Ni-BASED ALLOY FOR NONMAGNETIC STRUCTURAL MEMBER

Abstract

To provide a Ni-based alloy for a nonmagnetic structural member, where the Ni-based alloy has high corrosion resistance and low magnetic permeability, and maintains these properties in a stable manner when subjected to mechanical processing as the structural member.SOLUTION: This Ni-based alloy has a specific component composition, the component composition satisfying formulas (1)-(4) in the following: [C%]≤0.23×([Nb%]+[V%])+0.008 (1); [C%]≤0.092×([Nb%]+[V%])+0.022 (2); [N%]≤0.85×[Ti%]+0.013 (3); and [N%]≤0.73×[Ti%]+0.041 (4).SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a Ni-based alloy for non-magnetic structural members, and more particularly to a Ni-based alloy for non-magnetic structural members suitable for use in corrosive environments. [Background technology]

[0002] To ensure the accurate operation of electrical and electronic devices, non-magnetic structural members are used for the equipment components used to install them. Corrosion resistance is also required in corrosive environments such as offshore and coastal areas, and chemical plants, and so non-magnetic structural members made of Ni-based alloys are used. Known examples of such corrosion-resistant alloys include Ni-based alloys such as Hastelloy® 22 and Hastelloy® 276, which contain large amounts of Cr and Mo.

[0003] For example, Patent Document 1 discloses a non-magnetic Ni-based alloy that can be hardened by precipitation of precipitates through aging treatment while improving corrosion resistance through the addition of Cr. The alloy typically has a composition containing, by weight, 0.1% or less of C, 2.0% or less of Si, 2.0% or less of Mn, 0.03% or less of P, 0.01% or less of S, 30-45% of Cr, and 1.5-5.0% of Al in Ni, and is purported to have a magnetic permeability of 1.05 or less, as well as high corrosion resistance and hardness, through cold or warm plastic working and then aging treatment. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2006-274443 Summary of the Invention [Problem to be solved by the invention]

[0005] In recent years, with the advancement of high performance in electrical and electronic devices, Ni-based alloys for structural components used in the corrosive environments mentioned above are required to be more non-magnetic without sacrificing corrosion resistance. At the same time, they are also required to be stable enough to maintain their corrosion resistance and magnetic properties even when subjected to various mechanical processes such as welding as structural components.

[0006] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a Ni-based alloy for non-magnetic structural members that has high corrosion resistance and low magnetic permeability, and that retains these properties stably even when machined into structural members. [Means for solving the problem]

[0007] The Ni-based alloy for non-magnetic structural members according to the present invention contains, in mass %, C: 0.001 to 0.015%, Si: 0.01 to 0.10%, Mn: 0.10 to 1.5%, P: 0.020% or less, S: 0.0001 to 0.0015%, Cr: 14.0 to 23.0%, Mo: 12.0 to 17.5%, Cu: 0.03 to 3.5%, and Al: 0.005 to 0.20%. 0%, Ti: 0.001 to 0.035%, Fe: 2.5 to 7.2%, Co: 0.05 to 1.20%, W: 1.80 to 3.80%, N: 0.001 to 0.022%, and one or two of V and Nb in the ranges of V: 0.01 to 0.12%, Nb: 0.01 to 0.12%, and (Nb+V)≦0.12%, Mg: 0.0266% or less, B: 0.0059% or less, Sn: 0.052% or less, Ca: 0.0029% or less, O: 0.0040% or less, the balance being Ni and unavoidable impurities, and the composition of the composition is expressed by the following formulas (1) to (4): [C%]≦0.23×([Nb%]+[V%])+0.008 (1) [C%]≦-0.092×([Nb%]+[V%])+0.022 (2) [N%]≦0.85×[Ti%]+0.013 (3) [N%]≦-0.73×[Ti%]+0.041 (4) The present invention is characterized in that:

[0008] According to these characteristics, as a Ni-based alloy for non-magnetic structural members, it has high corrosion resistance and low magnetic permeability, and these properties are stable even when machined into structural members.

[0009] The above invention may be characterized in that the composition contains 0.0001 to 0.0025% Ca. The composition may also contain 0.0005 to 0.0250% Mg. The composition may also contain 0.0050% or less B, 0.050% or less Sn, and 0.0035% or less O. These characteristics provide a Ni-based alloy for non-magnetic structural members with excellent workability. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is a table showing the range of alloy components used in preliminary tests. [Figure 2] This is a scatter diagram showing the results of polishability and corrosion resistance in a mirror polishing test, organized by [Nb%] + [V%] and [C%]. [Figure 3] This is a scatter diagram showing the results of polishability in a mirror polishing test, organized by [Ti%] and [N%]. [Figure 4] 1 is a graph showing the relationship between the number of polishing sets in a mirror polishing test and the contents of Cu, Co, and W. [Figure 5] 1 is a list of component compositions of examples used in manufacturing tests. [Figure 6] 1 is a list of component compositions of comparative examples used in manufacturing tests. [Figure 7] 1 is a table showing the results of manufacturing tests of Examples and Comparative Examples. DETAILED DESCRIPTION OF THE INVENTION

[0011] Cr and Mo are considered as additive elements for improving the corrosion resistance of Ni-based alloys. Mo, on the other hand, lowers the Curie point and reduces magnetic properties at room temperature, making it effective for obtaining low-permeability Ni-based alloys for non-magnetic structural components. We investigated the relationship between the amounts of these elements and permeability and found that Ni-based alloys containing 12 to 20 mass% Cr and 12 to 20 mass% Mo can achieve low permeability, generally less than 1.003. The metallographic structure of these alloys was a single-phase structure consisting of a face-centered cubic (FCC) lattice. The present invention was based on these alloys. We evaluated their high corrosion resistance and low permeability, and their ability to withstand various mechanical processes, including welding, as structural components. For example, we evaluated the wear resistance and mechanical strength of welds during welding.

[0012] [Preliminary Examination] Prior to the production tests using large steel ingots, preliminary tests were conducted using small specimens.

[0013] Figure 1 summarizes the range of the elemental composition of the alloy used in the preliminary test. The base alloy was a Ni-based alloy containing, by mass, 18% Cr, 14.5% Mo, and 4.5% Fe, and the elemental composition was varied within the range shown in the figure to create the samples for the preliminary test. In Figure 1, "0" means that no intentional addition was made, and unavoidable trace amounts are allowed.

[0014] <Sample preparation> First, alloys with compositions within the ranges shown in Figure 1 were melted in a high-frequency induction furnace to obtain 10 kg alloy ingots. The alloy ingots were hot forged, annealed, and cold rolled to produce cold-rolled sheets with a thickness of 3.2 mm. The cold-rolled sheets were solution-treated at 1150°C for 1 minute, then water-cooled and pickled.

[0015] Welded test pieces were used to evaluate the stability of the test pieces against welding. These test pieces were prepared by finishing the end faces of the cold-rolled sheets so that they were perpendicular to the main surface, then butt-welding them. The welding was performed by TIG welding without filler, with a welding current of 16 A and a welding speed of 70 mm / min, while Ar+3%H2 was blown as a shield gas at 2.15 L / min.

[0016] <Evaluation> The welded test pieces were evaluated for polishability in the weld zone by a dry polishing test, a wet polishing test, and a mirror polishing test, which will be described later, and the corrosion resistance of the weld zone and base metal was evaluated by a salt spray cycle test.

[0017] Dry grinding test Since welded pipes, one of the applications of the alloys of this example, require bead removal on the production line, a polishing test was conducted to evaluate polishability as a measure of processability on the production line. Here, the surface bead of a welded test specimen was removed by dry grinding using a surface grinder (Cosmo Machinery Co., Ltd.: SG-300S). Specifically, a #46 alumina abrasive grinding wheel was rotated at 2900 rpm, the cutting depth per pass was set to 0.005 mm, and the table was manually fed for a polishing time of approximately 10 seconds per pass. Polishing was considered complete when visually observed differences in the surface roughness between the bead and the surrounding base material were eliminated, and polishability was evaluated based on the number of passes required to complete the polishing. Evaluations were as follows: excellent for 3 or fewer passes, good for 4 to 6 passes, fair for 7 to 9 passes, and poor for 10 or more passes.

[0018] Wet abrasive test The welded test specimens after bead removal were subjected to a wet full-surface polishing test. The polishing equipment used was a wet automatic polishing machine (Sankyo Rikagaku Co., Ltd.: MB-1-NYK01). A tape-shaped abrasive paper was brought into contact with the welded test specimen with a constant pressure using a roll and moved at a constant speed to polish the specimen. After one polishing pass, the abrasive paper was automatically advanced, allowing unused portions to be used in the next pass. The abrasive paper had a grit size of #400, the polishing pressure was 0.2 MPa, and the welded test specimen was fed at a speed of 85 mm / sec. Water was replenished as needed. The polishing direction was perpendicular to the dry polishing test direction. The polishing was completed when the polishing marks from the dry polishing test disappeared, both on the weld and the base metal, by visual and microscopic observation. The polishing was evaluated based on the number of passes required to complete the polishing. The evaluation was as follows: excellent if the number of passes to complete polishing was 2 or less, good if it was 3 to 5 passes, fair if it was 6 to 9 passes, and poor if it was 10 passes or more.

[0019] Mirror polishing test Following the wet polishing test, a mirror polishing test was performed. The polishing equipment used was a BUEHLER Auto-Met250. A small amount of 9 μm diamond paste was applied to a hard buffing cloth, and an aqueous developing solution was added and rotary polishing was performed. Polishing was performed for 3 minutes at a polishing plate rotation speed of 180 rpm and a sample holder rotation speed of 40 rpm. Next, a 3 μm diamond paste was used and polishing was performed under the same conditions for 2 minutes. The buffing cloth was then replaced with a soft buffing cloth, and polishing was performed for 2 minutes using a polishing solution containing a metal oxide abrasive at a polishing plate rotation speed of 180 rpm and a sample holder rotation speed of 40 rpm. The polishing was then continued for 30 seconds while running tap water over the polishing plate to wash away the polishing solution. If polishing scratches remained, a final polishing was performed using a 3 μm diamond paste and a soft buffing cloth. This final polishing was counted as one set, and polishing was considered complete when a mirror-like surface free of polishing scratches was obtained. Polishability was evaluated based on the number of sets (passes) required to complete the process. Whether a mirror-like surface was obtained was determined by visually inspecting the entire surface and by observing the area around the weld under a microscope at 400x magnification to confirm that no polishing marks or pinholes remained. The evaluation was based on whether two or fewer final polishing sets were required to complete the polishing process, excellent; good for three to five sets; fair for six to eight sets; and poor for nine or more sets.

[0020] Salt spray cycle test Considering the use of the alloy of this example in a corrosive environment, a salt spray cycle test was conducted under relatively severe conditions. The above-mentioned pickled cold-rolled sheets were butt-welded to obtain welded test specimens. After wet polishing with #240 abrasive paper, the specimens were cut out so that the weld was located at the center of the width direction. The cut-out dimensions were 3.2 mm thick x 40 mm wide x 80 mm long. The specimens were then bent 90° at a length of 25 mm to obtain test pieces. In the salt spray test, the test pieces were bent downward at a 20° inclination, sprayed with 5% salt water in a 70°C atmosphere for 1 hour, and then dried for 23 hours. This cycle was repeated 50 times. The corrosion resistance of the test pieces was then evaluated based on the presence or absence of corrosion and the maximum depth of pitting corrosion that occurred. The evaluation was excellent if no corrosion occurred, good if the maximum pitting depth was 10 μm or less, fair if it was 10 to 25 μm, and poor if it was more than 25 μm or there were many corrosion marks.

[0021] <Test Results> First, we will discuss the results of the salt spray test. While individual evaluations are omitted, the corrosion resistance of the base metal was good in all cases. Generally, corrosion is considered to decrease according to the pitting corrosion resistance index (PRE = [Cr%] + 3.3 [Mo%] + 16 [N%]). In this case, it is believed that the addition of Cr and Mo resulted in high corrosion resistance. Meanwhile, the corrosion resistance of the welded zones tended to be significantly lower in specimens with high C content than in the base metal, but was relatively improved by the addition of Nb and V. Microstructural observation revealed the presence of carbide precipitates, primarily composed of Mo and Cr, in the welded zones, which were believed to be the cause of the reduced corrosion resistance. In other words, suppressing these precipitates may be effective in maintaining high corrosion resistance even during welding, a mechanical process for structural components.

[0022] Next, we will explain the results of the dry polishing test, wet polishing test, and mirror polishing test. Regarding polishing properties, small scratches remained in samples with a high C content, or a high Nb or V content, and there was a tendency for the number of passes and sets required to complete polishing to be increased. When these scratches were investigated, Nb and V carbides were confirmed, and it was found that polishing properties decreased when these elements were contained above a certain level.

[0023] Figure 2 shows the results of the polishability and corrosion resistance measured by the mirror polishing test, organized by mass percent (Nb% + V%) and C content (C%). In the figure, "◇" indicates poor corrosion resistance, "△" indicates poor polishability in the mirror polishing test, and "●" indicates a rating of pass or better in both tests. Boundaries (dotted lines) were drawn between poor and pass corrosion resistance, and between poor and pass polishability. The boundary lines were [C%] = 0.23 × ([Nb%] + [V%]) + 0.008 and [C%] = -0.092 × ([Nb%] + [V%]) + 0.022, respectively. In other words, the range below these two boundary lines, where the C content is lower, is preferred because it consistently exhibits good corrosion resistance and polishability. That is, when the following two formulas (1) and (2) are satisfied, the material has stable and good corrosion resistance and polishability. [C%]≦0.23×([Nb%]+[V%])+0.008 … Formula (1) [C%]≦-0.092×([Nb%]+[V%])+0.022 … Formula (2)

[0024] Furthermore, no significant differences in polishing properties due to these elemental components were found in dry and wet polishing tests. It was also confirmed that Si has an effect on bead shape, and that adding an appropriate amount can improve the bead shape.

[0025] Figure 3 also shows the results of the polishability test, organized by mass percent (wt%) of Ti content [Ti%] and N content [N%], based on the Ti and N content. Because Ti nitrides were also observed in addition to the Nb carbides described above, the results were also organized by Ti and N content. In the figure, "□" indicates pinhole formation and poor polishability, "◯" indicates Ti compound formation and poor polishability, and "●" indicates a rating of pass or better in both tests. Boundaries (dotted lines) were drawn between poor and pass polishability due to pinholes and Ti compounds, respectively. The boundary lines were [N%] = 0.85 × [Ti%] + 0.013 and [N%] = -0.73 × [Ti%] + 0.041, respectively. In other words, in the figure, the range below these two boundary lines, where the N content is lower, is preferable because good polishability can be maintained. That is, when the following two formulas (3) and (4) are satisfied, the material has stable and good corrosion resistance and polishability. [N%]≦0.85×[Ti%]+0.013 … Formula (3) [N%]≦-0.73×[Ti%]+0.041 … Formula (4)

[0026] In other words, by controlling the C and N contents to appropriate levels, it is possible to suppress the precipitation of Mo and Cr carbides, stably obtaining high corrosion resistance even during welding, and to suppress the formation of Nb and V carbides and Ti nitrides, thereby suppressing a decrease in polishability. On the other hand, by including appropriate amounts of Nb, V, and Ti, the remaining C and N are fixed, ensuring the corrosion resistance and polishability of the weld, and furthermore, the structure of the weld is refined, contributing to ensuring strength.

[0027] Furthermore, as shown in Figure 4, it was found that the addition of Cu, Co, and W can improve polishability. Although the effects in the dry and wet polishing tests were not significant, a significant effect was confirmed in the polishability evaluation in the mirror polishing test. The addition of Cu, Co, and W improved the welded joints, which tend to have small scratches and unevenness, and polishing was completed with fewer sets. These elements do not deteriorate corrosion resistance or magnetic permeability, and W also improves corrosion resistance. The addition of Mo was also considered to improve corrosion resistance, but it was found that the addition of Mo tends to deteriorate polishability. Instead, W was added, which improved polishability while consistently achieving high corrosion resistance.

[0028] [Manufacturing test] Based on the above preliminary test, a production test was further carried out to produce a rolled plate.

[0029] <Sample preparation> First, alloys with the chemical compositions shown in Figure 5 (Examples 1 to 21) and Figure 6 (Comparative Examples 1 to 17) were melted in a 60-ton electric furnace. Ni alloy scrap, iron scrap, stainless steel scrap, ferrochrome, and other raw materials were used. The melted alloys were decarburized and refined in the AOD process by blowing oxygen and argon. Quicklime and fluorite were added to generate a CaO-SiO2-Al2O3-MgO-F slag, which was then desulfurized and deoxidized by adding Al and Si. These alloys were then cast into slabs by continuous casting. X-ray fluorescence analysis was used to analyze the chemical compositions. N was analyzed by an inert gas impulse heating and melting method, and C and S were analyzed by oxygen flow combustion and infrared absorption spectroscopy. The "-" in these figures indicates that no intentional additions were made.

[0030] The slab was then hot-rolled to form a hot-rolled coil with a thickness of 6.5 mm. The hot-rolled coil was then solution-treated, cold-rolled, annealed, and pickled to form a cold-rolled coil with a thickness of 3.2 mm. The annealing was performed at 1150°C for 1 minute, followed by water cooling. A cold-rolled sheet was cut out from the cold-rolled coil and butt-welded in the same manner as in the preliminary test described above to form a welded test piece.

[0031] <Evaluation> The welded test pieces were evaluated for polishability in the weld zone through dry polishing tests and mirror polishing tests, and for corrosion resistance in the weld zone through a salt spray cycle test. Furthermore, magnetic permeability and hardness measurements were also conducted to evaluate whether the welded test pieces maintained high corrosion resistance and low magnetic permeability even after mechanical processing by welding and polishing. Abrasiveness testing and evaluation As in the preliminary test, the welded test piece was subjected to dry polishing, wet polishing, and mirror polishing in that order, and the polishability was evaluated after dry polishing and after mirror polishing using the same criteria as in the preliminary test. Note that in the wet polishing, one additional pass was added after the dry polishing marks had disappeared visually, completing the polishing and proceeding to the next process; the polishability of the wet polishing was not evaluated.

[0032] Salt spray cycle test The salt spray cycle test was carried out in the same manner as the preliminary test, and the corrosion resistance was evaluated. The evaluation criteria were also the same as those of the preliminary test.

[0033] Magnetic permeability measurement The magnetic permeability was measured in both the base material and the welded portion. The magnetic permeability was measured by cutting a 10 mm wide x 10 mm long test piece from the above-mentioned wet-polished welded test piece. The welded portion test piece was cut so as to include the bead. The measurement was performed using a vibrating sample magnetometer (Riken Denshi Co., Ltd.: BVH-55) at room temperature, with the external magnetic field set to 2500 Oe. The magnetic permeability μ was calculated by dividing the slope of the magnetization curve by the magnetic permeability in vacuum. Here, B is the magnetic flux density, and H is the magnetic field. For both the base material and the welded portion, a magnetic permeability of less than 1.003 was evaluated as good, and a magnetic permeability of 1.003 or greater was evaluated as poor.

[0034] Hardness measurement Since high mechanical strength is desirable for use as a structural member, the hardness of the welded portion, which is more likely to have low strength, was measured and evaluated for mechanical strength. The evaluation was based on the hardness of the base material, and a welded portion with a hardness of +15HV or more was rated as excellent, +14 to -5HV as good, -6 to -15HV as fair, and -16HV or less as poor.

[0035] About the ingredients The values on the left side of the following formulas (1') to (4') were evaluated by removing the constant terms on the right side of the formulas (1) to (4) for the above-mentioned component compositions and shifting them to the left side. In other words, if the value on the left side is equal to or less than the constant term on the right side, it is evaluated that each condition is satisfied. [C%]-0.23×([Nb%]+[V%])≦0.008 … Formula (1') [C%]+0.092×([Nb%]+[V%])≦0.022 … Formula (2') [N%]-0.85×[Ti%]≦0.013 … Formula (3') [N%]+0.73×[Ti%]≦0.041 … Formula (4')

[0036] <Test Results> The evaluation results of each test and the values of the left side of each of the above formulas (1') to (4') are shown in Figure 7. The evaluations are indicated by "Excellent" (◎), "Good" (◯), "Fair" (△), and "Poor" (×).

[0037] For Examples 1 to 21, the value of the left side of formula (1') was 0.008 or less, the value of the left side of formula (2') was 0.022 or less, the value of the left side of formula (3') was 0.013 or less, and the value of the left side of formula (4') was 0.041 or less, and the component compositions satisfied the four formulas. Furthermore, the polishability, corrosion resistance, magnetic permeability, and hardness in the dry polishing test and mirror polishing test were all evaluated as acceptable or better.

[0038] On the other hand, in Comparative Example 1, the C content was relatively high and the Nb + V content was relatively low, so the value of the left side of formula (1') exceeded 0.008. As a result, the corrosion resistance and hardness were evaluated as poor. It is believed that the low hardness was due to the coarsening of the structure of the weld.

[0039] In Comparative Example 2, the C content was relatively high, and the value of the left side of formula (2') exceeded 0.022. As a result, the number of passes required to complete mirror polishing was large, and the polishability of the mirror polishing was evaluated as poor. This is thought to be due to the formation of large amounts of NbC and VC in the welded joint.

[0040] In Comparative Example 3, the Ti content was relatively low and the N content was relatively high, so the value of the left side of formula (3') exceeded 0.013. As a result, pinholes were formed in the welded joint, and the polishability in the mirror polishing test was evaluated as poor.

[0041] In Comparative Example 4, the contents of Ti and N were relatively high, and the value of the left side of formula (4') exceeded 0.041. As a result, many scratches were generated during mirror polishing, and the polishability of mirror polishing was evaluated as poor.

[0042] In Comparative Example 5, the Cu content was low and the Co and W contents were also relatively low. As a result, the number of passes in mirror polishing was increased, and the polishability of mirror polishing was evaluated as poor.

[0043] In Comparative Example 6, the content of Co was low and the contents of Cu and W were also relatively low. As a result, the number of passes in mirror polishing was increased, and the polishability of mirror polishing was evaluated as poor.

[0044] In Comparative Example 7, the content of W was low and the contents of Cu and Co were also relatively low, resulting in a large number of passes in mirror polishing and an evaluation of poor polishability in mirror polishing.

[0045] In Comparative Example 8, the C content was high and the Nb content was relatively low. As a result, the corrosion resistance was evaluated as poor. It is believed that the large amount of Cr carbide formed made the specimen sensitive to corrosion.

[0046] In Comparative Example 9, the content of Si was high. As a result, cracks occurred in the welded portion, and these cracks could not be removed by dry polishing, so the polishability of dry polishing was evaluated as poor. In addition, since the cracks could not be removed, wet polishing and mirror polishing were not performed, and the salt spray cycle test was not performed.

[0047] In Comparative Example 10, the Cr content was high. As a result, the corrosion resistance was evaluated as "fail." It is believed that an intermetallic compound was formed in the welded portion.

[0048] In Comparative Example 11, the content of Mo was high. As a result, the corrosion resistance was evaluated as "fail." It is believed that an intermetallic compound was formed in the welded portion.

[0049] The Cu content was high in Comparative Example 12. As a result, cracks occurred in the welded portion, which increased the number of passes in dry polishing, and the polishability in dry polishing was evaluated as poor.

[0050] The Al content was low in Comparative Example 13. As a result, the degree of oxidation in the welded portion was large, and the polishability in dry polishing was evaluated as fair, but the polishability in mirror polishing was evaluated as poor.

[0051] The Ti content was high in Comparative Example 14. As a result, cracks occurred in the welded portion, and the polishability in dry polishing was evaluated as poor.

[0052] In Comparative Example 15, the content of W was high. As a result, the corrosion resistance was evaluated as poor. It is believed that an intermetallic compound was formed in the welded portion.

[0053] In Comparative Example 16, the N content was high, and the value of the left side of formula (3') exceeded 0.013. As a result, the polishability in dry polishing was evaluated as "Fair." This is thought to be due to the formation of a large amount of TiN. In addition, the large amount of N caused many pinholes in the weld, and the polishability in mirror polishing was evaluated as "Fail."

[0054] In Comparative Example 17, the content of Fe was high. As a result, the magnetic permeability exceeded 1.003 and the sample was evaluated as unacceptable. It is believed that Fe reduced the phase stability.

[0055] In Comparative Example 18, the Mg content was high. As a result, the polishability of mirror polishing was evaluated as poor. This is thought to be due to the formation of a large amount of inclusions.

[0056] Comparative Example 19 had a high content of B. As a result, solidification cracking during welding was significant, and the abradability in dry polishing was evaluated as poor.

[0057] In Comparative Example 20, the B content was high and the Nb content was also relatively high. As a result, solidification cracking during welding was significant, and the polishability of dry polishing was evaluated as poor. In addition, since the cracks could not be removed, wet polishing and mirror polishing were not performed, and a salt spray cycle test was not performed.

[0058] Comparative Example 21 had a high Sn content, which resulted in significant solidification cracking during welding and resulted in an unacceptable rating for dry polishing abradability.

[0059] In Comparative Example 22, the Ca content was high. As a result, the bead shape was poor, and although the polishability in dry polishing was evaluated as acceptable, the polishability in mirror polishing was evaluated as unacceptable. This is thought to be due to the formation of a large amount of inclusions.

[0060] In Comparative Example 23, the content of O was high. As a result, the polishability in mirror polishing was evaluated as poor. This is thought to be due to the formation of a large amount of inclusions.

[0061] As described above, according to Examples 1 to 21, it was possible to obtain a Ni-based alloy for non-magnetic structural members that stably exhibits high corrosion resistance and low magnetic permeability even when processed by welding and polishing, as well as processability including polishability and hardness in dry polishing and mirror polishing.

[0062] The composition range of an alloy that can provide properties substantially equivalent to those of the Ni-based alloys including those of the above-mentioned examples is determined as follows.

[0063] C is an element that is effective in stabilizing the FCC phase and magnetic permeability. Furthermore, it is an important element that combines with Nb and V to control the fine grain size of the weld and contribute to maintaining strength. However, excessive C content forms carbides and carbonitrides of Cr and Mo, which can cause deterioration of corrosion resistance. Taking these factors into consideration, the C content is set within the range of 0.001 to 0.015%, preferably 0.002 to 0.012%, and more preferably 0.003 to 0.008%, by mass.

[0064] Silicon acts as a deoxidizer, ensures penetration during welding, and is an effective element for smoothing irregularities in the bead during welding. However, excessive silicon content promotes the precipitation of intermetallic compounds, increasing the susceptibility of welds to cracking. Furthermore, silicon promotes the formation of an oxide film, which causes temper color during welding, that is, promotes the formation of the oxide film, adversely affecting polishability. Taking these factors into consideration, the silicon content is set within the range of 0.01 to 0.10%, preferably 0.02 to 0.09%, and more preferably 0.03 to 0.08%, by mass.

[0065] Mn acts as a deoxidizer and stabilizes the FCC phase, maintaining a stable low magnetic permeability. However, excessive Mn content forms MnS, which acts as a starting point for pitting corrosion and reduces corrosion resistance. Taking these factors into consideration, the Mn content is set within the range of 0.10 to 1.5%, preferably 0.15 to 1.25%, and more preferably 0.20 to 1.00%, by mass.

[0066] P deteriorates hot workability, solidification cracking resistance of welds, and corrosion resistance, so it is desirable to reduce it as much as possible. Therefore, the P content is set to 0.020% or less, preferably 0.018% or less, and more preferably 0.015% or less, by mass%.

[0067] S is an element that deteriorates hot workability and promotes cracking in welds, so it is desirable to reduce its content. However, it has the effect of improving the penetration of welds and reducing bead irregularities during welding. Taking these factors into consideration, the S content is set within the range of 0.0001 to 0.0015%, preferably 0.0002 to 0.0013%, and more preferably 0.0003 to 0.0010%, by mass.

[0068] Cr is essential for ensuring corrosion resistance in severe corrosive environments, such as pitting corrosion resistance and crevice corrosion resistance. However, excessive Cr content promotes the precipitation of intermetallic compounds and Cr carbides, deteriorating corrosion resistance and the polishability of welds. Taking these factors into consideration, the Cr content is set within the range of 14.0 to 23.0%, preferably 14.5 to 22.5%, and more preferably 15.0 to 22.0%, by mass%.

[0069] Like Cr, Mo is an element that improves corrosion resistance, particularly pitting corrosion resistance and crevice corrosion resistance, and also contributes to stabilizing magnetic permeability. However, excessive Mo content causes precipitation of carbides and intermetallic compounds, deteriorating corrosion resistance and worsening the polishability of welds. Taking these factors into consideration, the Mo content is set within the range of 12.0 to 17.5% by mass, preferably 12.5 to 17.0%, and more preferably 13.0 to 16.5%.

[0070] Cu is an element that stabilizes the FCC phase and maintains a stable low magnetic permeability. Furthermore, when present together with Co and W, it improves the polishability of welds, improving workability, especially when achieving a mirror finish. However, excessive Cu content not only promotes cracking during welding, but also increases costs and reduces strength. Taking these factors into consideration, the Cu content is set within the range of 0.03 to 3.5% by mass, preferably within the range of 0.04 to 3.0%, and more preferably within the range of 0.05 to 2.5%.

[0071] Al functions as a deoxidizer and, in the presence of CaO-SiO2-Al2O3-MgO slag, promotes desulfurization through deoxidation, making it an important element for controlling the amount of sulfur during refining. However, excessive Al content can facilitate the formation of oxide scale during welding, or can form AlN and Al2O3, which can deteriorate polishability. Taking these factors into consideration, the Al content is set within the range of 0.005 to 0.200%, preferably 0.010 to 0.180%, and more preferably 0.020 to 0.150%, by mass.

[0072] Ti not only acts as a deoxidizer, but also combines with nitrogen to refine the structure of the weld, thereby contributing to ensuring strength. However, excessive Ti content promotes cracking during welding, deteriorates polishability due to the precipitation of TiN, and also deteriorates polishability by inducing the generation of temper color during welding. For this reason, Ti is an important element whose content must be strictly controlled. Taking these factors into consideration, the Ti content is set within the range of 0.001 to 0.035%, preferably 0.002 to 0.030%, and more preferably 0.003 to 0.025%, by mass.

[0073] Co stabilizes the FCC phase, lowering and stabilizing the magnetic permeability, and also contributes to ensuring the hardness of the weld. Furthermore, when coexisting with W and Cu, it improves the polishability of the weld, and is an essential element for improving the workability, particularly when mirror finishing is required. However, excessive Co content increases the manufacturing cost. Taking these factors into consideration, the Co content is set within the range of 0.05 to 1.20%, preferably 0.10 to 1.00%, and more preferably 0.15 to 0.90%, by mass.

[0074] Like Mo and Cr, W improves corrosion resistance, particularly pitting corrosion resistance and crevice corrosion resistance, and also contributes to stabilizing magnetic permeability. Furthermore, when coexisting with Co and Cu, W improves the polishability of welds, making it an important element for improving workability, particularly when achieving a mirror finish. However, excessive W content precipitates carbides and intermetallic compounds, degrading corrosion resistance and worsening the polishability of welds. Taking these factors into consideration, the W content is set within the range of 1.80 to 3.80%, preferably 2.20 to 3.60%, and more preferably 2.50 to 3.40%, by mass.

[0075] Fe is an element that improves hot workability. However, excessive content tends to promote weld cracking and destabilize the FCC phase, increasing magnetic permeability. Taking these factors into consideration, the Fe content is set within the range of 2.5 to 7.2%, preferably 3.0 to 7.0%, and more preferably 3.5 to 6.9%, by mass.

[0076] N stabilizes the FCC phase and, like Cr and Mo, significantly improves pitting and crevice corrosion resistance. Furthermore, by forming nitrides with Ti, it contributes to maintaining the fine grain size of the weld, similar to C, contributing to maintaining strength. However, excessive N content leads to the precipitation of large amounts of nitrides and facilitates the formation of pinholes, significantly deteriorating polishability. Taking these factors into consideration, the N content is within the range of 0.001 to 0.022%, preferably 0.002 to 0.015%, and more preferably 0.004 to 0.014%, by mass.

[0077] V and Nb are important elements that contribute to improving the strength of the base metal and, by suppressing the formation of Cr and Mo carbides and carbonitrides in welds, improve corrosion resistance and ensure strength. They also refine the structure of welds, improving polishability. However, excessive inclusion of either element can lead to cracking during welding and promote the precipitation of intermetallic compounds, resulting in reduced corrosion resistance. Taking these factors into consideration, the V content is within the range of 0.01 to 0.12%, preferably 0.02 to 0.11%, and more preferably 0.03 to 0.10%, by mass. Furthermore, the Nb content is within the range of 0.01 to 0.12%, preferably 0.02 to 0.11%, and more preferably 0.03 to 0.10%, by mass. The effects of Nb and V can be obtained whether they are contained alone or in combination, so it is sufficient to selectively include at least one of them. However, the total content of Nb and V is set to 0.12% or less by mass.

[0078] [C%]-0.23×([Nb%]+[V%])≦0.008 … Formula (1') Equation (1') is an experimentally determined relationship between the amount of C and the amounts of Nb and V. By controlling the content of each element in the composition so as to satisfy this equation, carbide precipitation can be suppressed and corrosion resistance of the weld can be ensured. In other words, the value of the left side of equation (1') is 0.008 or less. Furthermore, the value of the left side of equation (1') is preferably 0.007 or less, more preferably 0.005 or less.

[0079] [C%]+0.092×([Nb%]+[V%])≦0.022 … Formula (2') Equation (2') is the experimentally determined relationship between the amount of C and the amounts of Nb and V. By controlling the respective contents in the component composition so as to satisfy this equation, it is possible to suppress the deterioration of abrasiveness due to Nb, V, or their composite carbides, and ensure the abrasiveness of the weld. In other words, the value of the left side of equation (2') is 0.022 or less. Furthermore, the value of the left side of equation (2') is preferably 0.020 or less, and more preferably 0.018 or less.

[0080] [N%]-0.85×[Ti%]≦0.013 … Formula (3') Equation (3') is a relationship between the amount of N and Ti determined by experiment. By controlling the content of each element in the component composition so as to satisfy this equation, it is possible to suppress the occurrence of pinholes and ensure the polishability of the weld. In other words, the value of the left side of equation (3') is 0.013 or less. Furthermore, the value of the left side of equation (3') is preferably 0.012 or less, more preferably 0.010 or less.

[0081] [N%]+0.73×[Ti%]≦0.041 … Formula (4') Equation (4') is the relationship between the amount of N and Ti determined by experiment. By controlling the content of each element in the composition so as to satisfy this equation, the deterioration of abrasiveness due to TiN can be suppressed and the abrasiveness of the weld can be ensured. In other words, the value of the left side of equation (4') is 0.041 or less. Furthermore, the value of the left side of equation (4') is preferably 0.039 or less, more preferably 0.037 or less.

[0082] Mg is an element that improves hot workability when added in an appropriate amount, and may be added as needed. However, excessive content increases inclusions, worsening polishability and significantly deteriorating hot workability. In consideration of these, Mg can be added as needed within the range of 0.0266% or less, preferably within the range of 0.0005 to 0.0250%, more preferably within the range of 0.0010 to 0.0150%, and even more preferably within the range of 0.0015 to 0.0130% by mass.

[0083] B is an element that improves hot workability and may be added as needed. However, excessive B content promotes the occurrence of solidification cracking and cracking during welding. This is particularly noticeable when the Nb content is high. Taking these factors into consideration, B can be added as needed within the range of 0.0059% or less, preferably 0.0050% or less, more preferably 0.0030% or less, and even more preferably 0.0025% or less, by mass %. It is even more preferable to add B within the range of 0.0001% to 0.025%.

[0084] Sn has the effect of reducing the corrosion rate and may be added as needed. However, excessive Sn content promotes weld cracking. In addition, Sn forms a compound with Cu, reducing the effects of Cu in stabilizing magnetic permeability and improving the polishability of welds. Taking these factors into consideration, Sn can be added at will within the range of 0.052% or less, preferably 0.050% or less, more preferably 0.030% or less, and even more preferably 0.010% or less, by mass. It is even more preferable to add Sn within the range of 0.001% to 0.010%.

[0085] Ca is an element that improves hot workability by combining with S, which is harmful to hot workability, to form CaS, and may be added as needed. However, excessive Ca content degrades the surface quality of the weld bead and forms oxides that reduce polishability. Taking these factors into consideration, Ca can be added as needed within the range of 0.0029% or less, preferably within the range of 0.0001 to 0.0025%, more preferably within the range of 0.0002 to 0.0020%, and even more preferably within the range of 0.0003 to 0.015%, by mass%.

[0086] O is an unavoidable impurity element. It forms oxides with Si, Al, Mg, etc. to become inclusions, which adversely affect polishability. Therefore, to ensure stable performance, it is desirable to control the content as low as possible. It is recommended that the O content be reduced to 0.0040% or less, preferably 0.0035% or less, more preferably 0.0025% or less, and even more preferably 0.0018% or less, by mass.

[0087] While typical embodiments of the present invention and modifications based thereon have been described above, the present invention is not necessarily limited to these, and a person 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 appended claims.

Claims

1. In mass%, C: 0.001-0.015%, Si: 0.01 to 0.10%, Mn: 0.10 to 1.5%, P: 0.020% or less, S: 0.0001-0.0015%, Cr: 14.0-23.0%, Mo: 12.0-17.5%, Cu: 0.03 to 3.5%, Al: 0.005-0.200%, Ti: 0.001 to 0.035%, Fe: 2.5-7.2%, Co: 0.05-1.20%, W: 1.80-3.80%, N: 0.001 to 0.022%, One or both of V and Nb V: 0.01 to 0.12%, Nb: in the range of 0.01 to 0.12%, and (Nb+V)≦0.12%, Mg: 0.0266% or less, B: 0.0059% or less, Sn: 0.052% or less, Ca: 0.0029% or less, O: 0.0040% or less, The composition of the alloy is represented by the following formulas (1) to (4): [C%]≦0.23×([Nb%]+[V%])+0.008 (1) [C%]≦-0.092×([Nb%]+[V%])+0.022 (2) [N%]≦0.85×[Ti%]+0.013 (3) [N%]≦-0.73×[Ti%]+0.041 (4) A Ni-based alloy for non-magnetic structural members, characterized by satisfying the following:

2. In the above-mentioned component composition, Ca: 0.0001-0.0025%, 2. The Ni-based alloy for non-magnetic structural members according to claim 1, wherein:

3. In the above-mentioned component composition, Mg: 0.0005-0.0250%, 3. The Ni-based alloy for non-magnetic structural members according to claim 2, wherein:

4. In the above-mentioned component composition, Mg: 0.0005-0.0250%, 2. The Ni-based alloy for non-magnetic structural members according to claim 1, wherein:

5. In the above-mentioned component composition, B: 0.0050% or less, Sn: 0.050% or less, O: 0.0035% or less 5. The Ni-based alloy for non-magnetic structural members according to claim 1, wherein:

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