JPS59104431A - Method for baking phosphate coating of directional silicon steel plate - Google Patents

Abstract

PURPOSE:To prevent the red discoloration on the surface of a steel plate and to prevent the lowering of a product value, in baking a phosphate top coating, by bringing the baking atmosphere to a weak reductive atmosphere comprising N2 containing a specific amount of H2 or H2 and CO. CONSTITUTION:A forsterite film is formed on the surface of a directional silicon steel plate while a phosphate top insulating coating is further applied thereon and annealing is applied to perform the flattening of the steel plate and the baking of the insulating coating. In this case, an annealing atmosphere is constituted from a weak reductive atmosphere containing 15vol% or less of H2 or H2 and CO and the remainder consisting substantially of N2. By this method, the red discoloration on the surface of the steel plate is effectively prevented and the embrittlement thereof is prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for baking a phosphate-based top insulating coating during the manufacturing process of grain-oriented silicon steel sheets, and particularly relates to an annealing atmosphere when baking is performed in conjunction with flattening the steel sheet. It is.

As is well known, grain-oriented silicon steel sheets contain Si of about 4% or less.
Usually, a silicon steel material containing about 3% is melted and made into a slab by continuous casting or ingot-blowing rolling method, then hot rolled, and then cold rolled twice with intermediate annealing in between. It is manufactured by applying rough decarburization annealing and high-temperature box annealing to a thick plate, and its crystal texture is called a Goss texture in which the axis of easy magnetization is aligned in the rolling direction (110). It has a [001] orientation and is widely used as an iron core material for various power distribution transformers.

By the way, when such grain-oriented silicon steel sheets are used in transformers, they are usually laminated with a large number of thin steel sheets (copper strips), and in this case, the surface of the steel sheets is coated with metal to reduce eddy current loss. Generally, an electrically insulating film is formed on the surface. In order to form such an insulating film on the surface of a steel plate, the following series of steps are generally adopted. That is, an oxide film mainly consisting of Si 02 is generated on the steel plate surface by decarburization annealing after cold rolling during the manufacturing process of the grain-oriented silicon steel plate mentioned above, and then an annealing separator mainly composed of MgO is applied. Wrap it into a coil for 120 minutes.
Final box annealing was carried out in H2 at around 0°C, and (110
) [001] orientation, and at the same time, forsterite (2Mgo-810
2) Form a quality base film. Furthermore, a phosphate-based coating treatment solution is applied to form a phosphate-based top insulating coating on top of this forsterite coating, and after drying, the coating is baked at the same time as flattening annealing of the coil. Product.

As mentioned above, the insulation coating on grain-oriented silicon steel sheets is usually a double coating consisting of a forsterite base coating and a phosphate-based top coat. It occupies an important part in the process after cold rolling in the production of silicon steel sheets.

It goes without saying that products made of grain-oriented silicon steel sheets are required to have excellent magnetic properties, that is, high magnetic flux density and low iron loss, but the film properties also need to be improved for the insulation coating. Excellence is required. Insulating coating properties require that the coating has a uniform appearance, and
It is required to be excellent in a wide variety of properties such as adhesion, interlayer resistance, and space factor. The characteristics of the insulating coating of such a grain-oriented silicon steel sheet include those determined by the forsterite base coating and those determined by the top coat of phosphate.

Regarding the formation of a forsterite base film, various manufacturing technologies have been established in a series of processes from before and after decarburization annealing to application of an annealing separator and final box annealing, and the film has a uniform appearance and good adhesion. Formation of a base film has been achieved to some extent. On the other hand, various proposals have been made regarding the formation of phosphate-based top insulating coatings. In recent years, so-called tension-applied phosphate coatings have been introduced, which use a magnesium phosphate-anhydrous colomate-based coating solution to increase the tension applied to the steel sheet by the coating film, thereby reducing magnetostriction. It has become. However, when forming such a phosphate-based insulating topcoat,
Conventionally, this seizure sometimes caused the surface of the steel plate to turn red (discoloration), damaging its commercial value.In addition, when this red discoloration not only impairs its commercial value, but also makes the steel plate itself brittle. There was a problem.

The formation of phosphate-based insulating overcoat coatings and their problems as described above will be described in further detail below.

First, to explain in more detail the process of forming a phosphate-based insulating topcoat, after forming a forsterite base film by final box annealing,
A coating treatment solution based on magnesium phosphate or colloidal silica-magnesium phosphate-chromic acid anhydride as mentioned above is applied to the surface using a roll coaster, etc., and after drying, annealing is performed at around 800℃ for about 1 minute. , a process of straightening the coil set during final box annealing, that is, flattening the steel plate, and baking at the same time is usually adopted. Conventionally, the atmosphere for such baking was to introduce N2 gas into the furnace, but in this case, as the coil progresses at the entrance side of the steel plate of the annealing furnace, outside air flows into the furnace. The atmosphere inside the furnace became highly oxidizing, causing the base iron on the surface of the steel plate to oxidize and turn red. In other words, when the coverage of the forsterite film as the base is not good, the surface of the bare iron is exposed in places, and the phosphoric acid-based coating is sometimes oxidized by O2 brought into the atmosphere and becomes hematite (Fe).
203) is generated, and as a result, the surface of the steel sheet turns red, and at the same time, there is a problem that the steel sheet becomes brittle due to grain boundary oxidation. As a method to deal with such problems, as disclosed in Japanese Patent Application Laid-open No. 55-138024, flattening of the steel plate and phosphate treatment in a neutral atmosphere with an 02 content of 5% or less are proposed. A method of baking a system coating has been proposed. This proposed method uses a neutral gas such as N2 gas as the atmosphere gas introduced into the furnace, and controls the flow of gas in the furnace to reduce the actual atmosphere in the furnace to zero.
The aim is to limit the content of 2 to 5% or less. However, when the present inventor conducted more detailed experiments, it was found that even if the 02 content in the furnace atmosphere was restricted to 5% or less, some red discoloration occurred.

That is, the inventors of the present invention have developed a grain-oriented silicon steel sheet on which a base film of forsterite is formed, in JP-A [52-2]
The colloidal silica-magnesium phosphate-based phosphate-based coating treatment solution described in Japanese Patent No. 5296 was applied, and after drying, the furnace atmosphere was changed in a tube furnace.
An experiment was conducted in which the film was baked at 00° C. for 1 minute, and the incidence of discoloration in the appearance of the film was investigated, and the results shown in Table 1 were obtained.

As shown in Table 1, if the atmosphere has an 02 content of 5% or less, the incidence of discoloration will certainly be considerably reduced. However, even if the content of 02 is 5% or less, if the coverage of the forsterite base film is poor, there is still a few percent that turns red as shown in Table 1, 3 to 5.

From the above experimental results, the present inventors found that it is difficult to completely prevent red discoloration simply by regulating the 02 content in the baking atmosphere to 5% or less; rather, it is difficult to prevent red discoloration by regulating the atmosphere to a reducing atmosphere. We newly recognized that it was necessary to reduce the amount of phosphoric acid, and conducted experiments. We found that by creating a reducing atmosphere, red discoloration can be almost completely prevented. It was found that the moisture absorption resistance of salt-based coatings deteriorated. If the moisture absorption resistance of the phosphate-based coating deteriorates in this way, a moisture absorption pattern will appear on the coating surface after baking, which will not only impair the appearance of the coating but also cause a problem of lower interlayer resistance. As a result of more detailed experiments, we found that there is a limit to the degree of reducing properties that can be used to prevent red discoloration but also to prevent a decrease in moisture absorption resistance.
Alternatively, they discovered that the decrease in moisture absorption resistance can be prevented by reducing the content of N2 + GO to 15% (volume %) or less, leading to the completion of the present invention.

Therefore, the present invention provides a method for preventing discoloration when baking a phosphate-based topcoat, and at the same time
! The purpose of this is to prevent a decrease in i1 hygroscopicity, and the atmosphere during baking of the phosphorus M base top coating is adjusted to 1"2 or 15% N2 and CO.
The atmosphere is characterized by a weakly reducing atmosphere containing the following and the balance being N2.

The method of the present invention will be explained in more detail below.

In the method of the present invention, as described above, the atmosphere during baking is a weakly reducing atmosphere containing 15% or less of N2 or N2 and co. In this way, N2 etc. j! The red discoloration of the steel sheet surface can be almost completely prevented only by creating a weakly reducing atmosphere that contains a small amount of oxidizing gas components. Alternatively, if only a 100% N2 atmosphere is used, some red discoloration will occur.

On the other hand, if the reducing atmosphere during baking of a phosphate-based coating is too strong, the moisture absorption resistance of the phosphate-based coating will deteriorate significantly. That is, the present inventors applied a colloidal silica-magnesium phosphate coating treatment solution disclosed in JP-A-52-25296 to a silicon steel sheet on which a forsterite base film had been previously formed, The moisture absorption resistance after baking at 800° C. for 1 minute in various atmospheres was investigated using the analytical value of P elution from the steel sheet surface as an index, and the results shown in FIG. 1 were obtained. However, here, the baking atmosphere is a 20% 02-N2 atmosphere (in air), and a reducing gas component with a 1:1 mixing ratio of N2 + CO with various contents within the range of 0 to 25%. Various atmospheres containing N2 and the remainder N2 were set. In addition, as a method for analyzing the amount of P elution, three test pieces of 50+um x 50 mm were sheared and collected from the test pieces baked in each atmosphere.
A steel plate of 150 cn was immersed in distilled water at 100°C for 5 minutes and then boiled to quantitatively analyze the P dissolved in the distilled water.
It was expressed as P28 output per t' (unit: μQ).

As is clear from FIG. 1, if N2 + CO exceeds 15%, the amount of P eluted increases to more than 100 uO/150 C, and the moisture absorption resistance decreases. It was also found that the same holds true when containing more than 15% N2. In this way, if P elution d exceeds 100 μg/150 c♂,
After a long period of time, a moisture-absorbing pattern appears on the surface of the steel plate, which not only significantly reduces commercial value but also reduces interlayer resistance. On the other hand, the content of H2 or N2 and CO is 15
% or less, the moisture absorption resistance will hardly decrease and there will be no practical problem. Therefore, in the method of the present invention, the phosphate coating is baked in a weakly reducing atmosphere in which the content of N2 or N2 and CO is 15% or less, and the remainder is substantially N2.

As mentioned above, the reason why the moisture absorption resistance deteriorates when baking a phosphate-based coating in a strongly reducing atmosphere is as follows.
Although the details are not yet clear, it can be inferred as follows.

That is, in the film structure of the amorphous phosphate coating, which is composed of three-dimensional connections of 5i04 or PO4 tetrahedrons, baking in a strongly reducing atmosphere is strongly reduced to S: 04 or PO4. It is thought that moisture absorption is likely to occur because the bridges connecting the tetrahedrons are cut.

It should be noted that baking of phosphate-based coatings is caused by the presence of N in the atmosphere.
The lower limit of the content of N2 or N2 and CO is not particularly specified, but in order to effectively prevent red discoloration, it is preferably about 3% or more.

In the actual baking process, a mixture of N2 gas and pure H2 gas may be used as the atmosphere gas introduced into the furnace, but from a cost standpoint, it is advantageous to use DX gas. DX gas is a cracked gas generated when air is mixed with butane or propane and combusted, and usually contains about 70 to 80% N2, with the remainder being H2 and CO-
Although it is composed of CO2, etc., the composition changes depending on the air mixing ratio during decomposition, so by adjusting the air mixing ratio, it can be adjusted to the optimal N2 + CO containing H. In the case of X gas, in addition to N2, N2, and CO, CO2 is usually contained, but CO2 has little effect on red discoloration or moisture absorption resistance, so the inclusion of a small amount of CO2 does not pose any particular problem.

The above-mentioned component ranges in the method of the present invention are not defined by the atmospheric gas that is sometimes introduced into the furnace when baking the phosphate-based coating, but are determined by the actual atmosphere in the furnace.

The method of the present invention will be explained below based on examples.

9i 3.04%, CO,043%, 30,004%,
A silk material containing 3e0゜018%, SbO,022% and the remainder substantially Fe was hot-rolled to a thickness of 2.7nrn+, then intermediately annealed at 900°C for 3 minutes, and then cooled twice. Inter-rolling was performed to give a final plate thickness of 0.30 m. Then, after decarburizing annealing at 820 °C for 3 minutes in wet hydrogen, an annealing separator consisting of tQO was applied, and a final box annealing was performed at 1200 °C for 5 hours in a N2 atmosphere to remove excess annealing. The separating agent was removed to obtain a grain-oriented silicon steel sheet having a forsterite coating. A large number of test pieces were cut out from adjacent positions of the steel plate to serve as experimental materials, and a tension-applying phosphate-based coating treatment solution shown below was applied to each side to a film thickness of 2 μm.

Coating treatment liquid composition: 20% colloidal silicone water dispersion (S, G 1.12
)...100mff1 35% monobasic magnesium phosphate solution (S, G 1.3
5) ...60m1
Chlorohydric anhydride (CrO3)...5
g Then, the steel plate coated with the coating treatment solution as described above was heated at 800 °C in the annealing atmosphere shown in each column of Table 2.
Baking was performed at ℃ for 1 minute. The appearance of the silicon steel plate having the phosphorus-based top coating obtained in this way and the moisture absorption resistance such as the analytical value of the amount of P elution were investigated. The results are also shown in Table 2.

As is clear from Table 2, H2 or H below 15%
In the case of the method of the present invention in which baking is performed in an atmosphere containing 2 and CO, the film appearance is uniform and there is no red discoloration.
It can be seen that a oriented silicon-copper plate having a phosphate-based top coating that also has good moisture absorption resistance can be obtained. Based on this result, a seizure test was conducted on an actual production line at a factory, and the same results as above were reproduced on an actual coil.

As is clear from the above explanation, according to the method of this invention,
It can effectively prevent the red discoloration of the surface when baking the phosphate coating, and therefore effectively prevent the deterioration of product value due to discoloration and prevent embrittlement, and at the same time improve the moisture absorption resistance of the phosphate coating. It is also possible to prevent deterioration of the film, and therefore, a remarkable effect can be obtained in that it can effectively prevent loss of product value and decrease in interlayer resistance due to the occurrence of moisture absorption patterns.

[Brief explanation of drawings]

Figure 1 is a graph showing the influence of the atmosphere on the moisture absorption resistance of phosphate-based coatings. Applicant Kawasaki Steel Co., Ltd. agent Patent attorney Takehisa Toyota (and one other person)

Claims (1)

[Claims] After forming a forsterite film on the surface of a grain-oriented silicon steel plate, a phosphate-based top insulating coating is applied, and during annealing to flatten the disc and bake the phosphate-based coating, N2 or N2 and C
Contains 15% (volume %) or less of O, and the remainder is substantially N2
Baking of phosphate-based coatings on grain-oriented silicon steel waves is characterized by annealing in a weakly reducing atmosphere consisting of a method.