JPH1129824A - Manufacture of grain oriented silicon steel sheet with excellent glass film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of manufacture of a grain oriented silicon steel sheet having excellent glass film prepared by modifying an oxiode film layer, at the surface of a steel sheet, formed in a decarburizing annealing stage in the process of manufacture of the grain oriented silicon steel sheet. SOLUTION: The grain oriented silicon steel sheet is produced by subjecting a slab for grain oriented silicon steel sheet, containing <=4.5% Si, to heating up to a temp. not exceeding 1280 deg.C and to hot rolling, subjecting the resultant steel plate, after or without hot rolled plate annealing, to cold rolling once or two cold rolling two or more times while performing annealing between cold rolling stages to final sheet thickness, applying decarburizing annealing to the resultant steel sheet in a wet hydrogen atmosphere, further applying a separation agent at annealing to the steel sheet, and then performing final finish annealing. In this case, in the nitriding stage at the time of decarburizing annealing, nitriding is carried out while regulating the degree of oxidation of the nitriding atmosphere, that is, the value of PH2 O/PH2 to 0.00018-0.0225.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the modification of an oxide film layer on the surface of a steel sheet obtained in a decarburizing annealing step in a manufacturing process of a grain-oriented electrical steel sheet, and more particularly, to the modification of a steel sheet during final finish annealing. The present invention relates to a method for producing a grain-oriented electrical steel sheet having an excellent glass coating, which simultaneously satisfies both stabilization of secondary recrystallization and improvement of glass coating characteristics.

[0002]

2. Description of the Related Art A grain-oriented electrical steel sheet has a (110)
[001] A sheet thickness of 0.1%, usually containing 4.5% or less of Si, which is composed of crystal grains having an orientation (Goth orientation grains).
A steel plate of 0 to 0.35 mm, mainly used for transformer cores. The surface of the grain-oriented electrical steel sheet is covered with a forsterite film mainly composed of an oxide film usually called a glass film for the purpose of ensuring insulation and the like. That is,
Grain-oriented electrical steel sheets are extremely highly integrated (110) [001]
It can be said to be a composite material comprising a high Si-containing thin steel sheet having a texture (goss structure) and forsterite which is an oxide ceramic having a surface layer of 0.1 to several μm.

On the other hand, in the production of the above-mentioned grain-oriented electrical steel sheet, the conventional production method based on high-temperature slab heating of 1350 ° C. or higher has been replaced.
No. 27, slab heating at the ordinary steel level, that is, slab heating at a temperature of 1280 ° C. or lower, and an inhibitor of AlN, (Al.S
i) A manufacturing method has been developed in which finely dispersed precipitates such as N are formed in a nitriding treatment step performed after decarburizing annealing. However, even with this new manufacturing method, many problems still remain in order to constantly manufacture a grain-oriented electrical steel sheet excellent in glass coating and secondary recrystallization on an actual production line.

[0004] In particular, the formation of the glass film is a kind of oxide film formed on the surface of the steel sheet in the decarburizing annealing process.
Are those produced by the reaction of MgO in the annealing separator, and SiO ₂ in the oxide film, SiO ₂ as a component of oxide film
It is important to be rich and to secure a certain thickness or more. The stabilization of secondary recrystallization, which is closely related to the oxide film, is required because it is necessary to precipitate and disperse the above-mentioned inhibitors such as AlN and (Al.Si) N in processes other than the hot-rolled sheet annealing process. In order to nitride and precipitate Al in the steel at the stage before the next recrystallization, the oxide film generated on the steel sheet surface in the above-described decarburization annealing process is an oxide film that is stable with respect to the atmosphere gas in the final finish annealing. There is a problem that the above-mentioned secondary recrystallization cannot be stabilized unless an oxide film having a completely different function from the conventional one is formed.

In order to address this problem, Japanese Patent Laid-Open No.
Japanese Patent No. 22227 discloses a technology for manufacturing a grain-oriented electrical steel sheet using a decarburizing annealing furnace for forming an inhibitor such as AlN without performing high-temperature slab heating. In this prior art, a steel sheet is reduced in a reduction zone before the steel sheet enters a nitriding zone in order to nitride the steel sheet, and then a nitriding gas such as ammonia alone or mixed with a dry gas such as hydrogen is sprayed on the steel sheet. This is a method of completing the nitriding treatment in a short time. Regarding the above-described reduction, Japanese Patent Application Laid-Open No. 1-119621 discloses that the atmosphere after the decarburization annealing is set to a value of PH ₂ O / PH ₂ of 0.02 or less so that both the magnetic characteristics and the glass film are simultaneously obtained. A technique for producing a satisfactory grain-oriented electrical steel sheet is disclosed.

[0006] Japanese Unexamined Patent Application Publication No. Hei 4-329831 discloses a decarburization method.
After annealing, up to nitriding treatment and subsequent nitriding treatment
And the oxidation degree PH in the furnace_TwoO / PH_TwoIs 5 × 10^-FourAnd above
By doing, (oxidation) of the steel sheet generated in the decarburization annealing
Prevents the reduction of FeO (in the layer)
It is disclosed to obtain a coating. And as an explanation
And usually, after decarburizing annealing until nitriding
In the nitriding treatment, H_TwoAnd atmosphere containing nitridable gas
Gas is used, and the dew point of the gas is -50 to -6.
It is stated to be 0 ° C. Oxidation degree PH
_TwoO / PH_TwoWhen converted to_Two: N_TwoOf 75:25
5.2 × 10 in case^-Five~ 1.4 × 10^-FiveBecomes Again
In the embodiment, the degree of oxidation PH after decarburizing annealing to nitriding is_Two
O / PH _TwoAs 1.1 × 10^-Five~ 12.5 × 10^-Four
Are described. Thus, Japanese Patent Application Laid-Open No. 4-329
831 shows the degree of oxidation from decarburizing annealing to nitriding.
Is 12.5 × 10^-FourOperated in the low range of
It is disclosed.

[0007]

However, even when the above conditions are applied, there is a case where a good glass film cannot be formed. From this, the present inventors have conducted intensive studies, and as shown below, it is necessary to adjust the degree of oxidation of the atmosphere during nitriding treatment to a very narrow range as compared with conventionally known techniques. It was found that there was.

[0008] In general, a continuous annealing furnace for performing decarburizing annealing and nitriding treatment has its inner wall covered with a refractory. This refractory,
Absorbs moisture in the atmosphere when the temperature is lowered and opened during regular repairs. This moisture is released during the operation, but the rate is very slow. For example, as shown in FIG. 2, it takes several tens of hours for the degree of oxidation of the atmosphere to become 20.0 × 10 ⁻⁴ or less. is there.

Furthermore, a part of the oxide layer formed during the decarburizing annealing is reduced by hydrogen in the atmosphere after the decarburizing annealing to during nitriding, and a small amount of water is generated.
For the reasons described above, the degree of oxidation of the atmosphere after decarburizing annealing to nitriding tends to be higher than that shown in JP-A-4-329831. And, as described above, the degree of oxidation in the atmosphere has been increased, which has not been particularly problematic in the past.
It has been found that corrosion of the steel sheet surface by the nitriding gas has a great effect on glass film formation.

Generally, NH ₃ or NOx is used as a nitriding gas.
Although these are known, when they combine with moisture in the atmosphere, NH ₄ OH, HNO _{3 and the} like are formed. When these gases come into contact with the steel sheet after decarburizing annealing, it exhibits a strong corrosion effect on the oxide layer on the surface. As a result, a good glass film is not formed in the finish annealing. Since the generation amount of NH ₄ OH, HNO _{3, and the} like is almost proportional to the moisture concentration in the atmosphere, the deterioration of the glass film increases as the degree of oxidation of the atmosphere increases.

On the other hand, when the degree of oxidation in the atmosphere decreases, the nitriding gas tends to combine with the oxide on the surface layer of the steel sheet to form NH ₄ OH and HNO ₃ . For this reason, the reduction of the oxide layer is promoted as compared with the case where no nitriding gas is contained, and as a result, it becomes difficult to form a good glass film in the finish annealing. The present invention solves the above problems,
An object of the present invention is to provide a method for stably producing a grain-oriented electrical steel sheet having an excellent glass coating.

[0012]

According to the present invention, Si ≦ 4.5 is provided.
% Is heated to a temperature not exceeding 1280 ° C., and then subjected to hot rolling and hot-rolled sheet annealing, or once without hot-rolled sheet annealing or twice through annealing After performing the above cold rolling to a final sheet thickness, decarburizing annealing in a wet hydrogen atmosphere, further applying an annealing separator, and final finish annealing in a method for producing a unidirectional electrical steel sheet,
In the nitriding treatment step of the decarburizing annealing, the value of the degree of oxidation of the nitriding atmosphere: PH ₂ O / PH ₂ is set to 0.0016 to 0.022.
This is a method for producing a grain-oriented electrical steel sheet having an excellent glass coating, which is adjusted to a range of 5 (as the dew point is −20 ° C. to + 15 ° C.) and subjected to nitriding.

As a specific method, the adjusting means introduces steam and / or dry gas in accordance with the degree of atmospheric oxidation in the nitriding furnace.
Since the moisture content in the furnace in the nitriding furnace changes with time due to the moisture released from the refractory in the furnace, if the degree of oxidation of the atmosphere in the nitriding furnace is low, steam is introduced simultaneously with the gas to raise the moisture.
On the other hand, when the degree of oxidation is high, the introduction amount of a dry gas, for example, a dry gas such as a nitrogen gas or a hydrogen gas, is adjusted to maintain a desired required degree of oxidation.

[0014]

DETAILED DESCRIPTION OF THE INVENTION The nitriding annealing furnace used in the present invention is:
As shown in FIG. 1, since the atmospheric gas is introduced from the inlet divided into four in the traveling direction of the steel sheet to be treated, the dew point distribution in the nitriding zone and the steel sheet traveling direction in the furnace is substantially uniform. The furnace atmosphere gas is sampled from the center of the nitriding zone and passed through gas chromatography to constantly measure the furnace dew point.

This measured value is a single point measurement value because the atmosphere dew point in the furnace is substantially constant, but can be used as a value representative of the atmosphere dew point in the entire furnace. When the bricks in the furnace are exposed to the outside air for a long time due to periodic repairs, etc., the bricks absorb moisture in the atmosphere. The water absorbed by the brick during the repair is released into the atmosphere gas in the furnace. As a result, as shown in FIG. 2, the ambient gas dew point rises to +20 to 30 ° C. immediately after the periodic repair,
Affects the coating.

On the other hand, if the introduction of the dry gas is continued, the atmosphere gas in the furnace is replaced successively, and the dew point in the furnace decreases over several hours. After about one week, the measurement cannot be performed by gas chromatography. Dry to a possible dew point of -30 ° C or less. FIG. 3 shows a state of the glass film when the nitriding treatment is performed in such a situation. As can be seen from FIG.
As for the glass coating on the steel sheet surface, various coating defects may occur on the entire surface of the steel sheet, such as a thin and uneven glass coating, a metal surface exposed in a spot shape, and the extreme part turning black. I understand.

According to the present invention, the degree of oxidation is constantly supplemented in response to an increase or decrease in the degree of oxidation due to a temporal change in the value of the degree of oxidation PH ₂ O / PH ₂ in the nitriding treatment step. As shown in FIG. 5, the value of the degree of oxidation: PH ₂ O / PH ₂ is 0.0016 to 0.022 at any point.
5 (as a dew point, −20 ° C. to + 15 ° C.), preferably 0.0025 to 0.0080 (as a dew point, −15 ° C.)
(5 ° C.), a stable glass film can be obtained by performing the nitriding treatment. FIG. 5 (a)
FIG. 5 shows the relationship between the degree of oxidation (PH ₂ O / PH ₂ ) in a nitriding zone atmosphere and the film defect area ratio, and FIG. 5B shows the dew point (° C.) and the film defect area ratio at the nitriding zone dew point. As a specific method, as described above, water vapor and / or dry gas is introduced in accordance with the degree of atmospheric oxidation in the nitriding furnace. When the atmospheric oxidation degree in the nitriding furnace is low, water vapor is introduced simultaneously with the gas to raise the temperature, and when the oxidation degree is high, a dry gas such as nitrogen is used. The purpose is to maintain the desired required degree of oxidation by adjusting the amount of dry gas such as gas or hydrogen gas introduced.

In the above-mentioned oxidation degree, if the value of the oxidation degree in the nitriding atmosphere: PH ₂ O / PH ₂ is higher than 0.0225, the ammonia gas becomes NH ₄ OH, corrodes the steel sheet and makes it difficult to form a glass film. On the other hand, the value is 0.001
If it is lower than 6 or less, it is presumed that the formation of a glass film becomes similarly difficult to reduce oxides on the steel sheet.

As described above, in the present invention, the fluctuation of the atmospheric dew point over a long period of time is adjusted by introducing steam and dry gas. The steam introduced when the dew point is lowered is mixed with an atmosphere gas (mixed gas of H ₂ + N ₂ + NH ₃ ) in the introduction pipe before being introduced into the nitriding annealing furnace. In addition, the method of mixing the vapor with the atmospheric gas is as follows. If the target dew point is currently −10 ° C., if the measured dew point of the gas chromatography is lower than −10 ° C., the vapor pressure of 2.1 kg / hr is only 50 seconds. Into the furnace. Since the frequency of gas chromatography dew point measurement is once every 360 seconds (6 minutes), the measurement value of gas chromatography is determined again after 360 seconds, and if it is lower than the target -10 ° C, the same amount of steam is discharged. If the gas is blown in or has reached the target dew point or higher, the system is configured to wait until a gas chromatography measurement result is obtained after 360 seconds without introducing steam. As a result, the transition of the atmosphere dew point of the nitrided zone after the periodic repair is stabilized between -15 ° C and -20 ° C as shown in FIG.

[0020]

EXAMPLES In weight%, C: 0.050%, Si: 3.2
5%, acid-soluble Al: 0.034%, N: 0.0075
%, Mn: 0.10%, S: 0.0060%, balance Fe
A slab for a grain-oriented electrical steel sheet containing a component consisting of unavoidable impurities is heated at a temperature of 1150 ° C., and the hot-rolled 2.3 mm-thick hot-rolled sheet is annealed at 1120 ° C. , Then rapidly cooled. Then 0.28
Cold rolled to a final thickness of 5 mm, at a temperature of 840 ° C. for 170 seconds in a mixed atmosphere of hydrogen and nitrogen at a dew point of 70
Decarburization annealing was performed in an annealing atmosphere at a temperature of ° C. Then 770
A nitriding treatment was performed at a temperature of ° C. for 30 seconds in a mixed atmosphere of hydrogen, nitrogen and ammonia gas. Here, the sheet was passed while changing the oxidation degree of the nitriding atmosphere gas as shown in Table 1.

The steel sheet subjected to the decarburizing annealing and nitriding treatment is subjected to finish annealing after applying an annealing separator containing 5% TiO ₂ to 100 parts by weight of MgO as an annealing separator. gave. The condition of the finish annealing is 12
Heating was performed at a heating rate of 20 ° C./Hr with an atmosphere gas of N ₂ : 25% + H ₂ : 75% up to 00 ° C., and final annealing was performed by switching to H ₂ : 100% at 1200 ° C. and holding for 20 hours or more. Further, the steel sheet after the above treatment was subjected to strain relief annealing and a phosphoric acid + chromic acid-based tension coating was baked and applied, and the appearance of the forsterite film was confirmed. Table 1 shows the results.

[0022]

[Table 1]

Next, under the conditions described above, a dew point of -15 ° C., a degree of oxidation (PH ₂ O / PH ₂ ): 0.016 and a temperature of 770 ° C. in a mixed atmosphere of hydrogen, nitrogen and ammonia gas 3
When the nitriding treatment for holding for 0 second was performed by adopting the oxidation degree adjustment according to the present invention, as shown in FIG. 6 when the dew point of the nitrided zone after the periodic repair was in the range of −20 ° C. to + 15 ° C. within the range of the present invention. Only slight film defects have occurred. However, considering the fact that high-grade products are always required for unidirectional electrical steel sheets, it is best that film defects are naturally close to zero, and from this viewpoint, the range of the dew point according to the present invention is −15 ° C. When the temperature is in the range of from 0 ° C. to 0 ° C., it is possible to reduce the film defect to 1% or less, which is practically not a problem.

[0024]

As described above, the present invention is based on low-temperature slab heating to improve the oxide film layer on the steel sheet surface obtained in the decarburizing annealing step in the manufacturing process of the grain-oriented electrical steel sheet. By keeping the degree of oxidation (dew point) in the nitriding furnace constant for the formation defects of the glass film due to the nitriding treatment, both the stabilization of the secondary recrystallization of the steel sheet and the improvement of the glass film characteristics at the time of final finish annealing can be achieved. At the same time, it has become possible to produce satisfactory grain-oriented electrical steel sheets having a satisfactory glass coating.

[Brief description of the drawings]

FIG. 1 is a diagram showing a nitriding furnace structure used in the present invention.

FIG. 2 is a diagram showing a change in the atmospheric dew point of a nitrided zone after a periodic repair by a conventional method.

FIG. 3 is a view showing a state of a glass film of a product obtained by a conventional method.

FIG. 4 is a diagram showing a transition of the atmospheric dew point of the nitrided zone after the periodic repair according to the present invention.

5A is a diagram showing a nitriding zone atmosphere and a film defect rate, and FIG. 5B is a diagram showing a nitriding zone dew point and a film defect rate.

FIG. 6 is a view showing a state of a glass film of a product obtained by the method of the present invention.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Osamu Tanaka 46 Nishihara, Ohara 46, Tobata-ku, Kitakyushu-shi, Fukuoka Inside Nippon Steel Plant Design Co., Ltd. (72) Inventor Kenichi Yatsushiro 1-1 Hibata-cho, Tobata-ku, Kitakyushu-shi, Fukuoka Prefecture Inside Nippon Steel Corporation Yawata Works (72) Inventor Kizui Ishibashi Kitakyushu-shi, Fukuoka 1-1 Yawata-cho, Tobata-ku Nippon Steel Corporation Yawata Works

Claims (2)

[Claims] 1. A slab for a grain-oriented electrical steel sheet containing Si ≦ 4.5% is heated to a temperature not exceeding 1280 ° C. and then subjected to hot rolling and hot-rolled sheet annealing or no hot-rolled sheet annealing. One or two or more times of cold rolling with annealing interposed to make the final sheet thickness, then decarburizing annealing in a wet hydrogen atmosphere, further applying an annealing separator and final finishing annealing In the method for producing a conductive magnetic steel sheet, the degree of oxidation of the nitriding atmosphere in the nitriding treatment step after the decarburizing annealing is PH ₂ O / PH.
_2. A method for producing a grain-oriented electrical steel sheet having an excellent glass coating, wherein nitriding treatment is performed while adjusting the value of ₂ to a range of 0.0016 to 0.0225. 2. The method for producing a grain-oriented electrical steel sheet with excellent glass coating according to claim 1, wherein said adjusting means introduces steam and / or dry gas according to the degree of atmospheric oxidation in a nitriding furnace. .