CN110267754B - Mixed powder for powder metallurgy, sintered body, and method for producing sintered body - Google Patents

Mixed powder for powder metallurgy, sintered body, and method for producing sintered body Download PDF

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CN110267754B
CN110267754B CN201780085068.6A CN201780085068A CN110267754B CN 110267754 B CN110267754 B CN 110267754B CN 201780085068 A CN201780085068 A CN 201780085068A CN 110267754 B CN110267754 B CN 110267754B
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powder
iron
alloy steel
mass
sintered
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CN110267754A (en
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小林聪雄
中村尚道
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JFE Steel Corp
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
    • B22F1/00—Metallic powder; Treatment of metallic powder, e.g. to facilitate working or to improve properties
    • B22F1/10—Metallic powder containing lubricating or binding agents; Metallic powder containing organic material
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C33/00—Making ferrous alloys
    • C22C33/02—Making ferrous alloys by powder metallurgy
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
    • B22F1/00—Metallic powder; Treatment of metallic powder, e.g. to facilitate working or to improve properties
    • B22F1/09—Mixtures of metallic powders
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
    • B22F3/00—Manufacture of workpieces or articles from metallic powder characterised by the manner of compacting or sintering; Apparatus specially adapted therefor ; Presses and furnaces
    • B22F3/12—Both compacting and sintering
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C33/00—Making ferrous alloys
    • C22C33/02—Making ferrous alloys by powder metallurgy
    • C22C33/0207—Using a mixture of prealloyed powders or a master alloy
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C33/00—Making ferrous alloys
    • C22C33/02—Making ferrous alloys by powder metallurgy
    • C22C33/0257—Making ferrous alloys by powder metallurgy characterised by the range of the alloying elements
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
    • B22F1/00—Metallic powder; Treatment of metallic powder, e.g. to facilitate working or to improve properties
    • B22F1/0003—Metallic powders per se; Mixtures of metallic powders; Metallic powders mixed with a lubricating or binding agent
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
    • B22F2301/00—Metallic composition of the powder or its coating
    • B22F2301/35—Iron
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
    • B22F2998/00—Supplementary information concerning processes or compositions relating to powder metallurgy
    • B22F2998/10—Processes characterised by the sequence of their steps
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C1/00—Making alloys
    • C22C1/04—Making alloys by powder metallurgy
    • C22C1/0425—Copper-based alloys
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C1/00—Making alloys
    • C22C1/04—Making alloys by powder metallurgy
    • C22C1/045—Alloys based on refractory metals
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C1/00—Making alloys
    • C22C1/04—Making alloys by powder metallurgy
    • C22C1/05—Mixtures of metal powder with non-metallic powder
    • C22C1/051—Making hard metals based on borides, carbides, nitrides, oxides or silicides; Preparation of the powder mixture used as the starting material
    • C22C1/053—Making hard metals based on borides, carbides, nitrides, oxides or silicides; Preparation of the powder mixture used as the starting material with in situ forming of the hard compound
    • C22C1/055—Making hard metals based on borides, carbides, nitrides, oxides or silicides; Preparation of the powder mixture used as the starting material with in situ forming of the hard compound using carbon

Abstract

A mixed powder for powder metallurgy which has a higher compressibility than a partially diffused alloy steel powder and can provide a high molding density. A mixed powder for powder metallurgy, which contains: (a) contains Si: 0-0.2 mass% and Mn: 0 to 0.4 mass% of an iron-based powder, the balance being Fe and unavoidable impurities, and (b) a powder containing Mo: 2.0 to 21.0 mass%, Si: 0-0.2 mass% and Mn: 0 to 0.4 mass% of an alloy steel powder, the balance being Fe and unavoidable impurities, and the ratio of (b) the alloy steel powder to the total of the iron-based powder (a) and the alloy steel powder (b) being 50 to 90 mass%, and the ratio of Mo to the total of the iron-based powder (a) and the alloy steel powder (b) being 2.2 to 6.2 mass%.

Description

Mixed powder for powder metallurgy, sintered body, and method for producing sintered body
Technical Field
The present invention relates to a mixed powder for powder metallurgy (mixed powder for powder metallurgy), and particularly relates to a mixed powder for powder metallurgy having excellent compressibility (compressibility). The present invention also relates to a sintered body (sintered body) using the above mixed powder for powder metallurgy and a method for producing the sintered body.
Background
The powder metallurgy technique is a method that can form a part having a shape extremely close to the shape of a product (so-called near-net forming) and can produce a part having a complicated shape with high dimensional accuracy, and can significantly reduce cutting costs. Therefore, powder metallurgy products are used in many fields as various machines and parts.
In recent years, in order to reduce the size and weight of parts, it has been strongly desired to improve the strength of powder metallurgy products, and in particular, there has been a strong demand for an iron-based powder press-molded product and an iron-based powder sintered product to have higher strength.
In order to meet the demand for higher strength, it has been carried out to add an alloy element having an effect of improving hardenability or the like to the iron-based powder. For example, (1) pre-alloyed steel powder (pre-alloyed steel powder) and (2) partially diffused-alloyed steel powder (partially diffused-alloyed steel powder) are known as powders to which alloying elements are added at the stage of raw material powder.
(1) The prealloyed steel powder is a powder in which alloying elements are completely alloyed in advance. By using this prealloyed steel powder, segregation of the alloying elements can be completely prevented, and therefore the structure of the sintered body becomes uniform. As a result, the mechanical properties of the press-molded article and the sintered article can be stabilized. However, since complete alloying causes solid solution hardening of the entire powder particle, the compressibility of the powder is low, and as a result, there is a problem that it is difficult to increase the molding density during press molding.
(2) The partially diffused alloy steel powder is a powder in which alloy element powders are partially attached and diffused to the surface of pure iron powder or prealloyed steel powder. The partially diffused alloy steel powder is produced by mixing a metal powder of an alloying element or an oxide thereof with a pure iron powder or a prealloyed steel powder and heating the mixture in a non-oxidizing or reducing atmosphere to diffusion bond the alloying element powder to the surface of the pure iron powder or the prealloyed steel powder. Since the structure can be made relatively uniform by partially diffusing the alloy steel powder, the mechanical properties of the product can be stabilized as in the case of using the prealloyed steel powder. Further, the partially diffused alloy steel powder has a portion containing no alloying element or a small amount of alloying element in the inside thereof, and therefore, is superior in compressibility at the time of press forming compared with the prealloyed steel powder.
As a basic alloy component used in the prealloyed steel powder or the partially diffused alloy steel powder, Mo having an effect of improving hardenability is widely used. This is because: as alloying elements having an effect of improving hardenability, Mn, Cr, Si, and the like are known in addition to Mo, but among these elements, Mo is relatively less likely to be oxidized, and therefore, the production of alloy steel powder is easy. For example, prealloyed steel powder can be easily produced by powdering molten steel to which Mo is added as an alloying element by a water atomization method and subjecting the molten steel to final reduction in a normal hydrogen atmosphere. Further, when Mo oxide is mixed with pure iron powder or alloy steel powder and subjected to final reduction in a normal hydrogen atmosphere, it is possible to easily produce partially diffused alloy steel powder.
By adding Mo having an effect of improving hardenability in this way, the formation of ferrite during quenching treatment is suppressed, bainite or martensite is formed, and the parent phase is phase-transformed and strengthened. Further, Mo is distributed in the matrix phase to cause solid-solution strengthening of the matrix phase, and fine carbides are formed in the matrix phase to cause precipitation strengthening of the matrix phase. Further, Mo has a good gas carburizability and is a non-intergranular oxidation element, and therefore, also has a carburizing and strengthening effect.
Examples of the alloy steel powder using Mo include patent documents 1 and 2.
Patent document 1 proposes an alloyed steel powder in which Mo is further diffused and adhered to the surface of a prealloyed steel powder containing Mo as an alloying element.
Patent document 2 proposes applying a secondary forming-secondary sintering method to further improve the strength of a sintered body when Mo prealloyed steel powder is used. In the secondary forming-secondary sintering method, the alloy steel powder is formed and pre-sintered first, and then formed and primarily sintered again.
Documents of the prior art
Patent document
Patent document 1: japanese patent No. 4371003
Patent document 2: japanese laid-open patent publication No. H04-231404
Disclosure of Invention
Problems to be solved by the invention
However, the demand for high strength of iron-based powder press-molded products and iron-based powder sintered products is increasing, and the methods proposed in patent documents 1 and 2 cannot sufficiently meet the demand for high strength. The reason for this is as follows.
One measure for increasing the strength of iron-based powder press-molded products and iron-based powder sintered products is to increase the density. By increasing the density, rearrangement of the iron powder particles proceeds, the void volume ratio in the molded article decreases, and the area where the iron powder particles are entangled by contacting with each other increases, so that the mechanical properties such as tensile strength, impact value, fatigue strength, and the like of the iron-based powder press-molded article and the iron-based powder sintered article improve. In addition, in order to increase the density of an iron-based powder sintered product or an iron-based powder press-molded product, the compressibility of the alloy steel powder as a raw material for press molding can be increased, and the molding density can be easily increased.
Therefore, in patent document 1, a partially diffused alloy steel powder is used. As described above, since the partially diffused alloy steel powder has a portion containing no alloying element or a small amount of alloying element (hereinafter referred to as "low alloy portion") inside the particles, it is superior in compressibility in press forming compared to prealloyed steel powder. Although it is considered that the compressibility can be further improved by increasing the proportion of the low alloy portion, since it is necessary to diffuse and adhere the alloying element in a certain amount in order to bring the properties such as hardenability into a desired range, the proportion of the low alloy portion cannot be increased more than a certain amount, and therefore, sufficient compressibility cannot be secured.
Further, even if the secondary forming-secondary sintering method of patent document 2 is applied to the partially diffused alloy steel powder of patent document 1, diffusion of the alloy element proceeds in the first sintering, and thus compressibility in the second forming becomes insufficient, and thus sufficient compressibility cannot be obtained.
The present invention has been made in view of the above circumstances, and an object thereof is to provide a mixed powder for powder metallurgy which has a higher compressibility than a conventional partially diffused alloy steel powder and can obtain a high molding density. Further, the present invention aims to provide a sintered body using the above mixed powder for powder metallurgy and a method for producing the sintered body.
Means for solving the problems
The present inventors have conducted studies to solve the above problems, and as a result, have found the following.
The reason why the partially diffused alloy steel powder exhibits high compressibility is that a low alloy portion, that is, a portion containing no alloying element or containing little alloying element is present inside the particles constituting the partially diffused alloy steel powder. In the low alloy portion, the solid solution strengthening by the alloy element is weak, and deformation is easy at the time of press forming. On the other hand, since the alloying element is diffused and adhered to the surface of the particle, the concentration of the alloying element is high and the deformation is difficult.
As described above, the partially diffused alloy steel powder has a property that the surface is hardly deformed and the inside is easily deformed. By having such an internal structure of the particles, the partially diffused alloy steel powder is more likely to have rearrangement of the particles than the pre-alloyed powder, and therefore, the molding density is more likely to be improved. However, in practice, considering the state of the alloy steel powder when it is formed, it is found that the surface of the particles, not the inside of the particles, is preferably deformable in accordance with the shape of the particles existing around the particles in order to fill the gaps between the particles and rearrange the particles.
However, in both of the prealloyed steel powder and the partially diffused alloy steel powder, the particle surfaces contain alloy components, and therefore, the soft state of the particle surfaces as described above cannot be achieved.
Therefore, the present inventors thought to use an iron-based powder containing no Mo and an alloy steel powder containing Mo in combination instead of softening the particle surface. By using an iron-based powder having low hardness and containing no Mo in combination, the compressibility during press forming is improved even in the case of normal primary forming, and further, in the case of the secondary forming-secondary sintering method, a sufficient portion containing no Mo remains even if the alloy element is diffused in the primary sintering, and therefore, high compressibility is maintained in the secondary forming. However, when the amount of the iron-based powder containing no Mo is too small, such an effect is insufficient, and on the contrary, when it is too large, the mechanical properties are deteriorated.
Based on the above findings, various studies have been made on conditions that can achieve both compressibility and mechanical properties, and the present invention has been conceived as a result. That is, the gist of the present invention is as follows.
1. A mixed powder for powder metallurgy, which is prepared from a raw material,
it comprises the following components:
(a) contains Si: 0-0.2 mass% and Mn: 0 to 0.4 mass% of an iron-based powder, the balance being Fe and unavoidable impurities; and
(b) contains Mo: 2.0 to 21.0 mass%, Si: 0-0.2 mass% and Mn: 0 to 0.4 mass% of alloy steel powder with the balance being Fe and inevitable impurities,
(b) the ratio of the alloy steel powder to the total of the iron-based powder (a) and the alloy steel powder (b) is 50 to 90% by mass,
the ratio of Mo to the total of the iron-based powder (a) and the alloy steel powder (b) is 2.2 to 6.2 mass%.
2. The mixed powder for powder metallurgy as described in the above 1, wherein,
also contains (c) Cu powder and (d) graphite powder,
(c) the ratio of the Cu powder to the total of the iron-based powder (a), the alloyed steel powder (b), the Cu powder (c) and the graphite powder (d) is 0.5 to 4.0% by mass,
(d) the ratio of graphite powder to the total of the iron-based powder (a), the alloy steel powder (b), the Cu powder (c) and the graphite powder (d) is 0.2 to 1.0 mass%.
3. The mixed powder for powder metallurgy as described in the above 2, wherein,
also contains (e) a lubricant which is,
(e) the ratio of the lubricant to the total of the iron-based powder (a), the alloy steel powder (b), the Cu powder (c) and the graphite powder (d) is 0.2 to 1.5% by mass.
4. A sintered body obtained by molding and sintering the mixed powder for powder metallurgy according to any one of 1 to 3.
5. A method for producing a sintered body, wherein the mixed powder for powder metallurgy according to any one of 1 to 3 is molded and sintered to produce a sintered body.
Effects of the invention
The mixed powder for powder metallurgy of the present invention is superior in compressibility to conventional partially diffused alloy steel powder, and can give a press-formed article having a high forming density not only in the case of the ordinary primary forming-primary sintering method but also in the case of the secondary forming-secondary sintering method. In addition, according to the present invention, a sintered body having high strength can be obtained.
Detailed Description
The method for carrying out the present invention will be specifically explained. Unless otherwise specified, "%" in the following description means "% by mass".
A mixed powder for powder metallurgy (hereinafter, sometimes simply referred to as "mixed powder") according to an embodiment of the present invention contains, as essential components, (a) an iron-based powder and (b) an alloy steel powder.
(a) Iron-based powder
As the iron-based powder, a powder containing Si: 0-0.2% and Mn: 0 to 0.4% and the balance Fe and inevitable impurities. The iron-based powder has a function of ensuring compressibility during press molding by mixing with the alloy steel powder (b). Therefore, the iron-based powder is preferably as soft as possible. Since the iron-based powder contains elements other than Fe, which causes a reduction in compressibility, it is preferable to use an iron powder (also referred to as "pure iron powder") composed of Fe and unavoidable impurities as the iron-based powder.
In general, an iron-based powder contains Si and Mn as impurities. Si and Mn are elements having an effect of improving hardenability in addition to an effect of improving strength by solid solution strengthening. Therefore, when Si and Mn are contained, depending on cooling conditions, quenching, tempering, and the like at the time of sintering the press-formed product, the strength of the sintered body may be improved, and the sintered body may function effectively. For the above reasons, the above iron-based powder allows one or both of Si and Mn to be contained within the ranges described below.
Si：0～0.2％
Si is an element having an effect of improving the strength of steel by improving hardenability, solid-solution strengthening, and the like. However, if the Si content in the iron-based powder exceeds 0.2%, the formation of oxides increases, the compressibility decreases, and the oxides serve as starting points of fracture in the sintered body, and fatigue strength and toughness decrease. Therefore, the Si content of the iron-based powder is set to 0.2% or less. On the other hand, as described above, the lower the Si content, the better from the viewpoint of compressibility, and therefore, the Si content may be 0%. Therefore, the Si content of the iron-based powder is set to 0% or more.
Mn：0～0.4％
Mn is an element having an effect of improving the strength of steel by improving hardenability, solid solution strengthening, and the like, similarly to Si. However, if the Mn content in the iron-based powder exceeds 0.4%, the formation of oxides increases, the compressibility decreases, and the oxides serve as starting points of fracture in the sintered body, and fatigue strength and toughness decrease. Therefore, the Mn content of the iron-based powder is set to 0.4% or less. On the other hand, as described above, the lower the Mn content, the better from the viewpoint of compressibility, and therefore, the Mn content may be 0%. Therefore, the Mn content of the iron-based powder is set to 0% or more.
The amount of unavoidable impurities contained in the iron-based powder is not particularly limited, but is preferably 1.0 mass% or less, more preferably 0.5 mass% or less, and still more preferably 0.3 mass% or less in total. The content of P in the element contained as an inevitable impurity is preferably set to 0.020% or less. The S content is preferably set to 0.010% or less. The O content is preferably set to 0.20% or less. The N content is preferably set to 0.0015% or less. The Al content is preferably set to 0.001% or less. The Mo content is preferably 0.010% or less.
(b) Alloy steel powder
As the alloy steel powder, a powder containing Mo: 2.0 to 21.0%, Si: 0-0.2% and Mn: 0 to 0.4% and the balance of Fe and inevitable impurities. The alloy steel powder has a function of supplying Mo as an alloy element. By using the alloy steel powder (b) containing Mo and the iron-based powder (a) containing no Mo in a mixed manner, the excellent compressibility of the powder and the high mechanical strength of the sintered body can be achieved at a high level.
Mo：2.0～21.0％
As described above, Mo is less likely to be oxidized and is easily reduced to the same extent as Fe, and therefore, Mo-containing alloy steel powder can be relatively easily produced. Mo has an effect of enhancing hardenability to cause phase transformation strengthening of the matrix phase at the time of quenching treatment, and also has an effect of dispersing in the matrix phase to cause solid-solution strengthening of the matrix phase and an effect of forming fine carbides in the matrix phase to cause precipitation strengthening of the matrix phase. In addition, Mo has a good carburizing property and is a non-intergranular oxidation element, and therefore, also has a carburizing and strengthening effect. Therefore, Mo is also very useful as a reinforcing element.
However, in the present invention, since the iron-based powder is used in combination with the alloy steel powder, the Mo content of the powder metallurgy mixed powder as a whole is lower than that of the original alloy steel powder. For example, when the mixed powder for powder metallurgy is composed of only an iron-based powder and an alloy powder, the ratio of the alloy steel powder is 50 to 90% as described later, and therefore, the Mo content of the entire mixed powder is 1/2 to 9/10 of the Mo content in the alloy steel powder. In view of the above, the Mo content of the alloy steel powder is set to 2.0% or more. When the Mo content is less than 2.0%, the effect of Mo as a reinforcing element as described above cannot be sufficiently obtained. On the other hand, when the Mo content of the alloy steel powder exceeds 21.0%, the toughness is lowered. Therefore, the Mo content of the alloy steel powder is set to 21.0% or less.
Since the alloying elements other than Mo are not substantially used, the balance of the alloyed steel powder other than Mo may be Fe and inevitable impurities. In general, alloy steel powder contains Si and Mn as impurities. As also described above, Si and Mn are elements having an effect of improving hardenability in addition to an effect of improving strength by solid solution strengthening. Therefore, when Si and Mn are contained, depending on cooling conditions, quenching, tempering, and the like at the time of sintering the press-formed product, the strength of the sintered body may be improved, and the sintered body may function effectively. For the above reasons, the above alloy steel powder is allowed to contain one or both of Si and Mn within the range described below.
Si：0～0.2％
Si is an element having an effect of improving the strength of steel by improving hardenability, solid-solution strengthening, and the like. However, if the Si content in the alloy steel powder exceeds 0.2%, the formation of oxides increases, the compressibility decreases, and the oxides serve as starting points of fracture in the sintered body, and the fatigue strength and toughness decrease. Therefore, the Si content of the alloy steel powder is set to 0.2% or less. On the other hand, as described above, the lower the Si content, the better from the viewpoint of compressibility, and therefore, the Si content may be 0%. Therefore, the Si content of the alloy steel powder is set to 0% or more.
Mn：0～0.4％
Mn is an element having an effect of improving the strength of steel by improving hardenability, solid solution strengthening, and the like, similarly to Si. However, if the Mn content in the alloy steel powder exceeds 0.4%, the formation of oxides increases, the compressibility decreases, and the oxides serve as starting points of fracture in the sintered body, and the fatigue strength and toughness decrease. Therefore, the Mn content of the alloy steel powder is set to 0.4% or less. On the other hand, as described above, the lower the Mn content, the better from the viewpoint of compressibility, and therefore, the Mn content may be 0%. Therefore, the Mn content of the alloy steel powder is set to 0% or more.
The amount of unavoidable impurities contained in the alloy steel powder is not particularly limited, and is preferably 1.0 mass% or less, more preferably 0.5 mass% or less, and still more preferably 0.3 mass% or less in total. The content of P in the element contained as an inevitable impurity is preferably set to 0.020% or less. The S content is preferably set to 0.010% or less. The O content is preferably set to 0.20% or less. The N content is preferably set to 0.0015% or less. The Al content is preferably set to 0.001% or less.
The alloy steel powder is not particularly limited, and any alloy steel powder may be used as long as it has the above-described composition. For example, the above alloy steel powder may be set to one or both of pre-alloy steel powder and partially diffused alloy steel powder. As the above-mentioned partially diffused alloy steel powder, one or both of a partially diffused alloy steel powder in which an alloy element is diffused and adhered to the surface of an iron powder (pure iron powder) and a partially diffused alloy steel powder in which an alloy element is diffused and adhered to the surface of a prealloyed steel powder can be used.
Ratio of alloy steel powder: 50 to 90 percent
(b) The ratio of the mass of the alloy steel powder to the total mass of the iron-based powder (a) and the alloy steel powder (b) (hereinafter, simply referred to as "the ratio of the alloy steel powder") is set to 50 to 90%. When the ratio of the alloy steel powder is less than 50%, that is, the ratio of the iron-based powder exceeds 50%, the iron-based powder portion having low strength is connected to the inside of the sintered body, and when the sintered body is subjected to stress, cracks develop in the portion having low strength, and the sintered body is likely to be broken. Therefore, the alloy steel powder is set to 50% or more. On the other hand, when the ratio of the alloyed steel powder exceeds 90%, that is, the ratio of the iron-based powder is less than 10%, soft portions contributing to compressibility decrease, and compressibility of the entire mixed powder is insufficient. Therefore, the ratio of the alloyed steel powder is set to 90% or less.
Ratio of Mo: 2.2 to 6.2 percent
If the ratio of the mass of Mo to the total mass of the iron-based powder (a) and the alloy steel powder (b) (hereinafter, simply referred to as "Mo ratio") is less than 2.2%, the effect of Mo as a reinforcing element is insufficient. Therefore, the Mo content is set to 2.2% or more. On the other hand, excessive addition of Mo leads to an increase in alloy cost, and therefore, the ratio of Mo is set to 6.2% or less.
The mixed powder for powder metallurgy according to one embodiment of the present invention may be a mixed powder for powder metallurgy (iron-based powder + alloy steel powder: 100%) composed of only (a) an iron-based powder and (b) an alloy steel powder, but may contain optional other components. However, if the ratio of the total mass of the iron-based powder (a) and the alloy steel powder (b) to the mass of the entire mixed powder is too low, the mechanical properties of the sintered body deteriorate. Therefore, the ratio of the total mass of the iron-based powder (a) and the alloy steel powder (b) to the mass of the entire mixed powder is preferably set to 90% or more, and preferably 95% or more.
In one embodiment of the present invention, (c) Cu powder and (d) graphite powder may be further added to the mixed powder for powder metallurgy. By adding the Cu powder and the graphite powder, the strength of the sintered body can be further improved.
(c) Cu powder
Cu is an element having an action of promoting solid solution strengthening and improvement of hardenability of the iron-based powder to improve the strength of the sintered body. When the addition amount of the Cu powder is less than 0.5%, the above-described effect cannot be sufficiently obtained, and therefore, the addition amount of the Cu powder is set to 0.5% or more. The addition amount of the Cu powder is preferably set to 1.0% or more. On the other hand, if the amount of Cu powder added exceeds 4.0%, not only the strength-improving effect of the sintered part is saturated, but also the sintered density is rather lowered. Therefore, the amount of Cu powder added is set to 4.0% or less. The addition amount of the Cu powder is preferably set to 3.0% or less. Here, the "amount of addition of the Cu powder" is defined as a ratio of the mass of the (c) Cu powder to the total mass of the (a) iron-based powder, the (b) alloy steel powder, the (c) Cu powder, and the (d) graphite powder.
(d) Graphite powder
Graphite (Graphite) is an effective component for improving strength. When the amount of graphite powder added is less than 0.2%, the above-mentioned effects cannot be sufficiently obtained. Therefore, the amount of graphite powder added is set to 0.2% or more. The amount of graphite powder added is preferably set to 0.3% or more. On the other hand, when the amount of graphite powder added exceeds 1.0%, precipitation of cementite due to hypereutectoid increases, resulting in a decrease in strength. Therefore, the amount of graphite powder added is set to 1.0% or less. The amount of graphite powder added is preferably set to 0.8% or less. Here, the "amount of graphite powder added" is defined as a ratio of the mass of (d) graphite powder to the total mass of (a) iron-based powder, (b) alloy steel powder, (c) Cu powder, and (d) graphite powder.
In one embodiment of the present invention, (e) a lubricant may be further added to the mixed powder for powder metallurgy. By adding the lubricant, friction at the time of press molding the mixed powder for powder metallurgy can be reduced to prolong the life of the die, and the density of the molded body can be further increased.
(e) Lubricant agent
When the amount of the lubricant added is less than 0.2%, the above-described effect is hardly exhibited. Therefore, the amount of the lubricant added is set to 0.2% or more. The amount of the lubricant added is preferably set to 0.3% or more. On the other hand, if the amount of the lubricant added exceeds 1.5%, the non-metallic portion in the mixed powder increases, and the molding density is hard to increase, resulting in a decrease in strength. Therefore, the amount of lubricant added is set to 1.5% or less. The amount of the lubricant added is preferably 1.2% or less. Here, the "amount of lubricant added" is defined as a ratio of the mass of the lubricant (e) to the total mass of the iron-based powder (a), the alloy steel powder (b), the Cu powder (c), and the graphite powder (d).
The lubricant is not particularly limited, and any lubricant may be used. As the lubricant, for example, one or two or more selected from the group consisting of fatty acids, fatty acid amides, fatty acid bisamides, and metal soaps can be used. Among them, metal soaps such as lithium stearate and zinc stearate, and amide-based lubricants such as ethylene bis-stearamide are preferably used.
In addition to the method of adding and mixing the lubricant to the mixed powder, a method of directly applying the lubricant to the mold may be used, or a method of combining both methods may be used.
In one embodiment of the present invention, the sintered body can be produced using the above-described mixed powder for powder metallurgy. The method for producing the sintered body is not particularly limited, and the sintered body can be produced by any method, and generally, a mixed powder for powder metallurgy is press-molded by a conventional method for powder metallurgy to form a compact, which is then sintered.
The density of the molded article (which may be referred to as "molding density") is not particularly limited, but is preferably set to 6.85Mg/m from the viewpoint of ensuring sufficient mechanical properties (toughness and the like)3The above. The tensile strength required for the sintered body varies depending on the application, and the like, and is preferably 620MPa or more.
Examples
(example 1)
Mixed powders for powder metallurgy were produced using an iron-based powder and an alloy steel powder containing only Si and Mn as inevitable impurities, and their properties were evaluated. The specific steps are as follows.
(a) The iron-based powder was made as follows: the iron powder produced by the water atomization method was subjected to a final reduction treatment at 900 ℃ for 60 minutes in a hydrogen atmosphere to decarburize and deoxidize, and the obtained briquette was crushed, thereby producing the iron powder. The composition of the resulting iron-based powder is shown in table 1. Each of the elements shown in table 1 is an element contained as an inevitable impurity in the iron-based powder.
As the alloy steel powder (b), two kinds of prealloyed steel powder and composite alloy steel powder are used. The prealloyed steel powder is produced by the same method as the iron-based powder described above, except that a raw material containing Mo is used as a melt for water atomization. In this way, alloyed steel powder in which Mo as an alloying element was added in a prealloyed form was obtained. The composition of the obtained prealloyed steel powder is shown in table 1.
The composite alloy steel powder is manufactured as follows: the pre-alloyed steel powder containing 5.0 mass% of Mo was produced in the same manner as the above-described pre-alloyed steel powder, and Mo was further diffused and adhered to the surface of the obtained pre-alloyed steel powder. In the diffusion adhesion, MoO corresponding to Mo contents of 1.0 mass%, 1.7 mass%, 3.6 mass%, 7.0 mass%, and 15.0 mass% was added to the prealloyed steel powder3The powders were mixed and heat-treated at 900 ℃ for 60 minutes in a hydrogen atmosphere. By the above heat treatment, the prealloyed steel powder is decarburized and deoxidized while the MoO is reduced3Mo generated by the reduction of (a) is diffused and attached to the prealloyed steel powder. The blocks obtained by the above treatment are crushed to prepare the composite alloy steel powder with Mo diffused and adhered on the surface of the pre-alloy steel powder. The composition of the obtained composite alloy steel powder is shown in table 1.
Next, the obtained iron-based powder (a) and alloy steel powder (b) were mixed for 15 minutes by a V-type mixer in the combination and ratio shown in table 2 to obtain a mixed powder of the iron-based powder and the alloy steel powder. The mixing ratio of the iron-based powder (a) and the alloy steel powder (b) is a ratio intended to make the ratio of Mo to the total of the iron-based powder (a) and the alloy steel powder (b) 2.3 mass% or 6.0 mass%, and the calculated value of the ratio of Mo is shown in table 2.
Next, to the mixed powder of the iron-based powder and the alloy steel powder, Cu powder, graphite powder, and Wax-based lubricant powder were further added in the proportions shown in table 2, and mixed for 15 minutes by a V-type mixer to obtain a mixed powder for powder metallurgy. In nos. 1 to 3, no Cu powder or graphite powder was used, and only a lubricant was added.
The properties of the obtained mixed powder for powder metallurgy were evaluated by the following procedures.
Density of press-formed article
Using each powder mixture for powder metallurgy, a press-molded body as a test piece was produced, and the density thereof was evaluated. The press-molding body was formed into a ring shape having an outer diameter of 38 mm. phi. times.inner diameter of 25 mm. phi. times.height of 10mm, and the molding pressure was 686 MPa. The density was determined by measuring the weight of the obtained molded article and dividing by the volume calculated from the size. The results are shown in Table 2.
Tensile Strength of sintered body
Sintered bodies as tensile test pieces were prepared from the respective powder mixtures for powder metallurgy, and the tensile strength was measured. The tensile test piece was produced by molding the powder metallurgy mixture powder into a tensile test piece having parallel portions of 5.8mm in width × 5mm in height and subjecting the tensile test piece to a sintering treatment at 1130 ℃ for 20 minutes in an RX gas atmosphere. The results are shown together in table 2.
From the results shown in table 2, the following tendency was found: as the mixing ratio of the iron-based powder increases, the forming density increases, and the tensile strength increases first and then decreases. In addition, for the examples satisfying the conditions of the present invention, 6.85Mg/m was obtained3The above molding density and a tensile strength of 620MPa or more. On the other hand, when the mixing ratio of the iron-based powder was 0 mass%, the tensile strength was not 620MPa when the Mo content of the mixed powder was 2.3 mass%, and the molding density and tensile strength were not 6.85Mg/m when the Mo content of the mixed powder was 6.0 mass%3And 620 MPa. When the blending ratio of the pure iron powder is 70 mass% or more, the tensile strength is at any of 2.3 mass% and 6.0 mass% of the Mo content in the blended powderNone of them reached 620 MPa.
(example 2)
A mixed powder for powder metallurgy was produced in the same manner as in example 1 except that an iron-based powder containing Mn and an alloyed steel powder were used, and the performance thereof was evaluated. The compositions of the iron-based powder and the alloy steel powder used are shown in table 3, and the blending ratios of the respective components and the evaluation results are shown in table 4.
From the results shown in table 4, it is understood that as the mixing ratio of the iron-based powder is increased, the molding density is increased, and the tensile strength is increased and then decreased, as in the case of example 1. In addition, for the examples satisfying the conditions of the present invention, 6.85Mg/m was obtained3The above molding density and a tensile strength of 620MPa or more.
[ Table 3]
TABLE 3
The balance being Fe and other unavoidable impurities
[ Table 4]
TABLE 4
Ratio of 1 to the total of (a) iron-based powder and (b) alloyed steel powder
A ratio of 2 to a total of (a) the iron-based powder, (b) the alloy steel powder, (c) the Cu powder, and (d) the graphite powder
(example 3)
A mixed powder for powder metallurgy was produced in the same manner as in example 1 except that an iron-based powder containing Si and Mn and an alloy steel powder were used, and the performance thereof was evaluated. The compositions of the iron-based powder and the alloy steel powder used are shown in table 5, and the blending ratios of the respective components and the evaluation results are shown in table 6.
From the results shown in table 6, it is understood that as the mixing ratio of the iron-based powder is increased, the molding density is increased, and the tensile strength is increased and then decreased, as in the cases of examples 1 and 2. In addition, for the examples satisfying the conditions of the present invention, 6.85Mg/m was obtained3The above molding density and a tensile strength of 620MPa or more. It is also found that, in examples 2 and 3 in which the raw material powder containing one or both of Si and Mn was used, the tensile strength of the sintered body was improved as compared with example 1 while maintaining a high density of the formed body. Therefore, it can be said that when importance is attached to the strength, it is preferable to add one or both of Si and Mn.

Claims (5)

1. A mixed powder for powder metallurgy, which is prepared from a raw material,
it comprises the following components:
(a) contains Si: 0-0.2 mass% and Mn: 0 to 0.4 mass% of an iron-based powder, the balance being Fe and unavoidable impurities; and
(b) the alloy is prepared from Mo: 2.0 to 21.0 mass%, Si: 0-0.2 mass%, Mn: 0 to 0.4 mass% and the balance Fe and inevitable impurities,
the alloy steel powder (b) is pre-alloy steel powder containing Mo as an alloy element,
(b) the ratio of the alloy steel powder to the total of the iron-based powder (a) and the alloy steel powder (b) is 50 to 90% by mass,
the amount of unavoidable impurities in the (b) alloy steel powder is 1.0 mass% or less in total,
the ratio of Mo to the total of the iron-based powder (a) and the alloy steel powder (b) is 2.2 to 6.2 mass%.
2. The mixed powder for powder metallurgy according to claim 1, wherein,
also contains (c) Cu powder and (d) graphite powder,
(c) the ratio of the Cu powder to the total of the iron-based powder (a), the alloyed steel powder (b), the Cu powder (c) and the graphite powder (d) is 0.5 to 4.0 mass%,
(d) the ratio of graphite powder to the total of the iron-based powder (a), the alloy steel powder (b), the Cu powder (c) and the graphite powder (d) is 0.2 to 1.0 mass%.
3. The mixed powder for powder metallurgy according to claim 2, wherein,
also contains (e) a lubricant which is,
(e) the ratio of the lubricant to the total of the iron-based powder (a), the alloy steel powder (b), the Cu powder (c) and the graphite powder (d) is 0.2 to 1.5% by mass.
4. A sintered body obtained by molding and sintering the mixed powder for powder metallurgy according to any one of claims 1 to 3.
5. A method for producing a sintered body, wherein the mixed powder for powder metallurgy according to any one of claims 1 to 3 is molded and sintered to produce a sintered body.
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