Transformer core and transformer equipped therewith

Abstract

We provide transformer cores that offer superior improvements in building factors. A transformer core made of grain-oriented electrical steel sheet, wherein the grain-oriented electrical steel sheet has a grain density of 0.3 kgf / mm² in the steel sheet rolling direction. 2 The magnetostriction amplitude λp-p(B-) when compressive stress is applied, and the steel sheet rolled in the rolling direction after strain relief annealing at 800°C for 3 hours in an Ar atmosphere, at a rate of 0.3 kgf / mm 2 A transformer core in which the difference Δλp-p(-) between the magnetostriction amplitude λp-p(A-) and the compressed stress applied is 3.0 ppm or less.

Description

[Technical Field]

[0001] This invention relates to a transformer core and a transformer equipped with the same. [Background technology]

[0002] Grain-oriented electrical steel sheets are used as the core material for transformers. In these transformers, the heat loss (iron loss) that occurs when grain-oriented electrical steel sheets are magnetized with alternating current affects the efficiency of the transformer, so the development of grain-oriented electrical steel sheets with low iron loss is underway. Here, the iron loss of grain-oriented electrical steel sheets mainly consists of hysteresis loss and eddy current loss.

[0003] Methods to improve hysteresis loss include highly orienting the (110)

[0001] orientation, known as the GOSS orientation, in the rolling direction of the steel sheet, and reducing impurities in the steel sheet. Furthermore, methods 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, these methods have manufacturing limitations when pursuing further reductions in iron loss in grain-oriented electrical steel sheets.

[0004] Therefore, as a method to further reduce iron loss in grain-oriented electrical steel sheets, magnetic domain subdivision technology has been developed. Magnetic domain subdivision technology is a technique that introduces non-uniformity of magnetic flux into steel sheets after finish annealing or after the baking of an insulating coating, using physical methods such as forming grooves or introducing localized strain. This subdivides the width of the 180° magnetic domains (main magnetic domains) formed along the rolling direction, thereby reducing iron loss in grain-oriented electrical steel sheets, especially eddy current loss.

[0005] For example, Patent Document 1 proposes a technique to improve 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 on the surface of a steel plate. Patent Document 2 proposes a method of introducing localized thermal strain by irradiating the surface of a steel plate after secondary recrystallization with a plasma flame in the width direction of the plate. This allows for a reduction in iron loss (W) when the steel plate is excited with a magnetization force of 800 A / m and the magnetic flux density (B8) is 1.935 T, compared to when it is excited with a maximum magnetic flux density of 1.7 T and a frequency of 50 Hz. 17 / 50 This can improve the power consumption to 0.680 W / kg.

[0006] Furthermore, 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 if strain-relieving 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 strain-relieving annealing.

[0007] The iron loss of grain-oriented electrical steel sheets is measured by exciting them with a sinusoidal wave in the rolling direction while they are stationary. However, it is known that inside actual transformers, residual strain due to processing and magnetization behavior more complex than a sinusoidal wave increase the transformer loss (transformer iron loss) compared to 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 grain-oriented electrical steel sheet used as material). Reducing the building factor is essential for manufacturing more efficient transformers.

[0008] Here, the widely manufactured wound core employs a so-called cut-core type, in which a single layer is obtained by shearing a grain-oriented electrical steel sheet for each turn, and multiple such layers are stacked (wound) and inserted into a coil. In contrast, a stacked core is made by stacking multiple single layers, each consisting of multiple pieces (angled members) obtained by shearing a grain-oriented electrical steel sheet, combined in a frame-like manner within the same plane, or by stacking multiple such combinations. Therefore, in each of the above single layers, the sheared ends of the grain-oriented electrical steel sheets are joined to each other via an air gap. Near such joints, when the core is energized, the transfer of magnetic flux in the direction of stacking the single layers occurs, resulting in increased iron loss due to in-plane eddy current loss, which is known to be one of the factors contributing to the increase in the building factor.

[0009] Various methods have been considered for improving the above-mentioned building factors. For example, Patent Document 3 proposes a method of applying a magnetic domain subdivision treatment by strain to the grain-oriented electrical steel sheet near the joint. Also, Patent Document 4 proposes a method of tilting the joint in the winding direction in order to improve iron loss due to magnetic flux transfer. [Prior art documents] [Patent Documents]

[0010] [Patent Document 1] Special Publication No. 6-22179 [Patent Document 2] Japanese Patent Application Publication No. 7-192891 [Patent Document 3] Japanese Patent Publication No. 2020-96100 [Patent Document 4] Japanese Patent Publication No. 2005-150507 [Overview of the project] [Problems that the invention aims to solve]

[0011] Although all of the above proposals are effective, they are insufficient from the perspective of the improvement amount of the building factor, and the development of further building factor improvement methods is desired. In addition, the above proposals are premised on performing additional processing during the core design, and there is a concern that it is likely to increase the manufacturing cost, such as an increase in the manufacturing man-hours and a change in the shearing direction during manufacturing.

[0012] The present invention has been made in view of the above circumstances, and an object thereof is to provide a transformer core having an excellent effect of improving the building factor.

Means for Solving the Problems

[0013] The inventors of the present invention have intensively studied to solve the above problems.

[0014] First, the iron loss measurement environments of the oriented electromagnetic steel sheet constituting the building factor and the transformer core were compared and sorted out. Regarding the iron loss measurement of the oriented electromagnetic steel sheet, as defined in JIS C 2550-1:2011 and JIS C 2556:2015, Epstein test and single sheet test (SST) are performed. In all of these tests, the measurement is carried out by removing or ignoring the processing strain introduced into the steel sheet during the processing of the test piece. In addition, in all of these tests, the measurement frame is placed in a state of being stationary parallel to the ground.

[0015] On the other hand, the iron loss measurement of the transformer core is affected as follows. That is, slitting and shearing are performed on the oriented electromagnetic steel strip that is the material of the transformer core. Further, in a large transformer core, fixing holes are opened by punching in a part of the leg, and residual stress is generated by such processing. Further, the iron loss measurement of the transformer core is performed in a state where the core is erected, and at this time, a bending moment is generated in the core.

[0016] From the above, it can be seen that more compressive stress is generated in the transformer core than in the grain-oriented electrical steel sheet used as the material. Such compressive stress is considered to increase the building factor because it increases the iron loss of the grain-oriented electrical steel sheet. Based on the above findings, it has been newly found that the iron loss of the transformer core can be improved by manufacturing the core using a grain-oriented electrical steel sheet with excellent stress resistance.

[0017] The present invention has been made based on the above findings.

[0018] That is, the gist configuration of the present invention is as follows. [1] A transformer core made of a grain-oriented electrical steel sheet, wherein the grain-oriented electrical steel sheet has a difference Δλp-p(-) between the magnetostriction amplitude λp-p(B-) when a compressive stress of 0.3 kgf / mm 2 is applied in the steel sheet rolling direction and the magnetostriction amplitude λp-p(A-) when a compressive stress of 0.3 kgf / mm is applied in the steel sheet rolling direction after stress relief annealing at 800°C for 3 hours in an Ar atmosphere, and the difference Δλp-p(-) is 3.0 ppm or less. 2 [2] The grain-oriented electrical steel sheet according to [1], wherein the difference Δλp-p(+) between the magnetostriction amplitude λp-p(B+) when a tensile stress of 0.3 kgf / mm 2 is applied in the steel sheet rolling direction and the magnetostriction amplitude λp-p(A+) when a tensile stress of 0.3 kgf / mm is applied in the steel sheet rolling direction after stress relief annealing at 800°C for 3 hours in an Ar atmosphere is 0.40 ppm or less. 2 [3] The transformer core according to [1] or [2], which is a stacked core or a wound core. [4] A transformer comprising the transformer core according to any one of [1] to [3].

Effect of the Invention

[0019] According to the present invention, it is possible to provide a transformer core that has an excellent effect in improving the building factor.

[0020] According to the present invention, the building factor of the transformer core can be improved by manufacturing the transformer core using grain-oriented electrical steel sheets with excellent stress resistance. According to the present invention, by manufacturing the transformer core using grain-oriented electrical steel sheets with excellent stress resistance, it is possible to suppress iron loss and noise increase due to compressive stress caused by processing strain and bending moment during uprighting, thereby providing a transformer core with low iron loss and low noise. [Brief explanation of the drawing]

[0021] [Figure 1] Figure 1 is a schematic diagram showing the structure of a transformer core. [Figure 2] Figure 2 shows the relationship between λp-p(B-) and the building factor (BF). [Figure 3] Figure 3 shows the relationship between λp-p(-) and the building factor (BF). [Figure 4] Figure 4 shows the relationship between λp-p(B-) and transformer noise. [Figure 5] Figure 5 shows the relationship between λp-p(-) and transformer noise. [Figure 6] Figure 6 shows the relationship between λp-p(+) and the building factor (BF). [Figure 7] Figure 7 shows the relationship between λp-p(+) and transformer noise. [Modes for carrying out the invention]

[0022] The experimental results that led to the completion of this invention are described below.

[0023] (Experiment 1) The test material used was a 0.20 mm thick grain-oriented electrical steel strip, manufactured using a general manufacturing process with a steel slab consisting of the component systems shown in Table 1.

[0024]

Table 1

[0025] Regarding the above steel strip, in the baking process of the tension insulating film, a laser device that irradiates a laser beam having a top-hat type energy distribution is arranged on the outlet side of the furnace, and the laser beam is irradiated in a stripe shape parallel to the rolling direction of the steel strip to partially modify the tension insulating film. As a result, a striped structure (hereinafter, simply referred to as a stripe) modified by the laser beam irradiation is formed in the tension insulating film. Steel strips (directional electromagnetic steel sheets serving as core materials, hereinafter, also referred to as "material electromagnetic steel sheets") with various changes in the formation interval of these stripes (distance between stripes) were prepared.

[0026] From each of these material electromagnetic steel sheets, test pieces with a size of 280 mm in the rolling direction and 100 mm in the sheet width direction were cut out, and the iron loss (single-sheet iron loss: W 17 / 50 ) of the material electromagnetic steel sheet was measured by the single-sheet magnetic measurement method described in JIS C 2556:2015. Here, W 17 / 50 means the heat loss when a single-sheet test piece of the material electromagnetic steel sheet is magnetized in alternating current with a magnetic flux density of 1.7 T and a frequency of 50 Hz in the longitudinal direction of the test piece.

[0027] Furthermore, for the same test piece, the magnetostriction amplitude (λp-p(B-)) when a compressive stress of 0.3 kgf / mm 2 is applied in the steel plate rolling direction was measured. Specifically, for the test piece, the amplitude (λp-p(B-)) of the vibration displacement (magnetostriction) of the steel plate in the steel plate rolling direction when it is excited in alternating current with a magnetic flux density of 1.5 T and a frequency of 50 Hz in a state where a compressive stress of 0.3 kgf / mm 2 is applied parallel to the steel plate rolling direction was measured.

[0028] Subsequently, after subjecting the same test piece to stress relief annealing, a compressive stress of 0.3 kgf / mm 2The magnetostriction amplitude (λp-p(A-)) was measured when a compressive stress was applied. Specifically, the specimen was subjected to a stress-relieving annealing treatment at 800°C for 3 hours in an Ar atmosphere. It was then cooled to room temperature. The cooling rate was kept sufficiently slow to prevent residual cooling strain. For example, the cooling rate was 80°C / h. Afterward, the stress-relieving annealed specimen was subjected to a pressure of 0.3 kgf / mm parallel to the steel sheet rolling direction. 2 The amplitude (λp-p(A-)) of the vibration displacement (magnetostriction) of the steel sheet in the rolling direction was measured when the steel sheet was AC-excited at a magnetic flux density of 1.5T and a frequency of 50Hz while compressive stress was applied.

[0029] Next, using the above-mentioned electromagnetic steel sheet material, a stacked core was fabricated as schematically shown in Figure 1(a). Specifically, for the stacked core, a steel strip was cut into 100 mm wide beveled bars, and multiple of these beveled bars were combined in a frame-like manner within the same plane to form a single layer. Multiple of these single layers were stacked to a thickness of 30 mm to fabricate a three-phase tripod stacked core with a height of 420 mm and an overall width of 420 mm. For each of the fabricated stacked cores, 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 of the core legs was 1.7 T at a frequency of 50 Hz. The iron loss characteristics at 1.7 T and 50 Hz were measured as no-load loss using a wattmeter. From the ratio of this transformer iron loss to the single-plate iron loss described above, the building factor (BF) of each stacked core transformer was calculated. Furthermore, each core-mounted transformer was excited in a soundproof room under conditions of maximum magnetic flux density Bm = 1.7T and frequency 50Hz, and the noise level: dBA (transformer noise) was measured using a sound level meter. Specifically, for each core-mounted transformer, during excitation, measurements were taken at eight points surrounding the core-mounted transformer, at a position half the height of the core-mounted transformer and 30cm from the surface of the core-mounted transformer. The average value of these measurements was defined as the transformer noise.

[0030] The measurement results for the building factor and transformer noise described above are shown in Figures 2 to 5. Figure 2 shows the relationship between the magnetostrictive amplitude λp-p(B-) measured for the raw electrical steel sheet before strain relief annealing and the building factor (BF). Figure 3 shows the relationship between the change in magnetostrictive amplitude Δλp-p(-) of the raw electrical steel sheet before and after strain relief annealing [i.e., the difference between λp-p(B-) measured for the raw electrical steel sheet before strain relief annealing and λp-p(A-) measured for the raw electrical steel sheet after strain relief annealing] and the building factor (BF). Figure 4 shows the relationship between the magnetostrictive amplitude λp-p(B-) measured for the raw electrical steel sheet before strain relief annealing and the transformer noise. Figure 5 shows the relationship between the change in magnetostrictive amplitude Δλp-p(-) of the raw electrical steel sheet before and after strain relief annealing and the transformer noise.

[0031] As shown in Figures 3 and 5, significant improvements in BF and transformer noise were observed in the region where Δλp-p(-) is 3.0 ppm or less. On the other hand, as shown in Figures 2 and 4, the influence of λp-p(B-) on BF and transformer noise could not be fully understood. In other words, no clear correlation was observed between λp-p(B-) and BF or transformer noise.

[0032] The reason for this trend is still unclear, but the inventors of this invention hypothesize the following: Grain-oriented electrical steel sheets have a texture in which Goss orientations are highly concentrated in the rolling direction, resulting in a magnetic domain structure with magnetization components 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 compressive stress above a certain level is applied, it changes to a magnetic domain structure (stress pattern) with magnetization components in the thickness direction. The amount of expansion and contraction of the steel sheet when grain-oriented electrical steel sheets are magnetized in the rolling direction has a positive correlation with the volume of the magnetic domains that have undergone magnetization rotation during the magnetization process. Therefore, in a stress pattern magnetic domain structure, the amplitude of magnetostriction increases rapidly, and at the same time, the permeability in the rolling direction also decreases, leading to an increase in iron loss. The compressive stress that causes the steel sheet to change to a stress pattern is influenced by innate material-derived factors such as the crystal orientation and texture of the steel sheet, and acquired factors such as the state of the tension film and residual stress due to processing strain and cooling strain. Of these, innate factors change little within the base electrical steel sheet during the manufacturing process of the transformer core, while acquired factors change within the base electrical steel sheet due to manufacturing variations. Therefore, λp-p(B-) is thought to represent the stress resistance performance of the base electrical steel sheet when both factors are included. This explains why no clear correlation was observed with the characteristics of transformers made using a large amount of base electrical steel sheet. On the other hand, among the above factors, the state of the tension coating and residual stress, which are acquired and have a significant influence, can be removed by strain-relieving annealing (SRA). In other words, λp-p(A-) is thought to represent the innate stress resistance performance of the base electrical steel sheet. Therefore, Δλp-p(-) is thought to indicate the degree to which the grain-oriented electrical steel sheet used as the base material is susceptible to acquired factors.

[0033] Based on the above, we believe that by using grain-oriented electrical steel sheets with a Δλp-p(-) of 3.0 ppm or less to manufacture the transformer core, we were able to suppress the deterioration of building factors and noise caused by external disturbances such as bending moment during uprighting and processing strain during transformer manufacturing. Preferably, the transformer core should be manufactured using grain-oriented electrical steel sheets with a Δλp-p(-) of 2.5 ppm or less, and more preferably, with a Δλp-p(-) of 2.0 ppm or less. The lower limit of Δλp-p(-) is not particularly limited, but one example is 0.1 ppm.

[0034] (Experiment 2) Next, using steel slabs with the component systems shown in Table 1 above, grain-oriented electrical steel strips with a thickness of 0.27 mm, manufactured using a general manufacturing process, were used as test materials. For this steel strip, during the baking process of the tension insulating coating, a grinding device was installed on the furnace entrance side to create fine grooves parallel to the rolling direction on the coating substrate, after which the tension insulating coating was formed. The coating substrate was an undercoat such as a forsterite coating or other ceramic coatings, and in this experiment, a forsterite coating was used. Steel strips (raw material electrical steel sheets) were prepared with various groove spacings (distance between grooves).

[0035] From each of these electrical steel sheets, test pieces measuring 280 mm in the rolling direction and 100 mm in the sheet width direction were cut out, and the iron loss of the electrical steel sheet (single sheet iron loss: W) was measured using the single-sheet magnetic measurement method described in JIS C 2556:2015. 17 / 50 ) was measured.

[0036] Furthermore, using the same test specimen, in the same manner as in Experiment 1, 0.3 kgf / mm² was applied in the steel sheet rolling direction. 2 The magnetostriction amplitude (λp-p(B-)) was measured when compressive stress was applied.

[0037] Next, for the same test piece, 0.3 kgf / mm² was applied in the steel sheet rolling direction. 2 The magnetostriction amplitude (λp-p(B+)) was measured when a tensile stress of 0.3 kgf / mm² was applied to the test specimen. Specifically, a tensile stress of 0.3 kgf / mm² was applied parallel to the rolling direction of the steel sheet. 2The amplitude (λp-p(B+)) of the vibrational displacement (magnetostriction) of the steel sheet in the rolling direction was measured when the steel sheet was AC-excited at a magnetic flux density of 1.5T and a frequency of 50Hz while a tensile stress was applied.

[0038] Next, the same specimen was subjected to stress-relieving annealing in the same manner as in Experiment 1. Then, the specimen after stress-relieving annealing was subjected to 0.3 kgf / mm² in the steel sheet rolling direction. 2 The magnetostriction amplitude (λp-p(A-)) was measured when a compressive stress was applied. Subsequently, the test specimen after the strain-relieving annealing was subjected to a pressure of 0.3 kgf / mm² in the steel sheet rolling direction. 2 The magnetostriction amplitude (λp-p(A+)) was measured when a tensile stress of 0.3 kgf / mm² was applied to the test specimen. Specifically, a tensile stress of 0.3 kgf / mm² was applied to the specimen parallel to the rolling direction of the steel sheet. 2 The amplitude (λp-p(A+)) of the vibrational displacement (magnetostriction) of the steel sheet in the rolling direction was measured when the steel sheet was AC-excited at a magnetic flux density of 1.5T and a frequency of 50Hz while a tensile stress was applied.

[0039] Next, using electrical steel sheets from the above-mentioned steel strips with a Δλp-p(-) of 2 ppm, a three-phase tripod wound core, schematicly shown in Figure 1(b), was fabricated. Specifically, the steel strip was slit to a width of 100 mm, wound in the direction of the steel strip rolling, and multiple of these single layers were stacked to a thickness of 30 mm to fabricate a three-phase tripod wound core with a height of 300 mm and a total width of 210 mm. Here, the wound core was fabricated as a type (unicore) having two 45-degree bends as corner sections. For each of the fabricated wound cores, 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 of the core leg section was 1.7 T at a frequency of 50 Hz. The iron loss characteristics at 1.7 T and 50 Hz were measured using a wattmeter to determine the no-load loss. The building factor (BF) of each core-wound transformer was calculated from the ratio of the transformer iron loss to the single-plate iron loss mentioned above. Furthermore, the noise level: dBA (transformer noise) was measured for each core-wound transformer in the same manner as in Experiment 1.

[0040] The measurement results for the building factor and transformer noise described above are shown in Figures 6 and 7. Figure 6 shows the relationship between the change in magnetostrictive amplitude Δλp-p(+) of the raw electrical steel sheet before and after strain relief annealing [that is, the difference between Δλp-p(B+) measured for the raw electrical steel sheet before strain relief annealing and Δλp-p(A+) measured for the raw electrical steel sheet after strain relief annealing] and the building factor (BF). Figure 7 shows the relationship between the change in magnetostrictive amplitude Δλp-p(+) of the raw electrical steel sheet before and after strain relief annealing and the transformer noise.

[0041] As shown in Figures 6 and 7, significant improvements in BF and transformer noise were observed in the region where Δλp-p(+) is 0.40 ppm or less.

[0042] The cause of this phenomenon is unclear, but the inventors have hypothesized the following: Similar to Δλp-p(-) above, Δλp-p(+) is thought to indicate how susceptible the grain-oriented electrical steel sheet used as the material is to acquired factors (tensile stress applied later). When compressive stress due to bending moment is applied inside the transformer, a similar amount of tensile stress is applied to the back surface of the material electrical steel sheet. Therefore, in grain-oriented electrical steel sheets with a large Δλp-p(+), a large difference in characteristics occurs between the part under compressive stress (compressive stress area) and the part under tensile stress (tensile stress area). In other words, in grain-oriented electrical steel sheets with a large Δλp-p(+), it is thought that the noise emitted increases as a result of localized iron loss due to magnetic flux concentration in the tensile stress area with higher permeability, and as the core vibration becomes more complex due to non-uniform vibration.

[0043] Based on the above, we believe that by manufacturing the transformer core using grain-oriented electrical steel sheets with Δλp-p(+) of 0.40 ppm or less, magnetic flux concentration in tensile stress areas and vibration non-uniformity are suppressed, thereby suppressing deterioration of building factors and noise. Preferably, the transformer core is manufactured using grain-oriented electrical steel sheets with Δλp-p(+) of 0.30 ppm or less, more preferably 0.25 ppm or less, and even more preferably 0.20 ppm or less. The lower limit of Δλp-p(+) is not particularly limited, but one example is 0.01 ppm.

[0044] Preferred embodiments of the present invention will be described in detail below. However, the present invention is not limited to the configurations disclosed in these embodiments, and various modifications are possible without departing from the spirit of the invention.

[0045] [Grain-oriented electrical steel sheet] First, the grain-oriented electrical steel sheet used as the material for the transformer core of the present invention will be described. In the present invention, the component composition of the slab for the grain-oriented electrical steel sheet is sufficient as long as it is a component composition that produces secondary recrystallization. Furthermore, when using inhibitors, for example, if using an AlN-based inhibitor, Al and N should be included in appropriate amounts, and if using a MnS·MnSe-based inhibitor, Mn and Se and / or S should be included in appropriate amounts. Of course, both inhibitors may be used in combination. In this case, the preferred content of Al, N, S, and Se is Al: 0.010~0.065 mass%, N: 0.0050~0.0120 mass%, S: 0.005~0.030 mass%, and Se: 0.005~0.030 mass%, respectively.

[0046] Furthermore, the present invention can also be applied to grain-oriented electrical steel sheets that do not use inhibitors and have limited Al, N, S, and Se content. In this case, it is preferable to limit the Al, N, S, and Se content to 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.

[0047] This section specifically describes the basic components and optional additives for slabs used in grain-oriented electrical steel sheets.

[0048] C: 0.08% by mass or less Carbon (C) is added to improve the structure of the hot-rolled sheet. However, if the C content exceeds 0.08% by mass, it becomes difficult to decarburize to 50 ppm by mass or less during the manufacturing process, so it is preferable to keep the C content at 0.08% by mass or less. Furthermore, since secondary recrystallization occurs even in materials without C, there is no particular lower limit set for the C content. In other words, the C content may be 0% by mass.

[0049] Si:2.0~8.0% by mass Si is an effective element for increasing the electrical resistance of steel and improving iron loss. A Si content of 2.0 mass% or more further enhances the iron loss reduction effect. On the other hand, a Si content of 8.0 mass% or less makes it easier to suppress the decrease in workability and sheet treadability, as well as the decrease in magnetic flux density. Therefore, it is preferable to have a Si content in the range of 2.0 to 8.0 mass%.

[0050] Mn:0.005~1.0% by mass Mn is an essential element for improving hot workability. This effect is more easily achieved when the Mn content is 0.005% by mass or higher. On the other hand, when the Mn content is 1.0% by mass or lower, the decrease in magnetic flux density of the product plate is more easily suppressed. Therefore, it is preferable to have a Mn content in the range of 0.005% to 1.0% by mass.

[0051] The slab for the grain-oriented electrical steel sheet described above preferably has the above components as its basic components. In addition to the above basic components, the slab may optionally contain the following elements. The following elements are effective in improving magnetic properties.

[0052] One or more elements 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%.

[0053] Ni is an effective element for improving the structure of hot-rolled sheets and enhancing their magnetic properties. A Ni content of 0.03% by mass or more further enhances the magnetic property improvement effect. A Ni content of 1.50% by mass or less suppresses the instability of secondary recrystallization, reducing the risk of deterioration in the magnetic properties of the finished sheet. Therefore, when Ni is included, it is preferable to keep the Ni content in the range of 0.03% to 1.50% by mass.

[0054] Furthermore, Sn, Sb, Cu, P, Mo, and Cr are also elements that improve magnetic properties, and the improvement in magnetic properties is more easily obtained when the content of each of these elements is above the lower limit mentioned above. On the other hand, when the content of each of these components is below the upper limit mentioned above, the risk of suppressing the growth of secondary recrystallized grains is reduced, and the deterioration of magnetic properties is more easily suppressed. For this reason, when Sn, Sb, Cu, P, Mo, and Cr are included, it is preferable that the content of each of these elements be within the above ranges.

[0055] Furthermore, the remainder of the components other than those mentioned above consists of Fe and unavoidable impurities.

[0056] A slab having the above-described component composition is subjected to hot rolling, followed by hot-rolled sheet annealing. Then, it is cold-rolled once or twice to finish it into a steel strip of the final thickness. After that, the steel strip is subjected to decarburization annealing, an annealing separating agent is applied, and then it is wound into a coil and subjected to final annealing for the purpose of secondary recrystallization. After final annealing, the steel strip is subjected to planarization annealing to form an insulating film (tension insulating film). Furthermore, in this embodiment, as will be described later, a treatment is applied to give anisotropy to the film tension that the tension insulating film imparts to the steel sheet.

[0057] In this embodiment, the process after planarization annealing may include a magnetic domain refinement step in which thermal strain is formed on the surface of the grain-oriented electrical steel sheet (steel strip) by energy beam irradiation. Alternatively, the process after cold rolling may include a magnetic domain refinement step on the steel sheet, such as electrolytic etching or groove formation by laser irradiation.

[0058] [Magnetostriction measurement method (Δλp-p(-), Δλp-p(+))] The magnetostrictive measurement method, which is crucial in this invention, will now be described. In this invention, magnetostrictive measurement was performed using a TRS-200 manufactured by Toei Kogyo. A test specimen was taken from the grain-oriented electrical steel sheet (steel strip) to be measured, sheared 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 measurement laser reflection, the test specimen was set in the device's measurement frame, and one side of the test specimen was fixed with an air-pressure operated clamp. After fixing, a glass plate was placed on top to prevent buckling of the test specimen, and then the specified 3.0 kgf / mm² was measured in the steel sheet rolling direction. 2 A compressive stress is applied. In this state, the test piece is AC-excited with a magnetic flux density of 1.5T and a frequency of 50Hz, and the vibration at that time is measured with a laser Doppler meter, and the measured value is taken as λp-p(B-). Next, a specified 3.0kgf / mm² is applied in the steel plate rolling direction. 2 A tensile stress is applied. In this state, the test specimen is AC-excited with a magnetic flux density of 1.5T and a frequency of 50Hz, and the vibration at that time is measured with a laser Doppler meter, and the measured value is taken as λp-p(B+).

[0059] Subsequently, the test specimen is subjected to stress-relieving annealing. Stress-relieving annealing is a heat treatment in which the test specimen is held at 800°C for 3 hours in an Ar atmosphere. Then, magnetostriction measurements are performed on the test specimen after stress-relieving annealing in the same manner as above to obtain the magnetostriction amplitude λp-p(A-) when a predetermined compressive stress is applied and the magnetostriction amplitude λp-p(A+) when a predetermined tensile stress is applied. Then, Δλp-p(-) is 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+). The test specimen is taken from the leading and trailing ends of the electromagnetic steel sheet (coil) used to manufacture the transformer core, and the largest values ​​of Δλp-p(-) and Δλp-p(+) from the measured values ​​are treated as representative values ​​for that coil.

[0060] In the present invention, the method for adjusting Δλp-p(-) and Δλp-p(+) is not particularly limited. Examples include a method of irradiating the tension insulating coating with a laser or electron beam in a streaky pattern to give the coating tension anisotropy in the direction of steel plate rolling, a method of forming minute grooves or ridges on the coated surface of the tension insulating coating (the coating substrate on which the tension insulating coating is formed) to give the tension anisotropy in the direction of steel plate rolling, a method of increasing the rigidity of the base metal by applying shot peening or laser peening to the outermost surface of the base metal before baking the tension insulating coating, and methods that combine these as appropriate. Any of these methods may be used in the present invention.

[0061] By partially modifying the tension insulating coating by irradiating it with a laser or electron beam in a streaky pattern parallel to the steel sheet rolling direction, or by forming grooves or ridges parallel to the steel sheet rolling direction on the coating substrate on which the tension insulating coating is formed, anisotropy is introduced into the film tension that the tension insulating coating imparts to the steel sheet. By introducing anisotropy into the film tension imparted by the tension insulating coating, the coating becomes less susceptible to subsequent cooling strain and processing strain. For example, when a tension insulating coating is irradiated with a laser or electron beam, the irradiated area hardens (crystallization progresses in the irradiated area, and the Young's modulus increases). When a streaky structure is formed in the tension insulating coating by partially modifying it by irradiating it with a laser or electron beam in a streaky pattern parallel to the steel sheet rolling direction, a difference in Young's modulus occurs between the steel sheet rolling direction and the direction perpendicular to the steel sheet rolling direction. Therefore, a stronger film tension is generated in the steel sheet rolling direction. This increases the compressive stress resistance in the steel sheet rolling direction and suppresses the 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, generating stronger film tension in the steel sheet rolling direction as described above can also suppress the change in magnetostriction when tensile stress is applied.

[0062] The following describes 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 tension insulating coating using laser or electron beam irradiation.

[0063] (Laser or electron beam output: 50W to 5000W) The higher the output of the laser or electron beam, the more the crystallization of the coating is promoted, making the coating harder and increasing the anisotropy of the coating tension. Therefore, it is preferable that the output of the laser or electron beam be 50W or higher. On the other hand, if the output of the laser or electron beam is too high, excessive energy is supplied to the coating, causing damage to the coating. From this viewpoint, it is preferable that the output of the laser or electron beam be between 50W and 5000W.

[0064] (Laser or electron beam spot diameter: 300 μm or less) A smaller spot diameter for the laser or electron beam is preferable because it allows for more localized coating modification. Therefore, in this invention, it is preferable to set the spot diameter of the laser or electron beam to 300 μm or less. It is more preferable to set the spot diameter of the laser or electron beam to 280 μm or less, and even more preferable to set it to 260 μm or less. In this invention, the spot diameter refers to the full width at half maximum of the beam profile obtained by the slitting method using a slit with a width of 30 μm. As mentioned above, a smaller spot diameter for the laser or electron beam is preferable, and there is no lower limit to the spot diameter, but as an example, the spot diameter may be 2 μm or more.

[0065] (Muscle formation interval) By irradiating a steel strip with a laser or electron beam in a streaky pattern parallel to the rolling direction, a streaky structure (streaks) modified by the laser or electron beam irradiation is formed in the tension insulating coating. 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, leading to thermal strain on the steel plate and damage to the coating due to excessive heat input, which degrades the magnetism. Therefore, it is preferable that the lower limit of the streak spacing be approximately the same as the spot diameter of the laser or electron beam. As an example, a streak spacing of 0.1 mm or more is preferable. On the other hand, if the streak spacing is excessively large, anisotropy in the coating tension will not occur, so it is preferable that the streak spacing be 2 mm or less.

[0066] (Scanning speed: 5-400 m / s) A slower scanning speed for the laser or electron beam is preferable because it increases the amount of heat incident per unit length of the coating. However, if the scanning speed is excessively slow, the processing area per unit time decreases, reducing manufacturing efficiency. Therefore, a scanning speed of 5 m / s or higher is preferable. On the other hand, if the scanning speed is excessively high, the power supply capacity required to provide the heat input necessary for coating modification becomes larger, leading to larger equipment. Therefore, a scanning speed of 400 m / s or less is preferable.

[0067] The following describes 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 steel sheet rolling direction by hairline processing or other methods on the coating substrate for forming a tension insulating coating, or by forming ridges by powder bed additive manufacturing or other methods. Grooves or ridges can be formed, for example, during the baking process of the tension insulating coating, by installing a grinding device or additive manufacturing device on the furnace entrance side and performing a process to form grooves or ridges parallel to the rolling direction on the coating substrate.

[0068] (Spacing between furrows or ridges) A narrower spacing between grooves (distance between grooves) or ridges (distance between ridges) is preferable because it increases the anisotropy of the coating tension. However, if the groove spacing is excessively narrow, the insulation performance deteriorates due to increased grinding of the base coating. Similarly, if the ridge spacing is excessively narrow, the packing efficiency decreases due to an increase in the total film thickness of the tensile insulating coating and the base coating. Therefore, it is preferable that the lower limit of the groove or ridge spacing be approximately the same as the width of the groove or ridge. As an example, a groove or ridge spacing of 0.1 mm or more is preferable. On the other hand, if the groove or ridge spacing is excessively large, the anisotropy of the coating tension will not occur, so it is preferable that the groove or ridge spacing be 2 mm or less.

[0069] (Width of furrow or ridge) A narrower groove or ridge width is preferable because it increases the anisotropy of the film tension. Therefore, in this embodiment, it is preferable to set the groove or ridge width to 300 μm or less. It is more preferable to set the groove or ridge width to 280 μm or less, and even more preferable to set it to 260 μm or less. On the other hand, if the groove or ridge width becomes excessively narrow, the anisotropy of the film tension decreases, so it is preferable that the groove or ridge width be 2 μm or more.

[0070] (Depth of furrow formation or height of ridge formation) A larger groove formation depth (groove depth) or ridge formation height (ridge height) is preferable because it increases the anisotropy of the coating tension. On the other hand, if the groove formation depth is too large, the groove will reach the base metal and degrade the magnetism, which is undesirable. Similarly, if the ridge formation height is too large, it will exceed the film thickness of the tension insulating coating, leading to a deterioration of insulation and space factor, which is also undesirable. Therefore, it is preferable that the groove formation depth is less than the film thickness of the base coating, and the ridge formation height is less than the film thickness of the tension insulating coating. More preferably, the groove formation depth is less than 80% of the film thickness of the base coating, and the ridge formation height is less than 80% of the film thickness of the tension insulating coating.

[0071] As described above, grain-oriented electrical steel sheets that have been treated to give anisotropy to the film tension provided by the tension-insulating coating may subsequently undergo a non-heat-resistant magnetic domain refinement treatment. This non-heat-resistant magnetic domain refinement treatment can be carried out, for example, by irradiating the tension-insulating coating with a laser or electron beam according to known conditions.

[0072] Furthermore, in this invention, known methods for manufacturing grain-oriented electrical steel sheets can be used as appropriate, in addition to the processes and manufacturing conditions described above.

[0073] [Transformer core] The transformer core of the present invention is constructed using grain-oriented electrical steel sheets with a Δλp-p(-) of 3.0 ppm or less as the material electrical steel sheets constituting the transformer core. Examples of the transformer core include stacked cores and wound cores. An example of a stacked core is the three-phase tripod stacked core shown in Figure 1(a). Examples of wound cores include the Unicore shown in Figure 1(b) and the Trunco ​​shown in Figure 1(c). A wound core may also be a Duocore. These cores can be manufactured, for example, by known manufacturing methods. For example, a stacked core can be manufactured by laminating a frame-like combination of material electrical steel sheets slit at an angle. A Unicore can be manufactured by stacking material electrical steel sheets whose corner portions have been pre-bent. A Trunco ​​can be manufactured by winding material electrical steel sheets and then pressing the corner portions of the core to a predetermined curvature to form a rectangular shape.

[0074] Furthermore, as an example of a method for manufacturing a transformer core according to the present invention, the method includes the steps of: measuring λp-p(B-) and λp-p(A-) of a grain-oriented electrical steel sheet and calculating Δλp-p(-) from the difference between λp-p(B-) and λp-p(A-); selecting a grain-oriented electrical steel sheet in which the calculated Δλp-p(-) is within a predetermined range; and manufacturing a transformer core using the selected grain-oriented electrical steel sheet. Alternatively, the manufacturing method may include the steps of: measuring λp-p(B+) and λp-p(A+) of a grain-oriented electrical steel sheet and calculating Δλp-p(+) from the difference between λp-p(B+) and λp-p(A+); selecting a grain-oriented electrical steel sheet in which the calculated Δλp-p(+) is within a predetermined range; and manufacturing a transformer core using the selected grain-oriented electrical steel sheet. 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 manufacturing the transformer core, the transformer core can be manufactured using the grain-oriented electrical steel sheet selected in this way. [Examples]

[0075] Next, the present invention will be specifically described based on examples. The following examples illustrate preferred examples of the present invention and do not limit the invention in any way. It is also possible to implement the invention with modifications to the extent that they are in line with the spirit of the invention, and such modifications are also included within the technical scope of the present invention.

[0076] Using steel slabs with the component systems shown in Table 2, grain-oriented electrical steel sheets were manufactured using a general manufacturing process. Before forming a tension insulating coating on the grain-oriented electrical steel sheet, grooves were introduced parallel to the steel sheet rolling direction using a grinding device on the coating surface (forsterite, which is the coating base). Steel strip A, on which the tension insulating coating was then formed, was prepared as a test material. In addition, steel strips B (laser irradiated) and C (electron beam irradiated), on which the tension insulating coating was partially modified by irradiating the grain-oriented electrical steel sheet manufactured above with a laser or electron beam parallel to the steel sheet rolling direction, were prepared as test materials.

[0077] [Table 2]

[0078] Regarding steel strip A, in the baking process for the tension insulating coating, a grinding device was placed on the furnace entrance side, and grooves were machined into the coating substrate parallel to the rolling direction before forming the tension insulating coating. Furthermore, regarding steel strips B and C, in the baking process for the tension insulating coating, a device that irradiates a laser beam or electron beam with a top-hat-shaped energy distribution was placed on the furnace exit side. The laser beam or electron beam was then irradiated in a streak parallel to the rolling direction of the steel sheet, partially modifying the tension insulating coating. In this way, a process was performed to introduce anisotropy to the coating tension imparted by the tension insulating coating. Steel strips with varying groove or streak spacing, laser or electron beam spot diameter, and output were prepared as raw electrical steel sheets. By forming the grooves or streaks more densely, and by forming the streaks harder, the anisotropy of the coating tension is increased, and the coating tension in the steel sheet rolling direction is strengthened. Therefore, by changing the spacing between the grooves or grooves, the spot diameter of the laser or electron beam, and the output, Δλp-p(-) and Δλp-p(+) can be adjusted. Furthermore, after applying the above treatment to give anisotropy to the film tension provided by the tension insulating coating, some of the steel strips were further subjected to a non-heat-resistant magnetic domain subdivision treatment (DR).

[0079] From each of these steel strips, test pieces measuring 280 mm in the rolling direction and 100 mm in the width direction were cut out, and the iron loss of the raw electrical steel sheet (single sheet iron loss: W) was measured using the single-sheet magnetic measurement method described in JIS C2556:2015. 17 / 50 The following values ​​were measured: λp-p(B-), λp-p(B+), λp-p(A-), and λp-p(A+) were measured for the same test specimen using the method described above. Then, 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+).

[0080] Next, using the aforementioned electrical steel sheet material, a three-phase tripod stacked core was fabricated, as schematically shown in Figure 1(a). Specifically, as the stacked core, a steel strip was cut into 100 mm wide beveled bars, and multiple of these beveled bars were combined in a frame-like manner within the same plane to form a single layer. Multiple of these single layers were stacked to a thickness of 30 mm to produce a three-phase tripod stacked core with a height of 420 mm and a total width of 420 mm. Furthermore, using the aforementioned electrical steel sheet material, a three-phase tripod wound core was fabricated, as schematically shown in Figures 1(b) and (c). Specifically, as the wound core, a steel strip was slit to a width of 100 mm and wound in the direction of steel strip rolling to form a single layer. Multiple of these single layers were stacked to a thickness of 30 mm to produce a three-phase tripod wound core with a height of 300 mm and a total width of 210 mm. Here, two types of wound cores were fabricated: a type in which the core corners are wound in an arc shape (Trunco ​​(Figure 1(c))) and a type with two 45-degree bends as corners (Unicore (Figure 1(b))). The Trunco ​​and some of the Unicores were subjected to strain-relieving annealing at 800°C for 3 hours in an Ar atmosphere. For the Trunco, since the effect of non-heat-resistant magnetic domain subdivision is lost by strain-relieving annealing, only electromagnetic steel sheets that had not undergone non-heat-resistant magnetic domain subdivision treatment were used. In Tables 3 to 7, if non-heat-resistant magnetic domain subdivision treatment was not performed, "None" is written in the DR (Magnetic Domain Subdivision Treatment) column. If non-heat-resistant magnetic domain subdivision treatment was performed by laser, "Laser" is written in the DR column. If non-heat-resistant magnetic domain subdivision treatment was performed by electron beam, "Electron Beam" is written in the DR column.

[0081] For each fabricated iron 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 at the iron core leg portion was 1.7T at a frequency of 50Hz. The iron loss characteristics at 1.7T and 50Hz were measured using a wattmeter to determine the no-load loss. The building factor (BF) of each transformer was calculated from the ratio of this transformer iron loss to the single-plate iron loss mentioned above. Furthermore, for each transformer, as described above, it was excited in a soundproof room under the conditions of maximum magnetic flux density Bm=1.7T and frequency of 50Hz, and the noise level: dBA (transformer noise) was measured using a sound level meter.

[0082] Tables 3-7 confirm that by satisfying the requirements of the present invention, a transformer core with excellent building factor improvement effects 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 an even better improvement effect is obtained when a transformer core is made of electromagnetic steel sheet material that has undergone a non-heat-resistant magnetic domain subdivision treatment in addition to a treatment that gives anisotropy to the film tension imparted by the tension insulating coating.

[0083] [Table 3]

[0084] [Table 4]

[0085] [Table 5]

[0086] [Table 6]

[0087] [Table 7]

Claims

1. A transformer core made of grain-oriented electrical steel sheet, The aforementioned grain-oriented electrical steel sheet is 0.3 kgf / mm² in the steel plate rolling direction ２ The magnetostriction amplitude λp-p(B-) when compressive stress is applied, After annealing at 800°C for 3 hours in an Ar atmosphere to relieve strain, the steel sheet was subjected to a pressure of 0.3 kgf / mm² in the rolling direction. ２ A transformer core in which the difference Δλp-p(-) between the magnetostriction amplitude λp-p(A-) and the compressive stress applied is 3.0 ppm or less.

2. The aforementioned grain-oriented electrical steel sheet is 0.3 kgf / mm² in the steel plate rolling direction ２ The magnetostriction amplitude λp-p(B+) when a tensile stress is applied, After annealing at 800°C for 3 hours in an Ar atmosphere to relieve strain, the steel sheet was subjected to a pressure of 0.3 kgf / mm² in the rolling direction. ２ The transformer core according to claim 1, wherein the difference Δλp-p(+) between the magnetostriction amplitude λp-p(A+) and the tensile stress applied is 0.40 ppm or less.

3. The transformer core according to claim 1 or 2, wherein the transformer core is a stacked core or a wound core.

4. A transformer comprising the transformer core described in claim 1 or 2.

5. A transformer comprising the transformer core described in Claim 3.

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