Fe-Ni based permalloy and method of producing the same and cast slab

Abstract: A Fe-Ni based permalloy comprises Ni: 70-85 wt%, C: not more than 0.015 wt%, Si: not more than 1.0 wt%, Mn: not more than 1.0 wt%, P: not more than 0.01 wt%, S: not more than 0.005 wt%, O: not more than 0.0060 wt%, Al: not more than 0.02 wt% and, if necessary, not more than 15 wt% of at least one selected from the group consisting of Mo, Cu, Co and Nb within a range of not more than 20 wt% in total.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a Fe-Ni based permalloy suitable for use in a magnetic head, a magnetic shielding material, an iron core of a transformer or the like and having excellent magnetic properties and a method of producing the same. 2. Description of Related Art As the Fe-Ni based high magnetic permeability alloy or so-called permalloy, there are usually typified PB material (40-50 wt% Ni), PC material (70-85 wt% Ni-Mo-Cu), PD material (35-40 wt%-Ni-Fe) and the like, which are defined according to JIS C2531. Among these alloys, the PB material is mainly used in applications utilizing the characteristic that saturated magnetic flux density is large, such as stator in a watch, pole piece in an electromagnetic lens and the like, while the PC material is used as a high sensitivity transformer or a magnetic shielding material at a high frequency zone utilizing an excellent permeability. Among these alloys, it is designed to cope with applications such as a magnetic head, a shield case and the like by adding an additional element such as Nb, Cr or the like to provide the abrasion resistance and corrosion resistance (for example, JP-A-60-2651). As another example of improving the properties of these alloys, JP-A-62-142749 and the like disclose that the permeability and the punching property are improved by adjusting impurity elements such as S, O and the like. Recently, the movement from PC material to PB material or from PB material to PD material is observed for reducing the cost, or there is adopted a method of supplementing for the lack of material properties by designing a fabricator. In the material makers, therefore, it is strongly noticed to develop materials such as PB material having properties corresponding to those of PC material or PD material having properties corresponding to those of PB material, This increases a degree of freedom in the design of fabricator and hence is effective to give products having higher performances to markets. Prior art document US-A-5 669 989 discloses a magnetic Ni-Fe alloy and a method for producing said Ni-Fe magnetic alloy. In the method known from this prior art document, an alloy-based material is heated to 1200-1300°C for 10-30 hours and is subjected to slabbing at the finishing temperature of 950°C or more and is heated to 1150-1270°C for 1-5 hours and is hot rolled at the finishing temperature of 950°C or more. In the method known from US-A-5 669 989, a Mo-segregation is reduced by heating an ingot for a long time (i.e. for 10-14 hours). SUMMARY OF THE INVENTION It is, therefore, an object of the invention to provide a Fe-Ni based permalloy satisfying the above demand and to provide a method for its production. That is, the invention is to improve the magnetic properties of PC material and to develop materials capable of coping with applications of high sensitivity and frequency and to provide a method for the production of such Fe-Ni based permalloys. The above objective is achieved by a product as defined in claim 1 and by a method as defined in claim 4. Preferred embodiments and further improvments of the inventive product and method are defined in the depending subclaims 2-3 and 5-7, respectively. Moreover, the cold rolling step as defined in the subclaims 5 to 7 may include usually used steps such as annealing, BA, pickling and the like. And also, the cast slab used herein may include a cast ingot for the formation of usual ingot in addition to the continuously cast slab. BRIEF DESCRIPTION OF THE DRAWINGS The invention will be described with reference to the accompanying drawings: wherein FIG. 1 is a schematic view illustrating a method of measuring Ni segregation amount of Ni; FIG. 2 is a graph showing found data of results measured on Ni segregation amount in PB material; and FIG. 3 is a diagrammatically section view of a cast slab. DETAILED DESCRIPTION OF THE INVENTION As a result that the inventors have made many experiments, it has been found that it is effective to adopt the following means for solving the above matters, and the invention has been developed. That is, the invention is characterized by an alloy as defined in claim 1. The reason why the Ni segregation amount is particularly noticed in the invention is due to the fact that Ni is a most important component among the constitutional components and is slow in the diffusion rate in the alloy and serves as a rate-determining of the homogenizing, In the invention, therefore, the continuously cast slab is subjected to a homogenizing heat treatment at a higher temperature for a long time as mentioned later as a method of providing a desired Ni segregation amount. Moreover, when the slab is hot rolled without being subjected to the homogenizing heat treatment, the Ni segregation amount of the hot rolled material is usually about 0.4%. According to the inventors' studies, it has been found that when the homogenizing heat treatment is carried out so as to satisfy the following temperature and time conditions, there can be obtained materials having the segregation amount lower than the initially anticipated value. That is, according to the inventors' various experiments, it has been found that the Ni segregation amount of the hot rolled material after the hot rolling can be decreased to 0.15 wt% by conducting the homogenizing heat treatment under conditions that the value (D•t) 1/2 of the Ni diffusion distance D Ni represented by the following equation (1) is not less than 39 and the heat treating temperature T is within a range of 1100-1375°C: Ni diffusion distance D Ni = (D•t) 1/2 /µm wherein D: diffusion coefficient, D = Do x exp (-Q/RT), D 0 : vibration number item = 1.63x10 8 /µm 2 s -1 Q: activation energy of Ni diffusion = 2.79x10 5 /J mol -1 R: gas constant = 8.31/J mol -1 K -1 T: temperature/K t: annealing time/s. In the above equation (1), the value (D•t) 1/2 is an indication showing a degree of decreasing Ni segregation. As the temperature becomes higher and the time becomes longer, the value becomes larger and the segregation becomes decreased. Moreover, as an indication showing the degree of Ni segregation, a standard deviation is determined from the data of Ni concentration distribution obtained by linear analysis of EPMA (X-ray microanalyzer), which is used as Ni segregation amount. In the above homogenizing heat treatment, when the temperature is lower than 1100°C, the treating time becomes undesirably too long, while when it exceeds 1375°C, the yield is lowered due to the oxidation loss and there is caused a risk of brittle crack through heating. In the invention, therefore, the heat treating temperature is within a range of 1100-1375°C. And also, non-metal inclusions included in the alloy are noticed in the invention, and the size and number thereof are defined. That is, the ratio of the non-metal inclusion having a diameter of not less than 0.1 µm is controlled to not more than 20 particles/mm 2 , preferably not more than 15 particles/mm 2 , more particularly not more than 10 particles/mm 2 . As a method of controlling the distribution of the non-metal inclusions, it is advantageous to use a high cleaning technique such as smelting through dissolution under vacuum, deoxidizing with C or the like. Moreover, the Ni segregation amount C Ni s (wt%) at section of plate is calculated according to the following equation (2) based on FIG. 1 after the section of the plate is subjected to mirror polishing in usual manner and analyzed through EPMA (X-ray microanalyzer) under conditions shown in Table 1. In this case, the scanning distance is substantially a full length of the plate in thickness direction: C Ni s (wt%) = analytical value of Ni component (wt%) x C Ni S (c..p.s.)/Ci Ni ave. (c.p.s.) wherein Ci Ni s: standard deviation of X-ray intensity at section of plate (c.p.s.) represented by Ci Ni ave.: average intensity of total X-ray intensities at section of plate (c.p.s.). The above analytical value of Ni component (wt%) is a Ni content included in the starting material and an analytical value by a chemical or physical method. FIG. 2 is a graph of found data showing results measured on Ni segregation amount of PB material in a hot rolled plate having a thickness of 5 mm. The same measurement is carried out with respect to cold rolled sheet or magnetic heat-treated sheet having a thickness of about 0.2 mm. Probe diameter 1 µm Irradiated current 5.0x10-7 A Acceleration voltage 20 kV Measuring time 0.5 sec/point Measuring interval 2 µm Spectrocrystal LIF And also, the measurement of the number of non-metal inclusions is carried out by the following method. Firstly, a surface of a product is subjected to a mechanical polishing and finished by buffing and thereafter the polished surface is subjected to an electrolysis at a constant potential field (Speed process) in a nonaqueous solvent (10 v/v% acetylacetone + 1 w/v% tetramethyl ammonium chloride + methanol solution). The electrolysis is carried out in a potential field of 10 C (Coulomb)/cm 2 at 100 mV. As the observation is conducted by a scanning type electron microscope (SEM), non-metal inclusions having a diameter corresponding to circle of not less than 0.1 µm are counted at 1 mm 2 . Moreover, the term "diameter corresponding to circle" means a diameter when individual inclusion is converted into a true circle. As seen from the above, the characteristics of the alloy are considerably improved without largely changing the component composition. This can be considered as follows. That is, there are various factors dominating the soft magnetic properties of the alloy. For example, there are well-known size of crystal grain, crystal orientation, impurity component, non-metal inclusion, vacancy and the like. In the silicon steel sheets, however, it is known that the soft magnetic properties in a particular direction are considerably improved to highly improve power efficiency of al alternating current transformer by controlling the crystal orientation. On the contrary, according to the invention, it has been found that the magnetic properties of the Fe-Ni based permalloy can largely be improved by noticing the segregation of Ni, which has never been considered up to the present time, and controlling it. And also, adequate production conditions are found out therefor. In the invention, the alloy characteristics are controlled by controlling the segregation of Ni, which is particularly slow in the diffusion rate among segregations of the components. However, as a result of various examinations, it has been found that it is effective to simultaneously control the non-metal inclusions and crystal grain size for improving the characteristics to desirable levels. The control of such non-metal inclusions is carried out by rationalizing vacuum dissolution and deoxidation method and reducing elements producing elements producing oxide and sulfide. On the other hand, the control of the crystal grain (coarsening) can be realized by mitigating the component segregation and decreasing the amount of the non-metal inclusion such as sulfide, oxide and the like, for example, MnS, CaS and so on. In this case, the control of the non-metal inclusion is effective in view of two points such as the improvement of magnetic properties by reducing the inclusion itself and the improvement of magnetic properties by controlling the crystal grain. Moreover, the degree of influence differs in accordance with the components of the alloy in these control factors. For example, the influence of grain size, segregation is large in the PD material and PB material, while the influence of non-metal inclusion and component segregation is large in the PC material. As a method of reducing Ni segregation, which is inevitable for realizing the function and effect of the invention, it is effective to conduct a diffusion heat treatment at a high temperature for a long time as previously mentioned. According to the inventors' studies, it has been found that the segregation of Ni is closely related to a dendrite arm interval of solidification texture and it is advantageous to mitigate Ni segregation as the dendrite arm interval is small. In this case, it has been confirmed that when the continuously cast material is compared with the usual ingot material, the dendrite arm interval is as very small as 1/5-1/10 and in case of using the continuously cast material, Ni segregation can be mitigated at a small energy. In case of the alloys according to the invention satisfying the above crystal grain size and the amount and shape of the non-metal inclusion, when the magnification of Ni segregation amount is restricted to not more than 0.15 wt%, the permeability can be made to 2-5 times that of the conventional alloy and the coercive force can be made to about 1/2-1/7 thereof, and hence the improving effect becomes higher as the Ni segregation amount becomes small. As a result, the invention can provide PC material having higher magnetic properties. That is in order to improve the characteristics of the PC material (70-85 wt% Ni), it is intended to more improve the permeability and reduce the coercive force. As a numerical target value, there are maximum permeability µm = not less than 400000, initial permeability µi = not less than 200000, and coercive force Hc = not more than about 0.006 (Oe). The reason why the composition of the alloy components according to the invention is limited to the above range will be described below. (1) C: not more than 0.015 wt%; C is an element degrading soft magnetic properties because when the amount exceeds 0.015 wt%, carbide is formed to control the crystal growth. Therefore, the C amount is limited to not more than 0.015 wt%. (2) Si: not more than 1.0 wt%; Si is added as a deoxidizing component, but when the amount exceeds 1.0 wt%, a silicate based oxide is formed as a start point of forming sulfide such as MnS or the like. The resulting MnS is harmful for the soft magnetic properties and forms a barrier for the movement of domain wall, so that the Si amount is desirable to be as small as possible. Therefore, the Si amount is limited to not more than 1.0 wt%. (3) Mn: not more than 1.0 wt%; Mn is added as a deoxidizing component, but when the amount exceeds 1.0 wt%, the formation of MnS is promoted to degrade the soft magnetic properties likewise Si. In the PC material or the like, however, Mn acts to control the formation of ordered lattice against the magnetic properties, so that it is desired to add it at an adequate content. Therefore, the Mn amount is limited to not more than 1.0 wt%, preferably a range of 0.01-1.0 wt%. (4) P: not more than 0.01 wt%; When the P amount is too large, it is precipitated in the grains as a phosphoride to degrade the soft magnetic properties, so that the P amount is limited to not more than 0.01 wt%. (5) S: not more than 0.005 wt%; When the S amount exceeds 0.005 wt%, it easily forms a sulfide inclusion and diffuses as MnS or CaS. Particularly, these sulfides have a diameter of about 0.1 µm to about few µm, which is substantially the same as the thickness of the domain wall in case of the permalloy and is harmful against the movement of the domain wall to degrade the soft magnetic properties, so that the S amount is limited to not more than 0.005 wt%. (6) Al: not more than 0.02 wt%; Al is an important deoxidizing component. When the amount is too small, the deoxidation is insufficient and the amount of non-metal inclusion increases and the form of sulfide is easily changed into MnS by the influence of Mn, Si to control the grain growth. On the other hand, when it exceeds 0.02 wt%, constant of magnetostriction and constant of magnetic anisotropy becomes high to degrade the soft magnetic properties. Therefore, an adequate range of Al added is not more than 0.02 wt%, preferably 0.001-0.02 wt%. (7) O: not more than 0.0060 wt%; O is decreased by deoxidation to finally remain in steel, but it is divided into O remaining in steel as a solid solution and O remaining as an oxide of non-metal inclusion or the like. It is known that as the O amount becomes large, the amount of the non-metal inclusion necessarily increases to badly affect the magnetic properties, and at the same time it affects the existing state of S. That is, when the amount of remaining O is large, the deoxidation is insufficient, and the sulfide is easily existent as MnS to obstruct the movement of domain wall and the grain growth. From these facts, the O amount is limited to not more than 0.0060 wt%. (8) Mo: optionally not more than 15 wt%; Mo is an effective component for providing the magnetic properties of PC material under practical production conditions and has a function of controlling the forming condition of ordered lattice exerting upon the crystal magnetic anisotropy and magnetostriction. The ordered lattice is influenced by cooling conditions after the magnetic heat treatment. If Mo is not included, a very fast cooling rate is required, while if Mo is included in a certain amount, maximum properties can be obtained under a practical cooling condition in industry. However, when the amount is too large, an optimum cooling rate becomes too late or the Fe content becomes small and the saturated magnetic flux density becomes low. Therefore, the Mo amount is preferable to be 1-15 wt%. (9) Cu: optionally not more than 15 wt%; Cu has an action of mainly controlling the forming condition of the ordered lattice in the PC material likewise Mo, but acts to decrease the influence of the cooling rate to stabilize the magnetic properties as compared with the effect of Mo. And also, it is known that the addition of Cu in an adequate amount enhances the electric resistance and improves the magnetic properties under alternating current. However, when the Cu amount is too large, the Fe content becomes small and the saturated magnetic flux density becomes low. Therefore, the Cu amount is not more than 15 wt%, preferably 1-15 wt%. (10) Co: optionally not more than 15 wt%; Co enhances the magnetic flux density and at the same time acts to improve the permeability by addition of an adequate amount. However, when the Co amount is too large, the permeability lowers and also the Fe content becomes small and the saturated magnetic flux density becomes low. Therefore, the Co amount is not more than 15 wt%, preferably 1-15 wt%. (11) Nb: optionally not more than 15 wt%; Nb is less in the effect on the magnetic properties, but enhances the hardness of the material and improves the abrasion resistance, so that it is an essential component for use in a magnetic head or the like. And also, it is effective to reduce the magnetic degradation due to molding or the like. However, when the amount is too large, the Fe content becomes small and the saturated magnetic flux density becomes low. Therefore, the Nb amount is not more than 15 wt%, preferably 1-15 wt%. The production method of Fe-Ni based permalloy according to the invention will be described below. Firstly, an alloy having the above composition is melted and subjected to a continuous casting process to form a continuously cast slab. In this case, it is desirable to conduct the continuous casting without electromagnetic agitation. Then, the thus obtained continuously cast slab is subjected to a homogenizing heat treatment and further to a hot rolling after the surface treatment of the slab. In the thus obtained hot rolled sheet, the Ni segregation amount C Ni s can be made to not more than 0.15 wt%. The above homogenizing heat treatment is suitable to be carried out under a condition that the value D Ni (D•t)1/2 of Ni diffusion distance represented by the equation (1) is not less than 39 at a heat treating temperature T of 1100-1375°C. It is favorable that the slab subjected to the homogenizing heat treatment is repeatedly subjected to cold rolling and annealing after the hot rolling to obtain a product. The thickness of the product is dependent upon the use application, but it is usually not more than 0.1 mm as a thin sheet for lamination in the application requiring high frequency characteristic such as coiled core or the like, and about 0.2-1.0 mm in magnetic yoke, transformer, shielding machine or the like. As the slab to be subjected to the hot rolling, according to the present invention a slab having an equiaxed crystal of not more than 1% as an area ratio of slab section (area of equiaxed crystal/area of slab x 100) as shown in FIG. 3a is used because it is more easy to reduce Ni segregation. In case of a slab containing a large equiaxed crystal (20%) as shown in FIG. 3b, it is more difficult to reduce Ni segregation. As to the slab used in the invention, the reason why the use of the continuously cast slab without using the electromagnetic agitation is favorable is due to the fact that the continuously cast slab is relatively fast in the solidification rate and less in the equiaxed crystal. And also, when the electromagnetic agitation is not used, the growth of columnar dendrite texture produced in the solidification step is not obstructed and the equiaxed crystal becomes further small. Moreover, FIG. 3 is a diagrammatic view of a section perpendicular to the casting direction of the cast slab. It is possible to use slabs produced by usual ingot forming process if such a slab contains less equiaxed crystal. The following examples are given in illustration of the invention and are not intended as limitations thereof. In Table 2 are shown compositions of test materials used in the examples. Among the test materials, 10 tons of a starting material corresponding to PC material is melted under vacuum, while 60 tons of starting materials corresponding to PD and PB materials are melted in air, and then these melts are continuously cast. A part of the continuously cast slabs is subjected to a homogenizing heat treatment, and the remaining slabs are not subjected thereto, which are then hot rolled, and subjected repeatedly to cold rolling and annealing and finally to a temper rolling of few % to obtains products having a thickness of 0.35 mm. Thereafter, the thus obtained test materials are subjected to a magnetic heat treatment in a hydrogen atmosphere at 1100°C for 3 hours to measure direct current magnetization property and alternating current magnetization property (effective permeability e). The Ni segregation is measured in the hot rolled sheet, cold rolled sheet and magnetic heat-treated sheet at a section in a thickness direction, respectively. The degree of Ni segregation in the hot rolled sheet is approximately equal to that of the cold rolled sheet after the magnetic heat treatment. The Ni segregation amount is a measured value of the magnetic heat-treated sheet. The measurement of the direct current magnetization property is carried out by winding wire around a ring-shaped test specimen of JIS 45 x 33 50 turns on each of primary and secondary sides and measuring through a reversed magnetic field of 20 Oe, while the alternating current magnetization property is evaluated by winding 70 turns and measuring an effective permeability at a current of 0.5 mA and a frequency of 1 kHz. As the initial permeability i, the intensity of magnetic field is measured at 0.01 Oe in case of PB material and 0.005 Oe in case of PC material according to the definition of JIS C2531. The test results are shown in Table 3 for PD corresponding material (36Ni alloy: Table 2 1 ○), Table 4 for PB corresponding material (46Ni alloy: Table 2 2 ○) and Table 5 for PC corresponding material (JIS alloy: Table 23 ○), respectively. As seen from Table 5, the cast slab having an equiaxed crystal ratio of not more than 1% is used in the alloys according to the invention, so that the Ni segregation amount is small and hence the direct current magnetization property and alternating current magnetization property are largely improved. And also, the similar tendency is observed in the alloys 4 ○, 5 ○ of Table 2. That is, it has been confirmed that the permeability is further improved and the coercive force is lowered in the PC material. Ni Mo Cu Nb Co Fe Alloy 1 ○ corresponding to PD 35.5 - - - - bal. Alloy 2 ○ corresponding to PB 46.5 - - - - bal. Alloy 3 ○ corresponding to PC (JIS) 77.4 4.2 4.7 - - bal. Alloy 4 ○ corresponding to PC (hard permalloy) 79.0 4.0 - 4.5 - 12.5 Alloy 5 ○ corresponding to PC (high permeability) 80.1 4.5 - 2.0 1.5 11.9 As mentioned above, according to the invention, there can be provided Fe-Ni based permalloys having magnetic properties considerably higher than those of the conventional technique. Particularly, PC materials having excellent magnetic properties and indicating high sensitivity and frequency characteristics can be obtained.

Claims: A Fe-Ni based permalloy made of a hot rolled sheet comprising Ni: 70-85 wt%, C: not more than 0.015 wt%, Si: not more than 1.0 wt%, Mn: not more than 1.0 wt%, P: not more than 0.01 wt%, S: not more than 0.005 wt%, O: not more than 0.006 wt%, Al: not more than 0.02 wt%, and optionally comprising not more than 15 wt% of at least one selected from the group consisting of Mo, Cu, Co and Nb within a range of not more than 20 wt% in total and the remainder being Fe and inevitable impurities and having such magnetic properties that a maximum magnetic permeability µm is not less than 400000, an initial magnetic permeability pi is not less than 200000 and a coercive force Hc is not more than 0.006 (Oe), and having such a cast texture that an area ratio of equiaxed crystal is not more than 1% and having an amount of non-metallic inclusion having a diameter corresponding to a circle of not less than 0.1µm to not more than 20 particles/mm 2 , provided that Ni segregation amount C Ni s represented by the following equation is not more than 0.15 wt%: C Ni s = analytical value of Ni component (wt%) x Ci Ni s (c.p.s.)/Ci Ni ave. (c.p.s.) wherein Ci Ni s: a standard deviation of X-ray intensity (c.p.s.) Ci Ni ave.: an average intensity of total X-ray intensities (c.p.s.). A Fe-Ni based permalloy according to claim 1, wherein the Ni segregation amount C Ni s is not more than 0.10 wt%. A Fe-Ni based permalloy according to claim 1 or 2, wherein an amount of non-metallic inclusion having a diameter corresponding to a circle of not less than 0.1µm to not more than 10 particles/mm 2 . A method of producing a Fe-Ni based permalloy, which comprises continuously casting an alloy comprising Ni: 70-85 wt%, C: not more than 0.015 wt%, Si: not more than 1.0 wt%, Mn: not more than 1.0 wt%, P: not more than 0.01 wt%, S: not more than 0.005 wt%, O: not more than 0.0060 wt%, Al: not more than 0.02 wt% and optionally comprising not more than 15 wt% of at least one selected from the group consisting of Mo, Cu, Co and Nb within a range of not more than 20 wt% in total and the remainder being Fe and inevitable impurities into a slab having such a cast texture that an area ratio of equiaxed crystal is not more than 1% without electromagnetic agitation, and subjecting the cast slab to a homogenizing heat treatment and further to a hot rolling, wherein the homogenizing heat treatment of the cast slab is conducted at a temperature of 1100-1375°C under a condition that Ni diffusion distance D Ni represented by the following equation is not less than 39: D Ni = (D t) 1/2 /µm wherein D: diffusion coefficient, D = Do x exp (-Q/RT), D 0 : vibration number item = 1.63x10 8 /µm 2 s -1 Q: activation energy of Ni diffusion = 2.79x10 5 /J mol -1 R: gas constant = 8.31/J mol -1 K -1 T: temperature/K t: annealing time/s A method according to claim 4, wherein a cold rolling is carried out after the hot rolling. A method according to claim 4, wherein a cold rolling is carried out after the hot rolling and thereafter a magnetic heat treatment is carried out at 1100-1200°C. A method according to claim 4, wherein a cold rolling is carried out after the hot rolling and thereafter a magnetic heat treatment is carried out at 1100-1200°C in a hydrogen atmosphere.

Inventors: ITOH TATSUYA, OMORI TSUTOMU, ITOH, TATSUYA, OMORI, TSUTOMU, ITOH, Tatsuya, Nippon Yakin Kogyo Co., Ltd., OMORI, Tsutomu, Nippon Yakin Kogyo Co., Ltd.
Applicants: NIPPON YAKIN KOGYO CO LTD, NIPPON YAKIN KOGYO CO., LTD.
Application year: 2001
Main classification: C22C19
IPC codes: C22C38, C21D8, C22F1, C22C19, B22D11, H01F1