JPH09324246A - Austenitic stainless steel for heat exchanger excellent in high temperature corrosion resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce an austenitic stainless steel for a heat exchanger excellent in oxidation resistance, corrosion resistance, structural stability, weldability and high temp. strength. SOLUTION: This steel is the one having a compsn. contg., by weight, <=0.12% C, <=1.0% Si, <=5.0% Mn, <=0.04% P, <=0.03% S, 14 to 22% Cr, 10 to 25% Ni, 1.0 to 3.5% Al, <=0.02% N and Y, La and Ce by 0 to 0.07% as the total content, furthermore contg. one or >= two kinds among 0.05 to 0.5% Ti, 0.1 to 1.0% V, 0.1 to 1.0% Nb, 0.1 to 1.0 % Zr and 0.5 to 4.0% Cu, and the balance Fe with inevitable impurities and also satisfying the following inequalities I and II: the inequality I: α=(1.5Si+Cr+3Al)-(0.5Mn+Ni+30C+30 N)<9 and the inequality II: MCI=C/5-12×(Zr/91+Nb/93+Ti/48+V/68)>=0, where MCI denotes the index showing the ratio of MC type carbide in the whole carbide.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to an austenitic stainless steel applicable to a boiler for thermal power generation, a steel pipe for a heat exchanger in a chemical plant and the like.

[0002]

2. Description of the Related Art In recent years, mainly for the purpose of improving the efficiency of thermal power plants, proposals of new types of combined power plants, renewal of conventional plants, and high-temperature / high-pressure boiler operating conditions have been made. Increasing the operating temperature and pressure of boilers tends to continue to be promoted, and the ultra-supercritical pressure operation planned for general coal-fired thermal power plants will be applied to the operation of combined power plant steam boilers in the near future.
The scope of application is expected to expand in the future.

Conventionally, 18Cr-8Ni type stainless steel has been widely used as a tube material as a steel type for high temperature members such as heat exchangers, and its mainstream is JIS SUS304.
H (18Cr-10Ni-C), and further T
There are JIS SUS321H and the like to which i is added, and basically, 18-8 system is used in consideration of long-term phase stability.

[0004]

However, in order to cope with the high temperature and high pressure of the thermal power plant in the future as described above, at a high temperature of 600 to 800 ° C. for a long time of ten years or more. It is required to have a corrosion resistance at a level that can be used for a long time and high-temperature strength that exceeds the currently used 18Cr-8Ni type stainless steel, and at present, almost no usable steel type is found.

From the viewpoint of improving the steam oxidation resistance in the high-pressure water contact portion on the inner surface, Al and Si which are effective elements for improving the high temperature oxidation resistance disclosed in JP-A-53-63210 are used. The contained Cr-Ni type stainless steel is good. Not only that, it has been found that such Al- and Si-containing steels also have extremely good corrosion resistance to harsh environments in the presence of coal ash and the like. However, such a steel is insufficient as a pipe material for a pressure-resistant heat exchanger in terms of high-temperature strength, and further, its properties are not sufficient in terms of maintaining long-term phase stability and weldability.

Japanese Unexamined Patent Publication (Kokai) No. 6-271992 discloses a high Al content steel in consideration of oxidation resistance, but such a high Al content steel has the same problem. Furthermore, as another solution, JIS SUS3
It is also possible to use an austenitic stainless heat resistant steel based on a high alloy 25Cr-20Ni system typified by 10S, to which Nb, N, etc. are added to improve high temperature strength and long-term phase stability. Showa 57-16497
No. 1 and No. 164972 disclose steels aiming at such a thing. However, such Cr-N
i-type steel is basically expensive and is 600 to 800 ° C.
In the above, the sigma phase tends to precipitate after 500 to 5000 hours of use, and a sharp drop in toughness is feared.

Another problem is that the austenite solidifying component system has a high sensitivity to hot cracking during welding. Therefore, in the current high-temperature steel technology, it is strongly desired to establish an alloy design guideline that satisfies these three points of high-temperature corrosion resistance, long-term phase stability, and weldability at the same time and has high high-temperature strength. There is.

The present invention has been made in view of the above circumstances, and is for a heat exchanger excellent in oxidation resistance or corrosion resistance at 600 to 800 ° C., long-term structural stability, weldability and high temperature strength. It is intended to provide an austenitic stainless steel.

[0009]

Means for Solving the Problems First, the inventors selected inexpensive Al and Si as elements for improving high temperature oxidation resistance and examined the phase stability when they were heated at 600 to 800 ° C. for a long time. , The effect on brittle sigma phase precipitation was investigated. Generally, regarding the suppression of delta ferrite during rolling of austenitic stainless steel, or the amount of delta ferrite, for example, de Long in 1960, Meta.
It has been found that the concept of Cr equivalent and Ni equivalent can be used as proposed in I. Progress. However, the above equation could not be used as it is for predicting the amount of sigma phase precipitation. That is, in the Cr-Ni-based stainless steel, in the state diagram shown in FIG. 1, the precipitation region of the sigma phase is wider than the precipitation region of the delta ferrite. In other words, in order to suppress the precipitation of the sigma phase, It has been empirically revealed that a large amount of expensive Ni must be added to suppress the precipitation of delta ferrite.

Further, when an experiment was conducted on a Si-added system, the sigma phase precipitation region in the state diagram of FIG.
It was clarified that it expands as compared with Cr-Ni-based stainless steel containing no i. However, in the system containing Al, the sigma phase forming ability of Al is smaller than the delta ferrite forming ability. Therefore, the sigma phase precipitation region in the phase diagram of FIG. It was newly found that it is almost equal to the ferrite precipitation area.

That is, when Al is added as an element for improving the oxidation resistance, precipitation of delta ferrite during rolling and 600 to 800 within the range of the components satisfying the formula (1) below.
It was found that both precipitation of sigma phase when heated for a long time at ℃ can be suppressed.

Α = (1.5Si + Cr + 3Al) − (0.5Mn + Ni + 30C + 30N) <9 (1) By suppressing the precipitation of delta ferrite during rolling, ear cracks during rolling can be prevented and yield can be improved.

Next, regarding the weldability, various means for suppressing the hot crack susceptibility were examined. Generally, the hot cracking susceptibility of austenitic stainless steel can be reduced by the presence of several% of delta ferrite during solidification. However, if the amount of Cr is increased to increase the oxidation resistance, the stability of the austenite phase must be increased excessively with respect to delta ferrite in order to suppress the precipitation of sigma phase, and the austenite solidification is completely completed during solidification. And increase the sensitivity to hot cracking. Also,
When the amount of Si is increased, the Ni equivalent must be further increased in order to further suppress the sigma phase precipitation, which increases the hot cracking susceptibility.

However, since Al has a lower sigma phase forming ability than a delta ferrite forming ability, it is not necessary to excessively increase the stability of the austenite phase with respect to delta ferrite in order to suppress sigma phase precipitation. That is, in the system containing Al, even if the Ni equivalent is increased to stabilize the austenite phase as much as possible to sufficiently suppress the precipitation of the sigma phase, delta ferrite is present during the solidification of the weld, and the hot crack sensitivity of the weld is high. It has been found that can be suppressed to an extremely low level.

Regarding the high temperature strength, as a result of examination by conducting a number of laboratory dissolutions on solid solution strengthening, nitride precipitation strengthening, carbide precipitation strengthening, Cu enriched phase precipitation strengthening, intermetallic compound precipitation strengthening, etc. as strengthening factors, Carbide precipitation strengthening and Cu
It was found that the enriched phase precipitation strengthening was the best from the viewpoints of creep rupture strength, long-term toughness, and creep ductility. Therefore, as a result of further intensive studies on these, Cu is added in the range of 0.5 to 4.0%, and carbide forming elements are M ₂₃ C ₆ type which can be precipitated. Iron / Chromium carbide, MC type Z
It has been clarified that r, Nb, Ti, and V carbide satisfying the following formula (2 ′) can most effectively improve the creep rupture strength.

(CasMC) / (CasM ₂₃ C ₆ + CasMC) ≧ 0.2 (2 ′) (wherein CasMC and CasM ₂₃ C ₆ are carbon amounts forming MC type carbide and M ₂₃ C ₆ type carbide, respectively) In this case, in order to satisfy the above equation (2 ′), Z
It is necessary to add r, Nb, Ti and V within the range of the following formula (2).

MCI = C / 5-12 × (Zr / 91 + Nb / 93 + Ti / 48 + V / 68) ≦ 0 (2) (where MCI is an index showing the ratio of MC type carbides in all the carbides) The present invention has been made on the basis of such findings. Firstly, in% by weight, C: 0.12% or less, Si:
1.0% or less, Mn: 5.0% or less, P: 0.04% or less, S: 0.03% or less, Cr: 14 to 22%, Ni:
10-25%, Al: 1.0-3.5%, N: 0.02
% Or less, Y, La, Ce as a total content of 0 to 0.0
7% (including the case of no addition), and Ti:
0.05-0.5%, V: 0.1-1.0%, Nb:
0.1-1.0%, Zr: 0.1-1.0%, and C
u: 0.5 to 4.0% of one or more kinds are contained, the balance is Fe and inevitable impurities, and the following expressions (1) and (2) are satisfied. Provided is an austenitic stainless steel for a heat exchanger, which has excellent high-temperature corrosion resistance.

Secondly, in% by weight, C: 0.12% or less,
Si: 1.0% or less, Mn: 5.0% or less, P: 0.0
4% or less, S: 0.03% or less, Cr: 14 to 22%,
Ni: 10-25%, Al: 1.0-3.5%, N:
0.02% or less, 0 as the total content of Y, La and Ce
~ 0.07% (including no addition), B: 0.001
.About.0.01% and further Ti: 0.05-0.
5%, V: 0.1 to 1.0%, Nb: 0.1 to 1.0
%, Zr: 0.1 to 1.0%, and Cu: 0.5 to
It contains one or more of 4.0% and the balance is F.
Provided is an austenitic stainless steel for a heat exchanger, which is characterized by comprising e and unavoidable impurities and satisfying the following formulas (1) and (2). α = (1.5Si + Cr + 3Al)-(0.5Mn + Ni + 30C + 30N) <9 ... (1) MCI = C / 5-12x (Zr / 91 + Nb / 93 + Ti / 48 + V / 68) ≤0 ... (2)

[0019]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described specifically. The austenitic stainless steel for heat exchangers according to the present invention has a weight percentage of C: 0.12% or less and Si:
1.0% or less, Mn: 5.0% or less, P: 0.04% or less, S: 0.03% or less, Cr: 14 to 22%, Ni:
10-25%, Al: 1.0-3.5%, N: 0.02
% Or less, Y, La, Ce as a total content of 0 to 0.0
7% (including the case of no addition), and Ti:
0.05-0.5%, V: 0.1-1.0%, Nb:
0.1-1.0%, Zr: 0.1-1.0%, and C
u: 0.5 to 4.0% of the composition containing one or more. Moreover, B: 0.001 to 0.
It has a composition containing 01%.

The reasons why each component is contained and the range thereof is limited will be described below. C: C gives the high temperature strength of the parent phase of the steel of the present invention, and is an element effective for phase stability. However, when it is contained in excess of 0.12%, coarse carbides are formed in the form of longitudinal cutting through the crystal grains. Since it precipitates, its content was set to 0.12% or less.

Si: Si is an element effective for deoxidation, but if it exceeds 1.0%, the sigma phase has a large forming ability and it becomes difficult to maintain phase stability. The content was set to 1.0% or less in order to enhance the sensitivity.

Mn: Mn is an element effective for phase stability, but if it exceeds 5.0%, it becomes harmful to the high temperature corrosion resistance, so the content is made 5.0% or less. P: P is an element that segregates at grain boundaries and impairs ductility during rolling, and the smaller the content, the better. Therefore, in order to prevent cracking due to a decrease in ductility during rolling, its content is set to 0.04% or less.

S: S, like P, is an element that segregates at the grain boundaries and impairs ductility during rolling. The smaller the content, the better. Therefore, in order to prevent cracking due to a decrease in ductility during rolling, its content is set to 0.03% or less.

Cr: Cr is important as a basic element that imparts oxidation resistance at high temperatures. When the content is less than 14%, it is 600 even if Al which is effective for high temperature oxidation resistance is contained.
At ˜800 ° C., high temperature oxidation resistance or high temperature corrosion resistance cannot be significantly improved. On the other hand, when the content exceeds 22%, a large amount of expensive Ni is required to maintain the stability of the austenite phase, which impairs the economical efficiency, and has a small contribution to the improvement of high temperature oxidation resistance or high temperature corrosion resistance. Become. Therefore, the Cr content is 14 to
22%.

Ni: Ni is an essential element for obtaining a stable austenite structure. The content thereof needs to be 10% or more in view of the relationship between other contained elements, particularly Cr and Al. On the other hand, if the Ni content exceeds 25%, the effect of stabilizing austenite becomes small, and even if the Ni content is increased, elements such as Cr that improve high temperature oxidation resistance or high temperature corrosion resistance can be greatly increased. become unable. Also, N
When the amount of i is excessively increased, the stability of the austenato phase with respect to the ferrite phase becomes excessively high, and the hot cracking susceptibility during welding is increased. Therefore, the Ni content should be 1
It was set to 0 to 25%.

Al: Al alone forms A in an oxidizing environment.
It forms a very dense protective film of l ₂ O ₃ and is contained as a complex oxide in the presence of Cr oxide to increase the denseness of the oxide. In such a case, the surface protection property is very high, and it is an element that provides excellent high temperature oxidation resistance and high temperature corrosion resistance. However, this improvement in high temperature oxidation resistance and high temperature corrosion resistance has some effect when the Al content is less than 1.0%, but a significant effect cannot be observed.
However, when the Al content is 1.0% or more, such high temperature oxidation resistance and high temperature corrosion resistance are significantly improved. However, if Al exceeds 3.5%, it becomes difficult to maintain phase stability. Therefore, the Al content is 1.0
˜3.5%.

N: Similar to C, N is an element which gives the high temperature strength of the parent phase of the steel of the present invention and is effective for phase stability.
When the content exceeds 02%, a nitride is formed, which is harmful to the toughness, so the content of N is set to 0.02% or less.

Y, La, Ce: Y which is a rare earth element,
La and Ce dissolve in the Al ₂ O ₃ oxide film and increase the general resistance to high temperature oxidation, so one or more of these may be contained. These are 0.07 in total
%, The hot workability is impaired, so the total content is made 0.07% or less. Moreover, since these are added as needed, it is decided to include the case of no addition.

Ti, V, Nb, Zr: Ti, V, N
b and Zr finely disperse and precipitate as carbonitrides, thus contributing to the improvement of high temperature strength, but less than 0.05% in Ti,
If each of V, Nb, and Zr is less than 0.1%, the effect is not sufficient. Further, if added excessively, the amount of undissolved Ti, V, Nb, and Zr carbonitrides after solution heat treatment increases, impairs high-temperature strength, and further deteriorates weldability. The content of these is Ti: 0.0
5 to 0.5%, V: 0.1 to 1.0%, Nb: 0.1
1.0% and Zr: 0.1 to 1.0%.

Cu: Cu is an additive element that finely precipitates in the mother phase as a Cu-rich phase during use at high temperatures and contributes to the improvement of creep strength. However, if it is less than 0.5%, it hardly precipitates, while if it exceeds 4.0%, the fracture ductility is remarkably reduced. Therefore, the content of Cu is set to 0.5 to 4.0%.

As described above, since the carbide precipitation strengthening and the Cu-rich phase precipitation strengthening by Zr, Nb, Ti, V, etc. are effective for increasing the high temperature strength, T
It is decided to add one or more of i, V, Nb, Zr and Cu.

B: B is an element effective in strengthening the grain boundaries and improving the high temperature strength characteristics, but if added in excess, it deteriorates weldability. It was set to 0.001 to 0.01%. In addition, B is added as needed.

Next, the reason for limiting the equation (1) will be described. The main factor that determines the relative stability of the austenite phase and the sigma or ferrite phase that affects sigma brittleness resistance, hot workability, and weldability is the relationship between the Cr equivalent and the Ni equivalent. Each of these is expressed by the following equations.

Cr equivalent = 1.5 Si + Cr + 3Al Ni equivalent = 0.5 Mn + Ni + 30C + 30N When the following formula (1) using these Cr equivalent and Ni equivalent is established, the inventors of the present invention use after 600 to 800 ° C. for a long time. It was also found that there is almost no precipitation of brittle sigma phase, that is, good toughness is maintained even when used for a long time. Further, at the same time, it was found that when the formula (1) is satisfied, there is no ferrite phase after rolling, the structure is an austenite single-phase structure, and there is almost no cracking at the edges of the steel sheet. Cracking at the edges of the steel plate reduces the yield and is not economically preferable. Therefore, the requirement is to satisfy the following expression (1).

Α = (1.5Si + Cr + 3Al) − (0.5Mn + Ni + 30C + 30N) <9 (1) From the viewpoint of weldability, if the stability of the austenite phase with respect to the ferrite phase is excessively increased, the hot cracking susceptibility is increased. Therefore, it is preferable to use a component system in which the value on the left side of Expression (1) is as large as possible.

Next, the reason why the formula (2) is limited will be described. As described above, although carbide precipitation strengthening is effective for high temperature strength, as for the carbide forming elements, M ₂₃ C ₆ type iron / chromium carbide, MC type Zr, Nb, T which can be precipitated can be used.
The creep rupture strength can be most effectively improved when the i and V carbides satisfy the following expression (2 ′).

(CasMC) / (CasM ₂₃ C ₆ + CasMC) ≧ 0.2 (2 ′) In this case, in order to satisfy the above expression (2 ′), Z
It is necessary to add r, Nb, Ti and V within the range of the following formula (2).

MCI = C / 5−12 × (Zr / 91 + Nb / 93 + Ti / 48 + V / 68) ≦ 0 (2) The austenitic stainless steel of the present invention defined as above has a predetermined composition. It is melted in a state where it is contained in a predetermined amount in the form of a simple substance or a mother alloy, and then formed into a desired shape by a usual process such as casting or hot rolling.
Also, when forming into a steel pipe, it is not particularly limited and various commonly used methods can be adopted, but after melting, casting, a final shape is obtained through a tube manufacturing process by a hot extrusion method or a Mannesmann rolling method. The method is usually used preferably.

[0039]

Next, an embodiment of the present invention will be described. Table 1
Table 2 shows the chemical composition of the steel used in this example. No. 1 in Table 1. No. 1 to No. No. 19 is an invention steel satisfying the composition range of the first invention, and No. 19 20 to No. No. 25 is an invention steel satisfying the composition range of the second invention. On the other hand, No. 26 to No. 50 is a comparative steel.
In both tables, the value of α defined by equation (1) and (2)
The value of MCI defined by the formula is also shown.

[0040]

[Table 1]

[0041]

[Table 2]

Steel of these components is melted by an electric furnace, cast,
After manufacturing by hot rolling to a thickness of 12 mm and solution treatment at 1100 ° C., a corrosion test piece of steel plate total thickness × 30 mm × 50 mm was cut out and subjected to a coal ash coating high temperature corrosion test and a continuous oxidation test.

The high temperature corrosion test was carried out under the conditions of 700 ° C. coal ash corrosion test simulating an ultra-supercritical boiler superheater. That is, the composition is 30% Na ₂ SO ₄ + 40% K ₂ SO ₄
The mixed ash adjusted to + 30% Fe ₂ O ₃ was applied to the test piece, and 1% SO ₂ + 5% O ₂ + 10% CO ₂ + 84% N ₂
After heating to 700 ° C. in a mixed gas flow and holding for 100 hours, descaling treatment was performed to measure the corrosion weight loss.

The high temperature oxidation resistance test was carried out under the atmospheric oxidation conditions of 800 ° C., after keeping it for 480 hours in a humid atmospheric flow controlled to a dew point of 30 ° C., followed by air cooling, and after sufficient cooling, weight loss was measured. The number of repetitions was set to 3 for each steel type, and the average value of these was evaluated. The variation between individual test pieces is approximately 1
It was within 0 to 20%. All of these results were organized by the amount of thinning (mm / year). Table 3 shows these results.
And shown in Table 4.

On the other hand, for the purpose of evaluating the microstructure stability after long-term aging, the rolled and tempered material was heated at 800 ° C. for 4000 hours, and then a V-notch Charpy test piece was cut out at 0 ° C.
The absorbed energy in was measured.

Further, a creep rupture test was conducted for the purpose of grasping high temperature strength. Creep rupture test is 650 ℃, 75
The rupture test was performed at 0 ° C. for up to 10,000 hours, and the rupture strength of 100,000 hours was extrapolated from the result to determine the creep rupture strength.

Further, as a characteristic which becomes a problem when it is used as a structural member, susceptibility to high temperature cracking during welding was evaluated by a Balestrain test. Here, a bending strain of 1.0% was applied to the test piece while simulating welding with a heat input of 18 kJ / cm with the non-filler TIG, and the total crack length was measured after cooling. Tables 3 and 4 also show the absorbed energy of the aging material, the creep rupture strength, and the Vallestrain crack length.

[0048]

[Table 3]

[0049]

[Table 4]

Regarding the high temperature corrosion resistance, as can be seen from these tables, No. No. 1 to No. No. 25 invention steels are all comparative steel Nos. Working 18Cr tested as 26
-8Ni stainless steel SUS304H, and No.
It was confirmed that the corrosion weight loss was 1/2 or less as compared with SUS321H tested as No. 27, and good high temperature corrosion resistance was exhibited. It is considered that the inclusion of a certain amount or more of Al and Cr increases the denseness of the oxide film, stabilizes the oxide film, and improves the internal protection.
On the other hand, comparative steel No. In No. 28, the amount of added Cr is not sufficient, so that a large corrosion weight loss is exhibited. In No. 40, the high temperature corrosion resistance deteriorated due to the addition of excessive Mn.

Regarding the high temperature oxidation resistance, as can be seen from these tables, No. No. 1 to No. No. 25 invention steels are all comparative steel Nos. Working 18Cr tested as 26
-8Ni stainless steel SUS304H, and No.
As compared with SUS321H tested as No. 27, the weight loss due to oxidation was 1/2 or less, and good characteristics were exhibited. On the other hand, comparative steel No. In No. 28, sufficient high temperature oxidation resistance could not be obtained due to lack of Al amount.

Regarding the toughness of the aged material which is an index of the structural stability at high temperature, No. No. 1 to No. The invention steels of No. 25 are all No. 25. It showed better toughness than SUS321H tested as No. 27. It is considered that this is because in the steel of the present invention, generation of intermetallic compounds such as sigma phase was suppressed as a result of considering the austenite stability. On the other hand, comparative steel No. 32 has an excessive amount of Si,
No. Since 39 has an excessive amount of Al, the comparative steel N
o. Since the values of α of Nos. 43 and 50 were 9 or more, deterioration of toughness was observed in both cases due to precipitation of sigma phase due to long-term heat treatment. Comparative steel No. In No. 38, a decrease in toughness was observed due to excessive precipitation of carbide.

Regarding the creep rupture strength, no. No. 1 to No. No. 25 invention steels are all comparative steel Nos. 18Cr-8Ni series stainless steel S currently tested as No. 26
US304H, and No. SUS tested as 27
A value better than 321H was obtained. On the other hand, Comparative Steel No. 3 in which the addition amount of the precipitation strengthening element is not sufficient 29, 30, 4
In Nos. 8 and 49, the creep rupture strength was insufficient. Further, conversely, the addition of No. 35, 36, 37
In Comparative Steel No. 3, the fracture ductility was lowered and the N addition was excessive. 42, comparative steel No. 42 in which the addition of B was excessive.
Even with No. 47, the creep rupture strength was insufficient as a result of the decrease in fracture ductility. In addition, the comparative steel No. In No. 33, the addition of Cu was excessive, and No. 33 was used. In No. 46, since the total amount of Y and La was excessive, the fracture ductility was significantly reduced and the creep rupture strength was insufficient, and the 100,000-hour rupture strength extrapolated value could not be obtained.

Regarding hot cracking during welding, the steel of the present invention No. No. 1 to No. Regarding No. 25, no hot cracking was observed. However, the comparative steel No. 41, 44,
For No. 45, large hot cracking occurred. This is a comparative steel No. In Nos. 44 and 45, since the values of P and S were high, the resistance to hot cracking deteriorated. 41, relatively high Cr, N
It is considered that the component system containing i became an austenite solidification system and the hot cracking susceptibility was enhanced.

As is clear from the above results, according to the steel of the present invention, the high temperature corrosion resistance and the high temperature oxidation resistance of the body can be improved, and the toughness is deteriorated due to the excellent structural stability during high temperature use. It was confirmed that there was no cracking, the creep rupture strength was excellent, and the weldability at high temperature was good.

[0056]

According to the present invention, there is provided an austenitic stainless steel for a heat exchanger, which is excellent in oxidation resistance or corrosion resistance at 600 to 800 ° C., long-term structure stability, weldability and high temperature strength. It The stainless steel according to the present invention is 18Cr-8Ni which is a conventional general-purpose stainless steel.
These properties are significantly superior to the stainless steels,
It has the feature of good workability because it has low sensitivity to hot cracking during welding.

[Brief description of drawings]

FIG. 1 is a graph showing the C of a conventional Cr-Ni-based stainless steel containing no Al and an Al-containing Cr-Ni-based stainless steel of the present invention.
The figure which showed the difference of sigma phase precipitation area | region on r equivalent Ni equivalent binary system phase diagram.

Claims (2)

[Claims] 1. By weight%, C: 0.12% or less, Si:
1.0% or less, Mn: 5.0% or less, P: 0.04% or less, S: 0.03% or less, Cr: 14 to 22%, Ni:
10-25%, Al: 1.0-3.5%, N: 0.02
% Or less, Y, La, Ce as a total content of 0 to 0.0
7% (including the case of no addition), and Ti:
0.05-0.5%, V: 0.1-1.0%, Nb:
0.1-1.0%, Zr: 0.1-1.0%, and C
u: 0.5 to 4.0% of one or more kinds are contained, the balance is Fe and inevitable impurities, and the following expressions (1) and (2) are satisfied. Austenitic stainless steel for heat exchangers with excellent high temperature corrosion resistance. α = (1.5Si + Cr + 3Al)-(0.5Mn + Ni + 30C + 30N) <9 ... (1) MCI = C / 5-12x (Zr / 91 + Nb / 93 + Ti / 48 + V / 68) ≤0 ... (2) 2. By weight%, C: 0.12% or less, Si:
1.0% or less, Mn: 5.0% or less, P: 0.04% or less, S: 0.03% or less, Cr: 14 to 22%, Ni:
10-25%, Al: 1.0-3.5%, N: 0.02
% Or less, Y, La, Ce as a total content of 0 to 0.0
7% (including the case of no addition), B: 0.001 to 0.0
1%, Ti: 0.05-0.5%,
V: 0.1-1.0%, Nb: 0.1-1.0%, Z
r: 0.1 to 1.0%, and Cu: 0.5 to 4.0%
For a heat exchanger excellent in high-temperature corrosion resistance, characterized by containing one or more of the above, the balance consisting of Fe and unavoidable impurities, and satisfying the following formulas (1) and (2) Austenitic stainless steel. α = (1.5Si + Cr + 3Al)-(0.5Mn + Ni + 30C + 30N) <9 ... (1) MCI = C / 5-12x (Zr / 91 + Nb / 93 + Ti / 48 + V / 68) ≤0 ... (2)