JPH07310124A - Production of thick grain-oriented silicon steel plate excellent in magnetic characteristic and film coating characteristic - Google Patents

Abstract

PURPOSE:To produce a thick grain-oriented silicon steel plate excellent in magnetic characteristics and film coating characteristics by executing final finish annealing under specified conditions at the time of producing a thick grain-oriented silicon steel plate from a silicon steel slab having a specified componental compsn. CONSTITUTION:A thick grain-oriented silicon steel plate having 0.36 to 1.00 mm thickness is produced from a silicon steel slab having a compsn. contg., by weight, 0.021 to 0.075% C, 2.5 to 9.5% Si, 0.010 to 0.060% acid soluble Al, 0.0010 to 0.0130% N, S+0.405Se; <=0.014%, 0.05 to 0.8% Mn, and the balance Fe with inevitable impurities. At this time, the average grain size of primarily recrystallized grains till the start of final finish annealing after the completion of decarburizing annealing is regulated to 18 to 35mum, nitriding treatment is executed to the steel plate till the start of hot rolling-final finish annealing, and in the case the content of N at the start of the final finish annealing for the steel plate after the nitriding treatment is defined as N(%) and the plate thickness of the product as (t) (mm), the N and (t) are controlled by the inequality, and the dew point in the temp. range of <=800 deg.C at the time of temp. rising in the final finish annealing is regulated to 0 to 30 deg.C.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a thick unidirectional electrical steel sheet having excellent magnetic properties and coating properties, which is used as an iron core of a transformer or the like.

[0002]

2. Description of the Related Art Unidirectional magnetic steel sheets are mainly used as iron core materials for transformers and other electrical equipment, and are required to have excellent magnetic characteristics such as excitation characteristics and iron loss characteristics. The magnetic field strength is 800A as a numerical value showing the excitation characteristics.
The magnetic flux density B _{8 at} / m is usually used. As the numerical value showing the iron loss characteristic, the iron loss W _17/50 per 1 kg when magnetized to 1.7 Tesler (T) at a frequency of 50 Hz is used. The magnetic flux density is the most dominant factor of the iron loss characteristics, and generally speaking, the higher the magnetic flux density, the better the iron loss characteristics. Generally, when the magnetic flux density is increased, the secondary recrystallized grains become large, which may result in poor iron loss characteristics.
On the other hand, by controlling the magnetic domains, the iron loss characteristics can be improved regardless of the grain size of the secondary recrystallized grains.

This unidirectional electrical steel sheet is manufactured by causing secondary recrystallization in the final finishing annealing step to develop a so-called Goss structure having {110} axis on the steel sheet surface and <001> axis in the rolling direction. Has been done. In order to obtain good magnetic properties, it is necessary to highly align the <001> axis, which is the easy magnetization axis, with the rolling direction. Typical methods for producing such a high magnetic flux density unidirectional electrical steel sheet are methods described in Japanese Patent Publication No. 40-15644 by Satoru Taguchi et al. And Japanese Patent Publication No. 51-13469 by Takuichi Imanaka. . MnS and AlN are used in the former and Mn are used in the latter.
S, MnSe, Sb, etc. are used as main inhibitors.

Therefore, in the present technology, it is indispensable to appropriately control the size, morphology and dispersion state of the precipitates functioning as these inhibitors. Speaking of MnS, in the current process, MnS is generated when the slab is heated before hot rolling.
A method is used in which the above is completely solid-solved and then precipitated during hot rolling. A temperature of about 1400 ° C. is necessary to completely form a solid solution of the required amount of MnS for secondary recrystallization. This is higher than the slab heating temperature of ordinary steel by 200 ° C. or more, and this high-temperature slab heating treatment has the following disadvantages. That is, 1) A high temperature slab heating furnace dedicated to grain-oriented electrical steel is required. 2) The energy intensity of the heating furnace is high. 3) The amount of molten scale increases, and the adverse effect on operation is large, as seen in so-called shaving.

In order to avoid such a problem, the slab heating temperature should be lowered to the level of ordinary steel, but this is also effective as an inhibitor of MnS.
It means that the amount of is reduced or is not used at all, which necessarily causes destabilization of secondary recrystallization. Therefore, in order to realize low temperature slab heating, M
It is necessary to strengthen the inhibitor with precipitates other than nS to sufficiently suppress normal grain growth during finish annealing. Such inhibitors include sulfides, nitrides, oxides, grain boundary precipitation elements, and the like, and known techniques include, for example, the following.

In Japanese Patent Publication No. 54-24685, A
By containing grain boundary segregation elements such as s, Bi, Sn, and Sb in the steel, the slab heating temperature is increased to 1050 to 1350 ° C.
The method of making into the range was disclosed. JP-A-52-2411
No. 6, in addition to Al, Zr, Ti, B, Nb, T
A method has been disclosed in which the slab heating temperature is set in the range of 1100 to 1260 ° C. by containing a nitride forming element such as a, V, Cr and Mo. Also, JP-A-57-15832
No. 2, the Mn content is reduced and the Mn / S ratio is set to 2.
A technique has been disclosed in which the slab is heated at a low temperature by setting the ratio to 5 or less, and the secondary recrystallization is stabilized by adding Cu.

On the other hand, a technique has also been disclosed in which improvements are made from the metallographic side in combination with the reinforcement of these inhibitors. That is, in JP-A-57-89433, elements such as S, Se, Sb, Bi, Pb, Sn, and B are added to Mn, and the columnar crystal ratio of the slab and the secondary cold rolling reduction are combined with this. As a result, low temperature slab heating of 1100 to 1250 ° C is realized. Further, JP-A-59-1903
In JP-A-24-24, there is disclosed a technique in which, in addition to S or Se, an inhibitor is mainly composed of Al and B and nitrogen, and the secondary recrystallization is stabilized by performing pulse annealing during primary recrystallization annealing after cold rolling. It was published. Thus, in order to realize low temperature slab heating in the production of grain-oriented electrical steel sheets, great efforts have been made so far.

[0008] First, in JP-A-59-56522, Mn is 0.08 to 0.45% and S is 0.00.
A technique has been disclosed which enables low temperature slab heating by making the content 7% or less. By this method, the problem of defective linear secondary recrystallization of the product due to coarsening of the slab crystal grains during heating of the high temperature slab was solved.

On the other hand, labor saving of transformer manufacturers in recent years,
From the viewpoint of cost reduction, particularly in the field of laminated iron cores, there is an increasing need for grain-oriented electrical steel sheets having a large thickness in order to reduce the number of laminations. In the field of large rotators, there has been a long-standing desire to use grain-oriented electrical steel sheets. In this field as well, there is a strong need for grain-oriented electrical steel sheets with a large thickness in order to reduce the number of stacks. On the other hand, increasing the plate thickness
In general, since it leads to deterioration of iron loss characteristics, there is an increasing expectation for development of thick grain-oriented electrical steel sheets with excellent magnetic characteristics that can solve this problem.

[0010]

SUMMARY OF THE INVENTION The present invention provides a method for manufacturing a grain-oriented electrical steel sheet which has a large thickness and has excellent magnetic characteristics and coating characteristics.

[0011]

The gist of the present invention is that C: 0.021 to 0.075% by weight and Si:
2.5-4.5%, acid-soluble Al: 0.010-0.0
60%, N: 0.0010 to 0.0130%, S + 0.
405 Se: 0.014% or less, Mn: 0.05-0.
1% slab consisting of 8% and the balance Fe and unavoidable impurities
After heating at a temperature of less than 280 ° C., hot rolling, hot-rolled sheet annealing is performed, and final cold rolling with a rolling reduction of 80% or more is continuously performed. If necessary, one or more cold rollings with intermediate annealing are performed. Performed, then decarburized and annealed, annealed separator is applied, and finish annealed,
In a method of manufacturing a thick unidirectional electrical steel sheet having a thickness of 0.36 to 1.00 mm, which is applied with an insulating coating agent, after decarburization annealing is completed,
The average grain size of the primary recrystallized grains until the start of the final finish annealing is set to 18 to 35 μm, and the steel sheet is nitrided before the start of the final finish annealing after hot rolling, and the final finish annealing of the steel sheet after the nitriding treatment is started. When the amount of N at that time is N (%) by weight and the product plate thickness is t (mm), N (%) and t (mm) are controlled within the range of the following formula, and -0.0078 × t + 0. [0148] ≤N≤-0.0156 × t + 0.0406 A thick plate having excellent magnetic properties and coating properties, which has a dew point of 0 to 30 ° C. in a temperature range of 800 ° C. or less at the time of temperature increase in final finishing annealing. It is a method for manufacturing a unidirectional electrical steel sheet.

[0012]

The grain-oriented electrical steel sheet targeted by the present invention is a slab obtained by casting a molten steel obtained by a conventional steelmaking method by a continuous casting method or an ingot-making method, and optionally sandwiching a slabbing step. Then, hot rolling is carried out to form a hot-rolled sheet, and then this hot-rolled sheet is annealed as required, and if necessary, one or more cold rollings with intermediate annealing, decarburization annealing, and final finishing annealing are performed. It is manufactured by sequentially performing.

The inventors of the present invention have extensively studied the necessary conditions for obtaining good magnetic properties and coating properties in the manufacturing process of thick grain-oriented electrical steel sheets,
It was found that it is effective to control the amount of nitriding according to the plate thickness and to control the dew point at the time of temperature rise during finish annealing.

A detailed description will be given below based on the experimental results. Figure 1
FIG. 4 is a graph showing the influence of the sheet thickness after final cold rolling and the N content at the start of final finish annealing on the magnetic properties and coating properties of the product. Here, C: 0.050 wt% (hereinafter abbreviated as%), Si: 3.34%, acid-soluble Al: 0.029.
%, N: 0.0060%, S: 0.007%, Mn:
A slab of 40 mm thickness containing 0.10% and the balance Fe and unavoidable impurities is heated to 1150 ° C.
A hot rolled sheet having a thickness of 5.0 mm was used.

This hot-rolled sheet was kept at 1100 ° C. for 30 seconds, then kept at 900 ° C. for 30 seconds, then subjected to rapid hot-rolled sheet annealing, then cold-rolled at a rolling reduction of 80 to 88%, and then 830. Decarburization annealing was performed at 400 ° C. for 400 seconds and at 850 ° C. for 20 seconds. Then, during the heat treatment of holding at 750 ° C. for 30 seconds, various amounts of NH ₃ gas were mixed into the atmosphere gas to cause the steel sheet to absorb nitrogen. At this time, the N content of the steel sheet was 0.0061 to 0.0357%. The average grain size of the primary recrystallized grains in the entire thickness of this steel sheet was measured using an optical microscope and an image analyzer, and it was 23 to 28 μm. Then, an annealing separator containing MgO as a main component was applied to the plate after the nitriding treatment, and final finish annealing was performed.

The final finish annealing is carried out at a temperature rise of N ₂ : 25%, H
₂ : In a 75% atmosphere, the dew point is 10 ° C from room temperature to 800 ° C, and the dew point is Dry from 800 ° C to 1200 ° C.
(-30 ° C). The heating rate was 15 ° C./hour. 1
At 200 ° C., H _{2 was} 100%, and the steel sheet was kept in a Dry (−30 ° C.) atmosphere for 20 hours. After final finishing annealing, an insulating coating is applied and baked to form 0.36 to 1.00 mm
It was a thick product.

As is apparent from FIG. 1, the N amount at the start of final finish annealing is N (%), and the plate thickness of the final product is t (mm).
When N (%) and t (mm) are controlled within the range of the following formula, -0.0078 × t + 0.0148 ≦ N ≦ −0.0156 × t + 0.0406 good magnetic properties and good coating properties are simultaneously obtained. Was obtained.

The present inventors examined the influence of the atmosphere of final finish annealing on this phenomenon in relation to the phenomenon shown in FIG. FIG. 2 shows the influence of the dew point of 800 ° C. or lower at the time of temperature rise in final annealing on the magnetic properties and coating properties of the product. In this case, among the experimental materials whose results are shown in FIG. 1, a material having a hot-rolled plate thickness of 2.8 mm was used, and the product plate thickness was 0.46 mm (reduction ratio: about 84%).

At this time, the nitrogen content of the steel sheet after the nitriding treatment using NH ₃ gas was 0.0215%, and the average grain size of the primary recrystallized grains of the steel sheet was 25 μm. In addition, the dew point from room temperature to 800 ° C during the final finish annealing is -35 to
Experiments were carried out under various conditions in the range of 55 ° C. Other process conditions were the same as those of the experiment whose results are shown in FIG.

As is apparent from FIG. 2, when the dew point of 800 ° C. or lower at the time of temperature rise in the final annealing is 0 to 30 ° C., good magnetic properties and good coating properties were obtained at the same time. The reason why the phenomenon as shown in FIGS. 1 and 2 occurs is not necessarily clear, but the present inventors presume as follows.

In the present invention, it is premised that the steel sheet is subjected to a nitriding treatment after the hot rolling is completed and before the final finish annealing is started. By this nitriding treatment, a nitride is formed in the steel sheet, and when the temperature of the final finish annealing is increased, the nitride is AlN, (Al,
It becomes a state of Si) N and functions as an inhibitor that emphasizes the grain boundary character dependence of grain boundary migration during secondary recrystallization.

This nitride is produced during the temperature increase of the final annealing (1
(000 ° C or higher) and decomposes during purification at high temperature (up to 1200 ° C), and nitrogen is released from the steel. Since this nitrogen passes through the forsterite formed on the surface, a defect due to the passage of nitrogen is generated in the forsterite, and when the size is such that it can be visually observed, it becomes a defective film portion. Therefore, it is considered that the higher the nitrogen content in the steel at the start of the final finish annealing, the higher the proportion of the coating defect portion.

On the other hand, the grain boundary character dependence of the grain boundary migration during the secondary recrystallization increases as the inhibitor strength (precipitate volume fraction / precipitate size) increases. As the dependence of the grain boundary movement on the grain boundary character becomes stronger, the proportion of the {110} <001> oriented grains having the highest corresponding grain boundary density to the secondary recrystallization becomes higher, and the sharp { 110} <001> secondary recrystallization texture is obtained.

Therefore, increasing the amount of inhibitor at the time of secondary recrystallization leads to improvement of the magnetic flux density, and in this sense, the larger the amount of nitrogen at the time of secondary recrystallization, the more preferable. For this reason, if the amount of nitrogen at the start of final finish annealing is too large, film defects increase, and if it is too small, the magnetic flux density decreases.

On the other hand, when the plate thickness is large, the amount of nitrogen per unit area that emerges from the steel during final finish annealing increases, and film defects are likely to occur. On the other hand, the thicker the plate,
It takes time to release this nitrogen from the steel. For this reason,
Since the decrease in the inhibitor strength at the time of secondary recrystallization is relieved, a thicker plate is advantageous in that the magnetic flux density is increased. From the relationship between these two phenomena and the plate thickness, it is considered that the appropriate range of the amount of nitrogen in the steel at the time of final finish annealing that achieves both good magnetic properties and good coating properties becomes lower as the plate thickness increases. The results in FIG. 1 are believed to be due to this phenomenon.

Further, the dew point of 800 ° C. or lower at the time of temperature rise in the final finish annealing affects the surface oxidation at that time. The oxide formed at this time is mainly SiO _2. However, when this oxide is formed on the surface, it suppresses the absorption of nitrogen into the steel sheet during the temperature rise of the final finish annealing. In this sense, increasing the dew point of 800 ° C. or lower is considered to reduce the frequency of occurrence of film defects. On the other hand, when the dew point is high, oxidation of Al on the surface is likely to occur, and from the equilibrium reaction of AlN with solid solution Al and solid solution N, the situation is substantially the same as when the amount of Al is reduced. Is decomposed to increase the amount of solid solution N.

At this point, the temperature during the final finishing annealing is 800 ° C.
The higher the dew point below, the lower the volume fraction of the inhibitor during secondary recrystallization, and the {110} <0 of the secondary recrystallization texture.
01> The degree of integration is reduced. Therefore, there is an appropriate range of the dew point at the time of the final finish annealing temperature rise in order to achieve both the magnetic properties and the film properties.

The sheet thickness also influences this phenomenon, and the thicker the sheet thickness, the more advantageous it is in terms of magnetic properties, but it is disadvantageous in terms of film defects. The appropriate range is also affected by the plate thickness, and as in the present invention, 0.36 to
In the case of a product with a thickness of 1.00 mm, it is estimated that the range shown in FIG.

Next, the reasons for limiting the constituent features of the present invention will be described. First, the reasons for limiting the components of the slab and the slab heating temperature will be described in detail. When C is less than 0.021%, the secondary recrystallization becomes unstable, and even when secondary recrystallization is performed, it is difficult to obtain B ₈ > 1.80 (T).
It was set to 1% or more. On the other hand, if the amount of C is too large, the decarburization annealing time becomes long and it is not economical, so the content was made 0.075% or less. When Si exceeds 4.5%, cracking during cold rolling becomes significant, so the content of Si is set to 4.5% or less. If it is less than 2.5%, the specific resistance of the material is too low, and the low iron loss required for the transformer core material cannot be obtained. It is preferably 3.2% or more.

Al is AlN necessary for stabilizing the secondary recrystallization.
Alternatively, in order to secure (Al, Si) nitrides, 0.010% or more is required as acid-soluble Al. When acid-soluble Al exceeds 0.060%, A of hot rolled sheet
Since 1N becomes unsuitable and the secondary recrystallization becomes unstable, the content was made 0.060% or less. Regarding N, it is difficult to make it less than 0.0010% in ordinary steelmaking work, and it is economically unfavorable, so it is 0.0010% or more, while when it exceeds 0.0130%, the steel plate surface blistering called blister Therefore, 0.0130% or less is set.

Even if MnS and MnSe are present in the steel, it is possible to improve the magnetic characteristics by properly selecting the manufacturing process conditions. However, if S or Se is high, secondary recrystallization defective portions called linear fine grains tend to be generated, and in order to prevent the generation of this secondary recrystallization defective portion, (S + 0.405Se) ≦ 0.014 % Is desirable. If S or Se exceeds the above value, the probability of occurrence of defective secondary recrystallization is increased no matter how the manufacturing conditions are changed, which is not preferable. In addition, the time required for purification in the final finish annealing becomes too long, which is not preferable, and it is meaningless to increase S or Se unnecessarily from this viewpoint.

The lower limit of Mn is 0.05%. 0.0
If it is less than 5%, the shape (flatness) of the hot-rolled sheet obtained by hot rolling, especially the side edge portion of the strip becomes corrugated, which causes a problem of lowering the product yield. On the other hand, when the Mn content exceeds 0.8%, the magnetic flux density of the product is lowered, which is not preferable, so the upper limit of the Mn content was set to 0.8%. In addition, Sn and S, which are known as inhibitor constituent elements,
It is permissible to contain a trace amount of b, Cr, Cu, Ni, B, Ti and the like.

The slab heating temperature is limited to less than 1280 ° C. for the purpose of cost reduction in the same manner as ordinary steel. It is preferably 1200 ° C or lower. The subsequent hot-rolling step usually comprises rough hot-rolling and finish hot-rolling, in which a slab having a thickness of 100 to 400 mm is heated, and then each is performed in a plurality of passes.
The methods of rough hot rolling and finish hot rolling are not particularly limited, and ordinary methods are used. After hot rolling, hot-rolled sheet is annealed, and subsequently, final cold rolling with a rolling reduction of 80% or more is performed, and if necessary, one or more cold rollings with an intermediate annealing are performed to obtain 0.36.
A cold-rolled sheet with a thickness of up to 1.00 mm is used.

The thickness of the cold rolled sheet is defined to be 0.36 to 1.00 mm for the purpose of the present invention to obtain a thick unidirectional electrical steel sheet. If it exceeds 1.0 mm, decarburization annealing takes too long, which is not preferable. The reduction ratio of the final cold rolling is set to 80% or more in this range in the primary recrystallization texture,
An appropriate amount of {110} <001> oriented grains and its {11}
This is because an appropriate amount of corresponding oriented grains ({111} <112>, etc.) that are easily silkworm eroded when the 0} <001> oriented grains undergo secondary recrystallization is obtained.

The steel sheet after the final cold rolling was subjected to decarburization annealing to obtain Mg.
An annealing separator containing O as a main component is applied, and final finishing annealing is performed. Here, the average grain size of the primary recrystallized grains after decarburization annealing until the start of final finish annealing is set to 18 to 35 μm, in this range, a good magnetic flux density of B ₈ ≧ 1.88 T is obtained. Because it will be done. It is defined that the steel sheet is subjected to the nitriding treatment after the hot rolling and before the start of the final finish annealing, in the process premised on the low temperature slab heating as in the present invention, the inhibitor strength necessary for the secondary recrystallization tends to be insufficient. Because.

The method of nitriding is not particularly limited, and any method such as a method of mixing NH ₃ gas in an annealing atmosphere after decarburization annealing and nitriding or a method of using plasma may be used. The N content at the start of final finishing annealing of the steel sheet after the nitriding treatment is N (%) in weight ratio, and the product thickness of the steel sheet is t.
(Mm), it is necessary to control N (%) and t (mm) within the range of the following formula. −0.0078 × t + 0.0148 ≦ N ≦ −0.0156 × t + 0.0406 This is because good magnetic characteristics and film characteristics can be obtained in this range as shown in FIG.

The final finish annealing is usually performed in the form of a coil of 3 to 50 TON, and this final finish annealing causes secondary recrystallization, glass film formation and purification. Secondary recrystallization is usually 800
~ 1200 ℃, the glass film formation is usually 800 ~
It occurs at 1200 ℃, purification is mainly 1000 ~ 1300 ℃
Occurs in the temperature range of. The temperature of this final finish annealing is 800 ℃
The dew point in the temperature range up to is specified.

It is necessary to set the dew point in the temperature range of 800 ° C. or less at the time of temperature rise of the final finish annealing to 0 to 30 ° C. This is Figure 2
This is because good magnetic properties and film properties can be obtained in this range, as shown in FIG. In the temperature range from room temperature to 800 ° C., if the dew point for one period is 0 to 30 ° C. at least, it is effective in improving the magnetic properties and the film properties. The dew point in the temperature range over 800 ° C is not particularly limited.

The method of controlling the dew point in the temperature range from room temperature to 800 ° C. is not particularly limited.
Any of the method of adjusting the water content in the annealing separator, the method of adjusting the flow rate of the atmosphere gas at the time of final finishing annealing, the method of adjusting the coil winding method, the method of adding water to the atmosphere gas at the time of final finishing annealing, etc. The method does not matter.

[0040]

【Example】

Example 1 C: 0.054% by weight, Si: 3.26%, Mn:
0.13%, S: 0.006%, acid-soluble Al: 0.0
Slab containing 29%, N: 0.0078%, 11
After heating to a temperature of 50 ° C., hot rolling was performed to obtain a hot rolled plate of 2.8 mm. After this hot rolled sheet is annealed at 1120 ° C. for 30 seconds and subsequently kept at 900 ° C. for 30 seconds, it is cold rolled to a final thickness of 0.38 mm at a reduction rate of 86%. And hold it at 830 ° C for 150 seconds for decarburization annealing (25%
N ₂ + 75% H ₂ , dew point 65 ° C.), then 75
At the time of annealing for holding at 0 ° C. for 30 seconds, NH ₃ gas was mixed into the annealing atmosphere in three levels to cause nitriding of the steel sheet.

The amount of N after nitriding was 0.0101% by weight, 0.0240% by weight and 0.0370% by weight. The average grain size of the primary recrystallized grains of this steel sheet was 24 μm. After that, an annealing separator containing MgO as a main component was applied to the steel sheet, the temperature was raised to 1200 ° C. at 15 ° C./hr, and final finishing annealing was performed at 1200 ° C. for 20 hours. At this time, the atmosphere gas during temperature increase is 25% N ₂ + 75%
And H _2, to 800 ° C. from room temperature is set to 10% dew point,
After that, it was set to Dry (-30 ° C). The atmosphere gas held at 1200 ° C. was set to 100% H ₂ . Then, an insulating film was formed by coating and baking by a known method. The plate thickness of the final product was 0.40 mm. N amount number after nitriding,
Table 1 shows the magnetic properties and coating properties of the products.

[0042]

[Table 1]

Example 2 C: 0.040% by weight, Si: 3.01%, Mn:
0.12%, S: 0.008%, acid-soluble Al: 0.0
A slab consisting of 34%, N: 0.0064%, the balance Fe and unavoidable impurities is heated to a temperature of 1150 ° C.,
The hot rolled sheet had a thickness of 3.0 mm. This hot-rolled sheet was cold-rolled to a final sheet thickness of 0.48 at a reduction rate of 84% without annealing.
mm cold rolled sheet. The cold-rolled sheet was subjected to decarburization annealing at 830 ° C. for 300 seconds (25% N ₂ + 75% H ₂ , dew point 62 ° C.), and the subsequent steps were processed under the conditions described in Example 1. The amount of N after nitriding is 0.0105% by weight, 0.0
It was 218% by weight and 0.0401% by weight. The average grain size of the primary recrystallized grains of this steel sheet was 25 μm.
The plate thickness of the final product was 0.50 mm. Table 2 shows the process conditions and the magnetic properties and coating properties of the product plate.

[0044]

[Table 2]

Example 3 C: 0.043% by weight, Si: 3.21%, Mn:
0.12%, S: 0.009%, acid-soluble Al: 0.0
A slab consisting of 30%, N: 0.0080%, balance Fe and unavoidable impurities is heated to a temperature of 1200 ° C.,
The hot rolled sheet had a thickness of 3.0 mm. 1000 ° C on this hot rolled sheet
The hot rolled sheet was annealed for 3 minutes and cold rolled to a final sheet thickness at a rolling reduction of about 81% to obtain a cold rolled sheet of 0.58 mm. The cold-rolled sheet was subjected to 830 ° C. × 450 seconds (25% N ₂ + 75% H ₂ , dew point 6
(2 ° C.) decarburization annealing, and then, during annealing at 770 ° C. for 30 seconds, NH ₃ gas was mixed in the annealing atmosphere to cause nitrogen absorption in the steel sheet.

The amount of N after nitriding was 0.0218%, and the average grain size of primary recrystallized grains of this steel sheet was 26 μm. An annealing separator containing MgO as a main component was applied to this steel sheet, and the temperature was raised to 1200 ° C. at 20 ° C./hr, and 1200
Final finishing annealing was carried out at 20 ° C. for 20 hours. At this time, when the temperature was raised, an atmosphere gas prepared with a ratio of 25% N ₂ + 75% H ₂ was used, and 100% H ₂ gas was used as the atmosphere gas when held at 1200 ° C.

As the dew point condition at the time of temperature rise, Dry (dew point: -30 ° C.) from room temperature to 1200 ° C., dew point 10 ° C. from room temperature to 700 ° C., 700 ° C. to 1200 ° C.
Dry (dew point: -30 ° C), dew point 5 ° C from room temperature to 900 ° C, Dry (dew point: -30 ° C) from 900 ° C to 1200 ° C, dew point 10 ° C from room temperature to 600 ° C,
Dry (dew point: -30 from 600 ° C to 1200 ° C)
Dew point 20 ° C from room temperature to 800 ° C, Dry (dew point -30 ° C) from 800 ° C to 1200 ° C, dew point 40 ° C from room temperature to 800 ° C, Dry (dew point -30 ° C) from 800 ° C to 1200 ° C The final finish annealing was performed under the following six conditions.

Then, an insulating coating was formed by coating and baking by a known method. The plate thickness of the final product was 0.60 mm. Table 3 shows the process conditions and the magnetic properties and coating properties of the product.

[Table 3]

[0049]

INDUSTRIAL APPLICABILITY In the present invention, the average grain size of primary recrystallized grains is controlled after the completion of decarburization annealing and before the start of final finish annealing, and the steel sheet is subjected to nitriding treatment after hot rolling and before the start of final finish annealing. By maintaining the thickness of the final product and the amount of N at the start of final finishing annealing within a predetermined relation range and controlling the dew point at the time of temperature increase in final finishing annealing, a thick plate having good magnetic properties and coating properties Since a thick grain-oriented electrical steel sheet can be obtained,
Its industrial effect is extremely large.

[Brief description of drawings]

FIG. 1 is a graph showing the influence of the plate thickness of the final product and the N content at the start of final finish annealing on the magnetic properties and coating properties of the product.

FIG. 2 is a graph showing the influence of a dew point of 800 ° C. or lower at the time of temperature increase in final finish annealing on the magnetic properties and coating properties of products.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Nozomi Ishibashi 1-1 Tobahata-cho, Tobata-ku, Kitakyushu City Nippon Steel Co., Ltd. Yawata Works

Claims (1)

[Claims] 1. By weight ratio, C: 0.021 to 0.075%, Si: 2.5 to 4.5%, acid-soluble Al: 0.010 to 0.060%, N: 0.0010 to 0. 0.0130%, S + 0.405Se: 0.014% or less, Mn: 0.05 to 0.8%, and a balance of 1280 with a slab consisting of Fe and inevitable impurities.
After heating at a temperature of less than ℃, hot-rolled, subjected to hot-rolled sheet annealing, including final cold rolling of 80% or more rolling reduction, if necessary perform one or more cold rolling with intermediate annealing sandwiched, Next, in the method for producing a thick unidirectional electrical steel sheet having a thickness of 0.36 to 1.00 mm, in which decarburization annealing is performed, an annealing separator is applied, finish annealing is performed, and an insulating coating agent is applied. The average grain size of the primary recrystallized grains until the start of the final finish annealing is 18 to
35 μm, the steel sheet is subjected to nitriding treatment after hot rolling until the start of final finish annealing, and the N amount at the start of final finish annealing of the steel sheet after the nitriding treatment is defined as N (%) by weight ratio, and the product sheet thickness is t. (M
m), N (%) and t (mm) are controlled within the range of the following formula, and -0.0078 × t + 0.0148 ≦ N ≦ −0.0156 × t + 0.0406 800 ° C. or less at the time of temperature rise in final annealing. The dew point in the temperature range of 0 to 30 ° C. is set, and the method for producing a thick unidirectional electrical steel sheet having excellent magnetic properties and coating properties.