JPS6227122B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention uses a melting and atomizing method to minimize the C and N content in water-sprayed stainless steel powder, particularly chromium stainless steel powder, used in powder metallurgy, thereby suppressing the formation of retained austenite and improving the compressibility of the powder. The present invention relates to metallurgical stainless steel powder. In recent years, sintered stainless steel parts have been widely used not only in sintered machine parts but also in the chemical industry and pollution control equipment. Among them, chromium stainless steel powders such as SUS410L and 430L are relatively inexpensive compared to austenitic stainless steels containing Ni. In addition, since the green compact is easily densified during sintering, it has material advantages unique to powder metallurgy, such as improving the accuracy and toughness of the sintered compact. Stainless steel is generally used to provide corrosion resistance.
Cr is required to be at least 11%, but as water-sprayed stainless steel powder for press forming is used, the higher the Cr content, the more easily the surface oxidizes, and solid solution hardening makes the powder difficult to compress. [Kato, Soka: Denki Steel, VOL 48 (1977) p. 137] Therefore, for powder metallurgy, powders equivalent to relatively low Cr SUS410L, 430L, or 18Cr-2Mo with enhanced acid resistance are preferred. . In these stainless steel powders, C and other impurities are also reduced as much as possible because they undergo solid solution hardening. However, with normal water spray, the N pick-up due to Cr in the molten metal is 0.05 ~
The content is about 0.1%, and therefore, the high-temperature γ phase remains in the powder that is rapidly solidified at a rate of 10 ⁴ °C/sec, causing loss of compressibility. This state is C, N and Fe-
This is clear from the relationship between α/α+γ phase boundaries in Cr systems [Hasegawa: Stainless Steel Handbook, 1973, Nikkan Kogyo Shimbun]. From this relationship, even if the C content in the stainless steel powder is at most 0.04%, if N is included as much as 0.05%, the tip of the γ loop region will expand to a region with Cr of 20% or more. That is, when a molten metal with a lower Cr than this is sprayed with water, the powdered decomposed particles always pass through the above-mentioned γ phase region at a temperature of 1100° C. or higher, where the atomic diffusion rate is still high during cooling. Once generated in the α phase, the diffusion of the γ phase is suppressed at low temperatures, and it remains in the powder as it is. The present inventors have noticed that the γ phase remaining due to this phenomenon peculiar to water spraying (quenching) impairs the plastic deformability of the powder. On the other hand, if you prevent the γ phase from being included in the powder by melting and spraying it in a non-contaminating manner that suppresses the amount of N in the molten metal as much as possible (for example, in an argon gas atmosphere), it is possible to prevent the γ phase from being included in the powder. We thought that the compressibility could be improved without decomposing the γ phase and bringing it into an equilibrium state. Therefore, when various press property tests were conducted by varying the amounts of Cr and N, it was found that stainless steel powder consisting of the following constituent elements had a large effect in reducing N. That is, the present invention provides: (1) C: 0.04% or less, Si: 0.5 to 20%, Mn: 0.6 in stainless steel powder obtained by water spraying molten metal.
% or less, Cr: 11.0 to 21.0%, N: 0.03% or less, with the remainder essentially consisting of Fe. (2) In stainless steel powder obtained by water spraying molten metal, C: 0.04% or less, Si: 0.5-20%, Mn: 0.6
% or less, Cr: 11.0 to 21.0%, N: 0.03% or less, and Mo, which is a ferrite forming element,
A stainless steel powder for powder metallurgy, characterized in that one or more of Ti, V, Zr, Nb and Sn is added in an amount of 3.0% or less. Among these, Mo, Ti, V, Zr, Nb, and Sn in (2) are elements that improve the strength and corrosion resistance of the sintered body, but they are also ferrite-forming elements that suppress the formation of the γ phase in the future. is also valid. The reasons for limiting the composition of the stainless steel powder for powder metallurgy of the present invention will be described below. C: 0.04% or less If it exceeds 0.04%, the powder will be solid solution hardened, and as an austenite-forming element, a γ phase will be formed, making it difficult to compress, and the corrosion resistance after sintering will also deteriorate, so it is preferably 0.04% or less. Si: 0.5-20% It is necessary to add at least 0.50% or more to prevent surface oxidation and spheroidization of the powder caused by water spraying. However, if it is too large, the powder will harden as a solid solution and become difficult to compress, so it should be kept at 20% or less. Mn: 0.6% or less If the amount is too large, the surface oxidation of the powder will increase, and it will become spheroidized, making it difficult to press-form, so it should be 0.6% or less. Cr: 11.0-21.0% It is necessary to add at least 11% or more to ensure the inherent corrosion resistance of stainless steel. However, if the amount is too high, the powder will be more likely to be work hardened during press molding, and the surface oxidation of the powder will increase.
% or less. Mo, Ti, V, Zr, Nb, Sn: 3.0% or less of one or more of these elements Adding small amounts of these elements not only improves the strength and corrosion resistance of the sintered body, but also has the effect of suppressing the γ phase as a ferrite-forming element. big. However, if it is too large, the powder will harden as a solid solution, making it even more difficult to compress, so it is preferably 3.0% or less. N: 0.03% or less Since N is an austenite-forming element like C, it is reduced to 0.03% or less to minimize the residual γ phase in the chromium stainless steel powder and eliminate any adverse effects on compressibility. Hereinafter, the powder of the present invention will be explained in detail with reference to Examples. Example 1 Seven types of stainless steel powders equivalent to SUS410L and 430L were obtained by dissolving pure iron ferrochrome and spraying with water. In this case, N pick-up is avoided as much as possible by using low N chromium as a raw material or melting it in an Ar atmosphere to avoid contact with the atmosphere, and for comparison, chromium nitride is added to the molten metal melted in the atmosphere. A powder in which N was forcibly dissolved in solid solution was also produced. These powders were dehydrated, dried, and then sieved to -100 mesh to obtain test powders having seven types of component compositions as shown in Table 1. Note that Table 1 also shows the content of Ni contained as an inevitable impurity in each powder. Lubricating each powder

[Table] A press test was conducted in accordance with JSPM Standard 1-64 with 1.0% of the agent mixed in. The density of the obtained green compact was measured and divided into powders corresponding to SUS410L and 430L in relation to the pressing force, and these are shown in the figure. However, the broken lines in the figure (sample materials No. 5' and No. 7') are those obtained by annealing the powders of specimen materials No. 5 and 7 in a reducing atmosphere at 800°C for comparison. For both powders, the smaller the amount of N, the higher the green density becomes, but at 0.03% or less, the difference in compressibility becomes on the order of being virtually negligible. However, if the N content exceeds 0.03%, it becomes difficult to compress rapidly, and it is difficult to put it into practical use unless annealing treatment is performed as shown by the broken line to reduce the amount of γ phase. This type of powder containing a large amount of Cr has a large reducing ability and must be heated in an atmosphere with a low dew point, which is industrially disadvantageous. Example 2 In the same manner as Example 1, in addition to the powder equivalent to SUS430L, Cr20.25% was added, and
A powder was obtained by adding elements such as Mo and Ti to SUS430L. These powders were dehydrated and dried and then mesh-sieved to obtain test powders having 14 types of component compositions as shown in Table 2. Each powder was mixed with 1.0% lubricant and press-molded at a pressure of 7t/cm ³ in accordance with JSPM Standard 1-64. The density of the obtained green compact was measured and added to the right column of Table 2. Note that Table 2 also shows the content of Ni contained as an unavoidable impurity in each powder. When the amount of Cr exceeds 20%, the difference in green density due to the amount of N becomes small, and only a slight difference due to solid solution hardening of N can be seen, and the advantage of lowering N by non-contaminating dissolution is not substantially seen. Furthermore, even when elements such as Mo and Ti are added singly or in combination to SUS430L powder, differences in green density appear depending on the amount of N, as seen in test materials No. 12 and 13. It has been confirmed that the effect on compressibility is relatively small.

【table】

【table】 [Brief explanation of the drawing]

The figure is a diagram showing the relationship between compaction pressure and green powder density.

Claims (1)

[Claims] 1. In stainless steel powder obtained by water spraying molten metal, C: 0.04% or less, Si: 0.5 to 2.0%, Mn: 0.6
% or less, Cr: 11.0 to 21.0%, N: 0.03% or less, with the remainder essentially consisting of Fe. 2. C: 0.04% or less, Si: 0.5-2.0%, Mn: in stainless steel powder obtained by water spraying molten metal.
0.6% or less, Cr: 11.0 to 21.0%, N: 0.03% or less, and further contains ferrite-forming elements Mo, Ti,
One or more of V, Zr, Nb and Sn
A stainless steel powder for powder metallurgy, characterized in that it is added in an amount of 3.0% or less.