JP4234619B2 - Method for manufacturing sintered body - Google Patents

Description

  The present invention relates to a sintered body manufacturing technique based on a powder metallurgy method, and more particularly to a sintered body manufacturing technique capable of preventing abnormal expansion during sintering.

  The sintered body is, for example, iron powder (water atomized iron powder) manufactured by a water atomizing method, metal powder (powder such as Cu, Ni, Mo, etc.) for improving the mechanical strength of the sintered body, C It is manufactured by mixing with a source (such as graphite) and, if necessary, a binder, a lubricant, etc., forming, and then sintering.

  When manufacturing a sintered body, it is important to appropriately control the dimensional change rate before and after sintering. For example, Patent Document 1 describes as follows. In other words, the dimensional change is complicated by the shrinkage due to the sintering reaction, the expansion due to the carburization reaction from the graphite powder mixed with the iron powder into the iron powder in the γ region, the expansion due to the diffusion of Cu, etc. Since it appears in the expansion direction, the sizing process is applied after sintering to match the specified dimensions, but if the rate of dimensional change at this time is too large, the load of the sizing process increases. This causes various inconveniences in the manufacture of sintered parts, such as being impossible, so it is necessary to accurately control the dimensional change rate so that it can cope with the shape of the sintered parts and the circumstances of the sizing machine. Dimensional change rate includes sintering conditions (temperature, time, atmosphere gas composition, etc.), auxiliary material conditions such as binder (chemical properties, brands, mixing amount, etc.), iron powder component conditions (O, Mn, P, Content of S, etc. However, the dimensional change rate may fluctuate greatly even if these known requirements are controlled to a certain level, and depending on the case, a large amount of iron powder raw material or Sometimes the product must be disposed of. "

  And this patent document 1 elucidates that the dimension after sintering becomes large (expands), so that content of Na in iron powder decreases, and the dimensional change of a sintered compact by controlling Na quantity. Has succeeded in controlling the rate. The reason why the size of the sintered body increases as the Na content decreases is as follows: “C atoms of graphite powder infiltrate into the high-temperature γ crystal during the sintering process. Then, the shrinkage effect due to sintering prevails over the expansion due to the penetration of C, and the dimensional change rate tends to decrease, and “sodium atoms have the effect of hindering the penetration of C atoms. If it is suppressed and sufficient penetration of C atoms is performed, the expansion effect due to the penetration of C is exhibited and the dimensional change rate increases.

For example, Patent Document 1, Fe-2.0% Cu- 0.9% graphite -0.75% sintering material consisting of zinc stearate, and sintered under a 75% H ₂ -25% AX gas N ₂ , The rate of dimensional change is increased by making sodium in the iron powder 30 ppm or less.
Japanese Patent No. 3201070

  When the present inventors have made various attempts to produce a sintered body, they have encountered a phenomenon in which the sintered body expands abnormally. Then, when Na content rate in iron powder was measured, Na was fully contained in iron powder. When the Na content is high, the penetration of C atoms is prevented as described above, and the shrinkage action is won and the dimensional change rate decreases, so it is assumed that the high Na content is not the cause of abnormal expansion. It was.

  The present invention has been made paying attention to the above-described circumstances, and its purpose is to prevent abnormal expansion of the sintered body when molding and sintering a mixed powder containing iron powder. To provide technology.

  As a result of intensive studies to solve the above problems, the present inventors have unexpectedly found that the sintered body expands as the Na content increases in the present invention, contrary to the teaching of Patent Document 1. The reason is that the Cu content is increased to 2.8% by mass or more compared to Patent Document 1 in which the Cu content in the mixed powder is 2.0% by mass. As a result, the present invention was completed.

  That is, the method for producing a sintered body according to the present invention comprises a mixed powder containing iron powder as a base material and 2.8 to 6.0% by mass of Cu powder (making the total mixed powder 100% by mass). It relates to a method for producing a sintered body which is sintered in an RX gas atmosphere after being molded, and the content of Na in the iron powder (mass ratio based on iron powder) is 30 ppm or less. Has a gist. According to the present invention, abnormal expansion during sintering can be prevented.

  After forming a mixed powder containing iron powder, which is a base material, and Cu powder of 2.8 to 6.0% by mass (the total mixed powder is 100% by mass), it is sintered and fired in an RX gas atmosphere. When producing a knot, it may swell abnormally. If iron powder having a Na content (mass ratio based on iron powder) of 30 ppm or less is used, abnormal expansion during sintering can be prevented.

In the present invention, after forming a mixed powder containing iron powder as a base material and Cu powder of 2.8% by mass or more (preferably 3.0% by mass or more), RX gas (for example, CO: 20 to 30 volumes) % or so, H _2: about 30 to 40 vol%, CO _2: 0.1 to 0.8 vol% about, CH _4: 0 to 0.3 vol% of the remainder is sintered in N ₂₎ atmosphere The purpose is that. The amount of the Cu powder is a value when the entire mixed powder is 100% by mass. The reason why the amount of Cu powder is larger than 2.0% by mass of Patent Document 1 is to further improve the mechanical strength of the sintered body. And when the compounding quantity of Cu powder is 2.8 mass% or more, contrary to the case of 2.0 mass%, as the Na content in the iron powder increases, the dimensional change rate of the sintered body tends to increase. And abnormal expansion is likely to occur. Therefore, in the present invention, the content of Na in the iron powder (mass ratio based on the iron powder) is suppressed to 30 ppm or less (preferably 20 ppm or less, more preferably 15 ppm or less). By suppressing the Na content in the iron powder, abnormal expansion during sintering can be prevented.

  The expansion / contraction behavior when the Cu content is 2.8% by mass or more will be described with reference to FIG. FIG. 1 shows the temperature of the furnace for sintering under RX gas after molding a mixed powder composed of Fe-3.0 mass% Cu-0.9 mass% graphite-0.75 mass% zinc stearate. The change of the dimensional change rate when changing is shown.

For example, the solid line (Experimental Example 1) in FIG. 1 shows the transition of the dimensional change rate when the Na content in the iron powder is as small as about 10 ppm. From the temperature rising start point (A in the figure) Until the decomposition start point (B in the figure), it expands linearly as the temperature rises, expands discontinuously at the RX gas decomposition start temperature (B in the figure), and then further reaches the austenitization start temperature (C in the figure) Expands linearly. Thereafter, shrinkage associated with austenitization and expansion associated with the progress of carburization caused by an increase in the amount of C solid solution due to austenitization occur simultaneously, and as a whole, shrinkage tends to austenite finish temperature (D in the figure) After completion of austenite formation (D in the figure), large expansion due to carburization is observed until the end of carburization (E _{1 in the} figure). Thereafter, the process proceeds to linear expansion due to temperature rise, and after discontinuous expansion (F in the figure) due to dissolution of Cu, sintering is performed, and then linear contraction occurs due to temperature decrease.

However, when the Na content in the iron powder increases to about 18 ppm [see the dotted line in FIG. 1 (Experimental Example 2)], the end of carburization (E _{2 in the} figure) is delayed, and the Na content increases to about 300 ppm. [Refer to the alternate long and short dash line in FIG. 1 (Experimental Example 3)] The dissolution of Cu (F in the figure) starts without completing the carburization.

  From the above, in the present invention, contrary to Patent Document 1, it is presumed that the larger the Na content, the more likely it is to expand, for the following reason. That is, as the Na content is higher, the infiltration of C during the sintering process is hindered, and the expansion caused by carburization is hindered as in Patent Document 1. However, if the penetration of C is hindered, the contact angle between Fe atoms and Cu atoms becomes wider, and the wettability between Fe and Cu is improved, so that expansion due to Cu diffusion tends to occur. In the case of Patent Document 1, since Cu was as small as 2.0% by mass, the influence of the expansion suppressing action by carburization prevention was stronger than the expansion action by Cu, whereas it was difficult to expand as a whole. In the case of the present invention, since Cu is increased to 2.8% by mass or more, the influence of the expansion action by Cu is stronger than the expansion suppression action by carburization prevention, and the whole is easily expanded. It is assumed that there is.

  The upper limit of the Cu content is 6.0% by mass or less (particularly 4.0% by mass or less).

  Further, the mixed powder is usually obtained by adding auxiliary materials (for example, a lubricant and a binder) to the iron powder as a base material in addition to the Cu powder, and is modified within a range that does not adversely affect the present invention. Quality raw materials (other metal powders etc.) may be added.

  By the way, as a cause which Na content rate increases in iron powder, the factor as described in patent document 1, ie, Na content rate in atomized water used when manufacturing iron powder, is mentioned. . Therefore, the Na content in the iron powder can be suppressed by suppressing the Na content in the atomized water.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

Experimental example 1
Cu, graphite, zinc stearate, and water atomized iron powder for powder metallurgy [“Atomel 300M” manufactured by Kobe Steel, Ltd .; Na content: 10 ppm (mass basis)], Cu: 3.0 mass%, graphite: 0.00. 9% by mass, zinc stearate: 0.75% by mass, and iron powder: the balance was sufficiently mixed. The mixed powder was filled in a mold and pressed to form a rectangular parallelepiped having a size of 8 mm × 8 mm × 33 mm and a density of 6.80 ± 0.02 g / cm ³ . The molded body was sintered under the following conditions, and the dimensional change rate of the molded body during sintering (the change rate of the external dimensions when the pre-sintering was used as a reference) was measured.

Temperature rising rate: 15 ° C./min (however, the temperature was kept at 660 ° C. for 10 minutes for dewaxing)
Sintering: 1120 ° C. × 10 minutes Cooling rate: 15 ° C./minute Atmospheric gas: RX gas (CO: 24 vol%, H ₂ : 32 vol%, CO ₂ : 0.3 vol%, CH ₄ : 0.1 vol %, The rest is N ₂ )

Experimental example 2
The water atomized iron powder for powder metallurgy was replaced with that having an Na content of 18 ppm in the same manner as in Experimental Example 1. The Na content is different from that of Experimental Example 1 because of the difference in quality between lots.

Experimental example 3
The same procedure as in Experimental Example 1 was conducted except that 0.1% by mass of Na ₂ SO ₄ was further added to the water atomized iron powder for powder metallurgy. The Na content is equivalent to 334 ppm (mass basis).

  The results are shown in FIG. When Cu is mixed in an amount of 3.0% by mass or more with respect to 100% by mass of iron powder (Experimental Examples 1 to 3), as is apparent from FIG. Expands abnormally. On the other hand, when there is little Na content rate (Experimental Examples 1-2), the abnormal expansion | swelling of a molded object can be prevented.

Experimental Example 4
Using water atomized iron powder "Atomel 300M" for powder metallurgy in various lots, blended in a proportion of Fe-3 mass% Cu-0.9 mass% graphite-0.75 mass% zinc stearate and then filled in the mold Thus, a ring-shaped body was formed (outer diameter 64 mm, inner diameter 24 mm, height 20 mm, density 6.80 g / cm ³ ). This molded body was sintered (sintering temperature 1120 ° C., sintering time 20 minutes, RX gas), and the dimensional change rate after sintering (change rate of the external dimensions with respect to before sintering) was measured. .

  The above sintering was performed on various ring-shaped bodies having different Na contents, and the difference (Sx−Sstd) from the dimensional change rate Sx of the other example was obtained with reference to the dimensional change rate Sstd of one example. .

  FIG. 2 shows the relationship between the Na content in the iron powder and the dimensional change rate difference (Sx−Sstd).

  As is clear from FIG. 2, as the Na content in the iron powder increases, the dimensional change rate of the molded body increases, and abnormal expansion easily occurs.

FIG. 1 is a graph showing the behavior of the dimensional change rate during sintering. FIG. 2 is a graph showing the relationship between the Na content in iron powder and the dimensional change rate difference (Sx−Sstd).

Claims (1)

A sintered body that is sintered in an RX gas atmosphere after forming a mixed powder containing iron powder as a base material and Cu powder of 3.0 to 6.0 mass% (with the entire mixed powder being 100 mass%) A manufacturing method of
The method for producing a sintered body capable of preventing abnormal expansion during sintering, wherein the iron powder has a Na content (mass ratio based on iron powder) of 30 ppm or less.

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