Note: Descriptions are shown in the official language in which they were submitted.
~2~ 8
GRAIN-ORIENTED ELECTRICAL STEEL S~IEET HA~ING
ST~3LE MAGNETIC PROPERTIES RESISTANT TO
STRESS-RELIEF ANNEALING, AND_METHO~ AND
APPARATUS FOR PRODUCING THE SAME
BACXGROUND OF THE INVENTION
Field of the Invention
The present invention related to a grain-oriented
electrical steel sheet, the magnetic properties of which
are only slightly deteriorate by stress relief annealing,
and to a method and apparatus for producing the same.
Description of the Related Arts
It is important in the light of energy saving to
lessen the watt loss of grain-oriented elec~rical steel
sheet.
Japanese Unexamined Patent Publication No. 58-26406
discloses a watt-loss reduction method~ wherein th~
magnetic domains are subdivided by laser irradiation.
The strain induced by the laser irradiation causes a
watt loss reduction. The above mentioned method can
therefore be applied to the grain-oriented electrical
steel sheet for use in the production of laminated
cores, which need no stress relief annealing, ~ut cannot
be applied to grain-oriented electrical steel sheet for
use in the production of wound cores, which need stress
relief annealing.
Japanese Unexamined Patent Publication No. 56-130454
discloses a watt loss reduction method wherein strain is
induced to a steel sheet after the secondary recrystal-
lization annealing and the cluster of minute crystalgrains, which are formed during the heat treatment after
the secondary recyrstallization annealing due to the
strain, are utilized for watt loss reduction. Since the
cluster of minute crystal grains is formed on the surface
of secondary recrystallized steel sheets, the watt loss
characteristic is not impaired by the stress relief
~6~
- 2 -
annealing. It is, however, difricult, by the method of
Japanese Unexamined Patent Publication No. 56-13045~, to
obtain such a low watt loss as attained by the laser
irradiation method.
SU~MARY OF THE INVENTION
The first object of the present invention is to
provide a grain-oriented electrical steel sheet, in
which a disadvantage of the laser irradiation method,
according to which a low watt loss attained is increased
by the stress relief annealing and hence the grain-
oriented electrical steel sheet can not be stress relief
annealed, and a disadvantage of the method for forming
the minute crystals, according to which a watt loss
attained is not increased by the stress relief annealing
but is high, can be overcome.
~ second object of the present invention is to
provide a method for producing a grain-oriented elec-
trical steel sheet, wherein the watt loss chaxacteristic
which is only slightly impaired by the stress relief
annealing, and a low watt loss is stably attained.
The third object of the present invention is to
provide an apparatus for production of the grain-
oriented electrical steel sheet, according to which
apparatus the watt loss characteristic is only slightly
impaired by the stress relief annealing, and a low watt
loss can be stably attained.
The grain-oriented electrical steel sheet herein
consists of the steel sheet body, in the most limited
sense, the steel sheet body and a layer or coating
formed on the steel sheet body during the final texture
annealing in a less limited sense, and the steel sheet
body, the above layer or coating, and the insulating
coating formed thereon in a broad sense.
In accordance with the present invention there is
provided a grain oriented electrical steel sheet having
a low watt loss after the stress relief annealing,
wherein the grain-oriented electrical steel sheet, which
~4~
is final te~ture annealed, and to which tension i5
imparted, has a number of recessed parts on the surface
thereof ~orming indentations into the steel sheet body
having a depth in the range of from 0.01 to 0.1 mm, the
recessed parts are filled with a composition having a
coefficient of thermal expansion smaller than that of
the steel sheet body, and the magnetic domains are
subdivided from the recessed parts and the composition.
BRIEF DESCRIPTION OF THE DRAWINGS
Figures lA and B graphically illustrate the
relationships between the magnetic properties and the
width and depth of grooves formed on the steel sheet
body;
Figure 2 shows the scanning type electron microscope
photographs of the grain-oriented electrical steel
sheet, in which the magnetic domains are subdivided;
Figure 3A through G show schematically the struc~
tures of the grain-oriented electrical steel sheet
according to the present invention;
Figure 4 illustrates a process for forming and
filling the recessed parts according to the present
invention;
Figure 5 graphically illustrates the relationship
between the etching time and the removal depth of the
steel;
Figure 6 shows the relationships between the HNO3
concentration and etching time;
Figure 7 are graphs showing the variation in
removal depth of the steel in the three parts separated
as seen along the longitudinal direction of a coil;
Figure 8 shows graphs of -the variation in watt loss
W17/50 at the parts of a coil corresponding to those
of Fig. 6;
Figure 9 illustrates a spraying method according to
an example of the present invention; and,
Figure 10 illustrates an immersion method according
to a comparative method.
96~3
-- 4 --
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In accordance with the present invention, the
grain-oriented electrical steel sheet which has been
final te~ture annealed or may further be subjected to
formation of the insulating coating, is subjected to
partial removal of steel of the steel sheet body and a
phosphate coating or imparting tensile stress to a
steel sheet tusually referred -to as the tension coating)
is formed on the grain oriented electrical steel sheet.
The recessed parts formed by the partial removal of
steel are therefore filled with the compound which
exhibits, after a baking heat-treatment of the tension
coating, a coefficient of thermal expansion smaller than
that of the steel sheet body. The filling of the
recessed parts by such compound improves the magnetic
properties and prevents deterioration of the magnetic
properties from occurring during stress relief annealing.
The present invention is now explained in detail
with regard to the process beginning at the hot-rolling.
The silicon-steel slab containing 4% or less of Si is
heated and then hot-rolled to obtain a steel strip having
an intermediate thickness. The hot-rolled steel strip
is pickled and is heat-treated, if necessary. The hot-
rolled strip is cold-rolled by one stage-or two stage-
cold rolling with an intermediate annealing to obtainthe cold-rolled strip having the final sheet thickness.
The strip is then decarburization annealed. The
annealing separator is then applied on the decar-
burization annealed steel strip, which is then final
texture annealed. The above is an ordinary process for
producing a grain-oriented electrical steel sheet. The
grain-oriented electrical steel sheet may be further
coated with the tension coating.
The grain-oriented electrical steel sheet, the
steel of which is partly removed, is that with or
without the tension coating. The removal methods of the
steel are as follows. Mechanical method, such as
imparting the mechanical stress to the surface of the
grain-oriented electrical steel may be used. Marking
off and groove formation by a grooved roll can be used
as the mechanical removal method. In addition, laser,
electron beam or infrared ray irradiation may be used.
The insulating coating of the grain-oriented electrical
steel sheet is selectively removed or peeled by the
mechanical or irradiation method and then the selectively
exposed steel sheet body is dissolved and removed by
acid, such as chloric acid, or nitric acid. The dis-
solution procedure may be any of, for example, immersion,
spraying, electrolytic pickling, or the like. As a
result, the recessed parts indented into the steel sheet
body in the form of a groove are formed. If such
recessed parts can be formed only by marking off, this
method can be used.
The grooves extend preferably perpendicular to the
rolling direction (~001> orientation3 but may be slanted
by an angle of, for example, 45 to the rolling direc-
tion. If the slant angle is extremely great, thegrooves become undesirable in the light of wat-t loss
reduction. The laser irradiation mentioned above causes
the watt loss reduction and the strain induction as in
Japanese Unexamined Patent Publication No. 58-26405.
The laser irradiation mentioned above is, however,
different from this publication in the point that the
local strain remaining after the stress relief annealing
is advantageously utilized for the watt loss reduction.
The distance between the grooves as seen in the rolling
direction is preferably from 2.5 to 10 mm, since the
watt loss reduction is greatest at this distance. This
distance is the same as that disclosed in Japanese
Unexamined Patent Publication No. 58-26406.
Each groove may be formed by either a line or
spots~ In the case of spots, the distance between the
spots is preferably 0.7 mm or less, more preferably
0.3 mm or less. When the distance between the spots is
g~ ~
greater than 0.7 mm, the watt loss reduction becomes
small.
The present invention is further explained with
reference to the drawings.
Figure lA shows the dependence of the magnetic flux
density (B8) and watt loss Wl7/50 upon the width of
a groove. In the tests, the results of which are shown
in Fig. lA, 0.23 mm thick grain-oriented electrical
steel sheets having a phosphate coating or a coating
comprised of semi-organic coating were used, and grooves
0.05 mm in depth, spaced from one another by a distance
of 5 mm were formed while varying the groove width. The
tension coating was used as the composition having a
coefficient of thermal expansion different than that of
the steel sheet body. After application the tension
coating it was baked at 850C for 5 minutes.
The phosphate coating herein is liquid consisting
of colloidal silica, phosphate~ such as aluminum phos-
phate, and chromic acid anhydride or chromate. The
colloidal silica and phosphate are the major components
in the coating, and when these components are baked, the
composition resulting therefrom has a coefficient of
thermal expansion dilferent from that of steel sheet
body and imparts a tensional stress to that body. Note,
however, in implementing the present invention, any
coating composition having the same function as phosphate
coating can be used. It may contain further magnesium
phosphate.
Now again referring to Fig. lA, one of the magnetic
properties, i.e., the magnetic flux density B8 '
lessens in accordance with an increase in the groove
width, and the other magnetic property, i.e., the watt
loss W17/50 , is high at an extremely narrow width of
a groove- The high watt loss W17/50 results from a
small generation quantity of strain due to extremely
narrow width of a groove. It is therefore preferred
that the groove width be 0.1 mm or more. The grooves in
:~29L~ 8
-- 7 --
the light o-E the magnetic properties are preferably
approximately ~.4 mm or less.
Figure lB shows the results of tests similar to
those in the case of Fig. lA. In these tests, the
0.3 mm wide grooves were formed on 0.23 mm thick grain-
oriented electrical steel sheets, at a distance of 5 mm,
while varying the depth of the grooves. The above
mentioned coating solution was applied on the grain-
oriented electrical steel sheets and baked at 850~ for
5 minutes, to form a film of the coating composition in
the grooves. An improvement in the magnetic properties
over those of the prior art was attained at the groove
depth in the range of from 0.01 to 0.1 mm. At a depth
greater than 0.1 mm, the watt loss W17/50 is not
impaired but the magnetic flux density B8 is greatly
lessened. At a groove depth of 0.01 mm, the watt loss
W17/50 is approximately 0.9 W/kg and is lower than
that of the conventional products. The improvement in
the wa~t loss W17/50 is outstanding when the groove
depth is 0.02 mm or more. Accordingly, a preferred
groove depth is from 0.02 to 0.08 mm.
As is described with reference to Figs. lA and B,
steel is partly removed from a surface of the steel
sheet body and subsequently the so formed recessed parts
are filled with the compound in accordance with the
present invention. The coefficient of thermal expansion
of such a compound is smaller than that (approximately
13 x 10 6) of the steel sheet body and can impart
tensional stress to the steel sheet. In order to fill
the recessed parts with the compound, the coating
liquid, such as a phosphate coating, is applied on the
grain-oriented electrical steel sheet. From an
industrial point of view, the application of a coating
liquid over the entire grain-oriented electrical steel
sheet is preferred to partial application. In the tests
explained with reference to Figs. lA and B, the entire
application was conducted, but the local application of
69~8
-- 8 --
coating liquid is also efective for attaining the
advantages of the present invention. Regarding the
filling method of the compound, any method may be used,
provided that the compound is filled in the previously
recessed parts of the steel sheet body. In order to
enhance the bonding force between the filled composition
and steel of steel sheet body, a bonding-reinforcing
material, such as Ni plating or vaporization deposition
of silica is advantageously used. This f~lrther enhances
the improvement in the magnetic properties. The Ni
plating layer preferably has a thickness of 1 ~m or
less, when the coating liquid contains colloidal silica.
Referring to Figure 2 shows the result of obser-
vation o~ the magnetic domains by means of a scanning
type electron microscope. The sample subjected to the
observation was prepared as follows. A grain-oriented
elec-trical steel sheet having a tension coating was
irradiated with laser and was then corroded in a ~itric
solution to form holes approximately 0.025 mm in depth.
The coatin~ liquid consisting of alumina phosphate,
colloidal silica, and chromic acid was baked at 350C.
Heat treatment was then carried out in air at 850~C for
2 minutes. Stress relief annealing was then carried out
at 850C for 4 hours.
As is evident, the magnetic domains generate at the
parts where the holes were formed. It is believed that
the parts of the steel sheet body where the steel is
removed provide the generation sites of the subdivided
magnetic domains, which pass the rolling direction and
which lessen the watt loss.
In the foregoing descriptions, the grain-oriented
steel sheets in the less limited and the broad senses
were described, since the most economical products can
be obtained by using the same. Nevertheless, the
grain-oriented electrical steel sheet in the narrowest
sense also can be subjected to the method described
hereinabove. That is, final texture annealing is
6~3
_ 9 _
carried out and then secondarily recrys~allized steel
sheet having neither surface layer nor a film can be
subjected to the method of the present invention.
The magnetrostriction of the grain-oriented elec-
trical steel sheet according to the present inventionwas measured and found to be equivalent to that of the
conventional products.
Referring to Figs. 3A-G, various surface-layer
structures of the secondarily recrystallized steel sheet
according to the present invention are shown by a
partial cross sectional view.
In Fig. 3A, neither coating nor layer are formed
on the secondarily recrystallized steel sheet l. The
recessed parts 3 are formed on the surface 2 of the
secondarily recrystallized steel sheet l.
The composition for imparting tensional stress 6 to
the steel sheet is entirely applied on the surface 2 of
the steel sheet. The composition 6 is placed on the
major surface 4 of the steel sheet and is filled in the
recessed parts 3. Metal plating may be sandwiched
between the surface 2 of the steel sheet and the above-
mentioned composition 6 to enhance the bonding strength
therebetween.
In Fig. 3B, the grain-oriented electrical steel
sheet includes the secondary recrystallization film 7
usually referred to as the forsterite layer or film.
The recessed parts 3 are filled with the composition as
indicated by reEerence numeral 5. The composition 5 is
partly applied on the surface 2 of the steel sheet and
is filled in the recessed parts 3.
Referring to Fig. 3C the composition having the
same components and contents as that of composition 5 is
applied entirely on the surface 2 of the steel sheet (as
indicated by "6") and is filled in the recessed parts of
the grain-oriented electrical steel sheet having the
forsterite film such as shown in Fig. 3B. The grain-
oriented electrical steel sheets explained with reference
~6~
-- 10 --
to Fig. 3B and C can be used, as having the structure as
shown in the drawings, for producing the second core of
transformers.
The composition for imparting the tensional force 6,
which composition is partly applied and is fil-led in the
recessed parts, can undergo any thermal process, e.g.,
baking or stress relief annealing. After the thermal
process, local stress is generated in the recessed parts
due to a shrinkage fitting effect of the composition 6
and steel at the recessed parts 3. And, also the
recessed parts 3 have the poles therearound due to a
magnetic shape effect. The intensity of magne-tic poles
is influenced by the groove depth and width. Due to the
formation of magnetic poles, the 90 domains generate
around the recessed parts 3.
In Fig. 3~, the semi-organic film 8 is applied on
the entire top surface of the grain-oriented electrical
steel sheet.
In Fig. 3F, the recessed parts 3 are formed on the
grain~oriented electrical steel sheet having the
forsterite film 7 and the tension coating 9, and the
composition for imparting the tensional stress 5 is
filled within the recessed parts 3.
In Fig. 3G, the composition 5 mentioned above is
applied on the entire top surface of the grain-oriented
electrical steel sheet explained with reference to
Fig. 3F.
In the case of partial application of the above
mentioned composition, the composition is filled within
the recessed parts and may be applied in the vicinity of
the recessed parts. The grain-oriented electrical steel
sheet according to the present invention can have a
structure wherein the above mentioned composition is
filled in and located in the vicinity of the recessed
parts.
In Fig. 3D, a semi-organic film 8 which does not
impart tension to the steel sheet is applied on the
forsterite film 7. The composition for imparting the
tensional stress 5 is filled within the recessed parts 3
of the steel sheet 1.
The subdivided, 180 magnetic domains are formed
due to the forsterite film or tension coating, which
applies a tensional stress to the 90 magnetic domains.
When the above mentioned composition is applied on the
entire surface of a steel sheet, the subdivision of the
lC magnetic domains is promoted as compared with the
partial application, due to the tensional stress gener-
ated on the major surface by the above mentioned com-
position.
Example 1
A 0.23 mm thick grain-oriented electrical steel
sheet was prepared by a one stage cold-rolling method.
The grain-oriented electrical steel sheet which had
undergone the final texture annealing, was subjected to
marking-off by means of a knife. The linear grooves
were formed, by the tip end of a knife, in a direction
perpendicular to the rolling direction, and with a
distance of 5 mm therebetween~ The grooves had a width
of 0.2 mm and protruded into the underlying metal to a
depth of approximately 0.03 mm. Subsequently, a coating
solution was applied on the steel sheet. The coating
solution consisted of 100 cc of 20% colloidal silica-
aqueous dispersion liquid, 60 cc of 50% aluminum phos-
phate aqueous solution r and 6 g of chromic acid
anhydride. The coating solution was baked at 830C for
3 minutes. After baking the coating solution, the steel
sheet was stress-relief annealed at 850C for ~ hours.
The watt loss of the steel sheet was measured prior to
and subsequent to the stress relief annealing, in the
rolling direction.
For comparison purposes, the above mentioned
grain-oriented steel sheet was subjected to the same
procedure as explained above, except that the marking
off was not carried out. In addition, the grain-
- 12 ~ 6~
oriented electrical steel sheet was subjected to the
same procedure as explained above, except that the
marking off was carried out Inot after the final texture
annealing) after the baking of coating solution. The
distance between the grooves, and the depth and width of
the grooves were the same as explained above. However,
the grooves are formed on the phosphate coating. The
watt loss measured is given in Table 1.
Table 1
. _
Process W17/50
- - (W/kg)
Con~7en- Final texture annealing, 0.93
tional imparting a phosphate coating,
and then stress
relief annealing
After imparting the phosphate 0.85
coating in the above
process, marking off and
then stress relief annealing
Inven- Final texture annealing, Before 0.80
tion then marking off, and stress
application of a relief
tension-imparting coating, annealing
baking at 830C for 3
minutes After 0.82
stress
relief
annealing
:~ 2 ~
~ 13 -
As is apparant from Table l, the watt loss Wl7/50
is lessened only bv means of marking off. This is
because the marking off generates minute grains in the
secondary recrystallized grains. The steel sheet
treated according to the present invention exhibits a
Wl7/50 which is lower, by 0.11 to 0.13 W/kg, than the
Wl7/50 attained by the final texture annealing and then
tension-imparting by the phosphate coating. The steel
sheet treated according to the present invention exhibits
a W17/50 which is lower, by 0.03 to 0.05 W/kg than the
Wl7/50 attained by marking off and stress relief
annealing. It is therefore evident that when the
material of phosphate coating, which has a low coef-
ficient of thermal expansion and ~hich imparts tension
upon the film-formation, is filled in the grooves, such
filling is effective for improving the watt loss charac-
teristics.
xample 2
A 0.23 mm thick grain-oriented electrical steel
sheet was produced by a one stage cold-rolling process.
A phosphate coating was applied on the surface of the
grain oriented electrical steel sheet on a forsterite
film. Subsequently, the coated surface was subjected to
exfoliation by irradiation of a YAG laser, which was
pulsed at an intensity of about 4 mJ, thereby forming
spot-like holes 0.2 mm in diameter on the coated surface,
aligned at intervals of 0.3 mm in the direction per-
pendicular to the rolling direction. The holes were
arranged in the form of spot-lines, each being spaced at
5 mm. The steel sheet was then immersed in 61% nitric
acid at 25C for 90 sec to give a hole depth about
0.0~ mm.
Then the steel sheet was Ni-plated in a Watts bath
containing 240 g/l of nickel sulphate, 45 g/l of nickel
chloride and 30 g/l of boric acid, at 60C for 5 sec,
with a current density of 5 A/dm .
A coating liquid, composed of lO0 cc of 20%
6~3
- 14 -
colloidal silica-aqueous dispersion liquid, 60 cc of 50%
aluminum phosphate aqueous solution, 15 cc of 25~
magnesium chromate aqueous solution and 3 g of boric
acid, was applied on the Ni-plated steel sheet, followed
by baking at 850C for 3 mins. Then the steel sheet was
stress relief annealed at 800C for 4 hrs.
Another steel sheet was prepared in the same way as
above, but the Ni-plating was not applied.
The watt loss values in the rolling direction are
shown for both steels in Table 2.
Table 2
Art Process ~W/k
... . .. _ . , . . . _
Conven- Final texture annealed, 0.92
tional phosphate coating
imparted and stress-
relief annealed
Inven- Without Ni-plating Before SRA* 0.79
tion After SRA 0.82
Ni-plated Before SRA 0.78
- After SRA 0.80
,
* SRA: stress-relief annealing
As can clearly be seen in the Table 2, the steel
sheets according to the present invention have greatly
improved watt loss values compared to that of a con
ventional steel sheet.
Ni plating resulted in a slight difference in the
deterioration of the wa-tt loss due to the stress relief
- 15 _ ~Z~9~
annealing. The watt loss values of the products with
and without Ni plating were far superior to that of the
conventional product.
Example 3
A 0.175 mm thick grain-oriented electrical steel
sheet was produced by a two stage cold-rolling process.
A phosphate coating film was applied on the forsterite
film of the above mentioned steel shee~. Subsequently,
the coated surface was subjected to exfoliation by
irradiation of a Y~G laser, which was pulsed at an
intensity of about 4 mJ, and thereby forming spot-like
holes 0.3 m~ in diameter on the coated surface. The
holes were arranged in the form of spot lines in the
direction perpendicular to the rolling direction. ~ach
holes are spaced at 0.4 mm, and the lines of the holes
were spaced at 6 mm.
The steel sheet was then immersed in 61% nitric
acid at ~0C for 60 seconds to form holes about 0.~25 mm
in depth.
A coating liquid, composed of 100 cc of 25~
colloidal silica-aqueous dispersion li~uid, 60 cc of 50%
aluminum phosphate aqueous solution and 6 g of chromic
acid anhydride was applied on both surfaces ~2 g/m2)
of the immersion treated steel sheet, followed by baking
at 450C for 5 minutes. Then, the steel sheet was
stress-relief annealed at 850C for 4 hours.
The watt loss values in the rolLing direction of
the thus prepared steel sheet are shown in Table 3.
The steel sheet according to the present invention
shows greatly improved watt loss values compared to
those of the conventional sheet.
- 16 ~ 6~
Table 3
m_.__
Art Process13/50 W17/50
tW/kg) (W/kg~
Conven- Final texture annealed, 0.43 0.83
tional phosphate coating imparted
and then stress~relief
annealed.
Inven - 0.39 0.76
tion
Example 4
A 0.23 mm thick grain-oriented electrical steel
sheet was produced by a one stage cold-rolling process.
A phosphate coating was applied on the forsterite film
of the above mentioned steel sheet. Subsequently, the
coated surface was subjected to exfoliation by
irradiation of a CO2 laser (1.50 KW of power, 0.2 mm
beam diame-ter, and 12 m/sec scanning speed), thereby
exfoliating linearly in the direction perpendicular to
the rolling direction. The lines were spaced at 5 mm.
The steel was then immersed in 61~ nitric acid at
40C for 70 seconds to form grooves about 0.03 mm in
depth.
A coating liquid, 100 cc of which contained col-
loidal silica (SiO2 content, 14 g), 25 g of magnesium
phosphate and 4 g of chromic acid anhydride, was applied
by means of a rubber toothed wheel at 5 mm pitch to fill
tne underlying-iron removed part of the steel surface,
followed by baking at 450C for 5 minutes. Then, the
steel sheet was stress-relief annealed at 850C for 2
hours.
The wat-t loss value in the rolling direc~ion of the
- 17 ~ 6~6~
thus prepared steel sheet is shown in Table 4.
The steel sheet according to the present invention
shows a greatly improved watt loss value compared to
that of conventional steel sheet.
It is evident from this result that the local
filling of a tension imparting substance of phosphate
coating is effective in decreasing in watt loss.
Table 4
_
Art Process (W/kg)
Conven- Final texture annealed, phosphate 0.92
tional coating imparted and then
stress-relief annealed
Inven- - 0.83
tion
~ _ ~ ,
Exam~le 5
A 0.~3 mm thick, glass-free grain-oriented electri-
cal steel sheet was produced by a one stage cold-rolling
process. This steel sheet was mirror finished in a
solution containing lO0 volume part of hydrogen peroxide
(30% aqueous solution) and 5 volume part of fluoric
acid. ~oth surfaces of the steel sheet were mirror
finished. The samples were taken from this sheet and an
acid-resistant tape was adhered entirely to one of the
surfaces of samples. Acid-resistant tapes were adhered
on the other surface so that the mirror finished steel
was exposed at distances of 5 mm and widths of 0.3 mm.
The samples were then immersed in the 61% nitric acid
solution (~0C) for 60 seconds to form grooves approxi-
mately 0.025 mm in depth. After formation of the
- 18 -
grooves, all of the tapes were peeled off. The samples
were then subjected to Ni plating in a Watts bath at a
current density of 5 A/dm2 for 5 seconds, and subsequent-
ly to application of a coating solution on both surfaces
thereof. The coating solution consisted of lO0 cc of
20~ colloidal silica dispersion-aqueous solution, 60 cc
of 50~ aluminum phosphate-aqueous solution, 15 cc of 25%
magnesium chromate-aqueous solution, and 3 g of boric
acid. The application amount was 3 g/m2. The coating
solution was baked at 850C for 3 minutes. The samples
were the stress relief annealed at 800C for 4 hours.
For comparison purposes, the same procedure as
explained above was carriad out except that the grooves
were not formed and the baking was carried out at 850C
for 4 hours.
The results of the test are given in Table 5.
Table 5
Process Wl7/50
(W/kg)
. . _ . . _ . .
Conven- Final texture annealing, mirror 0.76
tional finishing of both surfaces,
Ni-plating, phosphate coating
and stress relief annealing
Invention Final texture annealing, mirror 0 71
finishing of both surfaces,
local removal of steel sheet
body Ni-plating, phosphate
coating and stress relief
annealing
As is apparent from Table 5, the Wl7/50 according
to the present invention is less than the conventional
Wl7/50 by 0.05 W/kg.
The method for treating the grain-oriented electri-
- 1 9
cal steel sheet provided in accordance with the third
object of the present invention comprises irradiating
the surface of the grain-oriented electrical steel sheet
having an insulating film thereon with a laser beam,
thereby forming a number of removed parts of the insu-
lating film, and then removing the steel of the steel
sheet body exposed through the above-mentioned recessed
parts, by etching. According to a feature of the
present invention, etching is performed by spraying a
nitric acid solution so as to obtain a removal depth of
steel of the steel sheet body, which depth is uniform
over the recessed parts of steel sheet body. The nitric
acid solution for etching the steel of the steel sheet
body is advantageous over the sulfuric acid solution and
hydrochloric acid solution, since the dissolution
~uantity of insulating film by the former solution, is
considerably smaller than that of the latter solution.
Below a concentration of nitric acid of 20% ~y wei~ht,
the etching rate is low, while at a concentration of
more than 70% weight, a problem of smoke generation
arises.
Referring to Fig. 4, an apparatus for performing
the method described abo~e is illustrated. The apparatus
comprises an uncoiling device 10, a laser beam-irradia-
tion device 12, an etching device 13 for spraying theacid liquid, a rinsing (water-cleaning~ and drying
device 14, a device for recoating the insulating film 15,
a baking device ~furnace) 16, and a coiling device 17.
The grain-oriented electrical steel strip S is treated
during uncoiling and coiling thereof by means of the
apparatuses and the like 12, 13, 14, 15, and 16. The
laser beam-irradiation device 12 forms marks on the
grain-oriented electrical steel strip S. These marks
are most preferably formed perpendicular to the rolling
direction. The marks may be linear or spot-like as
explained hereinabove~ and may be formed one or both
surfaces of the grain-oriented electrical steel sheets.
~6~368
20 -
In Fig. ~ the marks are formed on one surface, i.e.,
the upper surface of the grain-oriented electrical steel
sheets. These steel sheets were treated by an etching
device 13 to dissolve and remove the steel under the
laser marks. The etching liquid used for dissolution
and removal of the steel preferably does not dissolve
the electrical insulating film and is most preferably a
nitric acid solution. The etching device 13 is equipped
with a number of spray nozzles 18 positioned above the
steel strip S. The nitric acid solution is sprayed
through the spray nozzles for etching of the steel
strip S and is once transferred into the circulation
tank 20 via the connection circuit 21. The etching
solution is then fed and circulated by the action of a
pump 23 to each of the spray nozzles 18. The conduits
at the entrance side of the spray nozzles 18 are equipped
with valves 19. The heater 22 mai~tains the liquid
temperature within a predetermined range of, for e~ample,
from 30 to 70C.
The dispersion in the watt loss values is lessened
by spraying the nitric acid solution on to the grain-
oriented electrical steel strip S having a number of
removed parts of the insulating film due to the laser
beam irradiation in the device 12, as compared with the
case of dipping the above mentioned strip S in the case
of dip pickling. As a result the watt loss character-
istics are stable in the case of spraying the nitric
acid solution. The reasons for reduction in the disper-
sion of the watt loss values and stabilization of the
watt loss characteristics are considered to be as
follows.
Bubbles are formed by the acid during etching but
are washed out by the spraying. The bubbles therefore
do not impede a uniform and direct contact of the nitric
acid solution with steel exposed through the removed
parts of the insulating film. As a result, the depth of
recesses or indentations becomes uniform over the entire
- 21 ~ 6~
recesses or indentations, and hence the watt loss
characteristics become uniform.
In addition, the etching efficiency is higher in
the spraying method than in the dipping method. The
reason for this is because the fresh acid solution is
uninterruptedly and continuously fed over the steel
strip and hence the etching time is shortened (c.f.
Fig. 5).
Referring to Fig. 6, the relationship between the
concentration of nitric acid and the etching time, for
obtaining an optimum etching depth of from approximately
0.02 to 0.08 mm, is shown. The horizontal dot line
indicates the etching time of 50 seconds, which is the
longest etching time industrially employable. In order
to obtain such an etching time, the concentration of
nitric acid must be 20% by weight or more. The highest
concentration of nitric acid is limited, not Erom the
viewpoint of the etching time but from the point of
smoke generation which impairs the working environments.
The highest concentration of nitric acid is 70~ by
weightO A preferred concentration of nitric acid is
from 30 to 60~ by weight.
Example 6
~ 10 ton coil 0.23 mm thick, high magnetic flux
density, grain-oriented electrical steel strip coated
with a tension insulation film (5 g/m2) was separated
into binary parts along the longitudinal direction. One
of the separated parts of the coil was processed through
the line shown in Fig. 4 to form underlying metal
exposed sites, i.e., coating-film removed sites, on the
surface of the strip by means of irradiation by a YAG
laser. The strips then underwent etching of the
underlying-metal exposed sites in an etching apparatus
having multiple rows of nitric acid sprays. The strips
were subsequently rinsed by water, dried, and finally
subjected to a tension insulation coating trea~ment
(2 g/m2) in order to repair the underlying-metal
- 22 -
exposed sites.
The laser irradiation and etching conditions were
as follows:
(1) Laser irradiation:
(a) Irradiated surface: one surface
(bl Energy density: 2 mJ/mm
(c) Irradlation marks:
Diameter of spots ~holes3: O.Z ~ 0.3 mm
Distance between the centers of
spots in C direction (direction
along width): 0.5 mm
Distance between the rows
of spots in L direction
(longitudinal direction): 5 mm
(2) Etching:
~a) Mode: spraying
(b) Medium: 60 wt% nitric acid held at ~0C
(c) Number of rows of sprays: 20
(d~ Duration of spraying: 30 sec
~e3 Depth of etching- 25 ~m
Comparative_example
The other part of the separated coil was irradiated
with a YAG laser in the same manner as mentioned in
example 6 according to the present invention. This coil
strip was then immersed in 60% nitric acid at 40C for
60 sec to give a desired etching depth of 25 ~m. The
subsequent processes were the same as in example 6.
The etching by immersion took 60 sec, in con-trast
with the etching by spraying according to the present
invention, which took 30 sec. Thus, there is a great
advantage in etching by spraying compared to that by
immersion. The graphs in Fig. 7 show the scattered
values of the etched depth of underlying-metal in the
longitudinal direction for Example 6 according to the
present invention and for the Comparative Example. The
graphs in Fig. 8 show the scattered values of the watt
loss of the stress relief annealed strips corresponding
6~36~3
- 23 -
to Fig. ~.
The watt loss values were determined by a single
sheet magnetic measuring instrument.
The above results evidently confirm that the
present invention can provide a product having an
exceedingly small scatter in magnetic properties.
The process, according to the present invention,
for producing low watt loss, grain oriented electrical
steel sheet free from deterioration in the watt loss
characteristics in the stress relief annealed state can
markedly improve the pickling characteristics and watt
loss scatter of the steel sheet and can greatly contrib-
ute to producing grain oriented electrical steel sheet,
mainly for wound core transformer use.
lS It is one of the objects of the present invention
to provide products having a small variation in the watt
loss values, which products are obtained on an industrial
production scale.
As is well known, the iron components of the
pickling article are dissolved in the pickling solution,
with the result that, during the pickling procedure, the
Fe concentration increases in the pickling solution and
the pickling ability thereof gradually lessens. The
heretofore proposed measures against this usually
employed are a method of decreasing the conveying speed
of a strip and hence increasing the etching time when
the pickling ability decreases, and a method for
replenishing with fresh liquid when the pickling ability
decreases. If the former method is applied for the
treatment of grain-oriented electrical steel sheet in
accordance with the present invention, the conveying
speed of the strip is lessened not only in a pickling
station but also in a laser-irradiation station. As a
result, the irradiation distances and other irradiation
conditions of the laser beam will vary. ~his in turn
leads to variation in the baking condi-tion of -the film.
Accordingly the former method is disadvantageous for one
~69~
- 24 -
of the above objects, since the various properties of
the product become unstable. It is possible to conceive
that, depending upon the variation in -the conveying
speed of a strip, the conditions of laser-beam irradia-
tion and baking conditions of film are also varied.However, this is extremely difficult in practice.
Accordingly, the above object of the present invention
is therefore accomplished by providing an efficient
etching method, wherein the conveying speed of a grain-
oriented electrical steel sheet can be always keptconstant, i.e., is not decreased even if there is a
gradual decrease of the etching ability. Such etching
method allows the conditions for laser-beam irradiation
and baking to be kept constant. More specifically, this
object is attained by arranging a number of spraying
nozzles of acid liquid subsequent to the laser-irradia-
tion step, circulating the once-sprayed, etching liquid
to the spraying nozzles, subjecting a grain-oriented
electrical steel strip to the continuous laser-beam
irradiation and etching under a constant strip conveying
speed, and selecting the number of spraying nozzles used
for etching in accordance with the decrease in the
etching ability during the circulated use of the acid
liquid.
The etching method is described in detail with
reference to Fig. 4. The grain-oriented electrical
steel strip S is conveyed always at a constant speed.
The spray nozzles 18 consist of, for example, eight
groups. The etching ability is detected by analyzing
the Fe concentration in the nitric acid solution.
Alternatively, it is detected by measuring the dissolu-
tion depth of samples which are taken from the product.
The dissolution depth becomes shallow upon a reduction
in the etching ability. When the Fe concentration
becomes higher than the predetermined value or the
dissolution depth becomes less than the requisite value,
the valves 19 are operated to increase the number of
:~2~
- 25 -
spraying nozzles 18 used for etching. Such an increase
in the nozzle number is carried out s~ that addikional
nozzles as seen in the conveying direction of strip can
spray the etching solution. Accordingly the dissolution
depth can be maintained constant notwithstanding the
decrease in the etching ability. This in turn allows
the maintaining of a constant laser-beam irradiation
condition for forming the marks, and the maintaining of
a constant condition for dissolving the steel of steel
sheet body. It is, accordingly, easy to produce an
industrially grain-oriented electrical steel sheet
having magnetic properties stable at a high level.
The spraying method is also advantageous in the
case of an emergency stop of pickling line. In the case
of an emergency, the spraying is interrupted to immedi-
ately stop the feeding of nitric acid solution to the
steel strip S. The subsequent water rinsing effectively
prevents piercing or rupturing of the steel strip S due
to an excess of acid.
Example 7
0.23 mm thick high magnetic flux density, grain-
oriented electrical steel strips coated with tension
insulation film (5 g/m ) were processed through the
line shown in Fig. 4 to form underlying-metal exposed
sites, i.e., coating film removed sites, on the surfaces
of the strips by means of irradiation by a YAG laser.
The strips then underwent etching of the underlying-metal
exposed sites in a etching apparatus having multiple rows
of nitric acid sprays. The strips were subsequently
washed by water, dried and finally subjected to a
tension insulation coating treatment (2 g/m2) in order
to repair the underlying-metal exposed sites.
The laser irradiation and etching conditions were
as follows:
(1) Line speed: 40 m/min (constant)
(2) Laser irradiation:
(a) Irradiated surface: one surface
6~3
- 26 -
~b) Energy density: 2 mJ/mm2
(c) Irradiation marks:
Diameter of spots: 0.2 ~ 0.3 mm
Distance between the centers of
spots in C direction (direction
along width): 0.5 mm
Distance between the rows
of spots in L direction
(longitudinal direction): 5 mm
(3) Etching:
(a) Mode: spraying
(b~ Medium: 60 wt~ nitric acid held at 40C
(c) Number of rows of sprays: 20
(d) Duration of spraying: 30 sec or more
(e) Depth of etching: 25 ~m
250 tons of steel strips were treated under the
above mentioned conditions, though the number of rows of
working sprays were increased from an initial number
of 4 to a final number of 20 according to a dropping
concentration of the nitric acid and a rising concentra-
tion of Fe during etching, as shown in Fig. 3. The thus
treated strips were stress relief annealed at 800C for
2 hrs in an N2 atmosphere and then subjected to watt
loss measurement. The determined watt loss values are
also shown in Fig. 9. The watt loss values were deter-
mined by a single sheet magnetic measuring instrument.
As is seen in Fig. 9, even depths of etching and
resultant even, low watt loss values were obtained.
Comparative Example
The starting steel strips were equivalent to those
used in Example 7.
Laser irradiation on the strips was carried out at
the same line speed as in Example 7. After the irradi-
ation, the strips were then immersed in 60 wt% nitric
acid at 40C for a constant time of 60 sec. The subse-
quent processes were the same as in Example 7.
Though the concentrations of nitric acid and Fe
- 27 -
showed similar tendencies during etching to those in
Example 7, the etched depth decreased with an increase
in the steel weight etched. Particularly, for a weight
exceeding 100 tons, the tendency became stronger, and
accordingly, the watt loss values of the stress relief
annealed steel strips were raised.
The process, according to the present invention,
for producing low watt loss, grain-oriented electrical
steel sheet free from deterioration in the watt loss
characteristics in a stress relief annealed state, in
which process a laser irradiation treatment and an
etching treatment are carried out continuously and at a
constant line speed realized by a special etching
method, can provide a product with a low and stable watt
loss value.
