Silicon steel sheets having low iron loss and method of producing the same

Abstract: A silicon-containing steel sheet having a low iron loss has such a crystal structure that crystal grains having an inclination angle of {110} face of not more than 10° with respect to the sheet surface are included in an amount of not less than 80 vol% and exhibit a graining surface pattern in which boundaries of these crystal grains form stepwise difference or groove of not less than 0.4 µm as a maximum height. This sheet is produced by subjecting a grain oriented silicon steel sheet after final annealing to a magnetically smoothening treatment by electrolysis in an aqueous solution containing at least one of water soluble halides.

Description: This invention relates to silicon-containing steel sheets having not only excellent magnetic properties but also good adhesion property to a coating and a method of producing the same. Since an energy crisis has been imminent for several years, there has been a strong need for electrical machinery and apparatus to have less power loss. For this purpose, there has been a demand to develop electromagnetic steel sheets having much lower iron loss for use as a core material for these machineries and apparatuses. As the conventional method of producing grain oriented silicon steel sheets, there is usually performed a method wherein a starting material containing, for example, 2.0-4.0% by weight (hereinafter shown by % simply) of Si is hot rolled and subjected to a heavy cold rolling either once or twice with an intermediate annealing to provide a final sheet thickness, and then the resulting cold rolled sheet is decarburization-annealed, coated with a slurry of an annealing separator composed mainly of MgO and wound in the form of a coil, and thereafter the coil is subjected to secondary recrystallization annealing and purification annealing (these two annealing steps are usually performed in one process: hereinafter, the term "final annealing" is used) and further to a phosphate insulation coating if necessary. In the above purification annealing, a forsterite (Mg₂SiO₄) coating is formed by reacting an oxide layer of SiO₂ produced on the surface of the steel sheet after the decarburization annealing with MgO contained in the annealing separator. The grain oriented silicon steel sheets are obtained by aligning secondary recrystallized grains into (110) [001] orientation or Goss orientation through the above production steps and are mainly used as a core for transformers and other electrical machineries. To this end, they are required to have a high magnetic flux density (exemplified by B₁₀ value) and a low iron loss (exemplified by W 17/50 value). Particularly, it has recently been demanded to further reduce the iron loss for lessening the power loss of the transformer or the like from the viewpoint of energy-saving. The iron loss of the silicon steel sheet is the sum of the eddy current loss and the hysteresis loss. As an effective means for reducing the iron loss of the silicon steel sheet, there is a method of reducing the sheet thickness, which mainly reduces the eddy current loss and largely contributes to the reduction of iron loss and hence energy-saving. However, as the sheet thickness reduces to not more than 0,262 mm (11 mil), the proportion of the hysteresis loss in the total iron loss rapidly increases. As factors influencing the hysteresis loss, mention may be made of the orientation of the crystal grain, the amount of impurities, the influence of surface coating, the roughness of the sheet surface, and the like. As a method of reducing the hysteresis loss by particularly improving the surface properties of the steel sheet, for instance, Japanese Patent Application Publication No. 52-24,499 proposes a method wherein a grain oriented silicon steel sheet after final annealing is pickled to remove oxides from the surface and then rendered into a mirror state by subjecting it to chemical polishing or electrolytic polishing. Furthermore, Japanese Patent Application Publication No. 56-4,150 discloses a technique wherein the surface of the grain oriented silicon steel sheet is subjected to chemical or electrolytic polishing after the removal of non-metallic substance and then coated with a ceramic thin film. And also, Japanese Patent laid open No. 60-89,589 discloses a technique wherein the surface of the grain oriented silicon steel sheet after the secondary recrystallization using an annealing separator composed mainly of alumina is subjected to chemical or electrolytic polishing after the removal of oxides from the surface. Moreover, Japanese Patent laid open No. 60-39,123 discloses a technique wherein the grain oriented silicon steel sheet is subjected to chemical or electrolytic polishing without direct pickling after the amount of oxide formed on the surface is controlled by using an annealing separator composed mainly of alumina. Although, these techniques clearly show the effect of reducing the iron loss, they are not yet practised in industry. Because, in the case of chemical polishing, HF+H₂O₂, H₃PO₄+H₂O₂ or the like used as a polishing solution is expensive, resulting in an increase in cost. On the other hand, in the case of electrolytic polishing, phosphoric acid baths, sulfuric acid baths, phosphoric acid-sulfuric acid baths, perchloric acid baths, and the like have a high concentration of acid as a main ingredient and also contain a chromate, fluoric acid, organic compound or the like as an additive, so that they are high in cost and there are many unsolved problems of homogeniety, productivity, premature degradation of solution and the like when treating a great amount of steel sheet. Furthermore, a great drawback obstructing the industriallization is that the insulation coating is hardly adhered onto the mirror finished surface of the sheet. That is, the conventionally known phosphate coating, ceramic coating and the like have poor adhesion due to the mirror surface and are not durable in practical use. It is, therefore, an object of the invention to advantageously solve the aforementioned problems and to provide silicon-containing steel sheets having a magnetically smooth surface, i.e. a surface not obstructing the movement of magnetic domain walls which causes the hysteresis loss without performing the mirror finishing treatment through the electrolytic or chemical polishing, and an excellent adhesion property to a coating and a method of producing the same. The inventors have made various studies with respect to the influence of the surface upon the iron loss and ascertained the following: Firstly it has been learned that a factor largely influencing the hysteresis loss is oxide existing on the surface and the mirror state is not necessarily required to facilitate movement of magnetic domain walls. The term "mirror state" used herein is an optical concept and is not quantitatively defined, but usually indicates that the surface roughness is not more than 0.4 µm, particularly not more than 0.1 µm as a center-line average roughness. Fig. 2 of the accompanying drawings shows a comparison in iron loss between a conventional grain oriented silicon steel sheet having oxide in its surface, a grain oriented silicon steel sheet obtained by subjecting a conventional sheet to a mirror finishing treatment, and a grain oriented silicon steel sheet where the mirror finished surface has been subjected to pickling. As can be seen from Fig. 2, the iron loss property is not so degraded even if the mirror state is lost by the pickling. Thus, in order to obtain a low hysteresis loss silicon steel sheet, a mirror surface is not always required, and it is sufficient for the surface of the steel sheet to be a magnetically smooth surface, i.e. a surface not obstructing the movement of magnetic domains which causes the hysteresis loss. Therefore, the electrolytic polishing and the chemical polishing are not indispensable requirements and thus the surface treating means may be selected more freely. However, the introduction of strain into the surface of the silicon steel sheet during the magnetically smoothening process degrades the iron loss property, so that it should be avoided as far as possible, and hence the chemically strain-free polishing process is suitable. The mirror finishing phenomenon characterized by the electrolytic polishing method will be described below. In the electrolytic polishing, when current is passed in an electrolytic solution of strong acid or strong alkali by using the surface to be polished as an anode, metal is dissolved out from the surface as an ion by the electrolytic reaction, while a viscous film is formed between the metal surface and the electrolytic solution. Since such a viscous film is thin at the convex portion of the surface and much current flows thereto, the convex portion is more dissolved out as compared with the concave portion and finally the metal surface is formed into an even mirror finished surface. Therefore, chemical or electrolytic polishing is a method of smoothening the metal surface independently of crystal grain size and crystal orientation. In other words, the surface obtained by the chemical or electrolytic polishing provides a smooth surface having a high gloss irrespective of the crystal orientation of the base metal. Secondly, it has been ascertained that the surface state of the silicon steel sheet largely differs in accordance with the difference in crystal orientation when the sheet is subjected to an anodic electrolytic treatment in an aqueous halide solution. Heretofore, electrolytic treatment using halide has scarcely been carried out because its efficacy in obtaining a mirror polished surface is poor. However, the inventors have widely examined the electrolytic treatment and found the above mentioned peculiar phenomenon as a result of confirmation experiments using halide. Fig. 3 of the accompanying drawings shows microphotographs of sheet surfaces having different crystal face morphologies after an anodic electrolytic treatment in an aqueous NaCl solution as a halide, wherein A, B, and C are enlarged photographs of various morphologies of the crystal grains, respectively. In Fig. 3, A is the case where the {110} face of the crystal grains is inclined at an angle of 5° with respect to the rolling surface and exhibits a peculiar network surface morphology. This network surface is called a graining pattern surface because it closely resembles the grained surface obtained by electrolytic etching and is characterized by dispersing and adjoining recesses each apparently seeing the crystal grain into the grains. B is the case where the crystal face is inclined at an angle of 11° with respect to the rolling surface and exhibits a scale-like morphology. C is the case where the crystal face is inclined at an angle of 25° with respect to the rolling surface and exhibits a fine-grained texture. As shown in A to C in Fig. 3, the surfaces having these peculiar morphologies are not mirror surfaces even in the case of the network texture A, and exhibit an aspect similar to that of a pickled surface resembling a crystal grain boundary in macro appearance. Further, it is important that the surface having such a peculiar network texture is obtained by subjecting the silicon steel sheet having {110} face to an electrolytic treatment with an aqueous chloride solution as the electrolytic solution and that the network texture is a magnetically smooth surface which means that the hysteresis loss is very small. Thirdly, it has been ascertained that the graining pattern surface has a larger magnetic flux density as compared with the mirror surface obtained by the conventional electrolytic polishing treatment. Therefore, silicon-containing steel sheets based on the above knowledge have low production costs and have excellent magnetic properties as compared with the case using the conventional mirror finishing treatment. In the silicon-containing steel sheet, an insulation coating is frequently provided on the surface of the sheet. Furthermore, a tension may be applied to the insulation coating or a double coating of tension applied coat and insulation coat may be formed in order to further improve the magnetic properties such as magnetostriction, iron loss and the like. However, the surface obtained using conventional mirror polishing as a means for obtaining a magnetically smooth surface is difficult to provide with these coatings and also has poor adhesion to the coatings. On the contrary, it has been confirmed that the surface of the steel sheet obtained by the anodic electrolytic treatment in aqueous halide solution has excellent adhesion to the insulation coating as compared with the mirror surface obtained by chemical or electrolytic polishing. However, since there is caused a scattering in the adhesion to the coating depending upon the kind and thickness of the insulation coating, an improvement of such a surface state has been attempted by subjecting it to the usual brushing treatment, but satisfactory results have not yet been obtained. Now, the inventors have examined the cause of degrading the adhesion to the coating and have found that hydrated oxide of Fe and smut which are not removed by the usual brushing treatment and remain on the sheet surface influence the adhesion to the coating. Furthermore, it has been found that it is very effective to subject the sheet surface after the electrolysis to the brushing treatment with an aqueous solution or suspension of a hydrogen carbonate for removing the hydrated oxide and smut. Also the clean a surface obtained by this treatment appears to have improved adhesion to the coating. The invention will be described with reference to the accompanying drawings, wherein: Figs. 1a and 1b are graphs showing the improved margins of iron loss and magnetic flux density when the surface of a grain oriented silicon steel sheet is subjected to an anodic electrolytic treatment in phosphoric acid-chromic acid bath or halide bath or further provided thereon with a coating of TiN, respectively; Fig. 2 is a graph showing a comparison of iron loss value when the surface of a grain oriented silicon steel sheet is subjected to a mirror finishing treatment and when the mirror finished surface is subjected to a pickling treatment; Fig. 3 is a microphotograph of the surface of grain oriented silicon steel sheets after anodic electrolytic treatment in a chloride bath, wherein A, B and C are enlarged photographs of respective portions, respectively; Fig. 4 is a graph showing the dissolved-out thickness of the grain oriented silicon steel sheet and the improved margin of iron loss thereof when the sheet is subjected to an anodic electrolytic treatment in a chloride bath or a polyether containing-chloride bath; Fig. 5 is a graph showing the improved margin of iron loss when a grain oriented silicon steel sheet is subjected to an anodic electrolytic treatment in a polyether-containing chloride bath or a phosphoric acid-chromic acid bath and when the electrolysed surface is subjected to a coating of TiN; and Fig.6 is a graph showing iron loss values after a grain oriented silicon steel sheet is subjected to a mechanical polishing by means of a nonwoven cloth or a belt, or after the polished surface is subjected to an electrolytic treatment, and after the electrolysed surface is subjected to a coating of TiN. The invention is based on the aforementioned discoveries. That is, according to a first aspect of the present invention, there is provided a silicon-containing steel sheet having a crystal structure wherein crystal grains having an inclination angle of {110} face of not more than 10° with respect to the sheet surface are included in an amount of not less than 80 vol%, characterised in that the surfaces of these crystal grains exhibit at said sheet surface a graining pattern wherein the boundaries between these crystal grains are in the form of steps or groove-like concave portions having a maximum height Rmax of not less than 0.4µm and in that a tension-applying insulation coating is provided on said sheet surface. According to a second aspect of the invention, there is provided a method of producing a silicon-containing steel sheet having a low iron loss, which method comprises subjecting a grain oriented silicon steel sheet after final annealing to a magnetically smoothening treatment by electrolysis in an aqueous solution containing at least one water soluble halide selected from HCl, NaCl, KCl, NH₄Cl, MgCl₂, CaCl₂, AlCl₃, NaF, KF, NH₄F, HBr, NaBr, KBr, MgBr₂, CaBr₂, NH₄Br, HI, NaI, KI, NH₄I, CaI₂, MgI₂, H₂SiF₆, MgSiF₆, (NH₄)₂SiF₆, HBF₄, NH₄BF₄ and NaBF₄. In an embodiment of the invention, the aqueous solution further contains a polyether or a corrosion preventative agent. In a further embodiment of the invention, the sheet surface after the magnetically smoothening treatment is subjected to a brushing treatment in an aqueous solution or suspension of a hydrogen carbonate, or the final annealed sheet is subjected to a mechanical polishing treatment by means of an elastic polishing member giving a small strain to the base metal surface before the magnetically smoothening treatment. In accordance with the invention, it is necessary for the silicon-containing steel sheet to have a crystal structure where crystal grains having an inclination angle of the {110} face of not more than 10° with respect to the sheet surface (or base metal surface) are included in an amount of not less than 80 vol% per total volume. When the inclination angle of the {110} face exceeds 10°, the surface after the electrolytic treatment in the halide bath changes from the network texture to scale-like or further fine-grained texture and the magnetic smoothness is lost. Furthermore, when the ratio of crystal grains in such a preferred orientation is less than 80 vol%, the magnetically non-smooth surface becomes large and the iron loss is increased by the electrolytic treatment. Moreover, the starting sheet for the production of such silicon-containing steel sheets is obtained by subjecting a slab for silicon steel sheet to a hot rolling and further to cold rolling with intermediate annealing where appropriate to provide a final sheet thickness in the usual manner and then subjecting the cold rolled sheet to decarburization annealing and further to a final annealing. In the final annealing, an annealing separator composed mainly of MgO is used for simultaneously forming a forsterite coating, but a separator consisting essentially of Al₂O₃ and containing inert MgO, Ca or Sr compound may be used so as not to form the forsterite coating. Further, in the sheet surface according to the invention, the crystal grain boundaries are in the form of steps or groove-like concave portions of not less than 0.4 µm as Rmax, and the surfaces of these crystal grains exhibit a pattern comprising adjoining recesses bordered by convex portions, i.e. a graining pattern as obtained by electrolytic etching. Thus, the adhesion to the coating formed on the sheet surface is increased by the convex portion border and the crystal grain boundary of the concave portion and also the width of the magnetic domain becomes fine by means of the stepped or groove-like grain boundary thus improving the iron loss value. Sheets having such a graining pattern are characterized by having a magnetic flux density (as measured at 1,000 Am) higher by about 200-300 gauss as compared with sheets having a mirror surface obtained by the conventional electrolytic polishing. Moreover, the reason why the depth of the step or groove-like concave portion at the crystal grain boundary is limited to not less than 0.4 µm as Rmax is due to the fact that when the depth is less than 0.4 µm, little improvement in the iron loss property and the adhesion property occurs. The magnetically smooth graining pattern (or texture) is easily obtained by subjecting the silicon steel sheet to an anodic electrolytic treatment in an aqueous solution containing at least one water soluble halide or an electrolytic solution containing at least one water soluble halide and a polyether. The presence of the water soluble halides has a magnetically smoothening effect on the final annealed grain oriented silicon steel sheet having {110} crystal face, so that it is desirable to select a proper substance from among these halides bearing in mind the desirability of preventing the precipitation of metal onto the cathode and the like in the actual operation. Further, the concentration of the halide is desirably not less than 20 g/ℓ for ensuring the conductivity of the bath. Moreover, the use of sea water is possible from the viewpoint of its composition and concentration. The polyether can be added for effectively improving the iron loss property when the steel sheet is subjected to the anodic electrolysis while the concentration of the halide is much reduced. This polyether is a linear high polymer compound containing the ether bond (-O-) in its main chain and generally consisting of a repeated unit [MO], wherein M is usually a methylene group, a poly-methylene group or its derivative. Polyethylene glycol -(CH₂CH₂O-) is a typical example of the polyether. The amount of the polyether added is desirably not less than 2 g/ℓ. On the other hand, when the amount is too large, the conductivity of the bath lowers and also the addition effect can not be expected so the upper limit is about 300 g/ℓ. The bath temperature may optionally be selected from room temperature or more. However, when the bath temperature is too high, the evaporation of water becomes conspicuous, so that it is preferably within the range of from room temperature to about 90°C. Futhermore, the current density may be set within a range of from about 5 A/dm² to several hundred A/dm². However, when the bath temperature is low, if the current density exceeds 100 A/dm², the treated surface is apt to become uneven. Thus if it is intended to widen the range of current density, it is sufficient for the bath temperature to be not lower than 40°C. From the viewpoint of reducing the iron loss, it is preferable that the electric quantity of the electrolysis and the removal amount through the electrolysis are not less than 300 coulomb/dm² and not less than 1 µm per surface, respectively. As mentioned above, by means of the invention, the magnetically smoothening effect can be obtained under a very wide range of conditions as compared with the conventional method, which makes the invention advantageous in industrially practical use. The change of the bath as a result of electrolysis reaction will be described by referring to an aqueous solution of NaCl as follows. Anode: Fe+2Cl⁻ → FeCl₂+2e⁻ (1) Cathode: 2Na⁺+2H₂O+2e⁻ → 2NaOH+H₂↑ (2) Bulk: FeCl₂+2NaOH → 2NaCl+Fe(OH)₂↓ (3) That is, FeCl₂ produced by equation (1) and NaOH produced by equation (2) are reacted according to equation (3) to automatically reproduce NaCl. Therefore, the control of the bath composition is fundamentally carried out by removal of Fe(OH)₂ precipitate produced by equation (3), by supplementing the water, and by compensating for the NaCl taken out of the bath by the steel sheet so that it is fairly easy and low in cost as compared with conventional chemical or electrolytic polishing. This is a merit of the invention in industrial practice. In a preferred embodiment of the invention, after the anodic electrolytic treatment in the aqueous halide solution, the halide is washed out from the sheet surface with water, and then the surface is subjected to a brushing treatment with an aqueous solution or suspension of a hydrogen carbonate for further improving the adhesion to a coating as a result of surface cleaning. Suitable hydrogen carbonates include sodium hydrogen carbonate, ammonium hydrogen carbonate, potassium hydrogen carbonate and the like. In the case where an aqueous solution is used, the concentration is desirably not less than 10 g/ℓ because when it is less than 10 g/ℓ, the surface cleaning effect is not sufficient. Moreover, the cleaning effect becomes large as the concentration becomes high, so that the effect is conspicuous when using an aqueous suspension. However, the cleaning effect can be obtained at a concentration of not less than 10 g/ℓ as compared with a brushing treatment with water. For the brushing, a brush roll made of synthetic fiber or natural fiber, a nonwoven cloth roll or the like may advantageously be used. After the brushing, the surface is immediately washed with water and dried, whereby the clean surface can be maintained. Moreover, the surface of the grain oriented silicon steel sheet after the anodic electrolytic treatment in the aqueous halide solution is very active, so that when it is exposed to air, rust is apt to be easily produced. The occurrence of rust degrades not only the appearance but also the adhesion to the coating and hence brings about a degradation of magnetic properties. In order to prevent the occurrence of rust, therefore, it is effective to add a corrosion preventing agent (inhibitor) to the electrolytic bath. Inhibitors can be roughly classified into inorganic substances and organic substances, and either substance may be used. As inorganic inhibitor, mention may be made of chromates, nitrites, phosphates and so on while, as the organic inhibitor, mention may be made of organic sulfur compounds, amines having a polar amino group (-NH₂) in its molecule, and so on. The concentration of the inhibitor varies in accordance with the kind of inhibitor used, but it is usually within a range of about 0.1-50 g/ℓ. Moreover, when the grain oriented silicon steel sheet is subjected to the anodic electrolytic treatment in the aqueous halide solution, a great amount of Fe(OH)₂ precipitate is produced in the bath. If the precipitated amount exceeds about 2%, the viscosity of the solution is too high and normal electrolysis becomes impossible. Particularly, when using an electrolytic solution consisting mainly of an alkali metal halide, a constant amount of halogen ion is caught by the precipitate of Fe(OH)₂, so that the pH of the bath tends to increase. When the pH exceeds 13, a uniform electrolyzed surface can not be obtained. In order to prevent the occurrence of these problems, it is effective to add a pH buffering agent or a chelating agent chelating Fe ion. As the pH buffering agent, mention may be made of phosphoric acid, citric acid, boric acid, acetic acid, glycine, maleic acid and so on. As the chelating agent for Fe ion, mention may be made of oxyacids such as citric acid, tartaric acid, glycolic acid and the like; various amines; polyaminocarboxylic acids such as EDTA and the like; polyphosphoric acids and so on. The amount of this agent added is preferably within a range of about 1-100 g/ℓ. Further, in order to prevent the rise of pH in the bath during the electrolysis, it is effective to oxidize the precipitate of Fe(OH)₂ into Fe(OH)₃. In this case, there can be adopted air oxidation by forcedly enhancing contact between the bath and air, the addition of oxidising agent such as H₂O₂ or the like to the bath, and the like. Moreover, according to the invention, it is preferred that prior to the anodic electrolytic treatment the oxide layer produced on the sheet surface through the final annealing is removed by subjecting the sheet to a pretreatment to thereby provide a uniform surface. This is because the presence of the oxide layer is very harmful for promoting the electrolysis reaction when the steel sheet is subjected to the anodic electrolytic treatment and prevent the object of the invention being achieved. Although pickling can be considered as a means for removing the oxide layer, the unevenness of the surface increases and consequently surface smoothening should be carried out for such an uneven surface. Thus pickling is not favorable in industry because the thickness of the base metal is required to be several times more than the usual thickness. Furthermore, smoothening through mechanical polishing other than pickling can be considered. However, when the oxide layer is removed from the sheet surface by conventional mechanical polishing with a polishing roll or brush, or conventional shot blasting, strain is undesirably produced on the surface of the base metal which strain considerably degrades the magnetic properties of the silicon steel sheet. Therefore, in accordance with an embodiment of invention, mechanical polishing using an elastic polishing member, which produces little strain and thus does not cause degradation of the magnetic properties as in conventional mechanical polishing, is preferably adopted as a means for removing the oxide layer. The term "elastic polishing member" used herein means a roll or brush consisting of an elastic substrate having a compressive Young's modulus of not more than 10⁴ kg/cm² and abrasive grains carried thereon. In the elastic polishing member, it is preferred for the abrasive grains to have a grain size number of not less than #100 (according to JIS R6001). Furthermore, it is advantageous to vertically apply a pressure of not more than 3 kg/cm² to the steel sheet surface. Such a pressure value can not be attained when using the conventional mechanical polishing. Moreover, the abrasive grains are not necessarily bonded to the substrate. For instance, these abrasive grains may be dispersed into a polishing liquid as a free abrasive grain. By means of the invention, an effective improvement of the magnetic properties can be attained by subjecting the silicon-containing steel sheet to such a series of the above treatments. Furthermore, the magnetic properties can be much improved by forming a tension applying type coating on the graining pattern surface produced according to the invention. The tension applying type coating may be the conventionally known phosphate series coating containing collidal silica, or may be formed by a dry or wet plating. That is, a coating of at least one layer composed of at least one of nitrides and/or carbides of Ti, Nb, Si, V, Cr, Al, Mn, B, Ni, Co, Mo, Zr, Ta, Hf and W and oxides of Al, Si, Mn, Mg, Zn and Ti is strongly adhered to the steel sheet surface by CVD process, PVD process (ion plating, ion implantation or the like), plating or the like. Moreover, any substances having a low thermal expansion coefficient and strongly bonding to the steel sheet maybe used as a material of the above coating in addition to the above coatings. That is, such a substance is sufficient to have a function giving a tension to the steel sheet surface owing to the difference of thermal expansion coefficient. If the layer of this substance has poor insulating properties, an insulation coating may be further formed as a top coat. Moreover, a tension applying type, low thermal expansion insulation coating may be formed on the steel sheet surface, if necessary. In Fig. 1a there are shown the improved margin of iron loss after the silicon steel sheet mainly consisting of {110} crystal face is subjected to an anodic electrolytic treatment in an aqueous NaCl solution as the water soluble halide. For comparison, the improved margin of iron loss in the grain oriented silicon steel sheet mirror-finished by conventional electrolytic polishing (100 A/dm², 20 seconds) with a mixed acid (CrO₃+10% H₃PO₄) is also shown in Fig. 1a. Furthermore, the change of magnetic flux density is shown in Fig. 1b. As seen from Figs. 1a and 1b, the improved margins of the iron loss and the magnetic flux density are large with the treatment using the halide bath as compared with the conventional electrolytic polishing. Further, when the coercive force Hc before and after the electrolytic treatment is measured in the specimen of fine-grained texture in which the ratio of crystal face existing within 10° from the {110} face is low, Hc was found to be lower by 5% after the electrolytic treatment. In this case, the electrolytic treatment was carried out at a current density of 100 A/dm² for 10 seconds by using an aqueous 10% NaCl solution. Moreover, the improved margins when TiN coating is formed on the sheet surface through ion plating are also shown in Figs. 1a and 1b, from which the good improvement of iron loss and magnetic flux density can be recognized. Although the improvement of iron loss and magnetic flux density has been confirmed from Figs. 1a and 1b, in order to further improve the iron loss and the magnetic flux density, it is necessary that the anodic electrolytic treatment is carried out in the aqueous solution of the halide at a smaller dissolved amount. In this connection, the inventors have made studies with respect to the additives to be added to the aqueous halide solution and found that it is effective to use an electrolytic bath of the halide containing polyether. Fig. 4 shows the relationship between the dissolved thickness of steel sheet and the change of iron loss (W 17/50 ) (i.e. improved amount of iron loss) when a grain oriented silicon steel sheet of 0.23 mm in thickness after the final annealing and containing no forsterite coating is subjected to an anodic electrolytic treatment at a current density of 100 A/dm² in an aqueous solution of 100 g/ℓ NaCl as an electrolytic bath (bath temperature 60°C). Moreover, the dissolved thickness is changed by varying the electrolytic time. Furthermore, there are used three electrolytic baths, VIZ. a first one containing no additive, a second one containing 25 g/ℓ of polyethylene glycol having a molecular weight of about 600, and a third one containing 26 g/ℓ of polyethylene glycol having a molecular weight of about 2,000. As can be seen from Fig. 4, the dissolved thickness of the steel sheet required for obtaining the same improved amount of iron loss by the addition of polyethylene glycol can be reduced to about 1/2 that required when there is no additive. As a result, the reduction of the necessary dissolved thickness brings about industrially large merits such as reduction of power cost, increase of product yield, improvement of productivity, reduction of bath maintenance cost accompanied with reduction in the increase of Fe content in the bath and the like. Moreover, Fig. 4 shows the effect of using polyethylene glycol with molecular weight of 600 or 2,000, but it has been confirmed that a similar result is obtained by using polyethylene glycol of different molecular weight. Therefore, the molecular weight of the polyethylene glycol is not particularly restricted in accordance with this embodiment of the invention. As to the improved margin of iron loss in the case where the electrolytic bath contains aqueous halide solution and polyether, the same experiment as in Fig. 1 was repeated to obtain the results shown in Fig. 5. In this case, the aqueous NaCl solution (concentration 100 g/ℓ) containing 25 g/ℓ of polyethylene glycol with a molecular weight of 600 is used as an electrolytic bath and the electrolytic conditions are 100 A/dm² and 20 seconds. The other conditions are the same as in the experiment of Fig. 1. Furthermore, the improved margin of iron loss in the case of the formation of TiN coating after the electrolytic treatment is also shown in Fig. 5. In any case, the good effect of improving the iron loss is recognized. Although the mechanism of improving the iron loss by the addition of polyether is not clear, judging from the fact that the effect is developed irrespective of the molecular weight, it is believed that the polyether shows surface activity and promotes the magnetically smoothening of the steel sheet through chlorine ions, which is not dependent upon the mere viscosity rise of the bath or the like. In the use of the silicon-containing steel sheet, an insulation coating is frequently provided on the sheet surface. Furthermore, in order to further improve the magnetic properties such as magneto-striction, iron loss and the like, tension may be applied to the insulation coating, or a double layer of tension coating and insulation coating may be formed on the sheet surface. However, it is difficult to apply these coatings to the surface of sheets obtained by conventional mirror finishing as a means for obtaining a magnetically smooth surface and the adhesion of the surface to these coatings is poor. In this connection, the sheet surface according to the invention not only has a convex portion at the boundary of the network grains but also exhibits a step- or groove-like concave portion at the boundary of the crystal grain, so that it has very excellent adhesion to coatings. In the following Table 1 there are shown the results of the adhesion property measured when a phosphate tension coating or a TiN coating by ion plating (thickness: 0.30 mm) is formed on each of a grain oriented silicon steel sheet obtained by electrolytic polishing in a solution of H₃PO₄+CrO₃ (comparative mirror-finished product) and a grain oriented silicon steel sheet obtained by the electrolytic treatment in NaCl (invention product). Moreover, the adhesion property is evaluated by winding the sheet on a cylinder of 20 mm in diameter as follows: that is, no peeling of the coating indicates good adhesion property (100%), while the local occurrence of peeling of the coating indicates poor adhesion property. Adhesion property (%) phosphate tension coating TiN Invention product 100 100 Comparative product 9 77 As can be seen from Table 1, according to the invention, the adhesion to the coating is very excellent. Although the reason why the iron loss of the products according to the invention are low as compared with those of the products obtained by the conventional electrolytic or chemical polishing is not completely elucidated, it is believed that a highly geometrical smoothness is not always required for obtaining a magnetically smooth surface and that according to the invention, the grain boundary is in the form of a step or groove-like concave portion to cause magnetic domain refinement and hence gives rise to the reduction in iron loss. Furthermore, the reason why adhesion to the coating is improved by the brushing treatment using a hydrogen carbonate after the electrolytic treatment is due to the fact that the sheet surface is cleaned as previously mentioned. Since the reaction of equation (3) is caused even on the sheet surface after the electrolytic treatment, amorphous hydrated iron oxide is thinnly produced on the whole surface of the sheet and this has a loose chemical bond to the base metal, so that it can not be completely removed by the simple brushing treatment. Furthermore, acid insoluble components called smut also exist on the sheet surface. Moreover, since the grain oriented silicon steel sheet used as a starting sheet contains a large amount of Si, it is apt to be easily oxidized and a slight amount of chlorine ion adsorbed on the sheet surface always tends to promote the corrosion of this surface. For these reasons, the surface after the electrolytic treatment is not a complete metallic surface. On the other hand, the cleaning effect of the sheet surface can not be obtained merely by immersing the steel sheet after the electrolytic treatment in an aqueous solution or suspension of a hydrogen carbonate. As mentioned above, it is difficult to completely remove the surface stain even by a simple brushing treatment with water. Therefore, a means for removing the hydrated iron oxide from the sheet surface is provided by using hydrogen carbonate whereby the brushing treatment can be performed to sufficiently clean the surface. Fig. 6 shows the values of iron loss at each stage when the final annealed grain oriented silicon steel sheet is subjected to a mechanical polishing with a nonwoven cloth roll at a vertical polishing pressure of not more than 2 kg/cm² or a belt at a vertical polishing pressure of 6 kg/cm² using different grain size of abrasive grains to remove the oxide, subjected to an anodic electrolytic treatment in NaCl solution (dissolved amount 4 µm; concentration 100 g/ℓ; current density 300 A/dm²), and further provided on the surface with a tension coating of TiN (thickness 1 µm). As seen from Fig. 6, there is a great difference in the iron loss after the electrolytic treatment between the use of the nonwoven cloth roll (elastic polishing member) according to an embodiment of the invention and the use of the belt (non-elastic polishing member) as a comparative method. According to an embodiment of the invention, the sheet is preferably polished at an amount of not less than 0.5 µm per surface by the above mechanical polishing. The following examples are given in illustration of the invention and are not intended as limitations thereof. Example 1 A hot rolled sheet of silicon steel containing C: 0.03%, Si: 3.3%, Mn: 0.06%, Se: 0.02% and Sb: 0.02% was cold rolled to a thickness of 0.23 mm and then subjected to a decarburization annealing. A part of the thus annealed sheet was left as a comparative sheet A, while the remaining sheet was coated with a slurry of an annealing separator consisting essentially of Al₂O₃ (containing 0.1% of NaCl), coiled and subjected to a final annealing as a comparative sheet B. A part of the comparative sheet B was rendered into a mirror finished surface by emery and buff polishing as a comparative sheet C, while another part of the comparative sheet B was rendered into a mirror finished surface by the electrolytic polishing in a mixed solution of chromic acid and phosphoric acid (1:9) as a comparative sheet C', and a further part of the comparative sheet B was pickled with sulfuric acid to remove the surface layer by 4 µm as a comparative sheet D. Further, a part of the sheet B was immersed in an electrolytic solution of NaCl having a concentration of 75% (comparative sheet E), while the remaining portion of the sheet B was immersed in the above electrolytic solution and subjected to an anodic electrolytic treatment at 100 A/dm² for 10 seconds by using a stainless steel cathode (acceptable sheet). Moreover, the comparative sheet A was subjected to the same electrolytic treatment. The magnetic properties were measured with respect to these sheets. Furthermore, the morphology of the sheet surface was also observed. The measured results are shown below. Comparative sheet A: Since Hc increased 5% before and after the electrolytic treatment, magnetically smoothening could not be achieved. Further, the surface morphology was substantially a fine-grained texture (not less than 90%). Comparative sheet B: The iron loss of the sheet after the final annealing was W 17/50 =0.95 W/kg. As a result of the examination of 30 secondary grains, crystal grains existing within 10° with respect to {110} face were 100%. Comparative sheet C: The iron loss W 17/50 of the sheet after the mirror polishing with emery and buff is was 1.32 W/kg. Comparative sheet C': The iron loss after the electrolytic polishing was 0.86 W/kg. Comparative sheet D: The iron loss was 1.01 W/kg. Comparative sheet E: The iron loss was 0.97 W/kg. Acceptable sheet: The iron loss was 0.80 W/kg and the texture was a network pattern (graining pattern). Then, TiN of 1 µm in thickness was deposited on each of the comparative sheets B, C, C', D and the acceptable sheet by ion plating to obtain the following results: Sheet B Sheet C Sheet C' Sheet D Acceptable sheet W 17/50 (W/kg) 0.87 1.00 0.76 0.93 0.69 As to the adhesion property, the acceptable sheet and the comparative sheets B and D were good, but peeling was observed in the comparative sheets C and C' according to the bending test of 20 mmΦ. Example 2 A hot rolled sheet of silicon steel containing C: 0.03%, Si: 3.2%, Mn: 0.08%, S: 0.02% and Al: 0.02% was cold rolled to a thickness of 0.30 mm, subjected to a decarburization annealing, coated with an annealing separator of MgO and subjected to a final annealing. The iron loss W 17/50 after the final annealing was 1.02 W/kg. Further, when 30 crystal grains were measured by X-ray, the displacement of orientation from {110} face was not more than 10°. After the forsterite coating was removed from the surface of the final annealed sheet by pickling, the sheet was subjected to an anodic electrolytic treatment in a 100% solution of NH₄Cl by using the sheet as an anode under conditions of 50 A/dm² and 2,000 coulomb/dm², whereby a sheet having a beautiful graining surface texture and an iron loss W 17/50 of 0.83 W/kg was obtained. Further, when a Si₃N₄ coating (thickness 1 µm) was formed by ion plating, the iron loss W 17/50 reduced to 0.71 W/kg. Moreover, the adhesion property to the coating was good. Example 3 A hot rolled sheet of steel containing C: 0.043%, Si: 3.35%, Se: 0.018%, Mo: 0.013% and Sb: 0.025% was subjected to two cold rolling stages with an intermediate annealing to a thickness of 0.23 mm. Then, the cold rolled steel sheet was subjected to decarburization and primary recrystallization annealing in a wet hydrogen atmosphere at 830°C, coated with a slurry of an annealing separator consisting essentially of MgO and Al₂O₃, coiled and subjected to final annealing. After oxide coating was removed from the surface of the test sheet by pickling, the test sheet was subjected to electrolysis in an aqueous solution of a chloride as shown in the following Table 2 and then the iron loss (W 17/50 ) was measured. For comparison, there were conducted a mirror polishing process using phosphoric acid and chromic acid (Comparative Example 14), a mirror polishing process using only phosphoric acid (Comparative Example 15) and a mechanical polishing process (emery #1000 finish: Comparative Example 16). As is well-known, the process using phosphoric acid and chromic acid exhibits a large improvement of iron loss, which is not still better than that of the invention. Furthermore, the mirror finished surface using phosphoric acid is fairly poor in iron loss as compared with that of the invention. On the other hand, the iron loss is rather degraded by the mechanical polishing process. After a tension coating of TiN was formed on the surface of each of these sheets by ion plating, the bending adhesion test using a rod of 20 mm in diameter was carried out, and consequently the acceptable examples No. 1-13 were good (100% no peeling), the acceptable example No. 14 was slightly poor (20% peeling), and the comparative examples No. 15 and 16 were poor (No. 15 80% peeling, No. 16 100% peeling). The measured results are shown in Table 2. As can be seen from Table 2, the improvement of iron loss was large in all acceptable examples according to the invention. On the contrary, in the comparative examples treated outside the conditions of the invention, the electrolytic treating effect was small, and the improvement of iron loss was slight. Example 4 A hot rolled sheet of steel containing C: 0.059%, Si: 3.35%, Mn: 0.077%, Al: 0.024%, S: 0.023%, Cu: 0.1% and Sn: 0.015% was cold rolled twice with an intermediate annealing to a thickness of 0.23 mm. Then, the cold rolled sheet was subjected to decarburization and primary recrystallization annealing in a wet hydrogen atmosphere at 840°C, coated with a slurry of an annealing separator consisting essentially of Al₂O₃ and MgO, coiled, and subjected to a final annealing. Thereafter, the unreacted annealing separator was removed and the sheet was subjected to a flat annealing to correct the curling of the coil, whereby a test sheet was prepared. After the oxide coating was removed from the surface of the test sheet by pickling, the sheet was subjected to an electrolysis treatment in an aqueous solution of a chloride shown in the following Table 3, and then the iron loss (W 17/50 ) was measured. The measured results are shown in Table 3. No. 21 is a comparative example showing the case where the surface was rendered into a mirror state by electrolytic polishing with phosphoric acid and chromic acid, wherein the iron loss was fairly poor as compared with that of the invention. Also, No. 22 is a comparative example showing mirror electrolytic polishing with phosphoric acid and having a very narrow improved margin of iron loss. Example 5 The same test sheet as in Example 3 was provided. It was pickled to remove the oxide coating from the surface of the sheet, subjected to an electrolytic treatment in an aqueous solution of a chloride containing polyethylene glycol as shown in the following Table 4, and then the iron loss (W 17/50 ) was measured. For comparison, electrolytic polishing with phosphoric acid and chromic acid was also performed. The measured results of iron loss are also shown in Table 4. As can be seen from Table 4, the products according to the invention have a greatly improved margin of iron loss as compared with the product obtained by conventionally known electrolytic polishing with phosphoric acid and chromic acid. Furthermore, when each of these sheets was provided on its surface with a tension coating of TiN by ion plating and subjected to a bending adhesion test using a rod of 20 mm in diameter, the acceptable examples No. 1-13 according to the invention had good adhesion (no peeling), while the adhesion of comparative No. 14 was poor. Example 6 The same test sheet as in Example 4 was provided. It was pickled to remove the oxide coating from the surface of the sheet and subjected to an electrolytic treatment in an aqueous solution of a chloride as shown in the following Table 5, and then the iron loss (W 17/50 ) was measured. The measured results are also shown in Table 5. Moreover, No. 9 is a comparative example of mirror finishing by electrolytic polishing with phosphoric acid and chromic acid. As can be seen from Table 5, the iron loss value in the acceptable examples No. 1-8 according to the invention is considerably low as compared with the comparative No. 9. Example 7 The same test sheet as in Example 3 was provided. It was pickled to remove the oxide coating from the surface of the sheet and then subjected to an anodic electrolytic treatment in an aqueous solution of a chloride as shown in the following Table 6. Thereafter, each sheet was washed with water and then subjected to a brushing treatment with a nylon brushing roll while applying an aqueous solution or suspension of a hydrogen carbonate to the sheet. Then, each sheet was washed with water, dried, subjected to a coating as shown in Table 6, and then subjected to a strain relief annealing at 800°C for 3 hours. The magnetic properties and adhesion property of the thus obtained products were evaluated to obtain the results as shown in Table 6. For comparison, the same measurement was performed in cases where no brushing treatment was carried out (No. 8), brushing with water was carried out (No. 9), and electrolytic polishing with phosphoric acid and chromic acid was carried out (No. 10) to obtain the results as shown in Table 6. The acceptable examples were carried out according to the invention and, in these cases, the adhesion property was excellent and the iron loss value was good. Comparative Nos. 8 and 9 were also carried out in accordance with the invention but included no brushing treatment with hydrogen carbonate. In this case the adhesion was poor and the magnetic properties were slightly poor. Comparative No. 10 involved electrolytic polishing with phosphoric acid and chromic acid and was not in accordance with the invention. In this case the adhesion property and the magnetic properties were much poorer. Example 8 The same test sheet as in Example 4 was provided. It was pickled to remove the oxide coating from the surface of the sheet and then subjected to an anodic electrolytic treatment in an aqueous solution of a chloride as shown in the following Table 7. Thereafter, the sheet was washed with water and subjected to a brushing treatment with a nylon brushing roll while applying an aqueous solution or suspension of a hydrogen carbonate to the sheet. Then, the sheet was washed with water, dried, subjected to a coating as shown in Table 7 and further to a strain relief annealing at 800°C for 3 hours. The magnetic properties and adhesion property of the thus obtained product were evaluated to obtain the results shown in Table 7. For comparison, the same measurement was carried out without conducting a brushing treatment (No. 8), conducting the brushing with water (No. 9), and conducting chemical polishing with a mixed solution of H₂O₂ and HF (No. 10) to obtain the results shown in Table 7. The acceptable examples were carried out according to the invention. In these cases the adhesion property was excellent and the iron loss value was good. Comparative Nos. 8 and 9 were also carried out in accordance with the invention but included no brushing treatment with hydrogen carbonate. In this case the adhesion property was poor and the magnetic properties were slightly poor. Further in the case of Comparative No 10 which was not in accordance with the invention since chemical polishing with a mixed solution of H₂O₂ and HF (No. 10) was carried out, the adhesion property and the magnetic properties were very poor. Example 9 The same test sheets as in Examples 3 and 4 were provided. They were pickled to remove the oxide coating from the surface of the sheet and subjected to an anodic electrolytic treatment in an aqueous solution of a chloride containing polyethylene glycol as shown in the following Table 8. Thereafter, the sheets were washed with water and subjected to a brushing treatment with a nylon brushing roll while applying an aqueous solution or suspension of a hydrogen carbonate. Then, the sheets were washed with water, dried, subjected to a coating as shown in Table 8 and further to a strain relief annealing at 800°C for 3 hours. The magnetic properties and adhesion property of the thus obtained products were evaluated to obtain the results shown in Table 8. For comparison, the same measurement was carried out in cases where the brushing treatment was conducted only with water (Nos. 9 and 10) and where electrolytic polishing with phosphoric acid and chromic acid was effected (Nos. 11 and 12) to obtain the results shown in Table 8. The acceptable examples were carried out according to the invention and, in these cases, the adhesion property was excellent and the iron loss value was good. Comparative Nos. 9 and 10 were also in accordance with the invention but included no brushing treatment with hydrogen carbonate. In this case the adhesion property was poor and the magnetic properties were slightly poor. In the case of Comparative Nos 11 and 12 which were not in accordance with the invention since electrolytic polishing with phosphoric acid and chromic acid was carried out (Nos. 11 and 12), the adhesion property and the magnetic properties were very poor. Example 10 The same test sheet as in Example 3 was provided. It was pickled to remove the oxide coating from the surface of the sheet and then subjected to an anodic electrolytic treatment in an aqueous solution of a halide as shown in the following Table 9, and thereafter the iron loss (W 17/50 ) was measured. For comparison, electrolytic polishing with phosphoric acid and chromic acid (No. 9) was carried out to obtain the iron loss shown in Table 9. As can be seen from Table 9, the improved margin of iron loss is large in the acceptable examples according to the invention as compared with that of the comparative example. Example 11 The same test sheet as in Example 3 was provided. It was pickled to remove the oxide coating from the surface of the sheet and then subjected to an anodic electrolytic treatment in an aqueous solution of a halide containing polyethylene glycol as shown in the following Table 10, and thereafter the iron loss (W 17/50 ) was measured. For comparison, electrolytic polishing with phosphoric acid and chromic acid (No. 7) was carried out to obtain the iron loss shown in Table 10. As can be seen from Table 10, the improved margin of iron loss was large in the acceptable examples according to the invention as compared with that of the comparative product obtained by the conventionally known electrolytic polishing with phosphoric acid and chromic acid. Example 12 The same test sheet as in Example 3 was provided. It was pickled to remove the oxide coating from the surface of the sheet and then subjected to an anodic electrolytic treatment in an aqueous solution of a halide as shown in the following Table 11. Thereafter, the sheet was washed with water and subjected to a brushing treatment with a nylon brushing roll while applying an aqueous solution or suspension of a hydrogen carbonate. Then, the sheet was washed with water, dried, subjected to a coating as shown in Table 11 and further to a strain relief annealing at 800°C for 3 hours. The magnetic properties and adhesion property of the thus obtained products were evaluated to obtain the results shown in Table 11. For comparison, the same measurement was carried out without a brushing treatment (No. 6) and with a brushing treatment using only water (No. 7) to obtain the results shown in Table 11. In Examples 1 to 5 which were carried out according to the invention, the adhesion property was excellent and the iron loss value was good. In Examples 6 and 7, which are also in accordance with the invention, the adhesion property is poor and the iron loss value is not quite so good because of the omission of the brushing with hydrogen carbonate. Example 13 The same test sheet as in Example 3 was provided. It was pickled to remove the oxide coating from the surface of the sheet, subjected to an anodic electrolytic treatment in an aqueous solution of a halide containing an inhibitor as shown in the following Table 12, washed with water and dried, and thereafter the iron loss (W 17/50 ) was measured and also the corrosion resistance in wet air was examined. The same measurement was carried out with respect to sheets treated in the bath containing no inhibitor (Nos. 6 and 7). The measured results are shown in Table 12. As can be seen from Table 12, when the inhibitor is added to the bath, there is no problem in the improved margin of the iron loss, and particularly the corrosion resistance is excellent and rust hardly occurs. Example 14 The same test sheet as in Example 3 was provided. It was pickled to remove the oxide coating from the surface of the sheet and subjected to an anodic electrolytic treatment of a halide containing a pH buffering agent or a chelating agent as shown in the following Table 13, and then the iron loss (W 17/50 ) was measured and also the total electrolytic time until the surface became non-uniform and the gloss was lessened, (i.e. when the electrolytic treating capability was reduced) was measured. For comparison, the same measurement was carried out in the case where the bath contained no pH buffering agent or chelating agent (No. 6 and 7). The measured results are shown in Table 13. As can be seen from Table 13, when adding the pH buffering agent or the chelating agent, there is no problem in the improved margin of the iron loss value, and particularly stable electrolysis can be attained over a long time. Example 15 The same test sheet as in Example 3 was provided. It was pickled to remove the oxide coating from the surface of the sheet and subjected to an anodic electrolytic treatment in an aqueous solution of a halide containing an inhibitor or a pH buffering agent as shown in the following Table 14. Thereafter, the sheet was washed with water and subjected to a brushing treatment with a nylon brushing roll while applying an aqueous solution or suspension of a hydrogen carbonate. Then, the sheet was washed with water, dried, subjected to a coating as shown in Table 14 and further to a strain relief annealing at 800°C for 3 hours. The magnetic properties, adhesion property, corrosion resistance and electrolytic time of the thus obtained products were evaluated to obtain the results shown in Table 14. For comparison, the same measurement was carried out in the case where no brushing treatment was carried out (No. 11) and where the brushing treatment was effected only with water (No. 12) to obtain the results shown in Table 14. When the brushing treatment with hydrogen carbonate was carried out the adhesion property was very excellent and the iron loss value was good. Further, when the inhibitor was added, the corrosion resistance became particularly good, and also when the pH buffering agent or the chelating agent was added, stable electrolysis could be conducted over a long time. Example 16 A hot rolled sheet of silicon steel containing C: 0.032 wt% and Si: 3.3 wt% and MnSe and Sb as an inhibitor was cold rolled to a thickness of 0.23 mm in the usual manufacturing process for grain oriented silicon steel sheet and subjected to a final annealing using alumina as an annealing separator. When 50 crystal grains were examined after the final annealing, the crystal grains of (110) [001] orientation (displacement angle within 5°) were 94%. Then, the sheet was subjected to a mechanical polishing with an elastic polishing member in the form of a nonwoven cloth roll using abrasive alumina grains (vertical pressure: 1 kg/cm²) or to mechanical polishing with a belt or to pickling (10% H₂SO₄, 80°C) to thereby remove the oxide from the surface. Then, the sheet was subjected to an electrolytic treatment in an aqueous solution of 100 g/ℓ of NaCl (current density: 100 A/dm²) by using this sheet as an anode for 10 or 20 seconds, and then a tension coating of TiN was formed thereon. The iron loss after each treatment was measured to obtain the results shown in the following Table 15. As can be seen from Table 15, the sheets mechanically polished with a brush roll or non woven roll and abrasive of size number not less than #100 exhibit good properties even after the electrolytic treatment and the formation of the tension coating. On the other hand, when pickling is carried out as a treatment for the removal of oxide, the same level of the properties can be obtained by taking a long electrolytic time, but in this case the dissolved thickness of the sheet becomes very large. Example 17 A hot-rolled sheet of silicon containing C: 0.31 wt% and Si: 3.2 wt% and AlSn and MnS as an inhibitor was cold rolled to a thickness of 0.23 mm in the usual manufacturing process for grain oriented silicon steel sheet and subjected to a final annealing using MgO as an annealing separator. When 50 crystal grains were examined after the final annealing, the crystal grains of (110) [001] orientation (displacement angle within 5°) were 100%. Then, the sheet was subjected to a mechanical polishing with a nonwoven cloth roll using #1500 abrasive grains (vertical pressure: 1 kg/cm²) to thereby remove the oxide from the surface. Then, the sheet was subjected to an electrolytic treatment in an aqueous solution of 100 g/ℓ of NaCl or 50 g/ℓ of NH₄Cl (current density: 80 A/dm²) by using this sheet as an anode for 10 seconds, and then a tension coating of Si₃N₄ was formed thereon. For comparison, the same final annealed sheet as mentioned above was subjected to a mechanical polishing with a nonwoven cloth roll containing #60 abrasive grains or a belt roll bonded with #1000 abrasive grains and then treated in the same manner as mentioned above. The iron loss after each treatment was measured to obtain the results shown in the following Table 16. As can be seen from Table 16, the sheet polished with an elastic polishing member and abrasive of grain size number not less than #100 exhibits good properties even after the electrolytic treatment and the formation of the tension coating. As mentioned above, by means of the invention, silicon-containing steel sheets having excellent iron loss properties can be obtained stably and cheaply, so that their industrialization can easily be realized. Furthermore, the adhesion property of the sheet to coatings is good.

Claims: A silicon-containing steel sheet having a crystal structure wherein crystal grains having an inclination angle of {110} face of not more than 10° with respect to the sheet surface are included in an amount of not less than 80 vol%, characterised in that the surfaces of these crystal grains exhibit at said sheet surface a graining pattern wherein the boundaries between these crystal grains are in the form of steps or groove-like concave portions having a maximum height Rmax of not less than 0.4µm and in that a tension-applying type of insulation coating is provided on said sheet surface. A method of producing a silicon-containing steel sheet having a low iron loss, which method comprises subjecting a grain oriented silicon steel sheet after final annealing to a magnetically smoothening treatment by electrolysis in an aqueous solution containing at least one water soluble halide selected from HCl, NaCl, KCl, NH₄Cl, MgCl₂, CaCl₂, AlCl₃, NaF, KF, NH₄F, HBr, NaBr, KBr, MgBr₂, CaBr₂, NH₄Br, HI, NaI, KI, NH₄I, CaI₂, MgI₂, H₂SiF₆, MgSiF₆, (NH₄)₂SiF₆, HBF₄, NH₄BF₄ and NaBF₄. A method according to claim 2, wherein said aqueous solution further contains a polyether. A method according to claim 2 or 3, wherein said aqueous solution further contains a corrosion preventative agent. A method according to claim 2, 3 or 4, wherein said sheet is subjected to a brushing treatment in an aqueous solution or suspension of a hydrogen carbonate after said magnetically smoothening treatment. A method according to claim 2, 3, 4 or 5, wherein said sheet is subjected to a mechanical polishing treatment by means of an elastic polishing member to give a small strain to said sheet before said magnetically smoothening treatment.

Inventors: ISHITORI HIROTAKE C O KAWASAKI, KAMI TIKARA C O KAWASAKI STEEL, KOBAYASHI YASUHIRO C O KAWASAK, NISHIIKE UJIHIRO C O KAWASAKI, SUJITA SHIGEKO C O KAWASAKI ST, ISHITORI, HIROTAKE, KAMI, TIKARA, KOBAYASHI, YASUHIRO, NISHIIKE, UJIHIRO, SUJITA, SHIGEKO, Ishitori, Hirotake, c/o Kawasaki Steel Corp., Kami, Tikara, c/o Kawasaki Steel Corp., Kobayashi, Yasuhiro, c/o Kawasaki Steel Corp., Nishiike, Ujihiro, c/o Kawasaki Steel Corp., Sujita, Shigeko, c/o Kawasaki Steel Corp.
Applicants: KAWASAKI STEEL CO, KAWASAKI STEEL CORPORATION
Application year: 1988
Main classification: C21D8
IPC codes: H01F1, C21D8