JPS6229482B2 - - Google Patents

Description

[Detailed description of the invention]

The invention relates to a low apparent density atomized iron powder for powder metallurgy that has excellent fluidity, moldability, and compressibility. Recently, iron powder for powder metallurgy is often used as a raw material for sintered materials for machine structures, and many of them contain Ni powder, Cu powder, graphite powder, powder lubricant, etc. to increase strength and toughness. For example, (2Cu−0.8C−1Zn・
St.−96.2Fe) wt%, compression molded, and sintered. Traditionally, in the case of atomized iron powder for powder metallurgy, the apparent density
It is common to use iron powder with excellent compressibility as high as 2.80 to 3.20 g/cm ³ as raw material iron powder for high-density iron-based sintered materials. Conventionally, the shape of the atomized iron powder particles is tuft-like, but the surfaces of the individual particles or protrusions that make up the tufts are smooth spherical surfaces, so there are few pools of fine graphite powder and powder lubricant to be mixed, so the mixed powder According to the JIS Z2502 test method, the fluidity of the product was non-flow, and there was a major problem regarding the ability to fill the mold. In particular, this conventional iron powder has been limited in its application to thin-walled sintered materials with complex shapes. Moreover, the formability evaluated by the Rattler value and the transverse rupture strength of the green compact is insufficient, and the mechanical strength of the sintered material is low despite its high density, for example, 6.80 g/cm ³
The reality is that it is not used as a medium or low density sintering agent. Therefore, reduced iron powder (mill scale reduced iron powder, ore reduced iron powder) is used as the raw material powder for medium to low density sintered materials. Therefore, recently there has been a demand for atomized iron powder that has both excellent fluidity and formability while maintaining good compressibility, and the development of atomized iron powder that is particularly oriented toward improving formability is progressing. It is being For example, JP-A-54-114467 and JP-A-50-115161.
The publication discloses a method for producing water-sprayed iron powder whose apparent density is reduced to 2.50 g/cm ³ . That is, the technology disclosed in JP-A-54-114467 has a water pressure of 81.6 kg.
f/cm ² , high-pressure water jetted in an inverted cone shape with a spray angle of 80 to 120 degrees is injected into the falling flow of molten iron under a negative pressure atmosphere of 20 to 204 gf/cable to produce atomized iron powder with a large amount of fine powder. (hereinafter referred to as raw iron powder), and then 1000 ~
This is a method of reducing annealing in a reducing atmosphere at a temperature of 1200°C and crushing the obtained sintered cake to obtain a product having a larger particle size than raw iron powder. On the other hand, in the method disclosed in JP-A-50-115161, the particle size distribution is defined by a PSC value, and raw iron powder with a PSC value in the range of 1.0 to 2.7 is reduced and annealed at a temperature of 760 to 1149°C.
This method involves crushing the sintered cake using a disc mill. Both of the above-mentioned known methods simply increase the proportion of fine powder in the raw iron powder to sinter and agglomerate the particles during reduction annealing, and then crush the sintered cake under a relatively small load. This is a technology that aims to improve formability by lowering the apparent density, and only examples of producing water-sprayed iron powder with an apparent density of up to 2.50 g/cm ³ have been shown. Moreover, in general, as the amount of fine powder in raw iron powder increases and as the reduction annealing temperature increases, sintering of the particles progresses, and the apparent density of the crushed iron powder decreases, improving formability. The reality is that they do not have fluidity, formability, and sinterability that are equivalent to or better than reduced iron powder. In addition to the above-mentioned two prior arts, there is also the Powder and Powder Metallurgy Association; Collection of Autumn Lecture Summaries in 1974, Lecture No. 1.
11 discloses that the apparent density of ring-annealed iron powder is more strongly influenced by the apparent density of the as-sprayed powder (raw iron powder) itself than by the reducing conditions. There is no mention of atomized iron powder with a density of less than 2.50 g/cm ³ . The object of the present invention is to have fluidity and formability that are equal to or better than reduced iron powder without impairing the excellent compressibility of the atomized iron powder itself, and in particular, to have an apparent density of 2.00 to 2.50 g/ The purpose is to provide atomized iron powder with a low apparent density of ^cm3 . The details of the configuration of the present invention will be explained below. The present invention was completed based on the following four findings. (1) Iron powder for powder metallurgy is covered with a thick layer of mixed fine graphite powder and powder lubricants such as zinc stearate (Zn/St), but the excess is unevenly distributed in nodules. This deteriorates the fluidity and moldability of the mixed powder. Therefore, in order to ensure excellent fluidity and moldability, the iron powder must have a particle shape that has many reservoirs for the fine graphite powder and powder lubricant. In other words, it is necessary to have a low apparent density, and according to the JIS-Z-2502 test method, the apparent density of a single iron powder that flows smoothly is lower than that of atomized iron powder.
2.50g/ ^cm3 or less (reduced iron powder is 2.60g/cm3 or less
g/ ^cm2 or less). (2) Compared to conventional atomized iron powder, which has an apparent density of 2.80 to 3.20 g/cm ³ , the apparent density obtained by the present invention is
When the amount is 2.50 g/cm ³ or less, the Rattler value and transverse rupture strength of the green compact are significantly superior. For example, 2Cu−0.8C−1Zu・St−96.2wt%Fe
In the case of mixed powder, the Stoller value (according to JSPM standard 4-69 test method) at a molding pressure of 5t/ ^cm2 is
In the case of single iron powder (100wt%Fe), the transverse rupture strength of the green compact is 250Kg/cm according to the ASTM-B-312-64 test method at a compacting pressure of 4.2t/ ^cm2 . ² , which indicates that it has even better moldability. (3) Regarding the amount of fine powder of -325 mesh (-45 μm), if the amount exceeds 50 wt%, the apparent density will exceed 2.50 g/cm ³ and fluidity, compressibility and moldability will deteriorate. On the other hand, if it is less than 10 wt%, coarse pores and sharp pores along the grain interfaces of the sintered body will increase, and the mechanical strength of the sintered body will decrease significantly. (4) In order to achieve compressibility superior to reduced iron powder with an apparent density equivalent to that of reduced iron powder, that is, a green powder density of 6.70 g/cm ³ or more at a compacting pressure of 5 t/cm ² ,
It is necessary to limit the amount of impurities such as C, N, Si, P, and S, and also to ensure the same strength of the sintered body as when reduced iron powder is used when molded at the same molding pressure. The amount of impurities such as Si, Mn, and O must be limited. The sprayed iron powder of the present invention was produced based on the above-mentioned knowledge, and the reason why the chemical composition and characteristics of the iron powder were limited in accordance with the description of the examples will be explained below. What is important in obtaining the iron powder of the present invention is to use raw iron powder with a reasonably low apparent density as a starting material in order to achieve an apparent density of 2.0 to 2.5 g/cm ³ in the final product. . This means that the sintered cake is crushed with an impact force that maintains the irregular grain shape, which is the original characteristic of liquid atomized iron powder, and the aggregated irregular grain shape, which is sintered and agglomerated during reduction annealing. This is because it will be convenient for That is, according to the research of the present inventors, the single particle size of raw iron powder sieved with a Tyler standard sieve; di
Between [μm] and apparent density; x [g/cm ³ ],
x = approximately constant for particle sizes exceeding 150 μm, 150
It was found that there is a relationship of di = ae ^{- bx} for particle sizes of μm or less. It has also been found that this relationship also holds true for atomized iron powder obtained by crushing a reduction-annealed sintered cake with an impact force of 10 ^-5 Kgf·m/sec ² or less. Therefore, the apparent density of reduction annealed iron powder is limited by the apparent density of raw iron powder. For example, the apparent density of reduction annealed iron powder using raw iron powder with a particle size exceeding 150 μm is limited. It is difficult to make the apparent density lower than that of raw iron powder, and on the other hand, if strong crushing is used to solve this problem, the apparent density will be increased. Further, the apparent density of iron powder obtained by reduction annealing using raw iron powder with a particle size of 150 μm or less is lower than that of raw iron powder because the particles are sintered and agglomerated to form irregular particle sizes. In this case, the above particle shape constant;
b always has a positive value, and it is difficult to industrially realize a negative value. Therefore, in order to produce atomized iron powder with an apparent density of 2.50 g/cm ³ or less by reduction annealing, x 2.60 g/cm ³ is preferably x at a particle size exceeding 150 μm.
2.50g/cm ³ , di= for particle size below 150μm
ae ^-bx , where di: average diameter of three sieves of Tyler standard sieve (di=d ₁ + d ₂ /2) [μm], x: apparent density [g/
cm ³ ], a, b; It is important to use raw iron powder that satisfies the relationship of b1.5 in particle shape constant as a raw material to obtain the iron powder of the present invention, and this point has excellent fluidity. This is effective in ensuring moldability. Table 1 shows the properties of the raw iron powder mentioned above.
The relationship between its apparent density: x [g/cm ³ ] and the single particle size sieved with a Tyler standard sieve: di=d ₁ +d ₂ /2 [μm] is shown.

【table】

[Table] Next, the reason for limiting the chemical composition and particle size of the low apparent density sprayed iron powder of the present invention will be explained. FIG. 1 shows the relationship between the amount of impurities and the green density of iron powder 1 of the present invention. One of the characteristics of atomized iron powder is that its compressibility is superior to that of reduced iron powder, so it is at least necessary for the iron powder of the present invention to have a green density higher than that of reduced iron powder at the same apparent density. It is.
In this sense, the iron powder of the present invention must have a green density of 6.70 g/cm ³ or more at a compacting pressure of 5 t/cm ² , and in order to ensure this green density, as can be seen from FIG. 1, C; 0.05wt% or less, N: 0.0050wt% or less,
Si; 0.10wt% or less, P; 0.050wt% or less, S;
It must have a chemical composition of 0.10wt% or less. Furthermore, Mn and O were limited for the following reasons. That is, Mn is an element that spheroidizes the sprayed iron powder particles and increases the apparent density, but when it exceeds 0.40 wt%, the apparent density becomes higher than 2.50 g/cm ³ . Also, like Si, Mn is an oxygen scavenger for molten iron and is an element that preferentially oxidizes during spraying to form an oxide film on the surface of iron powder particles, making it difficult to reduce.
If it exceeds 0.10wt%), sinterability will be inhibited. Since 80% or more of O consists of oxides on the surface of iron powder, if it exceeds 0.25 wt%, the oxide film becomes thick, inhibiting sinterability, and the mechanical strength of the sintered body deteriorates significantly. Next, the iron powder of the present invention must contain particles of 45 μm or less in a range of 10 to 50 wt%, and the reason for this limitation will be explained below. Generally, the particle shape of atomized iron powder differs between coarse particles and fine particles, and is said to be dependent on particle size. In particular, -325 mesh (45 μm) particles are
This has a large effect on the powder metallurgical properties of spheroidized powder. Figure 2 is -325 mesh (-45μm)
It shows the relationship between the amount of fine powder and the apparent density of
Figure 3 also shows the amount of fine powder and 2Cu−0.8C−1Zn・
St-Relationship with fluidity of 96.2wt%Fe mixed powder, 4th
The figure also shows the relationship between the amount of fine powder and the green density at a compacting pressure of 5t/ ^cm2 , and Figure 5 shows the relationship between the amount of fine powder and the compacting pressure of 4.2t/cm2.
Figure 6 shows the relationship between the transverse rupture strength of a green compact in cm ² and the tensile strength of the iron powder of the present invention, which was formed with the same amount of fine powder and a pressure of 5 t/cm ² and sintered at 1120°C for 30 minutes in an ammonia decomposition gas atmosphere. As shown in these relationships, the amount of fine powder of -325 mesh (-45 μm) is
When it exceeds 50wt%, the apparent density exceeds 2.50g/ ^cm3 , the fluidity of the 2Cu-0.8C-1Zn・St-96.2wt%Fe mixed powder is nonflow, and the amount of N is
The green density at 50ppm and compacting pressure of 5t/ ^cm2 is
Molding pressure reduced to less than 6.70g/ ^cm3 with mold lubrication
The compact transverse rupture strength at 4.2t/cm ² decreases to less than 250Kg/cm ² . On the other hand, with +325 mesh (+45 μm) coarse grains, the apparent density can be lower than 2.00 g/cm ³ , but
The sintered body is coarse and has many irregularly shaped pores that become sharp along the particle interface, resulting in a significant decrease in mechanical strength, particularly fatigue strength. Therefore, it is necessary to increase the amount of -325 mesh (-45 μm) fine powder to 10 wt% or more to reduce coarse pores and improve the mechanical strength of the sintered body. Next, the reason why the apparent density is limited to the range of 2.00 to 2.50 g/cm ³ will be explained. In general, various powder metallurgical properties can be evaluated using the apparent density as one of the indicators of the particle shape of iron powder. Figure 7 shows the apparent density of iron powder at each level and 2Cu−0.8C−1Zn.
Figure 8 shows the relationship between the fluidity of St-96.2wt%Fe mixed powder and the apparent density iron powder and molding pressure at each level.
The relationship between ^the Rattler value of 1Zn・St-99wt%Fe mixed powder at 5t/cm ² is shown in Figure 9. This shows the relationship between the powder compact transverse rupture strength of 100wt%Fe). As shown in the relationships in Figures 7 to 9, when fine graphite powder or Zn/St powder is mixed, the fluidity of iron powder is extremely deteriorated, and the apparent density of the sprayed iron powder is 2.50 g/
If the density exceeds cm ³ , the fluidity becomes nonflow, and by reducing the apparent density to 2.50 g/cm ³ or less, the tip shape retention property, which is one of the moldability of the green compact, is evaluated. The Rattler value becomes even more excellent and low, resulting in improved fluidity and moldability. On the other hand, −
If 10 wt% or more of 325 mesh (-4 μm) fine powder is contained, it becomes impossible to produce atomized iron powder with an apparent density of less than 2.00 g/cm ³ , so this is set as the lower limit. Next, examples of the present invention will be described. Table 1 shows an example of the iron powder of the present invention produced under the conditions described below, and the powder characteristics of the raw iron powder measured by the JIS test method related to powder metallurgy and the powder of the iron powder obtained by reduction annealing. It is a characteristic. The angular difference between α and β can be reduced to
-β range of 4° (α: convergence angle of multiple jet impulses, β: convergence angle of membranous flow obtained by deflecting multiple jet impulses with a guide), water pressure 100 to 180 Kg/cm ²
The raw iron powders (starting raw material) of iron powders 1 and 2 of the present invention were produced under conditions of a water flow rate of 230/min and adjusting the G range and the molten metal nozzle diameter within a range of 8 to 14 mmφ. Next, select the raw iron powder with a -80 mesh using a Tyler standard sieve, soak it in ammonia decomposition gas for 45 minutes at a temperature of 950°C for reduction annealing, and cool it to below 180°C in a furnace. , the sintered cake taken out into the atmosphere is crushed with a hammer mill, and -80
It was sieved into mesh. The sintered cake is crushed using a hammer mill with mechanical specifications of rotation speed: 2800 r.pm, distance from the main shaft axis to the tip of the hammer: 150 mm, number of hammers: 20, and unit weight of hammers: 500 grf. A screen with a diameter of 100 mm was attached to crush the material, and the material was sieved using an 80-mesh sieve. Next, regarding this +80 mesh iron powder, 1mmφ
It was crushed using a screen with a hole diameter of 100 mm, and then sieved using an 80-mesh sieve. 0.5mm in the same way
A screen with a hole diameter of 0.3 mm was installed, and the crushing and sieving were repeated, and a total of four times of -80 mesh iron powder was blended in a V-type mixer. Table 2 shows the powder metallurgy of the iron powders 1 and 2 of the present invention produced in this way.
These are green compact properties measured using the JSPM (Society of Powder Metallurgy Standard) test method. Table 2 shows commercially available water-sprayed iron powder (commercially available water-sprayed iron powder) with an apparent density of 2.80 to 3.20 g/ ^cm3 , which is mainly used as a raw material for high-density iron-based sintered materials, as a comparison material. Symbol: A) and apparent density of 2.30 to 2.60 g/
The powder and compact characteristics of a commercially available mill scale reduced iron powder (symbol: B, °C) with high moldability of cm ³ are shown.

[Table] As shown in Tables 1 and 2, the iron powder of the present invention has been sufficiently decarburized and denitrified by reduction annealing, so it has the same compressibility as commercially available water-sprayed iron powder (symbol: A). , that is, the green density at a compacting pressure of 5t/ ^cm2 is
It has a particle size of 6.80 g/cm ³ or more and consists of particles made irregular by water spraying and particles made irregular by sintering and agglomeration of particles by reduction annealing, so it is not commercially available mill scale reduced iron powder ( Symbols: B and C) have superior moldability. In other words, the transverse rupture strength of the green compact at a molding pressure of 4.2t/ ^cm2 is 250Kg/cm2 when using only iron powder (100wt%Fe) in mold lubrication.
^cm2 or more, 2Cu−0.8C−1Z・St−96.2wt with zinc stearate (Zn・St) added as a powder lubricant
%Fe mixed powder has 150Kg/cm ² or more. As explained above, the present invention exhibits especially excellent fluidity and formability in a mixed powder mixed with fine graphite powder and powder lubricant, and has good compressibility and sinterability. It is suitable as a raw material iron powder for low-density iron-based sintered materials and thin-walled parts with complex shapes, and can be substituted for reduced iron powder. Of course, it can also be widely used for high-density iron-based sintered materials. Although the present invention has been explained using water-sprayed iron powder as an example, it is applicable to all iron powders manufactured using a liquid as a spray medium.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the relationship between the amount of impurities and green powder density of water-sprayed iron powder aimed at low apparent density, Figures 2 to 6
Both figures show the relationship to -325 mesh (-45 μm) in -80 mesh water sprayed iron powder. Figure 2 is a relationship with apparent density, Figure 3 is a relationship with fluidity, and FIG. 4 is a relationship diagram with green powder density, FIG. 5 is a relationship diagram with green compact transverse rupture strength, and FIG. 6 is a relationship diagram with tensile strength. Figures 7 to 9 are all diagrams showing the relationship between the apparent density of each level and -80 mesh water sprayed iron powder; Figure 7 is a diagram showing the relationship with fluidity;
The figure shows the relationship with the Rattler value, and FIG. 9 shows the relationship with the transverse rupture strength of the compact.

Claims (1)

[Claims] 1% by weight, O: 0.25% or less, C: 0.05% or less, N: 0.0050% or less, Si: 0.10% or less, Mn:
Contains 0.40% or less, P: 0.050% or less, S: 0.10% or less, with the remainder consisting of other unavoidable impurities and Fe, and has a particle size of 45 μm or less.
A low apparent density atomized iron powder for powder metallurgy that has a particle size distribution of 10 to 50 wt% and an apparent density of 2.00 to 2.50 g/cm ³ and has excellent fluidity, moldability, and compressibility.