JPH0762551A - Magnetic steel sheet excellent in rust resistance after stress relieving annealing - Google Patents

Abstract

PURPOSE:To improve the rust resistance after stress relieving annealing of a magnetic steel sheet having a chromic acid-org. resin insulating film on the surface. CONSTITUTION:A processing soln. formed by mixing an aq. chromic acid soln. contg. soluble aluminum hydroxide, boric acid, a compd. of bivalent metal (Mg, Ca, Zn, etc.), if necessary, an aq. emulsion resin and an org. reducing agent (e.g. glycerin) is applied on the surface of a steel sheet consisting of, by weight, <=0.01% C, <=4% Si, 0-2.0% Al, <=5% (Si+Al), <=0.5% Mn, 0.01-0.1% Cu and the balance Fe with inevitable impurities and baked to form an insulating film. In this case, the H3BO3-to-CrO3 molar ratio in the soln. is controlled to 0.1-1.5, the Me<2+>-to-Al<3+> molar ratio to <=7.0 and (Al<3+>+Me<2+>)-to-CrO3 molar ratio to 0.2-0.5. A magnetic steel sheet satisfying electric insulating properties, magnetic characteristic and other requirements and excellent in rust resistance is obtained in this way at a low cost.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic steel sheet having excellent resistance to rust after stress relief annealing.

[0002]

2. Description of the Related Art Generally, electromagnetic steel plates (also called electric iron plates) are used as iron cores for motors, transformers, etc. after undergoing processing steps such as laminating and TIG welding the ends. An electrical insulating coating is applied to the surface of the electromagnetic steel sheet in order to reduce iron loss due to eddy currents. The electrical steel sheet coated with the electric insulation film is punched into a predetermined shape and subjected to strain relief annealing as necessary.

The characteristics of this electric insulation coating are, in addition to electric insulation, punching resistance and seizure resistance (sticking resistance).
, Heat resistance, adhesion, corrosion resistance, oil resistance, CFC resistance, weldability, skewability, automatic caulking, high space factor, and resistance to rust after stress relief annealing are required. It
The chromium compound-organic resin-based coating has particularly good punchability and is often used as an electrical insulating coating for electromagnetic steel sheets.

In recent years, the tendency toward higher efficiency has been strengthened, especially in small equipment, and strain relief annealing has been adopted in most cases. When stress relief annealing is performed, the corrosion resistance of conventional insulation films is lost, and the occurrence of rusting is becoming a problem, and there is a strong demand for improvement in rust resistance after stress relief annealing. With respect to the compound-organic resin-based electric insulating film, some methods have been proposed to improve the rust resistance after stress relief annealing.

For example, in Japanese Patent Laid-Open No. 63-84671, in order to improve resistance to rusting after stress relief annealing, impurities such as halogen element and alkali metal in a chromic acid aqueous solution for forming an electric insulating coating are polarized. Limited to a very small amount. However, this method has a drawback that the cost becomes high because industrial chemicals and industrial water cannot be used.

Japanese Unexamined Patent Publication (Kokai) No. 62-124228 proposes a method of improving the rust resistance after strain relief annealing by applying Ni plating as a base treatment before forming an insulating film. However, an additional Ni plating step is required as a base treatment, which also has a drawback of high cost.

JP-A-3-36284 discloses a readily soluble aluminum compound (and, if necessary, a divalent metal compound).
There has been proposed a method of forming an electrically insulating coating by applying and baking a mixed solution of an aqueous chromic acid solution containing carboxylic acid and boric acid, an aqueous emulsion resin, and an organic reducing agent. This method is advantageous in terms of cost because it requires no base treatment, and the formed electrical insulating film has the above-mentioned necessary characteristics, and the rust resistance after stress relief annealing is also improved to some extent. There is. However, depending on the use environment, the occurrence of rusting may be observed even in the electrical insulating film formed by this method, and further improvement in rusting resistance after strain relief annealing has been demanded.

[0008]

As described above, the conventional electromagnetic steel sheets are not always satisfactory in terms of both cost and resistance to rusting after stress relief annealing. An object of the present invention is to provide an electrical steel sheet which can be manufactured at low cost and which has characteristics required for an electric insulating coating of an electrical steel sheet, and in which the rust resistance after pre-annealing is remarkably improved. A more specific object of the present invention is to provide an electrical steel sheet having further improved resistance to rusting after strain relief annealing by utilizing the electric insulating film described in JP-A-3-36284. .

[0009]

DISCLOSURE OF THE INVENTION As a result of investigations aimed at achieving the above object, the inventors of the present invention have found that the characteristics of the electric insulating film described in Japanese Patent Laid-Open No. 36284/1990, particularly the resistance to stress after annealing. We have noticed that the rust property varies greatly depending on the steel composition (chemical composition) of the base steel sheet. Fluctuating the rust resistance after stress relief annealing due to such a steel composition has hardly been studied so far.

More specifically, the higher the Mn content, the lower the rust resistance. Furthermore, when various alloy elements were added to the experiment, only when a small amount of copper (Cu) was added, various properties such as electrical insulation and magnetic properties were not affected, and the rust resistance was remarkable. It was an unexpected finding that it would be improved. The present invention is based on these findings.

The present invention is an electromagnetic steel sheet having a chromic acid-organic resin-based insulating coating on the surface, which comprises the following (1)
And (2) is a magnetic steel sheet having excellent resistance to rust after strain relief annealing.

(1) Steel plate, in% by weight, C: 0.01% or less,
Si: 4.0% or less, Al: 0 to 2.0%, (Si + Al): 5% or less,
Mn: 0.5% or less, Cu: 0.01 to 0.1%, balance: Fe and a steel composition composed of unavoidable impurities.

(2) The insulating film is formed by applying and baking a treatment liquid prepared by mixing an aqueous chromic acid solution, an aqueous emulsion resin, and an organic reducing agent, and the aqueous chromic acid solution is free chromic acid. A boric acid (H ₃ BO ₃ ) containing an easily soluble aluminum compound that dissolves by a neutralization reaction with, and the molar ratio of Al ³⁺ / CrO _{3 in} this aqueous solution is within the range of 0.2 to 0.5, H ₃ BO ₃ The molar ratio of / CrO ₃ is within the range of 0.1 to 1.5.

In one embodiment of the present invention, the chromic acid aqueous solution is further selected from the group consisting of divalent metal oxides, hydroxides, carbonates, chromates and dichromates. Alternatively, two or more kinds of compounds may be used in which the molar ratio of Me ²⁺ / Al ³⁺ is 7.0 or less (where Me ²⁺ is the divalent metal ion), (A
It is contained in an amount such that the molar ratio of l ³⁺ + Me ²⁺ ) / CrO ₃ is within the range of 0.2 to 0.5.

[0015]

In the present invention, the reason why each constituent element is limited as described above will be explained. In the following explanation,%
Is weight% unless otherwise specified.

Chemical Composition of Steel Sheet Since the present invention is an electromagnetic steel sheet, the base steel sheet must have good magnetic properties. C causes magnetic aging,
Since it is harmful to magnetic properties, it is limited to 0.01% or less.

Si is added in a relatively large amount in magnetic steel sheets in order to improve magnetic properties. Even in the present invention,
Add Si in an amount up to 4.0%. If Si exceeds 4.0%, cold rolling cannot be performed. The preferred Si content is 0.1
~ 3.0%.

Since Al also exhibits an effect of improving magnetic properties, Al can be added in an amount of 2.0% or less in combination with Si, if desired. If the Al content exceeds 2.0%, improvement in magnetic properties cannot be expected. When adding Al, (Si +
If the total amount of Al) exceeds 5%, cold rolling cannot be performed, so the total amount of (Si + Al) is set to 5% or less. Preferably, the Al content is 0.1-1.0%, and the total amount of (Si + Al) is 4.5.
% Or less.

Mn is also effective in improving the magnetic properties, but if it is added in a large amount, rusting tends to occur after stress relief annealing.
Therefore, the limit value at which the occurrence of rusting is remarkable is 0.5%
Add Mn in the following amounts. The preferred Mn content is 0.1-0.
3%.

In the present invention, Cu is added for the purpose of improving rust resistance. That is, only in the case where the insulating film is the one described in (2) above, by adding a small amount of Cu, it is possible to prevent the rust resistance after stress relief annealing without deteriorating the magnetic properties and the electrical insulation properties. Can be significantly improved. This effect is remarkably obtained when the Cu content is 0.01% or more. On the other hand, Cu
If the content exceeds 0.1%, although the effect of improving the rust resistance can be obtained, the magnetic properties deteriorate. Therefore, in the present invention,
Cu is added in an amount within the range of 0.01 to 0.1%. Preferred Cu
The content is 0.03 to 0.1%.

Insulating Film Chromic Acid Aqueous solution Chromic acid can form an insulating film by being reduced from Cr ⁶⁺ to Cr ³⁺ . The chromic acid aqueous solution used in the present invention can be prepared by dissolving chromic anhydride in water. The concentration of the chromic acid aqueous solution is not particularly limited, but normally, CrO ₃ is preferably in the range of 50 to 100 g / l.

In the preparation of the chromic acid aqueous solution, a divalent metal chromate and / or a dichromate, which will be described later, can be used instead of part of the chromic anhydride. In this case, chromate and dichromate are chromic acid (CrO ₃ )
At the same time, it is a source of divalent metal ions.

In the following explanation, the amount of CrO ₃ is not limited to the amount of chromic anhydride, but is also the total amount of chromic acid added as chromate or dichromate (amount converted to CrO ₃ amount). It is a thing. That is, when a part of chromic acid is added as chromate and / or dichromate, Cr
The O ₃ amount means an amount obtained by converting the total amount of Cr ⁶⁺ into the CrO ₃ amount.

Aqueous Emulsion Resin The treatment liquid used in the present invention contains an appropriate amount of an organic resin for improving punchability. An aqueous emulsion resin is used because the treatment liquid is an aqueous system. The compounding amount is not particularly limited, but is usually in the range of 10 to 100 parts by weight as a resin solid content with respect to 100 parts by weight of CrO ₃ in the chromic acid aqueous solution. If the amount of resin is too large, properties such as rust resistance and corrosion resistance deteriorate.

Any aqueous emulsion resin may be used, and one or more selected from various aqueous emulsion resins conventionally used in chromic acid-based treatment liquids can be used. Examples of such resins include acrylic resins and copolymers thereof, vinyl acetate resins and copolymers thereof, styrene resins and copolymers thereof, amino resins, alkyd resins, phenol resins, silicone resins, fluororesins, melamine. Examples thereof include resins, epoxy resins, polyvinyl alcohol, polyacrylamide, polyvinyl ether-maleic acid copolymers, polyhydroxyl ethyl cellulose and the like.

The preferred aqueous emulsion resin is vinyl acetate, acrylic resin, styrene resin, or copolymers thereof, and commercially available products can be used as they are.
Examples of such commercial products include Bonncoat 3515 and EC-818 manufactured by Dainippon Ink and Chemicals, and Acreset 210E manufactured by Nippon Shokubai Kagaku for acrylic resins;
EC710 and Axreset 250E manufactured by Nippon Shokubai Kagaku;
For vinyl acetate resin, Dainippon Ink and Chemicals Boncoat 9301
Etc. (These are all product names).

Organic reducing agent A reducing agent for reducing Cr ⁶⁺ to Cr ³⁺ for forming an insulating film is contained in the treatment liquid. An organic reducing agent containing no metal ion is used for this reduction. Examples of such organic reducing agents include polycarboxylic acids (eg, maleic acid, succinic acid, adipic acid, etc.), polyhydric alcohols (ethylene glycol, glycerin, polyethylene glycol, etc.), alkanolamines (monoethanol). Amine, diethanolamine, triethanolamine, etc.),
Examples thereof include oxycarboxylic acids (tartaric acid, etc.), but are not limited thereto. The amount of the organic reducing agent used may be an amount necessary for reducing Cr ⁶⁺ in the chromic acid aqueous solution by Cr ^3+, but is usually 10 parts with respect to 100 parts by weight of CrO _3.
Within the range of up to 100 parts by weight. If this amount is less than 10 parts by weight, reduction does not occur sufficiently, which may adversely affect the punching die, and Cr ⁶⁺ may remain to cause toxicity problems. On the other hand, if this amount exceeds 100 parts by weight, the punchability may deteriorate.

The presence of Al ³⁺ ions in the chromic acid aqueous solution of the readily soluble aluminum compound increases the insulation resistance of the formed film and improves the electrical insulation, so that the easily soluble aluminum compound is added to the chromic acid aqueous solution. Include. A typical example of the easily soluble aluminum compound is easily soluble aluminum hydroxide, but any aluminum compound that is easily soluble in an aqueous chromic acid solution can be used as long as it does not contain metal ions other than Al ³⁺ and divalent metal ions. Can be used.

The easily soluble aluminum hydroxide is disclosed in JP-A-50-
As described in JP-A Nos. 153799 and 59-205780, at least one of which is subjected to a neutralization reaction with an acidic substance and a basic substance which are aluminum compounds, and the precipitated aluminum hydroxide is immediately separated and washed with water to be used. It is obtained by removing alkali metal cations and inorganic anions as impurities derived from the raw materials. This easily soluble aluminum hydroxide has a structure cross-linked with CO ₂ groups or SO ₂ groups. The details are described in the above publication, so please refer to it.

Even if the divalent metal compound is added to the aqueous solution of the divalent metal compound chromic acid and Me ²⁺ is present, the insulation resistance of the film is increased. Therefore, in some cases, a divalent metal compound may be contained in the chromic acid aqueous solution in addition to Al ³⁺ . As the divalent metal compound, Ca,
One or more selected from oxides, hydroxides, carbonates, chromates and dichromates of metals such as Mg, Ba and Zn can be used. As mentioned above, chromate and dichromate are sources of both Me ²⁺ and CrO ₃ .

(Al ³⁺ + Me ²⁺ ) / CrO ₃ Molar Ratio (Al ³⁺ + Me ²⁺ ) / CrO ₃ Molar Ratio (Al ³⁺ / CrO ₃ Molar Ratio When Chromic Acid Aqueous Solution Does Not ^Contain Me ²⁺ ) Is less than 0.2 or more than 0.5, the stability of the resulting processing liquid deteriorates, causing nozzle clogging in the coating machine and adversely affecting punchability.Therefore, this molar ratio should be within the range of 0.2 to 0.5. And

The molar ratio of H ₃ BO ₃ / CrO ₃ boric acid (H ₃ BO ₃ ) is added to the chromic acid aqueous solution for the purpose of improving the rust resistance after strain relief annealing. It is considered that boric acid forms a vitreous film on the surface of a magnetic steel sheet by strain relief annealing, which leads to improvement of rust resistance. To obtain this effect, it is necessary to add boric acid in an amount such that the molar ratio of H ₃ BO ₃ / CrO ₃ is 0.1 or more. However, if this molar ratio exceeds 1.5, undissolved boric acid remains, and the adhesion of the insulating film deteriorates. Therefore, the addition amount of boric acid is within the range of 0.1 to 1.5 in terms of H ₃ BO ₃ / CrO ₃ molar ratio.

When a divalent metal compound having a molar ratio of Me ²⁺ / Al ³⁺ is added to the chromic acid aqueous solution, Me ²⁺
If the / Al ³⁺ molar ratio exceeds 7.0, the rust resistance after strain relief annealing deteriorates. This is because oxides of divalent metals produced by baking deteriorate their properties. Therefore,
In order to improve resistance to rusting, it is preferable that the addition amount of the divalent metal compound is as small as possible. When a divalent metal compound is added, the Me ²⁺ / Al ³⁺ molar ratio is 7.0 or less, preferably 5.0 or less.

Coating and baking treatment method The treatment liquid is a mixture of the above-mentioned chromic acid aqueous solution, an aqueous emulsion resin and an organic reducing agent, and, if necessary, diluted with water so as to have a viscosity suitable for the coating means adopted. It can be prepared by

The coating of the treatment liquid is carried out by any conventional coating means such as roll coating, spraying, dipping and curtain flow coating so that the surface of the steel sheet is uniformly coated. After application, 150 to 400 ℃, preferably 250 to 350 ℃
After baking for a short time (usually about 20 seconds to 2 minutes) in the oven, an insulating film is formed.

The thickness of the insulating film is not particularly limited, but is generally in the range of 0.1 to 2 μm. If it exceeds 2 μm, the adhesion tends to deteriorate, and if it is less than 0.1 μm, uniform coating tends to be difficult.

The magnetic steel sheet of the present invention having the insulating film thus formed is punched into a predetermined shape and then subjected to strain relief annealing before use. The strain relief annealing is a box annealing method or a continuous annealing method according to a conventional method, and is 720 to 80 in an inert gas atmosphere.
It can be carried out by heating to a temperature of 0 ° C. for 1 to 3 hours. As described above, the electromagnetic steel sheet of the present invention forms a chromium compound-organic resin-based insulating film formed from a specific treatment liquid on a steel sheet having a specific steel composition, so that rust does not occur even after stress relief annealing. It has the characteristic that it is unlikely to occur.

[0038]

EXAMPLES Next, the present invention will be described in detail based on examples. In the examples, parts and% are, unless otherwise specified,
Parts by weight and% by weight.

Example 1 A chromic acid aqueous solution prepared by dissolving 100 parts of chromic anhydride (CrO ₃ ) in 500 parts of pure water (ion-exchanged water) was added to easily soluble aluminum hydroxide, magnesium hydroxide, and borohydride. Acid was added. The amounts of these components added were: (Mg ²⁺ + Al ³⁺ ) / CrO ₃ molar ratio 0.4, H ₃ BO ₃ / CrO ₃ molar ratio 0.4, and Mg ²⁺ / Al ³⁺ molar ratio Is 0 (Al
³⁺ alone), 1.0, 3.0, 5.0, 7.0, 8.0, 9.0, 10.
Varyed to 0 and ∞ (Mg ²⁺ alone).

The readily soluble aluminum hydroxide used in this example was obtained by adding and stirring an aqueous solution of aluminum sulfate to an aqueous solution of sodium carbonate, washing the precipitated slurry aluminum hydroxide with water, and then dehydrating it. Aluminum hydroxide cross-linked with CO ₂ groups containing 8.20% ₂ O ₃ .

Acrylic-styrene type aqueous emulsion resin (Boncoat S410) was added to the aqueous solution of chromic acid in an amount of 15 parts as a solid content and 20 parts of ethylene glycol as a reducing agent to prepare a treatment liquid.

This treatment solution was applied by a roll coater to both sides of each of four types of steel sheets (each having a thickness of 0.5 mm) having the following chemical components, and baked for 1 minute in an oven at 280 ° C. An insulating film was formed. The thickness of the insulating film after baking was 0.5 μm. C: 0.005%, Si: 1.5%, Al: 0.3%, Mn: 0.2%, Cu: 0.0
5%, C: 0.005%, Si: 1.5%, Al: 0.3%, Mn: 0.8%, Cu: 0.0
5%, C: 0.005%, Si: 1.5%, Al: 0.3%, Mn: 0.2%, Cu: 0
%, C: 0.005%, Si: 1.5%, Al: 0.3%, Mn: 0.2%, Cu: 0.5
%.

Various evaluation tests described below were conducted on the magnetic steel sheet having the electric insulating film thus formed.
The test results are summarized in Table 1.

Strain relief annealing Each of the obtained magnetic steel sheets was cut into a predetermined size (30 × 280 mm)
Was divided into a large number of test materials. The length direction of the test material was set to be approximately the same as that in the rolling direction and that in the direction perpendicular to the rolling direction. A part of this test material was subjected to stress relief annealing at 750 ° C. for 2 hours in a 100% nitrogen atmosphere to obtain an annealed material.

Punching property Using a test material before annealing, mold material SKD-11, clearance 8%, meshing 0.5 mm, stroke number 350 spm, using kerosene as a punching oil, repeatedly punching a shape of 17 × 17 mm. It was The evaluation was performed according to the following criteria based on the number of punches when the burr height on any surface reached 50 μm. ◎: 2 million times or more, ○: 1.5 to 2 million times, △: 100
~ 1.5 million times, ×: less than 1 million times.

Interlayer insulation resistance JIS C 2550 was used by using both test materials before and after annealing.
The interlayer insulation resistance was measured according to (1986).

Adhesiveness The test material before and after annealing was used to bend the test material at various bending diameters by 180 °, and the minimum diameter at which the insulating film on the outside of the bent portion did not peel off with cellophane tape.
It was evaluated by (mm).

Corrosion resistance The test material before annealing was subjected to a salt spray test at 35 ° C. for 20 hours, and the rust generation condition was evaluated according to the following criteria.

⊚: No rust generated, ◯: Rust generated in an area ratio of less than 30%, Δ: Rust generated in an area ratio of 30% or more,
×: All surface rust occurred.

CFC resistance The test material before annealing was evaluated in accordance with the following criteria by weight increase / decrease after immersion in CFC R-22 at 120 ° C. for 100 hours.

⊚: Weight increase / decrease is less than 0.05 g / m ² , X: Weight increase / decrease is 0.05 g / m ² or more.

Oil resistance The test materials before annealing were immersed in No. 2 insulating oil at 120 ° C. for 72 hours, and evaluated by the following criteria by weight increase / decrease.

⊚: Weight change was less than 0.05 g / m ² , X: Weight change was 0.05 g / m ² or more.

Seizure resistance Two test materials before annealing were laminated to form a laminate, and a load of 0.5 kgf / cm ² was applied to the laminate to obtain 75% in 100% nitrogen.
After performing stress relief annealing at 0 ° C. for 2 hours, the fusion state of the steel sheets was evaluated according to the following criteria. ⊚: No fusion at all, Δ: Slight fusion, ×: Full fusion.

Resistance to rust resistance Two pieces of the test materials after annealing were overlapped and kept in a thermo-hygrostat at a temperature of 30 ° C and a relative humidity of 80% for 1500 hours, and then the rust generated on the superposed surfaces The area ratio was evaluated according to the following criteria.

◎: 0 to 10%, ○: over 10% to 20%,
△: over 20% to 30%, x: over 30%.

Magnetic characteristics Using test materials after annealing, iron loss (W _15/50 , maximum magnetic flux density 1.5 T, frequency 50 H was _measured according to JIS C 2550 (1986).
Iron loss per 1 kg for z) and magnetic flux density (B ₅₀ , 5000
The magnetic flux density when magnetized at A / m) was measured.

[0058]

[Table 1]

As can be seen from Table 1, when Cu is added to the steel sheet in an amount of 0.1% or less according to the present invention, various characteristics including magnetic characteristics and insulation resistance are hardly changed, and anti-corrosion resistance after stress relief annealing is performed. Sex was significantly improved. However, the added amount of Cu is 0.
When it exceeds 1%, the magnetic properties are conspicuously deteriorated and the insulation resistance is also decreased. Even if the Mn content exceeds 0.5%, the rust resistance is lowered. The addition of divalent metal ions (Mg ^{2+ in} this example) had the effect of increasing the insulation resistance, but on the other hand, there was a tendency for the rust resistance to decrease. In particular, the addition of a large amount of Mg ²⁺ having a Mg ²⁺ / Al ³⁺ molar ratio of more than 7.0 caused a remarkable decrease in the rust resistance.

Example 2 A treatment solution was prepared in the same manner as in Example 1. However, in this example, the Mg ²⁺ / Al ³⁺ molar ratio in the treatment liquid is
0.3, (Mg ²⁺ + Al ³⁺ ) / CrO ₃ molar ratio is also a constant value of 0.3,
H ₃ BO ₃ / CrO ₃ molar ratio of 0, 0.2, 0.6, 1.0, 1.4, 1.8
, 2.2. For 100 parts of CrO _3, vinyl acetate resin (Boncoat 930
1) as a solid content of 20 parts and glycerin 20 as a reducing agent
Parts were used individually.

This treatment solution was applied by a roll coater on both sides of each of the four types of steel plates (each having a plate thickness of 0.5 mm) having the chemical components shown in the following (1) to (30) in an oven at 310 ° C.
After baking for 2 seconds, an electric insulating film was formed. The film thickness after baking was 0.5 μm in all cases. C: 0.003%, Si: 0.5%, Mn: 0.2%, Cu: 0.04%, C: 0.003%, Si: 0.5%, Mn: 1.0%, Cu: 0.04%, C: 0.003%, Si: 0.5%, Mn: 0.2%, Cu: 0%, C: 0.003%, Si: 0.5%, Mn: 0.2%, Cu: 0.3%. Table 2 shows the results of testing the characteristics of the obtained electromagnetic steel sheet in the same manner as in Example 1.

[0062]

[Table 2]

As can be seen from Table 2, when boric acid was not added, the insulating coatings formed were significantly deteriorated in seizure resistance and rust resistance. On the other hand, when the amount of boric acid added increased and the H ₃ BO ₃ / CrO ₃ molar ratio exceeded 1.5, the punchability was greatly reduced. Also in this example, when Cu was added to the steel sheet in an amount of 0.1% or less, various properties including magnetic properties and insulation resistance were hardly changed, and the rust resistance after stress relief annealing was significantly improved. However, when the addition amount of Cu exceeds 0.1%, the magnetic properties are conspicuously deteriorated. Even if the Mn content exceeds 0.5%, the rust resistance is significantly reduced.

Claims (2)

[Claims] 1. An electromagnetic steel sheet having a chromium compound-organic resin-based insulating coating on its surface, wherein (1) the steel sheet is, by weight%, C: 0.01% or less, Si: 4.0% or less, Al: 0 to 2.0. %, (Si + Al): 5% or less, Mn: 0.5%
Hereinafter, Cu: 0.01 to 0.1%, the balance: having a steel composition consisting of Fe and unavoidable impurities, (2) The treatment liquid obtained by mixing the chromic acid aqueous solution, the aqueous emulsion resin, and the organic reducing agent in the insulating film. The chromic acid aqueous solution contains a readily soluble aluminum compound and boric acid (H ₃ BO ₃ ) which are dissolved by a neutralization reaction with free chromic acid, and the chromic acid aqueous solution contains Al in the aqueous solution. ³⁺ / CrO ₃ molar ratio in the range of 0.2-0.5, H ₃ BO ₃
A magnetic steel sheet having excellent rust resistance after stress relief annealing, characterized in that the molar ratio of / CrO ₃ is in the range of 0.1 to 1.5. 2. The chromic acid aqueous solution further contains one or more compounds selected from the group consisting of divalent metal oxides, hydroxides, carbonates, chromates and dichromates, The molar ratio of Me ²⁺ / Al ³⁺ is 7.0 or less (however, Me ²⁺ is the divalent metal ion), and the molar ratio of (Al ³⁺ + Me ²⁺ ) / CrO ₃ is
The electromagnetic steel sheet according to claim 1, which is contained in an amount within the range of 0.2 to 0.5.