HIGH Ni ALLOY HAVING SUPERIOR HIGH-TEMPERATURE CREEP STRENGTH AND METHOD FOR PRODUCING THE SAME

Abstract

To provide a high Ni alloy containing Nb and N that has superior high-temperature creep strength.SOLUTION: A high Ni alloy having superior high-temperature creep strength, the alloy comprising, in mass%, C: 0.08% 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: 0.0001-0.0050%, at least one of Mo and W in an amount of Mo+0.5×W: 0.01-1.50% (less than 1.50%), Nb: 0.10-0.65%, and at least one of V and Ta in a total amount of 0.01-0.60%, and satisfying the following [Limiting Expression A], and characterized in that the total amount (Nb) of Nb precipitated in the steel material is 0.11 mass% or less. [Limiting Expression A] 0.65≤Nb%+2×V%+Ta%≤1.60.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a high-Ni alloy having excellent high-temperature creep strength and used in high-temperature applications where high-temperature creep strength is required, and to a method for producing the same. [Background technology]

[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 enable the supply of products with low cost, good surface quality, and excellent usability. To this end, a shift in manufacturing methods from the conventional ingot method to continuous casting is underway. High-Ni alloys are highly susceptible to internal cracking of the slab during casting, edge cracking during hot working, and surface defects in the product. Therefore, efforts have been made to improve the manufacturability of continuous casting by improving and developing the design of the alloy's chemical composition, refining, casting, and hot working technologies.

[0003] On the other hand, its main application is in high-temperature reactors in 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 used.

[0004] In recent years, there has been an increasing use of materials containing Nb and N, such as ASTM N08120, which utilize precipitation strengthening by 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 mentioned above, these alloys are designed to 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. To increase creep strength due to the precipitation strengthening of Nb, the alloys are heat-treated at a maximum of 1300°C, which is higher than that of ordinary Fe-based high alloys. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Patent No. 6675846 [Patent Document 2] Patent No. 7174192 [Non-patent literature]

[0007] [Non-Patent Document 1] ASTM B409 Summary of the Invention [Problem to be solved by the invention]

[0008] Both Patent Documents 1 and 2 improve creep strength by controlling the amount of Nb carbonitride precipitation. However, as 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 solubility product with C and N. Even when the heat treatment temperature is increased to around 1300°C, coarse undissolved carbonitrides remain. As noted in Patent Document 1 in particular, excessive undissolved carbonitrides actually reduce creep strength. Therefore, further improvement of creep strength was considered difficult 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 issues described in Patent Documents 1 and 2 and further improve creep strength using only the components specified in ASTM N08120.

[0009] Furthermore, as suggested in Patent Document 1, to impart sufficient creep strength to these chemical compositions, it is desirable to heat the steel at an ultra-high temperature range of 1250°C to 1300°C in the final heat treatment. However, heat treatment above 1250°C causes significant damage to the furnace body, leading to increased furnace maintenance costs. Furthermore, only a limited number of steel sheet manufacturers are equipped with equipment capable of this, so the chemical composition must be designed according to the capabilities of the manufacturing equipment. Therefore, there has been a demand for steel whose chemical composition can be designed according to the capabilities of the heat treatment furnace.

[0010] The present invention aims to solve the above problems and to provide a high Ni alloy that has excellent high-temperature creep strength without excessively increasing the final heat treatment temperature in the production of the high Ni alloy, and a method for producing the same. [Means for solving the problem]

[0011] As a means for solving the above problems, the present inventors discovered that by incorporating V or Ta in combination with Nb and appropriately controlling (Nb) <total amount of Nb precipitated in steel>, creep strength can be significantly improved compared to steel containing Nb alone without excessively increasing the final heat treatment temperature in the production of high Ni alloys, and arrived at the present invention.

[0012] Because V and Ta both have a larger solid solubility product with N than Nb, adding them in the same amount as Nb does not produce as much carbonitride as Nb. 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 age-precipitate as carbonitrides, improving creep strength. The inventors discovered that adding V and / or Ta, which age-precipitate, in combination with Nb significantly improves creep strength compared to conventional high-Ni alloys containing Nb and N, which led to the present invention.

[0013] 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: 0.0001 to 0.0050%, Mo and W A high-Ni alloy with excellent high-temperature creep strength, characterized in that it contains at least one of Mo+0.5×W: 0.01% or more but less than 1.50%, Nb: 0.10 to 0.65%, and at least one of V and Ta in a total of 0.01 to 0.60%, with the balance consisting of Fe and impurities, and satisfies the following <Limiting Formula A>, and the total amount of Nb precipitated in the steel (Nb) is 0.11 mass% or less. <Limited formula A> 0.65 ≦ Nb%+2×V%+Ta% ≦ 1.60 In the limiting formula A, the element symbols with % indicate the content (mass %) of each element. [2] A high Ni alloy with excellent high-temperature creep strength according to [1], characterized in that it further contains, in 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%, in place of a portion of the Fe. [3] A high-Ni alloy with excellent high-temperature creep strength according to [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. [4] A method for producing a high Ni alloy having excellent high-temperature creep properties according to any one of [1] to [3], characterized in that the final heat treatment temperature T (K) during the final heat treatment satisfies the following limiting formula X: <Limited formula X> Nb%×N%-10 α ≦ 0.030 α = 4.1826 - 7948.9 ÷ T In the limiting formula X, the element symbols with % indicate the content (mass %) of each element. [Effects of the Invention]

[0014] 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 made larger or thinner and lighter, which is expected to contribute greatly to the development of the chemical and semiconductor industries. Furthermore, the high-Ni alloy of the present invention is primarily available 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. [Brief explanation of the drawings]

[0015] [Figure 1] FIG. 1 is a graph showing the relationship between the middle part of <Constraint Formula A> (A value) and the mass % of (Nb), which is the total amount of precipitated Nb, on the creep rupture time. DETAILED DESCRIPTION OF THE INVENTION

[0016] In the production of high-Ni alloys, a final heat treatment is performed after hot rolling. This final heat treatment is generally called a solution heat treatment or a solution heat treatment, and involves water cooling after the heat treatment. This heat treatment can impart high creep strength. It is necessary for high-Ni alloys produced in this way to achieve high creep strength at operating temperatures of 900°C or less, around 800°C.

[0017] 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>

[0018] C: 0.08% or less In heat-resistant materials, carbon is usually 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 carbon 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 carbon added is limited to 0.08% or less.

[0019] Si: 0.05 to 1.0% Si is added at 0.05% or more to improve deoxidation and oxidation resistance. However, because it also lowers the melting point of steel, adding more than 1.0% 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 is set to 1.0%. A preferred upper limit is 0.7%, and an even more preferred upper limit is 0.5%.

[0020] Mn: 0.05 to 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. 0.05% or more is added to improve heat resistance. However, adding more than 1.5% of Mn makes intermetallic compounds more likely to precipitate, deteriorating 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%.

[0021] 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 its content is limited to 0.030% or less, preferably 0.025% or less.

[0022] 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 it 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 at 0.0001%.

[0023] 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, and should be present in an amount of 20% or more, preferably 22% or more. In addition, the present invention utilizes the Z-phase, which precipitates during aging, to improve creep strength, and Cr is essential for ensuring stable formation of this Z-phase. 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, and intermetallic compounds precipitate, deteriorating heat resistance. Therefore, the content is limited to 30% or less. The preferred upper limit is 28%, and even more preferably 26%.

[0024] Ni: 23-60% Ni stabilizes the austenite structure at high temperatures and improves corrosion resistance to various acids, high-temperature corrosion resistance to chlorides, toughness, etc., so the Ni content is set to 23% or more, preferably 28% or more, and more preferably more than 35%. Increasing the Ni content makes it possible to incorporate 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 from the perspective of cost, the upper limit of Ni content in the steel of the present invention is set to 60% or less. The preferred upper limit is 50%.

[0025] 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 improving high-temperature strength. In the present invention, the addition of 0.01% or more, preferably 0.03% or more, is necessary to promote deoxidation and desulfurization and improve hot workability. On the other hand, if the Al content exceeds 0.20%, AlN precipitation occurs, which impairs manufacturability and heat resistance. For this reason, the upper limit of the Al content is set at 0.20%. The preferred upper limit is 0.15%, and the more preferred upper limit is 0.10%.

[0026] Ti: 0.05% or less In high 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 to be 0.05%. The preferred upper limit is 0.03%. Ti may not be contained.

[0027] B: 0.0002 to 0.0050% B improves hot workability and high-temperature creep strength in the region II embrittlement zone (near 1000°C), so it is actively added, especially in applications where the steel is used in high-temperature environments. B is an element that segregates at grain boundaries, thereby increasing grain boundary strength. The improvement in hot workability due to B content is obtained at 0.0002% or more, so the lower limit was set at 0.0002%. However, because B is an element that lowers the melting point of steel and is prone to segregation, excessive addition promotes solidification cracking and liquation cracking, significantly adversely affecting hot workability, especially in the region I embrittlement zone (near 1200°C). For this reason, the upper limit of the B content was set at 0.0050%. The preferred upper limit is 0.0030%.

[0028] 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 by V or Ta. It is the most stable and abundant nitride among the nitrides that remain in solution or precipitate during aging in the high-Ni alloy of the present invention. To improve high-temperature creep strength through Z-phase precipitation, the addition of 0.10% or more N is required. The preferred lower limit of N is N (atomic %) ≥ Nb (atomic %) + V (atomic %) + Ta (atomic %). On the other hand, excessive addition of more than 0.30% (mass %) not only causes the generation of bubbles during refining but also reduces creep strength by leaving coarse Z-phase undissolved during heat treatment, which inhibits the fine dispersion of the Z-phase that precipitates during aging, so the upper limit is set to 0.30%. The preferred upper limit is 0.25%.

[0029] 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 satisfied and nozzle clogging and inclusion-induced surface defects during continuous casting are 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.

[0030] Ca: 0.0001 to 0.0050% Ca fixes S in the alloy as CaS, improving the alloy's hot workability, resistance to welding hot cracking, and high-temperature oxidation resistance. This 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, and then the remaining Ca reacts with S in the alloy to form CaS. To improve the above properties, 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. However, excessive Ca addition reduces hot ductility around 1100°C. For this reason, the upper limit of the Ca content is set at 0.0050%. The preferred upper limit of the Ca content is 0.0045%.

[0031] One or more of Mo and W: Mo + 0.5 × W: 0.01% or more but less than 1.50% Both Mo and W enhance the high-temperature strength and high-temperature corrosion resistance of heat-resistant alloys. To achieve these effects, the present invention adds either one or a combination of elements so that the value of Mo + 0.5 × W 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%. Addition of Mo in excess of 0.3% is even more preferable. On the other hand, Mo has a greater adverse effect on hot workability than W, and a value of Mo + 0.5 × W greater than 1.50% significantly reduces hot workability. Therefore, the upper limit of Mo + 0.5 × W in the present invention's chemical composition is set to less than 1.50%. The preferred upper limits for Mo and W are 0.78% and 1.95%, respectively.

[0032] Nb: 0.10 to 0.65% Like N, Nb is an essential element for exerting the precipitation strengthening mechanism of the Z-phase utilized in the present invention, and its effect is exerted when the Nb content is 0.10% or more. The preferred lower limit is 0.20%. On the other hand, if the Nb content exceeds 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. Therefore, the upper limit of Nb content 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%.

[0033] One or more of V and Ta: 0.01 to 0.60% in total Both V and Ta contribute to the aging precipitation strengthening of the Z-phase utilized in the present invention. Both precipitate as Cr(Nb,V,Ta)N, substituting Nb in the Z-phase, and increasing the total amount of Z-phase increases creep strength. Since the effects of either element are manifested when added at 0.01% or more, the present invention adds one or both in a total of 0.01% or more. The preferred lower limit is a total of 0.02% or more, and a more preferred lower limit is a total of 0.05% or more. On the other hand, because addition of more than 0.60% 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%.

[0034] <Limited formula A> 0.65 ≦ Nb%+2×V%+Ta% ≦ 1.60 In the limiting formula A, the element symbols with % indicate the content (mass %) of each element. The present inventors conducted extensive research to improve high-temperature creep strength to a level difficult to achieve with conventional Nb-added 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 incorporating Nb in combination with V and / or Ta, which are group V metals like Nb but have a larger solid solubility limit with N than Nb. This led to 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, disperses and precipitates finely during aging. For example, when maintained at a relatively high operating temperature of approximately 800°C for approximately 500 hours, the Z-phase disperses and precipitates 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, so if the Nb content exceeds 0.65%, coarse Z phase remains undissolved during final heat treatment and undergoes Ostwald ripening in the operating temperature range. This prevents the fine Z phase that precipitates during aging from dispersing uniformly, resulting in a decrease in creep strength. V and Ta have a larger solid solubility limit than Nb, so almost all of them can be dissolved in solid solution in the final heat treatment temperature range. Therefore, adding V and Ta has almost no effect on the total amount of Z phase that remains undissolved during final heat treatment.

[0035] On the other hand, 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. Further intensive investigations revealed that the strengthening capacity per unit mass% of V, which has a smaller atomic weight than Nb, can be considered to be about twice that of Nb, while Ta has a larger atomic weight than Nb, but the size of the Z phase that precipitates during aging tends to be relatively small, so it is safe to consider its strengthening capacity per unit mass% to be about the same as Nb.

[0036] That is, in the component system of the present invention, the creep rupture time is A value = Nb% + 2 × V% + Ta It was discovered that the relationship between A and Nb can be expressed as follows. When the A value is below 0.65, creep strength remains at the level achievable with conventional steels containing only Nb. On the other hand, when the A value exceeds 1.50, the hot workability, aging embrittlement susceptibility, and microstructural stability of the alloy are significantly reduced. For this reason, the lower and upper limits of A were set at 0.65 and 1.50, respectively, and the above <Limiting Formula A> was established. The preferred lower limit of A is 0.75, and the preferred upper limit is 1.30.

[0037] (Nb) <Total amount of Nb precipitated in steel>: 0.11 mass% or less Even if the constituent elements satisfy the above chemical composition ranges and constraint formula A, if more undissolved precipitates remain after the final heat treatment than necessary, they will adversely affect creep strength by preventing uniform dispersion of the Z phase and normal grain growth (coarsening and regulating grain size). Therefore, it is necessary to control the amount of undissolved precipitates. The undissolved phase is mainly CrNbN (Z phase), but some MX-type (M: mainly Nb, X: carbonitrides with C or N) is also present. The amount of undissolved precipitates remaining after the final heat treatment depends on the steel's composition and the final heat treatment temperature. Even with the same composition, the lower the final heat treatment temperature, the greater the amount of undissolved precipitates.

[0038] Heat treatment furnaces owned by steel sheet manufacturers handling austenitic stainless steels or high-Ni alloys typically have a maximum heat treatment temperature in the range of 1160°C to 1250°C. When final heat treatment is performed within this temperature range, high-Ni alloys containing Nb and N often exhibit numerous coarse undissolved precipitates with equivalent circle diameters of 0.1 μm or more, some exceeding 1 μm, and averaging diameters of 0.2 to 0.8 μm. These coarse undissolved precipitates undergo Ostwald ripening at the service temperature, preventing the uniform dispersion of the fine Z phase that precipitates during aging. Furthermore, these undissolved precipitates tend to localize around Nb segregation sites, preventing uniform and normal grain growth and resulting in the formation of localized fine and mixed grains equivalent to GSN 6 or larger. Therefore, if a large amount of undissolved precipitates is present, even if <Constraint A> is satisfied, the creep rupture time will not increase, but will instead decrease.

[0039] Therefore, in this invention, we focused on Nb, a component of undissolved precipitates, as an index for controlling the amount of undissolved precipitates. Specifically, we expressed the total amount of Nb precipitated in a steel as (Nb), and used this index to represent the amount of undissolved precipitates in the steel after final heat treatment. We found that controlling the thus-defined (Nb) content to 0.11 mass% or less after final heat treatment promotes grain growth and granularity regulation during final heat treatment, resulting in the uniform, finely dispersed precipitation of solute Nb, V, and Ta as Z-phase within grains and at grain boundaries at operating temperatures. As a result, a creep rupture time exceeding 800 hours can be achieved at 800°C and 100 MPa. Since the amount of undissolved precipitates is more preferable in terms of creep properties, we do not set a lower limit for the (Nb) content in this invention.

[0040] <Limited formula X> Nb%×N%-10 α ≦ 0.030 α = 4.1826 - 7948.9 ÷ T Where, T: Final heat treatment temperature (K) In the limiting formula X, the element symbols with % indicate the content (mass %) of each element. As mentioned above, the amount of undissolved precipitates remaining after final heat treatment depends on the steel's chemical composition and the final heat treatment temperature. Even with the same chemical composition, the lower the final heat treatment temperature, the greater the amount of undissolved precipitates. In the production of the alloy of the present invention, the final heat treatment after hot rolling is performed at 1160°C to 1300°C, preferably 1180°C to 1250°C. This corresponds to Non-Patent Document 1 (ASTM B409), which is the ASTM standard for Ni-Fe-Cr alloys. As mentioned above, the final heat treatment is generally called solution heat treatment or solution heat treatment, and is followed by water cooling. This final heat treatment can impart high creep strength.

[0041] As described above, it has become clear that by controlling the defined Nb content to 0.11 mass% or less, a creep rupture time exceeding 800 hours can be achieved under conditions of 800°C and 100 MPa. Therefore, next, a means for controlling the Nb content to 0.11 mass% or less while providing the composition of the present invention was investigated. As a result, it became clear that the Nb content can be controlled to 0.11 mass% or less by determining the final heat treatment temperature T under the conditions of the above-mentioned limiting formula X based on the Nb content and N content in the steel and then performing the final heat treatment. The 10 on the left side of limiting formula X α corresponds to the solubility product of Nb·N at the final heat treatment (solution heat treatment) temperature, [Nb]·[N]; the higher the final heat treatment temperature, the greater the amount of Nb dissolved in solid solution [Nb], and the less (Nb) (total amount of Nb precipitated in the steel) there is. By performing heat treatment under the final heat treatment (solution treatment) conditions of limiting formula X, (Nb) is reduced to 0.11 mass% or less, and as a result, it is possible to achieve quality that allows a creep rupture time of over 800 hours in creep tests (based on JIS Z2271) at 800°C and 100 MPa.

[0042] As a result of these findings, it has become possible to produce steel with good creep properties within the capacity of the heat treatment furnace by adjusting the chemical composition of the steel according to the capacity of the heat treatment furnace used. That is, the Nb and N contents are determined so as to satisfy the constraint formula X within the capacity of the heat treatment furnace owned by the steel manufacturer, and the V and Ta contents are determined within the constraint formula A. In addition to creep strength, to prevent deterioration of aging toughness and liquation cracking resistance during welding due to localized grain coarsening, it is preferable to adjust the heat treatment temperature range so that the value of the left side of formula X is 0.030 to 0.015. For applications specializing in creep strength, it is preferable to set the heat treatment temperature so that the value of the left side of formula X is 0.020 or less.

[0043] 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 reasons for the limitations set forth in claim 2 will be described. Furthermore, instead of a portion of the Fe, the alloy may selectively contain the following components (mass %):

[0044] 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 and corrosion resistance properties, the Cu content is set at 0.01% or more, preferably 0.02% or more, and more preferably 0.05% or more. However, if the Cu content exceeds 0.50%, embrittlement-related defects will occur during solidification, so the upper limit is set at 0.50%.

[0045] 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 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 more preferably 0.50%.

[0046] 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, since addition of more than 0.05% reduces hot workability, the upper limit is set at 0.05%.

[0047] Zn+Pb+Bi:0.0010% or less Since Zn, Pb, and Bi all significantly reduce the hot workability of austenitic single-phase alloys, their upper limits must be strictly defined. Preferably, the upper limits are Pb≦0.0010%, Zn≦0.0010%, and Bi≦0.0010%, with the total of Pb, Zn, and Bi being 0.0010% or less.

[0048] Mg: 0.0050% or less Since Mg is an element that exhibits a desulfurization effect, even small amounts can improve the hot workability of the alloy, but excessive addition significantly reduces 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%, preferably 0.0040%, and more preferably 0.0030%.

[0049] 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 set at 0.10%.

[0050] La+Ce+Nd+Pr: 0.001 to 0.050% Adding La, Ce, Nd, and Pr in a total amount of 0.001% or more, preferably 0.005% or more, fixes P and S, improving the oxidation resistance and hot workability of steel. However, adding more than 0.050% of these elements in total can significantly impair manufacturability by forming coarse oxides and nitrides, which can cause nozzle clogging during refining and increase 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. [Example]

[0051] 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 rectangular 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.

[0052] [Table 1-1]

[0053] [Table 1-2]

[0054] The slabs cast from the melted material were tapered, measuring approximately 105mm square to 90mm square, and had a height of approximately 280mm. These slabs were hot forged in the temperature range of 1200℃ to 1000℃ and processed into 50mm thick x 120mm wide pieces. Subsequently, they were subjected to a heat treatment at 1250℃ for 3 hours at a soaking temperature. Immediately after removal from the furnace, the alloys were hot-rolled to a thickness of 16 mm at temperatures ranging from 1200°C to 900°C. Further, a final heat treatment was performed at either 1180°C, 1200°C, 1220°C, or 1250°C, depending on the alloy content, for 1 hour, followed by water quenching. Creep test specimens measuring 6 mm in diameter, 80 mm in length, and 12 mm in fixed area were prepared from the alloy plates parallel to the rolling direction. Blocks for Nb content analysis were also prepared, measuring 12 mm in thickness, 30 mm in length, and 20 mm in width, with the entire surface polished. The creep test was performed in accordance with JIS Z 2271, measuring the time to rupture at 800°C and 100 MPa. Specimens with a creep rupture time exceeding 800 hours were deemed to have passed. Quantitative analysis of Nb content was performed using the constant current electrolysis method. Specifically, the specimens were subjected to a 10% acetylacetone-1% tetramethylammonium chloride-methanol solution at a current density of 20 mA / cm. 2 The residue was filtered through a Nuclepore filter with a mesh size of 0.1 μm, and the amount of Nb in the residue was measured by ICP method and converted to mass%. The results are shown in Table 2 and Figure 1.

[0055] The creep test results are shown in Table 2 and Figure 1. In Figure 1, the horizontal axis represents the Nb content (mass%), and the vertical axis represents the creep rupture time. Circles indicate that the central side of <Constraint Formula A> (A value) is 0.65% or greater, and black squares indicate that the central side of <Constraint Formula A> (A value) is less than 0.65%. As is clear from Figure 1, when a circle (A value 0.65 or greater) and the Nb value is 0.11% or less by mass, i.e., when both <Constraint Formula A> and (Nb) ≦ 0.11% by mass are satisfied, a creep rupture time of 800 hours or greater is achieved.

[0056] [Table 2]

[0057] In Table 2, sub-number 1 indicates a comparative example, and sub-number 2 indicates an example of the invention. The final heat treatment temperature T' (°C) in Table 2 means T' (°C) = T (K) - 273. As shown in Table 2 and Figure 1, among Nos. 1 to 17 and 22 that all satisfy the component ranges and the ranges of <Restriction Formula A>, the one that was heat treated under conditions that satisfied Restriction Formula X had (Nb) ≦ 0.11 mass% (listed as an example of the invention in Table 2) (sub-number 2), and satisfied a time to rupture of 800 hours or more under creep test conditions of 800°C and 100 MPa.

[0058] On the other hand, Comparative Examples Nos. 18 to 21, in which the <limiting formula A> middle part (A value) was less than 0.65, had creep rupture times of less than 800 hours. Furthermore, Comparative Examples Nos. 2, 3, 7 to 10, 12, 13, 15 to 17 (sub-number 1) and Nos. 21 to 25, in which (Nb) exceeded 0.11 mass%, regardless of the A value, also had creep rupture times of less than 800 hours.

[0059] In addition, No. 26, whose Mo+0.5W value exceeded 1.5, and No. 27, whose limiting formula midpoint (A value) exceeded 1.50, both showed edge cracks at the plate end after hot rolling. Furthermore, No. 28, whose Cu content exceeded 0.5%, showed cracks propagating in the width direction at the center of the plate thickness at the tip after hot forging. Nos. 26 to 28 were judged to have inferior hot workability compared to the other steels and were therefore excluded from the test.

[0060] 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. [Industrial Applicability]

[0061] The present invention allows for the design of Nb- and N-containing high-Ni alloys for applications requiring high-temperature creep strength to be thinner, or for larger equipment and longer 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.

[0062] This will enable us to provide stable quality to meet the expanding demand for high-Ni alloys, which will make a significant contribution to the development of the industry.

Claims

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: 0.0001 to 0.0050%, Mo and W A high-Ni alloy with excellent high-temperature creep strength, characterized in that it contains at least one of Mo+0.5×W: 0.01% or more but less than 1.50%, Nb: 0.10 to 0.65%, and at least one of V and Ta in a total amount of 0.01 to 0.60%, with the balance consisting of Fe and impurities, and satisfies the following <Limiting Formula A>, and the total amount of Nb (Nb) precipitated in the steel material is 0.11 mass% or less. <Limited formula A> 0.65≦Nb%+2×V%+Ta%≦1.60 In the limiting formula A, the element symbols with % indicate the content (mass %) of each element.

2. 2. The high Ni alloy having excellent high-temperature creep strength according to claim 1, further comprising, in 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%, in place of a portion of the Fe.

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

4. 3. The method for producing a high Ni alloy having excellent high-temperature creep properties according to claim 1, wherein the final heat treatment temperature T (K) satisfies the following limiting formula X: <Limited formula X> Nb%×N%-10 α ≦ 0.030 α=4.1826-7948.9÷T In the limiting formula X, the element symbols with % signify the content (mass %) of each element.

5. 4. The method for producing a high Ni alloy having excellent high temperature creep properties according to claim 3, wherein the final heat treatment temperature T (K) in the final heat treatment satisfies the following limiting formula X: <Limited formula X> Nb%×N%-10 α ≦ 0.030 α=4.1826-7948.9÷T In the limiting formula X, the element symbols with % signify the content (mass %) of each element.

Images