EP4692418A1

EP4692418A1 - Insulating film processing liquid for grain-oriented electrical steel sheets, method for manufacturing same, and grain-oriented electrical steel sheet manufacturing method

Info

Publication number: EP4692418A1
Application number: EP24784971.4A
Authority: EP
Inventors: Shinsuke TAKATANI; Kazutoshi Takeda; Atsushi Shibayama
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 2023-04-05
Filing date: 2024-04-05
Publication date: 2026-02-11
Also published as: JP7810939B2; JPWO2024210189A1; KR20250170637A; CN121002224A; WO2024210189A1

Abstract

Provided is an insulation coating treatment liquid for a grain-oriented electrical steel sheet including: metal phosphate of one or more metals selected from among K, Al, Fe, Mg, Mn, Ni, Zn, Co, Mo, V, W, and Zr, and colloidal silica, wherein a K₂O/SiO₂ ratio in mass% between an amount of K in terms of K₂O and an amount of Si in terms of SiO₂ in the insulation coating treatment liquid is 0.5% or more and 10% or less.

Description

TECHNICAL FIELD

The present invention relates to an insulation coating treatment liquid for a grain-oriented electrical steel sheet, a method for producing the same, and a method for producing a grain-oriented electrical steel sheet by applying the aforementioned insulation coating treatment liquid.

BACKGROUND ART

Grain-oriented electrical steel sheets are steel sheets mainly used as the iron core of transformers and the like. Usually, a two-layer surface coating is formed on such a grain-oriented electrical steel sheet: a forsterite layer (also called primary coating), which is formed during high-temperature final annealing; and a phosphate coating, which is formed by baking after applying a treatment liquid composed mainly of phosphate during heat flattening of the steel sheet.
Phosphate coatings are required to provide electrical insulation to grain-oriented electrical steel sheets and to improve iron loss by reducing eddy current loss. In addition to insulation, various other properties such as corrosion resistance, heat resistance, slipperiness, and adhesion are also required for the phosphate coating. This is in order to facilitate various production processes when the grain-oriented electrical steel sheets are processed to make iron cores for transformers and other devices. For instance, if the phosphate coating has poor heat resistance, slipperiness, or adhesion, the phosphate coating may peel off during stress relief annealing in the production of the iron core, which prevents the phosphate coating from exhibiting its original insulation properties or worsens processability because the steel sheets cannot be stacked smoothly.
Furthermore, an important property of the insulation coating of the grain-oriented electrical steel sheet is that it applies tension to the steel sheets. When tension is applied to steel sheets, the iron loss of grain-oriented electrical steel sheets can be improved by facilitating magnetic wall movement. Tension can also reduce magnetostriction (one of the main causes of noise in transformers).
In order to improve the various properties of grain-oriented electrical steel sheets as described above, specifically, technologies such as those disclosed in the following Patent Documents 1 to 9 have been researched and developed.
For example, Patent Document 1 discloses that an insulation coating treatment liquid with a specific composition composed mainly of aluminum phosphate, chromate, and colloidal silica is applied to the forsterite film formed on the steel sheet surface after final annealing, and then baked. According to the technology disclosed in Patent Document 1, a high-tension insulation coating is formed on the steel sheet surface, thereby reducing the iron loss and magnetostriction of the grain-oriented electrical steel sheet.
In addition, Patent Document 2 discloses a method of applying a treatment liquid containing ultrafine colloidal silica particles with a particle diameter of 8 µm or less, primary phosphate, and chromate in specific proportions to a steel sheet, followed by baking the sheet. According to the technology disclosed in Patent Document 2, the high tension of the insulation coating can be maintained and, in addition, the lubricity of the coating can be improved.
Furthermore, Patent Document 3 discloses a technology for forming a high-tension insulation coating on the surface of a grain-oriented electrical steel sheet by applying a specific amount of an insulation coating composed mainly of phosphate, chromate, and colloidal silica with a glass transition point of 950°C to 1200°C.
According to the technologies disclosed in Patent Documents 1 to 3 above, it has been possible to form an insulation coating with various coating properties that are much better and with improved coating tension. However, all of the technologies disclosed in Patent Documents 1 to 3 contain chromate, a chromium compound, in the insulation coating. In recent years, with the focus on environmental issues, there has been societal demands to prohibit or restrict the use of compounds such as lead, chromium, and cadmium.
For this reason, technologies that enable the formation of effective insulation coatings without the abovementioned chromium compounds have been investigated. However, a problem with insulation coatings without chromium compounds is that they do not impart enough tension to the steel sheet.
As a method for solving the abovementioned problems, for example, Patent Document 4 discloses an insulation coating treatment method for a grain-oriented electrical steel sheet, in which a treatment liquid containing 20 parts by weight of colloidal silica in terms of SiO₂ content, 10 to 120 parts by weight of aluminum phosphate, 2 to 10 parts by weight of boric acid, and 4 to 40 parts by weight in total of one or more types selected from among sulfates of Mg, Al, Fe, Co, Ni, and Zn is baked at 300°C or higher.
In addition, Patent Document 5 discloses a technology relating to a coating agent for forming a coating that contains a mixture of boric acid and alumina sol, and a water-compatible organic solvent; this coating has a tension-imparting effect on grain-oriented electrical steel sheets.
In addition, Patent Document 6 discloses a technology in which one or more of organic acid salts of Ca, Mn, Fe, Mg, Zn, Co, Ni, Cu, B and Al are contained in a surface treatment agent for grain-oriented electrical steel sheets containing primary phosphates of Al, Mg, and Ca and colloidal silica. In addition, formate, acetate, oxalate, tartrate, lactate, citrate, succinate, and salicylate are exemplified as organic acid salts in Patent Document 6.
In addition, Patent Document 7 discloses a technology in which, in an insulation coating treatment agent for grain-oriented electrical steel sheets containing phosphate and colloidal silica, the metal components in the phosphate are a combination of specific proportions of divalent metal elements, trivalent metal elements, and metal elements having a valence of tetravalent or more.
Furthermore, Patent Document 8 discloses a grain-oriented electrical steel sheet including a steel sheet and an insulation coating that contains: a first metal phosphate which is metal phosphate of one or more metals selected from among Al, Fe, Mg, Mn, Ni, and Zn; a second metal phosphate which is metal phosphate of one or more metals selected from among Co, Mo, V, W, and Zr; and colloidal silica.
In addition, Patent Document 9 discloses an aqueous composition for coating grain-oriented electrical steel; the composition contains aluminum cations; manganese cations; dihydrogen phosphate, hydrogen phosphate and/or phosphate anions; colloidal silica; and optionally iron cations.

LIST OF PRIOR ART DOCUMENTS

PATENT DOCUMENT

Patent Document 1: JP48-39338A
Patent Document 2: JP61-41778A
Patent Document 3: JP11-071683A
Patent Document 4: JP54-143737A
Patent Document 5: JP7-278828A
Patent Document 6: JP2000-178760A
Patent Document 7: JP2010-13692A
Patent Document 8: WO2017-057513 A1
Patent Document 9: JP2022-519691A

SUMMARY OF INVENTION

TECHNICAL PROBLEM

These proposals have improved the application of tension to steel sheets. However, the present inventors' research to date has shown that insulation coatings that do not contain a chromium compound also deteriorate in terms of moisture absorption resistance. Even with all of the above techniques, it has not been possible to achieve the same level of moisture absorption resistance as conventional coatings containing chromic acid, and there is still room for improvement.
The insulation coating on grain-oriented electrical steel sheet has to be capable of imparting a large tension to the surface of the steel sheet. In addition, the insulation coating on a grain-oriented electrical steel sheet is also required to have good moisture absorption resistance and good productivity.
The present invention was made to solve the aforementioned problems, and objectives thereof are to provide an insulation coating treatment liquid for a grain-oriented electrical steel sheet that exhibit good moisture absorption resistance and, in addition, good productivity even when chromate is not included, and a method for producing the same, and to provide a method for producing a grain-oriented electrical steel sheet by applying the aforementioned insulation coating treatment liquid.

SOLUTION TO PROBLEM

The gist of the present invention is as follows

(1) An insulation coating treatment liquid for a grain-oriented electrical steel sheet according to an embodiment of the present invention includes metal phosphate of one or more metals selected from among K, Al, Fe, Mg, Mn, Ni, Zn, Co, Mo, V, W, and Zr, and colloidal silica,
wherein a K₂O/SiO₂ ratio in mass% between an amount of K in terms of K₂O and an amount of Si in terms of SiO₂ in the insulation coating treatment liquid is 0.5% or more and 10% or less.
(2) A method for producing an insulation coating treatment liquid for a grain-oriented electrical steel sheet according to another embodiment of the present invention includes:
- an adding process in which a K compound is added to metal phosphate of one or more metals selected from among Al, Fe, Mg, Mn, Ni, Zn, Co, Mo, V, W, and Zr, and
- a mixing process in which the colloidal silica and the metal phosphate with the added K compound are mixed to make a K₂O/SiO₂ ratio in mass% between an amount of K in terms of K₂O and an amount of Si in terms of SiO₂ in the insulation coating treatment liquid 0.5% or more and 10% or less.
(3) A method for producing a grain-oriented electrical steel sheet according to another embodiment of the present invention includes:
- a coating process in which an insulation coating treatment liquid is applied to a surface of a steel sheet, and
- a baking process in which the insulation coating treatment liquid is baked to form an insulation coating,
- wherein the insulation coating treatment liquid is the insulation coating treatment liquid for a grain-oriented electrical steel sheet as described in (1) above, and
- wherein in the baking process, the baking soaking temperature is 800 to 1000°C and the soaking holding time is 10 to 60 seconds.

ADVANTAGEOUS EFFECT OF INVENTION

According to the present invention, a grain-oriented electrical steel sheet with excellent moisture absorption resistance and productivity can be stably obtained.

BRIEF DESCRIPTION OF DRAWINGS

Figure 1 shows a relationship between the K₂O/SiO₂ ratio of the insulation coating treatment liquid and the amount of moisture absorption.

DESCRIPTION OF EMBODIMENTS

The results of the preliminary experiments leading to the present invention are described below.
A final annealed grain-oriented electrical steel sheets with a thickness of 0.23 mm produced by a known method were sheared to a width of 60 mm and a length of 300 mm, and the annealing separator adhering to the surface was removed by rinsing, and the resulting sheets were used as base metals. Next, 1 to 20 parts by mass of potassium phosphate was added to 100 parts by mass of aluminum primary phosphate, and then mixed with 70 parts by mass of colloidal silica to prepare 10 types of insulation coating treatment liquid. In this manner, the K₂O/SiO₂ ratio in the insulation coating treatment liquid was adjusted within a range of 0.6 to 10%. Note that the "K₂O/SiO₂ ratio" is a percentage (%) obtained by quantifying the amount of K in terms of K₂O (hereinafter referred to as the "K₂O amount", unit: mass%) and the amount of Si in terms of SiO₂ (hereinafter referred to as the "SiO₂ amount", unit: mass%) contained in the insulation coating treatment liquid respectively, and dividing the K₂O amount by the SiO₂ amount.
Then, the insulation coating treatment liquid was applied to both sides of the base metals prepared above using a roll coater so that the coating amount after baking would be 4.5 g/m² per side. They were then baked at a temperature of 850°C for 30 seconds.
For comparison, an insulation coating treatment liquid (the K₂O/SiO₂ ratio = 0.8%) consisting of 50 parts by mass of aluminum primary phosphate which does not contain K, 40 parts by mass of colloidal silica, and 10 parts by mass of chromic anhydride was similarly applied to the base metal and baked.
The resulting steel sheet was used as a test specimen to evaluate the moisture absorption resistance. First, the test specimen was placed in a thermo-hydrostat (temperature: 50°C, humidity: 90%) for one week, and the difference in weight before and after the thermo-hydrostat treatment was determined. The quantified weight difference was divided by the area of both sides of the test specimen, 0.036 m², which was defined as the amount of moisture absorption (unit: g/m²), and the moisture absorption resistance was evaluated using the amount of moisture absorption as an index.
Figure 1 shows the results of the above evaluation, organizing the relationship between the amount of moisture absorption and the K₂O/SiO₂ ratio. As shown in Figure 1, the steel sheet with a chromium-containing coating has a small K₂O/SiO₂ ratio; the amount of moisture absorption is 0.1 g/m² or less and almost no moisture is absorbed. In contrast, in the case of steel sheets having a coating that does not contain chromium, in the region where the K₂O/SiO₂ ratio is less than 0.5%, the moisture absorption amount exceeds 1.0 g/m², which is large, and sufficient moisture absorption resistance is not achieved.
However, in regions where the K₂O/SiO₂ ratio is large, especially where the K₂O/SiO₂ ratio is 0.5% or more, the amount of moisture absorption is 1.0 g/m² or less. Furthermore, in the region where the K₂O/SiO₂ ratio is 5.0% or more, almost no moisture is absorbed, and good moisture absorption resistance equivalent to that of steel the sheet coated with chromium-containing coating is achieved. On the other hand, when the K₂O/SiO₂ ratio exceeds 10%, it was confirmed that the dispersibility of colloidal silica deteriorates, resulting in an uneven appearance after baking.
Based on the above, the present inventors have considered the mechanism by which the K₂O/SiO₂ ratio in an insulation coating that does not contain chromium affects moisture absorption resistance as follows.
K₂O is known as a "Net-Work Modifier: NWM" that significantly changes the properties of glass when it is incorporated into glasses such as phosphate. K compound is generally thought to have the property of softening glass, but it is considered that potassium strengthens the network structure of phosphate, thereby significantly improving the moisture absorption resistance.
Based on the above experimental results, the present inventors have found that in an insulation coating treatment liquid containing metal phosphate and colloidal silica, it is possible to provide high moisture absorption resistance by limiting the K₂O/SiO₂ ratio in the insulation coating treatment liquid to 0.5% or more and 10% or less and thereby strengthening the network structure of phosphate that makes up the insulation coating.

This section describes the configuration of the insulation coating treatment liquid for a grain-oriented electrical steel sheet (hereinafter simply referred to as "insulation coating treatment liquid") according to the present embodiment and the reasons for its limitations.
The insulation coating treatment liquid of the present embodiment includes metal phosphate of one or more metals selected from among K, Al, Fe, Mg, Mn, Ni, Zn, Co, Mo, V, W, and Zr, and colloidal silica. It is preferred that the metal phosphate be metal phosphate of one or more metals selected from among Al, Mg, Ni, V, and W. This is because, when these phosphates are selected, a flat and uniform appearance can be achieved over a wide range of baking conditions. As described below, it is preferable to add potassium phosphate as the K compound, and therefore the insulation coating treatment liquid of the present embodiment preferably contains a metal phosphate of K.
The K₂O/SiO₂ ratio in the insulation coating treatment liquid of the present embodiment is 0.5% or more and 10% or less. If the K₂O/SiO₂ ratio is less than 0.5%, the effect of strengthening the network structure of phosphate that makes up the insulation coating is small, resulting in insufficient moisture absorption resistance. On the other hand, if the K₂O/SiO₂ ratio exceeds 10%, the appearance after baking deteriorates. The K₂O/SiO₂ ratio is preferably 1% or more, more preferably 3% or more, and even more preferably 5% or more. The K₂O/SiO₂ ratio is preferably 8% or less, more preferably 7% or less, and even more preferably 6% or less.
In the present embodiment, the analysis of the K₂O/SiO₂ ratio in the insulation coating treatment liquid is performed as follows. The amount of K contained in the insulation coating treatment liquid is measured by atomic absorption spectrophotometry and converted to the K₂O amount. The amount of Si contained in the insulation coating treatment liquid is determined by an elemental analysis method such as ICP and converted into the SiO₂ amount. Then, the ratio of K₂O/SiO₂ can be calculated from the obtained amounts of K₂O and SiO₂ and expressed as a percentage.

This section describes the configuration of the method for producing an insulation coating treatment liquid according to the present embodiment and the reasons for its limitations.
The method for producing an insulation coating treatment liquid of the present embodiment includes an adding process for adding a K compound to metal phosphate and a mixing process for mixing colloidal silica and the metal phosphate.
In the adding process, a K compound is added to metal phosphate of one or more metals selected from among Al, Fe, Mg, Mn, Ni, Zn, Co, Mo, V, W, and Zr. It is preferable to add potassium phosphate as the K compound. It is also possible to add potassium permanganate as the K compound. However, potassium permanganate is a strong oxidizing agent and is highly reactive, so its use is not preferable from a safety standpoint. Therefore, it is preferable to exclude potassium permanganate as the K compound.
One possible method of adding K to the insulation coating treatment liquid is to add a K compound to colloidal silica in advance. However, adding a K compound to colloidal silica carries the risk of the colloidal silica aggregating. Therefore, in the method for producing an insulation coating treatment liquid of the present embodiment, a K compound is added to metal phosphate in advance.
In the mixing process, the colloidal silica and the metal phosphate with the K compound added in the adding process are mixed. Then, the K₂O/SiO₂ ratio in mass% between the K₂O amount and the SiO₂ amount in the insulation coating treatment liquid is made 0.5% or more and 10% or less. The reason why the K₂O/SiO₂ ratio is set within the above range is as described above, and therefore will not be repeated here.
Furthermore, as long as the K₂O/SiO₂ ratio of the insulation coating treatment liquid is within the above range, a compound containing Si and/or K may be further mixed in the mixing process. For example, a K compound may be added to the colloidal silica in advance. In this case, for example, a method of adding potassium hydroxide and/or potassium chloride to the colloidal silica in advance can be used. In the mixing process, it is preferable not to actively add chromates, taking environmental issues into consideration.
The size of colloidal silica (silica particles) used in the present embodiment is not particularly limited; however, the average particle diameter (average primary particle diameter) is preferably 4 to 35 nm. If the average particle diameter of the colloidal silica is less than 4 nm, the colloidal silica tends to aggregate and the stability of the insulation coating treatment liquid may deteriorate, or the insulation coating may become a porous one with large gaps, reducing the adhesion of the insulation coating, which is undesirable. On the other hand, if the average particle diameter of the colloidal silica is more than 35 nm, the colloidal silica becomes less reactive and the mixing of the phosphate as a binder and the colloidal silica may not be sufficient, or cracks may appear in the insulation coating to reduce adhesion, which is undesirable.
It is further preferred that the upper limit of the average particle diameter of the colloidal silica be set to 31 nm, 22 nm, 18 nm, or 12 nm because smaller particle diameters of colloidal silica form denser coatings and increase coating tension. In addition, it is even more preferred that the surface of the colloidal silica be chemically treated with aluminum. The average particle diameter (average primary particle diameter) of the colloidal silica can be determined, for example, by conversion from the specific surface area measured by the BET adsorption method (according to JIS Z8830).
In the method for producing an insulation coating treatment liquid according to the present embodiment, the ratio of metal phosphate and colloidal silica is not particularly limited. As long as the K₂O/SiO₂ ratio in the insulation coating treatment liquid is 0.5% or more and 10% or less, the insulation coating of a grain-oriented electrical steel sheet using the insulation coating treatment liquid produced by the method of the present embodiment exhibits excellent properties. The following are examples of preferred values.
The content of colloidal silica in the insulation coating treatment liquid in terms of SiO₂ is preferably 25.0 to 65.0 mass% of the total mass of the insulation coating treatment liquid in terms of solid content. If the content of colloidal silica in the insulation coating treatment liquid is less than 25.0 mass%, the coating tension of the insulation coating may not be sufficient, which is undesirable. If the content of colloidal silica in the insulation coating treatment liquid exceeds 65.0 mass%, the adhesion of the insulation coating may decrease, which is undesirable. The content of colloidal silica in the insulation coating treatment liquid is more preferably 27.0 mass% or more, and even more preferably 35.0 mass% or more, 40.0 mass% or more, or 45.0 mass% or more in terms of solid content, relative to the total mass of the insulation coating treatment liquid. The content of colloidal silica in the insulation coating treatment liquid is more preferably 58.0 mass% or less, even more preferably 55.0 mass% or less, or even more preferably 50.0 mass% or less.
In addition, the insulation coating treatment liquid according to the present embodiment is composed mainly of metal phosphate and colloidal silica, and is used to form a phosphate coating. Therefore, the content of the metal phosphate in the insulation coating treatment liquid in terms of solid content is preferably 28.0 to 75.0 mass% of the total mass of the insulation coating treatment liquid in terms of solid content.
In the above mixing process, inorganic compounds including various oxides such as titanium oxide and molybdenum oxide, boric acid, sodium borate, pigments, and barium titanate may be further mixed. However, the content of components other than colloidal silica and phosphate in the insulation coating treatment liquid in terms of solid content is preferably 20.0 mass% or less, more preferably 15.0 mass% or less, even more preferably 10.0 mass% or less, and even more preferably 5.0 mass% or less, based on the total mass of the insulation coating treatment liquid in terms of solid content.
In addition, as mentioned above, it is preferable not to actively add chromate, and the content of chromate in the insulation coating treatment liquid is preferably not more than the impurity level. Specifically, the content of chromate in the insulation coating treatment liquid in terms of CrO₃ is preferably 0.1 mass% or less of the total mass of the insulation coating treatment liquid in terms of solid content.

Next, the method for producing a grain-oriented electrical steel sheet according to the present embodiment is described. The method for producing a grain-oriented electrical steel sheet (insulation coating treatment method) of the present embodiment includes a coating process in which an insulation coating treatment liquid is applied to the surface of the steel sheet and a baking process in which the insulation coating treatment liquid is baked. By baking, an insulation coating is formed.
In the method for producing a grain-oriented electrical steel sheet according to the present embodiment, the steel sheet on which the insulation coating is formed may be a normal grain-oriented electrical steel sheet with a forsterite film or a grain-oriented electrical steel sheet without a forsterite film. In either case, after final annealing followed by rinsing to remove excess annealing separator, the grain-oriented electrical steel sheet is subjected to pickling in a sulfuric acid bath or the like, and then rinsing in water. After the surface of the steel sheet is cleaned and activated in this manner, an insulation coating treatment liquid is applied to the steel sheet in the coating process. The grain-oriented electrical steel sheet coated with the insulation coating treatment liquid is then subjected to a baking process under the conditions described below to form an insulation coating on the surface.
In the baking process, the grain-oriented electrical steel sheet coated with the insulation coating treatment liquid is heated to the baking soaking temperature, held at the baking soaking temperature, and then cooled. The baking soaking temperature (°C) is the sheet temperature reached in the baking process (maximum sheet temperature) and must be 800°C or higher and 1000°C or lower. If the baking soaking temperature is less than 800°C, the insulation coating may not impart sufficient tension to the steel sheet. On the other hand, if the baking soaking temperature exceeds 1000°C, cracks may appear in the insulation coating, resulting in a decrease in coating tension, insulation properties, etc., and defects may also appear on the steel sheet. The baking soaking temperature is more preferably 850°C or higher and 950°C or lower.
The soaking holding time (seconds) indicates the holding time at the baking soaking temperature. The soaking holding time is to be 10 seconds or more. If the soaking holding time is less than 10 seconds, baking of the insulation coating is insufficient and the moisture absorption resistance may deteriorate (the amount of moisture absorption may increase). The soaking holding time is desirably 20 seconds or more. On the other hand, the soaking holding time is to be 60 seconds or less. If the soaking holding time is longer than 60 seconds, not only does the moisture absorption resistance hardly change, but excessive crystallization of the insulation coating may cause cracks and reduce the coating tension. The soaking holding time is more preferably 45 seconds or less to achieve necessary and sufficient coating properties.
The type of steel sheet to which the above insulation coating treatment is applied is not limited. The reason is that the main feature of the grain-oriented electrical steel sheet of the present embodiment is the composition of the insulation coating, and the effects of the insulation coating of the grain-oriented electrical steel sheet of the present embodiment, namely, the ability to impart a large tension to the surface of the steel sheet; good adhesion, corrosion resistance and productivity; and good moisture absorption resistance, even when the chromate is not included, can be achieved regardless of the type of steel sheet.
Preferably, the above-mentioned insulation coating treatment can be applied to grain-oriented electrical steel sheets, such as those produced using the technology disclosed in, for example, JP7-268567A , thereby making it possible to obtain a further effect of reducing iron loss. The above insulation coating treatment can further reduce iron loss by applying the above insulation coating treatment, specifically, to a grain-oriented electrical steel sheet containing at least C: 0.005% or less and Si: 2.5 to 7.0% by mass, and optionally further containing other alloying elements within the ranges not impairing the properties (e.g., Mn: 0 to 1.0%, Al: 0 to 0.03%, N: 0.01% or less, P: 0.01% or less, and S: 0.01% or less), and the balance Fe and impurities, and having an average grain size of 1 to 10 mm, and an average angle of 8° or less between the (110) [001] crystal orientation and the rolling direction.
Although there is no particular limit to the coating amount of the insulation coating of the grain-oriented electrical steel sheet produced by the method for producing a grain-oriented electrical steel sheet according to the present embodiment, the coating amount of 2.0 to 7.0 g/m² is suitable. If the coating amount of the insulation coating is less than 2.0 g/m², it becomes difficult to impart high tension to the grain-oriented electrical steel sheet, and the insulation properties and corrosion resistance of the grain-oriented electrical steel sheet may also deteriorate, which is undesirable. On the other hand, if the coating amount of the insulation coating is more than 7.0 g/m², the space factor of the grain-oriented electrical steel sheet may decrease, resulting in a deterioration of the transformer characteristics, which is undesirable. The coating amount of the insulation coating is preferably 3.0 g/m² or more, and even more preferably 4.0 g/m² or more. The coating amount of the insulation coating is preferably 6.0 g/m² or less, and even more preferably 5.0 g/m² or less.

EXAMPLE

Next, the effects of an embodiment of the present invention will be described in more concrete detail by means of examples. The conditions in the examples are examples of conditions adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to these examples of conditions. Various conditions may be employed in the present invention as long as they do not depart from the gist of the present invention and achieve the purpose of the present invention.
A slab containing, by mass%, C: 0.082%, Si: 3.25%, Mn: 0.084%, sol.Al: 0.026%, N: 0.0088%, P: 0.008%, and S: 0.023%, with the balance being Fe and impurities was cast, heated to 1150°C, and subjected to hot rolling to prepare a hot-rolled steel sheet with a thickness of 2.6 mm. The hot-rolled steel sheet was then annealed as necessary, and cold-rolled once, or several times with intermediate annealing, to obtain a cold-rolled steel sheet with a final thickness of 0.23 mm. The cold-rolled steel sheet underwent decarburization annealing and nitriding in an ammonia-containing atmosphere during the temperature reduction step. Known conditions were used in the processes, from slab heating to nitriding.
An annealing separator consisting mainly of MgO was applied to the decarburization-annealed sheet after decarburization annealing described above and left to dry. The decarburization-annealed sheet coated with the annealing separator was subjected to final annealing at 1200°C for 20 hours.
After that, the excess annealing separator was rinsed off with a scrubber, and then the insulation coating treatment liquids with the components adjusted as shown in Table 1 were applied and baked under the conditions shown in Table 2 to form insulation coatings. Note that the K₂O amount in the metal phosphate was adjusted by adding potassium phosphate.

[Table 1]

Table 1

Treatment liquid No.	Insulation coating treatment liquid
	Metal phosphate				Colloidal silica			Chromate	Inorganic compound		K₂O/SiO₂ ratio (%)
	Metal	Metal ratio (mol ratio)	Content (mass%)	K₂O amount (mass%)	Content (mass%)	SiO₂ amount (mass%)	Na₂O amount (mass%)	Content (mass%)	Type	Content (mass%)	K₂O/SiO₂ ratio (%)
A1	Al	100	74.8	0.13	25.2	25.1	0.09	-	-	-	0.5
A2	Al	100	72.8	0.54	27.2	27.1	0.09	-	-	-	2
A3	Al	100	64.7	1.1	35.3	35.1	0.14	-	-	-	3
A4	Al	100	59.6	2.0	40.4	40.2	0.18	-	-	-	5
A5	Al	100	54.6	2.3	45.4	45.2	0.19	-	-	-	5
A6	Al	100	49.6	2.5	50.4	50.2	0.19	-	-	-	5
A7	Al	100	44.6	3.3	55.4	55.2	0.21	-	-	-	6
A8	Al	100	41.6	4.7	58.4	58.2	0.19	-	-	-	8
A9	Al	100	34.5	6.5	65.5	65.3	0.27	-	-	-	10
A10	Al	100	52.6	2.3	45.4	45.2	0.20	-	Boric acid	2.0	5
A11	Al	100	52.7	2.3	45.3	45.1	0.14	-	Sodium borate	2.0	5
A12	Al	100	52.7	2.3	45.3	45.1	0.14	-	Titanium oxide	2.0	5
A13	Al	100	52.6	2.3	45.4	45.2	0.20	-	Molybdenum oxide	2.0	5
A14	Al	100	52.7	2.3	45.3	45.1	0.14	-	Barium titanate	2.0	5
A15	Al / Fe	80 / 20	54.7	2.3	45.3	45.1	0.15	-	-	-	5
A16	Al / Mg	80 / 20	54.6	2.3	45.4	45.2	0.18	-	-	-	5
A17	Al / Mn	80 / 20	54.6	2.3	45.4	45.2	0.19	-	-	-	5
A18	Al / Ni	80 / 20	54.6	2.3	45.4	45.2	0.19	-	-	-	5
A19	Al / Zn	80 / 20	54.7	2.3	45.3	45.1	0.14	-	-	-	5
A20	Al / Co	80 / 20	54.6	2.3	45.4	45.2	0.18	-	-	-	5
A21	Al / Mo	80 / 20	54.7	2.3	45.3	45.2	0.17	-	-	-	5
A22	Al / V	80 / 20	54.7	2.3	45.3	45.2	0.15	-	-	-	5
A23	Al / W	80 / 20	54.6	2.3	45.4	45.2	0.20	-	-	-	5
A24	Al / Zr	80 / 20	54.6	2.3	45.4	45.2	0.20	-	-	-	5
a1	Al	100	54.6	0.19	45.4	45.2	0.18	-	-	-	0.4
a2	Al	100	54.7	5.4	45.3	45.1	0.14	-	-	-	12
a3	Al	100	44.7	0.09	45.3	45.1	0.14	10.0	-	-	0.2

[Table 2]

Table 2

Producing method No.	Baking process
	Soaking temperature	Holding time
	(°C)	(sec)
B1	800	30
B2	1000	30
B3	850	10
B4	850	60
B5	850	30
B6	870	30
B7	950	30
b1	750	30
b2	1050	30
b3	850	5
b4	850	90

[Amount of moisture absorption]

The amount of moisture absorption of the resulting grain-oriented electrical steel sheet with an insulation coating was measured using the method described above.

[Appearance]

The appearance of the insulation coating after baking was evaluated. The appearance of the insulation coating after baking was judged by visual inspection based on the presence or absence of clouding of the insulation coating. A non-clouded area ratio of less than 10% of the coating surface was rated as "Very Good," 10% to less than 20% was rated as "Good," and 20% or more was rated as "Poor." When the surface of the clouded sample was observed using an SEM, fine cracks were found in the coating. Therefore, it is considered that the occurrence of cracks causes diffuse reflection of light, which causes the sample to be observed as cloudy.
The results of these evaluations are shown in Tables 3 to 5.

[Table 3]

Table 3

Test No.	Treatment liquid No.	Producing method No.	Property evaluation
Test No.	Treatment liquid No.	Producing method No.	Amount of moisture absorption (g/m²)	Appearance after baking
C1	A1	B5	0.95	good	Inventive example
C2	A2	B5	0.54	good
C3	A3	B5	0.35	good
C4	A4	B5	0.02	very good
C5	A5	B5	0.04	very good
C6	A6	B5	0.03	very good
C7	A7	B5	0.04	very good
C8	A8	B5	0.04	good
C9	A9	B5	0.03	good
C10	A10	B5	0.05	very good
C11	A11	B5	0.04	very good
C12	A12	B5	0.05	very good
C13	A13	B5	0.04	very good
C14	A14	B5	0.06	very good
C15	A15	B5	0.04	very good
C16	A16	B5	0.05	very good
C17	A17	B5	0.03	very good
C18	A18	B5	0.04	very good
C19	A19	B5	0.04	very good
C20	A20	B5	0.06	very good
C21	A21	B5	0.03	very good
C22	A22	B5	0.04	very good
C23	A23	B5	0.06	very good
C24	A24	B5	0.04	very good
c1	a1	B5	1.16	good	Comparative example
c2	a2	B5	0.03	poor
c3	a3	B5	0.01	good

[Table 4]

Table 4

Test No.	Treatment liquid No.	Producing method No.	Property evaluation
Test No.	Treatment liquid No.	Producing method No.	Amount of moisture absorption (g/m²)	Appearance after baking
D1	A4	B1	0.06	good	Inventive example
D2	A4	B2	0.04	good
D3	A4	B3	0.76	good
D4	A4	B4	0.05	good
D5	A4	B5	0.04	very good
D6	A4	B6	0.05	very good
D7	A4	B7	0.04	good
d1	A4	b1	1.35	good	Comparative example
d2	A4	b2	0.03	poor
d3	A4	b3	1.44	good
d4	A4	b4	0.05	poor

[Table 5]

Table 5

Test No.	Producing method No.	Insulation coating treatment liquid									Property evaluation
		Metal phosphate			Colloidal silica				Chromate	K₂O/SiO₂ ratio (%)	Amount of moisture absorption (g/m²)	Appearance after baking
		Metal	Content (mass%)	K₂O amount (mass%)	Average particle diameter (nm)	Content (mass%)	SiO₂ amount (mass%)	Na₂O amount (mass%)	Chromate	K₂O/SiO₂ ratio (%)	Amount of moisture absorption (g/m²)	Appearance after baking
E1	B5	Al	54.8	2.3	4	45.2	45.0	0.20	-	5	0.05	very good
E2	B5	Al	54.8	2.3	12	45.2	45.0	0.21	-	5	0.06	very good
E3	B5	Al	54.8	2.3	18	45.2	45.0	0.19	-	5	0.04	good	Inventive example
E4	B5	Al	54.8	2.3	22	45.2	45.0	0.20	-	5	0.05	good	Inventive example
E5	B5	Al	54.8	2.3	31	45.2	45.0	0.21	-	5	0.04	good
E6	B6	Al	54.8	2.3	35	45.2	45.0	0.19	-	5	0.06	good

As can be seen from Tables 3 to 5, in the inventive examples, the insulation coating treatment liquid satisfied the specified ranges of the present invention, and the grain-oriented electrical steel sheet on which the insulation coating was formed using the insulation coating treatment liquid of the inventive example had good moisture absorption resistance and excellent coating appearance.
In contrast, in the comparative examples, at least one item in the insulation coating treatment liquid was outside the scope of the present invention, and the moisture absorption resistance or the appearance of the insulation coating of the grain-oriented electrical steel sheet was inferior.

Claims

An insulation coating treatment liquid for a grain-oriented electrical steel sheet, comprising:
metal phosphate of one or more metals selected from among K, Al, Fe, Mg, Mn, Ni, Zn, Co, Mo, V, W, and Zr, and colloidal silica,

wherein a K₂O/SiO₂ ratio in mass% between an amount of K in terms of K₂O and an amount of Si in terms of SiO₂ in the insulation coating treatment liquid is 0.5% or more and 10% or less.
A method for producing an insulation coating treatment liquid for a grain-oriented electrical steel sheet, comprising:
an adding process in which a K compound is added to metal phosphate of one or more metals selected from among Al, Fe, Mg, Mn, Ni, Zn, Co, Mo, V, W, and Zr, and

a mixing process in which the colloidal silica and the metal phosphate with the added K compound are mixed to make a K₂O/SiO₂ ratio in mass% between an amount of K in terms of K₂O and an amount of Si in terms of SiO₂ in the insulation coating treatment liquid 0.5% or more and 10% or less.
A method for producing a grain-oriented electrical steel sheet, comprising:
a coating process in which an insulation coating treatment liquid is applied to a surface of a steel sheet, and

a baking process in which the insulation coating treatment liquid is baked to form an insulation coating,

wherein the insulation coating treatment liquid is the insulation coating treatment liquid for a grain-oriented electrical steel sheet according to claim 1, and

wherein in the baking process, the baking soaking temperature is 800 to 1000°C and the soaking holding time is 10 to 60 seconds.