JPH09272983A - Production of low core loss grain oriented silicon steel sheet excellent in corrosion resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the magnetic properties of a steel sheet and to make better its water resistance and corrosion resistance as well by coating a silicon steel sheet with a fine particle-dispersed soln. contg. a specified amt. of aluminum oxide precursory compounds and soluble boric acid, raising its temp. to a specified one at a specified rate, drying and solidifying the same, executing baking at a specified temp. in a specified atmosphere and forming insulating coating of aluminum boride. SOLUTION: The solid content of a fine particle-dispersed soln. is regulated to 10 to 40wt.%, and the balance is composed of a solvent. The solid content is composed of aluminum oxide precursory compounds by 74 to 88wt.% expressed in terms of aluminum oxide and soluble boric acid by 12 to 26wt.% expressed in terms of boron oxide. The surface of a grain-oriented silicon steel sheet contg. 5wt.% Si and subjected to finish annealing is coated with the dispersed soln. In a heat treating stage, its sheet temp. is raised to 300 deg.C at 15 to 150K/sec, and drying and solidifying are executed. It is baked at 500 to 1350 deg.C in an atmosphere of an inert gas or in a reducing atmosphere contg. hydrogen to form insulating coating of aluminum boride. Since the crystals of boric acid in the coating is fine, it is chemically stable.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention reduces iron loss by providing an aluminum borate coating on the surface of a steel sheet, which imparts a higher tension to the steel sheet, and also provides chemical stability such as water resistance and corrosion resistance. The present invention relates to a method for manufacturing an excellent unidirectional silicon steel sheet.

[0002]

2. Description of the Related Art One-directional silicon steel sheets are (100) [00
1) has a crystal structure with the main direction being [1] and is often used as a magnetic iron core material. In particular, a material having a small iron loss is required to reduce energy loss. To reduce the iron loss of a unidirectional silicon steel sheet containing 5% or less of silicon, it is effective to apply a tension to the steel sheet, and the iron loss can be effectively reduced by applying a tension up to about 1.5 kgf / mm ^2. It is known that it can be reduced. This tension is usually applied by a film formed on the surface.

Conventionally, for unidirectional silicon steel sheets, a primary coating mainly composed of forsterite formed by a reaction between an oxide on the surface of the steel sheet and an annealing separating agent in the finish annealing step, and Japanese Patent Laid-Open No. 48-39338. 1.0 kgf / in the case of a plate thickness of 0.23 mm due to the two-layer coating formed by baking the coating liquid mainly containing colloidal silica and phosphate disclosed in Japanese Patent Publication No. Tension of about mm ² is applied.

Therefore, in the case of these current coatings, although there is still room for improvement of iron loss by imparting a larger tension, increasing the imparting tension by thickening the coating is not preferable because it causes a decrease in space factor. A new high-strength coating was needed.

On the other hand, the inventors of the present invention disclosed in Japanese Patent Laid-Open No. 6-65.
No. 754, JP-A-6-65755, etc., a method for forming an oxide film by coating a fine particle dispersion containing alumina sol and boric acid, drying and gelling, and baking, and aluminum oxide obtained thereby. -We have proposed boron oxide composite coatings and aluminum borate coatings.

It has been found that this coating can impart a higher tensile strength to the steel sheet than ever before and, as a result, can improve the magnetic properties of the steel sheet. Further, according to this manufacturing method, good adhesion can be ensured even for a steel sheet without a primary coating, which is expected to have a lower iron loss, or a steel sheet subjected to a mirror finish, and a remarkable improvement in magnetic properties is achieved. I'm finding out what I can do.

However, this coating has a drawback that the chemical stability such as water resistance and corrosion resistance is not sufficient depending on the production conditions. To solve this problem, the inventors have already disclosed in JP-A-7-252666 that a compound such as iron, cerium, or silicon is added and reacted with residual B ₂ O ₃ in the coating to replace it with a stable compound. Further, JP-A-7-278829 discloses that water resistance can be improved by controlling the oxygen potential in the baking process.

[0008] Although these proposals improve the water resistance of the coating film, the corrosion resistance, that is, the generation of rust is greatly improved, but when exposed to an atmosphere of relatively high temperature and high humidity for a long time, rust is still generated. May be recognized,
This improvement was needed.

[0009]

DISCLOSURE OF THE INVENTION The present invention is a method of improving chemical stability such as water resistance and corrosion resistance of an aluminum borate coating film which can obtain a unidirectional silicon steel sheet having a lower iron loss than the conventional one by the effect of imparting high tension. The purpose is to provide.

[0010]

The present invention provides an aluminum oxide precursor compound having a solid content residual ratio after solvent removal of 10 to 40% by weight and having a solid content of 74 to 88% by weight in terms of aluminum oxide. 12 to 2 in terms of boron oxide
A fine particle dispersion containing 6% by weight of soluble boric acid was added to 5
After being applied to the surface of the finish-annealed unidirectional silicon steel sheet containing Si by weight or less, the temperature of the sheet is raised to 300 ° C. at 15 to 150 K / sec in a series of heat treatment steps to cause drying and solidification. And a method of forming an insulating coating film of boron aluminum by baking at 500 to 1350 ° C. in a reducing atmosphere containing inert gas or hydrogen.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION In order to improve the water resistance and corrosion resistance of an aluminum borate coating, the inventors have investigated the cause. As a result, it was found that the cause is that when the coating surface is exposed to an environment where it is wet with water, or when it is exposed to a wet atmosphere, the acidic component gradually elutes from there and becomes an acidic environment.

There are several possible causes for the elution of the acidic component from the steel sheet having the coating film. One of the causes is to find out that it is the boron oxide / boric acid remaining without reacting with the aluminum oxide component in the coating film. There is. JP 7
The technology disclosed in Japanese Patent Application Laid-Open No. 252666/1992 is based on this residual boron oxide
It is based on the idea of reacting boric acid with an additive to form a chemically stable compound and improving the stability of the entire film.

On the other hand, according to the present invention, by rapidly drying the coating solution, the boric acid crystals generated during drying are made into a fine state,
This is based on the technical idea of suppressing fluctuations in the concentrations of boron and aluminum as much as possible to realize a uniform mixed state, enabling reactions at lower temperatures and in a shorter time, and eliminating unreacted boron oxide / boric acid. It is a thing.

As a relatively preferred embodiment for forming an aluminum borate coating, the inventors have shown that a water-soluble boric acid component is used. In the case of a coating raw material in which this and an aluminum oxide precursor compound, and also additives are mixed as necessary, except when a water-soluble aluminum compound such as aluminum salts is used as the aluminum oxide precursor compound, depending on the drying conditions, it may be a single substance during drying. Of coarse boric acid crystals are precipitated and the above-mentioned concentration fluctuation occurs.

The adverse effects caused by the precipitation of coarse boric acid crystals are, in addition to the chemical stability of the film, a decrease in the film tension and a decrease in the space factor due to an increase in surface irregularities. It is an important characteristic. As described above, the present invention suppresses the precipitation of coarse boric acid crystals that cause concentration fluctuations, and aims to precipitate in the finest possible state, and the basis thereof is to change the temperature of the steel sheet from the application temperature to 300 ° C. Up to the temperature range in which boric acid precipitates as the coating dries, 15 to 150 K /
The point is that the temperature rises at a relatively fast rate of seconds.

Originally, it is the temperature history of the coating solution, that is, the rate of temperature rise, that determines the degree of precipitation of boric acid. However, according to the results of studies by the inventors, the temperature of the coating solution and the temperature of the steel sheet at the time of temperature rise are It is almost the same, and in the present invention, it is defined by the heating rate of the steel sheet which is easier to measure.

Strictly speaking, the temperature defining the rate of temperature rise of the present invention is 100 ° C., and the temperature rise curve (time-temperature curve)
It is desirable to define it by the slope at 100 ° C. in (1) that the measurement is practically difficult, and (2) the state of precipitation of boric acid in the dry film, which is the object of the present invention, is Since the object is sufficiently achieved even if the average heating rate of the claims is substituted, the average heating rate of the steel sheet from the coating temperature to 300 ° C. is defined.

It was confirmed that boric acid was finely dispersed in the dry coating within this temperature raising rate range, and as a result, the aluminum borate forming reaction was promoted and the water resistance and corrosion resistance were improved. The present invention has been completed. Conventionally, it was difficult to raise the temperature within the above range because the film was formed at a solid content concentration of about 5 to 10%, depending on the composition.

The fine particle dispersion of the present invention contains 10 to 40
Contains wt% solids. In this case, the solid content is 1
It is the concentration of the solid content remaining after heating to about 00 ° C. to remove the solvent, and the boric acid dissolved and present in the fine particle dispersion is also calculated as the solid content.

When the heating rate is less than 15 K / sec, a single coarse boric acid crystal is likely to be formed depending on the boric acid concentration in the fine particle dispersion. In a relatively wide range of boric acid concentration, the preferable heating rate for suppressing the formation of boric acid crystals is 20.
K / sec or more. On the other hand, when the heating rate is higher than 150 K / sec, the corrosion resistance of the present invention is not impaired at all, but the evaporation of the solvent during drying becomes too rapid and boiling causes boiling to cause defects on the coating surface. .

The value of the heating rate is 3 from the coating temperature.
The temperature rise (temperature difference) ΔT up to 00 ° C. is divided by the time t during which the actual temperature rise was occurring. FIG.
As shown in FIG. 5, during the temperature increase of ΔT, in addition to t, there is actually a time t ′ during which the temperature is kept constant during the evaporation of the solvent, and t + t ′ has elapsed. However,
The temperature rising rate of the present invention is not ΔT / (t + t ′) but Δ
It is defined by T / t.

The solid content concentration of 10 to 40% by weight is not particularly limited to this, but if it is less than 10% by weight, it is difficult to achieve the above temperature rising rate due to boiling and bumping of the solvent.
If it exceeds 0% by weight, the stability of the liquid will be impaired, and application will be difficult. A particularly preferable upper limit of the concentration is 35% by weight.

The proportion of aluminum oxide precursor compound and boric acid contained in the solid content is 7 in terms of aluminum oxide.
4 to 88% by weight and 12 to 26% by weight in terms of boron oxide. This is based on the idea that no boron oxide / boric acid component is left after baking as described above.

In order to reduce unreacted boron oxide / boric acid in the coating after baking to the extent that rusting is not affected, the molar ratio of Al ₂ O ₃ / B ₂ O ₃ About 2 or more, 74% by weight in terms of aluminum oxide conversion
The inventors have found that it is necessary to set the composition of boehmite and boric acid of 26% by weight or less in terms of boron oxide.

When the content of boric acid is higher than this, the amount of boron oxide remaining without reacting with the aluminum oxide component is large, and the corrosion resistance is not improved even if baking is performed at the heating rate of the present invention. Further, in the composition of more than 88% by weight of boehmite in terms of aluminum oxide and less than 12% by weight of boric acid in terms of boron oxide, γ-Al ₂ O ₃ becomes a coating film containing γ-Al ₂ O ₃ as a main component especially when baked at a low temperature. Therefore, high tension cannot be obtained.

Here, the aluminum oxide precursor compound is a general term for compounds that become aluminum oxide after baking, and not only aluminum oxide but also aluminum oxide represented by Al ₂ O ₃ .XH ₂ O such as boehmite. Refers to hydrates, aluminum hydroxide and the like. Various aluminum salts such as aluminum nitrate and aluminum chloride are also preferably used.

The coating liquid may contain components other than the aluminum oxide precursor compound and soluble boric acid, for example, trace amounts of additives. Additives that are commonly used include silicon oxide precursor compounds, iron compounds, alkali metal compounds, alkaline earth metal compounds, and rare earth element compounds.

The fine particle dispersion liquid prepared as described above is added to 5
It is applied to the surface of the unidirectional silicon steel sheet which contains Si by weight or less and which has been subjected to finish annealing, by a conventionally known method such as a coater such as a roll coater, a dipping method, spraying, or electrophoresis. The coating method is not particularly limited, and an optimal method may be selected according to the properties of the coating liquid and the like.

The steel sheet which has been subjected to the finish annealing as referred to herein means (1) a steel sheet having a forsterite primary coating formed on its surface by performing finish annealing by a conventionally known method.
(2) Steel sheet obtained by immersing the primary coating and incidentally formed interfacial oxide layer in an acid to remove it, (3) Steel sheet obtained by flattening annealing the steel sheet obtained in (2) above in hydrogen, or chemical A steel sheet that has been subjected to polishing, such as polishing or electrolytic polishing, (4) a conventionally known annealing separator to which a trace amount of additives such as alumina powder or the like which is inert to film formation, or chloride is added,
It refers to a steel sheet that has been subjected to finish annealing under conditions that do not produce a primary coating.

The coating amount is 2% or less of the steel plate thickness per one surface after baking. When the coating film exceeds 2% of the steel plate thickness, the iron loss reducing effect by applying tension is almost constant and saturated, and the space factor is rather reduced. A more preferable film thickness is 1.5% or less of the steel plate thickness per one surface.

The coated steel sheet is heated up to 300 ° C. in the above-mentioned heating rate range to cause gelation and solidification, and subsequently, heat treated, and finally, reducing property containing inert gas or hydrogen. In an atmosphere, baking is performed at 500 to 1350 ° C. to form an insulating film made of boric acid and an oxide of aluminum oxide. Baking in an oxidizing atmosphere is not preferable because the steel sheet may be oxidized.

If the baking temperature is less than 500 ° C., the applied boehmite and / or aluminum hydroxide may not be sufficiently decomposed to form an oxide.
If it exceeds 350 ° C, it is not economical although there is no particular inconvenience. More preferably, the temperature range is 550 to 1250 ° C.

When baking is carried out at a temperature near 500 ° C., crystalline aluminum borate is not produced in many cases, and it is often a mixture of aluminum borate precursor substance and / or boric acid and aluminum oxide. Even in this case, as compared with the coating film containing aluminum phosphate and colloidal silica as the main components, a larger tension is applied to the steel sheet.

[0034]

【Example】

(Example 1) A boric acid reagent on the market and a primary particle size of 6
Hexagonal plate boehmite sol with almost no aggregation at 0 nm,
A mixture with a 10 × 100 nm fibrous boehmite sol (mixing is 1: 1 by weight) was mixed at 25 parts by weight and 75 parts by weight in terms of respective oxides to prepare a fine particle dispersion. . The solid content concentration in the fine particle dispersion is 18.
It was 3%.

This was made to have a thickness of 0.1% containing 3.2% of Si.
It was applied to a unidirectional silicon steel sheet having a 2 mm forsterite coating and having undergone finish annealing so that the film thickness after baking would be about 2 μm per side. The coating temperature is
It is about 50 ° C.

Immediately thereafter, by loading into a furnace set to various temperatures, it is heated up to 300 ° C. at a heating rate of several conditions, dried and gelled, and finally H ₂ is added to 3 ° C. v
The aluminum borate film was formed by baking for 1 minute at 850 ° C. in an N ₂ atmosphere containing ol%. The rate of temperature increase was measured by directly attaching a thermocouple to the steel plate and changing the setting conditions of the furnace.

The obtained coating contains 70% or more of aluminum borate, most of which is crystalline (2Al ₂
O ₃ · B ₂ O ₃ ). Table 1 shows the characteristics of the surface coating and the unidirectional silicon steel sheet on which the coating is formed. The adhesion of the coating is 180 mm around a 20 mmφ cylinder.
The wrapping test was performed so that the degree of contact was constant, and the peeling condition was evaluated.

Among the corrosion resistance, the water resistance was measured by measuring the weight change of the test material immersed in boiling distilled water for 60 minutes, and expressed as the weight reduction ratio with respect to the amount of the immersion coating. The rust resistance was evaluated by the rust generation start time in a constant temperature and constant humidity atmosphere of 80 ° C. and 98% RH. Water resistance was a weight reduction of about 1 wt% or less, and corrosion resistance was a pass level unless rust was generated for 48 hours or more. The coating tension, B ₈ and W _17/50 were average values of 10 test materials. In Table 1, No. 1 to 5 are examples of the present invention, and the other two types are comparative examples.

[0039]

[Table 1]

From Table 1, it can be seen that the corrosion resistance of the coating is mainly improved by setting the rate of temperature increase up to 300 ° C. after coating to 15 to 150 K per second. The film tension and magnetic properties show good values irrespective of the heating rate, and the heating rate from the coating temperature to 300 ° C. is 15 to 15
By setting it to 0 K / sec, a unidirectional silicon steel sheet in which the corrosion resistance of the coating film and the magnetic characteristics of the steel sheet are compatible is obtained.

Example 2 Commercial boric acid reagent, and 1
Hexagonal plate-shaped boehmite sols having a secondary particle diameter of 60 nm and almost no aggregation were mixed at 15 parts by weight and 85 parts by weight in terms of respective oxides to prepare a fine particle dispersion. The solid content concentration in the fine particle dispersion was about 30%. This was applied to a unidirectional silicon steel sheet having a 0.2 mm thick forsterite coating containing 3.2% Si and having undergone finish annealing, and the film thickness after baking was about 2 μm per side.
It applied so that it might become. The coating temperature was about 60 ° C.

Thereafter, as in Example 1, the sample was heated to 300 ° C. at various heating rates, dried and gelled, and finally 850 ° C. in an N ₂ atmosphere containing 1 vol% of H _2. ,
It was baked for 30 seconds to form an aluminum borate coating film. The obtained coating film contained 70% or more of aluminum borate and was partially crystallized (2Al ₂ O ₃ .B ₂ O ₃ or 9Al ₂ O ₃ .2B ₂ O ₃ ). Surface coating,
The characteristics of the coated unidirectional silicon steel sheet are shown in Table 2. The coating adhesion, water resistance and rust resistance were evaluated in the same manner as in Example 1. The film tension, B ₈ and W _17/50 were the average values of 10 test materials. In Table 2, No. 1-5
Is an example of the present invention, and the other two types are comparative examples.

[0043]

[Table 2]

According to Table 2, when the present alumina sol is used, the temperature rising rate up to 300 ° C. after coating is 15 per second.
It can be seen that the corrosion resistance and the film tension of the film are improved by setting the value to 150 K. Also under the conditions of this example, a unidirectional silicon steel sheet having both the corrosion resistance of the coating film and the magnetic properties of the steel sheet was obtained by setting the heating rate from the coating temperature to 300 ° C. to 15 to 150 K / sec.

[0045]

EFFECTS OF THE INVENTION The unidirectional silicon steel sheet produced by the method of the present invention exhibits good magnetic properties by having an aluminum borate coating on its surface, which gives a greater tension than before.
In addition, it has excellent chemical stability such as water resistance and corrosion resistance.
In addition, the method for producing a unidirectional silicon steel sheet of the present invention is the same as the conventional method for forming a coating, which involves the same coating and baking, and is simple. The effect is enormous from the viewpoint of mass productivity and versatility.

[Brief description of drawings]

FIG. 1 is a schematic diagram of a temperature rising curve of a steel sheet, showing an example in which water is used as a dispersion medium.

Front page continuation (72) Inventor Shuichi Yamazaki 20-1 Shintomi, Futtsu City Nippon Steel Co., Ltd. Technical Development Division

Claims (1)

[Claims] 1. A solid content residual ratio after solvent removal is 10 to 4
5% by weight of a fine particle dispersion containing 0% by weight and a solid content of 74 to 88% by weight of aluminum oxide precursor compound in terms of aluminum oxide and 12 to 26% by weight of soluble boric acid in terms of boron oxide. % Of Si up to 300 ° C. in a series of heat treatment steps after being applied to the surface of the finish-annealed unidirectional silicon steel sheet.
In a reducing atmosphere containing an inert gas or hydrogen, which is heated and dried at 150 K / sec to cause drying and solidification,
A method for producing a low iron loss unidirectional silicon steel sheet having excellent corrosion resistance, which comprises forming an insulating coating made of boron aluminum by baking at 500 to 1350 ° C.