JP2008240031A - Preform for pressing using iron powder as raw material, and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a preform for pressing by which a high-density, high-strength iron-based sintered member can be produced without addition of expensive alloy components and also to provide its manufacturing method. <P>SOLUTION: An iron powder is mixed with a graphite powder or further a lubricating material and the resulting iron-based powder mixture is subjected to pressing and pre-sintering to manufacture the preform for pressing. In this manufacturing method, holding is done, as the pre-sintering, at 1,000 to 1,200°C for ≤60 sec in an inert atmosphere. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention is a machine part suitable for use in various applications (hereinafter referred to as an iron-based sintered member) through pressure molding and sintering from a mixed powder mainly composed of iron powder (hereinafter referred to as an iron-based mixed powder). In particular, the present invention relates to a molding material suitable for a compression molding method and a manufacturing method thereof for manufacturing a high-density and high-strength iron-based sintered member.

  Powder metallurgy technology can finish a complicated shape part into a predetermined shape with high accuracy, and can greatly reduce the cutting cost. For this reason, parts manufactured by powder metallurgy technology using iron powder as a raw material are widely used for various applications (for example, automobiles). Moreover, recently, in order to reduce the size and weight of various devices, it is required to increase the strength of parts, and particularly, there is a strong demand for increasing the strength of iron-based sintered members.

When manufacturing iron-based sintered parts,
(1) Iron powder is mixed with alloy powder (eg, graphite powder, copper powder, etc.) and lubricant (eg, zinc stearate, lithium stearate, etc.) to form an iron-based mixed powder.
(2) Filling the mold with iron-based mixed powder, and further pressing to form a molded body,
(3) A step of sintering the formed body to obtain a sintered body is employed. The obtained sintered body is subjected to sizing and cutting as necessary to become an iron-based sintered member having a predetermined shape and dimensions. Further, when high strength is required for the iron-based sintered member, carburizing heat treatment or bright heat treatment is performed.

Since the density of the molded body obtained in the above (2) is about 6.6 to 7.1 Mg / m ³ , the sintered body obtained in (3) has the same density.
In order to increase the strength of the iron-based sintered member, it is necessary to increase the density of the iron-based sintered member. Therefore, it is necessary to mix an additive for promoting high density and an alloy element for promoting high strength into the iron-based mixed powder obtained in (1). If the density of the iron-based sintered member is increased, the voids in the iron-based sintered member are reduced, and mechanical properties such as tensile strength, toughness and fatigue strength are improved. In addition, the addition of alloying elements such as Mo, Mn, Ni, and Cr improves the hardenability and improves the strength of the iron-based sintered member.

Therefore, various techniques for increasing the density and strength of the iron-based sintered member have been studied.
For example, Patent Documents 1, 2, 3, and 4 disclose a technique for forming iron powder while heating (so-called warm forming). In these technologies, 0.5% by mass of graphite powder and 0.6% by mass of lubricant are blended into Fe-4Ni-0.5Mo-1.5Cu based partially alloyed steel powder by applying warm forming technology. The obtained iron-based mixed powder is press-molded at 150 ° C. and 686 MPa to produce a molded body having a density of about 7.30 Mg / m ³ .

However, since the techniques disclosed in Patent Documents 1 to 4 require equipment for heating the iron-based mixed powder in order to apply the warm forming technique, the molding cost increases. In addition, the dimensional accuracy of the molded body decreases due to the influence of expansion or cooling due to heating of the molded body.
Patent Document 5 discloses a technique for manufacturing an iron-based sintered member having a nearly true density by combining powder metallurgy technique and cold forging technique. In this technique, iron powder is pressure-molded, and the resulting molded body is pre-sintered and further cold-forged, and then finish-sintered to produce a high-density iron-based sintered member.

  In the technique disclosed in Patent Document 5, after applying a liquid lubricant to the surface of the molded body and cold forging in the die, a negative pressure is applied to the die to suck and remove the liquid lubricant. Cold forging is performed again in the die and finish sintering is performed. That is, since cold forging is performed after sucking the liquid lubricant that has entered the pores of the compact, the pores are crushed and the high-density iron-based sintered member is obtained.

However, according to the study by the present inventors, even if the powder metallurgy technique and the cold forging technique are combined as disclosed in Patent Document 5, the density of the obtained iron-based sintered member is about 7.5 Mg / m ³ . There is no significant increase in density.
On the other hand, to increase the strength of the iron-based sintered member, it is effective to increase the C concentration. Specifically, iron powder to which graphite powder is added as a C source is pressure-molded in a mold, the resulting molded body is pre-sintered, and the molding material is compression molded as a molding material. Further, finish sintering is performed. In general, when an iron-based sintered member is manufactured by mixing graphite powder with iron powder, such a series of procedures is employed, but C diffuses throughout the forming material by pre-sintering. As a result, the hardness of the molding material increases and the deformation resistance of compression molding increases, so it is difficult to manufacture a high-density and high-strength iron-based sintered member.

  In order to solve such a problem, Patent Document 6 discloses that a mixed powder obtained by mixing iron powder, alloy powder, graphite powder and a lubricant is pressure-molded and pre-sintered (temperature 1100 ° C., holding time 15 to 20 minutes) A technology for producing iron-based sintered parts by forming a molding material and cold forging the molding material to cause at least 50% plastic deformation, followed by finish sintering, annealing and roll processing Is disclosed. That is, since presintering is performed under conditions that suppress the diffusion of C, high deformability can be expressed by cold forging.

However, according to the study by the present inventors, when pre-sintering as disclosed in Patent Document 6 is performed, C diffuses throughout the molding material, so that the hardness of the molding material increases and compression molding is performed. Deformation resistance increases. Therefore, it is difficult to manufacture a high-density and high-strength iron-based sintered member.
Patent Document 7 discloses a technique for improving the deformability of a molding material by controlling the pre-sintering conditions and adjusting the N content of the molding material. Since this technology regulates the C content of iron powder to 0.5% by mass or less, the deformation resistance of the molding material can be reduced, and the density of 7.7 Mg / m ³ or more can be achieved by compression molding the molding material. An iron-based sintered member can be produced.

However, in order to obtain a high hardness (about HRC60) of about HRC60 by the technique disclosed in Patent Document 7, it is necessary to add an expensive alloy element such as Mo and further perform carburizing and quenching heat treatment.
JP-A-2-156002 Japanese Patent Publication No.7-103404 U.S. Pat.No. 5,256,185 US Pat. No. 5,368,630 Japanese Unexamined Patent Publication No. 1-123005 U.S. Pat. Japanese Patent No. 3729764

  An object of the present invention is to provide a molding material for producing a high-density and high-strength iron-based sintered member by a compression molding method without adding an expensive alloy component, and a method for producing the same. To do.

The present invention is a molding material obtained by applying pressure molding to iron-based mixed powder obtained by mixing iron powder with graphite powder, or further with a lubricant, and pre-sintering, This molding material contains less than 0.2% by mass of Mn, 0.5 to 0.8% by mass of C, 0.02% by mass or less of free graphite, and the balance of Fe and inevitable impurities.
The present invention also relates to a method for producing a molding material in which pre-sintering is performed after pressure-forming iron-based mixed powder obtained by mixing iron powder with graphite powder or further with a lubricant. This is a method for producing a molding material that is held in a temperature range of 1000 to 1200 ° C for 60 seconds or less in an inert atmosphere as sintering.

  According to the present invention, C in the molding material is segregated at the crystal grain boundaries, so that the C content is reduced in the crystal grains and the hardness of the molding material is lowered. Therefore, a molding material with high deformability can be obtained, and the molding material can be densified by compression molding to produce a high-density and high-strength iron-based sintered member. In addition, since the deformation resistance of the molding material is small, an iron-based sintered member having a complicated shape can be manufactured.

  Further, after compression molding is performed on the molding material, finish sintering is performed to uniformly diffuse C. Accordingly, after the finish sintering, induction hardening is further performed, and an iron-based sintered member with further enhanced strength can be manufactured without adding an expensive alloy element.

FIG. 1 is a flowchart showing a procedure for producing an iron-based sintered member using a molding material produced by applying the present invention.
In the present invention, as shown in FIG. 1, graphite powder is mixed with iron powder to obtain an iron-based mixed powder. If necessary, a lubricant may be added to the iron powder. The obtained iron-based mixed powder is filled into a mold and further subjected to pressure molding to obtain a preform. The pressing force applied by pressure molding is adjusted so that the density of the preform is 7.1 Mg / m ³ or more. This is because by setting the density to 7.1 Mg / m ³ or more, the bonding area between the iron powder particles is increased, and the deformability of the preform is improved. However, if an excessive pressing force is applied during pressure molding, the durability of the mold is reduced. Therefore, the pressing force applied by pressure molding is preferably in the range of 7.1 to 7.5 Mg / m ³ . More preferably, it is in the range of 7.3 to 7.5 Mg / m ³ .

In the above (2), an iron-based mixed powder filled in a mold and further press-molded is referred to as a molded body, but here it is referred to as a preformed body in order to distinguish it from a finished molded body described later.
The preform thus obtained is pre-sintered to obtain a sintered body. Since this sintered body becomes a material for compression molding for producing an iron-based sintered member, it is referred to as a molding material.
The molding material is subjected to compression molding to obtain a finished molded body having a predetermined shape. Next, the finished compact is subjected to finish sintering to produce a high-density and high-strength iron-based sintered member. Further, by performing quenching (for example, induction quenching, bright quenching, etc.) as necessary, an iron-based sintered member (hereinafter referred to as a high-strength iron-based sintered member) with higher strength can be manufactured.

As described above, the present invention manufactures a molding material that is a compression molding material for manufacturing an iron-based sintered member.
If the C content of the molding material is less than 0.5% by mass, sufficient strength cannot be obtained even by finish sintering or induction hardening. On the other hand, if it exceeds 0.8% by mass, there is a possibility that cracking may occur in the iron-based sintered member or the high-strength iron-based sintered member by finish sintering or induction hardening. Therefore, the C content of the molding material is in the range of 0.5 to 0.8 mass%.

Mn is an element that improves the hardenability of the molding material, but if it is contained in an amount of 0.2% by mass or more, the deformation resistance of the molding material increases, so it is limited to less than 0.2% by mass.
When the free graphite of the molding material exceeds 0.02% by mass, an extended layer of graphite is formed along the flow of the material during compression molding, and C is diffused from the graphite to the base by finish sintering. As a result, the extension layer of graphite remains as voids, resulting in a decrease in density and strength of the molding material. Therefore, the free graphite of the molding material is 0.02% by mass or less.

The balance is Fe and inevitable impurities. Inevitable impurities mainly include N and O.
By setting the N content of the molding material to 0.005% by mass or less, the deformation resistance of the molding material can be reduced. Therefore, it is preferable to reduce the N content as much as possible. However, in order to reduce the molding material to less than 0.0005% by mass, the N concentration in the atmosphere must be suppressed to a low level, resulting in a decrease in productivity and an increase in manufacturing cost. Therefore, the N content of the molding material is preferably 0.005% by mass or less, and more preferably in the range of 0.0005 to 0.005% by mass.

  By setting the O content of the molding material to 0.3% by mass or less, the deformation resistance of the molding material can be reduced. Therefore, it is preferable to reduce the O content as much as possible. However, in order to reduce it to less than 0.02% by mass of the molding material, iron powder having a comparable O content must be used. The iron-based metal powder having such a reduced O content inevitably increases the production cost. Accordingly, the O content of the molding material is preferably 0.3% by mass or less, more preferably 0.02 to 0.3% by mass.

The particle size of the iron powder used in the present invention is not particularly limited, but it is preferable to use iron powder having a size that can be produced industrially at low cost (that is, within an average particle size of 30 to 120 μm). The average particle diameter is the value of the middle point (D50) of the weight integrated particle size distribution.
In addition, a lubricant may be added to the iron-based mixed powder in order to increase the density of the preformed body by effectively applying the pressing force in pressure molding and to reduce the pulling force when taking out from the mold. good. As the lubricant, zinc stearate, lithium stearate, ethylene bisstearamide, polyethylene, polypropylene, thermoplastic resin, polyamide, stearamide, oleic acid, calcium stearate and the like can be used. When the lubricant is added, it is preferable to add 0.1 to 0.6 parts by mass of the lubricant with respect to 100 parts by mass of the iron-based mixed powder (that is, the total amount of iron powder and graphite powder). Furthermore, wax or spindle oil may be added to the iron-based mixed powder so that the graphite powder can easily adhere to the surface of the iron powder.

As a mixing device for mixing iron powder and graphite powder (or further, if necessary, a lubricant), conventionally known mixing devices such as a Henschel mixer, a cone mixer, and a V mixer can be used.
When a preform is obtained by filling a mold with the iron-based mixed powder obtained in this way and performing pressure molding, a conventionally known mold lubrication method, multistage molding method using split molds , CNC press method, isostatic press method, warm forming method, roll forming method and the like can be used. However, since the warm forming method has the problems described above, it is preferable to employ a technique other than the warm forming method from the viewpoint of reducing the forming cost of the preform and improving the dimensional accuracy.

When pre-sintering the preform, if the temperature is less than 1000 ° C., C does not diffuse from the graphite powder to the base, and a large amount of free graphite is generated. On the other hand, when the temperature exceeds 1200 ° C., C is excessively diffused and the hardness of the preform is increased, so that the deformation resistance of the preform is increased. Therefore, the pre-sintering temperature is set within a range of 1000 to 1200 ° C. Preferably it is 1000-1100 degreeC.
When the pre-sintering holding time exceeds 60 seconds, C is excessively diffused from graphite to the base and the hardness of the preform is increased, so that the deformation resistance of the preform is increased. Therefore, the pre-sintering holding time is 60 seconds or less. On the other hand, if it is less than 10 seconds, C does not diffuse sufficiently and a large amount of free graphite is produced. Therefore, it is preferable that the holding time is in the range of 10 to 60 seconds. More preferably, it is 10 to 30 seconds. In order to perform pre-sintering in such a short time, it is preferable to employ plasma heating or high-frequency heating.

As the pre-sintering atmosphere gas, conventionally known nitrogen gas, hydrogen gas, mixed gas of nitrogen and hydrogen, argon gas, ammonia decomposition gas, RX gas and the like can be used.
Hereinafter, a method for producing an iron-based sintered member or a high-strength iron-based sintered member from the molding material will be described.
The molding material becomes a finished molded body having a predetermined size and shape by compression molding. For compression molding, conventionally known cold forging, roll forming and the like can be used. However, since the molding material of the present invention has excellent deformability, cold forging capable of producing a finished molded body with high dimensional accuracy at low cost is preferable.

When the final sintered body is subjected to finish sintering, if the temperature is lower than 1050 ° C., C does not diffuse from the graphite powder to the base, and a sufficiently strong iron-based sintered member cannot be obtained. On the other hand, when the temperature exceeds 1300 ° C., the crystal grains become coarse and an iron-based sintered member having sufficient strength cannot be obtained. Therefore, the finish sintering temperature is preferably in the range of 1050 to 1300 ° C.
When the holding time of finish sintering is less than 600 seconds, C does not diffuse from the graphite powder to the base, and a sufficiently strong iron-based sintered member cannot be obtained. On the other hand, even if it exceeds 3600 seconds, the effect of graphite diffusion is not improved so much and it is not economical. Accordingly, the holding time of finish sintering is preferably in the range of 600 to 3600 seconds.

The atmosphere for finish sintering is preferably an inert atmosphere, a reducing atmosphere or a vacuum in order to prevent oxidation of the iron-based sintered member.
In this way, a high-density and high-strength iron-based sintered member can be manufactured.
In order to manufacture a high-strength iron-based sintered member with higher strength, quenching (for example, induction quenching, bright quenching, etc.) is performed. The heating temperature during quenching is preferably in the range of 800 to 950 ° C., and the cooling medium may be water (so-called water quenching), but oil is used to prevent cracking (so-called oil quenching) ) Is preferable.

  Graphite powder and a lubricant were added to the iron powder and mixed with a V-type mixer to obtain an iron-based mixed powder. As the iron powder, iron powder (JIP301A made by JFE Steel) containing C: 0.007 mass%, Mn: 0.12 mass%, O: 0.15 mass%, and N: 0.001 mass% was used. Zinc stearate was used as the lubricant, and the amount added was 0.3 parts by mass with the total amount of iron-based metal powder and graphite powder being 100 parts by mass. The amount of graphite powder added is as shown in Table 1.

得られた鉄基混合粉を金型に充填して加圧成形を行ない、タブレット状（直径30mm，高さ15mm）の予備成形体とした。加圧成形は、油圧式圧縮成形機を用い、押圧力を調整して予備成形体の密度を7.4Mg／ｍ³ ，7.1Mg／ｍ³ ，6.9Mg／ｍ³ とした。

The obtained iron-based mixed powder was filled into a mold and subjected to pressure molding to obtain a tablet-shaped (diameter 30 mm, height 15 mm) preform. In the pressure molding, a hydraulic compression molding machine was used to adjust the pressing force so that the density of the preform was 7.4 Mg / m ³ , 7.1 Mg / m ³ , and 6.9 Mg / m ³ .

  This preform was pre-sintered to obtain a sintered body (that is, a molding material). The pre-sintering was performed at an atmospheric pressure equivalent to atmospheric pressure using a mixed gas of 90% by volume hydrogen and 10% by volume nitrogen as the atmospheric gas. The presintering temperature and holding time are as shown in Table 1. Test pieces were collected from the obtained molding materials and measured for C content, O content, and N content. The results are shown in Table 1. The C content was measured by a combustion infrared absorption method, the O content was measured by an inert gas melting infrared absorption method, and the N content was measured by an inert gas melting thermal conductivity method.

The amount of free graphite was determined by dividing the amount of C in the residue not dissolved in acid by the mass of the test piece when measuring the C content. The measured free graphite was over 0.02% by mass in Comparative Example 8, and the other was less than 0.02% by mass.
Next, compression molding (that is, cold forging by a backward extrusion method) with a cross-section reduction rate of 60% was performed using these molding materials to obtain a cup-shaped finished molded body. The cold forging load is as shown in Table 2. The appearance of the finished molded product thus obtained was visually observed for the presence or absence of cracks, and the density was measured by the Archimedes method. The results are as shown in Table 2.

  Further, the finished molded body was subjected to finish sintering and induction hardening to obtain an iron-based sintered member. The finish sintering was performed by using a mixed gas of 20% by volume hydrogen and 80% by volume nitrogen as an atmospheric gas and holding at 1140 ° C. for 1800 seconds. In the induction hardening, after heating to the temperature shown in Table 1, water quenching was performed. The appearance of the obtained iron-based sintered member was visually observed to investigate the presence or absence of cracks, and the hardness (HRC) was measured in accordance with JIS standard Z2245. The results are as shown in Table 2.

表２から明らかなように、発明例（No.２〜４，６，９，10）の成形用素材に圧縮成形を施した仕上げ成形体の密度は、いずれも7.77Mg／ｍ³ 以上であった。したがって、その仕上げ成形体に仕上げ焼結と高周波焼入れを施した鉄基焼結部材の密度も、同程度の密度が維持される。

As is clear from Table 2, the density of the finished molded body obtained by compression-molding the molding materials of the inventive examples (No. 2 to 4, 6, 9, 10) was 7.77 Mg / m ³ or more. It was. Therefore, the density of the iron-based sintered member obtained by subjecting the finished molded body to finish sintering and induction hardening is maintained at the same level.

  On the other hand, since the molding material of the comparative example (No. 1) has a low C content, the hardness of the iron-based sintered member was also low. In addition, since the molding material of the comparative example (No. 5) has a high C content, the deformation resistance is increased and compression molding cannot be performed to a predetermined shape. Since the molding material of the comparative example (No. 7) has a long pre-sintering holding time, C was excessively diffused from the graphite to the matrix, resulting in an increase in deformation resistance and compression molding to a predetermined shape. Since the molding material of Comparative Example (No. 8) has a large amount of free graphite, there is a possibility that defects due to the extended layer of graphite may occur, and the test was stopped. Since the density of the molding material of the comparative example (No. 11) was the lowest, the density of the finished molded body was also low.

It is a flowchart which shows the procedure at the time of manufacturing an iron-based sintered member using the raw material for shaping | molding manufactured by applying this invention.

Claims (2)

  A molding material obtained by pre-sintering iron-base mixed powder obtained by mixing iron powder with graphite powder or further with a lubricant, followed by pre-sintering, and 0.2 mass of Mn A molding material characterized in that it has a composition comprising less than 1%, 0.5 to 0.8% by mass of C, 0.02% by mass or less of free graphite, and the balance consisting of Fe and inevitable impurities.   In the method of manufacturing a molding material in which pre-sintering is performed after iron-based mixed powder obtained by mixing iron powder with graphite powder or further with a lubricant is pre-sintered. A method for producing a molding material, wherein the molding material is maintained in a temperature range of 1000 to 1200 ° C for 60 seconds or less in an active atmosphere.

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