JP2767169B2 - Fe-Cr-Ni-Si based shape memory alloy with excellent intergranular corrosion resistance and stress corrosion cracking resistance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a Fe-Cr-Ni-Si based shape memory alloy having excellent grain boundary corrosion resistance and stress corrosion cracking resistance, and relates to a grain boundary resistance in nitric acid of a nuclear fuel reprocessing plant or the like. Fe-Cr-Ni-Si excellent in corrosion resistance and stress corrosion cracking resistance and intergranular erosion resistance in high temperature and high pressure pure water such as light water reactor
It is intended to obtain a system shape memory alloy.

[0002]

2. Description of the Related Art Shape memory alloys undergo plastic deformation at a predetermined temperature near a martensitic transformation point, and are then heated to a predetermined temperature equal to or higher than a temperature at which the alloy is inversely transformed into its parent phase. An alloy exhibiting the property of recovering its shape before plastic deformation. By subjecting this shape memory alloy to plastic deformation at a predetermined temperature, the crystal structure of the alloy changes from its parent phase to martensite.

[0003] When the alloy subjected to plastic deformation as described above is heated to a predetermined temperature equal to or higher than the temperature at which the matrix is inversely transformed into the parent phase, martensite is inversely transformed into the original parent phase. The alloy exhibits shape memory characteristics, whereby the plastically deformed alloy returns to its original shape before plastic deformation. Many non-ferrous shape memory alloys have been known as alloys having such shape memory characteristics (for example, “Shape Memory Alloys”, edited by Teruyasu Funakubo, 1984 Sangyo Tosho).

[0004] Among these conventional non-ferrous shape memory alloys, Ni-Ti and Cu-based shape memory alloys have already been put into practical use, and pipe joints, clothing, medical equipment, actuators and the like have been used. It is manufactured using a non-ferrous shape memory alloy, and other technologies for applying the shape memory alloy to various uses have been actively developed in recent years.

[0005] However, from the viewpoint of application to structural members, Ni-Ti based shape memory alloys generate remarkable hydrogen compounds when used in high-temperature pure water, for example, and cannot be used in such an environment. It is. Further, a Cu—Zn—Al based shape memory alloy has insufficient corrosion resistance. In addition, these non-ferrous shape memory alloys are expensive and are therefore limited in terms of economy. Under these circumstances, iron-based shape memory alloys that are less expensive than non-ferrous shape memory alloys are being developed, and instead of non-ferrous shape memory alloys that are economically limited, the application range of iron-based shape memory alloys Expansion is expected.

[0006] By applying plastic deformation, the iron-based shape memory alloy transforms martensite from its parent phase into fct (face-centered tetragonal), bct (body-centered cubic) and hcp ( It is an iron-based shape memory alloy that undergoes plastic deformation to transform from its parent phase to dense hexagonal ε-martensite and has excellent corrosion resistance. 77554 (hereinafter referred to as “first
Conventional technology) has been proposed. That is, according to the first prior art, Cr: 5.0 to 20.0 wt%, Si: 2.0
88.0 wt%, at least one element selected from the group consisting of: Mn: 0.1 to 14.8 wt%, Ni: 0.1 to 20.0 wt%
%, Co: 0.1 to 3.0 wt%, Cu: 0.1 to 0.3 wt%, N:
It is a system containing 0.001 to 0.400% by weight, and has excellent shape memory characteristics and corrosion resistance.

[0007] Japanese Patent Application Laid-Open No. 2-301514 discloses a high Mn-based shape memory alloy having a high Cr content and improved corrosion resistance.
Cr: 10-17 wt%, Si: 3.0-6.0 wt%, Mn: 6.0-6.0
25.0 wt%, Ni: 7.0 wt% or less, Co: 2.0 to 10.0 wt%
In addition, an alloy system further containing Ti, Zr, V, Nb, Mo, Cu and the like has been proposed (hereinafter, referred to as "second conventional technology").

On the other hand, as an iron-based alloy having excellent stress corrosion cracking resistance, for example, BEWILDE, CORROSION-NACE (1986), V
ol.42, No.11, p.678. That is, this report includes C
r: 17.0 to 19.0 wt%, Si: 0.35 to 4.79 wt%, N
i: 8.83 to 9.07 wt%, Mn: 1.30 to 1.53 wt%, C
u: 0.009 to 0.20 wt%, N: 0.011 to 0.040 wt%
%, Mo: 0.019 to 0.21 wt%, and exhibit an excellent stress corrosion cracking resistance in high-temperature pure water (hereinafter referred to as "third prior art").

[0009]

When a shape memory alloy is used in nitric acid such as a nuclear fuel reprocessing plant and in high-temperature pure water (primary cooling water) such as a light water reactor, the shape memory characteristics and the resistance to the shape memory are as characteristics of the alloy. It is necessary to be excellent in intergranular corrosion resistance and stress corrosion cracking resistance, but none of the above-mentioned prior arts satisfy such requirements.

[0010] The iron-based shape memory alloy disclosed in the first prior art is an iron-based alloy to which both Cr and Si are added for the purpose of improving shape memory characteristics and corrosion resistance. , Co and N are added to the alloy, but have the following problems. In other words, although it is a shape memory alloy with excellent corrosion resistance,
The corrosion resistance was evaluated by a two-year atmospheric exposure test, and the above-mentioned intergranular corrosion resistance in nitric acid and the stress corrosion cracking resistance in high-temperature pure water cannot always be sufficient. As can be seen in the examples, the basic alloy system is Fe-13Cr
-6Si type and Fe-18Cr-2Si type are roughly classified. The former alloy type has a Cr content of 15.1 wt% or less, and the latter alloy type has a Si content of 2.8 wt% or less. is there. For this reason,
The effects of improving the intergranular corrosion resistance in nitric acid and the stress corrosion cracking resistance in high-temperature pure water, which are expected as effects of the addition of Cr and Si, are insufficient.

In the first prior art, the C and N contents are reduced.
Although the content is limited to 0.1 wt% or less, according to the study of the present inventors, in the case of an alloy system in which the total content of C and N exceeds 0.01 wt%, processing which is indispensable for enhancing shape memory characteristics is required. Heat treatment (for example, 500-700 ° C. after deformation at room temperature)
Heat treatment), the Cr carbide or Cr
Due to the lack of Cr from the grain boundaries accompanying the precipitation of nitrides at the grain boundaries, or the segregation of C or N at the grain boundaries even when the precipitates are not present, the grain boundaries in nitric acid Deterioration of corrosiveness and stress corrosion cracking resistance in high temperature pure water. However, in order to make the total content of C and N 0.01% by weight or less in the alloy component system defined by the prior art, there is no other means but to use expensive raw materials in the current production technology, which is extremely costly. Become.

Further, in the first prior art, Co is added as an optional element, but as described in Examples, the Co content is 1.0 wt% or more. Therefore, it is unsuitable for use in high-temperature pure water (primary cooling water) in the field of nuclear power from the viewpoint of activation, and its application range is limited.

The iron-based shape memory alloy disclosed in the second prior art has a high Cr content for the purpose of improving corrosion resistance, and further contains Ti, Zr, V and Nb to enhance the shape memory characteristics. Although it is of the Mn type, the second prior art has the following problems. That is, first, the content of Cr is 10-1.
Although the content is 7 wt%, in the example, the Cr content is 16 wt%.
%. For this reason, the effect of improving the intergranular corrosion resistance in nitric acid and the stress corrosion cracking resistance in high-temperature pure water, which are expected as effects of the addition of Cr, are insufficient.

Further, since the content of Mn is 6.0 wt% or more, stress corrosion cracking resistance in high-temperature pure water is deteriorated due to an increase in nonmetallic inclusions, and intergranular corrosion resistance in nitric acid is also improved. to degrade. Furthermore, since the Co content is 2.0 wt% or more, it is not suitable for use in high-temperature pure water (primary cooling water) in the field of nuclear power from the viewpoint of activation, and its application range is limited.

Further, the alloy disclosed in the third prior art is an alloy excellent in stress corrosion cracking resistance, but has an Si content of 2.9.
The alloys are roughly classified into alloys with wt% or less and alloys with Si content of 3.8 wt% or more. Since the former has a Si content of 2.9 wt% or less, the intergranular corrosion resistance and the stress corrosion cracking resistance under the above-mentioned environment are insufficient. In the latter, the ratio of the total content of austenite-forming elements to the total content of ferrite-forming elements is not appropriate, so that the shape memory properties are insufficient.

Accordingly, iron having excellent shape memory characteristics, intergranular corrosion resistance and stress corrosion cracking resistance, which can be used in nitric acid such as a nuclear fuel reprocessing plant or in high-temperature pure water (primary cooling water) such as a light water reactor. Although there is a strong demand for the development of a base shape memory alloy, such an iron base shape memory alloy has not been obtained yet.

[0017]

SUMMARY OF THE INVENTION The present invention has been studied to solve the above-mentioned technical problems in the prior art, and it has been found that any of shape memory characteristics, intergranular corrosion resistance and stress corrosion cracking resistance can be achieved. It has succeeded in providing an excellent Fe-Cr-Ni-Si based shape memory alloy.

When plastic deformation is applied to an hcp-type iron-based shape memory alloy at a predetermined temperature, the phase of the alloy becomes its parent phase,
That is, austenite is transformed into ε martensite. The alloy whose matrix was transformed to ε martensite in this manner was then transformed into an austenite transformation point (hereinafter “Af
Point)) and heating to a temperature near the Af point,
The ε-martensite is transformed back into its parent phase, ie austenite, so that the plastically deformed alloy recovers its original shape before plastic deformation.

In order for the hcp-type iron-based shape memory alloy to exhibit excellent shape memory characteristics, the following conditions must be satisfied.

(1) Before subjecting the alloy to plastic deformation at a predetermined temperature, the parent phase of the alloy needs to be mainly composed of austenite. The above-mentioned predetermined temperature, when plastic deformation is applied to the alloy at that temperature,
The temperature at which the parent phase can be transformed into ε martensite.

(2) The stacking fault energy of austenite must be low. Further, by subjecting the alloy to plastic deformation, it is necessary to transform from its parent phase to only ε martensite and not to α ′ martensite.

(3) The yield strength of austenite must be high. Furthermore, when plastic deformation is applied to the alloy, no slip deformation must occur in the crystal structure of the alloy.

The present invention satisfies each of the above conditions,
In addition, the present invention has succeeded in solving the problems of the prior art as described above, and is as follows.

(1) In wt%, Cr: 16.0 to 21.0%, Si:
3.0 to 7.0%, Ni: 11.0 to 21.0%, Ti: 0.01 to 1.0%, Zr: 0.01 to 2.0%, Hf: 0 .
01-2.0%, V: 0.01-1.0%, Nb: 0.01-2.0
%, Ta: 0.01 to 2.0%, and Niwt% ≧ ｛0.67 (Cr + 1.2 (Si + Ti + Zr +
Hf + V + Nb + Ta))-3｝ wt% and 0.02wt% ≦
｛Ti + V + 0.5 (Zr + Nb) +0.25 (Hf + Ta)｝ wt% ≦ 2.0
An Fe-Cr-Ni-Si based shape memory alloy excellent in intergranular corrosion resistance and stress corrosion cracking resistance, which is wt%, with the balance being Fe and unavoidable impurities.

(2) Cr: 16.0 to 21.0% in wt%, Si:
3.0 to 7.0% Ni: contained 11.0 to 21.0%, yet, Mn:. 0 1 ~5.0% , Cu: 0.1~1.0%, N: 0 .001-
0.1%, Mo: 0.1 to 3.0%, W: 0.1 to 3.0%, and Ti: 0.01 to 1.0. %, Zr: 0.01 to 2.0%, Hf: 0.01
22.0%, V: 0.01 to 1.0%, Nb: 0.01 to 2.0%,
Ta: 0.01 to 2.0% and any one or more of them, and (Ni + 0.5Mn + 0.06Cu + 0.002 (C + N)) wt% ≧ ｛0.67 (Cr + 1.2 (Si + Ti + Zr + Hf + V + Nb + Ta) + Mo + W) -3｝ wt%, and 0.02wt% ≦ {Ti + V + 0.5 (Zr + Nb) +0.25 (Hf + Ta)} wt% ≦ 2.0wt%, with the balance being Fe and inevitable impurities. Fe-Cr-Ni-Si based shape memory alloy with excellent intergranular corrosion resistance and stress corrosion cracking resistance.

[0026]

The reasons why the chemical composition of the iron-based shape memory alloy of the present invention is limited to the above range are as follows.

(1) Cr: Cr has the effect of lowering the stacking fault energy of austenite and increasing the yield strength of austenite, improving the shape memory characteristics.
Further, Cr has an effect of improving the intergranular corrosion resistance and stress corrosion cracking resistance of the alloy. This Cr content is 16.0wt%
If it is less than 1, desired results cannot be obtained for these effects, so the lower limit was set to 16.0 wt%. On the other hand, the content of Cr is 21.
Since exceeding 0 wt% is economically disadvantageous, the Cr content should be limited to the range of 16.0 to 21.0 wt%.

(2) Si: Si has the effect of lowering the stacking fault energy of austenite and increasing the yield strength of austenite, and thus improves the shape memory characteristics. Further, Si also has the effect of increasing the intergranular corrosion resistance and stress corrosion cracking resistance. However, if the Si content is less than 3.0% by weight, desired effects cannot be obtained in the above-described operation. on the other hand,
If the Si content exceeds 7.0 wt%, the ductility of the alloy is significantly reduced, and the hot workability and cold workability of the alloy are significantly deteriorated. Therefore, the Si content is 3.0 to 7.0.
Limited to the range of wt%.

(3) Ni: Ni is a powerful element that forms austenite, and this Ni has the function of mainly converting the parent phase of the alloy to austenite before plastic deformation is applied to the alloy. If the Ni content is less than 11.0% by weight, the desired effects due to the above-described effects cannot be obtained.
The lower limit was 11.0 wt%. On the other hand, Ni content is 21.0wt%
Exceeds the transformation point of ε-martensite (hereinafter “Ms point”).
) Significantly shifted to a low temperature range, whereby the temperature at which plastic deformation of the alloy was applied was significantly lowered, and the shape memory property was deteriorated. Therefore, the upper limit was set to 21.0 wt%.
Therefore, it is necessary to limit the Ni content within the range of 11.0 to 21.0 wt%.

In the present invention, at least one of the following elements can be added in addition to Cr, Si and Ni described above.

(4) Mn: Mn is a powerful element that forms austenite, and this Mn has an action of mainly turning a parent phase before plastic deformation is applied to the alloy into austenite. However, if the Mn content is less than 0.1 wt%, such an effect cannot be obtained properly, while the Mn content is 5.0 wt%.
%, The intergranular corrosion resistance is deteriorated, and the formation of the σ phase is remarkably facilitated to deteriorate the shape memory characteristics.
The upper limit was 5.0 wt%. That is, the Mn content is 0.1 to 5.0.
It is necessary to limit it to the range of wt%.

(5) Cu: Cu is an austenite-forming element, and Cu has an effect of mainly converting the parent phase of the alloy to austenite before plastic deformation is applied.
The addition of a small amount of Cu has an effect of improving the pitting corrosion resistance of the alloy. However, if the Cu content is less than 0.1% by weight, desired effects due to these effects cannot be obtained.
On the other hand, if the Cu content exceeds 1.0% by weight, the formation of ε martensite is hindered, and the shape memory characteristics deteriorate. The reason is that Cu has an effect of increasing the stacking fault energy of austenite, and therefore, the Cu content should be limited to the range of 0.1 to 1.0 wt%.

(6) N: N is an austenite-forming element, and this N has the function of mainly converting the parent phase of the alloy to austenite before plastic deformation is applied to the alloy.
Furthermore, the addition of a small amount of N improves the pitting resistance of the alloy,
It has the effect of increasing the yield strength of austenite. If the N content is less than 0.001 wt%, these effects cannot be obtained properly. On the other hand, if the N content exceeds 0.100 wt%, nitrides of Cr and Si tend to be formed, and Deteriorates the shape memory characteristics. At the same time, the intergranular corrosion resistance in nitric acid and the stress corrosion cracking resistance in high-temperature pure water deteriorate, and Ti, Zr, Hf, V, Nb, and Ta described below are added within the scope of the present invention. However, sufficient improvement cannot be obtained. Therefore, the N content was limited to the range of 0.001 to 0.100 wt%.

(7) Mo: Mo is an element effective for improving intergranular corrosion resistance and stress corrosion cracking resistance.
If the amount is less than wt%, these effects are insufficient.
0.1 wt%. However, the addition exceeding 3.0 wt% deteriorates shape memory characteristics. Therefore, the upper limit is set to 3.0 wt%.

(8) W: W is an element effective for improving intergranular corrosion resistance and stress corrosion cracking resistance.
If it is less than 1 wt%, the effect is insufficient, while 3.0 wt%
Addition of more than 0.1 degrades shape memory properties,
3.0 wt%.

(9) Ti, Zr, Hf, V, Nb, Ta: These elements are all strong C and N stabilizing elements and suppress the precipitation of Cr carbide or Cr nitride at crystal grain boundaries. This has the effect of suppressing deterioration of intergranular corrosion resistance and stress corrosion cracking resistance. Furthermore, the present inventors consider that when the total content of C and N exceeds 0.01 wt%, the total content is sufficiently low (for example, 0.02 wt%) to prevent precipitation of Cr carbide or Cr nitride. However, the processing heat treatment (for example,
Deformation at room temperature, followed by heating to 500 to 700 ° C) results in deterioration of intergranular corrosion resistance in nitric acid and stress corrosion cracking resistance in high-temperature pure water. It has been found that the addition of C and N stabilizing elements is effective. In order to obtain a sufficient improvement effect by the addition of these elements, it is necessary to add 0.01% by weight or more of all elements, and 0.02% by weight ≦ Ti + V + 0.5
(Zr + Nb) +0.25 (Hf + Ta)｝ wt% ≦ 2.0wt%. However, since all of these elements are ferrite forming elements, if added in large amounts, the shape memory characteristics deteriorate and the hot workability and weldability also deteriorate. Therefore, the upper limit of Ti and V is 1.0 wt%. The upper limit of the element is 2.0 wt%.

(10) P and S which are impurity elements, if contained in large amounts, degrade hot workability and toughness. Therefore, it is necessary that both of them be 0.1 wt% or less. As for C, which is an impurity element, the content of Ti, Zr,
Even if Hf, V, Nb, and Ta are added within the range of the present invention, sufficient improvement in intergranular corrosion resistance in nitric acid and stress corrosion cracking resistance in high-temperature pure water cannot be obtained. It must be 0.1 wt% or less. When N is contained as an impurity element, it must be limited to 0.1 wt% or less for the same reason as that of C described above. Considering the problem of activation in an environment such as high-temperature pure water (primary cooling water) typified in the field of nuclear power, Co, which is an impurity element, is desirably 0.1 wt% or less.

(11) The ratio of the total content of austenite-forming elements to the total content of ferrite-forming elements:
In the present invention, as described above, it is absolutely necessary that the parent phase of the alloy be mainly composed of austenite before plastic deformation is applied to the alloy at a predetermined temperature.
Therefore, in the present invention, it is necessary to satisfy the following expression in addition to the above-mentioned limitation on the chemical component composition. Ni wt% ≧ {0.67 (Cr + 1.2 (Si + Ti + Zr + Hf + V + Nb + Ta))-3} wt% (Ni + 0.5Mn + 0.06Cu + 0.002 (C + N)) wt% ≧ {0.67 (Cr + 1.2 (Si + Ti + Zr + Hf + V + Nb + Ta) + Mo + W) -3} wt% That is, by satisfying the above equation, the parent phase of the alloy before subjecting the alloy to plastic deformation at a predetermined temperature is
It can be mainly austenite.

[0039]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Specific embodiments of the present invention will be described. The present inventors prepared an alloy steel according to the present invention (No. 1 to 20) as shown in Table 1 below and a comparative alloy steel (No. 21 to 31) having a chemical composition outside the scope of the present invention. It was melted in a melting furnace under vacuum and cast into ingots. The ingot obtained in this way is 1100 to 1200 ° C.
, And then hot-rolled to a thickness of 12 mm to obtain a specimen. The shape memory characteristics, intergranular corrosion resistance and stress corrosion cracking resistance of each specimen were examined by the tests described below. Table 2 shows the results of these tests.
As shown in FIG.

[0040]

[Table 1]

[0041]

[Table 2]

Each characteristic in Table 2 is as follows.

(1) Shape Memory Characteristics A round bar-shaped test piece having a diameter of 6 mm and a distance between gauges of 30 mm was cut out from each of the present invention Nos. 1-20 and comparative alloys Nos. 21-30. Each of the test pieces cut as described above was subjected to a tensile strain of 4% at -196 ° C,
Next, each test piece was heated to a predetermined temperature (300 ° C. or more) at or above the Af point and near the Af point, the tensile strain was applied, and the distance between the gauge points of each test piece after heating was measured. Based on the measurement results between the benchmarks, the shape recovery rate was calculated by the following formula, and the shape memory characteristics of each match were evaluated (Ms point was slightly different for each match. Although different, the optimal temperature for applying plastic deformation was -196 ° C, which was standardized for each test).

The results of the above shape memory property test are shown in the column of "shape memory property" in Table 2 and in FIG. 1 for alloys Nos. 1 to 8 of the present invention and comparative alloys Nos. 21 to 24. The evaluation criteria of the shape memory characteristics used here are as follows. ◎: Shape recovery rate is 70% or more ：: Shape recovery rate is 30 to less than 70% ×: Shape recovery rate is less than 30% Here, L ₀ : distance between the first gauges of the test piece L ₁ : distance between the gauges of the test piece after tensile strain is applied L ₂ : distance between the gauges of the test piece after heating

In FIG. 1, the horizontal axis is the Si content (wt%),
The vertical axis indicates the Cr content (wt%). The range surrounded by the dotted line in FIG. 1 indicates that the Cr content and the Si content are within the range of the present invention, Has a shape recovery rate of 7
0% or more was recognized, and the mark “O” indicates that the shape recovery rate was 30.
% Or more and less than 70%, "X"
The mark indicates that the shape recovery rate is less than 30%. As is apparent from FIG. 1, the Ni content in the range of 11.0 to 21.0 wt%, the Cr content in the range of 6.0 to 21.0 wt%, and 3.0.
A match with a Si content in the range of 0-7.0 wt%
It shows excellent shape memory properties.

The comparative alloy "23" containing 1.8 wt% Si, which is outside the scope of the present invention, has very low shape memory properties. The comparative alloy “24” containing 2.6 wt% of Si shows the shape memory characteristics marked with “○”, but is inferior to the evaluation of the alloy of the present invention, and has the intergranular corrosion resistance or the Poor stress corrosion cracking. Although the comparative alloys "21" and "22" which are out of the range of the present invention in FIG. 1 also have sufficient shape memory characteristics, they have poor intergranular corrosion resistance or stress corrosion cracking resistance described later. .

(2) Intergranular Corrosion Resistance For each of the alloy Nos. 1 to 20 of the present invention and the comparative alloys Nos. 21 to 30, plate specimens having a thickness of 4 mm, a width of 20 mm, and a length of 100 mm were cut out, 4% tensile strain at 600 ° C
Was repeated three times. Thereafter, a plate test piece for corrosion test having a thickness of 2 mm, a width of 15 mm and a length of 20 mm was cut out from the plate test piece, the surface was wet-polished to # 600, and immersed in boiling 40% nitric acid. After immersion for 5 days, the cross section of the test piece was observed with an optical microscope, and the maximum erosion depth of the grain boundary was examined to evaluate the intergranular corrosion resistance. The results of the intergranular corrosion resistance test are shown in the column of “Intergranular corrosion resistance” in Table 2 above and FIG. 2 for the alloys Nos. 1 to 8 of the present invention and comparative alloys Nos. 21 to 24.

The evaluation criteria for intergranular corrosion resistance are as follows. ：: Maximum pit depth is less than 10 μm ○: Maximum pit depth is 10 μm or more and less than 30 μm ×: Maximum pit depth is 30 μm or more

As is clear from Tables 1 and 2 and FIG. 2, the Ni content in the range of 11.0 to 21.0 wt%, 16.0 to
Alloys with a Cr content in the range of 21.0 wt% and a Si content in the range of 3.0-7.0 wt% show excellent intergranular corrosion resistance.

Less than 16.0 wt% Cr outside the scope of the present invention
Alloys "21" and "22" containing 1.8% by weight
Comparative alloys containing Si "23", comparative alloys "25", "26" and "27" with insufficient or unadded amounts of C and N stabilizing elements such as Ti, and 5.
All of the comparative alloys "28" to which Mn was added in excess of 0 wt% had poor intergranular corrosion resistance. The evaluation of the comparative alloy “24” having a Si content of 2.6 wt% is indicated by “○”, but the stress corrosion cracking resistance described later is inferior. Mo and W are each 3.0
Comparative alloys “29” and “30” added at wt% or more are:
Although it shows excellent intergranular corrosion resistance, its shape memory properties are not sufficient.

(3) Resistance to Stress Corrosion Cracking For each of the alloys Nos. 1 to 20 of the present invention and comparative alloys Nos. 21 to 30, a plate-like test piece having a thickness of 4 mm, a width of 20 mm and a length of 100 mm was cut out and subjected to room temperature. 4% tensile strain at 600 ° C
Was repeated three times. Thereafter, the test piece shown in FIG. 3 was cut out from the plate-like test piece, and each of the test pieces cut out in this manner was set in a holder 3 as shown in FIG. 4, and then a strain gauge 2 was attached to the test piece 1. Then, the fastening bolt 4 is pushed in to apply a strain corresponding to a predetermined stress (yield stress). Then, the specimen was immersed under the stress corrosion cracking test conditions shown in the following Table 3, and after 3000 hours, the presence or absence of cracks on the test piece surface was confirmed, and the stress corrosion cracking resistance of each alloy was evaluated.

[0052]

[Table 3]

The results of the stress corrosion cracking test described above are also shown in the column of “stress corrosion cracking resistance” in Table 2 and the alloy No. 1 of the present invention.
FIG. 3 shows Nos. 8 to 8 and Comparative Alloy Nos. 21 to 24. The evaluation criteria for crack generation in the stress corrosion cracking test are as follows. ：: No crack generation was observed (no crack). X: Crack generation is observed (crack generation).

FIG. 5 shows a Fe—Cr—Ni according to an embodiment of the present invention.
FIG. 5 shows the effect of Cr and Si contents on intergranular corrosion resistance in the Si-based shape memory alloy.
The horizontal axis shows the Si content (wt%), and the vertical axis shows the Cr content (wt%).
5), the range surrounded by the dotted line in FIG. 5 indicates that the Cr content and the Si content are within the range of the present invention. Further, in FIG. 5, a mark “○” indicates that no crack was recognized, and a mark “x” indicates that a crack was recognized.

As apparent from Tables 1 and 2 and FIG. 5, the Ni content in the range of 11.0 to 21.0 wt%, 16.0 to
All alloys with a Cr content in the range of 21.0 wt% and a Si content in the range of 3.0-7.0 wt% show excellent stress corrosion cracking resistance. On the other hand, the comparative alloy “21” containing less than 16.0 wt% of Cr, which is outside the scope of the present invention.
And “22”, a comparative alloy “2” containing 1.8 wt% Si
3 ", comparative alloy" 24 "containing 2.6 wt% Si, comparative alloys" 25 "," 26 "and" 2 "in which the amounts of C and N stabilizing elements such as Ti are insufficient or not added.
7 "and the comparative alloy" 28 "to which Mn was added in excess of 5.0 wt% both had poor stress corrosion cracking resistance. Also, Mo
Alloy “2” containing 3.0 wt% or more of W and W, respectively.
"9" and "30" show excellent stress corrosion cracking resistance, but have insufficient shape memory properties.

[0051]

As described above, the Fe-Cr of the present invention can be used.
-Ni-Si based shape memory alloys are excellent in shape memory properties, intergranular corrosion resistance in nitric acid, and stress corrosion cracking resistance in high-temperature pure water. High temperature pure water in nitric acid and light water reactors (primary cooling water)
It is suitable for use as a structural material in the middle and other fields, and is also suitable for use in a wide range of fields other than nuclear power, such as materials for pipe joints and various fastening devices, actuators, and biomaterials. This is an invention that has effects such as cost reduction and is industrially significant.

[Brief description of the drawings]

FIG. 1 is a table showing the effect of Cr and Si contents on shape memory characteristics in an Fe—Cr—Ni—Si shape memory alloy according to an example of the present invention.

FIG. 2 is a chart showing the effect of the Cr and Si contents on the intergranular corrosion resistance in the Fe—Cr—Ni—Si based shape memory alloy according to the example of the present invention.

FIG. 3 is an explanatory diagram of a shape of a stress corrosion cracking test piece used in an example of the present invention.

FIG. 4 is an explanatory diagram of a method of applying a stress to a stress corrosion cracking test piece used in an example of the present invention.

FIG. 5 is a table showing the influence of Cr and Si contents on stress corrosion cracking resistance in the Fe—Cr—Ni—Si shape memory alloy according to the example of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Test piece 2 Strain gauge 3 Holder 4 Tightening bolt

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Naotake Ito 1-1-1, Wadazakicho, Hyogo-ku, Kobe City, Hyogo Prefecture Inside Mitsubishi Heavy Industries, Ltd.Kobe Shipyard (72) Inventor Toru Inazumi Marunouchi, Chiyoda-ku, Tokyo 1-2, Nippon Kokan Co., Ltd. (72) Inventor Yutaka Moriya 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Japan (72) Inventor Haruo Suzuki 1-2-1, Marunouchi, Chiyoda-ku, Tokyo No. Nippon Kokan Co., Ltd. (72) Inventor Katsumi Masamura 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nippon Kokan Co., Ltd. (72) Takemi Yamada 1-1-2, Marunouchi, Chiyoda-ku, Tokyo Japan (58) Field surveyed (Int. Cl. ⁶ , DB name) C22C 38/00-38/60

Claims (2)

(57) [Claims] 1. Cr: 16.0 to 21.0%, and Si: 3. wt%.
0 to 7.0%, Ni: 11.0 to 21.0%, Ti: 0.01 to 1.0%, Zr: 0.01 to 2.0%, Hf: 0.01
22.0%, V: 0.01 to 1.0%, Nb: 0.01 to 2.0%,
Ta: One or more of 0.01 to 2.0% is contained, and Niwt% ≧ ｛0.67 (Cr + 1.2 (Si + Ti + Zr + Hf + V + Nb + T)
a))-3% by weight and 0.02% by weight ≦ Ti + V + 0.5 (Zr + Nb) +0.25 (Hf + Ta))% by weight ≦ 2.0% by weight, with the balance being Fe and inevitable impurities. Fe-Cr-Ni-Si based shape memory alloy with excellent intergranular corrosion resistance and stress corrosion cracking resistance. 2. Cr: 16.0 to 21.0%, Si: 3. wt%
0 to 7.0% Ni: contained 11.0 to 21.0%, yet, Mn:. 0 1 ~5.0% , Cu: 0.1~1.0%, N: 0.001 ~
0.1%, Mo: 0.1 to 3.0%, W: 0.1 to 3.0%, and one or more of Ti: 0.01 to 1.0. %, Zr: 0.01 to 2.0%, Hf: 0.01
22.0%, V: 0.01 to 1.0%, Nb: 0.01 to 2.0%,
Ta: contains any one or more of 0.01 to 2.0%, and (Ni + 0.5Mn + 0.06Cu + 0.002 (C + N)) wt% ≧ ｛0.67 (Cr + 1.2 (Si + Ti + Zr + Hf + V + Nb + Ta) + Mo + W) -3｝ wt%, and 0.02wt% ≦ {Ti + V + 0.5 (Zr + Nb) +0.25 (Hf + Ta)} wt% ≦ 2.0wt%, with the balance being Fe and inevitable impurities. Fe-Cr-Ni-Si based shape memory alloy with excellent intergranular corrosion resistance and stress corrosion cracking resistance.