High-ni alloy having excellent high temperature creep strength

Abstract

A high-Ni alloy having excellent high-temperature creep strength, includes, in mass%: C: 0.08% 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%; at least one of Mo or W in an amount such that Mo + 0.5 × W is 0.01% or more and less than 1.50%; Nb: 0.10 to 0.65%; at least one of V or Ta in a total amount of 0.01 to 0.60%, and the alloy satisfying Inequalities A and B below, Inequality A: 0.65≤Nb%+2×V%+Ta%≤1.50 Inequality B: Nb%+0.15×V%+0.4×Ta%×C%+N%≤0.170.

Description

TECHNICAL FIELD

[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.BACKGROUND ART

[0002] As high-Ni alloys used in heat-resistant applications, Alloy 800H (ASTM N08810, N08811) is a representative commercial alloy. In response to recent increase in demands in developing countries, it has been requested to develop technologies for supplying commercial products that are inexpensive and excellent in surface quality and usage characteristics. In view of the above request, the steel manufacturing process has been shifted from a typical ingot casting process to a continuous casting process. Since high-Ni alloys are highly susceptible to slab internal cracks during casting, edge cracks during hot working, and surface defects on the product, efforts have been made to improve and develop design for chemical composition, refining, casting, and hot working techniques for the high-Ni alloys, in order to enhance manufacturability in a continuous casting process.

[0003] On the other hand, in the primary applications of the high-Ni alloys, namely high-temperature reaction vessels in chemical plants, the alloys are often used at a temperature of 600 degrees C or higher and, moreover, under high pressure in order to improve the efficiency of chemical reactions. In such applications, the higher the creep strength of the high-Ni alloys, the more readily the alloys can be used in thinner sections.

[0004] In recent years, there has been an increasing number of cases in which materials such as ASTM N08120, in which Nb and N are added and precipitation strengthening is achieved by Nb-based carbonitride precipitates, are employed. Non-Patent Literature 1 is an ASTM standard specification relating to Ni-Fe-Cr alloys, and also covers ASTM N08120. In this specification, the final heat-treatment temperature is described as being 1,177 degrees C or higher.

[0005] For example, related-art documents such as Patent Literatures 1 and 2 describe high-Ni alloys utilizing precipitation strengthening by addition of Nb and N (hereinafter also referred to as "Nb- and N-containing high-Ni alloys"). As described above, in each of these patent literatures, the alloy design makes combined use of precipitation strengthening by carbonitride precipitates of Nb and the like, solid-solution strengthening by the addition of Mo, and grain-boundary strengthening by the addition of B. In order to enhance the creep strength through precipitation strengthening by Nb, the final heat-treatment temperature after hot rolling is set as high as 1,300 degrees C at maximum, which is higher than that employed for ordinary Fe-based high alloys.CITATION LISTPATENT LITERATURE(S)

[0006] Patent Literature 1: JP Patent No. 6,675,846 Patent Literature 2: JP Patent No. 7,174,192 NON-PATENT LITERATURE(S)

[0007] Non-Patent Literature 1: ASTM B409SUMMARY OF THE INVENTIONPROBLEM(S) TO BE SOLVED BY THE INVENTION

[0008] In both Patent Literatures 1 and 2, the creep strength is enhanced by controlling the amount of Nb carbonitride precipitates. However, as is apparent from the fact that both Patent Literatures 1 and 2 set the upper limit on the added amount based on the product of the Nb and N contents, the solubility product of Nb with C and N is small. Accordingly, even when the heat treatment temperature is raised to around 1,300 degrees C during the final heat treatment after hot rolling (hereinafter also simply referred to as "final heat treatment"), coarse undissolved carbonitrides remain. In particular, as described in Patent Literature 1, there has been a limitation in that an excessive amount of these undissolved carbonitrides causes a decrease in creep strength. Accordingly, in the compositional systems described in Patent Literatures 1 and 2, further improvement of the creep strength had been considered difficult. Although the compositional systems described in Patent Literatures 1 and 2 partially fall within the ASTM N08120 specification, it has been difficult to solve the issues of Patent Literatures 1 and 2 and further enhance the creep strength by using only the components whose ranges are specified in ASTM N08120.

[0009] An object of the invention is to provide a high-Ni alloy capable of solving the above-described problem and having excellent high-temperature creep strength.MEANS FOR SOLVING THE PROBLEM(S)

[0010] The inventors, as a means for solving the above-described problem, have found that the creep strength is markedly improved, compared with steels containing Nb alone, by allowing V or Ta to be contained in combination with Nb, and thus completed the invention.

[0011] Since V and Ta each have a larger solubility product with N than Nb does, V and Ta do not form as large an amount of carbonitrides as Nb does even when V and Ta are contained in an amount approximately equal to the amount of Nb. On the other hand, at a service temperature of 900 degrees C or lower, the solubility product of each of V and Ta with N becomes small, similar to that of Nb with N, so that carbonitrides precipitate during aging, resulting in improved creep strength. The inventors have found that the creep strength is markedly improved, compared with typical Nb- and N-containing high-Ni alloys, by allowing V and / or Ta that precipitate during aging to be contained in combination with Nb, and thus completed the invention.

[0012] Specifically, main features of the invention are as follows. (1) A high-Ni alloy having excellent high-temperature creep strength, including, 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%; at least one of Mo or W in an amount such that Mo + 0.5 × W is 0.01% or more and less than 1.50%; Nb: 0.10 to 0.65%; at least one of V or Ta in a total amount of 0.01 to 0.60%; and a balance consisting of Fe and impurities, and the alloy satisfying Inequalities A and B below, Inequality A: 0.65 ≤ Nb % + 2 × V % + Ta % ≤ 1.50 Inequality B: Nb % + 0.15 × V % + 0.4 × Ta % × C % + N % ≤ 0.170 , where in Inequality A and Inequality B, the element symbols accompanied by "%" indicate a content of each element in mass%. (2) The high-Ni alloy according to (1), further including, in mass%, in place of a part of the Fe: 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 according to (1) or (2), in which, in a creep test conducted in accordance with JIS Z 2271 under conditions of 800 degrees C and 100 MPa, a creep rupture time exceeds 800 hours.

[0013] A high-Ni alloy containing Nb and N is used in applications such as reaction vessels in chemical plants, including polycrystalline silicon production equipment, where high levels of high-temperature creep strength and high-temperature corrosion resistance are required. By markedly improving the creep strength beyond typical levels through the combined addition of V and / or Ta in addition to Nb, it becomes possible to increase a size of such equipment or to reduce a wall thickness and weight thereof, which is expected to make a significant contribution to development of the chemical industry and the semiconductor industry. Further, the high-Ni alloy of the invention, which is mainly supplied in a product form of thick plates for the above plant applications, can also be supplied in various other product forms, including thin plates, tubes, coils, bars, and wire rods. A thickness of the thick plate is preferably from 5 mm to 80 mm.BRIEF DESCRIPTION OF DRAWING(S)

[0014] Fig. 1 is a diagram illustrating an influence of an A value on the middle term of Inequality A and a B value on the left-hand side of Inequality B on a creep rupture time.DESCRIPTION OF EMBODIMENT(S)

[0015] First, the reason for the limitation defined in claim 1 of the invention will be described below. It should be noted that the content of each component is expressed in mass%.Chemical CompositionC: 0.08% or Less

[0016] In heat-resistant materials, C is ordinarily an element that is actively added in order to secure high-temperature strength. However, in the invention, as will be described later, the creep strength is enhanced by aging precipitation strengthening due to a Z phase (a Cr-Nb-N-type nitride) that precipitates during aging at the service temperature. Excessive addition of C consumes Nb, which is intended to be used for forming the Z phase, in the form of MX-type carbonitrides (M: metal, X: C or N), thereby decreasing the creep strength. Accordingly, in the invention, the amount of C added is limited to 0.08% or less.Si: 0.05 to 1.0%

[0017] Si is added in an amount of 0.05% or more for deoxidation and improvement in oxidation resistance. However, since Si is also an element that lowers the melting point of steel, adding Si in an amount exceeding 1.0% reduces the hot ductility in the vicinity of 1,200 degrees C and deteriorates the susceptibility to solidification cracking and the susceptibility to liquation cracking during welding. In addition, intermetallic compounds become more likely to precipitate, resulting in deterioration of high-temperature properties. Therefore, the upper limit of the Si content is limited to 1.0%. The upper limit of the Si content is preferably 0.7%, more preferably 0.5%.Mn: 0.05 to 1.5%

[0018] 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 invention. In order to improve the heat resistance, Mn is added in an amount of 0.05% or more. However, when the added amount of Mn exceeds 1.5%, contrary to expectations, intermetallic compounds are likely to precipitate, which deteriorates the heat resistance and adversely affects the susceptibility to solidification cracking. Therefore, the upper limit of the Mn content is set to 1.5%. The upper limit of the Mn content is preferably 1.3%, more preferably 1.0%.P: 0.030% or Less

[0019] P is an element inevitably mixed from a raw material and has an effect of increasing the susceptibility to solidification cracking. Therefore, the P content is limited to 0.030% or less, preferably 0.025% or less.S: 0.0015% or Less

[0020] S is an element inevitably mixed from a raw material and deteriorates hot workability and oxidation resistance. Therefore, the S content is limited to 0.0015% or less, preferably 0.0010% or less. S is an element whose content can be lowered by refining. However, a large cost is required in order to extremely lower the S content. Therefore, the lower limit of the S content is preferably 0.0001%.Cr: 20 to 30%

[0021] Cr is a requisite element in order for the heat-resistant alloy used as a high-temperature material to exhibit oxidation resistance and high-temperature corrosion resistance. Therefore, the Cr content is 20% or more, preferably 22% or more. In addition, in the invention, the Z phase that precipitates during aging is utilized to improve the creep strength, and Cr is indispensable for ensuring the stable formation of this Z phase. When the Cr content falls below 20%, a Cr-depleted layer is formed at grain boundaries during aging, so that the Z phase does not precipitate in the vicinity of the grain boundaries, resulting in the formation of a precipitation-free zone. As a result, the creep rupture time decreases due to the resulting reduction in grain-boundary strength. On the other hand, when the Cr content exceeds 30%, the high-temperature microstructural stability decreases even with a large content of Ni, and intermetallic compounds begin to precipitate, thereby deteriorating the heat resistance. Therefore, the Cr content is limited to 30% or less. The upper limit of the Cr content is preferably 28%, more preferably 26%.Ni: 23 to 60%

[0022] Ni is an element that stabilizes the austenitic microstructure at high temperatures and improves corrosion resistance to various acids, high-temperature corrosion resistance to chlorides, and toughness. Therefore, the Ni content is 23% or more, preferably 28% or more, and still more preferably more than 35%. By increasing the Ni content, it becomes possible to contain larger amounts of ferrite-forming elements such as Cr, W, Mo, V, and Nb, which are required to ensure heat resistance. However, because Ni is an expensive element, the upper limit of the Ni content in the steel of the invention is set to 60% or less, preferably 50%, in view of the cost.Al: 0.01 to 0.20%

[0023] Al is a deoxidizing element and has an effect of forming a NiAl ordered phase in a high-Ni alloy to enhance high-temperature strength. In the invention, Al needs to be added in an amount of 0.01% or more, preferably 0.03% or more, in order to promote deoxidation and desulfurization to enhance hot workability. On the other hand, when the Al content exceeds 0.20%, manufacturability and heat resistance are impaired due to the precipitation of AIN. Therefore, the upper limit of the Al content is set to 0.20%. The upper limit of the Al content is preferably 0.15%, more preferably 0.10%.Ti: 0.05% or Less

[0024] In high-Ni alloys containing N, Ti forms coarse TiN. The formation of coarse TiN not only adversely affects manufacturability, surface quality of steel, and aging toughness, but also inhibits the fine precipitation of the Z phase, resulting in a decrease in creep strength. Therefore, the upper limit of the Ti content in the invention is limited to 0.05%, preferably 0.03%. Ti may not be contained.B: 0.0002 to 0.0050%

[0025] B is actively added, particularly for applications in which the material is used at high temperatures, in order to improve hot workability and high-temperature creep strength in the Region II embrittlement range (in the vicinity of 1,000 degrees C). B is an element that increases grain-boundary strength by segregating to the grain boundaries. The effect of improving the hot workability by containing B is exerted when the B content is 0.0002% or more. Therefore, the lower limit of the B content is set to 0.0002%. On the other hand, B is an element that lowers the melting point of steel and is also highly prone to segregation; therefore, excessive addition of B promotes solidification cracking and liquation cracking, and has a significantly adverse effect on hot workability, particularly in the Region I embrittlement range (in the vicinity of 1,200 degrees C). Therefore, the upper limit of the B content is set to 0.0050%. The upper limit of the B content is preferably 0.0030%.N: 0.10 to 0.30%

[0026] N is an indispensable element for exhibiting the precipitation strengthening mechanism by the Z phase utilized in the invention. The Z phase utilized in the invention is a CrNbN-type nitride or a Cr(Nb,V,Ta)N-type nitride in which a portion of Nb is substituted with V or Ta. In the high-Ni alloy of the invention, the Z phase is the most stable and the most abundant nitride among the nitrides that remain undissolved or precipitate during aging. In order to improve the high-temperature creep strength by precipitation of the Z phase, N needs to be added in an amount of 0.10% or more. The lower limit of the N content is preferably defined by N (atomic%) ≥ Nb (atomic%) + V (atomic%) + Ta (atomic%). On the other hand, excessive addition of N exceeding 0.30% (by mass%, hereinafter the same) not only causes bubble formation during refining but also results in a coarse Z phase remaining undissolved during the final heat treatment, which inhibits the fine dispersion of the Z phase that precipitates during aging and thereby decreases the creep strength. Therefore, the upper limit of the N content is set to 0.30%. The upper limit of the N content is preferably 0.25%.O: 0.006% or Less

[0027] Oxygen (O) reacts with Ca, Mg, Al, and Ti in the alloy of the invention to form oxide inclusions. The oxygen content, which depends on the total amount of the oxide inclusions, is an important factor as an index for deoxidation of the alloy. When the oxygen content exceeds 0.006%, the desired deoxidation equilibrium can no longer be satisfied, and nozzle clogging during continuous casting as well as surface defects caused by inclusions are more likely to occur. Therefore, the upper limit of the oxygen content is set to 0.006%. The upper limit of the oxygen content is preferably 0.005%, more preferably 0.004%. On the other hand, reduction in the oxygen content, which advantageously reduces nozzle clogging and weld hot cracking by reducing oxide inclusions, generates excessive Ca and excessive Mg in the alloy to lower the hot workability. Therefore, the oxygen content is preferably 0.0002% or more.Ca: 0.0001 to 0.0050%

[0028] Ca fixes S in the alloy in a form of CaS, thereby improving the hot workability, resistance to weld hot cracking, and resistance to high-temperature oxidation of the alloy. The reaction progresses as follows. Ca is bonded with oxygen in the alloy to form CaO and CaO-Al 2 O 3 to substantially annihilate oxygen dissolved in the alloy (free oxygen). Subsequently, residual Ca reacts with S in the alloy to form CaS. In order to improve the above properties in the alloy of the invention, the Ca content is 0.0001 % or more, preferably 0.0003% or more, and still more preferably 0.0005% or more. On the other hand, excessive addition of Ca decreases the hot ductility in the vicinity of 1,100 degrees C. Therefore, the upper limit of the Ca content is set to 0.0050%. The upper limit of the Ca content is preferably 0.0045%.Mo + 0.5 × W: from 0.01% to less than 1.50% (with at least one of Mo or W present)

[0029] Both Mo and W are elements that enhance the high-temperature strength and high-temperature corrosion resistance of heat-resistant alloys. In order to exert these effects, in the invention, one or both of Mo and W are added such that the value of Mo + 0.5 × W is 0.01% or more, preferably 0.05% or more, and still more preferably 0.10% or more. The value of Mo + 0.5 × W may exceed 0.3%. Still further more preferably, Mo in an amount exceeding 0.3% is added. On the other hand, with regard to the hot workability, Mo has a greater adverse effect than W, and the hot workability is markedly degraded when the value of Mo + 0.5 × W is 1.50% or more. Therefore, in the component system of the invention, the upper limit of the value of Mo + 0.5 × W is set to less than 1.50%. The value of Mo + 0.5 × W is more preferably less than 1.20%. The upper limit of the Mo content is preferably 0.78%. The upper limit of the W content is preferably 1.95%.Nb: 0.10 to 0.65%

[0030] Nb, as with N, is an indispensable element for exhibiting the precipitation strengthening mechanism by the Z phase utilized in the invention, and this effect is exerted when Nb is added in an amount of 0.10% or more. The lower limit of the Nb content is preferably 0.20%. On the other hand, when Nb is added in an amount exceeding 0.65%, a coarse Z phase remains undissolved even after the final heat treatment. Such coarse undissolved precipitates inhibit the fine dispersion of the Z phase that precipitates during aging at the service temperature, and consequently not only decrease the creep strength but also deteriorate the aging toughness and increase the susceptibility to weld hot cracking. Therefore, the upper limit of the Nb content in the invention is set to 0.65%. The upper limit of the Nb content is preferably 0.60%, more preferably 0.55%.Total of at least one of V or Ta: 0.01 to 0.60%

[0031] Both V and Ta are elements that contribute to the aging precipitation strengthening by the Z phase utilized in the invention. Both V and Ta precipitate in the form of Cr(Nb,V,Ta)N in which Nb in the Z phase is substituted with V or Ta, thereby increasing the total amount of the Z phase to improve the creep strength. Since either element exerts its effect when added in an amount of 0.01% or more, one or both of the two elements are added in a total amount of 0.01% or more in the invention. The lower limit of the total amount is preferably 0.02%, more preferably 0.05%. On the other hand, the addition in the total amount exceeding 0.60% adversely affects hot workability and aging toughness. Therefore, in the invention, the upper limit of the total amount is set to 0.60%. The upper limit of the total amount is preferably 0.50%. Inequality A: 0.65 ≤ Nb % + 2 × V % + Ta % ≤ 1.50

[0032] The inventors have conducted extensive studies in order to improve the high-temperature creep strength to a level that has been difficult to achieve with typical heat-resistant high-strength Fe-Cr-Ni alloys containing Nb and N, such as those disclosed in Cited Literatures 1 and 2. As a result, the inventors have found that further improvement in creep strength can be achieved by adding V and / or Ta, which are Group V metals like Nb and have a larger solubility limit with N than Nb, in combination with Nb, and thus completed the invention. The most important strengthening mechanism of the high-Ni alloy of the invention is aging precipitation strengthening, which is achieved by the fine dispersion of the Z phase, a stable precipitate phase at the service temperature, during aging. For example, when the high-Ni alloy is maintained at a relatively high temperature of around 800 degrees C for approximately 500 hours, the Z phase precipitates in a finely dispersed form with a diameter of about 0.1 µm or less, thereby markedly increasing the creep rupture time. In the case of typical steels in which only Nb is added, Nb has a small solubility product with N, and therefore, when Nb is added in an amount exceeding 0.65%, a coarse Z phase remains undissolved during the final heat treatment and undergoes Ostwald growth in the service temperature range. As a result, the uniform fine dispersion of the Z phase that precipitates during aging is hindered, and consequently, the creep strength is instead reduced. V and Ta have larger solubility limits than Nb and can be almost completely dissolved in the temperature range of the final heat treatment. Therefore, even when V and Ta are added, V and Ta have little effect on the total amount of undissolved Z phase remaining after the final heat treatment. On the other hand, at the service temperature, most of the V and Ta precipitate during aging as Cr(Nb,V,Ta)N in which Nb in the Z phase is substituted.

[0033] Furthermore, as a result of further intensive studies, it was found that the strengthening capability per unit mass percent of V, whose atomic weight is smaller than that of Nb, can be regarded as approximately twice that of Nb, while Ta, although having a larger atomic weight than Nb, tends to form a Z phase, which precipitates during aging, of relatively smaller size, and strengthening capability of Ta per unit mass percent can be considered to be approximately equal to that of Nb.

[0034] That is, in the component system of the invention, it was found that the creep rupture time can be correlated by the relationship of A Value = Nb% + 2 x V% + Ta%. When the A value is less than 0.65, the creep strength remains at a level attainable with typical steels in which only Nb is added. On the other hand, when the A value exceeds 1.50, the hot workability, susceptibility to aging embrittlement, and microstructural stability of the alloy are markedly degraded. Accordingly, the lower and upper limits of the A value are set to 0.65 and 1.50, respectively, and the above Inequality A is established. The lower limit of the A value is preferably 0.75. The upper limit of the A value is preferably 1.30. Inequality B: Nb % + 0.15 × V % + 0.4 × Ta % × C % + N % ≤ 0.170

[0035] Inequality B is defined so as to serve as an index, because, within the compositional range of the alloy of the invention, undissolved phases remaining during the final heat treatment adversely affect the creep strength.

[0036] B value = (Nb% + 0.15 × V% + 0.4 × Ta%) × (C% + N%) is determined. In the formula for the B value, the coefficients 0.15 for V and 0.4 for Ta are coefficients set in accordance with the differences in the solubility products of each of V and Ta with N or C. The undissolved phases are mainly the Z phase, but a portion of MX-type carbonitrides (where M is a metal and X is C or N) is also present. In a heat-treatment furnace with relatively high output, heat treatment can be performed at a heat-treatment temperature of 1,200 to 1,300 degrees C. However, even when the final heat treatment is carried out at such a high temperature, if the B value exceeds 0.170, a large number of undissolved phases remain, having a diameter of 0.2 µm or more and, in some cases, exceeding 1 µm. These coarse undissolved precipitates undergo Ostwald growth at the service temperature, thereby hindering the uniform fine dispersion of the Z phase that precipitates during aging. Consequently, even if Inequality A is satisfied, the creep rupture time does not increase and instead decreases. Accordingly, the above Inequality B is established. Considering that the smaller the left-hand side (B value) of Inequality B is, the more the dissolution of coarse Z phase is promoted, the upper limit of the B value is preferably 0.120, more preferably 0.100.

[0037] In producing the alloy of the invention, as the final heat treatment after hot rolling, a heat treatment at a temperature from 1,180 to 1,300 degrees C is performed. This corresponds to Non-Patent Literature 1 (ASTM B409), which is an ASTM standard specification for Ni-Fe-Cr alloys. The final heat treatment is generally referred to as solution heat treatment or solutionizing heat treatment, and cooling is performed by water quenching after the heat treatment. By performing the above final heat treatment, high creep strength can be imparted. That is, by performing the final heat treatment under the above condition after providing the alloy with the compositional requirements of the invention, it is possible to achieve the creep rupture time exceeding 800 hours under the conditions of 800 degrees C and 100 MPa in a creep test in accordance with JIS Z 2271. If a satisfactory creep rupture time is not obtained under the selected final heat-treatment temperature condition, a satisfactory creep rupture time can be achieved by further increasing the heat-treatment temperature within the final heat-treatment temperature range of 1,300 degrees C or lower. In addition to improving the creep strength, the optimum final heat-treatment temperature condition for preventing deterioration in aging toughness and resistance to liquation cracking during welding, due to grain refinement and grain coarsening, are in the range of 1,200 to 1,250 degrees C.

[0038] The chemical composition of the high-Ni alloy of the invention includes the above components and a balance consisting of Fe and impurities. Next, the reason for the limitation defined in claim 2 of the invention will be described below. Further, in place of a part of Fe, the following component(s) (mass%) is optionally contained.Cu: 0.01 to 0.50%

[0039] Cu is an element that improves the corrosion resistance of the alloy against acids and dew point corrosion resistance, which is a frequent issue of concern in high-temperature equipment, and also enhances the high-temperature strength and the microstructural stability. Accordingly, Cu may be added as required. In order to enhance the heat resistance and corrosion resistance, the Cu content is 0.01% or more, preferably 0.02% or more, and still more preferably 0.05% or more. However, when the Cu content exceeds 0.50%, defects caused by embrittlement occur during solidification. Therefore, the upper limit of the Cu content is set to 0.50%.Co: 0.01 to 1.0%

[0040] Co is an element that is effective for improving the high-temperature microstructural stability and the corrosion resistance of the alloy. In order to enhance these properties, the Co content is 0.01% or more, preferably 0.02% or more, and still more preferably 0.10% or more. When the Co content exceeds 1.0%, the effect is not commensurate with the cost because Co is an expensive element; therefore, the upper limit of the Co content is set to 1.0%. The upper limit of the Co content is preferably 0.8%, more preferably 0.50%.Sn: 0.001 to 0.05%

[0041] Sn is an element that improves the corrosion resistance and the high-temperature creep strength of steel at a content of 0.001% or more, preferably 0.005% or more. Sn may be added as required. However, the hot workability is deteriorated when Sn is added at a content exceeding 0.05%; therefore, the upper limit of the Sn content is set to 0.05%.Zn + Pb + Bi: 0.0010% or Less

[0042] Zn, Pb, and Bi considerably lower the hot workability in an austenitic single-phase alloy. Accordingly, the upper limit of each of the contents of these elements has to be strictly set. Pb ≤ 0.0010%, Zn ≤ 0.0010%, and Bi ≤ 0.0010% are preferable. A total of the contents of Pb, Zn, and Bi is set to 0.0010% or less.Mg: 0.0050% or Less

[0043] Since Mg is an element that exerts a desulfurization effect, a small amount of Mg can improve the hot workability of alloys. However, excessive addition of Mg markedly reduces the hot workability in the vicinity of 900 degrees C. Therefore, when Mg is added in the invention, the upper limit of the Mg content is set to 0.0050%. The upper limit of the Mg content is preferably 0.0040%, more preferably 0.0030%.Zr: 0.001 to 0.10%Hf: 0.001 to 0.10%

[0044] Both Zr and Hf, when added in an amount of 0.001% or more, preferably 0.005% or more, have the effect of improving resistance to solidification cracking, hot workability, and resistance to high-temperature oxidation of steel by fixing P and S, and may be added as required. On the other hand, addition in a large amount exceeding 0.10% forms coarse nitrides, resulting in a decrease in creep strength and also adversely affecting manufacturability. Accordingly, the upper limit of the content of each of Zr and Hf is set to 0.10%.La + Ce + Nd + Pr: 0.001 to 0.050%

[0045] La, Ce, Nd, and Pr are elements that fix P and S when added in a total amount of 0.001% or more, preferably 0.005% or more, to improve oxidation resistance and hot workability of steel. On the other hand, when these elements are added in a total amount exceeding 0.050%, coarse oxides and nitrides are formed, which significantly impairs manufacturability, such as by causing nozzle clogging during refining and increasing surface defects. Therefore, the upper limit of the total of contents of these elements is set to 0.050%. It should be noted that these elements may be added either in elemental form or in the form of an alloy, including misch metal.Examples

[0046] Examples will be described below. The inventors melted a high-Ni alloy in an MgO crucible using a 50-kg vacuum melting furnace, added Al, Ti, Ca, and Mg as required, and cast the melt into a 25-kg square mold, thereby obtaining high-Ni alloys having the compositions shown in Tables 1-1 and 1-2. It should also be noted that blank columns in Table 1 show that the corresponding components are at an impurity level. In the tables below, the components and the Inequalities that fall outside the scope of the invention are underlined.

[0047] Cast steels obtained by casting the melted material had a tapered shape with dimensions of approximately 105-mm square to approximately 90-mm square and a height of approximately 280 mm. These cast steels were subjected to hot forging in the temperature range of 1,200 to 1,000 degrees C and were processed into a thickness of 50 mm and a width of 120 mm. Subsequently, a heat treatment of soaking at 1,250 degrees C for 3 hours was performed, and immediately after removal from a heat treatment furnace, hot rolling was performed in the temperature range of 1,200 to 900 degrees C to obtain a plate thickness of 16 mm. Thereafter, a final heat treatment of soaking at 1,250 degrees C for 1 hour was performed, followed by water quenching. A creep test specimen was prepared from each alloy plate, with a gauge section of 6 mm in diameter and 80 mm in length, and a grip section of 12 mm in diameter, taken parallel to the rolling direction. The creep test was conducted in accordance with JIS Z 2271, under the conditions of 800 degrees C and 100 MPa, and the time to rupture was measured. A specimen having the creep rupture time exceeding 800 hours was regarded as acceptable. The results of the creep test are shown in Table 2 and Fig. 1.

[0048] In Fig. 1, the horizontal axis represents the middle term (A value) of Inequality A, and the vertical axis represents the creep rupture time. In addition, the white circles indicate cases in which the left-hand side (B value) of Inequality B is 0.170 or less, and the white squares indicate cases in which the left-hand side (B value) of Inequality B exceeds 0.170. As is apparent from Fig. 1, in the cases indicated by white circles (B value of 0.170 or less) in which the A value is 0.65 or more, that is, in the cases satisfying both Inequality A and Inequality B, it can be seen that the creep rupture time of 800 hours or more is achieved.

[0049] As shown in Table 2 and Fig. 1, Examples Nos. 1 to 16 of the invention, which satisfied all of the compositional ranges, Inequality A, and Inequality B, satisfied creep rupture times of 800 hours or more under the creep test conditions of 800 degrees C and 100 MPa. On the other hand, Comparatives Nos. 17 to 19, in which the middle term (A value) of Inequality A was less than 0.65, showed creep rupture times of less than 800 hours. In addition, Comparatives Nos. 20 to 23, in which the A value was 0.65 or more but the left-hand side (B value) of Inequality B exceeded 0.170, also showed creep rupture times of less than 800 hours. It should be noted that in No. 24, in which the value of Mo + 0.5W exceeded 1.5, and in No. 25, in which the middle term (A value) of the Inequality exceeded 1.50, edge cracking was observed at the leading end portion of each plate after hot rolling. Furthermore, in No. 26, in which the Cu content exceeded 0.5%, a crack propagating in the width direction was observed at the mid-thickness of the leading end portion after hot forging. Nos. 24 to 26 were judged to have inferior hot workability compared with the other steel materials and were therefore excluded from the creep test.

[0050] As is apparent from the above Examples, the compositional range of the steels having the creep strength of more than 800 hours at 800 degrees C and 100 MPa has been clarified by the invention.INDUSTRIAL APPLICABILITY

[0051] According to the invention, it becomes possible to reduce the required design plate thickness of Nb- and N-containing high-Ni alloys applied to uses requiring high-temperature creep strength, or to increase a size and extend a service life of equipment, thereby improving design flexibility. Further, these alloys are not only usable for high-temperature application but also widely usable for structures that are to be used for high corrosion resistant application.

[0052] It thus becomes possible to provide high-Ni alloys with stable quality, in response to increasing demands for high-Ni alloys, which greatly contributes to industrial development.

Examples

examples

[0046]Examples will be described below. The inventors melted a high-Ni alloy in an MgO crucible using a 50-kg vacuum melting furnace, added Al, Ti, Ca, and Mg as required, and cast the melt into a 25-kg square mold, thereby obtaining high-Ni alloys having the compositions shown in Tables 1-1 and 1-2. It should also be noted that blank columns in Table 1 show that the corresponding components are at an impurity level. In the tables below, the components and the Inequalities that fall outside the scope of the invention are underlined.

[0047]Cast steels obtained by casting the melted material had a tapered shape with dimensions of approximately 105-mm square to approximately 90-mm square and a height of approximately 280 mm. These cast steels were subjected to hot forging in the temperature range of 1,200 to 1,000 degrees C and were processed into a thickness of 50 mm and a width of 120 mm. Subsequently, a heat treatment of soaking at 1,250 degrees C for 3 hours was performed, and im...

TECHNICAL FIELD

[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.BACKGROUND ART

[0002] As high-Ni alloys used in heat-resistant applications, Alloy 800H (ASTM N08810, N08811) is a representative commercial alloy. In response to recent increase in demands in developing countries, it has been requested to develop technologies for supplying commercial products that are inexpensive and excellent in surface quality and usage characteristics. In view of the above request, the steel manufacturing process has been shifted from a typical ingot casting process to a continuous casting process. Since high-Ni alloys are highly susceptible to slab internal cracks during casting, edge cracks during hot working, and surface defects on the product, efforts have been made to improve and develop design for chemical composition, refining, casting, and hot working techniques for the high-Ni alloys, in order to enhance manufacturability in a continuous casting process.

[0003] On the other hand, in the primary applications of the high-Ni alloys, namely high-temperature reaction vessels in chemical plants, the alloys are often used at a temperature of 600 degrees C or higher and, moreover, under high pressure in order to improve the efficiency of chemical reactions. In such applications, the higher the creep strength of the high-Ni alloys, the more readily the alloys can be used in thinner sections.

[0004] In recent years, there has been an increasing number of cases in which materials such as ASTM N08120, in which Nb and N are added and precipitation strengthening is achieved by Nb-based carbonitride precipitates, are employed. Non-Patent Literature 1 is an ASTM standard specification relating to Ni-Fe-Cr alloys, and also covers ASTM N08120. In this specification, the final heat-treatment temperature is described as being 1,177 degrees C or higher.

[0005] For example, related-art documents such as Patent Literatures 1 and 2 describe high-Ni alloys utilizing precipitation strengthening by addition of Nb and N (hereinafter also referred to as "Nb- and N-containing high-Ni alloys"). As described above, in each of these patent literatures, the alloy design makes combined use of precipitation strengthening by carbonitride precipitates of Nb and the like, solid-solution strengthening by the addition of Mo, and grain-boundary strengthening by the addition of B. In order to enhance the creep strength through precipitation strengthening by Nb, the final heat-treatment temperature after hot rolling is set as high as 1,300 degrees C at maximum, which is higher than that employed for ordinary Fe-based high alloys.CITATION LISTPATENT LITERATURE(S)

[0006] Patent Literature 1: JP Patent No. 6,675,846 Patent Literature 2: JP Patent No. 7,174,192 NON-PATENT LITERATURE(S)

[0007] Non-Patent Literature 1: ASTM B409SUMMARY OF THE INVENTIONPROBLEM(S) TO BE SOLVED BY THE INVENTION

[0008] In both Patent Literatures 1 and 2, the creep strength is enhanced by controlling the amount of Nb carbonitride precipitates. However, as is apparent from the fact that both Patent Literatures 1 and 2 set the upper limit on the added amount based on the product of the Nb and N contents, the solubility product of Nb with C and N is small. Accordingly, even when the heat treatment temperature is raised to around 1,300 degrees C during the final heat treatment after hot rolling (hereinafter also simply referred to as "final heat treatment"), coarse undissolved carbonitrides remain. In particular, as described in Patent Literature 1, there has been a limitation in that an excessive amount of these undissolved carbonitrides causes a decrease in creep strength. Accordingly, in the compositional systems described in Patent Literatures 1 and 2, further improvement of the creep strength had been considered difficult. Although the compositional systems described in Patent Literatures 1 and 2 partially fall within the ASTM N08120 specification, it has been difficult to solve the issues of Patent Literatures 1 and 2 and further enhance the creep strength by using only the components whose ranges are specified in ASTM N08120.

[0009] An object of the invention is to provide a high-Ni alloy capable of solving the above-described problem and having excellent high-temperature creep strength.MEANS FOR SOLVING THE PROBLEM(S)

[0010] The inventors, as a means for solving the above-described problem, have found that the creep strength is markedly improved, compared with steels containing Nb alone, by allowing V or Ta to be contained in combination with Nb, and thus completed the invention.

[0011] Since V and Ta each have a larger solubility product with N than Nb does, V and Ta do not form as large an amount of carbonitrides as Nb does even when V and Ta are contained in an amount approximately equal to the amount of Nb. On the other hand, at a service temperature of 900 degrees C or lower, the solubility product of each of V and Ta with N becomes small, similar to that of Nb with N, so that carbonitrides precipitate during aging, resulting in improved creep strength. The inventors have found that the creep strength is markedly improved, compared with typical Nb- and N-containing high-Ni alloys, by allowing V and / or Ta that precipitate during aging to be contained in combination with Nb, and thus completed the invention.

[0012] Specifically, main features of the invention are as follows. (1) A high-Ni alloy having excellent high-temperature creep strength, including, 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%; at least one of Mo or W in an amount such that Mo + 0.5 × W is 0.01% or more and less than 1.50%; Nb: 0.10 to 0.65%; at least one of V or Ta in a total amount of 0.01 to 0.60%; and a balance consisting of Fe and impurities, and the alloy satisfying Inequalities A and B below, Inequality A: 0.65 ≤ Nb % + 2 × V % + Ta % ≤ 1.50 Inequality B: Nb % + 0.15 × V % + 0.4 × Ta % × C % + N % ≤ 0.170 , where in Inequality A and Inequality B, the element symbols accompanied by "%" indicate a content of each element in mass%. (2) The high-Ni alloy according to (1), further including, in mass%, in place of a part of the Fe: 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 according to (1) or (2), in which, in a creep test conducted in accordance with JIS Z 2271 under conditions of 800 degrees C and 100 MPa, a creep rupture time exceeds 800 hours.

[0013] A high-Ni alloy containing Nb and N is used in applications such as reaction vessels in chemical plants, including polycrystalline silicon production equipment, where high levels of high-temperature creep strength and high-temperature corrosion resistance are required. By markedly improving the creep strength beyond typical levels through the combined addition of V and / or Ta in addition to Nb, it becomes possible to increase a size of such equipment or to reduce a wall thickness and weight thereof, which is expected to make a significant contribution to development of the chemical industry and the semiconductor industry. Further, the high-Ni alloy of the invention, which is mainly supplied in a product form of thick plates for the above plant applications, can also be supplied in various other product forms, including thin plates, tubes, coils, bars, and wire rods. A thickness of the thick plate is preferably from 5 mm to 80 mm.BRIEF DESCRIPTION OF DRAWING(S)

[0014] Fig. 1 is a diagram illustrating an influence of an A value on the middle term of Inequality A and a B value on the left-hand side of Inequality B on a creep rupture time.DESCRIPTION OF EMBODIMENT(S)

[0015] First, the reason for the limitation defined in claim 1 of the invention will be described below. It should be noted that the content of each component is expressed in mass%.Chemical CompositionC: 0.08% or Less

[0016] In heat-resistant materials, C is ordinarily an element that is actively added in order to secure high-temperature strength. However, in the invention, as will be described later, the creep strength is enhanced by aging precipitation strengthening due to a Z phase (a Cr-Nb-N-type nitride) that precipitates during aging at the service temperature. Excessive addition of C consumes Nb, which is intended to be used for forming the Z phase, in the form of MX-type carbonitrides (M: metal, X: C or N), thereby decreasing the creep strength. Accordingly, in the invention, the amount of C added is limited to 0.08% or less.Si: 0.05 to 1.0%

[0017] Si is added in an amount of 0.05% or more for deoxidation and improvement in oxidation resistance. However, since Si is also an element that lowers the melting point of steel, adding Si in an amount exceeding 1.0% reduces the hot ductility in the vicinity of 1,200 degrees C and deteriorates the susceptibility to solidification cracking and the susceptibility to liquation cracking during welding. In addition, intermetallic compounds become more likely to precipitate, resulting in deterioration of high-temperature properties. Therefore, the upper limit of the Si content is limited to 1.0%. The upper limit of the Si content is preferably 0.7%, more preferably 0.5%.Mn: 0.05 to 1.5%

[0018] 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 invention. In order to improve the heat resistance, Mn is added in an amount of 0.05% or more. However, when the added amount of Mn exceeds 1.5%, contrary to expectations, intermetallic compounds are likely to precipitate, which deteriorates the heat resistance and adversely affects the susceptibility to solidification cracking. Therefore, the upper limit of the Mn content is set to 1.5%. The upper limit of the Mn content is preferably 1.3%, more preferably 1.0%.P: 0.030% or Less

[0019] P is an element inevitably mixed from a raw material and has an effect of increasing the susceptibility to solidification cracking. Therefore, the P content is limited to 0.030% or less, preferably 0.025% or less.S: 0.0015% or Less

[0020] S is an element inevitably mixed from a raw material and deteriorates hot workability and oxidation resistance. Therefore, the S content is limited to 0.0015% or less, preferably 0.0010% or less. S is an element whose content can be lowered by refining. However, a large cost is required in order to extremely lower the S content. Therefore, the lower limit of the S content is preferably 0.0001%.Cr: 20 to 30%

[0021] Cr is a requisite element in order for the heat-resistant alloy used as a high-temperature material to exhibit oxidation resistance and high-temperature corrosion resistance. Therefore, the Cr content is 20% or more, preferably 22% or more. In addition, in the invention, the Z phase that precipitates during aging is utilized to improve the creep strength, and Cr is indispensable for ensuring the stable formation of this Z phase. When the Cr content falls below 20%, a Cr-depleted layer is formed at grain boundaries during aging, so that the Z phase does not precipitate in the vicinity of the grain boundaries, resulting in the formation of a precipitation-free zone. As a result, the creep rupture time decreases due to the resulting reduction in grain-boundary strength. On the other hand, when the Cr content exceeds 30%, the high-temperature microstructural stability decreases even with a large content of Ni, and intermetallic compounds begin to precipitate, thereby deteriorating the heat resistance. Therefore, the Cr content is limited to 30% or less. The upper limit of the Cr content is preferably 28%, more preferably 26%.Ni: 23 to 60%

[0022] Ni is an element that stabilizes the austenitic microstructure at high temperatures and improves corrosion resistance to various acids, high-temperature corrosion resistance to chlorides, and toughness. Therefore, the Ni content is 23% or more, preferably 28% or more, and still more preferably more than 35%. By increasing the Ni content, it becomes possible to contain larger amounts of ferrite-forming elements such as Cr, W, Mo, V, and Nb, which are required to ensure heat resistance. However, because Ni is an expensive element, the upper limit of the Ni content in the steel of the invention is set to 60% or less, preferably 50%, in view of the cost.Al: 0.01 to 0.20%

[0023] Al is a deoxidizing element and has an effect of forming a NiAl ordered phase in a high-Ni alloy to enhance high-temperature strength. In the invention, Al needs to be added in an amount of 0.01% or more, preferably 0.03% or more, in order to promote deoxidation and desulfurization to enhance hot workability. On the other hand, when the Al content exceeds 0.20%, manufacturability and heat resistance are impaired due to the precipitation of AIN. Therefore, the upper limit of the Al content is set to 0.20%. The upper limit of the Al content is preferably 0.15%, more preferably 0.10%.Ti: 0.05% or Less

[0024] In high-Ni alloys containing N, Ti forms coarse TiN. The formation of coarse TiN not only adversely affects manufacturability, surface quality of steel, and aging toughness, but also inhibits the fine precipitation of the Z phase, resulting in a decrease in creep strength. Therefore, the upper limit of the Ti content in the invention is limited to 0.05%, preferably 0.03%. Ti may not be contained.B: 0.0002 to 0.0050%

[0025] B is actively added, particularly for applications in which the material is used at high temperatures, in order to improve hot workability and high-temperature creep strength in the Region II embrittlement range (in the vicinity of 1,000 degrees C). B is an element that increases grain-boundary strength by segregating to the grain boundaries. The effect of improving the hot workability by containing B is exerted when the B content is 0.0002% or more. Therefore, the lower limit of the B content is set to 0.0002%. On the other hand, B is an element that lowers the melting point of steel and is also highly prone to segregation; therefore, excessive addition of B promotes solidification cracking and liquation cracking, and has a significantly adverse effect on hot workability, particularly in the Region I embrittlement range (in the vicinity of 1,200 degrees C). Therefore, the upper limit of the B content is set to 0.0050%. The upper limit of the B content is preferably 0.0030%.N: 0.10 to 0.30%

[0026] N is an indispensable element for exhibiting the precipitation strengthening mechanism by the Z phase utilized in the invention. The Z phase utilized in the invention is a CrNbN-type nitride or a Cr(Nb,V,Ta)N-type nitride in which a portion of Nb is substituted with V or Ta. In the high-Ni alloy of the invention, the Z phase is the most stable and the most abundant nitride among the nitrides that remain undissolved or precipitate during aging. In order to improve the high-temperature creep strength by precipitation of the Z phase, N needs to be added in an amount of 0.10% or more. The lower limit of the N content is preferably defined by N (atomic%) ≥ Nb (atomic%) + V (atomic%) + Ta (atomic%). On the other hand, excessive addition of N exceeding 0.30% (by mass%, hereinafter the same) not only causes bubble formation during refining but also results in a coarse Z phase remaining undissolved during the final heat treatment, which inhibits the fine dispersion of the Z phase that precipitates during aging and thereby decreases the creep strength. Therefore, the upper limit of the N content is set to 0.30%. The upper limit of the N content is preferably 0.25%.O: 0.006% or Less

[0027] Oxygen (O) reacts with Ca, Mg, Al, and Ti in the alloy of the invention to form oxide inclusions. The oxygen content, which depends on the total amount of the oxide inclusions, is an important factor as an index for deoxidation of the alloy. When the oxygen content exceeds 0.006%, the desired deoxidation equilibrium can no longer be satisfied, and nozzle clogging during continuous casting as well as surface defects caused by inclusions are more likely to occur. Therefore, the upper limit of the oxygen content is set to 0.006%. The upper limit of the oxygen content is preferably 0.005%, more preferably 0.004%. On the other hand, reduction in the oxygen content, which advantageously reduces nozzle clogging and weld hot cracking by reducing oxide inclusions, generates excessive Ca and excessive Mg in the alloy to lower the hot workability. Therefore, the oxygen content is preferably 0.0002% or more.Ca: 0.0001 to 0.0050%

[0028] Ca fixes S in the alloy in a form of CaS, thereby improving the hot workability, resistance to weld hot cracking, and resistance to high-temperature oxidation of the alloy. The reaction progresses as follows. Ca is bonded with oxygen in the alloy to form CaO and CaO-Al 2 O 3 to substantially annihilate oxygen dissolved in the alloy (free oxygen). Subsequently, residual Ca reacts with S in the alloy to form CaS. In order to improve the above properties in the alloy of the invention, the Ca content is 0.0001 % or more, preferably 0.0003% or more, and still more preferably 0.0005% or more. On the other hand, excessive addition of Ca decreases the hot ductility in the vicinity of 1,100 degrees C. Therefore, the upper limit of the Ca content is set to 0.0050%. The upper limit of the Ca content is preferably 0.0045%.Mo + 0.5 × W: from 0.01% to less than 1.50% (with at least one of Mo or W present)

[0029] Both Mo and W are elements that enhance the high-temperature strength and high-temperature corrosion resistance of heat-resistant alloys. In order to exert these effects, in the invention, one or both of Mo and W are added such that the value of Mo + 0.5 × W is 0.01% or more, preferably 0.05% or more, and still more preferably 0.10% or more. The value of Mo + 0.5 × W may exceed 0.3%. Still further more preferably, Mo in an amount exceeding 0.3% is added. On the other hand, with regard to the hot workability, Mo has a greater adverse effect than W, and the hot workability is markedly degraded when the value of Mo + 0.5 × W is 1.50% or more. Therefore, in the component system of the invention, the upper limit of the value of Mo + 0.5 × W is set to less than 1.50%. The value of Mo + 0.5 × W is more preferably less than 1.20%. The upper limit of the Mo content is preferably 0.78%. The upper limit of the W content is preferably 1.95%.Nb: 0.10 to 0.65%

[0030] Nb, as with N, is an indispensable element for exhibiting the precipitation strengthening mechanism by the Z phase utilized in the invention, and this effect is exerted when Nb is added in an amount of 0.10% or more. The lower limit of the Nb content is preferably 0.20%. On the other hand, when Nb is added in an amount exceeding 0.65%, a coarse Z phase remains undissolved even after the final heat treatment. Such coarse undissolved precipitates inhibit the fine dispersion of the Z phase that precipitates during aging at the service temperature, and consequently not only decrease the creep strength but also deteriorate the aging toughness and increase the susceptibility to weld hot cracking. Therefore, the upper limit of the Nb content in the invention is set to 0.65%. The upper limit of the Nb content is preferably 0.60%, more preferably 0.55%.Total of at least one of V or Ta: 0.01 to 0.60%

[0031] Both V and Ta are elements that contribute to the aging precipitation strengthening by the Z phase utilized in the invention. Both V and Ta precipitate in the form of Cr(Nb,V,Ta)N in which Nb in the Z phase is substituted with V or Ta, thereby increasing the total amount of the Z phase to improve the creep strength. Since either element exerts its effect when added in an amount of 0.01% or more, one or both of the two elements are added in a total amount of 0.01% or more in the invention. The lower limit of the total amount is preferably 0.02%, more preferably 0.05%. On the other hand, the addition in the total amount exceeding 0.60% adversely affects hot workability and aging toughness. Therefore, in the invention, the upper limit of the total amount is set to 0.60%. The upper limit of the total amount is preferably 0.50%. Inequality A: 0.65 ≤ Nb % + 2 × V % + Ta % ≤ 1.50

[0032] The inventors have conducted extensive studies in order to improve the high-temperature creep strength to a level that has been difficult to achieve with typical heat-resistant high-strength Fe-Cr-Ni alloys containing Nb and N, such as those disclosed in Cited Literatures 1 and 2. As a result, the inventors have found that further improvement in creep strength can be achieved by adding V and / or Ta, which are Group V metals like Nb and have a larger solubility limit with N than Nb, in combination with Nb, and thus completed the invention. The most important strengthening mechanism of the high-Ni alloy of the invention is aging precipitation strengthening, which is achieved by the fine dispersion of the Z phase, a stable precipitate phase at the service temperature, during aging. For example, when the high-Ni alloy is maintained at a relatively high temperature of around 800 degrees C for approximately 500 hours, the Z phase precipitates in a finely dispersed form with a diameter of about 0.1 µm or less, thereby markedly increasing the creep rupture time. In the case of typical steels in which only Nb is added, Nb has a small solubility product with N, and therefore, when Nb is added in an amount exceeding 0.65%, a coarse Z phase remains undissolved during the final heat treatment and undergoes Ostwald growth in the service temperature range. As a result, the uniform fine dispersion of the Z phase that precipitates during aging is hindered, and consequently, the creep strength is instead reduced. V and Ta have larger solubility limits than Nb and can be almost completely dissolved in the temperature range of the final heat treatment. Therefore, even when V and Ta are added, V and Ta have little effect on the total amount of undissolved Z phase remaining after the final heat treatment. On the other hand, at the service temperature, most of the V and Ta precipitate during aging as Cr(Nb,V,Ta)N in which Nb in the Z phase is substituted.

[0033] Furthermore, as a result of further intensive studies, it was found that the strengthening capability per unit mass percent of V, whose atomic weight is smaller than that of Nb, can be regarded as approximately twice that of Nb, while Ta, although having a larger atomic weight than Nb, tends to form a Z phase, which precipitates during aging, of relatively smaller size, and strengthening capability of Ta per unit mass percent can be considered to be approximately equal to that of Nb.

[0034] That is, in the component system of the invention, it was found that the creep rupture time can be correlated by the relationship of A Value = Nb% + 2 x V% + Ta%. When the A value is less than 0.65, the creep strength remains at a level attainable with typical steels in which only Nb is added. On the other hand, when the A value exceeds 1.50, the hot workability, susceptibility to aging embrittlement, and microstructural stability of the alloy are markedly degraded. Accordingly, the lower and upper limits of the A value are set to 0.65 and 1.50, respectively, and the above Inequality A is established. The lower limit of the A value is preferably 0.75. The upper limit of the A value is preferably 1.30. Inequality B: Nb % + 0.15 × V % + 0.4 × Ta % × C % + N % ≤ 0.170

[0035] Inequality B is defined so as to serve as an index, because, within the compositional range of the alloy of the invention, undissolved phases remaining during the final heat treatment adversely affect the creep strength.

[0036] B value = (Nb% + 0.15 × V% + 0.4 × Ta%) × (C% + N%) is determined. In the formula for the B value, the coefficients 0.15 for V and 0.4 for Ta are coefficients set in accordance with the differences in the solubility products of each of V and Ta with N or C. The undissolved phases are mainly the Z phase, but a portion of MX-type carbonitrides (where M is a metal and X is C or N) is also present. In a heat-treatment furnace with relatively high output, heat treatment can be performed at a heat-treatment temperature of 1,200 to 1,300 degrees C. However, even when the final heat treatment is carried out at such a high temperature, if the B value exceeds 0.170, a large number of undissolved phases remain, having a diameter of 0.2 µm or more and, in some cases, exceeding 1 µm. These coarse undissolved precipitates undergo Ostwald growth at the service temperature, thereby hindering the uniform fine dispersion of the Z phase that precipitates during aging. Consequently, even if Inequality A is satisfied, the creep rupture time does not increase and instead decreases. Accordingly, the above Inequality B is established. Considering that the smaller the left-hand side (B value) of Inequality B is, the more the dissolution of coarse Z phase is promoted, the upper limit of the B value is preferably 0.120, more preferably 0.100.

[0037] In producing the alloy of the invention, as the final heat treatment after hot rolling, a heat treatment at a temperature from 1,180 to 1,300 degrees C is performed. This corresponds to Non-Patent Literature 1 (ASTM B409), which is an ASTM standard specification for Ni-Fe-Cr alloys. The final heat treatment is generally referred to as solution heat treatment or solutionizing heat treatment, and cooling is performed by water quenching after the heat treatment. By performing the above final heat treatment, high creep strength can be imparted. That is, by performing the final heat treatment under the above condition after providing the alloy with the compositional requirements of the invention, it is possible to achieve the creep rupture time exceeding 800 hours under the conditions of 800 degrees C and 100 MPa in a creep test in accordance with JIS Z 2271. If a satisfactory creep rupture time is not obtained under the selected final heat-treatment temperature condition, a satisfactory creep rupture time can be achieved by further increasing the heat-treatment temperature within the final heat-treatment temperature range of 1,300 degrees C or lower. In addition to improving the creep strength, the optimum final heat-treatment temperature condition for preventing deterioration in aging toughness and resistance to liquation cracking during welding, due to grain refinement and grain coarsening, are in the range of 1,200 to 1,250 degrees C.

[0038] The chemical composition of the high-Ni alloy of the invention includes the above components and a balance consisting of Fe and impurities. Next, the reason for the limitation defined in claim 2 of the invention will be described below. Further, in place of a part of Fe, the following component(s) (mass%) is optionally contained.Cu: 0.01 to 0.50%

[0039] Cu is an element that improves the corrosion resistance of the alloy against acids and dew point corrosion resistance, which is a frequent issue of concern in high-temperature equipment, and also enhances the high-temperature strength and the microstructural stability. Accordingly, Cu may be added as required. In order to enhance the heat resistance and corrosion resistance, the Cu content is 0.01% or more, preferably 0.02% or more, and still more preferably 0.05% or more. However, when the Cu content exceeds 0.50%, defects caused by embrittlement occur during solidification. Therefore, the upper limit of the Cu content is set to 0.50%.Co: 0.01 to 1.0%

[0040] Co is an element that is effective for improving the high-temperature microstructural stability and the corrosion resistance of the alloy. In order to enhance these properties, the Co content is 0.01% or more, preferably 0.02% or more, and still more preferably 0.10% or more. When the Co content exceeds 1.0%, the effect is not commensurate with the cost because Co is an expensive element; therefore, the upper limit of the Co content is set to 1.0%. The upper limit of the Co content is preferably 0.8%, more preferably 0.50%.Sn: 0.001 to 0.05%

[0041] Sn is an element that improves the corrosion resistance and the high-temperature creep strength of steel at a content of 0.001% or more, preferably 0.005% or more. Sn may be added as required. However, the hot workability is deteriorated when Sn is added at a content exceeding 0.05%; therefore, the upper limit of the Sn content is set to 0.05%.Zn + Pb + Bi: 0.0010% or Less

[0042] Zn, Pb, and Bi considerably lower the hot workability in an austenitic single-phase alloy. Accordingly, the upper limit of each of the contents of these elements has to be strictly set. Pb ≤ 0.0010%, Zn ≤ 0.0010%, and Bi ≤ 0.0010% are preferable. A total of the contents of Pb, Zn, and Bi is set to 0.0010% or less.Mg: 0.0050% or Less

[0043] Since Mg is an element that exerts a desulfurization effect, a small amount of Mg can improve the hot workability of alloys. However, excessive addition of Mg markedly reduces the hot workability in the vicinity of 900 degrees C. Therefore, when Mg is added in the invention, the upper limit of the Mg content is set to 0.0050%. The upper limit of the Mg content is preferably 0.0040%, more preferably 0.0030%.Zr: 0.001 to 0.10%Hf: 0.001 to 0.10%

[0044] Both Zr and Hf, when added in an amount of 0.001% or more, preferably 0.005% or more, have the effect of improving resistance to solidification cracking, hot workability, and resistance to high-temperature oxidation of steel by fixing P and S, and may be added as required. On the other hand, addition in a large amount exceeding 0.10% forms coarse nitrides, resulting in a decrease in creep strength and also adversely affecting manufacturability. Accordingly, the upper limit of the content of each of Zr and Hf is set to 0.10%.La + Ce + Nd + Pr: 0.001 to 0.050%

[0045] La, Ce, Nd, and Pr are elements that fix P and S when added in a total amount of 0.001% or more, preferably 0.005% or more, to improve oxidation resistance and hot workability of steel. On the other hand, when these elements are added in a total amount exceeding 0.050%, coarse oxides and nitrides are formed, which significantly impairs manufacturability, such as by causing nozzle clogging during refining and increasing surface defects. Therefore, the upper limit of the total of contents of these elements is set to 0.050%. It should be noted that these elements may be added either in elemental form or in the form of an alloy, including misch metal.Examples

[0046] Examples will be described below. The inventors melted a high-Ni alloy in an MgO crucible using a 50-kg vacuum melting furnace, added Al, Ti, Ca, and Mg as required, and cast the melt into a 25-kg square mold, thereby obtaining high-Ni alloys having the compositions shown in Tables 1-1 and 1-2. It should also be noted that blank columns in Table 1 show that the corresponding components are at an impurity level. In the tables below, the components and the Inequalities that fall outside the scope of the invention are underlined.

[0047] Cast steels obtained by casting the melted material had a tapered shape with dimensions of approximately 105-mm square to approximately 90-mm square and a height of approximately 280 mm. These cast steels were subjected to hot forging in the temperature range of 1,200 to 1,000 degrees C and were processed into a thickness of 50 mm and a width of 120 mm. Subsequently, a heat treatment of soaking at 1,250 degrees C for 3 hours was performed, and immediately after removal from a heat treatment furnace, hot rolling was performed in the temperature range of 1,200 to 900 degrees C to obtain a plate thickness of 16 mm. Thereafter, a final heat treatment of soaking at 1,250 degrees C for 1 hour was performed, followed by water quenching. A creep test specimen was prepared from each alloy plate, with a gauge section of 6 mm in diameter and 80 mm in length, and a grip section of 12 mm in diameter, taken parallel to the rolling direction. The creep test was conducted in accordance with JIS Z 2271, under the conditions of 800 degrees C and 100 MPa, and the time to rupture was measured. A specimen having the creep rupture time exceeding 800 hours was regarded as acceptable. The results of the creep test are shown in Table 2 and Fig. 1.

[0048] In Fig. 1, the horizontal axis represents the middle term (A value) of Inequality A, and the vertical axis represents the creep rupture time. In addition, the white circles indicate cases in which the left-hand side (B value) of Inequality B is 0.170 or less, and the white squares indicate cases in which the left-hand side (B value) of Inequality B exceeds 0.170. As is apparent from Fig. 1, in the cases indicated by white circles (B value of 0.170 or less) in which the A value is 0.65 or more, that is, in the cases satisfying both Inequality A and Inequality B, it can be seen that the creep rupture time of 800 hours or more is achieved.

[0049] As shown in Table 2 and Fig. 1, Examples Nos. 1 to 16 of the invention, which satisfied all of the compositional ranges, Inequality A, and Inequality B, satisfied creep rupture times of 800 hours or more under the creep test conditions of 800 degrees C and 100 MPa. On the other hand, Comparatives Nos. 17 to 19, in which the middle term (A value) of Inequality A was less than 0.65, showed creep rupture times of less than 800 hours. In addition, Comparatives Nos. 20 to 23, in which the A value was 0.65 or more but the left-hand side (B value) of Inequality B exceeded 0.170, also showed creep rupture times of less than 800 hours. It should be noted that in No. 24, in which the value of Mo + 0.5W exceeded 1.5, and in No. 25, in which the middle term (A value) of the Inequality exceeded 1.50, edge cracking was observed at the leading end portion of each plate after hot rolling. Furthermore, in No. 26, in which the Cu content exceeded 0.5%, a crack propagating in the width direction was observed at the mid-thickness of the leading end portion after hot forging. Nos. 24 to 26 were judged to have inferior hot workability compared with the other steel materials and were therefore excluded from the creep test.

[0050] As is apparent from the above Examples, the compositional range of the steels having the creep strength of more than 800 hours at 800 degrees C and 100 MPa has been clarified by the invention.INDUSTRIAL APPLICABILITY

[0051] According to the invention, it becomes possible to reduce the required design plate thickness of Nb- and N-containing high-Ni alloys applied to uses requiring high-temperature creep strength, or to increase a size and extend a service life of equipment, thereby improving design flexibility. Further, these alloys are not only usable for high-temperature application but also widely usable for structures that are to be used for high corrosion resistant application.

[0052] It thus becomes possible to provide high-Ni alloys with stable quality, in response to increasing demands for high-Ni alloys, which greatly contributes to industrial development.

Examples

examples

[0046]Examples will be described below. The inventors melted a high-Ni alloy in an MgO crucible using a 50-kg vacuum melting furnace, added Al, Ti, Ca, and Mg as required, and cast the melt into a 25-kg square mold, thereby obtaining high-Ni alloys having the compositions shown in Tables 1-1 and 1-2. It should also be noted that blank columns in Table 1 show that the corresponding components are at an impurity level. In the tables below, the components and the Inequalities that fall outside the scope of the invention are underlined.

[0047]Cast steels obtained by casting the melted material had a tapered shape with dimensions of approximately 105-mm square to approximately 90-mm square and a height of approximately 280 mm. These cast steels were subjected to hot forging in the temperature range of 1,200 to 1,000 degrees C and were processed into a thickness of 50 mm and a width of 120 mm. Subsequently, a heat treatment of soaking at 1,250 degrees C for 3 hours was performed, and im...

Claims

1. A high-Ni alloy having excellent high-temperature creep strength, comprising, 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%; at least one of Mo or W in an amount such that Mo + 0.5 × W is 0.01% or more and less than 1.50%; Nb: 0.10 to 0.65%; at least one of V or Ta in a total amount of 0.01 to 0.60%; and a balance consisting of Fe and impurities, and the alloy satisfying Inequalities A and B below, Inequality A: 0.65 ≤ Nb % + 2 × V % + Ta % ≤ 1.50 Inequality B: Nb % + 0.15 × V % + 0.4 × Ta % × C % + N % ≤ 0.170 , where in Inequality A and Inequality B, the element symbols accompanied by "%" indicate a content of each element in mass%.

2. The high-Ni alloy according to claim 1, further comprising, in mass%, in place of a part of the Fe: 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 according to claim 1 or 2, wherein, in a creep test conducted in accordance with JIS Z 2271 under conditions of 800 degrees C and 100 MPa, a creep rupture time exceeds 800 hours.

Images