HIGH Ni ALLOY HAVING SUPERIOR HIGH-TEMPERATURE CREEP STRENGTH AND HCl GAS CORROSION RESISTANCE

Abstract

To provide a high Ni alloy containing Nb and N that has superior high-temperature creep strength and HCl gas corrosion resistance.SOLUTION: A high Ni alloy having superior high-temperature creep strength and HCl gas corrosion resistance, the alloy comprising, in mass%, C: 0.08% or less, Cr: 18-30%, Ni: 23-60%, Al: 0.01-0.20%, B: 0.0002-0.0050%, N: 0.10-0.30%, 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], [Limiting Expression B], and [Limiting Expression C]: [Limiting Expression A] 0.65≤Nb%+2×V%+Ta%≤1.50. [Limiting Expression B] (Nb%+0.15×V%+0.4×Ta%)×(C%+N%)≤0.170. [Limiting Expression C] Ni%+2.7×Cr%≥100.0.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a high-Ni alloy having excellent high-temperature creep strength and HCl gas corrosion resistance, which is used in high-temperature applications where high-temperature creep strength and HCl gas corrosion resistance are required. [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 addition, since high-temperature corrosion by HCl gas occurs on the furnace walls of equipment where HCl gas is generated during high-temperature reactions, such as the reaction vessel of a polysilicon manufacturing equipment, corrosion resistance to high-temperature HCl gas is also required for application in such environments. Ni is well known as a metal with excellent resistance to HCl gas corrosion, but because Ni is expensive and has poor high-temperature strength, it is common to prepare a clad material by bonding Ni to a component with high-temperature strength, and then place the Ni side of the clad material in the area that will come into contact with HCl gas.

[0005] 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.

[0006] 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. In order to increase creep strength due to the precipitation strengthening of Nb, the final heat treatment temperature after hot rolling is set to a maximum of 1300°C, which is a higher temperature than that of ordinary Fe-based high alloys. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Patent No. 6675846 [Patent Document 2] Patent No. 7174192 [Non-patent literature]

[0008] [Non-Patent Document 1] ASTM B409 Summary of the Invention [Problem to be solved by the invention]

[0009] Both Patent Documents 1 and 2 improve creep strength by controlling the amount of Nb carbonitride precipitation. However, as can be seen from the fact that both Patent Documents 1 and 2 set the upper limit of the Nb content as the product of the Nb and N contents, Nb has a small solid solubility product with C and N. Therefore, even when the heat treatment temperature is raised to around 1300°C during the final heat treatment after hot rolling (hereinafter simply referred to as "final heat treatment"), undissolved coarse carbonitrides remain. As noted in Patent Document 1 in particular, excessive undissolved carbonitrides 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. In addition, neither Patent Document 1 nor Patent Document 2 takes into consideration corrosion resistance to HCl gas at high temperatures, and there has been a demand for steel that ensures high levels of both HCl gas corrosion resistance and creep strength.

[0010] The present invention aims to solve the above problems and to provide a high-Ni alloy that is excellent in high-temperature creep strength and HCl gas corrosion resistance. [Means for solving the problem]

[0011] The present inventors discovered that, as a means for solving the above problems, the creep strength is significantly improved by including V or Ta in combination with Nb compared to steel containing Nb alone, and arrived at the present invention.

[0012] Because V and Ta both have a larger solid solubility product with N than Nb, they do not form as many carbonitrides as Nb even when contained in the same amount. On the other hand, at temperatures below 900°C, which is the operating temperature range, V and Ta have a small solid solubility product with N, just like Nb, and therefore age-precipitate as carbonitrides, improving creep strength. It was discovered that the inclusion of 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. Furthermore, the present invention was achieved by optimizing the Cr and Ni contents to improve corrosion resistance to HCl gas compared to pure Ni.

[0013] That is, the gist of the present invention is as follows. [1] By 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: 18 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% A high-Ni alloy with excellent high-temperature creep strength and HCl gas corrosion resistance, characterized in that it contains at least one of Mo and W (Mo+0.5×W: 0.01% or more but less than 1.50%), Nb: 0.10 to 0.65%, at least one of V and Ta (0.01 to 0.60%) in total, and the balance consisting of Fe and impurities, and satisfies the following <Limiting Formula A>, <Limiting Formula B>, and <Limiting Formula C>. <Limited formula A> 0.65 ≦ Nb%+2×V%+Ta% ≦ 1.50 <Limited formula B> (Nb%+0.15×V%+0.4×Ta%)×(C%+N%) ≦ 0.170 <Limited expression C> Ni% + 2.7 × Cr% ≧ 100.0 In Restrictive Formula A, Restrictive Formula B, and Restrictive Formula C, the element symbols with % indicate the content (mass %) of each element. [2] A high-Ni alloy having excellent high-temperature creep strength and HCl gas corrosion resistance according to [1], characterized in that a part of the Fe is replaced by one or more of, by mass%, 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%.

[0014] [3] A high-Ni alloy with excellent high-temperature creep strength and HCl gas corrosion resistance 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] The corrosion weight loss after removing corrosion scale by exposing to a 90% HCl-10% N2 atmosphere at 600°C for 48 hours was 7.0 mg / cm 2 A high-Ni alloy having excellent high-temperature creep strength and HCl gas corrosion resistance according to any one of [1] to [3], characterized in that:

[0015] [5] A high-Ni alloy having excellent high-temperature creep strength and HCl gas corrosion resistance according to any one of [1] to [4], characterized in that it is used in applications where it is exposed to an atmosphere containing HCl gas at 500°C or higher. [Effects of the Invention]

[0016] High-Ni alloys containing Nb and N are used in applications requiring high levels of high-temperature creep strength and HCl gas corrosion resistance, such as reactor vessels for chemical plants, such as polycrystalline silicon manufacturing equipment. The addition of V and / or Ta in addition to Nb significantly improves creep strength compared to conventional products, enabling these facilities to be enlarged or thinned and lightweight. Furthermore, the improved HCl gas corrosion resistance achieved by optimizing the Ni and Cr content allows for use at higher temperatures, improving the productivity of these facilities and ultimately contributing significantly 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 supplied 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]

[0017] [Figure 1] FIG. 1 is a diagram showing the influence of the middle term (A value) of <Constraint Equation A> and the left term (B value) of <Constraint Equation B> on creep rupture time. [Figure 2] FIG. 1 is a diagram showing the effect of <Limiting Formula C> on HCl gas corrosion resistance. DETAILED DESCRIPTION OF THE INVENTION

[0018] 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>

[0019] 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.

[0020] 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%.

[0021] 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%.

[0022] 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.

[0023] 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%.

[0024] Cr: 18~30% Cr is an essential element for imparting oxidation resistance and high-temperature corrosion resistance to heat-resistant alloys used as high-temperature materials. Its content is set at 18% or more, preferably 20% or more. In addition, the present invention utilizes the Z-phase, which precipitates during aging, to improve creep strength. Cr is essential for ensuring stable formation of this Z-phase. If the Cr content is below 18%, Cr-depleted zones form at grain boundaries during aging, preventing Z-phase precipitation near the grain boundaries and forming precipitate-free zones. This results in reduced grain boundary strength and reduced HCl gas corrosion resistance, resulting in reduced creep rupture time and corrosion resistance to HCl gas. On the other hand, if the Cr content exceeds 30%, high-temperature structural stability is impaired, 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%.

[0025] Ni: 23-60% Ni stabilizes the austenite structure at high temperatures and improves corrosion resistance to various acids, high-temperature corrosion resistance to chlorides such as HCl, and toughness. Ni is contained in an amount of 23% or more, preferably 28% or more, and more preferably more than 35%. Increasing the Ni content allows for the inclusion of 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%, and the more preferred upper limit is 45%.

[0026] 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%.

[0027] Ti: 0.05% or less In high-Ni alloys containing N, Ti forms coarse TiN. The formation of coarse TiN not only adversely affects manufacturability, the surface quality, and aging toughness of the steel, but also inhibits the fine precipitation of Z phase, resulting in a decrease in creep strength. Therefore, in the present invention, the upper limit of the Ti content is specified to be 0.05%. The preferred upper limit is 0.03%. Ti may not be contained.

[0028] 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%.

[0029] N: 0.10 to 0.30% N is an essential element for utilizing 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 precipitation of the Z-phase within crystal grains and at grain boundaries, 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 final 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%.

[0030] 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.

[0031] 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%.

[0032] 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 these 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%. Adding more than 0.3% of Mo is even more preferable. However, 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 composition is set to less than 1.50%. It is more preferable for Mo + 0.5 × W to be less than 1.20%. The preferred upper limits for Mo and W are 0.78% and 1.95%, respectively.

[0033] 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 by adding 0.10% or more. The preferred lower limit is 0.20%. On the other hand, if added in excess of 0.65%, coarse Z-phase remains undissolved even after final heat treatment. These coarse undissolved precipitates inhibit the fine dispersion of the Z-phase that precipitates during aging at the service temperature, thereby not only reducing creep strength but also worsening aging toughness and weld hot cracking resistance. For this reason, the upper limit of Nb 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%.

[0034] 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 elements precipitate as Cr(Nb,V,Ta)N, substituting for Nb in the Z-phase, within grains and at grain boundaries, increasing the total amount of Z-phase and thereby improving creep strength. Since the effects of both elements are manifested when added at 0.01% or more, the present invention adds one or both of them in a total amount 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. However, 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%.

[0035] <Limited formula A> 0.65 ≦ Nb%+2×V%+Ta% ≦ 1.50 The present inventors conducted extensive research to improve high-temperature creep strength to a level difficult to achieve with conventional Nb-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 improvements in creep strength are possible by adding V and / or Ta, which are group V metals like Nb but have a larger solid solubility limit with N than Nb, in combination with Nb, leading 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, finely disperses during aging precipitation. For example, when held at a relatively high temperature of approximately 800°C for approximately 500 hours, the Z phase disperses and precipitates with a diameter of approximately 0.1 μm or less, significantly increasing the creep rupture time. In conventional steels containing only Nb, Nb has a small solid solubility product with N. Therefore, adding more than 0.65% Nb leaves coarse Z phase undissolved during final heat treatment and undergoes Ostwald ripening at operating temperatures. This hinders the uniform dispersion of the fine Z phase that precipitates during aging, resulting in a decrease in creep strength. V and Ta have a higher solid solubility limit than Nb, allowing almost all of them to be dissolved in the final heat treatment temperature range. Therefore, the addition of V and Ta has little effect on the total amount of Z phase remaining undissolved during final heat treatment. However, at operating temperatures, most of the V and Ta precipitate during aging as Cr(Nb,V,Ta)N, substituting Nb in the Z phase. Further investigation revealed that the strengthening capacity per unit mass% of V, which has a smaller atomic weight than Nb, can be considered to be approximately twice that of Nb. Meanwhile, Ta, although having a larger atomic weight than Nb, tends to produce a relatively small size of the Z phase that precipitates during aging, so its strengthening capacity per unit mass% can be considered to be similar to that of 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, resistance to aging embrittlement, 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] <Limited formula B> (Nb%+0.15×V%+0.4×Ta%)×(C%+N%) ≦ 0.170 Restriction formula B is defined as an index for the creep strength of the alloy of the present invention, since the undissolved phase remaining at the final heat treatment has an adverse effect on the creep strength within the composition range of the alloy. B value = (Nb% + 0.15 × V% + 0.4 × Ta%) × (C% + N%) In the B-value equation, the coefficients 0.15 and 0.4 for V and Ta correspond to the difference in the solid solubility product of each element with N or C. The undissolved phase is primarily Z-phase, but some MX-type (M: metal, X: carbonitrides with C or N) is also present. Although relatively high-power heat treatment furnaces can perform heat treatment at temperatures between 1200°C and 1300°C, even with final heat treatment at such high temperatures, if the B-value exceeds 0.170, numerous undissolved phases with diameters of 0.2 μm or larger, some even exceeding 1 μm, remain. These coarse undissolved precipitates undergo Ostwald ripening at the operating temperature, preventing the uniform dispersion of the fine Z-phase that precipitates during aging. Therefore, even if <Constraint Equation A> is satisfied, the creep rupture time does not increase, but rather decreases. Therefore, the above <Constraint Equation B> was established. Considering that the smaller the left side of the <limiting formula B> (B value), the more the solid solution of the coarse Z phase is promoted, the preferable upper limit of the B value is 0.120, and the more preferable upper limit is 0.100.

[0038] <Limited expression C> Ni% + 2.7 × Cr% ≧ 100.0 Restriction formula C is defined to ensure that the alloy of the present invention has HCl gas corrosion resistance equal to or greater than that of pure Ni within the composition range of the alloy. C value = Ni% + 2.7 × Cr% The coefficient for Cr in the formula for the C value is a coefficient set by comparing the effect per unit mass% on corrosion resistance to HCl gas with that of Ni. None of the metallic component elements of the steel of the present invention are corrosion-resistant to HCl gas, and in particular, they are unable to form a protective oxide film when HCl gas does not contain oxygen. Therefore, it is effective to increase the amount of metallic elements that have the lowest possible reaction rate with HCl gas. Most metallic elements have a slower reaction rate than Ni. In the case of Cr, its reaction rate is slower than Ni at temperatures above 500°C, and its effect per unit mass% can be estimated to be 2.7 times that of Ni. When the value of the limiting formula C exceeds 100.0, HCl gas corrosion resistance equivalent to or better than that of pure Ni can be imparted.

[0039] In the production of the alloy of the present invention, the final heat treatment after hot rolling is a heat treatment at 1180°C to 1300°C. This corresponds to Non-Patent Document 1 (ASTM B409), which is the ASTM standard for Ni-Fe-Cr alloys. The final heat treatment is generally called a solution heat treatment or a solution heat treatment, and is followed by water cooling.

[0040] The above-described final heat treatment can impart high creep strength. That is, by providing the composition of the present invention and performing the final heat treatment under the above-described conditions, a quality can be achieved in which the creep rupture time exceeds 800 hours in a creep test (based on JIS Z2271) under conditions of 800°C and 100 MPa. If an appropriate creep rupture time cannot be obtained under the selected temperature conditions of the final heat treatment, it can be achieved by further increasing the heat treatment temperature within a range of 1300°C or less. The optimal final heat treatment conditions for preventing a decrease in creep strength, aging toughness due to grain refinement and grain coarsening, and liquation cracking resistance during welding are 1200°C to 1250°C.

[0041] The high Ni alloy of the present invention has the composition of the present invention, and by satisfying the above-mentioned <Limiting Formula C> in particular, it can realize excellent HCl gas corrosion resistance in addition to high-temperature creep strength. That is, when exposed to a 90% by volume HCl-10% by volume N2 atmosphere at 600°C for 48 hours and the corrosion scale is removed, the corrosion weight loss is 7.0 mg / cm 2 The present invention is characterized by the following:

[0042] As described above, the high Ni alloy of the present invention has excellent HCl gas corrosion resistance, and therefore can be suitably used in applications where it is exposed to an atmosphere containing HCl gas at 500° C. or higher.

[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, significantly improving the oxidation resistance of steel, especially scale spallation resistance. These elements also improve hot workability through the above-mentioned effects. However, adding more than 0.050% in total generates coarse oxides and nitrides, significantly impairing manufacturability by causing nozzle clogging during refining and increasing 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 mischmetal. [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 needed, 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 Table 1 indicate impurity levels. In the following tables, elements and limiting formulas outside the ranges of the present invention are underlined. In Tables 1-1, 1-2, and Table 2 (described later), Steel Nos. 1 to 16, designated "Invention Examples," fall within the ranges of the present invention in terms of the content of each element and the formulas <Limiting Formula A>, <Limiting Formula B>, and <Limiting Formula C>. Steel Nos. 17 to 23, designated "Comparative Example 1," fall outside the ranges of the present invention in terms of either <Limiting Formula A> or <Limiting Formula B>. Steel Nos. 24 to 27, designated "Comparative Example 2," fall outside the range of the present invention in terms of <Limiting Formula C>. Steel Nos. 28 to 30, designated "Comparative Example 3," have specific element compositions outside the ranges of the present invention.

[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 approximately 280mm high. These slabs were hot forged in the temperature range of 1200°C to 1000°C and processed into 50mm thick x 120mm wide pieces. They were then soaked at 1250°C for three hours and hot rolled to a thickness of 16mm in the temperature range of 1200°C to 900°C immediately after removal from the heat treatment furnace. They then underwent a final heat treatment, soaking at 1250°C for one hour, before being water-cooled.

[0055] Creep test specimens were prepared from the alloy plates parallel to the rolling direction, measuring 6mmφ in diameter, 80mm in length, and 12mmφ in the fixed section. The creep test conformed to JIS Z2271, measuring the time to rupture under conditions of 800℃ and 100MPa, and specimens with a creep rupture time of over 800 hours were deemed to have passed. The creep test results are shown in Table 2 and Figure 1, respectively.

[0056] In Figure 1, the horizontal axis represents the middle part of <Constraint Formula A> (A value), and the vertical axis represents creep rupture time. "Invention Examples" are marked with an ◯, "Comparative Example 1" with an X, and "Comparative Example 2" with a △. For the "Invention Examples" and the "△" with an X (Comparative Example 2), the left side of <Constraint Formula B> (B value) is 0.170 or less. For the "X" with an X (Comparative Example 1), the left side of <Constraint Formula B> (B value) is greater than 0.170, or the A value of <Constraint Formula A> is less than 0.65. As is clear from Figure 1, when the B value is 0.170 or less and the A value is 0.65 or greater, i.e., when both <Constraint Formula A> and <Constraint Formula B> are satisfied, a creep rupture time of 800 hours or more is achieved.

[0057] For the HCl gas corrosion resistance test, each alloy plate was cut into a thickness of 6 mm, width of 14 mm, and length of 15 mm, and the entire surface was wet-polished with #600. The specimens were then held for 48 hours in a gas atmosphere of 90% by volume of HCl and 10% by volume of N2 at 600°C, after which the corrosion scale was thoroughly removed using a 10% citric acid solution, and the corrosion weight loss (mg / cm) was calculated by dividing the difference between the mass before the test and the mass after scale removal by the surface area of ​​the entire specimen. 2 For comparison, pure Ni polished to the same size and under the same conditions as the test material was also tested on the same test machine. The results of the corrosion weight loss measurements are shown in Table 2 and Figure 2, respectively.

[0058] In Figure 2, the horizontal axis is the left side of the <Limiting Formula C> (C value), and the vertical axis is the corrosion weight loss. "Invention Example" is marked with an ◯, "Comparative Example 1" with an X, and "Comparative Example 2" with a △. The ◯ and X marks indicate that the left side of the <Limiting Formula C> (C value) is 100.0 or more, and the △ mark indicates that the left side of the <Limiting Formula C> (C value) is less than 100.0. The corrosion weight loss of pure Ni was 6.96 mg / cm 2 Therefore, the corrosion weight loss was set at 7.0 mg / cm as the pass / fail criterion. 2 Passed the following: 7.0 mg / cm 2 As is clear from Figure 2, when the circle and cross marks, i.e., when the limiting formula C, was satisfied, the corrosion weight loss was 7.0 mg / cm 2 If the mark △, i.e., limiting formula C, is not satisfied, the corrosion weight loss is 7.0 mg / cm 2 It can be seen that the corrosion weight loss is greater than that of pure Ni.

[0059] [Table 2]

[0060] As shown in Table 2 and Figure 1, Examples 1 to 16 of the present invention and Examples 24 to 27 of Comparative Example 2, which satisfied all of the ranges of composition, <Constraint Formula A>, and <Constraint Formula B>, achieved a time to rupture of 800 hours or more under creep test conditions of 800°C and 100 MPa. On the other hand, Examples 17 to 19 of Comparative Example 1, in which the middle term (A value) of <Constraint Formula A> was less than 0.65, also achieved a time to rupture of less than 800 hours. Furthermore, Examples 20 to 23 of Comparative Example 1, in which the left term (B value) of <Constraint Formula B> (B value) exceeded 0.170 despite having an A value of 0.65 or more, also achieved a time to rupture of less than 800 hours.

[0061] Furthermore, as shown in Table 2 and FIG. 2, Nos. 1 to 16 of the present invention examples and Nos. 17 to 23 of Comparative Example 1, which satisfy <Limiting Formula C>, have a corrosion weight loss of 7.0 mg / cm 2 In Comparative Example 2, Nos. 24 to 27, whose left side of the <limiting formula C> (C value) was less than 100.0, exhibited a corrosion weight loss of 7.0 mg / cm, which is equivalent to pure Ni.2 It became bigger than that.

[0062] In addition, No. 28, whose Mo+0.5W value exceeded 1.5, and No. 29, whose limiting formula midpoint (A value) exceeded 1.50, both showed edge cracks at the plate edge after hot rolling. Furthermore, No. 30, 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. 28 to 30 were judged to have inferior hot workability compared to the other steels and were therefore excluded from the test.

[0063] As can be seen from the above examples, the present invention provides a steel sheet having a creep strength of more than 800 hours at 800°C and 100 MPa, and a corrosion weight loss of 7.0 mg / cm after exposure to a 90% by volume HCl-10% by volume N atmosphere at 600°C for 48 hours. 2 The range of steel components below this has become clear. [Industrial Applicability]

[0064] The present invention allows for the design of Nb- and N-containing high-Ni alloys for applications requiring high-temperature creep strength and HCl gas corrosion resistance to be thinner, or for larger equipment, higher performance, or 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. 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: 18 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%, A high-Ni alloy having excellent high-temperature creep strength and HCl gas corrosion resistance, characterized in that it contains at least one of Mo and W in the amount of Mo+0.5×W: 0.01% or more but less than 1.50%, Nb: 0.10 to 0.65%, at least one of V and Ta in a total amount of 0.01 to 0.60%, and the balance consisting of Fe and impurities, and satisfies the following <Limiting Formula A>, <Limiting Formula B>, and <Limiting Formula C>. <Limited formula A> 0.65≦Nb%+2×V%+Ta%≦1.50 <Limited type B> (Nb%+0.15×V%+0.4×Ta%)×(C%+N%) ≦ 0.170 <Limited formula C> Ni%+2.7×Cr% ≧ 100.0 In Restriction Formula A, Restriction Formula B, and Restriction Formula C, the element symbols with % indicate the content (mass %) of each element.

2. 2. The high-Ni alloy according to claim 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. 3. A high-Ni alloy having excellent high-temperature creep strength and HCl gas corrosion resistance 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 at 800°C and 100 MPa.

4. 90% by volume HCl-10% by volume N at 600°C ２ After 48 hours of exposure to the atmosphere and removal of corrosion scale, the corrosion weight loss was 7.0 mg / cm ２ 3. The high-Ni alloy having excellent high-temperature creep strength and HCl gas corrosion resistance according to claim 1 or 2, characterized in that:

5. 3. The high-Ni alloy having excellent high-temperature creep strength and HCl gas corrosion resistance according to claim 1 or 2, characterized in that it is used in applications where it is exposed to an atmosphere containing HCl gas at 500° C. or higher.

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