JP2021095599A - Sintered alloy and method for producing sintered alloy - Google Patents

Abstract

To provide a sintered alloy having excellent high-temperature strength and a method for producing the same.SOLUTION: A sintered alloy has a whole composition comprising, in mass%, Cr: 11.75-39.98%, Ni: 5.58-24.98%, Si: 0.16-2.54, P: 0.1-1.5%, C: 0.58-5.55% with the balance being Fe and unavoidable impurities, with an average crystal grain size of 30 μm-100 μm.SELECTED DRAWING: Figure 1

Description

One embodiment of the present invention relates to a sintered alloy and a method for producing a sintered alloy.

A sintered alloy can be preferably used for turbo parts for turbochargers, particularly for nozzle bodies and the like that require heat resistance, corrosion resistance, and wear resistance.
Generally, in a turbocharger attached to an internal combustion engine, the turbine is rotatably supported by a turbine housing connected to an exhaust manifold of the internal combustion engine, and a plurality of nozzle vanes are rotatably supported so as to surround the outer peripheral side of the turbine. Has been done. The exhaust gas that has flowed into the turbine housing flows into the turbine from the outer peripheral side and is discharged in the axial direction, at which time the turbine is rotated. Then, the compressor provided on the same shaft on the opposite side of the turbine rotates to compress the air supplied to the internal combustion engine.

Here, the nozzle vane is rotatably supported by a ring-shaped component called a nozzle body or a mount nozzle. The shaft of the nozzle vane penetrates the nozzle body, where it is connected to the link mechanism. Then, by driving the link mechanism, the nozzle vane rotates, and the opening degree of the flow path through which the exhaust gas flows into the turbine is adjusted. The object of the present invention is a turbo component provided in a turbine housing, such as a nozzle body (mount nozzle) or a plate nozzle mounted on the nozzle body (mount nozzle).

The turbo parts for turbochargers as described above are required to have heat resistance and corrosion resistance because they come into contact with exhaust gas, which is a high-temperature corrosive gas, and are also required to have wear resistance because they are in sliding contact with nozzle vanes. For this reason, conventionally, for example, high Cr cast steel, or a wear resistant material obtained by subjecting SCH22 type specified in JIS standard to Cr surface treatment for the purpose of improving corrosion resistance has been used. Further, as a wear-resistant part having excellent heat resistance, corrosion resistance and wear resistance, and at a low price, a wear-resistant sintered part in which carbides are dispersed in a ferrite stainless steel base has been proposed (Patent Documents). 1).

However, since the sintered part of Patent Document 1 is obtained by liquid phase sintering, it is necessary to machine it when the demand for dimensional accuracy is strict. However, since a large amount of hard carbide is deposited, the machinability is poor and the work piece is covered. Improvement of shaving property is desired. Further, the components of the turbocharger are generally made of an austenitic heat-resistant material, but the turbo parts for a turbocharger described in Patent Document 1 are made of a ferrite material. In this case, since the coefficient of thermal expansion is different from that of the surrounding members, a gap is created between the components made of both materials, and the connection between them is insufficient. It is desired that the coefficient of thermal expansion is equivalent to that of the material.

Patent Document 2 discloses a sintered alloy having a specific overall composition in which a phase A in which metal carbides having a relatively large size are precipitated and a phase B in which relatively small metal carbides are precipitated are distributed in a patchy manner. There is. The sintered alloy of Patent Document 2 has heat resistance, corrosion resistance, and wear resistance, and by adding Ni, the coefficient of thermal expansion becomes equivalent to that of the surrounding austenitic heat-resistant material, which simplifies component design. it can.

Japanese Patent No. 3784003 Japanese Unexamined Patent Publication No. 2013-57094

As described above, turbo parts for turbochargers are used in a high temperature environment, so that high temperature strength is required in addition to a heat resistant layer, corrosion resistance and wear resistance. In Patent Document 2, in order to form two phases of phase A and phase B in the iron matrix, sintering is performed at a relatively low temperature so that carbon elements and metal elements do not diffuse between the phases A and B. It is in progress. In the sintered alloy obtained through such a sintering process, there is a problem that the high temperature strength decreases in a higher temperature usage environment.
An object of the present invention is to provide a sintered alloy having excellent high-temperature strength and a method for producing the same.

One embodiment has the following gist.
[1] The overall composition is, in mass%, Cr: 11.75 to 39.98%, Ni: 5.58 to 24.98%, Si: 0.16 to 2.54, P: 0.1 to 1. A sintered alloy containing 5.5%, C: 0.58 to 5.55%, the balance consisting of Fe and unavoidable impurities, and an average crystal grain size of 30 μm to 100 μm.
[2] The sintered alloy according to [1], which further contains at least one selected from the group consisting of Mo, V, W, Nb, and Ti in mass% and 5% or less in total.
[3] In mass%, Cr: 25 to 45%, Ni: 5 to 15%, Si: 1.0 to 3.0%, C: 0.5 to 4.0%, and the balance is Fe and unavoidable. Iron alloy powder A composed of impurities,
Iron alloy powder B, which contains Cr: 12 to 25% and Ni: 5 to 15% in mass%, and the balance is composed of Fe and unavoidable impurities.
Prepare iron-phosphorus alloy powder, nickel powder, and graphite powder having a composition of P: 10 to 30% by mass and the balance consisting of Fe and unavoidable impurities;
The iron alloy powder A and the iron alloy powder B are added so that the ratio of the iron alloy powder A to the total amount of the iron alloy powder A and the iron alloy powder B is 20 to 80% by mass, and the raw material powder is added. The iron-phosphorus alloy powder is added in an amount of 1.0 to 5.0% by mass, the nickel powder is added in an amount of 1 to 12% by mass, and the graphite powder is added in an amount of 0.5 to 2.5% by mass based on the total amount of the raw material powder. To get;
A method for producing a sintered alloy, which comprises molding the raw material powder to obtain a molded product; and sintering the molded product at a temperature of more than 1200 ° C. and lower than 1250 ° C.
[4] At least one of the iron alloy powder A and the iron alloy powder B contains at least one selected from the group consisting of Mo, V, W, Nb, and Ti in a total amount, which is the total amount of the raw material powder. The method for producing a sintered alloy according to [3], which further comprises 5% by mass or less based on the composition.

According to one embodiment of the present invention, it is possible to provide a sintered alloy having excellent high temperature strength and a method for producing the same.

FIG. 1 (a) is a photograph of the crystal grains of Example 5 observed with a microscope, and FIG. 1 (b) is a photograph of the crystal grains of Example 3 observed with a microscope. FIG. 2 is a graph showing the relationship between creep time and strain for Examples 5 and 3.

Hereinafter, an embodiment of the present invention will be described, but the present invention is not limited by the following examples.

As the sintered alloy according to one embodiment, the overall composition is mass%, Cr: 11.75 to 39.98%, Ni: 5.58 to 24.98%, Si: 0.16 to 2.54, It contains P: 0.1 to 1.5% and C: 0.58 to 5.55%, the balance is composed of Fe and unavoidable impurities, and the average crystal grain size is 30 μm to 100 μm.
According to this, it is possible to provide a sintered alloy having excellent high temperature strength and a method for producing the same.
Further, in applications where heat resistance, corrosion resistance, wear resistance, and machinability are required, high temperature strength can be further improved. Further, this sintered alloy can have a coefficient of thermal expansion equivalent to that of an austenitic heat-resistant material, and the range of material design can be expanded.
This sintered alloy can be preferably used, for example, in turbo parts for turbochargers, particularly nozzle bodies that require heat resistance, corrosion resistance, and wear resistance.

The sintered alloy according to one embodiment can improve high-temperature strength in a range of 850 ° C. or higher, particularly in a range of 900 ° C. or higher, and further in a range of 950 ° C. or higher.
Stress from the mating material is applied to turbo parts for turbochargers in a high-temperature usage environment, but by using this sintered alloy, the amount of strain due to stress can be further reduced even in a high-temperature environment. In turbo parts for turbochargers, the exhaust gas becomes hotter as fuel efficiency improves, so it is required to reduce the amount of strain due to stress in a higher temperature environment.

The sintered alloy according to one embodiment preferably has an average crystal grain size of 30 to 100 μm.
In a normal temperature usage environment, the strength tends to decrease as the crystal grains of the sintered alloy become larger. Therefore, the conventional sintered alloy is manufactured so as to suppress the coarsening of the crystal grains. The present invention has been made based on the finding that the strength peaks between the smaller and larger crystal grains of the sintered alloy under a high-temperature usage environment. More specifically, under a high temperature usage environment suitable for turbo parts for turbochargers, the crystal grains of the sintered alloy can be further increased in strength in the range of 30 μm to 100 μm.

The average crystal grain size of the sintered alloy is preferably 30 μm or more, more preferably 35 μm or more. As a result, when the crystal grains have a certain size, the ability to absorb stress by the interface between the crystal grains is enhanced, and the occurrence of strain against the stress can be suppressed. In particular, in a high temperature environment, the occurrence of this distortion can be suppressed.
The average crystal grain size of the sintered alloy is preferably 100 μm or less, more preferably 80 μm or less, still more preferably 50 μm or less. As a result, it is possible to prevent the formation of coarse crystal grains and prevent the occurrence of interfacial deviation between the coarse crystal grains. In particular, in a high temperature environment, it is possible to prevent the occurrence of deviation of this interface and suppress the occurrence of strain against stress.

Here, the average crystal grain size of the sintered alloy can be measured by the following procedure.
The metallographic structure of the sintered body sample can be observed by adjusting the optical microscope so that the crystal grain boundaries can be seen, for example, by magnifying it at a magnification of 500 times. A photograph of the metal structure of the sintered body sample is taken in a predetermined region, an arbitrary number of crystal grains observed in the field of view are randomly extracted, and the longest size a and the shortest size b are measured. Assuming that the average value of the longest size a and the shortest size b is the particle size of one crystal grain, the average value of the measured values measured for an arbitrary number of crystal grains is defined as the average crystal grain size. The visual field region may be, for example, 148 μm × 196 μm. The number to be measured may be, for example, five. In this case, if a predetermined number of crystal grains are not observed in one field of view, it is advisable to increase the field of view for measurement. Commercially available products such as image analysis software (“WinROOF” manufactured by Mitani Corporation) can be used for measuring the size of crystal grains.

The overall composition of the sintered alloy according to one embodiment is, for example, in terms of mass%, Cr: 11.75 to 39.98%, Ni: 5.58 to 24.98%, Si: 0.16 to 2.54. , P: 0.1 to 1.5%, C: 0.58 to 5.55%, and the balance is preferably composed of Fe and unavoidable impurities.
In the following description, unless otherwise specified, "%" indicating the element ratio represents "mass%".

The sintered alloy according to one embodiment preferably has a uniform overall composition, and preferably has a uniform composition among a plurality of crystal grains.
The sintered alloy according to one embodiment may contain metal carbides. This metal carbide is contained in the sintered alloy by bonding and precipitating C and a metal element such as Cr in the sintered alloy. Since the metal carbide is hard, the wear resistance of the sintered alloy can be further improved.

The sintered alloy preferably contains Cr: 11.75 to 39.98%.
Cr contributes to the improvement of heat resistance and corrosion resistance of the iron matrix, and combines with C to form carbides to improve wear resistance. For example, Cr combines with C to form chromium carbide, a composite carbide of chromium and iron, and the like (hereinafter, these are also simply referred to as “chromium carbide”). Cr is preferably 11.75% or more, more preferably 15% or more, still more preferably 20% or more. As a result, the amount of chromium carbide precipitated can be increased to further enhance the wear resistance, and the heat resistance and corrosion resistance of the sintered alloy can be further improved. From the viewpoint of compressibility of the raw material powder, Cr is preferably 39.98% or less, more preferably 35% or less, still more preferably 30% or less. Further, when Cr is excessively blended, the amount of chromium carbide precipitated becomes excessive, and while the wear resistance is improved, the machinability tends to decrease. Further, if excess Cr remains without being dissolved in the sintered alloy, Cr is an element that is easily oxidized, so that the corrosion resistance tends to decrease. From this viewpoint as well, Cr is preferably 39.98% or less.
In order to uniformly exert the effect of Cr in the iron matrix, it is preferable to impart Cr in the form of iron alloy powder.

The sintered alloy preferably contains Ni: 5.58 to 24.98%.
Ni diffuses into the iron base to strengthen the solid solution, and at the same time, austenite the iron base to improve the high temperature strength of the wear-resistant parts. Ni is preferably 5.58% or more, more preferably 8% or more. Thereby, the high temperature strength can be further increased and the corrosion resistance can be further improved. Ni is preferably 24.98% or less, more preferably 20% or less. Thereby, high temperature strength can be sufficiently obtained.
In order to make such an effect of Ni act uniformly in the iron matrix, it is preferable to dissolve Ni in a solid solution and impart it in the form of an iron alloy powder. Since Ni does not increase in hardness of the iron alloy powder even if it is given as a solid solution in the iron alloy powder, it is preferable to give Ni as a solid solution in the iron alloy powder from this viewpoint as well. Further, Ni may be added as nickel powder from the viewpoint of promoting densification of the sintered alloy.

The sintered alloy preferably contains Si: 0.16 to 2.54%.
Si can be blended as a deoxidizer. Further, Si can improve the sinterability. Since the sintered alloy contains Cr which is easily oxidized, it is preferable to add Si as a deoxidizer to the raw material powder. When Si is 0.16% or more, the action can be sufficiently obtained. From the viewpoint of compressibility of the raw material powder, Si is preferably 2.54% or less, more preferably 2% or less, still more preferably 1% or less.
In order to uniformly impart such an action of Si into the iron matrix, it is preferable to impart Si in the form of an iron alloy powder.

The sintered alloy preferably contains P: 0.1 to 1.5%.
P, together with C, generates a Fe—PC liquid phase in sintering to promote densification of the sintered body. This makes it possible to achieve a high density ratio of the sintered alloy, particularly a density ratio of 90% or more. When P is 0.1% or more, the generation of a liquid phase can be promoted in sintering. From the viewpoint of sufficiently obtaining the action, P is preferably 1.5% or less, preferably 1% or less. Further, P is preferably 1.5% or less from the viewpoint that a dimensional error occurs in the sintered body when the liquid phase is excessively generated.
In order to promote liquid phase formation and achieve densification in sintering, P is preferably added in the form of iron-phosphorus alloy powder. When the amount of P is 10% or more with respect to the entire iron-phosphorus alloy powder, a sufficient liquid phase can be generated and densification of the sintered body can be promoted. The amount of P with respect to the total amount of the iron-phosphorus alloy powder is preferably 30% or less in order to prevent the iron-phosphorus alloy powder from becoming too hard and reducing the compressibility.

The sintered alloy preferably contains C: 0.58 to 5.55%.
In order to lower the liquid phase liquefaction temperature, C generates a Fe—P—C liquid phase in sintering and promotes densification of the sintered body. Further, C produces a metal element such as Cr and a metal carbide to contribute to wear resistance. When C is 0.58% or more, it is possible to promote densification of the sintered body and contribute to wear resistance. C is preferably 5.55% or less, more preferably 3.62% or less, still more preferably 3% or less. Thereby, the amount of metal carbides precipitated in the sintered alloy can be limited, and deterioration of machinability can be prevented. Further, when C is 5.55% or less, the compressibility of the raw material powder can be further improved.
C can be applied in the form of graphite powder. When the entire amount of C is applied in the form of graphite powder, the iron alloy powder becomes a powder in which Cr is solid-solved in the iron matrix, and the iron alloy powder may become too hard and the compressibility may be impaired. In addition, the use of a large amount of graphite powder also causes a decrease in the compressibility of the mixed powder. From this point of view, it is preferable to add a part of C in the form of iron alloy powder and the remaining C in the form of graphite powder.

The balance of the sintered alloy consists of Fe and unavoidable impurities.
The sintered alloy may further contain at least one selected from the group consisting of Mo, V, W, Nb and Ti.
Since the carbide-forming elements Mo, V, W, Nb, and Ti have a stronger carbide-forming ability than Cr, they preferentially form carbides over Cr. By containing these elements, there is an action of preventing a decrease in Cr concentration of the iron matrix, so that it can contribute to improvement of heat resistance and corrosion resistance of the iron matrix. Further, it can be combined with C to form an alloy carbide to improve wear resistance. From this point of view, Mo, V, or a combination thereof is more preferable.
However, when at least one element selected from the group consisting of Mo, V, W, Nb and Ti is added to the raw material powder in the form of a pure metal powder, each alloy diffuses slowly in sintering, so that the iron matrix Difficult to spread evenly throughout. Therefore, it is preferable to add these elements in the form of iron alloy powder. The total amount of Mo, V, W, Nb, and Ti dissolved in the iron alloy powder is preferably 5% or less. The amount of Mo, V, W, Nb, and Ti dissolved in the iron alloy powder is preferably 5% or less in terms of the amount of each single element.

As described above, by adding C to the sintered alloy in order to improve wear resistance, metal carbides in which C and a metal element such as Cr are bonded can be precipitated in the iron matrix of the sintered alloy. ..
The amount and size of carbides greatly contribute to wear resistance. Abrasion resistance improves as the amount of carbide increases. However, when the amount of carbide increases, the wear resistance of the self is improved, but the aggression to the mating material is increased, and the wear amount as a whole may be rather increased. Further, when only large carbides are dispersed in the iron matrix, more C is required to secure a certain degree of dispersion frequency of carbides in order to improve wear resistance, and hard carbides are dispersed with a certain degree of dispersion frequency. As a result, the machinability may decrease.
Therefore, in one embodiment, the sintered alloy contains C: 0.5 to 4.0% in the overall composition, so that the amount of metal carbides deposited can be adjusted and the wear resistance of the sintered alloy can be adjusted. It is possible to improve the machinability as well as the property. In addition, the amount of metal carbide precipitated can be adjusted more appropriately by going through the method for producing a sintered alloy, which will be described later.

In one embodiment, the average particle size of the metal carbide deposited on the sintered alloy is preferably 3 μm to 50 μm.
Here, the average particle size of the metal carbide is determined by mirror-polishing the cross section of the sintered body, corroding with royal water, and then observing with a microscope and analyzing the image to observe a plurality of metal carbide particles in a predetermined region. The diameter can be measured and obtained from the average value.

Hereinafter, an example of a method for producing a sintered alloy according to one embodiment will be described. The sintered alloy according to one embodiment is not limited to the one produced by the following production method.
The sintered alloy according to one embodiment contains, for example, iron alloy powder, iron-phosphorus alloy powder, nickel powder, and graphite powder having a composition of P: 10 to 30% in mass% and the balance consisting of Fe and unavoidable impurities. , Cr: 11.75 to 39.98%, Ni: 5.58 to 24.98%, Si: 0.16 to 2.54, P: 0.1 to 1.5 by mass%. %, C: 0.58 to 5.55%, the balance of which comprises Fe and unavoidable impurities to prepare a raw material powder, and after molding the raw material powder, sintering at a temperature of more than 1200 ° C. and lower than 1250 ° C.
In this method, the raw material powder contains 1.0 to 5.0% by mass of iron-phosphorus alloy powder, 1 to 12% by mass of nickel powder, and 0.5 to 2.5% by mass of graphite powder, and the balance. Is preferably an iron alloy powder.
According to this method, by sintering using the above raw material powder at a temperature exceeding 1200 ° C. and lower than 1250 ° C., a sintered alloy having an overall composition within the above range and an average crystal grain size of 30 to 100 μm is obtained. be able to.

More specifically, as a method for producing a sintered alloy according to one embodiment, in terms of mass%, Cr: 25 to 45%, Ni: 5 to 15%, Si: 1.0 to 3.0%, C: 0. Iron alloy powder A containing 5 to 4.0% and the balance consisting of Fe and unavoidable impurities, mass%, Cr: 12 to 25%, Ni: 5 to 15%, the balance being Fe and unavoidable impurities Prepare iron-phosphorus alloy powder, nickel powder, and graphite powder having a composition consisting of iron alloy powder B having a composition of B, mass%, P: 10 to 30%, and the balance consisting of Fe and unavoidable impurities; iron alloy powder. A and iron alloy powder B are added so that the ratio of iron alloy powder A to the total amount of iron alloy powder A and iron alloy powder B is 20 to 80% by mass, and the iron-phosphorus alloy is added to the total amount of raw material powder. Raw material powder is obtained by adding powder in an amount of 1.0 to 5.0% by mass, nickel powder in an amount of 1 to 12% by mass, and graphite powder in an amount of 0.5 to 2.5% by mass; the raw material powder is molded into a molded product. Obtaining; and preferably comprising sintering the compact at> 1200 ° C. and <1250 ° C.

The iron alloy powder A preferably contains Cr: 25 to 45% and C: 0.5 to 4.0% in mass%.
The amount of Cr in the iron alloy powder A is preferably 25% by mass or more, more preferably 30% by mass or more. As a result, the amount of Cr in the iron base portion becomes sufficient, and the heat resistance and corrosion resistance of the iron base portion can be further improved. The amount of Cr in the iron alloy powder A is preferably 45% by mass or less, more preferably 40% by mass or less. Thereby, the compressibility of the raw material powder can be further improved.

The amount of C in the iron alloy powder A is preferably 0.5% by mass or more, more preferably 1% by mass or more. As a result, it is possible to secure the formation of carbides that are the core in sintering, and to obtain carbides of a more preferable size. The amount of C in the iron alloy powder A is preferably 4.0% by mass or less, more preferably 3% by mass or less. As a result, it is possible to prevent an increase in the powder hardness of the iron alloy powder A and further improve the compressibility of the raw material powder.

The iron alloy powder B preferably contains Cr: 12 to 25% in mass%. Further, the iron alloy powder B preferably does not contain C. In addition, in the iron alloy powder B, "C-free" means that it is not positively added, and an amount that is contained as an unavoidable impurity is allowed.

The amount of Cr in the iron alloy powder B is preferably 12% by mass or more. As a result, it is possible to suppress the amount of chromium carbide produced during sintering, prevent a decrease in the amount of Cr in the iron matrix portion, and further improve the heat resistance and corrosion resistance of the sintered alloy. The amount of Cr in the iron alloy powder B is preferably 25% by mass or less. Since Cr finely disperses carbides that contribute to wear resistance, it is preferable to limit the amount of Cr to 25% by mass or less.

The raw material powder preferably contains graphite powder. The graphite powder is preferably 0.5 to 2.5% by mass with respect to the entire raw material powder.
It is preferable that C for precipitating and dispersing carbides in sintering is added to a mixed powder of iron alloy powder A and iron alloy powder B in the form of graphite powder. Since a part of the graphite powder is spent on reducing the oxide film on the surface of the iron alloy powder in sintering, it is preferable to add the graphite powder in anticipation of that amount. Specifically, since the iron alloy powder A and the iron alloy powder B each contain Cr, which is an easily oxidizing element, as described above, an oxide film of Cr is formed on the surface of each powder, and each of them is subjected to sintering. Excess graphite powder is spent on the reduction reaction of oxides on the surface of the powder. Since the amount of graphite lost by reduction or the like in sintering is about 0.2%, the amount of graphite powder added is preferably 0.5% by mass or more in anticipation of that amount. In this case, the amount of C supplied from the graphite powder and dissolved in the iron matrix can be 0.3% by mass or more. On the other hand, if graphite powder is added excessively, the amount of carbides precipitated becomes excessive, the sintered alloy becomes embrittled, and the aggression to the mating material increases remarkably, causing the mating material to wear or shrink bonding. It deteriorates the machinability of gold. Further, if the amount of carbide precipitated is excessive, the amount of Cr contained in the iron matrix of the sintered alloy is reduced by that amount, which may reduce the heat resistance and corrosion resistance of the sintered alloy. From this viewpoint, the graphite powder is preferably 2.5% by mass or less, more preferably 2% by mass or less.

In addition to the above-mentioned carbide forming action, the graphite powder, together with the iron-phosphorus alloy powder described later, generates an Fe-PC liquid phase in sintering to reduce the liquid phase formation temperature and reduce the liquid phase formation temperature of the sintered alloy. It also has the effect of contributing to the promotion of densification.

It is preferable that the iron matrix of the sintered alloy has heat resistance and corrosion resistance, and also has a coefficient of thermal expansion equivalent to that of the surrounding austenitic heat-resistant material. Therefore, it is preferable that the iron base contains Ni in order to improve the heat resistance and corrosion resistance of the iron base by solid solution to the iron base and to make the iron base austenite. In the sintered alloy according to one embodiment, it is preferable that the iron alloy powder A and the iron alloy powder B each contain Ni in order to obtain a more uniform austenite matrix, and nickel powder may be separately added to the raw material powder. preferable.

The iron alloy powder A and the iron alloy powder B preferably contain Ni: 5 to 15% by mass, respectively. This Ni is preferably contained in a solid solution in the iron alloy powder.
When the iron alloy powder contains Ni, the iron base of the iron alloy powder becomes an austenite base, and it is possible to obtain the effect of reducing the hardness of the iron alloy powder and improving the compressibility of the raw material powder. The amount of Ni in the iron alloy powder is preferably 5% by mass or more, more preferably 8% by mass or more. Thereby, austenitization of the iron alloy powder can be sufficiently obtained. On the other hand, even if the amount of Ni in the iron alloy powder exceeds 15% by mass, it is difficult to expect a further improvement in compressibility. Further, Ni is more expensive than Fe and Cr, and the price of bullion has been rising in recent years. From this, the amount of Ni in the iron alloy powder may be 15% by mass or less, and may be 10% by mass or less.

The raw material powder preferably contains 1 to 12% by mass of nickel powder with respect to the total amount of the raw material powder.
In addition to the Ni contained in the iron alloy powder A and the iron alloy powder B described above, it is preferable that nickel powder is separately added to the raw material powder. Thereby, densification of the sintered alloy can be promoted. This effect of promoting densification can be sufficiently obtained when the amount of nickel powder added is 1% by mass or more. On the other hand, if the amount of nickel powder added is excessive, the amount of Ni added in the form of powder becomes excessive, the nickel powder is not completely diffused into the iron base, and Ni tends to remain as a simple substance. Since carbides do not precipitate in the Ni phase formed by residual nickel powder in this way, they tend to adhere to the mating material, and wear progresses from there, resulting in a decrease in wear resistance of the sintered alloy. is there. From this, the nickel powder added to the raw material powder is preferably 12% by mass or less, more preferably 10% by mass or less.

Further, as the nickel powder to be added, the smaller the particle size of the nickel powder, the more difficult it is for the Ni phase to remain after sintering, which is preferable. Further, as the particle size of the nickel powder becomes smaller, the specific surface area of the powder increases, diffusion at the time of sintering is promoted, and the action of densification of the sintered alloy becomes larger. Therefore, the nickel powder is preferably a powder that passes through a 200 mesh sieve, that is, a nickel powder having a maximum particle size of 74 μm or less, and more preferably a powder that passes through a 325 mesh sieve, that is, a nickel powder having a maximum particle size of 43 μm or less. preferable.

The iron alloy powder A preferably contains Si: 1.0 to 3.0%.
Si is added as a deoxidizer in the production of iron alloy powder containing an easily oxidizing element such as Cr. Further, Si has an effect of enhancing the oxidation resistance and heat resistance of the iron matrix when it is given as a solid solution in the iron matrix. However, when Si dissolves in the iron matrix, it has an unfavorable effect of hardening the iron matrix. Here, the iron alloy powder A originally has a high powder hardness because carbides are precipitated in advance, while the iron alloy powder B is soft and can be mixed by mixing the two. It ensures the compressibility of the raw material powder. Therefore, by giving Si having the above effect to the iron alloy powder A which originally contains a large amount of Cr which is an easily oxidizing element and has a high powder hardness, the action of the above Si is imparted to the sintered alloy. be able to.

When the amount of Si in the iron alloy powder A is 1.0% by mass or more, the above-mentioned action can be more sufficiently obtained. Further, when the amount of Si in the iron alloy powder A is 3.0% by mass or less, it is possible to suppress an increase in the powder hardness of the iron alloy powder A and further improve the compressibility of the raw material powder. ..

The iron alloy powder B preferably does not contain Si. Since the iron alloy powder B contains Cr, which is an easily oxidizing element, Si may be used as a deoxidizer in the production of the iron alloy powder. Therefore, in the iron alloy powder B, mixing of less than 1.0% of Si can be tolerated as an unavoidable impurity.

The raw material powder preferably contains an iron-phosphorus alloy powder. The iron-phosphorus powder is preferably 1.0 to 5.0% by mass with respect to the total amount of the raw material powder.
Further, the iron-phosphorus alloy powder preferably has a composition of P: 10 to 30% in mass%, and the balance is Fe and unavoidable impurities.
The raw material powder preferably contains P in the state of an iron-phosphorus alloy powder in order to generate a liquid phase in sintering and promote densification of the sintered alloy. P can generate a Fe—PC liquid phase in sintering together with C to promote densification of the sintered body. This makes it possible to obtain a sintered alloy having a high density ratio, and in particular, it is possible to obtain a sintered alloy having a density ratio of 90% or more.
When the amount of P in the iron-phosphorus alloy powder is 10% by mass or more, more preferably 15% by mass or more, a liquid phase is more reliably generated in sintering, and densification of the sintered body is further promoted. be able to. When the amount of P in the iron-phosphorus alloy powder is 30% by mass or less, more preferably 25% by mass or less, the increase in powder hardness of the iron-phosphorus alloy powder is prevented, and the compressibility of the raw material powder is further improved. Can be improved.

When the amount of iron-phosphorus alloy powder added to the total amount of raw material powder is 1.0% by mass or more, it is possible to more reliably generate a liquid phase in sintering and further promote densification of the sintered alloy. it can. This makes it possible to obtain a sintered alloy having a high density ratio, particularly a density ratio of 90% or more. The amount of the iron-phosphorus alloy powder added to the total amount of the raw material powder is 5.0% by mass or less, more preferably 3% by mass or less. Can be prevented.
The Fe-PC liquid phase generated from the iron-phosphorus alloy powder in sintering is diffused and absorbed in the iron matrix formed by the iron alloy powder A and the iron alloy powder B. As a result, the obtained sintered alloy preferably contains P, and more preferably contains P in an amount of 0.1 to 1.5% by mass.

The iron alloy powder A and the iron alloy powder B may further contain at least one selected from the group consisting of Mo, V, W, Nb and Ti, respectively. These elements are preferably contained as a solid solution in each of the iron alloy powder A and the iron alloy powder B.
The total amount of Mo, V, W, Nb and Ti is preferably 5% by mass or less with respect to the total composition of the raw material powder. Further, the individual elements of Mo, V, W, Nb and Ti are preferably 5% by mass or less based on the total composition of the raw material powder.
Mo, V, W, Nb and Ti are preferably contained in at least one or both of the raw material powder A and the raw material powder B, respectively. In this case, the total amount of Mo, V, W, Nb and Ti is preferably 5% by mass or less based on the total amount of the iron alloy powder A. Further, the individual elements of Mo, V, W, Nb and Ti are preferably 5% by mass or less based on the total amount of the iron alloy powder A. The total amount of Mo, V, W, Nb and Ti is preferably 5% by mass or less based on the total amount of the iron alloy powder B. Further, the individual elements of Mo, V, W, Nb and Ti are preferably 5% by mass or less based on the total amount of the iron alloy powder B.

The iron alloy powder A and the iron alloy powder B are added to the above-mentioned raw material powder so that the ratio of the iron alloy powder A to the total amount of the iron alloy powder A and the iron alloy powder B is 20 to 80% by mass. Is preferable. This ratio is more preferably 30 to 70% by mass, further preferably 40 to 60% by mass. The raw material powder includes iron-phosphorus alloy powder in an amount of 1.0 to 5.0% by mass, nickel powder in an amount of 1 to 12% by mass, and graphite powder in an amount of 0.5 to 2.5 based on the total amount of the raw material powder. It is preferable to add% by mass. As the raw material powder, it is preferable that the balance other than the iron-phosphorus alloy powder, the nickel powder, and the graphite powder is the iron alloy powder A and the iron alloy powder B.

By molding the above-mentioned raw material powder and sintering it at a temperature exceeding 1200 ° C. and lower than 1250 ° C., a sintered alloy having the above-mentioned overall composition and having an average crystal grain size of 30 to 100 μm can be obtained.
The method for molding the above-mentioned raw material powder is not particularly limited, and a molded product can be produced according to a usual method. For example, a mold having a mold hole for forming the outer peripheral shape of the product, a lower punch that is slidably fitted with the mold hole of the mold to form the lower end surface of the product, and in some cases, the inner peripheral shape of the product. Alternatively, the raw material powder is filled in the cavity formed from the core rod for forming the lightening portion, and the raw material powder is compression-molded by the upper punch and the lower punch for forming the upper end surface of the product, and then the mold hole of the mold is formed. It can be molded into a molded product by a method of extracting from (so-called stamping method).

The method of sintering the obtained molded product is not particularly limited, and a sintered body can be produced by sintering using a sintering furnace.
The sintering temperature is preferably over 1200 ° C., more preferably 1210 ° C. or higher. In this temperature range, the growth of crystal grains of the sintered alloy is promoted, the average crystal grain size of the sintered alloy is set to 30 μm or more, and the high temperature strength can be further increased.
The sintering temperature is preferably less than 1250 ° C, more preferably 1240 ° C or lower. In this temperature range, excessive growth of crystal grains can be suppressed, the average crystal grain size of the sintered alloy can be set to 100 μm or less, and the high temperature strength can be further increased.

The sintering time is not particularly limited, but the sintering time is the holding time at the maximum temperature, and may be, for example, 10 min to 120 min. The cooling rate after sintering is not particularly limited, but is preferably 5 ° C./min to 10 ° C./min.

Sintering may be performed in a normal pressure atmosphere, a vacuum atmosphere, a reduced pressure atmosphere, or the like. In the production method according to one embodiment, since iron-phosphorus alloy powder can be added to generate a liquid phase in sintering to promote sintering, it is not necessary to use an expensive vacuum atmosphere or reduced pressure atmosphere.
Sintering may be performed by any of air atmosphere, active gas, non-reducing gas, non-oxidizing gas, non-active gas and the like, but non-oxidizing gas is preferably used in order to suppress the formation of an oxide film. Can be done. Examples of the non-oxidizing gas include a mixed gas of nitrogen and hydrogen containing 10% or more of nitrogen, a nitrogen gas, and the like.

(Preparation of sintered sample)
Table 1 shows the mixing ratio of the raw material powder and the overall composition of the sintered sample. Table 2 shows the sintering temperature and the evaluation result of the sintered body sample.
By mass%, Cr: 34%, Ni: 10%, Si: 2%, C: 2%, iron alloy powder A with the balance consisting of Fe and unavoidable impurities, by mass%, Cr: 18%, Ni: 8% , Iron alloy powder B with the balance composed of Fe and unavoidable impurities, P: 20% by mass%, iron-phosphorus alloy powder with the balance composed of Fe and unavoidable impurities, nickel powder and graphite powder were prepared, and these powders were prepared. Raw material powder was obtained by adding and mixing at the ratios shown in Table 1. This raw material powder is formed into a columnar shape having an outer diameter of 10 mm and a height of 10 mm; JIS G0567 (1988) II-6 type rod-shaped (parallel part diameter 6 mm, gauge distance 30 mm, parallel part length 33 to 45 mm, shoulder radius ≧) 3 mm); and formed into a thin plate shape with an outer diameter of 24 mm and a height of 8 mm, and sintered in a non-oxidizing atmosphere at the sintering temperature (holding time of the maximum temperature) shown in Table 2 for 30 minutes or more. A sample was prepared. The overall composition of the obtained sintered body sample is also shown in Table 1.

(Evaluation)
Using the obtained columnar sintered body sample, the average crystal grain size of the sintered body was evaluated by the following procedure.
An optical microscope (“ECLIPSE” manufactured by Nikon Corporation) was adjusted so that the crystal grain boundaries of the metal structure of the sintered body sample could be seen, and the sample was observed at a magnification of 500 times. A photograph of the metal structure of a 148 μm × 196 μm sintered body sample was taken, 5 crystal grains observed in the field of view were randomly sampled, and the longest size a and the shortest size b were softened (Mitani Shoji Co., Ltd. “ It was measured by WinROOF ")). Assuming that the average value of the longest size a and the shortest size b is the particle size of one crystal grain, the average value of a total of 10 measured values measured for 5 crystal grains was taken as the average crystal grain size. When 5 crystal grains were not observed in one visual field, the visual field was increased and the measurement was performed.

Using the obtained rod-shaped sintered body sample, the high temperature strength of the sintered body was evaluated at 900 ° C. using a tensile strength tester equipped with a heating furnace.
Device name: Precision universal testing machine "AG-100KNE / XR" manufactured by Shimadzu Corporation
High temperature holding time: 10 min or more Test speed: 0.09 mm / min until proof stress acquisition, then 2.25 mm / min

The obtained thin plate-shaped sintered body sample was used as a disc material, and a roll equivalent to JIS standard SUS316L was subjected to chromate treatment and had an outer diameter of 15 mm and a length of 22 mm as a mating material at a test temperature of 700 ° C. A roll-on disk friction and wear test in which reciprocating sliding was performed for 15 minutes was performed, and the wear depth of the disk material after the test was measured. The average wear depth is the average depth of the sliding surface shape curve drawn by the surface roughness measuring instrument.

As shown in each table, it was confirmed that the average crystal grain size of the sintered body was 30 to 100 μm when the overall composition was within the preferable range and the sintering temperature was more than 1200 ° C and less than 1250 ° C. In addition, the result was that the high temperature strength was excellent.
In Example 1 where the sintering temperature was 1250 ° C., the growth of crystal grains proceeded excessively, the sintered body melted during sintering, and a sample could not be produced.
In Examples 4 and 5 where the sintering temperatures were 1200 ° C. and 1140 ° C., the growth of crystal grains did not proceed sufficiently, the average crystal grain size was 26 μm and 10 μm, respectively, and the high temperature strength was lowered.

(Observation of crystal grains)
For Example 3 and Example 5, crystal grains were observed using an optical microscope. The result is shown in FIG. FIG. 1 (a) is Example 5, and FIG. 1 (b) is Example 3. From this result, it was confirmed that the average crystal grain size was less than 30 μm in Example 5 sintered at 1200 ° C. or lower, but the average crystal grain size was 30 to 100 μm in Example 3 sintered above 1200 ° C. ..

(Relationship between stress and strain)
For Examples 5 and 3, the relationship between creep time and strain was measured under the conditions of 950 ° C., a load of 10 MPa, a maximum test time of 24 hours, a test piece shape II-6 type, and a parallel portion φ6 to 30 mm. The results are shown in FIG. From this result, it can be seen that at 950 ° C., which is a higher temperature environment, the amount of strain of Example 3 in which the crystal grains are 30 to 100 μm is small. Specifically, when the creep time was 15 hours, the strain of Example 5 was 26.4%, but the strain of Example 3 was 2.7%.
The measurement conditions were as follows.
Test temperature: 950 ° C., load: 10 MPa, test time: maximum 24 hours, test piece shape: II-6 type, parallel part φ6 to 30 mm.

Although the present invention has been described in detail based on the above specific examples, the present invention is not limited to the above specific examples, and any modification or modification is possible as long as it does not deviate from the scope of the present invention.

The sintered alloy according to one embodiment has excellent heat resistance, corrosion resistance and abrasion resistance at high temperatures, has excellent machinability, and has an austenitic iron matrix structure, and is therefore equivalent to an austenitic heat resistant material. Since it has a coefficient of thermal expansion and also has excellent high-temperature strength, it can be suitably used for, for example, turbo parts for turbochargers, particularly nozzle bodies that require corrosion resistance and abrasion resistance as well as heat resistance.

Claims (4)

The overall composition is by mass%, Cr: 11.75 to 39.98%, Ni: 5.58 to 24.98%, Si: 0.16 to 2.54, P: 0.1 to 1.5%. , C: A sintered alloy containing 0.58 to 5.55%, the balance consisting of Fe and unavoidable impurities, and an average crystal grain size of 30 μm to 100 μm. The sintered alloy according to claim 1, further comprising at least one selected from the group consisting of Mo, V, W, Nb, and Ti in mass% and 5% or less in total. By mass%, it contains Cr: 25 to 45%, Ni: 5 to 15%, Si: 1.0 to 3.0%, C: 0.5 to 4.0%, and the balance consists of Fe and unavoidable impurities. Iron alloy powder A of composition,
Iron alloy powder B, which contains Cr: 12 to 25% and Ni: 5 to 15% in mass%, and the balance is composed of Fe and unavoidable impurities.
Prepare iron-phosphorus alloy powder, nickel powder, and graphite powder having a composition of P: 10 to 30% by mass and the balance consisting of Fe and unavoidable impurities;
The iron alloy powder A and the iron alloy powder B are added so that the ratio of the iron alloy powder A to the total amount of the iron alloy powder A and the iron alloy powder B is 20 to 80% by mass, and the raw material powder is added. The iron-phosphorus alloy powder is added in an amount of 1.0 to 5.0% by mass, the nickel powder is added in an amount of 1 to 12% by mass, and the graphite powder is added in an amount of 0.5 to 2.5% by mass based on the total amount of the raw material powder. To get;
A method for producing a sintered alloy, which comprises molding the raw material powder to obtain a molded product; and sintering the molded product at a temperature of more than 1200 ° C. and lower than 1250 ° C. At least one of the iron alloy powder A and the iron alloy powder B contains at least one selected from the group consisting of Mo, V, W, Nb, and Ti in a total amount with respect to the overall composition of the raw material powder. The method for producing a sintered alloy according to claim 3, further comprising 5% by mass or less.

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