Grain-oriented electrical steel sheet having improved glass film properties and low watt loss and a process for producing same

Abstract: The adhesiveness of the glass film and the watt loss of a grain-oriented electrical steel sheet are improved by partially protruding the glass film into the steel sheet. To attain the partial protrusion, the steel sheet is subjected, prior or subsequent to decarburization annealing, to a treatment of the surface thereof to form unevennesses, by optical or mechanical means, e.g., brush rolling, buff polishing, marking-off, sand papering, and grinding.

Description: The present invention relates to a grain-oriented electrical steel sheet having improved glass film properties and a low watt loss, and a method for producing the same. Grain-oriented electrical steel sheet is mainly used for the cores of electrical appliances, such as transformers and power generators. For such usage, it is important that the grain-oriented electrical steel sheet have excellent magnetic properties such as watt-loss characteristics and excitation characteristics, and excellent glass film properties. Usually, the grain-oriented electrical steel sheet is produced by the steps of hot-rolling a silicon-steel slab containing 4% or less of silicon, and if necessary, hot-coil annealing; cold-rolling once or twice or more with an intermediate annealing therebetween to obtain a cold-rolled sheet having a final sheet thickness; decarburization-annealing; applying an annealing separator mainly composed of MgO; finishing annealing to develop secondary recrystallized grains having a Goss texture; removing impurities such as S and N; forming a glass film; and finally, heat-flattening and treating with an insulating coating. An improvement of the magnetic properties, particularly the watt loss, together with an improvement of the glass film has been investigated, and it is known, as shown in J. Appl. Phys. 38, (1967), pp 1104 ∼ 1108, that a reduction in the sheet thickness and grain-refinement of a grain-oriented electrical steel sheet effectively reduces the watt loss. Reducing the sheet gauge is an effective method of reducing the watt loss, but the watt loss is increased due to an increase in the eddy current loss when the sheet thickness becomes less than a predetermined thickness. An improvement of the watt loss by grain-refinement is inherently limited by the secondary recrystallization phenomenon, which is utilized to attain a growth of grains having the Goss texture and enhance the orientation degree. For an improvement of a glass film, Japanese Unexamined Patent Publication (Kokai) No. 50-71526, for example, describes the pickling of a grain-oriented electrical steel sheet, which was cold-rolled to a final thickness, in such a manner that 3 g/m² or more of its surface layer is uniformly removed, thereby removing the surface deposits and a superficial part of the steel part thereof, and thus enabling a uniform progression in the decarburization reaction and the oxide-formation reaction. This, in turn, leads to a formation of an MgO-SiO₂ series insulating film having an improved uniformity and adhesiveness after the decarburization annealing, application of an annealing separator, and finishing annealing. Japanese Unexamined Patent Publication (Kokai) No. 57-101673 discloses that, after the decarburization annealing of a grain-oriented electrical steel strip cold-rolled to a final thickness and before the application of the annealing separator, such as MgO and the like, the surface of the steel strip is subjected to grinding or pickling so as to remove 0.025 to 0.5 g/m² of the surface per one side, thereby removing the oxide film constituting the surface layer of a grain-oriented electrical steel sheet. Subsequently, the annealing separator is applied, and finishing annealing is carried out. The thus-formed glass film has a uniform, grey appearance, and an improved adhesiveness. Japanese Unexamined Patent Publication (Kokai) No. 61-96082 proposes to grind and clean the surface of a steel sheet, without forming unevennesses, by a grinding means consisting of soft materials including a carborundum abrasive and an alundum abrasive, thereby enabling a uniform subscale of SiO₂ to be formed during the decarburization annealing and a uniform and dense film to be formed during the finishing annealing. US-A-4 213 804 relates to a process for producing electromagnetic silicon steel having a cube-on-edge orientation. The process comprises the step of removing at least 0.02 µm of surface from each side of the steel. This removal is made to render the steel more susceptible to decarburization and can be accomplished by either mechanical or chemical means. US-A-4 213 804 describes the removing of an oxide film in order to improve the magnetic properties of the steel and consequently aims at a uniform removal of the oxide film from the surface of the steel. In a preferred embodiment, the oxide film is removed by pickling. The prior methods attained improvements in the glass film properties, such as adhesiveness, and in the magnetic properties, but are not satisfactory. When improvements in the glass film properties are attempted by thickening the film, this can be effectively attained by thickening the oxide layer consisting mainly of SiO₂ in the decarburization annealing. In this case, measures such as increasing the ratio of P H₂O/P H₂ and elongating the soaking time become necessary. These measures inevitably lead to an increase in the amount of Fe series oxide formed, such as fayalite (Fe₂SiO₄), FeO, and the like, and thus to a degradation of the qualities of a glass film and an adverse influence on the inhibitors. Particularly, the high Si materials for improving the magnetic properties, especially reducing the watt loss, and materials with a special additive-composing element or compound as inhibitors, are concentrated in the surface layer or are selectively oxidized, with the result that a decarburization failure may occur or the formation of a decarburization-oxidized film may be impaired. An object of the present invention is to provide a grain-oriented electrical steel sheet having improved glass film properties and a low watt loss, and a method for producing the same. A further object of the present invention is to provide a method for producing a grain-oriented electrical steel sheet, which method enables an improvement in the glass film properties and a reduction of the watt loss of high Si materials and materials with special additives, these materials being difficult to produce with a high productivity by the prior art methods. The above objects are achieved by the grain-oriented electrical steel sheet and the method according to the claims. The present inventors discovered that, when an oxide is formed in such a way that it partially protrudes into the steel part or side of a grain-oriented electrical steel sheet, an anchoring effect is generated, thereby dramatically improving the adhesiveness of the glass film and greatly enhancing the tension effect of a film. The discoveries made by the present inventors are hereinafter described in detail. The present inventors carried out investigations into the influence of the shapes of the oxide layer formed on the steel sheet during the decarburization annealing, and of the glass film formed due to the reactions between the oxide layer and the annealing separator, upon the adhesiveness of a glass film, tension at the steel sheet, and the watt loss. The layer, which is constituted at the steel sheet part or side by an oxide(s) of either SiO₂-enriched Fe oxide, an ordinary oxide, or an oxide partially containing forsterite, is hereinafter referred to as the inner oxide layer. The invention is described in detail in connection with the drawings in which: Figures 1(A) and (B) are metal-microscope photographs of the inner oxide layers formed by the method of the present invention and by the comparative method, respectively; Figure 2 illustrates the influence exerted by the depth of the protrusion of the inner oxide layer upon the adhesiveness of a glass film; Figure 3 illustrates the influence exerted by the depth of the protrusion of the inner oxide layer upon the tension of the steel sheet; Figure 4 illustrates the influence exerted by the depth of the protrusion of the inner oxide layer upon the watt loss; Figure 5 illustrates the influence of the distance between the unevennesses formed on a steel sheet upon the watt loss; Figures 6(A) and (B) are similar photographs to those shown in Figs. 1(A) and 1(B), respectively, with regard to the effect of activation by polishing and light pickling; Figure 7 is a drawing of curves of the potential of oxide films in a dilute sulfuric acid; Figure 8 illustrates the influence of polishing the roughness of a steel sheet surface and decarburization annealing-conditions upon the adhesiveness of a glass film; Figure 9 illustrates the influence of polishing the roughness of a steel sheet surface and decarburization annealing-conditions upon the tension of a glass film; and Figure 10 illustrates the influence of polishing the roughness of a steel sheet surface and decarburization annealing-conditions upon the watt loss. In an experiment by the present inventors, the surfaces of cold-rolled steel sheets of grain-oriented electrical steel, which were cold-rolled to a thickness of 0.225 mm, were polished by sheets of sandpaper having different grades to form sharp and minute unevennesses, and then decarburization annealed to form the depth and shapes of the inner oxide layer. Subsequently, an annealing separator mainly composed of MgO was applied and finishing annealing was carried out. The inner oxide layer was, as shown in Fig. 1(B), virtually uniformly thick on steel which had not been polished. Conversely, on steel which had been polished, parts of the inner oxide layer were thicker than the average thickness, and were thick enough to protrude into the steel sheet side. The adhesiveness of the glass film was tested, after the application of the annealing separator and then finishing annealing, by bending to around 10 mm⊘, i.e., more severe than the usual condition of bending to around 20 ∼ 50 mm⊘, to investigate the peel area percentage of the glass film. The results are shown in Fig. 2. In the samples A and B, in which the formed inner oxide layer partially protrudes into the steel sheet side, no peeling occurs and the adhesiveness is extremely good. In addition, the tension imparted to the steel sheet is greatly increased, as shown in Fig. 3. Figure 4 shows that the watt loss is greatly decreased to attain a low watt loss. When a deep inner oxide layer was formed, the glass film formed by the finishing annealing was also deep. The unevenesses do not lead to refinement of secondary recrystallized grains at parts of a grain-oriented electrical steel sheet where the unevenesses are formed. The present invention was completed based on the discoveries as described above and in more detail hereafter. In the present invention, the inner oxide layer partially protrudes into the steel sheet side of a grain-oriented electrical steel sheet by a depth of from 2 to 15 µm, exceeding the average thickness thereof. The term "partially" herein indicates a continuous or discontinuous state of an inner oxide layer having protruding parts at an equal-distance or non-equi distance. Preferably, the above mentioned surface treatment is carried out prior to the decarburization-annealing, by an optical means, particularly irradiation of laser, e.g., YAG or CO₂ laser, and/or mechanical means, particularly brush rolling, buff polishing, marking-off, sand papering, and grinding, and further, sharp and minute unevennesses are formed by the mechanical and/or optical means on the entire surface of the steel sheet within ± 30 degrees to the direction perpendicular to the rolling direction, and at a distance of less than 1 mm. The surface treatment is carried out on one or both of the surfaces of the sheet to form the unevennesses on at least 35%, preferably 50%, by area of the steel sheet. The surface of the steel sheet is activated due to this formation of unevennesses, and a thick oxide layer is formed during the decarburization annealing and finishing annealing and protrudes into the steel sheet via the activated parts. The SiO₂ is enriched in the oxide formed during the decarburization annealing and finishing annealing due to the activating, with the result that the glass film properties are improved, and further, the steel sheet is shielded from the atmosphere during the finishing annealing, thereby suppressing reaction between the inhibitors, such as MnS and AlN, and the annealing atmosphere, and stably maintaining them to a high temperature. Therefore, a stable secondary recrystallization takes place. The SiO₂ enriched layer tends to impede decarburization and may lead to a reduction of the watt loss. Therefore, it is necessary to provide annealing conditions more favourable than those of the conventional method without the activation. The preferred annealing conditions are a temperature of 800 ∼ 860°C, an atmosphere of N₂ , H₂ , or a mixture of N₂ + H₂, and a ratio of P H₂O/P H₂ ≧ 0.40. When sharp and minute unevennesses are formed, the surface layer of the steel sheet is removed by an amount of generally 2.0 g/m² or more, which is greater than the amount of from 0.025 to 0.5 g/m² incurred when removing the oxide film on the surface of a steel sheet as described in Japanese Unexamined Patent Publication (Kokai) No. 57-101673. Therefore, the yield is a little decreased in the present invention, but this is negligible in the light of the dramatic improvement in the glass film properties and watt loss characteristics. A further reduction of the watt loss is attained by setting the distance between adjacent sharp and minute unevennesses to an extremely narrow distance of less than 1 mm, and preferably orienting them to within ± 30 degrees relative to the direction perpendicular to the rolling direction. The unevennesses should be formed before the completion of the decarburization annealing, preferably before starting the decarburization annealing or during the temperature-elevation period in the decarburization annealing process. Note, it is known to form minute marks, such as linear flaws, on a grain-oriented electrical steel sheet with a space between the marks, so as to subdivide the magnetic domains. The formation distance of the marks is allegedly more than 1 mm, but in practice, is from 3 to 12 mm. Allegedly, the watt loss increases at a minute mark distance of less than 1 mm when subdividing the magnetic domains, contrary to the case of the present invention. Figure 5 shows that sharp and minute unevennesses formed at a distance of less than 1 mm, preferably less than 0.5 mm, are advantageous for reducing the watt loss. The adhesivity of a glass film is also enhanced when the distance between the unevennesses is less than 1 mm. The distance is between the adjacent concave parts of the unevenesses. In an experiment by the present inventors, cold-rolled steel sheets of a grain-oriented electrical steel sheet, which were cold-rolled to a final thickness of 0.30 mm, were polished by a brush roll having abrasive grains embedded therein. The average roughness Ra and maximum roughness R T were 0.5 µm and 4.5 µm, respectively. Subsequently, light pickling by a dilute sulfuric acid was carried out to attain a weight loss of approximately 1 g/m², and activate the surfaces of the steel sheets. These steel sheets were decarburization annealed at 850°C in an N₂ + H₂ wet atmosphere having a ratio of P H₂O/P H₂ of 0.4. The annealing separator mainly composed of MgO was then applied and finishing annealing at 1200°C for 20 hours carried out. Figure 6(A) shows the inner oxide layer of the comparative sample, which has not been polished and lightly pickled. The inner oxide layer of the comparative sample is virtually uniformly thick. The inner oxide layer of the sample shown in Fig. 6(B) has a thickness such that parts thereof are thicker than the average thickness and protrude into the steel sheet part. Figure 7 shows the solution curves (potential curve) of oxide films on the decarburization annealed sheets in dilute sulfuric acid. As shown in Fig. 7 for the material B treated by polishing and then light pickling (activated), the potential peak corresponding to the SiO₂ layer is high, which indicates that a thick SiO₂ layer has been formed. Table 1 shows the magnetic properties of grain-oriented electrical steel sheets treated by the different processes. The amount removed by light pickling is preferably 2.5 g/m² or less. When the amount removed exceeds 2.5 g/m², the pickling is so severe that the surface of the steel sheet is roughened, and further, the sharp and minute unevennesses formed by a mechanical means or the like are deformed. In this case, the unevennesses do not have the function of forming sharp oxide protrusions. The depth of the unevennesses is preferably from 0.3 to 5 µm, in terms of the average roughness Ra, and approximately 15 µm in terms of the maximum roughness R T . Prior or subsequent to the decarburization annealing, preferably strain is imparted to a steel sheet by laser irradiation, marking off, a knife, or a tooth form roll. The distance between the strained regions is preferably from approximately 1 to 20 mm, and the angle of the strained regions relative to the rolling direction is preferably from 30 to 90 degrees. The strain, in combination with the activation of the surface of the steel sheet due to sharp and minute unevennesses, contributes to a further reduction of the watt loss. The direction of, for example, polishing for forming the sharp and minute unevenness, is not limited in any way. The processes for producing the grain-oriented electrical steel sheet according to the present invention are described hereinafter. The steel composition of a grain-oriented electrical steel sheet and production conditions until cold-rolling need not be specified since they are well known. The steels used may contain from 0.04 to 0.10% of C and from 2.0 to 4.0% of Si. Any adequate inhibitors, such as AlN, MnS, MnSe, BN, Cu₂S, and the like, may be used. If necessary, elements such as Cu, Sn, Cr, Ni, Mo, Sn, and the like may be added. Note, conventional industrially produced grain-oriented electrical steel sheets have a thickness of 0.30 mm, but 0.23 mm, 0.20 mm, 0.175 mm, and 0.150 mm thick grain-oriented electrical steel sheets have been developed and are now produced, to reduce the eddy current loss. One of the greatest hindrances to the production of thin grain-oriented electrical steel sheets is the instability of the secondary recrystallization. Japanese Unexamined Patent Publication (Kokai) No. 58-217630 proposed the addition of Sn and Cu for stabilizing the secondary recrystallization, and Japanese Unexamined Patent Publication proposed predecarburization annealing. In the present invention, however, the secondary recrystallization of 0.23 mm or less thin grain-oriented electrical steel sheets is advantageously stabilized. After the cold-rolling for obtaining the final thickness, decarburization annealing is carried out. Preferably, the decarburization annealing promotes the decarburization and oxidation reaction. This is attained by enhancing the dew point, for example, from 60 to 70°C, in the presence of a 25% N₂ + 75% H₂ atmosphere at 850°C. In an experiment by the present inventors, the surfaces of cold-rolled sheets of a grain-oriented electrical steel, which were cold-rolled to a final thickness of 0.225 mm, were polished by sheets of sand paper having different grades to form sharp and minute unevennesses. Subsequently, decarburization annealing was carried out at 850°C in an N₂ + H₂ atmosphere while varying the P H₂O/P H₂ ratio to 0.30, 0.40, and 0.50. Subsequently, an annealing separator composed mainly of MgO was applied and the finishing annealing was then carried out. Referring to Fig. 8, oxide peeling does not occur in the samples which are decarburization-annealed at a ratio P H₂O/P H₂ = 0.40 and 0.50. Polishing has a tendency to considerably enhance the tension of a film, as shown in Fig. 9, and the watt loss is improved considerably at a ratio of P H₂O/P H₂ ≧ 0.40, but is degraded when compared with the unpolished sample at a P H₂O/P H₂ < 0.40, as shown in Fig. 10. After the decarburization annealing, an annealing separator, which is mainly composed of MgO and in which additives, TiO₂ , B compounds, such as H₃BO₃ , Na₂B₄O₇ , and the like, SrS, SnS, CuS, and the like are added, is applied and dried. The finishing annealing is then carried out, and the oxide, having a thickness exceeding the average thickness and partially protruding into the steel sheet side, and the annealing separator are caused to react with each other, and thus a glass film is formed. The glass film is contiguous to the oxide which partially deeply protrudes into the steel sheet side. Alternatively, the glass film per se deeply protrudes into the steel sheet side. Therefore, the adhesiveness of the glass film is considerably enhanced, and furthermore, the tension which the glass film imparts to the steel sheet is drastically enhanced, to obtain steel sheets having an extremely low watt loss. The secondary recrystallization is satisfactory even in thin material, for example, 0.15 mm thick material, because the decomposition and disappearance of the inhibitors is suppressed due to the shielding effect of the oxide formed in the decarburization annealing. Subsequently, a flattening annealing is carried out, and then an insulating coating solution, which contains one or more of phosphoric acid, phosphates, such as aluminum phosphate, magnesium phosphate, zinc phosphate, and calcium phosphate, chromic acid, chromates, such as magnesium chromate and the like, bichromates, and colloidal silica, is applied on the steel sheet, followed by baking at a temperature of 350°C or more to form an insulating film. The advantages of the present invention will be further clarified by the following examples, which in no way limit the present invention. Example 1 A silicon steel-slab containing 0.060% of C, 2.95% of Si, 0.070% of Mn, 0.029% of Al, 0.025% of S, and a balance of iron was subjected, by a known method, to hot-rolling, annealing, and cold-rolling to obtain 0.27 mm thick sheets. On the sheets, sharp and minute unevennesses were formed in a direction perpendicular to the rolling direction, with a distance of 0.8 mm or less and 5 mm and an average roughness of 0.5 µm and 2.0 µm, by brush rolling and buff polishing. Then, decarburization annealing was carried out at 850°C for 120 seconds in an N₂ + H₂ humid atmosphere (P H₂O/P H₂ = 0.40). Subsequently, the application of an annealing separator, and a finishing annealing at 1200°C for 20 hours, were carried out. The glass film properties and the magnetic properties in this state were as shown in Table 2. As is apparent from Table 2, according to the present invention, grain-oriented electrical steel sheets having a high film tension, an improved adhesiveness, and a low watt loss were obtained. Example 2 A silicon steel-slab containing 0.065% of C, 3.25% of Si, 0.068% of Mn, 0.027% of Al, 0.023% of S, 0.07% of Cu, 0.12% of Sn, and a balance of iron was subjected, by a known method, to hot-rolling, annealing, and cold-rolling to obtain 0.225 mm thick sheets. Note, sheets which were not further subjected to a polishing-treatment are designated as "without treatment". An area of 50% of the steel sheets was polished by sand paper, while varying the grade thereof, to form unevennesses in terms of 12 µm, 9 µm, 7 µm, 5 µm, and 3 µm of the surface roughness of the steel sheet. Then, the processes of decarburization annealing, application of an annealing separator, and finishing annealing were carried out, and subsequently, product sheets were obtained by heat-flattening after the application of an insulating coating. The properties of the films and the magnetic properties of the product sheets were then measured and the results were as shown in Table 3. Note, an investigation of the adhesiveness of the films under an ordinary condition of bending to around 20 ∼ 50 mm⊘ revealed that no peeling occurred even for materials that were "without treatment". Accordingly, a more severe bending to 10 mm⊘ was carried out. No. Surface Roughness (µm) Magnetic Properties Adhesivity of Film (%) W 17/50 (W/kg) B₁₀ (T) 1 3 0.87 1.94 20 2 5 0.80 1.94 3 3 7 0.79 1.93 0 4 9 0.83 1.92 0 5 12 0.88 1.92 0 6 Comparative Material (without treatment) 0.91 1.93 75 Remarks: Area percentage of a film peeled by bending around 10 mm⊘. Example 3 A silicon steel-slab containing 0.060% of C, 3.15% of Si, 0.070% of Mn, 0.030% of Al, 0.024% of S, 0.07% of Cu, 0.13% of Sn, and a balance of iron was subjected, by a known method, to hot-rolling, annealing, and cold-rolling to obtain 0.29 mm thick sheets. An area of 80% of the steel sheets was treated by square shot-blasting to form unevennesses from 2.5 to 10 µm in depth. Then, the processes of decarburization annealing, application of an annealing separator, and finishing annealing were carried out, and subsequently, the product sheets were obtained by heat-flattening after the application of an insulating coating. The properties of the films and the magnetic properties of the product sheets were measured, and the results were as shown in Table 4. No. Indentation by Shot Treatment (µm) Magnetic Properties Adhesivity of Film (%) W 17/50 (W/kg) B₁₀ (T) 1 2.5 0.97 1.94 20 2 5 0.95 1.95 5 3 7.5 0.94 1.94 0 4 10 0.97 1.93 0 5 Comparative Material (without treatment) 1.04 1.94 70 Remarks: Area percentage of a film peeled by bending around 10 mm⊘. Example 4 A silicon steel-slab containing 0.058% of C, 3.10% of Si, 0.065% of Mn, 0.0010% of Al, 0.024% of S, and balance of iron was subjected to a well known double rolling method to obtain 0.265 mm thick steel sheets. Samples of these sheets were designated as "without treatment". An area of approximately 70% of the steel sheets was polished by a brush roll, to form unevennesses in terms of 3 ∼ 4 µm, 8 ∼ 10 µm, and 12 ∼ 15 µm of the surface roughness of the steel sheet. Then, the processes of decarburization annealing, application of an annealing separator, and finishing annealing were carried out, and subsequently, the product sheets were obtained by heat-flattening after the application of an insulating coating. The properties of the films and the magnetic properties of the product sheets were measured, and the results were as shown in Table 5. No. Brush Roll Treatment Conditions (Treatment Depth: µm) Magnetic Properties Adhesivity of Film (%) Tension of Glass film (kg/cm²) W 17/50 (W/kg) B₁₀ (T) 1 3 ∼ 4 1.10 1.87 10 0.48 2 5 ∼ 6 1.12 1.86 0 0.52 3 8 ∼ 10 1.15 1.85 0 0.60 4 12 ∼ 15 1.18 1.85 0 0.65 5 Comparative Material (without treatment) 1.20 1.86 60 0.35 Remarks: Area percentage of a film peeled by bending around 10 mm⊘. Example 5 The 0.225 mm thick cold-rolled steel sheets prepared in the same manner as in Example 2 were decarburization-annealed at 850°C for 3 minutes in an N₂ + H₂ humid atmosphere. An area of approximately 50% of the decarburization-annealed steel sheets was polished, by a brush roll, to form unevennesses in terms of 12 ∼ 15 µm, 8 ∼ 10 µm, 4 ∼ 6 µm, and 2 ∼ 3 µm of the surface roughness of the steel sheet. Subsequently, with regard to the samples that were decarburization-annealed alone and the samples decarburization-annealed and then polished, the application of an annealing separator and then finishing annealing at 1200°C for 20 hours were carried out and subsequently, the product sheets were obtained by heat-flattening after the application of an insulating coating. The properties of the films and the magnetic properties of the product sheets were measured, and the results were as shown in Table 6. No. Surface Roughness (µm) Magnetic Properties W 17/50 (W/kg) B₁₀ (T) 1 2 ∼ 3 0.84 1.95 2 4 ∼ 6 0.77 1.93 3 8 ∼ 10 0.79 1.92 4 12 ∼ 15 0.83 1.92 5 Comparative Material (without treatment) 0.92 1.95 Example 6 A silicon steel-slab containing 0.080% of C, 3.20% of Si, 0.065% of Mn, 0.035% of Al, 0.024% of S, 0.060% of Cu, 0.11% of Sn, and a balance of iron was subjected, by a known method, to hot-rolling, annealing, and cold-rolling to obtain 0.225 mm thick sheets. Sheets which were not polished are designated as "without treatment". The steel sheets were polished, while varying the area percentage of the parts polished to 20%, 50%, 70%, and 95%, by sand paper, to form unevennesses in terms of 5 µm of the surface roughness of the steel sheet. The steel sheets were then decarburization-annealed in an N₂ + H₂ humid atmosphere, and subsequently, the application of an annealing separator, in which 6.5 parts by weight of TiO₂ was blended with respect to 100 parts by weight of MgO, and then finishing annealing at 1200°C for 20 hours, were carried out. The properties of the films and the magnetic properties were then measured, and the results were as shown in Table 7. Example 7 Cold-rolled steel sheets 0.18 mm thick were prepared and decarburization-annealed in the same manner as in Example 6. The decarburization-annealed steel sheets were then polished, while varying the area percentage of the polished parts to 15%, 50%, 80%, and 95%, by a brush roll, to form polished parts 3 µm in depth. Subsequently, the application of an annealing separator, in which 6.5 parts by weight of TiO₂ was blended with respect to 100 parts by weight of MgO, and then finishing annealing at 1200°C for 20 hours, were carried out. The properties of the films and the magnetic properties were then measured, and the results were as shown in Table 8. Example 8 A silicon steel-slab containing 0.078% of C, 3.28% of Si, 0.065% of Mn, 0.033% of Al, 0.023% of S, 0.070% of Cu, 0.10% of Sn, and a balance of iron was subjected, by a known method, to hot-rolling, annealing, and cold-rolling to obtain 0.30 mm thick sheets. Sheets which were not polished are designated as "without treatment". Two surface activation treatments were carried out, as follows: samples of the steel sheets were polished, while varying the area percentage of the polished parts to 50%, and 85%, by sand paper, to form polished parts 3 µm in roughness, and in addition to these samples, polished and marked-off samples were prepared by treatment by a knife edge to introduce 10 µm deep strains at a distance of 5 mm and in a direction perpendicular to the rolling direction. The steel sheets were then decarburization-annealed in a humid atmosphere, and subsequently, the application of an annealing separator, and then finishing annealing at 1200°C for 20 hours, were carried out. The properties of the films and the magnetic properties were then measured, and the results were as shown in Table 9. In all of the samples on which surface sharp and minute unevennesses were formed, the film and magnetic properties were improved, and a further improvement in the watt loss was recognized in the samples which were further subjected to the strain-introduction by a knife. Example 9 A silicon steel-slab containing 0.073% of C, 3.20% of Si, 0.065% of Mn, 0.030% of Al, 0.024% of S, 0.075% of Cu, 0.11% of Sn, and a balance of iron was subjected, by a known method, to hot-rolling, annealing, and cold-rolling to obtain 0.225 mm thick sheets. The steel sheets were polished, while varying the area percentage of the polished parts to 60%, and 90%, by a brush roll, to form polished parts 3 µm in depth. Decarburization annealing was then carried out in an N₂ + H₂ humid atmosphere, and then, by using a marking-off needle, marking-off in a direction perpendicular to the rolling direction was carried out at a distance of 5 mm, so as to introduce the strain. Subsequently the application of an annealing separator, and finishing annealing were carried out, and subsequently, the product sheets were obtained by heat-flattening after the application of an insulating coating. The properties of the films and the magnetic properties of the product sheets were measured, and the results were as shown in Table 10. As in Example 8, the polished samples exhibited improved film properties and magnetic properties. In the samples which were further subjected to the strain-introduction by a knife, a further improvement of the watt loss was obtained. Example 10 A silicon steel-slab containing 0.068% of C, 3.15% of Si, 0.070% of Mn, 0.028% of Al, 0.025% of S, and a balance of iron was subjected, by a known method, to hot-rolling, annealing, and cold-rolling to obtain 0.27 mm thick sheets. The steel sheets were treated by a knife edge to introduce 15 µm deep strains at a distance of from 5 mm to 20 mm and in a direction perpendicular to the rolling direction. The steel sheets were then decarburization-annealed in an N₂ + H₂ wet atmosphere, and then activation was carried out by polishing with sand paper to form 2.5 µm deep polished parts over an area of 75%. Subsequently, the application of an annealing separator, and then finishing annealing at 1200°C for 20 hours, were carried out. The properties of the films and the magnetic properties were then measured, and the results were as shown in Table 11. The decarburization-annealed and then polished samples exhibited improved adhesiveness, film-tension, and magnetic properties. In the samples which were further subjected to strain-introduction by marking-off, a further improvement of the watt loss was obtained. Example 11 A silicon steel-slab containing 0.076% of C, 3.20% of Si, 0.072% of Mn, 0.026% of Al, 0.026% of S, and a balance of iron was subjected, by a known method, to hot-rolling, annealing, and cold-rolling, thereby finishing the slab to sheet thicknesses of 0.200 mm, 0.175 mm, 0.150 mm, and 0.125 mm. Samples were taken from the sheets having these thicknesses and several were activated by sand paper having a grade of #100 to form sharp and minute unevennesses. The remaining sheets were not activated. With regard to the activated and non-activated samples, the decarburization-annealing, application of annealing separator, and finishing annealing were carried out. Further, the application of an insulating coating and a measurement of the magnetic properties were then carried out. Subsequently, after pickling, the macro-structure was observed. The results were as shown in Table 12. Example 12 A silicon steel-slab containing 0.060% of C, 3.30% of Si, 0.065% of Mn, 0.030% of Al, 0.023% of S, 0.06% of Cu, 0.10% Sn, and a balance of iron was subjected, by a known method, to hot-rolling, annealing, and cold-rolling, to obtain 0.30 mm thick sheets. These sheets are designated as "before treatment". The steel sheets were polished, by sand paper, while varying the roughness thereof, to form polished, uneven parts 10 µm, 6 µm, and 3 µm in terms of surface roughness, over a 60% area of the steel sheets. Subsequently, decarburization-annealing of the sheets before treatment and of the polished sheets was carried out at 830°C for 3 minutes in N₂ + H₂ gas, while varying the P H₂O/P H₂ ratio to 0.3, 0.4, 0.5, and 0.6. After the application of an annealing separator, the finishing annealing was carried out at 1200°C for 20 hours. Subsequently, the product sheets were obtained by heat-flattening after the application of an insulating coating. The properties of the films and magnetic properties of the product sheets were measured, and the results were as shown in Table 13.

Claims: A grain-oriented electrical steel sheet having a glass film applied on the steel sheet, characterized by having sharp and minute unevennesses located at a depth of from 2 to 15 µm and at a distance of less than 1 mm from one another on the surface of the steel sheet, and by having an inner oxide layer partially protruding into the adjacent sharp concave portions of said unevennesses, thereby improving the adhesiveness of the glass film and the watt loss. A grain-oriented electrical steel sheet according to claim 1, wherein said oxide is a silica-enriched oxide. A method of producing a grain-oriented electrical steel sheet having an improved glass film adhesiveness and an improved watt loss, comprising the steps of hot-rolling a silicon-steel slab, annealing, cold-rolling the sheet once or twice or more often, with an intermediate annealing therebetween, decarburization-annealing, applying an annealing separator, and finishing annealing wherein a glass film is formed on the silicon steel sheet, characterized by subjecting said steel sheet, prior, during or subsequent to decarburization annealing, to a treatment of a surface thereof to form sharp and minute unevennesses over on area of at least 50% of the steel sheet, wherein adjacent concave portions of said unevennesses are located at a depth of from 2 to 15 µm and at a distance of less than 1 mm from one another, and which forms an inner oxide layer partially protruding into the adjacent sharp concave portions of said unevenness during finishing annealing or durig decarburization annealing and finishing annealing. A method according to claim 3, wherein said surface treatment is carried out prior to decarburization-annealing, by a mechanical means, particularly brush rolling, buff polishing, marking-off, sand papering, and grinding, and further, said unevennesses are formed over the surface of the steel sheet within ± 30 degrees to the direction perpendicular to the rolling direction. A method according to claim 3 or 4, wherein, after the surface treatment, pickling is carried out to attain a weight loss of 2.5 g/m² or less. A method according to any of claims 3 to 5, wherein the decarburization annealing is carried out, after the surface treatment, at a temperature of from 800 to 860°C under a ratio of P H₂O/P H₂ ≧ 0.4, in which P H₂O is the partial pressure of H₂O in the decarburization-annealing atmosphere, and P H₂ is the partial pressure of H₂ in the decarburization-annealing atmosphere.

Inventors: TAKASHIMA KUNIHIDE YAWATA WORK, TAKATA TOSHIHIKO YAWATA WORKS, TANAKA OSAMU, WADA TOSHIYA YAWATA WORKS, YASUMOTO HIROMICHI YAWATA WORK, TAKASHIMA, KUNIHIDE YAWATA WORKS, TAKATA, TOSHIHIKO YAWATA WORKS, TANAKA, OSAMU, WADA, TOSHIYA YAWATA WORKS, YASUMOTO, HIROMICHI YAWATA WORKS
Applicants: NIPPON STEEL CORP, NIPPON STEEL CORPORATION
Application year: 1986
Main classification: C21D8
IPC codes: H01F1, C21D8, C21D3