JP2015151586A - Method for producing sintered metal component - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a production method capable of stably producing a sintered metal component having high strength.SOLUTION: In the method for producing a sintered metal component including: a compressive molding step S2 of obtaining the green compact of metal powder; and a sintering step S3 of heating the green compact to obtain a sintered compact, before the performance of the compressive molding step S2, a surface modifying step S1 of performing surface modifying treatment such as reduction treatment, plasma treatment or the like of removing a passive state and its film formed on the surface of the metal powder is provided.

Description

  The present invention relates to a method for manufacturing a sintered metal part.

  In recent years, mainly for cost reduction, various machine parts such as automobile parts are sintered products (sintered metal parts) obtained by heating and sintering metal powder compacts from so-called molten products. Is being replaced. However, since sintered metal parts are porous bodies having innumerable internal pores, they are often inferior to molten products made of the same kind of material in terms of strength. Therefore, the scope of application of sintered metal parts is still limited. Accordingly, various attempts have been made to increase the strength of sintered metal parts in order to expand the application range of sintered metal parts.

  By the way, the sintered metal part is a sintered body by manufacturing a metal powder, a compression molding process for obtaining a green compact of a metal powder (a mixed powder obtained by adding an appropriate filler to the metal powder), and heating the green compact. For example, when manufacturing an iron-based sintered metal part, the metal powder surface (surface layer part) is subjected to metal before the sintering process is performed. Passivities such as oxides, hydroxides, or oxyhydroxides and films thereof (hereinafter simply referred to as “passive films”) are formed. This passive film inhibits the formation of a neck between metal powders in the initial stage of the sintering process. That is, the passive film is a factor that hinders the strength of the sintered metal part from being increased. Such a problem can be avoided as much as possible by employing the technical means disclosed in Patent Documents 1 and 2 below.

  Patent Document 1 discloses a method for manufacturing a sintered metal part in which raw material powder obtained by adding scaly graphite powder and earthy graphite powder in a predetermined ratio to iron-based powder is compression-molded and sintered. Soil-like graphite has lower crystallinity than scale-like graphite, and is rich in reactivity in a temperature range of 900 ° C. or lower where carbon solid-phase diffusion is activated. Therefore, if earth powder is included in the raw material powder, the passive film present on the surface layer of the iron-based powder can be reduced and removed as the green compact is heated and sintered. Thereby, the neck intensity | strength of iron type powder can be raised and a high intensity | strength sintered metal component can be obtained.

  Patent Document 2 discloses a metal containing zirconium (Zr) and silicon (Si) in a predetermined ratio, and the balance including at least one selected from the group consisting of iron (Fe), cobalt (Co), and nickel (Ni). And compression molding and sintering of metal powder composed of inevitable elements. Since Zr and Si function as a deoxidizing agent for removing oxygen, it is possible to improve the sinterability by removing oxides that hinder the sintering of the metal powder.

Japanese Patent Laid-Open No. 2-185944 JP 2012-7205 A

  However, in the technique disclosed in Patent Document 1, it is necessary to use a raw material powder having a high volume ratio of carbon powder. Since the carbon powder usually disappears as the green compact is heated and sintered, voids are formed in the sintered body where the carbon powder was present at the green compact stage. The Therefore, if the raw material powder having a high volume ratio of the carbon powder is used, the relative density of the sintered body is lowered. This may exceed the neck strength improvement enjoyed by reduction and removal of the passive film, resulting in a decrease in strength of the sintered metal part.

  Further, according to the technical means disclosed in Patent Document 2, although the oxide of the base material component that hinders the improvement of the sinterability can be removed, a newly generated oxide containing Zr or Si (new oxide) ) Remains in the sintered body. This new oxide is an impurity and may be a starting point of destruction. Therefore, it is difficult to say that the technical means of Patent Document 2 is a suitable technical means for stably obtaining a high-strength sintered metal part with high reliability.

  In view of the above circumstances, a main object of the present invention is to increase the neck strength between metal powders, and to stably manufacture a sintered metal component having high strength and high reliability.

  The present invention created to achieve the above object is a sintered metal part including a compression molding process for obtaining a green compact of a metal powder and a sintering process for obtaining a sintered body by heating the green compact. In this manufacturing method, prior to the compression molding step, the surface modification treatment for removing the passivation of the metal powder surface and the film thereof is performed.

  In this way, in the compression molding process, the passive surface and its film (hereinafter, also simply referred to as “passive film”) are removed by the surface modification treatment described above, so that a new surface (active surface) is exposed. Metal powder (hereinafter, this powder is also referred to as “surface modified powder”) can be used, and the sintering process is substantially performed on the green compact of the surface modified powder. Can do. Therefore, in the sintering process, since the sintering proceeds in a state where the new surfaces are in contact with each other, a high-strength sintered body in which the neck strength between the powders is increased can be obtained. In addition, since it is not necessary to use a mixed powder containing a large amount of carbon powder as a filler or a metal powder containing a component that has a high possibility of remaining in the sintered body as an impurity, it is possible to do so. Product quality is stable. Therefore, it is possible to stably manufacture a sintered metal part having high strength and high reliability.

  As said surface modification process, the method of removing the passive film of a powder surface layer part by making a medium (abrasive grain) collide with metal powder, for example is employable. However, if it does in this way, since work hardening will arise in the new surface exposed with the removal of a passive film, surface modification powder becomes difficult to carry out plastic deformation at the time of compression molding. That is, it becomes difficult to compression-mold the raw material powder at a high density, and it becomes difficult to obtain a high-strength sintered metal part. Therefore, as the surface modification treatment, a method that does not cause work hardening on the new surface accompanying this treatment is preferable, and specifically, reduction treatment or plasma treatment is preferably employed.

The above reduction treatment can be broadly classified into wet and dry processes, and either wet or dry processes may be adopted. As a wet reduction treatment method, for example, a method of crushing and removing the passive film by immersing the metal powder in the reducing solution and stirring the reducing solution can be employed. As the reduction method of a dry, for example, it is possible to employ a method of removing the passivation film by heating the metal powder was placed in a reducing atmosphere to a temperature above the A ₁ transformation point (reduction annealing method) .

  On the other hand, as the above plasma treatment, for example, a metal powder placed in a vacuum chamber is irradiated with Ar gas or the like gasified by plasma (ion bombardment method), or for discharge plasma sintering (SPS). It is possible to employ a method of subjecting the metal powder filled in the mold to SPS treatment under predetermined conditions.

  By the way, when the surface-modified powder is exposed to the atmosphere, the new surface gradually re-oxidizes and a new passivation is formed and grows, so that the sinterability gradually decreases. For this reason, in order to prevent the deterioration of sinterability over time as much as possible, it is desirable to take measures such as storing the surface-modified powder in a vacuum and compressing it under vacuum. Such a measure is difficult to say in view of workability and the like. On the other hand, if the protective layer forming process that prevents reoxidation of the new surface is performed after the surface modification process and before the compression molding process, the surface modified powder can be handled easily and better. Sinterability can be ensured.

  As long as the protective layer has a function to prevent reoxidation of the new surface, it can be adopted without any problem. However, if the protective layer has a lubricating component, it is possible to improve the compression moldability in the compression molding process and to increase the density of the powder. It is preferable to obtain a body because, in turn, a high-strength sintered metal part can be obtained. However, it is not preferable that the protective layer remains over the entire period when the sintering process is performed because the protective layer can be an obstacle to formation of a neck bond. Therefore, it is preferable that the protective layer is formed mainly of a substance that disappears at the initial stage of the sintering process when the sintering process is performed.

  In the manufacturing method described above, it is possible to further provide a strengthening step for subjecting the sintered body to a strengthening treatment. There are no particular restrictions on the method of strengthening treatment, and depending on the type of metal powder used and the required mechanical properties, for example, heat treatment such as carburizing, quenching and tempering, shot peening and cold working (cold working) One or more types of plastic working such as forging) can be employed. Note that, depending on the method of increasing the strength selected, the surface roughness of the sintered body increases, and there is a possibility that a stress concentration point (breaking origin) may occur. In such a case, it is preferable to perform a finishing treatment for reducing the surface roughness of the sintered body after the high-strength treatment. There is no particular limitation on the method of finishing treatment, and for example, one kind or plural kinds of machining such as grinding and polishing, plastic working such as sizing, rolling, and burnishing, electric discharge machining, and laser machining can be employed.

  The present invention relates to a sintered powder made of a sintered metal powder mainly composed of at least one metal selected from the group consisting of iron, stainless steel, copper, aluminum and titanium. This can be preferably applied when manufacturing a metal part. That is, the present invention can be preferably applied particularly when producing iron-based, stainless-based, copper-based, aluminum-based or titanium-based sintered metal parts.

  As described above, according to the present invention, a sintered metal component having high strength and high reliability can be stably manufactured.

It is a perspective view which shows an example of a sintered metal component. It is a principal part expanded sectional view of FIG. It is process drawing which shows one Embodiment of this invention. It is a principal part expanded sectional view of the sintered metal component which concerns on a modification. It is process drawing which shows other embodiment of this invention. It is a figure which shows the test result of a comparative test.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

  FIG. 1 shows an example of a sintered metal part A manufactured using the manufacturing method according to the present invention. A sintered metal part A shown in FIG. 1 is a gear 1 made of, for example, a sintered body of iron-based powder and having tooth surfaces 2 in which concave portions and convex portions are alternately arranged on the outer peripheral portion. As shown in FIG. 2, a hardened layer 3 formed by a strengthening process after sintering is provided on the surface layer portion of the gear 1. Further, the hardness of each part of the gear 1 is highest on the surface, and gradually decreases toward the core part (central part in the thickness direction). Therefore, the mechanical strength, wear resistance, and the like of the tooth surface 2 are the highest among the parts constituting the gear 1.

  As shown in FIG. 3, the gear 1 as the sintered metal part A is manufactured through a surface modification step S1, a compression molding step S2, a sintering step S3, and a high strength step S4 in this order. Hereinafter, each process is explained in full detail.

[Surface modification step S1]
In the surface modification step S1, for the metal powder that is the base material of the sintered metal component A (in this embodiment, iron-based powder containing iron as a main component), the metal oxide formed on the surface layer portion, etc. A treatment for removing the passivation and its film (hereinafter referred to as passive film) is carried out, thereby obtaining a metal powder (hereinafter also referred to as surface-modified powder) in which a new surface (active surface) is exposed.

  As a specific method of the surface modification treatment, for example, the metal powder is immersed in a reducing solution having a reducing action such as a rust remover or a reducing agent, and then the reducing solution is agitated. A so-called wet reduction treatment in which the passive film formed on the surface layer portion is crushed and removed to expose a new surface (active surface) can be employed. The reducing solution in which the metal powder is immersed may be stirred manually, but is preferably stirred using an ultrasonic generator such as an ultrasonic homogenizer. This is because the metal powder is smoothly flowed to prevent the powders from aggregating as much as possible, and the passive film of each metal powder can be efficiently crushed and removed.

  After the surface modified powder is taken out from the reducing solution, it may be subjected to a drying treatment for forcibly removing the liquid component adhering to the surface. The drying process may be performed in the atmosphere, but if this is done, a new passive film is likely to be formed on the surface (new surface) of the surface-modified powder along with the drying process. Therefore, in order to reduce the possibility that a new passive film is formed on the new surface, the drying treatment is preferably performed under vacuum or in an inert gas atmosphere.

[Compression molding step S2]
In this compression molding step S2, a mixed powder obtained by adding and mixing an appropriate filler to the surface-modified powder is compression-molded with a molding die, thereby substantially completing the shape corresponding to the target shape, here the shape of the gear 1. A compact green compact is obtained. As the filler, for example, a solid lubricant can be preferably used. This is to reduce the frictional force between the powders during compression molding and the frictional force between the powder and the inner wall surface of the mold, thereby improving the compression moldability and obtaining a high-density green compact.

  There is no particular limitation on the solid lubricant that can be used, for example, a metal soap represented by zinc stearate or calcium stearate, or a powder containing an amide wax represented by stearic acid amide or ethylenebisstearic acid amide, One or more selected from the group of carbon powder, powder containing molybdenum disulfide, and the like can be used. Among these, since the melting temperature (melting point) is as low as about 70 ° C. to 150 ° C., it melts and volatilizes in the initial stage of the sintering step S3 to be described later, affecting the neck formation between the metal powders (surface modified powders). Particular preference is given to powders containing no metal soap or amide wax.

  The molding pressure at the time of compression molding is set to a pressure at which the surface-modified powder is plastically deformed and can increase the contact area between adjacent surface-modified powders, for example, 980 MPa or more. Thereby, a high-density green compact in which the surface-modified powders are firmly adhered to each other is obtained.

[Sintering step S3]
In this sintering step S3, the green compact obtained in the compression molding step S2 is placed in an inert gas atmosphere or in a vacuum, and at a temperature equal to or higher than the sintering temperature of the main component metal (here, iron). Heat for hours. Thereby, the solid lubricant contained in the green compact is melted and vaporized, and a sintered body formed by sintering bonding (neck bonding) between adjacent surface modified powders is obtained. In addition, it is preferable that the heating temperature of the green compact of this embodiment formed mainly with iron-based powder is 1200 ° C. or higher and 1300 ° C. or lower. When the green compact is heated below 1200 ° C (for example, 1120 ° C, which is a temperature for forming a general iron-based metal sintered body), the surface-modified powders are sintered with sufficient neck strength. This is because the strength improvement effect is saturated when the temperature exceeds 1300 ° C.

[Strengthening step S4]
In this strengthening step S4, the sintered body obtained in the sintering step S3 is subjected to an appropriate strengthening process to form a hardened layer 3 (see FIG. 2) on the surface layer portion of the sintered body. Thereby, the mechanical strength and wear resistance of the gear 1 as the sintered body and, as a result, the sintered metal part A, are further enhanced. There is no particular restriction on the method of increasing the strength, and heat treatment such as carburizing quenching, submerged quenching, induction quenching, or plastic working such as shot peening or cold working (cold forging) can be employed. When heat treatment is adopted as the strengthening treatment, it is preferable to perform a tempering treatment after quenching in order to impart high toughness to the sintered body (gear 1).

  If there is no problem in terms of cost, both the heat treatment and the plastic working may be performed on the sintered body. In this case, it may be appropriately selected which heat treatment or plastic working is performed first. The same applies to other embodiments described later. As described above, when both the heat treatment and the plastic working are performed on the sintered body, the hardened layer 3 formed on the surface layer portion of the sintered body (gear 1) is subjected to heat treatment or A first hardened layer 3a formed by any one of the plastic processing (for example, heat treatment) and a second hardened layer 3b formed by the other processing (for example, plastic processing), Moreover, the second hardened layer 3b is formed so as to overlap the surface layer portion of the first hardened layer 3a. Therefore, the sintered body, and thus the gear 1 as the sintered metal part A can be further strengthened.

  Through the steps described above, the gear 1 as the sintered metal part A shown in FIGS. 1 and 2 is completed. When the strengthening process is performed in the strengthening process S4, the surface roughness of the sintered metal part A increases depending on the type of the process to be selected, and there is a possibility that a stress concentration point (breaking start point) may occur. If such an increase in surface roughness is a problem, mechanical processing such as grinding, polishing, lapping, super-finishing, plastic processing such as sizing, rolling, burnishing, electric discharge processing, laser processing, etc. You may further provide the finishing process which implements 1 type, or 2 or more types of finishing processing selected from a group with respect to the sintered compact after a high strengthening process.

  As described above, according to the present invention, the compression molding step S2 is performed in the metal powder having a new surface exposed by removing the passive film on the surface portion of the metal powder by surface modification treatment (reduction treatment) ( In addition, the sintering step S3 can be performed on the green compact of the surface-modified powder. Therefore, in the sintering step S3, since the sintering proceeds in a state where the active surfaces are in contact with each other, a high-strength sintered body in which the neck strength between the powders is increased can be obtained. In this way, it is not necessary to use raw material powder containing a large amount of carbon powder or metal powder containing components that are likely to remain inside the sintered body as impurities, so the product quality is stable. To do. Therefore, the gear 1 as the sintered metal part A having high strength and high reliability can be stably manufactured and mass-produced.

  As the treatment method employed in the surface modification step S1, for example, a method of removing the passive film on the powder surface layer by colliding a medium (abrasive grains) with the metal powder can be employed. Since work hardening occurs on the new surface that is exposed as a result of the removal, the surface-modified powder is difficult to be plastically deformed during compression molding. That is, it becomes difficult to compression-mold the surface-modified powder with high density, and it becomes difficult to stably mass-produce the high-strength sintered metal part A. On the other hand, in the present invention, since the passive film is removed by adopting a reduction treatment method which is a kind of treatment method in which work hardening does not occur on the new surface, it is appropriate to increase the strength of the sintered metal part A. Can be realized.

  As mentioned above, although one Embodiment of the manufacturing method of the sintered metal component A based on this invention was described, this invention is not limited to this, A various change can be given.

  For example, in the above-described embodiment, the high-strength step S4 is performed after the sintering step S3. If a desired mechanical strength is obtained in the stage after the sintering step S3, the high-strength step S4 is performed. The strengthening step S4 may be omitted.

  In addition, as shown in FIG. 5, a protective layer forming step S1 'for covering the surface (new surface) of the surface modified powder may be provided between the surface modifying step S1 and the compression molding step S2. In this way, it is possible to prevent as much as possible a situation in which the surface of the surface modified powder is reoxidized and the sinterability is lowered while facilitating the handling of the surface modified powder.

  The protective layer can be used without any problem as long as it has a function of preventing reoxidation of the new surface exposed by the surface modification treatment. However, if the protective layer has a lubricating component, the surface modification in the compression molding step S2 is possible. It is preferable to obtain a high-density green compact by improving the compression moldability of the powder, which in turn can provide a sintered metal part A having high strength. However, it is not preferable that the protective layer remains over the entire period when the sintering step S3 is performed because the protective layer can be an obstacle to formation of a neck bond. Therefore, the protective layer is preferably formed mainly of a material that disappears at the initial stage of the sintering step S3 (a material that volatilizes and decomposes at a temperature lower than the heat treatment temperature during sintering) during the sintering step S3. .

  Further, in the embodiment described above, by adopting a so-called wet reduction treatment method in the surface modification step S1, a surface modified powder having a new surface exposed is obtained. In the surface modification step S1, The processing method that can be adopted is not limited to this.

For example, it is possible to obtain surface-modified powder using a reduction annealing method which is a kind of dry reduction treatment method. The reduction annealing method, Briefly, a method of removing the passivation film by heating the metal powder was placed in a reducing atmosphere to a temperature above the A ₁ transformation point. The surface-modified powder can be obtained not only by reduction treatment but also by plasma treatment. Examples of the plasma treatment that can be employed include a method of irradiating a metal powder placed in a vacuum chamber with argon gas, nitrogen ion, hydrogen ion, etc. gasified by plasma (ion bombardment method), or discharge plasma sintering. Examples thereof include a method of subjecting the metal powder filled in the (SPS) mold to SPS treatment under predetermined conditions. If dry reduction treatment or plasma treatment is adopted, there is no advantage of drying treatment that may be performed afterwards when wet reduction treatment method is adopted, so that the process can be simplified. is there.

  Further, in the compression molding step S2, a molding die in which a lubricant is applied to a partial area or the entire area of the cavity inner wall surface, or a DLC film or a nitride film on a partial area or the entire area of the cavity inner wall surface, etc. Compression molding may be performed using a molding die on which a low friction hard coating is formed. In this way, the frictional force between the surface modified powder and the inner wall surface of the cavity can be reduced, so that the amount of filler (particularly solid lubricant) added to the surface modified powder can be reduced or the surface modified. It is possible to omit the addition of a solid lubricant to the powder. Thereby, since the ratio of the metal powder (surface modified powder) to the mixed powder increases, a high-density green compact, and thus a high-strength sintered metal part A can be obtained.

  Moreover, you may implement the compression molding implemented by compression molding process S2 using the shaping | molding die and / or surface modification powder which were heated at about 50-200 degreeC, for example. By adopting such a so-called warm forming method, the surface-modified powder is easily plastically deformed (the compression moldability is increased). Therefore, a high-density green compact, and thus a high-strength sintered metal part A is formed. It becomes easy to obtain.

  In the above, the present invention is applied when the sintered metal part A is obtained using the metal powder (iron-based powder) containing iron as a main component. However, the present invention forms a passive film on the surface. The present invention can also be preferably applied to the case where the metal sintered body A is obtained using other metal powders such as stainless steel powder, copper powder, aluminum powder, or titanium powder. Further, the present invention provides a sintered metal part A using a premix powder in which two or more selected from the group of iron-based powder, stainless-based powder, copper-based powder, aluminum-based powder, titanium-based powder and the like are mixed in advance. It can be preferably applied also when obtaining

  In the above description, the present invention is applied when the gear 1 as the sintered metal part A shown in FIG. 1 is manufactured. However, the present invention is applied to other mechanical parts made of a sintered body (metal sintered body), for example, a cam. It can also be preferably applied to the manufacture of bearings and bearings.

In order to demonstrate the usefulness of the present invention, a ring-shaped test piece prepared by applying the present invention (Examples 1 to 13) and a ring-shaped test piece manufactured without applying the present invention (Comparative Examples 1 to 3) ) The crushing strength for each is calculated by a method in accordance with the provisions of JIS Z 2507, and “D”, “C”, “B”, “A ⁻ ”, shown below, according to the obtained crushing strength values. The crushing strength of each test piece was evaluated in six stages of “A ⁺ ” and “S”. The compressive force in the direction of diameter reduction to be applied to the test piece when calculating the crushing strength was applied using an autograph AG-5000A manufactured by Shimadzu Corporation.
D: less than 1000 MPa C: 1000 MPa or more 1200MPa less B: 1200MPa or more 1400 MPa less than A ^-: 1400 MPa or more 1500 MPa less than A ^+: 1500 MPa or more 1600MPa less S: 1600MPa or more

  The ring-shaped test pieces according to Examples 1 to 13 and Comparative Examples 1 to 3 were produced as follows.

[Example 1]
First, a prealloy type iron-based powder containing 0.5% by mass and 1.0% by mass of nickel (Ni) and molybdenum (Mo), respectively, and the balance being iron (Fe) and inevitable impurities (Kobe Steel Corporation) After the oxide film removing agent as a reducing solution in which 46F4H) was immersed was manually stirred for 1 minute, the iron-based powder taken out from the reducing solution was washed with ethanol. Thereby, the passive film of the surface layer portion was removed, and an iron-based powder (surface modified powder) in which a new surface was exposed was obtained. Next, 0.5% by mass of an amide wax solid lubricant was added to the surface-modified powder, and these were mixed by a V-type mixer to produce a mixed powder. Next, this mixed powder was filled in a molding die and pressed in the axial direction at a molding pressure of 980 MPa to obtain a ring-shaped green compact having an outer diameter of 23 mm × inner diameter of 16 mm × height of 7 mm. Finally, the ring-shaped green compact was heated and sintered at 1250 ° C. in a nitrogen / hydrogen mixed gas atmosphere to obtain a ring-shaped test piece as Example 1.
[Example 2]
A ring-shaped test piece according to Example 2 is produced in the same manner as the test piece according to Example 1 except that a mixed powder to which 0.5% by mass of artificial graphite powder as carbon powder is further added is used. did.
[Example 3]
A ring-shaped test piece according to Example 3 was obtained by performing carburizing, quenching and tempering on the test piece according to Example 2.
[Example 4]
A ring-shaped test piece according to Example 4 was obtained by subjecting the test piece according to Example 3 to shot peening using an iron-based medium having an average particle diameter D50 = 180 μm.
[Example 5]
The ring-shaped test piece which concerns on Example 5 was obtained by performing burnishing processing to the part roughened by shot peening among the test pieces which concern on Example 4.
[Example 6]
The test piece according to Example 3 is prepared except that the stirring work of the oxide film removing agent in which the iron-based powder is immersed is performed using an ultrasonic generator (ultrasonic homogenizer) instead of human power. In the same manner as described above, a ring-shaped test piece according to Example 6 was produced.
[Example 7]
Powder obtained by spraying the above-described solid lubricant onto the above-mentioned surface-modified powder and attaching the solid lubricant to the surface of the surface-modified powder so that the surface of the surface-modified powder is coated with a lubricating film as a protective layer Got. A ring-shaped test piece according to Example 7 was prepared in the same manner as the test piece according to Example 3 except that this powder was used.
[Example 8]
The iron-based powder was held in a nitrogen / hydrogen mixed gas atmosphere at 800 ° C. for 60 minutes and then slowly cooled (by subjecting the iron-based powder to reduction annealing) to obtain a surface-modified powder. Thereafter, a ring-shaped test piece according to Example 8 was produced in the same manner as the test piece according to Example 3.
[Example 9]
Surface-modified powder was obtained by subjecting the iron-based powder to plasma treatment for 1 hour using a plasma irradiation apparatus. Thereafter, a ring-shaped test piece according to Example 9 was produced in the same manner as the test piece according to Example 3.
[Example 10]
After the iron-based powder is put into the cavity of the sintering mold set in the discharge plasma sintering apparatus, the iron-based powder is pressed in the axial direction at a surface pressure of 2 MPa in vacuum for a predetermined time at 800 ° C. By holding this, the above-mentioned surface modified powder was obtained. Thereafter, a ring-shaped test piece according to Example 10 was produced in the same manner as the test piece according to Example 3.
[Example 11]
As the raw material powder, a material obtained by reducing the amount of the solid lubricant added to the surface modified powder to 0.2% by mass and using the raw material powder as an ethanol solution in which the solid lubricant is dissolved is used. Compression molding was performed using a molding die applied to the inner wall surface of the cavity. A ring-shaped test piece according to Example 11 was produced in the same manner as the test piece according to Example 3 except for this.
[Example 12]
The ring shape according to Example 12 is the same as that for producing the test piece according to Example 3 except that the raw material powder is compression molded using a molding die having a DLC film (lubricating film) formed on the inner wall surface of the cavity. A test piece was prepared.
[Example 13]
A ring-shaped test piece according to Example 13 was produced in the same manner as the test piece according to Example 3 except that the raw material powder was compression-molded while the molding die was heated to 100 ° C.
[Comparative Examples 1-3]
Among the preparation processes of the test pieces according to Examples 1 to 3, except for omitting the removal process of the passive film, Comparative Examples 1 to 3 are performed in the same manner as the preparation of the test pieces according to Examples 1 to 3. Each such ring-shaped test piece was produced.

  The production conditions for each of the test pieces according to Examples 1 to 13 and Comparative Examples 1 to 3 described above and the evaluation of the crushing strength of each test piece are collectively shown in FIG. As is clear from the figure, all of the test pieces according to Examples 1 to 13 have higher crushing strength, that is, mechanical strength than the test pieces according to Comparative Examples 1 to 3. Therefore, it is possible to obtain a green compact and further a sintered body by using a metal powder that has been subjected to a surface modification treatment to remove the passive film formed on the surface (surface layer portion). It is understood that it is useful for increasing the strength of the steel.

  Further, from the determination results of Example 1 and Example 2, it is advantageous to increase the strength of the sintered metal part by performing compression molding after adding carbon powder to the surface-modified powder. From the judgment results of Example 2 and Examples 3 and 4, it is advantageous to further increase the strength of the sintered metal part by subjecting the sintered body to heat treatment such as carburizing, quenching and tempering, and further to plastic processing such as shot peening. It is understood that there is. Further, from the determination results of Example 3 and Example 6, stirring of the reducing solution in which the metal powder was immersed was performed by ultrasonic vibration when the metal powder was subjected to wet reduction treatment as surface modification treatment. It is understood that it is advantageous to increase the strength of the sintered metal part by using a container. Furthermore, from the determination results of Example 3 and Examples 11 to 13, it is possible to apply so-called mold lubrication molding method and warm molding method at the time of compression molding of raw material powder (green compact). It is understood that it is advantageous in increasing the strength.

1 Gear (sintered metal parts)
2 Fracture surface 3 Hardened layer A Sintered metal part S1 Surface modification process S1 'Protective layer formation process S2 Compression molding process S3 Sintering process S4 Strengthening process

Claims (9)

In a method for producing a sintered metal part, comprising a compression molding step of obtaining a green compact of a metal powder, and a sintering step of obtaining a sintered body by heating the green compact,
Prior to performing the compression molding step, a surface reforming process is performed to remove the passivation formed on the surface of the metal powder and its film, and a method for producing a sintered metal part.   The method for manufacturing a sintered metal part according to claim 1, wherein the surface modification treatment is a reduction treatment.   The method for manufacturing a sintered metal part according to claim 1, wherein the surface modification treatment is a plasma treatment.   A process of forming a protective layer that covers the new surface exposed on the metal powder by removing the passivation and the film after the surface modification treatment and before the compression molding step. Item 4. A method for producing a sintered metal part according to any one of Items 1 to 3.   The method for producing a sintered metal part according to claim 4, wherein the protective layer has a lubricating component.   The said protective layer is a manufacturing method of the sintered metal component of Claim 4 or 5 which has as a main component the compound which volatilizes when the said green compact is heated.   The manufacturing method of the sintered metal component as described in any one of Claims 1-6 which further has the strengthening process which performs the strengthening process to the said sintered compact.   The manufacturing method of the sintered metal component of Claim 7 which further has a finishing process which reduces the surface roughness of the secondary sintered compact obtained by the said high strengthening process.   The method for producing a sintered metal part according to any one of claims 1 to 8, wherein the sintered body is mainly composed of at least one metal selected from the group consisting of iron, stainless steel, copper, aluminum, and titanium. .