JP7727575B2 - How to clean hot working dies - Google Patents

Description

This disclosure relates to a method for cleaning a hot working mold.

Hot-deformed magnets, manufactured by hot plastic working, are known as R-T-B-based permanent magnets with excellent magnetic properties. One known method for manufacturing hot-deformed magnets is the forward extrusion method, in which a compact is extruded into a high-temperature die while undergoing plastic working (see, for example, Patent Documents 1 and 2).

Japanese Patent Application Laid-Open No. 2020-92167

When hot-formed magnets are manufactured using the forward extrusion method, material from the compact remains inside the mold after processing. As a result, cleaning the inside of the mold requires a lot of time.

This disclosure has been made in light of the above, and aims to provide technology that enables efficient cleaning of the inside of a mold after hot working.

To achieve the above-mentioned objective, one embodiment of the present disclosure provides a method for cleaning a hot-working die, which includes injecting a remover into an extrusion die after hot working a molded body made of a magnetic material, and removing the magnetic material by pushing the remover in with a punch while heating the extrusion die with the remover inside to a temperature equal to or higher than the plastic workable temperature of the molded body.

According to the above-mentioned method for cleaning hot working dies, a remover is poured into the extrusion die, and then the remover is pushed in while heated to the heating temperature, thereby removing any residue remaining in the extrusion die after hot working. This makes it easier to clean the inside of the die with the remover.

The molded body may be a molded body for producing an R-T-B system permanent magnet.

The material used for R-T-B permanent magnets has an anisotropic coefficient of thermal expansion, expanding in the direction of the hard magnetization axis upon cooling. This makes it difficult to remove any material remaining in the extrusion die after hot working. The above method is particularly effective for extrusion dies used to manufacture R-T-B permanent magnets, which are difficult to remove.

The remover may be made of a material that does not sinter at the plastic workable temperature.

The above configuration prevents the remover from sintering when heated at the heating temperature, thereby preventing the remover from remaining in the mold.

The weight of the remover poured into the extrusion die may be equal to or greater than the weight obtained by multiplying the true density of the remover by the difference between the volume of the extrusion die and the volume of the portion of the punch that is inserted into the extrusion die during the hot working.

The above configuration makes it possible to reliably remove any magnetic material remaining in areas that the punch cannot reach, making it possible to more reliably clean the inside of the mold with a remover.

This disclosure provides technology that enables efficient cleaning of the inside of a mold after hot working.

FIG. 1 is a flowchart illustrating an example of a method for manufacturing a hot-worked magnet, including a method for cleaning a hot-working die according to one embodiment. FIG. 2 is a diagram illustrating an example of the configuration of an extrusion die. 3(a) and 3(b) are diagrams illustrating an example of the configuration of an extrusion die. 4(a), 4(b), and 4(c) are diagrams illustrating an example of a method for cleaning an extrusion die using a remover.

Hereinafter, embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. Note that in the description of the drawings, identical elements will be designated by the same reference numerals, and duplicate explanations will be omitted.

[Method for manufacturing hot-worked magnets]
1 is a flowchart illustrating a method for manufacturing a hot-worked magnet, including a method for cleaning a hot-working mold according to one embodiment. Below, we will explain the method for manufacturing a neodymium magnet (neodymium-iron-boron magnet) whose main phase is an R ₂ T ₁₄ B crystal, which is a type of R-T-B permanent magnet.

In R-T-B system permanent magnets, R represents a rare earth element. The permanent magnet contains at least neodymium (Nd) as a rare earth element. The permanent magnet may contain other rare earth elements in addition to Nd. The other rare earth elements may be at least one selected from the group consisting of scandium (Sc), yttrium (Y), lanthanum (La), cerium (Ce), praseodymium (Pr), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu). In R-T-B system permanent magnets, T represents a transition metal element. The permanent magnet contains at least iron (Fe) as a transition metal element. The permanent magnet may contain only Fe as a transition metal element. Permanent magnets may contain both Fe and cobalt (Co) as transition metal elements. In R-T-B permanent magnets, B is boron.

When manufacturing a hot-processed magnet, the raw magnetic material is first pulverized into magnetic powder (Step S1). Pulverization can be performed, for example, using a cutter mill or propeller mill, in an argon gas atmosphere (or nitrogen gas atmosphere). The particle size of the magnetic powder obtained by pulverization is, for example, approximately 100 to 300 μm. The magnetic powder is not pulverized to the size level of neodymium magnet crystals (1 μm or less, for example, tens to hundreds of nm), and has a polycrystalline structure composed of multiple neodymium magnet crystals.

The magnetic powder obtained in step S1 is compacted in a compression molding machine to obtain a green body (step S2). Compaction is carried out in a nitrogen gas atmosphere (or argon gas atmosphere) at a high temperature of 800°C or less (for example, 750°C) and a press pressure of 200 MPa or less for several tens of seconds. A dense green body is obtained through compaction. However, in this green body state, the magnetic particles are randomly oriented and the axis of easy magnetization is not aligned.

The compact obtained in step S2 is hot-worked using forward extrusion to obtain a hot-worked magnet (step S3). Hot working is carried out, for example, in an argon gas atmosphere at a high temperature of approximately 500°C to 1000°C (e.g., 750°C) and a press pressure of 100 MPa or less for several tens of seconds. The heating temperature of the compact during hot working is the temperature at which plastic working of the magnetic material used in the hot-worked magnet is possible, and is called the plastic working temperature.

An extrusion die 1 (hot working die) used for hot working will be described with reference to Figures 2 and 3. In this embodiment, as an example, the extrusion die 1 has a cylindrical outer shape. However, the outer shape of the extrusion die is not limited to a circular shape and may be other shapes such as a rectangular column. The extrusion die 1 is made of a highly heat-resistant material (such as a nickel-based superalloy (e.g., Inconel (registered trademark)), molybdenum, etc.). The extrusion die 1 is made up of a cylindrical introduction section 10 and a cylindrical plastic processing section 20, with the introduction section 10 and plastic processing section 20 arranged in this order from top to bottom.

The introduction section 10 has a starting end face 10a and a terminal end face 10b that face each other. The introduction section 10 also has a through hole 12 that extends in the vertical direction to connect the starting end face 10a and the terminal end face 10b. The cross-sectional shape of the through hole 12 in a cross section perpendicular to the vertical direction in which the starting end face 10a and the terminal end face 10b face each other, i.e., in a horizontal plane, is the same as the end face shape of the starting end 22a of the plastically processed hole 22 described below. The cross-sectional shape of the through hole 12 is the same from the starting end face 10a to the terminal end face 10b.

The plastically processed portion 20 has a starting end surface 20a and an ending end surface 20b that face each other. The starting end surface 20a and the ending end surface 20b are parallel to each other. The ending end surface 10b of the introduction portion 10 substantially coincides with the starting end surface 20a of the plastically processed portion 20. The plastically processed portion 20 has a plastically processed hole 22 that continues from the above-mentioned through hole 12. The plastically processed hole 22 has a starting end 22a at the starting end surface 20a of the plastically processed portion 20 and a ending end 22b at the ending end surface 20b.

The starting end 22a of the plastically processed hole 22 has an end face shape that extends in one direction when viewed from the facing direction (vertical direction) between the starting end face 20a and the terminal end face 20b. As an example, the end face shape of the starting end 22a is rectangular. Furthermore, the starting end 22a of the plastically processed hole 22 is positioned so that the end face of the starting end 22a overlaps with the through hole 12 of the introduction section 10 when viewed from the facing direction (vertical direction) between the starting end face 20a and the terminal end face 20b.

For ease of explanation, the direction in which the starting end face 20a and the ending end face 20b of the plastically processed portion 20 face each other will be referred to as the Z direction, the direction in which the end face shape of the starting end 22a of the plastically processed hole 22 extends will be referred to as the X direction, and the direction perpendicular to the Z direction and the X direction will be referred to as the Y direction.

In the extrusion die 1, the cross-sectional area of the plastically processed hole 22 in the X-Y cross section gradually decreases from the starting end 22a to the terminal end 22b.

When viewed from the direction in which the starting end face 20a and the ending end face 20b of the plastically processed hole 22 face each other, the end face shape of the end face 22b is rectangular. The end face shape of the starting end 22a extends in the X direction (i.e., the long side is aligned with the X axis), while the end face shape of the end face 22b extends in the Y direction (i.e., the long side is aligned with the Y axis). When viewed from the direction in which the starting end face 20a and the ending end face 20b face each other, the X direction (first direction) in which the end face shape of the starting end 22a extends and the Y direction (second direction) in which the end face shape of the end face 22b extends intersect, or more specifically, are perpendicular to each other. The plastically processed hole 22 can also be described as having its long side (or major axis) and short side (or minor axis) interchanged between the rectangular end face of the starting end 22a and the rectangular end face of the terminal end 22b. The end faces of the starting end 22a and the terminal end 22b are in a twisted positional relationship.

As shown in Figure 2, the extrusion die 1 uses a punch 30 with a cross-sectional shape that is the same dimensions as the end face shape of the starting end 22a of the plastically processed hole 22 (or with the length of each side being slightly shorter) to extrude the above-mentioned compact introduced into the introduction section 10 forward toward the end face 20b of the plastically processed section 20, i.e., in the Z direction. This results in a strip-shaped hot-processed magnet having the same cross-sectional shape as the end face shape of the ending end 22b of the plastically processed hole 22. The strip-shaped hot-processed magnet is then cut to the desired width.

As an example, within the plastically processed portion 20 of the extrusion die 1, the contour of the plastically processed hole 22 may be formed by a curve, as shown in Figures 2 and 3.

When the extrusion die 1 is heated to a desired temperature and the compact is extruded forward in the Z direction from the starting end 22a of the plastically processed portion 20 toward the terminal end 22b, i.e., the Z direction, the grain boundary phase in the heated compact inside the die liquefies, generating a liquid phase. The cross-sectional shape of the plastically processed portion 20 changes, causing the compact to plastically deform. The generation of the liquid phase and plastic deformation accompany anisotropic growth in a direction perpendicular to the c-axis (axis of easy magnetization) of the main phase grains. The plastic deformation of the compact and the anisotropic growth of the main phase grains exert stress in a specific direction on the main phase grains. The liquid phase also lubricates each crystal grain, facilitating the movement of individual main phase grains. As a result, the crystal grains rotate due to grain boundary sliding, and the c-axis of each main phase grain becomes aligned approximately parallel to the stress direction. This results in a hot-worked magnet aligned along the easy axis of magnetization.

The mold temperature is 500°C to 1000°C, more preferably 600°C to 800°C. When the mold temperature is within the above range, the magnetic material can be plastically processed.

As mentioned above, the cross-sectional shape of the punch 30 inserted into the extrusion die 1 along the extrusion direction (Z direction) is substantially the same as the end face shape of the through hole 12 and the starting end 22a of the plastically processed hole 22. Therefore, the punch 30 cannot be inserted below the starting end 22a of the plastically processed portion 20, where the cross-sectional shape changes. Therefore, even after the compact is extruded by the punch 30, magnetic material remains within the extrusion die 1 (particularly within the plastically processed hole 22). This state will be explained with reference to Figure 4(a). Figures 4(a) to 4(c) show the through hole 12 and the plastically processed hole 22 within the extrusion die 1 as viewed from the direction corresponding to Figure 3(b). As shown in Figure 4(a), if the insertion length of the punch 30 into the extrusion die 1 is L, the magnetic material M is removed from the upper region of the extrusion die 1 by length L, while the magnetic material M remains in the region below. In particular, since the punch 30 is not inserted into the plastically processed hole 22, the magnetic material M inside the plastically processed hole 22 is not removed and remains inside.

In the method for manufacturing a hot-worked magnet according to this embodiment, as shown in FIG. 1, after the above-mentioned hot working (S3), a remover is poured into the extrusion die 1 (step S4), and while the extrusion die 1 is heated, a punch 30 is pressed into the extrusion die 1 to remove the magnetic material from within the extrusion die 1 (step S5). These steps correspond to a method for cleaning the extrusion die 1.

These steps will be explained with reference to Figures 4(b) and 4(c). As shown in Figure 4(a), magnetic material M remains inside the extrusion die 1 after thermal processing. Therefore, as shown in Figure 4(b), a remover R is poured into the interior of the extrusion die 1 from above the remaining magnetic material M.

The remover R is selected from materials that are solid when added and heated, and that do not sinter at the plastic workable temperature. In other words, the remover R is selected from materials that are stable even when heated at the plastic workable temperature of the compact (magnetic material), and that do not sinter inside the extrusion die 1. Note that the plastic workable temperature of the compact may vary depending on the magnetic material. In this case, an appropriate remover R material is selected depending on the plastic workable temperature.

In addition, the remover R is selected to have a lower hardness than the material forming the inner surface of the extrusion die 1. This prevents the remover R from damaging the extrusion die 1.

There are no particular restrictions on the particle size of the remover R, but if the particle size is relatively small, for example, if the remover R is in a powder or granular state, it will be easily dispersed inside the extrusion die 1.

Examples of removers R that meet these conditions include graphite, boron nitride, and talc. These materials do not sinter at the plastic processing temperatures (600°C to 900°C) of the above-mentioned molded body, and are less hard than a typical extrusion die 1, so they can meet the above conditions.

The amount (weight) of remover R introduced into the extrusion die 1 is not particularly limited. However, if the volume of the remover R compressed by the punch 30 is greater than the difference between the volume of the extrusion die 1 and the volume of the portion of the punch 30 inserted into the extrusion die 1 during hot working, the entire amount of remaining magnetic material M can be more easily replaced with remover R by extrusion by the punch 30. In other words, if the "filling amount of remover R" is greater than the product of the "true density of remover R" and the "difference between the volume of the extrusion die 1 and the volume of the portion of the punch inserted into the extrusion die during hot working," the removal of the magnetic material M remaining inside the plastically worked hole 22 can be facilitated. The "true density of remover R" refers to the particle density of remover R and is measured in accordance with JIS R 1620:1995, "Method for measuring particle density of fine ceramic powders."

Next, with the remover R added, the extrusion die 1 is heated while the punch 30 is inserted into the die. The extrusion die 1 is heated to a temperature at which the molded body can be plastically processed. The extrusion die 1 is surrounded by a gas atmosphere appropriate for the remover R. An example of a gas atmosphere appropriate for the remover R is an inert gas atmosphere to prevent the remover R from reacting with the extrusion die 1 or the magnetic material M when heated. In this state, the punch 30 is inserted into the extrusion die 1 at the same pressing pressure and processing speed as during hot processing, as shown in Figure 4(c). As a result, as shown in Figure 4(c), the magnetic material M inside the extrusion die 1 is pushed out from below by the insertion of the punch 30 and ejected from the extrusion die 1. Meanwhile, the remover R added in the upper layer replaces the magnetic material M and remains in the space within the extrusion die 1 that the punch 30 does not reach. As a result, the magnetic material M inside the extrusion die 1 is ejected to the outside.

If magnetic material M remains inside after hot working, when the extrusion die 1 cools, the magnetic material M also cools and solidifies with the magnetic material M remaining. This makes it difficult to remove the magnetic material M from the extrusion die 1. In contrast, by forcing the remover R into the interior as described above and expelling the magnetic material M and other residues to the outside, it is easier to remove the magnetic material M from inside the extrusion die 1.

In addition, any remover R remaining inside after inserting the punch 30 (S5) can be easily discharged into the extrusion die 1. As mentioned above, the remover R does not sinter inside the extrusion die 1 during the above process (S5), and therefore is less likely to adhere to the inner surface of the extrusion die 1. Therefore, the remover R can be easily removed from inside the extrusion die 1 using, for example, an air blower.

[evaluation]
After the hot-worked magnet was manufactured, the extrusion die was cleaned by the above-described method, and the internal condition of the die was evaluated according to the following procedure.

Using the apparatus shown in Figure 2, a hot-worked magnet was processed and then pressed with remover R. The extrusion die 1 used was made of Inconel (registered trademark). The internal volume of the extrusion die 1 was 8 ^cm3 . The type of remover R injected into the extrusion die 1 after producing the hot-worked magnet was changed as shown in Table 1 below, and the extrusion die 1 was cleaned under the conditions of Examples 1 to 4.

The magnetic material used to manufacture the hot-worked magnet was Nd ₂ Fe ₁₄ B. A predetermined amount of the magnetic material was placed in an extrusion die 1 and hot-worked in an argon gas atmosphere at a die temperature of 750°C and a pressing pressure of 100 MPa using a punch 30. These manufacturing conditions for the hot-worked magnet were the same for Examples 1 to 4.

Then, the powder of remover R shown in each example in Table 1 was poured into the extrusion die 1 in the amount shown in Table 1. The extrusion die 1 was then heated to 750°C in a nitrogen gas atmosphere. With the extrusion die 1 heated, the punch 30 was inserted into the extrusion die 1 at the same processing speed as during hot working, with the press pressure of the punch 30 set to 100 MPa, the same as during hot working. After extrusion by the punch 30, the powder remaining inside the extrusion die 1 was removed by air blowing. When the inside of the extrusion die 1 was observed after the powder had been removed by air blowing, no magnetic material was found adhering to the inner surface.

Under each condition, the surface roughness of the inner surface of the plastically worked hole 22 of the extrusion die 1 was measured before the hot-worked magnet was manufactured (before extrusion), and after extrusion with the punch 30 was performed (after cleaning) after adding remover R. The arithmetic mean roughness (Ra) specified in JIS B0601:2001 was measured as the surface roughness. The type, true density, and filling amount of remover, as well as the measurement results of the surface roughness of the inner surface of the plastically worked hole 22 of the extrusion die 1, are shown in Table 1.

As shown in Table 1, it was confirmed that the surface roughness after extrusion changed significantly when alumina was used as in Example 4 compared to when graphite, boron nitride, or talc was used as remover R as in Examples 1 to 3. However, it was confirmed that if the arithmetic mean roughness Ra was 50 μm or less, the extrusion die 1 could be used without any problems as an extrusion die for manufacturing hot-worked magnets, and could be used repeatedly even after magnetic material was removed using remover R.

[Modification]
Although the embodiments have been described above, the present disclosure is not necessarily limited to the above-described embodiments, and various modifications are possible without departing from the spirit of the present disclosure.

For example, the shape of the extrusion die 1 is not particularly limited to the shapes described in the above embodiment. For example, the end face shapes of the starting and ending ends of the plastically processed portion 20 of the extrusion die 1 are not limited to rectangular shapes, but may also be oval shapes extending in one direction, perfect circles, U-shapes, or V-shapes.

There are no particular restrictions on the magnetic material of the permanent magnet, as long as it can be manufactured by hot working. Therefore, the above configuration can also be applied to magnetic materials for permanent magnets other than R-T-B permanent magnets.

1...extrusion die, 10...introduction section, 12...through hole, 20...plastic processing section, 22...plastic processing hole, 30...punch.

Claims (4)

Injecting a remover into an extrusion die after hot working of a compact made of a magnetic material;
removing the magnetic material by pushing the remover in with a punch while heating the extrusion die with the remover introduced therein to a heating temperature that is equal to or higher than the plastic workable temperature of the compact;
1. A method for cleaning a hot working mold, comprising: The method for cleaning a hot working mold according to claim 1, wherein the green body is a green body for producing an R-T-B system permanent magnet. The method for cleaning a hot working die according to claim 1 or 2, wherein the remover is made of a material that does not sinter at the plastic workable temperature. 4. The method for cleaning a hot working die according to claim 1, wherein a weight of the remover introduced into the extrusion die is equal to or greater than a weight obtained by multiplying the true density of the remover by the difference between the volume of the extrusion die and the volume of the portion of the punch that is inserted into the extrusion die during the hot working.