ES2843268T3 - Ni-Cr-Fe Alloy - Google Patents

Abstract

A NiCrFe alloy, comprising a chemical composition consisting of: in% by mass, C: from 0.03 to 0.15%, Si: not more than 1.00%, Mn: not more than 2.00% , P: not more than 0.040%, S: not more than 0.0050%, Cr: 18.0 to 25.0%, Ni: 25.0 to 40.0%, Ti: 0.10 to 1.60%, Al: 0.05 to 1.00%, N: not more than 0.020%, O: not more than 0.008%, Rare Earth Metal (MTR): 0.001 to 0.100%, B: from 0 to 0.010%, Ca: 0 to 0.010%, Mg: 0 to 0.010%, V: 0 to 0.5%, Nb: 0 to 1.0%, Ta: 0 to 1.0% , Hf: 0 to 1.0%, Mo: 0 to 1.0%, W: 0 to 2.0%, Co: 0 to 3.0%, and Cu: 0 to 3.0 %, the rest being Fe and impurities, the chemical composition complying with the following Formulas (1) to (3): 0.50 <= Ti + 48A1 / 27 <= 2.20 (1) 0.40 <= Ti / ( Ti + 48A1 / 27) <= 0.80 (2) Σ [MTR / (A (MTR))] - S / 32 - 2/3-O / 16> = 0 (3) where, each element symbol of Formulas (1) to (3) is replaced by the content,% by mass, of the corresponding element, and A (MTR) of Formula (3) is replaced by the atomic weight of each rare earth metal.

Description

DESCRIPTION

Ni-Cr-Fe Alloy

Technical field

The present invention relates to a heat resistant austenitic alloy, and more specifically to a NiCrFe alloy.

Background of the technique

Conventionally, facilities such as thermal power generation boilers and chemical plants operate in high temperature environments (such as 400 to 800 ° C) and, furthermore, come into contact with process fluids that include sulfides and / or or chlorides. Therefore, the materials to be used in such installations require their creep resistance and corrosion resistance at high temperatures.

Examples of the material for use in such installations include 18-8 stainless steel such as SUS304H, SUS316H, SUS321H, and SUS347H, and NiCrFe alloys represented by alloy 800H, which is specified as NCF800H by the JIS standard.

A NiCrFe alloy excels in corrosion resistance and high temperature resistance compared to 18-8 stainless steel. Furthermore, a NiCrFe alloy stands out in terms of its economic efficiency compared to a Ni-based alloy represented by Alloy617. Therefore, NiCrFe alloys are widely used in severe usage environment regions.

NiCrFe alloys used in such severe usage environments are proposed in Japanese Patent Application Publication No. 2013-227644 (Patent Bibliography 1), Japanese Patent Application Publication No. 06-264169 (Bibliography Patent Application 2), Japanese Patent Application Publication No. 2002-256398 (Patent Bibliography 3) and Japanese Patent Application Publication No. 08-13104 (Patent Bibliography 4).

An austenitic heat resistant alloy disclosed in Patent Bibliography 1 consists of, in mass%, C: less than 0.02%, Si: not more than 2%, Mn: not more than 2%, Cr: not less 20% and less than 28%, Ni: more than 35% and not more than 50%, W: 2.0 to 7.0%, Mo: less than 2.5% (including 0%), Nb : less than 2.5% (including 0%), Ti: less than 3.0% (including 0%), Al: not more than 0.3%, P: not more than 0.04%, S : not more than 0.01%, and N: not more than 0.05%, the remainder being Fe and impurities, where f1 = (1/2) W Mo is 1.0 to 5.0, f2 = (1/2) W Mo Nb 2Ti is 2.0 to 8.0, and f3 = Nb 2Ti is 0.5 to 5.0.

A heat resistant and corrosion resistant alloy disclosed in Patent Bibliography 2 consists of, in weight%, 55 to 65% nickel, 19 to 25% chromium, 1 to 4.5% aluminum 0.045 to 0.3% yttrium, 0.15 to 1% titan, 0.005 to 0.5% carbon, 0.1 to 1.5% silicon, not more than 1% manganese , a total of 0.005% of at least one element selected from the group consisting of magnesium, calcium and cerium, a total of less than 0.5% of magnesium and calcium, less than 1% of cerium, from 0.0001 to 0 , 1% boron, not more than 0.5% zirconium, 0.0001 to 0.2% nitrogen and not more than 10% cobalt, the rest being Fe and the impurities that accompany it.

An austenitic alloy disclosed in Patent Bibliography 3 contains, in mass%, C: from 0.01 to 0.1%, Mn: from 0.05 to 2%, Cr: from 19 to 26% and Ni: from 10 to 35%, with the Si content fulfilling a formula of 0.01 <Si <(Cr 0.15 x Ni - 18) / 10.

A heat resistant alloy disclosed in Patent Bibliography 4 consists of, in% by weight, C: 0.02 to 0.15%, Si: 0.70 to 3.00%, Mn: not more than 0 , 50%, Ni: from 30.0 to 40.0%, Cr: from 18.0 to 25.0%, Al: from 0.50 to 2.00% and Ti: from 0.10 to 1.00 %, the rest being Fe and unavoidable impurities. Furthermore, also the patent application publication US2010 / 034690 discloses an austenitic NiCrFe alloy.

Appointment listing

Patent bibliography

Patent Bibliography 1: Japanese Patent Application Publication No. 2013-227644

Patent Bibliography 2: Japanese Patent Application Publication No. 06-264169

Patent Bibliography 3: Japanese Patent Application Publication No. 2002-256398

Patent Bibliography 4: Japanese Patent Application Publication No. 08-13104

Non-patent bibliography

Non-patent bibliography 1: Hans van Wortel: "Control of Relaxation Cracking in Austenitic High Temperature Components ", CORROSION2007 (2007), NACE, Item # 07423

Summary of the invention

Technical problem

The austenitic heat resistant alloy disclosed in Patent Bibliography 1 controls the formation of the Laves phase by specifying the content of W, Mo, Nb and Ti, thus improving creep resistance and toughness. The heat resistant and corrosion resistant alloy disclosed in Patent Bibliography 2 improves resistance to high temperature oxidation by causing it to precipitate during creep. The austenitic alloy disclosed in Patent Bibliography 3 improves carburizing properties by suppressing exfoliation of the oxide film that is predominantly composed of C2O3 and forms on the surface of the material. The heat resistant alloy disclosed in Patent Bibliography 4 contains a specific amount of Cr, a reduced amount of Mn and a fixed amount of Si, whereby it is possible to obtain excellent oxidation resistance even in a case where Ni content is reduced.

On the other hand, Non-patent Bibliography 1 discloses that a NiCrFe alloy has a high susceptibility to stress relaxation cracking. This means that a NiCrFe alloy requires post-work stress relief heat treatment for a bent part and a welded part, where residual stress is present. Therefore, a NiCrFe alloy requires not only excellent creep resistance, but also excellent resistance to stress relaxation cracking.

An object of the present invention is to provide a NiCrFe alloy that excels in creep strength and stress relaxation cracking resistance.

Problem solution

A NiCrFe alloy according to the present invention has a chemical composition consisting of, in mass%, C: 0.03 to 0.15%, Si: not more than 1.00%, Mn: not more than 2.00%, P: not more than 0.040%, S: not more than 0.0050%, Cr: 18.0 to 25.0%, Ni: 25.0 to 40.0%, Ti: of 0.10 to 1.60%, Al: 0.05 to 1.00%, N: not more than 0.020%, O: not more than 0.008%, rare earth metal (MTR): 0.001 to 0.100% , B: 0 to 0.010%, Ca: 0 to 0.010%, Mg: 0 to 0.010%, V: 0 to 0.5%, Nb: 0 to 1.0%, Ta: 0 to 1.0%, Hf: 0 to 1.0%, Mo: 0 to 1.0%, W: 0 to 2.0%, Co: 0 to 3.0%, and Cu: 0 3.0%, with the rest being Fe and impurities, the chemical composition that complies with formulas (1) to (3):

0.50 <Ti 48A1 / 27 <2.20 (1)

0.40 <Ti / (Ti 48A1 / 27) <0.80 (2)

I [MTR / (A (MTR))] - S / 32 - 2/3 * O / 16> 0 (3)

where, each element symbol in the formulas described above is replaced by the content (% by mass) of the corresponding element. A (MTR) of Formula (3) is replaced by the atomic weight of each rare earth metal.

Advantageous effect of the invention

A NiCrFe alloy according to the present invention excels in its creep strength and resistance to stress relaxation cracking.

Brief description of the drawings

[Fig. 1] Fig. 1 is a diagram to show a relationship between fn2 of each Reference Mark of the examples and a sum (mass%) of phase y 'and phase n after aging treatment.

Description of the achievements

The present inventors have made a detailed study on the creep strength and stress relaxation cracking resistance of NiCrFe alloys. As a result, the present inventors have obtained the following findings.

(A) To obtain excellent creep resistance, the precipitation amount of y '(intermetallic compound: Nia (Ti, Al)), which precipitates during creep in a high temperature environment, can be increased. If y 'precipitates sufficiently during creep in a high temperature environment, the creep resistance of the alloy is increased by precipitation hardening. However, if y 'precipitates excessively, the deformability within an austenite grain deteriorates, thus causing the concentration of stresses on the surfaces. grain limiting. As a result, the stress relaxation cracking resistance of the alloy deteriorates. Therefore, to achieve excellent creep strength and excellent stress relaxation cracking resistance at the same time, it is necessary to adjust the amount of y 'that precipitates during creep in a high temperature environment. To obtain a suitable amount of precipitation of y ', the content of Ti and Al, which constitute y', can be adjusted.

Specifically, the chemical composition of the NiCrFe alloy meets Formula (1) to maintain resistance to stress relaxation cracking while ensuring creep resistance:

0.50 <Ti 48A1 / 27 <2.20 (1)

where, each element symbol of Formula (1) is replaced by the content (mass%) of the corresponding element. Now fn1 is defined as fn1 = Ti 48A1 / 27. fn1 is an index to indicate the amount of y 'that precipitates during creep. fn1 is a total content of Al and Ti, the content of Al being converted into the amount of Ti. When fn1 is less than 0.50, a sufficient amount of precipitation will not be obtained from y '. For that reason, NiCrFe alloy cannot obtain excellent creep resistance. On the other hand, when fn1 is greater than 2.20, the stress relaxation cracking resistance of the NiCrFe alloy will deteriorate due to a large amount of precipitation of y '.

(B) The y 'that has precipitated during creep in a hot temperature environment can change shape over time. Specifically, while y 'fine precipitates at an early stage of creep, y' can change to a thick and acicular n phase (Ni3Ti) during creep in a high temperature environment over time. The formation of the n phase will decrease the creep strength of the NiCrFe alloy.

Then, the present inventors have investigated in detail a case where the y 'phase changes to the n phase in a high temperature environment. As a result, they assumed that the content of Ti with respect to the total content of Al and Ti, the content of Al being converted into the amount of Ti, is related to the change from phase y 'to phase n. Accordingly, the present inventors have investigated in detail the content of Ti relative to the total content of Al and Ti, the content of Al being converted into the amount of Ti, and the microstructure during creep.

Now fn2 is defined as fn2 = Ti / (Ti 48A1 / 27). fn2 is a ratio of the Ti content to the total content of Al and Ti, the Al content being converted into the amount of Ti. Figure 1 shows a relationship between fn2 and a sum of phase y 'and phase n after the aging treatment. Figure 1 is obtained by the following method. It is created using fn2, and the content of Ti, Al and Ni in phase y 'and phase n after aging treatment, which are obtained by the method described below, for NiCrFe alloys whose chemical compositions are within the range of the present invention, and wherein Formula (1) described above and Formula (3) described below are within the range of the present invention. Furthermore, Phase y 'and phase n are differentiated using a method that will be described below. The symbol "O" in Figure 1 indicates an example where the numerical density of phase n after the aging treatment is less than 5/100 | jm2. On the other hand, the symbol "•" in Figure 1 indicates an example in which the numerical density of phase n after the aging treatment is not less than 5/100 jm 2.

Referring to Figure 1, when fn2 is less than 0.40, a sufficient amount of precipitation of y 'will not be obtained. In this case, the NiCrFe alloy cannot obtain excellent creep resistance. On the other hand, when fn2 is greater than 0.80, y 'changes to phase n. As a result, NiCrFe alloy cannot obtain excellent creep resistance. Therefore, when fn2 is 0.40 to 0.80, it is possible to increase the creep strength of the NiCrFe alloy.

As described so far, if the chemical composition of the NiCrFe alloy of the present invention meets Formula (2), and 'precipitates in a suitable amount, and the n-phase precipitation will be suppressed even after the time has elapsed. , so that an excellent resistance to creep will be obtained:

0.40 <Ti / (Ti 48A1 / 27) <0.80 (2)

where, each element symbol of Formula (2) is replaced by the content (mass%) of the corresponding element.

(C) One of the causes of stress relaxation cracking is segregation of S at the grain boundaries. Therefore, it is possible to increase the resistance to stress relaxation cracking of the NiCrFe alloy by decreasing an impurity of S, which is secreted at the grain boundaries, thus causing the grain boundaries to brittle. On the other hand, rare earth metals (MTR) combine with a small amount of S, which cannot be removed by refining, in the alloy thus forming inclusions. In other words, an MTR can immobilize S as inclusions.

Therefore, adjusting the MTR content to be an appropriate amount will improve the stress relaxation cracking resistance of the NiCrFe alloy. MTR combines with S and is also likely to easily combine with O. Therefore, to immobilize S with MTR, the MTR content must be adjusted taking into account the amount of MTR that is combined with O.

If the chemical composition of the NiCrFe alloy of the present invention meets Formula (3), S will be sufficiently immobilized by MTR, and excellent resistance to stress relaxation cracking will be obtained:

£ [MTR / (A (MTR))] - S / 32 - 2/3 ^ O / 16> 0 (3)

where, each element symbol of Formula (3) is replaced by the content (mass%) of the corresponding element, and A (MTR) is replaced by the atomic weight of each rare earth metal.

£ [MTR / (A (MTR))] is replaced by a sum of values obtained by dividing each MTR content (mass%) contained in the NiCrFe alloy by the atomic weight of MTR.

We now define fn3 as fn3 = £ [MTR / (A (MTR))] - s / 32 - 2/3 ^ O / 16. MTR is a generic name of a total of 17 elements of Sc, Y and lanthanides. When fn3 is not less than 0, MTR can sufficiently immobilize S as inclusions, thus improving resistance to stress relaxation cracking.

The NiCrFe alloy according to the present invention, which has been completed based on the findings described above, has a chemical composition consisting of, in mass%, C: 0.03 to 0.15%, Si: no more 1.00%, Mn: not more than 2.00%, P: not more than 0.040%, S: not more than 0.0050%, Cr: 18.0 to 25.0%, Ni: 25 , 0 to 40.0%, Ti: 0.10 to 1.60%, Al: 0.05 to 1.00%, N: not more than 0.020%, O: not more than 0.008%, rare earths (MTR): 0.001 to 0.100%, B: 0 to 0.010%, Ca: 0 to 0.010%, Mg: 0 to 0.010%, V: 0 to 0.5%, Nb: 0 to 1.0%, Ta: from 0 to 1.0%, Hf: from 0 to 1.0%, Mo: from 0 to 1.0%, W: from 0 to 2.0%, Co: from 0 to 3.0%, and Cu: from 0 to 3.0%, with the rest being Fe and impurities, the chemical composition that complies with formulas (1) to (3):

0.50 <Ti 48A1 / 27 <2.20 (1)

0.40 <Ti / (Ti 48A1 / 27) <0.80 (2)

X [MTR / (A (MTR))] - S / 32 - 2/3 ^ O / 16> 0 (3)

where, each element symbol in Formulas (1) to (3) is replaced by the content (% by mass) of the corresponding element. A (MTR) of Formula (3) is replaced by one atomic weight of each rare earth metal.

The chemical composition described above can contain B: from 0.0001 to 0.010%.

The chemical composition described above may contain one or two types selected from the group consisting of Ca: 0.0001 to 0.010% and Mg: 0.0001 to 0.010%.

The chemical composition described above may contain one or more types selected from the group consisting of V: 0.01 to 0.5%, Nb: 0.01 to 1.0%, Ta: 0.01 to 1, 0% and Hf: from 0.01 to 1.0%.

The chemical composition described above may contain one or more types selected from the group consisting of Mo: 0.01 to 1.0%, W: 0.01 to 2.0%, Co: 0.01 to 3, 0%, and Cu: 0.01 to 3.0%.

The NiCrFe alloy according to the present invention has excellent creep strength and excellent stress relaxation cracking resistance. To be more specific, NiCrFe alloy will not break for 300 hours or more even if it is subjected to 10% tensile strain at a strain rate of 0.05 min_1 and held as is under an air atmosphere of 650 ° C after cold rolling at 20% area reduction.

Hereinafter, the NiCrFe alloy according to the present invention will be described in detail. The symbol "%" with respect to the elements means, unless otherwise specified,% by mass.

[Chemical composition]

The chemical composition of the NiCrFe alloy of the present invention contains the following elements.

C: 0.03 to 0.15%

Carbon (C) stabilizes the austenite and increases the high temperature creep strength of the alloy. When the C content is too low, these effects cannot be obtained. On the other hand, when the C content is too high, coarse carbide will precipitate in large quantity, thus deteriorating the ductility of the grain boundaries. In addition, the toughness and creep resistance of the alloy are reduced. Therefore, the C content is 0.03 to 0.15%. The lower limit of the C content is preferably 0.04%, more preferably more than 0.04%, and even more preferably 0.05%, and still more preferably 0.06%. The upper limit of the C content is preferably 0.12%, and more preferably 0.10%.

Yes: no more than 1.00%

Inevitably, there is silicon content (Si). Si deoxidizes the alloy, and improves corrosion resistance and high temperature oxidation of the alloy. However, when the Si content is too high, the stability of the austenite deteriorates, and the toughness and creep resistance of the alloy are reduced. Therefore, the Si content is not more than 1.00%. The upper limit of the Si content is preferably 0.80%, more preferably 0.60%, and even more preferably less than 0.60%. Excessive reduction of the Si content deteriorates the deoxidation effect, thus deteriorating the corrosion resistance and high temperature oxidation resistance of the alloy. And furthermore, the cost of production increases significantly. Therefore, the lower limit of the Si content is preferably 0.02%, and more preferably 0.05%.

Mn: not more than 2.00%

Inevitably, there is manganese (Mn) content. The Mn deoxidizes the alloy and stabilizes the austenite. However, when the Mn content is too high, brittleness is caused, and the toughness and creep ductility of the alloy deteriorate. Therefore, the Mn content is not more than 2.00%. The upper limit of the Mn content is preferably 1.80%, and more preferably 1.50%. Excessive reduction of the Mn content impairs the deoxidation effect and stabilization of austenite, and further causes a significant increase in production cost. Therefore, the lower limit of the Mn content is preferably 0.10%, more preferably 0.30%, and even more preferably more than 0.50%.

P: no more than 0.040%

Phosphorus (P) is an impurity. P deteriorates the hot workability and weldability of the alloy, and also deteriorates the yield ductility of the alloy after long hours of use. Therefore, the P content is not more than 0.040%. The upper limit of the P content is preferably 0.035%, and more preferably 0.030%. The P content is preferably as low as possible. However, excessive reduction of the P content will increase the cost of production. Therefore, the lower limit of the P content is preferably 0.0005%, and more preferably 0.0008%.

S: not more than 0.0050%

Sulfur (S) is an impurity. S deteriorates the stress relaxation cracking resistance of the alloy and also deteriorates the hot workability, weldability and creep ductility of the alloy. Therefore, the S content is not more than 0.0050%. The upper limit of the S content is preferably 0.0030%. The content of S is preferably as low as possible. However, excessive reduction of the S content will increase the cost of production. Therefore, the lower limit of the S content is preferably 0.0002%, and more preferably 0.0003%.

Cr: 18.0 to 25.0%

Chromium (Cr) improves the oxidation resistance and high temperature corrosion resistance of the alloy. When the Cr content is too low, these effects cannot be obtained. On the other hand, when the Cr content is too high, the stability of austenite at high temperatures deteriorates and the creep resistance of the alloy is reduced. Therefore, the Cr content is 18.0 to 25.0%. The lower limit of the Cr content is preferably 18.5%, and more preferably 19.0%. The upper limit of the Cr content is preferably 24.5%, and more preferably 24.0%.

Ni: 25.0 to 40.0%

Nickel (Ni) stabilizes the austenite structure. Furthermore, Ni forms y ', thus increasing the creep resistance of the alloy. When the Ni content is too low, y 'is unlikely to be formed, and the above-mentioned effect cannot be obtained. On the other hand, when the Ni content is too high, the production cost increases. Therefore, the Ni content is 25.0 to 40.0%. The lower limit of the Ni content is preferably 26.0%, and more preferably 27.0%. The upper limit of the Ni content is preferably 37.0%, and more preferably 35.0%.

Ti: 0.10 to 1.60%

Titanium (Ti) combines with Ni to form y '. Additionally, Ti combines with C to form TiC, thus increasing the yield strength and tensile strength of the alloy at high temperatures. When the Ti content is too low, such effects cannot be obtained. On the other hand, when the Ti content is too high, Y 'precipitates excessively, thus deteriorating the stress relaxation cracking resistance of the alloy. Therefore, the Ti content is 0.10 to 1.60%. The lower limit of the Ti content is preferably 0.20%, more preferably 0.30%, and even more preferably greater than 0.60%. The upper limit of the Ti content is preferably 1.50%, more preferably less than 1.50%, and even more preferably 1.40%.

Al: 0.05 to 1.00%

Aluminum (Ai) deoxidizes the alloy. In addition, Al combines with Ni to form y ', and increases the yield strength and tensile strength of the alloy at high temperatures. When the Al content is too low, such effects cannot be obtained. On the other hand, when the Al content is too high, and it precipitates in a large quantity, thus deteriorating the resistance to stress relaxation cracking, the yield ductility and the toughness of the alloy. Therefore, the Al content is 0.05 to 1.00%. The lower limit of the Al content is preferably 0.08%, and more preferably 0.10%. The upper limit of the Al content is preferably 0.90%, and more preferably 0.80%.

N: not more than 0.020%

Nitrogen (N) is an impurity. N precipitates as coarse TiN and reduces the amount of dissolved Ti, thus reducing the yield strength of the alloy. Furthermore, N deteriorates the toughness and hot workability of the alloy. Therefore, the N content is not more than 0.020%. The upper limit of the N content is preferably 0.017%, and more preferably 0.015%. The N content is preferably as low as possible. However, reducing it excessively will increase the cost of production. Therefore, the lower limit of the N content is preferably 0.002%, and more preferably 0.004%.

Or: not more than 0.008%

Oxygen (O) is an impurity. Oxygen deteriorates the hot workability of the alloy, and also deteriorates the toughness and ductility of the alloy. Therefore, the O content is not more than 0.008%. The upper limit of the O content is preferably 0.006%, and more preferably 0.005%. The O content is preferably as low as possible. However, reducing it excessively will increase the cost of production. Therefore, the lower limit of the O content is preferably 0.0005%, and more preferably 0.0008%.

MTR: 0.001 to 0.100%

The rare earth metal (MTR) forms a compound with S, thus reducing the content of S that has dissolved in the matrix and improving the resistance to stress relaxation cracking of the alloy. In addition, MTR improves the hot workability and oxidation resistance of the alloy. When the MTR content is too low, these effects cannot be obtained. On the other hand, when the MTR content is too high, the hot workability and weldability of the alloy will deteriorate. Therefore, the MTR content is 0.001 to 0.100%. The lower limit of the MTR content is preferably 0.003%, and more preferably 0.005%. The upper limit of the MTR content is preferably 0.090%, and more preferably 0.080%.

MTR is a generic name of a total of 17 elements of Sc, Y and lanthanides, and the content of MTR refers to a total content of one or more elements of MTR. Also, the composite metal generally has MTR content. For that reason, the MTR can be added to the molten metal as a composite metal, and it can be adjusted so that the MTR content is within the range described above.

The remainder of the chemical composition of the NiCrFe alloy according to the present invention consists of Fe and impurities. In this case, the term impurity means an element that is introduced from minerals and waste as raw material, or from a production environment, etc., when the NiCrFe alloy is produced industrially, and is allowed within a range that does not adversely affect the NiCrFe alloy of the present embodiment.

[Optional items]

The NiCrFe alloy according to the present invention may contain B instead of part of Fe.

B: 0 to 0.010%

Boron (B) is an optional element and may not be contained. When contained, B increases the yield strength of the alloy, causing the carbides at the grain boundaries to be finely dispersed. Additionally, B is secreted at the grain boundaries to aid MTR effects. When B is contained in a small amount, the aforementioned effects will be obtained to some extent. However, when the B content is too high, the weldability and hot workability of the alloy will deteriorate. Therefore, the content of B is 0 to 0.010%. The upper limit of the B content is preferably 0.008%. The lower limit of the B content to effectively obtain the above-mentioned effects is preferably 0.0001%, and more preferably 0.0005%.

The NiCrFe alloy according to the present invention may contain one or two types selected from the group consisting of Ca and Mg instead of part of Fe. Each of these elements forms a compound with S, thus aiding the effects of MTR .

Ca: 0 to 0.010 %

Calcium (Ca) is an optional element and may not be contained. When contained, Ca forms a compound with S, thus aiding the S-immobilizing effect of MTR. If Ca is contained in a small amount, the effect mentioned above will be obtained to some extent. However, when the Ca content is too high, the Ca forms oxide and deteriorates the hot workability of the alloy. Therefore, the Ca content is 0 to 0.010%. The upper limit of the Ca content is preferably 0.008%. The lower limit of the Ca content to effectively obtain the aforementioned effect is preferably 0.0001%, more preferably 0.0002%, and even more preferably 0.0003%.

Mg: 0 to 0.010%

Magnesium (Mg) is an optional element and may not be contained. When contained, Mg forms a compound with S, thus aiding the S-immobilizing effect of MTR. When Mg is contained in a small amount, the aforementioned effect will be obtained to some extent. However, when the Mg content is too high, the Mg forms oxide, thus deteriorating the hot workability of the alloy. Therefore, the Mg content is 0 to 0.010%. The upper limit of the Mg content is preferably 0.008%. The lower limit of the Mg content to effectively obtain the aforementioned effect is preferably 0.0001%, more preferably 0.0002%, and even more preferably 0.0003%.

The NiCrFe alloy according to the present invention may contain one or more types selected from the group consisting of V, Nb, Ta and Hf instead of part of Fe. Each of these elements forms carbide and carbonitride, thus increasing strength. to the creep of the alloy.

V: 0 to 0.5%

Vanadium (Va) is an optional element and may not be contained. When contained, V forms fine carbide and carbonitride with C and N, thus increasing the yield strength of the alloy. When the V is contained in a small amount, the aforementioned effect will be obtained to some extent. However, when the V content is too high, a large amount of carbide and carbonitride will precipitate, thus deteriorating the yield ductility of the alloy. Therefore, the V content is 0 to 0.5%. The upper limit of the V content is preferably 0.4%. The lower limit of the V content to effectively obtain the aforementioned effect is 0.01%.

Nb: 0 to 1.0%

Niobium (Nb) is an optional element and may not be contained. When contained, Nb forms fine carbide and carbonitride with C and N, thus increasing the yield strength of the alloy. When the Nb is contained in a small amount, the aforementioned effect will be obtained to some extent. However, when the Nb content is too high, a large amount of carbide and carbonitride will precipitate, thus deteriorating the yield ductility and toughness of the alloy. Therefore, the Nb content is 0-1.0%. The upper limit of the Nb content is preferably 0.4%. The lower limit of the Nb content to effectively obtain the above-mentioned effect is 0.01%.

Ta: 0 to 1.0%

Tantalum (Ta) is an optional item and may not be contained. When contained, Ta forms fine carbide and carbonitride with C and N, thus increasing the yield strength of the alloy. When Ta is contained in a small amount, the aforementioned effect will be obtained to some extent. However, when the Ta content is too high, a large amount of carbide and carbonitride will precipitate, thus deteriorating the yield ductility and toughness of the alloy. Therefore, the content of Ta is 0-1.0%. The upper limit of the Ta content is preferably 0.4%. The lower limit of the content of Ta to effectively obtain the aforementioned effect is 0.01%.

Hf: 0 to 1.0%

Hafnium (Hf) is an optional element and may not be contained. When contained, e1Hf forms fine carbide and carbonitride with C and N, thus increasing the yield strength of the alloy. When e1Hf is contained in a small amount, the aforementioned effect will be obtained to some extent. However, when the Hf content is too high, a large amount of carbide and carbonitride will precipitate, thus deteriorating the yield ductility and toughness of the alloy. Therefore, the Hf content is 0-1.0%. The upper limit of the Hf content is preferably 0.4%. The lower limit of the Hf content to effectively obtain the above-mentioned effect is 0.01%.

The NiCrFe alloy according to the present invention may contain one or more types selected from the group consisting of Mo, W, Co, and Cu instead of part of Fe.

Mo: 0 to 1.0%

Molybdenum (Mo) is an optional element and may not be contained. When contained, Mo dissolves in the alloy, thus increasing the yield strength of the alloy at high temperatures. When Mo is contained in a small amount, such an effect will be achieved to a certain extent. However, when the Mo content is too high, the stability of the austenite will be lost, thus deteriorating the toughness of the alloy. Therefore, the Mo content is 0-1.0%. The upper limit of the Mo content is preferably 0.9%. The lower limit to effectively obtain the aforementioned effect is preferably 0.01%.

W: 0 to 2.0%

Tungsten (W) is an optional item and may not be contained. When contained, W dissolves in the alloy, thus increasing the yield strength of the alloy at high temperatures. When W is contained in a small amount, such an effect will be achieved to a certain extent. However, when the W content is too high, the stability of the austenite will be lost, thus deteriorating the toughness of the alloy. Therefore, the W content is 0 to 2.0%. The upper limit of the W content is preferably 1.8%. The lower limit of the W content to effectively obtain the aforementioned effect is preferably 0.01%.

Co: 0 to 3.0%

Cobalt (Co) is an optional element and may not be contained. When contained, the Co stabilizes the austenite and dissolves in the alloy, thus increasing the yield strength of the alloy at high temperatures. When Co is contained in a small amount, such effects will be obtained to some extent. However, when the Co content is too high, the production cost increases. Therefore, the Co content is 0 to 3.0%. The upper limit of the Co content is preferably 2.8%. The lower limit of the Co content to effectively obtain the above-mentioned effects is preferably 0.01%.

Cu: 0 to 3.0%

Copper (Cu) is an optional element and may not be contained. When contained, Cu stabilizes the austenite and suppresses the precipitation of the brittle phase, such as the a phase, during use at high temperatures. When Cu is contained in a small amount, such effects will be obtained to some extent. However, when the Cu content is too high, the hot workability of the alloy deteriorates. Therefore, the Cu content is 0 to 3.0%. The upper limit of the Cu content is preferably 2.5%, and more preferably less than 2.0%. The lower limit of the Cu content to effectively obtain the above-mentioned effects is preferably 0.01%.

[Formula 1)]

The NiCrFe alloy according to the present invention also fulfills Formula (1):

0.50 <Ti 48A1 / 27 <2.20 (1)

where, each element symbol of Formula (1) is replaced by the content (mass%) of the corresponding element.

fn1 = Ti 48A1 / 27 is an index to indicate the amount of precipitation of y '. fn1 indicates a total amount of Ti when the amount of Al is converted to the amount of Ti. When fn1 is less than 0.50, a sufficient amount of precipitation of y 'will not be obtained, so that the NiCrFe alloy cannot obtain excellent creep resistance. On the other hand, when fn1 is greater than 2.20, the stress relaxation cracking resistance, yield ductility and toughness of the alloy will deteriorate due to excessive amount of precipitation of y '. Therefore, fn1 is 0.50 to 2.20. In this range, an appropriate amount of y 'precipitates and excellent creep resistance is obtained. The upper limit of fn1 is preferably 2.00. The lower limit of fn1 is preferably 0.65.

[Formula (2)]

The chemical composition described above additionally complies with Formula (2):

0.40 <Ti / (Ti 48A1 / 27) <0.80 (2)

where, each element symbol of Formula (2) is replaced by the content (mass%) of the corresponding element.

fn2 = Ti / (Ti 48A1 / 27) is a ratio of the Ti content with respect to the total content of Al and Ti, the content of Al being converted into the amount of Ti. When fn2 is less than 0.40, the Ti content is too low relative to the Al content, and the precipitation amount of y 'is reduced. As a result, NiCrFe alloy cannot obtain excellent creep resistance. On the other hand, when fn2 is greater than 0.80, the Ti content becomes excessive with respect to the Al content, so that, although the fine y 'precipitates at an early stage of creep, the y' changes to a thick and acicular n phase over time. As a result, the yield strength and toughness of the alloy deteriorate. Therefore, fn2 is 0.40 to 0.80. In this range, y 'precipitates in an appropriate amount, and will not change to phase n even after a longer time, so that excellent creep resistance is obtained. The upper limit of fn2 is preferably 0.75.

[Formula (3)]

The chemical composition described above additionally complies with Formula (3):

£ [MTR / (A (MTR))] - S / 32 - 2/3 * O / 16> 0 (3)

where, each element symbol of Formula (3) is replaced by the content (mass%) of the corresponding element, and A (MTR) is replaced by the atomic weight of each MTR.

fn3 = £ [MTR / (A (MTR))] - S / 32 - 2/3 * O / 16 is an index to indicate the amount of S that is segregated at the grain boundaries. When fn3 is a negative value, S is segregated at the grain boundaries, leading to embrittlement of the grain boundaries, so that the stress relaxation cracking resistance of the alloy deteriorates. On the other hand, when fn3 is not less than 0, the MTR immobilizes the S in the form of inclusions, thus reducing the content of S in the matrix. As a result, it is possible to improve the stress relaxation cracking resistance of the alloy. Therefore, fn3 is not less than 0.

[Method of production]

An example of the production method of the NiCrFe alloy of the present embodiment will be described. The production method of the present embodiment comprises an ingot production process (steel making process) and a hot rolled plate production process (hot working process). Hereinafter, each process will be described in detail.

[Steelmaking process]

First, alloys having the chemical compositions described above are cast. Melting is carried out using, for example, high frequency induction vacuum melting. Next, an ingot is produced by an ingot manufacturing method.

[Hot work process]

In hot work process, normally hot work is done one or more times. First, the ingot is heated, and then hot work is done. Hot working refers to, for example, hot forging and hot rolling. Hot work can be done by a well known method.

In addition, hot worked NiCrFe alloy can be cold worked. Cold working is, for example, cold rolling.

In addition, the NiCrFe alloy, which has undergone the work described above, can be subjected to heat treatment. The heat treatment temperature is preferably 1050 to 1200 ° C. Furthermore, after being heated and held, the NiCrFe alloy is preferably cooled with water.

In the illustrative production method described above, a method of producing a NiCrFe alloy plate has been described. However, the NiCrFe alloy can be an alloy rod or pipe. In other words, the shape of the product will not be limited. Also, in the case of the alloy pipe, it is preferable that hot work is done by hot extrusion.

The NiCrFe alloy produced by the processes described so far has excellent creep resistance and excellent stress relaxation cracking resistance.

[Microstructure]

In the NiCrFe alloy according to the present invention, the y 'phase and the n phase precipitate in a high temperature environment of use. In other words, the microstructure of the NiCrFe alloy according to the present The invention after being kept at 650 ° C for 3000 hours contains a total of 2 to 6 % by mass of phase y 'yn, wherein the number density of phase n is less than 5/100 | jm2. It should be noted that the y 'phase and n phase are also collectively referred to herein as "aging precipitates".

In case the NiCrFe alloy according to the present invention is subjected to an aging treatment to keep the alloy at 650 ° C for 3000 hours and then the total of phase y 'and phase n is less than 2% by mass, the amount of precipitation of y 'in the alloy will be reduced. As a result, NiCrFe alloy cannot obtain excellent creep resistance. On the other hand, if the same aging treatment is carried out and after the total of phase y 'and phase n is greater than 6% by mass, the amount of precipitation of y' may increase excessively. In that case, the alloy cannot obtain excellent resistance to stress relaxation cracking. Therefore, the total of phase y 'and phase n after the aging treatment is 2 to 6% by mass.

Specifically, the total of phase y 'and phase n can be measured by the following method. The NiCrFe alloy according to the present invention is subjected to an aging treatment to keep the alloy at 650 ° C for 3000 hours. A 10mm x 5mm x 50mm test sample is taken from the NiCrFe alloy after the aging treatment. When the alloy is an alloy plate, the test sample is taken from a middle part of the plate thickness of the alloy pipe. On the other hand, when the alloy is an alloy pipe, the test sample is taken from a middle part of the wall thickness. It should be noted that the weight of the test sample is measured beforehand.

The test sample taken is electrolyzed in a solution of 1% tartaric acid, 1% (NH4) 2SO4 and water to sample the electrolyte residue. The sampled residue is melted with a solution of HCl (1 + 4) and 20% tartaric acid at 60 ° C, and the solution is filtered. The filtrate is measured by ICP ( Inductively Coupled Plasma) emission spectrometry to determine the concentrations of Ti, Al and Ni in the residue. From the concentrations of Ti, Al and Ni determined in the residue and from the weight of the test sample, the content of Ti, Al and Ni of the y 'phase and the n phase of the test sample is determined. The sum of the contents of Ti, Al and Ni, which have been determined by the method described so far, is defined as a sum of the phase y 'and the phase n (% by mass).

In case the NiCrFe alloy according to the present invention is subjected to an aging treatment to keep the alloy at 650 ° C for 3000 hours and then the numerical density of phase n is not less than 5/100 jm 2, the part of y 'has changed to phase n. For that reason, NiCrFe alloy cannot obtain excellent creep resistance. Therefore, the numerical density of phase n after the aging treatment is less than 5/100 jm 2.

Specifically, the numerical density of phase n can be measured by the following method. The NiCrFe alloy according to the present invention is subjected to an aging treatment to keep the alloy at 650 ° C for 3000 hours. Microscopic observation is performed on the NiCrFe alloy after the aging treatment. Specifically, a microscopic test sample is taken from the NiCrFe alloy after the aging treatment. When the alloy is an alloy plate, the test sample is taken from a middle part of the plate thickness. On the other hand, when the alloy is an alloy pipe, the microscopic test sample is taken from a middle part of the wall thickness of the alloy pipe. The microscopic test sample taken is subjected to mechanical polishing. After mechanical polishing, the surface of the microscopic test sample is electrolytically corroded with 10% oxalic acid. Following electrolytic corrosion, the microscopic test sample is viewed through a Scanning Electron Microscope (SEM) in 5 fields of view, and an SEM image is created for each field of view. The magnification of the observation is 10,000 times, and the field of observation is, for example, 12 jm x 9 jm.

Phase y 'and phase n differ in their forms. Specifically, it is observed that y 'is spherical and that phase n is acicular. More specifically, an aspect ratio of y 'is less than 3, and a phase aspect ratio n is not less than 3. In this case, the expression "aspect ratio" means a value obtained by dividing the length of the major axis between the length of the minor axis for each aged precipitate.

In the SEM image described above of each visual field, the aging precipitates (y 'phase and n phase) are identified from the contrast. In addition, using image processing, the aspect ratios for the identified aging precipitates are calculated. To calculate an aspect ratio, a general purpose application software can be used. When a calculated aspect ratio is not less than 3, the aged precipitate is identified as phase n.

For an SEM image of each visual field, the number of phases n identified is counted to determine a sum of the numbers in all visual fields. Using the number of phases n in all visual fields and the area of all visual fields, the numerical density of phase n in an observation field of 100 jm 2 (number / 100 jm 2) is determined.

Examples

The alloys having chemical compositions indicated by the reference marks 1 to 15 shown in Table 1 were cast by the high frequency induction vacuum melting method.

[Table 1]

A 50 kg ingot was produced using one alloy from each reference brand. The ingot was hot forged and hot rolled to obtain a plate material with a thickness of 15 mm. Each plate material was kept at 1150 ° C for 30 minutes and subsequently the plate material was rapidly cooled (water cooling) and subjected to solution treatment. By the production processes described so far, NiCrFe alloy plate materials were produced. Using the NiCrFe alloy plate materials thus produced, the following tests were performed.

[Creep rupture test]

A test sample was made from the produced alloy plate material. The test sample was taken from a central part of the thickness of the alloy plate material parallel to the longitudinal direction (rolling direction). The sample was a round bar-shaped test sample, the parallel part of which had a diameter of 6 mm, and which had a gauge length of 30 mm. Using the test sample, a creep failure test was performed. The creep failure test was carried out under a tensile load of 70 MPa in an air atmosphere of 750 ° C. A test sample whose breakdown time was not less than 3000 hours was evaluated as "E" (Excellent), and those whose breakdown time was less than 3000 hours, as "NA" (Not Acceptable).

[Table 2]

TABLE 2

[Observation of microstructure]

From the alloy plate materials thus produced, test samples were manufactured by the method described above. The manufactured test samples were subjected to an aging treatment to keep them at 650 ° C for 3000 hours, and the sum (mass%) of the y 'phase and the n phase of each test sample was determined by the method described. previously. Furthermore, the number density of phase n (number / 100 pm2) was determined by the method described above. A sum of phase y 'and phase n less than 2% by mass was evaluated as "M" (Lower), that of 2 to 6% by mass, as "E" (Excellent), and that of more than 6 % by mass, such as "D" (Too much). Furthermore, those that showed a numerical density of phase n not less than 5/100 pm2 were evaluated as "n".

[Stress relaxation cracking test]

The alloy plate material produced was further cold worked. Specifically, the alloy plate material was cold rolled until its thickness became 12mm. The area reduction of this cold rolled was 20%. A test sample was manufactured from this plate material. alloy. The test sample was taken from a central part of the thickness of the alloy plate material parallel to the longitudinal direction (rolling direction). The sample was a round bar-shaped test sample, the parallel part of which had a diameter of 6 mm, and which had a gauge length of 30 mm. Using the sample, a stress relaxation cracking test was performed. The stress relaxation cracking test was conducted in such a way that the test sample was subjected to 10% tensile strain at a strain rate of 0.05 min-1 and was kept as is for 300 hours in an atmosphere. 650 ° C air pressure. A sample that did not break after being kept for 300 hours was evaluated as "E" (Excellent) and one that did break, as "NA" (Not Acceptable).

[Test results]

The test results are shown in Table 2.

Referring to Table 2, the chemical compositions of reference marks 1 to 8 were suitable, so that fn1 was 0.50 to 2.20, fn2 was 0.40 to 0.80, and fn3 was not less. a 0. For that reason, in the microstructure, the sum of the y 'phase and the n phase was 2 to 6% by mass. Furthermore, the numerical density of phase n was less than 5/100 | jm2. As a result, the creep break time was not less than 3000 hours, thus exhibiting excellent creep strength. Furthermore, none of the samples broke in the stress relaxation cracking test, exhibiting excellent resistance to stress relaxation cracking.

On the other hand, at benchmark 9, the value of fn1 was too low. For that reason, in the microstructure, the sum of phase y 'and phase n was less than 2% by mass, which was too low. As a result, the creep rupture time was less than 3000 hours, not exhibiting excellent creep resistance.

At benchmark 10, the value of fn1 was too high. For that reason, in the microstructure, the sum of the y 'phase and the n phase was greater than 6% by mass. Furthermore, the number density of phase n was less than 5/100 jm 2. In other words, in the microstructure, y 'was greater than 6% by mass, which was too high. As a result, the test sample broke in the stress relaxation cracking test, thus not exhibiting excellent resistance to stress relaxation cracking.

At benchmarks 11 and 12, the value of fn2 was too low. For that reason, in the microstructure, the sum of phase y 'and phase n was less than 2% by mass, which was too low. As a result, the creep rupture time was less than 3000 hours, not exhibiting excellent creep resistance.

At benchmark 13, the value of fn2 was too high. For that reason, in the microstructure, the numerical density of phase n was not less than 5/100 jm 2. As a result, the creep rupture time was less than 3000 hours, not exhibiting excellent creep resistance.

At benchmark 14, the value of fn3 was too low. As a result, the sample broke in the stress relaxation cracking test, thus not exhibiting excellent resistance to stress relaxation cracking. This is considered because the S of the matrix could not be immobilized.

At reference mark 15, the MTR content was too low. Also, the value of fn3 was too low. As a result, the test sample broke in the stress relaxation cracking test, not exhibiting excellent resistance to stress relaxation cracking. This is considered because the S of the matrix could not be immobilized.

So far, embodiments of the present invention have been described. However, the embodiments described above are merely an illustration for practicing the present invention. Therefore, the present invention will not be limited to the embodiments described above, and can be practiced by appropriately modifying the embodiments described above within a range that does not depart from the scope of the appended claims.

Industrial applicability

The present invention can be widely applied to uses requiring high creep strength and high stress relaxation cracking resistance. Particularly, the present invention can be suitably used for high temperature elements of thermal power generating boilers, oil refining plants and chemical industry, or the like.

Claims (6)

1. An alloy of NiCrFe, comprising a chemical composition consisting of: in % by mass, C: 0.03 to 0.15%, If: no more than 1.00%, Mn: not more than 2.00%, P: not more than 0.040%, S: not more than 0.0050%, Cr: from 18.0 to 25.0%, Ni: 25.0 to 40.0%, Ti: 0.10 to 1.60%, Al: from 0.05 to 1.00%, N: not more than 0.020%, Or: not more than 0.008%, Rare Earth Metal (MTR): 0.001 to 0.100%, B: 0 to 0.010%, Ca: 0 to 0.010%, Mg: 0 to 0.010%, V: 0 to 0.5%, Nb: 0 to 1.0%, Ta: from 0 to 1.0%, Hf: 0 to 1.0%, Mo: 0 to 1.0%, W: 0 to 2.0%, Co: 0 to 3.0%, and Cu: from 0 to 3.0%, the rest being Fe and impurities, the chemical composition complying with the following Formulas (1) to (3): 0.50 <Ti 48A1 / 27 <2.20 (1) 0.40 <Ti / (Ti 48A1 / 27) <0.80 (2) £ [MTR / (A (MTR))] - S / 32 - 2/3 * O / 16> 0 (3) where, each element symbol of Formulas (1) to (3) is replaced by the content, mass%, of the corresponding element, and A (MTR) of Formula (3) is replaced by the atomic weight of each metal rare earth. 2. The NiCrFe alloy according to claim 1, wherein the chemical composition contains B: 0.0001 to 0.010%. 3. The NiCrFe alloy according to claim 1 or 2, wherein the chemical composition contains one or two types selected from the group consisting of Ca: from 0.0001 to 0.010%, and Mg: from 0.0001 to 0.010. 4. The NiCrFe alloy according to any one of claims 1 to 3, wherein the chemical composition contains one or more types selected from the group consisting of V: 0.01 to 0.5%, Nb: 0.01 to 1.0%, Ta: from 0.01 to 1.0%, and Hf: 0.01 to 1.0%. 5. The NiCrFe alloy according to any one of claims 1 to 4, wherein the chemical composition contains one or more types selected from the group consisting of Mo: 0.01 to 1.0%, W: 0.01 to 2.0%, Co: 0.01 to 3.0%, and Cu: 0.01 to 3.0%. 6. The NiCrFe alloy according to any one of claims 1 to 5, wherein NiCrFe alloy does not break for 300 hours or more in a stress relaxation test, in which NiCrFe alloy is subjected to 10% tensile strain at a strain rate of 0.05 min '1 and it is kept as is in an air atmosphere of 650 ° C after cold rolling with an area reduction of 20%.