JP5113555B2 - Iron-based sintered alloy and method for producing the same - Google Patents

Description

  The present invention relates to an iron-based sintered alloy having excellent strength and toughness and a method for producing the same.

  An iron-based sintered alloy consisting of an iron-based sintered alloy obtained by further heat-sintering a powder molded body obtained by pressure-molding a raw material powder containing iron as a main component in order to reduce the manufacturing costs of structural members such as machine parts Members are conventionally used. If an iron-based sintered alloy member is used, a product close to the final shape can be obtained, and the manufacturing cost and material cost of the structural member can be reduced by reducing machining and improving yield.

  However, conventionally, a high-density powder compact could not be obtained industrially. For this reason, only a low-density iron-based sintered alloy can be obtained, or even when the density is high, the dimensional stability is poor and often does not meet the gist of adopting the sintering method. Therefore, iron-based sintered alloy members are rarely used for structural members that require high strength.

  However, according to the mold lubrication warm pressure molding method developed by the present inventor, ultra-high pressure molding is possible industrially, and a powder compact having a higher density than ever before can be obtained. Moreover, a sintered body having excellent dimensional stability and high density can be obtained. For this reason, the use of the iron-based sintered alloy is rapidly expanding to various structural members (see the following cited references 1 to 3).

Furthermore, the present inventor has already developed an iron-based sintered alloy having further enhanced strength and toughness, and uses of the iron-based sintered alloy member are increasingly expanding (see Patent Document 4 below).
Japanese Patent No. 3309970 JP 2005-336608 A JP-A-2005-336609 JP 2006-233331 A JP 2001-98301 A Japanese Patent No. 3952006 Japanese Patent No. 3904112

(1) As described above, the iron-based sintered alloy developed by the present inventor has a sufficiently high strength as described in Patent Document 4, etc., but the weight of the member is reduced, the size is reduced, and the cost is reduced. Therefore, an iron-based sintered alloy having higher mechanical properties such as strength and toughness is required. In particular, when an iron-based sintered alloy is actually used for a structural member, it is very important not only to have high tensile strength (bending strength) but also high fatigue strength.

The present invention has been made in view of such circumstances, and an object thereof is to provide an iron-based sintered alloy having improved strength, particularly fatigue strength, and a method for producing the same.
(2) Patent Document 5 discloses a high density of granulated powder in which small-diameter particles made of stainless steel are attached to the surface of large-diameter particles having the same composition as that of the steel using 1-2% by weight of a binder. A sintered body is disclosed. However, the sintered body of Patent Document 5 is only intended to have a high density that improves airtightness and yield, and is not intended to significantly increase the strength as in the present invention. Further, as can be seen from the fact that the binder used for the granulation is removed by dewaxing before sintering, the density and dimensional stability of the green compact are reduced by at least the binder, and at least Due to the dewaxing process, the manufacturing cost of the sintered body also increases. In addition, the strength of the sintered body, particularly the fatigue strength, can be reduced by the residual binder and pores formed by the binder discharge.

  In Patent Document 6, a high-density sintered body is obtained using a granulated powder obtained by granulating only a fine powder having an average particle diameter of 8.5 μm or less with a binder. However, the sintered body of Patent Document 6 is not intended to significantly increase the strength as in the present invention, and similarly to Patent Document 5, has a problem associated with the use of a binder.

  Patent Document 7 also discloses a sintered body of austenitic stainless steel made of fine powder. However, this case is not intended to significantly increase the strength as in the present invention, and still has problems associated with the use of the binder as described above.

  As a result of intensive research and trial and error to solve the above problems, the present inventor succeeded in improving the strength of a sintered body obtained by sintering a high-density powder molded body, in particular, fatigue strength. The present invention has been completed.

<Iron-based sintered alloy>
(1) The iron-based sintered alloy of the present invention comprises one or more coarse powders made of pure iron or an iron alloy and one or more fine powders made of pure iron or an iron alloy having an average particle size smaller than that of the coarse powder. In an iron-based sintered alloy composed of a sintered body obtained by heating and sintering a powder molded body obtained by pressure molding a mixed powder containing raw material powders,
The average particle size (d) of the fine powder is 1 ≦ d ≦ 25 (μm), and the average particle size (D) of the coarse powder is d <D ≦ 50 (μm), The proportion of fine powder is 5 to 45% by mass, and the sintered body has a sintered body density ratio (ρ ′ / ρ0 ′) which is a ratio of the bulk density (ρ ′) of the sintered body to the theoretical density (ρ0 ′). x100%) is 96% or more and is characterized by high strength.

(2) The present inventor sinters a powder compact obtained by high-density molding of a mixed powder whose average particle diameter and blending ratio are specified, thereby achieving high strength of an iron-based sintered alloy that has been conventionally high, The inventors have newly found that the fatigue strength, which is very important in practical use, can be further increased, and have completed the present invention.

  The mixed powder as used in the present invention includes at least one or more coarse powders and at least one or more fine powders within the above average particle size range, and the ratio of the fine powders to the whole raw material powder is in the above range. This is a powder. However, the reason why the mixed powder is specified by such expression is to limit the particle size distribution in the raw material powder to a specific range, and from an industrial viewpoint, it is common to mix a plurality of types of powders. . Therefore, conversely, if a particle size distribution similar to the particle size distribution specified by the average particle size and blending ratio as described above can be obtained, a substantially single type of powder having a moderately distributed particle size can be obtained. It will be included in the mixed powder of the invention. Further, the mixed powder is composed of a coarse powder and a fine powder, but is not limited to two kinds, and may be a mixture of three or more kinds of powders. That is, two or more kinds of powders included in the category of the coarse powder may be mixed from the viewpoint of the average particle diameter, or two or more kinds of powders included in the category of the fine powder may be mixed from the viewpoint of the average particle diameter. May be.

By the way, the reason and mechanism for improving the strength and the like of the iron-based sintered alloy by using such a mixed powder is not necessarily clear, but it is considered as follows.
By optimizing the average particle size (particle size) and blending ratio of the coarse powder and fine powder (hereinafter referred to as “main raw material powder” together), the fine powder is filled and introduced between the coarse powder particles. The For this reason, it is considered that not only the residual pores remaining in the sintered body are reduced in diameter, but also the frequency of scattered coarse residual pores is remarkably reduced.

  The coarse residual pores and the like are portions that are likely to be the starting point of fracture, as in the case where fragile inclusions exist at the bonding interface of the particles. Therefore, the strength, particularly fatigue strength, of the iron-based sintered alloy of the present invention can be greatly improved by significantly reducing coarse residual pores.

  Here, the iron-based sintered alloy of the present invention does not simply mean that the sintered body has a high density and few residual pores and coarse residual pores therein. The powder compact itself, which is the previous stage before reaching the sintered body, has a very high density. That is, the iron-based sintered alloy of the present invention reaches a high-density sintered body through a high-density powder compact. Moreover, not only the powder compact has a high density, but also the particle size distribution of the raw material powder (mixed powder) constituting the powder compact has the characteristics described above. For this reason, the powder compact which is the predecessor of the iron-based sintered alloy of the present invention has a very small residual pore diameter in the interior compared to a powder compact obtained by high-density molding of ordinary powder, and it is It seems that it became a thing which has virtually no coarse residual pores as a starting point.

  The iron-based sintered alloy of the present invention in which the powder compact is sintered has a tendency that the pores are further reduced, and the coarse residual pores are further reduced. Thus, it is considered that the iron-based sintered alloy of the present invention is extremely superior in mechanical properties such as strength (particularly fatigue strength) and toughness, which has not been found in the past.

  At first glance, it is obvious that the density of powder compacts and sintered bodies can be improved and the strength of iron-based sintered alloys can be improved by using a powder having a smaller average particle diameter than before. Seem. However, in the first place, the smaller the average particle size, the harder the powder is, and it tends to be difficult to obtain a high-density powder compact. Also, the smaller the average particle size, the more easily it becomes clayey, so that powder molding itself tends to be difficult. That is, if the raw material powder is simply refined, the fine powders are easily agglomerated or bridging. For this reason, conventionally, the finer the raw material powder, the more likely it is that coarse residual pores are frequently formed. Therefore, a fine powder having a small average particle diameter is rarely used. Even when a fine powder is used, the fine powder is once made into granulated powder by using a binder and used.

  However, as described above, there are various problems such as sacrificing the shape maintenance property and dimensional stability originally expected for the iron-based sintered alloy, and further reducing the manufacturing cost. Incidentally, in the case of the present invention, it has succeeded in obtaining a uniform mixed powder while using a fine powder, and further, a compact density ratio of 95 is obtained without using a binder, an internal lubricant or the like as described above. % Or more of a high-density powder molded body is obtained. As a result, it is considered that the residual pores including coarse residual pores in the iron-based sintered alloy can be reduced in diameter, and the mechanical properties of the iron-based sintered alloy are greatly improved.

(3) “Coarse residual pores” in the present specification are coarse residual pores that exist specifically, and the size thereof is different from the average diameter of the residual pores. Such coarse residual pores include, for example, (i) that carbon in agglomerated graphite powder diffuses into pure iron or iron alloy powder during sintering, or (ii) voids formed by bridging of fine powder. It seems to be caused by remaining without being compressed in the molding process.

  The size of the coarse residual pores is about 100 to 250 μm, but according to the present invention, the size is 63 μm or less, further 50 μm or less.

  As described above, the coarse powder and the fine powder in the present invention are not limited to one type, but may be two or more types. A mixture of a coarse powder having a different average particle diameter within the above range and a fine powder having a different average particle diameter within the above range may be used. Moreover, even if average particle diameter is the same, what mixed 2 or more types of coarse powder or fine powder from which a composition differs may be used. In addition, the average particle diameter of such a main raw material powder is a particle diameter with a cumulative frequency of 50% in the powder particle diameter distribution curve.

  The mechanical properties of the iron-based sintered alloy of the present invention vary depending on the composition of the raw material powder, the compact density (or compacting pressure), the sintering conditions (temperature, time, atmosphere, etc.), and cannot be specified in general. . In other words, regarding the “strength”, the tensile strength is preferably 2200 MPa or more, 2300 MPa or more, and further 2350 MPa or more. Speaking of the bending strength, it is preferably 3000 MPa or more, 3200 MPa or more, 3500 MPa or more, 3700 MPa or more, further 3800 MPa or more. Speaking of the endurance fatigue limit (rotary bending fatigue strength), it is preferably 550 MPa or more, 600 MPa or more, 650 MPa or more, and further 670 MPa or more. In addition, the “toughness” is preferably, for example, a deflection amount in a later-described bending test of 1.0 mm or more, 1.2 mm or more, and further 1.4 mm or more. Further, the elongation in the tensile test is preferably 1% or more, 2% or more, 3% or more, and more preferably 4% or more.

  The “iron-based sintered alloy” referred to in the present specification may be a material such as an ingot shape, a rod shape, a tubular shape, a plate shape, etc. It may be. Therefore, this iron-based sintered alloy can be rephrased as “iron-based sintered alloy member”.

Unless otherwise specified, “x to y” in the present specification includes the lower limit x and the upper limit y. Moreover, the lower limit and the upper limit described in this specification can be arbitrarily combined. The upper limit and the lower limit include an upper limit x and a lower limit y in a description format such as “x to y”.
<Method for producing iron-based sintered alloy>

(1) Said iron-based sintered alloy is obtained by the manufacturing method of the present invention as follows, for example. That is, the method for producing an iron-based sintered alloy according to the present invention comprises one or more coarse powders made of pure iron or an iron alloy and one or more fine powders made of pure iron or an iron alloy having an average particle size smaller than that of the coarse powder. A mixing step of mixing a raw material powder containing a mixture powder, a molding step of filling the mixed powder into a mold and pressing to form a powder compact, and heating and sintering the powder compact in an antioxidant atmosphere And a sintering step to form a sintered body, wherein the average particle size (d) of the fine powder is 1 ≦ d ≦ 25 (μm), and the average particle size (D) of the coarse powder is d < D ≦ 50 (μm), and the high-strength iron-based sintered alloy of the present invention is obtained after the sintering step.

(2) By the way, as the finer main raw material powder is used, the total surface area, which is the sum of the surface areas of the particles, increases. Since oxygen or the like is usually attached to the surface of each powder particle, oxygen or moisture introduced into the powder compact increases as the powder having a smaller average particle diameter is used. . Such deposits such as oxygen, moisture, and oil adhering to the surface of the powder particles are not particularly problematic at the stage of the powder compact, but especially when the powder compact is sintered, Thus, when a high-density powder compact is sintered, the strength and shape stability of the iron-based sintered alloy can be affected.

For example, when a high-density powder compact is formed, the pores formed between the particles of the raw material powder are closed pores (sealed pores). For this reason, if the deposits (especially gases such as oxygen) adhering to the powder are not sufficiently removed before molding, they are trapped in the closed pores and thermally expanded due to the high temperature during sintering. obtain. Furthermore, oxygen or the like adhering to the particle surface reacts with an alloy component (for example, carbon (C)) of the powder or an alloy powder (for example, graphite powder) mixed in the raw material powder to react with another gas (for example, In some cases, volume expansion may occur as CO or CO ₂ ). Such a case may be a factor for expanding the coarse residual pore diameter in the sintered body.

  Therefore, when producing the iron-based sintered alloy of the present invention, it is of course possible to sinter under an oxidation-preventing atmosphere, as well as under a high vacuum, particularly in an atmosphere where the oxygen concentration is as low as possible. preferable. However, even if sintering is performed in such an atmosphere, the powder molded body according to the present invention is dense and often has closed pores. For this reason, it is hard to expect that gas etc. will escape from the inside of a powder compact in a sintered compact. In consideration of industrial mass production, it is not realistic to form a powder compact after pre-processing the raw material powder in a vacuum or in an extremely low oxygen atmosphere. In addition, this does not necessarily remove oxygen and the like adhering from the surface of the powder particles at room temperature.

  Then, the problem mentioned above is solved by taking in oxygen etc. which adhered to the powder surface in a sintered compact by the form control which changes to the solid of an oxide. Here, when an oxide or the like is formed at the neck portion, which is a joint portion between the particles, the inclusion is also expected to be a starting point for new destruction. However, in reality, the inclusions are small compared to the residual pores, and there is almost no formation of large inclusions that serve as starting points for destruction at the neck portion. Therefore, mechanical properties such as strength and toughness of the iron-based sintered alloy are rarely reduced by the formation of such oxides.

  Based on the above, the iron-based sintered alloy and the method for producing the same according to the present invention are such that the raw material powder is separated from the coarse powder and the elements contained in the fine powder separately from the coarse powder and the fine powder. It is preferable to include an easily oxidizable powder made of an easily oxidizable element that easily binds to oxygen (O).

  As a result, for example, even when a fine powder containing a large amount of oxygen is used as the main raw material powder to form a high-density sintered body, the addition of a small amount of easily oxidizable (base) element reduces the strength characteristics due to oxygen. Sufficiently suppressed. In other words, the addition of a small amount of easily oxidizable (base) element reduces the frequency of coarse residual pores, while ensuring the effect of reducing the diameter of the iron-based sintered alloy with excellent strength and toughness. can get.

(3) The iron-based sintered alloy of the present invention is manufactured from fine raw material powder, but unlike the above-described prior art, granulation or the like is not necessary in the manufacturing process, and it is necessary to use a binder or the like. Nor. In the present invention, it is only necessary to mix the raw material powders described above so that the particle size distribution of the raw material powder as a whole becomes appropriate. However, as the powder having a smaller average particle diameter is used, fine powder particles are more likely to be secondarily aggregated or bridged in the present invention as well.

  Therefore, the present inventor efficiently imparts secondary agglomeration and bridging of the powder even when fine raw material powder is used by applying mechanical energy and plasma energy to the raw material powder in addition to normal rotational mixing energy. It was newly found that a uniform mixed powder can be obtained while suppressing it.

  Therefore, it is preferable that the mixing step of the present invention includes an energy application step of applying mechanical energy or excitation energy to the interface of the particles constituting the raw material powder. Of course, each of these energy application steps can be performed independently, but is more efficient when combined with the conventional rotation mixing energy application step.

  Accordingly, a uniform powder compact made of a uniform mixed powder can be obtained while using fine raw material powder without granulation, etc., and finally the residual pores can be reduced in size and coarse residual pores. Therefore, the strength and toughness of the iron-based sintered alloy can be reliably improved.

<Additional configuration>
In addition to the above-described configuration, the iron-based sintered alloy of the present invention or the method for producing the same is preferably added with one or two or more arbitrarily selected from the configurations listed below. . It should be noted that a configuration selected from the following can be added to a plurality of inventions in a superimposed manner and arbitrarily.

  In addition, for convenience, the iron-based sintered alloy itself and its manufacturing method are described separately, but any of the configurations shown below can be combined with each other across categories as appropriate. For example, if it is the structure regarding the composition of raw material powder, it is clear that it is related to the iron-based sintered alloy itself and its manufacturing method. Also, at first glance, it may seem like a configuration related to “method”, but if it is understood as a product-by-process, it can also be related to “product”.

(1) Iron-based sintered alloy
(i) The powder compact has a powder compact density ratio (ρ / ρ ₀ x100%) which is a ratio of the bulk density (ρ) to the theoretical density (ρ ₀ ) of 95% or more, 96% or more, 97% or more. Furthermore, it has a high density of 98% or more.
(ii) The raw material powder or sintered body is Cr: 0.1-10% by mass, Mo: 0.1-2.5% by mass, Mn: 0.01- It contains a modifying element that modifies any one or more of 1.2% by mass, Si: 0.01 to 1.2% by mass, and C: 0.01 to 1% by mass.
In addition, in the case of raw material powder (mixed powder), the whole composition is obtained by converting from various powders and their blending ratio. On the other hand, since the composition of the sintered body (iron-based sintered alloy) is not a uniform composition when viewed at the particle level of the raw material powder, a method for understanding an average composition of at least about 1 cm ³ (for example, chemical wet analysis) Determines the overall composition.
(iii) The component composition of the coarse powder is different from the component composition of the fine powder.
(iv) The coarse powder is composed of a plurality of types of powders having different average particle sizes or component compositions.
(v) The fine powder is composed of a plurality of types of powders having different average particle sizes or component compositions.
(vi) At least one of the coarse powder and the fine powder is made of low carbon steel having a C content of 0.4% by mass or less when the total powder is 100% by mass.
(vii) At least one powder contained in the raw material powder is Cr: 0.1 to 15% by mass, Mo: 0.2 to 3.0% by mass, Mn: 100% by mass of the whole powder It consists of an iron alloy containing one or more modifying elements of 0.03 to 1.5 mass%, Si: 0.01 to 1.5 mass%, and C: 0.01 to 0.4 mass%.
(viii) The raw material powder includes a modified powder composed of the modified element.
(ix) The modified powder is either a simple substance (pure substance) powder, an iron alloy (ferroalloy) powder, or a compound powder.
(x) The modifying element is prealloyed to the coarse powder or fine powder.
(xi) The raw material powder is a Cu-free powder substantially free of Cu.
(xii) The coarse powder is an iron alloy powder comprising 0.1 to 3.0% by mass of Mo, the balance being Fe and inevitable impurities.
(xiii) The fine powder is 0.5-3.5 mass% Cr, 0.1-0.6 mass% Mo, 0.01-1 mass% Mn, 0.01-0.5 mass. % Of iron, 0.001 to 1% by mass of C, and the balance is Fe alloy powder composed of Fe and inevitable impurities.
(ix) The raw material powder includes, apart from the coarse powder and the fine powder, an easily oxidizable powder composed of an easily oxidizable element that is more easily bonded to oxygen (O) than the elements contained in the coarse powder and the fine powder. .
(xv) The coarse powder and the fine powder are low carbon steel powder having a C content of 1.0% by mass or less when the total amount of each powder is 100% by mass, and the raw material powder is further a carbon powder including.

(2) Manufacturing method of iron-based sintered alloy
(i) The molding step includes a filling step in which the mixed powder is filled into the mold in which a higher fatty acid-based lubricant is applied to the inner surface, and the mixed powder is pressed in a warm state on the inner surface of the mold. It consists of a warm pressure forming step for forming a metal soap film different from the higher fatty acid lubricant on the surface of the mixed powder in contact therewith.
(ii) The sintering step is a step performed in an inert gas atmosphere having an extremely low oxygen partial pressure corresponding to an oxygen partial pressure of 10 ⁻¹⁹ Pa or less.
(iii) The easily oxidizable powder is any of a simple substance (pure substance) powder, an iron alloy (ferroalloy) powder, or a compound powder containing the easily oxidizable element.
(iv) The easily oxidizable powder is either a Mn—Si based powder made of an alloy or compound of Mn and Si or an Fe—Mn—Si powder made of an alloy or compound of Fe, Mn and Si.
(v) The mixing step includes at least one of rotational mixing, stirring mixing, and plasma mixing.
(vi) In the mixing step, either the stirring mixing or the plasma mixing is added to the rotating mixing.
(vii) The molding step includes a filling step in which the mixed powder is filled into the mold coated with a higher fatty acid-based lubricant on the inner surface, and the mixed powder is pressed in a warm state on the inner surface of the mold. This is a warm pressure forming process in which a metal soap film different from the higher fatty acid-based lubricant is formed on the surface of the mixed powder in contact therewith.
(viii) The sintering step is a step performed in an inert gas atmosphere having an extremely low oxygen partial pressure corresponding to an oxygen partial pressure of 10 ⁻¹⁹ Pa or less.
(ix) The raw material powder contains C as a whole, and further includes a heat treatment step for quenching and tempering the sintered body after the sintering step.

  Next, the present invention will be described in more detail with reference to embodiments. In addition, the content demonstrated by this specification including the following embodiment is applicable not only to the ferrous sintered alloy which concerns on this invention but its manufacturing method suitably. Which embodiment is the best depends on the target, required performance, and the like.

<Raw material powder>
(1) Average particle size and blending ratio
(a) The raw material powder of the iron-based sintered alloy of the present invention comprises at least two kinds of coarse powder and fine powder (main raw material powder) having different average particle diameters.

  If the average particle diameter (d) of the fine powder is too small, powder molding is difficult, secondary aggregation and bridging are likely to occur, and the surface area increases, resulting in increased adhesion of oxygen, moisture, etc. May be incurred. Further, if it is excessive, the residual pores cannot be sufficiently reduced in diameter. The average particle diameter of the fine powder is preferably 1 μm or more, 2 μm or more, 3 μm or more, 4 μm or more, further 5 μm or more, and preferably 20 μm or less, 15 μm or less, or 13 μm or less.

  If the average particle size (D) of the coarse powder is not larger than the average particle size (d) of the fine powder, a desired particle size distribution cannot be obtained in terms of strength. On the other hand, if the average particle size of the coarse powder is excessive, the fine powder and the coarse powder are separated during mixing, and it is not possible to reduce the residual pore size, and secondary aggregation and bridging of the fine powder are likely to occur. Absent.

The average particle size of the coarse powder is at 50μm or less, and further, more than 26 .mu.m, more 28 .mu.m, preferable to be 30μm or more, and preferably a 40μm or less.

  (b) A suitable blending ratio of the coarse powder and the fine powder may vary depending on the average particle diameter of the powder, but within the range of the above average particle diameter, the ratio of the fine powder to the whole raw material powder is 5 to 45% by mass. This is preferable. If the amount of fine powder is too small, it will not be possible to achieve a sufficient residual pore size reduction. An excessive amount of fine powder is not preferable because secondary aggregation and bridging of the fine powder are likely to occur and powder molding becomes difficult.

  The ratio of the fine powder to the whole raw material powder is 7 mass% or more, 9 mass% or more, 13 mass% or more in order to obtain an appropriate particle size distribution in order to improve the mechanical properties of the iron-based sintered alloy. Is preferably 17% by mass or more, and is preferably 42% by mass or less, 35% by mass or less, 32% by mass or less, and further preferably 25% by mass or less.

(2) Powder composition and composition
(a) The main raw material powder is an iron-based powder and is made of pure iron or an iron alloy. The alloying element contained in the iron alloy powder is one or more modifying elements such as C, Mn, Si, Cr, Mo, V, Ti, Nb, and Ni. These alloy elements are effective for improving the heat treatment property of the iron-based sintered alloy and the strength thereof.

  Elements other than Fe may exist as modified powders other than the main raw material powder, in addition to the case where they exist as alloy elements in the main raw material powder. For example, C, which is important when heat treating an iron-based sintered alloy, may be blended into the main raw material powder as a C-based powder such as graphite powder.

  There is also a powder containing an element (an easily oxidizable element) that serves as an oxygen getter that takes in oxygen adhering to the particle surface of the main raw material powder as an oxide and suppresses the formation of coarse residual pores in the iron-based sintered alloy. In addition to the alloying element in the main raw material powder, it can be blended as a modified powder. Examples of such easily oxidizable elements include Si and Mn. These Si, Mn, etc. have a very high affinity with O, and the oxide formation free energy is sufficiently low. Further, it is a basic element of steel and an element that can be obtained at a relatively low cost, and does not hinder the recyclability of the iron-based sintered alloy.

  Such an easily oxidizable element may be blended into the main raw material powder as a simple substance, an alloy or a compound powder in addition to the case where it is contained as an alloy element of the main raw material powder. Taking Mn and Si as an example, there are also Mn-Si-based powders composed of alloys or compounds thereof, but Fe composed of an alloy or intermetallic compound of Fe, which is the main component of an iron-based sintered alloy, and Mn and Si. -Mn-Si powder (hereinafter, this powder is referred to as "FeMS powder" as appropriate) is preferable.

  FeMS powder has higher affinity with O and lower free energy of oxide formation than Mn and Si alone. Moreover, they are available at a lower cost than those alone. Therefore, if an Fe—Mn—Si powder (FeMS powder) or the like is used, an iron-based sintered alloy having high density and excellent mechanical properties can be obtained at low cost.

  FeMS powder is 100% by mass of the entire FeMS powder, for example, Si is 15 to 75% by mass, Mn is 15 to 75% by mass, or the total of Si and Mn is 35 to 95% by mass, and the main balance Is preferably Fe. If the amount of Si or Mn is too small, it becomes a ductile iron alloy and it becomes difficult to pulverize it into fine powder. In addition, in order to secure the necessary amount of Si, the amount of FeMS powder added to the raw material powder increases, which increases the cost of the iron-based sintered alloy member, which is not preferable. However, too much Si or Mn is not preferable because the cost for adjusting the components increases. More preferably, Si is 20 to 65 mass%, Mn is 20 to 65 mass%, and the total of Mn and Si is 50 to 90 mass%.

  As the particle diameter of the easily oxidizable powder is smaller, the density ratio of the compact and the density ratio of the sintered body are improved, and a homogeneous iron-based sintered alloy member with less component variation and segregation can be obtained. However, powders with an excessively small particle size are difficult to obtain and costly. Aggregation and the like are likely to occur, and the handleability is poor. Therefore, Si-based powder having a particle size of 63 μm or less, further 45 μm or less, or 25 μm or less may be used. In addition, the particle size as used in this specification is specified by sieving.

  (b) The composition of the raw material powder as a whole or the iron-based sintered alloy as a whole is desired for the entire iron-based sintered alloy in consideration of mechanical properties such as heat treatment, strength or toughness. Depends on the composition. In addition to Fe, there are the following compositions of elements contained in the raw material powder or the iron-based sintered alloy.

  For example, when the whole raw material powder or the whole sintered body is 100% by mass, Cr is preferably 0.1 to 10% by mass, and more preferably 0.1 to 3.0% by mass. Cr is an element that improves hardenability and improves mechanical properties. If it is too small, the effect of increasing the strength is poor, and if it is too large, precipitation of a low-strength phase and a brittle phase is not preferable.

  Mo is preferably 0.1 to 2.5 mass%, more preferably 0.1 to 2.0 mass%. Mo is an element that improves hardenability and improves mechanical properties. If this amount is too small, the effect of increasing the strength is poor, and if it is too large, precipitation of a low-strength phase and a brittle phase is caused and the cost is increased.

  Mn is preferably 0.01 to 1.2% by mass, more preferably 0.05 to 0.6% by mass. Mn is an element that improves the mechanical properties (strength, ductility, etc.) of the iron-based sintered alloy together with Si. If the amount is too small, the effect is poor. If the amount is too large, the strength is reduced.

  Si is preferably 0.01 to 1.2% by mass, more preferably 0.05 to 0.5% by mass. Si is an element that acts as a deoxidizer and improves ductility and toughness. If it is too small, its effect is poor, and if it is too large, it can cause embrittlement and a decrease in strength.

  C is preferably 0.01 to 1% by mass, more preferably 0.1 to 0.7% by mass. C is an element that improves mechanical properties such as strength and hardness. If C is too small, a high-strength iron-based sintered alloy cannot be obtained, and if C is excessive, ductility decreases, which is not preferable.

  Incidentally, C introduced into the iron-based sintered alloy is preferably mixed as carbon powder such as graphite powder in the raw material powder from the viewpoint of formability of the raw material powder and ease of adjustment of the C amount. This is because it is not preferable to use high carbon steel having a high C amount for coarse powder or fine powder, because powder molding becomes difficult. When carbon powder is contained in the raw material powder, C diffuses from the carbon powder during sintering, and the iron-based sintered alloy is strengthened by solid solution. Furthermore, an iron-based sintered alloy containing an appropriate amount of C is significantly improved in mechanical properties by performing a heat treatment step such as quenching and tempering. It should be noted that carburization or the like can be used for introducing C into the iron-based sintered alloy.

  By the way, the iron-based sintered alloy of the present invention does not need to contain Cu, and is excellent in mechanical properties. That is, the iron-based sintered alloy of the present invention is Cu-free substantially free of Cu that is difficult to remove by smelting or the like, and has sufficient mechanical properties. Therefore, the iron-based sintered alloy of the present invention improves recyclability and is preferable for environmental measures. Further, by not using Cu, it is possible to reduce the material cost of the iron-based sintered alloy and avoid hot brittleness of the iron-based sintered alloy due to Cu.

(c) What kind of composition and what kind of powder is mixed as the raw material powder is first determined depending on what kind of composition the iron-based sintered alloy is desired. In addition, the properties of the iron-based sintered alloy can be affected by the availability and cost of the powder and the composition and form of the powder.
For example, the coarse powder and the fine powder, which are the main raw material powders, are more preferably the following powders in addition to those as described above.

  First, the coarse powder contains at least one of 0.25 to 2.0% by mass of Cr and 0.1 to 3.0% by mass of Mo, with the entire powder being 100% by mass, with the balance being iron and inevitable impurities. A certain iron alloy powder is preferable because it is excellent in heat treatment and availability.

  The fine powder contains at least one of 0.5 to 3.5% by mass of Cr and 0.1 to 0.6% by mass of Mo, with the rest of the powder being 100% by mass, with the balance being iron and inevitable impurities. The alloy powder is preferable because it is excellent in heat treatment property, availability, and the like. Further, the fine powder preferably contains at least one of 0.01 to 1% by mass of Mn, 0.01 to 0.5% by mass of Si, and 0.01 to 1% by mass of C.

  The mechanical characteristics of the iron-based sintered alloy can be further improved by individually adjusting the composition of the coarse powder and the fine powder. For example, when the composition of the fine powder is Cr-rich, the neck portion is strengthened and the strength and fatigue strength of the iron-based sintered alloy can be improved. On the other hand, by adjusting the composition of the coarse powder, the strength, toughness, or ductility inside the neck portion and the particles can be optimized.

<Production method>
The method for producing an iron-based sintered alloy according to the present invention mainly includes a mixing step, a forming step, and a sintering step, and a heat treatment step is appropriately performed. Hereinafter, each step will be described in detail.

(1) Mixing process A mixing process is a process of mixing raw material powder containing a coarse powder and a fine powder into a mixed powder. By this mixing step, various raw material powders are uniformly mixed, and a homogeneous iron-based sintered alloy can be obtained stably.

  The mixing method of the raw material powder is not particularly limited as long as the raw material powder is uniformly mixed. However, the raw material powder of the present invention contains a considerable amount of fine powder having an average particle size of 25 μm or less. The powder particles tend to become more agglomerated as the size of the particles increases. For this reason, in the conventional rotary mixing method using a ball mill or the like, it is not easy to sufficiently loosen the aggregation and obtain a mixed powder in which fine powder particles are finely dispersed in a short time.

  Therefore, the present inventor first came up with the idea of not only rotating and mixing the raw material powder but also positively applying a strong force to the raw material powder to enhance the stirring effect. As a result, each particle of the powder or the powder particle and a rigid body such as a rotor blade are rubbed strongly, and not only simple mixing but also crushing, crushing, surface modification, smoothing, shape control (spherical shape) of the powder particles Etc.) becomes possible. In this specification, such mixing is referred to as “stir mixing”. This can also be referred to as an energy application step of applying mechanical energy to the interface of the constituent particles of the raw material powder.

  Next, the present inventor has come up with the idea of imparting electrical energy to the interface of the constituent particles of the raw material powder in addition to rotational mixing and stirring mixing. For example, plasma discharge is performed between the electrodes provided in the container that accommodates the raw material powder, and discharge energy is applied to the interface of the constituent particles of the raw material powder. In this specification, such mixing is referred to as “plasma mixing”. This can also be referred to as an energy application step for applying excitation energy to the interface of the constituent particles of the raw material powder.

(2) Molding process The molding process of the present invention is a process in which the above-mentioned mixed powder is filled into a mold and pressed to form a powder compact. As long as the high-density powder compact referred to in the present invention can be obtained, the shape of the powder compact and the molding pressure itself are not a problem.

  But this inventor has established the shaping | molding method which can obtain such a high-density molded object (refer patent document 1). According to this molding method, it is possible to perform ultra-high pressure molding exceeding the conventional level, such as a molding pressure of 1000 MPa or more, 1200 MPa or more, 1500 MPa or more, or about 2000 MPa, at an industrial level. The density of the powder compact thus obtained can reach 95% or more, 96% or more, 97% or more, 98% or more, and even 99%. Hereinafter, this excellent molding method (hereinafter, this molding method will be referred to as “mold lubrication warm pressure molding method” as appropriate) will be described.

The mold lubrication warm pressure molding method (molding process) consists of a filling process in which the raw material powder is filled in a mold coated with a higher fatty acid-based lubricant, and the raw material powder in the mold is warm. It consists of a warm pressure forming step in which a metal soap film different from the higher fatty acid-based lubricant is formed on the surface of the raw material powder that is pressed and in contact with the inner surface of the mold. According to this molding method, even if the molding pressure is considerably increased, general problems such as galling between the raw material powder and the inner surface of the mold, an excessive release pressure, and a decrease in the mold life do not occur. .
Hereinafter, the filling step and the warm pressure forming step of this forming method will be described in more detail.

(a) Filling step Before the raw material powder is filled into the mold (cavity), a higher fatty acid-based lubricant is applied to the inner surface of the mold (application step). The higher fatty acid-based lubricant used here may be a metal salt of a higher fatty acid in addition to the higher fatty acid itself. Examples of the higher fatty acid metal salts include lithium salts, calcium salts, and zinc salts. In particular, lithium stearate, calcium stearate, zinc stearate and the like are preferable. In addition, barium stearate, lithium palmitate, lithium oleate, calcium palmitate, calcium oleate, and the like can also be used.

  The coating step is performed, for example, by spraying a higher fatty acid lubricant dispersed in water, an aqueous solution, an alcohol solution or the like in a heated mold. When the higher fatty acid lubricant is dispersed in water or the like, it is easy to spray the higher fatty acid lubricant uniformly on the inner surface of the mold. When it is sprayed into the heated mold, moisture and the like are quickly evaporated, and the higher fatty acid-based lubricant uniformly adheres to the inner surface of the mold. The heating temperature of the mold is preferable in consideration of the temperature in the warm pressure molding process described later. For example, heating to 100 ° C. or higher is sufficient. However, in order to form a uniform film of a higher fatty acid-based lubricant, it is preferable that the heating temperature be lower than the melting point of the higher fatty acid-based lubricant. For example, when lithium stearate is used as the higher fatty acid-based lubricant, the heating temperature is preferably less than 220 ° C.

  When the higher fatty acid-based lubricant is dispersed in water or the like, when the total weight of the aqueous solution is 100% by mass, the higher fatty acid-based lubricant is 0.1 to 5% by mass, If it is contained at a ratio of 2% by mass, a uniform lubricating film is preferably formed on the inner surface of the mold.

  Further, when the higher fatty acid-based lubricant is dispersed in water or the like, if the surfactant is added to the water, the higher fatty acid-based lubricant can be uniformly dispersed. Examples of such surfactants include alkylphenol surfactants, polyoxyethylene nonylphenyl ether (EO) 6, polyoxyethylene nonyl phenyl ether (EO) 10, anionic nonionic surfactants, and boric acid. Ester-based Emulbon T-80 or the like can be used. Two or more of these may be used in combination. For example, when lithium stearate is used as a higher fatty acid-based lubricant, three types of polyoxyethylene nonylphenyl ether (EO) 6, polyoxyethylene nonylphenyl ether (EO) 10 and borate ester Emulbon T-80 are available. It is preferable to use a surfactant at the same time. This is because the dispersibility of lithium stearate in water or the like is further activated as compared with the case where only one of them is added. In order to obtain an aqueous solution of a higher fatty acid-based lubricant having a viscosity suitable for spraying, it is preferable that the total amount of the aqueous solution is 100% by volume and the ratio of the surfactant is 1.5 to 15% by volume.

  In addition, a small amount of an antifoaming agent (for example, a silicon-based antifoaming agent) may be added. This is because when the foaming of the aqueous solution is intense, it is difficult to form a uniform film of higher fatty acid lubricant on the inner surface of the mold when sprayed. The addition ratio of the antifoaming agent may be, for example, about 0.1 to 1% by volume when the total volume of the aqueous solution is 100% by volume.

  The higher fatty acid-based lubricant particles dispersed in water or the like preferably have a maximum particle size of less than 30 μm. When the maximum particle size is 30 μm or more, the higher fatty acid-based lubricant particles are likely to precipitate in the aqueous solution, making it difficult to uniformly apply the higher fatty acid-based lubricant to the inner surface of the mold.

  Application of the aqueous solution in which the higher fatty acid-based lubricant is dispersed can be performed using, for example, a spray gun for painting, an electrostatic gun, or the like. In addition, as a result of investigating the relationship between the coating amount of the higher fatty acid-based lubricant and the extraction pressure of the powder molded body, the present inventor has found that the higher fatty acid-based lubricant has a film thickness of about 0.5 to 1.5 μm. It is preferable to apply a lubricant to the inner surface of the mold.

(b) Warm pressure molding process When the filled powder is warm-pressed into a mold coated with a higher fatty acid-based lubricant on the inner surface, the mixed powder (or powder compact) is in contact with the mold inner surface. ), A metal soap film different from the applied higher fatty acid lubricant is formed. The presence of this metal soap film is considered to enable ultra-high pressure molding that can be mass-produced at an industrial level.

  This metal soap film is firmly bonded to the surface of the powder molded body, and exhibits a lubricating performance far superior to the higher fatty acid-based lubricant adhered to the inner surface of the mold. As a result, the frictional force between the contact surfaces of the inner surface of the mold and the outer surface of the powder molded body is remarkably reduced, and no galling or the like occurs despite high-pressure molding. In addition, the powder compact can be taken out from the mold with a very low depressurization pressure, and there is no extreme shortening of the mold life.

  The metal soap film is, for example, an iron salt film of a higher fatty acid formed by causing a mechanochemical reaction between a higher fatty acid-based lubricant and Fe in the mixed powder under a warm high pressure. A typical example is an iron stearate film formed by reacting lithium stearate or zinc stearate, which is a higher fatty acid lubricant, with Fe.

  The “warm” in this step may be a heated state that can accelerate the reaction between the raw material powder and the higher fatty acid-based lubricant. Generally speaking, the molding temperature may be 100 ° C. or higher. However, the molding temperature is preferably set to 200 ° C. or less from the viewpoint of preventing deterioration of the higher fatty acid-based lubricant. It is more preferable that the molding temperature is 120 to 180 ° C.

  “Pressurization” in this step may be appropriately determined within a range in which a metal soap film is formed in consideration of the specifications of the iron-based sintered alloy. Considering the mold life and productivity, the upper limit of the molding pressure is preferably 2000 MPa. When the molding pressure is about 1500 MPa, the density of the obtained powder compact approaches the true density, and even if the pressure is increased to 2000 MPa or more, further increase in density cannot be expected.

  In addition, when this mold lubrication warm pressure molding method is used, since it is not necessary to use an internal lubricant, a higher-density powder molded body can be obtained. When an internal lubricant used in general powder molding is used, it is difficult to obtain a powder molded body having a molded body density ratio of 95% or more as referred to in the present invention at least on an industrial level. Of course, even if it is the powder compact of the present invention, it does not exclude the inclusion of a small amount of internal lubricant, but by not including the internal lubricant, when the powder compact is sintered, Contamination in the furnace due to decomposition and release of the lubricant can be prevented.

(3) Sintering process A sintering process is a process which heats and sinters the powder compact obtained at the formation process in antioxidant atmosphere.
The sintering temperature and the sintering time are appropriately selected in consideration of desired characteristics and productivity of the iron-based sintered alloy. The higher the sintering temperature, the higher the strength of the iron-based sintered alloy can be obtained in a short time. However, when the sintering temperature is too high, a liquid phase is generated or dimensional shrinkage is increased, which is not preferable. If the sintering temperature is too low, the diffusion of alloying elements and strengthening elements contained in various blended powders is not preferable. In addition, the sintering time becomes longer, and the productivity of the iron-based sintered alloy decreases.

The sintering temperature is preferably 1100 to 1400 ° C, more preferably 1150 to 1350 ° C. In particular, when obtaining a high-strength iron-based sintered alloy member, the sintering temperature is preferably 1200 ° C. or higher.
The sintering time is preferably 0.1 to 3 hours, more preferably 0.1 to 2 hours in consideration of the sintering temperature, the specifications of the iron-based sintered alloy, productivity, cost, and the like.

  The sintering atmosphere is preferably an antioxidant atmosphere. Here, Mn and Si contained in the easily oxidizable powder are elements that have extremely strong affinity with O and are very easily oxidized. In particular, when FeMS powder is used, the free energy of oxide formation is lower than that of Mn and Si alone, so it combines with a small amount of O in the heating furnace to form oxides of Mn and Si in the sintered body. There is a fear. Such inclusion of oxides is not preferable because it degrades the mechanical properties of the iron-based sintered alloy. Therefore, the sintering atmosphere is preferably an anti-oxidation atmosphere such as a vacuum atmosphere, an inert gas atmosphere, or a nitrogen atmosphere.

  Even in such an atmosphere, when residual oxygen (oxygen partial pressure) in the atmosphere becomes a further problem, high-purity hydrogen purified to nitrogen gas and hydrogen gas (low dew point (eg, −30 ° C. or lower)) You may employ | adopt the reducing atmosphere which mixed several volume% (for example, 5-10%).

However, the use of hydrogen gas is not very desirable industrially. Therefore, it is more preferable that the sintering step of the present invention is performed in an inert gas atmosphere having an extremely low oxygen partial pressure corresponding to an oxygen partial pressure of 10 ⁻¹⁹ Pa or less (CO concentration of 100 ppm or less).

  In such an inert gas atmosphere with an extremely low oxygen partial pressure, even if a complex oxide or the like is formed by the reaction between FeMS powder and O adhering to the raw material powder during sintering, it is further decomposed. Will be. As a result, an iron-based sintered alloy having a sound structure free from inclusions such as oxides can be obtained. In addition, the continuous sintering furnace which implement | achieves the inert gas (N2 gas) atmosphere of a very low oxygen partial pressure is marketed (Oxynon furnace by Kanto Yakin Kogyo Co., Ltd.).

<Sintered body>
(1) Density The density after sintering is preferably 7.5 g / cm ³ or more, more preferably 7.6 g / cm ³ or more. By sintering an ultra-high-density powder compact having a compact density ratio (ratio of the bulk density of the compact to the theoretical density) of 95% or more, and more than 96%, such ultra-high-density sintered compact (Iron-based sintered alloy) can be easily obtained.

(2) Structure By applying an appropriate heat treatment to the sintered body after sintering, the structure of the iron-based sintered alloy is partially or entirely modified, and the mechanical properties of the iron-based sintered alloy are improved. Can be improved. The heat treatment includes nitriding treatment, carburizing treatment, and the like, but industrially, quenching and tempering performed on an iron-based sintered alloy containing C are typical. The amount of C to be contained, the type and temperature of quenching, the temperature and time of tempering, and the like are appropriately adjusted according to mechanical properties such as strength and toughness desired for the iron-based sintered alloy.

  Here, unlike the melted carbon steel, the mechanical properties of the iron-based sintered alloy of the present invention are relatively greatly affected by the type of raw material powder (average particle size, composition, etc.). In particular, the mechanical properties of iron-based sintered alloys can be greatly influenced by the bonding relationship between the particles of the coarse powder. For this reason, in order to improve the mechanical characteristics of the iron-based sintered alloy as a whole, it is desirable to improve the mechanical characteristics of the joint portion (neck portion) between the particles. This is the same regardless of whether the particle interface is partially softened or melted during the sintering process or whether the alloy element is diffused and bonded. Therefore, as already described with respect to the raw material powder, for example, the composition of the coarse powder and fine powder of the present invention and other additive powders so that the neck portion is strengthened by heat treatment such as quenching and tempering. It is preferable to select the composition as appropriate.

<Others>
The iron-based sintered alloy of the present invention may be further subjected to heat treatment steps such as annealing, normalizing, aging, tempering (quenching, tempering), carburizing, and nitriding. Of course, the iron-based sintered alloy preferably has a composition (C, Mo, Cr, etc.) according to the type of heat treatment.

  The form and application of the iron-based sintered alloy of the present invention are not limited. An example of an iron-based sintered alloy in which the iron-based sintered alloy of the present invention can be used is as follows. In the automotive field, various pulleys, transmission synchro hubs, engine connecting rods, hub sleeves, sprockets, ring gears, parking gears, There are pinion gears. In addition, there are sun gears, drive gears, driven gears, reduction gears and the like.

Next, the present invention will be described more specifically with reference to examples.
<Manufacture of test pieces>
(1) Mixing process
(a) First, as iron-based powder that can be a coarse powder, which is one of the main raw material powders, Astroy Mo (Fe-1.5% Mo) manufactured by Höganäs, Astroy CrM (Fe-3% Cr--) manufactured by the company 0.5% Mo) and Astaloy CrL (Fe-1.5% Cr-0.2% Mo) manufactured by the same company were prepared. The unit of composition is mass%, and hereinafter, it is simply indicated by “%”.

  The average particle size of these iron-based powders was 80 μm (D50). However, for Astloy Mo, powders of 75 μm, 70 μm, 55 μm, 37 μm, 32 μm, 32 μm and 27 μm were prepared in addition to the average particle size of 80 μm. These powders having different average particle diameters were prepared by sieving. The average particle size was determined from the particle size of the sieved powder.

  (b) Next, as an iron-based powder that becomes one of the main raw material powders, SCM415 (Fe-1% Cr-0.2% Mo-0.75% Mn-0) manufactured by Epson Atmix Co., Ltd. .25% Si-0.33% C). The average particle size of the prepared SCM415 powder is 10 μm and 6 μm.

  (c) As other modified powders, graphite powder (Gr powder) and Si-based powder were prepared. The Gr powder was JCPB manufactured by Nippon Graphite Co., Ltd., and its particle size was 45 μm or less. The Si-based powder is obtained by pulverizing an ingot of Fe-50% Mn-33% Si or Fe-76% Si melted in an Ar gas atmosphere in the atmosphere and having a particle size of 25 μm or less (−500 mesh). Sieving into a powder. Hereinafter, the powder having the composition of Fe-50% Mn-33% Si is referred to as FeMS powder, and the powder having the composition of Fe-76% Si is referred to as FS powder.

(d) Various mixed powders were prepared by appropriately selecting and combining one or more of the above iron-based powders with Gr powder and Si-based powder. The mixed powder was prepared by rotating and mixing with a ball mill until the various powders blended so as to have a desired composition were uniformly mixed (mixing step).
(2) Molding process Molding of the prepared raw material powder was performed by a mold lubrication warm pressure molding method. Specifically, it is as follows.

  (a) Two types of cemented carbide molds each having a φ23 mm cylindrical cavity (cylindrical specimen cavity) and a 10 × 55 mm plate cavity (plate specimen cavity) were prepared. The inner peripheral surface of each mold was previously subjected to TiN coating treatment, and the surface roughness was set to 0.4 Z in terms of 10-point average roughness. Each mold was preheated to 150 ° C. with a band heater.

An aqueous solution in which lithium stearate (LiSt), which is a higher fatty acid-based lubricant, was dispersed was uniformly applied to the inner peripheral surface of the heated mold with a spray gun at a rate of about 1 cm ³ / second (application step). Accordingly, a LiSt film of about 1 μm was formed on the inner peripheral surface of each mold.

  The aqueous solution used here is obtained by dispersing LiSt in water obtained by adding a surfactant and an antifoaming agent. As the surfactant, polyoxyethylene nonylphenyl ether (EO) 6, (EO) 10 and boric acid ester Emulbon T-80 were used. Each of these surfactants was added in an amount of 1% by volume based on the entire aqueous solution (100% by volume). FS antifoam 80 was used as an antifoaming agent. This defoamer was added in an amount of 0.2% by volume based on the entire aqueous solution (100% by volume).

The LiSt used had a melting point of about 225 ° C. and an average particle size of 20 μm. The dispersion amount was 25 g with respect to 100 cm ^{3 of the} aqueous solution. The aqueous solution in which LiSt was dispersed was further refined with a ball mill pulverizer (Teflon-coated steel balls: 100 hours). The stock solution thus obtained was diluted 20 times, and an aqueous solution having a final concentration of 1% was subjected to the coating step.

  (b) The above-mentioned various mixed powders were naturally filled in the cavities of the respective molds on which the uniform LiSt film was formed on the inner surface (filling step). The mixed powder was preheated with a dryer to 150 ° C., the same temperature as the mold.

  Each mixed powder filled in the mold was molded at a pressure of 1568 MPa or 1960 MPa to obtain a powder compact (warm pressure molding step). At any molding pressure, no galling or the like occurred on the inner surface of the mold, and the powder compact could be easily taken out from the mold with a low output.

(3) Sintering process
(a) Each obtained powder compact was sintered in a nitrogen atmosphere using a continuous sintering furnace (Oxynon furnace manufactured by Kanto Metallurgical Industry) (sintering step). The sintering temperature was 1350 ° C., and the sintering time was 60 minutes or 120 minutes.

The soaking time was 30 minutes, and the cooling rate after sintering was 40 ° C./min. The inside of the sintering furnace was an extremely low oxygen partial pressure atmosphere having a CO concentration of 50 to 100 ppm (corresponding to 10 ⁻¹⁹ to 10 ⁻²¹ Pa in terms of oxygen partial pressure).
(b) Thus, φ23 mm cylindrical test pieces and □ 10 × 55 mm plate test pieces made of various iron-based sintered alloys were obtained.

  About a plate-shaped test piece, the heat processing of quenching and tempering was suitably performed after sintering. Quenching was performed by heating at 860 ° C. for 45 minutes in a nitrogen atmosphere and then rapidly cooling in oil at 60 ° C. The subsequent tempering was performed by heating in the atmosphere at 190 ° C. for 1 hour.

<Measurement of test piece>
(1) Using the cylindrical test piece, from the weight and dimensions before and after the sintering, the green body density (GD) and its density ratio, the sintered body density (SD) and its density ratio are obtained. It was.

(2) Using the plate-shaped test piece, a bending test was performed by three-point bending with a fulcrum distance of 40 mm. Also subjected to a tensile test using a tensile test specimen having a parallel part of φ4x ^L 15mm fabricated by machining from the plate specimen. The strength (bending strength) and deflection of each test piece until breaking were determined by a bending test, and the strength (tensile strength) and elongation until each test piece were broken by a tensile test.

(3) it was also carried out rotating bending fatigue test using a tensile test specimen having a parallel part of φ5x ^L 10mm fabricated by machining from above plate specimen. This fatigue test was performed at a test speed of 3000 rpm using an Ono type rotating bending fatigue tester.

<Evaluation of various test pieces>
The results of the above various tests are shown in Tables 1 to 4B and FIGS.
(1) Average particle size of main raw material powder and characteristics of iron-based sintered alloy (Evaluation 1)
(a) As described above, a coarse powder composed of various Astaroy Mo powders having an average particle diameter of 27 to 80 μm, a fine powder composed of SCM415 powder having an average particle diameter of 6 μm or 10 μm, Gr powder, and FeMS powder An iron-based sintered alloy was produced using a mixed powder consisting of 1-A1 to 1-A5 and Sample No. 1-B1 to 1-B7 were obtained.

  In addition, an iron-based sintered alloy was manufactured using a mixed powder composed of any one of the above-mentioned Astaroy CrM powder, Astaroy CrL powder or Astaroy Mo powder, and Gr powder and FeMS powder. 1-C1 to 1-C3.

All these iron-based sintered alloys are obtained by sintering powder compacts molded at 1568 MPa at 1350 ° C. for 30 minutes. The data shown in Table 1 and FIG. 1 are measured by subjecting the test pieces made of these iron-based sintered alloys to the heat treatment under the conditions described above.
(b) From Table 1 and FIG.

  That is, the bending force and the amount of deflection suddenly increase from the vicinity where the average particle diameter of the coarse powder changes from 70 μm to 55 μm, and this tendency may increase as the average particle diameter of the coarse powder decreases. I understand. Furthermore, the sample No. 2 in which the average particle diameter of the coarse powder is 80 μm. Sample Nos. 1-A1 and 1-B1, and the average particle size of the coarse powder of 55 μm or less. As is clear when comparing 1-A2 to 1-A5 and 1-B4 to 1-B7, the influence of the average particle size of the fine powder on the strength characteristics is such that the average particle size of the coarse powder is around 70 μm. I understood that it would reverse. That is, when the average particle size of the coarse powder is smaller than 63 μm as in the present invention, the strength characteristic is more favorable when the average particle size of the fine powder is also smaller. On the other hand, when the average particle diameter of the coarse powder was about 80 μm, the strength characteristics were more favorable when the average particle diameter of the fine powder was larger.

  In the range where the average particle diameter of the coarse powder is less than 63 μm, the strength of the main raw material powder is larger than that of one kind of iron-based sintered alloy (sample No. 1-C1 to 1-C3). I also understood.

  (c) Further, the density of the powder compact mainly depends on the average particle size of the fine powder, and it becomes smaller as the average particle size of the fine powder becomes smaller. On the other hand, the density of the sintered body tends to depend on the average particle size of the coarse powder, and it increases as the average particle size of the coarse powder decreases.

(2) Mixing ratio of fine powder and characteristics of iron-based sintered alloy (Evaluation 2)
(a) An iron-based sintered alloy is produced using a mixed powder composed of a coarse powder having an average particle size of 80 μm, 32 μm, and 27 μm, a fine powder having an average particle size of 10 μm, a Gr powder, and an FeMS powder, Sample No. 2-A1 to 2-A6, Sample No. 2-B1 to 2-B6 and Sample No. 2-C1-2-C6 was obtained. The conditions of molding, sintering and heat treatment are the same as in the case of evaluation 1 above. The measurement results of each sample are shown in Table 2 and FIG.

(b) From Table 2 and FIG.
That is, it was found that the bending strength of all the samples made of a powder obtained by mixing a fine powder with a coarse powder having an average particle size smaller than 63 μm increased rapidly. On the other hand, in a sample made of a powder obtained by mixing a fine powder with a coarse powder having an average particle size larger than 63 μm, the bending strength tended to decrease. And within the range where the blending ratio of the fine powder is 5 to 45% by mass, the bending strength of the sample with the average particle diameter of the coarse powder less than 63 μm is the bending resistance of the sample with the average particle diameter of the fine powder of more than 63 μm. It became bigger overall than power.

(c) Next, with respect to the deflection amount, within the range where the blending ratio of the fine powder is 5 to 45% by mass, the deflection amount of the sample having the average particle diameter of the coarse powder of less than 63 μm is the average particle diameter of the fine powder. Was larger than the deflection amount of the sample exceeding 63 μm.
(3) Mixing of oxidizable powder (FeMS powder) and characteristics of iron-based sintered alloy (Evaluation 3)

  (a) An iron-based sintered alloy is produced using a mixed powder composed of a coarse powder having an average particle diameter of 32 μm, a fine powder having an average particle diameter of 10 μm, a Gr powder, and an FeMS powder (easily oxidizable powder). Sample No. 3-1 to 3-7 were obtained.

  All these iron-based sintered alloys are obtained by sintering powder compacts molded at 1960 MPa at 1350 ° C. for 120 minutes. The measurement results of each sample are shown in Table 3 and FIG. 3A. Each data shown here is measured by subjecting each test piece to the heat treatment under the conditions described above.

(b) The following can be understood from Table 3 and FIG. 3A.
That is, by adding the FeMS powder to the mixed powder even a little, the tensile strength and elongation of the iron-based sintered alloy are rapidly increased. This tendency appeared when the blending amount of the FeMS powder was 1.5% by mass or less of the whole, and was remarkable when the FeMS powder was 1.0% by mass or less and further 0.75% by mass or less.

  (c) Further, the results of EPMA analysis of the composition of oxides contained in the iron-based sintered alloy in the case where the blending amount of FeMS powder is 0.1% by mass and 0.5% by mass are shown in FIG. 3B. Show.

  Comparing each case, when the blending amount of FeMS powder is 0.1% by mass, Cr oxide and Mn oxide are observed in addition to Si oxide, whereas the blending amount of FeMS powder is In the case of 0.5% by mass, it was found that almost no Cr oxide or Mn oxide was observed, but only Si oxide.

  (d) Based on this result, a test piece made of an iron-based sintered alloy in which the FeMS powder in the mixed powder is replaced with FS powder (Fe-76% Si) or Si powder (pure Si) is used as described in (3 ) Manufactured under the same conditions as shown in (a). FIG. 3C shows the results of measuring the bending strength and the deflection amount of each sample in which the blending amount of the powder was changed. Note that FIG. 3C is a summary of the measurement results of each sample, focusing on the amount of Si converted from the composition of the used Si-based powder (FeMS powder, FS powder, and Si powder). Incidentally, the particle size of the Si-based powder used was set to 25 μm or less.

  From FIG. 3C, it was first found that the characteristics of both the bending strength and the amount of deflection were improved by including the Si-based powder.

  It was found that the iron-based sintered alloy exhibits excellent characteristics in a range where the converted Si amount is 0.02 to 0.1% by mass, and further 0.05 to 0.09% by mass. In particular, it was found that when the converted Si amount is 0.06 to 0.08% by mass, the iron-based sintered alloy has excellent bending strength and deflection amount regardless of the type of Si-based powder used.

(4) Powder mixing method and characteristics of iron-based sintered alloy (Evaluation 4)
(a) An iron-based sintered alloy was produced using a mixed powder composed of a coarse powder having an average particle diameter of 32 μm, a fine powder having an average particle diameter of 10 μm, a Gr powder, and an FeMS powder. 4-A1 to 4-A6 and Sample No. 4-B1 to 4-B7 and Sample No. 4-C1-4-C3 and sample no. 4-D1 to 4-D4 were obtained. Each sample measured the mechanical characteristic about the iron-based sintered alloy which changed the mixing method of raw material powder, and the compounding quantity of Gr powder. The raw material powders were mixed in two ways: only the rotary mixing by the ball mill described above, and stirring and plasma mixing in addition to the rotary mixing.

  This stirring and mixing imparts mechanical energy such as shearing force and frictional force to the powder particle interface. Further, plasma mixing imparts electrical excitation energy to the powder particle interface. These agitation and plasma mixing were performed using a plasma irradiation type nanocula (NC-LAB-P) manufactured by Hosokawa Micron Corporation. Incidentally, rotary mixing by a ball mill was performed for 30 minutes. The above stirring and plasma mixing was performed for 5 minutes.

  As a test piece for measuring a bending property comprising a bending force and a bending amount, a powder molded body molded at 1568 MPa was sintered at 1350 ° C. for 30 minutes. On the other hand, as a test piece for measuring tensile properties consisting of tensile strength and elongation, a powder compact molded at 1960 MPa was sintered at 1350 ° C. for 120 minutes. And each sample was measured after heat-treating each test piece under the above-mentioned conditions. The respective measurement results are shown in Tables 4A and 4B and FIGS. 4A and 4B. In FIGS. 4A and 4B, the amount of C in the entire mixed powder calculated in consideration of the blending amount of the Gr powder in the mixed powder and the amount of carbon contained in the main raw material powder (converted C amount: mass%) It showed paying attention to.

  (b) From Tables 4A and 4B and FIGS. 4A and 4B, it was found that the strength and toughness of the iron-based sintered alloy are affected by the raw material powder mixing method. In particular, the iron-based sintered alloy consisting of a mixed powder obtained by adding agitation and plasma mixing to the conventional rotary mixing is stronger than the iron-based sintered alloy that is simply rotary mixed. It was also found that the toughness was improved overall and irrespective of the converted C amount.

  (c) It was confirmed from Tables 4A and 4B and FIGS. 4A and 4B that the strength and toughness of each iron-based sintered alloy are greatly influenced by the content of C. And it turned out that a bending strength is the most excellent when C amount exists in 0.4-0.6 mass%, especially 0.45-0.55 mass%. On the other hand, it has been found that the tensile strength is most excellent when the C content is 0.45 to 0.65 mass%, particularly 0.5 to 0.6 mass%.

  (d) When the raw material powder is mixed by adding agitation mixing and plasma mixing to the rotational mixing, the position where the intensity peak appears is higher in C than in the case of mixing the raw material powder only by simple rotational mixing. It turns out that it shifts to the quantity side. The reason for this is not clear, but since the ductility of the sample is improved when plasma mixing is added, sufficient elongation is ensured even on the higher C side, and the intensity peak position has shifted to the higher C side. It is done.

(5) Rotating bending fatigue strength and fracture starting point (Evaluation 5)
(a) Iron-based sintered bonding using a mixed powder composed of a coarse powder composed of Astloy Mo powder having an average particle diameter of 32 μm, a fine powder composed of SCM415 powder having an average particle diameter of 10 μm, a Gr powder, and an FeMS powder Produced gold. A sample containing 0.5% by mass of Gr powder was prepared as Sample No. Sample No. 5 was prepared by blending 5-A1, 0.55 mass% Gr powder. 5-A2.

Further, an iron-based sintered alloy was produced using a mixed powder composed of any one of Astloy CrM powder, Astloy CrL powder or Astloy Mo powder having an average particle size of 80 μm, and Gr powder and FeMS powder. 5-B1 to 1-B3. All of these samples were obtained by sintering a powder molded body molded at 1960 MPa at 1350 ° C. for 120 minutes and further performing heat treatment under the conditions described above.
FIG. 5 shows the rotational bending fatigue strength measured for each sample and the fatigue fracture surface observed with a scanning electron microscope (SEM).

(b) From FIG. 5, in the case of a sample made of a main raw material powder in which a coarse powder having an average particle size of less than 63 μm and a fine powder having an average particle size of 25 μm or less are mixed, It was found that the rotational bending fatigue strength of all of the samples consisting of only 700 MPa was as high as 150 to 180 MPa. As is apparent from the micrograph of the fracture surface, the coarse powder and the fine powder sample having a specific average particle diameter have almost no coarse residual pores as the starting point of fatigue fracture, and the size of the residual pores is small. It was confirmed.

It is a graph which shows the relationship between the average particle diameter of main raw material powder, and an intensity | strength characteristic. It is a graph which shows the relationship between the mixture ratio of a fine powder, and intensity | strength characteristics. It is a graph which shows the influence on the tensile characteristics by the mixing | blending of an easily oxidizable powder. It is an EPMA photograph which shows the change of an oxide composition with the increase in the amount of FeMS. It is a graph which shows the influence of Si amount which acts on a bending strength characteristic. It is a graph which shows the influence of the mixing method on a bending characteristic. It is a graph which shows the influence of the mixing method on a tensile characteristic. It is the microscope picture which shows the graph which shows the rotation bending fatigue strength of each sample, and a fracture surface.

Claims (6)

Press-molding a mixed powder in which one or more coarse powders made of pure iron or an iron alloy and one or more fine powders made of pure iron or an iron alloy having an average particle size smaller than the coarse powder are mixed. In an iron-based sintered alloy consisting of a sintered body obtained by heating and sintering a powder molded body,
The average particle diameter (d) of the fine powder is 1 ≦ d ≦ 25 (μm),
The average particle diameter (D) of the coarse powder is d <D ≦ 50 (μm),
The ratio of the fine powder to the whole mixed powder is 5 to 45% by mass,
The sintered body has a sintered body density ratio (ρ ′ / ρ0 ′ × 100%) which is a ratio of a bulk density (ρ ′) of the sintered body to a theoretical density (ρ0 ′) of 96% or more,
An iron-based sintered alloy characterized by high strength. The sintered body is
When the total is 100% by mass,
Chromium (Cr): 0.1 to 10% by mass,
Molybdenum (Mo): 0.1 to 2.5% by mass,
Manganese (Mn): 0.01-1.2% by mass,
Silicon (Si): 0.01 to 1.2% by mass,
Carbon (C): 0.01-1% by mass,
The iron-based sintered alloy according to claim 1, comprising a modifying element that modifies any one or more of the sintered bodies. Sintering density is 7.6 g / cm ³ The iron-based sintered alloy according to claim 1 or 2, which is as described above. A mixing step of mixing raw material powder containing one or more coarse powders made of pure iron or an iron alloy and one or more fine powders made of pure iron or an iron alloy having an average particle size smaller than the coarse powder into a mixed powder When,
A molding step of filling the mixed powder into a mold and pressing to form a powder compact;
A sintering step of heating and sintering the powder compact in an antioxidant atmosphere to form a sintered compact,
The average particle diameter (d) of the fine powder is 1 ≦ d ≦ 25 (μm),
The average particle diameter (D) of the coarse powder is d <D ≦ 50 (μm),
A method for producing an iron-based sintered alloy, wherein the high-strength iron-based sintered alloy according to any one of claims 1 to 3 is obtained after the sintering step. The raw material powder, according to claim 4 wherein Apart from coarse powder and the fine powder, comprising the oxidizable powder consisting of easily oxidizable elements combined with oxygen (O) than elements contained in the crude powder and fine powder A method for producing an iron-based sintered alloy according to 1. The said mixing process is a manufacturing method of the iron-based sintered alloy of Claim 4 or 5 including the energy provision process which provides mechanical energy or excitation energy to the interface of the particle | grains which comprise the said raw material powder.