Patent Application
20150310973
The present invention relates to a grain-oriented electrical steel sheet for use in an iron core of a transformer or the like and to a method for manufacturing the grain-oriented electrical steel sheet.
In recent years, energy use has become more efficient, and demand has emerged for a reduction in energy loss at the time of operation, for example in a transformer.
The loss occurring in a transformer is mainly composed of copper loss occurring in conducting wires and iron loss occurring in the iron core.
Iron loss can be further divided into hysteresis loss and eddy current loss. To reduce the former, measures such as improving the crystal orientation of the material and reducing impurities have proven effective. For example, JP 2012-1741 A (PTL 1) discloses a method for manufacturing a grain-oriented electrical steel sheet with excellent flux density and iron loss properties by optimizing the annealing conditions before final cold rolling.
On the other hand, in addition to reducing sheet thickness and increasing the added amount of Si, the eddy current loss is also known to improve dramatically by the formation of a groove or the introduction of strain on the surface of the steel sheet.
For example, JP H06-22179 B2 (PTL 2) discloses a technique for forming a linear groove, with a groove width of 300 μm or less and a groove depth of 100 μm or less, on one surface of a steel sheet so as to reduce the iron loss W17/50, which was 0.80 W/kg or more before groove formation, to 0.70 W/kg or less.
JP 2011-246782 A (PTL 3) discloses a technique for irradiating a secondary recrystallized steel sheet with a plasma arc so as to reduce the iron loss W17/50, which was 0.80 W/kg or more before irradiation, to 0.65 W/kg or less.
Furthermore, JP 2012-52230 A (PTL 4) discloses a technique for obtaining material for a transformer with low iron loss and little noise by optimizing the coating thickness and the average width of a magnetic domain discontinuous portion formed on the surface of a steel sheet by electron beam irradiation.
It is known, however, that the iron loss reduction effect achieved by such groove formation or introduction of strain differs depending on the sheet thickness of the material. For example, IEEE TRANSACTIONS ON MAGNETICS, VOL. MAG-20, NO. 5, p.1557 (NPL 1) describes how, as the sheet thickness increases, the amount of reduction in iron loss due to laser irradiation tends to decrease and notes a difference of approximately 0.05 W/kg in the amount of reduction in iron loss (ΔW17/50) between sheet thicknesses of 0.23 mm and 0.30 mm for a material with a flux density of 1.94 T.
Against this background, studies have been made of whether the effect of reducing iron loss of thick sheet material can be improved even slightly by adjusting the magnetic domain refining method. For example, JP 2000-328139 A (PTL 5) and JP 4705382 B2 (PTL 6) disclose techniques for improving the effect of reducing iron loss of a grain-oriented electrical steel sheet from thick sheet material by optimizing the laser irradiation conditions in accordance with the sheet thickness of the material. In particular, PTL 6 discloses having obtained extremely low iron loss by setting the strain ratio η to 0.00013 or more and 0.013 or less.
This strain ratio η is the ratio of the strain area within a rolling direction cross-section of the steel sheet and is expressed by the formula η/8×(w×w)/(t×PL), where t is the thickness of the steel sheet, w is the closure domain width in the rolling direction, and PL is the laser irradiation spacing in the rolling direction.
PTL 1: JP 2012-1741 A
PTL 2: JP H06-22179 B2
PTL 3: JP 2011-246782 A
PTL 4: JP 2012-52230 A
PTL 5: JP 2000-328139 A
PTL 6: JP 4705382 B2
PTL 7: JP H11-279645 A
PTL 8: JP 4344264 B2
NPL 1: IEEE TRANSACTIONS ON MAGNETICS, VOL. MAG-20, NO. 5, p. 1557
The inventors of the present invention conjectured that such a technique used in a laser method could also be applied to an electron beam method and therefore investigated the relationship between the strain ratio and iron loss in order to reduce the iron loss of a steel sheet.
Furthermore, when the strain ratio is in a range of 0.013 or less and 0.00013 or more as well, the iron loss is sometimes a high value of 0.78 W/kg or more, clearly showing that low iron loss is not always obtained.
The inventors assumed that the above results stem from a difference in principle between the electron beam method and the laser method and assumed that in the case of the electron beam method, a different strain distribution than that disclosed in PTL 6 would be formed.
The present invention has been conceived in light of the above circumstances and proposes a grain-oriented electrical steel sheet, and method for manufacturing the same, with reduced iron loss over a wide range of sheet thickness by forming a closure domain shape advantageous for iron loss reduction that utilizes electron beam characteristics and forming a closure domain that is appropriate for the sheet thickness.
Based on the above-described experiment results, the inventors conceived of separately controlling the width and the depth of the portion where the closure domain is formed in the portion irradiated during electron beam irradiation.
The inventors estimated, based on conventional knowledge, that to be advantageous for reducing iron loss, the portion where the closure domain is formed should be deep in the sheet thickness direction and have a small volume. The reason is that, for example in JP H11-279645 A (PTL 7), an increase in the depth in the sheet thickness direction has been shown to be advantageous for reducing the eddy current loss of material. Furthermore, JP 4344264 B2 (PTL 8) shows that since strain is accumulated in the portion where the closure domain is formed, shrinking the portion where the closure domain is formed is useful for suppressing deterioration of hysteresis loss.
The inventors realized that, as illustrated in
Furthermore, through numerous experiments, the inventors identified that for a constant average beam scanning rate, the width of the portion where the closure domain is formed increases as the irradiation energy per unit scanning length of the beam and the beam diameter increase (where P>45 (J/m/mm)) and that the depth of the portion where the closure domain is formed is affected by the “irradiation energy per unit length/beam diameter” of the beam and by the acceleration voltage.
Additionally,
Based on these experiment results illustrated in
The present invention is based on the above-described findings.
Specifically, primary features of the present invention include the following.
1. A grain-oriented electrical steel sheet with an actually measured sheet thickness t (mm), comprising a closure domain region extending linearly in a direction from 60° to 120° with respect to a rolling direction on a surface of the steel sheet, the closure domain region being formed periodically at a spacing s (mm) in the rolling direction, wherein
h≧74.9t+39.1(0.26≧t),
h≧897t−174.7(t>0.26),
(w×h)/(s×1000)≦−12.6t+7.9(t>0.22), and
(w×h)/(s×1000)≦−40.6t+14.1(t≦0.22),
where h (μm) is a depth and w (μm) is a width of the closure domain region, s (mm) is the spacing, and t (mm) is the actually measured sheet thickness.
2. A method for manufacturing the grain-oriented electrical steel sheet with an actually measured sheet thickness t (mm) of 1, comprising forming a closure domain region extending linearly in a direction from 60° to 120° with respect to a rolling direction on a surface of the steel sheet, the closure domain region being formed periodically at a spacing s (mm) in the rolling direction, by using an electron beam emitted at an acceleration voltage Va (kV), wherein
(w×h)/(s×1000)≦−12.6t+7.9(t>0.22), and
(w×h)/(s×1000)≦−40.6t+14.1(t≦0.22),
where h (μm) is a depth and w (μm) is a width of the closure domain region, s (mm) is the spacing, and t (mm) is the actually measured sheet thickness, and
Va≧580t+270−6.7P(0.26≧t),
Va≧6980t−1390−6.7P(t>0.26), and
P>45,
where P is irradiation energy per unit scanning length/beam diameter (J/m/mm).
3. The method of 2, wherein the beam diameter of the electron beam is 400 μm or less.
4. The method of 2. or 3, wherein a LaB6 cathode is used as an irradiation source of the electron beam.
According to the present invention, a closure domain shape advantageous for iron loss reduction that utilizes electron beam characteristics can be formed, and by forming a closure domain that is appropriate for the sheet thickness, iron loss can be reduced in a grain-oriented electrical steel sheet over a wide range of sheet thickness. Accordingly, the present invention allows for an increase in energy usage efficiency of a transformer produced with a grain-oriented electrical steel sheet of any sheet thickness and is therefore industrially useful.
Exemplary embodiments of the present invention will be further described below with reference to the accompanying drawings, wherein:
The present invention will be described in detail below with reference to exemplary embodiments.
The present invention provides a grain-oriented electrical steel sheet, and a preferable method for manufacturing the grain-oriented electrical steel sheet, that has a magnetic domain refined by irradiation with an electron beam.
An insulating coating may be formed on the electrical steel sheet irradiated with an electron beam, yet omitting the insulating coating poses no problem. As illustrated in
The grain-oriented electrical steel sheet of the present invention preferably has a linearly extending closure domain shape, as described below. Note that simply referring to a closure domain below designates a region with a linearly extending closure domain shape. Also note that a unit adjustment term has been included in the coefficient for the letters into which numerical values are substituted in the equations below. Therefore, the numerical values may be substituted as non-dimensional values, ignoring units.
[Volume of Portion where the Closure Domain is Formed]
As illustrated in
(w×h)/(s×1000)≦−12.6t+7.9(t>0.22), and
(w×h)/(s×1000)≦−40.6t+14.1(t≦0.22),
and more preferably
(w×h)/(s×1000)≦−12.3t+6.9(t>0.22), and
(w×h)/(s×1000)≦−56.1t+16.5(t≦0.22),
where h (μm) is the depth of the closure domain, w (μm) is the width of the closure domain, s (mm) is the RD line spacing, and t (mm) is the actually measured thickness of the steel sheet (the same letters being used below).
Since strain is introduced, the portion where the closure domain is formed is not preferable from the perspective of reducing hysteresis loss, and the volume thereof is preferably small. The volume of the portion where the closure domain is formed is proportional to the value yielded by dividing the area of the closure domain shape in a rolling direction cross-section parallel to the sheet thickness direction, obtained by observing a sheet thickness cross-section in the rolling direction (i.e. the area of the cross-sectional shape), by the spacing of the closure domain formed periodically in the rolling direction (RD line spacing: s). Therefore, in the present invention, this area of the cross-sectional shape/RD line spacing is used as a volume index.
Considering how this area of the cross-sectional shape can vary along the line of the electron beam irradiation, the average area is preferably used. When variation in the area of the cross-sectional shape is large, it is possible to make measurement of only the closure domain shape observed in a sheet thickness cross-section in the rolling direction for a characteristic portion. For example, in test material irradiated with an electron beam in a dot pattern in the transverse direction (direction orthogonal to the rolling direction), the closure domain shape in a dot-centered portion may differ from the closure domain shape between dots, yet in this case, the average of the widths and depths yielded by observing cross-sections are preferably used.
[Depth of Portion where the Closure Domain is Formed]
As illustrated in
h≧74.9t+39.1(0.26≧t), and
h≧897t−174.7(t>0.26)
and more preferably the following relationships (rate of improvement in eddy current loss: 5%):
h≧168t+29.0(0.26≧t), and
h≧1890t−418.7(t>0.26).
In the present invention, the shape of the cross-sectional closure domain can be measured with a Kerr effect microscope. The (100) face of the crystal is set as the observation face. The reason is that if the observation face is misaligned from the (100) face, a different domain structure is more easily expressed due to a surface magnetic pole occurring on the observation face, making it more difficult to observe the desired closure domain.
When the crystal orientation is accumulated in the ideal Goss orientation, a rolling direction cross-section parallel to the sheet thickness direction is rotated 45° with the rolling direction as the axis of rotation to yield the observation face, and the shape of the closure domain in a rolling direction cross-section parallel to the sheet thickness direction is calculated by conversion from the observed shape of the closure domain.
Since the region of the closure domain shape corresponds to the region of induced strain, a minute strain distribution in which a closure domain is formed may be observed by x-ray or electron beam and quantified.
As described above, a low closure domain volume is good, yet for a large sheet thickness, deterioration of hysteresis loss due to electron beam irradiation becomes more pronounced, making an even smaller closure domain preferable. Therefore, in the present invention, the sheet thickness is included as a parameter for the appropriate closure domain volume.
As the depth of the closure domain in the sheet thickness direction is larger, the closure domain is more advantageous for improving eddy current loss. For a large sheet thickness, however, domain refinement is difficult, perhaps because the domain wall energy is large. Accordingly, in order to obtain a sufficient magnetic domain refining effect, it is preferred to form a deeper closure domain.
[Electron Beam Generation Conditions]
The following describes the electron beam generation conditions preferred in the present invention.
Va≧580t+270−6.7P(0.26≧t)
Va≧6980t−1390−6.7P(t>0.26)
It is preferred for the acceleration voltage Va (kV) of the electron beam and P (J/m/mm) in embodiments of the present invention to satisfy the above expressions. The reason is that the above-described depth of the portion where the closure domain is formed can be adjusted easily.
As the acceleration voltage is higher, the penetration depth of the electrons in the steel increases, which is advantageous for a deeper closure domain shape. Furthermore, high acceleration voltage is preferable for obtaining a high magnetic domain refining effect in thick sheet material. The depth of the portion where the closure domain is formed also depends, however, on the irradiation energy per unit scanning length/beam diameter (P). When P is large, a narrow region is irradiated with extremely high-density energy. Hence, the electrons penetrate more easily in the sheet thickness direction. For this reason, when P is large, the lower limit on the acceleration voltage decreases.
[P>45(J/m/mm)]
When the irradiation energy per unit scanning length/beam diameter: P is excessively small, i.e. when the irradiation energy is low to begin with, or when the irradiation energy density is low since both the irradiation energy and the beam diameter are large, then the steel sheet cannot be provided with strain, and the effect of reducing iron loss is lessened. Therefore, in the present invention, P is preferably set to exceed 45. While there is no restriction on the upper limit of P, an excessively large P significantly damages the coating and makes it impossible to ensure an anti-corrosion property. Therefore, the upper limit preferably is approximately 300.
[RD Line Spacing: 3 mm to 12 mm]
The steel sheet is irradiated with the electron beam linearly from one edge in the width direction to the other edge, and the irradiation is repeated periodically in the rolling direction. The spacing (line spacing) s is preferably 3 mm to 12 mm. The reason is that if the line spacing is narrow, the strain region formed in the steel becomes excessively large, and iron loss (hysteresis loss) worsens. On the other hand, if the line spacing is too wide, the magnetic domain refining effect lessens no matter how much the closure domain extends in the depth direction, and iron loss does not improve. Accordingly, in the present invention, the RD line spacing s is preferably set in a range of 3 mm to 12 mm.
[Line Angle: 60° to 120°]
When irradiating the steel sheet linearly from one edge in the width direction to the other edge, the direction from the starting point to the ending point is set to be from 60° to 120° with respect to the rolling direction.
The reason is that if the line angle is less than 60° or more than 120°, the irradiation width increases, causing a drop in productivity. Moreover, the strain region grows large, causing hysteresis loss to worsen.
In the present invention, “linear” refers not only to a straight line, but also to a dotted line or a discontinuous line, and the line angle refers to the angle between the rolling direction and a straight line connecting the starting point with the ending point. In the case of a dotted line or a discontinuous line, the length of the portion not irradiated with the beam between dots along the line or between continuous line segments is preferably 0.8 mm or less. The reason is that if irradiated region is excessively small, the effect of improving the eddy current loss may be lessened.
[Processing Chamber Pressure: 3 Pa or Less]
If the processing chamber pressure is high, electrons emitted from the electron gun scatter, and the energy of the electrons forming the closure domain is reduced. Therefore, the magnetic domain of the steel sheet is not sufficiently refined, and iron loss properties do not improve. Accordingly, in the present invention, the processing chamber pressure is preferably set to 3 Pa or less. In terms of practical operation, the lower limit on the processing chamber pressure is approximately 0.001 Pa.
[Beam Diameter: 400 μm or Less]
The closure domain width and the beam diameter are correlated, and as the beam diameter is smaller, the closure domain width tends to decrease. Accordingly, a small (narrow) beam diameter is good, with a beam diameter of 400 μm or less being preferable. If the beam diameter is too small, however, the steel substrate and coating at the irradiated portion are damaged, dramatically decreasing the insulation properties of the steel sheet. Furthermore, in order to significantly reduce the beam diameter, the WD (distance from the focusing coil to the steel sheet) must be shortened, yet doing so causes the beam diameter to vary excessively in the deflection direction (sheet transverse direction) of the beam. The quality of the steel sheet thus easily becomes uneven in the width direction. Accordingly, the beam diameter is preferably 150 μm or more.
[Material for Source of Thermionic Emission: LaB6]
In general, a LaB6 cathode is known to be advantageous for outputting a high-intensity beam, and since the beam diameter is easily focused, LaB6 is preferably used as the emission source for the electron beam in the present invention.
[Regarding Beam Focusing]
When irradiating by deflecting in the width direction, the focusing conditions (focusing current and the like) are of course preferably adjusted in advance so that the beam is uniform in the width direction.
In the present invention, typical, well-known methods suffice for adjustment of conditions other than those listed above, such as the size of the portion where the closure domain is formed, the irradiation energy, the beam diameter, and the like.
In the grain-oriented electrical steel sheet used in the present examples, materials with W17/50 of 0.80 W/kg to 0.90 W/kg (t: 0.19 mm, 0.26 mm) and 0.90 W/kg to 1.00 W/kg (t: 0.285 mm) were irradiated with an electron beam. The electron beam had a line angle of 90° and a processing chamber pressure of 0.1 Pa. Table 1 lists the other irradiation conditions and the closure domain shape after irradiation.
Next, the closure domain shape of these steel sheets, No. 1 to 18, was evaluated according to the assessments below, and the iron loss W17/50 was measured. The measurement results and the like are shown in Table 2. Note that the depth and the width of the closure domain are respectively h (μm) and w (μm), and the RD line spacing is s (mm). The iron loss is the average of measurements for 15 sheets under each set of conditions.
Assessment 1:
Volume: (w×h)/(s×1000)≦−12.6t+7.9(t: 0.26 mm, 0.285 mm)
(w×h)/(s×1000)≦−40.6t+14.1(t: 0.19 mm)
Depth: h≧74.9t+39.1 (actually measured sheet thickness (t): 0.19 mm, 0.26 mm)
Depth: h≧897t−174.7 (actually measured sheet thickness (t): 0.285 mm)
Assessment 2:
Volume: (w×h)/(s×1000)≦−12.3t+6.9(t:0.26 mm, 0.285 mm)
(w×h)/(s×1000)≦−56.1t+16.5(t: 0.19 mm)
Depth: h≧168t+29.0 (actually measured sheet thickness (t): 0.19 mm, 0.26 mm)
Depth: h≧1890t−418.7 (actually measured sheet thickness (t): 0.285 mm)
Table 2 shows that applying the present technique yields a grain-oriented electrical steel sheet with low iron loss, such that W17/50 is 0.68 W/kg or less (t: 0.19 mm), 0.74 W/kg or less (t: 0.26 mm), or 0.85 W/kg or less (t: 0.285 mm).
| Number | Date | Country | Kind |
|---|---|---|---|
| 2012-240667 | Oct 2012 | JP | national |
This is the U.S. National Phase application of PCT/JP2013/006401, filed Oct. 29, 2013, which claims priority to Japanese Patent Application No. 2012-240667, filed Oct. 31, 2012, the disclosures of these applications being incorporated herein by reference in their entireties for all purposes.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2013/006401 | 10/29/2013 | WO | 00 |
