TECHNICAL FIELD
[0001] The present invention relates to grain-oriented electrical sheet superior in watt
loss which uses laser irradiation or the like to introduce residual stress for magnetic
domain control.
BACKGROUND ART
[0002] Grain-oriented electrical sheet having an axis of easy magnetization in the rolling
direction of the steel sheet is mainly being used for iron cores of transformers etc.
In recent years, it has been strongly demanded to reduce the watt loss of iron cores
from the viewpoint of energy savings.
[0003] The watt loss of electrical sheet may be roughly divided into hysteresis loss and
eddy current loss. It is known that the hysteresis loss is influenced by the crystal
orientation, defects, grain boundaries, etc., while the eddy current loss is influenced
by the sheet thickness, electrical resistance, magnetic domain width, etc. There are
limits to the technique of controlling and improving the crystal orientation so as
to reduce the hysteresis loss, so in recent years many proposals have been made of
the art of subdivision of the magnetic domain width so as to reduce the eddy current
loss accounting for most of the watt loss, that is, the art of magnetic domain control.
[0004] As a method for this, Japanese Patent Publication (B2) No.
6-19112 discloses a method of production of grain-oriented electrical sheet which uses YAG
laser irradiation to introduce lines of strain substantially perpendicular to the
rolling direction cyclically in the rolling direction and thereby reduce the watt
loss. The principle behind this method, called laser magnetic domain control, is to
use a laser beam to scan the surface and produce surface strain due to which the 180°
magnetic domain width is subdivided and the watt loss is reduced.
[0005] Further, Japanese Patent Publication (A) No.
2005-248291 makes a new proposal taking note of the maximum value of the rolling direction residual
stress formed at the steel sheet surface.
DISCLOSURE OF THE INVENTION
[0006] Almost all proposals up to now relating to the introduction of local strain to steel
sheet surfaces and subdivision of the 180° magnetic domain width to reduce the watt
loss, that is, laser magnetic domain control, including the prior art first patent
document, use trial and error to limit the type of the laser, the shape of the focused
spot of the laser beam, the laser energy density, the laser irradiation pitch, and
other laser irradiation parameters. The proposals are extremely fragmentary and lack
uniformity. The reason is that no allusion is made to a quantitative discussion of
the main factors causing magnetic domain subdivision and watt loss reduction, that
is, strain or residual stress. Inherently, in improvement of watt loss by laser irradiation,
even under the same laser irradiation conditions, due to the absorption rate of the
steel sheet (determined by laser wavelength or surface properties, shape, and film
composition) or film thickness, the conversion from laser energy to heat energy (temperature
distribution and temperature history) will differ, so even if the laser irradiation
conditions are the same, the strain introduced will differ depending on the properties
of the steel sheet. Further, even with the same heat energy (temperature distribution
or temperature history), due to the composition of the steel sheet (for example, amount
of Si), the physical property values (for example, Young's modulus or yield stress
value) will differ, so the residual stress will also differ. Therefore, even if the
optimal laser irradiation conditions with respect to steel sheet of certain conditions
are obtained, even a small change in the state of the film will cause the way the
strain is introduced due to the laser to differ and the watt loss value to change,
so the laser irradiation conditions and reduction in watt loss do not correspond to
each other on a 1 to 1 basis. Therefore, attempts have been made to find the inherent
factors influencing the watt loss. The second patent document quantitatively alludes
to the strain and residual stress, but there were limits to reduction of the watt
loss by just control of the strain or tensile residual stress of the steel sheet surface.
[0007] The object of the present invention is to provide grain-oriented electrical sheet
more superior in watt loss compared with the past by dividing the watt loss of grain-oriented
electrical sheet into hysteresis loss and eddy current loss and, in particular from
the viewpoint of the eddy current loss, quantitatively controlling the distribution
of the strain and residual stress not only at the surface, but also inside in the
sheet thickness direction under suitable conditions.
[0008] The inventors ran experiments on magnetic domain control introducing strain and residual
stress into grain-oriented electrical sheet by laser irradiation etc. and engaged
in in-depth research to investigate the distribution of residual stress introduced
into the obtained low watt loss grain-oriented electrical sheet. As a result, the
inventors discovered a correlation between the residual stress and eddy current loss
and discovered that if controlling the compressive stress value and the strain pitch,
it is possible to realize a grain-oriented electrical sheet superior in watt loss.
The gist of the present invention is as follows.
- (1) A grain-oriented electrical sheet obtained by irradiating a continuous wave laser
beam to introduce strain uniformly in a sheet width direction perpendicular to a rolling
direction, cyclically in the rolling direction, and in lines substantially perpendicular
to the rolling direction, characterized in that in the two-dimensional distribution of a rolling direction compressive residual stress
occurring near one location of the introduction of strain in a cross-suction perpendicular
to the sheet width direction, the value of the rolling direction compressive residual
stress integrated in the region of the cross-section where there is compressive residual
stress is 0.20N to 0.80N.
- (2) A grain-oriented electrical sheet as set forth in said (1), characterized in that a cyclic pitch in said rolling direction of the strain uniform in said sheet width
direction due to irradiation of the laser beam is 2 mm to 8 mm.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009]
FIG. 1 is a schematic view of an apparatus used for a method of production of a grain-oriented
electrical sheet of the present invention.
FIG. 2 shows a two-dimensional distribution of a rolling direction residual stress
near a laser irradiation position at a rolling direction/sheet thickness direction
cross-section.
FIG. 3 is a view of a relationship between a maximum value of a rolling direction
tensile residual stress and a watt loss W17/50.
FIG. 4 is a view of a relationship between a cumulative compressive stress value σS
and an eddy current loss We (laser irradiation pitch of fixed 4 mm).
FIG. 5 is a view of a relationship between a cumulative compressive stress value σS
and a watt loss W17/50 (laser irradiation pitch of fixed 4 mm).
FIG. 6 is a view of a relationship between a laser irradiation pitch PL and a watt
loss W17/50 (rolling direction laser irradiation diameter DL of 0.1 mm and scan direction laser
irradiation diameter DC of fixed 0.5 mm).
FIG. 7 is a view of a relationship between a maximum value of a rolling direction
compressive residual stress and a watt loss W17/50.
BEST MODE FOR CARRYING OUT THE INVENTION
[0010] The inventors took note of the two-dimensional distribution of rolling direction
residual stress in the cross-section vertical to the sheet width direction and the
rolling direction laser irradiation pitch for various laser irradiation conditions
in the method of irradiating a laser at the surface of grain-oriented electrical sheet
so as to introduce lines of strain substantially vertical to the rolling direction
at a constant pitch in the rolling direction so as to improve the watt loss and discovered
conditions by which grain-oriented electrical sheet superior in watt loss can be obtained.
Here, the "sheet width direction" is a direction perpendicular to the rolling direction.
As the method for introducing lines of strain like the above at the surface of grain-oriented
electrical sheet, in addition to the laser irradiation method, ion injection, electrodischarge
machining, local plating, ultrasonic vibration, etc. may be mentioned. The conditions
can be applied to grain-oriented electrical sheet introducing strain by any method.
Below, drawings will be used to explain the grain-oriented electrical sheet obtained
by laser irradiation of the present invention.
[0011] FIG. 1 is an explanatory view of the method of irradiating a laser beam according
to the present invention. In the present embodiment, the continuous wave (CW) laser
beam LB output from the laser device 3 is used to scan a grain-oriented electrical
sheet 1 using a polygonal mirror 4 and fθ lens 5. By changing the distance between
the fθ lens 5 and grain-oriented electrical sheet 1, the rolling direction focused
diameter dl of the laser beam was changed. 6 is a cylindrical lens or a plurality
of cylindrical combination lenses. This is used in accordance with need to change
the focused diameter (scan direction length) dc of the scan direction of the beam
(sheet width direction perpendicular to rolling direction) for the focused spot of
the laser beam so as to control the focused shape from a circular shape to an elliptical
shape. The average irradiation energy density Ua [mJ/mm
2] is defined using the laser power P [W], sheet width direction scan speed Vc of the
laser beam in the sheet width direction [m/s], and rolling direction laser irradiation
pitch PL (mm) as
The laser scan speed is determined by the rotational speed of the polygonal mirror,
so the average laser irradiation energy density can be adjusted by changing the laser
power, polygonal mirror rotational speed, and laser irradiation pitch. FIG. 1 is an
example of use of one set of a laser and laser beam scan device. It is also possible
to set a plurality of similar devices in the sheet width direction in accordance with
the width of the steel sheet.
[0012] The inventors ran experiments using a 10 µm fiber core diameter continuous wave fiber
laser device, changed the laser irradiation conditions while combining the focused
spot shape and average laser irradiation energy density Ua in various ways, and made
the laser beam scan the surface of the grain-oriented electrical sheet in lines in
a direction substantially vertical to the rolling direction so as to laser it. They
measured the two-dimensional distribution of the residual stress in the rolling direction
in the cross-section vertical to the sheet width direction and the watt loss and hysteresis
loss and divided the watt loss into hysteresis loss and eddy current loss for study.
For measurement of the two-dimensional distribution of the residual stress in the
rolling direction in the cross-section vertical to the sheet width direction, they
used the X-ray diffraction method to measure the lattice intervals and used the modulus
of elasticity and other physical property values to convert this to stress. The watt
loss was measured as W
17/50 by an SST (Single Sheet Tester) measuring device. N
17/50 is the watt loss at the time of a frequency of 50Hz and a maximum magnetic flux density
of 1.7T. In the grain-oriented electrical sheet sample used in this example, when
the sheet thickness is 0.23 mm, the W
17/50 before laser irradiation was 0.86W/kg. The hysteresis loss was calculated by a hysteresis
loop, while the eddy current was made the value of the watt loss minus the hysteresis
loss.
[0013] FIG. 2 shows a typical example of the two-dimensional distribution of the compressive
residual stress of the rolling direction occurring near the laser irradiated position
in a cross-section vertical to the sheet width direction. For steel sheet where improvement
in the watt loss is seen, there are differences in the absolute value of the residual
stress depending on the laser irradiation conditions, but there is a large tensile
stress near the surface of the steel sheet and there is compressive stress directly
under the sheet thickness direction. Note that the width of the rolling direction
in which the residual stress and plastic strain are present is substantially proportional
to the rolling direction diameter dl of the focused spot of the laser.
[0014] The inventors investigated the relationship between the maximum value of the tensile
residual stress and compressive residual stress of the surface of the steel sheet
and the watt loss. The relationship between the maximum value of the tensile residual
stress and the watt loss is shown in FIG. 3, while the relationship between the maximum
value of the compressive residual stress and the watt loss is shown in FIG. 7. For
the maximum value of the tensile residual stress, no correlation with the watt loss
or optimal value is seen. On the other hand, for the maximum value of the compressive
residual stress, the watt loss is good above the 100 MPa shown by the one-dot chain
line, but the upper limit value is not clear. As a result, the watt loss in magnetic
domain control by laser irradiation cannot be explained by the maximum value of the
tensile residual stress and cannot be completely explained even by the maximum value
of the compressive residual stress. The possibility of the presence of separate particularly
fine amounts may be considered.
[0015] Therefore, the inventors studied the data in detail and as a result noted, as a first
point, that the maximum value of the tensile residual stress is greater than the compressive
residual stress and the tensile residual stress concentrates in a narrow region, that
depending on the irradiation conditions, the yield stress, that is, plastic strain
region, is reached, that, on the other hand, some relationship was seen between the
maximum value of the compressive residual stress and the watt loss, and, as a second
point, even if the maximum value of the compressive residual stress is the same, there
is a difference in the spread of the distribution of compressive residual stress in
the depth direction. That is, they began to believe that as the main factors behind
the realization of reduction of watt loss and realization of magnetic domain subdivision
are, from the first point, not the tensile stress, but the compressive stress has
important meaning and, from the second point, not the maximum value of the residual
stress, but the spread of the distribution has important meaning.
[0016] To express the distribution of compressive stress for realizing reduction of the
watt loss, the inventors defined the characterizing quantity of the "cumulative compressive
stress value σS" as in the following formula (1):
That is, in the two-dimensional distribution of the rolling direction compressive
residual stress occurring near a lasered part, that is, near a part where strain is
introduced, in the cross-section vertical to the sheet width direction, they defined
the cumulative compressive stress value σS [N] as the value of the stress σ integrated
in the region S where the rolling direction compressive residual stress is σ [MPa],
the region in the cross-section in which there is compressive residual stress is S
[mm
2], and the area element_is ds. That is, the cumulative compressive stress value is
the sum of the compressive residual stress introduced by laser irradiation.
[0017] The inventors found the cumulative compressive stress by the above method for grain-oriented
electrical sheet obtained by setting the rolling direction laser irradiation pitch
PL at 4 mm (fixed), setting the shape of the laser focused spot at 20×2500 µm, 100×500
µm, 100×2000 µm, and 300×200 µm, and changing the laser power for each in stages for
the laser irradiation. On the other hand, they subtracted the hysteresis loss from
the watt loss measured for each to find the eddy current loss. FIG. 4 shows the relationship
between the two for each electrical sheet obtained by plotting the cumulative compressive
stress value σS on the abscissa and the eddy current loss We on the ordinate. From
the result, the cumulative compressive stress value and the eddy current loss are
in an inversely proportional relationship regardless of the shape of the focused spot.
This means that the reduction in the eddy current loss, that is, the magnetic domain
subdivision effect, is proportional to the sum of the introduced compressive residual
stresses. If considering this phenomenon from the physical principles, the result
becomes as follows. The magnetic elasticity energy E is
where C is a constant, σ is the residual stress, M is the magnetic moment, and θ is
the angle formed by σ and M. At this time, when there is compressive residual stress
in the rolling direction, since E becomes smallest when θ is 90 degrees, σ is a negative
value. If taking note of this, the orientation of the magnetic moment becomes vertical
to the rolling direction. Therefore, due to the compressive stress, the axis of easy
magnetization can be made not only the rolling direction, but also the vertical direction.
In general, this is called a "reflux magnetic domain". If there is a reflux magnetic
domain, the magnetostatic energy becomes higher and unstable, so it may be considered
to further divide the magnetic domains to lower the magnetostatic energy and stabilize
it. Accordingly, it is believed, the greater the reflux magnetic domains, that is,
the stronger and broader the compressive residual stress generated, the higher the
magnetic domain subdivision effect becomes and the more the eddy current loss is reduced.
[0018] FIG. 5 shows the relationship when using the data used in FIG. 4 and the measured
watt loss and plotting the cumulative compressive stress value σS on the abscissa
and the peak watt loss W
17/50 on the ordinate. From the results, in the range of 0.20N≤σS≤0.80N shown by the dot-chain
line, compared with the watt loss W
17/50=0.86 W/kg before magnetic domain control, a good watt loss of a watt loss improvement
rate of 13% or more (W
17/50≤0.75 W/kg) shown by the dotted line can be realized. Note that, the watt loss improvement
rate η is defined as η(%)={(watt loss of material-peak watt loss)/watt loss of material}×100.
If the cumulative compressive stress value σS is smaller than 0.20N, the eddy current
loss is high, so the watt loss is not reduced. It is believed that, when the cumulative
compressive stress value σS is larger than 0.80N, the eddy current loss is reduced,
but the hysteresis loss increases due to the plastic strain due to the tensile residual
stress near the surface, so the watt loss is not reduced. In the above way, it is
learned that if adjusting the cumulative compressive stress value σS to the range
of
a good improvement in the watt loss is obtained. More preferably, it is learned that
if adjusting the value to the range of 0.40N≤σS≤0.70N, a further effect of improvement
of the watt loss can be obtained.
[0019] In the above, the rolling direction laser irradiation pitch PL was fixed at 4 mm,
but the inventors further investigated the effects by changing the rolling direction
laser irradiation pitch PL. At this time, they made the shape of the focused spot
of the laser beam a rolling direction diameter of 0.1 mm and a scan direction (sheet
width direction) diameter of 0.5 mm and adjusted Ua so that the cumulative compressive
stress σS fell in the range of 0.20N≤σS≤0.80N. FIG. 6 plots the rolling direction
laser irradiation pitch PL on the abscissa and the watt loss W
17/50 on the ordinate and shows the relationship between the two. From the results, with
a PL of 2 mm to 8 mm, a good watt loss of a watt loss improvement rate of 13% can
be realized. In a range where PL is smaller than 2 mm, the hysteresis loss increases,
so the watt loss is not reduced. In a range where PL is larger than 8 mm, the eddy
current loss is not reduced, so the watt loss is not reduced. In the above way, it
is learned that if adjusting the rolling direction laser irradiation pitch PL to the
range of
a good improvement in the watt loss can be obtained.
Example 1
[0020] Using 0.23 mm thick grain-oriented electrical sheet, the surface of the steel sheet
was scanned using a continuous wave laser under the laser irradiation conditions as
shown in Table 1, the residual stress was measured, then the cumulative compressive
stress value was calculated and the watt loss (W
17/50) was measured. The results are shown in together in the same Table 1. Example 1 was
performed fixing the laser power at 200W and the laser irradiation pitch in the rolling
direction at 4 mm. The cumulative compressive stress value was calculated by using
the X-ray diffraction method to measure the rolling direction residual stress (strain)
and finding the value with respect to the compressive stress by formula (2).
[0021] As clear from Table 1, the electrical sheets shown in Test No. 1 to No. 8 (invention
examples) all had a rolling direction cumulative compressive stress value σS in the
range prescribed by the present invention, that is, 0.20N≤σS≤0.80N, so could be reduced
in watt loss to a low watt loss value (W
17/50) of 0.75W/kg, for a watt loss improvement rate of 13%, or less. On the other hand,
the electrical sheets shown in Test No. 9 to No. 12 (comparative examples) outside
the range of conditions 0.20N≤σS≤0.80N failed to achieve a low watt loss value (W
17/50) of 0.75W/kg or less. In this way, if using the present invention, it is possible
to obtain grain-oriented electrical sheet superior in watt loss.
Table 1
|
Test No. |
Rolling direction diameter DL mm |
Scan direction diameter DC mm |
Average energy density Ua mJ/mm2 |
Strain pitch PL mm |
Maximum tensile stress MPa |
Cumulative compressive stress value σS N |
Watt loss value W17/50 W/kg |
Watt loss improvement rate % |
Not lasered |
0 |
- |
- |
- |
- |
0 |
0 |
0.860 |
0 |
Inv.ex. |
1 |
0.020 |
2.50 |
2.5 |
4 |
370 |
0.30 |
0.730 |
15.1 |
Inv.ex. |
2 |
0.020 |
2.50 |
3.5 |
4 |
350 |
0.50 |
0.716 |
16.7 |
Inv.ex. |
3 |
0.100 |
0.50 |
1 |
4 |
460 |
0.45 |
0.725 |
15.7 |
Inv.ex. |
4 |
0.100 |
0.50 |
2 |
4 |
450 |
0.55 |
0.715 |
16.9 |
Inv.ex. |
5 |
0.100 |
2.00 |
2 |
4 |
400 |
0.38 |
0.730 |
15.1 |
Inv.ex. |
6 |
0.100 |
2.00 |
2.5 |
4 |
400 |
0.45 |
0.710 |
17.4 |
Inv.ex. |
7 |
0.300 |
0.20 |
2 |
4 |
420 |
0.58 |
0.730 |
15.1 |
Inv.ex. |
8 |
0.300 |
0.20 |
3 |
4 |
410 |
0.70 |
0.735 |
14.5 |
Comp.ex. |
9 |
0.020 |
2.50 |
1 |
4 |
330 |
0.10 |
0.820 |
4.7 |
Comp.ex. |
10 |
0.100 |
0.50 |
4 |
4 |
440 |
0.85 |
0.755 |
12.2 |
Comp.ex. |
11 |
0.100 |
2.00 |
1 |
4 |
390 |
0.14 |
0.800 |
7.0 |
Comp.ex, |
12 |
0.300 |
0.20 |
4 |
4 |
410 |
0.90 |
0.765 |
11.0 |
Example 2
[0022] The surface of 0.23 mm thick grain-oriented electrical sheet was scanned by a continuous
wave laser beam under the laser irradiation conditions as shown in Table 2, the residual
stress of the lasered part was measured, then the cumulative compressive stress value
was calculated and the watt loss (W
17/50) was measured. These values are shown in together in Table 2. Example 2 was performed
fixing the laser power at 200W the same as Example 1.
[0023] As clear from Table 2, the electrical sheets shown in Test No. 1 to No. 6 (invention
examples) all have a rolling direction cumulative compressive stress value σS and
a rolling direction laser irradiation pitch (strain pitch) PL in the ranges prescribed
in the present invention, that is, 0.20N≤σS≤0.80N and 2 mm≤PL≤8 mm, so could be reduced
in watt loss to a low watt loss value (W
17/50) of 0.75W/kg, for a watt loss improvement rate of 13%, or less. On the other hand,
the electrical sheet shown in Test No. 7 and No. 8 having a cumulative compressive
stress value σS satisfying the conditions, but off from the conditions of the irradiation
pitch PL failed to achieve a low watt loss value (W
17/50) 0.75W/kg or less. In this way, if using the present invention, it is possible to
obtain grain-oriented electrical sheet superior in watt loss.
Table 2
|
Test No. |
Rolling direction diameter DL mm |
Scan direction diameter DC mm |
Average energy density Ua mJ/mm2 |
Strain pitch PL mm |
Maximum tensile stress MPa |
Cumulative compressive stress value σS N |
Watt loss value W17/50 W/kg |
Watt loss improvement rate % |
Not lasered |
0 |
- |
- |
- |
- |
0 |
0 |
0.860 |
0 |
Inv.ex. |
1 |
0.100 |
0.20 |
1.5 |
2 |
340 |
0.45 |
0.735 |
14.5 |
Inv.ex. |
2 |
0.100 |
0.50 |
1.5 |
2 |
450 |
0.22 |
0.740 |
14.0 |
Inv.ex. |
3 |
0.100 |
0.50 |
1.5 |
4 |
440 |
0.50 |
0.720 |
16.3 |
Inv.ex. |
4 |
0.100 |
0.50 |
1.5 |
6 |
460 |
0.65 |
0.730 |
15.1 |
Inv.ex. |
5 |
0.100 |
0.50 |
1.5 |
8 |
450 |
0.75 |
0.745 |
13.4 |
Inv.ex. |
6 |
0.100 |
2.00 |
3 |
8 |
390 |
0.23 |
0.748 |
13.0 |
Inv.ex. |
7 |
0.100 |
0.50 |
1.5 |
1 |
330 |
0.21 |
0.755 |
12.2 |
Inv.ex. |
8 |
0.100 |
0.50 |
1.5 |
10 |
430 |
0.80 |
0.760 |
11.6 |
INDUSTRIAL APPLICABILITY
[0024] According to the present invention, by quantitatively suitably controlling the residual
stress introduced to the grain-oriented electrical sheet, in particular the compressive
residual stress, it is possible to obtain to stably obtain grain-oriented electrical
sheet superior in watt loss compared with the past. If using the grain-oriented electrical
sheet of the present invention as an iron core, a high efficiency, small-sized transformer
can be produced. The value of industrial application of the present invention is extremely
high.