JP3127759B2 - Oxide dispersion-strengthened ferritic steel having recrystallized structure and method for producing same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an oxide dispersion strengthened ferritic steel and a method for producing the same. More specifically, core components (e.g., fuel assemblies, control rods, reflectors, etc., consisting of fuel cladding tubes and wrapper tubes) and equipment structures that are used for a long time in the core environment of nuclear reactors, especially fast breeder reactors The present invention relates to an oxide dispersion-strengthened ferritic steel for members requiring excellent neutron irradiation resistance such as (e.g., equipment container members and cooling system piping members) and a method for producing the same.

[0002]

2. Description of the Related Art Conventionally, as a core component of a fast breeder reactor, SUS316 or an improved austenitic steel (hereinafter referred to as PNC316) having improved irradiation resistance characteristics of SUS316 has been used.
Or steel with high Ni (for example, PNC1520
15Cr-20Ni steel). The prototype fast breeder reactor "Monju" uses PNC316.

However, although austenitic steels have excellent high-temperature strength, their durability against high-speed neutrons such as swelling resistance and irradiation creep characteristics is limited. It has proven unsuitable. At present, efforts are being made to develop materials for improving the neutron irradiation resistance of austenitic steels.

On the other hand, although ferritic steel has excellent neutron irradiation resistance, it is inferior in high-temperature strength, and is suitable for a member to which a high temperature and a high load (internal pressure stress) are applied, such as a fuel cladding tube of a nuclear reactor. I know there isn't.

Therefore, in order to improve the high-temperature strength of ferritic steel, an oxide dispersion-strengthened ferritic steel in which fine oxide particles are dispersed in ferritic steel is expected as a material having excellent neutron resistance and high-temperature strength. Has been researched and developed. At present, MA957 (14Cr-0.3 Mo-1Ti-0.25
Y ₂ O ₃₎ and PNC-ODS (13Cr-3W- 0.5 Ti-0.35Y 2 O 3) has been developed. However, workability and mechanical properties (ductility,
Internal pressure creep rupture strength).

[0006]

SUMMARY OF THE INVENTION Oxide dispersion strengthened ferritic steel contains fine oxide particles (for example, Y ₂ O ₃ ) in the steel.
Is a steel in which is dispersed. These fine oxide particles obtain excellent high-temperature strength by suppressing the movement of dislocations.
However, such fine oxide particles also have the property of hardly causing recovery and recrystallization at the same time because they suppress the movement of dislocations.

Therefore, the harder the work, the harder the work
Sufficient recovery is not performed and processing becomes difficult. For this reason, in the case of conventional steel (PNC-ODS, MA957, etc.), when working into a pipe such as a cladding pipe, warm working had to be applied.
In addition, even when warm working is applied, cracks frequently occur during manufacturing, and the prototype cladding tube has many scratches and the yield is very low.

[0008] Even if the cladding tube is manufactured in such a time-consuming manner and at a high cost, recrystallization does not occur, so that the crystal grains become extremely elongated together with the processing. For example, the microscope shown in FIGS. 4 (a) and 4 (b) is used. A fibrous structure like bamboo is obtained as shown in the structure photograph. 4 (a) and 4 (b) are photographs of the same tissue except for the magnification. It has been found that a cladding tube having such a fibrous structure (hereinafter referred to as a bamboo structure) has a large anisotropy in creep rupture strength (difference between uniaxial creep rupture strength and internal pressure creep rupture strength). That is, the uniaxial creep rupture strength is very good, but the internal pressure creep rupture strength is about half of the uniaxial creep rupture strength. Furthermore, the uniform elongation in the circumferential direction of such a cladding tube decreases at around 400 ° C., and it is clear that improvement is required for nuclear pipe materials such as cladding tubes.

The present invention solves the above problems,
Swelling resistance and high temperature internal pressure creep strength (650 ℃, 1000
0h rupture strength: 190 MPa or more) and an oxide dispersion-strengthened ferritic steel having a circumferential ductility around 400 ° C. (uniform elongation in the circumferential direction at 800 ° C. or less: 1% or more) and a method for producing the same. Aim.

[0010]

Means for Solving the Problems The present inventor has conducted various trial productions and evaluations, and as a result of intensive research based on the results, the internal pressure creep rupture strength of the problematic oxide dispersion strengthened ferritic steel is as follows: I found out that it was caused by the bamboo-like structure elongated by processing, and found that a solution could be achieved by using a recrystallized structure with less directivity.

Furthermore, the present inventors have found that in oxide dispersion strengthened ferritic steel, Ti and oxygen play an important role through the formation of a composite oxide, and the amounts of Ti, oxygen and Y ₂ O ₃ depend on workability and We focused on the fact that it greatly affects the recrystallization characteristics.
By paying attention to the excess oxygen concentration and the amount of Y ₂ O ₃ , the present inventors have found that a range in which a recrystallized structure can be generated at a considerably lower temperature of 1300 ° C. or lower than the conventional one can be specified, and completed the present invention.

Of course, recrystallization can be achieved by heat treatment at a high temperature of 1350 ° C. or more even outside the range of the region specified as above. In this case, however, the oxide particles are coarsened, and the oxide enhances the dispersion. Since the function is significantly reduced, the significance of the recrystallized structure is lost. Desirably, the recrystallization heat treatment is performed at 1250 ° C. or less. Here, the present invention is as follows.

(1) In weight%, Cr: 7 to 18%, 1 / 2W + Mo:
An oxide dispersion-strengthened ferritic steel in which yttria is dispersed in a ferrite-based metal matrix mainly composed of Fe—Cr mainly containing Fe and unavoidable impurities, with the content being 0.1 to 3% and Ti: 0.10 to 1.0%. Excess O
Is in the range of 0.10% <Y ₂ O ₃ ≦ 0.30% 0.03% ≦ Excess O ≦ 0.15% Excess O ≦ 0.25−0.5 × Y ₂ O ₃ (%) by weight and has a recrystallized structure An oxide dispersion-strengthened ferritic steel, characterized in that:

Here, the excess oxygen amount is the total oxygen amount (Total oxygen amount).
O) from which the oxygen (O in Y ₂ O ₃ ) which is bound to yttrium as yttria (Y ₂ O ₃ ) is excluded from (O) ((Exces
s O] = [Total O] − [O in Y ₂ O ₃ ]).

(2) By weight%, Cr: 7 to 18%, 1 / 2W + Mo:
A method for producing an oxide dispersion-strengthened ferritic steel in which yttria is dispersed in a ferrite-based metal matrix mainly composed of Fe-Cr mainly containing 0.1 to 3%, Ti: 0.10 to 1.0%, and the balance being Fe and unavoidable impurities. , the range of yttria and excess oxygen amount (excess O), in _{weight%, 0.10% <Y 2 O} 3 ≦ 0.30% 0.03% ≦ excess O ≦ 0.15% excess O ≦ 0.25-0.5 × Y 2 O 3 (%) A method for producing an oxide dispersion-strengthened ferritic steel having excellent internal pressure creep strength and ductility, wherein the ferrite steel is subjected to recrystallization heat treatment at a temperature of 1300 ° C. or lower and at the same time.

Here, the excess oxygen amount is as defined in (1).

[0017]

Next, the present invention will be described in detail. In addition,
In the present specification, unless otherwise specified, "%"
Is "% by weight". The steel according to the present invention is Fe-Cr-Y ₂
The main feature is that it is made mainly of O ₃ ferrite steel, the amount of excess oxygen is limited, and the structure is a recrystallized structure.

FIG. 1 shows regions of recrystallized structures of various oxide dispersion-strengthened ferritic steels shown in Examples which will be described later when subjected to heat treatment at 1300 ° C. or less in relation to excess oxygen amount and Y ₂ O ₃ amount. It is a graph, and in the figure, a hatched region is a region of a recrystallized structure.

Conventionally, the concept of excess oxygen and the relationship between the concept and the recrystallization phenomenon have not been known at all.
Excess oxygen content was greater than 0.15%. In the present invention, “excess oxygen amount” and “Y ₂ O ₃ amount” are both calculated values obtained from yttrium (Y) analysis values.

Y ₂ O ₃ plays a leading role in strengthening dispersion and is an important additive. The more the addition, the higher the strength in the working direction, but on the other hand, the harder it is to cause recovery and recrystallization, and the lower the workability. When a material to which more than 0.30% of Y ₂ O ₃ is added is processed, the crystal grains elongate with the processing, forming a bamboo-like structure. This oxide dispersion-strengthened ferritic steel (ODS ferrite steel) having a bamboo-like structure has large anisotropy in strength,
The strength does not improve in the direction perpendicular to the processing direction (for example, in the case of a cladding tube, the internal pressure creep rupture strength).

Therefore, it is necessary to limit the amount of Y ₂ O ₃ so as to recrystallize. If it is 0.30% or less, recrystallization is possible, so this is the upper limit. When the content is 0.10% or less, recrystallization is relatively easy, but the effect of dispersion strengthening is small and the strength is low. Therefore, in the present invention, the lower limit is set to more than 0.10%.

The amount of excess oxygen plays an important role in refining oxide particles as in Ti. Complex oxide (Y ₂ Ti ₂ O ₇ , Y ₂ TiO
_{This is because} when forming ₅ ), not only Ti but also oxygen is required. However, the number of oxide particles increases as the oxide particles become finer, and the movement of dislocations is suppressed, so that recrystallization becomes difficult. It can also cause extra inclusions,
Decreases workability. If the amount of excess oxygen is within the range of 0.15% or less, recrystallization is possible, so this is the upper limit. Preferably it is 0.13% or less. On the other hand, if it is less than 0.03%, the strength is not obtained, so this is set as the lower limit. More preferably 0.04 ~
0.11%.

However, if the amount of Y ₂ O ₃ is in the range of 0.20 to 0.30%, the recrystallized structure will not be obtained by the heat treatment at 1300 ° C. or lower if the amount of Y ₂ O ₃ is out of the range of Excess O ≦ 0.25−0.5 × Y ₂ O _3. Since it cannot be obtained, it is set within the range satisfying the above expression.

Thus, in the present invention, the excess oxygen amount
The amount of Y ₂ O ₃ is limited to the above range, but the metal component of the parent phase at that time is, in terms of% by weight, Cr: 7 to 18%, 1 / 2W + Mo: 0.
1 to 3%, Ti: 0.10 to 1.0%, the balance being Fe and unavoidable impurities.

As other components, C, Si, Mn and the like can be considered, but they are usually present as impurities, and even if they are added as required, they are added in 0.1% respectively.
Below, 0.5% or less and about 1.0% or less are sufficient. Hereinafter, the reason for limiting the composition of the matrix steel as described above in the present invention will be described.

C is inevitably present to some extent and is mixed in from a steel ball or a container during the mechanical alloying treatment. On the other hand, C may be positively added to ferritic steels in some cases because it may also improve the strength. To this end, C is allowed up to about 0.1%, but usually as little as possible as an impurity. And

If necessary, Si is added as a deoxidizing agent. However, if it exceeds 0.5%, harmful intermetallic compounds are liable to precipitate during irradiation, resulting in embrittlement. Also,
Reacts with oxide particles (Y ₂ O ₃ ) to aggregate oxide particles,
Significantly reduces strength. Therefore, it is set to 0.5% or less.
Preferably it is 0.1% or less.

Mn is added as a deoxidizing agent and as necessary for improving hot workability. However, if added in excess of 1.0%, a hardened phase is formed, and toughness and workability are impaired. Therefore, the Mn content is set to 1.0% or less.

Cr is an essential component for improving sodium corrosion resistance and decarburization resistance.
% Or more is required. However, if it exceeds 18%, the structure becomes unstable, and the aging treatment at a high temperature for a long time tends to cause embrittlement. In particular, it is known that under neutron irradiation, embrittlement is caused by the α 'phase mainly composed of Cr. Therefore, the Cr content is 7 to 18
% Range. Preferably it is 7 to 14%.

Mo and W are important as solid solution strengthening elements. For this purpose, the Mo equivalent (Mo + 1 / 2W) needs to be 0.1% or more. However, if more than 3.0% is added, the μ phase [Fe ₆ (Mo,
W) ₇ ] and Laves [Fe ₂ (Mo, W)] phase are formed, and the ductility is reduced. Therefore, the amount of (Mo + 1 / 2W) should be 0.1% or more and 3.0% or less.

Ti plays an important role in strengthening the dispersion of Y ₂ O ₃ . Ti reacts with Y ₂ O ₃ to form a composite oxide called Y ₂ Ti ₂ O ₇ or Y ₂ TiO ₅ , and has a function of reducing oxide particles. Since this effect tends to saturate when Ti exceeds 1.0%, the upper limit is 1.0%. If it is less than 0.10%, the effect of miniaturization is small, so 0.10% is made the lower limit.

Ni is effective for improving toughness, and is added as necessary. However, if added over 2.0%, it changes to austenite during irradiation, and that portion swells. Therefore, the addition of Ni is limited to 2.0% or less in the present invention.

In the production of the oxide dispersion-strengthened ferritic steel according to the present invention, the method is not limited to a specific method as long as the excess oxygen amount and the yttria amount are specified as described above and the recrystallization treatment at 1300 ° C. or lower is performed. For example, after preparing the respective raw material powders, forming oxides by so-called mechanical alloying, or adjusting the powder composition by alloying, and then, through a molding and sintering process, at 1300 ° C. or less It is manufactured by recrystallization treatment. Such a manufacturing method may follow a conventional technique.

By the way, it is known that swelling resistance can be improved by finishing by cold working even in the past. For example, about 20% of cold working is applied to PNC316. However, in the case of the present invention, in particular, cold working with a rolling reduction of more than 50% increases the anisotropy of creep rupture strength, and therefore, oxide dispersion finished by cold working with a working rate of 50% or less. Reinforced ferritic steel is particularly excellent in swelling resistance. Next, the operation and effect of the present invention will be described more specifically with reference to examples.

[0035]

【Example】

(Example 1) Mechanical alloying treatment of alloy powder and oxide powder (Y ₂ O ₃ ), and Fe ₂ O ₃ powder to adjust excess oxygen amount
(Mechanical alloying), the mixture was filled in an extrusion capsule, degassed and sealed at 400 ° C., and hot extruded at 1150 ° C. The extruded bar was cold worked 60% and heat treated at 1200 ° C. In the case of the steel of the present invention, recrystallization was performed even at 1200 ° C. Table 1 shows the components of the test material and the comparative material (PNC-ODS, MA957).

FIG. 2 shows the creep rupture test characteristics at 650 ° C. Does not recrystallize at 1200 ℃ PNC-ODS1, PNC-ODS
2, MA957 has remarkably large anisotropy in creep rupture strength. On the other hand, invention steel 1 was recrystallized at 1200 ° C, so that the anisotropy of creep rupture strength was lower than that of the conventional material, and the internal pressure creep rupture strength, which is one index of the performance of the cladding tube, was oxidized more than that of the conventional steel. It is high despite the small amount of material powder (Y ₂ O ₃ ).

FIG. 3 shows the test results of uniform elongation in the circumferential direction.
As can be seen, the uniform elongation in the circumferential direction required for a cladding tube is reduced at around 400 ° C for the conventional material, while the steel of the present invention has good ductility even at 400 ° C. I understand.

Example 2 A dispersion-strengthened ferritic steel was produced in the same manner as in Example 1, and the relationship between the excess oxygen amount and the Y ₂ O ₃ amount was examined. However, the heat treatment temperature was 1300 ° C. or less. At 1350 ° C. or more, recrystallization occurs even with an oxide addition amount of 0.30% or more, but the oxide particles are coarsened and the strength is extremely reduced.

The component analysis values and results are shown in Tables 2 and 3, and FIG. However, this includes the results of the inventive steel 1 of Example 1. From these results, the weight%
In, 0.10% <it in a range of _{Y 2 O 3 ≦ 0.30% 0.03} % ≦ Excess O ≦ 0.15% Excess O ≦ 0.25-0.5 × Y 2 O 3 (%) can be recrystallized from a temperature of 1300 ° C. or less I understand.

FIG. 4 is a photograph of the metal structure, FIG. 4 (a) is a conventional bamboo structure, FIG. 4 (b) is an enlarged view thereof, and FIG. 4 (c) is an inventive steel 2 according to the present invention. 4 (d) is an enlarged view of FIG. According to the present invention, since the conventional bamboo-like structure disappears and a recrystallized structure is formed, it is understood that the anisotropy of the creep rupture strength is eliminated.

[0041]

[Table 1]

[0042]

[Table 2]

[0043]

[Table 3]

[0044]

As described above, according to the present invention, an oxide dispersion-strengthened ferritic steel having both excellent swelling resistance, excellent internal pressure creep rupture strength and uniform elongation in the circumferential direction can be provided. Furnace members, particularly members used at high temperatures of about 700 ° C. and under high stress, such as fuel cladding tubes, can be extended in life.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram showing a region that can be recrystallized by an excess oxygen amount and Y ₂ O ₃ .

FIG. 2 is a graph comparing the creep rupture strengths of a conventional steel and an inventive steel.

FIG. 3 is a graph comparing the circumferential uniform elongation of the conventional steel and the inventive steel.

4 is a microscopic metallographic view of a conventional material and an inventive material by an optical microscope. FIG. 4 (a) shows a conventional bamboo-like structure, and FIG. 4 (b)
Is an enlarged view thereof, FIG. 4 (c) is a recrystallized structure according to the present invention,
FIG. 4D is an enlarged view of FIG.

──────────────────────────────────────────────────の Continued on the front page (72) Inventor Shigeharu Ukai 4002 Narita-cho, Oarai-machi, Higashiibaraki-gun, Ibaraki Power reactor and nuclear fuel development corporation Oarai Engineering Center (72) Inventor Kenki Inoue Oarai-cho, Higashiibaraki-gun, Ibaraki 4002 Narita-cho, Oarai Engineering Center, Power Reactor and Nuclear Fuel Development Corp. (72) Koichi Okada 4002 Narita-cho, Oarai-machi, Higashiibaraki-gun, Ibaraki Pref. 4002 Narita-cho, Oarai-cho, Higashiibaraki-gun, Japan Power reactor and nuclear fuel development corporation Oarai Engineering Center (72) Inventor: Yasuyuki Fujiwara 2222-1 Ikeda, Ogami-cho, Kakogawa-shi, Hyogo Pref. Inventor Takanari Okuda 2222-1 Ikeda, Onoe-cho, Kakogawa City, Hyogo Prefecture Inside Kobe Steel, Ltd. 72) Inventor Kazutaka Asabe 4-5-33 Kitahama, Chuo-ku, Osaka-shi, Sumitomo Metal Industries, Ltd. (72) Inventor Yuyoshi Yamamoto 4-5-33 Kitahama, Chuo-ku, Osaka-shi, Sumitomo Metal Industries, Ltd. ( 56) References JP-A-1-287252 (JP, A) JP-A-63-186853 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C22C 38/00-38/60 C22C 33/02 103

Claims (2)

(57) [Claims] (1) In weight%, Cr: 7 to 18%, 1 / 2W + Mo: 0.
1 to 3%, Ti: 0.10 to 1.0%, the balance between yttria and yttria in an oxide dispersion-strengthened ferritic steel in which yttria is dispersed in a ferrite-based metal matrix mainly composed of Fe-Cr composed of Fe and unavoidable impurities. The amount of oxygen (Excess O)
In weight percent, in the range of _{0.10% <Y 2 O 3 ≦} 0.30% 0.03% ≦ Excess O ≦ 0.15% Excess O ≦ 0.25-0.5 × Y 2 O 3 (%), and that it has a recrystallized structure Oxide dispersion strengthened ferritic steel with excellent internal pressure creep strength and ductility. Here, the excess oxygen amount is the total oxygen amount (Tota
l O) from which the oxygen (O in Y ₂ O ₃ ), which is bound to yttrium as yttria (Y ₂ O ₃ ), is excluded from calculation ([Exc
ess O] = [Total O] − [O in Y ₂ O ₃ ]). 2. In weight%, Cr: 7 to 18%, 1 / 2W + Mo: 0.
1 to 3%, Ti: 0.10 to 1.0%, and a method for producing an oxide dispersion-strengthened ferritic steel in which yttria is dispersed in a ferritic metal matrix mainly composed of Fe-Cr composed of Fe and unavoidable impurities. Yttria and excess oxygen (Exce
Re at 1300 ° C. or less while adjusting the range of ss O) 0.10% by weight percent of _{<Y 2 O 3 ≦ 0.30%} 0.03% ≦ Excess O ≦ 0.15% Excess O ≦ 0.25-0.5 × Y 2 O 3 (%) A method for producing an oxide dispersion-strengthened ferritic steel having excellent internal pressure creep strength and ductility characterized by performing a crystal heat treatment. Here, the excess oxygen amount is as defined in claim 1.