SEAMLESS STAINLESS STEEL PIPE AND METHOD FOR MANUFACTURING SAME

Abstract

 Provided is a stainless steel seamless pipe having high strength and excellent low-temperature toughness and corrosion resistance. The stainless steel seamless pipe comprises: a predetermined chemical composition; 30 % or more martensite phase, 50 % or less ferrite phase, and 40 % or less retained austenite phase in volume fraction; a yield stress of 758 MPa or more; and a Charpy absorbed energy vE_-10 at -10 °C of 40 J or more.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a stainless steel seamless pipe suitable for use in oil wells and gas wells (hereafter simply referred to as oil wells). The present disclosure especially relates to a stainless steel seamless pipe with improved corrosion resistance in high-temperature severe corrosion environments containing carbon dioxide (CO₂) and chloride ions (Cl^-), environments containing hydrogen sulfide (H₂S), and the like.

BACKGROUND

[0002] Stainless steel seamless pipes are widely used for applications such as steel pipes for oil wells. Steel pipes for oil wells are required to have not only excellent yield stress but also excellent low-temperature toughness in view of recent oil field development in cold regions.

[0003] Given the possible depletion of energy resources in the near future, oil wells are actively developed in severe corrosion environments such as deep oil fields, environments containing carbon dioxide, and environments containing hydrogen sulfide called sour environments, which have not received much attention in the past. Hence, steel pipes for oil wells are also required to have high corrosion resistance.

[0004] Conventionally, 13Cr martensitic stainless steel pipes are commonly used as steel pipes for oil wells for mining in oil and gas fields in environments containing CO₂, Cl^-, etc. Recently, however, the development of oil wells of higher temperatures (up to 200 °C) is underway, and 13Cr martensitic stainless steel pipes sometimes lack corrosion resistance. There is thus a demand for a steel pipe for oil wells with higher corrosion resistance, which can be used even in such environments.

[0005] In response to this demand, for example, WO 2013/146046 A1 (PTL 1) proposes a stainless steel for oil wells having a composition containing, in mass%, C: 0.05 % or less, Si: 1.0 % or less, Mn: 0.01 % to 1.0 %, P: 0.05 % or less, S: less than 0.002 %, Cr: 16 % to 18 %, Mo: 1.8 % to 3 %, Cu: 1.0 % to 3.5 %, Ni: 3.0 % to 5.5 %, Co: 0.01 % to 1.0 %, Al: 0.001 % to 0.1 %, O: 0.05 % or less, and N: 0.05 % or less where Cr, Ni, Mo, and Cu satisfy a specific relationship.

[0006] WO 2017/168874 A1 (PTL 2) proposes a high-strength stainless steel seamless pipe for oil wells having a composition containing, in mass%, C: 0.005 % to 0.05 %, Si: 0.05 % to 0.50 %, Mn: 0.20 % to 1.80 %, P: 0.030 % or less, S: 0.005 % or less, Cr: 12.0 % to 17.0 %, Ni: 4.0 % to 7.0 %, Mo: 0.5 % to 3.0 %, Al: 0.005 % to 0.10 %, V: 0.005 % to 0.20 %, Co: 0.01 % to 1.0 %, N: 0.005 % to 0.15 %, and O: 0.010 % or less where Cr, Ni, Mo, Cu, C, Si, Mn, and N satisfy a specific relationship.

[0007] WO 2018/155041 A1 (PTL 3) proposes a high-strength stainless steel seamless pipe for oil wells having: a composition containing, in mass%, C: 0.05 % or less, Si: 0.5 % or less, Mn: 0.15 % to 1.0 %, P: 0.030 % or less, S: 0.005 % or less, Cr: 14.5 % to 17.5 %, Ni: 3.0 % to 6.0 %, Mo: 2.7 % to 5.0 %, Cu: 0.3 % to 4.0 %, W: 0.1 % to 2.5 %, V: 0.02 % to 0.20 %, Al: 0.10 % or less, N: 0.15 % or less, and B: 0.0005 % to 0.0100 % where C, Si, Mn, Cr, Ni, Mo, Cu, N, and W satisfies a specific relationship; and a microstructure containing, in volume fraction, more than 45 % martensite phase as a primary phase, 10 % to 45 % ferrite phase as a secondary phase, and 30 % or less retained austenite phase. According to PTL 3, such a high-strength stainless steel seamless pipe for oil wells has a strength of 862 MPa or more in yield stress YS, and exhibits sufficient corrosion resistance even in high-temperature severe corrosion environments containing CO₂, Cl^-, and H₂S.

[0008] WO 2021/065263 A1 (PTL 4) proposes a high-strength stainless steel seamless pipe for oil wells having: a composition containing, in mass%, C: 0.06 % or less, Si: 1.0 % or less, P: 0.05 % or less, S: 0.005 % or less, Cr: more than 15.7 % and 18.0 % or less, Mo: 1.8 % or more and 3.5 % or less, Cu: 1.5 % or more and 3.5 % or less, Ni: 2.5 % or more and 6.0 % or less, Al: 0.10 % or less, N: 0.10 % or less, O: 0.010 % or less, W: 0.5 % or more and 2.0 % or less, and Co: 0.01 % or more and 1.5 % or less where C, Si, Mn, Cr, Ni, Mo, Cu, and N satisfy a specific relationship; and a microstructure containing, in volume fraction, 25 % or more martensite phase, 65 % or less ferrite phase, and 40 % or less retained austenite phase. According to PTL 4, such a high-strength stainless steel seamless pipe for oil wells has a strength of 758 MPa or more in yield stress YS, and exhibits sufficient corrosion resistance even in high-temperature severe corrosion environments containing CO₂, Cl^-, and H₂S.

[0009] WO 2021/187331 A1 (PTL 5) proposes a high-strength stainless steel seamless pipe for oil wells having: a composition containing, in mass%, C: 0.06 % or less, Si: 1.0 % or less, Mn: 0.01 % or more and 1.0 % or less, P: 0.05 % or less, S: 0.005 % or less, Cr: 15.2 % or more and 18.5 % or less, Mo: 1.5 % or more and 4.3 % or less, Cu: 1.1 % or more and 3.5 % or less, Ni: 3.0 % or more and 6.5 % or less, Al: 0.10 % or less, N: 0.10 % or less, O: 0.010 % or less, and Sb: 0.001 % or more and 1.000 % or less where C, Si, Mn, Cr, Ni, Mo, Cu, and N satisfy a specific relationship; and a microstructure containing, in volume fraction, 30 % or more martensite phase, 65 % or less ferrite phase, and 40 % or less retained austenite phase. According to PTL 5, such a high-strength stainless steel seamless pipe for oil wells has a strength of 758 MPa or more in yield stress YS, and exhibits sufficient corrosion resistance even in high-temperature severe corrosion environments containing CO₂, Cl^-, and H₂S.

CITATION LIST

Patent Literature

[0010] 

PTL 1: WO 2013/146046 A1

PTL 2: WO 2017/168874 A1

PTL 3: WO 2018/155041 A1

PTL 4: WO 2021/065263 A1

PTL 5: WO 2021/187331 A1

SUMMARY

(Technical Problem)

[0011] The conventional techniques proposed in PTL 1 to PTL 5 can improve the corrosion resistance of stainless steel, but the performance is still insufficient.

[0012] As mentioned above, steel pipes for oil wells are required to have not only excellent yield stress and low-temperature toughness but also high corrosion resistance to withstand use in severe corrosion environments.

[0013] For example, steel pipes for oil wells are required to have excellent corrosion resistance in carbon dioxide environments (CO₂ corrosion resistance), and particularly required to have excellent CO₂ corrosion resistance even in high-temperature environments.

[0014] Steel pipes for oil wells are also required to have resistance (SSC resistance) to sulfide stress cracking (SSC) in hydrogen sulfide environments. Particularly in offshore oil fields, cold seawater has a high specific gravity and stays near the seabed, so that the pipes for oil wells are exposed to temperatures lower than the atmospheric temperature of the region. Accordingly, steel pipes for oil wells are required to have excellent SSC resistance even in low-temperature environments.

[0015] There are cases where, when extracting petroleum, the properties (mainly permeability) of the layer in which petroleum is stored (petroleum reservoir) are poor and sufficient production volume cannot be obtained, or the expected production volume cannot be obtained due to, for example, clogging in the reservoir. One way of improving productivity is acidizing, i.e. a treatment of injecting an acid such as hydrochloric acid into the reservoir. Steel pipes for oil wells are therefore also required to have excellent corrosion resistance in acid environments.

[0016] With the conventional techniques, however, it is impossible to obtain a steel pipe having sufficient levels of yield stress, low-temperature toughness, high-temperature CO₂ corrosion resistance, low-temperature SSC resistance, and corrosion resistance in acid environments.

[0017] In particular, the technique described in PTL 5 is supposed to provide high strength, high-temperature corrosion resistance, and corrosion resistance in acid environments, but the SSC resistance is not necessarily sufficient. The reason for this is considered as follows: If the phase fraction during steel pipe production is not appropriate, the hot workability is insufficient, and cracking occurs on the inner and outer surfaces of the steel pipe. In the case where such a steel pipe is used in an oil well, corrosive ions stay inside the cracks and further concentrate with the progress of corrosion. This causes insufficient SSC resistance.

[0018] It could therefore be helpful to provide a stainless steel seamless pipe having high strength of 758 MPa (110 ksi) or more in yield stress and excellent low-temperature toughness and corrosion resistance.

[0019] In the present disclosure, "excellent corrosion resistance" means excellent in all of high-temperature CO₂ corrosion resistance, low-temperature SSC resistance, and corrosion resistance in acid environments.

[0020] Herein, "excellent high-temperature CO₂ corrosion resistance" means that the corrosion rate when a test piece is immersed in a test liquid: 20 mass% NaCl aqueous solution (liquid temperature: 200 °C, 30 atm CO₂ gas atmosphere) held in an autoclave for an immersion time of 336 hours is 0.127 mm/y or less.

[0021] Moreover, "excellent low-temperature SSC resistance" means that, when a C-shaped test piece conforming to NACE TM0177 Method C is immersed in an aqueous solution obtained by adding acetic acid + sodium acetate to a 0.165 mass% NaCl aqueous solution (liquid temperature: 7 °C, 0.995 atm CO₂ gas, 0.005 atm H₂S atmosphere) to adjust the pH to 3.0 for an immersion time of 720 hours under load of 100 % of yield stress as load stress, there is no cracking in the test piece after the test.

[0022] Moreover, "excellent corrosion resistance in acid environments" means that the corrosion rate when a test piece is immersed in a 15 mass% hydrochloric acid solution heated to 80 °C for an immersion time of 40 minutes is 600 mm/y or less.

[0023] Moreover, "excellent low-temperature toughness" means that the Charpy absorbed energy vE_-10 at -10 °C is 40 J or more. The Charpy absorbed energy vE_-10 is measured by the following procedure. First, three V-notch test pieces (10 mm thick) whose longitudinal direction is perpendicular to the pipe axis and whose notch is on a plane perpendicular to the pipe axis are collected per one stainless steel seamless pipe in accordance with ASTM E23. These test pieces are then subjected to a Charpy impact test at a test temperature of -10 °C, and the lowest value of the absorbed energies of the three test pieces is taken to be the Charpy absorbed energy vE_-10 at -10 °C.

[0024] Upon careful examination on various factors influencing the corrosion resistance, especially the SSC resistance and the corrosion resistance in acid environments, of stainless steel, we discovered that excellent corrosion resistance can be achieved by containing at least predetermined amounts of Cr, Mo, Sb, Co, and Ca and limiting the amount of Ni, which affects the phase fraction of steel, to a predetermined range.

[0025] The present disclosure is based on these discoveries and further studies. We thus provide the following.

1. A stainless steel seamless pipe comprising: a chemical composition containing (consisting of), in mass%, C: 0.06 % or less, Si: 1.0 % or less, Mn: 0.01 % or more and 1.0 % or less, P: 0.05 % or less, S: 0.005 % or less, Cr: 15.2 % or more and 18.0 % or less, Mo: 1.5 % or more and 4.3 % or less, Cu: 1.2 % or more and 3.5 % or less, Ni: 3.5 % or more and 5.2 % or less, V: 0.5 % or less, Al: 0.10 % or less, N: 0.10 % or less, O: 0.010 % or less, Sb: 0.001 % or more and 1.000 % or less, Co: 0.01 % or more and 1.00 % or less, and Ca: 0.001 % or more and 0.030 % or less, with a balance consisting of Fe and inevitable impurities; 30 % or more martensite phase, 50 % or less ferrite phase, and 40 % or less retained austenite phase in volume fraction; a yield stress of 758 MPa or more; and a Charpy absorbed energy vE_-10 at -10 °C of 40 J or more.

2. The stainless steel seamless pipe according to 1., wherein the chemical composition further contains, in mass%, at least one selected from the group consisting of Nb: 0.07 % or less, Ti: 0.2 % or less, W: 0.9 % or less, B: 0.01 % or less, Ta: 0.3 % or less, Zr: 0.3 % or less, REM: 0.3 % or less, Mg: 0.01 % or less, and Sn: 1.0 % or less.

3. The stainless steel seamless pipe according to 1. or 2., comprising: 50 % or more martensite phase, 50 % or less ferrite phase, and 25 % or less retained austenite phase in volume fraction; and a yield stress of 862 MPa or more.

4. A production method for a stainless steel seamless pipe, the production method comprising: making a seamless steel pipe from a steel material having the chemical composition according to 1. or 2.; heating the seamless steel pipe to a quenching temperature of 850 °C to 1150 °C; cooling the seamless steel pipe after the heating to a cooling stop temperature of 50 °C or less at a cooling rate of 0.01 °C/s or more; and heating the seamless steel pipe after the cooling to a tempering temperature of 500 °C to 650 °C, to produce a stainless steel seamless pipe having: 30 % or more martensite phase, 50 % or less ferrite phase, and 40 % or less retained austenite phase in volume fraction; a yield stress of 758 MPa or more; and a Charpy absorbed energy vE_-10 at -10 °C of 40 J or more.

5. The production method for a stainless steel seamless pipe according to 4., wherein the stainless steel seamless pipe has: 50 % or more martensite phase, 50 % or less ferrite phase, and 25 % or less retained austenite phase in volume fraction; and a yield stress of 862 MPa or more.

(Advantageous Effect)

[0026] It is thus possible to provide a stainless steel seamless pipe having high strength of 758 MPa (110 ksi) or more in yield stress and excellent low-temperature toughness and corrosion resistance.

DETAILED DESCRIPTION

[0027] The presently disclosed techniques will be described in detail below.

[Chemical composition]

[0028] A stainless steel seamless pipe according to the present disclosure has the foregoing chemical composition. First, the reasons for limiting the chemical composition will be described below. Hereafter, "mass%" is simply written as "%" unless otherwise noted.

C: 0.06 % or less

[0029] C is an element inevitably contained in the steelmaking process. If the C content is more than 0.06 %, the corrosion resistance decreases. The C content is therefore 0.06 % or less. The C content is preferably 0.05 % or less, more preferably 0.04 % or less, and further preferably 0.03 % or less. Since the C content is desirably as low as possible from the viewpoint of the corrosion resistance, no lower limit is placed on the C content. From the viewpoint of decarburization cost, however, the C content is preferably 0.002 % or more, more preferably 0.003 % or more, and further preferably 0.005 % or more.

Si: 1.0 % or less

[0030] Si is an element that acts as a deoxidizer. If the Si content is more than 1.0 %, the hot workability and the corrosion resistance decrease. The Si content is therefore 1.0 % or less, preferably 0.7 % or less, more preferably 0.5 % or less, and further preferably 0.4 % or less. Although no lower limit is placed on the Si content, the Si content is preferably 0.03 % or more, more preferably 0.05 % or more, and further preferably 0.1 % or more from the viewpoint of enhancing the deoxidizing effect.

Mn: 0.01 % to 1.0 %

[0031] Mn is an element that acts as a deoxidizing material and a desulfurizing material and improves the hot workability. To achieve its effect as a deoxidizing and desulfurizing material and improve the strength, the Mn content is 0.01 % or more, preferably 0.03 % or more, more preferably 0.05 % or more, and further preferably 0.1 % or more. If the Mn content is more than 1.0 %, the effect is saturated. The Mn content is therefore 1.0 % or less, preferably 0.8 % or less, more preferably 0.6 % or less, and further preferably 0.4 % or less.

P: 0.05 % or less

[0032] P is an element that decreases the CO₂ corrosion resistance and the SSC resistance. To achieve the desired corrosion resistance, the P content is 0.05 % or less, preferably 0.04 % or less, and more preferably 0.03 % or less. Since it is desirable to reduce the P content as much as possible, no lower limit is placed on the P content, and the P content may be 0 %. Since excessive reduction causes an increase in cost, the P content is preferably 0.005 % or more and more preferably 0.010 % or more from the viewpoint of cost.

S: 0.005 % or less

[0033] S is an element that significantly decreases the hot workability and hinders stable operation in the hot pipe making process. Moreover, S exists as sulfide-based inclusions in the steel, and decreases the corrosion resistance. The S content is therefore 0.005 % or less, preferably 0.004 % or less, more preferably 0.003 % or less, and further preferably 0.002 % or less. Since it is desirable to reduce the S content as much as possible, no lower limit is placed on the S content, and the S content may be 0 %. Since excessive reduction causes an increase in cost, the S content is preferably 0.0003 % or more and more preferably 0.0005 % or more from the viewpoint of cost.

Cr: 15.2 % to 18.0 %

[0034] Cr is an element that forms a protective coating on the steel pipe surface and contributes to improved corrosion resistance. If the Cr content is less than 15.2 %, the desired CO₂ corrosion resistance and SSC resistance cannot be ensured. The Cr content is therefore 15.2 % or more, preferably 15.5 % or more, more preferably 16.0 % or more, and further preferably 16.30 % or more. If the Cr content is more than 18.0 %, the ferrite fraction is excessively high, and the desired strength cannot be ensured. The Cr content is therefore 18.0 % or less, preferably 17.5 % or less, more preferably 17.2 % or less, and further preferably 17.0 % or less.

Mo: 1.5 % to 4.3 %

[0035] Mo stabilizes the protective coating on the steel pipe surface and increases the resistance to pitting corrosion caused by Cl^- and low pH, thus enhancing the corrosion resistance. To achieve the desired corrosion resistance, the Mo content is 1.5 % or more, preferably 1.8 % or more, more preferably 2.0 % or more, and further preferably 2.3 % or more. If the Mo content is more than 4.3 %, the ferrite fraction is excessively high, and the desired strength cannot be ensured. The Mo content is therefore 4.3 % or less, preferably 4.0 % or less, more preferably 3.5 % or less, and further preferably 3.0 % or less.

Cu: 1.2 % to 3.5 %

[0036] Cu has the effect of enhancing the CO₂ corrosion resistance and the SSC resistance by strengthening the protective coating on the steel pipe surface. To achieve the desired strength and corrosion resistance, in particular CO₂ corrosion resistance, the Cu content is 1.2 % or more, preferably 1.8 % or more, more preferably 2.0 % or more, and further preferably 2.3 % or more. If the Cu content is excessively high, the hot workability of the steel decreases and outer surface flaws occur during pipe making, making it impossible to achieve the desired SSC resistance. The Cu content is therefore 3.5 % or less, preferably 3.2 % or less, more preferably 3.0 % or less, and further preferably 2.7 % or less.

Ni: 3.5 % to 5.2 %

[0037] Ni improves the low-temperature toughness of the steel. Ni also contributes to an increased austenite fraction, and thus influences the hot workability during hot rolling. To achieve the desired toughness, the Ni content is 3.5 % or more, preferably 3.8 % or more, more preferably 4.0 % or more, and further preferably 4.3 % or more. If the Ni content is more than 5.2 %, the austenite fraction is excessively high, and the hot workability of the steel decreases. Consequently, flaws tend to occur during hot rolling, and the desired SSC resistance may not be achieved. The Ni content is therefore 5.2 % or less, and preferably 5.0 % or less.

V: 0.5 % or less

[0038] V is an element that forms carbonitrides to thus increase the strength without impairing the toughness. V also has the effect of improving the corrosion resistance. This is because, as a result of V preferentially forming carbonitrides, corrosion-resistant elements such as Cr are prevented from forming carbonitrides and consequently a decrease in the amount effective for the corrosion resistance is suppressed. If the V content is more than 0.5 %, the effect is saturated. The V content is therefore 0.5 % or less, preferably 0.2 % or less, and further preferably 0.1 % or less. Although no lower limit is placed on the V content, the V content is preferably 0.01 % or more, and more preferably 0.03 % or more.

Al: 0.10 % or less

[0039] Al is an element that acts as a deoxidizer. If the Al content is more than 0.10 %, the corrosion resistance decreases. The Al content is therefore 0.10 % or less, preferably 0.07 % or less, and more preferably 0.05 % or less. Although no lower limit is placed on the Al content, the Al content is preferably 0.005 % or more, more preferably 0.01 % or more, and further preferably 0.015 % or more from the viewpoint of enhancing the deoxidizing effect.

N: 0.10 % or less

[0040] N is an element that is inevitably contained in the steelmaking process but also enhances the strength of the steel. If the N content is more than 0.10 %, the amount of nitride formed is excessive and the corrosion resistance decreases. The N content is therefore 0.10 % or less, preferably 0.07 % or less, more preferably 0.05 % or less, and further preferably 0.03 % or less. Although no lower limit is placed on the N content, excessive reduction of the N content causes an increase in steelmaking cost. The N content is therefore preferably 0.002 % or more, more preferably 0.003 % or more, and further preferably 0.005 % or more.

O: 0.010 % or less

[0041] O (oxygen) exists as oxides in the steel, and accordingly adversely affects various properties. In the present disclosure, it is desirable to reduce the O content as much as possible. In particular, if the O content is more than 0.010 %, the hot workability and the corrosion resistance decrease. The O content is therefore 0.010 % or less. Since excessive reduction causes an increase in cost, the O content is preferably 0.00005 % or more and more preferably 0.001 % or more from the viewpoint of cost.

Sb: 0.001 % to 1.000 %

[0042] Sb is an element necessary to improve the corrosion resistance in acid environments. To achieve the desired corrosion resistance, the Sb content is 0.001 % or more, preferably 0.003 % or more, more preferably 0.005 % or more, and further preferably 0.010 % or more. If the Sb content is more than 1.000 %, the effect is saturated. The Sb content is therefore 1.000 % or less, preferably 0.500 % or less, more preferably 0.100 % or less, and further preferably 0.050 % or less.

Co: 0.01 % to 1.00 %

[0043] Co is an element that improves the corrosion resistance. To achieve the desired corrosion resistance, the Co content is 0.01 % or more, preferably 0.03 % or more, and more preferably 0.05 % or more. If the Co content is more than 1.00 %, the effect is saturated. The Co content is therefore 1.00 % or less, preferably 0.50 % or less, more preferably 0.30 % or less, and further preferably 0.10 % or less.

Ca: 0.001 % to 0.030 %

[0044] Ca is an element that improves the hot workability through sulfide morphological control, and suppresses flaws during pipe making to thus contributes to improved SSC resistance of the steel pipe. To achieve the effect, the Ca content is 0.001 % or more, preferably 0.003 % or more, more preferably 0.005 % or more, even more preferably more than 0.010 %, further preferably 0.012 % or more, and most preferably 0.014 % or more. If the Ca content is more than 0.030 %, the effect is saturated, and the effect commensurate with the amount cannot be expected. The Ca content is therefore 0.030 % or less, preferably 0.025 % or less, and more preferably 0.020 % or less.

[0045] In one embodiment of the present disclosure, a stainless steel seamless pipe has a chemical composition containing the foregoing components with the balance consisting of Fe and inevitable impurities.

[0046] In another embodiment of the present disclosure, the chemical composition may optionally further contain at least one selected from the group consisting of Nb, Ti, W, B, Ta, Zr, REM, Mg, and Sn. Nb, Ti, W, B, Ta, Zr, REM, Mg, and Sn are optional steel components, and the contents of these components may be 0 %.

Nb: 0.07 % or less

[0047] Nb is an element that forms carbonitrides and further improves the strength and the corrosion resistance, and may be optionally added. Since Nb carbonitrides tend to decrease the low-temperature toughness, in the case of adding Nb, the Nb content is 0.07 % or less, preferably 0.03 % or less, and more preferably 0.01 % or less. Regarding the lower limit, the Nb content may be 0 %, but is preferably 0.001 % or more from the viewpoint of enhancing the effect of the addition of Nb.

Ti: 0.2 % or less

[0048] Ti is an element that further improves the strength and the corrosion resistance, and may be optionally contained. If the Ti content is more than 0.2 %, the low-temperature toughness decreases. Accordingly, in the case of adding Ti, the Ti content is 0.2 % or less, preferably 0.05 % or less, and more preferably 0.01 % or less. Regarding the lower limit, the Ti content may be 0 %, but is preferably 0.001 % or more from the viewpoint of enhancing the effect of the addition of Ti.

W: 0.9 % or less

[0049] W is an element that contributes to improved strength of the steel and stabilizes the protective coating on the steel pipe surface to further enhance the corrosion resistance. If the W content is more than 0.9 %, the low-temperature toughness decreases. Accordingly, in the case of adding W, the W content is 0.9 % or less, preferably 0.5 % or less, and more preferably 0.3 % or less. No lower limit is placed on the W content and the W content may be 0 %, but the W content is preferably 0.05 % or more and more preferably 0.1 % or more.

B: 0.01 % or less

[0050] B is an element that contributes to improved hot workability and also has the effect of suppressing cracking in the pipe making process. If the B content is more than 0.01 %, the low-temperature toughness decreases. Accordingly, in the case of adding B, the B content is 0.01 % or less, preferably 0.007 % or less, and more preferably 0.005 % or less. No lower limit is placed on the B content and the B content may be 0 %, but the B content is preferably 0.0005 % or more and more preferably 0.001 % or more.

Ta: 0.3 % or less

[0051] Ta is an element that has the effect of further improving the strength and further improving the corrosion resistance, and may be optionally contained. If the Ta content is more than 0.3 %, the effect is saturated. Accordingly, in the case of adding Ta, the Ta content is 0.3 % or less. No lower limit is placed on the Ta content and the Ta content may be 0 %, but the Ta content is preferably 0.001 % or more.

Zr: 0.3 % or less

[0052] Zr is an element that further improves the strength, and may be optionally contained. Zr also has the effect of further improving the SSC resistance. If the Zr content is more than 0.3 %, the effect is saturated. Accordingly, in the case of adding Zr, the Zr content is 0.3 % or less. No lower limit is placed on the Zr content and the Zr content may be 0 %, but the Zr content is preferably 0.0005 % or more.

REM: 0.3 % or less

[0053] REM (rare earth metal) is an element that contributes to further improved SSC resistance through sulfide morphological control, and may be optionally contained. If the REM content is more than 0.3 %, the effect is saturated, and the effect commensurate with the amount cannot be expected. Accordingly, in the case of adding REM, the REM content is 0.3 % or less. No lower limit is placed on the REM content and the REM content may be 0 %, but the REM content is preferably 0.0005 % or more. In the present disclosure, "REM" includes scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanoids from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. The chemical composition of the stainless steel seamless pipe according to the present disclosure may optionally contain at least one of these REMs. In the present disclosure, the "REM content" is the total content of these elements.

Mg: 0.01 % or less

[0054] Mg is an element that further improves the corrosion resistance, and may be optionally contained. If the Mg content is more than 0.01 %, the effect is saturated, and the effect commensurate with the amount cannot be expected. Accordingly, in the case of adding Mg, the Mg content is 0.01 % or less. No lower limit is placed on the Mg content and the Mg content may be 0 %, but the Mg content is preferably 0.0005 % or more.

Sn: 1.0 % or less

[0055] Sn is an element that further improves the corrosion resistance, and may be optionally contained. If the Sn content is more than 1.0 %, the effect is saturated, and the effect commensurate with the amount cannot be expected. Accordingly, in the case of adding Sn, the Sn content is 1.0 % or less. No lower limit is placed on the Sn content and the Sn content may be 0 %, but the Sn content is preferably 0.001 % or more.

[Microstructure]

[0056] Next, the reasons for limiting the microstructure of the stainless steel seamless pipe according to the present disclosure will be described below.

[0057] The stainless steel seamless pipe according to an embodiment of the present disclosure contains 30 % or more martensite phase, 50 % or less ferrite phase, and 40 % or less retained austenite phase in volume fraction.

Martensite phase: 30 % or more

[0058] If the volume fraction of martensite phase is less than 30 %, the desired strength cannot be ensured. The volume fraction of martensite phase is therefore 30 % or more, preferably 40 % or more, more preferably 45 % or more, and further preferably 50 % or more. Although no upper limit is placed on the volume fraction of martensite phase, the volume fraction of martensite phase is preferably 90 % or less, and more preferably 85 % or less.

Ferrite phase: 50 % or less

[0059] As a result of ferrite phase being contained, the propagation of sulfide stress corrosion cracking and sulfide stress cracking can be suppressed, and excellent corrosion resistance can be achieved. If the volume fraction of ferrite phase is more than 50 %, the desired strength cannot be ensured. The volume fraction of ferrite phase is therefore 50 % or less, preferably 40 % or less, and more preferably 35 % or less. Although no lower limit is placed on the volume fraction of ferrite phase, the volume fraction of ferrite phase is preferably 10 % or more, more preferably 15 % or more, and further preferably 20 % or more.

Retained austenite phase: 40 % or less

[0060] The presence of retained austenite phase improves the ductility and the low-temperature toughness. However, if a large amount of austenite phase with a volume fraction exceeding 40 % precipitates, the desired strength cannot be ensured. The volume fraction of retained austenite phase is therefore 40 % or less, preferably 30 % or less, and more preferably 25 % or less. Although no lower limit is placed on the volume fraction of retained austenite phase, the volume fraction of retained austenite phase is preferably 3 % or more, and more preferably 5 % or more.

[0061] The volume fraction of each phase can be measured by the following method: First, a test piece for microstructure observation collected from a stainless steel seamless pipe is etched with Vilella's reagent (a reagent obtained by mixing picric acid, hydrochloric acid, and ethanol in proportions of 2 g, 10 ml, and 100 ml respectively). Following this, the microstructure of the test piece for microstructure observation is imaged using a scanning electron microscope (SEM) with 1000 magnification to obtain a SEM image. The obtained SEM image is analyzed using image analysis software (ImageJ 1.52p, National Institute of Health), and the microstructure proportion (area ratio (%)) of ferrite phase is calculated. In the analysis, the SEM image is binarized, and each region with low brightness is regarded as ferrite phase. The area ratio obtained by this procedure is defined as the volume fraction (%) of ferrite phase.

[0062] Next, a test piece for X-ray diffraction collected from the stainless steel seamless pipe is ground and polished so that a cross section (C section) orthogonal to the pipe axis direction will be the measurement plane, and the microstructure proportion of retained austenite (y) phase is measured using an X-ray diffraction method. Specifically, the volume fraction of retained austenite phase is calculated from the integrated intensity of each of (220) plane of austenite and (211) plane of ferrite using the following formula:

where Vy is the volume fraction of retained austenite phase, Iα is the integrated intensity of (211) plane of ferrite, Iy is the integrated intensity of (220) plane of austenite, Rα is the crystallographic theoretical calculation value (34.15) of α, and Ry is the crystallographic theoretical calculation value (22.33) of y.

[0063] The balance other than ferrite phase and retained y phase calculated by the foregoing measurement method is the fraction of martensite phase. The method of observing each microstructure will be described in detail in the EXAMPLES section below.

[0064] The microstructure of the stainless steel seamless pipe according to the present disclosure substantially consists of martensite phase, ferrite phase, and retained austenite phase. That is, the microstructure may contain other microstructures as long as the effects according to the present disclosure are not impaired. Examples of the other microstructures include intermetallic compounds and inclusions.

[0065] In other words, in one embodiment of the present disclosure, a stainless steel seamless pipe may comprise a microstructure containing, in volume fraction, 50 % or less ferrite phase and 40 % or less retained austenite phase with the balance substantially consisting of martensite phase, wherein the volume fraction of martensite phase is 30 % or more.

[0066] In another embodiment of the present disclosure, a stainless steel seamless pipe may comprise a microstructure consisting of, in volume fraction, 30 % or more martensite phase, 50 % or less ferrite phase, and 40 % or less retained austenite phase.

[0067] In either case, it is permissible for the microstructure to contain inevitable impurity phase.

Yield stress: 758 MPa or more

[0068] The stainless steel seamless pipe according to the present disclosure has a yield stress of 758 MPa or more. Although no upper limit is placed on the yield stress, the yield stress is preferably 1034 MPa or less. The yield stress can be measured by a tensile test. More specifically, the yield stress can be measured by the method described in the EXAMPLES section.

vE_-10: 40 J or more

[0069] The stainless steel seamless pipe according to the present disclosure has a Charpy absorbed energy vE_-10 at -10 °C of 40 J or more. Since the Charpy absorbed energy vE_-10 is desirably as high as possible, no upper limit is placed on the Charpy absorbed energy vE_-10. For example, the Charpy absorbed energy vE_-10 may be 300 J or less, and may be 250 J or less. The Charpy absorbed energy can be measured by a Charpy impact test. More specifically, the Charpy absorbed energy can be measured by the method described in the EXAMPLES section.

[0070] The stainless steel seamless pipe according to the present disclosure may be used for any purpose without limitation. The stainless steel seamless pipe according to the present disclosure is particularly suitable for use in oil wells. That is, the stainless steel seamless pipe according to an embodiment of the present disclosure is a stainless steel seamless pipe for oil wells (high-strength stainless steel seamless pipe for oil wells).

[Production method]

[0071] Next, a preferred production method for a stainless steel seamless pipe according to the present disclosure will be described below.

[0072] The stainless steel seamless pipe according to the present disclosure can be produced by making a seamless steel pipe from a steel material and subjecting the seamless steel pipe to quenching-tempering treatment under specific conditions.

[0073] The steel material is not limited, and may be any material. A billet is typically used as the steel material. As the steel material, a material having the foregoing chemical composition can be used.

[0074] The production method for the steel material is not limited, and the steel material can be produced by any method. For example, molten steel having the foregoing chemical composition is obtained by conventional steelmaking using a converter or the like, and then a steel material such as a billet is produced by a method such as continuous casting or ingot casting and blooming.

[Pipe making]

[0075] The steel material is made into a seamless steel pipe. The pipe making method is not limited, and any method may be used. The pipe making is preferably performed by hot working. In the case of performing the pipe making by hot working, the steel material is heated and then hot worked into a seamless steel pipe. The heating temperature in the heating is not limited, but is preferably 1100 °C to 1350 °C from the viewpoint of achieving both the hot workability during the pipe making and the low-temperature toughness of the finished product at high levels.

[0076] The method of working the steel material into the seamless steel pipe is not limited, and any method may be used. For example, the seamless steel pipe can be obtained by any of the Mannesmann plug mill process and the Mannesmann mandrel mill process.

[0077] In the case of performing the pipe making by hot working, cooling treatment may be performed after the pipe making. The cooling treatment is not limited, and may be performed under any conditions. For example, it is preferable to, after the hot working, cool the seamless steel pipe to room temperature. The cooling rate in the cooling is not limited, and may be any rate. For example, the seamless steel pipe may be cooled at a cooling rate similar to that of air cooling.

[Quenching-tempering treatment]

[0078] The obtained seamless steel pipe is then subjected to heat treatment (quenching-tempering treatment) composed of quenching treatment and tempering treatment under specific conditions. The conditions in the quenching-tempering treatment will be described below.

- Quenching treatment

[0079] First, the seamless steel pipe is heated to a quenching temperature of 850 °C to 1150 °C, and the heated seamless steel pipe is cooled to a cooling stop temperature of 50 °C or less at a cooling rate of 0.01 °C/s or more.

Quenching temperature: 850 °C to 1150 °C

[0080] If the heating temperature (quenching temperature) in the quenching treatment is less than 850 °C, reverse transformation from martensite to austenite does not occur, and transformation from austenite to martensite does not occur during cooling, so that the desired strength cannot be ensured. The quenching temperature is therefore 850 °C or more, and preferably 900 °C or more. If the quenching temperature is more than 1150 °C, crystal grains coarsen, as a result of which the low-temperature toughness degrades. The quenching temperature is therefore 1150 °C or less, and preferably 1100 °C or less.

[0081] In the quenching treatment, after heating the seamless steel pipe to the quenching temperature, soaking treatment of holding the seamless steel pipe at the quenching temperature may be performed. The soaking treatment can make the temperature in the thickness direction of the seamless steel pipe uniform, and reduce variation in material property. The time (soaking time) for holding at the quenching temperature is not limited, but is preferably 5 minutes to 30 minutes.

Cooling rate: 0.01 °C/s or more

[0082] If the cooling rate in the quenching treatment is less than 0.01 °C/s, the desired microstructure cannot be obtained. The cooling rate is therefore 0.01 °C/s or more, preferably 1.0 °C/s or more, more preferably 5.0 °C/s or more, and further preferably 10.0 °C/s or more. Although no lower limit is placed on the cooling rate, the cooling rate is preferably 100 °C/s or less, more preferably 50 °C/s or less, and further preferably 30 °C/s or less.

[0083] The cooling is not limited, and may be performed by any method. For example, the cooling is preferably performed by at least one of air cooling and water cooling, and more preferably performed by water cooling.

Cooling stop temperature: 50 °C or less

[0084] If the cooling stop temperature is more than 50 °C, the desired microstructure cannot be obtained. In detail, if the cooling stop temperature is high, transformation from austenite to martensite is insufficient, and the volume fraction of retained austenite is excessively high. The cooling stop temperature in the quenching treatment is therefore 50 °C or less. No lower limit is placed on the cooling stop temperature, and the cooling stop temperature may be, for example, 0 °C or more. The cooling stop temperature herein is the surface temperature of the seamless steel pipe.

- Tempering treatment

[0085] The seamless steel pipe after the quenching treatment is then subjected to tempering treatment of heating to a tempering temperature of 500 °C to 650 °C.

Tempering temperature: 500 °C to 650 °C

[0086] If the tempering temperature is less than 500 °C, the tempering effect is insufficient, as a result of which the low-temperature toughness degrades. The tempering temperature is therefore 500 °C or more, and preferably 520 °C or more. If the tempering temperature is more than 650 °C, a large amount of intermetallic compounds precipitates, and excellent low-temperature toughness cannot be obtained. The tempering temperature is therefore 650 °C or less, and preferably 630 °C or less.

[0087] In the tempering treatment, after heating the seamless steel pipe to the tempering temperature, the seamless steel pipe may be held at the tempering temperature. The time (holding time) for holding at the tempering temperature is not limited, but is preferably 5 minutes or more from the viewpoint of making the temperature in the thickness direction uniform and preventing variation in material property. No upper limit is placed on the holding time, but the holding time is preferably 90 minutes or less.

[0088] After the tempering treatment, the seamless steel pipe can be allowed to naturally cool.

[0089] As a result of the quenching-tempering treatment, a stainless steel seamless pipe having the desired strength and excellent low-temperature toughness and corrosion resistance can be obtained.

EXAMPLES

[0090] The presently disclosed techniques will be described in more detail below by way of examples. The present disclosure is not limited to the following examples.

[0091] First, seamless steel pipes were made from steel materials having the chemical compositions shown in Tables 1 to 3 by the following procedure.

[0092] Each steel material was cast using molten steel having the chemical composition shown in Tables 1 to 3. The steel material was then heated, and hot worked using a model seamless mill into a seamless steel pipe of 177.8 mm in outer diameter and 16.0 mm in thickness, followed by air cooling. The heating temperature of the steel material before the hot working was 1250 °C.

[0093] The obtained seamless steel pipe was then subjected to quenching-tempering treatment under the following conditions to obtain a stainless steel seamless pipe.

- Quenching

[0094] The obtained seamless steel pipe was subjected to quenching treatment under the conditions shown in Tables 4 to 6. In detail, the seamless steel pipe was heated to the quenching temperature shown in Tables 4 to 6, and held at the quenching temperature for the soaking time shown in Tables 4 to 6. The seamless steel pipe was then cooled to the cooling stop temperature of 5 °C at the cooling rate shown in Tables 4 to 6. The cooling was performed by water cooling.

- Tempering

[0095] After this, the cooled seamless steel pipe was heated to the tempering temperature shown in Tables 4 to 6, and held at the tempering temperature for the holding time shown in Tables 4 to 6. The seamless steel pipe was then air-cooled (allowed to naturally cool). The cooling rate in the air cooling was 0.04 °C/s.

[0096] A test piece was collected from each obtained stainless steel seamless pipe, and microstructure observation, a tensile test, a Charpy impact test, and a corrosion resistance test were conducted. The test methods are as follows.

(1) Microstructure observation

[0097] A test piece for microstructure observation was collected from the obtained stainless steel seamless pipe so that a cross section including the pipe axis direction and the thickness direction would be the observation plane. The collected test piece for microstructure observation was etched with Vilella's reagent (a reagent obtained by mixing picric acid, hydrochloric acid, and ethanol in proportions of 2 g, 10 ml, and 100 ml respectively). The microstructure was imaged using a scanning electron microscope (SEM) with 1000 magnification to obtain a SEM image. The obtained SEM image was analyzed using image analysis software (ImageJ 1.52p, National Institute of Health), and the microstructure proportion (area ratio (%)) of ferrite phase was calculated. In the analysis, the SEM image was binarized, and each region with low brightness was regarded as ferrite phase. The area ratio obtained by this procedure was taken to be the volume fraction (%) of ferrite phase.

[0098] Moreover, a test piece for X-ray diffraction was collected from the obtained stainless steel seamless pipe, and ground and polished so that a cross section (C section) orthogonal to the pipe axis direction would be the measurement plane. The microstructure proportion of retained austenite (y) phase was measured using an X-ray diffraction method. Specifically, the volume fraction of retained austenite phase was calculated from the integrated intensity of each of (220) plane of austenite and (211) plane of ferrite using the following formula:

where Vy is the volume fraction of retained austenite phase, Iα is the integrated intensity of (211) plane of ferrite, Iy is the integrated intensity of (220) plane of austenite, Rα is the crystallographic theoretical calculation value (34.15) of α, and Ry is the crystallographic theoretical calculation value (22.33) of y.

[0099] The volume fraction of each phase obtained by the foregoing procedure is shown in Tables 4 to 6. The symbols M, F, and y of the microstructures in Tables 4 to 6 respectively denote the following phases:

M:

martensite phase

F:

ferrite phase

γ:

retained austenite phase.

(2) Tensile test

[0100] An arc-shaped tensile test piece was collected from the obtained stainless steel seamless pipe so that the pipe axis direction would be the tensile direction in accordance with API (American Petroleum Institute)-5CT. A tensile test was then conducted to determine the yield stress (YS). The stainless steel seamless pipe was rated pass as having high strength in the case where the yield stress YS was 758 MPa or more, and rated fail in the case where the yield stress YS was less than 758 MPa.

(3) Charpy impact test

[0101] A Charpy impact test was conducted by the following procedure to evaluate the low-temperature toughness.

[0102] First, a V-notch test piece (10 mm thick) whose longitudinal direction is perpendicular to the pipe axis and whose notch is on a plane perpendicular to the pipe axis was collected from the obtained stainless steel seamless pipe in accordance with ASTM E23. The test piece was subjected to a Charpy impact test at a test temperature of -10 °C. In the test, three test pieces were collected per one stainless steel seamless pipe, and the absorbed energy was measured for each test piece. The lowest value of the absorbed energies of the three test pieces was taken to be the Charpy absorbed energy vE_-10 at -10 °C. The stainless steel seamless pipe was rated pass in the case where vE_-10 was 40 J or more.

(4) CO₂ corrosion resistance test

[0103] The following test was conducted to evaluate the high-temperature CO₂ corrosion resistance.

[0104] A corrosion test piece of 3 mm in thickness, 30 mm in width, and 40 mm in length was machined from the obtained stainless steel seamless pipe. A corrosion test was conducted using the corrosion test piece, and the corrosion rate was measured as an index of the CO₂ corrosion resistance. In the corrosion test, the corrosion test piece was immersed in a test liquid: 20 mass% NaCl aqueous solution (liquid temperature: 200 °C, 30 atm CO₂ gas atmosphere) held in an autoclave for an immersion time of 14 days (336 hours). The weight of the corrosion test piece after the corrosion test was measured, and the premeasured weight of the test piece before the corrosion test was subtracted from the measured weight to yield the weight loss by the corrosion test. The weight loss was then divided by the surface area of the test piece used and the immersion time to obtain the weight loss per unit time and per unit area. The weight loss per unit time and per unit area was divided by the density of the steel to convert it into the corrosion thickness per unit time and per unit area. The corrosion thickness per unit time and per unit area (mm/y) thus obtained was taken to be the corrosion rate in carbon dioxide. The stainless steel seamless pipe was rated pass in the case where the corrosion rate was 0.127 mm/y or less, and rated fail in the case where the corrosion rate was more than 0.127 mm/y.

(5) SSC resistance test

[0105] The following test was conducted to evaluate the low-temperature SSC resistance.

[0106] A C-shaped test piece was machined from the obtained stainless steel seamless pipe in accordance with NACE TM0177 Method C, and an SSC resistance test was conducted. The curved surfaces corresponding to the inner and outer surfaces of the steel pipe were not ground or polished.

[0107] In the SSC resistance test, the C-shaped test piece conforming to NACE TM0177 Method C was immersed in an aqueous solution obtained by adding acetic acid + sodium acetate to a 0.165 mass% NaCl aqueous solution (liquid temperature: 7 °C, 0.995 atm CO₂ gas, 0.005 atm H₂S atmosphere) to adjust the pH to 3.0 for an immersion time of 720 hours under load of 100 % of yield stress as load stress. The stainless steel seamless pipe was rated pass in the case where there was no cracking, and rated fail in the case where there was cracking. In Tables 4 to 6, "1" denotes pass and "2" denotes fail.

(6) Corrosion test in acid environment

[0108] The following test was conducted to evaluate the corrosion resistance in acid environments.

[0109] First, a rectangular parallelepiped test piece was machined from the obtained stainless steel seamless pipe. The test piece was 50 mm in length in the longitudinal direction of the steel pipe, 3 mm in thickness in the thickness direction of the steel pipe, and 25 mm in width. After measuring the weight of the test piece, a corrosion test was conducted by immersing, for 40 minutes, the test piece in a 15 mass% hydrochloric acid solution heated to 80 °C. The weight of the test piece after the corrosion test was measured, and the weight of the test piece before the corrosion test was subtracted from the measured weight to yield the weight loss by the corrosion test. The weight loss was then divided by the surface area of the test piece used and the immersion time in the corrosion test to obtain the weight loss per unit time and per unit area. The weight loss per unit time and per unit area was divided by the density of the steel to convert it into the corrosion thickness per unit time and per unit area. The corrosion thickness per unit time and per unit area (mm/y) thus obtained was taken to be the corrosion rate in acid environments. The stainless steel seamless pipe was rated pass in the case where the corrosion rate was 600 mm/y or less, and rated fail in the case where the corrosion rate was more than 600 mm/y.

(7) Hot workability

[0110] The following evaluation was conducted to evaluate the hot workability of the obtained stainless steel seamless pipe. The flaw depth on the surface of the test piece in the SSC resistance test corresponding to the outer surface of the steel pipe was measured by ultrasonic testing. Based on the maximum value of the flaw depth measured, the hot workability was evaluated in the following three levels:

1: maximum flaw depth of 0.1 mm or less

2: maximum flaw depth of more than 0.1 mm and 0.2 mm or less

3: maximum flaw depth of more than 0.2 mm.

[0111] The results are shown in Tables 4 to 6. As can be seen from the results in Tables 4 to 6, each stainless steel seamless pipe satisfying the conditions according to the present disclosure had high strength of 758 MPa (110 ksi) or more in yield stress and excellent low-temperature toughness and corrosion resistance. Regarding the corrosion resistance in particular, all of the high-temperature CO₂ corrosion resistance, the low-temperature SSC resistance, and the corrosion resistance in acid environments were excellent. Such a stainless steel seamless pipe according to the present disclosure is very suitable for various applications such as steel pipes for oil wells.

[0112] In each Example in which the Ca content was more than 0.010 %, the maximum value of the flaw depth was 0.1 mm or less. This results indicates that the hot workability is remarkably excellent especially in the case where the Ca content is more than 0.010 %. Hence, the low-temperature SSC resistance is even more excellent if the Ca content is more than 0.010 %.

[Table 1]

[0113] 

Table 1

Steel sample ID	Chemical composition (mass%) *																	Remarks
	C	Si	Mn	P	S	Cr	Mo	Cu	Ni	V	Al	N	O	Sb	Co	Ca	Others
A	0.013	0.28	0.28	0.015	0.0010	16.57	2.56	2.46	4.79	0.050	0.022	0.016	0.002	0.017	0.08	0.0076	-	Conforming steel
B	0.013	0.32	0.31	0.017	0.0018	16.71	2.52	2.60	4.66	0.048	0.022	0.016	0.002	0.015	0.08	0.0163	-	Conforming steel
C	0.009	0.28	0.31	0.016	0.0020	16.62	2.56	2.52	4.57	0.049	0.027	0.013	0.002	0.018	0.08	0.0172	-	Conforming steel
D	0.052	0.32	0.30	0.016	0.0007	16.60	2.45	2.47	4.67	0.048	0.024	0.013	0.003	0.015	0.10	0.0107	-	Conforming steel
E	0.010	0.45	0.31	0.016	0.0009	16.75	2.50	2.41	4.41	0.051	0.021	0.012	0.002	0.015	0.11	0.0118	-	Conforming steel
F	0.010	0.81	0.33	0.015	0.0007	16.50	2.31	2.35	4.75	0.038	0.029	0.010	0.002	0.012	0.08	0.0102	-	Conforming steel
G	0.007	0.29	0.53	0.015	0.0008	16.68	2.51	2.40	4.48	0.050	0.026	0.012	0.002	0.017	0.10	0.0117	-	Conforming steel
H	0.008	0.28	0.83	0.014	0.0010	16.57	2.42	2.40	4.58	0.058	0.029	0.011	0.002	0.010	0.09	0.0087	-	Conforming steel
I	0.007	0.31	0.04	0.015	0.0007	16.64	2.54	2.56	4.41	0.052	0.030	0.014	0.002	0.017	0.11	0.0077	-	Conforming steel
J	0.012	0.30	0.27	0.014	0.0009	17.53	2.41	2.54	4.63	0.053	0.023	0.013	0.002	0.016	0.08	0.0127	-	Conforming steel
K	0.007	0.27	0.31	0.014	0.0012	15.56	2.45	2.41	4.65	0.049	0.025	0.011	0.002	0.018	0.11	0.0109	-	Conforming steel
L	0.007	0.31	0.33	0.015	0.0013	16.63	2.80	2.54	4.63	0.050	0.023	0.012	0.003	0.018	0.12	0.0098	-	Conforming steel
M	0.011	0.32	0.31	0.015	0.0013	16.73	1.78	2.40	4.59	0.048	0.024	0.012	0.002	0.012	0.11	0.0111	-	Conforming steel
N	0.012	0.30	0.33	0.014	0.0008	16.61	2.56	2.95	4.63	0.050	0.022	0.011	0.003	0.014	0.12	0.0078	-	Conforming steel
O	0.008	0.32	0.28	0.013	0.0010	16.70	2.51	1.64	4.46	0.050	0.020	0.012	0.002	0.016	0.12	0.0090	-	Conforming steel
P	0.011	0.28	0.27	0.016	0.0009	16.61	2.40	2.49	4.96	0.049	0.028	0.012	0.003	0.018	0.11	0.0092	-	Conforming steel
Q	0.010	0.31	0.28	0.016	0.0009	16.67	2.40	2.57	3.87	0.048	0.024	0.015	0.002	0.017	0.08	0.0079	-	Conforming steel
R	0.012	0.15	0.17	0.013	0.0010	16.58	2.29	2.31	4.58	0.360	0.025	0.013	0.003	0.009	0.10	0.0066	-	Conforming steel
S	0.011	0.28	0.31	0.014	0.0008	16.57	2.47	2.58	4.50	0.048	0.026	0.048	0.002	0.016	0.09	0.0124	-	Conforming steel
T	0.010	0.29	0.28	0.016	0.0009	16.60	2.46	2.44	4.60	0.051	0.026	0.011	0.002	0.052	0.12	0.0084	-	Conforming steel

* Balance consisting of Fe and inevitable impurities

[Table 2]

[0114] 

Table 2

Steel sample ID	Chemical composition (mass%) *																	Remarks
	C	Si	Mn	P	S	Cr	Mo	Cu	Ni	V	Al	N	O	Sb	Co	Ca	Others
U	0.007	0.11	0.13	0.015	0.0013	16.33	2.19	2.22	4.39	0.038	0.053	0.023	0.003	0.689	0.016	0.0093	-	Conforming steel
V	0.009	0.29	0.29	0.016	0.0013	16.58	2.44	2.52	4.63	0.048	0.023	0.014	0.002	0.005	0.11	0.0079	-	Conforming steel
W	0.011	0.28	0.27	0.017	0.0012	16.63	2.45	2.58	4.43	0.049	0.023	0.014	0.002	0.014	0.32	0.0114	-	Conforming steel
X	0.010	0.32	0.30	0.014	0.0007	16.61	2.59	2.50	4.41	0.048	0.030	0.010	0.002	0.016	0.05	0.0072	-	Conforming steel
Y	0.007	0.31	0.31	0.015	0.0008	16.62	2.55	2.55	4.71	0.048	0.028	0.011	0.003	0.012	0.09	0.0037	-	Conforming steel
Z	0.011	0.28	0.29	0.014	0.0008	16.69	2.40	2.46	4.46	0.051	0.027	0.014	0.003	0.015	0.11	0.0088	Nb:0.005, Ti:0.004	Conforming steel
AA	0.007	0.33	0.31	0.014	0.0009	16.72	2.40	2.52	4.69	0.050	0.028	0.010	0.003	0.015	0.12	0.0083	W:0.45, B:0.0026	Conforming steel
AB	0.010	0.29	0.28	0.016	0.0013	16.58	2.46	2.41	4.57	0.051	0.024	0.013	0.003	0.012	0.12	0.0087	REM:0.023	Conforming steel
AC	0.013	0.29	0.30	0.015	0.0007	16.60	2.54	2.54	4.62	0.049	0.022	0.013	0.002	0.013	0.11	0.0081	Nb:0.002, Sn:0.019	Conforming steel
AD	0.011	0.29	0.29	0.017	0.0007	16.62	2.40	2.52	4.57	0.049	0.025	0.015	0.002	0.014	0.11	0.0103	Ta:0.14, Zr:0.22 Mg:0.004	Conforming steel
AE	0.067	0.32	0.30	0.013	0.0013	16.64	2.58	2.44	4.57	0.047	0.025	0.014	0.003	0.016	0.11	0.0091	-	Comparative steel
AF	0.007	0.31	0.33	0.016	0.0007	18.41	2.51	2.53	4.66	0.047	0.021	0.015	0.002	0.012	0.11	0.0078	-	Comparative steel
AG	0.009	0.31	0.31	0.017	0.0010	14.89	2.43	2.43	4.64	0.048	0.025	0.013	0.003	0.014	0.08	0.0113	-	Comparative steel
AH	0.012	0.32	0.32	0.016	0.0012	16.64	4.61	2.49	4.42	0.047	0.024	0.012	0.002	0.016	0.11	0.0103	-	Comparative steel
AI	0.007	0.31	0.29	0.017	0.0011	16.62	1.32	2.42	4.61	0.047	0.025	0.015	0.002	0.014	0.09	0.0080	-	Comparative steel
AJ	0.009	0.32	0.29	0.017	0.0011	16.70	2.54	3.83	4.70	0.053	0.022	0.014	0.002	0.017	0.12	0.0087	-	Comparative steel
AK	0.010	0.27	0.32	0.016	0.0009	16.72	2.57	0.85	4.57	0.051	0.028	0.013	0.003	0.013	0.08	0.0084	-	Comparative steel
AL	0.013	0.30	0.33	0.013	0.0011	16.58	2.60	2.42	5.61	0.048	0.030	0.010	0.002	0.017	0.10	0.0120	-	Comparative steel
AM	0.007	0.29	0.29	0.014	0.0010	16.70	2.50	2.52	3.14	0.053	0.029	0.011	0.003	0.014	0.09	0.0077	-	Comparative steel
AN	0.012	0.27	0.32	0.015	0.0011	16.69	2.58	2.60	4.45	0.053	0.023	0.132	0.003	0.015	0.09	0.0075	-	Comparative steel
AO	0.011	0.29	0.33	0.017	0.0010	16.58	2.40	2.57	4.71	0.052	0.021	0.013	0.002	1.319	0.10	0.0072	-	Comparative steel

* Balance consisting of Fe and inevitable impurities

[Table 3]

[0115] 

Table 3

Steel sample ID	Chemical composition (mass%) *																	Remarks
	C	Si	Mn	P	S	Cr	Mo	Cu	Ni	V	Al	N	O	Sb	Co	Ca	Others
AP	0.013	0.28	0.28	0.015	0.0010	16.57	2.56	2.46	4.79	0.050	0.022	0.016	0.002	0.017	0.08	0.0106	-	Conforming steel
AQ	0.008	0.28	0.83	0.014	0.0010	16.57	2.42	2.40	4.58	0.058	0.029	0.011	0.002	0.010	0.09	0.0131	-	Conforming steel
AR	0.007	0.31	0.04	0.015	0.0007	16.64	2.54	2.56	4.41	0.052	0.030	0.014	0.002	0.017	0.11	0.0111	-	Conforming steel
AS	0.007	0.31	0.33	0.015	0.0013	16.63	2.80	2.54	4.63	0.050	0.023	0.012	0.003	0.018	0.12	0.0130	-	Conforming steel
AT	0.012	0.30	0.33	0.014	0.0008	16.61	2.56	2.95	4.63	0.050	0.022	0.011	0.003	0.014	0.12	0.0123	-	Conforming steel
AU	0.008	0.32	0.28	0.013	0.0010	16.70	2.51	1.64	4.46	0.050	0.020	0.012	0.002	0.016	0.12	0.0135	-	Conforming steel
AV	0.011	0.28	0.27	0.016	0.0009	16.61	2.40	2.49	4.96	0.049	0.028	0.012	0.003	0.018	0.11	0.0125	-	Conforming steel
AW	0.010	0.31	0.28	0.016	0.0009	16.67	2.40	2.57	3.87	0.048	0.024	0.015	0.002	0.017	0.08	0.0118	-	Conforming steel
AX	0.012	0.15	0.17	0.013	0.0010	16.58	2.29	2.31	4.58	0.360	0.025	0.013	0.003	0.009	0.10	0.0101	-	Conforming steel
AY	0.010	0.29	0.28	0.016	0.0009	16.60	2.46	2.44	4.60	0.051	0.026	0.011	0.002	0.052	0.12	0.0127	-	Conforming steel
AZ	0.007	0.11	0.13	0.015	0.0013	16.33	2.19	2.22	4.39	0.038	0.053	0.023	0.003	0.689	0.016	0.0127	-	Conforming steel
BA	0.009	0.29	0.29	0.016	0.0013	16.58	2.44	2.52	4.63	0.048	0.023	0.014	0.002	0.005	0.11	0.0120	-	Conforming steel
BB	0.010	0.32	0.30	0.014	0.0007	16.61	2.59	2.50	4.41	0.048	0.030	0.010	0.002	0.016	0.05	0.0103	-	Conforming steel
BC	0.007	0.31	0.31	0.015	0.0008	16.62	2.55	2.55	4.71	0.048	0.028	0.011	0.003	0.012	0.09	0.0073	-	Conforming steel
BD	0.011	0.28	0.29	0.014	0.0008	16.69	2.40	2.46	4.46	0.051	0.027	0.014	0.003	0.015	0.11	0.0133	Nb:0.005, Ti:0.004	Conforming steel
BE	0.007	0.33	0.31	0.014	0.0009	16.72	2.40	2.52	4.69	0.050	0.028	0.010	0.003	0.015	0.12	0.0133	W:0.45, B:0.0026	Conforming steel
BF	0.010	0.29	0.28	0.016	0.0013	16.58	2.46	2.41	4.57	0.051	0.024	0.013	0.003	0.012	0.12	0.0136	REM:0.023	Conforming steel
BG	0.013	0.29	0.30	0.015	0.0007	16.60	2.54	2.54	4.62	0.049	0.022	0.013	0.002	0.013	0.11	0.0128	Nb:0.002, Sn:0.019	Conforming steel
BH	0.067	0.32	0.30	0.013	0.0013	16.64	2.58	2.44	4.57	0.047	0.025	0.014	0.003	0.016	0.11	0.0122	-	Comparative steel
BI	0.007	0.31	0.33	0.016	0.0007	18.41	2.51	2.53	4.66	0.047	0.021	0.015	0.002	0.012	0.11	0.0110	-	Comparative steel
BJ	0.007	0.31	0.29	0.017	0.0011	16.62	1.32	2.42	4.61	0.047	0.025	0.015	0.002	0.014	0.09	0.0119	-	Comparative steel
BK	0.009	0.32	0.29	0.017	0.0011	16.70	2.54	3.83	4.70	0.053	0.022	0.014	0.002	0.017	0.12	0.0133	-	Comparative steel
BL	0.010	0.27	0.32	0.016	0.0009	16.72	2.57	0.85	4.57	0.051	0.028	0.013	0.003	0.013	0.08	0.0119	-	Comparative steel
BM	0.007	0.29	0.29	0.014	0.0010	16.70	2.50	2.52	3.14	0.053	0.029	0.011	0.003	0.014	0.09	0.0123	-	Comparative steel
BN	0.012	0.27	0.32	0.015	0.0011	16.69	2.58	2.60	4.45	0.053	0.023	0.132	0.003	0.015	0.09	0.0115	-	Comparative steel
BO	0.011	0.29	0.33	0.017	0.0010	16.58	2.40	2.57	4.71	0.052	0.021	0.013	0.002	1.319	0.10	0.0107	-	Comparative steel
BP	0.013	0.32	0.31	0.017	0.0018	16.71	2.52	2.60	4.66	0.048	0.022	0.016	0.002	0.015	0.08	-	-	Comparative steel

* Balance consisting of Fe and inevitable impurities

[Table 4]

[0116] 

Table 4

No.	Steel sample ID	Production conditions					Measurement results									Remarks
		Quenching treatment			Tempering treatment		Microstructure (volume fraction)			Strength	Low-temperature toughness	Corrosion resistance			Hot workability
		Quenching temperature (°C)	Soaking time (min)	Cooling rate (°C/s)	Tempering temperature (°C)	Holding time (min)	M (%)	F (%)	γ (%)	Yield stress YS (MPa)	vE-10 (J)	Corrosion rate in carbon dioxide (mm/y)	SSC resistance	Corrosion rate in acid environment (mm/y)
1	A	930	20	11	575	30	73	22	5	974	220	0.053	1	586	2	Example
2	B	930	20	11	575	30	69	25	6	967	212	0.055	1	587	1	Example
3	C	930	20	11	575	30	72	23	5	980	208	0.057	1	587	1	Example
4	D	930	20	11	575	30	67	20	13	932	209	0.078	1	593	1	Example
5	E	930	20	11	575	30	59	27	14	934	103	0.067	1	590	1	Example
6	F	930	20	11	575	30	50	31	19	905	185	0.051	1	577	1	Example
7	G	930	20	11	575	30	67	18	15	920	209	0.043	1	583	1	Example
8	H	930	20	11	575	30	64	15	21	911	189	0.048	1	567	2	Example
9	I	930	20	11	575	30	69	27	4	979	189	0.051	1	585	2	Example
10	J	930	20	11	575	30	55	29	16	916	109	0.038	1	581	1	Example
11	K	930	20	11	575	30	73	21	6	968	217	0.080	1	594	1	Example
12	L	930	20	11	575	30	56	29	15	921	123	0.045	1	584	2	Example
13	M	930	20	11	575	30	76	20	4	977	199	0.069	1	591	1	Example
14	N	930	20	11	575	30	70	17	13	939	212	0.044	1	583	2	Example
15	O	930	20	11	575	30	69	26	5	980	187	0.077	1	593	2	Example
16	P	930	20	11	575	30	63	20	17	918	200	0.052	1	586	2	Example
17	Q	930	20	11	575	30	67	29	4	977	73	0.059	1	588	2	Example
18	R	930	20	11	575	30	51	31	18	970	123	0.054	1	587	2	Example
19	S	930	20	11	575	30	52	30	18	913	218	0.077	1	593	1	Example
20	T	930	20	11	575	30	71	23	6	973	200	0.050	1	585	2	Example
21	U	930	20	11	575	30	58	22	20	922	109	0.043	1	563	2	Example
22	V	930	20	11	575	30	71	24	5	976	200	0.061	1	588	2	Example
23	W	930	20	11	575	30	73	22	5	973	209	0.052	1	586	1	Example
24	X	930	20	11	575	30	72	24	4	981	215	0.067	1	590	2	Example
25	Y	930	20	11	575	30	70	24	6	968	103	0.055	1	587	2	Example
26	Z	930	20	11	575	30	72	23	5	978	219	0.050	1	585	2	Example

[Table 5]

[0117] 

Table 5

No.	Steel sample ID	Production conditions					Measurement results									Remarks
		Quenching treatment			Tempering treatment		Microstructure (volume fraction)			Strength	Low-temperature toughness	Corrosion resistance			Hot workability
		Quenching temperature (°C)	Soaking time (min)	Cooling rate (°C/s)	Tempering temperature (°C)	Holding time (mm)	M (%)	F (%)	γ (%)	Yield stress YS (MPa)	vE-10 (J)	Corrosion rate in carbon dioxide (mm/y)	SSC resistance	Corrosion rate in acid environment (mm/y)
27	AA	930	20	11	575	30	70	24	6	968	204	0.061	1	588	2	Example
28	AB	930	20	11	575	30	72	22	6	967	205	0.053	1	586	2	Example
29	AC	930	20	11	575	30	70	24	6	971	202	0.046	1	584	2	Example
30	AD	930	20	11	575	30	58	29	13	908	172	0.053	1	580	1	Example
31	AE	930	20	11	575	30	44	18	38	814	223	0.130	2	609	2	Comparative Example
32	AF	930	20	11	575	30	26	33	41	735	62	0.033	1	580	2	Comparative Example
33	AG	930	20	11	575	30	76	19	5	980	220	0.141	2	612	1	Comparative Example
34	AH	930	20	11	575	30	26	31	43	750	65	0.035	1	581	1	Comparative Example
35	AI	930	20	11	575	30	73	23	4	979	207	0.129	2	609	2	Comparative Example
36	AJ	930	20	11	575	30	60	22	18	913	185	0.040	2	582	2	Comparative Example
37	AK	930	20	11	575	30	71	24	5	976	168	0.131	2	609	2	Comparative Example
38	AL	930	20	11	575	30	70	18	12	940	224	0.053	2	586	1	Comparative Example
39	AM	930	20	11	575	30	61	35	4	985	31	0.055	1	587	2	Comparative Example
40	AN	930	20	11	575	30	45	20	35	825	165	0.135	2	611	2	Comparative Example
41	AO	930	20	11	575	30	72	24	4	979	218	0.048	1	631	2	Comparative Example
42	A	930	20	11	620	30	47	27	26	853	227	0.051	1	585	2	Example
43	B	930	20	11	620	30	44	28	28	855	231	0.049	1	585	1	Example
44	C	930	20	11	620	30	43	27	30	856	232	0.053	1	586	1	Example
45	B	930	10	11	575	20	69	25	6	967	221	0.056	1	587	1	Example
46	B	930	50	11	575	20	69	24	7	962	215	0.053	1	586	1	Example
47	B	930	20	0.07	575	20	56	24	20	869	135	0.063	1	590	1	Example
48	B	930	20	11	575	10	70	24	6	974	210	0.058	1	587	1	Example
49	B	930	20	11	575	80	61	23	16	889	225	0.054	1	586	1	Example
50	B	800	20	11	575	30	14	35	51	514	159	0.053	1	586	1	Comparative Example
51	B	1200	20	11	575	30	55	39	6	971	29	0.049	1	585	1	Comparative Example
52	B	930	20	11	470	30	71	25	4	984	25	0.051	1	585	1	Comparative Example
53	B	930	20	11	680	30	38	27	35	827	32	0.044	1	583	1	Comparative Example

[Table 6]

[0118] 

Table 6

No	Steel sample ID	Production conditions					Measurement results									Remarks
		Quenching treatment			Tempering treatment		Microstructure (volume fraction)			Strength	Low-temperature toughness	Corrosion resistance			Hot workability
		Quenching temperature (°C)	Soaking time (min)	Cooling rate (°C/s)	Tempering temperature (°C)	Holding time (min)	M (%)	F (%)	γ (%)	Yield stress YS (MPa)	vE-10 (J)	Corrosion rate in carbon dioxide (mm/y)	SSC resistance	Corrosion rate in acid environment (mm/y)
54	AP	930	20	11	575	30	74	21	5	972	223	0.053	1	590	1	Example
55	AQ	930	20	11	575	30	63	16	21	916	192	0.049	1	568	1	Example
56	AR	930	20	11	575	30	71	26	3	977	189	0.046	1	582	1	Example
57	AS	930	20	11	575	30	55	30	15	918	123	0.042	1	588	1	Example
58	AT	930	20	11	575	30	69	18	13	944	217	0.044	1	580	1	Example
59	AU	930	20	11	575	30	68	27	5	978	189	0.072	1	598	1	Example
60	AV	930	20	11	575	30	63	19	18	915	202	0.049	1	586	1	Example
61	AW	930	20	11	575	30	67	29	4	975	71	0.06	1	587	1	Example
62	AX	930	20	11	575	30	51	30	19	970	124	0.051	1	590	1	Example
63	AY	930	20	11	575	30	71	24	5	974	204	0.052	1	587	1	Example
64	AZ	930	20	11	575	30	58	22	20	918	106	0.039	1	566	1	Example
65	BA	930	20	11	575	30	70	25	5	972	201	0.059	1	585	1	Example
66	BB	930	20	11	575	30	72	24	4	986	213	0.065	1	593	1	Example
67	BC	930	20	11	575	30	71	23	6	963	100	0.056	1	585	1	Example
68	BD	930	20	11	575	30	70	24	6	981	215	0.045	1	589	1	Example
69	BE	930	20	11	575	30	70	25	5	967	202	0.065	1	589	1	Example
70	BF	930	20	11	575	30	72	22	6	972	205	0.048	1	586	1	Example
71	BG	930	20	11	575	30	70	24	6	971	207	0.045	1	579	1	Example
72	BH	930	20	11	575	30	46	17	37	817	225	0 135	2	614	1	Comparative Example
73	BI	930	20	11	575	30	27	33	40	739	64	0.031	1	585	1	Comparative Example
74	BJ	930	20	11	575	30	75	22	3	975	209	0 132	2	610	1	Comparative Example
75	BK	930	20	11	575	30	61	22	17	912	185	0037	2	585	1	Comparative Example
76	BL	930	20	11	575	30	69	25	6	972	172	0.127	2	610	1	Comparative Example
77	BM	930	20	11	575	30	62	34	4	989	29	0.056	1	592	1	Comparative Example
78	BN	930	20	11	575	30	43	21	36	823	168	0 133	2	606	1	Comparative Example
79	BO	930	20	11	575	30	73	24	3	979	217	0.045	1	629	1	Comparative Example
80	BP	930	20	11	575	30	71	25	5	968	241	0.050	2	583	3	Comparative Example

Claims

1. A stainless steel seamless pipe comprising:

a chemical composition containing, in mass%,

C: 0.06 % or less,

Si: 1.0 % or less,

Mn: 0.01 % or more and 1.0 % or less,

P: 0.05 % or less,

S: 0.005 % or less,

Cr: 15.2 % or more and 18.0 % or less,

Mo: 1.5 % or more and 4.3 % or less,

Cu: 1.2 % or more and 3.5 % or less,

Ni: 3.5 % or more and 5.2 % or less,

V: 0.5 % or less,

Al: 0.10 % or less,

N: 0.10 % or less,

O: 0.010 % or less,

Sb: 0.001 % or more and 1.000 % or less,

Co: 0.01 % or more and 1.00 % or less, and

Ca: 0.001 % or more and 0.030 % or less,

with a balance consisting of Fe and inevitable impurities;

30 % or more martensite phase, 50 % or less ferrite phase, and 40 % or less retained austenite phase in volume fraction;

a yield stress of 758 MPa or more; and

a Charpy absorbed energy vE_-10 at -10 °C of 40 J or more.

2. The stainless steel seamless pipe according to claim 1, wherein the chemical composition further contains, in mass%, at least one selected from the group consisting of

Nb: 0.07 % or less,

Ti: 0.2 % or less,

W: 0.9 % or less,

B: 0.01 % or less,

Ta: 0.3 % or less,

Zr: 0.3 % or less,

REM: 0.3 % or less,

Mg: 0.01 % or less, and

Sn: 1.0 % or less.

3. The stainless steel seamless pipe according to claim 1 or 2, comprising:

50 % or more martensite phase, 50 % or less ferrite phase, and 25 % or less retained austenite phase in volume fraction; and

a yield stress of 862 MPa or more.

4. A production method for a stainless steel seamless pipe, the production method comprising:

making a seamless steel pipe from a steel material having the chemical composition according to claim 1 or 2;

heating the seamless steel pipe to a quenching temperature of 850 °C to 1150 °C;

cooling the seamless steel pipe after the heating to a cooling stop temperature of 50 °C or less at a cooling rate of 0.01 °C/s or more; and

heating the seamless steel pipe after the cooling to a tempering temperature of 500 °C to 650 °C, to produce a stainless steel seamless pipe having: 30 % or more martensite phase, 50 % or less ferrite phase, and 40 % or less retained austenite phase in volume fraction; a yield stress of 758 MPa or more; and a Charpy absorbed energy vE_-10 at -10 °C of 40 J or more.

5. The production method for a stainless steel seamless pipe according to claim 4, wherein the stainless steel seamless pipe has: 50 % or more martensite phase, 50 % or less ferrite phase, and 25 % or less retained austenite phase in volume fraction; and a yield stress of 862 MPa or more.