JPS60165363A - Highly corrosion resistant and high yield strength two- phase stainless steel - Google Patents

Abstract

PURPOSE:To provide the titled two-phase stainless steel excellent in stress corrosion cracking resistance and pitting resistance in a corrosive environment, by respectively prescribing the contents of C, Si, Mn, Cr, Ni, Mo, Cu, Co and N and providing a delta-ferrite phase with a specific areal ratio. CONSTITUTION:The titled two-phase stainless steel consists of, on a wt. basis, 0.08% or less C, 0.2-2.0% Si, 0.2-2.0% Mn, 19.0% or more - less than 24% Cr, 3.0-8.0% Ni, 1.0-5.0% Mo, 0.5-3.0% Cu, 0.2-4.0% Co, 0.05-0.3% N and the remainder of substantially Fe. The areal ratio of the delta-ferrite phase in the metal structure of this stainless steel is set to be 30-70% as a prerequisite condition. This stainless steel has excellent pitting resistance, stress corrosion cracking resistance and hydrogen sulfide cracking resistance and high strength and high ductility in a corrosive environment, especially, in an environment containing chloride, CO2 or H2S, for example, under a high temp. and high pressure condition of 300 deg.C and 6,000psi.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to duplex stainless steel, which has improved resistance to corrosion such as stress corrosion cracking and pitting corrosion, particularly in corrosive environments containing chlorides, carbon dioxide, etc., and improved strength, ductility, etc. It has improved mechanical properties.

As a corrosion-resistant material, austenitic stainless steel such as SUS 304 steel or 5US329J1.5C5
18A, 5C3I4A, 5FSA (Steel pou
derS'5ociety of America)
Stainless steel having a two-phase structure of ferrite and austenite, such as CD-4MCu, is used.

Austenitic stainless steels such as 5US304 steel exhibit excellent corrosion resistance due to the main alloy components Cr and Ni, but a major drawback is that they are susceptible to stress corrosion cracking in environments containing chlorine ions (Cl-), and pitting corrosion is a major drawback. It also has very low resistance to localized corrosion such as crevice corrosion.

On the other hand, those with a two-phase structure of ferrite and austenite phases have excellent general corrosion resistance, and the characteristics of the two phases combine to provide appropriate strength and toughness, as well as relatively good weldability. In recent years, various chemical industry plants,
Widely used as a material for seawater equipment. However, these materials also lack pitting corrosion resistance, crevice corrosion resistance, etc. under severe corrosive environments, especially in the presence of increased chlorine ions, carbon dioxide gas, and hydrogen sulfide gas, and often cause corrosion damage. It is known that the resistance to stress corrosion cracking and sulfide corrosion cracking is insufficient, and there are many cases in which early failure occurs. For example, in order to secure energy, oil and natural gas wells are forced to mine in increasingly poor environments, and as the wells get deeper, they are exposed to more corrosive substances such as chlorine ions, carbon dioxide gas, and hydrogen sulfide gas. The environment in which oil is used has become significantly harsher, with factors increasing, temperature and pressure increasing, and carbon dioxide gas, seawater, etc. being injected into wells to restore oil wells. Conventional materials cannot withstand such usage environments, and it is difficult to guarantee stability and sufficient service life as structural materials.

The present invention has been made in view of the above, and is capable of resisting pores in corrosive environments at high temperatures and high pressures (e.g., AOO°C, 6,000 psi), especially in environments containing chloride, carbon dioxide, or hydrogen sulfide gas. Provided is a ferritic-austenitic duplex stainless steel having excellent corrosion resistance, stress corrosion cracking resistance, hydrogen sulfide cracking resistance, etc., as well as high strength and high ductility.

The duplex stainless steel of the present invention has C: 008% or less, Si
: 0.2-2.0%, Mn: 0.2-2.0%
, Cr: 19.0% or more, less than 24.0%, Ni: 3.
0-8.0%, Mo: 1.0-5.0%, Cu:
0.5-3.0%, CO: 0.2-4.0%, N: 0
．． 05 to 0.3%, and the remainder substantially consists of Fe (component content is weight %), and the δ-ferrite phase in the metal structure occupies 30 to 70% in terms of area ratio.

The reasons for limiting the composition of the steel of the present invention are as follows.

c: o, os% C is an austenite-forming element and has a remarkable effect on improving strength, but if the content is too large, chromium carbide tends to precipitate, and the Cr concentration near the carbide increases. As a result, resistance to localized corrosion such as pitting corrosion, crevice corrosion, and intergranular corrosion decreases, and stress corrosion cracking resistance deteriorates. Therefore, the upper limit is set at 0.08%.

Si: 0.2-2.0% Si is required to be at least 0.2% in order to deoxidize molten steel and ensure castability. However, since a large amount of content impairs toughness and weldability, the upper limit is set at 2.0%.

Mn: 0.2-2.0% Mn is contained in an amount of about 0.2% during normal deoxidation and desulfurization processes, and is an effective element for stabilizing the austenite phase of the steel base. A content of up to 2% is sufficient for this purpose and need not exceed it. Therefore, 0.2 to 2.
Set to 0%.

Cr: 19.0% or more, less than 24.0% Cr has corrosion resistance,
It is particularly effective in improving intergranular corrosion resistance, and also contributes to improving stress corrosion cracking resistance. Further, Cr is a ferrite-forming element, and increases strength by forming a ferrite phase in a two-phase structure. In the steel of the present invention, it is difficult to secure the required amount of ferrite (area ratio of 30% or more) unless it contains 19.0% or more of Cr, which is correlated with the amount of Nl described later. Therefore, from the viewpoint of corrosion resistance and ferrite content, the lower limit of the Cr content is set to 19.0%.

On the other hand, if the amount of Cr is too large, the toughness of the steel will be significantly reduced and a hard and brittle σ phase will be formed during casting. Furthermore,
Due to the correlation with the amount of Nl, the amount of ferrite exceeds 70%,
The two-phase structure loses its balance with the austenite phase, impairing corrosion resistance, especially resistance to pitting corrosion and crevice corrosion. Therefore, the Cr content is set to less than 24.0%.

Ni: 3. O~8.0% Ni is an element that stabilizes the austenite phase and improves the toughness of steel. It is also a necessary element from the viewpoint of corrosion resistance. If the content is less than 3.0%, these effects will be insufficient. From the relationship with the above-mentioned Cr content, it is necessary to contain 3.0% or more in order to reduce 4g of Cr to 70% or less.

However, even if a large amount of Ni is added, the effect of improving corrosion resistance and mechanical properties is small compared to the Ni content, which is economically disadvantageous. lose balance.

Therefore, the upper limit of the Nl amount is 8.0%. In addition, C
O is also an austenite-forming element like Ni, so in order to take into consideration the contribution of CO to austenite formation and to ensure the lower limit (30%) of the ferrite amount, the Ni amount should be 8.
．． It is required that it does not exceed 0%.

Mo: 1.0-5.0% MO has a great effect on improving the corrosion resistance of stainless steel. It is particularly effective in improving resistance to pitting corrosion and crevice corrosion. At 1.0% or more, significant improvements in corrosion resistance to non-oxidizing acids, as well as resistance to pitting corrosion, intergranular corrosion and stress corrosion cracking in chloride-containing solutions are observed. However, if a large amount is added, the corrosion resistance improvement effect will be saturated and embrittlement during casting due to the precipitation of the σ phase will become significant, so 5.
The upper limit is 0%.

Cu: 0.5 to 8.0% Cu improves corrosion resistance, particularly stress corrosion cracking resistance, in an environment containing low concentrations of chlorine ions, and strengthens the austenite phase by solid solution. In order to obtain these effects sufficiently, it is necessary to contain at least 0.5%, but if the content is too large, the toughness will decrease due to the formation of intermetallic compounds, so 8.0% is required. Upper limit.

Co: 0.2-4.0% CO is the element that most strongly characterizes the steel of the present invention. C.O.
Like N1, is a substitutional austenite-forming element, but in the case of Ni, its addition tends to lower the 0.2% yield strength, whereas the addition of CO, on the contrary, lowers the 0.2% yield strength. It was found that this resulted in an improvement in As mentioned above, there is a strong demand for duplex stainless steel that has high mechanical strength and corrosion resistance that can withstand the harsh corrosive environment. The addition makes it possible to ensure sufficient mechanical properties to meet this requirement.

It has also been revealed that the addition of Co to duplex stainless steel significantly increases corrosion resistance in environments containing chlorine ions, such as seawater. Furthermore, Co has been found to have the effect of suppressing the agglomeration of precipitates while remaining in solid solution in the matrix.
Therefore, it greatly contributes to alleviating the major problems of conventional duplex stainless steels, such as σ-phase embrittlement and 475° C. embrittlement, especially the embrittlement caused by these precipitates in the heat-affected zone of the weld. Note that since CO is an austenite-forming element like Ni, ferrite R (30 to 70%) specified in the present invention is
In order to ensure this, the amount of Ni can be reduced in consideration of the increase in the amount of austenite phase due to the addition of CO.

The Co content in order to exhibit the above effects is at least 0.
．． 2% is required. The effect increases as the content increases, but sufficient improvement effects on mechanical properties, corrosion resistance, microstructure, etc. can be obtained by adding up to 4.0%.
There is no need to add more than that. CO is an expensive element, and adding more than that is disadvantageous in terms of cost. Therefore, it is set to 0.2 to 4.0%.

N: 0.05-0.3% N is normally treated as a harmful impurity element, but in the present invention it is added within the above range for the purpose of improving strength and corrosion resistance.

Like C, N is a strong austenite-forming element,
Since it is an interstitial solid solution element, it causes strong lattice distortion in the crystal lattice of the steel base, contributing significantly to improving the strength.

In addition, N is included in Cr, Ni, and M in a two-phase structure.

Element C, which affects the distribution ratio of major elements such as to the ferrite phase and austenite phase, and particularly contributes to corrosion resistance.
Corrosion resistance of duplex stainless steel is improved by distributing r, Mo, etc. in high concentration into the austenite phase. That is, duplex stainless steel usually contains Cr, MoS
Ferrite-forming elements such as i form the ferrite phase, and C
5 Austenite-forming elements such as Mn and Ni are distributed in high concentrations in the austenite phase, but as mentioned above, ferrite-forming elements such as Cr and Mo, which contribute to corrosion resistance, are distributed in high concentrations in the austenite phase due to the presence of N. This improves the corrosion resistance of duplex stainless steel, especially its resistance to localized corrosion such as crevice corrosion and pitting corrosion.

In particular, like the steel of the present invention, the Cr and Mo concentrations are high,
In alloy systems where the difference in the distribution ratio between the ferrite phase and the austenite phase is significant, in other words, the degree of segregation is large, the addition of N has the effect of distributing these corrosion-resistant elements to the austenite phase at a higher concentration. Therefore, corrosion resistance, especially local corrosion resistance, is significantly improved.

In order to fully exhibit the above effects, the amount of N is at least 0.
05% is required. The effect increases as the amount of N increases, but if it exceeds 0.3%, it will precipitate as nitrides, which will actually worsen the corrosion resistance. N has a significant effect on improving the strength and corrosion resistance as described above only when it is in a solid solution state. Therefore, the amount of N is set to 0.05 to 0.3%.

The steel of the present invention contains each of the above-mentioned component elements, and the remainder consists essentially of Fe, excluding impurity elements that are unavoidably mixed.

Next, to explain the structure of the steel of the present invention, the steel of the present invention is characterized by having a ferrite-austenite two-phase structure in which the amount of δ-ferrite occupies 30 to 70% in terms of area ratio. Figure 3 shows the organization. The quantitative balance of these two phases ensures mechanical properties that are in harmony with strength and toughness.If the amount of ferrite is less than 30%, the strength will be insufficient, while if it exceeds 70%, Ductility and toughness decrease significantly.

Further, the amount of ferrite in the two-phase structure is closely related to corrosion resistance. That is, the resistance to stress corrosion cracking in a corrosive environment, particularly in an environment containing chlorine ions, is significantly improved when the amount of ferrite is 30% or more. Conversely, in an environment containing hydrogen sulfide (H2S), the amount of ferrite decreases to 70%.
If the value exceeds 0.05, the susceptibility of the ferrite phase to sulfide stress corrosion cracking increases, and selective pitting corrosion, crevice corrosion, etc. of the ferrite phase are likely to occur. Therefore, also from the viewpoint of corrosion resistance, the amount of ferrite is specified to be 30 to 70%.

The quantitative balance in this two-phase structure is achieved by adjusting the composition of each alloy component within the specified range.

After casting, the steel of the present invention is subjected to solution treatment according to a conventional method. The heat treatment is performed at a temperature of 1000 to 1200, for example.
This is achieved by heating and maintaining the temperature at °C and then rapidly cooling it (for example, by water cooling).

Examples Mechanical property measurements, welding tests, and various corrosion resistance tests were conducted on test steels having the compositions and ferrite amounts shown in Table 1.

Perfume 2 to 4.6.7.14.15 are examples of the present invention, Perfume 1.
5 and 8 to 13 are comparative examples. Among the comparative examples, fragrance notes 10.11 are each JIS G3459SUS829J1.
and 5US316, fragrance 12 is JIS G5121
5C814A, and perfume 13 is 5FSA CD-4MC
It is u.

Perfumes 1 to 9 and 12 to 15 are molded centrifugally cast tubes (outer diameter 1
35+++m, length 600mm) was used as the sample material, and perfume 1
0.11 used a commercially available product. Each sample material was heat treated by holding it at 1100° C. for 1 hour per wall thickness of 25 mm and then cooling it with water.

[ADI Mechanical Properties (1) Table 2 shows the room temperature tensile properties, hardness, and absorbed energy by Charpy impact test.

The mechanical properties and 0.2% yield strength of perfumes 2 to 4.6.7.14 and 15 of the invention examples are different from those of the comparative example perfume 1 (the composition of components other than N and the amount of ferrite are as specified in the invention). (within the range). The degree of increase is approximately 8 when the amount of ferrite is kept constant at N50%.
．． It is recognized that there is a proportional relationship corresponding to 3 Ky 7mm2/0.1%N. This improvement in mechanical properties shows the remarkable effect of N addition in duplex stainless steel.

Perfume 8.9 has a ferrite amount within the specified range of the present invention (30~
This is an example of deviating from the 0.2% yield strength of perfume 8 (27% ferrite amount) with insufficient ferrite amount.
．． 8 Kg/mm2, which is low, while the amount of ferrite is excessive (
73%) fragrance 9 has a shock absorption energy of 12.7.
Kg・? It is inferior to that of 7Z and the example of the present invention. For this reason, the amount of ferrite in duplex stainless steel is also a major factor that affects mechanical properties, and from the viewpoint of strength, it is required to be 30% or more, and from the viewpoint of ensuring toughness, the upper limit is 70%. Also, as mentioned later, if the amount of ferrite is too large,
Since the toughness decreases significantly after aging, from this point of view as well, the upper limit of the amount of ferrite in the steel of the present invention is set at 70%.

Also, by comparing perfumes 3.14.15 of the present invention example,
Ni was kept constant at around 0.18%, and the amount of ferrite was 50%.
%, a remarkable increase in 0.2% yield strength was observed with the addition of Co, and the degree of increase was 2Kg/%.
It was found that there is a proportional relationship corresponding to ynmV1%Co. The tensile strength also increases. However, compared to these improvements in strength, the decrease in ductility and toughness is small. Two points are that you can improve strength while suppressing the decline in ductility and toughness.
This is one of the very excellent effects of Co addition in phase stainless steel.

In addition, the present invention example and the conventional material 5US316 (Perfume 11)
, SC5l 4A (Perfume 12), and CD-4MCu (Perfume 13), it shows far superior strength in mechanical properties 9, 0.2% proof stress, and tensile strength. This is mainly due to the synergistic effect of controlling the amount of ferrite and adding Co and N as alloying elements.

(2) Toughness after thermal aging Table 3 shows the absorbed energy (1 (1.77L)) in the Charpy impact test (2 mm V notch, 0°C) when thermally aged at 475°C. Perfume 3 of the invention example is made of 5US329J1 (steel type 10), which is a conventional duplex steel.
), the decrease in toughness after aging at 475°C for 1000 hours is extremely small. That is, in the steel of the present invention, 475°C embrittlement, which is the greatest weakness of conventional duplex stainless steels, is significantly improved.

Further, when comparing Perfume 3 of the present invention example with Perfume 1 of the comparative example, which has a very low N content of 0.02%, Perfume 3 shows excellent toughness against thermal aging. Therefore, N has a significant improvement effect on the deterioration of toughness due to thermal aging in duplex stainless steel.

Perfume 1 exhibits deterioration in toughness when subjected to thermal aging at 201° as described above, but the degree of deterioration is greater than that of the conventional material Perfume 1.
This is much better than 0, clearly demonstrating the influence of CO, which is one of the main effects of the present invention.

Therefore, due to the synergistic effect of the addition of CO and N, the inventive steel 15 has very little tendency for the impact absorption energy to deteriorate after aging, which can be seen in Table 3 even after aging for 1000 hr. It retains an extremely high absorbed energy of 12.7 kg・m. Thus, it has been found that the addition of N and CO is an extremely effective element for improving the 475°C brittleness, which is the weak point of conventional duplex stainless steel.

Incidentally, perfume 9 (73%) with an excessive amount of ferrite showed a significant decrease in toughness. The presence of a ferrite phase is advantageous in terms of stress corrosion cracking resistance, but from the perspective of toughness, an upper limit should be set in consideration of ensuring safety as a structural material. The upper limit is 70%.

CB) Weldability Regarding the steel part 2.3.4.6.7.14.15 of the present invention example, a groove shape with a groove angle of 20° and a root thickness of 1.6 mm was prepared, and the first layer and the second layer were prepared. The second layer was welded with TtG, and the third to final layers were butt welded using covered arc welding, and as a result of non-destructive inspection after welding and liquid penetration inspection of the cut surface of the weld, there were no defects such as cracks. It was confirmed that the weldability was good and there were no problems as a piping material.

[CD Corrosion resistance fl) Test 1 (pitting corrosion test) Ferric chloride (as specified in ASTM G48 A method)
Pitting corrosion test using FeC13) solution (TotaiImme)
rsion Ferric Chloride Te5
t) was carried out, and the results shown in Table 4 were obtained. The example of the present invention (perfume 2.3.4.6.7.14.15) is a conventional material 5
LjS829J1 (perfume 10), 5US316 (perfume 1
1), 5C3I4A (Fragrance 12) and CD-4MCu (
It exhibits much better pitting corrosion resistance than Perfume No. 13), and no corrosion loss is observed at all.

Furthermore, as is clear from the comparison with Perfume 1, which has a very low amount of N, the contribution of N to the improvement of pitting corrosion resistance is significant, clearly demonstrating the significance of N addition in the present invention.

Furthermore, Perfume 1, Perfume 2, which has a small amount of N, and 5, which is a conventional material.
US829J1 (Fragrance 10), 5US816 (Fragrance 1)
1), 5C8I4A (Fragrance 12) and CD-4MC
As is clear from the comparison with u (Fragrance 18), it was found that the addition of Co made a significant contribution to improving the pitting corrosion resistance.

Furthermore, from the results of Invention Example 4 and Comparative Example 5, the maximum amount of N is 0.3%.
is sufficient, and it is recognized that adding more than this does not improve the pitting corrosion resistance.

(2) Test 2 (crevice corrosion test) Crevice corrosion test using ferric chloride solution specified in ASTM G48 B method
eCrevice Te5t) was carried out, and the results shown in Table 4 were obtained. Invention steel (perfume 2.3.4.6.7.14
．． 15) is a conventional material 5US329J1 (perfume 10
), SUS 316 (Fragrance 11) and 5O814A (
Perfume 12) and CD-4MCu (Perfume 13) show significantly superior crevice corrosion resistance. It is clear that this is mainly due to Co and H as alloy components. Furthermore, as is clear from the comparison with Perfume 1, the effect of the addition of N on improving the crevice corrosion resistance is remarkable, and it is recognized that this reduces the corrosion weight (legally 1/~115).

Furthermore, looking at the results of Fragrance 8.9, the amount of ferrite is also a factor that affects the crevice corrosion resistance, and from this point of view, the appropriate range for the amount of ferrite in the steel of the present invention is defined as 30 to 70%. is recognized.

Perfume 1 and Perfume 2 with low N content and 5US32 which is a conventional material
9J1 (Fragrance 10), SUS 316 (Fragrance 11),
When compared with 5C5I4A, (Fragrance 12) and CD-4MCu (Fragrance 13), it is clearly recognized that the addition of C○ makes a significant contribution to the improvement of pitting corrosion resistance.

Also, from the results of Invention Example 4 and Comparative Example 5, the amount of N is 0.3% at maximum.
is sufficient, and it is recognized that adding more than this does not improve the crevice corrosion resistance.

(3) Resistance to Stress Corrosion Cracking The results of stress corrosion cracking tests using a constant load method in a boiling 42% magnesium chloride (MgC12) solution are shown in FIG.

The example of the present invention (perfume 3) is the conventional material 5US329Jl (
Perfume 10), 5US316 (Perfume 11), CD-4MC
It can be seen that it has much better stress corrosion cracking resistance than u (Fragrance 13). For example, 30KFI/Wei m2
The rupture time of 5US329J1 is about 2 hours for the applied stress of 2 hours, whereas that of Perfume 3, which is an example of the present invention, is about 50 hours, which is a significant improvement.

The effect of adding N in the steel of the present invention becomes clear by comparing Perfume 1 and Perfume 3. The amount of ferrite is N when the level is the same (approximately 50% for both fragrances 1.3)
It can be seen that stress corrosion cracking resistance is improved by adding . Therefore, the steel of the present invention is suitable for applications requiring stress corrosion cracking resistance in an environment where CI- is present.

Looking at the influence of the amount of ferrite, the stress corrosion cracking resistance of perfume 8, which has a low amount of ferrite of 27%, is 5US329J1 (
It is only on the same level as perfume 10). In order to ensure stress corrosion cracking resistance, the amount of ferrite must be at least 30%.

On the other hand, perfume 9 with a high ferrite content of 73% exhibits stress corrosion cracking resistance superior to perfume 3 of the present invention example, but on the other hand, as mentioned above, it is inferior in toughness and ductility after aging, so the ferrite content is low. The upper limit is set at 70%.

Next, looking at the results of Perfume 1, the effect of Co on stress corrosion cracking resistance is clearly recognized. In other words, in perfumery 1, the amount of N is 0.
．． Very low at 03%, but fragrance level 10 (SUS329J1)
, Perfume 1 B (CD-4MCu) shows more resistance to stress corrosion cracking. This is clearly an effect of Co when viewed from the constituent elements, and clearly shows the significance of the addition of C in the present invention.

Therefore, it is clear that Perfume 3 and Perfume 15 exhibit excellent stress corrosion cracking resistance, which is a major feature of the present invention.
It depends on the synergistic effect of adding 01N as an alloying element and controlling the level of ferrite content in the range of 30% to 70%.

(4) Corrosion fatigue strength Figure 2 shows the results of the Ono rotary bending fatigue test in artificial seawater (testing machine rotation speed: 300 orpm).

Artificial seawater was prepared according to methods prescribed by the US Navy.

Perfume 3, which is an example of the present invention, is CD-4, which is a conventional two-phase alloy.
MCu (Fragrance 13) and 5US316, which is an austenitic stainless steel. It has superior fatigue strength in seawater compared to (Fragrance 11). In particular, the corrosion fatigue strength of perfume 13 at 4 x 107 cycles is about 22 Kg/mu, whereas that of the example of the present invention is about 80 Kg/mm2.
This is approximately 8 Kg/mm2 higher.

Furthermore, by comparing Perfume 1 and Perfume 13, the effect of Co becomes clear. In other words, the amount of N in Perfume 1 is at a very low level of 0.03%, and the only difference in composition from Perfume 13 is basically Co. It is recognized that this is effective in improving the corrosion fatigue strength of

Furthermore, by comparing Perfume 1 and Perfume 3, the effect of N becomes clear. This shows that the addition of N is extremely effective in improving the corrosion fatigue strength of a two-phase alloy in an environment containing C1-, and is one of the greatest features of the steel of the present invention.

The above results demonstrate the synergistic effect of the addition of N and Co as alloying elements in Perfume 3 exhibiting high corrosion fatigue strength in seawater.

Table 3 Shock absorption energy when subjected to thermal aging a<g・77
L) As shown in Table 4 and above, the duplex stainless steel of the present invention
Compared to e-Cr-Ni-based duplex stainless steel, it has better general corrosion resistance under harsh operating conditions, especially in environments containing large amounts of corrosive factors such as chlorine ions, hydrogen sulfide, and carbon dioxide, as well as stress corrosion cracking and porosity. It has strong resistance to corrosion and crevice corrosion, and has excellent mechanical properties such as strength and ductility. Therefore, it provides greater durability and stability than conventional materials in applications requiring corrosion resistance and mechanical properties, such as tubing and line pipes for oil, natural gas, and sea water.

[Brief explanation of drawings]

FIG. 1 is a graph showing stress corrosion cracking resistance, FIG. 2 is a graph showing corrosion fatigue strength in a rotating bending fatigue test, and FIG. 3 is a photomicrograph in place of a drawing showing the metallographic structure of the steel of the present invention. Agent Patent Attorney Arata Miyazaki Department Figure 1 12 Figure 5 [ , 4゛ku013 Permission Application No. 021389 2, Title of the invention: Highly corrosion-resistant and highly resistant duplex stainless steel 3, Person making the amendment Relationship to the case: Patent applicant 4, Attorney 8, Contents of the amendment (11, 13th, 6th line directly below r(1) Deleted the text J. (2) Delete the text "(2) Toughness after thermal aging...is the upper limit" from line 3 directly below No. 15 to line 11 of page 17. (3 ) On page 18, line 6, “Table 4” was corrected to “Table 3.” (4) On page 19, line 8, “Table 4” was corrected to “Table 3.” (5 ) "Table 3" on the first line of page 27, [Thermal aging... (kg-m)J] on the second line of the same page and the table on the same page are deleted. " has been corrected to "Table 3."(That's all)

Claims (1)

[Claims] +11 C: 0.08% or less, Si: 0.2-2.0%
, Mn: 0.2~'2.0%, Cr: 19.0
% or more, less than 24.0%, Ni: 3.0 to 8.0%
, Mo: 1.0-5.0%, Cu: 0.5-3.0
%, C○: 0.2-4.0%, N: 0.05-0.3%
, the remainder essentially consists of Fe, and δ in the metal structure
- Highly corrosion-resistant and highly carbon-resistant duplex stainless steel in which the area ratio of ferrite phase is 30 to 70%.