Stainless steel pipes and oil well pipes

Abstract

The purpose of this invention is to provide a stainless steel tube with high strength and excellent resistance to hydrogen embrittlement. "Excellent resistance to hydrogen embrittlement" means that in a tensile test under hydrogen embrittlement, the elongation at break is 9.0% or more, and in said tensile test, true strain is taken on the horizontal axis, true stress on the vertical axis, and the true strain under the maximum load is set as ε. max When the true strain is 0.8ε max Above and ε max Within the following range, the rate of change of work hardening rate dσ / dε relative to ε is d 2 σ / dε 2 The strength is -200,000 MPa or higher. σ is the true stress (MPa), and ε is the true strain (unitless). "High strength" refers to a yield strength of 450 MPa or higher. A stainless steel pipe having a specific microstructure, wherein the average KAM (nuclear average orientation difference) value of the steel microstructure is 2.0° or higher and 4.0° or lower, and the standard deviation of the KAM value distribution is 1.5° or lower.

Description

Technical Field

[0001] This invention relates to stainless steel pipes and oil well pipes suitable for use in oil wells. Background Technology

[0002] In recent years, driven by the soaring price of crude oil and the anticipated depletion of oil resources in the near future, the development of previously neglected deep oil fields, as well as oil and gas fields with harsh corrosive environments containing carbon dioxide, chloride ions, and hydrogen sulfide, has become prevalent.

[0003] In hydrogen sulfide environments, hydrogen embrittlement caused by corrosion becomes a problem, thus increasing the demand for oil well tubing with hydrogen embrittlement resistance.

[0004] Patent document 1 proposes a martensitic stainless steel seamless pipe, which improves corrosion resistance by controlling the amount of added elements to keep the amount of residual austenite, repassivation potential and pitting potential within an appropriate range.

[0005] Patent document 2 proposes a duplex stainless steel pipe with improved corrosion resistance by controlling the distribution of austenite and ferrite.

[0006] Existing technical documents

[0007] Patent documents

[0008] Patent Document 1: Japanese Patent No. 6315159

[0009] Patent Document 2: Japanese Patent Application Publication No. 2022-111733 Summary of the Invention

[0010] The problem that the invention aims to solve

[0011] However, the steel pipes described in the aforementioned patent documents 1 and 2 sometimes do not have sufficient resistance to hydrogen embrittlement, and there is room for improvement in the resistance to hydrogen embrittlement.

[0012] The present invention was made in view of the above circumstances, and its object is to provide a stainless steel tube with high strength and excellent resistance to hydrogen embrittlement.

[0013] It should be noted that the "excellent resistance to hydrogen embrittlement" mentioned in this invention refers to an elongation at break of 9.0% or more in the tensile test under hydrogen charging described later, and in the tensile test, the true strain is taken on the horizontal axis, the true stress is taken on the vertical axis, and the true strain under the maximum load is set as ε. max When the true strain is 0.8ε max Above and ε max Within the following range, the rate of change of work hardening rate dσ / dε relative to ε is d 2 σ / dε 2It is above -200,000 MPa.

[0014] Here, σ is the true stress (MPa) and ε is the true strain (unitless).

[0015] In addition, the term "high strength" as used in this invention refers to a yield strength of 450 MPa or higher.

[0016] Methods for solving problems

[0017] The inventors conducted in-depth research and found that if there are regions with high dislocation density in the steel structure, the resistance to hydrogen embrittlement is reduced.

[0018] It was also found that the aforementioned regions tend to form near the interface between ferrite and martensite, or the interface between ferrite and austenite, i.e., near the interface between two phases with a large strength difference.

[0019] This invention is based on the above insights and is constituted by the following main points.

[0020] [1] A stainless steel pipe, wherein in the steel structure in the center of its wall thickness, the total volume fraction of ferrite and martensite is more than 30% and less than 99%, the volume fraction of σ phase is more than 0% and less than 3%, and the balance is austenite, wherein the average KAM (Kernel Average Misorientation) value of the steel structure is more than 2.0° and less than 4.0°, and the standard deviation of the KAM value distribution is less than 1.5°.

[0021] [2] The stainless steel pipe according to [1] has the following composition: by mass % containing C: 0.060% or less, Si: 1.00% or less, Mn: 6.00% or less, P: 0.050% or less, S: 0.0300% or less, Al: 0.005% or more and 0.100% or less, N: 0.400% or less, Cr: 11.00% or more and 30.00% or less, Mo: 5.00% or less, Ni: 15.00% or less, and further optionally containing Cu: 4.00% or more. It contains one or more of the following: less than % %, V: less than 0.300%, Nb: less than 0.300%, Ti: less than 0.300%, B: less than 0.0050%, W: less than 3.00%, Ca: less than 0.0050%, Co: less than 0.500%, Sn: less than 0.100%, Mg: less than 0.020%, Zr: less than 0.020%, REM: less than 0.020%, Ta: less than 0.10%, and Sb: less than 0.100%, with the balance consisting of Fe and unavoidable impurities.

[0022] [3] The stainless steel pipe according to [1] or [2], wherein the stainless steel pipe is a seamless steel pipe.

[0023] [4] An oil well pipe that uses the stainless steel pipe described in any one of [1] to [3] above.

[0024] Invention Effects

[0025] According to the present invention, it is possible to provide stainless steel pipes and oil well pipes with high strength and excellent resistance to hydrogen embrittlement. Attached Figure Description

[0026] Figure 1 This diagram shows a curve obtained by plotting the true strain ε on the horizontal axis and the true stress σ on the vertical axis during a tensile test with hydrogen charging at the cathode. Detailed Implementation

[0027] The stainless steel pipe of the present invention will be described below.

[0028] In the stainless steel pipe of the present invention, at the center of the wall thickness, the total volume fraction of ferrite and martensite is 30% or more and 99% or less, the volume fraction of σ phase is 0% or more and 3% or less, the balance is austenite, the average KAM (nuclear average orientation difference) value is 2.0° or more and 4.0° or less, and the standard deviation of the above KAM value distribution is 1.5° or less.

[0029] It should be noted that "volume fraction" will also be referred to as "fraction" below.

[0030] The microstructure of the steel in the center of the wall thickness consists of a combined volume fraction of ferrite and martensite of ≥30% and ≤99%, a volume fraction of σ phase of ≥0% and ≤3%, with the balance being austenite.

[0031] Ferrite is a soft microstructure, but its strength and / or corrosion resistance can be improved by utilizing hot working, heat treatment for refinement, and dislocation strengthening through cold working. Martensite is a hard microstructure that contributes to the strength of stainless steel. Austenite is a soft microstructure at room temperature but exhibits high corrosion resistance.

[0032] If the total volume fraction of ferrite and martensite is less than 30%, the yield strength decreases. Therefore, the total volume fraction of ferrite and martensite is preferably 30% or more. More preferably, it is 35% or more, further preferably 37% or more, and most preferably 40% or more. If the total volume fraction of ferrite and martensite exceeds 99%, austenite decreases, and corrosion resistance decreases. Therefore, the total volume fraction of ferrite and martensite is preferably 99% or less. More preferably, it is 95% or less, further preferably 93% or less, and most preferably 90% or less.

[0033] The σ phase is an intermetallic compound that reduces the corrosion resistance and toughness of stainless steel, and decreases the elongation at break and d under hydrogen charging. 2 σ / dε 2 The smaller the amount, the better. Specifically, the volume fraction of the σ phase is 0% or more and 3% or less. The volume fraction of the σ phase is 3% or less, preferably 2% or less, more preferably 1% or less, and even more preferably 0%.

[0034] The volume fractions of austenite and σ phase were determined using SEM / EBSD. The measurement surface was set as a cross-section parallel to both the wall thickness direction and the tube axis. The measurement area (one field of view) was set to 400 μm × 400 μm, and the measurement step size was set to 0.1 μm. The measurements from five fields of view were averaged. Based on the obtained EBSD data, the phase distribution diagram was obtained using the crystal orientation analysis software OIM Analysis (trademark). The area fractions of austenite and σ phase obtained therefrom were used as their respective volume fractions.

[0035] It should be noted that the interface between two phases with a large strength difference locally forms a region with high dislocation density, which reduces the resistance to hydrogen embrittlement. Therefore, it is preferable to minimize the presence of such interfaces. Consequently, the microstructure described above is further preferred to be a single-phase microstructure. Furthermore, by controlling the temperature history during cooling after heat treatment in the manufacture of stainless steel pipes, the formation of such regions can be suppressed, thereby improving the resistance to hydrogen embrittlement.

[0036] The volume fractions of ferrite and martensite were determined by optical microscopy. The test surfaces were electrolytically etched using a potassium hydroxide aqueous solution. Using an optical microscope at 400x magnification, ten fields of view were observed. Each phase was identified as follows, and its volume fraction was calculated as the average of the ten fields of view. At this point, ferrite, austenite, and the σ phase exhibited bright contrast, while martensite exhibited dark contrast. The microstructure was photographed using an optical microscope, and the total area fraction of ferrite, austenite, and the σ phase, as well as the area fraction of martensite, were calculated from the obtained images. Furthermore, the area fractions of austenite and the σ phase, determined by SEM / EBSD, were subtracted from the total area fraction of ferrite, austenite, and the σ phase to obtain the area fraction of ferrite. The area fractions of ferrite and martensite obtained above were used as their respective volume fractions.

[0037] Average KAM value: above 2.0° and below 4.0°

[0038] KAM (Kernel Average Orientation Difference) values ​​represent local orientation differences. They tend to show the following trend: the higher the KAM value, the higher the dislocation density and the higher the hardness at that measurement point. If the average KAM value is less than 2.0°, there is more soft ferrite or austenite with low dislocation density, and stress concentration occurs at the interface with the surrounding hard phase, becoming the initiation point of cracks. This results in lower elongation at break under hydrogen charging and lower d... 2 σ / dε 2The average KAM value is reduced. Therefore, the average KAM value is 2.0° or higher. The average KAM value is preferably 2.1° or higher, more preferably 2.2° or higher, even more preferably 2.3° or higher, and most preferably 2.4° or higher. On the other hand, if the average KAM value exceeds 4.0°, there is more hard martensite with high dislocation density, more work-hardened structure, and decreased elongation at break under hydrogen charging, as well as d... 2 σ / dε 2 The average KAM value is reduced. Therefore, the average KAM value is 4.0° or less. The average KAM value is preferably 3.9° or less, more preferably 3.8° or less, even more preferably 3.7° or less, and most preferably 3.6° or less.

[0039] Standard deviation of KAM value distribution: below 1.5°

[0040] If the KAM value fluctuates greatly, soft and hard parts coexist, and their hardness difference increases. Therefore, stress concentrates at their interface, becoming the initiation point of cracks, and the elongation at break under hydrogen charging decreases. Thus, the standard deviation of the KAM value distribution is 1.5° or less. Preferably, the standard deviation of the KAM value distribution is 1.4° or less, more preferably 1.3° or less, even more preferably 1.2° or less, and most preferably 1.1° or less. A smaller standard deviation of the KAM value distribution is better, but excessive reduction will lead to increased manufacturing costs and manufacturing load. Therefore, the standard deviation of the KAM value distribution is preferably 0.4° or more, more preferably 0.5° or more, and even more preferably 0.6° or more.

[0041] The average KAM value and the standard deviation of the KAM value distribution were determined using the SEM / EBSD method. The measurement area (one field of view) was set to 400 μm × 400 μm, and the measurement step size was set to 0.1 μm. The measured values ​​of five fields of view were averaged. Based on the obtained EBSD data, the distribution image of the KAM value (KAM map) was obtained using the crystal orientation analysis software OIM Analysis (trademark). Here, the KAM value was obtained by the following method. At each measurement point (a pixel in a regular hexagon), the orientation difference between each pixel was calculated using the pixel centered at the point and extending to the three neighboring pixels (all 37 pixels). The average value of the calculated orientation difference was taken as the KAM value of the central pixel. This operation was performed on all pixels in the field of view to obtain the KAM map. Based on the obtained distribution of KAM values, the average KAM value and the standard deviation of the KAM value distribution were calculated using equations (1) and (2), respectively.

[0042]

[0043] Where n is the number of measurement points in the field of view.

[0044]

[0045] Where n is the number of measurement points in the field of view.

[0046] Furthermore, the stainless steel pipe of the present invention preferably contains, by mass%, C: 0.060% or less, Si: 1.00% or less, Mn: 6.00% or less, P: 0.050% or less, S: 0.0300% or less, Al: 0.005% or more and 0.100% or less, N: 0.400% or less, Cr: 11.00% or more and 30.00% or less, Mo: 5.00% or less, Ni: 15.00% or less, and further optionally contains Cu: 4.00% or less. V: less than 0.300%, Nb: less than 0.300%, Ti: less than 0.300%, B: less than 0.0050%, W: less than 3.00%, Ca: less than 0.0050%, Co: less than 0.500%, Sn: less than 0.100%, Mg: less than 0.020%, Zr: less than 0.020%, REM: less than 0.020%, Ta: less than 0.10%, Sb: less than 0.100%, with the balance consisting of Fe and unavoidable impurities.

[0047] Unless otherwise specified, the percentage of steel composition in this specification refers to "mass percentage".

[0048] C: Below 0.060%

[0049] C is an element that increases the strength of steel through solid solution strengthening. Additionally, C increases the strength of steel by refining the grain size and lowering the phase transformation initiation temperature. Furthermore, it is an element that stabilizes austenite and increases the austenite and martensite fractions. To achieve these effects, a C content of 0.002% or more is preferred. More preferably, 0.003% or more, even more preferably 0.004% or more, and most preferably 0.005% or more is preferred. However, if the C content is excessive, Cr carbides form at the grain boundaries, increasing the corrosion susceptibility at the grain boundaries and reducing hydrogen embrittlement resistance. Therefore, the C content is preferably 0.060% or less. More preferably, 0.050% or less is preferred. More preferably, 0.040% or less is preferred. Most preferably, 0.035% or less is preferred.

[0050] Si: below 1.00%

[0051] Si acts as a deoxidizer and also stabilizes ferrite, increasing its fraction. To achieve this effect, a Si content of 0.01% or more is preferred. A Si content of 0.05% or more is more preferred, 0.07% or more is even more preferred, and 0.10% or more is most preferred. However, if the Si content exceeds 1.00%, the hot workability of intermediate products (such as steel billets) generated during the manufacturing process decreases. Therefore, a Si content of 1.00% or less is preferred. A Si content of 0.90% or less is more preferred. A Si content of 0.80% or less is even more preferred. A Si content of 0.70% or less is most preferred.

[0052] Mn: below 6.00%

[0053] Mn is an element that increases the strength of steel through solid solution strengthening. Additionally, Mn increases the strength of steel by refining the grain size and lowering the phase transformation initiation temperature. Furthermore, it stabilizes austenite and increases the austenite and martensite fractions. To achieve these effects, a Mn content of 0.05% or more is preferred. A Mn content of 0.10% or more is more preferred, 0.15% or more is even more preferred, and 0.20% or more is most preferred. However, if the Mn content is excessive, the strength of the steel becomes too high, and its resistance to hydrogen embrittlement decreases. Therefore, the Mn content is preferably 6.00% or less. A Mn content of 5.00% or less is more preferred, 3.00% or less is even more preferred, and 2.50% or less is most preferred.

[0054] P: below 0.050%

[0055] Phosphorus (P) segregates at grain boundaries, reducing toughness. Therefore, as an unavoidable impurity, it is preferable to minimize its content as much as possible, preferably to keep it below 0.050%. More preferably, the P content is below 0.040%, further preferably below 0.030%, and most preferably below 0.025%. It should be noted that there is no specific lower limit for the P content, but excessive reduction will lead to a significant increase in smelting costs; therefore, P is preferably set to 0.001% or more. More preferably, P is 0.002% or more, and further preferably 0.003% or more.

[0056] S: Below 0.0300%

[0057] Sulfur (S) is an element that significantly degrades hot workability during pipe manufacturing. It is preferable to keep the content as low as possible, but if it is reduced to below 0.0300%, pipe manufacturing in typical processes becomes possible; therefore, S is preferably set to below 0.0300%. More preferably, the S content is below 0.0100%, further preferably below 0.0050%, and most preferably below 0.0030%. It should be noted that there is no specific lower limit for S, but excessive reduction will lead to high smelting costs; therefore, S is preferably set to 0.0001% or more. More preferably, S is 0.0002% or more, and further preferably 0.0003% or more.

[0058] Al: Above 0.005% and below 0.100%

[0059] Al is an element that functions as a powerful deoxidizer. It also stabilizes ferrite and increases its ferrite content. To achieve this effect, it is preferable to contain 0.005% or more Al. More preferably, the Al content is 0.010% or more, even more preferably 0.015% or more. Most preferably, it is 0.018% or more. However, if the content is excessive, weldability deteriorates, and alumina inclusions increase, resulting in poor surface properties. Therefore, the Al content is preferably 0.100% or less. More preferably, it is 0.080% or less. More preferably, it is 0.070% or less. Most preferably, it is 0.065% or less.

[0060] N: below 0.400%

[0061] Nitrogen (N) improves resistance to hydrogen embrittlement. It also stabilizes austenite and increases both the austenite and martensite content. To achieve this effect, a N content of 0.002% or more is preferred. A more preferred N content is 0.005% or more, further preferred is 0.010% or more, and most preferably is 0.020% or more. On the other hand, if the N content exceeds 0.400%, hot workability decreases. Furthermore, nitrides form at grain boundaries, reducing resistance to hydrogen embrittlement. Therefore, a N content of 0.400% or less is preferred. A more preferred N content is 0.380% or less, even more preferred is 0.360% or less, and most preferably is 0.340% or less.

[0062] Cr: ≥11.00% and ≤30.00%

[0063] Cr has the effect of forming a protective film on the surface of steel, inhibiting hydrogen intrusion into the steel, and improving hydrogen embrittlement resistance. In addition, it is an element that stabilizes ferrite and increases the ferrite content. When the Cr content is less than 11.00%, the corrosion resistance becomes insufficient, and the hydrogen embrittlement resistance objective of this invention cannot be ensured. Therefore, the Cr content is set to 11.00% or more. The Cr content is preferably 11.20% or more, more preferably 11.50% or more, further preferably 11.70% or more, and most preferably 11.90% or more. On the other hand, if the Cr content is higher than 30.00%, the ferrite content becomes too high, and the strength decreases. Therefore, the Cr content is set to 30.00% or less. The Cr content is preferably 28.00% or less, more preferably 26.00% or less, more preferably 24.00% or less, and most preferably 20.00% or less.

[0064] Mo: 5.00% or less

[0065] Mo has the effect of stabilizing the protective film on the steel surface, inhibiting hydrogen intrusion into the steel, and improving hydrogen embrittlement resistance. Additionally, it is an element that stabilizes ferrite and increases the ferrite content. To obtain this effect, it is preferable to contain 0.01% or more Mo. More preferably, the Mo content is 0.05% or more, further preferably 0.10% or more, and most preferably 0.15% or more. On the other hand, if the Mo content exceeds 5.00%, the ferrite content becomes too high, and the strength decreases. Therefore, the Mo content is preferably 5.00% or less. More preferably, the Mo content is 4.50% or less, further preferably 4.30% or less, and most preferably 4.00% or less.

[0066] Ni: Below 15.00%

[0067] Ni has the effect of stabilizing the protective film on the steel surface, inhibiting hydrogen intrusion into the steel, and improving hydrogen embrittlement resistance. Furthermore, it is an element that stabilizes austenite and increases the austenite and martensite content. To obtain such an effect, it is preferable to contain 0.01% or more Ni. More preferably, the Ni content is 1.00% or more, further preferably 3.00% or more, and most preferably 3.50% or more. On the other hand, if it exceeds 15.00%, the austenite content becomes high, and the strength decreases. Therefore, the Ni content is preferably 15.00% or less. More preferably, the Ni content is 10.00% or less, further preferably 9.00% or less, and most preferably 8.50% or less.

[0068] In addition to the components mentioned above, one or more of the following can be selected: Cu, V, Nb, Ti, B, W, Ca, Co, Sn, Mg, Zr, REM, Ta, and Sb.

[0069] Cu: below 4.00%

[0070] Cu has the effect of stabilizing the protective film on the steel surface, inhibiting hydrogen intrusion into the steel, and improving hydrogen embrittlement resistance. Furthermore, it is an element that stabilizes austenite and increases the austenite and martensite fractions. To obtain such effects, it is preferable to contain 0.01% or more Cu. The Cu content is more preferably 0.05% or more, further preferably 0.10% or more, most preferably 0.20% or more, and even more preferably 0.30% or more. On the other hand, if it exceeds 4.00%, it leads to CuS grain boundary precipitation, reducing hot workability. Therefore, when Cu is present, the Cu content is set to 4.00% or less. The Cu content is preferably 3.50% or less, more preferably 3.00% or less, further preferably 2.80% or less, and most preferably 2.50% or less.

[0071] V: below 0.300%, Nb: below 0.300%, Ti: below 0.300%

[0072] V, Nb, and Ti are elements that contribute to improving the strength of steel by forming fine carbides and nitrides within it. Furthermore, they improve resistance to hydrogen embrittlement by capturing hydrogen atoms when hydrogen generated due to corrosion penetrates the steel. Additionally, V and Ti are elements that stabilize ferrite and increase its content. To achieve these effects, it is preferable to contain V: 0.002% or more, Nb: 0.002% or more, and Ti: 0.002% or more. More preferably, V: 0.005% or more, Nb: 0.005% or more, and Ti: 0.005% or more; even more preferably, V: 0.010% or more, Nb: 0.010% or more, and Ti: 0.010% or more. Most preferably, V: 0.015% or more, Nb: 0.015% or more, and Ti: 0.015% or more. However, if the content is excessive, the strength of the steel becomes too high, and the resistance to hydrogen embrittlement decreases. Additionally, the toughness decreases. Therefore, when V, Nb, and Ti are present, the content of V is set to 0.300% or less, Nb to 0.300% or less, and Ti to 0.300% or less. Preferably, the content of V, Nb, and Ti is 0.200% or less. More preferably, the content of V, Nb, and Ti is 0.150% or less. Even more preferably, the content of V, Nb, and Ti is 0.150% or less. Further preferably, the content of V, Nb, and Ti is 0.140% or less. Most preferably, the content of V, Nb, and Ti is 0.120% or less.

[0073] B: Below 0.0050%

[0074] Bo (B) is an element that helps refine the microstructure and increase the strength of steel by lowering the phase transformation initiation temperature. It also has the effect of suppressing sulfur (S) grain boundary segregation and improving hot workability. To achieve these effects, it is preferable to contain 0.0002% or more of B. More preferably, 0.0005% or more, further preferably 0.0008% or more, and most preferably 0.0010% or more. However, if the content is excessive, nitrides will form at the grain boundaries, reducing resistance to hydrogen embrittlement. Therefore, when B is present, the B content is set to 0.0050% or less. Preferably, it is 0.0045% or less, more preferably 0.0040% or less, more preferably 0.0035% or less, further preferably 0.0030% or less, and most preferably 0.0025% or less.

[0075] W: Below 3.00%

[0076] W is an element that contributes to the increase of steel strength through solid solution strengthening, and can stabilize the protective film on the steel surface and improve resistance to hydrogen embrittlement. Additionally, it is an element that stabilizes ferrite and increases the ferrite content. To obtain the above effects, it is preferable to contain 0.02% or more of W. More preferably, 0.05% or more, further preferably 0.10% or more, and most preferably 0.15% or more. However, if the content is excessive, the toughness will decrease due to the formation of intermetallic compounds. Therefore, when W is present, the W content is set to 3.00% or less. Preferably, it is 2.50% or less, more preferably 2.40% or less, further preferably 2.20% or less, and most preferably 2.00% or less.

[0077] Ca: below 0.0050%

[0078] Ca is an element that improves resistance to hydrogen embrittlement by spheroidizing sulfides such as MnS. To achieve this effect, it is preferable to contain 0.0005% or more Ca. More preferably, 0.0008% or more, further preferably 0.0010% or more, and most preferably 0.0012% or more. However, if the content is excessive, Ca oxide clusters will form in the steel, resulting in poor toughness. Therefore, when Ca is present, the Ca content is set to 0.0050% or less. Preferably, 0.0040% or less, more preferably 0.0035% or less, further preferably 0.0030% or less, and most preferably 0.0025% or less.

[0079] Co: below 0.500%

[0080] Co has the effect of stabilizing the protective film on the steel surface, inhibiting hydrogen intrusion into the steel, and improving hydrogen embrittlement resistance. Additionally, it is an element that stabilizes austenite and increases the austenite and martensite fractions. To obtain the above effects, it is preferable to contain 0.002% or more Co. More preferably, 0.005% or more, further preferably 0.010% or more, and most preferably 0.015% or more. However, if the content exceeds 0.500%, the effect is saturated, only increasing manufacturing costs. Therefore, when Co is present, the Co content is set to 0.500% or less. Preferably, it is 0.450% or less, more preferably 0.400% or less, further preferably 0.0300% or less, and most preferably 0.200% or less.

[0081] Sn: below 0.100%

[0082] Sn has the effect of stabilizing the protective film on the steel surface, inhibiting hydrogen intrusion into the steel, and improving hydrogen embrittlement resistance. To obtain the above effect, it is preferable to contain 0.001% or more of Sn. More preferably, 0.002% or more, and even more preferably 0.005% or more. However, if the content exceeds 0.100%, the hot workability decreases. Therefore, when Sn is present, the Sn content is set to 0.100% or less. Preferably, it is 0.070% or less, more preferably 0.050% or less, even more preferably 0.040% or less, and most preferably 0.030% or less.

[0083] Mg: below 0.020%

[0084] Mg has the effect of improving resistance to hydrogen embrittlement by controlling the morphology of inclusions. To obtain the above effect, it is preferable to contain 0.001% or more of Mg. More preferably, 0.002% or more, and even more preferably 0.005% or more. However, if the content exceeds 0.020%, the hot workability decreases. Therefore, when Mg is present, the Mg content is 0.020% or less. Preferably, it is 0.015% or less, more preferably 0.010% or less, even more preferably 0.009% or less, and most preferably 0.008% or less.

[0085] Zr: below 0.020%

[0086] Zr is an element that helps improve the strength of steel by forming fine carbides and nitrides within it. To achieve this effect, a Zr content of 0.001% or more is preferred. More preferably, 0.002% or more is preferred, and even more preferably, 0.005% or more is preferred. However, if the content is excessive, the strength of the steel becomes too high, and its resistance to hydrogen embrittlement decreases. Furthermore, its toughness decreases. Therefore, when Zr is present, the Zr content is set to 0.020% or less. Preferably, it is 0.015% or less, more preferably 0.010% or less, even more preferably 0.009% or less, and most preferably 0.008% or less.

[0087] REM: below 0.020%

[0088] REM has the effect of improving resistance to hydrogen embrittlement by controlling the morphology of inclusions. To obtain the above effect, it is preferable to contain 0.001% or more of REM. More preferably, 0.002% or more, further preferably 0.005% or more, and most preferably 0.006% or more. However, if the content exceeds 0.020%, the hot workability decreases. Therefore, when REM is present, the REM content is set to 0.020% or less. Preferably, it is 0.015% or less, more preferably 0.010% or less, further preferably 0.009% or less, and most preferably 0.008% or less.

[0089] It should be noted that, here, REM refers to the collective term for 17 elements, including Sc, Y, and the lanthanides. REM content refers to the total content of these elements.

[0090] Ta: Below 0.10%

[0091] Ta is an element that helps improve the strength of steel by forming fine carbides and nitrides in it. To achieve this effect, it is preferable to contain 0.01% or more Ta. More preferably, 0.02% or more, and even more preferably, 0.03% or more. Most preferably, 0.04% or more. However, if the content is excessive, the strength of the steel becomes too high, and its resistance to hydrogen embrittlement decreases. Furthermore, its toughness decreases. Therefore, when Ta is present, the Ta content is set to 0.10% or less. Preferably, it is 0.08% or less, more preferably 0.07% or less, even more preferably 0.06% or less, and most preferably 0.05% or less.

[0092] Sb: below 0.100%

[0093] Sb has the effect of stabilizing the protective film on the steel surface, inhibiting hydrogen intrusion into the steel, and improving hydrogen embrittlement resistance. To obtain the above effect, it is preferable to contain 0.001% or more of Sb. More preferably, 0.002% or more, further preferably 0.005% or more, and most preferably 0.008% or more. However, if the content exceeds 0.100%, the hot workability decreases. Therefore, when Sb is present, the Sb content is set to 0.100% or less. Preferably, it is 0.070% or less, more preferably 0.050% or less, further preferably 0.040% or less, and most preferably 0.030% or less.

[0094] The balance consists of Fe and unavoidable impurities. Examples of unavoidable impurities in the balance include As, Bi, Pb, Zn, O, Te, Hf, Ge, Sr, and Cs. However, without impairing the effects of the invention, it may also contain less than 0.10% of As, Te, Hf, Ge, Sr, and Cs, and less than 0.005% of Bi, Pb, Zn, and O.

[0095] The stainless steel tube of the present invention exhibits an elongation at break of 9.0% or more under a tensile test with cathodic hydrogen charging, and the true strain under the maximum load is set as ε. max When the true strain is 0.8ε max Above and ε max Within the following range, the rate of change of work hardening rate dσ / dε relative to ε is d 2 σ / dε 2 It is above -200,000 MPa. Here, σ is the true stress (MPa) and ε is the true strain (unitless).

[0096] Figure 1 The diagram shows a schematic of the curve obtained by plotting the true strain ε on the horizontal axis and the true stress σ on the vertical axis during a tensile test under cathodic hydrogen charging. The true strain is 0.8ε. max Above and ε max The following range represents the stage where the mode changes from elastic deformation (elastic region) to plastic deformation (plastic region), commonly referred to as uniform elongation. Since work hardening behavior is stable, the focus is on 0.8ε. max Above and ε max The following range is limited to the ranges described above. Regarding the ranges described above, the focus is primarily on those ranges, but they can also be set to 0.9ε. max above.

[0097] Stainless steel pipes that can achieve the above-mentioned properties will have excellent resistance to hydrogen embrittlement.

[0098] As a method of hydrogen charging, for example, in a solution at room temperature (21–27°C) in which 3.0 g of ammonium thiocyanate has been added to 1 L of a 3.0% by mass aqueous solution of sodium chloride, a current density of 0.05 mA / cm² is used. 2 The cathode was charged with hydrogen for 24 hours. Then, the tensile test was carried out while the cathode was charged with hydrogen under the above conditions.

[0099] The method of hydrogen charging is not limited to the methods described above, as long as the hydrogen content in the steel at the time of fracture in the tensile test is 1.5 ppm by mass or more. More preferably, the hydrogen content is 1.6 ppm by mass or more, and even more preferably, the hydrogen content is 1.7 ppm by mass or more. Furthermore, there is no particular upper limit, but it is preferable to set the hydrogen content to 10.0 ppm by mass or less. The hydrogen content in the steel is determined, for example, by a temperature-induced desorption method, with a heating rate of 100°C / s, heating to 600°C, and a gas chromatograph used as the analyzer.

[0100] The strain rate in the tensile test is set to, for example, 1.7 × 10⁻⁶. -5 s -1 There is no particular limitation on the strain rate in the tensile test, but it is preferably set to 1.0 × 10⁻⁶. -5 s -1 The above is more preferably set to 1.5 × 10 -5 s -1 The above is further optimized to be 1.8 × 10 - 5 s -1 The optimal value is set to 1.9 × 10. -5 s -1 That's all. Additionally, a preferred setting is 9.0 × 10. -5 s -1 Hereinafter, it is more preferable to set it to 6.0×10 -5 s -1 the following.

[0101] In the tensile test, a round bar test piece with a parallel section diameter of 3.8 mm and a parallel section length of 15 mm was used. The test piece was cut from the center of the wall thickness so that the tensile direction was parallel to the tube axis.

[0102] Elongation at break: ≥9.0%

[0103] If the elongation at break in the tensile test under hydrogen purging is low, the ductility in the hydrogen environment becomes insufficient. Therefore, in this invention, the elongation at break is set to 9.0% or more. Preferably, it is 9.5% or more, more preferably 10.0% or more, further preferably 10.2% or more, and most preferably 10.4% or more. A higher elongation at break is preferred, but if it is excessively increased, it will lead to an increase in manufacturing costs and manufacturing load. Therefore, the elongation at break is preferably 30.0% or less. More preferably, it is 28.0% or less. Further preferably, it is 25.0% or less, and most preferably 24.0% or less.

[0104] The elongation at break (%) is calculated as ((total length of the test piece after the test) - (total length of the test piece before the test)) / (total length of the test piece before the test) × 100.

[0105] When the true strain is 0.8ε max Above and ε max Within the following range d 2 σ / dε 2 Above -200000MPa

[0106] During work hardening in the plastic region, the work hardening rate dσ / dε decreases as ε increases.

[0107] That is, d 2 σ / dε 2 It is a negative value. d 2 σ / dε 2 The smaller the value, the more drastically dσ / dε decreases, thus leading to early necking, a reduced maximum load, and decreased ductility. This is particularly true at a true strain of 0.8ε. max Above and ε max It is important to control within the following range; therefore, in this invention, the true strain will be 0.8ε. max Above and ε max Within the following range, the tensile test under hydrogen charging described above, d 2 σ / dε 2 Set to -200,000 MPa or higher. Preferably -180,000 MPa or higher, more preferably -160,000 MPa or higher, even more preferably -150,000 MPa or higher, and most preferably -140,000 MPa or higher. 2 σ / dε 2 A larger value is preferable, but excessive increases lead to increased manufacturing costs and workload. Therefore, d 2 σ / dε 2 Preferably less than 0 MPa. More preferably -10000 MPa or less, and even more preferably -15000 MPa or less.

[0108] dσ / dε and d2 σ / dε 2 The values ​​are determined based on the time history data of the load and crosshead displacement during the tensile test. First, σ and ε are calculated at each time step based on the load and crosshead displacement. Then, to reduce the variation in σ, it is averaged at each time step within a range equivalent to ε = 0.001. Specifically, for example, at a strain rate of 1.7 × 10⁻⁶... -5 s -1 In this case, we calculate σ at a certain time t(s) as the average value of σ from time (t-30)(s) to time (t+30)(s). We perform this operation for all times. Then, we calculate dσ / dε at each time. Specifically, we calculate dσ / dε at a certain time t(s) as the average rate of change from time (t-60)(s) to time t(s). We perform this operation for all times. Furthermore, we calculate dσ / dε at each time. 2 σ / dε 2 Specifically, we calculate (d / dε)(dσ / dε) at a certain time t(s) as the average rate of change from time (t-60)(s) to time t(s). We perform this operation for all times.

[0109] It should be noted that in this invention, since the tensile test is performed in solution, it is difficult to directly measure the displacement of the parallel section using an elongation meter or similar device. Therefore, ε is calculated based on the crosshead displacement. In this case, the value of ε includes the error of the elastic deformation of the testing machine, but dσ / dε and d 2 σ / dε 2 The calculation is based on the change in ε, therefore the error mentioned above in this invention is so small as to be negligible.

[0110] Furthermore, the stainless steel pipe of the present invention has a yield strength of 450 MPa or more to withstand internal pressure and its own weight. Preferably, it has a yield strength of 550 MPa or more, more preferably 580 MPa or more, and even more preferably 600 MPa or more. On the other hand, if the yield strength increases, the corrosion resistance decreases. Therefore, the yield strength is preferably 1100 MPa or less, more preferably 1000 MPa or less, even more preferably 990 MPa or less, and most preferably 980 MPa or less.

[0111] Yield strength was determined by tensile testing at room temperature (10–38°C) in atmospheric conditions. Test pieces were cut according to ASTM E8 / E8M (2021), using round bars cut from the center of the wall thickness with the tensile direction parallel to the tube axis. The size of the round bar test piece was, for example, 8.9 mm in diameter at the parallel section and 35.6 mm between the markings. If a round bar test piece could not be cut from the steel tube, an arc-shaped test piece was cut. The size of the arc-shaped test piece was, for example, the thickness of the total wall thickness, the width of 25.4 mm, and the distance between the markings of 50.8 mm. Using the cut tensile test piece, tensile tests were performed according to ASTM E8 / E8M (2021). The resulting 0.2% offset endurance (MPa) was taken as the yield strength (MPa).

[0112] Furthermore, the stainless steel pipes mentioned above are preferably seamless steel pipes.

[0113] Furthermore, oil well tubing using the aforementioned stainless steel tubing is preferred.

[0114] Next, a method for manufacturing a stainless steel tube according to one embodiment of the present invention will be described.

[0115] The stainless steel tube of the present invention is manufactured, for example, by heating and hot-working a steel raw material having the above-mentioned composition to form a cylindrical shape, and then performing heat treatment after cooling.

[0116] It should be noted that in the following description of the manufacturing method, unless otherwise specified, the temperature-related "°C" is referred to as surface temperature. These surface temperatures can be measured using a radiation thermometer, etc. Furthermore, the temperature at the center of the wall thickness can be obtained by calculating the temperature distribution along the wall thickness using heat transfer analysis and correcting the results using surface temperatures.

[0117] In this invention, the smelting method for the steel raw material is not particularly limited; known smelting methods such as converters, electric furnaces, and vacuum melting furnaces are all suitable. The casting method is also not particularly limited; known casting methods such as continuous casting can be used to produce the desired dimensions. It should be noted that there is no problem even if the ingot-bill rolling method is used instead of the continuous casting method. Secondary refining, such as ladle refining, can also be performed on the molten steel. The steel raw material is preferably cast into round bar shapes such as steel billets, which are used as raw materials for steel pipes. However, steel sheets of the desired size and shape can also be produced by further hot rolling of the cast steel as raw materials for steel pipes.

[0118] Next, the obtained steel pipe raw material is heated for hot processing to produce stainless steel pipes of the specified shape. The hot forming process is preferably performed using a Mannesmann-plug mill process or a Mannesmann-mandrel mill process. It should be noted that stainless steel pipes can also be produced by hot extrusion using a stamping method. In the hot forming process, there are no special requirements as long as the specified shape of the stainless steel pipe can be produced.

[0119] In the heating process, the heating temperature of the steel pipe raw material (e.g., steel billet) is preferably set to a temperature in the range of 1100–1350°C. When the heating temperature is below 1100°C, the hot workability of the steel billet decreases, and defects are more likely to occur during pipe making. Furthermore, the load on the equipment becomes excessive. Therefore, the heating temperature is preferably set to 1100°C or higher. More preferably, it is 1150°C or higher, even more preferably 1160°C or higher, and most preferably 1170°C or higher. On the other hand, if the heating temperature exceeds 1350°C and becomes high, the grains coarsen, the fluctuation of the KAM value within the grains increases, and the standard deviation of the KAM value distribution increases. Therefore, the heating temperature in the heating process is preferably set to 1350°C or lower. More preferably, it is 1300°C or lower, even more preferably 1290°C or lower, and most preferably 1280°C or lower.

[0120] In the hot-forming process, the reduction of area (%) of the cross-section perpendicular to the tube axis is preferably 25% or more. Here, the reduction of area (%) is calculated as ((cross-sectional area of ​​the original steel tube) - (cross-sectional area after hot forming)) / (cross-sectional area of ​​the original steel tube) × 100. If the reduction of area is small, the grains become coarser, the fluctuation of the KAM value within the grains increases, and the standard deviation of the KAM value distribution becomes larger. A reduction of area of ​​30% or more is more preferable, further preferably 35% or more, and most preferably 40% or more is more preferable. On the other hand, if the reduction of area is too high, the effect becomes saturated, and the equipment load becomes excessive. Therefore, a reduction of area of ​​75% or less is preferably preferred. More preferably 70% or less, further preferably 65% ​​or less, and most preferably 60% or less is more preferable.

[0121] Following the aforementioned hot forming process, the resulting stainless steel tube undergoes a cooling treatment. During this cooling treatment, it is preferable to maintain an average cooling rate of 10°C / s or more within the range of 650–300°C at the center of the wall thickness, and a minimum cooling rate of 3°C / s or more. It should be noted that the minimum cooling rate is determined by dividing the time within the 650–300°C range into 3-second intervals, calculating the average cooling rate within each interval, and using the minimum of these intervals. Furthermore, the average cooling rate within each interval can be obtained by dividing the temperature obtained by subtracting the cooling end temperature from the cooling start temperature of the specified interval by the time required for cooling within the specified interval. Additionally, the average cooling rate within the 650–300°C range can be calculated as (650–300) / (time required for cooling within the 650–300°C range) (°C / s).

[0122] If the average cooling rate and minimum cooling rate are low, the grains become coarser, the fluctuation of KAM values ​​within the grains increases, and the standard deviation of the KAM value distribution becomes larger. Additionally, the formation of the σ phase as an embrittlement phase leads to changes in the elongation at break and d under hydrogen charging. 2 σ / dε 2 Therefore, the average cooling rate within the range of 650–300°C is preferably 10°C / s or more. More preferably, it is 15°C / s or more. Further preferably, it is 18°C / s or more, and most preferably, it is 20°C / s or more. On the other hand, if the average cooling rate exceeds 100°C / s, the load on the cooling device becomes too large, so the average cooling rate is preferably 100°C / s or less. More preferably, it is 70°C / s or less. Further preferably, it is 60°C / s or less, and most preferably, it is 50°C / s or less. In addition, the minimum cooling rate is preferably 3°C / s or more. The minimum cooling rate is more preferably 4°C / s or more, further preferably 5°C / s or more, and most preferably 6°C / s or more. On the other hand, if the minimum cooling rate exceeds 15°C / s, the load on the cooling device becomes too large, so the minimum cooling rate is preferably 15°C / s or less. The minimum cooling rate is more preferably 12°C / s or less, further preferably 11°C / s or less, and most preferably 10°C / s or less. To ensure the necessary cooling rate, water cooling is preferred.

[0123] Next, in this invention, it is preferable to perform a heat treatment consisting of quenching and tempering on the seamless steel pipe. However, when the Cr content is 20.00% or more, the austenite is stable at room temperature and the martensite fraction becomes lower, so the tempering treatment can be omitted.

[0124] During the quenching process, the stainless steel tube is reheated to a temperature ranging from 850 to 1150°C (heating temperature), preferably held for at least 300 seconds, and then the average cooling rate within the range of 650 to 300°C in the center of the wall thickness is at least 10°C / s, with a minimum cooling rate of at least 3°C / s. It should be noted that regarding the minimum cooling rate, the time within the 650 to 300°C range is divided into intervals of 3 seconds, and the average cooling rate in each interval is calculated, with the minimum value being used to determine the minimum cooling rate. Furthermore, the average cooling rate in each interval can be obtained by dividing the temperature obtained by subtracting the cooling end temperature from the cooling start temperature of the specified interval by the time required for cooling within the specified interval. Additionally, the average cooling rate within the 650 to 300°C range can be calculated as (650 - 300) / (time required for cooling within the 650 to 300°C range) (°C / s).

[0125] If the quenching temperature is low, the austenite fraction decreases and the ferrite fraction increases, thus lowering the average KAM value. Additionally, the yield strength decreases. Furthermore, due to the presence of undissolved σ phase residues, the elongation at break and d under hydrogen charging conditions also decrease. 2 σ / dε 2 To reduce [the impact of temperature fluctuations], the quenching temperature is preferably set to 850°C or higher. More preferably, it is 880°C or higher. Even more preferably, it is 900°C or higher, and most preferably, it is 920°C or higher. If the quenching temperature is high, the grains will coarsen, the fluctuation of the KAM value within the grains will increase, and the standard deviation of the KAM value distribution will increase. Therefore, the quenching temperature is preferably set to 1150°C or lower. More preferably, it is 1130°C or lower. Even more preferably, it is 1100°C or lower, and most preferably, it is 1080°C or lower.

[0126] It should be noted that if the average cooling rate and the minimum cooling rate are low, the grains will become coarser, the fluctuation of the KAM value within the grains will increase, and the standard deviation of the KAM value distribution will increase.

[0127] In addition, the formation of the σ phase, the elongation at break under hydrogen charging, and d 2 σ / dε 2Therefore, the average cooling rate within the range of 650–300°C is preferably 10°C / s or more. More preferably, it is 15°C / s or more. Further preferably, it is 18°C / s or more, and most preferably, it is 20°C / s or more. If the average cooling rate exceeds 100°C / s, the load on the cooling device becomes too large, so the average cooling rate is preferably 100°C / s or less. More preferably, it is 70°C / s or less. Further preferably, it is 60°C / s or less, and most preferably, it is 50°C / s or less. In addition, the minimum cooling rate is preferably 3°C / s or more. The minimum cooling rate is more preferably 4°C / s or more, further preferably 5°C / s or more, and most preferably 6°C / s or more. If the minimum cooling rate exceeds 15°C / s, the load on the cooling device becomes too large, so the minimum cooling rate is preferably 15°C / s or less. More preferably, it is 12°C / s or less, further preferably 11°C / s or less, and most preferably 10°C / s or less. To ensure the necessary cooling rate, water cooling is preferred.

[0128] For stainless steel pipes that have undergone quenching, it is preferable to then perform tempering treatment.

[0129] The tempering process is performed as follows: heating to 500°C or higher and 750°C or lower, preferably holding for 30 minutes or more, and then cooling to room temperature, preferably at a cooling rate of air cooling or higher. If the heating temperature of the tempering process is low, dislocation recovery cannot be fully achieved, the fluctuation of KAM values ​​within the grains becomes larger, and the standard deviation of the KAM value distribution becomes larger. Therefore, the heating temperature of the tempering process is preferably set to 500°C or higher. More preferably, it is 550°C or higher. Further preferably, it is 560°C or higher, and most preferably, it is 570°C or higher. On the other hand, if the heating temperature of the tempering process is high, the grains become coarser, the fluctuation of KAM values ​​within the grains becomes larger, and the standard deviation of the KAM value distribution becomes larger. In addition, the formation of the σ phase, the elongation at break under hydrogen charging, and d 2 σ / dε 2 Therefore, the heating temperature for tempering is preferably set to 750°C or below. More preferably, it is 720°C or below. Even more preferably, it is 700°C or below, and most preferably, it is 680°C or below.

[0130] Furthermore, in this invention, cold working can be performed immediately after tempering to correct defects in the steel pipe shape and adjust the yield strength caused by work hardening. Cold drawing is an example of a cold working method. If the reduction of area (%) of the cross-section perpendicular to the pipe axis during cold working exceeds 40%, work hardening increases, and therefore the average KAM value increases. Therefore, the reduction of area (%) during cold working is preferably 40% or less. More preferably, it is 35% or less. Further preferably, it is 30% or less, and most preferably, it is 25% or less. Furthermore, the lower limit is not particularly limited, but the reduction of area (%) during cold working is preferably set to 5% or more. Here, the reduction of area (%) during cold working is calculated by ((cross-sectional area before processing) - (cross-sectional area after processing)) / (cross-sectional area before processing) × 100.

[0131] Example

[0132] The present invention will be further described in detail below based on embodiments. It should be noted that the present invention is not limited to the following embodiments.

[0133] Molten steel with the composition shown in Table 1 is melted and made into steel billets (raw material for steel pipes).

[0134] The obtained steel billet was heated and hot-worked under the conditions shown in Table 2, and then cooled to obtain stainless steel pipes with outer diameter (mm) and wall thickness (mm) as shown in Table 2.

[0135] Test pieces were cut from the obtained stainless steel tubes and subjected to the following tests: KAM value determination, tensile test, and tensile test under cathodic hydrogen charging. It should be noted that the microstructure evaluation was performed based on the methods described in the implementation steps.

[0136] [KAM value determination]

[0137] The average KAM value and the standard deviation of the KAM value distribution were determined using the SEM / EBSD method. The accelerating voltage was set to 15 kV. The measurement area (one field of view) was set to 400 μm × 400 μm, and the measurement step size was set to 0.1 μm. The measured values ​​of five fields of view were averaged. Based on the obtained EBSD data, the distribution image of the KAM value (KAM map) was obtained using the crystal orientation analysis software OIM Analysis (trademark). Here, the KAM value was obtained by the following method. At each measurement point (a pixel in a regular hexagon), the orientation difference between each pixel was calculated using the pixel centered at the point and extending to the three neighboring pixels (all 37 pixels). The average value of the calculated orientation difference was taken as the KAM value of the central pixel. This operation was performed on all pixels in the field of view to obtain the KAM map. Based on the obtained distribution of KAM values, the average KAM value and the standard deviation of the KAM value distribution were calculated using equations (1) and (2), respectively.

[0138]

[0139] Where n is the number of measurement points in the field of view.

[0140]

[0141] Where n is the number of measurement points in the field of view.

[0142] [Tension test]

[0143] The test pieces were cut according to ASTM E8 / E8M (2021), with the tensile direction parallel to the tube axis to form an arc shape. The arc-shaped test pieces were sized as follows: thickness equal to the total wall thickness, width 25.4 mm, and spacing between markings 50.8 mm. Tensile tests were performed using the cut tensile test pieces according to ASTM E8 / E8M (2021). The obtained 0.2% offset endurance (MPa) was taken as the yield strength (MPa).

[0144] [Tension test under hydrogen charging at the cathode]

[0145] A round bar test piece was cut from the center of the wall thickness, with the stretching direction parallel to the tube axis. The diameter of the parallel section was set to 3.8 mm, and the length of the parallel section was set to 15 mm. For the test piece, in a room temperature solution containing 3.0 g of ammonium thiocyanate added to 1 L of a 3.0 wt% sodium chloride aqueous solution, a current density of 0.05 mA / cm² was applied. 2 The cathode was charged with hydrogen for 24 hours, and then tensile tests were conducted while continuing to charge the cathode with hydrogen under the same conditions. The strain rate in the tensile tests was set to 2.0 × 10⁻⁶. -5 s -1 .

[0146] The elongation at break (%) is calculated as ((total length of the test piece after the test) - (total length of the test piece before the test)) / (total length of the test piece before the test) × 100.

[0147] dσ / dε and d 2 σ / dε 2The values ​​are determined based on the time history data of the load and crosshead displacement during the tensile test. First, σ and ε are calculated for each time step based on the load and crosshead displacement. Then, to reduce the variation in σ, it is averaged at each time step within a range equivalent to ε = 0.001. Specifically, σ at a given time t(s) is calculated as the average value of σ from time (t-30)(s) to time (t+30)(s). This operation is performed for all time steps. Next, dσ / dε is calculated at each time step. Specifically, dσ / dε at a given time t(s) is calculated as the average rate of change from time (t-60)(s) to time t(s). This operation is performed for all time steps. Finally, dσ / dε is calculated at each time step. 2 σ / dε 2 Specifically, we calculate (d / dε)(dσ / dε) at a certain time t(s) as the average rate of change from time (t-60)(s) to time t(s). We perform this operation for all times.

[0148] The results are shown in Table 3.

[0149] In Table 3, stainless steel pipes No.1, No.3, No.5, No.8, No.10, No.12, No.15, No.16, No.17, No.20, No.26, and No.27 are examples of the present invention, while stainless steel pipes No.2, No.4, No.6, No.7, No.9, No.11, No.13, No.14, No.18, No.19, and No.21 to No.25 are comparative examples.

[0150] The stainless steel pipes of this invention all possess the following characteristics: in the steel microstructure at the center of the wall thickness, the combined volume fraction of ferrite and martensite is 30% or more and 99% or less, the volume fraction of σ phase is 0% or more and 3% or less, the balance is austenite, the average KAM value is 2.0° or more and 4.0° or less, and the standard deviation of the KAM value distribution is 1.5° or less. Furthermore, the yield strength is 450 MPa or more, the elongation at break is 9.0% or more, and the true strain is 0.8ε. max Above and ε max Within the following range, the rate of change of work hardening rate dσ / dε relative to ε is d 2 σ / dε 2 It is above -200,000 MPa.

[0151] On the other hand, for the stainless steel pipe of Comparative Example No. 2, the standard deviation of the KAM value distribution became larger, and as a result, the elongation at break required by the present invention was not obtained.

[0152] For the stainless steel pipe of Comparative Example No. 4, the standard deviation of the KAM value distribution increased, and the required elongation at break was not obtained as a result of the present invention.

[0153] For the stainless steel pipe of Comparative Example No. 6, the standard deviation of the KAM value distribution increased, and the required elongation at break was not obtained as a result of the present invention.

[0154] For the stainless steel pipe of Comparative Example No. 7, the standard deviation of the KAM value distribution increased, and the volume fraction of the σ phase was high, resulting in the failure to obtain the elongation at break required by the present invention.

[0155] For the stainless steel pipe of Comparative Example No. 9, the average KAM value was lower, and as a result, the required elongation at break and d were not obtained according to the present invention. 2 σ / dε 2 In addition, there is a tendency for the yield strength to be low.

[0156] For the stainless steel pipe of Comparative Example No. 11, the standard deviation of the KAM value distribution increased, and the required elongation at break was not obtained as a result of the present invention.

[0157] For the stainless steel pipe of Comparative Example No. 13, the standard deviation of the KAM value distribution increased, the volume fraction of the σ phase was high, and the elongation at break required by the present invention was not obtained.

[0158] For the stainless steel pipe of Comparative Example No. 14, the average KAM value was higher, resulting in the failure to obtain the elongation at break and d required by the present invention. 2 σ / dε 2 .

[0159] For the stainless steel pipe of Comparative Example No. 18, the standard deviation of the KAM value distribution increased, and the required elongation at break was not obtained as a result of the present invention.

[0160] For the stainless steel pipe of Comparative Example No. 19, the average KAM value was lower, resulting in the failure to obtain the elongation at break and d required by the present invention. 2 σ / dε 2 .

[0161] For the stainless steel pipe of Comparative Example No. 21, the standard deviation of the KAM value distribution increased, and the required elongation at break was not obtained as a result of the present invention.

[0162] For the stainless steel pipe of Comparative Example No. 22, the average KAM value was higher, and as a result, the required elongation at break and d were not obtained according to the present invention. 2 σ / dε 2 .

[0163] For the stainless steel tube of Comparative Example No. 23, the combined volume fraction of ferrite and martensite was higher, resulting in the failure to obtain the elongation at break and d required by the present invention. 2σ / dε 2 .

[0164] For the stainless steel pipe of Comparative Example No. 24, the combined volume fraction of ferrite and martensite is lower, resulting in a tendency for lower yield strength.

[0165] For the stainless steel tube of Comparative Example No. 25, the volume fraction of the σ phase was high, and the desired d phase of this invention was not obtained. 2 σ / dε 2 .

[0166]

Claims

1. A stainless steel pipe, wherein the steel structure in the center of its wall thickness, The combined volume fraction of ferrite and martensite is ≥30% and ≤99%, the volume fraction of σ phase is ≥0% and ≤3%, and the balance is austenite. The average KAM (nuclear average orientation difference) value of the steel structure is above 2.0° and below 4.0°. The standard deviation of the KAM value distribution is below 1.5°.

2. The stainless steel pipe according to claim 1, comprising the following components: Contains, by mass%, C: 0.060% or less, Si: 1.00% or less, Mn: 6.00% or less, P: 0.050% or less, S: 0.0300% or less, Al: 0.005% or more and 0.100% or less, N: 0.400% or less, Cr: 11.00% or more and 30.00% or less, Mo: 5.00% or less, Ni: 15.00% or less, and further optionally contains, by mass%, C: 4.00% or less, V: 0.300% or more, and Si: 1.00% or less, Mn: 6.00% or less, P: 0.050% or less, S: 0.0300% or less, Al: 0.005% or more and 0.100% or less, N: 0.400% or less, Cr: 11.00% or more and 30.00% or less, Mo: 5.00% or less, Ni: 15.00% or less, and further optionally contains, by mass%, C: 4.00% or less, Si: 1.00% or less, Mn: 0.300% or less, Mn: 1.00% or less, P: 0.050% or less, S: 0.0300% or less, Al: 0.005% or more and 0.100% or less, N: 0.400% or less, Cr: 11.00% or more and 30.00% or less, Mo: 5.00% or less, Ni: 15.00% or less, and further optionally contains, by mass%, C: 4.00% or less, S: 1.00% or less, Mn: 6.00% or less, P: The content of the following is: Nb: less than 0.300%, Ti: less than 0.300%, B: less than 0.0050%, W: less than 3.00%, Ca: less than 0.0050%, Co: less than 0.500%, Sn: less than 0.100%, Mg: less than 0.020%, Zr: less than 0.020%, REM: less than 0.020%, Ta: less than 0.10%, Sb: less than 0.100%, with the balance consisting of Fe and unavoidable impurities.

3. The stainless steel pipe according to claim 1 or 2, wherein The stainless steel pipe is a seamless steel pipe.

4. An oil well pipe that uses the stainless steel pipe according to any one of claims 1 to 3.

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