Title of Invention: Method for producing amorphous alloy ribbon, and method for producing nano-crystalline alloy ribbon with same Abstract: An amorphous alloy ribbon free from embrittlement and crystallization and having excellent surface conditions and shape in edge portions is produced by (a) preparing an alloy melt having a composition comprising 13 atomic % or less of B and 15 atomic % or less of at least one selected from the group consisting of transition elements of Groups 4A, 5A and 6A, the balance being substantially Fe; (b) ejecting the alloy melt at a temperature from the melting point of the alloy +50xc2x0 C. to the melting point of the alloy +250xc2x0 C. through a nozzle onto the cooling roll rotating at a peripheral speed of 35 m/second or less, a distance between a tip end of the nozzle and the cooling roll being 200 xcexcm or less; (c) starting to supply a gas based on CO2 to the alloy melt after the surface temperature of the cooling roll has become substantially constant; and (d) grinding the cooling roll while supplying the gas based on CO2. Brief Summary: The present invention relates to a method for producing an amorphous alloy ribbon having excellent surface conditions and shape in edge portions, and a method for producing a nano-crystalline alloy ribbon using such an amorphous alloy ribbon. Liquid-quenching methods are widely known as methods for producing amorphous alloy ribbons for use in magnetic cores, magnetic shields, etc. The liquid-quenching methods include a single roll method, a double roll method, a centrifugal method, etc., and preferable among them from the aspect of productivity and the maintenance of an apparatus is a single roll method in which a molten metal is supplied onto a cooling roll rotating at a high speed and rapidly quenched to form an alloy ribbon. FIG. 1 shows one example of apparatuses for carrying out the single roll method. An alloy ingot in a crucible 1 is melted by a high-frequency coil 2, and the resultant alloy melt 3 is ejected through a nozzle 4 onto a cooling roll 5 rotating at a high speed and rapidly quenched to form an amorphous alloy ribbon 6. As shown in FIG. 1, for instance, a high-pressure gas such as nitrogen, a compressed air, etc. is supplied from a peeling-gas nozzle 7 in an opposite direction to the rotation direction of the cooling roll 5 immediately after the casting, thereby forcedly peeling the amorphous alloy ribbon 6 from the cooling roll 5. The amorphous alloy ribbon 6 produced by the above method tends to be provided with small dents called xe2x80x9cair pocketsxe2x80x9d on a side in contact with the cooling roll 5. This is because a gas is entrained into a boundary between a melt pool portion 10 (hereinafter referred to as xe2x80x9cpaddlexe2x80x9d) and the cooling roll 5 by the rotation of the cooling roll 5, so that it expands in the paddle 10 until the melt is solidified. Because the formation of such air pockets leads to the roughing of a surface of the ribbon 6, the air pockets should be as few as possible. Proposed by German Patent DD266046A1, Japanese Patent Laid-Open No. 6-292950, etc. to suppress the formation of air pockets is a method in which a CO2 gas is supplied to the paddle in various directions. This method is advantageous in that it can suppress the formation of air pockets to reduce the surface roughness of a ribbon on a side in contact with the cooling roll. Alternatively, Japanese Patent Laid-Open No. 59-209457, Japanese Patent Publication No. 1-501924, etc. propose a method for producing an amorphous alloy ribbon in vacuum or in a He atmosphere, a method for producing an amorphous alloy ribbon while flowing a gas having a lower density than the air at normal temperature, such as a heated CO gas, a He gas at normal temperature etc., to the paddle from rearward. Why the formation of air pockets can be suppressed by these methods seems to be the fact that a gas entrained by the rotation of the cooling roll has a reduced density, resulting in decrease in the kinetic pressure of the gas impinging on the paddle, thereby suppressing the vibration of the paddle. Among the above methods, the method of flowing a CO2 gas to the paddle from rearward (from a side opposite to the side on which the ribbon is formed) is suitable for the mass production of amorphous alloy ribbons from the aspect of production cost and safety. The total length of an amorphous alloy ribbon continuously produced in one casting lot by a liquid-quenching method generally exceeds 3,000 m. When the resultant amorphous alloy ribbon is wound around a reel after the completion of casting, the ribbon is likely to be twisted. Accordingly, the quenched ribbon should continuously be wound immediately after peeling from the cooling roll. For instance, Japanese Patent Laid-Open Nos. 8-318352 and 11-188458 disclose a method in which a magnetized reel rotating in an opposite direction to a cooling roll is positioned near the cooling roll to magnetically attract the peeled ribbon, which is continuously wound around the reel. It is also known that the heat treatment of an amorphous alloy ribbon produced by the above-described methods at a temperature equal to or higher than the crystallization temperature of the alloy can provide a nano-crystalline alloy ribbon having an average particle size of 100 nm or less. Typical alloys capable of forming nano-crystalline alloy ribbons are Fexe2x80x94Sixe2x80x94Bxe2x80x94(Nb, Ti, Hf, Mo, W, Ta)xe2x80x94Cu alloys, Fexe2x80x94(Co, Ni)xe2x80x94Cuxe2x80x94Sixe2x80x94Bxe2x80x94(Nb, W, Ta, Zr, Hf, Ti, Mo) alloys, Fexe2x80x94(Hf, Nb, Zr)xe2x80x94B alloys, Fexe2x80x94Cuxe2x80x94(Hf, Nb, Zr)xe2x80x94B alloys, etc. described in Japanese Patent Publication Nos. 4-4393 and 7-74419, Japanese Patent 2,812,574, etc. The nano-crystalline alloys are not only substantially free from thermal instability unlike the amorphous alloy, but also are subjected to less change with time and have lower magnetostriction and higher permeability than the amorphous alloys, they are used for common-mode choke coils, pulse transformers, leakage breakers, etc. As a result of experiment to produce an amorphous alloy ribbon 6 using a laboratory-scale apparatus whose casting time is less than 30 seconds while flowing a CO2 gas, the resultant ribbon had excellent surface conditions. However, in a production experiment using a mass-production-scale apparatus, it was found that as the casting time passed, there arose the problems of embrittlement and crystallization in the formed amorphous alloy ribbon that were not observed in the short casting process, though the surface conditions of the amorphous alloy ribbon was improved by the supply of a CO2 gas. In addition to these problems, it has also be found that a new problem of serrated irregular shapes in their edge portions takes place. This phenomenon never occurs even in the long casting process of an amorphous alloy ribbon unless a CO2 gas is supplied. Because the total length of an amorphous alloy ribbon continuously produced in one casting step by a mass-production apparatus exceeds 3,000 m, the amorphous alloy ribbon is continuously wound around a large reel during the casting from the aspect of efficiency. The ribbon is then divided to proper length that can easily be handled to produce wound cores, etc., and wound around a large number of small reels. At this time, if the ribbon had a serrated irregular shape in its edge portions, the edge portion of the ribbon engages a reel, resulting in extreme difficulty in handling. The irregular shape of the ribbon in its edge portions also poses inconveniences in the production of a wound core. In the continuous production of a wound core from a ribbon, the winding of the ribbon is often carried out with the edge portions of the ribbon abutting against a plate to make the resultant wound core have a constant height. In this case, too, if the ribbon had a serrated irregular shape in its edge portions, the ribbon engages the abutting plate, thereby making the production of a wound core difficult. If the ribbon were brittle, breakage, cracking, etc. would be likely to occur in the production of wound cores and laminated cores. In addition, if the ribbon contains coarse crystals, it would have large crystal magnetic anisotropy, resulting in the deterioration of its soft magnetic properties. Further, if an amorphous alloy ribbon having coarse crystals were heat-treated at a temperature equal to or higher than the crystallization temperature of the alloy, the resultant nano-crystalline alloy ribbon would have deteriorated soft magnetic properties. Accordingly, an object of the present invention is to provide a method for continuously producing an amorphous alloy ribbon having improved surface conditions on a side in contact with a cooling roll and excellent edge shapes, free from embrittlement and crystallization. Another object of the present invention is to provide a method for producing a nano-crystalline alloy ribbon by heat-treating such an amorphous alloy ribbon. As a result of investigating problems such as irregular edge shapes, embrittlement and crystallization occurring as the amount of a ribbon cast increases (as casting continues longer) in the production of an amorphous alloy ribbon with a CO2 gas supplied, the inventors have found that the above problems can be overcome by grinding a cooling roll during the casting. The present invention has been completed based on this finding. Thus, the method for producing an amorphous alloy ribbon by ejecting an alloy melt onto a cooling roll and rapidly quenching it according to the present invention comprises carrying out the grinding of the cooling roll while supplying a gas based on CO2 near a paddle of said alloy melt during the casting. The alloy used in the present invention preferably has a composition comprising 13 atomic % or less of B and 15 atomic % or less of at least one selected from the group consisting of transition elements of Groups 4A, 5A and 6A, the balance being substantially Fe. Also, when the amorphous alloy ribbon is to be heat-treated for nano-crystallization, the alloy melt preferably contains 3 atomic % or less of at least one of Cu, Ag and Au. When a gas based on CO2 is supplied near a paddle of an alloy melt ejected from a nozzle onto a cooling roll immediately after the start of casting, the ribbon tends to be broken. However, when the gas based on CO2 starts to be supplied after the surface temperature of the cooling roll has become substantially constant, the possibility of the breakage of the ribbon substantially decreases. Here, xe2x80x9csurface temperature has become substantially constantxe2x80x9d means that the variation range of the surface temperature of the cooling roll has become within 10xc2x0 C. relative to its average temperature. Though the surface temperature of the cooling roll generally starts to elevate immediately after the start of casting, it becomes substantially constant in several seconds to ten and several seconds because heat from the alloy melt gets balanced with heat dissipating from the cooling roll. The peripheral speed of the cooling roll is preferably 35 m/second or less, more preferably 20-30 m/second. The temperature of the alloy melt is preferably from the melting point of the alloy +50xc2x0 C. to the melting point of the alloy +250xc2x0 C., more preferably from the melting point of the alloy +100xc2x0 C. to the melting point of the alloy +200xc2x0 C. In addition, the distance from a tip end of a nozzle to a cooling roll is preferably 200 xcexcm or less, more preferably 100-180 xcexcm, further preferably 100-150 xcexcm. Under such casting conditions, it is possible to stably produce an amorphous alloy ribbon having a thickness of 8-25 xcexcm, particularly 8-19 xcexcm. The preferred method of the present invention for producing an amorphous alloy ribbon by ejecting an alloy melt onto a cooling roll and rapidly quenching it comprises (a) preparing an alloy melt having a composition comprising 13 atomic % or less of B and 15 atomic % or less of at least one selected from the group consisting of transition elements of Groups 4A, 5A and 6A, the balance being substantially Fe; (b) ejecting the alloy melt at a temperature from the melting point of the alloy +50xc2x0 C. to the melting point of the alloy +250xc2x0 C. through a nozzle onto the cooling roll rotating at a peripheral speed of 35 m/second or less, a distance between a tip end of the nozzle and the cooling roll being 200 xcexcm or less; (c) starting to supply a gas based on CO2 to the alloy melt after the surface temperature of the cooling roll has become substantially constant; and (d) grinding the cooling roll while supplying the gas based on CO2. The heat treatment of the amorphous alloy ribbon produced by the above method at a temperature equal to or higher than the crystallization temperature of the alloy can provide a nano-crystalline alloy ribbon having a nano-crystalline structure having an average particle size of 100 nm or less. Detailed Description: [1] Supply of CO2 Gas and the Grinding of the Cooling Roll An important feature of the present invention is that the grinding of the cooling roll is carried out while supplying a gas based on CO2, in the production process of an amorphous alloy ribbon for a long period of time. The gas based on CO2 is a pure CO2 gas or a mixture of a CO2 gas and another gas, and another gas should not exert adverse effects on the resultant amorphous alloy ribbon. As a result of investigation of the problems inherent in the casting of an amorphous alloy ribbon in the presence of a CO2 gas for a long period of time, it has been found that the influence of the deterioration of the surface roughness of a cooling roll on the ability of the cooling roll to cool a melt is extremely larger when a CO2 gas is supplied during the casting than when no CO2 gas is supplied, and that the deterioration of the surface roughness of the cooling roll also affects the shape of resultant ribbon in its edge portions. The deterioration of the surface roughness of the cooling roll is caused by the formation of dents by incessant impingement by a high-temperature melt, the adhesion of an alloy to the surface of the cooling roll, etc. Further, to investigate the surface roughness of a cooling roll necessary for avoiding the above problems, comparison has been carried out on the surface roughness of cooling rolls according to JIS B 0601. As a result, it has been found that good amorphous alloy ribbons are obtained, when the cooling roll is ground such that it has an average surface roughness Ra of 0.5 xcexcm or less and a ten-point average surface roughness Rz of 4 xcexcm or less during the casting. It has also been found that better ribbons are obtained by having the cooling roll keep an average surface roughness Ra of 0.3 xcexcm or less and a ten-point average surface roughness Rz of 2 xcexcm or less. Investigation and consideration will be given in detail below with respect to the influence of the supply of a CO2 gas and the grinding of the cooling roll on the edge shape, embrittlement and crystallization of the resultant amorphous alloy ribbon, and the reasons why the grinding of the cooling roll should be carried out while supplying a CO2 gas. (1) Embrittlement and Crystallization of Ribbon When an amorphous alloy ribbon is produced by a mass-production-scale apparatus for a long period of time while supplying a CO2 gas without grinding the cooling roll, the cast amorphous alloy ribbon tends to suffer from extreme embrittlement and crystallization. On the other hand, when a CO2 gas is supplied while grinding the cooling roll, substantially no embrittlement and crystallization take place in the amorphous alloy ribbon even after a long period of casting by a mass-production-scale apparatus. (2) Shape of Ribbon Edge When the amorphous alloy ribbon is cast by a mass-production-scale apparatus for a long period of time while supplying a CO2 gas without grinding the cooling roll, extreme irregularity such as serration occurs in the shape of ribbon edges as the casting process proceeds. The serrated shape in the edge portions of the cast amorphous alloy ribbon becomes increasingly remarkable as the casting time passes. On the other hand, when a CO2 gas is supplied with the cooling roll ground, substantially no irregular shapes are formed in ribbon edges even in the long casting process. Thus, the problems caused by the supply of a CO2 gas can be overcome by grinding the cooling roll during the casting of a ribbon. The first reason why the air pockets of an alloy melt decrease by a CO2 gas is the thermal decomposition of the CO2 gas. When the CO2 gas is brought into contact with an alloy melt in the shape of a ribbon ejected from a nozzle, CO2 is thermal decomposed by the heat of the alloy melt to generate CO and O2, thereby forming a uniform oxide layer on the paddle surface. As a result, the viscosity of the paddle increases, thereby suppressing the vibration of the paddle. This in turn makes it difficult to entrain a CO2 gas in the paddle, resulting in decrease in air pockets. The second reason is that a CO2 gas has larger specific heat than the air, nitrogen, etc. With the cooling roll subjected to contact and friction with a high-temperature melt, the cooling roll surface is roughened, and a gas highly tends to be entrained into a paddle by the rotation of the cooling roll. In such a case, air pockets are unlikely to be formed in the presence of a CO2 gas, because a CO2 gas having a large specific heat is not so much thermally expanded in the paddle as other gases. Though a CO2 gas had the above advantage, it is more heated by radiant heat from the paddle than the air and nitrogen, because it has large heat absorption than the air and nitrogen. Therefore, the CO2 gas entrained into a boundary between the paddle and the cooling roll is at a higher temperature than the air and nitrogen. With a high-temperature CO2 gas entrained into the above boundary, it is presumed that the cooling speed of the ribbon on the cooling roll is lowered, making it more likely to cause embrittlement and crystallization in the ribbon. With dents on the cooling roll surface, the gas is likelier to be entrained in the paddle, and particularly in the case of using a CO2 gas having large heat absorption, the formation of dents on the cooling roll directly leads to increase in the embrittlement and crystallization of the ribbon. Therefore, the suppression of the formation of dents on the cooling roll surface is important to reduce air pockets by a CO2 gas in the long casting process of an amorphous alloy ribbon, and to suppress the embrittlement and crystallization of the amorphous alloy ribbon that start to occur after the surface temperature of the cooling roll has become substantially constant. It is generally known that the kinetic pressure of a gas is proportional to the square of the specific gravity of a gas. When a CO2 gas having a larger specific gravity than the air, nitrogen, etc. impinges on a paddle, the momentum of the CO2 gas to the paddle 10 is extremely larger than that of the air, etc. Taking a nitrogen gas having a specific gravity of 1.1 and a CO2 gas having a specific gravity of 1.8 at 300 K for instance, the kinetic pressure of the CO2 gas is about 2.7 times that of nitrogen at the same flow rate at 300 K. When there are few dents on the surface of a cooling roll 5 with little entrainment of a gas into the paddle 10, no problem is caused by a large momentum given by the CO2 gas in FIG. 2. However, in a state where a large number of dents are formed on the cooling roll 5, a large amount of a CO2 gas is entrained into the paddle 10. In this case, it is presumed that the paddle 10 is likely to be vibrated because of a large momentum of a CO2 gas, resulting in irregularities such as serration in the shape of edge portions of the resultant amorphous alloy ribbon. Accordingly, to suppress irregularities such as serration in the shape of edge portions of the ribbon that occurs when using a CO2 gas, it is important to suppress dents of the cooling roll 5, which increase as the casting time passes. Because the cooling roll 5 without deteriorated surface roughness is free from the above problems, the grinding of the cooling roll 5 may start at any time during the casting process. Also, though the grinding may be carried out intermittently with a certain interval, it is preferably carried out continuously from the aspect of easiness of control. Though the grinding of the cooling roll may be conducted with grinding papers, it is preferably carried out with a brush that does not generate dust, because the insufficient collection of dust generated by grinding causes such problem that pores, etc. are formed in the ribbon. As shown in FIG. 2, it is preferable to use as a brush 11 a roll-shaped wire brush rotating in the same direction as the cooling roll 5. The rotation speed of the brush 11 is preferably ⅓ or less, particularly {fraction (1/10)}-⅕, of the peripheral speed of the cooling roll 5. When the brush 11 used for grinding is too hard, the surface of the cooling roll 5 is deeply scratched by grinding, resulting in such problems as the breakage of the ribbon 6 during the casting and decrease in the improvement of surface roughness of the cooling roll 5. Accordingly, the brush 11 for grinding preferably has the same hardness or less as that of the cooling roll 5. Such brush 11 is preferably constituted by thin wires of stainless steel, brass, copper, etc. Each wire for the brush 11 preferably has a diameter of 0.03-0.1 mm. The materials of the cooling roll 5 are preferably alloys having high thermal conductivity, such as Cu, Cuxe2x80x94Be alloys, Cuxe2x88x92Cr alloys, etc. Also, when a cooling medium such as water, etc. is caused to flow in the cooling roll 5 in a circumferential or axial direction, the control of the surface temperature of the cooling roll 5 becomes easier. [2] Amorphous Alloy Ribbon (1) Alloy Composition The alloy to which the method of the present invention is applicable contains B from the aspect of capability of forming an amorphous structure. However, an excessively large content of B not only leads to the embrittlement and decrease in a magnetic flux density of an amorphous alloy ribbon, but also causes the problem of a higher cost because B is an expensive element. Accordingly, the content of B is preferably 13 atomic % or less. The more preferred content of B 4-13 atomic %. Particularly in the case of an Fe-based amorphous alloy for forming a nano-crystalline alloy, the content of B is preferably 10 atomic % or less, more preferably 4-10 atomic %. With the content of B of 10 atomic % or less, magnetically hard compounds such as Fe3B and Fe2B that hinder the movement of magnetic domain walls are difficult to be precipitated because of a nano-crystalline structure in the heat treatment, thereby making it likely to obtain a uniform nano-crystalline structure based on a bcc-Fe solid solution. The transition elements of Groups 4A, 5A and 6A are effective for the adjustment of magnetostriction and the improvement of permeability of an amorphous alloy. The preferred transition elements in Groups 4A, 5A and 6A are Ti, Zr, Hf, V, Nb, Ta, Cr, Mo and W. These elements are concentrated in an amorphous phase remaining around bcc-Fe crystal grains formed in the heat treatment of an amorphous alloy ribbon at a temperature equal to or higher than the crystallization temperature of the alloy, thereby stabilizing the remaining amorphous phase and thus suppressing the growth of bcc-Fe crystal grains. The amount of at least one of these transition elements added is preferably 15 atomic % or less. When the amount of the transition element added exceeds 15 atomic %, embrittlement and decrease in a magnetic flux density are likely to occur in the amorphous alloy ribbon. Though the effect of suppressing the formation of bcc-Fe crystal grains differs depending on elements, the preferred content of the transition element is 1-10 atomic %. When at least one of Cu, Ag and Au is added to the amorphous alloy ribbon together with at least one of elements of Groups 4A, 5A and 6A, the number of primary crystal nuclei precipitated increases in the heat treatment at a temperature equal to or higher than the crystallization temperature of the alloy, thereby making the precipitated crystal grains finer. The amount of at least one of Cu, Ag and Au added is preferably 3 atomic % or less. When the amount of this element added exceeds 3 atomic %, the resultant amorphous alloy ribbon becomes brittle. Because Cu, Ag and Au are elements that are easily separated from Fe, they would be separated from Fe even with a liquid-quenching method, failing to obtain a uniform solid solution, if their contents are excessive. Incidentally, the lower limits of the amounts of Cu, Ag and Au added are preferably 0.1 atomic % for each. (2) Production Method The method of the present invention for producing an amorphous alloy ribbon is characterized by carrying out the grinding of the cooling roll while supplying a gas based on CO2 near a paddle of an alloy melt ejected onto the cooling roll. When the gas based on CO2 is supplied to a paddle 10 in FIG. 2, there is a large likelihood that the amorphous alloy ribbon 6 is broken at the initial stage of casting. As a result of investigation of a ribbon 6 broken at the very initial stage of casting with a CO2 gas supplied since before the start of casting, it has been found that the breakage of the ribbon 6 occurs while the ribbon 6 is still on the cooling roll 5, and that no cracking and breakage occur in the edge portions of the ribbon 6 after winding. Because the surface temperature of a cooling roll is unstable several seconds to ten and several seconds after the start of casting, regardless of whether or not a CO2 gas is supplied to improve the surface conditions of the ribbon 6, the paddle 10 does not have a stable shape. Accordingly, the ribbon 6 obtained immediately after the start of casting tends to be relatively thin and weak and have a large number of pores, etc., whereby it is easily broken. Because the supply of a gas based on CO2 to a paddle 10 immediately after the start of casting leads to decrease in air pockets generated in the ribbon 6 on a side in contact with the cooling roll 5, a melt and a solidified ribbon 6 are in direct contact with the cooling roll 5 in a larger area, resulting in a higher cooling speed of the ribbon 6. As a result, the solidified ribbon 6 is subjected to rapid thermal shrinkage, undergoing higher internal stress than when a gas based on CO2 is not supplied. This seems to be why cracking and breakage are observed in edge portions of the ribbon 6 after winding. It is thus presumed that high stress in the ribbon 6 results in higher likelihood of breakage. For the reasons described above, to prevent the ribbon 6 from being broken at the initial stage of casting, it is effective to start supplying a gas based on CO2 near a paddle of the alloy melt, after the surface temperature of the cooling roll 5 becomes constant and the state of the ribbon 6 is stabilized. The peripheral speed of the cooling roll 5 is preferably 35 m/second or less. With the peripheral speed of the cooling roll 5 set at 35 m/second or less, the ribbon 6 can be provided with improved surface conditions and shape in its edge portions. Particularly the shape of the ribbon 6 in edge portions can remarkably be improved. Though thinner ribbons are usually cast at higher roll peripheral speed than thicker ribbons, the peripheral speed of the cooling roll 5 is preferably as low as possible even for thin ribbons from the aspect of easiness of control. In this sense, the peripheral speed of the cooling roll 5 is more preferably 20-30 m/second, particularly preferably 27-30 m/second. The temperature of a melt is preferably from the melting point of its alloy +50xc2x0 C. to the melting point of its alloy +250xc2x0 C. When the temperature of a melt is lower than the melting point of its alloy +50xc2x0 C., the nozzle 4 tends to be clogged. On the other hand, when the temperature of the melt is higher than the melting point of its alloy +250xc2x0 C., the cooling speed of the melt is slow near the solidification temperature of the melt, resulting in the brittleness and crystallization of alloy ribbons. The high melt temperature also provides the resultant alloy ribbons with rough surface because of large wear of the cooling roll 5. The more preferred temperature of the melt is from the melting point of its alloy +100xc2x0 C. to the melting point of its alloy +200xc2x0 C. The distance D between the tip end of the melt-ejecting nozzle 4 and the cooling roll 5 is preferably 200 xcexcm or less. When a thin ribbon is produced at a low peripheral speed, it is preferable to reduce the size of a slit of the melt-ejecting nozzle 4 or the distance D between the melt-ejecting nozzle 4 and the cooling roll 5. The more preferred distance D is 100-180 xcexcm, particularly 100-150 xcexcm. As shown in FIG. 2, the position of a nozzle 9 blowing a gas based on CO2 is preferably on the upstream side of the melt-ejecting nozzle 4. Here, xe2x80x9con the upstream side of a nozzle 4xe2x80x9d means a front side in a direction of the relative movement of the nozzle 4 to the cooling roll 5. The ejection direction of a CO2 gas, which is identical to the direction of the nozzle 9, is preferably the radial direction of the cooling roll 5. Also, the ejection direction of a gas based on CO2 is preferably set substantially upstream of the paddle. With the ejection of a gas based on CO2 directed to the cooling roll 5 substantially upstream of the paddle 10, the conditions of the ribbon 6 on a freely solidified surface are improved, and the formation of air pockets are more remarkably suppressed, than when the gas is directly supplied to the tip end of the nozzle 4 or to the paddle 10. (3) Amorphous Alloy Ribbon In the amorphous alloy ribbon produced under the above conditions, substantially no xcex1-Fe crystal phase is precipitated. Crystals precipitated in the amorphous alloy ribbon because of improper casting conditions are those in the form of dendrite as large as 0.2-1 xcexcm in particle size not nano-crystalline, which are much larger than fine crystals precipitated from the amorphous phase by the heat treatment. The existence of such large crystals makes the nano-crystalline structure obtained by the heat treatment non-uniform, resulting in larger crystal magnetic anisotropy and thus deteriorated magnetic properties of the nano-crystalline alloy ribbon. The method of the present invention can generally provide an amorphous alloy ribbon having a thickness in a range of 8-25 xcexcm. Particularly even at a thickness of 19 xcexcm or less, the amorphous alloy ribbon obtained by the present invention has excellent surface conditions and shape in edge portions. In addition, the method of the present invention is suitable for stably producing an amorphous alloy ribbon having a length of about 3,000 m or more and excellent surface conditions and shape in edge portions. [3] Nano-Crystalline Alloy Ribbon The amorphous alloy ribbon 6 with low crystallinity obtained by the method of the present invention can be converted to a nano-crystalline alloy ribbon by a heat treatment. Here, xe2x80x9cnano-crystallinexe2x80x9d means a metal structure containing fine crystals having an average particle size of 100 nm or less. As described above, to obtain a nano-crystalline alloy ribbon with excellent magnetic properties, it is important that the amorphous alloy ribbon 6 to be heat-treated should not contain coarse crystals. This is because the slightest amount of course crystals in the amorphous alloy ribbon 6 to be heat-treated causes extreme deterioration in magnetic properties in a nano-crystalline alloy ribbon obtained by a heat treatment at a crystallization temperature or higher. The heat treatment for obtaining a nano-crystalline alloy ribbon comprises heating at 400-700xc2x0 C. for 1 minute to 24 hours. When the heat treatment temperature is lower than 400xc2x0 C., fine crystallization cannot substantially be achieved. On the other hand, when it exceeds 700xc2x0 C., coarse crystal grains are likely to be precipitated. The preferred heat treatment temperature is 500-650xc2x0 C. Though the heat treatment time may vary depending on the heat treatment temperature, the heat treatment time of less than 1 minute generally fails to achieve the stable precipitation of fine crystals. On the other hand, when it exceeds 24 hours, only low production efficiency of forming a nano-crystalline alloy ribbon can be achieved. Incidentally, because the heat treatment of the amorphous alloy ribbon in the air oxidizes a ribbon surface, resulting in deterioration in the magnetic properties of the resultant nano-crystalline alloy ribbon, the heat treatment is preferably carried out in an inert atmosphere such as nitrogen, argon, etc. The present invention will be described in detail referring to EXAMPLES below without intention of limiting the present invention thereto. Production and Evaluation of Amorphous Alloy Ribbon An ingot of an Fe-based alloy having a composition of Cu1Nb2.5Si13.5B7Febal. by atomic % was introduced into a crucible 1 shown in FIG. 1, and melted by induction heating by a high-frequency coil 2. The resultant alloy melt 3 was ejected onto a cooling roll 5 made of a Cuxe2x80x94Be alloy and rapidly quenched under the conditions shown in Table 1 below, to form an amorphous alloy ribbon 6 of EXAMPLE 1 having a width of 30 mm and a thickness of 19 xcexcm. As shown in FIG. 1, the resultant amorphous alloy ribbon 6 was caused to peel off from the cooling roll 5 by supplying a nitrogen gas from a nozzle 7 onto the cooling roll 5 in an opposite direction to the rotation direction of the cooling roll 5, and a reel 8 having a permanent magnet embedded therein and rotating in an opposite direction to the rotation direction of the cooling roll 5 was brought close to the cooling roll 5 to wind the amorphous alloy ribbon 6. The winding of the amorphous alloy ribbon 6 onto the reel 8 was started after about 2 seconds from the start of casting. Portions of the resultant amorphous alloy ribbon were taken as samples when 1 minute passed from the start of casting, and when 10 minutes passed from the start of casting, respectively. The samples of EXAMPLE 1 and COMPARATIVE EXAMPLES 1 and 2 were ribbon portions obtained when 1 minute passed from the start of casting, and the samples of EXAMPLE 2 and COMPARATIVE EXAMPLES 3 and 4 were ribbon portions obtained when 10 minutes passed from the start of casting. Incidentally, the grinding of the cooling roll 5 was carried out, while supplying a CO2 gas and rotating a brush 11 in the same direction as the cooling roll 5 in the production process of the ribbon 6 of EXAMPLES 1 and 2. The grinding of the cooling roll 5 was not carried out in the production process of the ribbon 6 of COMPARATIVE EXAMPLES 1 and 3, though a CO2 gas was supplied. The supply of a CO2 gas and the grinding of the cooling roll 5 were not carried out in the production process of the ribbon 6 of COMPARATIVE EXAMPLES 2 and 4. With respect to each of the resultant ribbons, an average surface roughness Ra on both of the freely solidified surface and the surface in contact with the cooling roll 5, the presence of crystals and embrittlement were examined. The average surface roughness Ra was measured according to JIS B 0601. The presence of crystals was evaluated by the intensity of a peak of xcex1-Fe (200) in the X-ray diffraction pattern of each ribbon. The presence of embrittlement was evaluated by cracking caused by a 180xc2x0 bending test according to JIS Z 2248. In addition to the above items, a reflection electron image of an edge portion of each ribbon was taken using a scanning electron micrograph to evaluate the serrated irregularities of the edge portion. The evaluation results of the amorphous alloy ribbons of EXAMPLE 1 and COMPARATIVE EXAMPLES 1 and 2 are shown in Tables 2 and 3. As is clear from Table 2, when 1 minute passed from the start of casting, the ribbons of EXAMPLE 1 and COMPARATIVE EXAMPLE 1 produced while supplying a CO2 gas had substantially the same surface roughness, which was smaller than that of the ribbon of COMPARATIVE EXAMPLE 2 produced without supplying a CO2 gas, regardless of whether or not the grinding of the cooling roll 5 was carried out. With respect to embrittlement, crystallization and edge shape, there were no appreciable differences between the ribbon of EXAMPLE 1 and those of COMPARATIVE EXAMPLES 1 and 2. As is clear from Table 3, when 10 minutes passed from the start of casting, too, the ribbons 6 of EXAMPLE 2 and COMPARATIVE EXAMPLE 3 produced while supplying a CO2 gas, regardless of whether or not the grinding of the cooling roll 5 was carried out, had substantially the same surface roughness, which was extremely smaller than that of the ribbon of COMPARATIVE EXAMPLE 4. However, it was confirmed that the ribbon 6 of COMPARATIVE EXAMPLE 3 produced only by supplying a CO2 gas without grinding the cooling roll 5 was poorer than the ribbon of EXAMPLE 2 in crystallization, embrittlement and edge shape. On the other hand, the ribbon 6 of EXAMPLE 2 produced by carrying out the grinding of the cooling roll 5 during the casting was clearly improved in crystallization, embrittlement and edge shape. The electron photomicrographs of the amorphous alloy ribbons 6 of EXAMPLE 2 and COMPARATIVE EXAMPLES 3 and 4 in their edge portions are shown in FIGS. 3, 4 and 5. As is clear from FIGS. 3 and 4, the ribbon of EXAMPLE 2 produced while supplying a CO2 gas and grinding the cooling roll 5 was flatter in a shape in edge portions than the ribbon of COMPARATIVE EXAMPLE 3 produced while supplying a CO2 gas without grinding the cooling roll 5. Also, as is clear from FIG. 5, the ribbon of COMPARATIVE EXAMPLE 4 produced without supplying a CO2 gas was rife with air pockets, though it had a flat edge shape. Production and Evaluation of Nano-Crystalline Alloy Ribbon Using the same amorphous alloy ribbon portions as above, wound cores each having an outer diameter 20 mm and an inner diameter of 15 mm were produced, and then heat-treated at 550xc2x0 C. for 60 minutes to form wound cores of nano-crystalline alloy ribbons. Each of the resultant nano-crystalline alloy ribbons was measured with respect to an initial permeability at a frequency of 1 kHz. The results are shown in Tables 2 and 3. As is clear from Tables 2 and 3, a nano-crystalline alloy ribbon having excellent soft magnetic properties could be produced stably for a long period of time from an amorphous alloy ribbon 6 cast while supplying a CO2 gas and grinding the cooling roll 5. The observation of the metal structure of the nano-crystalline alloy ribbon with a transmission electron micrograph revealed that in a case where nano-crystalline alloy ribbons were obtained from the amorphous alloy ribbon portion of EXAMPLE 1 obtained when 1 minute passed from the start of casting and the amorphous alloy ribbon portion of EXAMPLE 2 obtained when 10 minutes passed, respectively, both of them had an average particle size of about 20 nm. The nano-crystalline alloy ribbons of COMPARATIVE EXAMPLES 1 and 2 also had an average particle size of about 20 nm, as long as they were obtained from the amorphous alloy ribbon portions obtained when 1 minute passed from the start of casting. However, crystal particles exceeding a particle size of 0.1 xcexcm were observed in the nano-crystalline alloy ribbons of COMPARATIVE EXAMPLES 3 and 4 obtained from the amorphous alloy ribbon portions obtained when 10 minutes passed from the start of casting. An ingot of an alloy having a composition of 9 atomic % of Si W and 13 atomic % of B, the balance being substantially Fe, was introduced into a crucible 1 shown in FIG. 1 and melted by induction heating by a high-frequency coil 2. The resultant melt 3 was ejected onto the cooling roll 5 made of a Cuxe2x80x94Cr alloy and rapidly quenched to produce an amorphous alloy ribbon 6 having a width of 40 mm and a thickness of 20 xcexcm under the conditions shown in Table 4. Taken as samples were amorphous alloy ribbon portions obtained when 1 and 10 minutes, respectively, passed from the start of casting. The samples of EXAMPLE 3 and COMPARATIVE EXAMPLES 5 and 6 were ribbon portions obtained when 1 minute passed from the start of casting, and the samples of EXAMPLE 4 and COMPARATIVE EXAMPLES 7 and 8 were ribbon portions obtained when 10 minutes passed from the start of casting. Incidentally, the grinding of the cooling roll 5 was carried out by rotating a brush 11 in the same direction as the cooling roll 5 in the production process of the ribbons 6 of EXAMPLES 3 and 4 in the presence of a CO2 gas supplied. The grinding of the cooling roll 5 was not carried out in the production process of the ribbons 6 of COMPARATIVE EXAMPLES 5 and 7 in the presence of a CO2 gas supplied. The grinding of the cooling roll 5 were also not carried out in the production process of the ribbons 6 of COMPARATIVE EXAMPLES 6 and 8 in the absence of a CO2 gas. Each of the resultant amorphous alloy ribbons 6 started to be wound around a reel 8 after about 2 seconds from the start of casting in the same manner as in EXAMPLE 1. Each of the amorphous alloy ribbons of EXAMPLE 3 and COMPARATIVE EXAMPLES 5 and 6 were evaluated with respect to surface roughness, a crystal phase, embrittlement and a shape in edge portions in the same manner as in EXAMPLE 1. The evaluation results are shown in Tables 5 and 6. As is clear from Table 5, regardless of whether or not the grinding of the cooling roll 5 was carried out, the amorphous alloy ribbon portions of EXAMPLE 3 and COMPARATIVE EXAMPLE 5 obtained when 1 minute passed from the start of casting with a CO2 gas supplied had substantially the same surface roughness, which was smaller than the surface roughness of the ribbon of COMPARATIVE EXAMPLE 6 produced without supplying a CO2 gas. With respect to embrittlement, crystallization and a shape in edge portions, there were no appreciable differences among the ribbons of EXAMPLE 3 and COMPARATIVE EXAMPLES 5 and 6. As shown in Table 6, among the amorphous alloy ribbon portions obtained when 10 minutes passed from the start of casting, the ribbon portions of EXAMPLE 4 and COMPARATIVE EXAMPLE 7 produced while supplying a CO2 gas had smaller surface roughness than the ribbon portions of COMPARATIVE EXAMPLE 8 produced without supplying a CO2 gas, regardless of whether or not the grinding of the cooling roll 5 was carried out. With respect to crystallization and embrittlement, there were no appreciable differences among the ribbons of EXAMPLE 4 and COMPARATIVE EXAMPLES 7 and 8. This seems to be because the alloys used in EXAMPLE 4 and COMPARATIVE EXAMPLES 7 and 8 had higher capability of forming an amorphous structure than the alloy used in EXAMPLE 1 However, the ribbon of COMPARATIVE EXAMPLE 7 produced while supplying a CO2 gas without grinding the cooling roll 5 had a poor shape in its edge portions. FIGS. 6 and 7 are electron photomicrographs showing the ribbon shapes of EXAMPLE 4 and COMPARATIVE EXAMPLE 7 in their edge portions. It was found from FIGS. 6 and 7 that the ribbon of EXAMPLE 4 had an improved shape in edge portions than the ribbon of COMPARATIVE EXAMPLE 7. Each ingot of alloys having compositions shown in Table 8 was introduced into a crucible 1 shown in FIG. 1, and melted by induction heating by a high-frequency coil 2. Each of the resultant alloy melts 3 was ejected onto a cooling roll 5 made of a Cuxe2x80x94Be alloy and rapidly quenched to produce an amorphous alloy ribbon 6 having a width of 30 mm and a thickness of 22 xcexcm under the conditions shown in Table 7. The casting of the amorphous alloy ribbon was repeated 10 times. In the production process of the ribbon with a CO2 gas supplied, the grinding of the cooling roll 5 was carried out by rotating a brush 11 in the same direction as the cooling roll 5. The winding method of the resultant ribbon was the same as in EXAMPLE 1. Each of the resultant ribbons was measured with respect to an average surface roughness Ra according to JIS B 0601 both on a freely solidified surface and on a side in contact with the cooling roll 5. Also, a 180xc2x0 bending test was carried out according to JIS Z 2248 to evaluate the percentage of cracking in each sample. The percentage of cracking was determined on 10 samples for each composition as a ratio of samples with cracking to all samples, each sample being a ribbon portion obtained 10 minutes after the start of casting. The results are shown in Table 8. As is clear from Table 8, the average surface roughness Ra was as small as 0.5 xcexcm or less in any samples, though the ribbon of EXAMPLE 7 containing more than 13 atomic % of B became brittle. The observation of ribbon edge shapes by an electron micrograph revealed that no ribbons suffered serrated irregularities in their edge portions. Each of the amorphous alloy ribbons of EXAMPLES 5 and 6 was heat-treated under the same conditions as in EXAMPLE 1 except for the heat treatment temperature of 600-650xc2x0 C. The observation of each heat-treated ribbon by a transmission electron micrograph revealed that any of the heat-treated ribbons had a nano-crystalline structure containing fine crystals having an average particle size of 100 nm or less. It was confirmed from these results that these amorphous alloy ribbons were converted to nano-crystalline alloy ribbons by a heat treatment. An ingot of an alloy having a composition shown in Table 10 was introduced into a crucible 1 shown in FIG. 1, and melted by induction heating by a high-frequency coil 2. Each of the resultant alloy melts 3 was ejected onto a cooling roll 5 made of a Cuxe2x80x94Be alloy and rapidly quenched to produce an amorphous alloy ribbon 6 having a width of 40 mm and a thickness of 20 xcexcm under the conditions shown in Table 9 below. In the production process of the ribbon 6 with a CO2 gas supplied, the grinding of the cooling roll 5 was carried out by rotating a brush 11 in the same direction as the cooling roll 5. The winding method of the resultant ribbon was the same as in EXAMPLE 1. Using as samples amorphous alloy ribbon portions obtained when 20 minutes passed from the start of casting, the peak of an xcex1-Fe crystal phase on a side in contact with the cooling roll 5 was evaluated by X-ray diffraction. As a result, it was confirmed that any of the amorphous alloy ribbons had no precipitated crystal phase. Each sample of the amorphous alloy ribbons was wound around a reel 8 to form a wound core having an outer diameter of 19 mm and an inner diameter of 15 mm, which was heat-treated by keeping at 500-600xc2x0 C. for 1 hour and then cooling. The observation of the heat-treated ribbons by a transmission electron micrograph revealed that any of the heat-treated ribbons had a nano-crystalline structure containing fine crystals having an average particle size of 100 nm or less. No irregular edge shapes were observed in any amorphous alloy ribbons. Each nano-crystalline alloy ribbon was measured with respect to the maximum value of a specific initial permeability xcexci at a frequency of 1 kHz. As a result, it was found that the ribbons having compositions with the most preferred range of
{'bibliographic_information': "{'document_kind': 'B2', 'document_number': '06749700', 'document_date': '20040615', 'publishing_country_or_organization': 'US', 'title_of_invention': 'Method for producing amorphous alloy ribbon, and method for producing nano-crystalline alloy ribbon with same', 'term_extension': '17'}", 'source_file': './grant/2004/pg040615.xml', 'source_data': 'USPTO-Grant'}