JPH09241810A - Austenitic stainless steel for high temperature equipment with welded structure - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an austenitic stainless steel for high temp. equipment with welded structure, excellent in oxidation resistance at 800-1000 deg.C, stability of structure for a long time, weldability, and high temp. strength. SOLUTION: This austenitic stainless steel has a composition consisting of, by weight, <=0.12% C, <=1.0% Si, <=5.0% Mn, <=0.04% P, <=0.03% S, 14-22% Cr, 10-25% Ni, 1.0-3.5% Al, <=0.02 N, 0.001-0.010% Ca, 0-0.05% (including 0%) Mg, 0-0.07% (including 0%), in total, of Y, La, and Ce, one or >=2 kinds among 0.05-0.5% Ti, 0.1-1.0% V, 0.1-1.0% Nb, and 0.1-1.0% Zr, and the balance Fe with inevitable impurities and satisfying inequalities α=(1.5Si+Cr+3Al)-(0.5Mn+Ni+30C+30N)<9 and MCI=C/2-12×(Zr/91+Nb/93+Ti/48+V/68)<=0.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an austenitic stainless steel for welded high-temperature equipment, which is required to have high-temperature oxidation resistance, heat resistance, microstructure stability, and high-temperature strength, which are typified by uncooled members in thermal power generation boilers. Regarding steel.

[0002]

2. Description of the Related Art With the progress of high temperature plant technology in recent years, a new type of combined cycle power plant, namely coal gasification combined plant, PFBC (Pressurised Fluidized Bed Combustion,
That is, a pressurized fluidized bed combustion) power generation system, a topping cycle, etc. have been proposed, and a test plant is being operated. In these new-type plants, unlike conventional boilers, the range of non-cooling members exposed to high temperatures is greatly expanded in addition to steam pipes, and heat-resistant steel for non-cooling members that can be used at a maximum of about 1000 ° C. Is also newly demanded.

[0003] Conventionally, for high temperature members, steel grades have been intensively developed with a focus on high temperature strength, and 18Cr-8Ni type stainless steel is often used as a tube material. The mainstream is JIS SUS304H (18Cr-10Ni-
C), and JIS SUS with Ti added to it.
321H, etc., and basically, it was possible to use 18-8 series stainless steel which is said to have long-term stability.

[0004]

However, in the parts such as the non-cooling part in the new type plant, the operating temperature is high, and almost no usable steel type is found. When the above-mentioned 18-8 type stainless steel is applied to such an application, as disclosed in JP-A-53-63210, it is an element that improves the high temperature oxidation resistance. Al, S
It is also conceivable to apply Cr-Ni-based stainless steel containing i. However, such steel grades cannot be said to be sufficient in terms of maintaining long-term phase stability and weldability as a structural member in a plant requiring a certain structural strength.

Further, Japanese Unexamined Patent Publication (Kokai) No. 6-271992 discloses a high Al content steel in consideration of oxidation resistance. But,
Although the steel disclosed herein considers oxidation resistance, it cannot be said that consideration is sufficient in terms of maintaining long-term phase stability, weldability, and high-temperature strength.

On the other hand, the expensive 25Cr-20Ni austenitic stainless heat-resisting steel represented by JIS SUS310S, which is about to be used for this purpose, is easily used at 800 to 900 ° C. for 200 to 700 hours. It is feared that the sigma phase will precipitate and cracking will occur just by hammering during the periodic inspection. Another problem is the high sensitivity to hot cracking during welding.

Therefore, it is strongly desired to establish an alloy design guideline that simultaneously satisfies the four points of high temperature oxidation resistance, long-term phase stability, weldability, and high temperature strength. The present invention has been made in view of such circumstances, and is 800 ° C to 1000 ° C.
An object of the present invention is to provide an austenitic stainless steel for welded high-temperature equipment, which is excellent in oxidation resistance at ℃, structural stability for a long time, weldability and high-temperature strength.

[0008]

In order to solve this problem, the inventors first select inexpensive Al and Si as elements for improving the high temperature oxidation resistance, and heat at 800 ° C. to 1000 ° C. for a long time. The effect of the phase stability on receiving the brittle sigma phase precipitation was investigated. Generally, for the suppression of delta ferrite during rolling of austenitic stainless steel, or the amount of delta ferrite, for example, as proposed by de Long in 1960 in Metal Progress,
It has been found that the concepts of Cr equivalent and Ni equivalent can be used. However, the above equation could not be used as it is for predicting the amount of sigma phase precipitation. That is, Cr-Ni
In the system stainless steel, the precipitation region of the sigma phase is wider than the precipitation region of delta ferrite on the state diagram shown in FIG. Therefore, Ni is added in order to suppress the precipitation of delta ferrite, but in order to suppress the precipitation of the sigma phase, it is necessary to add more expensive Ni than is added to suppress the precipitation of delta ferrite. Was empirically revealed.

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 oxidation resistance, precipitation of delta ferrite during rolling and 800 ° C. to 10 ° C. within the range of components in which the following formula (1) is satisfied:
It has been found that both precipitation of sigma phase when heated at 00 ° C. for a long time 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 the 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 in order to increase the oxidation resistance, the sigma phase is likely to precipitate. as a result,
In order to suppress sigma phase precipitation, the stability of the austenite phase must be increased excessively with respect to delta ferrite, resulting in complete austenite solidification during solidification,
It increases the sensitivity to hot cracking. Further, when the Si amount is increased, the Ni equivalent must be further increased in order to further suppress the sigma phase precipitation, and the hot cracking susceptibility is increased.

However, since Al has a low sigma phase forming ability with respect to 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.

Finally, regarding high temperature strength, solid solution strengthening, nitride precipitation strengthening, carbide precipitation strengthening, Cu
-As a result of conducting a number of laboratory melting tests on rich phase precipitation strengthening, intermetallic compound precipitation strengthening, etc., carbide precipitation strengthening is the best from the viewpoints of creep rupture strengthening, long-term toughness and creep ductility. It turned out that
With respect to the carbide-forming element, the ratios of M ₂₃ C ₆ type iron / chromium carbide, MC type Zr, Nb, Ti, and V carbide that can be precipitated satisfy the formula (2 ′). That is, Zr, Nb, T
It was clarified that the addition of i and V in the range of the formula (2) can most effectively improve the creep rupture strength.

(CasMC) / (CasM ₂₃ C ₆ + CasMC) ≧ 0.5 (2 ′) MCI = C / 2 −12 × (Zr / 91 + Nb / 93 + Ti / 48 + V / 68) ≦ 0 (2) The first invention is based on such knowledge, and the first invention is, in% by weight, C: 0.12% or less, S
i: 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% to 25%, Al: 1.0% to 3.5%,
N: 0.02% or less, Ca: 0.001% to 0.010
%, Further Ti: 0.05% to 0.5%, V: 0.1%
~ 1.0%, Nb: 0.1% to 1.0%, Zr: 0.1
% To 1.0% of 1 type or 2 types or more, and the balance is F
The present invention provides an austenitic stainless steel for welded high-temperature equipment, characterized by comprising e and unavoidable impurities and satisfying the following expressions (1) and (2).

Α = (1.5Si + Cr + 3Al) − (0.5Mn + Ni + 30C + 30N) <9 (1) MCI = C / 2 −12 × (Zr / 91 + Nb / 93 + Ti / 48 + V / 68) ≦ 0 (2) Second invention Is based on the first invention, further B: 0.001% to
It is an austenitic stainless steel for welded high temperature equipment containing 0.01%. The stainless steel of the present invention may contain 0.05% or less of Mg and 0.07% or less of Y, La, and Ce as a total content.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION The reasons why each component is contained in the stainless steel according to the present invention and the range thereof is limited will be described below. C: C is an element that gives the high temperature strength of the parent phase of the steel of the present invention and is effective for phase stability, but when it is contained in excess of 0.12%,
Since coarse carbide precipitates in the form of longitudinally cutting through the crystal grains,
The content was 0.12% or less.

Si: Si is an element effective for deoxidation, so it may be contained in an amount of 1.0% or less. However, if it is contained in an amount of more than 1.0%, the sigma phase forming ability is large and phase stability is maintained. Therefore, the content thereof is set to 1.0% or less in order to further increase the hot crack sensitivity during welding.

Mn: Mn is an element effective for phase stability, so it may be 5.0% or less. However, if it is contained in excess of 5.0%, it becomes harmful to the high temperature corrosion resistance. 5.
0% or less.

P: P is an element that segregates at 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 0.04% 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. If its content is less than 14%, it is 800 ° C even if it contains Al effective for high temperature oxidation resistance.
At ˜1000 ° C., it is not possible to obtain a significant improvement in high temperature oxidation resistance. On the other hand, if 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 also contributes little to the improvement in high temperature oxidation resistance. Therefore,
The Cr content was 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 the austenite becomes small, and even if the Ni content is increased, it is impossible to greatly increase the elements such as Cr that improve the high temperature oxidation resistance. Further, if the amount of Ni is excessively increased, the stability of the austenite phase with respect to the ferrite phase becomes excessively high, and the susceptibility to hot cracking during welding is increased. Therefore, the Ni content is set to 10 to 25%.

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

N: N, like 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.
Although it may be contained in an amount of not more than 02%, if it is contained in an amount of more than 0.02%, it forms a nitride and is harmful to toughness.
Content of 0.02% or less.

Ca: Ca is effective as an element for improving hot workability by adding a trace amount, but 0.001
If it is less than 1.0%, the effect is not sufficient, and if it exceeds 0.010%, the cleanability is impaired and the hot workability is deteriorated, so the Ca content was made 0.001% to 0.010% or less.

Ti, V, Nb, Zr: Ti, V, Nb,
Zr finely disperses and precipitates as a carbide, and contributes to the improvement of high temperature strength. However, if the content is 0.01% or less, the effect is not sufficient. Also, if added excessively, the amount of undissolved Ti, V, Nb, and Zr carbides after solution heat treatment increases, which impairs high-temperature strength, and further reduces weldability. Ti is 0.05 to 0.5%, V is 0.1 to 1.0%, Nb is 0.1 to 1.0%, and Zr is 0.1 to 1.0%. Accordingly, one or more of these are included.

B: B is an element effective for strengthening grain boundaries and improving high temperature strength properties, but if less than 0.001%, no effect is obtained, so 0.001% or more is required. is there. However, excessive addition deteriorates the weldability, so the content was made 0.03% or less.

In the present invention, Mg and / or Y, L are further added.
You may add a and Ce. Mg: Mg is effective as an element for improving hot workability by adding a trace amount, but if it exceeds 0.05%, the hot workability is deteriorated, so the content is made 0.05% or less. .

Y, La, Ce: Y, L which are rare earth elements
Since a and Ce dissolve in the Al ₂ O ₃ oxide film to increase the general resistance to high temperature oxidation, one or more of these may be contained. These are 0.07% in total
If it is contained in excess of 1.0, the hot workability is impaired, so the total content is made 0.07% or less.

Next, the reason for limiting α 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 satisfied, the present invention is 800 ° C. to 1000 ° C.
It has been found that there is almost no precipitation of brittle sigma phase even after long-term use in, that is, good toughness is maintained even after long-term use. 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 edge of the steel plate is not economically preferable because it lowers the yield. Therefore, it is necessary 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.

Finally, the reason for limiting the MCI will be described. Regarding the carbide-forming element, the ratio of M ₂₃ C ₆ type iron / chromium carbide, MC type Zr, Nb, Ti, and V carbide that can be precipitated satisfies the formula (2 ′), that is, Zr, N
The addition of b, Ti and V within the range of the formula (2) can most effectively improve the creep rupture strength. Therefore,
The requirement is to satisfy the following expression (2). (CasMC) / (CasM ₂₃ C ₆ + CasMC) ≧ 0.5 (2 ′) MCI = C / 2 −12 × (Zr / 91 + Nb / 93 + Ti / 48 + V / 68) ≦ 0 (2)

[0035]

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. 11 is an invention steel satisfying the composition range of the first invention, and No. 11 12 to No. 12 No. 15 is an invention steel satisfying the composition range of the second invention. On the other hand, No. No. 16 to No. 32 is a comparative steel.
In both tables, the value of α defined by the equation (1) and the value of MCI defined by the equation (2) are also shown.

These components were produced by melting in an electric furnace, casting, and hot rolling to a thickness of 12 mm, subjected to solution treatment at 1100 ° C., and then subjected to the following tests. First, 900 ° C
The results of the oxidation test in Table 3 are shown in Tables 3 and 4. In the oxidation test, a 20 mm × 30 mm × 5 mm corrosion test piece was used and heated in an annular furnace having a mullite tube as an inner tube. After soaking in air whose dew point was humidified and adjusted to 30 ° C. by a humidity control system for 240 hours, the side wall was moved to a water-cooled cooling chamber and cooled to room temperature. Measured values of temperature rising rate and temperature falling rate are
It was about 130 ° C./min and 150 ° C./min, respectively. Oxidation resistance is 5% KMnO ₄
After descaling with a + 18% NaOH aqueous solution and a 10% diammonium citrate aqueous solution, the weight was measured and evaluated by the weight loss due to oxidation with respect to the weight before the test. The number of repetitions (n number) was 3 for each steel type, and the average value was evaluated. The variation between individual test pieces was generally within 10 to 50%.

As can be seen from these tables, No. No. 1 to No. The invention steel of No. 15 is comparative steel No. General-purpose 18Cr-8Ni-based stainless steel (SUS3
It was confirmed that the corrosion weight loss was 1/7 or less as compared with 04H), and that good oxidation 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. 16, No.
No. 17 does not have sufficient high temperature oxidation resistance due to lack of Al content. Next, since the tissue stability is important as a problematic property at the time of use at high temperature, in order to grasp the tissue stability, aging at 850 ° C. for 400 hours was performed.
The absorbed energy of the Charpy impact test at 0 ° C. by the S4 Charpy impact test piece was measured. The values are also shown in Table 3 and Table 4.

The invention steel has an absorbed energy of 3 at 0 ° C.
It was as good as 0 J or more, and no reduction in toughness due to the formation of carbonitrides, intermetallic compounds such as sigma phase was observed. On the other hand, comparative steel No. Since 18 had an α value of 9 or more, deterioration of toughness was observed due to precipitation of sigma phase due to long-term heat treatment.

Next, the hot workability during rolling was evaluated. Here, the workability was evaluated by the presence or absence of edge cracks in the rolled plate material. The results are also shown in Tables 3 and 4. The steel of the present invention is heated at 1200 ° C. and the finishing temperature is 900
In the rolling process in which the temperature was set to ℃, no edge cracking occurred and good rolling results were obtained.

On the other hand, the comparative steel Nos. In which the value of α in the equation (1) is 9 or more. In No. 28, an edge crack occurred on the plate end surface. In addition, the comparative steel Nos. In which the amount of Ca is not appropriate. 31 and comparative steel No. 3 in which the amount of Mg is not appropriate. Also in No. 32, the edge of the plate was cracked.

Next, regarding the high-temperature strength, a creep rupture test was performed at 900 ° C., and the rupture strength of 100,000 h was extrapolated from the results of the rupture test up to 10,000 h.
The values are also shown in Table 3 and Table 4.

The steels of the present invention are comparative steel Nos. It exhibited creep rupture strength superior to that of the general-purpose 18Cr-8Ni steel stainless steel tested as 16. On the other hand, the comparative steel No. 16, N
o. 19, no. 20, no. 21, no. 22 is T
Due to lack of i, V, Nb, and Zr amounts, comparative steel No.
27, no. 28, no. No. 29 has a creep rupture strength of 18Cr-8 because the MCI value of the formula (2) exceeds 0.
It was about the same as Ni steel and stainless steel. Also, comparative steel N
o. Comparative Steel No. 18 has a sigma phase precipitated. Two
3, No. 24, no. 25, no. 26, no. 3
0, No. 31, No. 32 is Ti, V, N respectively
Since the amounts of b, Zr, B, Ca, and Mg were excessive, the ductility at break was remarkably reduced and the creep rupture strength was insufficient.
The extrapolated value of the breaking strength at 0,000 h could not be obtained.

Finally, the hot cracking susceptibility during welding, which is an important characteristic which becomes a problem when used as a structural member, was evaluated. The susceptibility to hot cracking during welding was evaluated by the Balestrain test. The test is non-filler TIG
While simulating a welding with a heat input of 18 kJ / cm, a bending strain of 1.0% was applied to the test piece, and the total crack length was measured after cooling.

As a result, in the steel of the present invention, no crack was found after the test. On the other hand, comparative steel No. 1
9, No. 20, no. 21, no. 22, no. 30
Since the Ti, V, Nb, Zr, and B contents were excessive, respectively, cracking was observed after welding.

As is clear from the above examples, according to the composition setting of the present invention, the oxidation resistance performance at high temperature can be improved, and the toughness is deteriorated because of excellent structure stability at high temperature use. It is confirmed that it is possible to obtain a heat-resistant stainless steel that is excellent in weldability because it has no weldability, excellent hot workability during rolling manufacturing, high strength at high temperature, and high susceptibility to hot cracking during welding. It was

[0047]

[Table 1]

[0048]

[Table 2]

[0049]

[Table 3]

[0050]

[Table 4]

[0051]

According to the present invention, 800 ° C to 1000 ° C
A heat-resistant austenitic stainless steel that can withstand the usage conditions characteristic of a new type power plant such as an uncooled part is provided. Moreover, it has much better oxidation resistance than the conventional general-purpose stainless steel, 18Cr-8Ni series stainless steel, shows good structural stability for long-term use, has high reliability, high temperature strength, and As a welded structure, it is useful for manufacturing members having low workability and good workability.

[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%, N
i: 10% to 25%, Al: 1.0% to 3.5%, N:
0.02% or less, Ca: 0.001% to 0.010%,
Further, Ti: 0.05% to 0.5%, V: 0.1% to
1.0%, Nb: 0.1% to 1.0%, Zr: 0.1%
~ 1.0% of 1 type or 2 types or more, with the balance being Fe
And an austenitic stainless steel for welded high-temperature equipment, characterized by comprising unavoidable impurities and satisfying the following expressions (1) and (2). α = (1.5Si + Cr + 3Al) − (0.5Mn + Ni + 30C + 30N) <9 (1) MCI = C / 2 −12 × (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%, N
i: 10% to 25%, Al: 1.0% to 3.5%, N:
0.02% or less, Ca: 0.001% to 0.010%,
Further, Ti: 0.05% to 0.5%, V: 0.1% to
1.0%, Nb: 0.1% to 1.0%, Zr: 0.1%
To 1.0% of one or more, and B:
Austenitic stainless steel for welded high temperature equipment, containing 0.001% to 0.01%, the balance consisting of Fe 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 / 2 −12 × (Zr / 91 + Nb / 93 + Ti / 48 + V / 68) ≦ 0 (2)