JPS60165362A - 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 stress 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:Highly corrosion resistant and high yield strength 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, 24.0-30.0% Cr, 4.0-9.0% 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 Fe. In this case, the areal ratio of the delta-ferrite phase in the metal structure of this stainless steel is set to 30-70% as an essential 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 5US304 steel or 5US829J1.5C51
3AS 5C314A15FSA (Steel Fou
nders'5oci'ety of Amer'i
ca) Stainless steel having a two-phase structure of ferrite and austenite, such as CD-4MCu, is used.

Austenitic stainless steels such as SUS 804 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 (CC1), and are susceptible to pitting corrosion and 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 lack pitting corrosion resistance and crevice corrosion resistance under severe corrosive environments, particularly in the presence of increased chlorine ions and carbon dioxide gas and hydrogen sulfide gas, often causing corrosion damage. Furthermore, 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 provides pitting corrosion resistance in a corrosive environment at high temperature and high pressure (e.g., 300°C, 6000 psi), especially in an environment containing chloride, carbon dioxide gas, or hydrogen sulfide gas. Provided is a fully austenitic duplex stainless steel having excellent stress corrosion cracking resistance, hydrogen sulfide cracking resistance, etc., and high strength and high ductility.

The duplex stainless steel of the present invention has co, os% or less, Si
02-2.0%, Mn 0.2-2.0%, Cr24
．． 0-80.0%, Ni 4.0-9.0%, Mo1
．． 0-5.0%, Cu O, 5-8.0%, Co 0.
2-4.0%, N0105-0.3%, remainder substantially F
(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% or less C is an austenite-forming element and has a remarkable effect on improving strength, but if the content is too large, chromium carbides tend to precipitate, resulting in a decrease in the Cr concentration near the carbides. , 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 at least 0 to deoxidize molten steel and ensure castability.
．． 2% is required. However, the inclusion of a large amount deteriorates the toughness.
Since it also impairs 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: 24.0 to 80.0% Cr has a remarkable effect on improving corrosion resistance, especially intergranular corrosion resistance, and also contributes to improving stress corrosion cracking resistance. Also,
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, the required amount of ferrite (area ratio of 30% or more) is required if the steel does not contain 24.0% or more of Cr, depending on the Ni content described below.
difficult to secure. Therefore, from the viewpoint of corrosion resistance and ferrite content, the lower limit of the Cr content is set to 24.0%.

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,
The amount of ferrite exceeds 70% due to the correlation with the amount of Ni,
It loses the balance with the austenite phase in the two-phase structure, impairing corrosion resistance, especially resistance to pitting corrosion and crevice corrosion. Therefore, the upper limit of the Cr content is set to 30.0%.

Ni: 4.0 to 9.0% 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 occupied amount is less than 4.0%, these effects will be insufficient. From the relationship with the amount of Cr, it is necessary to contain 4.0% or more in order to keep the ferrite amount 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 (not only is it economically disadvantageous, but the austenite phase in the two-phase structure becomes excessive and the amount of the two-phase structure increases. loss of balance.

Therefore, the upper limit of the Ni content is 9.0%. In addition, C
Since o is also an austenite-forming element like Ni, in order to take into consideration the contribution of co to austenite formation and to ensure the lower ferrite content (80%), the Ni content should be set at 9.
．． It is required that it does not exceed 0%.

Mo: 1.0 to 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 effect of improving corrosion resistance will be saturated and embrittlement during casting due to precipitation of σ phase will become significant.
The upper limit is 0%.

Cu: 0.5 to 3.0% Cu improves corrosion resistance, particularly stress corrosion cracking resistance, in an environment containing low concentrations of chlorine ions, and strengthens the austenite phase as a 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 3.0% is required. Upper limit.

Co: 0.2-4.0% Co is the element that most strongly characterizes the steel of the present invention. Co
Like Ni, C is a substitutional austenite forming element, but in the case of Ni, there is a tendency for the yield strength to decrease by 0.2% with the addition of C.

It was found that the addition of , on the contrary, resulted in an improvement in yield strength by 0.2%. 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 improves 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 as a solid solution in the matrix. Therefore, it has been found that Co has the effect of suppressing the agglomeration of precipitates, which is a major problem with conventional duplex stainless steels, such as σ-phase brittleness and 475°C brittleness, especially in welding. This greatly contributes to alleviating the brittleness caused by these precipitates in the heat-affected zone. In addition, C.

Since, like Ni, is an austenite-forming element, in order to secure the ferrite amount (30 to 70%) specified in the present invention, it is necessary to reduce the Ni amount by considering the increase in the amount of austenite phase due to the addition of co. can.

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%.

NO, 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 present in the two-phase structure in CrXN4. M.

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 rXMo and the like into the austenite phase at high concentrations. That is, in normal duplex stainless steel, Cr, MO, S
Ferrite-forming elements such as i form the ferrite phase, and C
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 due to the presence of N, are distributed in high concentrations in the austenite phase. This increases the corrosion resistance of the duplex stainless steel, particularly 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 4 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.

Perfumes 2 to 4, 6, 7, 14 and 15 are examples of the present invention, and Perfumes 1.5 and 8 to 13 are comparative examples.

Among the comparative examples, perfume notes 10 and 11 are each JIS G345.
9 5US329J1 and 5US316, fragrance 12 is JIS G51.21 5C5I4A.

Moreover, perfume 13 is 5FSA CD-4MCu.

Perfumes 1 to 9 and 12 to 15 are molded centrifugally cast tubes (outer diameter L
85mm, length 600cm) was used as the test material, and fragrance adjustment 10.1
1 used a commercially available product. In addition, each sample material is all 110
After being held at 0°C for 1 hour per wall thickness of 25 mm, heat treatment was performed by cooling with water.

[A] 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 4.6.7.14.15 of the invention examples are the same as those of the comparative example perfume 1 (components other than N and the amount of ferrite are as specified in the invention). (within the range).

It is recognized that the degree of increase is proportional to approximately 3.5 k'j/ma/0.1%N when the amount of ferrite is kept constant at approximately 50%. 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 deviation from 70%), and perfume 8 (ferrite amount 28%) with insufficient ferrite amount has a 02% yield strength of 54.
”akg/-, while the amount of ferrite is excessive (74%
)'s perfume 9 has a shock absorption energy of 11.8 kg・m
and is inferior to that of 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%.

Further, as described later, if the amount of ferrite is too large, the toughness after aging will be significantly reduced, so 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%.

By comparing perfumes 3.14.15 of the present invention example,
The amount of N was kept constant around 0.18%, and the amount of ferrite was 50%.
%, the addition of Co causes a noticeable increase in 0.0%.
A 2% increase in yield strength was observed, and the degree of increase was approximately 2kq.
It was found that there is a proportional relationship equivalent to /rd/1%Co. Moreover, the tensile strength also increases. Moreover, compared to these improvements in strength, the decrease in ductility and toughness is small.

One of the outstanding effects of Co addition in duplex stainless steel is that it can suppress the decline in ductility and toughness and increase strength.
It is one.

In addition, in the example of the present invention, the conventional material 5US816 (fragrance 11)
, 5C514A (Fragrance 12), CD-4MCu (Fragrance 1)
As is clear from the comparison with 8), the mechanical properties, especially 0.
It shows far superior strength in terms of 2% yield strength and tensile strength. This is mainly due to the control of the amount of ferrite in the steel of the present invention and the synergistic effect of the addition of Co and N as alloying elements.

(2) Toughness after thermal aging Table 8 and Figure 1 show the Charpy impact test (2NMv notch, 0°C) when thermally aged at 475°C.
It shows the absorbed energy (kti・m) by

First, the perfume 3 of the example of the present invention is made of conventional duplex steel 5US8.
Compared to 29J1 (steel type 10), 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 Comparative Example, which has a very low N content of 0.02%, Perfume 8 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. Furthermore, perfume 1 exhibits deterioration in toughness as described above (when subjected to thermal aging, but the degree of deterioration is much better than that of perfume 10, which is a conventional material, and is a key feature of the present invention). This clearly shows the influence of Co, which is one of the effective effects of Co. Therefore, the inventive steel 15 suffers from the synergistic effect of the addition of Co and N, which reduces the deterioration of shock absorption energy after aging. Even after aging for 1000 hours, the absorption energy is very high at 11.9 #722 as seen in Table 3.In this way, NSC.

The addition of 475° is a weak point of conventional duplex stainless steel.
It has been discovered that C is an extremely effective element for improving brittleness. - Incidentally, perfume 9 (73%) with an excessive amount of ferrite has 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%.

[B] Weldability Regarding the inventive example perfume 2.3.4.6.7.14.15, a groove shape with a groove angle of 20 and a root thickness of 1.6 was prepared, and the first layer and the second layer were prepared. The second layer is TIG welded, the third layer to the final layer is butt welded using covered arc welding,
As a result of post-weld non-destructive inspection and liquid penetration inspection of the cut surface of the welded part, it was confirmed that there were no defects such as cracks, that the weldability was good, and that there were no problems as a piping material.

(q Corrosion resistance (1) Test 1 (pitting corrosion test) Ferric chloride (as specified in ASTM G48 A method)
Pitting corrosion test with Fe(J?3) solution (-Totai II
mlersion Ferric Chloride
Te5t) was carried out, and the results shown in Table 4 were obtained. Examples of the present invention (perfume 2.8.4.6.7.14.15) are conventional materials 5US829J1 (perfume 10) and 5US816.
(Perfume 11), SC514A (Perfume 12) and CD-4
It exhibits much better pitting corrosion resistance than MCu (Fragrance 13), and no corrosion weight loss is observed.

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. .

In addition, Perfume 1 and Perfume 2, which have a small amount of N, and conventional materials 5US829J1 (Perfume io) and 5US316 (Perfume 1
1), SC5I 4A (Fragrance 12) and CD-4MCu
As is clear from the comparison with (Fragrance 18), it was found that the contribution of Co addition to the improvement of pitting corrosion resistance was significant.

Furthermore, from the results of Invention Example 4 and Comparative Example 5, the maximum amount of N was 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 (Ferric Chlo-r
ide Crevice Te5t) and the fourth
The results shown in the table were obtained. Invention steel (Mei number 2.3.4.6.
7.14.15) is the conventional material 5US829J1 (
Meiban io), 5US316 (Meiban 11) and 5C514
A (light number 12), compared to CD-4MCu (light number 13),
Shows exceptional crevice corrosion resistance. It is clear that this is mainly due to Co and N as alloy components.

Furthermore, as is clear from the comparison with Aki No. 1, the effect of adding N on improving the crevice corrosion resistance is remarkable, and the corrosion weight loss is reduced by about 115 to 1/6.

Furthermore, looking at the results of Akira No. 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 of the amount of ferrite in the steel of the present invention should be defined as 80 to 70%. I understand that.

Ai No. 1 and Ai No. 2 with a small amount of N, and the conventional material 5US8
29J1 (number 10), 5US816 (number 11), 5
C6I4A (light number 12) and CD-4MCu (light number 18)
), it is clearly recognized that the contribution of Co addition to the improvement of pitting corrosion resistance is significant.

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

(3) Resistance to stress corrosion cracking The results of a stress corrosion cracking test conducted in a boiling 42% magnesium chloride (MgCn 2 ) solution using a constant load method are shown in FIG.

The example of the present invention (number 3) is SUS 829Jl, which is a conventional material.
(Mei No. 10), 5US816 (Mei No. 11), CD-4
It can be seen that it has much better stress corrosion cracking resistance than MCu (Amber No. 18). For example, the rupture time of SUS 829J1 is approximately 2 for a load stress of 80 to 9/-.
In contrast, the time for light number 3, which is an example of the present invention, is approximately 80 hours, which is a significant improvement.

The effect of adding N in the steel of the present invention becomes clear by comparing Bright No. 1 and Bright No. 3. It can be seen that stress corrosion cracking resistance is improved by adding N when the amount of ferrite is at approximately the same level (approximately 50% for both numbers 1 and 3). Therefore, the steel of the present invention is suitable for applications requiring stress corrosion cracking resistance in an environment where Cβ- is present.

Looking at the influence of the ferrite content, the stress corrosion cracking resistance of Aki No. 8, which has a low ferrite content of 28%, is 5US829J1 (
It is only about the same level as that of Meiban 10). In order to ensure stress corrosion cracking resistance, the amount of ferrite must be at least 30%.

On the other hand, light number 9, which has a high ferrite content of 74%, exhibits stress corrosion cracking resistance superior to the present invention example, light number 3, 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 for Aki No. 1, it is recognized that the addition of Co has a significant effect on stress corrosion cracking resistance.

In other words, the N content of light number 1 is very low at 0.02%, but light number 10 (SUS329J1), light number 18 (CD-4MCu)
) shows superior 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 Co in the present invention.

Therefore, the fact that Ai No. 3 and Ai No. 15 exhibit excellent stress corrosion cracking resistance is due to the above-mentioned effect of adding Co and N as alloying elements and controlling the level of ferrite content within the range of 30% to 70%. It depends on the synergistic effect of

(4) Corrosion fatigue strength Figure 3 shows the results of the Ono rotary bending fatigue test in artificial seawater (testing machine rotation speed: 800 rpm).

Artificial seawater was prepared according to methods prescribed by the United States Maritime Administration.

Ai number 8, which is an example of the present invention, is CD-4, which is a conventional two-phase alloy.
It has superior fatigue strength in seawater compared to MCu (Fragrance 13) and 5US816 (Fragrance 11), which is an austenitic stainless steel. In particular, the corrosion fatigue strength of perfume 13 at 4 x 107 cycles is about 22 ktj/-, whereas that of the example of the present invention is about 82 kq/-, which is about 10
A: Shows a high value of g/-.

Furthermore, by comparing Perfume 1 and Perfume 13, the effect of Co becomes clear. In other words, it can be seen that the amount of N in Perfume 1 is at a very low level of 0.03%, which is effective in improving the fatigue strength of Perfume 13.

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 indicate that Perfume 3 has high corrosion fatigue strength in seawater because the addition of N and Co as alloying elements has a synergistic effect. Shock absorption energy (k'j −
m) 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 the drawing]

Figure 1 is a graph showing changes in impact properties due to thermal aging, Figure 2 is a graph showing stress corrosion cracking resistance, Figure 3 is a graph showing corrosion fatigue strength in rotary bending fatigue tests, and Figure 4 is a graph showing changes in impact properties due to thermal aging.
The figure is a photomicrograph substituted for a drawing showing the metallographic structure of the steel of the present invention. Agent Patent Attorney Miyazaki Shin VIII Part Figure 1 Procedural Amendment April 26, 1985 1, Case Description 1988 Patent Application No. 021388 2, Title of Invention Highly Corrosion-Resistant Highly Potassium-Resistant Duplex Stainless Steel 3. Relationship with the case of the person making the amendment Patent applicant 4, Agent 6, Number of inventions increased by amendment None 7, Subject of amendment 8, Contents of amendment (1) “Detailed description of the invention” in the specification Column +11 1st
In the 6th line directly below 1, the text ``Figure 4'' has been corrected to ``Figure 1 3''. (2) Delete r(1) J at the beginning of the 7th line directly below No. 13. (3) From line 3 directly below No. 15 to line 11 on page 17, the text "(2) Toughness after thermal aging... shall be the upper limit." has been deleted. (4) On page 18, line 6, "Table 4" was corrected to "Table 3." (5) On page 19, line 8, replace “Table 4” with “Table 3.”
Corrected. (6) Corrected "Figure 2" to "Figure 1" in lines 6-5 directly below No. 20. (7) In the 5th line directly below No. 22, "Figure 3" was corrected to "Figure 2." (8) “Table 3” on page 27, line 1, “Table 3” on page 27, lines 2-3
Thermal aging... (kg-m)J and the table on the same page have been deleted. (9) On page 28, line 1, "Table 4" was corrected to "Table 3." (II) "Brief explanation of one drawing of the specification" column (1) On page 29, lines 13-14, "Figure 1 is a graph showing..."
Deleted a certain thing. (2) On page 29, line 14, replace “Figure 2” with “Figure 1”
In line 15 of the same page, ``Figure 3'' was corrected to ``Figure 2,'' and on line 16 of the same page, ``Figure 4'' was corrected to ``Figure 3.'' (l[[) As shown in the attached drawing sheet (Deletion of Figure 1, correction of Figure 2 to Figure 1, Figure 3 to Figure 2, and Figure 4 to Figure 3 as indicated in red) . (that's all)

Claims (1)

[Claims] (1) C0.08% or less, Si 0.2-2.0%,
Mn0, 2-2.0%, Cr24.0-80.0%, N
i4.0~9.0%, Mo1.0~5.0%, Cu
O, 5-8.0%, Co 0.2-4.0%, NO, 0
5 to 0.8%, and the remainder substantially consists of Fe, and the area ratio of the δ-ferrite phase in the metal structure is 30 to 70%.