Manufacturing method of grain-oriented electrical steel sheet

Abstract

To provide a method for producing a grain-oriented electromagnetic steel sheet for obtaining a high magnetic flux density and suppressing dispersion of the magnetic flux density.SOLUTION: In a production method of a grain-oriented electromagnetic steel sheet of an embodiment, a cold rolling step includes a tandem rolling step and a reverse rolling step. In the tandem rolling step, an average diameter D1 of a plurality of work rolls used in a plurality of passes is set to 200 mm or more, and a cumulative reduction ratio CR1 is set to 30-87%. In the reverse rolling step, an average diameter D2 of the plurality of work rolls used in the plurality of passes is set to 100 mm or less, and a cumulative reduction ratio CR2 is set to 24-86%. In a decarburization annealing step, an average heating rate HR in the range of 550-750°C of the cold-rolled steel sheet is 100°C / sec or more.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] Grain-oriented electrical steel sheets have a crystal orientation of {110} <001> Grain-oriented electrical steel is a steel sheet that has been assembled in a specific orientation (Goss orientation). Grain-oriented electrical steel is used as a soft magnetic material in the iron cores of transformers and other electrical equipment.

[0003] A method for producing grain-oriented electrical steel sheet is, for example, as follows: A slab is heated and hot-rolled to produce a hot-rolled steel sheet; The produced hot-rolled steel sheet is annealed; The hot-rolled steel sheet is cold-rolled to produce a cold-rolled steel sheet; The cold-rolled steel sheet is decarburization annealed to induce primary recrystallization; The cold-rolled steel sheet after decarburization annealing is finish annealed to induce secondary recrystallization. Grain-oriented electrical steel sheet is produced by the above steps.

[0004] As mentioned above, grain-oriented electrical steel sheets are used as iron core materials, so they require high magnetic properties. Specifically, there is a demand for improved magnetic flux density B8 at a magnetic field strength of 800 A / m.

[0005] One method for increasing magnetic flux density is to increase the degree of concentration in the Goss orientation. To increase the degree of concentration in the Goss orientation, high-temperature, long-term finish annealing is usually performed. During the finish annealing, Goss-oriented grains, which have excellent magnetic properties, grow to centimeter-order sizes while encroaching on surrounding crystal grains of other orientations (secondary recrystallization). This coarsening of the Goss-oriented grains aligns the crystal orientation, increasing the degree of concentration in the Goss orientation.

[0006] As Goss-oriented grains become coarser, their concentration in the Goss orientation increases. As a result, the magnetic flux density also increases. However, the coarsening of Goss-oriented grains causes variations in magnetic flux density. Specifically, in the above-mentioned manufacturing method, a coiled steel sheet is loaded into a heat treatment furnace and subjected to finish annealing. In this case, secondary recrystallization occurs when the steel sheet is given a certain curvature. Therefore, when the coiled steel sheet is unwound and flattened, a continuous deviation in the crystal orientation occurs according to the curvature of the coil. This deviation in crystal orientation may cause the crystal orientation of the crystal grains to deviate from the Goss orientation. In this case, the magnetic flux density decreases. This deviation in crystal orientation causes variations in magnetic flux density. Means for increasing the degree of integration in the Goss orientation have been proposed in Japanese Patent Laid-Open Nos. 6-049543 (Patent Document 1), 7-62436 (Patent Document 2), 10-280040 (Patent Document 3), and 2003-096520 (Patent Document 4). In these documents, rapid heating is carried out during the temperature rise process of annealing treatment to induce primary recrystallization, which enriches the Goss orientation grains that act as nuclei for secondary recrystallization in the primary recrystallized structure. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Publication No. 6-049543 [Patent Document 2] Japanese Patent Application Publication No. 7-62436 [Patent Document 3] Japanese Patent Application Publication No. 10-280040 [Patent Document 4] Japanese Patent Application Laid-Open No. 2003-096520 Summary of the Invention [Problem to be solved by the invention]

[0008] The manufacturing methods described in Patent Documents 1 to 4 can certainly enrich the fine Goss-oriented grains that serve as nuclei for secondary recrystallization in the primary recrystallized structure, thereby increasing the concentration of Goss-oriented grains. However, if the Goss-oriented grains become excessively coarse during secondary recrystallization, the above-mentioned misalignment of the crystal orientation occurs. This can result in variations in magnetic flux density between the outermost and innermost portions of a coiled grain-oriented electrical steel sheet.

[0009] An object of the present disclosure is to provide a method for manufacturing a grain-oriented electrical steel sheet that can obtain a high magnetic flux density and suppress variation in magnetic flux density. [Means for solving the problem]

[0010] The method for manufacturing a grain-oriented electrical steel sheet according to the present disclosure includes the following steps.

[0011] The chemical composition is, in mass%, C: 0.01 to 0.20%, Si: 2.0 to 4.5% Mn: 0.01 to 0.30%, S: 0.01 to 0.05%, sol.Al: 0.01~0.05%, N: 0.01 to 0.02%, Cr: 0.00~0.50%, Sn: 0.00~0.30% Sb: 0.00 to 0.30% Ni: 0.00 to 0.50% Mo: 0.00 to 0.20% P: 0.00~0.15%, Cu: 0.00~0.50% Se: 0.00 to 0.03%, V: 0.00 to 0.15%, and Bi: 0.0000~0.0100%, a hot rolling step of producing a hot-rolled steel sheet by hot rolling a slab containing a hot-rolled sheet annealing step of annealing the hot-rolled steel sheet; A cold rolling process in which the hot-rolled steel sheet after the hot-rolled sheet annealing process is cold-rolled at a cumulative reduction rate CR0 of 90% or more to manufacture a cold-rolled steel sheet; a decarburization annealing step of performing decarburization annealing on the cold-rolled steel sheet to manufacture a decarburization annealed steel sheet; a finish annealing step of applying an annealing separator to the decarburization-annealed steel sheet and performing finish annealing on the decarburization-annealed steel sheet to which the annealing separator has been applied, to manufacture a finish-annealed steel sheet; an insulating film forming step of applying an insulating film forming liquid to the finish annealed steel sheet and performing a heat treatment on the finish annealed steel sheet to which the insulating film forming liquid has been applied, thereby forming an insulating film on the finish annealed steel sheet, The cold rolling step includes: a tandem rolling process in which the hot-rolled steel sheet is subjected to continuous rolling by a plurality of passes using a tandem rolling mill including a plurality of rolling stands arranged in a row, thereby producing an intermediate steel sheet; A reverse rolling process in which the intermediate steel plate that has not been heat treated after the tandem rolling process is subjected to reverse rolling with a plurality of passes using a multi-stage rolling mill to manufacture the cold-rolled steel plate, In the tandem rolling process, The average diameter D1 of the work rolls used in the plurality of passes is 200 mm or more, The cumulative reduction rate CR1 is set to 30-87%. In the reverse rolling step, The average diameter D2 of the work rolls used in the plurality of passes is 100 mm or less, The cumulative reduction rate CR2 is set to 24-86%. In the decarburization annealing step, The average heating rate HR in the temperature range of the cold-rolled steel sheet from 550°C to 750°C is 100°C / sec or more. Manufacturing method for grain-oriented electrical steel sheets. [Effects of the Invention]

[0012] The method for manufacturing a grain-oriented electrical steel sheet according to the present disclosure can produce a grain-oriented electrical steel sheet that can obtain a high magnetic flux density and suppress variation in magnetic flux density. [Brief explanation of the drawings]

[0013] [Figure 1] FIG. 1 is a flow diagram of the method for manufacturing a grain-oriented electrical steel sheet according to this embodiment. [Figure 2] FIG. 2 is a schematic diagram of a tandem rolling mill. [Figure 3] FIG. 3 is a schematic diagram of a multi-stage rolling mill used in the reverse rolling process. DETAILED DESCRIPTION OF THE INVENTION

[0014] The inventors of the present invention considered that suppressing excessive coarsening of Goss-oriented grains after secondary recrystallization would be effective in obtaining high magnetic flux density and suppressing variation in magnetic flux density. Therefore, they developed a steel with a chemical composition of C: 0.01-0.20%, Si: 2.0-4.5%, Mn: 0.01-0.30%, S: 0.01-0.05%, sol.Al: 0.01-0.05%, N: 0.01-0.02%, Cr: 0.00-0.50%, Sn: 0.00-0.30%, Sb: 0.00-0.30%, Ni: 0.00-0.50%, and Mo: 0.00-0. The magnetic flux density and the variation in magnetic flux density between the outer and inner peripheral portions of the coil were investigated when grain-oriented electrical steel sheets were manufactured using a slab containing 20% ​​Fe, 0.00 to 0.15% P, 0.00 to 0.50% Cu, 0.00 to 0.50% Se, 0.00 to 0.03%, 0.00 to 0.15% V, and 0.0000 to 0.0100% Bi, with the remainder being Fe and impurities.

[0015] First, rapid heating was carried out during the decarburization annealing process, which is when primary recrystallization occurs. As a result, it was found that if the heating rate HR was 100°C / s or higher when the steel sheet temperature was raised from 550°C to 750°C, a sufficient amount of fine Goss-oriented grains would be generated in the primary recrystallized structure, and the degree of accumulation of Goss-oriented grains would increase during secondary recrystallization.

[0016] However, although rapid heating during the decarburization annealing step can increase the concentration of Goss orientation grains during secondary recrystallization, it is not possible to sufficiently suppress the coarsening of Goss orientation grains, and therefore it is not possible to sufficiently suppress the variation in magnetic flux density.

[0017] Therefore, the present inventors have investigated methods for suppressing the formation of Goss-oriented crystal grains during secondary recrystallization, and have obtained the following findings.

[0018] The inventors focused on the cold rolling process. In the cold rolling process during the manufacturing process of grain-oriented electrical steel sheets, reverse rolling using a multi-stage rolling mill is often adopted to increase the reduction ratio. In a multi-stage rolling mill, a pair of work rolls is supported by multiple backup rolls to apply a high reduction to the steel sheet to be rolled. This allows the diameter of the pair of work rolls to be reduced, thereby minimizing roll deflection. This allows for a high reduction ratio to be achieved.

[0019] However, when cold rolling is performed using work rolls with small diameters, a large amount of shear strain is introduced into the surface layer of the steel sheet. When a large amount of shear strain is introduced, α-fiber orientations, one of the stable orientations in cold rolling, develop in the surface layer of the steel sheet. The crystal orientation grains that undergo primary recrystallization from the α-fiber orientations tend to coarsen. Therefore, if a large number of Goss-oriented grains are generated during primary recrystallization, the number of Goss-oriented grains that serve as nuclei for secondary recrystallization decreases during the grain growth process up to just before the onset of secondary recrystallization in the final annealing process, making the secondary recrystallized grains more likely to coarsen. As a result, grain-oriented electrical steel sheets that undergo final annealing in coil form are prone to variations in magnetic flux density.

[0020] Therefore, the inventors of the present invention considered that the coarsening of Goss-oriented grains during secondary recrystallization could be suppressed if the introduction of shear strain could be reduced during the cold rolling process. If the introduction of shear strain during the cold rolling process is suppressed, γ-fiber orientation groups are more likely to remain in the cold-rolled steel sheet instead of α-fiber orientation groups. The γ-fiber orientation groups generate {111} recrystallization groups during primary recrystallization. The {111} recrystallization groups have a low driving force for grain growth and are less likely to coarsen. Therefore, if a large number of Goss-oriented grains are generated during primary recrystallization, the number of Goss-oriented grains that serve as nuclei for secondary recrystallization is less likely to decrease during the grain growth process up to just before the onset of secondary recrystallization in the final annealing process. Therefore, a large number of nuclei (Goss-oriented grains) undergo secondary recrystallization, suppressing the grain growth of each secondary recrystallized grain. As a result, grain-oriented electrical steel sheets that undergo final annealing in a coiled state are more likely to have variable magnetic flux density.

[0021] Based on the above considerations, the present inventors have investigated the cold rolling process, and as a result, have found that a high hourly speed density can be obtained in a coil-shaped grain-oriented electrical steel sheet and variation in magnetic flux density can be suppressed by performing a tandem rolling process and a reverse rolling process in the cold rolling process and satisfying the following conditions in each process.

[0022] The method for manufacturing a grain-oriented electrical steel sheet according to this embodiment, which has been completed based on the above findings, includes the following steps.

[0023] [1] The chemical composition is, in mass%, C: 0.01 to 0.20%, Si: 2.0 to 4.5% Mn: 0.01 to 0.30%, S: 0.01 to 0.05%, sol.Al: 0.01~0.05%, N: 0.01 to 0.02%, Cr: 0.00~0.50%, Sn: 0.00~0.30% Sb: 0.00 to 0.30% Ni: 0.00 to 0.50% Mo: 0.00 to 0.20% P: 0.00~0.15%, Cu: 0.00~0.50% Se: 0.00 to 0.03%, V: 0.00 to 0.15%, and Bi: 0.0000~0.0100%, a hot rolling step of producing a hot-rolled steel sheet by hot rolling a slab containing a hot-rolled sheet annealing step of annealing the hot-rolled steel sheet; A cold rolling process in which the hot-rolled steel sheet after the hot-rolled sheet annealing process is cold-rolled at a cumulative reduction rate CR0 of 90% or more to manufacture a cold-rolled steel sheet; a decarburization annealing step of performing decarburization annealing on the cold-rolled steel sheet to manufacture a decarburization annealed steel sheet; a finish annealing step of applying an annealing separator to the decarburization-annealed steel sheet and performing finish annealing on the decarburization-annealed steel sheet to which the annealing separator has been applied, to manufacture a finish-annealed steel sheet; an insulating film forming step of applying an insulating film forming liquid to the finish annealed steel sheet and performing a heat treatment on the finish annealed steel sheet to which the insulating film forming liquid has been applied, thereby forming an insulating film on the finish annealed steel sheet, The cold rolling step includes: a tandem rolling process in which the hot-rolled steel sheet is subjected to continuous rolling by a plurality of passes using a tandem rolling mill including a plurality of rolling stands arranged in a row, thereby producing an intermediate steel sheet; A reverse rolling process in which the intermediate steel plate that has not been heat treated after the tandem rolling process is subjected to reverse rolling with a plurality of passes using a multi-stage rolling mill to manufacture the cold-rolled steel plate, In the tandem rolling process, The average diameter D1 of the work rolls used in the plurality of passes is 200 mm or more, The cumulative reduction rate CR1 is set to 30-87%. In the reverse rolling step, The average diameter D2 of the work rolls used in the plurality of passes is 100 mm or less, The cumulative reduction rate CR2 is set to 24-86%. In the decarburization annealing step, The average heating rate HR in the temperature range of the cold-rolled steel sheet from 550°C to 750°C is 100°C / sec or more. Manufacturing method for grain-oriented electrical steel sheets.

[0024] [2] A method for producing the grain-oriented electrical steel sheet according to [1], The slab is Cr: 0.01 to 0.50%, Sn: 0.01 to 0.30% Sb: 0.01 to 0.30%, Ni: 0.01 to 0.50% Mo: 0.01 to 0.20%, P: 0.01-0.15%, Cu: 0.01 to 0.50% Se: 0.01 to 0.03%, V: 0.01 to 0.15%, and Bi: 0.0001 to 0.0100%, Contains one or more elements selected from the group consisting of Manufacturing method for grain-oriented electrical steel sheets.

[0025] The method for producing a grain-oriented electrical steel sheet according to this embodiment will be described in detail below. In this specification, % regarding the content of an element means % by mass unless otherwise specified.

[0026] [Manufacturing process flow] 1 is a flow diagram of a method for producing a grain-oriented electrical steel sheet according to the present embodiment. Referring to FIG. 1, this production method includes the following steps S1 to S6. Hot rolling process S1 ·Hot rolled plate annealing process S2 Cold rolling process S3 Decarburization annealing process S4 Finishing annealing process S5 ·Insulating film formation process S6

[0027] The cold rolling step S3 further includes the following two steps. Tandem rolling process S31 Reverse rolling process S32

[0028] The manufacturing method of this embodiment is characterized in that the following manufacturing conditions are satisfied in the cold rolling step S3 (tandem rolling step S31, reverse rolling step S32) and the decarburization annealing step S4. (Conditions of tandem rolling process S31) Condition 1: A tandem rolling mill is used to carry out continuous rolling with a plurality of passes. Condition 2: The average diameter D1 of the work rolls is 200 mm or more. Condition 3: The cumulative rolling reduction rate CR1 is set to 30 to 87%. (Conditions of the reverse rolling process S32) Condition 4: Reverse rolling is performed using a multi-stage rolling mill with multiple passes. Condition 5: The rolling target is an intermediate steel plate that has not been heat treated after the tandem rolling process. Condition 6: The average diameter D2 of the work rolls is 100 mm or less. Condition 7: The cumulative rolling reduction rate CR2 is set to 24 to 86%. (Conditions for the entire cold rolling process S3) Condition 8: The cumulative reduction rate CR0 throughout the entire cold rolling process S3 is set to 90% or more. (Conditions in decarburization annealing process S4) Condition 9: The average heating rate HR in the range of 550°C to 750°C is set to 100°C / sec or more. Each of steps S1 to S6 will be explained below.

[0029] [Hot rolling process (S1)] In the hot rolling step (S1), the prepared slab is hot rolled to produce a steel sheet. The chemical composition of the slab contains the following elements:

[0030] [Essential elements in the chemical composition of the slab] C: 0.01 to 0.20% Carbon (C) is effective for controlling the structure until the completion of the decarburization annealing step during the manufacturing process. If the C content is less than 0.01%, the above effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the C content exceeds 0.20%, and the contents of other elements are within the ranges of this embodiment, even if the decarburization annealing step described below is performed, decarburization will be insufficient, causing magnetic aging, and in this case, sufficient iron loss characteristics will not be obtained. Therefore, the C content is 0.01 to 0.20%. The lower limit of the C content is preferably 0.02%, more preferably 0.05%, and even more preferably 0.07%. The upper limit of the C content is preferably 0.18%, more preferably 0.16%, and even more preferably 0.14%.

[0031] Si: 2.0 to 4.5% Silicon (Si) increases the resistivity of grain-oriented electrical steel sheets and reduces eddy current loss, which is one of the iron losses. If the Si content is less than 2.0%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Si content exceeds 4.5%, the cold workability of the steel will decrease even if the contents of other elements are within the ranges of this embodiment. Therefore, the Si content is 2.0 to 4.5%. The lower limit of the Si content is preferably 2.2%, more preferably 2.5%, and even more preferably 2.8%. The upper limit of the Si content is preferably 4.3%, more preferably 4.0%, and even more preferably 3.7%.

[0032] Mn: 0.01 to 0.30% Manganese (Mn) increases the resistivity of grain-oriented electrical steel sheets and reduces iron loss. Mn also improves hot workability and suppresses the occurrence of cracks during hot rolling. Mn also combines with S to form fine MnS during the hot rolling process. The fine MnS acts as a precipitation nucleus for fine AlN, which acts as an inhibitor. Therefore, if a large amount of fine MnS precipitates during the hot rolling process, a sufficient amount of fine AlN can be obtained during the hot-rolled sheet annealing process. If the Mn content is less than 0.01%, the above effects cannot be fully achieved. On the other hand, if the Mn content exceeds 0.30%, the magnetic flux density of the grain-oriented electrical steel sheet decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the Mn content is 0.01 to 0.30%. The lower limit of the Mn content is preferably 0.02%, and more preferably 0.03%. The upper limit of the Mn content is preferably 0.20%, and more preferably 0.15%.

[0033] S: 0.01 to 0.05% During the hot rolling process, sulfur (S) combines with Mn to form the above-mentioned fine MnS. As described above, the fine MnS serves as a precipitation nucleus for fine AlN, which is used as an inhibitor. Therefore, if the amount of fine MnS precipitated in the hot rolling process is large, a sufficient amount of fine AlN can be obtained. If the S content is less than 0.01%, the above-mentioned effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the S content exceeds 0.05%, even if the contents of other elements are within the ranges of this embodiment, MnS may remain in the steel sheet after the finish annealing process, which may result in a deterioration in the magnetic properties of the grain-oriented electrical steel sheet. Therefore, the S content is 0.01 to 0.05%. The lower limit of the S content is preferably 0.02%, and the upper limit of the S content is preferably 0.04%.

[0034] sol.Al: 0.01-0.05% Aluminum (Al) bonds with N to form AlN during the manufacturing process of grain-oriented electrical steel sheet, and functions as an inhibitor. If the sol. Al content is less than 0.01%, a sufficient amount of AlN to function as an inhibitor cannot be obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the sol. Al content exceeds 0.05%, even if the contents of other elements are within the ranges of this embodiment, the inhibitor function becomes excessive, and good secondary recrystallization does not occur. Therefore, the sol. Al content is 0.01 to 0.05%. The lower limit of the sol. Al content is preferably 0.02%, and the upper limit of the sol. Al content is preferably 0.04%. In this specification, the sol. Al content means the content of acid-soluble Al.

[0035] N: 0.01 to 0.02% Nitrogen (N) combines with Al to form AlN during the manufacturing process of grain-oriented electrical steel sheets, acting as an inhibitor. To keep the N content below 0.01%, excessive refining is required in the steelmaking process, which increases manufacturing costs. Therefore, the lower limit of the N content is 0.01%. On the other hand, if the N content in the steel material exceeds 0.02%, even if the contents of other elements are within the ranges of this embodiment, a large number of blisters (voids) are likely to be generated in the steel sheet during cold rolling. Therefore, the N content is 0.01 to 0.02%.

[0036] The balance of the chemical composition of the slab according to this embodiment is composed of Fe and impurities. Here, the term "impurities" refers to substances that are mixed in from raw materials such as ore and scrap, or from the manufacturing environment, during industrial production of slabs, which are the raw material for grain-oriented electrical steel sheets, and are acceptable within a range that does not adversely affect the grain-oriented electrical steel sheets manufactured by the manufacturing method according to this embodiment.

[0037] [Optional elements in the chemical composition of the slab] The chemical composition of the above slab further includes, instead of a part of Fe, Cr: 0.00~0.50%, Sn: 0.00~0.30% Sb: 0.00 to 0.30% Ni: 0.00 to 0.50% Mo: 0.00 to 0.20% P: 0.00~0.15%, Cu: 0.00~0.50% Se: 0.00 to 0.03%, V: 0.00 to 0.15%, and Bi: 0.0000~0.0100%, It may contain one or more elements selected from the group consisting of:

[0038] [Group 1: Cr, Sn and Sb] Cr, Sn, and Sb all improve the magnetic properties of grain-oriented electrical steel sheets and reduce variations in the magnetic properties. Cr, Sn, and Sb will be described below.

[0039] Cr: 0.00~0.50% Chromium (Cr) is an optional element and may not be contained, that is, the Cr content may be 0.00%. When contained, Cr improves the properties of the oxide layer formed during the decarburization annealing process. In this case, the properties of the primary coating formed using this oxide layer during the final annealing process are improved. Furthermore, Cr stabilizes the formation of the oxide layer and the primary coating, thereby improving the magnetic properties of the grain-oriented electrical steel sheet and suppressing variations in the magnetic properties. Even if even a small amount of Cr is contained, the above effects can be obtained to some extent. However, if the Cr content exceeds 0.50%, the formation of the primary coating may become unstable even if the contents of other elements are within the ranges of this embodiment. Therefore, the Cr content is 0.00 to 0.50%. In order to more effectively obtain the above effects, the lower limit of the Cr content is preferably 0.01%, and more preferably 0.02%. The upper limit of the Cr content is preferably 0.40%, more preferably 0.30%, even more preferably 0.20%, and still more preferably 0.15%.

[0040] Sn: 0.00~0.30% Tin (Sn) is an optional element and may not be contained, that is, the Sn content may be 0.00%. When contained, Sn improves the properties of the oxide layer formed during the decarburization annealing process. In this case, the properties of the primary coating formed using this oxide layer during the final annealing process are improved. Furthermore, Sn stabilizes the formation of the oxide layer and the primary coating, thereby improving the magnetic properties of the grain-oriented electrical steel sheet and suppressing variations in the magnetic properties. Even if even a small amount of Sn is contained, the above effects can be obtained to some extent. However, if the Sn content exceeds 0.30%, the surface of the steel sheet becomes difficult to oxidize, and the formation of the primary coating may become insufficient. Therefore, the Sn content is 0.00 to 0.30%. In order to more effectively obtain the above effects, the lower limit of the Sn content is preferably 0.01%, and more preferably 0.02%. The upper limit of the Sn content is preferably 0.25%, and more preferably 0.20%.

[0041] Sb: 0.00 to 0.30% Antimony (Sb) is an optional element and may not be contained, that is, the Sb content may be 0.00%. When contained, Sb improves the properties of the oxide layer formed during the decarburization annealing process. In this case, the properties of the primary coating formed using this oxide layer during the final annealing process are improved. Furthermore, Sb stabilizes the formation of the oxide layer and the primary coating, thereby improving the magnetic properties of the grain-oriented electrical steel sheet and suppressing variations in the magnetic properties. Even if even a small amount of Sb is contained, the above effects can be obtained to some extent. However, if the Sb content exceeds 0.30%, the surface of the steel sheet becomes difficult to oxidize, and the formation of the primary coating may become insufficient. Therefore, the Sb content is 0.00 to 0.30%. In order to more effectively obtain the above effects, the lower limit of the Sb content is preferably 0.01%, and more preferably 0.02%. The upper limit of the Sb content is preferably 0.25%, and more preferably 0.20%.

[0042] [Group 2: Ni, Mo and P] Ni, Mo, and P all reduce the iron loss of grain-oriented electrical steel sheets. Ni, Mo, and P will be explained below.

[0043] Ni: 0.00 to 0.50% Nickel (Ni) is an optional element and may not be contained, that is, the Ni content may be 0.00%. When contained, Ni homogenizes the hot-rolled structure and improves the primary recrystallization texture. As a result, Ni reduces the iron loss of the grain-oriented electrical steel sheet (final product). Even if even a small amount of Ni is contained, the above effects can be obtained to some extent. However, if the Ni content exceeds 0.50%, the secondary recrystallization may become unstable even if the contents of other elements are within the ranges of this embodiment. Therefore, the Ni content is 0.00 to 0.50%. In order to more effectively obtain the above effects, the lower limit of the Ni content is preferably 0.01%, and more preferably 0.02%. The upper limit of the Ni content is preferably 0.45%, more preferably 0.40%, and even more preferably 0.35%.

[0044] Mo: 0.00 to 0.20% Molybdenum (Mo) is an optional element and may not be contained, that is, the Mo content may be 0.00%. When contained, Mo increases the electrical resistance of grain-oriented electrical steel sheets and reduces iron loss. Even if even a small amount of Mo is contained, these effects can be obtained to some extent. However, if the Mo content exceeds 0.20%, the workability of the steel sheet decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the Mo content is 0.00 to 0.20%. In order to more effectively obtain the above effects, the lower limit of the Mo content is preferably 0.01%, more preferably 0.02%, and even more preferably 0.05%. The upper limit of the Mo content is preferably 0.18%, more preferably 0.15%, and even more preferably 0.13%.

[0045] P: 0.00 to 0.15% Phosphorus (P) is an optional element and may not be contained, that is, the P content may be 0.00%. When contained, P improves the primary recrystallization texture of grain-oriented electrical steel sheets and reduces the iron loss of the grain-oriented electrical steel sheets (final products). Even if even a small amount of P is contained, the above effects can be obtained to some extent. However, if the P content exceeds 0.15%, the workability of the steel sheet will decrease even if the contents of other elements are within the ranges of this embodiment. Therefore, the P content is 0.00 to 0.15%. In order to more effectively obtain the above effects, the lower limit of the P content is preferably 0.01%, more preferably 0.02%, and even more preferably 0.03%. The upper limit of the P content is preferably 0.13%, more preferably 0.11%, and even more preferably 0.09%.

[0046] [Group 3: Cu, Se, V and Bi] Cu, Se, V, and Bi all promote the production of inhibitors or function as inhibitors. Cu, Se, V, and Bi will be explained below.

[0047] Cu: 0.00 to 0.50% Copper (Cu) is an optional element and may not be contained, that is, the Cu content may be 0.00%. When contained, Cu promotes the precipitation of fine MnS, which acts as an inhibitor and becomes the nucleus for the formation of AlN in the hot rolling process.Even if even a small amount of Cu is contained, the above effect can be obtained to some extent. However, if the Cu content exceeds 0.50%, CuS precipitates may form and remain after final annealing even if the contents of other elements are within the ranges of this embodiment. If CuS precipitates remain in the steel, the magnetic properties of the grain-oriented electrical steel sheet may deteriorate. Therefore, the Cu content is 0.00 to 0.50%. In order to more effectively obtain the above effects, the lower limit of the Cu content is preferably 0.01%, more preferably 0.03%, and even more preferably 0.05%. The upper limit of the Cu content is preferably 0.40%, and more preferably 0.30%.

[0048] Se: 0.00 to 0.03% Selenium (Se) is an optional element and may not be contained, that is, the Se content may be 0.00%. When Se is contained, it combines with Mn to form fine MnSe during the hot rolling process. The fine MnSe acts as an inhibitor and becomes the precipitation nucleus for fine AlN. Therefore, it promotes the formation of fine AlN during the hot rolling process. Even if even a small amount of Se is contained, the above effect can be obtained to some extent. However, if the Se content exceeds 0.03%, even if the contents of other elements are within the ranges of this embodiment, MnSe may remain in the steel sheet after the finish annealing process, which may result in a deterioration in the magnetic properties of the grain-oriented electrical steel sheet. Therefore, the Se content is 0.00 to 0.03%. In order to more effectively obtain the above effects, the lower limit of the Se content is preferably 0.01%, and the upper limit of the Se content is preferably 0.02%.

[0049] V: 0.00 to 0.15% Vanadium (V) is an optional element and may not be contained, that is, the V content may be 0.00%. When contained, it functions as an inhibitor by binding to C or N. If even a small amount of V is contained, the above effect can be obtained to some extent. However, if the V content exceeds 0.15%, V inhibitors may remain in the steel sheet after the finish annealing process even if the contents of other elements are within the ranges of this embodiment, which may result in deterioration of the magnetic properties of the grain-oriented electrical steel sheet. Therefore, the V content is 0.00 to 0.15%. In order to more effectively obtain the above effects, the lower limit of the V content is preferably 0.01%, more preferably 0.02%, and even more preferably 0.03%. The upper limit of the V content is preferably 0.13%, more preferably 0.11%, and even more preferably 0.09%.

[0050] Bi: 0.0000 to 0.0100% Bismuth (Bi) is an optional element and may not be contained, that is, the Bi content may be 0.0000%. When contained, Bi stabilizes MnS and MnSe and enhances their inhibitor function. Even if even a small amount of Bi is contained, the above effect can be obtained to some extent. However, if the Bi content exceeds 0.0100%, the adhesion of the primary coating formed on the steel sheet decreases even if the contents of other elements are within the ranges of this embodiment. If the Bi content exceeds 0.0100%, edge cracks are more likely to occur. Therefore, the Bi content is 0.0000 to 0.0100%. In order to more effectively obtain the above effects, the lower limit of the Bi content is preferably 0.0001%, more preferably 0.0003%, even more preferably 0.0005%, even more preferably 0.0007%, and even more preferably 0.0010%. The upper limit of the Bi content is preferably 0.0070%, more preferably 0.0050%, and even more preferably 0.0040%.

[0051] [Method of manufacturing slabs having the above chemical composition] An example of a method for producing a slab having the above chemical composition is as follows: Molten steel having the above chemical composition is produced (melted), and a slab is produced using the molten steel by continuous casting.

[0052] [Hot rolling using the above slab] The prepared slab having the above chemical composition is hot-rolled using a hot rolling mill to produce a steel sheet (hot-rolled steel sheet). Specifically, the hot-rolling step S1 includes the following steps. ·Heating process S11 ·Rough rolling process S12 Finishing rolling process S13

[0053] [Heating process S11] In the heating step, the slab is heated. For example, the slab is charged into a known heating furnace or a known soaking furnace and heated. The preferred heating temperature of the slab is 1100 to 1450°C.

[0054] [Rough rolling process S12] In the rough rolling step S12, rough rolling is performed on the heated slab to produce a rough bar. Here, rough rolling means hot rolling the slab using a known rough rolling mill. The rough bar means a steel plate after rough rolling is completed and before finish rolling begins. In the rough rolling step, the rough rolling mill is used to apply a plurality of passes of reduction to the slab to produce a rough bar.

[0055] [Finishing rolling process S13] In the finish rolling step S13, the rough bar produced in the rough rolling step S12 is subjected to a known finish rolling process to produce a hot-rolled steel sheet. Here, finish rolling means hot-rolling the rough bar using a known finish rolling mill. In the finish rolling step S13, a continuous rolling mill consisting of multiple tandem finishing rolling stands arranged in a row on a pass line is used to apply a rolling reduction to the rough bar through multiple passes to produce a hot-rolled steel sheet.

[0056] [Hot rolled plate annealing process S2] In the hot-rolled sheet annealing step S2, an annealing treatment is performed on the hot-rolled steel sheet produced in the hot rolling step S1. By performing the hot-rolled sheet annealing step S2, recrystallization occurs in the steel sheet structure, and magnetic properties are improved.

[0057] In the hot-rolled sheet annealing step S2, it is sufficient to carry out well-known hot-rolled sheet annealing. The method for heating the hot-rolled steel sheet in the hot-rolled sheet annealing is not particularly limited, and any well-known heating method may be adopted. The hot-rolled sheet annealing temperature is, for example, 900 to 1200°C. The holding time at the hot-rolled sheet annealing temperature is, for example, 10 to 300 seconds. Note that, when the hot-rolled sheet annealing step S2 is carried out, the hot-rolled steel sheet may be subjected to pickling treatment after the hot-rolled sheet annealing step S2 and before the cold-rolling step S3.

[0058] [Cold rolling process S3] In the cold rolling step S3, the produced hot-rolled steel sheet is cold-rolled to produce a cold-rolled steel sheet. As described above, the cold rolling step S3 includes the following two steps. Tandem rolling process S31 Reverse rolling process S32 Each step S31 and S32 will be described below.

[0059] [Tandem rolling process S31] In the tandem rolling step S31, cold rolling is carried out using a tandem rolling mill. Fig. 2 is a schematic diagram of a tandem rolling mill. Referring to Fig. 2, the tandem rolling mill CM is arranged between a payoff reel (unwinding device) 11 and a tension reel (winding device) 12 from upstream to downstream.

[0060] The payoff reel 11 unwinds the wound hot-rolled steel sheet ST0. The tension reel 12 winds up the intermediate steel sheet ST1 produced by the tandem rolling mill CM. The tandem rolling mill CM performs continuous rolling on the rewound hot-rolled steel sheet ST over a plurality of passes to produce an intermediate steel sheet ST1.

[0061] The tandem rolling mill CM consists of multiple rolling stands CMS1 to CMS2 arranged in a row from upstream to downstream. j(j is a natural number of 2 or more). Each rolling stand CMS is equipped with a pair of work rolls WR1 extending horizontally. The pair of work rolls WR1 come into contact with the hot-rolled steel sheet to be cold-rolled, and cold-roll the hot-rolled steel sheet. The rolling stand CMS may be equipped with multiple backup rolls BR1. The backup rolls BR1 support the work rolls WR1 and suppress deflection of the work rolls WR1 during rolling.

[0062] In continuous rolling using a tandem rolling mill CM, reducing the hot-rolled steel sheet at each rolling stand CMS as it passes through that rolling stand CMS is called "one pass." Continuous rolling means using a tandem rolling mill CM to reduce the hot-rolled steel sheet in multiple passes. Note that it is not necessary for all rolling stands CMS in the tandem rolling mill CM to reduce the hot-rolled steel sheet. For example, if six rolling stands CMS1 to CMS6 are arranged in the tandem rolling mill CM, and the hot-rolled steel sheet passes through rolling stand CMS6 without being reduced, this means that five passes of continuous rolling have been performed.

[0063] [Reverse rolling process S32] Fig. 3 is a schematic diagram of a multi-stage rolling mill used in the reverse rolling step S32. Referring to Fig. 3, in the reverse rolling step S32, the multi-stage rolling mill SM is used to perform reverse rolling with a plurality of passes on the intermediate steel sheet ST1 after the tandem rolling step S31 to produce a cold-rolled steel sheet ST2.

[0064] An example of a multi-stage rolling mill is a Sendzimir rolling mill. The multi-stage rolling mill SM is equipped with a pair of work rolls WR2 and a plurality of backup rolls BR2. In the multi-stage rolling mill SM, the pair of work rolls WR2 are supported by a plurality of backup rolls BR2, thereby minimizing deflection of the work rolls WR2. This allows for a high reduction.

[0065] Here, when a reduction is applied to the intermediate steel sheet ST1 as it passes through the multi-stage rolling mill SM, this means that one pass of reduction has been performed. In the case of reverse rolling, a reduction is applied to the intermediate steel sheet ST1 as it moves from upstream to downstream, and also as it moves from downstream to upstream. More specifically, one pass of reduction is applied to the intermediate steel sheet ST1 as it passes through the multi-stage rolling mill SM from upstream to downstream. Also, one pass of reduction is applied to the intermediate steel sheet ST1 as it passes through the same multi-stage rolling mill SM from downstream to upstream. In other words, when reduction is applied in both directions, two passes of reduction are performed on the intermediate steel sheet ST1. Note that there are cases where no reduction is applied to the intermediate steel sheet ST1 as it passes through the multi-stage rolling mill SM.

[0066] In the cold rolling step S3, the above-described tandem rolling step S31 and reverse rolling step S32 are carried out to produce a cold-rolled steel sheet.

[0067] [Manufacturing conditions in cold rolling process S3] In the cold rolling step S3, the following conditions 1 to 8 are satisfied. (Conditions of tandem rolling process S31) Condition 1: Continuous rolling is carried out using a tandem rolling mill CM with multiple passes. Condition 2: The average diameter D1 of the work roll WR1 is set to 200 mm or more. Condition 3: The cumulative rolling reduction rate CR1 is set to 30 to 87%. (Conditions of the reverse rolling process S32) Condition 4: Reverse rolling is performed with a plurality of passes using a multi-stage rolling mill SM. Condition 5: The rolling target is an intermediate steel plate that has not been heat treated after the tandem rolling process. Condition 6: The average diameter D2 of the work rolls WR2 is set to 100 mm or less. Condition 7: The cumulative rolling reduction rate CR2 is set to 24 to 86%. (Conditions for the entire cold rolling process S3) Condition 8: The cumulative reduction rate CR0 throughout the entire cold rolling process S3 is set to 90% or more. Conditions 1 to 8 will be explained below.

[0068] [Conditions for tandem rolling process S31] [Regarding Condition 1] In the tandem rolling process S31, the intermediate steel sheet ST1 is produced by performing continuous rolling with multiple passes using the above-mentioned tandem rolling mill CM. As described above, in tandem rolling, rolls with larger diameters can be used for rolling down compared to reverse rolling. Therefore, the shear strain imparted to the steel sheet can be reduced compared to reverse rolling. As a result, the size of Goss-oriented crystal grains can be minimized in the final annealing process. Therefore, not only can sufficient magnetic flux density be obtained in the grain-oriented electrical steel sheet, but the difference between the magnetic flux density at the outermost and innermost portions of the coil of the grain-oriented electrical steel sheet can also be reduced. In other words, the variation in magnetic flux density of the grain-oriented electrical steel sheet can be reduced.

[0069] [Regarding Condition 2] In tandem rolling, the average diameter D1 of the multiple work rolls used in each pass is set to 200 mm or more. In the multi-high rolling mill (Sendzimir rolling mill) used in the reverse rolling process described below, the diameter of the work rolls is set to 100 mm or less to suppress deflection of the work rolls, and the work rolls are supported by multiple backup rolls. In a multi-high rolling mill, a high pressure is exerted by the combination of such a pair of work rolls and multiple backup rolls that support the work rolls. However, because the diameter of the work rolls is small, rolling using a Sendzimir rolling mill imparts a large shear strain to the steel plate being rolled.

[0070] As mentioned above, when a large shear strain is applied, α-fiber orientation groups develop in the surface layer of the cold-rolled steel sheet. When α-fiber orientation groups develop, grain growth immediately prior to the onset of secondary recrystallization reduces the number of Goss-oriented grains that serve as nuclei for secondary recrystallization. This causes the secondary recrystallized grains to coarsen. As a result, the magnetic properties of the grain-oriented electrical steel sheet deteriorate or become inconsistent.

[0071] In this embodiment, tandem rolling is employed for part of the cold rolling, and the average diameter D1 of the multiple work rolls used in each pass is set to 200 mm or more. In this case, the amount of shear strain imparted to the surface layer of the steel sheet during cold rolling can be reduced. In this case, gamma fiber orientations are stabilized more than alpha fiber orientations during cold rolling. Therefore, the alpha fiber orientations are suppressed in the surface layer of the cold-rolled steel sheet, and the gamma fiber orientations remain. The gamma fiber orientations generate {111} recrystallized grains when primary recrystallization occurs. In the finish annealing process, the {111} recrystallized grains suppress the encroachment of Goss orientation grains by other crystal grains before secondary recrystallization occurs. Therefore, the degree of concentration of Goss orientation grains after secondary recrystallization can be increased. As a result, the magnetic properties of the grain-oriented electrical steel sheet can be improved, and the variation in magnetic properties can be suppressed.

[0072] The average diameter D1 is the arithmetic mean value of the diameters (mm) of each of the pairs of work rolls used in each pass.

[0073] The lower limit of the average diameter D1 is preferably 250 mm, more preferably 300 mm, even more preferably 325 mm, and even more preferably 350 mm. The upper limit of the average diameter D1 is preferably 1000 mm, more preferably 900 mm, even more preferably 800 mm, even more preferably 700 mm, and even more preferably 650 mm.

[0074] [Regarding condition 3] The cumulative reduction CR1 in the tandem rolling process is set to 30 to 87%, where the cumulative reduction CR1 (%) is defined by the following formula (1). CR1 = (thickness of hot-rolled steel plate before tandem rolling process - thickness of intermediate steel plate after tandem rolling process) / thickness of hot-rolled steel plate before tandem rolling process × 100 (1)

[0075] If the cumulative reduction rate CR1 is less than 30%, the reduction in the tandem rolling process is insufficient. In this case, the reduction in the reverse rolling process is increased. As a result, the amount of shear strain imparted to the cold-rolled steel sheet becomes excessive. This reduces the concentration of Goss orientation after secondary recrystallization. As a result, the magnetic properties of the grain-oriented electrical steel sheet deteriorate. On the other hand, if the cumulative reduction rate CR1 exceeds 87%, the reduction amount in the reverse rolling process is too small. In this case, the amount of shear strain imparted to the cold-rolled steel sheet is insufficient. If the shear strain is insufficient, the non-uniform deformation region known as the shear band, which is the origin of primary recrystallization in the Goss orientation, decreases. As a result, the number of secondary recrystallization nuclei (Goss orientation grains) decreases. As a result, the magnetic properties of grain-oriented electrical steel sheet vary. Therefore, the cumulative rolling reduction CR1 is set to 30 to 87%. The lower limit of the cumulative rolling reduction CR1 is preferably 35%, more preferably 40%, and even more preferably 45%. The upper limit of the cumulative rolling reduction CR1 is preferably 85%, more preferably 80%, and even more preferably 75%.

[0076] [Conditions for the reverse rolling process S32] [Regarding condition 4] In the reverse rolling step S32, a multi-stage rolling mill SM is used to perform reverse rolling with a number of passes on the intermediate steel sheet ST1 that has not been heat treated after the tandem rolling step S31, to produce a cold-rolled steel sheet. Reverse rolling using the multi-stage rolling mill SM can apply a higher reduction to the steel sheet compared to tandem rolling, which allows the cold-rolled steel sheet to be thinner.

[0077] [Regarding condition 5] The object to be rolled in the reverse rolling step S32 is an intermediate steel sheet that has not been heat treated after the tandem rolling step S31. That is, in the reverse rolling step S32, the intermediate steel sheet is an as-tandem rolled material. Here, "heat treatment" refers to a process of heating the steel sheet to a temperature of 500°C or higher and / or holding it for a predetermined period of time. The heat treatment is, for example, annealing.

[0078] If the intermediate steel sheet ST1 produced by the tandem rolling process S31 is subjected to heat treatment such as annealing, recovery and recrystallization within the intermediate steel sheet ST1 progress, reducing the total strain introduced by cold rolling. In this case, even if the cumulative reduction CR0 after the reverse rolling process S32 is 90% or more, the magnetic properties of the grain-oriented electrical steel sheet deteriorate. This is because, when the intermediate steel sheet ST1 is heat treated, the strain decreases as described above, resulting in deterioration of the primary recrystallization texture in the center layer of the steel sheet. Specifically, the center layer of the steel sheet inherits the α-fiber orientation groups formed in the hot rolling process S1. Even after the reverse rolling process S32, the α-fiber orientation groups in the center layer of the steel sheet are significantly more developed than in the surface layer of the steel sheet. To promote the recrystallization of these α-fiber orientation groups, a cumulative reduction CR0 of 90% or more is required.

[0079] When the intermediate steel sheet ST1 is subjected to heat treatment such as annealing, the recrystallization of the α-fiber orientation group in the central layer of the steel sheet cannot be promoted even if the cumulative reduction rate CR0 is 90% or more. If the recrystallization of the α-fiber orientation group in the central layer of the steel sheet is not promoted sufficiently, the {411} <148> The recrystallization orientation decreases. {411} <148> The recrystallization orientation has a Σ9 corresponding orientation relationship with the Goss orientation, which promotes secondary recrystallization in the Goss orientation.

[0080] [Regarding condition 6] In reverse rolling using a multi-stage rolling mill, the average diameter D2 of the multiple work rolls WR2 used in each pass is set to 100 mm or less. If the average diameter D2 exceeds 100 mm, it is not possible to impart sufficient rolling reduction to the intermediate steel sheet ST1 being reverse rolled. In this case, it is not possible to impart sufficient shear strain to the intermediate steel sheet ST1. If the average diameter D2 is 100 mm or less, sufficient shear strain can be imparted to the intermediate steel sheet ST1 during each pass of reverse rolling, resulting in sufficient shear bands that serve as the origin of primary recrystallization in the Goss orientation, and a sufficient amount of Goss orientation grains being produced in the decarburization annealing step S4.

[0081] When reverse rolling is performed using one multi-high rolling mill SM, the average diameter D1 is the arithmetic mean value of the diameters (mm) of each pair of work rolls used in each pass. When reverse rolling is performed using multiple multi-high rolling mills SM, the average diameter D2 is the arithmetic mean value of the diameters (mm) of each pair of work rolls used in each pass.

[0082] The upper limit of the average diameter D2 is preferably 95 mm, more preferably 90 mm, and even more preferably 85 mm. The lower limit of the average diameter D2 is not particularly limited, and the lower limit of the average diameter D2 is preferably 50 mm, more preferably 55 mm, and even more preferably 60 mm.

[0083] [Regarding condition 7] The cumulative reduction CR2 in the reverse rolling step S32 is set to 24 to 86%, where the cumulative reduction CR2 (%) is defined by the following formula (2). CR2 = (thickness of intermediate steel plate before reverse rolling process - thickness of cold-rolled steel plate after reverse rolling process) / thickness of intermediate steel plate before reverse rolling process × 100 (2)

[0084] If the cumulative reduction rate CR2 is less than 24%, the reduction in the reverse rolling process is insufficient. In this case, there are insufficient shear bands that serve as the origin of primary recrystallization in the Goss orientation, and in the subsequent decarburization annealing step S4, a sufficient number of nuclei for Goss orientation grains formed by primary recrystallization are not generated. This results in a low concentration of Goss orientation grains after secondary recrystallization. As a result, the magnetic properties of the grain-oriented electrical steel sheet are degraded. On the other hand, if the cumulative rolling reduction CR2 exceeds 86%, the applied shear strain becomes large, and α-fiber orientation groups develop in the surface layer of the cold-rolled steel sheet. When α-fiber orientation groups develop, the number of Goss-oriented grains that serve as nuclei for secondary recrystallization decreases due to grain growth up to just before the onset of secondary recrystallization in the final annealing process S5. This results in coarsening of the secondary recrystallized grains. As a result, the magnetic properties of the grain-oriented electrical steel sheet become poor or vary. Therefore, the cumulative rolling reduction CR2 is set to 24 to 86%.

[0085] The lower limit of the cumulative rolling reduction CR2 is preferably 30%, more preferably 35%, and even more preferably 40%. The upper limit of the cumulative rolling reduction CR2 is preferably 80%, more preferably 75%, and even more preferably 70%.

[0086] [Regarding Condition 8] The cumulative reduction CR0 in the entire cold rolling process S3 (the tandem rolling process S31 and the reverse rolling process S32) is set to 90% or more. Here, the cumulative reduction CR0 (%) is defined by the following formula (3). CR0 = (thickness of hot-rolled steel sheet before tandem rolling process - thickness of cold-rolled steel sheet after reverse rolling process) / thickness of hot-rolled steel sheet before tandem rolling process × 100 (3)

[0087] If the cumulative reduction rate CR0 is less than 90%, the reduction in the cold rolling process is insufficient. In this case, the total amount of strain accumulated in the cold-rolled steel sheet is small, resulting in deterioration of the magnetic properties of the grain-oriented electrical steel sheet. This is because if the cumulative reduction rate CR0 is less than 85%, the total amount of strain accumulated in the cold-rolled steel sheet is too small, resulting in deterioration of the primary recrystallization texture in the center layer of the steel sheet. Specifically, the center layer of the steel sheet inherits the α-fiber orientations formed in the hot rolling process. Therefore, even after the reverse rolling process S32, the α-fiber orientations in the center layer of the steel sheet are significantly more developed than in the surface layer of the steel sheet. To promote the recrystallization of these α-fiber orientations, a cumulative reduction rate CR0 of 90% or more is required.

[0088] When the intermediate steel sheet ST1 is subjected to heat treatment such as annealing, it is not possible to promote the recrystallization of the α-fiber orientation group in the central layer of the steel sheet even if the cumulative reduction rate CR0 is 90% or more. <148> The recrystallization orientation decreases. <148> The recrystallization orientation has a corresponding orientation relationship of Σ9 with the Goss orientation, and plays a role in promoting secondary recrystallization in the Goss orientation. Therefore, the cumulative rolling reduction CR0 is set to 90% or more.

[0089] The lower limit of the cumulative rolling reduction CR0 is preferably 91%, more preferably 92%, and even more preferably 93%. The upper limit of the cumulative rolling reduction CR0 is preferably 97%, and more preferably 95%.

[0090] In the cold rolling step S3, tandem rolling and reverse rolling are performed to produce a cold-rolled steel sheet in which the amount of shear strain in the surface layer of the steel sheet is controlled so as to satisfy the above-mentioned conditions 1 to 8. By performing subsequent processes using this cold-rolled steel sheet, it is possible to produce a grain-oriented electrical steel sheet in which variation in magnetic flux density is suppressed.

[0091] [Decarburization annealing process S4] In the decarburization annealing step S4, the cold-rolled steel sheet after the cold rolling step S3 is subjected to decarburization annealing to induce primary recrystallization.

[0092] The decarburization annealing step S4 includes the following steps. Temperature rising process S41 ·Decarburization process S42 ·Cooling process S43

[0093] In the temperature-raising step S41, the steel sheet is heated to a desired temperature (ultimate temperature) between 750°C and 950°C. In the decarburization step S42, the steel sheet is held at a decarburization annealing temperature of 750°C to 950°C to perform decarburization annealing, thereby causing primary recrystallization. In the cooling step S43, the steel sheet after the decarburization step S42 is cooled by a well-known method. Note that the ultimate temperature and the decarburization annealing temperature may be the same temperature, or the ultimate temperature may be higher than the decarburization annealing temperature.

[0094] In this embodiment, in the heating step S41, the average heating rate HR is significantly increased in the temperature range of 550°C to 750°C, which corresponds to the recrystallization temperature range of the steel sheet. This promotes recrystallization in the Goss orientation. This increases the degree of concentration in the Goss orientation after secondary recrystallization. As a result, the magnetic properties of the grain-oriented electrical steel sheet can be improved and the variation in the magnetic properties can be suppressed.

[0095] Each step will be described in detail below.

[0096] [Temperature increasing step S41] In the temperature-raising step S41, first, the cold-rolled steel sheet after the cold-rolling step (S3) is loaded into a heat treatment furnace. In the heat treatment furnace for decarburization annealing in this embodiment, the cold-rolled steel sheet is heated to an arbitrary temperature between 750°C and 950°C by, for example, high-frequency induction heating or electric heating. The temperature-raising step S41 satisfies the following condition 9. Condition 9: Average heating rate in the range of 550°C to 750°C: 100°C / sec or more

[0097] [Regarding Condition 9] In the temperature increasing step S41, the average temperature increasing rate when the temperature of the cold-rolled steel sheet is in the range of 550°C to 750°C is defined as the average temperature increasing rate HR (°C / sec).

[0098] As mentioned above, strain accumulates in cold-rolled steel sheets. If the average heating rate HR is less than 100°C / s, the strain energy that drives recrystallization is released before recrystallization begins. In this case, a sufficient number of Goss-oriented grains cannot be generated in the cold-rolled steel sheet after primary recrystallization (i.e., the cold-rolled steel sheet after the decarburization annealing process).

[0099] If the average heating rate HR is 100°C / sec or higher, primary recrystallization occurs in a state where sufficient strain energy is accumulated in the cold-rolled steel sheet. Therefore, a sufficient amount of Goss-oriented grains can be generated in the cold-rolled steel sheet after primary recrystallization. Therefore, in the subsequent finish annealing step S5, many Goss-oriented grains remain when secondary recrystallization occurs. Therefore, the degree of concentration of Goss-oriented grains after secondary recrystallization can be increased. As a result, the magnetic properties of the grain-oriented electrical steel sheet can be improved, and the variation in magnetic properties can be suppressed.

[0100] The upper limit of the average heating rate HR is not particularly limited. However, even if the average heating rate HR is set to be faster than 2000°C / sec, the above effect saturates. Therefore, the upper limit of the average heating rate HR is set to 2000°C / sec.

[0101] The lower limit of the average heating rate HR is preferably 150°C / sec, more preferably 200°C / sec, even more preferably 300°C / sec, even more preferably 400°C / sec, even more preferably 500°C / sec, even more preferably 600°C / sec, even more preferably 700°C / sec, even more preferably 800°C / sec, and even more preferably 900°C / sec.

[0102] The average heating rate HR is measured using the following method. Multiple thermometers are installed in a heat treatment furnace to measure the surface temperature of the steel plate. The multiple thermometers are arranged from upstream to downstream of the heat treatment furnace. The average heating rate HR is calculated based on the temperature of the steel plate measured by the thermometers and the time it takes for the steel plate temperature to rise from 550°C to 750°C.

[0103] [Decarburization process S42] In the decarburization step S42, the cold-rolled steel sheet after the temperature-raising step S41 is held at the decarburization annealing temperature Ta to perform decarburization annealing. This causes primary recrystallization to occur in the cold-rolled steel sheet. The atmosphere during the decarburization step may be a well-known atmosphere, for example, a wet nitrogen-hydrogen mixed atmosphere containing hydrogen and nitrogen. By performing decarburization annealing, carbon in the steel sheet is removed from the steel sheet, causing primary recrystallization to occur. The decarburization annealing temperature Ta and the holding time at the decarburization annealing temperature Ta are not particularly limited. The decarburization annealing temperature Ta is, for example, 800 to 950°C. The holding time at the decarburization annealing temperature Ta is, for example, 15 to 150 seconds.

[0104] [Cooling process S43] In the cooling step S43, the cold-rolled steel sheet after the decarburization step S42 is cooled to room temperature by a well-known method to obtain a decarburization-annealed steel sheet. The cooling method may be natural cooling or water cooling. Preferably, the cold-rolled steel sheet after the decarburization step S42 is natural cooling. Through the above steps, a decarburization-annealed steel sheet is produced in the decarburization-annealing step (S4).

[0105] [Finishing annealing process S5] In the finish annealing step S5, an annealing separator is applied to the decarburization-annealed steel sheet, and the decarburization-annealed steel sheet to which the annealing separator has been applied is subjected to finish annealing to produce a finish-annealed steel sheet. The finish annealing step S5 includes the following steps. Annealing separator application process S51 Annealing process S52 Each step will be described below.

[0106] [Annealing separator application process S51] In the annealing separator application step S51, an annealing separator is applied to the decarburized annealed steel sheet. Specifically, an aqueous slurry containing the annealing separator is applied to the decarburized annealed steel sheet. The aqueous slurry is prepared by adding water to the annealing separator and stirring. The annealing separator contains magnesium oxide (MgO). Preferably, MgO is the main component of the annealing separator. Here, "main component" means that the MgO content in the annealing separator is 60.0% by mass or more. The annealing separator may contain well-known additives in addition to MgO.

[0107] In the annealing separator application step S51, an annealing separator in the form of an aqueous slurry is applied to the surface of a decarburized annealed steel sheet. The steel sheet with the annealing separator applied to its surface is wound into a coil. After the steel sheet is coiled, the annealing step S52 is carried out.

[0108] [Annealing process S52] The steel sheet after the annealing separator application step S51 is subjected to the annealing step S52 to induce secondary recrystallization. The final annealing step (S5) is performed by charging the coiled steel sheet into a heat treatment furnace. The manufacturing conditions for the annealing step S52 are, for example, as follows. The atmosphere in the furnace during the annealing step S52 is a well-known atmosphere.

[0109] Final annealing temperature: 1000~1300℃ Holding time at final annealing temperature: 5 to 60 hours If the final annealing temperature is less than 1000°C, sufficient secondary recrystallization does not occur, and the purification to remove the precipitates used in the secondary recrystallization is insufficient. As a result, the magnetic properties of the manufactured grain-oriented electrical steel sheet are poor. On the other hand, if the final annealing temperature exceeds 1300°C, the effects of secondary recrystallization and purification are low, and problems such as deformation of the steel sheet occur. If the final annealing temperature is 1000 to 1300°C, sufficient secondary recrystallization occurs and the magnetic properties are improved, provided that the above-mentioned holding time is appropriate. Furthermore, a primary coating containing forsterite is formed on the steel sheet surface. Through the above manufacturing steps, a finish-annealed steel sheet is manufactured in the finish annealing step S5.

[0110] The final annealing step S5 removes some of the elements in the chemical composition of the steel sheet. In particular, S, Al, N, and other elements that function as inhibitors are largely removed. Furthermore, a primary coating containing forsterite is formed on the surface of the grain-oriented electrical steel sheet after the final annealing step S5.

[0111] [Insulating film formation process S6] In the insulating film forming step S6, an insulating film forming liquid is applied to the finish annealed steel sheet, and the finish annealed steel sheet to which the insulating film forming liquid has been applied is subjected to a heat treatment to form an insulating film on the finish annealed steel sheet.

[0112] Specifically, a known insulating film-forming solution containing colloidal silica and phosphate as the main components is applied to the surface (on the primary coating) of a finish-annealed steel sheet. The finish-annealed steel sheet to which the insulating film-forming solution has been applied is then baked. As a result, a known insulating film is formed on the primary coating.

[0113] [Other optional processes] [Nitriding process] The method for producing the grain-oriented electrical steel sheet 1 according to this embodiment may further include a nitriding step after the decarburization annealing step and before the finish annealing step, if necessary. The nitriding step may be carried out under well-known conditions. Preferred nitriding conditions are, for example, as follows: Nitriding temperature: 700~850℃ Atmosphere inside the nitriding furnace (nitriding atmosphere): An atmosphere containing gases with nitriding properties such as hydrogen, nitrogen, and ammonia

[0114] If the nitriding temperature is 700°C or higher, or 850°C or lower, nitrogen easily penetrates into the steel sheet during nitriding. If nitriding is performed within this temperature range, the amount of nitrogen inside the steel sheet can be preferably secured. Therefore, fine AlN is preferably formed in the steel sheet before secondary recrystallization. As a result, secondary recrystallization is preferably exhibited during finish annealing. The time for which the steel sheet is held at the nitriding temperature is not particularly limited, but is, for example, 10 to 60 seconds.

[0115] [Magnetic domain refining process] The grain-oriented electrical steel sheet according to this embodiment may further be subjected to a magnetic domain refining treatment step after the finish annealing step S5 or the insulating film forming step S6, as necessary. In the magnetic domain refining treatment step, the surface of the grain-oriented electrical steel sheet is irradiated with a laser beam that has a magnetic domain refining effect, or grooves are formed in the surface. In this case, a grain-oriented electrical steel sheet with even better magnetic properties can be produced.

[0116] [Summary of manufacturing process] As described above, in the manufacturing method of this embodiment, by satisfying conditions 1 to 8 in the cold rolling step S3, the development of α-fiber orientation groups in the steel sheet surface layer in the cold-rolled steel sheet is suppressed and a sufficient amount of γ-fiber orientation groups is left. Then, the cold-rolled steel sheet is subjected to the decarburization annealing step S4 that satisfies condition 9. As a result, in the decarburization annealing step S4, a sufficient amount of {111} recrystallization groups are generated along with a sufficient amount of fine Goss-oriented grains in the steel sheet surface layer of the decarburization annealed steel sheet after the onset of primary recrystallization.

[0117] In the final annealing step S5, the {111} recrystallized grains suppress the coarsening of other crystal grains than the fine Goss-oriented grains before secondary recrystallization occurs. This prevents other crystal grains from coarsening and encroaching on the Goss-oriented grains. As a result, a sufficient number of Goss-oriented grains undergo secondary recrystallization, increasing the concentration of the Goss-oriented grains. As a result, the magnetic flux density of the grain-oriented electrical steel sheet can be increased.

[0118] Furthermore, since a sufficient amount of Goss-oriented grains undergo secondary recrystallization, it is possible to suppress the increase in the diameter of the secondary recrystallized grains (Goss-oriented grains). By suppressing the grain size of the Goss-oriented grains after secondary recrystallization, the texture of the coiled steel sheet can be prevented from varying from the Goss orientation, specifically, the grains parallel to the rolling direction can be prevented from varying from the Goss orientation. <100> This reduces deviation from the orientation, thereby reducing variations in magnetic flux density between the outer and inner peripheries of the coiled grain-oriented electrical steel sheet.

[0119] In the manufacturing method of this embodiment, even if the in-process time, which is the period from the hot-rolled sheet annealing step S2 until the cold rolling step S3 is performed, is long, the grain-oriented electrical steel sheet to be manufactured can have a sufficiently high magnetic flux density and the variation in magnetic flux density between the portion near the outer periphery and the portion near the inner periphery of the grain-oriented electrical steel sheet can be suppressed. Furthermore, as in the case where the in-process time is long, even if annealing is performed to suppress the occurrence of brittle cracking in the cold rolling step S3, the grain-oriented electrical steel sheet to be manufactured can have a sufficiently high magnetic flux density and the variation in magnetic flux density between the portion near the outer periphery and the portion near the inner periphery of the grain-oriented electrical steel sheet can be suppressed.

[0120] Hereinafter, aspects of the present invention will be described in detail with reference to examples. These examples are examples for confirming the effects of the manufacturing method of the grain-oriented electrical steel sheet according to the present embodiment, and are not intended to limit the present invention. [Example]

[0121] In Example 1, grain-oriented electrical steel sheets were manufactured by varying Conditions 1 to 9 in the manufacturing process described above.

[0122] Specifically, a slab was prepared whose chemical composition, in mass %, was C: 0.08%, Si: 3.3%, Mn: 0.08%, S: 0.02%, sol. Al: 0.03%, N: 0.01%, with the remainder being Fe and impurities.

[0123] The hot rolling step S1 was carried out on the slabs with each test number shown in Tables 1-1 and 1-2. Specifically, the slabs with each test number were heated to 1340°C in a heating furnace. The heated slabs were hot rolled to produce hot-rolled steel sheets with thicknesses of 1.8 to 3.5 mm.

[0124] The hot-rolled steel sheet after the hot rolling step was subjected to a hot-rolled sheet annealing step S2 at a hot-rolled sheet annealing temperature of 900 to 1200°C for a holding time of 10 to 300 seconds. After the hot-rolled sheet annealing step S2, a cold-rolling step S3 was carried out to produce a cold-rolled steel sheet having a sheet thickness of 0.22 mm. Specifically, a first cold-rolling step was carried out, and then a second cold-rolling step was carried out. Conditions 1 to 9 were as shown in Tables 1-1 and 1-2.

[0125] [Table 1-1]

[0126] [Table 1-2]

[0127] Specifically, in Table 1-1, "reverse" in the "condition 1" column of the "first cold rolling step" column means that a reverse rolling step was performed as the first cold rolling step. "tandem" means that a tandem rolling step was performed as the first cold rolling step.

[0128] The "Condition 2" column in the "First Cold Rolling Process" section of Table 1-1 lists the average diameter D1 (mm) of the work rolls used in each pass. The "Condition 3" column in the "First Cold Rolling Process" section of Table 1-1 lists the cumulative reduction rate CR1 (%) in the first cold rolling process.

[0129] In Table 1-1, "Reverse" in the "Condition 4" column of the "Second Cold Rolling Step" column means that a reverse rolling step was performed as the second cold rolling step. "Tandem" means that a tandem rolling step was performed as the second cold rolling step. "-" means that a second cold rolling step was not performed.

[0130] In Table 1-1, the "Condition 5" column in the "Second Cold Rolling Process" column indicates whether the cold-rolled steel sheet to be rolled has been heat-treated or not. "No" means that the cold-rolled steel sheet to be rolled in the second cold rolling process was an unannealed, as-rolled material. "Yes" means that the cold-rolled steel sheet to be rolled in the second cold rolling process was an annealed cold-rolled steel sheet. During annealing, the cold-rolled steel sheet was held at 1100°C for 80 seconds.

[0131] The "Condition 6" column in the "Second Cold Rolling Process" section of Table 1-1 lists the average diameter D2 (mm) of the work rolls used in each pass. The "Condition 7" column in the "Second Cold Rolling Process" section of Table 1-1 lists the cumulative reduction rate CR2 (%) in the second cold rolling process.

[0132] The "Condition 8" column in Table 1-1 lists the cumulative reduction rate CR0 (%) for the entire cold rolling process S3.

[0133] The cold-rolled steel sheet after the cold rolling step S3 was subjected to a decarburization annealing step S4. Specifically, the temperature was raised to an ultimate temperature of 870°C, and then held at the decarburization annealing temperature of 830°C for 80 seconds. The cold-rolled steel sheet was then allowed to cool to room temperature to obtain a decarburization annealed steel sheet. During the temperature rise, the average heating rate HR from 550°C to 750°C was set as shown in the "Condition 9" column of "Decarburization annealing step 4" in Table 1-2.

[0134] An annealing separator mainly composed of MgO was applied to the surface of the decarburized annealed steel sheet. The decarburized annealed steel sheet coated with the annealing separator was then wound into a coil. Hereinafter, the coiled steel sheet will also be simply referred to as a "coil." The radius of curvature of the inner circumference of the coil was 250 mm, and the radius of curvature of the outer circumference of the coil was 850 mm.

[0135] The coil was subjected to finish annealing to produce a finish annealed steel sheet. The finish annealing temperature was set to 1100°C to 1200°C, and the holding time at the finish annealing temperature was set to 5 to 30 hours.

[0136] The steel sheets after the finish annealing step S5 were subjected to the insulating film forming step S6. Specifically, an insulating film forming solution mainly composed of colloidal silica and phosphate was applied to the surface of the finish annealed steel sheets of each test number. The finish annealed steel sheets to which the insulating film forming solution had been applied were then baked under the same conditions to form an insulating film on the primary coating. Coil-shaped grain-oriented electrical steel sheets of each test number were manufactured by the above manufacturing steps.

[0137] [Evaluation test] Of the grain-oriented electrical steel sheets manufactured with each test number, test specimens were taken from the innermost and outermost parts of the coil during finish annealing. The size of the test specimens was 60 mm x 300 mm x sheet thickness. Hereinafter, the test specimens from the innermost part of the coil will be referred to as "inner test specimens" and the test specimens from the outermost part of the coil will be referred to as "outer test specimens."

[0138] The magnetic flux density B8 of the inner peripheral test piece and the outer peripheral test piece was determined in accordance with JIS C2556: 2015. Furthermore, the magnetic flux density difference ΔB was calculated using the following formula. ΔB = magnetic flux density B8 of outer test piece - magnetic flux density B8 of inner test piece

[0139] [Evaluation results] The obtained magnetic flux density B8 is shown in Table 1-2. Conditions 1 to 9 were appropriate for test numbers 3 to 6, 8 to 16, 18 to 23, 25 to 33, 42, and 43. Therefore, the magnetic flux density B8 was 1.911 T or more for both the inner peripheral test piece and the outer peripheral test piece. Furthermore, ΔB was 0.006 T or less, and the variation in magnetic flux density was small.

[0140] On the other hand, in Test No. 1, the second cold rolling step was not performed. Therefore, the magnetic flux density B8 of the inner peripheral test piece and the outer peripheral test piece was low. Furthermore, ΔB exceeded 0.006 T, and the variation in magnetic flux density was large.

[0141] Test No. 2 did not satisfy condition 2. Therefore, the magnetic flux density B8 of the inner peripheral test piece and the outer peripheral test piece was low. Furthermore, ΔB was 0.006 T or more, and the variation in magnetic flux density was large.

[0142] Test Nos. 7 and 24 did not satisfy condition 8. Therefore, the degree of integration in the Goss orientation was low. As a result, the magnetic flux density B8 of both the inner peripheral test piece and the outer peripheral test piece was low.

[0143] Test No. 17 did not meet the lower limit of Condition 3. Condition 7 exceeded the upper limit. As a result, the magnetic flux density B8 of the inner and outer peripheral test pieces was low. Furthermore, ΔB exceeded 0.006 T, and the magnetic flux density variation was large.

[0144] Test No. 34 did not satisfy the upper limit of Condition 3. Therefore, Condition 7 was below the lower limit. Therefore, the degree of integration in the Goss orientation was low. As a result, the magnetic flux density B8 of both the inner and outer peripheral test pieces was low. Furthermore, ΔB exceeded 0.006 T, and the magnetic flux density variation was large.

[0145] In test numbers 35 to 39, the rolling target in the reverse rolling process was a heat-treated intermediate steel sheet. Therefore, the degree of concentration in the Goss orientation was low. As a result, the magnetic flux density B8 of both the inner and outer peripheral test pieces was low, and ΔB sometimes exceeded 0.006 T.

[0146] Test Nos. 40 and 41 did not satisfy condition 9. Therefore, the degree of concentration in the Goss orientation was low. As a result, the magnetic flux density B8 of both the inner and outer test pieces was low. Furthermore, ΔB exceeded 0.006 T, and the variation in magnetic flux density was large. [Example]

[0147] Grain-oriented electrical steel sheets were manufactured using slabs having various chemical compositions in Example 2. Specifically, slabs having the chemical compositions shown in Tables 2-1 and 2-2 were prepared.

[0148] [Table 2-1]

[0149] [Table 2-2]

[0150] The symbol "-" next to the element content in Table 2-1 means that the corresponding element content is 0% when rounded to the nearest significant figure (the lowest digit) as specified in the above embodiment. For example, the Cr content specified in this embodiment is specified as a numerical value up to two decimal places. Therefore, in test number 1 in Table 2-1, the measured Cr content was 0% when rounded to the nearest three decimal places. Furthermore, the Sn content specified in this embodiment is specified as a numerical value up to two decimal places. Therefore, in test number 1 in Table 2-1, the measured Sn content was 0% when rounded to the nearest three decimal places. Rounding off means that if the digit (fraction) below the specified minimum digit is less than 5, it is rounded down, and if it is 5 or more, it is rounded up.

[0151] The hot rolling step S1 was performed on the slabs of each test number listed in Tables 2-1 and 2-2. Specifically, the slabs of each test number were heated to 1340°C in a heating furnace. The heated slabs were hot rolled to produce hot-rolled steel sheets with a thickness of 2.3 mm.

[0152] The hot-rolled steel sheet after the hot rolling step was subjected to a hot-rolled sheet annealing step S2 at a hot-rolled sheet annealing temperature of 900 to 1200°C for a holding time of 10 to 300 seconds.

[0153] After the hot-rolled sheet annealing process S2, a cold-rolling process S3 was carried out to produce a cold-rolled steel sheet with a thickness of 0.22 mm. Specifically, the hot-rolled steel sheet was first subjected to a tandem rolling process S31 to produce an intermediate steel sheet. Five passes of reduction were carried out in the tandem rolling process. The average diameter D1 of the work rolls in the tandem rolling was 500 mm, and the cumulative reduction rate CR1 was 60%. A reverse rolling process was carried out on the intermediate steel sheet without heat treatment. Three passes of reduction were carried out in the reverse rolling process. The average diameter of the work rolls in the reverse rolling process was 70 mm, and the cumulative reduction rate CR2 was 76%. The cumulative reduction rate CR0 of the entire cold-rolling process S3 was 90%.

[0154] The cold-rolled steel sheet after the cold rolling step S3 was subjected to a decarburization annealing step S4. Specifically, the temperature was raised to an ultimate temperature of 850°C, and then held at the decarburization annealing temperature of 830°C for 80 seconds. The cold-rolled steel sheet was then allowed to cool to room temperature to obtain a decarburization annealed steel sheet. During the temperature rise, the average heating rate HR from 550°C to 750°C was 1300°C / s.

[0155] An annealing separator mainly composed of MgO was applied to the surface of the decarburized annealed steel sheet. The decarburized annealed steel sheet coated with the annealing separator was then wound into a coil. Hereinafter, the coiled steel sheet will be simply referred to as the "coil." The inner curvature radius of the coil was 250 mm, and the outer curvature radius of the coil was 850 mm.

[0156] The coil was subjected to finish annealing to produce a finish annealed steel sheet. The finish annealing temperature was set to 1100°C to 1200°C, and the holding time at the finish annealing temperature was set to 5 to 30 hours.

[0157] The steel sheets after the finish annealing step S5 were subjected to the insulating film forming step S6. Specifically, an insulating film forming solution mainly composed of colloidal silica and phosphate was applied to the surface of the finish annealed steel sheets of each test number. The finish annealed steel sheets to which the insulating film forming solution had been applied were then baked under the same conditions to form an insulating film on the primary coating. Coil-shaped grain-oriented electrical steel sheets of each test number were manufactured by the above manufacturing steps.

[0158] [Evaluation test] As in Example 1, inner and outer peripheral test pieces were taken from the coil-shaped grain-oriented electrical steel sheets of each test number. The magnetic flux densities B8 of the inner and outer peripheral test pieces were determined in accordance with JIS C2556:2015. Furthermore, the magnetic flux density difference ΔB was determined.

[0159] [Test Results] The evaluation results are shown in Table 2-2. The chemical composition of the slab was appropriate for all grain-oriented electrical steel sheets of test numbers 1 to 15. Furthermore, conditions 1 to 9 during the manufacturing process were also appropriate. Therefore, the magnetic flux density B8 was 1.911 T or more for both the inner peripheral test piece and the outer peripheral test piece. Furthermore, ΔB was 0.006 T or less, and the variation in magnetic flux density was small. [Example]

[0160] In Example 3, the influence of the period (in-process time) from the completion of the hot-rolled sheet annealing process S2 to the cold rolling process S3, and the influence of the annealing temperature when annealing treatment is performed before the cold rolling process S3 were investigated.

[0161] Specifically, a slab was prepared whose chemical composition, in mass %, was C: 0.08%, Si: 3.4%, Mn: 0.08%, S: 0.02%, sol. Al: 0.03%, N: 0.01%, with the remainder being Fe and impurities.

[0162] The hot rolling step S1 was performed on the slabs with each test number shown in Tables 3-1 and 3-2. Specifically, the slabs with each test number were heated to 1340°C in a heating furnace. The heated slabs were hot rolled to produce hot-rolled steel sheets with a thickness of 2.3 mm.

[0163] [Table 3-1]

[0164] [Table 3-2]

[0165] The hot-rolled steel sheet after the hot rolling step was subjected to a hot-rolled sheet annealing step S2 at a hot-rolled sheet annealing temperature of 900 to 1200°C for a holding time of 10 to 300 seconds.

[0166] After the hot-rolled sheet annealing step S2, the hot-rolled steel sheets were placed in an indoor yard for the number of days (days) shown in Table 3-2. After the number of days had elapsed, the hot-rolled steel sheets of test numbers 7 to 14 were annealed (heat treated) at the annealing temperature (°C) shown in Table 3-2 immediately before being subjected to the cold rolling step S3. Note that no annealing was performed on the hot-rolled steel sheets of test numbers 1 to 6. Thereafter, the hot-rolled steel sheets of test numbers 1 to 14 were subjected to a cold-rolling step S3 to produce cold-rolled steel sheets with a thickness of 0.19 mm. Specifically, after the first cold-rolling step, a second cold-rolling step was performed. Conditions 1 to 4, 6, and 7 were as shown in Table 3-1. Specifically, in Table 3-1, "reverse" in the "condition 1" column of the "first cold rolling step" column means that a reverse rolling step was performed as the first cold rolling step. "tandem" means that a tandem rolling step was performed as the first cold rolling step.

[0167] The "Condition 2" column in the "First Cold Rolling Process" section of Table 3-1 lists the average diameter D1 (mm) of the work rolls used in each pass. The "Condition 3" column in the "First Cold Rolling Process" section of Table 3-1 lists the cumulative reduction rate CR1 (%) in the first cold rolling process.

[0168] In Table 3-1, "Reverse" in the "Condition 4" column of the "Second cold rolling step" column means that a reverse rolling step was performed as the second cold rolling step. "-" means that the second cold rolling step was not performed.

[0169] The "Condition 6" column in the "Second Cold Rolling Process" section of Table 3-1 lists the average diameter D2 (mm) of the work rolls used in each pass. The "Condition 7" column in the "Second Cold Rolling Process" section of Table 3-1 lists the cumulative reduction CR2 (%) in the second cold rolling process. The "Condition 8" column in Table 3-1 lists the cumulative reduction CR0 (%) for the entire cold rolling process S3.

[0170] In addition, for Condition 5, in all test numbers, intermediate steel sheets that had not been subjected to heat treatment were the target of rolling in the second cold rolling process. In addition, when Condition 1 was reverse rolling, three passes of reduction were performed. When Condition 1 was tandem rolling, five passes of reduction were performed.

[0171] The cold-rolled steel sheet after the cold rolling step S3 was subjected to a decarburization annealing step S4. Specifically, the temperature was raised to an ultimate temperature of 850°C, and then held at the decarburization annealing temperature of 830°C for 80 seconds. The cold-rolled steel sheet was then allowed to cool to room temperature to obtain a decarburization annealed steel sheet. During heating, the average heating rate HR (Condition 9) from 550°C to 750°C was 1300°C / s.

[0172] An annealing separator mainly composed of MgO was applied to the surface of the decarburized annealed steel sheet. The decarburized annealed steel sheet coated with the annealing separator was then wound into a coil. Hereinafter, the coiled steel sheet will be simply referred to as the "coil." The inner curvature radius of the coil was 250 mm, and the outer curvature radius of the coil was 850 mm.

[0173] The coil was subjected to finish annealing to produce a finish annealed steel sheet. The finish annealing temperature was set to 1100°C to 1200°C, and the holding time at the finish annealing temperature was set to 5 to 30 hours.

[0174] The steel sheets after the finish annealing step S5 were subjected to the insulating film forming step S6. Specifically, an insulating film forming solution mainly composed of colloidal silica and phosphate was applied to the surface of the finish annealed steel sheets of each test number. The finish annealed steel sheets to which the insulating film forming solution had been applied were then baked under the same conditions to form an insulating film on the primary coating. Coil-shaped grain-oriented electrical steel sheets of each test number were manufactured by the above manufacturing steps.

[0175] [Evaluation test] As in Example 1, inner and outer peripheral test pieces were taken from the coil-shaped grain-oriented electrical steel sheets of each test number. The magnetic flux densities B8 of the inner and outer peripheral test pieces were determined in accordance with JIS C2556:2015. Furthermore, the magnetic flux density difference ΔB was determined.

[0176] [Test Results] The evaluation results are shown in Table 3-2.

[0177] Conditions 1 to 9 were appropriate for Test Nos. 1 to 3 and 7 to 10. Therefore, even if the in-process time was one day or more, and even if annealing was performed before the first cold rolling step, an excellent magnetic flux density B8 was obtained on both the inner and outer circumferences of the coil.

[0178] On the other hand, for test numbers 4 to 6 and 11 to 14, a conventional cold rolling process was performed. Therefore, when the production time was one day or more, the magnetic flux density B8 was low on both the inner and outer circumferences of the coil. In addition, when annealing was performed before the first cold rolling process, the magnetic flux density B8 was also low on both the inner and outer circumferences of the coil. Furthermore, ΔB exceeded 0.006 T, and the variation in magnetic flux density was large.

[0179] The embodiments of the present invention have been described above. However, the above-described embodiments are merely examples for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiments, and the above-described embodiments can be appropriately modified and carried out without departing from the spirit of the present invention.

Claims

1. The chemical composition, in mass%, is C: 0.01-0.20%, Si: 2.0 to 4.5%, Mn: 0.01 to 0.30%, S: 0.01-0.05%, sol. Al: 0.01 to 0.05%, N: 0.01-0.02%, Cr: 0.00-0.50%, Sn: 0.00-0.30%, Sb: 0.00 to 0.30%, Ni: 0.00 to 0.50%, Mo: 0.00-0.20%, P: 0.00-0.15%, Cu: 0.00-0.50%, Se: 0.00-0.03%, V: 0.00 to 0.15%, and Bi: 0.0000-0.0100%, a hot rolling step of hot rolling a slab containing the above-mentioned alloy and the balance consisting of Fe and impurities to produce a hot-rolled steel sheet; a hot-rolled sheet annealing step of annealing the hot-rolled steel sheet; A cold rolling process in which the hot-rolled steel sheet after the hot-rolled sheet annealing process is cold-rolled at a cumulative rolling reduction rate CR0 of 90% or more to manufacture a cold-rolled steel sheet; a decarburization annealing step of performing decarburization annealing on the cold-rolled steel sheet to manufacture a decarburization annealed steel sheet; a finish annealing step of applying an annealing separator to the decarburization-annealed steel sheet and performing finish annealing on the decarburization-annealed steel sheet to which the annealing separator has been applied, to manufacture a finish-annealed steel sheet; an insulating film forming step of applying an insulating film forming liquid to the finish annealed steel sheet and performing a heat treatment on the finish annealed steel sheet to which the insulating film forming liquid has been applied, thereby forming an insulating film on the finish annealed steel sheet, The cold rolling step includes: a tandem rolling process in which the hot-rolled steel sheet is subjected to continuous rolling by a plurality of passes using a tandem rolling mill including a plurality of rolling stands arranged in a row, thereby producing an intermediate steel sheet; A reverse rolling process in which the intermediate steel plate that has not been heat treated after the tandem rolling process is subjected to reverse rolling with a plurality of passes using a multi-stage rolling mill to manufacture the cold-rolled steel plate, In the tandem rolling process, The average diameter D1 of the work rolls used in the plurality of passes is 200 mm or more, The cumulative rolling reduction rate CR1 is set to 30 to 87%, In the reverse rolling step, The average diameter D2 of the work rolls used in the plurality of passes is 100 mm or less, The cumulative rolling reduction rate CR2 is set to 24 to 80%, In the decarburization annealing step, The average heating rate HR in the temperature range of the cold-rolled steel sheet from 550°C to 750°C is 100°C / sec. That is all. Manufacturing method for grain-oriented electrical steel sheets.

2. A method for producing the grain-oriented electrical steel sheet according to claim 1, The slab is Cr: 0.01-0.50%, Sn: 0.01-0.30%, Sb: 0.01-0.30%, Ni: 0.01-0.50%, Mo: 0.01-0.20%, P: 0.01-0.15%, Cu: 0.01 to 0.50%, Se: 0.01-0.03%, V: 0.01 to 0.15%, and Bi: 0.0001-0.0100%, Contains one or more elements selected from the group consisting of Manufacturing method for grain-oriented electrical steel sheets.

Images