JP6586210B2 - Method for producing chromium for powder metallurgy - Google Patents

Description

  The present invention relates to a method for producing chromium for powder metallurgy used as a raw material for powder metallurgy.

  Powder metallurgy is known in which a metal powder is put into a mold, compressed and hardened, and sintered at a high temperature to produce a highly accurate part. The manufacturing method of powder metallurgy includes a mixing step of mixing metal powders, a forming step of compressing the mixed metal powders with a press to form a product, and a sintering step of baking and solidifying the molded product. Powder metallurgy is characterized in that it can be molded even in complex shapes, and parts with various properties can be produced by freely combining metal powders.

  By the way, the chromium-containing metal has a characteristic of excellent oxidation resistance even at high temperatures. As a method for producing a chromium-containing metal, an electric furnace reduction method in which chromium oxide is reduced with an electric furnace to obtain metal chromium (see Patent Document 1), and an aluminum thermite method in which chromium oxide is reduced with metal aluminum to obtain metal chromium ( Patent Document 2) is known.

JP 2003-49235 A JP 60-36632 A

  As described above, the chromium-containing metal has a characteristic of excellent oxidation resistance even at high temperatures. For this reason, it is desired to use chromium-containing metal powder as a raw material for powder metallurgy. The molten metal produced by the electric furnace reduction method or the aluminum thermit method is pulverized through the following steps. First, the molten metal in the furnace is poured into a mold, cooled to an ingot, then shot blasted to remove the slag on the ingot surface, and then the ingot is roughened by a crusher. Grind and then pulverize the coarse particles.

  The applicant tried to use a chromium-containing metal powder finely pulverized by a rod mill as a raw material for powder metallurgy using a rod mill for fine pulverization of coarse particles. A rod mill is a pulverizer suitable for pulverizing hard metal.A steel rod is placed in a drum as a pulverizing medium, and the drum is rotated to compress coarse particles interposed between the steel rods. The particles are pulverized. However, when the coarse particles are finely pulverized using a rod mill, the strength of the sintered product produced by powder metallurgy is reduced.

  An object of this invention is to provide the manufacturing method of the chromium for powder metallurgy which can prevent the intensity | strength of the product after sintering manufactured by powder metallurgy falling.

  In order to solve the above-mentioned problems, the present inventor has studied the cause of the decrease in strength of the product after sintering. As a result, when finely pulverized using a rod mill, coarse particles are crushed between the rods, so that the powder tends to be flattened. It was found that sufficient strength could not be secured. Then, using an impact-type grinding mill suitable for grinding soft things such as wheat, soybeans, rice, etc., the chromium-containing metal powder was crushed by impacting and pulverizing, so that the powder of the chromium-containing metal was not flattened. It was found that a block shape can be obtained, thereby ensuring the contact area of the powder and the strength of the product after sintering. The impact pulverization mill is a pulverizer that pulverizes coarse particles by impacting them with a pin or hammer that rotates at high speed. As the impact pulverization mill, a pin mill provided with a pin on a high-speed rotating disk, a hammer mill provided with a hammer on a high-speed rotating rotor, or the like can be used.

  The present invention is based on the above knowledge, and chromium-containing metal is finely pulverized by an impact-type pulverization mill, and the chromium-containing metal powder finely pulverized by the impact-type pulverization mill is a predetermined particle diameter or more. Chromium that has been screened to a particle size of less than a predetermined particle size or more than a predetermined particle size is used as a raw material for powder metallurgy. This is a method for producing chromium for powder metallurgy, in which a powder of a metal containing is mixed with a powder of a metal containing chromium crushed by a vibromill to form a briquette.

  In a preferred aspect of the present invention, the chromium-containing metal is produced by roughly pulverizing an ingot of chromium-containing metal that has been discharged from a furnace and cooled and solidified.

  In a further preferred aspect of the present invention, the coarse particles are finely pulverized using a first impact pulverization mill, and the finely pulverized fine particles are further finely pulverized using a second impact pulverization mill. To do.

  In a further preferred aspect of the present invention, the chromium-containing metal is obtained by reducing chromium oxide with an electric furnace.

  According to the present invention, the powder of chromium-containing metal can be formed into a square block shape that is not flat by pulverizing with an impact using an impact pulverization mill. Also, the surface of the finely pulverized chromium-containing metal powder can be uneven, and the contact area of the chromium-containing metal powder can be secured, preventing the strength of the sintered product produced by powder metallurgy from being reduced. it can.

  Since the chromium-containing metal powder is screened to a predetermined particle size or more and less than the predetermined particle size, impurities such as carbon, oxide, and nitride attached to the chromium-containing metal powder can be removed.

  Since the chromium-containing metal powder screened to be smaller than the predetermined particle diameter or larger than the predetermined particle diameter is mixed with the chromium-containing metal powder pulverized by vibro mill to form a briquette, the yield of metal chromium is improved.

  According to a preferred embodiment of the present invention, the ingot of the chromium-containing metal that has been cooled and solidified is coarsely pulverized, and then the coarse particles are finely pulverized. Further, it is possible to prevent excessively pulverization and rounding of the powder, thereby reducing the contact area of the powder interface.

  According to a further preferred embodiment of the present invention, the coarse particles are finely pulverized using a first impact pulverization mill, and the finely pulverized fine particles are further finely pulverized using a second impact pulverization mill. The grinding efficiency can be improved. In addition, it is possible to prevent excessively finely pulverizing and rounding off the corners of the powder and reducing the contact area of the powder interface.

  According to a further preferred aspect of the present invention, the chromium-containing metal powder is screened to a predetermined particle size or more and less than a predetermined particle size, so that impurities such as carbon, oxides, and nitrides attached to the chromium-containing metal powder Can be removed.

According to a further preferred aspect of the present invention, since the chromium-containing metal is obtained in an electric furnace, there are fewer inclusions such as Al ₂ O ₃ compared to the case of obtaining the chromium-containing metal by an aluminum thermite method, Excellent sinterability.

It is process drawing of the manufacturing method of the chromium for powder metallurgy of one Embodiment of this invention. It is detail drawing of the grinding | pulverization process of this embodiment. It is sectional drawing of a pin mill. It is sectional drawing of a pin mill with a classification function. It is sectional drawing of a hammer mill. It is a picked-up image of the metal chromium powder of an Example. It is a picked-up image of metal chromium powder of a comparative example. It is the graph which compared the quality of the powder of metal chromium with a pin mill and a hammer mill.

  FIG. 1 shows a process chart of a method for producing chromium for powder metallurgy according to an embodiment of the present invention. The chromium-containing metal of this embodiment is metallic chromium or low carbon ferrochrome. Metal chromium is a metal having a chromium content of 90% by mass or more. Ferrochrome is an alloy containing chromium and iron. FIG. 1 shows an example of producing metallic chromium as an example of a chromium-containing metal.

As shown in FIG. 1, first, chromium oxide, metal silicon, and a CaO-based flux are charged into an electric furnace as raw materials (S1). Chromium oxide (Cr ₂ O ₃ ) is obtained by refining chromite (FeO · Cr ₂ O ₃ ). Metallic silicon is used as a reducing agent that reduces chromium oxide (Cr ₂ O ₃ ) to metallic chromium. CaO-based flux is used to promote the reduction reaction.

  After charging the raw material into the electric furnace, the electric furnace is energized to heat and melt the raw material (S2). When the raw material is melted, a reduction reaction of metallic silicon and chromium oxide proceeds, and a molten metal chromium and primary slag are generated in the electric furnace. After discharging the primary slag, a CaO-based flux is added, re-energized to generate secondary slag, and the molten metal is refined with the secondary slag.

  Next, the molten metal is poured out into the mold together with the secondary slag (S3). The cooled and solidified ingot is taken out of the mold and subjected to shot blasting on the surface of the ingot to remove slag adhering to the surface of the ingot.

  Next, the ingot of metal chromium is coarsely pulverized (S4). Since metallic chromium produced in an electric furnace contains carbon, it is easily broken. For coarse pulverization, for example, three types of crushers including a hydraulic excavator, a jaw crusher, and a roll crusher are used. For example, the metallic chrome ingot is primarily crushed with a hydraulic excavator, the primary crushed ingot is secondary crushed with a jaw crusher, and the secondary crushed ingot is tertiary crushed with a roll crusher. The size of the ingot after coarse pulverization is 10 mm or less.

  Next, the coarse particles are finely pulverized (S5). For the fine pulverization, a pin mill or a hammer mill which is an impact pulverization mill is used. By applying an impact using an impact pulverization mill and pulverizing, the metal chromium powder can be formed into a non-flat square block shape.

  As shown in FIG. 2, the fine pulverization is performed twice (S5 in FIG. 2). First, coarse particles are finely pulverized using a first impact pulverization mill, and then finely pulverized fine particles are further finely pulverized using a second impact pulverization mill. Here, pin mills are used as the first and second impact grinding mills. The size of the particles after primary pulverization is 2 mm or less, and the size of the particles after secondary pulverization is 0.2 mm or less. The first impact pulverization mill and the second impact pulverization mill may be used in the same impact pulverization mill, or may be separate impact pulverization mills. Thus, grinding efficiency can be improved by carrying out fine grinding in two stages. Further, it is possible to prevent the powder contact area from being reduced by excessively finely pulverizing and rounding the powder.

  FIG. 3 shows a pin mill as an example of an impact grinding mill used for fine grinding. A disk 1 as a rotor that rotates at high speed is provided with a number of pins 2 that protrude in a direction parallel to the rotation center 1c. The stator 3 surrounding the disk 1 is also provided with a number of pins 4 protruding in a direction parallel to the rotation center 1c. The material of the disk 1, the pin 2 on the disk 1 side, the stator 3, and the pin 4 on the stator 3 side is cemented carbide such as tungsten carbide or ceramics.

  The raw material is supplied from the upper part of the case 5 to the vicinity of the rotation center 1c of the disk 1. When the disk 1 is rotated at a high speed (for example, 5000 rpm to 8000 rpm), the raw material jumps out in the radial direction by centrifugal force. The raw material jumping out in the radial direction is hit (impacted) between the pin 2 on the disk 1 side rotating at a high speed and the pin 4 on the stator 3 side, and the raw material is crushed by the impact. The crushed raw material is lowered by gravity and discharged through the screen 6. By changing the rotational speed of the disk 1 of the pin mill and / or the diameter of the disk 1, the particle size distribution of the pulverized particles can be adjusted.

  FIG. 4 shows a pin mill with a classification function as another example of an impact pulverization mill used for fine pulverization. The raw material is put into the liner 12 by the screw feeder 11. A disk 13 that rotates at high speed is provided in the liner 12. The disk 13 is provided with a number of pins 14 protruding in a direction parallel to the rotation center 13c. The classification rotor 15 is for sorting powder. Between the disk 13 and the classification rotor 15, a cylindrical guide ring 16 that promotes convection in the liner 12 and classifies the raw material crushed by the pins 14 is provided. The material of the disk 13, the pin 14, and the liner 12 is cemented carbide such as tungsten carbide or ceramics.

  The raw material charged into the liner 12 is hit (impacted) between the pin 14 of the disk 13 rotating at a high speed and the liner 12, and is crushed by the hit. The pulverized powder is guided to the classification rotor 15 by the air flow supplied from the air supply pipe 17. The classification rotor 15 sorts the powder and discharges the fine powder to the outside through the discharge pipe 18. The remaining coarse powder is pulverized again by the pins 14 of the disk 13 rotating at high speed. The particle size distribution of the pulverized particles can be adjusted by changing the rotation speed of the pin mill disk 13 and the classification rotor 15 and / or the diameter of the disk 13.

  FIG. 5 shows a hammer mill as another example of an impact-type grinding mill used for fine grinding. The raw material is put into the liner 22 by the screw feeder 21. A rotor 23 that rotates at high speed around a horizontal axis is provided in the liner 22. A large number of swingable hammers 24 are provided on the outer periphery of the rotor 23. For the material of the rotor 23, the hammer 24, and the liner 22, a cemented carbide such as tungsten carbide or ceramics is used.

  When the rotor 23 is rotated at a high speed (for example, 5000 rpm to 8000 rpm), the hammer 24 jumps out in the radial direction, and a hammer (impact) is applied to the raw material between the hammer 24 and the liner 22 rotating at a high speed, and the raw material is pulverized. By changing the rotational speed of the rotor 23 and / or the diameter of the rotor 23 of the hammer mill, the particle size distribution of the pulverized particles can be adjusted.

As shown in FIG. 2, the finely pulverized metal chromium powder is screened to 45 μm or more and 200 μm or less. For sieving of 45 μm or more, a standard sieve having a nominal size of 45 μm and a mesh (number of eyes / inch) 330 is used. For sieving of 200 μm or less, a standard sieve with a nominal size of 200 μm and a mesh (number of eyes / inch) 74 is used. The powder sieved to 45 μm or more and 200 μm or less is used as a raw material for powder metallurgy. By sieving, the average particle diameter of the powder becomes D ₅₀ = 90 to 180 μm. In order to maintain compression moldability and high density after firing, D ₅₀ is adjusted to 90 to 180 μm. For the measurement of the average particle diameter, a measuring apparatus using a laser diffraction / scattering method is used.

  Impurities such as carbon, oxides and nitrides adhering to the grain boundaries of the chromium metal powder are removed by sieving. The carbon content of the metal chromium in the powder (product) after sieving is 0.2 mass% or less, the oxide content is 0.2 mass% or less, and the nitride content is 0.1 mass% or less. is there. Carbon, oxides, and nitrides cause a decrease in compression density, a decrease in strength, and poor sintering of powder metallurgical compacts. For this reason, content of carbon, an oxide, and nitride is made into the said mass% or less.

  As shown in FIG. 2, in this embodiment, fine grinding using a vibro mill (a kind of rod mill, a grinding device that vibrates the case) is performed in parallel with fine grinding using an impact grinding mill. And after grind | pulverizing metal chromium to the powder of 0.2 mm or less with a vibromill (S6), the PVA (polyvinyl alcohol) solution as a lump forming agent is mixed with this, and it compression-molds to briquette form (S7). Thereafter, the briquette is held at, for example, 1350 ° C. and 13 Pa (0.1 Torr) for 50 hours, and vacuum heat treatment is performed (S8). By carrying out the vacuum heat treatment, impurities such as carbon and oxygen in the briquette are removed. Metal chromium powder pulverized by vibromill is not screened, and impurities such as carbon and oxygen cannot be removed by screening. For this reason, these impurities are removed by vacuum heat treatment.

  In the sieving step after fine pulverization of S5 in FIG. 2, the metal chromium powder screened to less than 45 μm or 200 μm or more is mixed with the powder pulverized by vibromill in S6. And it briquettes like the powder grind | pulverized with the vibro mill (S7), and vacuum-heat-processes (S8). Briquetted metal chromium cannot be used as a raw material for powder metallurgy, but is sold as a product. The yield of metal chromium improves by making the powder of metal chromium screened to less than 45 micrometers or 200 micrometers or more into a briquette.

  As described above, the metal chromium powder of the present embodiment has a feature that the strength of a sintered product manufactured by powder metallurgy can be improved. Moreover, since the product manufactured by powder metallurgy contains chromium, it has oxidation resistance and good electrical conductivity even at a high temperature (for example, 800 ° C. to 1000 ° C.). Products manufactured by powder metallurgy are not particularly limited, and examples include products such as welding rods, vacuum deposition materials, and fuel cell interconnectors.

  Note that the present invention is not limited to being embodied in the above-described embodiment, and can be variously modified without changing the gist of the present invention.

  For example, in the above embodiment, metal chromium is obtained by the electric furnace reduction method, but metal chromium can also be obtained by the aluminum thermit method.

  In the above embodiment, the ingot of metal chromium having a chromium content of 90% or more is pulverized, but the ingot of ferrochrome such as low carbon ferrochrome can be pulverized. Ferrochrome is produced by melting chromium iron ore, carbon, and calcined lime in an electric furnace, pouring hot water, and refining outside the furnace.

  In the above embodiment, the metal chromium powder is used as a raw material for the interconnector of the fuel cell, but it can also be used for a chromium copper (Cr—Cu) alloy for automobiles.

(Example)
Chromium oxide was reduced with metallic silicon in an electric furnace, and the molten metal was poured into a mold. The ingot of cooled and solidified metal chromium was coarsely pulverized with a crusher and then finely pulverized with a pin mill. The pulverization was carried out in two portions with the same pin mill. The rotational speed of the pin mill disk was 5500 rpm. After pulverization, the metal chromium powder was sieved to 45 μm or more and 200 μm or less.

(Comparative example)
The ingot of metal chromium produced by the silicon reduction method as in the example was roughly pulverized by the same crusher as in the example. Then, it pulverized using a vibro mill. The pulverization was performed twice in the same vibro mill. After pulverization, the metal chromium powder was sieved to 45 μm or more and 200 μm or less.

(Comparison of powder shape between Example and Comparative Example)
A secondary electron image (SEI) was taken using an electron beam microanalyzer (EPMA, JAX-8200 manufactured by JEOL Ltd.). FIG. 6 shows an image of the example, and FIG. 7 shows an image of the comparative example. As shown in FIGS. 7 (a) and 7 (b), the metal chromium powder of the comparative example is flat, whereas the metal chromium powder of the embodiment is not flat as shown in FIGS. 6 (a) and 6 (b). It became a shape.

  Further, in the comparative example as shown in the enlarged view of FIG. 7 (c), inclusions adhere to the metal chromium powder even when sieving, whereas in the example as shown in FIG. 6 (b), sieving is performed. By this, inclusions adhering to the metal chromium powder were removed. When finely pulverized with a vibromill as in the comparative example, the inclusions are crushed into the particles, and it is assumed that the inclusions cannot be removed even by sieving.

(Measurement of powder characteristics)
The fluidity, apparent density, and tap density of the metal chromium powder of the example were measured. Also, the Rattler value was measured by the Rattler test method. The results are shown in Table 1. It was found that the metal chromium powder of the example had a low Rattler value and had good moldability.

The fluidity, apparent density, tap density, compression density, and Rattler value of the metal chromium powder of the comparative example (fine pulverization with vibromill) were measured. The results are shown in Table 2. It was found that the Latler value of the metal chromium powder of the comparative example was high and inferior in formability.

Samples of cylindrical shaped bodies were made from the metal chromium powder of the example. The molding conditions are as shown in Table 3 below. When the compression density of the sample was calculated, a large compression density of 6 g / cc or more was obtained as shown in the calculation density column of Table 3.

(Quality comparison between pin mill grinding and hammer mill grinding)
The quality (compressive strength, Rattler value) of the metal chromium powder was compared between when pulverized with a pin mill and when pulverized with a hammer mill. As shown in FIG. 8, when pulverizing with a pin mill, the compression density is higher and the Rattler value is lower than when pulverizing with a hammer mill. Even when pulverized with a hammer mill, sufficient compressibility and moldability that can be used as a raw material for powder metallurgy can be obtained.

1, 13 ... disk 2, 14 ... pin 23 ... rotor 24 ... hammer

Claims (4)

Chromium-containing metal is finely pulverized by an impact pulverization mill,
The chromium-containing metal powder finely pulverized by the impact-type pulverization mill is sieved to a predetermined particle size or more and less than a predetermined particle size,
Utilizing a chromium-containing metal powder that has been screened to a predetermined particle size or more and less than a predetermined particle size as a raw material for powder metallurgy,
A method for producing chromium for powder metallurgy, in which a chromium-containing metal powder screened to a particle size of less than a predetermined particle size or larger than a predetermined particle size is mixed with a chromium-containing metal powder pulverized by a vibromill to form a briquette.   2. The method for producing chromium for powder metallurgy according to claim 1, wherein the chromium-containing metal is produced by roughly pulverizing an ingot of chromium-containing metal that is discharged from a furnace and solidified by cooling. 3. First pulverizing the chromium-containing metal using a first impact grinding mill,
The method for producing chromium for powder metallurgy according to claim 1 or 2, wherein the finely pulverized fine particles are further finely pulverized by using a second impact pulverizing mill.   The method for producing chromium for powder metallurgy according to any one of claims 1 to 3, wherein the chromium-containing metal is obtained by reducing chromium oxide in an electric furnace.