Grain-oriented electrical steel sheet

Abstract

The present invention proposes a method that can reduce the noise generated by a transformer core and the like when formed by laminations of a grain-oriented electrical steel sheet in which core loss has been reduced by a magnetic domain refinement process. In this steel sheet, linear distortion extending with an orientation in which an angle formed with a direction perpendicular to the rolling direction of the steel sheet is an angle of 30 or less is periodic in the direction of rolling of the steel sheet, core loss (W.sub.17/50) is 0.720 W/kg or less, and magnetic flux density (B.sub.8) is 1.930 T. The volume of the closure domain arising in the distortion part is 1.00-3.00% of the total magnetic domain volume within the steel sheet.

Claims

1. A grain-oriented electrical steel sheet, comprising: periodic linear strain in a rolling direction of the steel sheet, the linear strain extending in a direction that forms an angle of 30 or less with a direction orthogonal to the rolling direction of the steel sheet, iron loss W.sub.17/50 being 0.720 W/kg or less, a magnetic flux density B.sub.8 being 1.930 T or more, and a volume fraction of a closure domain occurring in the strain portion being 1.00% or more and 3.00% or less of a total magnetic domain volume in the steel sheet, wherein the volume fraction is defined by following formula (A) using a magnetic strain constant .sub.100 in [100] orientation, 2310.sup.6, and a difference .sub.P-P between the maximum and minimum of the magnetic strain measurement with an alternating magnetic field under saturated flux density $\begin{array}{cc}=-\frac{2}{3}\frac{{}_{p-p}}{{}_{100}}.& \left(A\right)\end{array}$

2. The grain-oriented electrical steel sheet according to claim 1, wherein the linear strain is applied by continuous laser beam irradiation.

3. The grain-oriented electrical steel sheet according to claim 1, wherein the linear strain is applied by irradiation with an electron beam.

4. The grain-oriented electrical steel sheet according to claim 1, wherein a deviation of the [001] orientation is 4 or less.

5. The grain-oriented electrical steel sheet according to claim 1, wherein a contribution of magnetization rotation to magnetic strain is (610.sup.4).sub.100 or less.

6. The grain-oriented electrical steel sheet according to claim 1, wherein the steel comprises by mass %, C: 0.002% to 0.10%, Si: 1.0% to 7.0%, and Mn: 0.01% to 0.8%.

7. The grain-oriented electrical steel sheet according to claim 6, wherein the steel further comprises at least one element selected from the group consisting of Al: 0.005% to 0.050%, N: 0.003% to 0.020%, Se: 0.003% to 0.030%, and S: 0.002% to 0.03%.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) The present invention will be further described below with reference to the accompanying drawings, wherein:

(2) FIG. 1 illustrates a preferable range for the volume fraction of the closure domain in the present invention.

DESCRIPTION OF EMBODIMENTS

(3) First, regarding transformer noise, i.e. magnetostrictive vibration of the steel sheet, the oscillation amplitude becomes smaller as the density of crystal grains of the material along the easy axis of magnetization is higher. Therefore, to suppress noise, a magnetic flux density B.sub.8 of 1.930 T or higher is necessary. If the magnetic flux density B.sub.8 is less than 1.930 T, rotational motion of magnetic domains becomes necessary to align magnetization in parallel with the excitation magnetic field during the magnetization process, yet such magnetization rotation yields a large change in the magnetic strain, causing the transformer noise to increase.

(4) In addition, changing the orientation, interval, or region of the applied strain changes the resulting iron loss reduction effect. When appropriate strain is not applied, the iron loss properties might not be sufficiently reduced, resulting in a good magnetic property not being attained, and even if the volume fraction of the closure domain is controlled, the magnetic strain might not decrease, preventing suppression of transformer noise. Therefore, by using a steel sheet to which strain has been appropriately applied and for which the iron loss W.sub.17/50 is 0.720 W/kg or less, a noise reduction effect via control of the closure domain can be obtained.

(5) Next, as the method for applying strain, continuous laser beam irradiation, electron beam irradiation, or the like is suitable. The irradiation direction is a direction intersecting the rolling direction, preferably a direction within 60 to 90 with respect to the rolling direction (a direction that forms an angle of 30 or less with the direction orthogonal to the rolling direction). Irradiation is performed at intervals of approximately 3 mm to 15 mm in the rolling direction. The amount of applied strain can be assessed by measuring the magnetic strain in the rolling direction under an alternating magnetic field that provides saturated magnetic flux density and then calculating the volume fraction of the closure domain with equation (A) above. Measurement of the magnetic strain is preferably performed with a method to prepare a single electrical steel sheet and use a laser Doppler vibrometer or a strain gauge.

(6) Here, preferable irradiation conditions when using a continuous laser beam are a beam diameter of 0.1 mm to 1 mm and a power density, which depends on the scanning rate, in a range of 100 W/mm.sup.2 to 10,000 W/mm.sup.2. With respect to the condenser diameter of the laser beam, directly irradiating the surface of the steel sheet with a narrow beam, such that the minimum diameter determined by the configuration of the laser irradiation device is 0.1 mm or less, increases the amount of applied strain. The volume fraction of the closure domain also increases, causing the noise in the iron core of the transformer to increase. Accordingly, the volume fraction of the closure domain is adjusted by changing the diameter of the laser beam striking the condenser lens for focusing the laser. For example, irradiation is preferably performed under the condition that the beam diameter on the surface of the steel sheet is increased to approximately twice the minimum diameter. If the condenser diameter becomes too large, the magnetic domain refining effect lessens, suppressing the improvements in iron loss properties. Therefore, expansion of the condenser diameter is preferably limited to a factor of approximately five. Effective excitation sources include a fiber laser excited by a semiconductor laser.

(7) On the other hand, preferable irradiation conditions when using an electron beam are an acceleration voltage of 10 kV to 200 kV and a beam current of 0.005 mA to 10 mA. By adjusting the beam current, the volume fraction of the closure domain can be adjusted. While the acceleration voltage is also a factor, if the current exceeds this range, the amount of applied strain increases, causing the noise in the iron core of the transformer to increase.

(8) Note that as long as the grain-oriented electrical steel sheet has iron loss W.sub.17/50 of 0.720 W/kg or less and a magnetic flux density B.sub.8 of 1.930 T or more, the chemical composition is not particularly limited. However, an example of a preferable chemical composition includes, by mass %, C: 0.002% to 0.10%, Si: 1.0% to 7.0%, and Mn: 0.01% to 0.8%, and further includes at least one element selected from Al: 0.005% to 0.050%, N: 0.003% to 0.020%, Se: 0.003% to 0.030%, and S: 0.002% to 0.03%.

Example 1

(9) A steel slab including, by mass %. C: 0.07%, Si: 3.4%, Mn: 0.12%, Al: 0.025%, Se: 0.025%, and N: 0.015%, and the balance as Fe and incidental impurities was prepared by continuous casting. The slab was heated to 1400 C. and then hot-rolled to obtain a hot-rolled steel sheet. The hot-rolled steel sheet was subjected to hot-band annealing, and subsequently two cold-rolling operations were performed with intermediate annealing therebetween to obtain a cold-rolled sheet for a grain-oriented electrical steel sheet having a final sheet thickness of 0.23 mm. The cold-rolled sheet for grain-oriented electrical steel sheets was then decarburized, and after primary recrystallization annealing, an annealing separator containing MgO as the primary component was applied, and final annealing including a secondary recrystallization process and a purification process was performed to yield a grain-oriented electrical steel sheet with a forsterite film. An insulating coating containing 60% colloidal silica and aluminum phosphate was then applied to the grain-oriented electrical steel sheet, which was baked at 800 C. Next, magnetic domain refining treatment was performed to irradiate with a continuous fiber laser in a direction orthogonal to the rolling direction. For the laser irradiation, the average laser power was set to 100 W and the beam scanning rate to 10 m/s, and a variety of conditions were adopted by changing the beam diameter on the surface of the steel sheet. W.sub.17/50 measurement with an SST measuring instrument was performed on the resulting samples, which were sheared into rectangles 100 mm wide by 280 mm long. Using a laser Doppler vibrometer, the magnetic strain in the rolling direction was measured, and the volume fraction of the closure domain in each steel sheet was calculated in accordance with equation (A) above. As bevel-edged material with a width of 100 mm, the samples were stacked to a thickness of 15 mm to produce the iron core of a three-phase transformer. A capacitor microphone was used to measure the noise at a maximum magnetic flux density of 1.7 T and a frequency of 50 Hz. At this time, A-scale weighting was performed as frequency weighting.

(10) Table 1 lists the measured noise of the iron core of the transformer along with the conditions on the focus of the laser beam and the beam diameter on the surface of the steel sheet, as well as the B.sub.8 value of the steel sheet and the results of calculating the volume fraction of the closure domain. As is clear from Table 1, a steel sheet with B.sub.81.930 T and with the volume fraction of the closure domain within the designated range yielded good characteristics, with the noise from the iron core of the transformer being lower than 36 dBA and the W.sub.17/50 value also being equal to or lower than 0.720 W/kg.

(11) By contrast, in a region where the beam diameter was too narrow, the volume fraction of the closure domain deviated from the range of the present invention, and the noise also worsened. Furthermore, when the beam diameter was too wide, the volume fraction of the closure domain was within the range of the present invention and the noise property was also good, yet the W.sub.17/50 value was high. Even when the volume fraction of the closure domain was within the range of the present invention and the iron loss properties were good, a steel sheet with a B.sub.8 value lower than 1.930 T had worse noise from the iron core of the transformer. Based on these results, it is essential for all three of the following to fall within the range of the present invention in order to achieve a grain-oriented electrical steel sheet suitable as the iron core of a transformer or the like: the magnetic flux density B.sub.8, the iron loss W.sub.17/50, and the volume fraction of the closure domain.

(12) TABLE-US-00001 TABLE 1 Beam diameter Volume on fraction of Iron Steel surface of closure loss sheet steel sheet domain W.sub.17/50 Noise No. (mm) (%) B.sub.s (T) (W/kg) (dBA) Notes 1 0.08 4.47 1.931 0.711 40.2 Comparative example 2 0.11 4.11 1.934 0.713 39.3 Comparative example 3 0.17 3.42 1.932 0.714 37.0 Comparative example 4 0.19 3.00 1.935 0.715 35.9 Inventive example 5 0.21 2.93 1.924 0.716 37.2 Comparative example 6 0.21 2.81 1.930 0.717 35.4 Inventive example 7 0.24 2.48 1.921 0.717 36.6 Comparative example 8 0.24 2.48 1.935 0.719 35.0 Inventive example 9 0.28 1.58 1.933 0.720 34.7 Inventive example 10 0.30 1.00 1.934 0.720 34.5 Inventive example 11 0.40 0.79 1.936 0.726 34.1 Comparative example

Example 2

(13) The same samples as the electrical steel sheets that, before laser irradiation, were used for laser beam irradiation in Example 1 were irradiated with an electron beam, adopting a variety of conditions by changing the beam current under the conditions of an acceleration voltage of 60 kV and a beam scanning rate of 30 m/s. Like Example 1, the volume fraction of the closure domain in the steel sheet, the W.sub.17/50 value, and the noise from the iron core of the transformer were measured for the resulting samples.

(14) Table 2 lists the measured noise from the iron core of the transformer, along with the beam current, the B.sub.8 value, and the volume fraction of the closure domain. For the electron beam as well, reduced noise was achieved, with noise of 36 dBA or less, in samples for which B.sub.81.930 T and the beam current was lowered so that the volume fraction of the closure domain was within the designated range.

(15) By contrast, when the current density was raised, the volume fraction of the closure domain exceeded the range of the present invention, resulting in increased noise, whereas when the current density was lowered, the volume fraction of the closure domain fell below the range of the present invention, and the W.sub.17/50 value worsened. Furthermore, even when the volume fraction of the closure domain was within the range of the present invention, and the W.sub.17/50 value was 0.720 W/kg or less, the samples had noise greater than 36 dBA when B.sub.8<1.930 T. Hence, for electron beam irradiation as well, the magnetic property can be made compatible with the noise property only by all three of the following falling within the range of the present invention: the magnetic flux density B.sub.8, the iron loss W.sub.17/50, and the volume fraction of the closure domain.

(16) TABLE-US-00002 TABLE 2 Volume Iron Steel Beam fraction of loss sheet current closure W.sub.17/50 Noise No. (mA) domain (%) B.sub.s (T) (W/kg) (dBA) Notes 1 10 4.70 1.932 0.704 41.4 Comparative example 2 9 3.76 1.930 0.707 41.1 Comparative example 3 8 3.45 1.934 0.711 38.6 Comparative example 4 7.5 3.00 1.936 0.712 35.8 Inventive example 5 7 2.88 1.920 0.720 36.7 Comparative example 6 7 2.46 1.930 0.714 35.5 Inventive example 7 6 2.12 1.935 0.717 35.2 Inventive example 8 4 1.24 1.933 0.719 35.0 Inventive example 9 3.5 1.00 1.934 0.720 34.7 Inventive example 10 3 0.86 1.931 0.731 34.5 Comparative example