Cr-Ni alloy pipe

Abstract

A Cr-Ni alloy tube in which the tensile yield stress (YSLT) and the compressive yield stress (YSLC) in the tube axis direction of the Cr-Ni alloy tube, and the tensile yield stress (YSHT) of a test piece after high-frequency induction heating and quenching, which are acquired from the center position of the thickness of the Cr-Ni alloy tube so that the longitudinal direction is parallel to the tube axis direction, satisfy [0.80 < = YSLC / YSLTlt; [YSHT-0. 24 * YSLT > = 50.0] and [YSLT > = 443 * YSLT / YSLC + 2.28 * YSHT-350] are provided in the invention.

Description

Technical Field

[0001] This invention relates to Cr-Ni alloy pipes. Background Technology

[0002] In recent years, the extraction conditions of oil wells and gas wells (hereinafter collectively referred to as "oil wells") have become more demanding. Therefore, oil well pipes used for oil wells sometimes use austenitic alloy pipes containing Cr and Ni and having excellent corrosion resistance (hereinafter also referred to as "Cr-Ni alloy pipes").

[0003] In addition to excellent corrosion resistance, oil well tubing also requires high strength. The strength grade of oil well tubing is generally defined by the tensile yield stress in the pipe axis direction. Based on trial production and geological surveys, oil well tubing users calculate the environment of the well to be exploited (formation pressure, temperature and pressure of the production fluid) and select oil well tubing with a durable strength grade.

[0004] However, it is known that in cold-worked corrosion-resistant alloy pipes, the compressive yield strength in the pipe axis direction is less than the tensile yield stress in the pipe axis direction. As mentioned above, the strength grade of oil well pipes is generally evaluated based on tensile yield strength. Therefore, it is preferable that the difference between the compressive yield stress and the tensile yield stress is small.

[0005] Patent document 1 discloses an austenitic alloy tube that is durable even when subjected to different stress distributions depending on the usage environment due to its small anisotropy in yield strength.

[0006] Existing technical documents

[0007] Patent documents

[0008] Patent Document 1: International Publication No. 2012 / 128258 Summary of the Invention

[0009] The problem the invention aims to solve

[0010] On the other hand, Cr-Ni alloy tubing used as oil well tubing is sometimes welded together with other alloy tubing or steel tubing. In this case, the weld joint of the Cr-Ni alloy tubing softens due to the welding heat. Therefore, it is necessary to select oil well tubing with a strength grade that can withstand the target oil well environment, taking into account the softening of the weld joint.

[0011] To maintain high strength even when the weld softens, it is conceivable to increase the degree of cold working, but this results in a significant reduction in compressive yield stress compared to tensile yield stress, and is therefore not preferred.

[0012] The purpose of this invention is to solve the above-mentioned problems and provide a Cr-Ni alloy tube that minimizes the difference between compressive yield stress and tensile yield stress and has high strength.

[0013] Solution for solving the problem

[0014] The present invention was made to solve the above-mentioned problems, and its main purpose is the Cr-Ni alloy tube described below.

[0015] (1) A Cr-Ni alloy pipe, wherein,

[0016] The tensile yield stress YS in the axial direction of the aforementioned Cr-Ni alloy tube LT and compressive yield stress YS LC And the tensile yield stress YS of the following test pieces. HT Satisfying equations (i) to (iii) below,

[0017] The test piece was obtained as follows: it was collected from the center of the wall thickness of the aforementioned Cr-Ni alloy tube, with the length direction parallel to the tube axis. Using a high-frequency induction heating device, it was heated from room temperature to 1150°C at a heating rate of 100°C / s, then heated to 1200°C at a heating rate of 25°C / s, and held at 1200°C for 180s. Then, by blowing Ar gas, it was rapidly cooled to room temperature at a cooling rate of 50°C / s within a temperature range of 1200°C to 800°C, thus obtaining the test piece.

[0018] 0.80≤YS LC / YS LT <0.95…(i)

[0019] YS HT -0.24×YS LT ≥50.0…(ii)

[0020] YS LT ≥443×YS LT / YS LC +2.28×YS HT -350…(iii)

[0021] The meanings of the symbols in the above formula are as follows.

[0022] YS LC Compressive yield stress (MPa) along the tube axis

[0023] YS LT Tensile yield stress (MPa) in the axial direction of the tube.

[0024] YS HTTensile yield stress (MPa) of the test specimens after high-frequency induction heating and rapid cooling.

[0025] (2) According to the Cr-Ni alloy tube described in (1) above, wherein the tensile yield stress YS in the axial direction of the aforementioned Cr-Ni alloy tube is LT It is above 780MPa.

[0026] (3) The Cr-Ni alloy tube according to (1) or (2) above has the following chemical composition in mass %:

[0027] C: Below 0.030%

[0028] Si: less than 0.50%

[0029] Mn: below 1.00%

[0030] P: below 0.030%

[0031] S: below 0.0050%

[0032] Cr: 19.00~32.00%

[0033] Ni: 29.50~55.00%

[0034] Mo: 2.50–12.00%

[0035] Cu: below 3.00%

[0036] V: 0.01~0.50%

[0037] Nb+Ti: 0.002~1.000%

[0038] Al: 0.001~0.500%

[0039] N: 0.005~0.400%

[0040] W: 0~1.00%

[0041] Co: 0-1.00%

[0042] Sn: 0~0.010%

[0043] As: 0~0.010%

[0044] Zn: 0~0.010%

[0045] Pb: 0~0.010%

[0046] Sb: 0~0.010%

[0047] B: 0~0.0050%

[0048] Ca: 0~0.0200%

[0049] Mg: 0~0.0200%

[0050] REM: 0~0.100%,

[0051] The balance consists of Fe and impurities.

[0052] (4) The Cr-Ni alloy tube according to (3) above, wherein the aforementioned chemical composition contains, by mass%, a variety selected from...

[0053] W: 0.01~1.00%

[0054] Co: 0.01~1.00%

[0055] Sn: 0.001~0.010%

[0056] As: 0.001~0.010%

[0057] Zn: 0.001~0.010%

[0058] Pb: 0.0005~0.010%

[0059] Sb: 0.0005~0.010%

[0060] B: 0.0001~0.0050%

[0061] Ca: 0.0005~0.0200%

[0062] Mg: 0.0001–0.0200%, and

[0063] REM: One or more of the following: 0.001% to 0.100%.

[0064] (5) The Cr-Ni alloy tube according to (3) or (4) above, wherein the aforementioned chemical composition also satisfies the following formulas (iv) and (v).

[0065] 9Mo + 100N ≥ 45.0…(iv)

[0066] Ni+1.4Cu+Mo-0.9Cr-2Si-1.2Mn-5.7C+20N+90(0.15Nb+0.27V+0.3Ti)≤42.0…(v)

[0067] In the above formula, the element symbols refer to the content (mass%) of each element in the alloy tube, and zero is substituted if the element is not present.

[0068] (6) The Cr-Ni alloy tube according to any one of (3) to (5) above, wherein the aforementioned chemical composition also satisfies the following formula (vi).

[0069] 0.001×YS LT -0.820 <N<0.001×YS LT -0.740…(vi)

[0070] The meanings of the symbols in the above formula are as follows.

[0071] YS LC Compressive yield stress (MPa) along the tube axis

[0072] N: N content (mass%) in the alloy tube

[0073] The effects of the invention

[0074] According to the present invention, it is possible to obtain a Cr-Ni alloy tube that minimizes the difference between compressive yield stress and tensile yield stress and has high strength. Detailed Implementation

[0075] The features of the present invention will now be described in detail.

[0076] 1. Mechanical properties

[0077] The mechanical properties of the Cr-Ni alloy tube of this embodiment will be described. The Cr-Ni alloy tube of this embodiment has mechanical properties that satisfy equations (i) to (iii) below. It should be noted that, in this invention, the Cr-Ni alloy tube refers to an austenitic alloy tube containing Cr: 19.00 to 32.00% and Ni: 29.50 to 55.00%.

[0078] <Intensity Anisotropy>

[0079] The Cr-Ni alloy tube of this embodiment improves its strength through work hardening caused by cold working. However, if high-temperature heat treatment is subsequently performed, the strength will decrease and it will be unable to maintain the strength required for the service environment. That is, the Cr-Ni alloy tube of this embodiment is in a state of being either cold-worked or has undergone low-temperature heat treatment after cold working. Therefore, the tensile yield stress in the axial direction of the Cr-Ni alloy tube is necessarily higher than the compressive yield stress in the axial direction.

[0080] However, if the difference between the compressive yield stress and the tensile yield stress in the pipe axis direction is excessive, then even if the tensile yield stress is sufficient, the compressive yield stress will be insufficient, making it unusable as an oil well pipe. Therefore, the compressive yield stress and tensile yield stress in the pipe axis direction need to satisfy the following equation (i).

[0081] 0.80≤YS LC / YS LT <0.95…(i)

[0082] The meanings of the symbols in the above formula are as follows.

[0083] YS LC Compressive yield stress (MPa) along the tube axis

[0084] YS LT Tensile yield stress (MPa) in the axial direction of the tube.

[0085] YS LC / YS LT The value is preferably 0.82 or higher, more preferably 0.84 or higher, and even more preferably 0.86 or higher. Additionally, YS LC / YS LT The value can also be set to below 0.94.

[0086] It should be noted that the tensile yield stress YS in the tube axis direction... LT Without requiring special restrictions, as described above, the Cr-Ni alloy tube of this embodiment is in a state of cold working or cold working followed by low-temperature heat treatment, therefore its tensile yield stress is 780 MPa or higher. The tensile yield stress YS in the tube axial direction... LT Preferably, it is 800 MPa or higher, more preferably 820 MPa or higher, even more preferably 840 MPa or higher, and even more preferably 850 MPa or higher.

[0087] Specifically, the compressive yield stress in the tube axis direction is determined using the following method. A compression test is performed according to ASTM E9 (2019). A cylindrical test specimen for the compression test is prepared from the center of the wall thickness of the Cr-Ni alloy tube. The dimensions of the cylindrical test specimen are, for example, a diameter of 4 mm and a length of 8 mm at the parallel section. It should be noted that the length direction of the cylindrical test specimen is parallel to the tube axis direction of the Cr-Ni alloy tube. Using the cylindrical test specimen, a compression test is performed at room temperature (25°C) in atmospheric conditions. The obtained 0.2% conditional yield strength is defined as the compressive yield stress YS in the tube axis direction. LC (MPa).

[0088] Similarly, the tensile yield stress in the tube axis direction is determined by the following method. Specifically, a tensile test is performed according to the method of ASTM E8 / E8M (2021). A round bar specimen for tensile testing is prepared from the center of the wall thickness of the Cr-Ni alloy tube. The dimensions of the round bar specimen are, for example, a parallel section diameter of 4 mm and a gauge length of 20 mm. It should be noted that the length direction of the round bar specimen is parallel to the tube axis direction of the Cr-Ni alloy tube. Using the round bar specimen, a tensile test is performed at room temperature (25°C) in atmospheric conditions, and the obtained 0.2% conditional yield strength is defined as the tensile yield stress YS. LT (MPa).

[0089] <Basic Strength>

[0090] As mentioned above, the strength of Cr-Ni alloy tubes can be improved through cold working, but excessive working can lead to increased anisotropy. Therefore, to avoid excessive working, it is necessary to ensure sufficient strength before cold working, i.e., strength in the state without work hardening (hereinafter also referred to as "basic strength"). The inventors have found that the basic strength index can be obtained based on the tensile yield stress of test pieces that have eliminated the effects of work hardening through high-frequency induction heating and quenching, and the tensile yield stress in the axial direction of the Cr-Ni alloy tube.

[0091] Specifically, by satisfying the following formula (ii), it will have sufficient basic strength for use as an oil well pipe.

[0092] YS HT -0.24×YS LT ≥50.0…(ii)

[0093] The meanings of the symbols in the above formula are as follows.

[0094] YS LT Tensile yield stress (MPa) in the axial direction of the tube.

[0095] YS HT Tensile yield stress (MPa) of the test specimens after high-frequency induction heating and rapid cooling.

[0096] It should be noted that the test pieces after high-frequency induction heating and quenching are obtained through the following steps. First, the test piece is collected from the center of the Cr-Ni alloy tube wall, with its length parallel to the tube axis. This test piece is prepared according to ASTM E8 / E8M (2021). Specifically, for alloy tubes with a wall thickness of 6 mm or more, a cylindrical test piece with a diameter of 6 mm and a gauge length of 30 mm is used. Alternatively, for alloy tubes with a wall thickness less than 6 mm, an arc-shaped test piece with the same thickness as the alloy tube wall and a gauge length of 50 mm is used.

[0097] Next, the cylindrical test piece was heated from room temperature to 1150°C at a heating rate of 100°C / s using a high-frequency induction heating device, and then heated to 1200°C at a heating rate of 25°C / s. The heating area based on the high-frequency induction heating device was defined as a region extending 3.5 mm from the center of the test piece along its length to both ends, with a length of 7.0 mm. Furthermore, the temperature of the test piece was measured using a thermocouple positioned at the center of the test piece along its length.

[0098] The test piece was then held at 1200°C for 180 s. Immediately after holding, Ar gas was blown through, thereby rapidly cooling to room temperature at a rate of 50°C / s over a temperature range of 1200°C to 800°C. Then, using the test piece after high-frequency induction heating and rapid cooling, a tensile test was performed at room temperature (25°C) in atmospheric conditions according to JIS Z 2241 (2011). The obtained 0.2% conditional yield strength was defined as the tensile yield stress YS of the test piece after high-frequency induction heating and rapid cooling. HT (MPa).

[0099] <Work hardening ability>

[0100] To suppress the increase of anisotropy and improve strength through cold working, excellent work hardening ability is needed, which can efficiently perform work hardening even with small machinability. The inventors conducted in-depth research and found that YS, as an indicator of anisotropy... LC / YS LT The reciprocal of YS LT / YS LC It also became an indicator of workability. Furthermore, it was found that the tensile yield stress in the axial direction of Cr-Ni alloy pipes, and YS, which serves as an indicator of workability... LT / YS LC The tensile yield stress YS of the test piece whose work hardening effect was eliminated by high-frequency induction heating and rapid cooling. HT The relationship becomes an indicator of work hardening capacity.

[0101] Specifically, by satisfying the following equation (iii), it will have sufficient work hardening capability for use as an oil well pipe.

[0102] YS LT ≥443×YS LT / YS LC +2.28×YS HT -350…(iii)

[0103] The meanings of the symbols in the above formula are as follows.

[0104] YSLC Compressive yield stress (MPa) along the tube axis

[0105] YS LT Tensile yield stress (MPa) in the axial direction of the tube.

[0106] YS HT Tensile yield stress (MPa) of the test specimens after high-frequency induction heating and rapid cooling.

[0107] As described above, based on the three mechanical properties of Cr-Ni alloy pipes—the tensile yield stress and compressive yield stress in the pipe axis direction, and the tensile yield stress of the test pieces after high-frequency induction heating and rapid cooling—Cr-Ni alloy pipes can maintain high strength even when the weld softens, making them suitable as blanks for high-reliability oil well pipes.

[0108] 2. Chemical composition

[0109] Regarding the chemical composition of the Cr-Ni alloy tube of this embodiment, there are no particular limitations as long as it contains a specified amount of Cr and Ni and possesses the aforementioned mechanical properties. The Cr-Ni alloy tube of this embodiment preferably has, for example, the chemical composition shown below. The reasons for limiting each element are explained below. It should be noted that in the following description, "%" for content refers to "mass %".

[0110] C: Below 0.030%

[0111] Carbon (C) is an element that improves the strength of alloys. However, if C is present in excess, Cr carbides will form at the grain boundaries. Cr carbides increase the susceptibility to stress corrosion cracking (SCC) at the grain boundaries. Therefore, the C content is 0.030% or less. Preferably, the C content is less than 0.030%, more preferably 0.025%, and even more preferably 0.020% or less. The lower limit of the preferred C content is 0.001%, more preferably 0.003%, and even more preferably 0.005%.

[0112] Si: below 0.50%

[0113] Silicon (Si) is an element that deoxidizes alloys. However, if Si is present in excess, the hot workability of the alloy decreases. Therefore, the Si content is 0.50% or less. Preferably, the Si content is less than 0.50%. The upper limit of the preferred Si content is 0.45%, more preferably 0.40%. The lower limit of the preferred Si content is 0.05%, more preferably 0.10%.

[0114] Mn: below 1.00%

[0115] Manganese (Mn) is an element that deoxidizes alloys. Additionally, Mn is an austenite-forming element, stabilizing the austenite phase. However, excessive Mn content reduces the hot workability of the alloy. Therefore, the Mn content is 1.00% or less. Preferably, the Mn content is less than 1.00%. The upper limit of the preferred Mn content is 0.90%, more preferably 0.80%. The lower limit of the preferred Mn content is 0.10%, more preferably 0.30%.

[0116] P: below 0.030%

[0117] Phosphorus (P) is an impurity element. P increases the stress corrosion cracking susceptibility of alloys in a hydrogen sulfide environment. Therefore, a low P content, preferably 0.030% or less, is preferred. A more preferred P content is 0.025% or less, and even more preferably 0.020% or less.

[0118] S: below 0.0050%

[0119] Sulfur (S) is an impurity element. S reduces the hot workability of alloys. Therefore, it is preferable to have a low S content, 0.0050% or less. More preferably, the S content is 0.0030% or less, and even more preferably 0.0010% or less.

[0120] Cr: 19.00~32.00%

[0121] Chromium (Cr) is an element that improves the resistance to stress corrosion cracking (SCC) of alloys when coexisting with Ni. Cr also enhances the strength of alloys through solid solution strengthening. On the other hand, if Cr is present in excess, its effect saturates, thereby reducing the hot workability of the alloy. Therefore, the Cr content is 19.00% to 32.00%. The preferred lower limit of the Cr content is 21.00%, more preferably 23.00%. The preferred upper limit of the Cr content is 30.00%, more preferably 28.00%.

[0122] Ni: 29.50~55.00%

[0123] Nickel (Ni) is an austenite-forming element that stabilizes the austenite phase. Ni also improves resistance to SCC (sulfate-corrosion cracking). However, its effect saturates if Ni is present in excess. Therefore, the Ni content is 29.50% to 55.00%. The preferred lower limit of the Ni content is 34.00%, more preferably 40.00%. The preferred upper limit of the Ni content is 53.00%, more preferably 50.00%.

[0124] Mo: 2.50–12.00%

[0125] Molybdenum (Mo) is an element that improves the SCC resistance of alloys when coexisting with Cr and Ni. Mo further enhances the strength of the alloy through solid solution strengthening. However, if Mo is present in excess, its effect saturates, leading to a decrease in the hot workability of the alloy. Therefore, the Mo content is 2.50% to 12.00%. The preferred lower limit of the Mo content is 3.50%, more preferably 5.00%. The preferred upper limit of the Mo content is 10.00%, more preferably 8.00%.

[0126] Cu: below 3.00%

[0127] Copper (Cu) is effective in stabilizing passivation coatings and is an effective element in improving resistance to pitting and general corrosion. However, if Cu is present in excess, its effect becomes saturated, leading to reduced hot workability. Therefore, the Cu content is 3.00% or less. Preferably, the Cu content is 2.00% or less, more preferably 1.20% or less. The lower limit of the preferred Cu content is 0.10%, more preferably 0.50%.

[0128] V: 0.01~0.50%

[0129] Vanadium (V) is an element that enhances the strength of alloys by finely precipitating within the grains in the form of carbides, nitrides, or carbonitrides. It also refines grains by pinning grain boundaries. V increases the solubility of nitrogen (N) in the alloy, thus suppressing pinhole formation near the surface when the N content is increased to improve alloy strength. However, excessive V content leads to excessive precipitates, resulting in reduced toughness and increased risk of weld cracking in the heat-affected zone. Therefore, the V content is typically 0.01–0.50%. A lower limit for the preferred V content is 0.02%, more preferably 0.03%. An upper limit for the preferred V content is 0.35%, more preferably 0.20%.

[0130] Nb+Ti: 0.002~1.000%

[0131] Niobium (Nb) and titanium (Ti) are elements that enhance the strength of alloys by finely precipitating within the grains as carbides, nitrides, or carbonitrides. They also refine the grains by pinning grain boundaries. Nb and Ti increase the solubility of nitrogen in the alloy, thus suppressing pinhole formation near the surface when the nitrogen content is increased to improve alloy strength. However, excessive Nb and / or Ti results in excessive precipitates, leading to reduced toughness and increased risk of weld cracking in the heat-affected zone. Therefore, the total content of one or both of Nb and Ti is 0.002–1.000%. A lower limit of the preferred total content is 0.010%, more preferably 0.030%. An upper limit of the preferred total content is 0.500%, more preferably 0.300%.

[0132] Al: 0.001~0.500%

[0133] Aluminum (Al) is an element that deoxidizes alloys. However, excessive Al content can impair purity, processability, and ductility. Therefore, the Al content is 0.001–0.500%. The preferred lower limit of the Al content is 0.005%, more preferably 0.010%. The preferred upper limit of the Al content is 0.300%, more preferably 0.100%. It should be noted that in this invention, the Al content refers to the content of acid-soluble Al (so-called "sol.Al").

[0134] N: 0.005~0.400%

[0135] Nitrogen (N) enhances the strength of alloys through solid solution strengthening. By including N and performing solid solution heat treatment, high-strength alloy tubes can be obtained. Using high-strength alloy tubes, even with low-degree cold working, the desired strength can be achieved after cold working. However, if N is present in excess, pinholes are easily formed near the surface during alloy solidification. Furthermore, N reduces the hot workability of the alloy. Therefore, the N content is 0.005–0.400%. The preferred lower limit of the N content is 0.010%, more preferably 0.030%, and even more preferably 0.060%. The preferred upper limit of the N content is 0.350%, more preferably 0.300%.

[0136] In addition to the elements mentioned above, the Cr-Ni alloy tube of this embodiment may also contain one or more elements selected from W, Co, Sn, As, Zn, Pb, Sb, B, Ca, Mg, and REM. It should be noted that there is no lower limit to the content of any element; it can be 0%.

[0137] W: 0~1.00%

[0138] Tungsten (W) is a selective element. Therefore, it may be optional to omit W. W has the effect of improving SCC resistance, so it can be included as needed. However, if W is present in excess, its effect will saturate, thereby reducing hot workability. Therefore, the W content is 1.00% or less. Preferably, the W content is 0.50% or less, more preferably 0.30% or less. The preferred lower limit of the W content is 0.01%, more preferably 0.05%.

[0139] Co: 0~1.00%

[0140] Cobalt (Co) is a preferred element. Therefore, it may be omitted. Co, like Ni, has the effect of improving the stability of the austenitic phase, and therefore can be included as needed. However, Co is an extremely expensive element, so increasing its content leads to increased costs. Therefore, the Co content is 1.00% or less. Preferably, the Co content is 0.50% or less, more preferably 0.30% or less. The lower limit of the preferred Co content is 0.01%, more preferably 0.05%.

[0141] Sn: 0~0.010%

[0142] Tin (Sn) is the selected element. Therefore, it may be optional to omit Sn. Sn can be included as needed because it influences convection in the weld pool, increases weld penetration, and improves weldability. However, excessive Sn content not only increases weld crack susceptibility but also reduces corrosion resistance. Therefore, the Sn content is 0.010% or less. Preferably, the Sn content is 0.008% or less, more preferably 0.006% or less. The preferred lower limit for the Sn content is 0.001%, more preferably 0.002%.

[0143] As: 0~0.010%

[0144] Arsenic (As) is the selected element. Therefore, it is also possible to omit As. As can be included as needed because it affects the convection of the molten pool during welding, increases the penetration depth, and improves weldability. However, excessive As content will not only increase the susceptibility to weld cracking but also lead to a decrease in corrosion resistance. Therefore, the As content is 0.010% or less. Preferably, the As content is 0.008% or less, more preferably 0.006% or less. The preferred lower limit of the As content is 0.001%, more preferably 0.002%.

[0145] Zn: 0~0.010%

[0146] Zinc (Zn) is the selected element. Therefore, it may be omitted. Zn can be included as needed because it influences the convection of the molten pool during welding, increases weld penetration, and improves weldability. However, excessive Zn content not only increases weld crack susceptibility but also reduces corrosion resistance. Therefore, the Zn content is 0.010% or less. Preferably, the Zn content is 0.008% or less, more preferably 0.006% or less. The lower limit of the preferred Zn content is 0.001%, more preferably 0.002%.

[0147] Pb: 0–0.010%

[0148] Lead (Pb) is the selected element. Therefore, it may be optional to omit Pb. Pb can be included as needed because it influences convection in the weld pool, increases weld penetration, and improves weldability. However, excessive Pb not only increases weld crack susceptibility but also reduces corrosion resistance. Therefore, the Pb content is 0.010% or less. Preferably, the Pb content is 0.008% or less, more preferably 0.006% or less. The lower limit of the preferred Pb content is 0.0005%, more preferably 0.001%.

[0149] Sb: 0~0.010%

[0150] Antimony (Sb) is the selected element. Therefore, it may be optional to omit Sb. Sb can be included as needed because it influences convection in the weld pool, increases weld penetration, and improves weldability. However, excessive Sb not only increases weld crack susceptibility but also reduces corrosion resistance. Therefore, the Sb content is 0.010% or less. Preferably, the Sb content is 0.008% or less, more preferably 0.006% or less. The preferred lower limit for the Sb content is 0.0005%, more preferably 0.001%.

[0151] B: 0~0.0050%

[0152] Boron (B) is a selective element. Therefore, it may be omitted. B is an element that contributes to grain boundary strengthening and can be included as needed to improve hot workability. However, if B is present in excess, its effect becomes saturated, leading to liquefaction cracking at the grain boundaries in the heat-affected zone during welding. Therefore, the B content is 0.0050% or less. Preferably, the B content is 0.0030% or less, more preferably 0.0015% or less. The lower limit of the preferred B content is 0.0001%, more preferably 0.0003%.

[0153] Ca: 0~0.0200%

[0154] Calcium (Ca) is a selective element. Therefore, it may be omitted. Ca has the effect of fixing sulfur in the form of sulfides, improving the hot workability of the alloy, and therefore can be included as needed. However, if Ca is present in excess, coarse oxides will form, which will reduce the hot workability of the alloy. Therefore, the Ca content is 0.0200% or less. Preferably, the Ca content is 0.0100% or less, more preferably 0.0050% or less. The lower limit of the preferred Ca content is 0.0005%, more preferably 0.0010%.

[0155] Mg: 0~0.0200%

[0156] Magnesium (Mg) is a selective element. Therefore, it may be omitted. Like Ca, Mg has the effect of fixing sulfur in the form of sulfides, improving the hot workability of the alloy, and therefore may be included as needed. However, if Mg is present in excess, coarse oxides will form, which will actually reduce the hot workability of the alloy. Therefore, the Mg content is 0.0200% or less. Preferably, the Mg content is 0.0100% or less, more preferably 0.0050% or less, and even more preferably 0.0020% or less. The lower limit of the preferred Mg content is 0.0001%, more preferably 0.0005%.

[0157] REM: 0~0.100%

[0158] Rare earth elements (REMs) are optional. Therefore, REMs may not be present. Like Ca and Mg, REMs improve the hot workability of the alloy by fixing sulfur in the form of sulfides, and can therefore be included as needed. However, excessive REMs will form coarse oxides, which will actually reduce the hot workability of the alloy. Therefore, the REM content is 0.100% or less. Preferably, the REM content is 0.050% or less, more preferably 0.030% or less. The lower limit of the preferred REM content is 0.001%, more preferably 0.005%.

[0159] In this embodiment, "REM" refers to a total of 17 elements, including Sc, Y, and the lanthanides. "REM content" refers to the content of a single REM element, and to the total content of two or more REM elements. Furthermore, REM is generally supplied in the form of alloys of multiple REM elements, i.e., mixed rare earth alloys. Therefore, it is possible to include one or more individual elements, or to add them in the form of, for example, mixed rare earth alloys.

[0160] The Cr-Ni alloy tube of this embodiment contains all the elements described above, with the balance being Fe and impurities. Here, "impurities" refers to components that may be mixed in during the industrial manufacturing of alloy materials due to various reasons related to raw materials such as ores and waste, as well as manufacturing processes. These components are permissible within the scope that do not adversely affect the present invention.

[0161] The chemical composition of the Cr-Ni alloy tube in this embodiment preferably also satisfies the following formulas (iv) and (v).

[0162] 9Mo + 100N ≥ 45.0…(iv)

[0163] Ni+1.4Cu+Mo-0.9Cr-2Si-1.2Mn-5.7C+20N+90(0.15Nb+0.27V+0.3Ti)≤42.0…(v)

[0164] In the above formula, the element symbols refer to the content (mass%) of each element, and zero is substituted if the element is not present.

[0165] The inventors analyzed the relationship between the mechanical properties and chemical composition of various Cr-Ni alloy pipes, and the results clearly showed that Mo and N are particularly helpful in improving the basic strength in the above chemical composition. Moreover, it was determined that high basic strength can be stably obtained by satisfying the above equation (iv).

[0166] It should be noted that the value of 9Mo+100N does not need to be set with an upper limit, but from the viewpoint of suppressing the reduction of hot workability, it is preferred to be 85.0 or less, and more preferably 83.0 or less.

[0167] Furthermore, focusing on the elements that affect stacking fault energy, the relationship between mechanical properties and chemical composition was analyzed. The results show that by satisfying the above equation (v), excellent work hardening ability can be stably obtained.

[0168] There is no need to set a lower limit for the value on the left side of equation (v), and the lower the value, the better. However, considering the range of the content of each element, the actual lower limit is 1.2.

[0169] The chemical composition of the Cr-Ni alloy tube in this embodiment preferably also satisfies the following formula (vi). As mentioned above, there is a tendency for increased anisotropy when the cold workability is high. However, by adjusting the N content to satisfy the following formula (vi) in relation to the tensile yield stress in the axial direction of the Cr-Ni alloy tube, the increase in anisotropy can be easily suppressed even when the strength is above 900 MPa.

[0170] 0.001×YS LT -0.820 <N<0.001×YS LT -0.740…(vi)

[0171] YS LC Compressive yield stress (MPa) along the tube axis

[0172] N: N content (mass%) in the alloy tube

[0173] 3. Manufacturing method

[0174] An example of the manufacturing method of the Cr-Ni alloy tube of this embodiment will be described.

[0175] First, a molten austenitic alloy is produced by melting it. The alloy can be melted using an electric furnace, an Ar-O2 mixed gas bottom-blown decarburization furnace (AOD furnace), a vacuum decarburization furnace (VOD furnace), etc.

[0176] Next, the molten metal is used to manufacture casting materials. These materials can be, for example, ingots, slabs, or large billets. Specifically, ingots are manufactured using the ingot casting method, or slabs or large billets are manufactured using the continuous casting method.

[0177] Next, the casting material is hot-worked to produce a round billet. Hot working methods include, for example, hot rolling or hot forging. The produced round billet is then hot-worked to produce a tube blank. Specifically, tube blanks are produced from round billets using extrusion tube making methods, such as high-speed glass lubricant extrusion. Alternatively, tube blanks are produced from round billets using the Mannesmann tube making process.

[0178] The manufactured tube blank is subjected to cold working. Cold working includes cold drawing and cold rolling, represented by Pilger rolling. In this invention, either cold drawing or cold rolling can be used. Compared with cold rolling, cold drawing imparts a large tensile strain to the alloy tube in the tube axis direction. Cold rolling imparts a large strain not only in the tube axis direction but also in the tube circumferential direction. Therefore, compared with cold drawing, cold rolling imparts a large compressive strain to the tube blank in the tube circumferential direction.

[0179] The preferred reduction of area during cold working is 15% or more. Here, the reduction of area is defined by equation (I) below. The lower limit of the preferred reduction of area is 20%. On the other hand, if the reduction of area is too high, not only will the roundness of the alloy tube decrease, but the anisotropy will also become significant. Therefore, the upper limit of the preferred reduction of area during cold working is 50%.

[0180] Reduction of area (%) = 100 × (Cross-sectional area of ​​the tube blank before cold working - Cross-sectional area of ​​the tube blank after cold working) / Cross-sectional area of ​​the tube blank before cold working…(I)

[0181] Other treatments can also be performed between hot working and cold working. For example, solution heat treatment can be applied to the hot-worked tube blank. In this case, the tube blank after solution heat treatment is descaled, and after descaling, cold working is performed. Furthermore, multiple cold workings can also be performed. In the case of multiple cold workings, solution heat treatment can be performed as a softening heat treatment between the cold working and the next cold working.

[0182] Furthermore, after cold working, straightening processing using an inclined roller straightener and / or low-temperature heat treatment can be performed to reduce anisotropy. Both straightening processing and low-temperature heat treatment can be performed first. However, if the heat treatment temperature is too high, the yield stress will decrease excessively, making it impossible to maintain the strength required for the service environment. Therefore, when performing low-temperature heat treatment, the heating temperature is set to 300–550°C, preferably 400–500°C.

[0183] The dimensions of the alloy tube obtained through the above process are not particularly limited. From the viewpoint of maintaining the strength required in the service environment, the wall thickness of the alloy tube is preferably 4 mm or more, more preferably 6 mm or more, even more preferably 8 mm or more, and even more preferably 10 mm or more.

[0184] The present invention will be described in more detail below through embodiments, but the present invention is not limited to these embodiments.

[0185] Example

[0186] The alloy with the chemical composition shown in Table 1 was melted and cast in a vacuum to obtain an ingot with an outer diameter of 120 mm and a weight of 30 kg. This ingot was then hot-forged to produce a billet with a thickness of 50 mm. The temperature of the ingot before hot forging was 1200°C. The billet was then hot-rolled. To mitigate the strength variation of the product, the thickness of the intermediate alloy material (alloy plate) was determined by back-calculation, ensuring that the thickness after cold rolling was uniformly 11.5 mm after applying the cold working rate shown in Table 2. Then, a softening heat treatment was performed, with a 10-minute heat treatment at the temperature shown in Table 2 followed by water cooling. Next, the thickness was uniformly 11.5 mm after applying the cold working rate shown in Table 2. For experiments No. 22 and 23, a further 10-minute heat treatment at the temperature shown in Table 2, followed by water cooling solution heat treatment, was performed after cold working. For experiment No. 24, no cold working was performed. It should be noted that in this embodiment, from the viewpoint of ease of experimentation, an alloy plate was used instead of an alloy tube.

[0187] [Table 1]

[0188]

[0189] [Table 2]

[0190]

[0191] Next, the compressive yield stress YS in the tube axis direction is calculated using the above method. LC (MPa) and tensile yield stress YS LT (MPa). Specifically, cylindrical test specimens for compression testing and round bar test specimens for tensile testing were collected from the center of the thickness of the alloy plate for each test number, with the length direction parallel to the rolling direction. The parallel portion of the cylindrical test specimen had a diameter of 4 mm and a length of 8 mm. The parallel portion of the round bar test specimen had a diameter of 4 mm and a gauge length of 20 mm. For the cylindrical test specimens used in the compression test, the compression test was conducted at room temperature (25°C) in atmospheric conditions according to the method of ASTM E9 (2019). The 0.2% conditional yield strength obtained through the compression test was taken as the compressive yield stress YS in the tube axis direction. LC(MPa). Furthermore, for the round bar specimens used in the tensile test, tensile tests were conducted at room temperature (25°C) in atmospheric conditions using the method according to ASTM E8 / E8M (2013). The 0.2% conditional yield strength obtained through the tensile test was taken as the tensile yield stress YS in the tube axis direction. LT (MPa).

[0192] Furthermore, the tensile yield stress YS of the test piece after high-frequency induction heating and rapid cooling was determined using the above method. HT (MPa). Specifically, cylindrical test pieces for high-frequency induction heating and quenching were collected from the center of the thickness of the alloy plate for each test number, with the length direction parallel to the rolling direction. It should be noted that the cylindrical test pieces were manufactured according to ASTM E8 / E8M (2021), with a diameter of 6 mm and a gauge length of 30 mm. Using a high-frequency induction heating device, the cylindrical test pieces for high-frequency induction heating and quenching were heated from room temperature to 1150°C at a heating rate of 100°C / s, and then heated to 1200°C at a heating rate of 25°C / s. At this point, the heating area based on the high-frequency induction heating device was set as a region extending 3.5 mm from the center of the test piece along its length, with a length of 7.0 mm. Furthermore, the temperature of the test piece was measured using a thermocouple positioned at the center of the test piece along its length.

[0193] The test piece was then held at 1200°C for 180 s. Immediately after holding, Ar gas was blown through, thereby rapidly cooling to room temperature at a rate of 50°C / s over a temperature range of 1200°C to 800°C. Then, using the test piece after high-frequency induction heating and rapid cooling, a tensile test was performed at room temperature (25°C) in atmospheric conditions according to JIS Z 2241 (2011). The obtained 0.2% conditional yield strength was defined as the tensile yield stress YS of the test piece after high-frequency induction heating and rapid cooling. HT (MPa).

[0194] The results are shown in Table 2. As shown in Table 2, the alloy materials that meet all the requirements of this invention have a high tensile yield stress of 780 MPa or more, and can make YS LC / YS LT It is above 0.80.

[0195] Industrial availability

[0196] According to the present invention, a Cr-Ni alloy pipe with high strength can be obtained, which minimizes the difference between compressive yield stress and tensile yield stress. Therefore, by selecting the Cr-Ni alloy pipe of the present invention as the blank for oil well pipe, a highly reliable oil well pipe that can maintain high strength even when softening occurs at the weld is obtained.

Claims

1. A Cr-Ni alloy pipe, wherein, The tensile yield stress YS in the axial direction of the Cr-Ni alloy tube LT and compressive yield stress YS LC And the tensile yield stress YS of the following test pieces. HT Satisfying equations (i) to (iii) below, The test piece was obtained as follows: A sample was taken from the center of the wall thickness of the Cr-Ni alloy tube, with the length direction parallel to the tube axis. Using a high-frequency induction heating device, the sample was heated from room temperature to 1150°C at a heating rate of 100°C / s, then heated to 1200°C at a heating rate of 25°C / s, and held at 1200°C for 180s. Then, by blowing Ar gas, the sample was rapidly cooled to room temperature at a cooling rate of 50°C / s within a temperature range of 1200°C to 800°C, thus obtaining the test piece. 0.80≤YS LC / YS LT <0.95…(i) YS HT −0.24×YS LT ≥50.0…(ii) YS LT ≥443×YS LT / YS LC +2.28×YS HT -350…(iii) The meanings of the symbols in the above formula are as follows. YS LC Compressive yield stress in the axial direction of the tube, in MPa. YS LT Tensile yield stress in the axial direction of the tube, in MPa. YS HT Tensile yield stress of the test piece after high-frequency induction heating and rapid cooling, in MPa.

2. The Cr-Ni alloy pipe according to claim 1, wherein, The tensile yield stress YS in the axial direction of the Cr-Ni alloy tube LT It is above 780MPa.

3. The Cr-Ni alloy pipe according to claim 1 or claim 2, wherein its chemical composition, by mass%, is C: less than 0.030%. Si: less than 0.50% Mn: below 1.00% P: below 0.030% S: below 0.0050% Cr：19.00～32.00％、 Ni: 29.50~55.00% Mo: 2.50–12.00% Cu: below 3.00% V：0.01～0.50％、 Nb+Ti: 0.002~1.000% Al：0.001～0.500％、 N：0.005～0.400％、 W：0～1.00％、 Co: 0-1.00% Sn: 0~0.010% As: 0~0.010% Zn: 0~0.010% Pb: 0~0.010% Sb: 0~0.010% B：0～0.0050％、 Ca: 0~0.0200% Mg: 0~0.0200% REM: 0~0.100%, The balance consists of Fe and impurities.

4. The Cr-Ni alloy pipe according to claim 3, wherein, The chemical composition contains, by mass%, a selection from […]. W：0.01～1.00％、 Co: 0.01~1.00% Sn: 0.001~0.010% As: 0.001~0.010% Zn: 0.001~0.010% Pb: 0.0005~0.010% Sb: 0.0005~0.010% B：0.0001～0.0050％、 Ca: 0.0005~0.0200% Mg: 0.0001–0.0200%, and REM: One or more of the following: 0.001% to 0.100%.

5. The Cr-Ni alloy pipe according to claim 3 or claim 4, wherein, The chemical composition also satisfies the following equations (iv) and (v). 9Mo + 100N ≥ 45.0…(iv) Ni+1.4Cu+Mo-0.9Cr-2Si-1.2Mn-5.7C+20N+90(0.15Nb+0.27V+0.3Ti)≤42.0…(v) In the formula, the element symbols refer to the mass percentage content of each element in the alloy tube, and zero is substituted if the element is not present.

6. The Cr-Ni alloy tube according to any one of claims 3 to 5, wherein, The chemical composition also satisfies the following equation (vi). 0.001×YS LT -0.820 <N<0.001×YS LT -0.740…(we) The meanings of the symbols in the formula are as follows: YS LC Compressive yield stress in the axial direction of the tube, in MPa. N: The nitrogen content in the alloy tube, expressed as a percentage by mass.

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