Iron-based sintered alloy and method for manufacturing the same

Abstract

The objective is to provide an iron-based sintered alloy with excellent surface pressure fatigue strength. [Solution] An iron-based sintered alloy having an overall composition of Cr: 1-5% and C: 0.6-1.5% by mass, with the remainder being Fe and unavoidable impurities, comprising a matrix and metal carbides dispersed in the matrix, with the number of metal carbides having a particle size of 0.2-2.0 μm being 10,000-25,000 in the range of 100 μm × 100 μm.

Description

[Technical Field]

[0001] This disclosure relates to an iron-based sintered alloy and a method for producing the same. [Background technology]

[0002] Sintered materials are inferior to melted materials in terms of strength, which can limit their applications. To address this issue, efforts have been made to improve the strength of sintered materials. Patent Document 1 discloses a sintered material with excellent strength due to the presence of an appropriate amount of compound particles such as oxides and a relative density of 93% or more. Patent Document 2 discloses an iron-based sintered alloy with excellent heat resistance, corrosion resistance, wear resistance, and machinability due to having a metallic structure in which carbides are uniformly precipitated and dispersed within the iron alloy matrix. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] International Publication No. 2020 / 157880 [Patent Document 2] International Publication No. 2013 / 199695 [Overview of the project] [Problems that the invention aims to solve]

[0004] When carbides in sintered alloys become coarse, they can become points of stress concentration, leading to cracking and potentially reducing material strength. Such points of stress concentration can affect the fatigue strength under surface pressure over time. On the other hand, material strength can be increased by homogeneously forming a hard structure through solid solution diffusion of carbon. However, if solid solution diffusion of carbon occurs locally, coarse metal carbides may precipitate in the matrix. These coarse metal carbides can cause cracking and other problems due to stress concentration.

[0005] One of the objectives of this disclosure is to provide an iron-based sintered alloy with excellent surface pressure fatigue strength. [Means for solving the problem]

[0006] One embodiment of the present disclosure provides an iron-based sintered alloy having an overall composition of Cr: 1-5% and C: 0.6-1.5% by mass, with the remainder being Fe and unavoidable impurities, comprising a matrix and metal carbides dispersed in the matrix, wherein the number of metal carbides with a particle size of 0.2-2.0 μm is 10,000-25,000 in the range of 100 μm × 100 μm.

[0007] Another embodiment of the present disclosure provides a method for producing an iron-based sintered alloy, which includes using a raw material powder containing carbon powder such that the mass percentage is 0.6 to 1.5% C, with the remainder being Fe and unavoidable impurities in an iron alloy powder containing 1 to 5% Cr by mass, the remainder being Fe and unavoidable impurities, molding the raw material powder at 500 MPa or higher to obtain a molded body, sintering the molded body to obtain a sintered alloy, and quenching the sintered alloy. [Effects of the Invention]

[0008] According to embodiments of this disclosure, an iron-based sintered alloy exhibiting excellent surface pressure fatigue strength can be provided. [Brief explanation of the drawing]

[0009] [Figure 1] This is an image of the iron-based sintered alloy 1 of the example, obtained by observation using a scanning electron microscope (SEM). [Figure 2] This is an image of comparative example sintered alloy 1 obtained by observation using a scanning electron microscope (SEM). [Modes for carrying out the invention]

[0010] Some embodiments of this disclosure are described in detail below, but these are merely illustrative examples, and the present invention is not limited to these examples.

[0011] In this disclosure, numerical ranges indicated using "~" represent a range that includes the numbers before and after "~" as the minimum and maximum values, respectively. In numerical ranges described in stages in this disclosure, the upper or lower limit of a numerical range in one stage can be arbitrarily combined with the upper or lower limit of a numerical range in another stage. In numerical ranges described in this disclosure, the upper or lower limit of that numerical range may be replaced with the values ​​shown in the examples. In this disclosure, the term "process" is included not only in the sense of an independent process, but also in the sense of a process that is not clearly distinguishable from other processes, as long as the intended function of that process is achieved. Unless otherwise specified, "element" refers to one or more elements.

[0012] <Iron-based sintered alloy> The present disclosure provides an iron-based sintered alloy having an overall composition of Cr: 1-5% and C: 0.6-1.5% by mass, with the remainder being Fe and unavoidable impurities, comprising a matrix and metal carbides dispersed in the matrix, wherein the number of metal carbides with a particle size of 0.2-2.0 μm is 10,000-25,000 in the range of 100 μm × 100 μm.

[0013] This iron-based sintered alloy can possess high surface pressure fatigue strength. In the following description, unless otherwise specified, percentages indicating the overall composition represent mass percentages. Also, unless otherwise specified, the density of the sintered alloy is expressed as a value measured in accordance with JIS Z 2501:2000.

[0014] The inclusion of 1-5% Cr in iron-based sintered alloys enhances hardenability, allowing for a harder structure and increased material strength. The inclusion of 0.6-1.5% C in iron-based sintered alloys promotes solid solution diffusion of carbon, resulting in sufficient hardness after quenching and further increasing material strength. On the other hand, the inclusion of carbon in iron-based sintered alloys can lead to the precipitation of metal carbides in the matrix. These metal carbides can precipitate as coarse particles in the matrix during the heat treatment history from sintering to quenching. For example, they can precipitate as linear metal carbides in the matrix after quenching, originating from pearlite formed after sintering. In a hard structure, coarse metal carbides, especially linear metal carbides, can become points of stress concentration, potentially leading to a decrease in the material strength of the iron-based sintered alloy.

[0015] Iron-based sintered alloys have a matrix and metal carbides dispersed within the matrix. Due to their manufacturing method, iron-based sintered alloys contain both a matrix and pores, but at least a portion of the metal carbides precipitate within the matrix. In the matrix, the metal carbides can be identified and observed from differences in image density under scanning electron microscopy. Examples of metal carbides include carbides with Cr, Fe, etc. Also, carbides with Mo, V, W, Nb, Ti, etc. Furthermore, composite carbides of these metals may also exist. These may be one or a combination of two or more.

[0016] In the iron-based sintered alloy, the number of particles with a particle size of 0.2 to 2.0 μm among the metal carbides is preferably 10,000 to 25,000 in the range of 100 μm × 100 μm. The number is preferably 10,000 or more, more preferably 12,000 or more, and even more preferably 14,000 or more in the range of 100 μm × 100 μm. When the number of metal carbides is within these ranges, smaller metal carbides are finely distributed in the matrix, the proportion of coarse metal carbides becomes small, and sufficient surface contact fatigue strength can be achieved. When the number of metal carbides is within these ranges, smaller metal carbides are included in the matrix, so metal carbides that serve as stress concentration points are reduced, and cracks and the like are less likely to occur. Judging from the C content of the overall composition of the iron-based sintered alloy, when metal carbides are included within these ranges, the influence of coarse metal carbides can be eliminated.

[0017] The number is preferably 25,000 or less, more preferably 21,000 or less, and even more preferably 17,000 or less in the range of 100 μm × 100 μm. As the metal carbides become smaller, their number increases and the distance between the metal carbides becomes closer, so cracks may occur starting from there. When the number of metal carbides is within these ranges, the distance between the metal carbides can be appropriately maintained, and sufficient surface contact fatigue strength can be achieved. For example, the number may be 10,000 to 25,000, 12,000 to 21,000, or 14,000 to 17,000.

[0018] In the present disclosure, the method for measuring the number of particles with a particle size of 0.2 to 2.0 μm among the metal carbides follows the following procedure: Cut the iron-based sintered alloy, mirror-polish the cross-section, observe the cross-section with a scanning electron microscope (SEM), and measure and obtain the number observed in the range of 100 μm × 100 μm. Image analysis software (WinROOF manufactured by Mitani Corporation) is preferably used for the measurement of the number. The particle size of the carbide shall be the equivalent circle diameter.

[0019] By identifying the number of metal carbide particles with a particle size of 0.2 to 2.0 μm, it is possible to provide an iron-based sintered alloy suitable for surface pressure fatigue strength. In an iron-based sintered alloy, the maximum particle size of the metal carbide may be 2.0 μm or less, 1.5 μm or less, or 1.0 μm or less, from the viewpoint of preventing crack formation. The maximum particle size is determined by cutting the iron-based sintered alloy, mirror-polishing the cross-section, observing the cross-section with a scanning electron microscope (SEM), and taking the longest diameter of the metal carbide particles observed in a 100 μm × 100 μm area.

[0020] The overall composition of the iron-based sintered alloy is described below. The iron-based sintered alloy should contain C and Cr, with the remainder being Fe and unavoidable impurities. Fe should be the largest component element.

[0021] Iron-based sintered alloys contain 0.6-1.5% carbon (C). A portion of the carbon dissolves in Fe, improving strength. The carbon content may be 0.6% or more, preferably 0.7% or more, and more preferably 0.8% or more. Within these ranges, solid solution diffusion is promoted, increasing strength. Solid solution diffusion of carbon into the matrix facilitates the dispersion and precipitation of other elements and metal carbides within the matrix. The finely dispersed metal carbides precipitated in the matrix reduce the likelihood of stress concentration points, potentially improving surface pressure fatigue strength. The carbon content may be 1.5% or less, 1.3% or less, 1.1% or less, or 1% or less. Within these ranges, while maintaining malleability and ductility, the sintered alloy can be easily softened before quenching, improving machinability.

[0022] Iron-based sintered alloys contain 1-5% Cr. Cr improves the hardenability of the sintered alloy and, after sintering and quenching, imparts a hard structure to the iron-based sintered alloy. The Cr content should ideally be 1% or more, 1.2% or more, 1.5% or more, or 2% or more. Within these ranges, solid solution diffusion of C and the precipitation of fine metal carbides become easier. Furthermore, improved hardenability can increase the strength of the iron-based sintered alloy through its hard structure. It can also improve wear resistance and corrosion resistance. The Cr content should ideally be 5% or less, 4.5% or less, 4% or less, 3.5% or less, or 3% or less. Within these ranges, the coarse precipitation of chromium carbides in the matrix is ​​suppressed, allowing for better dispersion of fine metal carbides in the matrix.

[0023] The iron-based sintered alloy may contain at least one element selected from the group consisting of Mo, V, W, Nb, and Ti. The total amount of at least one element selected from the group consisting of Mo, V, W, Nb, and Ti should be 2% or less. These elements are carbide-forming elements and have a stronger carbide-forming ability than Cr, thus preferentially forming metal carbides over Cr. Because these elements tend to precipitate as metal carbides in the matrix rather than at grain boundaries, using these elements allows for the dispersion of finer metal carbides in the matrix. Furthermore, using these elements prevents a decrease in the Cr concentration in the matrix of the iron-based sintered alloy, thus improving the wear resistance and strength of the matrix. The inclusion of Mo is particularly desirable.

[0024] When using at least one of Mo, V, W, Nb, and Ti, if the amount dissolved in the iron alloy powder exceeds 2%, it can harden the powder itself, reducing its compressibility. Furthermore, these additional components are expensive, so excessive use leads to increased manufacturing costs. Therefore, when using at least one of Mo, V, W, Nb, and Ti, their total amount should ideally be 2% or less, 1% or less, or 0.8% or less, and preferably between 0.1-2%, 0.2-1%, or 0.5-0.8%. If Mo is included, its individual amount should ideally be 0.1-2%, 0.2-1%, or 0.5-0.8%.

[0025] Mn is an element that improves hardenability and toughness. Mn may be present in iron-based sintered alloys at concentrations of 0-5%, 0.05-1%, or 0.1-0.3%. Ni is an element that improves low-temperature toughness and corrosion resistance. Ni may be included in iron-based sintered alloys at concentrations of 0-5%, 0.05-1%, or 0.1-0.3%. To promote the precipitation of carbides such as Cr and Mo and to refine the metal carbides, Mn and Ni may be substantially omitted. Si can be included in iron base materials as a deoxidizing agent. While Si is an element that improves oxidation resistance and heat resistance, it also hardens iron alloy powder and reduces its compressibility. Si may be included in concentrations of 0-5%, 0.05-1%, or 0.1-0.3%, but may be substantially absent.

[0026] According to some embodiments, the iron-based sintered alloy may have a Rockwell hardness of 40 to 70 on the HRC scale. Here, the Rockwell hardness HRC is measured in accordance with JIS Z 2245:2016. The Rockwell hardness HRC may be 40 or higher, 45 or higher, or 50 or higher. In these ranges, surface pressure fatigue strength can be obtained more sufficiently. Because the metal carbides of the iron-based sintered alloy are finely dispersed in the matrix, and the Cr and C content is further specified to sufficiently form a hard structure and increase material strength, the iron-based sintered alloy may have Rockwell hardness in these ranges. The Rockwell hardness HRC may be 70 or lower, 68 or lower, or 66 or lower.

[0027] <Manufacturing method for iron-based sintered alloys> The following describes several embodiments of methods for manufacturing iron-based sintered alloys. Iron-based sintered alloys can be obtained by mixing raw material powders to achieve the desired composition, pressurizing them to produce a molded body, and then sintering the molded body.

[0028] According to several embodiments, a method for producing an iron-based sintered alloy can be provided, which involves using a raw material powder containing carbon powder such that the mass percentage is 0.6 to 1.5% C, with the remainder being Fe and unavoidable impurities in an iron alloy powder containing 1 to 5% Cr by mass, the remainder being Fe and unavoidable impurities; molding the raw material powder at 500 MPa or higher to obtain a molded body; sintering the molded body to obtain a sintered alloy; and quenching the sintered alloy. This manufacturing method can provide an iron-based sintered alloy with high surface pressure fatigue strength.

[0029] First, the molding process will be explained. The raw material powder is preferably an iron alloy powder containing 1-5% Cr by mass, with the remainder being Fe and unavoidable impurities, and containing carbon powder so that the mass percentage is 0.6-1.5% C. As the raw material powder, pure iron powder, metal powder, iron alloy powder, other alloy powders, carbon powder, etc., can be used, blended to achieve the overall composition of an iron-based sintered alloy. Examples of iron alloy powder include Cr-Fe alloy powder. Alternatively, pure iron powder may be used in combination with Cr powder or Cr-Fe alloy powder. Examples of carbon powder include graphite powder. For example, iron alloy powders such as Fe-Cr alloys and Fe-Cr-Mo alloys can be used in combination with carbon powder such as graphite powder. By using a raw material powder in which carbon powder, preferably graphite powder, is added to iron alloy powder, solid solution diffusion of carbon is promoted during the heat treatment history, and the metal carbides precipitated in the matrix can be made finer. By adding graphite powder to an alloy powder in which Cr and Fe are pre-alloyed, when carbon diffuses through solid solution into the iron matrix to form metal carbides with Cr, the metal carbides can be distributed more uniformly within the iron matrix. This suppresses the formation of coarse metal carbides, resulting in a finer dispersion of metal carbides within the iron matrix.

[0030] The raw material powder may further contain at least one element selected from the group consisting of Mo, V, W, Nb, and Ti. It is even better if the total amount of at least one element selected from the group consisting of Mo, V, W, Nb, and Ti is 2% by mass or less in the raw material powder. These elements are carbide-forming elements and can preferentially form metal carbides over Cr. These elements may be used as individual metal powders or as alloy powders pre-alloyed with Fe. When used as alloy powders, they may be composite alloy powders of Fe and two or more of these elements, or even composite alloy powders of Fe, these elements, and Cr. When using composite alloy powders, adding carbon as graphite powder allows carbon to solid-solve and diffuse into the iron matrix, forming metal carbides with these elements, thereby enabling a more uniform distribution of metal carbides in the iron matrix. Since these elements preferentially form metal carbides over Cr, the amount of Cr consumed by chromium carbides can be reduced, preventing a decrease in wear resistance and corrosion resistance due to the reduction in Cr content. Furthermore, since metal carbides can be produced more finely, it is preferable that the raw material powder contains at least one selected from the group consisting of Mo, V, W, Nb, and Ti, and it is even preferable that it contains Mo. It is even preferable to use, for example, Fe-3%Cr alloy or Fe-3%Cr-0.5%Mo alloy as the iron alloy powder.

[0031] The particle size of the raw material powder is preferably 5 to 100 μm or 10 to 80 μm at the particle diameter (D50) where the cumulative value in the volume-based particle size distribution becomes 50%. Here, the particle diameter (D50) can be measured using a laser diffraction particle size distribution analyzer or the like.

[0032] The raw material powder may contain a molding lubricant. The molding lubricant can prevent seizing when removing the compacted product from the mold. On the other hand, the inclusion of a molding lubricant may cause a decrease in density. Therefore, the amount of molding lubricant may be 0.1 to 5% by mass, 0.2 to 1% by mass, or 0.3 to 0.5% by mass relative to the total mass of the raw material powder. Examples of molding lubricants include metal soaps such as lithium stearate, zinc stearate, barium stearate, calcium stearate, and magnesium stearate. Other examples include fatty acid amides such as lauric acid amide, stearic acid amide, and palmitic acid amide; and higher fatty acid amides such as ethylenebisstearic acid amide.

[0033] Molding can be performed by filling a mold with raw material powder and applying pressure. The molding pressure should preferably be 500 MPa or higher, 600 MPa or higher, or 700 MPa or higher. By using a molding pressure within these ranges, the resulting sintered alloy can have an appropriate porosity, thereby increasing its material strength. There is no particular upper limit to the molding pressure. Furthermore, the density of the molded body should be 6.5 to 7.8 g / cm³. 3 6.8~7.5g / cm³ 3 , or 7.0~7.3 g / cm³ 3 It would be good if that were the case.

[0034] The molding can be carried out, for example, using a press device having a mold capable of uniaxial pressing. To prevent the raw material powder from sticking to the mold, an external lubricant may be applied to the inner surface of the mold. The same type of external lubricant as the molding lubricant described above can be used.

[0035] Next, we will explain the sintering process. Sintering should be carried out in a non-oxidizing atmosphere, preferably in a nitrogen atmosphere or a reducing atmosphere. The sintering temperature should be between 900°C and 1250°C. The maximum holding temperature should be within this range. The sintering temperature should be above 900°C, above 1000°C, or above 1100°C. Within these ranges, the diffusion of elements such as C and Cr into the matrix can be promoted, further improving sinterability. Alternatively, the sintering temperature should be below 1250°C, below 1200°C, or below 1150°C. Within these ranges, the excessive diffusion of elements such as C and Cr into Fe can be suppressed, the growth of coarse carbon nuclei can be suppressed, and the formation of coarse metal carbides can be prevented. The sintering time should be between 10 minutes and 3 hours.

[0036] After sintering, the iron-based sintered alloy may be slowly cooled by furnace cooling, air cooling, etc., or rapidly cooled by water cooling, oil cooling, etc. The structure after sintering may be a soft structure. Sintered alloys with a carbon content of 0.6% or more and a Cr content of 1% or more can provide high-strength materials. However, if rapid cooling follows sintering, some of the carbon is formed as hard metal carbides, while the remaining carbon is supersaturated and dissolved in the iron, making it easier to form a hard matrix structure. Therefore, it is preferable to have a soft structure after sintering and to precipitate fine metal carbides before quenching by heat treatment such as annealing, as described later, and then perform quenching.

[0037] Annealing may be performed before quenching the sintered alloy. The annealing process is described below. Note that quenching may be performed on the sintered alloy without annealing. Also, annealing may be performed simultaneously with the sintering process.

[0038] Annealing is preferably carried out by heating and holding the sintered alloy at an annealing temperature above the austenitization initiation temperature, followed by furnace cooling. For example, the annealing temperature is preferably 750°C to 900°C, followed by furnace cooling to a temperature in the range of 500°C to 700°C from the annealing temperature, and then air cooling. The holding time at the annealing temperature is preferably 10 minutes to 3 hours. The annealing atmosphere is preferably a non-oxidizing atmosphere, and preferably a vacuum, nitrogen atmosphere, or reducing atmosphere. By furnace cooling at the above temperature, the sintered alloy can be cooled slowly, and the carbides can be dispersed in a finer state.

[0039] By performing the above-described heat treatment on a sintered alloy, the precipitation of carbides dissolved in the matrix as fine metal carbide particles can be promoted. If pearlite is formed during sintering, the precipitation of the layered carbides in the pearlite as fine carbide particles can be promoted. By precipitating fine metal carbides, it is possible to prevent coarse metal carbides from being mixed into the hard structure after quenching. In addition, since a soft structure such as ferrite is formed throughout after annealing, the base material becomes softer, allowing for precise machining and densification before quenching. By quenching the annealed sintered alloy, it is possible to retain fine metal carbides in the matrix while transforming the matrix into a hard structure such as martensite, thereby further increasing the material strength.

[0040] From this perspective, the annealing temperature may be 750°C or higher, 780°C or higher, or 800°C or higher. The annealing temperature may be 900°C or lower, 880°C or lower, or 850°C or lower.

[0041] Forging may be performed before quenching the sintered alloy. The forging process is described below. If annealing is performed, it is recommended to anneal the sintered alloy, forge it, and then quench it in that order. Alternatively, quenching may be performed on the sintered alloy without forging.

[0042] Cold forging is preferable. Cold forging increases the density of the sintered alloy, thereby increasing its material strength. The density of the sintered alloy should ideally be increased to 1%, 3%, 5%, or 7% or higher before and after cold forging. For example, the density of the sintered alloy after cold forging should be 7.0 g / cm³. 3 More than 7.2g / cm 3 Above, or 7.5 g / cm³ 3 That's all.

[0043] As an example of cold forging, a forging die is prepared, a lubricant such as zinc stearate is applied to the surface of the die hole or the surface of the sintered alloy, the sintered alloy is then inserted into the die hole, and pressure is applied from above and below with a punch at a pressure of 700 to 2500 MPa. The forging may be cold forging without heating the sintered alloy, but hot forging or warm forging may also be used.

[0044] Machining may be performed on the sintered alloy before quenching. If annealing is performed, it is advisable to perform machining between annealing and quenching of the sintered alloy. If forging is performed, it is advisable to perform machining between forging and quenching of the sintered alloy. For example, annealing, forging, machining, and quenching of the sintered alloy may be performed in this order. Note that quenching of the sintered alloy may be performed without machining.

[0045] Machining a sintered alloy that has a soft structure before quenching can further improve its machinability. In annealed sintered alloys, the structure is softened, which can further improve machinability. When forging is performed, the shape accuracy of the final product can be improved by removing shape defects such as burrs that occur after forging, and the surface pressure fatigue strength of the iron-based sintered alloy can be further improved.

[0046] Next, the quenching process will be described. By quenching, the metal structure of the sintered alloy becomes a hard structure such as martensite, and the material strength can be increased. When the sintered alloy before quenching has a metal structure such as ferrite and pearlite, the metal carbides dispersed in the matrix can maintain their shape in the hard structure after quenching. Thereby, in the obtained iron-based sintered alloy, the number of metal carbides dispersed in the matrix can be controlled. For example, in the sintered alloy before quenching, the number of particles with a particle diameter of 0.2 to 2.0 μm among the metal carbides may be set to 10,000 to 25,00 for a range of 100 μm × 100 μm. The number of metal carbides before quenching may be controlled by performing forming, sintering, and quenching within the range of the contents of Cr and C in the raw material powder. At this time, the number can also be controlled by sintering conditions, the heat treatment history from after sintering to before quenching, and the like. Also, by using a raw material powder obtained by adding graphite powder to an iron alloy powder containing Cr, after the heat treatment history from sintering to before quenching, metal carbides are likely to precipitate and disperse in the matrix, and the number can be controlled.

[0047] The density of the sintered alloy to be quenched is 7.0 g / cm 3 or more, 7.2 g / cm 3 or more, or 7.5 g / cm 3 or more. The density within these ranges may be adjusted by reducing pores through a forging process after sintering. Although the sintered alloy may contain a certain amount of pores after sintering, through the forging process, a low porosity can be achieved, and the surface contact fatigue strength can be further increased.

[0048] ]>The sintered alloy to be quenched may have a Rockwell hardness of 50 to 100 in the HRB scale. Here, the Rockwell hardness HRB is measured in accordance with JIS Z 2245:2016. The Rockwell hardness HRB may be 50 or more, 60 or more, or 70 or more. Within these ranges, pores in the sintered alloy are reduced before quenching, and the surface contact fatigue strength can be further increased after quenching. The Rockwell hardness HRB may be 100 or less, 98 or less, or 95 or less. Within these ranges, the machinability of the sintered alloy can be obtained before quenching.

[0049] The sintered alloy to be quenched is preferably soft in structure, for example, ferrite, pearlite, or a combination thereof. However, it may also contain hard structures; for example, hard structures may make up 0-30% by area ratio. The sintered alloy to be quenched should preferably have a soft structure with finely dispersed metal carbides. In addition, it should have a density of 7.0 g / cm³. 3 The above conditions are met, and it is even better if at least one of the following conditions is met: a Rockwell hardness of 50 to 100 on the HRB scale. By quenching such a sintered alloy, metal carbides are finely dispersed in the hard structure, and the surface pressure fatigue strength of the iron-based sintered alloy can be further improved.

[0050] Quenching is preferably performed by heating and holding the sintered alloy at a temperature above the austenitization initiation temperature, followed by rapid cooling. For example, the quenching temperature is preferably 750°C to 900°C, and even better if it is 800°C to 860°C. The holding time at the quenching temperature is preferably 10 minutes to 3 hours. The quenching atmosphere is preferably a non-oxidizing atmosphere, preferably a nitrogen atmosphere or a reducing atmosphere. A reduced-pressure nitrogen atmosphere may also be used. After holding the sintered alloy at the quenching temperature, it is preferable to rapidly cool it from the quenching temperature to a predetermined temperature within the temperature range of 25 to 150°C. Cooling is preferably performed by rapid cooling such as water cooling or oil cooling. For example, the cooling rate is preferably 50°C / second to 300°C / second. Quenching may also be performed by carburizing quenching, nitriding quenching, carburizing-nitriding quenching, etc. Carburizing quenching, etc., can form a carburized layer, a nitrided layer, etc. on the surface of the sintered alloy, further strengthening the hard structure of the surface and improving the surface pressure fatigue strength.

[0051] After quenching the sintered alloy, tempering may be performed. The tempering temperature may be between 100 and 300°C. The holding time at the tempering temperature may be between 10 minutes and 3 hours. Tempering the sintered alloy further relieves stress, stabilizes the structure, and imparts toughness. If forging is performed before quenching the sintered alloy, it is even better to perform tempering to relieve the stress caused by forging.

[0052] After quenching or tempering the sintered alloy, further machining may be performed. This can further improve the shape accuracy of the final iron-based sintered alloy product. The improved shape accuracy can lead to further improvements in surface pressure fatigue strength.

[0053] Iron-based sintered alloys have excellent surface pressure fatigue strength and can be used in gears such as sprockets, spur gears, helical gears, and racks, as well as pulleys, bearings, and the like.

[0054] Some embodiments of this disclosure are shown below. [1] An iron-based sintered alloy having an overall composition of Cr: 1-5% and C: 0.6-1.5% by mass, with the remainder being Fe and unavoidable impurities, comprising a matrix and metal carbides dispersed in the matrix, wherein the number of metal carbides with a particle size of 0.2-2.0 μm is 10,000-25,000 in the range of 100 μm × 100 μm.

[0055] [2] The iron-based sintered alloy according to [1], wherein the overall composition comprises at least one selected from the group consisting of Mo, V, W, Nb, and Ti in a total amount of 2% or less by mass. [3] An iron-based sintered alloy as described in [1] or [2], having a Rockwell hardness of 40 to 70 on the HRC scale.

[0056] [4] A method for producing an iron-based sintered alloy, comprising using a raw material powder containing carbon powder such that the mass percentage is 0.6 to 1.5% C, with the remainder being Fe and unavoidable impurities in an iron alloy powder containing 1 to 5% Cr by mass, the remainder being Fe and unavoidable impurities, molding the raw material powder at 500 MPa or more to obtain a molded body, sintering the molded body to obtain a sintered alloy, and quenching the sintered alloy.

[0057] [5] A method for producing an iron-based sintered alloy according to [4], further comprising forging the sintered alloy before quenching. [6] The density of the sintered alloy to be quenched is 7.0 g / cm³ 3 The method for producing an iron-based sintered alloy as described in [4] or [5] above.

[0058] [7] A method for producing an iron-based sintered alloy according to any one of [4] to [6], wherein the sintered alloy to be quenched comprises a matrix and metal carbides dispersed in the matrix, and the number of metal carbides having a particle size of 0.2 to 2.0 μm is 10,000 to 25,000 in the range of 100 μm × 100 μm. [8] A method for producing an iron-based sintered alloy according to any one of [4] to [7], wherein the sintered alloy subjected to quenching has a Rockwell hardness of 50 to 100 on the HRB scale. [Examples]

[0059] The present disclosure will be described in detail below with reference to examples, but the present disclosure is not limited to these examples.

[0060] <Fabrication of iron-based sintered alloy 1> A raw material powder (composition: Fe-3%Cr-0.5%Mo-1.28%C) was obtained by adding 1.3 parts by mass of graphite powder and 0.5 parts by mass of molding lubricant to 100 parts by mass of iron alloy powder with a composition of Fe-3%Cr-0.5%Mo (particle size: average particle diameter: 80 μm). This raw material powder was then molded to produce a molded body. The density of the molded body was 7.0 g / cm³. 3 Next, this molded body was sintered in a reducing atmosphere at 1130°C for 30 minutes to obtain sintered alloy 1.

[0061] The obtained sintered alloy 1 was annealed in a reducing atmosphere at 830°C for 120 minutes. Cold forging was performed on the annealed sintered alloy 2. The forged sintered alloy 3 was maintained at 860°C for 60 minutes in a reduced-pressure nitrogen atmosphere, and then quenched by rapid cooling at 100°C / second. The quenched sintered alloy 4 was tempered by maintaining it at 180°C for 60 minutes in an air atmosphere to obtain iron-based sintered alloy 1.

[0062] <Rating> In the manufacturing process of the iron-based sintered alloy 1 described above, the density, Rockwell hardness, and Vickers hardness of sintered alloys 1-3 and iron-based sintered alloy 1 were measured according to the following procedure. The results are shown in Table 1.

[0063] (density) Density was measured in accordance with JIS Z 2501:2000.

[0064] (Rockwell hardness) Rockwell hardness was measured according to JIS Z 2245:2016, with HRB measured for sintered alloys 1-3 and HRC measured for iron-based sintered alloy 1.

[0065] (Vickers hardness) The Vickers hardness was measured at HV0.03 in accordance with JIS Z 2244. Equipment: Mitutoyo MH-200 Micro Vickers Hardness Tester Condition: Test load 0.03N

[0066] [Table 1]

[0067] Cross-sections of sintered alloys 1-3 and iron-based sintered alloy 1 were observed using a scanning electron microscope (SEM). A cross-sectional image of iron-based sintered alloy 1 is shown in Figure 1, and cross-sectional images of sintered alloy 1 are shown in Figure 2. In sintered alloy 1, pearlite was observed, and a layered structure of carbides and the matrix was observed (region B in Figure 2). In sintered alloy 2, the amount of pearlite decreased, and ferrite was observed, with carbide particles finely dispersed in the ferrite. In sintered alloy 3, a decrease in pores was observed while maintaining the microstructure of sintered alloy 2. In iron-based sintered alloy 1, carbide particles were finely dispersed in the matrix with reduced pores (region A in Figure 1).

[0068] <Number of metal carbide particles with a particle size of 0.2 to 2.0 μm> In iron-based sintered alloy 1, the number of metal carbides with a particle size of 0.2 to 2.0 μm was measured and found to be 15,900.

[0069] The number of metal carbides with a particle size of 0.2 to 2.0 μm was determined by cutting an iron-based sintered alloy, mirror-polishing the cross-section, observing the cross-section with a scanning electron microscope (SEM), and counting the number of particles observed within a 100 μm × 100 μm area. Image analysis software (WinROOF, manufactured by Mitani Corporation) was used for particle counting. The particle size of the carbides was defined as the equivalent diameter of a circle.

[0070] <Surface pressure fatigue strength> For iron-based sintered alloy 1, the surface pressure fatigue strength was measured by a three-ball pitting test (temperature: room temperature, rotation speed: 600 min). -1 (Oil used: MTF-III, ball material: SUJ-2). The surface pressure fatigue strength was as follows. Iron-based sintered alloy 1:600 ​​GPa

Claims

1. The overall composition consists of Cr: 1-5% and C: 0.6-1.5% by mass, with the remainder being Fe and unavoidable impurities, and comprises a matrix and metal carbides dispersed in the matrix. An iron-based sintered alloy in which the number of metal carbides with a particle size of 0.2 to 2.0 μm is 10,000 to 25,000 in the range of 100 μm × 100 μm.

2. The iron-based sintered alloy according to claim 1, wherein the overall composition contains, in mass%, at least one selected from the group consisting of Mo, V, W, Nb, and Ti in a total amount of 2% or less.

3. The iron-based sintered alloy according to claim 1 or 2, wherein the Rockwell hardness is 40 to 70 on the HRC scale.

4. Using a raw material powder containing carbon powder such that the mass percentage of C is 0.6 to 1.5% in an iron alloy powder consisting of 1 to 5% Cr by mass and the remainder being Fe and unavoidable impurities, the raw material powder is molded at a pressure of 500 MPa or higher to obtain a molded body. The molded body is sintered to obtain a sintered alloy, and The sintered alloy is quenched, A method for manufacturing iron-based sintered alloys.

5. A method for producing an iron-based sintered alloy according to claim 4, further comprising forging the sintered alloy before quenching.

6. The density of the sintered alloy to be quenched is 7.0 g / cm³. ３ The method for producing an iron-based sintered alloy according to claim 4 or 5.

7. A method for producing an iron-based sintered alloy according to claim 4 or 5, wherein the sintered alloy subjected to quenching comprises a matrix and metal carbides dispersed in the matrix, and the number of particles of the metal carbides having a particle size of 0.2 to 2.0 μm is 10,000 to 25,000 in the range of 100 μm × 100 μm.

8. A method for producing an iron-based sintered alloy according to claim 4 or 5, wherein the sintered alloy subjected to quenching has a Rockwell hardness of 50 to 100 on the HRB scale.

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