JP7322880B2 - Iron-based sintered member, iron-based powder mixture, and method for producing iron-based sintered member - Google Patents

Description

Embodiments of the present invention relate to iron-based sintered components, iron-based powder mixtures, and methods of making iron-based sintered components.

An iron-based sintered member manufactured by a powder metallurgy method is applied to various uses because it is possible to manufacture an iron-based member with a special metal structure that cannot be obtained by a smelting method.

One application that requires a special metal structure is iron-based sintered parts for bearing caps. A bearing cap is a member that holds a bearing that rotatably supports a 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 bearing cap attached to a cylinder block of an automobile engine. The cylinder block 2 is formed with a rectangular recess 2a for positioning and housing the bearing cap 1 and a semicircular recess 2b for housing the bearing 3. As shown in FIG. The bearing cap 1 is formed with a semicircular recess for accommodating the bearing 3 and has an arch-shaped outer shape. The bearing cap 1 is positioned at the fitting portion of the rectangular concave portion 2a of the cylinder block 2 and fixed with bolts 4, 4 for use. The iron-based sintered member applied to such a bearing cap 1 is required to have mechanical strength because it needs to hold the crankshaft 5 . In addition, in order to make the semicircular arcs of the bearing cap 1 and the aluminum alloy cylinder block 2 concentric when bolted, the inner circumference of the circle formed by the semicircular arcs is usually machined. is applied. Therefore, iron-based sintered members applied to bearing caps are required to have machinability equivalent to that of aluminum alloys.

As a measure for improving the machinability of an iron-based sintered member, it is common to disperse manganese sulfide or graphite, which has a solid lubricating effect, into the pores of the iron-based sintered member. However, in the iron-based sintered member manufactured using manganese sulfide, the manganese sulfide powder added to the iron-based powder mixture serving as the raw material hinders the bonding due to the diffusion of the iron-based powders, resulting in a decrease in mechanical strength. tends to be low. In addition, since graphite easily diffuses into the iron matrix and disappears, if an iron-based sintered member with graphite dispersed in the pores is to be obtained, the diffusion of graphite into the iron matrix is unlikely to occur. It should be sintered at a temperature below °C. However, at a temperature at which the diffusion of graphite is difficult to occur, the iron-based sintered member tends to have low mechanical strength because the iron-based powders are poorly bonded by diffusion.

Under these circumstances, sintering is performed at a temperature range of 1,000 to 1,200°C, which is the sintering temperature of ordinary iron-based sintered members, to promote bonding due to diffusion of iron-based powders to promote high temperature. A technique has been developed for improving machinability by suppressing diffusion of graphite into an iron matrix and dispersing graphite in the iron matrix while obtaining mechanical strength (see Patent Document 1).

Patent Document 1 describes a powder mixture for an iron-based sintered material with excellent machinability, in which 0.01 to 1.0% by weight of boron oxide powder and 0.1 to 2.0% by weight of graphite powder are added. , And iron with excellent machinability added with 0.1 to 2.5% by weight of boron nitride powder containing 10 to 40% by weight of boron oxide and 0.1 to 2.0% by weight of graphite powder It relates to powder mixtures for system sintered materials.

JP-A-09-241701

In Patent Document 1, boron oxide or boron nitride containing boron oxide is used to suppress the diffusion of graphite into an iron matrix. It's expensive if not. For this reason, the iron-based sintered member obtained by Patent Document 1 has a high raw material cost, and an inexpensive technique for improving machinability is desired.

In view of the above, it is an object of embodiments of the present invention to provide an inexpensive iron-based sintered member having both high mechanical strength and excellent machinability. Another object of the present invention is to provide an inexpensive iron-based powder mixture that can produce iron-based sintered parts having both high mechanical strength and excellent machinability. Another object of the present invention is to provide an inexpensive manufacturing method capable of manufacturing an iron-based sintered member having both high mechanical strength and excellent machinability.

The 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 has a metallographic structure comprising an iron matrix and pores dispersed in the iron matrix, containing iron, 0.1-3.5% by weight carbon, aluminum, sodium, and sulfur. and an iron-based sintered member in which aluminum, sodium, and sulfur are concentrated on the surface of the iron matrix in contact with the pores, and free carbon is contained in the pores.
The iron-based sintered member contains 0.1 to 3.5% by mass of carbon, 0.5 to 6.0% by mass of copper, aluminum, sodium, and sulfur, and the balance is iron. It is preferably composed of unavoidable impurities.

Another embodiment of the present invention has a metallographic structure comprising an iron matrix and pores dispersed in the iron matrix, comprising: iron, 0.1-3.5% by weight carbon, aluminum, sodium, and sulfur and having 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 surface of the iron matrix that is in contact with the pores. It relates to a base sintered member.
The iron-based sintered member contains 0.1 to 3.5% by mass of carbon, 0.5 to 6.0% by mass of copper, aluminum, sodium, and sulfur, and the balance is iron. It is preferably composed of unavoidable impurities.

Another 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% by mass of graphite powder, aluminum salt of higher fatty acid and sodium sulfate. 0.05 to 1.5% by mass of the mixed powder, and the content of the sodium sulfate is 0.5% by mass or more based on the mass of the mixed powder.
The higher fatty acid preferably contains 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 comprises 0.1 to 3.5% by mass of graphite powder, 0 to 10% by mass of copper powder, 0.05 to 1.5% by mass of the mixed powder, and the balance of iron powder. and iron alloy powder, and inevitable impurities.

Another embodiment of the present invention is that the iron-based powder mixture is filled in a mold and compression-molded to obtain a green compact, and the green compact is subjected to carbon iron in a non-oxidizing gas atmosphere. The present invention relates to a method for producing an iron-based sintered component, including sintering at a temperature above the diffusion temperature into the matrix.
The temperature above the diffusion temperature of carbon into the iron matrix is preferably 1,000 to 1,200.degree.

According to the embodiment of the present invention, an inexpensive iron-based sintered member having both high mechanical strength and excellent machinability can be provided. Further, according to another embodiment of the present invention, it is possible to provide an inexpensive iron-based powder mixture from which an iron-based sintered member having both high mechanical strength and excellent machinability can be produced. Furthermore, according to another embodiment of the present invention, it is possible to provide an inexpensive manufacturing method capable of manufacturing an iron-based sintered member having both 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 Examples and a mapping image showing elemental distribution by energy dispersive X-ray analysis (EDS) (magnification: 200). FIG. 3 is an optical microscope photograph of the cross section of the sintered bodies obtained in Examples and Comparative Examples (magnification: 200). FIG. 4 is a mapping image showing the elemental distribution of the surface of the sintered body obtained in Example by an electron probe microanalyzer (EPMA) (magnification: 3,000). FIG. 5 is a schematic diagram showing a lathe working process performed in Examples and Comparative Examples. FIG. 6 is a schematic perspective view showing a cutting tool used in Examples and Comparative Examples. FIG. 7 is a graph showing evaluation results of the machinability of sintered bodies obtained in Examples and Comparative Examples.

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

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

When the carbon content in the iron-based sintered member is 0.1% by mass or more, an excellent effect of improving the machinability of the iron-based sintered member can be obtained. On the other hand, when the carbon content is 3.5% by mass or less, it is possible to prevent the pores from increasing too much, and the mechanical strength of the iron-based sintered member can be maintained. Therefore, the carbon content in the iron-based sintered member should be 0.1 to 3.5% by mass. The carbon content is preferably 0.3 to 2.5% by mass, more preferably 0.5 to 1.5% by mass, and 0.6 to 1.0% by mass. More preferred.

In general, iron powder and/or iron alloy powder used for manufacturing iron-based sintered members (in this specification, at least one selected from the group consisting of iron powder and iron alloy powder is referred to as "iron-based powder" ) contains aluminum, sodium, and sulfur that are mixed in during steelmaking as impurities. For example, aluminum, sodium, and sulfur contained in the iron-based powder as impurities are all about 0.01% by mass based on the mass of the iron-based powder.

According to one embodiment, the iron-based sintered member contains, in addition to impurities derived from the iron-based powder, 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 is referred to as the "pore surface" or It has concentrated aluminum, sodium, and sulfur at 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). "Aluminum, sodium, and sulfur are concentrated on the surface of the iron matrix in contact with the pores" means that the concentration of each of aluminum, sodium, and sulfur on the "surface of the iron matrix in contact with the pores" is "at the stomata." Concentration higher than that detected on the non-contact iron base surface. Specifically, the detected amounts of aluminum, sodium, and sulfur on the "surface of the iron matrix in contact with pores" by surface analysis are respectively It means higher than the detected amount, and can be confirmed by a mapping image obtained by surface analysis. For confirmation, an electron probe microanalyzer (eg, "EPMA-1600W" manufactured by Shimadzu Corporation) can be used, and the conditions are, for example, an acceleration voltage of 15 kV and a sample current of 100 nA.

Further, 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 on the pore surface. That's it. The concentrations of aluminum, sodium, and sulfur can be measured by surface analysis of the pore surfaces on the surface of the iron-based sintered member using, for example, 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, 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, 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, even more preferably 0.15 to 0.25% by mass. . These contents (% by mass) are based on the mass of the iron-based sintered member within the range of surface analysis of the pore surfaces. The “range of surface analysis of pore surfaces” can be a field of view at a magnification of 3,000 that includes the pore surfaces. Planar analysis of the pore surface can be performed on any arbitrary portion of the surface of the iron-based sintered member. As the content, a value measured using an electron probe microanalyzer (for example, "EPMA-1600W" manufactured by Shimadzu Corporation) under the conditions of an acceleration voltage of 15 kV and a sample current of 100 nA can be adopted. Preferably, 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, a sulfur has at least one location where the concentration of is 0.05% by mass or more.

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

The concentrations of aluminum, sodium, and sulfur contained in the iron matrix are not particularly limited, but are usually less than 0.05% by mass, preferably less than 0.03% by mass, based on the mass of the iron matrix. It is more preferably less than 0.0% by mass. The concentration of aluminum, sodium, and sulfur can be determined, for example, by scanning electron microscopy (SEM) observation at a magnification of 1,000 and by energy dispersive X-ray spectroscopy (EDS) in the areas of the surface of the iron matrix that are not in contact with pores. can be measured by surface analysis. The surface analysis of the iron matrix that is not in contact with the pores may be performed on an arbitrary portion of the surface of the iron-based sintered member.

In the iron-based sintered member, aluminum, sodium, and sulfur at the interface between the pores and the iron matrix are considered to exist in the form of oxides and/or composite oxides. It is speculated that the presence of aluminum, sodium, and sulfur at the interface allows the iron-based sintered component to retain carbon within the pores, resulting in excellent machinability.

From the viewpoint of obtaining excellent machinability, the content of free carbon in the iron-based sintered member is preferably 0.1 mass% or more based on the mass of the iron-based sintered member, and is preferably 0.15 mass. % or more, more preferably 0.3 mass % or more, and particularly preferably 0.35 mass % or more. On the other hand, from the viewpoint of obtaining sufficient strength, the content of free carbon is preferably 0.6% by mass or less, more preferably 0.5% by mass or less, based on the mass of the iron-based sintered member. More preferably, it is 0.45% by mass or less. 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 quantification method (JIS G 1211-5:2011).

The iron-based sintered member may contain copper. In a preferred embodiment, the iron-based sintered component contains iron, 0.1-3.5% by weight carbon, 0.5-6.0% by weight copper, aluminum, sodium, and sulfur. contains. More preferably, the iron-based sintered member contains 0.1 to 3.5% by mass of carbon, 0.5 to 6.0% by mass of copper, aluminum, sodium, and sulfur, the balance being Consists of iron and unavoidable impurities. The content of copper is the content based on the mass of the iron-based sintered member, and can be measured, for example, according to the iron and steel-ICP emission spectroscopic analysis method (JIS G 1258:2014).

The method for producing the iron-based sintered member is not particularly limited. A manufacturing method using an iron-based powder mixture, which will be described later, and a method using a manufacturing method for an iron-based sintered member, which will be described later, are preferable.

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

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

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. Aluminum salts of higher fatty acids may be used singly or in combination. Aluminum salts of higher fatty acids are extremely inexpensive compared to boron oxide powders or boron nitride powders containing boron oxide, so that raw material costs can be greatly 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. If the content of sodium sulfate is 0.5% by mass or more, the pore surfaces of the iron-based sintered member are enriched with aluminum, sodium, and sulfur, and/or an appropriate amount of aluminum is added to the pore surfaces; Sodium and sulfur can be present. On the other hand, when the content of sodium sulfate is 10% by mass or less, aluminum, sodium, and sulfur existing on the pore surfaces of the iron-based sintered member do not become excessive, and the bonding between the iron-based powders is hindered. sintering can proceed without 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. Also, 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 aluminum salt of higher fatty acid and unavoidable impurities as balance. The content of the higher fatty acid aluminum salt is preferably 95% by mass or more based on the mass of the mixed powder. The mixed powder is obtained by mixing higher fatty acid aluminum salt powder and sodium sulfate powder. Further, it is also possible to use a powder of a higher fatty acid aluminum salt containing a small amount of sodium sulfate, which is commercially available as a "higher fatty acid aluminum salt", as the mixed powder.

The content of the mixed powder is 0.05 to 1.5% by mass based on the mass of the iron-based powder. When the content of the mixed powder is 0.05% by mass or more, the pore surfaces of the iron-based sintered member are enriched with aluminum, sodium, and sulfur, and/or an appropriate amount of aluminum is added to the pore surfaces. Sodium and sulfur can be present. On the other hand, when the content of the mixed powder is 1.5% by mass or less, aluminum, sodium, and sulfur existing on the pore surfaces of the iron-based sintered member do not become excessive, and the iron-based powders are bonded together. Sintering can proceed without hindrance. The content of the mixed powder is preferably 0.1% by mass or more, more preferably 0.5% by mass or more. Moreover, the content of the mixed powder is preferably 1.2% by mass or less, more preferably 1% by mass or less.

In general, higher fatty acid metal salt powders are used as molding lubricants added to raw material powders and are inexpensive. Further, since the metal salt of the higher fatty acid is decomposed and removed during the heating process during sintering, it does not interfere with the diffusion bonding of the iron-based powders by sintering. When an aluminum salt of a higher fatty acid is used as such a metal salt of a higher fatty acid and sodium sulfate is used, aluminum generated by decomposition of the aluminum salt of a higher fatty acid during the temperature rising process, sodium and sulfur are combined into an iron base. and remain concentrated at the interface between the 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 to the iron matrix, and the iron-based sintered member obtained after sintering has pores inside the pores. is thought to contain free carbon.

(iron-based powder)
The iron-based powder contains at least one selected from the group consisting of iron (Fe) powder and iron alloy powder. Elements contained in iron alloys include copper (Cu), nickel (Ni), chromium (Cr), molybdenum (Mo), vanadium (V), manganese (Mn), titanium (Ti), aluminum (Al), carbon ( C) and the like. 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 0.1-3.5% by weight of graphite powder, based on the weight of the iron-based powder. When the graphite powder content is 0.1% by mass or more, a sufficient amount of free carbon is dispersed in the pores of the iron-based sintered member obtained after sintering, and the effect of improving the machinability is obtained. On the other hand, when the amount of graphite powder added is 3.5% by mass or less, the amount of pores in the iron-based sintered member obtained after sintering does not become excessive, and the mechanical strength of the iron-based sintered member is improved. is maintained. Graphite powder is inhibited from diffusing into the iron matrix by aluminum, sodium, and sulfur, and mostly remains as free carbon in the pores. Part of the graphite powder diffuses into the iron matrix and contributes to the improvement of the mechanical strength of the iron matrix as a pearlite or a mixed structure of pearlite and ferrite. Improves mechanical strength.

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

In addition, the iron-based powder mixture used in the stamping method in the molding process usually contains a molding lubricant powder. A powder of a higher fatty acid salt, a powder of a commonly used molding lubricant such as wax, etc. may be used in combination.

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

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

Iron-based powder mixtures can be widely used in the production of sintered structural components. In particular, it is suitable for producing iron-copper-carbon-based sintered parts.

[Manufacturing method of iron-based sintered member]
According to one embodiment, a method for producing an iron-based sintered member includes filling a mold with the iron-based powder mixture described above and performing compression molding to obtain a green compact (molding step); is sintered 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 manufacturing an iron-based sintered member may further have an arbitrary step.

(Molding process)
In the molding step, the iron-based powder mixture is filled into a desired mold and compression-molded to obtain a green compact. The molding method is not particularly limited, and for example, a stamping method can be applied. In a general stamping method, a raw material powder is filled into a mold cavity of a mold, compression-molded by upper and lower punches, and the obtained green compact is extracted from the mold. 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 galling between the green compact and the mold cavity when the green compact is extracted from the mold cavity. . The higher fatty acid aluminum salt is preferably dispersed uniformly inside the green compact.

(Sintering process)
In the sintering step, the green compact obtained is sintered in a sintering furnace under a predetermined atmosphere and temperature. Although the mechanism by which excellent machinability and strength are obtained has not been elucidated, it is considered as follows. That is, when the green compact is sintered at a temperature higher than the diffusion temperature of carbon into the iron matrix, the higher fatty acid aluminum salt powder and sodium sulfate decompose during the temperature rising process (200 to 600 ° C.) for sintering. do. At least part of aluminum, which is a metal component, and sodium and sulfur generated by decomposition of sodium sulfate are adsorbed on the surface of the iron-based powder, and the moisture adsorbed on the surface of the iron-based powder is at a high temperature (about 500 ° C.). Oxygen generated by desorption at or oxygen bound to the surface of the iron-based powder is reduced at high temperature (about 900 to 1,000 ° C) and combines with oxygen generated to form oxides of aluminum, sodium, and sulfur. and/or become a composite oxide and remain on the surface of the iron matrix after the iron-based powder is bonded, that is, on the interface between the iron matrix and pores. 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. It is presumed that diffusion is prevented.

In the temperature rising process of the sintering process, from about 850° C., diffusion of some carbon from the graphite powder to the iron-based powder and/or the iron matrix begins, and as the temperature rises, the amount of carbon diffusion increases. At 1,000 to 1,200° C., carbon diffusion proceeds remarkably. When the green compact containing the aluminum salt of a higher fatty acid and sodium sulfate is sintered, oxides of aluminum, sodium, and sulfur or 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 a temperature at which carbon diffuses. A complex oxide is formed. The oxide and/or composite oxide functions as a barrier for the diffusion of carbon from the graphite powder to the iron-based powder (and the iron matrix), the diffusion bonding of the iron-based powder progresses, and the carbon diffuses significantly. Even at a sintering temperature of 1,200° C., diffusion of carbon from the graphite powder to the iron-based powder (and iron matrix) is suppressed. Therefore, the iron-based sintered member obtained after sintering has a metal structure in which free carbon remains and is dispersed in the pores. Also, at this time, the iron-based powders are sufficiently bonded by diffusion, and the sintering proceeds sufficiently. As a result, the obtained sintered body is sufficiently sintered to have high mechanical strength, and on the other hand, has high machinability because carbon is dispersed in the pores.

When the metallographic structure of the surface of the iron-based sintered member is analyzed by an electron probe microanalyzer (EPMA), compared to the amounts of aluminum, sodium, and sulfur detected on the surface of the iron-based sintered material that is not in contact with the pores, the pore surface higher amounts of aluminum, sodium, and sulfur detected in Iron-based powder, which is one of the raw materials, contains aluminum, sodium, and sulfur as unavoidable impurities during steelmaking. Aluminum, sodium, and sulfur detected on the surface of the iron matrix not in contact with the pores originate from unavoidable impurities contained in the iron-based powder. Aluminum, sodium, and sulfur detected on the surface of the pores are derived from higher fatty acid aluminum. It is believed to be derived from salt and sodium sulphate.

When an iron-based powder mixture containing higher fatty acid aluminum salt powder without sodium sulfate is used as the raw material powder, aluminum does not remain in the iron matrix, pores and interfaces. Although the details are unknown, in the embodiment of the present invention, it is considered that aluminum, sodium and sulfur are present in the iron matrix, the pores and the interfaces due to the coexistence of these. The reason explained above is speculation and does not limit the present invention.

The sintering temperature is a temperature equal to or higher than the diffusion temperature of carbon, preferably 1,000 to 1,200.degree. In general, when sintering at a temperature of about 850° C. or higher at which carbon begins to diffuse from the graphite powder and a temperature of less than 1,000° C., the diffusion of carbon from the graphite powder to the iron matrix can be further suppressed. . However, when sintered at 1,000° C. or higher, the iron-based powders are sufficiently bonded by diffusion to each other, and the resulting iron-based sintered member has a high mechanical strength. On the other hand, if the sintering temperature is 1,200° C. or lower, wear of the sintering furnace can be suppressed. The gas atmosphere for heat treatment is a non-oxidizing gas atmosphere such as nitrogen gas.

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

Embodiments of the present invention will be specifically described with reference to Examples. Embodiments of the invention are not limited to the following examples.

<Preparation of iron-based powder mixture>
[Example 1]
Mixed powder (A) containing iron powder, copper powder, graphite powder, and aluminum stearate and sodium sulfate is put into a 10 kg V-type mixer so that the content ratio shown in Table 1 is obtained, 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 mixed powder (A) was changed to 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 zinc stearate powder was used as the mixed powder (A).

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): Mixed powder (B) shown in Table 1 below : Zinc stearate powder shown in Table 1 below: average particle size 13 μm

The content of sodium sulfate in the mixed powder was measured according to the following procedure.
(1) About 1.0 g of mixed powder is precisely weighed in a magnetic crucible, carbonized with a small flame so as not to scatter the mixed powder, and then completely incinerated in an electric furnace at 950 to 1,000°C.
(2) After standing to cool in a desiccator for 30 minutes, the ashed material is accurately weighed once, placed in a 300 ml beaker, added with 200 ml of distilled water and boiled for 30 minutes.
No. It is filtered using 5A quantitative filter paper (manufactured by ADVANTEC Group) and washed three times with 20 ml of distilled water while pouring the sediment into the beaker.
(3) The sediment remaining on the filter paper is put into a crucible together with the filter paper, dried and incinerated again.
(4) After standing to cool in a desiccator for 30 minutes, the ash is accurately weighed.
(5) Calculate the content (% by mass) of sodium sulfate (Na ₂ SO ₄ ) from the following formula.
Sodium sulfate content (% by mass) =
(Mass (g) of ash after first ashing - Mass (g) of ash after second ashing)/Mass of mixed powder (g) x 100

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

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

<Production 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.

(Production of sintered body)
Molding process The iron-based powder mixture obtained in Example 1 was filled in a mold, and the pressure was adjusted so that the density of the green compact was 6.75 g/cm ³ . A powder was obtained.
In the molding process, the powder compact could be extracted from the mold in good condition, and defects such as galling and chipping did not occur in the green compact. A sufficient lubricating effect was obtained by the mixed powder (A).

Sintering step The green compact is heated in a sintering furnace under a non-oxidizing atmosphere (N ₂ +5 vol% H ₂ atmosphere) at 1,130 ° C. for 20 minutes to obtain a sintered body (iron-based A sintered member) was obtained. An optical microscope photograph of the cross section of the obtained sintered body is shown in FIG. 3 (magnification: 200).

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

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

[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, respectively. FIG. 3 shows an optical microscope photograph of the cross section of the obtained sintered body.
Further, in the same manner as in Example 3, (1) concentrations of aluminum, sodium, and sulfur, (2) carbon content, and (3) free carbon content were measured. Table 3 shows the results. Further, an image showing the concentration distribution of aluminum (Al), sodium (Na), sulfur (S), carbon (C), and oxygen (O) in the sintered body of Example 4 is shown in FIG. ,000 times). Dark portions in the SEM image are pores. Bright areas in the image are locations where each element was detected. The thickness of the portion where aluminum, sodium, and sulfur were concentrated was about 2 μm.

<Evaluation of machinability and strength>
The machinability of the sintered bodies was evaluated by cutting the sintered bodies of Examples 3 and 4 and Comparative Example 2 and confirming the amount of wear of the cutting tool. Moreover, the strength of the sintered bodies of Examples 3 and 4 and Comparative Example 2 was evaluated by measuring the surface hardness of the sintered bodies.

(Machinability of sintered body)
The end face of the sintered body was cut by lathe processing. Lathe processing was performed according to the following conditions. FIG. 5 is a schematic diagram showing a lathe machining process. In FIG. 5, 11 denotes a cutting tool, 14 denotes a holder, and 15 denotes 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 being used for cutting. In FIG. 6, 11 is the cutting tool, 12 is the rake face, 13 is the flank face, 13a is the worn portion of the flank face, and 13b is the width of the worn portion.

Cutting machine: NC lathe (Numerical Control lathe)
Cutting tool: Cermet indexable tip (Material: NX2525, "TNMG160404" manufactured by Mitsubishi Materials Corporation)
Cutting speed: 350m/min
Feed: 0.03mm/rev
Machining allowance: 0.10mm

FIG. 7 is a graph showing the relationship between the cutting distance and the amount of flank wear. As shown in FIG. 7, the sintered body produced using the mixed powder containing the aluminum salt of higher fatty acid and sodium sulfate can greatly suppress wear of the cutting tool and has excellent machinability. was The use of a mixed powder containing an aluminum salt of a higher fatty acid and sodium sulfate can prevent deterioration of the cutting tool, thereby reducing the manufacturing cost of the iron-based sintered member.

(Strength of sintered body)
The Vickers hardness of the surface of the sintered body was measured according to JIS Z 2244:2009.
Table 4 shows the measurement results.

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

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

The disclosure of the present application relates 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.

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 part 13b Width of wear part 14 Holder 15 Sintered body

Claims (3)

Mixed powder 0.05 to 1.5 containing at least one selected from the group consisting of iron powder and iron alloy powder, 0.1 to 3.5% by mass of graphite powder, aluminum salt of higher fatty acid and sodium sulfate % by mass, and the content of the sodium sulfate is 0.5% by mass or more based on the mass of the mixed powder. The iron group according to claim 1 , wherein the higher fatty acid contains 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. powder mixture. Select from the group consisting of 0.1 to 3.5% by mass of graphite powder, 0 to 10% by mass of copper powder, 0.05 to 1.5% by mass of the mixed powder, and the balance of iron powder and iron alloy powder and unavoidable impurities .

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