JP2674916B2 - Method for manufacturing mirror-finished high magnetic flux density grain-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 grain-oriented silicon steel sheet having a high magnetic flux density and extremely low iron loss (hereinafter referred to as a grain-oriented electrical steel sheet). Especially, in the secondary recrystallization process (finishing annealing process), the forsterite (hereinafter referred to as glass) coating is not formed on the steel plate surface, and at the same time, the same process is completed with the steel plate surface mirror-finished by thermal etching. After that, treatments such as magnetic domain subdivision and tension coating are performed to improve the core loss characteristics.

[0002]

2. Description of the Related Art Grain-oriented electrical steel sheets are frequently used as magnetic iron cores for electric equipment, and have been repeatedly improved to reduce energy loss. As a means for reducing iron loss of grain-oriented electrical steel sheets, a method of irradiating a laser beam to a material surface after finish annealing to impart local strain, thereby subdividing magnetic domains to reduce iron loss is disclosed in, for example, 58-264
No. 05 is disclosed.

[0003] Further, since local strain is removed by stress relief annealing (strain relief annealing) after processing which is usually performed, the domain refining effect disappears. A means for improving this, that is, means for preventing the domain refining effect from disappearing even by stress relief annealing, is disclosed in, for example, Japanese Patent Application Laid-Open No. 62-8617.

In order to further reduce the iron loss value, it is important to remove (smooth) the irregularities on the surface of the base steel which hinder the movement of magnetic domains near the surface of the steel sheet. The highest level of smoothing is specular. As a method for smoothing (mirror finishing) the surface of a material after finish annealing, see Japanese Patent Application Laid-Open No. 64-83620.
There are chemical polishing, electrolytic polishing and the like disclosed in Japanese Patent Application Publication No.

[0005]

Conventionally, as a method of mirror-finishing (smoothing) the surface of a steel sheet, in addition to the above chemical polishing and electrolytic polishing, chemical polishing such as brush polishing, sandpaper polishing, grinding or the like is performed. There is a way. However, these methods are suitable for producing small specimens and small samples, but have various difficulties for surface mirroring (smoothing) of industrially produced metal strips and the like.

In the chemical method that is said to be the most smooth, that is, in chemical polishing, environmental problems such as chemical concentration control and wastewater treatment, and in the physical method, a surface having an industrially large area is used as the same standard. It is extremely difficult to smooth (mirror-finish) with.

An object of the present invention is to eliminate these problems and to provide a method for mirror-finishing (smoothing) the surface of a grain-oriented electrical steel sheet on an industrial production scale. Of course, the magnetic properties must not be lost due to mirroring (smoothing).

In the present invention, it is intended to simultaneously achieve the objects in the finish annealing step. That is, the orientation of secondary recrystallization is controlled to obtain an extremely high magnetic flux density and to obtain a mirror surface (smooth surface).

[0009]

A feature of the present invention is that, as described above, a mirror surface (smooth surface) is used during finish annealing.
Is about to get. That is, Mg that is normally used
Without using an annealing separator mainly composed of O, a substance that does not react with or hardly reacts with SiO ₂ such as Al ₂ O ₃ is used as an annealing separator, and the N ₂ partial pressure (%) in the atmosphere is controlled. Specialized finish annealing is performed to obtain grain-oriented electrical steel sheet with high magnetic flux density and at the same time glass (forsterite) on the surface of the steel sheet.
It is characterized in that the metal surface is subjected to secondary recrystallization without forming a coating film, and at the same time, the metal surface is mirror-finished (smoothed) by thermal etching.

The means is Si: 2.0 to 4.8% by weight, acid-soluble Al: 0.008 to 0.05% by weight, N ≦.
A silicon hot-rolled steel strip consisting of 0.010% by weight and the balance Fe and unavoidable impurities is annealed as required, and then cold rolled once or twice or more with intermediate annealing sandwiched, to obtain a predetermined plate thickness. , Then, after performing the primary recrystallization annealing, applying an annealing separator, in the production of the grain-oriented silicon steel sheet subjected to finish annealing, after the primary recrystallization annealing, to remove the oxide film on the steel sheet surface generated in the annealing step, Al ₂ O ₃ alone or coated with annealing separator composed mainly of Al ₂ O _3, a neutral one have a reducing atmosphere, the Atsushi Nobori rate of finish annealing at 50 ° C. / hr or higher 92
It is a method for producing a grain-oriented silicon steel sheet in which the temperature is raised to 0 to 1150 ° C., when the temperature is reached, the N ₂ % of the atmosphere is made higher than the N ₂ % at the time of raising the temperature, and the temperature is maintained for 5 hours or more.

Here, the method of removing the oxide film on the surface of the steel sheet is preferably acid pickling, and in particular, pickling with an acid mixed with fluorine causes an oxide layer on the surface of the steel sheet generated during primary recrystallization annealing. Is effective in removing. Before entering the step of removing the oxide film on the surface of the steel sheet from the primary recrystallization annealing, perform a nitriding treatment with ammonia and N in the atmosphere at the time of temperature rise.
_{Setting 2} to 5% or more is effective from the viewpoint of strengthening the inhibitor.

[0012] It is also effective to apply the annealing separating agent by electrostatic coating from the viewpoint of not forming a glass film and preventing the deterioration of the inhibitor. It is also possible to use an oxide of an alkaline earth metal other than MgO as the annealing separator.

Hereinafter, the present invention will be described in detail. The present inventors have investigated in detail the rate-determining process of inhibitor deterioration during finish annealing, and the largest factor is the oxidation process of Al at the steel sheet interface, and the oxide layer on the surface of the steel sheet that occurs during primary recrystallization annealing is It was found that it is greatly involved in the deterioration.

Si: 3.3% by weight, acid-soluble Al: 0.
028% by weight, N: 0.008% by weight, Mn: 0.14
% By weight, S: 0.007% by weight, C: 0.05% by weight,
1 hot rolled steel strip consisting of the balance Fe and unavoidable impurities
After annealing at 100 ° C for 2 minutes, cold-rolled, 0.23mm
It was thick. These cold-rolled sheets were annealed at 800 ° 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, and the total nitrogen amount was 180 pp.
m and strengthened the inhibitor.

After that, Al ₂ O was added to the two materials which were pickled as they were and with a 0.5% hydrofluoric acid-5% sulfuric acid mixed solution.
₃ was electrostatically applied, and finish annealing was performed in a 100% H ₂ atmosphere while maintaining a temperature rising rate of 15 ° C./hr. When the inhibitors (AlN, (Al, Si) N, etc.) during the final annealing were examined, as shown in FIG. 1, the material having an oxide layer generated on the steel sheet surface during the primary recrystallization annealing had no oxide layer. It was found that the inhibitor strength deteriorated faster than the material.

That is, if the oxide layer formed on the surface of the steel sheet during the primary recrystallization annealing is removed, strong inhibitor strength can be maintained up to a high temperature. The acid-soluble Al in the steel sheet removes oxygen from the oxide layer mainly composed of SiO ₂ during finish annealing to become Al ₂ O ₃ etc. and precipitates in the oxide layer. Therefore, the acid-soluble Al in the steel sheet decreases.

Although the acid-soluble Al concentration in steel is shown as the inhibitor strength in FIG. 1, Al is AlN,
Since a compound (precipitate) such as (Al, Si) N is formed and acts as an inhibitor, the amount of acid-soluble Al can be considered as an index showing the inhibitor strength.

Further, the inventors of the present invention have conducted a detailed investigation on the rate-determining process of inhibitor deterioration, and found that the amount of nitrogen in the steel and the amount of nitrogen in the annealing atmosphere are also influenced in addition to the oxidation of Al at the steel sheet interface. It was The amount of nitrogen in the annealing atmosphere increases the amount of nitrogen in the steel through the steel sheet interface, and the effect is the same as that of nitrogen contained in the steel from the beginning.

Nitrogen in steel and nitrogen in the annealing atmosphere are Al
Oxidation of Al is suppressed in order to increase the number of precipitates such as N to fix Al and reduce the movement of Al to the steel sheet interface.

Therefore, acid-soluble Al in steel during finish annealing
As for the amount, the higher the nitrogen partial pressure, the less the deterioration, and the higher the temperature, the stronger the inhibitor. In order to obtain a high magnetic flux density, which is one of the gist of the present invention, it is preferable that the inhibitor is strong, but it is preferable that the inhibitor is strongly and constantly maintained.

This is for growing only crystals (GOSS grains) having a good orientation from the start to the end of the secondary recrystallization, and if the inhibitor is weakened from the start to the end of the secondary recrystallization, grains with bad orientation grow. However, the magnetic flux density of the product steel sheet decreases. The solubility of AlN, which is an inhibitor, naturally increases as the steel sheet temperature rises, and the inhibitor inevitably deteriorates.

As a measure for this, it is desirable to raise the partial pressure of nitrogen as the temperature rises to increase the amount of nitrogen in the steel sheet and keep AlN as a precipitate constant. However, in order to obtain a mirror surface, which is one of the gist of the present invention, the nitrogen partial pressure must not be too high.

Secondary recrystallization at a constant temperature at which the solubility of AlN does not change, that is, the inhibitor strength does not change, is extremely effective in that the inhibitor is secondarily recrystallized at a constant strength.

As described above, the solubility of AlN does not change if it is kept at a constant temperature, but acid-soluble Al is more oxygen than oxygen in the atmosphere or an oxide of an element having a smaller oxygen affinity than Al on the surface of the steel sheet. Then, it becomes Al ₂ O ₃ and decreases, and the inhibitor deteriorates. Therefore, also in this case, the partial pressure of nitrogen is increased to suppress the dissolution of AlN, and the acid-soluble A
The decrease in l must be suppressed.

The implementation conditions will be described below. At the time of primary recrystallization annealing, as a method of removing the oxide layer formed on the surface of the steel sheet, there are mechanical polishing, for example, brush polishing, sandpaper polishing, grinding, etc., which are effective for this purpose, but are industrially various. Difficult and not very practical. The present inventors have found that the pickling method is extremely effective.

This is because a continuous pickling line for hot-rolled steel strips or steel sheets has already been put to practical use. As the pickling solution (pickling solution), mineral acids such as hydrochloric acid, sulfuric acid and nitric acid are effective, but the oxide layer formed on the surface of the steel sheet is mainly SiO.
_Since it is an oxide consisting mainly of ₂ , it is difficult to pickle it with only mineral acids such as hydrochloric acid, sulfuric acid and nitric acid. When hydrofluoric acid is mixed with these acids, the oxide layer can be removed very efficiently, that is, at a high speed. Further, it is effective to strengthen the inhibitor by performing a nitriding treatment with ammonia after the primary recrystallization annealing and before the finish annealing. This is due to the inhibitor strength at the time of completion of primary recrystallization,
Insufficient, and even if the partial pressure of nitrogen during finish annealing is increased to strengthen the inhibitor or prevent deterioration, sufficient inhibitor cannot be secured for secondary recrystallization. Therefore, generally, the reinforcement of the inhibitor by the ammonia treatment improves the magnetic properties.

Nitrogen gas was added at a rate of 5% in the annealing atmosphere at the time of temperature increase in order to secure an inhibitor necessary for the progress of secondary recrystallization.
It is desirable that the hydrogen gas be contained in an amount of at least
%, But if the nitrogen gas is less than 5%, the effect of strengthening the inhibitor or preventing deterioration is small.

The neutral or reducing atmosphere is an oxidation-reduction of silicon by one or a mixture of two or more gases selected from inert gases such as nitrogen, oxygen, water, hydrogen and argon. A gas composition that is neutral or reducing with respect to. In general, nitrogen and hydrogen gas are used in the finish annealing of the magnetic steel sheet, so that the combination of these two gases is 0% to 100%.

There is no bearing even if an inert gas such as argon or helium is mixed with the combination of both gases in order to adjust the nitrogen partial pressure. The neutral or reducing atmosphere is used to prevent the decrease of Al in the steel and not to oxidize silicon in the steel to form SiO ₂ on the surface or to increase it.

In the present invention, since the oxide layer on the surface of the steel sheet is removed after the primary recrystallization annealing, SiO ₂ is not present on the surface of the steel sheet before finish annealing, but a slight amount of oxygen in the finish annealing furnace is A small amount of SiO during finish annealing
₂ produces. The presence of MgO forms a forsterite film (glass film), so that a material that does not react with or is difficult to react with SiO ₂ is required as an annealing separator. One that meets this purpose is Al
₂ O ₃ (alumina) is the best.

Further, from the viewpoint of obtaining a mirror surface, which is the gist of the present invention, oxide powders of alkaline earth metals such as BaO, CaO, SrO were effective. Further, in order to prevent the formation of a coating film on the surface of the steel sheet, it was effective to use a material state having low reactivity, for example, to increase the particle size of the powder or to apply it to the surface of the steel sheet without forming a hydrate. Electrostatic coating was extremely effective as a method of coating the annealing separator on the surface of the steel sheet without forming a hydrate.

The higher the rate of temperature rise up to the temperature at which secondary recrystallization is possible in the finish annealing, the more convenient the inhibitor is. When the heating rate was less than 15 ° C./hr, the inhibitor was remarkably deteriorated, and the inhibitor necessary for secondary recrystallization was not secured, sufficient secondary recrystallization was not obtained, and the magnetic flux density (B ₈ ) of the steel sheet was low. In terms of obtaining a high magnetic flux density, which is one of the gist of the present invention,
A temperature rising rate of not less than ° C / hr was required.

Holding at a constant temperature for secondary recrystallization is particularly effective for obtaining a high magnetic flux density. This is to prevent deterioration of the inhibitor and carry out secondary recrystallization with an appropriate inhibitor strength.

If this temperature is less than 920 ° C., the time until the completion of secondary recrystallization becomes too long and it becomes impractical.
On the other hand, if the temperature exceeds 1150 ° C., the inhibitor is so much deteriorated that the necessary inhibitor cannot be secured until the secondary recrystallization is completed.

A holding time of 5 hours or more is required to carry out the secondary recrystallization, and if the time is shorter than this, it will not be completed within the holding time. When the holding temperature is reached, the nitrogen partial pressure is made higher than that at the time of temperature increase in order to suppress the deterioration of the inhibitor and keep the inhibitor strength constant as described above.

However, the higher the nitrogen partial pressure, the higher the better. If it is too high, the inhibitor becomes too strong and it takes too much time to complete the secondary recrystallization, and the secondary recrystallization does not occur. The nitrogen partial pressure at the time of maintaining the temperature must be increased by linking with the nitrogen partial pressure at the time of raising the temperature. After the completion of secondary recrystallization, increasing the hydrogen concentration in order to complete the purification and the mirror-finishing of the steel sheet surface, and maintaining the temperature at around 1200 ° C. for several hours,
Extremely effective.

The steel component in the present invention is Si: 2.0 to
4.8 wt%, acid-soluble Al: 0.008 to 0.05 wt%, N ≦ 0.010 wt%, balance: Fe and inevitable impurities, and other elements are not particularly limited.

[0038] Si is important in increasing electric resistance and reducing iron loss, but if it exceeds 4.8%, it is easily cracked during cold rolling. On the other hand, if it is less than 2.0%, there is a problem in lowering electric resistance and lowering iron loss.

Acid-soluble Al is an important inhibitor constituent element, and is combined with nitrogen, silicon, etc. to form AlN, (A
l, Si) N, etc. are formed to serve as an inhibitor. In terms of inhibitor strength, that is, the range in which the magnetic flux density increases, the content is 0.008 to 0.05% by weight.

When the content of nitrogen exceeds 0.010% by weight, vacancies called blisters are formed in the steel sheet, so this range is suitable. In addition, Mn,
S, Se, Sn, B, Bi, Nb, Ti, P and the like can be added.

Hereinafter, embodiments of the present invention will be described. Si:
2.0-4.8% by weight, acid-soluble Al: 0.008-
Molten steel consisting of 0.05% by weight, N ≦ 0.010% by weight, the balance: Fe and inevitable impurities is cast into a hot-rolled steel sheet or a hot-rolled steel strip in a usual process or by continuous casting.

The hot rolled steel sheet or hot rolled steel strip is 750 ° C.
Annealing for improving the magnetic flux density is performed in a temperature range of ˜1200 ° C. for 30 seconds to 30 minutes. Then, these hot rolled steel sheets or hot rolled steel strips are cold rolled. The cold rolling is performed at a final cold rolling rate of 80% or more, as disclosed in Japanese Patent Publication No. 40-15644.

The material after cold rolling is usually subjected to primary recrystallization annealing in a temperature range of 750 ° C. to 900 ° C. in a wet water atmosphere in order to remove carbon in steel. At this time, an oxide layer is formed on the surface of the steel sheet together with decarburization and primary recrystallization. This oxide layer forms a so-called internal oxide layer and has a thickness of 0.1 to 6.0 μm from the surface of the steel sheet, depending on the degree of water content (usually represented by dew point) in a wet water atmosphere, that is, an atmosphere containing water. Therefore, SiO ₂ mainly exists as an oxide here.

Note that 80 to 90% or more of the oxygen amount of the oxide formed during the primary recrystallization annealing is in the form of SiO ₂ . The oxide layer on the surface of the steel sheet or steel strip after primary recrystallization is removed.

This method includes physical and chemical methods as described above, but is generally carried out by pickling. Performing nitriding treatment with ammonia to strengthen the inhibitor prior to removal of the oxide layer on the surface of the steel sheet is extremely effective for improving the magnetic flux density.

The primary recrystallized plate from which the oxide layer on the surface has been removed is coated with an annealing separator and enters the finish annealing furnace. Heating rate of finish annealing is higher magnetic prevent inhibitor degradation
In order to obtain a flux density, as large as possible, preferably 50
℃ / hr or more.

The atmosphere at the time of raising the temperature of the finish annealing is neutral or reducing, and when the temperature reaches 920 ° C. to 1150 ° C., the nitrogen partial pressure is made higher than that at the time of raising the temperature and maintained for 5 hours or more. It should be noted that mixing an inert gas such as argon or helium for adjusting the nitrogen partial pressure does not matter.

After the secondary recrystallization is completed, the product is kept at a high temperature (about 1200 ° C.) with 100% hydrogen for purification. Note that, unlike the commonly used annealing separator (mainly MgO),
Since it is possible to prevent water or hydrated water from being brought into the annealing separator, a step for removal is not required during the finish annealing, and the finish annealing can be shortened accordingly. After finishing annealing, a magnetic domain refinement treatment such as laser beam irradiation is performed, and a tension coating treatment is further performed.

[0049]

【Example】

(Example 1) Si: 3.2% by weight, acid-soluble Al: 0.
029 wt%, N: 0.008 wt%, Mn: 0.13
% By weight, S: 0.007% by weight, C: 0.05% by weight,
1 hot rolled steel strip consisting of the balance Fe and unavoidable impurities
After annealing at 100 ° C for 2 minutes, cold-rolled, 0.23mm
It was thick. These cold-rolled sheets were annealed at 820 ° C. for 2 minutes in an annealing furnace in a humid water atmosphere in order to perform decarburization, and were subjected to primary recrystallization.

Next, in order to stabilize the secondary recrystallization, a nitriding treatment is performed in an ammonia atmosphere to reduce the total nitriding amount to 160.
ppm, and the inhibitor was strengthened. After that, the oxide layer formed on the surface of the steel sheet was removed with sulfuric acid mixed with hydrofluoric acid.

An annealing separator consisting of Al ₂ O ₃ : 100% is electrostatically applied, and up to 1100 ° C., 10% N ₂ -90% H.
_{In 2} atmospheres, the temperature was raised to 1100 ° C. while maintaining the temperature rising rate of 150 ° C./hr.

After reaching 1100 ° C., 50% N ₂ -50%
The atmosphere was changed to H ₂ atmosphere, and the temperature was maintained for 10 hours without changing the atmosphere. Then 100% hydrogen,
The temperature was further raised to 1200 ° C., and the temperature was maintained for 10 hours.

After finishing annealing, a laser beam was irradiated to perform tension coating treatment of phosphoric acid-chromic acid system. Table 1 shows the properties of the obtained product.

[0054]

[Table 1] (Example 2) The hot-rolled silicon steel strip of Example 1 was subjected to 2 at 1100 ° C.
After annealing for a minute, it was cold rolled to a thickness of 0.20 mm. These cold-rolled sheets were annealed at 820 ° C. for 2 minutes in an annealing furnace in a wet water atmosphere for double decarburization, and primary recrystallization was performed.

Next, in order to stabilize the secondary recrystallization, a nitriding treatment is performed in an ammonia atmosphere, and the total nitrogen amount is adjusted to 170.
ppm, and the inhibitor was strengthened. After that, the oxide layer formed on the surface of the steel sheet was removed with sulfuric acid mixed with hydrofluoric acid.

An annealing separator consisting of Al ₂ O ₃ : 100% is electrostatically applied, and up to 1100 ° C., 10% N ₂ -90% H.
_{In 2} atmospheres, the temperature was raised to 1100 ° C. while maintaining the temperature rising rate of 150 ° C./hr.

After reaching 1100 ° C., 75% N ₂ -25%
The atmosphere was changed to H ₂ atmosphere, and the temperature was maintained for 10 hours without changing the atmosphere. Then 100% hydrogen,
The temperature was further raised to 1200 ° C., and the temperature was maintained for 10 hours. After the finish annealing, a laser beam was irradiated to perform a phosphoric acid-chromic acid-based tension coating treatment. Table 2 shows the properties of the obtained product.

[0058]

[Table 2]

[0059]

According to the present invention, a grain-oriented electrical steel sheet having a high magnetic flux density and a small irregularity (a mirror surface) on the steel sheet surface, which is a factor inhibiting magnetic properties, can be easily obtained. The subdivision and tension coating treatment provided a magnetic material with extremely low iron loss. In the production of the grain-oriented electrical steel sheet, the mirror finishing treatment of the steel sheet is performed in a normal finish annealing furnace, so that it is extremely easy and has great industrial value.

[Brief description of the drawings]

FIG. 1 is a table showing changes in finish annealing temperature and steel sheet inhibitor (acid-soluble Al).

Claims (9)

(57) [Claims] 1. By weight ratio, Si: 2.0 to 4.8%, acid-soluble Al: 0.008 to 0.05%, N ≦ 0.010%, balance: heat composed of Fe and unavoidable impurities the rolled silicon steel strip once or is subjected to two or more cold rolling sandwiching the intermediate annealing to a final thickness and then after primary recrystallization annealing, applying an annealing separating agent, oriented silicon subjected to finish annealing In the method for producing a steel sheet, after the primary recrystallization annealing, after removing the oxide film on the surface of the steel sheet (strip) produced in the annealing step, an annealing separator containing Al ₂ O ₃ alone or Al ₂ O ₃ as a main component. was applied, then, in a neutral some have a reducing atmosphere, 920 to 1,150 ° C. in the above heating rate 50 ° C. / hr
It was heated to a temperature range, after reaching the temperature range, and higher than N _2% to _2% N in the atmosphere during the heating,
A method for producing a mirror-finished high magnetic flux density grain-oriented silicon steel sheet, characterized by performing finish annealing for holding the temperature in the reached temperature range for 5 hours or more. 2. The method according to claim 1, wherein the means for removing the oxide film on the surface of the steel sheet (strip) produced in the primary recrystallization annealing step is pickling. 3. The steel sheet (strip) is pickled by an acid solution containing hydrofluoric acid as a means for removing an oxide film on the surface of the steel sheet (strip) produced in the primary recrystallization annealing step. the method of. 4. By weight ratio, Si: 2.0 to 4.8%, acid-soluble Al: 0.008 to 0.05%, N ≦ 0.010%, balance: heat composed of Fe and unavoidable impurities the rolled silicon steel strip once or is subjected to two or more cold rolling sandwiching the intermediate annealing to a final thickness and then after primary recrystallization annealing, applying an annealing separating agent, oriented silicon subjected to finish annealing In the method for producing a steel sheet, after the primary recrystallization annealing, the steel sheet (strip) is nitrided in an atmosphere containing ammonia gas,
Then, after removing the oxide film on the surface of the steel sheet (strip) produced in the primary recrystallization annealing step , Al ₂ O ₃ alone
The or coated with annealing separator composed mainly of Al ₂ O _3, then neutral some have a reducing atmosphere, 50 ° C. / hr
Heated to above the temperature range of from 920 to 1,150 ° C. at a heating rate, after reaching the temperature range, and higher than N _2% to _2% N in the atmosphere during the Atsushi Nobori, the reached temperature range A method for producing a mirror-finished high magnetic flux density grain-oriented silicon steel sheet, characterized in that finish annealing is carried out for 5 hours or more. 5. The method according to claim 4, wherein the means for removing the oxide film on the surface of the steel sheet (strip) produced in the primary recrystallization annealing step is pickling. 6. The means for removing an oxide film on the surface of a steel sheet (strip) produced in the primary recrystallization annealing step is to pickle the steel sheet (strip) with an acid solution mixed with hydrofluoric acid. the method of. 7. The method according to claim 1, wherein the N ₂ content in the atmosphere during the temperature rising process of the material in the finish annealing step is 5% or more. 8. A steel plate (strip) according to claim 1 to 7 either applying the annealing separator certain There oxides of alkaline earth metals other than MgO is mainly composed of them after removing the oxide film on the surface The method described. 9. The method according to claim 1, wherein the application of the annealing separator is electrostatic application.