JP6209998B2 - Method for producing grain-oriented electrical steel sheet - Google Patents

Description

  The present invention relates to a method for producing a grain-oriented electrical steel sheet capable of obtaining a grain-oriented electrical steel sheet having excellent magnetic properties at a 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 increases the size of crystal grains having a (110) [001] orientation, so-called Goss orientation, during secondary recrystallization annealing during the production process of grain-oriented electrical steel sheets. Growing, formed through secondary recrystallization.

  Conventionally, such grain-oriented electrical steel sheets were heated to 1300 ° C or higher by heating a slab containing Si of 4.5 mass% or less and an inhibitor component such as MnS, MnSe, and AlN to temporarily dissolve the inhibitor component. After that, it is hot-rolled and subjected to hot-rolled sheet annealing as necessary, and then to the final sheet thickness by one or more cold rollings sandwiching intermediate annealing, and primary recrystallization annealing is performed in a wet hydrogen atmosphere. The primary recrystallization and decarburization are performed, and an annealing separator mainly composed of magnesia (MgO) is applied, and then 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 (see, 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 inhibitor components are added by high-temperature slab heating exceeding 1300 ° C. A process in which secondary recrystallization is developed by once forming a solid solution and finely precipitating in a subsequent process has been adopted.
In other words, the conventional manufacturing process for grain-oriented electrical steel sheets required slab heating at a high temperature exceeding 1300 ° C, so the manufacturing cost has to be extremely high, and in recent years, there has been a demand for a reduction in manufacturing cost. I left a problem where I couldn't respond.

  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 to suppress slab heating to a low temperature, and in an appropriate nitriding atmosphere in the decarburization annealing process. A method of using (Al, Si) N as an inhibitor by performing nitriding has been proposed.

  However, even if (Al, Si) N is finely dispersed in the steel and functions as an effective inhibitor, the inhibitor strength is determined by the Al content. There is a big problem that secondary recrystallization becomes unstable without obtaining sufficient grain growth inhibiting power.

  In this way, a number of methods have been proposed in which nitriding treatment is performed in the middle of steel plate manufacturing and (Al, Si) N or AlN is used as an inhibitor. Recently, manufacturing methods in which the slab heating temperature exceeds 1300 ° C have also been proposed. It is disclosed.

In the manufacturing method described in Patent Document 5, precipitates (Si ₃ N ₄ or (Si, Mn) N) mainly composed of silicon nitride are formed only on the surface layer of the steel sheet after nitriding. In the subsequent secondary recrystallization annealing, the precipitate mainly composed of silicon nitride changes to a more thermally stable Al-containing nitride ((Al, Si) N or AlN).

  However, in order for precipitates mainly composed of silicon nitride formed only on the surface layer to precipitate uniformly in steel as Al-containing nitrides, first, silicon nitride dissolves and nitrogen diffuses in the steel. Since it is necessary to take the process of forming the contained nitride, the control may become unstable.

  On the other hand, a technique for developing secondary recrystallization without containing an inhibitor component in the slab has been studied from the beginning, and Patent Document 6 discloses a technique (inhibitor) that allows secondary recrystallization without containing an inhibitor component. Less method).

  The inhibitorless method is a technique in which a higher-purity steel is used and secondary recrystallization is expressed by texture (control of texture). This inhibitor-less method does not require high-temperature slab heating, and enables production of grain-oriented electrical steel sheets at a low cost. However, because it does not have an inhibitor, temperature variation during production or in the middle of the process, etc. As a result, there is a problem that the magnetic characteristics of the product are likely to vary.

  Further, since texture control is an important factor in the inhibitorless method, many techniques have been proposed such as using warm rolling for texture control. In the inhibitorless method, if the texture cannot be controlled sufficiently, the degree of integration in the Goss orientation ((110) [001]) after secondary recrystallization is low and the magnetic flux density is low compared to the technique using inhibitors. In many cases, it was lowered.

U.S. Pat. No. 1,965,559 Japanese Patent Publication No. 40-15644 Japanese Patent Publication No. 51-13469 Japanese Patent No. 2782086 Japanese Patent Laid-Open No. 04-235222 JP 2000-129356 A

  As described above, in the various methods of manufacturing grain-oriented electrical steel sheets that have been proposed so far, it has been difficult to stably achieve good characteristics.

  In addition, the inventors used a component in accordance with an inhibitorless component in which Al is suppressed to less than 100 ppm, and while performing nitriding treatment while avoiding high-temperature slab heating, mainly at the time of secondary recrystallization. Grain-oriented electrical steel sheet that can avoid destabilization of secondary recrystallization due to deviation in the amount of Al in steelmaking when AlN is used as an inhibitor by using a technique that precipitates the deposited precipitate and uses it as an inhibitor It has been found that it is possible to manufacture.

  However, in the technique described above, when the precipitate mainly composed of silicon nitride cannot maintain a stable inhibitor effect up to a high temperature during the secondary recrystallization, the secondary recrystallization becomes unstable, and the magnetic properties of the steel sheet are reduced. There still remained the problem of varying characteristics.

  The present invention has been made in view of the above circumstances, and by using a component according to the inhibitorless component, the magnetic properties can be improved and secondary recrystallization is stable without performing high-temperature slab heating. It is an object of the present invention to propose a manufacturing method that can produce a grain-oriented electrical steel sheet that is manifested and has greatly reduced variations in magnetic properties of the steel sheet.

In order to solve the above-mentioned problems, the inventors conducted a detailed investigation on the behavior of a precipitate mainly composed of silicon nitride (or simply referred to as a precipitate) as an inhibitor during secondary recrystallization.
As a result, silicon nitride is thermodynamically unstable compared to AlN, so it dissolves at a lower temperature than AlN and loses its effect as an inhibitor. It has been found that increasing the solubility product of silicon and nitrogen is effective for exerting the inhibitor effect. This is because the silicon nitride is thermodynamically stabilized even at high temperatures because the solid solution temperature of silicon nitride increases due to the increase in the solubility product.

At the same time, intensive studies were conducted on methods for efficiently increasing the solubility product of silicon and nitrogen.
Since the solubility of silicon is determined by the slab component, there are various restrictions such as rollability, and it is not easy to change greatly. Therefore, the inventors have studied to increase the solubility product of nitrogen by increasing the solubility of nitrogen to improve the thermodynamic stability of silicon nitride.
As a result, in order to increase the solubility product, the nitrogen content of the steel sheet increases uniformly in the thickness direction after the nitriding treatment of the steel sheet and before the finish annealing starts, especially only in the vicinity of the steel sheet surface. It was found that it is important not to increase the amount of nitrogen alone.

For this reason, the inventors presume as follows.
If there is a large amount of nitrogen near the surface of the steel sheet before finish annealing, the nitrogen diffuses toward the center of the sheet thickness during finish annealing due to the concentration difference between the central thickness of the sheet and the nitrogen near the surface. Diffusion into the center of the plate thickness is due to the fact that the grain boundary inside has a high nitrogen migration speed and that silicon nitride has a large lattice misfit with iron, so it grows more easily at the grain boundary than within the crystal grain. The nitrogen that is consumed is consumed by growing silicon nitride at the grain boundaries, and does not contribute to improving the solubility product in the grains.

Accordingly, although silicon nitride at the crystal grain boundary stabilizes and grows at the center of the plate thickness, the silicon nitride within the grain becomes unstable.
In addition, the silicon nitride at the grain boundaries is coarsened in this process, and the pinning force for abnormal grain growth is weakened. As a result, it was found that when the amount of nitrogen increased only in the vicinity of the surface, the inhibitor effect of silicon nitride during the secondary recrystallization was weakened, and the magnetic properties of the product deteriorated.
Based on these findings, further experimental studies were conducted to complete the present invention.

That is, the gist configuration 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 less than 100 mass ppm, N is It is controlled to 80 mass ppm or less, and satisfies the relationship of sol.Al (mass ppm) -N (mass ppm) × (26.98 / 14.00) ≦ 30 (mass ppm), with the balance being the composition of Fe and inevitable impurities A steel slab consisting of either a re-heated or re-heated hot-rolled sheet, and then a cold-rolled sheet with the final thickness by annealing and rolling, followed by primary recrystallization annealing. In the method for producing a grain-oriented electrical steel sheet that is subjected to nitriding after being performed or during primary recrystallization annealing, and further subjected to secondary recrystallization annealing by applying an annealing separator,
The steel sheet when the secondary recrystallization annealing is performed has a composition of 50 mass ppm or more in terms of an average nitrogen amount increment with respect to the average nitrogen amount of the entire plate thickness before nitriding: ΔN (total thickness), and the average nitrogen The amount increment: ΔN (total thickness) and the average nitrogen amount increment from the both surfaces of the steel sheet to the depth of 10% of the plate thickness: ΔN (surface) among the average nitrogen amount increments is expressed by the following formula (1). A method for producing a grain-oriented electrical steel sheet characterized by satisfying
ΔN (surface) / ΔN (total thickness) ≦ 2.0 (1)
ΔN (surface): Increment of average nitrogen amount (mass ppm) from both surfaces of the steel sheet to a depth of 10% of the plate thickness
ΔN (total thickness): Average nitrogen content increment (mass ppm) of the entire plate thickness

2. 2. The method for producing a grain-oriented electrical steel sheet according to 1 above, characterized in that the nitriding treatment conditions are adjusted such that nitrogen that has entered through nitriding treatment diffuses to a thickness center (plate thickness 1/2) in a solid solution state. A method for producing a grain-oriented electrical steel sheet.

3. The grain-oriented electrical steel sheet manufacturing method according to 1 or 2, wherein the sheet temperature is kept at least 800 ° C. during nitriding, and then the sheet temperature is maintained for at least 10 seconds. Production method.

  According to the present invention, it is possible to obtain a grain-oriented electrical steel sheet that significantly reduces the variation in magnetic properties without industrially high-temperature slab heating, and that has industrially stable and good magnetic properties.

The value of .DELTA.N (surface) / .DELTA.N (total thickness) is a diagram showing the relationship between the magnetic flux density _{(B 8).}

Hereinafter, the present invention will be specifically described.
First, the reason why the component composition of the steel slab is limited to the above range in the present invention will be described. Unless otherwise specified, “%” and “ppm” for steel components mean mass% and mass ppm.
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 improving 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 electrical resistance. However, if the content exceeds 4.5%, the cold rolling property deteriorates significantly, so Si is limited to 4.5% or less. Further, since Si needs to function as a nitride forming element, it is necessary to contain 2.0% or more. Further, from the viewpoint of improving iron loss characteristics, a desirable addition amount 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 the content is not particularly limited, but is preferably about 0.005% for improving hot workability.

S, Se, and O: less than 50 ppm If any one of S, Se, and O is 50 ppm or more, secondary recrystallization becomes difficult. 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 100 ppm In the present invention, the amount of Al is suppressed because silicon nitride is used as an inhibitor instead of AlN. Al forms a dense oxide film on the surface, and during nitriding, it may be difficult to control the amount of nitriding or inhibit decarburization, so the amount of sol.Al is suppressed to less than 100 ppm. However, Al with high oxygen affinity reduces the amount of dissolved oxygen in steel by adding a small amount in steelmaking, and can reduce oxide inclusions that lead to deterioration of properties, so if it is in the range of less than 100 ppm, By adding, magnetic deterioration can be suppressed.

N: 80 ppm or less In the present invention, an inhibitorless manufacturing method is applied and texture formation is performed. Therefore, N in steel needs to 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 blisters during slab heating, it must be suppressed to 80 ppm or less. Further, it is desirably 60 ppm or less.

sol.Al (ppm) -N (ppm) × (26.98 / 14.00) ≦ 30ppm
It is important for the present invention to deposit silicon nitride uniformly in the steel sheet after nitriding. However, if the situation in which excess Al remains after nitriding is left, the thermodynamically more stable (Al, Si) N crystal in which Si is dissolved in AlN precipitates, and the desired silicon nitride is obtained. I can't. That is, if excessive Al remains after nitriding, N is consumed more than purely bound to Al, and precipitation in the form of silicon nitride cannot be stably obtained.
Therefore, if the value of sol.Al (ppm) -N (ppm) × (26.98 / 14.00) is controlled to 0 or less, the steel sheet always contains more than the N amount necessary for precipitation as AlN. Therefore, it is possible to deposit Al as AlN before the nitriding treatment, and ΔN added by the nitriding treatment without effectively remaining excessive Al is effectively used for silicon nitride formation. be able to.
Therefore, in terms of stable precipitation of silicon nitride, it is most desirable to control the value of sol.Al (ppm) −N (ppm) × (26.98 / 14.00) to 0 or less. In the range where the value of sol.Al (ppm) −N (ppm) × (26.98 / 14.00) is 0 or less, silicon nitride can be formed by nitriding with ΔN of approximately 50 ppm or more.

In addition, when the value of sol.Al (ppm) −N (ppm) × (26.98 / 14.00) is in the range of 0 to 30, a more excessive nitrogen increment (ΔN) is required to form pure silicon nitride. Although necessary, since the amount of residual Al that contributes to precipitation is very small, pure silicon nitride can be deposited.
On the other hand, when the value of sol.Al (ppm) -N (ppm) × (26.98 / 14.00) exceeds 30 ppm, the effect of additional finely precipitated AlN and (Al, Si) N increases and precipitates. Silicon nitride cannot be obtained stably, or secondary recrystallization temperature becomes excessively high due to precipitation of more thermodynamically stable AlN or (Al, Si) N, resulting in secondary recrystallization failure. Sometimes.
Therefore, in the present invention, at least the value of sol.Al (ppm) −N (ppm) × (26.98 / 14.00) needs to be 30 ppm or less.

The essential components in the slab 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.
Ni: 0.005-1.5%
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 1.5%, secondary recrystallization will occur. Since it becomes difficult 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 to 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. For this purpose, it is preferable to contain 0.01% or more. On the other hand, if the content exceeds 1.50%, secondary recrystallization becomes difficult and the magnetic properties are reduced. Since it deteriorates, 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. This effect cannot be obtained unless it is contained in excess of the lower limit. On the other hand, when the upper limit is exceeded, if the final product remains by forming carbides or nitrides, there is a risk of causing iron loss deterioration. Therefore, when adding, it is desirable to set it as the above-mentioned range.

Next, the manufacturing method of this invention is demonstrated.
The steel slab adjusted to any one of the above component composition ranges 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 has no effect in the present invention, which contains almost no inhibitor in the steel at the slab stage, and only increases the cost, while heating below 1000 ° C increases the rolling load. This is because the desired rolling becomes difficult.

  Next, the hot-rolled sheet obtained by hot rolling is subjected to hot-rolled sheet annealing as necessary, and then subjected to two or more cold rollings sandwiching one cold rolling or intermediate annealing, The 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 that the annealing temperature of the primary recrystallization annealing is in a range of about 800 ° C. or more and less than 950 ° C. The annealing atmosphere at this time can also serve as decarburization annealing by making it a wet hydrogen nitrogen atmosphere or a wet hydrogen argon atmosphere.

Furthermore, in the present invention, it is important to perform nitriding during or after the primary recrystallization annealing.
The nitriding process for performing the nitriding treatment can be applied before primary recrystallization annealing, during annealing, or after annealing, but a part of AlN is dissolved in the annealing before the final cold rolling, so that sol.Al Therefore, when applied before the primary recrystallization annealing, the precipitation state of silicon nitride becomes different from the expected state due to the influence of the remaining sol.Al.
For this reason, in the present invention, the nitriding treatment is performed after the primary recrystallization annealing heat-up when the solid solution Al is precipitated again as AlN, that is, during or after the primary recrystallization annealing.

The method of nitriding treatment in the present invention is not particularly limited as long as nitrided precipitates can be controlled. Gas nitriding may be performed using NH ₃ atmosphere gas in the past in the form of a coil, or nitriding may be continuously performed on a running strip. It is also possible to use salt bath nitriding, plasma nitriding or the like, which has a higher nitriding ability than gas nitriding. However, when using a cyan salt bath nitriding method, care should be taken because toxic cyanide is formed at a high temperature of 600 ° C. or higher.

  The important point is to increase the amount of nitrogen uniformly in the thickness direction of the steel sheet in the nitriding treatment. This is because silicon nitride works as a thermodynamically stable inhibitor in secondary recrystallization by not biasing the amount of nitrogen only in the vicinity of the surface after the nitriding treatment. Therefore, an experimental study was conducted to determine the range of the magnetic improvement effect after secondary recrystallization by uniform nitriding of the steel sheet in the thickness direction.

  First, an index representing uniform nitriding in the thickness direction is defined. The amount of nitrogen contained in the steel sheet before nitriding is measured by chemical analysis at any stage before nitriding. After the nitriding treatment, before the final annealing, the nitrogen amount is also measured by chemical analysis, and the difference from that before the nitriding treatment is defined as ΔN (total thickness): average nitrogen amount increment (ppm) of the entire plate thickness To do.

Similarly, 10% of the plate thickness on both sides of the sample after nitriding treatment was reduced by mechanical polishing or chemical polishing, and then the nitrogen content was measured by chemical analysis, and nitriding for 80% of the plate thickness Find the increment ΔN (80%). Using the nitriding increment ΔN (80%) for 80% of the plate thickness, ΔN (surface): the average nitrogen amount increment (ppm) from both surfaces to the plate thickness: 10% is expressed by the following formula (2 ). In addition, the depth from the steel plate surface to a thickness of 10% is referred to as the vicinity of the surface in the present invention.
ΔN (surface) = (ΔN (total thickness)-0.8 x ΔN (80%)) / 0.2 (2)
Then, the ratio between ΔN (surface) and ΔN (total thickness) obtained as described above, that is, ΔN (surface) / ΔN (total thickness) is an index representing uniform nitriding in the thickness direction in the present invention. It is defined as

Next, the results of studies conducted on the nitriding treatment to determine the range of the index representing the uniform nitriding in the plate thickness direction will be described.
The steel plate that has been decarburized and annealed by the components and production method shown in Example 1 described later is subjected to NH ₃ gas nitriding under various conditions, and the processing temperature and the time of exposure to the nitriding atmosphere are adjusted. Thus, samples having ΔN (total thickness) of about 120 ppm and different ΔN (surface) / ΔN (total thickness) were produced.

Twenty-two steel sheets with the same processing conditions are prepared per condition, and two of them are used to perform chemical analysis of the nitrogen content when the total thickness and 10% of the surface are chemically polished. Was used to determine ΔN (surface) and ΔN (total thickness).
Further, the remaining 20 sheets were coated with an annealing separator containing MgO ₂ and 5% TiO ₂ as a water slurry, dried, and baked on the steel sheet. Subsequently, the temperature was raised between 300 and 800 ° C. in 20 hours, and a holding and soaking treatment was performed at 920 ° C. for 40 hours. Further, until 1050 ° C., the heating rate was uniformly 20 ° C./h, and after 1050 ° C. was exceeded. Was subjected to final finish annealing up to 1200 ° C. at a rate of temperature increase of 10 ° C./h.
Subsequently, after applying and baking a phosphate-based insulating tension coating, the magnetic flux density B ₈ (T) at a magnetizing force of 800 A / m was evaluated. FIG. 1 shows the relationship between the value of ΔN (surface) / ΔN (total thickness) and the average B ₈ (T) of 20 sheets under each nitriding condition.

As shown in FIG. 1, it has been found that in the region of ΔN (surface) / ΔN (total thickness) ≦ 2.0, B ₈ after finish annealing is particularly high, and a product having excellent magnetic properties can be obtained. Therefore, in the present invention, the nitriding treatment is performed so as to satisfy ΔN (surface) / ΔN (total thickness) ≦ 2.0 (referred to as equation (1) in the present invention).

Furthermore, the amount of nitriding at the end of nitriding is also important for manufacturing. This is because when the amount of nitriding is small, the solubility product is not increased in finish annealing, and the effect of stabilizing the desired inhibitor cannot be expected.
Specifically, when the amount of nitriding at the end of nitriding is obtained, in the present invention, ΔN (total thickness) needs to be 50 ppm or more based on the experiment of Example 1 described later.

  Next, a method for realizing uniform nitriding in the thickness direction will be described. Based on thermodynamic calculations and experimental investigations, when the nitriding temperature is less than 800 ° C., part of the nitrogen forms silicon nitride at the same time as the nitriding, and therefore does not diffuse to the center of the steel sheet. On the other hand, when the nitriding temperature is 800 ° C. or higher, nitrogen diffuses through the steel to the center of the steel sheet in a solid solution state. An increase in quantity is avoided.

Further, at that time, holding at a temperature of 800 ° C. or higher for 10 seconds or more allows nitrogen to sufficiently diffuse to the plate thickness center (plate thickness ½). The upper limit of the temperature is not particularly defined, but 1000 ° C. or less is preferable. This is because when the temperature is higher than 1000 ° C. during nitriding, the decomposition of ammonia in the gas phase is promoted in the nitriding using NH ₃ gas, and the ammonia reaching the steel sheet is reduced. In addition to reducing the performance, the primary recrystallized grains become larger in size, reducing the driving force that causes secondary recrystallization, making the secondary recrystallization unstable and providing excellent magnetic properties. Because there is no.

  Of the heat treatment of holding at 800 ° C. or higher and holding for at least 10 seconds, setting the plate temperature to 800 ° C. or higher needs to be performed at least during the nitriding treatment. This is because once silicon nitride or iron nitride is formed in the vicinity of the surface layer by nitriding, it takes time for the nitrogen of the silicon nitride or iron nitride to be dissolved. Therefore, by setting the processing temperature to 800 ° C. or higher during nitriding, in the present invention, nitrogen can be diffused in a solid solution state up to the center of the plate thickness without forming silicon nitride or iron nitride.

  In order to diffuse nitrogen in a sufficiently solid solution state up to the center of the plate thickness, it is preferable to hold at a plate temperature of 800 ° C. or higher for 10 seconds or more, but this holding is intermittent even if continuous. There may be. Further, there is no problem even if the nitriding process is finished during this holding and the atmosphere is switched to a nitrogen atmosphere.

An annealing separator is applied to the steel sheet surface after the primary recrystallization annealing and nitriding treatment. 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 agent, 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.

  Following this, finish annealing is performed. In addition, the finish annealing in this invention should just follow a conventional method.

  Since the precipitate mainly composed of silicon nitride used in the present invention is coarse unlike the conventionally used inhibitor (precipitate particle size is 0.1 μm or less), the precipitate is thermodynamically dissolved or Ostwald. It has the feature that the time required for growth becomes longer. In other words, unlike the technique of finely precipitating AlN or (Al, Si) N by including Al in the slab at 100 ppm or more, it is longer for the suppression of normal grain growth in the primary recrystallized structure to be smaller. It has the feature that it takes time. Therefore, it takes time as described below to reach the secondary recrystallization.

The technique of isothermal holding near the secondary recrystallization temperature is generally applied when time is required for nucleation and grain growth of secondary recrystallization. In the present invention, however, silicon nitride is different from this. This is to secure the time required for the thermodynamically changing shape.
Therefore, when the residence time in the vicinity of the secondary recrystallization temperature is less than 10 hours, the effect of suppressing normal grain growth by silicon nitride continues to a high temperature, and sufficient magnetic properties cannot be obtained. Therefore, the residence time in the vicinity of the secondary recrystallization temperature is 10 hours or more. Further, after the retention (residence) near the secondary recrystallization temperature, purification is performed at 1200 ° C. As the annealing atmosphere, any of N ₂ , Ar and H ₂ or a mixed gas thereof is suitable.

After the above-described finish annealing, an insulating film can be further applied and baked on the steel plate 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 method is preferred.
Further, the shape of the steel sheet can be adjusted by flattening annealing, and this flattening annealing can be combined with the baking treatment of the insulating coating.

Silicon steel slab containing Si: 3.25%, solAl: 0.006%, N: 0.004%, C: 0.04%, Mn: 0.08%, Cr: 0.005%, Cu: 0.1%, Sb: 0.01%, After 30 minutes of heating, hot-rolled to a hot-rolled sheet with a thickness of 2.2 mm, annealed at 1050 ° C. for 1 minute, and then cold-rolled to a final thickness of 0.27 mm. A sample of 100 mm × 400 mm size was taken from the center of the rolled coil and annealed in the laboratory for both primary recrystallization and decarburization.
Thereafter, nitriding was performed under the nitriding conditions shown in Tables 1 to 3 in a mixed atmosphere of ammonia, hydrogen, and nitrogen. At that time, the conditions other than the steel plate temperature, ammonia concentration, and each holding time were the same. 23 sheets of steel under the same conditions were produced per condition. Using two of them, perform a chemical analysis of the nitrogen amount before polishing and the nitrogen amount when 10% of the surface is chemically polished. From the nitrogen amount before nitriding treatment and the nitrogen amount before polishing after nitriding treatment , ΔN (total thickness) was obtained, and further, ΔN (surface) was obtained using the above formula (2).
In addition, an annealing separator containing MgO as a main component and containing 5% of TiO ₂ was made into a water slurry for the remaining 21 sheets, applied to a steel plate, dried, and baked on the steel plate. One of them was subjected to secondary recrystallization annealing with a temperature gradient in the sample, and the temperature at which secondary recrystallization occurred was determined.
Furthermore, the remaining 20 sheets are subjected to a holding heat treatment for 40 hours in the vicinity of the previously determined secondary recrystallization temperature, and further up to 1050 ° C. at a rate of temperature increase of 20 ° C./h and exceeding 1050 ° C. After that, final finish annealing was performed in which the temperature was increased to 1200 ° C. at a rate of temperature increase of 10 ° C./h.
Subsequently, a phosphate-based insulation tension coating was applied and baked, and the magnetic flux density (B ₈ , T) at a magnetizing force of 800 A / m was evaluated. The magnetic characteristics were evaluated by the average value of 20 sheets for each condition.

  Regarding the nitriding treatment, the condition No. In Nos. 1 to 55, nitriding was performed while maintaining the mixed atmosphere of ammonia, hydrogen, and nitrogen for the time indicated in Tables 1 and 2. Tables 1 and 2 show the time maintained at 800 ° C. or higher including the nitriding process to the cooling process. Furthermore, condition no. 56 to 60 were nitrided under the conditions shown in Table 3, then cooled to room temperature, held at 850 ° C. for 30 seconds, reheated, and then cooled to finish annealing.

As can be seen from Tables 1 and 2, in the inventive examples, B ₈ is significantly increased as compared with the comparative examples, and it is clear that the magnetic characteristics are improved.
In addition, as shown in Table 3, during the nitriding treatment, after raising the steel plate to a temperature of 800 ° C. or higher, if not held for 10 seconds or more, silicon nitride is precipitated in the vicinity of the surface layer, and the subsequent annealing starts. In the above process, even if reheating is performed at 800 ° C or higher, the above-mentioned formula (1) (ΔN (surface) / ΔN (total thickness) ≤ 2.0) cannot be satisfied, and good magnetic properties are obtained. Not obtained.

Claims (4)

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 less than 100 mass ppm, N is It is controlled to 80 mass ppm or less, and further satisfies the relationship of sol.Al (mass ppm) −N (mass ppm) × (26.98 / 14.00) ≦ 0 (mass ppm), with the balance being the composition of Fe and inevitable impurities A steel slab consisting of either a re-heated or re-heated hot-rolled sheet, and then a cold-rolled sheet with the final thickness by annealing and rolling, followed by primary recrystallization annealing. In the method for producing a grain-oriented electrical steel sheet that is subjected to nitriding after being performed or during primary recrystallization annealing, and further subjected to secondary recrystallization annealing by applying an annealing separator,
The steel sheet when the secondary recrystallization annealing is performed has a composition of 50 mass ppm or more in terms of an average nitrogen amount increment with respect to the average nitrogen amount of the entire plate thickness before nitriding: ΔN (total thickness), and the average nitrogen The amount increment: ΔN (total thickness) and the average nitrogen amount increment from the both surfaces of the steel sheet to the depth of 10% of the plate thickness: ΔN (surface) among the average nitrogen amount increments is expressed by the following formula (1). Further, in the secondary recrystallization annealing, the residence time in the vicinity of the secondary recrystallization temperature is set to 10 hours or more .
ΔN (surface) / ΔN (total thickness) ≦ 2.0 (1)
ΔN (surface): Increment of average nitrogen amount (mass ppm) from both surfaces of the steel sheet to a depth of 10% of the plate thickness
ΔN (total thickness): Average nitrogen content increment (mass ppm) of the entire plate thickness   2. The method for producing a grain-oriented electrical steel sheet according to claim 1, wherein nitriding treatment conditions are adjusted so that nitrogen that has entered through nitriding treatment diffuses to a thickness center (plate thickness 1/2) in a solid solution state. A method for producing a grain-oriented electrical steel sheet.   3. The grain-oriented electrical steel sheet according to claim 1 or 2, wherein the sheet temperature is kept at least 800 [deg.] C. during nitriding, and then the sheet temperature is maintained for at least 10 seconds. Manufacturing method.   The method for manufacturing a grain-oriented electrical steel sheet according to any one of claims 1 to 3, wherein the composition further includes, in mass%, Ni: 0.005-1.5%, Sn: 0.01-0.50%, Sb: 0.005-0.50. %, Cu: 0.01 to 0.50%, Cr: 0.01 to 1.50%, P: 0.0050 to 0.50%, Nb: 0.0005 to 0.0100%, and Mo: 0.01 to 0.50%. A method for producing a grain-oriented electrical steel sheet.

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