JP2579714B2 - Method for forming insulating film on mirror-oriented silicon steel sheet - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of forming an insulating film on a silicon iron plate with good adhesion in a process of manufacturing a grain-oriented silicon steel sheet without a forsterite film (hereinafter referred to as a grain-oriented electrical steel sheet). Thus, the grain-oriented electrical steel sheet obtains the tensile tension due to the coating, thereby providing an electromagnetic material with low iron loss.

[0002]

2. Description of the Related Art Grain-oriented electrical steel sheets are widely used for iron cores of transformers and the like. It has been required to reduce iron loss from the viewpoint of energy saving. Although many improvements have been made to Fe-Si metal itself, it is known that grain-oriented electrical steel sheets reduce iron loss when tension is applied to the steel sheet. A method of applying tension using the difference has been put to practical use.

For example, Japanese Patent Publication No. 59-15988 discloses that MgO is used as an annealing separator for finish annealing, and colloidal silica is added thereto to improve a forsterite film and improve magnetic properties. The addition of this colloidal silica has an effect on so-called secondary recrystallization, but increases the amount of forsterite film formed, increases the tension of the steel sheet by this film, and improves (reduces) iron loss. .

A method has also been proposed in which a phosphoric acid-chromic acid-colloidal silica-based insulating film is applied on the forsterite film and the tension of the film is increased (Japanese Patent Publication No. 53-28375). ).

In recent years, finish annealing has been completed on a metal surface without forming a forsterite film by finish annealing of a grain-oriented electrical steel sheet, or the forsterite film generated by the finish annealing has been removed by pickling or the like to remove the metal surface. Attempts have been made to form a tension coating on the surface where the surface is exposed (JP-B-63-54767, JP-A-3-130377).

[0006]

The iron loss of a grain-oriented electrical steel sheet decreases as the tension increases. As a method of imparting tension by the coating, a substance having a smaller thermal expansion coefficient than that of the Fe-Si alloy may be formed on the steel plate. That is, after cooling from a high temperature, such as strain relief annealing after forming the coating or after processing the magnetic steel sheet, a tensile stress is generated in the Fe-Si alloy due to a difference in thermal expansion between the coating and the Fe-Si alloy. In order to increase the tensile stress, the amount of the film having a small coefficient of thermal expansion may be increased.

Heretofore, there has been a problem that when a large amount of phosphoric acid-chromic acid-colloidal silica-based insulating film is applied to increase the tension due to the film, the film is peeled off. The ion plating method and the like proposed in Japanese Patent Publication No.
Although a film having good adhesion can be formed on the -Si alloy base iron, a special apparatus is required, and there is a problem that the manufacturing cost is increased. An object of the present invention is to form an insulating coating film having good adhesion at low cost and to manufacture a grain-oriented electrical steel sheet having low iron loss.

[0008]

SUMMARY OF THE INVENTION The gist of the present invention is that the temperature of the steel sheet during the cooling process after the completion of the purification annealing in the finish annealing of the grain-oriented electrical steel sheet on which the forsterite film is not formed is 10%.
Exposure to an atmosphere containing oxygen or water vapor between 00 ° C and 200 ° C, and after cooling, applying a tension coating, such as phosphoric acid or phosphate, chromic anhydride or chromate and colloidal silica It is effective to apply a solution containing, and bake and dry at 300 to 950 ° C. for 10 seconds or more.

The details will be described below. Grain-oriented electrical steel sheets are usually made by decarburizing and annealing a magnetic steel sheet cold-rolled to a constant thickness, applying an annealing separator mainly composed of MgO, and finish-annealing the steel sheet. There is an annealing method in which a glass coating mainly composed of forsterite is formed, or a metal surface is formed by finish annealing without forming a forsterite coating using alumina or the like as an annealing separator.

The present invention aims at improving the film adhesion during the latter tension coating. According to the study of the present inventors, in an electrical steel sheet having forsterite on the steel sheet surface, the lower Fe-Si alloy interface of forsterite has significantly unevenness magnetically remarkably obstructing the movement of the domain wall, thereby reducing iron loss. It turned out to be worse. As is well known, the Fe-Si alloy interface needs to be smooth in order to reduce the iron loss of the grain-oriented electrical steel sheet, and various methods have been proposed as a manufacturing method.

For example, Si: 2.0 to 4.8% by weight, acid-soluble Al: 0.008 to 0.05% by weight, N ≦ 0.0
10% by weight, a silicon hot-rolled steel strip comprising the balance of Fe and unavoidable impurities is annealed as necessary, and then cold-rolled once or twice or more with intermediate annealing to a predetermined thickness. After performing primary recrystallization annealing, apply an annealing separator, and then perform finish annealing.In the production of grain-oriented electrical steel sheets, after primary recrystallization annealing, the oxide film on the steel sheet surface generated in the annealing step is removed by pickling or the like. , atmosphere annealing separator as Al ₂ O ₃ (alumina) is N _2: 0 to 95%, grain-oriented electrical steel sheet having metallic surface (mirror surface) is obtained by performing finish annealing of the balance H _2.

A method of applying an ordinary annealing separator mainly composed of MgO and finish annealing, and removing the forsterite film by pickling and mechanically polishing the obtained electromagnetic steel sheet having forsterite, for example, a method of smoothing. However, there are defects such as complicated processes and high costs.

[0013] Based on these facts, the present inventors consider tension coating of a material (magnetic steel sheet) having a metal surface after finish annealing without forming a forsterite film using an annealing separator mainly composed of alumina. It was to be. The following methods are available for the finish annealing method that does not form forsterite.

That is, the forsterite forming reaction is stopped (delayed) without forsterite raw material being placed on the steel sheet surface. In this case, a surface oxide layer mainly composed of SiO ₂ (silica) formed by decarburizing annealing is removed, or MgO is not used as an annealing separator.
In this case, the MgO particles are made larger and the reaction area is made smaller.
As another modification, a substance that does not easily react with SiO ₂ , for example, alumina, beryllia, or the like is used as the annealing separator.

The present inventors have proposed a combination with the method for perfection, that is, decarburization annealing and then pickling to remove the decarboxylation film, and apply a material which is difficult to react with silicon such as alumina as an annealing separator. It is recommended to perform finish annealing.

The grain-oriented electrical steel sheet having a metal surface is coated with a tension for interlayer insulation and for imparting tension. However, there is a disadvantage that the coating is peeled off when the coating amount is increased to increase the tension. The present inventors have studied this improvement and found an extremely effective means.

As is well known, in the finish annealing of grain-oriented electrical steel sheets, the temperature is raised to around 1200 ° C. after the completion of the secondary recrystallization, where it is kept for about 20 hours for purification and then cooled.
Attempt was made to improve the adhesion of the tension coating by modifying the metal surface and specializing the atmosphere in a specific temperature range of this temperature drop process.

FIG. 1 is an example showing a change in the temperature in the coil during finish annealing of the grain-oriented electrical steel sheet 11T coil. The figure shows the outer peripheral portion of the coil and the temperatures at the coldest point during heating and heating and the hottest point during cooling. The positions of the coldest point at the time of heating up and the hottest point at the time of cooling down are almost the same. The inventors reproduced the heat cycle at the time of cooling in a laboratory and made various studies.

In the normal annealing, 100% hydrogen is used in the purification at around 1200 ° C., but when the temperature of the coil is lowered to a certain extent at the time of cooling, the temperature is switched to N ₂ , and furthermore, a step in which oxidation of the steel sheet does not occur. To switch to air. Fig. 2 shows the temperature cycle of Fig. 1 in the laboratory. After completion of the purification annealing (1200 ° C), a 100% nitrogen atmosphere was added, 0.5% air was mixed in the atmosphere, and after cooling, tension coating was applied. It shows the adhesiveness of the film.

The horizontal axis represents the temperature of the steel sheet when air was mixed, and the vertical axis represents the adhesion of the coating. As an index of the adhesion of the film, the film was bent 180 degrees while being wound around a round bar having a diameter of 20 mm, and the state of peeling of the film at the bent portion was observed. The larger the proportion of the film remaining area, the better the adhesion.

The tension coating has a dry weight of 4 g / m ^2.
Then, a treatment liquid having a composition of a 20% aqueous dispersion of colloidal silica: 120 g, a 50% aqueous solution of aluminum phosphate: 70 g, and a chromic anhydride: 6 g was applied and baked at 800 ° C. for 90 seconds and dried. It can be seen that the adhesion of the tension coating is good when the steel sheet temperature is from 200 ° C. to 1000 ° C.

The inventors of the present invention have found that the reason why the adhesion is good is that the third layer, which is located between the metal and the insulating film and has a coefficient of thermal expansion including iron between the metal and the insulating film, reduces the distortion. I think it's because. Note that, even after the purification annealing at 1200 ° C., the temperature was lowered with 100% hydrogen without switching to the nitrogen atmosphere, and water vapor was added, the same result as in FIG. 2 was obtained. The mixing amount of air and water vapor is 0.05% by volume or more in the case of air, and 0.01% by volume or more in the case of water vapor.

The tension coating will be described. A solution containing phosphoric acid or phosphate, chromic anhydride or chromate and colloidal silica, that is, a coating solution is a solution or slurry in which these components are mixed in an appropriate amount.

As the phosphate, Ca, Mg, Sr, A
Salts such as 1 are used. As chromates, Na,
Salts such as K, Ca, Mg, and Sr are used. The colloidal silica may be from SiO ₂ colloidal particles close to a solution to particles as large as a slurry, and the size of the particles is not particularly limited.

From the working properties during coating and the properties of the coating, phosphoric acid or phosphate: 3 to 24% by weight, chromic anhydride or chromate: 0.2 to 4.5% by weight, colloidal silica: 4 to 4% by weight. A solution consisting of 16% by weight and the balance: water (hereinafter simply referred to as a treatment liquid) is effective. It should be noted that these solutions may contain a slight amount of a surfactant, boric acid, etc. for improving workability and film properties.

Further, Ca is added to the phosphoric acid solution as a phosphate.
There is no problem in adding a hydroxide such as (OH) ₂ or an oxide such as CaO to form a phosphate.

Although the above-mentioned tension coating is most effective, a solution mainly composed of phosphate and chromic acid is also effective as a coating solution, but the tension seems to be slightly lower.
Further, a coating solution mainly composed of chromic acid and phosphate alone is also effective, but there is a point that various properties as a coating of a grain-oriented electrical steel sheet are insufficient.

For example, a film mainly composed of phosphate alone has a decrease in tension and a decrease in corrosion resistance, and a film mainly composed of chromic acid alone has a decrease in interlayer insulation resistance and a decrease in tension. From the above viewpoints, the most excellent coating solution is a solution containing phosphoric acid or phosphate, chromic anhydride or chromate and colloidal silica as described above, that is, a coating solution.

The film formation is achieved at a baking and drying temperature of 300 ° C. to 950 ° C., and a temporary film is formed.
If the temperature is less than 0 ° C., the baking / drying temperature is preferably 700 ° C. or more in consideration of the reduction of the tension effect due to the reduction of the temperature difference and the following strain relief annealing temperature, and the optimal range is 700 ° C. to 950 ° C. The reason why the temperature is set to 700 ° C. or higher is to completely form the coating film.
Since the film is kept at 0 ° C. for several hours, inadequate formation of a film at this time causes inconvenience such as image sticking. On the other hand, if the temperature exceeds 950 ° C., the steel sheet is oxidized, and the adhesion of the film due to the scale (iron oxide) is deteriorated.

Other examples of the tension coating method include ion plating, low-pressure plasma spraying, and CVD, which are effective because they can form a film with good adhesion on Fe-Si alloy base iron without forsterite, but are special. There is a problem that a device is required and the manufacturing cost is increased.

[0031]

【Example】

Example 1 Si: 3.2% by weight, acid-soluble Al: 0.027% by weight, N: 0.008% by weight, Mn: 0.013% by weight,
S: 0.007% by weight, C: 0.05% by weight, balance Fe
And hot rolled steel strip consisting of unavoidable impurities at 1100 ° C
And then cold-rolled to a thickness of 0.23 mm. These cold-rolled sheets were annealed at 830 ° C. for 2 minutes in an annealing furnace in a humid water atmosphere to serve as decarburization, and primary recrystallized. Next, in order to stabilize the secondary recrystallization, nitriding treatment was performed in an ammonia atmosphere to make the total nitrogen amount 200 ppm, and the inhibitor was strengthened. Thereafter, the resultant was pickled with a mixed solution of 0.5% hydrofluoric acid and 5% sulfuric acid.

This material is electrostatically coated with an annealing separator consisting of 100% Al ₂ O ₃ , and is heated to 1200 ° C. and 25% N ₂
In a -75% H ₂ atmosphere, the temperature was raised while maintaining a temperature rising rate of 30 ° C./Hr. After reaching 1,200 ° C., the temperature was changed to 100% hydrogen and maintained at that temperature for 10 hours. After completion of holding at 1200 ° C., the temperature was lowered, the atmosphere was switched to 100% nitrogen at 800 ° C., the temperature was further lowered to 700 ° C., 1% of air was mixed in, and 100 ° C.
For comparison, the atmosphere was further cooled to 100 ° C. even after 700 ° C. with nitrogen only. Then, it was uncovered, exposed to air, and cooled to room temperature.

After cooling, the steel sheet is coated with 20% colloidal silica.
An aqueous dispersion: 120 g, a 50% aqueous solution of aluminum phosphate: 70 g, and a treatment liquid having a composition of 6 g of chromic anhydride were baked and dried at 800 ° C. for 90 seconds to a coating dry weight (coating amount) of 5 g / m ^2. Bake and dry. Table 1 shows the peeled state of the coating.

[0034]

[Table 1]

As described above, according to the method of the present invention, the adhesion of the coating is extremely good.

Further, the treatment liquid is applied and baked and dried for 8 hours.
90 ° C., 90 seconds, dry weight of coating (coating amount) 3 g / m ²
It was baked and dried. The total amount of the insulating coating was 8 g / m ² , and peeling started without bending. However, according to the present invention, the adhesion of the coating was good, and the iron loss of the magnetic steel sheet was reduced due to the increase in tension. This situation is shown in Table 2.

[0037]

[Table 2]

[0038]

Industrial Applicability According to the present invention, there is provided a grain-oriented electrical steel sheet having improved insulating properties, capable of imparting a large tension to the electrical steel sheet and having improved magnetic properties, that is, having low iron loss. Is overwhelming.

[Brief description of the drawings]

FIG. 1 is a chart showing a relationship between a temperature in a coil and a time in finish annealing of a grain-oriented electrical steel sheet 11T coil.

FIG. 2 is a chart showing a relationship between a steel sheet temperature and a film remaining amount when a 0.5% air is mixed in a 100% nitrogen atmosphere.

Claims (2)

(57) [Claims] In a finish annealing of a grain-oriented silicon steel sheet on which a forsterite film is not formed, after completion of purification annealing,
A method for forming an insulating coating on a grain-oriented silicon steel sheet, comprising exposing the steel sheet to an atmosphere containing oxygen or water vapor at a temperature of 1000 ° C. to 200 ° C. in a cooling step, and applying a tension coating after cooling. 2. A patent wherein the tension coating is performed by applying a solution containing phosphoric acid or phosphate, chromic anhydride or chromate and colloidal silica, and baking and drying at 300 to 950 ° C. for 10 seconds or more. The method for forming an insulating coating on a mirror-oriented silicon steel sheet according to claim 1.