Method for producing grain-oriented electromagnetic steel sheet

Abstract

Proposed is a method for producing a grain-oriented electromagnetic steel sheet, with which it is possible to further improve magnetic properties and suppress fluctuations in magnetic properties and film properties in a coil without using an inhibitor. This method for producing a grain-oriented electromagnetic steel sheet includes: a hot rolling step for hot rolling a steel roll material having a constituent composition which contains C, Si, Mn, acid-soluble Al, N and Sb and in which S and Se are contained so as to satisfy a prescribed relational expression; an optional hot-rolled sheet annealing step; a cold rolling step; an acid washing step; a primary recrystallization annealing step; and a finish annealing step. In intermediate annealing in the hot-rolled sheet annealing step or cold rolling step before a final cold rolled steel sheet is obtained, the γ-phase fraction in the steel structure at the annealing temperature is 1.0-25.0 vol%, the average cooling speed after annealing is 20-50ºC / s, acid washing is carried out using a solution which contains 2-15 mass% of Fe ions and has an acid concentration of 1-20 mass% in the acid washing step, and an annealing separating agent contains a Ti compound at a quantity of 0.3-8 parts by mass in terms of Ti relative to 100 parts by mass of magnesia in the finish annealing step.

Description

Manufacturing method of grain-oriented electrical steel sheet

[0001] The present invention relates to a method for manufacturing grain-oriented electrical steel sheets used as iron core materials for transformers. In this specification, the term "x to y" representing a numerical range means x or more and y or less, and includes the boundary value.

[0002] Grain-oriented electrical steel sheets are soft magnetic materials used as iron core materials for transformers and large generators. Grain-oriented electrical steel sheets have a crystal texture in which the <001> crystallographic axis, which is the axis of easy magnetization of iron, is highly aligned in the rolling direction of the steel sheet. This texture is achieved by preferentially growing grains with the {110}<001> orientation, known as the Goss orientation, through secondary recrystallization. Therefore, a commonly used technique for manufacturing grain-oriented electrical steel sheets is to use a precipitate called an inhibitor to induce secondary recrystallization of grains with the Goss orientation during secondary recrystallization annealing.

[0003] Patent Document 1 discloses a method using AlN or MnS as an inhibitor, and Patent Document 2 discloses a method using MnS or MnSe as an inhibitor, both of which have been put into industrial use. These methods using inhibitors require slab heating at high temperatures of 1300°C or higher to completely dissolve the inhibitor components. However, these methods are extremely useful for stably developing secondary recrystallized grains.

[0004] Furthermore, in order to enhance the action of these inhibitors, Patent Document 3 discloses a method of using Pb, Sb, Nb, or Te, and Patent Document 4 discloses a method of using Zr, Ti, B, Nb, Ta, V, Cr, or Mo.

[0005] Furthermore, Patent Document 5 proposes a method in which acid-soluble Al (sol. Al) is added in an amount of 0.010 to 0.060 mass %, slab heating is kept at a low temperature, and nitriding is performed in an appropriate nitriding atmosphere in the decarburization annealing step. As a result, (Al, Si)N is precipitated during secondary recrystallization annealing and used as an inhibitor. Many methods have been proposed, called nitriding methods, in which such nitriding treatment is performed during the intermediate steps of manufacturing grain-oriented electrical steel sheets and (Al, Si)N or AlN is used as an inhibitor.

[0006] On the other hand, Patent Document 6 and other publications disclose a technique for developing Goss-oriented crystal grains by secondary recrystallization in a material that does not contain inhibitor components. This technique minimizes impurities such as inhibitor components, thereby revealing the grain boundary misorientation angle dependence of the grain boundary energy of the grain boundaries during primary recrystallization and allowing Goss-oriented crystal grains to undergo secondary recrystallization without the use of inhibitors. This effect is called the texture inhibition effect. This method does not require the fine dispersion of inhibitors in the steel, and therefore does not require high-temperature slab heating, which was previously essential. This method has significant advantages in terms of both cost and maintenance.

[0007] Furthermore, Patent Document 7 discloses a technique for achieving both magnetic properties and film properties using a material that does not contain an inhibitor component.

[0008] Japanese Patent Publication No. 40-15644 Japanese Patent Publication No. 51-13469 Japanese Patent Publication No. 38-08214 Japanese Patent Laid-Open No. 52-24116 Japanese Patent Laid-Open No. 03-02324 Japanese Patent Laid-Open No. 2000-129356 Japanese Patent Laid-Open No. 2007-138199

[0009] However, the prior art disclosed in the above patent document has the following problems. Grain-oriented electrical steel sheets manufactured from materials containing no inhibitor components still have inferior stability in magnetic properties and coating properties compared to grain-oriented electrical steel sheets manufactured using conventional inhibitors. In particular, the magnetic properties and coating properties can vary in the width or length direction of the steel strip due to the finish annealing performed after the steel sheet is wound into a coil. To combat global warming, various countries and regions are tightening regulations to improve energy efficiency, creating a high demand for grain-oriented electrical steel sheets with high magnetic properties. However, when efforts were made to improve magnetic properties to meet these demands, a new problem was discovered: the magnetic properties and coating properties also varied significantly within the coil. Therefore, when the technology for achieving both magnetic properties and coating properties was applied to steel types using Al-based inhibitors, the technology described in Patent Document 7 alone did not result in any improvement in magnetic properties.

[0010] The present invention has been made in view of the above problems, and aims to solve the above problems by proposing a method for manufacturing a grain-oriented electrical steel sheet that can further improve magnetic properties and suppress fluctuations in magnetic properties and coating properties within a coil without using an inhibitor.

[0011] To solve the above-mentioned problems, the inventors conducted extensive research focusing on the annealing conditions before final cold rolling and the primary recrystallization annealing conditions for materials that do not contain inhibitor components. As a result, they found that grain-oriented electrical steel sheets that combine excellent magnetic properties and coating properties can be consistently obtained by specifying the γ phase fraction of the material during annealing before final cold rolling and the cooling conditions after annealing. In addition, they found that grain-oriented electrical steel sheets that combine excellent magnetic properties and coating properties can be consistently obtained by specifying the pickling conditions before primary recrystallization annealing and the auxiliary agent conditions for magnesia in the annealing separator. Combining these findings led to the development of the present invention.

[0012] <Experiment 1> A steel slab containing, by mass, 0.015-0.070% C, 2.98-3.16% Si, 0.10-0.12% Mn, 0.0035-0.0052% N, 0.0040-0.065% sol. Al, 0.0015-0.0025% S, and 0.04-0.08% Sb, with the balance consisting of Fe and unavoidable impurities, was produced by continuous casting. The steel slab was subjected to slab heating at 1200°C and then hot-rolled to a 2.6 mm thick hot-rolled steel sheet. In this experiment, the objective was to change the γ-phase fraction of the steel structure at the annealing temperature by significantly changing the C content. The hot-rolled steel sheet was then subjected to hot-rolled sheet annealing at 1050°C for 30 seconds to obtain a hot-rolled annealed sheet. Next, the hot-rolled annealed steel sheet was subjected to pickling to remove surface scale, and then cold-rolled to obtain a final cold-rolled steel sheet with a thickness of 0.27 mm. The final cold-rolled steel sheet was then immersed in a 7 mass% hydrochloric acid solution containing 5 mass% Fe ions at 50°C for 10 seconds to pickle the surface, thereby obtaining a pickled cold-rolled steel sheet. Further, the pickled cold-rolled steel sheet was subjected to a pickling treatment at 840°C for 150 seconds with 55% by volume H ２ -45% by volume N ２ The steel sheet was subjected to primary recrystallization annealing accompanied by decarburization in a humid atmosphere with a dew point of 60°C to obtain a primary recrystallization annealed steel sheet. ２The resulting mixture was applied to a primarily recrystallized annealed sheet, which was then subjected to secondary recrystallization annealing at 1200°C for 15 hours, resulting in a finish-annealed sheet. The secondary recrystallization annealing was performed in a nitrogen atmosphere below 1000°C and in an Ar atmosphere above 1000°C. During the temperature increase process of the secondary recrystallization annealing, the residence time in the temperature range of 800 to 900°C was 75 hours. The finish-annealed sheet was then washed with water to remove any unreacted annealing separator, and a coating primarily composed of magnesium phosphate, colloidal silica, and chromic acid was applied.

[0013] The magnetic flux density B of the obtained sample ８ was measured by the method described in JIS C2550. ８ refers to the magnetic flux density when excited at 800 A / m. In this experiment, the conditions were changed for each coil, and the final coil was divided into four parts and the properties were evaluated at each end, with the worst value being taken as the representative value for the original coil. If the property variation is large, the value will also be poor, so if good properties are obtained, it can be determined that the property variation within the coil is also small. The following experiments were also evaluated by obtaining representative values ​​in the same way. In addition, the γ phase fraction at 1050°C, the annealing temperature of the hot-rolled sheet, was calculated using the component composition of each steel slab, and the magnetic flux density B obtained above was used. ８ The results of this analysis are shown in Figure 1. The results in Figure 1 show that good magnetic properties can be obtained when the γ phase fraction is in the range of 1.0 to 25.0 volume percent. The γ phase fraction can be calculated using equilibrium calculation software such as Thermo-Calc (Thermo-Calc software AB). In this experiment, Thermo-Calc ver. 2020b was used, and the database used was TCFE10: Steels / Fe Alloys v10.1. Only elements available in this database were used in the calculation.

[0014] <Experiment 2> Steel slab A, which contained, by mass, 0.042% C, 3.35% Si, 0.07% Mn, 0.0041% N, 0.0070% sol. Al, 0.0032% S, and 0.04% Sb, with the balance consisting of Fe and unavoidable impurities, was produced by continuous casting. Steel slab A had a γ-phase fraction of 9.9 vol% at 1070°C. Similarly, steel slab B, which contained, by mass, 0.064% C, 3.30% Si, 0.13% Mn, 0.0040% N, 0.0064% sol. Al, 0.0030% S, and 0.04% Sb, with the balance consisting of Fe and unavoidable impurities, was produced by continuous casting. Steel slab B had a γ-phase fraction of 26.8 vol% at 1070°C. Steel slab A and steel slab B were subjected to slab heating at 1220°C, followed by hot rolling to obtain a hot-rolled steel sheet with a thickness of 2.4 mm. The hot-rolled steel sheet was then subjected to hot-rolled sheet annealing at 1070°C for 20 seconds to obtain a hot-rolled annealed steel sheet. At this time, the cooling rate between 800 and 400°C during the cooling process after annealing was variously changed. Next, the hot-rolled annealed steel sheet was subjected to pickling to remove surface scale, and then cold-rolled to obtain a final cold-rolled steel sheet with a thickness of 0.27 mm. Thereafter, the final cold-rolled steel sheet was immersed in a 5 mass% hydrochloric acid solution at 65°C containing 7 mass% Fe ions for 5 seconds to pickle the surface, thereby obtaining a pickled cold-rolled steel sheet. Furthermore, the pickled cold-rolled steel sheet was subjected to pickling at 850°C for 120 seconds, 55% by volume H ２ -45% by volume N ２ The steel sheet was subjected to primary recrystallization annealing accompanied by decarburization in a humid atmosphere with a dew point of 55°C to obtain a primary recrystallization annealed steel sheet. ２ The resulting mixture was applied to a primary recrystallization annealed sheet, with 1.0 part by mass of the mixture added per 100 parts by mass of magnesia, calculated as Ti, and then subjected to secondary recrystallization annealing at 1200°C for 5 hours in a hydrogen atmosphere to obtain a finish-annealed sheet. During the secondary recrystallization annealing, an Ar atmosphere was generally used during temperature increase and decrease. During this temperature increase process, the residence time in the temperature range of 800 to 900°C was 100 hours, and a nitrogen atmosphere was used during this period. The finish-annealed sheet was then washed with water to remove any unreacted annealing separator, and a coating composed primarily of magnesium phosphate, colloidal silica, and chromic acid was applied.

[0015] The magnetic flux density B of the obtained sample ８ was measured in the same manner as in Experiment 1. The obtained magnetic flux density B８ The results of arranging the magnetic flux density B in the cooling rate of the hot-rolled sheet annealing are shown in Figure 2. From the results, it can be seen that for steel slab A, the magnetic flux density B ８ In addition, the magnetic flux density B of steel slab B was 1.92 T or more even when the cooling rate was changed. ８ It can be seen that the value is below 1.92T.

[0016] <Experiment 3> The final cold-rolled steel sheet obtained from steel slab A in Experiment 2 was immersed in a 5 mass% hydrochloric acid solution containing 0 to 20 mass% Fe ions at 50°C for 5 seconds to pickle the surface, thereby obtaining a pickled cold-rolled steel sheet. ２ -50% by volume N ２ The steel sheet was subjected to primary recrystallization annealing accompanied by decarburization in a humid atmosphere with a dew point of 52°C to obtain a primary recrystallization annealed steel sheet. ２ The steel sheets were coated with 1.5 parts by mass of titanium equivalent to 100 parts by mass of magnesia, and then subjected to secondary recrystallization annealing at 1200°C for 5 hours in a hydrogen atmosphere to obtain finish-annealed steel sheets. During the temperature-raising process of secondary recrystallization annealing, the residence time in the temperature range of 800 to 900°C was 100 hours. The finish-annealed steel sheets were then rinsed with water to remove unreacted annealing separator, and a coating composed primarily of magnesium phosphate, colloidal silica, and chromic acid was applied. The coating adhesion of the resulting samples was evaluated. Coating adhesion was evaluated by wrapping the steel sheets around cylinders of various diameters and determining the minimum diameter at which the coating did not peel. A smaller minimum diameter indicates better coating adhesion. The results of the coating adhesion evaluation, organized by the amount of Fe ions in the hydrochloric acid solution, are shown in Figure 3. These results indicate that good coating properties can be obtained when the Fe ions in the hydrochloric acid solution are in the range of 2 to 15% by mass.

[0017] <Experiment 4> The final cold-rolled steel sheet obtained from steel slab A in Experiment 2 was pickled under different pickling conditions to obtain pickled cold-rolled steel sheets. In pickling condition A, the steel sheet was immersed in a 10 mass% hydrochloric acid solution containing 7 mass% Fe ions at 75°C for 3 seconds. In pickling condition B, the steel sheet was immersed in a 10 mass% hydrochloric acid solution containing 20 mass% Fe ions at 75°C for 3 seconds. The obtained pickled cold-rolled steel sheet was then immersed in a 55% by volume H solution at 835°C for 180 seconds. ２ -45% by volume N２ The steel sheet was subjected to primary recrystallization annealing accompanied by decarburization in a humid atmosphere with a dew point of 56°C to obtain a primary recrystallization annealed steel sheet. ２ The resulting mixture was applied to a primarily recrystallized annealed sheet in an amount of 0 to 12 parts by mass per 100 parts by mass of magnesia, calculated as Ti, and then subjected to secondary recrystallization annealing at 1200°C for 5 hours to obtain a finish-annealed sheet. The secondary recrystallization annealing was performed in a nitrogen atmosphere below 1000°C and in an Ar atmosphere above 1000°C. During the temperature rise process of the secondary recrystallization annealing, the residence time in the temperature range of 800 to 900°C was 60 hours. The finish-annealed sheet was then washed with water to remove unreacted annealing separator, and a coating containing magnesium phosphate, colloidal silica, and chromic acid as its main components was applied.

[0018] The coating adhesion of the obtained sample was evaluated in the same manner as in Experiment 3. The evaluation results of the coating adhesion were calculated as TiO (Ti equivalent) per 100 parts by mass of magnesia. ２ The results arranged by the amount of addition are shown in Figure 4. From the results in Figure 4, under pickling conditions A, TiO in terms of Ti was ２ When the amount of addition is in the range of 0.3 to 8.0 parts by mass per 100 parts by mass of magnesia, good coating adhesion is obtained. ２ It is clear that good coating adhesion cannot be obtained even if the amount added is changed.

[0019] Experiments 1 and 2 identified conditions for obtaining good magnetic properties. While the reasons for this are not entirely clear, the inventors believe the following: In Experiment 1, good magnetic properties were obtained by controlling the γ-phase fraction during annealing before cold rolling to obtain the final cold-rolled steel sheet. Even in a composition system that does not use inhibitors, small amounts of Al and N, or Mn and S and Se, form precipitates and are presumed to affect grain growth. The γ-phase formed by heating a steel strip at high temperatures has a higher amount of dissolved elements than the α-phase. That is, the γ-phase has a higher amount of dissolved inhibitor-forming elements than the α-phase, resulting in a heterogeneous state. Because the amount of inhibitor-forming elements is originally low, a high γ-phase fraction promotes this heterogeneous state. Therefore, it is possible that inhibitors are formed only in the γ-phase region, resulting in non-uniform grain growth and degraded magnetic properties. Therefore, it is presumed that the γ-phase fraction during the annealing process must be kept low, at 25% by volume or less. Furthermore, if the γ phase fraction is extremely low, the grain size becomes coarse during annealing before cold rolling to obtain the final cold-rolled steel sheet, which is thought to significantly change the texture before secondary recrystallization and affect the magnetic properties.

[0020] The influence of the average cooling rate between 800 and 400°C in the cooling process after annealing before cold rolling to obtain the final cold-rolled steel sheet is believed to be as described in Patent Document 7. Here, in this experiment, the influence of the γ phase fraction was also newly confirmed, and it was newly discovered that, unless it is within the above-mentioned appropriate range, good magnetic properties cannot be obtained even if the average cooling rate between 800 and 400°C is within the appropriate range described in Patent Document 7. In other words, it was newly discovered that good magnetic properties cannot be obtained unless these two conditions are specified.

[0021] In Experiments 3 and 4, conditions for obtaining good coating adhesion were identified. Experiment 3 showed that the amount of Fe ions in the pickling performed before the primary recrystallization annealing accompanied by decarburization had a significant effect on coating adhesion. The reason for this is not entirely clear, but the inventors speculate as follows: It is believed that Fe ions promoted cleaning of the steel sheet surface with acid through their catalytic effect. That is, when there were few Fe ions, the reactivity in the acid solution was poor, and cleaning of the steel sheet surface was insufficient. Furthermore, when there were too many Fe ions, the pickling effect was reduced, and cleaning of the steel sheet surface was also insufficient. This surface cleaning is believed to have affected the coating properties. The effect obtained in Experiment 4 is believed to be as described in Patent Document 7. It was newly discovered in this experiment that pickling must be performed with an acid solution containing a certain range of Fe ions as a prerequisite.

[0022] The present invention has been made based on this finding, and the method for producing a grain-oriented electrical steel sheet according to the present invention, which advantageously solves the above-mentioned problems, is configured as follows.

[0023] [1] In mass%, C: 0.01 to 0.10%, Si: 2.5 to 4.0%, Mn: 0.04 to 0.50%, acid-soluble Al: 0.0030 to 0.0100%, N: 0.0020 to 0.0060%, and Sb: 0.03 to 0.30%, and the contents of S and Se satisfy S + 0.405Se ≦ 0.0050%, and optionally, Group A: at least one selected from Sn: 0.005 to 0.500%, Cr: 0.005 to 0.500%, Cu: 0.01 to 0.50%, Ni: 0.01 to 0.50%, Bi: 0.005 to 0.500%, P: 0.005 to 0.200%, Mo: 0.005 to 0.500%, and Co: 0.001 to 0.500%; Group B: at least one selected from B: 0.0001 to 0.0025, Nb: 0.001 to 0.020%, Ti: 0.0005 to 0.0400%, V: 0.001 to 0.020%, and W: 0.001 to 0.020%; Group C:containing at least one component selected from the group consisting of Zn: 0.0005 to 0.020%, Zr: 0.001 to 0.020%, Pb: 0.0001 to 0.0100%, As: 0.001 to 0.020%, Ag: 0.001 to 0.050%, Au: 0.001 to 0.050%, Ga: 0.0001 to 0.0050%, Ge: 0.0001 to 0.0050%, Ca: 0.0005 to 0.020%, Mg: 0.0005 to 0.020%, REM: 0.0005 to 0.0200%, and Hf: 0.001 to 0.020%, A hot rolling process in which a steel material having a composition with the balance consisting of Fe and unavoidable impurities is heated to a temperature of 1250°C or less and hot-rolled to form a hot-rolled steel sheet; a hot-rolled sheet annealing process in which the hot-rolled steel sheet is optionally subjected to hot-rolled sheet annealing to form a hot-rolled annealed sheet; a cold rolling process in which the hot-rolled steel sheet or the hot-rolled annealed sheet is cold-rolled once or twice or more times with intermediate annealing in between to form a final cold-rolled steel sheet; a pickling process in which the final cold-rolled steel sheet is pickled to form a pickled cold-rolled steel sheet; a primary recrystallization annealing process in which the pickled cold-rolled steel sheet is subjected to primary recrystallization annealing accompanied by decarburization to form a primary recrystallization annealed sheet; and a finish annealing step of applying an annealing separator mainly composed of TiO to the final cold-rolled steel sheet and performing secondary recrystallization annealing, wherein hot-rolled sheet annealing in the hot-rolled sheet annealing step or intermediate annealing in the cold rolling step is performed before cold rolling to obtain the final cold-rolled steel sheet, and during annealing before obtaining the final cold-rolled steel sheet, the γ phase fraction of the steel structure at the annealing temperature is 1.0 to 25.0 volume % and the average cooling rate between 800 and 400°C in the cooling process after annealing is 20 to 50°C / s, and in the pickling step, the final cold-rolled steel sheet is pickled with a solution containing 2 to 15 mass % of Fe ions and having an acid concentration of 1 to 20 mass %, and in the finish annealing step, ２ , TiO ３ ・H ２ O, TiO・(OH) ２ and Ti(OH) ４[2] The method for producing a grain-oriented electrical steel sheet according to the above [1], wherein the temperature rise process in the final annealing step involves a residence time in the temperature range of 800 to 900°C of 40 to 150 hours. [3] The method for producing a grain-oriented electrical steel sheet according to the above [1] or [2], wherein the annealing separator in the final annealing step further comprises SrSO ４ , Sr(OH) ２ ・8H ２ O, SrCO ３ and Sr(NO) ３ The method for producing a grain-oriented electrical steel sheet contains at least one Sr compound selected from the group consisting of Sr, Sr-equivalent, in an amount of 0.2 to 8 parts by mass per 100 parts by mass of magnesia.

[0024] According to the present invention, by using a material that does not contain inhibitor components, the γ phase fraction of the material during annealing before final cold rolling, the cooling conditions after annealing, the pickling conditions before primary recrystallization annealing, and the auxiliary agent conditions for magnesia in the annealing separator during finish annealing are specified, and the resulting grain-oriented electrical steel sheet can achieve good magnetic properties and coating properties.

[0025] The γ phase fraction of the material is the magnetic flux density B ８ 1 is a graph showing the effect of the cooling rate of the hot-rolled sheet annealing on the magnetic flux density B ８ 1 is a graph showing the effect of Fe ion concentration in a hydrochloric acid solution on coating adhesion. ２ 1 is a graph showing the effect of the content on the coating adhesion.

[0026] Hereinafter, a method for manufacturing a grain-oriented electrical steel sheet according to an embodiment of the present invention will be described.

[0027] <Steel material for manufacturing grain-oriented electrical steel sheet> First, the chemical composition of the steel material used to manufacture the grain-oriented electrical steel sheet according to this embodiment and the reasons for limiting this composition will be described. In the following description, unless otherwise specified, the notation "%" in the chemical composition of the steel material means "% by mass."

[0028] C: 0.01 to 0.10% If the C content is less than 0.01%, the steel structure of the material will be a single α phase, with no γ phase appearing, resulting in poor magnetic properties. In addition, the steel will become embrittled during casting or hot rolling, resulting in defects that interfere with manufacturing. On the other hand, if the C content exceeds 0.10%, it will be difficult to reduce it to 0.005% or less, at which point magnetic aging does not occur, by annealing accompanied by decarburization. Therefore, the C content is set to the range of 0.01 to 0.10%. Preferably, the C content is in the range of 0.025 to 0.06%.

[0029] Si: 2.5 to 4.0% Si is an element necessary for increasing the resistivity of steel and reducing iron loss. The above effects are not sufficient if the Si content is less than 2.5%. On the other hand, if the Si content exceeds 4.0%, workability decreases, making it difficult to manufacture by rolling. Therefore, the Si content is set to the range of 2.5 to 4.0%. Preferably, the Si content is in the range of 2.8 to 3.6%.

[0030] Mn: 0.04 to 0.50% Mn is an element necessary for improving the hot workability of steel. The above effect is not sufficient if the Mn content is less than 0.04%. On the other hand, if the Mn content exceeds 0.50%, the magnetic flux density of the finished sheet decreases. Therefore, the Mn content is set to the range of 0.03 to 0.50%. Preferably, the Mn content is in the range of 0.05 to 0.30%.

[0031] Acid-soluble Al: 0.0030 to 0.0100%. When producing grain-oriented electrical steel sheets by inducing secondary recrystallization using a composition system without using an inhibitor, Al is an impurity element. Therefore, the acid-soluble Al content must be 0.0100% or less. However, if the acid-soluble Al content is 0.0030% or more, it densifies the oxide film formed on the steel sheet surface during primary recrystallization annealing, which involves decarburization. This suppresses nitrogen absorption during secondary recrystallization annealing, improving the concentration of secondary recrystallized grains in the Goss orientation and improving magnetic properties. Therefore, in this embodiment, the acid-soluble Al content is set to a range of 0.0030 to 0.0100%. Preferably, the acid-soluble Al content is set to a range of 0.0040 to 0.0080%.

[0032] N: 0.0020 to 0.0060% When producing grain-oriented electrical steel sheet by inducing secondary recrystallization in a component system without using an inhibitor, N (nitrogen) is an impurity element. Therefore, the N content must be 0.0060% or less. However, in order to suppress nitrogen absorption during secondary recrystallization annealing, the N content must be 0.0020% or more. In this embodiment, the N content is in the range of 0.0020 to 0.0060%. Preferably, the N content is in the range of 0.0030 to 0.0050%.

[0033] Sb: 0.03 to 0.30% Sb very effectively suppresses the increase in nitrogen content in steel sheet during secondary recrystallization annealing. Sb is an essential element for obtaining excellent magnetic properties and stabilizing those properties. To fully exert this effect, an Sb content of 0.03% or more is required. However, if the Sb content exceeds 0.30%, decarburization during primary recrystallization annealing becomes very poor, making the steel unsuitable for industrial mass production. Therefore, the Sb content is limited to the range of 0.03 to 0.30%. Preferably, the Sb content is in the range of 0.04 to 0.15%.

[0034] The S and Se contents satisfy S + 0.405Se ≦ 0.0050%. When producing grain-oriented electrical steel sheets by inducing secondary recrystallization in a component system without using an inhibitor, S and Se are impurity elements. Here, the S equivalent is expressed as S + 0.405Se. The element symbols in the formula represent the contents. In this embodiment, the S equivalent must be 0.0050% or less. If the S equivalent exceeds 0.0050%, secondary recrystallization becomes difficult, resulting in deterioration of magnetic properties. Preferably, the S equivalent is 0.0040 mass% or less.

[0035] The above is the basic composition of the steel material for manufacturing grain-oriented electrical steel sheets according to this embodiment. Optionally, the steel material may further contain at least one component from Groups A to C below.

[0036] Group A: At least one selected from Sn: 0.005-0.500%, Cr: 0.005-0.500%, Cu: 0.01-0.50%, Ni: 0.01-0.50%, Bi: 0.005-0.500%, P: 0.005-0.200%, Mo: 0.005-0.500%, and Co: 0.001-0.500%. The inclusion of at least one of Sn, Cr, Cu, Ni, Bi, P, Mo, and Co improves the recrystallization texture, thereby improving the final magnetic properties. Addition of less than the lower limit of each element produces little effect. On the other hand, addition exceeding the upper limit saturates the effect, resulting in the disadvantage of increased costs. Therefore, when at least one of Sn, Cr, Cu, Ni, Bi, P, Mo and Co is added, the content thereof is set to be within the above range.

[0037] Group B: at least one selected from B: 0.0001-0.0025%, Nb: 0.001-0.020%, Ti: 0.0005-0.0400%, V: 0.001-0.020%, and W: 0.001-0.020%. By including at least one of B, Nb, Ti, V, and W, the carbides and nitrides of these elements are finely formed, resulting in a refined grain size after annealing. This results in improved bending characteristics and reduced threading problems. Addition below the lower limit of each element produces little effect. On the other hand, addition above the upper limit saturates the effect, resulting in the disadvantage of increased costs. Therefore, when at least one of B, Nb, Ti, V, and W is added, the content must be within the above range.

[0038] Group C: at least one selected from Zn: 0.0005 to 0.020%, Zr: 0.001 to 0.020%, Pb: 0.0001 to 0.0100%, As: 0.001 to 0.020%, Ag: 0.001 to 0.050%, Au: 0.001 to 0.050%, Ga: 0.0001 to 0.0050%, Ge: 0.0001 to 0.0050%, Ca: 0.0005 to 0.020%, Mg: 0.0005 to 0.020%, REM: 0.0005 to 0.0200%, and Hf: 0.001 to 0.020%. By including at least one of Zn, Zr, Pb, As, Ag, Au, Ga, Gr, Ca, Mg, REM, and Hf, these elements concentrate at grain boundaries or form compounds at the grain boundaries, strengthening the grain boundaries. As a result, defects caused by grain boundary fracture can be suppressed. Addition of less than the lower limit of each element's content results in a small effect. On the other hand, addition of more than the upper limit saturates the effect, resulting in a disadvantage that causes increased costs. Therefore, when at least one of Zn, Zr, Pb, As, Ag, Au, Ga, Gr, Ca, Mg, REM, and Hf is added, the content should be within the above range.

[0039] The chemical composition of the steel material used to manufacture the grain-oriented electrical steel sheet according to this embodiment is such that the remainder other than the above-mentioned component composition is substantially Fe and unavoidable impurities.

[0040] <Method for Manufacturing Grain-Oriented Electrical Steel Sheet> Next, a method for manufacturing a grain-orientated electrical steel sheet according to this embodiment will be described. The method for manufacturing a grain-orientated electrical steel sheet according to this embodiment includes a hot rolling step, and optionally a hot-rolled sheet annealing step, a cold rolling step, a pickling step, a primary recrystallization annealing step, and a finish annealing step. A steel material for the grain-orientated electrical steel sheet according to this embodiment can be produced by a method for manufacturing a general electrical steel sheet. That is, molten steel with a predetermined composition adjustment may be used as a steel material by a normal ingot-making method or a continuous casting method to produce, for example, a steel slab. Alternatively, a thin cast piece having a thickness of 100 mm or less may be produced by a direct casting method. Of the above-mentioned additive elements, those elements that are difficult to add in intermediate steps after casting are preferably added at the molten steel stage.

[0041] Hot Rolling Process: A steel material, such as a steel slab, is heated by a conventional method and hot-rolled to form a hot-rolled steel sheet. The composition of this embodiment does not require high-temperature heating to dissolve the inhibitor. Therefore, it is essential to keep the slab heating temperature low, at 1250°C or less, in order to reduce energy costs.

[0042] Hot-rolled sheet annealing process: Next, if necessary, the hot-rolled steel sheet is annealed to obtain a hot-rolled annealed sheet. When the hot-rolled sheet annealing is performed before the final cold rolling, for the reasons described above, the calculated γ phase fraction at the annealing temperature of the steel sheet as a raw material is set to a range of 1.0 to 25.0 volume % in order to suppress property fluctuations within the steel sheet coil. The average cooling rate between 800 and 400 °C during the cooling process after annealing is set to a range of 20 to 50 °C / s. The annealing temperature is preferably in the range of 900 to 1200 °C. If the annealing temperature is less than 900 °C, the recrystallized grains become finer, the Goss nuclei in the primary recrystallized structure decrease, and the magnetic properties deteriorate. On the other hand, if the annealing temperature exceeds 1200 °C, the crystal grains become coarse, making cracks more likely to occur during the subsequent rolling process, which is disadvantageous from the viewpoint of manufacturability.

[0043] Cold Rolling Process: The hot-rolled steel sheet or hot-rolled annealed sheet is cold-rolled once or twice or more times with intermediate annealing in between to produce the final cold-rolled steel sheet. Cold rolling with intermediate annealing in between is performed as necessary. The intermediate annealing temperature is also preferably in the range of 900 to 1200°C for the same reasons as for annealing of hot-rolled sheets. In the final cold rolling to produce the final cold-rolled steel sheet, it is preferable to increase the cold rolling temperature to 100 to 300°C and to perform aging treatment in the range of 100 to 300°C once or multiple times during the cold rolling, as this changes the primary recrystallization texture and improves the magnetic properties.

[0044] Pickling Process The final cold-rolled steel sheet is pickled in a solution containing 2 to 15% by mass of Fe ions and having an acid concentration of 1 to 20% by mass to obtain a pickled cold-rolled steel sheet. It is essential to pickle the steel sheet with an acid solution containing Fe ions before primary recrystallization annealing. It is believed that Fe ions promote cleaning of the steel sheet surface with acid through their catalytic effect. The Fe ion concentration can be calculated, for example, by measuring the conductivity, liquid temperature, and ultrasonic propagation velocity of the acid solution using an ultrasonic concentration meter. This method was also adopted in this embodiment. It can also be measured by an extraction method using a chelating agent. The type of acid is not limited, and hydrochloric acid, phosphoric acid, nitric acid, sulfuric acid, etc. can be used depending on the purpose.

[0045] Primary recrystallization annealing process: The pickled cold-rolled steel sheet is subjected to primary recrystallization annealing accompanied by decarburization to obtain a primary recrystallization annealed sheet. In the primary recrystallization annealing accompanied by decarburization, the annealing temperature is preferably in the range of 800 to 900°C from the viewpoint of decarburization. Furthermore, the atmosphere is preferably a humid atmosphere. From the viewpoint of texture, the heating rate to the holding temperature is preferably 100°C / s or more.

[0046] Finish Annealing Step In the finish annealing, an annealing separator containing magnesia as a main component is applied to the surface of the primary recrystallization annealed sheet. Here, "containing magnesia as a main component" means that the content of magnesia (MgO) in the annealing separator is 60 mass % or more in terms of solid content. The content of magnesia in the annealing separator is preferably 80 mass % or more in terms of solid content. Magnesia is applied to the steel sheet as a slurry solution in which magnesia is suspended in water. In this case, in order to suppress an increase in viscosity, it is preferable that the slurry solution be maintained at a constant temperature within the range of 5 to 30°C. The annealing separator contains TiO ２ , TiO ３ ・H ２ O, TiO・(OH) ２ and Ti(OH) ４ It is essential to contain at least one Ti compound selected from the following in an amount of 0.3 to 8 parts by mass, calculated as Ti, per 100 parts by mass of magnesia in order to obtain good coating adhesion.

[0047] It is also preferable to combine 0.2 to 8 parts by mass of a Sr compound, calculated as Sr, with 100 parts by mass of magnesia in combination with the Ti compound. This is effective in further improving and stabilizing the magnetic properties and film properties. Examples of Sr compounds include SrSO ４ , Sr(OH) ２ ・8H ２ O, SrCO ３ , Sr(NO) ３ At least one selected from the following can be used.

[0048] In order to further improve the uniformity of the coating, the annealing separator may further contain oxides such as CaO, MgSO ４ ・7H ２ O and SnSO ４ Sulfides such as Na ２ B ４ O ７ B-based compounds such as Sb ２ O ３ and Sb ２ (SO4) ３ It is more preferable to add at least one selected from the above Sb-based compounds, either singly or in combination.

[0049] Furthermore, the magnesia (MgO) used in the annealing separator preferably has a hydration amount, i.e., a weight loss upon ignition at 1000°C for 1 hour, in the range of 1 to 5 mass% after hydration at 20°C for 60 minutes. If the hydration amount of magnesia is less than 1 mass%, the formation of a forsterite coating film becomes insufficient. On the other hand, if the hydration amount of magnesia exceeds 5 mass%, too much moisture is carried between the coil layers, resulting in a large amount of additional oxidation of the steel sheet. This is because there is a risk that a good forsterite coating film will not be obtained.

[0050] Thereafter, secondary recrystallization annealing is performed for the purpose of secondary recrystallization and purifying impurities in the steel. Here, it is preferable to set the residence time in the temperature range of 800 to 900°C during the temperature rise process to 40 hours or more and 150 hours or less. In a composition system that does not use an inhibitor, secondary recrystallization occurs in this temperature range, so a longer residence time tends to improve the magnetic properties. By performing finish annealing, it is possible to develop the secondary recrystallization structure and form a forsterite film. It is preferable to perform finish annealing at 800°C or higher to induce secondary recrystallization.

[0051] From the viewpoint of purification, it is preferable to hold the temperature at 1100°C or higher for 1 hour or more. More preferably, it is 1150°C or higher for 3 hours or more. ２ The introduction of an atmosphere promotes purification, so in the high temperature range, H ２ However, the bendability of the product may be deteriorated in some cases, and in such cases, it is preferable to use an Ar atmosphere in the high temperature region.

[0052] After the secondary recrystallization annealing, it is useful to perform water washing, brushing, or pickling to remove any adhering annealing separator.

[0053] After that, further flattening annealing and shape correction are effective for reducing iron loss.

[0054] When steel sheets are used in a stack, it is effective to apply an insulating coating to the surface of the steel sheet before or after planarization annealing in order to improve iron loss. A coating that can impart tension to the steel sheet is preferred to reduce iron loss. The use of a tension coating method using a binder, or a coating method in which an inorganic substance is vapor-deposited on the surface of the steel sheet by physical vapor deposition or chemical vapor deposition is preferred because these methods provide excellent coating adhesion and a significant iron loss reduction effect.

[0055] The embodiments of the present invention will be further explained by way of examples. Note that the present invention is not limited to the manufacturing conditions and product performance shown in the following examples. The embodiments can achieve the desired performance within the scope of the present invention.

[0056] (Example 1) A steel slab containing, by mass, 0.042% C, 3.22% Si, 0.0040% acid-soluble Al, 0.0042% N, 0.06% Sb, 0.15% Mn, and 0.0030% Se, with the balance being Fe and unavoidable impurities, was produced by continuous casting. The steel slab was subjected to slab heating by soaking at 1200 ° C for 40 minutes, and then hot-rolled to obtain a hot-rolled steel sheet with a thickness of 2.6 mm. The hot-rolled steel sheet was then subjected to hot-rolled annealing at 1100 ° C for 45 seconds to obtain a hot-rolled annealed sheet. The hot-rolled annealed sheet was pickled to remove surface scale, and then cold-rolled to obtain a cold-rolled steel sheet with a thickness of 1.8 mm. The hot-rolled annealed sheet was then subjected to intermediate annealing at 1000 ° C for 50 seconds. At this time, a compositional analysis of the sample before intermediate annealing was performed, and the obtained values ​​were used to calculate the γ phase fraction at an intermediate annealing temperature of 1000°C using Thermo-Calc ver. 2020b, which was 11.6% by volume. Furthermore, the cooling rate between 800 and 400°C after intermediate annealing was changed as shown in Table 1. Subsequently, a final cold-rolled steel sheet with a thickness of 0.27 mm was obtained by cold rolling. Subsequently, the steel sheet was immersed in an acid containing the amount of Fe ions shown in Table 1 to obtain a pickled cold-rolled steel sheet. The pickling conditions were uniform, with an acid concentration of 5% by mass, a solution temperature of 60°C, and immersion for 5 seconds. The pickled cold-rolled steel sheet was subjected to a 50% by volume H treatment at 840°C for 180 seconds. ２ -50% by volume N ２ The specimens were subjected to primary recrystallization annealing accompanied by decarburization in a humid atmosphere with a dew point of 55°C, to obtain a primarily recrystallized annealed sheet. An annealing separator mainly composed of magnesia was then applied to the primarily recrystallized annealed sheet. The Ti compounds listed in Table 1 were added to the annealing separator. Subsequently, secondary recrystallization annealing was performed by holding the specimen at 1200°C for 5 hours. A nitrogen atmosphere was used below 1000°C, and an Ar atmosphere was used above 1000°C. During this temperature increase process, the residence time in the temperature range of 800 to 900°C was 75 hours. The specimens were then washed with water to remove unreacted annealing separator, and a coating composed primarily of magnesium phosphate, colloidal silica, and chromic acid was applied.

[0057] Magnetic properties of the obtained sample B ８The magnetic flux density (when excited at 800 A / m) was measured using the method described in JIS C2550, and the results are shown in Table 1. In this example, the conditions were changed for each coil, and the final coil was divided into four parts, and the magnetic properties were evaluated at each end, with the worst value being taken as the representative value for the original coil. In addition, the coating adhesion was evaluated by wrapping the steel sheet around cylinders of various diameters and using the smallest diameter at which the coating did not peel off. The results are also shown in Table 1. Table 1 shows that good magnetic properties and coating properties can be obtained by using conditions that are compatible with the present invention.

[0058]

[0059] (Example 2) A steel slab containing, by mass, 0.029% C, 2.98% Si, 0.0060% acid-soluble Al, 0.0035% N, 0.12% Sb, 0.08% Mn, and 0.0030% Se, with the balance being Fe and unavoidable impurities, was produced by continuous casting. The steel slab was subjected to slab heating by soaking at 1200 ° C for 40 minutes, and then hot-rolled to obtain a 2.6 mm thick hot-rolled steel sheet. The γ phase fraction at the hot-rolled sheet annealing temperature of 1050 ° C was calculated from the composition of the steel slab using Thermo-Calc ver. 2020b, and was found to be 8.8 vol%. The hot-rolled steel sheet was then subjected to hot-rolled sheet annealing at 1050 ° C for 45 seconds to obtain a hot-rolled annealed sheet. The cooling rate between 800 and 400°C after annealing the hot-rolled sheet was 25°C / s. The hot-rolled annealed sheet was pickled to remove surface scale, and then cold-rolled to a final cold-rolled steel sheet with a thickness of 0.23 mm. Next, the sheet was immersed in nitric acid containing 5% by mass of Fe ions to obtain a pickled cold-rolled steel sheet. The pickling conditions were 5% by mass nitric acid, a solution temperature of 60°C, and immersion for 5 seconds. The pickled cold-rolled steel sheet was then immersed in 5% by mass H at 840°C for 150 seconds. ２ -50% by volume N ２ The primary recrystallization annealing was performed in a humid atmosphere with a dew point of 52°C, which was accompanied by decarburization, to obtain a primary recrystallization annealed sheet. Furthermore, an annealing separator mainly containing magnesia was applied to the primary recrystallization annealed sheet, and the annealing separator contained TiO ２was added in an amount of 3 parts by mass, calculated as Ti, per 100 parts by mass of magnesia. Furthermore, a Sr compound shown in Table 2 was also added. Thereafter, secondary recrystallization annealing was performed by holding at 1220°C for 10 hours in a hydrogen atmosphere. During this temperature increase process, the residence time in the temperature range of 800 to 900°C was variously changed as shown in Table 2. Thereafter, the unreacted annealing separator was removed by rinsing with water, and a coating containing magnesium phosphate, colloidal silica, and chromic acid as its main components was applied.

[0060] The magnetic flux density B of the obtained sample ８ The magnetic flux density (when excited at 800 A / m) was measured using the method described in JIS C2550, and the results are also shown in Table 2. In this example, the conditions were changed for each coil, and the final coil was divided into four parts, and the magnetic properties were evaluated at each end, with the worst value being taken as the representative value for the original coil. In addition, the coating adhesion was evaluated by wrapping the steel sheet around cylinders of various diameters and using the smallest diameter at which the coating did not peel off. The results are also shown in Table 2. Table 2 shows that good magnetic properties and coating properties can be obtained by using conditions that are compatible with the present invention.

[0061]

[0062] (Example 3) A steel slab having the chemical composition shown in Table 3, with the balance consisting of Fe and unavoidable impurities, was produced by continuous casting. The steel slab was subjected to slab heating by soaking at 1230°C for 40 minutes, and then hot-rolled to obtain a 2.2 mm thick hot-rolled steel sheet. The γ phase fraction at the hot-rolled sheet annealing temperature of 1050°C was calculated from the chemical composition of the steel slab using Thermo-Calc ver. 2020b, and the results are also shown in Table 3. The hot-rolled steel sheet was then annealed at 1050°C for 50 seconds to obtain a hot-rolled annealed sheet. The cooling rate between 800 and 400°C after hot-rolled sheet annealing was 50°C / s. The hot-rolled annealed sheet was pickled to remove surface scale, and then cold-rolled to obtain a final cold-rolled steel sheet with a thickness of 0.23 mm. Next, the final cold-rolled steel sheet was immersed in hydrochloric acid containing 11 mass% Fe ions to obtain a pickled cold-rolled steel sheet. The pickling conditions were immersion in 11 mass% hydrochloric acid at a solution temperature of 60°C for 3 seconds. The pickled cold-rolled steel sheet was then immersed in 52% by volume H at 840°C for 150 seconds. ２ -48% by volume N ２The primary recrystallization annealing was performed in a humid atmosphere with a dew point of 54°C, which was accompanied by decarburization, to obtain a primary recrystallization annealed sheet. Further, an annealing separator mainly containing magnesia was applied to the primary recrystallization annealed sheet, and the annealing separator contained TiO ２ was added in an amount of 0.5 parts by mass, calculated as Ti, per 100 parts by mass of magnesia. Subsequently, secondary recrystallization annealing was performed by holding at 1220°C for 10 hours in a hydrogen atmosphere. During this temperature increase process, the residence time in the temperature range of 800 to 900°C was 100 hours. Subsequently, the unreacted annealing separator was removed by rinsing with water, and a coating containing magnesium phosphate, colloidal silica, and chromic acid as its main components was applied.

[0063] The magnetic flux density B of the obtained sample ８ The magnetic flux density (when excited at 800 A / m) was measured using the method described in JIS C2550, and the results are also shown in Table 3. As in the experiment, the conditions were changed for each coil, and the final coil was divided into four parts and the properties were evaluated at each end, with the worst value being taken as the representative value for the original coil. In addition, the coating adhesion was evaluated by wrapping the steel sheet around cylinders of various diameters and using the smallest diameter at which the coating did not peel off. The results are also shown in Table 3. Table 3 shows that good magnetic properties and coating properties can be obtained by using conditions that are compatible with the present invention.

[0064]

Claims

1. Containing, by mass%, C: 0.01 to 0.10%, Si: 2.5 to 4.0%, Mn: 0.04 to 0.50%, acid-soluble Al: 0.0030 to 0.0100%, N: 0.0020 to 0.0060%, and Sb: 0.03 to 0.30%, wherein the S and Se contents satisfy S + 0.405Se ≦ 0.0050%, and optionally Group A: at least one selected from Sn: 0.005 to 0.500%, Cr: 0.005 to 0.500%, Cu: 0.01 to 0.50%, Ni: 0.01 to 0.50%, Bi: 0.005 to 0.500%, P: 0.005 to 0.200%, Mo: 0.005 to 0.500%, and Co: 0.001 to 0.500%; Group B: at least one selected from B: 0.0001 to 0.0025, Nb: 0.001 to 0.020%, Ti: 0.0005 to 0.0400%, V: 0.001 to 0.020%, and W: 0.001 to 0.020%; Group C:containing at least one component selected from the group consisting of Zn: 0.0005 to 0.020%, Zr: 0.001 to 0.020%, Pb: 0.0001 to 0.0100%, As: 0.001 to 0.020%, Ag: 0.001 to 0.050%, Au: 0.001 to 0.050%, Ga: 0.0001 to 0.0050%, Ge: 0.0001 to 0.0050%, Ca: 0.0005 to 0.020%, Mg: 0.0005 to 0.020%, REM: 0.0005 to 0.0200%, and Hf: 0.001 to 0.020%, the hot rolling process includes a hot rolling step of heating a steel material having a composition with the balance consisting of Fe and unavoidable impurities to a temperature of 1250°C or less and hot rolling it to obtain a hot rolled steel sheet; a hot rolled sheet annealing step of optionally subjecting the hot rolled steel sheet to hot rolled sheet annealing to obtain a hot rolled annealed sheet; a cold rolling step of cold rolling the hot rolled steel sheet or the hot rolled annealed sheet once or two or more times with intermediate annealing in between to obtain a final cold rolled steel sheet; a pickling step of pickling the final cold rolled steel sheet to obtain a pickled cold rolled steel sheet; a primary recrystallization annealing step of subjecting the pickled cold rolled steel sheet to primary recrystallization annealing with decarburization to obtain a primary recrystallization annealed sheet; and a finish annealing step of applying an annealing separator mainly composed of magnesia to the surface of the primary recrystallization annealed sheet and subjecting it to secondary recrystallization annealing. hot-rolled sheet annealing in the hot-rolled sheet annealing step or intermediate annealing in the cold rolling step is performed before cold rolling to obtain the final cold-rolled steel sheet; during annealing before obtaining the final cold-rolled steel sheet, the γ phase fraction of the steel structure at the annealing temperature is set to 1.0 to 25.0 volume % and the average cooling rate between 800 and 400°C in the cooling process after annealing is set to a range of 20 to 50°C / s; in the pickling step, the final cold-rolled steel sheet is pickled with a solution containing 2 to 15 mass % of Fe ions and having an acid concentration of 1 to 20 mass %; and in the finish annealing step, TiO; ２ , TiO ３ ・H ２ O, TiO・(OH) ２ and Ti(OH) ４ and a method for producing a grain-oriented electrical steel sheet, the method comprising using an annealing separator containing, in terms of Ti, at least one Ti compound selected from the group consisting of the following in an amount of 0.3 to 8 parts by mass per 100 parts by mass of magnesia.

2. The method for producing grain-oriented electrical steel sheet according to claim 1, wherein the temperature rise process in the final annealing step is performed with a residence time in the temperature range of 800 to 900°C of 40 to 150 hours.

3. In the final annealing step, the annealing separator further contains SrSO ４ , Sr(OH) ２ ・8H ２ O, SrCO ３ and Sr(NO) ３ 3. The method for producing a grain-oriented electrical steel sheet according to claim 1, wherein the magnesia contains at least one Sr compound selected from the group consisting of Sr, Sr-equivalent, and Sr-based compounds in an amount of 0.2 to 8 parts by mass per 100 parts by mass of magnesia.

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