JPH0892700A - γ ', γ "phase precipitation strengthened high Ni steel - Google Patents

Abstract

(57) [Summary] [Purpose] Swelling resistance, high temperature creep strength, ductility,
Provide a high Ni austenitic steel balanced in phase stability under irradiation. [Structure] Si: 0.5% or less by weight%, Mn: 1.0
% Or less, Cr: 13% to 18%, Ni: 30 to 50%,
Mo + W = 2.0 to 6.0, Al = 0.05 to 1.0
%, Ti = 0.05 to 0.6%, Nb = 2 to 5%, and the balance being Fe and inevitable impurities, γ ′, γ ″ phase precipitation strengthening high Ni steel.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high Ni steel, in particular, a core component (for example, a fuel assembly including a fuel cladding tube or a trumpet tube) that is used for a long time in a core environment of a nuclear reactor (especially fast breeder reactor) The present invention relates to a high Ni steel such as a rod, a reflector or the like) or a device structure (for example, a device container member, a cooling system piping member, etc.) that requires excellent neutron resistance.

[0002]

2. Description of the Related Art Conventionally, SUS has been used as a core constituent member.
In addition to 316, an improved steel for increasing the Ni content of the SUS316 has been used. This is to improve the irradiation resistance property of SUS316, and the conventional Fe-Cr-N
In the irradiation test result of the i-based alloy, the compositional dependence of Ni on the void swelling is recognized, and for example, Fe-15Cr
This is because it has been reported that the alloy has the lowest swelling amount at 45% Ni.

On the other hand, on the other hand, when the amount of Ni is increased, the high temperature strength decreases, so it is not appropriate to simply increase the Ni in order to improve the irradiation resistance characteristics. Therefore, in order to improve the high temperature strength while improving the swelling resistance, it is necessary to add some measure other than the high Ni content. Currently, a material that combines strengthening by fine precipitation of carbonitride and solid solution strengthening (Japanese Patent Application No. 6-39391).
Alternatively, a material such as PE16 reinforced by precipitation of γ ′ is considered as a candidate material for a practical furnace.

[0004]

However, at present, no material has been developed which completely satisfies the conditions of a severe environment such as high temperature, fast neutrons and high stress, which are assumed in a practical reactor. For example, when the amount of Ni is small, the swelling resistance becomes a problem, and conversely, N is added to improve the swelling resistance.
When the i amount is 50% or more, He embrittlement becomes a problem.
Further, although the carbonitride precipitation strengthened steel (Japanese Patent Application No. 6-39391) developed by the present inventors has excellent phase stability, carbon (C) and nitrogen (N) are solid-dissolved in Ni. Since it is difficult to do so, it becomes difficult to increase the amount of Ni while maintaining the high temperature strength, and the strength achieved by the invention is the limit. Meanwhile, Ninom
ic PE16 is a material obtained by precipitation-strengthening an Fe-Ni-based material with γ'and has excellent high-temperature strength, but irradiation-induced grain boundary precipitation embrittlement poses a problem (note that Ninom).
Irradiation-induced grain boundary precipitation embrittlement in ic PE16 is a phenomenon in which γ'reprecipitates at grain boundaries upon neutron irradiation, and ductility is completely lost). It is known that in INCONEL 706 containing a large amount of Ti, γ ′ is melted by irradiation and the η phase (Ni ₃ Ti) is reprecipitated at the grain boundaries to cause embrittlement. Further, in order to solve these problems, it is possible to simply suppress the amount of γ '+ γ "to 10 vol.% Or less, but the solution temperature of γ', γ" decreases, and when it exceeds 750 ° C, the temperature rapidly increases. There is a problem that the strength is lowered.

An object of the present invention is to solve the above problems and to provide a high Ni austenitic steel in which swelling resistance, high temperature creep strength, ductility and phase stability under irradiation are well balanced. To do.

[0006]

In order to solve the above problems, in the present invention, in view of phase stability and high temperature strength, d electronic alloy design theory (see Japanese Patent Application No. 2-25622), trial production-evaluation. As a result of intensive research based on the above, the following high Ni steel was invented.

(1) Si by weight%: 0.5% or less, M
n: 1.0% or less, Cr: 13-18%, Ni: 30-
50%, Mo + W = 2.0 to 6.0%, Al = 0.05
~ 1.0%, Ti = 0.05-0.6%, Nb = 2-5
%, The balance being Fe and unavoidable impurities, and a γ ′, γ ″ phase precipitation strengthened high Ni steel excellent in irradiation resistance, high temperature strength and ductility.

(2) A high Ni steel according to item 1 above, wherein 0.08% or less of P is added.

(3) High Ni according to item 1 or 2 above
A high Ni steel, characterized in that 0.01% or less of B is added to the steel.

(4) In the γ ', γ "phase precipitation strengthened high Ni steel described in any one of items 1 to 3, after grain size adjustment, solution treatment is performed at 1060 ° C to 1150 ° C, and then 10 to 30. % Cold-worked, high Ni steel aged at 700 ° C to 900 ° C.

The present invention will be described in detail below. In addition, "γ ', γ""used in the present specification means
means γ'and / or γ ", and γ'is Ni ₃ A
l, Ni ₃ Ti, γ ″ is an intermetallic compound with Ni containing Ni ₃ Nb as a component, and means a fine precipitate of 0.01 μm or less.

The material of the present invention is mainly composed of Fe-Cr-Ni system steel, to which Mo and W are added as solid solution strengthening elements, and Ti, Al and Nb as precipitation strengthening elements of γ ', γ ".
The compound addition of Ti, Al, and Nb is characterized by having a component that particularly adds a large amount of Nb.

Although Si is added as a deoxidizer, Si.
If it exceeds 5%, harmful intermetallic compounds tend to precipitate during irradiation, causing embrittlement, so the content is made 0.5% or less.

Mn improves hot workability and is effective in stabilizing the structure, but if it exceeds 1.0%, a hardened phase is formed and toughness and workability are impaired. Below.

Cr is an essential component for improving sodium corrosion resistance and decarburization resistance, and 13% or more is required for that purpose. However, if it exceeds 18%,
The structure becomes unstable and harmful intermetallic compounds are easily deposited. In particular, grain boundary precipitation embrittlement due to phase instability easily occurs under neutron irradiation. Therefore, the Cr content is 13% to 18
The range is%.

Ni is an austenite stabilizing element and an important element for improving the swelling resistance, and at least 30% is required for that purpose. But 5
If it exceeds 0%, the residual radioactivity due to irradiation becomes extremely high, which poses a problem in storage and reprocessing of waste. In addition, the sodium corrosion resistance is also reduced. Further, Ni also causes He embrittlement (Ni reacts with neutrons to generate He).
Therefore, the Ni content is 30 to 50%.

Mo and W are important as solid solution strengthening elements and must be added in a total amount of 2.0% or more. But,
If it is added in an amount of 6.0% or more, the neutron absorption cross section becomes large, which causes a problem of proliferation. In addition, μ phase (Fe ₇ (Mo,
W) ₆ ) occurs and reduces ductility. Therefore, (Mo +
The amount of W) is 2.0 to 6.0%.

Ti, Al and Nb form an intermetallic compound (γ ', γ ") and are important as precipitation strengthening elements.

That is, it is common to increase the strength by γ ', γ "even in the similar steels on the market, but in the case of the high Ni steel according to the present invention, the operating temperature is Since the ductility at 700 ° C. and under neutron irradiation is required, the amount of γ ′, γ ″ is set to 10 vol. % Or less, and at least the solution temperature (γ ',
The temperature at which γ ″ melts) must be 800 ° C. or higher.
To obtain such solution temperature, Ti, Al, Nb
It has been confirmed by experiments that it is effective to contain a large amount of any one of the components. That is, a sample having a high γ, γ + γ ′, γ ″ phase boundary temperature related to the solution temperature is T
The component (mol amount) of any one of i, Al, and Nb is high. However, it was found that the sample having a large amount of Ti was inferior in tensile ductility and the sample having a large amount of Al was inferior in creep strength. On the other hand, the sample with a large amount of Nb does not have such inconvenience, and the sample with a large amount of Nb exhibits excellent characteristics in both creep strength and tensile ductility. In the present invention, in order to secure sufficient ductility, the amounts of γ ′ and γ ″ are set to 10 vo
l. %, In consideration of the need to maintain the content of Al at most,
= 0.05-1.0%, Ti = 0.05-0.6%, N
b = 2 to 5%.

Since the precipitation upper limit temperature of γ ', γ "is about 1050 ° C, the solutionizing temperature is at least 1060 ° C.
If the amount of γ ″ increases, the upper limit temperature of precipitation rises, but if the heat treatment temperature is too high, the crystal grains become large and ultrasonic flaw inspection cannot be performed, so the upper limit is set to 1150 ° C. Next, γ ′, γ It is necessary to introduce dislocations by cold working and precipitate them on the dislocations in order to finely precipitate ". A minimum of 10% cold work is required to precipitate on dislocations. Three
If it exceeds 0%, the creep strength decreases due to the introduced dislocations, so this is made the upper limit. Regarding the aging temperature, since the operating temperature of the fast reactor is about 700 ° C, it is desirable to precipitate it at a temperature higher than that. Therefore, the lower limit of the aging temperature is 700 ° C. In addition, γ'precipitated above 900 ° C,
Since γ ″ becomes coarse, the upper limit is set to 900 ° C.

[0021]

[Example] Vacuum melting, homogenizing heat treatment (1200 ° C x 20
h), hot forging, hot rolling, intermediate annealing (1100 ° C x 3
0min WQ) to make a plate with a thickness of 20mm, and
0% cold rolling, solution treatment (1040 to 1080 × 30
min WQ) to adjust the grain size and make a plate material of 8t. Finally, the test material is finished by 20% cold working and aging treatment at 750 ° C. for 8 hours. Table 1 shows the composition of the test material
Shown in.

[0022]

[Table 1] The results of the creep rupture test of each of the test materials shown in Table 1 are shown in FIG. From the results shown in FIG. 1, it can be seen that the test material according to the present example shows that the material in which a large amount of Ti and Nb are added exhibits an excellent creep strength exceeding that of PNC1520 which is the candidate material for the cladding tube of the demonstration reactor. .

FIG. 2 shows the elongation in the tensile test. From the results shown in FIG. 2, it can be seen that the material to which a large amount of Ti has been added exhibits poor creep strength, but is poor in tensile strength and ductility. It can be seen that the material of the invention steel containing a large amount of Nb has good tensile properties and ductility.

No. 1 shown in Table 1 For the sample of No. 7, the effect of heat treatment was also examined. 60g button ingot was melted in a tri-arc furnace and homogenized (1
A plate of 4 t is formed by hot rolling (1200 ° C.) at 200 ° C. for 24 hours. Intermediate heat treatment (1100 ℃ × 30min W
Q), 20% cold rolling, solution heat treatment (1080 ° C x 4
A test piece was prepared according to h). Next, the following four heat treatments and the precipitation state of γ ′ and γ ″ were investigated.

(A) 1080 ° C. × 1 h → 20% CW → 900 ° C. × 1 h → 750 ° C. × 8 h (B) 1080 ° C. × 1 h → 900 ° C. × 1 h → 750 ° C. × 8 h (C) 1080 ° C. × 1 h → 20 % CW → 750 ° C. × 8 h (D) 1080 ° C. × 1 h → 750 ° C. × 8 h Table 2 shows the results of examination of the precipitation state by electron microscope observation.

[0026]

[Table 2] It can be seen from Table 2 that the heat treatment of (C) is most effective.

[0027]

As described above, according to the present invention, it is possible to provide a high Ni steel having a well-balanced swelling resistance, high temperature creep strength and phase stability under irradiation. That is, according to the present invention, it is possible to provide a γ ′, γ ”phase precipitation strengthened high Ni steel excellent in irradiation resistance, high temperature strength and ductility. 70, such as structural members (especially fast breeder reactor members), especially fuel cladding
It is possible to achieve a long service life of a structural member used at a high temperature of about 0 ° C. and under a high stress.

[Brief description of drawings]

FIG. 1 is a diagram showing the results of a creep rupture test at 700 ° C. for a trial material (test material).

FIG. 2 is a diagram showing a result of comparison of tensile elongations of trial materials (test materials).

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Shigeharu Ukai 4002 Narita-cho, Oarai-cho, Higashi-Ibaraki-gun, Ibaraki-ken Within the Oarai Engineering Center, Power Reactor and Nuclear Fuel Development Corp. Oarai-cho Narita-cho 4002 Power Reactor and Nuclear Fuel Development Corporation Oarai Engineering Center (72) Inventor Makoto Harada 5-8-1, 527-205 Gakuen Nishimachi, Nishi-ku, Kobe-shi, Hyogo (72) Inventor Toshio Nishida Ibaraki 4002 Narita-cho, Oarai-cho, Higashi-Ibaraki-gun, Japan Oarai Engineering Center, Reactor and Nuclear Fuel Development Corporation

Claims (4)

[Claims] 1. Si: 0.5% or less by weight%, Mn:
1.0% or less, Cr: 13-18%, Ni: 30-50
%, Mo + W = 2.0 to 6.0%, Al = 0.05 to
1.0%, Ti = 0.05 to 0.6%, Nb = 2 to 5
%, The balance being Fe and unavoidable impurities, and a γ ′, γ ″ phase precipitation strengthening high Ni steel. 2. The high Ni steel according to claim 1, wherein
High N, characterized by the addition of 08% or less P
i steel. 3. The high Ni steel according to claim 1 or 2, wherein B is added in an amount of 0.01% or less. 4. The γ ′ according to any one of claims 1 to 3,
After adjusting the grain size in γ "phase precipitation strengthened high Ni steel, 104
Solution treatment is performed at 0 ° C to 1150 ° C, and then 10
High Ni steel that has been cold worked at 30% and aged at 700 ° C to 900 ° C.