Ni-fe-cr-based alloy pipe and method for producing same

Abstract

Provided is an Ni-Fe-Cr-based alloy pipe having high creep strength and excellent smoothness. In this Ni-Fe-Cr-based alloy pipe, the average grain size number in a cross-section parallel to the axial direction and the thickness direction of the alloy pipe is 5.0 or less, and the root mean square roughness RMS of the inner surface and the outer surface of the alloy pipe is 1.60 μm or less.

Description

Ni-Fe-Cr based alloy tube and manufacturing method thereof

[0001] The present invention relates to a Ni-Fe-Cr based alloy pipe and a method for producing the same.

[0002] Heat transfer tubes for steam generators used in nuclear power plants (hereinafter simply referred to as "heat transfer tubes") are exposed to high-temperature water environments, and therefore Ni-Fe-Cr-based alloy tubes, which have good creep strength, are often used for the heat transfer tubes (see, for example, Patent Document 1).

[0003] Japanese Patent Application Laid-Open No. 2021-11591

[0004] Increasing the grain size of heat transfer tubes is an effective way to improve creep strength. Furthermore, heat transfer tubes are exposed to corrosive environments, so corrosion products adhere to the tube. If the corrosion products remain attached, the heat transfer performance of the heat transfer tube may deteriorate. Therefore, the surface of the heat transfer tube must be smooth so that the corrosion products peel off naturally in the operating environment.

[0005] SUMMARY OF THE INVENTION An object of the present invention is to solve the above problems and to provide a Ni--Fe--Cr based alloy pipe having high creep strength and excellent smoothness, and a method for producing the same.

[0006] The present invention has been made to solve the above problems, and is summarized as follows: a Ni-Fe-Cr based alloy pipe and a method for manufacturing the same.

[0007] (1) A Ni-Fe-Cr based alloy pipe, wherein the average grain size number in a cross section parallel to the axial direction and the wall thickness direction of the alloy pipe is 5.0 or less, and the root mean square roughness (RMS) of the inner surface and the outer surface of the alloy pipe is 1.60 μm or less.

[0008] (2) The Ni-Fe-Cr based alloy tube according to (1) above, having a chemical composition, in mass%, of C: 0.040 to 0.110%, Si: 1.10% or less, Mn: 1.60% or less, S: 0.016% or less, P: 0.028% or less, Ni: 29.5 to 35.5%, Cr: 18.5 to 23.5%, Ti: 0.10 to 0.70%, Al: 0.10 to 0.70%, and N: 0.0100% or less, the balance being Fe and impurities.

[0009] (3) The Ni-Fe-Cr based alloy pipe according to (1) or (2) above, wherein the chemical composition of the Ni-Fe-Cr based alloy pipe contains, in place of a portion of the Fe, one or more elements selected from, by mass%, Cu: 0.76% or less, V: 0.500% or less, W: 1.00% or less, Nb: 1.00% or less, Co: 1.00% or less, Mo: 1.00% or less, B: 0.0400% or less, Ca: 0.0100% or less, Mg: 0.0500% or less, and REM: 0.100% or less.

[0010] (4) A method for producing the Ni—Fe—Cr base alloy pipe according to (1) above, comprising the steps of: subjecting a hot-extruded mother pipe to a first cold rolling step, a softening heat treatment step, a second cold rolling step, and a solution heat treatment step in that order; the softening heat treatment step is carried out in a hydrogen furnace or an atmospheric furnace; and the solution heat treatment step is carried out in a hydrogen furnace; in the softening heat treatment step, the mother pipe is held at a softening heat treatment temperature of 1000°C or higher for 5 minutes or longer; and in the solution heat treatment step, the mother pipe is held at a solution heat treatment temperature of 1140 to 1170°C for 10 minutes or longer; and the softening heat treatment temperature minus the solution heat treatment temperature is 0°C or lower.

[0011] (5) The method for producing a Ni—Fe—Cr-based alloy pipe according to (4) above, wherein the softening heat treatment temperature−the solution heat treatment temperature is −20° C. or less.

[0012] (6) A method for producing the Ni—Fe—Cr base alloy pipe according to (2) above, wherein the raw pipe has a chemical composition, in mass%, of C: 0.040 to 0.110%, Si: 1.10% or less, Mn: 1.60% or less, S: 0.016% or less, P: 0.028% or less, Ni: 29.5 to 35.5%, Cr: 18.5 to 23.5%, Ti: 0.10 to 0.70%, Al: 0.10 to 0.70%, N: 0.0100% or less, and the balance: Fe and impurities.

[0013] (7) A method for producing the Ni—Fe—Cr base alloy pipe according to (3) above, wherein the chemical composition of the mother pipe contains, in mass %, one or more elements selected from Cu: 0.76% or less, V: 0.500% or less, W: 1.00% or less, Nb: 1.00% or less, Co: 1.00% or less, Mo: 1.00% or less, B: 0.0400% or less, Ca: 0.0100% or less, Mg: 0.0500% or less, and REM: 0.100% or less, in place of a portion of the Fe.

[0014] (8) A method for producing the Ni—Fe—Cr base alloy pipe according to (3) above, wherein the chemical composition of the mother pipe contains, in mass %, one or more elements selected from Cu: 0.76% or less, V: 0.500% or less, W: 1.00% or less, Nb: 1.00% or less, Co: 1.00% or less, Mo: 1.00% or less, B: 0.0400% or less, Ca: 0.0100% or less, Mg: 0.0500% or less, and REM: 0.100% or less, in place of a portion of the Fe.

[0015] According to the present invention, it is possible to obtain a Ni--Fe--Cr based alloy pipe having high creep strength and excellent smoothness.

[0016] Figure 1 is a schematic diagram of the metal structure of an alloy pipe produced by heat treatment method A. Figure 2 is a schematic diagram of the metal structure of an alloy pipe produced by heat treatment method B. Figure 3 is a microphotograph of the alloy pipe of Test No. 1. Figure 4 is a microphotograph of the alloy pipe of Test No. 2. Figure 5 is a microphotograph of the alloy pipe of Test No. 5.

[0017] The present inventors have conducted extensive research to solve the above-mentioned problems. When manufacturing an alloy tube for use as a heat transfer tube for a steam generator, a mother tube manufactured by hot extrusion is subjected to first cold rolling, softening heat treatment, second cold rolling, and solution heat treatment. In a conventional method for manufacturing such an alloy tube, the softening heat treatment and solution heat treatment are performed in an atmospheric furnace. In this case, scale is formed on the surface of the alloy tube during the softening heat treatment and solution heat treatment stages, necessitating pickling. As a result, the surface roughness increases. Therefore, to eliminate the need for pickling to remove the scale and achieve excellent smoothness, at least the solution heat treatment must be performed in a hydrogen furnace.

[0018] On the other hand, in order to ensure high creep strength, it is necessary to have coarse crystal grains with an average grain size number of 5.0 or less. To adjust the crystal grains to such a size, it is necessary to increase the heating temperature in the softening heat treatment and / or solution heat treatment to coarsen the crystal grains. However, due to equipment limitations, it is difficult to use a hydrogen furnace in a temperature range exceeding 1200°C, and it is generally used in a temperature range of 1160°C or less. With heat treatment in such a temperature range, it is generally difficult to coarsen the crystal grains to an average grain size number of 5.0 or less.

[0019] Therefore, the inventors considered performing a softening heat treatment at a high temperature to coarsen the grains as much as possible. Specifically, the inventors decided to perform a first cold rolling on a mother pipe manufactured by hot extrusion, followed by a softening heat treatment at 1280°C in an atmospheric furnace, a second cold rolling, and then a solution heat treatment at 1160°C in a hydrogen furnace. Hereinafter, this type of heat treatment will be referred to as "heat treatment method A." By performing heat treatment method A on a mother pipe, a softening heat treatment at a high temperature is performed, resulting in coarsening of the grains. Therefore, it was expected that the size of the coarsened grains would be maintained to a certain extent even after the second cold rolling and solution heat treatment.

[0020] FIG. 1 is a schematic diagram of the metal structure of an alloy pipe manufactured by heat treatment method A. (a) shows the metal structure after softening heat treatment, (b) shows the metal structure after second cold rolling, and (c) shows the metal structure after solution heat treatment. When softening heat treatment was performed at 1280°C, the average grain size number was approximately 0, and it was found that the crystal grains were coarse as expected. However, when second cold rolling and solution heat treatment were performed, the proportion of crystal grains with grain size numbers 4 to 6 was approximately 50%, and the proportion of crystal grains with grain size numbers 7 to 9 was approximately 50%, and many fine crystal grains were also formed. This suggests that precipitates were formed during solution heat treatment, and the crystal grains became finer due to the pinning effect.

[0021] Therefore, focusing on Ti, the Ti content in the precipitates of the alloy pipe after softening heat treatment and after solution heat treatment was investigated. Ti is one of the elements that precipitates as carbide and / or nitride and exhibits a pinning effect. As a result of electrolytic extraction residue analysis, the Ti content in the precipitates of the alloy pipe after softening heat treatment was 0.028 mass %, while that of the alloy pipe after solution heat treatment was 0.068 mass %. The Ti content in the precipitates after solution heat treatment was higher than that after softening heat treatment.

[0022] From the above experimental results and investigations, it is believed that the crystal grains were refined through the following mechanism. When softening heat treatment was performed at 1280°C, Ti was almost completely dissolved, and Ti-containing precipitates (hereinafter referred to as "Ti precipitates") were scarce. However, when second cold rolling was performed, strain accumulated at the grain boundaries, as shown by the hatching in Figure 1(b). When solution heat treatment was then performed at a temperature lower than the softening heat treatment temperature, much of the dissolved Ti precipitated at the grain boundaries as Ti precipitates, as shown by the black dots in Figure 1(c). Furthermore, recrystallization occurred at the grain boundaries where strain accumulated, but the pinning effect of the Ti precipitates prevented the crystal grains from growing, resulting in the formation of fine crystal grains at the grain boundaries. The hatching in Figure 1(b) and the black dots in Figure 1(c) are shown as representatives in the central area of ​​the field of view.

[0023] Next, based on the above results, the present inventors investigated a method for suppressing the precipitation of Ti precipitates during solution treatment and obtaining crystal grains having an average grain size number of 5.0 or less. As a result, the present inventors decided to perform a first cold rolling on a mother pipe produced by hot extrusion, followed by a softening heat treatment at 1,120°C in an atmospheric furnace, and then a second cold rolling, followed by a solution heat treatment at 1,160°C in a hydrogen furnace. This type of heat treatment will hereinafter be referred to as "heat treatment method B." It was thought that if softening heat treatment was performed at a temperature lower than the solution heat treatment temperature, Ti precipitates would precipitate during the softening heat treatment, and the precipitation of new Ti precipitates during the solution heat treatment would be suppressed.

[0024] Figure 2 is a schematic diagram of the metallographic structure of an alloy pipe manufactured by heat treatment method B. (a) shows the metallographic structure after softening, (b) shows the metallographic structure after second cold rolling, and (c) shows the metallographic structure after solution heat treatment. By performing these heat treatments, the proportion of crystal grains with grain size numbers 4 to 6 was approximately 50%, and the proportion of crystal grains with grain size numbers 7 to 9 was approximately 50%, resulting in a large proportion of fine crystal grains. However, after solution heat treatment, the average grain size number was 3.3, indicating that the crystal grains had become coarse. Furthermore, the Ti content in the precipitates of the alloy pipes after softening and solution treatments was investigated. Analysis of the electrolytic extraction residue revealed that the Ti content in the precipitates was 0.069 mass% for the alloy pipe after softening and 0.070 mass% for the alloy pipe after solution treatment. The Ti content in the precipitates after the softening heat treatment was similar to that after the solution heat treatment.

[0025] From the above experimental results, it is believed that the coarse crystal grains were obtained through the following mechanism. When softening heat treatment was performed at 1120°C, the crystal grains did not coarsen, but Ti precipitates precipitated within the crystal grains and at the grain boundaries, as shown by the black dots in Figure 2(a). Even if strain accumulated at the grain boundaries due to the second cold rolling, Ti precipitates were already precipitated during the softening heat treatment, so it is believed that there was almost no new Ti precipitate precipitated during the solution heat treatment. It is also believed that the crystal grains were coarsened by performing solution heat treatment at 1160°C.

[0026] As a result of the above investigations and further improvements, it was found that by performing heat treatment in a hydrogen furnace, it was possible to obtain an alloy pipe with excellent smoothness, with a root mean square roughness RMS of 1.60 μm or less, on the inner and outer surfaces. It was also found that by performing solution heat treatment in a temperature range of 1140 to 1170°C for 10 minutes or more, softening heat treatment in a temperature range of 1000°C or higher for 5 minutes or more, and further setting the softening heat treatment temperature - solution heat treatment temperature to 0°C or lower, it is possible to obtain an alloy pipe with excellent creep strength and an average grain size number of 5.0 or less throughout the entire wall thickness of the alloy pipe.

[0027] The present invention was made based on the above findings. Each of the features of the present invention will be described in detail below.

[0028] (A) Average Grain Size Number The alloy pipe according to the present invention has an average grain size number of 5.0 or less in a cross section parallel to the axial direction and wall thickness direction (radial direction of the alloy pipe) of the alloy pipe. The cross section parallel to the axial direction and wall thickness direction of the alloy pipe is, in other words, a cross section perpendicular to the circumferential direction of the alloy pipe.

[0029] As mentioned above, creep strength can be improved by coarsening the crystal grains throughout the entire thickness. Therefore, the average grain size number is set to 5.0 or less. The average grain size number is preferably set to 4.0 or less. The average grain size number is preferably -2.0 or more, more preferably -1.0 or more, and even more preferably 0 or more.

[0030] The average grain size number is calculated in accordance with ASTM E112:2013. Specifically, a test piece for microstructure observation is taken so that the cross section parallel to the axial direction and wall thickness direction of the alloy pipe serves as the observation surface, and the observation surface is mirror-polished. After polishing, the specimen is corroded with mixed acid and observed under an optical microscope. First, assuming that the wall thickness of the alloy pipe is d, a total of 10 measurement areas are set, each 1 / 10 d apart, from the outer surface to the inner surface of the alloy pipe. Each measurement area is then observed at 100x magnification, and the grain size number of each measurement area is determined using the comparison method specified in ASTM E112:2013. The average value is taken as the average grain size number of the alloy pipe.

[0031] However, if coarse grains and fine grains are mixed within one measurement area, the comparison method cannot identify the average grain size number. Also, there is a risk that the creep strength of the fine grains will decrease. Therefore, in such cases, the average grain size number cannot be evaluated.

[0032] (B) Root Mean Square Roughness RMS The alloy pipe according to the present invention has a root mean square roughness RMS of 1.60 μm or less on the inner and outer surfaces of the alloy pipe.

[0033] As mentioned above, the surface of the heat transfer tube is required to be smooth so that corrosion products will naturally peel off in the operating environment. Therefore, the root mean square roughness (RMS) is set to 1.60 μm or less. The root mean square roughness (RMS) can be reduced by performing heat treatment in a hydrogen furnace in the manufacturing method described below.

[0034] The root mean square roughness (RMS) is measured in accordance with ASTM B46.1 (2019). Specifically, a 100 mm measurement sample is taken from the alloy pipe as is, and the longitudinal (axial) direction is measured. The tip of the stylus used for measurement is spherical with a radius of 10 μm, and the maximum measurement load is 15 mN. The scanning speed of the jig holding the stylus is 0.1 mm / sec, and the scanning distance is 4 mm. The measured profile Z(x) is used in the following equation (i) to calculate Rq(RMS).

[0035]

[0036] (C) Chemical Composition There are no particular limitations on the chemical composition of the Ni-Fe-Cr based alloy tube according to the present invention. A preferred chemical composition of the Ni-Fe-Cr based alloy tube according to the present invention will be described below. The reasons for limiting each element are as follows. In the following description, "%" in relation to the content means "% by mass."

[0037] C: 0.040 to 0.110% C is an element that stabilizes austenite and contributes to high strength. However, excessive C content reduces toughness. Therefore, the C content is preferably 0.040 to 0.110%. The C content is more preferably 0.050% or more, and even more preferably 0.060% or more. Furthermore, the C content is more preferably 0.100% or less, and even more preferably 0.090% or less.

[0038] Si: 1.10% or less Si is an element that has a deoxidizing effect. However, excessive Si content reduces the stability of austenite, resulting in a decrease in toughness. Therefore, the Si content is preferably 1.10% or less. The Si content is more preferably 0.90% or less, and even more preferably 0.70% or less. Furthermore, the Si content is preferably 0.30% or more, and more preferably 0.50% or more.

[0039] Mn: 1.60% or less Like Si, Mn is an element that not only has a deoxidizing effect but also contributes to the stabilization of austenite. However, excessive Mn content makes the alloy hard and brittle, and actually reduces toughness. Therefore, the Mn content is preferably 1.60% or less. The Mn content is more preferably 1.40% or less, and even more preferably 1.20% or less. Furthermore, the Mn content is preferably 0.30% or more, and more preferably 0.50% or more.

[0040] S: 0.016% or less S is an element contained in the alloy as an impurity and reduces the hot workability of the alloy. Therefore, the S content is preferably 0.016% or less. The S content is more preferably 0.014% or less, and even more preferably 0.012% or less. On the other hand, it is preferable to reduce the S content as much as possible, but excessive reduction of the S content increases the manufacturing cost. Therefore, the S content is preferably 0.001% or more, more preferably 0.003% or more, and even more preferably 0.005% or more.

[0041] P: 0.028% or less P is an element contained in the alloy as an impurity and reduces the hot workability of the alloy. Therefore, the P content is preferably 0.028% or less. The P content is more preferably 0.025% or less, and even more preferably 0.020% or less. On the other hand, it is preferable to reduce the P content as much as possible, but excessive reduction of the P content increases the manufacturing cost. Therefore, the P content is preferably 0.001% or more, more preferably 0.003% or more, and even more preferably 0.005% or more.

[0042] Ni: 29.5 to 35.5% Ni is an element that stabilizes the austenite structure and is an essential element for ensuring structural stability during long-term use. However, excessive Ni content increases manufacturing costs. Therefore, the Ni content is preferably 29.5 to 35.5%. The Ni content is more preferably 30.0% or more, and even more preferably 30.5% or more. Furthermore, the Ni content is more preferably 35.0% or less, and even more preferably 34.5% or less.

[0043] Cr: 18.5 to 23.5% Cr is an element that has the effect of improving strength and corrosion resistance. However, excessive Cr content deteriorates the stability of austenite at high temperatures. Therefore, the Cr content is preferably 18.5 to 23.5%. The Cr content is more preferably 19.0% or more, and even more preferably 19.5% or more. Furthermore, the Cr content is more preferably 23.0% or less, and even more preferably 22.5% or less.

[0044] Ti: 0.10 to 0.70% Ti is an element that forms stable carbides and contributes to improving intergranular corrosion resistance. However, excessive Ti content leads to a decrease in toughness. Therefore, the Ti content is preferably 0.10 to 0.70%. The Ti content is more preferably 0.11% or more, 0.13% or more, 0.15% or more, 0.17% or more, or 0.20% or more, and even more preferably 0.30% or more. Furthermore, the Ti content is more preferably 0.60% or less, and even more preferably 0.50% or less.

[0045] Al: 0.10 to 0.70% Al is an element that has a deoxidizing effect. However, if the Al content is excessive, the cleanliness of the alloy will be significantly deteriorated, and the hot workability and ductility will decrease. Therefore, the Al content is preferably 0.10 to 0.70%. The Al content is more preferably 0.20% or more, and even more preferably 0.30% or more. Furthermore, the Al content is more preferably 0.60% or less, and even more preferably 0.50% or less.

[0046] N: 0.0100% or less N is an element that contributes to increasing strength. However, excessive N content reduces toughness. Therefore, the N content is preferably 0.0100% or less. The N content is more preferably 0.0070% or less, and even more preferably 0.0050% or less. Furthermore, the N content is preferably 0.0010% or more, and more preferably 0.0020% or more.

[0047] In the chemical composition of the present invention, the balance is Fe and impurities. Here, the Fe content is preferably 35% or more and 50% or less. Furthermore, "impurities" refer to components that are mixed in during industrial steel production due to raw materials such as ore and scrap, and various factors in the manufacturing process, and are acceptable within a range that does not adversely affect the present invention.

[0048] The alloy tube of the present invention may further contain one or more elements selected from Cu, V, W, Nb, Co, Mo, B, Ca, Mg, and REM in the ranges shown below. Note that these elements are not necessarily essential for the member, so the lower limit of the content is 0%. The reasons for limiting each element will be explained below.

[0049] Cu: 0.76% or less Cu is an austenite-forming element. Therefore, Cu may be contained. However, excessive Cu content reduces hot workability. Therefore, the Cu content is preferably 0.76% or less. The Cu content is more preferably 0.60% or less, and even more preferably 0.50% or less. On the other hand, if the above effects are to be obtained, the Cu content is preferably 0.05% or more, and more preferably 0.10% or more.

[0050] V: 0.500% or less V is an element that contributes to improving strength. Therefore, V may be contained. However, since V is an expensive element, excessive V content increases costs. Therefore, the V content is preferably 0.500% or less. The V content is more preferably 0.400% or less, and even more preferably 0.300% or less. On the other hand, if the above effects are to be obtained, the V content is preferably 0.010% or more, and more preferably 0.050% or more.

[0051] W: 1.00% or less W is an element that contributes to improving strength. Therefore, W may be contained. However, since W is an expensive element, excessive W content increases costs. Therefore, the W content is preferably 1.00% or less. The W content is more preferably 0.80% or less, and even more preferably 0.60% or less. On the other hand, if the above effects are to be obtained, the W content is preferably 0.01% or more, and more preferably 0.05% or more.

[0052] Nb: 1.00% or less Nb is an element that generates stable carbides and contributes to improving intergranular corrosion resistance. Therefore, Nb may be contained. However, if the Nb content is excessive, toughness decreases. Therefore, the Nb content is preferably 1.00% or less. The Nb content is more preferably 0.80% or less, and even more preferably 0.60% or less. On the other hand, if the above effects are to be obtained, the Nb content is preferably 0.05% or more, and more preferably 0.10% or more.

[0053] Co: 1.00% or less Co is an element that contributes to improving strength. Therefore, Co may be contained. However, since Co is an extremely expensive element, excessive Co content will result in a significant increase in costs. Therefore, the Co content is preferably 1.00% or less. The Co content is more preferably 0.80% or less, and even more preferably 0.60% or less. On the other hand, if the above effects are to be obtained, the Co content is preferably 0.01% or more, and more preferably 0.05% or more.

[0054] Mo: 1.00% or less Mo is an element that contributes to improving corrosion resistance. Therefore, Mo may be contained. However, since Mo is an expensive element, excessive inclusion of Mo leads to increased costs. Therefore, the Mo content is preferably 1.00% or less. The Mo content is more preferably 0.80% or less, and even more preferably 0.70% or less. On the other hand, if the above effects are to be obtained, the Mo content is preferably 0.01% or more, and more preferably 0.05% or more.

[0055] B: 0.0400% or less B is an element that contributes to improving hot workability. Therefore, B may be contained. However, if the B content is excessive, not only will the weldability deteriorate, but the hot workability will also deteriorate. Therefore, the B content is preferably 0.0400% or less. The B content is more preferably 0.0300% or less, and even more preferably 0.0200% or less. On the other hand, if the above effects are to be obtained, the B content is preferably 0.0010% or more, and more preferably 0.0020% or more.

[0056] Ca: 0.0100% or less Ca may be added as needed because it forms a compound with S to reduce the amount of S in the matrix and improve hot workability. However, excessive Ca content combines with O, significantly reducing cleanliness and actually deteriorating hot workability. Therefore, the Ca content is preferably 0.0100% or less. The Ca content is more preferably 0.0080% or less. To achieve the above effects, the Ca content is preferably 0.0001% or more, more preferably 0.0005% or more, and even more preferably 0.0010% or more.

[0057] Mg: 0.0500% or less Like Ca, Mg forms a compound with S to reduce the amount of S in the matrix and improve hot workability, so it may be added as needed. However, if the Mg content is excessive, it bonds with O, significantly reducing cleanliness and actually deteriorating hot workability. Therefore, the Mg content is preferably 0.0500% or less. The Mg content is more preferably 0.0450% or less. To achieve the above effects, the Mg content is preferably 0.0001% or more, more preferably 0.0005% or more, and even more preferably 0.0010% or more.

[0058] REM: 0.100% or less Like Ca, REM forms compounds with S to reduce the amount of S in the matrix and improve hot workability, so it may be added as needed. However, if the REM content is excessive, it combines with O, significantly reducing cleanliness and actually deteriorating hot workability. Therefore, the REM content is preferably 0.100% or less. The REM content is more preferably 0.080% or less. To achieve the above effects, the REM content is preferably 0.0001% or more, more preferably 0.0002% or more, and even more preferably 0.0003% or more.

[0059] REM is a collective term for 17 elements, including Sc, Y, and lanthanides, and the REM content refers to the total content of one or more REM elements. REM is generally contained in misch metal. Therefore, for example, REM may be added in the form of misch metal, and the amount of REM may be adjusted to fall within the above range.

[0060] (D) Dimensions The dimensions of the alloy tube according to the present invention are not particularly limited. However, when used as a heat transfer tube, the outer diameter of the alloy tube is preferably 20.00 to 30.00 mm, and the wall thickness is preferably 2.00 to 5.00 mm.

[0061] (E) Manufacturing Method The manufacturing method of an alloy pipe according to the present invention involves subjecting a hot-extruded mother pipe to a first cold-rolling step, a softening heat treatment step, a second cold-rolling step, and a solution heat treatment step, in that order. The mother pipe preferably has the chemical composition described above. Each step will be described in detail below.

[0062] An ingot is produced by continuous casting or the like, and the produced ingot is then bloomed to produce a billet. The various conditions for producing the billet may be in accordance with conventional methods. The resulting billet is then hot extruded into a pipe shape. In the present invention, a pipe formed by hot extrusion in this manner is referred to as a mother pipe. The hot extrusion conditions are not particularly limited, but in order to prevent harmful defects from occurring during pipe production, the pipe is heated to a temperature in the range of 900 to 1300°C, for example, before hot extrusion.

[0063] <First Cold Rolling Step> In the first cold rolling step, the hot-extruded mother pipe is subjected to first cold rolling. The various conditions for the first cold rolling may be in accordance with conventional methods.

[0064] <Softening Heat Treatment Step> In the softening heat treatment step, the mother tube is subjected to softening heat treatment. The softening heat treatment step is carried out in an atmospheric furnace or a hydrogen furnace. In the softening heat treatment step, the mother tube is held at a softening heat treatment temperature of 1000°C or higher for 5 minutes or longer. In this case, as will be described later, the softening heat treatment temperature is set to be lower than the solution heat treatment temperature of 0°C.

[0065] Whether or not excellent smoothness can be ensured is greatly influenced by whether the solution heat treatment is performed in a hydrogen furnace, and is less influenced by the conditions of the softening heat treatment. Therefore, the softening heat treatment step may be performed in either an atmospheric furnace or a hydrogen furnace.

[0066] If the softening heat treatment temperature is less than 1000°C, softening will be insufficient, and Ti precipitates will not be sufficiently precipitated. Furthermore, if the holding time at the softening heat treatment temperature is less than 5 minutes, Ti precipitates will not be sufficiently precipitated. Therefore, in the softening heat treatment step, the mother tube is held at a softening heat treatment temperature of 1000°C or higher for 5 minutes or more. There is no particular upper limit to the holding time at the softening heat treatment temperature. However, if the holding time exceeds 15 minutes, the manufacturing cost will increase. Therefore, the holding time is preferably 15 minutes or less.

[0067] If the softening heat treatment temperature is higher than the solution heat treatment temperature, Ti precipitates will form in the solution heat treatment step, as described above for heat treatment method A, making it impossible to obtain crystal grains with an average grain size number of 5.0 or less. Therefore, the softening heat treatment temperature minus the solution heat treatment temperature is set to 0°C or less. The softening heat treatment temperature minus the solution heat treatment temperature is preferably -20°C or less, and more preferably -50°C or less.

[0068] In the present invention, the softening heat treatment temperature refers to the temperature inside the furnace in the softening heat treatment step. Holding at a softening heat treatment temperature of 1000°C or higher for 5 minutes or more means placing in a furnace at 1000°C or higher for 5 minutes or more. Furthermore, the solution heat treatment temperature refers to the temperature inside the furnace in the solution heat treatment step. Holding at a solution heat treatment temperature of 1140 to 1170°C for 10 minutes or more means placing in a furnace at 1140 to 1170°C for 10 minutes or more.

[0069] <Second Cold Rolling Step> In the second cold rolling step, the mother pipe is subjected to second cold rolling. The conditions for the second cold rolling may be in accordance with conventional methods.

[0070] <Solution Heat Treatment Step> In the solution heat treatment step, the mother pipe after the second cold rolling is subjected to solution heat treatment. The solution heat treatment step is performed in a hydrogen furnace. In the solution heat treatment step, the mother pipe is held at a solution heat treatment temperature of 1140 to 1170°C for 10 minutes or more.

[0071] When solution heat treatment is performed in an atmospheric furnace, scale forms on the surface of the alloy pipe, making pickling necessary. When alloy pipe is pickled, the surface roughness increases. Therefore, solution heat treatment is performed in a hydrogen furnace.

[0072] As mentioned above, hydrogen furnaces are generally used in a temperature range of 1170°C or less. On the other hand, if the solution heat treatment temperature is less than 1140°C, the crystal grains cannot be coarsened. Therefore, the solution heat treatment temperature is set to 1140 to 1170°C. Furthermore, if the holding time at the solution heat treatment temperature is less than 10 minutes, the crystal grains cannot be coarsened. Therefore, the holding time at the solution heat treatment temperature is set to 10 minutes or more. There is no particular upper limit to the holding time at the solution heat treatment temperature. However, if the holding time exceeds 15 minutes, the manufacturing cost increases. Therefore, it is preferable that the holding time be 15 minutes or less.

[0073] The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to these examples.

[0074] A mother pipe having the chemical composition shown in Table 1 was produced by hot extrusion. The mother pipe was then subjected to first cold rolling with the area reduction shown in Table 2, and softening heat treatment was performed at the softening heat treatment temperature in the type of furnace shown in Table 2. After second cold rolling with the area reduction shown in Table 2, solution heat treatment was performed at the solution heat treatment temperature in the type of furnace shown in Table 2. After water quenching, alloy pipes having the outer diameter and wall thickness shown in Table 2 were obtained.

[0075]

[0076]

[0077] The alloy pipes thus obtained were subjected to measurement of the average grain size number and root mean square roughness RMS, and a creep test was carried out, according to the procedures described below.

[0078] <Average grain size number> The average grain size number was calculated in accordance with ASTM E112:2013. Specifically, a test piece for microstructure observation was taken so that the cross section parallel to the axial direction and wall thickness direction of the alloy pipe was the observation surface, and the observation surface was mirror-polished. After polishing, the specimen was corroded with mixed acid and observed under an optical microscope. First, when the wall thickness of the alloy pipe is d, a total of 10 measurement areas were set, each 1 / 10 d, from the outer surface to the inner surface of the alloy pipe. Then, each measurement area was observed at a magnification of 100 times, and the grain size number of each measurement area was determined using the comparison method specified in ASTM E112:2013, and the average value was taken as the average grain size number of the alloy pipe.

[0079] However, when coarse grains and fine grains are mixed within one measurement area, the average grain size number cannot be determined using the comparison method. Also, there is a risk that the creep strength of the fine grains will decrease. Therefore, in such cases, the average grain size number cannot be evaluated.

[0080] <Root Mean Square Roughness RMS> The root mean square roughness RMS was measured in accordance with ASTM B46.1:2019. Specifically, a 100 mm measurement sample was taken from the alloy pipe as is, and the longitudinal direction (axial direction) was measured. The tip shape of the stylus used for the measurement was spherical with a radius of 10 μm, and the maximum measurement load was 15 mN. The scanning speed of the jig holding the stylus was 0.1 mm / sec, and the scanning distance was 4 mm. The measured profile Z(x) was used in the above formula (i) to calculate Rq(RMS).

[0081] The results are summarized in Table 2. Note that the "-" in the root mean square roughness RMS for Test Nos. 2 and 4 means that the values ​​were expected to be the same as those for Test Nos. 1 and 3, respectively, and therefore measurement was not performed.

[0082] Test Nos. 5, 8, and 9, which satisfied the requirements of the present invention, had an average grain size number of 5.0 or less and a root mean square roughness RMS of 1.6 μm or less. On the other hand, Test Nos. 1 to 4, 6, and 7, which did not satisfy the requirements of the present invention, were inferior in both or one of the average grain size number and the root mean square roughness RMS.

[0083] In particular, the average grain size numbers of Test Nos. 1 to 4 were undeterminable. Figures 3 to 5 are diagrams illustrating cases where the average grain size number is undeterminable. Figure 3 is a microphotograph of the alloy pipe of Test No. 1, Figure 4 is a microphotograph of the alloy pipe of Test No. 2, and Figure 5 is a microphotograph of the alloy pipe of Test No. 5. As shown in Figure 5, the crystal grains of Test No. 5, an example of the present invention, are approximately the same size. On the other hand, as shown in Figures 3 and 4, the comparative examples of Test Nos. 1 and 2 have areas where coarse crystal grains exist and areas where fine crystal grains are concentrated. Thus, in addition to Test Nos. 1 and 2, Test Nos. 3 and 4, the average grain size numbers could not be determined using the above-mentioned comparison method, and therefore were deemed undeterminable.

[0084] According to the present invention, it is possible to obtain a Ni--Fe--Cr based alloy pipe having high creep strength and excellent smoothness.

Claims

1. A Ni-Fe-Cr based alloy pipe, wherein the average grain size number in a cross section parallel to the axial direction and wall thickness direction of said alloy pipe is 5.0 or less, and the root mean square roughness (RMS) of the inner and outer surfaces of said alloy pipe is 1.60 μm or less.

2. The Ni-Fe-Cr base alloy tube according to claim 1, wherein the chemical composition of the Ni-Fe-Cr base alloy tube is, in mass%, C: 0.040 to 0.110%, Si: 1.10% or less, Mn: 1.60% or less, S: 0.016% or less, P: 0.028% or less, Ni: 29.5 to 35.5%, Cr: 18.5 to 23.5%, Ti: 0.10 to 0.70%, Al: 0.10 to 0.70%, and N: 0.0100% or less, with the balance being Fe and impurities.

3. A Ni-Fe-Cr based alloy pipe according to claim 1 or claim 2, wherein the chemical composition of the Ni-Fe-Cr based alloy pipe contains, in place of a portion of the Fe, one or more elements selected from, by mass%, Cu: 0.76% or less, V: 0.500% or less, W: 1.00% or less, Nb: 1.00% or less, Co: 1.00% or less, Mo: 1.00% or less, B: 0.0400% or less, Ca: 0.0100% or less, Mg: 0.0500% or less, and REM: 0.100% or less.

4. A method for producing a Ni-Fe-Cr based alloy pipe as set forth in claim 1, comprising the steps of: subjecting a hot-extruded mother pipe to a first cold rolling step, a softening heat treatment step, a second cold rolling step, and a solution heat treatment step in that order; the softening heat treatment step is carried out in a hydrogen furnace or an atmospheric furnace; and the solution heat treatment step is carried out in a hydrogen furnace; in the softening heat treatment step, the mother pipe is held at a softening heat treatment temperature of 1000°C or higher for 5 minutes or more; and in the solution heat treatment step, the mother pipe is held at a solution heat treatment temperature of 1140 to 1170°C for 10 minutes or more; and the softening heat treatment temperature - the solution heat treatment temperature is 0°C or lower.

5. A method for producing a Ni-Fe-Cr based alloy pipe according to claim 4, wherein the softening heat treatment temperature minus the solution heat treatment temperature is −20° C. or less.

6. A method for producing a Ni-Fe-Cr based alloy pipe according to claim 2, wherein the raw pipe has a chemical composition, in mass%, of C: 0.040 to 0.110%, Si: 1.10% or less, Mn: 1.60% or less, S: 0.016% or less, P: 0.028% or less, Ni: 29.5 to 35.5%, Cr: 18.5 to 23.5%, Ti: 0.10 to 0.70%, Al: 0.10 to 0.70%, N: 0.0100% or less, and the balance: Fe and impurities.

7. A method for producing a Ni-Fe-Cr based alloy pipe as set forth in claim 3, wherein the chemical composition of the mother pipe contains, in place of a portion of the Fe, one or more elements selected from, by mass%, Cu: 0.76% or less, V: 0.500% or less, W: 1.00% or less, Nb: 1.00% or less, Co: 1.00% or less, Mo: 1.00% or less, B: 0.0400% or less, Ca: 0.0100% or less, Mg: 0.0500% or less, and REM: 0.100% or less.

8. A method for producing a Ni-Fe-Cr based alloy pipe according to claim 3, wherein the chemical composition of the mother pipe contains, in place of a portion of the Fe, one or more elements selected from, by mass%, Cu: 0.76% or less, V: 0.500% or less, W: 1.00% or less, Nb: 1.00% or less, Co: 1.00% or less, Mo: 1.00% or less, B: 0.0400% or less, Ca: 0.0100% or less, Mg: 0.0500% or less, and REM: 0.100% or less.

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