JPS633009B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Field of Application] The present invention particularly relates to a method for manufacturing a unidirectional electrical steel sheet with a good forsterite film. [Prior Art] A unidirectional electrical steel sheet is a soft magnetic material mainly used for transformer cores. This type of steel sheet is composed of crystal grains ((110) [001] oriented grains) whose axes of easy magnetization are aligned within a range of several degrees with respect to the rolling direction, in order to significantly increase magnetic permeability only in the direction of magnetic flux flow.
Furthermore, the inclusion of Si increases the specific resistance and reduces iron loss caused by Joule heat. On its surface, forsterite, MgSiO ₄ , which is produced by a solid phase reaction between SiO ₂ selectively oxidized during decarburization annealing in the steel sheet manufacturing process and MgO coated on it, is several μm thick. It is attached to the surface and acts as an insulating film, as well as
Perform subdivision of magnetic domains. It also plays an important role in terms of magnetic properties. Usually, it is used as a transformer core after being coated with a secondary coating mainly made of colloidal silica for the purpose of adding tension. From this point of view, a unidirectional electrical steel sheet is a composite material that is covered with crystal grains with (110) [001] orientation in the rolling direction, and has a ceramic film and a secondary film mainly composed of forsterite on the surface layer. It can be said that Achieving a high degree of integration of (110) [001] orientation in unidirectional electrical steel sheets and forming a forsterite film in oxide-based ceramics is usually achieved through a box called final finish annealing.
They occur almost at the same time during annealing. The former is called secondary recrystallization, which is the abnormal grain growth of (110)[001] oriented grains, and the latter is achieved by the SiO ₂ -MgO solid phase reaction on the surface of the steel sheet, and these two reactions are essentially completely different. Despite being a phenomenon, real reactions proceed while indirectly interfering with each other. As is well known, forsterite, Mg ₂ SiO ₄ ,
is produced by MgO penetrating into _SiO2 in the form of Mg ⁺⁺ and ^O-- (GW Brindley & R. Hayami
Phil, Mag. 12, 505 (1965)). In the field of research related to the production of grain-oriented electrical steel sheets, this reaction system
It is empirically known that the presence of trace amounts of substances such as MnO ₂ , TiO ₂ , B ₂ O ₃ , CaO, and Sr compounds changes the properties of the produced forsterite. For example, in Japanese Patent Publication No. 51-12450,
The addition of MnO ₂ was reported in Publication No. 51-12451.
The addition of TiO ₂ was disclosed respectively. In addition, the effects of Sr compounds on the properties of forsterite films have been disclosed in Japanese Patent Publications No. 32716/1980, No. 89422/1989, and No. 75577/1983. JP-A-54-66935 discloses a method for controlling the amount of water brought in, mainly in the form of magnesium hydroxide, depending on the amount of CaO component contained in an annealed and separated product containing MgO as the main component. What is the mechanism behind these trace additives?
It affects the reaction mechanism of the MgO-SiO ₂ system solid phase reaction, and also changes various properties such as the crystal grain size of the forsterite produced and the adhesion between the so-called forsterite film composed of forsterite crystal grains and the steel plate. It is not clear at this point whether this is the case. However, these substances commonly include MgO, SiO ₂ , Mg ₂ SiO ₄ etc.
These additives have the property of lowering the melting point of MgO−SiO ₂ -based oxides, so these additives ^are
It is thought that it contributes to the reaction by accelerating the diffusion rate of O ^-- . In some cases, the maximum temperature for finish annealing of normal grain-oriented electrical steel sheets is 1200℃.
It is speculated that the reaction rate of the system is significantly increased by partially forming a liquid phase in the vicinity. Now, the present inventors first achieved a small forsterite crystal grain size by controlling the Mn activity in the steel and the oxygen partial pressure (expressed as PH ₂ O / PH ₂ ) in the atmosphere during this forsterite formation reaction. (Japanese Patent Application No. 59-53819) disclosed a method for forming a forstellite film which has good adhesion and the tension exerted by the film on a steel plate is greater than that obtained by conventional techniques. This method differs from previous methods in which the properties of the film are improved by adding small amounts of additives to the annealed separator.This method uses the ingredients in the steel, namely the Mn activity and the oxygen partial pressure in the external atmosphere, to improve the properties of the film. The aim is to improve. The key point of the mechanism is thought to be that MnO is generated by high-temperature oxidation during the MgO-SiO ₂ solid phase reaction.
It can be said that it has a different feature from the conventional technology from an industrial point of view as it requires fewer types of trace additives. [Problems to be Solved by the Present Invention] However, when the technology disclosed by the present inventors was applied to coils with large unit weights of 10 tons and 20 tons, general It was discovered that there were many coating defects called ``Nishimofuri'' where the bare metal was exposed. These film defects not only deteriorate the appearance of the finished product, but also have an unfavorable effect on the magnetic properties, such as deterioration of the insulation properties and the tension applied to the steel sheet. [Means for Solving the Problems] The purpose of the present invention is to obtain a patent application filed in 1983 by the present inventors, which aims to produce a good forsterite film.
The object of the present invention is to provide a final annealing method that eliminates and improves these problems that subsequently occurred in No. 53819 and forms a good forsterite film in a coil having a large unit weight. That is, in the present invention, C: 0.025% to 0.075 by weight
%, Si: 3.0%~4.5%, Acid soluble Al: 0.010~
0.060%, N: 0.0030% to 0.0130%, S≦0.010%,
and Mn 0.8%≧Mn≧0.05+7×(S+
0.405Se), and
Starting from a slab containing Cr: 0.07-0.25% as necessary, the balance consisting of Fe and unavoidable impurities,
In the conventional method for producing unidirectional electrical steel sheets, the oxygen partial pressure (PH ₂ O / PH ₂ ) in the atmosphere in the temperature range from the start of finish annealing to 650°C is limited to 0.015 or less, and decarburization annealing is performed. In the process of applying an annealing separator mainly composed of magnesia to a plate, the annealing separator is made into a slurry, the amount of undercoat applied and dried is 4 g/ ^m2 or less on one side, and 3~
The present invention provides a method for removing defects occurring in a forsterite film by simultaneously applying a so-called electrostatic coating method in which 6 g/m ² (one side) of magnesia is electrostatically applied. In addition to this method, during finish annealing
By holding the steel plate in the temperature range of 800 to 900°C for 10 to 30 hours, film defects can be removed more completely, and the oxygen partial pressure can be reduced when the temperature is increased from 800 to 900°C to 1100°C. It has become clear that the film properties can be improved by regulating the amounts of Mn, S, and Se in the steel within specific ranges. The present invention will be explained in detail below. Figure 2 shows C: 0.055%, Si: 3.25%, Mn: 0.18
%, S: 0.006%, P: 0.02%, Al: 0.03%, N:
After decarburizing a cold-rolled sheet of 8 cm x 8 cm containing 0.007% TiO ₂ in a wet hydrogen atmosphere,
The frequency of occurrence of lame-like film defects that appeared on the forsterite film obtained after applying 7 g/m ² of annealing separation material mainly composed of magnesia containing parts by weight on one side and performing final annealing is shown. It is something. In this experiment, the final annealing atmosphere was 25% N ₂ and 75% H ₂ .
The oxygen partial pressure up to 650℃ was set to various values as a parameter. This experiment revealed that the frequency of occurrence of chilblain-like film defects is particularly remarkable when the oxygen partial pressure in the atmosphere during the first half of final annealing exceeds 0.015 in terms of PH ₂ O/PH ₂ . In addition, when similar experiments were conducted by changing the dew point during decarburization annealing for materials with different Mn activities in steel, it was found that this type of film defect occurs more frequently in materials with higher Mn activities. Furthermore, it was found that decarburization tends to occur when the oxygen partial pressure during decarburization annealing is high. The present inventors investigated in detail the occurrence of these film defects and came to the following conclusion. In other words, these film defects are caused by MgO and SiO ₂ , which constitute forsterite, or TiO ₂ , which is intentionally added to the annealing separator, during high-temperature annealing at around 1200°C.
and B, alkali or alkaline earth oxides such as Na ₂ O that are unavoidably mixed in, and FeO, MnO, which is produced by high-temperature oxidation of components in steel,
This is due to the formation of a melt resulting from the composition of these composite oxide systems such as Al ₂ O ₃ satisfying a eutectic composition with a eutectic point of 1200° C. or less. An excessive increase in the oxygen partial pressure in the annealing atmosphere up to 650°C in the first half of the final annealing causes excessive production of (Fe, Mn)O-based oxides, and due to the presence of these oxides, even after the formation of forsterite, the composite It was concluded that the melting point of the oxide system becomes below 1200℃, resulting in the formation of chimney-like film defects. Now, based on these findings, the present inventors conducted a similar experiment using a 10-ton coil while maintaining the final annealing atmosphere at PH ₂ O/PH ₂ at 0.015 or less. However, as a result, the present inventors faced a new problem in that the grain-like film defects that disappeared in the small sample test were not removed in the 10-ton coil test. In order to solve this problem, the present inventors continued to conduct experiments based on the following considerations. The usual method for applying the annealing separation material is to dissolve magnesia and various additives in pure water, apply it to the steel plate as a slurry suspension, and dry it. On this occasion,
The hydration reaction of magnesia progresses and Mg(OH) ₂ is partially formed. This hydration component is 400 to 500 during finish annealing.
It is released in the temperature range of °C and increases the partial pressure of oxygen between the coil plates. Oxygen partial pressure in the atmosphere outside the coil
Even if PH ₂ O/PH ₂ is controlled to 0.015 or less, it takes time for the oxygen partial pressure in the atmosphere directly above the steel plate to actually reach such a value, which results in the formation of chilblain film defects. . FIG. 3 shows an example in which a computer is used to simulate the change over time in the oxygen partial pressure (in this case, the water vapor partial pressure) between the coil plates. In order to prevent such an excessive increase in the oxygen partial pressure between the coil plates, there are two measures: to reduce as much as possible the hydration components that are unnecessarily brought in between the coil plates, and to increase the gas permeability between the plates. Basically valid. The method disclosed in JP-A No. 58-67871 by the present inventors, in which non-hydratable dead burnt magnesia powder is electrostatically attached after applying a suspension liquid containing magnesia as a main component ( Electrocoating (also called electrocoating) serves the purpose of controlling the amount of hydration components introduced into the coil. Table 1 shows the usual magnesia coating method (coating amount: 7 g/m ² on one side) and the electrostatic coating method (coating amount: undercoating 3 g/m 2 on one side) on a 10 ton coil with a coil width of ¹ m while satisfying the above external atmosphere conditions. , 5 g of electrostatic coating on one side) and final annealing. By employing the electrostatic coating method in this way, it has become possible to prevent the occurrence of scallop-like film defects even when final annealing is performed on an actual coil.

[Table] As mentioned above, it has become clear that a forsterite film can be stably formed by adopting the electrostatic coating method. steel plate in the temperature range
By holding it for 10 to 30 hours, defects in the forsterite film are more reliably reduced. This will be explained next.

[Table] Figure 4 shows that a cold-rolled sheet containing 0.2% Mn and 0.004% S was decarburized and annealed in an atmosphere of PH ₂ O/PH ₂ 0.366, then coated with magnesia containing 5 parts by weight of TiO ₂ and finished annealed. GDS (Glow Discharge Optical
The results are shown below.
As can be seen from this figure, Si
gathers on the surface. When we investigated the existence form using infrared reflection spectra, we found that these Si
It was found that it exists as SiO ₂ . From these results, it became clear that the oxide film formed during decarburization annealing undergoes alteration in which SiO ₂ becomes concentrated in the temperature range of 800 to 900°C during final annealing. Therefore, such enrichment of SiO ₂ , that is, a change in the composition of the complex oxide system to the SiO ₂ rich side, increases the eutectic point of the system.
The conclusion was reached that by setting the temperature to 1200°C or higher, the occurrence of chilblain film defects was suppressed. Note that such concentration of SiO ₂ at 800 to 900° C. during final annealing is particularly noticeable in materials with high Mn activity. The inventors' views regarding this reason will be described below. As is well known, the high-temperature oxidation behavior of the 3% Si-Fe system varies depending on the Mn activity, and when the Mn content exceeds a certain level, the oxidation rate of the material increases significantly (N. Morito & T. Ichida 17 961 (1977)) ). By exposing the film thus formed to a low oxygen partial pressure atmosphere such as during final annealing,
The film composition is now in a new equilibrium state, i.e. SiO ₂ /
(Fe, Mn) ₂ It is thought that the SiO ₄ ratio is approaching a high state. These changes occur in the initial film.
It is thought that the lower the SiO ₂ /(Fe, Mn) ₂ SiO ₄ ratio, the more strongly it appears. The final annealing method described above is based on the Patent Application No. 53819/1986 filed by the present inventors, whose main point is to control the oxygen partial pressure in the final annealing atmosphere according to the Mn activity in the steel.
By combining it with the number, the effect can be even more effectively demonstrated. Mn in Table 3:
0.18%, S: 0.006% material is decarburized and annealed, TiO ₂
The graph shows the incidence of film defects and the average forsterite crystal grain size of materials obtained by applying magnesia containing 5 parts by weight by a conventional method and an electrostatic coating method and final annealing.

[Table] Upper row: Shimofuri defect occurrence rate Lower row: Forsterite crystal grain size In this way, by increasing the dew point of the atmosphere at 850-1100°C to some extent during finish annealing, the crystal grain size of the forsterite film can be reduced. . The reason for this is not necessarily clear, but as described in Japanese Patent Application No. 59-53819, the present inventors found that an appropriate amount of MnO is generated on the oxide film side by combining the Mn activity in the steel and the oxygen partial pressure in the atmosphere. And this MnO is MgO
-It is thought to have a catalytic effect on SiO ₂ -based solid phase reactions. The method of applying a certain amount of oxygen partial pressure in the second half of final annealing is based on the idea of increasing the SiO ₂ /(Fe, Mn) ₂ SiO ₄ ratio by holding the temperature at 800 to 900°C, which was mentioned earlier, and eliminating the defects. At first glance, it seems like a premonition. However, as already mentioned, these methods have been experimentally shown to be effective in producing good forsterite films on materials with high Mn activity. The present inventors' views regarding these phenomena will be described below. First, from the viewpoint of promoting the solid phase reaction, an appropriate amount of MnO is effective in forming a MgO-SiO ₂ film. Therefore, the stability of MnO becomes a problem for materials with high Mn activity. Figure 5 shows the thermodynamic stability of MnO using Mn activity as a parameter. The numerical values required to create this diagram are (“Metallurgical Thermochemistry” (5
th edition, 1979) Kubaschewski & Alcock
(Pergamon Press). As is clear from this figure, the equilibrium stability of MnO strongly depends on the oxygen partial pressure of the temperature atmosphere and the Mn activity in the steel. Moreover, kinetic factors should act even more strongly to reduce MnO once formed. The present inventors' view is that an appropriate amount of MnO is necessary to improve the properties of the forsterite film, and that excess MnO, FeO, or (Fe, Mn)O-based oxides in general, can reduce MgO-SiO ₂ . This is undesirable in the sense that it lowers the eutectic point of the main composite oxide system and induces furi defects. As is clear from Figure 5, if a thermodynamic equilibrium state is always achieved, in order to secure an appropriate amount of MnO,
For example, the transition may be made as shown by the arrow in FIG. 5 depending on the Mn activity. Simply put, as the temperature rises, the partial pressure of oxygen in the atmosphere also increases little by little. However, in the actual manufacturing process of grain-oriented electrical steel sheets, this does not apply from the initial stage. This is because the oxygen partial pressure during decarburization annealing is (1) to perform decarburization, and ( ₂ ) log (PH ₂ O／
PH ₂ )-0.3, and as can be seen from FIG. 5, this value is much higher than the oxygen partial pressure range that is a problem during final annealing. In order to form a good forsterite film starting from the oxide film thus formed, it is necessary to use the method described in the present invention, that is, to suppress additional oxidation in the first half of the final annealing (PH ₂ O /PH ₂
0.015), especially in actual coils, the amount of hydration components brought in as Mg(OH) ₂ is limited (electrostatic coating method), and the composition of the oxide film is enriched to some extent with SiO ₂ (maintaining 800-900℃). is valid. Once this state is reached, conversely, a certain amount of oxygen partial pressure is applied as the temperature rises.
Securing the necessary amount of MnO works effectively. Due to the technological advances mentioned above, S+
0.405Se: 0.010 or less and Mn: {0.05×7(S+
0.405Se)} to 0.8%, a material with a high Mn activity, it has become possible to stably obtain a forstellite film without any frizz defects. Next, the limiting conditions of the constituent factors of the present invention will be described. The regulations for Mn, S and Se in steel mainly apply to steel.
This was determined from the viewpoint of securing Mn activity and improving the properties of the forsterite film. In other words, as mentioned above, MnO produced by oxidation of Mn in steel improves the properties of the forsterite film in the finished product, but in order to secure the amount of free Mn necessary for this purpose, the S content must be 0.010% or less. It is desirable that In terms of trapping Mn, Se also has a similar effect, so increasing Se is also not desirable. From the above points, the upper limit value for S and Se was set as S+0.405Se, which is 0.010%. If S is increased further than this, secondary recrystallization defects called linear fine grains will occur in terms of material quality, and the characteristics of the surface film will also deteriorate. Mn
The lower limit of S and
It was set as {0.05+7 (S+0.405Se)}% with respect to the amount of Se.
If Mn is less than this value, the film will deteriorate and the secondary recrystallization will become unstable, which is not preferable. The upper limit of Mn was set at 0.8%. If the amount of Mn increases more than this, the magnetic flux density of the finished product will deteriorate, which is not preferable. If C is less than 0.025%, secondary recrystallization becomes unstable, and even if secondary recrystallization is performed, the magnetic flux density (only 1.80T or less can be obtained with B ₁₈ ) is poor, so 0.025
% or more. On the other hand, if the C content is too large, the decarburization annealing time becomes long, which is not economical, so it is set to 0.075% or less. If Si exceeds 4.5%, cracking during cold rolling becomes significant, so it was set to 4.5% or less. In addition, if it is less than 3.0%, it will not be possible to obtain the highest grade iron loss of W _17/58 of 1.05W/Kg or less with a product plate thickness of 0.30mm, so it was set to be 3.0% or more. It is preferably 3.2% or more. In the present invention, as a precipitate necessary for secondary recrystallization,
Use AlN. Therefore, in order to ensure the minimum amount of AlN required, 0.010% or more of acid-soluble Al,
N is required to be 0.0030% or more. acid soluble Al
If it exceeds 0.060%, AlN in the hot rolled sheet becomes inappropriate and secondary recrystallization becomes unstable, so it was set to 0.060% or less. Regarding N, if it exceeds 0.0130%, "blistering" on the surface of the steel plate will occur, so it was set to 0.0130% or less. Cr is effective in stabilizing magnetism. Rule
By including Cr, the range of acid-soluble Al amount that can provide high magnetic flux density can be expanded. moreover,
Improves iron loss characteristics under the same magnetic flux density.
This iron loss reduction effect becomes noticeable when the Cr content is 0.07% or more, and is saturated at 0.25%. Furthermore, if Cr is contained in an amount exceeding 0.25%, a problem arises in that the decarburization rate is reduced when the decarburization temperature is increased, so the Cr content is set to 0.07% to 0.25%. The oxygen partial pressure in the atmosphere from the start of final annealing to 650°C must be PH ₂ O/PH ₂ 0.015. If it is more than this, a chirping defect will occur in the finished forsterite film. The amount of coating in the electrostatic coating method is limited as follows. First, the coating amount of the annealing separation material is 4 g/m ² or less on one side, which is applied as an undercoat in the form of a slurry using the usual method and dried. If it is more than this, the amount of hydration components brought into the space between the coil plates will be unnecessarily increased, and the original purpose of controlling the oxygen partial pressure in the atmosphere during final annealing will not be achieved. In addition, the amount of magnesia applied electrostatically to prevent burn-in is 3 to 3 times per side.
6g/ ^m2 . If it is less than 3 g/m ² , coil seizure may occur, and if it is applied in an amount exceeding 6 g/m ² , the anti-seizure effect remains the same and it is not economical. When holding a steel plate in a temperature range of 800°C to 900°C and aiming to improve film properties, the holding time is limited to 10 to 30 hours. If it is shorter than this, no significant effect on improving film properties can be expected, and even if it is longer than this, the effect is not much different from that of 30-hour retention.
Therefore, it is not economical to hold the coil for more than 30 hours. Next, the reason for limiting the atmospheric oxygen partial pressure in the temperature range from 800 to 900°C in the latter half of the final annealing to 110°C as shown in claims 2 and 4 of the present invention will be explained (FIG. 1). The purpose of this invention is to improve the characteristics of a forsterite film formed on the surface of a product, and the detailed content of the invention is the same as that of Japanese Patent Application No. 59-53819 filed by the present inventors. That is, the gist of the present invention is to secure a necessary amount of MnO, which has a catalytic function in the MgO-SiO ₂ solid phase reaction, which is a forsterite production reaction, in an oxide film mainly composed of SiO ₂ . The method is to adjust the Mn activity in the steel, specifically the amount of Mn in the steel and the amounts of S and Se that trap Mn, from the temperature range of 800-900℃ where the MgO- _SiO2 solid phase reaction proceeds. ℃
The oxygen partial pressure in the final annealing atmosphere, specifically PH ₂ O/PH ₂ , was limited to the range shown in FIG. 1 in the temperature range up to . In Figure 1, AB and DC are limitations regarding the components (as mentioned above), and the reasons for these limitations have been explained, but to reiterate:
If Mn-1.719 (S + 0.405Se) is less than 0.05 (AB), the necessary amount of MnO that is effective in improving the film properties cannot be obtained, and if it is more than 0.8 (DC), the magnetic flux density of the product will deteriorate, which is undesirable. . BC is the oxygen partial pressure necessary to secure the MnO necessary to improve the properties of the forsterite film, and this is the oxygen partial pressure required if a _Mo・PH ₂ O/PH ₂ (a _Mo・Mn activity) is not above a certain value. Since this is not the case, a straight line with the slope shown in the figure was created. If PH ₂ O/PH ₂ is less than this, the average grain size of the forsterite crystals will exceed 0.5 μm, which is inappropriate. The upper limit value DE of PH ₂ O/PH ₂ is such that if the Mn activity is increased further or the atmosphere is made oxidizing, film defects will occur that are thought to be caused by the excessively generated liquid phase, and the interlayer resistance of the product will decrease. Since this causes deterioration, the settings were made as shown in the figure.
Further, the upper limit value AE is limited due to problems in on-site operations. In other words, in order to perform finish annealing at PH ₂ O/PH ₂ of 5×10 ^-2 or more, a large-capacity humidifier is required, and it is also difficult to apply oxygen partial pressure evenly in the width and length directions of the coil. The upper limit value needs to be set to AE because the product yield also decreases.
Due to the above constraints on PH ₂ O / PH ₂ and a _Mo , the oxygen partial pressure from the temperature range of 800 to 900℃ to 1100℃ during final annealing must be maintained within the range shown in Figure 1 ABCDE. . Example 1 C: 0.054%, Si: 3.3%, Mn: 0.15%, P:
0.02%, S: 0.006%, Se: 0.0006%, acid soluble
A slab was made by a continuous casting method from molten steel containing Al: 0.030% and N: 0.007%, and after heating the slab to 1150°C, it was hot rolled into a hot rolled plate with a thickness of 1.8 mm. This hot rolled plate
After hot-rolled sheet annealing at 1100°C for 2 minutes, it was cold rolled to 0.18 mm and decarburized annealed in warm hydrogen at a temperature of 810°C. This board is coated using the usual method (coating amount, 6g/side).
m ² ) and electrostatic coating method (undercoat 3 g/m ² on _one side + electrostatic coating method (MgO only) 4 g/m ² on one side).
Parts by weight of magnesia were applied. The 10 ton coil created in this way was heated to 25% nitrogen, 75% hydrogen, dew point -20℃ (PH ₂ O / PH ₂ = 0.002), 0℃ (PH ₂ O /
PH ₂ = 0.008) and 20℃ (PH ₂ O/PH ₂ = 0.032)
The temperature was raised to 650℃ in an atmosphere of
The temperature was changed to 1200°C at a heating rate of 10°C/hr. Table 4 shows the occurrence rate of shingle-like film defects in the obtained steel sheets.

[Table] 5 5 indicates the present invention

Claims (1)

[Claims] 1% by weight, C: 0.025% to 0.075%, Si: 3.0%
~4.5%, acid soluble Al: 0.010~0.060%, N:
0.0030%~0.0130%, S≦0.010%, and Mn
A silicon steel slab containing so as to satisfy the relationship 0.8%≧Mn≧0.05+7×(S+0.405Se), with the balance consisting of Fe and unavoidable impurities, was hot rolled, cold rolled, and then heated in a wet hydrogen atmosphere. In a method for producing a unidirectional electrical steel sheet in which finish annealing is performed after decarburization annealing and application of an annealing separator, applying a slurry annealing separator of 4 g/m ² or less per side of the steel sheet to the steel sheet after decarburization annealing, After drying, apply 3 to 6 coats per side of the steel plate on top of it.
g/m ² of magnesia is applied electrostatically, and the oxygen partial pressure (expressed in PH ₂ O / PH ₂ ) of the atmosphere in the temperature range from the start of final annealing to 650 ℃ is 0.015 or less, and at 800 to 900 ℃. 1. A method for producing a unidirectional electrical steel sheet with a good forsterite film, characterized by holding the steel sheet in a temperature range of 10 to 30 hours. 2. During final annealing, the oxygen partial pressure when the temperature is raised from 800 to 900℃ to 1100℃ is set to the value in the area surrounded by ABCDE in Figure 1, corresponding to the amount of Mn and S in the steel. A method according to claim 1, characterized in that: 3 In weight%, C: 0.025% to 0.075%, Si: 3.0%
~4.5%, acid soluble Al: 0.010~0.060%, N:
0.0030%~0.0130%, S≦0.010%, Cr:0.07~
0.25% and Mn 0.8%≧Mn≧0.05+7×(S+
0.405Se), and the remainder
Manufacture of unidirectional electrical steel sheets by hot rolling and cold rolling silicon steel slabs containing Fe and unavoidable impurities, decarburizing annealing in a wet hydrogen atmosphere, applying an annealing separator, and post-finish annealing. In the method, a slurry annealing separator of 4 g/m ² or less per side of the steel plate is applied to the steel plate after decarburization annealing, and after drying, magnesia of 3 to 6 g/m ² per side of the steel plate is electrostatically applied thereon. And the oxygen partial pressure of the atmosphere in the temperature range from the start of final annealing to 650℃ (expressed as PH ₂ O / PH ₂ )
A method for producing a unidirectional electrical steel sheet with a good forsterite film, characterized in that the steel sheet is maintained in a temperature range of 800 to 900°C for 10 to 30 hours. 4 During final annealing, set the oxygen partial pressure during temperature rise from 800 to 900℃ to 1100℃ to the value in the area surrounded by ABCDE in Figure 1, corresponding to the amount of Mn and S in the steel. The method according to claim 3, characterized in that: