JPH08337850A - Austenitic stainless steel for welding structural high temperature apparatus - Google Patents

Abstract

PURPOSE: To impart oxidation resistance at a high temp., structural stability for a long time and weldability to a steel by specifying value expressed by αin a Cr-Ni austenitic stainless steel having a specified compsn. contg. Al. CONSTITUTION: The compsn. of an austenitic stainless steel for welding structural high temp. apparatus is composed of the one 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, 0.001 to 0.010% Ca, 0 to 0.05% Mg, Y, La and Ce by 0 to 0.07% in total, and the balance Fe with inevitable impurities, and also, the conditions of α=(1.5Si+Cr+3Al)-(0.5Mn+Ni+30C+30N)<9 are satisfied. Thus, the heat resistant austenitic stainless steel capable of withstanding using conditions characteristic of new type power plants such as nonpressure withstanding and noncooling parts at 800 to 900 deg.C can be obtd.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a heat-resistant austenitic stainless steel for a welded high-temperature equipment member requiring structural stability represented by a non-pressure resistant non-cooling member in a thermal power generation boiler.

[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 (Pressurized Fluid)
An sized bed combustion (pressurized fluidized bed combustion) power generation system, a topping cycle, etc. have been proposed, and a test plant is also being operated. In these new type plants, unlike conventional type boilers, the range of non-cooling members exposed to high temperatures other than steam pipes has been greatly expanded, and heat-resistant steel for non-cooling members that can be used at a maximum of about 900 ° 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 and the like, and basically it was possible to cope with the 18-8 system, which is said to have long-term stability.

[0004]

However, in general, the parts such as the non-cooling part in a new-type plant have a high operating temperature, and there are steel types that can be used even when they are designed as non-pressure resistant members to be stored in a pressure vessel. I can hardly find it.

When the 18-8 system is applied to such applications, it is an element which improves high temperature oxidation resistance, as disclosed in JP-A-53-63210. It is also conceivable to apply Cr-Ni type stainless steel containing Al and Si. 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 No. 6-271992 discloses a high Al content steel in consideration of oxidation resistance.
However, although the steel disclosed herein takes into consideration the oxidation resistance, it cannot be said that consideration is sufficient in terms of maintaining long-term phase stability and weldability.

On the other hand, the expensive 25Cr-20Ni austenitic stainless heat-resisting steel represented by JIS SUS310S, which is about to be used for the above purpose, can be 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, in the current high temperature technology, it is strongly desired to establish an alloy design guideline that simultaneously satisfies the three points of high temperature oxidation resistance, long-term phase stability and weldability.

The present invention has been made in view of the above circumstances, and is an austenitic stainless steel for welding high temperature equipment excellent in oxidation resistance at 800 ° C. to 900 ° C., long-term microstructure stability, and weldability. Intended to provide steel.

[0010]

Means for Solving the Problems The inventors first selected inexpensive Al and Si as elements for improving high temperature oxidation resistance, and phase stability when subjected to heating at 800 ° C. to 900 ° C. for a long time. Was investigated for its influence on brittle sigma phase precipitation. In general, regarding the suppression of delta ferrite during rolling of austenitic stainless steel, or the amount of delta ferrite, de Long reported in 1960, Met.
Cr, as proposed in al Progress
It has been found that the concept of equivalent weight, Ni equivalent weight can be used. 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, it was found that 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 oxidation resistance improving element, precipitation of delta ferrite during rolling and 800 ° C. to 90 ° 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 for a long time at 0 ° C. 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 weldability, various means for suppressing hot cracking 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, the stability of the austenite phase must be increased excessively with respect to the delta ferrite in order to suppress the sigma phase precipitation when the Cr content is increased in order to increase the oxidation resistance, and the austenite solidification is completely completed during solidification. And increase 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.

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%. Less than,
P: 0.04% or less, S: 0.03% or less, C
r: 14% to 22%, Ni: 10% to 25%, Al:
1.0% to 3.5%, N: 0.02% or less, Ca:
0.001% to 0.010%, Mg: 0% to 0.05%
(Including the case of no addition), Y, La, and Ce as a total content of 0 to 0.07% (including the case of no addition), the balance consisting of Fe and inevitable impurities, and The present invention provides an austenitic stainless steel for welded high temperature equipment, characterized by satisfying the formula (1).

Α = (1.5Si + Cr + 3Al) − (0.5Mn + Ni + 30C + 30N) <9 (1) Secondly, in 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% to 25%, Al: 1.0% to 3.5
%, N: 0.02% or less, Ca: 0.001% to
0.010%, Mg: 0% to 0.05% (including the case of no addition), Y, La, and Ce as a total content of 0 to 0.
07% (including the case of no addition),
i: 0.01% to 0.5%, V: 0.01% to 1.0
%, Nb: 1% or more of 0.01% to 1.0%, B: 0% to 0.03% (including the case of no addition), and Zr: 0% to 0.3% ( The present invention provides an austenitic stainless steel for welded high-temperature equipment, characterized in that it contains (including the case of no addition), the balance consists of Fe and unavoidable impurities, and satisfies the following formula (1). α = (1.5Si + Cr + 3Al)-(0.5Mn + Ni + 30C + 30N) <9 (1)

[0018]

The reason 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 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 cuts within the crystal grains. Since it precipitates, its content was set to 0.12% or less.

Si: Si is an element effective for deoxidation, so Si may be contained in an amount of 1.0% or less, but if it is contained in excess of 1.0%, the sigma phase forming ability is large and the 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: Since Mn is an element effective for phase stability, it may be 5.0% or less, but if it is contained in excess of 5.0%, it becomes harmful to the high temperature corrosion resistance. It was set to 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. If its content is less than 14%, it is 800 even if it contains Al effective for high temperature oxidation resistance.
It is not possible to obtain a significant improvement in high temperature oxidation resistance in the range of ℃ to 900 ℃. 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 austenato phase with respect to the ferrite phase becomes excessively high, and the hot cracking susceptibility at the time of welding is increased. Therefore, the Ni content is set to 10 to 25%.

Al: Al alone forms A in an oxidizing environment.
l ₂ O ₃ forms a very dense oxide film,
In the presence of oxide, it is contained as a complex oxide in it,
Improves oxide compactness. 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: Similar to C, N gives the high temperature strength of the parent phase of the steel of the present invention and is an element effective for phase stability.
The content of N may be 0.02% or less, but if it exceeds 0.02%, a nitride is formed and it is harmful to the toughness, so the content of N is set to 0.02% or less.

Ca: Ca is effective as an element for improving hot workability by adding a trace amount, but 0.00
If it is less than 1%, the effect is not sufficient, and if it exceeds 0.010%, the cleanability is impaired and the hot workability is deteriorated, so Ca
The content was set to 0.001 to 0.010%.

Mg: Mg is effective as an element for improving hot workability by adding a trace amount, but 0.05
%, The hot workability deteriorates, so the content was made 0.05% or less. Moreover, since it has the same effect as Ca, it is decided to include the case of no addition.

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: Ti, V, Nb finely disperse and precipitate as carbonitrides and contribute to the improvement of high temperature strength. However, if each of them is 0.01% or less, the effect is not sufficient. Further, if added excessively, the amount of undissolved carbonitrides of Ti, V, and Nb after solution heat treatment increases, which impairs high-temperature strength, and further deteriorates weldability. The content of Ti is 0.01 to
0.5% or less, V is 0.01 to 1.0% or less, and Nb is 0.01 to 1.0% or less.
Seed or two or more kinds are included.

B, Zr: B and Zr are effective elements for strengthening grain boundaries and improving high temperature strength characteristics, but if added in excess, the weldability deteriorates, so B is 0.03% or less. , Zr was 0.30% or less. Moreover, since these are added as needed, it is decided to include the case of no addition.

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 inventors use it for a long time at 800 ° C. to 900 ° C. It was found that even after that, there was almost no precipitation of brittle sigma phase, that is, good toughness was 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 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 will be 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.

The austenitic stainless steel of the present invention defined as described above is manufactured by containing a predetermined amount of predetermined components in molten steel in the form of a simple substance or a master alloy.

[0036]

EXAMPLES Next, examples of the present invention will be described. Table 1
Table 2 shows the chemical composition of the steel used in this example. No. of Table 1 1 to No. No. 15 is an invention steel satisfying the composition range of the first invention, and No. 15 16 to No. No. 25 is an invention steel satisfying the composition range of the second invention. On the other hand, No. 26 to No. 52 is a comparative steel.
In both tables, the value of α defined by the formula (1) is also shown.

[0037]

[Table 1]

[0038]

[Table 2]

The results of an oxidation test at 900 ° C. for these steels are shown in Tables 3 and 4. The oxidation test is 20
A corrosion test piece of mm × 30 mm × 5 mm 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. The measured values of the temperature rising rate and the temperature falling rate are about 1 each.
It was 30 ° C./min and 150 ° C./min. The oxidation resistance is 5% KMnO ₄ + 18% Na for the test piece after the test.
After descaling with an OH aqueous solution and a 10% diammonium citrate aqueous solution, the weight was measured, and the weight loss before the test was evaluated by oxidation weight loss. 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. 1 to No. The invention steel No. 25 is comparative steel No. General-purpose 18Cr-8Ni stainless steel (SUS3
It was confirmed that the corrosion weight loss was ⅕ or less compared to that of (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. 26, No. Four
4, no. No. 45 is a comparative steel N due to lack of Al amount
o. No. 46 does not have sufficient resistance to high temperature oxidation due to lack of Cr content. Comparative steel No. No. 47 contains a large amount of Mn, so that the high temperature oxidation resistance is lowered, and as a result, a large corrosion weight loss is exhibited.

Next, since the tissue stability is important as a characteristic that becomes a problem when used at high temperatures, in order to grasp the tissue stability, it is aged at 850 ° C. for 400 hours, and then subjected to JI.
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 σ phase, etc. was observed. On the other hand, comparative steel No. No. 27 has an excessive amount of Cr, so
Comparative steel No. 28 and No. Comparative steel No. 29 has an excessive amount of Si. 33 and No. 33. In No. 34, since the amount of Al was excessive, the sigma phase was precipitated and the toughness was decreased. In addition, comparative steel No. In No. 32, since the Ni content was insufficient, the sigma phase was precipitated and the toughness was decreased. Comparative steel No. 38 and No. 38.
Since the value of α of 39 is 9 or more, deterioration of toughness was observed due to precipitation of sigma phase by heat treatment for a long time.

Comparative Steel No. No. 31 was caused by excessive precipitation of carbides, and the comparative steel No. In No. 35, the toughness was decreased due to excessive precipitation of nitride. Next, the hot workability during rolling was evaluated. Here, the workability was evaluated based on 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 was heated at 1200 ° C., and in the rolling process in which the finishing temperature was 900 ° C., no edge crack was generated and a good rolling effect was obtained.

On the other hand, Comparative Steel No. 6 in which the amount of Ca is not appropriate. Three
6 and No. 37, comparative steel No. 40
In the case of No. 2, an ear crack was formed on the plate end surface. Also, Y, L
Comparative steel No. in which the total amount of a and Ce is not appropriate. 41 and N
o. 42 and No. 42. Also in No. 43, the edge of the plate was cracked. Comparative steel No. in which the value of α in the formula (1) is 9 or more. 38 and N
o. No. 39 also had a crack in the edge of the plate.

Finally, the hot cracking susceptibility at the time of 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. Two
8 and No. 29 is comparative steel N because the amount of Si is excessive.
o. In No. 30, since the Ni content was excessive, the ferrite phase was unstable and the austenite phase was excessively stable in all cases, and cracking was observed after welding. Also, comparative steel N
o. Comparative Steel No. 48 has an excessive Ti content. 49 is V
Comparative steel No. Comparative steel No. 50 has an excessive Nb content. No. 51 has an excessive B content, so comparative steel N
o. No. 52 had an excessive amount of Zr, so cracking was observed after welding.

As is clear from the above Examples and Comparative Examples, according to the component setting of the present invention, it is possible to improve the oxidation resistance performance at high temperature, and it is also excellent in the stability of the structure when used at high temperature. It was confirmed that it is possible to obtain a heat-resistant stainless steel that has no deterioration in toughness, has excellent hot workability during rolling manufacturing, and has high susceptibility to hot cracking during welding, and that also has excellent weldability.

[0049]

[Table 3]

[0050]

[Table 4]

[0051]

According to the present invention, there is provided a heat resistant austenitic stainless steel capable of withstanding the use conditions of 800 ° C. to 900 ° C., which are characteristic of a new type power plant such as a non-breakdown pressure / uncooled part. 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, and is highly reliable. It is useful for manufacturing a member having low hot cracking sensitivity 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, 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
% -0.010%, Mg: 0% -0.05% (including the case of no addition), Y, La, Ce as a total content of 0-
Contains 0.07% (including the case of no addition), the balance is F
An austenitic stainless steel for welded high-temperature equipment, characterized by comprising e and unavoidable impurities and satisfying the following formula (1). α = (1.5Si + Cr + 3Al)-(0.5Mn + Ni + 30C + 30N) <9 (1) 2. C, 0.12% or less by weight%, 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
% -0.010%, Mg: 0% -0.05% (including the case of no addition), Y, La, Ce as a total content of 0-
0.07% (including the case of no addition), further, Ti: 0.01% to 0.5%, V: 0.01%
˜1.0%, Nb: 0.01% to 1.0%, one or more kinds, B: 0% to 0.03% (including the case of no addition), and Zr: 0% to 0%. Austenitic stainless steel for welded high-temperature equipment, containing 3% (including the case of no addition), the balance consisting of Fe and unavoidable impurities, and satisfying the following formula (1). α = (1.5Si + Cr + 3Al)-(0.5Mn + Ni + 30C + 30N) <9 (1)