JPH0625378B2 - Manufacturing method of ferritic structural members for fast reactor core - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for manufacturing a structural member for a fast reactor core having excellent creep strength.

(Prior art and its problems) Structural materials for fast reactor cores, that is, materials such as fuel cladding tubes and wrapper tubes, are subjected to high-temperature 650 ° C while being irradiated with fast neutrons in liquid sodium, and force from multiple directions. The operating conditions are extremely harsh because it is used under.

Therefore, the properties required for materials used for such applications include corrosion resistance to liquid sodium and swelling resistance to high-speed and high-density neutron irradiation (swelling is the ejection of the atoms that compose the crystal due to fission, which causes bubbles in the material. , A phenomenon in which cavities occur and expand), and excellent high-temperature strength characteristics are required. Further, there is also a demand for dimensional accuracy when it is used as a final finish member, and it is also extremely severe as compared with general members, so that excellent cold workability is also required.

Conventionally, as a structural member for a fast reactor core, 316 type austenitic heat-resistant steel, which has excellent corrosion resistance to liquid sodium, high temperature strength and workability, has been used. However, this steel type has a problem that swelling is large under fast neutron irradiation, and the end of use (neutron irradiation amount, about 2.0 ×
The amount of swelling at 10 ²³ n / cm ² ) may exceed about 15%. In core structural members having different neutron irradiation doses, the amount of swelling varies depending on the position, and the members are deformed.
Therefore, for example, it is assumed that the wrapper pipe may be hindered by the deformation during fuel exchange, and the fuel clad pipe may have a narrowed space between the fuel pins due to the bulging and deformation of the outer diameter to cause a decrease in the cooling capacity.

Various proposals have been made for the purpose of improving the swelling resistance of the above austenitic materials. (For example, JP-A-56-126794, JP-A-61-87853, and Japanese Patent Publication
See 58-56024. ) However, in future demonstration reactors and commercial reactors, the fast neutron irradiation dose is 2.5 to 5.0 × 10 ²³ n / cm ² , and the temperature is 650 to 675 ℃ and 3 to 5 from the viewpoint of economic efficiency.
While it is necessary to develop structural members for cores that can withstand years of use, there is a limit to the improvement of conventional austenitic heat-resistant steels, and alternative structural members for fast reactor cores with excellent swelling resistance are available. The development of is awaited.

By the way, it is known that ferritic heat-resistant steel (hereinafter referred to as ferritic steel) has significantly better swelling resistance than the austenitic heat-resistant steel (hereinafter referred to as austenitic steel).

Figure 4 shows the swelling properties of typical austenitic steels and ferritic steels (BAChin et al, Nuclear.
Technology, Vol.57,1982, P.426), it is clear that ferritic steel is far superior to austenitic steel in swelling resistance. This is due to the crystal structure of both steel types, and although there is a difference in life due to differences in composition and manufacturing method, it is said that the superiority of ferritic steel remains the same.

Another property required for structural members for fast reactor cores is corrosion resistance to liquid sodium.

Fig. 5 shows an example of a study on the decarburizing behavior of ferritic steel (9Cr-1Mo steel) and austenitic steel in liquid sodium (C. Tyzack, AWThorley: Proceedings of Internation
al Conference on Ferritic Steels for Fast Reactor
Steam Generators, London, 1977, P.190), but as shown here, the decarburizing limit in liquid sodium is almost the same for both ferritic steel and austenitic steel. It is reported to be few.

Further, ferritic steel has advantages of high thermal conductivity and small thermal expansion coefficient as compared with austenitic steel, and since it is inexpensive, it is attracting attention as a structural member for a fast reactor core.

However, although the ferritic steel has the above-mentioned excellent properties, it is inferior to austenite in terms of high-temperature strength, structural stability during high-temperature use, and toughness. For example, among the existing ferritic steels developed in the United States as materials for boilers and nuclear power, ASTM A213-T19 is one of the strongest steels.
Even with (Mod. 9Cr-1Mo steel), the creep rupture strength at 650 ℃ × 10 ⁴ h is 7.5 kgf / mm ² , and the crease rupture at 650 ℃ × 10 ⁴ h of DIN X20CrMoWV 121 widely used as 12Cr series steel. The strength is about 6 kgf / mm ² at most. These materials are
Since strength is emphasized at relatively low temperatures of 600 ° C or less, it cannot necessarily be applied to structural members for fast reactor cores.

In order to use ferritic steel as a structural member for a fast reactor core, improvement of its high temperature strength is a major issue.

The object of the present invention is to select a component system that can also be used as a structural member for a fast reactor core in ferritic steel, and in particular, creep strength (specific target is 650 ℃ × 10 ⁴ h creep rupture strength of 8 kgf / mm ² or more) is to propose a manufacturing method. Further, it is to propose a method for manufacturing a structural member for a fast reactor core having a strict size specification which is not required for ordinary industrial products.

(Means for Solving Problems) The inventors of the present invention have adjusted various alloy components, in particular, determined appropriate values of Mo and W contents in order to improve the high temperature strength of the ferritic heat resistant steel. The heat treatment for maximizing this steel type is normalization-tempering, but a large strain occurs during this heat treatment. As a method of removing this strain, we adopted a cold working method,
The range of practical application of this cold working was determined. That is, the gist of the present invention is as follows.

% By weight, C: 0.04 to 0.2%, Si: 0.3% or less, Mn: 1.5% or less, Ni: 0.1 to 1% Cr: 8 to 13%, Mo: 0.1 to 2.5%, W: 0.1 to 4%, V: 0.1-0.4%, Nb: 0.01-0.2%, sol. Al: 0.03% or less, N: 0.01 to 0.08%, balance Fe and unavoidable impurities, Mo + 1 / 2W: 1 to 2.5%, and P and S in the impurities are 0.02% or less each, ferritic steel From the Ac ₃ point in the final heat treatment process of the member
High-speed characterized by normalizing in the temperature range up to 1200 ℃ and tempering in the temperature range from 780 ℃ to Ac ₁ point, and then finishing by cold working 5-20% Manufacturing method of ferrite-based structural members for reactor core.

The ferritic steel used as the material of the method of the present invention contains B: 0.001 to 0.01% in addition to the above-mentioned components, or
At least one of the group of Ti, Zr, La, Ce, Y, and Ca in total
Containing 0.05 to 0.3% or B: 0.001 to 0.01%
And Ti, Zr, La, Ce, Y, and Ca in a total amount of 0.05 to 0.3%. (In this specification, all "%" mean% by weight.) (Function) First, the reason why the content of the alloy component is specified in the present invention will be described together with the function and effect.

C: Austenite stabilizing element, which stabilizes the martensite structure. A large number of dislocation structures due to martensitic transformation contribute to improving swelling resistance. Further, it combines with alloy elements Nb, V, Cr, etc. to form fine carbides and improves creep rupture strength and swelling resistance.
If it is less than 04%, the effect is insufficient, and the strength and toughness are significantly impaired by the increase in the amount of δ-ferrite. Toughness must be good from the viewpoint of cold workability, which is particularly necessary in the production of structural members for fast reactor cores, and it is also a property that should be provided from the viewpoint of suppressing embrittlement during long-time heating. Is one. On the other hand, if it exceeds 0.2%, carbides increase, the steel hardens,
Its workability and weldability are impaired. Therefore, 0.04 to 0.2
% Is an appropriate content of C.

Si: It is added as a deoxidizer, but when the content exceeds 0.3%, embrittlement during heating at high temperature is remarkable. Therefore, 0.3%
The content is set as follows, but if the content is preferably 0.1% or less, the toughness improving effect is great.

Mn: Improves hot workability and is effective in stabilizing the structure, but if it exceeds 1.5%, a hardened phase is formed, and toughness and workability are impaired. Therefore, the proper content of Mn is 1.
It is 5% or less.

Ni: A component that stabilizes the martensite structure as an austenite stabilizing element. In the present invention, the amount of δ-ferrite is adjusted to 0.1% in order to impart strength, toughness and workability.
Although it is contained as described above, if the content exceeds 1%, the creep strength is impaired, and the transformation point becomes too low, degrading the heat treatment property and workability. Therefore, the proper content of Ni is 0.1 to 1%.

Cr: Indispensable element for improving the corrosion resistance and decarburization resistance in liquid sodium.
It is not suitable for high temperature use above 0 ° C. On the other hand, if it exceeds 13%, δ-ferrite will increase and the strength and toughness will be impaired.
Therefore, the proper content of Cr is 8 to 13%.

Mo and W: both are solid solution strengthening elements and contribute to the improvement of creep strength as constituent elements of carbides and intermetallic compounds. If both are less than 0.1%, the above effect cannot be obtained.
If the Mo content exceeds 2.5% or the W content exceeds 4%, not only the amount of δ-ferrite increases and the toughness is impaired, but also a large amount of intermetallic compound precipitates and embrittles at high temperature. Therefore, the proper content of Mo is 0.1 to 2.5%, and the proper content of W is 0.1 to 4%.

Furthermore, according to the knowledge of the present inventor, Mo and W are set to 1% ≦ Mo +
The combined effect of 1/2 W ≦ 2.5% has a remarkable effect of improving the high temperature strength. In particular, considering the usage conditions of up to about 650 ° C as structural members for the fast reactor core, we decided to use both components together within the above range.

V: Combined with C, N to form a fine precipitate of V (C, N),
Contributes to creep strength. If it is less than 0.1%, a sufficient effect cannot be obtained, and if it exceeds 0.4%, the strength is rather deteriorated. Therefore, the proper content of V is 0.1 to 0.4%.

Nb: Like V, it combines with C and N to form fine precipitates of Nb (C, N), which contributes to creep strength. It is also effective for making the structure finer and improving the toughness. If it is less than 0.01%, no effect is obtained, while if it exceeds 0.2%, a large amount of undissolved precipitates remain during heat treatment, impairing the creep strength. Therefore, the proper Nb content is 0.01 to 0.2%.

Sol.Al: Although added as a deoxidizer, if the content exceeds 0.03%, the creep strength is impaired. Therefore, Sol.
The appropriate Al content is 0.3% or less.

N: Combines with V and Nb to form carbonitrides and contributes to the improvement of creep strength, but if its content is less than 0.01%, it has no effect, and if it exceeds 0.08%, workability, toughness, and welding Reduce sex. Therefore, the proper N content is 0.01
~ 0.08%.

P and S: Both are impurity elements harmful to toughness, workability, and weldability. In consideration of unavoidable content, the upper limit of each is set to 0.02%, but the content is preferably as low as possible.

Ferrite steel containing the above alloy components and consisting of the balance Fe and unavoidable impurities is the basic subject of the method of the present invention, but for the purpose of further improving its properties, the following components are added and contained. be able to.

B: Addition of fine particles disperses and stabilizes carbides and contributes to improvement of creep strength. If the content is less than 0.001%, the above effect does not occur, and if it exceeds 0.01%, workability and weldability are impaired. Therefore, when B is contained, its amount is 0.001 to 0.01%.

Ti, Zr, La, Ce, Y, Ca: Addition of these elements alone or in a small amount to P, S which is harmful to the morphology control and toughness of inclusions
There is a cleaning action. The total content of these elements is 0.05
If it is less than 0.3%, the above effect cannot be obtained, and if it exceeds 0.3%, it is rather harmful to the toughness and workability. Therefore, 00.5
~ 0.3% is a proper content range.

From the viewpoint of strength and toughness, the heat treatment of the steel according to the present invention is a normalizing treatment in the temperature range from the Ac ₃ transformation point to 1200 ° C and the subsequent 7
Tempering in the temperature range from 80 ℃ to Ac ₁ transformation point.

The normalizing treatment has an action of strengthening the structure into martensite, and the tempering treatment has a function of stabilizing the structure (that is, mechanical properties) of the martensite structure at high temperature.

The normalizing temperature is Ac ₃ in order to obtain a martensitic structure.
It must be above the point. The upper limit is 1200 ° C. from the viewpoint of suppressing the amount of δ-ferrite and suppressing high temperature oxidation.
That is, the proper range of the normalizing temperature is Ac ₃ point to 1200 ° C.

The tempering temperature is sufficiently higher than the use temperature, and it is necessary to be 780 ° C or higher in order to stabilize the structure and obtain high creep strength. To prevent transformation at high temperature, the tempering temperature is set to Ac ₁ point or lower. That is,
The proper temperature for normalizing is 780 ℃ ~ Ac ₁ point.

Next, cold working will be described. The heat-treated member is
When the size is finished by machining after that, it may not pass the strict size standard. For example, in the case of a fuel cladding tube, heat treatment causes deformation such as bending and flattening, and when this is machined, the wall thickness fluctuates and the quality deteriorates. In this case, the dimensions can be made accurate by cold working such as drawing or rolling. However, this characteristic changes in the case of a fuel cladding tube that requires creep strength. That is, although the creep strength does not change with light cold working,
When the cold workability increases, recrystallization of the martensite structure occurs and the creep strength decreases. For extremely strict members that require dimensional accuracy, such as structural members for fast reactor cores,
If the processing is less than 5%, the required dimensional correction effect cannot be obtained.
If the processing exceeds%, there is a concern that the creep strength will decrease.
Therefore, the proper range of cold working is 5 to 20%, preferably 5 to 15%.

(Example 1) In order to specifically confirm the effect of the method of the present invention, a test was conducted using steels having various structures shown in Table 1 as test materials.

In Table 1, A to F are existing ferritic steels for boilers, chemical industry, and nuclear power, A is STBA26 steel, and B is STBA27 for thermal power generation.
Steel, C is ASTM / A213-T91 steel, D is for boiler 9Cr-2Mo-V-Nb
Steel, E is DIN / X20Cr-MoV121 steel, F is DIN / X20CrMoWV1
It is 21 steel (commonly called HT9).

G to R are target steels of the present invention, and S to Z are comparative steels adjusted to see the combined effect of Mo and W.

These test materials were melted in a 50 kg vacuum melting furnace, forged into a 20 mm thick plate, and then subjected to the following heat treatment and cold working, respectively.

A to F: 950 to 1050 ℃ normalizing +800 ℃ tempering. (For heat treatment) G to Z: Normalizing at 1050 ° C + tempering at 800 ° C. As cold as 10%. However, only J is 5-30% cold processed.

A 6 mmφ × 30 mm GL round bar tensile test piece was prepared from these test materials, and the room temperature tensile test, 650 ° C tensile test, and 650 ° C tensile test were performed.
A ° C creep rupture test was performed. In addition, a JIS No. 4 Charpy impact test piece was sampled and subjected to a Charpy impact test at 0 ° C. The results are shown in Table 2. As is clear from Table 2, in the creep rupture strength of 650 ° C. × 10 ⁴ h, all the steels according to the present invention exceed 8.0 kgf / mm ² and far exceed the existing steels A to F.

FIG. 3 compares the results of the creep rupture test of Steel I to which the present invention is applied with those of typical existing steels. As clearly shown in FIG. 3, the steel to which the present invention is applied has a creep strength much higher than that of C steel, which is the highest strength among existing steels, and is extremely stable for a long time.

FIG. 1 shows the proper range of the combined addition of Mo and W. The Mo + 1 / 2W content (%) of A to Z steels in Table 1 and 650 ° C.
10 ⁴ h Shows the relationship of creep rupture strength (kgf / mm ² ). The steel to which the present invention is applied surely achieves the target strength of 8 kgf / mm ² at 650 ° C. × 10 ⁴ h.

(Example 2) FIG. 2 shows the results of examining the effect of cold working using the steel J of the present invention. 650 ℃ × 10 ⁴ h Creep rupture strength 8kgf / mm ²
It can be seen that the cold working rate should be 20% or less in order to exceed the target of.

Materials that have undergone cold work in excess of 20% experience a significant reduction in strength. It is considered that this is because the accumulation of high cold work strain accelerated the recrystallization of the tempered martensite structure during high temperature creep of 650 ° C or higher.

As is clear from these results, cold working is essential to secure strict dimensional accuracy like structural members for fast reactor cores, and the working rate is 20% or less for members that require high creep strength. You should choose. The lower limit of the cold workability is 5% in order to make the dimensional accuracy of the member appropriate as described above. In addition, the member to which the present invention is applied can maintain the required creep rupture strength even if cold working of 5 to 20% is applied.

(Effect of the Invention) According to the method of the present invention, a ferrite-based structural member having a high high-temperature strength of 650 ° C. × 10 ⁴ h creep rupture strength of 8 kgf / mm ² or more can be obtained. Since this member is made of ferrite, it has much better swelling resistance than austenitic heat-resistant steel and is suitable for cores of fast reactors. Further, the proper range was confirmed for the cold working which is indispensable when used for such an application. Therefore, the method of the present invention is suitable as a method for producing a structural member for a fast reactor core, and can be expected to be used as a method for producing a fuel cladding tube for a core, a wrapper tube, and the like.

[Brief description of drawings]

FIG. 1 is a diagram showing the relationship between the combined addition amount of Mo and W and 650 ° C. × 10 ⁴ h creep rupture strength, and FIG. 2 is the cold workability of the steel to which the present invention is applied and 650 ° C. × 10 ⁴ h FIG. 3 is a diagram showing the relationship with creep rupture strength, FIG. 3 is a diagram showing an example of the results of a creep rupture test at 650 ° C. of a steel to which the present invention is applied and an existing steel, and FIG. 4 is a neutron for ferritic steel and austenitic steel. A reference diagram showing the relationship between irradiation dose (fluence) and swelling amount, Fig. 5 shows ferritic steel and austenitic steel.
It is a reference diagram showing the relationship between temperature, the amount of carbon in sodium, and the carburizing / decarburizing region.

Front page continuation (72) Inventor Atsuro Iseta 1-3-3 Nishi-Nagasumotodori, Amagasaki, Hyogo Prefecture Sumitomo Metal Industries, Ltd. Research Institute (72) Inventor Kazuhiko Yoshikawa 1-chome Nishinagasumoto, Amagasaki, Hyogo Address 3: Sumitomo Metal Industries, Ltd.

Claims (4)

[Claims] 1. By weight%, C: 0.04 to 0.2%, Si: 0.3% or less, Mn: 1.5% or less, Ni: 0.1 to 1% Cr: 8 to 13%, Mo: 0.1 to 2.5%, W: 0.1 to 4%, V: 0.1 to 0.4%, Nb: 0.01 to 0.2%, Sol.Al: 0.03% or less, N: 0.01 to 0.08%, balance Fe and unavoidable impurities, Mo + 1 / 2W: 1 to 2.5% And P and S in the impurities are 0.02% or less, respectively, from the Ac ₃ point in the final heat treatment process of the ferritic steel member.
High-speed characterized by normalizing in the temperature range up to 1200 ℃ and tempering in the temperature range from 780 ℃ to Ac ₁ point, and then finishing by cold working 5-20% Manufacturing method of ferrite-based structural members for reactor core. 2. In% by weight, C: 0.04 to 0.2%, Si: 0.3% or less, Mn: 1.5% or less, Ni: 0.1 to 1%, Cr: 8 to 13%, Mo: 0.1 to 2.5%, W : 0.1 to 4%, V: 0.1 to 0.4%, Nb: 0.01 to 0.2%, Sol. Al: 0.03% or less, N: 0.01 to 0.08%, B: 0.001 to 0.01%, balance Fe and unavoidable impurities, Mo + 1 / 2W: 1 to 2.5%, and P and S in the impurities are 0.02% each. The following ferritic steel members were selected from the Ac ₃ point in the final heat treatment process of the members.
High-speed characterized by normalizing in the temperature range up to 1200 ℃ and tempering in the temperature range from 780 ℃ to Ac ₁ point, and then finishing by cold working 5-20% Manufacturing method of ferrite-based structural members for reactor core. 3. By weight%, C: 0.04 to 0.2%, Si: 0.3% or less, Mn: 1.5% or less, Ni: 0.1 to 1%, Cr: 8 to 13%, Mo: 0.1 to 2.5%, W : 0.1 to 4%, V: 0.1 to 0.4%, Nb: 0.01 to 0.2%, Sol. Al: 0.03% or less, N: 0.01 to 0.08%, one or more components of the group of Ti, Zr, La, Ce, Y and Ca: in total
The final heat treatment process of a ferritic steel member consisting of 0.05 to 0.3% balance Fe and unavoidable impurities, Mo + 1 / 2W: 1 to 2.5%, and P and S in the impurities each 0.02% or less. At Ac from ₃ points
High-speed characterized by normalizing in the temperature range up to 1200 ℃ and tempering in the temperature range from 780 ℃ to Ac ₁ point, and then finishing by cold working 5-20% Manufacturing method of ferrite-based structural members for reactor core. 4. By weight%, C: 0.04 to 0.2%, Si: 0.3% or less, Mn: 1.5% or less, Ni: 0.1 to 1%, Cr: 8 to 13%, Mo: 0.1 to 2.5%, W : 0.1 to 4%, V: 0.1 to 0.4%, Nb: 0.01 to 0.2%, Sol. Al: 0.03% or less, N: 0.01 to 0.08%, B: 0.001 to 0.01%, one or more components of the group of Ti, Zr, La, Ce, Y, and Ca: in total
The final heat treatment process of a ferritic steel member consisting of 0.05 to 0.3% balance Fe and unavoidable impurities, Mo + 1 / 2W: 1 to 2.5%, and P and S in the impurities each 0.02% or less. At Ac from ₃ points
High-speed characterized by normalizing in the temperature range up to 1200 ℃ and tempering in the temperature range from 780 ℃ to Ac ₁ point, and then finishing by cold working 5-20% Manufacturing method of ferrite-based structural members for reactor core.