JP7598166B2 - Magnet alloy, bonded magnet and manufacturing method thereof - Google Patents

Description

The present invention relates to magnetic alloys, bonded magnets and methods for manufacturing same.

In recent years, microcrystalline isotropic magnets have been developed that are composed of hard magnetic phases such as Nd-Fe-B and Sm-Fe-N, which are made up of fine crystal grains with sizes on the order of nanometers to submicrometers, or nanocomposite isotropic magnets (hereafter referred to as "nanocomposite magnets"), in which hard magnetic phases such as Nd-Fe-B and Sm-Fe-N, which are made up of fine crystal grains, and soft magnetic phases such as Fe-B and α-Fe exist in the same metal structure. However, it has been clarified by computer simulations using micromagnetics that these isotropic iron-based rare earth magnets composed of crystal grains with sizes on the order of nanometers to submicrometers have excellent magnetic properties because the crystal grains are magnetically bonded through exchange interactions in addition to static magnetic interactions due to their fine crystal grains, and they have been put to practical use as high-performance permanent magnet materials.

Until now, fine-crystalline isotropic iron-based rare earth magnets have taken advantage of their isotropic properties by being crushed to an average particle size of around 50 μm to 200 μm, and then mixed with epoxy resin-based thermosetting resins or thermoplastic resins such as nylon and polyphenylene sulfide (PPS) to form resin-bonded magnets (commonly known as bonded magnets), which have a high degree of freedom in shape and have been used primarily in the electronic parts industry for applications such as optical drives, spindle motors for hard disks, vibration motors (pager motors) for mobile phones, and various sensors. However, in recent years, the improved magnetic properties of these fine-crystalline isotropic iron-based rare earth magnets have raised hopes for their use in brushless DC motors of around 1 horsepower (750 W) or less for automobiles (including electric vehicles and hybrid vehicles) and white goods.

In particular, in order to improve the performance and efficiency of small motors in the several tens to several hundreds of watts class, there is a shift from brushed motors using conventional ferrite magnets (iron-based oxide permanent magnets) to brushless DC motors using isotropic rare earth bonded magnets. In the past, isotropic rare earth bonded magnets using fine crystal-type isotropic iron-based rare earth magnetic materials were limited to spindle motors and vibration motors, etc., and in consideration of replacing ferrite magnets, magnetic materials for isotropic rare earth bonded magnets with better corrosion resistance are required. Specifically, in a bonded magnet state, the flux loss after 1000 hours in an 80℃/5% NaCl (salt water) immersion test is less than -20% (0 to -20%), and at that time, the magnetic flux amount (Open There is a demand for magnetic materials for isotropic rare earth bonded magnets that have extremely excellent corrosion resistance, such as a flux of 0.5 mWb (milli-Weber) or more.
In order to ensure this required performance, it is most important to suppress the reduction in the volume ratio of the RE2FE14B type tetragonal compound, which is the main phase responsible for the permanent magnetic performance of the magnet material alloy, due to the oxidation of the rare earth elements (hereinafter referred to as RE), which are the basic constituent elements contained in the magnet material, especially Nd, Pr, Dy, and Tb (forming RE2O3), and the oxidation of Fe, which is also a basic constituent element (forming Fe2O3).
In addition, it is necessary to suppress the oxidation of the subphases, such as the RE-rich phase and RE-Fe phase, which surround the grain boundaries of the main phase, the RE2Fe14B compound. If this subphase is oxidized, the metal structure consisting of the main phase and subphase cannot be maintained, and in the worst case, the magnet itself will collapse and will no longer be able to retain its shape as a bonded magnet.
In this regard, there are hopes for highly corrosion-resistant isotropic iron-based rare earth magnet materials for isotropic rare earth bonded magnets that have the corrosion resistance to replace ferrite magnets (iron-based oxide magnets) and can be used in small DC brushless motors in the tens to hundreds of watts class.

Iron-based rare earth magnets, whose main phase is a Nd2Fe14B-type tetragonal compound made up of fine crystal grains from which high magnetic properties are expected, have a basic stoichiometric composition of Nd:Fe:B=11.76:remainder:5.88. However, because rare earth elements (RE), such as Nd, are extremely reactive to oxygen, iron oxide-based ferrite magnets have been used up until now for automotive motors and other applications that require high corrosion resistance.
However, amid the drive to conserve energy, the emergence of hybrid and electric vehicles has led to greater electrification than ever before, demanding motors with higher performance and efficiency. There is strong market demand for a shift from brushed DC motors using ferrite magnets to brushless DC motors using rare earth magnets, but there are no iron-based rare earth magnets that have the excellent corrosion resistance to replace ferrite magnets in a wide range of applications.

Because there are limitations to the compositional approach in improving the corrosion resistance of the above-mentioned iron-based rare earth magnets, iron-based rare earth magnets have been resin-molded to suppress oxidation of the magnet body, and are therefore used in motors for automotive fuel pumps, inverters, and water pumps for cooling batteries. However, resin molding widens the gap between the rotor and stator, significantly reducing the performance of the iron-based rare earth magnets, which have high magnetic properties. Not only cannot the magnet torque be effectively utilized, but the motor body also becomes larger, making it unsuitable for home appliance and electrical equipment applications where miniaturization is also required. As an alternative to resin molding, heat-resistant injection-bonded magnets have begun to be used, which are made by mixing and kneading a compound made by isotropic iron-based rare earth magnet powder with a thermoplastic resin with excellent heat resistance, typically polyphenylene sulfide (PPS) resin. However, even with these injection-bonded magnets, no isotropic iron-based rare earth magnet alloys have been found that have an extremely excellent corrosion resistance such that the flux loss after 1000 hours in an 80°C/5% NaCl (salt water) immersion test is less than -20%, and that the magnet alone has a diameter of 10 mm, height of 7 mm, and a permeance coefficient (Pc) of 2 and an open flux of 0.5 mWb or more.

Patent Document 1 discloses an anisotropic sintered magnet whose main phase is an RE2Fe14B tetragonal crystal structure. This magnet has a metal structure composed of RE2Fe14B tetragonal crystal grains on the order of microns, and the magnet exhibits good magnetic properties by aligning the magnetic moment in the C-axis direction of the RE2Fe14B tetragonal crystals through magnetic orientation. However, because an RE-rich phase is essential in the grain boundary phase, corrosion will progress even in an atmospheric room temperature environment without surface treatment, and no matter what kind of rust prevention treatment is applied, a significant deterioration in the magnetic properties cannot be avoided in an 80°C/5% NaCl (salt water) immersion test, with the demagnetization rate (flux loss) falling well below -20%.

Patent Document 2 discloses an isotropic permanent magnet with a hard magnetic phase as the main phase, which has a RE2Fe14B tetragonal crystal structure consisting of at least 10 atomic % rare earth elements (RE), approximately 0.5 atomic % to approximately 10 atomic % boron, and the remainder iron. This magnet has a grain boundary-less metal structure, but because it has a fine metal structure, it is possible to realize a permanent magnet through exchange bonds between the main phases. Although it is superior in corrosion resistance to the anisotropic sintered magnet with the aforementioned RE2Fe14B tetragonal crystal structure as the main phase, oxidation of the RE, which is a constituent element of the main phase, is unavoidable. Even for an injection-bonded magnet made of PPS resin, the flux loss after 1000 hours in an 80°C/5% NaCl (salt water) immersion test is less than -20%, and at that time, the magnetic flux amount (Open It is not possible to ensure extremely excellent corrosion resistance such that the flux is 0.5 mWb or more.

Patent Document 3 and Patent Document 4 disclose iron-based rare earth isotropic nanocomposite magnets. These iron-based rare earth isotropic nanocomposite magnets can have a lower ratio of RE compared to other iron-based rare earth magnets, so the deterioration of magnetic properties due to oxidation of RE is suppressed. However, the soft magnetic phase α-Fe present as a subphase causes red rust when immersed in salt water, and even injection-bonded magnets made of PPS resin cannot ensure extremely excellent corrosion resistance, such as a flux loss of less than -20% after 1000 hours in an 80°C/5% NaCl (salt water) immersion test, and a magnetic flux amount (open flux) of 0.5 mWb or more for a magnet alone with a diameter of 10 mm, height of 7 mm, and permeance coefficient (Pc) of 2.

On the other hand, in the iron-based rare earth isotropic nanocomposite magnets described in Patent Document 5, which mainly contain an iron-based halide phase as the soft magnetic phase, the addition of Ti suppresses the precipitation and growth of the α-Fe phase during the cooling process of the molten alloy, and allows the precipitation and growth of the Nd2Fe14B phase to proceed preferentially. However, since Ti easily bonds with boron (B) and crystallizes the TiB2 phase during the crystallization process, the metal structure is finer than that of the iron-based rare earth magnets described in Patent Documents 1, 2, 3, and 4, and corrosion resistance also tends to improve. However, the absolute amount of boron required to generate the main phase, Nd2Fe14B phase, is reduced, and only a small amount of RE and iron exists alone or in the form of an Fe-RE alloy, and rust progresses from this point, so even injection-bonded magnets made of PPS resin have a low demagnetization rate (Flux) after 1,000 hours in an 80°C/5% NaCl (salt water) immersion test. It is not possible to ensure extremely excellent corrosion resistance with a loss of less than -20%.

Furthermore, Patent Document 6 describes how excellent wear resistance, heat resistance, and corrosion resistance can be achieved by forming a high-density hydrocarbon polymerized film by plasma polymerization on the surface of iron-based rare earth sintered magnets and bonded magnets molded using iron-based rare earth magnet powder. However, even in this document, the corrosion resistance test is limited to a constant temperature and humidity test at 85°C and 95% RH, and there is no mention of corrosion resistance in the extremely harsh environment of immersion in 80°C/5% NaCl (salt water).

Similarly, Patent Document 7 discloses that a highly corrosion-resistant magnet can be obtained by surface treatment of iron-based rare earth sintered magnets using a three-layer coating in which the first layer of the surface coating is a Ni-P film formed by electroless or a combination of electroless and electrolytic coating, the second layer is an electrolytic Cu film, and the third layer is an electrolytic Ni film. However, this merely shows that the rusting condition was improved in a PCT test in which the sample was held for 24, 72, 120 and 168 hours at 120°C, 100% relative humidity and 1kgf/cm2, and it cannot be said to be a highly corrosion-resistant iron-based rare earth magnet that can be used in the extremely harsh environment of immersion in 80°C/5% NaCl (salt water).

Patent Document 8 discloses that a coating layer is formed on the surface of Sm-Fe-N magnet powder, which is said to have better corrosion resistance than Nd-Fe-B alloys, by treating it with plasma gasification of CF4, argon, nitrogen or air, and then forming an anisotropic bonded magnet, thereby obtaining a magnet with excellent corrosion resistance. However, the corrosion resistance was evaluated using a constant temperature and humidity test of 85°C x 85% RH x 200 hours, and this document also does not produce a highly corrosion-resistant magnet that can be used in a saltwater immersion environment.

Patent Document 9 discloses Sm-Fe-N alloy powder and an isotropic bonded magnet using said powder, but this document also makes no mention of a highly corrosion-resistant magnet that can be used in a saltwater immersion environment, and none of the methods described in Patent Documents 1 to 9 disclose a method for producing an iron-based rare earth magnet with extremely excellent corrosion resistance, in which the flux loss after 1,000 hours in an 80°C/5% NaCl (saltwater) immersion test is less than -20%, and the magnetic flux (open flux) of the magnet alone, which is 10 mm in diameter, 7 mm in height, and has a permeance coefficient (Pc) of 2, is 0.5 mWb or more.

Japanese Patent Publication No. 59-046008 Japanese Patent Publication No. 60-009852 Japanese Patent Application Laid-Open No. 8-162312 Japanese Patent Application Laid-Open No. 10-53844 JP 2002-175908 A Japanese Patent Application Laid-Open No. 1-280303 JP 2001-176709 A JP 2020-50904 A JP 2002-57017 A

Electrical equipment fuel pumps, water pumps, and DC brushless motors for EVs require iron-based rare earth magnets with excellent corrosion resistance that can withstand immersion in salt water due to the environmental conditions in which they are used. However, conventional methods only ensure corrosion resistance by forming a treatment film with excellent weather resistance on the surface of the magnet material. If the surface treatment film peels off or the magnet body is chipped or scratched, the base material (fresh surface) of the magnet material is exposed, which becomes the starting point for rust. Therefore, the only way to achieve higher corrosion resistance is to use methods such as resin molding. However, even with iron-based rare earth magnets that have magnetic properties superior to ferrite magnets, the magnet torque that contributes to high motor efficiency is significantly reduced, resulting in the problem that the cost-effectiveness of using expensive RE as a raw material is not achieved. The inventors of the present application thought that by substituting some of the Fe sites of an isotropic Nd-Fe-B magnet alloy with Cr, which contributes to improving corrosion resistance, to make it Fe-Cr, it would be possible to significantly improve the corrosion resistance of the Nd-(Fe,Cr)-B alloy without relying on surface treatment of the magnet. However, they found that simply adding Cr significantly reduces the magnetic properties, especially magnetization, making it difficult to obtain magnetic performance suitable for use in DC brushless motors.

The present invention has been made in consideration of the above circumstances, and its main objective is to provide a magnetic alloy, a bonded magnet, and a manufacturing method thereof, which have magnetic properties applicable to small DC brushless motors while having the corrosion resistance to replace ferrite magnets.

The magnet alloy according to the present invention is an isotropic iron-based rare earth boron magnet alloy having a RE2Fe14B type tetragonal compound phase (RE is a rare earth element) as a main phase, and is expressed by a composition formula _{T100-xyzm ( B1 -nCn ) xREyCrzMm (} T is at least one element selected from the group consisting of Fe, Co, and Ni, and is a transition metal element that always contains Fe, RE is a rare earth element that always contains Nd or Pr, and M is one or more metal elements selected from the group consisting of Al, Si, V, Ti, Mn, Cu, Zn, Ga, Zr, Nb, Mo, Ag, Hf, Ta, W, Pt, Au, and Pb), and the composition ratios x, y, z, m, and n are respectively:
5.6≦x≦6.4 atomic %,
11.2≦y≦12.0 atomic %,
2.3≦z≦5.4 atomic %,
0.0≦m≦3.0 atomic %
0.0≦n≦0.5
The present invention is characterized in that the magnet alloy has a composition that satisfies the above requirement and contains Cr as an essential component.

In this magnet alloy, it is preferable that the average crystal grain size of the main phase, the RE2Fe14B type tetragonal compound, is 20 nm or more and less than 100 nm, and the standard deviation (σ) is within 50% of the average crystal grain size.

It is preferable that this magnet alloy has permanent magnet properties such as a residual magnetic flux density Br of 0.7 T or more, an intrinsic coercivity HcJ of 800 kA/m or more, and a maximum energy product (BH) max of 80 kJ/m3 or more.

This magnet alloy can be made into a powder form with high corrosion resistance and an average powder grain size of 20 μm or more and less than 200 μm.

The above-mentioned object of the present invention is also achieved by a bonded magnet obtained by mixing and kneading the above-mentioned powdered magnetic alloy with a thermoplastic resin or a thermosetting resin and then molding it.

This bonded magnet is a bonded magnet with a diameter of 10 mm, height of 7 mm and a permeance coefficient (Pc) of 2, which uses at least one of the thermoplastic resins polyamide, polyphenylene sulfide (PPS) and polyether ether ketone (PEEK) as the mixed resin, and it is preferable that when immersed in 80°C/5% NaCl (salt water) for 1000 hours, the demagnetization rate (flux loss) is less than -20% (0 to -20%) and the magnetic flux of the magnet alone (open flux) is 0.5 mWb or more.

The reason why the demagnetization rate (flux loss) after 1000 hours in the 80°C/5% NaCl (salt water) immersion test was set to less than -20% (0 to -20%) is that if it were to be demagnetized beyond this, and the 80°C/5% NaCl (salt water) immersion test was then continued, the demagnetization rate would exceed -30% before reaching 2000 hours, and the surface magnetic flux (flux) would be equal to or less than that of a ferrite magnet.

In addition, the magnetic flux amount (open flux) of the magnet alone after 1,000 hours in an 80°C/5% NaCl (salt water) immersion test as a bonded magnet with a diameter of 10 mm, height of 7 mm and permeance coefficient (Pc) of 2 is set to 0.5 mWb or more because if it is lower than 0.5 mWb, there is not much difference in the surface magnetic flux from oxide-based ferrite sintered magnets and ferrite bonded magnets, and it becomes difficult to achieve the goal of the present invention, which is to replace ferrite magnets by exhibiting higher magnetic properties than ferrite magnets and promote their application in small DC brushless motors in the tens to hundreds of watts class for electrical equipment fuel pumps and water pumps.

The object of the present invention is to provide a method for producing a crystalline silicon nitride semiconductor laser, comprising the steps of: providing a crystalline silicon _{nitride semiconductor laser} having a composition represented by the formula _{T100-xyzm (} B1 _{- nCn} ) _xREyCrzMm (T is at least one element selected from the group consisting of Fe, Co and Ni, and is a transition metal element which necessarily includes Fe; RE is a rare earth element which necessarily includes Nd or Pr; and M is one or more metal elements selected from the group consisting of Al, Si, V, Ti, Mn, Cu, Zn, Ga, Zr, Nb, Mo, Ag, Hf, Ta, W, Pt, Au and Pb), in which the composition ratios x, y, z, m and n are respectively
5.6≦x≦6.4 atomic %,
11.2≦y≦12.0 atomic %,
2.3≦z≦5.4 atomic %,
0.0≦m≦3.0 atomic %
0.0≦n≦0.5
preparing a molten alloy having a composition that satisfies the above;
This can be achieved by a method for producing a magnet alloy, comprising the steps of: spraying the molten alloy onto the surface of a rotating roll whose main raw material is any of copper, copper alloy, Mo, and W at an average pouring rate of 200 g/min or more and less than 2000 g/min per orifice disposed at the tip of a nozzle, thereby producing a rapidly solidified alloy having 1 volume % or more of an amorphous phase or a crystalline phase including the RE2Fe14B phase.

In this method for manufacturing a magnetic alloy, it is preferable that the rotating roll has a surface roughness of arithmetic mean roughness (Ra) of 0.1 μm or more and less than 0.6 μm.

It is preferable that the manufacturing method of this magnetic alloy further includes a step of producing a magnetic alloy having a main phase of an RE2Fe14B type tetragonal compound by subjecting the rapidly solidified alloy to a heat treatment (flash annealing) in which the alloy is immediately quenched after 0.01 sec to less than 7 min has elapsed after the alloy has reached a constant temperature range of from the crystallization temperature to 850°C at a heating rate of 10°C/sec to less than 200°C/sec.

The object of the present invention is also achieved by a method for producing a bonded magnet, comprising the steps of: pulverizing the magnet alloy obtained by the above-mentioned method for producing a magnet alloy to an average powder particle size of 100 μm or more and less than 200 μm to obtain a magnet alloy powder; adding a thermosetting resin to the magnet alloy powder, filling the magnet alloy powder into a molding die and forming a compression molded body by press molding; and heat treating the compression molded body at a temperature equal to or higher than the polymerization temperature of the thermosetting resin to obtain a bonded magnet. Alternatively, the object of the present invention is achieved by a method for producing a bonded magnet, comprising the steps of pulverizing the magnet alloy obtained by the above-mentioned method for producing a magnet alloy to an average powder particle size of 20 μm or more and less than 100 μm to obtain a magnet alloy powder; and injection molding the injection molding compound prepared by adding a thermoplastic resin to the magnet alloy powder.

According to the present invention, it is possible to provide a magnet alloy, a bonded magnet, and a manufacturing method thereof that have the corrosion resistance to replace ferrite magnets, while having magnetic properties applicable to small DC brushless motors in the tens to hundreds of watts class for use in fuel pumps and water pumps for electrical equipment.

FIG. 2A is a diagram showing the equipment configuration of a heat treatment furnace for implementing flash annealing, and FIG. 2B is a diagram showing the state of a molten rapidly solidified alloy moving inside a furnace tube. FIG. 2 is a conceptual diagram of a thermal history by flash annealing carried out in the present invention. 1 is a powder X-ray diffraction profile of the rapidly solidified alloy (as-spun) obtained in Example 3. 1 shows a powder X-ray diffraction profile of the isotropic iron-based rare earth boron magnet obtained in Example 3. 1 is a powder X-ray diffraction profile of the rapidly solidified alloy (as-spun) obtained in Comparative Example 14. 1 shows a powder X-ray diffraction profile of the isotropic iron-based rare earth boron magnet obtained in Comparative Example 14. 1 shows the change in flux loss in Example 1, Example 2, Example 3, and Comparative Example 16 in an 80° C./5% NaCl (salt water) immersion test. 1 shows the change in magnetic flux (open flux) in an 80° C./5% NaCl (salt water) immersion test for Examples 1, 2, and 3, and Comparative Example 16. This shows the relationship between the amount of Cr added and the flux loss after 1000 hours in an 80℃/5% NaCl (salt water) immersion test. 1 is a photograph showing the state of rusting in an 80° C./5% NaCl (salt water) immersion test in Example 2. 1 is a photograph showing the state of rusting in an 80° C./5% NaCl (salt water) immersion test in Comparative Example 16.

The magnet alloy according to one embodiment of the present invention is an isotropic iron-based rare earth boron magnet alloy that requires the addition of Cr, and in the alloy composition range in which a magnet alloy having the RE2Fe14B phase as the main phase is obtained, some of the Fe sites are replaced with Cr to produce an RE2(Fe,Cr)14B phase containing Fe-Cr with excellent corrosion resistance. This magnet alloy has a metal structure that can make maximum use of the exchange interaction acting between each main phase grain by making the average crystal grain size of the RE2(Fe,Cr)14B phase 20 nm or more and less than 100 nm and making it a uniform fine metal structure with a standard deviation of the crystal grain size within 50% of the total crystal grain size.

The inventors discovered that by achieving the above-mentioned uniform and fine metal structure, the main phase, RE2(Fe,Cr)14B phase, is bound by strong exchange interactions in addition to static magnetostatic interactions, and although the addition of Cr reduces the saturation magnetization Js, the squareness of the demagnetization curve (Br/Js) is improved due to strong interparticle interactions, and as a result, the reduction in residual magnetic flux density Br is suppressed. This discovery led to the invention of the present application, as it was discovered that, despite being a Cr-added material, it exhibits magnetic properties that can be applied to small DC brushless motors in the tens to hundreds of watts class for use in fuel pumps and water pumps for electrical equipment.

When the amount of Cr added is less than 2.3 atomic %, an isotropic rare earth injection bonded magnet with a diameter of 10 mm, height of 7 mm, and permeance coefficient (Pc) of 2, produced using a known injection molded bonded magnet manufacturing process, is tested at 80°C/5% NaCl (salt water) and the flux loss after 1000 hours exceeds -20%, meaning that magnetic properties that can replace ferrite magnets cannot be maintained. Also, when the amount of Cr added is 5.4 atomic % or more, although a flux loss of less than -20% after 1000 hours can be ensured, the initial residual magnetic flux density Br is 0.7 T or less, and the magnetic properties after 1000 hours of immersion in 80°C/5% NaCl (salt water) cannot be maintained at a level that can replace ferrite magnets.

In contrast, by adding Cr in an amount between 2.3 atomic % and 5.4 atomic %, and by creating a magnet alloy in which the average crystal grain size of the main phase, RE2(Fe,Cr)14B, is between 20 nm and 100 nm, with a standard deviation (σ) within 50% of the average crystal grain size, it is possible to achieve magnetic properties that make it suitable for small DC brushless motors in the several tens to several hundreds of watts class for use in fuel pumps and water pumps for electrical equipment. At the same time, using an isotropic rare earth injection bonded magnet sample with a diameter of 10 mm, height of 7 mm, and a permeance coefficient (Pc) of 2, the flux loss after 1,000 hours in an 80°C/5% NaCl (salt water) immersion test is less than -20%, and the magnetic flux of the magnet alone (open flux) is 0.5 mWb or more, achieving both extremely excellent corrosion resistance and magnetic properties that make it possible to replace ferrite magnets.

None of Patent Documents 1, 2, 3, 4, 5, 6, 7, 8, and 9 disclose a magnetic material that has extremely excellent corrosion resistance as an iron-based rare earth magnet, such as an iron-based rare earth magnet sample with a diameter of 10 mm, height of 7 mm, and a permeance coefficient (Pc) of 2, in which the flux loss after 1,000 hours in an 80°C/5% NaCl (salt water) immersion test is less than -20%, and the magnetic flux amount (open flux) of the magnet alone is 0.5 mWb or more.

The isotropic iron-based rare earth boron magnet of the present invention, characterized by the addition of Cr, has a portion of the Fe sites substituted with Cr in the alloy composition range in which a magnet alloy with the RE2Fe14B phase as the main phase is obtained, resulting in an RE2(Fe,Cr)14B phase containing Fe-Cr with excellent corrosion resistance. In addition, the average crystal grain size of the RE2(Fe,Cr)14B phase is set to 20 nm or more and less than 100 nm, and a uniform fine metal structure with a standard deviation of the crystal grain size within 50% of the grain size in the alloy is created. This allows the crystal grains of the RE2(Fe,Cr)14B phase to be bound together by strong exchange interaction in addition to static magnetostatic interaction, thereby suppressing the decrease in residual magnetic flux density Br in Cr-added materials, and resulting in an isotropic iron-based rare earth boron magnet that can combine excellent corrosion resistance with magnetic properties that can be used in small DC brushless motors in the tens to hundreds of watts class for fuel pumps and water pumps for electrical equipment.

In addition, the isotropic iron-based rare earth boron magnet of the present invention, which is characterized by the addition of Cr, can have its corrosion resistance further improved by replacing some of the B with C.

A preferred embodiment of the present invention is described below.

[Alloy composition]
The transition metal T, which contains Fe as an essential element, occupies the remainder of the above-mentioned elements. The desired hard magnetic properties can be obtained by substituting a part of Fe with one or both of Co and Ni, which are ferromagnetic elements like Fe. However, if the amount of substitution for Fe exceeds 30%, the magnetic flux density is significantly reduced, so the amount of substitution is limited to the range of 0% to 30%. If Co is added, it not only contributes to improving magnetization, but also has the effect of reducing the viscosity of the molten metal and stabilizing the tapping rate from the nozzle during quenching of the molten metal, so the amount of Co substitution is preferably 0.5% to 30%, and more preferably 0.5% to 10% from the viewpoint of cost-effectiveness.

If the B+C composition ratio x is less than 5.6 atomic percent, the alloy's amorphous forming ability is greatly reduced, and α-Fe precipitates during quenching and solidification of the molten metal, impairing the squareness of the demagnetization curve. If the B+C composition ratio x exceeds 6.4 atomic percent, the B+C concentration required to form the RE2Fe14B phase is exceeded, and the excess B+C becomes a grain boundary component and does not contribute to the expression of magnetization, resulting in a decrease in magnetic properties. Therefore, the composition ratio x is set to a range of 5.6 atomic percent to 6.4 atomic percent, and the composition ratio x is preferably set to a range of 5.6 atomic percent to 6.2 atomic percent, and more preferably set to a range of 5.8 atomic percent to 6.2 atomic percent.

The corrosion resistance of the RE2Fe14B phase is improved by replacing part of B with C, but if the substitution rate of C for B exceeds 50%, the ability to form amorphous structures drops significantly, which is not preferable, and the substitution rate is limited to 0% to 50%. From the viewpoint of improving corrosion resistance, the substitution rate is preferably 2% to 30%, and more preferably 3% to 15%.

In the present invention, the rare earth element y, which always includes Nd or Pr, is set to less than 12.0 atomic % because if it is less than 11.2 atomic %, the grain boundary phase consisting of iron and rare earth elements is not generated and the target permanent magnet characteristics cannot be secured, and if it exceeds 12.0 atomic %, the RE concentration required for the formation of the RE2Fe14B phase is exceeded, and an RE-rich phase that is extremely active against oxygen is generated at the main phase grain boundary, resulting in a decrease in corrosion resistance. In addition, y is preferably 11.4 atomic % to 11.9 atomic % in terms of ensuring a stable intrinsic coercive force HcJ, and more preferably 11.4 atomic % to 11.8 atomic % in terms of ensuring as high Br as possible.

In the present invention, Cr is essential to ensure excellent corrosion resistance, but if the added amount z is less than 2.3 atomic %, the desired corrosion resistance cannot be guaranteed, and if it exceeds 5.4 atomic %, the residual magnetic flux density Br decreases significantly and the desired magnetic properties cannot be ensured, so the Cr added amount z is set to 2.3 atomic % or more and 5.4 atomic % or less. From the viewpoint of corrosion resistance, z is preferably 2.5 atomic % or more and 5.4 atomic % or less, and furthermore, taking into consideration the decrease in magnetic properties, 2.5 atomic % or more and 5.0 atomic % or less is more preferable.

In the present invention, one or more additive elements M selected from the group consisting of Al, Si, V, Ti, Mn, Cu, Zn, Ga, Zr, Nb, Mo, Ag, Hf, Ta, W, Pt, Au and Pb may be added. The additive elements improve the amorphous generation ability, improve HcJ by uniformly refining the metal structure after crystallization heat treatment, and improve the squareness of the demagnetization curve, thereby improving the permanent magnet properties. However, if the composition ratio m of these elements M exceeds 3.0 atomic %, it will lead to a decrease in magnetization, so z is limited to 0 atomic % or more and 3.0 atomic % or less, preferably 0 atomic % or more and 2.0 atomic % or less, and more preferably 0 atomic % or more and 1.5 atomic % or less.

[Metal structure]
The isotropic iron-based rare earth boron magnet obtained by the present invention is characterized in that the average crystal grain size of the RE2Fe14B tetragonal compound, which is the main phase, is 20 nm or more and less than 100 nm, but an average crystal grain size less than 20 nm leads to a decrease in HcJ, and a grain size of 100 nm or more reduces the exchange interaction between the crystal grains, reducing the squareness of the demagnetization curve, making it impossible to obtain the necessary magnetic properties of a residual magnetic flux density Br of 0.7 T or more, an intrinsic coercive force HcJ of 800 kA/m or more, and a maximum energy product (BH)max of 80 kJ/m3 or more. From the viewpoint of improving the magnetic properties, the average crystal grain size is preferably 20 nm or more and 80 nm or less, and more preferably 20 nm or more and 70 nm or less.

Note that if the standard deviation (σ) of the crystal grains of the above RE2Fe14B tetragonal compound exceeds 50% of the crystal grain size, the uniformity of the metal structure is lost and the exchange interaction between each crystal grain of the RE2(Fe,Cr)14B phase decreases, resulting in a decrease in the residual magnetic flux density Br, so σ is set to 50% or less. To achieve a more uniform and fine structure and improved magnetic properties, σ is preferably 40% or less, and even more preferably 30% or less.

[Magnetic properties]
The isotropic iron-based rare earth boron magnet obtained by the present invention can exhibit permanent magnet performance such as a residual magnetic flux density Br of 0.7 T or more, an intrinsic coercivity HcJ of 800 kA/m or more, and a maximum energy product (BH)max of 80 kJ/m3 or more, but when considering application as an injection bonded magnet in small DC brushless motors in the several tens to several hundreds of W class for electrical fuel pumps and water pumps, an HcJ of 850 kA/m or more is preferable, and 900 kA/m or more is even more preferable.
Moreover, Br is preferably 0.72 T or more, and more preferably 0.75 T or more.

[Quenching of molten metal]
In the present invention, raw materials prepared to obtain a predetermined alloy composition are melted, and then the molten metal is sprayed onto the surface of a rotating roll made mainly of copper, copper alloy, Mo, or W at an average pouring rate of 200 g/min or more and less than 2000 g/min per orifice at the tip of the nozzle, to produce a rapidly solidified alloy having 1 volume % or more of an amorphous phase or a crystalline phase including the RE2Fe14B phase. However, if the average pouring rate is less than 200 g/min, the manufacturing productivity is poor, and if it is more than 2000 g/min, the molten metal quenched alloy structure containing coarse α-Fe is formed, so that the desired permanent magnet properties cannot be obtained even if a crystallization heat treatment is performed, so the pouring rate per orifice at the tip of the nozzle is limited to 200 g/min or more and less than 2000 g/min. This average pouring rate is preferably 300 g/min or more and 1500 g/min or less, and more preferably 400 g/min or more and 1300 g/min or less.

When producing the above-mentioned rapidly solidified alloy, the adhesion between the molten alloy and the rotating roll is important, and since this adhesion of the molten alloy is largely dependent on the surface roughness of the roll, the surface roughness of the rotating roll is set to an arithmetic mean roughness (Ra) of 0.1 μm or more and less than 0.6 μm in order to ensure adhesion of the molten alloy and maintain a stable quenched state of the molten alloy. If Ra is less than 0.1 μm, the molten alloy will slip on the surface of the rotating roll and sufficient cooling will not be possible, and if Ra is 0.6 or more, there is a risk that the quenched alloy will stick to the rotating roll. Ra is preferably 0.1 μm or more and less than 0.55 μm, and more preferably 0.15 μm or more and less than 0.5 μm.

When producing the above-mentioned rapidly solidified alloy, an oxygen-free or low-oxygen atmosphere is preferable for the rapidly solidified atmosphere, since oxidation of the molten alloy can be prevented, thereby suppressing an increase in the viscosity of the molten alloy and maintaining a stable tapping rate. To achieve this atmosphere, the inside of the rapid solidification apparatus must be evacuated to 20 Pa or less, preferably 10 Pa or less, and more preferably 1 Pa or less, and then an inert gas must be introduced into the rapid solidification apparatus to reduce the oxygen concentration inside the apparatus to 500 ppm or less, preferably 200 ppm or less, and more preferably 100 ppm or less, before rapid solidification is performed. As the inert gas, a rare gas such as helium or argon or nitrogen can be used, but since nitrogen is relatively reactive with rare earth elements and iron, it is preferable to use a rare gas such as helium or argon, and argon gas is even more preferable from the standpoint of cost.

In the process of producing the rapidly solidified alloy, the material of the rotating roll for rapidly cooling the molten alloy is preferably a base material made of copper, molybdenum, tungsten, or a similar alloy. This is because these base materials have excellent thermal conductivity and durability. In addition, plating the surface of the base material of the rotating roll with chromium, nickel, or a combination of these increases the heat resistance and hardness of the roll surface, and prevents melting and deterioration of the roll surface during rapid solidification.
The diameter of the rotating roll is, for example, Φ200 mm to Φ20,000 mm. If the quenching and solidification time is short, 10 seconds or less, it is not necessarily necessary to water-cool the rotating roll. However, if the quenching and solidification time is 10 seconds or more, it is preferable to run cooling water inside the rotating roll to suppress a rise in temperature of the rotating roll base material, and it is further preferable that the water-cooling capacity of the rotating roll is calculated according to the latent heat of solidification and the molten metal tapping rate per unit time and optimally adjusted as appropriate.

[Flash annealing]
If the heating rate during the crystallization heat treatment is less than 10°C/sec, excessive grain growth will result in a fine metal structure, which will result in a decrease in HcJ and Br. If the heating rate is 200°C/sec or more, the grains will not grow in time, and a uniform fine metal structure will not be obtained, with the RE2Fe14B type tetragonal compound as the main phase, which has an average crystal grain size of 20nm or more and less than 100nm, which is necessary for the development of a permanent magnet. This will result in a decrease in magnetic properties, just as in the case of a heating rate of less than 10°C/sec. Therefore, the heating rate should be 10°C/sec or more and less than 200°C/sec, preferably 30°C/sec or more and less than 200°C/sec, and more preferably 40°C/sec or more and 180°C/sec or less.

In the crystallization heat treatment of the present invention, in order to obtain good permanent magnetic properties, it is preferable to immediately quench after the heat treatment temperature reaches a constant temperature range of from the crystallization temperature to 850°C. In detail, a holding time of substantially 0.01 sec or more until quenching after reaching the heat treatment temperature is sufficient, and holding for more than 7 min is not preferable because it damages the uniform fine metal structure and leads to a deterioration of various magnetic properties. Therefore, the holding time should be 0.01 sec to 7 min, preferably 0.01 sec to 2 min, and more preferably 0.01 sec to 30 sec.

In the crystallization heat treatment of the present invention, it is preferable to cool the molten rapidly solidified alloy powder to 400°C or less at a cooling rate of 2°C/sec or more and 200°C/sec or less. If the cooling rate is less than 2°C/sec, the crystal structure will become coarse, and if it exceeds 200°C/sec, the alloy may oxidize. This cooling rate is more preferably 5°C/sec or more and 200°C/sec or less, and even more preferably 5°C/sec or more and 150°C/sec or less.

The crystallization heat treatment is preferably performed in an inert gas atmosphere to prevent oxidation of the molten alloy. As the inert gas, rare gases such as helium or argon or nitrogen can be used. However, since nitrogen reacts relatively easily with rare earth elements and iron, it is preferable to use rare gases such as helium or argon, and argon gas is even more preferable from the viewpoint of cost.

[Crushing and molding]
The rapidly solidified alloy obtained through the above process may be cut or crushed into a ribbon shape, for example to a size of 50 mm or less, before the crystallization heat treatment. By crushing the magnet of the present invention after the crystallization heat treatment into a magnetic powder having an appropriate average powder particle size in the range of 20 μm to 200 μm, it is possible to manufacture various resin-bonded magnets (commonly known as plastic magnets) using the magnetic powder through known processes.

When producing the resin-bonded magnets, magnet powder is mixed with epoxy, polyamide, polyphenylene sulfide (PPS), polyether ether ketone (PEEK), liquid crystal polymer, acrylic, polyether, etc., and molded into the desired shape.

The resin to be mixed in the resin-bonded magnet is a thermoplastic resin such as polyamide, polyphenylene sulfide (PPS), or polyether ether ketone (PEEK), and magnet powder is mixed and kneaded so that the content is between 50% and 80% by volume, and then made into a compound for injection molding. This compound is used to produce an isotropic rare earth injection bonded magnet with a diameter of 10 mm, height of 7 mm, and a permeance coefficient (Pc) of 2. In an 80°C/5% NaCl (salt water) immersion test, the flux loss after 1,000 hours is less than -20%, and the magnetic flux of the magnet alone (open flux) is 0.5 mWb or more, making it an isotropic iron-based rare earth injection bonded magnet with extremely excellent corrosion resistance. Furthermore, if the magnetic powder is 45% by volume or less, the desired magnetic properties cannot be obtained, and if it is 80% by volume or more, the compound has poor fluidity and cannot be injection molded, so the mixing ratio of the magnetic powder should be 45% by volume or more and 80% by volume or less, preferably 50% by volume or more and 80% by volume or less, and even more preferably 50% by volume or more and 75% by volume or less.

When the magnetic powder of the present invention is used for injection molded bonded magnets, it is preferable to pulverize it so that the average powder particle size is 100 μm or less, and more preferably the average powder particle size is 20 μm or more and 100 μm or less. When it is used for compression molded bonded magnets, it is preferable to pulverize it so that the average powder particle size is 100 μm or more and 200 μm or less, and more preferably the average powder particle size is 50 μm or more and 150 μm or less. Even more preferably, the particle size distribution has two peaks and the average powder particle size is 80 μm or more and 130 μm or less.

Furthermore, by subjecting the surface of the magnet powder of the present invention to surface treatments such as coupling treatment or chemical conversion treatment (including phosphate treatment and glass coating treatment), it is possible to improve the moldability during molding of the resin-bonded magnet and the corrosion resistance and heat resistance of the resulting resin-bonded magnet, regardless of the molding method. In addition, when the surface of the resin-bonded magnet after molding is subjected to surface treatments such as resin painting, chemical conversion treatment, and plating, it is also possible to improve the corrosion resistance and heat resistance of the resin-bonded magnet, in the same way as with the surface treatment of the magnet powder.

The following describes an embodiment of the present invention.

(Example)
To obtain the alloy composition shown in Table 1, 100 g of raw material containing the main elements Nd, Pr, B, Cr and Fe with a purity of 99.5% or more, as well as additive elements such as C, Co, Ga, Si, Ti and Mo, was placed into an alumina melting crucible and then set in the work coil inside a vacuum melting furnace. The vacuum melting furnace was then evacuated to 0.02 Pa or less, and argon gas was introduced until the pressure reached normal pressure. The alloy was then molten by high-frequency induction heating, and the molten alloy was then poured into a water-cooled copper mold to produce the master alloy.

Next, the obtained mother alloy was divided into appropriate sizes, and 40 g was inserted into a transparent quartz nozzle with an orifice having a suitable diameter (0.7 mm to 1.2 mm) at the bottom so that the melting rate shown in Table 1 would be obtained. The nozzle was then set in the work coil of a single roll quenching device. The vacuum melting furnace was then evacuated to 0.02 Pa or less, and argon gas was introduced until the quenching atmosphere pressure in Table 1 was reached. The mother alloy was remelted by high-frequency induction heating, and the molten metal was ejected from the nozzle orifice at a spray pressure of 30 kPa onto the surface of a pure copper rotating roll with the surface roughness shown in Table 1, which rotated at the roll surface speed (Vs) shown in Table 1, to produce a molten metal quenched and solidified alloy. The distance between the nozzle tip and the rotating roll surface was 0.8 mm. Figure 3 shows the powder XRD profile of the rapidly solidified alloy of Example 3 as a representative example. From FIG. 3, it was confirmed that the Nd2Fe14B phase was already present in the as-spun rapidly solidified state, and that a small amount of α-Fe was mixed into the structure.

The rapidly solidified alloy obtained in the above process was coarsely crushed to a few mm or less to obtain a molten rapidly solidified alloy powder. The coarse powder was then fed into the raw material hopper of a crystallization heat treatment furnace (flash annealing furnace, furnace tube: transparent quartz, outer diameter 15 mm x inner diameter 12.5 mm x length 1000 mm, heating zone 300 mm, cooling zone by cooling fan 500 mm) shown in Figure 1 (a), and heat treatment was performed at a work cutting speed of 20 g/min. The crystallization heat treatment furnace shown in Figure 1 (a) is equipped with a raw material hopper 1, a raw material supply feeder 2, a furnace tube 3, a tubular furnace 4, a cooling tower 5, a recovery hopper 6, a vibrator 7, a motor for rotating the furnace tube 8, a furnace tube rotating shaft 9, and an apparatus stand 10. The furnace tube inclination angle 11, the furnace tube rotation speed, and the furnace tube vibration frequency were appropriately adjusted along with the heat treatment temperature and heat treatment time listed in Table 2 so as to obtain the heating rate in Table 2. As a result, as shown in the enlarged view of the inside of the core tube 3 in FIG. 1( b ), the rapidly solidified powder 13 passes through the core tube in the direction of the arrow 14 while undergoing a combination of stirring due to rotational motion caused by the operation of the core tube rotating motor 8 and hopping phenomenon 15 of the rapidly solidified powder caused by vibration of the core tube by the operation of the vibrator 7, while the rapidly solidified powder 13 is subjected to unique heat treatment conditions in which the powder is subjected to a thermal history not as a whole but individually as powder.

When the constituent phases of the molten rapidly solidified alloy powder after the crystallization heat treatment were confirmed by powder X-ray diffraction, the presence of the Nd2Fe14B phase was confirmed. Figure 4 shows a powder XRD profile after the crystallization heat treatment of Example 3 as a representative example. In addition, in Figure 4, the intensity of the crystal peak of the Nd2Fe14B phase tends to increase compared to Figure 3, and it was confirmed that the crystallinity of the Nd2Fe14B phase has improved due to the heat treatment. It was also confirmed that the structure contained a small amount of α-Fe, similar to Figure 3 before the heat treatment.

After the crystallization heat treatment described in Table 2 was applied, the resulting isotropic iron-based rare earth boron magnet was cut into a sample for evaluating magnetic properties measuring approximately 7 mm in length, approximately 0.9-2.3 mm in width, and 18-25 μm in thickness. The sample was then magnetized in the longitudinal direction using a pulsed magnetic field of 3.2 MA/m, and the magnetic properties were then set in the longitudinal direction to suppress the effects of the demagnetizing field. The magnetic properties at room temperature were measured using a vibrating sample magnetometer (VSM), and the results are shown in Table 3. From Table 3, it can be seen that the target magnetic property levels of Br≧0.7T, HcJ≧800kA/m, and (BH)max≧80kJ/m3 were achieved using the alloy composition and manufacturing method described in the examples.

The isotropic iron-based rare earth boron magnets obtained after the crystallization heat treatment shown in Table 4 were observed with a transmission electron microscope. A fine metal structure with the RE2Fe14B phase as the main phase was confirmed in the bright field image. Table 4 shows the average crystal grain size of the main phase and the standard deviation σ of the crystal grain size. Table 4 shows that the average crystal grain size of the RE2Fe14B tetragonal compound is 20 nm or more and less than 100 nm, and σ is within 50% of the average crystal grain size. The crystal grain size of the RE2Fe14B tetragonal compound, which is the main phase, was evaluated by image analysis based on the JIS standard (JIS G 0551:2005) after binarizing the bright field image taken with a transmission electron microscope to separate the main phase from the grain boundaries.

Next, the heat-treated magnetic powder obtained in Examples 1 to 13 was pulverized in a pin disc mill to an average particle size of 70 μm. After the pulverized magnetic powder and PPS resin were weighed out to a specified weight, they were mixed uniformly using a universal mixer. The resulting mixture was then kneaded in a twin-screw extrusion kneader to produce a compound for bonded magnets for injection molding.

The compound for injection-molded bonded magnets was injection-molded in an injection molding machine to produce an isotropic injection-molded bonded magnet. The shape of the resulting injection-molded bonded magnet was 10 mm in diameter x 7 mm in height, and the density of the molded body was 4.4 g/cm3 (true specific gravity of magnetic powder 7.5 g/cm3), giving a magnetic powder filling rate of 58.7% by volume.

The magnetic properties of the isotropic injection molded bonded magnets obtained using the magnetic powders of Examples 1 to 13 were measured using a BH tracer after they were magnetized in the longitudinal direction with a pulsed magnetic field of 3.2 MA/m. The results are shown in Table 5.

Next, the isotropic injection molded bonded magnets obtained using the magnetic powders of Examples 1 to 13 were subjected to an 80°C/5% NaCl (salt water) immersion test to investigate the rusting state and the change in magnetic flux of the magnet alone over time. Table 6 shows the flux loss and open flux after 1000 hours. In addition, Figure 7 shows the flux loss change in the 80°C/5% NaCl (salt water) immersion test, Figure 8 shows the change in open flux in the 80°C/5% NaCl (salt water) immersion test, Figure 9 shows the relationship between the amount of Cr added and the flux loss in the 80°C/5% NaCl (salt water) immersion test, and Figure 10 shows a photograph showing the rusting state in the 80°C/5% NaCl (salt water) immersion test of Example 2.

Comparative Example
To obtain the alloy composition shown in Table 1, 100 g of raw material containing the main elements Nd, Pr, B and Fe with a purity of 99.5% or more, as well as additive elements Si, Cr, Nb, Ti and Zr, was placed into an alumina melting crucible and then set in the work coil inside a vacuum melting furnace. The vacuum melting furnace was then evacuated to 0.02 Pa or less, and argon gas was introduced until the pressure reached normal pressure. After that, the alloy was made molten by high-frequency induction heating, and the molten alloy was then poured into a water-cooled copper mold to produce the master alloy.

Next, the obtained mother alloy was divided into appropriate sizes, and 40 g was inserted into a transparent quartz nozzle with an orifice having a suitable diameter (0.7 mm to 1.2 mm) at the bottom so that the melting rate shown in Table 1 would be obtained. The nozzle was then set in the work coil of a single roll quenching device. After that, the vacuum melting furnace was evacuated to 0.02 Pa or less, and argon gas was introduced until the quenching atmosphere pressure in Table 1 was reached. The mother alloy was remelted by high-frequency induction heating, and the molten metal was ejected from the nozzle orifice at a spray pressure of 30 kPa onto the surface of a pure copper rotating roll with the surface roughness shown in Table 1, which rotates at the roll surface speed (Vs) shown in Table 1, to produce a molten metal quenched and solidified alloy. At this time, the distance between the nozzle tip and the rotating roll surface was 0.8 mm. Figure 5 shows the powder XRD profile of the rapidly solidified alloy of Comparative Example 14 as a representative example. Figure 5 shows the presence of the Nd2Fe14B phase already in the rapidly solidified state (as-spun). It is clear that the metal structure is a single phase of Nd2Fe14B.

The rapidly solidified alloy obtained in the above process was coarsely pulverized to a few mm or less to obtain a molten rapidly solidified alloy powder, which was then fed into a crystallization heat treatment furnace (flash annealing furnace, furnace tube: transparent quartz outer diameter 15 mm x inner diameter 12.5 mm x length 1000 mm, heating zone 300 mm, cooling zone by cooling fan 500 mm) and heat treated at a workpiece cutting speed of 20 g/min. The furnace tube inclination angle, furnace tube rotation speed, and furnace tube vibration frequency were appropriately adjusted along with the heat treatment temperature and heat treatment time listed in Table 2 to achieve the heating rate in Table 2.

When the constituent phases of the molten rapidly solidified alloy powder after the crystallization heat treatment were confirmed by powder X-ray diffraction, the presence of the Nd2Fe14B phase was confirmed. Figure 6 shows the powder XRD profile after the crystallization heat treatment of Comparative Example 14 as a representative example. As shown in Figure 6, as in Figure 5, it can be seen that the Nd2Fe14B single phase remains even after the heat treatment.

The isotropic iron-based rare earth boron magnets obtained by the crystallization heat treatment described in Table 2 were made into samples for magnetic property evaluation measuring approximately 7 mm in length x approximately 0.9 mm to 2.3 mm in width x 18 μm to 25 μm in thickness. They were then magnetized in the longitudinal direction using a pulsed magnetic field of 3.2 MA/m. The samples for magnetic property evaluation were then set in the longitudinal direction to reduce the effects of the demagnetizing field, and the magnetic properties at room temperature were measured using a vibrating sample magnetometer (VSM). The results are shown in Table 3.

The isotropic iron-based rare earth boron magnets obtained after the crystallization heat treatment shown in Table 4 were observed with a transmission electron microscope. A fine metal structure with the RE2Fe14B phase as the main phase was confirmed in the bright field image. Table 4 shows the average crystal grain size of the main phase and the standard deviation σ of the crystal grain size.

Next, the heat-treated magnetic powder obtained in Comparative Examples 14 to 18 was pulverized in a pin disc mill to an average particle size of 70 μm. After weighing out a specified weight of the pulverized magnetic powder and PPS resin, they were mixed uniformly using a universal mixer. The resulting mixture was then kneaded in a twin-screw extrusion kneader to produce a compound for bonded magnets for injection molding.

The compound for injection-molded bonded magnets was injection-molded in an injection molding machine to produce an isotropic injection-molded bonded magnet. The shape of the resulting injection-molded bonded magnet was 10 mm in diameter x 7 mm in height, and the density of the molded body was 4.4 g/cm3 (true specific gravity of magnetic powder 7.5 g/cm3), giving a magnetic powder filling rate of 58.7% by volume.

The magnetic properties of the isotropic injection molded bonded magnets obtained using the magnetic powders of Comparative Examples 14 to 18 were measured using a BH tracer after being magnetized in the longitudinal direction with a pulsed magnetic field of 3.2 MA/m. The results are shown in Table 5.

Next, the isotropic injection molded bonded magnets obtained using the magnetic powders of Comparative Examples 14 to 18 were subjected to an 80°C/5% NaCl (salt water) immersion test to investigate the rusting condition and the change in magnetic flux of the magnet alone over time. Table 6 shows the flux loss and open flux after 1000 hours. In addition, Figure 7 shows the change in flux loss in the 80°C/5% NaCl (salt water) immersion test, Figure 8 shows the change in open flux in the 80°C/5% NaCl (salt water) immersion test, and Figure 11 shows a photograph showing the rusting condition of Comparative Example 16 in the 80°C/5% NaCl (salt water) immersion test.

1 Raw material hopper 2 Raw material supply feeder 3 Furnace tube 3a Enlarged view of furnace tube 3b Enlarged view of cross section of furnace tube 4 Tubular furnace 5 Cooling tower 6 Recovery hopper 7 Oscillator 8 Motor for rotating furnace tube 9 Furnace tube rotating shaft 10 Device stand 11 Inclination angle of furnace tube 12 Cooling fan air 13 Molten metal rapidly solidified alloy powder (workpiece)
14 Workpiece moving direction 15 Workpiece hopping phenomenon 16 Temperature rise rate 17 Holding temperature 18 Temperature fall rate

Claims (11)

_{The present invention} relates to an isotropic iron-based rare earth boron magnet alloy having a RE2Fe14B type tetragonal compound phase (RE is a rare earth element) as the main phase, and is expressed by the composition formula _T100-xyzm (B1 _{- nCn} ) _xREyCrzMm (T is at least one element selected from the group consisting of Fe, Co, and Ni, and is a transition metal element that always contains Fe, RE is a rare earth element that always contains Nd or Pr, and M is one or more metal elements selected from the group consisting of Al, Si, V, Ti, Mn, Cu, Zn, Ga, Zr, Nb, Mo, Ag, Hf, Ta, W, Pt, Au, and Pb), and the composition ratios x, y, z, m, and n are respectively:
5.6≦x≦6.4 atomic %,
11.2≦y≦12.0 atomic %,
2.3≦z≦5.4 atomic %,
0.0≦m≦3.0 atomic %
0.0≦n≦0.5
and wherein Cr is an essential component. A magnet alloy as described in claim 1, in which the average crystal grain size of the main phase, the RE2Fe14B type tetragonal compound, is 20 nm or more and less than 100 nm, and the standard deviation (σ) is within 50% of the average crystal grain size. A magnet alloy as described in claim 1 or 2, having permanent magnet properties of a residual magnetic flux density Br of 0.7 T or more, an intrinsic coercivity HcJ of 800 kA/m or more, and a maximum energy product (BH) max of 80 kJ/m3 or more. A magnet alloy according to any one of claims 1 to 3, in powder form having high corrosion resistance and an average powder particle size of 20 μm or more and less than 200 μm. A bonded magnet obtained by mixing and kneading the powdered magnetic alloy according to claim 4 with a thermoplastic resin or a thermosetting resin and then molding the mixture. 6. A bonded magnet according to claim 5, which uses at least one thermoplastic resin selected from the group consisting of polyamide, polyphenylene sulfide (PPS) and polyether ether ketone (PEEK) as the resin to be mixed, has a diameter of 10 mm, a height of 7 mm, and a permeance coefficient (Pc) of 2,
A bonded magnet whose flux loss after 1000 hours of immersion in 80℃/5% NaCl (salt water) is less than -20% (0 to -20%) and whose magnetic flux amount by itself (open flux) is 0.5mWb or more. _{It is expressed} by the composition formula _T100-xyzm (B1 _{- nCn} ) _xREyCrzMm (T is at least one element selected from the group consisting of Fe, Co, and Ni, and is a transition metal element that always includes Fe, RE is a rare earth element that always includes Nd or Pr, and M is one or more metal elements selected from the group consisting of Al, Si, V, Ti, Mn, Cu, Zn, Ga, Zr, Nb, Mo, Ag, Hf, Ta, W, Pt, Au, and Pb), and the composition ratios x, y, z, m, and n are respectively:
5.6≦x≦6.4 atomic %,
11.2≦y≦12.0 atomic %,
2.3≦z≦5.4 atomic %,
0.0≦m≦3.0 atomic %
0.0≦n≦0.5
preparing a molten alloy having a composition that satisfies the above;
and spraying the molten alloy onto the surface of a rotating roll whose main raw material is any of copper, copper alloy, Mo, and W at an average pouring rate of 200 g/min or more and less than 2000 g/min per orifice disposed at the tip of a nozzle, thereby producing a rapidly solidified alloy having 1 volume % or more of an amorphous phase or a crystalline phase including the RE2Fe14B phase. A method for producing a magnetic alloy as described in claim 7, wherein the rotating roll has a surface roughness of arithmetic mean roughness (Ra) of 0.1 μm or more and less than 0.6 μm. The method for producing a magnetic alloy according to claim 7 or 8 further comprises a step of producing a magnetic alloy having a main phase of an RE2Fe14B type tetragonal compound by subjecting the rapidly solidified alloy to a heat treatment in which the alloy is immediately quenched after 0.01 sec to less than 7 min has elapsed since the alloy reached a constant temperature range of not less than the crystallization temperature and not more than 850°C at a heating rate of not less than 10°C/sec and not more than 200°C/sec. A step of pulverizing the magnet alloy obtained by the method for producing a magnet alloy according to claim 9 to obtain a magnet alloy powder having an average powder particle size of 100 μm or more and less than 200 μm;
a step of adding a thermosetting resin to the magnet alloy powder, filling the mixture into a molding die, and forming a compression molded body by press molding;
and heat-treating the compression molded body at a temperature equal to or higher than the polymerization temperature of the thermosetting resin to obtain a bonded magnet. A step of pulverizing the magnet alloy obtained by the method for producing a magnet alloy according to claim 9 to obtain magnet alloy powder having an average powder particle size of 20 μm or more and less than 100 μm;
and injection molding the injection molding compound prepared by adding a thermoplastic resin to the magnet alloy powder.