JP7233257B2 - Method for producing ferrous powder for sintered metal parts - Google Patents

Description

The present invention relates to a method for producing ferrous powder for sintered metal parts.

As methods for evaluating powder properties of powders used in powder metallurgy, JIS includes (1) a method for measuring metal powder fluidity (JIS Z2502: 2012), (2) a method for measuring apparent density of metal powder (JIS Z2504: 2012), and (3) a method for measuring the tap density of metal powder (JIS Z2512: 2012), respectively.

Of these, the above (1) evaluates the fluidity of the metal powder from the time it takes for the powder supplied to a funnel of a predetermined size to flow down. If the fluidity of the metal powder is low, the speed at which the metal powder is filled into the mold for molding the green compact before sintering is reduced, resulting in a reduction in the production speed and green density of the green compact. There is (Patent Document 1). Under such circumstances, in the field of powder metallurgy, the fluidity of the raw material powder is an important index for judging the moldability and mold fillability when molding a green compact.

Japanese Unexamined Patent Application Publication No. 2007-2340

However, the evaluation of fluidity by the above test method relies on the free fall of powder from the funnel. There are cases where the body becomes clogged and measurement becomes impossible. Even with such unmeasurable powders, there are many cases where there is no problem with mold filling properties and moldability, and compacting can be performed without problems. As described above, the powder used in powder metallurgy cannot be sufficiently evaluated only by the fluidity test specified in JIS.

In recent years, there is an increasing need to replace molten steel parts with ferrous sintered metal parts. For this replacement, it is desirable to reduce the porosity and pore diameter contained in the sintered body as much as possible in order to improve the mechanical properties of the metal parts. In order to realize these, it is effective to sufficiently densify the green compact at the completion of the green compact molding process, but such densification cannot be achieved only with the flowability data of the raw material powder. can be achieved or not cannot be predicted.

Therefore, the quality of the powder cannot be defined by the powder characteristics (performance index) obtained by the evaluation of the above conventional method.

In view of the above circumstances, the present invention provides an iron-based powder for sintered metal parts and an iron-based powder that can obtain products having performance and quality above a certain level by using powder characteristics and indices obtained by introducing a new method. The Company provides sintered metal parts.

The method for producing an iron-based powder for sintered metal parts according to the present invention, which has been made to solve the above-mentioned problems , is characterized in that Fe—Ni—Mo alloy powder and Fe—Ni—Mo—Cu alloy powder are subjected to Graphite powder as a carbon solid solution source and a wax-based solid lubricant for reducing friction during molding are glued with a binder to form various segregation-preventing treated powders, and the above-mentioned segregation-preventing treated powders are molded at room temperature. By uniaxially pressing and molding at a pressure of 980 MPa to prepare a disk-shaped test piece, and using the volume calculated from the dry weight, diameter and thickness of the test piece, the green density of the test piece is derived. , By arranging the data of each segregation-preventing treated powder such that the uniaxial collapse stress in the single-sided shear test at a normal stress of 22 to 26 kPa is in ascending order, and using the correlation in which the green density at a molding pressure of 980 MPa is reversed, Under a normal stress of 22 to 26 kPa, the uniaxial collapse stress is 4.0 kPa or less and the compact density is 7.2 g/cm ³ or more. It is something to do. That is, the present invention relates to a powder that is used as a raw material for producing sintered metal parts, and the powder means pure iron powder or alloyed steel powder as a base material, or an alloyed steel powder to which an additive is added. It refers to raw material powders in general, including mixed powders and segregation-preventing treated powders to which additives are adhered via a binder or the like.

Fluidity index evaluated by a single-sided shear test (ring-type shear test), specifically, the value of uniaxial collapse stress measured under a normal stress of 22 to 26 kPa, the obtained green compact, and by extension the sintered compact By obtaining the uniaxial collapse stress of the raw material powder based on the correlation between the density of the powder, it is possible to predict the density of the green compact and sintered compact manufactured (molded and sintered) under the same conditions. .

The base powder is preferably Fe--Ni--Mo alloy powder and Fe--Ni--Mo--Cu alloy powder.

Further, the iron-based sintered metal part of the present invention uses the iron-based powder for sintered metal parts of the present invention as a raw material and has a green density of 7.2 g/cm ³ or more.

As described above, according to the present invention, the fluidity index (specifically, the value of the uniaxial collapse stress measured under a normal stress of 22 to 26 kPa) evaluated by a single-sided shear test (ring-type shear test), Green compacts manufactured (molded and sintered) under the same conditions by obtaining the uniaxial collapse stress of the raw material powder based on the correlation between the density of the resulting green compact and, by extension, the density of the sintered compact. and the density of the sintered body can be predicted. As a result, when comparing and examining steel powders with similar compositions in the development stage, it is possible to predict the quality and order of steel powders without actually manufacturing samples, and to screen the materials to be examined. . In addition, at the manufacturing stage, the uniaxial collapse stress value measured at the development stage may be applied to incoming inspections of raw material powders and to investigate deterioration of materials due to long-term storage. It can contribute to stabilization.

FIG. 2 is a cross-sectional view (cross-sectional view taken along the line AA in FIG. 2) of the ring-type shear tester according to the embodiment of the present invention; It is a bottom view of the upper cell of the ring shear tester. It is a figure explaining how to obtain|require simple collapse stress (sigma)c. 4 is a graph showing the results of normal stress and uniaxial collapse stress measured for the segregation-preventing treated powder of Example 1. FIG.

An embodiment of a method for evaluating a powder for powder metallurgy according to the present invention will be described below with reference to FIGS.

In addition to the base metal powder (iron powder, copper powder, etc.), the powder for powder metallurgy in the present embodiment includes various additives for improving any of powder characteristics, green compact characteristics, and sintered compact characteristics. For example, graphite powder as a carbon solid solution source, solid lubricants (graphite powder, _MoS2 powder, etc.) as slidability improvers, solid lubricants (zinc stearate powder, wax powder, etc.), a machinability improving agent (MnS powder), and the like. Powder metallurgy powders also include metal powders (Ni, Mo, Cu, Cr, Mn, Sn, etc.) that are added to base metal powders in small amounts. These additive metal powders may be used as individual powders, or may be used after being plated on the surface of other metal powders or alloyed with other metal powders. The method of alloying is not particularly limited, and complete alloying (pre-alloying), diffusion alloying, simple mixing (alloying during sintering), etc. can be employed. Moreover, the method for producing the base metal powder is not particularly limited, and various powders produced by a reduction method, a water atomization method, a gas atomization method, a carbonyl method, a stamp method, etc. can be used. Plural types of metal powders and alloy powders can also be used as the base metal powder.

In powder metallurgy, these powders (raw material powders) are mixed in a mixer such as a V-shaped mixer, the mixed powders are supplied to a mold, and a compact is compacted and then sintered. Various sintered parts such as gears, connecting rods, and bearings are manufactured by subjecting the sintered body after sintering to post-treatments such as sizing, heat treatment, and machining as necessary.

Characteristic evaluation of the powder for powder metallurgy in the present embodiment is performed by a technique called a direct shear test. This single-sided shear test measures the shear stress that occurs when the powder layer is pressed vertically and then slid horizontally, and the details of the test method are specified in JIS Z8835: 2016. there is This single-sided shear test is a test method that is mainly used in the field of soil engineering to evaluate the characteristics of soil, but in this embodiment, this test is diverted to the evaluation of the powder characteristics of powders for powder metallurgy. The test procedure at that time basically conforms to the above JIS regulations.

In the direct shear test in this embodiment, a ring shear tester 1 shown in FIG. 1 is used as a tester. A ring-type shear tester 1 has, as main components, a lower cell 2 (movable cell) on the rotating side and an upper cell 3 (fixed cell) on the stationary side.

Lower cell 2 is rotationally driven by motor 4 and speed reducer 5 . The rotational torque of the lower cell 2 is measured by a torque meter (not shown). The upper surface of the lower cell 2 is provided with an outer wall 2a and an inner wall 2b each having a cylindrical shape. formed. A shaft portion 2c protruding upward is provided at the center of the lower cell 2, and the lower cell 2 is driven by the motor 4 and the speed reducer 5 to rotate about the rotation axis O in a certain direction.

The upper cell 3 is formed in a substantially disk-like shape with a hole, and the central hole is fitted to the shaft portion 2c of the lower cell 2 so as to be relatively rotatable. The lower surface of the upper cell 3 is provided with a protruding portion 3a that protrudes downward. The projecting portion 3a has an annular shape centered on the rotation axis O, and is fitted to the inner peripheral surface of the outer wall 2a and the outer peripheral surface of the inner wall 2b of the lower cell 2 in such a manner as to allow relative rotation. A lower end portion of the protruding portion 3a is inserted into the housing portion S. As shown in FIG.

In the test using this ring-type shear tester, after filling a specified amount of the powder to be measured in the storage part S to form a powder layer 8, the uneven type upper cell 3 is set on the lower cell 2, and the upper cell This is done by driving the motor 4 while applying a predetermined vertical load W to the motor 3 . Since the blades 7 of the upper cell 3 bite into the powder layer 8 due to the vertical load W, a sheared surface is formed in the powder layer 8 . The lower cell 2 is rotated at a constant rotational speed, and when it is rotated by a predetermined angle (for example, about 15°), the test is terminated, and the vertical stress σ is calculated from the vertical load W at that time, and the torque meter at that time. Calculate the shear stress τ from the measured values. The above tests are performed multiple times with different vertical loads W, and the vertical stress σ and shear stress τ for each time are plotted in a normal stress σ-shear stress τ diagram as shown in FIG.

The approximate straight line connecting the plotted points represents the limit state line CSL, and the point where a circle (Mohr's circle) passing through the origin and in contact with the limit state line CSL intersects the vertical stress axis represents the uniaxial collapse stress σc. The inclination angle δi of the limit state line CSL with respect to the vertical stress axis represents the internal friction angle of the interparticle friction index.

The uniaxial collapse stress σc is the stress value required to crush and flow the powder layer (cylinder) compacted and compacted by each vertical stress, and the smaller the value, the easier the powder to flow. do. Therefore, the value of the uniaxial collapse stress σc can be used as a fluidity index of powder. As an example of the effect of using a powder having a small uniaxial collapse stress, it is possible to shorten the supply time of the raw material powder to the mold and improve the production efficiency.

According to the one-sided shear test as described above, it is possible to evaluate the properties of the powder not only in the state where only gravity acts, but also in the state where vertical stress acts from above. Therefore, it becomes possible to diversify the method of evaluating the characteristics of powders used in powder metallurgy, and to measure the characteristics of powders in accordance with the actual behavior during compression molding in a mold. Further, according to the above-mentioned single shear test, it is possible to evaluate the fluidity and frictional properties of the powder in detail.

The present invention particularly focuses on fluidity. The index of fluidity (which can be evaluated by a single-sided shear test) employed in this evaluation method is uniaxial collapse stress (unit: kPa). In the conventional evaluation method (evaluation method of fluidity using a funnel specified by JIS), the performance difference between powders cannot be classified with high repeatability, so the performance of sintered parts is defined as in the present invention. No index is available for The main reason for this is that it is the result of reflecting only the gravitational flow due to free fall.

On the other hand, in the single-sided shear test (ring-type shear test) adopted in this embodiment, if the vertical stress during the test is reduced, it is possible to evaluate the fluidity similar to the JIS method, which reflects only gravity flow. By increasing the vertical stress at time, the fluidity under compression in the elastic deformation region in the mold, that is, the "rearrangement behavior" in which other particles enter the voids composed of particles and are compacted The fluidity (= ease of powder filling by compression) can be evaluated.

Therefore, there is a correlation between the uniaxial collapse stress value measured under a high normal stress, specifically at a normal stress of 22 to 26 kPa, and the green density after compression molding (see Examples below). In order to obtain a green compact having a green density of 7.2 g/cm ³ or ^more , the uniaxial collapse stress value must be 4.0 kPa or less. , the uniaxial collapse stress value should be approximately 1.2 kPa or less (see Examples 1 and 2 below).

Graphite powder as a carbon solid solution source during sintering and a wax-based solid lubricant for reducing friction during molding are added to Fe-Ni-Mo alloy powder and Fe-Ni-Mo-Cu alloy powder. Various segregation-preventing treated powders pasted with a binder were subjected to a single-sided shear test (ring-shaped shear test) to determine the uniaxial collapse stress [kPa], which is an index of fluidity. As a representative example, the measurement results of the segregation-preventing treated powders of Example 1 and Comparative Example 1 (graph of correlation between normal stress and uniaxial collapse stress during measurement) are shown in FIG.

Further, each segregation-preventing treatment powder was uniaxially press-molded at room temperature (approximately 25° C.) at a molding pressure of 980 MPa to prepare a disc-shaped test piece of φ20×t5 mm. Using the dry weight of the test piece and the volume calculated from the diameter and thickness, the green density [g/cm ³ ] of the test piece was derived. For the diameter and thickness of the test piece, the average value of the dimensions measured four times using a micrometer was used.

Focusing on the relationship between the uniaxial collapse stress (numerical value at normal stress of 22 to 26 kPa), which is a fluidity index in a single-sided shear test, and the compact density, when the compact density is 7.3 g / cm ³ or more, ◎, 7 A case of 2 g/cm ³ or more and less than 7.3 g/cm ³ was evaluated as ○, and a case of less than 7.2 g/cm ³ was evaluated as x. Table 1 summarizes the evaluation results.

As a result of arranging the data of each segregation-preventing treated powder so that the uniaxial collapse stress in the one-sided shear test at a normal stress of 22 to 26 kPa is in ascending order, the green density at a compacting pressure of 980 MPa was in descending order. .

Judging based on the above criteria, Example 1 is ⊙ because the green density is 7.3 g/cm ³ or more, and Examples 2 to 5 are ◯ because the green density is 7.2 g/cm ³ or more and less than 7.3 g/cm ³ , and Comparative Examples 1 and 2 were evaluated as x because they were less than 7.2 g/cm ³ .

With the conventional JIS method using a funnel, it is very difficult to clarify the difference in flowability between powder types, and the result reflects only gravity flow due to free fall. No clear correlation with green density is seen.

On the other hand, in the direct shear test, if the vertical stress during the test is reduced, it is possible to evaluate the flowability in the same way as the JIS method, which reflects only gravity flow. In other words, fluidity during rearrangement behavior when other particles enter the voids composed of particles and are compacted (= ease of powder filling due to compression ) can be evaluated. Therefore, it is considered that there is a correlation with the green density after compression molding.

From this result, there is a clear correlation between the uniaxial collapse stress and the green density under high normal stress, specifically at a normal stress of 22 to 26 kPa, and it has a green density of 7.2 g / cm ³ or more. In order to obtain a green compact, the uniaxial collapse stress value must be 4.0 kPa or ^less . .2 kPa or less.

1 Ring type shear tester 2 Lower cell (movable cell)
3 upper cell (fixed cell)
σc Uniaxial collapse stress δi Internal friction angle

Claims (1)

Graphite powder as a carbon solid solution source during sintering and a wax-based solid lubricant for reducing friction during molding are added to Fe-Ni-Mo alloy powder and Fe-Ni-Mo-Cu alloy powder. Various segregation-preventing treated powders are pasted with a binder, and each of the segregation-preventing treated powders is uniaxially press-molded at room temperature at a molding pressure of 980 MPa to prepare a disk-shaped test piece, and the dry weight of the test piece. And, by deriving the compacted density of the test piece using the volume calculated from the diameter and thickness, each segregation prevention such that the uniaxial collapse stress in the one-sided shear test at a normal stress of 22 to 26 kPa is in ascending order. By arranging the data of the processed powder, using the correlation that the green density is reversed at a compacting pressure of 980 MPa, under a normal stress of 22 to 26 kPa, the green density is 4.0 kPa or less with the uniaxial collapse stress . A method for producing an iron-based powder for sintered metal parts, characterized in that the iron-based powder for sintered metal parts is produced as the segregation-preventing powder having a grain size of 7.2 g/cm ³ or more.

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