JPH03264642A - Production of iron-based high-strength sintered body - Google Patents

Abstract

PURPOSE:To produce the iron-based high-strength sintered body having a high density and high strength by diffusing and sticking powders of specific ratios of Cu, Mo, W, and Ni to the surfaces of prealloy steel powder particles contg. a specific ratio of Cr, mixing a specific ratio of graphite therewith and molding and sintering the mixture. CONSTITUTION:The prealloy steel powder contg. 0.5 to 5.0wt.% Cr and further, contg. >=1 kinds of 0.01 to 0.5% V, 0.005 to 0.1 % Nb and 0.001 to 0.01 % B at need is prepd. by a water atomization method, etc. The powders consisting of >=1 kinds of 0.1 to 1.0% Cn, 0.5 to 2.0% Mo and 0.05 to 1.0% W and 0.5 to 5.0% Ni having <=45mum grain size are partially diffused and stuck to the surfaces of such steel powder particles. The resulted alloy steel is controlled to <=10.0% Ni+Cu+Mo+W and <=0.20% O and is constituted of the balance Fe and unavoidable impurities. Such powder is used as powder metallurigical alloy steel powder, with which 0.5 to 1.2% graphite is mixed. The mixture is molded and the molding is sintered at about 1250 deg.C. The as-sintered iron-based high-strength sintered body having about >=7.00g/cm<2> density and about >=100kgf/ mm<2> tensile strength is obtd. in this way.

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention provides a method for achieving a sintered density of 7 without heat treatment by powder metallurgy.
, 0 Og7cm3 or more, and a tensile strength of 100 Kgf/mm2 or more as-sintered.

<Conventional technology> Iron-based sintered materials are widely used in automobile parts.

Recently, these parts have been required to have high density and high strength. In order to increase the strength, high alloying as shown in JP-A No. 61-231102 or optimization of the alloy composition as shown in JP-A-57-164901 has been carried out. ing.
The above techniques are extremely effective in increasing strength and can be applied to strong members. However, from the viewpoint of fatigue properties, the density etc. are not sufficient. To increase the density of alloy steel powder sintered bodies, there is a method of using sinter forging as disclosed in JP-A No. 61-44104, but there are many restrictions in terms of mold life and product shape. .

In addition, the double pressurization method, in which after sintering is performed again in the mold and pressurized in a cold state in the same way as the first pressurization, compared to sintered forging, the method is more effective in terms of mold life and product shape. It has few restrictions and is practical. However, in order to obtain the final strength, heat treatment must be performed after the second pressurization and sintering, which is not economical. On the other hand, when pure iron powder is mixed with alloying elements in powder form, it is superior to Brialloy steel powder in terms of density, but segregation of the added alloying element powder becomes a problem.

<Problems to be Solved by the Invention> High strength is generally achieved by adding alloying elements, making the composition martensite by heat treatment, precipitation strengthening, etc.
However, in the case of powder metallurgy products, excessive addition of alloying elements reduces the compressibility of the powder and does not increase the density of the sintered body.

Furthermore, when alloying elements are mixed in powder form with pure iron powder, segregation of the added alloying element powder becomes a problem. The most effective way to increase the strength of a sintered body in powder metallurgy is to control the structure by quenching and tempering.

However, this treatment is effective in combination with the addition of alloying elements. Therefore, the present inventors have developed a method for manufacturing a sintered body with unprecedented high density and high strength by controlling the structure of the sintered body.

An object of the present invention is to provide a method for producing such an iron-based high-strength sintered body.

<Means for Solving the Problems> In order to achieve the above object, according to a first aspect of the present invention, Cu : 0.1-1.0wt%, Mo: 0.5-2.
At least one selected from 0 wt% and W: 0.05 to 1.0 wt% and Ni: 0.5 to 5.0 wt%, both of which are made of alloyed steel powder in which powder with a particle size of 45 μm or less is partially diffused and adhered. , and N i +Cu+Mo+W≦10.0wt%, including 0 controlled to 0.20wt% or less, and the remainder is Fe.
A method for manufacturing an iron-based high-strength sintered body is characterized by using powder metallurgy alloyed steel powder consisting of steel powder and inevitable impurities, mixing 0.5 to 1.2 wt% of graphite with the powder, forming and sintering the powder. provided.

Further, according to the second aspect of the present invention, Cr20.5 to 5
．． 0wt% and V: 0.01-0.5wt%, Nb:
0.005-0.1 wt% and B: 0.001-0.
Cu: 0.1-1.0 wt%, Mo: 0.5-2.
Alloy steel powder containing at least one selected from 0 wt% and W: 0.05 to 10 wt% and Ni: 0.5 to 5.0 wt%, both of which have a particle size of 45 μm or less, partially diffused and attached, and N i +Cu+Mo+W≦10.0wt%, including 0 controlled to 0.20wt% or less, and the remainder is Fe.
A method for manufacturing an iron-based high-strength sintered body is provided, which comprises using a powder metallurgy alloyed steel powder containing unavoidable impurities, mixing 05 to 1.2 wt% of graphite to the powder, shaping and sintering the powder. Ru.

Here, the austenite phase contained in the structure of the sintered body is 5 to 20 voJ2%, and 85% or more of the austenite phase is transformed into a martensite structure when plastic deformation is applied. is preferred.

The present invention will be explained in more detail below.

As a result of intensive research on increasing the density and strength of sintered bodies, the present inventors found that the composition of the copper powder used and the amount of austenite in the sintered bodies significantly affect the increase in strength of sintered bodies. Ta.

The reason why the alloy components are limited to the above range in the first aspect of the present invention will be explained.

In the present invention, the prealloying components and the components to be attached to the surface of the prealloyed steel powder particles by diffusion (hereinafter referred to as composite alloying) are selected based on the required functions.

In other words, as a prealloy component, it has little effect on the compressibility of the copper powder, improves the hardenability of the sintered body by adding a small amount, and is relatively difficult to reduce, making it difficult to reduce the oxide with hydrogen. Therefore, the target component is one that is difficult to form into a composite alloy without impairing compressibility by diffusion adhesion, and in the present invention, Cr is selected as such a component. Since Cr has high hardenability, twice as hardenability as Ni, it was used as the main component to improve the strength of the sintered body. Furthermore, pre-alloying this is extremely beneficial in the following respects.

i.e. (1) Pre-alloying improves the uniformity of the sintered steel structure.

(2) When Cr is prealloyed into iron, its activity is reduced, so oxidation resistance is improved, and it becomes possible to form a composite alloy of oxides of elements that are more easily reduced than Cr.

(3) Since hardenability is improved with a small amount of Cr, the amount added can be reduced and the compressibility of the copper powder is less likely to be lowered.

(4) Cr is cheaper and more economical than Ni.

The upper limit of the amount of Cr added was set at 5.0 wt% (hereinafter simply expressed as %) in consideration of the upper limit of the amount of steel powder and compressibility after composite alloying with easily reducible oxides. . On the other hand, the lower limit is 0, 5, where the above Cr addition effect can be obtained.
%.

The reason why Ni, CulMo, and W were selected as alloy components to be compositely alloyed on the particle surface of the above-mentioned prealloyed steel powder particles is as follows. All of these elements can form a composite alloy by adhering to steel powder particles without impairing compressibility, and are effective elements for increasing the strength of the sintered body.

That is, the addition of N1 not only improves the sinterability of iron powder, but also has a remarkable effect on improving the strength and toughness of sintered steel. Furthermore, Ni is an austenite phase forming element and improves ductility. However, if the amount added is less than 0.5%, high strength due to solid solution strengthening and hardenability improvement, ductility improvement effect due to the austenite phase, and matrix toughness improvement effect cannot be obtained. On the other hand, if it is added in an excessive amount exceeding 5.0%, excessive austenite phase will be generated, resulting in a decrease in strength. Therefore, 05
It was limited to a range of 5.0%.

The lower limit of Cu is 0.1% at which the addition effect appears, and the upper limit is 1.0% at which the compressibility is not impaired, and is in the range of 0.1 to 10%.

Mo improves the hardenability of the sintered body. However, M.O.
If the addition amount is less than 0.5%, the addition effect will be poor, while if it is added in excess of 2.0%, the mechanical properties will deteriorate. It was decided to add within the following range.

W also has the same effect as Mo, and effectively contributes to improving the hardenability of the sintered body. However, the amount added is 0.0
If the amount is less than 5%, the effect of adding it will be poor; on the other hand, if it is 1.0%
If it exceeds 0.05 to 1.0%, the compressibility will be inhibited.
It was decided to add within the following range.

Each of them has the effect of improving the properties of sintered steel even when used alone, but when Ni, which is an austenite-forming element, is added, and at least one of Mo, Cu, and W is added in combination, the effect becomes even more effective. be enhanced.
However, if excessive addition occurs, a reaction between the composite components will occur during the production of copper powder, forming compounds and reducing compressibility, so the total amount of these (N i + Cu + Mo + W) will be 10
．． It is important to keep it below 0%.

Since the amount of steel powder O has the effect of reducing the compressibility of copper powder, it is desirable to reduce its contamination as much as possible, but 020
% or less is acceptable.

1 By using the above-mentioned composite alloy steel powder, it is possible to achieve high density of the sintered body. Furthermore, since it contains copper powder or Cr, it has excellent hardenability, and with the addition of graphite, a martensitic structure can be obtained only by sintering, resulting in a high-strength sintered body. If the amount of graphite added is less than 0.5%, ferrite is present in the structure of the sintered body, making it impossible to obtain strength. On the other hand, if it exceeds 1.2%, the martensitic transformation start temperature will be below room temperature and a large amount of austenite will remain, reducing the strength.
It was limited to a range of 0.5 to 1.2%.

Due to the action of Ni diffused onto the surface of the prealloyed steel powder, retained austenite also exists in the sintered body at the same time.
However, even higher strength can be obtained by applying plastic deformation to the sintered body and transforming it into martensite (hereinafter, the martensite produced by this transformation will be referred to as strained martensite).

The present invention is a sintered body using composite alloy steel powder with excellent compressibility and hardenability, and the austenite phase contained in the structure mainly composed of martensite is 5 to 20V.
ou%, and when plastic deformation is applied, 85% or more of the austenite phase transforms into a martensitic structure.

Controlling the amount of austenite in the sintered body is one of the features of the present invention. If the amount of austenite is less than 5%, high strength will not be achieved even if the entire amount undergoes strain-induced martensitic transformation. On the other hand, if it exceeds 20%, the ratio of transformation to martensite during plastic deformation will decrease, resulting in a failure to obtain high strength. Note that the amount of austenite in the sintered body can be controlled to 5 to 20% by controlling the martensitic transformation start temperature by changing the amount of graphite added and the amount of composite alloying elements. Next, when tensile or compressive stress is applied, 85% or more of the total amount of austenite must be strain-induced transformed into martensite. If the amount of transformation is less than 85%, high strength cannot be obtained due to residual austenite.

The components that are partially diffused and adhered to the particle surfaces of prealloyed steel powder particles by the composite alloy method are Cu and M.

and at least one selected from W and Ni,
As the component, a fine powder having a particle size (average particle size of 3 to 10 μm) of 45 μm or less is used. If the composite alloying component particles exceed 45 μm, austenite becomes stable and difficult to transform into martensite, making it impossible to obtain high strength.

By the manufacturing method of the present invention, the sintered density of the sintered body is 7.
, OOg/cm3 or more and tensile strength of 100
An iron-based high-strength sintered body having a high strength of Kgf/mm2 or more can be obtained.

Next, a second aspect of the present invention will be described, but a description of parts that overlap with the first aspect will be omitted.

In addition to Cr, pre-alloying components include V, Nb, and B, which are difficult to form into a composite alloy because it is difficult to reduce the oxide with hydrogen. There is. As a result of various studies, the amount added was limited as follows.

(2) is effective in improving hardenability. However, if the amount added is less than 0.01%, the effect is poor;
．． If it exceeds 5%, the hardenability will decrease, so 0.0
It was limited to a range of 1 to 0.5%.

Nb has the effect of refining crystal grains and contributes to improving the strength and toughness of the sintered body. However, the amount added is 0.00
If the amount is less than 5%, the effect of adding it will be poor; on the other hand, 0.1%
If it exceeds 0005~0, it will actually reduce the toughness.
．． It was limited to a range of 1%.

B effectively contributes to increasing the hardenability of sintered steel, but
If the amount added is less than 0.001%, the addition effect will be poor,
On the other hand, if it exceeds 0.01%, the toughness will deteriorate;
It was limited to a range of 1% to 0.01%.

Sintering can be performed by mixing graphite for powder metallurgy and zinc stearate as a lubricant with each steel powder, preparing a compact 5 and performing sintering at 1250° C. for 6 Qmin.

The atmosphere at that time may be a 75% N2-25% H2 atmosphere.

By limiting the amounts of V, Nb, and B added as described above, the function of Cr can be further enhanced.

<Example> The present invention will be specifically explained below based on Examples.

(Example 1) Water atomized steel powder containing Cr in the range of 0.3 to 5.8%, and V+CI in addition to Cr: 0.3 to 5.8%
No, s%, Nb: 0-0.15%, BO-0015
At least 1f selected from among! Each water atomized steel powder containing 1 was annealed at 1150°C in a reduced pressure atmosphere of 0.01 Torr for 240 m1n, and after removing oxides on the surface of the water atomized steel powder with C in the steel powder, normal powder metallurgy was performed. The crushed and sieved portion used in the production of industrial steel powder was subjected to operations to obtain various Cr-containing steel powders.

Next, Ni metal powder and Cu metal powder are added to the copper powder so that the final steel powder contains Ni: O ~ 5.8% and Cu: O ~ 15%, and Mo oxide powder, W oxide powder M in the final steel powder.

After mixing in various combinations so that O ~ 25% and W: 0 ~ 15%, a composite alloying treatment is performed to partially diffuse and adhere Ni, CuMo, and W to the surface of the prealloyed steel powder particles. provided.

The composite alloying process was carried out in a hydrogen atmosphere (dew point -30°C) at 7°C.
The test was carried out under conditions of heating at 50° C. for 50 ml.

After such composite alloying treatment, the crushed and sieved portions were subjected to operations to obtain alloyed steel powders having various component compositions.

After that, each steel powder was mixed with 0.9% of graphite for powder metallurgy and 1% of zinc stearate as a lubricant, and then heated with a pressure cuff.
cm2 and formed into a tensile test piece.

This green compact was heated to 1250°C in a 75%N, -25%H2 atmosphere.
Sintering was performed at ℃ for 60 ml.

The cooling rate after sintering was 17° C./min.

Table 1 shows the sintered densities of Examples 1 to 18 and Comparative Examples 1 to 13,
The results of investigating the amount of austenite before the test, the amount of austenite after the test, and the tensile strength in the sintered body are shown.

In all of the invention examples 1 to 3, high strength was obtained. In Comparative Example 1, the Cr content was 0.3%, which was less than the lower limit of 0.5%, so the hardenability was not improved and no strength was obtained. In Comparative Example 2, the Cr content was 58%, which exceeded the upper limit of 5.0%, so the compressibility decreased due to solid solution hardening of Cr. Since Comparative Example 3.4 is a pre-alloyed steel powder, the sintered density is lower than that of the example, and the retained austenite is not present or even if it is present, it is stable, so that no strength can be obtained.

Among the present invention examples 4 to 18 and comparative examples 5 to 13, the present invention examples 4 to 18 have high strength because the content satisfies the appropriate range of this invention, and the comparative examples outside the range 5~
In No. 13, the strength decreased for the same reason as Comparative Examples 1 to 4.

(Example 2) As in Example 1, 0.3 to 1.5% by weight of graphite was added.
After mixing and producing a sintered body, it was subjected to a tensile test. The chemical composition of the alloy steel powder used in the experiment was 3%Cr-2%Ni-
Table 2 shows the austenite amount before the test, the austenite amount after the test, and the tensile strength at 1% Mo.

It can be seen that in the present invention Examples 19 to 22, in the austenite amount within the range limited in the present invention, 85vOρ% or more of the austenite was transformed after being subjected to plastic deformation, and thus had excellent properties.

(Example 3) Two types of Ni with different particle sizes of 45 μm or less and more than 45 μm
A 3% Cr-2% Ni-1% MO alloy steel powder was produced in the same manner as in Example 1 using metal powder.

Thereafter, each copper powder was mixed with 0.9% graphite for powder metallurgy and 1% zinc stearate as a lubricant, and then formed into a tensile test piece using a pressure cuff t/cm2.

This green compact was heated to 1250°C in a 75%N2-25%H2 atmosphere.
Sintering was performed at a temperature of 60 ml.

The cooling rate after sintering was 17° C./min.

Table 3 shows the amount of austenite before the tensile test, the amount of austenite after the test, and the tensile strength.

When Ni metal powder with a diameter exceeding 45 μm was used, austenite became stable, strain-induced transformation did not occur, and high strength could not be obtained.

That is, Comparative Example 1 using Ni metal powder exceeding 45 μm
In Example 6, high strength cannot be obtained because the transformation rate of retained austenite is as low as 6885%, whereas inventive example 23, which uses Ni metal powder with a diameter of 45 μm or less, has a high transformation rate of 89.0%, resulting in poor bow tensile strength. It has been possible to obtain a sintered body with a high strength of 130.3 Kgf/mm2.

0 24 <Effects of the Invention> Since the present invention is configured as described above, by utilizing the phenomenon of strain-induced transformation of the second austenite phase into martensite during stress loading, the present invention This made it possible to produce a sintered body with unprecedented high density and strength, further expanding the range of uses for sintered parts.

Claims (1)

[Claims] (1) On the surface of prealloyed steel powder particles containing Cr: 0.5-5.0 wt%, Cu: 0.1-1.0 wt%, Mo: 0.5-2. 0w
At least one selected from t% and W: 0.05 to 1.0 wt% and Ni: 0.5 to 5.0 wt%, both of which are made of alloyed steel powder to which powder with a particle size of 45 μm or less is partially diffused and adhered. , and Ni+Cu+Mo+W≦10.0wt%, and 0
．． It is characterized by using powder metallurgy alloy steel powder containing O controlled to 20 wt% or less and the remainder consisting of Fe and unavoidable impurities, mixed with 0.5 to 1.2 wt% of graphite, and then molded and sintered. A method for manufacturing an iron-based high-strength sintered body. (2) The austenite phase contained in the structure of the sintered body is 5 to 20 vol%, and 85% or more of the austenite phase transforms into a martensite structure when plastic deformation is applied. Item 1. A method for producing an iron-based high-strength sintered body. (3) Cr: 0.5-5.0wt%, V: 0.01-0.5wt%, Nb: 0.005-0.1w
Cu: 0.1-1.0 wt%, Mo: 0.5-2. 0w
At least one selected from t% and W: 0.05 to 1.0 wt% and Ni: 0.5 to 5.0 wt%, both of which are made of alloyed steel powder to which powder with a particle size of 45 μm or less is partially diffused and adhered. , and Ni+Cu+Mo+W≦10.0wt%, and 0
．． A powder metallurgy alloy steel powder containing O controlled to 20 wt% or less and the balance consisting of Fe and unavoidable impurities is used, and 0.5 to 1.2 wt% of graphite is mixed with this powder, which is then molded and sintered. A method for manufacturing an iron-based high-strength sintered body. (4) The austenite phase contained in the structure of the sintered body is 5 to 20 vol%, and 85% or more of the austenite phase transforms into a martensite structure when plastic deformation is applied. Item 3. A method for producing an iron-based high-strength sintered body.