JP5923023B2 - Mixed powder for powder metallurgy and method for producing sintered material - Google Patents

Description

  The present invention relates to a powder mixture for powder metallurgy in order to obtain a sintered material by powder metallurgy, and a sintered material obtained from such a powder mixture for powder metallurgy, and particularly high strength and high toughness even under normal sintering conditions. The present invention relates to a mixed powder for powder metallurgy for obtaining a sintered material exhibiting the above, and such a sintered material.

  In powder metallurgy, which uses iron powder as a main raw material to produce a sintered material, usually the main raw material powder and auxiliary raw material powder (such as graphite powder and alloy component powder) for improving the physical properties of the sintered material. And a mixed powder containing a lubricant or the like. Sintered materials by powder metallurgy are widely used because of their good material yield and the ability to manufacture parts at low cost. In addition, since pores exist in the sintered material, there is an advantage that the weight of the part can be reduced. However, the pores present in the sintered material exhibit the above-described characteristics, but cause the impact value (toughness) and tensile strength of the sintered material to be lower than that of a normal steel material.

  In order to improve the impact value and tensile strength of the sintered material, (1) increase the density of the sintered material, (2) increase the temperature during sintering, and (3) alloy the sintered material with an alloy. It is known that means such as (4) a light-quenching quenching process and a heat treatment such as carburizing quenching and tempering are effective. However, each of these treatments has advantages and disadvantages, and the actual situation is that further improvement is desired.

  Among the above-described means, increasing the density of the sintered material acts in the direction of decreasing the number of pores, so that it can be expected that the impact value and the tensile strength are improved. However, in order to increase the density of the sintered material, it is necessary to adopt a special molding method.

  As one of the methods for increasing the density, a powder forging method combining powder forming and hot forging is known. However, this method has a drawback that special equipment is required and the production cost is increased. As a method for increasing the density, for example, Patent Document 1 discloses a method in which a green compact is once molded, temporarily sintered, then molded again (recompressed), and then sintered (main sintering). ing. Even in this method, the process of manufacturing the sintered material is almost doubled, and an increase in cost is inevitable.

  In addition to the above-mentioned methods for increasing the density, warm molding in which powder is molded in a heated state and mold lubrication molding in which a lubricant is applied to a mold are also proposed. Or a device for applying the lubricant to the mold (a technique for uniformly applying the lubricant to the die is also required), which increases the cost. Further, it is conceivable to increase the density by increasing the molding pressure, but there is a limit in terms of the capacity of the molding machine (press).

  In order to increase the bonding between powder (iron powder) particles and increase the density, it can be easily predicted that the higher sintering temperature is better. Increasing the sintering temperature (high temperature sintering) is because elements between the powders diffuse to each other and integration becomes easy. As a result, the impact value (toughness) and strength of the sintered material are improved. From such a viewpoint, a technique for sintering at a high temperature of 1180 ° C. or higher has also been proposed (for example, Patent Document 2).

  In order to carry out high-temperature sintering, equipment that forms a high-temperature atmosphere of 1140 ° C. or higher is usually required, and special equipment such as a pusher-type sintering furnace or a vacuum sintering furnace is known as such equipment. ing. However, these facilities have a drawback that the facility cost is extremely high. In addition, there is a problem that productivity is lowered as compared with the case of employing a normal sintering facility.

  Therefore, without using special equipment as described above, using ordinary sintering equipment such as a mesh belt sintering furnace, excellent characteristics (impact Realization of a sintered material capable of exhibiting value and strength is desired.

  On the other hand, the method of alloying the sintered material is roughly divided into two types depending on the form of the raw material powder. One of them is a method (prealloy type steel powder) in which a strengthening element is alloyed in the iron powder production stage (melting process). As another method, a metal powder such as nickel (Ni), molybdenum (Mo), or copper (Cu) is mixed with iron powder, and the mixed powder is sintered to alloy the sintered material. (Premix method). In the latter method, the metal powder to be mixed is generally mixed with pure iron powder, but the metal powder is added to the pre-alloyed steel powder that has been alloyed in advance to become a raw material powder. Yes. Alternatively, from the viewpoint of preventing segregation of these metal powders, iron powder in which heat is applied to the metal powder or the metal powder is adhered to the surface of the iron powder with an organic binder has been proposed.

  When a sintered material is manufactured using prealloyed steel powder, the strength of the sintered material basically shows a tendency to improve. However, increasing the strength generally decreases the impact value in inverse proportion to it. In addition, when using a mixed powder in which a metal powder such as nickel, molybdenum or copper is mixed with iron powder, these alloy elements diffuse into the iron powder during sintering, strengthening the alloy and improving the strength of the sintered material. Will do. However, even in this case, the impact value tends to decrease.

The tendency to improve characteristics by alloying varies slightly depending on the type of element to be alloyed, and in the case of using raw material powder (mixed powder) in which Ni powder is mixed at a ratio of 4% by mass, both strength and impact value are obtained. Excellent characteristics. As a composition of such a mixed powder, 4% Ni diffusion type mixed powder (chemical component composition: 4% Ni-1.5% Cu-0.5% Mo) is known. In such a mixed powder, Ni does not become uniform, and a portion with a high Ni concentration becomes retained austenite, so that the impact value is also excellent. In the sintered material obtained using this mixed powder, the impact value (Charpy impact value): 17 J / cm ² or more and the tensile strength: 1100 MPa or more are exhibited. However, in order to exhibit good characteristics, it is necessary to use a large amount of high-purity and high-priced Ni powder. It is difficult to reduce the amount of Ni powder, and the amount of Ni powder is reduced as much as possible. It is hoped that.

  As a method for increasing the strength, it is also effective to perform a heat treatment such as a fluoric quenching process or a carburizing quenching tempering process. By performing such heat treatment, the metal structure becomes martensite and the tensile strength is improved. However, although heat treatment is effective for improving the strength, the impact value (toughness) of the sintered material is reduced. In particular, when carburizing, quenching, and tempering are performed, the impact value is significantly reduced. As a method for preventing a decrease in impact value due to carburizing, quenching and tempering, there is no use other than the 4 mass% Ni diffusion mixed powder as a raw material powder, and no other effective method has been established. Is the actual situation.

JP-A-2-97602 JP 2001-295915 A

  The present invention has been made under these circumstances, and its purpose is not to use a special molding method or apparatus for increasing the density, without requiring special sintering that increases the sintering temperature, Powder metallurgy mixing that uses conventional facilities and can suppress the amount of expensive Ni powder mixed, and can obtain sintered materials with high strength and toughness at low cost. An object of the present invention is to provide a sintered material that can exhibit good strength and toughness even after carburizing, quenching, and tempering using the powder and the mixed powder.

  The mixed powder for powder metallurgy of the present invention that has solved the above-mentioned problems comprises a pre-alloyed steel powder containing Mo in an amount of 0.6 to 1.0% by mass and having a particle size of 200 μm or less, Ni powder, and graphite powder. In addition, the ratio of Ni powder is 2.3 to 3.0 mass% and the ratio of graphite powder is 0.2 to 0.4 mass% with respect to the total 100 mass% of these pre-alloyed steel powder, Ni powder and graphite powder. It is characterized by being.

On the other hand, the sintered material of the present invention that has achieved the above object comprises prealloy-type steel powder containing Mo in an amount of 0.6 to 1.0% by mass and having a particle size of 200 μm or less, Ni powder, and graphite powder. A powder having a Ni powder ratio of 2.3 to 3.0 mass% and a graphite powder ratio of 0.2 mass% or more with respect to a total of 100 mass% of the pre-alloyed steel powder, Ni powder and graphite powder. A sintered material obtained by sintering and carburizing, quenching, and tempering a mixed powder for metallurgy, and the maximum particle size D (μm) of the pre-alloyed steel powder and the amount of graphite powder (mass%) satisfy the relationship of the following formula (1). It is characterized by that.
Graphite powder amount (% by mass) ≦ −2.0 × 10 ⁻³ × D (μm) +0.85 (1)

  According to the present invention, a predetermined amount of Mo is contained in iron powder as a pre-alloy type, and a mixed powder for powder metallurgy in which the amount of Ni powder is reduced is reduced to a low-cost, high-strength, high-toughness firing. Sintered material that can produce a mixed powder for powder metallurgy capable of obtaining a binder, and can exhibit good strength and toughness even after carburizing and tempering by producing a sintered material using such a mixed powder Is obtained.

FIG. 1 is a graph showing the relationship between the Mo content in the pre-alloyed steel powder and the Charpy impact value of the sintered material. FIG. 2 is a graph showing the relationship between the Mo content in the pre-alloyed steel powder and the tensile strength of the sintered material. FIG. 3 is a graph showing the relationship between the amount of Ni powder added to the mixed powder and the Charpy impact value of the sintered material. FIG. 4 is a graph showing the relationship between the amount of Ni powder added to the mixed powder and the tensile strength of the sintered material. FIG. 5 is a graph showing the relationship between the amount of graphite powder added to the mixed powder and the Charpy impact value of the sintered material. FIG. 6 is a graph showing the relationship between the amount of graphite powder added to the mixed powder and the tensile strength of the sintered material. FIG. 7 is a graph showing the influence of the particle diameter (sieving mesh) of prealloyed steel powder and the amount of graphite powder added on the Charpy impact value of the sintered material.

  The present inventors examined raw material powder (mixed powder for powder metallurgy) that can achieve the above object from various angles. As a result, if a predetermined amount of Mo is prealloyed with iron powder to form a pre-alloyed steel powder, and a mixed powder with a reduced amount of Ni powder added thereto, a sintered material exhibiting desired characteristics can be obtained. The present invention was completed by finding that it could be a mixed powder for powder metallurgy. In the mixed powder of the present invention, a chemical component composition that does not oxidize even in an endothermic gas is basically selected, and there is an advantage that a commonly used sintering gas or carburizing gas can be used. Hereinafter, each requirement prescribed | regulated by this invention is demonstrated.

  The mixed powder of the present invention contains pre-alloyed steel powder containing 0.6 to 1.0 mass% of Mo as an alloy component (ratio in steel powder). Mo is an element that does not deteriorate the compressibility of iron powder relatively among the strengthening elements, and is suitable as a strengthening element for pre-alloyed steel powder. Such Mo is effective in improving the strength of the sintered material. In particular, when heat treatment such as carburizing, quenching and tempering is performed, the effect becomes remarkable.

  When the present inventors confirmed the effect about Mo, in what pre-alloy type steel powder alloyed Mo with content of 0.6 mass% or more, even if the addition amount of Ni powder to add decreases, it is excellent. It was found that a sintered material exhibiting a high impact value can be obtained. However, if the Mo content exceeds 1.0 mass%, the impact value is decreased and the cost is increased. In addition, the minimum with preferable Mo content is 0.7 mass% or more, and a preferable upper limit is 0.9 mass% or less.

It is an important requirement that the prealloyed steel powder used in the present invention has a particle size of 200 μm or less. The finer the particle diameter, the larger the impact value of the obtained sintered material. Since the impact value (Charpy impact value) of the sintered material so far has been 17 J / cm ² or more, in order to surpass these characteristics, the particle size needs to be 200 μm or less. This particle size is preferably 180 μm or less, more preferably 150 μm or less. The “particle size of the pre-alloyed steel powder” means the size of the sieve mesh, for example, “the particle size is 200 μm or less” means that the sieve mesh is a particle size that passes through a 200 μm mesh. . The “maximum particle size” means the size of the mesh sieve through which the powder has passed.

  The prealloy type steel powder used in the present invention contains Mo as an alloy component, and the balance is basically iron and unavoidable impurities (for example, 0.3% by mass or less of Mn, Ni, Cu, etc.). In the present invention, the prealloy type steel powder is a prealloy type steel powder obtained from molten steel containing Mo as an alloy component. This pre-alloyed steel powder is usually produced by atomizing molten steel.

  The mixed powder for powder metallurgy according to the present invention includes the above prealloyed steel powder, Ni powder, and graphite powder. Ni powder exhibits the effect | action which improves the tensile strength and impact value of a sintered material. Such an effect is effectively exhibited when the proportion of Ni powder is 2.3% by mass or more with respect to 100% by mass in total of the pre-alloyed steel powder, Ni powder and graphite powder. However, when the proportion of Ni powder exceeds 3.0% by mass, the cost increases. In other words, even if the proportion of Ni powder (the Ni content in the sintered material finally) is 3.0% by mass or less, the effect of improving characteristics by Ni is effectively achieved. The preferable lower limit of the proportion of Ni powder is 2.5% by mass or more, and the preferable upper limit is 2.8% by mass or less. About the average particle diameter of Ni powder, it is about 2-5 micrometers normally (preferably about 2.5-4.5 micrometers).

On the other hand, graphite powder diffuses into steel powder (mother powder) and becomes carbon. This carbon exhibits the effect of increasing the strength of the sintered material, but tends to reduce the impact value. The sintered material subjected to the carburizing and quenching treatment has a high carbon content on the material surface and has a concentration gradient unlike the material center. When the present inventors examined the relationship between the impact value of the sintered material and the ratio of the graphite powder, in order to ensure an impact value of 17 J / cm ² or more in the sintered material, the ratio of the graphite powder is: It has been found that the content may be 0.4% by mass or less with respect to a total of 100% by mass of the pre-alloyed steel powder, Ni powder and graphite powder. However, if the ratio of the graphite powder becomes too low, the effect of improving the strength is not exhibited (preferably the tensile strength is 1200 MPa or more), so it is necessary to set it to 0.2% by mass or more. Incidentally, the graphite powder becomes a carbon component in the sintered material, but does not completely diffuse into the iron powder, for example, it reacts with oxygen in the iron powder during the sintering and becomes CO gas, and the above range (0.2 to 0.4 mass%) is about 0.15 to 0.37 mass% in terms of the amount of carbon in the sintered material. About the average particle diameter of graphite powder, it is about 2-10 micrometers normally (preferably about 2.5-7.5 micrometers).

  If necessary, the mixed powder for powder metallurgy of the present invention is also blended with a lubricant. This lubricant exhibits the effect of suppressing the occurrence of mold galling and damage to the mold by reducing the coefficient of friction between the green compact and the mold. Examples of such lubricants include ethylene bisstearylamide, stearamide, zinc stearate, and lithium stearate. The blending amount when the lubricant is contained is preferably about 0.2 to 2.0% by mass with respect to the entire mixed powder. If the blending amount is less than 0.2% by mass, the effect of the lubricant is not exhibited, and if it exceeds 2.0% by mass, the strength of the sintered material may be reduced.

The present inventors examined the influence of the particle diameter of pre-alloyed steel powder and the ratio of graphite powder (graphite powder amount) on the impact value of the sintered material. As a result, in order to ensure an impact value (Charpy impact value) of 17 J / cm ² or more in the sintered material, the maximum particle diameter D (μm: 200 μm or less) of prealloy type steel powder and the amount of graphite powder (mass%) ) Satisfies the following equation (1) (see FIG. 7). In this case, the amount of graphite powder may be higher than 0.4% by mass (preferably 0.6% by mass or less).
Graphite powder amount (% by mass) ≦ −2.0 × 10 ⁻³ × D (μm) +0.85 (1)

  The sintered material of the present invention is formed (green compact molding), sintered and carburized, quenched and tempered using the mixed powder for powder metallurgy as described above. However, it is only necessary to follow normal conditions currently used, and no special equipment is required.

  For example, the pressure in compacting is about 588 to 686 MPa. The sintering temperature may be 1050 ° C. or higher and 1130 ° C. or lower. Further, the atmosphere in the furnace at the time of sintering and carburizing need not be controlled to a vacuum atmosphere or a special gas atmosphere, and usually used sintering gas and carburizing gas (for example, RX gas) can be applied.

  Hereinafter, the present invention will be described in more detail with reference to examples. However, the following examples are not intended to limit the present invention, and may be implemented with appropriate modifications within a range that can meet the purpose described above and below. These are all possible and are within the scope of the present invention.

[Experiment 1]
For the mixed powder prepared by mixing the raw material powder with a mixer (30 minutes) so as to have the component composition shown in Tables 1 to 3 below, molding (compact molding), sintering, processing, immersion under the following conditions Charcoal quenching and tempering treatment was performed to obtain a sintered material (carburized material). Mechanical properties (density, Charpy impact value, tensile strength, hardness) of the obtained sintered material were evaluated. In the following Tables 1-3, the content of the alloy components (Mn, Ni, Mo) in the pre-alloyed steel powder is the content in the steel powder, and the ratio of the additive is the ratio to the entire mixed powder (Table 4 below). , 6 and 8). In preparing the mixed powder, zinc stearate (Zn-St) as a lubricant was blended so as to be 0.75% by mass (ratio to the total mixed powder).

  The pre-alloyed steel powder used at this time has a particle size (size of sieve mesh passed through) of 250 μm. In addition, each powder when used as an additive is Ni powder: Carbonyl Ni powder (with an average particle size of 2.5 μm), Cu powder: Atomized Cu powder (with 100 mesh under), and added Mo powder The particle size (average particle size) of graphite powder is 3 μm and 5 μm, respectively.

<Molding process>
Molding pressure: 588 MPa (6 t / cm ² )
Shape: 10 × 10 × 60 (mm)

<Sintering process>
Sintering temperature: 1120 ° C (30 minutes)
Sintering atmosphere: (nitrogen atmosphere containing 10% by mass of hydrogen)

<processing>
Tensile test piece: Machining to produce a JIS No. 4 (φ5 mm) tensile test piece Impact test piece: Unprocessed state (10 × 10 × 60 (mm) remains)

<Carburizing and tempering>
Carburizing and quenching: 920 ° C x 90 minutes (RX gas atmosphere: carbon potential 0.8%)
→ 850 ° C x 60 minutes (oil quenching)
Tempering treatment: 200 ° C x 60 minutes (in air)

  For the sintered material (carburized material) obtained as described above, tensile strength is measured at room temperature under the condition of tensile speed: 0.5 mm / min (processed into a test piece shape), and Charpy at room temperature. An impact test was performed to measure an impact value (Charpy impact value) [with a shape of 10 × 10 × 60 (mm)]. Moreover, the hardness of the test piece surface used for the impact test was measured by the Rockwell A scale (HRA). The results are also shown in Tables 1 to 3 below.

  As is apparent from Table 1, it can be seen that the inclusion of Mo as an alloy component of the pre-alloyed steel powder is effective in improving the strength without reducing the impact value (toughness) (Test Nos. 1 and 2). . On the other hand, it can be seen that those containing Ni as an alloy component of the pre-alloyed steel powder tend to lower the impact value and strength (Test Nos. 3 and 4).

As is apparent from Table 2, when Mo is added as an alloy component of the pre-alloy type steel powder and a predetermined amount of Ni powder is added, the density of the sintered material is increased and effective in improving the impact value and strength. I understand that there is. In particular, the prealloy type steel powder containing 0.85% by mass of Mo (Test No. 9) can be expected to be most effective in improving the impact value and strength. However, in these data, the particle diameter (sieving mesh) of the pre-alloy type steel powder is 250 μm, and the Charpy impact value does not achieve 17 J / cm ² or more. In addition, when the Mo content in the pre-alloy type steel powder is 0.5% by mass or 1.5% by mass (Test Nos. 8 and 10), the impact value is not improved or decreased. Therefore, it is expected that the expected impact value will not be achieved even if the particle size (screening) of the pre-alloy type steel powder is made finer.

  As is apparent from Table 3, Mo is not contained as an alloy component of the pre-alloy type steel powder ("pre-alloy type steel powder" shown in Table 3 substantially corresponds to "pure iron powder"), powder (additional) When added as a material, it can be seen that the improvement in impact value is not achieved.

[Experiment 2]
The mixed powder prepared by mixing the raw material powder with a mixer (30 minutes) so as to have the component composition (and particle size) shown in Table 4 below was molded (green compact molding) and fired in the same manner as in Experiment 1. Sintering, processing, carburizing, quenching, and tempering were performed to obtain a sintered material (carburized material). The mechanical properties (density, Charpy impact value, tensile strength, hardness) of the obtained sintered material were evaluated in the same manner as in Experiment 1.

  The results are shown in Table 5 below. Based on the results in Table 5, the relationship between the Mo content in the pre-alloy type steel powder and the Charpy impact value of the sintered material is shown in FIG. 1, and the relationship between the Mo content in the pre-alloy type steel powder and the tensile strength of the sintered material is shown in FIG. FIG. 2 shows them respectively. In FIGS. 1 and 2, the Ni content in the prealloy-type steel powder is indicated by ○, □, and Δ, respectively, when the Ni content is 0 mass%, 0.5 mass%, and 2 mass%.

  From these results, it can be considered as follows. It can be seen that the Charpy impact value can be increased when the Mo content in the pre-alloyed steel powder is 0.6 to 1.0 mass% (FIG. 1). However, when the Mo content exceeds 1.0% by mass, the Charpy impact value is lowered. On the other hand, even if Ni is contained as an alloy component of the pre-alloyed steel powder, the Charpy impact value cannot be improved, but tends to decrease.

  Moreover, regarding tensile strength, it turns out that high intensity | strength can be ensured irrespective of Mo content (FIG. 2). However, when Ni is contained in the prealloy type steel powder, the tensile strength tends to decrease as the content increases.

[Experiment 3]
The mixed powder prepared by mixing the raw material powder with a mixer (30 minutes) so as to have the component composition (and particle size) shown in Table 6 below was molded (green compact molding) and fired in the same manner as in Experiment 1. Sintering, processing, carburizing, quenching, and tempering were performed to obtain a sintered material (carburized material). The mechanical properties (density, Charpy impact value, tensile strength, hardness) of the obtained sintered material were evaluated in the same manner as in Experiment 1.

  The results are shown in Table 7 below. Based on the results of Table 7, FIG. 3 shows the relationship between the Ni powder addition amount of the mixed powder and the Charpy impact value of the sintered material, and FIG. 4 shows the relationship between the Ni powder addition amount of the mixed powder and the tensile strength of the sintered material. Respectively. In addition, in FIG. 3, 4, the thing with Cu powder addition amount of 0 mass%, 1 mass%, 2 mass%, 3 mass% was shown by (circle), (square), (triangle | delta), and (◇), respectively.

  From these results, it can be considered as follows. That is, it can be seen that the Charpy impact value increases as the Ni powder addition amount increases regardless of the Cu powder addition amount (FIG. 3). On the other hand, when Ni powder is added to the mixed powder, the tensile strength acts in the direction of improving as the addition amount increases, but when Cu powder is added, the tensile strength decreases instead. A tendency will be shown (FIG. 4). Moreover, in order to ensure both the characteristics of Charpy impact value and tensile strength by adding Ni powder, it was confirmed that the addition may be about 2.3 to 3.0% by mass.

The data shown in Table 7 show that the prealloy type steel powder has a particle size (sieving mesh) of 250 μm, but in particular, a Charpy impact value of 17 J / cm ² or more is achieved (Test Nos. 28 and 34). ). In particular, test no. No. 28 satisfies both the Charpy impact value and the tensile strength (1200 MPa or more). When the pre-alloy type steel powder having a particle size (sieve) of 250 μm is used, there is a variation in the characteristics of the sintered material due to non-uniformity of the mixed powder due to the large particle size difference of the raw material powder. It tends to occur (the tensile strength decreases in Test No. 34). Even from the viewpoint of exhibiting stable characteristics when the amount of graphite powder added is 0.2% by mass or more, it is necessary that the particle diameter (screened) of the pre-alloy type steel powder be 200 μm or less.

  Moreover, the larger the particle size, the larger (coarse) particles are included, and the powder is not uniform and the pores are not uniform. Therefore, it is considered that variations during carburizing (carburizing gas intrusion) are likely to occur. Test No. In the case of No. 34, the total amount of Ni powder and Cu powder becomes 4.5% by mass, and when combined with the amount of Mo contained in the prealloy type steel powder, the intended amount of alloy is not reduced.

[Experiment 4]
The mixed powder prepared by mixing the raw material powder with a mixer (30 minutes) so as to have the component composition (and particle size) shown in Table 8 below was molded (green compact molding) and fired in the same manner as in Experiment 1. Sintering, processing, carburizing, quenching, and tempering were performed to obtain a sintered material (carburized material). The mechanical properties (density, Charpy impact value, tensile strength, hardness) of the obtained sintered material were evaluated in the same manner as in Experiment 1.

  The results are shown in Table 9 below. Based on the results of Table 9, FIG. 5 shows the relationship between the amount of graphite powder added to the mixed powder and the Charpy impact value of the sintered material, and FIG. 6 shows the relationship between the amount of graphite powder added to the mixed powder and the tensile strength of the sintered material. Respectively. In FIGS. 5 and 6, the prealloy-type steel powders having a particle size (sieve) of 250 μm, 180 μm, and 150 μm are indicated by ○, □, and Δ, respectively.

From this result, it can be considered as follows. That is, both the Charpy impact value and the tensile strength can be satisfied by setting the particle size of the pre-alloyed steel powder (screening) to 200 μm or less and the graphite powder addition amount to 0.2 to 0.4 mass%. I understand. In addition, in the data shown in Table 7, the particle size (sieve) of the pre-alloy type steel powder is 250 μm, and the Charpy impact value of 17 J / cm ² or more is also observed (Test Nos. 49 to 51). . In particular, test no. 50 and 51 satisfy both characteristics of Charpy impact value and tensile strength. As described above, these materials are apt to cause variations in characteristics of the sintered material (the tensile strength decreases in Test No. 49). Moreover, the larger the particle size, the larger (coarse) particles are included, and the powder is not uniform and the pores are not uniform. Therefore, it is considered that variations during carburizing (carburizing gas intrusion) are likely to occur.

Based on the results of Table 9 above, FIG. 7 shows the influence of the particle size (screening) of the pre-alloyed steel powder and the added amount of graphite powder on the Charpy impact value of the sintered material. In FIG. 7, the case where the impact value of the sintered material satisfies 17 J / cm ² or more is indicated by ◯, and the case where the impact value is not satisfied is indicated by □. As is clear from this result, when the graphite powder addition amount is 0.2% by mass or more, the smaller the particle size (sieve) of the pre-alloyed steel powder, the more the graphite powder addition amount for exhibiting good characteristics. I understand that I can do it.

Based on this result, the equation (1) was obtained. That is, when the particle diameter (sieving mesh) of the pre-alloy type steel powder is 150 μm and 250 μm, the sintered material has an impact value of 17 J / cm ² or more and the intermediate value of the graphite powder addition amount of the unsatisfactory one, 0.55% by mass and 0.35% by mass are selected, and these two points (the particle size is 150 μm and the amount of graphite powder added is 0.55% by mass, the particle size is 250 μm and the amount of graphite powder added is 0.00. Based on a straight line connecting the points of 35% by mass, the equation (1) was obtained.

Claims (2)

  When the size of the sieve mesh through which the pre-alloy type steel powder passes is defined as the particle size, the pre-alloy type steel powder containing Mo at 0.6 to 1.0% by mass and the particle size is 200 μm or less, Ni powder, The ratio of Ni powder is 2.3 to 3.0% by mass, and the ratio of graphite powder is 0.2 to 0 with respect to 100% by mass of the total of these prealloy type steel powder, Ni powder and graphite powder. 4% by mass of mixed powder for powder metallurgy When the size of the sieve mesh through which the pre-alloy type steel powder passes is defined as the particle size, the pre-alloy type steel powder containing Mo at 0.6 to 1.0% by mass and the particle size is 200 μm or less, Ni powder, The ratio of Ni powder is 2.3 to 3.0% by mass and the ratio of graphite powder is 0.2% by mass with respect to the total of 100% by mass of these prealloy type steel powder, Ni powder and graphite powder. What der above, the maximum particle diameter D ([mu] m) and mixed powder for powder metallurgy of graphite powder amount (mass%) satisfies the following relationship (1) of the prealloy type steel powder, sintering and carburizing quenching and tempering A method for producing a sintered material , comprising:
Graphite powder amount (% by mass) ≦ −2.0 × 10 ⁻³ × D (μm) +0.85 (1)