High-Ni alloy having excellent high-temperature creep strength

Abstract

A high-Ni alloy having excellent high-temperature creep strength, which is characterized by containing, in mass%, 0.08% or less of C, 20-30% of Cr, 23-60% of Ni, 0.01-0.20% of Al, 0.05% or less of Ti, 0.0002-0.0050% of B, 0.10-0.30% of N, 0.006% or less of O, 0.0001-0.0050% of Ca, either one or more of Mo and W: Mo + 0.5 * W being 0.01-1.50%, 0.10-0.65% of Nb, and either one or more of V and Ta being 0.01-0.60% in total, and satisfying the following lt; a defined formula: Agt; , lt; defining formula Bgt; . Lt; a defined formula: Agt; 0.65 < = Nb% + 2 * V% + Ta% < = 1.50 lt; defining formula Bgt; (Nb% + 0.15 * V% + 0.4 * Ta%) * (C% + N%) < = 0.170.

Description

Technical Field

[0001] This invention relates to high-Ni alloys with excellent high-temperature creep strength for use in high-temperature applications requiring high-temperature creep strength. Background Technology

[0002] Alloy 800H (ASTM N08810, N08811), a high-Ni alloy used in heat-resistant applications, is a representative commercial alloy. In recent years, demand has been expanding in developing countries, requiring technological development to supply inexpensive products with good surface quality and performance characteristics. Therefore, a shift from the traditional ingot method to continuous casting is underway. High-Ni alloys are highly sensitive to internal cracks in the slab during casting, edge cracks during hot working, and surface defects in the finished product. Therefore, improvements and developments have been made in the design of the alloy's chemical composition, refining, casting, and hot working techniques, all based on the perspective of improving manufacturability in continuous casting.

[0003] On the other hand, in high-temperature reaction vessels in chemical plants, where this is the primary application, temperatures of 600°C or higher are often used, and furthermore, high pressures are applied to optimize chemical reaction efficiency. In these applications, higher creep strength allows for use with thinner wall thicknesses.

[0004] In recent years, there has been an increasing use of materials that incorporate Nb and N, and utilize Nb-based carbonitride precipitates for precipitation strengthening, as described in ASTM N08120. Non-Patent Literature 1 is an ASTM standard specification for Ni-Fe-Cr alloys, which also includes ASTM N08120 as an object. In this specification, the final heat treatment temperature is described as 1177°C or higher.

[0005] As for high-Ni alloys that utilize precipitation strengthening from the presence of Nb and N (hereinafter also referred to as "Nb- and N-containing high-Ni alloys"), there are prior art documents such as Patent Documents 1 and 2. These all employ alloy designs that, as described above, comprehensively utilize precipitation strengthening from carbonitride precipitates such as Nb, solid solution strengthening from the addition of Mo, and grain boundary strengthening from the addition of B. To improve the creep strength resulting from Nb precipitation strengthening, the final heat treatment temperature after hot rolling is set to a maximum of 1300°C, a higher temperature than that of typical Fe-based high-Ni alloys.

[0006] Existing technical documents Patent documents Patent Document 1: Japanese Patent No. 6675846 Patent Document 2: Japanese Patent No. 7174192 Non-patent literature Non-Patent Document 1: ASTM B409 Summary of the Invention

[0007] The problem that the invention aims to solve Both Patent Documents 1 and 2 improve creep strength by controlling the precipitation of Nb carbonitrides. However, both Patent Documents 1 and 2 set an upper limit for the amount added based on the product of the Nb and N content, indicating that the solubility product of Nb with C and N is small. Therefore, in the final heat treatment after hot rolling (hereinafter referred to as "final heat treatment"), even if the heat treatment temperature is increased to around 1300°C, large, undissolved carbonitrides will remain. In particular, as described in Patent Document 1, if there is too much undissolved carbonitride, it may actually reduce creep strength. Therefore, it is believed that the composition systems of Patent Documents 1 and 2 are unlikely to further improve creep strength. Some parts of Patent Documents 1 and 2 conform to the ASTM N08120 standard, but it is difficult to solve the problems of Patent Documents 1 and 2 and further improve creep strength simply by using compositions within the range specified in ASTM N08120.

[0008] The purpose of this invention is to solve the above-mentioned problems and provide a high-Ni alloy with excellent high-temperature creep strength.

[0009] Methods for solving problems The inventors discovered that, as a means to solve the above-mentioned problems, by combining V or Ta with Nb, the creep strength is significantly improved compared with steel containing Nb alone, thus completing the present invention.

[0010] For V and Ta, since their solid solubility products with N are both greater than those with Nb, even if V and Ta are contained in amounts equal to Nb, they will not generate as many carbonitrides as Nb. On the other hand, below 900°C, which is the operating temperature range, the solid solubility products of V and Ta with N are similarly small to those of Nb, thus they precipitate as carbonitrides over time, resulting in improved creep strength. It was found that by combining these age-precipitated V and / or Ta with Nb, the creep strength was significantly improved compared to conventional high-Ni alloys containing Nb and N, thus completing the present invention.

[0011] That is, the main idea of ​​this invention is as follows.

[0012] (1) A high-Ni alloy with excellent high-temperature creep strength, characterized in that, by mass%, it contains: C: less than 0.08%, Si: 0.05~1.0%, Mn: 0.05~1.5%, P: less than 0.030%, S: less than 0.0015%, Cr: 20~30%, Ni: 23~60%, Al: 0.01~0.20%, Ti: less than 0.05%, and B: 0.0002~0.0050%. %, N: 0.10~0.30%, O: less than 0.006%, Ca: 0.0001~0.0050%, any one or more of Mo and W: Mo + 0.5 × W is more than 0.01% and less than 1.50%, Nb: 0.10~0.65%, any one or more of V and Ta: total 0.01~0.60%, the remainder consists of Fe and impurities, and satisfies the following <limiting formula A> and <limiting formula B>.

[0013] <Limited Form A> 0.65≤Nb%+2×V%+Ta%≤1.50 <Limited Form B> (Nb%+0.15×V%+0.4×Ta%)×(C%+N%)≤0.170 In formulas A and B, the element symbol with % indicates the content (mass %) of each element.

[0014] (2) The high-Ni alloy with excellent high-temperature creep strength according to (1) is characterized in that, instead of a portion of the above-mentioned Fe, it further contains, by mass%, one or more of the following: Cu: 0.01~0.50%, Co: 0.01~1.0%, Sn: 0.001~0.05%, Zn+Pb+Bi: less than 0.0010%, Mg: less than 0.0050%, Zr: 0.001~0.10%, Hf: 0.001~0.10%, La+Ce+Nd+Pr: 0.001~0.050%.

[0015] (3) The high-Ni alloy with excellent high-temperature creep strength according to [1] or [2] is characterized in that when a creep test according to JIS Z2271 is carried out at 800℃ and 100MPa, the creep fracture time exceeds 800 hours.

[0016] High-Ni alloys containing Nb and N are used in chemical plants, such as those for polycrystalline silicon manufacturing equipment, and in applications requiring high levels of high-temperature creep strength and resistance to high-temperature corrosion. In addition to Nb, the addition of V and / or Ta significantly improves creep strength compared to conventional methods, enabling the scaling up or thinning and lightweighting of these devices, and is expected to make a significant contribution to the development of the chemical and semiconductor industries. Furthermore, while the high-Ni alloys of this invention are primarily produced in the form of thick plates for the aforementioned plant applications, they can also be provided in a wide range of forms, including thin plates, tubes, coils, bars, and wires. The preferred thickness for thick plates is 5 mm to 80 mm. Attached Figure Description

[0017] Figure 1 This is a graph showing the effect of the side (A value) in <Constraint A> and the left side (B value) of <Constraint B> on creep fracture time. Detailed Implementation

[0018] Hereinafter, the reasons for the limitation described in claim 1 of the present invention will be explained first. Furthermore, the content of each component is expressed as a percentage by mass.

[0019] <Ingredients and Composition> C: Below 0.08% In heat-resistant materials, carbon (C) is typically added actively to ensure high-temperature strength. However, in this invention, as described later, creep strength is improved through aging precipitation strengthening of the Z-phase (CrNbN-based nitrides) that precipitates at the service temperature. Adding more than necessary amounts of C consumes Nb, which would otherwise be utilized as the Z-phase, as MX-based (M: metal, X: C or N) carbonitrides, thus actually reducing creep strength. Therefore, the amount of C added is limited to 0.08% or less in this invention.

[0020] Si: 0.05~1.0% Si is added at 0.05% or more to deoxidize and improve oxidation resistance. However, Si is also an element that lowers the melting point of steel; therefore, if the addition exceeds 1.0%, the hot ductility around 1200°C decreases, and the susceptibility to solidification cracking and liquefaction cracking during welding worsens. Furthermore, intermetallic compounds are more likely to precipitate, deteriorating high-temperature properties. Therefore, the upper limit of Si content is limited to 1.0%. A preferred upper limit is 0.7%, and a more preferred upper limit is 0.5%.

[0021] Mn: 0.05~1.5% Mn has the effect of increasing the stability of the austenite phase and improving heat resistance. Therefore, it is preferable to actively add Mn to the alloy of the present invention. To improve heat resistance, more than 0.05% Mn is added. However, when more than 1.5% Mn is added, intermetallic compounds are more likely to precipitate, the heat resistance deteriorates, and it also has an adverse effect on the susceptibility to solidification cracking. Therefore, the upper limit of Mn content is specified as 1.5%. The preferred upper limit is 1.3%, and the more preferred upper limit is 1.0%.

[0022] P: below 0.030% Phosphorus (P) is an element that inevitably mixes into the raw materials and has the effect of increasing the sensitivity to solidification cracks. Therefore, the P content is limited to 0.030% or less, preferably 0.025% or less.

[0023] S: below 0.0015% Sulfur (S) is an element that inevitably mixes into raw materials, deteriorating their heat workability and oxidation resistance. Therefore, its content is limited to 0.0015% or less, preferably 0.0010% or less. While S can be reduced through refining, extreme reductions in content lead to increased costs. Therefore, it is preferable to set the lower limit of S content to 0.0001%.

[0024] Cr: 20~30% Cr is an essential element for the oxidation resistance and high-temperature corrosion resistance of heat-resistant alloys used in high-temperature applications, and its content is 20% or more, preferably 22% or more. Furthermore, in this invention, the Z-phase precipitated during aging is utilized to improve creep strength, but Cr is indispensable to ensure the stability of this Z-phase. If Cr is less than 20%, a Cr-deficient layer is formed at the grain boundaries during aging, thus preventing the precipitation of the Z-phase near the grain boundaries and forming a precipitation-free zone. As a result, the grain boundary strength decreases, and the creep rupture time decreases. On the other hand, if the Cr content exceeds 30%, even with a large amount of Ni, the high-temperature microstructure stability decreases, intermetallic compounds precipitate, and the heat resistance deteriorates; therefore, the content is limited to 30% or less. A preferred upper limit for the Cr content is 28%, and a more preferred upper limit is 26%.

[0025] Ni: 23~60% Ni is an element that stabilizes the austenitic structure at high temperatures, improves resistance to various acids, high-temperature corrosion resistance to chlorides, and enhances toughness. Therefore, the steel contains 23% or more, preferably 28% or more, and more preferably over 35% Ni. By increasing the Ni content, more ferrite-forming elements such as Cr, W, Mo, V, and Nb, which are necessary to ensure heat resistance, can be included. On the other hand, Ni is an expensive alloy, and from a cost perspective, the upper limit in the steel of this invention is set at 60% or less. A preferred upper limit is 50%.

[0026] Al: 0.01~0.20% Al is a deoxidizing element and forms an ordered NiAl phase in high-Ni alloys, which improves high-temperature strength. In this invention, to promote deoxidation and desulfurization and improve hot workability, it is necessary to add 0.01% or more, preferably 0.03% or more of Al. On the other hand, when Al exceeds 0.20%, AlN precipitation hinders manufacturability and heat resistance. Therefore, the upper limit of Al content is set at 0.20%. The preferred upper limit of Al content is 0.15%, and a more preferred upper limit is 0.10%.

[0027] Ti: below 0.05% In high-Ni alloys containing nitrogen, Ti forms coarse TiN. The formation of coarse TiN not only negatively impacts manufacturability, surface quality, and aging toughness of the steel, but also hinders the fine precipitation of the Z phase, leading to a decrease in creep strength. Therefore, the upper limit of Ti content is specified as 0.05% in this invention. A preferred upper limit is 0.03%. Ti may also be absent.

[0028] B: 0.0002~0.0050% Boron (B) improves the hot workability and high-temperature creep strength of the embrittlement region II (around 1000°C), and is therefore actively added, especially for applications in high-temperature environments. Boron is an element that improves grain boundary strength through segregation at grain boundaries. The improvement in hot workability resulting from B content is obtained at 0.0002% or higher, therefore a lower limit of 0.0002% is set. On the other hand, Boron lowers the melting point of steel and is prone to segregation; therefore, excessive addition promotes solidification cracking and liquefaction cracking, particularly significantly negatively impacting the hot workability of the embrittlement region I (around 1200°C). Therefore, an upper limit of 0.0050% of the B content is specified. A preferred upper limit is 0.0030%.

[0029] N: 0.10~0.30% Nitrogen (N) is an essential element for utilizing the precipitation strengthening mechanism of the Z-phase employed in this invention. The Z-phase used in this invention is a Cr(Nb,V,Ta)N type nitride obtained by replacing a portion of Nb with V or Ta. In the high-Ni alloy of this invention, it is the most stable and abundant nitride among undissolved or age-precipitated nitrides. To improve high-temperature creep strength through Z-phase precipitation, at least 0.10% N needs to be added. The preferred lower limit for N is N (atomic %) ≥ Nb (atomic %) + V (atomic %) + Ta (atomic %). On the other hand, adding excess N exceeding 0.30% (mass %, hereinafter the same) not only causes bubble formation during refining but also results in coarse Z-phase remaining undissolved during final heat treatment, hindering the fine dispersion of age-precipitated Z-phase, thereby reducing creep strength. Therefore, the upper limit for N content is set at 0.30%. A preferred upper limit is 0.25%.

[0030] O: below 0.006% Oxygen (O) forms oxide inclusions with Ca, Mg, Al, and Ti in the alloys of this invention. The oxygen content corresponds to the total amount of oxide inclusions and is an important indicator of the deoxidation state of the alloy. When its content exceeds 0.006%, the desired deoxidation balance is not met, and nozzle clogging during continuous casting and surface defects caused by inclusions are easily generated. Therefore, the upper limit of the oxygen content is specified as 0.006%. The preferred upper limit of the oxygen content is 0.005%, and a more preferred upper limit is 0.004%. On the other hand, although reducing the oxygen content is beneficial for nozzle clogging and the suppression of high-temperature welding cracks by reducing oxide inclusions, it also generates excess Ca or Mg in the alloy, which becomes the main reason for the reduction of hot workability. Therefore, the oxygen content is preferably 0.0002% or more.

[0031] Ca: 0.0001~0.0050% Ca fixes sulfur (S) in the alloy as CaS, improving the alloy's hot workability, resistance to high-temperature weld cracking, and resistance to high-temperature oxidation. The reaction occurs as follows: Ca combines with oxygen in the alloy to form CaO and CaO-Al2O3, reducing the dissolved oxygen (free oxygen) in the alloy to almost zero. The remaining Ca then reacts with S in the alloy to form CaS. In the alloy of the present invention, to improve the above-mentioned properties, it contains 0.0001% or more, preferably 0.0003% or more, and more preferably 0.0005% or more of Ca. On the other hand, excessive Ca addition reduces the thermal ductility around 1100°C. Therefore, the upper limit of the Ca content is set to 0.0050%. The preferred upper limit of the Ca content is 0.0045%.

[0032] One or more of Mo and W: Mo + 0.5 × W is 0.01% or more and less than 1.50%. Mo and W are both elements that improve the high-temperature strength and high-temperature corrosion resistance of heat-resistant alloys. To achieve these effects, in this invention, either Mo or W, or a combination thereof, is added such that the value of Mo + 0.5 × W is 0.01% or more, preferably 0.05% or more, and more preferably 0.10% or more. Mo + 0.5 × W can exceed 0.3%. Regarding Mo, it is more preferable to add more than 0.3%. On the other hand, regarding hot workability, Mo has a greater negative impact than W. If the value of Mo + 0.5 × W is 1.50% or more, it leads to a significant reduction in hot workability. Therefore, in the composition system of this invention, the upper limit of Mo + 0.5 × W is set to be less than 1.50%. The value of Mo + 0.5 × W is more preferably less than 1.20%. A preferred upper limit for Mo is 0.78%, and a preferred upper limit for W is 1.95%.

[0033] Nb: 0.10~0.65% Like nitrogen (N), nitrogen (Nb) is an essential element for utilizing the precipitation strengthening mechanism of the Z-phase employed in this invention, and its effect is achieved by adding 0.10% or more. The preferred lower limit for Nb content is 0.20%. On the other hand, if Nb content exceeds 0.65%, coarse Z-phase remains undissolved after the final heat treatment. This coarse, undissolved precipitate hinders the fine dispersion of the Z-phase precipitated during aging at the operating temperature, thus not only reducing creep strength but also worsening aging toughness and weld cracking resistance. Therefore, the upper limit for Nb in this invention is specified as 0.65%. A preferred upper limit is 0.60%, and a more preferred upper limit is 0.55%.

[0034] One or more of V and Ta: The total is 0.01~0.60%. Both V and Ta are elements that contribute to the age-induced precipitation strengthening of the Z-phase utilized in this invention. Both precipitate as Cr(Nb,V,Ta)N, replacing Nb in the Z-phase, thus increasing creep strength by increasing the total amount of the Z-phase. Each element exhibits its effect by adding 0.01% or more; therefore, in this invention, any one or both are added in a total of 0.01% or more. A preferred lower limit is 0.02% or more, and a more preferred lower limit is 0.05% or more. On the other hand, additions exceeding 0.60% in total negatively impact hot workability and age-induced toughness; therefore, in this invention, an upper limit of 0.60% in total is specified. A preferred upper limit is 0.50%.

[0035] <Limited Form A> 0.65≤Nb%+2×V%+Ta%≤1.50 The inventors conducted in-depth research to improve the high-temperature creep strength to a level that was difficult to achieve in conventional Nb- and N-containing heat-resistant high-strength Fe-Cr-Ni alloys as described in references 1 and 2. The results showed that by adding V and / or Ta, which are the same Group V metals as Nb and have a larger solid solution limit with N than Nb, the creep strength could be further improved, thus completing this invention. The most important strengthening mechanism of this high-Ni alloy is age-induced precipitation strengthening, which manifests as the fine dispersion of the Z phase, a stable precipitate at the service temperature, during aging precipitation. For example, when held at a relatively high temperature of around 800°C for about 500 hours, the Z phase precipitates with a fine size of about 0.1 μm or less in diameter, significantly increasing the creep rupture time. In conventional steels where only Nb is added, the solid solubility product of Nb and N is small; therefore, if more than 0.65% Nb is added, the coarse Z phase remains undissolved during the final heat treatment and undergoes Ostwald growth in the service temperature range. Therefore, the fine Z-phase that hinders the uniform dispersion of aging-precipitated particles results in a decrease in creep strength. V and Ta have larger solubility limits than Nb, allowing for near-complete solubility in the final heat treatment temperature range. Therefore, even with the addition of V and Ta, the total amount of undissolved Z-phase remaining after the final heat treatment is hardly affected. On the other hand, at the operating temperature, most of V and Ta precipitate during aging as Cr(Nb,V,Ta)N, replacing Nb in the Z-phase.

[0036] Further in-depth research revealed that the strengthening ability of V, which has a smaller atomic weight than Nb, can be considered to be about twice that of Nb per unit mass%. On the other hand, although Ta has a larger atomic weight than Nb, it tends to have a smaller size of the Z phase precipitated during aging, and its strengthening ability per unit mass can be considered to be the same as that of Nb.

[0037] That is, it was found that the creep rupture time in the composition system of the present invention can be adjusted according to the relationship A value = Nb% + 2 × V% + Ta. When the A value is below 0.65, the creep strength remains at the level that can be achieved by adding Nb alone in conventional steel. On the other hand, when the A value is above 1.50, the hot workability, resistance to aging embrittlement, and microstructure stability of the alloy are significantly reduced. Therefore, the lower limit and upper limit of the A value are set to 0.65 and 1.50, respectively, and the above-mentioned <limiting formula A> is defined. The preferred lower limit of the A value is 0.75, and the preferred upper limit is 1.30.

[0038] <Limited Form B> (Nb%+0.15×V%+0.4×Ta%)×(C%+N%)≤0.170 Within the compositional range of the alloy of this invention, the residual unsolvated material after final heat treatment negatively impacts the relative creep strength; therefore, a limiting formula B is specified as an indicator of this. Let it be: B value = (Nb% + 0.15 × V% + 0.4 × Ta%) × (C% + N%) In the formula for the B value, the coefficients 0.15 and 0.4 for V and Ta are set to correspond to the different solid solubility products of each element with N or C. The undissolved phase is mainly the Z phase, but there is also a portion of the MX system (M: metal, X: carbonitrides that become C or N). In a high-power heat treatment furnace, heat treatment can be performed at a temperature of 1200℃~1300℃. However, even when the final heat treatment is performed at such a high temperature, when the B value exceeds 0.170, a large number of undissolved phases with a diameter of 0.2 μm or more, and some with a diameter exceeding 1 μm, remain. These coarse undissolved precipitates undergo Ostwald growth at the operating temperature, thus hindering the uniform dispersion of the fine Z phase precipitated during aging. For example, even if <Constraint A> is satisfied, the creep rupture time does not increase, but instead decreases. Therefore, <Constraint B> is set as described above. If we consider that a smaller value on the left side of <limiting formula B> (B value) promotes the solid solution of the coarse Z phase, then the preferred upper limit of the B value is 0.120, and the further preferred upper limit of the B value is 0.100.

[0039] In the manufacture of the alloy of the present invention, a heat treatment at 1180°C to 1300°C is performed as the final heat treatment after hot rolling. This corresponds to Non-Patent Document 1 (ASTM B409), which is the standard specification for Ni-Fe-Cr alloys in ASTM. The final heat treatment is generally referred to as solution heat treatment or solution annealing heat treatment, and is followed by water cooling. By performing the above-described final heat treatment, high creep strength can be imparted. That is, based on the composition of the present invention, by performing the final heat treatment under the above conditions, a creep rupture time exceeding 800 hours can be achieved in a creep test (according to JIS Z2271) at 800°C and 100 MPa. If a suitable creep rupture time cannot be obtained under the selected final heat treatment temperature conditions, a suitable creep rupture time can be achieved by further increasing the heat treatment temperature within the range below 1300°C. Besides creep strength, the optimal final heat treatment conditions to prevent the reduction of aging toughness and resistance to liquefaction cracks during welding caused by the granulation and coarsening of the microstructure are 1200℃~1250℃.

[0040] The high-Ni alloy of the present invention comprises the aforementioned components, with the remainder consisting of Fe and impurities. Next, the limiting reasons for claim 2 will be explained. Furthermore, the components shown below (mass %) can be selectively included to replace a portion of the aforementioned Fe.

[0041] Cu: 0.01~0.50% Cu is an element that improves the alloy's resistance to acid corrosion and its resistance to dew point corrosion, which is often a problem in high-temperature equipment. It also improves high-temperature strength and structural stability, and can therefore be added as needed. To improve these heat and corrosion resistance properties, the Cu content is 0.01% or more, preferably 0.02% or more, and more preferably 0.05% or more. On the other hand, if the Cu content exceeds 0.50%, embrittlement defects will occur during solidification; therefore, the upper limit is set at 0.50%.

[0042] Co: 0.01~1.0% Co is an effective element for improving the high-temperature structural stability and corrosion resistance of alloys. To improve these properties, it is contained at least 0.01%, preferably at least 0.02%, and more preferably at least 0.10%. When the content of Co exceeds 1.0%, it is an expensive element and therefore cannot achieve an effect commensurate with the cost; therefore, the upper limit is specified as 1.0%. The preferred upper limit for Co is 0.8%, more preferably 0.50%.

[0043] Sn: 0.001~0.05% Sn is an element that improves the corrosion resistance and high-temperature creep strength of steel by adding 0.001% or more, preferably 0.005% or more, and can be added as needed. However, the addition of Sn exceeding 0.05% reduces hot workability, so the upper limit is specified as 0.05%.

[0044] Zn+Pb+Bi: below 0.0010% Zn, Pb, and Bi all significantly reduce hot workability in austenitic single-phase alloys, therefore strict upper limits need to be specified. Preferably, Pb ≤ 0.0010%, Zn ≤ 0.0010%, and Bi ≤ 0.0010%, with the total of Pb, Zn, and Bi specified to be below 0.0010%.

[0045] Mg: below 0.0050% Since Mg is an element that exhibits desulfurization effects, even trace amounts can improve the hot workability of the alloy. However, excessive addition significantly reduces the hot workability around 900°C. Therefore, in this invention, the upper limit of Mg content is set at 0.0050%. A preferred upper limit is 0.0040%, and a more preferred upper limit is 0.0030%.

[0046] Zr: 0.001~0.10% Hf: 0.001~0.10% Both Zr and Hf can improve the steel's resistance to solidification cracking, hot workability, and high-temperature oxidation resistance by fixing P and S at a concentration of 0.001% or more, preferably 0.005% or more, and can be added as needed. On the other hand, excessive addition exceeding 0.10% forms coarse nitrides, leading to reduced creep strength and adversely affecting manufacturability. Therefore, the upper limit for their addition is specified as 0.10%.

[0047] La+Ce+Nd+Pr: 0.001~0.050% La, Ce, Nd, and Pr are elements that fix P and S by adding a total of 0.001% or more, preferably 0.005% or more, to improve the oxidation resistance and hot workability of steel. On the other hand, if the total addition of these elements exceeds 0.050%, coarse oxides and nitrides are formed, leading to manufacturability problems such as nozzle clogging during refining and a significant increase in surface defects. Therefore, the upper limit of their content is set at 0.050% based on the sum of these elements. Furthermore, methods for adding these elements include adding various metals or alloys, and adding a mixture of rare earth metals.

[0048] Example The following describes the embodiments. The inventors melted a high-Ni alloy in an MgO crucible using a 50kg vacuum melting furnace, added Al, Ti, Ca, and Mg as needed, and cast it into a 25kg square mold to obtain the high-Ni alloys with the compositions shown in Tables 1-1 and 1-2. Regarding the compositions shown in Table 1, blank columns indicate impurity levels. Furthermore, in the following tables, components and limiting formulas that deviate from the scope of this invention are indicated by underlining.

[0049] The cast billet, formed from the molten material, is conical in shape, approximately 105 mm square to 90 mm square, with a height of approximately 280 mm. This billet is hot-forged in a temperature range of 1200°C to 1000°C to achieve a thickness of 50 mm × 120 mm. Next, it undergoes a heat treatment at 1250°C for 3 hours, and immediately after removal from the heat treatment furnace, it is hot-rolled in a temperature range of 1200°C to 900°C to achieve a plate thickness of 16 mm. A final heat treatment is then performed at 1250°C for 1 hour, followed by water cooling. Creep test pieces with a measuring section diameter of 6 mm φ, a length of 80 mm, and a fixing section diameter of 12 mm φ are fabricated from this alloy plate, parallel to the rolling direction. The creep test is conducted according to JIS Z2271, measuring the time until fracture at 800°C and 100 MPa; a creep fracture time exceeding 800 hours is considered acceptable. The creep test results are shown in Table 2 and... Figure 1 .

[0050] exist Figure 1 In the graph, the horizontal axis represents the middle edge of <Constraint A> (A value), and the vertical axis represents the creep rupture time. Additionally, a white circle indicates that the left side of <Constraint B> (B value) is below 0.170, while a white square indicates that the left side of <Constraint B> (B value) exceeds 0.170. Figure 1 It can be seen that when the white circle symbol is used (B value is below 0.170) and the A value is above 0.65, that is, when both <limiting formula A> and <limiting formula B> are satisfied, a creep rupture time of more than 800 hours is achieved.

[0051] As shown in Table 2 and Figure 1 As shown, Examples No. 1 to 16 of the present invention, which all satisfy the composition range, <Limited Formula A>, and <Limited Formula B>, have a creep rupture time of 800 hours or more under creep test conditions of 800°C and 100 MPa. On the other hand, Comparative Examples No. 17 to 19, whose <Limited Formula A> middle edge (A value) is less than 0.65, have a creep rupture time of less than 800 hours. In addition, Comparative Examples No. 20 to 23, whose A value is 0.65 or more, but whose <Limited Formula B> left side (B value) exceeds 0.170, also have a creep rupture time of less than 800 hours. Furthermore, No. 24, with a Mo+0.5W value greater than 1.5, and No. 25, with a <Limited Formula> middle edge (A value) greater than 1.50, were both found to have edge cracks at the plate ends after hot rolling. Furthermore, in No. 26, with a Cu content exceeding 0.5%, cracks that propagate in the width direction from the center of the plate thickness at the front end were confirmed after hot forging. Steels No. 24-26 were judged to have poorer hot workability than other steels and were excluded from the creep test.

[0052] As can be seen from the above embodiments, the composition range of steel with creep strength exceeding 800 hours at 800℃ and 100MPa is clearly defined according to the present invention.

[0053] Industrial availability According to the present invention, it is possible to achieve thinner plate thicknesses or larger, longer-life equipment using high-Ni alloys containing Nb and N, suitable for applications requiring high-temperature creep strength, thereby increasing design freedom. Furthermore, these alloys can be widely used not only in high-temperature applications but also in structures used in applications requiring high corrosion resistance.

[0054] It can provide stable quality to meet the growing demand for high-Ni alloys, making a significant contribution to the development of the industry.

Claims

1. A high-Ni alloy with excellent high-temperature creep strength, characterized in that, The substance, by mass%, contains: C: less than 0.08%, Si: 0.05~1.0%, Mn: 0.05~1.5%, P: less than 0.030%, S: less than 0.0015%, Cr: 20~30%, Ni: 23~60%, Al: 0.01~0.20%, Ti: less than 0.05%, B: 0.0002~0.0050%, N: 0.10~0.30%, O: less than 0.006%, Ca: 0.0001~0.0050%, any one or more of Mo and W: Mo + 0.5 × W is more than 0.01% and less than 1.50%, Nb: 0.10~0.65%, any one or more of V and Ta: total 0.01~0.60%, with the remainder consisting of Fe and impurities, and satisfies the following <Constraint A> and <Constraint B>. <Limited Form A> 0.65≤Nb%+2×V%+Ta%≤1.50 <Limited Form B> (Nb%+0.15×V%+0.4×Ta%)×(C%+N%)≤0.170 In formulas A and B, the element symbol with % indicates the content (mass %) of each element.

2. The high-Ni alloy with excellent high-temperature creep strength according to claim 1, characterized in that, Instead of a portion of the Fe, it further contains, by mass%, one or more of the following: Cu: 0.01-0.50%, Co: 0.01-1.0%, Sn: 0.001-0.05%, Zn+Pb+Bi: less than 0.0010%, Mg: less than 0.0050%, Zr: 0.001-0.10%, Hf: 0.001-0.10%, and La+Ce+Nd+Pr: 0.001-0.050%.

3. The high-Ni alloy with excellent high-temperature creep strength according to claim 1 or claim 2, characterized in that, When a creep test according to JIS Z2271 is conducted at 800℃ and 100MPa, the creep rupture time exceeds 800 hours.

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