Manufacturing method of grain-oriented electrical steel sheets

Abstract

This method for producing a grain-oriented electrical steel sheet comprises a hot rolling step, a hot rolled sheet annealing step, a cold rolling step, a decarburization annealing step, a separation agent coating step, a finish annealing step, a surface treatment step, a thermal oxidation-reduction annealing step and a tension-imparting insulating film forming step. The thermal oxidation-reduction annealing step comprises: a primary heating process in which the temperature of a surface-treated steel sheet is raised by heating; a primary annealing process in which the temperature of the steel sheet is maintained at a certain temperature within the range of 800°C to 900°C for a certain retention time within the range of 10 to 200 seconds in an atmosphere in which the oxygen potential PH2O / PH2 is at a certain value within the range of 0.10 to 10.00; and a secondary annealing process in which the temperature of the steel sheet is maintained at a certain temperature within the range of 900°C to 1100°C for a certain retention time within the range of 3 to 100 seconds in an atmosphere in which the oxygen potential PH2O / PH2 is less than 0.1000 and the dew point is at a certain value less than -20°C.

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a grain-oriented electrical steel sheet. This application claims priority based on Japanese Patent Application No. 2023-013751 filed in Japan on February 1, 2023, and incorporates its content herein.

Background Art

[0002] A grain-oriented electrical steel sheet contains about 0.5 mass% to 7 mass% of silicon (Si), and utilizes a phenomenon called secondary recrystallization to accumulate crystal orientations in the {110}<001> orientation (Goss orientation). It is mainly used as a soft magnetic material for cores such as transformers. Since the properties of grain-oriented electrical steel sheets greatly affect the performance of transformers, intensive studies have been carried out to achieve good magnetization characteristics and low iron loss for grain-oriented electrical steel sheets.

[0003] A general method for manufacturing a grain-oriented electrical steel sheet is as follows. First, a steel slab having a predetermined chemical composition is heated and hot-rolled to produce a hot-rolled steel sheet. After performing hot-rolled sheet annealing on the obtained hot-rolled steel sheet as necessary, the hot-rolled steel sheet is pickled. Cold rolling is performed on the pickled hot-rolled steel sheet to produce a cold-rolled steel sheet. Decarburization annealing is performed on the obtained cold-rolled steel sheet to induce primary recrystallization. Then, an aqueous slurry containing an annealing separating agent mainly composed of MgO is applied to the surface of the cold-rolled steel sheet after decarburization annealing and dried. Thereafter, the steel sheet is wound into a coil shape and finish annealing is performed to induce secondary recrystallization. During finish annealing, simultaneously with the occurrence of secondary recrystallization in the steel sheet, MgO in the annealing separating agent reacts with SiO2 in the internal oxide layer formed on the surface of the cold-rolled steel sheet during decarburization annealing, and a glass film mainly composed of forsterite (Mg2SiO4) (hereinafter, also referred to as "primary film") is formed on the surface of the base steel sheet. After finish annealing (after formation of the primary film), a tension-applied insulating film (hereinafter, also referred to as "secondary film") is formed by applying and baking a chemical solution mainly composed of, for example, silica and phosphate on the upper layer of the primary film.

[0004] Incidentally, the primary coating, in addition to functioning as an insulating coating, also has the function of improving the adhesion of the secondary coating formed on top of the primary coating. Furthermore, iron loss is reduced by the tension exerted by both the primary and secondary coatings. However, since the primary coating is a non-magnetic phase, it is not desirable from the viewpoint of magnetic properties. In addition, the interface between the base steel plate and the primary coating has an embedded structure in which the roots of the primary coating are intertwined with the base steel plate, which can sometimes cause obstruction of magnetic domain wall movement and lead to an increase in iron loss. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 8-269560 [Patent Document 2] International Publication No. 2019 / 182149 [Patent Document 3] International Publication No. 2020 / 149345 [Overview of the project] [Problems that the invention aims to solve]

[0006] Given the above background, many inventions have been made concerning grain-oriented electrical steel sheets without a primary coating. For example, Patent Document 1 discloses a manufacturing method aimed at suppressing the formation and peeling off of the primary coating by adding chloride to the annealing separating agent in the annealing separating agent application step before the secondary recrystallization annealing step. This manufacturing method has high industrial value due to its simplicity. However, because no primary film is formed, the adhesion of the secondary film is still insufficient. For example, in Patent Document 2, irregularities are formed on the surface of the base steel sheet after secondary recrystallization annealing and before the coating and baking process of the secondary film. With this manufacturing method, film adhesion is ensured by the anchoring effect that occurs at the interface between the base steel sheet and the secondary film. However, these irregularities at the interface can hinder the movement of magnetic domain walls when the grain-oriented electrical steel sheet is magnetized, and can be a factor that prevents the reduction of iron loss. Furthermore, in the aforementioned Patent Document 3, intermediate annealing is performed on the base steel sheet prior to the application of the chemical solution for the secondary coating. This manufacturing method generates an oxide film on the surface of the base steel sheet, which is used as a buffer layer when the secondary coating adheres to it, making it possible to achieve both high magnetic properties and high coating adhesion. In this way, high coating adhesion can be obtained by intermediate annealing. On the other hand, in order to further improve the performance of transformers, there has been a demand for grain-oriented electrical steel sheets with even higher coating adhesion.

[0007] The present invention has been made in view of the above circumstances, and aims to provide a method for manufacturing grain-oriented electrical steel sheets that can produce grain-oriented electrical steel sheets having higher coating adhesion (secondary coating adhesion) without impairing magnetic properties. [Means for solving the problem]

[0008] In order to solve the above problems and achieve the above objectives, the present invention employs the following embodiments. (1) A method for manufacturing grain-oriented electrical steel sheets according to one aspect of the present invention is: base steel plate and formed on the surface of the base steel plate Fe-PO compound A method for manufacturing a grain-oriented electrical steel sheet having an oxide layer containing and a tension-imparting insulating coating formed on the surface of the oxide layer, In terms of chemical composition, in mass%, C: 0.020%~0.150% Si: 3.00%~4.00% Mn: 0.01%~0.50% S: 0.0010%~0.0400%, Acid soluble Al: 0.010%~0.050%, N: 0.002%~0.020% Bi: 0.0000%~0.0200%, P: 0.000%~0.100%, Sn: 0.00%~0.50% Cu: 0.00%~0.50%, Cr: 0.00%~0.50%, Sb: 0.00%~0.20% Mo: 0.00%~0.10%, Nb: 0.0000%~0.0200%, B: 0.0000%~0.0200%, Te: 0.0000%~0.0200%, Ni: 0.00%~0.20%, Se: 0.0000%~0.0200%, A hot rolling process to obtain a hot-rolled steel sheet by heating and hot-rolling a slab containing the above, with the remainder being Fe and impurities, The process involves hot-rolled steel sheet being subjected to hot-rolled sheet annealing to obtain a hot-rolled sheet, and then immersing the hot-rolled sheet in an acid pickling solution, A cold rolling process to obtain a cold-rolled steel sheet by cold rolling the aforementioned hot-rolled sheet and annealed sheet, A decarburization annealing step is performed on the cold-rolled steel sheet to obtain a decarburized annealed steel sheet, A separation agent coating step is performed by applying an annealing separation agent containing MgO, Al2O3, and chloride to the surface of the decarburized annealed steel sheet. A finish annealing step is performed on the decarburized annealed steel sheet to which the annealing separating agent has been applied to obtain a finish annealed sheet. A surface treatment step is performed by pickling the surface of the finished annealed plate to obtain a surface-treated steel plate. A thermal oxidation-reduction annealing process is performed on the surface-treated steel sheet in a two-stage annealing process to obtain a reduction-annealed steel sheet, A tension-imparting insulating coating forming step, in which the tension-imparting insulating coating containing phosphate and silica is formed on the surface of the reduction-annealed steel sheet, It has, The aforementioned thermal oxidation-reduction annealing process, A primary heating process is performed after the surface treatment process, in which the surface-treated steel sheet is heated to raise its temperature. Following the aforementioned first heating process, a first annealing process is performed in which the steel plate temperature is maintained at a constant temperature within the range of 800°C to 900°C for a constant holding time within the range of 10 seconds to 200 seconds in an atmosphere with a constant oxygen potential (PH2O / PH2) within the range of 0.10 to 10.00, After the primary annealing process, in an atmosphere with a constant oxygen potential PH2O / PH2 less than 0.1000 and a dew point less than -20°C, the temperature of the steel sheet is maintained at a constant temperature within the range of 900°C to 1100°C for a certain holding time within the range of 3 seconds to 100 seconds, including a secondary annealing process. (2) In the method for manufacturing a grain-oriented electrical steel sheet according to (1), in the annealing separator, the content of MgO may be 0.0% by mass or more and 79.5% by mass or less, the content of Al2O3 may be 20.0% by mass or more and 99.5% by mass or less, and the balance may be the chloride. (3) In the method for manufacturing a grain-oriented electrical steel sheet according to (1), in the primary heating process, the oxygen potential PH2O / PH2 when the temperature of the steel sheet is 100°C to 800°C is 0.5000 or less, the average heating rate when the temperature of the steel sheet is 550°C to 800°C may be 100°C / second to 2000°C / second. (4) In the method for manufacturing a grain-oriented electrical steel sheet according to (2), in the primary heating process, the oxygen potential PH!O / PH! when the temperature of the steel sheet is 100°C to 800°C is 0.5000 or less, the average heating rate when the temperature of the steel sheet is 550°C to 800°C may be 100°C / second to 2000°C / second. (5) The method for manufacturing a grain-oriented electrical steel sheet according to any one of (1) to (4) may further include a nitriding annealing process of nitriding the decarburized annealed steel sheet after the decarburizing annealing process and before the separator coating process. (6) In the method for manufacturing a grain-oriented electrical steel sheet according to any one of (1) to (4), in the surface treatment process, the finished annealed sheet is immersed in a first treatment liquid containing at least one of hydrochloric acid, sulfuric acid, nitric acid, and phosphoric acid and having a total acid concentration of 0.1% by volume to 20.0% by volume and a liquid temperature of 50°C to 90°C for 3 seconds to 60 seconds to obtain the surface-treated steel sheet. (7)(5) The method for manufacturing a grain-oriented electrical steel sheet described above may obtain the surface-treated steel sheet by immersing the finish annealed sheet in a first treatment liquid containing at least one of hydrochloric acid, sulfuric acid, nitric acid, and phosphoric acid, having a total acid concentration of 0.1% by volume to 20.0% by volume, and a liquid temperature of 50°C to 90°C for 3 seconds to 60 seconds. (8) The method for manufacturing a grain-oriented electrical steel sheet according to any one of (1) to (7) is such that the slab has the following chemical composition in mass%: Bi: More than 0.0000%, 0.0200% or less, P: More than 0.000%, 0.100% or less, Sn: More than 0.00%, 0.50% or less, Cu: More than 0.00%, 0.50% or less, Cr: More than 0.00%, 0.50% or less, Sb: More than 0.00%, 0.20% or less, Mo: More than 0.00%, 0.10% or less, Nb: More than 0.0000%, 0.0200% or less, B: More than 0.0000%, 0.0200% or less, Te: More than 0.0000%, 0.0200% or less, Ni: More than 0.00%, 0.20% or less, Se: More than 0.0000%, 0.0200% or less, and may contain at least one selected from the group consisting of.

Advantages of the Invention

[0009] According to each of the above aspects of the present invention, a grain-oriented electrical steel sheet having higher film adhesion can be manufactured without impairing magnetic properties.

Brief Description of the Drawings

[0010] [Figure 1] It is a schematic cross-sectional view of a grain-oriented electrical steel sheet obtained by the method for manufacturing a grain-oriented electrical steel sheet according to an embodiment of the present invention. [Figure 2] It is a flowchart showing the method for manufacturing a grain-oriented electrical steel sheet according to the same embodiment. [Figure 3]This diagram illustrates the thermal oxidation-reduction annealing process in the manufacturing method, with the horizontal axis representing time and the vertical axis representing the annealing temperature (base steel sheet temperature). [Modes for carrying out the invention]

[0011] A method for manufacturing grain-oriented electrical steel sheets according to one embodiment of the present invention is described below. However, the present invention is not limited to the configuration and process disclosed in this embodiment, and various modifications are possible without departing from the spirit of the invention. Furthermore, the numerical limits described below include both lower and upper limits. On the other hand, numerical values ​​indicated as "greater than" or "less than" do not include the numerical range. Also, unless otherwise specified, percentages related to chemical composition mean mass percentage.

[0012] Figure 1 is a schematic cross-sectional view of a grain-oriented electrical steel sheet (grain-oriented electrical steel according to this embodiment) obtained by the manufacturing method of grain-oriented electrical steel sheet according to this embodiment. As shown in Figure 1, when viewed from a cross-section where the cutting direction is parallel to the thickness direction, the grain-oriented electrical steel sheet 1 according to this embodiment has a base steel sheet 2 and a secondary coating 3 which is an insulating coating (tension-imparting insulating coating) disposed on the surface of the base steel sheet 2. At the interface between the base steel sheet 2 and the secondary coating 3, only an oxide layer 4 containing an Fe-PO compound is formed, and the glass coating (hereinafter also referred to as the "primary coating") is substantially absent. The average thickness of the base steel sheet 2 can be exemplified as 0.17 mm to 0.29 mm. The average film thickness of the secondary coating 3 can be exemplified as 1 μm to 6 μm.

[0013] To improve iron loss characteristics, it is effective to smooth the surface of the base steel sheet 2 to facilitate magnetic domain wall movement. It is also effective to ensure electrical insulation between multiple grain-oriented electrical steel sheets 1 that are stacked on top of each other by ensuring close contact between the base steel sheet 2 and the secondary coating 3, thereby applying tension to the base steel sheet 2. In the grain-oriented electrical steel sheet 1 according to this embodiment, the surface smoothness of the base steel sheet 2 is ensured by the secondary coating 3 being placed in contact with the base steel sheet 2 (there is no primary coating). Furthermore, as will be explained in the manufacturing method described later, by precisely controlling the atmospheric conditions, soaking temperature, and holding time of the soaking temperature in each step of the two-stage thermal oxidation-reduction annealing process, an Fe-PO compound is formed at the interface between the base steel sheet 2 and the secondary coating 3, thereby ensuring the adhesion of the secondary coating 3. As a result, the grain-oriented electrical steel sheet 1 according to this embodiment has excellent iron loss characteristics and coating adhesion.

[0014] In the Fe-PO compound, Fe originates from the base metal component of the steel base plate 2, and P originates from the secondary coating solution. Fe ions diffuse from the base metal side towards the coating side, while P ions diffuse and concentrate from the coating side towards the base metal side. Chemical bonding occurs when Fe ions and P ions associate, resulting in the formation of the Fe-PO compound. In other words, as a result of chemical bonding between the ions in the coating and the base metal (as a result of the coating adhering closely to the base metal), the Fe-PO compound is observed at the interface between the coating and the base metal.

[0015] Next, the manufacturing method of grain-oriented electrical steel sheets according to this embodiment will be explained using Figures 2 and 3. The manufacturing method described below is illustrative, and appropriate modifications may be made as long as they do not affect the formation of the Fe-PO compound. Figure 2 is a flowchart showing the manufacturing method of grain-oriented electrical steel sheets according to this embodiment. Figure 3 is a diagram illustrating the thermal oxidation-reduction annealing process in the same manufacturing method, where the horizontal axis represents time and the vertical axis represents the annealing temperature (base steel sheet temperature).

[0016] The manufacturing method for grain-oriented electrical steel sheets according to this embodiment includes: a hot rolling step of heating and hot rolling a slab (steel billet) having a predetermined chemical composition to obtain a hot-rolled steel sheet; a hot-rolled annealing step of hot-rolled steel sheet annealing to obtain a hot-rolled annealed sheet, and then immersing the hot-rolled annealed sheet in an acid pickling solution (acid pickling); a cold rolling step of cold rolling the hot-rolled annealed sheet to obtain a cold-rolled steel sheet; a decarburization annealing step of decarburizing the cold-rolled steel sheet to obtain a decarburized annealed steel sheet; and annealing the surface of the decarburized annealed steel sheet. The process mainly includes: a separation agent application step of applying a separation agent; a finish annealing step of applying finish annealing to the decarburized annealed steel sheet coated with the annealing separation agent to obtain a finish annealed sheet; a surface treatment step of pickling the surface of the finish annealed sheet to obtain a surface-treated steel sheet; a thermal oxidation-reduction annealing step of performing a two-stage annealing on the surface-treated steel sheet after the surface treatment step to obtain a reduction annealed steel sheet; and a tension-imparting insulating film formation step of forming a tension-imparting insulating film containing phosphate and silica on the surface of the reduction annealed steel sheet after the thermal oxidation-reduction annealing step. Furthermore, a nitriding annealing step is included as needed after the decarburization annealing step and before the separation agent application step. Additionally, a pickling step is included as needed after the thermal oxidation-reduction annealing step and before the tension-imparting insulating film formation step. The thermal oxidation-reduction annealing process includes a primary heating process, a primary annealing process, and a secondary annealing process. The tension-applying insulating film formation process includes a secondary coating solution application process and a baking process.

[0017] The details of each of the above processes will be explained below. In the following explanation, if the conditions for each process are not specified, publicly known conditions should be applied as appropriate. [Hot rolling process] In the hot rolling process, a steel billet (for example, a steel ingot such as a slab) having a predetermined chemical composition is hot-rolled. For example, the slab (steel billet) subjected to the hot rolling process has a chemical composition of, in mass%, C: 0.020%~0.150% Si: 3.00%~4.00% Mn: 0.01%~0.50% S: 0.0010%~0.0400%, Acid soluble Al: 0.010%~0.050%, N: 0.002%~0.020% Bi:0.0000~%0.0200%, P: 0.000%~0.100%, Sn: 0.00%~0.50% Cu: 0.00%~0.50%, Cr: 0.00%~0.50%, Sb: 0.00%~0.20% Mo: 0.00%~0.10%, Nb: 0.0000%~0.0200%, B: 0.0000%~0.0200%, Te: 0.0000%~0.0200%, Ni: 0.00%~0.20%, Se: 0.0000%~0.0200% It should contain [the specified substance], with the remainder consisting of Fe and impurities.

[0018] Furthermore, the slab (steel billet) described above has a chemical composition in mass%, Bi: more than 0.0000%, less than 0.0200%, P: more than 0.000%, less than 0.100%, Sn: more than 0.00%, less than 0.50%, Cu: more than 0.00%, less than 0.50%, Cr: more than 0.00%, less than 0.50%, Sb: more than 0.00%, less than 0.20%, Mo: more than 0.00%, less than 0.10%, Nb: more than 0.0000%, less than 0.0200%, B: More than 0.0000%, less than 0.0200%, Te: more than 0.0000%, less than 0.0200%, Ni: more than 0.00%, less than 0.20%, Se: more than 0.0000%, less than 0.0200%, It may contain at least one selected from the group consisting of the following.

[0019] C: 0.020%~0.150% Carbon (C) is a fundamental element for steel billets (slabs). C is included to increase the concentration of Goss orientation in secondary recrystallization. The C content required for improving magnetic properties is 0.020% or more, preferably 0.040% or more, and more preferably 0.050% or more, in the slab. However, if excessive C remains in the final product, it can become a factor in iron loss degradation. Therefore, decarburization treatment is necessary in the decarburization annealing process, but if the C content in the slab exceeds 0.150%, decarburization treatment becomes difficult. The C content in the slab is 0.150% or less, preferably 0.120% or less, and more preferably 0.100% or less.

[0020] Si: 3.00%~4.00% Silicon (Si) is a fundamental element for steel slabs. If the Si content is less than 3.00%, eddy current losses cannot be sufficiently reduced, and good magnetic properties cannot be obtained. Therefore, the Si content should be 3.00% or more. Preferably, the Si content is 3.10% or more, and more preferably 3.20% or more. On the other hand, if the Si content exceeds 4.00%, the steel sheet becomes brittle, and the passability during manufacturing deteriorates significantly, so the Si content should be 4.00% or less. Preferably, the Si content is 3.80% or less, and more preferably 3.60% or less.

[0021] Mn: 0.01%~0.50% Manganese (Mn) is a fundamental element for steel slabs. If the Mn content is less than 0.01%, it is difficult to form MnS (or MnSe if Se is used as part of S) which functions as an inhibitor, secondary recrystallization does not proceed sufficiently, and good magnetic properties cannot be obtained. Therefore, the Mn content should be 0.01% or more. Preferably, the Mn content is 0.03% or more, and more preferably 0.06% or more. On the other hand, if the Mn content exceeds 0.50%, the steel undergoes a phase transformation during finish annealing, secondary recrystallization does not proceed sufficiently, and good magnetic properties cannot be obtained. Therefore, the Mn content should be 0.50% or less. Preferably, the Mn content is 0.30% or less, and more preferably 0.20% or less.

[0022] S: 0.0010%~0.0400% Se: 0.0000%~0.0200% S (sulfur) is a fundamental element for steel billets (slabs). S is the element that forms the inhibitor MnS. The S content of a slab is preferably 0.0010% or more, more preferably 0.0100% or more, and more preferably 0.0150% or more. On the other hand, if the S content of a slab exceeds 0.0400%, it can cause hot brittleness, making hot rolling difficult. The S content of a slab is preferably 0.0400% or less, more preferably 0.0350% or less, and more preferably 0.0300% or less. If S remains in excess in the final product, it can cause magnetic degradation. Therefore, S also needs to be removed (purified) from the base steel sheet during finish annealing. Since selenium (Se) is also an element that forms MnSe, Se may be used as part of the sulfur (the Se content may exceed 0.0000%). If there is an excess of Se, secondary recrystallization may become unstable, potentially causing magnetic degradation. Therefore, the Se content must be between 0.0000% and 0.0200%. Preferably, the Se content is between 0.0000% and 0.0150%, and more preferably between 0.0000% and 0.0100%.

[0023] Acid soluble Al: 0.010%~0.050% Acid-soluble aluminum (Al) (sol.Al) is a fundamental element for steel billets (slabs). Acid-soluble Al forms the inhibitor AlN, which is necessary to enhance magnetic properties. The acid-soluble Al content of the slab is 0.010% or more, preferably 0.015% or more, and more preferably 0.020% or more. On the other hand, if the slab contains an excess of acid-soluble Al, embrittlement may become significant. The acid-soluble Al content of the slab is 0.050% or less, preferably 0.040% or less, and more preferably 0.030% or less. Similar to N, acid-soluble Al needs to be removed (purified) from the base steel sheet during finish annealing.

[0024] N: 0.002%~0.020% Nitrogen (N) is a fundamental element for steel billets (slabs). N is necessary for forming the inhibitor AlN and for increasing the concentration of Goss orientation during secondary recrystallization. The N content required for inhibitor formation in a slab is 0.002% or more, preferably 0.004% or more, and more preferably 0.006% or more. On the other hand, if the N content in a slab exceeds 0.020%, blisters (voids) may form in the steel sheet during cold rolling, the strength of the steel sheet may increase, and the passability during manufacturing may deteriorate. The N content in a slab is 0.020% or less, preferably 0.015% or less, and more preferably 0.010% or less. Like carbon (C), excess N remaining in the final product can cause magnetic degradation. Therefore, N needs to be purified during finish annealing.

[0025] P: 0.000%~0.100% Phosphorus (P) is a selective element for steel billets (slabs). If the P content exceeds 0.100%, the workability of the steel sheet may decrease significantly. Therefore, the P content should be 0.100% or less. Preferably, the P content is 0.070% or less, and more preferably 0.030% or less. On the other hand, the lower limit of the P content is not particularly limited and may be 0.000%. However, since P has the effect of improving the texture and the magnetic properties of the steel sheet, the P content may be greater than 0.000% and may be 0.005% or more.

[0026] Bi: 0.0000%~0.0200% Bi (bismuth) is a selective element for steel billets (slabs). If the Bi content exceeds 0.0200%, the passability during cold rolling may deteriorate. Also, if the purification during finish annealing is insufficient and an excess of Bi remains, it may adversely affect the magnetic properties. Therefore, the Bi content should be 0.0020% or less. Preferably, the Bi content is 0.0150% or less, and more preferably 0.0100% or less. On the other hand, the lower limit of the Bi content is not particularly limited and may be 0.0000%. However, since Bi has the effect of improving magnetic properties, the Bi content may exceed 0.0000% and may be 0.0005% or more.

[0027] Sn: 0.00%~0.50% Tin (Sn) is a preferred element for steel billets (slabs). If the Sn content exceeds 0.50%, secondary recrystallization becomes unstable, which can adversely affect the magnetic properties. Therefore, the Sn content should be 0.50% or less. Preferably, the Sn content is 0.40% or less, and more preferably 0.30% or less. On the other hand, there is no particular lower limit to the Sn content, and it may be 0.00%. However, since Sn has the effect of improving magnetic properties by increasing the concentration of Goss orientations, the Sn content may be greater than 0.00%, and may be 0.01% or more, or 0.03% or more.

[0028] Cu: 0.00%~0.50% Copper (Cu) is a preferred element for steel billets (slabs). If the Cu content exceeds 0.50%, the steel sheet may become brittle during hot rolling. Therefore, the Cu content should be 0.50% or less. Preferably, the Cu content is 0.40% or less, and more preferably 0.30% or less. On the other hand, there is no particular lower limit to the Cu content, and it may be 0.00%. However, since Cu has the effect of improving magnetic properties by increasing the concentration of Goss orientations, the Cu content may be greater than 0.00%, and may be 0.01% or more, or 0.03% or more.

[0029] Cr: 0.00%~0.50% Cr (chromium) is a selective element for steel billets (slabs). If the Cr content exceeds 0.50%, Cr oxide may form, which can adversely affect the magnetic properties. Therefore, the Cr content should be 0.50% or less. Preferably, the Cr content is 0.40% or less, and more preferably 0.30% or less. On the other hand, there is no particular lower limit to the Cr content, and it may be 0.00%. However, since Cr has the effect of improving magnetic properties by increasing the concentration of Goss orientation, the Cr content may be greater than 0.00%, and may be 0.01% or more, or 0.03% or more.

[0030] Sb: 0.00%~0.20% Antimony (Sb) is a preferred element for steel slabs. If the Sb content exceeds 0.20%, it may adversely affect the magnetic properties. Therefore, the Sb content should be 0.20% or less. Preferably, the Sb content is 0.15% or less, and more preferably 0.10% or less. On the other hand, there is no particular lower limit to the Sb content, and it may be 0.00%. However, since Sb functions as an inhibitor and has the effect of stabilizing secondary recrystallization, the Sb content may be greater than 0.00%, and may be 0.01% or more.

[0031] Mo: 0.00%~0.10% Mo (molybdenum) is a preferred element for steel billets (slabs). If the Mo content exceeds 0.10%, problems may arise with the rolling properties of the steel sheet. Therefore, the Mo content should be 0.10% or less. Preferably, the Mo content is 0.05% or less, and more preferably 0.03% or less. On the other hand, the lower limit of the Mo content is not particularly limited and may be 0.00%. However, since Mo has the effect of improving magnetic properties by increasing the concentration of Goss orientations, the Mo content may be greater than 0.00%, and may be 0.01% or more.

[0032] Nb: 0.0000%~0.0200% Niobium (Nb) is a preferred element for steel billets (slabs). If the Nb content exceeds 0.0200%, secondary recrystallization may become unstable. Therefore, the Nb content should be 0.0200% or less. Preferably, the Nb content is 0.0100% or less, and more preferably 0.0050% or less. On the other hand, the lower limit of the Nb content is not particularly limited and may be 0.0000%. However, since Nb has the effect of stabilizing secondary recrystallization, the Nb content may exceed 0.0000% and may be 0.0005% or more.

[0033] B: 0.0000%~0.0200% Boron (B) is a preferred element for steel billets (slabs). If the B content exceeds 0.0200%, secondary recrystallization may become unstable. Therefore, the B content should be 0.0200% or less. Preferably, the B content is 0.0100% or less, and more preferably 0.0050% or less. On the other hand, there is no particular lower limit to the B content, and it may be 0.0000%. However, since B has the effect of stabilizing secondary recrystallization, the B content may exceed 0.0000%, and may be 0.0005% or more.

[0034] Te: 0.0000%~0.0200% Tellurium (Te) is a preferred element for steel billets (slabs). If the Te content exceeds 0.0200%, fracture may occur during hot rolling or cold rolling. Therefore, the Te content should be 0.0200% or less. Preferably, the Te content is 0.0150% or less, and more preferably 0.0100% or less. On the other hand, there is no particular lower limit to the Te content, and it may be 0.0000%. However, since Te has the effect of stabilizing secondary recrystallization, the Te content may be greater than 0.0000%, and may be 0.0005% or more.

[0035] Ni: 0.00%~0.20% Nickel (Ni) is a selective element for steel billets (slabs). Ni influences the crystal orientation rotation that occurs during cold rolling and is an effective element for obtaining a desirable texture for secondary recrystallization. It is also an effective element for increasing resistivity and reducing iron loss. Therefore, it may be included. When Ni is included, it is preferable to have a Ni content of 0.01% or more in order to obtain these effects. On the other hand, if the Ni content exceeds 0.20%, secondary recrystallization may become unstable. Therefore, if Ni is included, the Ni content should be 0.20% or less. Preferably, the Ni content is 0.15% or less, and more preferably 0.10% or less.

[0036] Steel billets (slabs) subjected to the hot rolling process may contain impurities. "Impurities" refer to substances that are introduced into the steel during the industrial manufacturing process from the raw materials such as ore and scrap, or from the manufacturing environment. The chemical composition of steel billets (slabs) used in the hot rolling process can be measured using general analytical methods. For example, it can be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). Acid-soluble aluminum can be measured using ICP-AES with the filtrate obtained after heating and decomposing the sample with acid. In addition, carbon and sulfur can be measured using the combustion-infrared absorption method, and nitrogen can be measured using the inert gas fusion-thermal conductivity method.

[0037] In the hot rolling process, the steel billet is first heat-treated. The heating temperature can be, for example, between 1200°C and 1600°C. Preferably, the lower limit of the heating temperature is 1280°C, and the upper limit is 1500°C. Next, the heated steel billet is hot-rolled. The thickness of the hot-rolled steel sheet after hot rolling is preferably in the range of, for example, 2.0 mm to 3.0 mm.

[0038] [Hot-rolled sheet annealing process] In the hot-rolled sheet annealing process, the hot-rolled steel sheet obtained in the hot-rolling process is annealed. This hot-rolled sheet annealing causes recrystallization within the steel sheet, ultimately enabling the realization of good magnetic properties. The conditions for hot-rolled sheet annealing are not particularly limited, but for example, the hot-rolled steel sheet can be annealed at a temperature range of 900°C to 1200°C for 10 seconds to 5 minutes. After hot-rolled sheet annealing, pickling is performed. In this pickling process, the surface of the hot-rolled steel sheet after annealing is immersed in a pickling solution to pickle it and obtain a hot-rolled sheet annealed.

[0039] [Cold rolling process] In the cold rolling process, the hot-rolled and annealed sheet is subjected to either one cold rolling pass or multiple cold rolling passes with an intermediate annealing in between. Here, "one pass" means performing one or more passes of cold rolling or intermediate annealing once. When intermediate annealing is performed between cold rolling passes, the heating method for intermediate annealing is not particularly limited. Furthermore, cold rolling may be performed in three or more separate passes with intermediate annealing in between, but since this increases manufacturing costs, it is preferable to perform cold rolling in one or two passes.

[0040] The final cold rolling reduction ratio in cold rolling (cumulative cold rolling reduction ratio without intermediate annealing, or cumulative cold rolling reduction ratio after intermediate annealing) should be, for example, in the range of 80% to 95%. By setting the final cold rolling reduction ratio within the above range, the final {110} <001> This method increases the degree of concentration in a particular orientation and suppresses the destabilization of secondary recrystallization. The thickness of the cold-rolled steel sheet is usually the same as the thickness of the base steel sheet (final thickness) of the grain-oriented electrical steel sheet that is ultimately produced. The thickness of the cold-rolled steel sheet after cold rolling is preferably in the range of 0.17 mm to 0.29 mm.

[0041] [Decarburization annealing process] In the decarburization annealing process, the cold-rolled steel sheet obtained in the cold-rolling process is decarburized and annealed. This decarburization annealing removes the carbon contained in the cold-rolled steel sheet, resulting in primary recrystallization. Decarburization annealing is preferably performed in a humid atmosphere to remove the carbon contained in the cold-rolled steel sheet. For example, annealing can be performed in a humid atmosphere at a temperature range of 700°C to 1000°C for 10 seconds to 10 minutes.

[0042] [Nitriding annealing process] If necessary, a nitriding annealing step may be performed after the decarburization annealing step and before the separation agent application step to obtain a nitrided steel sheet. In this nitriding annealing step, the decarburized annealed steel sheet is annealed for 10 to 60 seconds at a temperature range of 700°C to 850°C in an atmosphere containing a gas with nitriding ability, such as hydrogen, nitrogen, and ammonia. When a nitriding annealing step is performed, the amount of AlN, which acts as an inhibitor, increases, which has the advantage of stabilizing secondary recrystallization and improving magnetism.

[0043] [Separation material application process] In the separation agent coating process, in order to prevent the coiled steel sheet from sticking during the subsequent finish annealing process, an annealing separation agent is applied to the decarburized annealed steel sheet obtained in the decarburized annealing process and dried before the finish annealing process. The annealing separation agent contains magnesia (MgO), alumina (Al2O3), and chloride. Here, it is preferable that the total content of MgO and Al2O3 in the annealing separation agent, calculated on a solid content basis, is 80.0% to 99.5% by mass, with the remainder being chloride. That is, the chloride content in the annealing separation agent is the value obtained by subtracting the total content of MgO and Al2O3 from 100% by mass, and is preferably 0.5% to 20% by mass. The remainder may contain impurities. In the annealing separating agent, it is preferable that the content of MgO as elemental is 0.0% by mass or more and 79.5% by mass or less, and the content of Al2O3 as elemental is 20.0% by mass or more and 99.5% by mass or less. The total content of MgO and Al2O3 is more preferably 85.0% by mass or more, and even more preferably 90.0% by mass or more. The total content of MgO and Al2O3 is more preferably 99.0% by mass or less, and even more preferably 95.0% by mass or less. On the other hand, the content of the remaining chloride is more preferably 1.0% by mass or more, and even more preferably 5.0% by mass or more. The content of the remaining chloride is more preferably 15.0% by mass or less, and even more preferably 10.0% by mass or less. Although there are no specific requirements regarding chlorides, examples include bismuth oxychloride (BiOCl), bismuth trichloride (BiCl3), calcium chloride, iron chloride, cobalt chloride, and nickel chloride.

[0044] [Finishing annealing process] In the finish annealing process following the separation agent application process, the decarburized annealed steel sheet, which has been pre-coated with an annealing separation agent, is subjected to finish annealing. Finish annealing is carried out for an extended period of time with the steel sheet wound into a coil. The annealing conditions for finish annealing are not particularly limited, and known conditions may be used as appropriate. For example, in finish annealing, a decarburized annealed steel sheet coated with an annealing release agent and dried may be held at a temperature range of 1000°C to 1300°C for 10 to 60 hours. The atmosphere during finish annealing may be, for example, a nitrogen atmosphere or a mixed atmosphere of nitrogen and hydrogen. After finish annealing, the surface of the finish annealed sheet may be washed with water to remove dust.

[0045] This finishing annealing process causes secondary recrystallization in the steel sheet, resulting in a crystal orientation of {110}. <001> The crystals are oriented in a specific direction. In this secondary recrystallized structure, the easy magnetization axes are aligned in the rolling direction, and the crystal grains are coarse. Excellent magnetic properties are obtained due to this secondary recrystallized structure. In this embodiment, since the annealing separating agent contains chloride, the formation of the primary film is suppressed, resulting in a smooth surface on the finished annealed sheet. Alternatively, the atmosphere during finish annealing may be changed to a hydrogen atmosphere to perform a purification treatment. This purification treatment removes elements such as Al, N, S (and Se if Se is used as part of S) contained in the steel sheet as part of the steel composition, thereby purifying the steel sheet.

[0046] [Surface treatment process] In this process, the surface of the finish annealed sheet obtained in the finish annealing process is pickled to obtain a surface-treated steel sheet. The pickling conditions at this time are not specifically specified, but for example, the finish annealed sheet may be immersed in an acid of a specific concentration (first treatment solution). The first treatment solution may contain at least one of hydrochloric acid, sulfuric acid, nitric acid, and phosphoric acid, with a total acid concentration of 0.1% to 20.0% by volume, and a liquid temperature of 50°C to 90°C. The finish annealed sheet may be surface-treated using this first treatment solution for 3 to 60 seconds.

[0047] In this process, it is preferable to perform surface treatment under conditions that activate the surface of the finished annealed plate while preventing the formation of etch pits on the surface of the finished annealed plate. To achieve this, the above conditions should be controlled in a complex and inseparable manner. For example, if the pickling strength is increased for one of the above conditions, the pickling strength of the other conditions should be decreased to achieve both an activated surface and a smooth surface. A person skilled in the art can perform surface control including pickling behavior, and by considering the influence of each of the above conditions on the pickling strength, it is possible to control the surface state by combining the above conditions.

[0048] If the total acid concentration of the first treatment solution is less than 0.1% by volume, it is difficult to achieve an active surface state on the surface of the finished annealed plate, and internal oxidation of SiO2 is unlikely to form in the subsequent thermal oxidation-reduction annealing step. Preferably, the total acid concentration is 1.0% by volume or higher. On the other hand, if the total acid concentration of the first treatment solution exceeds 20.0% by volume, etch pits are likely to form on the surface of the finished annealed plate. Similarly, if the temperature of the first treatment solution is less than 50°C, an active surface state cannot be obtained, and if the temperature of the first treatment solution exceeds 90°C, etch pits are likely to form. Similarly, if the surface treatment time is less than 3 seconds, an active surface state cannot be obtained, and if the surface treatment time exceeds 60 seconds, etch pits are likely to form.

[0049] [Thermal oxidation-reduction annealing process] In the thermal oxidation-reduction annealing process, the surface-treated steel sheet after the surface treatment process undergoes a two-stage annealing treatment corresponding to thermal oxidation annealing and reduction annealing. As shown in Figures 2 and 3, the thermal oxidation-reduction annealing process of this embodiment includes a primary heating process P1, a primary annealing process P2 (first stage), and a secondary annealing process P3 (second stage). Thus, a particularly distinctive feature of the thermal oxidation-reduction annealing process of this embodiment is that the soaking treatment is performed in two stages instead of the conventional single-stage heating.

[0050] <Primary heating process P1> In the first heating process P1, after the surface treatment process and before the subsequent first annealing process P2, the surface-treated steel sheet is heated from room temperature to the target temperature T1. Specifically, the target temperature T1 is selected from within the range of 800°C to 950°C. If the target temperature T1 is less than 800°C, it is difficult for an SiO2 oxide film (an oxide film mainly composed of SiO2) to form. Therefore, the target temperature T1 is set to 800°C or higher. Preferably, the target temperature T1 is 820°C or higher, and more preferably 840°C or higher. On the other hand, if the target temperature T1 is greater than 950°C, internal oxidized SiO2 is not formed, and the entire surface of the steel sheet tends to be excessively oxidized. Therefore, the target temperature T1 is set to 950°C or lower. Preferably, the target temperature T1 is 920°C or lower, and more preferably 900°C or lower.

[0051] In this first heating process P1, it is preferable to control the atmospheric dew point to 0°C or below when the steel plate temperature is between 100°C and 800°C. Furthermore, it is preferable that the oxygen potential PH2O / PH2, defined by the ratio of the partial pressure of water vapor PH2O to the partial pressure of hydrogen PH2 in the atmosphere, be 0.5000 or less. If the oxygen potential PH2O / PH2 exceeds 0.5000, it becomes difficult to form an SiO2 oxide film. Therefore, it is preferable to set the oxygen potential PH2O / PH2 to 0.5000 or less. More preferably, the oxygen potential PH2O / PH2 is 0.4000 or less, and even more preferably 0.3000 or less. From the viewpoint of ensuring a uniform SiO2 oxide film, it is preferable to set the oxygen potential PH2O / PH2 to 0.0001 or more. More preferably, it is 0.0010 or more, and even more preferably 0.0050 or more.

[0052] Furthermore, in this primary heating process P1, it is preferable to control the average heating rate when the steel plate temperature is between 550°C and 800°C to within the range of 100°C / second to 2000°C / second. If the average heating rate is below 100°C / second, there is a concern that oxide films other than SiO2, such as Fe2SiO4, may be formed. The average heating rate is more preferably 150°C / second or higher, and even more preferably 200°C / second or higher. Conversely, if the average heating rate exceeds 2000°C / second, the generation of SiO2 itself decreases, and the effect of improving film adhesion becomes difficult to enjoy. Therefore, an average heating rate of 2000°C / second is preferred. The average heating rate is more preferably 1800°C / second or lower, and even more preferably 1600°C / second or lower. By controlling the heating conditions to the above-mentioned conditions, a homogeneous SiO2 oxide film is preferably formed on the surface of the steel sheet. That is, the SiO2 oxide film is formed by the reaction of Si in the steel sheet surface with O2 in the atmosphere, and by performing the above control, a uniform SiO2 oxide film of uniform thickness can be formed evenly over the entire surface of the steel sheet. A uniform SiO2 oxide film has the effect of stabilizing the oxidation behavior of the steel sheet in the subsequent primary annealing process P2, and contributes to improving the adhesion of the film.

[0053] To precisely control the average heating rate within the narrow temperature range of 550°C to 800°C, special equipment is required. Therefore, the starting point for controlling the average heating rate of the steel sheet may be set to a relatively low temperature range of around 100°C. However, even in this case, the average heating rate from 550°C to 800°C should be set to 100°C / second to 2000°C / second. This allows for the formation of sufficient internal SiO2 oxide on the surface of the base steel sheet without requiring special equipment.

[0054] <First annealing process P2> The primary annealing process P2 follows the primary heating process P1. In this primary annealing process P2, the steel sheet temperature is maintained at the annealing temperature T2 for a constant holding time between 10 and 200 seconds in an atmosphere with a constant oxygen potential (PH2O / PH2) within the range of 0.10 to 10.00. The annealing temperature T2 is a constant temperature within the range of 800°C to 900°C. If the oxygen potential PH2O / PH2 is less than 0.10, Fe2SiO4 may not precipitate. Fe2SiO4 is reduced to pure iron, which is useful for film adhesion, by the subsequent secondary annealing process P3. In other words, if Fe2SiO4 does not precipitate in the primary annealing process P2, it leads to a deterioration of film adhesion. Therefore, the lower limit of the oxygen potential PH2O / PH2 is set to 0.10. Preferably, the lower limit of the oxygen potential PH2O / PH2 is 0.15, and more preferably 0.20. Conversely, if the oxygen potential PH2O / PH2 exceeds 10.00, an unreducible amount of Fe oxide may precipitate during the subsequent secondary annealing P3. Therefore, the upper limit of the oxygen potential PH2O / PH2 is set to 10.00. The oxygen potential PH2O / PH2 is preferably 1.00 or less, and more preferably 0.60 or less.

[0055] If the annealing temperature T2 is below 800°C, sufficient Fe2SiO4 may not be formed, which can lead to a deterioration in film adhesion. Therefore, the annealing temperature T2 should be 800°C or higher. Conversely, if the annealing temperature T2 exceeds 900°C, excess Fe2SiO4 may be generated, leading to a deterioration in film adhesion. Therefore, the annealing temperature T2 should be 900°C or lower. More preferably, the annealing temperature T2 is 880°C or lower, and even more preferably 850°C or lower.

[0056] If the holding time is less than 10 seconds, Fe2SiO4 may not be sufficiently generated, and the adhesion of the coating may deteriorate. Therefore, the holding time should be 10 seconds or more. Preferably, the holding time is 30 seconds or more, and more preferably 50 seconds or more. Conversely, if the holding time exceeds 200 seconds, excessive Fe2SiO4 may be generated, potentially degrading the adhesion of the coating. Therefore, the holding time should be 200 seconds or less. Preferably, the holding time is 180 seconds or less, and more preferably 150 seconds or less. By controlling the soaking conditions to the conditions described above, the SiO2 oxide film generated in the first heating process P1 reacts with Fe derived from the steel sheet, forming a uniform Fe2SiO4 oxide film on the steel sheet surface.

[0057] <Secondary annealing process P3> In the second annealing process P3, the steel sheet is held at an annealing temperature T3 for a fixed holding time within the range of 3 to 100 seconds in a constant atmosphere with an oxygen potential PH2O / PH2 of less than 0.1000 and a dew point of less than -20°C. The annealing temperature T3 is a constant temperature within the range of 900°C to 1100°C. If the oxygen potential PH2O / PH2 is 0.1000 or higher, Fe2SiO4 may not be sufficiently reduced, potentially degrading the film adhesion. Therefore, the oxygen potential PH2O / PH2 should be less than 0.1000. Preferably, the oxygen potential PH2O / PH2 is 0.0800 or lower, and more preferably 0.0600 or lower. The oxygen potential (PH2O / PH2) should be as low as possible, with a lower limit of 0.0001 being acceptable for operational purposes. Furthermore, along with the oxygen potential PH2O / PH2, the dew point is controlled to below -20°C. This is because if the dew point is above -20°C, iron-based oxides such as FeO are formed, which negatively affects the adhesion of the coating.

[0058] If the annealing temperature T3 is less than 900°C, the reduction of Fe2SiO4 may not proceed sufficiently, and the adhesion of the coating may deteriorate. Therefore, the annealing temperature T3 should be 900°C or higher. The annealing temperature T3 is preferably 920°C or higher, and more preferably 940°C or higher. There is no particular upper limit set for the annealing temperature T3, but if T3 exceeds 1100°C, strain may occur due to grain boundary slip and other factors, which may degrade the magnetic properties. Therefore, the annealing temperature T3 should be 1100°C or lower. The annealing temperature T3 is preferably 1000°C or lower, and more preferably 980°C or lower.

[0059] If the holding time is less than 3 seconds, the reduction of Fe2SiO4 may not proceed sufficiently, and the adhesion of the coating may deteriorate. Therefore, the holding time should be 3 seconds or more. Preferably, the holding time is 10 seconds or more, and more preferably 20 seconds or more. Conversely, if the holding time exceeds 100 seconds, the magnetic properties may deteriorate due to grain boundary slip. Therefore, the holding time should be 100 seconds or less. Preferably, the holding time is 80 seconds or less, and more preferably 60 seconds or less. By controlling the soaking conditions in the secondary annealing process P3 to the above conditions, α-Fe can be uniformly deposited on the steel sheet surface from the Fe2SiO4 oxide film formed in the primary annealing process P2 (Fe2SiO4 → 2Fe + SiO2 + O2). By uniformly depositing α-Fe on the steel sheet surface in this way, the α-Fe reacts with the insulating film forming liquid applied in the subsequent tension-applying insulating film formation process, promoting the formation of a layer that contributes to film adhesion.

[0060] The transition from the primary annealing process P2 to the secondary annealing process P3 may be carried out consecutively with another primary heating process in between, as shown in Figure 3. That is, after the primary annealing process P2 is completed, the steel sheet temperature may be raised from the annealing temperature T2, and the secondary annealing process P3 may be started at the annealing temperature T3. Alternatively, after the primary annealing process P2 is completed, the steel sheet temperature may be lowered from the annealing temperature T2 back to room temperature, and then the steel sheet may be reheated and the secondary annealing process P3 may be started at the annealing temperature T3.

[0061] [Pickling process] If necessary, a pickling step may be performed after the thermal oxidation-reduction annealing step and before the subsequent tension-imparting insulating film formation step. In this pickling step, the reduction-annealed steel sheet obtained in the thermal oxidation-reduction annealing step is immersed for 3 to 60 seconds in an acid (second treatment solution) containing at least one of hydrochloric acid, sulfuric acid, nitric acid, and phosphoric acid, with a total acid concentration of 0.1% to 20.0% by volume, and a liquid temperature of 50°C to 90°C. If the total acid concentration of the second treatment solution is less than 0.1% by volume, an active surface state cannot be obtained. On the other hand, if the total acid concentration of the second treatment solution exceeds 20.0% by volume, surface irregularities may be created, and the magnetic properties may deteriorate. Therefore, the acid concentration of the second treatment solution should be between 0.1% by volume and 20.0% by volume. Similarly, if the surface treatment time is less than 3 seconds, an active surface state cannot be obtained, and if the surface treatment time exceeds 60 seconds, surface irregularities may be created, and the magnetic properties may deteriorate. The pickling step may be omitted; in that case, the tension-applying insulating film formation step will be performed immediately after the thermal oxidation-reduction annealing step.

[0062] [Tension-applying insulating coating formation process] The tension-imparting insulating film formation process is carried out following the thermal oxidation-reduction annealing process or the pickling process. In this tension-imparting insulating film formation process, a secondary film (tension-imparting insulating film) 3 is formed on the surface of the reduction-annealed steel sheet obtained in the thermal oxidation-reduction annealing process, or on the surface of the pickled sheet obtained in the pickling process. The secondary film 3 is a film containing phosphate and silica, and its specific components and their content may be under known conditions. The tension-applying insulating coating formation process includes a secondary coating solution application process and a baking process. In the following description, reduction-annealed steel sheets and pickled steel sheets will be collectively referred to as "steel sheets."

[0063] <Secondary coating solution application process> In the secondary coating solution application process, an insulating film-forming solution (hereinafter also referred to as the "secondary coating solution") mainly composed of aluminum phosphate or silica is applied to the surface of the steel sheet. It is preferable that the secondary coating solution does not contain chromium. In addition to aluminum phosphate, phosphates such as Ca, Al, and Sr may also be used. The silica is not particularly limited to silica with specific properties. The particle size is also not particularly limited to a specific size, but 200 nm (number-average particle size) or less is preferred. For example, 5 nm to 30 nm is acceptable. If the particle size exceeds 200 nm, it may settle in the coating solution.

[0064] <Baking process> In the baking process following the secondary coating solution application process, an insulating film (secondary film 3) is formed on the surface of the steel sheet by applying heat treatment to the steel sheet coated with the secondary coating solution. This insulating film reduces iron loss as a single steel sheet by applying tension to the steel sheet, and also reduces iron loss as an iron core by ensuring electrical insulation between each steel sheet when grain-oriented electrical steel sheets are used in a laminated manner. Examples of the heating conditions for the heat treatment include, for instance, heat treatment at a constant temperature within the range of 350°C to 1150°C for a constant time within the range of 5 to 300 seconds. Furthermore, the oxygen potential of the atmosphere (PH2O / PH2) may be controlled as needed. In this embodiment, the thermal oxidation-reduction annealing process is performed in two stages: a primary annealing process P2 and a secondary annealing process P3, thereby uniformly pre-depositing α-Fe on the surface of the steel sheet. As a result, this α-Fe dissolves into the secondary coating solution, and Fe atoms diffuse into the secondary coating solution. The Fe dissolves in the phosphate, and an Fe-PO compound is formed at the interface between the base steel sheet 2 and the secondary coating 3. Because this Fe-PO compound is present at the interface, the grain-oriented electrical steel sheet 1 according to this embodiment has high coating adhesion and excellent iron loss characteristics.

[0065] Through the above processes, the grain-oriented electrical steel sheet 1 shown in Figure 1 is manufactured. After forming the secondary coating 3 on the steel sheet, flattening annealing may be performed as needed to correct its shape. Performing this flattening annealing makes it possible to further reduce the iron loss of the grain-oriented electrical steel sheet 1. Furthermore, if necessary, magnetic domain control processing may be performed before or after the tension-applying insulating film formation process. By performing magnetic domain control processing, iron loss in the grain-oriented electrical steel sheet can be further reduced. If the magnetic domain control process is performed before the tension-applying insulating film formation process, linear or dot-shaped grooves extending in a direction intersecting the rolling direction should be formed at predetermined intervals along the rolling direction. If the magnetic domain control process is performed after the tension-applying insulating film formation process, linear or dot-shaped stress-strain areas extending in a direction intersecting the rolling direction should be formed at predetermined intervals along the rolling direction. The magnetic domain control process narrows the width of the 180° magnetic domains (the 180° magnetic domains are subdivided). When forming grooves, mechanical groove formation methods using gears, chemical groove formation methods using electrolytic etching, and thermal groove formation methods using laser irradiation can be applied. Furthermore, when forming stress-strained areas, laser beam irradiation and electron beam irradiation can be applied.

[0066] According to the manufacturing method for grain-oriented electrical steel sheets described above, grain-oriented electrical steel sheets with higher coating adhesion can be manufactured without impairing their magnetic properties. [Examples]

[0067] Next, the effects of one aspect of the present invention will be described in more detail by reference to examples. However, the various conditions in these examples are illustrative examples adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to these examples. The present invention can adopt various conditions as long as it does not depart from the spirit of the invention and achieves the objectives of the present invention. First, in the hot rolling process, slabs (steel billets) a to j having the chemical compositions shown in Table 1 below were prepared.

[0068] [Table 1]

[0069] Specifically, for all slabs No. a to j, the chemical composition is given in mass percent. C: 0.020%~0.150% Si: 3.00%~4.00% Mn: 0.01%~0.50% S: 0.0010%~0.0400%, Acid soluble Al: 0.010%~0.050%, N: 0.002%~0.020% It contained [the specified component], with the remainder consisting of Fe and impurities. Furthermore, for slabs No. c to j, the chemical composition is expressed in mass percent as follows: Bi:0.0200% or less, P: 0.100% or less, Sn: 0.50% or less, Cu: 0.50% or less, Cr: 0.50% or less, Sb: 0.20% or less, Mo: 0.10% or less Nb: 0.0200% or less, B: 0.0200% or less, Te: 0.0200% or less, Ni: 0.20% or less, Se: 0.0200% or less, It further contains at least one selected from the group consisting of the following. Then, each of these slabs a to j was heated to a constant temperature within the range of 1100°C to 1400°C and subjected to hot rolling to produce hot-rolled steel sheets with a thickness of 2.3 mm.

[0070] Next, in the hot-rolled steel annealing process, the hot-rolled steel sheet obtained in the hot-rolling process was annealed at 1100°C for 100 seconds, and then its surface was pickled by immersion in a pickling solution. In this way, a hot-rolled annealed sheet was obtained. Next, in the cold rolling process, the hot-rolled and annealed sheets after pickling were subjected to either a single cold rolling or multiple cold rolling processes with intermediate annealing in between, to obtain cold-rolled steel sheets with a base steel sheet thickness of 0.22 mm to 0.23 mm as shown in Table 2A below.

[0071] Next, in the decarburization annealing process, the cold-rolled steel sheet obtained in the cold-rolling process was subjected to decarburization annealing at 820°C for 140 seconds using a mixture of nitrogen and hydrogen gas to obtain a decarburized annealed steel sheet. The aforementioned mixture of nitrogen and hydrogen gas was used in a humidified atmosphere. Next, some of the decarburized annealed steel sheets underwent a nitriding treatment after the decarburization annealing process. Specifically, for tests No. 1, 2, 9, 10, 18, 19, and 20, nitriding treatment was performed after decarburization annealing. Next, in the separation agent application process, an annealing separation agent mainly composed of MgO and alumina in slurry form was applied to the surface of the decarburized annealed steel sheet after the decarburization annealing process, or the nitrided annealed steel sheet after the nitrided annealing process. The composition of the annealing separation agent used is shown in Table 2B. Next, in the finish annealing process, the decarburized annealed steel sheet coated with an annealing separating agent was subjected to finish annealing at 1200°C for approximately 20 hours. The annealing atmosphere during the heating process of the finish annealing was a mixture of nitrogen and hydrogen gas, and the annealing atmosphere during the soaking process at 1200°C was hydrogen gas. Next, in the surface treatment process, the finished annealed steel sheet obtained in the finish annealing process was surface-treated by immersing it in a treatment solution containing sulfuric acid, having the concentration (acid concentration) and temperature shown in Table 2C, for the time shown in Table 2C, to obtain a surface-treated steel sheet.

[0072] [Table 2A]

[0073] [Table 2B]

[0074] [Table 2C]

[0075] Next, in the thermal oxidation-reduction annealing process, the surface-treated steel sheet was subjected to an intermediate annealing heat treatment after the surface treatment process. This thermal oxidation-reduction annealing process included a primary heating process P1, a primary annealing process P2, and a secondary annealing process P3. Then, in the first heating process P1, the surface-treated steel sheet was heated to a higher temperature after the surface treatment process and before the subsequent first annealing process P2. In this first heating process P1, the heating rate and oxygen potential PH2O / PH2 between 550°C and 800°C were as shown in Table 2A. Furthermore, the first annealing process P2 was carried out immediately following the first heating process P1. Specifically, in this first annealing process P2, the annealing temperature T2, annealing time (holding time), and oxygen potential PH2O / PH2 were set as shown in Table 2A. Furthermore, after the primary annealing process P2, a secondary annealing process P3 was performed by increasing the heating temperature. Specifically, in this secondary annealing process P3, the annealing temperature T3, annealing time (holding time), and oxygen potential PH2O / PH2 were set as shown in Table 2B.

[0076] Following the thermal oxidation-reduction annealing process described above, tests No. 6 to 8 underwent an acid pickling process using a 1% sulfuric acid solution at 60°C for 10 seconds. On the other hand, tests No. 1 to 5 and 9 to 30 did not undergo the acid pickling process after the thermal oxidation-reduction annealing.

[0077] Furthermore, a tension-imparting insulating coating formation process was carried out following either the thermal oxidation-reduction annealing process or the pickling process. Specifically, in this tension-imparting insulating coating formation process, a secondary coating (tension-imparting insulating coating) containing aluminum phosphate and silica was formed on the surface of the reduction-annealed steel sheet obtained in the thermal oxidation-reduction annealing process, or on the surface of the pickled sheet obtained in the pickling process. The tension-imparting insulating coating formation process includes a secondary coating chemical application process and a baking process. In the following description, reduction-annealed steel sheets and pickled sheets will be collectively referred to as "steel sheets."

[0078] In the secondary coating solution application process, an insulating coating-forming solution containing aluminum phosphate and silica (hereinafter also referred to as the "secondary coating solution") was applied to the surface of the steel plate. In the baking process following the secondary coating solution application process, an insulating coating (secondary coating) was formed on the surface of the steel sheet by applying heat treatment to the steel sheet coated with the secondary coating solution. The heating conditions for this heat treatment were maintained at 900°C for a certain period of time within the range of 10 to 20 seconds. The annealing atmosphere was a humidified atmosphere using a mixture of nitrogen and hydrogen gas with a dew point controlled to be 20°C or higher. For each of the tests No. 1 to 26 obtained through the above processes, the coating adhesion, iron loss, and magnetic flux density were evaluated as product characteristics.

[0079] Specifically, to evaluate coating adhesion, the test specimen was wrapped around a cylinder with a diameter of 20 mm and bent 180°. The area ratio of the remaining coating surface to the area of ​​the steel plate in contact with the cylinder was calculated. The area of ​​the steel plate in contact with the roll was determined by calculation. The area of ​​the remaining surface was determined by taking photographs of the steel plate after the test and performing image analysis on the photographic images. The coating surface area percentage was evaluated as follows: Excellent (EX) if it was 95% or more, Very Good (VG) if it was 90% or more but less than 95%, Good (G) if it was 85% or more but less than 90%, Fair (F) if it was 80% or more but less than 85%, and Not Good (NG) if it was less than 80%. A coating surface area percentage of 80% or more was judged to be acceptable in terms of coating adhesion. The results are shown in Table 2C.

[0080] Next, the iron loss characteristics were evaluated using the Single Sheet Tester (SST) method. Under conditions of AC frequency: 50 Hz and excitation magnetic flux density: 1.7 T, the iron loss W17 / 50 (W / kg), defined as the power loss per unit weight (1 kg) of the steel sheet, was measured. A value of less than 0.75 W / kg for iron loss W17 / 50 was considered acceptable. For magnetic flux density, a magnetic field of 800 A / m was applied to the test piece, and the magnetic flux density B8 (T) in the rolling direction was measured. The results are shown in Table 2C.

[0081] In the inventive examples of steels No. 1-11, 25, 26, 29, and 30, during the primary annealing process P2, the steel plate temperature was maintained at a constant temperature within the range of 800°C to 900°C for a constant holding time within the range of 10 to 200 seconds in an atmosphere with a constant oxygen potential PH2O / PH2 within the range of 0.10 to 10.00. In addition, in the secondary annealing process P3 of steels No. 1-11, the steel plate temperature was maintained at a constant temperature within the range of 900°C to 1100°C for a constant holding time within the range of 3 to 100 seconds in an atmosphere with a constant oxygen potential PH2O / PH2 less than 0.1000. As a result, all of these steels, No. 1-11, 25, 26, 29, and 30, met the passing criteria in terms of both coating adhesion and iron loss. In particular, steels No. 1-8, 25, and 26 showed high coating adhesion, with steels No. 3-8 exhibiting the highest coating adhesion among them. In addition, no significant decrease in magnetic flux density was observed in any of the tests No. 1-11, 25, 26, 29, and 30. Therefore, the results met the passing criteria for both coating adhesion and iron loss without compromising magnetic flux density.

[0082] On the other hand, in the comparative examples of steels No. 12-24, 27, and 28, the coating adhesion did not meet the acceptance criteria in any case, and therefore, iron loss could not be measured. Specifically, for steel No. 12, the annealing temperature T2 was 780°C, which was below the lower limit of 800°C. As a result, the coating adhesion was insufficient. Furthermore, for steel No. 13, the annealing temperature T2 was 920°C, exceeding the upper limit of 900°C. As a result, the coating adhesion was insufficient. Furthermore, with steel No. 14, the annealing time was 8 seconds, which was below the lower limit of 10 seconds. As a result, the coating adhesion was insufficient. Furthermore, with steel No. 15, the annealing time was 210 seconds, exceeding the upper limit of 200 seconds. As a result, the coating adhesion was insufficient. Furthermore, in steel No. 16, the oxygen potential during the first annealing process P2 was 0.09, which was below the lower limit of 0.10. As a result, the coating adhesion was insufficient. Furthermore, in steel No. 17, the oxygen potential during the primary annealing process P2 was 10.52, exceeding the upper limit of 10.00. Additionally, no surface treatment process was performed. As a result, the coating adhesion was insufficient.

[0083] Furthermore, in steel No. 18, the annealing temperature T3 in the secondary annealing process P3 was 880°C, which was below the lower limit of 900°C. Also, no surface treatment process was performed. As a result, the coating adhesion was insufficient. Furthermore, in steel No. 19, the annealing temperature T3 in the secondary annealing process P3 was 1110°C, exceeding the upper limit of 1100°C. As a result, the coating adhesion was insufficient. Furthermore, in steel No. 20, the annealing time in the secondary annealing process P3 was 2 seconds, which was below the lower limit of 3 seconds. As a result, the coating adhesion was insufficient. Furthermore, in the case of steel No. 21, the annealing time in the secondary annealing process P3 was 110 seconds, exceeding the upper limit of 100 seconds. As a result, the coating adhesion was insufficient. Furthermore, in steel No. 22, the oxygen potential during the secondary annealing process P3 was 0.1255, exceeding the upper limit of 0.1000. As a result, the coating adhesion was insufficient. In steel No. 23, the dew point during the secondary annealing process P3 was above -20°C, exceeding the upper limit of -20°C. As a result, the coating adhesion was insufficient. In the case of steel No. 24, no surface treatment process was performed. As a result, the coating adhesion was insufficient. In steel No. 27, the oxygen potential during the first annealing process P2 was 10.52, exceeding the upper limit of 10.00. As a result, the coating adhesion was insufficient. In steel No. 28, the annealing temperature T3 during the secondary annealing process P3 was 880°C, which was below the lower limit of 900°C. As a result, the coating adhesion was insufficient.

[0084] From the above results, it was confirmed that in order to meet the acceptance criteria for both coating adhesion and iron loss, in the first annealing process P2, the steel plate temperature must be maintained at a constant temperature within the range of 800°C to 900°C for a constant holding time within the range of 10 seconds to 200 seconds in an atmosphere with a constant oxygen potential PH2O / PH2 within the range of 0.10 to 10.00, and in addition, in the second annealing process P3, the steel plate temperature must be maintained at a constant temperature within the range of 900°C to 1100°C for a constant holding time within the range of 3 seconds to 100 seconds in an atmosphere with a constant oxygen potential PH2O / PH2 less than 0.1000. [Industrial applicability]

[0085] According to the present invention, grain-oriented electrical steel sheets with higher coating adhesion can be manufactured without impairing magnetic properties. Therefore, it has high potential for industrial application. [Explanation of Symbols]

[0086] 1 Grain-oriented electrical steel sheet 3. Secondary coating (tension-imparting insulating coating) P1 Primary heating process P2 Primary annealing process P3 Secondary annealing process

Claims

1. A method for manufacturing a grain-oriented electrical steel sheet, comprising a base steel sheet, an oxide layer containing an Fe-P-O compound formed on the surface of the base steel sheet, and a tension-imparting insulating coating formed on the surface of the oxide layer, In terms of chemical composition, in mass percent, C: 0.020% to 0.150%, Si: 3.00% to 4.00%, Mn: 0.01% to 0.50%, S: 0.0010% to 0.0400%, Acid-soluble Al: 0.010% to 0.050%, N: 0.002% to 0.020%, Bi: 0.0000% to 0.0200%, P: 0.000% to 0.100%, Sn: 0.00% to 0.50%, Cu: 0.00% to 0.50%, Cr: 0.00% to 0.50%, Sb: 0.00% to 0.20%, Mo: 0.00% to 0.10%, Nb: 0.0000% to 0.0200%, B: 0.0000% to 0.0200%, Te: 0.0000% to 0.0200%, Ni: 0.00% to 0.20%, Se: 0.0000% to 0.0200%, A hot rolling process to obtain a hot-rolled steel sheet by heating and hot-rolling a slab containing the above, with the remainder being Fe and impurities, The process involves hot-rolled steel sheet being subjected to hot-rolled sheet annealing to obtain a hot-rolled sheet annealed sheet, and then immersing the hot-rolled sheet annealed sheet in an acid pickling solution, A cold rolling process to obtain a cold-rolled steel sheet by cold rolling the aforementioned hot-rolled sheet and annealed sheet, A decarburizing annealing step is performed on the cold-rolled steel sheet to obtain a decarburized annealed steel sheet, On the surface of the decarburized annealed steel sheet, MgO and Al ２ O ３ A separation agent application step involves applying an annealing separation agent containing chlorides, A finish annealing step is performed on the decarburized annealed steel sheet to which the annealing separating agent has been applied to obtain a finish annealed sheet. A surface treatment step is performed by pickling the surface of the finished annealed plate to obtain a surface-treated steel plate. A thermal oxidation-reduction annealing step is performed on the surface-treated steel sheet in a two-stage annealing process to obtain a reduction-annealed steel sheet, A tension-imparting insulating coating forming step, in which the tension-imparting insulating coating containing phosphate and silica is formed on the surface of the reduction-annealed steel sheet, It has, The aforementioned thermal oxidation-reduction annealing process, A primary heating step is performed after the surface treatment step, in which the surface-treated steel sheet is heated to raise its temperature. This is carried out following the aforementioned first heating process, and the oxygen potential PH ２ O / PH ２ A primary annealing process in which the steel plate temperature is maintained at a constant temperature within the range of 800°C to 900°C for a constant holding time within the range of 10 seconds to 200 seconds in an atmosphere with a constant value within the range of 0.10 to 10.00, This is performed after the first annealing process, with an oxygen potential of PH ２ O / PH ２ The process includes a secondary annealing step in which the steel plate temperature is maintained at a constant temperature within the range of 900°C to 1100°C for a constant holding time within the range of 3 seconds to 100 seconds in an atmosphere with a constant value of less than 0.1000 and a dew point below -20°C. A method for manufacturing grain-oriented electrical steel sheets, characterized by the following features.

2. In the annealing separating agent, the content of MgO is 0.0% by mass or more and 79.5% by mass or less, and the Al ２ O ３ The content of is 20.0% by mass or more and 99.5% by mass or less, with the remainder being the aforementioned chloride. The method for manufacturing grain-oriented electrical steel sheets according to claim 1.

3. In the aforementioned first heating process, The oxygen potential PH at the temperature of the steel sheet being 100°C to 800°C ２ O / PH ２ is 0.5000 or less, The average heating rate of the steel plate at temperatures between 550°C and 800°C is 100°C / second to 2000°C / second. The method for manufacturing grain-oriented electrical steel sheets according to claim 1.

4. In the aforementioned first heating process, The oxygen potential PH when the steel plate temperature is between 100°C and 800°C ２ O / PH ２ It is 0.5000 or less, The average heating rate of the steel plate at temperatures between 550°C and 800°C is 100°C / second to 2000°C / second. The method for manufacturing grain-oriented electrical steel sheets according to claim 2.

5. The process further includes a nitriding annealing step, which involves nitriding the decarburized annealed steel sheet after the decarburization annealing step and before the separation agent application step. A method for manufacturing grain-oriented electrical steel sheets according to any one of claims 1 to 4.

6. In the surface treatment step, the surface-treated steel sheet is obtained by immersing the finished annealed sheet in a first treatment solution containing at least one of hydrochloric acid, sulfuric acid, nitric acid, and phosphoric acid, with a total acid concentration of 0.1% to 20.0% by volume, and a liquid temperature of 50°C to 90°C, for 3 to 60 seconds. A method for manufacturing grain-oriented electrical steel sheets according to any one of claims 1 to 4.

7. In the surface treatment step, the surface-treated steel sheet is obtained by immersing the finished annealed sheet in a first treatment solution containing at least one of hydrochloric acid, sulfuric acid, nitric acid, and phosphoric acid, with a total acid concentration of 0.1% to 20.0% by volume, and a liquid temperature of 50°C to 90°C, for 3 to 60 seconds. The method for manufacturing grain-oriented electrical steel sheets according to feature 5.

8. The slab has the following chemical composition in mass%: Bi: more than 0.0000%, less than 0.0200%, P: more than 0.000%, less than 0.100%, Sn: more than 0.00%, less than 0.50%, Cu: more than 0.00%, less than 0.50%, Cr: more than 0.00%, less than 0.50%, Sb: more than 0.00%, less than 0.20%, Mo: more than 0.00%, less than 0.10%, Nb: more than 0.0000%, less than 0.0200%, B: more than 0.0000%, less than 0.0200%, Te: more than 0.0000%, less than 0.0200%, Ni: more than 0.00%, less than 0.20%, Se: more than 0.0000%, less than 0.0200%, It contains at least one selected from the group consisting of A method for manufacturing grain-oriented electrical steel sheets according to claim 1 or 2.

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