JPH0681039A - Production of mirror-finished grain-oriented silicon steel sheet with high magnetic flux density - Google Patents

Abstract

PURPOSE:To obtain a grain oriented silicon steel sheet having high magnetic flux density by means of short-time finish annealing. CONSTITUTION:This steel has a composition which consists of 2-4.8% Si, 0.008-0.020% acid-soluble Al, <=0.010% N, and the balance Fe with inevitable impurities and where the amount of acid-soluble Al, as the remainder obtained by subtracting Al of N equivalent from the acid-soluble Al in a hot rolled steel strip, is regulated to <=0.01%. After cold rolling, primary recrystallization annealing is done and the oxide layer formed in the above stage is removed from the surface of the resulting steel sheet by means of pickling, etc. After a separation agent at annealing, composed essentially of alumina, is applied, the steel sheet is subjected to temp. rise from 920 up to 1150 deg.C at >=50 deg.C/Hr temp. rise rate in a neutral or reducing atmosphere and held at the above temp. for >=5hr. Then, purification annealing is done. Moreover, nitriding by using ammonia can be done before the removal of the oxide layer in order to perform inhibitor strengthening. By this method, the grain oriented silicon steel sheet with high magnetic flux density can be obtained in a short time, and, an ultralow iron loss material can be formed by applying magnetic domain fractionizing treatment and tension coating.

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 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 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. This remedy, that is, means for preventing the magnetic domain refining effect from disappearing even when stress-relief annealing is disclosed, for example, in 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 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 the annealing separator mainly composed of MgO, which is usually used, 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 to specialize the heating cycle. Finish annealing is performed to obtain grain-oriented electrical steel sheet with high magnetic flux density, and at the same time, the glass (forsterite) film is not formed on the surface of the steel sheet, but secondary recrystallization is performed with the metal surface exposed, and at the same time, thermal It is characterized in that the metal surface is mirror-finished (smoothed) by etching.

That is, in the present invention, Si: 2.0 to 4.8% by weight, acid-soluble Al: 0.008 to 0.020% by weight, N
≦ 0.010% by weight, the remaining hot-rolled steel strip made of Fe and unavoidable impurities is annealed as needed, and then cold rolled once or twice or more with intermediate annealing, to obtain a predetermined plate thickness. Then, in the production of a grain-oriented silicon steel sheet which is then subjected to primary recrystallization annealing and then applied with an annealing separator, and subjected to finish annealing, acid-soluble Al obtained by subtracting N equivalent of Al from acid-soluble Al in the hot-rolled steel strip. Content of 0.010% by weight or less, after the primary recrystallization annealing, the oxide film on the surface of the steel sheet generated in the same annealing step is removed, and an annealing separator mainly composed of Al ₂ O ₃ (alumina) is applied. Or a reducing atmosphere, the temperature increase rate of finish annealing is raised to 920 to 1150 ° C. at 50 ° C./hr or more, and after reaching this temperature, the temperature is maintained for 5 hours or more.

Here, the method for removing the oxide film on the surface of the steel sheet is preferably acid pickling, and in particular, pickling with an acid mixed with hydrofluoric acid oxidizes the surface of the steel sheet that occurs during primary recrystallization annealing. Effective for removing layers. It is effective from the aspect of inhibitor strengthening to perform nitriding treatment with ammonia before starting the step of removing the oxide film on the steel sheet surface from the primary recrystallization annealing and to make N _{2 in} the atmosphere during finish annealing 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. It is also possible to use an oxide of an alkaline earth metal other than MgO 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.

Regarding the oxidation of Al at the steel sheet interface, oxygen in the oxidized surface of the steel sheet generated during the primary recrystallization annealing and oxygen in the annealing atmosphere serve as oxygen supply sources to oxidize Al.
Therefore, if the oxide layer generated during the primary recrystallization annealing is removed,
The source of oxygen is reduced, and strong inhibitor strength can be maintained even at high temperatures. The inhibitor strength indicates the concentration of acid-soluble Al in steel. Al is AlN, (A
Since a compound (precipitate) such as l, Si) N is formed to serve as an inhibitor, the amount of acid-soluble Al can be considered as an index indicating the inhibitor strength.

As described above, the acid-soluble Al becomes Al ₂ O ₃ by capturing a trace amount of oxygen (water) in the annealing atmosphere.
The temperature is raised to the temperature required for secondary recrystallization in a short time and acid-soluble A
It is effective to shorten the reaction time between 1 and oxygen in the annealing atmosphere to suppress inhibitor deterioration. In the experiments conducted by the present inventors, rapid heating of 50 ° C./Hr 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.

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 are Al
Oxidation of Al is suppressed in order to increase the amount of precipitates such as N to fix Al and reduce the migration of Al to the steel sheet interface.

Therefore, acid-soluble Al in steel during finish annealing
Regarding the amount, the higher the nitrogen partial pressure is, the less the deterioration is, and the stronger the inhibitor is at high temperature. 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 by the strength of secondary recrystallization.

Here, the present inventors calculated the amount of acid-soluble Al obtained by subtracting N equivalent of Al from the acid-soluble Al in the hot-rolled steel strip as 0.
The reason for setting the content to 010% by weight or less will be described.

FIG. 1 is a photograph in which precipitates inside the steel sheet after nitriding with ammonia after primary recrystallization annealing 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 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 has grown to 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
In order to optimize the amount of N, acid-soluble Al other than AlN existing in the hot-rolled steel strip was specified. As a result, A produced mainly after the primary recrystallization as an inhibitor
In could be set to some proper 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, the nitrogen in the steel increases and the dissolution of Al decreases, so
Although the decrease of acid-soluble Al is suppressed, if uncombined Al that does not form compound AlN more than necessary is present, the amount of AlN produced after nitriding as an inhibitor becomes excessive, and secondary recrystallization is completed. Takes a long time, or the magnetic flux density decreases, which is a disadvantage. 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.

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 does not contain oxygen in the atmosphere or oxygen more than the oxide of the 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, 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. During primary recrystallization annealing, methods for removing the oxide layer formed on the surface of the steel sheet include mechanical polishing, for example, brush polishing, sandpaper polishing, grinding, etc., which are effective for this purpose, but are industrially difficult. Therefore, it is not very practical.

The present inventors have found that the method using pickling 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), hydrochloric acid, sulfuric acid,
Although a mineral acid such as nitric acid is effective, the oxide layer formed on the surface of the steel sheet is an oxide mainly composed of SiO ₂ , and therefore it is difficult to pickle it with only a mineral acid such as hydrochloric acid, sulfuric acid or nitric acid. Mixing these acids with hydrofluoric acid is very efficient, namely:
The oxide layer can be removed at 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 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 was 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, and argon, and is used for redox of silicon. On the other hand, it means 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 steel and to oxidize silicon in steel so as not to produce SiO ₂ on the surface or to increase SiO ₂ .

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 or moisture in the finish annealing furnace causes Small amount of SiO during finish annealing
₂ produces. If MgO is present here, a forsterite film (glass film) is formed, so a substance that does not react with or is difficult to react with SiO ₂ is required as an annealing separator.

Al ₂ is suitable for this purpose.
O ₃ (alumina) is the best. There is no problem in adding a small amount of the following substances to alumina. 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 and SrO were effective.

Further, in order to prevent the formation of a coating on the surface of the steel sheet, a method in which a substance having a low reactivity, for example, increasing the particle size of the powder or applying it to the surface of the steel sheet without forming a hydrate has been 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 increase up to the temperature at which secondary recrystallization is possible in finish annealing, the more favorable the inhibitor is because the deterioration of the inhibitor is small. When the temperature rising rate was less than 15 ° C./Hr, the inhibitor was remarkably deteriorated and the inhibitor required 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 ℃ / 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. Generally, the higher the partial pressure of nitrogen, the better, but if it is too high, it will take too long to complete the secondary recrystallization, and the secondary recrystallization will not occur. After the completion of secondary recrystallization, it is extremely effective to increase the hydrogen concentration and maintain the temperature at around 1200 ° C. for several hours in order to completely purify and mirror-finish the surface of the steel sheet. The steel composition in the present invention is S
i: 2.0 to 4.8% by weight, acid-soluble Al: 0.008
~ 0.020% by weight, 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 an important constituent element of the inhibitor, 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.

When the content of nitrogen exceeds 0.010% by weight, vacancies called blisters are generated 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, the 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 done in Japanese Patent Publication No. 40-
As disclosed in Japanese Patent No. 15644, the final cold rolling rate is 80% or more.

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 content 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 the primary recrystallization is removed. This method includes physical and chemical methods as described above, but is generally carried out by pickling. It is extremely effective to improve the magnetic flux density by nitriding with ammonia for strengthening the inhibitor before removing the oxide layer on the surface of the steel sheet. 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.

The temperature rising rate of finish annealing is as large as possible in order to prevent inhibitor deterioration, and is preferably 50.
℃ / Hr or more. The atmosphere during the temperature rise of finish annealing is neutral or reducing, and after reaching 920 ° C to 1150 ° C, the temperature is 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 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.

[0047]

【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 inevitable impurities at 1100 ° C
After annealing 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 for double decarburization, and primary recrystallization was performed.

Next, in order to stabilize the secondary recrystallization, nitriding treatment is performed in an ammonia atmosphere, and the total nitrogen content is adjusted to 160
ppm to enhance the inhibitor. 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 in an atmosphere of 25% N ₂ -75% H ₂ at 150 ° C. /
The temperature is raised while maintaining the heating rate of Hr and 30 ° C / Hr,
The temperature was raised to 1100 ° C.

After reaching 1100 ° C., the temperature was maintained for 10 hours. After that, 100% hydrogen is added and further 1200 ° C.
The temperature was raised to and maintained at this temperature 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.

[0050]

[Table 1]

Example 2 The hot-rolled silicon steel strip of Example 1 was annealed at 1100 ° C. for 2 minutes and then cold-rolled to a thickness of 0.20 mm. These cold-rolled sheets were annealed in an annealing furnace in a wet water atmosphere to also serve as decarburization.
It was annealed at 0 ° C. for 2 minutes for primary recrystallization. Next, in order to stabilize the secondary recrystallization, nitriding treatment was performed in an ammonia atmosphere to adjust the total nitrogen amount to 155 ppm and strengthen the inhibitor.

After that, the oxide layer formed on the surface of the steel sheet was removed with sulfuric acid mixed with hydrofluoric acid. Al ₂ O ₃ : 100
Of 10% N ₂ -90
In a% H ₂ atmosphere, the temperature was raised while maintaining the temperature rising rates of 150 ° C./Hr and 15 ° C./Hr to 1050 ° C. After reaching 1050 ° C., the temperature was maintained for 10 hours.
Then, 100% hydrogen was used, the temperature was further raised to 1200 ° C., and the temperature was maintained for 10 hours. After finishing annealing,
Irradiation with a laser beam was performed for phosphoric acid-chromic acid-based tension coating treatment. The characteristics of the obtained product are shown in Table 2.
Is the street.

[Table 2]

Example 3 Si: 3.3% by weight, acid-soluble Al: 0.020% by weight, N: 0.008% 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: 0.008% 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, the total nitrogen content was set to 180 ppm in both steel sheets, 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% was electrostatically applied, and the atmosphere was removed in an atmosphere of 25% N ₂ -75% H ₂ for 15 minutes.
The temperature was raised to 1050 ° C. while maintaining the temperature rising rate of 0 ° C./Hr. After reaching 1050 ° C., the temperature was maintained for 10 hours.

After that, 100% hydrogen was added, and then 120
The temperature was raised to 0 ° C. and kept at this temperature for another 10 hours.
After finishing annealing, irradiate with laser beam, phosphoric acid-
Chromic acid-based tension coating treatment was performed. The characteristics of the obtained product are as shown in Table 3.

[0057]

[Table 3]

[0058]

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.

─────────────────────────────────────────────────── ───

[Procedure amendment]

[Submission date] December 2, 1992

[Procedure Amendment 1]

[Document name to be amended] Statement

[Correction target item name] 0019

[Correction method] Change

[Correction content]

FIG. 1 is a schematic view of a photograph in which precipitates inside a steel sheet after nitriding with ammonia after primary recrystallization annealing are extracted and magnified 5000 times with an electron microscope.
The large precipitates are the Al and N that have already existed at the time of hot-rolled steel strip, and have grown to AlN. The small precipitate is AlN newly formed by ammonia nitriding.

[Procedure Amendment 2]

[Document name to be amended] Statement

[Name of item to be corrected] Figure 1

[Correction method] Change

[Correction content]

FIG. 1 is a schematic view of a 5000 × electron micrograph of a precipitate extracted from a steel sheet subjected to ammonia nitriding after primary recrystallization annealing.

Front Page Continuation (72) Inventor Hiroyasu Fujii 20-1 Shintomi, Futtsu City Nippon Steel Corp. Technical Development Division

Claims (7)

[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 oxide film on the surface of the steel sheet generated in the same annealing step is removed by 0.010% by weight or less.
An annealing separator containing l ₂ O ₃ (alumina) as a main component is applied, and the temperature is raised from 920 to 1150 ° C. at a heating rate of 50 ° C./hr or more in finish annealing in a neutral or reducing atmosphere, For 5 hours or more, a method for producing a specular high magnetic flux density grain-oriented silicon steel sheet. 2. The amount of acid-soluble Al obtained by subtracting N equivalent of Al from acid-soluble Al in the hot-rolled steel strip is 0.010% by weight or less, and after primary recrystallization annealing, nitriding treatment with ammonia strengthens the inhibitor. Then, the oxide film on the surface of the steel sheet generated in the same annealing step is removed, and then an annealing separator mainly composed of Al ₂ O ₃ (alumina) is applied to increase the temperature rising rate of finish annealing in a neutral or reducing atmosphere. 920 to 1 at 50 ° C / Hr or higher
The method for producing a specular high magnetic flux density grain-oriented silicon steel sheet according to claim 1, wherein the temperature is raised to 150 ° C. and the temperature is maintained for 5 hours or more. 3. The method for producing a specular high magnetic flux density grain-oriented silicon steel sheet according to claim 1 or 2, wherein the method of removing the oxide film on the surface of the steel sheet is pickling. 4. The method for producing a specular high magnetic flux density grain-oriented silicon steel sheet according to claim 1 or 2, wherein the method for removing the oxide film on the surface of the steel sheet is pickling with an acid mixed with hydrofluoric acid. . 5. An N ₂ gas content of 5% in the atmosphere during finish annealing.
It is above, The manufacturing method of the mirror surface high magnetic flux density grain oriented silicon steel plate of Claim 1 or Claim 2 characterized by the above-mentioned. 6. The method for producing a specular high magnetic flux density grain-oriented silicon steel sheet according to claim 1 or 2, wherein the annealing separator is applied electrostatically. 7. The method for producing a specular high magnetic flux density grain-oriented silicon steel sheet according to claim 1 or 2, wherein an oxide of an alkaline earth metal other than MgO is used as the annealing separator.