Grain-oriented silicon steel sheet having a low iron loss free from deterioration due to stress-relief annealing and a method of producing the same

A grain-oriented silicon steel sheet having a low iron loss free from deterioration due to the stress-relief annealing, can be obtained by forming on its surface a forsterite film locally having regions, which have a thickness different from that of the remaining regions in the film, or locally having filmless regions which do not coat the steel sheet surface.

BACKGROUND OF THE INVENTION 
(1) Field of the Invention 
The present invention relates to a grain-oriented silicon steel sheet 
having a low iron loss, and a method of producing the steel sheet, and 
more particularly relates to a technique for lowering the iron loss of a 
grain-oriented silicon steel sheet by giving ununiformity to a coating 
film formed on the steel sheet surface so as to define and form, on the 
steel sheet surface, local (or localized) regions subjected to a tensile 
force different from that acting upon the remaining region of the steel 
sheet surface. 
(2) Description of the Prior Art 
Grain-oriented silicon steel sheets are mainly used in the iron cores of a 
transformer and other electric instruments, and are required to have 
excellent magnetic properties, particularly to have a low iron loss 
represented by W.sub.17/50. 
In order to meet the requirement, it is necessary that the &lt;001&gt; 
orientation of secondary recrystallized grains in a steel sheet is highly 
aligned to the rolling direction of the steel sheet, and further that 
impurities and precipitates contained in the final product are decreased 
as possible. The iron loss value of grain-oriented silicon steel sheets 
produced so as to meet the requirements becomes lower year and year by 
laborous investigations and efforts, and recently a grain-oriented silicon 
steel sheet having a low iron loss value of W.sub.17/50 of 1.05 W/kg in a 
sheet thickness of 0.30 mm has been obtained. 
However, since the energy crisis several years ago, electrical instruments 
having a lower iron loss are greatly in demand, and a grain-oriented 
silicon steel sheet having a lower iron loss has been demanded in order to 
use the steel sheet as an iron core material of the instruments. 
As a means for lowering the iron loss of grain-oriented silicon steel 
sheet, there have generally been known metallurgical methods; for example, 
a method wherein the Si content is increased, a method wherein the 
thickness of a product steel sheet is made small, a method wherein the 
secondary recry stallization grains are made fine, a method wherein the 
content of impurities are decreased, a method wherein secondary 
recrystallization grains having a (110)[001] orientation are highly 
aligned, and the like. However, these means have been fully investigated, 
and the improvement of these means is very difficult, and even when the 
means are somewhat improved, the effect for lowering iron loss is very 
little. 
A part from these means, Japanese Patent Application Publication No. 
23,647/79 discloses a method, wherein secondary recrystallization-checking 
regions are formed on a steel sheet surface, thereby finely divided 
secondary recrystallization grains are made. However, this technique 
cannot control in a stable manner the size of the secondary 
recrystallization grain, and is not a practical method. 
Japanese Patent Application Publication No. 5,968/83 disclose a technique, 
wherein slight strain is introduced into the surface of a secondarily 
recrystallized steel sheet by means of a ball-point pen-like small globe 
to subdivide the magnetic domain wall spacing, thereby the iron loss is 
lowered. Japanese Patent Application Publication No. 2,252/82 discloses a 
technic, wherein laser beams are irradiated to the surface of a final 
product steel sheet at an interval of several mm in a direction 
substantially perpendicular to the rolling direction to introduce high 
dislocation density regions into the surface layer of the steel sheet, 
thereby the magnetic domain wall spacing is subdivided to lower the iron 
loss. Japanese Patent Laid-open Application No. 188,810/82 discloses a 
technique, wherein slight strain is introduced into a steel sheet surface 
layer by means of an electric spark, thereby the magnetic domain wall 
spacing is subdivided to lower the iron loss. In these methods, slight 
plastic strain is introduced into the surface layer of a secondarily 
recrystallized steel sheet matrix, whereby the magnetic domain wall 
spacing is subdivided to lower the iron loss. These methods are practical 
methods, and are excellent in the effect for lowering the iron loss. 
However, the effect attained by the introduction of plastic strain into 
the steel sheet is lost by the heat treatments, such as stress-relief 
annealing and baking treatment of coating, which are carried out after 
punching, shearing, coiling and the like of the steel sheet. When it is 
intended to introduce very slight plastic strain into a steel sheet after 
a coating treatment, an insulating coating must be again applied to the 
steel sheet in order to maintain the insulating property. Also, additional 
steps, such as a strain-giving step and a recoating step, are required, 
resulting in a high production cost of grain-oriented silicon steel 
sheets. Japanese Patent Application Publication No. 17,757/78 discloses a 
technique for lowering magnetostriction of a grain-oriented silicon steel 
sheet by forming inorganic coating films having a stripe-shaped pattern or 
checkered pattern on both matrix surfaces of the steel sheet. 
The object of the present invention is to provide a grain-oriented silicon 
steel sheet having excellent magnetic properties by subdividing the 
magnetic domain wall spacing based on a technical idea different from that 
of the above described prior art, which steel sheet can secure its 
excellent magnetic properties obtained by the subdivision of magnetic 
domain wall spacing, even after the stress-relief annealing at high 
temperatures. 
The present invention is based on the discoveries that, when a forsterite 
film constituting a surface film of a grain-oriented silicon steel sheet 
has locally regions having a thickness different from that of the 
remaining regions in the forsterite film, the magnetic domain wall spacing 
can be very advantageously subdivided in the resulting grain-oriented 
silicon steel sheet; and that, when a tension-giving type insulating 
coating is applied onto the above described forsterite film locally having 
regions having a thickness different from that of the remaining regions in 
the film, the effect for subdividing the magnetic domain width can be more 
improved by their synergistic effect. 
Hereinafter, in the specification, claims and drawings, the regions in a 
forsterite film, which have a thickness different from that of the 
remaining regions in the film, may be called as "forsterite film 
different-thickness regions", or merely called as "different-thickness 
regions". 
The present invention is further based on the discoveries that, when a 
forsterite film constituting a surface film of a grain-oriented silicon 
steel sheet has locally filmless regions which do not coat the steel sheet 
surface, the magnetic domain width of the resulting grain-oriented silicon 
steel sheet can be very advantageously subdivided similarly to the 
presence of the forsterite film different-thickness regions; and that, 
when a tension-giving type insulating coating is applied onto the above 
described forsterite film locally having the filmless regions, the effect 
for subdividing magnetic domain width can be more improved by their 
synergistic effect. 
Hereinafter, in the specification, claims, and drawings, the filmless 
regions in a forsterite film which do not coat the steel sheet surface may 
be called as "non-forsterite film regions" or merely called as "filmless 
regions". 
In the production of grain-oriented silicon steel sheets, a cold rolled 
steel sheet having a final gauge is generally subjected to a 
decarburization annealing to remove harmful carbon. The decarburized steel 
sheet has a primary recrystallization texture containing an inhibitor, 
which forms a fine second phase dispersed in the interior of the steel 
sheet, and at the same time the surface layer of the steel sheet has a 
subscale structure consisting of the matrix and fine SiO.sub.2 grains 
dispersed in the matrix. After the decarburized and primary recrystallized 
sheet is applied on the surface with an annealing separator consisting 
mainly of MgO, the steel sheet is subjected to a secondary 
recrystallization and purification annealing (a final annealing) at a high 
temperature of about 1,200.degree. C. By this secondary recrystallization, 
the crystal grains in the steel sheet grow into coarse grains having a 
{110}&lt;001&gt; orientation. Moreover, by the high temperature purification, a 
part of inhibitors, such as S, Se, N, etc., which remain in the steel 
sheet, is removed from the steel sheet matrix. 
Furthermore, in this purification, SiO.sub.2 in the subscale of the surface 
layer of the steel sheet and MgO in the annealing separator coated on the 
steel surface are reacted with each other according to the following 
equation: 
EQU 2MgO+SiO.sub.2 .fwdarw.Mg.sub.2 SiO.sub.4 
to form a coating film consisting of a polycrystal of forsterite (Mg.sub.2 
SiO.sub.4) on the surface layer of the steel sheet. In this case, 
unreacted excess MgO serves to prevent the fusing between the fellow steel 
sheets. After the final annealing, the unreacted annealing separator is 
removed from the steel sheet, and if necessary, an insulating coating is 
finally coated or a coil set is removed to obtain a product steel sheet. 
The inventors have reinvestigated the role of forsterite film, and found 
that the film gives a tensile force to a steel sheet to subdivide the 
magnetic domain wall spacing and the subdivision effect of the magnetic 
domain wall spacing in the steel sheet varies finely depending upon 
positions. As the result, the inventors have reexamined carefully about 
the subdivision effect of the magnetic domain wall spacing in a steel 
sheet, and found that the above mentioned effect is remarkable in a place 
where the thickness of the forsterite film changes. 
SUMMARY OF THE INVENTION 
The first aspect of the present invention lies in a grain-oriented silicon 
steel sheet having a low iron loss free from deterioration due to the 
stress-relief annealing, said steel sheet having no plastically strained 
regions in the matrix surface layer and having a forsterite film, the 
forsterite film locally having regions, which have a thickness different 
from that of the remaining regions in the film. 
The second aspect of the present invention lies in a grain-oriented silicon 
steel sheet having a low iron loss free from deterioration due to the 
stress-relief annealing, the steel sheet having no plastically strained 
regions in the matrix surface layer and having a forsterite film, the 
forsterite film locally having regions, which have a thickness different 
from that of the remaining regions in the film, and the steel sheet 
further having a tension-giving type insulating coating film having a 
thermal expansion coefficient of not higher than 9.8.times.10.sup.-6 
1/.degree.C. formed on the forsterite film. 
The third aspect of the present invention lies in a grain-oriented silicon 
steel sheet having a low iron loss free from deterioration due to the 
stress-relief annealing, the steel sheet having no plastically strained 
regions in the matrix surface layer and having a forsterite film, the 
forsterite film locally having filmless regions which do not coat the 
steel sheet surface. 
The fourth aspect of the present invention lies in a grain-oriented silicon 
steel sheet having a low iron loss free from deterioration due to the 
stress-relief annealing, the steel sheet having no plastically strained 
regions in the matrix surface layer and having a forsterite film, the 
forsterite film locally having filmless regions which do not coat the 
steel sheet surface, and the steel sheet further having a tension-giving 
type insulating coating film having a thermal expansion coefficient of not 
higher than 9.8.times.10.sup.-6 1/.degree.C. formed on the forsterite film 
.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
In the present invention, the starting material steel sheets are limited to 
ones having no plastically strained regions. The reason is that the 
subdivision of the magnetic domain wall spacing by the introduction of a 
plastic strain into the steel sheet causes a serious deterioration in the 
properties due to the stress-relief annealing as described later. 
The steel sheets having a forsterite film in the present invention include 
not only a steel sheet having a forsterite film alone but also a steel 
sheet having a general top coating film formed on the forsterite film as a 
surface coating. 
The present invention will be explained in more detail. 
The inventors have changed locally the thickness of a forsterite film or 
have removed locally a forsterite film and have investigated the influence 
of the shape, thickness difference, direction, etc. of the regions, 
wherein the thickness of the film has been changed or the film has been 
removed, upon the subdivision of the magnetic domain wall spacing, and 
have studied the relation between the shape, thickness difference, 
direction, etc. of the region upon the iron loss of the product steel 
sheet. 
In this experiment, in order to decrease locally the thickness of the film 
or remove locally the film, forsterite was chemically dissolved in HF 
solution, and in order to obtain a large thickness region, chemical 
reaction to make SiO.sub.2 on the surface was locally enhanced by coating 
oxidizing agent on that region. 
It has been found that, as to the shape of the different-thickness region, 
a continuous or discontinuous linear groove or land as illustrated in FIG. 
1A is especially effective for lowering the iron loss. However, in a 
discontinuous linear groove or land formed of recesses or protrusions 
arranged in a row, when the distance between adjacent recesses or 
protrusions is more than 0.5 mm, the effect is low. In case of the 
discontinuous linear groove or land party having missing portions like a 
dot-dashed line, the effect for lowering the iron loss is almost the same 
as that of the continuous linear groove or land. 
Regarding the direction of the different-thickness region of the forsterite 
film, as illustrated in FIGS. 1B and 2, it is especially effective in case 
of an inclination angle of 60.degree..about.90.degree. with respect to the 
rolling direction (measuring condition in FIG. 2: sheet thickness: 0.30 
mm; dotted line-like different-thickness region, interval: 4 mm, width: 1 
mm, decreased thickness: 1.5 mm). Further, regarding the thickness 
difference between the forsterite film differnt-thickness region and the 
remaining region, as illustrated in FIG. 3, both the larger thickness 
region and the smaller thickness region exhibit almost same effect. In any 
case, it has been found that it is effective when the thickness difference 
is not less than 0.3 .mu.m (measuring condition in FIG. 3: sheet 
thickness: 0.30 mm, linear groove or land, interval: 5 mm, width: 0.5 mm, 
angle: 90.degree.). Regarding the width of the continuous or discontinuous 
linear groove or land, an excellent effect is obtained in a width within 
the range of 0.05-2.0 mm, preferably 0.5-2.0 mm, particularly preferably 
0.8-1.5 mm, as illustrated in FIG. 4 (measuring condition in FIG. 4; sheet 
thickness: 0.30 mm; linear land, interval: 3 mm, angle: 90.degree., 
thickness difference: 0.4 .mu.m). 
Further, it is effective in order to lower the iron loss of the whole steel 
sheet that the different-thickness region of the forsterite film is formed 
repeatedly in a direction crossing the rolling direction. In this case, 
the interval between adjacent regions as illustrated in FIG. 1C is 
desirably within the range of 1-30 mm as illustrated FIG. 5 (measuring 
condition in FIG. 5: sheet thickness: 0.30 mm; linear groove, angle; 
90.degree., width: 1 mm, thickness difference: 0.5 .mu.m). 
The effect of the different-thickness regions in the forsterite film is 
almost the same when the regions are formed on both surfaces of a steel 
sheet and when the regions are formed only on one surface thereof. 
It has been found that, when a steel sheet having a forsterite film locally 
having such different-thickness regions is coated with a coating liquid, 
which form a coating film having a thermal expansion coefficient of 
5.times.10.sup.-6 1/.degree.C., and baked with the liquid to form a 
tension-giving type coating film on the forsterite film, the iron loss of 
the steel sheet having the forsterite film and the tension-giving type 
insulating coating film is remarkably lower than that of the steel sheet 
merely having the forsterite film having different-thickness regions as 
illustrated in FIG. 6 (measuring condition in FIG. 6: sheet thickness: 
0.28 mm; linear groove, interval: 3 mm, angle: 90.degree., thickness 
difference: 0.8 .mu.m). Further, it has been found that the effect of the 
tension-giving type coating film is higher in the case where a forsterite 
film has different-thickness regions than in the case where a forsterite 
film has no different-thickness regions. 
When various coating films having different thermal expansion coefficients 
were coated on a grain-oriented silicon steel sheet having a forsterite 
film locally having the thickness-different regions, and the effect of the 
coating films was examined in the same manner as described above, it has 
been found that the use of a coating film having a thermal expansion 
coefficient of not higher than 9.8.times.10.sup.-6 1/.degree.C. results in 
a satisfactorily low iron loss. 
An explanation will be made with respect to the method for forming 
different-thickness regions in a forsterite film. The same primarily 
recrystalled steel sheet is used, and different-thickness regions are 
formed by the following methods. 
(1) A method wherein the thickness of a forsterite film is controlled by 
utilizing a reaction for forming a forsterite film during a final 
annealing. For example, a method, wherein uncoated regions of an annealing 
separator are formed on the surface of a steel sheet after the 
decarburization annealing; and methods, wherein an inhibitor for the 
forsterite forming reaction, a substance having a water-repelling property 
to an annealing separator slurry, or an oxidizing agent for Si contained 
in the steel is locally adhered to the steel sheet surface. 
(2) A method, wherein a forsterite film after a final annealing, is 
subjected to a chemical dissolving treatment to decrease the film 
thickness. 
(3) A method, wherein a uniform forsterite film after a final annealing, is 
weakly contacted with a rotating grindstone to remove forsterite and to 
decrease the film thickness. 
(4) A method, wherein a uniform forsterite film after a final annealing, is 
applied with a tension-giving type coating, and pulse-shaped high-power 
laser beams are irradiated to the steel sheet to volatilize the coating 
and the forsterite and to decrease the film thickness. 
(5) A method, wherein a forsterite film after a final annealing, is applied 
with a tension-giving type coating, and an iron needle having a sharp 
point is lightly pushed to the steel sheet under a low pressure to remove 
the coating film together with a part of the forsterite film and to 
decrease the thickness of the forsterite film. 
The steel sheets treated with the above described methods (1), (2) and (3) 
were coated with the same tension-giving type coating as that described in 
methods (4) and (5). 
In the above described experiments, the following results were obtained. 
Any of methods (1)-(5) resulted in grain-oriented silicon steel sheets 
having a very low iron loss of W.sub.17/50 of 0.96-0.99 W/kg. When these 
steel sheets were subjected to a stress-relief annealing at 800.degree. C. 
for 1 hour, the steel sheets treated with methods (1), (2), (3) and (5) 
still had a low iron loss of 0.96-0.99 W/kg, but the iron loss of the 
steel sheet treated with method (4) was deteriorated to 1.04 W/kg. The 
inventors have ascertained the reason as follows. Among the steel sheets 
treated with methods (1)-(5) before the stress-relief annealing, only the 
steel sheet treated with method (4) had a plastically strained region 
formed in the matrix surface layer just beneath the decreased thickness 
region of the forsterite film, and this plastic strain is released and 
extinguished by the stress-relief annealing. Accordingly, in order not to 
deteriorate the iron loss due to the stress-relief annealing, it is 
important that plastically strained regions are not introduced into the 
steel sheet matrix surface layer. 
In the stress-relief annealed steel sheet (5), the coating film located 
around the removed portion of the coating film is flowed into the removed 
portion of the coating film by the stress-relief annealing so as to repair 
the removed portion of the coating film into a uniform surface, and the 
coating film has excellent insulating property and corrosion resistance. 
It has been found that the annealing temperature necessary for repairing 
such coating film is preferably within the range of 
600.degree.-900.degree. C. 
There have been found out the following facts with respect to the shape, 
direction and the like of the non-forsterite film region in the present 
invention. 
It has been found that, as the shape of the non-forsterite film region, a 
continuous or discontinuous linear non-forsterite region as illustrated in 
FIG. 1A is especially effective for lowering the iron loss. However, in a 
discontinuous linear filmless region formed of recesses arranged in a row, 
when the distance between adjacent recesses is more than 0.5 mm, the 
effect is low. In case of the discontinuous linear filmless region partly 
having missing portions like a dot-dashed line, the effect for lowering 
the iron loss is almost the same as that of the continuous linear filmless 
region. 
Regarding the direction of the non-forsterite film region, as illustrated 
in FIGS. 1B and 7, it is especially effective in case of an inclination 
angle of 60.degree.-90.degree. with respect to the rolling direction 
(measuring condition in FIG. 7: sheet thickness: 0.30 mm; dotted line-like 
filmless region, interval: 4 mm, width: 1 mm). Regarding the width of the 
continuous or discontinuous linear filmless region, an excellent effect is 
obtained within the range of 0.05-2.0 mm, preferably 0.8-1.5 mm, as 
illustrated in FIG. 8 (measuring condition in FIG. 8: sheet thickness: 
0.30 mm; linear filmless region, interval: 3 mm, angle: 90.degree.). 
Further, it is effective in order to lower the iron loss of the whole steel 
sheet that the filmless region of the forsterite film is formed repeatedly 
in a direction crossing the rolling direction. In this case, the distance 
between adjacent regions as illustrated in FIG. 1C is desirably within the 
range of 1-30 mm as illustrated FIG. 9 (measuring condition in FIG. 9: 
sheet thickness: 0.30 mm; linear filmless region, angle: 90.degree., 
width: 1 mm). 
The effect of the filmless regions in a forsterite film is almost the same 
in the case where the film is formed on both surfaces of a steel sheet and 
in the case where the film is formed only on one surface thereof. 
It has been found that, when a steel sheet having a forsterite film locally 
having such filmless regions is coated with a coating liquid, which forms 
a coating film having a thermal expansion coefficient of 5.times.10.sup.-6 
1/.degree.C., and baked with the liquid to form a tension-giving type 
coating film on the forsterite film, the iron loss of the steel sheet 
having the forsterite film and the tension-giving type insulating coating 
film is remarkably lower than that of the steel sheet merely having the 
forsterite film having filmless regions as illustrated in FIG. 10 
(measuring condition in FIG. 10: sheet thickness: 0.28 mm, linear filmless 
region, interval; 4 mm, angle: 90.degree.). Further it has been found that 
the effect of the tension-giving type coating film is higher in the case 
where a forsterite film has filmless regions than in the case where a 
forsterite film has no filmless regions. 
When various coating films having different thermal expansion coefficients 
were coated on a grain-oriented silicon steel sheet having a forsterite 
film locally having the filmless regions, and the effect of the coating 
films was examined in the same manner as described above, it has been 
found that the use of a coating film having a thermal expansion 
coefficient of not higher than 9.8.times.10.sup.-6 1/.degree.C. results in 
a satisfactorily low iron loss. 
An explanation will be made with respect to the method for forming a 
forsterite film locally having filmless regions which do not coat a steel 
sheet surface. 
A finally annealed silicon steel sheet having a forsterite film on the 
matrix surface and further having a tension-giving type coating film 
having a thermal expansion coefficient of 5.6.times.10.sup.-6 1/.degree.C. 
formed on the forsterite film was divided into 4 steel sheets, and 
non-forsterite film regions, each having a width of 1.0 mm were formed at 
an inclination angle of 90.degree. with respect to the rolling direction 
and at a repeating interval of 4 mm by the following methods. 
(1) The forsterite film is locally dissolved in a concentrated NaOH 
solution to form linear filmless regions. 
(2) A disc-shaped grinding stone is weakly contacted with the steel sheet 
to form linear filmless regions. 
(3) Pulse-shaped high-power laser beams are irradiated to the steel sheet 
to volatilize both the coating and the forsterite and to form dotted 
line-like filmless regions (distance between adjacent recesses: 0.4 mm). 
(4) An iron needle having a sharp point is lightly pushed to the steel 
sheet under a light pressure to form dotted line-like filmless regions 
(distance between adjacent recesses: 0.4 mm). 
Any of the above described methods (1)-(4) resulted in grain-oriented 
silicon steel sheets having a very low iron loss of W.sub.17/50 of 
0.97-0.98 W/kg. When these steel sheets were subjected to a stress-relief 
annealing at 800.degree. C. for 3 hours, the steel sheets treated with 
methods (1), (2) and (4) still had a low iron loss of 0.97-0.98 W/kg, but 
the iron loss of the steel sheet treated with method (3) was noticeably 
deteriorated to 1.05 W/kg. 
The inventors have ascertained the reason as follows. Among the steel 
sheets treated with methods (1)-(4) before the stress-relief annealing, 
only the steel sheet treated with method (3) had a plastically strained 
region formed in the matrix surface layer just beneath the region, wherein 
the forsterite film has been removed, and this plastic strain is released 
and extinguished by the stress-relief annealing. Accordingly, in order not 
to deteriorate the iron loss due to the stress-relief annealing, it is 
important that plastically strained regions are not introduced into the 
steel sheet matrix surface layer. 
In the stress-relief annealed steel sheets (1)-(4), the coating film 
located around the removed portion of the coating film is flowed into the 
removed portion of the coating film by the stress-relief annealing so as 
to repair the removed portion of the coating film into a uniform surface, 
and the coating film has excellent insulating property and corrosion 
resistance. It has been found that the annealing temperature necessary for 
repairing such coating film is preferably within the range of 
600.degree.-900.degree. C. 
An explanation will be made hereinafter with respect to the method of 
producing the grain-oriented silicon steel sheet of the present invention. 
As the starting material in the present invention, there is used a hot 
rolled coil produced by a method, wherein a molten steel is produced by a 
commonly known steel-making process, for example, by a converter, an 
electric furnace, etc., the molten steel is subjected to an ingot 
making-slabbing process or a continuous casting process etc. to produce a 
slab, and the slab is subjected to a hot rolling. 
It is necessary that the hot rolled sheet has a composition containing 
about 2.0-4.0% by weight of Si. The reason is that the Si content of less 
than 2.0% results in a grain-oriented silicon steel sheet having a very 
poor iron loss, and the Si content of more than 4.0% results in a poor 
cold workability of the hot rolled sheet. As to other constituents, any 
constituents for grain-oriented silicon steel sheets are applicable. 
The hot rolled sheet is subjected to one cold rolling or two or more cold 
rollings with an intermediate annealing between them to produce a cold 
rolled sheet having a final gauge. In this case, if necessary, a 
normalizing annealing of a hot rolled sheet or a warm annealing instead of 
the cold rolling may be carried out. 
The cold rolled sheet having a final gauge is subjected to a primary 
recrystallization annealing under an oxidizing atomsphere capable of 
decarburization or under a weak oxidizing atmosphere capable of forming a 
subscale. Then, an annealing separator consisting mainly of MgO is applied 
to the steel sheet surface. In this application step, regions not applied 
with the annealing separator are locally formed on the steel sheet 
surface, whereby the object aimed in the present invention is 
advantageously achieved. 
That is, by carrying out the final annealing, an ordinary forsterite film 
is formed on the surface applied with the annealing separator. On the 
contrary, merely a thin forsterite film is formed on the surface not 
applied with the annealing separator, so that a small thickness region 
directed in the first and second aspects of the present invention is 
formed. 
Further, as methods of adhering the annealing separator to the steel sheet, 
commonly known methods of application by a roll or a brush, spraying and 
electrostatic painting can be used. 
As the other methods for forming the different-thickness region directed in 
the first and second aspects of the present invention, there are four 
methods as described hereinafter besides the above-mentioned method. 
(1) A method, wherein, before applying an annealing separator to a steel 
sheet surface after the primary recrystallization annealing, an inhibitor 
(an obstacle) for the forsterite forming reaction is locally adhered to 
the steel sheet surface in an amount within the range of not more than 1 
g/m.sup.2. 
In this method, as the inhibitor, there can be used oxides such as 
SiO.sub.2, Al.sub.2 O.sub.3, ZrO.sub.2, etc. and metals such as Zn Al, Sn, 
Ni, Fe, etc. When the inhibitor is adhered to the steel sheet surface in 
an amount of more than 1 g/m.sup.2, the inhibiting effect for the reaction 
becomes excessive and a forsterite film is not formed. Hence, it is 
necessary that the thickness of a forsterite film to be decreased is 
controlled by using the inhibitor in an amount persistently within the 
range of not more than 1 g/m.sup.2. As a means for applying these reaction 
inhibitors to the steel sheet, any of application, spraying, plating, 
printing, electrostatic painting, etc. are available. 
(2) A method, wherein, before applying an annealing separator to a steel 
sheet surface after the primary recrystallization annealing, a substance 
having a water repelling property to an annealing separator slurry (a 
suspension of an annealing separator in water) is locally adhered to the 
steel sheet surface in an amount within the range of not more than 0.1 
g/m.sup.2. 
As such water-repelling substances, oil paint and varnish, etc. are 
advantageously used. The water-repelling substances prevent the contact of 
the steel sheet surface with the annealing separator to delay the 
forsterite forming reaction and to form the smaller thickness region. 
However, when the substances are adhered to the steel sheet in an amount 
of more than 0.1 g/m.sup.2, the reaction-delaying effect becomes 
excessive, and a forsterite film is not at all formed. Therefore, it is 
necessary that the thickness of a forsterite film to be decreased is 
controlled by using the water-repelling substance in an amount 
persistently within the range of not more than 0.1 g/m.sup.2. Further, as 
a means for adhering the water-repelling substance to a steel sheet, there 
can be used application, spraying, printing, electrostatic painting, etc. 
similarly to the above described reaction inhibitors. 
(3) A method, wherein, before applying an annealing separator to a steel 
sheet surface after the primary recrystallization annealing, an oxidizing 
agent for Si contained in the steel is locally adhered to the steel sheet 
surface in an amount within the range of not more than 2 g/m.sup.2. 
The oxidizing agent oxidizes Si in the steel at a high temperature during 
the following final annealing to increase the amount of SiO.sub.2 
particles in the subscale of the steel sheet surface layer and to 
increases the thickness of a forsterite film after the final annealing. 
Hence, the larger thickness film can be locally formed on the steel sheet 
surface. As the oxidizing agent, there can be advantageously used oxides, 
such as FeO, Fe.sub.2 O.sub.3, TiO.sub.2, etc., easily reducible 
silicates, such as Fe.sub.2 SiO.sub.4, etc. hydroxides, such as 
Mg(OH).sub.2, etc. However, when the adhered amount of these oxidizing 
agents to the steel sheet surface exceeds 2 g/m.sup.2, the thickness of 
the resulting oxide films becomes too large, and the adhesive force of the 
film to the steel sheet is lost, and the film peels away. As the result, 
the expected object can not be attained. 
(4) A method, wherein a forsterite film formed on the steel sheet surface 
after the final annealing, is removed without causing a plastic strain in 
the matrix steel sheet surface layer, thereby smaller thickness regions 
are formed. 
As the method, besides a chemical polishing and an electrolyte polishing, 
there are methods of removing the forsterite film by a rotating disc-like 
grindstone, by an iron needle under a light pressure, and by an optical 
means, for example laser beams, etc. having a power properly adjusted, and 
other methods. Especially, when laser beams are used as the optically 
removing method, multiple beams may be taken out from one light source or 
the whole irradiation may be effected in the presence of an appropriate 
masking, whereby a plural number of different-thickness regions can be 
advantageously formed efficiently by one operation. 
Further, in the third and the fourth aspects of the present invention, in 
order to produce the non-forsterite film regions, among the above 
mentioned methods of (1), (2), (3) and (4), methods of (1), (2) and (4) 
are available. However, in these 3 methods, when the methods of (1) and 
(2) are used, it is necessary to determine the amount of the treating 
agents as described hereinafter. 
(1) When an inhibitor for the forsterite forming reaction is used, the 
inhibitor must be locally adhered to the surface of a steel sheet after 
the primary recrystallization annealing, in an amount within the range of 
more than 1 g/m.sup.2 before the annealing separator is applied to the 
steel sheet surface. When the adhered amount of the reaction inhibitor to 
the steel sheet surface is not more than 1 g/m.sup.2, there is a risk of 
forming a forsterite film, and hence the adhered amount of the reaction 
inhibitor has been determined within the range of more than 1 g/m.sup.2 in 
order to avoid the risk. 
(2) When a substance having a water-repelling property to an annealing 
separator slurry (a suspension of an annealing separator in water) is 
used, the water-repelling substance must be locally adhered to the surface 
of a steel sheet after the primary recrystallization annealing, in an 
amount within the range of more than 0.1 g/m.sup.2 before the annealing 
separator is applied to the steel sheet surface. When the adhered amount 
of the water-repelling substance to the steel sheet surface is not more 
than 0.1 g/m.sup.2, there is a risk of forming a forsterite film, and 
hence the adhered amount has been determined within the range of more than 
0.1 g/m.sup.2 in order to avoid the risk. 
In the method of forming the different-thickness region or filmless region 
by the above described removal method, special care must be taken not to 
form a plastically strained region on the matrix surface during the 
removal treatment. The reason is that, when the plastic strain is 
introduced into the matrix surface, the properties of the steel sheet 
after the stress-relief annealing is noticeably deteriorated as described 
hereinafter. 
As to the shape of the different-thickness region or filmless region, a 
continuous linear groove or land is especially effective. The continuous 
linear groove or land can be replaced by a discontinuous linear groove or 
land, that is, by recesses or protrusions arranged in a row. However, in 
case of such a discontinuous linear groove or land, when the distance 
between adjacent recesses or protrusions is more than 0.5 mm, the effect 
is low. Further, when the width of the linear different-thickness region 
or linear filmless region is about 0.05-2.0 mm, the effect is high. 
Regarding the direction of the linear groove or land, an inclination angle 
within the range of 60.degree.-90.degree. with respect to the rolling 
direction is especially preferable. When the direction is parallel to the 
rolling direction, there is no effect, and when the direction is 
perpendicular to the rolling direction, the highest effect is obtained. 
The inclination angle with respect to the rolling direction of the steel 
sheet is especially important. The reason why the effect for lowering the 
iron loss is poor in case of excessively large width of the 
different-thickness region or filmless region, or in case of isolated 
recesses or protrusions, is probably that the directional effect of the 
whole regions does not sharply appear. 
It is preferable that the continuous or discontinuous linear groove or land 
is arranged repeatedly with respect to the rolling direction. In this 
case, it is especially effective that the interval between adjacent 
grooves or lands is within the range of 1.0-30 mm. The continuous or 
discontinuous linear groove or land may have different shapes and widths, 
and may be arranged in different angles with respect to the rolling 
direction. 
Further, the forsterite film different-thickness region or the 
non-forsterite film region exhibits almost the same effect in the case 
where the region is present on both surfaces of a steel sheet and in the 
case where the region is present only on one surface of the steel sheet. 
When a tension-giving type insulating coating film having a thermal 
expansion coefficient of not higher than 9.8.times.10.sup.-6 1/.degree.C. 
as a top coating is formed on the grain-oriented silicon steel sheet 
locally having the above described forsterite film different-thickness 
region or non-forsterite regions, grain-oriented silicon steel sheets 
having more excellent magnetic properties of the present invention can be 
obtained. 
Alternatively, the silicon steel sheet having more excellent magnetic 
properties of the present invention can be produced in the following 
manner. A tension-giving type insulating coating film having a thermal 
expansion coefficient of not more than 9.8.times.10.sup.-6 1/.degree.C. as 
a top coating is fomred on a grain-oriented silicon steel sheet having a 
forsterite film, and then the top coating and a part of the forsterite 
film or the top coating and the forsterite film are locally removed to 
form small thickness regions of the forsterite film or to form 
non-forsterite film regions on the steel sheet, and then the steel sheet 
is subjected to an annealing at a temperature of 600.degree.-900.degree. 
C. to repair the final coating-absent portion. 
The top coating gives a surface tension to a steel sheet surface by the 
difference in thermal expansion coefficient between the steel sheet and 
the coating film, and therefore it is necessary that the top coating film 
has a thermal expansion coefficient somewhat different from that of the 
steel sheet. The inventors have ascertained that a top coating film having 
a thermal expansion coefficient of not higher than 9.8.times.10.sup.-6 
1/.degree.C. gives a satisfactorily low iron loss value to the product 
steel sheet by the synergistic effect of the effect caused by the presence 
of different-thickness region or filmless region in the forsterite film 
and the surface tension-giving effect of the top coating film. 
The thickness of the coating film is preferably within the range of about 
0.5-10 g/m.sup.2 (per one surface) in view of corrosion resistance and 
space factor. 
Moreover, in the steel sheet of the present invention, only the shape of 
the coating film portion is changed and therefore the change of the shape 
is small, and the lowering of space factor does not substantially occur. 
As described above, the grain-oriented silicon steel sheets having a 
forsterite film locally having different-thickness regions or locally 
having filmless regions exhibit excellent magnetic properties in both 
cases, wherein the steel sheets are directly used in a practical apparatus 
similar to the commonly used grain-oriented silicon steel sheets, and 
wherein the steel sheets are used in a practical apparatus after the 
sheets are subjected to the top insulation coating. 
According to the present invention, the iron loss value is lowered by 
defining and forming different-thickness regions or filmless regions in a 
forsterite film. The reason is probably that regions, which are subjected 
to a tension different from that acting upon the remaining region of a 
steel sheet surface, are formed on the steel sheet surface by forming 
thickness-different regions or filmless regions in a forsterite film, and 
the plastic strain is introduced into the steel sheet surface by the 
action of the different tension, so that the magnetic domain wall spacing 
is effectively subdivided. 
In the grain-oriented silicon steel sheet having a elastic strain caused by 
the different tension, there is no artificially and plastically strained 
region in the steel sheet matrix surface layer contrary to the 
conventional methods, wherein plastically strained regions or high 
dislocation density regions, such as laser beam marks, are formed in the 
matrix surface layer, and therefore deterioration of iron loss does not 
substantially occur even when a stress-relief annealing is carried out 
under a commonly used condition of about 800.degree. C. and of from 1 
minute to several hours. In the conventional grain-oriented silicon steel 
sheet, the plastic strain in the surface layer of a matrix is extinguished 
at a high temperature. Therefore, the conventional steel sheet has a fetal 
defect that the iron loss deteriorates. However, the grain-oriented 
silicon steel sheet of the present invention has satisfactorily low iron 
loss regardless of stress-removing annealing. 
The present invention will be explained with reference to specific 
examples. 
EXAMPLE 1 
A cold rolled steel sheet of 0.30 mm in thickness was prepared from a hot 
roled silicon steel sheet containing 3.2% of Si according to the ordinary 
method, and subjected to a decarburization-primary recrystallization 
annealing. Then, before an annealing separator was applied to the surface 
of the annealed sheet, Al.sub.2 O.sub.3 powder as an inhibitor for the 
forsterite forming reaction was adhered linearly to the steel sheet 
surface under a condition that the adhesion amount: 0.5 g/m.sup.2, the 
inclination angle with respect to the rolling direction: 90.degree., the 
adhesion width: 2 mm, and the repeating interval in the rolling direction: 
4 mm. Thereafter, the annealing separator was applied onto the thus 
treated steel sheet, and then the steel sheet was subjected to a final 
annealing at 1,200.degree. C. for 5 hours. 
For comparison, a grain-oriented silicon steel sheet was prepared as a 
Comparative Example according to the ordinary method wherein Al.sub.2 
O.sub.3 powder was not adhered. 
Examination of the film properties showed that, in the Comparative Example, 
a grey film of a uniform thickness was formed, while in this Example 1, a 
forsterite film having a thickness smaller by 0.8 .mu.m was formed in the 
regions to which the Al.sub.2 O.sub.3 powder was applied, than the 
thickness at the region to which the Al.sub.2 O.sub.3 powder was not 
applied. The iron loss values of Example and Comparative Example were as 
follows: 
Comparative Example: W.sub.17/50 =1.06 W/kg 
Example: W.sub.17/50 =1.02 W/kg 
An ordinary phosphate type top coating was applied to the above treated 
steel sheets, and iron loss values of the top-coated steel sheets were 
measured. The following results were obtained. 
Comparative Example: W.sub.17/50 =1.06 W/kg 
Example: W.sub.17/50 =1.01 W/kg 
Further, the above top coated steel sheets were subjected to a 
stress-relief annealing at 800.degree. for 2 hours, and the iron loss 
values of the annealed sheets were measured. The obtained values are as 
follows. 
Comparative Example: W.sub.17/50 =1.06 W/kg 
Example: W.sub.17/50 =1.01 W/kg 
EXAMPLE 2 
A cold rolled steel sheet of 0.28 mm in thickness was prepared from a hot 
rolled silicon steel sheet containing 3.0% of Si according to the ordinary 
method, and subjected to a decarburization-primary recrystallization 
annealing. After an annealing separator consisting mainly of MgO was once 
applied onto the surface of the annealed steel sheet, the annealing 
separator was removed linearly by a plastic bar with a fine tip under a 
condition that the inclination angle with respect to the rolling 
direction: 90.degree., the width: 0.5 mm, and the repeating interval in 
the rolling direction: 2 mm. Then, the steel sheet was subjected to a 
final annealing at 1,200.degree. C. for 5 hours. A steel sheet treated up 
to the final annealing step according to the ordinary steps, wherein the 
annealing separator was not removed, was adopted as a Comparative Example. 
Examination of the film properties of both the samples showed, that, in 
Comparative Example, a grey forsterite film of a uniform thickness was 
formed, while in Example 2, a forsterite film having a small thickness was 
formed at the regions at which the annealing separator was removed. The 
iron loss values of Example 2 and Comparative Example were as follows: 
Comparative Example: W.sub.17/50 =1.07 W/kg 
Example: W.sub.17/50 =1.01 W/kg 
When these samples steel sheets were subjected to a stress-relief annealing 
at 800.degree. C. for 5 hours, and the iron loss values of the steel 
sheets were measured. The following valuves were obtained. 
Comparative Example: W.sub.17/50 =1.07 W/kg 
Example: W.sub.17/50 =1.01 W/kg 
EXAMPLE 3 
A cold rolled steel sheet of 0.23 mm in thickness was prepared from a hot 
rolled silicon steel sheet containing 3.0% of Si according to the ordinary 
method, and subjected to a decarburization.primary recrystallization 
annealing. After an annealing separation consisting mainly of MgO was once 
applied onto the surface of the annealed steel sheet, the annealing 
separator was removed linearly by a plastic bar with a fine tip under a 
condition that the inclination angle with respect to the rolling 
direction: 90.degree., the width: 0.5 mm, and the repeating interval in 
the rolling direction: 5 mm. Then, the steel sheet was subjected to a 
final annealing at 1,200.degree. C. for 5 hours. A steel sheet treated up 
to the final annealing step according to the ordinary steps, wherein the 
annealing separator was not removed was adopted as a Comparative Example. 
Examination of the film properties of both the samples showed that, in 
Comparative Example, a grey forsterite film of a uniform thickness was 
formed; while in Example 3, a forsterite film having a small thickness was 
formed at the regions at which the annealing separator was removed. The 
iron loss values of Example 3 and Comparative Example were as follows: 
Comparative Example: W.sub.17/50 =0.93 W/kg 
Example: W.sub.17/50 =0.84 W/kg 
When these sample steel sheets were subjected to a stress-relief annealing 
at 800.degree. C. for 5 hours, and the iron loss values of the steel 
sheets were measured. The following values were obtained. 
Comparative Example: W.sub.17/50 =0.93 W/kg 
Example: W.sub.17/50 =0.84 W/kg 
EXAMPLE 4 
A cold rolled steel sheet of 0.30 mm in thickness was prepared from a hot 
rolled silicon steel sheet containing 3.0% of Si according to the ordinary 
method, and subjected to a decarburization.primary recrystallization 
annealing. Then, an annealing separator was applied to the surface of the 
steel sheet by means of a rubber roll with ridges. At this time, the 
annealing separator was applied to the steel sheet surface such that 
applied regions and non-applied regions were alternatively defined and 
formed with respect to the rolling direction under a condition that the 
width of the non-applied region: 1.5 mm, and the repeating interval in the 
rolling direction: 5 mm. Thereafter, the steel sheet was subjected to a 
final annealing at 1,200.degree. C. for 5 hours. For comparison, a 
grain-oriented silicon steel sheet as a Comparative Example was prepared 
according to the ordinary production steps in which the forsterite film 
was uniformly formed over the whole surface of the steel sheet. 
Examination of the film properties of both the samples showed that, in the 
Comparative Example, a grey forsterite film of a uniform thickness was 
formed; while in Example 4, a forsterite film of a small thickness was 
formed at the regions at which no annealing separator was applied. The 
iron loss values of these samples were as follows: 
Comparative Example: W.sub.17/50 =1.05 W/kg 
Example: W.sub.17/50 =1.03 W/kg 
When these samples steel sheets were subjected to a stress-relief annealing 
at 800.degree. C. for 1 hour and the iron loss values of the steel sheets 
were measured. The following values were obtained. 
Comparative Example: W.sub.17/50 =1.08 W/kg 
Example: W.sub.17/50 =1.03 W/kg 
EXAMPLE 5 
A cold rolled steel sheet of 0.30 mm in thickness was prepared from a hot 
rolled silicon steel sheet containing 3.2% of Si according to the ordinary 
method, and subjected to a decarburization.primary recrystallization 
annealing. Then, prior to the application of an annealing separator, FeO 
as an oxidizing agent for Si contained in the steel was linearly applied 
to the surface of the steel sheet under a condition that the amount of 
FeO: 0.5 g/m.sup.2, the inclination angle with respect to the rolling 
direction: 90.degree., the width: 2 mm, and the repeating interval in the 
rolling direction: 10 mm. Thereafter, the annealing separator was applied 
onto the surface of the thus treated steel sheet, and then the steel sheet 
was subjected to a final annealing at 1,200.degree. C. for 5 hours. For 
comparison, a grain-oriented silicon steel sheet was prepared as a 
Comparative Example according to the ordinary steps in which no oxidizing 
agent was applied prior to the application of the annealing separator. The 
iron loss values were as follows: 
Comparative Example: W.sub.17/50 =1.04 W/kg 
Example: W.sub.17/50 =0.99 W/kg 
After a stress-relief annealing was performed for the above treated steel 
sheets at 800.degree. C. for 2 hours, the iron loss values thereof were 
measured. The following values were obtained. 
Comparative Example: W.sub.17/50 =1.04 W/kg 
Example: W.sub.17/50 =0.99 W/kg 
EXAMPLE 6 
A cold rolled steel sheet of 0.20 mm in thickness was prepared from a hot 
rolled silicon steel sheet containing 3.2% of Si according to the ordinary 
method, and subjected to a decarburization.primary recrystallization 
annealing. Then, prior to the application of an annealing separator, the 
surface of the steel sheet was printed with an oil paint having 
water-repellent property to the annealing separator slurry in an amount of 
0.05 g/m.sup.2 by a printing process in the form of a discontinuous 
straight line under a condition that the inclination angle of the printed 
regions with respect to the rolling direction: 90.degree., the width: 0.3 
mm, the distance between adjacent spots arranged in a row: 0.3 mm, and the 
interval of the adjacent printed regions in the rolling direction: 3 mm. 
Thereafter, the annealing separator was applied to the printed steel sheet, 
the applied steel sheet was dried under heating, and then subjected to a 
final annealing at 1,200.degree. C. for 10 hours. For comparison, a 
grain-oriented silicon steel sheet was prepared as a Comparative Example 
according to the oridinary steps in which the above mentioned printing 
treatment of the water-repelling substance was not performed. 
The iron loss values of both the samples were as follows: 
Comparative Example: W.sub.17/50 =0.92 W/kg 
Example: W.sub.17/50 =0.87 W/kg 
The following values were obtained with respect to the iron loss values 
after a stress-relief annealing was performed at 800.degree. C. for 2 
hours. 
Comparative Example: W.sub.17/50 =0.92 W/kg 
Example: W.sub.17/50 =0.87 W/kg 
EXAMPLE 7 
A cold rolled steel sheet of 0.30 mm in thickness was prepared from a hot 
rolled silicon steel sheet containing 3.2% of Si according to the ordinary 
method, and subjected to a decarburization.primary recrystallization 
annealing. Then, an annealing separator consisting mainly of MgO was 
applied onto the surface of the steel sheet, and the applied steel sheet 
was subjected to a final annealing at 1,200.degree. C. for 5 hours to form 
a grain-oriented silicon steel sheet with a grey forsterite film on the 
surface thereof. 
The iron loss value of this steel sheet was 1.06 W/kg at W.sub.17/50. 
Then, an iron needle with a fine tip was pushed to the steel surface under 
a light pressure and moved thereon to draw a line and to remove the 
forsterite film, whereby linear decreased thickness regions of the 
forsterite film were formed in the forsterite film, which regions were 
arranged under a condition that the depth: 0.5 .mu.m, the width: 0.5 mm, 
the inclination angle with respect to the rolling direction: 90, and the 
interval between adjacent regions in the rolling direction: 6 mm. 
As a result, the iron loss of the steel sheet thus obtained was 1.02 W/kg 
at W.sub.17/50. The iron loss value after a stress-relief annealing of the 
above obtained steel sheet at 850.degree. C. for 2 hours was 1.01 W/kg at 
W.sub.17/50. 
EXAMPLE 8 
A cold rolled steel sheet of 0.30 mm in thickness was prepared from a hot 
rolled silicon steel sheet containing 3.2% of Si according to the ordinary 
method, and subjected to a decarburization.primary recrystallization 
annealing. The resulting steel sheet was divided into two pieces, and one 
of them as such was coated with an annealing separator consisting mainly 
of MgO, and then subjected to a final annealing at 1,200.degree. C. for 5 
hours, which was used as a Comparative Example. The other steel sheet 
piece was adhered linearly on its surface with Al.sub.2 O.sub.3 powder as 
an inhibitor for the forsterite forming reaction under a condition that 
the adhesion amount: 0.5 g/m.sup.2, the inclination angle with respect to 
the rolling direction: 90.degree., the adhesion width: 2 mm, and the 
repeating interval in the rolling direction: 4 mm, and then the annealing 
separator was applied thereon, followed by a final annealing. 
As a result, in the former case, a uniform grey film was formed; while, in 
the latter case, a thin forsterite film having a thickness smaller by 0.8 
.mu.m than that of the forsterite film formed at the regions, to which the 
Al.sub.2 O.sub.3 powder was not applied, was formed at the regions to 
which the Al.sub.2 O.sub.3 powder was applied. The iron loss values of 
these steel sheets were as follows: 
Steel sheet with a uniform film: W.sub.17/50 =1.06 W/kg 
Steel sheet with a film having reduced thickness regions: W.sub.17/50 =1.02 
W/kg 
Next, each of coating liquids I-VII shown in Table 1 was applied and baked 
onto each of the above steel sheets to form a top coat insulating film 
thereon. The iron loss values of the thus obtained products are shown in 
Table 2. Then, the iron loss values of the steel sheets, after a 
stress-relief annealing at 800.degree. C. for 2 hours, were measured and 
the obtained results are also shown in Table 2. 
It can be seen from Table 2 that the iron loss of the steel sheets having a 
forsterite film locally having different-thickness regions defined and 
formed therein are conspicuously improved by the coating film having a 
thermal expansion coefficient of not higher than 9.8.times.10.sup.-6 
1/.degree.C. 
TABLE 1 
__________________________________________________________________________ 
Amount in 100 ml solution 
Ingredient 
Thermal 
Amount of 
expansion 
Kind SiO.sub.2 fine 
coefficient 
of Sodium 
Aluminum 
Magnesium 
SiO.sub.2 content 
Chromic 
particle 
of coating 
coating 
hydroxide 
phosphate 
phosphate 
in colloidal 
anhydride 
(50-1000.ANG.) 
film 
liquid 
(g) (g) (g) silica (g) 
(g) (g) (1/.degree.C.) 
__________________________________________________________________________ 
I 5 -- 20 8 -- 0.5 15 .times. 10.sup.-6 
II -- -- 20 -- 3 -- 10 .times. 10.sup.-6 
III 1 25 -- 6 1 3 9.8 .times. 10.sup.-6 
IV 0.5 10 15 8 -- 1 8.3 .times. 10.sup.-6 
V -- 12 16 10 2 0.5 6.7 .times. 10.sup.-6 
VI -- -- 25 13 3 1 5.6 .times. 10.sup.-6 
VII -- 20 -- 12 4 -- 4.8 .times. 10.sup.-6 
__________________________________________________________________________ 
TABLE 2 
__________________________________________________________________________ 
Decreased Iron loss 
thickness 
Iron loss 
Iron loss 
value after 
regions in 
value before 
value after 
stress-relief 
Kind of 
forsterite 
coating coating annealing 
coating 
film W.sub.17/50 (W/kg) 
W.sub.17/50 (W/kg) 
W.sub.17/50 (W/kg) 
Remarks 
__________________________________________________________________________ 
I No 1.06 1.06 1.06 Comparative example 
Present 
1.02 1.02 1.02 Comparative example 
II No 1.06 1.06 1.06 Comparative example 
Present 
1.02 1.01 1.01 Comparative example 
III No 1.06 1.05 1.05 Comparative example 
Present 
1.02 0.99 0.99 Example 
IV No 1.06 1.05 1.05 Comparative example 
Present 
1.02 0.98 0.98 Example 
V No 1.06 1.04 1.04 Comparative example 
Present 
1.02 0.97 0.97 Example 
VI No 1.06 1.04 1.04 Comparative example 
Present 
1.02 0.96 0.96 Example 
VII No 1.06 1.04 1.04 Comparative example 
Present 
1.02 0.96 0.96 Example 
__________________________________________________________________________ 
EXAMPLE 9 
A grain-oriented silicon steel sheet containing 2.8% of Si and having a 
thickness of 0.28 mm, having an iron loss value of 1.08 W/kg at 
W.sub.17/50 and having a uniform forsterite film on the surface thereof 
was divided into three pieces A, B and C. Then, the coating liquid II and 
coating liquid V shown in Table 1 were applied and baked onto the piece A 
and the pieces B and C respectively to produce grain-oriented silicon 
steel sheets each having a top coating film. Among them, further in the 
piece C, linear decreased thickness regions of the forsterite film were 
formed under an arrangement condition that the width: 0.5 mm, the 
inclination angle with respect to the rolling direction: 90.degree., and 
the interval between adjacent regions in the rolling direction: 3 mm, 
without forming stretches on the steel sheet matrix surface by a method in 
which an iron needle with a fine tip was pushed to the steel sheet surface 
and moved thereon under a light pressure to remove the coating film and a 
part of the forsterite film. 
Thereafter, the pieces A, B and C were subjected to the annealing at 
700.degree. C. for 1 minute, and it was found that the filmless regions of 
the coating film adhered onto the surface of the piece C was repaired. The 
iron loss values of the steel sheets thus obtained were: 
A: W.sub.17/50 =1.08 W/kg (Comparative Example) 
B: W.sub.17/50 =1.06 W/kg (Comparative Example) 
C: W.sub.17/50 =1.01 W/kg (Example) 
After these steel sheets were subjected to a stress-relief annealing at 
800.degree. C. for 5 hours, the iron loss values thereof were measured. 
The following results were obtained. 
A: W.sub.17/50 =1.08 W/kg (Comparative Example) 
B: W.sub.17/50 =1.06 W/kg (Comparative Example) 
C: W.sub.17/50 =1.00 W/kg (Example) 
EXAMPLE 10 
A cold rolled steel sheet of 0.30 mm in thickness was prepared from a hot 
rolled silicon steel sheet containing 3.2% of Si according to the ordinary 
method, and subjected to a decarburization.primary recrystallization 
annealing. Then, the resulting steel sheet was divided into two pieces, 
and one of them as such was coated with an annealing separator consisting 
mainly of MgO, and then subjected to a final annealing at 1,200.degree. C. 
for 5 hours to prepare a Comparative Example. The other steel sheet piece 
was adhered linearly on the surface with Al.sub.2 O.sub.3 powder as an 
inhibitor for the reaction of the annealing separator with SiO.sub.2 
contained in the subscales of the steel sheet under a condition that the 
adhesion amount: 1.5 g/m.sup.2, the inclination angle with respect to the 
rolling direction: 90.degree., the adhesion width: 2 mm, and the repeating 
interval in the rolling direction: 4 mm, and then coated with the 
annealing separator, and subjected to a final annealing. 
As a result, in the former case, a uniform grey film was formed, while in 
the latter case, no forsterite film was formed at the regions to which the 
Al.sub.2 O.sub.3 powder was applied. the iron loss values of these steel 
sheets are as follows: 
Comparative Example: W.sub.17/50 =1.06 W/kg 
Example: W.sub.17/50 =1.02 W/kg 
Each of coating liquids I-VII shown in Table 1 was applied and baked onto 
each of the above steel sheets to form a top coat insulating film thereon. 
The iron loss values of the thus obtained articles are shown in Table 3. 
Further, after a stress-relief annealing of the articles was performed at 
800.degree. C. for 2 hours, the iron loss values thereof were measured. 
The obtained results are also shown in Table 3. 
It can be seen from Table 3 that the iron loss of the steel sheet having a 
forsterite film locally having filmless regions was conspicously improved 
by the coating film having a thermal expansion coefficient of not higher 
than 9.8.times.10.sup.-6 1/.degree. C. 
TABLE 3 
__________________________________________________________________________ 
Iron loss 
Non- Iron loss 
Iron loss 
value after 
forsterite 
value before 
value after 
stress-relief 
Kind of 
film coating coating annealing 
coating 
region 
W.sub.17/50 (W/kg) 
W.sub.17/50 (W/kg) 
W.sub.17/50 (W/kg) 
Remarks 
__________________________________________________________________________ 
1 No 1.06 1.06 1.06 Comparative example 
Present 
1.02 1.02 1.02 Example 
II No 1.06 1.06 1.06 Comparative example 
Present 
1.02 1.01 1.01 Example 
III No 1.06 1.06 1.06 Comparative example 
Present 
1.02 1.00 0.99 Example 
IV No 1.06 1.06 1.05 Comparative example 
Present 
1.02 0.98 0.98 Example 
V No 1.06 1.04 1.04 Comparative example 
Present 
1.02 0.97 0.96 Example 
VI No 1.06 1.05 1.04 Comparative example 
Present 
1.02 0.96 0.96 Example 
VII No 1.06 1.05 1.05 Comparative example 
Present 
1.02 0.96 0.96 Example 
__________________________________________________________________________ 
EXAMPLE 11 
A cold rolled steel sheet of 0.30 mm in thickness was prepared from a hot 
rolled silicon steel sheet containing 3.2% of Si, and subjected to a 
decarburization primary recrystallization annealing according to the 
ordinary method. 
Then, prior to application of an annealing separator, the surface of the 
steel sheet was coated with Fe.sub.2 SiO.sub.4 as an oxidizing agent for 
Si contained in the steel under a condition that the adhesion amount: 4 
g/m.sup.2, the inclination angle with respect to the rolling direction: 
90.degree., the width: 2 mm, and the repeating interval in the rolling 
direction: 10 mm, and further coated with the annealing separator, and 
subjected to a final annealing at 1,200.degree. C. for 5 hours. For 
comparison, a grain-oriented silicon steel sheet was prepared as 
Comparative Example by the ordinary steps in which no oxiding 
agent-adhering treatment was performed prior to the application of the 
annealing separator. The iron loss values of the resulting steel sheets 
are as follows: 
Comparative Example: W.sub.17/50 =1.04 W/kg 
Example: W.sub.17/50 =0.99 W/kg 
After a stress-relief annealing of the steel sheets was performed at 
800.degree. C. for 2 hours, iron loss values of the steel sheets were 
measured. The obtained results were as follows: 
Comparative Example: W.sub.17/50 =1.04 W/kg 
Example: W.sub.17/50 =0.99 W/kg 
EXAMPLE 12 
A cold rolled steel sheet of 0.30 mm in thickness was prepared from a hot 
rolled silicon steel sheet containing 3.2% of Si, and subjected to 
decarburization.primary recrystallization annealing. Then, the surface of 
the resulting steel sheet was coated with an annealing separator 
consisting mainly of MgO, and subjected to a final annealing at 
1,200.degree. C. for 5 hours to obtain a grain-oriented silicon steel 
sheet having a uniform grey forsterite film on the surface thereof. 
The iron loss value of this steel sheet was 1.06 W/kg at W.sub.17/50. Then, 
filmless regions were formed in the forsterite film under an arrangement 
condition that the width: 0.5 mm, the inclination angle with respect to 
the rolling direction: 90.degree., and the interval between adjacent 
regions in the rolling direction: 6 mm, by a method, in which an iron 
needle with a fine tip was pushed to the steel sheet surface and moved 
thereon under a light pressure to draw a line and to remove the forsterite 
film. 
As a result, the iron loss of the above treated steel sheet was 1.02 W/kg 
at W.sub.17/50. The iron loss value after a stress-relief annealing at 
850.degree. C. for 2 hours, was 1.01 W/kg at W.sub.17/50. 
EXAMPLE 13 
A grain-oriented silicon steel sheet containing 2.8% of Si, having a 
thickness of 0.28 mm, having an iron loss value of 1.08 W/kg at 
W.sub.17/50, and having a uniform forsterite film formed on the surface 
thereof was divided into three pieces A, B and C. Then, the coating liquid 
II and the coating liquid VI shown in Table 1 were applied and baked onto 
the piece A and the pieces B and C respectively to produce grain-oriented 
silicon steel sheets each having a top coating film. Further, in the piece 
C, linear filmless regions were formed in the forsterite film under an 
arrangement condition that the width: 0.5 mm, the inclination angle with 
respect to the rolling direction: 90.degree., and the interval between 
adjacent regions in the rolling direction: 5 mm, without forming scratches 
on the steel sheet matrix surface by a method in which an iron needle with 
a fine tip was pushed to the steel sheet surface and moved thereon under a 
light pressure to remove the coating film and forsterite film. 
The pieces A, B and C were subjected to an annealing at 800.degree. C. for 
10 minutes. In the piece C, the filmless regions of the coating film on 
the surface of the piece C were repaired by such an annealing treatment. 
The iron loss values of the thus treated steel sheets were: 
A: W.sub.17/50 =1.09 W/kg 
B: W.sub.17/50 =1.06 W/kg 
C: W.sub.17/50 =1.02 W/kg 
After a stress-reflief annealing was performed for the above treated steel 
sheets at 800.degree. C. for 5 hours, the iron loss values thereof were 
measured. The obtained results were as follows: 
A: W.sub.17/50 =1.09 W/kg 
B: W.sub.17/50 =1.06 W/kg 
C: W.sub.17/50 =1.02 W/kg