Method of producing permalloy cores

Abstract: A method of producing permalloy cores comprising the steps of applying an annealing separator coating 0.1 to 50 µm thick to at least one surface of wide permalloy strip, slitting the wide permalloy strip to a final width, winding or punching the final width strip, and annealing the strip at a temperature in the range 1000° to 1300°C, and a permalloy core producible with this method. The method allows the efficient manufacturing of permalloy cores with highly stable magnetic properties.

Description: This invention relates to a method of producing permalloy (high permeability Ni-Fe alloy) cores. High permeability Ni-Fe magnetic alloys are widely used to form magnetic cores for light electrical equipment applications. The cores are produced by slitting wide permalloy strip to a final width and winding the strip to form wound cores, or by the steps of punching, bending and drawing core plates to a final shape. After annealing at 1000 to 1300°C to remove internal stresses and impurities, the material is assembled into the device concerned. The processed core pieces are stacked in the furnace for the annealing operation. The high temperature can cause the core pieces to seize together or to be burned at points of contact with the metal vessel. To prevent this, taking wound cores as an example, after being slit to the prescribed final width the core strips are degreased, immersed in a slurry consisting of water and alumina or magnesia, dried, wound to the prescribed diameter, and then are annealed. When manufacturing E and I magnetic cores, after the cores have been punched out the punching fluid is removed and the cores are then coated with finely powdered alumina or magnesia to prepare them for the annealing. Magnetic shielding materials are first degreased, bent and drawn to the prescribed shape, and are then coated with an annealing separator at specified points to prepare for the annealing process. Thus, in the prior art the core pieces, for example core strips for wound cores, are slit to the final width, degreased, coated, dried and wound, a series of steps that has to be carried out core by core, which is extremely time-consuming and inefficient. Moreover, the thickness of the coating can vary from core to core or place to place, which can easily lead to non-uniform pressure during the winding operation and degrade the magnetic properties of the end product. Also in the case of E and I cores and magnetic shielding material, usually after the small cores have been formed they are individually degreased, coated and dried, which in practice is a highly complex and inefficient task. An object of the present invention is to provide a method of efficiently manufacturing permalloy wound cores, E cores, I cores and other such magnetic cores with highly stable magnetic properties. The basic feature of the present invention that distinguishes it from the conventional methods is that the annealing separator is applied at a different time. Specifically, the method of producing permalloy cores according to the present invention comprises the step of applying an annealing separator coating 0.1 to 50 µm thick to at least one surface of wide permalloy strip, and subsequent steps for which there are the two modes of application described below. The invention is defined in claims 1 and 2. Preferred embodiments are shown in claims 3 - 8. A first method, comprising the steps of slitting the wide permalloy strip to final width followed by winding or punching, applying additional annealing separator as required to cut surface portions following any bending or drawing that is required, and annealing at a temperature range of from 1000° to 1300°C. The method of producing permalloy cores according to the present invention makes it possible to efficiently manufacture wound cores, E cores, I cores and other such cores in addition to which the products have highly stabilized magnetic properties, and as such has high commercial value. In the prior art the annealing separator is applied after the core material has been slit to the final width. The feature of the first method of the present invention, however, is that by the time the strips are slit to the final width they have already been coated with the annealing separator, an arrangement that was found to give rise to a number of advantages. For the purposes of this first method of the invention the permalloy may be any Ni-Fe alloy. However, for manufacturing high permeability magnetic cores and magnetic shielding materials, it is preferable to use a Ni-Fe alloy having a nickel content within the range of 40 to 90%. Elements such as molybdenum, copper, cobalt, chromium, manganese, boron, vanadium, niobium, and titanium may be added. There is no specific limitation on the thickness of the permalloy strip, which usually ranges from around 0.01 mm to 5.0 mm. Similarly, there is no specific limitation on the width of the strip, other than that it should be of a width that enables it to be slit into a multiplicity of strips of the final width. In practice there is a wide range of widths, from around 10 mm to 1200 mm, but most commonly widths range from 50 mm to 700 mm. For convenience, the starting coil strip will be referred to as wide coil strip. The annealing separation coating is applied to one or both surfaces of the strip. The thickness of the coating is limited to 0.1 to 50 µm because if it is thinner than 0.1 µm the coating will not provide sufficient separation, while a thickness that exceeds 50 µm will produce a marked decrease in the space factor. There is no specific limitation on the main constituent components of the annealing separator coating; a conventional composition may be used. The wide coil strip is immersed in a solution that is a suspension of water and a fine powder of one or more substances selected from among alumina, magnesia, magnesium hydroxide, calcium oxide, calcium hydroxide and titanium oxide, for example, after which the moisture is evaporated. For narrow strip, the method disclosed by JP-A-63-121670 may be used comprising immersing the strip in a solution constituted of at least one substance selected from among organo-metallic compounds and decomposition products thereof, then drying the strip. One application of permalloy strip is wound cores, which are formed by slitting the permalloy into strips of the final service width ranging from several millimeters to several tens of millimeters, coating the strips with a slurry of fine alumina powder, drying the coating, winding the strips into coils of specified diameter ranging from several millimeters to several hundred millimeters and annealing these coils. Coating the wide strip starting material with the annealing separator in accordance with this first method of the invention reduces the time needed to manufacture wound cores, the reason being that while coating and drying limited the winding speed of the conventional method, with the present invention the strips can be wound at high speed to form the cores after the strips have been slit to the final width. This speeds up the operation, while among other advantages are that it results in a uniform coating thickness and winding pressure that make it easier to achieve more stable magnetic properties, and there is little variation in the space factor. Another application of permalloy strip is E and I cores for use in small transformers. To form these, the permalloy is slit into strips of the final service width ranging from several millimeters to several tens of millimeters, the E and I core pieces are punched, then coated with a fine powder of Al₂O₃ or the like as the annealing separator, and the cores are then annealed. Producing E and I cores from permalloy strip that has been coated with the annealing separator eliminates the type of difficult, time-consuming operation of the prior art method which involved individually applying the Al₂O₃ powder to each of the small punched-out core pieces. Instead, with the first method of the present invention, the punching and mechanical stacking of the core pieces for annealing can be implemented on a continuous basis, greatly enhancing operating efficiency. The first method of the present invention basically eliminates the need to apply the annealing separator immediately prior to annealing. However, optionally additional annealing separator may be applied when burning of the cut or punched edges may be a problem. Compared to the prior art procedure, this additional application of the annealing separator involves far less work. In this method of the present invention, the reason for specifying a lower limit of 1000°C for the annealing temperature is that at a lower temperature stress release and elimination of impurities will be inadequate, while temperatures above 1300°C soften the permalloy, making it unable to retain its strength. Therefore a range of 1000 - 1300°C has been specified for the annealing temperature. The annealing time will normally be 1 - 3 hours, but at high temperatures may be a matter of a few minutes. The magnesium hydroxide (Mg(OH)₂) specified as the main component of the annealing separator of this first method of the invention enhances the effect of the method. The reason for specifying Mg(OH)₂ as the main component of the annealing separator is that hydrolysis during the annealing heating produces MgO, which has good annealing separation properties, while the film formed by Mg(OH)₂ has better adherence to the permalloy than other separators such as alumina (Al₂O₃). The most important reason for using Mg(OH)₂ is that it possesses a good solid-lubricant effect, so that during slitting, punching, bending and drawing operations, rather than having the adverse abrasive effect of the Al₂O₃ that is generally used, its lubricating effect extends the service life of the machine tools and lessens or eliminates the need for the usual slitting and punching lubricating fluids. What this means is that in some cases the step of washing off lubricating fluids is no longer required, the effect of which is considerable. The coating consisting mainly of Mg(OH)₂ may be applied in the form of a slurry of water and Mg(OH)₂. A small amount of a finely powdered ceramics substance such as, for example, MgO, Al₂O₃, CaCO₃ may be added. Optionally, one or more selected from a binder, a thickening agent and a defoaming agent may be added to the slurry to improve permalloy adhesion, spreadability and other such properties. It is particularly useful to improve adhesion in cases where slitting, punching, bending and drawing operations exert a frictional force on the surface of the permalloy strip that causes peeling of the coating. Preferably the binder should be one developed for ceramics applications, with as few organic components as possible, and only the minimum amount required should be used. Adding a large amount of binder increases the viscosity, decreasing the spreadability, and in the annealing process organic components present in the Ni-Fe become included as impurities, degrading the magnetic properties. There is no specific limitation on the composition of the binder. Any substance that provides the requisite function may be used as the main constituent, such as a water-soluble emulsion type acrylic ester copolymer resin or ethylene-vinyl acetate copolymer resin. Generally, in a slurry consisting of water and Mg(OH)₂, or of water, Mg(OH)₂ and MgO, with the addition of a small amount of binder, solids in the slurry such as the Mg(OH)₂ and MgO settle, degrading the spreadability. To a considerable extent this can be remedied by adding a small amount of a thickening agent to enable the slurry to maintain the right viscosity. Adding too much thickening agent can cause gelling, markedly decreasing the spreadability. Such a thickening agent may be constituted of a substance having a smectitic structure that is composed mainly of SiO₂; this is not limitative, however, and any substance having the above effect may be used. The addition of a binder tends to produce foaming in the separator solution, reducing the spreadability. In most cases this can be solved by adding minute amounts of a commercial defoaming agent. During fabrication of wound cores, for example, after the permalloy strip has been slit to the final width the separator is coated and dried on the strip, but this immersion method does not always produce a coating having the uniform prescribed thickness. A roll coater or bar coater can be used to apply a coating of a thickness in the range 0.1 - 50 µm, more preferably 0.5 - 10 µm. Roll or bar coater application is a known method of applying a uniform coating to thin sheet materials, and is also well suited to the object of this first method of the invention. Roll coaters are suitable for sheet thicknesses of 0.1 - 5 mm and bar coaters for thicknesses of 0.01 - 0.1 mm. The applied slurry is dried until the water content evaporates and it is not sticky to the touch. Drying takes a short time at 100°C and is usually done on a continuous basis. The thickness of the coating is controlled according to the thickness of the permalloy strip and the intended application, but applied with a roll coater or bar coater can be in the range 0.1 - 50 µm, a 50 µm coating being for thick sheet and a 0.1 µm coating for very thin sheet. In practice the thickness will usually be 0.5 - 10 µm. As the coating comprised mainly of Mg(OH)₂ has lubricating properties, as mentioned, the amount of lubricating fluid generally used for slitting, punching, bending and drawing operations can be reduced or omitted. This also means that the task of washing off the conventional lubricating fluid is shortened or eliminated, showing one of the invention's major effects. When a very thin coating is used, all or part of the annealing separator can be utilized as interlaminar insulation, as the annealing does not result in any large loss of adhesion. The production of permalloy cores according to the first method of the present invention markedly improves production efficiency and provides magnetic cores with excellent magnetic properties and space factors. The above is a description of first method of the present invention. The second method of this invention will now be described, with specific reference to the manufacture of wound cores from wide permalloy strip that has been coated with an annealing separator. In accordance with this second method of the invention, the wound cores are formed by winding unslit permalloy strip, that is permalloy strip in its wide state, that has been coated with an annealing separator into coils having a prescribed inner diameter and thickness. A further improvement in efficiency can be realized by arranging this winding operation on the same production line used to apply and dry the coating. It is also possible to apply and dry the coating and wind the strip into large starting coils, then afterwards uncoil the strip and rewind it to form wound cores having the prescribed inner diameter and thickness. The wide coils thus formed are slit to the final width of the wound cores. Compared to the conventional method in which the strip for each wound core is individually coated, dried and wound, there is far less variation among finished wound cores obtained in accordance with the second method of this invention, comprising uniformly coating an entire coil of wide-strip material, winding the wide strip at a constant pressure, and then slitting the wound strip into sections to form the individual wound cores. Slitting methods vary according to the diameter of the core and the intended application, but include high-speed rotary fine-tooth slitters, saws, and laser-beam cutters. Care should be taken not to cut strip where it sags. The lubricating properties of the main constituent of the coating, Mg(OH)₂, come in useful when a saw is used. The wound cores in their final form are then annealed. At this point, annealing separator may optionally be applied where metallic surface luster portions are exposed at the edge of the cut portions. This additional application is very easy, compared to the conventional arrangement. Annealing takes place at a temperature of 1000 - 1300°C for a period of 1 - 3 hours. The production of the coils wound to a specified diameter and thickness for cutting into sections will now be described. There are two methods. In the first method the cores are wound on the same line used to apply the coating, and in the second method the strip is rolled into large-diameter starting coils, and is then uncoiled and wound into cores. The coating used for the second method has to have a stronger adhesion than the coating used for the first method. This can be achieved by adding a small amount of binder. It is preferable to use a binder that has a very low organic content the major part of which will be eliminated in the course of the annealing heating process. A binder developed for ceramics application may be used such as one constituted mainly of a water-soluble emulsion type acrylic ester copolymer resin or ethylene-vinyl acetate copolymer resin, but this not limitative as any substance having the above-described requisite effect may be used. With a slurry of finely powdered ceramics, spreadability may be adversely affected by settling of the solids in the slurry. This can be prevented by adding a small amount of a thickening agent. The thickening agent may be constituted of a substance with a smectitic structure that is composed mainly of SiO₂. However, this is not limitative, and any substance having the above effect may be used. The addition of a binder tends to produce foaming in the separator solution, reducing the spreadability. In most cases this can be solved by adding small amounts of a commercial defoaming agent. The production of permalloy cores according to this second method of the invention improves productivity and provides permalloy wound cores of excellent quality. The efficacy of the first and second methods according to the present invention will now be described with reference to the following examples. Example 1 PC grade permalloy consisting of 77.0% Ni, 3.6% Cu, 4.3% Mo, 0.007% C, 0.4% Si, 0.6% Mn and the balance of Fe and unavoidable impurities was cold-rolled to form coils of wide permalloy sheet 350 mm wide and 0.1 mm thick. These coils were divided into types (1) and (2). Type (1) are coils with a final width of 10 mm obtained by slitting the wide permalloy strip, in accordance with the conventional method. After degreasing with trichlene the narrow strip coil (1) and wide strip coil (2) were immersed in a slurry of distilled water and alumina powder with an average particle size of 0.2 µm and are then dried by being passed through a furnace at 150°C. At this point, in both cases the thickness of the applied coating was about 5 µm but in the case of the narrow coils (1) there was variation in thickness in the widthwise direction, with the thickness at the center portion being 4.5 - 5.5 µm while the thickness at the edges was 5 - 15 µm. In the case of the wide strip coil (2), while the thickness right at the edges was 5 - 15 µm, the thickness at most of the center portion was 4.5 - 5.5 µm. The narrow coils (1) formed a total of 30 wound cores, each 50 turns thick and having an inside diameter of 40 mm, and these cores were subjected to annealing. A slitter was used to cut the wide strip coil (2) material into strips 10 mm wide which used to form 30 wound cores, each 50 turns thick and having an inside diameter of 40 mm, and these cores were subjected to annealing. The wound cores obtained from the narrow coils (1) and the wide strip coil (2) were annealed for 30 minutes at 1150°C in a stream of dry hydrogen. The magnetic properties and outside diameter (which has a bearing on the space factor) were measured and are listed in Table 1. From the results listed in Table 1 it can be seen that wound cores produced by the method of the present invention exhibit highly stable magnetic properties and an excellent space factor. Effective relative permeability (average of 30 cores) and variation (1 kHz) Outside diameter (mm) (min - max) (1) Prior art method 25,300±9,000 54 - 58 (2) Present invention 28,500±4,000 53 - 55 Example 2 PCS grade permalloy consisting of 79.3% Ni, 5.1% Mo, 0.003% C, 0.33% Si, 0.9% Mn, 0.0004% S, 0.002% P, 0.0007% N and the balance of Fe and unavoidable impurities was cold-rolled to form wide permalloy strip coil 250 mm wide and 0.01 mm thick. After degreasing with trichlene, a coating 3 µm thick consisting mainly of magnesium hydroxide was applied to each surface of the strip sheet by the following method. 150 g of highly active magnesium hydroxide with an average particle size of 0.1 µm was mixed into 5 liters of distilled water and the mixture was stirred vigorously for 30 minutes at room temperature. Then, a small amount of a binder consisting of water-soluble emulsion type acrylic ester copolymer resin developed for ceramics applications was added to the mixture, together with a small amount of a smectitic thickening agent composed mainly of SiO₂, and the mixture was stirred for a further 30 minutes. The resultant slurry was then applied to the sheet with a rubber bar-coater and then furnace-dried at 150°C to form permalloy sheet with an annealing separator coating consisting mainly of Mg(OH)₂ and featuring high adhesion. The sheet was then slit into strips 15 mm wide, the final width, and a high-speed automatic coiling machine was then used to wind the strips into wound cores 100 turns thick and having an inside diameter of 50 mm. Finely powdered Al₂O₃ was sprinkled over the cut edges of the cores, which were then vacuum-annealed for 2 hours at 1100°C. The ten wound cores thus obtained had an average effective relative permeability at 1 kHz of 47,500. In the case of the prior art method in which the annealing separator is applied and dried on strips that have already been slit to the final width, non-uniformity of the coating thickness makes it impossible to use a high-speed automatic winder to wind the strip. With the method of the present invention, however, as there is no such problem the winding proceeds smoothly. Example 3 PC grade permalloy consisting of 77.5% Ni, 3.4% Cu, 4.4% Mo, 0.008% C, 0.2% Si, 0.5% Mn and the balance of Fe and unavoidable impurities was cold-rolled to form wide permalloy strip coil 400 mm wide and 0.25 mm thick. These wide strip coils were divided into types (3) and (4). The type (3) strip was slit into strips having a final width of 20 mm, in accordance with the conventional method. E and I core pieces were punched from these strips, degreased with trichlene and coated with alumina powder having an average particle size of 0.3 µm. The wide type (4) strip was degreased with trichlene, immersed in a tank of 5 weight-percent butyl acetate solution of Zr(OC₄H₂)₄ and then passed through the drying furnace. This cycle of operations was repeated a number of times to form a surface coating 3 µm thick. The strip was then slit into 20-mm widths from which E and I core pieces were punched. 100 kg of cores thus obtained from each of the types (3) and (4) were then annealed for 1 hour at 1120°C in a stream of dry hydrogen. Table 2 lists the time required to produce the cores (comparative time required from wide strip to annealing) and the magnetic properties (measurements were conducted on sets of four hollow-square-shaped specimens punched from each final-width strip; the specimens measured 20 mm by 20 mm, thereby including the strip edge portions, and had a center hole measuring 12 mm by 12 mm.). As shown by Table 2, with the method of this invention it takes less time to produce the cores compared to the time required by the conventional method, and there was less variation among the cores. Process Time required (hr) (comparitive) Initial average permeability and variance (3) Prior art Wide strip - slitting - punching - degreasing - Al₂O₃ application 10 130,000 ±25,000 (4) Inventive method Wide strip - degreasing - immersion/drying - slitting - punching 7 155,000 ±12,000 Example 4 PCS grade permalloy consisting of 79.5% Ni, 5.0% Mo, 0.004% C, 0.24% Si, 0.8% Mn, 0.0003% S, 0.001% P, 0.0005% N and the balance of Fe and unavoidable impurities was cold-rolled to form wide permalloy strip coil sheet 250 mm wide and 0.35 mm thick. This wide strip coil was divided into types (5) and (6). Type (5) material corresponds to strip prepared by the prior art method, and was used to prepare annealed samples by the same method applied to the type (3) strip of Example 3. The type (6) wide strip coil was degreased with trichlene and a coating about 2 µm thick consisting mainly of magnesium hydroxide was applied to each surface of the strip by the following method. 150 g of highly active magnesium hydroxide with an average particle size of 0.1 µm was mixed into 5 liters of distilled water and the mixture was stirred vigorously for 20 minutes at room temperature. A small amount of a binder consisting of water-soluble emulsion type acrylic ester copolymer resin developed for ceramics applications was then added to the mixture, together with a small amount of a smectitic thickening agent composed mainly of SiO₂, and the mixture was stirred for a further 30 minutes. The resultant slurry was then applied to the sheet with a rubber roll-coater and then furnace-dried at 250°C to form permalloy sheet with an annealing separator coating consisting mainly of Mg(OH)₂ and featuring high adhesion. A round-tooth slitter was then used to cut the wide strip thus prepared into strips 20 mm wide, the final width, and E and I cores were punched from these strips. Slitting or punching fluid is not required, so the following degreasing process can be omitted. 100 kg of E and I cores thus obtained from each of the types (5) and (6) were then vacuum-annealed for 3 hours at 1100°C. In the case of cores obtained from the type (6) strip, even after the annealing the coating had some adhesion so the cores could be installed as they were; thus the coating can function as interlaminar insulation. Table 3 shows measured values obtained by the same method used in Example 3. Process Time required (hr) (comparitive) Initial average permeability and variance (5) Prior art Wide strip - slitting - punching - degreasing - Al₂O₃ application 10 150,000 ±30,000 (6) Inventive method Wide strip - degreasing - Mg(OH)₂ coating/drying - slitting - punching 6 185,000 ±12,000 Example 5 PB grade permalloy consisting of 45% Ni, 0.013% C, 0.3% Si, 0.5% Mn, 0.0005% S and the balance of Fe and unavoidable impurities was cold-rolled to form wide permalloy strip coil 250 mm wide and 0.1 mm thick. These wide strip coils were divided into types (7) to (10). Type (7) was slit to form final width strips 8 mm wide, in accordance with the conventional method, and after degreasing with trichlene the strips were immersed in a slurry consisting of distilled water and alumina powder with an average particle size of 0.5 µm, thoroughly mixed by means of a high-speed stirrer, and then dried by being passed through a furnace at 180°C, forming a coating 3 µm thick. Twenty conventional wound cores were then produced by winding these strips twenty times around a bobbin 30 mm in diameter. The wide strip coils (8), (9) and (10), produced by the method of this invention, were degreased, immersed in their original widths in the same alumina slurry used for type (7), and after the drying step, thereby forming a 3 µm coating, were each wound twenty times around a long bobbin 30 mm in diameter. The wound coils were then each cut into sections 8 mm long, using a high-speed cutter in the case of (8), a fine-tooth saw in the case of (9) and a laser-beam cutter in the case of (10), thereby producing 20 wound cores per coil, and these cores were annealed for 3 hours at 1100°C in a stream of dry hydrogen. Table 4 lists the time required up to the annealing, and the magnetic properties, in respect of each type. As shown by Table 4, with the method of this invention it takes about half the time to produce the cores compared to the time required by the conventional method, and the cores were better quality, with less variation. Process Time required (hr) (comparitive) Initial average permeability and variance (1kHz) (7) Prior art Wide strip - slitting - degreasing - alumina application/drying - winding 10 4,800 ±1,400 (8) Inventive method Wide strip - degreasing - alumina application/drying - full-width winding - cutting with high-speed cutter 3 5,550 ±400 (9) Inventive method Wide strip - degreasing - alumina application/drying - full-width winding - cutting with saw 6 5,400 ±550 (10) Inventive method Wide strip - degreasing - alumina application/drying - full-width winding - cutting with laser-beam cutter 5 5,500 ±500 Example 6 PC grade permalloy consisting of 77.5% Ni, 3.4% Cu, 4.4% Mo, 0.008% C, 0.2% Si, 0.5% Mn and the balance of Fe and unavoidable impurities was cold-rolled to form wide permalloy strip coils (11) and (12), each 400 mm wide and 0.1 mm thick. Type (11) was slit to form final width strips 20 mm wide. After degreasing with trichlene the narrow strip coil (11) and wide strip coil (12) were immersed in a tank of 5 weight-percent butyl acetate solution of Zr(OC₄H₂)₄, and then dried in a furnace. This cycle of operations was repeated a number of times to form a surface coating 2.5 µm thick. The (11) strips were then wound to form 15 wound cores each 8 mm thick and having an inside diameter of 50 mm. The (12) strip was wound in its full width state to an inside diameter of 50 mm and a thickness of 8 mm, and was then cut into sections 20 mm long, using a high-speed cutter, thereby producing 15 finished wound cores. Al₂O₃ powder was sprinkled over the cut edges of the cores which were then, together with the (11) cores, annealed for 2 hours at 1150°C in a stream of hydrogen. Table 5 lists the time required up to the annealing, and the magnetic properties. The results listed in Table 5 show that the method of the present invention enables the efficient manufacture of cores possessing good magnetic properties. Process Time required (hr) (comparitive) Initial average permeability and variance (11) Prior art Wide strip - slitting - degreasing - immersion in tank of 5 weight-percent butyl acetate solution/drying - winding 10 28,000 ±7,000 (12) Inventive method Wide strip - slitting - degreasing - immersion in tank of 5 weight-percent butyl acetate solution/drying - full-width winding - cutting with high-speed cutter 7 29,500 ±3,000 Example 7 PCS grade permalloy consisting of 79.5% Ni, 5.0% Mo, 0.004% C, 0.24% Si, 0.8% Mn, 0.0003% S, 0.001% P, 0.0005% N and the balance of Fe and unavoidable impurities was cold-rolled to form wide permalloy strip coil 300 mm wide and 0.05 mm thick. In its original wide state the strip was degreased with trichlene and a coating about 2 µm thick consisting mainly of magnesium hydroxide was applied to each surface of the strip by the following method. 150 g of highly active magnesium hydroxide with an average particle size of 0.1 µm was mixed into 5 liters of distilled water and the mixture was stirred vigorously for 20 minutes at room temperature. A small amount of a binder consisting of water-soluble emulsion type acrylic ester copolymer resin developed for ceramics applications was then added to the mixture, together with a small amount of a smectitic thickening agent composed mainly of SiO₂, and the mixture was stirred for a further 30 minutes. The resultant slurry was then applied to the sheet with a rubber roll-coater and then furnace-dried at 200°C to form large-diameter permalloy coils with an annealing separator coating consisting mainly of Mg(OH)₂ and featuring high adhesion. The coils were then rewound onto bobbins to form two rolls, one with a diameter of 300 mm and a thickness of 3 mm, and the other with a diameter of 100 mm and a thickness of 3 mm. A laser-beam cutter was then used to cut the coils into sections 8 mm wide. Al₂O₃ powder was sprinkled over the cut edges of the cores which were then vacuum-annealed for 1 hour at 1150°C. The wound cores thus obtained had an average effective relative permeability at 1 kHz of 48,000 - 52,000, exceeding the JIS standard (40,000 or more).

Claims: A method of producing permalloy cores comprising the steps of: applying an annealing separator coating 0.1 to 50 µm thick to at least one surface of wide permalloy strip; slitting the wide permalloy strip to a final width; winding or punching the final width strip; annealing at a temperature in the range 1000° to 1300°C. A method of producing permalloy wound core comprising the steps of: applying an annealing separator coating 0.1 to 50 µm thick to at least one surface of wide permalloy strip; winding the permalloy strip in its wide state to form core of a prescribed final inside diameter and thickness; slitting the core to form wound cores of a final width; annealing at a temperature in the range 1000° to 1300°C. The method according to claim 1 in which the wound or punched strip is subjected to bending or drawing. The method according to any of claims 1 to 3 in which the annealing is carried out after annealing separator has been applied to cut surfaces of the strip. The method according to any of claims 1 to 4 in which the main component of the annealing separator coating is magnesium hydroxide (Mg(OH)₂). The method according to any of claims 1 to 5 further comprising the application of an annealing separator coating agent constituted of a slurry of water, Mg(OH)₂ and MgO to which has been added one or more selected from a binder, a thickening agent and a defoaming agent. The method according to any of claims 1 to 6 in which a roll coater or bar coater is used to apply the annealing separator coating solution to the permalloy strip and the coating solution is dried to form a coating with a thickness in the range 0.1 - 50 µm and more preferably within the range 0.5 - 10 µm. The method according to any of claims 1 to 7 in which all or part of the annealing separator adhering to the surface after the annealing is utilized as an interlaminar insulation layer.

Inventors: AMEMURA AKIRA C O NIPPON STEEL, IWAYAMA KENZO C O NIPPON STEEL, SHIMIZU TSUNEHIRO C O NIPPON S, SUMITOMO HIDEHIKO C O NIPPON S, TAKASHIMA KUNIHIDE C O NIPPON, TANAKA OSAMU C O NIPPON STEEL, AMEMURA, AKIRA, C/O NIPPON STEEL CORPORATION, IWAYAMA, KENZO, C/O NIPPON STEEL CORP., TECHNICAL, SHIMIZU, TSUNEHIRO, C/O NIPPON STEEL CORPORATION, SUMITOMO, HIDEHIKO, C/O NIPPON STEEL CORPORATION, TAKASHIMA, KUNIHIDE, C/O NIPPON STEEL CO., TECHN., TANAKA, OSAMU, C/O NIPPON STEEL CO., TECHN.
Applicants: NIPPON STEEL CORP, NIPPON STEEL CORPORATION
Application year: 1991
Main classification: C21D1
IPC codes: C21D1, H01F41