JP2014156619A - METHOD FOR PRODUCING GRAIN-ORIENTED ELECTROMAGNETIC STEEL SHEET, AND MgO FOR ANNEALING SEPARATING AGENT - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing grain-oriented electromagnetic steel sheet that can effectively remove nitrogen and produce a grain-oriented electromagnetic steel sheet having a good magnetic property in an industrially stable manner, even though it is a method for producing grain-oriented electromagnetic steel sheet in which nitrogen is added to the steel sheet before secondary recrystallization for the purpose of forming silicon nitride.SOLUTION: In a method for producing grain-oriented electromagnetic steel sheet comprising a series of processes including: subjecting a steel slab whose N is controlled within a range of [acid soluble Al (sol. Al) mass ppm/26.98]×14.00≤N≤80 mass ppm and the remainder is controlled within a predetermined composition, to hot rolling, without re-heating or after re-heating, to yield a hot rolled sheet; followed by annealing and rolling to yield a cold rolled sheet having a final sheet thickness; subjecting the same to nitriding treatment such that the increased nitrogen amount became 50 to 1000 mass ppm until before secondary recrystallization annealing; followed by coating the same with an annealing separating agent; and controlling the residence time during the interval between 300°C and 800°C in the temperature rising process of secondary recrystallization annealing within 5 hours or more and 150 hours or less, an annealing separating agent that contains MgO by 50 mass% or more and that satisfies the condition that the citric acid activity of the MgO is 60 seconds≤CAA30≤90 seconds as well as 150 seconds≤CAA70≤250 seconds, is used as the annealing separating agent.

Description

  The present invention relates to a method for producing a grain-oriented electrical steel sheet having excellent magnetic properties and an MgO for annealing separator, which can obtain a grain-oriented electrical steel sheet having excellent magnetic properties at low cost.

  A grain-oriented electrical steel sheet is a soft magnetic material used as a core material for transformers and generators, and has a crystal structure in which the <001> orientation, which is the easy axis of iron, is highly aligned in the rolling direction of the steel sheet. . Such a texture preferentially grows grains of the {110} <001> orientation called the Goss orientation during secondary recrystallization annealing during the production process of grain-oriented electrical steel sheets. Formed through secondary recrystallization.

  Conventionally, such a grain-oriented electrical steel sheet is heated to 1300 ° C or higher by heating a slab containing about 4.5 mass% or less of Si and an inhibitor component such as MnS, MnSe, and AlN to once dissolve the inhibitor component. After that, it is hot-rolled and subjected to hot-rolled sheet annealing as necessary, to obtain the final sheet thickness by one or more cold rollings sandwiching intermediate annealing, followed by primary recrystallization annealing in a wet hydrogen atmosphere After performing primary recrystallization and decarburization, and then applying an annealing separator mainly composed of magnesia (MgO), the secondary recrystallization and the inhibitor component are purified at 1200 ° C. for about 5 hours. It has been manufactured by performing final finish annealing (for example, Patent Document 1, Patent Document 2, and Patent Document 3).

  As described above, when manufacturing conventional grain-oriented electrical steel sheets, precipitates (inhibitor components) such as MnS, MnSe, and AlN are included in the slab stage, and these are heated at a high temperature exceeding 1300 ° C. The step of causing secondary recrystallization by once dissolving the inhibitor component of this compound and then precipitating it finely in a subsequent step has been adopted.

  As described above, in the manufacturing process of conventional grain-oriented electrical steel sheets, slab heating at a high temperature exceeding 1300 ° C. is necessary, so the manufacturing cost has to be extremely high, and in recent years the manufacturing cost has been reduced. He left a problem where he was unable to meet the demand.

  In order to solve such a problem, for example, in Patent Document 4, 0.010 to 0.060% of acid-soluble Al (sol. Al) is contained, slab heating is suppressed to a low temperature, and an appropriate nitriding atmosphere is used in the decarburization annealing process. A method has been proposed in which (Al, Si) N is precipitated during secondary recrystallization by nitriding and used as an inhibitor. (Al, Si) N functions as an effective inhibitor by being finely dispersed in the steel, but since the inhibitor strength is determined by the Al content, it is sufficient if the accuracy of Al in steelmaking is insufficient. In some cases, it was not possible to obtain a sufficient grain growth inhibiting force. Many methods have been proposed in which nitriding treatment is performed in the middle of the process and (Al, Si) N or AlN is used as an inhibitor. Recently, however, a manufacturing method in which the slab heating temperature exceeds 1300 ° C has been disclosed. ing.

On the other hand, as a technique for causing secondary recrystallization without containing an inhibitor component in the slab, Patent Document 5 discloses a technique (inhibitorless method) that allows secondary recrystallization without containing an inhibitor component. ing.
Here, the inhibitorless method is a technique in which secondary recrystallization is manifested by texture (control of texture) using higher-purity steel. However, the inhibitorless method does not require high-temperature slab heating and enables production of grain-oriented electrical steel sheets at a low cost. As a result, there is a disadvantage that the magnetic characteristics of the product are likely to vary.

  In addition, since texture control is an important factor for magnetic properties, many conditions have been proposed for warm rolling and the like in which texture control is performed. If this texture control cannot be performed sufficiently, the degree of integration in the Goss orientation ({110} <001>) after secondary recrystallization will be lower and the magnetic flux density will tend to be lower than in the technique using inhibitors. Become.

U.S. Patent No. 1965559 Japanese Patent Publication No. 40-15644 Japanese Patent Publication No.51-13469 Japanese Patent No. 2782086 Japanese Unexamined Patent Publication No. 2000-129356

As described above, it has often been difficult to stably achieve good magnetic properties in the method of manufacturing grain-oriented electrical steel sheets that has been proposed so far.
On the other hand, the inventors deposited silicon nitride instead of AlN by applying nitrogen augmentation while avoiding high-temperature slab heating using a component according to the inhibitorless component in which Al was suppressed to less than 100 ppm. Thus, by making this silicon nitride function as a suppressive force for normal grain growth, the inventors have come up with a method for producing grain-oriented electrical steel sheets that have significantly reduced variations in magnetic properties and have industrially stable and good properties.

  However, in the above method, as an additional inhibitor, it is necessary to add nitrogen to the steel sheet before the secondary recrystallization for the purpose of forming silicon nitride, but this nitrogen is unnecessary after the secondary recrystallization. Therefore, a new problem has been revealed that the amount of nitrogen that must be removed from the steel by the end of annealing increases, so that there is nitrogen that cannot be removed by the conventional method and the iron loss increases.

  The present invention has been developed in view of the above situation, and even in a method for producing a grain-oriented electrical steel sheet in which nitrogen is added to a steel sheet before secondary recrystallization for the purpose of forming silicon nitride, nitrogen is efficiently used. An object of the present invention is to provide a method for producing a grain-oriented electrical steel sheet that is removed and industrially stable and has good magnetic properties.

  Nitrogen is removed (purified) from the steel by releasing nitrogen into the annealing atmosphere before the forsterite film is formed at high temperature (gas phase emission), and after forming the forsterite film in the film as a nitride. It is known that there is a method of fixing nitrogen.

  In this manufacturing method, since so-called nitrogen increase is performed, the amount of nitrogen that needs to be purified increases and exceeds the amount of nitrogen that can be fixed in the coating. Therefore, the inventors diligently studied a method for purifying nitrogen by promoting the vapor phase release of nitrogen by slowing the formation of the forsterite film. As a result, it is possible to increase the film formation rate after vapor phase release while slowing the film formation, that is, by suppressing the film formation reaction at low temperature and promoting the film formation reaction at high temperature, The present invention was completed by finding a method for maintaining a good coating appearance after secondary recrystallization annealing while securing time.

The gist of the present invention is as follows.
1. In addition to containing C: 0.08% or less, Si: 2.0 to 4.5%, and Mn: 0.5% or less, S, Se, and O are each suppressed to less than 50 ppm by mass, and sol.Al is suppressed to less than 100 ppm by mass. Further, N is controlled in the range of acid-soluble [Al (sol.Al) mass ppm / 26.98] × 14.00 ≦ N ≦ 80 mass ppm, and the remainder is reheated to a steel slab having a composition of Fe and inevitable impurities. Without re-heating or after re-heating, hot-rolled sheet is used to make a cold-rolled sheet with the final thickness by annealing and rolling. A series of directions in which an annealing separator is applied after nitriding to a mass ppm or less, and the residence time between 300 ° C. and 800 ° C. is set to 5 hours or more and 150 hours or less in the temperature raising process of secondary recrystallization annealing. In the production of heat-resistant electrical steel sheets,
The annealing separator contains MgO in an amount of 50 mass% or more, and the MgO has a citric acid activity of 60 seconds ≦ CAA30 ≦ 90 seconds and 150 seconds ≦ CAA70 ≦ 250 seconds. A method for producing grain-oriented electrical steel sheets.

2. _{2. The} method for producing a grain-oriented electrical steel sheet according to 1 above, further comprising, as an annealing separator, titanium oxide in terms of TiO ₂ in an amount of 5 parts by mass to 20 parts by mass with respect to 100 parts by mass of the annealing separator. .

3. Furthermore, the said steel slab is mass%,
Ni: 0.005-1.50%,
Sn: 0.01 to 0.50%,
Sb: 0.005-0.50%,
Cu: 0.01 to 0.50%,
Cr: 0.01 to 1.50%,
P: 0.0050 to 0.50%,
Mo: 0.01-0.50% and
Nb: 0.0005-0.0100%
The method for producing a grain-oriented electrical steel sheet according to 1 or 2 above, comprising one or more selected from among the above.

4). MgO for annealing separator used in the method for producing a grain-oriented electrical steel sheet according to any one of 1 to 3 above, wherein the citric acid activity satisfies 60 seconds ≦ CAA30 ≦ 90 seconds and 150 seconds ≦ CAA70 ≦ 250 seconds.

  According to the present invention, even if high temperature slab heating is not performed, the magnetic property variation is greatly reduced, and a grain-oriented electrical steel sheet having industrially stable and good characteristics can be obtained.

After decarburization annealing, nitriding treatment is performed so as to obtain 100 ppm and 500 ppm nitriding increase, and the residence time from 300 ° C. to 800 ° C. during secondary recrystallization annealing is 8 hours. It is the identification result by EDX (energy dispersive X-ray spectroscopy) of the structure | tissue photograph which immediately cooled with water after heating up to 800 degreeC, and observed the structure | tissue with the electron microscope, and the deposit in the structure | tissue.

Hereinafter, the present invention will be specifically described.
First, the reason for limiting the component composition of the steel slab in the present invention will be described. Unless otherwise specified, “%” and “ppm” in relation to ingredients mean mass% and mass ppm, respectively.
C: 0.08% or less C is an element useful for improving the primary recrystallization texture. However, if the content exceeds 0.08%, the primary recrystallization texture is deteriorated. % Or less. A desirable addition amount from the viewpoint of magnetic properties is in the range of 0.01 to 0.06%. If the required magnetic property level is not so high, C may be set to 0.01% or less in order to omit or simplify the decarburization in the primary recrystallization annealing.

Si: 2.0-4.5%
Si is a useful element that improves iron loss by increasing electric resistance. However, if the content exceeds 4.5%, the cold rolling property deteriorates remarkably, so Si is limited to 4.5% or less. In the present invention, Si needs to function as a nitride-forming element, so it is important to contain 2.0% or more. In addition, the desirable addition amount from the viewpoint of iron loss is in the range of 2.0 to 4.5%.

Mn: 0.5% or less
Mn has the effect of improving hot workability during production, but if the content exceeds 0.5%, the primary recrystallized texture deteriorates and causes deterioration of magnetic properties, so Mn is 0.5 % Or less. The lower limit of Mn is not particularly defined, but is preferably about 0.01%.

S, Se, and O: less than 50 ppm each When the amount of S, Se, and O is 50 ppm or more, it becomes difficult to obtain a desired secondary recrystallization. This is because coarse oxides and MnS and MnSe coarsened by slab heating make the primary recrystallized structure non-uniform. Accordingly, S, Se, and O are all suppressed to less than 50 ppm.

sol.Al: less than 100ppm
Al forms a dense oxide film on the surface, which may make it difficult to control the amount of nitridation during nitridation or inhibit decarburization. Therefore, Al is suppressed to less than 100 ppm in terms of sol.Al. did. However, Al with high oxygen affinity can be added in a range of less than 100ppm because it can be added in a small amount in steelmaking to reduce the amount of dissolved oxygen in the steel and reduce oxide inclusions that lead to deterioration of the steel sheet properties. By doing so, magnetic deterioration can be suppressed.

N: (sol.Al/26.98)×14.00 ppm ≦ N ≦ 80 ppm
Since the present invention is characterized by precipitating silicon nitride in the steel sheet structure after nitriding, it is important to previously contain N equal to or more than N that precipitates as AlN with respect to the amount of Al contained. . That is, since each atom of AlN is bonded at a ratio of 1: 1, it is possible to add N in the amount of (sol.Al) × [N atomic weight (14.00) / Al atomic weight (26.98)] in the steel. A small amount of Al contained in can be deposited before nitriding. On the other hand, since it may cause defects such as “swelling” at the time of slab heating, it must be suppressed to 80 ppm or less. Further, it is desirably 60 ppm or less.

The essential components have been described above. In the present invention, the following elements can be appropriately contained as components that improve the magnetic properties more stably industrially. The balance is Fe and inevitable impurities.
Ni: 0.005-1.5%
Ni works to improve the magnetic properties by increasing the uniformity of the hot-rolled sheet structure. For this purpose, it is preferable to contain 0.005% or more, but if the content exceeds 1.5%, the desired secondary Since it becomes difficult to obtain recrystallization and the magnetic properties deteriorate, it is desirable to contain Ni in the range of 0.005 to 1.5%.

Sn: 0.01-0.50%
Sn is a useful element that suppresses nitriding and oxidation of steel sheets during secondary recrystallization annealing and promotes secondary recrystallization of grains having good crystal orientation to improve magnetic properties. However, if it exceeds 0.50%, the cold rolling property deteriorates, so it is desirable to contain Sn in the range of 0.01 to 0.50%.

Sb: 0.005-0.50%
Sb is a useful element that effectively suppresses nitridation and oxidation of steel sheets during secondary recrystallization annealing, promotes secondary recrystallization of grains with good crystal orientation, and effectively improves magnetic properties. For the purpose, it is preferable to contain 0.005% or more, but if it exceeds 0.50%, the cold rolling property deteriorates, so Sb is preferably contained in the range of 0.005 to 0.50%.

Cu: 0.01-0.50%
Cu suppresses oxidation of the steel sheet during secondary recrystallization annealing, promotes secondary recrystallization of grains having a good crystal orientation, and effectively improves magnetic properties. However, if it exceeds 0.50%, the hot rolling property is deteriorated, so it is desirable to contain Cu in the range of 0.01 to 0.50%.

Cr: 0.01 to 1.50%
Cr has a function of stabilizing the formation of the forsterite film, and for that purpose, it is preferable to contain 0.01% or more, but when the content exceeds 1.50%, a desired secondary recrystallization can be obtained. Since it becomes difficult and the magnetic properties deteriorate, it is desirable to contain Cr in the range of 0.01 to 1.50%.

P: 0.0050 ~ 0.50%
P has a function of stabilizing the formation of the forsterite film. For that purpose, P is preferably contained in an amount of 0.0050% or more. However, if the content exceeds 0.50%, the cold rolling property deteriorates, so P is 0.0050 to It is desirable to make it contain in 0.50% of range.

Nb: 0.0005-0.0100%, Mo: 0.01-0.50%
Nb and Mo have an effect of suppressing sag after hot rolling through suppression of cracking due to temperature change during slab heating. If these elements do not contain at least one of the above lower limit value or more, the effect of suppressing heges is small. On the other hand, if either of these elements exceeds the above upper limit, carbide or nitride is formed, and the final product. In order to cause deterioration of the iron loss when remaining up to, it is desirable to be in the above range.

Next, the manufacturing method of this invention is demonstrated.
The steel slab adjusted to the above preferable component composition range is subjected to hot rolling without being reheated or after being reheated. When the slab is reheated, the reheating temperature is preferably about 1000 ° C. or higher and 1300 ° C. or lower. This is because slab heating above 1300 ° C is meaningless for the present invention, which contains almost no inhibitor in the steel at the slab stage, and only increases costs. On the other hand, when the slab is heated at a temperature lower than 1000 ° C., the rolling load at the time of hot rolling becomes high and it becomes difficult to perform rolling.

  Next, the hot-rolled sheet is annealed and rolled to obtain a cold-rolled sheet having a final thickness. Specifically, after performing hot-rolled sheet annealing as necessary, it is subjected to one or more cold rollings or two or more cold rollings sandwiching intermediate annealing to obtain a final cold-rolled sheet. This cold rolling may be performed at normal temperature, or may be warm rolling in which the steel sheet temperature is raised to a temperature higher than normal temperature, for example, about 250 ° C.

  Subsequently, primary recrystallization annealing is applied to the final cold rolled sheet. The purpose of this primary recrystallization annealing is to adjust the primary recrystallization grain size optimal for secondary recrystallization by primary recrystallization of a cold rolled sheet having a rolled structure. For that purpose, it is desirable to set the annealing temperature of the primary recrystallization annealing to about 800 ° C. or more and less than 950 ° C. Note that the annealing atmosphere at this time may be wet hydrogen nitrogen or wet hydrogen argon atmosphere, and may also serve as decarburization annealing.

The nitriding treatment for increasing the amount of nitrogen in the present invention is performed after the cold rolling and before the application of the annealing separator after the primary recrystallization annealing. The nitriding method is not particularly limited as long as the amount of nitriding to be increased can be controlled. For example, the nitriding method has been implemented in the past, for example, a method of performing gas nitriding using NH ₃ atmosphere gas in a coil form, Thus, it is possible to adopt a method of continuously nitriding. It is also preferable to use salt bath nitriding, which has a higher nitriding ability than gas nitriding.

In the nitriding process, an important point is to form a nitride layer on the surface layer. In particular, it is desirable to perform nitriding at a temperature of 800 ° C. or less in order to suppress diffusion into the steel, but by reducing the time to a short time (for example, about 30 seconds), the nitride is applied to the surface even at high temperatures A layer can be formed. In addition, it is important that the nitrogen increase by nitriding is 50 ppm or more and 1000 ppm or less. Note that the nitrogen concentration is a value averaged to the average in the thickness direction of the steel plate even if it is concentrated in a part of the steel plate.
This is because if the nitrogen increase is less than 50 ppm, the effect cannot be obtained sufficiently, while if it exceeds 1000 ppm, the amount of silicon nitride deposited becomes excessive and secondary recrystallization does not occur. Desirably, it is 200 ppm or more and 1000 ppm or less.

After the nitriding treatment, an annealing separator is applied to the surface of the steel sheet. The annealing separator is 50% by mass in the annealing separator to form a forsterite film on the steel sheet surface after the secondary recrystallization annealing. The above is MgO. In the present invention, titanium oxide can be further contained in an amount of 5 parts by mass or more and 20 parts by mass or less in terms of TiO ₂ with respect to 100 parts by mass of the annealing separator. This is because by controlling the titanium oxide to such a content, there is an advantage that the appearance of the coating is improved or the adhesion of the coating is improved.

Furthermore, in the present invention, it is necessary to suppress the low-temperature reactivity of the forsterite coating in order to purify nitrogen that has become unnecessary after the secondary recrystallization into the gas phase, and for this purpose, the citric acid activity of MgO is 60 seconds ≦ CAA30 ≦ 90 seconds. In order to promote the reactivity of the forsterite film after nitrogen purification, the citric acid activity of MgO needs to be adjusted to 150 seconds ≦ CAA70 ≦ 250 seconds.
By using such MgO, it is possible to effectively achieve both nitrogen purification and good coating appearance as described below.

Here, citric acid activity CAA30 is the time required for 0.4N citric acid and 30% MgO to react at 30 ° C. CAA70 is also 0.4N citric acid and 70% MgO 30 Time to react at ℃. It is not particularly unusual to define MgO reactivity by citric acid activity. For example, in Patent 3650525, CAA 70% is 250 to 1000 seconds, CAA 70% / CAA 40% is 1.5 to 6.0, Further, magnesia having a particle size 20% value of 1.2 μm or less and a BET value of 20.5 to 35 has been proposed.
Japanese Examined Patent Publication No. 57-45472 discloses a technique for forming a good film by keeping the concentration of impurities such as CaO, SO ₃ and B, the specific surface area, the particle diameter, and the distribution of citric acid activity within a predetermined range. Proposed.
However, there is no suggestion about MgO suitable for the production of the grain-oriented electrical steel sheet of the present invention using silicon nitride by increasing the amount of nitrogen. In addition, MgO using a known technique does not purify nitrogen. It was confirmed that a good film appearance could not be achieved.

  Therefore, in the present invention, CAA30 is used as an index of low-temperature reactivity, that is, a highly active ingredient. That is, in CAA20, since the activity to be evaluated is too high, an optimal value for purifying nitrogen cannot be obtained. In addition, the CAA 40 that has been widely used in the past has too low activity, and even if the CAA 70 described later is controlled, the optimum value for obtaining a good coating cannot be stably obtained. Therefore, in the present invention, CAA30 is defined.

  On the other hand, one-point evaluation of CAA30 was able to obtain a range of values optimal for nitrogen purification, but still could not stably obtain a good film. Therefore, by controlling CAA70 as an index related to high temperature reactivity, that is, an index of a low active ingredient of MgO, it has succeeded in stably obtaining a good film. In addition, since the reactivity evaluated by CAA80, CAA90, etc. is too low, a favorable film cannot be obtained even if any value is adjusted.

Here, an example of the manufacturing method of MgO used for this invention is shown.
Starting materials include mainly bitter juice (main component MgCl ₂ ) and seawater. This starting material is reacted with lime milk (Ca (OH) ₂ ) to obtain Mg (OH) ₂ . MgO having various activities can be produced by firing the obtained Mg (OH) ₂ at a temperature of about 700 ° C. to 1000 ° C. using a rotary kiln or a muffle furnace. At this time, MgO having a small CAA value (that is, high activity) can be obtained by firing at a low temperature for a short time, for example, 700 ° C. for 30 minutes. On the other hand, MgO having a large CAA value (ie, low activity) can be obtained by baking at a high temperature for a long time, for example, 1000 ° C. for 1 hour.

In addition, for the purpose of improving the purity of MgO and suppressing the dispersion of production, the MgO obtained once is reacted with water again to return to the form of Mg (OH) ₂ (rehydration), and again. There are also methods of firing in a rotary kiln or muffle furnace. MgO suitable for the present invention can be easily obtained by measuring the CAA value for each production lot and mixing lots having different CAA values. In particular, those fired in a muffle furnace are susceptible to uneven baking, and depending on the production lot, MgO suitable for the present invention can be obtained without mixing with other lots.
In addition, there are a method of baking magnesium carbonate, a method of obtaining MgO by directly baking MgCl ₂ , etc. Even if any method is used, the citric acid activity of MgO is 60 seconds ≦ CAA 30 ≦ 90 seconds It is important to select a condition that satisfies 150 seconds ≦ CAA70 ≦ 250 seconds.

  Following the application of the annealing separator, secondary recrystallization annealing is performed. In the secondary recrystallization annealing in the present invention, the residence time between 300 and 800 ° C. needs to be 5 hours or more and 150 hours or less. During this time, the surface nitride layer decomposes and N diffuses into the steel. In particular, in this component system in which Al that can form AlN does not remain, N which is a grain boundary segregation element diffuses into steel using the grain boundary as a diffusion path. Since silicon nitride has a high misfit rate with steel, its precipitation rate is extremely slow. Here, since precipitation of silicon nitride is intended to suppress normal grain growth, a sufficient amount needs to be selectively deposited on the grain boundary at the stage of 800 ° C. where normal grain growth proceeds.

And by setting the residence time in the temperature region to 5 hours or more, silicon nitride cannot be precipitated in the grains, but is connected to N that has diffused through the grain boundaries, and selectively on the grain boundaries. It can be deposited. Although it is not always necessary to set an upper limit, even if annealing is performed for more than 150 hours, only the energy required for annealing is required. As the annealing atmosphere, any of N ₂ , Ar, H ₂ or a mixed gas thereof is suitable.

  Produced through the process according to the present invention described above for slabs in which the amount of Al is suppressed and slabs that contain excess N with respect to AlN precipitation and hardly contain other inhibitor components typified by MnS, MnSe, etc. In the oriented grain-oriented electrical steel sheet, silicon nitride of coarser size (100 nm or more) is selectively formed at grain boundaries compared to conventional inhibitors in the secondary recrystallization annealing temperature raising process and the stage until the start of secondary recrystallization. can do.

  1 shows that after decarburization annealing, nitriding treatment is performed so that nitriding increase of 100 ppm and 500 ppm can be obtained, and the temperature is raised to 800 ° C. at a heating rate of 300 ° C. to 800 ° C. for a residence time of 8 hours. Immediately cooled with water, the structure was observed and identified with an electron microscope. As is clear from the figure, it is confirmed that coarse silicon nitride exceeding 100 nm is precipitated on the grain boundaries even if it is the smallest, unlike the fine precipitates (<100 nm) used conventionally. Is done.

  From the viewpoint of energy efficiency, it is clear that the secondary recrystallization heating process is most effective for precipitation of silicon nitride in terms of production. However, if a similar heat cycle is used, Since grain boundary selective precipitation is possible, a separate heat treatment can be performed as silicon nitride dispersion annealing before the long-time secondary recrystallization annealing.

After the secondary recrystallization annealing, an insulating film can be further applied and baked on the steel sheet surface. The type of the insulating coating is not particularly limited, and any conventionally known insulating coating is suitable. For example, a coating solution containing phosphate-chromate-colloidal silica described in JP-A-50-79442 and JP-A-48-39338 is applied to a steel plate and baked at about 800 ° C. The attaching method is suitable.
Further, the shape of the steel sheet can be adjusted by flattening annealing, and this flattening annealing can also be combined with the baking treatment of the insulating film.

Example 1
A silicon steel plate slab containing, in mass%, Si: 3.3%, C: 0.06%, Mn: 0.08%, S: 0.001%, Al: 0.002%, N: 0.002%, Cu: 0.05% and Sb: 0.01% After heating at 1100 ° C for 30 minutes, hot rolled to a hot rolled sheet with a thickness of 2.2 mm, after annealing at 1000 ° C for 1 minute, it was cold rolled to a final thickness of 0.23 mm, A sample having a size of 100 mm × 400 mm was taken from the center of the obtained cold rolled coil and subjected to primary recrystallization annealing, decarburization and nitriding (continuous nitriding treatment) in a laboratory. Nitriding treatment was performed by gas nitriding treatment with H ₂ —NH ₃ mixed gas at 750 ° C. for 20 seconds. The amount of nitrogen was 300 ppm for the total thickness, and 25 ppm when the surface layer (both sides) was 3 μm each shaved with sandpaper and the surface layer was removed. Subsequently, an annealing separator mixed at a ratio of 95% MgO and 5% TiO ₂ was applied to a water slurry, and then applied and dried. At this time, MgO adjusted to the CAA value described in Table 1 was used. MgO was prepared by firing magnesium hydroxide and adjusted by mixing MgO whose firing temperature was changed. The steel sheet was heated from 300 ° C. to 800 ° C. over 100 hours, then heated to 1200 ° C. at 50 ° C./h and subjected to final finish annealing at 1200 ° C. for 5 hours.
Subsequently applying baking of the insulating tension coating Phosphate, magnetic flux density at a magnetizing force _{800A / m (B 8, T} ), 1.7T, iron loss at 50Hz excited (W _17/50, W / kg) Measurements were made to evaluate the appearance of the film and the amount of nitrogen in the steel after the film was peeled off. The appearance of the film was determined to be good when the film coverage by visual observation was 99% or more and the color tone of the appearance was uniform.
The measurement and evaluation results are also shown in Table 1.

  As described in the table, in the invention examples according to the present invention, the film properties and nitrogen purification are good.

(Example 2)
After heating the silicon steel plate slab containing the components shown in Table 2 at 1200 ° C. for 20 minutes, hot rolling to a hot-rolled sheet having a thickness of 2.0 mm, annealing at 1000 ° C. for 1 minute, Thickness by cold rolling: Cold rolling to 1.5mm, intermediate annealing at 1100 ° C for 2 minutes, and final rolling thickness of 0.27mm by cold rolling shown below, PH ₂ O / PH ₂ = In an atmosphere of 0.3, decarburization annealing was performed by maintaining the annealing temperature at 820 ° C. for 2 minutes. Thereafter, nitriding treatment (under NH ₃ atmosphere) is performed on some coils by batch treatment to increase the amount of N in the steel by 550 ppm, and then MgO having CAA listed in Table 2 is the main component, and TiO ₂ After applying a slurry of 10% annealing separator mixed with water, it is wound into a coil, and the final finish annealing is performed at a temperature increase rate of 30 hours between 300 and 800 ° C. Subsequently, the product was subjected to flattening annealing for the purpose of applying and baking a phosphate-based insulating tension coating and flattening the steel strip.
The Epstein test piece was collected from the product coil thus obtained, the magnetic flux density (B ₈ ) was measured, and the appearance of the coating and the amount of ground nitrogen after stripping were evaluated.
The measurement and evaluation results are also shown in Table 2.

  As shown in the table, it can be seen that in the inventive examples according to the present invention, a high magnetic flux density is obtained, and the appearance of the coating film and the purification of the ground iron nitrogen are good.

(Example 3)
A silicon steel plate slab containing, in mass%, Si: 3.3%, C: 0.06%, Mn: 0.08%, S: 0.001%, Al: 0.002%, N: 0.002%, Cu: 0.05% and Sb: 0.01% After heating at 1100 ° C for 30 minutes, hot-rolled to a hot-rolled sheet with a thickness of 2.2 mm, annealed at 1000 ° C for 1 minute, and then cold-rolled to a final thickness of 0.23 mm. A sample with a size of 100 mm × 400 mm was taken from the center of the cold-rolled coil and annealed for both primary recrystallization and decarburization in a laboratory. Some samples were subjected to primary recrystallization annealing and decarburization annealing. Thereafter, nitriding treatment (batch treatment) was performed under the conditions shown in Table 3 to increase the amount of nitrogen in the steel. Nitrogen was quantified by chemical analysis for the total thickness and for the surface layer (both sides) 3 μm each with sandpaper and the sample excluding the surface layer. 21 steel plates with the same conditions were prepared for each condition, and MgO with CAA30 of 70 seconds and CAA70 of 210 seconds was formed into a water slurry using an annealing separator, and then applied, dried, and baked on the steel plate. Twenty of them were subjected to final finish annealing under the conditions shown in Table 3, followed by application and baking of a phosphate-based insulating tension coating, magnetic flux density (B ₈ , T) at a magnetizing force of 800 A / m, 1.7 T The iron loss (W _17/50 , W / kg) at 50 Hz excitation was evaluated. The magnetic characteristics were evaluated by the average value of 20 sheets for each condition.
The measurement and evaluation results are also shown in Table 3.

As shown in the table, the inventive examples according to the present invention show good results in both magnetic properties and film properties.

Claims (4)

In addition to containing C: 0.08% or less, Si: 2.0 to 4.5%, and Mn: 0.5% or less, S, Se, and O are each suppressed to less than 50 ppm by mass, and sol.Al is suppressed to less than 100 ppm by mass. Further, N is controlled in the range of [acid-soluble Al (sol.Al) mass ppm / 26.98] × 14.00 ≦ N ≦ 80 mass ppm, and the remainder is reheated to a steel slab having a composition of Fe and inevitable impurities. Without re-heating or after re-heating, hot-rolled sheet is used to make a cold-rolled sheet with the final thickness by annealing and rolling. A series of directions in which an annealing separator is applied after nitriding to a mass ppm or less, and the residence time between 300 ° C. and 800 ° C. is set to 5 hours or more and 150 hours or less in the temperature raising process of secondary recrystallization annealing. In the production of heat-resistant electrical steel sheets,
The annealing separator contains MgO in an amount of 50 mass% or more, and the MgO has a citric acid activity of 60 seconds ≦ CAA30 ≦ 90 seconds and 150 seconds ≦ CAA70 ≦ 250 seconds. A method for producing grain-oriented electrical steel sheets. The production of the grain-oriented electrical steel sheet according to claim 1, further comprising, as an annealing separator, titanium oxide in an amount of 5 to 20 parts by mass in terms of TiO ₂ with respect to 100 parts by mass of the annealing separator. Method. Furthermore, the said steel slab is mass%,
Ni: 0.005-1.50%,
Sn: 0.01 to 0.50%,
Sb: 0.005-0.50%,
Cu: 0.01 to 0.50%,
Cr: 0.01 to 1.50%,
P: 0.0050 to 0.50%,
Mo: 0.01-0.50% and
Nb: 0.0005-0.0100%
The method for producing a grain-oriented electrical steel sheet according to claim 1, comprising one or more selected from among the above.   MgO for annealing separator used in the method for producing a grain-oriented electrical steel sheet according to any one of claims 1 to 3, wherein the citric acid activity satisfies 60 seconds ≤ CAA 30 ≤ 90 seconds and 150 seconds ≤ CAA 70 ≤ 250 seconds. .