Stainless steel pipe and method of manufacturing the same

Abstract

The present invention aims to provide a stainless steel pipe having excellent corrosion resistance, high tensile yield strength in the pipe axis direction, a small difference between the tensile yield strength and the compression yield strength in the pipe axis direction, and further excellent fatigue properties of the threaded portion, and a method for manufacturing the same. A stainless steel pipe having a prescribed composition, a tensile yield strength in the pipe axis direction of 689 MPa or more, a compression yield strength in the pipe axis direction / tensile yield strength in the pipe axis direction of 0.85 to 1.15, and a structure having a ferrite phase of 20 to 80% by volume fraction and an austenite phase as the remainder, and a fastening portion having an external thread or an internal thread at at least one of the pipe end portions, wherein the curvature radius R of the corner portion formed by the thread root surface and the pressure side surface of the pipe axis cross section of the fastening portion is 0.2 mm or more.

Description

[0001] This application is a divisional application of patent application 202180016166.0 (international application date: January 20, 2021, invention title: stainless steel pipe and manufacturing method thereof). Technical Field

[0002] This invention relates to a stainless steel tube exhibiting excellent corrosion resistance, high axial tensile yield strength, a small difference between axial tensile yield strength and compressive yield strength, and excellent fatigue characteristics of the threaded portion, as well as a method for manufacturing the same. It should be noted that high axial tensile yield strength refers to a axial tensile yield strength of 689 MPa or higher, and a small difference between axial tensile yield strength and compressive yield strength refers to a axial compressive yield strength / axial tensile yield strength ratio in the range of 0.85 to 1.15. Background Technology

[0003] Duplex stainless steel pipes possess excellent corrosion resistance, strength, and low-temperature toughness, making them suitable for piping in chemical equipment, oil and gas well extraction, and transportation. In recent years, duplex stainless steel pipes have been researched and applied in geothermal power generation for extraction and heat exchange, and have been partially implemented. Duplex stainless steel pipes are used for extraction of resources such as gases, oils, or hot water at high depths; therefore, high strength to withstand heavy loads and high pressures, as well as corrosion resistance to harsh corrosive environments such as gases, oils, and hot water, are crucial. For corrosion resistance, the amount of corrosion-enhancing elements such as Cr, Mo, W, and N added to the steel is important. Examples include duplex stainless steels such as SUS329J3L containing 22% Cr, SUS329J4L containing 25% Cr, and ASTM UNS S32750 and S32760 with a large amount of Mo.

[0004] On the other hand, regarding strength characteristics, the most important is the axial tensile yield strength, which is the representative value of the product's strength specifications. The reason is that when connecting pipes to high depths in resource and hot water extraction, the ability to withstand the tensile stress of the pipe's own weight is paramount. By possessing a sufficiently large axial tensile yield strength to withstand the tensile stress caused by its own weight, plastic deformation can be suppressed, preventing damage to the passivation coating on the pipe surface, which is crucial for maintaining corrosion resistance.

[0005] Among the strength specifications of products, the most important is the axial tensile yield strength, while the axial compressive yield strength of the pipe joint is also important. For pipes used in oil and gas wells, considering fire prevention and the repeated insertion and removal of pipes for hot water extraction, welding is not used in the joints; instead, threaded fastening is employed. Furthermore, for chemical piping and resource transportation piping, threaded fastening is sometimes also used to simplify welding operations that require time and labor. Therefore, the axial compressive stress in the thread due to the fastening force is repeatedly generated due to thread re-tightening and bending deformation of the fastening part. Therefore, the axial compressive yield strength capable of withstanding this compressive stress is also important.

[0006] Duplex stainless steel tubes consist of a ferrite phase and an austenite phase with low yield strength in their crystalline structure. However, the strength required for various applications cannot be guaranteed in the hot-formed and heat-treated state during the manufacturing process. Therefore, to obtain the required strength, dislocation strengthening based on various cold rolling processes is utilized to increase the tensile yield strength in the tube axis direction. Cold rolling methods for duplex stainless steel tubes are limited to cold drawing and Pilger rolling. In the NACE (National Association of Corrosion Engineers) international standard concerning oil well tubing applications, only cold drawing and cold pilgering are permitted. Both types of cold rolling involve thinning and shrinking the tube to extend it along its long side, thus dislocation strengthening based on deformation is more effective in increasing the tensile yield strength in the long side direction. On the other hand, it is known that in these cold rolling processes that deform the tube along its long side, the compressive yield strength in the tube axis direction decreases by about 20% due to the strong Bauschinger effect generated in the tube axis direction. Therefore, in threaded fasteners that require compressive yield strength characteristics in the axial direction, the Bauschinger effect is generally taken as a prerequisite, resulting in a low yield strength design. This affects the overall product specifications in the strength design of the threaded fastener.

[0007] Regarding these issues, Patent Document 1 proposes a duplex stainless steel tube, characterized by containing, by mass%, 0.008–0.03% C, 0–1% Si, 0.1–2% Mn, 20–35% Cr, 3–10% Ni, 0–4% Mo, 0–6% W, 0–3% Cu, and 0.15–0.35% N, with the remainder consisting of iron and impurities. The duplex stainless steel tube exhibits a tensile yield strength YS of 689.1–1000.5 MPa in the axial direction. LT Tensile yield strength YS LT Compressive yield strength YS in the axial direction LCThe tensile yield strength YS in the circumferential direction of the above-mentioned duplex stainless steel pipe CT Compressive yield strength YS in the circumferential direction CC It satisfies the prescribed formula.

[0008] Furthermore, fatigue strength is also important in duplex stainless steel pipes. Fatigue-induced failure occurs under repeated stresses below the yield stress, and duplex stainless steel pipes experience fatigue-induced failure due to stresses such as bending during construction or pressure from fluids flowing inside. Therefore, to improve fatigue strength, it is important to reduce stress concentration points such as surface depressions, marks, and large inclusions that could become initiators of fatigue cracks. On the other hand, when threads are used to fasten steel pipes, stress concentration points inevitably arise. That is, the threads are tightened by pressure generated on the sides of the external and internal threads. At the root of the convex portion forming the side threads, a bending moment is generated during tightening according to the reaction force, resulting in repeated compressive and tensile stresses with each tightening and loosening of the threads. In addition, for example, in the extraction of oil, gas, and hot water, the threaded steel pipe is inserted into the well while being rotated, and thus, due to the eccentricity of the rotating shaft, the entire steel pipe, including the threaded portion, is repeatedly bent. At this time, compressive and tensile stresses are repeatedly applied to the root of the convex portion of the threaded portion, which is a stress concentration point. Therefore, special care must be taken at the root of the protrusion of the threaded portion, which is a stress concentration point, to prevent damage caused by fatigue.

[0009] Regarding the issue of fatigue strength of the threaded portion, reference 2 discloses a seamless steel pipe for oil wells made of stainless steel with a specified composition and matrix structure, wherein the radius of curvature R of each arc surface is set to 0.3 mm or more, and the threaded portion exhibits excellent fatigue strength.

[0010] Existing technical documents

[0011] Patent documents

[0012] Patent Document 1: Japanese Patent No. 5500324

[0013] Patent Document 2: Japanese Patent No. 6604093 Summary of the Invention

[0014] The threaded portion experiences both compressive and tensile stresses. Therefore, in duplex stainless steel tubes, due to conventional cold rolling, the compressive yield stress decreases due to the Bauschinger effect, leading to a reduction in fatigue life. To suppress the decrease in compressive yield strength caused by the Bauschinger effect in the threaded portion, the low-temperature heat treatment disclosed in Patent Document 1 is effective. However, if such a low-temperature heat treatment is used, elements important for corrosion resistance will precipitate as embrittlement phases containing carbonitrides and nitrogen, thus losing their corrosion resistance effect. Therefore, in duplex stainless steel tubes, it is impossible to simultaneously achieve both the fatigue characteristics of the threaded portion and corrosion resistance.

[0015] Furthermore, Patent Document 2 limits the materials to those with chemical compositions solidified by quenching heat treatment, and does not include duplex stainless steel tubes that require dislocation strengthening through cold working. In other words, Patent Document 2 does not consider the problem of reduced fatigue characteristics of the threaded portion due to the decrease in compressive yield strength associated with the Bauschinger effect in duplex stainless steel tubes.

[0016] The present invention was made in view of the above-mentioned actual situation, and its purpose is to provide a stainless steel pipe with excellent corrosion resistance, high tensile yield strength in the axial direction, small difference between tensile yield strength and compressive yield strength in the axial direction, and excellent fatigue characteristics of the threaded part, as well as a method for manufacturing the same.

[0017] Duplex stainless steel achieves a highly corrosion-resistant film and inhibits the progression of localized corrosion by increasing the solid solution content of Cr and Mo in the steel. Furthermore, maintaining a suitable duplex phase ratio of ferrite and austenite in the microstructure is crucial for protecting the material from various corrosion modalities. However, Cr and Mo, the main corrosion-resistant elements, are both ferrite phase-forming elements; simply increasing their amounts will not achieve the desired duplex phase ratio. Therefore, appropriate amounts of austenite phase-forming elements are necessary. These elements include C, N, Mn, Ni, and Cu. Increasing the amount of C in the steel deteriorates corrosion resistance, so its content should be limited to a minimum, typically below 0.08% in duplex stainless steel. Other austenite phase-forming elements are inexpensive to add, and in the solution-treated state, N, which enhances corrosion resistance, is often used.

[0018] Here, duplex stainless steel tubes incorporate corrosion-resistant elements dissolved in the steel, and with the phase fraction set to a suitable duplex state, a solution heat treatment at a high temperature of 1000°C or higher is performed after hot forming. Furthermore, dislocation strengthening is achieved through cold rolling when high strength is required. When products are manufactured in either the solution heat-treated or cold-rolled state, the corrosion-resistant elements are dissolved in the steel, exhibiting high corrosion resistance.

[0019] When it is necessary to suppress the reduction in compressive yield strength of threaded fasteners based on the Bauschinger effect, low-temperature heat treatment, as shown in Patent Document 1, is effective. However, in the case of low-temperature heat treatment, the elements dissolved in the steel diffuse due to the solidification heat treatment. As a result, elements important for corrosion resistance precipitate and are consumed in the form of embrittled phases containing carbonitrides or nitrogen, thus losing their corrosion resistance effect. In this case, it can be considered that the intentional addition of large amounts of nitrogen, either through atmospheric melting or in combination with other additive metallic elements, has a negative impact. That is, nitrogen has a small atomic size and easily diffuses even under low-temperature heat treatment, combining with surrounding corrosion-resistant elements and causing the corrosion-resistant effect to disappear.

[0020] Therefore, the inventors conducted various investigations into the precipitation of carbonitrides during low-temperature heat treatment, and the fact that a large amount of N added relative to a trace amount of C can lead to a decrease in corrosion resistance due to nitride formation. The results yielded the following insights.

[0021] First, the relationship between the amount of nitrogen and the amount of nitride during heat treatment was investigated. Figure 1 , 2 The figure shows the thermal balance calculation of SUS329J3L (22% Cr stainless steel). Figure 1 ) and SUS329J4L (25% Cr stainless steel, Figure 2 The values ​​were obtained by considering the nitrogen content and the amount of nitride precipitation of Cr and Mo after low-temperature heat treatment (590℃). Without heat treatment, no nitride formation with corrosion-resistant elements was observed; the nitrides remained completely dissolved in the steel. Furthermore, for heat treatment temperatures of 150–450℃, the... Figure 1 , 2 Similarly, nitride levels increase with increasing nitrogen (N). Low-temperature heat treatment confirmed that a large portion of the precipitated nitrides are Cr- and Mo-based, both crucial elements for corrosion resistance. Furthermore, nitride levels increase with increasing N in all steel grades, consuming more corrosion-resistant elements as precipitates. That is, when N is not heat-treated (only solution treatment), it dissolves in the steel and combines with other corrosion-resistant elements to improve corrosion resistance. However, with low-temperature heat treatment, nitride levels increase proportionally to the increase in N content. It is believed that the accompanying consumption of corrosion-resistant elements by N reduces its concentration in the steel, thus causing a decrease in corrosion resistance. Additionally, it is thought that excessive addition of N with corrosion-resistant elements other than Cr and Mo (such as W) also forms nitrides, reducing corrosion resistance.

[0022] According to Patent Document 1, in addition to cold drawing and rolling, low-temperature heat treatment is also a necessary condition. That is, the method in Patent Document 1 utilizes conventional cold drawing and Pierce cold rolling, thus failing to prevent the Bauschinger effect along the tube axis. Heat treatment is used to mitigate the yield strength anisotropy resulting from the Bauschinger effect. However, the method in Patent Document 1, which involves heat treatment in addition to cold drawing and rolling, results in a decrease in corrosion-resistant elements in the steel, leading to a reduction in corrosion resistance. Specifically, for the corrosion resistance of duplex stainless steel tubes, not only are the amounts of corrosion-resistant elements such as Cr, Mo, W, and N dissolved in the steel important, but also, due to the heat treatment performed to reduce the Bauschinger effect, these corrosion-resistant elements precipitate as nitrides. As a result, the amount of dissolved N decreases, leading to a decrease in corrosion resistance.

[0023] Furthermore, in order to clarify the relationship between nitrogen content and corrosion resistance, the inventors evaluated the stress corrosion resistance under varying nitrogen content. Figure 1 The composition system was modified by adjusting the nitrogen content to 0.050, 0.110, 0.149, 0.152, 0.185, and 0.252%, then melting and thermoforming the mixture. Following this, a solution heat treatment at 1050℃ and cold working were performed to adjust the yield strength to 865–931 MPa, resulting in 4-point bending corrosion test pieces. The stress corrosion resistance of each test piece was compared under two conditions: no heat treatment and heat treatment at 400℃.

[0024] The load stress condition based on four-point bending was 90% of the yield strength. The corrosion environment was an aqueous solution simulating a sulfide corrosion environment (20% NaCl + 0.5% CH3COOH + CH3COONa aqueous solution with added H2S gas, pH adjusted to 3.5, and test temperature 25°C). In the investigation, the samples were immersed in the corrosive solution for 720 hours under stress, and the nitrogen content was compared with the corrosion state after the test. The results showed that no corrosion occurred without heat treatment. On the other hand, with heat treatment, no corrosion occurred up to a nitrogen content of 0.149%, but at 0.152%, micro-pore corrosion and crack formation were confirmed, and further, at nitrogen contents above this level, large-scale fracture propagation was confirmed. Observing the corroded areas, fracture originated from nitride precipitation along the grain boundaries of the material microstructure. During heat treatment, corrosion-resistant elements near the grain boundaries, where diffusion rates are faster, preferentially became nitrides and were consumed, and the local reduction in the solid solution content of corrosion-resistant elements contributed to the formation of pore corrosion. Therefore, the maximum value of N is determined to be less than 0.150%.

[0025] For the fatigue strength of the threaded portion, the compressive yield strength in the tube axis direction is crucial. That is, stress concentration points are unavoidable in the threaded portion. Depending on the tightness of the thread and the utilization of the steel pipe in the tightened state, tensile and compressive stresses repeatedly act in the tube axis direction relative to the stress concentration points. If the compressive yield stress in the tube axis direction decreases due to the Bauschinger effect, the compressive yield stress in the tube axis direction is relatively lower relative to the stress concentration points, resulting in a decrease in fatigue strength. Furthermore, if the stress at the stress concentration points exceeds the reduced compressive yield stress in the tube axis direction due to the Bauschinger effect, plastic deformation occurs at the stress concentration points, further reducing fatigue life. On the other hand, as mentioned above, in the case of duplex stainless steel pipes, there is a problem that the fatigue characteristics of the threaded portion decrease due to the reduced compressive yield strength associated with the Bauschinger effect. Therefore, the inventors have conducted in-depth research to maintain the corrosion resistance of duplex stainless steel pipes while also satisfying the repeatability of fatigue characteristics in the threaded portion. The result is that the difference between the tensile yield strength and compressive yield strength in the axial direction of the tube is reduced and the strength ratio is controlled. Furthermore, in the threaded fastening section, the radius of curvature of the corner formed by the thread valley surface and the pressure side surface is set to 0.2 mm or more, thereby obtaining a stainless steel tube that combines corrosion resistance and fatigue characteristics of the threaded section.

[0026] The present invention was completed based on the above circumstances, and its main points are as follows.

[0027] [1] A stainless steel pipe having the following composition:

[0028] It contains, by mass percent, C: 0.005–0.08%, Si: 0.01–1.0%, Mn: 0.01–10.0%, Cr: 20–35%, Ni: 1.0–15%, Mo: 0.5–6.0%, and N: more than 0.005% but less than 0.150%, with the remainder consisting of Fe and unavoidable impurities.

[0029] The stainless steel tube has the following structure:

[0030] The axial tensile yield strength is above 689 MPa.

[0031] The ratio of compressive yield strength to tensile yield strength in the axial direction is 0.85–1.15.

[0032] The microstructure consists of a ferrite phase comprising 20–80% by volume and an austenite phase as the remainder.

[0033] The pipe end of at least one of the pipes has a fastening part with external or internal threads, and the radius of curvature R of the corner R formed by the thread valley surface and the pressure side surface of the fastening part is set to 0.2 mm or more.

[0034] [2] According to the stainless steel pipe described in [1], the ratio of the compressive yield strength in the circumferential direction to the tensile yield strength in the axial direction is 0.85 or higher.

[0035] [3] The stainless steel tube according to [1] or [2], wherein it further contains, by mass %, one or two of the following: W: less than 6.0% and Cu: less than 4.0%.

[0036] [4] The stainless steel tube according to any one of [1] to [3], wherein it further contains, by mass %, one or more of the following: Ti: less than 0.50%, Al: less than 0.30%, V: less than 0.55%, Nb: less than 0.75%.

[0037] [5] The stainless steel pipe according to any one of [1] to [4], wherein it further contains, by mass %, one or more of the following: B: less than 0.010%, Zr: less than 0.10%, Ca: less than 0.010%, Ta: less than 0.3%, REM: less than 0.10%, Mg: less than 0.10%.

[0038] [6] The stainless steel pipe according to any one of [1] to [5], wherein it further contains, by mass %, one or more of Sn: less than 0.30%, Sb: less than 0.30%, and Ag: less than 0.30%.

[0039] [7] The stainless steel pipe according to any one of [1] to [6], wherein the stainless steel pipe is a seamless steel pipe.

[0040] [8] The stainless steel tube according to any one of [1] to [7], wherein the radius of curvature of the corner R is 0.3 mm or more.

[0041] [9] The stainless steel tube according to [8], wherein the fastening part has a metal contact seal and a torque shoulder.

[0042]

[10] A method for manufacturing a stainless steel pipe is the method for manufacturing a stainless steel pipe as described in any one of [1] to [9], wherein a stretching process is performed in the direction of the pipe axis, and then a heat treatment is performed at a heating temperature of 150 to 600°C excluding 460 to 480°C.

[0043]

[11] A method for manufacturing a stainless steel pipe, which is the method for manufacturing a stainless steel pipe as described in any one of [1] to [9], wherein a stretching process is performed in the direction of the pipe axis at a processing temperature of 150 to 600°C, excluding 460 to 480°C.

[0044]

[12] According to the manufacturing method of stainless steel pipe described in

[11] , after the above stretching process, heat treatment is further performed at a heating temperature of 150 to 600°C, excluding 460 to 480°C.

[0045]

[13] A method for manufacturing a stainless steel pipe, which is the method for manufacturing a stainless steel pipe as described in any one of [1] to [9], wherein the pipe is bent and re-bent in the circumferential direction.

[0046]

[14] In the manufacturing method of stainless steel pipe according to

[13] , the processing temperature of the bending-back bending process in the circumferential direction of the pipe is below 600°C, excluding 460 to 480°C.

[0047]

[15] The manufacturing method of stainless steel pipe according to

[13] or

[14] , wherein after the above-mentioned bending-back bending process, it is further heat-treated at a heating temperature of 150 to 600°C excluding 460 to 480°C.

[0048] Invention Effects

[0049] According to the present invention, a stainless steel pipe with high corrosion resistance, high tensile yield strength in the axial direction, and a small difference between the tensile yield strength and compressive yield strength in the axial direction, resulting in excellent fatigue strength characteristics of the threaded portion, can be obtained. Therefore, with the stainless steel pipe of the present invention, thread fastening operations during use in severely corrosive environments and construction of oil wells and gas wells become easier, and the shape design of the thread fastening portion also becomes easier. Attached Figure Description

[0050] Figure 1 This is a graph showing the relationship between the amount of nitrogen (N) and the amount of Cr and Mo nitrides during low-temperature heat treatment in SUS329J3L (22% Cr stainless steel).

[0051] Figure 2 This is a graph showing the relationship between the amount of nitrogen (N) and the amount of Cr and Mo nitrides during low-temperature heat treatment in SUS329J4L (25% Cr stainless steel).

[0052] Figure 3 This is a sectional view (parallel to the pipe axis) of the fastening part of the external and internal threads. Figure 1 (a) is the case of an angular thread. Figure 1 (b) is the case of a trapezoidal thread. Figure 1 (c) is the case of a triangular thread.

[0053] Figure 4 This is a sectional view of the threaded connector along the pipe axis (a sectional view parallel to the pipe axis). Figure 4 (a) is the case of an API threaded connector. Figure 4 (b) is the case of a special thread.

[0054] Figure 5 This is a schematic diagram of the area near the head of the extension of the pin. Figure 5 (a) is a cut-off sectional view with the pin parallel to the tube axis of the coupling fastener. Figure 5 (b) is the torque shoulder of the threaded front end of the pin as viewed from the front.

[0055] Figure 6 This is a schematic diagram illustrating the bending-back bending process in the circumferential direction of the pipe. Detailed Implementation

[0056] The present invention will now be described.

[0057] First, the reasons for defining the composition of the steel pipe of the present invention will be explained. Hereinafter, unless otherwise specified, mass % will be expressed as %.

[0058] C: 0.005~0.08%

[0059] Carbon (C) is an austenite phase-forming element, and its appropriate content helps to rationalize the phase fraction. However, excessive content leads to a decrease in corrosion resistance due to carbide formation. Therefore, the upper limit for C is below 0.08%. The decrease in austenite phase associated with a decrease in C content can be compensated by other austenite phase-forming elements, so there is no need to set a specific lower limit. However, if the C content is too low, the decarburization cost during melting increases, so it is above 0.005%.

[0060] Si: 0.01~1.0%

[0061] Si has a deoxidizing effect on steel, and therefore its appropriate content in molten steel is very effective. However, the presence of a large amount of Si in the steel can impair workability and low-temperature toughness. Therefore, the upper limit for Si is 1.0% or less. Since excessively reducing the amount of Si after deoxidation would lead to increased manufacturing costs, the lower limit is set at 0.01% or more. It should be noted that, from the viewpoint of achieving sufficient deoxidation while suppressing the side effects caused by excessive Si residue in the steel, Si is preferably 0.2% or more, and more preferably 0.8% or less.

[0062] Mn: 0.01~10.0%

[0063] Mn is a strong austenite-forming element and is much cheaper than other austenite-forming elements. Furthermore, even with low-temperature heat treatment, it does not consume corrosion-resistant elements like C and N. In addition, Mn is very effective at neutralizing sulfur (S), an impurity element mixed into molten steel; even trace amounts can fix S, which causes significant deterioration in the corrosion resistance and toughness of steel, into the form of MnS. From these perspectives, the Mn content should be 0.01% or more. On the other hand, excessive Mn content reduces low-temperature toughness. Therefore, it is 10.0% or less. To avoid compromising low-temperature toughness, it is preferable to be less than 1.0%. Considering both low-temperature toughness and cost reduction, to fully utilize Mn as an austenite-forming element, it is preferable to be 2.0% or more, and more preferably 8.0% or less.

[0064] Cr: 20-35%

[0065] Cr is the most important element for stabilizing the passivation film on steel and improving its corrosion resistance. Stainless steel pipes used in harsh corrosive environments require a Cr content of 20% or more. While higher Cr content contributes to improved corrosion resistance, a content exceeding 35% leads to the precipitation of embrittled phases during the melting-to-solidification process, causing overall cracking and making subsequent forming and processing difficult. Therefore, the upper limit is 35% or less. It should be noted that, from the viewpoint of ensuring both corrosion resistance and manufacturability, a preferred range is 21.5% or more, and more preferably 28.5% or less.

[0066] Ni: 1.0~15%

[0067] Ni is a strong austenite-forming element and improves the low-temperature toughness of steel. When using Mn, an inexpensive austenite-forming element, Ni should be actively utilized when low-temperature toughness is a concern; the minimum Ni content is 1.0% or more. On the other hand, Ni is the highest-valence austenite-forming element, and increasing its content leads to higher manufacturing costs. Therefore, the maximum Ni content is 15% or less. It should be noted that for applications where low-temperature toughness is not a problem, a Ni content in the range of 1.0% to 5% is preferable, preferably added in combination with other elements. On the other hand, when high and low-temperature toughness are required, the active addition of Ni is very effective, preferably 5% or more, and more preferably 13% or less.

[0068] Mo: 0.5–6.0%

[0069] Mo (Mo) content improves the pitting corrosion resistance of steel. Therefore, it is added in appropriate amounts depending on the corrosive environment. On the other hand, excessive Mo content causes embrittlement phases to precipitate during solidification of the molten steel, resulting in numerous fractures in the solidified structure and significantly impairing subsequent forming stability. Therefore, the upper limit for Mo content is 6.0% or less. To maintain stable corrosion resistance in sulfide environments, the Mo content needs to be 0.5% or more. It should be noted that, from the viewpoint of balancing the required corrosion resistance and manufacturing stability of stainless steel pipes, the Mo content is preferably 1.0% or more, and more preferably 5.0% or less.

[0070] N: ≥0.005% and <0.150%

[0071] Nitrogen (N) is a strong austenite phase-forming element and is inexpensive. Furthermore, it is a corrosion-resistant element in monomers and can be actively utilized. However, when subjected to low-temperature heat treatment after solution treatment, excessive addition of N leads to nitride precipitation, resulting in decreased corrosion resistance due to the consumption of corrosion-resistant elements. Therefore, the upper limit for the N content is less than 0.150%. It should be noted that there is no particular limitation on the lower limit of the N content, but if the N content becomes too low, the melting process becomes complicated, leading to reduced productivity. Therefore, the lower limit is 0.005% or more. It should also be noted that the presence of N within a range where corrosion resistance is not a problem suppresses the content of Ni, Mn, and Cu, which are other austenite phase-forming elements, leading to cost reduction. Therefore, the N content is preferably 0.08% or more, and preferably 0.14% or less.

[0072] The remainder consists of Fe and unavoidable impurities. It should be noted that unavoidable impurities include P (below 0.05%), S (below 0.05%), and O (below 0.01%). P, S, and O are impurities that inevitably mix in during refining. Excessive residual amounts of these elements as impurities lead to various problems such as reduced hot workability, reduced corrosion resistance, and reduced low-temperature toughness. Therefore, they need to be controlled to below 0.05% for P, below 0.05% for S, and below 0.01% for O, respectively.

[0073] In addition to the above-mentioned components, the present invention may contain the following elements as needed.

[0074] Selected from one or two of the following: W: less than 6.0% and Cu: less than 4.0%.

[0075] W: Less than 6.0%

[0076] Like Mo, W improves pitting corrosion resistance depending on its content, but excessive W content can impair workability during hot working and manufacturing stability. Therefore, the upper limit for W content is less than 6.0%. No specific lower limit needs to be set for W content; for the sake of stabilizing the corrosion resistance of stainless steel pipes, a content of 0.1% or more is preferred. It should be noted that, from the viewpoint of the required corrosion resistance and manufacturing stability of stainless steel pipes, a W content of 1.0% or more is more preferred, and 5.0% or less is even more preferred.

[0077] Cu: less than 4.0%

[0078] Cu is a strong austenite-forming element and improves the corrosion resistance of steel. Therefore, it should be actively utilized among other austenite-forming elements such as Mn and Ni when corrosion resistance is insufficient. On the other hand, excessive Cu content leads to reduced hot workability and difficulty in forming. Therefore, when present, Cu content should be less than 4.0%. A lower limit for Cu content is not specifically specified; a content of 0.1% or more is sufficient to achieve corrosion resistance. It should be noted that, from the viewpoint of simultaneously improving corrosion resistance and hot workability, a Cu content of 1.0% or more is more preferably 3.0% or less.

[0079] The present invention may further include, as needed, the elements described below.

[0080] Selected from one or more of the following: Ti: less than 0.50%, Al: less than 0.30%, V: less than 0.55%, and Nb: less than 0.75%.

[0081] Adding appropriate amounts of Ti, Al, V, and Nb will combine with excess N, reducing the amount of N dissolved in the steel, inhibiting the combination of corrosion-resistant elements with N, and improving corrosion resistance. These components can be added individually or in combination, and can be utilized appropriately. There is no specific lower limit for the amount of these components added, but all can provide corrosion resistance at a concentration of 0.0001% or more. However, excessive addition increases the cost of the alloy; therefore, it is preferable to set upper limits for Ti: 0.50% or less, Al: 0.30% or less, V: 0.55% or less, and Nb: 0.75% or less, respectively. More preferably, the upper limits are Ti: 0.30% or less, Al: 0.20% or less, V: 0.30% or less, and Nb: 0.30% or less.

[0082] The present invention may further include, as needed, the elements described below.

[0083] Selected from one or more of the following: B: less than 0.010%, Zr: less than 0.10%, Ca: less than 0.010%, Ta: less than 0.3%, REM: less than 0.10%, and Mg: less than 0.10%.

[0084] Adding trace amounts of B, Zr, Ca, REM, and Mg improves grain boundary bonding and surface oxide morphology, thus enhancing hot workability and formability. Duplex stainless steel pipes are generally difficult to machine, easily resulting in rolling marks and shape defects due to variations in processing amount and method. These elements are effective under forming conditions that cause such problems. There is no specific lower limit for the addition of B, Zr, Ca, REM, and Mg; adding 0.0001% or more of each when present improves workability and formability. Conversely, increasing the addition amount worsens hot workability and increases alloy cost due to their rarity. Therefore, the upper limit for addition is 0.010% or less for B and Ca, and 0.10% or less for Zr, REM, and Mg. Adding small amounts of Ta suppresses phase transformation in the embrittled phase while improving hot workability and corrosion resistance. Ta is effective when hot working is followed by prolonged cooling within the temperature range where the embrittled phase is stable. Therefore, the content is above 0.0001% when Ta is present. On the other hand, if the amount added increases, the cost of the alloy increases, so the content is below 0.3% when Ta is present.

[0085] The present invention may further include, as needed, the elements described below.

[0086] Selected from one or more of Sn: less than 0.30%, Sb: less than 0.30%, and Ag: less than 0.30%.

[0087] Adding trace amounts of Sn, Sb, and Ag improves corrosion resistance. No specific lower limit needs to be set for the amount added, but when present, an improvement in corrosion resistance can be obtained by setting the amount to 0.0001% or more for each. On the other hand, excessive addition reduces hot workability. Therefore, when added, the amount is kept below 0.30% for each.

[0088] Next, the suitable phase fractions of the ferrite and austenite phases, which are important for corrosion resistance, will be explained. The microstructure of the present invention is a duplex microstructure consisting of a ferrite phase comprising 20-80% by volume fraction and an austenite phase as the remainder.

[0089] The various phases in duplex stainless steel play different roles in corrosion resistance, and these high corrosion resistance is achieved by the presence of both austenitic and ferrite phases within the steel. Both austenitic and ferrite phases must be present in duplex stainless steel, and their phase fractions are also important from the viewpoint of corrosion resistance. This invention pertains to stainless steel tubes used in applications requiring corrosion resistance; therefore, it is important to achieve a suitable duplex phase fraction from a corrosion resistance perspective. A suitable duplex phase fraction in this invention refers to a ferrite phase fraction in the stainless steel tube microstructure of 20–80% by volume. Furthermore, for applications requiring more stringent corrosion resistance, it is preferable to set the ferrite phase to 35–65% based on ISO 15156-3. It should be noted that the remainder is austenitic phase. The ferrite phase volume fraction is determined after various cold rolling and processing following solution heat treatment. Simple determination and prediction can be obtained by analyzing the chemical composition of the resulting steel and performing heat balance calculations. Alternatively, samples can be cut from the obtained steel pipe, and the results can be obtained by comparing the peak values ​​of the ferrite and austenite phases after X-ray diffraction and by determining the volume fractions of fcc and bcc after crystal orientation analysis.

[0090] For example, the strength grade of duplex stainless steel pipes, which are cold-worked to achieve high strength for oil and gas wells, is determined by the axial tensile yield strength, which generates the highest load. In the stainless steel pipe of this invention, the axial tensile yield strength is 689 MPa or higher. Typically, duplex stainless steel contains a soft austenitic phase in its microstructure, so the axial tensile yield strength cannot reach 689 MPa under solution heat treatment. Therefore, the axial tensile yield strength is adjusted by dislocation strengthening based on the aforementioned cold working (stretching in the axial direction or bending-back bending in the circumferential direction). It should be noted that a higher axial tensile yield strength allows for thinner wall designs, which is cost-effective. However, if the wall thickness is reduced without changing the outer diameter of the pipe, the flattening caused by external pressure is weakened, making it unusable. Based on the above reasons, even with a high axial tensile yield strength, it can be used within the range of 1033.5 MPa.

[0091] Furthermore, in this invention, the ratio of the compressive yield strength to the tensile yield strength in the axial direction, i.e., the ratio of compressive yield strength to tensile yield strength in the axial direction, is set to 0.85 to 1.15. By setting it to 0.85 to 1.15, higher stress can be withstood relative to the compressive stress in the axial direction generated when the steel pipe is bent in the well during thread tightening, thus reducing the wall thickness required to withstand compressive stress. In addition, the yield strength is high relative to the tensile and compressive stress repeatedly applied to the threaded fastener, thereby improving fatigue characteristics. The increased freedom in wall thickness, especially the expansion of the thinning range, leads to cost reduction and increased production volume due to reduced material costs. It should be noted that by setting the N amount to 0.005% or more and less than 0.150%, and by performing any one of low-temperature heat treatment, warm stretching, or bending-back bending after stretching in the axial direction, it is possible to maintain corrosion resistance while setting the ratio of compressive yield strength to tensile yield strength in the axial direction to 0.85 to 1.15. In addition, if the bending-back bending process is set to be warm, or if a low-temperature heat treatment is performed after the warm stretching process or after the bending-back bending process, the ratio of the tube axial compressive yield strength to the tube axial tensile yield strength can be made closer to 1, which has low anisotropy.

[0092] Furthermore, in this invention, the ratio of circumferential compressive yield strength to axial tensile yield strength is preferably 0.85 or higher. The strength representative value of duplex stainless steel tubes is mostly the axial tensile strength characteristic. If the circumferential yield strength is smaller than this value, the external pressure dependent on the circumferential strength characteristic weakens relative to the product's strength standard. It should be noted that there is no particular problem when the circumferential compressive yield strength is large compared to the axial tensile yield strength, but the ratio typically saturates around 1.50. On the other hand, if the strength ratio is not high enough, other mechanical properties in the axial and circumferential directions, such as low-temperature toughness, decrease significantly compared to the axial direction. Therefore, the circumferential compressive yield strength to axial tensile yield strength ratio is more preferably in the range of 0.85 to 1.25.

[0093] Furthermore, in this invention, the aspect ratio of the austenite grains divided by a crystallization orientation angle difference of 15° or more in the pipe axis direction wall thickness section is preferably 9 or less. In addition, the austenite grains with an aspect ratio of 9 or less are preferably 50% or more in terms of area fraction. The stainless steel pipe of this invention is adjusted to a suitable ferrite phase fraction by adjusting the solid solution heat treatment temperature. Here, in the remaining austenite phase interior, recrystallization is carried out during hot working and heat treatment to form a structure with multiple grains divided by an orientation angle of 15° or more. As a result, the aspect ratio of the austenite grains is small. The stainless steel pipe in this state does not have the pipe axis tensile yield strength required for oil well pipes, and on the other hand, the pipe axis compressive yield strength / pipe axis tensile yield strength is also close to 1. Subsequently, in order to obtain the pipe axis tensile yield strength required for oil well pipes, (1) stretching processing in the pipe axis direction: cold drawing rolling, Pierce cold rolling, and (2) bending-back bending processing in the pipe circumferential direction are performed. Through these processing steps, the ratio of compressive yield strength to tensile yield strength in the axial direction changes with the aspect ratio of austenite grains. Specifically, the aspect ratio of austenite grains is closely related to the ratio of compressive yield strength to tensile yield strength in the axial direction. In particular, during processing (1) or (2), the yield strength of the austenite grains in the axial wall thickness section increases in the tensile direction before and after processing, but decreases in the opposite direction due to the Bauschinger effect, resulting in a larger difference between the compressive yield strength and the tensile yield strength in the axial direction. Therefore, if the aspect ratio of austenite grains before and after processing (1) or (2) is controlled relatively well, steel pipes with low strength anisotropy in the axial direction can be obtained.

[0094] In this invention, if the aspect ratio of the austenite grains is 9 or less, a steel pipe with stable strength anisotropy can be obtained. Furthermore, if the area fraction of austenite grains with an aspect ratio of 9 or less is 50% or more, a steel pipe with stable strength anisotropy can be obtained. It should be noted that a steel pipe with low strength anisotropy can be obtained more stably by having an aspect ratio of 5 or less. If the aspect ratio decreases, the strength anisotropy is further reduced; therefore, the lower limit is not particularly limited and can be close to 1. Furthermore, the aspect ratio of the austenite grains can be determined, for example, by analyzing the crystal orientation of the austenite phase in the wall thickness section along the pipe axis, observing grains with a crystal orientation angle of 15° or more, and calculating the ratio of the long side to the short side of the grain when contained within a rectangular frame. It should be noted that the measurement error increases with small-grained austenite grains; therefore, if small-grained austenite grains are included, the aspect ratio may also be inaccurate. Therefore, when determining the aspect ratio of austenite grains, it is preferable to use the area of ​​the measured grains, and the diameter of the circle with the same area is preferably 10 μm or more.

[0095] In order to stably obtain a microstructure with a small aspect ratio of austenite grains in the tube axis section, it is effective not to stretch in the tube axis direction during processing (1) or (2), and thus not to reduce the wall thickness. For processing method (1), in principle, it is accompanied by stretching and thinning in the tube axis direction, so the aspect ratio increases compared with before processing, which easily leads to strength anisotropy. Therefore, reducing the amount of processing (setting the wall thickness reduction rate to 40% or less, or setting the stretching in the tube axis direction to 50% or less to suppress the stretching of the microstructure), reducing the outer perimeter of the tube while stretching and thinning (reducing the outer perimeter by more than 10% when stretching in the tube axis direction), and keeping the aspect ratio small, in addition, in order to mitigate the strength anisotropy, a low-temperature heat treatment after processing is required (if the heat treatment temperature is below 600°C, softening due to recrystallization and recovery will not occur). On the other hand, processing method (2) is a bending-back bending deformation in the tube circumferential direction, so the aspect ratio basically does not change. Therefore, the processing method of (2) limits the amount of shape change such as stretching and thinning of the tube, but it is extremely effective in maintaining a small aspect ratio and reducing strength anisotropy, and does not require the low-temperature heat treatment required in (1) after processing. It should be noted that by controlling the processing temperature and heat treatment conditions of (1) within the range of the present invention, or by using the processing method of (2), it is possible to control the austenite grains with an aspect ratio of 9 or less to be more than 50% in terms of area fraction.

[0096] In processing methods (1) or (2), even if heat treatment is performed after processing, the aspect ratio does not change. In addition, for the ferrite phase, for the same reason as the austenite phase, the one with a smaller aspect ratio is preferred, but the one with a smaller aspect ratio of the austenite phase has a low yield strength, which can easily affect the Bauschinger effect after processing.

[0097] In product strength standards, the most important is the tensile yield strength in the axial direction, but for pipe connections, the compressive yield strength in the axial direction is also important. From the perspective of preventing fires and repeated insertion and removal, pipe connections for oil wells, gas wells, or geothermal wells cannot be welded and can be fastened using threads.

[0098] A threaded joint consists of a pin with external threads and a housing with internal threads. As threaded joints, there are standard threaded joints as specified by API (American Petroleum Institute) standards, and high-performance special threaded joints with not only threaded sections but also metal-to-metal contact seals and torque shoulders. To achieve a secure fastening of the threaded section, it is typically designed to generate contact surface pressure in the diametrical direction; for example, tapered threads can be used. Along with the diametrical surface pressure, the pin (internal thread side) shrinks and deforms, stretching towards the pipe axis, while the housing (external thread side) expands and deforms, stretching towards the pipe axis, thus generating contact surface pressure on the sides at both ends of the threaded section. Therefore, axial compressive stress is generated in the thread teeth based on the fastening force. Therefore, axial compressive yield strength that can withstand this compressive stress is important. In special threads, a large axial compressive stress is generated at the torque shoulder; therefore, materials with high axial compressive yield strength are important in preventing plastic deformation of the torque shoulder.

[0099] As mentioned earlier, when using steel pipes with high corrosion resistance for oil wells, gas wells, or geothermal wells, improving the axial tensile yield strength of the steel pipe and the strength characteristics of the threaded portion used for fastening are extremely important. In special threads, the strength characteristics of the moment shoulder are also crucial. High corrosion-resistant materials, such as duplex stainless steel, generally include an austenitic phase with low yield strength at room temperature in their microstructure. Therefore, in order to obtain both high corrosion resistance and the high yield strength required for oil wells or geothermal wells, dislocation strengthening is necessary after solution heat treatment using cold drawing or Pilger rolling. These cold working methods significantly improve the axial tensile yield strength for use in oil and gas wells; however, they cannot simultaneously achieve the strength characteristics of the threaded portion used for fastening. That is, conventional cold drawing and Pilger rolling achieve a axial tensile shape by reducing the pipe wall thickness or by applying a drawing force, thus ultimately increasing the axial tensile yield strength of the steel pipe through tensile deformation in the axial direction. On the other hand, in metallic materials, the Bauschinger effect occurs, where the yield strength is significantly reduced relative to deformation in the direction opposite to the final deformation direction. Steel pipes obtained through conventional cold working methods possess the axial tensile yield strength required for oil wells, gas wells, or geothermal wells, but their axial compressive yield strength is reduced. Therefore, if steel pipes obtained through conventional cold working methods are subjected to low-temperature heat treatment to restore the reduced compressive yield strength, the threaded portion and torque shoulder cannot withstand axial compressive stress during threaded fastening, which is essential for oil well drilling. This results in plastic deformation, damage to the passivation film, reduced corrosion resistance, and loss of structural function as a threaded joint.

[0100] The stainless steel pipe of the present invention is a steel pipe that is directly or indirectly coupled to other steel pipes. At least one end of the pipe has a fastening portion with external or internal threads. The radius of curvature R of the corner R formed by the pressure side of the thread valley surface on the pipe axis section of the fastening portion is set to 0.2 mm or more. According to the present invention, regardless of the type of thread, the external and internal threads are brought into contact through fastening. By setting the radius of curvature R of the corner R formed by the pressure side (pressure side surface) generated by fastening and the thread valley surface to 0.2 mm or more, the fatigue characteristics of the threaded portion can be improved.

[0101] The stainless steel tube of this invention exhibits excellent compression resistance, and therefore can be used in threaded joints that are directly connected to other steel pipes (integral type) or in threaded joints that are connected via coupling (T&C type). In the threaded fastening portion, tensile and compressive stresses in the pipe axis are generated during tightening due to bending deformation after tightening. By using the stainless steel tube of this invention in threaded joints, threaded joints that maintain high corrosion resistance and performance can be achieved.

[0102] Figure 3 It is a sectional view of the fastening part of the external and internal threads along the pipe axis (a sectional view parallel to the pipe axis direction), and a schematic diagram showing the position of the radius of curvature R of the fastening part and corner of the thread. Figure 3 (a) is the case of an angular thread. Figure 3 (b) is the case of a trapezoidal thread. Figure 3 (c) is the case of a triangular thread. In this invention, at least one end of the pipe has a fastening portion with an external or internal thread, and the radius of curvature of the corner formed by the side surface of the fastening portion and the bottom surface of the thread is 0.2 mm or more. That is, according to this invention, regardless of the type of thread, the external thread and the internal thread can be brought into contact by fastening, and the radius of curvature of the corner R formed by the side surface where the pressure is generated by fastening and the bottom surface of the thread is set to 0.2 mm or more, thereby improving fatigue characteristics while maintaining high corrosion resistance. It should be noted that, for the side surface, the thread bevel on the side closer to the pipe end in the external thread (pin) is called the insertion tooth side, and the thread bevel on the side farther from the pipe end is called the bearing tooth side. In the internal thread (box), the thread bevel opposite to the insertion tooth side of the pin is called the insertion tooth side, and the thread bevel opposite to the bearing tooth side of the pin is called the bearing tooth side.

[0103] Figure 4 This is a sectional view of the threaded connector along the pipe axis (a sectional view parallel to the pipe axis). Figure 4 (a) is the case of an API threaded connector. Figure 4(b) is the case of a special thread. In a threaded joint consisting only of a threaded portion, such as an API threaded joint, the maximum surface pressure is generated at both ends of the threaded portion when the thread is tightened. The threaded portion on the front end of the pin contacts the insertion tooth side, and the threaded portion on the rear end of the pin contacts the bearing tooth side. In the case of a special thread, the reaction force from the moment shoulder also needs to be considered, and the maximum surface pressure is generated at both ends of the threaded portion on the bearing tooth side when the thread is tightened. In the past, due to the Bauschinger effect in the tube axis direction, the compressive yield strength in the tube axis direction, which is lower than the tensile yield strength in the tube axis direction, is low. If compressive stress is generated at the stress concentration point, the compressive yield strength is low, which easily leads to micro-deformation and reduced fatigue life. In order to reduce the Bauschinger effect, a method of low-temperature heat treatment has been disclosed. However, if low-temperature heat treatment is performed, it is not in the "solid solution state of the corrosion-resistant element", and high corrosion resistance cannot be obtained. It is impossible to obtain both corrosion resistance and improved fatigue characteristics of the threaded portion. According to the present invention, by setting the radius of curvature of the corner R to 0.2 mm or more, the fatigue characteristics of the threaded portion of the stainless steel seamless pipe are improved, and good corrosion resistance can be obtained.

[0104] Increasing the radius of curvature of the corner R to 0.2 mm or more is effective in mitigating further stress concentration. However, a large corner R deprives the design of the threaded portion of freedom, potentially limiting the size of the threadable steel pipe. Furthermore, increasing the corner R reduces the area of ​​the contacting external and internal threads, resulting in a decrease in sealing performance and tightening force. Therefore, a corner R of 0.2 mm or more is preferred, and 0.3 mm or more is preferable. Additionally, a corner R of 3.0 mm or less is preferable. Alternatively, given that the area of ​​the reduced side surface due to the size of the corner R is related to the thread height, it is suitable to define the radius of curvature of the corner R as a radial length (diameterally from the center of the pipe axis) less than 20% of the thread height, and the radius of curvature of the corner R can be set to 0.2 mm or more. Furthermore, since the area of ​​the pressure-side side surface reduced due to the size of the corner R is affected by the thread height, a length less than 10% of the thread height can be set as the radius of curvature of the corner R, and the radius of curvature of the corner R can be designed to be 0.3 mm or more.

[0105] Figure 4 (b) is a schematic diagram of a special thread that not only has a threaded portion but also a metal-to-metal contact seal and a torque shaft shoulder. Through Figure 4 The metal contact seal shown in (b) Figure 4 (b) Seal) ensures the sealing of the tightened tube. On the other hand, the torque shaft shoulder ( Figure 4(b) The Shoulder acts as a limiter during tightening, playing a crucial role in ensuring a stable tightened position. However, it generates high compressive stress during tightening. If the torque shaft shoulder deforms due to high compressive stress, the high sealing performance is compromised, or deformation towards the inner diameter side causes problems with inner diameter reduction. Therefore, it is necessary to increase the wall thickness and improve the compressive strength to prevent deformation of the torque shaft shoulder, making it impossible to design thin-walled steel pipes, or resulting in material waste due to excessive wall thickness.

[0106] In addition, when tightening threads, the tightening torque value (the torque value during thread tightening) is usually determined. Based on the closed torque value (since tightening exceeds a certain reference, it becomes the torque value representing the closed state, hence called the torque value during tightening), the torque value that does not deform at the torque shaft shoulder (if it exceeds a certain reference, the thread tip will deform when the torque value is increased, so it does not exceed the torque value of that reference) is used as the upper limit. Based on the closed torque value, tightening is managed and performed within the range of torque values ​​that do not deform at the torque shaft shoulder.

[0107] At this point, with weak compressive yield strength in the tube's axial direction, the upper limit of the torque value decreases to suppress deformation at the torque shoulder. Therefore, the control range of the torque value narrows, and the fastening cannot be stable. According to the present invention, which possesses excellent compressive yield strength in the tube's axial direction, deformation at the torque shoulder can be suppressed while maintaining high corrosion resistance. To suppress deformation at the torque shoulder and ensure stable fastening, it is possible to ensure... Figure 5 The thickness of the front end of the torque shaft shoulder (the portion bearing the coupling side of the external thread, (Ds1-Ds0) / 2) as shown in the diagram should be at least 25% of the cross-sectional area of ​​the blank tube (at least 0.25 based on the cross-sectional area ratio of the shoulder). Increasing the thickness of the front end of the torque shaft shoulder as the external thread would result in excessive head rigidity, potentially causing burning during tightening; therefore, a range of 25% to 60% is preferred. Designing a head that further enhances the compressive strength of the torque shaft shoulder allows for even higher torque performance (higher torque value without deformation, providing a higher tightening torque), which is therefore preferable. A schematic diagram of the extension of the pin, i.e., near the head, is shown. Figure 5 (a) and (b) show cross-sectional views parallel to the pipe axis of the pin and the coupling fastener, respectively, observing the torque shoulder of the threaded tip of the pin from the entire front end. To achieve high torque, the ratio x / L, where the sealing point position from the pipe end is set to x, relative to the head length L of the unthreaded portion at the pin tip, can be set to 0.01 to 0.1. By setting the sealing point position near the shoulder, the actual cross-sectional area of ​​the shoulder (cross-sectional area of ​​the shoulder: π / 4 × (Ds1)) is increased. 2 -Ds0 2As the head length increases, high torque performance can be obtained. However, if the head length is too long, the head rigidity decreases, making it unable to withstand high compressive forces; therefore, the head length can be less than 0.5 inches. On the other hand, if the head length is too short, there is no room for configuring a sealing portion; therefore, a length of 0.2 inches or more is preferred. It should be noted that in conventional stainless steels with low compressive yield strength in the axial direction, achieving arbitrarily high torque performance is not possible.

[0108] It should be noted that, Figure 5 middle,

[0109] δ: refers to the sealing interference amount, defined by the maximum value of the overlapping portion when the attached drawings are overlapped.

[0110] Ds1: Outer diameter of the shoulder contact area

[0111] Ds0: Inner diameter of the shoulder contact area.

[0112] The airtightness of the threaded portion is also important, and it is preferable to meet the compression ratio of 85% or more shown in the sealing test of ISO 13679:2019. To achieve high sealing performance, the head length of the unthreaded portion at the pin tip can be set to 0.3 inches or more, and the ratio x / L of the sealing point position from the pipe end to the head length L can be set to 0.2 to 0.5. However, if the head length is extended beyond what is necessary, cutting time is increased, head rigidity decreases, and performance becomes unstable; therefore, the head length is preferably 1.0 inch or less. It should be noted that in duplex stainless steel with a long head length and lower compressive yield strength than conventional designs, a design with a thinner head tip is inherently unacceptable, making it impossible to achieve this effect.

[0113] It should be noted that, from the viewpoint of obtaining uniform material properties and strength properties in the circumferential direction, the stainless steel pipe of the present invention is preferably a seamless steel pipe without joints such as welds in the circumferential direction.

[0114] Next, the manufacturing method of the stainless steel pipe of the present invention will be described.

[0115] First, steel billets with the aforementioned duplex stainless steel composition are prepared. Various melting processes can be used to melt duplex stainless steel without limitation. For example, when manufacturing by electro-melting iron scrap or blocks of various elements, vacuum melting furnaces or atmospheric melting furnaces can be used. Alternatively, when using molten steel based on the blast furnace method, bottom-blown decarburization furnaces with Ar-O2 mixed gas or vacuum decarburization furnaces can be used. The melted material is solidified by static casting or continuous casting to form steel ingots or slabs. Subsequently, hot rolling is used to form plate-shaped steel billets, or forging and rolling are used to form round steel billet shapes, thus producing steel billet products.

[0116] Next, in the case of a steel billet in the shape of a plate, after forming it into a roughly tubular shape, the ends are welded to form a steel pipe. There are no particular restrictions on the steel pipe forming process; forming techniques such as UOE forming, roll forming, welding with welding materials, and resistance welding using induction heating can be used. Alternatively, in the case of a round steel billet, it is heated in a furnace and formed into a steel pipe shape through various hot rolling processes. Hot forming (piercing process) is then performed to form a hollow tube from the round steel billet. As for hot forming, any method such as the Mannesmann method or extrusion tube forming can be used. Furthermore, depending on the needs, the hollow tube can be processed using hot rolling processes such as stretching mills, Assel mills, roll mills, mandrel mills, screening machines, and stretching diameter mills for thinning and outer diameter shaping.

[0117] Next, the formed steel pipe is preferably subjected to solution heat treatment. The steel pipe, formed by bending a plate-shaped steel billet, accumulates deformation caused by bending. Furthermore, in the process of forming the steel pipe shape through various hot rolling processes, the duplex stainless steel gradually decreases in temperature from its high heating temperature to the rolling temperature. Moreover, after hot forming, it is mostly air-cooled, and the temperature history varies depending on the size and grade, making it impossible to control. Therefore, corrosion-resistant elements are consumed as thermochemically stable precipitates in various temperature ranges during the temperature reduction process, potentially leading to a decrease in corrosion resistance. Additionally, the phase transformation to the embrittlement phase can significantly reduce low-temperature toughness. Furthermore, duplex stainless steel can withstand various corrosive environments, so maintaining an appropriate austenite and ferrite phase ratio is crucial. In particular, the cooling rate from the heating temperature cannot be controlled, making it difficult to control the sequentially changing duplex phase ratio by maintaining the temperature. Due to the aforementioned problems, in order to remove accumulated deformation, dissolve precipitates into the steel, and achieve a reverse phase transformation from embrittled to non-embrittled phases, thus achieving a suitable two-phase state, a solution heat treatment involving high-temperature heating followed by rapid cooling is often used. This treatment removes residual stress caused by accumulated deformation, melts precipitates and embrittled phases into the steel, and controls the phase fraction to a suitable two-phase state. The temperature for solution heat treatment varies depending on the amount of added elements, but is generally above 1000°C. Furthermore, since the solid solution state is maintained after heating, rapid cooling is performed, which can be achieved using compressed air, or various cooling catalysts such as water mist, oil, or water.

[0118] Since steel pipes treated with solution hardening contain an austenitic phase with low yield strength, they are unsuitable for applications requiring high strength, such as oil and gas well drilling. Therefore, dislocation strengthening is achieved through various cold rolling processes to increase the pipe's strength. It should be noted that the strength grade of the high-strength stainless steel pipe is determined based on its axial tensile yield strength.

[0119] In this invention, as described below, the tube is made stronger by either (1) stretching in the tube axis direction or (2) bending-back bending in the tube circumferential direction.

[0120] (1) Stretching process in the direction of tube axis: cold drawing and rolling, Pilgrimage cold rolling

[0121] In the cold rolling process for pipes, the two most standardized methods for oil and gas well production are cold drawing and rolling, and Pilgh cold rolling. Both methods can achieve high strength along the pipe axis and can be appropriately utilized. These methods primarily involve varying the reduction rate and the rate of change of outer diameter to achieve the required strength level. Furthermore, cold drawing and Pilgh cold rolling, due to their reduction of the outer diameter and wall thickness, involve a significant stretching of this portion along the long side of the pipe axis, thus easily achieving high strength along the long side. However, a known problem exists: a large Bauschinger effect occurs in the compression direction along the pipe axis, resulting in a maximum reduction of approximately 20% in the compressive yield strength compared to the tensile yield strength along the pipe axis.

[0122] Therefore, in this invention, after stretching in the tube axis direction, a heat treatment at 150–600°C (excluding 460–480°C) is performed. If the nitrogen content is less than 0.150%, even after the above heat treatment, the corrosion resistance performance will not decrease due to the consumption of corrosion-resistant elements, and the reduction in tube axis compressive yield strength caused by stretching in the tube axis direction will be improved.

[0123] Furthermore, it is effective to perform the stretching process at a temperature of 150–600°C (excluding 460–480°C) in the axial direction. If the nitrogen content is less than 0.150%, the heat treatment after stretching does not result in a decrease in corrosion resistance, and the reduction in axial compressive yield strength caused by axial stretching can be mitigated. Additionally, a reduction in processing load due to material softening can also be expected.

[0124] The upper limit of the processing temperature during stretching and the heating temperature during heat treatment must be such that the dislocation strengthening caused by the processing will not disappear, which can be applied up to 600°C. In addition, processing at 460-480°C, which is the embrittlement temperature of the ferrite phase, should be avoided because it can lead to the deterioration of product properties due to tube embrittlement and cracking during processing.

[0125] If the heating temperature during heat treatment and the processing temperature during tensile processing are less than 150°C, this becomes a temperature range where a sharp decrease in yield strength occurs. Therefore, to achieve a sufficient reduction in processing load, the temperature is set to 150°C or higher. Preferably, the temperature is set to 350–450°C to avoid the passage of the embrittled phase during heating and cooling.

[0126] (2) Bending-back bending process in the circumferential direction of the tube

[0127] If there is no standardized cold working method for duplex stainless steel seamless pipes used in oil and gas well production, high-strength pipes can be achieved by using dislocation strengthening through bending and back bending in the circumferential direction. This processing method will be described based on the accompanying drawings. In this method, the deformation caused by rolling differs from that of cold drawing and Pielschner cold rolling, which occur along the long side of the pipe axis. Figure 6 As shown, deformation is applied through a bending process caused by the flattening of the tube (the first flattening process), followed by a return bending process to a perfect circle (the second flattening process). In this method, the deformation is adjusted by repeating the bending-return bending process and varying the amount of bending, but the applied deformation is an additional shear deformation that does not change the shape before and after processing. Furthermore, almost no deformation occurs in the tube axis direction; high strength is achieved through dislocation strengthening caused by deformation in the tube circumferential and wall thickness directions, thus suppressing the Bauschinger effect in the tube axis direction. That is, because the reduction in tube axial compressive strength is minimal, as in cold drawing and rolling or Pielge cold rolling, the design freedom of the threaded fasteners can be increased. Moreover, if processing is performed by reducing the outer circumference of the tube, the compressive strength in the tube circumferential direction is increased, enabling the production of strong steel tubes even relative to the external pressure during high-depth oil and gas well extraction. Bending-back bending processes in the circumferential direction, such as cold drawing and rolling or Pilgrimage cold rolling, cannot provide large changes in outer diameter and wall thickness. However, they are effective when there is a particular requirement to reduce the anisotropy of strength in the axial direction and the circumferential direction of compression relative to the axial direction.

[0128] It should be explained that Figure 6 (a) and (b) are cross-sectional views showing the case where the tool contact area is divided into two parts. Figure 6 (c) is a cross-sectional view showing the case where the tool contact area has three parts. Additionally, Figure 6 The thick arrow indicates the direction of the force applied during the flattening process of the steel pipe. For example... Figure 6 As shown, during the second flattening process, the tool contacts the area that was not flattened in the first step by rotating the tool to make the steel pipe rotate, or by staggering the tool's position, etc. Figure 6 The slashed part in the middle indicates the first flattened part.

[0129] like Figure 6 As shown, by intermittently or continuously applying a bending-back process in the circumferential direction of the steel pipe to flatten it, deformation caused by bending is applied near the maximum curvature of the steel pipe, and deformation caused by back bending is applied towards the minimum curvature of the steel pipe. The result is the accumulation of deformation from the bending-back deformation required for the increase in the strength of the steel pipe (dislocation strengthening). Furthermore, this processing method differs from processing methods that compress the wall thickness and outer diameter of the pipe. Its characteristic is that it does not require a large amount of power, and because it is based on flattening deformation, the shape change before and after processing is minimized, and processing can be performed.

[0130] for Figure 6 The tool shape used in the flattening of steel pipes can be rollers. If two or more rollers are arranged circumferentially between the steel pipes to flatten and rotate them, the deformation resulting from repeated bending and rebending can be easily applied. Furthermore, by tilting the rollers' rotation axis within 90° relative to the pipe's rotation axis, the steel pipe undergoes flattening while moving along its rotation axis, thus facilitating continuous processing. Additionally, by appropriately varying the roller spacing to change the flattening amount relative to the steel pipe's movement during continuous processing using these rollers, the curvature (flattening amount) of the steel pipe can be easily changed for the first and second flattening operations. Therefore, by varying the roller spacing, the movement path of the neutral line can be altered, homogenizing deformation in the wall thickness direction. Similarly, the same effect can be achieved by varying not only the roller spacing but also the roller diameter to change the flattening amount. Furthermore, these methods can be combined. While the equipment is complex, setting the number of rollers to three or more can suppress pipe swaying during processing, enabling stable processing.

[0131] The processing temperature for bending-back bending towards the circumference of the tube can be room temperature. On the other hand, if the processing temperature is room temperature, all nitrogen (N) can be in a solid solution state, which is preferred from a corrosion resistance point of view. If the N content is less than 0.150%, it is effective to soften the material by increasing the processing temperature under conditions of high cold working load and difficult processing. The upper limit of the processing temperature needs to be based on the temperature at which dislocation strengthening does not disappear during processing, and can be applied up to 600°C or below. Furthermore, processing at 460–480°C, which is the embrittlement temperature of the ferrite phase, not only leads to deterioration of product properties due to tube embrittlement but also causes cracking during processing, and should therefore be avoided. Therefore, in the case of bending-back bending towards the circumference of the tube, the processing temperature is preferably below 600°C, excluding 460–480°C. The lower limit of the processing temperature is in the temperature range where a sharp decrease in yield strength occurs when the processing temperature is below 150°C, so the processing temperature is more preferably above 150°C. The upper limit of the processing temperature is more preferably 450°C to avoid energy saving and embrittlement of the phase during heating and cooling. In addition, performing bending-back bending at the specified processing temperature also has the effect of slightly reducing the strength anisotropy of the processed pipe, thus being effective even in cases where strength anisotropy is a problem.

[0132] After the processing described in (1) or (2) for dislocation strengthening, heat treatment can be further performed in this invention. By performing heat treatment, the strength anisotropy can be improved while maintaining corrosion resistance. If the heating temperature of the heat treatment is less than 150°C, it becomes a temperature range where the yield strength decreases sharply, so the heating temperature is preferably 150°C or higher. In addition, the upper limit of the heating temperature needs to be based on the temperature at which the dislocation strengthening of the processing does not disappear, and it can be applied to 600°C or lower. On the other hand, heat treatment at the embrittlement temperature of the ferrite phase, i.e., 460 to 480°C, will lead to the deterioration of product characteristics due to tube embrittlement, so it should be avoided. Therefore, when performing further heat treatment, it is preferable to perform heat treatment at a heating temperature of 150 to 600°C, excluding 460 to 480°C. In order to obtain the anisotropy improvement effect and avoid the passage of embrittled phases during heating and cooling, 350 to 450°C is more preferred. The cooling rate after heating can be either air cooling or water cooling.

[0133] After cold working, surface treatments such as plating can be performed as needed.

[0134] In this invention, for the stainless steel tube obtained above, the external and internal threads can be designed such that the radius of curvature of the corner R formed by the bottom and side surfaces of the threaded joint section (the section parallel to the tube axis) is 0.2 mm or more. The thread shape can be designed by cutting or rolling; cutting is preferred to reliably obtain the shape of the corner R. As a threaded joint, to further improve performance, a special thread that not only has a threaded portion but also a metal-to-metal contact seal and a torque shoulder is preferred. The stainless steel tube of this invention, by having a high compressive yield strength in the tube axis direction, can function as a joint without problems when the cross-sectional area of ​​the shoulder is 25% or more of the cross-sectional area of ​​the blank tube.

[0135] To achieve high torque performance (higher torque value without deformation, providing higher tightening torque), it will be used as Figure 5 The length of the unthreaded portion at the tip of the pin shown is set to 0.2 inches to 0.5 inches, and the ratio x / L of the sealing point position from the pipe end to the head length L can be set to 0.01 to 0.1. On the other hand, in order to achieve a metal-to-metal contact seal with high airtightness, the length of the unthreaded portion at the tip of the pin can be set to 0.3 inches to 1.0 inches, and the ratio x / L of the sealing point position from the pipe end to the head length L can be set to 0.2 to 0.5.

[0136] The stainless steel tube of the present invention can be obtained according to the above manufacturing method.

[0137] Example 1

[0138] The present invention will now be described based on embodiments. It should be noted that the properties of duplex stainless steel are homogenized after solution heat treatment, so there is no significant difference between steel pipes formed and welded from plate-shaped billets and seamless steel pipes manufactured from round billets. Therefore, in this embodiment, seamless steel pipes are used as billets before cold working.

[0139] The chemical compositions of A to S shown in Table 1 are melted using a vacuum melting furnace, and then hot-rolled into a φ60mm round steel billet.

[0140]

[0141] After hot rolling, the round steel billet is reinserted into the heating furnace and held at a high temperature above 1200℃. Then, it is hot-formed into a seamless tube with an outer diameter of φ70mm and an inner diameter of 58mm (wall thickness of 6mm) using a Menesmann piercing mill. The hot-formed tube is then subjected to solution heat treatment at a temperature suitable for a two-phase state with appropriate ferrite and austenite phase fractions, for high-strength processing. The processing methods are shown in Table 2, including two types: drawing rolling and bending-back bending, which are one of the stretching processes in the tube axis direction. It should be noted that after drawing rolling or bending-back bending, a portion is cut off, with a measured area of ​​1.5mm². 2 Crystallization orientation analysis was performed to determine the proportion of bcc (ferrite phase) relative to the overall microstructure, thus confirming the appropriate two-phase fraction state of ferrite and austenite phases.

[0142] Furthermore, for the wall thickness direction of the tube cross-section parallel to the tube axis, EBSD-based crystal orientation analysis was performed to determine the aspect ratio of austenite grains divided by a 15° crystal orientation angle. The measurement area was 1.2 mm × 1.2 mm, and the aspect ratio of austenite grains with a diameter of 10 μm or more was determined under the assumption of perfect spherical shape.

[0143] Drawing and rolling involve a wall thickness reduction of 10%–30%, with a 20% reduction in the outer circumference. Bending and back-bending are prepared using a mill with three cylindrical rolls spaced at 120° intervals on the outer circumference of the tube. Figure 6 (c) The outer circumference of the tube is clamped while the roller spacing is smaller than the outer diameter of the tube, and the tube is rotated. Additionally, under certain conditions, warm working at 300–570°C is performed. Furthermore, after each cold working and warm working, under certain conditions, a low-temperature heat treatment at 300–620°C is performed.

[0144] The obtained steel pipes were tested for tensile and compressive yield strength in the long side direction of the pipe axis and compressive yield strength in the circumferential direction. The tensile yield strength in the pipe axis, which is the strength grade of the steel pipes for oil wells and gas wells, the compressive yield strength / tensile yield strength in the pipe axis, and the compressive yield strength / tensile yield strength in the pipe axis, which are used to evaluate strength anisotropy, were also measured.

[0145] Furthermore, stress corrosion testing was conducted. The corrosive environment was an aqueous solution of chloride and sulfide (20% NaCl + 0.5% CH3COOH + CH3COONa) with H2S gas added at a pressure of 0.01–0.10 MPa, pH adjusted to 3.0, and test temperature 25°C. Stress was applied by cutting 5 mm thick, 4-point bending test pieces in a manner that could apply stress along the long side of the tube axis. Tensile yield strength was applied along the tube axis, and 90% stress was applied, followed by immersion in the corrosive solution. The corrosion condition was evaluated as follows: after immersion in the corrosive solution for 720 hours under stress, the surface immediately subjected to stress upon removal was evaluated as "○"; the surface that showed no cracks but was considered not to have reached fracture was evaluated as "fracture"; and the surface where the cracks extended and fractured was evaluated as "fracture".

[0146] Furthermore, at the ends of the obtained stainless steel tubes, angular threaded portions are formed by machining. After fastening two steel tubes with the threads, fatigue tests are conducted on the threaded portions with the two tube ends eccentrically rotated at a 3-10% angle, based on the axial tensile yield strength of the steel tubes. Additionally, at the ends of the obtained stainless steel tubes, trapezoidal and triangular threaded portions are formed by machining. After fastening two steel tubes with the threads or by coupling, fatigue tests are conducted on the threaded portions with the two tube ends eccentrically rotated at a 3-10% angle, based on the axial tensile yield strength of the steel tubes. It should be noted that for the threaded portions, the curvature radius R of the corner of the bearing tooth side and the insertion tooth side of the pin thread (which is a stress concentration point) and the curvature radius R of the corner of the bearing tooth side and the insertion tooth side of the coupling thread are varied at the same value to investigate whether there are fatigue cracks at stress concentration points and thread breakage due to the progression of fatigue cracks. Cases without fatigue cracks are rated as "○", cases that lead to breakage but have confirmed fatigue cracks at the corner R are rated as "cracks", and cases that lead to breakage of the thread teeth are rated as "breakage".

[0147] The manufacturing conditions and evaluation results are shown in Table 2. It should be noted that the processing methods, number of processing passes, and processing temperatures described here refer to further processing to obtain strength after heat treatment of the hot-rolled steel pipe, specifically drawing rolling and bending-back bending.

[0148] Table 2

[0149]

[0150] The underlined part is outside the scope of the invention.

[0151] *For No. 53, since it is a ferrite phase monomer, the aspect ratio of the austenite phase cannot be determined. Therefore, it is recorded as X.

[0152] As shown in Table 2, the composition system of the present invention exhibits excellent corrosion resistance in all examples, with a small difference between the tensile yield strength and compressive yield strength in the axial direction, and excellent fatigue characteristics in the threaded portion. On the other hand, the comparative examples do not meet the qualification criteria in any of the following aspects: corrosion resistance, tensile yield strength in the axial direction, compressive yield strength ratio, or fatigue characteristics.

[0153] Example 2

[0154] Next, the design of the torque shoulder was evaluated in the special thread. As shown in Table 3, a tightening test (Yield torque evaluation test) was carried out in a threaded joint (special thread) consisting of a pin with an outer diameter of Φ88.9 mm, a wall thickness of t6.5 mm, a tensile strength of 689 MPa and its corresponding coupling.

[0155]

[0156] Specifically, if the cross-sectional area of ​​the shoulder is less than 20% of the cross-sectional area of ​​the unmachined portion of the pin, yielding occurs at a tightening torque of 3000 N·m. Therefore, if the cross-sectional area of ​​the shoulder is 20% or more of the cross-sectional area of ​​the unmachined portion of the pin, the yield becomes 4000 N·m or more, ensuring a sufficiently high torque for tightening. This value is typically required to be 25% or more in duplex stainless steels with low compressive strength. Therefore, the cross-sectional area of ​​the shoulder of the duplex stainless steel of the present invention can be confirmed to be 20% or more of the cross-sectional area of ​​the unmachined portion of the pin, ensuring a superior torque. The results are shown in Table 3.

[0157] Furthermore, as a second high-performance threaded joint, the realization of a high-sealing threaded joint that passes the sealing test of ISO 13679:2019 can be cited. Therefore, as shown in Table 4, sealing tests were performed on threaded joints (special threads) consisting of a pin with an outer diameter of Φ88.9 mm, a wall thickness of t6.5 mm, a tensile strength of 689 MPa and its corresponding coupling, and threaded joints (special threads) consisting of a pin with an outer diameter of Φ244.5 mm, a wall thickness of t13.8 mm and its corresponding coupling.

[0158]

[0159] As shown in Tables 3 and 4, the application of the stainless steel tube of this invention enables the creation of fastenable threaded joints even with a lower shoulder cross-sectional area. This feature increases the design freedom of threaded joints, enabling the creation of the following two types of high-performance threaded joints.

[0160] Firstly, as a high-performance threaded connector, a high-torque threaded connector that ensures sealing performance even when high tightening torque is applied can be cited. High torque is achieved by applying the high compressive strength stainless steel seamless pipe of the present invention to the threaded connector. Furthermore, high torque can be further achieved through the rationalization of the threaded connector design. Specifically, the head length of the unthreaded portion at the pin tip is set to 0.2 inches to 0.5 inches, and the ratio x / L of the sealing point position from the pipe end to the head length L is set to 0.01 to 0.1.

[0161] Furthermore, based on the sealing test results, in order to achieve a highly airtight metal-to-metal contact seal, the length of the unthreaded portion serving as the pin tip can be set to 0.3 inches to 1.0 inches, and the ratio x / L of the sealing point position from the tube end to the head length L can be set to 0.2 to 0.5. As mentioned above, if the head length is extended and the sealing point is moved away from the tube end, the cross-sectional area of ​​the shoulder becomes smaller. In conventional materials, this cross-sectional area becomes a problem of yielding, making it highly undesignable. This problem is significant in thin-walled materials and cannot be achieved with a wall thickness of 6.5 mm. In the stainless steel tube of the present invention, due to its high compressive strength, if the cross-sectional area of ​​the shoulder is ensured to be 20% or more, the problem of yielding can be avoided, achieving both a large cross-sectional area of ​​the shoulder and a high-sealing appearance design. As shown in Table 4, when the ratio of the compressive yield strength in the tube axial direction to the tensile yield strength in the tube axial direction is 0.85 or more, it can be confirmed that the sealing test is qualified with a compression rate of 85% or more under the test load of ISO 13679:2019.

Claims

1. A stainless steel pipe having the following composition: It contains, by mass%, C: 0.005–0.08%, Si: 0.01–1.0%, Mn: 0.01–10.0%, Cr: 20–35%, Ni: 1.0–15%, Mo: 0.5–6.0%, N: ≥0.005% and <0.150%, Nb: ≥0.0001% and <0.75%, with the remainder consisting of Fe and unavoidable impurities. The axial tensile yield strength is above 689 MPa. The ratio of compressive yield strength in the axial direction to tensile yield strength in the axial direction is 0.85–1.

15. Furthermore, it has a microstructure consisting of 20-80% ferrite phase by volume fraction and the remainder austenite phase. Furthermore, at least one end of the pipe has a fastening part with external or internal threads, and the radius of curvature R of the corner R formed by the thread valley surface and the pressure side surface of the fastening part is 0.2 mm or more.

2. The stainless steel pipe according to claim 1, wherein, The ratio of the compressive yield strength in the circumferential direction to the tensile yield strength in the axial direction is greater than 0.

85.

3. The stainless steel pipe according to claim 1 or 2, wherein, It further contains, by mass%, one or two of the following: W: less than 6.0% and Cu: less than 4.0%.

4. The stainless steel pipe according to any one of claims 1 to 3, wherein, It further contains, by mass%, one or more of the following: Ti: less than 0.50%, Al: less than 0.30%, and V: less than 0.55%.

5. The stainless steel pipe according to any one of claims 1 to 4, wherein, Further, by mass%, it contains one or more of the following: less than 0.010% B, less than 0.10% Zr, less than 0.010% Ca, less than 0.3% Ta, less than 0.10% REM, and less than 0.10% Mg.

6. The stainless steel pipe according to any one of claims 1 to 5, wherein, It further contains, by mass%, one or more of the following: Sn: less than 0.30%, Sb: less than 0.30%, and Ag: less than 0.30%.

7. The stainless steel pipe according to any one of claims 1 to 6, wherein, The stainless steel pipe is a seamless steel pipe.

8. The stainless steel pipe according to any one of claims 1 to 7, wherein, The radius of curvature of the corner R is 0.3 mm or more.

9. The stainless steel pipe according to claim 8, wherein, The fastening part includes a metal contact seal and a torque shaft shoulder.

10. A method for manufacturing a stainless steel pipe, comprising the method for manufacturing a stainless steel pipe according to any one of claims 1 to 9, wherein the pipe is stretched in the direction of the pipe axis, and then subjected to heat treatment at a heating temperature of 150 to 600°C excluding 460 to 480°C.

11. A method for manufacturing a stainless steel pipe, comprising the method for manufacturing a stainless steel pipe according to any one of claims 1 to 9, wherein a stretching process is performed in the direction of the pipe axis at a processing temperature of 150 to 600°C, excluding 460 to 480°C.

12. The method for manufacturing a stainless steel pipe according to claim 11, wherein, After the stretching process, a heat treatment is further performed at a heating temperature of 150-600°C, excluding 460-480°C.

13. A method for manufacturing a stainless steel pipe, which is the method for manufacturing a stainless steel pipe according to any one of claims 1 to 9, wherein a bending-back bending process is performed in the circumferential direction of the pipe.

14. The method for manufacturing a stainless steel pipe according to claim 13, wherein, The processing temperature for the bending-back bending process in the circumferential direction of the tube is below 600°C, excluding 460-480°C.

15. The method for manufacturing a stainless steel pipe according to claim 13 or 14, wherein, After the bending-back bending process, the material is further heat-treated at a heating temperature of 150-600°C, excluding 460-480°C.

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