JPH0742503B2 - Method for forming high quality primary insulating film on unidirectional electrical steel sheet - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The most important characteristic of the grain-oriented electrical steel sheet mainly used as a core material of a transformer is magnetic characteristics. That iron loss is low (W _17/50 is being represented by values), it is essential magnetic flux density (represented by ₈ value B) is high. On the other hand, since the unidirectional electrical steel sheet is punched into an appropriate shape and used by laminating it, if the insulation between layers is poor, the eddy current loss increases and the iron loss deteriorates. Therefore, if the uniformity of the primary insulation film and the adhesion with the steel sheet are poor, the insulation film peels off at the punched part, and the iron loss deteriorates due to conduction in that part, so the properties of the primary insulation film are Extremely important.

The present invention relates to a method for forming a primary insulating coating on a unidirectional electrical steel sheet, and more particularly to a method for forming a high-quality primary insulating coating having extremely high uniformity and excellent adhesion to a steel sheet.

(Prior Art) In the conventional decarburization annealing process before finish annealing, the primary insulation film of conventional unidirectional electrical steel sheet is annealed and separated from the internal oxide layer mainly composed of SiO ₂ formed on the steel sheet surface during decarburization annealing. The MgO applied as agent reacts and forms during subsequent high temperature finish annealing. To date, many techniques regarding additives to MgO, etc. for forming a good primary insulating film have been presented, and as a method for forming an internal oxide layer, JP-B-58-46
Method for adhering Si, O to a steel sheet before decarburization annealing shown in Japanese Patent No. 547, method for applying metal sulfate before decarburization annealing shown in Japanese Patent Publication No. 55-65367, Japanese Patent Publication No. 62-54085 There is a temperature rising rate, a soaking time, an atmosphere control method and the like during decarburization annealing shown in the publication.

(Problems to be Solved by the Invention) However, in any of the above methods, the method of performing the pretreatment is disadvantageous in economical efficiency due to the additional process, and the condition control during the decarburization annealing is for controlling the internal oxide layer amount. There are limits,
It was insufficient for forming a good quality primary insulating film. The present inventors, in the hot-rolled sheet annealing step before the decarburization annealing step, to form a layer mainly composed of dense Si, O in the surface layer portion after the hot-rolled sheet annealing by appropriately controlling the annealing conditions. , And succeeded in obtaining a steel sheet excellent in adhesion with the steel sheet, which is an important commercial value of the grain-oriented electrical steel sheet, by significantly improving the properties of the primary insulating film in combination with the subsequent decarburization annealing conditions. .

(Means for Solving the Problems) In the present invention, first, an appropriate internal oxide layer necessary for forming an excellent primary insulating film is formed by annealing a hot-rolled sheet, unlike the conventional internal oxide layer formation by only a decarburizing annealing step. By forming a dense SiO ₂ rich internal oxide layer in the base in the process and forming a more appropriate internal oxide layer in the subsequent decarburization annealing process, an excellent SiO ₂ rich internal oxide layer is formed. Is the way. That is, in the hot-rolled sheet annealing step, the gas annealing conditions are set to the air ratio in the temperature range where the steel sheet temperature is 500 ° C or higher.
Hot-rolled sheet annealing is performed in a combustion gas atmosphere controlled to a range of 1.0 to 1.5, and the decarburizing annealing atmosphere is continuously set to 0.30 to 0.45 with PH ₂ O / PH ₂ in the decarburizing annealing process before finish annealing. The present invention provides an excellent method for forming a primary insulating coating on a grain-oriented electrical steel sheet.

The method for calculating the air ratio of the present invention is shown below.

Relationship between O ₂ (%) and air ratio α: Air ratio A ₀ : Theoretical air amount (Nm ³ ) N ₂ : Dry combustion N ₂ gas amount (Nm ³ ) n _n : (CO + CO ₂ ) generation amount (Nm ³ ) per 1Nm ^{3 of} fuel During hot-rolled sheet annealing The air ratio of the furnace atmosphere has the above relationship with the combustion waste gas atmosphere O ₂ (%) in the furnace. Specifically, the actual results of the applicant line are as follows.

Air ratio α O ₂ (%) 1.0 0 1.3 3 1.6 6 The present invention will be described in detail below.

First, the conditions for hot-rolled sheet annealing, which is a feature of the present invention, are essentially steps for obtaining excellent magnetic properties. That is, in order to obtain excellent magnetic properties, stable growth of highly oriented secondary recrystallized grains is essential,
In order to obtain such secondary recrystallized grains, it is an essential condition that precipitates of a proper size and amount are present in a proper dispersed state in the steel sheet. These precipitates are called inhibitors, but the ones currently used industrially include MnS and Al.
There are N, Cus, etc. The main role of this hot-rolled sheet annealing process is to uniformly disperse these precipitates in a proper size and amount in the steel sheet. In order to achieve such a dispersed morphology of precipitates, the hot rolled sheet annealing conditions, especially for thin materials with a product sheet thickness of 0.23 mm or less, are soaked and held at a steel sheet temperature of 1080 to 1200 ° C for 10 to 30 seconds. A known condition is a heat cycle in which cooling from a temperature range to 900 to 980 ° C. is controlled and quenching is performed from 900 to 980 ° C. to room temperature. This hot-rolled sheet is usually annealed in many cases by continuous annealing, and the inlet and outlet of the furnace are usually sealed so as to isolate the inside of the furnace from the outside air. In the present invention, the reason for limiting the steel plate temperature to a region of 500 ° C. or higher is that the steel plate temperature is lower than 500 ° C., so that the internal oxidation generation of the steel plate surface is insufficient and the internal oxide layer sufficient for the purpose of the present invention. Was not obtained, and the air ratio was 1.
This is because it is not necessary to set the combustion gas atmosphere in the range of 0 to 1.5. However, it goes without saying that even if the air ratio is controlled to 1.0 to 1.5 in the entire temperature range, the same good result can be obtained.

The air ratio of the combustion gas, which is one of the important constituent features of the present invention, is the reason for defining the air ratio to 1.0 to 1.5 for the following reason.

In the hot-rolled sheet annealing process, not only in the steel sheet, but also the interface state on the steel sheet surface changes in various ways. Especially, plate temperature 50
The internal oxide layer that reacts and forms on the surface of the steel sheet at an air ratio of 1.0 to 1.5 above 0 ℃ has an ideal shape in which the amount and distribution of Si and O are dense and deeply rooted in the depth direction of the steel sheet. The inventors have found that A typical example will be described.

Fig. 1 shows a comparison of changes in the steel sheet interface state depending on the air ratio conditions. This is C: 0.073%, Si: 3.15%, Mn: 0.0
Hot-rolled electrical steel sheet containing 75%, S: 0.024%, Al: 0.029%, N: 0.008% is gas burned with CH ₄ 50%, H ₂ 20%, N ₂ 10%, and CO 10% as main components. It is the result of GDS elemental analysis of the surface of the annealed plate which was subjected to cooling-pickling treatment after annealing the steel plate temperature of 1120 ° C. for a soaking time of 20 seconds under two conditions of an air ratio of 0.80 and 1.20 in an atmosphere.
Under the air ratio of 0.80 in Fig. 1 (a), the generation of Si, O, Fe at the steel plate interface is small, but under the air ratio of 1.20 in Fig. 1 (b), the Si, O steel plate interface is small. The amount produced in
It can be seen that the concentration ratio of Si and O in the outermost surface layer is increased to about twice the air ratio of 0.80. In addition, as a result of microscopic observation of the outermost layer cross section of the product obtained by annealing the hot rolled sheet under the decarburizing annealing atmosphere condition of the present invention, the sample with an air ratio of 1.20 is SiO ₂
A dense and deeply rooted ideal morphology was observed in the steel sheet depth direction. Further, if the air ratio is further increased, the amount of Si, O, Fe produced at the steel sheet interface further increases, but if the air ratio exceeds 1.5, the amount of FeO greatly increases beyond the amount of SiO ₂ , It was also found that the decarburization amount in the decarburization annealing step described above drastically decreases, which causes a magnetic property defect in the final product. The relationship between these air ratios, adhesion and decarburization is shown in FIG. The adhesion was measured by the same method as in Example 1 described later. This is the same as in Fig. 1 with C: 0.073%, Si: 3.15%, Mn: 0.075%, S: 0.024%, Al:
Electromagnetic steel hot rolled sheet containing 0.029%, N: 0.008% CH ₄ 50%, H
₂ 20%, N ₂ 10%, CO 10% in a gas combustion atmosphere with an air ratio of 0.80 to 1.8 Steel plate temperature 1120 ° C × soaking time 20
After carrying out second annealing, cooling-pickling-cold rolling (rolling rate 87%)
Made into a steel plate with a thickness of 0.300 mm, and this steel plate is N ₂ : 50% +
H ₂ : 50%, PH ₂ O / PH ₂ = 0.40 in atmospheric gas conditions 840 ℃ ×
It was decarburized and annealed for 2 minutes and passed through a plate. For the steel sheet after this decarburization annealing
An annealing separator containing 3 parts by weight of TiO ₂ was applied to 100 parts by weight of MgO, and final finish annealing was performed at 1200 ° C. for 20 hours. As shown in FIG. 2, the relationship between the air ratio and the adhesiveness and decarburizing property of the steel sheet treated under such stripping conditions is as shown in FIG. Good adhesion levels are obtained in the region. Further, in the region where the air ratio exceeds 1.5, decarburization defects are rapidly increasing. The above is the air ratio
This is the reason why it is set to 1.0 to 1.5. Now, as an invention relating to the air ratio in the hot-rolled sheet annealing step, a relation between the descaling pickling effect and the air ratio condition is pointed out as shown in JP-A-62-120427. It is said that the steel sheet surface condition changes drastically at the air ratio of 1.0, and the scale layer remarkably increases above 1.0, and the descaling and pickling effect decreases.

On the other hand, the present invention succeeds in producing a high-quality primary insulating film by combining the temperature condition and air ratio control in the hot-rolled sheet and the condition of PH ₂ O / PH ₂ during the subsequent decarburization annealing. The present invention is completely different from the invention described in JP-A-62-120427. JP 62-
It has been pointed out that the method described in the publication No. 120427 significantly reduces the descaling property at an air ratio of 1.0 or more, but it is already said that shot-plast descaling equipment has been introduced to the line to strengthen pickling. Many, there is no problem of descaling.

Now, in the air ratio range of 1.0 to 1.5, S of the surface layer of the final product is
As mentioned above, the amount and distribution of i and O have an ideal form that is dense and deeply rooted in the depth direction of the steel sheet. Then, it is difficult to realize an internal oxide layer having such a form. Although the reason for this phenomenon is not clear, firstly, a difference in steel plate temperature condition between the hot rolled sheet annealing step and the decarburization annealing step is considered. Under known conditions, the maximum steel plate temperature x soaking and holding time conditions are: 1080-1200 ° C x 10-30 seconds for hot-rolled sheet annealing, 800-900 ° C for decarburization annealing.
X 120 to 150 seconds, the hot rolled sheet annealing condition is higher than the decarburizing annealing condition by 200 to 400 ° C at the maximum steel sheet temperature and 140 to 90 seconds shorter at the soaking and holding time. In other words, it can be said that the hot rolled sheet annealing condition is a high temperature short time treatment, and the decarburizing annealing condition is a low temperature long time treatment. For this reason, the selective oxidation on the surface of the steel sheet that should be originally performed in the decarburization annealing step is performed at a high temperature for a short time during hot-rolled sheet annealing. Compared to the conditions, it is easier to proceed in a much shorter time, the amount of Si and O oxide layers in the steel sheet surface layer increases significantly, and the generated internal oxide layer disappears even after pickling and cold rolling. It is considered that there is a large difference in the internal oxide layer formation amount and morphology in the decarburization annealing process compared to the conventional material.

For this reason, in the present invention, unlike the conventional method of forming an internal oxide layer during decarburization annealing, a good quality internal oxide layer is formed by forming the internal oxide layer in the hot-rolled sheet annealing step. It is a thing. However, even if the internal oxide layer is simply formed in the hot-rolled sheet annealing step, in the subsequent decarburization annealing step, if it is annealed in the conventionally known range of PH ₂ O / PH ₂ = 0.2 to 0.6, good quality is obtained. It does not become an internal oxide layer. Therefore, as a result of various studies, the present inventors have found that PH ₂ O during decarburization annealing
It was found that by limiting / PH ₂ to 0.30 to 0.45 in particular, a good internal oxide layer formed in the hot-rolled sheet annealing process can be maintained until before finish annealing.

Next, the reason for the atmospheric conditions PH ₂ O / PH ₂ = 0.30 to 0.45 in the decarburization annealing process will be described.

The manufacturing conditions are as follows: C: 0.073%, Si: 3.15%, Mn: 0.0
Hot-rolled electrical steel sheet containing 75%, S: 0.024%, Al: 0.029%, N: 0.008% is gas burned with CH ₄ 50%, H ₂ 20%, N ₂ 10%, and CO 10% as main components. After annealing at a steel plate temperature of 1120 ° C and a soaking time of 20 seconds in an atmosphere with an air ratio of 1.20, cooling under the above-mentioned known conditions-pickling-
After cold rolling (rolling rate 87%) to make a 0.300 mm thick steel plate, this steel plate is N ₂ : 50% + H ₂ : 50%, PH ₂ O / PH ₂ = 0.2 to 0.5
Decarburization annealing was performed for 2 minutes at 840 ° C. in the atmosphere gas condition of 1) and the steel sheet was passed. After the decarburization annealing, the annealing separator containing 100 parts by weight of MgO and 3 parts by weight of TiO ₂ was applied to the steel sheet, and final annealing was performed at 1200 ° C. for 20 hours. If PH ₂ O / PH ₂ in the decarburizing annealing process exceeds 0.45 under these manufacturing conditions, F
An internal oxide layer with excess e, O is formed, which significantly impairs the appearance of the product coating.When PH ₂ O / PH ₂ is less than 0.30, decarburization is extremely slow, the carbon content of the product is extremely high, and the magnetic properties deteriorate. I found out that

Next, the reasons for limitation in the present invention will be described.

First, regarding the component composition, if Si exceeds 4.5%, cold rolling becomes difficult, which is not preferable. On the other hand, if it is less than 2.5%, the electric resistance is low and the iron loss value cannot be improved.

C must be contained in an amount sufficient to cause γ transformation in at least a part of steel depending on the amount of Si. When C exceeds 0.12%, not only products with high magnetic flux density cannot be obtained, but
Complete decarburization annealing becomes difficult, which is not preferable.

Al and N are basic elements for forming AlN for obtaining a product with a high magnetic flux density in the present invention, and if outside the range of the present invention described above, secondary recrystallization becomes unstable and a product with a high magnetic flux density is obtained. Will not be obtained.

Mn and S are elements necessary for forming Mns, and Mn
A suitable amount of n is 0.02-0.2%.

If S exceeds 0.04, desulfurization during purification annealing is unfavorably deteriorated. On the other hand, if it is less than 0.01%, the amount of Mns is insufficient.

Further, it is known that Sn, Cu, and Sb each have an effect as an inhibitor individually or by forming a compound, but it is not preferable that these elements are contained in a large amount. Sn, Cu, Sb
The reason why the maximum value was set to 0.4% is that if it exceeds this value, the magnetic properties deteriorate and the hot ductility, pickling property, decarburizing property, etc. are poor and the workability deteriorates.

Further, as a process condition, in the hot rolled sheet annealing step, the reason for setting the air ratio in the annealing atmosphere to 1.0 to 1.5 is that the air ratio is 1.0.
If the ratio is less than 1, the amount of the internal oxide layer, which is the base of the primary insulating film, which is the final purpose, will be insufficient, and a good quality primary insulating film will not be possible.If the air ratio exceeds 1.5, the amount of iron oxide in the internal oxide layer will rapidly increase. However, the decarburization amount sharply decreases in the subsequent decarburization annealing step, resulting in poor magnetic properties in the final product. In addition, the reason why the decarburization annealing atmosphere condition range in the decarburization annealing step was set to 0.30 to 0.45 as the PH ₂ O / PH ₂ partial pressure ratio was that decarburization was extremely slow when the partial pressure ratio was less than 0.30, and the magnetic properties were poor in the final product. What if 0.45 is exceeded?
This is because the internal oxide layer containing excess iron oxide is formed due to the peroxidation, and the appearance of the product film is significantly impaired.

Embodiments of the present invention will be described below.

<Example 1> Specimen: 2.3 mm thick and C: 0.078%, Si: 3.25%, M
n: 0.075%, S: 0.024 %, Al: 0.028%, N: CH 4 50% electromagnetic steel hot-rolled sheet containing _{0.008%, H 2 20%,} N 2 10%, mainly composed of CO 10% Air ratio = 0.80,1.00,1.20,1.5 in gas combustion atmosphere
After annealing at 1100 ° C. for 20 seconds under 5 conditions of 0.16.10, pickling and cold rolling were performed under known conditions to obtain a steel sheet having a thickness of 0.265 mm. This steel sheet was decarburized and annealed at 840 ° C. for 2 minutes in an atmosphere gas condition of N ₂ : 50% + H ₂ : 50% and PH ₂ O / PH ₂ = 0.5, 0.35, 0.2, and then passed. After the decarburization annealing, the annealing separator containing 100 parts by weight of MgO and 3 parts by weight of TiO ₂ was applied to the steel sheet, and the final annealing was performed at 1200 ° C. for 20 hours. Table 1 shows the film appearance of the steel sheet and the adhesion to the steel sheet.

The adhesion with the steel plate was measured by the following method.

A product steel plate is wound around a cylinder having a constant diameter, and the state of the peeling of the primary coating on the cylinder side is observed. Test with cylinders having smaller diameters in order, and use the smallest diameter that does not cause film peeling as an index of film adhesion. That is, the smaller the value, the better the adhesion.

It can be seen from Table 1 that the examples of the present invention have better film appearance and adhesion than the comparative examples.

<Example 2> Specimen: 2.3 mm in thickness and C: 0.078%, Si: 3.25%, Mn:
0.075%, S: 0.024%, Al: 0.027%, N: 0.008%, Cu: 0.08%,
Electromagnetic steel hot rolled sheet containing Sn: 0.12% was added to CH ₄ 50%, H ₂ 20%, N ₂
Air ratio = 10%, CO 10% in the gas combustion atmosphere
After carrying out annealing at 1100 ° C for 20 seconds under five conditions of 0.80, 1.00, 1.20, 1.50, 1.60, pickling-cold rolling by a known method to 0.230 mm.
It was a thick steel plate. This steel sheet is N ₂ : 50% + H ₂ : 50%, PH ₂ O / PH
Decarburization annealing was performed at 840 ° C for 2 minutes in the atmosphere gas conditions of ₂ = 0.5, 0.35 and 0.2, and the strip was passed. MgO 100 is added to the steel sheet after this decarburization annealing.
By applying an annealing separator containing 3 parts by weight of TiO ₂ to parts by weight,
Final finish annealing was performed at 1200 ° C for 20 hours. Table 2 shows the film appearance of the steel sheet and the adhesion to the steel sheet.

It can be seen from Table 2 that the examples of the present invention have better film appearance and adhesion than the comparative examples.

(Effect of the Invention) According to the present invention, only the air ratio and the decarburization annealing conditions in the hot-rolled sheet annealing step are appropriately selected for the formation of the internal oxide layer having the necessary and sufficient amount and composition for good primary film formation. As a result, it is possible to improve the appearance and adhesion of the product film without requiring decarburization annealing pretreatment as in the prior art and using only a small amount of TiO _{2 as} an additive to MgO.

[Brief description of drawings]

1 (a) and 1 (b) are comparisons of changes in the steel sheet interface state (using GDS) according to the air ratio in the annealing atmosphere of the hot rolled sheet of electromagnetic steel (Example H).
i-B 0.30 mm material (product plate thickness)), where (a) is an air ratio of 0.8 and (b) is an air ratio of 1.2. FIG. 2 is a diagram (Example Hi-B 0.30 mm material (product sheet thickness)) showing the relationship between the air ratio in the annealing atmosphere of the electromagnetic steel sheet and the adhesion of the primary insulating film.

Claims (1)

[Claims] 1. A weight ratio of Si: 2.5 to 4.5% and C: 0.020 to 0.12.
%, Mn: 0.02-0.2%, acid-soluble Al: 0.015-0.040%, N: 0.0
040〜0.0100%, S: 0.01〜0.04%, balance Fe and unavoidable impurities, and further annealing 0.4% or less of Sn, Cu, Sb one or more kinds of silicon steel hot rolled sheet In the process of producing a high magnetic flux density unidirectional electrical steel sheet in which final cold rolling performed in the range of 75 to 95% is performed once or more to obtain the final sheet thickness, decarburization, and finish annealing, the hot rolled sheet annealing is performed. , Steel plate temperature is 50
In the temperature range above 0 ° C, change the air ratio in the atmosphere
It is performed in a combustion gas atmosphere controlled in the range of 1.0 to 1.5, and the PH ₂ O / PH ₂ partial pressure ratio of the atmosphere of decarburization annealing in the decarburization annealing step before finish annealing is set to be in the range of 0.30 to 0.45. A method for forming a high-quality primary insulating coating on a characteristic grain-oriented electrical steel sheet.