JPH07118743A - Production of high magnetic flux density grain-oriented silicon steel sheet free from forsterite film - Google Patents

Abstract

PURPOSE:To obtain a high magnetic flux density grain-oriented silicon steel sheet by finish annealing in a short time. CONSTITUTION:This steel sheet has a compsn. contg. 2 to 4.8% Si, 0.08 to 0.020% acid soluble A1, <=0.010% N, and the balance Fe with inevitable impurities, and the amt. of acid soluble Al obtd. by subtracting Al equivalent to N from acid soluble Al in the hot rolled steel strip is regulated to <=0.01%. After cold rolling, it is subjected to primary recrystallization annealing and is coated with a separation agent for annealing essentially consisting of alumina and its temp. is raised to 920 to 1150 deg.C in a neutral or reducing atmosphere so as to regulate the temp. rising rate in finish annealing to >=50 deg.C/Hr, and it is held to the same temp. for >=5hr. Moreover, it is preferabe for obtaining a high magnetic flux density material to execute nitriding by ammonia before the start of the finish annealing and to straighten the inhibitor. Thus, the high magnetic flux density grain-oriented silicon steel sheet can be obtd. in a short time, and domain fractionating treatment and tension coating are executed to obtain an ultralow core loss material.

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 high magnetic flux density and extremely low iron loss (hereinafter referred to as grain-oriented electrical steel sheet). Especially, in the secondary recrystallization step (finish annealing step), the forsterite (hereinafter referred to as glass) film is not formed on the surface of the steel sheet, and at the same time, the steel sheet surface is smoothed (mirror surface) by thermal etching. After the process is completed, magnetic domain subdivision, tension coating, and other treatments are performed to improve the iron 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 the iron loss of grain-oriented electrical steel sheets, a method of irradiating a laser beam on the surface of the material after finish annealing to give local strain to thereby subdivide the magnetic domains to reduce the iron loss is disclosed in, for example, Japanese Patent Application Laid-Open No. 58-264
No. 05 publication.

Further, since the local strain is removed by the stress relief annealing (strain relief annealing) after the usual working, the magnetic domain refining effect disappears. For example, Japanese Patent Laid-Open No. 62-8617 discloses a remedy for this problem, that is, a means in which the magnetic domain refinement effect does not disappear even when stress relief annealing is performed.

In order to further reduce the iron loss value, it is important to remove (smooth) the irregularities on the surface of the base metal 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 material surface after finish annealing, there is disclosed in JP-A-64-83620.
There are chemical polishing, electrolytic polishing, and the like disclosed in the publication.

[0005]

Conventionally, as a method of mirror-finishing (smoothing) the surface of a steel sheet, chemical or physical methods such as brush polishing, sandpaper polishing and grinding other than the above chemical polishing and electrolytic polishing are used. There is. However, these methods are suitable for producing small test pieces and small samples, but are accompanied by various difficulties for the surface mirror-finishing (smoothing) of industrially mass-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.

[0007]

As a matter of course, the magnetic characteristics must not be lost due to mirroring (smoothing). In the present invention, it is intended to simultaneously achieve the object 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).

A feature of the present invention is to obtain a mirror surface (smooth surface) during finish annealing. That is,
Without using a commonly used annealing separator mainly composed of MgO, a substance which does not react with SiO ₂ such as Al ₂ O ₃ or which hardly reacts is used as the annealing separator, and a directional electromagnetic field having a high magnetic flux density is obtained. At the same time as obtaining the steel plate, without forming a glass (forsterite) film on the surface of the steel plate, secondary recrystallization is performed with the metal surface exposed, and at the same time, the metal surface is mirror-finished (smoothed) by thermal etching. Is characterized by.

In the present invention, Si: 2.0 to 4.8% by weight, acid-soluble Al: 0.008 to 0.020% by weight, N ≦ 0.
A silicon hot-rolled steel strip consisting of 010 wt%, balance Fe and unavoidable impurities is annealed if necessary, and then cold rolled once or twice or more with intermediate annealing, to a predetermined plate thickness, and then In the production of a grain-oriented silicon steel sheet, which is subjected to a primary recrystallization annealing, is applied with an annealing separator, and is then subjected to finish annealing,
The amount of acid-soluble Al obtained by subtracting N equivalent of Al from acid-soluble Al in the hot-rolled steel strip is set to 0.010% by weight or less, and after primary recrystallization annealing, annealing separation mainly composed of Al ₂ O ₃ (alumina) Apply the agent, raise the heating rate of finish annealing in a neutral or reducing atmosphere at 920 to 1150 ° C at 50 ° C / Hr or more, and when the temperature is reached, hold at that temperature for 5 hours or more. Is.

Here, it is effective from the viewpoint of inhibitor strengthening to perform a nitriding treatment with ammonia before starting from the primary recrystallization annealing to the finishing annealing step, and to make N _{2 in} the atmosphere at the time of temperature rising 5% or more. Is. It is also effective to apply the annealing separator as an electrostatic coating from the viewpoint of not forming a glass coating and preventing the deterioration of the inhibitor. An oxide of an alkaline earth metal other than MgO can be used as the annealing separator.

The present invention will be described in detail below. The present inventors have investigated the behavior of the inhibitor (precipitate) during finish annealing in detail, and when the amount of N in the steel is large, silicon nitride is generated as a precipitate in addition to AlN, and the amount of N in the steel is When the amount was small, it was found that the precipitate was only AlN. However, as can be seen from thermodynamics, silicon nitride is unstable at high temperatures and usually decomposes and does not exist stably at 900 ° C. or higher.

Therefore, silicon nitride is not suitable as an inhibitor for secondary recrystallization at 900 ° C. or higher.
1N is effective. However, it is known from experiments by the present inventors that AlN deteriorates due to oxidation of Al on the surface (interface) of the steel sheet.

As described above, the acid-soluble Al becomes Al ₂ O ₃ by capturing oxygen in the oxide layer on the surface of the steel sheet generated during the primary recrystallization annealing and a small amount of oxygen (water) in the annealing atmosphere. Acid-soluble Al is heated in a short time to the temperature required for secondary recrystallization.
It is effective to shorten the reaction time with oxygen in the annealing atmosphere and suppress inhibitor deterioration. In the experiments conducted by the present inventors, rapid heating of 50 ° C./H or more is effective, and if the heating temperature is less than 50 ° C., the inhibitor deterioration is large and a grain-oriented electrical steel sheet having a sufficient magnetic flux density cannot be obtained. The inhibitor strength indicates the concentration of acid-soluble Al in steel. 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 nitrogen contained in the steel from the beginning. Nitrogen in steel and nitrogen in the annealing atmosphere increase precipitates such as AlN to fix Al,
Oxidation of Al is suppressed in order to reduce the migration of 1 to the steel plate interface.

Therefore, acid-soluble Al in steel during finish annealing
The higher the nitrogen partial pressure, the less the amount of deterioration, and the stronger the inhibitor is at high temperatures. 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. However, it should not be so strong that secondary recrystallization does not occur.

It is a condition for obtaining a high magnetic flux density that it is strongly retained with the strength of secondary recrystallization. Here, the reason why the present inventors set the amount of acid-soluble Al obtained by subtracting N equivalent of Al from acid-soluble Al in the hot-rolled steel strip to 0.010% by weight or less will be described.

FIG. 1 is a photograph in which precipitates inside the steel sheet after the primary recrystallization annealing and nitriding treatment with ammonia were extracted and magnified 5000 times with an electron microscope. Large precipitates are formed by the combination of N and Al that are already present in the hot-rolled steel strip.
It is a precipitate that has grown to 1N. The small precipitate is AlN newly formed by ammonia nitriding. The AlN existing in the hot-rolled steel strip has been exposed to high temperature for a long time from the steel making which determines the steel composition, so that the so-called Ostwald growth was performed to grow into large AlN particles.
The AlN formed by the nitridation after the primary recrystallization annealing does not have sufficient Ostwald growth, and remains small.

As is well known, the inhibitor effect is proportional to the amount of precipitate and inversely proportional to its size. Therefore, the inhibitor effect of AlN existing in the hot-rolled steel strip is much smaller than that of AlN produced by nitriding after primary recrystallization.

The present inventors have found that Al generated after primary recrystallization
AlN present in hot-rolled steel strip to optimize the amount of N
Other acid-soluble Al is defined. As a result, it was possible to set AlN generated after primary recrystallization, which is the main inhibitor, to an appropriate value. This is extremely important in manufacturing, that is, the inhibitor can always be secured in a necessary and sufficient amount.

If the nitriding amount after the primary recrystallization is a certain amount or more, a necessary and sufficient inhibitor amount can be secured regardless of the nitriding amount. This control becomes extremely easy when controlling the nitrogen partial pressure during ammonia nitridation or finish annealing after primary recrystallization to secure the inhibitor.

As described above, if the nitrogen partial pressure during finish annealing is increased, nitrogen in the steel is increased and the dissolution of Al is decreased, so that the decrease of acid-soluble Al is suppressed, but the compound AlN is formed more than necessary. If unreacted Al is present, newly generated AlN as an inhibitor becomes excessive, and it takes a long time to complete the secondary recrystallization, or the secondary recrystallization becomes unstable. Such inconvenience becomes possible by limiting the acid-soluble Al that does not form AlN in the hot-rolled steel strip.

To keep the inhibitor strength strong and constant,
Crystals with good orientation from the start to the end of secondary recrystallization (GOSS
This is to grow only grains), and if the inhibitor weakens from the start to the end of secondary recrystallization, grains with bad orientation grow and the magnetic flux density of the product steel sheet decreases. As a measure for this, it is desirable to increase the partial pressure of nitrogen in the annealing atmosphere to increase the amount of nitrogen in the steel sheet and keep AlN as a precipitate constant. However, the nitrogen partial pressure must not be too high in order to obtain a mirror surface, which is one of the gist of the present invention.

The 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 maintained at a constant temperature, but the acid-soluble Al has more oxygen than oxygen in the atmosphere or an oxide of an element having a lower 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, it is necessary to raise the nitrogen partial pressure to suppress the dissolution of AlN and suppress the decrease of the acid-soluble Al.

The implementation conditions will be described below. It is effective to strengthen the inhibitor by nitriding with ammonia after the primary recrystallization annealing and before the finish annealing. This is because the inhibitor strength at the time of completion of primary recrystallization is not sufficient for secondary recrystallization, and even if the inhibitor is strengthened or deterioration is prevented by increasing the partial pressure of nitrogen during finish annealing. Sometimes we can't get enough inhibitors.
Therefore, the inhibitor strength obtained by the ammonia treatment generally improves the magnetic properties.

In order to secure an inhibitor necessary for the progress of secondary recrystallization, 5% or more of nitrogen gas is used in the annealing atmosphere.
It is desirable to add 5% or less, but 100% hydrogen gas may be used. If the nitrogen gas content is less than 5%, the effect of strengthening the inhibitor or preventing deterioration is small.

The neutral or reducing atmosphere is one or a mixture of two or more gases selected from inert gases such as nitrogen, oxygen, water, hydrogen, argon, etc., for oxidation-reduction of silicon. A gas composition that is neutral or reducing. Generally, nitrogen and hydrogen gas are used in the finish annealing of the electromagnetic steel sheet, and therefore the combination of these gases is 0% to 100%.

There is no problem 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 reduction of Al in the steel and to oxidize silicon in the steel so as not to increase SiO ₂ on the surface.

Si in the oxide layer on the surface of the steel sheet before finish annealing
Since SiO ₂ generated during finish annealing due to O ₂ and a slight amount of oxygen or water in the annealing furnace forms a forsterite film (glass film) when MgO is present therein, SiO ₂ is used as an annealing separator. A substance that does not react or is difficult to react is required. Al ₂ O ₃ (alumina) is the best one that meets this purpose.
In addition, even if the following substances are included in alumina, there is no problem.

Further, from the viewpoint of obtaining a mirror surface (smooth surface), which is the gist of the present invention, BaO, CaO,
An oxide powder of an alkaline earth metal such as SrO was effective. Further, in order to prevent the formation of a coating film on the surface of the steel sheet, a method of applying it to the surface of the steel sheet without reducing the reactivity of the substance, for example, increasing the particle size of the powder or forming a hydrate was effective. 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 finish annealing, the more favorable the inhibitor is because it is less likely to deteriorate. If the heating rate is less than 15 ° C./Hr, the inhibitor is remarkably deteriorated and the inhibitor required for secondary recrystallization is not secured, sufficient secondary recrystallization is not obtained, and the magnetic flux density (B ₈ ) of the steel sheet is low. . In terms of obtaining a high magnetic flux density, which is one of the main points of the present invention, it is 5
A heating rate of 0 ° C./hr or more 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 lower 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.

The holding time for secondary recrystallization is required to be 5 hours or more, and shorter time does not complete 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 better. If it is too high, the inhibitor becomes too strong, and it takes too long to complete the secondary recrystallization, and the secondary recrystallization does not occur. After the completion of secondary recrystallization, the hydrogen concentration was increased to 1200 ° C. in order to completely purify and mirror-finish (smooth) the steel plate surface.
Holding for several hours in the vicinity is extremely effective.

The steel composition in the present invention is Si: 2.0-
4.8% by weight, acid-soluble Al: 0.008 to 0.020
%, N ≦ 0.010% by weight, balance Fe and unavoidable impurities, and other elements are not particularly limited.
Si is important for increasing the electric resistance and reducing the iron loss, but if it exceeds 4.8%, it tends to crack during cold rolling. On the other hand, if it is less than 2.0%, there is a problem in that the electric resistance is low and the iron loss is lowered.

Acid-soluble Al is important as an inhibitor constituent element, and is combined with nitrogen, silicon, etc. to form AlN, (A
l, Si) N, etc. are formed to function as an inhibitor. In terms of the inhibitor strength, that is, the range in which the magnetic flux density is high, it is 0.008 to 0.020% by weight.

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

The embodiments of the present invention will be described below. Si:
2.0-4.8% by weight, acid-soluble Al: 0.008-
Molten steel consisting of 0.020% by weight, N ≦ 0.010% by weight, balance Fe and unavoidable impurities is cast into a hot-rolled steel sheet or a hot-rolled steel strip in a usual process or continuously cast.
This hot rolled steel sheet or hot rolled steel strip has a temperature of 750 ° C. to 1200 ° C.
Annealing for improving the magnetic flux density is performed in the temperature range of ° C for 30 seconds to 30 minutes.

Subsequently, these hot-rolled steel sheets or hot-rolled steel strips are cold-rolled. Cold rolling is conducted in Japanese Patent Publication No. 40-15
As disclosed in Japanese Patent No. 644, the final cold rolling rate 8
0% or more. The material after cold rolling is usually 750 ° C. to 900 ° C. in a wet water atmosphere to remove carbon in steel.
Primary recrystallization annealing is performed in the temperature range of. 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 depending on the degree of water content (usually expressed as a dew point) in a wet water atmosphere, that is, an atmosphere containing water.
The thickness is about 6.0 μm, and here, as an oxide,
SiO ₂ is mainly present. Note that 80 to 90% or more of the oxygen content of the oxide formed during the primary recrystallization annealing is in the form of SiO ₂ . Nitriding treatment with ammonia to strengthen the inhibitor is extremely effective in improving the magnetic flux density.

The primary recrystallized plate is coated with an annealing separator and enters the finish annealing furnace. The temperature rising rate of finish annealing is as large as possible and preferably 50 ° C./Hr or more in order to prevent inhibitor deterioration. The atmosphere of finish annealing is neutral or reducing, and after reaching 920 ° C to 1150 ° C,
Hold at that temperature 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 cause any problem.

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 moisture or hydrated water from being brought into the annealing separator, a step for removal is not required during the final annealing, and the final annealing can be shortened accordingly. After finishing annealing, magnetic domain subdivision processing such as laser beam irradiation is performed, and tension coating is further performed.

[0043]

【Example】

Example 1 Si: 3.3 wt%, acid-soluble Al: 0.020 wt%, N: 0.008 wt%, Mn: 0.13 wt%,
S: 0.007 wt%, C: 0.05 wt%, balance Fe
And hot-rolled silicon steel strip consisting of unavoidable impurities, and S
i: 3.3% by weight, acid-soluble Al: 0.031% by weight,
N: 0.008% by weight, Mn: 0.13% by weight, S:
Both steel strips of the hot-rolled silicon steel strip made of 0.007% by weight, C: 0.05% by weight, the balance Fe and inevitable impurities are 110
After annealing at 0 ° C. for 2 minutes, it was cold rolled to a thickness of 0.23 mm.

These cold-rolled sheets were annealed at 820 ° C. for 2 minutes in an annealing furnace in a wet water atmosphere in order to also serve as decarburization, and primary recrystallization was performed. Next, in order to stabilize the secondary recrystallization, nitriding treatment was performed in an ammonia atmosphere so that the total nitrogen content was 175 ppm in both steel sheets, and the inhibitor was strengthened.

After that, an annealing separating agent consisting of Al ₂ O ₃ : 100% is electrostatically applied, and the atmosphere of finish annealing is set to 10% N.
_The temperature was set to 2-90% H _2, and the temperature was raised to 1050 ° C. while maintaining the temperature rising rate of 150 ° C./Hr. After reaching 1050 ° C., the temperature was maintained for 10 hours. After that, the annealing atmosphere is further increased to 10
After switching to 0% 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 a phosphoric acid-chromic acid tension coating process. The characteristics of the obtained product are as shown in Table 1.

[0046]

[Table 1]

Example 2 Si: 3.3% by weight, acid-soluble Al: 0.020% by weight, N: 0.009% by weight, Mn: 0.13% by weight,
S: 0.007 wt%, C: 0.05 wt%, balance Fe
And hot-rolled silicon steel strip consisting of unavoidable impurities, and S
i: 3.3% by weight, acid-soluble Al: 0.031% by weight,
N: 0.009% by weight, Mn: 0.13% by weight, S:
Both steel strips of the hot-rolled silicon steel strip made of 0.007% by weight, C: 0.05% by weight, the balance Fe and inevitable impurities are 110
After annealing at 0 ° C. for 2 minutes, it was 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 wet water atmosphere in order to also serve as decarburization, and primary recrystallization was performed. Next, in order to stabilize the secondary recrystallization, nitriding treatment was performed in an ammonia atmosphere so that the total nitrogen content was 150 ppm in both steel sheets, and the inhibitor was strengthened.

After that, an annealing separator composed of Al ₂ O ₃ : 100% is electrostatically applied, and the atmosphere of finish annealing is set to 25% N.
_The temperature was set to 2-75% H _2, and the temperature was raised to 1050 ° C. while maintaining the rate of temperature increase of 150 ° C./Hr. After reaching 1050 ° C., the temperature was maintained for 10 hours.

Thereafter, the annealing atmosphere was further switched to 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 a phosphoric acid-chromic acid tension coating process. The characteristics of the obtained product are as shown in Table 2.

[0051]

[Table 2]

[0052]

According to the present invention, a grain-oriented electrical steel sheet having a high magnetic flux density and small irregularities on the surface of the steel sheet (which is a mirror surface) which is a factor that obstructs magnetic properties can be easily obtained, and magnetic domains for laser beam irradiation treatment or the like can be obtained. The subdivided, tension-coated treatment provided a magnetic material with extremely low iron loss. In the production of this grain-oriented electrical steel sheet, mirror finishing of the steel sheet is carried out in a normal finish annealing furnace, so that it is extremely easy and its industrial value is enormous.

[Brief description of drawings]

FIG. 1 is a 5000 × electron micrograph of precipitates extracted from a steel sheet subjected to ammonia nitridation after primary recrystallization annealing.

 ─────────────────────────────────────────────────── ─── Continued Front Page (72) Inventor Hiroyasu Fujii 20-1 Shintomi, Futtsu City Nippon Steel Corporation Technical Development Division

Claims (5)

[Claims] 1. Si: 2.0 to 4.8% by weight, acid-soluble Al: 0.008 to 0.020% by weight, N ≦ 0.010.
A silicon hot-rolled steel strip consisting of weight%, balance Fe and unavoidable impurities is annealed as required, and then cold rolled once or twice or more with intermediate annealing to obtain a predetermined plate thickness, and then primary In the production of a grain-oriented silicon steel sheet in which a recrystallization annealing is performed and then an annealing separator is applied and finish annealing is performed, the amount of acid-soluble Al obtained by subtracting N equivalent of Al from acid-soluble Al in a hot-rolled steel strip is 0. After the primary recrystallization annealing, the annealing separator containing Al ₂ O ₃ (alumina) as the main component is applied to the content of 0.010% by weight or less, and the temperature increase rate of the final annealing is 50 ° C./Hr in a neutral or reducing atmosphere. A method for producing a high magnetic flux density grain-oriented silicon steel sheet having no forsterite coating, which comprises heating to 920 to 1150 ° C. and holding the temperature for 5 hours or more. 2. The amount of acid-soluble Al obtained by subtracting N equivalent of Al from acid-soluble Al in a hot-rolled steel strip is set to 0.010 wt% or less, and after primary recrystallization annealing, nitriding treatment with ammonia is performed, and thereafter, , An annealing separator mainly composed of Al ₂ O ₃ (alumina) is applied, and the temperature rising rate of finish annealing in a neutral or reducing atmosphere is 50 ° C./hr or more at 920 to 1150 ° C.
The method for producing a high magnetic flux density grain-oriented silicon steel sheet having no forsterite coating according to claim 1, wherein the temperature is raised to and maintained for 5 hours or more. 3. N ₂ gas of 5% in the atmosphere during finish annealing.
The method for producing a high magnetic flux density grain-oriented silicon steel sheet having no forsterite coating according to claim 1 or 2, characterized by the above. 4. The method for producing a high magnetic flux density grain-oriented silicon steel sheet having no forsterite coating according to claim 1 or 2, wherein the annealing separator is applied electrostatically. 5. The method for producing a high magnetic flux density grain-oriented silicon steel sheet without a forsterite coating according to claim 1, wherein an oxide of an alkaline earth metal other than MgO is used as the annealing separator.