JP2014208907A - Method of producing grain oriented electrical steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of producing a grain oriented electrical steel which is produced by adding nitrogen to a steel plate before secondary recrystallization so as to form silicon nitride and has good magnetic characteristics due to efficient diffusion of nitrogen into the steel and resultant deposition of silicon nitride in the grain boundary.SOLUTION: The surface of a cold-rolled plate of a final plate thickness has an average roughness Ra of 0.20 μm or greater. In a nitrogen increase treatment which is based on ammonia gas and carried out at the timing of during primary recrystallization annealing or after annealing, the oxidative PH0/PHof the treatment atmosphere is 0.05 or lower, and the hydrogen concentration of the treatment atmosphere is 10 vol% or higher, and the nitrogen weighting (ΔN) of the nitrogen increase treatment is 50-1,000 ppm.

Description

  The present invention relates to a method for producing a grain-oriented electrical steel sheet suitable for a core material such as a transformer.

  A grain-oriented electrical steel sheet is a soft magnetic material mainly used as a core material of a transformer 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.
Further, in Patent Document 4, even when the inhibitor component is not contained in the slab, the secondary recrystallization is performed by increasing the amount of S in the ground iron after the primary recrystallization annealing and before the completion of the secondary recrystallization. A technique (“sulfurization method”) capable of expressing crystals is disclosed. However, in the above technique, it is difficult to uniformly disperse S that has entered the steel from the temperature increasing process of the secondary recrystallization annealing to immediately before the secondary recrystallization after the vulcanization treatment. Tended to be unstable. In particular, when the coil annealing is performed, it is difficult to make the temperature in the coil and the atmosphere between the steel sheet layers constant, so that the structure (orientation) of secondary recrystallization tends to vary.

In order to solve such a problem, for example, in Patent Document 5, 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, regarding a technique for expressing secondary recrystallization without containing an inhibitor component in the slab, Patent Document 6 discloses a technique (inhibitorless method) that enables 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.

  Also, since texture control is an important factor for magnetic properties, many conditions have been proposed for warm rolling and the like for texture control. 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 be lower than in the technique using inhibitors. .

U.S. Patent No. 1965559 Japanese Patent Publication No. 40-15644 Japanese Patent Publication No.51-13469 Japanese Patent No. 4321120 Japanese Patent No. 2782086 JP 2000-129356 JP Japanese Patent Laid-Open No. 11-29824

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.
In contrast, in order to manufacture a grain-oriented electrical steel sheet with reduced variation in magnetic properties while suppressing the slab heating temperature, the inventors have used a method for producing a grain-oriented electrical steel sheet that does not contain an inhibitor component to perform primary regeneration. A crystal texture was formed, and silicon nitride (Si ₃ N ₄ ) was deposited using nitridation in the middle of the process, and the use of this silicon nitride as an inhibitor was investigated.

  That is, by examining the grain boundary diffusion behavior of nitrogen from the surface nitrogen enriched layer and the precipitation behavior of silicon nitride, using a component similar to the inhibitorless component with Al suppressed to less than 100 mass ppm, By applying a nitrous treatment (nitrogen increase) while avoiding slab heating, silicon nitride is deposited instead of AlN, and this silicon nitride functions as a suppressive force for normal grain growth, resulting in variations in magnetic properties. This is a method for producing a grain-oriented electrical steel sheet that is greatly reduced and industrially stable and has good characteristics.

In addition, the inventors further examined the conditions for the nitrogen increase.
As a result, in particular, when the boosted nitrogen process gas having a nitriding ability such as ammonia in an atmosphere mainly composed, as a carrier gas, typically utilize N ₂ gas, but as in the present invention in the steel In the method of precipitating silicon nitride with a Si concentration of about 3% by mass, Si oxide such as SiO ₂ is easily generated even in a slight oxidizing atmosphere, and the silicon nitride is deposited. It has become clear that it may have a great influence on
When the oxidizing atmosphere is represented by PH ₂₀ / PH ₂ which is an atmospheric oxidizing index, the upper limit value was 0.05.

  Here, Patent Document 7 discloses a technique for appropriately reducing the oxidizability during nitriding in a system containing A1N as an inhibitor. However, as in the present invention, N is combined with Si to precipitate. In the case of forming an object, there has been a case where nitriding cannot be performed stably yet.

Therefore, the inventors have repeatedly investigated the nitriding treatment conditions for stably depositing silicon nitride, containing 10 vol% or more of H ₂ gas in the carrier gas, and keeping the dew point in the furnace low, that is, water vapor Not only is the atmospheric oxidation index obtained by dividing the partial pressure PH ₂ 0 divided by the hydrogen partial pressure PH ₂ , but PH ₂ 0 / PH ₂ is suppressed to 0.05 or less, and the surface roughness of the steel sheet before nitriding is adjusted. Thus, it was newly discovered that good magnetic properties can be obtained after secondary recrystallization by stably depositing silicon nitride.

  The present invention was developed on the basis of the above knowledge, and for the purpose of forming silicon nitride, the directionality in which nitrogen was added to the steel sheet before secondary recrystallization to the steel sheet before secondary recrystallization in a nitrogen-based atmosphere. Provided is a method for producing a grain-oriented electrical steel sheet that can obtain a grain-oriented electrical steel sheet having good magnetic properties by efficiently diffusing nitrogen into the steel and precipitating silicon nitride at grain boundaries. For the purpose.

First, the experimental results that led to the completion of the present invention will be described.
In mass% or mass ppm, C: 600ppm, Si: 3.30%, Mn: 0.08%, S: 10ppm, Al: 20ppm, N: 20ppm, Sb: 0.01% and Cu: 0.05%, the balance being Fe and inevitable After continuous casting, the steel slab with the composition of mechanical impurities is heated to 1100 ° C, then hot-rolled to a 2.2mm thick hot-rolled sheet, then hot-rolled sheet annealed at 1000 ° C, and then pickled Then, it was finished to a thickness of 0.23 mm by cold rolling.

  Next, each coil was degreased and decarburized and annealed in a wet hydrogen atmosphere at 850 ° C., and an Epstein sample was cut out along the rolling direction from the obtained decarburized and annealed plate.

Subsequently, using a gas nitriding furnace at 750 ° C., a nitrogen increase treatment of about ΔN = 300 ppm was performed in various mixed gas atmospheres mainly composed of ammonia. Thereafter, an annealing separator containing MgO as a main component was applied to the steel sheet, and a final finish annealing was performed at 1150 ° C. for 5 hours. As atmosphere gas for final finish annealing, purification was performed using N ₂ gas during the temperature rise and H ₂ gas after reaching 1150 ° C. Then, after removing the unreacted separating agent, an insulating coating mainly composed of colloidal silica and Mg phosphate was formed at 850 ° C. to obtain a product plate.

The iron loss value W _17/50 (W / kg) and the magnetic flux density B ₈ (T) at a magnetic flux density of 1.7 T and a frequency of 50 Hz were measured for the Epstein samples of the product plate, respectively. Among the measured results were plotted atmosphere oxidizing horizontal axis in FIG. 1 the magnetic flux density B ₈ as ordinate.

As shown in FIG. 1, in the case where the component system containing no low Al inhibitor component is subjected to nitrogen enrichment treatment and secondary recrystallization, the atmospheric oxidation property of nitrogen enrichment treatment: PH ₂ 0 / PH ₂ is 0.05 or less. It was found that a high magnetic flux density can be stably obtained.

  In addition, in the process of forming silicon nitride, unlike the conventional case where nitriding is applied to a component system having an Al content of 100 ppm or more, when the Si concentration of the base material is as high as about 3% by mass, It became clear that the influence of surface properties on nitride formation by ammonia gas or the like was larger than expected.

Furthermore, it is known that the latter of the eddy current loss and hysteresis loss responsible for iron loss can be reduced by increasing the smoothness of the surface. However, when performing nitriding treatment, Ra (centerline average roughness) ), The surface roughness of the so-called steel sheet is 0.20 μm or more, so that not only the processing time suitable for continuous processing can be realized, but also nitride formation near the surface and subsequent diffusion of nitrogen in the steel, It was also found that the Si ₃ N ₄ precipitation in the steel plate was stabilized and the magnetic flux density of the resulting steel plate could be stabilized at a high level.
The present invention is based on the above findings.

The gist of the present invention is as follows.
1. In mass%, C: 0.08% or less, Si: 2.0 to 4.5% and Mn: 0.5% or less, S, Se and O are each less than 50 mass ppm, sol.Al is 100 mass ppm or less, and N is After restraining to 80 mass ppm or less, the balance is a steel slab composed of Fe and inevitable impurities, without reheating or after reheating, after hot rolling at 1300 ° C or less, and hot-rolled sheet annealing Alternatively, without performing hot-rolled sheet annealing, a cold-rolled sheet with the final thickness is obtained by a single cold rolling, and further, a primary recrystallization annealing is applied and an annealing separator is applied, and a secondary recrystallization annealing is performed. In the manufacturing process of a series of grain-oriented electrical steel sheets to be applied,
The average roughness Ra of the surface of the cold rolled plate is 0.20 μm or more,
During the above-mentioned primary recrystallization annealing, when performing the nitrification treatment mainly composed of ammonia gas at any timing after the annealing, the atmospheric oxidation property of the nitrification treatment: PH ₂ 0 / PH ₂ is 0.05 or less, the hydrogen concentration is A method for producing a grain-oriented electrical steel sheet with 10% by volume or more and further a nitrogen increase (ΔN) in the above nitrous treatment of 50 to 1000 ppm by mass.

2. 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: 0.0050-0.50%
Mo: 0.01-0.50% and
Nb: 0.0005-0.0100%
The method for producing a grain-oriented electrical steel sheet according to 1 above, comprising one or more selected from among the above.

3. 3. The method for producing a grain-oriented electrical steel sheet according to 1 or 2, wherein the average roughness Ra of the surface of the cold-rolled sheet is further 0.25 μm or more.

  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.

It is the graph which showed the measurement result of the magnetic flux density with respect to each condition of cold rolling, with the atmosphere oxidation property of the nitriding treatment: PH ₂ 0 / PH ₂ as the horizontal axis and the magnetic flux density B ₈ as the vertical axis.

Hereafter, the point regarding manufacture in this invention is described.
In the present invention, basically, a conventionally known method for producing grain-oriented electrical steel sheets that does not utilize high-temperature slab heating is followed.
First, the reason for limiting the component composition of the steel slab in the present invention will be described. Unless otherwise specified, “%” and “ppm” described below mean mass% and mass ppm, respectively.

  About the molten steel component of this invention, sol.Al is suppressed to 100 ppm or less at the time of steel melting. Since the present invention assumes a slab heating temperature of 1300 ° C. or less on the premise of a process that does not use AlN as an inhibitor, when sol.Al is more than 100 ppm, it cannot be completely dissolved as AlN or the like, and coarse precipitation It becomes a disturbance factor of secondary recrystallization as a product. Therefore, it is necessary to suppress sol.Al to 100 ppm or less.

In the present invention, since it is important to deposit Si ₃ N ₄ after the nitriding treatment, if AlN is inevitably deposited by the Al component contained, it will be a factor that disturbs the subsequent precipitate control, so the less Is good. In addition, sol.Al is preferably suppressed to 80 ppm or less because it may cause defects such as blisters during slab heating.

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%. Note that the lower limit is required to be about 30 ppm or less in terms of magnetic characteristics. Therefore, when the required level of magnetic characteristics is not so high, it is not particularly necessary.

Si: 2.0-4.5%
Si is an element effective in increasing the electrical resistance of steel and improving iron loss. However, if its content is less than 2.0%, its additive effect is poor. On the other hand, if it exceeds 4.5%, the workability is remarkably high. The amount of Si needs to be in the range of 2.0 to 4.5% because the magnetic flux density also decreases.

Mn: 0.5% or less
Since Mn has the effect of improving hot workability during production, it is preferable to contain 0.005% or more. However, if the content exceeds 0.5%, the primary recrystallization texture deteriorates and magnetic properties are increased. Since the characteristics are deteriorated, Mn is limited to 0.5% or less.

S, Se and O: each less than 50 ppm As described above, in the present invention, since Al is reduced, there is no utilization of so-called inhibitors mainly composed of AlN. In this case, in order to obtain a grain-oriented electrical steel sheet having a high magnetic flux density, S: less than 50 ppm (0.005%) and Se: less than 50 ppm (0.005%) are required. This is because, in a steel component system that does not contain an inhibitor component that exhibits a strong suppressive force, the effect of impurities on the grain growth in primary recrystallization is large.
Further, the amount of O needs to be less than 50 ppm (0.005%). This is because oxides as inclusions adversely affect the magnetic properties.

  Here, the amount of S is better as the added amount is larger from the viewpoint of improving the magnetic flux density. However, when performing low-temperature slab heating, it is difficult to precipitate as an inhibitor such as MnS with good controllability. It should not be increased, and it is desirable to increase the steel sheet by performing a vulcanization treatment after the primary recrystallization annealing until the completion of the secondary recrystallization. Moreover, even if the vulcanization treatment is performed at such timing, since the secondary recrystallization annealing is performed in a batch manner, the rate of temperature rise is generally slow, and the S that has penetrated into the steel by the vulcanization treatment is uniform. There is no problem because it is suitable for dispersion.

N: 80 ppm or less In the present invention, an inhibitorless manufacturing method is applied to complete the formation of a texture. Therefore, N must be suppressed to 80 ppm or less. This is because if it exceeds 80 ppm, the effect of grain boundary segregation and the formation of a trace amount of nitrides will cause the adverse effect of deterioration of the texture. Moreover, since it may cause defects such as “fluff” at the time of slab heating, it is necessary to suppress it to 80 ppm or less. Desirably, it is 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: 0.005-1.50%
Ni works to improve the magnetic properties by increasing the uniformity of the hot-rolled sheet structure, and for that purpose, it is preferable to contain 0.005% or more, but if the content exceeds 1.50%, 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 1.50%.

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.

Mo: 0.01-0.50%, Nb: 0.0005-0.0100%
Mo and Nb 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 desirably about 1000 ° C. or higher, and it is necessary to set it to 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.

Subsequently, the hot-rolled sheet is subjected to hot-rolled sheet annealing as necessary, and then cold-rolled once to obtain a final cold-rolled sheet (cold-rolled sheet having a final thickness). 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.
In the present invention, the surface roughness of the final cold rolled sheet needs to be a surface roughness expressed by Ra (center line average roughness) of 0.20 μm or more, and more preferably 0.25 μm or more. . For the purpose of reducing the iron loss value of electrical steel sheets, it is effective to increase the surface smoothness, that is, to reduce the surface roughness Ra, from the viewpoint of reducing hysteresis loss. From the viewpoint of improving the stability and the high stability of the magnetic flux density, the larger the surface roughness Ra, the better. The former has the effect of increasing the reaction surface area of the nitriding reaction on the steel sheet surface. For the latter, the mechanism is not sufficiently clear, but when the surface roughness Ra is reduced to less than 0.20 μm, the surface nitride formed immediately after the nitriding treatment is likely to be unevenly formed, It affects the subsequent decomposition, diffusion of nitrogen in the steel and Si ₃ N ₄ precipitation, and the final magnetic flux density tends to vary. On the other hand, it is presumed that moderate surface roughness promotes the formation of macroscopically uniform surface nitride during nitriding, and works effectively for subsequent uniform precipitation of Si ₃ N ₄ .

  Subsequently, primary recrystallization annealing is performed on the cold-rolled sheet having the final thickness. 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.

  Further, in the present invention, the nitriding treatment for increasing nitrogen is premised on gas nitriding with ammonia gas, and is performed during or after the primary recrystallization annealing.

As shown in FIG. 1 above, under conditions where the oxidizing gas PH ₂ 0 / PH ₂ of the atmosphere gas in the nitriding furnace is larger than 0.05, a material having a high magnetic flux density can be obtained, but the variation in magnetic characteristics is large, As a result, it is understood that the secondary recrystallization behavior is unstable.
That is, when a nitrogen enrichment process is performed by gas nitriding on a component system that does not contain a low Al inhibitor component in the present invention, the atmospheric oxidation PH ₂ 0 / PH _{2 is set} to 0.05 or less, and oxidation of Si in steel is performed. It is necessary to suppress as much as possible the inhibition of silicon nitride formation due to. There is no particular lower limit to the value of PH ₂ 0 / PH ₂ , but it is about 0.002 from the industrially available dew point.

Here, although the cause of the variation in the magnetic characteristics is not clear, an oxide mainly composed of SiO ₂ is formed on the surface after the primary recrystallization annealing accompanied with decarburization. As a result, the Si concentration in the steel near the surface decreases. And it is estimated that the Si distribution non-uniformity caused by the Si concentration reduction affects the formation and distribution behavior of silicon nitride.

On the other hand, with respect to the H ₂ gas mixed as the atmospheric gas, it can be seen that when it is not mixed at all, a good magnetic flux density cannot be obtained, and it is necessary to add 10% or more in volume fraction. Although there is no particular upper limit, 80 vol% or less is desirable because mixing of ammonia gas having nitriding ability and N ₂ gas is necessary.

Therefore, in the present invention, it is necessary to mix 10 vol% or more of H ₂ gas as the atmospheric gas, and at the same time, it is necessary to reduce the atmospheric oxidation property defined by PH ₂ 0 / PH ₂ to 0.05 or less. For this purpose, it is important to keep the dew point in the nitriding furnace, that is, the water vapor partial pressure low.

  An important point in the above nitriding treatment is to obtain a nitride layer as a surface layer. In order to suppress diffusion into steel, it is desirable to perform nitriding at a temperature of 800 ° C. or less. However, the nitride layer is formed on the surface even at high temperatures by shortening the time (for example, about 30 seconds). It can be formed.

  Further, it is important that the nitrogen increase (ΔN) by nitriding is 50 ppm or more and 1000 ppm or less. If the nitrogen increase is less than 50 ppm, the effect of increasing nitrogen cannot be obtained sufficiently. On the other hand, if it exceeds 1000 ppm, the amount of silicon nitride deposited becomes excessive and secondary recrystallization does not occur effectively. The preferred range is 200 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.

After performing the primary recrystallization annealing and nitriding treatment, an annealing separator is applied to the steel sheet surface. In order to form a forsterite film on the steel sheet surface after secondary recrystallization annealing, it is necessary to use magnesia (MgO) as the main component of the annealing separator. As the separating agent main component, an appropriate oxide having a melting point higher than the secondary recrystallization annealing temperature, such as alumina (Al ₂ O ₃ ) or calcia (CaO), can be used.

In the secondary recrystallization annealing, the residence time between 300 and 800 ° C. is preferably 5 hours or more and 150 hours or less. Since the purpose of precipitation of silicon nitride is to suppress normal grain growth, a sufficient amount must be selectively deposited on the grain boundary at the stage of 800 ° C. where normal grain growth proceeds, and 300 to 800 By setting the residence time in the temperature range of 5 ° C. to 5 hours or more, silicon nitride cannot be precipitated in the grains, but N and Si diffused in the grain boundaries are selectively on the grain boundaries. It can be deposited in On the other hand, it is not always necessary to set an upper limit. However, 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.

  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.

When the grain-oriented electrical steel sheet is used for a core material such as a transformer, it is used by being laminated, so that an insulating layer for interlayer insulation is required. As the additionally applied insulating coating, a generally used inorganic coating can be used for the grain-oriented electrical steel sheet. In particular, a coating having a tension-imparting effect is extremely effective in combination with a grain-oriented electrical steel sheet in which the steel sheet surface is smoothed in order to achieve low iron loss.
As a type of tension-imparting coating, a coating containing silica that reduces the thermal expansion coefficient is effective, and the phosphate-colloidal silica-chromic acid system that has been used for grain-oriented electrical steel sheets having a forsterite film has been used. A coating or the like is preferable from the viewpoint of its effect and cost, uniform processability and the like. The thickness of the coating is preferably in the range of about 0.3 μm or more and 10 μm or less from the viewpoint of tension application effect, space factor, film adhesion, and the like.

Furthermore, in the present invention, the shape of the steel sheet can be adjusted by flattening annealing, and this flattening annealing can also serve as a baking treatment of the insulating coating.
Further, as the magnetic domain subdivision process, it is possible to further reduce the iron loss by applying a thermal strain introducing type magnetic domain subdivision process such as laser or electron beam irradiation after the insulating coating. It is also effective to reduce the iron loss by forming mechanical and electrochemical physical grooves to subdivide the magnetic domain.

  A slab including the components of steel symbols 1 to 4 shown in Table 1 and the balance being composed of Fe and inevitable impurities was heated to 1200 ° C. and hot-rolled to obtain a 2.6 mm thick hot rolled coil. Next, the hot rolled coil was annealed at 1000 ° C., pickled, and finished to a thickness of 0.27 mm by a tandem rolling mill. The surface roughness of the final cold-rolled sheet was adjusted by the surface roughness of the work roll, the amount of coolant for lubrication, and the like, and the average roughness Ra was adjusted between 0.16 and 0.26 μm.

Each coil was degreased and decarburized and annealed in a wet hydrogen atmosphere at 850 ° C., and an Epstein sample was cut out along the rolling direction from the obtained decarburized and annealed plate.
Next, nitrogen enrichment is performed in a nitriding furnace mainly composed of ammonia gas, and H ₂ gas concentration, N ₂ gas concentration, atmospheric dew point, nitriding temperature, time, etc., so that the amount of nitrogen in the steel becomes two levels of 250 ppm and 400 ppm. Changed. The ammonia gas concentration was fixed at 15 vol%. Subsequently, after applying an annealing separator mainly composed of MgO to the steel sheet, it was heated to 1075 ° C. in a mixed atmosphere of Ar and N ₂ , and purification annealing at 1200 ° C. was performed in an H ₂ atmosphere. Then, after removing the unreacted separating agent, an insulating coating composed mainly of colloidal silica and magnesium phosphate was formed at 800 ° C.
As an evaluation of the magnetic properties of the product thus obtained, the iron loss value W _17/50 and the magnetic flux density B ₈ at 50 Hz AC excitation at a magnetic flux density of _{1.7 T} were measured. Table 2 summarizes the nitriding treatment conditions when using the final cold-rolled sheet with the average roughness Ra in the range of 0.22 to 0.24 μm and the magnetic properties of the obtained steel sheet by the Epstein test method. It was.

As is apparent from Table 2, all of the conditions B, F and H satisfying the present invention exhibited good magnetic properties. On the other hand, in the conditions A, D, G and I in which C and E, which do not satisfy the steel components, the H ₂ gas concentration is less than 10 vol%, or the atmospheric oxidizing PH ₂ 0 / PH ₂ is greater than 0.05, all Good magnetic properties are not obtained.

Table 3, for the material of the steel No. 1 and 4, and the increasing nitriding condition to the material changing the surface roughness of the final cold rolled sheet, the average value of the resulting magnetic flux density B ₈ and the variation (maximum and minimum The magnetic flux density difference ΔB ₈ ) was summarized.
Here, the magnetic flux density B ₈ is what cut 10 sheets of test pieces of 400 L × 100W in coil width direction were evaluated by single plate magnetic testing method.

As apparent from Table 3, neither the present invention satisfies conditions L, N and R represents a good magnetic flux density, the variation .DELTA.B ₈ was small. On the other hand, conditions J, K, P and Q that do not satisfy the condition of surface roughness Ra tend to have a small increase in nitrogen itself, but the obtained magnetic flux density variation ΔB ₈ is large, resulting in an average of B ₈ also is low. In addition, under conditions M, O, S, and T that do not satisfy the H ₂ concentration and atmospheric oxidizing conditions, none of the average magnetic properties such as B ₈ has been obtained.

Claims (3)

In mass%, C: 0.08% or less, Si: 2.0 to 4.5% and Mn: 0.5% or less, S, Se and O are each less than 50 mass ppm, sol.Al is 100 mass ppm or less, and N is After restraining to 80 mass ppm or less, the balance is a steel slab composed of Fe and inevitable impurities, without reheating or after reheating, after hot rolling at 1300 ° C or less, and hot-rolled sheet annealing Alternatively, without performing hot-rolled sheet annealing, a cold-rolled sheet with the final thickness is obtained by a single cold rolling, and further, a primary recrystallization annealing is applied and an annealing separator is applied, and a secondary recrystallization annealing is performed. In the manufacturing process of a series of grain-oriented electrical steel sheets to be applied,
The average roughness Ra of the surface of the cold rolled plate is 0.20 μm or more,
During the above-mentioned primary recrystallization annealing, when performing the nitrification treatment mainly composed of ammonia gas at any timing after the annealing, the atmospheric oxidation property of the nitrification treatment: PH ₂ 0 / PH ₂ is 0.05 or less, the hydrogen concentration is A method for producing a grain-oriented electrical steel sheet with 10% by volume or more and further a nitrogen increase (ΔN) in the above nitrous treatment of 50 to 1000 ppm by mass. 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: 0.0050-0.50%
Mo: 0.01-0.50% and
Nb: 0.0005-0.0100%
The manufacturing method of the grain-oriented electrical steel sheet according to claim 1, comprising one or more selected from among the above.   The method for producing a grain-oriented electrical steel sheet according to claim 1 or 2, wherein the average roughness Ra of the surface of the cold-rolled sheet is further 0.25 µm or more.

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