Ni-Cr-Mo-Nb alloys

Abstract

To provide Ni-Cr-Mo-Nb alloys with excellent corrosion resistance. [Solution] In mass%, Cr: 18.0~24.0%, Mo: 7.50~9.00%, Nb: 2.50~4.00%, C: 0.002~0.020%, Si: 0.02~1.00%, Mn: 0.02~1.00%, Cu: 0.01~0.20%, Al: 0.005~0.400%, Ti: 0.10~1.00%, Fe: 3.00~6% A Ni-Cr-Mo-Nb alloy having a component composition of 0.00%, Co: 0.01~0.50%, V: 0.01~0.50%, W: 0.01~0.50%, Ag: 0.0001~0.0020%, N: 0.002~0.020%, P: 0.030% or less, S: 0.005% or less, Sn: less than 0.003%, with the remainder being Ni and unavoidable impurities.

Description

Technical Field

[0001] The present invention relates to a Ni-Cr-Mo-Nb alloy having excellent corrosion resistance.

Background Art

[0002] Ni-Cr-Mo alloys containing Ni (nickel) as the main component and Cr (chromium) and Mo (molybdenum) as the main alloying elements exhibit excellent corrosion resistance in various corrosive environments such as chloride-containing acidic environments, reducing acids, oxidizing acids, and even mixed acid environments. Therefore, they are widely used in chemical process equipment, natural gas field and oil field facilities, heat exchangers exposed to seawater environments, flue gas desulfurization equipment, and the like.

[0003] For example, in Patent Document 1, a Ni-Cr-Mo alloy is disclosed which contains, in mass%, 18.0 to 24.0% of Cr and 8.0 to 10.0% of Mo in Ni, and a predetermined amount of C, Si, Mn, P, S, Al, Ti, Fe, Nb, W, V is added, and which is said to have excellent corrosion resistance and excellent workability. Here, by controlling NbC carbide and (Ti,Nb)N nitride to a predetermined level, it is said that the machinability at room temperature, particularly the 0.2% proof stress which is the stress at which plastic deformation starts, can be controlled within a range suitable for processing, such as 270 to 400 MPa.

[0004] Even in such Ni-based alloys having excellent corrosion resistance, precipitates such as carbides, intermetallic compounds, and Laves phases may be formed at grain boundaries. If the formation of these precipitates causes a decrease in the concentration of Cr and Mo in the vicinity of grain boundaries, it can cause intergranular corrosion and intergranular stress corrosion cracking. Furthermore, since it is not only used as plates, pipes, and bars, but is also often used as welded structural members, weldability should also be considered from the viewpoint of ensuring the reliability of welded joints.

[0005] Patent Document 2 discloses a Ni-Cr-Mo alloy that can maintain workability and corrosion resistance even after welding, in which the alloy contains, by mass%, 18.0-24.0% Cr and 7.5-9.0% Mo in Ni, along with predetermined amounts of C, Si, Mn, P, S, Cu, Al, Ti, Fe, Co, V, Nb, N, Sn, and W. It states that controlling the amounts of Cu, Co, C, and N is effective in improving the ductility of the material itself, and that adding appropriate amounts of Nb, Ti, and V is effective in refining the solidification structure of the weld. It also states that crack occurrence is reduced in the weld by suppressing carbides, carbonitrides, and eutectic precipitates that lower the melting point. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2021-161528 [Patent Document 2] Patent No. 7009666 [Overview of the project] [Problems that the invention aims to solve]

[0007] Ni-Cr-Mo alloys containing a predetermined amount of Nb are known to have excellent workability and weldability. In such alloys, a decrease in the concentration of Cr and Mo near grain boundaries can cause intergranular corrosion and intergranular stress corrosion cracking. However, it has been observed that in the above-mentioned component composition system, the solidification segregation of Cr and Mo during the solidification of the molten metal has a strong influence. On the other hand, since high temperatures and long periods of time are required to resolve precipitates and eliminate solidification segregation through heat treatment, a new method was needed from the perspective of operational burden and environmental burden.

[0008] The present invention has been made in view of the above circumstances, and its objective is to provide a Ni-Cr-Mo-Nb alloy with excellent corrosion resistance. [Means for solving the problem]

[0009] The Ni-Cr-Mo-Nb alloy according to the present invention has the following composition in mass%, Cr: 18.0-24.0%, Mo: 7.50-9.00%, Nb: 2.50-4.00%, C: 0.002-0.020%, Si: 0.02-1.00%, Mn: 0.02-1.00%, Cu: 0.01-0.20%, Al: 0.005-0.400%, Ti: 0.10-1.00% It is characterized by having a component composition consisting of Fe: 3.00-6.00%, Co: 0.01-0.50%, V: 0.01-0.50%, W: 0.01-0.50%, Ag: 0.0001-0.0020%, N: 0.002-0.020%, P: 0.030% or less, S: 0.005% or less, Sn: less than 0.003%, with the remainder being Ni and unavoidable impurities.

[0010] These characteristics make it possible to create an alloy with improved local corrosion resistance and excellent corrosion resistance.

[0011] In the above-described characteristics, the component composition may also be characterized such that, if [M] is the mass % of element M, then 0.02 ≤ 10 × [Ag] + [W] ≤ 0.10. According to this characteristic, local corrosion resistance can be reliably improved, resulting in an alloy with excellent corrosion resistance.

[0012] In the above-described invention, the component composition may further be characterized by containing Te: 0.00001 to 0.002%. With this characteristic, local corrosion resistance can be reliably improved, and an alloy with excellent corrosion resistance can be obtained.

[0013] In the above-described invention, the critical crevice corrosion initiation temperature may be 45°C or higher when the test solution of ASTM G48 Method D is an aqueous solution containing 6% by mass of ferric chloride and 1.5% by mass of hydrochloric acid, and the test period is 90 hours, and the critical pitting corrosion temperature initiation temperature is 85°C or higher when the test solution of JIS G 0578:2000 is an aqueous solution containing 6% by mass of ferric chloride and 1.5% by mass of hydrochloric acid, and the test period is 90 hours. With such features, local corrosion resistance can be reliably improved, and an alloy with excellent corrosion resistance can be obtained. [Brief explanation of the drawing]

[0014] [Figure 1] This diagram illustrates the mechanism of localized corrosion in conventional Ni-Cr-Mo-Nb alloys. [Figure 2] This figure illustrates the mechanism for improving local corrosion resistance in a Ni-Cr-Mo-Nb alloy according to an embodiment of the present invention. [Figure 3] This is a list of the chemical components of the alloys used in the manufacturing tests. [Figure 4] This is a table showing the results of the manufacturing tests. [Modes for carrying out the invention]

[0015] The embodiments of the Ni-Cr-Mo-Nb alloy according to the present invention will be described in detail below.

[0016] First, based on Figure 1, we will explain the mechanism by which the local corrosion resistance, such as intergranular corrosion and intergranular stress corrosion cracking, is reduced in Ni-Cr-Mo-Nb alloys due to a decrease in the concentration of Cr and Mo near the grain boundaries. This mechanism is related to the normal solidification of molten alloy during the process of obtaining ingots such as slabs and ingots by continuous casting or ingot casting before hot working such as hot rolling.

[0017] As shown in Figure (a), during the solidification process of molten alloy, Cr and Mo, which improve corrosion resistance, tend to migrate from solid phase 1 to liquid phase 2 (see small arrows in the figure). As a result, the concentration in solid phase 1 decreases and the concentration in liquid phase 2 increases, while solid phase 1 grows (see white arrows in the figure). In other words, the concentration of Cr and Mo increases in line with the growth of solid phase 1. Then, as shown in Figure (b), the concentration of Cr and Mo increases in the grain boundary region 2', which remained as liquid phase 2 until the end, compared to the grain interior region 1' which solidified as solid phase 1 first. In other words, Cr and Mo undergo solidification segregation, and their concentration near the grain boundary increases excessively. Furthermore, as shown in Figure (c), numerous precipitates 3, such as carbides, nitrides, and intermetallic compounds containing Cr and Mo, are formed within the grain boundary region 2'. As a result of the formation of many precipitates 3, the concentration of Cr and Mo decreases around the precipitates 3 within the grain boundary region 2'. This Cr and Mo deficient phase with a concentration gap of 4 can cause intergranular corrosion and intergranular stress corrosion cracking. The graphs shown at the bottom of each of the figures (a) to (c) are images of the Cr and Mo concentration profiles along line segment AB.

[0018] Therefore, the inventors of this application diligently investigated whether it was possible to suppress the decrease in concentration of Cr and Mo near the grain boundaries. As a result, they came up with a Ni-Cr-Mo-Nb alloy with the following component composition.

[0019] In this embodiment, the Ni-Cr-Mo-Nb alloy is a Ni-based alloy having a component composition in mass % of Cr: 18.0 to 24.0%, Mo: 7.50 to 9.00%, Nb: 2.50 to 4.00%, C: 0.002 to 0.020%, Si: 0.02 to 1.00%, Mn: 0.02 to 1.00%, Cu: 0.01 to 0.20%, Al: 0.005 to 0.400%, Ti: 0.10 to 1.00%, Fe: 3.00 to 6.00%, Co: 0.01 to 0.50%, V: 0.01 to 0.50%, W: 0.01 to 0.50%, Ag: 0.0001 to 0.0020%, N: 0.002 to 0.020%, P: 0.030% or less, S: 0.005% or less, and Sn: less than 0.003. Such a component composition containing a trace amount of Ag suppresses a decrease in local corrosion resistance, and a Ni-Cr-Mo-Nb alloy excellent in corrosion resistance, workability, and weldability can be obtained.

[0020] That is, as shown in Fig. 2(a), in the case of the Ni-Cr-Mo-Nb alloy having the above-described component composition, particularly by containing Ag, it is considered that during solidification in the process of obtaining an ingot before hot working as described above, Ag preferentially moves from the solid phase 1 to the liquid phase 2 side, and the movement of Cr and Mo to the liquid phase 2 side is suppressed. As a result, as shown in Fig. 2(b), the difference in the concentrations of Cr and Mo in the grain boundary 2' with respect to the concentrations of Cr and Mo in the crystal grain 1' immediately after solidification is smaller than that in the conventional Ni-Cr-Mo-Nb alloy (see Fig. 1). That is, it is considered that the grain boundary segregation of Cr and Mo is suppressed. Further, as shown in Fig. 2(c), it is considered that while cooling, the formation of precipitates 3 such as carbides, nitrides, and intermetallic compounds containing Cr or Mo is suppressed in the region of the grain boundary 2'. As a result, the precipitates 3 generated in the grain boundary 2' are few, and the degree of concentration decrease of Cr and Mo around the precipitates 3 and the range in which the concentration decrease occurs are also slight. That is, it is considered that the concentration decrease of Cr and Mo in the vicinity of this grain boundary 2' can be suppressed, and intergranular corrosion, intergranular stress corrosion cracking, etc. can be suppressed.

[0021] (Reasons for limiting each element) The content of each element in the above-described Ni-Cr-Mo-Nb alloy is determined within a range in consideration of comprehensive factors such as corrosion resistance, mechanical properties, workability, weldability, and economy. The reasons are shown below.

[0022] C: 0.002 to 0.020 mass% C contributes to the precipitation strengthening of the alloy obtained by forming carbides with Nb, Ti, etc., and has the role of improving mechanical strength. However, if it is contained in excess, a large amount of carbides will precipitate at the grain boundaries, causing Cr deficiency around them and inducing intergranular corrosion and intergranular stress corrosion cracking. Considering these factors, C is within the range of 0.002 to 0.020% by mass, preferably within the range of 0.003 to 0.015% by mass, and more preferably within the range of 0.004 to 0.010% by mass.

[0023] Si: 0.02 to 1.00 mass% Si acts as a deoxidizer and contributes to obtaining a clean alloy ingot by removing oxygen during melting. Also, Si contributes to the improvement of oxidation resistance up to a certain amount, but if it is contained in excess, it will cause the formation of intermetallic compounds and embrittlement of the alloy, reducing workability and toughness. Considering these factors, Si is within the range of 0.02% to 1.00% by mass, preferably within the range of 0.05 to 0.40% by mass, and more preferably within the range of 0.10 to 0.20% by mass.

[0024] Mn: 0.02 to 1.00% Mn is an element effective for deoxidation and desulfurization. It combines with S in the molten metal to be fixed as MnS and has the effect of improving hot workability. However, if it is contained in excess, there is a risk of coarsening and segregation of precipitates. Considering these factors, Mn is within the range of 0.02 to 1.00% by mass, preferably within the range of 0.05 to 0.40% by mass, and more preferably within the range of 0.10 to 0.30% by mass.

[0025] P: 0.030% or less While phosphorus (P) can contribute to improving the mechanical strength of alloys through solid solution strengthening, it is a harmful element that easily leads to intergranular embrittlement and increases susceptibility to intergranular corrosion and stress corrosion cracking in corrosive environments. Therefore, it is desirable to control its content as much as possible. Accordingly, the amount of P is set to 0.030% or less by mass, preferably 0.015% or less, and more preferably 0.013% or less.

[0026] S: 0.005% or less While sulfur (S) can combine with manganese (Mn) to form MnS, which can contribute to improved machinability of alloy ingots, it tends to segregate at grain boundaries, becoming a starting point for localized corrosion and cracking. Therefore, in alloys requiring corrosion resistance, S should be reduced as much as possible. Accordingly, the amount of S is set to 0.005% or less by mass, preferably 0.0030% or less, and more preferably 0.0010% or less.

[0027] Cr: 18.0~24.0% Cr is one of the most important elements that imparts corrosion resistance to alloys through the formation of a passive film, and increasing its content improves oxidation resistance and localized corrosion resistance. However, excessive content may lead to the formation of brittle phases such as the σ phase and a decrease in workability. Taking these factors into consideration, the amount of Cr is set to be in the range of 18.0 to 24.0% by mass, preferably in the range of 20.0 to 23.0%, and more preferably in the range of 22.0 to 22.5%.

[0028] Mo: 7.50~9.00% Mo is an essential element in Ni-based alloys requiring corrosion resistance, as it significantly enhances local corrosion resistance, particularly in reducing environments and acidic environments containing chlorides, thereby improving resistance to pit corrosion and crevice corrosion. However, excessive amounts of Mo can easily precipitate intermetallic compounds such as the Laves phase, leading to decreased hot workability, reduced toughness of welds, and increased crack susceptibility. Considering these factors, the amount of Mo is set to within the range of 7.50 to 9.00% by mass, preferably within the range of 8.00 to 8.50%, and more preferably within the range of 8.10 to 8.30%.

[0029] Cu: 0.01~0.20% Cu has the effect of reducing the corrosion rate in certain acidic environments, such as reducing acids, and can supplementally enhance corrosion resistance when added in small amounts to Ni-based alloys. On the other hand, if present in excess, it can lead to increased segregation and decreased workability. Taking these factors into consideration, the amount of Cu is set to be in the range of 0.01 to 0.20% by mass, preferably in the range of 0.02 to 0.15%, and more preferably in the range of 0.03 to 0.10%.

[0030] Al: 0.005~0.400% A1 acts as a deoxidizing agent and also contributes to improving oxidation resistance on the alloy surface. Furthermore, in Ni-based alloys, it is involved in the formation of precipitation-strengthening phases such as the γ' phase, but in this embodiment, it is included mainly for the purpose of deoxidation and improvement of surface properties. On the other hand, if included in excess, it can lead to the formation of brittle phases and a decrease in workability. Taking these factors into consideration, the amount of Al is set to be in the range of 0.005 to 0.400% by mass, preferably in the range of 0.100 to 0.350%, and more preferably in the range of 0.150 to 0.250%.

[0031] Ti: 0.10~1.00% Ti combines with C and N to form carbonitrides, contributing to improved mechanical strength through precipitation strengthening, as well as the immobilization of harmful elements at grain boundaries. On the other hand, excessive amounts of Ti form coarse carbonitrides, reducing ductility and toughness, and causing welding cracks. Considering these factors, the amount of Ti is set to be in the range of 0.10 to 1.00% by mass, preferably in the range of 0.15 to 0.38%, and more preferably in the range of 0.20 to 0.30%.

[0032] Fe: 3.00~6.00% Fe is inevitably included because it originates from raw materials such as scrap, but since it can replace some of the Ni, a certain amount of Fe content contributes to cost reduction. On the other hand, excessive content may reduce corrosion resistance, especially in highly acidic environments. Taking these factors into consideration, the amount of Fe is set to be in the range of 3.00 to 6.00% by mass, preferably in the range of 3.50 to 4.95%, and more preferably in the range of 4.0 to 4.90%.

[0033] Nb: 2.50~4.00% Nb combines with C and N to form Nb carbonitrides, improving the high-temperature strength and creep strength of the alloy through precipitation strengthening, and also contributing to corrosion resistance under specific environmental conditions. On the other hand, excessive Nb content can lead to the formation of Nb-rich intermetallic compounds such as the Laves phase, resulting in a decrease in toughness and increased crack susceptibility of the welded joint. Considering these factors, the Nb content is set to be in the range of 2.50 to 4.00% by mass, preferably in the range of 3.00 to 3.60%, and more preferably in the range of 3.20 to 3.40%.

[0034] Co: 0.01~0.50% Co is an austenite-stabilizing element that improves mechanical strength and high-temperature properties through solid solution strengthening. On the other hand, excessive content leads to increased costs. Considering these factors, the amount of Co is set to be in the range of 0.01 to 0.50% by mass, preferably in the range of 0.02 to 0.30%, and more preferably in the range of 0.03 to 0.20%.

[0035] V: 0.01~0.50% V acts as a carbonitride-forming element, contributing to precipitation strengthening and grain boundary stabilization. On the other hand, excessive content can lead to embrittlement due to the formation of coarse carbonitrides. Considering these factors, the amount of V is set to be in the range of 0.01 to 0.50% by mass, preferably in the range of 0.015 to 0.30%, and more preferably in the range of 0.020 to 0.10%.

[0036] N: 0.002~0.020% N not only contributes to increasing the strength of alloys as a solid solution strengthening element, but also contributes to improving corrosion resistance through interaction with Cr. Furthermore, it contributes to precipitation strengthening through carbonitride formation with Nb and Ti. On the other hand, if present in excess, it can cause embrittlement due to nitride precipitation and welding cracks. Taking these factors into consideration, the amount of N is set to be in the range of 0.002 to 0.020% by mass, preferably in the range of 0.003 to 0.018%, and more preferably in the range of 0.004 to 0.016%.

[0037] Sn: Less than 0.003% Sn is a harmful element that, even in trace amounts in Ni-based alloys, has a strong tendency to segregate at grain boundaries, causing intergranular corrosion and embrittlement. Therefore, it should be reduced as much as possible. Accordingly, the amount of Sn is set to less than 0.003% by mass, preferably 0.002% or less, and more preferably 0.001% or less.

[0038] W: 0.01~0.50% W has a similar effect to Mo, contributing to local corrosion resistance and improved high-temperature strength. On the other hand, excessive content can lead to embrittlement due to the formation of the Laves phase. Taking these factors into consideration, as well as the balance with the Mo content, the amount of W is set to be in the range of 0.01 to 0.50% by mass, preferably in the range of 0.02 to 0.30%, and more preferably in the range of 0.03 to 0.10%.

[0039] Ag: 0.0001~0.0020% Even in trace amounts, Ag has the effect of improving local corrosion resistance. On the other hand, if included in excess, it may have adverse effects such as increased costs and excessive solidification segregation. Taking these factors into consideration, the amount of Ag is set to be in the range of 0.0001 to 0.0020% by mass, preferably in the range of 0.0002 to 0.0018%, and more preferably in the range of 0.0003 to 0.0015%.

[0040] (Relationship between Ag and W) In the alloy composition described above, it is preferable that the following relationship is satisfied when [M] is the mass % of element M. 0.02 ≤ 10 × [Ag] + [W] ≤ 0.10 This relationship is a composite inequality relating to the local corrosion resistance described above, due to Ag and W. If the value of the middle side of this composite inequality (10 × [Ag] + [W]) is less than 0.02, the overall effect of Ag and W is insufficient, the effect of suppressing grain boundary segregation of Mo and Cr is not fully realized, and local corrosion resistance becomes insufficient. On the other hand, if the value of the middle side exceeds 0.10, excessive Ag and W content may lead to excessive segregation of Ag and the formation of brittle phases such as Laves phase, potentially resulting in a decrease in toughness and increased crack susceptibility of welds and heat-affected zones. This relationship defines the overall "effective amount" of Ag and W, thereby improving local corrosion resistance. The value of the middle side of this relationship is preferably in the range of 0.025 to 0.08, and more preferably in the range of 0.03 to 0.06.

[0041] (Addition of Te) In addition to the component composition described above, it is also preferable to include Te. Te, when present in trace amounts in the Ni-based alloy, contributes to modifying the solidification structure and controlling grain boundary behavior, thereby improving corrosion resistance and workability in a balanced manner. On the other hand, excessive content may cause segregation and embrittlement. Taking these factors into consideration, Te may be included in a mass% range of 0.00001 to 0.002%, preferably in the range of 0.00005 to 0.0015%, and more preferably in the range of 0.0001 to 0.001%.

[0042] (Manufacturing Test) Next, we will describe the manufacturing tests conducted to specifically confirm the effects of the above-mentioned component composition.

[0043] First, cold-rolled sheets were produced from Ni-Cr-Mo-Nb alloys with the component compositions shown in Examples 1-17 and Comparative Examples 1-5 in Figure 3. In detail, raw materials such as scrap, Ni, Cr, and Mo were melted in an electric furnace and decarburized by oxygen blowing with AOD and VOD. Then, Al and limestone were added to reduce Cr, and further limestone and fluorite were added to form a CaO-SiO2-A12O3-MgO-F slag on the molten alloy to perform deoxidation and desulfurization. The molten alloy refined in this way was cast in a continuous casting machine to obtain slabs. Subsequently, the slabs were hot-rolled in a Steckel mill and then cold-rolled to produce cold-rolled sheets with a thickness of 3 mm. The chemical composition shown in the figure is the result of quantitative analysis of the obtained cold-rolled sheets using an X-ray fluorescence analyzer.

[0044] Next, we will describe the evaluation methods for crevice corrosion resistance, pitting corrosion resistance, and hot workability of the obtained cold-rolled sheets, as well as their overall evaluation.

[0045] Crevice corrosion resistance evaluation: For crevice corrosion resistance, test specimens were prepared from cold-rolled sheets, and the critical crevice corrosion temperature (CCT) was measured and evaluated according to ASTM G48 Method D. The CCT was measured by the following test. A multi-clevis fixture conforming to ASTM G48 Method D was used for the test, and the test specimen, which had been wet-polished beforehand with SiC#200 abrasive paper, was mounted with a tightening torque of 0.28 N·m. Next, the test specimen was immersed in an aqueous solution containing 6 mass% ferric chloride and 1.5 mass% hydrochloric acid, and the test temperature was changed from 40°C to 65°C in 5°C increments, and the specimen was held for a predetermined holding time at each test temperature. Although the holding time is 72 hours according to the ASTM standard, it was set to 90 hours in order to more precisely evaluate the difference in corrosion resistance. After the test, the test specimen was washed and dried, and the surface including the crevice was observed, and the maximum crevice corrosion depth was measured. Crevice corrosion was determined to have occurred when the maximum crevice corrosion depth was 25 μm or more, and the lowest test temperature at which crevice corrosion occurred was defined as the CCT (Critical Temperature). A higher CCT indicated superior resistance to crevice corrosion. A CCT of 60°C or higher was marked as "◎" for excellent performance, "○" for good performance between 60°C and 55°C, "△" for acceptable performance between 55°C and 45°C, and "×" for poor performance below 45°C.

[0046] Pitting corrosion resistance evaluation: For pitting corrosion resistance, test specimens were prepared by cutting cold-rolled sheets to specified dimensions and pre-polishing them wet with SiC#200 abrasive paper. The critical pitting temperature (CPT) was measured and evaluated in accordance with JIS G 0578:2000 Critical Pitting Temperature Test Method - Test Method (B). The CPT was measured using the following test. The test solution was an aqueous solution containing 6% by mass of ferric chloride and 1.5% by mass of hydrochloric acid. The test temperature was varied from 80°C to 103°C, and the test specimens were immersed at each test temperature. The holding time was 90 hours, instead of the 72 hours specified in the ASTM standard, in order to more precisely evaluate the difference in corrosion resistance. After the test, the test specimens were washed and dried, and the surface of the test specimens was observed to measure the maximum pitting depth. Pitting corrosion that occurred on the end faces of the test specimens was excluded from the measurement. Pitting corrosion was determined to have occurred when the maximum pitting depth was 25 μm or more. The lowest test temperature at which pitting corrosion occurred was defined as the critical pitting temperature (CPT) of the test specimen, and pitting resistance was evaluated using this CPT. A higher CPT was considered to indicate superior pitting resistance. Specifically, a CPT of 100°C or higher was marked "◎" for excellent performance, "〇" for good performance between 95°C and below 100°C, "△" for acceptable performance between 85°C and below 95°C, and "×" for poor performance below 85°C.

[0047] Hot workability evaluation: During the hot rolling process, which is an intermediate step in the manufacturing of cold-rolled sheets, the surface of the rolled sheet was observed along its entire length, and 10m 2 If there were 0 visible surface defects caused by hot working, the result was marked as "Excellent" (◎); if there was 1 defect, it was marked as "Good" (〇); if there were 2 defects, it was marked as "Acceptable" (△); and if there were 3 or more defects, it was marked as "Poor" (×). Surface defects included cracks, breaks, laps, deep slag inclusions, and other defects judged to be caused by insufficient hot plastic deformation capacity or structural non-uniformity.

[0048] comprehensive evaluation: First, if any of the crevice corrosion resistance, pitting corrosion resistance, or hot workability evaluations were rated as poor (×), the overall evaluation was recorded as poor (×). For all other evaluations, points were assigned to each of the crevice corrosion resistance, pitting corrosion resistance, and hot workability evaluations: 3 points for excellent (◎), 2 points for good (〇), and 1 point for acceptable (△). The total score was then ranked to determine the overall evaluation. If the total score was 8-9 points, the overall evaluation was recorded as excellent (◎); if it was 6-7 points, it was recorded as good (○); and if it was 3-5 points, it was recorded as acceptable (△).

[0049] As shown in Figure 3, Examples 1 to 17 all satisfied the above-mentioned range of component composition, exhibited excellent crevice corrosion resistance and pitting corrosion resistance, and had few surface defects due to hot workability, resulting in good overall evaluations of "acceptable" (△) or higher. In particular, Examples 15 to 17 satisfied the above-mentioned composite inequality and contained a predetermined amount of Te, exhibiting excellent crevice corrosion resistance, pitting corrosion resistance, and hot workability, and received good overall evaluations.

[0050] On the other hand, in Comparative Examples 1 to 5, the content of some elements all deviated from the range described above, and the evaluation results further supported the reasons for limiting each element in this embodiment, as detailed below.

[0051] In Comparative Example 1, the Mo content was high at 9.03%, which is thought to have led to the formation of intermetallic compounds (such as the Laves phase) due to the excess Mo, resulting in an increase in surface defects that appear to be caused by a decrease in hot workability.

[0052] In Comparative Example 2, the Mo content was low at 7.42%, which is thought to have resulted in insufficient local corrosion resistance, and both crevice corrosion resistance and pitting corrosion resistance were rated as poor.

[0053] In Comparative Example 3, the high Ag content of 0.0093% is thought to have created corrosion initiation points due to solidification segregation and localized galvanic cell formation, resulting in a low evaluation of corrosion resistance. Furthermore, the segregation is thought to have made the structure non-uniform, and the hot workability was also not well evaluated.

[0054] In Comparative Example 4, the Ag content was low at 0.00007%, and it is thought that the effect of Ag in suppressing the segregation of Cr and Mo was hardly observed, resulting in a low evaluation of corrosion resistance.

[0055] In Comparative Example 5, the high concentration of Sn (0.004%) is thought to have increased susceptibility to intergranular corrosion due to excessive segregation at the grain boundaries, leading to a decrease in corrosion resistance. Furthermore, a decrease in hot workability, likely due to intergranular embrittlement, was also observed, resulting in a "passable" rating. This result demonstrates that keeping Sn concentrations extremely low is highly effective in ensuring grain boundary stability and corrosion resistance.

[0056] These results indicate that the examples in which the component composition was controlled to within the specified range of each element's content were superior to the comparative examples in overall evaluation of crevice corrosion resistance, pitting corrosion resistance, and hot workability.

[0057] As described above, the Ni-Cr-Mo-Nb alloy in this embodiment exhibits extremely excellent resistance to localized corrosion (pit corrosion, crevice corrosion) in acidic environments containing chlorides or mixed acid environments. In particular, it is believed that controlling the Ag content within the above-mentioned range results in excellent resistance to localized corrosion. Although the detailed reasons are not entirely clear, as mentioned above, it is presumed that Ag preferentially segregates at the grain boundaries, suppressing excessive segregation of Mo and Cr at the grain boundaries, thereby suppressing the precipitation of carbides, nitrides, intermetallic compounds, etc., and inhibiting the formation of Cr and Mo deficient layers near the grain boundaries.

[0058] Furthermore, its excellent resistance to localized corrosion was demonstrated by a critical crevice corrosion initiation temperature of 45°C or higher when the ASTM G48 Method D test solution was an aqueous solution containing 6% by mass of ferric chloride and 1.5% by mass of hydrochloric acid, and the test period was 90 hours, and a critical pitting corrosion temperature of 85°C or higher when the JIS G 0578:2000 pitting corrosion critical temperature test method was an aqueous solution containing 6% by mass of ferric chloride and 1.5% by mass of hydrochloric acid, and the test period was 90 hours.

[0059] Furthermore, controlling the content of Ag and W to satisfy the above-mentioned composite inequality is also preferable, as it is believed that this can suppress the occurrence and progression of localized corrosion and maintain stable corrosion resistance over a long period of time. Although the detailed mechanism is not entirely clear, it is presumed that W, like Ag, has the effect of retaining Cr and Mo within the crystal grains, and contributes to suppressing the segregation of Cr and Mo at the grain boundaries.

[0060] Furthermore, as mentioned above, it is preferable to include a small amount of Te in the component composition, as this contributes to modifying the solidification structure and controlling grain boundary behavior, thereby reliably improving local corrosion resistance and resulting in an alloy with excellent corrosion resistance.

[0061] As described above, the Ni-Cr-Mo-Nb alloy according to this embodiment has excellent corrosion resistance, workability, and weldability, making it particularly suitable for use as structural members exposed to harsh environments containing chlorides and mixed acids, such as reaction vessels, towers and tanks, piping, heat exchangers, pumps, valves, and fittings for chemical plants, as well as seawater desalination plants, flue gas desulfurization equipment, oil and gas well-related equipment, geothermal well casings, and nuclear-related equipment. It is also useful as sheet metal, pipe metal, rod metal, forgings, castings, and welding materials (welding wire, welding rod, weld metal).

[0062] Although representative embodiments of the present invention and modifications thereof have been described above, the present invention is not necessarily limited thereto, and those skilled in the art will be able to find various alternative embodiments and modifications without departing from the spirit of the present invention or the scope of the attached claims.

Claims

1. In mass percent, Cr: 18.0-24.0%, Mo: 7.50-9.00%, Nb: 2.50-4.00%, C: 0.002-0.020%, Si: 0.02-1.00%, Mn: 0.02-1.00%, Cu: 0.01-0.20%, Al: 0.005-0.400%, Ti: 0.10-1.00%, Fe: 3.00-6.00%, Co: 0.01 to 0.50%, V: 0.01-0.50%, W: 0.01-0.50%, Ag: 0.0001-0.0020%, N: 0.002-0.020%, P: 0.030% or less, S: 0.005% or less, Sn: Less than 0.003% A Ni-Cr-Mo-Nb alloy characterized by having a component composition consisting of the remainder being Ni and unavoidable impurities.

2. In the above component composition, if [M] is the mass percentage of element M, 0.02≦10×[Ag]+[W]≦0.10 The Ni-Cr-Mo-Nb alloy according to claim 1, characterized in that it is provided as follows.

3. The Ni-Cr-Mo-Nb alloy according to claim 2, characterized in that the above component composition further contains Te: 0.00001 to 0.002%.

4. The Ni-Cr-Mo-Nb alloy according to one of claims 1 to 3, characterized in that, when the ASTM G48 Method D test solution is an aqueous solution containing 6% by mass of ferric chloride and 1.5% by mass of hydrochloric acid, and the test time is 90 hours, and the test temperature T1 is varied, the lowest test temperature T1 at which the maximum crevice corrosion depth measured by observing the surface including the crevice is 25 μm or more is 45°C or higher, and when the JIS G 0578:2000 pitting corrosion critical temperature test method is performed with an aqueous solution containing 6% by mass of ferric chloride and 1.5% by mass of hydrochloric acid, and the test time is 90 hours, and the test temperature T2 is varied, the lowest test temperature T2 at which the maximum pitting corrosion depth measured by observing the surface is 25 μm or more is 85°C or higher.

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