JPH08218140A - High chromium austenitic heat resistant alloy excellent in high temperature strength and high temperature corrosion resistance - Google Patents

Abstract

PURPOSE: To produce a heat resistant alloy having excellent strength in a severe high temp. environment by forming a specified compsn. in which Ti and B are compositely added, Y, La and Ce are incorporated and the content of Mn is reduced. CONSTITUTION: This high chromium austenitic heat resistant alloy has a compsn. contg., by weight, >0.02 to 0.10% C, <=1.0% Si, <=0.5% Mn, 28.0 to 38.0% Cr, 35.0 to 60.0% Ni, >0.5 to 1.5% Ti, <=0.05% N, 0.01 to 0.3% sol Al, 0.001 to 0.01% B and respectively 0 to 1.0% Zr and Nb, furthermore contg. one or more kinds of 0.5 to 3.0% Mo and 1.0 to 6.0% W and respectively 0.01 to 0.25% of one or more kinds among Y, La and Ce, and the balance Fe. This alloy may furthermore be incorporated with respectively 0.001 to 0.05% of one or both of Mg and Ca. By the same componental regulation, the precipitation of an α-Cr phase is promoted, and the growing and coarsening thereof are suppressed to improve its creep strength, and while a Cr2 O3 film preferentially grows, the adhesiveness between the Cr2 O3 film and the boundary of the metals improves.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high chromium austenitic heat resistant alloy which is excellent in high temperature strength and corrosion resistance under severe high temperature and corrosive environments such as boilers and chemical plants.

[0002]

2. Description of the Related Art In recent years, in a thermal power plant, an ultra-high temperature and high pressure boiler has been attracting attention for the purpose of improving thermal efficiency. In this boiler, the steam conditions are higher in temperature and pressure than in conventional boilers, and therefore the superheater tube material must meet even more stringent requirements for high temperature strength and corrosion resistance. Therefore, there is a demand for a high-strength and high-corrosion-resistant austenitic steel that has higher high-temperature strength than the 18-8 series stainless steel that has been widely used in the past, and that also has excellent steam oxidation resistance and high-temperature corrosion resistance.

Generally, in order to improve the corrosion resistance, it is effective to increase the Cr content in steel. However, the high-temperature strength at 600 to 700 ° C. is 1 as seen in, for example, SUS310-STB steel containing about 25% Cr.
Rather lower than 8-8 series stainless steel, and σ
There is a problem of toughness deterioration due to phase precipitation. Furthermore, when the Cr content is about 25%, the corrosion resistance is not sufficient under a severe corrosive environment.

As an alloy having relatively good corrosion resistance, C
The r content is increased to about 30%, for example, JP-A-59-1.
Although there is an alloy as disclosed in Japanese Patent No. 53858, the high temperature strength is insufficient to be applied as a single tube under the severe conditions as described above. Further, when the double pipe is used, there are many problems in terms of manufacturing cost and reliability.

Further, it is intended to improve the strength by adding Mo and W while increasing the Cr content to about 30%, as disclosed in JP-A-60-100640.
There are heat-resistant alloys such as those disclosed in JP-A-174350, JP-A-61-276948, and JP-A-64-55352, but the strengths thereof cannot be said to be sufficient.

Therefore, the present inventors have set the Cr content to 30
%, The strength of a Cr-Ni-Fe-based austenitic alloy having a high Cr content increased to about 10%, specifically, the high temperature creep rupture strength, is bc enriched with Cr by the combined addition of Ti and B.
A patent application was previously filed for an alloy that was significantly improved by promoting the fine precipitation of the α-Cr phase, which is the c phase (Japanese Patent Application No.
-9366).

However, in recent years, thermal power plants have responded to changes in power demand depending on the time of day by using DSS (Daily star).
The tendency to operate is increasing, and the frequency of use in a thermal cycle environment is increasing. When used in such a heat cycle environment, the alloy of Japanese Patent Application No. 6-9366 previously proposed by the applicant of the present invention is also Cr.
The deterioration of steam oxidation resistance and high temperature corrosion resistance due to peeling of the ₂ O ₃ film is unavoidable, and excellent creep rupture strength and excellent steam oxidation resistance and high temperature corrosion resistance under heat cycle environment are satisfied at the same time. It was newly found that I could not get it.

[0008]

The object of the present invention has been made in view of the above-mentioned circumstances, and it has excellent high-temperature strength in a more severe high-temperature environment and has corrosion resistance, especially in a heat cycle environment. To provide a high chromium austenite heat resistant alloy excellent in steam oxidation resistance and high temperature corrosion resistance.

[0009]

The gist of the present invention resides in the following high chromium austenitic heat resistant alloys (1) and (2).

(1) In% by weight, C: more than 0.02% and 0.10% or less, Si: 1.0% or less, Mn: 0.5%
Below, Cr: 28.0 to 38.0%, Ni: 35.0 to
60.0%, Ti: more than 0.5% and 1.5% or less, N:
0.05% or less, sol-Al: 0.01 to 0.3%,
B: 0.001-0.01%, Zr: 0-0.1% and Nb: 0-1.0% are contained, and Mo: 0.5-.
One or two of 3.0% and W: 1.0 to 6.0%, and Y: 0.01 to 0.25%, La: 0.
01 to 0.25% and Ce: 0.01 to 0.25%, and one or more of them is contained, and the balance is Fe and unavoidable impurities. High-chromium austenitic heat resistant alloy with excellent properties.

(2) In addition to the above-mentioned component (1), Mg: 0.001 to 0.05% and C by weight%.
a: A high chromium austenitic heat resistant alloy containing one or both of 0.001 to 0.05% and having excellent high temperature strength and high temperature corrosion resistance.

In the above (1), Zr and Nb may not be added. When these are positively added, the content range is 0.01 to 0.1% for Zr and 0.
It is desirable to be 1 to 1.0%.

In order to obtain sufficient corrosion resistance in the more severe high temperature corrosive environment as described above, it is necessary to contain 28.0% or more of Cr. In such a high Cr-containing Cr-Ni-Fe alloy, the α-Cr phase, which is a Cr-rich bcc phase, precipitates out in a predetermined Ni content range, resulting in high temperature strength, particularly creep strength. affect.

The present inventors have made a dramatic improvement in the creep rupture strength of the above-mentioned conventional high Cr content austenitic alloys, and have corrosion resistance under the above-mentioned thermal cycle environment, specifically, steam oxidation resistance and high temperature resistance. As a result of intensive research aimed at dramatically improving corrosivity, the following findings were obtained.

The combined addition of Ti and B significantly improves creep rupture strength. This is because Ti promotes the precipitation of α-Cr phase, B further promotes and stabilizes the fine precipitation of carbides to suppress the growth of α-Cr phase, and the α-Cr phase becomes coarse even after long-term use. That is because there is nothing to do.

High Cr with a composite addition of Ti and B
In the contained austenite alloy, while one or more kinds of Y, La, and Ce are contained in an appropriate amount, and when the content of Mn is reduced, the Cr ₂ O ₃ film grows preferentially, As a result of fixing S, which is a trace impurity that causes a decrease in adhesion, in the metal as a sulfide, Cr
The adhesion between the ₂ O ₃ film and the metal is remarkably improved, and steam oxidation resistance and high temperature corrosion resistance in a heat cycle environment are remarkably improved.

[0017]

The function and effect of the constituents of the alloy of the present invention and the reason why the proper content is determined as described above will be described below. In the following,% means% by weight.

C: more than 0.02% and not more than 0.10% C is an element effective for forming carbides and improving the tensile strength and creep rupture strength required as a heat resistant alloy. If the C content is 0.02% or less, these desired effects cannot be obtained. On the other hand, if it exceeds 0.10%, the ductility and toughness of the alloy are greatly deteriorated. Therefore, the C content is set to more than 0.02% and 0.10% or less.

Si: 1.0% or less Si is an element necessary for deoxidation and also contributes to improvement of oxidation resistance. However, if Si is 1.
If it exceeds 0% and is excessively present, weldability and structure stability are deteriorated. Therefore, the Si content is set to 1.0% or less.
It is preferably 0.5% or less. Incidentally, in order to surely obtain the effects of deoxidation and improvement of oxidation resistance, it is desirable to contain 0.05% or more.

Mn: 0.5% or less Mn is a spinel type oxide film formation promoting element, and Mn
Is present in excess of 0.5%, the spinel-type oxide film increases and the steam oxidation resistance of the alloy deteriorates. Therefore, in the present invention, the content is limited to 0.5% or less. Preferably 0.3%
It is the following.

From the viewpoint of preferentially growing and forming a Cr ₂ O ₃ film, the smaller the content of Mn, the better, and it is not necessary to set the lower limit of the content. However, Mn has a function as a deoxidizing agent and a function for improving hot workability, and the effect is obtained at 0.05% or more. Therefore, if these effects are desired, 0.05% or more is required. It is preferable to contain it.

Cr: 28.0 to 38.0% Cr exerts an excellent action in improving corrosion resistance such as oxidation resistance, steam oxidation resistance or high temperature corrosion resistance, and further bears high temperature strength in the present invention. It is an important element that forms an α-Cr phase. However, if the content is less than 28.0%, these desired effects cannot be obtained. On the other hand, if it exceeds 38.0%, workability is deteriorated and the structure is destabilized. Therefore, the Cr content is set to 28.0 to 38.0%.

Ni: 35.0-60.0% Ni is an essential element for obtaining a stable austenite structure. It is also an element having an action of suppressing the precipitation of the α-Cr phase. However, its content is 3
If it is less than 5.0%, the securing of the austenite structure becomes unstable. On the other hand, if it exceeds 60.0%, the precipitation of the α-Cr phase is suppressed, the high temperature strength becomes insufficient, and a great economical disadvantage is brought about. Therefore, the Ni content is 35.0 to 6
It was set to 0.0%. Preferably, it is 40.0 to 58.0%.

Ti: more than 0.5% and less than 1.5% Ti is an important element that has a great effect on the precipitation of the α-Cr phase effective for improving the high temperature strength. The inclusion of Ti promotes the precipitation of the α-Cr phase. A Ti content of more than 0.5% is necessary in order to secure the amount of α-Cr phase with which sufficient high temperature strength can be obtained. On the other hand, if it exceeds 1.5%, α
In addition to the excessive precipitation of the -Cr phase, the fracture ductility is deteriorated and the fracture life is shortened, and the toughness after long-term use is impaired. Therefore, the Ti content exceeds 0.5% and is 1.5
% Or less. Preferably, it is more than 0.5% and 0.9% or less.

N: 0.05% or less N is an element which improves the high temperature strength and stabilizes the austenite structure. Therefore, it is effective to substitute as a part of Ni, which is an expensive element. However, if it is present in excess of 0.05%, nitrides will precipitate during long-term use at high temperature, resulting in a decrease in toughness after long-term use. Therefore, the N content is set to 0.05% or less.
It is preferably 0.03% or less.

Sol-Al: 0.01 to 0.3% Al is an element effective as a deoxidizing element, and the sol-Al content is required to be 0.01% or more to obtain the effect. On the other hand, if the sol-Al content exceeds 0.3%, the workability decreases. Therefore, the sol-Al content is 0.
It was set to 01 to 0.3%.

B: 0.001 to 0.01% B is an element effective in improving creep rupture strength. B
By this, fine precipitation of carbide is promoted and stabilized, which greatly contributes to the improvement of high temperature strength, and the growth of α-Cr phase is suppressed by the finely precipitated carbide, and the α-Cr phase does not coarsen even after long-term use. . However, if the content is less than 0.001%, this effect cannot be obtained. On the other hand, if it exceeds 0.01%, the creep rupture strength decreases,
Weldability also deteriorates. Therefore, the B content is 0.001 to
It was set to 0.01%. Preferably 0.002-0.00
6%.

Mo and W: Mo is 0.5 to 3.0%,
W is 1.0 to 6.0% Mo and W are mainly effective as solid solution strengthening elements and improve creep rupture strength. However, if the content of Mo is less than 0.5% and the content of W is less than 1.0%, these effects cannot be obtained. On the other hand, the Mo content is 3.
If the W content exceeds 0% and the W content exceeds 6.0%, the corrosion resistance and workability deteriorate. Therefore, the Mo content is 0.5 to 3.0.
%, And the W content was 1.0 to 6.0%. Preferred, M
o content is 1.0 to 2.5%, and W content is 2.0 to 5.
It is 0%.

These elements may be contained alone or in combination of two. However, when using two kinds in combination, the total content is [Mo + (1
/ 2) W] is preferably suppressed to 3.0% or less.

Y, La and Ce: 0.01 to 0.25% for all Y, La and Ce are mainly metal and Cr by fixing S, which is a trace impurity in the alloy, as a sulfide in the metal.
Increases the adhesion of the ₂ O ₃ film, and Cr under heat cycle conditions
Prevents peeling of ₂ O ₃ film and improves steam oxidation resistance during use under temperature fluctuations. However, if the content of any element is less than 0.01%, the above effect cannot be obtained. On the other hand, if any of the elements exceeds 0.25%, the workability deteriorates and the effect of preventing the Cr ₂ O ₃ film peeling is saturated. Therefore, the contents of Y, La, and Ce are set to 0.01 to 0.25% for all the elements.
A preferable content is 0.02 to 0.20 for each element.
%.

These elements may be contained alone or in a combination of two or more.
However, in the case of using two or more kinds in combination, it is desirable to suppress the total content to 0.25% or less, preferably 0.20% or less, particularly from the viewpoint of maintaining workability.

The alloy of the present invention may further contain the following components in addition to the above components.

Zr: upper limit of 0.1% Zr mainly has the effect of strengthening the grain boundaries of the alloy to improve the creep rupture strength, and therefore, if desired, if necessary, Zr is contained. Since the effect is obtained when the Zr content is 0.01% or more, when it is contained, 0.01% or more, preferably 0.02% or more is desirable. However, if its content exceeds 0.1%, on the contrary, the creep rupture strength decreases and the weldability also deteriorates. Therefore, the upper limit of the content of Zr is 0.1%. A preferable upper limit is 0.06%.

Nb: upper limit 1.0% Nb has the effect of refining the crystal grains and improving the ductility, and at the same time forming a solid solution in the austenite phase and Cr carbide to improve the creep rupture strength. It is contained if necessary to obtain the effect. Since the effect is obtained when the Nb content is 0.1% or more, when it is contained, 0.1% or more, preferably 0.2% or more is desirable. However, if the content exceeds 1.0%, toughness is deteriorated. Therefore, the upper limit when Nb is contained is set to 1.0%. The preferred upper limit is 0.8
%.

Mg and Ca: Both upper limits are 0.05% Mg and Ca are effective elements for improving workability, and if desired, if necessary, they are contained. The effect is that the content of each element is 0.0
Since it is obtained in an amount of 01% or more, when contained, any element is 0.001% or more, preferably 0.002%
It is desirable to set it as above. However, if the content of any of the elements exceeds 0.05%, the workability is deteriorated. Therefore, the upper limit in the case of containing Mg and Ca is set to 0.05% for each element. The preferable upper limit of each element is 0.02%.

These elements may be contained alone or in a combination of two kinds. However, in the case of using two kinds in combination, it is desirable that the total content is suppressed to 0.05% or less, preferably 0.02% or less.

[0037]

EXAMPLE 20 kg of each ingot obtained by melting 30 kinds of alloys having the chemical compositions shown in Table 1 in a high frequency vacuum melting furnace were forged, cold rolled, and subjected to solution heat treatment at 1200 ° C. Each test piece was prepared from the test material and subjected to repeated steam oxidation test, repeated high temperature corrosion test and creep rupture test.

The repeated steam oxidation test was conducted with a thickness of 3 mm ×
A test piece having a width of 10 mm and a length of 20 mm is exposed to 700 ° C. steam for 100 hours and then cooled to room temperature.
After repeated application, the thickness of the inner scale (scale generated on the metal side of the metal surface) of the oxide scale generated on the surface of the test piece was measured by optical microscope observation, and the average value was obtained. In addition, about some match money, the scale residual rate after repeating 10, 20, 30 and 40 times was measured.

In the repeated high temperature corrosion test, synthetic coal ash (1.5 mol.Na ₂ SO ₄ -1.5 mol.K ₂ S) was applied to the surface of a test piece having a thickness of 3 mm, a width of 15 mm and a length of 15 mm.
O ₄ -1 mol · Fe ₂ O ₃ ) was applied and the test piece was coated with 1% SO ₂ -5% O ₂ -15% CO ₂ -bal. After the operation of exposing to a gas of N _{2 at} 700 ° C. for 20 hours and then cooling to normal temperature was repeated 10 times, the corrosion weight loss of the test piece was obtained.

The creep rupture test was carried out by using a test piece having an outer diameter of 6 mm and a gauge length of 30 mm at a load stress of 16 at 700 ° C.
The rupture time (hr) was determined by performing the test at kgf / mm ² .

The results of these tests are shown in Table 2. In Table 1, No. 1 to 22 are alloys of the present invention, No. A to
H is a comparative alloy, of which Nos. A to C are Japanese Patent Application No. 6
The invention alloy of No. 9366, No. F is disclosed in JP-A-61-174.
The conventional alloy, No. G, disclosed in Japanese Patent No. 350 is disclosed in Japanese Patent Laid-Open No.
Conventional alloys disclosed in JP-A 1-276948, No. H
Is a conventional alloy disclosed in JP-A-64-55352.

[0042]

[Table 1]

[0043]

[Table 2]

FIG. 1 is a graph showing the effects of Y, La and Ce contents on the repeated steam oxidation characteristics.
FIG. 2 is a diagram showing the influence of Mn content on the repeated steam oxidation characteristics, and FIG. 3 is a diagram showing the influence of Mn content and Y content on the scale residual rate after the repeated steam oxidation test.

As can be seen from the test results shown in Table 2, the alloys of the present invention (Nos. 1 to 22) are conventional alloys (No.
It has a very long creep rupture time as compared with H) and has excellent creep rupture strength. This is because the inclusion of an appropriate amount of Ti and B in combination promotes the precipitation of the α-Cr phase and further suppresses its growth coarsening.

Further, as can be seen from Table 2 and FIG.
The alloys of the present invention (Nos. 1 to 22) containing 0.01% or more of any one of Y, La and Ce have repeated steam oxidation resistance and repeated high temperature corrosion resistance Compared with the previously proposed alloys (Nos. A to C) and the conventional alloys (Nos. F and H), both are greatly improved.

On the other hand, as can be seen from Table 2 and FIG. 2, even in the alloy containing an appropriate amount of Y, a comparative example containing a large amount of Mn in excess of 0.5% specified in the present invention. The alloys (Nos. D and E) of No. 2 are markedly inferior in cyclic steam oxidation resistance and cyclic hot corrosion resistance.

Further, as can be seen from FIG. 3, in the alloy of the present invention (No. 1) having a Mn content of 0.5% or less and containing an appropriate amount of Y (No. 1), even after 40 repeated steam oxidation tests. Scale peeling does not occur at all, and scale adhesion is excellent, but a comparative example alloy (No. A) that does not contain Y although the Mn content is low, or the content of Mn that contains an appropriate amount of Y In the comparative alloy (No. E) having a high amount, the scale peeling was remarkably caused by the repeated steam oxidation test, and the scale adhesion was poor.

The effects of the alloy of the present invention described above are obtained by using appropriate amounts of Y and L.
While containing one or more of a and Ce, M
The Cr ₂ O ₃ film preferentially grows due to the synergistic effect of setting the n content to 0.5% or less, and
_This is because the adhesion between the ₂ O ₃ film and the metal interface is greatly improved.

The conventional alloy proposed in Japanese Patent Laid-Open No. 61-276948 does not contain Y as an essential component, but as shown by the conventional alloy (No. G), Y
In the case of containing, the alloy has almost the same characteristics as the alloy of the present invention with respect to the cyclic steam oxidation resistance and cyclic hot corrosion resistance. However, this Japanese Patent Laid-Open No. 61-276
Conventional alloy containing Y proposed in Japanese Patent No. 948 (No. G)
As described above, since it does not contain a proper amount of Ti and B as an essential component in combination, the creep rupture strength is extremely low, and the creep rupture strength and the cyclic steam oxidation resistance and cyclic hot corrosion resistance are satisfied at the same time. Absent.

[0051]

The high chromium austenitic heat-resistant alloy of the present invention has excellent creep rupture strength even in a more severe high temperature environment, and corrosion resistance, particularly excellent steam oxidation resistance and resistance in a heat cycle environment. It is an alloy with high temperature corrosion. Therefore, it can be used as a single pipe even in a severe high temperature and thermal cycle corrosive environment, and it is more cost-effective and more reliable than the conventional alloy double pipe.

[Brief description of drawings]

FIG. 1 is a diagram showing the effect of Y, La and Ce contents on repeated steam oxidation characteristics.

FIG. 2 is a diagram showing the effect of Mn content on repeated steam oxidation characteristics.

FIG. 3 is a diagram showing the effects of Mn content and Y content on the scale retention rate after the repeated steam oxidation test.

Claims (2)

[Claims] 1. By weight%, C: more than 0.02% and 0.10%
Below, Si: 1.0% or less, Mn: 0.5% or less, C
r: 28.0 to 38.0%, Ni: 35.0 to 60.0
%, Ti: more than 0.5% and less than 1.5%, N: 0.05%
Hereinafter, sol-Al: 0.01 to 0.3%, B: 0.0
01-0.01%, Zr: 0-0.1% and Nb: 0
.About.1.0%, and one or two of Mo: 0.5 to 3.0% and W: 1.0 to 6.0%, and Y: 0.01 to 0.25. %, La: 0.01-0.
25% and Ce: 0.01 to 0.25% of 1 type or 2 types or more, and the balance is Fe and inevitable impurities, and is excellent in high temperature strength and high temperature corrosion resistance. High chromium austenitic heat resistant alloy. 2. In addition to the components according to claim 1, further, in weight%, Mg: 0.001-0.05% and Ca:
A high chromium austenite heat resistant alloy containing 0.001 to 0.05% of one or both of them and having excellent high temperature strength and high temperature corrosion resistance.