JP2005120403A - Non-oriented electrical steel sheet with low core loss in high-frequency region - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a non-oriented electrical steel sheet having low core loss in a high-frequency region as well as in a low-frequency region by means of a technique different from the addition of specific-resistance-increasing elements and the reduction of sheet thickness. <P>SOLUTION: In the non-oriented electrical steel sheet, ≤4 mass% Si is contained and the size of crystalline grains is smaller in the surface layer part of the steel sheet than in the inner part of the steel sheet. This steel sheet has the following characteristics: when a fine crystalline grain region is defined as the steel-sheet surface layer part where the average crystalline grain size Dz in a sheet-thickness direction z is ≤1.5 times the minimum value Dmm of the above grain sizes and a coarse crystalline grain region is defined as the remaining inner part of the steel sheet, the thickness of the fine crystalline grain region per side is 8 to 25% of the whole sheet thickness and the average crystalline grain size in the fine crystalline grain region is 2 to 70% of that in the coarse crystalline grain region. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a non-oriented electrical steel sheet used for iron cores of electric motors, generators, stationary machines, and the like, and more particularly, to a non-oriented electrical steel sheet having excellent iron loss characteristics not only in a low frequency range but also in a high frequency range of 400 Hz or higher. Is.

  2. Description of the Related Art In recent years, motors used in the fields of electric equipment, electric vehicles, and the like have been adopted with higher rotational speeds than before for the purpose of reducing the size. In a magnet motor, a multipolar rotor tends to be used from the viewpoint of improving torque and reducing torque ripple. In such a high rotation speed, multi-pole motor, the frequency of the drive power supply tends to increase as the rotation speed and the number of poles increase. Therefore, an electromagnetic steel sheet that is a core material of a motor is required to have excellent magnetic properties at high frequencies. On the other hand, since motors used in electrical equipment and electric vehicles are frequently used in a low frequency range, it is desirable that the iron loss be low in the low frequency range as well as in the high frequency range.

  Now, the iron loss of the electrical steel sheet is the sum of hysteresis loss and eddy current loss, and the eddy current loss consists of classic eddy current loss and abnormal eddy current loss. It is known that the iron loss of the iron core increases due to the increase of eddy current loss. In order to suppress this increase, it is effective to reduce the classic eddy current loss by increasing the specific resistance of the electromagnetic steel sheet, which is a core material, or by reducing the plate thickness. However, in order to increase the specific resistance, it is necessary to add a large amount of specific resistance increasing elements such as Si, Al, Cr, Mn, etc., which increases the rolling load and makes it difficult to manufacture. There is a problem that the punching performance by the user deteriorates due to the rise in the height. The addition of the specific resistance increasing element is advantageous for reducing the eddy current loss, but is not preferable for the purpose of obtaining a high output at a low speed because the saturation magnetization is reduced. In addition, when the plate thickness is reduced, the rolling property is lowered and the annealing cost is increased, and problems such as difficulty in handling by the user due to the reduced rigidity of the steel plate occur.

  For this reason, development of a new iron loss reduction technique in a high frequency region, which is different from the above-described means of increasing specific resistance and reducing plate thickness, has been performed. For example, Patent Document 1 proposes a technique for reducing abnormal eddy current loss by reducing the plate thickness to 0.10 to 0.25 mm and controlling the average crystal grain size to 5 to 60 μm, which is smaller than the conventional appropriate value. ing. In this technique, the iron loss is the sum of the hysteresis loss that decreases as the crystal grain size increases and the eddy current loss that increases conversely. Then, it is based on the idea that it is necessary to make the grain size finer than the crystal grain size optimized in the low frequency range.

Further, when the magnetic material is magnetized at a high frequency, the magnetic flux concentrates on the surface layer portion of the plate thickness due to the skin effect, and the eddy current also concentrates on the surface layer portion of the plate thickness. Therefore, in order to reduce the iron loss in the high frequency range, it is effective to reduce the iron loss in the surface layer portion of the plate thickness. As a technique based on this idea, for example, in Patent Document 2 and Patent Document 3, the magnetic properties are improved by performing silicon dioxide diffusion treatment by chemical vapor deposition or the like, and increasing the Si concentration of the steel sheet surface layer from the center of the plate thickness. Patent Document 4 discloses a technique for improving the high-frequency magnetic characteristics by properly controlling the grain size of the crystal grain of the outermost layer thickness, focusing on the role of hysteresis loss. Discloses a technique for improving the texture of the plate thickness surface layer portion and reducing the hysteresis loss, focusing on the concentration of magnetic flux on the plate thickness surface layer portion.
Japanese Unexamined Patent Publication No. 03-223445 JP-A-62-227033 JP-A-11-209852 Japanese Unexamined Patent Publication No. 06-073511 Japanese Unexamined Patent Publication No. 07-150310

  However, since the technique of Patent Document 1 only defines an optimum crystal grain size for iron loss characteristics in a high frequency region, a decrease in magnetic permeability and hysteresis loss in a low frequency region as the crystal grain size becomes finer. The increase is unavoidable and the iron loss in the low frequency range is significantly deteriorated. For this reason, there is a problem that it is not suitable for a device that places importance on the driving efficiency at the same time as the high speed rotation. Further, the techniques of Patent Documents 2 and 3 have a problem that special manufacturing equipment is required for the siliconization treatment, resulting in an increase in manufacturing cost and a decrease in saturation magnetization as the amount of siliconization increases. Furthermore, the technique of Patent Document 4 has a problem that it is difficult to stably obtain a low iron loss because the influence of crystal grains on the steel sheet surface on the hysteresis loss is not always constant. Further, the technique of Patent Document 5 does not disclose an effective method for reducing eddy current loss, which is a problem in a high frequency range.

  The object of the present invention is to realize the low iron loss not only in the low frequency range but also in the high frequency range by a method different from the addition of the specific resistance increasing element and the reduction of the plate thickness in view of the above-mentioned problems of the prior art. It is to provide a grain-oriented electrical steel sheet.

  The inventors diligently studied a method for improving the iron loss in the high frequency range without deteriorating the magnetic characteristics in the low frequency range. As a result, since the magnetic flux concentrates on the plate thickness surface layer portion in the high frequency range, it is effective to reduce the iron loss to make the crystal grains in the plate thickness surface layer portion fine. Since the reduction of iron loss is disadvantageous for reducing iron loss, it is effective to reduce the iron loss by making the crystal grains coarse, so these two technologies are combined to make the steel plate surface layer fine particles and the steel plate inside coarse. It has been found that it is effective to obtain a low iron loss in both low and high frequency regions by properly controlling the size of fine grains and coarse grains and the thickness of those areas. It was.

  In addition, by coarsening the crystal grains inside the plate thickness, the permeability in a relatively low magnetic field increases, and as a result, it is possible to suppress a decrease in the magnetic permeability due to the refinement of the crystal grains in the surface layer portion. It has been found that it has an advantageous effect on reducing iron loss from high frequency to high frequency range. That is, due to the coarsening of the crystal grains inside the plate thickness, the average permeability is improved in the low frequency range compared to the case where the entire plate thickness is reduced, and in the high frequency range, the inside of the plate thickness is magnetized to some extent. The fact that the crystal grain size inside the plate thickness is large has an advantageous effect on ensuring the permeability in the high frequency range, reduces the hysteresis loss in the high frequency range, and reduces the iron loss. did.

  The present invention developed on the basis of the above knowledge is a non-oriented electrical steel sheet containing Si of 4 mass% or less and having a crystal grain size smaller in the steel sheet surface layer than in the steel sheet. When the surface portion of the steel sheet whose diameter Dz is 1.5 times or less the minimum value Dmin is a fine grain region, and the remaining steel sheet is a coarse grain region, the thickness per one side of the fine grain region is all Non-directional electromagnetic wave with low iron loss in high frequency region, characterized in that it is 8-25% of plate thickness and average crystal grain size of fine grain region is 2-70% of that of coarse grain region It is a steel plate.

  ADVANTAGE OF THE INVENTION According to this invention, the non-oriented electrical steel sheet which improved the iron loss of a high frequency region can be provided cheaply, without impairing the iron loss in a low frequency region largely.

  The method for measuring the average crystal grain size in the present invention will be described with reference to FIG. FIG. 1 schematically shows a cross section of a steel sheet having a crystal grain size different from that of the surface layer portion (particle size of the surface layer portion <internal particle size). After revealing the grain boundaries in the sheet thickness section (plane perpendicular to the rolling surface) in the rolling direction of the steel sheet by etching or the like, a line segment parallel to the rolling surface is plotted in the sheet thickness direction on this section. The number of grain boundaries intersecting this line is measured at a certain position z, and the value (L / n) obtained by dividing the length L of the line segment by the number n is the average grain size Dz in the plate thickness direction z. And Such measurement is measured at intervals of 30 μm or less over the entire plate thickness direction, and the smallest average crystal grain size among Dz obtained in this way is defined as Dmin. When there is a difference in the size of crystal grains between the rolling direction and the direction perpendicular to the rolling direction (sheet width direction), the average of both is taken as the average crystal grain size.

Further, the thickness of the fine crystal grain region is determined by the following method. When the above-mentioned line segment parallel to the rolling surface is placed at the center of the plate thickness and the position z is gradually moved to the front and back plate surface sides, the average crystal grain size Dz is the first to be the minimum average crystal grain size Dmin. when 1.5 times the size of the distance from the front and back of the plate thickness surface of the (Dz = Dmin × 1.5) and a position and t _1, t ₂ respectively, the respective distances t _1, t ₂ from each surface of the steel sheet 1.5 times the area is defined as the fine crystal grain region, the plate thickness central portion t ₃ when left is defined as a large crystal grain region. The average crystal grain size of the fine crystal grain region and the coarse crystal grain region is defined as an average of Dz in each region.

Next, the reason for limiting the non-oriented electrical steel sheet according to the present invention will be described.
Fine grain region thickness: 8-25% of plate thickness per side
As described above, the fine crystal grain region of the steel plate surface layer portion is necessary to reduce eddy current loss due to magnetic flux concentrated on the steel plate surface in the high frequency region. In general, the high-frequency characteristics of non-oriented electrical steel sheets are often evaluated at a frequency of 400 Hz. To reduce iron loss at 400 Hz, the surface thickness of the steel sheet is about 1/5 of the thickness per side. In a fine crystal grain region, specifically, in a magnetic steel sheet having a thickness of 0.15 to 0.5 mm, it is necessary to form a fine crystal grain region that is 8 to 25% of the plate thickness per side. If the thickness of the fine crystal grain region is less than 8%, the effect of reducing the iron loss in the high frequency range is small. On the other hand, if the thickness exceeds 25%, the permeability is lowered or the iron loss in the low frequency range is increased. This is not preferable. The thickness of the fine crystal grain region is preferably adjusted within this range according to the frequency used and the required magnetic permeability level. A preferable range of the fine grain region is a range of 10 to 20% of the plate thickness per side.
In addition, it is preferable that this fine crystal grain area | region exists in the surface layer part of the front and back of a steel plate. Further, it is desirable that the thicknesses of the fine crystal grain regions on the front and back sides are uniform, but even if they are not uniform, a corresponding effect of reducing iron loss can be obtained if each is within the range of 8 to 25% of the plate thickness.

Average crystal grain size of fine crystal grain region: 2 to 70% of average crystal grain size of coarse crystal grain region
The fine grain region aims to reduce the eddy current loss concentrated near the surface of the steel plate in the high frequency region, while the coarse grain region prevents an increase in iron loss in the low frequency region. The object is to ensure a high magnetic permeability from the low frequency range to the high frequency range. Therefore, the crystal grains in the fine crystal grain region of the steel sheet surface layer portion are sufficiently finer than the crystal grains in the coarse crystal grain region in the central part of the steel plate. It is necessary to be 2 to 70% of the average crystal grain size of the region. When the average grain size of the fine crystal grain region in the surface layer part is excessively refined below 2% of the average grain size of the coarse crystal grain region, an increase in hysteresis loss becomes remarkable, and the iron loss improvement effect cannot be obtained. On the other hand, if the average grain size of the fine crystal grain region in the surface layer portion exceeds 70% of the average grain size of the coarse crystal grain region, the effect of reducing the eddy current loss in the steel plate surface layer portion cannot be obtained. The average crystal grain size of the fine crystal grain region is preferably 2 to 60%, more preferably 4 to 40% of the average crystal grain size of the coarse crystal grain region.

Si: 4 mass% or less
Si is an element effective for increasing specific resistance and reducing eddy current loss. However, when the plate thickness is reduced to reduce eddy current loss, there is no need to add it. On the other hand, when trying to reduce the iron loss in a higher frequency range, it is preferable to add 2 mass% or more of Si according to the frequency range to be used. However, if the addition amount of Si exceeds 4 mass%, rolling becomes difficult, so the amount is limited to 4 mass% or less.

  In the present invention, components other than Si are not particularly limited, and can be added according to required characteristics. For example, a specific resistance increasing element such as Mn, Cr, Al or the like may be not added to increase saturation magnetization, or conversely, from the viewpoint of placing importance on high-frequency iron loss. When adding, it is preferable that they are the range of Mn: 0.1-4mass%, Cr: 0.05-10mass%, Al: 0.1-4mass%. In addition, in order to refine the crystal grains in the surface layer of the steel sheet, an element having a grain refinement effect such as Nb, P, S, Se, N, Sn, Sb, Ti, C or the like is used in the material slab manufacturing stage or You may make it add to a surface layer part in the stage of a slab or a steel plate. When added to the surface layer, Nb: 0.003-0.5 mass%, P: 0.02-1.0 mass%, S: 0.003-0.1 mass%, Se: 0.003-0.1 mass%, N: 0.003-0.02 mass%, Sn : 0.02 to 1 mass%, Sb: 0.01 to 0.2 mass%, Ti: 0.06 to 0.2 mass%, C: preferably 0.006 to 0.5 mass%. However, with respect to the coarse crystal grain region at the center of the plate thickness, Nb, S, Se, N, Ti, and C are each preferably 0.0050 mass% or less from the viewpoint of avoiding adverse effects on the hysteresis loss due to precipitates.

  The method for producing the electrical steel sheet according to the present invention, that is, the method for obtaining a fine crystal grain region having an appropriate thickness in the surface layer portion is not particularly limited. Casting slabs with different surface layer and internal components using clad technology, etc., and refining the crystal grains in the surface layer part, carburizing the surface of the slab before hot rolling and the steel sheet before annealing A method of refinement of surface layer grains using α-γ transformation, nitriding treatment or plating / diffusion treatment on the steel plate surface layer after final cold rolling, A method of refining grains can be used.

C: 0.003mass%, Si: 2.0mass%, Mn: 0.3mass%, Al: 0.3mass%, steel slab consisting of Fe and unavoidable impurities in the balance is manufactured by continuous casting. After heating, a hot rolled steel sheet having a thickness of 2.0 mm was obtained by hot rolling. In the above slab heating, carburizing treatment is performed, C is infiltrated from the slab surface to various depths, and the surface layer portion of the steel sheet is finely formed with various thicknesses by using α-γ transformation in the subsequent hot rolling. Turned into. The hot-rolled steel sheet is then pickled, cold-rolled to a thickness of 0.35 mm, and then subjected to decarburization annealing in a 25% H ₂ -75% N ₂ atmosphere. Annealing was performed. A total of 500 g of Epstein specimens were collected from the various steel plates obtained as described above, 250 g from each of the rolling direction and the direction perpendicular thereto, and subjected to the Epstein magnetic test.

The results of the Epstein magnetic test are shown in Table 1. The non-oriented electrical steel sheet of the present invention has an iron loss value that is almost equivalent in the low frequency range as compared with a comparative material that does not have a fine grain region in the surface layer portion, but is about 10% or more lower in the high frequency range. Loss values are obtained. The iron loss improvement rate in Table 1 is the following formula when the iron loss value of No. 1 or No. 2 in Table 1 is A and the iron loss value of No. 3 to 19 is B:
Iron loss reduction rate (%) = (A−B) / A × 100
The iron loss reduction rate given by is calculated for each iron loss value of W _15/50 , W _13/100 , W _10/400 , W _{5 / 1k} , W _{5 / 2k} , and shows the average value of them is there.

C: 0.003mass%, Si: 3.0mass%, Mn: 0.3mass%, Al: 0.3mass%, when the molten steel consisting of Fe and inevitable impurities is continuously cast into a steel slab, the slab is added by the wire addition method. Nb was concentrated in the surface layer portion of. This slab is reheated to 1200 ° C and hot-rolled to obtain a hot-rolled steel sheet with a thickness of 1.8 mm. Then, the hot-rolled steel sheet is subjected to hot-rolled sheet annealing, pickling, and then cold-rolled. After forming a cold rolled steel sheet having a thickness of 0.15 mm, final annealing was performed at 850 ° C. for 1 minute in a 25% H ₂ -75% N ₂ atmosphere. From the steel plate thus obtained, a total of 500 g of Epstein test specimens were collected from the rolling direction and the right-angled direction, 250 g each, and subjected to the Epstein magnetic test.

The results of the Epstein magnetic test are shown in Table 2. The non-oriented electrical steel sheet of the present invention has a low iron loss value in the high frequency range as compared with the comparative material, and the iron loss deterioration in the low frequency range is small. The iron loss improvement rate in Table 2 is the following formula when the iron loss value of No. 21 or No. 22 in Table 2 is A and the iron loss value of No. 23 is B:
Iron loss reduction rate (%) = (A−B) / A × 100
The iron loss reduction rate given by is obtained for each iron loss value of W _15/50 , W _10/400 , and W _{5 / 1k} , and the average value is shown.

It is the figure which showed in the schematic diagram the measuring method of the average crystal grain size and thickness of the fine grain area | region and coarse grain area | region in the non-oriented electrical steel sheet of this invention.

Claims (1)

In a non-oriented electrical steel sheet containing 4 mass% or less of Si and having a crystal grain size smaller in the surface layer than in the steel sheet, the average grain size Dz in the thickness direction z is 1.5 times the minimum value Dmin. When the following steel plate surface layer portion is a fine crystal grain region and the remaining steel plate is a coarse crystal grain region, the thickness per one side of the fine crystal grain region is 8 to 25% of the total plate thickness, and A non-oriented electrical steel sheet having a low iron loss in a high frequency region, characterized in that an average crystal grain size in a fine grain region is 2 to 70% of that in a coarse grain region.

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