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

[0001] This invention relates to a method of producing a low iron loss grain oriented silicon steel sheet not degrading properties by strain relief annealing, and more particularly to an improvement of iron loss value in the grain oriented silicon steel sheet after secondary recrystallization annealing without degrading properties through strain relief annealing which can be realized by giving ununiformity to an oxide layer formed on the surface of the sheet to provide regions acting different tension or magnetically different regions on the surface.

[0002] The grain oriented silicon steel sheets are mainly utilized as a core for transformer and other electrical machinery and equipment, and are required to have excellent magnetic properties, particularly a low iron loss (represented by W_17/50 value).

[0003] For this purpose, it is demanded to highly align <001> orientation of secondary recrystallized grains in the silicon steel sheet into the rolling direction and to reduce impurities and precipitates existent in steel of the final product as far as possible.

[0004] Under the above circumstances, there are attempted a great of efforts for improving the properties of the grain oriented silicon steel sheets up to the present. As a result, the iron loss value is also improved from year to year. Recently, the W_17/50 value of 1.05 W/kg is obtained in the product having a thickness of 0.30 mm.

[0005] However, it is strongly demanded to develop electrical machinery and equipment having less power loss on the border of energy crisis since several years ago. In this connection, the grain oriented silicon steel sheets having a lower iron loss become demanded as a core material.

[0006] As a general means for reducing the iron loss of the grain oriented silicon steel sheet, there are mainly known metallurgical means, such as increasing of Si content, thinning of product thickness, fining of secondary recrystallized grains, reducing of impurity content, highly aligning of secondary recrystallized grains into {110}<001> orientation and the like. These metallurgical means already reach to a limit in view of the existing production process, so that it is very difficult to attain the improvement of the properties exceeding the existing values. If somewhat improvement is recognized, the actual effect of improving the iron loss is slight for the effort.

[0007] Apart from the above general means, Japanese Patent Application Publication No. 54-23647 proposes a method of fining secondary recrystallized grains by forming secondary recrystallization inhibiting regions on the steel sheet surface. In this method, however, the control of secondary recrystallized grain size is unstable, so that such a method can not be said to be practical.

[0008] 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 for use in ballpen to the steel sheet surface to conduct 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 conduct 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 conduct refine­ment of magnetic domains.

[0009] All of these methods are to 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 conduct refinement of magnetic domains, and are evenly practical and have an excellent effect of reducing the iron loss. However, the effect by the introduction of plastic strain in these methods is undesirably reduced by the strain relief annealing after the punching, shearing work, coiling or the like of the steel sheet or by the heat treatment such as baking of coating layer or the like.

[0010] In Japanese Patent laid open No. 61-73886, there is proposed a technique for reducing the iron loss in which ununiform elastic strain is given to the steel sheet surface by locally removing off a surface coating through a vibration body forcedly performing reciprocal movement at a moving quantity of not less than 5×10⁻⁶ kg·m/s. Even in this technique, however, the effect is largely lost by the annealing above 600°C.

[0011] 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 the rise of cost.

[0012] 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.

[0013] There are described two methods for the forma­tion of deficient portions in the above publication, one of which methods being a method of locally forming no forsterite portion and the other of which being a method of locally forming the deficient portions after the formation of forsterite. Among them, the method of locally removing forsterite is an actually industrial and useful method because in the method of locally forming no forsterite portion, the process control is difficult due to the use of chemical means or means for obstructing the reaction.

[0014] On the other hand, as the means for locally removing forsterite after the secondary re­crystallization or forsterite formation, there are disclosed chemical polishing, electrolytic polishing, mechanical method of 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 chemical polishing and electrolytic polishing become considerably high in the cost. In the use of the rotational disc-­like grindstone, it is difficult to control the position following to the disc height in accordance with the surface properties, so that this is unsuitable for industrial production. Moreover, the optical method using the laser beam or the like becomes high in the cost.

[0015] On the other hand, the use of the iron needle under light pressure is low in the cost, but is difficult to remove only forsterite and finally removes a part of 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 to considerably lower the lamination factor and the like. That is, the use of the iron needle is difficult to industrially put into practical use.

[0016] As the technique for the refinement of magnetic domains, the formation of grooves in the surface of the silicon steel sheet 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. Since this technique utilizes a phenomenon of magnetic domain refinement through diamagnetic field in the groove space, however, there are many drawbacks that the magnetic flux density (represented by B₁₀ value) is largely decreased, and the mechanical properties are degraded and the lamination factor is considerably decreased in accordance with the groove form though the above technique is durable to the strain relief annealing.

[0017] It is, therefore, an object of the invention to provide a method for the production of low iron loss grain oriented silicon steel sheets which can provide a sheet having good surface properties in the lamination without decreasing not only B₁₀ value largely but also lamination factor and does not cause the degradation of magnetic properties, particularly iron loss property during the strain relief annealing and easily performs the actual operation without decreasing the efficiency.

[0018] According to the invention, there is the provision of a method of producing a low iron loss grain oriented silicon steel sheet not causing degradation of properties through strain relief annealing, charac­terized in that ultrasonic vibrations are applied to a surface of a grain oriented silicon steel sheet after secondary recrystallization annealing to locally remove an oxide layer from the surface of the sheet. Thus, the effect of magnetic domain refinement can be stably and cheaply obtained without largely decreasing the B₁₀ value and the lamination factor and disappearing the effect of reducing the iron loss through strain relief annealing.

[0019] In the method of the invention, a working tip of an ultrasonic vibrating member is pushed onto the surface of the sheet under a certain pressure. 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 are 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 this member is pushed to the sheet surface under a controlled pressure. At such a state, the head portion is reciprocatedly moved in the widthwise direction of the running sheet, whereby ultrasonic vibrations are applied to the sheet of the grain oriented silicon steel sheet to locally remove the oxide layer such as forsterite or the like produced by the secondary recrystallization from the sheet surface.

[0020] 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 as far as the oxide layer can locally be removed. Further, the material of the working tip may be hard crystals such as diamond, ruby and the like; ceramics; metals such as brass, copper and the like, grindstone, wood piece and the like.

[0021] The frequency of the ultrasonic vibration is desirable to be not less than 10 kHz.

[0022] The invention will be described with reference to the accompanying drawings, wherein:

Figs. 1a and 1b are charts showing locally removed tracks of the oxide layer as measured by means of a three dimensional roughness meter, respectively;

Figs. 2a and 2b are graphs showing the effect of improving magnetic properties, respectively;

Fig. 3 is a graph showing wearing loss of the working tip by the local removal of the oxide layer;

Fig. 4 is a graph showing the effect of improving magnetic properties through electrolytic etching;

Fig. 5 is a graph showing the effect by filling of foreign substance;

Figs. 6a and 6b are plan view and side view of a first embodiment for practicing the method of the invention, respectively;

Figs. 7a and 7b are plan view and side view of a second embodiment for practicing the method of the invention, respectively;

Fig.8 is a partially enlarged sectional view of the ultrasonic vibrating member used in the invention; and

Figs. 9 and 10 are schematic views showing the removing state of oxide layer from the surface of the steel sheet, respectively.

[0023] According to the 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 relating to the technique of locally forming grooves as the conventional magnetic domain refinement. That is, when ultrasonic vibrations are applied to the surface of the grain oriented silicon steel sheet, the working tip of the ultrasonic vibrating member is pushed to the sheet surface under a pressure of not more than 40 kg/mm². Because, when the pressure exceeds the above value, the plastic strain is given to the surface portion of the base metal, and 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.

[0024] Further, according to the invention, a large plastic strain as described in the conventional technique of forming grooves by 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, so that there are never caused the large decrease of B₁₀ value and the degradation of mechanical properties.

[0025] There will be described the form of worked track after the removal of oxide layer by applying ultrasonic vibrations according to the invention, wherein the working tip of the ultrasonic vibrating member is made from ruby, and by using the iron needle under a slightly light pressure as a comparative example below.

[0026] Fig. 1 shows the locally removed portion of the oxide layer as measured by means of a three dimensional roughness meter.

[0027] Fig. 1a is a case of applying ultrasonic vibrations, while Fig. 1b is a case of using the iron needle under a light pressure.

[0028] As 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 the 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 from the left-side edge of the groove in Fig. 1b. Such an upheaving of the base metal not only brings about the degradation of lamination factor in the electromagnetic steel sheet laminate, but also results in the insulation breakage, so that the validity as an industrial product is lost. On the contrary, according to the invention, the upheaving of the base metal is not caused as seen from Fig. 1a. That is, it is clear that the application of ultrasonic vibration has effects in addition to the decreasing of the pushing pressure at the working tip.

[0029] The improvement of magnetic properties according to the invention is shown by mark ○ in Fig. 2 together with a case (◇) of removing the surface coating with the iron needle and a case (♢) of forming the groove as comparative examples.

[0030] 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 the working diamond tip to form grooves each having a width of 80 µm and a depth of 0.2 µm at a spacing of 5 mm onto the sheet surface in parallel to each other in a direction perpendicular to the rolling direction of the sheet.

[0031] On the other hand, when a steel scriber as the iron needle was used under a light pressure, the 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, the 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 the groove having a depth of 2 µm results in the application of heavy pressure to the base metal. As a result, the iron loss is considerably reduced before the strain relief annealing in the use of iron needle under heavy pressure, but it is inversely degraded after the strain relief annealing. Because, strain is introduced into the base metal by the force applied for the formation of the groove having a depth of 2 µm to conduct the refinement of magnetic domain, so that the iron loss is reduced once but such an effect of reducing the iron loss is lost by the subsequent strain relief annealing (800°C × 3 hours). In this case, the decrease of B₁₀ value is large, so that the iron loss value is poor as compared with the iron loss value just after the secondary recrystallization annealing. Furthermore, the forsterite layer in the vicinity of the groove is 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) is substantially lost and hence the iron loss is largely degraded.

[0032] When the local removal of oxide layer up to a depth of 0.2 µm is carried out by the method of the invention, the improving ratio of iron loss before and after the removal of oxide layer is small as compared with the case of forming the groove under heavy pressure, but the degradation of iron loss is not caused after the strain relief annealing and the improving tendency is rather caused. Though the reason for such an improvement is not clear, it is considered that unnecessary strain slightly introduced by the application of ultrasonic vibrations is disappeared by the strain relief annealing or the oxide layer formed advantageously acts to the improvement of iron loss.

[0033] When the oxide layer is removed to a depth of 0.2 µm by the iron needle under a light pressure, the degradation of the iron loss and magnetic flux density is caused after the strain relief annealing. This is considered due to the fact that the leakage of magnetic flux becomes large by the upheaving of base metal at the worked portion.

[0034] In Japanese Patent laid open No. 56-130454, there is disclosed a technique that ultrasonic wave is applied to a gear-like roll and the roll is linearly contacted to the surface of the grain oriented silicon steel sheet after the secondary recrystallization annealing under a pressure in order to form fine recrystallized grain groups on the sheet surface. This technique is to give complicated strain to the sheet surface for obtaining fine recrystallized grains. Therefore, it is naturally required to apply a strain enough to enable the recrystallization, and consequently the gear roll is used.

[0035] On the contrary, the invention is to locally break and remove the oxide layer, which is entirely different from the formation of fine recrystallized grains. For this purpose, the working tip of needle-­like or plate-like form is used. As a result, the recrystallized grain groups are not newly formed in the method of the invention.

[0036] In a preferred embodiment of the invention, the sheet is subjected to an electrolytic etching after the local removal of the oxide layer. Thus, the effect of magnetic domain refinement can be more improved by utilizing diamagnetic field at groove formed after the local removal of the oxide layer. In another embodiment of the invention, a foreign substance is filled in the grooves after the electrolytic etching to further improve the magnetic properties by mark ● in Figs. 2a and 2b. Of course, the significance of lamination factor is sufficiently held in these cases.

[0037] The results of the iron loss and B₁₀ value in these preferred embodiments are also shown by 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 are obtained when the sheet is subjected to local removal of oxide layer, electrolytic etching in an NaCl aqueous solution (100 g/ℓ) at a current density of 20 A/dm² for 5 seconds, filling with colloidal silica and strain relief annealing (800°C × 3 hours).

[0038] According to the invention, the starting material is required to be a grain oriented silicon steel sheet after secondary recrystallization annealing. That is, the case of applying the method of the inven­tion to the sheet before secondary recrystallization annealing is meaningless, but when the method of the invention is applied to the sheet after the secondary recrystallization annealing, it develops an effect irrespective of previous history of the sheet such as kind of inhibitor, cold rolling number or the like.

[0039] Since the secondary recrystallization annealing is usually carried out at a temperature of 800∼1200°C, the oxide layer is existent onto the surface of the grain oriented silicon steel sheet.

[0040] 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 ultrasonic vibration in order to follow the working tip to the sheet surface. When the pressure exceeds this value, plastic strain is undesirably generated in the surface portion of the sheet.

[0041] The effect by the local removal of the oxide layer is usually unchangeable before or after the formation of insulation coating onto the oxide layer. In this case, the insulation coating may be a tension coating.

[0042] It is desired that the local removal of the oxide layer is carried out in dotted line form or continuous or discontinuous linear form across the rolling direction to repeatedly form the 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 favorable to be 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 the sufficient improvement of iron loss value is not obtained, while when it exceeds 30 mm, the effect of magnetic domain refinement is lost.

[0043] Further, the effect is substantially un­changeable even when the local removal is applied to either one-side surface or both-side surfaces of the sheet.

[0044] In the invention, the local removal of oxide layer is necessary to be carried out by using the working tip subjected to ultrasonic vibration. The shape of the working tip is desirable to be needle-­like. The width of the removed portion can be varied by the size or thickness of the working tip. The width of the removed portion is 10∼1000 µm, preferably about 100 µm. When the width of the removed portions is less than 10 µm, the 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 the improvement of iron loss value is not obtained. Since the ultrasonic vibrations are applied to the working tip in the local removal of oxide such as forsterite or the like, there are advantages that the working strain is small, the tool (working tip) is made small and the smooth surface without the upheaving of the base metal is obtained.

[0045] When the local removal of oxide layer is mechanically carried out by using the iron needle without application of ultrasonic vibration, the plastic deformation portion becomes larger, resulting the large decrease of lamination factor and B₁₀ value.

[0046] Vibrations having a frequency of not less than 10 kHz and an amplitude of not more than 50 µm and mainly containing a component in a perpendicular direction to the sheet surface are preferable as a condition for the application of ultrasonic vibration. When the frequency is less than 10 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 the large strain is caused to decrease the B₁₀ value.

[0047] In this case, pulse or continuous mode is used as a generation mode of ultrasonic vibration.

[0048] As the working tip for giving ultrasonic vibrations to the sheet surface, use 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. Because, when the material is not hard, it is worn to change the removing means of the oxide layer and badly affect the magnetic domain refinement. And also, the semicircular shape having a diameter of more than 2 mm or other shape badly affects the magnetic domain refinement due to the wearing.

[0049] Fig. 3 shows a wearing degree of the working tip together with results using the iron needle as a comparative example.

[0050] 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 the 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.

[0051] On the other hand, 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 of iron needle under a load of 100 kg/mm² as a comparative example.

[0052] As seen from Fig. 3, the iron needle is largest in the wearing degree of working tip, while the electrodeposited diamond used in the application of ultrasonic vibration according to the invention has no weight loss, but the tip of the electrodeposited diamond used under a load of 20 kg/mm² is broken to reduce the weight, which badly affects the state of oxide removal.

[0053] According to the invention, when the electrolytic etching is carried out after the local removal of oxide layer by application of ultrasonic vibration, the iron loss can be further reduced. In this case, the etching depth of the groove is desirable to be not more than 20 µm.

[0054] Fig. 4 shows a relation between the etching depth after the local removal of oxide layer and the magnetic properties.

[0055] 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 φ and forming grooves at a spacing of 8 mm in parallel to each other in a direction perpendicular to the rolling direction through this working tip. Then, the 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 to magnetic properties is shown in Fig. 4.

[0056] The iron loss value is further improved when a substance locally producing a different tension based on the difference of thermal expansion coefficient or a magnetically different substance producing diamagnetic field (for example, metal, silicate, phosphorus compound, oxide, nitride or the like) is filled 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.

[0057] Fig. 5 shows an effect of improving the iron loss value by the filling of foreign substance. In this case, the groove having a depth of 10 µm was formed by the local removal of oxide and the electrolytic etching in the same manner as in Fig. 4. Thereafter, the groove was subjected to Sb plating and further to a strain relief annealing at 800°C for 3 hours.

[0058] Then, the application of ultrasonic vibrations to the sheet surface according to the invention will be described in detail with reference to Figs. 6 to 10.

[0059] In Figs. 6a and 6b is shown a first embodiment of 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 surface of the running steel sheet around the roller 3 and is provided with plural 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 through a screw 6 supported at both ends by bearings 7 and a motor 8.

[0060] 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 injury the surface of the roller 2. Further, the pushing pressure of working tip 14 to the steel sheet 1 can be controlled by adjusting an air pressure applied from the air cylinder 15 to the ultrasonic vibrating member 5.

[0061] When the oxide layer is continuously and locally removed from the surface of the silicon steel sheet by applying ultrasonic vibrations through 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.

[0062] In Figs. 7a and 7b is shown a second embodiment of the apparatus for locally removing oxide layer from the surface of the grain oriented silicon steel sheet after the 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.

[0063] 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.

[0064] According to the above structure, the head portion 4 is moved in the running direction of the sheet or 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.

[0065] In any case, as the number of the ultrasonic vibrating members used increases, the efficiency in the formation of removed track (productivity) becomes naturally excellent. Moreover, in case of 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 is just opposite to that formed at the going stage and the parallel tracks can not be formed on the sheet surface. However, when 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 case of using the apparatus of Fig. 7, 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 is two times in the production efficiency as compared with 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 the ultrasonic vibrating members in the latter apparatus can be reduced to a half in the former apparatus.

[0066] 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 front in the running direction of the sheet.

[0067] The spacing between the adjoining ultrasonic vibrating members is preferable to be about 5 mm. The diameter of the roller 2 is not less than 300 mm for giving no bending strain to the sheet 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 case of the rigid rubber, the hardness is preferable to be not less than 60 (Hs).

[0068] The following examples are given in illustration of the invention and are not intended as limitations thereof.

Example 1

[0069] 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 two-times cold rolling through an intermediate annealing at 950°C to obtain a cold rolled sheet having a final thickness of 0.23 mm.

[0070] Thereafter, the cold rolled sheet was subjected to decarburization and primary recrystallization annealing at 800°C in a wet hydrogen atmosphere, coated at its surface with a slurry of an annealing separator consisting mainly of MgO and coiled, which 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.

[0071] After excessive annealing separator was merely removed from the sheet surface, the sheet was treated under conditions as shown in the following Table 1.

[0072] The iron loss W_17/50 (W/kg) of the thus obtained sheet was measured to obtain results as shown in Table 1.

Table 1

		Local removing treatment of oxide layer				Iron loss value after treatment W17/50(W/kg)	Magnetic properties after the formation of insulation coating and the annealing at 800°C for 2 hours		Lamination factor (%)
		Generation mode of ultrasonic vibration *	Working tip	Working pitch (mm)	Working mode		W17/50 (W/kg)	B₁₀ (T)
1	Acceptable Example	continuous	Electrodeposited diamond	10	linear	0.86	0.85	1.90	-
2	"	pulse	"	10	"	0.86	0.85	1.90	-
3	"	pulse	grindstone	10	"	0.87	0.86	1.90	-
4	"	pulse	Electrodeposited diamond	5	"	0.85	0.83	1.91	97
5	"	continuous	ruby	10	"	0.85	0.83	1.91	-
6	"	continuous	"	10	"	0.86	0.84	1.91	-
7	"	continuous	"	5	"	0.86	0.83	1.90	97
8	"	pulse	steel sheet	10	"	0.85	0.84	1.91	-
9	"	pulse	ruby	10	"	0.86	0.83	1.91	-
10	"	pulse	"	5	"	0.86	0.83	1.91	-
11	"	continuous	sintered diamond	10	"	0.87	0.85	1.90	-
12	"	pulse	"	10	"	0.88	0.86	1.90	-
13	Comparative Example	none	iron needle under heavy pressure	10	"	0.85	0.93	1.87	95
14	"	none	iron needle under light pressure	10	"	0.87	0.89	1.89	96
15	"	none	laser	10	"	0.84	0.91	1.90	97
16	standard	-	-	-	-	-	0.91	1.91	-

* Frequency: 28.5 kHz

Example 2

[0073] A hot rolled sheet of silicon steel containing Si: 3.05%, Mn: 0.073%, Se: 0.020% and Sb: 0.025% was subjected to two-times cold rolling through 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 at its surface with a slurry of an annealing separator consisting mainly of Al₂O₃ and coiled, which 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.

[0074] After the removal of the annealing separator, an insulation coating was formed on the sheet surface, which was then subjected to a flat annealing. Then, the thus treated sheet was subjected to a treatment for locally removing the oxide layer under conditions as 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.

[0075] 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.

Table 2

		Local removing treatment of oxide layer				Iron loss value after treatment W17/50 (W/kg)	Post-treatment	Magnetic properties after strain relief annealing followed by post-treatment		Lamination factor (%)
		Generation mode of ultrasonic vibration *	Working tip	Working pitch (mm)	Working mode			B₁₀ (T)	W17/50(W/kg)
1	Acceptable Example	continuous	ruby	10	linear	0.87	Electrolytic etching	1.91	0.84	97
2	"	continuous	ruby	"	"	0.86	"	1.92	0.84
3	"	pulse	ruby	"	"	0.86	"	1.91	0.83
4	"	pulse	ruby	"	"	0.87	"	1.91	0.84
5	"	continuous	Electrodeposited diamond	"	"	0.88	"	1.92	0.84
6	"	continuous	Electrodeposited diamond	"	"	0.87	"	1.92	0.83
7	"	pulse	Electrodeposited diamond	"	"	0.87	"	1.91	0.84
8	"	pulse	Electrodeposited diamond	"	"	0.86	"	1.91	0.83
9	Comparative Example	none	iron needle (under light pressure)	"	"	0.88	"	1.89	0.91	96
10	"	none	laser	"	"	0.86	"	1.90	0.89	97
11	"	none	scriber (under heavy pressure)	"	"	0.87	"	1.87	0.87	95

* Frequency: 28.5 kHz

Example 3

[0076] A hot rolled sheet of silicon steel containing Si: 3.25%, Mn: 0.072%, Se: 0.018% and Sb: 0.025% was subjected to two times cold rolling through 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 at its surface with a slurry of an annealing separator consisting mainly of MgO and coiled, which 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.

[0077] After the removal of excessive annealing separator and the flat annealing, the sheet was subjected to a treatment for local removal of oxide layer under conditions as shown in the following Table 3. As the post-treatment, the 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 conduct the baking. On the other hand, a part of the sheet was coated with antimony sol and dried at 100°C.

[0078] The iron loss values W_17/50 (W/kg) of the thus obtained sheets were measured to obtain results as 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

[0079] 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.

[0080] 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 at its surface with a slurry of an annealing separator consisting mainly of MgO and coiled, which 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.

[0081] After the removal of excessive annealing separator and the flat annealing, the sheet was subjected to a treatment for the local removal of oxide layer under conditions as shown in the following Table 4.

[0082] The iron loss values W_17/50 (W/kg) of the thus obtained sheets were measured to obtain results as 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.

Table 4

		Local removing treatment of oxide layer				Iron loss value W17/50 (W/kg)	Magnetic properties after the formation of insulation coating and the strain relief annealing	Lamination factor (%)
		Generation mode of ultrasonic vibration *	Working tip	Working pitch (mm)	Working mode		Iron loss value W17/50(Wkg)
1	Acceptable Example	continuous	ruby	10	linear	0.86	0.84	97
2	"	continuous	ruby	5	"	0.84	0.83
3	"	pulse	ruby	10	"	0.85	0.83
4	"	pulse	ruby	5	"	0.84	0.82
5	"	continuous	Electrodeposited diamond	10	"	0.86	0.84
6	"	continuous	"	5	"	0.84	0.83
7	"	pulse	"	10	"	0.85	0.83
8	"	pulse	"	5	"	0.84	0.82
9	Comparative Example	none	iron needle (under light pressure)	5	"	0.86	0.88	96
10	"	none	laser	5	"	0.82	0.89	97

* Frequency: 28.5 kHz

Example 5

[0083] The oxide layer was locally removed from the surface of the grain oriented silicon steel sheet after the 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 to 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.

[0084] Similarly, the oxide layer was removed by using a working tip of super-hard alloy with a sharp point without application of ultrasonic vibration. In this case, a load of 10 kg/mm² was applied to the working tip.

[0085] After the removal of oxide layer, the 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 an Ni plating and further to a strain relief annealing (800°C × 2 hours). The magnetic properties of the thus obtained sheets are shown in the following Table 5.

Table 5

▲ degradation

		Conditions for local removal of oxide
		Application of ultrasonic vibrations			No application of ultrasonic vibration
		after local removal	after etching	after strain relief annealing	after local removal	after etching	after strain relief annealing
no electrolytic etching and filling	Δ W17/50 (W/kg)	0.05	-	0.06	0.04	-	0.02
	Δ B₁₀ (T)	▲0.005	-	0	▲0.03	-	▲0.02
no filling after etching	Δ W17/50 (W/kg)	0.05	0.06	0.07	0.04	0.04	0.02
	Δ B₁₀ (T)	▲0.005	▲0.02	▲0.01	▲0.03	▲0.05	▲0.04
filling after etching	Δ W17/50 (W/kg)	0.05	0.06	0.08	0.04	0.04	0.04
	Δ B₁₀ (T)	▲0.005	▲0.02	▲0.01	▲0.03	▲0.05	▲0.04

[0086] As mentioned above, according to the invention, grain oriented silicon steel sheets having a very low iron loss and not losing the effect of magnetic domain refinement even after strain relief annealing can be produced without causing the decreases of lamination factor and B₁₀ value which have never been avoided in the conventional technique.

Claims

1. A method of producing a low iron loss grain oriented silicon steel sheet not causing degradation of properties through strain relief annealing, characterized in that ultrasonic vibrations are applied to a surface of a grain oriented silicon steel sheet after secondary recrystallization annealing to locally remove an oxide layer from the surface of the sheet.

2. The method according to claim 1, wherein after said local removal of oxide layer, an electrolytic etching is applied to said sheet.

3. The method according to claim 1, wherein after said local removal of oxide layer, an eletrolytic etching is applied to said sheet and then a foreign substance is filled in said etched portions.

4. The method according to claim 1, wherein said local removal of oxide layer is carried out by arranging a head portion of an apparatus for generating ultrasonic vibrations in opposition to said surface of the running sheet around a roller so as to reciprocatedly move in the widthwise direction of said running sheet, staggeredly arranging plural ultrasonic vibrating members movable toward and away from said surface of the running sheet in said head portion in up and down directions thereof, pushing a working tip of each ultrasonic vibrating member to said surface of the running sheet under a predetermined 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 form removed portions on said surface of the running sheet in a direction perpendicular to the rolling direction thereof at a given spacing.

5. The method according to claim 4, 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.

6. The method according to claim 1, wherein said oxide removed portions have a width of 10∼1000 µm in form of dot or continuous or discontinuous linear form and are formed in parallel to each other at a spacing of 1∼30 mm.

7. The method according to claim 1, wherein said ultrasonic vibration has a vibrating component in a direction perpendicular to said surface of the running sheet having a frequency of not less than 10 kHz and an amplitude of not more than 50 µm.

8. The method according to claim 1, 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.

9. The method according to claim 1, wherein said pushing pressure of said working tip is not more than 40 kg/mm².

10. The method according to claim 2, wherein an etching depth by said electrolytic etching is not more than 20 µm.

11. The method according to claim 3, wherein said foreign substance is selected from the group consisting of metal, silicate, phosphorus compound, oxide and nitride.

Drawing