Indian Patents. 280049:ALLOY STEEL POWDER FOR POWDER METALLURGY, IRON-BASED SINTERED MATERIAL, AND METHOD FOR PRODUCING THE IRON-BASED SINTERED MATERIAL

Title of Invention	ALLOY STEEL POWDER FOR POWDER METALLURGY, IRON-BASED SINTERED MATERIAL, AND METHOD FOR PRODUCING THE IRON-BASED SINTERED MATERIAL
Abstract	An alloy steel powder for powder metallurgy is produced by diffusion-bonding a molybdenum-containing powder in a molybdenum-based amount of 0.05% to 1.5% by mass onto surfaces of a steel powder prealloyed with 0.02% to 0.4% by mass of niobium.

Full Text	ALLOY STEEL POWDER FOR POWDER METALLURGY, IRON-BASED SINTERED MATERIAL, AND METHOD FOR PRODUCING THE IRON-BASED SINTERED MATERIAL BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to an alloy steel powder for powder metallurgy suitable for use in powder metallurgy. In particular, it relates to an alloy steel powder that improves the strength and toughness of a sintered material produced by using the alloy steel powder. The present invention also relates to a sintered material having excellent strength and toughness produced by using the alloy steel powder for powder metallurgy and a method for producing the sintered material. 2. Description of the Related Art [0002] Powder metallurgy can produce parts having complex shapes very close to product shapes (a.k.a., near-net shapes) at high dimensional accuracy and thus significantly reduces the cost for machining. Powder metallurgy products are thus used in various machines and parts in many fields. Recently, in order to reduce the size and weight of parts, improvements on the strength of powder metallurgy products are demanded. There is particularly strong demand for increasing the strength of iron-based powder products 'iron- based sintered compacts). [0003] Green compacts produced from iron-based powders for powder metallurgy are usually made by mixing an iron- based powder with an alloying powder such as a copper powder or a graphite powder and a lubricant such as stearic acid or lithium stearate to prepare an iron-based powder mixture, filling a die with the iron-based powder mixture, and compacting the iron-based powder mixture in the die. Iron based powders are classified into iron powders (e.g., pure iron powders), alloy steel powders, etc., in accordance with the component. Iron-based powders are also classified in accordance with the production method into atomized iron powders, reduced iron powders, etc. According to these classifications, the term "iron powder" is used in a broad sense and includes alloy steel powders. [0004] The density of green compacts obtained by a common powder metallurgy process is usually about 6.6 to 7.1 Mg/m3. The iron-based powder green compacts are sintered into sintered compacts and, if needed, sized or machined to prepare powder metallurgy products. If a higher strength is needed, the sintered compacts may be subjected to a carburizing heat treatment or a bright heat treatment. [0005] Examples of the powder in which alloy elements are added at the stage of raw material powder include the following: (1) a powder mixture containing a pure iron powder and alloy element powders, (2) a prealloyed steel powder in which elements are fully alloyed, and (3) a partially diffused alloy steel powder in which alloy element powders are partially diffusion-bonded onto surfaces of a pure iron powder or a prealloyed steel powder. [0006] The powder mixture (1) containing a pure iron powder and alloy element powders has an advantage that it can achieve a high compressibility comparable to that of pure iron powders. However, during sintering, Mn, Cr, V, Si, Nb, Ti, and the like which are metals having a higher activity than Fe undergo oxidation unless the CO2 concentration and the dew point in a sintering atmosphere or a carburizing atmosphere are stringently controlled to low levels. Thus, the amount of oxygen is not reduced, the alloy elements do not sufficiently diffuse into Fe, the microstructure remains inhomogeneous, and the strengthening of the matrix cannot be achieved. [0007] Due to these drawbacks, the powder mixture (1) containing a pure iron powder and alloy element powders has not been able to meet the recent demand for high strength and is thus no longer used. [0008] In contrast, the prealloyed steel powder (2) in which elements are fully alloyed and which is produced by atomizing molten steel has an advantage that the amount of oxygen can be reduced and a high compressibility comparable to that of pure iron powders can be achieved by limiting the types and amounts of the alloy elements, such as Mn, Cr, V, Si, Nb, and Ti, although oxidation still occurs during the process of atomizing the molten steel and solution hardening occurs due to full alloying. Moreover, there is a possibility that the strength of the matrix will be increased by full alloying and thus this powder has been developed to serve as a prealloyed steel powder for achieving high strength. [0009] The partially diffused alloy steel powder (3) is made by blending a pure iron powder or a prealloyed steel powder with metal powders of respective elements and heating the resulting mixture in a nonoxidative or reducing atmosphere to cause metal powders to partially diffusion- bond onto surfaces of the pure iron powder or the prealloyed steel powder. Thus, the partially diffused alloy powder combines the advantages of the iron-based powder mixture (1) and the prealloyed steel powder (2). Since the partially diffused alloy steel powder (3) reduces the amount of oxygen, achieves high compressibility comparable to that of a pure iron powder, and bears a possibility of increasing the strength of the matrix by developing a multi-phase microstructure constituted by a fully alloyed phase and a local high-concentration phase, the partially diffused alloy steel powder is being developed for high strength use. [0010] Molybdenum is frequently used as a basic alloy component of the prealloyed steel powders and the partially diffused alloy steel powders described above. The reason therefor is the same as the reason for using molybdenum (Mo) as a reinforcing element of iron and steel materials. That is, Mo not only suppresses generation of ferrite in iron and steel materials and forms bainite microstructures to strengthen the matrix by transformation, but also distributes itself to the matrix and carbides to achieve solution hardening of the matrix and precipitation hardening of the matrix by turning into fine carbides. Moreover, since Mo has high gas carburizability without grain boundary oxidizing, Mo also has an effect of carburization hardening. [0011] Elements, such as V, Nb, and Ti, having a high carbide-forming capability are also added since they form carbides that strengthen the sintered materials through precipitation hardening. [0012] For example, Japanese Unexamined Patent Application Publication No. 8-49047 (Patent Document 1) discloses an alloy steel powder for powder metallurgy containing Mo: 0.1 to 6.0%, V: 0.05 to 2.0%, and Nb: 0.10*; as a prealloy and Mo: 4% or less which is partially diffusion-bonded thereto. This alloy steel powder contains a reduced amount of oxygen and offers high compressibility comparable to that of a pure iron powder at the powder stage, and achieves a decrease in oxygen content and matrix strengthening when the powder is processed into sintered materials or carburized and quenched materials. [0013] Japanese Unexamined Patent Application Publication No. 7-331395 (Patent Document 2) discloses an alloy steel powder for high-strength sintered compacts, containing, on a weight basis, Cr: 0.5 to 2%, Mn: 0.08% or less, Mo: 0.1 to 0.6%, and V: 0.05 to 0.5%, at least one of Nb: 0.01 to 0.08- and Ti: 0.01 to 0.08%, and Mo: 0.05 to 3.5% diffusion-bonded thereto. According to this technique, an alloy steel powder controlled to exhibit excellent compressibility and an adequate hardenability can be obtained. When this alloy steel powder is used and the cooling rate after sintering is controlled, a fine pearlite microstructure can be produced in a sintered compact without generating a coarse upper bainite microstructure and a high-strength is achieved as sintered. [0014] However, the inventors have found that it is difficult even for sintered materials made from the alloy- steel powders set forth in Patent Documents 1 and 2 to achieve both strength and toughness. SUMMARY OF THE INVENTION [0015] Accordingly, an object of the present invention is to provide an alloy steel powder for powder metallurgy that can achieve both strength and toughness, as well as a sintered material having excellent strength and toughness produced by using the alloy steel powder and a method for producing the sintered material. [0016] The inventors have extensively studied the alloy components of iron-based powders and means for adding the alloy components and found the following. [0017] When an iron-based powder in which a carbide- forming element such as Nb is prealloyed in an iron powder and only Mo is diffusion-bonded to the iron powder is mixed with a carbon powder and the resulting mixture is compacted and sintered, the concentration of the alloy element, Mo is high in sintered neck portions between the grains of the iron-based powder. Accordingly, in the sintered neck portions, C and carbide-forming elements such as Mo and Nb are present and carbides containing Mo, Nb, and the like are precipitated and dispersed. [0018] Since the sintered neck portions contain a large number of pores, the strength of the sintered neck portions tends to be low. However, when carbides are precipitated around the pores, the sintered neck portions are strengthened. [0019] In contrast, since Mo is not contained in the matrix portion, formation of carbides is suppressed in the matrix portion compared to the sintered neck portions. Thus the matrix portion forms a microstructure with high toughness. The present invention has been made on the basis of these findings. [0020] A first aspect of the present invention provides an alloy steel powder for powder metallurgy produced by diffusion-bonding a molybdenum-containing powder in a molybdenum-based amount of 0.05% to 1.5% by mass onto surfaces of a steel powder prealloyed with 0.02% to 0.4% by mass of niobium. [0021] A second aspect of the present invention provides an alloy steel powder for powder metallurgy produced by diffusion-bonding a molybdenum-containing powder in a molybdenum-based amount of 0.05% to 1.5% by mass onto surfaces of a steel powder prealloyed with 0.02% to 0.4 V by mass of niobium and at least one of 0.01% to 0.4% by mass of vanadium and 0.01% to 0.4% by mass of titanium. [0022] A third aspect of the present invention provides an iron-based sintered material produced by compacting and sintering the alloy steel powder according to the first or second aspect, in which a carbide containing at least niobium and molybdenum is precipitated around a pore of the sintered material. [0023] The carbide containing at least niobium and molybdenum may be at least one of (Nb,Mo)C, (Nb,V,Mo)C, (Nb,Ti,Mo)C, and (Nb,Ti,V,Mo)C. [0024] A fourth aspect of the present invention provides a method for producing an iron-based sintered material. The method includes mixing the alloy steel powder according to Claim 1 or 2 with 0.1% to 1.0% by mass of a carbon powder to prepare a mixture; compacting the mixture at a pressure of 400 to 1000 MPa to prepare a green compact; and sintering the green compact at a temperature of 1100°C to 1300°C to precipitate a carbide containing at least niobium and molybdenum around a pore of the resulting sintered material. [0025] In this method, the carbide containing at least niobium and molybdenum may be at least one of (Nb,Mo)C, (Nb,V,Mo)C, (Nb,Ti,Mo)C, and (Nb,Ti,V,Mo)C [0026] A sintered material having both high strength and high toughness can be obtained by using an alloy steel powder for powder metallurgy in which Nb only or Nb and V and/or Ti are prealloyed and only Mo is diffusion bonded. BRIEF DESCRIPTION OF THE DRAWINGS [0027] Figure is a schematic view showing a sintered microstructure including sintered neck portions of a sintered compact obtained by the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS [0028] The present invention will now be described in detail. [0029] According to an alloy steel powder for powder metallurgy of the present invention, a Mo-containing powder is diffusion-bonded onto surfaces of a steel powder prealloyed either with Nb only or with Nb and V and/or Ti. [0030] When the iron-based powder of the present invention is mixed with a carbon powder and the resulting mixture is compacted and sintered, the concentration of the alloy element Mo is high in the sintered neck portions between the grains of the iron-based powder. Accordingly, Mo, Nb, V, Ti, and C are present and carbides containing Mo, Nb, V, Ti, and the like are precipitated and dispersed in the sintered neck portions. [0031] Since a large number of pores are present in the sintered neck portions, the strength of the sintered neck portions tends to be low. However, when the carbides are precipitated around the pores, the sintered neck portions are strengthened. [0032] In contrast, since Mo is not contained in the matrix portion, formation of carbides is suppressed in the matrix portion compared to the sintered neck portions. Thus the matrix portion forms a microstructure with high toughness. [0033] High strength and high toughness are presumably simultaneously achieved by controlling the carbide-formation regions as described above. [0034] The reason for pre-alloying Nb, V, and Ti in the composition ranges described above will now be described. In the description below, "%" indicates the fraction (mass%) relative to the entire alloy steel powder for powder metallurgy (after diffusion-bonding of Mo-containing powder; of the present invention. Nb: 0.02 to 0.4% [0035] Niobium (Nb) has an action of highly effectively- improving the strength once precipitated as a carbide in the matrix. However, when the Nb content is less than 0.02%, the amount of carbide generated is insufficient and the strength of the sintered compact is not sufficiently increased. In contrast, when the Nb content exceeds 0.4%, not only the carbide becomes coarse, the effect of improving the strength is lowered, and the compressibility is deteriorated due to hardening of alloy steel powder grains, but also an economic disadvantage results. The Nb content is more preferably 0.05% to 0.3%. At least one of V: 0.01 to 0.4% and Ti: 0.01 to 0.4% [0036] Vanadium (V) and titanium (Ti) are the next useful carbide-forming elements to Nb and contributes to further improving the strength when used in combination with Nb. However, neither of the elements exhibits sufficient hardening effect below the lower limit and both elements form coarse carbides at a content exceeding the upper limit, resulting in a decreased strength improving effect and deterioration of compressibility. Thus, V and Ti contents are set in the above-described ranges. More preferably, the content of each of these elements is 0.3% or less. [0037] When the total content of Nb, V, and Ti is within the range of 0.09% to 0.18%, a sintered material having outstanding strength and toughness can be obtained. [0038] Next, a method for producing an alloy steel powder for powder metallurgy according to the present invention is described. [0039] First, an iron-based powder (iron-based powder as a raw material) containing particular amounts of alloy elements as alloy components (i.e., as a prealloy), and a Mo raw material powder, which is a raw material of the Mo- containing powder, are prepared. [0040] An atomized iron powder is preferred as the iron- based powder. An atomized iron powder is an iron-based powder obtained by atomizing molten steel having alloy components adjusted to suite the purpose with water or gas. An atomized iron powder is usually heated in a reducing atmosphere (e.g., hydrogen atmosphere) after atomization to decrease the amounts of C and 0 in the iron powder. However, an as-atomized iron powder, i.e., an iron powder without such a heat treatment, may be used as the iron-based powder as the raw material of the present invention. [0041] The target Mo-containing powder itself may be used as the Mo raw material powder or a molybdenum compound that can be reduced to a Mo-containing powder may be used as the Mo raw material powder. [0042] Preferred examples of the Mo-containing powder include a pure metal powder of Mo, a Mo oxide powder, and a Mo alloy powder such as a ferro-molybdenum (FeMo) powder. Examples of the molybdenum compound include a Mo carbide, a Mo sulfide, and a Mo nitride. [0043] The iron-based powder and the Mo raw material powder are mixed with each other at a particular ratio. The mixing method is not particularly limited. For example, a Henschel mixer or a conical mixer can be used. [0044] The resulting mixture is held at a high temperature and Mo is diffused in iron at contact surfaces between the iron-based powder and the Mo raw material powder to conduct bonding. As a result, an alloy steel powder for powder metallurgy according to the present invention is obtained. [0045] The atmosphere for the heat treatment is preferably a reducing atmosphere or a hydrogen-containing atmosphere and more preferably a hydrogen-containing atmosphere. The heat treatment may be conducted under vacuum. The temperature for the heat treatment is preferably in the range 800°C to 1200°C and more preferably in the range of 800°C to 1000°C. [0046] When an as-atomized iron powder is used as the iron-based powder, the heat treatment is preferably conducted in a reducing atmosphere to decrease the amounts of C and 0 contained in large amounts in the iron powder. Decreasing the C content and the 0 content activates the iron-based powder surfaces and bonding caused by diffusion of the Mo-containing powder assuredly occurs even at a low temperature (about 800°C to 900°C). [0047] When the diffusion-bonding treatment is conducted as described above, the iron-based powder and the Mo- containing powder usually become sintered and consolidated. Thus, the mixture is pulverized and classified to a desired particle size and, if needed, annealed to prepare an alloy steel powder for powder metallurgy. [0048] In the present invention, fine particles of the Mo-containing powder are preferably uniformly bonded onto the iron-based powder surfaces. When the particles are not uniformly bonded, they tend to detach from the iron-based powder surfaces during transportation or pulverization of the alloy steel powder for powder metallurgy after the bonding treatment. Accordingly, the amount of the Mo- containing powder in a free state increases in particular. When a green compact formed of an alloy steel powder in that state is sintered, the carbide diffusion state tends to become segregated. Accordingly, in order to increase the strength and toughness of the sintered compact, it is preferable to have the Mo-containing powder uniformly bonded onto the surfaces of the iron-based powder to reduce the amount of Mo-containing powder in a free state generated by detachment or the like. [0049] The amount of Mo to be diffusion-bonded is 0.05% to 1.5% on a Mo basis relative to the entire alloy steel powder. At a content lower than 0.05%, the carbide-forming effect is small and the strength-improving effect is weakened. In contrast, at a content exceeding 1.5%, the carbide-forming effect is saturated and the microstructure of the sintered compact becomes inhomogeneous. Thus, the strength-improving effect is no longer obtained. Accordingly, the amount of Mo to be diffusion-bonded is set to 0.05% to 1.5%, preferably 0.05% to 0.5%, more preferably 0.1% to 0.5%, and yet more preferably 0.15% to 0.3%. [0050] The balance of the alloy steel powder includes iron and impurities. Impurities contained in the alloy steel powder are, for example, C, O, N, S, and the like. They pose no problem as long as the C content is 0.02% or less, the 0 content is 0.3% or less, the N content is 0.004 or less, and the S content is 0.03% or less. In particular, the 0 content is preferably 0.2% or less. [0051] In producing a sintered compact by using the alloy steel powder described above as a raw material, a carbon powder such as graphite is added in an amount of 0.1% to 1.0% on a C basis and mixed prior to the compaction since the carbon powder is effective for increasing the strength and enhancing the fatigue strength. This amount on a C basis is a mass ratio relative to the alloy steel powder mixture after the mixing. [0052] In the present invention, additives for improving characteristics may, as a matter of course, be added to meet the purpose. For example, a Cu powder or a Ni powder may be added to improve the strength of the sintered compact and a machinability improving powder such as MnS may be added to improve the machinability of the sintered compact. [0053] Impurities contained in the alloy steel powder mixture pose no particular problem as long as the 0 content is 0.3% or less, the N content is 0.004% or less, and the S content is 0.03% or less. Preferably, the 0 content is 0.2% or less. However, when any of these elements is intentionally added as an additive, such an element is no longer an impurity and the content thereof does not have to- be in the range described above. [0054] Next, preferable sintering conditions for producing a sintered compact by using the alloy steel powder for powder metallurgy according to the present invention are described. [0055] A powder lubricant may be mixed with the alloy steel powder in conducting the compaction. The lubricant may be applied or adhered to a die. In any case, a known lubricant such as a metal soap, e.g., zinc stearate, or an amide wax, e.g., ethylenebisstearamide, is preferably used as the lubricant. When a lubricant is mixed, the amount of the lubricant is preferably about 0.1 to 1.2 parts by mass relative to 100 parts by mass of the alloy steel powder mixture. [0056] Compaction needs to be conducted at a pressure of 400 to 1000 MPa. This is because, at a pressure less than 400 MPa, the density of the resulting compact is decreased and the characteristics of the sintered compact are degraded. In contrast, at a pressure exceeding 1000 MPa, the lifetime of the die is shortened and an economic disadvantage results. The temperature during the compaction is preferably in the range of room temperature (about 20°C) to about 160°C. [0057] Sintering needs to be conducted in a temperature range of 1100°C to 1300°C. This is because sintering does not proceed and the characteristics of the sintered compact are degraded at a sintering temperature less than 1100oC and the lifetime of the sintering furnace is shortened and an economic disadvantage results at a sintering temperature exceeding 1300°C. The duration of sintering (sintering time) is preferably 10 to 180 minutes. The sintering temperature range and the sintering time are suitable for precipitating at least carbides of Nb and Mo around the pores of the sintered material. [0058] The resulting sintered compact may be subjected to a strengthening treatment such as carburized quenching, bright quenching, high-frequency quenching, and carbonitriding as needed. The sintered compact exhibits improved strength and toughness without undergoing the strengthening treatment compared to existing sintered compacts (not subjected to a strengthening treatment). The strengthening treatment may be conducted according to a common method. [0059] When the sintering is conducted, regions with high Mo concentrations are formed in the sintered neck portions between grains of the iron-based powder. Since C is also present as well as the carbide-forming element such as Nb in these regions, carbides containing Mo, Nb, and the like are precipitated and dispersed in these regions. When the carbides are precipitated around the pores, the sintered neck portions are strengthened and thus a microstructure having excellent strength and toughness can be obtained. Figure is a schematic diagram of a sintered microstructure of a sintered compact obtained by the present invention. The sintered microstructure includes an iron-based powder 1 and sintered neck portions 2 around pores. [0060] Examples of the carbides containing Mo, Nb, and the like include (Nb,Mo)C, (Nb,V,Mo)C, (Nb,Ti,Mo)C, and (Nb,Ti,V,Mo)C. [0061] Preferably, about one to one hundred precipitates of such carbides are present in these Mo-rich regions of the sintered neck portions in a unit area of 1 µm. The Mo-rich regions refer to regions about 10 µm around the sintered neck portions. EXAMPLE [0062] The present invention will now be described in further detail by using Examples. These examples do not limit the scope of the present invention. [0063] Each of the molten metals containing alloy elements indicated in Nos. 1 to 17 in Table was atomized by a water atomization method and formed into an as-atomized iron-based powder. To the iron-based powder, a Mo oxide powder was added at a particular ratio. The resulting mixture was mixed in a V-type mixer for 15 minutes and heat- treated (holding temperature: 875°C, holding time: 1 h) in a hydrogen atmosphere with a dew point of 30°C to prepare an alloy steel powder for powder metallurgy constituted by the iron-based powder having surfaces onto which a particular amount Mo was diffusion-bonded. [0064] To the alloy steel powder for powder metallurgy, graphite in an amount indicated in Table was added. To 100 parts by mass of the resulting alloy steel powder mixture, 0.6 parts by mass of ethylenebisstearamide was added, followed by mixing in a V-type mixer for 15 minutes. Then the mixture was compacted at a pressure of 686 MPa to prepare a tablet-shape green compact having a length of 55 mm, a width of 10 mm, and a thickness of 10 mm. [0065] The tablet-shape green compact was sintered into a sintered compact. Sintering was performed in a N2-10% H2 atmosphere at a sintering temperature of 1130°C and a sintering time of 20 minutes. [0066] The sintered compact was processed into a round bar test piece 5 mm in diameter in a parallel portion for use in a tensile test. The sintered compact having a shape as sintered was prepared as a test piece for a Charpy impact test. Each test piece was subjected to gas-carburizing (holding temperature: 870°C, holding time: 60 minutes) at a carbon potential of 0.8%, followed by quenching (60°C, oil quenching) and tempering (180°C, 60 minutes). The tensile strength TS (MPa) and impact value (J/cm2) of each sintered compact were measured. The results are shown in Table. [0067] The tensile strength and impact value were compared between Examples and Comparative Examples as shown in Table. Whereas all of Examples achieved both high- strength, i.e., a tensile strength of 1150 MPa or more, and high toughness, i.e., an impact value of 10 J/cm2 or more, none of Comparative Examples achieved both. [0068] A section was taken from each sintered compact and observed with a scanning electron microscope and analyzed by image analysis. In all Examples, about 1 to 100 precipitates of (Nb,Mo)C, (Nb,V,Mo)C, (Nb,Ti,Mo)C, (Nb,Ti,V,Mo)C, and the like were observed in a unit area of 1 µm2 in the Mo-rich regions of the sintered neck portions. we claim: 1. An alloy steel powder for powder metallurgy produced by diffusion-bonding a molybdenum-containing powder in a molybdenum-based amount of 0.05% to 1.5% by mass onto surfaces of a steel powder prealloyed with 0.02% to 0.4% by- mass of niobium. 2. An alloy steel powder for powder metallurgy produced by diffusion-bonding a molybdenum-containing powder in a molybdenum-based amount of 0.05% to 1.5% by mass onto surfaces of a steel powder prealloyed with 0.02% to 0.4% by- mass of niobium and at least one of 0.01% to 0.4% by mass or vanadium and 0.01% to 0.4% by mass of titanium. 3. An iron-based sintered material produced by compacting and sintering the alloy steel powder according to Claim 1 or 2, wherein a carbide containing at least niobium and molybdenum is precipitated around a pore of the sintered material. 4. The iron-based sintered material according to Claim 3, wherein the carbide containing at least niobium and molybdenum is at least one of (Nb,Mo)C, (Nb,V,Mo)C, (Nb,Ti,Mo)C, and (Nb,Ti,V,Mo)C. 5. A method for producing an iron-based sintered material, comprising: mixing the alloy steel powder according to Claim 1 or 2 with 0.1% to 1.0% by mass of a carbon powder to prepare a mixture; compacting the mixture at a pressure of 400 to 1000 MPa to prepare a green compact; and sintering the green compact at a temperature of 1100°C to 1300°C to precipitate a carbide containing at least niobium and molybdenum around a pore of the resulting sintered material. 6. The method according to Claim 5, wherein the carbide containing at least niobium and molybdenum is at least one of (Nb,Mo)C, (Nb,V,Mo)C, (Nb,Ti,Mo)C, and (Nb,Ti,V,Mo)C. An alloy steel powder for powder metallurgy is produced by diffusion-bonding a molybdenum-containing powder in a molybdenum-based amount of 0.05% to 1.5% by mass onto surfaces of a steel powder prealloyed with 0.02% to 0.4% by mass of niobium.

Full Text

ALLOY STEEL POWDER FOR POWDER METALLURGY, IRON-BASED
SINTERED MATERIAL, AND METHOD FOR PRODUCING THE IRON-BASED
SINTERED MATERIAL
BACKGROUND OF THE INVENTION
1. Field of the Invention
[0001] The present invention relates to an alloy steel
powder for powder metallurgy suitable for use in powder
metallurgy. In particular, it relates to an alloy steel
powder that improves the strength and toughness of a
sintered material produced by using the alloy steel powder.
The present invention also relates to a sintered material
having excellent strength and toughness produced by using
the alloy steel powder for powder metallurgy and a method
for producing the sintered material.
2. Description of the Related Art
[0002] Powder metallurgy can produce parts having complex
shapes very close to product shapes (a.k.a., near-net
shapes) at high dimensional accuracy and thus significantly
reduces the cost for machining. Powder metallurgy products
are thus used in various machines and parts in many fields.
Recently, in order to reduce the size and weight of parts,
improvements on the strength of powder metallurgy products
are demanded. There is particularly strong demand for
increasing the strength of iron-based powder products 'iron-

based sintered compacts).
[0003] Green compacts produced from iron-based powders
for powder metallurgy are usually made by mixing an iron-
based powder with an alloying powder such as a copper powder
or a graphite powder and a lubricant such as stearic acid or
lithium stearate to prepare an iron-based powder mixture,
filling a die with the iron-based powder mixture, and
compacting the iron-based powder mixture in the die. Iron
based powders are classified into iron powders (e.g., pure
iron powders), alloy steel powders, etc., in accordance with
the component. Iron-based powders are also classified in
accordance with the production method into atomized iron
powders, reduced iron powders, etc. According to these
classifications, the term "iron powder" is used in a broad
sense and includes alloy steel powders.
[0004] The density of green compacts obtained by a common
powder metallurgy process is usually about 6.6 to 7.1 Mg/m3.
The iron-based powder green compacts are sintered into
sintered compacts and, if needed, sized or machined to
prepare powder metallurgy products. If a higher strength is
needed, the sintered compacts may be subjected to a
carburizing heat treatment or a bright heat treatment.
[0005] Examples of the powder in which alloy elements are
added at the stage of raw material powder include the
following:

(1) a powder mixture containing a pure iron powder and alloy
element powders,
(2) a prealloyed steel powder in which elements are fully
alloyed, and
(3) a partially diffused alloy steel powder in which alloy
element powders are partially diffusion-bonded onto surfaces
of a pure iron powder or a prealloyed steel powder.
[0006] The powder mixture (1) containing a pure iron
powder and alloy element powders has an advantage that it
can achieve a high compressibility comparable to that of
pure iron powders. However, during sintering, Mn, Cr, V, Si,
Nb, Ti, and the like which are metals having a higher
activity than Fe undergo oxidation unless the CO2
concentration and the dew point in a sintering atmosphere or
a carburizing atmosphere are stringently controlled to low
levels. Thus, the amount of oxygen is not reduced, the
alloy elements do not sufficiently diffuse into Fe, the
microstructure remains inhomogeneous, and the strengthening
of the matrix cannot be achieved.
[0007] Due to these drawbacks, the powder mixture (1)
containing a pure iron powder and alloy element powders has
not been able to meet the recent demand for high strength
and is thus no longer used.
[0008] In contrast, the prealloyed steel powder (2) in
which elements are fully alloyed and which is produced by

atomizing molten steel has an advantage that the amount of
oxygen can be reduced and a high compressibility comparable
to that of pure iron powders can be achieved by limiting the
types and amounts of the alloy elements, such as Mn, Cr, V,
Si, Nb, and Ti, although oxidation still occurs during the
process of atomizing the molten steel and solution hardening
occurs due to full alloying. Moreover, there is a
possibility that the strength of the matrix will be
increased by full alloying and thus this powder has been
developed to serve as a prealloyed steel powder for
achieving high strength.
[0009] The partially diffused alloy steel powder (3) is
made by blending a pure iron powder or a prealloyed steel
powder with metal powders of respective elements and heating
the resulting mixture in a nonoxidative or reducing
atmosphere to cause metal powders to partially diffusion-
bond onto surfaces of the pure iron powder or the prealloyed
steel powder. Thus, the partially diffused alloy powder
combines the advantages of the iron-based powder mixture (1)
and the prealloyed steel powder (2). Since the partially
diffused alloy steel powder (3) reduces the amount of oxygen,
achieves high compressibility comparable to that of a pure
iron powder, and bears a possibility of increasing the
strength of the matrix by developing a multi-phase
microstructure constituted by a fully alloyed phase and a

local high-concentration phase, the partially diffused alloy
steel powder is being developed for high strength use.
[0010] Molybdenum is frequently used as a basic alloy
component of the prealloyed steel powders and the partially
diffused alloy steel powders described above. The reason
therefor is the same as the reason for using molybdenum (Mo)
as a reinforcing element of iron and steel materials. That
is, Mo not only suppresses generation of ferrite in iron and
steel materials and forms bainite microstructures to
strengthen the matrix by transformation, but also
distributes itself to the matrix and carbides to achieve
solution hardening of the matrix and precipitation hardening
of the matrix by turning into fine carbides. Moreover,
since Mo has high gas carburizability without grain boundary
oxidizing, Mo also has an effect of carburization hardening.
[0011] Elements, such as V, Nb, and Ti, having a high
carbide-forming capability are also added since they form
carbides that strengthen the sintered materials through
precipitation hardening.
[0012] For example, Japanese Unexamined Patent
Application Publication No. 8-49047 (Patent Document 1)
discloses an alloy steel powder for powder metallurgy
containing Mo: 0.1 to 6.0%, V: 0.05 to 2.0%, and Nb: 0.10*;
as a prealloy and Mo: 4% or less which is partially
diffusion-bonded thereto. This alloy steel powder contains

a reduced amount of oxygen and offers high compressibility
comparable to that of a pure iron powder at the powder stage,
and achieves a decrease in oxygen content and matrix
strengthening when the powder is processed into sintered
materials or carburized and quenched materials.
[0013] Japanese Unexamined Patent Application Publication
No. 7-331395 (Patent Document 2) discloses an alloy steel
powder for high-strength sintered compacts, containing, on a
weight basis, Cr: 0.5 to 2%, Mn: 0.08% or less, Mo: 0.1 to
0.6%, and V: 0.05 to 0.5%, at least one of Nb: 0.01 to 0.08-
and Ti: 0.01 to 0.08%, and Mo: 0.05 to 3.5% diffusion-bonded
thereto. According to this technique, an alloy steel powder
controlled to exhibit excellent compressibility and an
adequate hardenability can be obtained. When this alloy
steel powder is used and the cooling rate after sintering is
controlled, a fine pearlite microstructure can be produced
in a sintered compact without generating a coarse upper
bainite microstructure and a high-strength is achieved as
sintered.
[0014] However, the inventors have found that it is
difficult even for sintered materials made from the alloy-
steel powders set forth in Patent Documents 1 and 2 to
achieve both strength and toughness.
SUMMARY OF THE INVENTION

[0015] Accordingly, an object of the present invention is
to provide an alloy steel powder for powder metallurgy that
can achieve both strength and toughness, as well as a
sintered material having excellent strength and toughness
produced by using the alloy steel powder and a method for
producing the sintered material.
[0016] The inventors have extensively studied the alloy
components of iron-based powders and means for adding the
alloy components and found the following.
[0017] When an iron-based powder in which a carbide-
forming element such as Nb is prealloyed in an iron powder
and only Mo is diffusion-bonded to the iron powder is mixed
with a carbon powder and the resulting mixture is compacted
and sintered, the concentration of the alloy element, Mo is
high in sintered neck portions between the grains of the
iron-based powder. Accordingly, in the sintered neck
portions, C and carbide-forming elements such as Mo and Nb
are present and carbides containing Mo, Nb, and the like are
precipitated and dispersed.
[0018] Since the sintered neck portions contain a large
number of pores, the strength of the sintered neck portions
tends to be low. However, when carbides are precipitated
around the pores, the sintered neck portions are
strengthened.
[0019] In contrast, since Mo is not contained in the

matrix portion, formation of carbides is suppressed in the
matrix portion compared to the sintered neck portions. Thus
the matrix portion forms a microstructure with high
toughness. The present invention has been made on the basis
of these findings.
[0020] A first aspect of the present invention provides
an alloy steel powder for powder metallurgy produced by
diffusion-bonding a molybdenum-containing powder in a
molybdenum-based amount of 0.05% to 1.5% by mass onto
surfaces of a steel powder prealloyed with 0.02% to 0.4% by
mass of niobium.
[0021] A second aspect of the present invention provides
an alloy steel powder for powder metallurgy produced by
diffusion-bonding a molybdenum-containing powder in a
molybdenum-based amount of 0.05% to 1.5% by mass onto
surfaces of a steel powder prealloyed with 0.02% to 0.4 V by
mass of niobium and at least one of 0.01% to 0.4% by mass of
vanadium and 0.01% to 0.4% by mass of titanium.
[0022] A third aspect of the present invention provides
an iron-based sintered material produced by compacting and
sintering the alloy steel powder according to the first or
second aspect, in which a carbide containing at least
niobium and molybdenum is precipitated around a pore of the
sintered material.
[0023] The carbide containing at least niobium and

molybdenum may be at least one of (Nb,Mo)C, (Nb,V,Mo)C,
(Nb,Ti,Mo)C, and (Nb,Ti,V,Mo)C.
[0024] A fourth aspect of the present invention provides
a method for producing an iron-based sintered material. The
method includes mixing the alloy steel powder according to
Claim 1 or 2 with 0.1% to 1.0% by mass of a carbon powder to
prepare a mixture; compacting the mixture at a pressure of
400 to 1000 MPa to prepare a green compact; and sintering
the green compact at a temperature of 1100°C to 1300°C to
precipitate a carbide containing at least niobium and
molybdenum around a pore of the resulting sintered material.
[0025] In this method, the carbide containing at least
niobium and molybdenum may be at least one of (Nb,Mo)C,
(Nb,V,Mo)C, (Nb,Ti,Mo)C, and (Nb,Ti,V,Mo)C
[0026] A sintered material having both high strength and
high toughness can be obtained by using an alloy steel
powder for powder metallurgy in which Nb only or Nb and V
and/or Ti are prealloyed and only Mo is diffusion bonded.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] Figure is a schematic view showing a sintered
microstructure including sintered neck portions of a
sintered compact obtained by the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0028] The present invention will now be described in
detail.
[0029] According to an alloy steel powder for powder
metallurgy of the present invention, a Mo-containing powder
is diffusion-bonded onto surfaces of a steel powder
prealloyed either with Nb only or with Nb and V and/or Ti.
[0030] When the iron-based powder of the present
invention is mixed with a carbon powder and the resulting
mixture is compacted and sintered, the concentration of the
alloy element Mo is high in the sintered neck portions
between the grains of the iron-based powder. Accordingly,
Mo, Nb, V, Ti, and C are present and carbides containing Mo,
Nb, V, Ti, and the like are precipitated and dispersed in
the sintered neck portions.
[0031] Since a large number of pores are present in the
sintered neck portions, the strength of the sintered neck
portions tends to be low. However, when the carbides are
precipitated around the pores, the sintered neck portions
are strengthened.
[0032] In contrast, since Mo is not contained in the
matrix portion, formation of carbides is suppressed in the
matrix portion compared to the sintered neck portions. Thus
the matrix portion forms a microstructure with high
toughness.
[0033] High strength and high toughness are presumably

simultaneously achieved by controlling the carbide-formation
regions as described above.
[0034] The reason for pre-alloying Nb, V, and Ti in the
composition ranges described above will now be described.
In the description below, "%" indicates the fraction (mass%)
relative to the entire alloy steel powder for powder
metallurgy (after diffusion-bonding of Mo-containing powder;
of the present invention.
Nb: 0.02 to 0.4%
[0035] Niobium (Nb) has an action of highly effectively-
improving the strength once precipitated as a carbide in the
matrix. However, when the Nb content is less than 0.02%,
the amount of carbide generated is insufficient and the
strength of the sintered compact is not sufficiently
increased. In contrast, when the Nb content exceeds 0.4%,
not only the carbide becomes coarse, the effect of improving
the strength is lowered, and the compressibility is
deteriorated due to hardening of alloy steel powder grains,
but also an economic disadvantage results. The Nb content
is more preferably 0.05% to 0.3%.
At least one of V: 0.01 to 0.4% and Ti: 0.01 to 0.4%
[0036] Vanadium (V) and titanium (Ti) are the next useful
carbide-forming elements to Nb and contributes to further

improving the strength when used in combination with Nb.
However, neither of the elements exhibits sufficient
hardening effect below the lower limit and both elements
form coarse carbides at a content exceeding the upper limit,
resulting in a decreased strength improving effect and
deterioration of compressibility. Thus, V and Ti contents
are set in the above-described ranges. More preferably, the
content of each of these elements is 0.3% or less.
[0037] When the total content of Nb, V, and Ti is within
the range of 0.09% to 0.18%, a sintered material having
outstanding strength and toughness can be obtained.
[0038] Next, a method for producing an alloy steel powder
for powder metallurgy according to the present invention is
described.
[0039] First, an iron-based powder (iron-based powder as
a raw material) containing particular amounts of alloy
elements as alloy components (i.e., as a prealloy), and a Mo
raw material powder, which is a raw material of the Mo-
containing powder, are prepared.
[0040] An atomized iron powder is preferred as the iron-
based powder. An atomized iron powder is an iron-based
powder obtained by atomizing molten steel having alloy
components adjusted to suite the purpose with water or gas.
An atomized iron powder is usually heated in a reducing
atmosphere (e.g., hydrogen atmosphere) after atomization to

decrease the amounts of C and 0 in the iron powder. However,
an as-atomized iron powder, i.e., an iron powder without
such a heat treatment, may be used as the iron-based powder
as the raw material of the present invention.
[0041] The target Mo-containing powder itself may be used
as the Mo raw material powder or a molybdenum compound that
can be reduced to a Mo-containing powder may be used as the
Mo raw material powder.
[0042] Preferred examples of the Mo-containing powder
include a pure metal powder of Mo, a Mo oxide powder, and a
Mo alloy powder such as a ferro-molybdenum (FeMo) powder.
Examples of the molybdenum compound include a Mo carbide, a
Mo sulfide, and a Mo nitride.
[0043] The iron-based powder and the Mo raw material
powder are mixed with each other at a particular ratio. The
mixing method is not particularly limited. For example, a
Henschel mixer or a conical mixer can be used.
[0044] The resulting mixture is held at a high
temperature and Mo is diffused in iron at contact surfaces
between the iron-based powder and the Mo raw material powder
to conduct bonding. As a result, an alloy steel powder for
powder metallurgy according to the present invention is
obtained.
[0045] The atmosphere for the heat treatment is
preferably a reducing atmosphere or a hydrogen-containing

atmosphere and more preferably a hydrogen-containing
atmosphere. The heat treatment may be conducted under
vacuum. The temperature for the heat treatment is
preferably in the range 800°C to 1200°C and more preferably
in the range of 800°C to 1000°C.
[0046] When an as-atomized iron powder is used as the
iron-based powder, the heat treatment is preferably
conducted in a reducing atmosphere to decrease the amounts
of C and 0 contained in large amounts in the iron powder.
Decreasing the C content and the 0 content activates the
iron-based powder surfaces and bonding caused by diffusion
of the Mo-containing powder assuredly occurs even at a low
temperature (about 800°C to 900°C).
[0047] When the diffusion-bonding treatment is conducted
as described above, the iron-based powder and the Mo-
containing powder usually become sintered and consolidated.
Thus, the mixture is pulverized and classified to a desired
particle size and, if needed, annealed to prepare an alloy
steel powder for powder metallurgy.
[0048] In the present invention, fine particles of the
Mo-containing powder are preferably uniformly bonded onto
the iron-based powder surfaces. When the particles are not
uniformly bonded, they tend to detach from the iron-based
powder surfaces during transportation or pulverization of
the alloy steel powder for powder metallurgy after the

bonding treatment. Accordingly, the amount of the Mo-
containing powder in a free state increases in particular.
When a green compact formed of an alloy steel powder in that
state is sintered, the carbide diffusion state tends to
become segregated. Accordingly, in order to increase the
strength and toughness of the sintered compact, it is
preferable to have the Mo-containing powder uniformly bonded
onto the surfaces of the iron-based powder to reduce the
amount of Mo-containing powder in a free state generated by
detachment or the like.
[0049] The amount of Mo to be diffusion-bonded is 0.05%
to 1.5% on a Mo basis relative to the entire alloy steel
powder. At a content lower than 0.05%, the carbide-forming
effect is small and the strength-improving effect is
weakened. In contrast, at a content exceeding 1.5%, the
carbide-forming effect is saturated and the microstructure
of the sintered compact becomes inhomogeneous. Thus, the
strength-improving effect is no longer obtained.
Accordingly, the amount of Mo to be diffusion-bonded is set
to 0.05% to 1.5%, preferably 0.05% to 0.5%, more preferably
0.1% to 0.5%, and yet more preferably 0.15% to 0.3%.
[0050] The balance of the alloy steel powder includes
iron and impurities. Impurities contained in the alloy
steel powder are, for example, C, O, N, S, and the like.
They pose no problem as long as the C content is 0.02% or

less, the 0 content is 0.3% or less, the N content is 0.004
or less, and the S content is 0.03% or less. In particular,
the 0 content is preferably 0.2% or less.
[0051] In producing a sintered compact by using the alloy
steel powder described above as a raw material, a carbon
powder such as graphite is added in an amount of 0.1% to
1.0% on a C basis and mixed prior to the compaction since
the carbon powder is effective for increasing the strength
and enhancing the fatigue strength. This amount on a C
basis is a mass ratio relative to the alloy steel powder
mixture after the mixing.
[0052] In the present invention, additives for improving
characteristics may, as a matter of course, be added to meet
the purpose. For example, a Cu powder or a Ni powder may be
added to improve the strength of the sintered compact and a
machinability improving powder such as MnS may be added to
improve the machinability of the sintered compact.
[0053] Impurities contained in the alloy steel powder
mixture pose no particular problem as long as the 0 content
is 0.3% or less, the N content is 0.004% or less, and the S
content is 0.03% or less. Preferably, the 0 content is 0.2%
or less. However, when any of these elements is
intentionally added as an additive, such an element is no
longer an impurity and the content thereof does not have to-
be in the range described above.

[0054] Next, preferable sintering conditions for
producing a sintered compact by using the alloy steel powder
for powder metallurgy according to the present invention are
described.
[0055] A powder lubricant may be mixed with the alloy
steel powder in conducting the compaction. The lubricant
may be applied or adhered to a die. In any case, a known
lubricant such as a metal soap, e.g., zinc stearate, or an
amide wax, e.g., ethylenebisstearamide, is preferably used
as the lubricant. When a lubricant is mixed, the amount of
the lubricant is preferably about 0.1 to 1.2 parts by mass
relative to 100 parts by mass of the alloy steel powder
mixture.
[0056] Compaction needs to be conducted at a pressure of
400 to 1000 MPa. This is because, at a pressure less than
400 MPa, the density of the resulting compact is decreased
and the characteristics of the sintered compact are degraded.
In contrast, at a pressure exceeding 1000 MPa, the lifetime
of the die is shortened and an economic disadvantage results.
The temperature during the compaction is preferably in the
range of room temperature (about 20°C) to about 160°C.
[0057] Sintering needs to be conducted in a temperature
range of 1100°C to 1300°C. This is because sintering does
not proceed and the characteristics of the sintered compact
are degraded at a sintering temperature less than 1100oC and

the lifetime of the sintering furnace is shortened and an
economic disadvantage results at a sintering temperature
exceeding 1300°C. The duration of sintering (sintering
time) is preferably 10 to 180 minutes. The sintering
temperature range and the sintering time are suitable for
precipitating at least carbides of Nb and Mo around the
pores of the sintered material.
[0058] The resulting sintered compact may be subjected to
a strengthening treatment such as carburized quenching,
bright quenching, high-frequency quenching, and
carbonitriding as needed. The sintered compact exhibits
improved strength and toughness without undergoing the
strengthening treatment compared to existing sintered
compacts (not subjected to a strengthening treatment). The
strengthening treatment may be conducted according to a
common method.
[0059] When the sintering is conducted, regions with high
Mo concentrations are formed in the sintered neck portions
between grains of the iron-based powder. Since C is also
present as well as the carbide-forming element such as Nb in
these regions, carbides containing Mo, Nb, and the like are
precipitated and dispersed in these regions. When the
carbides are precipitated around the pores, the sintered
neck portions are strengthened and thus a microstructure
having excellent strength and toughness can be obtained.

Figure is a schematic diagram of a sintered microstructure
of a sintered compact obtained by the present invention.
The sintered microstructure includes an iron-based powder 1
and sintered neck portions 2 around pores.
[0060] Examples of the carbides containing Mo, Nb, and
the like include (Nb,Mo)C, (Nb,V,Mo)C, (Nb,Ti,Mo)C, and
(Nb,Ti,V,Mo)C.
[0061] Preferably, about one to one hundred precipitates
of such carbides are present in these Mo-rich regions of the
sintered neck portions in a unit area of 1 µm. The Mo-rich
regions refer to regions about 10 µm around the sintered
neck portions.
EXAMPLE
[0062] The present invention will now be described in
further detail by using Examples. These examples do not
limit the scope of the present invention.
[0063] Each of the molten metals containing alloy
elements indicated in Nos. 1 to 17 in Table was atomized by
a water atomization method and formed into an as-atomized
iron-based powder. To the iron-based powder, a Mo oxide
powder was added at a particular ratio. The resulting
mixture was mixed in a V-type mixer for 15 minutes and heat-
treated (holding temperature: 875°C, holding time: 1 h) in a
hydrogen atmosphere with a dew point of 30°C to prepare an

alloy steel powder for powder metallurgy constituted by the
iron-based powder having surfaces onto which a particular
amount Mo was diffusion-bonded.
[0064] To the alloy steel powder for powder metallurgy,
graphite in an amount indicated in Table was added. To 100
parts by mass of the resulting alloy steel powder mixture,
0.6 parts by mass of ethylenebisstearamide was added,
followed by mixing in a V-type mixer for 15 minutes. Then
the mixture was compacted at a pressure of 686 MPa to
prepare a tablet-shape green compact having a length of 55
mm, a width of 10 mm, and a thickness of 10 mm.
[0065] The tablet-shape green compact was sintered into a
sintered compact. Sintering was performed in a N2-10% H2
atmosphere at a sintering temperature of 1130°C and a
sintering time of 20 minutes.
[0066] The sintered compact was processed into a round
bar test piece 5 mm in diameter in a parallel portion for
use in a tensile test. The sintered compact having a shape
as sintered was prepared as a test piece for a Charpy impact
test. Each test piece was subjected to gas-carburizing
(holding temperature: 870°C, holding time: 60 minutes) at a
carbon potential of 0.8%, followed by quenching (60°C, oil
quenching) and tempering (180°C, 60 minutes). The tensile
strength TS (MPa) and impact value (J/cm2) of each sintered
compact were measured. The results are shown in Table.

[0067] The tensile strength and impact value were
compared between Examples and Comparative Examples as shown
in Table. Whereas all of Examples achieved both high-
strength, i.e., a tensile strength of 1150 MPa or more, and
high toughness, i.e., an impact value of 10 J/cm2 or more,
none of Comparative Examples achieved both.

[0068] A section was taken from each sintered compact and
observed with a scanning electron microscope and analyzed by
image analysis. In all Examples, about 1 to 100
precipitates of (Nb,Mo)C, (Nb,V,Mo)C, (Nb,Ti,Mo)C,
(Nb,Ti,V,Mo)C, and the like were observed in a unit area of
1 µm2 in the Mo-rich regions of the sintered neck portions.

we claim:
1. An alloy steel powder for powder metallurgy
produced by diffusion-bonding a molybdenum-containing powder
in a molybdenum-based amount of 0.05% to 1.5% by mass onto
surfaces of a steel powder prealloyed with 0.02% to 0.4% by-
mass of niobium.
2. An alloy steel powder for powder metallurgy
produced by diffusion-bonding a molybdenum-containing powder
in a molybdenum-based amount of 0.05% to 1.5% by mass onto
surfaces of a steel powder prealloyed with 0.02% to 0.4% by-
mass of niobium and at least one of 0.01% to 0.4% by mass or
vanadium and 0.01% to 0.4% by mass of titanium.
3. An iron-based sintered material produced by
compacting and sintering the alloy steel powder according to
Claim 1 or 2, wherein a carbide containing at least niobium
and molybdenum is precipitated around a pore of the sintered
material.
4. The iron-based sintered material according to Claim
3, wherein the carbide containing at least niobium and
molybdenum is at least one of (Nb,Mo)C, (Nb,V,Mo)C,
(Nb,Ti,Mo)C, and (Nb,Ti,V,Mo)C.

5. A method for producing an iron-based sintered
material, comprising:
mixing the alloy steel powder according to Claim 1 or 2
with 0.1% to 1.0% by mass of a carbon powder to prepare a
mixture;
compacting the mixture at a pressure of 400 to 1000 MPa
to prepare a green compact; and
sintering the green compact at a temperature of 1100°C
to 1300°C to precipitate a carbide containing at least
niobium and molybdenum around a pore of the resulting
sintered material.
6. The method according to Claim 5, wherein the
carbide containing at least niobium and molybdenum is at
least one of (Nb,Mo)C, (Nb,V,Mo)C, (Nb,Ti,Mo)C, and
(Nb,Ti,V,Mo)C.

An alloy steel powder for powder metallurgy is produced
by diffusion-bonding a molybdenum-containing powder in a
molybdenum-based amount of 0.05% to 1.5% by mass onto
surfaces of a steel powder prealloyed with 0.02% to 0.4% by
mass of niobium.

Documents:

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Patent Number

280049

Indian Patent Application Number

1101/KOL/2011

PG Journal Number

06/2017

Publication Date

10-Feb-2017

Grant Date

08-Feb-2017

Date of Filing

23-Aug-2011

Name of Patentee

JFE STEEL CORPORATION

Applicant Address

2-3, UCHISAIWAI-CHO 2-CHOME, CHIYODA-KU TOKYO 100-0011 JAPAN

Inventors:

#	Inventor's Name	Inventor's Address
1	SHIGERU UNAMI	C/O. INTELLECTUAL PROPERTY DEPT., JFE STEEL CORPORATION, 2-3, UCHISAIWAI-CHO 2-CHOME, CHIYODA-KU TOKYO 100-0011, JAPAN
2	YUKIKO OZAKI	C/O. INTELLECTUAL PROPERTY DEPT., JFE STEEL CORPORATION, 2-3, UCHISAIWAI-CHO 2-CHOME, CHIYODA-KU TOKYO 100-0011, JAPAN

PCT International Classification Number

C22C33/02

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1	2010-280611	2010-12-16	Japan