JP2019026880A - Method for producing forging material - Google Patents

Abstract

To provide a method for producing a forging material possessing both excellent impact resistance and excellent flexure strength.SOLUTION: A method for producing a forging material includes a step of molding a base powder consisting of 0.5-2.0 mass% of Mo and 0.4 mass% or less of carbon and the remainder of iron and impurities, and obtaining a sintered body by sintering a compact having a density of 6.8 g/cmor higher, a step of forging the sintered body with a surface occupancy ratio of 99.7% or higher and a total density ratio of 98.5% or more, and a step of subjecting the forged sintered body to a carburization heat treatment.SELECTED DRAWING: None

Description

  The present invention relates to a method for producing a forged material.

  For automotive parts that require complex shapes and high strength, hot forging (partly cold forging) using melted material is the mainstream worldwide. In addition, sintered materials using metal powder as a raw material are widely used for small, complex-shaped parts such as precision machine parts and household appliance parts, while high-strength automobile parts with high mass production effects have high strength in North America. Sintered materials are actively used, and the use of high-strength sintered materials is also increasing in Europe and Japan.

  Hot forging of melted materials is currently mainstream in Japan, but post-treatment is necessary because of problems of size change and surface roughness, which increases the manufacturing cost of forged materials. Development of die materials and coating technology with high heat resistance and wear resistance, reduction of burden on dies and molding machines by lubrication and controlled forging, CAE to reduce lead time and manufacturing cost of forging materials due to process shortening effect, CAE (Computer Aided Engineering) Researches such as speeding up analysis are being conducted.

  On the other hand, in cold forging of melted materials, problems of size change and surface roughening are unlikely to occur, but as the size increases, heavy equipment that can withstand heavy loads is required, and multiple forging processes occur, resulting in molten material. Similar to the hot forging, there is a problem that the manufacturing cost of forged parts increases. Further, in the forging of the melted material, there is a problem that there are many discarded portions that become waste materials during the primary processing of the melted material before forging.

  Next, the sintered material using metal powder as a raw material has a small amount of waste material, and can be mass-produced at low cost, in a small size, with high accuracy, and with a complicated shape. However, there is a problem that a sintered material obtained by sintering metal powder is brittle and has low strength. This is because pores present on the surface and inside of the sintered material are the main factors, and the sintered material tends to have a non-uniform metal structure compared to the melted material.

  Here, Patent Documents 1 and 2 disclose a method for manufacturing a sintered member that can increase the strength of the sintered member (sintered material) and obtain the same strength as when the melted material is used.

JP 2012-77348 A JP2013-204080A

  However, in the sintered member manufacturing methods of Patent Documents 1 and 2, it is difficult to achieve both impact resistance and bending strength. Especially when the density ratio is set to 97.8% or more by forging, The bending strength of the binding member may not be sufficient.

  The present invention has been made in view of the above problems, and an object of the present invention is to provide a method for producing a forging material that achieves both excellent impact resistance and excellent bending strength.

One embodiment of the present invention relates to the following.
<1> A raw material powder containing 0.5% by mass to 2.0% by mass of Mo and 0.4% by mass or less of carbon and the balance of iron and impurities is formed, and the density is 6.8 g. The step of obtaining a sintered body by sintering a compact having a density of at least ³ cm / cm3, and the sintering so that the surface occupation ratio is 99.7% or higher and the overall density ratio is 98.5% or higher. A method for producing a forging material, comprising: forging a body; and carburizing heat treatment of the forged sintered body.
<2> The method for producing a forged material according to <1>, wherein the density of the formed body is 7.2 g / cm ³ or more.
<3> A raw material powder containing 0.5% by mass to 2.0% by mass of Mo and 0.4% by mass or less of carbon and the balance of iron and impurities is formed, and the overall density ratio is The step of obtaining a sintered body by sintering a molded body of 86.3% or more, and the sintering so that the surface occupation ratio is 99.7% or more and the overall density ratio is 98.5% or more. A method for producing a forging material, comprising: forging a bonded body; and carburizing heat treatment of the forged sintered body.
<4> The method for producing a forged material according to <3>, wherein an overall density ratio of the molded body is 91.4% or more.
<5> In the forging step, the sintered body is forged so that the surface occupation ratio is 99.9% or more and the total density ratio is 99.0% or more. The manufacturing method of the forging material of any one of Claims 1.
<6> The method for producing a forged material according to any one of <1> to <5>, wherein a sintering temperature when the compact is sintered is 750 ° C to 1250 ° C.
<7> The method for producing a forged material according to any one of <1> to <6>, wherein a sintering time when the molded body is sintered is 20 minutes or more.
<8> The method for producing a forging material according to any one of <1> to <7>, including a step of heat-treating the sintered body after the forging step and before the carburizing heat treatment step.

  ADVANTAGE OF THE INVENTION According to this invention, the manufacturing method of the forging material which was able to make the outstanding impact resistance and the outstanding bending strength compatible can be provided.

It is a schematic block diagram of the compression molding machine for manufacturing the molded object formed by compressing raw material powder used with the manufacturing method of this indication. It is a schematic block diagram of the compression molding machine for cold forging the sintered compact used with the manufacturing method of this indication. It is a graph (primary sintering time 20 minutes) which shows the relationship between the primary sintering temperature and the bending strength of a sintered compact when changing the density of a molded object. It is a graph (primary sintering time 20 minutes) which shows the relationship between the primary sintering temperature and the Charpy impact value of a sintered compact when changing the density of a molded object. It is a graph (primary sintering time 60 minutes) which shows the relationship between the primary sintering temperature and the Charpy impact value of a sintered compact when changing the density of a molded object. It is a graph (primary sintering time 20 minutes) which shows the relationship between the primary sintering temperature and the hardness of a sintered compact when changing the density of a molded object. When the primary sintered temperature (975 ° C., 1025 ° C., 1075 ° C.) was changed and the compact was sintered for 20 minutes, the load during the cold forging of the sintered compact and the sintering after the cold forging treatment It is a graph which shows the relationship with the whole body density. In the graph (the primary sintering temperature 1075 degreeC, the primary sintering time 20 minutes) which shows the relationship between the density of the sintered compact after cold forging, and flash (remaining wall) height when changing the density of a molded object is there. A graph (primary) showing the relationship between the density of the sintered body after cold forging and the flash (remaining wall) height when the molded body is sintered while changing the primary sintering time (20 minutes, 60 minutes) Sintering temperature 1075 ° C.). The load at the time of cold forging when the primary sintering temperature (975 ° C., 1025 ° C., 1075 ° C.) is changed, and the bending strength of the forging material (after cold forging and after gas carburizing) 3 is a graph showing the relationship (molded body density 6.8 g / cm ³ , primary sintering time 20 minutes). The load at the time of cold forging when the primary sintering temperature (975 ° C., 1025 ° C., 1075 ° C.) is changed, and the bending strength of the forging material (after cold forging and after gas carburizing) 3 is a graph showing the relationship (molded body density 7.2 g / cm ³ , primary sintering time 20 minutes). The load at the time of cold forging when the primary sintering temperature (975 ° C., 1025 ° C., 1075 ° C.) is changed, and the bending strength of the forging material (after cold forging and after gas carburizing) It is a graph (compact density 7.4 g / cm ³ , primary sintering time 20 minutes) showing the relationship. The relationship between the load at the time of the cold forging process when the primary sintering temperature (1000 ° C., 1075 ° C.) is changed and the bending strength of the forged material (after the cold forging process and after the gas carburizing process) are shown. It is a graph (molded body density of 7.4 g / cm ³ , primary sintering time 60 minutes). It is a reflection electron image of the surface layer cross section after the cold forging process in Example 3 (The density of a molded object 7.4g / cm < ³ >, sintering temperature 1050 degreeC, sintering time 20 minutes). In Example 3 (molded body density 7.4 g / cm ³ ), the secondary sintering temperature and the Charpy impact of the forging material when the primary sintering temperature (875 ° C., 975 ° C., 1050 ° C.) was changed. It is a graph which shows the relationship with a value. It is a graph which shows the relationship between the repetition number until the fracture | rupture of a forging material, and a load at the time of changing the load (600 kN, 800 kN, 1200 kN) at the time of cold forging.

Hereinafter, embodiments for carrying out the present invention will be described in detail. However, the present invention is not limited to the following embodiments. In the following embodiments, the components are not essential unless otherwise specified. The same applies to numerical values and ranges thereof, and the present invention is not limited thereto.
In the present disclosure, the numerical ranges indicated using “to” include numerical values described before and after “to” as the minimum value and the maximum value, respectively.
In the numerical ranges described stepwise in the present disclosure, the upper limit value or the lower limit value described in one numerical range may be replaced with the upper limit value or the lower limit value of another numerical description. . Further, in the numerical ranges described in the present disclosure, the upper limit value or the lower limit value of the numerical range may be replaced with the values shown in the examples.

Hereinafter, an embodiment of a method for producing a forged material of the present invention will be described. The method for producing a forged material according to the present disclosure includes forming a raw material powder containing 0.5 mass% to 2.0 mass% of Mo and 0.4 mass% or less of carbon, with the balance being iron and impurities. And a step of obtaining a sintered body by sintering a molded body having a density of 6.8 g / cm ³ or more, an occupation ratio of the surface of 99.7% or more, and an overall density ratio of 98.5% or more. The process includes forging the sintered body and subjecting the forged sintered body to carburizing heat treatment. By this manufacturing method, a forged material that has both excellent impact resistance and excellent bending strength can be manufactured.
The “impurities”, “surface occupation ratio”, and “total density ratio” are as described later.

  As the raw material powder used in the production method of the present disclosure, Mo includes 0.5 mass% to 2.0 mass% and carbon includes 0.4 mass% or less, with the balance being iron and impurities. That's fine. For example, as a raw material powder, iron powder, powder of various metal elements, mixed powder mixed with carbon powder, etc .; iron alloy powder alloyed with various metal elements, mixed powder mixed with carbon powder, etc .; iron alloy powder, various Examples thereof include mixed powders obtained by mixing metal element powders, carbon powders, and the like. More specifically, as raw material powder, Mo: 0.5% by mass to 2.0% by mass, balance: iron alloy powder composed of iron and impurities, and 0.4% by mass or less of carbon powder is mixed. Powder may be used. Examples of the carbon powder include graphite powder and carbon black.

  The raw material powder preferably contains 0.6% by mass to 1.5% by mass of Mo, and 0.8% by mass to 1.2% by mass from the viewpoint of promoting the element diffusion function during sintering of the molded body. More preferably, it is more preferable to contain 0.9 mass%-1.1 mass%.

  The raw material powder preferably contains 0.1% by mass to 0.4% by mass of carbon, more preferably 0.2% by mass to 0.4% by mass, and more preferably 0.3% by mass to 0.4% by mass. It is more preferable to include.

  Impurities are not particularly limited as long as they can be contained in iron powder, iron alloy powder, carbon powder, etc. In addition to so-called inevitable impurities, Ni, Cr, Mn, Cu, Zn, Co, Nb, Ti, V , W, Li and the like.

  Further, when the raw material powder is compressed into a compact, a lubricant may be added to the raw material powder from the viewpoint of lubrication between the raw material powder and the mold. Examples of the lubricant include metal soaps such as zinc stearate and lithium stearate, waxes such as EBS (amide wax), and composites thereof. The addition amount of the lubricant is, for example, preferably 0.1% by mass to 1.0% by mass with respect to 100% by mass of the raw material powder excluding the lubricant, and 0.1% by mass to 0.5% by mass. It is more preferable that When the addition amount of the lubricant is 0.1% by mass or more, there is a tendency to reduce friction generated when the raw material powder is formed into a molded body, and when the addition amount of the lubricant is 1.0% by mass or less, It tends to be easier to increase the density of the molded body.

  In the manufacturing method of the present disclosure, a molded body (a green compact) obtained by compressing raw material powder is used. The compact is obtained by supplying the raw material powder to a mold and compressing it. Examples of the raw material powder compression method include a one-pushing method, a floating die method, and a withdrawal method.

  In the floating die method effective for adjusting the density gradient (neutral zone position), for example, a raw material powder is compressed using a compression molding machine as shown in FIG. A compression molding machine 1 shown in FIG. 1 is held by a spring 11 and moves up and down by friction during molding, an upper punch 3 and a lower punch 3 that compress a raw material powder 6 supplied into a mold 8. And a punch 5. In the compression molding machine 1, the raw material powder 6 supplied into the mold 8 is sandwiched between the upper punch 3 and the lower punch 5 and compressed.

  The shape of the molded body is not particularly limited. For example, the shape according to the use of the forging material manufactured may be sufficient, and general shapes, such as a rectangular parallelepiped shape, a cylindrical shape, and a cube shape, may be sufficient.

When the raw material powder is compressed into a compact, the density of the compact is 6.8 g / cm ³ or more. Thereby, the crack of a sintered compact, a crack, a clearance gap, etc. can be suppressed, and finally high impact resistance and high bending strength are obtained. The compact density is preferably 7.0 g / cm ³ or more, and more preferably 7.2 g / cm ³ or more from the viewpoint of further increasing the density after forging the sintered body and excellent plastic deformability during forging. It is more preferable that Further, the raw material powder may be compressed to form a molded body so that the molded body density is 7.5 g / cm ³ or less or 7.4 g / cm ³ or less.
When the plastic deformability at the time of forging is excellent, it is easy to increase the density of the surface and surface layer of the sintered body after forging. Thereby, the destruction resulting from pores in the surface layer is suppressed, and the fatigue life, the friction life, and the like tend to be extended. Furthermore, when manufacturing a complex-shaped part as a forging material by the manufacturing method of this indication, it is easy to manufacture without a crack by one forging.

  The compact density can be measured by the Archimedes method. Moreover, the pressure at the time of molding may be adjusted to a predetermined molded body density. Specifically, the pressure at the time of molding may be 350 MPa to 1500 MPa, 600 MPa to 1200 MPa, or 800 MPa to 1000 MPa. .

The manufacturing method of this indication includes the process (primary sintering process) which obtains a sintered compact by sintering the compact | molding | casting whose density is 6.8 g / cm < ³ > or more as mentioned above. Thereby, the sintered compact formed by sintering a molded object is obtained.

  The sintered compact is preferably performed in a non-oxidizing gas atmosphere such as nitrogen gas or in a vacuum atmosphere from the viewpoint of suppressing the oxidation of the sintered body. The degree of vacuum may be, for example, 0.03 kPa or less.

  Moreover, as a sintering temperature (primary sintering temperature) when sintering a molded object, it is preferable that it is 750 degreeC-1250 degreeC, it is more preferable that it is 900 degreeC-1150 degreeC, and 1000 degreeC-1150 degreeC. It is further more preferable that it is 1050 to 1100 degreeC. By increasing the sintering temperature, precipitation of carbon such as graphite at grain boundaries is suppressed, and the forging material obtained by the production method of the present disclosure tends to have excellent impact resistance (for example, Charpy impact value). .

  Moreover, as sintering time (primary sintering time) when sintering a molded object, it is preferable that it is 20 minutes or more, It is more preferable that it is 30 minutes-80 minutes, It is 50 minutes-1 hour. More preferably. The longer sintering time suppresses the precipitation of carbon such as graphite at grain boundaries, and the sintered body after primary sintering tends to be excellent in bending strength and plastic deformability during forging.

  Moreover, from the point which is excellent in the bending strength of the sintered compact after primary sintering, and the plastic deformability at the time of forging, the sintering temperature (primary sintering temperature) when sintering a molded object becomes like this. Preferably it is 950 to 1050 degreeC. More preferably, the temperature is 975 ° C. to 1025 ° C., and the sintering time (primary sintering time) when the molded body is sintered is preferably 30 minutes to 80 minutes, more preferably 50 minutes to 1 hour.

In the manufacturing method of the present disclosure, when the raw material powder is compressed into a molded body, the overall density ratio may be 86.3% or more. Thereby, the crack of a sintered compact, a crack, etc. can be suppressed and high impact resistance and high bending strength are obtained. The overall density ratio of the molded body is 88.9% (the overall density is 7.0 g / cm ^{3) because} the density after forging the sintered body is higher and the plastic deformability during forging is excellent. Or more), more preferably 91.4% (corresponding to a total density of 7.2 g / cm ³ ) or more. Further, the raw material powder may be compressed to form a molded body so that the density ratio of the entire molded body is 94.0% (corresponding to the total density corresponding to 7.4 g / cm ³ ) or less.

In the forging step, the sintered body may be forged so that the occupation ratio of the surface is 99.7% or more and the overall density ratio is 98.5% or more. By this treatment, the forged sintered body has a high density in the surface layer region and a high density in the whole, and the forged material obtained after the carburizing heat treatment described later has a high density and a long life. Further, when the sintered body is forged so that the overall density ratio is 97.8% to 98.4%, extremely small cracks are likely to be formed along the grain boundary in the vicinity of the surface layer, and finally obtained. This is not preferable because the bending strength of the forged material is greatly reduced.
In addition, when forging a sintered compact, hot forging may be sufficient and cold forging may be sufficient. Cold forging is preferable from the viewpoint of suppressing size change and surface roughness.

  In the forging step, the sintered body may be forged so that the surface occupation ratio is preferably 99.8 (99.80)% or more, more preferably 99.9 (99.90)% or more. Good. In addition, you may forge a sintered compact so that the occupation ratio of the surface may be 99.91% or less, for example, Preferably it is 99.90% or less.

  The sintered body may be forged so that the overall density ratio is preferably 98.7% or more, more preferably 99.0% or more. Thereby, the crack which arises in a sintered compact is crimped | bonded, and it exists in the tendency for the bending strength of the forging material finally obtained after heat processing to improve. In addition, you may forge a sintered compact so that the whole density ratio may be 99.5% or less, for example.

The occupying ratio of the surface of the forged sintered body refers to the area ratio of the sintered body in the plane that penetrates 0.2 mm from the outermost surface to the area other than the pores with respect to the total cross-sectional area including the metal structure. .
In addition, since the plastic flow of the material occurs in the vicinity of the surface portion and the surface layer is densified, the occupation ratio of the surface of the forged sintered body becomes higher than the overall density ratio. Here, the occupation ratio of the surface is obtained by cutting out an area having a depth of 0.2 mm and a width of 0.3 mm from the outermost surface of the sintered body, and performing image analysis of the surface layer pore distribution on the cross section of the sintered body after being cut out. It can be confirmed by doing.

  The overall density ratio of the forged sintered body can be calculated by, for example, obtaining the apparent density by the Archimedes method and calculating the ratio of the apparent density to the true density (apparent density / true density).

  Forging is performed, for example, by supplying a sintered body to a mold hole of a forging die, and compressing and compressing the sintered body, for example. For example, the sintered compact is compressed and forged using a compression molding machine as shown in FIG. The compression molding machine 2 shown in FIG. 2 includes a die 10 held by a fixed rod 12, and an upper punch 4 and a lower punch 5 that compress a sintered body 7 supplied in an open mold 9. Prepare. Further, when the sintered body 7 is compressed by the upper punch 4, a gap is formed between the inner side surface of the open mold 9 and the side surface of the upper punch 4, and the sintered body 7 deformed into this gap. A protrusion that is a part of the protrusion is generated, and the height of the protrusion is called a flash height.

  As for the load when compressing and compressing the sintered body supplied to the mold hole of the forging die, the surface occupation ratio is 99.7% or more and the overall density ratio is 98.5 for the sintered body after forging. It is adjusted to be at least%. For example, the load when compressing and compressing the above-described sintered body may be 950 kN to 1400 kN, 1000 kN to 1300 kN, or 1100 kN to 1200 kN. In addition, the load when compressing and compressing the above-mentioned sintered body may be appropriately adjusted according to the shape, size, etc. of the sintered body, and is not limited to the above numerical range.

As the density of the compact increases, the load when adjusting the surface of the sintered body after forging to 99.7% or more and the overall density ratio to 98.5% or more tends to be reduced. It is in.
For example, when the density of the molded body is less than 6.8 g / cm ³ or more 7.2 g / cm ^3, the load when the pressure compressing the sintered body is preferably 1050KN～1400kN, in 1100kN~1300kN More preferably.
Further, when the density of the compact is 7.2 g / cm ³ or more and less than 7.4 g / cm ³ , the load when compressing and compressing the sintered body is preferably 1000 kN to 1400 kN, and 1050 kN to 1300 kN. More preferably.
Moreover, when the density of a molded object is 7.4 g / cm < ³ > or more, it is preferable that the load at the time of compressing and compressing a sintered compact is 1000 kN-1400 kN, and it is more preferable that it is 1000 kN-1300 kN.

  After the step of sintering the molded body in a vacuum atmosphere, before the forging step, the sintered body may be cooled and annealed in an inflow nitrogen atmosphere, for example, in an inflow nitrogen atmosphere It may be annealed by cooling to 50 ° C. at 10 ° C./min to 20 ° C./min, more specifically about 15 ° C./min.

  The manufacturing method of this indication may include the process of heat-treating a sintered compact after the process of forging, and before the process of carburizing heat treatment mentioned below. The step of heat treatment may be a step of sintering the sintered body after forging (secondary sintering step). By performing a heat treatment after the forging step, crack recovery becomes prominent, and the pressure-bonding cracks tend to be reduced and harmless.

  In the step of heat-treating the sintered body, the heat treatment temperature of the sintered body is preferably 850 ° C to 1250 ° C, more preferably 950 ° C to 1150 ° C, and 1000 ° C to 1100 ° C. Is more preferable. By increasing the sintering temperature, the forging material obtained by the production method of the present disclosure tends to be excellent in impact resistance (for example, Charpy impact value).

  Further, the heat treatment time of the sintered body is preferably 20 minutes or more, more preferably 30 minutes to 80 minutes, and further preferably 50 minutes to 1 hour.

  The manufacturing method of this indication includes the process of carburizing heat treatment of the forged sintered compact. Examples of the carburizing heat treatment include gas carburizing treatment and vacuum carburizing treatment. Among these, vacuum carburizing treatment is preferable because carburizing heat treatment at a higher temperature is easy. The forged material produced by carburizing heat treatment has excellent surface hardness and wear resistance, and mechanical strength, hardness, impact resistance, fatigue strength, wear resistance, etc., compared with the sintered body. The characteristics are greatly improved. Moreover, the process of carburizing heat treatment may serve as the above-mentioned secondary sintering.

  For example, as the vacuum carburizing treatment, it is preferable to perform the carburizing treatment under conditions of a degree of vacuum of 10 Pa or less, a temperature of 900 ° C. to 1100 ° C., and 20 minutes to 1 hour.

  The forged material obtained after the carburizing heat treatment preferably has an effective hardened layer having an HV (Vickers hardness) of 550 or more in terms of increasing fatigue strength by 0.7 mm or more. It is preferable to have 4 mm or more.

  The forged material obtained after the carburizing heat treatment is preferably tempered from the viewpoint of improving toughness and suppressing distortion. The tempering treatment may be performed in the atmosphere at a temperature of about 180 ° C., similarly to the tempering performed on general steel materials and sintered bodies.

The forged material obtained by the production method of the present disclosure preferably has a Charpy impact value of 15 J / cm ² or more, more preferably 40 J / cm ² or more, and more preferably 100 J / cm ² or more by heating immediately before carburizing. It is more preferable that it is 300 J / cm ² or more. The Charpy impact value of the forged material is a value obtained by preparing a test piece manufactured with the same composition and the same conditions as the forged material obtained by the manufacturing method of the present disclosure and performing a Charpy impact test on the test piece. is there.

  The forged material obtained by the production method of the present disclosure preferably has a bending strength of 1600 MPa or more, and more preferably 1650 MPa or more. The bending strength of the forged material is a value obtained by preparing a test piece manufactured under the same composition and the same conditions as the forged material obtained by the manufacturing method of the present disclosure, and conducting a three-point bending strength test on the test piece. It is.

  The forged material obtained by the manufacturing method of the present disclosure can be applied to all small complex shaped parts that require strength, such as various automobile parts such as levers, gears, sprockets, and bearings.

  EXAMPLES The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples.

<Examples 1-3>
(Preparation of raw material powder)
Atomized pure iron powder for sintering forging (main particle size of 100 μm or less, JFE Steel Corporation: SGM10MO) in which molybdenum (Mo) powder is diffused and adhered to the surface of the iron particles as raw material powder, and further graphite powder and wax-based lubricant are added. -CMX) was used. The content of each component in the raw material powder is as follows.
Mo ... 1.0 mass%
Carbon: 0.35 mass%
Wax-based lubricant: 0.5% by mass (amount based on 100% by mass of raw material powder excluding wax-based lubricant)
Iron ... balance

(Preparation of molded body)
Next, the raw material powder prepared as described above was supplied into a mold and compressed by a floating die method using a compression molding machine as shown in FIG. 1 to prepare a compact. In Examples 1 to 3, the molding pressure, the size of the obtained molded body, and the density of the molded body are as shown in Table 1. In addition, the density of a molded object is measured by the Archimedes method, and is substantially the same as the density of the sintered compact mentioned later.

(Manufacture of sintered body)
The molded bodies in Examples 1 to 3 were put into a sintering furnace, and sintered in a temperature range of 975 ° C. to 1100 ° C. for 20 minutes or 60 minutes in a vacuum atmosphere (pressure 0.03 kPa or less). (Primary sintered body) was obtained.

(Mechanical properties of sintered body)
Next, the mechanical properties of the sintered body obtained as described above were examined. Specifically, the bending strength, Charpy impact value, and hardness of the sintered body were measured. The results are shown in FIGS.

<Measurement method of bending strength>
The bending strength of a part of the sintered body obtained as described above was determined. Specifically, the sintered body is placed in a three-point bending jig so that the fracture occurs from the surface where the density of the sintered body is the densest (upper punch surface), and a three-point bending strength test is performed. The bending strength was measured. As shown in FIG. 3, the bending strength tends to increase by increasing the density (ρ) and the sintering temperature of the compact, and in particular, the density of the compact is 7.2 g / cm ³ and the sintering temperature is high. When the temperature is 1000 ° C. or higher, the bending strength tends to be higher.

<Measurement method of Charpy impact value>
The Charpy impact value of some of the sintered bodies obtained as described above was determined. Specifically, the sintered compact was arranged so that impact destruction occurred from the surface (upper punch surface) where the density of the sintered compact was the highest, and a Charpy impact test was performed to obtain a Charpy impact value. As shown in FIGS. 4 and 5, the Charpy impact value tends to increase by increasing the density and sintering temperature (Pre-sintering: PS temperature) of the molded body. When 2 g / cm ³ and the sintering temperature is 1000 ° C. or higher, the Charpy impact value tends to be higher.

<Measurement method of hardness>
The hardness of a part of the sintered body obtained as described above was measured. Specifically, the surface hardness of the side surface of the sintered body was measured with a Rockwell hardness tester. As shown in FIG. 6, the surface hardness tends to increase by increasing the density and sintering temperature of the compact, and in particular, the density of the compact is 7.2 g / cm ³ and the sintering temperature is 1000 ° C. At the above time, the surface hardness tends to be higher.

(Cold forging process)
The sintered body obtained as described above was cold forged. Specifically, the obtained sintered body is supplied to a die hole of a cold forging die, and this sintered body is placed at room temperature and a load of 400 kN to 1300 kN using a compression molding machine as shown in FIG. Compressed and compressed with a load time of 1 second.

In Examples 1 to 3, when the primary sintered temperature (975 ° C., 1025 ° C., 1075 ° C.) was changed and the molded body was sintered for 20 minutes, the load during cold forging of the sintered body, The relationship with the density of the sintered body after the forging process is shown in FIG. As shown in FIG. 7, the density (ρ _CF ) of the sintered body after the cold forging process tends to increase as the load during cold forging or the density of the formed body increases. Moreover, when the load at the time of cold forging was 1100 kN to 1300 kN, the density of the sintered body after the cold forging process was almost constant. Further, when the density 7.874 g / cm ³ of the melted material having the composition as in Examples 1 to ³ is 100%, the density ratio of the sintered body after the cold forging treatment is 98.5% or more (about 7.756 g / The load at the time of cold forging to be cm ³ or more is approximately as follows.
Example 1 (density of the molded body 6.8 g / cm ³ )... 1050 kN to 1300 kN
Example 2 (Density of molded body: 7.2 g / cm ³ ) ... 1000 kN to 1300 kN
Example 3 (Density of 7.4 g / cm ³ ) ··· 1000 kN to 1300 kN

  Moreover, in Examples 1-3, the occupation ratio of the surface of the sintered compact after the cold forging process was confirmed by surface layer region pore distribution image analysis. Specifically, an area having a depth of 0.2 mm and a width of 0.3 mm was cut out from the outermost surface of the sintered body, the cut surface was alumina buffed, photographed at 400 times with a metal microscope, and the entire image (about 300 μm width × The range of 200 μm depth) was binarized into white (metal structure) and black (pores), and the occupation ratio of the surface of the sintered body was determined from the ratio of black pixels to the whole. As a result, it was confirmed that the sintered compact after the cold forging process in which the overall density ratio was 98.5% or more had a surface occupation ratio of 99.7% or more.

In Examples 1 to 3, the relationship between the density of the sintered body after the cold forging process and the flash height (flash height) is shown in FIG. The larger the flash height, the better the sintered body has plastic deformability. As shown in FIG. 8, when the density of the molded body is high, for example, 7.2 g / cm ³ or more, surplus cracks can be suppressed by deformation due to forging. FIG. 9 shows the relationship between the density of the sintered body after the cold forging process and the flash height when the sintering time is changed. As shown in FIG. 9, when the sintering time is long (60 minutes), the density and flash height of the sintered body after the cold forging process are higher than when the sintering time is short (20 minutes).

(Gas carburizing treatment)
About the sintered compact after a cold forging process, the gas carburizing process which served as secondary sintering was performed at 900 degreeC for 180 minutes. Thereafter, oil quenching treatment was performed at 860 ° C. for 30 minutes, and then tempering treatment was performed at 180 ° C. for 90 minutes to produce a forged material.

About the manufactured forging material, the density of a molded object when a primary sintering temperature (975 degreeC, 1000 degreeC, 1025 degreeC, 1075 degreeC) and time (20 minutes, 60 minutes) are changed, and a cold forging process The relationship between the load at the time and the bending strength of the forged material (after the cold forging process and after the gas carburizing process) is shown in FIGS.
As shown in FIG. 10, in Example 1 where the density of the molded body is 6.8 g / cm ³ , the bending strength that has been on the rise until then is reduced when the load during cold forging is around 900 kN to 1000 kN. However, the bending strength of the forged material increases again when the load during cold forging is 1050 kN or more.
As shown in FIG. 11, in Example 2 where the density of the molded body is 7.2 g / cm ³ , the bending strength decreases when the cold forging load is around 950 kN, and the cold forging load is 1000 kN. As described above, the bending strength of the forged material is increased.
As shown in FIG. 12, in Example 3 in which the density of the molded body was 7.4 g / cm ³ , the bending strength that had been on the rise until then was reduced when the load during cold forging was around 800 kN to 900 kN. However, the bending strength of the forged material increases again when the load during cold forging is 1000 kN or more.
As shown in FIG. 13, in Example 3 in which the density of the compact was 7.4 g / cm ³ , the bending strength that had been on the rise until then was reduced when the load during cold forging was around 600 kN to 700 kN. However, the bending strength of the forged material increases again when the load during cold forging is 1000 kN or more.

Next, FIG. 14 shows a reflected electron image of the cross section of the surface layer after the cold forging treatment in Example 3 (the density of the compacted body is 7.4 g / cm ³ , the sintering temperature is 1050 ° C., and the sintering time is 20 minutes). In FIG. 14, (a) is the case where the load during cold forging is 600 kN, (b) is the case where the load during cold forging is 800 kN, and (c) is the load during cold forging 950 kN. (D) is a case where the load at the time of cold forging is 1200 kN. As shown in FIG. 14, it is considered that the larger the load during cold forging, the smaller the gap and the better the strength and wear resistance.
In addition, in FIG. 14C, micro cracks were observed. At this time, as shown in FIG. 7, the density ratio of the sintered body after the cold forging treatment is about 98%, and microcracks are generated before and after this density ratio. It is thought that the bending strength tends to decrease.

Considering FIGS. 10 to 13 and FIGS. 14A and 14B, for example, when the load during cold forging is 600 kN to 800 kN (the density ratio of the entire sintered body after cold forging is 98). 12, in the case of 600 kN to 700 kN in FIG. 12 and in the case of 500 kN to 600 kN in FIG. 13), the bending strength of the forged material tends to increase as the load increases, while the fatigue life of the forged material, It is presumed that the wear resistance is insufficient.
Further, considering (c) in FIGS. 10 to 13 and 14, for example, when the load during cold forging is over 800 kN to about 1000 kN (the density ratio of the entire sintered body after cold forging is 98). In FIG. 12, in the case of more than 700 kN to 900 kN, and in FIG. 13 in the case of more than 600 kN to 900 kN), microcracks are generated along the grain boundaries in the surface layer region, so It is estimated that the strength is insufficient.
Further, considering (d) in FIGS. 10 to 13 and 14, for example, when the load during cold forging increases from around 1000 kN (the density ratio of the entire sintered body after cold forging is 98.N). 5% or more) is presumed to be excellent in bending strength of the forged material, and excellent in fatigue life and wear resistance of the forged material.

Next, in the sintered body after cold forging obtained in Example 3 (density of compact 7.4 g / cm ³ , sintering temperature 875 ° C., 975 ° C., 1050 ° C., sintering time 20 minutes) When secondary sintering was performed, the relationship between the secondary sintering temperature and the Charpy impact value of the forged material was examined. Here, the secondary sintering temperature is 900 ° C. to 1100 ° C., and the secondary sintering time is 60 minutes. The results are shown in FIG.
As shown in FIG. 15, it was estimated that the Charpy impact value tends to be improved by increasing the primary sintering temperature or the secondary sintering temperature. In particular, it was confirmed that the Charpy impact value tends to be improved by increasing the primary sintering temperature.

<Three point bending fatigue test>
The sintered body after cold forging under the conditions of loads of 600 kN, 800 kN and 1200 kN is subjected to the above-mentioned gas carburizing treatment, oil quenching treatment and tempering treatment, and a forged material for a three-point bending fatigue test (effective A carburized depth of about 0.7 mm, a square bar) was produced.
Next, the three-point bending fatigue test is performed on the forged material for the three-point bending fatigue test under the conditions of both-end support, center one-point pressurization, the distance between fulcrums of 50 mm, the load range of 1020 MPa to 1180 MPa and the load speed of 20 Hz. It was. The results are shown in FIG.

As shown in FIG. 16, the forged material cold-forged under the condition of a load of 1200 kN (CF 1200 kN, the overall density ratio of the sintered body after cold forging is 98.5% or more) is a condition of a load of 600 kN or 800 kN. Compared with the forging material cold-forged at 600 (CF600 kN or 1000 kN, the total density ratio of the sintered body after cold forging is less than 98.5%), the number of repetitions until breakage was high.
Therefore, it was shown that the fatigue life of the forged material is excellent when the density ratio of the entire sintered body after cold forging is 98.5% or more.

1, 2 Compression molding machine 3, 4 Upper punch 5 Lower punch 6 Raw material powder 7 Sintered body 8 Mold 9 Open mold 10 Die 11 Spring 12 Fixed rod

Claims (8)

A raw material powder containing 0.5% by mass to 2.0% by mass of Mo and 0.4% by mass or less of carbon, the balance being iron and impurities, is formed, and the density is 6.8 g / cm ^3. Sintering the molded body as described above to obtain a sintered body,
Forging the sintered body so that the occupation ratio of the surface is 99.7% or more and the overall density ratio is 98.5% or more;
A method for producing a forging material, comprising: subjecting the forged sintered body to a carburizing heat treatment. The method for producing a forged material according to claim 1, wherein the density of the compact is 7.2 g / cm ³ or more. A raw material powder containing 0.5% by mass to 2.0% by mass of Mo and 0.4% by mass or less of carbon, with the balance being iron and impurities, is formed, and the overall density ratio is 86.3. % To obtain a sintered body by sintering a molded body that is at least%,
Forging the sintered body so that the occupation ratio of the surface is 99.7% or more and the overall density ratio is 98.5% or more;
A method for producing a forging material, comprising: subjecting the forged sintered body to a carburizing heat treatment.   The method for producing a forged material according to claim 3, wherein a density ratio of the entire compact is 91.4% or more.   5. The method according to claim 1, wherein, in the forging step, the sintered body is forged so that a surface occupation ratio is 99.9% or more and an overall density ratio is 99.0% or more. The manufacturing method of the forging material as described in a term.   The method for producing a forged material according to any one of claims 1 to 5, wherein a sintering temperature when the compact is sintered is 750C to 1250C.   The method for producing a forged material according to any one of claims 1 to 6, wherein a sintering time when the molded body is sintered is 20 minutes or more.   The method for producing a forged material according to any one of claims 1 to 7, further comprising a step of heat-treating the sintered body after the forging step and before the carburizing heat-treating step.