JPH08127848A - High chromium austenitic heat resistant alloy with excellent high temperature strength - Google Patents

Abstract

(57) [Summary] [Purpose] To provide a high chromium austenitic heat resistant alloy with excellent high temperature strength. [Constitution] C: more than 0.02% and 0.10% or less, Si:
1.0% or less, Mn: 2.0% or less, Cr: 28 to 38
%, Ni: 35-60%, Ti: over 0.5% 1.5%
Hereinafter, N: 0.05% or less, sol-Al: more than 0.3% and 1.5% or less, B: 0.001 to 0.01%, Zr:
0 to 0.1% and Nb: 0 to 1.0%, and further Mo: 0.5 to 3.0% and W: 1.0 to 6.0%.
A high chromium austenitic heat resistant alloy excellent in high temperature strength, characterized in that it contains at least one of the above, and the balance is Fe and inevitable impurities. This alloy is
g: 0.001-0.05% and Ca: 0.001-
One or more of 0.05% can be contained. [Effect] A creep rupture strength of 18 kgf / mm ² or more at 700 ° C. × 1000 hr and a toughness of 12 kgf-m / cm ² or more after aging at 700 ° C. × 100 hr are obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high chromium austenite heat resistant alloy which is excellent in high temperature strength under harsh 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 deterioration of toughness 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, JP-A-64-33532, etc., but their strengths are not 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 consideration of further harsh operating environment and thinning in order to reduce equipment cost, it was revealed that the alloy proposed in Japanese Patent Application No. 6-9366 does not have sufficient high temperature strength. .

[0008]

The object of the present invention has been made in view of the above-mentioned circumstances, and it has excellent high temperature strength even in a more severe high temperature environment, and further, in a high temperature corrosive environment. It is intended to provide a high chromium austenitic heat resistant alloy having excellent 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: 2.0%
Below, Cr: 28-38%, Ni: 35-60%, T
i: more than 0.5% and 1.5% or less, N: 0.05% or less,
sol-Al: more than 0.3% and 1.5% or less, B: 0.0
01-0.01%, Zr: 0-0.1% and Nb: 0
To 1.0%, and further contains at least one of Mo: 0.5 to 3.0% and W: 1.0 to 6.0%,
The balance is Fe and unavoidable impurities, which is a high chromium austenite heat resistant alloy excellent in high temperature strength.

(2) In addition to the above-mentioned component (1), Mg: 0.001 to 0.05% and C by weight%.
a: A high chromium austenite heat resistant alloy containing at least one of 0.001 to 0.05% and excellent in high temperature strength.

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 under the more severe high temperature corrosive environment as described above, it is necessary to contain 28% or more of Cr. Cr with high Cr content
In the —Ni—Fe alloy, the α-Cr phase, which is a bcc phase enriched with Cr, precipitates in the range of a predetermined Ni content and affects the high temperature strength, particularly the high temperature creep strength.

The inventors of the present invention have conducted earnest research aiming at a dramatic improvement in creep rupture strength of the conventional high Cr content austenitic alloy, and as a result, have obtained the following findings.

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. This is because there is nothing to do.

A content of more than 0.3% sol-Al is
Greatly improves creep rupture strength. This is because a large amount of sol-Al narrows the solid solubility limit of the α-Cr phase, further promotes the precipitation of the α-Cr phase, and precipitates the γ ′ phase, which greatly contributes to the improvement of the high temperature strength. is there.

By adding Ti, B and sol-Al in appropriate amounts in combination, it is possible to obtain
It is possible to obtain a highly corrosion-resistant alloy having a high-temperature strength far superior to the alloys disclosed in Japanese Patent Publication No. 0 and the like and the alloys previously filed by the present inventors.

[0018]

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 reduced. Therefore, the C content is 0.
It was set to more than 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: 2.0% or less Mn is an element necessary for deoxidation. However, if Mn exceeds 2.0%, the heat resistance is deteriorated. Therefore, the Mn content is set to 2.0% or less. Preferably 1.
0% or less.

In order to surely obtain the effects of deoxidation and tissue stabilization, it is desirable to contain 0.1% or more.

Cr: 28-38% Cr exhibits an excellent effect of improving corrosion resistance such as oxidation resistance, steam oxidation resistance or high temperature corrosion resistance. Further, in the present invention, the α-Cr phase responsible for high temperature strength. Is an important element that forms. However, if the content is less than 28%, these desired effects cannot be obtained. On the other hand, if it exceeds 38%, the workability is deteriorated and the structure is destabilized.

Therefore, the Cr content is set to 28 to 38%.

Ni: 35-60% 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 35
If it is less than%, securing of the austenite structure becomes unstable. On the other hand, if it exceeds 60%, the precipitation of the α-Cr phase is suppressed, the high temperature strength becomes insufficient, and a great disadvantage is economically brought about. Therefore, the Ni content is set to 35 to 60%. Preferably, it is 40 to 58%.

Ti: more than 0.5% and less than 1.5% Ti is an important element which 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 life decreases due to the reduction in fracture ductility, 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 0.5 to 0.9%.

N: 0.05% or less N is an element having an effect of improving the high temperature strength and stabilizing 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 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: more than 0.3% and less than 1.5% Al is generally contained as a deoxidizing element, but in the present invention, a large amount of Al is contained for the purpose of greatly improving creep rupture strength. Let That is, when a large amount of Al is contained, the solid solution limit of the α-Cr phase effective for improving the high temperature strength is narrowed, the precipitation of the α-Cr phase is promoted, and the γ'phase is precipitated, which improves the high temperature strength. Greatly contribute to. However, in order to obtain the effect, 0.3% or more of sol-A
However, if it exceeds 1.5%, the toughness is significantly reduced. Therefore, the sol-Al content is set to more than 0.3% and 1.5%. It is preferably 0.5 to 1.3%.

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

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%, the toughness deteriorates. 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 0.0
5% Mg and Ca are effective elements for improving the hot workability, and are contained as necessary in order to obtain this effect. Since the effect is obtained when the content of each element is 0.001% or more, when included, all the elements are 0.001% or more, preferably 0.1% or more.
It is desirable to be 002% or more. 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 38 kinds of alloys having the chemical compositions shown in Tables 1 and 2 in a high frequency vacuum melting furnace were forged, cold rolled, and solution heat treated at 1200 ° C. Each test piece was prepared from the provided test material and subjected to a creep rupture strength and a Charpy impact test after aging.

The creep rupture test was carried out at 700 ° C. using a test piece having an outer diameter of 6 mm and a gauge length of 30 mm,
The breaking strength at 0 hours was obtained. In addition, the Charpy impact test after aging was carried out at a temperature of 700 ° C. for 100 hours.
The test was performed at 0 ° C. using an impact test piece having a 2 mm V notch formed at 5 mm.

The results are also shown in Tables 1 and 2. Alloy Nos. 1 to 23 are examples of the present invention, Alloy No. A to
O is a comparative example, of which alloy Nos. E to H are Japanese Patent Application No. 6-
The invention alloys Nos. I to K of No. 9366 are disclosed in JP-A-61-1
Conventional alloy and alloy No. L disclosed in Japanese Patent No. 174350.
To O are conventional alloys disclosed in JP-A-61-276948.

[0040]

[Table 1]

[0041]

[Table 2]

FIG. 1 shows the effect of sol- on the creep rupture strength.
It is a figure which shows the influence of Al content, and FIG. 2 is a figure which shows the influence of the sol-Al content which affects the Charpy impact value (toughness) after aging. From FIG. 1 and FIG. 2, sol-
If the Al content exceeds 0.3%, the creep rupture strength is remarkably improved, but if the Al content exceeds 1.5%, the effect is almost saturated, but the Charpy impact value (toughness) is significantly reduced. It is clear.

FIG. 3 shows typical alloys of the present invention (Nos. 5, 13, 17, 19) based on the results of Tables 1 and 2.
Comparative alloys (No. E, F, G, H, I, J, K, L,
(M, N, O) is a diagram showing, in comparison, the creep rupture strengths of alloys having almost the same chemical composition except for sol-Al. As is clear from FIG. 3, the alloy of the present invention has a sol-Al content of 0.3% or less even if it contains Ti and B in combination (Nos. E to H).
It can be seen that the creep rupture strength is greatly improved as compared with. Further, even if the alloy contains up to 0.5% of sol-Al, the alloy of Comparative Example (No. I to K) having a Ti content of 0.5% or less, or the sol-Al content of 0. A conventional alloy of 3% or less and containing no B (N
It can be seen that the creep rupture strength is much higher than that of o.

This is because, within the composition range defined by the present invention, the combined addition of Ti and B promotes fine precipitation of the α-Cr phase and suppresses coarsening, and a proper amount of so.
The inclusion of l-Al causes the γ'phase to precipitate and greatly contribute to the improvement of high temperature strength.
This is because l-Al narrows the solid solubility limit of the α-Cr phase and further promotes the precipitation of the α-Cr phase.

[0045]

The high chromium austenite heat resistant alloy of the present invention is an alloy having excellent creep rupture strength even under a severer high temperature environment. Further, the alloy of the present invention has a high C
Since it is an r-containing alloy, it also has excellent corrosion resistance in severe high temperature corrosive environments. The single tube made of this alloy is more cost effective and more reliable than the double tube made of the conventional alloy.

[Brief description of drawings]

FIG. 1 is a diagram showing the effect of sol-Al content on creep rupture strength.

FIG. 2 sol-A on Charpy impact value after aging
It is a figure which shows the influence of 1 content.

FIG. 3 is a diagram showing the creep rupture strength of a representative alloy of the present invention and a conventional alloy in comparison.

Claims (2)

[Claims] 1. By weight%, C: more than 0.02% and 0.10%
Below, Si: 1.0% or less, Mn: 2.0% or less, C
r: 28-38%, Ni: 35-60%, Ti: 0.5
% To 1.5% or less, N: 0.05% or less, sol-A
1: 0.3% to 1.5% or less, B: 0.001 to 0.
01%, Zr: 0-0.1% and Nb: 0-1.0%
In addition, Mo: 0.5 to 3.0% and W:
It contains at least one of 1.0 to 6.0% and the balance is F
A high-chromium austenite heat-resistant alloy excellent in high-temperature strength, which is characterized by comprising e and inevitable impurities. 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 at least one of 0.001 to 0.05% and excellent in high temperature strength.