WO2025243610A1 - Transformer iron core and transformer provided with same - Google Patents

Abstract

Provided is a transformer iron core having an exceptional effect for improving a building factor.　This transformer iron core is obtained using a grain-oriented electromagnetic steel sheet, wherein the grain-oriented electromagnetic steel sheet is configured to have a difference Δλp−p(−) of 3.0 ppm or less between the magnetorestriction amplitude λp−p(B−) when compressive stress of 0.3 kgf/mm² is applied in a steel sheet rolling direction and the magnetorestriction amplitude λp−p(A−) when compressive stress of 0.3 kgf/mm² is applied in the steel sheet rolling direction under an Ar atmosphere after strain-relief annealing is implemented for 3 hours at 800°C.

Description

Transformer core and transformer including the same

The present invention relates to a transformer core and a transformer equipped with the same.

Grain-oriented electromagnetic steel sheets are used as the material for transformer cores. In these transformers, the heat loss (iron loss) that occurs when grain-oriented electromagnetic steel sheets are magnetized with AC current affects the efficiency of the transformer, so efforts are underway to develop grain-oriented electromagnetic steel sheets with low iron loss. Here, the iron loss of grain-oriented electromagnetic steel sheets is mainly composed of hysteresis loss and eddy current loss.

Methods that have been developed to improve hysteresis loss include highly orienting the (110) [001] orientation, known as the GOSS orientation, in the rolling direction of the steel sheet, and reducing impurities in the steel sheet. Furthermore, methods that have been developed to improve eddy current loss include increasing the electrical resistance of the steel sheet by adding Si, and applying coating tension in the rolling direction of the steel sheet. However, when pursuing even lower iron loss in grain-oriented electrical steel sheets, these methods have manufacturing limitations.

In response, magnetic domain refinement technology has been developed as a method for further reducing iron loss in grain-oriented electrical steel sheets. Magnetic domain refinement technology is a technique for introducing non-uniform magnetic flux through physical methods, such as forming grooves or introducing localized distortion, into steel sheets after final annealing or after the insulating coating has been baked. This refines the width of the 180° magnetic domains (main magnetic domains) formed along the rolling direction, making it possible to reduce iron loss, particularly eddy current loss, in grain-oriented electrical steel sheets.

For example, Patent Document 1 proposes a technology for improving iron loss from 0.80 W/kg or more to 0.70 W/kg or less by introducing linear grooves with a width of 300 μm or less and a depth of 100 μm or less into the surface of a steel sheet. Patent Document 2 also proposes a method for irradiating a plasma flame in the width direction of the steel sheet surface after secondary recrystallization to locally introduce thermal strain. This method improves iron loss (W _{17/50 ) to 0.680 W/kg when the steel sheet is excited at a maximum magnetic flux density of 1.7 T and a frequency of 50 Hz, when the magnetic flux density (B 8 )} of the steel sheet is 1.935 T when excited with a magnetizing force of 800 A/m.

The method of introducing linear grooves as disclosed in Patent Document 1 is called heat-resistant magnetic domain refinement because the magnetic domain refinement effect does not disappear even when stress relief annealing is performed after core forming. On the other hand, the method of introducing thermal strain as disclosed in Patent Document 2 is called non-heat-resistant magnetic domain refinement because the effect of introducing thermal strain is lost due to stress relief annealing.

The iron loss of grain-oriented electrical steel sheets is measured by leaving the sheet stationary and applying a sinusoidal excitation in the rolling direction. However, inside actual transformers, residual strain due to processing and magnetization behavior that is more complex than a sinusoidal wave are known to increase the transformer loss (transformer iron loss) beyond the iron loss of the grain-oriented electrical steel sheet used as the core material. This rate of increase is called the building factor (BF) (= transformer iron loss / iron loss of the grain-oriented electrical steel sheet used as material). Reducing the building factor is essential for manufacturing more efficient transformers.

Widely manufactured wound cores use the cut core method, in which grain-oriented electromagnetic steel sheets are sheared for each turn, resulting in a single layer, or multiple layers of these sheets stacked together, and then these single layers are stacked (wound) and inserted into a coil. Stacked cores, on the other hand, are made by stacking multiple single layers, each of which is made by shearing grain-oriented electromagnetic steel sheets and combining them in the same plane in a frame-like shape, or by stacking multiple such combinations. Therefore, in each of the single layers, the sheared ends of the grain-oriented electromagnetic steel sheets are joined via an air gap. Near such joints, magnetic flux transfer occurs in the lamination direction of the single layers when the core is excited, resulting in increased iron loss due to in-plane eddy current loss, which is known to be one of the factors behind the increase in the building factor.

Various methods for improving the building factor have been studied. For example, Patent Document 3 proposes a method of subjecting grain-oriented electrical steel sheets near the joint to a magnetic domain refinement process using distortion. Furthermore, Patent Document 4 proposes a method of tilting the joint in the winding direction to improve iron loss due to magnetic flux transfer.

Special Publication No. 6-22179 Japanese Unexamined Patent Publication No. 7-192891 Japanese Patent Application Laid-Open No. 2020-96100 Japanese Patent Application Laid-Open No. 2005-150507

While all of the above proposals are effective, they are insufficient in terms of the amount of improvement in the building factor, and further development of methods to improve the building factor is desired. In addition, the above proposals assume that additional processing is performed during the core design, which raises concerns that this could increase manufacturing costs by increasing manufacturing man-hours or changing the shear direction during manufacturing.

The present invention was made in consideration of the above circumstances, and aims to provide a transformer core that is highly effective in improving the building factor.

The inventors conducted extensive research to solve the above problems.

First, we compared and organized the iron loss measurement environments for grain-oriented electrical steel sheets that make up the building factor and transformer cores. To measure the iron loss of grain-oriented electrical steel sheets, the Epstein test and single sheet test (SST) are performed, as specified in JIS C 2550-1:2011 and JIS C 2556:2015. In both of these tests, measurements are performed by eliminating or negligibly reducing the processing strain introduced into the steel sheet during specimen processing. Furthermore, in both of these tests, the measurement frame is placed stationary and parallel to the ground.

Meanwhile, measurements of iron loss in transformer cores are affected by the following factors. The grain-oriented electromagnetic steel strips that form the material for transformer cores are slit and sheared. Furthermore, in large transformer cores, fixing holes are punched into some of the legs, and these processes create residual stress. Furthermore, iron loss in transformer cores is measured with the core standing upright, which creates a bending moment in the core.

From the above, it has been found that transformer cores experience greater compressive stress than the grain-oriented electromagnetic steel sheets that form their raw material. This compressive stress increases the iron loss of the grain-oriented electromagnetic steel sheets, and is therefore thought to increase the building factor. Based on the above findings, it has been newly discovered that it is possible to improve the iron loss of transformer cores by manufacturing the cores using grain-oriented electromagnetic steel sheets, which have excellent stress resistance.

The present invention was made based on the above findings.

That is, the gist and configuration of the present invention are as follows.
[1] A transformer core made of grain-oriented electromagnetic steel sheets,
The grain-oriented electrical steel sheet is
Magnetostriction amplitude λp-p(B-) when a compressive stress of 0.3 kgf/ ^mm2 is applied in the rolling direction of the steel sheet,
A transformer core in which the difference Δλp-p(-) between the magnetostriction amplitude λp-p(A-) when a compressive stress of 0.3 kgf/ ^mm2 is applied in the rolling direction of the steel sheet after stress relief annealing at 800°C for 3 hours in an Ar atmosphere is 3.0 ppm or less.
[2] The grain-oriented electrical steel sheet is
The magnetostriction amplitude λp-p(B+) when a tensile stress of 0.3 kgf/ ^mm2 is applied in the rolling direction of the steel sheet,
The transformer core according to [1], wherein the difference Δλp-p(+) between the magnetostriction amplitude λp-p(A+) when a tensile stress of 0.3 kgf/ ^mm2 is applied in the rolling direction of the steel sheet after stress relief annealing at 800°C for 3 hours in an Ar atmosphere is 0.40 ppm or less.
[3] The transformer core according to [1] or [2], wherein the transformer core is a stacked core or a wound core.
[4] A transformer comprising the transformer core according to any one of [1] to [3].

The present invention makes it possible to provide a transformer core that is highly effective in improving the building factor.

According to the present invention, by manufacturing a transformer core using grain-oriented electromagnetic steel sheets with excellent stress resistance, the building factor of the transformer core can be improved. According to the present invention, by manufacturing a transformer core using grain-oriented electromagnetic steel sheets with excellent stress resistance, it is possible to suppress increases in iron loss and noise due to compressive stress caused by processing distortion and bending moment during erection, and it is possible to provide a transformer core with low iron loss and low noise.

FIG. 1 is a schematic diagram showing the structure of a transformer core. FIG. 2 is a diagram showing the relationship between λp-p(B-) and the building factor (BF). FIG. 3 is a diagram showing the relationship between λp-p(-) and the building factor (BF). FIG. 4 is a diagram showing the relationship between λp-p(B-) and transformer noise. FIG. 5 is a diagram showing the relationship between λp-p(-) and transformer noise. FIG. 6 is a diagram showing the relationship between λp-p(+) and the building factor (BF). FIG. 7 is a diagram showing the relationship between λp-p(+) and transformer noise.

Below, we will explain the experimental results that led to the completion of this invention.

(Experiment 1)
A 0.20 mm thick grain-oriented electrical steel strip manufactured by a general manufacturing process using a steel slab having the chemical composition shown in Table 1 was used as a test material.

 

For the above steel strip, during the tensile insulation coating baking process, a laser device was placed on the furnace outlet side to emit a laser beam with a top-hat energy distribution. The laser beam was irradiated in stripes parallel to the rolling direction of the steel strip, partially modifying the tensile insulation coating. As a result, a stripe-like structure (hereinafter simply referred to as "streaks") modified by laser beam irradiation was created in the tensile insulation coating. Steel strips (grain-oriented electrical steel sheets that will become the core material, hereinafter also referred to as "raw electrical steel sheets") were prepared with various variations in the spacing between these stripes (the distance between stripes).

Test pieces measuring 280 mm in the rolling direction and 100 mm in the sheet width direction were cut out from each of these material electrical steel sheets, and the iron loss (single sheet iron loss: W _17/50 ) of the material electrical steel sheets was measured using the single sheet magnetic measurement method described in JIS C 2556: 2015. Here, W _17/50 refers to the heat loss when a single sheet test piece of the material electrical steel sheet was AC magnetized in the longitudinal direction of the test piece at a magnetic flux density of 1.7 T and a frequency of 50 Hz.

Furthermore, the magnetostriction amplitude (λp-p(B-)) was measured when a compressive stress of 0.3 kgf/ ^mm2 was applied to the same test piece in the rolling direction of the steel sheet. Specifically, the test piece was subjected to AC excitation at a magnetic flux density of 1.5 T and a frequency of ⁵⁰ Hz, and the amplitude (λp-p(B-)) of the vibration displacement (magnetostriction) of the steel sheet in the rolling direction of the steel sheet was measured in a state where a compressive stress of 0.3 kgf/mm2 was applied to the test piece parallel to the rolling direction of the steel sheet.

Next, the same test specimen was subjected to strain relief annealing, and then the magnetostriction amplitude (λp-p(A-)) was measured when a compressive stress of 0.3 kgf/ ^mm2 was applied in the rolling direction of the steel sheet. Specifically, the test specimen was subjected to heat treatment at 800°C for 3 hours in an Ar atmosphere as strain relief annealing. Thereafter, the test specimen was cooled to room temperature. At this time, the cooling rate was made sufficiently slow so that no cooling strain remained. The cooling rate was, for example, 80°C/h. Thereafter, the test specimen that had been subjected to the strain relief annealing was subjected to AC excitation at a magnetic flux density of 1.5 T and a frequency of 50 Hz while a compressive stress of ^0.3 kgf/mm2 was applied parallel to the rolling direction of the steel sheet, and the amplitude (λp-p(A-)) of the vibration displacement (magnetostriction) of the steel sheet in the rolling direction of the steel sheet was measured.

Next, stacked cores, as shown in the schematic diagram in Figure 1(a), were fabricated using the above-mentioned electrical steel sheets. Specifically, the steel strips were cut into 100 mm-wide beveled bars, and several of these beveled bars were combined in a frame-like pattern on the same plane to form a single layer. These single layers were then stacked to a thickness of 30 mm to create a three-phase, three-limbed stacked core with a height of 420 mm and a total width of 420 mm. Each of the fabricated stacked cores was then wound with 50 turns on both the primary and secondary sides of each leg, and the iron loss characteristics (transformer iron loss) were measured when the magnetic flux density in the core legs was 1.7 T at a frequency of 50 Hz. The no-load loss at 1.7 T and 50 Hz was measured using a wattmeter. The building factor (BF) of each stacked core transformer was calculated from the ratio of this transformer iron loss to the single-plate iron loss described above. Furthermore, each stacked core transformer was excited in a soundproof room under conditions of a maximum magnetic flux density Bm = 1.7 T and a frequency of 50 Hz, and the noise level (dBA) (transformer noise) was measured using a sound level meter. Specifically, for each stacked core transformer, measurements were taken at eight points surrounding the stacked core transformer while it was excited, at a position half the height of the stacked core and 30 cm from the surface of the stacked core transformer, and the average value was taken as the transformer noise.

The measurement results of the building factor and transformer noise described above are shown in Figures 2 to 5. Figure 2 shows the relationship between the building factor (BF) and the magnetostriction amplitude λp-p(B-) measured on the base electrical steel sheet before stress relief annealing. Figure 3 shows the relationship between the building factor (BF) and the change in magnetostriction amplitude Δλp-p(-) of the base electrical steel sheet before and after stress relief annealing [i.e., the difference between λp-p(B-) measured on the base electrical steel sheet before stress relief annealing and λp-p(A-) measured on the base electrical steel sheet after stress relief annealing]. Figure 4 shows the relationship between the magnetostriction amplitude λp-p(B-) measured on the base electrical steel sheet before stress relief annealing and the transformer noise. Figure 5 shows the relationship between the change in magnetostriction amplitude Δλp-p(-) of the base electrical steel sheet before and after stress relief annealing and the transformer noise.

As shown in Figures 3 and 5, significant improvements in BF and transformer noise were confirmed in the region where Δλp-p(-) was 3.0 ppm or less. On the other hand, as shown in Figures 2 and 4, the impact of λp-p(B-) on BF and transformer noise could not be fully determined. In other words, no clear correlation was observed between λp-p(B-), BF, and transformer noise.

While the reason for this trend is still unclear, the inventors speculate as follows: Grain-oriented electrical steel sheets have a texture in which the Goss orientation is highly concentrated in the rolling direction, resulting in a magnetic domain structure with a magnetization component in the rolling direction. When compressive stress is applied in the rolling direction, the magnetic anisotropy in the rolling direction decreases due to the magnetoelastic effect, and when a certain level of compressive stress is applied, the magnetic domain structure changes to one with a magnetization component in the thickness direction (stress pattern). When grain-oriented electrical steel sheets are magnetized in the rolling direction, the amount of expansion and contraction of the steel sheet is positively correlated with the volume of magnetic domains rotated by magnetization during the magnetization process. Therefore, in the stress pattern magnetic domain structure, the amplitude of magnetostriction increases sharply, and at the same time, the magnetic permeability in the rolling direction also decreases, resulting in increased iron loss. The compressive stress that changes into a stress pattern varies depending on innate factors inherent to the material, such as the crystal orientation and texture of the steel sheet, as well as acquired factors such as the state of the tensile coating and residual stress due to processing distortion and cooling distortion. Of these, innate factors change little within the raw electrical steel sheet during the transformer core manufacturing process, while acquired factors change within the raw electrical steel sheet due to manufacturing variations. Therefore, λp-p(B-) is thought to refer to the stress-resistance performance of the raw electrical steel sheet when both factors are taken into account. This is thought to be why no clear correlation was observed with the characteristics of transformers manufactured using many raw electrical steel sheets. On the other hand, of the above factors, the state of the tensile coating and residual stress, which have a significant acquired effect, can be removed by stress relief annealing (SRA). In other words, λp-p(A-) is thought to refer to the innate stress-resistance performance of the raw electrical steel sheet. Therefore, Δλp-p(-) is thought to indicate the susceptibility of the grain-oriented electrical steel sheet used as the raw material to acquired factors.

Based on the above, we believe that by manufacturing transformer cores using grain-oriented electrical steel sheets with a Δλp-p(-) of 3.0 ppm or less, it is possible to suppress deterioration in building factor and noise caused by disturbance factors such as bending moments during erection and processing distortion during transformer manufacturing. It is preferable to manufacture transformer cores using grain-oriented electrical steel sheets with a Δλp-p(-) of 2.5 ppm or less, and more preferably Δλp-p(-) of 2.0 ppm or less. The lower limit of Δλp-p(-) is not particularly limited, but an example is 0.1 ppm.

(Experiment 2)
Next, a 0.27 mm thick grain-oriented electrical steel strip manufactured using a steel slab with the chemical composition shown in Table 1 above and a standard manufacturing process was used as the test material. For this steel strip, a grinding device was installed on the inlet side of the furnace during the tensile insulating coating baking process, and fine grooves were machined parallel to the rolling direction on the coating base, followed by the formation of a tensile insulating coating. The coating base is a base coating such as a forsterite coating or other ceramic coating; in this experiment, a forsterite coating was used. Steel strips (raw electrical steel sheets) with various groove spacings (distance between grooves) were prepared.

Test pieces measuring 280 mm in the rolling direction and 100 mm in the sheet width direction were cut out from each of these material electrical steel sheets, and the iron loss (single sheet iron loss: W _17/50 ) of the material electrical steel sheets was measured using the single sheet magnetic measurement method described in JIS C 2556:2015.

Furthermore, in the same manner as in Experiment 1, a compressive stress of 0.3 kgf/mm ² was applied to the same test piece in the rolling direction of the steel sheet, and the magnetostriction amplitude (λp-p(B-)) was measured.

Next, the magnetostriction amplitude (λp-p(B+)) was measured when a tensile stress of 0.3 kgf/ ^mm2 was applied to the same test piece in the rolling direction of the steel sheet. Specifically, the test piece was subjected to AC excitation at a magnetic flux density of 1.5 T and a frequency of ⁵⁰ Hz, and the amplitude (λp-p(B+)) of the vibration displacement (magnetostriction) of the steel sheet in the rolling direction of the steel sheet was measured in a state where a tensile stress of 0.3 kgf/mm2 was applied to the test piece parallel to the rolling direction of the steel sheet.

Next, the same test specimens were subjected to strain relief annealing in the same manner as in Experiment 1. Then, for the test specimens after strain relief annealing, the magnetostriction amplitude (λp-p(A-)) was measured when a compressive stress of 0.3 kgf/ ^mm2 was applied in the rolling direction of the steel sheet. Next, for the test specimens after strain relief annealing, the magnetostriction amplitude (λp-p(A+)) was measured when a tensile stress of 0.3 kgf/ ^mm2 was applied in the rolling direction of the steel sheet. Specifically, the test specimens were subjected to AC excitation at a magnetic flux density of 1.5 T and a frequency of 50 Hz in a state where a tensile stress of 0.3 kgf/ ^mm2 was applied parallel to the rolling direction of the steel sheet, and the amplitude (λp-p(A+)) of the vibration displacement (magnetostriction) of the steel sheet in the rolling direction of the steel sheet was measured.

Next, a three-phase, three-limbed wound core, as shown in the schematic diagram in Figure 1(b), was fabricated using the raw material electrical steel sheet from the above steel strip, which had a Δλp-p(-) of 2 ppm. Specifically, the wound core was fabricated by slitting the steel strip into 100 mm widths and winding it in the rolling direction to form a single layer. Multiple such layers were stacked to a thickness of 30 mm to fabricate a three-phase, three-limbed wound core with a height of 300 mm and an overall width of 210 mm. The wound core was fabricated as a Unicore type with two 45-degree bends at the corners. Each wound core was then wound with 50 turns on both the primary and secondary sides, and the iron loss characteristics (transformer iron loss) were measured at a frequency of 50 Hz when the magnetic flux density in the core legs was 1.7 T. The iron loss characteristics at 1.7 T and 50 Hz were measured using a wattmeter to measure no-load loss. The building factor (BF) of each wound core transformer was calculated from the ratio of this transformer iron loss to the single-plate iron loss described above. Furthermore, the noise level (dBA) of each wound core transformer was measured in the same manner as in Experiment 1.

The measurement results of the building factor and transformer noise mentioned above are shown in Figures 6 and 7. Figure 6 shows the relationship between the change in magnetostriction amplitude Δλp-p(+) of the raw electrical steel sheet before and after stress relief annealing [i.e., the difference between Δλp-p(B+) measured for the raw electrical steel sheet before stress relief annealing and Δλp-p(A+) measured for the raw electrical steel sheet after stress relief annealing] and the building factor (BF). Figure 7 shows the relationship between the change in magnetostriction amplitude Δλp-p(+) of the raw electrical steel sheet before and after stress relief annealing and the transformer noise.

As shown in Figures 6 and 7, significant improvements in BF and transformer noise were confirmed in the region where Δλp-p(+) was 0.40 ppm or less.

The reason for this phenomenon is unclear, but the inventors speculate as follows. Similar to the above-mentioned Δλp-p(-), Δλp-p(+) is thought to indicate the susceptibility of the grain-oriented electrical steel sheet used as the raw material to acquired factors (acquired tensile stress). Furthermore, when compressive stress due to bending moment is applied inside the transformer, a similar level of tensile stress is applied to the backside of the raw material electrical steel sheet. Therefore, in grain-oriented electrical steel sheets with a large Δλp-p(+), a large difference in characteristics occurs between the areas where compressive stress is applied (compressive stress areas) and the areas where tensile stress is applied (tensile stress areas). In other words, grain-oriented electrical steel sheets with a large Δλp-p(+) are thought to emit more noise due to increased localized iron loss caused by magnetic flux concentration in the tensile stress areas with higher magnetic permeability, and complex core vibration caused by non-uniform vibration.

Based on the above, we believe that by manufacturing transformer cores using grain-oriented electrical steel sheets with a Δλp-p(+) of 0.40 ppm or less, magnetic flux concentration in tensile stress areas and uneven vibration can be suppressed, thereby suppressing deterioration of the building factor and noise. It is preferable to manufacture transformer cores using grain-oriented electrical steel sheets with a Δλp-p(+) of 0.30 ppm or less, more preferably a Δλp-p(+) of 0.25 ppm or less, and even more preferably a Δλp-p(+) of 0.20 ppm or less. There are no particular restrictions on the lower limit of Δλp-p(+), but one example is 0.01 ppm.

A preferred embodiment of the present invention is described in detail below. However, the present invention is not limited to the configuration disclosed in this embodiment, and various modifications are possible without departing from the spirit of the present invention.

[Grain-oriented electrical steel sheet]
First, we will explain the grain-oriented electrical steel sheet that serves as the material for the transformer core of the present invention. In the present invention, the composition of the slab for the grain-oriented electrical steel sheet may be any composition that allows secondary recrystallization to occur. Furthermore, when an inhibitor is used, for example, when an AlN-based inhibitor is used, appropriate amounts of Al and N are contained, and when an MnS/MnSe-based inhibitor is used, appropriate amounts of Mn and Se and/or S are contained. Of course, both inhibitors may be used in combination. In this case, the preferred contents of Al, N, S, and Se are, respectively, 0.010 to 0.065 mass% Al, 0.0050 to 0.0120 mass% N, 0.005 to 0.030 mass% S, and 0.005 to 0.030 mass% Se.

Furthermore, the present invention can also be applied to grain-oriented electrical steel sheets in which the Al, N, S, and Se contents are limited and no inhibitors are used. In this case, it is preferable to limit the Al, N, S, and Se contents to Al: less than 0.010 mass%, N: less than 0.0050 mass%, S: less than 0.0050 mass%, and Se: less than 0.0050 mass%, respectively.

This section provides a detailed description of the basic components and optional additive components of slabs for grain-oriented electrical steel sheets.

C: 0.08 mass% or less C is added to improve the hot-rolled sheet structure. However, if the C content exceeds 0.08 mass%, it becomes difficult to decarburize to 50 mass ppm or less, at which point magnetic aging does not occur during the manufacturing process. Therefore, the C content is preferably set to 0.08 mass% or less. Furthermore, since secondary recrystallization occurs even in materials that do not contain C, no lower limit for the C content is particularly set. In other words, the C content may be 0 mass%.

Si: 2.0 to 8.0% by mass
Si is an element effective in increasing the electrical resistance of steel and improving iron loss. When the Si content is 2.0 mass% or more, the iron loss reduction effect is further enhanced. On the other hand, when the Si content is 8.0 mass% or less, it becomes easier to suppress deterioration in workability and threading ability, and also to suppress deterioration in magnetic flux density. Therefore, it is preferable that the Si content be in the range of 2.0 to 8.0 mass%.

Mn: 0.005 to 1.0% by mass
Mn is an element necessary for improving hot workability. When the Mn content is 0.005% by mass or more, this effect is easily obtained. On the other hand, when the Mn content is 1.0% by mass or less, it is easy to suppress a decrease in the magnetic flux density of the product sheet. Therefore, the Mn content is preferably in the range of 0.005 to 1.0% by mass.

The above-mentioned slab for grain-oriented electrical steel sheet preferably has the above-mentioned components as its basic components. In addition to the above-mentioned basic components, the slab can optionally contain the following elements. The following elements are effective in improving magnetic properties.

One or more selected from the following: Ni: 0.03-1.50 mass%, Sn: 0.01-1.50 mass%, Sb: 0.005-1.50 mass%, Cu: 0.03-3.0 mass%, P: 0.03-0.50 mass%, Mo: 0.005-0.10 mass%, and Cr: 0.03-1.50 mass%

Ni is an element that is effective in improving the hot-rolled sheet structure and enhancing magnetic properties. When the Ni content is 0.03 mass% or more, the effect of improving magnetic properties is further enhanced. When the Ni content is 1.50 mass% or less, secondary recrystallization can be prevented from becoming unstable, making it easier to reduce the risk of deterioration of the magnetic properties of the finished sheet. Therefore, when Ni is contained, the Ni content is preferably in the range of 0.03 to 1.50 mass%.

Furthermore, Sn, Sb, Cu, P, Mo, and Cr are also elements that improve magnetic properties, and when the content of each element is above the lower limit of the above-mentioned range, the effect of improving magnetic properties is more likely to be achieved. On the other hand, when the content of each component is below the upper limit of the above-mentioned range, the risk of suppressing the growth of secondary recrystallized grains is more likely to be reduced, and deterioration of magnetic properties is more likely to be suppressed. Therefore, when Sn, Sb, Cu, P, Mo, and Cr are contained, it is preferable that the content of each of the above elements is within the above-mentioned range.

The remainder other than the above components consists of Fe and unavoidable impurities.

A slab having the above-mentioned chemical composition is hot-rolled and then hot-rolled sheet annealed. It is then cold-rolled once or twice to finish it into a steel strip of the final thickness. The steel strip is then decarburized and annealed, coated with an annealing separator, wound into a coil, and subjected to final annealing for secondary recrystallization. After final annealing, the steel strip is subjected to flattening annealing to form an insulating coating (tensile insulating coating). Furthermore, in this embodiment, as described below, a process is performed to impart anisotropy to the coating tension imparted to the steel sheet by the tensile insulating coating.

In this embodiment, a magnetic domain refinement process may be included after the flattening annealing process, in which thermal distortion is formed on the surface of the grain-oriented electrical steel sheet (steel strip) by irradiating it with an energy beam. Alternatively, a magnetic domain refinement process such as electrolytic etching or groove formation on the steel sheet by laser irradiation may be included after the cold rolling process.

[Magnetostriction measurement method (Δλp-p(-), Δλp-p(+))]
The magnetostriction measurement method, which is essential in the present invention, will be described. Magnetostriction measurements in the present invention were performed using a TRS-200 manufactured by Toei Kogyo Co., Ltd. A test piece was sheared from the grain-oriented electrical steel sheet (steel strip) to a width of 100 mm in the sheet width direction and a length of 280 mm in the rolling direction. After attaching a mirror for measuring laser reflection, the test piece was set in the measurement frame of the device, and one side of the test piece was fixed with an air-pressure-operated clamp. After fixing, a glass plate was placed on the test piece to prevent buckling, and a specified compressive stress of 3.0 kgf/ ^mm2 was applied in the rolling direction of the steel sheet. In this state, the test piece was excited with an AC magnetic flux density of 1.5 T and a frequency of 50 Hz, and the vibration at that time was measured with a laser Doppler meter, and the measured value was taken as λp-p(B-). Next, a specified tensile stress of 3.0 kgf/ ^mm2 was applied in the rolling direction of the steel sheet. In this state, the test piece is subjected to AC excitation at a magnetic flux density of 1.5 T and a frequency of 50 Hz, and the vibration at this time is measured with a laser Doppler meter, and the measured value is taken as λp-p(B+).

The test specimens are then subjected to stress relief annealing. Strain relief annealing involves heat treatment in which the test specimens are held at 800°C in an Ar atmosphere for three hours. Magnetostriction measurements are then performed on the test specimens after stress relief annealing in the same manner as above, to obtain the magnetostriction amplitude λp-p(A-) when a specified compressive stress is applied, and the magnetostriction amplitude λp-p(A+) when a specified tensile stress is applied. Δλp-p(-) is then calculated from the difference between λp-p(B-) and λp-p(A-), and Δλp-p(+) is calculated from the difference between λp-p(B+) and λp-p(A+). Test specimens are taken from the leading and trailing ends of the raw electromagnetic steel sheet (coil) used to manufacture the transformer core, and the largest measured values for Δλp-p(-) and Δλp-p(+) are treated as the representative values for that coil.

In the present invention, there are no particular limitations on the method for adjusting Δλp-p(-) and Δλp-p(+). Examples include irradiating the tension insulating coating with a laser or electron beam in stripes to impart anisotropy to the coating tension in the rolling direction of the steel sheet; forming minute grooves or ridges on the surface to which the tension insulating coating is applied (the coating base on which the tension insulating coating is formed) to impart anisotropy to the tension in the rolling direction of the steel sheet; increasing the rigidity of the base steel by subjecting the top surface of the base steel to shot peening or laser peening before baking the tension insulating coating; or a suitable combination of these methods. Any of these methods may be used in the present invention.

The tension insulation coating is partially modified by irradiating it with a laser or electron beam in stripes parallel to the rolling direction of the steel plate, or by forming grooves or ridges parallel to the rolling direction of the steel plate in the coating base on which the tension insulation coating is formed, thereby making the coating tension imparted to the steel plate anisotropic. By making the coating tension imparted by the tension insulation coating anisotropic, the coating is less susceptible to the effects of subsequent cooling distortion and processing distortion. For example, when a laser or electron beam is irradiated onto a tension insulation coating, the irradiated area hardens (crystallization of the irradiated area progresses, increasing the Young's modulus). When a striped structure is formed in the tension insulation coating, for example by irradiating it with a laser or electron beam in stripes parallel to the rolling direction of the steel plate to partially modify the tensile insulation coating, a difference in Young's modulus occurs between the rolling direction of the steel plate and the direction perpendicular to the rolling direction of the steel plate. As a result, a stronger coating tension is generated in the rolling direction of the steel plate. This improves compressive stress resistance in the rolling direction of the steel sheet and suppresses deterioration of magnetostriction when compressive stress is applied. Furthermore, since the change in magnetostriction when tensile stress is applied gradually decreases as the tensile stress increases, by generating stronger coating tension in the rolling direction of the steel sheet as described above, it is also possible to suppress changes in magnetostriction when tensile stress is applied.

Below, we explain an example of laser or electron beam irradiation conditions when adjusting the Δλp-p(-) and Δλp-p(+) of a steel strip by partially modifying the tensile insulation coating using laser or electron beam irradiation.

(Laser or electron beam output: 50W or more and 5000W or less)
The higher the output of the laser or electron beam, the more the crystallization of the coating is promoted, the harder the coating becomes, and the stronger the anisotropy of the coating tension becomes, so the output of the laser or electron beam is preferably 50 W or more. On the other hand, if the output of the laser or electron beam is too high, excessive energy is input into the coating, causing damage to the coating. From the above perspectives, the output of the laser or electron beam is preferably 50 W or more and 5000 W or less.

(Laser or electron beam spot diameter: 300 μm or less)
The smaller the spot diameter of the laser or electron beam, the more localized the coating modification can be, which is preferable. Therefore, in the present invention, the spot diameter of the laser or electron beam is preferably 300 μm or less. The spot diameter of the laser or electron beam is more preferably 280 μm or less, and even more preferably 260 μm or less. In the present invention, the spot diameter refers to the full width at half maximum of the beam profile obtained by the slit method using a slit with a width of 30 μm. As described above, the smaller the spot diameter of the laser or electron beam, the more preferable, and there is no lower limit to the spot diameter, but as an example, the spot diameter may be 2 μm or more.

(streak formation interval)
By irradiating a laser or electron beam in stripes parallel to the rolling direction of the steel strip, a streak-like structure (streaks) is formed in the tension insulation coating, modified by the laser or electron beam irradiation. In this case, a narrower spacing between the streaks is preferable because it increases the anisotropy of the coating tension. However, if the spacing between the streaks is excessively narrow, adjacent laser beams or electron beams interfere with each other, introducing thermal strain into the steel sheet or damaging the coating due to excessive heat input, resulting in deterioration of magnetic properties. Therefore, it is preferable that the lower limit of the spacing between the streaks be approximately the same as the spot diameter of the laser or electron beam. As an example, the spacing between the streaks is preferably 0.1 mm or more. On the other hand, if the spacing between the streaks is too large, anisotropy in the coating tension will not occur, so the spacing between the streaks is preferably 2 mm or less.

(Scanning speed: 5 to 400 m/s)
The scanning speed of the laser or electron beam is preferably slower because the amount of heat incident per unit length of the coating can be increased as the scanning speed becomes slower. However, if the scanning speed is too slow, the processing area per unit time decreases, resulting in a decrease in production efficiency. Therefore, the scanning speed is preferably 5 m/s or more. Furthermore, if the scanning speed is too fast, a power supply capacity is required to provide the heat input necessary for coating modification, leading to an increase in the size of the equipment. Therefore, the scanning speed is preferably 400 m/s or less.

Below, we will explain an example of groove or ridge formation conditions when adjusting the Δλp-p(-) and Δλp-p(+) of a steel strip by forming grooves parallel to the rolling direction of the steel sheet on the base material on which the tensile insulating coating is formed using hairline processing or other methods, or by forming ridges using powder bed additive manufacturing or other methods. Grooves or ridges can be formed, for example, by installing a grinding device or additive manufacturing device on the inlet side of the furnace during the baking process of the tensile insulating coating, and processing the base material to form grooves or ridges parallel to the rolling direction.

(Interval of furrow or ridge formation)
The narrower the groove spacing (the distance between grooves) or ridge spacing (the distance between ridges), the greater the anisotropy of the coating tension. However, if the groove spacing is too narrow, the amount of undercoating grinding increases, resulting in a deterioration of insulation properties. Furthermore, if the ridge spacing is too narrow, the total thickness of the tensile insulating coating and undercoating increases, resulting in a decrease in the space factor. Therefore, the lower limit of the groove or ridge spacing is preferably set to a value approximately equal to the width of the groove or ridge. As an example, the groove or ridge spacing is preferably 0.1 mm or more. On the other hand, if the groove or ridge spacing is too large, the coating tension anisotropy will not occur, so the groove or ridge spacing is preferably 2 mm or less.

(Width of furrow or ridge)
The narrower the groove or ridge width, the greater the anisotropy of the film tension, which is preferable. Therefore, in this embodiment, the groove or ridge width is preferably 300 μm or less. The groove or ridge width is more preferably 280 μm or less, and even more preferably 260 μm or less. On the other hand, if the groove or ridge width is excessively narrow, the anisotropy of the film tension decreases, so the groove or ridge width is preferably 2 μm or more.

(Depth of groove or height of ridge)
The greater the groove depth (groove depth) or ridge height (ridge height), the greater the anisotropy of the coating tension, which is preferable. On the other hand, excessive groove depth is undesirable because the grooves reach the base steel and degrade the magnetic properties. Furthermore, excessive ridge height is undesirable because it exceeds the thickness of the tension insulating coating, leading to deterioration of the insulation properties and space factor. Therefore, it is preferable that the groove depth is less than the thickness of the base coating and the ridge height is less than the thickness of the tensile insulating coating. More preferably, the groove depth is less than 80% of the thickness of the base coating and the ridge height is less than 80% of the thickness of the tensile insulating coating.

Grain-oriented electrical steel sheets that have been treated as described above to impart anisotropy to the coating tension imparted by the tensile insulating coating may then be subjected to a non-heat-resistant magnetic domain refinement treatment. The non-heat-resistant magnetic domain refinement treatment can be carried out, for example, by irradiating the tensile insulating coating with a laser or electron beam under known conditions.

In addition, in the present invention, known methods for manufacturing grain-oriented electrical steel sheets can be used as appropriate for the steps and manufacturing conditions other than those described above.

[Transformer core]
The transformer core of the present invention is constructed using grain-oriented electromagnetic steel sheets having a Δλp-p(-) of 3.0 ppm or less as the material electromagnetic steel sheets constituting the transformer core. Examples of the transformer core include stacked cores and wound cores. Examples of stacked cores include the three-phase, three-legged stacked core shown in FIG. 1(a). Examples of wound cores include the Unicore shown in FIG. 1(b) and the Trunco shown in FIG. 1(c). The wound core may also be a Duocore. These cores can be manufactured by, for example, known manufacturing methods. For example, a stacked core can be manufactured by stacking material electromagnetic steel sheets slit at an oblique angle and combining them in a frame shape. A Unicore can be manufactured by stacking material electromagnetic steel sheets that have been pre-bent at the corners of the core. A Trunco can be manufactured by winding and stacking material electromagnetic steel sheets, and then pressing the corners of the core to a predetermined curvature to form a rectangular shape.

An example of a method for manufacturing a transformer core according to the present invention includes the steps of measuring λp-p(B-) and λp-p(A-) of grain-oriented electromagnetic steel sheets and calculating Δλp-p(-) from the difference between λp-p(B-) and λp-p(A-), selecting grain-oriented electromagnetic steel sheets whose calculated Δλp-p(-) is within a predetermined range, and fabricating a transformer core using the selected grain-oriented electromagnetic steel sheets. The manufacturing method may also include the steps of measuring λp-p(B+) and λp-p(A+) of grain-oriented electromagnetic steel sheets and calculating Δλp-p(+) from the difference between λp-p(B+) and λp-p(A+), selecting grain-oriented electromagnetic steel sheets whose calculated Δλp-p(+) is within a predetermined range, and fabricating a transformer core using the selected grain-oriented electromagnetic steel sheets. In this case, in the step of selecting the grain-oriented electrical steel sheet, a grain-oriented electrical steel sheet can be selected in which the calculated Δλp-p(-) is within a predetermined range and the calculated Δλp-p(+) is within a predetermined range. Then, in the step of fabricating the transformer core, the transformer core can be fabricated using the grain-oriented electrical steel sheet selected in this manner.

Next, the present invention will be described in detail based on examples. The following examples are intended to illustrate preferred examples of the present invention, and are not intended to limit the scope of the present invention. Modifications may be made within the scope of the present invention, and such modifications are also within the technical scope of the present invention.

Grain-oriented electrical steel sheets were manufactured using a steel slab with the chemical composition shown in Table 2, using a standard manufacturing process. Before forming a tensile insulating coating on the grain-oriented electrical steel sheets, grooves were introduced parallel to the rolling direction of the steel sheet into the surface to which the tensile insulating coating was applied (the forsterite base material of the coating) using a grinding device, and steel strip A on which the tensile insulating coating was then formed was prepared as the test material. In addition, after forming a tensile insulating coating on the grain-oriented electrical steel sheets manufactured as described above, steel strip B (laser irradiated) and steel strip C (electron beam irradiated) were treated to partially modify the tensile insulating coating by irradiating them with a laser or electron beam parallel to the rolling direction of the steel sheet, and prepared as test materials.

 

For the above steel strip A, during the baking process of the tensile insulating coating, a grinding device was placed on the inlet side of the furnace, and grooves were machined into the coating base parallel to the rolling direction, after which the tensile insulating coating was formed. For the above steel strips B and C, during the baking process of the tensile insulating coating, a device was placed on the outlet side of the furnace to irradiate a laser beam or electron beam with a top-hat energy distribution. The laser beam or electron beam was then irradiated in stripes parallel to the rolling direction of the steel sheet, partially modifying the tensile insulating coating. In this way, a process was performed to impart anisotropy to the coating tension imparted by the tensile insulating coating. Steel strips with various groove or stripe spacings, laser or electron beam spot diameters, and output powers were prepared as base electrical steel sheets. By forming the grooves or stripes more densely and by making the stripes harder, the anisotropy of the coating tension is strengthened, and the coating tension in the rolling direction of the steel sheet is increased. Therefore, Δλp-p(-) and Δλp-p(+) can be adjusted by changing the spacing between the grooves or streaks, the spot diameter of the laser or electron beam, and the output. Furthermore, after the treatment described above to impart anisotropy to the coating tension imparted by the tension insulation coating, some steel strips were further subjected to a non-heat-resistant magnetic domain refinement treatment (DR).

Test pieces measuring 280 mm in the rolling direction and 100 mm in the sheet width direction were cut from each of these steel strips, and the iron loss (single sheet iron loss: W _17/50 ) of the raw electrical steel sheets was measured using the single sheet magnetic measurement method described in JIS C2556:2015. Furthermore, for the same test pieces, λp-p(B-), λp-p(B+), λp-p(A-), and λp-p(A+) were measured using the method described above. Δλp-p(-) was calculated from the difference between λp-p(B-) and λp-p(A-), and Δλp-p(+) was calculated from the difference between λp-p(B+) and λp-p(A+).

Next, using the above-mentioned raw material electromagnetic steel sheets, a three-phase three-legged stacked core was fabricated as shown in the schematic diagram of Figure 1(a). For the stacked core, a steel strip was cut into 100 mm wide bevel bars, and several of these bevel bars were combined in the same plane in a frame-like shape to form a single layer. Several of these single layers were stacked to a thickness of 30 mm to produce a three-phase three-legged stacked core with a height of 420 mm and an overall width of 420 mm. Furthermore, using the above-mentioned raw material electromagnetic steel sheets, a three-phase three-legged wound core was fabricated as shown in the schematic diagrams of Figures 1(b) and (c). For the wound core, a steel strip was slit into 100 mm wide pieces and wound in the rolling direction of the steel strip to form a single layer. Several of these single layers were stacked to a thickness of 30 mm to produce a three-phase three-legged wound core with a height of 300 mm and an overall width of 210 mm. Two types of wound cores were fabricated: one with arc-shaped core corners (Tranco (Figure 1(c))), and one with two 45-degree bends at the corners (Unicore (Figure 1(b))). The Tranco and some Unicores were subjected to stress relief annealing at 800°C for three hours in an Ar atmosphere. Because the effect of non-heat-resistant domain refinement is lost during stress relief annealing, only magnetic steel sheets that had not undergone non-heat-resistant domain refinement treatment were used for the Trancos. In Tables 3 to 7, if no non-heat-resistant domain refinement treatment was performed, "none" is listed in the DR (domain refinement treatment) column. If non-heat-resistant domain refinement treatment was performed using a laser, "laser" is listed in the DR column. If non-heat-resistant domain refinement treatment was performed using an electron beam, "electron beam" is listed in the DR column.

For each manufactured core, 50 turns of winding were applied to both the primary and secondary sides of each leg, and the iron loss characteristics (transformer iron loss) were measured when the magnetic flux density in the core leg section was 1.7 T at a frequency of 50 Hz. The no-load loss was measured using a wattmeter to determine the iron loss characteristics at 1.7 T and 50 Hz. The building factor (BF) of each transformer was calculated from the ratio of this transformer iron loss to the single-plate iron loss described above. Furthermore, each transformer was excited in a soundproof room as described above under conditions of maximum magnetic flux density Bm = 1.7 T and a frequency of 50 Hz, and the noise level: dBA (transformer noise) was measured using a sound level meter.

Tables 3 to 7 confirm that by satisfying the requirements of the present invention, a transformer core with excellent improvements in building factor can be obtained. By satisfying the requirements of the present invention, a transformer core with low iron loss and low noise can be obtained. Furthermore, it can be confirmed that even more excellent improvements can be achieved with transformer cores made from electrical steel sheets that have been subjected to a non-heat-resistant magnetic domain refinement process in addition to a process that imparts anisotropy to the coating tension imparted by the tensile insulating coating.

 

 

 

 

 

Claims (4)

A transformer core made of grain-oriented electromagnetic steel sheets,
The grain-oriented electrical steel sheet is
Magnetostriction amplitude λp-p(B-) when a compressive stress of 0.3 kgf/ ^mm2 is applied in the rolling direction of the steel sheet,
A transformer core in which the difference Δλp-p(-) between the magnetostriction amplitude λp-p(A-) when a compressive stress of 0.3 kgf/ ^mm2 is applied in the rolling direction of the steel sheet after stress relief annealing at 800°C for 3 hours in an Ar atmosphere is 3.0 ppm or less. The grain-oriented electrical steel sheet is
The magnetostriction amplitude λp-p(B+) when a tensile stress of 0.3 kgf/ ^mm2 is applied in the rolling direction of the steel sheet,
2. The transformer core according to claim 1, wherein the difference Δλp-p(+) between the magnetostriction amplitude λp-p(A+) when a tensile stress of 0.3 kgf/ ^mm2 is applied in the rolling direction of the steel sheet after stress relief annealing at 800°C for 3 hours in an Ar atmosphere is 0.40 ppm or less. The transformer core according to claim 1 or 2, wherein the transformer core is a stacked core or a wound core. A transformer comprising the transformer core according to any one of claims 1 to 3.