EP0143548B1

EP0143548B1 - 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

Info

Publication number: EP0143548B1
Application number: EP84307320A
Authority: EP
Inventors: Ujihiro C/O Research Laboratories Nishiike; Michiro C/O Research Laboratories Komatsubara; Yoshiaki C/O Research Laboratories Iida; Isao C/O Research Laboratories Matoba; Yukio C/O Research Laboratories Inokuti; Yo C/O Research Laboratories Ito
Original assignee: Kawasaki Steel Corp
Current assignee: JFE Steel Corp
Priority date: 1983-10-27
Filing date: 1984-10-24
Publication date: 1988-08-24
Also published as: DE3473679D1; EP0143548A1

Description

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 non-uniformity to a coating film formed on the steel sheet surface so as to define and form, on the steel sheet surface, local regions which are subjected to a tensile force different from that acting upon the remaining regions of the steel sheet surface.
Grain-oriented silicon steel sheets are mainly used in the iron cores of transformers and other electric instruments, and are required to have excellent magnetic properties and particularly to have a low iron loss represented by W_17/50.
In order to meet the requirements, it is necessary that the <001> orientation of the secondary recrystallized grains in the 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 far as possible. The iron loss value of grain-oriented silicon steel sheets produced so as to meet the requirements becomes lower year by year as a result of a laborous investigations and efforts, and recently a grain-oriented silicon steel sheet having a low iron loss value of W_17/50=1_-05 W/kg at 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 have been demanded more and more and grain-oriented silicon steel sheets having a lower iron loss have been demanded for use as the 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 recrystallization grains are made fine; a method wherein the content of impurities is 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 improvement of these means is very difficult. Even when the means are somewhat improved, only a slight lowering of the iron loss occurs.
Apart frm these means, Japanese Patent Application Publication No. 23,647/79 discloses a method wherein secondary recrystallization-checking regions are formed on the steel sheet surface whereby finely divided secondary recrystallization grains are produced. However, this technique cannot reliably control 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 the secondary recrystallized steel sheet by means of a ball-point pen-like small globe to subdivide the magnetic domain wall spacing whereby the iron loss is lowered. Japanese Patent Application Publication No. 2,252/82 discloses a technique wherein laser beams are irradiated on to the surface of the final steel sheet product at intervals 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 whereby 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 whereby 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 the secondarily recrystallized steel sheet matrix, whereby the magnetic domain wall spacing is subdivided to lower the iron loss, and these methods are practical methods, and are excellent for lowering the iron loss. However, the effect attained by the introduction of plastic strain into the steel sheet is lost by heat treatments, such as stress-relief annealing and the 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, and additional steps, such as a strain-giving step and a recoating step, are required, resulting in high production costs for the 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 retains its excellent magnetic properties, obtained by the subdivision of the magnetic domain wall spacing, even after stress-relief annealing at high temperatures.
EP-A-0 033 878 discloses that grain-oriented silicon treated sheets for transformers and the like are known wherein the surface of the sheets are coated with forsterite which is overcoated with an insulating film based on, for example, a phosphate.
According to the present invention there is provided a grain-oriented silicon steel sheet comprising a matrix surface layer coated with a forsterite film characterised in that the matrix surface layer is free of plastically strained zones and includes regions of first and second types wherein the first type is coated with forsterite film and the second type is free of forsterite film or carries forsterite film having a thickness which is different to the thickness of the forsterite film carried by the first type, the grain oriented sheet having a low iron loss which is free from deterioration due to stress-relief annealing.
The present invention is based on the discoveries that, when a grain-oriented silicon steel sheet has local regions coated with different thicknesses of forsterite film the magnetic domain wall spacing can be very advantageously subdivided in the resulting grain-oriented silicon steel sheet because there occurs local non-uniformity of magnetic field or elastic strain field; and that, when a tension-giving type insulating coating is applied onto a grain-oriented silicon steel sheet having such regions the subdividing of the magnetic domain width can be more improved as a result of their synergistic effect.
Hereinafter, in the specification, claims and drawings, steel sheets which include forsterite films of different thicknesses in the first and second types of regions are referred to as having "forsterite film different-thickness regions", or merely "different-thickness regions".
The present invention is further based on the discoveries that, when a grain-oriented silicon steel sheet has local regions which are coated with forsterite film and local regions which are free of forsterite film, the magnetic domain width of the resulting grain-oriented silicon steel sheet can be very advantageously subdivided similarly to the case where the sheet includes forsterite film different-thickness regions; and that, when a tension-giving type insulating coating is applied onto a grain oriented sheet having such regions the subdividing of the magnetic domain width can be more improved as a result of their synergistic effect.
Hereinafter, in the specification, claims and drawings, steel sheets which include regions of a first type coated with forsterite film and regions of a second type which are free of forsterite film are referred to as having "non-forsterite film regions" or merely "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 Si0₂ grains dispersed in the matrix. After the decarburized and primary recrystallized sheet has an annealing separator consisting mainly of MgO applied to its surface, the steel sheet is subjected to a secondary recrystallization and purification annealing (a final annealing) at a high temperature of about 1,200°C. By this secondary recrystallization the crystal grains in the steel sheet grow into coarse grains having a {100}<001> orientation. Moreover, by the high temperature purification, a part of the inhibitors, such as S, Se, N, etc. which remains in the steel sheet, is removed from the steel sheet matrix.
Furthermore, in this purification, Si0₂ in the subscale of the surface layer of the steel sheet and MgO in the annealing separator coated on the steel surface react with each other according to the following equation:
to form a coating film consisting of a polycrystal of forsterite (Mg₂Si0₄) on the surface layer of the steel sheet. In this case, unreacted excess MgO serves to prevent fusing between adjacent steel sheets. After the final annealing, the unreacted annealing separator is removed from the steel sheet, and if necessary, an insulating coating is finally applied or a coil set is removed to obtain a product steel sheet.
The inventors have reinvestigated the role of forsterite film, and have found that the film gives a tensile force to the steel sheet to subdivide the magnetic domain wall spacing and the subdivision effect of the magnetic domain wall spacing in the steel sheet varies. As a result, the inventors have reexamined carefully the subdivision effect of the magnetic domain wall spacing in a steel sheet, and have found that the above mentioned effect is remarkable in places where the tensile stress field or the magnetic field is changed by the thickness of the foresterite film.
In a first embodiment of the present invention the grain-oriented silicon steel sheet has a low iron loss free from deterioration due to the stress-relief annealing, no plastically strained zones in the matrix surface layer, and a forsterite film coated on the surface wherein first regions of the surface have a forsterite film having a thickness different from that of the forsterite film in second regions of the surface.
In a second embodiment of the present invention the grain-oriented silicon steel sheet has a low iron loss free from deterioration due to the stress-relief annealing, no plastically strained zones in the matrix surface layer, and a forsterite film coated on the surface wherein first regions of the surface have a forsterite film having a thickness different from that of the forsterite film in second regions of the surface, said steel sheet further having a tension-giving type insulating coating film having a thermal expansion coefficient of not higher than 9.8x10-⁶ 1/°C formed on the forsterite film.
In a third embodiment of the present invention the grain-oriented silicon steel sheet has a low iron loss free from deterioration due to the stress-relief annealing, no plastically strained zones in the matrix surface layer, and a discontinuous forsterite film coated on the surface whereby first regions of the surface have no forsterite film coating and second regions of the surface do have a forsterite film coating.
In a fourth embodiment of the present invention the grain-oriented silicon steel sheet has a low iron loss free from deterioration due to the stress-relief annealing, no plastically strained zones in the matrix surface layer, and a discontinuous forsterite film coated on the surface whereby first regions of the surface have no forsterite film coating and second regions of the surfaces do have a forsterite film coating said steel sheet further having a tension-giving type insulating coating having a thermal expansion coefficient of not higher than 9.8x 10-⁶ 1/°C formed on the forsterite film..
The present invention also provides a method of producing a grain oriented silicon steel sheet as claimed in claim 6.
For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, by way of example to the accompanying drawings, wherein:

Fig. 1A is an explanative view illustrating the shapes of the different-thickness regions or filmless regions of a sheet in accordance with the invention;
Fig. 18 is an explanative view illustrating the inclination angles of the different-thickness regions or the filmless regions with respect to the rolling direction of a steel sheet in accordance with the invention;
Fig. 1C is an explanative view of the intervals between adjacent different-thickness regions or adjacent filmless regions of a sheet in accordance with the invention;
Fig. 2 is a graph illustrating the influence of the inclination angle of linear different-thickness regions with respect to the rolling direction of a steel sheet of the invention upon the iron loss value of the product steel sheet;
Fig. 3 is a graph illustrating the relationship between the thickness difference of different-thickness regions of a steel sheet of the invention and the iron loss value of the product steel sheet;
Fig. 4 is a graph illustrating the relationship between the width of the different-thickness regions of a steel sheet of the invention and the iron loss value of the product steel sheet;
Fig. 5 is a graph illustrating the relationship between the interval of adjacent different-thickness regions of a steel sheet of the invention and the iron loss value of the product steel sheet;
Fig. 6 is a graph illustrating the relationship between the width of the different thickness regions of a steel sheet of the invention and the iron loss value of the product steel sheet in the presence or absence of a tension-giving type coating film formed on the forsterite film;
Fig. 7 is a graph illustrating the influence of the inclination angle of linear filmless regions with respect to the rolling direction of a steel sheet of the invention upon the iron loss value of the product steel sheet;
Fig. 8 is a graph illustrating the relationship between the width of the filmless regions of a steel sheet of the invention and the iron loss value of the product steel sheet;
Fig. 9 is a graph illustrating the relationship between the interval of adjacent filmless regions of a sheet of the invention and the iron loss value of the product steel sheet; and
Fig. 10 is a graph illustrating the relationship between the width of the filmless regions of a steel sheet of the invention and the iron loss value of the product steel sheet in the presence or absence of a tension-giving type casting film formed on the forsterite film.

In the present invention, the starting material steel sheets are limited to ones having no plastically strained zones. 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 accordance with the present invention include not only steel sheets having a forsterite film alone but also steel sheets 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 on the sheet surface or have removed locally forsterite film from the sheet surface to provide the sheet surface with different regions and have investigated the influence of the shape, thickness difference, direction, etc. of these regions, upon the subdivision of the magnetic domain wall spacing, and have studied the relationship between the shape, thickness difference, direction, etc. of the regions upon the iron loss of the product steel sheet.
In their experiments, 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 Si0₂ 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 the case of a discontinuous linear groove or land having missing portions like a dot-dashed line, the effect for lowering the iron loss is almost the same as that of a continuous linear groove or land.
Regarding the direction of the different-thickness regions, as illustrated in Figs. 1B and 2, it is especially effective in the case of an inclination angle of 60―90° 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 regions, as illustrated in Fig. 3, both the larger thickness region and the smaller thickness region exhibit almost the same effect. In any case, it has been found that it is effective when the thickness difference is not less than 0.3 um (measuring condition in Fig. 3: sheet thickness: 0.30 mm, linear groove or land, interval: 5 mm, width: 0.5 mm, angle: 90°). 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°, thickness difference: 0.4 pm).
Further, it is effective in order to lower the iron loss of the whole steel sheet that the different-thickness regions are 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°, width: 1 mm, thickness difference: 0.5 pm).
The effect of the different-thickness regions is almost the same when the regions are provided on both surfaces of the steel sheet and when the regions are provided only on one surface thereof.
It has been found that, when a steel sheet having a forsterite film locally having such different- thicknesses is coated with a coating liquid, which forms a coating film having a thermal expansion coefficient of 5x10-⁶ 1/°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 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°, thickness difference: 0.8 µm). Further, it has been found that the effect of the tension-giving type coating film is higher in the case where the sheet has different-thickness regions than in the case where the sheet has no different-thickness regions.
When various coating films having different thermal expansion coefficients were coated on a grain-oriented silicon steel sheet having different-thickness 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 9.8x 10-⁶ 1/°C results in a satisfactorily low iron loss.
An explanation will now be made with respect to the method for providing the sheet with the different-thickness regions.
The same primarily recrystallized steel sheet was used, and different-thickness regions were 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 final annealing. For example, a method, wherein uncoated regions of an annealing separator are formed on the surface of the 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 final annealing, is subjected to a chemical dissolving treatment to decrease the film thickness.
(3) A method, wherein a uniform forsterite film after 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 final annealing, is applied with a tension-giving type coating, and pulse-shaped high-power laser beams are irradiated on 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 final annealing, is applied with a tension-giving type coating, and an iron needle having a sharp point is lightly pushed against 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_17/50 of 0.96-0.99 W/kg. When these steel sheets were subjected to a stress-relief annealing at 800°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 zone 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 zones 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 600-900°C.
There have been found out the following facts with respect to the shape, direction and the like of the filmless regions of the sheets of the present invention.
It has been found that, with regard to the shape of the filmless regions, a continuous or discontinuous linear region free of forsterite 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 the case of a 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 a continuous linear filmless region.
Regarding the direction of the filmless region, as illustrated in Figs. 1 B and 7, it is especially effective in the case of an inclination angle of 60-90° 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 Fig. 8 (measuring condition in Fig. 8: sheet thickness: 0.30 mm; linear filmless region, interval: 3 mm, angle: 90°).
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 filmless 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; linearfilmless region, angle: 90°, width: 1 mm).
The effect of the filmless regions is almost the same in the case where a forsterite film having regions free of forsterite is formed on each surface of a steel sheet and in the case where a forsterite film having regions free of forsterite is formed only on one surface thereof.
It has been found that, when a steel sheet having a forsterite film including local regions free of forsterite is coated with a coating liquid, which forms a coating film having a thermal expansion coefficient of 5x10-⁶ 1/°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 including regions free of forsterite as illustrated in Fig. 10 (measuring condition in Fig. 10: sheet thickness: 0.28 mm, linear filmless region, interval: 4 mm, angle: 90°). 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 regions free of forsterite than in the case where a forsterite film has no regions free of forsterite.
When various coating films having different thermal expansion coefficients were coated on a grain-oriented silicon steel sheet having 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 9.8x10-⁶ 1/°C results in a satisfactorily low iron loss.
An explanation will be made with respect to the method for providing the steel sheet surface with filmless regions.
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 5.6x10-6 1/°C formed on the forsterite film was divided into 4 steel sheets, and regions free of forsterite film, each having a width of 1.0 mm, were formed at an inclination angle of 90° 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 on 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 against 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_17/50 of 0.97-0.98 W/kg. When these steel sheets were subjected to a stress-relief annealing at 800°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 zone 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 zones 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 600-900°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 a Si content of less than 2.0% results in a grain-oriented silicon steel sheet having a very poor iron loss, and a 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 the 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 atmosphere 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 coated with annealing separator are locally formed on the steel sheet surface, whereby the object aimed in the present invention are advantageously achieved.
That is, by carrying out the final annealing, an ordinary forsterite film is formed on the surface coated with the annealing separator. On the contrary, merely a thin forsterite film is formed on the surface which was not coated with the annealing separator, so that the sheet surface is provided with different thickness-regions as required by the first and second embodiments of the present invention.
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 other methods for providing the different-thickness regions as required by the first and second embodiments 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 the steel sheet surface after the primary recrystallization annealing, an inhibitor 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/m².
In this method, as the inhibitor, there can be used oxides such as Si0₂, AI _z0₃, Zr0₂, etc. and metals such as Zn AI, Sn, Ni, Fe, etc. When the inhibitor is adhered to the steel sheet surface in an amount of more than 1 g/m², the inhibiting effect for the reaction becomes excessive and a forsterite film is not formed. Hence, it is necessary that the degree to which the thickness of the forsterite film is to be decreased is controlled by using the inhibitor in an amount persistently of not more than 1 g/m^2. As a means for applying these reaction inhibitors to the steel sheet, any application technique e.g. spraying, plating, printing, and electrostatic painting is available.
(2) A method, wherein, before applying an annealing separator to the 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 of not more than 0.1 g/m².
As such water-repelling substances, oil paint and varnish, etc. are advantageously used. The water-repelling substances prevent contact of the steel sheet surface with the annealing separator to delay the forsterite forming reaction and to form a region of smaller thickness. However, when the substances are adhered to the steel sheet in an amount of more than 0.1 g/m², the reaction-delaying effect becomes excessive, and a forsterite film is not at all formed. Therefore, it is necessary that the degree to which the thickness of the forsterite film is to be decreased is controlled by using the water-repelling substance in an amount persistently of not more than 0.1 g/m². Further, as a means for adhering the water-repelling substance to a steel sheet, there can be used spraying, printing, electrostatic painting, etc. similarly to the above described reaction inhibitors.
(3) A method, wherein, before applying an annealing separator to the 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 of not more than 2 g/m².
The oxidizing agent oxidizes Si in the steel at a high temperature during the following final annealing to increase the amount of Si0₂ particles in the subscale of the steel sheet surface layer and to increase the thickness of the forsterite film after the final annealing. Hence, a 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 ₂0₃, TiO_z, etc., easily reducible silicates, such as Fe₂ Si0₄, etc. hydroxides, such as Mg(OH)₂, etc. However, when the adhered amount of these oxidizing agents to the steel sheet surface exceeds 2 g/m², 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 a result, the desired 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, whereby smaller thickness regions are formed.
As to the method, besides chemical polishing and electrolytic polishing, there are methods of removing the forsterite film by using a rotating disc-like grindstone, by using an iron needle under a light pressure, and by optical means, for example laser beams, etc. having their 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 mask, whereby a plural number of different-thickness regions can be advantageously formed efficiently by one operation.
Further, in the third and the fourth embodiments of the present invention, in order to produce the filmless regions, among the above mentioned methods of (1), (2), (3) and (4), methods of (1), (2) and (4) are available. However, 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 the steel sheet after the primary recrystallization annealing, in an amount within the range of more than 1 g/m² 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/m², there is a risk of forming a forsterite film, and hence the adhered amount of the reaction inhibitor has been determined to be more than 1 g/m² in order to avoid the risk.
(2) When a substance having a water-repelling property with respect 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 the steel sheet after the primary recrystallization annealing, in an amount within the range of more than 0.1 g/m² 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/m², there is a risk of forming a forsterite film, and hence the adhered amount has been determined to be more than 0.1 g/m² 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 zone on the matrix surface during the removal treatment. The reason is that, when plastic strain is introduced into the matrix surface, the properties of the steel sheet after the stress-relief annealing are 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 60-90° 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 the case of excessively large width of the different-thickness region or filmless region, or in the case of isolated recesses or protrusions, is probably because 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 effect of providing the different-thickness regions or the filmless regions is almost the same in the case where the regions are present on both surfaces of the steel sheet as in the case where the regions are 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 9.8x 10-6 1/°C as a top coating is formed on the grain-oriented silicon steel sheet having different-thickness regions or filmless regions, grain-oriented silicon steel sheets having more excellent magnetic properties can be obtained.
Alternatively, silicon steel sheets having more excellent magnetic properties can be produced in the following manner. A tension-giving type insulating coating film having a thermal expansion coefficient of not more than 9.8x10-⁶ 1/°C is formed as a top coating 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 all of the forsterite film are locally removed to form regions including a small thickness of forsterite film or regions which are free of forsterite film and then.the steel sheet is subjected to an annealing at a temperature of 600-900°C to repair the portions where the top coating is absent.
The top coating gives a surface tension to the 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 9.8x 10-⁶ 1/°C gives a satisfactorily low iron loss value to the product steel sheet by the synergistic effect of the effect caused by the different-thickness regions or fil mless regions 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/m² (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 shape is small, and a lowering of the space factor does not substantially occur.
As described above, the grain-oriented silicon steel sheets having different-thickness regions or filmless regions exhibit excellent magnetic properties in both cases. The steel sheets can be directly used in a practical apparatus similar to the commonly used grain-oriented silicon steel sheets or they can be used in a practical apparatus after they have been provided with 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 the forsterite film. The reason is probably that these regions are subjected to different tensions and plastic strain is introduced into the steel sheet surface by the action of these different tensions so that the magnetic domain wall spacing is effectively subdivided.
In grain-oriented silicon steel sheets having strain caused by the different tension, there are no artificially plastically strained zones in the steel sheet matrix surface layer as in conventional methods wherein plastically -strained zones or high dislocation density regions, such as laser beam marks, are formed in the matrix surface layer. Therefore deterioration of iron loss does not substantially occur even when a stress-relief annealing is carried out under the commonly used condition of about 800°C and for from 1 minute to several hours. In the conventional grain-oriented silicon steel sheet, the plastic strain in the surface layer of the matrix is extinguished at high temperature. Therefore, the conventional steel sheet has a fatal defect in that the iron loss deteriorates. However, the grain-oriented silicon steel sheet of the present invention has a satisfactory 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 rolled 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, A1 ₂0₃ powder as an inhibitor for the forsterite forming reaction was adhered linearly to the steel sheet surface under the following conditions:- adhesion amount: 0.5 g/m², the inclination angle with respect to the rolling direction: 90°, 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°C for 5 hours.
For comparison, a grain-oriented silicon steel sheet was prepared as a Comparative Example according to the ordinary method wherein A1 ₂0₃ 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, the forsterite film having a thickness smaller by 0.8 µm formed in the regions to which the A1 ₂0₃ powder was applied had a thickness smaller by 0.8 um than the thickness at the region to which the A1 ₂0₃ powder was not applied. The iron loss values of the Example and Comparative Example were as follows:
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.
Further, the above top coated steel sheets were subjected to a stress-relief annealing at 800° for 2 hours, and the iron loss values of the annealed sheets were measured. The obtained values are as follows.

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 the following conditions:―the inclination angle with respect to the rolling direction: 90°, 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°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 the 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 the Comparative Example were as follows:
When these samples steel sheets were subjected to a stress-relief annealing at 800°C for 5 hours, and the iron loss values of the steel sheets were measured, the following values were obtained.

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 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 the following αonditions:the inclination angle with respect to the rolling direction: 90°, 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°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 the 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 the Comparative Example were as follows:
When these sample steel sheets were subjected to a stress-relief annealing at 800°C for 5 hours, and the iron loss values of the steel sheets were measured, the following values were obtained.

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 the following conditions:- 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°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:
When these samples steel sheets were subjected to a stress-relief annealing at 800°C for 1 hour and the iron loss values of the steel sheets were measured, the following values were obtained

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 the following conditions:- the amount of FeO: 0.5 g/m², the inclination angle with respect to the rolling direction: 90°, 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°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:
After a stress-relief annealing was performed for the above treated steel sheets at 800°C for 2 hours, the iron loss values thereof were measured. The following values were obtained.

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/m² by a printing process in the form of a discontinuous straight line under the following αonditions: the inclination angle of the printed regions with respect to the rolling direction: 90°, 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°C for 10 hours. For comparison, a grain-oriented silicon steel sheet was prepared as a Comparative Example according to the ordinary 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:
The following values were obtained with respect to the iron loss values after a stress-relief annealing was performed at 800°C for 2 hours.

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°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_17/50.
Then, an iron needle with a fine tip was pushed against 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 fosterite film, which regions satisfied the following conditions:- the depth: 0.5 pm, 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_17/50. The iron loss value after a stress-relief annealing of the above obtained steel sheet at 850°C for 2 hours was 1.01 W/kg at W_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°C for 5 hours, which was used as a Comparative Example. The other steel sheet piece was adhered linearly on its surface with Al₂O₃ powder as an inhibitor for the forsterite forming reaction under the following conditions:- the adhesion amount: 0.5 g/m², the inclination angle with respect to the rolling direction: 90°, 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 µm than that of the forsterite film formed at the regions, to which the A1 ₂0₃ powder was not applied, was formed at the regions to which the A1 ₂0₃ powder was applied. The iron loss values of these steel sheets were as follows:
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°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 9.8x10^-6 1/°C.

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_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. Further in the piece C, linear decreased thickness regions of the forsterite film were formed under the following conditions:- 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: 3 mm, without forming scratches on the steel sheet matrix surface by a method in which an iron needle with a fine tip was pushed against 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 annealing at 700°C for 1 minute, and it was found that the filmless regions of the coating film adhered onto the surface of the piece C were repaired. The iron loss values of the steel sheets thus obtained were:
After these steel sheets were subjected to a stress-relief annealing at 800°C for 5 hours, the iron loss values thereof were measured. The following results were obtained.

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°C for 5 hours to prepare a Comparative Example. The other steel sheet piece was adhered linearly on the surface with AI ₂0₃ powder as an inhibitor for the reaction of the annealing separator with Si0₂ contained in the subscales of the steel sheet under the following conditions:- the adhesion amount: 1.5 g/m², the inclination angle with respect to the rolling direction: 90°, 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 A1 ₂0₃ powder was applied. The iron loss values of these steel sheets are as follows:
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°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 conspicuously improved by the coating film having a thermal expansion coefficient of not higher than 9.8x10^-6 1/°C.

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 Fe₂Si0₄ as an oxidizing agent for Si contained in the steel under the following conditions:- the adhesion amount: 4 g/m², the inclination angle with respect to the rolling direction: 90°, 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°C for 5 hours. For comparison, a grain-oriented silicon steel sheet was prepared as a Comparative Example by the ordinary steps in which no oxidizing 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:
After a stress-relief annealing of the steel sheets was performed at 800°C for 2 hours, iron loss values of the steel sheets were measured. The obtained results were as follows:

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°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_17/50. Then, filmless regions were formed in the forsterite film under the following αonditions: 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, by a method, in which an iron needle with a fine tip was pushed against 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_17/50. The iron loss value after a stress-relief annealing at 850°C for 2 hours, was 1.01 W/kg at W_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_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 the following conditions:- 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: 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 against 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°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:
After a stress-relief annealing was performed for the above treated steel sheets at 800°C for 5 hours, the iron loss values thereof were measured. The obtained results were as follows:

Claims

1. A grain-oriented silicon steel sheet comprising a matrix surface layer coated with a forsterite film characterised in that the matrix surface layer is free of plastically strained zones and includes regions of first and second types wherein the first type is coated with forsterite film and the second type is free of forsterite film or carries forsterite film having a thickness which is different to the thickness of the forsterite film carried by the first type, the grain oriented sheet having a low iron loss which is free from deterioration due to stress-relief.

2. A steel sheet according to claim 1, wherein the regions of the second type are in the form of continuous or discontinuous linear grooves or lands.

3. A steel sheet according to claim 2, wherein the continuous or discontinuous linear grooves or lands are inclined at an angle of 60-90° with respect to the rolling direction of the steel sheet.

4. A steel sheet according to claim 1, 2 or 3 wherein the regions of the second type carry a forsterite film having a thickness which differs by not less than 0.3 pm from the thickness of the forsterite film carried by the regions of the first type.

5. A grain-oriented silicon steel sheet as claimed in any one of the preceding claims wherein it additionally includes a tension-giving type insulating coating film having a thermal expansion coefficient of not higher than 9.8x10^-6 1/°C formed on the forsterite film.

6. A method of producing a grain-oriented silicon steel sheet as claimed in claim 1 wherein a hot rolled sheet produced from a silicon-containing steel slab by hot rolling 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, the cold rolled sheet is subjected to a decarburisation primary recrystallisation annealing, an annealing separator consisting mainly of MgO is applied to the surface of the decarburised and primarily recrystallised steel sheet, and the thus treated steel sheet is subjected to a final annealing to form a forsterite film on the steel sheet surface, characterised in that the sheet is provided with regions carrying no forsterite film or regions carrying forsterite films of different thicknesses.

7. A method according to claim 6 wherein said regions are provided by locally applying the annealing separator to the surface of the decarburised and primary recrystallised steel sheet.

8. A method according to claim 6 wherein said regions are provided by uniformly applying the annealing separator to the surface of the decarburised and primarily recrystallised steel sheet and then locally removing annealing separator from the steel sheet surface.

9. A method according to claim 6 wherein said regions are provided by locally adhering an inhibitor for the forsterite forming reaction to the surface of the decarburised and primarily recrystallised steel sheet in an amount of not more than 1 g/m² before the application of the annealing separator.

10. A method according to claim 6 wherein said regions are provided by locally adhering a water-repelling substance for a slurry of the annealing separator to the surface of the decarburised and primarily recrystallised steel sheet in an amount of not more than 0.1 g/m² before the application of the slurry of the annealing separator.

11. A method according to claim 6 wherein said regions are provided by locally adhering an oxidising agent for Si contained in the steel to the surface of the decarburised and primarily recrystallised steel sheet in an amount of not more than 2.0 g/m² before the application of the annealing separator.

12. A method according to claim 6 wherein said regions are provided by locally removing a part or all of the forsterite film formed during the final annealing without causing a plastic strain in the interior of the steel sheet matrix.

13. A method according to claim 6 and comprising the additional step of applying, onto the steel sheet surface carrying the forsterite film, a top coating liquid which forms a tension-giving type insulating top coating film having a thermal expansion coefficient of not higher than 9.8x10-⁶ 1/°C, and baking the coating liquid to the forsterite film at a temperature range of 600-900°C.

14. A method according to claim 13 wherein the top coating film and at least a part of the forsterite film are locally removed without causing a plastic strain in the interior of the steel sheet matrix to provide the sheet with said regions and then subjecting the thus treated steel sheet to an annealing at a temperature range of 600-900°C to repair the top coating film.