Transformer iron core and transformer provided with same

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

[0001] The present invention relates to a transformer core and a transformer including the same.

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

[0003] As methods for improving hysteresis loss, methods have been developed such as 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, as methods for improving eddy current loss, methods have been developed such as 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 further reduction in iron loss in grain-oriented electrical steel sheets, these methods have manufacturing limitations.

[0004] Therefore, 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 technology that introduces magnetic flux nonuniformity through physical methods, such as forming grooves or introducing localized strain, into steel sheets after final annealing or after baking of an insulating coating. This technology refines the width of the 180° magnetic domains (main magnetic domains) formed along the rolling direction, thereby reducing iron loss, especially eddy current loss, in grain-oriented electrical steel sheets.

[0005] For example, Patent Document 1 proposes a technique 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. Also, Patent Document 2 proposes a method for irradiating a plasma flame in the width direction of the surface of a steel sheet after secondary recrystallization to locally introduce thermal strain. This reduces the magnetic flux density (B ８ ) is 1.935T, the iron loss (W１７／５０ ) can be improved to 0.680 W / kg.

[0006] The method of introducing linear grooves as disclosed in Patent Document 1 is called heat-resistant magnetic domain refining because the magnetic domain refining effect is not lost 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 refining because the effect of introducing thermal strain is lost due to stress relief annealing.

[0007] The iron loss of grain-oriented electrical steel sheets is measured by applying a sinusoidal excitation in the rolling direction while the sheet is left stationary. However, due to residual strain caused by processing and magnetization behavior that is more complex than a sinusoidal wave, it is known that the transformer's loss (transformer iron loss) increases more than the iron loss of the grain-oriented electrical steel sheet used as the core material. This increase rate is called the building factor (BF) (= transformer iron loss / iron loss of the grain-oriented electrical steel sheet used as the material). Reducing the building factor is essential for manufacturing more efficient transformers.

[0008] Widely manufactured wound cores are manufactured using a so-called 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), which are then stacked (wound) and inserted into a coil. Stacked cores are manufactured by laminating multiple single layers, each of which is made by shearing grain-oriented electromagnetic steel sheets into multiple pieces (bevel bars) and combining them in a frame-like shape on the same plane, or by stacking multiple such combinations. Therefore, in each single layer, the sheared ends of the grain-oriented electromagnetic steel sheets are joined via an air gap. When the core is excited, magnetic flux transfer occurs in the lamination direction of the single layers, resulting in increased iron loss due to in-plane eddy current loss near such joints. This is known to be one of the causes of the increase in the building factor.

[0009] Various methods for improving the building factor have been studied. For example, Patent Document 3 proposes a method of applying a magnetic domain refinement treatment to grain-oriented electrical steel sheets near the joint by strain. Also, Patent Document 4 proposes a method of tilting the joint in the winding direction to improve iron loss due to magnetic flux transfer.

[0010] Japanese Patent Publication No. 6-22179 Japanese Patent Application Laid-Open No. 7-192891 Japanese Patent Application Laid-Open No. 2020-96100 Japanese Patent Application Laid-Open No. 2005-150507

[0011] Although all of the above proposals are effective, they are insufficient in terms of the improvement in the building factor, and further development of methods for improving the building factor is desired. In addition, the above proposals are premised on additional processing during the core design, which raises concerns that they may increase manufacturing costs due to an increase in manufacturing man-hours and changes in the shear direction during manufacturing.

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

[0013] The present inventors have conducted extensive research to solve the above problems.

[0014] First, we compared and organized the iron loss measurement environments for grain-oriented electrical steel sheets and transformer cores that make up the building factor. 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.

[0015] On the other hand, 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 measurements for transformer cores are performed with the core standing upright, which creates a bending moment in the core.

[0016] From the above, it has been found that transformer cores experience greater compressive stress than the grain-oriented electrical steel sheets that form their raw material. This compressive stress increases the iron loss of the grain-oriented electrical steel sheets, which is 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 electrical steel sheets, which have excellent stress resistance.

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

[0018] That is, the gist of the present invention is as follows: [1] A transformer core made of grain-oriented electrical steel sheet, wherein the grain-oriented electrical steel sheet has a strength of 0.3 kgf / mm in the rolling direction of the steel sheet. ２ The magnetostriction amplitude λp-p(B-) when a compressive stress of 0.3 kgf / mm was applied in the rolling direction of the steel sheet after annealing for 3 hours at 800 ° C in an Ar atmosphere. ２ [2] The grain-oriented electrical steel sheet has a magnetostriction amplitude Δλp-p(-) of 3.0 ppm or less when a compressive stress of 0.3 kgf / mm is applied to the steel sheet. ２ The magnetostriction amplitude λp-p(B+) when a tensile stress of 0.3 kgf / mm was applied in the rolling direction of the steel sheet after annealing for 3 hours at 800 °C in an Ar atmosphere. ２ [1] The transformer core according to [1], wherein the difference Δλp-p(+) between the magnetostriction amplitude λp-p(A+) when a tensile stress of 1000 kJ / s is applied and the magnetostriction amplitude λp-p(A+) when a tensile stress of 1000 kJ / s is applied 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].

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

[0020] According to the present invention, by manufacturing a transformer core using grain-oriented electrical 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 electrical 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.

[0021] 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.

[0022] The experimental results that led to the completion of the present invention will be described below.

[0023] (Experiment 1) A steel slab having the chemical composition shown in Table 1 was used, and a grain-oriented electrical steel strip having a thickness of 0.20 mm, manufactured by a general manufacturing process, was used as a test material.

[0024]

[0025] For the steel strip, in the baking process of the tensile insulating coating, a laser device that irradiates a laser beam with a top-hat energy distribution was placed on the outlet side of the furnace, and the laser beam was irradiated in stripes parallel to the rolling direction of the steel strip, thereby partially modifying the tensile insulating coating. As a result, a stripe-like structure (hereinafter simply referred to as "streaks") modified by laser beam irradiation was created in the tensile insulating coating. Steel strips (grain-oriented electrical steel sheets that will become the raw material for iron cores, hereinafter also referred to as "raw material electrical steel sheets") with various spacings between these streaks (distance between streaks) were prepared.

[0026] 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 ) was measured. １７／５０ The heat loss refers to the heat loss when a single sheet test piece of the material electrical steel sheet is 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.

[0027] Furthermore, the same test piece was subjected to a load of 0.3 kgf / mm in the rolling direction of the steel plate. ２ The magnetostriction amplitude (λp-p(B-)) was measured when a compressive stress of 0.3 kgf / mm was applied to the test piece in a direction parallel to the rolling direction of the steel sheet. ２ With a compressive stress of 1.5 T applied, the amplitude (λp-p(B-)) of the vibration displacement (magnetostriction) of the steel sheet in the rolling direction of the steel sheet was measured when the steel sheet was excited with an AC current at a magnetic flux density of 1.5 T and a frequency of 50 Hz.

[0028] Subsequently, the same test piece was subjected to stress relief annealing, and then subjected to a load of 0.3 kgf / mm in the rolling direction of the steel sheet. ２ The magnetostriction amplitude (λp-p(A-)) was measured when a compressive stress of 0.3 kgf / mm was applied to the test piece. Specifically, the test piece was subjected to a heat treatment at 800°C for 3 hours in an Ar atmosphere as strain relief annealing. Thereafter, the test piece was cooled to room temperature. At this time, the cooling rate was made sufficiently slow so that no cooling strain remained. As an example, the cooling rate was 80°C / h. Thereafter, a compressive stress of 0.3 kgf / mm was applied to the test piece that had been subjected to the strain relief annealing in a direction parallel to the rolling direction of the steel sheet. ２ With a compressive stress of 1.5 T applied, the amplitude (λp-p(A-)) of the vibration displacement (magnetostriction) of the steel sheet in the rolling direction of the steel sheet was measured when the steel sheet was excited with an AC current at a magnetic flux density of 1.5 T and a frequency of 50 Hz.

[0029] 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 to form a single layer. These single layers were then stacked to a thickness of 30 mm to produce a three-phase, three-limbed stacked core with a height of 420 mm and a total width of 420 mm. Each stacked core 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 at a frequency of 50 Hz when the magnetic flux density in the core legs was 1.7 T. 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 the 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, the transformer noise was measured for each stacked core transformer during excitation at eight positions surrounding the stacked core transformer at a position half the height of the stacked core and a distance of 30 cm from the surface of the stacked core transformer, and the average value was taken as the transformer noise.

[0030] The measurement results of the building factor and transformer noise described above are shown in Figures 2 to 5. Figure 2 is a graph showing the relationship between the building factor (BF) and the magnetostriction amplitude λp-p(B-) measured for the base electrical steel sheet before stress relief annealing. Figure 3 is a graph showing 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 for the base electrical steel sheet before stress relief annealing and λp-p(A-) measured for the base electrical steel sheet after stress relief annealing]. Figure 4 is a graph showing the relationship between the magnetostriction amplitude λp-p(B-) measured for the base electrical steel sheet before stress relief annealing and the transformer noise. Figure 5 is a graph showing 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.

[0031] 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 effects of λp-p(B-) on BF and transformer noise could not be fully clarified. In other words, no clear correlation was observed between λp-p(B-), BF, and transformer noise.

[0032] The reason for this trend is not yet clear, but 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. When compressive stress exceeds a certain level, the magnetic domain structure changes to one with a magnetization component in the thickness direction (stress pattern). The amount of expansion and contraction of grain-oriented electrical steel sheets when magnetized in the rolling direction is positively correlated with the volume of magnetic domains rotated during the magnetization process. Therefore, in the stress-pattern magnetic domain structure, the amplitude of magnetostriction increases rapidly, and at the same time, the magnetic permeability in the rolling direction also decreases, resulting in increased iron loss. The compressive stress that changes to a stress pattern depends on innate factors inherent to the steel sheet, such as the crystal orientation and texture, as well as acquired factors, such as the state of the tension coating and residual stress due to processing strain and cooling strain. Of these, the innate factors change little within the raw electrical steel sheet during the manufacturing process of transformer cores, while the acquired factors change within the raw electrical steel sheet due to manufacturing variations. Therefore, λp-p(B-) is considered to represent the stress-resistance performance of the raw electrical steel sheet in a state that includes both factors. 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, among 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 considered to represent the innate stress-resistance performance of the raw electrical steel sheet. Therefore, Δλp-p(-) is considered to represent the susceptibility of the grain-oriented electrical steel sheet used as the raw material to acquired factors.

[0033] Based on the above, we believe that by manufacturing a transformer core using grain-oriented electrical steel sheet 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 moment during erection and processing distortion during transformer manufacturing. It is preferable to manufacture a transformer core using grain-oriented electrical steel sheet with a Δλp-p(-) of 2.5 ppm or less, and more preferably 2.0 ppm or less. The lower limit of Δλp-p(-) is not particularly limited, but an example is 0.1 ppm.

[0034] (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 was 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.

[0035] 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 ) was measured.

[0036] Furthermore, the same test piece was subjected to a load of 0.3 kgf / mm in the rolling direction of the steel sheet in the same manner as in Experiment 1. ２ The magnetostriction amplitude (λp-p(B-)) was measured when a compressive stress of 1000 kJ / s was applied.

[0037] Next, the same test piece was subjected to a load of 0.3 kgf / mm in the rolling direction of the steel plate. ２ The magnetostriction amplitude (λp-p(B+)) was measured when a tensile stress of 0.3 kgf / mm was applied to the test piece in a direction parallel to the rolling direction of the steel sheet. ２With a tensile stress of 1.5 T applied, the amplitude (λp-p(B+)) of the vibration displacement (magnetostriction) of the steel sheet in the rolling direction of the steel sheet was measured when the steel sheet was excited with an AC current at a magnetic flux density of 1.5 T and a frequency of 50 Hz.

[0038] Subsequently, the same test specimens were subjected to stress relief annealing in the same manner as in Experiment 1. Then, the test specimens after stress relief annealing were subjected to a stress of 0.3 kgf / mm in the rolling direction of the steel sheet. ２ The magnetostriction amplitude (λp-p(A-)) was measured when a compressive stress of 0.3 kgf / mm was applied to the test piece after the strain relief annealing. ２ The magnetostriction amplitude (λp-p(A+)) was measured when a tensile stress of 0.3 kgf / mm was applied to the test piece in a direction parallel to the rolling direction of the steel sheet. ２ With a tensile stress of 1.5 T applied, the amplitude (λp-p(A+)) of the vibration displacement (magnetostriction) of the steel sheet in the rolling direction of the steel sheet was measured when the steel sheet was excited with an AC current at a magnetic flux density of 1.5 T and a frequency of 50 Hz.

[0039] Next, a three-phase, three-limbed wound core, as shown in the schematic diagram in Figure 1(b), was fabricated using a raw material electrical steel sheet with a Δλp-p(-) of 2 ppm. Specifically, the steel strip was slit into 100 mm widths and wound in the rolling direction to form a single layer. Multiple such single 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 cores were 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. 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 as no-load loss. The building factor (BF) of each wound core transformer was calculated from the ratio of this transformer iron loss to the above-mentioned single-plate iron loss. Furthermore, the noise level (dBA) of each wound core transformer was measured in the same manner as in Experiment 1.

[0040] The measurement results of the building factor and transformer noise described above are shown in Figures 6 and 7. Figure 6 is a graph showing the relationship between the change in magnetostriction amplitude Δλp-p(+) of the material electrical steel sheet before and after stress relief annealing [i.e., the difference between Δλp-p(B+) measured for the material electrical steel sheet before stress relief annealing and Δλp-p(A+) measured for the material electrical steel sheet after stress relief annealing] and the building factor (BF). Figure 7 is a graph showing the relationship between the change in magnetostriction amplitude Δλp-p(+) of the material electrical steel sheet before and after stress relief annealing and the transformer noise.

[0041] 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.

[0042] The cause of 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 compressive stressed portion (compressive stress portion) and the tensile stressed portion (tensile stress portion). In other words, grain-oriented electrical steel sheets with a large Δλp-p(+) are thought to emit increased noise due to increased localized iron loss caused by magnetic flux concentration in the tensile stressed portion with higher magnetic permeability, and complex core vibration caused by non-uniform vibration.

[0043] Based on the above, we believe that by fabricating a transformer core using grain-oriented electrical steel sheet with a Δλp-p(+) of 0.40 ppm or less, magnetic flux concentration in tensile stress areas and non-uniform vibration can be suppressed, thereby suppressing deterioration of the building factor and noise. It is preferable to fabricate a transformer core using grain-oriented electrical steel sheet with a 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 an 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 the present embodiments, and various modifications are possible without departing from the spirit of the present invention.

[0045] [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.

[0046] Furthermore, the present invention can also be applied to grain-oriented electrical steel sheets in which the contents of Al, N, S, and Se are limited and no inhibitor is used. In this case, the contents of Al, N, S, and Se are preferably limited 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.

[0047] The basic components and optional added components of the slab for grain-oriented electrical steel sheets will be specifically described.

[0048] 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%.

[0049] Si: 2.0 to 8.0 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%.

[0050] Mn: 0.005 to 1.0 mass% Mn is an element necessary for improving hot workability. When the Mn content is 0.005 mass% or more, this effect is easily achieved. On the other hand, when the Mn content is 1.0 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 mass%.

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

[0052] One or more selected from Ni: 0.03 to 1.50 mass%, Sn: 0.01 to 1.50 mass%, Sb: 0.005 to 1.50 mass%, Cu: 0.03 to 3.0 mass%, P: 0.03 to 0.50 mass%, Mo: 0.005 to 0.10 mass%, and Cr: 0.03 to 1.50 mass%

[0053] Ni is an element effective for improving the hot-rolled sheet structure and improving 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 suppressed from becoming unstable, and the risk of deterioration of the magnetic properties of the product sheet can be reduced. Therefore, when Ni is contained, the Ni content is preferably in the range of 0.03 to 1.50 mass%.

[0054] Furthermore, Sn, Sb, Cu, P, Mo, and Cr are also elements that improve magnetic properties, and when the content of each of these elements is equal to or greater than the lower limit of the respective element, the effect of improving magnetic properties is more easily obtained. On the other hand, when the content of each of these components is equal to or less than the upper limit of the respective element, the risk of suppressing the growth of secondary recrystallized grains is more easily reduced, and deterioration of magnetic properties is more easily suppressed. Therefore, when Sn, Sb, Cu, P, Mo, and Cr are contained, the content of each of these elements is preferably set within the respective ranges described above.

[0055] The balance other than the above components consists of Fe and unavoidable impurities.

[0056] A slab having the above-described chemical composition is hot-rolled and then hot-rolled sheet annealed. Then, one or two cold-rolled passes are performed to finish the steel strip to 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. The steel strip after final annealing is subjected to flattening annealing to form an insulating coating (tensile insulating coating). Furthermore, in this embodiment, as described below, a treatment is performed to impart anisotropy to the coating tension imparted to the steel sheet by the tensile insulating coating.

[0057] In this embodiment, a magnetic domain refining step may be included in a step after flattening annealing, 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 refining step such as electrolytic etching or groove formation on the steel sheet by laser irradiation may be included in a step after cold rolling.

[0058] [Magnetostriction Measurement Method (Δλp-p(-), Δλp-p(+))] The magnetostriction measurement method, which is essential in the present invention, will now be described. Magnetostriction measurements in the present invention were carried out using a TRS-200 manufactured by Toei Kogyo. A test piece was taken from the grain-oriented electrical steel sheet (steel strip) to be measured, by shearing it 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 reflecting the measurement laser, 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 top to prevent the test piece from buckling, and then the specified pressure of 3.0 kgf / mm was applied in the rolling direction of the steel sheet. ２ 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-). Next, a specified compressive stress of 3.0 kgf / mm is applied in the rolling direction of the steel sheet. ２ 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+).

[0059] The test specimens were then subjected to stress relief annealing. The stress relief annealing was a heat treatment in which the test specimens were held at 800°C in an Ar atmosphere for 3 hours. The test specimens after stress relief annealing were then subjected to magnetostriction measurements in the same manner as described above, to obtain the magnetostriction amplitude λp-p(A-) when a predetermined compressive stress was applied and the magnetostriction amplitude λp-p(A+) when a predetermined tensile stress was applied. 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+). Test specimens were collected from the leading and trailing ends of the raw electrical steel sheet (coil) used to manufacture the transformer core, and the largest values ​​of Δλp-p(-) and Δλp-p(+) were used as the 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 a tensile insulation coating with a laser or electron beam in stripes to impart anisotropy to the coating tension in the rolling direction of the steel sheet, a method of forming minute grooves or ridges on the surface to which the tensile insulation coating is applied (the coating base on which the tensile insulation coating is formed) to impart anisotropy to the tension in the rolling direction of the steel sheet, a method of increasing the rigidity of the base steel by subjecting the outermost surface of the base steel to shot peening or laser peening before baking the tensile insulation coating, or a suitable combination of these methods. Any of these methods may be used in the present invention.

[0061] The tension of the tensile insulating coating is imparted to the steel sheet by irradiating the tensile insulating coating with a laser or electron beam in stripes parallel to the rolling direction of the steel sheet to partially modify the coating, or by forming grooves or ridges parallel to the rolling direction of the steel sheet in the coating substrate on which the tensile insulating coating is formed. By imparting anisotropy to the coating tension imparted by the tensile insulating coating, the coating is less susceptible to subsequent cooling distortion and processing distortion. For example, when the tensile insulating coating is irradiated with a laser or electron beam, the irradiated area hardens (crystallization of the irradiated area progresses, increasing the Young's modulus). Forming a striped structure in the tensile insulating coating by irradiating the tensile insulating coating with a laser or electron beam in stripes parallel to the rolling direction of the steel sheet to partially modify the coating results in a difference in Young's modulus between the rolling direction of the steel sheet and the direction perpendicular to the rolling direction of the steel sheet. This results in a stronger coating tension in the rolling direction of the steel sheet. This improves the compressive stress resistance in the rolling direction of the steel sheet, 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, by generating a stronger coating tension in the rolling direction of the steel sheet as described above, the change in magnetostriction when tensile stress is applied can also be suppressed.

[0062] Below, an example of laser or electron beam irradiation conditions will be described when adjusting the Δλp-p(-) and Δλp-p(+) of a steel strip by partially modifying the tensile insulation coating through laser or electron beam irradiation.

[0063] (Laser or electron beam output: 50 W or more and 5000 W 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. Therefore, 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 to the coating, causing damage to the coating. From the above viewpoints, the output of the laser or electron beam is preferably 50 W or more and 5000 W or less.

[0064] (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.

[0065] (Striation Spacing) 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 streak spacing is preferable because it increases the anisotropy of the coating tension. However, if the streak spacing 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 streak spacing be approximately the same as the spot diameter of the laser or electron beam. As an example, the streak spacing is preferably 0.1 mm or more. On the other hand, if the streak spacing is too large, anisotropy in the coating tension will not occur, so the streak spacing is preferably 2 mm or less.

[0066] (Scanning speed: 5 to 400 m / s) The slower the scanning speed of the laser or electron beam, the more heat can be incident per unit length of the coating, so a slower speed is preferable. However, if the scanning speed is set too slow, the processed 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 set 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.

[0067] Below, an example of groove or ridge formation conditions will be described for adjusting the Δλp-p(-) and Δλp-p(+) of the 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 by hairline processing or the like, or by forming ridges by powder bed additive manufacturing or the like. The grooves or ridges can be formed, for example, by providing 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.

[0068] (Spacing of Grooves or Ridges) The spacing between grooves (the distance between grooves) or between ridges (the distance between ridges) is preferably narrower because it increases the anisotropy of the coating tension. However, if the spacing between grooves is too narrow, the amount of undercoating grinding increases, resulting in a deterioration of insulation properties. Furthermore, if the spacing between ridges is too narrow, the total thickness of the tensile insulating coating and undercoating increases, resulting in a decrease in the space factor. Therefore, it is preferable that the lower limit of the spacing between grooves or ridges be approximately the same as the width of the groove or ridge. As an example, the spacing between grooves or ridges is preferably 0.1 mm or more. On the other hand, if the spacing between grooves or ridges is too large, the anisotropy of the coating tension will not occur, so the spacing between grooves or ridges is preferably 2 mm or less.

[0069] (Width of grooves or ridges) The narrower the width of the grooves or ridges, the greater the anisotropy of the film tension, which is preferable. Therefore, in this embodiment, the width of the grooves or ridges is preferably 300 μm or less. The width of the grooves or ridges is more preferably 280 μm or less, and even more preferably 260 μm or less. On the other hand, if the width of the grooves or ridges is excessively narrow, the anisotropy of the film tension decreases, so the width of the grooves or ridges is preferably 2 μm or more.

[0070] (Groove Depth or Ridge Height) A larger groove depth (groove depth) or ridge height (ridge height) is preferable because it increases the anisotropy of the coating tension. 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 film thickness of the tensile insulating coating, leading to deterioration of the insulation properties and space factor. Therefore, it is preferable that the groove depth is less than the film thickness of the base coating and the ridge height is less than the film thickness of the tensile insulating coating. More preferably, the groove depth is less than 80% of the film thickness of the base coating and the ridge height is less than 80% of the film thickness of the tensile insulating coating.

[0071] The grain-oriented electrical steel sheet that has been subjected to the above-described treatment to impart anisotropy to the coating tension imparted by the tensile insulation coating may then be subjected to a non-heat-resistant magnetic domain refinement treatment, which can be carried out, for example, by irradiating the tensile insulation coating with a laser or electron beam under known conditions.

[0072] In the present invention, other than the steps and manufacturing conditions described above, known manufacturing methods for grain-oriented electrical steel sheets can be used as appropriate.

[0073] [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 material electromagnetic steel sheets and then pressing the corners of the core to a predetermined curvature to form a rectangular shape.

[0074] 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 electrical steel sheets, calculating Δλp-p(-) from the difference between λp-p(B-) and λp-p(A-), selecting grain-oriented electrical steel sheets whose calculated Δλp-p(-) is within a predetermined range, and fabricating a transformer core using the selected grain-oriented electrical steel sheets. The manufacturing method may also include the steps of measuring λp-p(B+) and λp-p(A+) of grain-oriented electrical steel sheets, calculating Δλp-p(+) from the difference between λp-p(B+) and λp-p(A+), selecting grain-oriented electrical steel sheets whose calculated Δλp-p(+) is within a predetermined range, and fabricating a transformer core using the selected grain-oriented electrical steel sheets. In this case, the step of selecting grain-oriented electrical steel sheets can select grain-oriented electrical steel sheets whose calculated Δλp-p(-) is within a predetermined range and whose calculated Δλp-p(+) is within a predetermined range, and the step of producing a transformer core can produce a transformer core using the grain-oriented electrical steel sheets selected in this manner.

[0075] 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 the present invention is not limited to these examples. The present invention can be practiced with modifications within the scope of the spirit of the present invention, and such modifications are also within the technical scope of the present invention.

[0076] Grain-oriented electrical steel sheets were manufactured using a steel slab with the chemical composition shown in Table 2, using a general manufacturing process. Before forming a tensile insulating coating on the grain-oriented electrical steel sheet, grooves were introduced parallel to the rolling direction of the steel sheet into the surface to which the tensile insulating coating was to be applied (the forsterite base material of the coating) using a grinding device, and then steel strip A on which the tensile insulating coating was formed was prepared as a test material. In addition, after forming a tensile insulating coating on the grain-oriented electrical steel sheet manufactured as described above, steel strip B (laser irradiated) and steel strip C (electron beam irradiated) were subjected to a treatment to partially modify the tensile insulating coating by irradiating the tensile insulating coating on the grain-oriented electrical steel sheet manufactured as described above with a laser or electron beam parallel to the rolling direction of the steel sheet, and prepared as test materials.

[0077]

[0078] For the steel strip A, a grinding device was installed on the inlet side of the furnace during the baking process of the tensile insulating coating. After grooves were formed parallel to the rolling direction on the coating base, a tensile insulating coating was formed. For the steel strips B and C, a device for irradiating a laser beam or electron beam with a top-hat energy distribution was installed on the outlet side of the furnace during the baking process of the tensile insulating coating. The laser beam or electron beam was then irradiated in stripes parallel to the rolling direction of the steel sheet to partially modify 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 was strengthened, and the coating tension in the rolling direction of the steel sheet was 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. After the treatment to impart anisotropy to the coating tension imparted by the tension insulating coating as described above was performed, some of the steel strips were further subjected to a non-heat-resistant magnetic domain refinement treatment (DR).

[0079] Test pieces measuring 280 mm in the rolling direction and 100 mm in the sheet width direction were cut out from each of these steel strips, and the iron loss of the material electrical steel sheet (single sheet iron loss: W １７／５０ ) was measured. Furthermore, λp-p(B-), λp-p(B+), λp-p(A-), and λp-p(A+) were measured for the same test piece 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+).

[0080] Next, using the above-mentioned raw material electromagnetic steel sheets, a three-phase three-limbed stacked core as shown in the schematic diagram of Figure 1(a) was produced. 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. A plurality of these single layers were stacked to a stacking thickness of 30 mm to produce a three-phase three-limbed 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-limbed wound core as shown in the schematic diagrams of Figures 1(b) and 1(c) was produced. 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. A plurality of these single layers were stacked to a stacking thickness of 30 mm to produce a three-phase three-limbed 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 (Fig. 1(c))), and one with two 45-degree bends at the corners (Unicore (Fig. 1(b))). The Tranco and some Unicores were subjected to stress relief annealing at 800°C for three hours in an Ar atmosphere. For the Tranco, only electrical steel sheets that had not undergone non-heat-resistant magnetic domain refinement treatment were used, because the effect of non-heat-resistant magnetic domain refinement is lost due to stress relief annealing. In Tables 3 to 7, if no non-heat-resistant magnetic domain refinement treatment was performed, the DR (magnetic domain refinement treatment) column is marked with "none." If non-heat-resistant magnetic domain refinement treatment was performed using a laser, the DR column is marked with "laser." If non-heat-resistant magnetic domain refinement treatment was performed using an electron beam, the DR column is marked with "electron beam."

[0081] For each manufactured core, 50 turns of winding were applied to each leg on both the primary and secondary sides, and the iron loss characteristics (transformer iron loss) were measured when the magnetic flux density in the core leg portion 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 as no-load loss. The building factor (BF) of each transformer was calculated from the ratio of this transformer iron loss to the above-mentioned single-plate iron loss. Furthermore, each transformer was excited in a soundproof room as described above 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.

[0082] Tables 3 to 7 confirm that satisfying the requirements of the present invention enables the production of transformer cores with excellent improvements in building factor. Satisfying the requirements of the present invention enables the production of transformer cores with low iron loss and low noise. Furthermore, it can be confirmed that transformer cores using raw electrical steel sheets that have undergone non-heat-resistant magnetic domain refinement treatment in addition to treatment that imparts anisotropy to the coating tension imparted by the tensile insulating coating provide even more excellent improvements.

[0083]

[0084]

[0085]

[0086]

[0087]

Claims

1. A transformer core made of grain-oriented electromagnetic steel sheet, wherein the grain-oriented electromagnetic steel sheet has a strength of 0.3 kgf / mm in the rolling direction of the steel sheet. ２ The magnetostriction amplitude λp-p(B-) when a compressive stress of 0.3 kgf / mm was applied in the rolling direction of the steel sheet after annealing for 3 hours at 800 ° C in an Ar atmosphere. ２ A transformer core in which the difference Δλp-p(-) between the magnetostriction amplitude λp-p(A-) when a compressive stress of 1000 kJ / s is applied and the magnetostriction amplitude λp-p(A-) is 3.0 ppm or less.

2. The grain-oriented electrical steel sheet has a strength of 0.3 kgf / mm in the rolling direction of the steel sheet. ２ The magnetostriction amplitude λp-p(B+) when a tensile stress of 0.3 kgf / mm was applied in the rolling direction of the steel sheet after annealing for 3 hours at 800 °C in an Ar atmosphere. ２ 2. The transformer core according to claim 1, wherein a difference Δλp-p(+) between the magnetostriction amplitude λp-p(A+) when a tensile stress of 1000 kJ / s is applied and the magnetostriction amplitude λp-p(A+) is 0.40 ppm or less.

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

4. A transformer comprising a transformer core according to any one of claims 1 to 3.

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