JPH05214444A - Production of nonoriented silicon steel sheet minimal inplane anisotropy of magnetic property - Google Patents

Abstract

PURPOSE:To minimize the inplane anisotropy of magnetic properties and further to improve the productivity of a nonoriented silicon steel sheet without deteriorating the average magnetic properties of a nonoriented silicon steel sheet containing Si and Al. CONSTITUTION:In the manufacturing process for a nonoriented silicon steel sheet having a composition consisting of, by weight, <=0.01% C, <=2.0% Mn, 2.0-5.0%, in total, of Si and Al, and the balance iron with inevitable impurities, heating at the time of annealing at least prior to final cold rolling is performed on an electric resistance heating system where direct electrification is applied to a steel strip. The heating may be done by performing the heating in a low temp. region by means of ordinary indirect heating and performing the heating only in a high temp. region of >=800 deg.C by means of electric resistance heating. By this method, rapid heating is made possible, and as a result, the crystalline grains at least before final cold rolling become extremely fine and the plane anisotropy of the nonoriented silicon steel sheet can remarkably be improved. Further, annealing time before cold rolling can also be shortened.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION This invention uses Si and Al in a total amount of 2.0
It relates to the manufacturing method of so-called high-grade non-oriented electrical steel sheet containing up to 5.0%, and in particular, at least the heating in the annealing before the final cold rolling is performed by the electric resistance heating method in which the steel strip is directly energized, the average magnetic properties are The present invention relates to a method for producing a non-oriented electrical steel sheet having a small in-plane magnetic property anisotropy, which is characterized by improving the in-plane anisotropy of magnetic property without damaging the magnetic property.

[0002]

2. Description of the Related Art Non-oriented electrical steel sheets are thin steel sheets used for iron cores of motors and transformers of various electric appliances such as home electric appliances and machine tools, and their magnetic properties are important to control the efficiency and size of these appliances. Is a factor. Iron loss is a heat loss generated when magnetizing iron, and if this value is small, the use efficiency of electric power increases. On the other hand, if the magnetic flux density is high, the cross-sectional area of the iron core can be reduced, which is advantageous for downsizing the equipment.

Iron loss is divided into eddy current loss and hysteresis loss. The eddy current loss decreases as the specific resistance of steel increases, and the specific resistance increases as the alloying element is added. Si and Al have a large rate of increase in specific resistance per addition amount and are relatively inexpensive. On the other hand, the hysteresis loss becomes smaller as the number of fine precipitates and grain boundaries that hinder the movement of the domain wall decreases. Therefore, in order to produce a high-grade non-oriented electrical steel sheet with low iron loss, Si and
It is common practice to add a large amount of Al, further reduce the impurity elements in the steel as much as possible, and further perform final annealing at a high temperature to coarsen the crystal grains.

The magnetic flux density generally tends to decrease as the amount of alloying element added increases. In order to reduce iron loss, the addition of alloying elements is essential, so the higher the quality of non-oriented electrical steel sheets, the more important it is to take measures to improve the magnetic flux density. In order to improve the magnetic flux density, it is effective to control the texture (the crystallographic orientation of crystal grains). The texture after the final annealing is governed by the grain size before cold rolling and the rolling reduction of cold rolling.

Usually, in the case of producing a high grade non-oriented electrical steel sheet by one cold rolling, the hot rolled sheet is annealed (hereinafter, referred to as
"Hot-rolled sheet annealing" is usually performed. The reason for this is that if the structure before cold rolling is a restructured structure that has not been recrystallized, a proper texture cannot be obtained after the final annealing, leading to a decrease in magnetic flux density. Further, if hot-rolled sheet annealing is not performed, a surface property defect called ridging tends to occur, and in that sense, hot-rolled sheet annealing is an essential step in the production of high-grade non-oriented electrical steel sheets. When manufacturing a high-grade non-oriented electrical steel sheet by cold rolling two or more times, the intermediate annealing serves a role equivalent to hot-rolled sheet annealing, and for the same reason, at least the structure before the final cold rolling is recrystallized. I need to keep it.

It is also well known that the larger the size of recrystallized grains in the structure before the final cold rolling, the better the magnetic properties. The reason for this is considered to be that if the recrystallized grains before the final cold rolling are coarse, a texture containing a large number of crystal orientations advantageous for magnetic properties called Goss orientation is likely to develop after the final annealing. There is. Therefore, a process for coarsening the recrystallized grains before the final cold rolling is adopted within a range that does not cause trouble such as breakage of the steel strip during cold rolling.

High-grade non-oriented electrical steel sheets have been conventionally manufactured based on the above various findings. That is, for the purpose of reducing the eddy current loss, the total amount of Si and Al is 2.0 to 5.0.
% Is added. For the purpose of reducing the hysteresis loss, the steel in which impurities such as S, N (nitrogen), O (oxygen) are minimized is smelted, and then the slab heating temperature and finishing temperature are adjusted so that fine precipitates do not occur. The coiling temperature is adjusted and hot rolling is performed, and further final annealing is performed at a high temperature to coarsen the crystal grains. For the purpose of improving the magnetic flux density, hot-rolled sheet annealing or intermediate annealing is performed to adjust the grain size before cold rolling, and the rolling reduction and intermediate annealing temperature in cold rolling are performed to obtain an appropriate texture. Choose.

[0008]

As described above, many studies have been made in the past regarding the method of manufacturing a high-grade non-oriented electrical steel sheet, and the basic manufacturing process has been generally determined. However, in the conventional manufacturing process,
The main focus is on improving the average value of the magnetic properties, and sufficient studies have not necessarily been made on reducing the in-plane anisotropy of the magnetic properties. For example, the development of the Goss orientation, which is advantageous for improving the average value of the magnetic properties, significantly increases the difference in the magnetic properties between the rolling direction of the steel sheet and the direction perpendicular thereto. Therefore, the in-plane anisotropy may be large even if the average value of the magnetic properties is good. When a non-oriented electrical steel sheet having a large in-plane anisotropy of magnetic properties is applied to manufacture an iron core, a problem may occur. That is, if there is a large difference in the magnetic properties between the rolling direction of the non-oriented electrical steel sheet and the direction perpendicular to the rolling direction or in the middle direction thereof, the punching direction of the non-oriented electrical steel sheet or the shape of the iron core is greatly restricted. Become. Therefore, it is desirable that the in-plane anisotropy of the magnetic properties be as small as possible, but at the present time, a method that is compatible with the improvement of the average value of the magnetic properties has not necessarily been established.

In order to improve the average value of magnetic properties and further reduce the in-plane anisotropy, at least the structure before the final cold rolling is completely recrystallized, and the recrystallized grains are made as fine as possible. Although it is important to control them to the above, it was impossible to control them sufficiently in the conventional manufacturing process.

The key to controlling the structure before cold rolling to be a fine recrystallized grain structure is how fast the heating rate of hot-rolled sheet annealing or intermediate annealing can be. In the case of box annealing in which a large coil is processed in a batch system, since the thermal inertia is large, an extremely slow heating rate of about several tens of degrees Celsius per hour can be obtained. Therefore, the recrystallization nuclei are generated less frequently in the heating process, and the recrystallized grains are inevitably coarsened because they are kept at a high temperature for a long time.

On the other hand, in the case of the continuous annealing method, which has recently become the mainstream due to its high productivity, the heating rate is overwhelmingly high, which is of the order of several tens of degrees per second in the usual method, and further the induction heating method. If applied, it will be possible to reach several hundred degrees Celsius per second. For this reason, the frequency of recrystallized nucleation increases and the holding time in the high temperature region is short, so that the recrystallized grains tend to be finer and more uniform than in box annealing.

However, the size of the recrystallized grains cannot reach the target level of the present invention even with continuous annealing by the conventional method. That is, the conventional continuous annealing is a system in which a steel strip is passed through at least a high temperature soaking part heated by a radiant tube, and the steel strip is indirectly heated through a heated atmosphere. Therefore, the heating rate becomes slower as the temperature of the steel strip approaches the ambient temperature, and as a result, the staying time of the steel strip in the high temperature region becomes longer and recrystallized grains grow. Growth of recrystallized grains results in deterioration of texture. Further, when the steel strip is kept in a high temperature region for a long time, internal oxidation is likely to occur in the steel strip and the magnetic properties of the steel strip are likely to deteriorate. If only the heating rate is to be increased, the ambient temperature may be raised, but this will further promote the growth of recrystallized grains and internal oxidation. On the other hand, lowering the ambient temperature increases the risk of the recrystallization itself becoming insufficient, although it suppresses the growth of recrystallized grains, and thus cannot be adopted.

After all, in the indirect heating method through the atmosphere,
The above-mentioned problems cannot be fundamentally solved.

There is also a method of applying high frequency induction heating in order to increase the heating rate in a high temperature region, but there is a drawback that it is difficult to ensure temperature uniformity in the width direction and the thickness direction due to the skin effect. If the temperature exceeds the magnetic transformation point (770 ° C for iron), the heating efficiency becomes extremely poor, and a sufficient heating rate cannot be secured. Therefore, even with this method, it is impossible for the size of the recrystallized grains to reach the target level of the present invention.

The present invention has been made for the purpose of improving both the average value of magnetic properties and the in-plane anisotropy thereof, and in particular the object of improving the heating rate in the steps of hot-rolled sheet annealing or intermediate annealing. It is a thing.

[0016]

SUMMARY OF THE INVENTION The present inventors have found that in order to reduce the in-plane anisotropy of a high-grade non-oriented electrical steel sheet without impairing the average value of its magnetic properties, at least before the final cold rolling. Recognizing that making the recrystallized grains of the steel strip extremely fine is the deciding factor, we found that it is optimal to use the resistance heating method in which the steel strip is directly energized at least during the annealing process before the final cold rolling. It was The present invention based on this finding is based on the following (1) and (2).

(1) C of 0.01% or less and Mn of 2.0% by weight
% Or less, the total of Si and Al is 2.0 to 5.0%, and the balance is at least directly heated in the annealing before the final cold rolling in the manufacturing process of the non-oriented electrical steel sheet consisting of inevitable impurities and iron. A method of manufacturing a non-oriented electrical steel sheet having a small in-plane anisotropy of magnetic properties, which is performed by an electric resistance heating method.

(2) At least the heating in the annealing before the final cold rolling should be performed by using both the indirect heating method and the electric resistance heating method in which the electric current is directly applied, and this electric resistance heating should be performed in the temperature range of 800 ° C. or higher. A method for producing a non-oriented electrical steel sheet having a small magnetic property in-plane anisotropy according to the above (1).

In the above-mentioned method of the present invention, there is no particular restriction on the method for producing a steel strip (coil) which is subjected to hot rolling sheet annealing or intermediate annealing by the electric resistance heating method. The methods usually used for manufacturing this type of electromagnetic steel sheet, or all the manufacturing methods of melting, hot rolling, cold rolling and finally finish annealing under various modified conditions set to target.

[0020]

[Action] First, the action and effect in the case where the heating in at least the annealing before the final cold rolling (the hot-rolled sheet annealing or the intermediate annealing in the cold rolling process) is performed by the electric resistance heating method in which the steel strip is directly energized explain.

In the resistance heating method, an electric current is directly applied to the steel strip to utilize Joule heat generated by the electric resistance of the steel strip.

FIG. 1 briefly shows one method of directly supplying an electric current to a steel strip. Two pairs of energizing pinch rolls 2-1 and 2-2 facing each other as shown in the drawing are used between them. Steel strip 1
The method in which a current is supplied from the power source 3 is adopted. The amount of electric current is determined by the cross-sectional area of the steel strip and the required maximum heating temperature. The energizing direction is the traveling direction of the steel strip (arrow X in FIG. 1).
Direction) or vice versa. The current may be direct current or alternating current, but in the case of alternating current, the surface and end of the steel strip are overheated by the skin effect in the high frequency region, so it is better to set it to a low frequency region where the skin effect does not appear. , It is desirable to use the commercial frequency (50Hz, 60Hz).

FIG. 2 compares the heating rates of indirect heating through an atmosphere (conventional method) and direct electrical heating (method of the present invention). The electric resistance of the steel strip increases as the temperature rises, so if the direct current heating method is used, the heating rate will increase at an increasing rate with increasing temperature, up to 100,000 ° C / se.
It is possible to increase to c. Therefore, the time required to reach the required maximum heating temperature is much shorter in the direct current heating method than in the conventional indirect heating method.

According to the method of the present invention, since the heating rate in the high temperature region is extremely high, the staying time of the steel strip in the high temperature region is remarkably shortened, which is extremely fine as previously impossible. A recrystallized structure is formed. By cold rolling the steel strip on which the extremely fine recrystallized structure is formed and then performing final annealing, a non-oriented electrical steel strip with a small in-plane anisotropy of magnetic properties and a good average value can be obtained. You can get it. When cold rolling is performed twice or more, at least the recrystallized structure before final cold rolling should be extremely fine.

Further, since the holding time of the steel strip in the high temperature range is remarkably shortened, there is almost no risk of internal oxidation. Further, as a result of significantly shortening the annealing time, it becomes possible to increase the striping speed of the continuous annealing device, and there is also a great effect of improving the productivity of the non-oriented electrical steel sheet. In addition,
Since the more the alloying element is added, the more the specific resistance of the steel strip increases, it can be said that direct current heating is more suitable for a high-grade non-oriented electrical steel sheet to which more alloying element is added.

The direct electric heating method has many advantages as described above. However, although the heating time is short, it is needless to say that the heating portion needs to be sealed in a non-oxidizing atmosphere because internal oxidation and surface oxidation occur when heating is performed in the atmosphere.

The direct current heating method may be applied to all heating steps in the annealing (hot-rolled sheet annealing or intermediate annealing) before the final cold rolling. However, as can be seen from FIG. 2, the advantage of the direct current heating becomes remarkable as the temperature becomes higher.
Therefore, it is rational to use the conventional indirect heating method in the low temperature range, and use the direct current heating method only in the high temperature range. The method that uses both methods together is effective in simplifying equipment and reducing overall energy costs. Specifically, direct current heating may be performed at 800 ° C. or higher at which an influence on the recrystallization structure and internal oxidation appears. Since recrystallization does not occur below this temperature, the influence on the structure is relatively small even if the indirect heating method in which the heating temperature is slow is used, and the risk of internal oxidation is also small.

The reason for selecting the content of the components of the non-oriented electrical steel sheet targeted by the method of the present invention is as follows. All percentages are by weight.

C: C is an element that forms carbides and deteriorates magnetic properties, and the smaller the content, the better. If the C content exceeds 0.01%, the adverse effect becomes remarkable, so the upper limit is made 0.01%. From the viewpoint of magnetic aging, 0.003% or less is desirable.

Mn: Mn is preferably added in an amount of 0.05% or more in order to prevent hot embrittlement due to S. However, if it exceeds 2.0%, the grain growth property deteriorates, so the upper limit is made 2.0%. Mn is also effective in increasing the specific resistance.

Si and Al: Si and Al are added to increase the specific resistance. When the total content of these elements is less than 2.0%, a transformation point appears, and the upper limit of the annealing temperature is determined by the transformation point, so that the effect of the method of the present invention cannot be sufficiently exhibited. Therefore, the lower limit is 2.0%. On the other hand, if the total content exceeds 5.0%, the cold rolling property deteriorates significantly, so the upper limit is made 5.0%. In addition, since Si is cheaper than Al, Al is often used as the main component and Al is used as a supplement, but theoretically Si is used.
Only one of Al and Al may be used.

In addition, inevitable impurities such as S, N and O
Each is 0.003% or less, and it is desirable to keep it as low as possible.

[0033]

【Example】

C: 0.002%, Si: 3.25%, Mn: 0.25%, P: 0.015
%, S: 0.001%, Al: 0.83%, N: 0.002%, O: 0.
A continuous cast slab containing 001% and the balance essentially iron is heated to 1200 ° C, finishing temperature 850 ° C, winding temperature 550
Hot rolling was performed at ℃ to obtain a hot rolled coil with a thickness of 2.3 mm.

After this hot rolled coil was pickled, hot rolled sheet annealing or intermediate annealing was carried out in the steps shown in (A) and (B) below.

Then, after measuring the grain size of the steel sheet before the final cold rolling, cold rolling was performed to a thickness of 0.5 mm. Further, final annealing was performed to evaluate the magnetic properties of the obtained non-oriented electrical steel sheet.

(A) In the case of single cold rolling: After pickling a hot-rolled coil that has been pickled to a width of 200 mm, the direct current heating method which is the method of the present invention and the conventional indirect heating method (atmosphere heating and high frequency induction heating) ) Was annealed.

(B) In the case of double cold rolling: The pickled hot-rolled coil is cold-rolled to a thickness of 1.2 mm and the primary cold-rolled coil is slit to a width of 200 mm. Intermediate annealing was performed by the electric heating method and the conventional indirect heating method (atmosphere heating and high frequency induction heating).

In the direct current heating, the coil is passed at a speed of 1 m / min, the power source voltage is 10 V between two sets of current-use pinch rolls arranged at an interval of 1 m, and in the case of the step shown in (A) above, 50
A current of 00 A, and a current of 2500 A in the case of the process shown in (B) above were applied to heat from room temperature. The maximum heating temperature is
At 1100 ° C, the atmosphere was 100% nitrogen.

The atmosphere was heated from room temperature by passing the coil through a continuous annealing furnace heated by an electric heater at a speed of 1 m / min. Maximum heating temperature is 1100 ℃, atmosphere is 1
00% nitrogen was used.

High frequency induction heating has a frequency of 60 Hz and power of 60 kW
The coil was passed through a continuous annealing furnace heated by the high-frequency induction heating device at 1 m / min and heated from room temperature. The maximum heating temperature was 1100 ° C and the atmosphere was 100% nitrogen.

To measure the crystal grain size, a micrograph is taken,
A linear analysis method was used.

The final annealing is carried out in a continuous annealing furnace heated by an infrared heating heater, and the atmosphere gas is 98% nitrogen +
2% hydrogen, the soaking temperature was 1000 ° C., and the holding time at the soaking temperature was 1 minute.

The magnetic properties were evaluated by measuring the iron loss W _15/50 and the magnetic flux density B ₅₀ using a single plate magnetometer.

Table 1 shows the case of single cold rolling (in the case of hot rolled sheet annealing), and Table 2 shows the case of double cold rolling (in the case of intermediate annealing).
Shows the crystal grain size and magnetic characteristics. Tables 1 and 2 show rolling direction (test direction “0 °” described in the table), perpendicular direction (90 °) and 45 ° direction (45 °).
The measured values of the iron loss W _15/50 and the magnetic flux density B ₅₀ of each sample are shown in “°”), and their average values are shown.

[0045]

[Table 1]

[0046]

[Table 2]

The direct current heating method of the method of the present invention has a significantly higher heating rate than the conventional indirect heating method, and at least the crystal grains of the steel sheet before the final cold rolling are extremely fine. The in-plane anisotropy of the characteristics is extremely small both in terms of iron loss and magnetic flux density. Further, the average value of the magnetic characteristics is also better in the direct current heating method of the method of the present invention than in the conventional indirect heating method.

Therefore, it can be seen that by using the direct current heating method of the method of the present invention, it is possible to manufacture a higher quality non-oriented electrical steel sheet than before.

[0049]

Industrial Applicability According to the manufacturing method of the present invention, a high-grade non-oriented electrical steel sheet having extremely small in-plane anisotropy of magnetic properties and a good average value can be manufactured industrially and stably.
By using this non-oriented electrical steel sheet having a small in-plane anisotropy of magnetic characteristics as a material, there is no restriction in processing or using the iron core.

Further, this non-oriented electrical steel sheet not only has better magnetic properties than those manufactured by the conventional method, but also has productivity higher than that of the conventional method, and its practical value is extremely high. ..

[Brief description of drawings]

FIG. 1 is a schematic diagram illustrating direct electric heating in the method of the present invention.

FIG. 2 is a diagram showing a comparison of temperature rising patterns of a steel sheet by direct current heating (method of the present invention) and indirect heating (conventional method).

Claims (2)

[Claims] 1. A process for producing a non-oriented electrical steel sheet comprising C: 0.01% or less, Mn: 2.0% or less, Si and Al: 2.0 to 5.0% in total, the balance being inevitable impurities and iron. 2. A method for producing a non-oriented electrical steel sheet having a small magnetic property in-plane anisotropy, wherein at least the heating in the annealing before the final cold rolling is performed by an electric resistance heating method in which a steel strip is directly energized. 2. At least the heating in the annealing before the final cold rolling is performed by using both the indirect heating method and the electric resistance heating method of directly energizing, and the electric resistance heating is performed in a temperature range of 800 ° C. or higher. The method for producing a non-oriented electrical steel sheet according to claim 1, wherein the magnetic property in-plane anisotropy is small.