JP7537491B2 - Manufacturing method of iron-based sintered body - Google Patents

Description

One embodiment of the present invention relates to a method for producing an iron-based sintered body.

The so-called powder metallurgy method, in which raw powder is compressed and molded in a die, and the resulting green compact is sintered, can produce near-net shapes, which means less cutting allowance and less material loss in subsequent machining, and it is economical because products of the same shape can be mass-produced once the die is made. Powder metallurgy also allows for a wide range of alloy design, as it can produce special alloys that cannot be obtained with alloys produced by normal melting. For this reason, it is widely used in mechanical parts, including automobile parts.

In the powder metallurgy method, a raw material powder such as an iron-based alloy is molded by various molding methods, and the obtained molded body is sintered to obtain a sintered body.
In one method of molding, pressure molding can obtain a molded body using a mixed powder in which a lubricant is added to a raw material powder in order to improve moldability. The lubricant contained in the molded body is mainly an organic component, so it can be thermally decomposed in the subsequent heat treatment and removed from the sintered body. There is a method of dewaxing the molded body by heat treating it at a relatively low temperature in order to thermally decompose and remove the lubricant, etc. from the molded body before heat treating it at a high temperature to sinter it.

One method of dewaxing is to supply butane gas to a sintering furnace, burn it with a burner, and directly heat the green body with the combustion exhaust gas. However, this method has the problem that it is difficult to control the atmosphere inside the furnace so that it is heated uniformly. If the atmosphere inside the furnace is uniform, the dewaxing can be controlled more accurately, making it possible to properly remove lubricants and the like from the green body during dewaxing, and furthermore, a homogeneous composition can be obtained in the resulting sintered body.

On the other hand, the type of gas supplied to the dewaxing furnace is also being considered.
Patent Document 1 (JP Patent Publication 8-260002 A) proposes controlling the dew point during dewaxing and sintering of an iron-based powder compact in order to prevent the generation of MnS-derived stains on the surface of the sintered compact.

Japanese Patent Application Publication No. 8-260002

However, in order to accelerate the thermal decomposition of zinc stearate during dewaxing and prevent stains from remaining on the surface of the sintered body, further consideration of the dewaxing conditions is required. In addition, simply strengthening the dewaxing conditions to accelerate the thermal decomposition of zinc stearate may have a problem of affecting the metal structure of the iron-based sintered body.
An object of one embodiment of the present invention is to produce an iron-based sintered body with little surface contamination.

One embodiment of the present invention is as follows.
[1] A method for producing an iron-based sintered body, comprising: dewaxing a molded body containing a raw material powder including an iron-based powder and zinc stearate; and sintering the dewaxed molded body, the dewaxing comprising: preparing a water vapor-containing nitrogen atmosphere containing nitrogen and water vapor and having a dew point of 0°C or higher and 30°C or lower; heating the water vapor-containing nitrogen atmosphere to 100°C or higher and supplying it to a dewaxing section; and heat-treating the molded body at 800°C or lower in the dewaxing section.
[2] The method for producing an iron-based sintered body according to [1], further comprising preparing the water vapor-containing nitrogen atmosphere by mixing water vapor with a nitrogen atmosphere, the nitrogen atmosphere containing 95 volume % or more of nitrogen.
[3] The method for producing an iron-based sintered body according to [1] or [2], wherein the atmosphere in the dewaxing section contains less than 5 volume % of hydrogen.
[4] The method for producing an iron-based sintered body according to any one of [1] to [3], wherein the dewaxing section includes heating the water vapor-containing nitrogen atmosphere to 250° C. or higher to perform dewaxing.

[5] The method for producing an iron-based sintered body according to any one of [1] to [4], wherein the raw powder of the compact contains Cu in an amount of more than 0 mass% and not more than 20 mass%, with the remainder being iron and unavoidable impurities.
[6] The method for producing an iron-based sintered body according to any one of [1] to [5], wherein the raw powder of the compact contains Mn and S in a total amount of 0.2 mass % or less.
[7] The method for producing an iron-based sintered body according to any one of [1] to [6], wherein the raw material powder of the compact further contains C of more than 0 mass % and 5 mass % or less.
[8] The method for producing an iron-based sintered body according to any one of [1] to [7], wherein the zinc stearate is 0.05 to 5 parts by mass with respect to 100 parts by mass of the raw material powder.
[9] The method for producing an iron-based sintered body according to any one of [1] to [8], wherein the dewaxing and sintering are carried out in a continuous sintering furnace.

According to one embodiment, an iron-based sintered body with minimal surface contamination can be produced.

FIG. 1 is a schematic cross-sectional view of a roller hearth dewaxing furnace used in the examples. FIG. 2 is a photograph of the metal structure of the sintered body of the example observed with a scanning electron microscope (SEM).

One embodiment of the present invention is described below, but the present invention is not limited to the following examples.

A method for producing an iron-based sintered body according to one embodiment includes dewaxing a molded body including a raw material powder including an iron-based powder and zinc stearate, and sintering the dewaxed molded body, and the dewaxing includes preparing a water vapor-containing nitrogen atmosphere including nitrogen and water vapor and having a dew point of 0° C. or more and 30° C. or less, heating the water vapor-containing nitrogen atmosphere to 100° C. or more and supplying it to a dewaxing section, and heat-treating the molded body at 800° C. or less in the dewaxing section.
According to this, an iron-based sintered body with little surface contamination can be manufactured. Furthermore, according to one embodiment, the strength of the sintered body can be further increased.

According to one embodiment, by specifying the type of gas and the dew point of the atmosphere in which the molded body is dewaxed, it is possible to promote the thermal decomposition of zinc stearate contained in the molded body and reduce the stains on the surface of the obtained sintered body. The stains remaining on the surface of the sintered body are mainly observed as organic components such as zinc stearate remaining on the surface of the sintered body.
Moreover, according to one embodiment, by specifying the type of gas and the dew point of the atmosphere in which the molded body is dewaxed, a certain amount of carbon components remain in the resulting sintered body, thereby suppressing the formation of ferrite phase and further increasing the strength of the sintered body.
According to one embodiment, since it is only necessary to specify the gas type and dew point of the atmosphere in which the molded body is dewaxed, it is possible to use an indirect heating method such as a radiant tube burner, which can heat the inside of the furnace more uniformly, in addition to burner combustion, as the heat source for the heat treatment of dewaxing, and therefore it becomes possible to more easily control the temperature inside the furnace.

In one embodiment, the compact may include a raw powder including an iron-based powder and zinc stearate.
The iron-based powder may be an iron powder, an iron alloy powder, or a combination thereof. The iron-based powder may include unavoidable impurities.

In the raw material powder, the overall composition preferably contains iron as a main component. For example, the iron content of the total amount of the raw material powder is preferably 50% by mass or more, more preferably 80% by mass or more, and even more preferably 90% by mass or more.

The raw material powder may have an overall composition containing Cu with the remainder being iron. In this case, it is preferable that the Cu content is more than 0 mass % and 20 mass % or less with respect to the total amount of the raw material powder.
Cu has the effect of diffusing into Fe to increase the strength of the material.
During sintering, part of Cu diffuses into the Fe matrix, and part of Cu dissolves and mixes with the Fe to form a copper alloy. Therefore, when the sintered alloy is cooled, the structure is in a state where Cu is dispersed in the form of a copper phase or a copper alloy phase in the matrix of the iron-based sintered body.
In addition, when Cu is used in combination with C, it can improve the hardenability of the iron-based material, refine pearlite to increase strength, and promote the formation of high-strength bainite or martensite at a normal cooling rate during sintering.
Cu is not an essential element, but is preferably 0.01% or more, more preferably 0.1% or more, and even more preferably 1% or more. From the viewpoint of preventing a decrease in material strength, Cu is preferably 20% or less, and more preferably 15% or less. Furthermore, Cu is preferably 10% or less, more preferably 5% or less, and even more preferably 3% or less. This makes it possible to suppress the generation of a soft Cu phase, thereby preventing a decrease in material strength, and also to suppress the generation of a Cu liquid phase during sintering, thereby improving the dimensional accuracy of the entire product.

The raw material powder may further contain C in its overall composition. In this case, it is preferable that C is contained in an amount of more than 0 mass % and 5 mass % or less.
C can improve strength by dissolving a part of it in Fe. If an excessive amount of C is added, there is a problem that brittle cementite precipitates in a network shape. For this reason, C is preferably 0 to 5%, more preferably 0.1 to 3%, and even more preferably 0.5 to 1%. It is also preferable that the entire amount of C is dissolved in the matrix or precipitated as metal carbide. It is preferable that the metal carbide is dispersed and contained in the matrix.

Furthermore, the raw material powder may have an overall composition that includes at least one element selected from the group consisting of Cr, Mn, Mo, W, Ni, Co, V, and Nb. In this case, each element is preferably included in a total amount of 15 mass% or less, more preferably 10 mass% or less, and even more preferably 5 mass% or less.
The raw material powder may further contain, in its overall composition, one or more of Si, P, S, O, N, etc., either singly or in combination.

Cu can be blended into the raw material powder as iron alloy powder, or Cu can be blended into the raw material powder as copper powder, copper alloy powder, or a combination thereof.
Metals other than Cu can be blended as iron alloy powder blended into the raw material powder, for example, as alloy powder of iron and at least one selected from the group consisting of Cr, Mn, Mo, W, Ni, Co, V, and Nb. Metals other than Cu can be blended as metal powder of these metals alone.

C can be blended into the raw powder in a state of solid solution in iron powder and/or iron alloy powder. C can also be blended into the raw powder as graphite powder to increase the compressibility of the compact. C can also be blended into the raw powder as an organic component such as zinc stearate. Organic components such as zinc stearate are thermally decomposed by dewaxing and sintering, but it is believed that some of the C remains in the sintered compact and becomes solid-soluble in the iron matrix.

The raw material powder may contain unavoidable impurities. The total amount of the unavoidable impurities is preferably 1 mass % or less, more preferably 0.5 mass % or less, and even more preferably 0.1 mass % or less, based on the total amount of the raw material powder.
In particular, when Mn and S are not added as raw materials to the raw material powder, the total amount of Mn and S is preferably 0.2 mass % or less, and more preferably 0.1 mass % or less, based on the total amount of the raw material powder.

The average particle size of the raw material powder is not particularly limited, but from the viewpoint of moldability and sinterability, it is more preferably 20 to 200 μm.
Here, the average particle size is a volume-based average particle size, and specifically, can be measured by a laser diffraction method.

The molding includes zinc stearate.
The molded body is preferably formed by filling a mixed powder state of zinc stearate and a raw material powder into a molding die.
By mixing zinc stearate into the raw material powder, friction between particles of the raw material powder is reduced, giving the raw material powder fluidity when it is filled into a molding die, preventing particle aggregation, and improving filling properties.
In addition, by mixing zinc stearate with the raw material powder, friction between the raw material powder and the wall of the mold during the compression process of the raw material powder filled in the mold can be reduced, preventing wear on the mold.
In addition, by mixing zinc stearate into the raw material powder, the pressure distribution becomes more uniform when the raw material powder is filled into a mold and compressed, making it possible to obtain a molded body with a more uniform density distribution and excellent strength.

Zinc stearate is preferably blended in an amount of 0.05 to 5 parts by mass, more preferably 0.1 to 2 parts by mass, and even more preferably 0.5 to 1 part by mass, per 100 parts by mass of the raw material powder.
The amount of zinc stearate is preferably 0.05 parts by mass or more, more preferably 0.1 parts by mass or more, and even more preferably 0.5 parts by mass or more, based on 100 parts by mass of the raw material powder, which can further improve the moldability as described above.
The amount of zinc stearate is preferably 5 parts by mass or less, more preferably 2 parts by mass or less, and even more preferably 1 part by mass or less, relative to 100 parts by mass of the raw material powder, so that the amount of organic components remaining in the sintered body can be reduced by thermally decomposing the zinc stearate through the heat treatments of dewaxing and sintering.

The molded body may contain other additives. Examples of other additives include amide-based lubricants as lubricants other than zinc stearate.

The molded body can be obtained by mixing a raw material powder containing an iron-based powder with zinc stearate and molding the mixed powder. The molding method may be either a dry method or a wet method, but a dry method is preferred. As a dry molding method, pressure molding such as powder compaction can be preferably used.

The molding conditions may be appropriately set depending on the composition, size, etc. of the molded body. For example, the molding pressure may be 300 to 1000 MPa, and the pressure holding time may be 0.2 to 2 seconds.
The apparent density of the molded body may be appropriately set depending on the composition of the molded body, and may be, for example, 1 to 8 g/ ^m3 , or 5 to 7 g/ ^m3 .

Next, the step of dewaxing the molded body will be described.
Preferably, winterization comprises the following steps:
(1) A water vapor-containing nitrogen atmosphere containing nitrogen and water vapor and having a dew point of 0° C. or higher and 30° C. or lower is prepared.
(2) Heating a water vapor-containing nitrogen atmosphere to 100°C or higher and supplying it to the dewaxing section.
(3) Heat treating the molded body at a temperature of 800°C or less in the dewaxing section.

The dewaxing is preferably carried out in a water vapor-containing nitrogen atmosphere containing nitrogen and water vapor.
The dew point of the water vapor-containing nitrogen atmosphere is preferably 0° C. or higher, more preferably 10° C. or higher, and even more preferably 12° C. or higher. This reduces the amount of undecomposed matter derived from zinc stearate remaining in the sintered body, and reduces the dirt on the surface of the sintered body derived from the undecomposed matter. This is thought to be because the inclusion of a certain amount of water vapor in the dewaxing atmosphere promotes the oxidation of organic components contained in the compact, and the thermal decomposition of zinc stearate progresses.
The dew point of the dewaxing atmosphere is preferably 30° C. or less, more preferably 20° C. or less, even more preferably 18° C. or less, and even more preferably 16° C. or less, which prevents the carbon component from being excessively removed from the sintered body and allows a sufficient amount of carbon to remain in the sintered body, suppressing the formation of a ferrite phase, and thereby enabling a sintered body with high strength to be obtained.
From the viewpoint of controlling the atmosphere supplied to the furnace, the dewaxing atmosphere has a dew point of preferably 15°C±5°C, more preferably 15°C±3°C, and even more preferably 15°C±2°C.

Here, the dew point of the atmosphere is the temperature at which condensation begins when cooling an atmosphere containing water vapor, and can be determined in accordance with the humidity measurement method of JIS Z8806:2001. Specifically, the dew point of the atmosphere can be measured using a dew point meter. By installing a dew point meter in the atmosphere supply section of the sintering furnace, it is possible to manage the dew point of the atmosphere supplied to the sintering furnace.

The water vapor-containing atmosphere is preferably prepared by mixing water vapor with a nitrogen atmosphere. In this case, the nitrogen atmosphere preferably contains 95% or more by volume of nitrogen, more preferably 98% or more by volume.

The dewaxing atmosphere may further contain an inert gas, such as Ar gas or He gas, in addition to the water vapor and nitrogen.
In order to promote the oxidation of organic components, it is preferable to limit the amount of reducing gas used in the dewaxing atmosphere. Examples of reducing gas include _H2 gas and CO gas. In particular, the dewaxing atmosphere preferably contains less than 5% by volume of _H2 gas, more preferably 3% by volume or less, and even more preferably 1% by volume or less.
When dewaxing and sintering are performed through a continuous sintering furnace, the atmosphere used in the sintering process may flow back into the dewaxing process and be mixed in. For example, when reducing _H2 gas is used in the sintering process, _H2 gas may be mixed in the dewaxing process. Even in this case, the dewaxing atmosphere preferably contains less than 5% by volume of _H2 gas, more preferably 3% by volume or less, and even more preferably 1% by volume or less.

It is preferable that the water vapor-containing nitrogen atmosphere supplied to the dewaxing section is heated before being supplied to the dewaxing section. For example, it is preferable that the water vapor-containing nitrogen atmosphere supplied to the dewaxing section is supplied to the dewaxing section in a state heated to 100°C or higher. This temperature is preferably 100 to 500°C, more preferably 300 to 500°C.

In the dewaxing, the flow rate of the atmosphere supplied to the sintering furnace is, for example, preferably 0.5 m ³ /h or more, more preferably 1 m ³ /h or more, and even more preferably 2 m ³ /h or more.
In the dewaxing, the flow rate of the atmosphere supplied to the sintering furnace is, for example, preferably 60 m ³ /h or less, more preferably 50 m ³ /h or less, and even more preferably 45 m ³ /h or less.
Here, the flow rate of the atmosphere supplied to the sintering furnace is the flow rate of the gas passing through the piping of the gas pipe supplying the gas to the sintering furnace.

In the dewaxing, the dewaxing temperature is preferably 100° C. or higher, more preferably 250° C. or higher, more preferably 350° C. or higher, and even more preferably 550° C. or higher, which can further promote the thermal decomposition of organic components such as zinc stearate contained in the molded body.
In the dewaxing, the dewaxing temperature is preferably 800° C. or less, more preferably 600° C. or less, and even more preferably 580° C. or less. This allows zinc stearate to be sufficiently pyrolyzed without consuming excessive energy. In the subsequent sintering step, sintering progresses, so the dewaxing temperature may be 800° C. or less.
The dewaxing temperature can be determined by measuring the temperature in the sintering furnace with a thermocouple. The thermocouple is preferably installed near the position where the molded body is placed. When dewaxing is performed in a continuous sintering furnace with a gradient of the dewaxing temperature, it is preferable that the dewaxing temperature at the highest temperature in the dewaxing section of the continuous sintering furnace is within the above-mentioned range.

The dewaxing time can be appropriately set depending on the composition, size, etc. of the molded body, and may be, for example, 10 to 60 minutes, or 20 to 40 minutes. It is preferable that the time required for the dewaxing temperature to reach 100° C. or higher is within this range.
The dewaxing pressure can be appropriately set depending on the composition, size, etc. of the molded body, and may be, for example, 1 to 500 Pa, or may be 5 to 100 Pa.

After dewaxing, a sintering step may be carried out without cooling using a continuous sintering furnace, or, when a batch-type sintering furnace is used, the sintering step may be carried out after cooling after dewaxing.
In consideration of the environmental aspects, the exhaust gas discharged from the sintering furnace during dewaxing can be subjected to a combustion treatment to further pyrolyze the organic components before being discharged.

Next, the step of sintering the dewaxed compact will be described.
In the sintering, the sintering temperature is preferably 1000° C. or higher, more preferably 1100° C. or higher, and even more preferably 1130° C. or higher. This can promote sintering between particles of the iron-based powder in the sintered body, thereby further improving the sinterability.
In the sintering, the sintering temperature is preferably 1400° C. or less, more preferably 1300° C. or less, and even more preferably 1200° C. or less. This makes it possible to obtain a sintered body with good sinterability without consuming excessive energy. In addition, since excessive heating is prevented, the carbon component is prevented from being excessively removed from the sintered body, the formation of a ferrite phase is suppressed, and a sintered body with higher strength can be obtained.
The sintering temperature can be determined by measuring the temperature in the sintering furnace with a thermocouple. The thermocouple is preferably installed near the position where the molded body is placed. When sintering is performed in a continuous sintering furnace with a gradient of sintering temperature, it is preferable that the sintering temperature at the point where the temperature is highest in the sintering section of the continuous sintering furnace is within the above-mentioned range.

The sintering atmosphere may be an oxidizing gas, a reducing gas, an inert gas, or a mixture of these gases. Among these, a reducing gas is preferably used to promote sintering.
The sintering atmosphere may be, for example, butane converted gas, ammonia decomposition gas, _H2 gas, etc. Among them, _H2 gas or a mixed gas of _H2 gas and an inert gas is preferable. Examples of the inert gas include _N2 gas and Ar gas.
For example, when a mixed gas of _H2 gas and an inert gas is used as the sintering atmosphere, the _H2 gas content is preferably 2 to 75% by volume, more preferably 4 to 50% by volume.

The dew point of the sintering atmosphere is preferably less than 10° C., more preferably 0° C. or lower, and even more preferably −10° C. or lower, in order to prevent oxidation of the sintered body due to the inclusion of water vapor.
The lower limit of the dew point of the sintering atmosphere is not particularly limited, but from the viewpoint that the lower the dew point, the more difficult it is to control the gas dew point and the higher the control costs become, the lower the dew point is preferably −80° C. or higher, and more preferably −60° C. or higher.

In sintering, the flow rate of the atmosphere supplied to the sintering furnace is, for example, preferably 0.5 m ³ /h or more, more preferably 1 m ³ /h or more, and even more preferably 2 m ³ /h or more.
In sintering, the flow rate of the atmosphere supplied to the sintering furnace is, for example, preferably 80 m ³ /h or less, more preferably 75 m ³ /h or less, and even more preferably 70 m ³ /h or less.
Here, the flow rate of the atmosphere supplied to the sintering furnace is the flow rate of the gas passing through the piping of the gas pipe supplying the gas to the sintering furnace.

The sintering time can be appropriately set depending on the composition, size, etc. of the molded body, and may be, for example, 15 to 120 minutes, or 30 to 60 minutes. It is preferable that the time required for the sintering temperature to reach 1000° C. or higher is within this range.
The sintering pressure can be appropriately set depending on the composition, size, etc. of the molded body, and may be, for example, 1 to 500 Pa, or may be 5 to 100 Pa.

After sintering, the mixture may be air-cooled or the cooling rate may be controlled. For example, the cooling rate after sintering may be 1 to 100° C./min, or 5 to 50° C./min.
In consideration of the environmental aspects, the exhaust gas discharged from the sintering furnace during sintering can be subjected to a combustion treatment to further pyrolyze the organic components before being discharged.

An example of the sintering furnace will be described below.
In one embodiment, the dewaxing and sintering may be carried out in a continuous or batch sintering furnace.
As the continuous sintering furnace, for example, a mesh belt furnace, a roller hearth furnace, a tray pusher furnace, a walking beam furnace, or the like can be used.
The continuous sintering furnace may be a sintering furnace in which a dewaxing section, a sintering section, and a cooling section are continuously arranged in a tunnel-shaped furnace, and the molded bodies are transported from the dewaxing section to the cooling section by a transport means. The molded bodies may be transported while being placed on a tray.
Between the dewaxing section and the sintering section, a partition should be provided to allow the conveying means and the compacts to pass through, so that the dewaxing section and the sintering section can be separated to allow temperature control and to prevent the atmospheres in the furnace from mixing with each other. The same applies between the sintering section and the cooling section.

The dewaxing section is equipped with a conveying means, a heating section, an atmosphere supplying section, an exhaust section, and a thermocouple.
The dewaxing section may have a plurality of heating sections to uniformly heat the inside of the furnace. Examples of the heating section include radiant tube burners, metal heaters, ceramic heaters such as SiC, and cup burners using butane gas, etc. Among them, radiant tube burners are preferably used because they are less deteriorated by exhaust gas, are indirect heating that is less affected by the atmosphere inside the furnace, and are easy to maintain.
The atmosphere supplying section of the dewaxing section supplies a nitrogen atmosphere containing water vapor at a predetermined temperature into the furnace, and a plurality of such sections may be provided to uniformly supply the atmosphere into the furnace.
It is advisable to provide a flow meter in the atmosphere supply section of the dewaxing section to adjust the flow rate of the atmosphere supplied into the furnace.

The dewaxing section may heat the molded body in a single space with uniform temperature and atmosphere, or may heat the molded body by passing it through multiple separated spaces with different temperatures and atmospheres.
In one example of the dewaxing section, a first dewaxing section, a second dewaxing section, and a third dewaxing section are arranged from the upstream side in the conveying direction of the molded bodies along the conveying direction. Each dewaxing section is separated by a partition to an extent that the conveying means and the molded bodies can pass through, and the temperature and atmosphere can be adjusted in each dewaxing section. Note that the number of spaces into which the dewaxing sections are separated is not limited to this.
By lowering the temperature of the dewaxing section upstream in the conveying direction and raising the temperature of the dewaxing section downstream in the conveying direction, thereby creating a temperature gradient, the molded body can be gradually heated, allowing the removal of organic components such as zinc stearate to proceed slowly, and preventing shape defects such as cracks in the molded body.
In addition, by reducing the flow rate of the atmosphere in the dewaxing section upstream in the conveying direction and increasing the flow rate of the atmosphere in the dewaxing section downstream in the conveying direction, thereby providing a gradient in the flow rate, the molded body can be gradually oxidized, and the amount of carbon removed that originates from organic components such as zinc stearate can be adjusted so that a certain amount of carbon remains in the sintered body, which contributes to improving the strength of the sintered body.

In addition, when multiple dewaxing sections are arranged in the conveying direction, the flow rate of the water vapor-containing nitrogen atmosphere may be reduced or no water vapor-containing nitrogen atmosphere may be supplied to the dewaxing section adjacent to the sintering furnace, which prevents an atmosphere with a high dew point from being mixed into the sintering section and suppresses excessive oxidation of the sintered body in the sintering section that is treated at a higher temperature.
In the multiple dewaxing sections, the water vapor-containing nitrogen atmosphere is preferably supplied from the same supply source in order to simplify the equipment, and the dew point and gas type may be the same.

In the atmosphere supply section of the dewaxing section, methods of generating steam that can be used include a constant-volume supply pump and electric heater heating, boiler heating, ultrasonic vibration, etc. Among these, a constant-volume supply pump and electric heater heating is preferably used because it is easy to adjust the amount of steam generated and maintenance is simple.

Another example of the water vapor generating device includes a water tank unit, a water constant volume supply pump unit, a vaporizer, a nitrogen gas supply unit that supplies nitrogen gas to the vaporizer, and a water vapor atmosphere exhaust unit.
In this example, first, water is supplied from a water tank to the vaporizer by a water constant-feed pump. It is preferable to use water from which ionic impurities such as pure water have been removed. Water vapor can be generated by heating the vaporizer to 100°C or higher, preferably 200°C or higher, and more preferably 250°C or higher. Nitrogen gas can be supplied from the nitrogen gas supply unit to the vaporizer to generate a nitrogen atmosphere containing water vapor. This nitrogen atmosphere containing water vapor can be discharged from the water vapor atmosphere discharge unit and supplied to the dewaxing unit.
The piping from the water vapor atmosphere exhaust section to the dewaxing section is preferably kept at a temperature of 60 to 100°C.

The sintering section includes a conveying means, a heating section, an atmosphere supplying section, an exhaust section, and a thermocouple.
The heating section of the sintering section is not particularly limited, and can be appropriately selected from those explained in the dewaxing section above.
The atmosphere supplying section of the sintering section supplies the atmosphere at a predetermined temperature into the furnace, and a plurality of such supplying sections may be provided in order to uniformly supply the atmosphere into the furnace.
It is advisable to provide a flow meter in the atmosphere supply section of the sintering section to adjust the flow rate of the atmosphere supplied into the furnace.

The cooling section is not particularly limited, and may be any cooling section that is applicable to a normal sintering furnace.
In the continuous sintering furnace, the conveying means can be driven by a drive motor. In addition, in the continuous sintering furnace, one or more exhaust sections can be provided in the dewaxing section and the sintering section, and exhaust gas containing organic components pyrolyzed from the molded body can be exhausted. In the continuous sintering furnace, one or more thermocouples can be provided in the dewaxing section and the sintering section, and the furnace temperature can be controlled in each section.

The sintered body produced in one embodiment preferably has the following physical properties, although this is not limited to these:

The sintered body preferably has a Rockwell hardness of 20 to 70 HRB.
The Rockwell hardness of the sintered body is preferably 20 HRB or more, more preferably 30 HRB or more, and even more preferably 40 HRB or more. This makes it possible to provide a sintered body with higher strength.
The Rockwell hardness of the sintered body is not particularly limited, but may be, for example, 100 HRB or less.

Here, the Rockwell hardness of the sintered body can be measured according to the method specified in JIS Z 2245.

The sintered body preferably has a tensile strength of 220 to 590 MPa.
The tensile strength of the sintered body is preferably 220 MPa or more, more preferably 250 MPa or more, and even more preferably 300 MPa or more. This makes it possible to provide a sintered body with higher strength.
The tensile strength of the sintered body is not particularly limited, but may be, for example, 600 MPa or less.

Here, the tensile strength of the sintered body can be measured by preparing a test piece of ASTM: E8-69 and following the test method specified in JIS Z2241.

In the sintered body, the area ratio of the ferrite phase in the metal structure is preferably 50 area % or less, more preferably 40 area % or less, even more preferably 30 area % or less, and even more preferably 20 area % or less. By further reducing the ferrite phase, a sintered body with higher strength can be obtained. In the sintered body, it is preferable that a pearlite phase is formed in the metal structure, and the pearlite phase is more preferably 60 to 90 area %.
The metal structure of the sintered body can be observed with a scanning electron microscope.

The present invention will now be described in detail with reference to examples, but the present invention is not limited to these examples.

After dewaxing and sintering, the test pieces were evaluated for microstructure, strength, and appearance.
"Molding"
As the raw material powder, an iron-based powder having a composition of 0.8% by mass of C, 2.0% by mass of Cu, and the remainder being iron was prepared. 0.8 parts by mass of zinc stearate ("ZNS-730" manufactured by ADEKA CORPORATION) was added to 100 parts by mass of the iron-based powder, and mixed for 30 minutes in a V-type mixer. This mixed powder was pressed into a circular ring having an outer diameter of 90 mm, an inner diameter of 15 mm, and a height of 15 mm to obtain a molded body. The mass of the molded body was 590 g, and the density was 6.9 g/ ^cm3 .

"Let's take it off."
Dewaxing and sintering were carried out by passing the compact through a roller hearth furnace (NR-1 furnace).
A schematic cross-sectional view of a roller hearth dewaxing furnace is shown in FIG.
In this figure, a dewaxing furnace 10 is equipped with conveying rollers 1, a carbon tray T for conveying test pieces placed on the conveying rollers 1, and, from the upstream side in the conveying direction, a dewaxing section 2a, a dewaxing section 2b, and a dewaxing section 2c. Each of the dewaxing sections 2a, 2b, and 2c is separated by a partition plate and equipped with radiant tube burners 3a, 3b, and 3c disposed in each of the dewaxing sections 2a, 2b, and 2c, gas inlet sections 4a, 4b, and 4c for supplying a water vapor-containing nitrogen atmosphere to each of the dewaxing sections 2a, 2b, and 2c, and an exhaust port section 5 for discharging exhaust gas from the dewaxing sections 2b and 2c.
The temperature inside the dewaxing furnace is controlled by radiant tube burners, and the molded bodies on the carbon trays are transported by transport rollers in the transport direction X. A nitrogen atmosphere containing water vapor is supplied from an inlet on the floor of the furnace to dewax the molded bodies. The exhaust gas discharged from the dewaxing section is combusted in an exhaust gas combustion device installed at the exhaust port, and is discharged from an exhaust hood duct 6.
The radiant tube burner, which is the heat source, is an indirect heating system and is isolated from the inside of the furnace, allowing the atmosphere inside the furnace to be kept constant.

The conditions of the roller hearth dewaxing furnace were as follows:
Water vapor was generated by a vaporizer (not shown), and the generated water vapor was mixed with a nitrogen atmosphere containing 100% by volume of nitrogen to prepare a water vapor-containing atmosphere having a dew point shown in Table 1.
The vaporizer temperature was constant at 300° C., and the temperatures of dewaxing sections 2a, 2b, and 2c were constant at 250° C., 350° C., and 550° C., respectively. The flow rates of the water vapor-containing nitrogen atmosphere from the vaporizer to dewaxing sections 2a, 2b, and 2c were constant at 2 m ³ /h, 4 m ³ /h, and 0 m ³ /h, respectively. The number of molded bodies charged was constant at 9 per tray.
Other dewaxing conditions were as follows:
Dewaxing pressure: 5 Pa.
Dewaxing time: 30 minutes.

"Sintering"
The compact was then passed through a roller hearth sintering furnace under the following sintering conditions:
Sintering temperature: 1130°C.
Sintering pressure: 10 Pa.
Sintering atmosphere: 4 vol.% _H2 + 96 vol.% _N2 .
Dew point of sintering atmosphere: -60°C.
Gas flow rate: _N2 gas 34 ^m3 /h. AX gas ² m3/h.
Sintering time: 40 min.
Cooling rate: 40° C./min to room temperature.

The hydrogen concentration in the dewaxing sections 2a, 2b, and 2c includes the _H2 gas contained in the atmosphere flowing in from the sintering furnace. The _H2 ratio of the AX gas flow rate was controlled to be less than 5% by volume relative to the total gas flow rate of the _N2 gas flow rate in the dewaxing section, the _N2 gas flow rate in the sintering furnace, and the AX gas flow rate in the dewaxing furnace. This made it possible to keep the hydrogen concentration in the dewaxing furnace less than 5% by volume, even taking into account the _H2 gas flowing from the sintering furnace to the dewaxing furnace. The AX gas is an ammonia decomposition gas containing 75% by volume _H2 and 25% _N2 .

"evaluation"
The obtained sintered bodies were subjected to the following evaluations. The results are shown in Table 1.
(Contamination of sintered body)
The surface of the sintered body was visually observed, and the staining of the sintered body was evaluated according to the following criteria.
A: No stains were observed.
B: A small amount of dirt was observed.

(Strength of sintered body)
The tensile strength of the sintered body was measured under the following conditions.
The tensile strength of the sintered body was measured by preparing a test piece of ASTM: E8-69 and following the test method specified in JIS Z2241.

(Microstructure of sintered body)
The metal structure of the sintered body was observed by a scanning electron microscope (SEM). The SEM photograph is shown in Figure 2. In addition, the area ratio of the ferrite phase, which is an index of decarburization, was calculated from the SEM photograph.

As shown in Table 1, the sintered bodies of each Example had little surface contamination and high strength. In Example 1, the dew point of the dewaxing atmosphere was low at 10°C, and some surface contamination occurred on the sintered body. In Examples 4 and 5, the dew points of the dewaxing atmosphere were high at 20°C and 30°C, and the strength of the sintered body was slightly reduced.

Claims (9)

The method includes dewaxing a molded body containing a raw material powder including an iron-based powder and zinc stearate, and sintering the dewaxed molded body,
The dewaxing step includes the steps of: preparing a water vapor-containing nitrogen atmosphere containing nitrogen and water vapor and having a dew point of 0°C or higher and 30°C or lower; heating the water vapor-containing nitrogen atmosphere to 300°C to 500°C and supplying the nitrogen atmosphere to a dewaxing section; and heat-treating the compact at 580 °C or lower in the dewaxing section. The method for producing an iron-based sintered body according to claim 1, comprising preparing the water vapor-containing nitrogen atmosphere by mixing water vapor and a nitrogen atmosphere, the nitrogen atmosphere containing 95% or more by volume of nitrogen. The method for producing an iron-based sintered body according to claim 1 or 2, wherein the atmosphere in the dewaxing section contains less than 5% by volume of hydrogen. The method for producing an iron-based sintered body according to any one of claims 1 to 3 , wherein the raw powder of the compact contains Cu in an amount of more than 0 mass % and not more than 20 mass %, with the remainder being iron and unavoidable impurities. The method for producing an iron-based sintered body according to any one of claims 1 to 4 , wherein the raw material powder for said compact contains Mn and S in a total amount of 0.2 mass % or less. The method for producing an iron-based sintered body according to any one of claims 1 to 5 , wherein the raw material powder for the compact further contains C in a range of from more than 0 mass % to 5 mass % or less. The method for producing an iron-based sintered body according to any one of claims 1 to 6 , wherein the zinc stearate is 0.05 to 5 parts by mass with respect to 100 parts by mass of the raw material powder. The method for producing an iron-based sintered body according to any one of claims 1 to 7 , wherein the dewaxing and the sintering are carried out in a continuous sintering furnace. 9. The method for producing an iron-based sintered body according to claim 8, wherein the dewaxing is performed by heating the molded body by passing it through a plurality of partitioned spaces having different temperatures and atmospheres, and the flow rate of the water vapor-containing nitrogen atmosphere in the dewaxing section on the upstream side of the conveying direction is reduced and the flow rate of the water vapor-containing nitrogen atmosphere in the dewaxing section on the downstream side of the conveying direction is increased along the conveying direction of the molded body.

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