阅读说明：本技术 稀土磁体粉末的制造方法 (Method for producing rare earth magnet powder ) 是由 山崎理央 杉本谕 于 2019-07-17 设计创作，主要内容包括：提供一种能够得到高磁特性的稀土磁体粉末的制造方法。本发明为一种稀土磁体粉末的制造方法,具有：歧化工序,其中,使磁体原料吸氢而发生歧化反应,所述磁体原料是将包含稀土元素(称为“R”)、硼(B)和过渡元素(称为“TM”)的铸造合金暴露于350℃～550℃的氢气气氛中而得到的；和再结合工序,其中,从该歧化工序后的磁体原料中脱氢而发生再结合反应。铸造合金暴露于的氢气气氛的氢气分压没有限制,例如可以是1kPa～250kPa。铸造合金优选包含在暴露于氢气气氛之前预先进行了固溶处理的铸锭。通过使该氢气气氛的温度(氢破碎温度)在预定范围内,裂纹主要在晶界相中产生,主相的裂纹得到抑制。这在HDDR后也得到反映,认为得到了高磁特性的磁体粉末。(Provided is a method for producing a rare earth magnet powder with high magnetic characteristics. The present invention is a method for producing a rare earth magnet powder, comprising: a disproportionation step in which a magnet raw material obtained by exposing a cast alloy containing a rare earth element (referred to as "R"), boron (B), and a transition element (referred to as "TM") to a hydrogen atmosphere at 350 to 550 ℃ is subjected to hydrogen absorption to cause a disproportionation reaction; and a recombination step of dehydrogenating the magnet material after the disproportionation step to cause recombination. The hydrogen partial pressure of the hydrogen atmosphere to which the cast alloy is exposed is not limited, and may be, for example, 1kPa to 250 kPa. The cast alloy preferably comprises an ingot that has been previously solution treated prior to exposure to a hydrogen atmosphere. When the temperature of the hydrogen atmosphere (hydrogen fracture temperature) is within a predetermined range, cracks are mainly generated in the grain boundary phase, and cracks in the main phase are suppressed. This is reflected also after HDDR, and it is considered that a magnetic powder having high magnetic characteristics is obtained.)

1. A method for producing a rare earth magnet powder, wherein the method comprises:

a disproportionation step in which a magnet raw material obtained by exposing a cast alloy containing a rare earth element (referred to as "R"), boron (B), and a transition element (referred to as "TM") to a hydrogen atmosphere at 350 to 585 ℃ is subjected to hydrogen absorption to cause a disproportionation reaction; and

and a recombination step in which the magnet material after the disproportionation step is dehydrogenated to cause recombination reaction.

2. A method for producing a rare earth magnet powder according to claim 1, wherein a hydrogen partial pressure of the hydrogen atmosphere is 1kPa to 250 kPa.

3. A method for producing a rare earth magnet powder according to claim 1 or 2, wherein the cast alloy includes an ingot subjected to the solution treatment.

4. A method for producing a rare earth magnet powder according to any one of claims 1 to 3, wherein when the entire cast alloy is 100 atomic%, R in the cast alloy is 11 atomic% to 15 atomic% and B is 5 atomic% to 9 atomic%.

Technical Field

The present invention relates to a method for producing a rare earth magnet powder for a bonded magnet or the like.

Background

Bonded magnets obtained by consolidating rare earth magnet powder with a binder resin are used in many applications such as electric appliances and various electromagnetic devices of automobiles, where energy saving and weight reduction are desired, because they have excellent shape flexibility and exhibit high magnetic characteristics. In order to further expand the use of bonded magnets, it is desirable to improve the magnetic properties of rare earth magnet powders. Therefore, various proposals have been made regarding the hydrogen treatment performed in the production process of the rare earth magnet powder, and the following patent documents describe the hydrogen treatment.

The hydrogen treatment mainly includes a Disproportionation reaction by hydrogen absorption (also simply referred to as "HD reaction") and a Recombination reaction by dehydrogenation (also simply referred to as "DR reaction"). The HD reaction and the DR reaction are collectively referred to as "HDDR reaction", and the hydrogen treatment is referred to as "HDDR (treatment)". Incidentally, unless otherwise specified, the HDDR referred to in the present specification includes an improved d-HDDR (dynamic-Hydrogenation-Desorption-recommendation) and the like.

Documents of the prior art

Patent document

Patent document 1: japanese patent No. 3871219

Patent document 2: japanese patent laid-open No. 2008-127648

Patent document 3: japanese patent laid-open No. 2008-305908

Disclosure of Invention

Problems to be solved by the invention

Patent document 1 describes the following subject matter: a magnet raw material having an average particle diameter of 10mm or less is obtained by mechanically crushing an ingot by a jaw crusher, and the magnet raw material is subjected to d-HDDR having a low-temperature hydrogenation step (room temperature × 100kPa), a high-temperature hydrogenation step, a structure stabilization step, and a controlled degassing step, thereby obtaining a rare earth anisotropic magnet powder having high magnetic characteristics.

Patent document 2 describes the following subject matter: the raw material alloy storing hydrogen (150 ℃ C.. times.250 kPa) is subjected to HDDR treatment in which the HD reaction is gradually progressed by introducing hydrogen gas after the temperature is raised, whereby a rare earth anisotropic magnet powder having high magnetic characteristics can be obtained.

Patent document 3 describes that the raw material alloy is hydrogen pulverized (about 300 ℃. times.130 kPa). However, patent document 3 is directed to a sintered magnet, and is not directed to a bonded magnet.

The present invention has been made under such circumstances, and an object thereof is to provide a method for producing a rare earth magnet powder which can obtain high magnetic properties suitable for a bonded magnet by a method different from the conventional method.

Means for solving the problems

As a result of intensive studies to solve the above problems, the present inventors have newly found that a rare earth magnet powder having higher magnetic properties than conventional ones can be obtained by subjecting a magnet raw material obtained by subjecting a cast alloy to hydrogen treatment (hydrogen crushing) under predetermined conditions to HDDR (including d-HDDR). By further developing this result, the invention described hereinafter is completed.

Method for producing rare earth magnet powder

(1) The present invention is a method for producing a rare earth magnet powder, wherein the method comprises: a disproportionation step in which a magnet raw material obtained by exposing a cast alloy containing a rare earth element (referred to as "R"), boron (B), and a transition element (referred to as "TM") to a hydrogen atmosphere at 350 to 585 ℃ is subjected to hydrogen absorption to cause a disproportionation reaction; and a recombination step of dehydrogenating the magnet material after the disproportionation step to cause recombination.

(2) According to the production method of the present invention, the magnet raw material obtained by exposing the cast magnet alloy (cast alloy) to a hydrogen atmosphere in a higher temperature range than in the conventional case is subjected to HDDR, whereby the rare earth magnet powder having high magnetic characteristics can be obtained. The reason for this is not clear, but the mechanism which is currently thought will be described later.

In the present specification, the treatment of exposing the cast alloy to a hydrogen atmosphere for obtaining a magnet raw material is simply referred to as "hydrogen fracturing" regardless of the state (bulk, particulate, powder, etc.) of the magnet raw material supplied to HDDR. The cast alloy after hydrogen fracturing is generally prone to spalling, forming coarse lumps or particles at a slight degree of fracturing. The magnet material may be supplied to the HDDR in a coarse state as it is, or may be supplied to the HDDR after being further pulverized into a fine state.

Magnet raw material, rare earth magnet powder, composite, and bonded magnet

The present invention can be understood as a magnet raw material obtained by hydrogen pulverization (a magnet raw material in a powder state is also referred to as "raw material powder"), and a magnet powder obtained by the above-described production method. The present invention can also be understood as a bonded magnet including a rare earth magnet powder and a resin for consolidating the powder particles. Further, the present invention can also be understood as a composite for manufacturing the bonded magnet. The composite is a material obtained by previously attaching a resin as a binder to the surface of each powder particle. The magnet powder used for the bonded magnet or the composite may be a composite powder in which a plurality of kinds of magnet powders having different average particle diameters, alloy compositions, and the like are mixed.

(others)

(1) The rare earth magnet powder according to the present invention may be an isotropic magnet powder, but is preferably an anisotropic magnet powder having higher magnetic properties. The anisotropic magnet powder contains magnet particles in which the magnetic flux density (Br) in one direction (the easy magnetization axis direction, the c-axis direction) is greater than the magnetic flux density in the other direction. Isotropy and anisotropy can be distinguished by the Degree of anisotropy (DOT: depth of Texture) ([ Br (/ /) -Br (#) ]/Br (/ /) obtained when a magnetic field is applied in parallel (/) and perpendicular (#) to the c-axis direction, and is isotropic if the value of DOT is 0 and anisotropic if it is greater than 0.

(2) In the present specification, "R" may be one or more of Y, lanthanoid, and actinoid, and is mainly Nd. TM is one or more of 3d transition elements (Sc to Cu) and 4d transition elements (Y to Ag), and further is any of group 8 to 10 elements (particularly Fe, Co, Ni), but is mainly Fe. A part of B may be replaced with C.

The magnet raw material or the magnet powder may contain a modifying element effective for improving the characteristics, and (inevitable) impurities. Examples of the modifier element include Cu, Al, Ti, V, Cr, Ni, Zn, Ga, Zr, Nb, Mo, Sn, Hf, Ta, W, Dy, Tb, and Co, which are effective for enhancing the coercive force.

(3) Unless otherwise specified, "x to y" referred to in the present specification include a lower limit value x and an upper limit value y. Any numerical value included in various numerical values or numerical value ranges described in the present specification may be newly set as a new lower limit value or an upper limit value in the range of "a to b". "x to ykPa" means an xkPa to ykPa, and the same applies to other units.

Drawings

FIG. 1A is a flowchart showing a process of manufacturing a magnet powder.

FIG. 1B is a diagram showing a hydrogen gas atmosphere at the time of hydrogen fragmentation.

FIG. 1C is a diagram showing another mode of the hydrogen atmosphere at the time of hydrogen fragmentation.

Fig. 2 is an SEM image obtained by observing the appearance of each magnet raw material (raw material block) obtained by hydrogen fracturing under different conditions.

Fig. 3A is an SEM image obtained by comparing the appearance and cross section of magnet raw material particles obtained by hydrogen fragmentation at different atmospheric temperatures.

FIG. 3B is a scatter diagram showing the relationship between the hydrogen fracturing temperature and the average particle diameter of the raw material powder.

Fig. 4A is an SEM image obtained by comparing the appearance and cross section of each powder (magnet powder) obtained by subjecting each raw material powder obtained by hydrogen crushing at different atmospheric temperatures to d-HDDR.

FIG. 4B is a scatter diagram showing the relationship between the hydrogen fracturing temperature of the raw material powder and the average particle diameter of the magnet powder.

Fig. 5A is a scatter diagram showing a relationship between the hydrogen fracture temperature of the raw material powder and the maximum energy product of the magnet powder.

Fig. 5B is a scatter diagram showing the relationship between the hydrogen fracture temperature of the raw material powder and the residual magnetic flux density of the magnet powder.

Fig. 5C is a scatter diagram showing the relationship between the hydrogen fracture temperature of the raw material powder and the coercive force of the magnet powder.

FIG. 6 is a scatter diagram showing the relationship between the hydrogen fracture temperature and the crack density.

FIG. 7 is an explanatory view of a mechanism of generating cracks in the main phase and the grain boundary phase.

FIG. 8A is a scatter diagram showing the relationship between the temperature of the ingot for dispersion treatment and the maximum energy product of the magnet powder.

FIG. 8B is a scatter diagram showing the relationship between the temperature of the ingot for dispersion treatment and the residual magnetic flux density of the magnet powder.

Fig. 8C is a scatter diagram showing the relationship between the dispersion treatment temperature of the ingot and the coercive force of the magnet powder.

Fig. 9A is a scatter diagram showing the maximum energy product of each magnet powder obtained by performing HDDR after hydrogen fracturing.

Fig. 9B is a scatter diagram showing the residual magnetic flux density of each of these magnet powders.

Fig. 9C is a scatter diagram showing the coercive force of each of these magnetic powders.

Detailed Description

One or two or more components arbitrarily selected from the present specification may be added to the components of the present invention. The content described in the present specification suitably corresponds to not only the production method of the present invention but also a magnet raw material, a rare earth magnet powder, a composite, a bonded magnet, and the like, and even a method component may be a component related to an object. Which embodiment is the best, varies according to the object, the required performance, and the like.

Magnet raw Material

(1) Casting alloy

The cast alloy may be an ingot alloy obtained by casting a melt of an R-TM-B alloy into a mold and solidifying the melt, or a quench-solidified alloy obtained by quenching the melt. The quenched solidified alloy can be obtained by, for example, a strip casting method (ストリップキャスト method) or the like.

The cast alloy preferably includes an ingot subjected to solution treatment (step) before hydrogen crushing. Since the ingot alloy is solidified slowly (cooling rate is low), the soft magnetic α Fe alloy is easily crystallized (remains). When the ingot alloy is subjected to solution treatment, the α Fe phase disappears, segregation or the like is eliminated, and a homogeneous structure (for example, a grain size: 50 μm to 250 μm) in which fine grains grow is obtained.

Since the rapidly solidified alloy solidifies faster (has a higher cooling rate) than the ingot alloy, the soft magnetic α Fe phase is hardly crystallized (remains) or is crystallized only in a small amount. Therefore, the crystalline structure of the quenched solidified alloy is relatively homogeneous compared to the ingot alloy. The rapidly solidified alloy has a structure (for example, a grain size of 50 to 250 μm) in which a fine grain grows mainly when subjected to a solution treatment.

In the ingot alloy and the quench-solidified alloy, the purpose of the solution treatment is not necessarily the same. However, the same applies to any alloy, in that the metal structure of the cast alloy before hydrogen fragmentation is brought into a desired form by solution treatment. The solution treatment may be referred to as a homogenization heat treatment.

The solution treatment can be performed by heating the cast alloy before hydrogen crushing at 1050 to 1250 ℃ and further at 1100 to 1200 ℃ in a treatment furnace (heating furnace). The treatment time may be, for example, 3 to 50 hours, and further 10 to 40 hours. The treatment atmosphere may be an inert gas atmosphere (an inert gas atmosphere such as Ar or a vacuum atmosphere).

The cast alloy (ingot) after the solution treatment may be subjected to a heat treatment (referred to as "R-rich dispersion treatment") by heating the cast alloy in a temperature range lower than the treatment temperature of the solution treatment and higher than a treatment temperature of hydrogen fragmentation (hydrogen fragmentation temperature) described later. The treatment temperature may be 650 to 900 ℃, 650 to 800 ℃, and further 680 to 750 ℃, for example. The treatment time may be, for example, 10 minutes to 10 hours, and further 0.5 hours to 3 hours. The treatment atmosphere may be, for example, an inert atmosphere (an inert gas (Ar or the like) atmosphere, a vacuum atmosphere, or the like). The R-rich dispersion treatment promotes dispersion (distribution) of the rare earth element (R) in the grain boundaries of the cast alloy, and the crystal grains of the cast alloy are uniformly covered with the R-rich phase. When such a cast alloy is subjected to hydrogen fracturing treatment, fracture (separation) occurs more preferentially at the grain boundaries of the cast alloy, and it is considered that a magnet material with few intergranular cracks is easily obtained.

(2) Alloy composition

Form R₂TM₁₄B₁The theoretical composition of the tetragonal compound of form crystal (main phase) is, in atomic% (at%), R: 11.8 atomic%, B: 5.9 atomic%, TM: and (4) the balance. The cast alloy is preferably R-rich in the theoretical composition value because the particle size distribution after hydrogen fracturing can be optimized and the coercivity of the rare earth magnet powder can be increased. Therefore, when the entire cast alloy is 100 atomic%, R of the cast alloy is preferably 11 atomic% to 15 atomic%, more preferably 12 atomic% to 13 atomic%, and B is preferably 5 atomic% to 9 atomic%, more preferably 6.2 atomic% to 7 atomic%.

(3) Hydrogen crushing (working procedure)

The magnet raw material is obtained by subjecting the cast alloy to a specific hydrogen treatment (raw material hydrogen treatment). The treatment (hydrogen crushing and further hydrogen crushing) is carried out by exposing the cast alloy placed in the treatment furnace to a hydrogen atmosphere at 350 to 585 ℃, 400 to 575 ℃, and further 425 to 550 ℃ (hydrogen absorption step). The atmosphere temperature is the temperature at which the cast alloy is maintained at a substantially constant temperature. The hydrogen partial pressure may be low or high as long as the atmospheric temperature is within a predetermined range. Of course, the hydrogen partial pressure is preferably 1kPa to 250kPa, and more preferably 5kPa to 150kPa, in view of efficiency and safety at the time of hydrogen fragmentation.

Hydrogen fragmentation is performed, for example, by evacuating a treatment furnace containing the cast alloy and then introducing hydrogen gas into the treatment furnace. The gas introduced into the treatment furnace may be hydrogen gas alone or may be introduced together with an inert gas. The latter case is preferable from the viewpoint of easier control of the hydrogen partial pressure. In addition, the gas introduced into the treatment furnace may be in a flowing (flow) state. The hydrogen fragmentation may be performed, for example, after the atmospheric temperature reaches the target temperature, for 0.5 to 10 hours, and further for 1 to 5 hours. It is preferable to introduce hydrogen into the furnace after the atmospheric temperature (or the temperature of the cast alloy) reaches a predetermined value.

The cast alloy exposed to the hydrogen atmosphere obtains a block having a maximum length of about several cm to about several mm to the extent of self-collapsing or slightly crushing by hydrogen absorption. Such magnet material is referred to as "material block". The magnet raw material, which is made into a powder (raw material powder) having a particle diameter (maximum diameter) of about 100 μm to about 1mm, can be supplied to the next step (HDDR) by separately crushing and further pulverizing the raw material block obtained by hydrogen crushing. The magnet material supplied to HDDR may be in a state of hydrogen absorption or may be a dehydrogenated magnet material. Incidentally, it is difficult to strictly distinguish between "crushing" and "pulverizing", but if it is hard to distinguish, the case where a shearing force is applied to intentionally miniaturize particles is "crushing", and the case where a slight impact or the like is applied to break up lumps is "crushing".

In the case where the production of the magnet raw material (hydrogen crushing treatment) and the production of the magnet powder (HDDR treatment) are performed discontinuously, the magnet raw material from which the absorbed hydrogen is released may be supplied to the HDDR. By inserting the hydrogen release step, deterioration of the magnet material before HDDR can be prevented. The hydrogen release step may be performed by releasing the hydrogen at the same temperature (350 ℃ to 585 ℃) as that during hydrogen absorption, and then cooling the hydrogen to around room temperature (R.T ℃).

(4) Supplying mode to HDDR

The main purpose of the hydrogen fragmentation of the present invention is not the fragmentation or micronization of the cast alloy itself. The purpose of the present invention is to minimize the occurrence of cracks in the crystal grains (single crystal grains) constituting a cast alloy. When the cast alloy is exposed to a high-temperature hydrogen atmosphere as in the hydrogen decrepitation of the present invention, hydrogen hardly invades into the inside of the grains, but mainly invades into the grain boundary phase (R-rich phase/Nd-rich phase) existing between grains (grain boundary). As a result, cracks generated by volume expansion of the grain boundary phase accompanying hydrogen intrusion preferentially occur at the grain boundary. Thus, it is considered that the cast alloy after hydrogen crushing yielded a magnet raw material containing crystal grains with almost no cracking. As a result of supplying the magnet raw material containing crystal grains with few cracks to HDDR, it is estimated that a magnet powder with high magnetic characteristics is obtained. The cast alloy (magnet raw material) after hydrogen fracturing may be particles (single crystal grains) containing the above-described single crystal grains, or may contain an aggregate (polycrystalline grains) of the crystal grains. The feedstock pieces already described generally comprise polycrystalline grains.

Therefore, as described above, the magnet raw material obtained by hydrogen-crushing the cast alloy can be supplied directly to the HDDR without performing special pulverization or the like (for example, keeping the state of the raw material block). That is, the cast alloy (magnet raw material) after hydrogen crushing does not necessarily have to be in a powder state.

Of course, magnet raw material (raw material powder) having been subjected to particle size adjustment such as slight crushing, pulverization, classification, etc. may be supplied to the HDDR in consideration of the specification of the magnet powder, the manufacturing process (equipment), the specification of the bonded magnet, etc. The particle size can be adjusted, for example, so that the average particle size is 30 to 200. mu.m. The average particle diameter is greatly influenced by the crystal grain size of the cast structure before hydrogen fracturing, but the magnet raw material after hydrogen fracturing may be further pulverized or the like to be adjusted to a desired average particle diameter.

The average particle diameter (also referred to as "average powder particle diameter") referred to in the present specification is defined as follows. First, a powder having a particle size of-212 μm was prepared by pulverizing or sieving. The powder was sieved (classified) into 0(μm) to 53(μm), 53(μm) to 75(μm), 75(μm) to 106(μm), 106(μm) to 150(μm), and 150(μm) to 212(μm). The weight ratio (referred to as "weight frequency") of each powder classified (y to x: μm) to the whole was determined. For each powder after classification, the product of the average particle size ((y + x)/2: μm) and the weight frequency was determined. The sum of the products is referred to as "average particle diameter" (average powder particle diameter).

The expression according to the screening method (see: JIS Z8801) means the following. -x μm: powder passing through a sieve having a mesh size of x (μm) (powder having a maximum particle diameter of less than x μm), (+) y μm: powder not passing through a sieve having a mesh size of y (μm) (powder having a minimum particle diameter larger than y μm), y (μm) to x (μm): a powder that passed through a sieve with a mesh size of x (μm) and did not pass through a sieve with a mesh size of y (μm).

Incidentally, the particle size measurement by the laser diffraction method was not performed. This is because the magnet material after hydrogen fragmentation is further micronized by the high-pressure gas injected before measurement, and appropriate measurement cannot be performed.

Magnetic powder

Subjecting the above-mentioned hydrogen-crushed magnet raw material (raw material powder/raw material block) to hydrogen treatment (HDDR) to obtain a magnet powder containing R fine therein₂TM₁₄B₁Polycrystalline bodies (magnet particles) in which crystal forms (average crystal grain size: 0.05 μm to 1 μm) are aggregated.

(1) When HDDR is roughly classified, a disproportionation step (HD) and a recombination step (DR) are included. The disproportionation step is a step of exposing the magnet material placed in the treatment furnace to a predetermined hydrogen gas atmosphere to cause disproportionation reaction of the magnet material after hydrogen absorption. The disproportionation process may be carried out, for example, at a hydrogen partial pressure: 10kPa to 300kPa, atmosphere temperature: 600-900 ℃, treatment time: 1 to 5 hours.

The recombination step is a step of dehydrogenating the magnet material after the disproportionation step to cause a recombination reaction of the magnet material. The recombination process may be carried out, for example, at a hydrogen partial pressure: 1kPa or less, atmosphere temperature: 600-900 ℃, treatment time: 1 to 5 hours.

(2) All or a part of the HD step or the DR step may be performed as the following steps.

(a) Low temperature hydrogenation process

The low-temperature hydrogenation step is a step of keeping the magnet material in the processing furnace in a hydrogen atmosphere at a temperature not higher than the temperature at which the disproportionation reaction occurs (for example, room temperature to 300 ℃ C., and further room temperature to 100 ℃ C.). In this step, the magnet material is in a state of previously storing hydrogen, and the disproportionation reaction by the subsequent high-temperature hydrogenation step (corresponding to the disproportionation step) proceeds slowly. This facilitates control of the reaction rate of the normal phase transformation. The hydrogen partial pressure in this case may be, for example, about 30kPa to about 100 kPa. As described above, the hydrogen atmosphere referred to in the present specification may be a mixed gas atmosphere of hydrogen and an inert gas (the same applies hereinafter).

(b) High temperature hydrogenation process

The high-temperature hydrogenation step is a step of maintaining the magnet raw material after the low-temperature hydrogenation step in a hydrogen atmosphere having a hydrogen partial pressure of 10 to 60kPa and a temperature of 750 to 860 ℃. In this step, the magnet material after the low-temperature hydrogenation step undergoes a disproportionation reaction (normal phase shift reaction) to form a three-phase decomposed structure (α Fe phase, RH phase)₂Phase, Fe₂Phase B).

In this step, the hydrogen partial pressure or the atmospheric temperature may not be always constant. For example, at the end of the step of decreasing the reaction rate, at least one of the hydrogen partial pressure and the temperature may be increased to adjust the reaction rate and promote the three-phase decomposition (tissue stabilization step).

(c) Controlled venting sequence

The controlled exhaust step is a step of holding the magnet material after the high-temperature hydrogenation step in a hydrogen atmosphere having a hydrogen partial pressure of 0.7 to 6kPa and a temperature of 750 to 850 ℃. In this step, the magnet material after the high-temperature hydrogenation step undergoes a recombination reaction (reverse phase transition reaction) accompanied by dehydrogenation. Thereby, the triphase decomposition structure becomes from RH₂In phase hydrogen is removed and Fe is transferred₂Crystal orientation of B phaseFine R of₂TM₁₄B₁Crystalline hydrides (RFeBH)_X). The recombination reaction in this step proceeds slowly because it proceeds under a relatively high hydrogen partial pressure. When the high-temperature hydrogenation step and the controlled-degassing step are performed at substantially the same temperature, the transition from the high-temperature hydrogenation step to the controlled-degassing step is facilitated only by changing the hydrogen partial pressure.

(d) Forced exhaust process

The forced exhaust step can be performed, for example, in a vacuum atmosphere at 750 to 850 ℃ and 1Pa or less. The hydrogen remaining in the magnet material is removed by this step, and dehydrogenation is completed.

The forced air discharging step and the controlled air discharging step need not be performed continuously. The controlled exhausting process may be followed by a cooling process of cooling the magnet raw material, and the forced exhausting process may be performed in a batch process. Cooling after the forced air-discharging step is preferably rapidly cooled in order to suppress the growth of crystal grains.

(3) Diffusion treatment for enhancing the coercive force can be performed. The diffusion treatment can be performed by, for example, heating a mixed raw material obtained by mixing a magnet raw material with a diffusion raw material. Thus, R may be₂TM₁₄B₁A nonmagnetic phase is formed on the surface or the grain boundary of the crystal form, and the coercive force of the magnet particles is improved. The diffusion treatment is performed by, for example, mixing a magnet powder obtained after HDDR with a diffusion raw material powder, and heating the resulting mixture in a vacuum atmosphere or an inert gas atmosphere. When the magnet material and the diffusion material are mixed in advance before the low-temperature hydrogenation step, before the high-temperature hydrogenation step, before the controlled exhaust step, or before the forced exhaust step, the subsequent steps are also used as the diffusion treatment. The diffusion raw material is, for example, a heavy rare earth element (Dy, Tb, etc.), an alloy or compound thereof (e.g., fluoride), an alloy of a light rare earth element (e.g., Cu alloy, Cu — Al alloy), or a compound.

(4) Magnet powder

The magnet powder obtained after HDDR (including d-HDDR) (rare earth magnet powder after the recombination step) also has an average particle size of, for example, 30 to 200 μm, (larger than) 50 to 190 μm, and further 55 to 180 μm.

Application

The rare earth magnet powder of the present invention can be used in various applications, and a bonded magnet is a typical example. The bonded magnet mainly contains a rare earth magnet powder and a binder resin. The binder resin may be a thermosetting resin or a thermoplastic resin. The bonded magnet may be a compression-molded bonded magnet or an injection-molded bonded magnet. A bonded magnet using the rare earth anisotropic magnet powder can exhibit high magnetic properties when formed in an oriented magnetic field.

Examples

As shown in fig. 1A, the ingot (casting alloy) after the solution treatment was subjected to various hydrogen crushings in different hydrogen atmospheres. Each magnet raw material after hydrogen crushing was gently pulverized and then classified by sieving. The raw material powders thus obtained were subjected to HDDR treatment to obtain magnet powders. Then, the magnetic properties and the like of each magnet powder were evaluated. The present invention will be specifically described below based on such examples.

Preparation of sample

[ example 1]

(1) Casting alloy

Raw materials weighed to have a desired alloy composition (Nd: 12.5 atomic%, B: 6.4 atomic%, Nb: 0.2 atomic%, Ga: 0.3 atomic%, and Fe: the balance) were melted in a high-frequency melting furnace to obtain an ingot (casting alloy).

(2) Solution treatment

The ingot was homogenized by heating at 1140 ℃ for 20 hours under an Ar gas atmosphere.

(3) Hydrogen fragmentation

The ingot after the solution treatment was subjected to hydrogen crushing as follows. First, a treatment furnace containing an ingot is evacuated (10)^-2Pa or less). Then, the treatment furnace as it was after the evacuation was heated. As shown in FIG. 1B, the treatment furnace was brought to the desired atmospheric temperature for 1 hour. Hydrogen is then introduced into the furnace to achieve the desired hydrogen partial pressure. This state was maintained for 5 hours (hydrogen absorption)Step (ii). At this time, hydrogen partial pressure: 10kPa or 100kPa, atmosphere temperature: room temperature (R.T.) to 600 ℃. The atmospheric temperature in the treatment furnace was measured by a thermocouple in contact with each ingot, and the hydrogen partial pressure was measured by a pressure gauge provided in the treatment furnace.

Then, the furnace was cooled to room temperature while maintaining the hydrogen partial pressure. The hydrogen gas in the treatment furnace was replaced with an inert gas (Ar at atmospheric pressure), and the magnet raw material after the hydrogen crushing treatment was taken out from the furnace in an Ar atmosphere. Slight crushing is applied to the magnet raw material whose atmosphere temperature is set to r.t. -500 ℃. The magnet raw material whose atmosphere temperature is set to 550 ℃ or 600 ℃ is difficult to be powdered by only slight crushing, and therefore mechanical pulverization is applied. Each of the obtained powders was classified by a sieve to obtain a raw material powder of-212. mu.m. At this time, the pulverization and classification are carried out in an inert gas atmosphere.

Incidentally, the hydrogen crushing treatment may also be performed in the mode shown in fig. 1C instead of the mode shown in fig. 1B. In the fig. 1C mode, after the hydrogen absorption step, 0.5 hour (30 minutes) has elapsed since the hydrogen gas in the treatment furnace was discharged, and then the furnace was cooled to room temperature. Otherwise, the same as the mode of fig. 1B. It was confirmed that magnet powder having the same characteristics can be obtained by using the magnet material processed in the mode of fig. 1C or the magnet material processed in the mode of fig. 1B.

(4) HDDR processing

Each raw material powder (15g) obtained by hydrogen pulverization at different temperatures was placed in a processing furnace, and vacuum was applied. The HDDR treatment was performed on each raw material powder by controlling the hydrogen partial pressure and temperature in the treatment furnace. Specifically, each raw material powder was subjected to a disproportionation reaction (normal phase change reaction) in a high-temperature hydrogenation step (820 ℃ C. times.30 kPa. times.3 hours) (disproportionation step).

Then, a controlled exhaust step (820 ℃ C.. times.5 to 1 kPa. times.1.5 hours) of continuously exhausting hydrogen gas from the inside of the treatment furnace and a subsequent forced exhaust step (820 ℃ C.. times.10 hours) are performed^-2Pa.times.0.5 hours). In this way, the respective raw material powders are subjected to recombination reaction (reverse phase transition reaction) (recombination step). Then, Ar gas is introduced to treatThe treated material in the furnace is quenched (cooling step). The treated material was gently crushed in Ar gas and then classified (sieved), thereby obtaining a particle size: magnet powder of 212 μm.

Observation

(1) Magnet raw material after hydrogen crushing

The raw material (raw material block) of each magnet as it is obtained by hydrogen-crushing an ingot under various conditions was observed by a Scanning Electron Microscope (SEM), and the morphology obtained by the observation was compared and shown in fig. 2.

The raw material block after the hydrogen crushing treatment was crushed (including crushing, abbreviated as "crushing") at room temperature (R.T.) × 100kPa or 500 ℃ × 100kPa in the hydrogen gas atmosphere in the treatment furnace, and the particles of the obtained raw material powder were observed by SEM. The appearance and cross section thereof are shown in fig. 3A.

(2) Magnet powder after HDDR

HDDR under the same conditions was performed on raw material powders having different atmospheric temperatures for hydrogen fracture (hydrogen fracture temperatures), and the particles of each of the obtained magnet powders were observed by SEM. The appearance and cross section thereof are shown in fig. 4A.

Measurement of

(1) Average particle diameter

Fig. 3B shows the average particle diameter of each raw material powder obtained by hydrogen pulverization and subsequent pulverization in different atmospheres. Fig. 4B also shows the average particle diameter of the magnet powder obtained by subjecting each of these raw material powders to HDDR under the same conditions. The average particle diameter of each powder after classification (-212 μm) was measured. The average particle diameter is calculated by the above-mentioned method.

(2) Magnetic characteristics

The magnetic properties of each magnet powder obtained by subjecting each raw material powder having different hydrogen fracture treatment conditions to HDDR under the same conditions were measured as follows. Each magnet powder was encapsulated, subjected to magnetic field orientation (1193kA/m) in molten paraffin (about 80 ℃ C.), and then magnetized (3580 kA/m). The magnetic properties of the magnetized magnet powder were measured using a Sample Vibrating Magnetometer (VSM). At this time, each magnet powder was pulverizedThe density of (A) is assumed to be 7.5g/cm³. The maximum energy product ((BH) of each magnet powder thus obtained was calculated_max) Fig. 5A, 5B, and 5C (which are collectively referred to simply as "fig. 5") show the remanence (Br) and the coercivity (Hc), respectively.

Evaluation

(1) Magnet raw material

As is clear from fig. 2, when hydrogen fracturing is performed in a hydrogen atmosphere in a room temperature range as in the prior art, many cracks having different sizes are generated regardless of the grain boundary phase and the main phase.

On the other hand, the temperature of the atmosphere at the time of hydrogen fragmentation (referred to as hydrogen fragmentation temperature (T)) is varied_HD) Increase in strength, decrease in cracking. It was also found that this tendency is hardly affected by the partial pressure of hydrogen, depending mainly on the hydrogen fragmentation temperature. However, it is considered that when the atmospheric temperature at the time of hydrogen fragmentation reaches 600 ℃, disproportionation reaction (HD reaction) and melting of the R-rich phase (Nd-rich phase) partially occur.

When the hydrogen fracture temperature reached 400 ℃ to 500 ℃, fine cracks were significantly reduced, cracks were mainly generated in the grain boundary phase, and were hardly seen in the main phase. This is also clear from the SEM image shown in fig. 3A. This tendency is reflected in the average particle diameter shown in fig. 3B.

(2) Magnet powder

When comparing fig. 3A and 3B (both figures are abbreviated as "fig. 3") and fig. 4A and 4B (both figures are abbreviated as "fig. 4"), it is clear that the particle morphology after hydrogen fragmentation is roughly reflected in the particle morphology after HDDR. As is clear from FIG. 5, the magnetic properties of the magnet powder (of example 1) increase with the hydrogen crushing temperature, Br or (BH)_maxShows a peak at a hydrogen fragmentation temperature of 450 ℃ to 500 ℃. Focusing on (BH) as an index of comprehensive magnetic properties_maxIt is clear that the hydrogen fragmentation temperature can be set to 350 ℃ to 585 ℃, 400 ℃ to 575 ℃, and further, 425 ℃ to 550 ℃. The magnet powder according to example 1 had a degree of anisotropy (DOT) of 0.69 to 0.73.

Examination of

As described above, it is found that by performing HDDR using a magnet raw material (raw material powder) obtained by hydrogen pulverization in a specific temperature range, a magnet powder having higher magnetic properties than the conventional one can be obtained. Although the reason is not necessarily clear, the following is currently considered.

(1) Density of cracks

In order to investigate the above reason, the crack density of each raw material block obtained by hydrogen crushing in different atmospheres was determined for the ingot after the solution treatment. The crack density is an index of whether or not the crystal grains after hydrogen fracture are easily broken in the grains (in the main phase).

The crack density was calculated as follows. Each raw material piece was observed by a field emission type scanning electron microscope (FE-SEM). The SEM image is processed by image analysis software to obtain the total of the crack lengths (referred to as "intra-grain crack lengths") within the crystal grains (main phase) within a specific field of view. The total of these was divided by the specific field area to obtain the crack density. The results thus obtained are shown in fig. 6. The grain boundaries are premised on cracking, and this part is not included in the calculation of the crack length.

As is clear from fig. 6, the crack density monotonously decreases with an increase in the hydrogen fracture temperature. When the hydrogen fracture temperature reached 600 ℃, neither intergranular nor intergranular cracks were observed due to the HD reaction (hydrogenation disproportionation reaction).

(2) Mechanism of

From fig. 2, 3A, 4A and 6, it is considered that the mechanism by which the magnetic powder having higher magnetic properties than the conventional one can be obtained by the production method of the present invention is as follows. The gist of which is schematically shown in fig. 7.

First, as shown in fig. 7, the cast alloy (ingot) after the solution treatment is formed of a main phase and a grain boundary phase surrounding the main phase. In the case where the casting alloy is a typical Nd-Fe-B-based magnet alloy, the main phase is Nd₂Fe₁₄The B phase and the grain boundary phase are Nd-rich phases (R-rich phases).

When the hydrogen fracture temperature is low as in the conventional case, hydrogen enters not only the grain boundary phase but also the main phase in the cast alloy (magnet material/material block) after hydrogen fracture, and cracks are generated. When this raw material piece is crushed (including crushed), magnet raw material particles are formed which break along any cracks inside and outside the main phase. As shown in the photograph located above fig. 3A, this form is also known from the fact that each particle has a plurality of protrusions formed by brittle fracture inside the particle.

Such magnet raw material particles are in a state where a plurality of crystal grains (main phases) having different easy magnetization axis directions (arrows in fig. 7) are present in a mixed manner. This state is also inherited by the magnet particles after HDDR. As a result, it is considered that even when HDDR is performed on a magnet raw material obtained by hydrogen fracturing at a low temperature, a magnet powder having high magnetic properties (particularly Br) cannot be obtained.

On the other hand, when the hydrogen fracture temperature is high as in the present invention, hydrogen mainly enters only the grain boundary phase in the cast alloy (raw material block) after hydrogen fracture, hydrogen hardly enters the main phase, and cracks are mainly generated in the grain boundary phase. When this raw material block is pulverized, magnet raw material particles that fracture along the grain boundary phase at the time of casting are formed. This form can also be seen in the photograph shown at the bottom of fig. 3A.

Such magnet material particles are mainly composed of single crystal grains (main phase), and are in a state in which the directions of easy magnetization axes are aligned. This state is also inherited by the magnet particles after HDDR. As a result, it is considered that when HDDR is performed on a magnet raw material obtained by hydrogen fracturing at a high temperature, a magnet powder having high magnetic characteristics (particularly Br/fig. 5B) can be obtained.

In addition, when HDDR is applied to magnet raw material particles having an intragranular crack as in the related art, a grain boundary phase (not shown) present on the surface of the magnet raw material particle, which is a grain boundary phase (Nd-rich phase/R-rich phase: white thick line portion at the bottom right of fig. 7) of an ingot, melts and penetrates into the intragranular crack (white thick single-dot chain line portion at the bottom right of fig. 7), thereby constituting a pool phase (Nd-rich phase/R-rich phase). Accordingly, it is difficult to form a sufficient grain boundary phase (Nd-rich phase/R-rich phase: black thin line portion at the bottom right of FIG. 7) between fine grains formed after HDDR. In this way, even if HDDR is performed on a magnet raw material obtained by hydrogen fracturing at a low temperature, a magnet powder having high magnetic characteristics (in particular, Hc/fig. 5C) cannot be obtained.

On the other hand, when HDDR is performed on magnet raw material particles having almost no in-grain cracks as in the present invention, the Nd-rich phase/R-rich phase is not accumulated wastefully, and a sufficient grain boundary phase (Nd-rich phase/R-rich phase: black thin line portion at the upper right most in FIG. 7) is formed between fine grains after HDDR. It is considered that when the magnet material obtained by hydrogen fracturing at high temperature is subjected to HDDR in this way, a magnet powder having high magnetic properties (in particular, Hc/fig. 5C) can be obtained.

Incidentally, as is clear from a comparison of FIGS. 5A, 5B and 6, Br and BHmax of the magnet powder showed a tendency to decrease in the hydrogen fracture temperature range (550 ℃ C. to 600 ℃ C.) after the initial peak, although the crack density decreased. The reason for this is considered as follows.

As described above, the ingot (magnet raw material) obtained by the hydrogen treatment at 550 ℃ or 600 ℃ is not pulverized to a slight degree of breakage, and is mechanically pulverized before being supplied to the HDDR to prepare a raw material powder having a predetermined particle size. As can be seen from the photograph shown in fig. 2, the reason why such a pulverization treatment is necessary is considered to be that, when hydrogen treatment is performed at 550 ℃ or 600 ℃, cracks are hardly generated not only in the grains but also in the grain boundaries, and the grains are in a state of being hardly broken.

By applying a grinding force considerably larger than that in the case of crushing at the time of the crushing treatment, breakage penetrating the inside of the grains is generated in the ingot subjected to the hydrogen treatment. As a result, polycrystalline grains (see fig. 7) in the raw material powder supplied to the HDDR increase again. It is considered that due to this influence, Br or BHmax of the magnet powder obtained from the magnet raw material whose hydrogen crushing temperature was set to 550 ℃ or 600 ℃ was lower than that at the peak.

[ example 2]

(1) The ingot subjected to the R-rich dispersion treatment after the solution treatment (before hydrogen crushing) was subjected to hydrogen crushing and HDDR treatment. The treatment was carried out in the same manner as in example 1 except for the R-rich dispersion treatment. The R-rich dispersion treatment was performed in the following manner.

The treatment furnace containing the ingot after the solution treatment is vacuumized (10)^-2Pa or less). Heating the vacuumized treatment furnace with 1Hour (partial pressure of hydrogen: 10)^-2Pa or less) is 500 to 900 ℃. This state was maintained for 1 hour (R-rich dispersion treatment step). Next, the treatment atmosphere (500 ℃ C.. times.100 kPa) was changed to hydrogen atmosphere for a predetermined period of time.

(2) The treatment temperature of the R-rich dispersion treatment (referred to as "dispersion treatment temperature (Tr)") and the magnetic properties ((BH) of each of the obtained magnet powders_maxBr, Hc) are shown in fig. 8A to 8C (these are collectively referred to simply as "fig. 8"). The magnetic properties were determined by the methods already described. The "untreated" shown in fig. 8 indicates a magnet powder (corresponding to the magnet powder of example 1) obtained by hydrogen crushing (500 ℃. times.100 kPa) an ingot which was not subjected to the R-rich dispersion treatment.

As is clear from fig. 8, the magnetic properties are further improved by performing the R-rich dispersion treatment. In particular, when the dispersion treatment temperature is more than 600 ℃ and 650 ℃ or more, (BH)_maxOr Br increased significantly. This tendency does not change even when the dispersion treatment temperature is set to 900 ℃. However, when the dispersion treatment temperature exceeds 750 ℃, Hc tends to decrease. When a magnet powder having a high coercive force is required, the dispersion treatment temperature may be 750 ℃ or less, and further 720 ℃ or less. The magnetic powder obtained at a dispersion treatment temperature of 700 ℃ had a degree of anisotropy (DOT) of 0.76.

[ example 3]

(1) Magnet powders (samples 31 and 32) were also produced by taking out the magnet raw material without hydrogen pulverization from the processing furnace and continuing the HDDR treatment after hydrogen pulverization (hydrogen partial pressure: 100kPa, atmosphere temperature: 500 ℃ C., 5 hours). At this time, evacuation before the HDDR treatment performed in example 1 was not performed. In addition, mechanical pulverization and classification are not performed after hydrogen crushing, but after HDDR treatment. Except for this, the treatment was performed in the same manner as in example 1.

Sample 31 is a magnet powder obtained by subjecting a magnet raw material obtained by cooling a furnace to room temperature while maintaining a hydrogen partial pressure (100kPa) after hydrogen crushing to HDDR treatment. Sample 32 was a magnet powder obtained by performing atmosphere adjustment without performing furnace cooling after hydrogen crushing and performing a transition to HDDR treatment. In addition, a magnet powder (sample C) produced by changing the atmospheric temperature of hydrogen fragmentation in sample 31 to room temperature (23 ℃) was also produced.

(2) The magnetic properties ((BH) of magnet powder) of each sample were measured_maxBr, Hc) are shown in FIGS. 9A to 9C (these are collectively referred to simply as "FIG. 9"). The magnetic properties were determined by the methods already described. The broken lines in fig. 9 indicate the respective magnetic properties of the magnet powder corresponding to example 1 obtained by hydrogen fracturing under the same conditions (500 ℃x100 kPa).

As is clear from FIG. 9, even when the HDDR treatment was continued after the hydrogen fracturing, high magnetic properties (particularly Br and (BH)) were obtained in the same manner as in example 1_max) The magnet powder of (1). The magnetic powder of samples 31 and 32 had a degree of anisotropy (DOT) of 0.71 to 0.74.