CN112368409B

CN112368409B - Sintered alloy and method for producing same

Info

Publication number: CN112368409B
Application number: CN201980045440.XA
Authority: CN
Inventors: 深江大辅; 河田英昭
Original assignee: Showa Denko KK
Current assignee: Lishennoco Co ltd
Priority date: 2018-07-11
Filing date: 2019-07-10
Publication date: 2022-07-26
Anticipated expiration: 2039-07-10
Also published as: CN112368409A; WO2020013227A1; EP3822379B1; EP3822379A1; JP7248027B2; JPWO2020013227A1; EP3822379A4

Abstract

The present invention provides a sintered alloy having an overall composition including, in mass%, Cr: 13.86-27.72%, Ni: 6.47-20.33%, Cu: 0.85-11.05%, Si: 0.46-2.77%, P: 0.15-1.95%, C: 0.2 to 1.0%, and the balance of Fe and unavoidable elements, and has a density of 6.8 to 7.4Mg/m ³ The sintered alloy has a metal structure including an iron alloy matrix having pores dispersed therein and carbides dispersed in the iron alloy matrix, and the average crystal grain size of the iron alloy matrix is 10 to 50 μm.

Description

Sintered alloy and method for producing same

Technical Field

The present invention relates to a sintered alloy suitable for a turbine member for a turbocharger and the like and a method for producing the same, and particularly relates to a sintered alloy suitable for a nozzle body and the like which require heat resistance, corrosion resistance, and abrasion resistance and a method for producing the same.

Background

In general, in a turbocharger attached to an internal combustion engine, a turbine is rotatably supported in 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 an outer peripheral side of the turbine. The exhaust gas flowing into the turbine housing flows into the turbine from the outer peripheral side and is discharged in the axial direction, and at this time, the turbine is rotated. Then, the air supplied to the internal combustion engine is compressed by the rotation of the compressor provided coaxially on the opposite side of the turbine.

The nozzle vanes are rotatably supported by an annular member called a nozzle body or a mounting nozzle (マウントノズル). The shaft of the nozzle vane penetrates the nozzle body and is connected to the link mechanism. Then, the nozzle vanes are rotated by driving the link mechanism, and the opening degree of the flow path through which the exhaust gas flows into the turbine is adjusted.

The turbine member for a turbocharger as described above, that is, the turbine member provided in the turbine housing, such as the nozzle body (mounting nozzle) and the plate nozzle mounted on the nozzle body (mounting nozzle), is in contact with the exhaust gas, which is a high-temperature corrosive gas. Therefore, these members are required to have heat resistance and corrosion resistance, and also have abrasion resistance for coping with sliding contact with the nozzle vanes. Therefore, conventionally, as a material constituting a turbine component, for example, high-chromium cast steel, heat-resistant steel specified as a type SCH22 in JIS standards, or an abrasion resistant material obtained by subjecting a heat-resistant alloy to a chromium surface treatment for the purpose of improving corrosion resistance, and the like have been used.

On the other hand, sintered alloys suitable for various machine parts have been developed in the powder metallurgy method, and a heat-resistant and wear-resistant sintered alloy for the above-mentioned turbine part has been proposed (see patent document 1). In the powder metallurgy method, a sintered alloy exhibiting a special metal structure which cannot be formed by a steel melt obtained by casting or the like can be obtained.

Documents of the prior art

Patent document

Patent document 1: japanese patent laid-open publication No. 2013-199695

Disclosure of Invention

Problems to be solved by the invention

However, in a turbocharger component which is in sliding contact with another member, such as a nozzle body in sliding contact with a nozzle vane, further improvement in wear resistance is desired in order to prevent adhesive wear with another member. Further, since oxidation is likely to proceed due to water vapor contained in the high-temperature exhaust gas, further improvement in corrosion resistance is also desired. Further, in order to respond to the requirement of a complicated shape, improvement of machinability (machinability) is desired.

As described above, an object of the present invention is to provide a sintered alloy which is suitable for application to a turbocharger component and has excellent wear resistance, corrosion resistance, and machinability, by further improving an iron-based sintered alloy, and a method for producing the same.

Means for solving the problems

In order to solve the above problem, according to one aspect of the present invention, the main point is that the entire composition of the sintered alloy contains, in mass%, Cr: 13.86-27.72%, Ni: 6.47-20.33%, Cu: 0.85-11.05%, Si: 0.46-2.77%, P: 0.15-1.95%, C: 0.20 to 1.00%, and the balance of Fe and unavoidable elements, the density of the sintered alloy being 6.8 to 7.4Mg/m ³ The sintered alloy has a metal structure comprising an iron alloy matrix in which pores are dispersed and a carbide dispersed in the iron alloy matrix, wherein the iron alloy matrix is composed of crystal grains having an average crystal grain diameter of 10 to 50 [ mu ] m.

According to another aspect of the present invention, the main point is that the entire composition of the sintered alloy contains, in mass%, Cr: 13.86-27.72%, Ni: 6.47-20.33%, Cu: 0.85-11.05%, Si: 0.46-2.77%, P: 0.15-1.95%, C: 0.20-1.00%, carbide-forming elements: less than or equal to 3.23%, and the balance of Fe and unavoidable elements, the density of the sintered alloy being 6.8 to 7.4Mg/m ³ The carbide-forming element is at least 1 element selected from the group consisting of Mo, V, W, Nb, and Ti, and the sintered alloy has a metal structure including a porous iron alloy matrix and a carbide dispersed in the iron alloy matrix, and the iron alloy matrix is composed of crystal grains having an average crystal grain diameter of 10 to 50 [ mu ] m.

In addition, according to an aspect of the present invention, in a method for producing a sintered alloy, a sintered alloy containing, in mass%, Cr: 15-30%, Ni: 7-24%, Si: 0.5 to 3.0%, and the balance of Fe and FeIron alloy powder of impurities; preparing a blending material for blending phosphorus and copper, wherein the blending material comprises 1 or more selected from iron-phosphorus alloy powder with the phosphorus content of 10-30 mass%, copper-phosphorus alloy powder with the phosphorus content of 5-20 mass% and copper powder; mixing the iron alloy powder, the compounding material, and the graphite powder to prepare a raw material powder containing 0.15 to 1.95 mass% of phosphorus, 0.85 to 11.05 mass% of copper, and 0.20 to 1.00 mass% of carbon; compressing the raw material powder to form a powder with a density of 6.0-6.8 Mg/m ³ The pressed powder of (1); and heating the green compact to 1050-1160 ℃ in a non-oxidizing atmosphere to sinter the green compact.

According to another aspect of the present invention, in a method for producing a sintered alloy, an iron alloy powder is prepared, the iron alloy powder containing, in mass%, Cr: 15-30%, Ni: 7-24%, Si: 0.5-3.0%, carbide-forming elements: 3 mass% or less, and the balance of Fe and inevitable impurities, the carbide-forming element being at least 1 element selected from the group consisting of Mo, V, W, Nb and Ti; preparing a blending material for phosphorus and copper, wherein the blending material comprises 1 or more selected from iron-phosphorus alloy powder with the phosphorus content of 10-30 mass%, copper-phosphorus alloy powder with the phosphorus content of 5-20 mass% and copper powder; mixing the iron alloy powder, the compounding material, and the graphite powder to prepare a raw material powder containing 0.15 to 1.95 mass% of phosphorus, 0.85 to 11.05 mass% of copper, and 0.20 to 1.00 mass% of carbon; compressing the raw material powder to form a powder with a density of 6.0-6.8 Mg/m ³ The pressed powder of (1); and heating the green compact to 1050-1160 ℃ in a non-oxidizing atmosphere to sinter the green compact.

The blending material is preferably a powder material containing phosphorus in the form of one or both of an iron-phosphorus alloy powder and a copper-phosphorus alloy powder, and containing copper in the form of one or both of a copper powder and a copper-phosphorus alloy powder. Any one of the following (1) to (5) can be used as the compounding material. (1) A combination of an iron-phosphorus alloy powder having a phosphorus content of 10 to 30 mass% and a copper powder, (2) a combination of an iron-phosphorus alloy powder having a phosphorus content of 10 to 30 mass% and a copper-phosphorus alloy powder having a phosphorus content of 5 to 20 mass%, (3) a combination of an iron-phosphorus alloy powder having a phosphorus content of 10 to 30 mass% and a copper-phosphorus alloy powder having a phosphorus content of 5 to 20 mass% and a copper powder, (4) a combination of a copper-phosphorus alloy powder having a phosphorus content of 5 to 20 mass% and a copper powder, and (5) a copper-phosphorus alloy powder having a phosphorus content of 5 to 20 mass%. The non-oxidizing atmosphere may be a mixed gas of nitrogen and hydrogen containing not less than 10 mass% of nitrogen, or an atmosphere at normal pressure containing nitrogen, and the sintered alloy in which nitrides are formed on the surface of the sintered alloy and the inner surface of the pores can be obtained.

ADVANTAGEOUS EFFECTS OF INVENTION

By designing the composition of the iron-based alloy, the machinability of the sintered alloy can be improved, and the influence on the characteristics of other materials can be suppressed, whereby the sintered alloy which is excellent in wear resistance, corrosion resistance and machinability and is suitable for application to a turbocharger component and a method for producing the same can be provided.

Detailed Description

Carbide particles dispersed in the metal structure of the sintered alloy tend to hinder cutting during machining, and if the amount of carbide particles is increased, the machinability of the sintered alloy is lowered. Therefore, it is considered that it is effective to suppress the generation of carbide particles for improving the machinability of the sintered alloy. According to this guideline, studies have been made on a sintered alloy having a composition in which the proportion of carbide is low, and as a result, it has been found that a reduction in the amount of carbide is effective for improving the machinability, and on the other hand, the pinning effect by the carbide is reduced and the crystal grains of the iron alloy matrix are coarsened, thereby reducing the oxidation resistance.

Therefore, as a result of examining a method for compensating for the above-described drawbacks, it has been found that if copper is dissolved in an iron alloy matrix, the oxidation resistance of the matrix in a steam atmosphere is improved, and the corrosion resistance can be improved. Further, it is also shown that solid solution of copper in the matrix solidifies the soft austenite to inhibit adhesion between the matrix and other members, thereby improving machinability. Further, it was also found that the oxidation resistance can be improved by reducing the sintering temperature to suppress the growth of crystal grains. In this regard, since the copper-phosphorus alloy powder can generate a liquid phase at a low temperature, if it is used as a raw material for introducing copper, it is an effective means for setting the sintering temperature low.

In view of the above, the present invention provides a sintered alloy having improved oxidation resistance, corrosion resistance, and machinability by devising the alloy composition so that the components constituting the sintered alloy function in a well-balanced manner. The composition of the sintered alloy and the raw material powder of the present invention will be described below.

< composition of sintered alloy and raw powder >

In the present invention, the matrix of the sintered alloy is an iron alloy matrix having a composition of austenitic stainless steel, and shows a metal structure in which pores are dispersed in the matrix and carbide particles are precipitated and dispersed. Austenitic stainless steel is an iron alloy in which chromium and nickel are solid-dissolved in gamma-iron, has high corrosion resistance and heat resistance, and has a coefficient of thermal expansion that is about the same as that of a general austenitic heat-resistant material.

The austenitic stainless steel has improved oxidation resistance in a water vapor atmosphere by suppressing the growth of crystal grains. In the present invention, the austenitic stainless steel constituting the matrix of the sintered alloy is composed of crystal grains having an average crystal grain size of 10 to 50 μm. The growth of crystal grains is suppressed to be in such a grain size range, and the crystal grains can be sintered at a temperature lower than the conventional sintering temperature, and the sintering temperature is set to be in the range of 1050 to 1160 ℃. In order to enable sintering at low temperatures, it is useful to use a composition that generates a liquid phase at low temperatures. In this regard, the copper-phosphorus alloy powder can be sintered at a relatively low temperature of 1050 to 1130 ℃. Therefore, the use of the copper-phosphorus alloy powder effectively acts on lowering the sintering temperature, and is useful in suppressing the growth of crystal grains. By performing appropriate sintering with such temperature setting, the density of the sintered alloy becomes 6.8 to 7.4Mg/m ³ Degree of swelling.

The austenite constituting the matrix of the sintered alloy can be produced using an iron alloy powder in which chromium or nickel is solid-dissolved in iron as a main raw material. That is, austenitizing elements (chromium and nickel) are alloyed into iron to prepare an iron alloy powder, and introduced into the raw material powder. Thereby, the austenitizing element is uniformly distributed in the iron alloy matrix, and corrosion resistance and heat resistance are exhibited. Silicon is another component alloyed with iron, and acts as an antioxidant for chromium in the preparation of an iron-chromium alloy powder.

If the iron alloy matrix contains chromium (Cr) in an amount of 12 mass% or more, the iron alloy matrix exhibits corrosion resistance to oxidizing acids. Considering that a part of chromium contained in the iron alloy powder used as a raw material is precipitated as carbide during sintering, the chromium content of the iron alloy powder is preferably 15 mass% or more in order to sufficiently retain chromium in the iron alloy matrix. However, if the chromium content of the iron alloy powder exceeds 30 mass%, a brittle σ phase is formed, and the compressibility of the iron alloy powder is significantly impaired. Therefore, the content of chromium in the iron alloy powder as a raw material is preferably about 15 to 30 mass%. In a suitable sintered alloy obtained by using such powder, the proportion of chromium in the entire composition is about 13.86 to 27.72 mass%. More preferably, it is not less than 16.88% by mass and not more than 23.10% by mass, and most preferably, it is not less than 18.48% by mass and not more than 20.33% by mass.

If the iron alloy matrix contains 3.5 mass% or more of nickel (Ni), it exhibits corrosion resistance to a non-oxidizing acid. Further, if the nickel content of the iron alloy powder is 7 mass% or more, such an effect is suitably exerted to impart oxidation resistance to the obtained sintered alloy. However, when the iron alloy powder containing nickel in an amount exceeding 24 mass% is used, the effects of corrosion resistance and oxidation resistance are not so much changed. Since nickel is also an expensive raw material, it is preferable that the nickel content of the iron alloy powder used as the raw material is 7 to 24 mass%, and it is preferable to use an iron alloy powder containing 10 to 22 mass% of nickel, in consideration of the production cost. The nickel content in the overall composition of a suitable sintered alloy obtained by using such a raw material is 6.47 to 20.33 mass%. In terms of oxidation resistance, it is preferably 7.39 mass% or more, and in terms of economy, it is preferably 18.48 mass% or less.

The steel exhibiting an austenitic structure has a high crystallographic atomic density, and therefore has excellent corrosion resistance as compared with the steel having a ferritic structure. Therefore, in order to allow the iron alloy matrix obtained after sintering to exhibit an austenite structure appropriately, it is preferable to use, as a raw material, an iron alloy powder in which the chromium content and the nickel content are appropriately adjusted. Specifically, in the annealed structure diagram of an Fe-Cr-Ni alloy in which the horizontal axis represents the amount of chromium (mass%) and the vertical axis represents the amount of nickel (mass%), the austenite structure is formed in a region having a large amount of nickel by a broken line connecting the A point (Cr: 15, Ni: 7.5), the B point (Cr: 18, Ni: 6.5) and the C point (Cr: 24, Ni: 18). Therefore, it is preferable to adjust the composition of the iron alloy powder used as the raw material so that the amount of chromium and the amount of nickel are in these ranges.

Since the iron alloy powder contains chromium which is easily oxidized, silicon is added as a deacidification agent to the molten metal for preparing the iron alloy powder. Therefore, the iron alloy matrix contains silicon (Si). Further, if silicon is dissolved in the iron alloy matrix, there is an effect of improving the oxidation resistance and heat resistance of the matrix, and the effect is remarkable at 0.46 mass% or more. However, since the iron alloy powder having a silicon content exceeding 3.0 mass% is hard, the compressibility is significantly impaired. If the oxide generated from silicon increases, the progress of sintering is hindered and the strength of the sintered alloy is reduced. Therefore, the silicon content of the iron alloy powder used as a raw material is preferably 0.5 to 3.0 mass%. The silicon content in the entire composition of the sintered alloy produced using the iron alloy powder is 0.46 to 2.77 mass%. From the viewpoint of oxidation resistance, the silicon content in the entire composition is preferably 0.74 mass% or more and 1.85 mass% or less.

Sintering of iron alloy powders with high chromium content is difficult. Therefore, in order to promote sintering, phosphorus (P) which produces a eutectic liquid phase of iron-phosphorus-carbon is used and blended in the form of a phosphorus alloy powder. In the present invention, as the phosphorus alloy powder, one or both of an iron-phosphorus alloy powder and a copper-phosphorus alloy powder can be used. When the iron-phosphorus alloy powder is used, if the phosphorus content is less than 10 mass%, the amount of liquid phase generated is small, and sintering is difficult to sufficiently proceed. On the other hand, if the phosphorus content exceeds 30 mass%, the iron-phosphorus alloy powder becomes hard, and thus the compressibility of the raw material powder is significantly impaired. Therefore, the phosphorus content of the iron-phosphorus alloy powder used is preferably 10 to 30 mass%. When the copper-phosphorus alloy powder is used, if the phosphorus content is less than 5 mass%, the amount of liquid phase generated is small, and sintering is difficult to sufficiently proceed. On the other hand, if the phosphorus content exceeds 25 mass%, the copper-phosphorus alloy powder becomes hard, significantly impairing the compressibility of the raw material powder. Further, the generated liquid phase may easily flow out to the outside of the sintered body before sufficiently diffusing. Therefore, the phosphorus content of the copper-phosphorus alloy powder used is preferably 5 to 25 mass%. In addition, if the proportion of phosphorus in the entire composition of the sintered alloy is less than 0.15 mass%, the amount of liquid phase produced is insufficient and the sintering promoting effect is small. On the other hand, if the phosphorus content exceeds 1.95 mass%, sintering proceeds excessively to densify the alloy, and if the content exceeds the upper limit of the density of the sintered alloy, the pores are reduced. This makes it difficult to suppress plastic flow of the base and lowers the abrasion resistance. Further, the excess phosphorus alloy powder is likely to flow out as a liquid phase, and if the liquid phase flows out, the portions where the phosphorus alloy powder exists become pores (so-called Kirkendall (Kirkendall) voids), and coarse pores are formed in the iron alloy matrix, thereby decreasing the corrosion resistance. Further, if the sintering proceeds excessively due to the increase in the formation of the eutectic liquid phase, the growth of chromium carbide is promoted, and the precipitated chromium carbide coarsens. Therefore, the phosphorus alloy powder is preferably blended with the raw material powder so that the proportion of phosphorus in the entire composition of the sintered alloy is 0.15 to 1.95 mass%. More preferably, the phosphorus content is 0.60 mass% or more and 1.50 mass% or less, and still more preferably 0.60 mass% or more and 1.05 mass% or less.

In the sintered alloy of the present invention, copper (Cu) is dissolved in a matrix, thereby improving oxidation resistance and corrosion resistance. At the same time, copper solidifies the soft austenite to inhibit adhesion of the matrix, thereby improving machinability. Copper can be incorporated into the raw material powder in the form of copper powder or copper-phosphorus alloy powder. When the total amount of phosphorus for promoting sintering is blended as an iron-phosphorus alloy powder, copper is blended in the form of copper powder. In the case where part or all of the phosphorus is not blended in the form of an iron-phosphorus alloy powder, a copper-phosphorus alloy powder is used so that the phosphorus content of the raw material powder becomes the above-mentioned appropriate composition ratio of phosphorus. Further, in the case where the amount of copper introduced by the copper-phosphorus alloy powder is insufficient, or in the case where the copper-phosphorus alloy powder is not used, the copper powder is used. The copper content in the entire composition of the sintered alloy is preferably 0.85 to 11.05 mass% because the oxidation resistance and corrosion resistance are good. More preferably, the copper content is 3.40 mass% or more and 8.50 mass% or less, and still more preferably, the copper content is 3.40 mass% or more and 5.95 mass% or less.

Therefore, as a blending material for blending phosphorus and copper, 1 kind of powder or a combination of plural kinds of powders as shown in the following 5 forms can be used. In the case of using the copper-phosphorus alloy powder (P: 5 to 25 mass%), if the copper-phosphorus alloy powder is blended in a proportion of 1.0 to 13 mass% of the raw material powder as the form (5), the copper amount can be adjusted in the range of 0.75 to 12.35 mass% and the phosphorus amount can be adjusted in the range of 0.05 to 3.25 mass% in the entire composition. The copper-phosphorus alloy powder generates a eutectic liquid phase at a lower temperature than the iron-phosphorus alloy powder, and therefore can lower the sintering temperature, and is suitable in suppressing the growth of crystal grains. The content of phosphorus in the copper-phosphorus alloy powder used is preferably 5 mass% or more and 25 mass% or less, more preferably 10 mass% or more and 20 mass% or less, from the viewpoint of good generation of a liquid phase. Further, since the machinability of the obtained sintered alloy tends to be improved in the copper-phosphorus alloy powder as compared with the case of using the iron-phosphorus alloy powder, the forms (2) to (5) of the copper-phosphorus alloy powder are preferably used. In the embodiments (1) and (5), the amount of the blending material to be used can be easily determined based on the composition ratio of phosphorus and copper.

(1) Combination of iron-phosphorus alloy powder and copper powder

(2) Combination of iron-phosphorus alloy powder and copper-phosphorus alloy powder

(3) Combination of iron-phosphorus alloy powder and copper powder

(4) Combination of copper-phosphorus alloy powder and copper powder

(5) Copper-phosphorus alloy powder

Carbon is added to the raw material powder in the form of graphite powder, and when heated, an iron-phosphorus-carbon eutectic liquid phase is generated to promote sintering. If carbon diffused from the eutectic liquid phase of iron-phosphorus-carbon into the iron alloy matrix is combined with chromium, it is precipitated as chromium carbides dispersed in the matrix. Since the machinability is deteriorated if the amount of carbide precipitation is large, in the present invention, the amount of graphite powder added is appropriately adjusted to control the precipitation of carbide. Specifically, the graphite powder is blended so that the ratio of carbon to the entire composition is 0.20 to 1.00 mass%. If the carbon content exceeds 1.00 mass% of the entire composition, a large amount of carbide is formed even if sintering progresses due to the formation of an iron-phosphorus-carbon eutectic liquid phase, and machinability is therefore degraded. Further, the amount of chromium dissolved in the matrix decreases, and the heat resistance and corrosion resistance decrease. When the proportion of carbon is less than 0.20 mass% of the entire composition, the effect of promoting sintering and the wear resistance due to carbide cannot be obtained. The carbon content is preferably 0.20 mass% or more and 1.00 mass% or less, more preferably 0.4 mass% or more and 0.80 mass% or less of the entire composition.

Further, if necessary, an element having a carbide forming ability higher than that of chromium (hereinafter referred to as a carbide forming element) is added to the raw material powder, whereby precipitation of chromium carbide can be suppressed. The carbide-forming element reacts preferentially with graphite to form carbide in comparison with chromium during sintering, and at least a part of the carbide-forming element is present in the sintered alloy in the form of carbide. This suppresses a decrease in the chromium concentration in the iron alloy matrix, and thus has the effect of improving the heat resistance and corrosion resistance of the matrix. Further, if the carbide-forming element reacts with carbon to form alloy carbide, it contributes to improvement of wear resistance. The carbide-forming element can be selected from the group consisting of molybdenum, vanadium, tungsten, niobium, and titanium, and 1 or 2 or more kinds can be used in combination. However, if the carbide-forming element is blended so as to exceed 3.23 mass% of the entire composition, the compressibility of the raw material powder is lowered, and therefore, the carbide-forming element may be blended arbitrarily within a range of 3.23 mass% or less of the entire composition of the sintered alloy. The content of the carbide-forming element is preferably 0.46 mass% or more and 2.77 mass% or less of the entire composition. When a plurality of elements are used in combination, the total amount thereof may be the above composition ratio.

Therefore, as embodiment 1 of the present invention, the sintered alloy preferably has a total composition including, in mass%, Cr: 13.86-27.72%, Ni: 6.47-20.33%, Cu: 0.85-11.05%, Si: 0.46-2.77%, P: 0.15-1.95%, C: 0.20 to 1.00%, and the balance of Fe and unavoidable elements. Further, as embodiment 2, the sintered alloy preferably has an overall composition including, in mass%, Cr: 13.86-27.72%, Ni: 6.47-20.33%, Cu: 0.85-11.05%, Si: 0.46-2.77%, P: 0.15-1.95%, C: 0.20-1.00%, carbide-forming elements: less than or equal to 3.23%, and the balance Fe and unavoidable elements. The carbide-forming element is at least 1 kind selected from the group consisting of molybdenum, vanadium, tungsten, niobium and titanium, and may be 1 kind or 2 or more kinds thereof.

In the sintered alloy prepared based on the above composition ratio, carbides are precipitated and dispersed in the iron alloy matrix in which the pores are dispersed, and the carbides can be formed of iron, chromium, and the above carbide-forming element. Can be suitably produced to have a density of 6.8 to 7.4Mg/m ³ A degree of sintering of the alloy. The carbide contributes to wear resistance by reducing contact between the iron alloy base and the mating member when the sintered alloy slides against the mating member and suppressing plastic flow of the base. In the present invention, in order to improve machinability, the alloy composition is designed in such a manner that the proportion of carbides is less than or equal to a predetermined amount. Therefore, in order to efficiently exert the function of carbideIt is preferable that the carbide is not coarsened but dispersed and precipitated as a large number of particles. Specifically, if the carbide is coarsened exceeding 10 μm, the area where the carbide is not present, that is, the area where plastic flow is not suppressed, increases due to localization of the carbide, and the wear resistance significantly decreases. Further, the machinability of the sintered alloy is also adversely affected. However, if the dispersed carbide particles have a size of less than 1 μm, the function of suppressing the flow of the matrix is substantially not provided. Thus, carbide particles having a maximum diameter (maximum value of particle diameter) in the range of 1 to 10 μm are preferable. In the structural cross section of the sintered alloy, the ratio (area ratio) of the area of the carbide particles having the maximum diameter in the range of 1 to 10 μm to the area of all the carbide particles is preferably 90% or more. In the present invention, the heating temperature during sintering can be set low by promoting sintering of a phosphorus alloy (particularly, a copper-phosphorus alloy), and the coarsening of carbide particles can be suppressed.

In addition, for the maximum value of the particle size of the carbide particles, the following length is applied: when the particle size is measured from an image of a cross section of the particle by image analysis of a cross section of the metal structure using image analysis software (WinROOF, manufactured by mitsubishi corporation), the image analysis software determines the length of the long side of the largest particle portion in the image. In addition, as for the average crystal grain size of the crystal grains of the iron alloy matrix, the following values are applicable: the area of the austenite matrix and the number of crystal grains in the cross section are measured by image analysis of the cross section of the metal structure, the average area (number average) of the crystal grains is calculated from these values, and the average area is converted into the area equivalent circle diameter by approximation.

< method for producing sintered alloy >

A mixed powder is prepared by blending raw materials so that the ratio of each component becomes the composition ratio of the sintered alloy, and the mixed powder is used as a raw material powder for molding. The green compact is obtained by compression molding, and a sintered body obtained by heating the green compact to a sintering temperature is the above-mentioned sintered alloy.

In embodiment 1, the iron alloy powder (1 st material) containing nickel, chromium and silicon as described above is mixed with a blending material (2 nd material) for blending phosphorus and copper in any of the above-described forms (1) to (5) and graphite powder uniformly to prepare a mixed powder. In this case, the mixing ratio is adjusted so that the mixed powder contains 0.15 to 1.95 mass% of phosphorus, 0.85 to 11.05 mass% of copper, and 0.20 to 1.00 mass% of carbon. The obtained mixed powder can be used as a raw material powder for molding. When a combination of a plurality of powders (forms of (1) to (4)) is used as the 2 nd raw material, the plurality of powders may be separately charged into the mixed powder for preparation, or may be uniformly mixed in advance and then charged.

In embodiment 2, a raw material powder containing a carbide-forming element further added so as to be 3.23 mass% or less of the entire composition is used. The carbide-forming element is selected from molybdenum, vanadium, tungsten, niobium and titanium, and 1 or 2 or more kinds thereof may be used in combination. The carbide-forming element can be used in an alloyed state with the iron alloy as the 1 st raw material. That is, in embodiment 2, it is preferable to use an iron alloy powder containing nickel, chromium, silicon, and a carbide-forming element as the 1 st raw material. If the proportion of the carbide-forming element exceeds 3.23 mass% of the entire composition, the compressibility of the iron alloy powder is reduced, and it becomes difficult to mold the raw material powder to a desired compact density. Therefore, the proportion of the carbide-forming element is preferably 3.23 mass% or less of the entire composition. The effect of the addition of the carbide-forming element is about 0.92 mass%, and therefore, it is preferably used in the range of 0.92 to 3.23 mass%. However, they are optional components and can be used in the range of 0 to 3.23 mass% in accordance with the design of the alloy composition, particularly, the composition ratio of chromium and carbon.

< Density of powder compact and Density of sintered alloy >

Pores between powder particles in the green compact obtained by compression molding also remain in the sintered alloy after sintering, and if the pore amount is large, the strength and the wear resistance are lowered. However, the sintered alloy used as a member for a turbocharger is made ofThe inert coating of chromium is formed on the surface and pore inner surface of the sintered alloy by oxygen contained in the high-temperature exhaust gas, thereby improving the wear resistance and corrosion resistance. The chromium passive film is hard and firmly adhered to the surface of the sintered alloy, and has an effect of preventing the iron alloy base from adhering to the mating member. Therefore, if an appropriate amount of pores are dispersed in the sintered alloy, the pores on the inner surface covered with the chromium passive film have the effect of preventing plastic flow of the iron alloy matrix, and the wear resistance of the sintered alloy is improved. When this is taken into consideration, the density of the sintered alloy is 6.80 to 7.40Mg/m ³ The degree is appropriate if it exceeds 7.40Mg/m ³ As the amount of pores decreases, the effect of the passive film is not obtained, and the wear resistance decreases. If it is less than 6.80Mg/m ³ The strength as a sintered alloy is lowered, and the wear resistance is lowered. Preferably greater than or equal to 7.00Mg/m ³ And less than or equal to 7.40Mg/m ³ Preferably, it is more preferably 7.20Mg/m or more ³ And less than or equal to 7.40Mg/m ³ The sintered density of (2).

< Molding and sintering >

In order to obtain a sintered alloy having such a density, the density of the green compact is set to 6.00 to 6.80Mg/m ³ The raw material powder is preferably compression-molded to a certain degree. Since sintering proceeds from liquid phase generation at low temperature by using the phosphorus alloy powder, sintering proceeds by heating the pressed powder having such a density to 1050 to 1160 ℃, and a density of 6.80 to 7.40Mg/m can be obtained ³ A degree of sintering of the alloy. By setting the heating temperature during sintering to the above temperature range, the growth of crystal grains in the iron alloy matrix is suppressed, and the average crystal grain size of the iron alloy matrix is about 10 to 50 μm. If the heating temperature is less than 1050 ℃, sintering is difficult, and if it exceeds 1160 ℃, coarse grains having a grain size of more than 50 μm tend to grow in the sintered iron alloy matrix. More preferably, the sintering temperature is 1100 ℃ or higher and 1140 ℃ or lower.

In general, in the production of sintered gold having a high chromium content, in order to actively perform sintering, alloy powder containing chromium from which a passive film on the surface has been removed is used as a raw material, and sintering is performed in a vacuum atmosphere or a reduced-pressure atmosphere. In this regard, in the present invention, since sintering proceeds well at a relatively low temperature due to generation of a liquid phase by the phosphorus alloy powder, the activity at the time of sintering can be maintained if the sintering is performed in a non-oxidizing atmosphere, and sintering can be performed even in an atmospheric pressure environment. Therefore, it is not necessary to adjust the pressure environment to vacuum or reduced pressure, and the turbocharger component can be produced inexpensively in a non-oxidizing environment similar to that in the production of a general sintered component.

Further, if a gas containing nitrogen in an amount of about 10 vol% or more is used as the sintering atmosphere, a hard nitride (mainly, chromium nitride) is formed on the surface of the sintered alloy and the inner surface of the pores, and the wear resistance of the sintered alloy can be improved, which is preferable. Examples of the atmosphere gas containing nitrogen include nitrogen gas, a mixed gas of nitrogen gas and hydrogen gas, an ammonia decomposition gas, a mixed gas in which nitrogen is mixed with an ammonia decomposition gas, a mixed gas in which hydrogen is mixed with an ammonia decomposition gas, and the like. In this case, the amount of nitrogen introduced into the sintered alloy from the atmosphere is very small, and is about the same as the amount of unavoidable impurities contained in the sintered alloy.

Based on the above, the density of the magnesium alloy can be 6.8-7.4 Mg/m ³ The sintered alloy of (4) has a structure in which precipitated particles of pores and carbides are dispersed in an iron alloy matrix having a composition of austenitic stainless steel. The solid solution of copper solidifies the austenite structure, thereby improving the wear resistance and corrosion resistance of the iron alloy matrix. Since the structure of the iron alloy matrix is low in sintering temperature, the iron alloy matrix becomes fine crystal grains having an average crystal grain size of about 10 to 50 μm, and the corrosion resistance and oxidation resistance of the matrix are improved. The size of the carbide particles is about 1 to 10 μm, and the area occupied by the carbide in the structure section is less than 10% of the area occupied by the carbide having a particle size of more than 10 μm. The carbide particles are produced at a composition ratio determined so as to suppress the amount of precipitation of the carbide, and therefore the carbide particles account for 10 area% or less of the cross section of the metal structure, and the sintered alloy is cutThe sexual performance is improved. Further, even if the precipitation amount of carbide itself is reduced, carbide particles are finely dispersed, whereby the adhesive wear of the matrix can be prevented, and it is also effective for suppressing the grain growth of the matrix due to the pinning effect of carbide.

Example 1

(sample No. 1 to 39)

As the iron alloy powder, an alloy powder (average particle diameter: 70 μm) containing chromium, nickel and silicon in the composition ratio shown in Table 1 was prepared. Further, as the copper-phosphorus alloy powder, copper-phosphorus alloy powders having phosphorus contents shown in Table 1 (average particle diameter: 40 μm, including copper and unavoidable impurities as the balance) were prepared. In addition, the average particle diameter of the powder is the median diameter determined based on the particle size distribution. These alloy powders and graphite powder (average particle diameter: 10 μm) were uniformly mixed in the mixing ratio shown in Table 1 to obtain a mixed powder having the overall composition shown in Table 2. This was used as a raw material powder for molding in the following operations.

The raw material powder was charged into a die hole, and compression-molded at a pressure of 600MPa using a punch press to form 2 kinds of powder compacts each having a cylindrical shape and a disk shape. The dimensions of the cylindrical green compact were outside diameter: 10mm, height: 10mm, the dimensions of the disk-shaped compact were the outer diameter: 24mm, height: 8 mm. In sample Nos. 4 and 9 to 39, the density of the green compact was calculated in advance to be 6.4Mg/m ³ The amount of the raw material powder of (1) to (3) and (5) to (8) was adjusted by weighing the amount of the raw material powder whose density of the green compact was the value shown in table 2. The density of the obtained green compact was confirmed by the archimedes method.

The two kinds of green compacts obtained were heated to 1130 ℃ in a mixed gas atmosphere of hydrogen and nitrogen, maintained at the temperature and sintered for 60 minutes, and then cooled to room temperature. At this time, the average cooling rate from the sintering temperature to 300 ℃ was 12 ℃/min. The sintered bodies of sample numbers 1 to 39 were produced in this manner.

The obtained sintered body was used as a sintered alloy sample, and the density, the amount of wear, the crystal grain size of the iron alloy matrix, and the thickness of the oxide film were measured by the following operations. The measurement results are shown in table 3.

< Density and abrasion loss of sintered alloy >

The density of the sintered alloy was measured by the sintered density test method of the metal sintered material specified in Japanese Industrial Standard (JIS) Z2505 using a cylindrical sintered alloy sample.

The wear amount of the sintered alloy was measured as the wear amount in a roll-disk (roll on disk) friction wear test using a disk-shaped sintered alloy sample. In the roll-disk friction wear test, a sintered alloy sample was used as a disk material, and the reciprocating sliding of the disk material against a mating member was performed at 700 ℃ for 15 minutes to measure the amount of wear of the disk material. As the engaging member, a roll (outer diameter: 15mm, length: 22mm) which was chromated with respect to a material equivalent to SUS316L in JIS standard was used.

< grains of ferroalloy matrix >

Further, a cylindrical sintered alloy sample was cut, the cross section of the sample was mirror-polished, the cross section was etched with aqua regia (nitric acid: hydrochloric acid: 1:3), and then the microstructure of the cross section was observed with a microscope at a magnification of 200 times, and the structure of the substrate was observed. At this time, WinROOF manufactured by mitsubishi co ltd was used as image analysis software to perform image analysis of the structure cross section and binarize the image, measure the area of the austenite matrix, and calculate the average area of the crystal grains by measuring the number of crystal grains of the matrix. The average crystal grain size of the crystal grains is determined by converting this value into an area equivalent circle diameter.

< thickness of oxide coating >

Further, from the outer diameter: 24mm, length: a cylindrical sintered body of 8mm was machined to cut out a long axis: 20mm, short axis: 10mm, height: 3mm strip-shaped sintered alloy specimens. The sintered alloy sample was left to stand in an atmosphere containing water vapor (temperature: 860 ℃ C., test atmosphere: 8% water vapor/air) for 100 hours, then collected and cut, and the sample cross section was processed in the same manner as the measurement of the crystal grains, and the microstructure of the cross section was observed with a microscope. In this cross-sectional observation, the thickness of the oxide film was measured. The measurement is performed by selecting 3 arbitrary places in the oxide film part of the cross-sectional image and measuring the thickness as an average value of the measured values.

[ Table 1]

[ Table 2]

[ Table 3]

The sintered alloys of sample numbers 1 to 8 have different chromium contents. In any of the samples, the grains of the iron alloy matrix were small, and the growth of the grains of the matrix was appropriately suppressed. It is observed that the crystal grain size, the amount of wear, and the thickness of the oxide film tend to decrease as the chromium content increases, and when the chromium content is 18.48 mass% or more of the entire composition, the crystal grain size is almost constant, and the amount of wear is also almost constant. Even if the chromium content exceeds 30 mass%, no increase in the amount of the wear is caused. This is considered to be because the amount of graphite added is small, and therefore, the formation of chromium carbide and the particle growth are suppressed, and it is considered that the amount of chromium in the matrix is maintained to prevent the strength of the matrix from being lowered, and therefore, abrasion can be suppressed. However, when the chromium content exceeds 20 mass%, the thickness of the oxide film increases. This is considered to be because the reduction in compressibility of the iron alloy powder having an excessively high chromium content reduces the density of the green compact and the sintered body, and oxidation from the surface is likely to proceed. Based on these results, from the viewpoint of achieving both the wear resistance and the oxidation resistance, a preferable chromium content can be regarded as a range of 13.86 mass% or more and 27.72 mass% or less, a more preferable chromium content can be set to 16.88 mass% or more and 23.10 mass% or less, and a most preferable chromium content can be set to 18.48 mass% and 20.33 mass% or less.

The sintered alloys of sample numbers 4 and 9 to 15 have different nickel contents, and in any of the samples, the grains of the iron alloy matrix are small, and the growth of the grains of the matrix is appropriately suppressed. Since the thickness of the oxide film is rapidly reduced by the addition of nickel, it is known that the oxidation resistance of the sintered alloy is improved by nickel. It is considered that the increase in density of the sintered body is due to the large specific gravity of nickel. From the results of table 3, it is expected that there is no problem in material characteristics even if the nickel content is 22.18 mass% or more (sample No. 15), and it is understood that a nickel content of 6.47 mass% or more makes it possible to obtain a sintered alloy having wear resistance and oxidation resistance. From the viewpoint of oxidation resistance, a nickel content of 7.39 mass% or more is more preferable.

The sintered alloys of sample numbers 4, 16 to 21 differ in silicon content. Since the thickness of the oxide film rapidly decreases with an increase in the silicon content, it is found that silicon is effective for improving the oxidation resistance. However, if the silicon content exceeds 2.77 mass%, the thickness of the oxide film increases sharply. The reason for this is considered to be a decrease in the density of the sintered alloy due to a decrease in the compressibility of the raw material powder and a decrease in the oxidation resistance due to coarsening of crystal grains, which can be understood from the tendency observed in the densities of the green compact and sintered body and the crystal grain size of the iron alloy matrix. The density of the green compact and the sintered body decreases with an increase in the silicon content, which is considered to be caused by a decrease in the compressibility of the iron alloy powder. Further, the crystal grain size of the iron alloy matrix increases with the silicon content. From these results, it is considered that the oxidation resistance is lowered due to insufficient density of the sintered body and coarsening of crystal grains. Therefore, the silicon content is suitably 0.46 mass% or more and 2.77 mass% or less, and is preferably set to 0.74 mass% or more and 1.85 mass% or less.

Since the blend ratios of the copper-phosphorus alloy powders of the sintered alloys of sample numbers 4 and 22 to 27 are different, the contents of copper and phosphorus in the alloy compositions vary depending on the blend ratios. It can be understood that: by adding the copper-phosphorus alloy powder and increasing the proportion thereof, the density of the resulting sintered alloy increases, and sintering of the base body can be promoted by the copper-phosphorus alloy powder. Furthermore, it can be understood that: the oxidation resistance is improved because the thickness of the oxide film is significantly reduced. Further, if the blending ratio of the copper-phosphorus alloy powder exceeds 4 mass%, the amount of wear is reduced and the wear resistance is improved. However, if the blending ratio of the copper-phosphorus alloy powder exceeds 13 mass%, the sintered density decreases because a liquid phase is excessively generated in sintering. Therefore, in sample No. 27, the measurement of the crystal grain size and the material properties is omitted. In the results shown in Table 3, the sintered alloys of sample No. 4 and 23 to 26 in which the blending ratio of the copper-phosphorus alloy powder was 1.00 to 13.00 mass% were excellent in wear resistance and oxidation resistance. Since the copper content in these sintered alloys is 0.85 to 11.05 mass%, and the phosphorus content is 0.15 to 1.95%, these ranges can be regarded as appropriate contents of copper and phosphorus. More preferably, the copper content is 3.40 mass% or more and 8.50 mass% or less, the phosphorus content is 0.60 mass% or more and 1.50 mass% or less, more preferably, the copper content is 3.40 mass% or more and 5.95 mass% or less, and still more preferably, the phosphorus content is 0.60 mass% or more and 1.05 mass% or less.

The sintered alloys of sample numbers 4, 28 to 33 were different in alloy composition among the copper-phosphorus alloy powders used. In these samples, the copper content ratio decreased as the phosphorus content ratio increased, but in any of the samples, the copper content of the obtained sintered alloy was within the above-described appropriate range. The sintered body of sample number 28 has a relatively low density and a thick oxide film, and therefore, it is considered that the sintering promoting effect is small because the phosphorus content is insufficient. Further, the reduction in the sintered density in sample No. 33 is due to excessive generation of a liquid phase during sintering. Therefore, for sample No. 33, the measurement of the crystal grain size and the material properties was omitted. Based on these results, the content of phosphorus in the copper-phosphorus alloy powder used is preferably 5 mass% or more and 25 mass% or less, more preferably 10 mass% or more and 20 mass% or less.

The sintered alloys of sample numbers 4 and 34 to 39 have different carbon contents, and the carbon content is designed to be as low as 0.10 to 1.50 mass% in order to improve the machinability of the sintered alloy. Within this range, it is observed that if the carbon content is reduced, the density of the sintered body is reduced and the crystal grain size of the iron alloy matrix tends to increase, but even if it is reduced to 0.20 mass%, the amount of the wear and the thickness of the oxide film are suppressed to be low. Namely, it is found that the wear resistance and oxidation resistance are maintained. In sample No. 39, it is considered that the amount of chromium dissolved in the matrix decreases when the iron-phosphorus-carbon eutectic liquid phase is formed, and therefore the oxidation resistance of the matrix decreases. Therefore, the carbon content is preferably 0.20 mass% or more and 1.00 mass% or less, more preferably 0.4 mass% or more and 0.80 mass% or less.

Example 2

(sample No. 40 to 46)

A mixed powder as shown in Table 4 was prepared in the same manner as in sample No. 4 of example 1 except that 3.00 mass% of an iron-phosphorus alloy powder (phosphorus content: 35.00 mass%, average particle diameter: 40 μm) and 0 to 13.00 mass% of a copper powder (average particle diameter: 30 μm) were blended in place of the copper-phosphorus alloy powder. Using this as a raw material powder for molding, disk-shaped and cylindrical compacts (density of compact: 6.4 Mg/m) as shown in Table 5 were molded in the same manner as in example 1 ³ ). These were sintered under the same conditions as in example 1 to prepare sintered alloy samples, and the density, the amount of wear, the crystal grain size of the iron alloy matrix, and the thickness of the oxide film were measured. The results are shown in table 6.

[ Table 4]

[ Table 5]

[ Table 6]

In sample No. 43 of Table 6, the density of the sintered alloy was 7.1Mg/m ³ The mean crystal grain size of the iron alloy matrix was 22 μm, the molar loss was 23 μm, and the thickness of the oxide film was 6 μm. The overall composition of sample No. 43 is almost the same as that of sample No. 4, and if these samples are compared, it is understood that the properties of the sintered alloy are also the same. Therefore, it is found that a sintered alloy having wear resistance and oxidation resistance can be obtained in the same manner even when an iron-phosphorus alloy powder is used in combination with a copper powder instead of the copper-phosphorus alloy powder.

In table 6, it is observed that the crystal grain size of the alloy matrix, the amount of wear, and the thickness of the oxide film tend to decrease as the composition ratio of copper increases. This is considered to be because copper is dissolved in the matrix to protect the passive film on the surface layer, and the oxidation resistance is improved. However, if the copper proportion is further increased, the crystal grain size of the matrix, the amount of wear, and the thickness of the oxide film all increase, and therefore the copper composition ratio is preferably set in the range of 0.85 to 11.05 mass%.

In sample numbers 40 to 46, the density of the sintered alloy decreased as the composition ratio of copper increased. Such a tendency is not observed in sample numbers 22 to 25 using the copper-phosphorus alloy powder, and is considered to be related to the balance between generation of liquid phase and progress of sintering. In this regard, it is considered that the use of the copper-phosphorus alloy powder is more preferable than the use of the iron-phosphorus alloy powder and the copper powder in combination.

Example 3

(sample No. 47 to 52)

A mixed powder was prepared in the same manner as in sample No. 4 of example 1. The same operation as in example 1 was repeated except that the amount of the raw material powder filled in the cavity was changed so that the molding density of the green compact became the value shown in table 7, and the green compact was used as a raw material powder for molding, and a disc-shaped or cylindrical green compact was molded. These were sintered under the same conditions to prepare sintered alloy samples. The density and the amount of wear of the sample, the crystal grain size of the iron alloy matrix, and the thickness of the oxide film were measured. The results are shown in Table 7.

(sample No. 53 to 58)

The same operation as in sample No. 4 of example 1 was repeated to prepare a mixed powder. This was used as a raw material powder for molding, and molded into a disk-shaped or cylindrical green compact in the same manner as in example 1. Sintered alloy samples were produced under the same conditions as in example 1, except that the sintering temperatures were changed to the temperatures shown in table 7 using these. The density and the amount of wear of the sample, the crystal grain size of the iron alloy matrix, and the thickness of the oxide film were measured. The results are shown in table 7.

[ Table 7]

From the results of sample numbers 4, 47 to 52, the passing density was 6.00 to 6.80Mg/m ³ The sintered alloy obtained by sintering the green compact of (A) has good abrasion resistance and oxidation resistance. If the density of the green compact is low, the oxidation resistance is lowered due to insufficient density of the sintered alloy. In sample No. 52, the molding of the green compact was difficult, and a density exceeding 6.80Mg/m could not be obtained ³ The green compact of (1). From the results in Table 7, it is seen that the density of the sintered body is 6.90Mg/m or more ³ And less than or equal to 7.40Mg/m ³ Is suitably adjusted, preferably to greater than or equal to 7.00Mg/m ³ And less than or equal to 7.40Mg/m ³ The preferred method is. More suitably greater than or equal to 7.20Mg/m ³ And less than or equal to 7.40Mg/m ³ The sintered density of (2).

As is understood from the results of sample No. 4 and 53 to 58, the average crystal grain size of the iron alloy matrix increases as the sintering temperature increases, and the progress of sintering is promoted by the increase in the sintering temperature. The sintered alloy sintered at a temperature of 1050 ℃ or higher and 1160 ℃ or lower is excellent in wear resistance and oxidation resistance. In sample No. 53 having a sintering temperature of less than 1050 deg.C, the wear resistance and oxidation resistance were low. This is considered to be because the eutectic liquid phase is not sufficiently generated, and the strength of the iron alloy matrix is not obtained. In sample No. 58 in which the sintering temperature exceeded 1160 ℃, a liquid phase was excessively generated during sintering, and therefore the sintering density was reduced, and therefore, the measurement of the crystal grain size and the material characteristics was omitted. More preferred sintering temperatures can be considered to be greater than or equal to 1100 ℃ and less than or equal to 1140 ℃.

Example 4

(sample No. 59 to 65)

A mixed powder was prepared in the same manner as in example 1 except that the iron alloy powder in sample No. 4 was changed to an iron alloy powder (average particle size: 70 μm) obtained by alloying molybdenum so as to have the total composition shown in Table 8. The same operation as in example 1 was repeated using the thus-obtained powder as a raw material powder for molding to obtain a disk-shaped or cylindrical green compact (molding pressure: 600 MPa). These were sintered under the same conditions as in sample No. 4 to prepare a sintered alloy sample, and the density, the amount of wear, the crystal grain size of the iron alloy matrix, and the thickness of the oxide film were measured. The results are shown in Table 8.

[ Table 8]

The results of sample nos. 4 and 59 to 65 showed stable wear resistance and excellent oxidation resistance in all of the samples. However, a tendency to decrease with increasing molybdenum content was observed for the density of the green compact and sintered body. This is considered because the compressibility of the iron alloy powder is reduced to some extent by alloying of molybdenum, and therefore it is considered that it is preferable to add molybdenum, that is, a carbide-forming element, in a range of 3.23 mass% or less. More preferably, the molybdenum content is 0.46 mass% or more and 2.77 mass% or less.

Example 5

In order to examine the machinability of the sintered alloys of sample numbers 4, 22 to 27, and 34 to 46, turning tools made of cemented carbide were prepared, and the following turning process was performed using cylindrical sintered alloy samples. That is, the end face of the sample was machined with a tool from the outer periphery to the inner periphery (cutting speed: 50 m/min, notch depth: 0.2mm, feed speed: 0.05 mm/revolution), and the amount of wear of the flank face of the tool (tool wear amount) was measured at a stage when the total cutting distance reached 1000 m. The measured values are shown in table 9 as indices for evaluating machinability.

[ Table 9]

Based on the results of sample Nos. 4, 22 to 27, and 40 to 46, it can be understood that the wear of the tool is reduced by adding copper, and the machinability is improved. When the copper content is 0.85 mass% or more, the machinability is good. However, if the addition amount of copper is further increased, the tool wear amount is shifted to increase, which is considered to be because: hardening of the matrix due to solid solution of copper proceeds excessively. In the results in table 9, it is considered that the machinability of the sintered alloy is improved in the range where the copper content is 0.85 to 11.05 mass%.

Further, from the results of sample nos. 4 and 34 to 39, it was observed that the tool wear amount tended to increase by the increase in the carbon content. That is, it is understood that setting the amount of graphite to be lower than the conventional amount is effective for improving the machinability, and that setting the amount to make the carbon content of the sintered alloy 1.00 mass% or less can improve the machinability and suppress the tool wear.

The disclosure of the present application is related to the subject matter described in japanese patent application No. 2018-131364, filed on 7/11/2018, the entire disclosure of which is incorporated by reference into the present application.

It should be noted that: various modifications, changes, and variations may be made to the embodiments described above, in addition to those already described, without departing from the novel and advantageous features of the present invention. Accordingly, it is intended that such modifications and variations be included in the appended claims.

Industrial applicability

Since a sintered alloy having excellent oxidation resistance, heat resistance, and abrasion resistance and improved machinability can be provided, the sintered alloy can be suitably used for turbine members for turbochargers, and can be favorably used for members such as nozzle bodies, which require durability against high-temperature corrosive gases.

Claims

1. A sintered alloy whose overall composition comprises, in mass%, Cr: 13.86-27.72%, Ni: 6.47-20.33%, Cu: 0.85-11.05%, Si: 0.46-2.77%, P: 0.15-1.95%, C: 0.20 to 1.00%, and the balance of Fe and unavoidable elements,

the density of the sintered alloy is 6.8-7.4 Mg/m ³ ，

The sintered alloy exhibits a metal structure having an iron alloy matrix in which pores are dispersed and carbides dispersed in the iron alloy matrix, and the iron alloy matrix is composed of crystal grains having an average crystal grain diameter of 10 to 50 [ mu ] m.

2. The sintered alloy according to claim 1, wherein nitrides are formed on the surface of the sintered alloy and on the inner surface of the pores.

3. A sintered alloy whose overall composition comprises, in mass%, Cr: 13.86-27.72%, Ni: 6.47-20.33%, Cu: 0.85-11.05%, Si: 0.46-2.77%, P: 0.15-1.95%, C: 0.20-1.00%, carbide-forming elements: less than or equal to 3.23%, and the balance Fe and unavoidable elements,

the density of the sintered alloy is 6.8-7.4 Mg/m ³ ，

The carbide-forming element is at least 1 element selected from the group consisting of Mo, V, W, Nb and Ti,

the sintered alloy exhibits a metallic structure having an iron alloy matrix in which pores are dispersed and a carbide dispersed in the iron alloy matrix, and the iron alloy matrix is composed of crystal grains having an average crystal grain diameter of 10 to 50 μm.

4. The sintered alloy according to claim 3, wherein nitrides are formed on the surface of the sintered alloy and on the inner surface of the pores.

5. A method for producing the sintered alloy as claimed in claim 1,

a material containing Cr: 15-30%, Ni: 7-24%, Si: 0.5 to 3.0%, and the balance Fe and inevitable impurities,

preparing a blending material for blending phosphorus and copper, the blending material comprising 1 or more selected from iron-phosphorus alloy powder with phosphorus content of 10-30 mass%, copper-phosphorus alloy powder with phosphorus content of 5-25 mass% and copper powder,

mixing the iron alloy powder, the compounding material, and graphite powder to prepare a raw material powder containing 0.15 to 1.95 mass% of phosphorus, 0.85 to 11.05 mass% of copper, and 0.20 to 1.00 mass% of carbon,

compressing the raw material powder to form a powder with a density of 6.0-6.8 Mg/m ³ The powder-pressed body of (a) is,

and heating the green compact to 1050-1160 ℃ in a non-oxidizing atmosphere for sintering.

6. The method for producing a sintered alloy according to claim 5, wherein the blending material is a powder containing phosphorus in the form of one or both of the iron-phosphorus alloy powder and the copper-phosphorus alloy powder, and containing copper in the form of one or both of the copper powder and the copper-phosphorus alloy powder.

7. The method of producing a sintered alloy according to claim 5 or 6, wherein the non-oxidizing atmosphere is an atmosphere at normal pressure formed of a mixed gas of nitrogen and hydrogen containing nitrogen at 10 mass% or more.

8. A method for producing the sintered alloy as claimed in claim 3,

preparing a ferroalloy powder containing, in mass%, Cr: 15-30%, Ni: 7-24%, Si: 0.5-3.0%, carbide-forming elements: less than or equal to 3 mass%, and the balance Fe and unavoidable impurities, the carbide-forming element being at least 1 element selected from the group consisting of Mo, V, W, Nb, and Ti,

mixing the iron alloy powder, the compounding material and graphite powder to prepare a raw material powder containing 0.15 to 1.95 mass% of phosphorus, 0.85 to 11.05 mass% of copper and 0.20 to 1.00 mass% of carbon,

compressing the raw material powder to form a powder with a density of 6.0-6.8 Mg/m ³ The powder-pressed body of (1) is,

9. The method of producing a sintered alloy according to claim 8, wherein the blending material is a powder containing phosphorus in the form of one or both of the iron-phosphorus alloy powder and the copper-phosphorus alloy powder, and containing copper in the form of one or both of the copper powder and the copper-phosphorus alloy powder.

10. The method for producing a sintered alloy according to claim 8 or 9, wherein the non-oxidizing atmosphere is an atmosphere at normal pressure formed from a mixed gas of nitrogen and hydrogen that contains nitrogen in an amount of 10 mass% or more, or nitrogen.