Grain-oriented electromagnetic steel strip, laminated iron core, and method for producing grain-oriented electromagnetic steel strip

Abstract

The present invention reduces BF. This grain-oriented electromagnetic steel strip includes magnetic domains subdivided by strain introduced linearly so as to intersect the rolling direction on at least one surface. The ratio of the region in which the maximum distance between magnetic walls of a reflux magnetic domain measured by an X-ray magnetic circularly polarized light-emitting microscope in the inner part of the strip thickness is 10 μm or more to the total length in the rolling direction of the grain-oriented electromagnetic steel strip is 50% or more.

Description

Grain-oriented electromagnetic steel strip, stacked iron core, and method for manufacturing grain-oriented electromagnetic steel strip

[0001] The present invention relates to grain-oriented electrical steel strip, a stacked iron core, and a method for manufacturing grain-oriented electrical steel strip.

[0002] Transformers that use grain-oriented electrical steel sheets are required to have low iron loss and low noise. Reducing the iron loss of grain-oriented electrical steel sheets themselves is an effective way to reduce iron loss in transformers, and one technology for this is to subdivide magnetic domains by irradiating the steel sheet surface with high-energy beams such as laser beams, plasma beams, and electron beams. This technology not only subdivides the main magnetic domains but also forms a new magnetic domain structure called a closure domain within the steel sheet, thereby achieving low iron loss.

[0003] FIG. 1 is a schematic diagram showing the magnetic domain structure of a grain-oriented electrical steel sheet that has been subjected to the magnetic domain refining treatment, observed from a direction perpendicular to the surface of the steel sheet. The grain-oriented electrical steel sheet has alternating main magnetic domains 10a-10h with different magnetization directions, each extending in the rolling direction (RD). Linear regions (strain-introduced regions 20) where strain has been introduced by high-energy beam irradiation are formed across the rolling direction, and closure domains 21a-21d are formed in the strain-introduced regions 20. Similar to the main magnetic domains, the closure domains also have alternating closure domains 21a-21d with different magnetization directions. The boundaries between adjacent magnetic domains are called domain walls, and the distance between the domain walls is called the domain wall distance (or domain width). As shown in this figure, by introducing strain across the main magnetic domain, the main magnetic domain can be subdivided, i.e., the magnetic domain width of the main magnetic domain can be reduced. As a result, the iron loss of the grain-oriented electrical steel sheet can be reduced. Note that Figure 1 is merely a schematic representation of the structure of an extremely narrow region of a grain-oriented electrical steel sheet, and the number and dimensional ratios of the magnetic domains shown in the figure do not correspond to the actual structure.

[0004] As described above, progress has been made in improving the iron loss of grain-oriented electrical steel sheets, but even if a transformer is manufactured using a grain-oriented electrical steel sheet with low iron loss as its iron core, the iron loss of the resulting transformer (transformer iron loss) is not necessarily low. This is because, whereas the excitation magnetic flux used to evaluate the iron loss of grain-oriented electrical steel sheets themselves has only a component in the rolling direction, when the steel sheet is actually used as a transformer iron core, the excitation magnetic flux has a component in both the rolling direction and a component perpendicular to the rolling direction.

[0005] The building factor (BF), defined as the ratio of the transformer's iron loss to the iron loss of the base steel sheet, is commonly used as an index to represent the difference in iron loss between the base steel sheet itself and the transformer manufactured using that steel sheet. A BF greater than 1 means that the transformer's iron loss is greater than the iron loss of the base steel sheet. Grain-oriented electrical steel sheets are materials whose iron loss is lowest when magnetized in the rolling direction. Therefore, when they are incorporated into a transformer that is magnetized in directions other than the rolling direction, the iron loss increases and the BF becomes greater than 1. To improve the energy efficiency of transformers, it is necessary not only to reduce the iron loss of the base steel sheet, but also to make this BF as low as possible, i.e., approach 1.

[0006] For example, Patent Document 1 discloses a method for improving BF by optimizing the total tension applied to a steel sheet by a forsterite film and a tension coating, even when the coating is deteriorated by laser irradiation or electron beam irradiation.

[0007] Furthermore, Patent Document 2 discloses a technique for obtaining good iron loss in a transformer by optimizing the intervals between points of an electron beam irradiated in a point sequence.

[0008] On the other hand, a technique has been proposed that focuses on closure domains formed during magnetic domain refinement using laser irradiation, and reduces iron loss by optimizing their shape and dimensions (Patent Documents 3 and 4).

[0009] Japanese Patent Application Laid-Open No. 2012-031498 Japanese Patent Application Laid-Open No. 2012-036450 Japanese Patent Application Laid-Open No. 11-279645 Japanese Patent Application Laid-Open No. 2002-012918

[0010] By applying the techniques described above, it is possible to improve the BF to some extent. However, the BF is still far from being 1, and further improvement of the BF is currently desired.

[0011] The present invention has been made in view of the above circumstances, and has an object to provide a grain-oriented electrical steel strip and a stacked core that can further reduce BF.

[0012] The inventors conducted extensive research to solve the above-mentioned problems. They particularly focused on the fact that BFs can vary even when the core loss (hereinafter referred to as "material core loss") of grain-oriented electrical steel strips used as raw materials is the same, and investigated the causes of this. As a result, they obtained the following findings: (1) The maximum domain wall distance (domain width) of closure domains within the strip thickness, as measured using an X-ray magnetic circularly polarized emission microscope, significantly affects BF. (2) In conventional general magnetic domain refining methods, the domain wall distance varies significantly in the rolling direction of a single steel strip, resulting in a deterioration of BF. (3) When performing magnetic domain refining by irradiating a high-energy beam, the domain wall distance can be appropriately controlled by introducing strain while dynamically controlling the focus of the high-energy beam, thereby effectively improving BF.

[0013] The main results of the experiments conducted to obtain the above findings will be described below.

[0014] Deterioration of BF over a Long Term Span Magnetic domain refinement treatment of grain-oriented electrical steel strips is usually performed by irradiating the surface of the steel strip with a high-energy beam while continuously transporting the steel strip after finish annealing in the rolling direction of the steel strip. Although the time required for magnetic domain refinement treatment of one steel strip is not very long, such magnetic domain refinement treatment is performed continuously on a large number of steel strips. Therefore, magnetic domain refinement treatment equipment in actual factories is used continuously for long periods of time, such as several months or more.

[0015] In order to examine the effect of such continuous magnetic domain refinement treatment over a long period on BF, magnetic domain refinement treatment was carried out continuously for a period of six months or more using one magnetic domain refinement treatment facility. During that period, magnetic domain refinement treatment by electron beam irradiation was carried out under the same conditions on steel strips after finish annealing produced under the same conditions every other month. The resulting steel strips that had undergone magnetic domain refinement treatment (grain-oriented electrical steel strips) are designated steel strips A to G in order of the timing of the magnetic domain refinement treatment, starting with the strips that were subjected to the treatment earliest.

[0016] Samples (single plate samples) for evaluating material iron loss were cut out from each of the steel strips A to G. １７／５０ The evaluation results are shown in Figure 2A.

[0017] A transformer core was also fabricated by stacking grain-oriented electromagnetic steel sheets cut from the grain-oriented electromagnetic steel strip. The core was a three-phase, three-legged stacked core, with a rectangular shape of 500 mm on each side, made of 100 mm-wide steel sheets. The grain-oriented electromagnetic steel sheets were bevel-cut so that the longitudinal direction was the rolling direction, and the sheets were stacked to a thickness of approximately 15 mm and a core weight of approximately 20 kg. The stacking method was a five-stage step-lap stack of two sheets. The core was stacked flat on a flat surface and further clamped and fixed between Bakelite pressure plates under a load of approximately 0.1 MPa.

[0018] Next, the transformer iron loss of the fabricated stacked core was measured. The excitation conditions for the measurement were a phase difference of 120°, a maximum magnetic flux density of 1.7 T, and a frequency of 50 Hz. From the obtained transformer iron loss and the previously calculated material iron loss, BF was calculated using the following formula. The results are shown in Figure 2B. BF = transformer iron loss / material iron loss

[0019] As can be seen from the results shown in FIGS. 2A and 2B, the material iron loss remained at approximately the same level over a long period of time, whereas BF gradually increased.

[0020] Correlation between maximum inter-domain wall distance and BF obtained using different measurement methods and measurement positions The inventors suspected that the deterioration in BF was due to differences in the closure domains introduced by the magnetic domain refinement process, and so observed the closure domains of each of the steel strips A to G and measured the average inter-domain wall distance of the closure domains. Note that "inter-domain wall distance" refers to the distance between adjacent domain walls and is also referred to as "domain width." In Figure 1, d denotes the inter-domain wall distance between individual closure domains, and this "inter-domain wall distance (domain width) d" is different from the overall width of the region (strain-introduced portion 20) where the closure domains are formed.

[0021] Observation of the closure domains was carried out using the following three methods, of which (2) and (3) were carried out at two locations: the surface of the steel sheet and the interior of the steel sheet. Note that the method using a domain viewer is only applicable to the surface of the steel sheet. (1) Observation using a domain viewer (2) Observation using a Kerr effect microscope (3) Observation using an X-ray magnetic circularly polarized light emission microscope

[0022] For the observation using the domain viewer in (1) above, a CMOS-Magview manufactured by Matesy GmbH was used. For the observation using the Kerr effect microscope in (2) above, a Kerr effect microscope BH-786V-JS manufactured by NeoArc was used. For the observation using the X-ray magnetic circularly polarized emission microscope in (3) above, synchrotron radiation was used. The specific observation method was as described in "Observation of transverse domains in grain-oriented electrical steel by means of X-ray magnetic circularly polarized emission microscopy" in the 46th Annual Conference on Magnetics in Japan, 2022, p. 58. In the observation using the X-ray magnetic circularly polarized emission microscope, the magnetic domains on the surface and inside the steel strip were observed by changing the incident position of the synchrotron radiation.

[0023] The average domain wall distance on the steel strip surface was measured as follows. First, the steel strip to be measured was divided into 50 equal parts in the rolling direction (longitudinal direction), and 50 samples were taken. Next, each sample was further divided into 10 parts in the strip width direction, and an observation sample measuring 30 mm × 30 mm was taken from the center of each divided area. The closure domains on the surface of the observation sample were then observed in a 1 mm × 1 mm field of view randomly selected from within the 30 mm × 30 mm area, and the domain wall distance of the closure domains was measured. The average value of the domain wall distances in the 10 observation samples divided into 10 parts in the strip width direction was defined as the average domain wall distance in the strip width direction. Similar measurements were performed on 50 samples divided into 50 parts in the rolling direction, and the average value of the obtained average domain wall distances in the strip width direction was defined as the "average domain wall distance on the steel strip surface."

[0024] For the observation of the interior of the steel sheet, the steel strip was further divided into sections at a pitch of 5 μm in the depth direction (thickness direction), and the average inter-domain wall distance at each depth was determined by the above procedure. The maximum value of the average inter-domain wall distances obtained at each depth was then defined as the "maximum inter-domain wall distance within the thickness direction" of the steel strip.

[0025] The above measurements were carried out for each of steel strips A to G, and the "average distance between domain walls on the steel strip surface" and the "maximum distance between domain walls within the thickness of the strip" were obtained. The correlation between the obtained "average distance between domain walls on the steel strip surface" or "maximum distance between domain walls within the thickness of the strip" and BF is shown in Figures 3 to 5. Each figure corresponds to the following observation results. - Figure 3: Measurement results of the steel strip surface using a domain viewer - Figure 4A: Measurement results of the steel strip surface using a Kerr effect microscope - Figure 4B: Measurement results of the interior of the strip using a Kerr effect microscope - Figure 5A: Measurement results of the steel strip surface using an X-ray magnetic circularly polarized light emission microscope - Figure 5B: Measurement results of the interior of the strip using an X-ray magnetic circularly polarized light emission microscope

[0026] As shown in Figure 5B, there was a correlation between the "maximum inter-domain wall distance within the thickness of the sheet" observed using an X-ray magnetic circularly polarized light emission microscope and BF. In contrast, no correlation was observed between the "maximum inter-domain wall distance within the thickness of the sheet" measured using a Kerr effect microscope and BF. Furthermore, there was no correlation observed between the "average inter-domain wall distance on the steel strip surface" and BF in either measurement method.

[0027] The reason for the differences due to the measurement method and measurement position is not clear, but is thought to be as follows.

[0028] (i) Cross-sectional observation using a Kerr effect microscope requires processing the steel strip to expose the observation surface. This processing releases the constraints inside the steel strip and changes the internal strain distribution. As a result, it is not possible to observe the magnetic domains as they are in the actual state used in the iron core.

[0029] (ii) In contrast, an X-ray magnetic circularly polarized light luminescence microscope allows for non-destructive observation of the inside of a steel strip, making it possible to observe the magnetic domains as they are in use in an actual iron core.

[0030] (iii) Because closure domains have magnetization components in directions other than the rolling direction, the presence of closure domains is advantageous in reducing BF. Furthermore, because closure domains are repeatedly created and annihilated during the magnetization process, the larger the domain wall distance (i.e., the wider the domain width), the less likely the closure domains are to annihilate, which is advantageous in reducing BF.

[0031] (iv) Since the strain caused by high-energy beam irradiation is introduced from the surface layer of the steel strip, the thermal energy that causes the strain is sufficiently large in the surface layer of the steel sheet. Therefore, the influence of fluctuations in the conditions during the magnetic domain refinement treatment on the magnetic domain wall distance is relatively small. As a result, no significant correlation was found between the "average magnetic domain wall distance on the steel strip surface" and BF.

[0032] (v) On the other hand, the thermal energy introduced by the high-energy beam irradiation attenuates as it propagates into the steel strip. In this state of attenuation, the influence of fluctuations in the conditions during the magnetic domain refinement treatment on the distance between magnetic domain walls is relatively large. As a result, a correlation was found between the "maximum distance between magnetic domain walls within the sheet thickness" and BF.

[0033] Fluctuations in Focus Conditions Fluctuations in focus conditions during high-energy beam irradiation were investigated as a cause of fluctuations in the "maximum inter-domain wall distance within the plate thickness" as shown in FIG. 5B.

[0034] During the magnetic domain refinement process by electron beam irradiation, the focus of the electron beam can be adjusted by controlling the convergence current. In the magnetic domain refinement process for the steel strips A to G described above, the initially set convergence current value I ０ was used without any changes.

[0035] At that time, a beam profiler was used to measure the convergence current I that becomes a JUST focus during the processing of each steel strip A to G (every month). Ａ ~I Ｇ Here, the just focus refers to the state in which the beam profile intensity on the processing surface (steel strip surface) is at its maximum (beam diameter is at its minimum). The obtained focusing current I Ａ ~I Ｇ and the initially set convergence current value I ０ The difference between these values ​​is taken as the convergence current deviation ΔI. The correlation between the obtained convergence current deviation ΔI and BF is shown in FIG.

[0036] As can be seen from Figure 6, a correlation was found between the convergence current deviation ΔI and BF. This indicates that when the magnetic domain refining process is performed continuously for a long period of time, the convergence current that results in a JUST focus becomes equal to the initially set convergence current value I ０ As a result, it can be seen that the BF gradually deviates from the normal.

[0037] This focus deviation is believed to occur for the following reasons: In the case of electron beams, continuous use of an electron beam source reduces the electron generation efficiency due to wear of the cathode. As a result, the electron generation area must be increased to extract the same number of electrons. This change in electron generation area is thought to cause fluctuations in the initial beam diameter, which in turn changes the convergence current that results in just focus. In addition to changes over time, the electron generation efficiency is also thought to change due to individual differences in the cathode, variations in the cathode heating temperature, and variations in the vacuum level in the electron gun chamber, which in turn causes fluctuations in just focus.

[0038] As mentioned above, the steel strips A to G have approximately the same material iron loss. Therefore, even if the above-mentioned fluctuations in the focus conditions occur, if product performance is evaluated based solely on the material iron loss, all of them will be at an acceptable level. In that sense, the fluctuations in the focus conditions are a minor issue. However, in order to improve BF, it is considered important to perform focus adjustment with higher precision in order to reduce the influence of such fluctuations in the focus conditions.

[0039] - Control based on bias voltage As mentioned above, fluctuations in focus conditions can be caused by a variety of factors. It is difficult to grasp and control all of these various factors. Therefore, we attempted to estimate fluctuations in focus conditions by monitoring the bias voltage, which reflects the ease of electron extraction from the cathode, and control the focus current value based on the results.

[0040] First, the electron generation efficiency was changed by changing the cathode heating temperature. The bias voltage changed with this change in electron generation efficiency. Therefore, the relationship between the bias voltage and the focusing current that results in a just focus was investigated. As a result, it was found that the focusing current that results in a just focus decreases as the bias voltage decreases, as shown in Figure 7.

[0041] Next, in order to confirm the effect of controlling the convergence current, the magnetic domain refining process was carried out at one-month intervals in the same manner as in the above-mentioned experiment under the following three conditions.

[0042] Condition 1: No adjustment of convergence current In condition 1, the convergence current was not adjusted. That is, after the convergence current was set when the first steel strip was subjected to the magnetic domain refinement treatment, the treatment was continued with the same convergence current.

[0043] Condition 2: Adjustment on a Steel Strip Basis Under Condition 2, when the magnetic domain refinement process was performed at monthly intervals as described above, the convergence current was adjusted based on the bias voltage at the start of processing for each steel strip. That is, when the magnetic domain refinement process was performed at each timing (every month), the bias voltage was checked, and the deviation in the convergence current was predicted from the difference between the bias voltage when the focus was set and the current bias voltage. The convergence current was adjusted to achieve JUST focus, and then the magnetic domain refinement process was performed.

[0044] Condition 3: Dynamic control Under condition 3, the focusing current was adjusted based on the bias voltage while the magnetic domain refinement process was being performed on one steel strip. Under this condition, the focus was dynamically controlled while the magnetic domain refinement process was being continuously performed on one steel strip while it was being transported, so irradiation was always performed in a focused state.

[0045] The obtained steel strips (grain-oriented electromagnetic steel strips) that have been subjected to the magnetic domain refinement treatment are designated as steel strips H to P in order of the timing of the magnetic domain refinement treatment, starting from the earliest.

[0046] When the iron loss of the steel strips obtained in the above experiment was measured, the iron loss W １７／５０ The magnetic field strength was about 0.68 W / kg, which was very good magnetic properties.

[0047] Meanwhile, when stacked cores were fabricated from each steel strip using the same procedure as in the previous experiment and the BF was measured, the results shown in Figure 8 were obtained. That is, under condition 1, where no control was performed, the BF deteriorated over time, similar to the results shown in Figure 2B above. On the other hand, under condition 2, where the focus was adjusted for each steel strip, the deterioration of BF was suppressed compared to condition 1, but was not completely suppressed. In contrast, under condition 3, where the focus was dynamically controlled, the deterioration of BF was significantly suppressed, even compared to condition 2.

[0048] These results demonstrate that when performing magnetic domain refinement by irradiating a high-energy beam, it is important to dynamically control the focus of the high-energy beam to introduce strain. This indicates that fluctuations in the focus conditions occur not only over long time spans such as one month, but also over extremely short time scales such as the time it takes to perform magnetic domain refinement on a single steel strip.

[0049] Next, the "maximum inter-domain wall distance within the thickness direction" of the steel strip H obtained under each of conditions 1 to 3 was measured using an X-ray magnetic circular polarization luminescence microscope. Figure 9 is a graph in which the measured "maximum inter-domain wall distance within the thickness direction" is plotted against the position in the rolling direction (longitudinal direction) of the steel strip. Note that the distance of zero on the horizontal axis of Figure 9 indicates the leading edge of the steel strip, i.e., the position where the magnetic domain refinement treatment was first performed.

[0050] These results show that under conditions 1 and 2, where no dynamic control was performed during the processing of a single steel strip, the maximum domain wall distance varied significantly depending on the position in the rolling direction of the steel strip. This variation is believed to be due to the variation in the focus conditions, as described above. Under condition 2, the convergence current was adjusted at the start of the magnetic domain refinement process for the steel strip, resulting in an overall increase in the maximum domain wall distance compared to condition 1. However, the slopes of the plots under conditions 2 and 1 were similar. This indicates that simply adjusting the convergence current at the start of the magnetic domain refinement process cannot prevent subsequent variations in the focus conditions. In contrast, under condition 3, where the focus was dynamically controlled, the variation in the maximum domain wall distance depending on the position in the rolling direction of the steel strip was significantly suppressed.

[0051] As described above, we found that the fluctuation of the maximum closure domain width can be suppressed by dynamically controlling the focus and introducing distortion during domain refinement by irradiating a high-energy beam. Therefore, we next investigated what range of the maximum closure domain width is necessary to suppress the increase in BF.

[0052] Specifically, we produced steel strips with various inter-domain wall distances by subjecting steel strips obtained under the same conditions after finish annealing to magnetic domain refinement treatment under different focus conditions. In the magnetic domain refinement treatment, the focus was dynamically controlled during the treatment of one steel strip to maintain a constant focus state over the entire length in the rolling direction. The focus state was varied from upper focus to under focus for each steel strip.

[0053] Next, the "maximum distance between magnetic domain walls within the thickness direction" and BF of each of the obtained steel strips were measured by the method described above. Note that for all steel strips, the "maximum distance between magnetic domain walls within the thickness direction" was constant and did not vary in the rolling direction.

[0054] 10 is a graph plotting the measured BF against the "maximum domain wall distance within the thickness of the sheet." This result shows that if the maximum domain wall distance of the closure domains measured with an X-ray magnetic circular polarization luminescence microscope is 10 μm or more, the BF is maintained in a good state.

[0055] Control based on orifice temperature: In magnetic domain refining processing using electron beam irradiation, focus control is possible not only by the bias voltage described above but also by monitoring the orifice temperature. The orifice is installed so that some of the electrons are cut off in the beam path. The orifice is installed between the cathode and the focusing coil.

[0056] When the ease of electron extraction from the cathode changes, the area within the cathode that emits electrons changes. When electrons are easy to extract, the emission area is small, and when electrons are difficult to extract, the emission area is large. The beam diameter up to the focusing coil changes depending on the emission area, and the focusing current value that achieves just focus on the steel sheet surface changes depending on the beam diameter when it reaches the focusing coil. When the beam diameter changes, the number of electrons cut by the orifice changes. The number of electrons cut is reflected in the orifice temperature, so fluctuations in beam diameter can be determined by monitoring the orifice temperature. Specifically, if the orifice temperature increases, the beam diameter increases, and conversely, if the orifice temperature decreases, the beam diameter decreases.

[0057] Therefore, if the relationship between the orifice temperature and the focusing current is known in advance, it is possible to dynamically control the focus based on the change in the orifice temperature during the magnetic domain refining process.

[0058] The position at which the orifice temperature is measured is not particularly limited, and it can be measured at any one point on the orifice. However, from the viewpoint of grasping the beam change in more detail, it is preferable to measure the temperature at multiple positions on the orifice. For example, in a substantially circular orifice, the temperature can be measured at multiple positions equally spaced in the angular direction (circumferential direction). When measuring at 120° intervals, the number of measurement points is three, and when measuring at 90° intervals, the number of measurement points is four. When measuring the temperature at multiple positions in this way, it is also possible to know in which direction the beam has changed. In focus control, the average value of the temperatures measured at multiple positions on the orifice may be used as the orifice temperature.

[0059] All of the above experiments used magnetic domain refining treatment using electron beam irradiation. However, it was found that similar fluctuations in focus conditions also occur when magnetic domain refining treatment is performed using laser irradiation. During laser irradiation, the thermally induced optical distortion generated in the focusing lens varies depending on the output power of the irradiated beam. This thermally induced optical distortion changes the focal length of the lens, becoming the primary cause of focus fluctuations. Although it is difficult to monitor the amount of distortion, by understanding the relationship between laser output and focal length fluctuations in advance, it is possible to dynamically control the focus based on the laser beam output during magnetic domain refining treatment. Laser output not only changes over time, but also changes with irradiation conditions (such as temperature and humidity). Therefore, it is highly likely that the conditions for achieving just focus will fluctuate even immediately after focus adjustment.

[0060] Dynamic focus control in laser irradiation can be performed based on the orifice temperature, as in electron beam irradiation. The orifice is installed so that a portion of the laser beam is cut off in the beam path. When the laser output is higher than the set value, the beam energy cut off by the orifice increases, resulting in a higher orifice temperature. Conversely, when the output is lower than the set value, the beam energy cut off decreases, resulting in a lower orifice temperature. Therefore, based on the previously determined relationship between the orifice temperature and focal length, the position of the focusing lens of the laser beam can be adjusted in accordance with the measured orifice temperature.

[0061] Next, we investigated the effect of the maximum magnetic domain wall distance within the thickness of grain-oriented magnetic steel sheets on the BF when manufacturing a stacked core by stacking multiple grain-oriented magnetic steel sheets. Specifically, grain-oriented magnetic steel sheets were cut from grain-oriented magnetic steel strips manufactured under various conditions, and stacked cores with various BFs were manufactured by combining these sheets.

[0062] 11 is a graph plotting BF against the "weight percentage of regions where the maximum inter-wall distance is 10 μm or more." Note that the "weight percentage of regions where the maximum inter-wall distance is 10 μm or more" on the horizontal axis refers to the "percentage of regions within the thickness of the grain-oriented electrical steel sheet where the maximum inter-wall distance of closure domains is 10 μm or more as measured with an X-ray magnetic circular polarization luminescence microscope, relative to the total weight of the multiple grain-oriented electrical steel sheets constituting the stacked core."

[0063] 11, the BF improved when the "weight ratio of the region where the maximum inter-domain wall distance is 10 μm or more" was 50% or more. Furthermore, the BF improved further when the "weight ratio of the region where the maximum inter-domain wall distance is 10 μm or more" was 80% or more, and the BF was lowest when it was 100%.

[0064] Furthermore, if the grain-oriented electrical steel strips used as the raw material for stacked cores also contain regions where BF increase can be suppressed at a similar rate, stacked cores with good BF can be obtained even if the steel strips to be used are selected arbitrarily without strict management of the grain-oriented electrical steel strips. Therefore, in grain-oriented electrical steel strips, it is desirable that the ratio of regions within the sheet thickness where the maximum inter-wall distance of closure domains is 10 μm or more as measured with an X-ray magnetic circularly polarized light emission microscope to the total length of the grain-oriented electrical steel strip in the rolling direction is 50% or more.

[0065] The present invention has been completed based on the above findings, and the gist and configuration of the present invention are as follows.

[0066] 1. A grain-oriented electrical steel strip in which magnetic domains are refined by linearly introducing strain on at least one surface so as to intersect with the rolling direction, wherein the ratio of a region within the sheet thickness where the maximum inter-wall distance of closure domains is 10 μm or more as measured with an X-ray magnetic circularly polarized light emission microscope to the total length of the grain-oriented electrical steel strip in the rolling direction is 50% or more.

[0067] 2. A stacked core formed by laminating a plurality of grain-oriented electromagnetic steel sheets, wherein the region within the thickness of the grain-oriented electromagnetic steel sheets, where the maximum inter-wall distance of closure domains is 10 μm or more as measured with an X-ray magnetic circularly polarized light emission microscope, accounts for 50% or more of the total weight of the grain-oriented electromagnetic steel sheets.

[0068] 3. A method for producing a grain-oriented electrical steel strip as set forth in 1 above, comprising: carrying out a magnetic domain refinement treatment by irradiating a surface of a steel strip after finish annealing with a high-energy beam while continuously transporting the steel strip in the rolling direction of the steel strip; and during the irradiation of the high-energy beam, introducing strain while dynamically controlling the focus of the high-energy beam, thereby setting the ratio to 50% or more.

[0069] 4. The method for producing grain-oriented electrical steel strip according to 3 above, wherein the high-energy beam is an electron beam, and the dynamic focus control comprises measuring a bias voltage, and adjusting the focusing current of the electron beam in accordance with the measured bias voltage based on a previously determined relationship between the bias voltage and the focusing current.

[0070] 5. A method for producing grain-oriented electrical steel strip as set forth in 3 above, wherein the high-energy beam is an electron beam, an orifice is installed in the beam path of the electron beam so as to cut off a portion of the electron beam, and the dynamic focus control comprises measuring the temperature of the orifice, and adjusting the focusing current of the electron beam in accordance with the measured temperature of the orifice based on a previously determined relationship between the orifice temperature and the focusing current.

[0071] 6. A method for producing grain-oriented electromagnetic steel strip as set forth in 3 above, wherein the high-energy beam is a laser beam, an orifice is installed in the beam path of the laser beam so as to cut off a portion of the laser beam, and the dynamic control of the focus comprises measuring the temperature of the orifice, and adjusting the position of a focusing lens of the laser beam in accordance with the measured temperature of the orifice based on a predetermined relationship between the orifice temperature and a focal length.

[0072] According to the present invention, the BF can be reduced.

[0073] 1 is a schematic diagram showing the magnetic domain structure of a grain-oriented electrical steel sheet that has been subjected to magnetic domain refinement treatment, observed from a direction perpendicular to the surface of the steel sheet. FIG. 2 is a graph showing the change in material iron loss when grain-oriented electrical steel strips are produced at monthly intervals. FIG. 3 is a graph showing the change in BF when grain-oriented electrical steel strips are produced at monthly intervals. FIG. 4 is a graph showing the correlation between BF and the "average domain wall distance on the steel strip surface" measured using a domain viewer. FIG. 5 is a graph showing the correlation between BF and the "average domain wall distance on the steel strip surface" measured using a Kerr effect microscope. FIG. 6 is a graph showing the correlation between BF and the "maximum domain wall distance within the sheet thickness" measured using a Kerr effect microscope. FIG. 7 is a graph showing the correlation between BF and the "average domain wall distance on the steel strip surface" measured using an X-ray magnetic circular polarization luminescence microscope. FIG. 8 is a graph showing the correlation between BF and the "maximum domain wall distance within the sheet thickness" measured using an X-ray magnetic circular polarization luminescence microscope. FIG. 9 is a graph showing the correlation between the convergence current shift ΔI and BF. 1 is a graph showing an example of the relationship between bias voltage and convergence current that results in JUST focus. FIG. 2 is a graph showing changes in BF when grain-oriented electrical steel strips are manufactured at one-month intervals. FIG. 3 is a graph plotting "maximum domain wall distance within the sheet thickness" against the position in the rolling direction (longitudinal direction) of the steel strip. FIG. 4 is a graph showing the relationship between "maximum domain wall distance within the sheet thickness" and BF. FIG. 5 is a graph plotting BF against "weight percentage of regions where the maximum domain wall distance is 10 μm or more." FIG. 6 is a graph showing an example of the relationship between orifice temperature and convergence current that results in JUST focus. FIG. 7 is a graph showing an example of the relationship between orifice temperature and focal length that results in JUST focus.

[0074] The present invention will be described in detail below. Note that the following description is of preferred embodiments of the present invention, and the present invention is not limited to the embodiments described below.

[0075] [Grain-oriented electrical steel strip] A grain-oriented electrical steel strip according to one embodiment of the present invention is a grain-oriented electrical steel strip in which magnetic domains are subdivided by linearly introducing strain on at least one surface so as to intersect with the rolling direction, and in which the ratio of a region within the sheet thickness where the maximum inter-domain wall distance of closure domains as measured with an X-ray magnetic circularly polarized light emission microscope is 10 μm or more to the total length of the grain-oriented electrical steel strip in the rolling direction is 50% or more. In the following description, the "maximum inter-domain wall distance of closure domains as measured with an X-ray magnetic circularly polarized light emission microscope within the sheet thickness" may be simply referred to as the "maximum inter-domain wall distance."

[0076] As the experimental results above show, to obtain a good BF, the ratio of the region where the "maximum inter-domain wall distance" is 10 μm or more to the total length of the grain-oriented electrical steel strip in the rolling direction must be 50% or more. The larger the maximum average inter-domain wall distance within the sheet thickness, the later the timing at which the closure domains completely disappear when used as a transformer core, and as a result, the increase in BF is suppressed. Note that there is no particular upper limit to the maximum inter-domain wall distance, but since the width of the strain-introducing region is the practical upper limit, it is approximately 100 μm.

[0077] From the viewpoint of further improving the BF, the ratio of the region where the maximum inter-domain distance is 10 μm or more to the total length of the grain-oriented electrical steel strip in the rolling direction is preferably 75% or more, more preferably 90% or more. On the other hand, the upper limit of this ratio is not particularly limited and may be 100%. Most preferably, this ratio is 100%.

[0078] The ratio of the area where the maximum inter-domain distance is 10 μm or more to the total length of the grain-oriented electrical steel strip in the rolling direction can be determined by the following procedure.

[0079] First, the steel strip is divided into 50 equal parts in the rolling direction (longitudinal direction), 10 parts in the width direction, and a 5 μm pitch in the depth direction (thickness direction). Each divided part is observed using an X-ray magnetic circular polarization luminescence microscope, and the distance between the domain walls of the closure domains is measured. Next, the measured domain wall distances are averaged at 10 points in the width direction to determine the "average domain wall distance in the width direction" for each depth. The maximum value of the "average domain wall distance in the width direction" obtained at each depth is defined as the "maximum domain wall distance." This gives the "maximum domain wall distance" for each of the 50 parts divided in the rolling direction. Then, the percentage of the 50 regions divided in the rolling direction where the maximum domain wall distance is 10 μm or more is calculated.

[0080] The distance between the closure domain walls in each divided region was measured using the method described above. Specifically, a 30 mm × 30 mm observation sample was taken from the center of each divided region. The closure domains on the surface of the observation sample were then observed in a 1 mm × 1 mm field of view randomly selected from within the 30 mm × 30 mm region, and the distance between the closure domain walls was measured.

[0081] From the viewpoint of reducing the variation in properties within the steel strip and further improving the BF, the difference Δd between the maximum and minimum values ​​of the maximum domain wall distance over the entire longitudinal direction of the steel strip is preferably 10 μm or less, and more preferably 5 μm or less. On the other hand, from the viewpoint of further reducing the BF, the smaller Δd, the better. Therefore, the lower limit of Δd is not particularly limited and may be 0 μm.

[0082] [Stacked core] In one embodiment of the present invention, the stacked core is a stacked core formed by stacking a plurality of grain-oriented electromagnetic steel sheets, and the proportion of the region within the thickness of the grain-oriented electromagnetic steel sheets where the maximum inter-wall distance of closure domains as measured with an X-ray magnetic circular polarization luminescence microscope is 10 μm or more to the total weight of the plurality of grain-oriented electromagnetic steel sheets is 50% or more.

[0083] As shown by the above experimental results, if the weight ratio is 50% or more, the BF can be improved. Furthermore, the weight ratio is preferably 80% or more, and more preferably 90% or more. On the other hand, the upper limit of the weight ratio is not particularly limited and may be 100%. The weight ratio is most preferably 100%.

[0084] Note that stacked cores that satisfy the above conditions can be manufactured by using grain-oriented electrical steel sheets with known weight ratios. Furthermore, when evaluating the weight ratios of existing stacked cores, the lamination direction can be used as the reference instead of the rolling direction. For example, each part in a stacked core can be divided evenly into 50 parts in the lamination direction, and 50 samples can be taken. Then, a 30 mm x 30 mm observation sample can be taken from the center of each sample and measured.

[0085] In the stacked core of the present invention, the desired effects can be achieved by using grain-oriented electrical steel sheets that satisfy certain conditions as described above. Therefore, the type of stacked core is not particularly limited. For example, the stacked core may be either a single-phase or three-phase core, and may be either a shell-type or a core-type core. Furthermore, the stacked core may have any number of legs, for example, either a three-legged core or a five-legged core.

[0086] [Manufacturing Method of Grain-Oriented Electrical Steel Strip] Next, a manufacturing method of grain-orientated electrical steel strip according to one embodiment of the present invention will be described. The manufacturing method of grain-orientated electrical steel strip according to one embodiment of the present invention includes performing a magnetic domain refinement treatment by irradiating a high-energy beam onto the surface of a steel strip after finish annealing while continuously transporting the steel strip in the rolling direction of the steel strip. During the irradiation of the high-energy beam, strain is introduced while dynamically controlling the focus of the high-energy beam, thereby setting the ratio at 50% or more.

[0087] The dynamic control method is not particularly limited. The focus control may be performed continuously or intermittently. Examples of suitable control methods are described below. Note that the control method is not limited to the methods described below.

[0088] In the case of electron beam irradiation, when magnetic domain refining is performed by electron beam irradiation, the bias voltage during the process can be monitored (measured) and the focus can be controlled accordingly. The bias voltage can be measured continuously or intermittently. The focus can be adjusted by changing the current (focus current) in the focus coil. To perform this control, it is sufficient to determine in advance the relationship between the bias voltage and the focus current that results in just focus at that bias voltage. When determining this relationship, the bias voltage can be changed by adjusting the heater current.

[0089] When performing magnetic domain refining processing by electron beam irradiation, it is possible to monitor not only the bias voltage but also the orifice temperature and control the focus accordingly. The orifice temperature can be measured continuously or intermittently. The focus can be adjusted by changing the current (focus current) in the focus coil. To perform this control, it is sufficient to determine in advance the relationship between the orifice temperature and the focus current that results in just focus at that orifice temperature. When determining this relationship, the orifice temperature can be changed by adjusting the heater current.

[0090] - In the case of laser irradiation, on the other hand, the main cause of focus changes in laser irradiation is thermal distortion of the lens due to output fluctuations. Therefore, when performing magnetic domain refining processing by laser irradiation, an orifice is installed in the beam path and part of the laser beam is cut off by the orifice. In this case, the temperature of the orifice changes depending on the cut laser energy. Therefore, it is sufficient to monitor (measure) the orifice temperature during processing and control the focus accordingly.

[0091] The orifice temperature can be measured continuously or intermittently. The focus can be adjusted by changing the position of the condenser lens. To perform this control, the relationship between the orifice temperature and the focal length that provides just focus at that orifice temperature is determined in advance, and the position of the condenser lens is changed so that the distance between the condenser lens and the object becomes the focal length that corresponds to that orifice temperature. When determining this relationship, the orifice temperature can be changed by intentionally changing the beam output setting.

[0092] As described above, in the present invention, it is important to dynamically control the focus during the magnetic domain refining process. Therefore, other magnetic domain refining process conditions are not limited, and general conditions can be applied.

[0093] For example, when irradiating a high-energy beam, the scanning direction of the beam may be a direction intersecting the rolling direction of the steel strip. In other words, the scanning direction may be greater than 0° and less than 90° relative to the rolling direction (RD) of the steel strip. By irradiating the steel strip while scanning the beam in a direction intersecting the rolling direction of the steel strip, linear strain can be introduced into the surface of the steel strip so as to intersect with the rolling direction. In other words, the angle of the linearly introduced strain is the same as the scanning direction. Note that the 90° direction corresponds to the transverse direction (TD). The scanning direction is preferably 60 to 90° relative to the rolling direction (RD) of the steel strip.

[0094] The output of the high-energy beam to be irradiated is not particularly limited, and may be adjusted so as to introduce the desired strain. Typically, the output can be 50 W to 5 kW for both electron beam irradiation and laser irradiation. From the viewpoint of productivity, the deflection speed (scanning speed) is preferably 10 m / s or higher. Meanwhile, there is no upper limit to the deflection speed. However, if the deflection speed is excessively high, larger equipment will be required to ensure the energy density required for magnetic domain refinement. Therefore, the deflection speed is preferably 400 m / s or lower, and more preferably 200 m / s or lower.

[0095] In the magnetic domain refinement treatment, the high-energy beam is repeatedly irradiated at intervals in the rolling direction (longitudinal direction) of the steel strip. This interval is generally referred to as the "irradiation line interval" or simply the "line interval." The line interval is not particularly limited, but from the viewpoint of further reducing the iron loss of the grain-oriented electrical steel sheet, it is preferably more than 3.0 mm and less than 8.0 mm.

[0096] The line spacing is equal to the spacing in the rolling direction of the linear strain introduced into the grain-oriented electrical steel strip. Therefore, in one embodiment of the grain-oriented electrical steel strip, the spacing in the rolling direction of the linear strain is preferably more than 3.0 mm and less than 8.0 mm.

[0097] Furthermore, from the viewpoint of further reducing iron loss, it is preferable that the beam diameter be 0.20 mm or less. On the other hand, although there is no particular lower limit for the beam diameter, it is industrially preferable that the beam diameter be 0.010 mm or more. Note that the beam diameter here refers to the diameter of the irradiated energy beam when it reaches the surface of the steel strip.

[0098] The steel strip to be subjected to the magnetic domain refinement treatment is not particularly limited and can be produced by a general method. An example of a method for producing a steel strip to be subjected to the magnetic domain refinement treatment will be described below.

[0099] The typical production process of grain-oriented electrical steel strips includes the following steps: a hot rolling process in which a steel material (steel slab) is hot-rolled to form a hot-rolled steel strip; a cold rolling process in which the hot-rolled steel strip is cold-rolled to form a cold-rolled steel strip; a primary recrystallization annealing process in which the cold-rolled steel strip is subjected to primary recrystallization annealing and decarburization; a coating process in which an annealing separator is applied to the surface of the steel strip after the primary recrystallization annealing process; and a finish annealing process in which the steel strip after the coating process is subjected to secondary recrystallization annealing and purification. The magnetic domain refinement treatment can be performed after this finish annealing process. Furthermore, a hot-rolled sheet annealing process can be optionally included after the hot rolling process and before the cold rolling process. Furthermore, the cold rolling process can include two or more cold rolling passes, with intermediate annealing between them.

[0100] Elemental Composition The steel strip to be subjected to the magnetic domain refinement treatment can be produced, for example, by subjecting a steel material (steel slab) to treatments such as rolling and annealing. Therefore, first, an example of the elemental composition of a steel material (steel slab) that can be suitably used will be described. However, the present invention is not limited to the content described below. In the description of the elemental composition, the units of content "%" and "ppm" refer to "mass %" and "mass ppm," respectively, unless otherwise specified.

[0101] First, the component composition of the steel material (steel slab) used can contain C, Si, and Mn as basic components in the following contents.

[0102] C: 0.08% or less C is an element added to improve the hot-rolled sheet structure. However, since residual C can cause magnetic aging, decarburization is generally performed during the manufacturing process of an electrical steel strip to reduce the C content in the steel to 50 ppm or less, at which point magnetic aging does not occur. However, if the initial C content (C content of the steel material) exceeds 0.08%, it becomes difficult to reduce the C content to 50 ppm or less even by decarburization. Therefore, the C content of the steel material is preferably 0.08% or less. On the other hand, since secondary recrystallization is possible even when the steel material does not contain C, the lower limit of the C content is not particularly limited and may be 0%. However, from the viewpoint of improving the hot-rolled sheet structure, the C content is preferably 0.01% or more.

[0103] Si: 2.0 to 8.0% Si is an element that has the effect of increasing the electrical resistance of steel and improving iron loss. To fully obtain this effect, the Si content is preferably 2.0% or more. On the other hand, if the Si content exceeds 8.0%, not only will workability decrease, but the magnetic flux density will also decrease. Therefore, the Si content is preferably 8.0% or less, and more preferably 4.0% or less.

[0104] Mn: 0.005 to 1.0% Mn is an element that has the effect of improving hot workability. To enhance this effect, the Mn content is preferably 0.005% or more, and more preferably 0.01% or more. On the other hand, if the Mn content exceeds 1.0%, the magnetic flux density of the final grain-oriented electrical steel strip decreases. Therefore, the Mn content is preferably 1.0% or less, and more preferably 0.1% or less.

[0105] In addition to the basic components, the steel material may optionally contain at least one component selected from the group listed below to further improve the magnetic properties: Ni: 0.03 to 1.50%, Sn: 0.01 to 1.50%, Sb: 0.005 to 1.50%, Cu: 0.03 to 3.0%, P: 0.03 to 0.50%, Mo: 0.005 to 0.10%, Cr: 0.03 to 1.50%.

[0106] Ni is an element that has the effect of improving the hot-rolled sheet structure and further enhancing magnetic properties, and can be added as desired. To enhance this effect, the Ni content is preferably 0.03% or more. On the other hand, if the Ni content exceeds 1.50%, secondary recrystallization becomes unstable, which may result in deterioration of magnetic properties. Therefore, the Ni content is preferably 1.50% or less.

[0107] Furthermore, Sn, Sb, Cu, P, Mo, and Cr are elements that have the effect of further improving magnetic properties, and each can be added arbitrarily. When adding at least one of these elements, it is preferable to set the content of the element to be equal to or greater than the above-mentioned lower limit in order to enhance the above-mentioned effect. On the other hand, excessive addition inhibits the development of secondary recrystallized grains. Therefore, when adding at least one of these elements, it is preferable to set the content of the element to be equal to or less than the above-mentioned upper limit.

[0108] Furthermore, the composition of the steel material may further contain an inhibitor element as desired. For example, it is preferable to add at least one selected from the following group as the inhibitor element: Al: 0.01 to 0.065%, N: 0.005 to 0.012%, S: 0.005 to 0.03%, Se: 0.005 to 0.03%.

[0109] For example, when an AlN-based inhibitor is used, Al and N can be added. When an MnS-based or MnSe-based inhibitor is used, Mn can be added in addition to either or both of Se and S. Multiple inhibitors can also be used in combination.

[0110] Conversely, in the present invention, grain-oriented electrical steel strip can also be produced without using an inhibitor. When an inhibitor is not used, it is preferable to control the contents of Al, N, S, and Se in the composition of the steel material to the following ranges. On the other hand, the lower limits of the contents of Al, N, S, and Se are not particularly limited and may be 0 ppm. Al: 100 ppm or less N: 50 ppm or less S: 50 ppm or less Se: 50 ppm or less

[0111] The balance other than the above components is unavoidable impurities and Fe. The unavoidable impurities may include, for example, elements that are mixed in as impurities during the manufacturing process.

[0112] On the other hand, in the manufacturing process of grain-oriented electrical steel strip, primary recrystallization annealing is performed after rolling. During this primary recrystallization annealing, the steel strip undergoes decarburization, resulting in a decrease in the C content. Furthermore, during final annealing, the steel strip undergoes purification, resulting in a decrease in Al, N, S, Se, and other elements. As a result, the contents of C, Al, N, S, and Se in the final grain-oriented electrical steel strip are reduced to impurity levels. Therefore, in the chemical composition of grain-oriented electrical steel strip, Al, N, S, and Se can be considered unavoidable impurities. Typically, the Al content is 0.01% or less, and the contents of C, N, S, and Se are each 0.005% or less.

[0113] It should be noted that the variations in the contents of elements other than those mentioned above that are reduced by decarburization and purification are negligible during the manufacturing process, and therefore the contents of other elements contained in the composition of the final grain-oriented electrical steel strip are the same as the composition of the steel material described above.

[0114] Next, the present invention will be described in more detail based on examples. The following examples are examples of preferred embodiments of the present invention, and the present invention is not limited to these examples. The embodiments of the present invention can be appropriately modified within the scope of the invention, and all such modifications are included in the technical scope of the present invention.

[0115] First, the same magnetic flux density (B ８ A plurality of finish-annealed steel strips with a thickness of 0.23 mm and a tensile strength of 1.92 T were prepared. The steel strips were subjected to a magnetic domain refinement treatment over the entire longitudinal direction to obtain grain-oriented electrical steel strips. The magnetic domain refinement treatment was carried out under the conditions shown in Table 1 while the steel strips were continuously transported.

[0116] In some examples, when the above-mentioned magnetic domain refinement process was performed, distortion was introduced while dynamically controlling the focus of a high-energy beam (electron beam or laser). In examples using electron beam irradiation, the dynamic control was performed based on the previously determined relationship between the bias voltage and the focusing current that results in a just focus (FIG. 7) or the relationship between the orifice temperature and the focusing current (FIG. 12). In examples using laser irradiation, the dynamic control was performed based on the previously determined relationship between the orifice temperature and the focal length that results in a just focus (FIG. 13). The focusing current that results in a just focus is the focusing current that maximizes the beam profile intensity (minimum beam diameter) at the treatment surface (steel strip surface), and was determined by changing the focusing current value and measuring the beam profile intensity using a known beam profiler.

[0117] For comparison, in some cases the magnetic domain refining process was carried out without dynamic focus control.

[0118] Next, for each of the obtained grain-oriented electrical steel strips, the material iron loss W１７／５０ The distance between the domain walls of the closure domains within the thickness direction of the grain-oriented electrical steel strip was also measured using an X-ray magnetic circular polarization luminescence microscope to determine the maximum distance between the domain walls and the difference Δd between the maximum and minimum values ​​of the maximum distance between the domain walls over the entire longitudinal direction of the steel strip. The measurement results are shown in Table 1.

[0119] Next, a three-phase stacked core was fabricated using the obtained grain-oriented electromagnetic steel strips. The stacked core had an overall width of 890 mm, a depth of 800 mm, a lamination thickness of 250 mm, a core mass of approximately 1.0 t, and a capacity of 2000 kVA. The joining method was a step-lap method.

[0120] The resulting loss of the stacked core W １７／５０ The BF was calculated by comparing the BF with the iron loss of the material. The results are also shown in Table 1.

[0121] As can be seen from the results shown in Table 1, even without dynamic focus control, if other irradiation conditions were the same, material iron loss was obtained that was equivalent to that obtained with dynamic control. However, in the inventive example in which dynamic focus control was performed, fluctuations in the magnetic domain wall distance in the longitudinal direction of the steel strip were suppressed, and a grain-oriented electrical steel strip that satisfied the conditions of the present invention was obtained. Furthermore, the stacked core manufactured using this grain-oriented electrical steel strip satisfied the conditions of the present invention and had an improved BF compared to the comparative example.

[0122]

[0123] 10a to 10h: main magnetic domain; 20: strain introduction portion; 21a to 21d: closure domain; RD: rolling direction.

Claims

1. A grain-oriented electrical steel strip in which magnetic domains are refined by linearly introducing strain that intersects the rolling direction on at least one surface, and in which the ratio of the area within the thickness of the strip where the maximum inter-wall distance of closure domains is 10 μm or more as measured with an X-ray magnetic circularly polarized light emission microscope to the total length of the grain-oriented electrical steel strip in the rolling direction is 50% or more.

2. A stacked core made by stacking multiple grain-oriented electromagnetic steel sheets, wherein the region within the thickness of the grain-oriented electromagnetic steel sheets where the maximum inter-wall distance of closure domains as measured with an X-ray magnetic circular polarization luminescence microscope is 10 μm or more accounts for 50% or more of the total weight of the grain-oriented electromagnetic steel sheets.

3. A method for producing grain-oriented electrical steel strip as described in claim 1, comprising: subjecting the surface of the steel strip after finish annealing to magnetic domain refinement treatment by irradiating the steel strip with a high-energy beam while continuously transporting the steel strip in the rolling direction of the steel strip; and during the irradiation of the high-energy beam, introducing distortion while dynamically controlling the focus of the high-energy beam, thereby setting the ratio to 50% or more.

4. A method for manufacturing grain-oriented electromagnetic steel strip as described in claim 3, wherein the high-energy beam is an electron beam, and the dynamic control of the focus comprises measuring a bias voltage and adjusting the focusing current of the electron beam in accordance with the measured bias voltage based on a previously determined relationship between the bias voltage and the focusing current.

5. A method for manufacturing grain-oriented electromagnetic steel strip as described in claim 3, wherein the high-energy beam is an electron beam, an orifice is installed in the beam path of the electron beam so as to cut off a portion of the electron beam, and the dynamic control of the focus comprises measuring the temperature of the orifice, and adjusting the focusing current of the electron beam in accordance with the measured temperature of the orifice based on a predetermined relationship between the orifice temperature and the focusing current.

6. A method for manufacturing grain-oriented electromagnetic steel strip as described in claim 3, wherein the high-energy beam is a laser beam, an orifice is installed in the beam path of the laser beam so as to cut off a portion of the laser beam, and the dynamic control of the focus comprises measuring the temperature of the orifice, and adjusting the position of the focusing lens of the laser beam in accordance with the measured temperature of the orifice based on a predetermined relationship between the orifice temperature and the focal length.

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