JP7501713B2 - Iron-based sintered member, iron-based powder mixture, and method for manufacturing iron-based sintered member - Google Patents

Description

Embodiments of the present invention relate to an iron-based sintered member, an iron-based powder mixture, and a method for manufacturing an iron-based sintered member.

Iron-based sintered components manufactured using powder metallurgy are used in a variety of applications because they can produce iron-based components with special metal structures that cannot be obtained using melting methods.

One of the applications requiring a special metal structure is an iron-based sintered member for a bearing cap. A bearing cap is a member that holds a bearing that rotatably supports the crankshaft of an internal combustion engine. The bearing cap, for example, has an approximately arch-shaped appearance and is used by being fastened and fixed to an aluminum alloy cylinder block with bolts. FIG. 1 is a schematic side view showing a state in which a bearing cap is attached to a cylinder block of an automobile engine. A rectangular recess 2a that positions and houses the bearing cap 1, and a semicircular recess 2b that houses the bearing 3 are formed in the cylinder block 2. The bearing cap 1 has a semicircular recess that houses the bearing 3 and has an arch-shaped outer shape. The bearing cap 1 is positioned at the fitting portion of the rectangular recess 2a of the cylinder block 2 and is fixed with bolts 4, 4. The iron-based sintered member applied to such a bearing cap 1 is required to hold the crankshaft 5, and therefore mechanical strength is required. In addition, in order to make each semicircular arc of the bearing cap 1 and the aluminum alloy cylinder block 2 concentric when bolted together, the inner circumference of the circle formed by each semicircular arc is usually machined. Therefore, the iron-based sintered material used in the bearing cap is required to have the same machinability as the aluminum alloy.

As a method for improving the machinability of iron-based sintered components, a common method is to disperse manganese sulfide or graphite, which has a solid lubricating effect, into the pores of the iron-based sintered component. However, iron-based sintered components manufactured using manganese sulfide tend to have low mechanical strength because the manganese sulfide powder added to the raw iron-based powder mixture inhibits bonding between the iron-based powders due to diffusion. In addition, since graphite easily diffuses into the iron matrix and disappears, in order to obtain an iron-based sintered component with graphite dispersed in the pores, it is necessary to sinter at a temperature at which diffusion of graphite into the iron matrix is difficult, for example, less than 1,000°C. However, at a temperature at which diffusion of graphite is difficult to occur, bonding between the iron-based powders due to diffusion is poor, and the iron-based sintered component tends to have low mechanical strength.

Under these circumstances, a method was developed in which sintering is performed in the temperature range of 1,000 to 1,200°C, which is the sintering temperature for normal iron-based sintered components, promoting bonding through diffusion between iron-based powders to obtain high mechanical strength, while suppressing the diffusion of graphite into the iron matrix and dispersing graphite in the iron matrix to improve machinability (see Patent Document 1).

Patent Document 1 relates to a powder mixture for iron-based sintered materials with excellent machinability, which contains 0.01 to 1.0 weight percent boron oxide powder and 0.1 to 2.0 weight percent graphite powder, and a powder mixture for iron-based sintered materials with excellent machinability, which contains 0.1 to 2.5 weight percent boron nitride powder containing 10 to 40 weight percent boron oxide and 0.1 to 2.0 weight percent graphite powder.

Japanese Patent Application Laid-Open No. 09-241701

Patent Document 1 uses boron oxide or boron nitride containing boron oxide to suppress the diffusion of graphite into the iron matrix, but boron oxide powder is expensive, and boron nitride containing boron oxide is also expensive, although not as expensive as boron oxide powder. For this reason, the iron-based sintered parts obtained by Patent Document 1 have high raw material costs, and an inexpensive method for improving machinability is desired.

In view of the above, an embodiment of the present invention aims to provide an inexpensive iron-based sintered member that combines high mechanical strength and excellent machinability. Another embodiment of the present invention aims to provide an inexpensive iron-based powder mixture that can produce an iron-based sintered member that combines high mechanical strength and excellent machinability. Still another embodiment of the present invention aims to provide an inexpensive manufacturing method that can produce an iron-based sintered member that combines high mechanical strength and excellent machinability.

The present invention includes various embodiments. Examples of embodiments are listed below. The present invention is not limited to the following embodiments.

One embodiment of the present invention relates to an iron-based sintered member having a metal structure including an iron matrix and pores dispersed in the iron matrix, the iron matrix containing 0.1 to 3.5 mass % of carbon, aluminum, sodium, and sulfur, wherein aluminum, sodium, and sulfur are concentrated on a surface of the iron matrix in contact with the pores, and the pores contain free carbon.
The iron-based sintered component preferably contains 0.1 to 3.5 mass % of carbon, 0.5 to 6.0 mass % of copper, aluminum, sodium, and sulfur, with the remainder being iron and unavoidable impurities.

Another embodiment of the present invention relates to an iron-based sintered member having a metal structure including an iron matrix and pores dispersed in the iron matrix, the iron matrix containing 0.1 to 3.5 mass% of carbon, aluminum, sodium, and sulfur, and wherein the aluminum concentration is 0.1 mass% or more, the sodium concentration is 0.05 mass% or more, and the sulfur concentration is 0.05 mass% or more on the surface of the iron matrix in contact with the pores.
The iron-based sintered component preferably contains 0.1 to 3.5 mass % of carbon, 0.5 to 6.0 mass % of copper, aluminum, sodium, and sulfur, with the remainder being iron and unavoidable impurities.

Another embodiment of the present invention relates to an iron-based powder mixture that contains at least one selected from the group consisting of iron powder and iron alloy powder, 0.1 to 3.5 mass % of graphite powder, and 0.05 to 1.5 mass % of a mixed powder containing an aluminum salt of a higher fatty acid and sodium sulfate, wherein the content of the sodium sulfate is 0.5 mass % or more based on the mass of the mixed powder.
The higher fatty acid preferably includes at least one selected from the group consisting of stearic acid, 12-hydroxystearic acid, lauric acid, myristic acid, palmitic acid, ricinoleic acid, and behenic acid.
The iron-based powder mixture preferably contains 0.1 to 3.5 mass % graphite powder, 0 to 10 mass % copper powder, 0.05 to 1.5 mass % of the mixed powder, and the remainder being at least one selected from the group consisting of iron powder and iron alloy powder, and unavoidable impurities.

Another embodiment of the present invention relates to a method for producing an iron-based sintered member, the method including: filling a die with the iron-based powder mixture, performing compression molding to obtain a green compact, and sintering the green compact in a non-oxidizing gas atmosphere at a temperature equal to or higher than a diffusion temperature of carbon into the iron matrix.
The temperature at which carbon diffuses into the iron matrix is preferably 1,000 to 1,200°C.

According to an embodiment of the present invention, it is possible to provide an inexpensive iron-based sintered member that combines high mechanical strength and excellent machinability. Also, according to another embodiment of the present invention, it is possible to provide an inexpensive iron-based powder mixture that can produce an iron-based sintered member that combines high mechanical strength and excellent machinability. Furthermore, according to another embodiment of the present invention, it is possible to provide an inexpensive manufacturing method that can produce an iron-based sintered member that combines high mechanical strength and excellent machinability.

FIG. 1 is a schematic side view showing an example of a state in which a bearing cap is attached to a cylinder block of an automobile engine. FIG. 2 is a scanning electron micrograph (SEM) of the mixed powder used in the examples and a mapping image (magnification: 200 times) showing the element distribution by energy dispersive X-ray analysis (EDS). FIG. 3 is an optical microscope photograph (magnification: 200 times) of the cross section of the sintered bodies obtained in the examples and comparative examples. FIG. 4 is a mapping image (magnification: 3,000 times) showing the element distribution on the surface of the sintered body obtained in the example, measured by an electron probe microanalyzer (EPMA). FIG. 5 is a schematic diagram showing the lathe processing steps carried out in the examples and comparative examples. FIG. 6 is a schematic perspective view showing a cutting tool used in the examples and comparative examples. FIG. 7 is a graph showing the evaluation results of the machinability of the sintered bodies obtained in the examples and comparative examples.

An embodiment of the present invention will be described below, but the present invention is not limited to the following embodiment.
[Iron-based sintered material]
An iron-based sintered member according to one embodiment of the present invention has a metal structure including an iron matrix and pores dispersed in the iron matrix. The iron matrix is formed from an iron-based powder, and the pores are formed by remaining gaps between the iron-based powder. The iron matrix is preferably a pearlite structure or a mixed structure of ferrite and pearlite. The iron-based sintered member contains free carbon in the pores that has not diffused into the iron matrix.

The iron-based sintered component contains iron, 0.1 to 3.5 mass% carbon, aluminum, sodium, and sulfur. The carbon content is based on the mass of the iron-based sintered component, and can be measured, for example, according to the combustion-infrared absorption method (JIS G 1211-3:2013).

In an iron-based sintered component, if the carbon content is 0.1 mass% or more, an excellent effect of improving the machinability of the iron-based sintered component can be obtained. On the other hand, if the carbon content is 3.5 mass% or less, it is possible to prevent an excessive increase in pores, and the mechanical strength of the iron-based sintered component can be maintained. For this reason, the carbon content in the iron-based sintered component is set to 0.1 to 3.5 mass%. The carbon content is preferably 0.3 to 2.5 mass%, more preferably 0.5 to 1.5 mass%, and even more preferably 0.6 to 1.0 mass%.

In general, iron powder and/or iron alloy powder (in this specification, at least one selected from the group consisting of iron powder and iron alloy powder may be referred to as "iron-based powder") used in the manufacture of iron-based sintered components contains aluminum, sodium, and sulfur as impurities that are mixed in during steelmaking. As an example, the aluminum, sodium, and sulfur contained in the iron-based powder as impurities are each about 0.01% by mass based on the mass of the iron-based powder.

According to one embodiment, the iron-based sintered member has concentrated aluminum, sodium, and sulfur on the surface of the iron matrix in contact with the pores (in this specification, the surface of the iron matrix in contact with the pores may be referred to as the "pore surface" or the "interface between the pores and the iron matrix"). The presence of concentrated aluminum, sodium, and sulfur can be confirmed by surface analysis of the surface of the iron-based sintered member using, for example, an electron probe microanalyzer (EPMA). "The presence of concentrated aluminum, sodium, and sulfur on the surface of the iron matrix in contact with the pores" means that the concentrations of aluminum, sodium, and sulfur are higher on the "surface of the iron matrix in contact with the pores" than on the "surface of the iron matrix not in contact with the pores". Specifically, the amounts of aluminum, sodium, and sulfur detected on the "surface of the iron matrix in contact with the pores" by surface analysis are higher than the amounts of aluminum, sodium, and sulfur detected on the "surface of the iron matrix not in contact with the pores", respectively, and can be confirmed by a mapping image obtained by surface analysis. To confirm, an electron beam microanalyzer (for example, Shimadzu Corporation's "EPMA-1600W") can be used, and the conditions are, for example, an acceleration voltage of 15 kV and a sample current of 100 nA.

According to one embodiment, the iron-based sintered member has an aluminum concentration of 0.1% by mass or more, a sodium concentration of 0.05% by mass or more, and a sulfur concentration of 0.05% by mass or more on the pore surface. The concentrations of aluminum, sodium, and sulfur can be measured by surface analysis of the pore surface on the surface of the iron-based sintered member, for example, using an electron probe microanalyzer (EPMA). The aluminum concentration is preferably 0.1 to 1.0% by mass, more preferably 0.12 to 0.5% by mass, and even more preferably 0.15 to 0.3% by mass. The sodium concentration is preferably 0.05 to 1.0% by mass, more preferably 0.15 to 0.6% by mass, and even more preferably 0.3 to 0.5% by mass. The sulfur concentration is preferably 0.05 to 0.5% by mass, more preferably 0.08 to 0.3% by mass, and even more preferably 0.15 to 0.25% by mass. These contents (mass%) are based on the mass of the iron-based sintered member in the area where the pore surface was analyzed. The "area where the pore surface was analyzed" can be a visual field of 3,000 times magnification that includes the pore surface. The pore surface area analysis can be performed on any part of the surface of the iron-based sintered member. The contents can be values measured using an electron beam microanalyzer (for example, "EPMA-1600W" manufactured by Shimadzu Corporation) under conditions of an acceleration voltage of 15 kV and a sample current of 100 nA. Preferably, the iron-based sintered member has at least one location where the aluminum concentration is 0.1 mass% or more, the sodium concentration is 0.05 mass% or more, and the sulfur concentration is 0.05 mass% or more in the visual field of 3,000 times magnification using EPMA under the above conditions.

According to a preferred embodiment, the iron-based sintered member has aluminum, sodium, and sulfur concentrated on the pore surfaces, and the aluminum concentration on the pore surfaces is 0.1 mass% or more, the sodium concentration is 0.05 mass% or more, and the sulfur concentration is 0.05 mass% or more.

The concentrations of aluminum, sodium, and sulfur contained in the iron matrix are not particularly limited, but are preferably less than 0.05 mass%, more preferably less than 0.03 mass%, and even more preferably 0.0 mass%, based on the mass of the iron matrix. The concentrations of aluminum, sodium, and sulfur can be measured, for example, by surface analysis of the parts of the surface of the iron matrix that are not in contact with the pores using energy dispersive X-ray analysis (EDS) at a 1,000x magnification using a scanning electron microscope (SEM). Surface analysis of the iron matrix that is not in contact with the pores may be performed on any part of the surface of the iron-based sintered member.

In iron-based sintered components, aluminum, sodium, and sulfur at the interface between the pores and the iron matrix are believed to exist in the form of oxides and/or composite oxides. It is presumed that the aluminum, sodium, and sulfur present at the interface enable the iron-based sintered components to retain carbon within the pores, resulting in excellent machinability.

From the viewpoint of obtaining excellent machinability, the free carbon content in the iron-based sintered member is preferably 0.1 mass% or more, more preferably 0.15 mass% or more, even more preferably 0.3 mass% or more, and particularly preferably 0.35 mass% or more, based on the mass of the iron-based sintered member. On the other hand, from the viewpoint of obtaining sufficient strength, the free carbon content is preferably 0.6 mass% or less, more preferably 0.5 mass% or less, and even more preferably 0.45 mass% or less, based on the mass of the iron-based sintered member. The free carbon content is the content based on the mass of the iron-based sintered member, and can be measured, for example, according to the free carbon determination method (JIS G 1211-5:2011).

The iron-based sintered member may contain copper. In a preferred embodiment, the iron-based sintered member contains iron, 0.1 to 3.5 mass% carbon, 0.5 to 6.0 mass% copper, aluminum, sodium, and sulfur. More preferably, the iron-based sintered member contains 0.1 to 3.5 mass% carbon, 0.5 to 6.0 mass% copper, aluminum, sodium, and sulfur, with the remainder being iron and inevitable impurities. The copper content is based on the mass of the iron-based sintered member, and can be measured, for example, according to the Iron and Steel - ICP Atomic Emission Spectroscopic Analysis Method (JIS G 1258:2014).

The method for producing the iron-based sintered member is not particularly limited. A method for producing the iron-based sintered member using an iron-based powder mixture described below, or a method for producing the iron-based sintered member using a method described below is preferred.

[Iron-based powder mixture]
An iron-based powder mixture according to one embodiment of the present invention contains at least one selected from the group consisting of iron powder and iron alloy powder, 0.1 to 3.5 mass% graphite powder, and a total of 0.05 to 1.5 mass% of an aluminum salt of a higher fatty acid and sodium sulfate. The content of sodium sulfate is 0.5 mass% or more based on the total mass of the aluminum salt of a higher fatty acid and sodium sulfate. As the aluminum salt of a higher fatty acid and sodium sulfate, a mixed powder containing the aluminum salt of a higher fatty acid and sodium sulfate can be used.
An iron-based powder mixture according to one embodiment of the present invention contains at least one selected from the group consisting of iron powder and iron alloy powder, 0.1 to 3.5 mass % of graphite powder, and 0.05 to 1.5 mass % of a mixed powder containing an aluminum salt of a higher fatty acid and sodium sulfate. The content of sodium sulfate is 0.5 mass % or more based on the mass of the mixed powder. The iron-based powder mixture contains a mixed powder containing an aluminum salt of a higher fatty acid and sodium sulfate as an effective alternative to boron oxide powder.

(Mixed powder)
The mixed powder contains an aluminum salt of a higher fatty acid and sodium sulfate. As the aluminum salt of a higher fatty acid, a commonly used molding lubricant powder can be used. From the viewpoint of obtaining a sufficient effect as a lubricant, the carbon number of the higher fatty acid is preferably 12 or more, more preferably 14 or more, and even more preferably 16 or more. Moreover, from the viewpoint of improving machinability and obtaining a high density of a green compact, the carbon number of the higher fatty acid is preferably 28 or less, more preferably 26 or less, and even more preferably 22 or less.

Examples of aluminum salts of higher fatty acids include powders of aluminum stearate, aluminum 12-hydroxystearate, aluminum laurate, aluminum myristate, aluminum palmitate, aluminum ricinoleate, and aluminum behenate. Aluminum stearate is preferred. The aluminum salts of higher fatty acids may be used alone or in combination. Aluminum salts of higher fatty acids are extremely inexpensive compared to boron oxide powder or boron nitride powder containing boron oxide, and therefore raw material costs can be significantly reduced.

The content of sodium sulfate is 0.5% by mass or more, preferably 0.5 to 10% by mass, based on the mass of the mixed powder. When the content of sodium sulfate is 0.5% by mass or more, aluminum, sodium, and sulfur can be concentrated on the pore surface of the iron-based sintered member, and/or appropriate amounts of aluminum, sodium, and sulfur can be present on the pore surface. On the other hand, when the content of sodium sulfate is 10% by mass or less, the aluminum, sodium, and sulfur present on the pore surface of the iron-based sintered member is not excessive, and sintering can be progressed without inhibiting the bonding between the iron-based powders. The content of sodium sulfate is preferably 0.5% by mass or more, more preferably 1% by mass or more, and even more preferably 1.5% by mass or more. In addition, the content of sodium sulfate is preferably 5% by mass or less, more preferably 3% by mass or less, and even more preferably 2% by mass or less.

The mixed powder contains the aluminum salt of a higher fatty acid and inevitable impurities as the remainder. The content of the aluminum salt of a higher fatty acid is preferably 95% by mass or more based on the mass of the mixed powder. The mixed powder is obtained by mixing the aluminum salt powder of a higher fatty acid with sodium sulfate powder. It is also possible to use, as the mixed powder, aluminum salt powder of a higher fatty acid containing a small amount of sodium sulfate, which is commercially available as "aluminum salt of a higher fatty acid."

The content of the mixed powder is 0.05 to 1.5 mass% based on the mass of the iron-based powder. If the content of the mixed powder is 0.05 mass% or more, aluminum, sodium, and sulfur can be concentrated on the pore surface of the iron-based sintered member, and/or appropriate amounts of aluminum, sodium, and sulfur can be present on the pore surface. On the other hand, if the content of the mixed powder is 1.5 mass% or less, the aluminum, sodium, and sulfur present on the pore surface of the iron-based sintered member is not excessive, and sintering can be progressed without inhibiting the bonding between the iron-based powders. The content of the mixed powder is preferably 0.1 mass% or more, more preferably 0.5 mass% or more. Also, the content of the mixed powder is preferably 1.2 mass% or less, more preferably 1 mass% or less.

Generally, powders of metal salts of higher fatty acids are used as molding lubricants added to raw powders, and are inexpensive. In addition, metal salts of higher fatty acids are decomposed and removed during the temperature rise process during sintering, so they do not hinder the diffusion bonding of iron-based powders by sintering. When aluminum salts of higher fatty acids are used as the metal salts of such higher fatty acids, and sodium sulfate is used, aluminum, sodium, and sulfur generated by the decomposition of the aluminum salts of higher fatty acids during the temperature rise process remain concentrated at the interface between the iron matrix and the pores. The remaining aluminum, sodium, and sulfur act as a barrier to prevent the diffusion of carbon from the graphite powder, suppressing the diffusion of carbon into the iron matrix, and it is believed that the iron-based sintered part obtained after sintering contains free carbon in the pores.

(Iron-based powder)
The iron-based powder includes at least one selected from the group consisting of iron (Fe) powder and iron alloy powder. Elements included in the iron alloy include copper (Cu), nickel (Ni), chromium (Cr), molybdenum (Mo), vanadium (V), manganese (Mn), titanium (Ti), aluminum (Al), carbon (C), etc. As the iron-based powder, one type of powder may be used alone, or two or more types of powder may be mixed and used.

(Graphite powder)
The iron-based powder mixture contains 0.1 to 3.5 mass% of graphite powder based on the mass of the iron-based powder. When the content of graphite powder is 0.1 mass% or more, the amount of free carbon dispersed in the pores of the iron-based sintered member obtained after sintering is sufficient, and the effect of improving machinability is obtained. On the other hand, when the amount of graphite powder added is 3.5 mass% or less, the amount of pores in the iron-based sintered member obtained after sintering is not excessive, and the mechanical strength of the iron-based sintered member is maintained. The diffusion of the graphite powder into the iron matrix is suppressed by aluminum, sodium, and sulfur, and most of the graphite powder remains in the pores as free carbon. A part of the graphite powder diffuses into the iron matrix and contributes to improving the mechanical strength of the iron matrix by making the structure of the iron matrix into pearlite or a mixed structure of pearlite and ferrite, and as a result, the mechanical strength of the iron-based sintered member is improved.

(any powder)
The iron-based powder mixture may contain any powder. Examples of the optional powder include metal powder and/or metal alloy powder other than the iron-based powder, molding lubricant powder, and the like. The optional powder can be used to modify or strengthen the sintered body. The optional powder can be appropriately selected according to the desired characteristics of the sintered body. The iron-based powder mixture preferably contains at least one selected from the group consisting of copper (Cu) powder and copper alloy powder, and more preferably contains copper powder. When the iron-based powder mixture contains at least one selected from the group consisting of copper (Cu) powder and copper alloy powder, the content is preferably 0.5 mass% or more based on the mass of the iron-based powder. The content is preferably 10 mass% or less based on the mass of the iron-based powder, and more preferably 6 mass% or less.

In addition, the iron-based powder mixture used in the pressing method in the molding process usually contains a molding lubricant powder, but a powder of an aluminum salt of a higher fatty acid may be used alone as the molding lubricant powder, or a powder of a higher fatty acid salt other than an aluminum salt, wax, or other commonly used molding lubricant powder may be used in combination.

(composition)
According to one embodiment, the iron-based powder mixture may be any powder mixture containing iron powder and/or iron alloy powder as a main component, 0.1-3.5% by mass of graphite powder, and 0.05-1.5% by mass of mixed powder.

For example, the iron-based powder mixture contains at least one selected from the group consisting of iron powder and iron alloy powder, 0.1 to 3.5 mass% graphite powder, 0 to 10 mass% copper powder, and 0.05 to 1.5 mass% of the mixed powder. Preferably, the iron-based powder mixture contains 0.1 to 3.5 mass% graphite powder, 0 to 10 mass% copper powder, 0.05 to 1.5 mass% of the mixed powder, and the remainder is at least one selected from the group consisting of iron powder and iron alloy powder, and unavoidable impurities. The copper powder content is preferably 0.5 to 6 mass%.

Iron-based powder mixtures can be widely used in the manufacture of structural sintered components. They are particularly suitable for the manufacture of iron-copper-carbon sintered components.

[Method of manufacturing iron-based sintered member]
According to one embodiment, the method for producing an iron-based sintered member includes filling a die with the above-mentioned iron-based powder mixture, compressing and molding the mixture to obtain a green compact (molding step), and sintering the green compact in a non-oxidizing gas atmosphere at a temperature equal to or higher than the diffusion temperature of carbon into the iron matrix (sintering step). The method for producing an iron-based sintered member may further include any optional step.

(Molding process)
In the molding step, the iron-based powder mixture is filled into a desired die and compressed to obtain a green compact. There is no particular limitation on the molding method, and for example, a pressing method can be applied. In a general pressing method, the raw material powder is filled into a die cavity of a die and compressed by upper and lower punches, and the obtained green compact is removed from the die. Since the iron-based powder mixture contains an aluminum salt of a higher fatty acid that functions as a molding lubricant powder, it is possible to prevent the green compact from galling the die cavity when the green compact is removed from the die cavity. It is preferable that the aluminum salt of a higher fatty acid is uniformly dispersed inside the green compact.

(Sintering process)
In the sintering process, the obtained green compact is sintered in a sintering furnace in a predetermined atmosphere at a predetermined temperature. The mechanism by which excellent machinability and strength are obtained is not clear, but is thought to be as follows. That is, when the green compact is sintered at a temperature equal to or higher than the diffusion temperature of carbon into the iron matrix, the powder of aluminum salt of higher fatty acid and sodium sulfate decompose during the temperature rise process (200 to 600°C) for sintering. At least a part of aluminum, which is a metal component, and sodium and sulfur generated by the decomposition of sodium sulfate are adsorbed on the surface of the iron-based powder, and are combined with oxygen generated by the desorption of moisture adsorbed on the surface of the iron-based powder at a high temperature (about 500°C) or oxygen generated by the reduction of oxygen bound to the surface of the iron-based powder at a high temperature (about 900 to 1,000°C), to become oxides and/or composite oxides of aluminum, sodium, and sulfur, which remain on the surface of the iron matrix after the iron-based powder is bound, i.e., on the interface between the iron matrix and the pores. It is presumed that the oxide and/or composite oxide coats the surface of the iron matrix prior to the diffusion of carbon from the graphite powder to the iron matrix, and as a result, the coating functions as a barrier to prevent the diffusion of carbon from the graphite powder in the pores to the iron matrix.

During the temperature rise in the sintering process, a small amount of carbon begins to diffuse from the graphite powder to the iron-based powder and/or the iron matrix at about 850°C, and as the temperature rises, the amount of carbon diffused increases, and at 1,000 to 1,200°C, carbon diffusion progresses significantly. When a compact containing an aluminum salt of a higher fatty acid and sodium sulfate is sintered, oxides or composite oxides of aluminum, sodium, and sulfur are formed at the interface between the iron-based powder and/or the iron matrix and the pores at the temperature at which carbon diffuses. The oxides and/or composite oxides function as a barrier to the diffusion of carbon from the graphite powder to the iron-based powder (and the iron matrix), and diffusion bonding of the iron-based powder progresses, and even at sintering temperatures of 1,000 to 1,200°C at which carbon diffuses significantly, the diffusion of carbon from the graphite powder to the iron-based powder (and the iron matrix) is suppressed. Therefore, the metal structure of the iron-based sintered member obtained after sintering has free carbon remaining and dispersed in the pores. In addition, at this time, the iron-based powder particles are sufficiently bonded together by diffusion, and sintering proceeds sufficiently. As a result, the resulting sintered body has high mechanical strength due to the sufficient sintering, and at the same time, it is highly machinable because carbon is dispersed in the pores.

When the surface metal structure of an iron-based sintered component is analyzed with an electron probe microanalyzer (EPMA), the amount of aluminum, sodium, and sulfur detected on the pore surface is greater than the amount of aluminum, sodium, and sulfur detected on the surface of the iron matrix that is not in contact with the pores. Iron-based powder, which is one of the raw materials, contains aluminum, sodium, and sulfur as unavoidable impurities during steelmaking. The aluminum, sodium, and sulfur detected on the surface of the iron matrix that is not in contact with the pores are thought to originate from the unavoidable impurities contained in the iron-based powder, and the aluminum, sodium, and sulfur detected on the pore surface are thought to originate from aluminum salts of higher fatty acids and sodium sulfate.

When an iron-based powder mixture that does not contain sodium sulfate and contains a powder of an aluminum salt of a higher fatty acid is used as the raw powder, aluminum does not remain at the interface between the iron matrix and the pores. Although the details are unclear, in the embodiment of the present invention, it is believed that the coexistence of aluminum, sodium, and sulfur results in these elements being present at the interface between the iron matrix and the pores. The reasons explained above are speculations and do not limit the present invention.

The sintering temperature is equal to or higher than the carbon diffusion temperature, and is preferably 1,000 to 1,200°C. In general, when sintering is performed at a temperature of 850°C or higher, at which carbon starts to diffuse from the graphite powder, and at a temperature less than 1,000°C, the diffusion of carbon from the graphite powder into the iron matrix can be suppressed. However, when sintering is performed at 1,000°C or higher, the bonding between the iron-based powders due to diffusion progresses sufficiently, and the mechanical strength of the resulting iron-based sintered part is increased. On the other hand, when the sintering temperature is 1,200°C or lower, wear on the sintering furnace can be suppressed. The gas atmosphere for the heat treatment is a non-oxidizing gas atmosphere such as nitrogen gas.

(Optional step)
Optional steps that may be included in the method for producing an iron-based sintered member include a cutting step for cutting the sintered body into a desired shape, a mixing step for mixing powders, a degreasing step for removing organic matter, a recompressing step for compressing the sintered body, and a surface treatment step for treating the surface of the sintered body. For example, the cutting process may be a turning process, a milling process, or both. Materials for cutting tools include cermets, ceramics, cemented carbide, high-speed tool steel, diamond sintered bodies, cBN sintered bodies, and the like.

The embodiments of the present invention will be described in detail with reference to examples. The embodiments of the present invention are not limited to the following examples.

<Preparation of iron-based powder mixture>
[Example 1]
Iron powder, copper powder, graphite powder, and mixed powder (A) containing aluminum stearate and sodium sulfate were charged into a 10 kg V-type mixer so as to have the content ratio shown in Table 1, and mixed for 30 minutes to obtain an iron-based powder mixture.

[Example 2]
An iron-based powder mixture was obtained in the same manner as in Example 1, except that the mixed powder (A) was changed to a mixed powder (B) containing aluminum stearate and sodium sulfate.

[Comparative Example 1]
An iron-based powder mixture was obtained in the same manner as in Example 1, except that the mixed powder (A) was changed to zinc stearate powder.

The powders used in Examples 1 and 2 and Comparative Example 1 are shown below.
Iron powder: Water atomized iron powder with a particle size of 180 μm or less Copper powder: Electrolytic copper powder with a particle size of 150 μm or less Graphite powder: Natural graphite powder with an average particle size of 20 μm Mixed powder (A): See Table 1 below Mixed powder (B): See Table 1 below Zinc stearate powder: Average particle size 13 μm

The content of sodium sulfate in the mixed powder was measured according to the following procedure.
(1) Approximately 1.0 g of the mixed powder is precisely weighed out into a magnetic crucible, and the powder is carbonized over a small flame so as not to scatter, and then completely incinerated in an electric furnace at 950 to 1,000°C.
(2) After cooling in a desiccator for 30 minutes, accurately weigh the ashed material, place it in a 300 ml beaker, add 200 ml of distilled water, and boil for 30 minutes.
The mixture was filtered using No. 5A quantitative filter paper (manufactured by ADVANTEC Group), and the precipitate was washed three times with 20 ml of distilled water while pouring it into a beaker.
(3) The precipitate remaining on the filter paper is placed in a crucible together with the filter paper, dried, and re-ashed.
(4) After cooling in a desiccator for 30 minutes, the ashed material is weighed accurately.
(5) The content (mass%) of sodium sulfate (Na ₂ SO ₄ ) is calculated using the following formula.
Sodium sulfate content (mass%) =
(Mass of ash material after first ashing (g)−Mass of ash material after second ashing (g))/Mass of mixed powder (g)×100

Figure 2 shows a scanning electron microscope (SEM) photograph of the mixed powder (A) and the results of evaluating the distribution of aluminum, sodium, and sulfur by energy dispersive X-ray analysis (EDS). In Figure 2, the upper part is an SEM photograph, and the lower part is an image showing the element distribution by EDS (magnification: 200 times).

Table 2 shows the content ratio (mass%) of each powder relative to the mass of iron powder.

<Preparation of sintered body>
[Example 3]
A sintered body was produced using the iron-based powder mixture obtained in Example 1 according to the following method.

(Preparation of sintered body)
Compaction Step The iron-based powder mixture obtained in Example 1 was filled into a die, and the pressure was adjusted so that the density of the compact was 6.75 g/cm ³ , to obtain a compact having an outer diameter of 40 mm and a total length of 20 mm.
In the compacting step, the green compact could be removed from the die in a good condition, and the green compact did not suffer from defects such as galling, chipping, etc. The mixed powder (A) provided a sufficient lubricating effect.

The green compact was heated in a sintering furnace under a non-oxidizing atmosphere ( _N2 + 5% by volume _H2 atmosphere) at 1,130°C for 20 minutes to obtain a sintered body (iron-based sintered member). An optical microscope photograph of the cross section of the obtained sintered body is shown in Figure 3 (magnification 200 times).

(Evaluation of sintered body)
(1) Concentrations of aluminum, sodium, and sulfur The surfaces of the obtained sintered bodies were subjected to area analysis using an electron probe microanalyzer ("EPMA-1600W" manufactured by Shimadzu Corporation, measurement conditions: acceleration voltage 15 kV, sample current 100 nA). The concentrations of aluminum, sodium, and sulfur on the surface of the iron matrix in contact with the pores are shown in Table 3.

(2) Carbon Content The carbon content of the sintered body was measured in accordance with JIS G 1211-3: 2013. The results are shown in Table 3.
(3) Amount of free carbon The amount of free carbon contained in the sintered body was measured according to JIS G 1211-5: 2011. The results are shown in Table 3.

[Example 4 and Comparative Example 2]
A sintered body was produced in the same manner as in Example 3, except that the iron-based powder mixture obtained in Example 1 was changed to the iron-based powder mixture obtained in Example 2 or Comparative Example 1. An optical microscope photograph of a cross section of the obtained sintered body is shown in Figure 3.
In addition, in the same manner as in Example 3, (1) the concentrations of aluminum, sodium, and sulfur, (2) the carbon content, and (3) the amount of free carbon were measured. The results are shown in Table 3. In addition, for the sintered body of Example 4, an image showing the concentration distribution of aluminum (Al), sodium (Na), sulfur (S), carbon (C), and oxygen (O) is shown in FIG. 4 (magnification: 3,000 times). Note that the dark areas in the SEM image are pores. The bright areas in the image are the locations where each element was detected. The thickness of the area where aluminum, sodium, and sulfur were concentrated was about 2 μm.

<Evaluation of machinability and strength>
The sintered bodies of Examples 3 and 4 and Comparative Example 2 were cut and the wear amount of the cutting tool was confirmed to evaluate the machinability of the sintered bodies. In addition, the surface hardness of the sintered bodies of Examples 3 and 4 and Comparative Example 2 was measured to evaluate the strength of the sintered bodies.

(Machinability of sintered body)
The end face of the sintered body was cut by lathe processing. The lathe processing was carried out under the following conditions. Fig. 5 is a schematic diagram showing the lathe processing process. In Fig. 5, 11 indicates a cutting tool, 14 indicates a holder, 15 indicates a sintered body, Fig. 5(a) is a schematic side view, and Fig. 5(b) is a schematic front view. Fig. 6 is a schematic perspective view showing the cutting tool after use in cutting. In Fig. 6, 11 indicates the cutting tool, 12 indicates the rake face, 13 indicates the flank face, 13a indicates the worn portion of the flank face, and 13b indicates the width of the worn portion.

Cutting machine: NC lathe (Numerical Control lathe)
Cutting tool: Cermet throw-away tip (material: NX2525, Mitsubishi Materials Corporation "TNMG160404")
Cutting speed: 350 m/min
Feed: 0.03 mm/rev
Removal allowance: 0.10 mm

Figure 7 is a graph showing the relationship between cutting distance and wear amount on the flank. As shown in Figure 7, the sintered body produced using a mixed powder containing an aluminum salt of a higher fatty acid and sodium sulfate was able to significantly reduce wear on the cutting tool and had excellent machinability. By using a mixed powder containing an aluminum salt of a higher fatty acid and sodium sulfate, deterioration of the cutting tool can be prevented, and the manufacturing cost of the iron-based sintered member can be reduced.

(Strength of sintered body)
The Vickers hardness of the surface of the sintered body was measured in accordance with JIS Z 2244:2009.
The measurement results are shown in Table 4.

As shown in Table 4, the sintered body produced using a mixed powder containing an aluminum salt of a higher fatty acid and sodium sulfate had sufficient strength.

The iron-based sintered member according to the embodiment of the present invention has high mechanical strength as the sintering progresses, has excellent machinability due to free carbon being dispersed in the pores, and is inexpensive. The iron-based sintered member according to the embodiment of the present invention is suitable, for example, for a bearing cap that is assembled into an aluminum alloy cylinder head and machined together with the aluminum alloy.

The disclosure of this application is related to the subject matter described in Japanese Patent Application No. 2018-128224, filed on July 5, 2018, the entire disclosure of which is incorporated herein by reference.

Reference Signs List 1 Bearing cap 2 Cylinder block 2a Rectangular recess 2b Semicircular recess 3 Bearing 4 Bolt 5 Crankshaft 11 Cutting tool 12 Rake face 13 Flank face 13a Wear portion 13b Width of wear portion 14 Holder 15 Sintered body

Claims (7)

The alloy has a metal structure including an iron matrix and pores dispersed in the iron matrix,
Contains iron, 0.1 to 3.5 mass % carbon, aluminum, sodium, and sulfur;
aluminum, sodium, and sulfur are concentrated on the surface of the iron matrix in contact with the pores;
The iron-based sintered member contains free carbon within the pores. The alloy has a metal structure including an iron matrix and pores dispersed in the iron matrix,
Contains iron, 0.1 to 3.5 mass % carbon, aluminum, sodium, and sulfur;
An iron-based sintered member, wherein the surface of the iron-based material in contact with the pores has an aluminum concentration of 0.1 mass % or more, a sodium concentration of 0.05 mass % or more, and a sulfur concentration of 0.05 mass % or more. The iron-based sintered member according to claim 1 or 2, containing 0.1 to 3.5 mass% carbon, 0.5 to 6.0 mass% copper, aluminum, sodium, and sulfur, with the remainder being iron and inevitable impurities. a powder mixture containing at least one selected from the group consisting of iron powder and iron alloy powder, 0.1 to 3.5 mass % of graphite powder, and 0.05 to 1.5 mass % of a mixed powder containing an aluminum salt of a higher fatty acid and sodium sulfate, the content of the sodium sulfate being 0.5 mass % or more based on the mass of the mixed powder , is filled into a die, and compression-molded to obtain a green compact;
sintering the green compact in a non-oxidizing gas atmosphere at a temperature equal to or higher than the diffusion temperature of carbon into the iron matrix;
The method for producing an iron-based sintered member includes the steps of : The method for producing an iron-based sintered member according to claim 4, wherein the higher fatty acid includes at least one selected from the group consisting of stearic acid, 12-hydroxystearic acid, lauric acid, myristic acid, palmitic acid, ricinoleic acid, and behenic acid . The method for producing an iron-based sintered member according to claim 4 or 5, wherein the iron-based powder mixture is composed of 0.1 to 3.5 mass% graphite powder, 0 to 10 mass% copper powder, 0.05 to 1.5 mass% of the mixed powder, and the remainder being at least one selected from the group consisting of iron powder and iron alloy powder, and inevitable impurities . The method for producing an iron-based sintered member according to any one of claims 4 to 6, wherein the temperature equal to or higher than the diffusion temperature of carbon into the iron matrix is 1,000 to 1,200°C.

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