JPH03219040A - High strength sintered steel and its manufacture - Google Patents

Abstract

PURPOSE:To manufacture a high strength sintered steel excellent in the fatigue limit of impact value or the like by sintering a green compact by the mixed powder of alloy steel powder, graphite powder and alloy iron powder, thereafter subjecting the sintered body to nitriding treatment and rapidly cooling it from a specified temp. CONSTITUTION:The powder of alloy iron contg., by weight, one or >=2 kinds among 0.05 to 1.5% Si, 0.5 to 2.0% Cr, 0.5 to 5.0% Ni, 0.1 to 1.0% Mo and 0.1 to 2.0% Mn and the balance Fe or the powder of an alloy steel constituted of the balance Fe and C is regulated as a raw material. As for the alloy iron powder, the powder of graphite is mixed thereto by 0.1 to 1.0%, and the mixed powder is compacted into a prescribed shape, is thereafter heated at a high temp. in a nonoxidizing atmosphere and is sintered. The sintered body is subjected to nitriding treatment or carbo-nitriding treatment at >=600 deg.C to diffuse N and C into the sintered body and is thereafter quenched in oil from the temp. of the A1 transformation point or above to form a retained austenite structure by 30 to 95vol.% on the surface layer of the sintered body and the void parts of the sintered body. Successively, the surface is subjected to shot peening treatment and is finished by the formation of compressed residual stress.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a high-strength sintered steel with improved fatigue limits such as rotary bending mechanical properties and impact values, and a method for manufacturing the high-strength sintered steel. Regarding.

[Conventional technology]

High-strength sintered steel for mechanical parts that generally require high strength and high toughness is an alloy containing appropriate amounts of carbon, nickel, molybdenum, chromium, manganese, copper, etc. as additive elements to improve hardenability. Copper or iron alloy powder, and Fe-Mn-Cr o-C alloy steel (4100 series), F
Powders such as e-Mn-Cr-Mo-C alloy steel (4600 series) are used, and these powders are compacted and pressed to a high density, sintered at a temperature range of 1200 to 1300"C, and then quenched and tempered. Alternatively, the alloy steel sintered body is mechanically strengthened by heat treatment such as carburizing, quenching, and tempering.

However, in these high-strength sintered steels, since notches exist in the voids in the surface layer, microcranks occur during quenching, reducing the fatigue limit strength. Therefore, in order to modify the surface layer of sintered steel, shot peening is applied to the sintered steel so that the fatigue limit strength does not decrease.

[Problem to be solved by the invention]

These high-strength sintered steels have a tensile strength of 90~90 through shot peening and improvements in manufacturing technology.
Although it is quite high at 200 kg/mm2, the fatigue limit that occurs due to the concentration of stress in the sintered steel voids, that is, the rotational bending value is 30-35 kg/mm2, and the impact value is 1-35 kg/mm2.
2 kg-m/cI! Therefore, there is a problem in that it is limited in its application to gear parts that require higher strength.

Therefore, in order to further improve the fatigue limit strength, there is a need for a means of increasing the density of sintered steel by reducing the voids by repressurization, resintering, forging, or the like. Therefore, there is a problem that the cost is high.

Further, if the strength of sintered steel is increased, there is a problem that it cannot be used in sizing, coining, etc., which have been conventionally used to correct the dimensions of sintered steel parts, due to the relationship between the hardness of the part and the life of the mold.

The present invention has been made in view of the above, and includes carbon, nickel, molybdenum, chromium, and other elements that improve hardenability.
By using carbon-containing alloy steel powder or carbon-free alloy iron powder containing various combinations of additive elements such as manganese in a low content range, we can produce products with good dimensional accuracy, improved void defects, and high strength. The object of the present invention is to easily obtain sintered mechanical parts having high toughness and to provide a method for manufacturing the same.

[Means to solve the problem]

As a result of various studies to solve the above problems, the present inventors noticed that the fatigue limit strength can be improved by relaxing the stress concentrated in the voids of the sintered body, which is a factor that affects fatigue. Furthermore, the present invention was completed based on the discovery that the fatigue limit strength of the sintered body can be improved by leaving a large amount of retained austenite structure in the surface layer of the sintered body by nitriding or carbonitriding.

The high-strength sintered steel of the present invention is produced by molding and pressing a mixed powder of carbon-containing alloy steel powder and/or carbon-free alloy iron powder and graphite powder into a predetermined shape, and then sintering it in a high-temperature non-oxidizing atmosphere. After sintering, nitrogen atoms are diffused into the sintered body by nitriding or carbonitriding at a temperature of 600'C or higher, and then the surface of the sintered body is improved by rapid cooling from a temperature higher than the A1 transformation line. It is characterized in that a retained austenite l-&11 weave having a volume ratio of 30 to 95 vol.chi. is formed in the layers and the voids.

Further, a step of molding and pressurizing a mixed powder of a carbon-containing alloy copper powder and/or a carbon-free alloy iron powder and graphite powder, a step of sintering the molded and pressurized compact in a non-oxidizing atmosphere, or After the step of sintering in a non-oxidizing atmosphere, the sintered steel is recompressed and sintered in a non-oxidizing atmosphere, the sintered steel is nitrided or carbo-nitrided at a temperature of 600° C. or higher, and The present invention proposes a method for producing high-strength sintered steel, which includes a step of rapidly cooling sintered steel from a temperature equal to or higher than the A1 transformation line and tempering it.

Next, the configuration of the present invention will be further explained.

The carbon-containing alloy steel powder used in the present invention is 0.05
~1.5wt! Silicon, 0.5-1. owtχ chromium,
0.5-5.0 wt% nickel, 0.1-1. owtχ
It is a composition powder containing one or more of molybdenum and 0.1 to 2.0 wt% manganese, and the balance is iron, unavoidable impurities, and carbon, and contains iron alloy powder and graphite powder that do not contain carbon. Mixed powder with 0.1-1.0w
In addition to t-carbon, the above-mentioned alloy components and the balance are a powder composition containing iron and unavoidable impurities.

The lower limits of the amounts of chromium, nickel, molybdenum, and manganese to be added are determined by taking into account the hardenability of the sintered body, and the upper limits of the amounts of chromium and molybdenum are determined from the viewpoint of cost. The addition of nickel and manganese contributes to the formation of a retained austenite phase, but since manganese is easily oxidized, the upper limit is set so that it has no effect, and nickel is set to a low nickel base from a cost perspective and in relation to the strength of sintered steel. . Here, it is preferable to mix carbon-free ferroalloy and graphite powder because the moldability is good and the carbon content of the sintered body can be freely adjusted. Carbon added is a component that increases the hardness and strength of sintered steel, but if the amount added is less than 0.1wt''A, it lacks strength, and on the other hand, if it exceeds 1.0iytχ, sintering will occur. It is not preferable because the toughness of the steel decreases.The preferable range is 0.1 to 1.0-L.
It is χ.

In the structure of the present invention, sintering is performed in a high-temperature non-oxidizing atmosphere because molybdenum, manganese, etc. are added to the alloy steel or iron powder used, so molybdenum and manganese oxides are Its presence impedes intergranular diffusion during sintering. Therefore, it is preferable to perform the sintering in a vacuum atmosphere of 10-2 torr or less at 1200 to 1300° C. for 60 to 120 minutes.

In the structure of the present invention, the nitriding or carbonitriding treatment is performed at a temperature of 600°C or higher, and the term nitriding is usually 550°C, as in nitriding.
When heat treated at about ~570"C, nitrides are generated on the particle surface and the strength of the sintered steel is reduced. Nitriding treatment is different from nitriding and is an essential condition of the present invention, and a temperature of 600"C or higher is required. The reason for this is to set the temperature at which nitrides such as γ and 8 layers are difficult to precipitate on the surface layer (referred to as white eyebrows) during nitriding treatment.Nitriding treatment is performed in an ammonia decomposition gas atmosphere. Medium (residual ammonia concentration 5-15%), 8
It is preferable to carry out the process at a temperature range of 00 to 900"C. In principle, only nitriding is performed when obtaining sintered steel from alloyed steel powder containing carbon, and carbo-nitriding is performed only when sintered steel is obtained from alloyed steel powder containing carbon. When obtaining sintered steel from a mixed powder of alloyed iron powder and graphite powder, the steel is first carburized according to a conventional method, and then nitrided to diffuse nitrogen into the sintered body.

With the configuration of the present invention, the retained austenite structure has a capacity ratio of 3
The reason for setting it to 0 to 95 volχ is to impart appropriate fatigue limit strength and toughness to the sintered steel, and less than 30 volχ is insufficient to relieve stress concentrated in the voids. Note that although a retained austenite phase of nearly 30 νolχ can be obtained by quenching with only carburizing treatment, fatigue limit strength and toughness cannot be improved.

On the other hand, when it exceeds 95νo1χ, the retained austenite phase is relatively soft in items such as gears where the local force is high, so there is a risk that the shape of the gear will collapse. However, even if it becomes 100volX, the fatigue limit strength,
As for the impact value, since the properties do not deteriorate significantly, a 100 vol'A retained austenite phase may be formed depending on the purpose and application.

In the configuration of the present invention, quenching from a temperature above the A1 transformation line refers to the A1 transformation line shown in the C-Fe phase diagram,
Its temperature is 721°C. Eutectoid sintered steel (C: 0.85
wtχ) allows rapid cooling from 740°C.

In sintered steels other than eutectoid sintered copper containing 0.85wtχ carbon, as understood from the C-Fe alloy phase diagram, austenite is formed by rapid cooling from a temperature above the liquidus depending on the carbon content. The condition is exactly the same, and it is formed as it is at room temperature.

[Function] The present invention leaves a large amount of retained austenite structure in the surface layer and voids of the sintered body through nitriding or carbonitriding treatment, so that the fatigue limit strength of the sintered steel can be significantly improved, and the toughness can be improved. is enhanced. In addition, by leaving a multilayered retained austenite phase in the surface layer and voids,
It is highly effective in preventing microcracks that are likely to occur from the gap cutouts exposed on the surface of the sintered body and the gap cutouts that exist near the surface. Furthermore, by shot peening the surface layer of the sintered body after the heat treatment of quenching and tempering to form a retained austenite phase, significant compressive residual stress is formed in the surface layer, and the voids slightly remaining in the surface layer are removed. Crushing reduces the effects of fatigue due to notches, resulting in a sintered steel with higher fatigue limit strength.Also, by leaving a large amount of retained austenite phase, the hardness of the surface layer can be made softer than that of conventional steel. Therefore, the deformation resistance of the sintered steel can be reduced, making sizing easier to correct.

(Effects of the Invention) Therefore, the high-strength sintered steel of the present invention has a large amount of retained austenite structure remaining in the voids of the surface layer of the sintered body by nitriding or carbonitriding treatment. Sintered steel has a higher fatigue limit strength than sintered steel and has toughness that can withstand large impacts. Sintered steel can be obtained at low cost, making it suitable for high-strength parts of gears used in engines, oil welding, connections, automobiles, etc. It becomes possible to apply to

〔Example〕

Examples of the present invention will be described below in comparison with comparative examples.

Table 1 shows the chemical composition of the iron alloy powder used in the practice of the present invention.

Table 1 Example 1 Alloy iron powder No. 1 in Table 1 (Shinko Atmel 4600
) with 0.3wtχ graphite powder (KS6 manufactured by Lonza) and 0.3wtχ graphite powder (KS6 manufactured by Lonza).
A mixed powder to which 5wtχ of Akrawanx (lubricant) was added was put into a mold, molded under a pressure of 6L/c+fl, and 1O-
Sintering was carried out at 1250° C. for 60 minutes in a vacuum atmosphere of 2 Lorr or less. The obtained sintered steel was machined into the shape of a JI32 rotary bending fatigue test piece, and then carburized under heat treatment conditions of 930°C for 5 hours so that the surface carbon potential of the test piece became 0.8wtχ carbon. After that, further 10% ammonia decomposition gas atmosphere at 850℃
A nitriding treatment was carried out under the conditions of x4 hours, quenching was performed directly in oil at 80°C, and the test piece was tempered at 150°C for 3 hours to obtain a sintered steel test piece (1) of the present invention. A test piece (2) was prepared in which the surface layer of the test piece (1) obtained in the same manner as above was subjected to shot peening. After quenching, a sintered steel test piece was obtained by tempering at 150°C for 3 hours.The rotating bending fatigue limit and impact value were determined for each of the obtained test pieces (1) and (2) and the comparative example. Table 2 shows the results of the investigation of residual austenite ffi, etc.

The amount of residual austenite is measured by observation using a microscope and
Obtained by line diffraction method.

From the results in Table 2, the fatigue limit strength and impact value of the sintered steel according to the present invention are significantly improved compared to the comparative example. Further, by applying short peening, the fatigue limit strength can be improved by about 10%.

Example 2 6t/c+I of No. 1 to 4 alloyed iron powder in Table 1
After sintering at 850°C for 60 minutes in a vacuum atmosphere of 10-''torr or less, the sintered body was further compressed and molded with a press at a pressure of 6t/all.
In a vacuum atmosphere of 0-2 torr or less, 1250℃ x 60
It was sintered under the same sintering conditions. The obtained sintered steel was carburized and nitrided under the same conditions as in Example 1 to obtain sintered steel test pieces (3), (4), (5), and (6) of the present invention.

A comparative example was prepared using the iron alloy powder No. 1 in Table 1 and treated under the same conditions except that the nitriding treatment was not performed.

Table 3 shows the results of examining the same characteristic values as in Table 2 for the obtained test pieces and comparative examples.

From the results in Table 3, it can be seen that by performing recompression molding and resintering in the sintering process, both the fatigue limit strength and impact value can be significantly improved compared to the comparative example even with the same sintered mixed powder. .

In the comparative example, the formation of retained austenite is small and no improvement in fatigue limit is observed.

Example 3 0.8wtx graphite powder (KS6 manufactured by Lonza) and 0.5w were added to Nol alloy iron powder (Shinko Atmel 4600) in Table 1.
A mixed powder to which Lχ Acra wax (lubricant) was added was placed in a mold and sintered under the same conditions as in Example 1.

The obtained sintered steel was machined into a JI No. 32 rotating bending fatigue test piece, and was subjected to a 10% ammonia cracking gas atmosphere.
Only nitriding treatment was performed under the conditions of 00°C x 4 hours, and directly 13
A sintered steel test piece (7) was obtained by quenching in oil at 0°C and tempering at 400'CX for 3 hours.

Among the above treatment conditions, a sample without nitriding treatment was used as a comparative example.

Table 4 shows the results of examining the same characteristic values as in Table 2 for the obtained test pieces.

Table 4 hand Continued 1.Display of the incident 2. Name of the invention 3. Person who makes corrections Relationship with the incident Residence (residence) Full name (name) 4. Agent address Patent Application No. 14381 Hei 2-14381 High strength sintered steel and its manufacturing method 1-4-21 Shinmachi, Nishi-ku, Osaka Ne city official book (spontaneous)

Claims (1)

[Claims] 1. A high-strength sintering method in which a mixed powder of carbon-containing alloy steel powder and/or carbon-free alloy iron powder and graphite powder is molded and pressed into a predetermined shape and sintered in a high-temperature non-oxidizing atmosphere. In sintered steel, after sintering, nitrogen atoms are diffused into the sintered body by nitriding or carbonitriding at a temperature of 600°C or higher, and then the surface layer of the sintered body is formed by rapid cooling from a temperature higher than the A_1 transformation line. and a high-strength sintered steel characterized in that a retained austenite structure with a volume ratio of 30 to 95 vol% is formed in the voids. 2. The high-strength sintered steel according to claim 1, wherein compressive residual stress is formed in the surface layer of the high-strength sintered steel by shot peening. 3. A step of molding and pressing a mixed powder of a carbon-containing alloy steel powder and/or a carbon-free carbon-free alloy iron powder and graphite powder, a step of sintering the molded and pressurized compact in a non-oxidizing atmosphere, or the above-mentioned step. , a step of recompression molding the sintered steel after the step of sintering in a non-oxidizing atmosphere and sintering it in a non-oxidizing atmosphere, a step of nitriding or carbonitriding the sintered steel at a temperature of 600°C or higher, and A_1 A method for producing high-strength sintered steel, comprising the steps of rapidly cooling sintered steel from a temperature above its transformation line and tempering it. 4 The carbon-containing alloy steel powder is 0.05 to 1.5w
t% silicon, 0.5-2.0wt% chromium, 0.5-5
．． It has a composition containing one or more of 0 wt% nickel, 0.1 to 1.0 wt% molybdenum, and 0.1 to 2.0 wt% manganese, with the balance being iron, inevitable impurities, and carbon. The product according to claim 1 or 3. 5 The mixed powder of carbon-free ferroalloy powder and graphite powder contains 0.1 to 1.0 wt% carbon and 0.05 to 1
．． 5wt% silicon, 0.5-2.0wt% chromium, 0.
A composition containing one or more of 5 to 5.0 wt% nickel, 0.1 to 1.0 wt% molybdenum, and 0.1 to 2.0 wt% manganese, with the remainder containing iron and unavoidable impurities. The product according to claim 1 or 3.