HIGH-Ni ALLOY MATERIAL AND HIGH-Ni ALLOY PLATE HAVING EXCELLENT INTERGRANULAR CORROSION RESISTANCE, AND METHODS FOR PRODUCING THEM

Abstract

To provide a high-Ni alloy that exhibits superior workability and superior resistance to intergranular corrosion after being held for a long period at a sensitization temperature.SOLUTION: A high-Ni alloy material comprises, in mass%, C: 0.015% or less, Si: 0.60% or less, Mn: 2.00% or less, P: 0.040% or less, S: 0.040% or less, Ni: 37.00% or more and 48.00% or less, Cr: 18.50% or more and 24.50% or less, Mo: 1.50% or more and 4.50% or less, Cu: 0.50% or more and 4.00% or less, Al: 0.30% or less, Ti: 0.50% or more and 1.30% or less, N: 0.0200% or less, Ca: 0.0060% or less, Nb: 0.003% or more and 0.250% or less, B: 0.003% or more and 0.0030% or less, with the balance being Fe and unavoidable impurities.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a high Ni alloy material and a high Ni alloy plate having excellent intergranular corrosion resistance, and to a method for producing the same. [Background technology]

[0002] High-Ni alloys are used in applications requiring oxidation resistance and corrosion resistance. Patent Document 1 discloses an Al- and Ti-containing Fe-Ni-Cr alloy with excellent surface properties, which contains 17 to 46 mass% Ni, 18 to 25 mass% Cr, more than 0.06 to 0.6 mass% Al, 0.2 to 1.0 mass% Ti, and further contains Ca and Mg.

[0003] UNS08825 is known as a high-Ni alloy. UNS08825 is an austenitic, highly corrosion-resistant alloy used in severely corrosive environments such as chemical plant components and heat exchangers, and is sometimes used in environments exposed to high-temperature nitric acid. For these applications, it is required that the alloy have good intergranular corrosion resistance even when maintained in the sensitization temperature range for long periods of time. This alloy contains 0.6% or more Ti and is designed to reduce solute C.

[0004] It is recommended that UNS08825 be annealed at 930 to 980°C (Non-Patent Document 1). When annealed within this temperature range, solute C in the austenite phase precipitates not only as Ti carbide but also as Cr carbide. This significantly reduces solute C and provides excellent intergranular corrosion resistance. It is believed that sufficient element diffusion occurs within this temperature range to eliminate Cr-depleted regions, so Cr carbide precipitation does not result in Cr-depleted regions. On the other hand, when annealing is performed at temperatures higher than 980°C, Cr carbide does not form, resulting in insufficient reduction of solute C. Therefore, if this material is held in the sensitization temperature range for a long period of time, Cr carbide precipitates on the austenite grain boundaries, creating Cr-depleted regions, resulting in reduced intergranular corrosion resistance.

[0005] Austenitic stainless steels are generally annealed at temperatures above 980°C. Therefore, when UNS08825 is produced in an annealing facility primarily used for the production of austenitic stainless steels, it is desirable to anneal at temperatures above 980°C from the standpoint of productivity. However, when annealing at temperatures above 980°C, the C content must be further reduced to ensure intergranular corrosion resistance, which significantly increases the decarburization load in the steelmaking process.

[0006] Given this background, there was a demand for a high-Ni alloy that could be annealed at higher temperatures, reduce the load on the steelmaking process, and have excellent resistance to intergranular corrosion. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2014-189826 [Non-patent literature]

[0008] [Non-Patent Document 1] ELRAYMOND: Corrosion, 1968, vol.24, pp.180-188. Summary of the Invention [Problem to be solved by the invention]

[0009] An object of the present invention is to provide a high Ni alloy material and a high Ni alloy plate that are easy to manufacture and have excellent resistance to intergranular corrosion after being held at a sensitization temperature for a long period of time, and a method for manufacturing the same. [Means for solving the problem]

[0010] The present inventors have conducted extensive research into the relationship between the composition of high-Ni alloys and the annealing temperature. As a result, they have found that the corrosion rate of alloys containing a specified range of B and Nb in combination is suppressed compared to alloys containing either element alone or neither element. Furthermore, they have found that this phenomenon occurs when the C content is below a specified level and the alloy is annealed at a temperature higher than the temperature range recommended in UNS08825.

[0011] Although the mechanism by which the above-mentioned measures improve intergranular corrosion resistance has not yet been elucidated, it is believed that the combined inclusion of B and Nb reduces the amount of dissolved C in austenite during sensitization heat treatment, and Cr carbides are uniformly dispersed in the austenite matrix. Furthermore, it is believed that Nb replaces part of the Cr, which is a substitutional element for the Cr carbides. It is presumed that these phenomena suppress the formation of Cr-deficient regions, thereby improving intergranular corrosion resistance.

[0012] The present invention has been made based on the above findings, and the gist of the present invention is as follows. [1] A high-Ni alloy material containing, by mass%, C: 0.015% or less, Si: 0.60% or less, Mn: 2.00% or less, P: 0.040% or less, S: 0.040% or less, Ni: 37.00% or more and 48.00% or less, Cr: 18.50% or more and 24.50% or less, Mo: 1.50% or more and 4.50% or less, Cu: 0.50% or more and 4.00% or less, Al: 0.30% or less, Ti: 0.50% or more and 1.30% or less, N: 0.0200% or less, Ca: 0.0060% or less, Nb: 0.003% or more and 0.250% or less, B: 0.0003% or more and 0.0030% or less, with the remainder being Fe and impurities. [2] The high Ni alloy material according to [1], characterized in that it contains, in mass%, one or more of V: ​​0.30% or less, Sn: 0.12% or less, W: 1.0% or less, Co: 0.5% or less, and Sb: 0.15% or less, in place of a portion of the Fe. [3] A high-Ni alloy plate having the composition described in [1] or [2].

[0013] [4] The method for producing a high Ni alloy material according to [1] or [2], characterized in that the annealing temperature after hot rolling is 980°C or higher. [5] The method for producing a high Ni alloy sheet according to [3], characterized in that the annealing temperature after hot rolling is 980°C or higher. [Effects of the Invention]

[0014] According to the present invention, a high Ni alloy having excellent intergranular corrosion resistance can be stably supplied without limiting the annealing temperature range after hot rolling. DETAILED DESCRIPTION OF THE INVENTION

[0015] 1.High Ni alloy material, high Ni alloy plate The high Ni alloy material and high Ni alloy plate of the present invention will be described in detail below.

[0016] (Chemical composition) First, the components contained in the high Ni alloy material and high Ni alloy plate of the present invention will be described. Note that the "%" for the content of each element means % by mass.

[0017] C is an element that affects intergranular corrosion resistance, as it is related to the amount of C dissolved in austenite and the formation of Cr deficiency due to the precipitation of Cr carbides when maintained in the sensitization temperature range. If the C content exceeds 0.015%, excessive Cr carbides precipitate, forming Cr-deficient regions around the grain boundaries of the austenite parent phase. Therefore, the C content must be 0.015% or less. Preferably, it is 0.013% or less, and more preferably, it is 0.010% or less. Since a lower C content results in better quality, there is no particular lower limit. However, excessively reducing the C content increases production costs. From the perspective of practical manufacturing, the C content is preferably 0.002% or more, and more preferably, 0.003% or more.

[0018] Si is an element that improves oxidation resistance. There is no particular lower limit. However, from the viewpoint of product characteristics, the Si content is preferably 0.01% or more, and more preferably 0.05% or more. On the other hand, if the Si content exceeds 0.60%, cracking during manufacturing is promoted. Therefore, the Si content is set to 0.60% or less, preferably 0.50% or less, and more preferably 0.40% or less.

[0019] Mn is used as a deoxidizing and desulfurizing element. There is no particular lower limit. From the viewpoint of stable production, the Mn content is preferably 0.01% or more, and more preferably 0.05% or more. On the other hand, if the Mn content exceeds 2.00%, the effect saturates. Therefore, the Mn content is set to 2.00% or less, preferably 1.40% or less, and more preferably 0.80% or less.

[0020] P is an unavoidable impurity element and also an element that reduces corrosion resistance. The lower its content, the better. Therefore, the P content is set to 0.040% or less, preferably 0.030% or less, and more preferably 0.025% or less. There is no particular lower limit.

[0021] S is an unavoidable impurity element that promotes cracking during manufacturing. Therefore, the S content is set to 0.040% or less, preferably 0.020% or less, and more preferably 0.010% or less. There is no particular lower limit.

[0022] Ni is an element that improves corrosion resistance, particularly sulfuric acid resistance, and the higher the content, the better the properties. If the Ni content is less than 37.00%, sufficient sulfuric acid resistance cannot be ensured. On the other hand, if the Ni content exceeds 48.00%, Ni is a rare and expensive element, and excessive content leads to increased alloy costs and impaired manufacturability. Therefore, the Ni content is set to 37.00% or more and 48.00% or less. Preferably, it is 39.00% or less and 47.00% or less, and more preferably, it is 41.00% or more and 46.00% or less.

[0023] Cr is an element that improves general corrosion resistance. It also combines with C to form Cr carbides during maintenance in the sensitization temperature range, affecting intergranular corrosion resistance. If the Cr content is less than 18.50%, sufficient general corrosion resistance cannot be ensured. On the other hand, if the Cr content exceeds 24.50%, Cr is a rare and expensive element, and excessive inclusion leads to increased alloy costs and impaired manufacturability. Therefore, the Cr content is set to 18.50% or more and 24.50% or less. Preferably, it is 20.30% or more and 23.50% or less, and more preferably, it is 22.10% or more and 23.30% or less.

[0024] Mo is an element that improves corrosion resistance. If the Mo content is less than 1.50%, sufficient corrosion resistance cannot be ensured. On the other hand, if the Mo content exceeds 4.50%, the effect saturates. Furthermore, since Mo is a rare and expensive element, excessive content leads to increased alloy costs and impaired manufacturability. Therefore, the Mo content is set to 1.50% or more and 4.50% or less. Preferably, it is 1.75% or more and 4.00% or less, and more preferably, it is 3.00% or more and 3.50% or less.

[0025] Cu is an element that improves sulfuric acid resistance. If the Cu content is less than 0.50%, sufficient acid resistance cannot be ensured. On the other hand, if the Cu content exceeds 4.00%, the effect saturates. Furthermore, excessive Cu content reduces hot workability and increases manufacturing costs. Therefore, the Cu content is set to 0.50% or more and 4.00% or less. Preferably, it is 1.00% or more and 3.00% or less, and more preferably, it is 1.50% or more and 2.00% or less.

[0026] Al is used as a deoxidizing element. There is no particular lower limit. The Al content is preferably 0.03% or more. If the Al content exceeds 0.30%, the effect saturates. Therefore, the Al content is set to 0.30% or less. It is preferably 0.20% or less, and more preferably 0.15% or less.

[0027] Ti is an element that plays a role in reducing the amount of solute carbon in austenite as Ti carbide. This action suppresses the precipitation of Cr carbide during holding in the sensitization temperature range, thereby suppressing the generation of Cr deficiency and contributing to intergranular corrosion resistance. If the Ti content is less than 0.50%, sufficient intergranular corrosion resistance cannot be ensured. On the other hand, even if the Ti content exceeds 1.30%, the effect saturates. It can also be a factor in reducing weldability. Therefore, the Ti content is set to 0.50% or more and 1.50% or less. Preferably, it is 0.60% or more and 1.20% or less, and more preferably, it is 0.65% or more and 0.80% or less.

[0028] N is an element that forms Ti nitrides by bonding with Ti. If the N content exceeds 0.0200%, a large amount of Ti nitrides is formed, which causes surface defects on the alloy sheet and increases manufacturing costs. Therefore, the N content is set to 0.0200% or less, preferably 0.0150% or less, and more preferably 0.0100% or less. The N content is preferably 0.0005% or more.

[0029] Ca is used as a deoxidizing and desulfurizing element. If the Ca content exceeds 0.0060%, the effect saturates. Therefore, the Ca content is set to 0.0060% or less. It is preferably 0.0050% or less, and more preferably 0.0040% or less. There is no particular lower limit. However, from the viewpoint of preventing nozzle clogging during casting, the Ca content is preferably 0.0005% or more, and more preferably 0.0010% or more.

[0030] Nb is an important element in the high-Ni alloy of the present invention, and its inclusion in combination with B exhibits the effect of improving intergranular corrosion resistance. It is believed that the inclusion of Nb reduces the amount of solute carbon in austenite and that Nb replaces some of the substitutional elements in Cr carbides precipitated during sensitization heat treatment, thereby reducing the likelihood of Cr-deficient regions. If the Nb content is less than 0.003%, the effect of reducing the amount of solute carbon is insufficient, and the replacement of substitutional elements in Cr carbides with Nb is also insufficient, resulting in the failure to exhibit the effect of improving intergranular corrosion resistance. On the other hand, if the Nb content exceeds 0.250%, an intermetallic compound phase mainly composed of Nb, which is easily soluble in high-temperature nitric acid, precipitates on the austenite grain boundaries, and its dissolution is believed to increase corrosion weight loss. Therefore, the Nb content is set to 0.003% or more and 0.250% or less. It is preferably 0.004% or more and 0.100% or less, and more preferably 0.006% or more and 0.050% or less.

[0031] B is an important element in the high-Ni alloy of the present invention, and its inclusion in combination with Nb enhances intergranular corrosion resistance. The inclusion of B facilitates uniform intragranular precipitation of Cr carbides during sensitization heat treatment, reducing the amount of Cr carbides on austenite grain boundaries. It is presumed that the addition of Nb, which replaces substitutional elements for Cr carbides with Nb, improves intergranular corrosion resistance. If the B content is less than 0.0003%, the effect of uniformly dispersing Cr carbides within grains is insufficient, and the effect of improving intergranular corrosion resistance is not achieved. On the other hand, if the B content exceeds 0.0030%, the corrosion-resistant element Cr is consumed by the grain boundary precipitation of boride, resulting in a decrease in intergranular corrosion resistance. Therefore, the B content is set to 0.0003% or more and 0.0030% or less. It is preferably 0.0004% or more and 0.0020% or less, and more preferably 0.0006% or more and 0.0015% or less.

[0032] As described above, it is important that the high Ni alloy of the present invention contains a predetermined range of Nb and B. If the content of either element is outside the predetermined range, the effect of improving intergranular corrosion resistance will not be achieved.

[0033] The high Ni alloy material and high Ni alloy plate of the present invention contain the above-mentioned components, with the balance being Fe and impurities. In addition to the above-mentioned basic composition, the present invention may further contain, in mass %, one or more of Sn: 0.12% or less, Sb: 0.15% or less, Co: 0.50% or less, V: 0.30% or less, and W: 1.0% or less, instead of a portion of the Fe.

[0034] Sn, Co, V, and W do not necessarily have to be contained. These elements have the effect of improving general corrosion resistance. When these elements are contained, the Sn content should be 0.12% or less, the Co content should be 0.50% or less, the V content should be 0.30% or less, and the W content should be 1.0% or less. To ensure the above effects, it is preferable that the Sn content be 0.01% or more, the Co content be 0.03% or more, and the V and W contents be 0.02% or more.

[0035] Sb does not have to be contained. This element has the effect of changing inclusions such as oxides and sulfides to suppress hot working defects. When Sb is contained, the Sb content should be 0.15% or less. To ensure the above effect, the Sb content is preferably 0.0001% or more.

[0036] 2. Manufacturing method for high Ni alloy material and high Ni alloy plate (Manufacturing method) A method for producing a high Ni alloy material and a high Ni alloy plate according to the present invention will now be described.

[0037] An alloy having the above composition is melted and cast to produce an ingot. The ingot is then heated. The heating temperature may be any of the methods commonly used in the production of high-Ni alloys. The heating temperature is 1150°C or higher, preferably 1180°C or higher, and more preferably 1200°C or higher. There is no particular upper limit. However, if the heating temperature is excessively high, there is a concern that uneven patterns may occur on the surface of the alloy sheet after pickling. From this perspective, the heating temperature is preferably 1350°C or lower.

[0038] Next, the heated ingot (alloy ingot) is hot-rolled. The hot-rolling conditions may be any of those commonly used in the production of high-Ni alloys. Before the hot-rolling, hot forging, breakdown rolling, and soaking may be performed to eliminate segregation and homogenize the structure, thereby obtaining an alloy ingot. When forging, breakdown rolling, or soaking is performed, the alloy ingot is reheated and hot-rolled.

[0039] Finish annealing is performed after the hot rolling. In conventional high-Ni alloys that do not contain Nb and B in combination, as mentioned above, when annealing is performed at a temperature higher than 980°C, Cr carbides are not formed, and the reduction of solute C is insufficient. When this material is held in the sensitization temperature range for a long time, Cr carbides precipitate on the austenite grain boundaries, causing Cr-deficient regions, resulting in a decrease in intergranular corrosion resistance.

[0040] In the production of the high Ni alloy of the present invention, the composite content of Nb and B within a predetermined range promotes a reduction in the amount of dissolved carbon, even when the annealing temperature is 980°C or higher, and Cr-deficient regions are less likely to occur during sensitization heat treatment, thereby suppressing a decrease in intergranular corrosion resistance. That is, in the present invention, it is important to maintain the composite content of Nb and B within a predetermined range and to control the annealing temperature to 980°C or higher. The annealing temperature is preferably 1020°C or higher, more preferably 1060°C or higher. There is no particular upper limit, but based on past experience, it is 1250°C or lower. It is preferably 1200°C or lower. 1150°C or lower is more preferable. The presence or absence of the effect of improving intergranular corrosion resistance is determined by comparing the corrosion rate with that in the case where Nb and B are not contained. Specifically, this is determined by the rate ratio defined by formula (B) according to the following procedure.

[0041] A high Ni alloy plate (hereinafter referred to as "subject alloy plate") having the specified components satisfying the present invention and a comparative alloy plate not containing Nb or B (both of which are unavoidable contaminant amounts) but containing the same other components as the subject alloy plate were prepared. The annealing temperature after hot rolling of the comparative alloy plate was the same as that of the subject alloy plate. Both the subject alloy plate and the comparative alloy plate were subjected to a sensitization heat treatment at 700°C for 900 minutes, and then subjected to a corrosion test in accordance with ASTM A262-C (immersion in 65% boiling nitric acid for 48 hours, five times) to measure the corrosion weight loss ΔW (g). The corrosion rate (g / m) was calculated using formula (A). 2 / h) is calculated, and the corrosion rate after sensitization heat treatment is calculated by the following formula (B), where X is the corrosion rate of the target alloy plate and Y is the corrosion rate of the comparative alloy plate. Formula (A): Corrosion rate (g / m 2 / h) = (7290 × ΔW) / (A × t) × K A: Surface area A (cm 2 ), t: immersion time (h), K: unit conversion factor (mm / month to g / m 2 / h): 1.39 Formula (B): Speed ​​ratio=X / Y

[0042] If the speed ratio is less than 1.00, it is determined that the effect of improving intergranular corrosion resistance is realized. On the other hand, if it is 1.00 or more, it is determined that the effect is not realized. If the Nb and B content is equal to or greater than the lower limit of the above-mentioned component range, it is determined that the alloy contains both Nb and B.

[0043] The corrosion rate when Nb and B are contained is smaller than the values ​​given by equations (α), (β), and (γ), which approximate the relationship between the corrosion rate and the C content when Nb and B are not contained. Equations (α), (β), and (γ) are selected according to the C content. Here, [C] is the C content (mass%). <<When [C]≦0.006>> Corrosion rate (g / m 2 / h)=0.002·EXP(613.39[C]) Formula (α) <When 0.006<[C]<0.008> Corrosion rate (g / m 2 / h)=0.04·EXP(154.08[C]) Formula (β) <<When 0.008≦[C]>> Corrosion rate (g / m 2 / h)=0.01·EXP(392.35[C]) Formula (γ)

[0044] On the other hand, when the annealing temperature is less than 980°C, the amount of solute C in austenite is sufficiently reduced, and therefore the intergranular corrosion resistance is good even without the combined inclusion of Nb and B within the specified range. Therefore, even if the combined inclusion of Nb and B within the specified range is made, the effect of improving the intergranular corrosion resistance is not realized.

[0045] After the annealing, the alloy sheet is subjected to pickling to remove oxide scale from the surface. This pickling is a process for removing oxide scale from the surface, and can be carried out by a method commonly used in the production of high-Ni alloys. Alternatively, oxide scale removal can be carried out by cutting.

[0046] The effects of the high Ni alloy of the present invention will be explained by the following Examples 1 and 2. [Example]

[0047] Example 1 illustrates the C content and annealing heat treatment temperature that exhibit the effects of the present invention. Table 1 shows the composition of alloys melted in the laboratory. Uppercase letters indicate alloys containing both Nb and B, while lowercase letters indicate alloys containing neither Nb nor B. The unavoidable contamination amounts in the laboratory melting were 0.001% or less for Nb and 0.0001% or less for B. Alloys containing neither Nb nor B had Nb and B contents at the above-mentioned unavoidable contamination levels. On the other hand, alloys containing Nb and B with contents above the lower limit of the present invention had Nb and B intentionally added during melting, and thus correspond to alloys containing both Nb and B. In Table 1 and Tables 2-1 to 4 described below, values ​​and items outside the range of the present invention are underlined.

[0048] In the laboratory melting, 60 kg was melted and cast for two minutes to produce two 30 kg ingots. The first ingot contained no Nb or B, while the second ingot contained Nb and B by actively adding them. These ingots were heated at 1230°C for 100 minutes and then hot forged to produce alloy ingots with a thickness of 50 mm. The alloy ingots were then subjected to homogenization heat treatment at 1200°C for 720 minutes and cooled to room temperature. After further heating at 1250°C for 120 minutes, they were hot-rolled to produce hot-rolled sheets with a thickness of 8 mm. The hot-rolled sheets were then annealed for 20 minutes at the heat treatment temperatures shown in Tables 2-1 and 2-2 to produce alloy sheets. In Table 1, alloy sheets containing both Nb and B, designated by uppercase letters, correspond to "target alloy sheets," and alloys containing neither Nb nor B, designated by lowercase letters, correspond to "comparison alloy sheets." As can be seen from Tables 2-1 and 2-2, in examples with the same parent number, both the child number 1 and the child number 2 were annealed at the same annealing temperature.

[0049] (Method for measuring corrosion rate) The corrosion rate, which indicates the intergranular corrosion resistance after sensitization, was measured using the following method. First, the target alloy plate and the comparative alloy plate were subjected to sensitization heat treatment at 700°C for 900 minutes to prepare measurement samples. Test pieces measuring 15 mm wide x 50 mm long x 2 mm thick were taken from the measurement samples. The test pieces were taken from 1 mm inside the surface of the alloy plate, with the longitudinal direction of the test pieces aligned with the rolling direction of the alloy plate. The surface of the test pieces was dry finished to #600.

[0050] The test pieces were subjected to a corrosion test in accordance with ASTM A262-C (immersion in 65% boiling nitric acid for 48 hours, five times). The corrosion weight loss ΔW (g) was measured after each immersion, and the corrosion rate (g / m 2 / h) was calculated. The average of five measurements was used as the representative value. The meanings of each symbol are as follows: Formula (A): Corrosion rate (g / m 2 / h) = (7290 × ΔW) / (A × t) × K A: Surface area A (cm 2), t: immersion time (h), K: unit conversion factor (mm / month to g / m 2 / h): 1.39 Here, the average value of the five measurements was defined as the representative corrosion rate for each alloy plate.

[0051] (Indicator of improvement in intergranular corrosion resistance) The improvement effect due to the combined inclusion of Nb and B was evaluated by the velocity ratio defined by the following formula (B). Formula (B): Rate ratio (-) = corrosion rate of element 1 x (g / m 2 / h) / corrosion rate Y (g / m 2 / h)

[0052] [Table 1]

[0053] [Table 2-1]

[0054] [Table 2-2]

[0055] In examples with the same parent number, both parent numbers 1 and 2 were annealed at the same annealing temperature. The "corrosion rate X of parent number 1" refers to the corrosion rate of a target alloy sheet containing both or either Nb and B, and the "corrosion rate Y of parent number 2" refers to the corrosion rate of a comparative alloy sheet containing neither Nb nor B. A rate ratio of less than 1.00 indicates that the combined inclusion of Nb and B improved intergranular corrosion resistance, which can contribute to expanding the annealing heat treatment temperature range and reducing the load in the steelmaking process. On the other hand, a rate ratio of 1.00 or more indicates that the combined inclusion of Nb and B did not exhibit the effect of improving intergranular corrosion resistance.

[0056] The results are shown in Tables 2-1 and 2-2. The speed ratio index for parent alloys Nos. 2-4, 6-8, 10-15, 17-19, and 24-30, which contained less than the specified amount of carbon and were annealed at heat treatment temperatures above the specified level, was less than 1.00. In contrast, for parent alloys Nos. 1, 5, 9, 16, and 20, which were heat treated at 950°C, both the target and comparative alloys contained very little dissolved carbon after annealing, so the combined addition of Nb and B was ineffective. Furthermore, for parent alloys Nos. 21-23, which contained excessive carbon, a significant amount of Cr carbide precipitated, resulting in Cr deficiency. Therefore, the combined addition of Nb and B within the specified range did not improve intergranular corrosion resistance. [Example]

[0057] Example 2 illustrates that the combined inclusion of Nb and B is essential for achieving the effects of the present invention.

[0058] Table 3 shows the chemical compositions of the alloys melted in the laboratory. Alloys indicated with two capital letters contain both Nb and B, alloys indicated with two lowercase letters contain neither Nb nor B, and alloys indicated with one capital and one lowercase letter contain only Nb or B. The method of melting in the laboratory, the amount of unavoidable Nb and B contamination, and the distinction between the presence and absence of Nb and B are the same as in Example 1.

[0059] [Table 3]

[0060] [Table 4]

[0061] A hot-rolled sheet having a thickness of 8 mm was obtained by the same method as in Example 1. The hot-rolled sheet was then annealed at 1050°C for 20 minutes to obtain an alloy sheet. This alloy was subjected to the same sensitization heat treatment as in Example 1, and the corrosion rate was measured.

[0062] The results are shown in Table 4. For parent Nos. 1, 4-5, and 8-19, which contained Nb and B within the specified range, the velocity ratio index was less than 1.00. For parent No. 2, which contained Nb above the specified range, an intermetallic compound that was easily soluble in the corrosion test solution was formed, and its dissolution is presumed to have increased the corrosion rate. For parent No. 3, which contained B above the specified range, boride, primarily composed of Cr, was formed, resulting in Cr deficiency, and therefore the effect of the composite inclusion was not apparent. For parent No. 6, which contained Nb below the specified range, there was insufficient Nb compared to the amount of Cr carbide formed, and therefore the effect of the composite inclusion was not apparent. For parent No. 7, which contained B below the specified range, precipitation of Cr carbide was concentrated at the austenite grain boundaries, and therefore the effect of the composite inclusion was not apparent. [Industrial Applicability]

[0063] The high Ni alloy material of the present invention is excellent in manufacturability and resistance to intergranular corrosion after being held at the sensitization temperature for a long period of time, and can contribute to improving the intergranular corrosion resistance of components for chemical plants and heat exchangers, for example.

Claims

1. A high Ni alloy material characterized by containing, by mass%, C: 0.015% or less, Si: 0.60% or less, Mn: 2.00% or less, P: 0.040% or less, S: 0.040% or less, Ni: 37.00% to 48.00%, Cr: 18.50% to 24.50%, Mo: 1.50% to 4.50%, Cu: 0.50% to 4.00%, Al: 0.30% or less, Ti: 0.50% to 1.30%, N: 0.0200% or less, Ca: 0.0060% or less, Nb: 0.003% to 0.250%, B: 0.0003% to 0.0030% or less, with the balance being Fe and impurities.

2. Instead of a part of the Fe, in mass%, The high Ni alloy material according to claim 1, characterized in that it contains one or more of the following: V: 0.30% or less, Sn: 0.12% or less, W: 1.0% or less, Co: 0.50% or less, and Sb: 0.15% or less.

3. A high Ni alloy plate having the composition according to claim 1 or 2.

4. 3. The method for producing a high Ni alloy material according to claim 1, wherein the annealing temperature after hot rolling is 980° C. or higher.

5. 4. The method for producing a high Ni alloy sheet according to claim 3, wherein the annealing temperature after hot rolling is 980° C. or higher.

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