Ni-BASED ALLOY TUBE

Abstract

Provided is an Ni-based alloy tube that has excellent corrosion resistance against high-temperature molten nitrates over a long period of time and excellent high-temperature strength over a long period of time. The chemical composition of the Ni-based alloy tube is, in terms of mass%, 0.005-0.100% C, 0.010-0.500% Si, 0.010-0.500% Mn, 20.00-23.50% Cr, 8.00-10.50% Mo, 0.010-0.400% Ti, 2.50-4.60% in total of Nb and Ta, 0.010-1.50% in total of Cu and Co, 0.010-0.400% Al, 0.0150% or less P, 0.0150% or less S, 0.0010-0.0400% N, and 0.0100% or less O, with the remainder being Ni and impurities. At a thick central portion of the Ni-based alloy tube, the number density of inclusions having an Mo content of 5.0% or greater and a maximum length of 10 μm or less is 10-100 / mm2, and the average crystal grain size thereof is 4.0-8.0.

Description

Ni-based alloy tube

[0001] The present invention relates to a Ni-based alloy pipe.

[0002] A tower-type concentrating solar power generation system has a tower located in the center and hundreds to thousands of reflecting mirrors arranged around the tower. A heat transfer tube is installed at the top of the tower and contains a heat transfer medium.

[0003] In tower-type concentrating solar power generation systems, sunlight is concentrated on the outer surface of a heat transfer tube by a reflector, heating the tube. This heat heats the heat transfer medium placed in the tube. The heat transfer medium is then sent to the bottom of the tower, where water is evaporated to generate electricity. Nitrate is sometimes used as the heat transfer medium, and inside the heat transfer tube during use, the nitrate becomes a molten salt at a high temperature of as much as 600°C.

[0004] Therefore, heat transfer tubes used in tower-type concentrating solar power generation systems are required to have excellent corrosion resistance against molten nitrates.

[0005] Patent Document 1 discloses a method for producing a Ni-based alloy and a Ni-based alloy in which soaking is performed before hot rolling to suppress the segregation of Mo and improve corrosion resistance. In particular, it has been confirmed that the alloy has excellent corrosion resistance through a 120-hour immersion test in a 50% sulfuric acid / ferric sulfate solution.

[0006] International Publication No. 2019 / 107456

[0007] Heat transfer tubes are required to have excellent corrosion resistance against high-temperature molten nitrates and to maintain the excellent corrosion resistance for a long period of time. In the prior art, there is still room for improvement in terms of ensuring excellent corrosion resistance against high-temperature molten nitrates and maintaining the excellent corrosion resistance for a long period of time.

[0008] Furthermore, Ni-based alloy tubes used for heat transfer tubes are required to maintain high-temperature strength for a long period of time in the harsh environment of exposure to heat from sunlight. Furthermore, from the viewpoint of efficiently transferring heat from sunlight to a heat transfer medium, it is conceivable to reduce the ratio of wall thickness to outer diameter (hereinafter also referred to as "t / D") of the transfer tube and make it thinner. However, in order to withstand the above-mentioned harsh environment even with a thin wall, a heat transfer tube with a thin wall is required to have excellent high-temperature strength in addition to the above-mentioned excellent corrosion resistance.

[0009] SUMMARY OF THE INVENTION An object of the present invention is to solve the above problems and to provide a Ni-based alloy pipe having excellent corrosion resistance against molten nitrates at high temperatures for a long period of time and excellent high-temperature strength for a long period of time.

[0010] The present invention has been made to solve the above problems, and the gist of the present invention is the following Ni-based alloy pipe.

[0011] (1) The chemical composition of the Ni-based alloy pipe is, in mass%, C: 0.005 to 0.100%, Si: 0.010 to 0.500%, Mn: 0.010 to 0.500%, Cr: 20.00 to 23.50%, Mo: 8.00 to 10.50%, Ti: 0.010 to 0.400%, Nb and Ta total: 2.50 to 4.60%, Cu and Co total: 0.010 to 1.50%, Al: 0.010 to 0.400%, P: 0.0150% or less, S: 0.0150% or less, N: 0.0010 to 0.0400%, O: 0.0100% or less, and the balance: Ni and impurities; In the center of the wall thickness in a cross section perpendicular to the longitudinal direction of the Ni-based alloy pipe, the Mo content is 5.0 mass % or more, and the number density of inclusions having a maximum length of 10 μm or less is 10 to 100 / mm ２ and an average grain size of 4.0 to 8.0.

[0012] (2) The chemical composition of the Ni-based alloy pipe is, in mass%, C: 0.005 to 0.080%, Si: 0.010 to 0.500%, Mn: 0.010 to 0.500%, Cr: 20.00 to 23.50%, Mo: 8.00 to 10.50%, Ti: 0.010 to 0.400%, Nb and Ta total: 2.50 to 4.60%, Cu and Co total: 0.010 to 1.50%, Al: 0.010 to 0.400%, P: 0.0150% or less, S: 0.0030% or less, N: 0.0010 to 0.0400%, O: 0.0100% or less, and the balance: Ni and impurities; In the center of the wall thickness in a cross section perpendicular to the longitudinal direction of the Ni-based alloy pipe, the Mo content is 5.0 mass % or more, and the number density of inclusions having a maximum length of 10 μm or less is 10 to 100 / mm ２ and an average grain size of 4.0 to 8.0.

[0013] (3) The Ni-based alloy pipe according to (1) or (2) above, wherein the chemical composition contains, in mass %, one or more elements selected from Fe: 5.50% or less, W: less than 0.150%, V: less than 0.150%, Sn: 0.010% or less, Ca: 0.0030% or less, Mg: 0.0030% or less, B: 0.0100% or less, and REM: 0.0100% or less, in place of a portion of the Ni.

[0014] According to the present invention, a Ni-based alloy tube can be obtained which has excellent corrosion resistance to molten nitrates at high temperatures for a long period of time and excellent high-temperature strength for a long period of time.

[0015] Fig. 1 is a diagram illustrating the shape of a circular-arc test piece used in measuring high-temperature strength. Fig. 2 is a diagram illustrating the shape of a circular-arc test piece used in measuring corrosion weight loss.

[0016] In order to solve the above-mentioned problems, the present inventors have conducted detailed investigations into corrosion resistance to high-temperature molten salts and high-temperature strength, and have come to the following findings.

[0017] In order to maintain corrosion resistance against molten nitrates for a long period of time, it is effective to add Cr and form a dense protective film on the surface of the alloy. To achieve this, it is effective to increase the grain boundaries, i.e., to make the crystal grains finer, so that Cr can be easily supplied to the surface of the alloy through the grain boundaries. However, if the crystal grains are made finer, it becomes impossible to maintain high-temperature strength for a long period of time. Therefore, it is necessary to adjust the crystal grain size to fall within a predetermined range.

[0018] Furthermore, in order to improve high-temperature strength, it is effective to finely precipitate inclusions. However, in the present invention, it is required to maintain a predetermined strength for a long period of time in a harsh high-temperature environment where sunlight is concentrated. Therefore, it is necessary for the inclusions to be able to exist stably at high temperatures for a long period of time. As a result of detailed studies by the present inventors, it was found that inclusions containing 5.0 mass% or more of Mo and having a maximum length of 10 μm or less (hereinafter referred to as "specific Mo inclusions") are stable in such an environment, and that by setting the number density of the specific Mo inclusions within a predetermined range, the high-temperature strength of the alloy can be maintained for a long period of time.

[0019] In this way, by strictly controlling the grain size and the number density of the specific Mo inclusions, it is possible to obtain excellent long-term high-temperature strength and excellent long-term corrosion resistance to high-temperature molten nitrates.

[0020] The present invention was made based on the above findings. Each of the features of the present invention will be described in detail below.

[0021] (A) Chemical Composition The reasons for limiting the content of each element are as follows: In the following description, "%" for the content means "% by mass."

[0022] C: 0.005 to 0.100% C is an element necessary for forming carbides and improving high-temperature strength. However, excessive C content combines with Cr to form Cr carbides. As a result, Cr-depleted layers are formed near grain boundaries, reducing corrosion resistance. Therefore, the C content is set to 0.005 to 0.100%. The C content is preferably 0.008% or more, more preferably 0.010% or more, and even more preferably 0.012% or more. The C content is preferably 0.080% or less, more preferably 0.060% or less, and even more preferably 0.050% or less.

[0023] Si: 0.010 to 0.500% Si has a deoxidizing effect. However, excessive Si content embrittles the alloy. Therefore, the Si content is set to 0.010 to 0.500%. The Si content is preferably 0.020% or more, more preferably 0.030% or more, and even more preferably 0.050% or more. Furthermore, the Si content is preferably 0.480% or less, more preferably 0.450% or less, and even more preferably 0.430% or less.

[0024] Mn: 0.010 to 0.500% Mn has the effect of improving strength through solid solution strengthening. However, excessive Mn content reduces hot workability. Therefore, the Mn content is set to 0.010 to 0.500%. The Mn content is preferably 0.030% or more, more preferably 0.050% or more, and even more preferably 0.080% or more. Furthermore, the Mn content is preferably 0.480% or less, more preferably 0.450% or less, and even more preferably 0.400% or less.

[0025] Cr: 20.00 to 23.50% Cr has the effect of forming a passive film on the surface of the alloy and improving corrosion resistance. Cr is also an element that is effective in maintaining high-temperature strength by forming Cr-containing carbonitrides. However, excessive Cr content reduces structural stability at high temperatures. As a result, high-temperature strength deteriorates. Therefore, the Cr content is set to 20.00 to 23.50%. The Cr content is preferably 20.50% or more, more preferably 21.00% or more, and even more preferably 21.20% or more. The Cr content is preferably 23.30% or less, more preferably 23.00% or less, and even more preferably 22.80% or less.

[0026] Mo: 8.00 to 10.50% Mo is an element necessary for maintaining high-temperature strength by containing 5.0 mass% or more of Mo in inclusions. Mo also has the effect of improving the high-temperature strength of the alloy through solid-solution strengthening. However, excessive Mo content reduces hot workability. Therefore, the Mo content is set to 8.00 to 10.50%. The Mo content is preferably 8.20% or more, more preferably 8.50% or more, and even more preferably 8.70% or more. The Mo content is preferably 10.30% or less, more preferably 10.00% or less, and even more preferably 9.80% or less.

[0027] Ti: 0.010 to 0.400% Ti has the effect of forming carbonitrides and contributing to improving high-temperature strength. Furthermore, by forming carbonitrides, Ti suppresses the formation of Cr carbides and reduces the deterioration of corrosion resistance at grain boundaries. However, excessive Ti content causes large amounts of Ti carbonitrides to precipitate, reducing ductility. Furthermore, the pinning effect of Ti carbonitrides refines crystal grains, reducing high-temperature strength. Therefore, the Ti content is set to 0.010 to 0.400%. The Ti content is preferably 0.050% or more, more preferably 0.080% or more, and even more preferably 0.100% or more. The Ti content is preferably 0.380% or less, more preferably 0.350% or less, and even more preferably 0.320% or less.

[0028] Sum of Nb and Ta: 2.50 to 4.60% Like Ti, both Nb and Ta have the effect of forming carbonitrides and contributing to improving high-temperature strength. Furthermore, by forming carbonitrides, Nb and Ta suppress the formation of Cr carbides and reduce the deterioration of corrosion resistance at grain boundaries. However, excessive Nb and Ta content causes large amounts of Nb and Ta carbonitrides to precipitate, reducing ductility. Furthermore, the pinning effect of the Nb and Ta carbonitrides refines crystal grains, reducing high-temperature strength. Therefore, the total content of Nb and Ta is set to 2.50 to 4.60%.

[0029] Nb and Ta may be contained singly or both may be contained simultaneously. The total content of Nb and Ta is preferably 2.70% or more, more preferably 3.00% or more. The total content of Nb and Ta is preferably 4.40% or less, more preferably 4.20% or less.

[0030] Sum of Cu and Co: 0.010 to 1.50% Both Cu and Co have the effect of increasing structural stability in high-temperature molten salts and improving corrosion resistance in non-oxidizing acid and chloride environments. However, excessive Cu and Co content reduces hot workability and increases manufacturing costs. They also reduce ductility, resulting in a deterioration of high-temperature strength. Therefore, the total content of Cu and Co is set to 0.010 to 1.50%.

[0031] Cu and Co may be contained singly or both may be contained simultaneously. The total content of Cu and Co is preferably 0.020% or more, more preferably 0.030% or more. The total content of Cu and Co is preferably 1.30% or less, more preferably 1.00% or less.

[0032] Al: 0.010 to 0.400% Al has a deoxidizing effect. Furthermore, Al contributes to improving oxidation resistance at high temperatures. However, excessive Al content generates brittle compounds with Ni, reducing hot workability. Therefore, the Al content is set to 0.010 to 0.400%. The Al content is preferably 0.020% or more, more preferably 0.030% or more, and even more preferably 0.050% or more. Furthermore, the Al content is preferably 0.350% or less, more preferably 0.300% or less, and even more preferably 0.280% or less.

[0033] P: 0.0150% or less P is contained in Ni-based alloys as an impurity. Therefore, the P content is set to 0.0150% or less. The P content is preferably set to 0.0130% or less, and more preferably set to 0.0120% or less. It is preferable to reduce the P content as much as possible, but excessive reduction increases manufacturing costs. Therefore, the P content is preferably set to 0.0010% or more, and more preferably set to 0.0020% or more.

[0034] S: 0.0150% or less S is contained in Ni-based alloys as an impurity. Therefore, the S content is set to 0.0150% or less. The S content is preferably set to 0.0120% or less, 0.0100% or less, 0.0080% or less, 0.0060% or less, or 0.0040% or less, and more preferably set to 0.0030% or less, or 0.0020% or less. It is preferable to reduce the S content as much as possible, but excessive reduction increases manufacturing costs. Therefore, the S content is preferably set to 0.0001% or more, and more preferably set to 0.0003% or more.

[0035] N: 0.0010 to 0.0400% N has the effect of promoting the formation of nitrides and carbonitrides, which improve high-temperature strength. However, excessive N content causes large amounts of nitrides, carbonitrides, and specific Mo inclusions to precipitate, reducing ductility. Therefore, the N content is set to 0.0010 to 0.0400%. The N content is preferably 0.0020% or more, more preferably 0.0030% or more, and even more preferably 0.0040% or more. Furthermore, the N content is preferably 0.0350% or less, more preferably 0.0300% or less, and even more preferably 0.0250% or less.

[0036] O: 0.0100% or less O is contained in Ni-based alloys as an impurity. Therefore, the O content is set to 0.0100% or less. The O content is preferably set to 0.0080% or less, and more preferably set to 0.0060% or less. It is preferable to reduce the O content as much as possible, but excessive reduction increases manufacturing costs. Therefore, the O content is preferably set to 0.0004% or more, and more preferably set to 0.0008% or more.

[0037] In the chemical composition of the Ni-based alloy pipe according to the present invention, the balance is Ni and impurities. Here, "impurities" refer to components that are not intentionally added but are mixed in due to various factors such as raw materials and manufacturing processes during industrial production of the Ni-based alloy pipe, and are acceptable within a range that does not adversely affect the present invention.

[0038] The alloy tube of the present invention may further contain one or more elements selected from Fe, W, V, Sn, Ca, Mg, B, and REM in the ranges shown below. Since these elements are not necessarily essential for the alloy tube, the lower limit of their content is 0%. The reasons for limiting the content of each element will be explained below.

[0039] Fe: 5.50% or less Fe has the effect of precipitating Laves phases at grain boundaries at high temperatures and strengthening the grain boundaries. Therefore, it may be added as needed. However, excessive Fe content reduces corrosion resistance. Therefore, the Fe content is set to 5.50% or less. The Fe content is preferably set to 5.30% or less, and more preferably set to 5.00% or less. To more reliably obtain the above effects, the Fe content is preferably set to 0.01% or more, more preferably set to 0.50% or more, and even more preferably set to 1.50% or more.

[0040] W: Less than 0.150% W combines with carbon to form carbides, improving high-temperature strength. Therefore, it may be added as needed. However, excessive W content causes the precipitation of large amounts of W-containing carbides and carbonitrides, reducing ductility. As a result, high-temperature strength is reduced. Furthermore, W is an expensive element, increasing manufacturing costs. Therefore, the W content is set to less than 0.150%. The W content is preferably set to 0.130% or less, and more preferably set to 0.100% or less. To ensure the above effects, the W content is preferably set to 0.010% or more, and more preferably set to 0.020% or more.

[0041] V: Less than 0.150% V combines with carbon to form carbides, which has the effect of improving high-temperature strength. Therefore, it may be added as needed. However, excessive V content causes large amounts of V carbides and carbonitrides to precipitate, reducing ductility. As a result, high-temperature strength is reduced. Therefore, the V content is set to less than 0.150%. The V content is preferably set to 0.130% or less, and more preferably set to 0.100% or less. Note that, to more reliably obtain the above effects, the V content is preferably set to 0.010% or more, and more preferably set to 0.020% or more.

[0042] Sn: 0.010% or less Sn segregates at grain boundaries at high temperatures, strengthening the grain boundaries and improving hot workability. Therefore, it may be contained as needed. However, excessive Sn content embrittles the alloy. Therefore, the Sn content is set to 0.010% or less. The Sn content is preferably set to 0.009% or less, and more preferably set to 0.008% or less. To more reliably obtain the above effects, the Sn content is preferably set to 0.001% or more, more preferably set to 0.002% or more, and even more preferably set to 0.003% or more.

[0043] Ca: 0.0030% or less Ca has the effect of improving hot workability. Therefore, it may be added as needed. However, if an excessive amount of Ca is added, it combines with oxygen and significantly reduces cleanliness. As a result, it actually reduces hot workability. Therefore, the Ca content is set to 0.0030% or less. The Ca content is preferably set to 0.0020% or less, and more preferably set to 0.0010% or less. Note that, to more reliably obtain the above effect, the Ca content is preferably set to 0.0001% or more, and more preferably set to 0.0003% or more.

[0044] Mg: 0.0030% or less Like Ca, Mg has the effect of improving hot workability. Therefore, it may be added as needed. However, if an excessive amount of Mg is added, it combines with oxygen and significantly reduces cleanliness. As a result, it actually reduces hot workability. Therefore, the Mg content is set to 0.0030% or less. The Mg content is preferably set to 0.0020% or less, and more preferably set to 0.0010% or less. Note that, to more reliably obtain the above effect, the Mg content is preferably set to 0.0001% or more, and more preferably set to 0.0003% or more.

[0045] B: 0.0100% or less B segregates at grain boundaries at high temperatures, strengthening the grain boundaries and improving hot workability. Therefore, B may be added as needed. However, excessive B content increases weld crack susceptibility. Therefore, the B content is set to 0.0100% or less. The B content is preferably set to 0.0080% or less, and more preferably set to 0.0060% or less. To more reliably obtain the above effects, the B content is preferably set to 0.0002% or more, and more preferably set to 0.0005% or more.

[0046] REM: 0.0100% or less Like Ca and Mg, REM has the effect of improving hot workability during manufacturing. Therefore, it may be added as needed. However, if an excessive amount of REM is added, it combines with oxygen and significantly reduces cleanliness. As a result, it actually reduces hot workability. Therefore, the REM content is set to 0.0100% or less. The REM content is preferably set to 0.0050% or less, and more preferably set to 0.0030% or less. Note that, to more reliably obtain the above effect, the REM content is preferably set to 0.0001% or more, and more preferably set to 0.0003% or more.

[0047] Here, REM is a general term for Sc, Y, and lanthanoids, a total of 17 elements, and the REM content means the total amount of the above elements. Note that lanthanoids are industrially added in the form of misch metals.

[0048] (B) Specific Mo inclusions In the Ni-based alloy pipe according to the present invention, in the center part of the wall thickness in a cross section perpendicular to the longitudinal direction of the Ni-based alloy pipe (hereinafter simply referred to as the "center part of the wall thickness"), the Mo content is 5.0 mass % or more, and the number density of inclusions (hereinafter also referred to as "specific Mo inclusions") having a maximum length of 10 μm or less is 10 to 100 / mm ２ In the present invention, the inclusions include precipitates and crystallized substances.

[0049] The reasons for specifying the Mo content to be 5.0 mass % or more and the maximum length to be 10 μm or less for the specific Mo inclusions will be explained below for each requirement.

[0050] If the Mo content in the inclusions is less than 5.0 mass%, the inclusions cannot stably exist for a long time in a severe high-temperature environment, and high-temperature strength cannot be maintained for a long time. Therefore, the Mo content in the inclusions is 5.0 mass% or more. There is no particular upper limit for the Mo content in the specific Mo inclusions. However, if the Mo content in the specific Mo inclusions is excessive, the number of inclusions with a Mo content of 5.0 mass% or more may decrease, and the number density of the specific Mo inclusions may become low. Therefore, the Mo content in the specific Mo inclusions is preferably 15.0 mass% or less.

[0051] If the maximum length of the inclusions exceeds 10 μm, it will be impossible to maintain high-temperature strength for a long period of time in a severe high-temperature environment. Therefore, the target inclusions are those with a maximum length of 10 μm or less. The lower limit of the maximum length of the specific Mo inclusions is set to 0.1 μm, which is the minimum value measurable by MQA, which will be described later.

[0052] The number density of specific Mo inclusions is 10 / mm ２ However, if the number density of the specific Mo inclusions is less than 100 / mm ２ If the number density of the specific Mo inclusions exceeds 10 to 100 / mm, the ductility deteriorates, and as a result, the high temperature strength cannot be maintained for a long period of time. ２ Let's say.

[0053] The number density of specific Mo inclusions is measured using a Metal Quality Analyzer (MQA). First, a cross section perpendicular to the longitudinal direction of the Ni-based alloy pipe is mirror-polished using diamond abrasive grains with a particle size of 1 μm. Then, the measurement field is set to 600.6 μm × 2005 μm, and the size of the inclusions present within the field is measured and the elements contained in the inclusions are analyzed so that the center of the wall thickness is the center of the measurement field. The number of inclusions with a Mo content of 5.0 mass% or more and a maximum length of 10 μm or less is counted and divided by the field area to determine the number density of the specific Mo inclusions. The acceleration voltage is 20 kV and the working distance is 17 mm. The above measurement is performed for a total of four fields at 90° intervals around the circumferential direction of the Ni-based alloy pipe, and the number density of the specific Mo inclusions is the average value of the number densities measured in the four fields.

[0054] In the MQA measurement, first, a scanning electron microscope (SEM) equipped in the MQA is used to capture backscattered electron images of the cross section of the Ni-based alloy pipe and the reference pure Al substrate. Then, the contrast and brightness of the substrate in the cross section of the Ni-based alloy pipe are matched to the pure Al substrate used for SEM observation. Next, among the 256 brightness levels, regions with brightness levels of 0 to 155 and 220 to 255 are searched for, and these regions are identified as inclusions. Then, for the identified inclusions with brightness levels of 0 to 165 and 220 to 255, the size of each inclusion and the elements contained in each inclusion are measured.

[0055] (C) Average Grain Size The Ni-based alloy pipe according to the present invention has an average grain size of 4.0 to 8.0 at the center of the wall thickness.

[0056] As mentioned above, the corrosion resistance of Ni-based alloy pipes can be improved by making the crystal grains finer. However, making the crystal grains finer results in a deterioration of high-temperature strength. Therefore, the average crystal grain size is set to 4.0 to 8.0. The average crystal grain size is preferably 5.0 or more. Furthermore, the average crystal grain size is preferably less than 8.0, and more preferably 7.9 or less, or 7.8 or less.

[0057] The average grain size is measured in accordance with ASTM E112:2013. Specifically, a test specimen for microstructure observation is taken so that the cross section perpendicular to the longitudinal direction of the alloy pipe serves as the observation surface, and the observation surface is mirror-polished. After polishing, the specimen is etched with mixed acid and observed under an optical microscope. Ten fields of view are observed, with the center of the field of view being the center of the thickness of the alloy pipe. The grain size of each field of view is then determined using the comparison method specified in ASTM E112, and the average value is taken as the average grain size. In this case, 100x is used as the standard observation magnification. If the grain size number is 5 or greater, the measurement is retaken at 200x, and if the grain size number is 7 or greater, the measurement is retaken at 500x. Furthermore, when the observation magnification is 200x or 500x, correction is performed in accordance with ASTM E112:2013 using the correction value Q defined by the following formula (i): Q = 6.64 log １０ (M / 100) (i) where M in the above formula is the observation magnification.

[0058] (D) Dimensions of Ni-Based Alloy Tube The Ni-base alloy tube according to the present invention preferably has a t / D x 100 value of 3.0 to 19.0%. By setting the t / D x 100 value to 3.0% or more, stable production is possible, and by setting it to 19.0% or less, heat transfer properties can be ensured. The t / D x 100 value is more preferably 5.0% or more, and even more preferably 7.0% or more. Furthermore, the t / D x 100 value is more preferably 15.0% or less, and even more preferably 13.0% or less.

[0059] (E) Manufacturing Method A preferred manufacturing method for the Ni-based alloy pipe according to the present invention will be described. The Ni-based alloy pipe according to the present invention can achieve the effects described above regardless of the manufacturing method, but can be manufactured stably by carrying out the following preparation step, hot extrusion step, cold working step, and heat treatment step, for example.

[0060] <Preparation Step> First, a Ni-based alloy ingot is produced as a raw material for the Ni-based alloy pipe. The Ni-based alloy ingot is preferably produced by melting an alloy having the above-mentioned chemical composition in an electric furnace or the like, removing impurities by refining, and then casting the melt. Next, the obtained ingot is preferably hot forged into a cylindrical billet.

[0061] <Hot Extrusion Step> In the hot extrusion step, a cylindrical billet is hot extruded. The area reduction rate during hot extrusion is set to 75.0% or more. In the present invention, the area reduction rate is the area reduction rate in a cross section perpendicular to the longitudinal direction of the Ni-based alloy pipe.

[0062] Area reduction rate: 75.0% or more Mo is an element that is prone to segregation, and there is a risk of segregation during the cooling stage after casting. Therefore, in order to uniformly precipitate specific Mo inclusions in the wall thickness direction of the Ni-based alloy pipe, a long period of soaking is required before the hot extrusion process. However, by increasing the area reduction rate in the hot extrusion process and the cold working process, processing strain increases. This increases the number of dislocations, making it easier for Mo to move through the gaps between dislocations. Therefore, it is recommended not to perform a long period of soaking before the hot extrusion process, and to reduce the heat treatment time t in the heat treatment process described below. ｒ As a result, the specific Mo inclusions are uniformly precipitated in the thickness direction of the Ni-based alloy pipe, and the number density of the specific Mo inclusions in the center of the wall thickness is 10 to 100 / mm ２ The range can be:

[0063] If the reduction in area ratio in the hot extrusion process is less than 75.0%, Mo cannot be sufficiently moved and remains in a segregated state, and the number density of specific Mo inclusions is 10 / mm ２ Therefore, the area reduction rate in the hot extrusion step is set to 75.0% or more. The upper limit of the area reduction rate in the hot extrusion step is not particularly limited, but from the viewpoint of manufacturability, it may be set to 85.0% or less.

[0064] <Cold Working Step> In the cold working step, the mother pipe after the hot extrusion is subjected to cold rolling or cold drawing (hereinafter collectively referred to as "cold working"). The area reduction rate during cold working is set to 70.0% or more.

[0065] Area reduction rate: 70.0% or more Even if the area reduction rate in the hot extrusion process is 75.0% or more, if the area reduction rate in the cold working process is less than 70.0%, the processing strain is insufficient, and Mo cannot be sufficiently moved in the wall thickness direction of the Ni-based alloy pipe. As a result, a large number of specific Mo inclusions are precipitated in the center of the wall thickness of the Ni-based alloy pipe, and the number density of the specific Mo inclusions in the center of the wall thickness is 100 / mm ２ Therefore, the area reduction rate in the cold working step is set to 70.0% or more.

[0066] The upper limit of the area reduction rate in the cold working step is not particularly limited, but from the viewpoint of manufacturability, it may be set to 80.0% or less. Furthermore, in the cold working step, cold working may be performed multiple times. In this case, the area reduction rate in each cold working step is set to 70.0% or more. Intermediate heat treatment may be performed between each cold working step. This can prevent cracking during cold working. The conditions for the intermediate heat treatment are not particularly limited, and may be conventional, for example, at 1150 to 1250°C for 1 to 5 minutes.

[0067] <Heat Treatment Step> In the heat treatment step, first, the Ni-based alloy pipe is heated to a heat treatment temperature T at a temperature increase rate of 50°C / min or more. Next, the heat treatment temperature T is increased to 900 to 1200°C for a heat treatment time t ｒ The temperature is kept at 5 min or less and the value of the following formula (ii) is kept within the range of 1480 to 1880. (T + 273.15) × Log(t ｒ × Mo) (ii) where the meanings of the symbols in the formula (ii) are as follows: T: heat treatment temperature (°C) t ｒ : Heat treatment time (min) Mo: Content of Mo contained in the Ni-based alloy (mass%)

[0068] Heating rate: 50°C / min or more If the heating rate is less than 50°C / min, crystal grains grow before the heat treatment temperature is reached, resulting in a decrease in corrosion resistance. Therefore, the heating rate is set to 50°C / min or more. The heating rate is preferably 70°C / min or more. On the other hand, from the viewpoint of manufacturing costs, the heating rate is set to 200°C / min or less. Note that, because the growth of specific Mo inclusions is slower than the growth of crystal grains, even if the heating rate is less than 50°C / min, there is no significant effect on the number density of the specific Mo inclusions.

[0069] Heat treatment temperature T: 900 to 1200°C If the heat treatment temperature T is less than 900°C, the crystal grain size cannot be adjusted and the crystal grain size remains small, resulting in a decrease in high-temperature strength. Furthermore, specific Mo inclusions precipitate in the temperature range of 900 to 1200°C, but below 900°C, the specific Mo inclusions cannot precipitate sufficiently. This results in a decrease in high-temperature strength. On the other hand, if the heat treatment temperature T exceeds 1200°C, the crystal grains become coarse, resulting in a decrease in corrosion resistance. Furthermore, the number density of the specific Mo inclusions becomes excessive, resulting in a decrease in ductility, and as a result, a decrease in high-temperature strength.

[0070] Heat treatment time t ｒ : 5 min or less Heat treatment temperature t ｒ If the heat treatment time is longer than 5 min, the grains become coarse, resulting in a deterioration in corrosion resistance. Furthermore, the number density of specific Mo inclusions becomes excessive, resulting in a deterioration in ductility, and as a result, a decrease in high-temperature strength. Furthermore, in the present invention, since the hot extrusion process involves extensive processing, there is a lot of processing strain. Therefore, Mo can be uniformly distributed in a short time. Therefore, the heat treatment time t ｒ The heat treatment time t ｒ is preferably 2 min or less.

[0071] (ii) Formula value: 1480 to 1880 The number density of specific Mo inclusions is 10 to 100 / mm ２ In order to achieve the above range, in addition to satisfying each of the above conditions individually, the heat treatment temperature T and the heat treatment time t ｒ The value of the above formula (ii), which is composed of the content of Mn and the Mo content, must be in the range of 1480 to 1880.

[0072] As described above, the number density of the specific Mo inclusions varies depending on the heat treatment temperature T and the heat treatment time t ｒ Furthermore, if the Mo content in the Ni-based alloy pipe is high, the number density of the specific Mo inclusions also tends to be high. ｒ It is necessary to strictly control the value of formula (ii), which takes into consideration the Mo content in the Ni-based alloy tube.

[0073] If the value of the formula (ii) is less than 1480, the number density of the specific Mo inclusions becomes insufficient, and high-temperature strength cannot be improved. On the other hand, if the value of the formula (ii) is more than 1880, the number density of the specific Mo inclusions becomes excessive, which deteriorates ductility and, as a result, reduces high-temperature strength. Therefore, the value of the formula (ii) needs to be 1480 to 1880.

[0074] After the heat treatment step is completed, it is preferable to perform water cooling, and the cooling rate is preferably 500 to 600° C. / min.

[0075] The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to these examples.

[0076] Alloys having the chemical compositions shown in Tables 1 and 2 were melted in a laboratory to prepare ingots. The obtained ingots were then hot forged into cylindrical billets, which were then subjected to the hot extrusion process, cold working process, and heat treatment process shown in Table 3 to obtain alloy tubes of Test Nos. 1 to 42. The t / D (%) of the obtained alloy tubes is shown in Table 3.

[0077]

[0078]

[0079]

[0080] <Number Density of Specific Mo Inclusions> The number density of the specific Mo inclusions was measured by MQA. First, a cross section perpendicular to the longitudinal direction of the Ni-based alloy pipe was mirror-polished using diamond abrasive grains with a particle size of 1 μm. Then, the measurement field was set to 600.6 μm × 2005 μm, and the size of the inclusions present within the field was measured and the elements contained in the inclusions were analyzed so that the center of the wall thickness was the center of the measurement field. The number of inclusions with a Mo content of 5.0 mass% or more and a maximum length of 10 μm or less was counted and divided by the field area to determine the number density of the specific Mo inclusions. The acceleration voltage was 20 kV and the working distance was 17 mm. The above measurement was performed for a total of four fields at 90° intervals around the circumferential direction of the Ni-based alloy pipe, and the number density of the specific Mo inclusions was taken as the average value of the number densities measured in the four fields.

[0081] In the MQA measurement, first, backscattered electron images of the cross section of the Ni-based alloy pipe and the reference Al substrate were taken using the SEM equipped in the MQA. Then, the contrast and brightness of the substrate in the cross section of the Ni-based alloy pipe were matched to those of the pure Al substrate used for SEM observation. Next, among the 256 brightness levels, regions with brightness levels of 0 to 155 and 220 to 255 were searched for, and these regions were identified as inclusions. Then, for the identified inclusions with brightness levels of 0 to 165 and 220 to 255, the size of each inclusion and the elements contained in each inclusion were measured.

[0082] <Average Grain Size> The average grain size was measured in accordance with ASTM E112:2013. Specifically, a test specimen for microstructure observation was taken so that the cross section perpendicular to the longitudinal direction of the alloy pipe served as the observation surface, and the observation surface was mirror-polished. After polishing, the specimen was etched with mixed acid and observed under an optical microscope. Ten fields of view were observed, with the center of the field of view being the center of the thickness of the alloy pipe. The grain size of each field of view was then determined using the comparison method specified in ASTM E112, and the average value was taken as the average grain size. In this case, 100x was used as the standard observation magnification. If the grain size number was 5 or greater, the measurement was retaken at 200x, and if the grain size number was 7 or greater, the measurement was retaken at 500x. Furthermore, when the observation magnification was 200x or 500x, correction was performed in accordance with ASTM E112:2013 using the correction value Q defined by the following formula (i): Q = 6.64 log １０ (M / 100) (i) where M in the above formula is the observation magnification.

[0083] <High-temperature strength> High-temperature strength was evaluated by creep rupture time. Circular arc-shaped test specimens with the shape and dimensions shown in Figure 1 were taken from each alloy pipe parallel to the longitudinal direction and subjected to creep rupture tests at 650°C and 350 MPa. Test specimens with creep rupture times exceeding 1500 hours were rated A, indicating good high-temperature strength. Test specimens with creep rupture times of 1500 hours or less were rated B, indicating poor high-temperature strength. When the wall thickness of the alloy pipe was 7 mm or less, the thickness was left as it was. When the wall thickness of the alloy pipe was more than 7 mm, the inner surface was machined to a thickness of 7 mm.

[0084] <Corrosion Resistance> Circular arc-shaped test pieces having the shape and dimensions shown in Fig. 2 were taken from each alloy pipe and polished with wet polishing #400 to prepare test pieces for corrosion resistance evaluation. The corrosion resistance was evaluated by a test in which the pieces were immersed in molten salt for 1000 hours. The molten salt was NaNO ３ and KNO ３ The test piece was immersed for 1000 hours, taking into consideration the operating environment of the heat transfer tube, which is in contact with the molten salt for a long time. The corrosion weight loss of the test piece was 15.0 mg / cm ２When the corrosion weight loss was 15.0 mg / cm or less, the corrosion resistance was evaluated as A, and the corrosion resistance was judged to be good. ２ When the value was greater than 100%, the corrosion resistance was rated as B, and the corrosion resistance was judged to be poor.

[0085] Table 4 shows the measurement results of the number density and average grain size of specific Mo inclusions, as well as the evaluation results of high-temperature strength and corrosion resistance.

[0086]

[0087] As shown in Table 4, Test Nos. 1 to 26, 40, and 41, which satisfied all of the requirements of the present invention, achieved excellent results in all performance evaluations. In contrast, Test Nos. 27 to 39 and 42, which are comparative examples, showed deterioration in either high-temperature strength or corrosion resistance.

[0088] According to the present invention, a Ni-based alloy tube can be obtained which has excellent corrosion resistance to molten nitrates at high temperatures for a long period of time and excellent high-temperature strength for a long period of time.

Claims

1. The chemical composition of the Ni-based alloy tube is, by mass%, C: 0.005 to 0.100%, Si: 0.010 to 0.500%, Mn: 0.010 to 0.500%, Cr: 20.00 to 23.50%, Mo: 8.00 to 10.50%, Ti: 0.010 to 0.400%, the total of Nb and Ta: 2.50 to 4.60%, the total of Cu and Co: 0.010 to 1.50%, Al: 0.010 to 0.400%, P: 0.0150% or less, S: 0.0150% or less, N: 0.0010 to 0.0400%, O: 0.0100% or less, the balance: Ni and impurities, and in the center of the wall thickness in a cross section perpendicular to the longitudinal direction of the Ni-based alloy tube, the Mo content is 5.0 mass% or more, and the number density of inclusions having a maximum length of 10 μm or less is 10 to 100 / mm ２ and the average crystal grain size is 4.0 to 8.0, Ni-based alloy tube.

2. The chemical composition of the Ni-based alloy tube is, by mass%, C: 0.005 to 0.080%, Si: 0.010 to 0.500%, Mn: 0.010 to 0.500%, Cr: 20.00 to 23.50%, Mo: 8.00 to 10.50%, Ti: 0.010 to 0.400%, the total of Nb and Ta: 2.50 to 4.60%, the total of Cu and Co: 0.010 to 1.50%, Al: 0.010 to 0.400%, P: 0.0150% or less, S: 0.0030% or less, N: 0.0010 to 0.0400%, O: 0.0100% or less, the balance: Ni and impurities, and in the center of the wall thickness in a cross section perpendicular to the longitudinal direction of the Ni-based alloy tube, the Mo content is 5.0 mass% or more, and the number density of inclusions having a maximum length of 10 μm or less is 10 to 100 / mm ２ and the average crystal grain size is 4.0 to 8.0, Ni-based alloy tube.

3. The chemical composition contains, in mass%, one or more selected from the following, in place of part of the Ni: Fe: 5.50% or less, W: less than 0.150%, V: less than 0.150%, Sn: 0.010% or less, Ca: 0.0030% or less, Mg: 0.0030% or less, B: 0.0100% or less, and REM: 0.0100% or less. The Ni-based alloy tube according to claim 1 or claim 2.

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