Method for producing grain-oriented electrical steel sheet

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 separating agent application step, a finished product annealing step, a surface treatment step, a thermal oxidation-reduction annealing step, and a tension-imparting insulating coating film formation step. The thermal oxidation-reduction annealing step includes: a primary temperature rise step in which the surface-treated steel sheet is heated and the temperature thereof is raised; a primary annealing process in which the temperature of the steel sheet is maintained at a certain temperature in the range of 800 DEG C to 900 DEG C for a certain holding time in the range of 10 seconds to 200 seconds in an atmosphere in which the oxygen potential PH2O / PH2 is a certain value in 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 in the range of 800 DEG C to 900 DEG C for a certain holding time in the range of 10 seconds to 200 seconds; and a secondary annealing process in which the temperature of the steel sheet is maintained at a certain temperature in the range of 900-1100 DEG C for a certain holding time in the range of 3-100 seconds in an atmosphere having a certain value such that the oxygen potential PH2O / PH2 is less than 0.1000 and the dew point is less than-20 DEG C.

Description

Technical Field

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

[0002] This application claims priority based on Japanese Patent Application No. 2023-013751 filed in Japan on February 1, 2023, the contents of which are incorporated herein by reference. Background Art

[0003] Grain-oriented electrical steel sheets contain about 0.5% to 7% by mass of silicon (Si) and utilize a phenomenon called secondary recrystallization to orient the crystals toward {110} <001> Grain-oriented (Goss-oriented) steel sheets are primarily used as soft magnetic materials in the cores of transformers and other applications. Because the properties of grain-oriented electromagnetic steel sheets significantly impact transformer performance, intensive research is underway to achieve superior magnetic properties and low iron loss.

[0004] A general method for producing a grain-oriented electrical steel sheet is as follows.

[0005] First, a steel slab with a predetermined chemical composition is heated and hot-rolled to produce a hot-rolled steel sheet. The resulting hot-rolled steel sheet is then annealed as needed and pickled. The pickled hot-rolled steel sheet is then cold-rolled to produce a cold-rolled steel sheet. The resulting cold-rolled steel sheet is then decarburized and annealed to induce primary recrystallization.

[0006] Then, an aqueous slurry containing an annealing separator containing MgO as a main component is applied to the surface of the cold-rolled steel sheet after decarburization annealing and dried. The steel sheet is then coiled into a coil and subjected to finish annealing to induce secondary recrystallization. During finish annealing, while secondary recrystallization occurs in the steel sheet, the MgO in the annealing separator reacts with SiO2 in the internal oxide layer formed on the surface of the cold-rolled steel sheet during decarburization annealing, forming a glass film (hereinafter also referred to as "primary film") containing forsterite (Mg2SiO4) as a main component on the surface of the base steel sheet.

[0007] After the finish annealing (after the formation of the primary film), a chemical solution containing, for example, silicon dioxide and phosphate as main components is applied to the upper layer of the primary film and baked to form a tension-imparting insulating film (hereinafter also referred to as "secondary film").

[0008] In addition, the primary film, in addition to functioning as an insulating film, also has the function of improving the adhesion of the secondary film formed on the upper layer of the primary film. Moreover, the iron loss is reduced by the tension generated by both the primary film and the secondary film. However, since the primary film is a non-magnetic phase, it is not preferred from the perspective of magnetic properties. In addition, the interface between the base steel plate and the primary film has an embedded structure in which the root of the primary film is intertwined with the base steel plate, which may hinder the movement of the magnetic domain wall and cause an increase in iron loss.

[0009] Prior art literature

[0010] Patent Literature

[0011] Patent Document 1: Japanese Patent Application Laid-Open No. 8-269560

[0012] Patent Document 2: International Publication No. 2019 / 182149

[0013] Patent Document 3: International Publication No. 2020 / 149345 Summary of the Invention

[0014] Problems to be solved by the invention

[0015] Given this background, numerous inventions have been developed for grain-oriented electrical steel sheets without a primary coating. For example, Patent Document 1 discloses a manufacturing method that aims to suppress the formation and delamination of the primary coating by adding a chloride to the annealing separator during the application step prior to the secondary recrystallization annealing step. This manufacturing method is highly industrially valuable due to its simplicity.

[0016] However, because the primary coating is not formed, the adhesion of the secondary coating is still insufficient. Therefore, for example, in Patent Document 2, after the secondary recrystallization annealing and before the secondary coating is applied and baked, irregularities are formed on the surface of the base steel sheet. This manufacturing method ensures the adhesion of the coating by the anchoring effect generated at the interface between the base steel sheet and the secondary coating. However, the irregularities at this interface hinder the movement of magnetic domain walls during magnetization of the grain-oriented electromagnetic steel sheet, potentially hindering the reduction of iron loss.

[0017] Furthermore, in Patent Document 3, the base steel sheet undergoes intermediate annealing before the secondary coating is applied. This manufacturing method forms an oxide film on the surface of the base steel sheet, which serves as a buffer layer for the secondary coating to adhere, achieving both high magnetic properties and high coating adhesion. This intermediate annealing achieves high coating adhesion. Meanwhile, to further improve transformer performance, there is a demand for grain-oriented electromagnetic steel sheets with even higher coating adhesion.

[0018] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a method for producing a grain-oriented electrical steel sheet capable of producing a grain-oriented electrical steel sheet having higher coating adhesion (secondary coating adhesion) without impairing magnetic properties.

[0019] Means for solving problems

[0020] In order to solve the above-mentioned problems and achieve the relevant objects, the present invention adopts the following means.

[0021] (1) A method for producing a grain-oriented electrical steel sheet according to one embodiment of the present invention includes the following steps:

[0022] The hot rolling step is to obtain a hot-rolled steel sheet by heating and hot-rolling a slab, wherein the slab contains, as a chemical composition, in mass %:

[0023] C: 0.020% to 0.150%,

[0024] Si: 3.00% to 4.00%,

[0025] Mn: 0.01% to 0.50%,

[0026] S: 0.0010% to 0.0400%,

[0027] Acid soluble Al: 0.010% to 0.050%,

[0028] N: 0.002% to 0.020%,

[0029] Bi: 0.0000%~0.0200%,

[0030] P: 0.000%~0.100%,

[0031] Sn: 0.00% to 0.50%,

[0032] Cu: 0.00% to 0.50%,

[0033] Cr: 0.00% to 0.50%,

[0034] Sb: 0.00% to 0.20%,

[0035] Mo: 0.00% to 0.10%,

[0036] Nb: 0.0000% to 0.0200%,

[0037] B: 0.0000%~0.0200%,

[0038] Te: 0.0000% to 0.0200%,

[0039] Ni: 0.00% to 0.20%,

[0040] Se: 0.0000%~0.0200%,

[0041] The remainder is Fe and impurities;

[0042] a hot-rolled plate annealing step of immersing the hot-rolled plate annealed in a pickling solution after annealing the hot-rolled steel plate to obtain the hot-rolled plate annealed plate;

[0043] a cold rolling step of cold-rolling the hot-rolled annealed steel sheet to obtain a cold-rolled steel sheet;

[0044] a decarburization annealing step of performing decarburization annealing on the cold-rolled steel sheet to obtain a decarburization annealed steel sheet;

[0045] a separator coating step of coating an annealing separator containing MgO, Al2O3 and chloride on the surface of the decarburized annealed steel sheet;

[0046] a finish annealing step of performing finish annealing on the decarburized annealed steel sheet coated with the annealing separator to obtain a finish annealed sheet;

[0047] a surface treatment step of pickling the surface of the finished annealed steel plate to obtain a surface-treated steel plate;

[0048] a thermal oxidation-reduction annealing step of performing two-stage annealing on the surface-treated steel sheet to obtain a reduction-annealed steel sheet; and

[0049] a tension-imparting insulating film forming step of forming a tension-imparting insulating film containing phosphate and silicon dioxide on the surface of the reduction-annealed steel sheet;

[0050] The thermal oxidation-reduction annealing process includes the following steps:

[0051] a primary heating process, in which the surface-treated steel plate is heated to increase its temperature after the surface treatment step;

[0052] a primary annealing process, which is performed subsequent to the primary heating process, in an atmosphere having an oxygen potential PH2O / PH2 of a constant value within a range of 0.10 to 10.00, and during a constant holding time within a range of 10 seconds to 200 seconds, wherein the temperature of the steel sheet is maintained at a constant temperature within a range of 800° C. to 900° C.; and

[0053] The secondary annealing process is performed after the primary annealing process, in an atmosphere having an oxygen potential PH2O / PH2 of less than 0.1000 and a dew point of less than -20°C, and the steel sheet 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.

[0054] (2) According to the method for manufacturing a grain-oriented electrical steel sheet described in (1), in the annealing separator, the content of MgO may be 0.0 mass% to 79.5 mass%, the content of Al2O3 may be 20.0 mass% to 99.5 mass%, and the remainder may be the chloride.

[0055] (3) According to the method for manufacturing a grain-oriented electromagnetic steel sheet described in (1), during the single heating process, the oxygen potential PH2O / PH2 when the steel sheet temperature is between 100°C and 800°C is less than 0.5000, and the average heating rate when the steel sheet temperature is between 550°C and 800°C is between 100°C / second and 2000°C / second.

[0056] (4) According to the method for manufacturing a grain-oriented electromagnetic steel sheet described in (2), during the single heating process, the oxygen potential PH2O / PH2 when the steel sheet temperature is between 100°C and 800°C is less than 0.5000, and the average heating rate when the steel sheet temperature is between 550°C and 800°C is between 100°C / second and 2000°C / second.

[0057] (5) The method for producing a grain-oriented electrical steel sheet according to any one of (1) to (4) may further include a nitriding annealing step of nitriding the decarburization annealed steel sheet after the decarburization annealing step and before the separator coating step.

[0058] (6) The method for manufacturing a grain-oriented electrical steel sheet according to any one of (1) to (4), wherein in the surface treatment step, the surface-treated steel sheet is obtained by immersing the finished annealed sheet in a first treatment liquid containing at least one of hydrochloric acid, sulfuric acid, nitric acid, and phosphoric acid, with a total acid concentration of 0.1% by volume to 20.0% by volume, and at a liquid temperature of 50°C to 90°C for 3 seconds to 60 seconds.

[0059] (7) According to the method for manufacturing a grain-oriented electrical steel sheet described in (5), the surface-treated steel sheet can also be obtained by immersing the finished annealed sheet in a first treatment liquid containing at least one of hydrochloric acid, sulfuric acid, nitric acid and phosphoric acid with a total acid concentration of 0.1 volume % to 20.0 volume % and a liquid temperature of 50°C to 90°C for 3 seconds to 60 seconds in the surface treatment step.

[0060] (8) The method for producing a grain-oriented electrical steel sheet according to any one of (1) to (7), wherein the slab may contain, as the chemical composition, at least one element selected from the following elements in terms of mass %:

[0061] Bi: more than 0.0000% and 0.0200% or less,

[0062] P: more than 0.000% and 0.100% or less,

[0063] Sn: more than 0.00% and 0.50% or less,

[0064] Cu: more than 0.00% and 0.50% or less,

[0065] Cr: more than 0.00% and 0.50% or less,

[0066] Sb: more than 0.00% and 0.20% or less,

[0067] Mo: more than 0.00% and 0.10% or less,

[0068] Nb: more than 0.0000% and 0.0200% or less,

[0069] B: more than 0.0000% and 0.0200% or less,

[0070] Te: more than 0.0000% and 0.0200% or less,

[0071] Ni: more than 0.00% and 0.20% or less,

[0072] Se: exceeds 0.0000% and is 0.0200% or less.

[0073] Effects of the Invention

[0074] According to the above-described aspects of the present invention, a grain-oriented electrical steel sheet having higher coating adhesion can be manufactured without impairing magnetic properties. BRIEF DESCRIPTION OF THE DRAWINGS

[0075] Figure 1 This is a schematic cross-sectional view of a grain-oriented electrical steel sheet obtained by the method for producing a grain-oriented electrical steel sheet according to one embodiment of the present invention.

[0076] Figure 2 This is a flowchart showing a method for manufacturing a grain-oriented electrical steel sheet according to this embodiment.

[0077] Figure 3 This is a diagram illustrating the thermal oxidation-reduction annealing step in the manufacturing method. The horizontal axis represents time, and the vertical axis represents annealing temperature (base material steel plate temperature). DETAILED DESCRIPTION

[0078] The following describes a method for manufacturing a grain-oriented electrical steel sheet according to one embodiment of the present invention. 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 present invention. Furthermore, for the numerical ranges described below, the lower and upper limits are included in the ranges. On the other hand, values indicated as "exceeding" or "below" are not included in the numerical ranges. Furthermore, "%" in chemical composition refers to "mass %" unless otherwise specified.

[0079] Figure 1 This is a schematic cross-sectional view of a grain-oriented electromagnetic steel sheet (grain-oriented electromagnetic steel of this embodiment) obtained by the method for manufacturing a grain-oriented electromagnetic steel sheet of this embodiment. Figure 1 As shown in FIG, the grain-oriented electrical steel sheet 1 of the present embodiment, when viewed in a cross section parallel to the sheet thickness direction, comprises a base steel sheet 2 and a secondary coating 3, which is an insulating coating (tension-applying 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 substantially no glass coating (hereinafter also referred to as "primary coating") is present.

[0080] The average thickness of the base steel plate 2 can be 0.17 mm to 0.29 mm, for example. The average thickness of the secondary coating 3 can be 1 μm to 6 μm, for example.

[0081] In order to improve the iron loss characteristics, it is effective to make the surface of the base steel plate 2 smooth so that the movement of the magnetic domain wall becomes easier. In addition, it is effective to make the base steel plate 2 and the secondary film 3 close together, apply tension to the base steel plate 2, and ensure the electrical insulation between the multiple overlapping directional electromagnetic steel plates 1. In the directional electromagnetic steel plate 1 of the present embodiment, the secondary film 3 is arranged in contact with the base steel plate 2 (no primary film is present), thereby ensuring the surface smoothness of the base steel plate 2. In addition, as described in the manufacturing method described later, by finely controlling the atmosphere conditions, soaking temperature, soaking temperature holding time, etc. of each process of the thermal oxidation-reduction annealing process performed in two stages, Fe-PO compounds are formed at the interface between the base steel plate 2 and the secondary film 3 to ensure the close adhesion of the secondary film 3. Therefore, the directional electromagnetic steel plate 1 of the present embodiment has excellent iron loss characteristics and film close adhesion.

[0082] The Fe in the Fe-PO compound originates from the base metal component of the parent steel plate 2, and the P originates from the secondary coating solution. Fe ions diffuse from the base metal side toward the coating side, while P ions diffuse / concentrate from the coating side toward the base metal side. Fe ions and P ions associate to form chemical bonds, resulting in the formation of Fe-PO compounds. That is, the result of chemical bonding between the ions in the coating and the base metal (the result of the coating and the base metal being tightly bonded) is that Fe-PO compounds are observed at the interface between the coating and the base metal.

[0083] Next, use Figure 2 and Figure 3 The following description will describe a method for producing a grain-oriented electrical steel sheet according to the present embodiment. The following production method is merely an example and may be modified as appropriate as long as it does not affect the formation of Fe-PO compounds. Figure 2 This is a flowchart showing a method for manufacturing a grain-oriented electrical steel sheet according to the present embodiment. Figure 3 This is a diagram illustrating the thermal oxidation-reduction annealing step in the manufacturing method. The horizontal axis represents time, and the vertical axis represents annealing temperature (base material steel plate temperature).

[0084] The method for producing a grain-oriented electrical steel sheet according to the present embodiment mainly includes the following steps: a hot rolling step of heating a slab (steel billet) having a predetermined chemical composition and hot rolling the slab to obtain a hot-rolled steel sheet; a hot-rolled sheet annealing step of annealing the hot-rolled steel sheet to obtain an annealed hot-rolled sheet, and then immersing the annealed hot-rolled sheet in a pickling solution (for pickling); a cold rolling step of cold rolling the annealed hot-rolled 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 a separator coating step of applying a separator. An annealing separator is applied to the surface of the decarburized annealed steel sheet; a finish annealing step is performed on the decarburized annealed steel sheet coated with the annealing separator to obtain a finished annealed steel sheet; a surface treatment step is performed on the surface of the finished annealed steel sheet to obtain a surface-treated steel sheet by pickling; a thermal oxidation-reduction annealing step is performed after the surface treatment step to obtain a reduction-annealed steel sheet by performing a second-stage annealing on the surface-treated steel sheet to obtain a reduction-annealed steel sheet; and a tension-imparting insulating film forming step is performed after the thermal oxidation-reduction annealing step to form a tension-imparting insulating film containing phosphate and silica on the surface of the reduction-annealed steel sheet. Furthermore, if necessary, a nitriding annealing step is included after the decarburization annealing step and before the separator application step. Furthermore, if necessary, a pickling step is included after the thermal oxidation-reduction annealing step and before the tension-imparting insulating film forming step.

[0085] The thermal oxidation-reduction annealing step includes a primary heating process, a primary annealing process, and a secondary annealing process. Furthermore, the tension-imparting insulating film forming step includes a secondary film chemical solution coating process and a baking process.

[0086] The following further describes the details of each of the above steps. In the following description, when the conditions of each step are not described, known conditions can be appropriately applied.

[0087] [Hot rolling process]

[0088] In the hot rolling process, a steel slab (for example, a steel ingot such as a slab) having a predetermined chemical composition is hot rolled.

[0089] For example, the slab (steel billet) subjected to the hot rolling process contains, as a chemical composition, the following in mass %.

[0090] C: 0.020% to 0.150%,

[0091] Si: 3.00% to 4.00%,

[0092] Mn: 0.01% to 0.50%,

[0093] S: 0.0010% to 0.0400%,

[0094] Acid soluble Al: 0.010% to 0.050%,

[0095] N: 0.002% to 0.020%,

[0096] Bi: 0.0000~0.0200%,

[0097] P: 0.000%~0.100%,

[0098] Sn: 0.00% to 0.50%,

[0099] Cu: 0.00% to 0.50%,

[0100] Cr: 0.00% to 0.50%,

[0101] Sb: 0.00% to 0.20%,

[0102] Mo: 0.00% to 0.10%,

[0103] Nb: 0.0000% to 0.0200%,

[0104] B: 0.0000%~0.0200%,

[0105] Te: 0.0000% to 0.0200%,

[0106] Ni: 0.00% to 0.20%,

[0107] Se: 0.0000%~0.0200%,

[0108] The remainder may contain Fe and impurities.

[0109] In addition, the above-mentioned slab (steel billet) may contain at least one selected from the following elements in terms of mass % as a chemical composition:

[0110] Bi: more than 0.0000% and 0.0200% or less,

[0111] P: more than 0.000% and 0.100% or less,

[0112] Sn: more than 0.00% and 0.50% or less,

[0113] Cu: more than 0.00% and 0.50% or less,

[0114] Cr: more than 0.00% and 0.50% or less,

[0115] Sb: more than 0.00% and 0.20% or less,

[0116] Mo: more than 0.00% and 0.10% or less,

[0117] Nb: more than 0.0000% and 0.0200% or less,

[0118] B: more than 0.0000% and 0.0200% or less,

[0119] Te: more than 0.0000% and 0.0200% or less,

[0120] Ni: more than 0.00% and 0.20% or less,

[0121] Se: exceeds 0.0000% and is 0.0200% or less.

[0122] C: 0.020%~0.150%

[0123] C (carbon) is an essential element for steel billets (slabs). C is included for the purpose of increasing the aggregation degree of Goss orientation in secondary recrystallization. Regarding the C content required for improving magnetic properties, it is 0.020% or more as a slab, preferably 0.040% or more, and more preferably 0.050% or more. However, if excessive C remains in the final product, it may become a factor in deterioration of iron loss. Therefore, decarburization treatment is required through a decarburization annealing process, but in the case of a slab containing more than 0.150% C, decarburization treatment becomes difficult. Regarding the C content, it is 0.150% or less as a slab, preferably 0.120% or less, and more preferably 0.100% or less.

[0124] Si: 3.00% to 4.00%

[0125] Si (silicon) is an essential element for steel billets (slabs). If the Si content is less than 3.00%, eddy current loss cannot be sufficiently reduced, and therefore good magnetic properties cannot be obtained. Therefore, the Si content is set to 3.00% or more. The Si content is preferably 3.10% or more, and more preferably 3.20% or more. On the other hand, if the Si content exceeds 4.00%, the steel plate becomes brittle and the sheet passability is significantly deteriorated during manufacturing, so the Si content is set to 4.00% or less. The Si content is preferably 3.80% or less, and more preferably 3.60% or less.

[0126] Mn: 0.01% to 0.50%

[0127] Mn (manganese) is an essential element for steel billets (slabs). If the Mn content is less than 0.01%, it is difficult to form MnS (MnSe when Se is used as part of S) that functions as an inhibitor, and secondary recrystallization will not proceed sufficiently, and good magnetic properties cannot be obtained. Therefore, the Mn content is set to 0.01% or more. The Mn content is preferably 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 phase transformation during the finish annealing, and secondary recrystallization will not proceed sufficiently, and good magnetic properties cannot be obtained. Therefore, the Mn content is set to 0.50% or less. The Mn content is preferably 0.30% or less, and more preferably 0.20% or less.

[0128] S: 0.0010%~0.0400%

[0129] Se: 0.0000%~0.0200%

[0130] S (sulfur) is an essential element for steel billets (slabs). S is an element that forms MnS, which serves as an inhibitor. Regarding the S content, as a slab, it is 0.0010% or more, preferably 0.0100% or more, and more preferably 0.0150% or more. On the other hand, as a slab, if the S content exceeds 0.0400%, it may cause hot brittleness and hot rolling becomes difficult. Regarding the S content, as a slab, it is 0.0400% or less, preferably 0.0350% or less, and more preferably 0.0300% or less. If S remains excessively in the final product, it may cause magnetic deterioration. Therefore, S also needs to be removed (purified) from the base steel plate during the finished product annealing.

[0131] Se (selenium) is also an element that forms MnSe, so Se can be used as part of the S (the Se content can also exceed 0.0000%). If Se is present in excess, secondary recrystallization may become unstable, causing magnetic degradation. Therefore, the Se content should be set to 0.0000% to 0.0200%. The Se content is preferably 0.0000% to 0.0150%, and more preferably 0.0000% to 0.0100%.

[0132] Acid-soluble Al: 0.010% to 0.050%

[0133] Acid-soluble Al (aluminum) (sol.Al) is an essential element for steel billets (slabs). Acid-soluble Al is an element required to form AlN as an inhibitor and to improve magnetic properties. Regarding the acid-soluble Al content, it is 0.010% or more as a slab, preferably 0.015% or more, and more preferably 0.020% or more. On the other hand, when an excessive amount of acid-soluble Al is contained in the slab, embrittlement may become significant. Regarding the acid-soluble Al content, it is 0.050% or less as a slab, 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 plate during the finished annealing.

[0134] N: 0.002%~0.020%

[0135] N (nitrogen) is an essential element for steel billets (slabs). N is an element required to form AlN as an inhibitor and to increase the aggregation degree of Goss orientation during secondary recrystallization. Regarding the N content required for inhibitor formation, as a slab, it is 0.002% or more, preferably 0.004% or more, and more preferably 0.006% or more. On the other hand, as a slab, if the N content exceeds 0.020%, blisters (voids) will be generated in the steel plate during cold rolling, and the strength of the steel plate will increase, and there is a possibility that the sheet passability during manufacturing will deteriorate. Regarding the N content, as a slab, it is 0.020% or less, preferably 0.015% or less, and more preferably 0.010% or less. Like C, N, if it remains excessively in the final product, it may cause magnetic deterioration. Therefore, N needs to be purified during the annealing of the finished product.

[0136] P: 0.000%~0.100%

[0137] P (phosphorus) is an optional element for steel billets (slabs). If the P content exceeds 0.100%, the workability of the steel sheet may be significantly reduced. Therefore, the P content only needs to be 0.100% or less. The P content is preferably 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, P has the effect of improving the texture and the magnetic properties of the steel sheet, so the P content may be set to more than 0.000%, or even 0.005% or more.

[0138] Bi: 0.0000%~0.0200%

[0139] Bi (bismuth) is an optional element for steel billets (slabs). If the Bi content exceeds 0.0200%, the sheet passability during cold rolling may deteriorate. In addition, if the purification during the finished product annealing is insufficient and an excessive amount of Bi remains, it may have an adverse effect on the magnetic properties. Therefore, the Bi content only needs to be 0.0020% or less. The Bi content is preferably 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 also be 0.0000%. However, Bi has the effect of improving magnetic properties, so the Bi content may also be set to more than 0.0000%, or may be set to more than 0.0005%.

[0140] Sn: 0.00%~0.50%

[0141] Sn (tin) is an optional element for steel billets (slabs). If the Sn content exceeds 0.50%, the secondary recrystallization becomes unstable, which may have an adverse effect on the magnetic properties. Therefore, the Sn content only needs to be 0.50% or less. The Sn content is preferably 0.40% or less, and more preferably 0.30% or less. On the other hand, the lower limit of the Sn content is not particularly limited and may also be 0.00%. However, Sn has the effect of increasing the aggregation degree of Goss orientation and improving the magnetic properties, so the Sn content may also be set to more than 0.00%, or may be set to more than 0.01% or more than 0.03%.

[0142] Cu: 0.00%~0.50%

[0143] Cu (copper) is an optional element for steel billets (slabs). If the Cu content exceeds 0.50%, the steel plate may become brittle during hot rolling. Therefore, the Cu content only needs to be 0.50% or less. The Cu content is preferably 0.40% or less, more preferably 0.30% or less. On the other hand, the lower limit of the Cu content is not particularly limited and may also be 0.00%. However, Cu has the effect of increasing the aggregation of Gossian orientation and improving magnetic properties, so the Cu content may also be set to more than 0.00%, or may be set to more than 0.01% or more than 0.03%.

[0144] Cr: 0.00%~0.50%

[0145] Cr (chromium) is an optional element for steel billets (slabs). If the Cr content exceeds 0.50%, Cr oxides may be formed, which may have an adverse effect on the magnetic properties. Therefore, the Cr content only needs to be 0.50% or less. The Cr content is preferably 0.40% or less, and more preferably 0.30% or less. On the other hand, the lower limit of the Cr content is not particularly limited and may also be 0.00%. However, Cr has the effect of increasing the aggregation degree of Goss orientation and improving magnetic properties, so the Cr content may also be set to more than 0.00%, or may be set to more than 0.01% or more or more than 0.03%.

[0146] Sb: 0.00%~0.20%

[0147] Sb (antimony) is an optional element for steel billets (slabs). If the Sb content exceeds 0.20%, it may have an adverse effect on the magnetic properties. Therefore, the Sb content only needs to be 0.20% or less. The Sb content is preferably 0.15% or less, and more preferably 0.10% or less. On the other hand, the lower limit of the Sb content is not particularly limited and may also be 0.00%. However, Sb has the effect of stabilizing secondary recrystallization by functioning as an inhibitor, so the Sb content may also be set to more than 0.00%, or may be set to more than 0.01%.

[0148] Mo: 0.00%~0.10%

[0149] Mo (molybdenum) is an optional element for steel billets (slabs). If the Mo content exceeds 0.10%, there is a possibility that the rollability of the steel plate will be affected. Therefore, the Mo content only needs to be 0.10% or less. The Mo content is preferably 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, Mo has the effect of increasing the aggregation degree of Goss orientation and improving magnetic properties, so the Mo content may be set to more than 0.00%, or may be set to more than 0.01%.

[0150] Nb: 0.0000%~0.0200%

[0151] Nb (niobium) is an optional element for steel billets (slabs). If the Nb content exceeds 0.0200%, secondary recrystallization may become unstable. Therefore, the Nb content only needs to be 0.0200% or less. The Nb content is preferably 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, Nb has the effect of stabilizing secondary recrystallization, so the Nb content may be set to more than 0.0000%, or even 0.0005% or more.

[0152] B: 0.0000%~0.0200%

[0153] B (boron) is an optional element for steel billets (slabs). If the B content exceeds 0.0200%, there is a possibility that secondary recrystallization will become unstable. Therefore, the B content only needs to be 0.0200% or less. The B content is preferably 0.0100% or less, and more preferably 0.0050% or less. On the other hand, the lower limit of the B content is not particularly limited and may be 0.0000%. However, B has the effect of stabilizing secondary recrystallization, so the B content may be set to more than 0.0000%, or may be set to more than 0.0005%.

[0154] Te: 0.0000%~0.0200%

[0155] Te (tellurium) is an optional element for steel billets (slabs). If the Te content exceeds 0.0200%, there is a possibility of fracture during hot rolling or cold rolling. Therefore, the Te content only needs to be 0.0200% or less. The Te content is preferably 0.0150% or less, and more preferably 0.0100% or less. On the other hand, the lower limit of the Te content is not particularly limited and may be 0.0000%. However, Te has the effect of stabilizing secondary recrystallization, so the Te content may be set to more than 0.0000%, or may be set to more than 0.0005%.

[0156] Ni: 0.00%~0.20%

[0157] Nickel (Ni) is an optional element for steel billets (slabs). Ni influences the crystal orientation rotation produced during cold rolling, effectively achieving a texture preferred for secondary recrystallization. It is also an effective element for increasing resistivity and reducing iron loss. Therefore, Ni may be included. When Ni is included, in order to achieve these effects, the Ni content is preferably set to 0.01% or more.

[0158] On the other hand, if the Ni content exceeds 0.20%, secondary recrystallization may become unstable. Therefore, when Ni is contained, the Ni content is set to 0.20% or less. The Ni content is preferably 0.15% or less, and more preferably 0.10% or less.

[0159] The steel slab (or billet) subjected to the hot rolling process may contain impurities. "Impurities" refer to substances that are mixed in from raw material ores, scrap, or the production environment during industrial steel production.

[0160] The chemical composition of the steel billet (slab) supplied to the hot rolling process can be measured by a general analytical method. For example, it can be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). Acid-soluble Al can be measured using the filtrate after the sample is heated and decomposed with acid and then measured by ICP-AES. In addition, C and S can be measured using the combustion-infrared absorption method, and N can be measured using the inert gas melting-thermal conductivity method.

[0161] In the hot rolling process, the steel slab is first heated. The heating temperature can be set to, for example, 1200°C to 1600°C. The lower limit of the heating temperature is preferably 1280°C, and the upper limit is preferably 1500°C. The heated steel slab is then hot rolled. The thickness of the hot-rolled steel sheet after hot rolling is preferably in the range of 2.0 mm to 3.0 mm, for example.

[0162] [Hot-rolled plate annealing process]

[0163] In the hot-rolled steel sheet annealing process, the hot-rolled steel sheet obtained in the hot-rolling process is annealed. This hot-rolled steel sheet annealing causes recrystallization in the steel sheet, ultimately achieving excellent magnetic properties. The hot-rolled steel sheet annealing conditions are not particularly limited; for example, annealing the hot-rolled steel sheet in a temperature range of 900°C to 1200°C for 10 seconds to 5 minutes is sufficient.

[0164] After the hot-rolled steel sheet is annealed, it is subsequently pickled. In this pickling, the surface of the hot-rolled steel sheet after the hot-rolled steel sheet annealing is dipped in a pickling solution to perform pickling, thereby obtaining a hot-rolled annealed steel sheet.

[0165] [Cold rolling process]

[0166] In the cold rolling process, the hot-rolled sheet annealed after the hot-rolled sheet annealing process is subjected to a single cold rolling pass or multiple cold rolling passes with intermediate annealing. The term "single pass" here refers to one or more cold rolling passes or intermediate annealing. When intermediate annealing is performed during cold rolling, the heating method for the intermediate annealing is not particularly limited. Alternatively, cold rolling can be performed in three or more passes with intermediate annealing, but due to increased manufacturing costs, a single or two cold rolling passes is preferred.

[0167] The final cold rolling reduction ratio in cold rolling (the cumulative cold rolling ratio without intermediate annealing or the cumulative cold rolling ratio after intermediate annealing) can be set in the range of 80% to 95%. By setting the final cold rolling reduction ratio in the above range, the {110} <001> The cold-rolled steel sheet generally has the same thickness as the parent steel sheet (final thickness) of the grain-oriented electrical steel sheet that will be 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, for example.

[0168] [Decarburization annealing process]

[0169] In the decarburization annealing step, the cold-rolled steel sheet obtained in the cold rolling step is subjected to decarburization annealing. This decarburization annealing removes carbon contained in the cold-rolled steel sheet, causing primary recrystallization. Decarburization annealing is preferably performed in a moist atmosphere to remove carbon contained in the cold-rolled steel sheet. For example, annealing in a moist atmosphere at a temperature range of 700°C to 1000°C for 10 seconds to 10 minutes is sufficient.

[0170] [Nitriding annealing process]

[0171] If necessary, a nitriding annealing step can be performed after the decarburization annealing step and before the separator coating step to obtain a nitrided steel sheet. In this nitriding annealing step, the decarburized annealed steel sheet is annealed in an atmosphere containing a nitriding gas such as hydrogen, nitrogen, and ammonia for 10 to 60 seconds at a temperature range of 700°C to 850°C. Performing the nitriding annealing step offers the advantage of increasing the amount of AlN, which acts as an inhibitor, thereby stabilizing secondary recrystallization and improving magnetic properties.

[0172] [Separating agent coating step]

[0173] In the separator coating step, in order to prevent the steel sheet wound into a coil in the subsequent finish annealing step from sintering, the decarburized annealed steel sheet obtained in the decarburization annealing step is coated with an annealing separator and dried before the finish annealing step.

[0174] The annealing separator contains magnesium oxide (MgO), aluminum oxide (Al2O3), and chloride. The total content of MgO and Al2O3 in the annealing separator is preferably 80.0% to 99.5% by mass, calculated as solids, with the remainder being chloride. Specifically, the chloride content in the annealing separator 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.

[0175] In the annealing separator, it is preferred that the content of MgO as a simple substance is 0.0 mass % to 79.5 mass %, and the content of Al 2 O 3 as a simple substance is 20.0 mass % to 99.5 mass %.

[0176] The total content of MgO and Al2O3 is more preferably 85.0 mass% or more, and further preferably 90.0 mass% or more. The total content of MgO and Al2O3 is more preferably 99.0 mass% or less, and further preferably 95.0 mass% or less.

[0177] On the other hand, the content of chloride as the remainder is more preferably 1.0 mass % or more, and further preferably 5.0 mass % or more. The content of chloride as the remainder is more preferably 15.0 mass % or less, and further preferably 10.0 mass % or less.

[0178] The chloride is not particularly specified, but for example, bismuth oxychloride (BiOCl), bismuth trichloride (BiCl 3 ), calcium chloride, iron chloride, cobalt chloride, nickel chloride, etc. can be considered.

[0179] [Finished product annealing process]

[0180] In the finish annealing step, which follows the separator coating step, the decarburized annealed steel sheet previously coated with the annealing separator is subjected to finish annealing. The finish annealing is performed for a long period of time while the steel sheet is coiled.

[0181] The annealing conditions for the finish annealing are not particularly limited; known conditions may be employed as appropriate. For example, the decarburized annealed steel sheet, after being coated with an annealing separator and dried, may be held in a temperature range of 1000°C to 1300°C for 10 to 60 hours. The atmosphere during the finish annealing may be, for example, a nitrogen atmosphere or a mixed atmosphere of nitrogen and hydrogen. Furthermore, after the finish annealing, the surface of the finish annealed sheet may be washed with water to remove powder.

[0182] Through this finish annealing, secondary recrystallization is induced in the steel sheet, and the crystal orientation is along {110} <001> Orientation is achieved. This secondary recrystallized structure tends to align the magnetization axes in the rolling direction, resulting in coarse grains. This secondary recrystallized structure results in excellent magnetic properties. In this embodiment, the annealing separator contains chloride, which suppresses the formation of a primary film and results in a smooth surface for the finished annealed sheet.

[0183] Alternatively, the atmosphere during the finish annealing can be changed to a hydrogen atmosphere for purification. This purification process allows elements such as Al, N, and S (including Se when Se is used as part of S) contained in the steel plate as part of the steel composition to be discharged out of the system, thereby purifying the steel plate.

[0184] [Surface treatment process]

[0185] In this process, the surface of the finished annealed sheet obtained in the finish annealing process is pickled to obtain a surface-treated steel sheet. The pickling conditions are not particularly specified, but for example, the finished annealed sheet can be immersed in an acid of a specific concentration (a first treatment solution). The first treatment solution contains 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 the solution temperature can be 50°C to 90°C. The finished annealed sheet can be surface treated using this first treatment solution for 3 to 60 seconds.

[0186] In this process, the surface of the finished annealed sheet is rendered active, but it is preferred that the surface treatment be performed under conditions that prevent corrosion pits from forming on the surface of the finished annealed sheet. To this end, the aforementioned conditions can be controlled in a complex and inseparable manner. For example, if the pickling intensity is increased for one of the aforementioned conditions, the surface active state and surface smoothness can be balanced by modifying the other conditions to reduce the pickling intensity. Those skilled in the art are capable of performing surface control that includes pickling. Therefore, by considering the impact of each of the aforementioned conditions on the pickling intensity, the aforementioned conditions can be combined to control the surface state.

[0187] If the total acid concentration of the first treatment liquid is less than 0.1% by volume, it will be difficult to achieve an active surface state on the surface of the finished annealed sheet, making it difficult to form internally oxidized SiO2 during the subsequent thermal oxidation-reduction annealing step. The total acid concentration is preferably 1.0% by volume or greater. On the other hand, if the total acid concentration of the first treatment liquid exceeds 20.0% by volume, etch pits are likely to form on the surface of the finished annealed sheet. Similarly, if the temperature of the first treatment liquid is less than 50°C, an active surface state cannot be achieved, while if the temperature of the first treatment liquid 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 achieved, while if the surface treatment time exceeds 60 seconds, etch pits are likely to form.

[0188] [Thermal oxidation-reduction annealing process]

[0189] In the thermal oxidation-reduction annealing step, a heat treatment equivalent to a two-stage annealing of thermal oxidation annealing and reduction annealing is performed on the surface treated steel sheet after the surface treatment step. Figure 2 and Figure 3 As shown in FIG, 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, the thermal oxidation-reduction annealing process of this embodiment is particularly characterized by the fact that the soaking treatment is performed in two stages instead of the conventional single stage.

[0190] <One heating process P1>

[0191] In the primary heating process P1, after the surface treatment step and before the subsequent primary annealing process P2, the surface treated steel sheet is heated and heated from room temperature to a heating arrival temperature T1. Specifically, the heating arrival temperature T1 is selected from the range of 800°C to 950°C. If the heating arrival temperature T1 is lower than 800°C, it is difficult to form a SiO2 oxide film (an oxide film mainly composed of SiO2). Therefore, the heating arrival temperature T1 is set to be above 800°C. The heating arrival temperature T1 is preferably above 820°C, and more preferably above 840°C. On the other hand, if the heating arrival temperature T1 exceeds 950°C, internal oxidized SiO2 will not be formed, and the entire surface of the steel sheet is easily excessively oxidized. Therefore, the heating arrival temperature T1 is set to be below 950°C. The heating arrival temperature T1 is preferably below 920°C, and more preferably below 900°C.

[0192] In the primary heating process P1, the dew point of the atmosphere is preferably controlled to be below 0°C when the steel plate temperature is between 100°C and 800°C. In addition, the oxygen potential PH2O / PH2 defined by the ratio of the water vapor partial pressure PH2O to the hydrogen partial pressure PH2 in the atmosphere is preferably below 0.5000.

[0193] If the oxygen potential PH2O / PH2 exceeds 0.5000, it becomes difficult to form a SiO2 oxide film. Therefore, the oxygen potential PH2O / PH2 is preferably set to 0.5000 or less. The oxygen potential PH2O / PH2 is more preferably 0.4000 or less, and even more preferably 0.3000 or less. From the perspective of ensuring a uniform SiO2 oxide film, the oxygen potential PH2O / PH2 is preferably set to 0.0001 or more. It is more preferably 0.0010 or more, and even more preferably 0.0050 or more.

[0194] Furthermore, in the primary heating process P1, it is preferable to control the average heating rate when the steel sheet temperature is 550°C to 800°C to be within a range of 100°C / second to 2000°C / second.

[0195] If the average heating rate is less than 100°C / second, oxide films other than SiO2, such as Fe2SiO4, may form. 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, making it difficult to achieve the effect of improving film adhesion. Therefore, the average heating rate is preferably 2000°C / second. The average heating rate is more preferably 1800°C / second or lower, and even more preferably 1600°C / second or lower.

[0196] By controlling the temperature rise conditions to the above conditions, a uniform SiO2 oxide film is preferably formed on the surface of the steel sheet. Specifically, the Si on the steel sheet surface reacts with the O2 in the atmosphere to form the SiO2 oxide film. However, by performing the above control, a SiO2 oxide film with uniform thickness can be formed evenly across the entire surface of the steel sheet. This uniform SiO2 oxide film stabilizes the oxidation behavior of the steel sheet during the subsequent primary annealing step P2, contributing to improved film adhesion.

[0197] Special equipment is required to precisely control the average heating rate within a narrow temperature range of 550°C to 800°C. Therefore, it is possible to initially control the average heating rate of the steel plate at a relatively low temperature of approximately 100°C. However, in this case, the average heating rate within the range of 550°C to 800°C is set to 100°C / second to 2000°C / second. This allows sufficient internally oxidized SiO2 to form on the surface of the base steel plate without requiring special equipment.

[0198] <Primary annealing process P2>

[0199] The primary annealing process P2 is performed following 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 hold time in the range of 10 seconds to 200 seconds in an atmosphere with an oxygen potential PH2O / PH2 of a constant value within the range of 0.10 to 10.00. The annealing temperature T2 is a constant temperature in the range of 800°C to 900°C.

[0200] If the oxygen potential PH2O / PH2 is less than 0.10, Fe2SiO4 may not precipitate. Fe2SiO4 is reduced to pure iron, which is beneficial for film adhesion, during the subsequent secondary annealing process P3. In other words, the inability to precipitate Fe2SiO4 during the primary annealing process P2 leads to deterioration in film adhesion. Therefore, the lower limit of the oxygen potential PH2O / PH2 is set to 0.10. The lower limit of the oxygen potential PH2O / PH2 is preferably 0.15, and more preferably 0.20.

[0201] On the other hand, if the oxygen potential PH2O / PH2 exceeds 10.00, there is a possibility that Fe oxides will precipitate in an amount that cannot be completely reduced 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.

[0202] If the annealing temperature T2 is lower than 800°C, Fe2SiO4 will not be generated sufficiently, which may lead to deterioration of the film adhesion. Therefore, the annealing temperature T2 is set to 800°C or higher.

[0203] On the other hand, if the annealing temperature T2 exceeds 900°C, Fe2SiO4 is excessively generated, which may lead to deterioration of the film adhesion. Therefore, the annealing temperature T2 is set to 900°C or less. The annealing temperature T2 is more preferably 880°C or less, and even more preferably 850°C or less.

[0204] If the holding time is less than 10 seconds, Fe2SiO4 will not be fully generated, and the film adhesion may be deteriorated. Therefore, the holding time is set to 10 seconds or more. The holding time is preferably 30 seconds or more, and more preferably 50 seconds or more.

[0205] On the contrary, if the holding time exceeds 200 seconds, Fe2SiO4 will be excessively generated, and the film adhesion may be deteriorated. Therefore, the holding time is set to 200 seconds or less. The holding time is preferably 180 seconds or less, and more preferably 150 seconds or less.

[0206] By controlling the soaking conditions to the conditions described above, the SiO2 oxide film generated in the primary heating process P1 can react with Fe from the steel sheet to form an Fe2SiO4 oxide film with a uniform film thickness on the steel sheet surface.

[0207] <Secondary Annealing Process P3>

[0208] In the secondary annealing step P3, the steel sheet is held at an annealing temperature T3 for a holding time in the range of 3 to 100 seconds in an atmosphere having an oxygen potential of PH2O / PH2 below 0.1000 and a dew point below -20°C. The annealing temperature T3 is a constant temperature in the range of 900°C to 1100°C.

[0209] If the oxygen potential PH2O / PH2 is above 0.1000, Fe2SiO4 cannot be fully reduced, and the film adhesion may be deteriorated. Therefore, the oxygen potential PH2O / PH2 is set to less than 0.1000. The oxygen potential PH2O / PH2 is preferably below 0.0800, and more preferably below 0.0600.

[0210] The lower the oxygen potential PH2O / PH2, the better. As the lower limit value in operation, 0.0001 can be shown.

[0211] In addition, the dew point is controlled to be lower than -20°C together with the oxygen potential PH2O / PH2. This is because if the dew point is -20°C or higher, iron oxides such as FeO are generated, which adversely affects the film adhesion.

[0212] If the annealing temperature T3 is lower than 900°C, the reduction of Fe2SiO4 will not proceed sufficiently, and the film adhesion may be deteriorated. Therefore, the annealing temperature T3 is set to 900°C or higher. The annealing temperature T3 is preferably 920°C or higher, and more preferably 940°C or higher.

[0213] There is no specific upper limit for the annealing temperature T3. However, if T3 exceeds 1100°C, strain may occur due to effects such as grain boundary sliding, potentially degrading magnetic properties. Therefore, the annealing temperature T3 is set to 1100°C or lower. The annealing temperature T3 is preferably 1000°C or lower, and more preferably 980°C or lower.

[0214] If the holding time is less than 3 seconds, the reduction of Fe2SiO4 will not proceed sufficiently, and the film adhesion may deteriorate. Therefore, the holding time is set to 3 seconds or more. The holding time is preferably 10 seconds or more, and more preferably 20 seconds or more.

[0215] On the other hand, if the holding time exceeds 100 seconds, the magnetic properties may deteriorate due to the influence of grain boundary sliding. Therefore, the holding time is set to 100 seconds or less. The holding time is preferably 80 seconds or less, and more preferably 60 seconds or less.

[0216] By controlling the soaking conditions in the secondary annealing step P3 to the above conditions, α-Fe can be uniformly precipitated from the Fe2SiO4 oxide film formed in the primary annealing step P2 onto the steel sheet surface (Fe2SiO4 → 2Fe+SiO2+O2). By pre-precipitating α-Fe uniformly onto the steel sheet surface in this manner, the insulating film-forming liquid applied in the subsequent tension-imparting insulating film-forming step can react with α-Fe, promoting the formation of a layer that contributes to film adhesion.

[0217] The transfer from the primary annealing process P2 to the secondary annealing process P3 can also be done as follows Figure 3 As shown in , the two processes may be performed continuously with another primary heating process interposed between them. That is, after the primary annealing process P2 is completed, the temperature of the steel sheet may be raised from the annealing temperature T2, and then the secondary annealing process P3 may be started at the annealing temperature T3. Alternatively, after the primary annealing process P2 is completed, the temperature of the steel sheet may be temporarily lowered from the annealing temperature T2 to room temperature, and then the steel sheet may be reheated and the secondary annealing process P3 may be started directly at the annealing temperature T3.

[0218] [Pickling process]

[0219] If necessary, a pickling step may be performed after the thermal oxidation-reduction annealing step and before the subsequent tension-applying insulating coating formation step. In this pickling step, the reduction-annealed steel sheet obtained in the thermal oxidation-reduction annealing step is immersed in an acid (second treatment liquid) 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, at a liquid temperature of 50°C to 90°C for 3 to 60 seconds.

[0220] If the total acid concentration of the second treatment liquid is less than 0.1% by volume, an active surface state cannot be achieved. On the other hand, if the total acid concentration of the second treatment liquid exceeds 20.0% by volume, the surface may become uneven, deteriorating the magnetic properties. Therefore, the acid concentration of the second treatment liquid is set to 0.1% to 20.0% by volume. Similarly, if the surface treatment time is less than 3 seconds, an active surface state cannot be achieved. If the surface treatment time exceeds 60 seconds, the surface may become uneven, deteriorating the magnetic properties.

[0221] The pickling step may be omitted. In this case, the tension-imparting insulating film forming step is performed after the thermal oxidation-reduction annealing step.

[0222] [Tension-Providing Insulation Film Forming Step]

[0223] The tension-imparting insulating coating forming step is performed following the thermal oxidation-reduction annealing step or the pickling step. In this tension-imparting insulating coating forming step, a secondary coating (tension-imparting insulating coating) 3 is formed on the surface of the reduction-annealed steel sheet obtained in the thermal oxidation-reduction annealing step or on the surface of the pickled sheet obtained in the pickling step. The secondary coating 3 contains phosphate and silica, and its specific composition and content can be based on known conditions.

[0224] The tension-applying insulating coating forming step includes a secondary coating chemical coating step and a baking step. In the following description, the reduction-annealed steel sheet and the pickled steel sheet are collectively referred to as "steel sheet."

[0225] <Secondary film coating process>;

[0226] In the secondary coating solution coating step, an insulating coating forming solution containing at least one of aluminum phosphate and silicon dioxide as a main component (hereinafter also referred to as "secondary coating solution") is coated on the surface of the steel sheet. The secondary coating solution preferably does not contain chromium.

[0227] In addition to aluminum phosphate, phosphates such as Ca, Al, and Sr may also be used. Silica is not particularly limited to silica of a specific property. The particle size is also not particularly limited to a specific particle size, but is preferably 200 nm or less (number average particle size). For example, 5 nm to 30 nm is sufficient. If the particle size exceeds 200 nm, sedimentation may occur in the coating solution.

[0228] <Baking process>

[0229] In the baking step following the secondary coating solution application step, the steel sheets coated with the secondary coating solution are heat treated to form an insulating film (secondary film 3) on the surface of the steel sheets. This insulating film applies tension to the steel sheets, thereby reducing the iron loss of the individual steel sheets. Furthermore, when grain-oriented electromagnetic steel sheets are stacked, it ensures electrical insulation between the individual steel sheets, thereby reducing the iron loss of the core.

[0230] The heat treatment conditions include, for example, heat treatment at a temperature within the range of 350° C. to 1150° C. for a time within the range of 5 to 300 seconds. Furthermore, the oxygen potential PH2O / PH2 of the atmosphere may be controlled as needed.

[0231] In this embodiment, the thermal oxidation-reduction annealing step is configured as a two-stage heating process consisting of a primary annealing step P2 and a secondary annealing step P3, thereby preliminarily uniformly precipitating α-Fe on the steel sheet surface. This α-Fe then dissolves into the secondary coating solution, where Fe atoms diffuse. Fe dissolves in the phosphate, forming an Fe-PO compound at the interface between the base steel sheet 2 and the secondary coating 3. The presence of this Fe-PO compound at the interface contributes to high coating adhesion and excellent iron loss characteristics in the grain-oriented electrical steel sheet 1 of this embodiment.

[0232] Through the above steps, Figure 1 The grain-oriented electromagnetic steel sheet 1 shown in FIG.

[0233] After forming the secondary film 3, the steel sheet may be subjected to flattening annealing for shape correction as needed. By performing this flattening annealing, the iron loss of the grain-oriented electrical steel sheet 1 can be further reduced.

[0234] Furthermore, before or after the tension-applying insulating film forming step, a magnetic domain control treatment may be performed as needed. The magnetic domain control treatment can further reduce the iron loss of the grain-oriented electrical steel sheet.

[0235] When magnetic domain control treatment is performed before the tension-applying insulating coating formation step, linear or dot-shaped grooves extending in a direction intersecting the rolling direction can be formed at predetermined intervals along the rolling direction. Alternatively, when magnetic domain control treatment is performed after the tension-applying insulating coating formation step, stress-strain regions extending in a direction intersecting the rolling direction can be formed at predetermined intervals along the rolling direction. The magnetic domain control treatment narrows the width of the 180° magnetic domain (180° domain refinement).

[0236] When forming the groove portion, mechanical groove forming methods using gears, chemical groove forming methods using electrolytic etching, and thermal groove forming methods using laser irradiation can be applied. In addition, when forming the stress strain portion, laser beam irradiation, electron beam irradiation, etc. can be applied.

[0237] According to the method for manufacturing a grain-oriented electrical steel sheet described above, a grain-oriented electrical steel sheet having higher coating adhesion can be manufactured without impairing magnetic properties.

[0238] Example

[0239] Next, the effects of one embodiment of the present invention will be described in more detail using examples. However, the various conditions in these examples are merely illustrative examples employed to confirm the feasibility and effects of the present invention, and the present invention is not limited to these examples. Various conditions may be employed within the scope of the present invention as long as they do not deviate from the spirit of the present invention and achieve the purpose of the present invention.

[0240] First, in a hot rolling process, slabs (steel billets) a to j having the chemical compositions shown in Table 1 below were prepared.

[0241] [Table 1]

[0242]

[0243] Specifically, in all slabs No. a to j, the chemical composition was set as follows: containing, in mass %, the following:

[0244] C: 0.020% to 0.150%,

[0245] Si: 3.00% to 4.00%,

[0246] Mn: 0.01% to 0.50%,

[0247] S: 0.0010% to 0.0400%,

[0248] Acid soluble Al: 0.010% to 0.050%,

[0249] N: 0.002%~0.020%,

[0250] The remainder includes Fe and impurities.

[0251] In addition, slabs No. c to j further contain, as a chemical composition, at least one selected from the following elements in terms of mass %:

[0252] Bi: 0.0200% or less,

[0253] P: 0.100% or less,

[0254] Sn: 0.50% or less,

[0255] Cu: 0.50% or less,

[0256] Cr: 0.50% or less,

[0257] Sb: 0.20% or less,

[0258] Mo: 0.10% or less,

[0259] Nb: 0.0200% or less,

[0260] B: 0.0200% or less,

[0261] Te: 0.0200% or less,

[0262] Ni: 0.20% or less,

[0263] Se: 0.0200% or less.

[0264] Then, each of these slabs a to j was heated to a constant temperature within a range of 1100° C. to 1400° C. and subjected to hot rolling to produce hot-rolled steel sheets having a thickness of 2.3 mm.

[0265] Next, in the hot rolled steel annealing step, the hot rolled steel sheet obtained in the hot rolling step is annealed at 1100°C for 100 seconds, and the surface is further pickled by immersing it in a pickling solution.

[0266] Next, in the cold rolling process, the pickled and annealed hot-rolled sheet was subjected to a single cold rolling or multiple cold rolling steps with intermediate annealing to obtain cold-rolled steel sheets having a base steel plate thickness of 0.22 mm to 0.23 mm as shown in Table 2A below.

[0267] Next, in the decarburization annealing step, the cold-rolled steel sheet obtained in the cold rolling step was subjected to decarburization annealing at 820° C. for 140 seconds in a nitrogen and hydrogen mixed gas atmosphere, where the nitrogen and hydrogen mixed gas atmosphere was humidified, to obtain a decarburized annealed steel sheet.

[0268] Next, for some of the decarburized annealed steel sheets, a nitriding treatment step was performed after the decarburization annealing step. That is, for Test Nos. 1, 2, 9, 10, 18, 19, and 20, a nitriding treatment was performed after the decarburization annealing step.

[0269] Next, in the separator coating step, a slurry of an annealing separator mainly composed of MgO and alumina is applied to the surface of the decarburized or nitrided steel sheet after the decarburization annealing step or the nitriding annealing step. The composition of the annealing separator used is shown in Table 2B.

[0270] Next, in the finish annealing step, the decarburized annealed steel sheet coated with the annealing separator was finish annealed at 1200°C for approximately 20 hours. The annealing atmosphere during the temperature rise step of the finish annealing step was a mixed gas of nitrogen and hydrogen, and the annealing atmosphere during the soaking step at 1200°C was hydrogen.

[0271] Next, in the surface treatment step, the finish annealed steel sheet obtained in the finish annealing step was immersed in a treatment liquid containing sulfuric acid and having the concentration (acid concentration) and temperature shown in Table 2C for the time shown in Table 2C to perform surface treatment and obtain a surface-treated steel sheet.

[0272] [Table 2A]

[0273]

[0274] [Table 2B]

[0275]

[0276] [Table 2C]

[0277]

[0278] Next, in the thermal oxidation-reduction annealing step, the surface-treated steel sheet after the surface treatment step is subjected to intermediate annealing. This thermal oxidation-reduction annealing step is configured to include a primary heating process P1, a primary annealing process P2, and a secondary annealing process P3.

[0279] Furthermore, in the primary heating process P1, after the surface treatment step and before the subsequent primary annealing process P2, the surface-treated steel sheet is heated to increase its temperature. In this primary heating process P1, the heating rate and the oxygen potential PH2O / PH2 at 550°C to 800°C are set as shown in Table 2A.

[0280] Furthermore, the primary annealing process P2 is performed following the primary temperature rising process P1. Specifically, in the primary annealing process P2, the annealing temperature T2, the annealing time (holding time), and the oxygen potential PH2O / PH2 are set as shown in Table 2A.

[0281] Furthermore, after the primary annealing process P2, the heating temperature is increased to perform the secondary annealing process P3. Specifically, in the secondary annealing process P3, the annealing temperature T3, the annealing time (holding time), and the oxygen potential PH2O / PH2 are each set as shown in Table 2B.

[0282] After the thermal oxidation-reduction annealing step, a pickling step was performed for 10 seconds using a 1% sulfuric acid solution at 60°C for Test Nos. 6 to 8. On the other hand, the pickling step after the thermal oxidation-reduction annealing step was not performed for the other Test Nos. 1 to 5 and 9 to 30.

[0283] After the thermal oxidation-reduction annealing step or the pickling step, a tension-imparting insulating coating forming step is performed. Specifically, in this tension-imparting insulating coating forming step, a secondary coating (tension-imparting insulating coating) containing aluminum phosphate and silicon dioxide is formed on the surface of the reduction-annealed steel sheet obtained in the thermal oxidation-reduction annealing step or the surface of the pickled sheet obtained in the pickling step. The tension-imparting insulating coating forming step includes a secondary coating chemical solution application step and a baking step. In the following description, the reduction-annealed steel sheet and the pickled sheet are collectively referred to as "steel sheet."

[0284] In the secondary coating liquid coating step, an insulating coating forming liquid containing aluminum phosphate and silicon dioxide (hereinafter also referred to as "secondary coating liquid") is coated on the surface of the steel sheet.

[0285] In the baking step following the secondary coating solution application step, the steel sheet coated with the secondary coating solution is heat treated to form an insulating coating (secondary coating) on the surface of the steel sheet. The heat treatment is performed at 900°C for a period of 10 to 20 seconds. The annealing atmosphere is a mixed gas of nitrogen and hydrogen, maintained in a humidified atmosphere with a dew point of 20°C or higher.

[0286] For each of Test Nos. 1 to 26 obtained through the above steps, film adhesion, iron loss, and magnetic flux density were evaluated as product characteristics.

[0287] Specifically, the film adhesion was evaluated by measuring the remaining film area ratio when the test piece was wrapped around a 20 mm diameter cylinder and bent 180°. The ratio of the remaining film area to the area of the steel sheet in contact with the cylinder was calculated. The area of the steel sheet in contact with the roller was calculated. The remaining film area was determined by photographing the steel sheet after the test and performing image analysis on the photographic image.

[0288] A film remaining area ratio of 95% or more was evaluated as excellent (EX), 90% or more but less was evaluated as very good (VG), 85% or more but less was evaluated as good (G), 80% or more but less was evaluated as acceptable (F), and less than 80% was evaluated as poor (NG). A film remaining area ratio of 80% or more was considered acceptable for film adhesion. The results are shown in Table 2C.

[0289] Next, regarding the iron loss characteristics, the test pieces were evaluated using the Single Sheet Tester (SST). Under the conditions of an AC frequency of 50 Hz and an excitation magnetic flux density of 1.7 T, the iron loss W17 / 50 (W / kg), defined as the power loss per unit weight (1 kg) of the steel plate, was measured. Iron loss W17 / 50 below 0.75 W / kg was considered acceptable. The magnetic flux density was determined by applying a magnetic field of 800 A / m to the test piece and measuring the magnetic flux density B8 (T) in the rolling direction. The results are shown in Table 2C.

[0290] For steels Nos. 1 to 11, 25, 26, 29, and 30, which are examples of the invention, the steel sheet temperature was maintained at a constant temperature within the range of 800°C to 900°C during a constant holding time within the range of 10 seconds to 200 seconds in an atmosphere having an oxygen potential PH2O / PH2 of a constant value within the range of 0.10 to 10.00 in the primary annealing process P2. Furthermore, for steels Nos. 1 to 11, the steel sheet temperature was maintained at a constant temperature within the range of 900°C to 1100°C during a constant holding time within the range of 3 seconds to 100 seconds in the secondary annealing process P3 in an atmosphere having an oxygen potential PH2O / PH2 of a constant value below 0.1000.

[0291] As a result, all of Steels Nos. 1 to 11, 25, 26, 29, and 30 met the acceptable standards for both film adhesion and iron loss. Steels Nos. 1 to 8, 25, and 26 showed particularly high film adhesion, with Steels Nos. 3 to 8 exhibiting the highest film adhesion.

[0292] Furthermore, no significant decrease in magnetic flux density was observed in all of Test Nos. 1 to 11, 25, 26, 29, and 30. Therefore, the results satisfy the acceptance criteria for both film adhesion and iron loss without compromising magnetic flux density.

[0293] On the other hand, regarding Steel Nos. 12 to 24, 27, and 28 as comparative examples, all of the coating adhesions failed to meet the acceptance criteria, and therefore the iron loss could not be measured.

[0294] Specifically, in Steel No. 12, the annealing temperature T2 was 780° C., which was lower than the lower limit of 800° C. As a result, the film adhesion became insufficient.

[0295] In addition, in Steel No. 13, the annealing temperature T2 was 920° C., which exceeded the upper limit of 900° C. As a result, the film adhesion became insufficient.

[0296] In addition, the annealing time of Steel No. 14 was 8 seconds, which was shorter than the lower limit of 10 seconds. As a result, the film adhesion became insufficient.

[0297] In addition, the annealing time of Steel No. 15 was 210 seconds, exceeding the upper limit of 200 seconds. As a result, the film adhesion became insufficient.

[0298] In addition, in Steel No. 16, the oxygen potential in the primary annealing process P2 was 0.09, which was lower than the lower limit value of 0.10. As a result, the film adhesion became insufficient.

[0299] In Steel No. 17, the oxygen potential in the primary annealing step P2 was 10.52, exceeding the upper limit of 10.00. Furthermore, no surface treatment was performed. As a result, the film adhesion was insufficient.

[0300] In Steel No. 18, the annealing temperature T3 in the secondary annealing process P3 was 880° C., which was lower than the lower limit of 900° C. Furthermore, no surface treatment step was performed. As a result, the film adhesion became insufficient.

[0301] In addition, in Steel No. 19, the annealing temperature T3 in the secondary annealing process P3 was 1110° C., which exceeded the upper limit of 1100° C. As a result, the film adhesion became insufficient.

[0302] In addition, in Steel No. 20, the annealing time in the secondary annealing process P3 was 2 seconds, which was shorter than the lower limit of 3 seconds. As a result, the film adhesion became insufficient.

[0303] In addition, in Steel No. 21, the annealing time in the secondary annealing process P3 was 110 seconds, which exceeded the upper limit of 100 seconds. As a result, the film adhesion became insufficient.

[0304] In addition, in Steel No. 22, the oxygen potential in the secondary annealing process P3 was 0.1255, exceeding the upper limit of 0.1000. As a result, the film adhesion became insufficient.

[0305] In Steel No. 23, the dew point in the secondary annealing process P3 was -20° C. or higher, exceeding the upper limit, that is, lower than -20° C. As a result, the film adhesion became insufficient.

[0306] Steel No. 24 was not subjected to a surface treatment process, resulting in insufficient film adhesion.

[0307] In Steel No. 27, the oxygen potential in the primary annealing process P2 was 10.52, exceeding the upper limit of 10.00. As a result, the film adhesion became insufficient.

[0308] In Steel No. 28, the annealing temperature T3 in the secondary annealing process P3 was 880° C., which was lower than the lower limit of 900° C. As a result, the film adhesion became insufficient.

[0309] Based on the above results, it was confirmed that in order to meet the acceptance criteria for both film adhesion and iron loss, it is necessary to maintain the steel sheet temperature 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 an oxygen potential PH2O / PH2 of a constant value within the range of 0.10 to 10.00 in the primary annealing process P2, and to maintain the steel sheet temperature 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 the secondary annealing process P3 in an atmosphere with an oxygen potential PH2O / PH2 of a constant value below 0.1000.

[0310] Industrial applicability

[0311] According to the present invention, a grain-oriented electrical steel sheet having higher film adhesion can be manufactured without impairing magnetic properties. Therefore, the present invention has high industrial applicability.

[0312] Explanation of symbols

[0313] 1. Grain-oriented electrical steel sheet

[0314] 3. Secondary coating (tension-giving insulation coating)

[0315] P1 primary heating process

[0316] P2 primary annealing process

[0317] P3 secondary annealing process

Claims

1. A method for manufacturing a grain-oriented electromagnetic steel sheet, characterized in that: The process includes the following steps: The hot rolling process is to obtain a hot-rolled steel sheet by heating and hot-rolling a slab, wherein the slab comprises, as a chemical composition, in mass %: C：0.020％～0.150％、 Si: 3.00% to 4.00%, Mn: 0.01% to 0.50%, S：0.0010％～0.0400％、 Acid soluble Al: 0.010% to 0.050%, N：0.002％～0.020％、 Bi: 0.0000%~0.0200%, P：0.000％～0.100％、 Sn: 0.00% to 0.50%, Cu: 0.00% to 0.50%, Cr：0.00％～0.50％、 Sb: 0.00% to 0.20%, Mo: 0.00% to 0.10%, Nb: 0.0000% to 0.0200%, B：0.0000％～0.0200％、 Te: 0.0000% to 0.0200%, Ni: 0.00% to 0.20%, Se: 0.0000%~0.0200%, The remainder is Fe and impurities; a hot-rolled plate annealing step of immersing the hot-rolled plate annealed in a pickling solution after annealing the hot-rolled steel plate to obtain the hot-rolled plate annealed plate; a cold rolling step of cold-rolling the hot-rolled annealed steel sheet to obtain a cold-rolled steel sheet; a decarburization annealing step of performing decarburization annealing on the cold-rolled steel sheet to obtain a decarburization annealed steel sheet; a separator coating step of coating an annealing separator containing MgO, Al2O3 and chloride on the surface of the decarburized annealed steel sheet; a finish annealing step of performing finish annealing on the decarburized annealed steel sheet coated with the annealing separator to obtain a finish annealed sheet; a surface treatment step of pickling the surface of the finished annealed steel plate to obtain a surface-treated steel plate; a thermal oxidation-reduction annealing step of performing two-stage annealing on the surface-treated steel sheet to obtain a reduction-annealed steel sheet; and a tension-imparting insulating film forming step of forming a tension-imparting insulating film containing phosphate and silicon dioxide on the surface of the reduction-annealed steel sheet; The thermal oxidation-reduction annealing process includes the following steps: a primary heating process, in which the surface-treated steel plate is heated to increase its temperature after the surface treatment step; a primary annealing process, which is performed subsequent to the primary heating process, in an atmosphere having an oxygen potential PH2O / PH2 of a constant value within a range of 0.10 to 10.00, and during a constant holding time within a range of 10 seconds to 200 seconds, wherein the temperature of the steel sheet is maintained at a constant temperature within a range of 800° C. to 900° C.; and The secondary annealing process is performed after the primary annealing process, in an atmosphere having an oxygen potential PH2O / PH2 of less than 0.1000 and a dew point of less than -20°C, and the steel sheet 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.

2. The method for manufacturing a grain-oriented electrical steel sheet according to claim 1, wherein: In the annealing separator, the content of MgO is 0.0 mass % to 79.5 mass %, the content of Al 2 O 3 is 20.0 mass % to 99.5 mass %, and the remainder is the chloride.

3. The method for manufacturing a grain-oriented electrical steel sheet according to claim 1, wherein: During the one heating process, the oxygen potential PH2O / PH2 of the steel plate when the temperature is 100°C to 800°C is less than 0.5000, and the average heating rate of the steel plate when the temperature is 550°C to 800°C is 100°C / second to 2000°C / second.

4. The method for manufacturing a grain-oriented electrical steel sheet according to claim 2, wherein: During the one heating process, the oxygen potential PH2O / PH2 of the steel plate when the temperature is 100°C to 800°C is less than 0.5000, and the average heating rate of the steel plate when the temperature is 550°C to 800°C is 100°C / second to 2000°C / second.

5. The method for producing a grain-oriented electrical steel sheet according to any one of claims 1 to 4, wherein: The method further includes a nitriding annealing step of subjecting the decarburized annealed steel sheet to nitriding annealing after the decarburization annealing step and before the separator coating step.

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

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

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

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