METHOD FOR PRODUCING LOW IRON LOSS GRAIN ORIENTED SILICON STEEL SHEETS

Abstract: A grain oriented silicon steel sheet having a low iron loss and not causing degradation of properties even through strain relief annealing is produced by applying ultrasonic vibrations to the surface of the sheet after secondary recrystallization annealing.

Description: This invention relates to a method of producing a low iron loss grain oriented silicon steel sheet whose properties are not degraded by strain relief annealing, and more particularly to a method of improving the iron loss value of a grain oriented silicon steel sheet after secondary recrystallization annealing by giving non-uniformity to the oxide layer on the surface of the sheet to provide regions exerting different tension or magnetically different regions on the surface. Grain oriented silicon steel sheets are mainly utilized as cores for transformers and other electrical machinery and equipment, and are required to have excellent magnetic properties and particularly a low iron loss (represented by W 17/50 value). For this purpose, it is necessary to highly align <001> orientation of the secondary recrystallized grains in the silicon steel sheet in the rolling direction and to reduce impurities and precipitates existing in the final product as far as possible. Hitherto there have been many efforts to improve the properties of grain oriented silicon steel. As a result, the iron loss value has improved from year to year. Recently, a W 17/50 value of 1.05 W/kg has been obtained in a product having a thickness of 0.30 mm. However, there is a strong demand for electrical machinery and equipment having less power loss because of the energy crisis several years ago. In this connection, grain oriented silicon steel sheets having a lower iron loss were required as a core material. As a general means for reducing the iron loss of grain oriented silicon steel sheet, there are mainly known metallurgical means, such as increasing the Si content, reducing the sheet thickness, making the secondary recrystallized grains finer, reducing the impurity content, highly aligning the secondary recrystallized grains into {110}<001> orientation and the like. These metallurgical means have already reached their limit in view of the existing production process, so that it is very difficult to attain an improvement of the properties so as to exceed the existing values. Even if some improvement can be achieved, the actual effect on the iron loss is slight for the effort involved. Apart from the above general means, Japanese Patent Application Publication No. 54-23647 proposes a method of making the secondary recrystallized grains finer by forming secondary recrystallization inhibiting regions on the steel sheet surface. In this method, however, the control of the secondary recrystallized grain size is unstable, so that such a method can not be said to be practical. In addition, Japanese Patent Application Publication No. 58-5968 proposes a technique for reducing the iron loss in which microstrain is introduced into the surface portion of the steel sheet after the secondary recrystallization by pushing a small ball, as used in ballpens, onto the steel sheet surface to effect refinement of magnetic domains, and Japanese Patent Application Publication No. 57-2252 proposes a technique for reducing the iron loss in which a laser beam is irradiated at an interval of several mm onto the surface of the final product in a direction perpendicular to the rolling direction to introduce high dislocation density regions into the surface portion of the sheet and effect refinement of magnetic domains. Further, Japanese Patent laid open No. 57-188810 proposes a technique of reducing the iron loss in which microstrain is introduced into the surface portion of the steel sheet by discharge working to effect refinement of magnetic domains. All of these methods reduce the iron loss by introducing micro plastic strain into the surface portion of the base metal in the steel sheet after the secondary recrystallization to effect refinement of the magnetic domains, and are equally practical and have an excellent effect on reducing the iron loss. However, the effect obtained by the introduction of plastic strain in these methods is undesirably reduced by the strain relief annealing effected after the punching, shearing work, coiling or like operations carried out on the steel sheet or by the heat treatment such as baking of the coating layer or the like. In Japanese Patent laid open No. 61-73886, there is proposed a technique for reducing the iron loss in which non-uniform elastic strain is given to the steel sheet surface by locally removing the surface coating by means of a vibration body forcedly performing reciprocal movement at a moving quantity of not less than 5x10⁻⁶ kg·m/s. Even using this technique, however, the effect is largely lost by annealing above 600°C. Moreover, when the introduction of micro plastic strain is carried out after the coating treatment, the reapplication of insulative coating should be carried out for maintaining the insulation property, so that the number of steps largely increases, resulting in increased cost. In order to solve the above drawbacks of the conventional techniques, the formation of deficient portions on forsterite film is proposed in Japanese Patent laid open No. 60-92481. There are described two methods for the formation of deficient portions in the above publication, one of which methods involves locally forming no forsterite portion and the other of which involves locally forming the deficient portions after the formation of forsterite. Of these, the method of locally removing forsterite is an actual industrial and useful method because in the method of locally forming no forsterite portion, process control is difficult due to the use of chemical means or means for obstructing the reaction. On the other hand, as a means for locally removing forsterite after the secondary recrystallization or forsterite formation, there are disclosed chemical polishing, electrolytic polishing, mechanical methods using a rotational disc-like grindstone or an iron needle under a light pressure, and further an optical method using an output-adjusted laser beam or the like. These methods exhibit an effect to a certain extent, respectively. However, the cost of chemical polishing and electrolytic polishing is considerable. When using the rotational disc-like grindstone, it is difficult to control the position according to the disc height in dependence on the surface properties, so that this is unsuitable for industrial production. Moreover, the optical method using the laser beam or the like is expensive. On the other hand, the use of the iron needle under light pressure is cheap, but it is difficult to remove only forsterite and it results in the removal of a part of the surface portion of the base metal together with forsterite. As a result, upheaving of the base metal is caused at both sides of the removed portion or deficient portion and this considerably lowers the lamination factor and the like. That is, the use of the iron needle is difficult to industrially put into practical use. As a technique for the refinement of magnetic domains, the formation of grooves in the surface of silicon steel sheets is disclosed in Japanese Patent Application Publication No. 50-35679, and Japanese Patent laid open Nos. 59-28525, 59-197520, 61-117218 and 61-117284 and is a well-known technique. However, since this technique utilizes the phenomenon of magnetic domain refinement by a diamagnetic field in the groove space, there are many drawbacks and the magnetic flux density (represented by B₁₀ value) is largely decreased. Also the mechanical properties are degraded and the lamination factor is considerably decreased by the groove formation even though the above technique gives results which withstand strain relief annealing. It is, therefore, an object of the present invention to provide a method for the production of low iron loss grain oriented silicon steel sheets which can provide sheets having good surface properties without decreasing not only the B₁₀ value but also the lamination factor and where the magnetic properties, and particularly the iron loss property, are not degraded during strain relief annealing and sheets can easily be produced in practice without decreasing efficiency. According to the present invention, there is provided a method of producing a low iron loss grain oriented silicon steel sheet by applying ultrasonic vibrations to the surface of a grain oriented silicon steel sheet after secondary recrystallization annealing to locally remove oxide layer from the surface of the sheet wherein the ultrasonic vibrations have a frequency of not less than 20KHz and an amplitude of not more than 50 µm and are produced by applying the working tip of an ultrasonic vibrating member to the surface of the sheet at a pressure of not more than 40 kg/mm². By means of the present invention the effect of magnetic domain refinement can be stably and cheaply obtained without largely decreasing the B₁₀ value and the lamination factor and, moreover, the reduction in iron loss is not impaired by strain relief annealing. According to a preferred embodiment of the invention, a head portion of an apparatus for generating ultrasonic vibrations is arranged opposite to the surface of the sheet extending and running about a roller so as to move in the widthwise direction of the sheet and a plurality of ultrasonic vibrating members is arranged in the head portion in a staggered form so as to move toward and away from the surface of the sheet. When the ultrasonic vibrating member is moved toward the sheet surface, the working tip of the member is pushed on to the sheet surface under controlled pressure. At such a position, the head portion is reciprocatedly moved in the widthwise direction of the running sheet, whereby ultrasonic vibrations are applied to the sheet of grain oriented silicon steel to locally remove the oxide layer, such as forsterite or the like produced by the secondary recrystallization, from the sheet surface. The shape of the working tip for applying ultrasonic vibrations to the surface of the grain oriented silicon steel sheet after the secondary recrystallization annealing may be plate-like or needle-like. Further, the working tip may be formed of hard crystals such as diamond, ruby and the like; ceramics; metals such as brass, copper and the like, grindstone, wood, and the like. For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, by way of example to the accompanying drawings, wherein: Figs. 1a and 1b are charts showing locally removed tracks of oxide layer as measured by means of a three dimensional roughness meter; Figs. 2a and 2b are graphs showing the effect of various treatments on the magnetic properties; Fig. 3 is a graph showing the wear loss of the working tip of a vibrating member used for the local removal of the oxide layer; Fig. 4 is a graph showing the improvement in magnetic properties by electrolytic etching; Fig. 5 is a graph showing the effect obtained by introducing a foreign substance into the track; Figs. 6a and 6b are plan and side views, respectively, of a first apparatus for practising the method of the invention, Figs. 7a and 7b are plan and side views, respectively, of a second apparatus for practising the method of the invention, Fig. 8 is a partially enlarged sectional view of an ultrasonic vibrating member of the apparatus of Figures 6a to 7b; and Figs. 9 and 10 are schematic views showing the removal of oxide layer from the surface of the steel sheet. According to the present invention, the oxide layer is effectively and locally broken and removed from the surface of the grain oriented silicon steel sheet by the shock of ultrasonic vibrations, so that it is not required to apply a large load as described in Japanese Patent laid open No. 61-117218 wherein conventional magnetic domain refinement is effected by the technique of locally forming grooves. When ultrasonic vibrations are applied to the surface of the grain oriented silicon steel sheet in accordance with the present invention, the working tip of the ultrasonic vibrating member is pushed to the sheet surface under a pressure of not more than 40 kg/mm². When the pressure exceeds the above value, plastic strain is given to the surface portion of the base metal. Also the lamination factor is decreased and the working tip is considerably worn due to the upheaving of the base metal around the removed portion of the oxide layer. Further, according to the present invention, a large plastic strain, as occurs in the conventional technique of forming grooves using an iron needle, is not given to the surface of the base metal and it is not required to form a deep groove in the base metal. Thus a large decrease of the B₁₀ value and degradation of the mechanical properties do not occur. There will now be described the form of the track obtained by removal of the oxide layer by applying ultrasonic vibrations according to the present invention using a working tip made from ruby and, as a comparison, by using an iron needle under a slightly light pressure. Fig. 1a illustrates the case of applying ultrasonic vibrations, while Fig. 1b illustrates the case where the iron needle under a light pressure was used. As can be seen from Figs. 1a and 1b, the depth of the removed portion in both cases is a few tenth µm, from which it is apparent that a deep groove is not formed in the base metal. However, when the oxide layer is mechanically removed by the iron needle, though the removed portion or groove is not so deep, the base metal upheaves around the groove as seen at the left-side edge of the groove in Fig. 1b. Such an upheaving of the base metal not only brings about a degradation of the lamination factor when an electromagnetic steel sheet laminate is formed, but also results in insulation breakage, so that it loses value as an industrial product. On the contrary, according to the invention, upheaving of the base metal is not caused as can be seen from Fig. 1a. Thus, it is clear that the application of ultrasonic vibration has an effect in addition to the merely allowing a reduction of the pushing pressure at the working tip. The improvement of magnetic properties according to the invention is shown by the mark ○ in Fig. 2 where the mark (⋄) is the result obtained by removing the surface coating with the iron needle under light pressure and the mark (♦) is the result obtained by forming the groove using an iron needle and heavy pressure, as comparative examples. According to the method of the invention, the oxide layer was locally removed from the surface of the grain oriented silicon steel sheet after the secondary recrystallization annealing by applying ultrasonic vibrations of 30 kHz to a diamond working tip to form grooves, each having a width of 80 µm and a depth of 0.2 µm at a spacing of 5 mm, on the sheet surface, the grooves being parallel to each other and extending in a direction perpendicular to the rolling direction of the sheet. On the other hand, when the iron needle, in the form of a steel scriber, was used under a light pressure, grooves having a depth of 0.2 µm were formed at a spacing of 5 mm in parallel to each other, while when the steel scriber was used under a heavy pressure, grooves having a depth of 2 µm (width 120 µm) were formed at a spacing of 5 mm in parallel to each other. In the latter method, the formation of grooves having a depth of 2 µm resulted in the application of heavy pressure to the base metal. As a result, the iron loss was considerably reduced before strain relief annealing when the iron needle under heavy pressure was used, but it was inversely degraded after strain relief annealing. This was because strain was introduced into the base metal by the force needed for the formation of grooves having a depth of 2 µm to effect the refinement of the magnetic domain. Thus, although the iron loss was reduced initially, the reduced iron loss was lost in the subsequent strain relief annealing (800°C x 3 hours). In this case, the decrease of the B₁₀ value was large, so that the iron loss value was poor as compared with the iron loss value just after the secondary recrystallization annealing. Furthermore, the forsterite layer in the vicinity of the groove was ununiformly broken under the heavy pressure, so that the effect of magnetic domain refinement by the removal of oxide layer such as forsterite or the like (which is also expected by the method of the invention) was substantially lost and hence the iron loss was largely degraded. When the local removal of oxide layer up to a depth of 0.2 µm was carried out by the method of the invention, the improving ratio of iron loss before and after the removal of oxide layer was small as compared with the case of forming the groove under heavy pressure, but degradation of iron loss was not caused by the strain relief annealing. Rather there was a tendency to improve the iron loss. Though the reason for such an improvement is not clear, it is believed that unnecessary strain slightly introduced by the application of the ultrasonic vibrations is removed by the strain relief annealing or that the oxide layer formed advantageously acts to improve the iron loss. When the oxide layer was removed to a depth of 0.2 µm by the iron needle under a light pressure, degradation of the iron loss and magnetic flux density occurred after strain relief annealing. This is believed to be due to the fact that the leakage of magnetic flux becomes large by the upheaving of base metal at the worked portion. In Japanese Patent laid open No. 56-130454, there is disclosed a technique where ultrasonic waves are applied to a gear-like roll and the roll is linearly contacted with the surface of the grain oriented silicon steel sheet after the secondary recrystallization annealing under pressure in order to form fine recrystallized grain groups on the sheet surface. This technique gives complicated strain to the sheet surface for obtaining fine recrystallized grains. Therefore, it is naturally required to apply a strain sufficient enough to enable recrystallization to occur, and consequently the gear roll is used. On the contrary, the present invention involves locally breaking and removing the oxide layer, which is entirely different from the formation of fine recrystallized grains. For this purpose, the working tip is preferably needle-like or of plate-like form. As a result, the recrystallized grain groups are not newly formed when carrying out the method of the invention. In a preferred embodiment of the invention, the sheet is subjected to electrolytic etching after the local removal of the oxide layer. Thus, the effect of magnetic domain refinement can be more improved by utilizing the diamagnetic field at the grooves formed after local removal of the oxide layer. In another embodiment of the invention, a foreign substance is introduced into the grooves after the electrolytic etching to further improve the magnetic properties as denoted by mark  in Figs. 2a and 2b. Of course, the lamination factor is sufficiently maintained in these cases. The results of the iron loss and B₁₀ value in these preferred embodiments are shown by the mark  in Figs. 2a and 2b, from which it is apparent that the iron loss is further reduced but the B₁₀ value is somewhat degraded. Such measured data was obtained when the sheet was subjected to local removal of oxide layer, to electrolytic etching in an NaCl aqueous solution (100 g/ℓ) at a current density of 20 A/dm² for 5 seconds, to filling with colloidal silica and to strain relief annealing (800°C x 3 hours). According to the invention, the starting material is a grain oriented silicon steel sheet after secondary recrystallization annealing. The application of the method of the invention to the sheet before secondary recrystallization annealing is meaningless. However, when the method of the invention is applied to the sheet after secondary recrystallization annealing, it develops an effect irrespective of the previous history of the sheet such as the kind of inhibitor, the cold rolling number or the like. Since the secondary recrystallization annealing is usually carried out at a temperature of 800∼1200°C, an oxide layer exists on the surface of the grain oriented silicon steel sheet. According to the invention, this oxide layer is locally removed by applying ultrasonic vibrations. In this case, the working tip of the ultrasonic vibrating member is contacted with the sheet surface under a pressure of not more than 40 kg/mm² at the time of applying the ultrasonic vibration in order for the working tip to follow the sheet surface. When the pressure exceeds this value, plastic strain is undesirably generated in the surface portion of the sheet. The effect obtained by the local removal of the oxide layer is usually the same before and after the formation of an insulation coating on the oxide layer. In this case, the insulation coating may be a tension coating. The local removal of the oxide layer may be carried out in dotted line form or in continuous or discontinuous linear form across the rolling direction to repeatedly form removed portions in parallel to each other on the sheet surface. Preferably, the removing direction is perpendicular to the rolling direction. The spacing between parallel removed portions is preferably within a range of 1∼30 mm. When the spacing between parallel removed portions is less than 1 mm, the surface properties are degraded by the resulting grooves and sufficient improvement of iron loss value is not obtained, while when it exceeds 30 mm, the effect of magnetic domain refinement is lost. Further, the effect is substantially the same when the local removal occurs on one surface or both surfaces of the sheet. In the invention, the local removal of the oxide layer is carried out by using a working tip subjected to ultrasonic vibration. The shape of the working tip is preferably needle-like. The width of the removed portion can be varied depending upon the size or thickness of the working tip. The width of the removed portion should generally be 10∼1000 µm, preferably about 100 µm. When the width of the removed portions is less than 10 µm, breaking of the sheet is apt to be caused by the notch action, while when it exceeds 1000 µm, the surface properties are degraded and also an improvement in the iron loss value is not obtained. Since the ultrasonic vibrations are applied to the working tip to locally remove oxide such as forsterite or the like, there are advantages in that the working strain is small, the working tip can be small, and a smooth surface is obtained without causing upheaving of the base metal. On the contrary, when local removal of oxide layer is mechanically carried out using an iron needle without application of ultrasonic vibration, the plastic deformation portion becomes larger, resulting in a large decrease of the lamination factor and B₁₀ value. Vibrations having a frequency of not less than 20 kHz and an amplitude of not more than 50 µm are used and vibrations containing a component in the direction perpendicular to the sheet surface are preferable. When the frequency is less than 20 kHz, the shock density by vibrations becomes small and the effect is less. On the other hand, when the amplitude is more than 50 µm,the shock force becomes large and a large strain occurs which decreases the B₁₀ value. Pulse or continuous mode can be used as the generation mode for the ultrasonic vibration. The working tip for giving ultrasonic vibrations to the sheet surface may be made of any materials capable of locally removing the oxide layer, but the use of diamond, ceramics or super-hard alloy having a semi-ball or columnar shape of not more than 2 mm in diameter is preferable. When the material is not hard, it wears and the resultant change in the means of removing the oxide layer badly affects the magnetic domain refinement. Also the use of a working tip of semicircular shape having a diameter of more than 2 mm or other shape badly affects the magnetic domain refinement due to wear. Fig. 3 shows the degree of wear of the working tip together with the wear results using an iron needle as a comparative example. In the method of the invention, the oxide layer was locally removed from the surface of the steel sheet after the secondary recrystallization annealing by applying ultrasonic vibrations of 30 kHz to a working tip of electrodeposited diamond and moving the working tip under a load of 10 kg/mm² in a direction perpendicular to the rolling direction to form groove portions at a spacing of 5 mm in parallel to each other. On the other hand, in the comparative example the grooves were formed at a spacing of 5 mm in parallel to each other by using a scriber of electrodeposited diamond under a load of 20 kg/mm² or a scriber in the form of an iron needle under a load of 100 kg/mm². As can be seen from Fig. 3, the iron needle was subject to the greatest wear while the electrodeposited diamond tip used in applying ultrasonic vibration according to the invention had no weight loss. However the tip of electrodeposited diamond used under a load of 20 kg/mm² broke which reduced its weight and badly affected the state of oxide removal. According to the invention, when electrolytic etching is carried out after the local removal of oxide layer by the application of ultrasonic vibration, the iron loss can be further reduced. In this case, the etching depth of the groove is preferably not more than 20 µm. Fig. 4 shows the relation between the etching depth after the local removal of oxide layer and the magnetic properties. In this case, the local removal of oxide was carried out by applying ultrasonic vibrations having a frequency of 20 kHz and an amplitude of 15 µm to the super-hard working tip of 1.5  to form grooves at a spacing of 8 mm in parallel to each other in a direction perpendicular to the rolling direction. Then, electrolytic etching was carried out in an aqueous solution of NH₄Cl-NaCl (100 g/ℓ - 100 g/ℓ) at a current density of 5 A/dm², during which the etching depth was determined by varying the etching time. The effect of the etching on the magnetic properties is shown in Fig. 4. The iron loss value is further improved when a substance locally producing a different tension because of the difference in thermal expansion coefficient or a magnetically different substance producing a diamagnetic field (for example, metal, silicate, phosphorus compound, oxide, nitride or the like) is introduced, as a foreign substance, in the grooves produced by the electrolytic etching. In this case, it is desirable that the foreign substance has a thermal expansion coefficient smaller than that of the silicon steel sheet in order to obtain the different tension effect. Fig. 5 shows the effect of improving the iron loss value by the introduction of foreign substance. In this case, grooves having a depth of 10 µm were formed by the local removal of oxide and electrolytic etching in the same manner as in Fig. 4. Thereafter, the grooves were subjected to Sb plating and further to a strain relief annealing at 800°C for 3 hours. The application of the ultrasonic vibrations to the sheet surface according to the invention will now be described in detail with reference to Figs. 6 to 10. In Figs. 6a and 6b there is shown a first embodiment for carrying out the method according to the invention. A grain oriented silicon steel sheet 1 after secondary recrystallized annealing extends about a roller 2 supported by a bearing 3. On the other hand, a head portion 4 of an apparatus for generating ultrasonic vibrations is arranged in opposition to the free surface of the steel sheet running around the roller 3 and is provided with a plurality of ultrasonic vibrating members 5 staggeredly arranged in the up and down directions of the head portion 4. Further, the head portion 4 is reciprocatedly moved in the widthwise direction of the running steel sheet 1 by means of a screw 6, supported at both ends by bearings 7, and a motor 8. The detail of the ultrasonic vibrating member 5 is shown in Fig. 8. Each of the ultrasonic vibrating members 5 staggeredly arranged in the head portion 4 is connected to an air cylinder 15 involved in or supported by the head portion 4 in such a manner that the ultrasonic vibrating member 5 is moved toward the surface of the running steel sheet 1 and away therefrom at both widthwise ends of the steel sheet by the action of the air cylinder 15 so as not to injure the surface of the roller 2. Further, the pushing pressure of working tip 14 on the steel sheet 1 can be controlled by adjusting the air pressure applied from the air cylinder 15 to the ultrasonic vibrating member 5. When the oxide layer is continuously and locally removed from the surface of the silicon steel sheet by applying ultrasonic vibrations by means of the apparatus shown in Fig. 6, the number of ultrasonic vibrating members 5 used and the moving speed of the head portion 4 are first determined so as to well balance the feeding speed of the steel sheet 1. In this case, the oxide removal is performed at the going stage of the head portion, while the ultrasonic vibrating member is moved away from the sheet surface at the coming back stage of the head portion. Such going and coming back stages of the head portion are continuously repeated to perform the local removal of oxide layer from the surface of the running steel sheet. The removed track of the oxide layer is shown in Fig. 9. Moreover, the removed track as shown in Fig. 10 can be obtained by intermittently feeding the steel sheet 1. In Figs. 7a and 7b there is shown a second embodiment of an apparatus for locally removing oxide layer from the surface of a grain oriented silicon steel sheet after secondary recrystallization annealing by application of ultrasonic vibrations according to the invention, wherein the removed track, as shown in Fig. 10, is obtained by continuously feeding the steel sheet. As shown in Figs. 7a and 7b, an end of an arm 9 is connected to each of bearings 3 located at both ends of the roller 2, and a segment gear is formed on the other end of the arm 9. This segment gear of the arm 9 is engaged with a pinion gear 12 of a pinion shaft 11 supported by a support 10 and connected to a driving motor 13. On the other hand, the screw shaft 6 supporting and moving the head portion 4 of the apparatus for generating ultrasonic vibrations is supported by the arm 9. By means of the above construction, the head portion 4 can be moved in the running direction of the sheet or in the peripheral direction of the roller 2 by synchronizing the engaging movement between the segment gear and the pinion gear with the feeding speed of the sheet by the driving motor 13, and at the same time the head portion 4 is moved in the widthwise direction of the sheet by the driving motor 8, whereby the removed track can be formed in a direction perpendicular to the running direction of the sheet as shown in Fig. 10. In any case, as the number of ultrasonic vibrating members used increases, the efficiency in the formation of removed track (productivity) becomes naturally excellent. Moreover, when using the apparatus of Fig. 6, the formation of the removed track is attained only at the going stage for the movement of the head portion 4 because if the formation of the removed track is also performed at the coming back stage, the slant of the removed track will be opposite to that formed at the going stage and parallel tracks can not be formed on the sheet surface. However, if the feeding of the sheet is intermittently stopped, the formation of removed track can be carried out even at the coming back stage. On the other hand, in the case where the apparatus of Fig. 7 is used, the formation of the removed track as shown in Fig. 10 can be achieved at both going and coming back stages while continuously feeding the sheet. Therefore, the latter apparatus has twice the production efficiency of the former apparatus when the number of the ultrasonic vibrating members and the feeding speed of the sheet are same. In other words, the number of ultrasonic vibrating members in the latter apparatus can be reduced to a half of the number in the former apparatus. The working tip 14 of the ultrasonic vibrating member 5 may be made from diamond, ruby, brass, steel, grindstone or the like as previously mentioned. Further, the frequency of vibrations to be applied is not less than 20 kHz, preferably 25∼50 kHz, and the pushing pressure of the working tip is not more than 40 kg/mm². The working tip 14 of the ultrasonic vibrating member 5 can easily be inclined in the running direction of the sheet. The spacing between adjoining ultrasonic vibrating members is preferably about 5 mm. The diameter of the roller 2 is preferably not less than 300 mm for ensuring the sheet is not subjected to bending strain and may be properly determined together with the number of the ultrasonic vibrating members and the feeding speed of the sheet. As the material of the roller, steel, rigid rubber and the like are suitable. In the case where rigid rubber is used, the hardness is preferably not less than 60 (Hs). The following examples are given to illustrate the invention and are not intended as limitations thereof. Example 1 A hot rolled sheet of silicon steel containing Si: 3.27 wt% (hereinafter shown by % simply), Mn: 0.070%, Se: 0.019% and Sb: 0.020% was subjected to cold rolling twice with an intermediate annealing at 950°C to obtain a cold rolled sheet having a final thickness of 0.23 mm. Thereafter, the cold rolled sheet was subjected to decarburization and primary recrystallization annealing at 800°C in a wet hydrogen atmosphere, coated on its surface with a slurry of an annealing separator consisting mainly of MgO and coiled. It was subjected to a secondary recrystallization annealing in a box furnace at 850°C for 50 hours and further to a purification annealing in a dry hydrogen atmosphere at 1200°C for 10 hours. After excessive annealing separator had merely been removed from the sheet surface, the sheet was treated under the conditions as shown in the following Table 1. The iron loss W 17/50 (W/kg) of the thus obtained sheet was measured to obtain the results shown in Table 1. Example 2 A hot rolled sheet of silicon steel containing Si: 3.05%, Mn: 0.073%, Se: 0.020% and Sb: 0.025% was twice subjected to cold rolling with an intermediate annealing at 950°C to obtain a cold rolled sheet having a final thickness of 0.23 mm. Thereafter, the cold rolled sheet was subjected to decarburization and primary recrystallization annealing at 810°C in a wet hydrogen atmosphere, coated on its surface with a slurry of an annealing separator consisting mainly of Al₂O₃, coiled, and subjected to a secondary recrystallization annealing in a box furnace at 850°C for 50 hours and to a purification annealing in a dry hydrogen atmosphere at 1200°C for 10 hours. After the removal of the annealing separator, an insulation coating was formed on the sheet surface, which was then subjected to flat annealing. Then, the thus treated sheet was subjected to a treatment for locally removing the oxide layer under conditions shown in the following Table 2. Next, the sheet was subjected to an electrolytic etching in an aqueous solution of NaCl (100 g/ℓ) at a current density of 30 A/dm² for 10 seconds and further to an insulation coating with a phosphate. The iron loss W 17/50 (W/kg) of the thus obtained sheets was measured to obtain results as shown in Table 2. Moreover, the standard sheet after the flat annealing had B₁₀=1.9T and W 17/50 =0.95 W/kg. Example 3 A hot rolled sheet of silicon steel containing Si: 3.25%, Mn: 0.072%, Se: 0.018% and Sb: 0.025% was subjected to cold rolling twice with an intermediate annealing at 950°C to obtain a cold rolled sheet having a final thickness of 0.23 mm. Then, the cold rolled sheet was subjected to decarburization and primary recrystallization annealing at 820°C in a wet hydrogen atmosphere, coated on its surface with a slurry of an annealing separator consisting mainly of MgO and coiled, and subjected to a secondary recrystallization annealing in a box furnace at 850°C for 50 hours and further to a purification annealing in a dry hydrogen atmosphere at 1200°C for 10 hours. After the removal of excess annealing separator and flat annealing, the sheet was subjected to a treatment for local removal of the oxide layer under the conditions shown in the following Table 3. As a post-treatment, electrolytic etching was carried out in an aqueous solution of NaCl (250 g/ℓ) at a current density of 30 A/dm² for 10 seconds. Then, the resulting grooves were filled with a solution of borosiloxane, which was gradually heated to 200∼400°C to effect baking. On the other hand, a part of the sheet was coated with antimony sol and dried at 100°C. The iron loss values W 17/50 (W/kg) of the thus obtained sheets were measured to obtain the results shown in Table 3. Moreover, the standard sheet after the flat annealing had magnetic properties of W 17/50 =0.92 W/kg and B₁₀=1.91T. Example 4 A hot rolled sheet of silicon steel containing Si: 3.28%, Mn: 0.074%, Se: 0.026%, sol.Al: 0.027% and N: 0.0083% was annealed at 1130°C for 4 minutes, quenched and pickled. Then, the sheet was subjected to a heavy cold rolling to obtain a cold rolled sheet having a final thickness of 0.23 mm. Thereafter, the cold rolled sheet was subjected to decarburization and primary recrystallization annealing in a wet hydrogen atmosphere at 840°C, coated on its surface with a slurry of an annealing separator consisting mainly of MgO and coiled, and subjected to a secondary recrystallization annealing in a box furnace at 850°C for 50 hours and further to a purification annealing in a dry hydrogen atmosphere at 1200°C for 10 hours. After the removal of excess annealing separator and flat annealing, the sheet was subjected to a treatment for the local removal of oxide layer under the conditions shown in the following Table 4. The iron loss values W 17/50 (W/kg) of the thus obtained sheets were measured to obtain the results shown in Table 4. Moreover, the standard sheet after the flat annealing had magnetic properties of W 17/50 =0.89 W/kg and B₁₀=1.92T. Example 5 The oxide layer was locally removed from the surface of a grain oriented silicon steel sheet after secondary recrystallization annealing having a thickness of 0.23 mm by linearly pushing a working tip of sintered diamond having a diameter of 1 mm against the sheet surface in a direction perpendicular to the rolling direction at a spacing of 8 mm. In this case, ultrasonic vibrations having a frequency of 25 kHz and an amplitude of 20 µm were applied to the working tip and the pushing pressure of the working tip was 10 kg/mm². Similarly, the oxide layer was removed by using a working tip formed of super-hard alloy and having a sharp point without the application of ultrasonic vibration. In this case, a load of 10 kg/mm² was applied to the working tip. After the removal of oxide layer, electrolytic etching was carried out in an aqueous solution of NaCl (200 g/ℓ) at a current density of 10 A/dm² for 8 seconds, and then the thus treated sheet was subjected to Ni plating and further to strain relief annealing (800°C x 2 hours). The magnetic properties of the thus obtained sheets are shown in the following Table 5. As mentioned above, according to the invention, grain oriented silicon steel sheets which have a very low iron loss and which do not lose the effect of magnetic domain refinement even after strain relief annealing can be produced without causing a significant decrease in lamination factor and B₁₀ value which has never been achieved by conventional techniques.

Claims: A method of producing a low iron loss grain oriented silicon steel sheet by applying ultrasonic vibrations to the surface of a grain oriented silicon steel sheet after secondary recrystallization annealing to locally remove oxide layer from the surface of the sheet wherein the ultrasonic vibrations have a frequency of not less than 20KHz and an amplitude of not more than 50 µm and are produced by applying the working tip (14) of an ultrasonic vibrating member (5) to the surface of the sheet at a pressure of not more than 40 kg/mm². A method according to claim 1 wherein after said local removal of oxide layer, the sheet is subjected to electrolytic etching. A method according to claim 2, wherein the electrolytic etching is carried out to obtain portions having an etching depth of not more than 20 µm. A method according to claim 2 or 3, wherein after electrolytic etching, a foreign substance is introduced into the etched portions. A method according to claim 4, wherein said foreign substance is selected from the group consisting of metals, silicates, phosphorus compounds, oxides and nitrides. A method according to any one of the preceding claims wherein the oxide layer is locally removed by arranging a head portion (4) of an apparatus for generating ultrasonic vibrations in opposition to the surface of the sheet running around a roller (2) so as to reciprocatedly move in the widthwise direction of said running sheet; staggeredly arranging, in said head portion in up and down directions thereof, a plurality of said ultrasonic vibrating members (5) movable toward and away from said surface of the running sheet; urging the working tip (14) of each ultrasonic vibrating member against said surface of the running sheet at said pressure while applying ultrasonic vibrations to said working tip; and repeating the reciprocative movement of said head portion in the widthwise direction of said running sheet to remove oxide layer from portions on said surface of the running sheet in a direction perpendicular to the rolling direction thereof at a given spacing. A method according to claim 6, wherein said head portion is moved up and down in the running direction of said sheet in synchronization with the running speed of said sheet while being moved in the widthwise direction of said sheet. A method according to any one of the preceding claims, wherein said oxide layer is removed from portions of the sheet having a width of 10 to 1000 µm and being in the form of dots or continuous or discontinuous lines parallel to each other at a spacing of 1 to 30 mm. A method according to any one of the preceding claims wherein said working tip is made from a material selected from the group consisting of diamond, ceramics, ruby and super-hard alloy and has a needle-like form or a plate-like form.

Inventors: NAGAMINE, TSUNEO C/O CHIBA WORKS, NISHIIKE, UJIHIRO TECHNICAL RESEARCH DIVISION, SUJITA, SHIGEKO TECHNICAL RESEARCH DIVISION
Applicants: KAWASAKI STEEL CORPORATION
Application year: 1988
Main classification: C21D8
IPC codes: B08B7, C25F1, C25F3, H01F1, C21D8