High-ni alloy having excellent high temperature creep strength

Abstract

This high-Ni alloy has excellent high temperature creep strength, is characterized by comprising, 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, one or more of Mo and W such that Mo+0.5×W is not less than 0.01% but less than 1.50%, 0.10-0.65% of Nb, and a total of 0.01-0.60% of one or more of V and Ta, and is characterized by satisfying <limitation formula A> and <limitation formula B>. <Limitation formula A> 0.65≤ Nb%+2×V%+Ta%≤1.50 <Limitation formula B> (Nb%+0.15×V%+0.4×Ta%)×(C%+N%)≤0.170

Description

High Ni alloy with excellent high temperature creep strength

[0001] The present invention relates to a high-Ni alloy having excellent high-temperature creep strength, which is used in high-temperature applications where high-temperature creep strength is required.

[0002] Alloy 800H (ASTM N08810, N08811) is a typical commercial high-Ni alloy used for heat-resistant applications. In recent years, demand has expanded in developing countries, necessitating technological developments to supply products with low cost, excellent surface quality, and excellent usability. To address this, a shift in manufacturing methods from the conventional ingot method to the continuous casting method is underway. High-Ni alloys are highly susceptible to internal slab cracking during casting, edge cracking during hot working, and surface defects in products. Therefore, improvements and developments in the design of alloy chemical compositions, refining, casting, and hot working technologies have been promoted with the aim of improving manufacturability in the continuous casting method.

[0003] On the other hand, its main application is in high-temperature reactors at chemical plants, where it is often used at temperatures of 600°C or higher, and also under high pressure to improve the efficiency of chemical reactions. In these applications, the higher the creep strength, the thinner the wall thickness can be.

[0004] In recent years, there has been an increase in the use of materials that incorporate Nb and N, such as ASTM N08120, and utilize precipitation strengthening through Nb carbonitride precipitates. Non-Patent Document 1 is an ASTM standard for Ni-Fe-Cr alloys, and includes ASTM N08120. The standard specifies that the final heat treatment temperature should be 1177°C or higher.

[0005] Prior art documents such as Patent Documents 1 and 2 describe high-Ni alloys that utilize precipitation strengthening due to the inclusion of Nb and N (hereinafter also referred to as "Nb- and N-containing high-Ni alloys"). As described above, these all have alloy designs that utilize a combination of precipitation strengthening due to carbonitride precipitates such as Nb, solid solution strengthening due to the addition of Mo, and grain boundary strengthening due to the addition of B. In order to increase creep strength due to the precipitation strengthening of Nb, the final heat treatment temperature after hot rolling is set to a maximum of 1300°C, which is a higher temperature than that of ordinary Fe-based high alloys.

[0006] Patent No. 6675846 Patent No. 7174192

[0007] ASTM B409

[0008] Both Patent Documents 1 and 2 improve creep strength by controlling the amount of Nb carbonitride precipitation. However, as can be seen from the fact that both Patent Documents 1 and 2 set the upper limit of the Nb content as the product of the Nb and N contents, Nb has a small solid solubility product with C and N. Therefore, even when the heat treatment temperature is raised to around 1300°C in the final heat treatment after hot rolling (hereinafter simply referred to as "final heat treatment"), undissolved coarse carbonitrides remain. As noted in Patent Document 1 in particular, excessive undissolved carbonitrides have the drawback of reducing creep strength. Therefore, it was considered difficult to further improve creep strength with the compositional systems described in Patent Documents 1 and 2. Although some of Patent Documents 1 and 2 fall under ASTM N08120, it was difficult to solve the problems described in Patent Documents 1 and 2 and further improve creep strength using only the components specified in ASTM N08120.

[0009] An object of the present invention is to solve the above problems and to provide a high-Ni alloy that is excellent in high-temperature creep strength.

[0010] The present inventors discovered that, as a means for solving the above problems, the creep strength is significantly improved by including V or Ta in combination with Nb compared to steel containing Nb alone, and arrived at the present invention.

[0011] Because V and Ta both have a larger solid solubility product with N than Nb, they do not form as many carbonitrides as Nb even when contained in the same amount. On the other hand, at temperatures below 900°C, which is the operating temperature range, V and Ta have a small solid solubility product with N, just like Nb, and therefore precipitate as carbonitrides during aging, improving creep strength. The inventors discovered that by incorporating V and / or Ta, which precipitate during aging, in combination with Nb, creep strength is significantly improved compared to conventional high-Ni alloys containing Nb and N, and this led to the present invention.

[0012] That is, the gist of the present invention is as follows: (1) In mass %, C: 0.08% or less, Si: 0.05 to 1.0%, Mn: 0.05 to 1.5%, P: 0.030% or less, S: 0.0015% or less, Cr: 20 to 30%, Ni: 23 to 60%, Al: 0.01 to 0.20%, Ti: 0.05% or less, B: 0.0002 to 0.0050%, N: 0.10 to 0.30%, O: 0.006% or less, Ca: 0.00 A high-Ni alloy with excellent high-temperature creep strength, characterized in that it contains 0.01 to 0.0050% of Ni, at least one of Mo and W (Mo + 0.5 × W: 0.01% to less than 1.50%), 0.10 to 0.65% of Nb, a total of 0.01 to 0.60% of at least one of V and Ta, with the balance being Fe and impurities, and satisfies the following <Limiting Formula A> <Limiting Formula B> (Nb% + 0.15 × V% + 0.4 × Ta%) × (C% + N%) ≦ 0.170 In Limiting Formula A and Limiting Formula B, the element symbols with % indicate the content (mass%) of each element. (2) The high-Ni alloy having excellent high-temperature creep strength according to (1), characterized in that, in place of a portion of the Fe, it further contains, by mass%, one or more of Cu: 0.01 to 0.50%, Co: 0.01 to 1.0%, Sn: 0.001 to 0.05%, Zn + Pb + Bi: 0.0010% or less, Mg: 0.0050% or less, Zr: 0.001 to 0.10%, Hf: 0.001 to 0.10%, and La + Ce + Nd + Pr: 0.001 to 0.050%. (3) The high-Ni alloy having excellent high-temperature creep strength according to (1) or (2), characterized in that, when a creep test in accordance with JIS Z2271 is carried out under conditions of 800°C and 100 MPa, the creep rupture time exceeds 800 hours.

[0013] High-Ni alloys containing Nb and N are used in applications requiring high levels of high-temperature creep strength and high-temperature corrosion resistance, such as reaction vessels in chemical plants, such as polycrystalline silicon manufacturing equipment. The combined addition of V and / or Ta in addition to Nb significantly improves creep strength compared to conventional methods, enabling these facilities to be enlarged or thinned and lightweight, which is expected to significantly contribute to the development of the chemical and semiconductor industries. Furthermore, the high-Ni alloy of the present invention is primarily produced in the form of thick plates used in the above-mentioned plant applications, but can also be provided in a wide range of product forms, including thin plates, pipes, coils, steel bars, and wire rods. The preferred thickness of the thick plates is 5 mm to 80 mm.

[0014] FIG. 1 is a diagram showing the influence of the middle term (A value) of <Constraint Formula A> and the left term (B value) of <Constraint Formula B> on creep rupture time.

[0015] First, the reasons for the limitations set forth in claim 1 of the present invention will be explained below. The content of each component is expressed in mass %. <Component Composition>

[0016] C: 0.08% or less In heat-resistant materials, C is usually an element that is actively added to ensure high-temperature strength. However, in the present invention, as described below, creep strength is improved by strengthening the Z phase (CrNbN-based nitrides) that age precipitates at the operating temperature. Adding more C than necessary consumes Nb, which should be utilized as the Z phase, as MX-based (M: metal, X: C or N) carbonitrides, which actually reduces creep strength. Therefore, in the present invention, the amount of C added is limited to 0.08% or less.

[0017] Si: 0.05-1.0% Si is added in an amount of 0.05% or more to improve deoxidation and oxidation resistance. However, because Si is also an element that lowers the melting point of steel, adding more than 1.0% Si reduces hot ductility at temperatures around 1200°C and worsens solidification cracking susceptibility and liquation cracking susceptibility during welding. In addition, intermetallic compounds are more likely to precipitate, deteriorating high-temperature properties. Therefore, the upper limit of the Si content is limited to 1.0%. A preferred upper limit is 0.7%, and a more preferred upper limit is 0.5%.

[0018] Mn: 0.05-1.5% Mn has the effect of increasing the stability of the austenite phase and improving heat resistance. For this reason, it is preferable to actively add Mn to the alloy of the present invention. To improve heat resistance, 0.05% or more Mn is added. However, adding Mn in excess of 1.5% conversely makes intermetallic compounds more likely to precipitate, degrading heat resistance and adversely affecting solidification cracking susceptibility. Therefore, the upper limit of the Mn content is set at 1.5%. A preferred upper limit is 1.3%, and a more preferred upper limit is 1.0%.

[0019] P: 0.030% or less P is an element that is inevitably mixed in from the raw materials and has the effect of increasing solidification cracking susceptibility, so the P content is limited to 0.030% or less, preferably 0.025% or less.

[0020] S: 0.0015% or less S is an element that is inevitably mixed in from raw materials and deteriorates hot workability and oxidation resistance, so the content is limited to 0.0015% or less, preferably 0.0010% or less. Although the S content can be reduced by refining, an extreme reduction in the content increases costs. For this reason, it is preferable to set the lower limit of the S content to 0.0001%.

[0021] Cr: 20-30%. Cr is an essential element for imparting oxidation resistance and high-temperature corrosion resistance to heat-resistant alloys used as high-temperature materials. Its content is 20% or more, preferably 22% or more. In addition, the present invention utilizes the Z-phase, which precipitates during aging, to improve creep strength. Cr is essential for ensuring stable Z-phase formation. If the Cr content is below 20%, a Cr-depleted zone forms at the grain boundaries during aging, preventing Z-phase precipitation near the grain boundaries and forming a precipitate-free zone. This results in a decrease in grain boundary strength and a decrease in creep rupture time. On the other hand, if the Cr content exceeds 30%, high-temperature structural stability is reduced, even with a high Ni content, leading to the precipitation of intermetallic compounds, which degrades heat resistance. Therefore, the Cr content is limited to 30% or less. The preferred upper limit of the Cr content is 28%, and even more preferably 26%.

[0022] Ni: 23 to 60% Ni is an element that stabilizes the austenite structure at high temperatures and improves corrosion resistance to various acids, high-temperature corrosion resistance to chlorides, toughness, and the like. Therefore, Ni is contained in an amount of 23% or more, preferably 28% or more, and more preferably more than 35%. Increasing the Ni content makes it possible to include more ferrite-forming elements such as Cr, W, Mo, V, and Nb, which are necessary to ensure heat resistance. However, Ni is an expensive alloy, and in the steel of the present invention, the upper limit is set to 60% or less from the viewpoint of cost. The preferred upper limit is 50%.

[0023] Al: 0.01 to 0.20% Al is a deoxidizing element and also has the effect of forming an NiAl ordered phase in high-Ni alloys, thereby increasing high-temperature strength. In the present invention, the addition of 0.01% or more, preferably 0.03% or more, of Al is necessary to promote deoxidation and desulfurization and improve hot workability. On the other hand, if Al exceeds 0.20%, AlN precipitation occurs, which impairs manufacturability and heat resistance characteristics. For this reason, the upper limit of the Al content is set at 0.20%. The preferred upper limit of the Al content is 0.15%, and the more preferred upper limit is 0.10%.

[0024] Ti: 0.05% or less In high-Ni alloys containing N, Ti forms coarse TiN. The formation of coarse TiN not only adversely affects manufacturability, the surface quality and aging toughness of the steel, but also inhibits the fine precipitation of Z-phase, resulting in a decrease in creep strength. Therefore, in the present invention, the upper limit of the Ti content is specified as 0.05%. The preferred upper limit is 0.03%. Ti may not be contained.

[0025] B: 0.0002 to 0.0050% B improves hot workability and high-temperature creep strength in the region II embrittlement (near 1000°C), so it is actively added, especially in applications involving high-temperature environments. B is an element that segregates at grain boundaries to enhance grain boundary strength. Since the improvement in hot workability due to B content is achieved at 0.0002% or more, the lower limit was set at 0.0002%. However, because B lowers the melting point of steel and is prone to segregation, excessive addition promotes solidification cracking and liquation cracking, significantly adversely affecting hot workability, particularly in the region I embrittlement (near 1200°C). Therefore, the upper limit of the B content was set at 0.0050%. A preferred upper limit is 0.0030%.

[0026] N: 0.10 to 0.30% N is an essential element for exerting the precipitation strengthening mechanism of the Z-phase utilized in the present invention. The Z-phase utilized in the present invention is CrNbN or a Cr(Nb,V,Ta)N-type nitride in which part of the Nb is replaced with V or Ta. Among the nitrides that remain in solution or precipitate during aging in the high-Ni alloy of the present invention, this is the most stable and abundant nitride. To improve high-temperature creep strength through Z-phase precipitation, the addition of 0.10% or more N is necessary. The preferred lower limit for N is N (atomic %) ≥ Nb (atomic %) + V (atomic %) + Ta (atomic %). On the other hand, excessive addition of N exceeding 0.30% (mass %, the same applies below) not only causes the generation of bubbles during refining, but also reduces creep strength by leaving coarse Z-phase undissolved during final heat treatment, inhibiting the fine dispersion of the Z-phase that precipitates during aging. Therefore, the upper limit of the N content is set to 0.30%. The preferred upper limit is 0.25%.

[0027] O: 0.006% or less. Oxygen (O) forms oxide-based inclusions with Ca, Mg, Al, and Ti in the alloy of the present invention. The oxygen content corresponds to the total amount of oxide-based inclusions and is an important indicator of the deoxidation state of the alloy. If the oxygen content exceeds 0.006%, the desired deoxidation equilibrium is not met and nozzle clogging and inclusion-induced surface defects during continuous casting are more likely to occur. Therefore, the upper limit of the oxygen content is set to 0.006%. The preferred upper limit of the oxygen content is 0.005%, and more preferably 0.004%. On the other hand, while a reduced oxygen content is advantageous in suppressing nozzle clogging and weld hot cracking by reducing oxide-based inclusions, it also generates excess Ca and Mg in the alloy, which can cause a decrease in hot workability. For this reason, the oxygen content is preferably 0.0002% or more.

[0028] Ca: 0.0001 to 0.0050% Ca fixes the S in the alloy as CaS, improving the hot workability, resistance to welding hot cracking, and resistance to high-temperature oxidation of the alloy. This reaction is as follows: Ca combines with oxygen in the alloy to form CaO, CaO-Al ２ O ３ After the dissolved oxygen (free oxygen) in the alloy is reduced to almost zero, the remaining Ca reacts with the S in the alloy to form CaS. To improve the above characteristics, the alloy of the present invention contains 0.0001% or more of Ca, preferably 0.0003% or more, and more preferably 0.0005% or more. On the other hand, excessive addition of Ca reduces hot ductility around 1100°C. For this reason, the upper limit of the Ca content is set to 0.0050%. The desirable upper limit of the Ca content is 0.0045%.

[0029] At least one of Mo and W is added to the alloy in a ratio of 0.01% to 1.50%. Mo and W both enhance the high-temperature strength and high-temperature corrosion resistance of heat-resistant alloys. To achieve these effects, the present invention adds either one of them alone or in combination so that the Mo + 0.5 × W value is 0.01% or greater, preferably 0.05% or greater, and more preferably 0.10% or greater. Mo + 0.5 × W may be greater than 0.3%. Adding more than 0.3% of Mo is even more preferable. On the other hand, Mo has a greater adverse effect on hot workability than W, and a Mo + 0.5 × W value of 1.50% or greater significantly deteriorates hot workability. Therefore, the upper limit of Mo + 0.5 × W in the present composition system is set to less than 1.50%. It is more preferable for Mo + 0.5 × W to be less than 1.20%. The upper limit of Mo is preferably 0.78%, and the upper limit of W is preferably 1.95%.

[0030] Nb: 0.10 to 0.65% Like N, Nb is an essential element for utilizing the precipitation strengthening mechanism of the Z-phase utilized in the present invention, and its effect is exerted by adding 0.10% or more. The preferred lower limit of the Nb content is 0.20%. On the other hand, if Nb is added in an amount exceeding 0.65%, coarse Z-phase remains undissolved even after final heat treatment. These coarse undissolved precipitates inhibit the fine dispersion of the Z-phase that precipitates during aging at the service temperature, thereby not only reducing creep strength but also worsening aging toughness and weld hot cracking resistance. For this reason, the upper limit of Nb is specified as 0.65% in the present invention. The preferred upper limit is 0.60%, and the more preferred upper limit is 0.55%.

[0031] A total of 0.01 to 0.60% of one or more of V and Ta. Both V and Ta contribute to the aging precipitation strengthening by the Z-phase utilized in the present invention. Both precipitate as Cr(Nb, V, Ta)N, substituting Nb in the Z-phase, and increase the total amount of Z-phase, thereby improving creep strength. Since the effects of each element are exhibited when added at 0.01% or more, the present invention adds one or both of them at a total of 0.01% or more. The preferred lower limit is a total of 0.02% or more, and the more preferred lower limit is a total of 0.05% or more. On the other hand, because addition of more than 0.60% in total adversely affects hot workability and aging toughness, the present invention specifies an upper limit of 0.60% in total. The preferred upper limit is 0.50%.

[0032] <Limiting Formula A> 0.65 ≦ Nb% + 2 × V% + Ta% ≦ 1.50 The present inventors conducted extensive research to improve high-temperature creep strength to a level difficult to achieve with conventional Nb- and N-containing heat-resistant, high-strength Fe—Cr—Ni alloys such as those described in References 1 and 2. As a result, they discovered that further improvement in creep strength is possible by adding Nb in combination with V and / or Ta, which are group V metals like Nb and have a larger solid solubility limit with N than Nb, and have arrived at the present invention. The most important strengthening mechanism of this high-Ni alloy is aging precipitation strengthening, which occurs when the Z-phase, a stable precipitate phase at operating temperatures, is finely dispersed during aging precipitation. For example, when held at a relatively high temperature of approximately 800°C for approximately 500 hours, the Z-phase is dispersed and precipitated in fine particles with a diameter of approximately 0.1 μm or less, significantly increasing the creep rupture time. In conventional steels containing only Nb, Nb has a small solid solubility product with N. Therefore, adding more than 0.65% Nb results in a coarse Z phase remaining undissolved during final heat treatment and undergoing Ostwald ripening at operating temperatures. This prevents the fine Z phase from being uniformly dispersed during aging, resulting in a decrease in creep strength. V and Ta have a larger solubility limit than Nb, allowing almost all of them to dissolve in the final heat treatment temperature range. Therefore, adding V and Ta has almost no effect on the total amount of Z phase remaining undissolved during final heat treatment. However, at operating temperatures, most of the V and Ta precipitate during aging as Cr(Nb, V, Ta)N, substituting for Nb in the Z phase.

[0033] Further investigations revealed that the strengthening capacity per unit mass% of V, which has a smaller atomic weight than Nb, can be considered to be approximately twice that of Nb. On the other hand, Ta, although having a larger atomic weight than Nb, tends to produce a relatively small Z phase during aging, and therefore its strengthening capacity per unit mass% can be considered to be approximately the same as that of Nb. In other words, it was discovered that in the composition system of the present invention, the creep rupture time can be expressed as follows: A value = Nb% + 2 × V% + Ta. When the A value is below 0.65, the creep strength remains at the level achievable with conventional steels containing only Nb. On the other hand, when the A value is above 1.50, the alloy's hot workability, aging embrittlement susceptibility, and microstructural stability are significantly reduced. For these reasons, the lower and upper limits of A value were set to 0.65 and 1.50, respectively, and the above-mentioned <Limiting Formula A> was established. The preferred lower limit of A value is 0.75, and the preferred upper limit is 1.30.

[0034] <Constraint Formula B> (Nb% + 0.15 x V% + 0.4 x Ta%) x (C% + N%) ≤ 0.170. Constraint Formula B was established as an index to measure the adverse effect of undissolved phases remaining at the final heat treatment on creep strength within the alloy composition range of the present invention. B-value = (Nb% + 0.15 x V% + 0.4 x Ta%) x (C% + N%). In the B-value formula, the V and Ta coefficients of 0.15 and 0.4 are set to correspond to the difference in the solid solubility product of each element with N or C. The undissolved phase is primarily a Z-phase, but some MX-based phases (M: metal, X: carbonitrides with C or N) are also present. In a relatively high-power heat treatment furnace, heat treatment can be performed at a heat treatment temperature of 1200°C to 1300°C. However, even when final heat treatment is performed at such high temperatures, if the B value exceeds 0.170, numerous undissolved phases with diameters of 0.2 μm or more, and even larger ones exceeding 1 μm, remain. These coarse undissolved precipitates undergo Ostwald ripening at the operating temperature, preventing the uniform dispersion of the fine Z phase that precipitates during aging. Even if <Constraint Equation A> is satisfied, the creep rupture time does not increase, but rather decreases. Therefore, the above <Constraint Equation B> was established. Considering that the smaller the left side of <Constraint Equation B> (B value), the more readily the coarse Z phase is dissolved, the more preferable the upper limit of B value is 0.120, and the even more preferable upper limit of B value is 0.100.

[0035] In the production of the alloy of the present invention, a final heat treatment after hot rolling is performed at 1180°C to 1300°C. This corresponds to ASTM B409, the ASTM standard for Ni—Fe—Cr alloys. This final heat treatment is generally called a solution heat treatment or a solution heat treatment, and is followed by water quenching. This final heat treatment imparts high creep strength. That is, by providing the composition of the present invention and performing the final heat treatment under the above conditions, a quality can be achieved in which the creep rupture time exceeds 800 hours in a creep test (based on JIS Z2271) at 800°C and 100 MPa. If a suitable creep rupture time cannot be obtained under the selected temperature conditions of the final heat treatment, a suitable creep rupture time can be achieved by further increasing the heat treatment temperature within a range of 1300°C or less. The optimum final heat treatment conditions for preventing a decrease in creep strength, aging toughness due to grain regulation and grain coarsening, and resistance to liquation cracking during welding, are 1200°C to 1250°C.

[0036] The composition of the high Ni alloy of the present invention contains the above-mentioned components, with the balance being Fe and impurities. Next, the reason for the limitation in claim 2 will be described. Furthermore, instead of a part of the Fe, the following components (mass %) can be selectively contained.

[0037] Cu: 0.01 to 0.50% Cu is an element that increases the alloy's acid corrosion resistance and dew-point corrosion resistance, which is often a problem in high-temperature equipment, and also has the effect of improving high-temperature strength and structural stability, so it can be added as needed. To improve these heat resistance and corrosion resistance, Cu is contained in an amount of 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-induced defects will occur during solidification, so the upper limit is set at 0.50%.

[0038] Co: 0.01 to 1.0% Co is an element effective in improving the high-temperature structural stability and corrosion resistance of the alloy, and in order to improve these properties, it is contained in an amount of 0.01% or more, preferably 0.02% or more, and more preferably 0.10% or more. Co is an expensive element, and if it is contained in an amount exceeding 1.0%, the effect will not be commensurate with the cost, so the upper limit is set at 1.0%. The preferred upper limit of Co is 0.8%, and the more preferred upper limit is 0.50%.

[0039] Sn: 0.001 to 0.05% Sn is an element that improves the corrosion resistance and high-temperature creep strength of steel when added in an amount of 0.001% or more, preferably 0.005% or more, and can be added as needed. However, because the addition of Sn in excess of 0.05% reduces hot workability, the upper limit is set at 0.05%.

[0040] Zn + Pb + Bi: 0.0010% or less Zn, Pb, and Bi all significantly reduce the hot workability of austenitic single-phase alloys, so their upper limits must be strictly defined. Preferably, Pb≦0.0010%, Zn≦0.0010%, and Bi≦0.0010%, with the total of Pb, Zn, and Bi being 0.0010% or less.

[0041] Mg: 0.0050% or less Mg is an element that exhibits a desulfurization effect, and therefore even a small amount can improve the hot workability of the alloy. However, excessive addition of Mg significantly reduces the hot workability at temperatures around 900°C. Therefore, in the present invention, when Mg is added, the upper limit of the Mg content is set to 0.0050%. A preferred upper limit is 0.0040%, and a more preferred upper limit is 0.0030%.

[0042] Zr: 0.001 to 0.10% Hf: 0.001 to 0.10% Addition of 0.001% or more, preferably 0.005% or more, of either Zr or Hf has the effect of fixing P and S, thereby improving the steel's solidification cracking susceptibility, hot workability, and high-temperature oxidation resistance, and can be added as needed. On the other hand, addition of a large amount exceeding 0.10% forms coarse nitrides, which reduces creep strength and has a negative effect on manufacturability. Therefore, the upper limit of these addition amounts is specified at 0.10%.

[0043] La + Ce + Nd + Pr: 0.001 to 0.050%. Adding a total of 0.001% or more, preferably 0.005% or more, of La, Ce, Nd, and Pr immobilizes P and S, improving the oxidation resistance and hot workability of steel. On the other hand, adding these elements in a total amount exceeding 0.050% significantly impairs manufacturability by producing coarse oxides and nitrides, leading to nozzle clogging during refining and increased surface defects. Therefore, the upper limit of the total content of these elements is set at 0.050%. These elements can be added as individual metals or alloys, or as misch metals.

[0044] Examples are described below. The inventors melted high-Ni alloys in MgO crucibles using a 50 kg vacuum melting furnace, added Al, Ti, Ca, and Mg as necessary, and cast the alloys into 25 kg square molds to obtain high-Ni alloys with the compositions shown in Tables 1-1 and 1-2. Blanks in the components shown in Table 1 indicate impurity levels. In the following tables, components and limiting formulas outside the scope of the present invention are underlined.

[0045]

[0046]

[0047] The slabs cast from the melted material were tapered, measuring approximately 105 mm square to 90 mm square, and approximately 280 mm high. These slabs were hot forged at temperatures ranging from 1200°C to 1000°C to produce 50 mm thick x 120 mm wide pieces. They were then soaked at 1250°C for 3 hours and hot rolled to a thickness of 16 mm at temperatures ranging from 1200°C to 900°C immediately after removal from the heat treatment furnace. A final heat treatment was then performed at 1250°C for 1 hour, followed by water quenching. Creep test specimens were prepared from the alloy plates parallel to the rolling direction, measuring 6 mm in diameter, 80 mm in length, and 12 mm in diameter at the fixed portion. Creep tests were performed in accordance with JIS Z2271, measuring the time to rupture at 800°C and 100 MPa. Specimens with creep rupture times exceeding 800 hours were deemed acceptable. The creep test results are shown in Table 2 and Figure 1.

[0048] In Figure 1, the horizontal axis represents the center of <Constraint Formula A> (A value), and the vertical axis represents creep rupture time. Furthermore, open circles indicate that the left side of <Constraint Formula B> (B value) is 0.170 or less, and open squares indicate that the left side of <Constraint Formula B> (B value) is greater than 0.170. As is clear from Figure 1, when a open circle (B value 0.170 or less) indicates that the A value is 0.65 or more, i.e., when both <Constraint Formula A> and <Constraint Formula B> are satisfied, a creep rupture time of 800 hours or more is achieved.

[0049]

[0050] As shown in Table 2 and Figure 1, Nos. 1 to 16 of the present invention, which satisfied all of the ranges of the composition, <Constraint Formula A>, and <Constraint Formula B>, achieved a rupture time of 800 hours or more under creep test conditions of 800°C and 100 MPa. On the other hand, Nos. 17 to 19 of the comparative examples, in which the middle term (A value) of <Constraint Formula A> was less than 0.65, also achieved a creep rupture time of less than 800 hours. Furthermore, Nos. 20 to 23 of the comparative examples, in which the left term (B value) of <Constraint Formula B> (B value) exceeded 0.170 despite having an A value of 0.65 or greater, also achieved a creep rupture time of less than 800 hours. Note that No. 24, in which the Mo+0.5W value exceeded 1.5, and No. 25, in which the middle term (A value) of the constraint formula exceeded 1.50, both exhibited edge cracking at the sheet edge after hot rolling. Furthermore, in No. 26, which had a Cu content exceeding 0.5%, cracks propagating in the width direction were observed in the center of the plate thickness at the tip end after hot forging. Nos. 24 to 26 were judged to have inferior hot workability compared to the other steel materials and were therefore excluded from the creep test.

[0051] As can be seen from the above examples, the present invention has clarified the range of chemical components for steels having creep strength exceeding 800 hours at 800° C. and 100 MPa.

[0052] The present invention enables the design thickness of Nb- and N-containing high-Ni alloys to be reduced when used in applications requiring high-temperature creep strength, or allows for larger equipment and longer service life, thereby improving design flexibility. Furthermore, these alloys can be widely used not only for high-temperature applications but also for structures used in highly corrosion-resistant applications. This will enable stable quality to be provided to meet the expanding demand for high-Ni alloys, greatly contributing to the development of the industry.

Claims

1. In mass%, C: 0.08% or less, Si: 0.05-1.0%, Mn: 0.05-1.5%, P: 0.030% or less, S: 0.0015% or less, Cr: 20-30%, Ni: 23-60%, Al: 0.01-0.20%, Ti: 0.05% or less, B: 0.0002-0.0050%, N: 0.10-0.30%, O: 0.006% or less, Ca: 0.0001 A high Ni alloy with excellent high-temperature creep strength, characterized in that it contains up to 0.0050%, at least one of Mo and W (Mo+0.5×W: 0.01% or more but less than 1.50%), 0.10-0.65% Nb, at least one of V and Ta in total 0.01-0.60%, with the balance consisting of Fe and impurities, and satisfies the following <Limiting Formula A> and <Limiting Formula B>. <Limiting Formula A> 0.65≦Nb%+2×V%+Ta%≦1.50 <Limiting Formula B> (Nb%+0.15×V%+0.4×Ta%)×(C%+N%)≦0.170 In Limiting Formula A and Limiting Formula B, the element symbols with % mean the content (mass%) of each element.

2. A high Ni alloy with excellent high temperature creep strength as claimed in claim 1, further comprising, in mass %, one or more of Cu: 0.01-0.50%, Co: 0.01-1.0%, Sn: 0.001-0.05%, Zn+Pb+Bi: 0.0010% or less, Mg: 0.0050% or less, Zr: 0.001-0.10%, Hf: 0.001-0.10%, and La+Ce+Nd+Pr: 0.001-0.050% in place of a portion of the Fe.

3. A high Ni alloy with excellent high temperature creep strength as claimed in claim 1 or 2, characterized in that the creep rupture time exceeds 800 hours when a creep test conforming to JIS Z2271 is carried out under conditions of 800°C and 100 MPa.

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