JP2008031500A - Method for evaluating compact and method for manufacturing sintered body - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To simply and precisely evaluate a compacted state (forming state of cracks) of a compact which has been compacted from a raw powder. <P>SOLUTION: This evaluation method includes: evaluating the compacted state (forming state of cracks) of the compact 1 based on a state of bubbles generated when the compact 1 of the raw powder has been immersed in a solution 2; or evaluating the compacted state of the compact 1 based on an intrusion state of the paint after having applied the paint on the compact 1 of the raw powder and immersing it in a solution 2. The evaluation method further includes combining the above methods to quantitatively evaluate the compacted state of the compact. These evaluation methods are applied to a compacting step in a manufacturing process for a sintered body (for instance, manufacturing process for a rare-earth sintered magnet). <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a method for evaluating a molded body for accurately grasping, for example, the state of occurrence of cracks in a molded body obtained by molding a raw material powder, and further relates to a method for manufacturing a sintered body to which this is applied. .

  For example, in a wide range of fields such as a hard disk drive voice coil motor and an automobile drive motor, miniaturization and high performance of the motor are required. In order to reduce the size and performance of motors, it is important to improve the performance of magnets built into motors. In recent years, rare earth sintered magnets such as neodymium iron boron-based sintered magnets that exhibit extremely high magnetic properties have been used. Widely used.

  As a method for producing a rare earth sintered magnet, a powder metallurgy method is generally used. Specifically, a raw material alloy having a desired composition is used, and then a coarse pulverization process, a fine pulverization process, a molding process, and a sintering process are performed. It is manufactured. That is, in order to produce the rare earth sintered magnet, the raw material alloy powder formed by pulverization is filled in a mold cavity of a molding apparatus to form a molded body of a predetermined shape, which is sintered and sintered. Let it be the body.

  As described above, rare earth sintered magnets are produced using powder metallurgy, but powder metallurgy is widely used in various fields as well as the production of rare earth sintered magnets. It has become an indispensable technique for producing the body.

  In a general powder metallurgy method, a raw material powder micronized to several hundred μm or less is subjected to pressure molding, and the obtained powder compact (green compact) is heat-treated to produce a sintered body. In particular, NdFeB rare earth sintered magnets, which are frequently used for motors, etc., are first pulverized and finely pulverized to a micron order, and the fine powder is pressed in a magnetic field. It is produced by performing sintering and aging treatment.

  In the above-mentioned powder metallurgy method, there is a problem that so-called mold galling occurs in the press molding process. When the galling occurs, the molded body is scratched, cracked, cracked, etc. This causes a problem that the quality of the product is greatly deteriorated. In particular, the rare earth alloy powder has poor fluidity, and the problem of deterioration of the molded product quality due to the die galling is remarkable.

As one countermeasure against such a problem, it has been studied to add a lubricant to the raw material powder (see, for example, Patent Document 1). In Patent Document 1, the extraction pressure of the molded body at the time of molding is reduced by mixing a lubricant (including at least one of stearic acid, zinc stearate, and bisamide) with Fe-R-B magnet powder. However, it is disclosed to suppress the occurrence of scratches and the like.
JP-A-4-214803

  By the way, for example, even if a lubricant is added to the raw material powder, cracks may occur due to variations in molding conditions, etc. Therefore, when mass production is considered, the molded state of the molded body (the state of occurrence of cracks, etc.) ) Must be accurately grasped. If it is possible to accurately grasp the occurrence state of cracks in the molded body, it is possible to prevent the occurrence of cracks by finely adjusting the molding conditions.

  However, it is not easy to evaluate cracks in the molded body. For example, cracks at such a level that the molded body breaks immediately after demolding can be easily determined, but normal cracks are difficult to visually evaluate. Ordinary cracks can hardly be found even when the molded body is observed from the outside.

  Since the portion where the crack is generated is not bound by sintering, a large crack is generated corresponding to the crack portion of the formed body after sintering, and in some cases, cracking or deformation is accompanied. Such a sintered body cannot be shipped as a product. Although it is conceivable to increase the size of the sintered body compared to the product size and to produce a product by processing the outer peripheral part after sintering and removing the cracked part, the processing itself increases as the processing cost increases. Since it becomes difficult and the processing allowance part is discarded, the utilization efficiency of the raw material powder is also lowered. In this way, when considering the final product of a sintered body, cracks cause a decrease in yield in any case, and if the crack can be properly evaluated at the stage of the molded body, it greatly contributes to yield improvement. It is considered a thing.

  The present invention has been proposed in view of such a conventional situation, and provides a molded body evaluation method capable of easily and reliably grasping a molded state (for example, a crack state) of a molded body. For the purpose. In addition, the present invention provides a method for manufacturing a sintered body that can suppress the occurrence of cracks in the sintered body after sintering and can greatly improve the product yield by applying the evaluation method. For the purpose.

  In order to achieve the above-described object, the molded body evaluation method according to the first invention of the present application involves immersing a molded body of raw material powder in a solution and evaluating the molded state of the molded body based on the generation state of bubbles. It is characterized by that. The molded body evaluation method of the second invention is characterized in that a molded body of raw material powder is immersed in a solution and the molded state of the molded body is evaluated based on the infiltration state of the paint. According to a third aspect of the present invention, there is provided a method for evaluating a molded article, comprising: immersing a molded body of raw material powder in a solution; grasping a molded state of the molded body based on an infiltration state of a paint; When another molded body molded under the same conditions as above is immersed in the solution, the foam generation state when the molded body of the raw material powder to be evaluated is immersed in the solution on the basis of the foam generation state at this time The molded state of the molded body is evaluated by comparing the above.

  Although it is very difficult to determine the cracks in the molded body with the naked eye, when the molded body is immersed in a solution, bubbles are generated from the crack portion. Bubbles are hardly generated from a portion where no crack is generated, and can be easily distinguished visually. The method for evaluating a molded body according to the first aspect of the present invention is to evaluate the molded state (the occurrence of cracks) of the molded body by utilizing this.

  When a solution containing a paint is used or a paint is applied to a molded body and the molded body is immersed in the solution, the paint also enters the crack as the solution enters the cracked portion. If the color of the paint is different from the color of the molded body, cracks are easily recognized by observing the cut surface of the molded body after immersion in the solution. The method for evaluating a molded body according to the second aspect of the present invention is to evaluate the molded state (the occurrence of cracks) of the molded body by utilizing this.

  The method for evaluating a molded body of the first invention (evaluating the molded state of the molded body based on the state of generation of bubbles) has the advantage that the molded body can be evaluated by a very simple technique, but is qualitative. It stops in the evaluation. On the other hand, the method for evaluating a molded body according to the second invention (evaluating the molded state of the molded body based on the infiltration state of the paint) can directly observe the occurrence of cracks. The number etc. can be evaluated quantitatively to some extent. However, it takes a little time compared to the evaluation method of the first invention. The molded body evaluation method according to the third aspect of the invention enables the occurrence of cracks to be quantitatively grasped to some extent by combining the first evaluation method and the second evaluation method. That is, in the method for evaluating a molded body of the third invention, first, the molded body is immersed in a solution containing a paint, or a molded body of a raw material powder coated with a paint is immersed in a solution, and the paint is infiltrated. Based on this, the molded state of the molded body is quantitatively grasped. Thereafter, a molded body molded under the same conditions as the molded body whose molding state has been grasped (simultaneously molded molded body) is immersed in a solution, and the generation state of bubbles is observed. Then, using the foam generation state at this time as a reference, the molded state of the molded body is evaluated by comparing the foam generation state when the molded body of the raw material powder to be evaluated is immersed in the solution. Therefore, in the evaluation method of the third invention, the molded state of the molded body is quantitatively evaluated while maintaining the simplicity of observing the bubble generation state.

  On the other hand, the method for producing a sintered body according to the present invention includes a molding step of forming a raw material powder to produce a molded body, a molded body evaluation step of evaluating the molded state of the molded body, and sintering the molded body. A molding step, and in the molded body evaluation step, the molded state of the molded body is evaluated by any of the evaluation methods of the first to third inventions described above.

  In the manufacturing method, the molding state of the molded body can be accurately grasped, and, for example, the occurrence of cracks in the molding process can be minimized by feeding back to the molding conditions. If the generation of cracks can be suppressed at the molding stage, the generation of cracks after sintering can be prevented, and the yield is greatly improved.

  In the manufacturing process of a rare earth sintered magnet, impregnation of a compact before sintering with an oil agent is also disclosed in, for example, Japanese Patent Application Laid-Open Nos. 2002-170728 and 2002-8935. However, the techniques disclosed in these publications focus on the prevention of oxidation of the molded body. When a molded body obtained by molding a rare earth alloy powder is exposed to the atmosphere, it reacts with oxygen to cause a decrease in magnetic properties, and in the worst case, heat generation and ignition occur. Therefore, in the inventions described in the above publications, the molded body is immersed in an oil agent, and the surface of the molded body is covered with the oil agent to prevent the oxidation. However, the above-mentioned publications do not describe anything about the molded state (the occurrence state of cracks) of the molded body, and its technical idea is greatly different from the present invention.

  According to each evaluation method of the present invention, it is possible to accurately evaluate a molding state (for example, a crack generation state) of a molded body that is difficult to grasp visually, and a crack generation state of the molded body by an extremely simple operation. Etc. can be accurately grasped. In addition, according to the method for producing a sintered body of the present invention, it is possible to suppress the occurrence of cracks in the molded body by applying the evaluation method. It is possible to suppress and significantly improve the product yield.

  Hereinafter, a molded body evaluation method and a sintered body manufacturing method to which the present invention is applied will be described in detail with reference to the drawings.

(First embodiment)
The present embodiment relates to a method for evaluating a molded body, and is characterized in that a molded state is grasped by generation of bubbles from a molded body immersed in a solution.

  FIG. 1 schematically shows an evaluation method in the present embodiment. In this embodiment, as shown in FIG. 1, the molded object 1 used as evaluation object is immersed in the solution 2, and the bubble generation state at this time is observed. The molded body 1 is placed on, for example, the mesh plate 3 and immersed in the solution 2. By using the net 3, for example, it is possible to observe the bubble generation state on the six surfaces of the rectangular parallelepiped shaped molded body 1. In addition, it is preferable to put the solution 2 in the transparent container 4, for example, and perform the said evaluation. This is because by using the transparent container 4, it is possible to observe the generation state of the foam of the molded body 1 from the side surface and the bottom surface.

  The molded object 1 to be evaluated may be any material as long as the raw material powder is molded, and can be applied to powder molded bodies in all fields. In particular, in a molded body that is sintered after molding to form a sintered body, it is significant to grasp the occurrence state of cracks and the like, and application of the evaluation method is effective. Specific examples include a compact formed from a raw material powder of a magnet. For example, a compact formed from a rare earth alloy powder that is a raw material powder of a NdFeB rare earth sintered magnet, or a raw material powder of an SmCo based magnet is formed. And a molded body of a raw material powder of a ferrite magnet. Alternatively, it can also be applied to compacts of these raw material powders in the production process of ceramic products, ceramics and the like.

  On the other hand, the solution 2 is preferably a solution suitable for generating bubbles. Here, a liquid substance having a relatively low viscosity such as various solvents is suitable for the generation of bubbles. When the viscosity of the solution 2 is high, the rate of penetration into the cracks is slow, the generation of bubbles is slow, and the amount of generation per unit time is small, making it difficult to judge.

As described above, since the state of bubble generation changes depending on the viscosity of the solution 2, the solution 2 is preferably set in a viscosity range suitable for crack evaluation. Specifically, the viscosity of the solution 2 is preferably 100 × 10 ⁻³ Pa · s or less. Even if the molded body 1 is immersed in a solution having a viscosity exceeding 100 × 10 ⁻³ Pa · s, it is difficult to determine the presence or absence of cracks from the generated bubbles. More preferably, the viscosity is 50 × 10 ⁻³ Pa · s or less, and further preferably, the viscosity is 10 × 10 ⁻³ Pa · s or less. Moreover, since the viscosity of acetone which is a typical organic solvent is 0.3 × 10 ⁻³ Pa · s, the practical lower limit value is 0.1 × 10 ⁻³ Pa · s. The viscosity of the solution is a value measured with a B-type viscometer.

  The solution 2 to be used is preferably colorless and transparent. If the solution 2 is colorless and transparent, it is possible to easily confirm the location where the crack is generated. For example, the abnormal portion of the molded body 1 having a complicated shape is recognized, and the design of the mold and powder characteristics is changed. Thus, feedback to the molding process becomes possible. Therefore, it is particularly useful in application to a molded product corresponding to a special shape such as a VCM magnet shape.

  Furthermore, it is important to select a solution that does not easily react with the evaluation material (that is, the material of the raw material powder constituting the molded body 1) as the solution 2. When the raw material powder constituting the molded body 1 and the solution 2 react, bubbles may be generated with the reaction. When bubbles are generated along with the reaction, it is difficult to distinguish the bubbles due to the reaction and the bubbles due to the occurrence of cracks, and it is preferable to select a solution that does not easily react with the molded body 1 as the solution 2 in order to prevent confusion. For example, when the evaluation method is used in a continuous molding process, since the evaluation is also performed at regular intervals, a highly volatile solution is not preferable.

  For example, when the molded body 1 is a molded body obtained by molding the raw material powders of the various magnets, as the solution 2 that satisfies the above-mentioned conditions, an alkaline cleaner (for example, Nikka Seiko Co., Ltd., trade name Devale), A terpineol solution, kerosene (kerosene), or the like can be used. The viscosity of these solutions is adjusted so as to fall within the viscosity range, and used as the solution 2.

  By immersing the molded body 1 in the solution 2 described above, bubbles are generated from the crack generation site. FIG. 2 is a photograph of the crack 5 generated in the sintered body, and FIG. 3 schematically shows the state of the crack 5 generated in the molded body 1. For example, in the rectangular parallelepiped shaped molded body 1, the crack 5 is formed so as to obliquely enter the inside of the molded body 1 along any side. When such a crack 5 is formed, as shown in FIG. 4 (photograph) and FIG. 5 (schematic diagram), generation of bubbles 6 is observed from a portion where the crack 5 is formed (one side of the molded body 1). It is done. This is a phenomenon that occurs when the solution 2 enters the voids in the molded body 1 and air is expelled instead. If the crack (crack) which arose in the molded object 1 is large, the penetration speed | rate and the penetration amount will become large, and a bubble will generate | occur | produce more intensely. The feature is that the bubbles generated from the crack site are large and intense.

  As described above, in the present embodiment, the molded state of the molded body 1 such as the presence or absence of cracks is determined from the state of occurrence of bubbles by an extremely simple operation of immersing the molded body 1 to be evaluated in a low viscosity solution prepared in advance. It becomes possible. The determination of the molding state based on the generation state of bubbles is basically qualitative, but it is also possible to quantitatively grasp, for example, the degree of occurrence of cracks, for example, from the degree of generation of bubbles.

(Second Embodiment)
The present embodiment also relates to a method for evaluating a molded body, and is characterized by grasping a molding state by soaking a paint.

  In this embodiment, a coating material is added to the solution 2, and the solution 2 itself is colored and penetrates into the crack. The type of paint to be added is arbitrary, and various paints, paints, and the like, pigment-based paints containing pigments as colorants, various inks and dye paints, and paints containing dyes as colorants can be used. Further, depending on the type of the solution 2, a water-based paint, an oil-based paint, a solvent-based paint or the like may be appropriately selected and used. However, it is preferable to use a color different from the color of the molded body 1. For example, when the molded body 1 is black, a paint such as white or yellow is used. Thereby, it is possible to clearly see the invasion of the paint.

  As described above, when the molded body 1 is immersed in the solution 2, if a crack is present, the solution 2 enters the crack. If the solution 2 containing a coating material is used, the coating material also enters as the solution 2 enters the crack. Then, if the molded object 1 is cut | disconnected, a crack can be visually confirmed by penetration | invasion of a coating material. When the paint enters, the color of the cracked part is different from the other parts, and when a crack is present, for example, a white or yellow line is clearly observed in the black molded body 1. Here, the phenomenon occurs in all the cracks, and therefore, it is possible to quantitatively grasp the number of cracks generated in the molded body 1, the length of each crack, and the like.

  Alternatively, it is possible to apply a paint in advance to the surface of the molded body 1 and to infiltrate it into the cracks by immersion in the solution 2. In this case, first, a paint is applied to the surface of the molded body 1. Then, the molded body 1 to which the paint is applied is immersed in the solution 2. Here, the requirements for the solution 2 include the viscosity. In the present embodiment, it is preferable to use a solution having a low viscosity in consideration of the penetration rate into the crack. Moreover, it is necessary to consider the solubility with respect to a coating material. If the solubility in the coating material is too high, the coating material applied to the surface of the molded body 1 diffuses into the solution 2 and the coating material may not sufficiently penetrate into the cracks. On the other hand, if the solubility in the paint is too low, even if the solution 2 penetrates, the paint does not dissolve and penetrate into the cracks, and there is a possibility that the paint may not sufficiently penetrate. Therefore, it is preferable that the solution 2 to be used has moderate solubility with respect to the paint.

  Although it is conceivable that the paint is soaked into the crack only by applying the paint to the molded body 1, it is difficult to sufficiently infiltrate the paint into the crack because the viscosity of the paint is high. Therefore, as described above, combining the application of the paint to the molded body 1 and the immersion in the solution 2 is optimal in evaluating the occurrence of cracks. It is also possible to combine the addition of a paint (dye or pigment) into the solution 2 and the application of the paint to the surface of the molded body 1.

(Third embodiment)
In this embodiment, by combining the evaluation method of the first embodiment and the evaluation method of the second embodiment, it is possible to quantitatively grasp the occurrence of cracks in the molded body 1 with minimal effort. It is.

  The evaluation method of the first embodiment (method of evaluating the molded state of a molded body based on the generation state of bubbles) is an extremely simple method, but has the disadvantage of being limited to qualitative evaluation. On the other hand, the evaluation method of the second embodiment (method of evaluating the molded state of the molded body based on the infiltration state of the paint) can quantitatively evaluate the length and number of cracks, etc. It is necessary to cut the body 1 and observe the cross section, and the operation is complicated compared to the evaluation method of the first embodiment.

  Therefore, in the present embodiment, by combining these, it is possible to grasp the occurrence of cracks in the molded body 1 quantitatively to some extent while being a simple method. Specifically, first, the occurrence of cracks in the molded body 1 is examined by the evaluation method of the second embodiment. And about the molded object 1 from which the generation condition of a crack differs, the generation condition of a bubble is investigated with the method of the evaluation method of 1st Embodiment. At this time, although the generation state of bubbles varies depending on the size of cracks, etc., since the generation state of cracks is grasped by the evaluation method of the second embodiment for each molded body 1, the bubbles corresponding to the generation state of cracks It is possible to grasp the occurrence status of

  Generation of bubbles when another molded body molded under the same molding conditions as the molded body 1 whose molding state is grasped by the evaluation (another molded body molded simultaneously with the molded body) is immersed in a solution. After grasping the state, the generation state of bubbles when the molded body 1 to be evaluated is immersed in the solution is compared with this as a reference. If it is determined by the above comparison that the bubble generation state is substantially the same, it is presumed that a crack equivalent to the molded body 1 corresponding to this is generated. Therefore, after the setting of the reference, it is possible to quantitatively grasp the occurrence of cracks simply by immersing the molded body 1 in the solution 2 and observing the generation state of bubbles.

(Fourth embodiment)
The present embodiment relates to a method for manufacturing a sintered body, and is an embodiment in which the evaluation method is applied to a method for manufacturing a rare earth sintered magnet.

  The rare earth sintered magnet has, for example, a rare earth element R, a transition metal element T, and boron as main components. However, the magnet composition is not particularly limited, and may be arbitrarily selected according to the application. For example, the rare earth element R specifically means Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb or Lu. 1 type (s) or 2 or more types can be used. Among these, it is preferable that the main component as the rare earth element R is Nd because it is abundant in resources and relatively inexpensive. Moreover, as the transition metal element T, any conventionally used transition metal element can be used. For example, one or more of Fe, Co, Ni and the like can be used. Among these, from the viewpoint of magnetic properties, Fe is the main component, and it is particularly preferable to add Co that is effective in improving the Curie temperature and improving the corrosion resistance of the grain boundary phase. In addition to the rare earth element R, transition metal element T, and boron B, the inclusion of other elements is allowed. For example, elements such as Al, Cu, Zr, Ti, Bi, Sn, Ga, Nb, Ta, Si, V, Ag, and Ge can be appropriately contained. On the other hand, it is desirable to reduce impurity elements such as oxygen, nitrogen, and carbon as much as possible. In particular, the amount of oxygen that impairs magnetic properties is preferably 7000 ppm or less, more preferably 5000 ppm or less. This is because when the amount of oxygen is large, the rare-earth oxide phase, which is a nonmagnetic component, increases and the magnetic properties are deteriorated. The rare earth sintered magnet to be manufactured is not limited to the RTB based rare earth sintered magnet. For example, the rare earth sintered magnet may be an SmCo-based sintered magnet.

  Rare earth sintered magnets are produced by powder metallurgy, but the manufacturing process basically consists of an alloying process, coarse pulverization process, fine pulverization process, molding process, sintering process, and aging process. . Further, in order to produce a sintered magnet with high characteristics, further oxidation prevention is essential, and most of the steps until after the sintering are performed in a vacuum or in an inert gas atmosphere (in a nitrogen gas atmosphere, It is desirable to carry out in an Ar gas atmosphere or the like.

  In the alloying step, a raw material metal or alloy is blended in accordance with a desired composition, melted in a vacuum or an inert gas, for example, Ar atmosphere, and cast into an alloy. As the casting method, any method can be adopted, but the strip casting method (continuous casting method) in which a molten high-temperature liquid metal is supplied onto a rotating roll and the alloy thin plate is continuously cast is a viewpoint of productivity and the like. From the viewpoint of the form of the resulting alloy.

  As the raw material metal (alloy) used in the alloying, pure rare earth elements, rare earth alloys, pure iron, ferroboron, and alloys thereof can be used. As the alloy, a single alloy having almost the final magnet composition may be used, or a plurality of types of alloys having different compositions may be mixed so as to have the final magnet composition.

  In the coarse pulverization step, the previously cast raw alloy thin plate, ingot, or the like is pulverized until the particle size is about several hundred μm. As the pulverizing means, a stamp mill, a jaw crusher, a brown mill, or the like can be used. In order to improve the coarse pulverization property, it is effective to perform coarse pulverization after occlusion of hydrogen and embrittlement.

  After the above-described coarse pulverization step is completed, a lubricant is added to the coarsely pulverized raw material alloy powder as necessary. As the lubricant, for example, a fatty acid compound can be used, and in particular, by using a fatty acid or fatty acid amide having a melting point of 60 ° C. to 120 ° C. as a lubricant, good magnetic properties, in particular, a high degree of orientation. A rare earth sintered magnet having high magnetization can be obtained, and the strength of the compact can be adjusted to a predetermined value depending on the type and amount of addition.

  After the coarse pulverization step, a fine pulverization step is performed. This fine pulverization step is performed using, for example, an airflow pulverizer. The conditions for the fine pulverization may be appropriately set according to the airflow pulverizer to be used, and the raw material alloy powder is finely pulverized until the average particle size becomes about 1 to 10 μm, for example, 2 to 7 μm. A jet mill or the like is suitable as the airflow pulverizer.

  After the pulverization step, the raw material alloy powder is formed in the magnetic field in the magnetic field forming step. Specifically, the raw material alloy powder obtained in the fine pulverization step is filled in a mold in which an electromagnet is arranged, and is molded in a magnetic field in a state where crystal axes are oriented by applying a magnetic field. The forming in the magnetic field may be either a parallel magnetic field forming in which the forming pressure and the magnetic field direction are parallel, or an orthogonal magnetic field forming in which the forming pressure and the magnetic field direction are orthogonal to each other. Further, a pulse power source and an air-core coil can be employed as the magnetic field applying means. The forming in the magnetic field may be performed in a magnetic field of 700 to 1600 kA / m, for example, at a pressure of 30 to 300 MPa, preferably about 130 to 160 MPa. The final relative density of the molded body obtained by molding in a magnetic field is usually 50 to 65%.

  In the molding step described above, the evaluation method disclosed as the first embodiment, the second embodiment, or the third embodiment is performed on the molded body to be molded. The evaluation may be performed on several molded bodies at an initial state at the start of molding and every predetermined time (for example, every 4 hours) at the time of continuous molding. Thereby, it is possible to manage the molding conditions and the like of the molded body during continuous molding and to prevent the occurrence of cracks after sintering.

  For example, it is possible to suppress cracks in the molded body by feeding back the evaluation result to the molding conditions in the molding process and adjusting the molding conditions. In the molding process, even if molding is performed under the same molding conditions using the same molding apparatus, cracks may occur due to slight fluctuation over time. In such a case, if the molding conditions are finely adjusted based on the evaluation, the occurrence of cracks can be reliably prevented. For example, the molding pressure can be adjusted, and the number of swings, swing amplitude, etc. when the raw material alloy powder is fed into the mold by the feeder box may be adjusted.

  In addition, when applying the above-described method for evaluating a molded body to a molded body of NdFeB-based rare earth alloy powder, it is preferable to select a solution according to the concentration of oxygen contained in the alloy powder, considering that the powder is likely to be oxidized. . Specifically, when the oxygen concentration of the alloy powder is 3000 ppm or more, the above-mentioned solutions (alkaline detergents, etc.) may be used. When the oxygen concentration of the alloy powder is 3000 ppm or less, other than the aqueous solution It is preferable to use an oil-based or solvent-based solution. This is because an alloy powder having an oxygen concentration of 3000 ppm or less has very high activity, and when immersed in an aqueous solution, bubbles are generated by an oxidation reaction, making crack determination difficult.

When performing crack evaluation of the molded article of NdFeB-based rare-earth alloy powder for example by the evaluation method described above, it is preferable that the compact density and ^{3.5g / cm 3 ~4.8g / cm 3} . More ^preferably, it is ^{4.0g / cm 3 ~4.8g / cm 3} . When the molded body density is less than 3.5 g / cm ³ , the shape retention is poor and it is difficult to carry out the evaluation method. On the other hand, when the density of the molded body exceeds 4.8 g / cm ³ , it is difficult for the solution to enter the molded body, which may increase the difficulty of crack evaluation. On the other hand, if the density of the green body is set within the above range, not only cracks on the surface of the green body but also cracks remaining inside the green body can be evaluated. Since the NdFeB rare earth alloy powder has poor fluidity, cracks are likely to occur during molding, and application of the evaluation method and adjustment of molding conditions based thereon are effective.

  In addition, it is preferable to make the molded object used for the said evaluation discard. When the molded body soaked with the solution is sintered, deformation and sintering are insufficient, resulting in a defective product. In order to avoid this, it is necessary to sufficiently remove the binder used in the evaluation and optimize the sintering conditions. However, if this is applied to a molded body not used in the evaluation, too much sintering will occur. Or cause deformation. Furthermore, when the molded body that has been evaluated and the molded body that has not been evaluated are debindered and sintered at the same time, the gasification solution or reaction gas generated from the molded body that has been evaluated during processing is evaluated. This may adversely affect the molded body that has not been formed and may cause deformation and poor sintering.

  Next, a sintering process is performed. In the sintering process, the sintering temperature must be adjusted according to various conditions such as composition, pulverization method, difference in particle size and particle size distribution, for example, 1000 ° C. to 1300. Sintering is performed at 1 ° C. for about 1 to 10 hours. The atmosphere during the sintering process is preferably a vacuum or an inert gas atmosphere (such as an argon gas atmosphere).

  After the sintering treatment, the obtained sintered body is preferably subjected to an aging treatment. This aging treatment is an important step in controlling the coercive force Hcj of the obtained rare earth magnet, and is performed, for example, in a vacuum or in an inert gas atmosphere. As the aging treatment, a two-stage aging treatment is preferable. The second stage aging treatment is held at a temperature of about 800 ° C. for 1 hour to 3 hours in the first stage aging treatment process, and is kept at a temperature of about 600 ° C. for 1 hour to 3 hours in the second stage aging treatment process. You can do it. Since the coercive force Hcj is greatly increased by heat treatment at around 600 ° C., when the aging treatment is performed in one stage, it is preferable to perform the aging treatment at around 600 ° C.

  After the sintering process, a machining process and a film forming process are performed to complete a rare earth sintered magnet. The machining process is a process of mechanically processing into a desired shape. The film forming step is a step performed for the purpose of suppressing oxidation of the obtained rare earth sintered magnet, and is a step of forming, for example, a plating film or a resin film on the surface of the rare earth sintered magnet.

  In the method for producing a rare earth sintered magnet as described above, the occurrence of cracks in the sintered rare earth sintered magnet can be minimized. Therefore, it is possible to manufacture a rare earth sintered magnet with high yield.

  Hereinafter, the present invention will be described based on specific experimental results.

Experiment 1
In this experiment, the difference in the generation state of bubbles depending on the presence or absence of cracks was examined. As a molded body for evaluation, a rare earth alloy powder was molded to produce a molded body. The composition of the rare earth alloy powder was Nd 30.2 mass%, Dy 1.4 mass%, B 1 mass%, Cu 0.1 mass%, Al 0.2 mass%, Co 0.5 mass%, and the balance Fe. The rare earth alloy powder has an average particle size of 4.0 μm, an oxygen content of 4000 ppm, and a carbon content of 1000 ppm. The size of the molded body was 60 mm × 10 mm × 20 mm, and the weight of the molded body was 50 g. Further, the set density of the formed body is 4.2 g / cm ³ .

  Molding conditions were adjusted for the molded body, and a molded body A with no cracks, a molded body B with a crack size of about 5 to 10 mm, and a molded body C with a crack size of 10 mm or more were produced. And these molded object AC was immersed in the solution (alkaline type cleaning agent aqueous solution: Nikka Seiko Co., Ltd. make, brand name Debale), and the bubble generation | occurence | production state was observed. As a result, in the molded body A where no crack was generated, the generation of bubbles was hardly observed, whereas in the molded bodies B and C where the crack was formed, bubbles were generated violently at the crack site. In addition, more bubbles were observed in the molded body C than in the molded body B, and the generated bubbles were larger.

Experiment 2
In this experiment, the viscosity of the solution was examined. The molded body used as the evaluation object is the same as in Experiment 1 above. Using the solutions shown in Table 1, the viscosity was adjusted and cracks were evaluated. The viscosity of the aqueous solution was adjusted by adjusting the concentration with water, and the viscosity of the organic solvent solution was adjusted by adjusting the concentration with ethanol. For the evaluation of the cracks, the bubbles from the crack part were visually confirmed, and the case where it was clearly identifiable was evaluated as ◎, the case where it was somehow identifiable as ◯, and the case where identification was difficult was evaluated as ×. The evaluation results are also shown in Table 1.

Identification was possible when the viscosity of the solution was 100 × 10 ⁻³ Pa · s or less, and distinction was possible when the viscosity was 10 × 10 ⁻³ Pa · s or less. On the other hand, when the viscosity of the solution exceeds 100 × 10 ⁻³ Pa · s, the generation of bubbles is very slow, and it is difficult to distinguish between the generation of bubbles at the cracked portion and the generation of bubbles at other portions. It was.

Experiment 3
In this experiment, the forming state of the compact of the raw material alloy powder of the SmCo magnet was evaluated. The composition of the raw material alloy powder is Sm 26.4 mass%, Fe 15.9 mass%, Cu 7.4 mass%, Zr 2.2 mass%, and the balance Co. The density of the molded body is 5.4 g / cm ³ . Using the solutions shown in Table 2, the viscosity was adjusted and cracks were evaluated. The evaluation results are also shown in Table 2.

  Even in the SmCo magnet, it was possible to identify the generation of bubbles in the crack portion of the molded body by using a solution having an appropriate viscosity.

Experiment 4
In this experiment, the molded state of the raw powder compact of the Sr ferrite magnet was evaluated. The density of the molded body is 3.0 g / cm ³ . Using the solutions shown in Table 3, the viscosity was adjusted and cracks were evaluated. The evaluation results are also shown in Table 3.

  Even in the ferrite magnet, it was possible to identify the occurrence of bubbles in the crack portion of the molded body by using a solution having an appropriate viscosity.

Experiment 5
About the molded object similar to Experiment 4, the generation | occurrence | production state of a crack was investigated using the solution containing a coating material. Specifically, the ferrite magnet molded body was immersed in a solution obtained by diluting white paint 20 times with thinner. Thereafter, the molded body was cut and the fracture surface was observed. As a result, a white line could be confirmed in the crack portion.

Experiment 6
For the same ferrite magnet molded body as in Experiment 4, a plurality of types of molded bodies having different cracking conditions were produced by changing the molding conditions. And these molded objects were immersed in the solution which diluted the white paint similar to Experiment 5 with the thinner, the fracture surface was observed, and the crack part was observed.

Next, the molded body formed at the same time as each of the molded bodies was immersed in an aqueous Debale solution (viscosity 1.2 × 10 ⁻³ Pa · s) in the same manner as in Experiment 4, and the state of foam generation was examined. As a result, it was found that as the molded body having a higher crack generation rate, more bubbles were generated, and the size of the cracks confirmed by the penetration of the paint and the bubble generation rate were almost proportional.

It is a schematic diagram explaining the evaluation method of this invention. It is a drawing substitute photograph which shows an example of the crack which generate | occur | produced in the sintered compact. It is a schematic diagram which shows an example of the crack which generate | occur | produced in the molded object. It is a drawing substitute photograph which shows the generation | occurrence | production state of the bubble in a crack site | part. It is a schematic diagram which shows the generation | occurrence | production state of the bubble in a crack site | part.

Explanation of symbols

1 Molded body, 2 solution, 3 mesh plate, 4 transparent container, 5 cracks, 6 bubbles

Claims (17)

  A method for evaluating a molded body, comprising: immersing a molded body of raw material powder in a solution and evaluating a molded state of the molded body based on a state of generation of bubbles.   The method for evaluating a molded body according to claim 1, wherein the state of occurrence of cracks in the molded body is evaluated based on the state of generation of bubbles from the molded body in a state immersed in a solution.   2. The method for evaluating a molded body according to claim 1, wherein the state of occurrence of cracks in the molded body is evaluated based on the state of generation of bubbles from the molded body after being taken out from the solution. The method for evaluating a molded article according to claim 1, wherein the viscosity of the solution is 100 × 10 ⁻³ Pa · s or less.   The method for evaluating a molded body according to any one of claims 1 to 4, wherein the molded body is immersed in a solution in a state in which a part of the bottom surface of the molded body is supported.   6. The method for evaluating a molded body according to claim 5, wherein the molded body is immersed in a solution with a bottom surface supported by a mesh member.   A method for evaluating a molded body, comprising: immersing a molded body of a raw material powder in a solution and evaluating a molded state of the molded body based on an infiltration state of a paint.   The method for evaluating a molded article according to claim 7, wherein a solution containing a paint is used as the solution.   The method for evaluating a molded body according to claim 7, wherein a coating is previously applied to the molded body of the raw material powder.   The method for evaluating a molded body according to any one of claims 7 to 9, wherein a cross-section of the molded body is observed, and the state of occurrence of cracks in the molded body is evaluated by visually recognizing a paint that has entered the crack. . After immersing the molded body of the raw material powder in the solution and grasping the molded state of the molded body based on the infiltration state of the paint, another molded body molded under the same conditions as the molded body for which the molded state was grasped Soak in the solution,
Based on the state of foam generation at this time, it is characterized in that the molded state of the molded body is evaluated by comparing the state of foam generation when the molded body of the raw material powder to be evaluated is immersed in a solution. Evaluation method for molded product   The method for evaluating a molded body according to any one of claims 1 to 11, wherein the raw material powder is a magnetic raw material powder.   13. The molded body evaluation method according to claim 12, wherein the raw material powder is a NdFeB-based rare earth alloy powder. A molding step of molding a raw material powder to produce a molded body,
A molded body evaluation step for evaluating the molded state of the molded body;
A sintering step of sintering the molded body,
In the said molded object evaluation process, the molding state of a molded object is evaluated with the evaluation method of the molded object of any one of Claim 1 to 13, The manufacturing method of the sintered compact characterized by the above-mentioned.   The method for producing a sintered body according to claim 14, wherein molding conditions in the molding step are adjusted based on an evaluation result in the molded body evaluation step.   The method for producing a sintered body according to claim 14 or 15, wherein the raw material powder is an NdFeB-based rare earth alloy powder, and a compact is produced by pressure molding in a magnetic field in the molding step. Method for producing a sintered body according to claim 16, wherein that the compact density of the green body and ^{3.5g / cm 3 ~4.8g / cm 3} .

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