JPS5935601A - Production of atomized steel powder having high compressibility - Google Patents

Abstract

PURPOSE:To improve the compressibility of atomized steel powder by heat- treating the atomized steel powder in a non-oxidative atmosphere then denitriding in the atmosphere of a specific compsn. CONSTITUTION:Atomized steel powder is held for 5-45min at 900-1,050 deg.C in a reducing or neutral atmosphere to reduce the oxide on the surface formed during the manufacture of the steel powder and to soften uniformly the texture generated in the stage of solidifying by spraying. The steel powder is slowly cooled at 5-20 deg.C/min cooling rate within a 900-550 deg.C range in an atmosphere of <=40 deg.C dew point of the compsn. consisting of 50-80vol% H2, <=400ppm NH3 and the balance N2. The N2 in the steel powder is denitrided by such heat treatment, whereby the compressibility of the steel powder is improved. If the steel powder is used as a raw material for the sintered product, the strength of the sintered product is improved and the production of a large-sized sintered product is made easy.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing highly compressible atomized steel powder for powder metallurgy, and particularly to a production method for improving the compressibility of atomized steel powder by improving the hot section (conditions) after atomization. Regarding proposals.

Atomized steel powder for powder metallurgy is usually produced by injecting a high-speed liquid or gas into a molten metal stream and causing the molten metal to atomize and scatter. After drying, the atomized steel powder as sprayed is subjected to finishing heat treatment in a reducing atmosphere to impart predetermined powder metallurgical properties, and then crushed to produce a product.

With this process, JI11! The atomized steel powder has less impurities than reduced iron powder, and is dense with no pores inside the particles, so it has better compressibility than reduced iron powder.

In recent years, with the increase in the size and strength of fixed parts, ↓Hara J
It is strongly desired to improve the compressibility of copper powder. By improving compressibility, it is possible to increase the size and strength of φ-bonded parts, and many other benefits are expected, such as sintering of electromagnetic parts and extending the life of molding molds. It is from.

The compressibility of atomized steel powder is determined in part by the composition of the copper powder. Therefore, it is important to select the raw material SFRAG level and IQ, and to lower unnecessary alloys, components, and components as much as possible. In addition, the work after blowing "O,N" in the second heat section
It is necessary to remove impurities such as O and 11N (C and N significantly deteriorate compressibility).

Therefore, even if the copper powder is the same, the finished hot part fIT! The compressibility of atomized steel powder is greatly influenced by the suitability of the process.

The purpose of finishing heat treatment of atomized steel powder is not just to reduce 0°N and 0, but ■! Hey-sama II! ? Another advantage is that the rapid solidification of F11 homogeneously softens the steel powder, making it easy to compression mold. Furthermore, in water atomized steel powder,
During spraying (the surface of the ζ steel powder is oxidized, one of the purposes of finishing heat treatment is to reduce and remove this).

On the other hand, in order to achieve the above purpose, finishing heat treatment of atomized steel powder is generally carried out by heating and holding at a temperature of 850 to 1050 °C for 120 minutes at RO in a reducing gas containing H2. ing. However, as mentioned above, the compressibility of gold 1 powder is significantly reduced by ON in steel powder. In other words, it is possible to reduce the N ratio of the steel powder by using the finishing heat zone P11, but it is possible to improve the royal tendency by reducing the N ratio of the steel powder using the finishing heat zone P11. The reality is that removal of the surface oxide layer is prioritized, and denitrification is not given much consideration.Therefore, if the N needle of copper powder produced by the usual finishing heat treatment as described above exceeds 40 ppm. Therefore, what is the green density of steel powder for shell powder metallurgy?
t-zinc stearate) was mixed and the molding pressure was 5t/a.
When molded with male, the yield remains at 6.75 to 6.85 f', /8 grain density.

Note that denitrification is thought to proceed mainly according to the following reaction formula.

1.1 + 75'H2-) NIP8(1), that is, the influence of the heat treatment atmosphere on denitrification is large, and pure H2
Denitrification progresses easily inside the atmosphere, but the N, IN of the atmosphere
As N2 in the atmosphere increases, the de9ation rate decreases and
penetrates into the copper powder. What is Tir?

In order to promote denitrification, it is desirable that the atmosphere be pure H2.

However, pure H2 is expensive and difficult to handle, and is not necessarily suitable for producing steel powder for general powder metallurgy. Therefore, the heat treatment atmosphere is generally ammonia decomposition gas (Hp, 75 vol, N.

(21'l vol) etc. are used, but NII not only suppresses denitrification, but also increases the dew point and denitrification due to the water aeration generated by the reduction of surface oxides of fine powder. Since the atmosphere inside the furnace is polluted by NH8 and the like, denitrification is further suppressed. Therefore, it cannot be said that the finishing heat treatment atmosphere generally used at present is the optimum atmosphere for zero denitrification of steel powder. As a result of the two piles, it is considered that a large amount of N remains in the copper powder as described above.

Several attempts have been made to actively carry out denitrification up to this point and improve compressibility. For example, the finishing heat treatment of reduced iron powder is divided into two stages: return reduction and denitrification.
f1. A heat treatment method using , , , etc. has been proposed.

Similarly, it is thought that it is possible to improve the compressibility of atomized copper powder by homogenizing the structure and performing two heat treatments that separate reduction and denitrification of surface oxides. Due to this, the number of times of crushing increases and the resulting deterioration of formability and production FAfitl.
The soil texture of i@i6 is avoided, and it cannot be said to be the best method for producing raw material copper powder for general powder metallurgy other than for special purposes.

A similar method is a two-stage heat treatment method in which thermal cycles are performed in stages. This method is a heat treatment method that adds a step of holding at a low temperature for the purpose of denitrification after homogenizing the structure and reducing the surface oxide layer at a high temperature. This method is
This method can be said to be superior as it causes less deterioration in formability compared to two-time heat treatment. However, the finishing heat treatment of atomized field powder
Usually, a belt-type continuous Pμ is used. In order to adopt a two-stage heat treatment method in such a continuous furnace, a stepwise heat cycle of heating the copper powder, holding it at a high temperature, cooling it, holding it at a low temperature, and then cooling it and then taking it out of the furnace is carried out in one furnace. This requires significant furnace modification, such as changing the furnace internal structure and extending the furnace length, and also limits belt speed and reduces furnace productivity. Therefore, the two-stage heat treatment method is
It cannot be said that the continuous furnace is the most suitable method for producing relatively inexpensive copper powder.

In other words, the improvement in the compressibility of atomized steel powder reduces the
Although it is possible by doing
It is said that the pressure (1) is moving towards a low value because no denitrification method has been proposed. i Chiru Chiru.

The present invention (1. Atomized steel powder for powder metallurgy with stagnation) By solving the disadvantages of the conventional method of heat treatment of steel powder, atomization for powder metallurgy that actively performs denitrification and has excellent compressibility. The purpose is to provide a method for producing silver powder with high productivity, and its gist is to improve the quality of atomized steel powder for powder metallurgy,
In the HIRL process, the structure of the atomized steel powder is made uniform and surface oxides are reduced by heating and maintaining the product temperature at 900°C or higher in a neutral atmosphere.
0~550℃ cooling Ll → Cooling rate of 5~20'C
There are 7 methods by which compressible atomized copper powder can be produced with a pressure of 7 min.

The groove of the present invention was developed as a result of various research and experiments on finishing heat treatment of sprayed atomized silver powder.
It is possible to improve the JE analysis of atomized copper powder by promoting the denitrification of atomized copper powder, and the effect of holding temperature and holding time is However, it has been found that by appropriately selecting the cooling rate after high-temperature heating and holding, the de-9 reaction can proceed relatively easily even in an atmosphere with poor denitrification properties.

In the present invention (based on this knowledge), in the heat treatment of the sprayed one-year-old atomized steel powder, 900-1 ( 1
After keeping the hot water temperature within the temperature range of 50℃ for 5 to 45 minutes, the -r tomize pot total H, +(] ~ 8Q vo/!・L
This is method Z), in which atomized steel powder is produced which can be cooled quickly within the temperature N-17 range of 90.0 to 550°C in an atmosphere with NH of 3400 ppm or less and a dew point of 40°C or less.

Below, the structure and matters of the method for dipping atomized steel powder according to the present invention will be explained in detail.

One of the purposes of finishing and processing is the character @Kuri LI! The purpose of this method is to homogeneously soften the structure of the IE, 1i11, into a powder with improved moldability.

As shown in Figure 1, the microhardness of the atomized silver powder after finishing heat treatment depends on the finishing heat treatment temperature, and is less than 900℃ ≠ 1.
In the blind hot part 1 "("), even after holding for 90 minutes, the soft f
It is f"t,, -7'j" that the Jr" ratio of 1; is small and the homogeneous softening is sufficiently progressing. Therefore, for uniform IFq softening, !1 fl tI C, on tLJ ,fL
There are certain things that must be done. When the atomized "1 Powder Q" is subjected to finishing heat treatment, the powder also becomes agglomerated into a spongy agglomerate of steel powder.・The product is made by crushing it using a hummer mill, etc. For this reason, when heat treatment is performed at a high temperature of 1050°C in a reducing atmosphere,
The sintering of the copper powder progresses excessively and it is difficult to crush it.
In order to maintain appropriate particle size, excessive crushing must be performed, resulting in (1) deterioration of compressibility due to work hardening; (2)
There are no negative effects such as deterioration of formability due to the aging of copper powder.

When the finishing heat treatment temperature of the present invention (ri, uniformity is sufficiently advanced I7 and no adverse effects due to particle sintering occur5) 00 to l fl 511°C and ξ + ( 4
It is essential.

In addition, the hardness of the trowel powder from A1 to the present-top finishing hot part J
ffi time 111) Shadow? fi'? ! :'nFl]
Held, @ '4'fi (1) Soft (104,9 (
At a temperature of 10° C. or higher, the shadow 111I' of a■ during holding is small. (7 Therefore, to homogenize the spray solidification structure, t-j: 45
A heat treatment of less than a minute is sufficient. Since at least 5 molecules of an l- are required for soaking the copper powder in practice, the holding time at high temperature in the finishing heat treatment of the present invention is set to 5 to 5 minutes.

In addition, the atomized steel powder of the present invention 4rtp x++
r homogeneously softens the solidified structure to improve compressibility. Figure 2 shows that it is strongly influenced by speed and atmosphere composition.

The drawing shows the effect of the heat treatment atmosphere composition on the amount of N in the metal powder during slow cooling after high-temperature treatment, and shows examples where the cooling rate is 5°C/m1n and 45°C/rr+ in. be. As mentioned above, the removal of nitrogen from copper powder is (1
) is the JH8 production reaction shown in the formula.

Therefore, naturally, what is the atmosphere like with high H2t and low N2?

Nitrogen content in copper powder decreases. Especially the cooling rate is 45C/m
In the case of a fast cooling rate of 5°C/min, denitrification cannot be seen unless the H2 temperature is high, but in the case of a slow cooling rate of 5°C/min, the atmosphere is relatively low in H, river and rich in N, It. Denitrification is also progressing.

The relationship between the cooling rate and the denitrification reaction is shown in Figure 8.
(Unfortunately, the stinginess of the cooling rate is noticeable, and the p
(F shows that denitrification is possible by slow cooling even in ambient air F.) Therefore, the conventional dew point is
, etc. include 'J1 filtration and denitrification properties are inferior! ? Surrounding air 1c %-
(In 8, the cooling rate of copper powder is delayed by 1゜(4) and the denitrification is accelerated (Ii''), and the atomized steel powder for general powder metallurgy, which has conventionally remained at a relatively low value, In the present invention, in the cooling process after finishing heat treatment, the cooling rate is limited to a range of ff5 to 20°C/ml within a temperature range of 900 to 55°C. If the cooling rate is less than 5°C/min, denitrification will progress, but since the cooling rate is slow, the heat treatment time will increase by 1, which is undesirable because productivity will deteriorate.
This is because denitrification progresses slowly in an atmosphere other than high-density H2 at a cooling rate exceeding 100%, and the N% of the copper powder cannot be sufficiently removed, resulting in no improvement in compressibility.

Note that denitrification is preferably carried out in the bα phase region where the diffusion rate of 01N in the steel is high, and denitrification does not proceed at low temperatures below 55Q1:. Therefore, in heat treatment for the purpose of denitrification, slow cooling should be performed within the temperature range of 900 to 550°C.
! :do.

As mentioned above, denitrification is closely related to the cooling rate and the atmosphere composition. Therefore, it is necessary to limit the atmosphere composition during cooling. First, as shown in FIG. 2, the amount of H3 and N deposit, which greatly affect denitrification, are limited. If the amount of H2 exceeds 80 vol f, the cooling rate should be increased to 45 vol.
Denitrification proceeds even at a relatively fast rate of ℃/ra1). Therefore, in a denitrifying, collapsed atmosphere, it is not necessary to use the present invention even at -4°, so the upper limit of the amount of H2 is set to 80 vo/? %. Also, if the amount of H is 5 (less than 1 vo/section),
Even if the cooling rate is 5° C./min, denitrification is insufficient and predetermined compressibility cannot be obtained. Therefore, the atmospheric composition during cooling in the present invention is Hz i k 50 vo/1
Limited to 1 or more and 80vo/1 or less. Also, as shown in Figure 4, the amount of NH8 in the atmosphere is 400% in the NH8 box.
In heat treatment in an atmosphere of ppm or higher, the reaction of formula (1) is suppressed, and slow cooling does not promote de9ation and does not provide the effect of improving the shrinkage property. 7() ■The risk shall be +00 ppt/or less.

++7! When a large amount of V misted copper powder with a size of 1 F is subjected to soil treatment in a continuous furnace in an atmosphere containing H2, the 4 F water vapor generated at the F and W sources of the oxide layer on the surface of the button powder is Although the upper row of the dew point of the atmosphere cannot be avoided, an increase in the dew point suppresses denitrification as shown in Figure 5 below. In the case of the present invention...
Even in such an atmosphere with poor denitrification properties, it is possible to completely promote denitrification by slow cooling. 1) Within the range (20℃/min), the dew point exceeds 4 u-c, the residual F of N in the copper powder
The de9ation will not be sufficient as the number of pods increases, so the dew point of the atmosphere is limited to 40°C or higher.

The method of the present invention will be specifically explained below with reference to Examples.

Fa of molten metal described in Japanese Patent Publication No. 52-19540
(F. The chemical formation of the 8-world powder, which was atomized using a crusher and used as a test sample, is shown in Table 1, and the apparent density, fluidity, and particle size distribution t@Table 2).

This 57 Hfl 75voe q6, N g* 2n
v o I! '14, heat treated under the conditions shown in Table 8 in an atmosphere with a dew point of 20° C., and the O, N and O buckets of the copper powder after heat treatment are also shown in Table 48.

Particles are sintered by heat treatment and become spongy. After crushing using a steel powder hammer mill, sieve 1+ using an 80#σ) tyler@m-, and compress using -FlO# steel powder. The sex was measured.

Table 2 Powder characteristics of thermal processing (Japanese Patent Application Laid-Open No. 59-35601 (5) Here, in Comparative Example 1, finishing heat treatment was performed at 950°C for 60 minutes, and - steel powder for shell powder metallurgy. These are the final heat treatment conditions.In Comparative Example 2, after heating and holding at 850°C for 45 minutes, slow cooling was performed.

In Comparative Example 1, denitrification did not proceed sufficiently due to the fast cooling rate.
The nine elements in the copper powder are large at 0.11042 wt%.

However, in Comparative Example 2 and Examples 1 to 5 of the present invention, slow cooling was performed (thus, dechambering was promoted, and the amount of nitrogen in the copper powder was 0.002'1 or less. Copper powder, 1qtr)H? aiR agent (Zn5t) vK: Added and mixed for 15 minutes with a V-type mixer, outer diameter 1], Rmm
φ, height 11. A cylindrical green compact of Fl ±0.25 m, m, h was molded at a compacting pressure of 5 t/cy", and the green compact density was measured from the outer diameter dimension and weight M to evaluate the compressibility.The results 5C is shown in Table 8. In Comparative Example 1, the amount of nitrogen in the steel powder was high, resulting in low green density and poor compressibility. In Comparative Example 2, denitrification progressed due to gradual cooling. Because the holding temperature was low, homogeneous softening of the structure was insufficient, and the green compact density was low.However, Examples 1 to 5 in which the heat treatment of the present invention was performed
All of the compressed powders exhibited excellent compressibility of 6.5 f/80π, demonstrating that the heat treatment method of the present invention is an excellent means for improving compressibility. 'After holding Samurai for a short time as in Examples 4 and 5, Xu? ′? By carrying out step f, it became possible to produce atomized copper powder with excellent productivity and high compressibility at a low cost.

As explained above, in the current state of finishing heat treatment of atomized steel powder, the removal of nitrogen, which significantly reduces the compressibility of copper powder, is not much considered, and even after the second heat treatment, 'rA large amount of 19 remains in the Tomized steel powder 11. However, the present invention provides a denitrification treatment method in which slow cooling is performed under timed conditions after high-temperature reduction heat treatment. The production method of atomized copper powder for powder metallurgy of the present invention has become possible to mass produce 6 pieces of powder and meet the recent demands for larger and stronger sintered parts. It is something that brings about an effect.

[Brief explanation of drawings]

1M is a graph showing the relationship between heat treatment temperature of atomized inertia powder for powder metallurgy and copper powder hardness, and Figure 2 is a graph showing the relationship between H2 in the heat treatment atmosphere at different cooling rates.
Figure 8 is a graph showing the relationship between cooling rate and copper powder Nt, Figure @4 shows the relationship between NH8 in the heat treatment atmosphere and steel powder N content. Graph, Figure 5 is a graph showing the relationship between the dew point of the heat treatment atmosphere and the amount of copper powder N. Patent applicant: Kawasaki Steel Corporation Figure 1 Figure 2 Figure 8 Figure 4 NH3 amount (PP")

Claims (1)

[Claims] 1. In finishing heat treatment of atomized steel powder for powder metallurgy, atomized steel powder is heated to 90% in a reducing or neutral atmosphere.
After maintaining the atomized i-powder within the temperature range of 0 to 1050°C for 5 to 45 minutes, the atomized i-powder was heated to HF+O to 80 vol! 1.Nu
s 400 ppm or less and a dew point of 40° C. or less, 9゛00~FI Is OY', /7) Cooling within the temperature range 17 at a cooling rate of 5~zO℃/min, and the ゛γ
A method for producing a C°6 compressible steel powder with excellent productivity, which is characterized by promoting denitrification of atomized steel powder.