JP2016056445A - Mixed powder for powder metallurgy, sintered metal component using the same, and method for producing the mixed powder for powder metallurgy - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide mixed powder for powder metallurgy capable of obtaining a sintered metal component having homogeneity and high density, in which the fluidity of the mixed powder is increased for increasing powder packing properties.SOLUTION: Provided is mixed powder for powder metallurgy containing iron-based powder and carbon powder, being the powder subjected to stirring and mixing under the conditions (the number of revolution and stirring time) where fluidity reaches below 30 s/50 g after charing to a mixing machine, in which the Vickers hardness (Hv) of the iron-based powder is 100 or lower, the particle diameter of the carbon powder D90 is 4 to 10 μm, and a sintered metal component formed using the mixed powder has a density of 7.6 g/cm3 or higher.SELECTED DRAWING: Figure 3

Description

  The present invention relates to a mixed powder for powder metallurgy, a sintered metal part using the same, and a method for producing a mixed powder for powder metallurgy.

  In powder metallurgy for producing sintered bodies mainly composed of iron, as raw materials, in addition to iron-based powders as main raw material powders, graphite powders and alloys for improving physical properties such as strength and hardness of sintered bodies A mixed powder in which an auxiliary raw material powder containing components and a lubricant for ensuring moldability are mixed is used. This mixed powder has different powder characteristics such as shape, particle size, and specific gravity, etc., so that specific components (for example, graphite powder) are segregated in the mixed powder and the fluidity of the mixed powder is reduced. There is. When segregation occurs in the mixed powder, the density and mechanical properties vary between products, and quality stability is impaired. Moreover, when the fluidity | liquidity of mixed powder falls, there exists a possibility that the powder flowability from a hopper to a metal mold | die may deteriorate, and the powder filling property to a metal mold | die may be reduced.

  For example, in Patent Document 1 below, graphite is refined (average particle size of 4 μm or less), and iron-base powder and graphite are mixed while applying a shearing force to prevent segregation of graphite in the mixed powder. Techniques to do this are disclosed.

JP 2012-102355 A

  However, the above-mentioned Patent Document 1 does not show an active technique for improving the fluidity of the mixed powder. For this reason, the powder filling property to a metal mold | die falls by lack of the fluidity | liquidity of mixed powder, and the concern that the density of a green compact and by extension, a sintered compact is insufficient cannot be wiped off.

  In view of the above circumstances, the problem to be solved by the present invention is to provide a mixed powder for powder metallurgy capable of obtaining a homogeneous and high-density sintered metal part.

  Conventionally, the purpose of stirring a mixed powder for powder metallurgy (hereinafter also simply referred to as “mixed powder”) using a mixer is to homogenize the mixed powder, particularly to use fine carbon powder (graphite powder). It was to disperse | distribute uniformly in a component raw material powder. Therefore, it was sufficient to stir under the stirring conditions (rotation speed and stirring time) in which the mixed powder becomes homogeneous. According to the verification by the present inventors, by mixing and mixing the mixed powder under predetermined conditions, the iron-based powder is rounded (see FIG. 3), thereby improving the fluidity of the mixed powder (the fluidity is small). It became clear. Therefore, when the mixed powder is stirred and mixed, not only the mixed components such as carbon powder are uniformly dispersed, but also if the stirring conditions are set so that the fluidity of the mixed powder is less than a predetermined value, the mixed powder is homogeneous and fluid. A highly mixed powder can be obtained.

  The present invention has been made on the basis of the above knowledge, and specifically, is a mixed powder for powder metallurgy containing an iron-based powder and a carbon powder, and the flow rate is less than 30 s / 50 g. Provided is a mixed powder for powder metallurgy that is stirred and mixed. The present invention is also a method for producing a powder mixture for powder metallurgy comprising an iron-based powder and a carbon powder, wherein the powder mixture for powder metallurgy is mixed with stirring under conditions where the fluidity is less than 30 s / 50 g. A manufacturing method is provided.

  As described above, if the stirring condition of the mixed powder is appropriately set, the fluidity of the mixed powder can be greatly reduced by stirring and mixing. For example, the fluidity of the mixed powder for powder metallurgy can be reduced by stirring and mixing. The fluidity of the iron-based powder alone before stirring and mixing can be reduced by 9 s / 50 g or more.

  When the mixed powder is vigorously stirred, work hardening occurs in the powder due to collision between the powders or between the powder and the mixer. If the powder is too hard, the compressibility may be reduced, and the density and dimensional accuracy may be reduced. Therefore, in the present invention, the Vickers hardness (HV) of the iron-based powder is 100 or less, or the increase rate of the Vickers hardness (HV) of the iron-based powder before and after mixing is less than 20%. It is preferable to set stirring conditions.

  When the particle diameter D90 of the carbon powder blended in the mixed powder is 4 μm or more, it is not necessary to use a very fine carbon powder, and the material availability and handling of the carbon powder are improved. The particle size D90 refers to the particle size when the cumulative volume from the small diameter side of the particle size distribution on the volume basis of the carbon powder is 90%.

By the way, since the carbon powder contained in the mixed powder is solid-solved in the iron-based powder at the time of sintering, the place where the carbon powder was present becomes a void. Normally, the amount of carbon powder blended is extremely small (for example, 1 wt% or less). Therefore, when sintered metal parts have a relatively low density, voids are formed by solid solution of carbon powder as described above. There is almost no problem. However, when manufacturing a sintered metal part having a high density (for example, 7.6 / cm ³ or more), voids due to solid solution of the carbon powder as described above cannot be ignored. In such a case, it is effective to make the carbon powder finer and to make fine pores formed by solid solution of the carbon powder. Specifically, the particle size D90 of the carbon powder is preferably 10 μm or less.

  As the iron-based powder blended in the mixed powder, for example, a diffusion alloy powder in which Ni is diffused and adhered to an Fe—Mo alloy powder can be used.

  If the mixed powder for powder metallurgy is stirred and mixed by shear mixing, the iron-based powder is easily rounded, and the fluidity of the mixed powder can be improved efficiently. In this case, if the rotational speed of the mixer is too small, the iron-based powder is not sufficiently rounded and the fluidity increases. On the other hand, if the rotational speed of the mixer is too large, a portion of the iron-based powder is pulverized to generate a large amount of fine powder, and the fluidity increases due to the presence of this fine powder around the iron-based powder. Therefore, in order to make the fluidity less than 30 s / 50 g, it is preferable to set the rotational speed of the mixer within a predetermined range (for example, 2000 to 9000 rpm).

  As described above, since the mixed powder according to the present invention is homogeneous and has high fluidity, it is possible to uniformly and densely fill the cavity of a mold for compression molding. Therefore, if this mixed powder is used, a homogeneous and high-density sintered metal part can be obtained.

It is a graph which shows the particle size distribution of iron-based powder. It is a SEM photograph of the iron base powder before a mixing process. It is a SEM photograph of mixed powder (iron base powder) after mixing on the stirring conditions concerning the example of the present invention.

  Hereinafter, the mixed powder for powder metallurgy according to an embodiment of the present invention will be described.

  The mixed powder according to the present embodiment includes an iron-based powder as a main raw material powder and a carbon powder, and further includes a molding aid (a lubricant or a binder) as necessary. The mixed powder of this embodiment contains 90 wt% or more of iron-based powder and 1 wt% or less of carbon powder.

  In the iron-based powder, each particle has iron as a main component. As the iron-based powder, in addition to pure iron powder, an alloy powder obtained by adding an alloy component to iron as a main raw material in order to improve hardenability and mechanical properties can be used. As a pure iron powder, what was manufactured by the atomizing method, the reduction method, the stamp method, the carbonyl method etc. can be used, for example. In particular, in view of availability, cost, and high density formability, pure iron powder produced by the water atomization method is preferable.

  When alloy powder is used as the iron-based powder, as the alloy component, for example, one or more kinds of metals among Ni, Mo, Mn, and Cr can be used. As the alloy powder, for example, a complete alloy powder obtained by mixing iron and an alloy component in a molten state and solidified, or a diffusion alloy powder in which an alloy component is diffused and adhered to the surface of the iron powder or the iron alloy powder may be used. it can. In this embodiment, iron-based alloy powder containing Ni and Mo as alloy components and the balance being iron and inevitable impurities is used. Specifically, a diffusion alloy powder in which Ni is diffused and adhered to the surface of the Fe—Mo alloy is used. In this way, the hardness of the iron-based powder before sintering can be suppressed by diffusion-attaching a metal such as Ni to the Fe alloy, compared with the case where Fe and Ni are completely alloyed. Formability at the time is improved. As a result, a relatively large amount of Ni can be blended.

As the carbon powder, graphite powder, carbon black powder, fullerene powder, nanocarbon powder, or the like can be used. Of these, graphite powder is most preferable from the viewpoint of availability and cost. The particle diameter D90 of the carbon powder is 10 μm or less, preferably 8 μm or less. If the particle size of the carbon powder is too large, coarse pores are formed by solid solution of the carbon powder in the iron-based powder, so that a sintered metal part formed with the mixed powder (particularly 7.6 g / cm ³ or more) This is because the density of the ultra-high density sintered metal part) is reduced, and the strength is reduced (details will be described later). The particle size D90 of the carbon powder is 3 μm or more, preferably 4 μm or more. This is because, if the particle size of the carbon powder is too small, the material availability and the handleability deteriorate.

  The lubricant is added for the purpose of reducing the friction between the powders when the mixed powder is compression-molded or between the mold and the powder. As the lubricant, for example, metal soap or amide wax is used, and specifically, for example, ethylene bisstearylamide (EBS) is used. Alternatively, the lubricant is dispersed in the solution, and the dispersion is sprayed onto the iron-based powder, or the iron-based powder is immersed in the dispersion, and the solvent component is volatilized and removed, whereby the lubricant is added to the iron-based powder. May be coated. The binder is for ensuring moldability, and for example, a hydrocarbon resin, wax, polyvinyl alcohol, or the like can be used. The binder is used by being added to and sprayed on the main raw material powder and plays a role of increasing the bonding force between the powders. However, care should be taken because the addition of a binder causes a decrease in the density of the molded body. If not particularly necessary, it is not necessary to add a molding aid such as a lubricant or a binder to the mixed powder.

  Said mixed powder is manufactured by throwing iron-base powder, carbon powder, and lubricant into a container of a mixer and mixing them. In the present embodiment, a mixer that performs shear mixing is used, and preferably, a mixer that simultaneously performs convective mixing and shear mixing is used. A mixer that performs shear mixing is a mixer that applies shearing force to powder charged in a container. Such a mixer has a stirring member that moves while shearing the powder, and includes, for example, a stirring blade that rotates in a container, particularly a stirring blade that rotates at the bottom of the container. When mixing with a mixer having such a stirring blade, not only a shearing force is applied to the mixed powder by the rotation of the stirring blade, but also the entire mixed powder convects in the container. The container may be a fixed type or a rotary type. Thus, specific examples of a mixer that simultaneously performs convective mixing and shear mixing include a high-speed mixer (Henschel) and a multipurpose mixer (Nihon Coke).

  By mixing with the mixer as described above, the mixed powder is stirred, the carbon powder is uniformly dispersed in the mixed powder, and the iron-based powder is rounded to improve the fluidity of the mixed powder. Specifically, the carbon powder is uniformly dispersed in the mixed powder to such an extent that an aggregate of carbon powder having a major axis of 10 μm or more (preferably 5 μm or more) is not observed in the mixed powder. In the present embodiment, the fluidity is less than 30 s / 50 g, preferably less than 25 s / 50 g, by setting the stirring conditions (such as the number of rotations of the stirring member and the stirring time) within an appropriate range. For example, if the rotation speed of the stirring member is too small or the stirring time is too short, the iron-based powder is not sufficiently rounded and the fluidity cannot be set in the above range. On the other hand, if the rotation speed of the stirring member is too large or the stirring time is too long, the iron-based powder is partially crushed and a large amount of fine powder is generated, and this fine powder exists around the iron-based powder. Since fluidity | liquidity falls by this, a fluidity | liquidity cannot be made into the said range. Specific stirring conditions may be set as appropriate based on the above findings, but in this embodiment, the fluidity of the mixed powder is set by setting the rotation speed of the stirring member to 2000 to 9000 rpm and stirring for 10 to 60 minutes. Is within the above range.

  Further, by stirring the mixed powder with a mixer, the powders or the powder and the stirring blade collide with each other. Due to this collision, work hardening occurs in the powder, and the hardness of the powder increases. If the hardness of the powder becomes excessively high, the moldability at the time of compression molding is lowered, which is not desirable. In this embodiment, the Vickers hardness (HV) of the mixed powder is 100 or less, preferably 95 or less, or the increase rate of the Vickers hardness (HV) of the iron-based powder before and after mixing is less than 20%. The stirring conditions are preferably set to be less than 10%.

  By using the above mixed powder, sintered metal parts, for example, sintered machine parts such as gears and cams can be manufactured. Sintered metal parts are manufactured through a compression molding process in which the powder mixture is compressed with a mold to form a green compact, a sintering process in which the green compact is fired to obtain a sintered body, and a surface treatment process. Is done.

  In the compression molding process, first, the mixed powder stored in the hopper is poured into a mold cavity and filled. In this embodiment, since the mixed powder is homogeneous, the mixed powder is filled in the cavity in a homogeneous state. In addition, since the fluidity of the mixed powder is high (specifically, the fluidity is less than 30 s / 50 g), the mixed powder flows smoothly from the hopper to the mold, so the filling property to the cavity is high, The mixed powder can be filled in the cavity in a high density state.

  By compressing the mixed powder filled in the cavity from above and below with the upper and lower punches, a green compact having a predetermined shape is formed. At this time, since the mixed powder is uniformly and densely filled in the cavity, a compact powder having a uniform and high density can be obtained. Moreover, since the Vickers hardness (HV) of mixed powder is 100 or less, the moldability at the time of compression is improved and a compact with high dimensional accuracy can be obtained.

Thereafter, the green compact is fired at a predetermined temperature to obtain a homogeneous and high-density sintered body (that is, a sintered metal part). The sintering temperature is set, for example, within a range of 1000 to 1300 ° C. The sintering process is performed in an inert atmosphere, for example, in an atmosphere of a mixed gas of nitrogen and hydrogen, argon gas, or the like. During the sintering, the carbon powder is solid-dissolved in the iron-based powder, so that the hardness of the iron-based powder is increased, so that the mechanical properties of the sintered metal part are further improved. On the other hand, if the carbon powder is added excessively, the carbon powder dissolves in the iron-based powder, so that the place where the carbon powder is located becomes voids, so that the density of the sintered metal parts is reduced and the strength is reduced. Invite. In particular, a density of 7.6 g / cm ³ or more, further in the case of sintered metal parts of 7.62 g / cm ³ or more high-density, negligible relatively coarse voids formed by the solid solution of carbon powder Disappears, causing a decrease in strength. In the present embodiment, as described above, since the average particle diameter D90 of the carbon powder is 10 μm or less, the pores formed by the solid solution of the carbon powder are refined, and the strength reduction of the sintered metal part can be prevented. .

  After the sintering step, the sintered body is subjected to surface treatment without performing a recompression step. In the present embodiment, the sintered body is subjected to a carburizing quenching and tempering process. As a result, the hardness of the surface is increased and the internal toughness is secured, so that the propagation of cracks is suppressed. The surface treatment process is not limited to the above carburizing quenching and tempering, but various heat treatments such as submerged quenching and tempering, induction quenching and tempering, carbonitriding and vacuum carburizing, nitriding, soft nitriding, sulfurizing, diamond-like Various surface modifications such as formation of hard coatings such as carbon (DLC) and resin coatings, various platings, rust prevention treatments such as black dyeing and steam treatment, etc. are applicable. It is also possible. Note that the surface treatment step may be omitted unless particularly necessary. Moreover, you may give a recompression process after a sintering process. Thus, the sintered machine part according to the embodiment of the present invention is completed.

For the above sintered machine parts, the square root of the estimated maximum pore envelope area in the region of 30% or more from the surface layer when the depth at which the tensile stress extends in the depth direction from the surface layer is 100% is √area _max is 50 μm. Less than, preferably less than 45 μm, more preferably less than 40 μm (a method for calculating √area _max will be described later). For example, if the sintered machine part is a gear, calculate the depth of the tensile stress in the depth direction from the tooth surface, and if the sintered machine part is a cam, calculate the depth from which the tensile stress is applied in the depth direction. The relationship between the depth and the square root √area _max of the estimated maximum hole envelope area is in the above range.

  As an iron-based powder, 500 g of 2 wt% Ni, 1 wt% Mo, partially diffused alloy powder (JEF Steel Sigmaloy 2010, average particle size of about 100 μm, powder hardness of about HV90) with the balance being Fe and inevitable impurities did. The flow distribution of this iron-based powder was investigated by sieving, and the results are shown in FIG. To this iron-based powder, 0.2 wt% of graphite powder (TIMREX F-10 manufactured by TIMCAL) as carbon powder was added with respect to 100 wt% of the iron-based powder. This raw material powder was put into a mixer and mixed under different stirring conditions (number of rotations and time) to produce a plurality of types of mixed powders. And the following items were evaluated about each mixed powder (Example and comparative example). The evaluation methods and evaluation criteria for each item are as follows.

(1) Dispersibility of graphite The evaluation standard of the dispersibility of graphite is x when there is at least one graphite having a major axis of 10 μm or more in the field of view of 320 μm × 250 μm. Although it is dispersed throughout the main raw material powder, △ indicates that segregation of 5 to 10 μm is observed in the major axis, and ○ indicates that segregation of 5 μm or more is not observed because graphite is dispersed throughout the main raw material powder. It was.

(2) Fluidity The fluidity was in accordance with JIS Z2502 (metal powder fluidity test method). Specifically, the time until 50 g of the mixed powder flows out through the orifice of φ2.5 mm was measured, and this time was defined as the fluidity of the mixed powder. The evaluation criteria of fluidity are as shown in Table 1 below.

(3) Bulk density Bulk density was in accordance with JIS Z2504 (metal powder-apparent density test method). Specifically, the mixed powder was poured into a cup from a 2.5 mm orifice, and the density was measured from the weight of the mixed powder and the volume of the cup when worn. The evaluation criteria for the bulk density are as shown in Table 2 below.

(4) Powder hardness The powder hardness measured the Vickers hardness of the powder (especially iron-based powder) exposed to the surface by carrying out resin embedding and surface polishing of the mixed powder. The powder hardness is evaluated by the rate of increase in Vickers hardness before and after mixing, and the evaluation criteria are as shown in Table 3 below. In addition, since the Vickers hardness of the mixed powder (especially iron-based powder) before the mixing process is about HV86, x is about HV100 or more, △ is about HV95-100, ○ is about HV95 or less. Become.

  The following Table 4 and Table 5 show the stirring conditions of the examples and comparative examples and the results of the respective evaluation items. Examples 1 to 15 and Comparative Examples 2 to 4 are mixed using a mixer (MP5B / I type multi-purpose mixer manufactured by Nippon Coke Co., Ltd.) that simultaneously performs convection mixing and shear mixing, and Comparative Example 1 is a convection. Mixing was performed using a mixer (V-type mixer S-3 type manufactured by Tsutsui Rika Kikai Co., Ltd.) that only performs mixing.

As shown in Tables 4 and 5, in Examples 1 to 15 and Comparative Examples 3 and 4 mixed by shear mixing, graphite was almost uniformly dispersed in the mixed powder, but mixed only by convection mixing. In Comparative Example 1 and Comparative Example 2 in which only 10 minutes were mixed at a rotational speed of 1000 min ⁻¹ by shear mixing, graphite was segregated in the mixed powder. Moreover, the fluidity of the mixed powder after mixing was less than 30 s / 50 g in Examples 1 to 15 which were shear mixed at a rotational speed of 2000 min ⁻¹ or more. On the other hand, Comparative Example 1 mixed only by convection mixing and Comparative Examples 2 to 4 shear mixed at a rotational speed of 1000 min ⁻¹ had a mixed powder fluidity of 30 s / 50 g or more after mixing. From the above, it became clear that the fluidity of the mixed powder can be reduced to less than 30 s / 50 g by adjusting the rotation speed of the mixer (specifically, by setting the rotation speed to 2000 min ⁻¹ or more). . In addition, since the fluidity | liquidity only with the iron-based powder before stirring mixing was 39 s / 50g, in Examples 1-15, it can be said that the fluidity fell 9 s / 50g or more by stirring mixing.

When mixing for the purpose of homogenizing the mixed powder, for example, in Comparative Examples 3 and 4 in which the rotational speed of the mixer is 1000 min ⁻¹ , a sufficiently homogeneous mixed powder is obtained. Therefore, for the purpose of homogenization only, a rotation speed of 1000 min ⁻¹ is sufficient, and it is not necessary to rotate at a higher speed. In the present invention, based on the knowledge that the iron-based powder is rounded by stirring and mixing, by adjusting the stirring conditions such as the rotational speed (specifically, 2000 min ⁻¹ or more), the fluidity is 30 s / It has been found that it can be less than 50 g.

  FIG. 2 is an SEM photograph of the mixed powder (particularly iron-based powder) before the mixing treatment, and FIG. 3 is an SEM photograph of the mixed powder after stirring under the conditions according to Example 9. From these photographs, it can be seen that the mixed powder stirred under appropriate conditions has iron-base powder (partial diffusion alloy powder) sufficiently rounded. Thus, it is thought that the fluidity | liquidity of mixed powder was improved by iron-base powder being rounded.

As described above, the powder characteristics (graphite dispersibility, fluidity, bulk density) were generally improved by increasing the rotation speed and time of the mixer. However, the rotational speed 10000 min ^-1 in Example 15 was mixed 60 minutes, than Example 14 were mixed for 30 minutes at a rotation speed of 10000 min ^-1, the fluidity decreases (flowability is increased). From this result, it can be seen that if the number of rotations and the stirring time are increased unnecessarily, the fluidity is lowered. This is because if the rotational speed or stirring time is too long, the iron powder is partially crushed and a large amount of fine powder is generated, and the flow of the powder is inhibited by the presence of this fine powder around the iron powder. it is conceivable that. Further, Examples 12 to 15 in which the rotation speed of the mixer was high and the stirring time was long resulted in the hardness of the powder being significantly increased by mixing. This is presumably because work hardening occurred in the powder due to an increase in energy applied to the powder.

From the above, the rotation speed and stirring time of the mixer may be set so that the characteristics (particularly fluidity and hardness) of the powder are in a desired range. In this example, if mixing at a rotational speed of 2000 to 9000 min ⁻¹ for 10 to 60 minutes, preferably about 30 to 60 minutes at a rotational speed of 2000 to 5000 min ⁻¹ , the fluidity is excellent and the hardness is increased. A suppressed mixed powder can be obtained.

  Next, the characteristics of a compact (test piece) obtained by compression molding the above powder at a predetermined pressure were investigated. The test piece was ring-shaped and adjusted so that the outer diameter was φ23.2 mm, the inner diameter was φ16.4 mm, and the axial dimension was 7 mm. The moldability was evaluated by visually observing the surface of the molded body, and x for those with cracks and ◯ for those without cracks. Sintering was performed in a furnace in an RX gas atmosphere at a maximum temperature of 1250 ° C. and a maximum temperature holding time of 150 minutes. The method for measuring the sintered density was in accordance with JIS Z2501.

In the test piece obtained by the above process, the sintered density and the square root of the estimated maximum pore envelope area were examined. Details of the √area _max value estimation method will be described below.

First, it is assumed that the extreme value distribution of pores in the sintered body follows a double exponential distribution. Thereby, the maximum value of the hole envelope area is estimated using extreme value statistics. Specifically, the square root √area _{max of} the estimated maximum hole envelope area is calculated through the following procedure.

The specimen subjected to mirror polishing is observed with a microscope, and an image of a y region having a predetermined reference area So (mm ² ) is acquired. The obtained image is binarized using image analysis software, and the envelope area of the holes is analyzed. The largest envelope area among the obtained envelope areas is defined as the maximum hole envelope area in the reference area So, and the square root thereof is defined as √area _max in the region. This measurement is repeated n times while changing the inspection region.

Measured n √area _max are arranged in order from the smallest, and √area _{max, j} (j = 1 to n), respectively. (See Formula 1)

For each j (j = 1 to n), a cumulative distribution function F _j (%) expressed by Expression 2 and a standardized variable y _j expressed by Expression 3 are calculated.

Taking √area _max on the coordinate horizontal axis of the extreme value probability sheet, the above result is plotted to obtain an extreme value distribution. (The vertical axis of the extreme probability sheet takes F or y)

An approximate straight line obtained by the least square method is extrapolated with respect to the extreme value distribution to obtain a and b represented by Expression 4. However, y is the normalization variable represented by Formula 5, T is the recursion period represented by Formula 6, V is the volume of the estimation target region (predicted volume: mm ³ ), and V ₀ is the standard represented by Formula 7. Volume (mm ³ ), h is an average value (mm) of the measured √area _{max, j} represented by Formula 8.

It is confirmed that the plot points at 10 to 85% of the F scale which is the vertical axis of the extreme value probability sheet are on an approximate straight line. Thereby, it can be confirmed that the obtained extreme value distribution follows a double exponential distribution. Substituting the volume V (predicted volume) of the estimation target region into Equation 6, the point where the recurring period T and the obtained extreme value distribution intersect is the square root √area _max of the estimated maximum hole envelope area.

In this embodiment, the reference area S ₀ is 0.39 mm ² , the number of inspections n is 32, and the volume V of the estimation target region is 200 mm ³ . The surface layer was a region having a depth of 0.54 mm from the inner peripheral surface of the test piece 1. As for the reference area, the radial dimension was 0.54 mm from the inner peripheral surface of the test piece, and the axial dimension was 0.74 mm. The estimation target area was a cylindrical area of 0.54 mm from the inner peripheral surface of the test piece, and the axial dimension was 7 mm.

The evaluation criteria for the sintered density and the square root (√area _max ) of the estimated maximum pore envelope area are shown in Tables 6 and 7, respectively. Table 8 shows the specifications and evaluation results of each test piece.

As shown in the test pieces 1 to 7, the characteristics of the test piece tend to be improved by increasing the processing rotation speed and time and increasing the molding pressure. Specifically, when the number of rotations when the mixed powder is stirred is 2,000 min ⁻¹ or more, the stirring time is 30 min or more, and the molding pressure during compression molding is 900 MPa or more (test pieces 3 to 7). Especially good results. On the other hand, as shown in the test pieces 11 and 12, the moldability deteriorated when the rotational speed and the stirring time during the stirring of the mixed powder were further increased. This is presumably because the entanglement between the powders decreased due to the modification of the powder surface by the treatment. From this result, it can be said that considering the moldability, it is preferable that the rotation speed of the mixed powder is 1,500 to 2,500 min ⁻¹ and the stirring time is 10 to 50 min.

Here, √area _max of the test piece 5 is 32 μm. On the other hand, the √area _max of the test piece 9 in which the lubricant is reduced to 0.1 wt.%, Warm molding at 120 ° C. is performed at 1,200 MPa, and die lubrication is used together is 35 μm. Since √area _{max of} both is equal, √area _max in the test piece can be reduced without applying a load to the mold by reducing the lubricant or through complicated processes such as warm molding and mold lubrication. It has been confirmed that it is possible to reduce.

Claims (11)

A mixed powder for powder metallurgy comprising iron-based powder and carbon powder,
A mixed powder for powder metallurgy that is stirred and mixed under a condition that the fluidity is less than 30 s / 50 g.   The mixed powder for powder metallurgy according to claim 1, wherein the iron-based powder has a Vickers hardness (HV) of 100 or less.   The mixed powder for powder metallurgy according to claim 1 or 2, wherein a particle diameter D90 of the carbon powder is 4 µm or more and 10 µm or less.   The mixed powder for powder metallurgy according to any one of claims 1 to 3, wherein the iron-based powder is a diffusion alloy powder in which Ni is diffused and adhered to an Fe-Mo alloy powder.   The sintered metal part formed using the mixed powder in any one of Claims 1-4. The sintered metal part according to claim 5, wherein the density is 7.6 g / cm ³ or more. In producing a mixed powder for powder metallurgy by charging a raw material powder containing iron-based powder and carbon powder into a mixer and stirring and mixing,
A method for producing a mixed powder for powder metallurgy, wherein the stirring conditions are set so that the fluidity is less than 30 s / 50 g.   The method for producing a mixed powder for powder metallurgy according to claim 7, wherein the fluidity of the powder mixture for powder metallurgy is reduced by 9 s / 50 g or more than the fluidity of the iron-based powder alone before stirring and mixing by stirring and mixing. .   The method for producing a mixed powder for powder metallurgy according to claim 7 or 8, wherein the mixer stirs and mixes the mixed powder for powder metallurgy by shear mixing.   The method for producing a mixed powder for powder metallurgy according to claim 9, wherein the rotational speed of the mixer is 2000 to 9000 rpm. The method for producing a mixed powder for powder metallurgy according to any one of claims 7 to 10, wherein the rate of increase in Vickers hardness (HV) of the iron-based powder before and after mixing is less than 20%.