Stainless steel pipes and oil well pipes

Abstract

The object of the present invention is to provide a stainless steel pipe that has high strength and excellent hydrogen embrittlement resistance. "Excellent hydrogen embrittlement resistance" means that in a tensile test under hydrogen charging, the fracture elongation is 9.0% or more, and in the tensile test, the horizontal axis is true strain and the vertical axis is true stress, and the true strain at the maximum load is ε max When the true strain is 0.8ε max ε or more max In the following range, the rate of change d of the work hardening rate dσ / dε relative to ε is 2 σ / dε 2 is -200,000 MPa or more. σ is true stress (MPa) and ε is true strain (no unit). "High strength" means that the yield strength is 450 MPa or more. A stainless steel pipe having a specific structure, an average KAM (Kernel Average Misorientation) value, which is the average of the KAM values ​​of the steel structure, of 2.0° or more and 4.0° or less, and a standard deviation of the KAM value distribution of 1.5° or less.

Description

[Technical Field]

[0001] The present invention relates to a stainless steel pipe suitable for use in oil country tubular goods, and to an oil country tubular goods. [Background technology]

[0002] In recent years, due to the soaring crude oil prices and the expected depletion of petroleum resources in the near future, there has been active development of deep oil fields that were previously ignored, as well as oil and gas fields with severe corrosive environments containing carbon dioxide gas, chloride ions, and hydrogen sulfide.

[0003] In hydrogen sulfide environments, hydrogen embrittlement due to corrosion becomes a problem, and therefore there is an increasing demand for oil country tubular goods that are resistant to hydrogen embrittlement.

[0004] Patent Document 1 proposes a martensitic stainless steel seamless pipe with improved corrosion resistance by controlling the amount of added elements so that the amount of retained austenite, repassivation potential, and pitting potential fall within appropriate ranges.

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

[0006] [Patent Document 1] Patent No. 6315159 [Patent Document 2] Japanese Patent Publication No. 2022-111733 Summary of the Invention [Problem to be solved by the invention]

[0007] However, the steel pipes described in Patent Documents 1 and 2 above may not be able to provide sufficient hydrogen embrittlement resistance, and there is room for improvement in hydrogen embrittlement resistance.

[0008] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a stainless steel pipe having high strength and excellent resistance to hydrogen embrittlement.

[0009] In the present invention, "excellent hydrogen embrittlement resistance" means that in a tensile test under hydrogen charging, which will be described later, the fracture elongation is 9.0% or more, and in the tensile test, the horizontal axis is true strain and the vertical axis is true stress, and the true strain at the maximum load is ε max When the true strain is 0.8ε max ε or more max In the following range, the rate of change d of the work hardening rate dσ / dε relative to ε is 2 σ / dε 2 This means that the stress is -200,000 MPa or more. Here, σ is the true stress (MPa) and ε is the true strain (no unit).

[0010] In addition, "high strength" as used in the present invention refers to a yield strength of 450 MPa or more. [Means for solving the problem]

[0011] As a result of extensive research, the present inventors have found that the presence of regions with locally high dislocation density in the steel structure reduces hydrogen embrittlement resistance.

[0012] It was also found that the above-mentioned region is likely to be formed near the interface between ferrite and martensite or the interface between ferrite and austenite, that is, near a two-phase interface where there is a large difference in strength.

[0013] The present invention has been completed based on the above findings and comprises the following gist. [1] The steel structure at the center of the wall is 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, and the remainder is austenite, The average KAM (Kernel Average Misorientation) value, which is the average of the KAM values ​​of the steel structure, is 2.0° or more and 4.0° or less, A stainless steel pipe in which the standard deviation of the KAM value distribution is 1.5° or less. [2] In 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: Contains 15.00% or less, Further optionally, Cu:4.00% or less, V: 0.300% or less, Nb: 0.300% or less, Ti: 0.300% or less, B: 0.0050% or less, W: 3.00% or less, Ca: 0.0050% or less, Co: 0.500% or less, Sn: 0.100% or less, Mg: 0.020% or less, Zr: 0.020% or less, REM: 0.020% or less, Ta: 0.10% or less, Sb: Contains one or more selected from 0.100% or less, The balance has a composition consisting of Fe and unavoidable impurities. [1] The stainless steel pipe according to [1]. [3] The stainless steel pipe is a seamless steel pipe. [1] or [2]. [4] An oil country tubular good using the stainless steel pipe according to any one of [1] to [3] above. [Effects of the Invention]

[0014] According to the present invention, it is possible to provide stainless steel pipes and oil country tubular goods that have high strength and excellent resistance to hydrogen embrittlement. [Brief explanation of the drawings]

[0015] [Figure 1] This shows a schematic diagram of a curve plotting true strain ε on the horizontal axis and true stress σ on the vertical axis when a tensile test was carried out under cathodic hydrogen charging. DETAILED DESCRIPTION OF THE INVENTION

[0016] The stainless steel pipe of the present invention will be described below.

[0017] The stainless steel pipe of the present invention has, at the center of the wall thickness, a total volume fraction of ferrite and martensite of 30% or more and 99% or less, a volume fraction of σ phase of 0% or more and 3% or less, with the remainder being austenite, an average KAM (Kernel Average Misorientation) value, which is the average of the KAM values, of 2.0° or more and 4.0° or less, and a standard deviation of the KAM value distribution of 1.5° or less.

[0018] In the following, "volume ratio" will also be referred to as "fraction."

[0019] Steel structure at the center of the thickness: total volume fraction of ferrite and martensite is 30% to 99%, volume fraction of σ phase is 0% to 3%, and the remainder is austenite Ferrite is a soft structure, but its strength and / or corrosion resistance can be increased by refinement through hot working or heat treatment, or by dislocation strengthening through cold working. Martensite is a hard structure that contributes to increased strength of stainless steel. Austenite is a soft structure at room temperature, but is highly corrosion-resistant.

[0020] 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. The total volume fraction of ferrite and martensite is more preferably 35% or more, even more preferably 37% or more, and most preferably 40% or more. If the total volume fraction of ferrite and martensite exceeds 99%, the amount of austenite decreases and the corrosion resistance decreases. Therefore, the total volume fraction of ferrite and martensite is preferably 99% or less. The total volume fraction of ferrite and martensite is more preferably 95% or less, even more preferably 93% or less, and most preferably 90% or less.

[0021] The σ phase is an intermetallic compound that reduces the corrosion resistance and toughness of stainless steels. It is also responsible for the fracture elongation and d 2 σ / dε 2 To reduce the volume fraction of the σ phase, a smaller volume fraction is preferable. 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%.

[0022] The volume fractions of austenite and σ phase are determined using the SEM / EBSD method. The measurement surface is a cross section parallel to both the wall thickness direction and the axial direction of the tube. The measurement area (one field of view) is 400 μm x 400 μm, the measurement step size is 0.1 μm, and the measured values ​​for five fields of view are averaged. Based on the obtained EBSD data, a phase distribution diagram is obtained using crystal orientation analysis software OIM Analysis (trademark). The area fractions of austenite and σ phase obtained in this way are taken as their respective volume fractions.

[0023] Furthermore, since two-phase interfaces with large strength differences form areas with high dislocation density locally, which reduces hydrogen embrittlement resistance, it is preferable to reduce the number of such interfaces as much as possible. Therefore, it is even more preferable that the above-mentioned structure is a single-phase structure. Furthermore, by controlling the temperature history during cooling after heat treatment during the production of stainless steel pipes, the formation of such regions can be suppressed, thereby improving hydrogen embrittlement resistance.

[0024] The volume fractions of ferrite and martensite are determined by optical microscope observation. The measurement surface is electrolytically etched using a potassium hydroxide aqueous solution. Ten fields of view are observed at a magnification of 400x using an optical microscope. Each phase is identified as follows, and the volume fractions of each are calculated as follows, taking the average value of the 10 fields. At this time, ferrite, austenite, and σ phase have a bright contrast structure, while martensite has a dark contrast structure. This is photographed using an optical microscope, and the total area fractions of ferrite, austenite, and σ phase, as well as the area fraction of martensite, are determined from the resulting image. Furthermore, the area fraction of ferrite is determined by subtracting the area fractions of austenite and σ phase determined by SEM / EBSD from the total area fraction of ferrite, austenite, and σ phase. The area fractions of ferrite and martensite determined in this manner are their respective volume fractions.

[0025] Average KAM value: 2.0° to 4.0° The KAM (Kernel Average Misorientation) value represents the local misorientation. The higher the KAM value, the higher the dislocation density at that measurement point, and the higher the hardness tends to be. If the average KAM value is less than 2.0°, there is a lot of soft ferrite or austenite with low dislocation density, and stress concentrates at the interface with the surrounding hard phase, becoming the starting point for cracks, resulting in low fracture elongation and d under hydrogen charging. 2 σ / dε 2 Therefore, the average KAM value is 2.0° or more. The average KAM value is preferably 2.1° or more, more preferably 2.2° or more, even more preferably 2.3° or more, and most preferably 2.4° or more. On the other hand, if the average KAM value exceeds 4.0°, the amount of hard martensite with a high dislocation density and work-hardened structures increases, and the fracture elongation and d 2 σ / dε 2Therefore, 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.

[0026] Standard deviation of KAM value distribution: 1.5° or less If the KAM value variation is large, soft and hard parts are mixed and the difference in hardness between them becomes large, so stress concentrates at the interface between them, which becomes the initiation point of cracks and reduces the fracture elongation under hydrogen charging. Therefore, the standard deviation of the KAM value distribution is 1.5° or less. The standard deviation of the KAM value distribution is preferably 1.4° or less, more preferably 1.3° or less, even more preferably 1.2° or less, and most preferably 1.1° or less. The smaller the standard deviation of the KAM value distribution, the better, but excessive reduction leads 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.

[0027] The average KAM value and the standard deviation of the KAM value distribution are measured using the SEM / EBSD method. The measurement area (one field of view) is 400 μm × 400 μm, the measurement step size is 0.1 μm, and the measured values ​​for five fields of view are averaged. Based on the obtained EBSD data, a distribution image of the KAM values ​​(KAM map) is obtained using the crystal orientation analysis software OIM Analysis™. Here, the KAM value is calculated using the following method. At each measurement point (a regular hexagonal pixel), the misorientation between each pixel is calculated using the center and the three neighboring pixels (37 pixels in total), and the average of the calculated misorientation is used as the KAM value of the central pixel. This operation is performed for all pixels in the field of view to obtain a KAM map. From the obtained KAM value distribution, the average KAM value and the standard deviation of the KAM value distribution are calculated using equations (1) and (2), respectively.

[0028]

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[0029]

number

[0030] The composition of the stainless steel pipe of the present invention is, in 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 optionally Cu: 4.00% or less, V: 0.300% or less, Nb: 0.300% or less, T It is preferable that the alloy contains one or more elements selected from i: 0.300% or less, B: 0.0050% or less, W: 3.00% or less, Ca: 0.0050% or less, Co: 0.500% or less, Sn: 0.100% or less, Mg: 0.020% or less, Zr: 0.020% or less, REM: 0.020% or less, Ta: 0.10% or less, and Sb: 0.100% or less, with the balance being Fe and unavoidable impurities.

[0031] In this specification, unless otherwise specified, "%" indicating the steel composition means "mass %".

[0032] C: 0.060% or less C is an element that increases the strength of steel through solid solution strengthening. It also refines crystal grains by lowering the transformation start temperature, thereby increasing the strength of steel. It also stabilizes austenite and increases the austenite and martensite fractions. To achieve these effects, it is preferable for C to be 0.002% or more. It is more preferably 0.003% or more, even more preferably 0.004% or more, and most preferably 0.005% or more. However, excessive C content forms Cr carbides at grain boundaries, increasing corrosion susceptibility at the grain boundaries and reducing hydrogen embrittlement resistance. Therefore, the C content is preferably 0.060% or less. The C content is more preferably 0.050% or less, even more preferably 0.040% or less, and most preferably 0.035% or less.

[0033] Si: 1.00% or less Silicon acts as a deoxidizer and also stabilizes ferrite, increasing the ferrite fraction. To achieve this effect, it is desirable to contain 0.01% or more of silicon. The silicon content is more preferably 0.05% or more, even more preferably 0.07% or more, and most preferably 0.10% or more. However, if the silicon content exceeds 1.00%, the hot workability of intermediate products (such as billets) produced during the manufacturing process of a product is reduced. For this reason, the silicon content is preferably 1.00% or less. The silicon content is more preferably 0.90% or less, even more preferably 0.80% or less, and most preferably 0.70% or less.

[0034] Mn: 6.00% or less Mn is an element that increases the strength of steel through solid solution strengthening. Furthermore, Mn refines crystal grains by lowering the transformation start temperature, thereby increasing the strength of steel. It also stabilizes austenite and increases the austenite and martensite fractions. To achieve these effects, Mn is preferably contained in an amount of 0.05% or more. The Mn content is more preferably 0.10% or more, even more preferably 0.15% or more, and most preferably 0.20% or more. However, excessive Mn content increases the strength of the steel and reduces its hydrogen embrittlement resistance. Therefore, the Mn content is preferably 6.00% or less. The Mn content is more preferably 5.00% or less, even more preferably 3.00% or less, and most preferably 2.50% or less.

[0035] P:0.050% or less Since P segregates at grain boundaries and reduces toughness, it is preferable to reduce it as an unavoidable impurity as much as possible, and the P content is preferably in the range of 0.050% or less. The P content is more preferably 0.040% or less, even more preferably 0.030% or less, and most preferably 0.025% or less. Although there is no particular lower limit for P, excessive reduction leads to an increase in smelting costs, so P is preferably 0.001% or more. P is more preferably 0.002% or more, and even more preferably 0.003% or more.

[0036] S: 0.0300% or less S is an element that significantly deteriorates hot workability during the pipe manufacturing process, so it is desirable to keep its content as low as possible. However, since pipe manufacturing using normal processes is possible if S is reduced to 0.0300% or less, S is preferably set to 0.0300% or less. The S content is more preferably 0.0100% or less, even more preferably 0.0050% or less, and most preferably 0.0030% or less. Although there is no particular lower limit for S, excessive reduction leads to increased smelting costs, so S is preferably set to 0.0001% or more. S is more preferably 0.0002% or more, and even more preferably 0.0003% or more.

[0037] Al: 0.005% or more and 0.100% or less Al is an element that acts as a powerful deoxidizer. It also stabilizes ferrite and increases the ferrite fraction. To achieve this effect, it is preferable to contain 0.005% or more of Al. The Al content is more preferably 0.010% or more, even more preferably 0.015% or more, and most preferably 0.018% or more. However, excessive Al content deteriorates weldability and increases alumina-based inclusions, deteriorating surface properties. For this reason, the Al content is preferably 0.100% or less. The Al content is more preferably 0.080% or less, even more preferably 0.070% or less, and most preferably 0.065% or less.

[0038] N: 0.400% or less N has the effect of improving hydrogen embrittlement resistance. It is also an element that stabilizes austenite and increases the austenite fraction and martensite fraction. To achieve this effect, it is preferable to contain 0.002% or more of N. The N content is more preferably 0.005% or more, even more preferably 0.010% or more, and most preferably 0.020% or more. On the other hand, if the N content exceeds 0.400%, hot workability deteriorates. Furthermore, nitrides are formed at grain boundaries, which deteriorates hydrogen embrittlement resistance. For this reason, the N content is preferably 0.400% or less. The N content is more preferably 0.380% or less, even more preferably 0.360% or less, and most preferably 0.340% or less.

[0039] Cr:11.00% or more and 30.00% or less Cr forms a protective film on the surface of steel, inhibiting hydrogen penetration into the steel and improving hydrogen embrittlement resistance. It also stabilizes ferrite and increases the ferrite fraction. If the Cr content is less than 11.00%, corrosion resistance will be insufficient, and the hydrogen embrittlement resistance targeted by the present invention will not 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, even more preferably 11.70% or more, and most preferably 11.90% or more. On the other hand, if the Cr content exceeds 30.00%, the ferrite fraction will be too high, resulting in a decrease in strength. 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, even more preferably 24.00% or less, and most preferably 20.00% or less.

[0040] Mo: 5.00% or less Mo has the effect of stabilizing the protective film on the steel surface, suppressing hydrogen penetration into the steel, and improving hydrogen embrittlement resistance. It is also an element that stabilizes ferrite and increases the ferrite fraction. To achieve this effect, it is preferable that the Mo content be 0.01% or more. The Mo content is more preferably 0.05% or more, even more 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 fraction becomes too high, resulting in a decrease in strength. For this reason, the Mo content is preferably 5.00% or less. The Mo content is more preferably 4.50% or less, even more preferably 4.30% or less, and most preferably 4.00% or less.

[0041] Ni: 15.00% or less Ni stabilizes the protective film on the steel surface, inhibits hydrogen penetration into the steel, and enhances hydrogen embrittlement resistance. It is also an element that stabilizes austenite and increases the austenite and martensite fractions. To achieve this effect, a Ni content of 0.01% or more is preferable. The Ni content is more preferably 1.00% or more, even more preferably 3.00% or more, and most preferably 3.50% or more. On the other hand, if the Ni content exceeds 15.00%, the austenite fraction increases and strength decreases. For this reason, the Ni content is preferably 15.00% or less. The Ni content is more preferably 10.00% or less, even more preferably 9.00% or less, and most preferably 8.50% or less. In addition to the above components, one or more elements selected from Cu, V, Nb, Ti, B, W, Ca, Co, Sn, Mg, Zr, REM, Ta, and Sb can be optionally selected.

[0042] Cu:4.00% or less Cu stabilizes the protective film on the steel surface, inhibits hydrogen penetration into the steel, and enhances hydrogen embrittlement resistance. It also stabilizes austenite, increasing the austenite and martensite fractions. To achieve this effect, a Cu content of 0.01% or more is desirable. The Cu content is more preferably 0.05% or more, even more preferably 0.10% or more, most preferably 0.20% or more, and most preferably 0.30% or more. On the other hand, a Cu content exceeding 4.00% leads to grain boundary precipitation of CuS, degrading hot workability. Therefore, when Cu is contained, the Cu content is limited to 4.00% or less. The Cu content is preferably 3.50% or less, more preferably 3.00% or less, even more preferably 2.80% or less, and most preferably 2.50% or less.

[0043] V: 0.300% or less, Nb: 0.300% or less, Ti: 0.300% or less V, Nb, and Ti are elements that contribute to improving the strength of steel by forming fine carbides and nitrides in the steel. They also have the effect of improving hydrogen embrittlement resistance by trapping hydrogen atoms when hydrogen generated by corrosion penetrates the steel. V and Ti also stabilize ferrite and increase the ferrite fraction. To achieve the above effects, V and Ti preferably contain 0.002% or more of V, 0.002% or more of Nb, and 0.002% or more of Ti. More preferably, V is 0.005% or more, Nb is 0.005% or more, and Ti is 0.005% or more. Even more preferably, V is 0.010% or more, Nb is 0.010% or more, and Ti is 0.010% or more. Most preferably, V is 0.015% or more, Nb is 0.015% or more, and Ti is 0.015% or more. However, excessive V and Ti contents result in excessively high strength and reduced hydrogen embrittlement resistance. Furthermore, the toughness decreases. Therefore, when V, Nb, and Ti are contained, the V contents are set to 0.300% or less, Nb contents to 0.300% or less, and Ti contents to 0.300% or less. Preferably, the V contents are set to 0.200% or less, Nb contents to 0.200% or less, and Ti contents to 0.200% or less. More preferably, the V contents are set to 0.150% or less, Nb contents to 0.150% or less, and Ti contents to 0.150% or less. Even more preferably, the V contents are set to 0.140% or less, Nb contents to 0.140% or less, and Ti contents to 0.140% or less. Most preferably, the V contents are set to 0.120% or less, Nb contents to 0.120% or less, and Ti contents to 0.120% or less.

[0044] B: 0.0050% or less B is an element that contributes to refining the structure by lowering the transformation start temperature and increases the strength of the steel. It also has the effect of suppressing grain boundary segregation of S and improving hot workability. To achieve the above effects, it is preferable that the B content be 0.0002% or more. More preferably, it is 0.0005% or more, even more preferably, it is 0.0008% or more, and most preferably, it is 0.0010% or more. However, excessive B content forms nitrides at grain boundaries, which reduces hydrogen embrittlement resistance. Therefore, when B is contained, the B content is set to 0.0050% or less. It is preferably 0.0045% or less, preferably 0.0040% or less, more preferably, it is 0.0035% or less, even more preferably, it is 0.0030% or less, and most preferably, it is 0.0025% or less.

[0045] W:3.00% or less W is an element that contributes to improving the strength of steel through solid solution strengthening and stabilizes the protective film on the steel surface, thereby enhancing hydrogen embrittlement resistance. It also stabilizes ferrite and increases the ferrite fraction. To achieve the above effects, it is preferable for W to be contained in an amount of 0.02% or more. It is more preferably 0.05% or more, even more preferably 0.10% or more, and most preferably 0.15% or more. However, excessive W content reduces toughness by forming intermetallic compounds. Therefore, when W is contained, the W content is set to 3.00% or less. It is preferably 2.50% or less, more preferably 2.40% or less, even more preferably 2.20% or less, and most preferably 2.00% or less.

[0046] Ca: 0.0050% or less Ca is an element that can improve hydrogen embrittlement resistance by spheroidizing sulfides such as MnS. To achieve the above-mentioned effects, it is preferable to contain 0.0005% or more of Ca. More preferably, it is 0.0008% or more, even more preferably, it is 0.0010% or more, and most preferably, it is 0.0012% or more. However, if it is contained in excess, Ca oxide clusters are formed in the steel, which deteriorates toughness. Therefore, if Ca is contained, the Ca content is set to 0.0050% or less. It is preferably 0.0040% or less, more preferably, it is 0.0035% or less, even more preferably, it is 0.0030% or less, and most preferably, it is 0.0025% or less.

[0047] Co:0.500% or less Co stabilizes the protective film on the steel surface, inhibits hydrogen penetration into the steel, and enhances hydrogen embrittlement resistance. It also stabilizes austenite, increasing the austenite and martensite fractions. To achieve the above-mentioned effects, it is preferable for the Co content to be 0.002% or more. It is more preferable for the Co content to be 0.005% or more, even more preferable for the Co content to be 0.010% or more, and most preferable for the Co content to be 0.015% or more. However, if the Co content exceeds 0.500%, the obtained effects become saturated and only the manufacturing cost increases. Therefore, if Co is contained, the Co content is set to 0.500% or less. It is preferably 0.450% or less, more preferably 0.400% or less, even more preferably 0.0300% or less, and most preferably 0.200% or less.

[0048] Sn: 0.100% or less Sn has the effect of stabilizing the protective film on the steel surface, inhibiting hydrogen penetration into the steel, and enhancing hydrogen embrittlement resistance. To achieve the above-mentioned effects, it is preferable that the Sn content be 0.001% or more. More preferably, it is 0.002% or more, and even more preferably, it is 0.005% or more. However, if the Sn content exceeds 0.100%, hot workability deteriorates. Therefore, when Sn is contained, the Sn content is set to 0.100% or less. It is preferably 0.070% or less, more preferably 0.050% or less, even more preferably 0.040% or less, and most preferably 0.030% or less.

[0049] Mg: 0.020% or less Mg has the effect of improving hydrogen embrittlement resistance by controlling the morphology of inclusions. To achieve the above effect, it is preferable that the Mg content be 0.001% or more. More preferably, it is 0.002% or more, and even more preferably, it is 0.005% or more. However, if the content exceeds 0.020%, hot workability deteriorates. Therefore, when Mg is contained, the Mg content is set to 0.020% or less. It is preferably 0.015% or less, more preferably 0.010% or less, even more preferably 0.009% or less, and most preferably 0.008% or less.

[0050] Zr: 0.020% or less Zr is an element that contributes to improving the strength of steel by forming fine carbides and nitrides in the steel. To achieve the above-mentioned effects, it is preferable that the Zr content be 0.001% or more. More preferably, it is 0.002% or more, and even more preferably, it is 0.005% or more. However, if it is contained in excess, the strength of the steel becomes too high, and the hydrogen embrittlement resistance decreases. Furthermore, the toughness decreases. For this reason, when Zr is contained, the Zr content is set to 0.020% or less. It is preferably 0.015% or less, more preferably 0.010% or less, even more preferably 0.009% or less, and most preferably 0.008% or less.

[0051] REM: 0.020% or less REM has the effect of improving hydrogen embrittlement resistance by controlling the morphology of inclusions. To achieve the above effect, it is preferable that the REM content be 0.001% or more. More preferably, it is 0.002% or more, even more preferably, it is 0.005% or more, and most preferably, it is 0.006% or more. However, if the content exceeds 0.020%, the hot workability deteriorates. Therefore, when REM is contained, the REM content is set to 0.020% or less. It is preferably 0.015% or less, more preferably, it is 0.010% or less, even more preferably, it is 0.009% or less, and most preferably, it is 0.008% or less. Here, REM is a general term for Sc, Y, and lanthanoid elements, a total of 17 elements, and the REM content refers to the total content of these elements.

[0052] Ta: 0.10% or less Ta is an element that contributes to improving the strength of steel by forming fine carbides and nitrides in the steel. To achieve the above-mentioned effects, it is preferable that the Ta content be 0.01% or more. More preferably, it is 0.02% or more, and even more preferably, it is 0.03% or more. Most preferably, it is 0.04% or more. However, if it is contained in excess, the strength of the steel becomes too high, and the hydrogen embrittlement resistance decreases. Furthermore, the toughness decreases. Therefore, if Ta is contained, the Ta content is set to 0.10% or less. It is preferably 0.08% or less, more preferably 0.07% or less, even more preferably 0.06% or less, and most preferably 0.05% or less.

[0053] Sb: 0.100% or less Sb stabilizes the protective film on the steel surface, inhibits hydrogen penetration into the steel, and enhances hydrogen embrittlement resistance. To achieve the above-mentioned effects, it is preferable to contain 0.001% or more of Sb. More preferably, it is 0.002% or more, even more preferably, it is 0.005% or more, and most preferably, it is 0.008% or more. However, if it contains more than 0.100%, hot workability deteriorates. Therefore, when Sb is contained, the Sb content is set to 0.100% or less. It is preferably 0.070% or less, more preferably, 0.050% or less, even more preferably, it is 0.040% or less, and most preferably, it is 0.030% or less.

[0054] The balance is Fe and unavoidable impurities. Examples of the unavoidable impurities in the balance include As, Bi, Pb, Zn, O, Te, Hf, Ge, Sr, and Cs. However, within the range that does not impair the effects of the present invention, As, Te, Hf, Ge, Sr, and Cs may each be contained in an amount of 0.10% or less, and Bi, Pb, Zn, and O may each be contained in an amount of 0.005% or less.

[0055] The stainless steel pipe of the present invention has a fracture elongation of 9.0% or more when subjected to a tensile test under cathodic hydrogen charging, and a true strain at the maximum load of ε max When the true strain is 0.8ε max ε or more max In the following range, the rate of change d of the work hardening rate dσ / dε relative to ε is 2 σ / dε 2 is greater than -200,000 MPa, where σ is the true stress (MPa) and ε is the true strain (no unit).

[0056] Figure 1 shows a schematic diagram of a curve plotting true strain ε on the horizontal axis and true stress σ on the vertical axis when a tensile test was conducted under cathodic hydrogen charging. max ε or more max In the range below, the mode changes from elastic deformation (elastic region) to plastic deformation (plastic region), and this is the stage generally referred to as uniform elongation. Since the work hardening behavior becomes stable, 0.8ε maxε or more max The following ranges are taken into consideration and limited to the above. Regarding the above ranges, the above ranges are basically taken into consideration, but 0.9ε max It may be more than that.

[0057] A stainless steel pipe that can achieve the above properties has excellent resistance to hydrogen embrittlement.

[0058] As a method of hydrogen charging, for example, a current density of 0.05 mA / cm is applied in a solution at room temperature (21 to 27°C) in which 3.0 g of ammonium thiocyanate is added per 1 L of a 3.0 mass% aqueous sodium chloride solution. 2 Cathodic hydrogen charging is performed for 24 hours under the above conditions. Thereafter, a tensile test is performed while continuing cathodic hydrogen charging under the above conditions.

[0059] The hydrogen charging method is not limited to the above method, and any condition may be used 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. Conditions that result in a hydrogen content of 1.6 ppm by mass or more are more preferable, and conditions that result in a hydrogen content of 1.7 ppm by mass or more are even more preferable. Furthermore, although there is no particular upper limit, it is preferable that the hydrogen content be 10.0 ppm by mass or less. The hydrogen content in the steel is measured, for example, by thermal desorption spectroscopy, with a heating rate of 100°C / s, the temperature raised to 600°C, and a gas chromatograph used as the analyzer.

[0060] The strain rate in the tensile test is, for example, 1.7 × 10 -5 s -1 The strain rate in the tensile test is not particularly limited, but is preferably 1.0 x 10 -5 s -1 It is preferable that the concentration is 1.5×10 or more. -5 s -1 More preferably, it is 1.8×10 or more. -5 s -1 More preferably, it is 1.9×10 or more. -5 s -1 It is most preferable to set the value to 9.0×10 or more. -5 s -1It is preferable that the value is 6.0×10 or less. -5 s -1 It is more preferable to set the following:

[0061] For the tensile test, a round bar specimen with a parallel part diameter of 3.8 mm and a parallel part length of 15 mm is used. The specimen is taken from the center of the wall thickness so that the tensile direction is parallel to the tube axis.

[0062] Breaking elongation: 9.0% or more If the fracture elongation in the tensile test under hydrogen charging described above is small, the ductility in a hydrogen environment will be insufficient. Therefore, in the present invention, the fracture elongation is set to 9.0% or more. It is preferably 9.5% or more, more preferably 10.0% or more, even more preferably 10.2% or more, and most preferably 10.4% or more. The greater the fracture elongation, the better, but an excessive increase in fracture elongation will lead to increased manufacturing costs and manufacturing load, so the fracture elongation is preferably 30.0% or less. More preferably, it is 28.0% or less. It is even more preferably 25.0% or less, and most preferably 24.0% or less. The breaking elongation (%) is calculated by ((total length when the test pieces are butted together after the test) - (total length of the test piece before the test)) / (total length of the test piece before the test) x 100.

[0063] True strain is 0.8ε max ε or more max In the following range, d 2 σ / dε 2 :-200,000 MPa or more While work hardening progresses in the plastic region, the work hardening rate dσ / dε decreases with increasing ε. That is, d 2 σ / dε 2 is a negative value. 2 σ / dε 2 The smaller the true strain, the more rapidly dσ / dε decreases, causing necking to occur earlier, reducing the maximum load and decreasing ductility. max ε or more max It is important to control the true strain within the following range.max ε or more max In the following range, d in the tensile test under hydrogen charging 2 σ / dε 2 is -200,000 MPa or more, preferably -180,000 MPa or more, more preferably -160,000 MPa or more, even more preferably -150,000 MPa or more, and most preferably -140,000 MPa or more. 2 σ / dε 2 The larger is the better, but if it is increased too much, it will lead to an increase in manufacturing costs and manufacturing load. 2 σ / dε 2 is preferably less than 0 MPa, more preferably -10,000 MPa or less, and even more preferably -15,000 MPa or less.

[0064] dσ / dε and d 2 σ / dε 2 is obtained from the time history data of the load and crosshead displacement in the tensile test. First, σ and ε at each time are obtained from the load and crosshead displacement. Then, to reduce the fluctuation of σ, σ is averaged within the range equivalent to ε = 0.001 at each time. Specifically, for example, when the strain rate is 1.7 × 10 -5 s -1 In this case, σ at a certain time t(s) is calculated as the average value of σ from time (t-30)(s) to time (t+30)(s). This is done for all times. Next, dσ / dε is calculated for each time. Specifically, dσ / dε at a certain time t(s) is calculated as the average rate of change from time (t-60)(s) to time t(s). This is done for all times. Furthermore, d 2 σ / dε 2 Specifically, (d / dε)(dσ / dε) at a certain time t(s) is calculated as the average rate of change from time (t-60)(s) to time t(s). This is done for all times.

[0065] In this invention, since the tensile test is performed in a solution, it is difficult to directly measure the displacement of the parallel part using an extensometer or the like. Therefore, ε is calculated from the crosshead displacement. In this case, the value of ε contains an error due to the elastic deformation of the testing machine, but dσ / dε and d 2 σ / dε 2 is calculated from the change in ε, the error in the present invention is so small that it can be ignored.

[0066] 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 is 550 MPa or more, more preferably 580 MPa or more, and even more preferably 600 MPa or more. On the other hand, as the yield strength increases, corrosion resistance decreases. Therefore, the yield strength is preferably 1100 MPa or less. More preferably, it is 1000 MPa or less, even more preferably 990 MPa or less, and most preferably 980 MPa or less.

[0067] Yield strength is measured by tensile testing at room temperature (10 to 38°C) in air. Test specimens are prepared in accordance with ASTM E8 / E8M(2021). Round bar test specimens are prepared from the center of the wall thickness so that the tensile direction is parallel to the pipe axis. The size of the round bar test specimen is, for example, 8.9 mm in diameter at the parallel portion and 35.6 mm in gauge length. If round bar test specimens cannot be prepared from the steel pipe, arc test specimens are prepared. The size of the arc test specimen is, for example, the full wall thickness, 25.4 mm in width, and 50.8 mm in gauge length. The prepared tensile test specimen is then subjected to tensile testing in accordance with ASTM E8 / E8M(2021). The obtained 0.2% offset yield strength (MPa) is taken as the yield strength (MPa).

[0068] The stainless steel pipe is preferably a seamless steel pipe.

[0069] Furthermore, it is preferable that the oil country tubular good be made of the above stainless steel pipe.

[0070] Next, a method for manufacturing a stainless steel pipe according to one embodiment of the present invention will be described.

[0071] The stainless steel pipe of the present invention is produced, for example, by heating and hot working a steel material having the above-mentioned composition into a cylindrical shape, cooling it, and then subjecting it to heat treatment.

[0072] In the following description of the manufacturing method, the temperature indicated in "°C" refers to the surface temperature unless otherwise specified. These surface temperatures can be measured using a radiation thermometer or the like. The temperature at the center of the wall thickness can be determined by calculating the temperature distribution in the wall thickness direction using heat transfer analysis and correcting the result by the surface temperature.

[0073] In the present invention, the method for melting the steel material is not particularly limited, and any of the known melting methods such as converters, electric furnaces, and vacuum melting furnaces is suitable. The casting method is also not particularly limited, and steel is produced to the desired dimensions by a known casting method such as continuous casting. Note that there is no problem if an ingot casting-blooming rolling method is used instead of continuous casting. The molten steel may further be subjected to secondary refining such as ladle refining. It is preferable that the steel material is cast into a round bar shape such as a billet, and this is used as a steel pipe material. However, the cast slab may also be further hot-rolled to have the desired dimensions and shape, and the resulting slab may be used as a steel pipe material.

[0074] The resulting steel pipe material is then heated and hot worked to form a stainless steel pipe of the desired shape. The hot pipe-making process is preferably a Mannesmann plug mill type or Mannesmann mandrel mill type hot pipe-making process. However, the stainless steel pipe may also be formed by hot extrusion using a press method. The hot pipe-making process does not require any particular conditions, as long as it can produce a stainless steel pipe of the desired shape.

[0075] In the heating step, the heating temperature of the steel pipe material (e.g., billet) is preferably in the range of 1100 to 1350°C. If the heating temperature is less than 1100°C, the hot workability of the billet decreases, making it more likely to have defects during pipe making. Furthermore, the load on the equipment becomes excessive. Therefore, the heating temperature is preferably 1100°C or higher. The heating temperature is more preferably 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 too high, the crystal grains become coarse, the variation in the KAM value within the crystal grains increases, and the standard deviation of the KAM value distribution increases. Therefore, the heating temperature in the heating step is preferably 1350°C or lower. The heating temperature is more preferably 1300°C or lower, even more preferably 1290°C or lower, and most preferably 1280°C or lower.

[0076] In the hot pipe-making process, the area reduction rate (%) of the cross section perpendicular to the pipe axis direction is preferably 25% or more. Here, the area reduction rate (%) is calculated by ((cross-sectional area of ​​steel pipe material) - (cross-sectional area after hot pipe-making)) / (cross-sectional area of ​​steel pipe material) × 100. If the area reduction rate is small, the crystal grains become coarse, the variation in KAM values ​​within the crystal grains increases, and the standard deviation of the KAM value distribution increases. The area reduction rate is more preferably 30% or more, even more preferably 35% or more, and most preferably 40% or more. On the other hand, if the area reduction rate is too high, the obtained effect saturates and the equipment load becomes excessive, so the area reduction rate is preferably 75% or less. More preferably, it is 70% or less, even more preferably 65% ​​or less, and most preferably 60% or less.

[0077] After the hot pipe-making process, the resulting stainless steel pipe is subjected to a cooling treatment. In the cooling treatment, it is preferable that the average cooling rate at the center of the wall thickness in the range of 650 to 300°C is 10°C / s or more and the minimum cooling rate is 3°C / s or more. The minimum cooling rate is determined by dividing the time in the range of 650 to 300°C into 3-second intervals, calculating the average cooling rate in each interval, and taking the minimum value of these. The average cooling rate in each interval can be calculated by subtracting the cooling end temperature from the cooling start temperature in the given interval and dividing the result by the time required for cooling in the given interval. The average cooling rate in the range of 650 to 300°C can be calculated by (650 - 300) / (time required for cooling in the range of 650 to 300°C) (°C / s). When the average cooling rate and the minimum cooling rate are low, the grains become coarse, the dispersion of the KAM value within the grains increases, and the standard deviation of the KAM value distribution increases. In addition, the σ phase, which is an embrittlement phase, is generated, and the fracture elongation and d 2 σ / dε 2 Therefore, the average cooling rate in the range of 650 to 300°C is preferably 10°C / s or more, more preferably 15°C / s or more, even more preferably 18°C / s or more, and most preferably 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 excessive, so the average cooling rate is preferably 100°C / s or less, more preferably 70°C / s or less, even more preferably 60°C / s or less, and most preferably 50°C / s or less. In addition, the minimum cooling rate is preferably 3°C / s or more, more preferably 4°C / s or more, even more 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 excessive, so the minimum cooling rate is preferably 15°C / s or less. The minimum cooling rate is more preferably 12°C / s or less, even more preferably 11°C / s or less, and most preferably 10°C / s or less. The cooling method is preferably water cooling in order to ensure the required cooling rate.

[0078] In the present invention, the seamless steel pipe is then preferably subjected to a heat treatment consisting of quenching and tempering. However, when the Cr content is 20.00% or more, the austenite is stable at room temperature and the martensite fraction is low, so the tempering treatment can be omitted.

[0079] In the quenching process, the stainless steel pipe is reheated to a temperature (heating temperature) in the range of 850 to 1150°C and held at that temperature for at least 300 seconds. The average cooling rate in the range of 650 to 300°C at the center of the wall thickness is set to at least 10°C / s, and the minimum cooling rate is set to at least 3°C / s. The minimum cooling rate is calculated by dividing the time in the range of 650 to 300°C into 3-second intervals, calculating the average cooling rate in each interval, and then taking the minimum value. The average cooling rate in each interval can be calculated by subtracting the cooling end temperature from the cooling start temperature in the given interval and dividing the result by the time required for cooling in the given interval. The average cooling rate in the range of 650 to 300°C can be calculated by (650 - 300) / (time required for cooling in the range of 650 to 300°C) (°C / s).

[0080] If the heating temperature during quenching is low, the austenite fraction will be low and the ferrite fraction will be high, resulting in a low average KAM value. The yield strength will also be low. Furthermore, the σ phase may remain undissolved, resulting in low fracture elongation and d 2 σ / dε 2 Therefore, the heating temperature for the quenching treatment is preferably 850°C or higher, more preferably 880°C or higher, even more preferably 900°C or higher, and most preferably 920°C or higher. If the heating temperature for the quenching treatment is high, the crystal grains become coarse, the variation in KAM values ​​within the crystal grains increases, and the standard deviation of the KAM value distribution increases. Therefore, the heating temperature for the quenching treatment is preferably 1150°C or lower, more preferably 1130°C or lower, even more preferably 1100°C or lower, and most preferably 1080°C or lower.

[0081] If the average cooling rate and the minimum cooling rate are low, the crystal grains become coarse, the variation in the KAM values ​​within the crystal grains increases, and the standard deviation of the KAM value distribution increases. In addition, the σ phase was generated, and the fracture elongation and d 2 σ / dε 2 Therefore, the average cooling rate in the range of 650 to 300°C is preferably 10°C / s or more, more preferably 15°C / s or more, even more preferably 18°C / s or more, and most preferably 20°C / s or more. If the average cooling rate exceeds 100°C / s, the load on the cooling device will become excessive, so the average cooling rate is preferably 100°C / s or less, more preferably 70°C / s or less, even more preferably 60°C / s or less, and most preferably 50°C / s or less. Furthermore, the minimum cooling rate is preferably 3°C / s or more, more preferably 4°C / s or more, even more 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 will become excessive, so the minimum cooling rate is preferably 15°C / s or less, more preferably 12°C / s or less, even more preferably 11°C / s or less, and most preferably 10°C / s or less. The cooling method is preferably water cooling in order to ensure the required cooling rate.

[0082] The quenched stainless steel pipe is preferably subsequently subjected to a tempering treatment.

[0083] In tempering, the material is heated to 500°C or higher and 750°C or lower, preferably held for 30 minutes or longer, and then cooled, preferably to room temperature, at a cooling rate faster than air cooling. If the heating temperature in tempering is low, dislocation recovery does not proceed sufficiently, resulting in a large variation in KAM values ​​within the crystal grains and a large standard deviation in the KAM value distribution. Therefore, the heating temperature in tempering is preferably 500°C or higher, more preferably 550°C or higher, even more preferably 560°C or higher, and most preferably 570°C or higher. On the other hand, if the heating temperature in tempering is high, the crystal grains become coarse, resulting in a large variation in KAM values ​​within the crystal grains and a large standard deviation in the KAM value distribution. In addition, a σ phase is generated, which reduces the fracture elongation and d under hydrogen charging. 2 σ / dε 2 Therefore, the heating temperature in the tempering treatment is preferably 750°C or less, more preferably 720°C or less, even more preferably 700°C or less, and most preferably 680°C or less.

[0084] Furthermore, in the present invention, if necessary, cold working may be performed following tempering to correct defects in the steel pipe shape or to adjust the yield strength through work hardening. Examples of cold working methods include cold drawing. If the area reduction rate (%) of the cross section perpendicular to the pipe axis direction during cold working exceeds 40%, work hardening increases, resulting in a high average KAM value. Therefore, the area reduction rate (%) during cold working is preferably 40% or less, more preferably 35% or less, even more preferably 30% or less, and most preferably 25% or less. Although there is no particular lower limit, the area reduction rate (%) during cold working is preferably 5% or more. Here, the area reduction rate (%) during cold working is calculated by ((cross-sectional area before working) - (cross-sectional area after working)) / (cross-sectional area before working) × 100. [Example]

[0085] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples.

[0086] Molten steel having the chemical composition shown in Table 1 was melted and made into billets (steel pipe materials). The obtained billet was heated and hot worked under the conditions shown in Table 2, and then cooled to obtain stainless steel pipes having the outer diameter (mm) and wall thickness (mm) shown in Table 2.

[0087] Test pieces were taken from the obtained stainless steel pipes, and the following KAM value measurements, tensile tests, and tensile tests under cathodic hydrogen charging were carried out. Note that the microstructure evaluation was carried out based on the details explained in the embodiment.

[0088] [KAM value measurement] The average KAM value and the standard deviation of the KAM value distribution were measured using the SEM / EBSD method. The acceleration voltage was 15 kV. The measurement area (one field of view) was 400 μm × 400 μm, the measurement step size was 0.1 μm, and the measured values ​​for five fields of view were averaged. Based on the obtained EBSD data, a distribution image of KAM values ​​(KAM map) was obtained using the crystal orientation analysis software OIM Analysis™. The KAM value was calculated using the following method. For each measurement point (a regular hexagonal pixel), the misorientation between each pixel was calculated using the center and the three neighboring pixels (37 pixels in total), and the average of the calculated misorientation values ​​was used as the KAM value of the central pixel. This operation was performed for all pixels in the field of view to obtain a KAM map. From the obtained KAM value distribution, the average KAM value and the standard deviation of the KAM value distribution were calculated using equations (1) and (2), respectively.

[0089]

number

[0090]

number

[0091] [Tensile test] Test specimens were prepared in accordance with ASTM E8 / E8M(2021), with arc-shaped specimens prepared so that the tensile direction was parallel to the pipe axis. The arc-shaped specimens had a thickness equal to the full wall thickness, a width of 25.4 mm, and a gauge length of 50.8 mm. Tensile tests were conducted using the prepared tensile test specimens in accordance with ASTM E8 / E8M(2021). The obtained 0.2% offset yield strength (MPa) was taken as the yield strength (MPa).

[0092] [Tensile test under cathodic hydrogen charging] A round bar test piece was taken from the center of the wall thickness so that the tensile direction was parallel to the tube axis. The diameter of the parallel part was 3.8 mm, and the length of the parallel part was 15 mm. The test piece was immersed in a solution of 3.0 mass% sodium chloride solution at room temperature, containing 3.0 g of ammonium thiocyanate per 1 L, at a current density of 0.05 mA / cm. 2 Cathodic hydrogen charging was performed for 24 hours under the same conditions, and then a tensile test was performed while continuing cathodic hydrogen charging under the same conditions. The strain rate in the tensile test was 2.0 × 10 -5 s -1 It was decided. The breaking elongation (%) was calculated by ((total length of the test pieces when butted together after the test) - (total length of the test piece before the test)) / (total length of the test piece before the test) x 100.

[0093] dσ / dε and d 2 σ / dε 2 was calculated from the time history data of the load and crosshead displacement in the tensile test. First, σ and ε at each time were calculated from the load and crosshead displacement. Then, to reduce the fluctuation of σ, σ was averaged in the range corresponding to ε = 0.001 at each time. Specifically, σ at a certain time t(s) was calculated as the average value of σ from time (t-30)(s) to time (t+30)(s). This was repeated for all times. Next, dσ / dε was calculated for each time. Specifically, dσ / dε at a certain time t(s) was calculated as the average rate of change from time (t-60)(s) to time t(s). This was repeated for all times. Furthermore, d 2 σ / dε 2Specifically, (d / dε)(dσ / dε) at a certain time t(s) was calculated as the average rate of change from time (t-60)(s) to time t(s). This was done for all times.

[0094] The results obtained are shown in Table 3.

[0095] In Table 3, stainless steel pipes Nos. 1, 3, 5, 8, 10, 12, 15, 16, 17, 20, 26, and 27 are examples of the present invention, and stainless steel pipes Nos. 2, 4, 6, 7, 9, 11, 13, 14, 18, 19, and 21 to 25 are comparative examples.

[0096] In the stainless steel pipes of the present invention, the steel structure at the center of the wall thickness had a total volume fraction of ferrite and martensite of 30% to 99%, a volume fraction of σ phase of 0% to 3%, and the remainder was austenite.The average KAM value was 2.0° to 4.0°, and the standard deviation of the KAM value distribution was 1.5° or less.Furthermore, the yield strength was 450 MPa or more, the fracture elongation was 9.0% or more, and the true strain was 0.8ε. max ε or more max In the following range, the rate of change d of the work hardening rate dσ / dε relative to ε is 2 σ / dε 2 was over -200,000 MPa.

[0097] On the other hand, the stainless steel pipe No. 2 of the comparative example had a large standard deviation in the KAM value distribution, and as a result, the breaking elongation required in the present invention was not obtained. The stainless steel pipe of Comparative Example No. 4 had a large standard deviation in the KAM value distribution, and as a result, was unable to obtain the breaking elongation required in the present invention. The comparative stainless steel pipe No. 6 had a large standard deviation in the KAM value distribution, and as a result, was unable to obtain the breaking elongation required in the present invention. The stainless steel pipe of Comparative Example No. 7 had a large standard deviation in the KAM value distribution and a high volume fraction of the σ phase, and therefore could not obtain the breaking elongation required in the present invention. The comparative stainless steel pipe No. 9 had a low average KAM value, and as a result, the breaking elongation and d required by the present invention were not achieved. 2 σ / dε 2 Furthermore, the yield strength also tended to be low. The comparative stainless steel pipe No. 11 had a large standard deviation in the KAM value distribution, and as a result, was unable to obtain the breaking elongation required in the present invention. The comparative stainless steel pipe No. 13 had a large standard deviation in the KAM value distribution and a high volume fraction of the σ phase, and as a result, the breaking elongation required in the present invention was not obtained. The comparative stainless steel pipe No. 14 had a high average KAM value, and as a result, the breaking elongation and d required by the present invention were not achieved. 2 σ / dε 2 was not obtained. The comparative stainless steel pipe No. 18 had a large standard deviation in the KAM value distribution, and as a result, was unable to obtain the breaking elongation required in the present invention. The comparative stainless steel pipe No. 19 had a low average KAM value, and as a result, the breaking elongation and d required by the present invention were not achieved. 2 σ / dε 2 was not obtained. The comparative stainless steel pipe No. 21 had a large standard deviation in the KAM value distribution, and as a result, the breaking elongation required in the present invention was not obtained. The comparative stainless steel pipe No. 22 had a high average KAM value, and as a result, the breaking elongation and d required by the present invention were not achieved. 2 σ / dε 2 was not obtained. The stainless steel pipe No. 23 of the comparative example has a high total volume fraction of ferrite and martensite, and as a result, the fracture elongation and d required by the present invention are 2 σ / dε 2 was not obtained. The stainless steel pipe No. 24 of the comparative example had a tendency to have a low yield strength as a result of the low total volume fraction of ferrite and martensite. The stainless steel pipe No. 25 of the comparative example had a high volume fraction of the σ phase, and therefore did not meet the d required by the present invention. 2 σ / dε2 was not obtained.

[0098] [Table 1]

[0099] [Table 2]

[0100] [Table 3]

Claims

1. In 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: Contains 15.00% or less, Further optionally, Cu: 4.00% or less, V: 0.300% or less, Nb: 0.300% or less, Ti: 0.300% or less, B: 0.0050% or less, W: 3.00% or less, Ca: 0.0050% or less, Co: 0.500% or less, Sn: 0.100% or less, Mg: 0.020% or less, Zr: 0.020% or less, REM: 0.020% or less, Ta: 0.10% or less, Sb: 0.100% or less, The balance has a composition consisting of Fe and unavoidable impurities, The steel structure at the center of the wall thickness is The total volume fraction of ferrite and martensite is 30% or more and 99% or less, the volume fraction of the σ phase is 0% or more and 3% or less, and the remainder is austenite, an average KAM (Kernel Average Misorientation) value, which is an average of the KAM values ​​of the steel structure, is 2.0° or more and 4.0° or less; A stainless steel pipe in which the standard deviation of the KAM value distribution is 1.5° or less.

2. The stainless steel pipe is a seamless steel pipe. The stainless steel pipe according to claim 1.

3. 3. An oil country tubular good using the stainless steel pipe according to claim 1 or 2.

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