JPS61201706A - Seamless sintered steel pipe and its production - Google Patents

Abstract

PURPOSE:To obtain a seamless sintered steel pipe having excellent stress corrosion cracking resistance, etc. by mixing desired steel kinds of stainless steel powders, packing the powder mixture into a metallic vessel and subjecting the mixture to pressure molding and sintering then to extrusion working, etc. CONSTITUTION:>=1 kinds of powders of ferritic stainless steel powder, austenitic stainless steel, two-phase stainless steel or three-phase stainless steel mixed with martensite therewith are mixed and the mixture composed thereof is packed into the metallic vessel. The inside of the vessel is degassed and gaseous nitrogen is preferably introduced therein. The vessel packed with such mixture is subjected to pressure molding by a hot hydrostatic method, etc. by which the stainless steel powders in the vessel are joined and sintered. The sintered body is subjected to hot extrusion, hot drawing, hot forging, hot rolling, cold extrusion, etc. to form the seamless sintered steel pipe. The SCC resistance is improved and the seamless stainless steel pipe having excellent toughness is obtd. by the above-mentioned method.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention relates to a seamless stainless steel pipe and a method for producing the same, particularly a seamless sintered steel pipe produced from stainless steel powder by a powder metallurgy method, and a method for producing the same. Regarding.

(Conventional technology) Stainless steel powder manufactured by water atomization, pulverization, intergranular corrosion, etc. has traditionally been used to make use of its excellent corrosion resistance and heat resistance to produce mechanical parts, watch parts, filter elements, etc. It was used for ornaments, etc.

However, the development of gas atomization in recent years has made it possible to supply high-quality stainless steel powder with a low oxygen content in large quantities at a relatively low cost, which has encouraged the development of various new applications. Month 17-2
Amer held in Detroit, Michigan, USA on the 0th.
1984 ASM International C sponsored by ican 5ociety For Metals
onference on New Development
As seen in the 8410-013 report by C1eaa Tornberg in the ENTA, carbon steel capsules were filled with stainless steel powder produced by gas atomization, and the capsules were densified by cold isostatic pressure and made into billets by hot extrusion. The 0-grade steel types that are made into seamless pipes include austenitic stainless steels such as TP304.304L, duplex stainless steels such as TP329, ferritic stainless steels such as ↑P446,
^l It is extremely widespread with Ni-based alloys such as 1o7625.

Sintering or compacting using stainless steel powder as raw material,
The characteristics of the case where it is bonded to increase density and have the same functions and properties as conventional agglomerates are (11) The compositional fluctuations caused by solidification segregation during agglomeration seen in melted materials are (2) SSP segregation in steel is small and nonmetallic inclusions are finely dispersed; (3) SSP has poor workability at high temperatures and is difficult to manufacture. (4) Composite materials containing steel powder can be produced, etc.Also, two or more different types of steel powder can be mixed and sintered. It is also possible.Shodoshima et al.
5US430L in 3.191111.31160
, created a mixed powder sintered body using steel powder equivalent to SO5304L, and reported on its high-temperature deformation behavior.

(Problems to be Solved by the Invention) Today, the environment in which stainless steel is used is becoming more severe year by year, and on the other hand, users are demanding cheaper products in order to reduce costs. Pipe manufacturers are currently making great efforts to develop inexpensive seamless steel pipes with excellent corrosion resistance. However, as long as conventional means are used, the improvement effect is only small, and it is considered that the above-mentioned demands cannot be met unless measures based on completely different ideas are adopted.

Here, conventional duplex stainless steel will be described in order to make the purpose and characteristics of the present invention clearer.

As is well known, there are three types of stainless steel: martensitic, ferritic, austenitic, and two-phase stainless steel, and each type is used depending on its characteristics.

Ferritic stainless steel is inexpensive and has excellent stress corrosion cracking resistance (hereinafter referred to as 'SCC resistance'), but it has the disadvantage of poor toughness and weldability. be.

Although austenitic stainless steel has excellent toughness and corrosion resistance, it is generally expensive because it contains a large amount of Ni, and also has the disadvantage of poor SCC resistance. High N
Although i-ization is effective in improving SCC resistance, there is a limit to the improvement, and furthermore, the addition of Ni increases the cost, which significantly impairs the versatility of the material.

On the other hand, duplex stainless steel was proposed to eliminate these drawbacks, and combines the advantages of both ferritic and austenitic stainless steels, with the excellent toughness and durability of austenitic stainless steel pipes. S
It has CC properties.

Regarding the SCC resistance of duplex stainless steel, please refer to Eda
Ieanu is J, Iron 5teel 1nst,
, 173. , 140 (1953) in 18cr-8N
Since the publication of research focusing on the amount of δ ferrite in i-Till, there have been many studies, and the effects of component elements, heat treatment conditions, and the amount of ferrite have been reported.

It is well known that duplex stainless steel has a high SCC critical stress value. Figures 1 and 2 were created by one of the inventors in the "Corrosion Prevention Technology J vol.
, 30. Nl 4.1981, pp, 218-226
This is what was reported. Of these, Figure 1 uses test materials of 25Cr stainless steel with varying Ni content. This is a summary of the results of evaluating SCC resistance in two melts. The vertical axis is the ratio of the SCC critical stress value to the proof stress (σth/σ・,
The higher the value, the better the SCC resistance. In ferritic stainless steel that does not contain Ni, cracks do not occur, but in ferritic stainless steel that contains a small amount of Ni, σth/'a, :L rapidly decreases. σth
/σ takes a minimum value at 2% Ni. σ at 6-8% Ni
The increase in th/17.2 is due to the fact that the structure is a two-phase structure of ferrite and austenite. However, the SCC resistance of duplex stainless steel is still inferior to that of ferritic stainless steel that does not contain N1.

This is considered to be because the ferrite phase of the duplex stainless steel contains a large amount of Ni according to the element distribution between the ferrite phase and the austenite phase.

Figure 2 shows the 25Cr-6Ni duplex stainless steel (0
28Cr-4 having a component corresponding to the ferrite phase
Three steels were melted separately: Ni ferritic stainless steel (*) and 21Cr 9Ni austenitic stainless steel with an austenite-equivalent component (Δ), and their SCC resistance was compared (test conditions). is the same as in Figure 1), and 28Cr-4Ni (α) steel, which is equivalent to the ferrite phase containing 4% Ni, has SCC resistance because it contains 4% Ni even though it is called a ferrite phase.
I can see that it's inferior. This is considered to be the reason why SCC propagates through the ferrite phase in conventional duplex stainless steel, bypasses the austenite phase, and hits the austenite phase again to be stopped.

Figure 3 schematically explains the above-mentioned SCC propagation mechanism in a conventional molten duplex stainless steel.
In the figure, the SCC propagation path is shown by the black line, and the SCC that has propagated by bypassing the T phase propagates while penetrating the Nj-containing α phase and partially crossing the r phase.

In other words, the SCC resistance of duplex stainless steel strongly depends on the SCC resistance of the ferrite phase, but the ferrite phase of ordinary duplex stainless steels is inevitably affected by the element distribution between the ferrite phase and the austenite phase during solidification. Because it contains approximately 4% Ni, its SCC resistance is inferior to that of ferritic stainless steel that does not contain Ni, and therefore the SCC resistance of duplex stainless steel is inferior to that of ferritic stainless steel that does not contain Ni. -ing In other words, in order to make the metal structure two-phase, conventional dual-phase stainless steel contains 4 to 8 wt% due to the relationship with the Ni balance.
The ferrite phase contains 3 to 6 wt% of Ni, depending on the element distribution between the ferrite phase and the austenite phase.
As a result of containing a certain amount of Ni, the SCC resistance was inevitably inferior to that of ferritic stainless steel that does not contain Ni. The same applies when sintering is performed using duplex stainless steel powder. Therefore, as long as duplex seamless stainless steel pipes are used because they are cheap, even if they are made from duplex stainless steel powder, they are better than duplex stainless steel pipes made by ordinary melting methods. No improvement in SCC resistance is made.

By the way, even in the past, Japanese Patent Publication No. 55-10643 discloses a method of manufacturing steel pipes using stainless steel powder, but this method simply uses powder produced by powder metallurgy to produce steel pipes using conventional melting methods. The purpose of this invention is to manufacture stainless steel pipes equivalent to lumber, and not only does the present invention have an objective of improving SCC resistance, but there is no disclosure of any means for significantly improving SCC resistance.

(Object of the Invention) The first object of the present invention is to provide a stainless steel pipe with significantly improved SCC resistance compared to conventional duplex stainless steel, and a method for manufacturing the same.

Another object of the present invention is to provide a stainless steel pipe having the excellent toughness of an austenitic stainless steel pipe and the excellent SCC resistance of a ferritic stainless steel pipe, and a method for manufacturing the same.

Another object of the present invention is to provide a method of manufacturing a stainless steel pipe made from stainless steel powder, which has dramatically improved SCC resistance and excellent toughness, using a powder metallurgy method.

(Summary of the Invention) As mentioned above, in the past, in duplex stainless steel, it was thought that it was unavoidable for the ferrite phase to contain several percent of Ni. Judging from the results shown in Figures 1 and 2, if the lit content in the ferrite phase of duplex stainless steel is lowered, its SC resistance will increase.
We have been pursuing means to arbitrarily control the amount of ferrite in the ferrite phase of duplex stainless steel, believing that it is possible to dramatically improve carbon properties. As a result, the above-mentioned problems were solved at once by melting the steels with compositions that make up each phase of duplex stainless steel separately, solidifying them as powder, mixing them in a predetermined ratio, and sintering them. After confirming that it can be done, we have arrived at the present invention.

Furthermore, according to the findings of the present inventors, if the powder is kept at a high temperature of over 1100°C during the powder compacting, joining, and densification processes after mixing, even if ferrite is used for the purpose of improving SCC resistance. The amount of Ni in the steel powder constituting the phase was set to 0.5
% or less, when held at such high temperatures, the Ni content in the ferrite phase increases due to NlvA loss to the ferrite phase side compared to austenitic stainless steel powder, two-phase stainless steel powder, or three-phase stainless steel powder. As a result, SCC resistance deteriorates. Therefore, the maximum heating temperature in the manufacturing process is 11
The temperature is preferably 00° C. or lower, and even if the temperature is heated to 1100° C. or higher, the shorter the heating time, the more preferable it is.

Further, it is more desirable that the ferritic stainless steel powder used in the steel of the present invention contains Mo in view of corrosion resistance problems, and preferably 0.5% or more.

Thus, the present invention mixes ferritic stainless steel pipe powder with one or more of austenitic stainless steel powder, two-phase stainless steel powder consisting of austenite and ferrite, and three-phase stainless steel powder consisting of austenite, ferrite, and martensite. A seamless sintered steel pipe with excellent stress corrosion cracking resistance and a steel composition containing 100% steel powder.
Preferably, the ferritic stainless steel powder has a Mo content of 0.5% or more, and the Ni content of the ferritic stainless steel powder is as low as possible (preferably limited to 0.5% or less).

The present invention also provides at least one of ferritic stainless steel powder, austenitic stainless steel powder, duplex stainless steel powder consisting of austenite and ferrite, and duplex stainless steel powder consisting of austenite, ferrite, and martensite. This is a method for producing a seamless sintered steel pipe with excellent stress corrosion cracking resistance, which includes the step of filling a metal container with a green body mixed with the following.

After filling each powder into the metal container, nitrogen gas may be filled after degassing if necessary.Preferably, when hot isostatic processing is performed, the maximum heating temperature is set to 1100°C or less during processing. In order to suppress the Ni diffusion as much as possible, the ferritic stainless steel that becomes the matrix and the shite are 511.
5444.5tlS KM-27,5tlS 44”l
In addition to standardized steel types such as JL, those with non-standard components can also be used.On the other hand, austenitic stainless steels that form the dispersed phase include SO5304, SO3304L, 5
IIS 316.5IIS 316L, 311331
In addition to standardized steel types such as 7.5113317L, steels with non-standard compositions can also be used. As the duplex stainless steel, in addition to standard steel such as SUS 329J1, it is also possible to use materials with non-standard components. As already mentioned, in order to further improve the corrosion resistance of the resulting sintered steel pipe, it is preferable to use at least the above-mentioned ferritic stainless steel containing 0.5% or more of Mo.

SCC resistance seen in the present invention as shown in FIGS. 4 and 5
The mechanism of sexual improvement is explained as follows.

That is, according to the present invention, since a ferrite phase having excellent SCC resistance is present in the matrix, for example, in a two-phase system in which the ferrite phase shown in FIG. 4 is used as a matrix and the austenite phase is dispersed in this, Even if SCC occurs on the austenite phase side, the ferrite phase side has little or no susceptibility to SCC, so the propagation of SCC stops on the ferrite phase side. This also applies when a two-phase stainless steel powder or a three-phase stainless steel powder such as ferrite + austenite + martensite is used as the dispersed phase. For example, in the example shown in FIG. 5, since the dispersed phase itself has a two-phase structure, crack propagation is prevented even in the dispersed phase in the same way as in conventional two-phase stainless steel. Since the crack propagation stopping effect as described above also exists between the dispersed phase and the matrix phase, the SCC resistance is further improved.

In other words, in the present invention, by making the ferrite phase with excellent SCC resistance derived from ferritic stainless steel powder exist as a matrix, in other words, the austenite phase dispersed in island shape, the two of ferrite and austenite By making the matrix exist so as to surround the phases, that is, by making the matrix exist as a continuous phase, even if SCC occurs, the presence of this ferrite phase, which has excellent SCC resistance, will prevent its propagation and make it resistant. The aim is to improve SCC characteristics. Although the ferrite phase that forms the matrix mentioned above is melted as ferritic stainless steel from the beginning and solidified into a powder, the Ni content can be freely selected (for example, NiO, 5% or less). 1 by reducing Ni
This makes it possible to utilize stainless steel powder consisting of a ferrite phase with significantly improved fscc properties. Therefore, according to the present invention, the Ni content of the matrix phase can be easily and freely selected by controlling the Ni content of the stainless steel powder as the raw material powder.

(Aspects of the Invention) Stainless steel pipes manufactured from the stainless steel powder according to the present invention include stainless steel pipes that have been heat-treated as necessary.

In addition, in addition to the additive elements and impurities of shark, MsJ, which are normally contained in stainless steel, S, Pbs, Ses, Te5
A machinability improving component such as Ca may be included.

Note that the manufacturing history of each stainless steel powder, as well as the form and particle size distribution of the stainless steel powder, are not particularly limited as long as they do not go against the spirit of the present invention.

As described above, the present invention uses one or more of ferritic stainless steel powder, austenitic stainless steel powder, two-phase stainless steel powder, or three-phase stainless steel powder, which have excellent SCC resistance. By mixing and sintering an appropriate amount according to the purpose, the SCC resistance is dramatically improved. Therefore, in the present invention, any combination of austenitic stainless steel powder and two-phase or three-phase stainless steel powder may be used as long as the combination contains at least ferritic stainless steel powder, and the combination may be any combination containing at least ferritic stainless steel powder. The most suitable composition may be selected depending on the composition. Preferably, the ferrite phase from which the ferritic stainless steel powder is derived is 20 to 80% by weight, more preferably 3% by weight.
It is preferable that the ferrite phase becomes a continuous phase at a blending ratio of 0 to 70% by weight.

Therefore, according to one aspect of the present invention, the ferrite phase derived from the ferritic stainless steel powder is 20 to 80%.
It is a seamless sintered steel pipe manufactured from stainless steel powder having a metallographic structure of

As one specific embodiment of the present invention, the metal structure is
20-80% ferrite phase derived from ferritic stainless steel powder, the remainder has a structure selected from single austenite phase, ferrite or two phases of martensite and austenite, or three phases of ferrite, martensite and austenite. This is a seamless sintered mesh pipe made from stainless steel powder that has dramatically improved SCC resistance.

As is clear from the above, the steel according to the present invention is equivalent to conventional inexpensive duplex stainless steel because the two-phase component ratio can be arbitrarily selected, unlike conventional duplex stainless steel made by melting. It is possible to select and prepare an appropriate component system depending on the purpose, from W1 type, which has corrosion resistance superior to conventional duplex stainless steel, to S11 type, which has better corrosion resistance than conventional duplex stainless steel. You get it. In addition, recent improvements in alloy steel powder manufacturing technology and 11.1.
Due to new technology in the field of powder metallurgy such as P, the mechanical properties of the alloys have become comparable to those of fused materials. The mechanical properties of stainless steel pipes are also not significantly different from those of melted steel pipes.

However, the seamless sintered steel pipe produced from the stainless steel powder of the present invention is compacted into the shape of the final product, sintered,
Not only can it be used as it is or in a heat-treated state after sintering, but it can also be processed into any shape by hot extrusion, hot drawing, cold extrusion, cold drawing, etc. This is particularly important as a practical effect.

It is particularly preferable to use the thus obtained seamless sintered steel pipe as an inexpensive material with dramatically improved SCC resistance.

Next, the present invention will be further explained by examples.

Implementation ■ Six kinds of stainless steel powders (-300 mesh) having the compositions shown in Table 1 were produced by a conventional atomization method. A steel powder, B
l [Powder and C8I powder are ferritic stainless steel, D
Steel powder and Em powder are austenitic stainless steel, F
iM powder corresponds to duplex stainless steel.

These six types of steel powder were mixed in the proportions shown in Table 2,
After filling a carbon steel capsule, the inside was evacuated and sealed while heating. The conditions for vacuuming are 1xx
It was carried out at 500°C x lhr at o-' mmHg. Also,
Although it is possible to maintain the temperature at room temperature, heating is more effective than removing internal moisture. However, heating at 500° C. or lower and 200° C. or higher is sufficient.

Note that nitrogen may be sealed in the capsule after vacuum degassing. These manufacturing steps are summarized in Table 3.

This was then subjected to a cold isostatic pressure method.
A carbon steel capsule was press-molded using a press machine.

This process is carried out using the hot isostatic pressure method (lot l5ostati).
It is also possible to complete the sintering at the same time as the pressure forming by pressing. However, H,1,P
, when performing, under conditions that suppress Ni diffusion from austenitic stainless steel powder or duplex stainless steel powder to ferritic stainless steel powder as much as possible, and under conditions that allow sufficient densification and sintering to proceed. The necessary 0-appropriate H1r, p, and conditions need to be considered depending on the specific composition and combination of the steel powder used, but in general, when kept at a high temperature exceeding 1100°C, Ni diffusion becomes noticeable and the resistance Significant deterioration in SCC properties.

The relationship between heating temperature and SCC resistance was determined by boiling 42% Mg (:
Evaluation was made by measuring the rupture time in an aqueous solution of No. 12. The test materials (test steel 12 and comparison steel 21.22) were heated to an appropriate temperature after completion of hot rolling, held for 40 minutes, and then cooled with water.

Figure 6 summarizes those results and shows the changes in SCC resistance due to differences in heating conditions for each specimen of steel number 12, 21, and 22 in Table 2. Deterioration is observed in some cases, and significant deterioration occurs when heated above 1100°C. In the figure, blacked-out areas indicate fractures, and depending on the alloy system, intermetallic defects such as σ phase may occur. It is necessary to consider the heating temperature taking into consideration the formation of compounds. Here, as long as the above conditions are satisfied, it goes without saying that a lower temperature is more desirable from the viewpoint of workability.

The carbon steel capsules pressure-formed by CIP were heated and held in the atmosphere at each temperature shown in Table 2 for 1 hour each, and then hot extruded to form seamless steel pipes with an outer diameter of 60su+ and an inner diameter of 3fJsu+. The obtained seamless steel pipe is heat treated as necessary, the carbon steel capsule is removed by pickling, and further cold drawn to have an outer diameter of 22+u+ and an inner diameter of 15-
Finish annealing and pickling were performed on seamless sintered steel pipes.

Test pieces were cut out from the seamless sintered steel pipe produced from the stainless steel powder thus obtained, and subjected to an SCC resistance test, a Charpy impact test, and a tensile test at room temperature.

In the SCC resistance test, the parallel part is 31111% in diameter and 2% in length.
A round bar tensile test piece of 0III was prepared, and a 42% magnesium chloride aqueous solution was boiled, and a constant load was applied and the time taken to break was measured by immersing it in the boiling solution. -1 A "double U-bend test piece" was prepared by stacking two strip-shaped test pieces with a length of 75+ am and bending them into a U shape, and the test piece was evaluated based on whether or not SCC occurred in high-temperature water containing 1000 pp+mCl-. Second result
They are summarized in the table.

As is clear from the results shown in Table 2, the seamless steel pipe manufactured from the stainless steel powder according to the present invention is produced in the same process as the conventional melt material (steel number 21.22) and the prototype pipe whose composition is shown in Table 4. It exhibits significantly superior SCC resistance compared to seamless steel pipes (steel number: 11.20) manufactured using only austenitic stainless steel powder or only duplex stainless steel powder (steel number: 11.20).

The rupture time in a 42% magnesium chloride aqueous solution is extremely long at all applied stresses, especially when the amount of ferrite phase is 70% or more, the applied stress is 40 kg.
f/+n! However, it did not break even after 1,000 hours, showing properties equivalent to ferritic stainless sintered steel (steel number 1). Furthermore, although conventional steels with the same composition generate SCC in high-temperature water containing IQOQPpmCl-, the invented steel exhibits extremely good SCC resistance without causing any through cracking due to SCC.

Figure 7 shows samples of steel numbers L to 11 in Table 2, and 35 kgf/n &
Figure 0 shows a graph summarizing the time to rupture when immersed under a constant load and the absorbed energy value in the Charpy impact test at 0°C.
Each number indicates the steel number in Table 2. The Charpy impact test was conducted using a JIS No. d type test piece having a thickness of 5 m+w.

From Figure 7, regarding SCC resistance, the amount of ferrite is 2.
It can be seen that the amount of ferrite is preferably 0% or more, and on the other hand, from the viewpoint of toughness, it is desirable that the amount of ferrite is 80% or less. However, as shown in Table 2, in the SCC resistance test, when the load stress is 40 kgf/J, the amount of ferrite is 2.
At 0%, the rupture time is significantly shortened to 250 hours, so
Preferably, the amount of ferrite is 30% or more.

As shown in Table 2, the seamless sintered steel pipe according to the present invention did not crack in the flattening test or under the completely adhesion condition as hot extruded, and passed the intergranular corrosion resistance test (JIS G 0
575) also showed good performance.

As described above, the sintered steel pipe according to the present invention has significantly superior SCC resistance compared to duplex stainless steel pipes manufactured from conventional melted materials and duplex stainless steel powder, and has other corrosion resistance and mechanical properties. Both are equivalent to conventional steel.

As described above, according to the present invention, a stainless steel pipe with excellent stress corrosion cracking resistance that cannot be obtained with conventional melted materials can be obtained.

The seamless sintered steel pipe according to the present invention can be used even in environments where conventional duplex stainless steel is likely to generate SCC, and its industrial applicability is extremely large.

Table 3 Table 4

[Brief explanation of drawings]

Fig. 1 is a graph showing the change in the amount of ferrite phase and the change in SCC critical stress value when the Ni content is changed in 2SCr stainless steel;
0 mark) and its ferrite phase equivalent component.
Graphs showing the SCC resistance of 4Ni ferritic stainless steel (marked with *) and 21Cr-9Ni austenitic stainless steel with a component equivalent to the austenite phase (Δ); Figures 3 to 5 show the SCC resistance of sintered stainless steel. S seen in
A schematic diagram showing the pattern of CC crack propagation; FIG. 6 is a graph showing the relationship between SCC resistance and heat treatment temperature in Examples;
and Figure 7 shows the mixing ratio of ferritic stainless steel powder, Charpy impact absorption energy and SC resistance in Examples.
It is a graph showing the relationship with C nature. Applicant Sumitomo Metal Industries Co., Ltd. Agent Patent Attorney Akira Hirose - Maku 1 Design 7N21 Fellatio 4 To Series Stainless Steel Bond Ribun AL Kai 1 Shizuka 1 Na (Content %
) Hou 2 Zuha vr custom c<5) Jo 3 Kobo 5 Ko #roko HILtr time (meeting)

Claims (11)

[Claims] (1) A mixture of ferritic stainless steel powder and one or more of austenitic stainless steel powder, two-phase stainless steel powder consisting of austenite and ferrite, and three-phase stainless steel powder consisting of austenite, ferrite, and martensite. Seamless sintered steel pipe with excellent stress corrosion cracking resistance, characterized by being manufactured from an unsintered body. (2) At least the ferritic stainless steel powder is M
The seamless sintered steel pipe according to claim 1, which has an o-containing steel composition. (3) The seamless sintered steel pipe according to claim 2, wherein the Mo content is 0.5% or more. (4) A mixture of ferritic stainless steel powder and one or more of austenitic stainless steel powder, two-phase stainless steel powder consisting of austenite and ferrite, and three-phase stainless steel powder consisting of austenite, ferrite, and martensite. A method for producing a seamless sintered steel pipe with excellent stress corrosion cracking resistance, which includes the step of filling a green body into a metal container. (5) Unfired combination of ferritic stainless steel powder and one or more of austenitic stainless steel powder, two-phase stainless steel powder consisting of austenite and ferrite, and three-phase stainless steel powder consisting of austenite, ferrite, and martensite. 5. The method of manufacturing a seamless sintered steel pipe with excellent stress corrosion cracking resistance according to claim 4, which includes the step of degassing the inside of the metal container filled with the cementing mixture. (6) Unfired combination of ferritic stainless steel powder and one or more of austenitic stainless steel powder, two-phase stainless steel powder consisting of austenite and ferrite, and three-phase stainless steel powder consisting of austenite, ferrite, and martensite. A seamless method with excellent stress corrosion cracking resistance according to claim 4, which includes the step of introducing a gas mainly consisting of nitrogen into the metal container after degassing the inside of the metal container filled with the coagulation mixture. Method for manufacturing sintered steel pipes. (7) Unfired combination of ferritic stainless steel powder and one or more of austenitic stainless steel powder, two-phase stainless steel powder consisting of austenite and ferrite, and three-phase stainless steel powder consisting of austenite, ferrite, and martensite. The stress corrosion resistance according to any one of claims 4 to 6, which includes the step of pressurizing a metal container filled with and sealed with a coagulation mixture by cold isostatic pressure to increase the packing density in the container. A method for manufacturing seamless sintered steel pipes with excellent crackability. (8) Unfired combination of ferritic stainless steel powder and one or more of austenitic stainless steel powder, two-phase stainless steel powder consisting of austenite and ferrite, and three-phase stainless steel powder consisting of austenite, ferrite, and martensite. Any one of claims 4 to 6, which includes the steps of press-forming a metal container filled and sealed with the solidifying mixture by hot isostatic pressing, and bonding and sintering the stainless steel powder in the container. A method for producing a seamless sintered steel pipe with excellent stress corrosion cracking resistance as described in the above. (9) The maximum heating temperature when press forming by hot isostatic pressing, joining and sintering stainless steel powder in a metal container is 110℃.
A method for manufacturing a seamless sintered steel pipe with excellent stress corrosion cracking resistance according to claim 8, wherein the temperature is 0° C. or lower. (10) Hot extrusion, hot drawing, hot forging, hot rolling,
Production of a seamless sintered steel pipe with excellent stress corrosion cracking resistance according to any one of claims 4 to 9, which includes one or more steps of cold extrusion and cold drawing. Method. (11) The method for producing a seamless sintered steel pipe with excellent stress corrosion cracking resistance according to claim 10, wherein the maximum heating temperature is 1100°C or less.