KR20030054695A - Method for magnetic annealing non- oriented electrical steel sheet - Google Patents

Abstract

PURPOSE: A method for magnetic annealing a non-oriented electrical steel sheet is provided to decrease iron loss while increasing magnetic flux. CONSTITUTION: The method includes the steps of reheating a non-oriented electrical steel sheet at higher than 200 deg.C; starting magnetic annealing from the temperature range of 350 and 600 deg.C at a cooling rate of 30 to 120 deg.C/sec; and finishing magnetic annealing at less than 200 deg.C, wherein vertical and horizontal magnetic field are alternatingly applied to the non-oriented electrical steel sheet.

Description

Magnetic field heat treatment method of non-oriented electrical steel sheet {Method for magnetic annealing non- oriented electrical steel sheet}

The present invention relates to a method for manufacturing a non-oriented electrical steel sheet used as an iron core of an electric device, such as a motor, and more particularly, by applying a horizontal magnetic field and a vertical magnetic field sequentially to control the magnetic field application start temperature and the cooling rate is low It relates to a magnetic field heat treatment method that can produce a high permeability non-oriented electrical steel sheet with high productivity.

Non-oriented electrical steel sheets used as iron cores in electric devices such as motors and transformers are the most energy-loss parts when electrical energy is converted into mechanical energy. Therefore, the use of low iron loss electrical steel sheet can reduce the energy loss by that much. Recently, the use of low-loss materials due to environmental problems, besides energy loss, can reduce the amount of power generation, and interest in low-loss materials is increasing in terms of suppressing power plant construction. If the power loss can be reduced by 10%, it is calculated that 20 power plants can be reduced based on 200 domestic power plants.

Magnetic properties in non-oriented electrical steel sheets include magnetic flux density, iron loss, permeability, and magnetostriction. Higher magnetic flux density and permeability are better, and lower iron loss and magnetostriction are better products. If the magnetic flux density and permeability are good, the torque (rotational power) of the motor will increase, which will bring it to a relatively small size, and the amount of copper wire wound on the iron core can be reduced, thereby reducing the copper wire. In order to increase the magnetic flux density and permeability, many researchers have been studying the aggregate structure according to the change of component design and processing conditions.

Iron loss in non-oriented electrical steel sheet has hysteresis loss and eddy current loss. Hysteresis losses are largely affected by grain size and steel cleanliness. Vortex losses are mainly affected by the resistivity, thickness and grain size of the steel sheet. In order to reduce the iron loss, the researchers have mainly attempted to increase the specific resistance by adding Si and Al in steelmaking or to reduce the thickness through cold rolling. However, in general, the method of adding Si or Al is accompanied with the effect of lowering the magnetic flux density but lowering the iron loss. Lowering iron loss while increasing the magnetic flux density is very difficult because the two properties are mutually opposite. To this end, there is a technique for reducing the thickness of the non-oriented electrical steel sheet. If the thickness of the steel sheet is taken thin, the vortex loss is reduced in proportion to the iron loss, and the magnetic flux density is affected by the components, and thus the magnetic flux density and the iron loss can be obtained at the same time. However, thinning the steel sheet has a great influence on the production volume for the steel sheet manufacturer. The thinner the thickness, the lower the reduction ratio, the harder the material, so it is not easy to make. In addition, in order to make the thickness thinner, the number of rolls increases, which inevitably decreases the yield.

Therefore, the technology for improving the iron loss and the magnetic flux density at the same time while reducing the thickness of the steel sheet is not very positive, and the introduction of a new technology is required.

There is not much interest in magnetostriction because there is much room for improvement of magnetic flux density, iron loss and permeability in magnetic properties of non-oriented electrical steel sheet. Magnetostriction causes noise generated when the steel sheet is processed with iron cores and used in applications such as motors. That is, the change of the magnetic core in the iron core in the length of the iron core causes a noise, which is called magnetostriction. Magnetostriction is known to be caused by the change of the length of the material as the distance between atoms is changed by the rotation of the magnetic domain in the material.

For a long time, many researchers have used magnetic field heat treatment to control the magnetic domains present in the material. Magnetic field heat treatment has been applied to a variety of materials since it was discovered by Pender and Jones in 1913. This is to change the magnetization curve by rearranging the magnetic domain of the material by imparting a magnetic field while cooling to room temperature below the Curie temperature. Magnetic field heat treatment is mainly used to improve magnetic properties in materials with low magnetic anisotropy energy. In the Physics of Ferromagnetism (S. Chikazumi. Oxford Press, NY, 1997) and Introduction to Magnetic Materials (B.Cullity. A. Wesley Publ., London, 1972) The heat treatment while applying in the axial direction, that is, the rolling direction, has been explained by the magnetic field heat treatment.

However, this magnetic field heat treatment technology has been mainly applied to oriented electrical steel sheet. 1964 V.A. ZAYKOVA et al. (Fiz. Metal. Metalloved. 18, 349 (1964)) was actively studied at that time. However, it was found that it is not easy to change the magnetic properties of the steel sheet by magnetic field heat treatment because the crystal magnetic anisotropy energy of the grain-oriented electrical steel sheet is too large.

Specific techniques for applying magnetic field heat treatment to a grain-oriented electrical steel sheet include (1) Japanese Patent Application Laid-open No. Hei 8-134543, (2) Hei 8-134551, and (3) Hei 7-197132.

(1) Japanese Patent Application Laid-Open No. 8-134543 shows a magnetic strain reduction effect according to the proper temperature and cooling rate of magnetic field heat treatment using a material having a magnetic flux density of 1.93 to 1.94 Tesla at 800 A / m (B ₈ ). Giving. In this case, the oriented electrical steel sheet is subjected to secondary recrystallization annealing, followed by coating and flattening. It is characterized by.

(2) In Japanese Patent Laid-Open No. 8-134551, a temperature gradient of 1.5 ° C in the width direction of the steel sheet from 700 ° C to 400 ° C was obtained using a material having a magnetic flux density of 800 A / m (B ₈ ) of 1.94 Tesla. At the same time, the tensile stress of the steel sheet is 0.30% or more, or 0.15% or less, and the DC magnetic field is applied to 50 ersted or more, thereby reducing the iron loss and the magnetic flux density.

(3) Japanese Patent Application Laid-Open No. 7-197132 shows an alternating magnetic field of more than 40 A / m in an effective magnetic field (sine wave, triangular wave and square wave magnetic fields) in a silicon steel sheet containing Si: 1 to 10% by weight in the region below the Curie temperature. A method for producing a grain-oriented electrical steel sheet is disclosed, which is cooled to 400 ° C. or lower in this magnetic field.

In the above prior arts, high magnetic flux density oriented electrical steel sheets (magnetic flux density of 1.90 Tesla or more at 800 A / m) or substantially high silicon (silicon content of 4% or more) steel sheets are used. Further, in the prior art of (1) (2) (3), the magnetic field heat treatment technique of applying the magnetic field in the biaxial direction for magnetization (to impart the magnetic field in the rolling direction of the steel sheet), and in (1) (2), the direct current magnetic field ( In 3), exchange magnetic field is given. In the case of the DC magnetic field, a lot of power is required according to the magnetic field strength, so it is very difficult to obtain a high magnetic field strength, and it is very inefficient because the magnetic field is not high compared to the power input. In addition, the alternating magnetic field has a good effect of reducing the magnetostriction according to the magnetic field strength, and has a better effect than the direct current magnetic field, but it also requires a lot of power to obtain a high magnetic field.

As described above, the prior art related to the technology for improving the magnetic properties through magnetic field heat treatment for non-oriented electrical steel sheet is difficult to find, because the magnetic field heat treatment so far is recognized as a technique for arranging magnetic domains in one direction. Application to non-oriented electrical steel sheets in which grains have different orientations was not considered at all.

The present invention provides a magnetic field heat treatment method capable of manufacturing a non-oriented electrical steel sheet having high iron loss and high permeability with high productivity by controlling the magnetic field application start temperature and cooling rate while sequentially applying horizontal and vertical magnetic fields. There is a purpose.

1 is a schematic diagram of a magnetic field heat treatment according to a magnetic field direction;

Fig. 1 (a) is a schematic diagram of magnetic field heat treatment (LDMA) for magnetically applying in the longitudinal direction (rolling direction) of the steel sheet.

Fig. 1 (b) is a schematic diagram of magnetic field heat treatment (TDMA) for magnetically applying in the width direction (vertical direction to the rolling direction of the steel sheet) of the steel sheet;

2 is a graph showing the magnetostriction improvement rate according to the magnetic field start temperature

3 is a graph showing the magnetostriction improvement rate according to the cooling rate

4 is a schematic diagram of a magnetic field heat treatment apparatus;

5 is a graph showing the change in iron loss and magnetic permeability according to the magnetic field strength

6 is a graph showing changes in iron loss and permeability with frequency

7 is a graph showing changes in iron loss and permeability according to pulse widths.

* Description of the symbols for the main parts of the drawings *

1 .. electrical steel sheet 2.. Heat treatment furnace

20 .. Horizontal magnetic field generating means 30. Vertical magnetic field generating means

40. Power supply 50. Control unit

52. Power switching means 60. Oscilloscope

Non-oriented magnetic field heat treatment method of the present invention for achieving the above object, the step of heating the non-oriented electrical steel sheet to a temperature of 200 ℃ or more,

Starting the application of the magnetic non-oriented electrical steel sheet at 350 ~ 600 ℃ by cooling at a rate of 30 ~ 120 ℃ / sec while applying the horizontal and vertical magnetic field in order to terminate the application of magnetic field at a temperature before 200 ℃ It consists of steps.

Hereinafter, the present invention will be described in detail.

There are many grains in the non-oriented electrical steel sheet, and each grain has different orientations, so the domains formed therein also face different directions. When the motor rotates, the magnetic domain at any point inside the iron core rotates or moves the wall according to the magnetic field given from the outside at the same time. At this time, the better the movement of the magnetic wall is less iron loss. The magnetic anisotropy energy must be low in order for the movement of the magnetic wall to move easily according to the external magnetic field.

Therefore, the present inventors realized that lowering the magnetic anisotropy energy could easily rotate the magnetic domains arranged in different directions or move the magnetic walls to lower the iron loss, thereby reducing the magnetic anisotropy energy in the oriented electrical steel sheet. An experiment was introduced to introduce the technology. However, as a result of applying the horizontal magnetic field (FIG. 1A) applying the magnetic field in the longitudinal direction of the steel sheet, which is a biaxial axis for magnetization, the effect was not obtained. In the ongoing research, the vertical magnetic field and the horizontal magnetic field were sequentially applied along with the vertical magnetic field (FIG. 1B) to apply the magnetic field in the width direction of the steel sheet. As a result, the magnetic anisotropy energy was significantly lowered.

In the present invention, while introducing a new magnetic field heat treatment technology to the non-oriented electrical steel sheet, there is a feature to apply the horizontal magnetic field and vertical magnetic field in sequence, and also to propose a magnetic field heat treatment apparatus used here. Therefore, the present invention will be described by dividing it into [1] object steel plate, [2] magnetic field heat treatment method, and [3] magnetic field heat treatment apparatus.

[1] sheet steels

The present invention is a non-oriented electrical steel sheet as a target steel. Non-oriented electrical steel sheet is produced by reheating a steel slab, hot rolling, winding, pickling and cold rolling, and final annealing of the cold rolled steel sheet. The final annealed cold rolled sheet may be insulated coated.

Therefore, in the present invention, magnetic field heat treatment is performed on the final annealed cold rolled steel sheet or insulation coated cold rolled steel sheet. Various kinds of components of non-oriented electrical steel sheet are suggested by manufacturers, and most steel grades contain Si of 4.5 wt% or less. In addition, it contains alloying elements such as Al, Mn, P, B, Sb, Cu, Ni and Cr for improving magnetic properties and processability.

[2] magnetic field heat treatment

In the present invention, the magnetic field heat treatment is performed on the final annealing material or the insulating coating material after the final annealing and the insulating coating. Therefore, these steel sheets may be heated and subjected to magnetic field heat treatment, or the magnetic field may be subjected to magnetic field heat treatment in the cooling process after the final annealing or in the cooling process after the curing of the insulating coating. The magnetic field heat treatment greatly improves iron loss, magnetic flux density, and magnetic strain by lowering magnetic anisotropy energy by optimizing magnetic field start temperature, cooling speed, magnetic field application pattern, and magnetic field conditions.

· Magnetic field-applied start temperature and cooling rate

The magnetic anisotropy energy of non-oriented electrical steel sheet is rapidly lowered at higher temperature, so it is better to apply magnetic field at high temperature. However, as the magnetic field heat treatment temperature approaches the magnetic transformation point (Curie temperature, about 720 ° C) of the steel sheet, the magnetic permeability of the non-oriented electrical steel sheet decreases, so that the magnetic flux is weakened. Do. Therefore, it is common to think that the magnetic field application start temperature is higher at 720 degrees C or less. However, even if the magnetic field application start temperature is lowered to 350 ° C., as shown in FIG. 2, the magnetostriction improvement rate is not large. Therefore, it can be seen that it is not necessary to perform magnetic field heat treatment at a high temperature while consuming heat energy. In the present invention, the application of the magnetic field is disclosed at a temperature of 350 ~ 600 ℃. The end of magnetic field application temperature is better than 200 ℃. As shown in Figure 2, applying the magnetic field at less than 200 ℃ has no effect.

Moreover, it is preferable to make the rate of cooling a steel plate 30-120 degreeC / sec, applying a magnetic field. As can be seen in Figure 3, the slower the cooling rate is better for the improvement of the magnetostriction, but even if the cooling rate is increased up to about 120 ℃ / sec, there is some degree of improvement in the magnetostriction. Therefore, in the present invention, the cooling rate of the steel sheet is set to 30 to 120 ° C / sec to improve the magnetic characteristics with productivity.

· Magnetic field-applied pattern

In the non-oriented electrical steel sheet, spontaneous spheres are formed according to the texture of grains during recrystallization through processing and heat treatment. As a way to lower the magnetic anisotropy energy of the spontaneous sphere formed as described above, the vertical magnetic field and the horizontal magnetic field are sequentially applied. A method of sequentially applying a magnetic field will be described with reference to FIG. 4.

The horizontal magnetic field is first applied to the heating steel sheet drawn out by the heat treatment through the first horizontal magnetic field applying means 20a. Then, immediately after the application of the horizontal magnetic field is applied, the vertical magnetic field is applied from the right side to the left side of the steel sheet in the conveying direction of the steel sheet through the first vertical magnetic field applying means 30a. Then, immediately after the application of the vertical magnetic field through the first vertical magnetic field applying means 30a, the horizontal magnetic field is applied in the reverse direction of the conveying direction of the steel sheet through the second horizontal magnetic field applying means 20b. Subsequently, the application of the horizontal magnetic field is terminated, and then the vertical magnetic field is applied from the left to the right through the second vertical magnetic field generating means 30b in the conveying direction of the steel sheet. In this pattern, horizontal and vertical magnetic fields are applied successively. Of course, this sequential application direction may be reversed. The application pattern may be any of various other methods. The possible method is expressed as the power supply method of the magnetic field generating means 20, 30 as follows.

(1) First horizontal magnetic field generating means 20a → First vertical magnetic field generating means 30a → Second horizontal magnetic field generating means 20b → Second vertical magnetic field generating means 30b: Circulation

(2) First horizontal magnetic field generating means 20a → Second vertical magnetic field generating means 30b → Second horizontal magnetic field generating means 20b → First vertical magnetic field generating means 30a: Circulation

(3) First horizontal magnetic field generating means (20a) → First vertical magnetic field generating means (30a) → First horizontal magnetic field generating means (20a) → First vertical magnetic field generating means (30a): Circulation

(4) First horizontal magnetic field generating means (20a) → Second vertical magnetic field generating means (30b) → First horizontal magnetic field generating means (20a) → Second vertical magnetic field generating means (30b): Circulation

(5) Second horizontal magnetic field generating means 20b → First vertical magnetic field generating means 30a → Second horizontal magnetic field generating means 20b → First vertical magnetic field generating means 30a: Circulation

(6) First horizontal magnetic field generating means (20b) → Second vertical magnetic field generating means (30b) → First horizontal magnetic field generating means (20b) → Second vertical magnetic field generating means (30b): Circulation

The most preferable example in the above six application patterns is (1) (2). When the magnetic field is applied to the non-oriented electrical steel sheet along the application patterns (1) and (2), the magnetic domain at any point rotates in response to the magnetic field. And the traveling speed of the steel sheet needs to be adjusted so that any part of the steel sheet is in the region of the magnetic field circulating at least once. At this time, it is preferable that each magnetic field generating means applies a magnetic field in one direction as indicated by arrows in FIG. This is because it is very effective in rotating the magnetic domain inside the steel sheet.

· Conditions for magnetic field

In the present invention, the magnetic field to be applied may be any one of direct current, alternating current, and pulse, but it is preferable to use alternating current or pulse. More preferably, a pulse magnetic field capable of instantaneously applying a high magnetic field is preferable. Pulsed magnetic field means that the time for applying the magnetic field is very short compared to the direct or alternating magnetic field. The alternating magnetic field may be square, triangular, or reflected wave. The magnetic field application condition at this time is preferably a pulse width of 10 ms or more, a frequency of 1 Hz or more, and a pulse intensity of 100 0e or more.

That is, as shown in FIG. 5 (pulse width 35 ms, frequency 8 Hz), the magnetic field strength increases in proportion to the iron loss and the magnetic permeability improvement rate when the magnetic field strength is 100 Oe or more. Invisible Therefore, the most preferable pulse magnetic field strength is 100 to 700 Oe, but the present invention is not necessarily limited thereto.

As shown in FIG. 6 (pulse width 35 ms, magnetic field strength 300 Oe), the higher the frequency, the greater the iron loss and permeability improvement. When the frequency is higher than 1 Hz, the magnetostriction improvement effect is large and the increase is slowed down to 8 Hz or more.

As shown in FIG. 7 (frequency 8 Hz, pulse intensity 600 Oe), the magnetic improvement effect starts to appear when it is 10 ms or more, and the improvement effect is not so large when it approaches 40 ms.

The magnetic field heat treatment apparatus to which the magnetic field heat treatment method of the present invention is applied is shown in FIG. 4. This is briefly explained. The magnetic field heat treatment includes a heat treatment furnace 2, a horizontal magnetic field applying means 20, a vertical magnetic field applying means 30, a power supply 40, a power switching means 52, and a controller 50.

, The heat treatment furnace (2)

The heat treatment furnace of the present invention is to heat the electrical steel sheet to the magnetic field heat treatment temperature, it can be replaced by the final annealing furnace of the non-oriented electrical steel sheet or the curing of the insulating coating.

And horizontal magnetic field applying means (20),

The horizontal magnetic field applying means is provided at the rear of the heat treatment as shown in FIG. 1, but may be disposed in whole or in part in the heat treatment furnace. Two of the first vertical magnetic field generating means (20a) and the second vertical magnetic field generating means (20b) are installed at regular intervals, and a vertical magnetic field generating means (30) is provided therebetween.

Means 30 generate a vertical magnetic field,

In Fig. 1, a first vertical magnetic field generating means 30a and a second vertical magnetic field generating means 30b are provided in close proximity to both side surfaces of the steel sheet between two horizontal magnetic field generating means 20, respectively.

The horizontal magnetic field generating means 20 and the vertical magnetic field generating means 30 may be a solenoid, a Helmholtz coil, or an electromagnet.

· Power supply (40)

A power supply 40 for supplying current to the magnetic field generating means is provided. Select a power source from the group of DC, AC and pulse. The power supply is preferably provided with a waveform controller (not shown) to adjust the frequency, width, and intensity.

, Power switching means 52,

Power supply switching means 52 is provided so that the current supplied from the power supply 40 is sequentially transmitted to the magnetic field generating means. The application pattern of (1) to (6) is implemented by opening and shutting off the power by the power switching means.

, The controller 50

The controller 50 controls the power switching means 52 to implement the application patterns 1 to 6 or various application patterns as necessary. The operation of the controller will be described taking the application pattern 1 as an example. Control first,

Opening the first power source switching means 52a and blocking the remaining power source switching means 52b, 52c, 52d to supply current to the first horizontal field applying means 20a;

Then, the second power source switching means 52b is opened, and the remaining power source switching means 52a, 52c, 52d are blocked to apply current to the first vertical magnetic field applying means 30a,

Then, the third power source switching means 52c is opened and the remaining power source switching means 52a, 52b, 52d are

Supplying a current to the second horizontal magnetic field applying means (20b) by blocking;

Subsequently, the fourth power source switching means 52d is opened and the remaining power source switching means 52a, 52b, 52c are cut off to sequentially supply current to the second vertical magnetic field applying means 30b. The horizontal and vertical magnetic fields are sequentially applied to the non-oriented electrical steel sheet.

On the other hand, oscilloscope 60 is shown in FIG. In the lower part of FIG. 1, which is an enlarged view of the oscilloscope, pulses may be sequentially applied.

Hereinafter, the present invention will be described in more detail with reference to Examples.

Example 1

In the embodiment of the present invention, the target steel used non-oriented electrical steel sheet manufactured by the following process. By weight percent oriented electrical steel slab consisting of C: 0.002%, Si: 1.05%, Mn: 0.27%, P: 0.013, S: 0.0024%, Al: 0.34%, N: 0.0020%, remaining Fe and other unavoidable impurities After heating to 1150 degreeC for 2 hours, it hot-rolled and obtained the hot rolled sheet whose plate | board thickness is 2.0 mm. The hot rolled sheet was wound in air at 700 ° C. and cooled. The cooled hot rolled sheet was cold rolled to a thickness of 0.50 mm after pickling the surface. The cold rolled plate was annealed for 2 minutes in a mixed gas of 20% H ₂ + 80% N ₂ in a furnace kept at 1000 ° C.

A magnetic field was applied to the steel sheet drawn out from the final annealing furnace 2 of FIG. As the magnetic field applying pattern, the magnetic field was sequentially applied by adopting the above (1). That is, the first horizontal magnetic field generating solenoid (20a) → the first vertical magnetic field generating solenoid (30a) → the second horizontal magnetic field generating solenoid (20b) → the second vertical magnetic field generating solenoid (30b): circulation

The magnetostriction improvement rate was measured by changing the temperature when the steel sheet exited the final annealing furnace into the magnetic field applying solenoid and the results are shown in FIG. 2. At this time, the cooling rate of the steel sheet was 30 ℃ / sec, the cooling end temperature was 100 ℃.

As shown in FIG. 2, when the cooling start temperature is set to 350 ° C. or higher, it was confirmed that the magnetostriction can be improved even when a magnetic field is applied in the width direction of the steel sheet.

Example 2

In Example 1, when the steel sheet exits the final annealing furnace and enters the magnetic field applying solenoid, the temperature is adjusted to 450 ° C., the cooling rate is terminated at 100 ° C. while the cooling rate is changed, and then the magnetostriction improvement rate is measured according to the cooling rate. The results are shown in FIG. 3. At this time, the pulse magnetic field was applied such that the maximum height was 600 Hersted (Oe) and the pulse width was 5 ms.

As shown in FIG. 3, when the cooling rate is 120 ° C./sec or less, it was confirmed that the magnetostriction can be improved even when a magnetic field is applied in the width direction of the steel sheet.

As described above, according to the present invention, there is a useful effect of providing a non-oriented electrical steel sheet having low iron loss and high permeability and small magnetic strain while maintaining high productivity.

Claims (5)

Heating the non-oriented electrical steel sheet to a temperature of 200 ° C. or higher, The application of the magnetic field at the temperature of the non-oriented electrical steel sheet 350 ~ 600 ℃ to apply the horizontal and vertical magnetic field sequentially while cooling at a rate of 30 ~ 120 ℃ / sec to terminate the magnetic field application at a temperature before 200 ℃ Magnetic field heat treatment method for a non-oriented electrical steel sheet comprising a step. The magnetic field heat treatment method of claim 1, wherein the heating is a final annealing process of the non-oriented electrical steel sheet or a curing heat treatment process of the insulation coating. The magnetic field heat treatment method of claim 1, wherein the applied magnetic field is one of an alternating magnetic field or a pulsed magnetic field selected from a group of square waves, triangle waves, and reflected waves. 4. The method of claim 3, wherein the alternating magnetic field and the pulse magnetic field have a pulse width of 10 ms or more, a frequency of 1 Hz or more, and a magnetic field strength of 100 to 700 Oe. The method of claim 1, wherein the sequential application of the horizontal and vertical magnetic fields, A first horizontal magnetic field applying step of applying a horizontal magnetic field in a conveying direction of the steel sheet, Terminating the application of the first horizontal magnetic field, and then applying a vertical magnetic field from the right side to the left side of the steel sheet in the conveying direction of the steel sheet; Terminating the application of the first vertical magnetic field, and then applying a second horizontal magnetic field to apply a horizontal magnetic field in a direction opposite to the conveying direction of the steel sheet; Terminating the application of the second horizontal magnetic field, and subsequently performing a second vertical magnetic field applying step of applying a vertical magnetic field from the left side to the right side of the steel sheet as viewed in the conveying direction of the steel sheet. .