secondary recrystallization of Goss-oriented grains. To achieve this effect, it is necessary to add 0.004% by mass or more. On the other hand, N can sometimes cause defects such as blistering when the billet is heated, so it is suppressed to 0.009% by mass or less. In addition, N combines with Al and precipitates as AlN. Al and N combine in an atomic weight ratio of 1:1. Therefore, even if N is contained in an atomic weight ratio of 1 or more relative to Al, that is, in the range of (14.00 / 26.98)×[%sol.Al] relative to the mass% of sol.Al, the inhibitor effect cannot be fully exerted. Therefore, the N amount is 0.009% by mass or less. Preferably, the N amount satisfies the condition of (14.00 / 26.98)×[%Sol.Al]-0.002% by mass or more. In addition, the amount of N preferably satisfies the condition of (14.00 / 26.98)×[%Sol.Al]+0.002 mass % or less.

[0073] At least one of S: 0.0005 to 0.02 mass% and Se: 0.0005 to 0.02 mass%

[0074] S and Se combine with Mn to form inhibitors, but if the content of one or two selected from S and Se is less than 0.0005 mass%, the absolute amount of the inhibitor is insufficient, resulting in insufficient inhibitory force on normal grain growth. On the other hand, if the content of one or two selected from S and Se exceeds 0.02 mass%, desulfurization and desegregation are incomplete during secondary recrystallization annealing, thus causing iron loss degradation. Therefore, the content of one or two selected from S and Se is in the range of 0.0005 to 0.02 mass%, respectively. The content of one or two selected from S and Se is preferably 0.001 mass% or more, more preferably 0.002 mass% or more, respectively. In addition, the content of one or two selected from S and Se is preferably 0.01 mass% or less, more preferably 0.008 mass% or less, respectively.

[0075] The remainder of the steel slab composition other than the above-mentioned components is Fe and unavoidable impurities.

[0076] The component composition may further contain one or more selected from the group consisting of Sb: 0.005 to 0.500 mass % and Sn: 0.005 to 0.50 mass %.

[0077] Sb: 0.005-0.500 mass%

[0078] Sb is an element required to improve the selective growth of Goss-oriented grains as an inhibitor. To achieve this effect, 0.005% by mass is added. On the other hand, excessive addition impairs rollability and hinders manufacturing, so the upper limit is 0.500% by mass. The Sb amount is preferably 0.010% by mass or more, more preferably 0.015% by mass or more. In addition, the Sb amount is preferably 0.20% by mass or less, more preferably 0.10% by mass or less.

[0079] Sn: 0.005-0.500 mass%

[0080] Sn is an element required to improve the selective growth of Gossian oriented grains as an inhibitor. To achieve this effect, 0.005% by mass is added. On the other hand, to improve rollability, the upper limit is 0.500% by mass. The amount of Sn is preferably 0.010% by mass or more, more preferably 0.015% by mass or more. In addition, the amount of Sn is preferably 0.20% by mass or less, more preferably 0.10% by mass or less.

[0081] It should be noted that in the present invention, in order to improve magnetic properties, etc., one or more selected from Ni: 0.01-1.50 mass%, Cr: 0.005-0.50 mass%, Cu: 0.03-0.50 mass%, P: 0.005-0.500 mass%, As: 0.0005-0.05 mass%, Bi: 0.005-0.500 mass%, Mo: 0.005-0.100 mass%, B: 0.0002-0.0025 mass%, Te: 0.0005-0.0100 mass%, Zr: 0.001-0.010 mass%, Nb: 0.001-0.010 mass%, V: 0.001-0.010 mass% and Ta: 0.001-0.010 mass% may be appropriately contained.

[0082] Adding Cr within the above range promotes film formation. When Cr is added, the amount added is preferably 0.01 mass % or more. Furthermore, when Cr is added, in order to keep the magnetic flux density B8 within a more appropriate range, the amount added is preferably 0.1 mass % or less.

[0083] Furthermore, if Ni is added within the above range, the γ phase fraction can be increased. When Ni is added, the amount added is more preferably 0.5 mass % or less in order to further reduce production costs and prevent embrittlement of the steel.

[0084] Next, the method for producing the grain-oriented electrical steel sheet of the present invention will be described.

[0085] After melting the steel raw material with the above composition through conventional refining steps, it is formed into steel slabs using conventional agglomeration and block rolling or continuous casting. Alternatively, thin steel slabs with a thickness of less than 100 mm can be produced by direct casting. The steel slab is then heated to a temperature above the γ phase precipitation temperature but below 1380°C and then hot rolled. The γ phase precipitation temperature can be estimated in advance using equilibrium calculation software such as Thermo-Calc (Thermo-Calc Software AB) or verified experimentally. When estimating the γ phase precipitation temperature using Thermo-Calc ver. 2017b, the TCFE7: TCS Steel and Fe-alloys Database v7.0 is used as the database. Only elements available in this database are used in the calculation. If γ phase precipitates during reheating, carbon is enriched in the γ phase, resulting in a non-uniform structure and an inability to achieve a high magnetic flux density. Furthermore, if the billet is heated at a temperature exceeding 1380°C, the ferrite grain size before hot rolling becomes excessively large, the recrystallization rate becomes low, and a high magnetic flux density cannot be achieved after final annealing. The billet heating temperature is preferably 1360°C or lower. Note that the billet heating temperature is based on the billet surface temperature.

[0086] Next, the heated billet is subjected to a true strain ε of plate thickness at a temperature not lower than (the temperature at which the γ phase fraction is maximum - 20°C) and at least two passes. t The rough rolling is carried out by rolling with a rolling speed of 0.50 or more to produce a rough rolled plate. Here, the plate thickness true strain ε t Calculated by the following formula (1).

[0087] ε t = -ln(plate thickness after rolling / plate thickness before rolling)…(1)

[0088] This is because rolling is performed at a higher temperature, thereby increasing the rolling rate per pass, thereby promoting strain introduction and making the ferrite structure more easily recrystallized. Therefore, it is believed that the ferrite structure before finish rolling can be refined, and the recrystallization of ferrite can be promoted in the subsequent finish rolling. As a result, the proportion of grains with less strain in the microstructure of the hot-rolled plate can be increased, and a high magnetic flux density can be obtained. The plate thickness true strain εt is more preferably greater than 0.60. The plate thickness true strain εt is preferably greater than 0.60. t The upper limit of is not particularly limited, but is preferably 0.80 or less.

[0089] Rough rolling preferably includes one or more rolling passes from (temperature of maximum γ phase fraction - 20°C) to (temperature of maximum γ phase fraction + 50°C). In the rolling from (temperature of maximum γ phase fraction - 20°C) to (temperature of maximum γ phase fraction + 50°C), the rolling is performed in a state where a large amount of hard γ phase is dispersed. Therefore, the strain introduction in ferrite can be promoted, the driving force for recrystallization can be increased, the microstructure before finish rolling can be refined, and the magnetic flux density B8 can be further increased. More preferably, rough rolling includes one or more rolling passes from (temperature of maximum γ phase fraction - 15°C). In addition, more preferably, rough rolling includes one or more rolling passes below (temperature of maximum γ phase fraction + 40°C). It should be noted that the rolling temperature of rough rolling is based on the temperature of the steel plate surface.

[0090] The number of rough rolling passes is preferably 4. By setting the number of rough rolling passes to 4, the number of recrystallizations can be increased, and the microstructure before finish rolling can be refined, thereby further improving the magnetic flux density B8.

[0091] In the finishing rolling, the finishing temperature is set to 900°C or above. It should be noted that the finishing temperature is the average temperature of the steel plate surface at the front end of the coil and the tail end of the coil. This is because if the finishing temperature is lower than 900°C, the inhibitor will precipitate during the finishing rolling, and the inhibitor of the hot-rolled plate will become too coarse. Since the finer the inhibitor, the more conducive it is to the selective growth of the Gossian orientation in the secondary recrystallization annealing, it is preferred to precipitate it finely at the stage of the hot-rolled plate. The finishing temperature is preferably above 950°C. The upper limit of the finishing temperature is not particularly limited, but in order to prevent the inhibitor from coarsely precipitating after rolling, it is preferably below 1000°C.

[0092] In order to prevent the coarsening of the inhibitor, the hot-rolled plate is cooled at a cooling rate of 70°C / s or more for more than 1 second within 2 seconds after the completion of the finish rolling, and the cooled hot-rolled plate is coiled at a coiling temperature of 600°C or less to complete the hot rolling process. Preferably, the hot-rolled plate is cooled within 1 second after the completion of the finish rolling. In addition, the cooling time is preferably 2 seconds or more. The upper limit of the cooling time is not particularly limited, but is preferably 8 seconds or less. The cooling rate is more preferably 80°C / s or more. The upper limit of the cooling rate is not particularly limited, but is more preferably 300°C / s or less. It should be noted that the cooling rate is based on the temperature of the steel plate surface. The lower limit of the coiling temperature is not particularly limited, but is preferably 450°C or more. The coiling temperature is 600°C or less. The coiling temperature is the average of the surface temperature of the steel plate at the front end and the surface temperature of the steel plate at the rear end of the hot-rolled plate strip.

[0093] Next, temper rolling can be performed after finishing rolling and before annealing the hot-rolled sheet. Temper rolling can enforce the shape of the steel sheet. The elongation of temper rolling is preferably 0.05% or more. By setting the elongation of temper rolling to 0.05% or more, strain is introduced into the hot-rolled sheet, the size of the ferrite grains is increased in the subsequent hot-rolled sheet annealing process, and the magnetic flux density B8 of the grain-oriented electromagnetic steel sheet is further increased by making the aggregate structure of the primary recrystallized sheet more preferable. However, if the recrystallization rate Y of the hot-rolled sheet is not 20% or more, the effect of introducing strain by temper rolling is low. The elongation of temper rolling is more preferably 0.1% or more. The elongation of temper rolling is more preferably 10% or less.

[0094] Next, the hot-rolled sheet after finish rolling or the hot-rolled sheet obtained by the temper rolling process is subjected to hot-rolled sheet annealing. During hot-rolled sheet annealing, the key point of the present invention is to appropriately precipitate inhibitors based on the recrystallization rate Y of the center layer of the hot-rolled sheet. The soaking temperature for hot-rolled sheet annealing is 1000°C or higher. This is because, at soaking temperatures below 1000°C, particularly in manufacturing methods without intermediate annealing during cold rolling, as in the present invention, the diffusion of inhibitor-forming elements such as Al is insufficient, and the precipitated inhibitors cannot austenite ripen to the appropriate size. Furthermore, at low soaking temperatures, residual strain in grains elongated in the rolling direction of the hot-rolled sheet cannot be removed, making it difficult to fully grow the precipitated inhibitors, hindering the development of secondary recrystallization. On the other hand, at high soaking temperatures, the inhibitors dissolve, increasing the amount of inhibitors that cannot be precipitated. In the present invention, the upper limit of the soaking temperature is determined by the recrystallization rate Y (%) of the hot-rolled sheet and is specifically set at or below (1150-2.5Y)°C. That is, when the recrystallization rate Y of the hot-rolled sheet is high, a lower soaking temperature allows for more inhibitor precipitation. Conversely, when the recrystallization rate Y of the hot-rolled sheet is low, hot-rolled sheet annealing is performed at a higher soaking temperature to prioritize strain removal in the ferrite structure. The soaking temperature for hot-rolled sheet annealing is more preferably 1050°C or higher. Furthermore, the soaking temperature for hot-rolled sheet annealing is more preferably (1150-2.8Y)°C or lower. It should be noted that the soaking temperature for hot-rolled sheet annealing is based on the temperature of the steel sheet surface.

[0095] Here, the recrystallization rate Y of the center layer of the plate thickness of the hot-rolled plate is calculated as follows. First, the microstructure of the L section of the hot-rolled plate is measured by the SEM-EBSD method (scanning electron microscope-electron back scattering diffraction). The L section of the hot-rolled plate is ground to form an observation surface. The measurement includes the center layer of the plate thickness from the 1 / 5 depth position of the plate thickness of the observation surface (the layer that penetrates 20% of the inner layer from a single surface of the steel plate in the plate thickness direction) to the 4 / 5 depth position of the plate thickness (the layer that penetrates 80% of the inner layer from the above single surface in the plate thickness direction). The measurement area in the rolling direction is more than 1 mm. The step size is 1.5 μm. The obtained data is analyzed by software such as OIM Analysis (v9) and the Kernel average misorientation (KAM) diagram analysis is performed. The calculation point of the KAM value is the second approach point. The KAM value reflects the local crystal orientation change caused by dislocations in the structure and is considered to have a good correlation with microstrain. It shows a low value of less than 0.5 in areas with less strain such as recrystallized grains. Here, the area ratio of the region with a KAM value of 0.4 or less in the region from 1 / 4 depth position to 3 / 4 depth position of the plate thickness is set as the recrystallization rate Y. It should be noted that in the evaluation of the KAM value, the range of the plate thickness to be measured is extremely important. Usually, during the hot rolling process, the surface side of the steel plate is subjected to large shear strain. Since strain becomes the driving force for the expression of recrystallization, the recrystallization rate of the hot-rolled plate shows a higher value on the surface side of the plate thickness. For example, for a sample in which the area ratio of the region with a KAM value of 0.4 or less from the 1 / 4 depth position to the 3 / 4 depth position of the plate thickness is 29%, when the area ratio of the region with a KAM value of 0.4 or less in the entire plate thickness is calculated, it is 50%.

[0096] In order to obtain a particularly excellent magnetic flux density B8, the recrystallization rate Y of the hot-rolled sheet is preferably 15% or more, more preferably 18% or more, further preferably 20% or more, and most preferably 24% or more.

[0097] After the hot-rolled sheet is annealed, the hot-rolled annealed sheet is cold-rolled to produce a cold-rolled sheet having the final sheet thickness. In this method without intermediate annealing, the soaking time of the hot-rolled sheet annealing is set to 60 seconds or more to promote the austenitic ripening of the precipitated inhibitors. After soaking, the hot-rolled annealed sheet is cooled to 80°C or less by any one of rapid cooling, slow cooling, isothermal holding, or a combination thereof without increasing the temperature of the steel sheet. Here, (1) the temperature range above 800°C is an important temperature range for the austenitic ripening of the inhibitors. Therefore, in order to promote the growth of the inhibitors, the first average cooling rate v1 from the soaking temperature to 800°C is preferably less than 40°C / s. The first average cooling rate v1 from the soaking temperature to 800°C is more preferably less than 30°C / s. (2) The temperature range of 650-800°C is a temperature range related to the precipitation of carbides. In order to suppress the formation of coarse carbides, the second average cooling rate v2 from 800°C to 650°C is preferably greater than the first average cooling rate v1. (3) The temperature range of 400-650°C is a temperature range related to the precipitation of silicon nitride. The residence time t3 of the hot-rolled sheet in the temperature range of 650°C to 400°C is preferably 10 seconds or more. By setting the residence time t3 to 10 seconds or more, N that cannot precipitate at high temperatures above 1000°C can be precipitated in the form of silicon nitride, thereby increasing the magnetic flux density of the final product sheet. There are many unknowns about the detailed mechanism, but when N precipitates in the form of silicon nitride in the hot-rolled annealed sheet, the amount of AlN precipitated during decarburization annealing increases compared to when N exists in a solid solution state, and the inhibitor effect becomes stronger, thereby increasing the magnetic flux density of the final product sheet. By isothermally holding the hot-rolled sheet in this temperature range for 10 seconds or more, or cooling the hot-rolled sheet for 10 seconds or more by a cooling method without using water, the residence time t3 of the hot-rolled sheet in the temperature range of 650°C to 400°C can be set to 10 seconds or more. More preferably, the residence time t3 of the hot rolled sheet in the temperature range of 650°C to 400°C is 15 seconds or more. (4) Below 400°C is a temperature range related to suppressing the coarsening of carbides or ensuring the amount of dissolved carbon. In this temperature range, cooling is preferably performed at a cooling rate of 50°C / s or more for 2 seconds or more. More preferably, cooling is performed at a cooling rate of 50°C / s or more for 3 seconds or more at a temperature below 400°C. It should be noted that the cooling temperatures and cooling rates for hot rolled sheet annealing are based on the temperature of the steel sheet surface.

[0098] Cold rolling can be performed by either tandem rolling (unidirectional rolling) or reverse rolling, or by using known warm rolling techniques or interpass aging techniques. The cold rolling reduction is 88% to 91%. If the cold rolling reduction is 88% to 91%, the microstructure of the primary recrystallized sheet can be optimized for Goss-oriented selective growth during secondary recrystallization.

[0099] From the viewpoint of reducing the rolling load, the final thickness of the cold-rolled sheet is preferably 0.15 mm or more. The upper limit of the final thickness of the grain-oriented electrical steel sheet is not particularly limited, but is preferably 0.30 mm.

[0100] The cold-rolled sheet, which has been reduced to its final thickness, is then subjected to a primary recrystallization annealing. When this primary recrystallization annealing also serves as a decarburization annealing, the annealing temperature is preferably in the range of 800-900°C for rapid decarburization reaction. Furthermore, a humid atmosphere is preferred. It should be noted that the decarburization annealing can also be performed separately from the primary recrystallization annealing. The annealing temperature for the primary recrystallization annealing is based on the surface temperature of the steel sheet.

[0101] Next, the primary recrystallization annealed sheet is subjected to secondary recrystallization annealing to obtain a grain-oriented electrical steel sheet. When the iron loss characteristics and the reduction of transformer noise are particularly important, it is preferred to apply an annealing separator mainly composed of MgO to the surface (one or both sides) of the primary recrystallization annealed sheet and perform secondary recrystallization annealing after drying. Here, mainly composed of MgO means that the annealing separator contains 80% or more of MgO by mass relative to the entire annealing separator. By applying the annealing separator to the surface of the primary recrystallization annealed sheet and then performing secondary recrystallization annealing, a highly integrated secondary recrystallized structure in the Gossian orientation can be developed, and a forsterite film can be formed on the surface of the steel sheet. On the other hand, when blanking workability is important and the formation of a forsterite film is not desired, it is preferred not to apply an annealing separator, or to perform secondary recrystallization annealing using an annealing separator mainly composed of silica or alumina. Here, mainly composed of silica or alumina means that the annealing separator contains 80% or more of silica or alumina by mass relative to the entire annealing separator. It should be noted that, when a forsterite film is not formed, it is also effective to apply the annealing separator by electrostatic coating without introducing moisture. Alternatively, a known heat-resistant inorganic material sheet can be used in place of the annealing separator. Examples of heat-resistant inorganic material sheets include silica, alumina, and mica.

[0102] As a condition for secondary recrystallization annealing, when a forsterite film is formed, it is preferred to maintain the temperature at around 800-1050°C for more than 20 hours, and then raise the temperature to 1100°C or higher after the secondary recrystallization is expressed and completed. In the case of implementing purification treatment with emphasis on iron loss characteristics, it is more preferred to further raise the temperature to around 1200°C. On the other hand, when no forsterite film is formed, it is sufficient as long as the secondary recrystallization is completed, so the annealing can be terminated by raising the temperature to 800-1050°C. It should be noted that the annealing temperature of the secondary recrystallization annealing is based on the temperature of the steel plate surface. Alternatively, when it is difficult to directly measure the temperature of the steel plate surface, the temperature of the steel plate surface inferred from the furnace temperature, etc. can be used as the annealing temperature of the secondary recrystallization annealing.

[0103] In addition, the unreacted annealing separator attached to the surface of the secondary recrystallization annealed sheet (grain-oriented electromagnetic steel sheet) after the secondary recrystallization annealing can be removed by water washing, flushing, pickling, etc. In addition, the secondary recrystallization annealed sheet can be further subjected to flattening annealing. Since the secondary recrystallization annealing is usually carried out in a coil state, the winding characteristics of the coil are generated. This winding characteristic has the risk of deteriorating the iron loss characteristics. By performing flattening annealing, shape correction can be performed to further reduce the iron loss. Furthermore, in the case of stacking steel sheets, it is effective to form an insulating film on the surface of the steel sheet during or before and after the above-mentioned flattening annealing. In particular, in order to reduce iron loss, it is preferred to form a tension-imparting film that imparts tension to the steel sheet as an insulating film. In order to form the tension-imparting film, in addition to the method of applying the tension-imparting film by adhesive coating, a method of using a physical vapor deposition method or a chemical vapor deposition method to deposit an inorganic substance on the surface of the steel sheet and forming an insulating film thereon instead of the forsterite film can also be used. According to these methods, an insulating film having excellent film adhesion and a significantly large iron loss reduction effect can be formed.

[0104] In order to further reduce iron loss, it is preferable to subject the grain-oriented electrical steel sheet to a magnetic domain refining treatment. Examples of methods for magnetic domain refining include forming grooves on the surface (front or back) of the grain-oriented electrical steel sheet (final product sheet), introducing thermal strain or impact strain linearly or point-wise through plasma irradiation, laser irradiation, electron beam irradiation, or the like, and forming grooves by etching the surface of a cold-rolled sheet or intermediate steel sheet after cold rolling to the final sheet thickness.

[0105] In addition, the manufacturing conditions other than the above conditions can be carried out by conventional methods.

[0106] According to the technology disclosed in this invention, even in a composition system that actively utilizes inhibitors by containing 0.008% by mass or more of Al, by appropriately managing the rough rolling pass schedule and increasing the frequency of low-strain grains in the hot-rolled sheet, a crystal aggregate structure that is conducive to increasing the magnetic flux density after secondary recrystallization can be formed in the primary recrystallized sheet. As a result, it is possible to produce grain-oriented electrical steel sheets with superior magnetic properties compared to conventional techniques. If grain-oriented electrical steel sheets produced according to this technology are used in transformers, not only can energy efficiency be improved, but transformer noise can also be reduced. According to the method for producing grain-oriented electrical steel sheets, not only can power equipment such as transformers be used efficiently, but also noise caused by magnetostriction during operation can be reduced.

[0107] The present invention can exhibit superior magnetic properties compared to conventional technologies. The manufacturing method of the present invention can produce a grain-oriented electrical steel sheet with a magnetic flux density B8 of 1.935 T or greater. The magnetic flux density B8 is measured using the Epstein method described in JIS C2550 using Epstein test pieces cut from the grain-oriented electrical steel sheet.

[0108] Example

[0109] Steel raw materials with the composition shown in Table 4, with the remainder consisting of Fe and inevitable impurities, were melted and formed into steel slabs by continuous casting. The slabs were heated under the conditions shown in Table 5 and subjected to rough rolling to produce rough-rolled sheets. The rough-rolled sheets were then finish-rolled to produce hot-rolled sheets. The hot-rolled sheets were cooled within 1.5 seconds after the completion of finish rolling, coiled, and annealed to produce hot-rolled annealed sheets. The γ phase precipitation temperature and the temperature at which the γ phase fraction reaches its maximum (γ phase maximum temperature) were calculated using Thermo-Calc ver. 2017b.

[0110] Here, the rough rolling condition (1) is "to reduce the introduced plate thickness true strain ε to 100°C at a temperature higher than (the temperature at which the γ phase fraction is the largest - 20°C)". t The rolling with a temperature of 0.50 or more is set to be more than 2 passes". Condition (2) is "rolling with a temperature of (maximum γ phase fraction - 20°C) to (maximum γ phase fraction + 50°C) for more than 1 pass". Condition (3) is "the number of rough rolling passes is 4 passes in total". In Table 5, when these conditions are met, it is recorded as 0, and when they are not met, it is recorded as ×. The finishing temperature (FDT) is the average of the surface temperature of the steel plate at the front end of the strip and the surface temperature of the steel plate at the tail end. The thickness of the plate after hot rolling is 2.2 to 2.3 mm in any case. After annealing the hot-rolled plate under the conditions shown in Table 5, it is cold rolled to a plate thickness of 0.22 mm at a rolling rate of 90%. Next, in 60 vol% H2-40 vol% N2, dew Point: In a humid atmosphere of 58°C, a primary recrystallization annealing is performed at 860°C for 120s to produce a primary recrystallization plate. After an annealing separator with MgO as the main component is applied to the surface of the primary recrystallization plate, a secondary recrystallization annealing is performed at 1200°C for 50 hours, followed by application of a phosphate-based insulating tension coating, sintering, and flattening annealing for the purpose of flattening the steel strip to produce a product plate. An Epstein test piece is cut out from the obtained product plate, and the magnetic flux density B8 is measured by the above method. In addition, the recrystallization rate Y of the hot-rolled plate after winding is measured by the above method. The results are shown in Table 5. It should be noted that if the magnetic flux density B8 is 1.935T or more, it is judged to be excellent in magnetic flux density.

[0111]

[0112]

Claims

1. A method for manufacturing a grain-oriented electrical steel sheet, A steel slab having the following composition, wherein the steel slab comprises 0.005-0.085 mass% of C, 2.00-4.50 mass% of Si, 0.03-1.00 mass% of Mn, 0.008 mass% or more and less than 0.030 mass% of sol. Al, and 0.004-0.009 mass% of N, and further comprises at least one of 0.0005-0.02 mass% of S and 0.0005-0.02 mass% of Se, with the remainder being Fe and unavoidable impurities, is heated to a temperature exceeding a γ phase precipitation temperature and not exceeding 1380° C. Next, the steel slab is subjected to rough rolling at a temperature not less than (temperature at which the γ phase fraction is maximum - 20°C) including two or more passes of rolling to introduce a true thickness strain εt of 0.50 or more to produce a rough-rolled sheet, wherein: The temperature at which the γ phase fraction is maximum is calculated using Thermo-Calc ver. 2017b. Next, the rough-rolled sheet is subjected to finish rolling at a rolling end temperature of 900° C. or higher to obtain a hot-rolled sheet. Next, within 2 seconds after the completion of the finish rolling, the hot rolled sheet is cooled at a cooling rate of 70°C / s or higher for 1 second or longer. The cooled hot-rolled sheet is coiled at a coiling temperature below 600°C. Next, the coiled hot-rolled sheet is subjected to hot-rolled sheet annealing at a soaking temperature of 1000° C. to (1150-2.5Y)° C. for 60 seconds or longer to produce a hot-rolled annealed sheet, wherein Y is the recrystallization rate in % of the center layer of the hot-rolled sheet after coiling. Next, the hot-rolled annealed sheet is cold-rolled at a rolling reduction of 88% to 91% to obtain a cold-rolled sheet having a final thickness. Then, the cold rolled sheet is subjected to primary recrystallization annealing to produce a primary recrystallization annealed sheet. Next, the primary recrystallization annealed sheet is subjected to secondary recrystallization annealing to obtain a grain-oriented electrical steel sheet; The magnetic flux density B8 in the rolling direction of the grain-oriented electromagnetic steel sheet is 1.940 T or more. The plate thickness true strain ε t Calculated by the following formula (1), ε t = -ln (plate thickness after rolling / plate thickness before rolling)…(1).

2. The method for producing a grain-oriented electrical steel sheet according to claim 1, wherein: The above-mentioned composition further contains one or two selected from the group consisting of Sb: 0.005 to 0.500 mass % and Sn: 0.005 to 0.500 mass %.

3. The method for producing a grain-oriented electrical steel sheet according to claim 1 or 2, wherein: The composition further contains one or more elements selected from the group consisting of Ni: 0.01-1.50 mass%, Cr: 0.005-0.50 mass%, Cu: 0.03-0.50 mass%, P: 0.005-0.500 mass%, As: 0.0005-0.050 mass%, Bi: 0.005-0.500 mass%, Mo: 0.005-0.100 mass%, B: 0.0002-0.0025 mass%, Te: 0.0005-0.0100 mass%, Zr: 0.001-0.010 mass%, Nb: 0.001-0.010 mass%, V: 0.001-0.010 mass% and Ta: 0.001-0.010 mass%.

4. The method for producing a grain-oriented electrical steel sheet according to any one of claims 1 to 3, wherein: The rough rolling includes one or more passes of rolling from (temperature at which the γ phase fraction is maximum - 20°C) to (temperature at which the γ phase fraction is maximum + 50°C).

5. The method for producing a grain-oriented electrical steel sheet according to any one of claims 1 to 4, wherein: The rough rolling is performed at least four times in total.

6. The method for producing a grain-oriented electrical steel sheet according to any one of claims 1 to 5, wherein: The soaked hot-rolled sheet is cooled with a first average cooling rate v1 from the soaking temperature to 800°C set to less than 40°C / s and a second average cooling rate v2 from 800°C to 650°C set to v1 or higher.

7. The method for producing a grain-oriented electrical steel sheet according to any one of claims 1 to 6, wherein: The recrystallization rate Y is 18% or more.

8. The method for producing a grain-oriented electrical steel sheet according to any one of claims 1 to 7, wherein: The recrystallization rate Y is 20% or more, and temper rolling with an elongation of 0.05% or more is performed after the finish rolling and before the hot-rolled sheet is annealed.

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