Seamless stainless steel pipe and production method therefor

Abstract

Provided are a stainless steel seamless pipe and a method for manufacturing the same. The stainless steel seamless pipe of the invention has a chemical composition containing, in mass %, C: 0.06% or less, Si: 1.0% or less, Mn: 0.01 to 1.0%, P: 0.05% or less, S: 0.005% or less, Cr: 15.2 to 18.0%, Mo: 1.5 to 4.3%, Cu: 0.5 to 3.5%, Ni: 3.5 to 5.2%, V: 0.5% or less, Al: 0.10% or less, N: 0.10% or less, and O: 0.010% or less, with the balance being Fe and incidental impurities and has a microstructure including a martensite phase at a volume fraction of 40% or more, a ferrite phase at a volume fraction of 15 to 55%, and a retained austenite phase at a volume fraction of 40% or less, with the average of ferrite filling factors being 0.80 or less. The yield strength of the stainless steel seamless pipe is 758 MPa or more.

Description

Technical Field

[0001] The present invention relates to a stainless steel seamless pipe suitably used in oil wells and gas wells (hereinafter referred to simply as oil wells). More particularly, the invention relates to a stainless steel seamless pipe having improved corrosion resistance in a severe high-temperature corrosive environment containing carbon dioxide (CO 2 ) and chlorine ions (Cl -< ), an environment containing hydrogen sulfide (H 2 S), etc.Background Art

[0002] Stainless steel seamless pipes are widely used for applications such as oil well steel pipes. The oil well steel pipes are required to have high yield strength. Moreover, from the viewpoint of the expected depletion of energy resources in the near future, oil wells are being actively developed in severe corrosive environments that have not received much attention such as deep oil fields, environments containing carbon dioxide gas, and so-called sour environments containing hydrogen sulfide. Therefore, oil well steel pipes are also required to have high corrosion resistance.

[0003] Examples of oil well steel pipes used for extraction in oil fields and gas fields in an environment containing CO 2 , Cl -< , etc. include 13Cr martensitic stainless steel pipes. However, in some corrosive environments, the corrosion resistance of the 13Cr martensitic stainless steel pipes is insufficient. There is therefore a need for oil well steel pipes having higher corrosion resistance and usable in such corrosive environments.

[0004] In recent years, to achieve a decarbonized society, attention has been given to a technology called carbon capture and storage (CCS) in which CO 2 is separated and collected from various CO 2 emission sources and air and stored underground. In the CCS, high pressure CO 2 is injected underground. Therefore, in some injection wells, steel pipes may be used in an environment containing CO 2 , Cl -< , hydrogen sulfide, etc. that is similar to the environments in the oil wells described above, and there may be a demand for steel pipes having high corrosion resistance such as those for oil wells.

[0005] To meet the above demand, for example, Patent Literature 1 to Patent Literature 4 propose the following techniques. Patent Literature 1 proposes stainless steel for oil wells that has a chemical composition containing, in mass %, C: 0.05% or less, Si: 0.5% or less, Mn: 0.01 to 0.5%, P: 0.04% or less, S: 0.01% or less, Cr: more than 16.0 to 18.0%, Ni: more than 4.0 to 5.6%, Mo: 1.6 to 4.0%, Cu: 1.5 to 3.0%, Al: 0.001 to 0.10%, and N: 0.05% or less, wherein Cr, Ni, Mo, Cu, C, N, and Mn satisfy a specific relation.

[0006] Patent Literature 2 proposes a high-strength stainless steel seamless pipe for oil wells that has a chemical composition containing, in mass %, C: 0.005 to 0.06%, Si: 0.05 to 0.5%, Mn: 0.2 to 1.8%, P: 0.03% or less, S: 0.005% or less, Cr: 15.5 to 18.0%, Ni: 1.5 to 5.0%, V: 0.02 to 0.2%, Al: 0.002 to 0.05%, N: 0.01 to 0.15%, and O: 0.006% or less and further containing one or two or more selected from Mo: 1.0 to 3.5%, W: 3.0% or less, and Cu: 3.5% or less, wherein Cr, Ni, Mo, W, Cu, C, Si, Mn, and N satisfy a specific relation.

[0007] Patent Literature 3 proposes stainless steel having a chemical composition containing, in mass %, C: 0.001 to 0.06%, Si: 0.05 to 0.5%, Mn: 0.01 to 2.0%, P: 0.03% or less, S: less than 0.005%, Cr: 15.5 to 18.0%, Ni: 2.5 to 6.0%, V: 0.005 to 0.25%, Al: 0.05% or less, N: 0.06% or less, O: 0.01% or less, Cu: 0 to 3.5%, Co: 0 to 1.5%, Nb: 0 to 0.25%, Ti: 0 to 0.25%, Zr: 0 to 0.25%, Ta: 0 to 0.25%, B: 0 to 0.005%, Ca: 0 to 0.01%, Mg: 0 to 0.01%, and REMs: 0 to 0.05%, wherein one or two selected from the group consisting of Mo: 0 to 3.5% and W: 0 to 3.5% satisfy a specific relation.

[0008] Patent Literature 4 proposes a seamless steel pipe containing, in mass %, C: 0.050% or less, Si: 0.50% or less, Mn: 0.01 to 0.20%, P: 0.025% or less, S: 0.0150% or less, Cu: 0.09 to 3.00%, Cr: 15.00 to 18.00%, Ni: 4.00 to 9.00%, Mo: 1.50 to 4.00%, Al: 0.040% or less, N: 0.0150% or less, Ca: 0.0010 to 0.0040%, Ti: 0.020% or less, Nb: 0.020% or less, V: 0 to 0.20%, Co: 0 to 0.30%, and W: 0 to 2.00%.Citation ListPatent Literature

[0009] PTL 1: International Publication No. WO2010 / 134498 PTL 2: Japanese Unexamined Patent Application Publication No. 2013-249516 PTL 3: International Publication No. WO2017 / 022374 PTL 4: International Publication No. WO2020 / 013197 Summary of InventionTechnical Problem

[0010] With the conventional techniques proposed in Patent Literature 1 to Patent Literature 4, 17Cr stainless steel having higher corrosion resistance than 13Cr steel can be obtained. However, the corrosion resistance, particularly the stress corrosion cracking resistance, is not always sufficient, and it has been found that the steel cannot be used in some environments.

[0011] It is an object of the present invention to solve the problem in the conventional techniques to thereby provide a stainless steel seamless pipe having high strength in terms of a yield strength of 758 MPa (i.e., 110 ksi) or more and good stress corrosion cracking resistance and to provide a method for manufacturing the stainless steel seamless pipe.

[0012] The "good stress corrosion cracking resistance" is defined as follows. A test specimen subjected to a stress corresponding to its yield stress at 200°C by four-point bending is immersed in a 20% by mass aqueous NaCl solution (the temperature of the aqueous solution: 200°C) in an autoclave for 720 hours. The aqueous NaCl solution has a pH of 4.5 adjusted by adding sodium hydrogencarbonate and is in contact with a 50 atm CO 2 -0.01 atm H 2 S gas in the autoclave. The stress corrosion cracking resistance is rated good when no cracking is found in the test specimen after the test and the corrosion rate determined by a weight loss method after removal of corrosion products is 0.10 mm / year or less.

[0013] The details of the above test will also be described later in Examples.Solution to Problem

[0014] To achieve the foregoing object, the present inventors have conducted extensive studies on various factors influencing the properties of stainless steel, particularly its stress corrosion cracking resistance. Then the inventors have noticed that, although elements such as Cr, Mo, and Cu generally known as corrosion resistant elements are effective for stress corrosion cracking resistance, the amount of a martensite phase that contributes to strength cannot be obtained stably in some cases when the amounts of these elements added are excessively large. Accordingly, the inventors have come up with an entirely new approach. Specifically, the piercing conditions in the manufacture of a seamless steel pipe are optimized and control the microstructural form of the steel. With this approach, good stress corrosion cracking resistance can be obtained without adding large amounts of corrosion resistant elements.

[0015] More specifically, the microstructural form of ferrite contained in the steel is controlled as follows. In an optical micrograph obtained by capturing an image of a microstructure of a steel pipe in a plane including the longitudinal and wall thickness directions of the steel pipe at a magnification of 400X, ferrite is extracted by image analysis in an area of 300 µm in the longitudinal direction × 200 µm in the wall thickness direction in actual size. The inventors have found that good stress corrosion cracking resistance can be obtained by controlling the microstructural form such that the average ferrite filling factor is 0.80 or less. The "average ferrite filling factor" will be described later, and its description is omitted here.

[0016] The "plane including the longitudinal and wall thickness directions of the steel pipe" in the invention is a cross section in the wall thickness direction that includes the pipe axis. The image analysis is performed in the "area of 300 µm in the longitudinal direction × 200 µm in the wall thickness direction in actual size" in any one point in the cross section.

[0017] The "filling factor" is an indicator that can be computed using, for example, image analysis free software ImageJ used in Examples in the invention. One of the main points of the invention is that the inventors have found that a microstructural form with good stress corrosion cracking resistance can be represented by this indicator.

[0018] Next, a method for manufacturing a steel pipe in which the average ferrite filling factor is 0.80 or less will be described.

[0019] Patent Literature 1 focuses on an improvement in stress corrosion cracking resistance and describes a method for manufacturing stainless steel in which a reduction in area of a steel material at 850°C to 1250°C is 50% or more. First, the inventors manufactured a steel pipe using the manufacturing method in Patent Literature 1. However, the average ferrite filling factor could not be adjusted to 0.80 or less using only the control described in Patent Literature 1, and the target stress corrosion cracking resistance of the invention was not achieved.

[0020] Accordingly, the inventors studied the relation between the manufacturing conditions of a seamless steel pipe and the average ferrite filling factor. The results are shown in Fig. 1. The inventors have found that, when piercing in a piercing step is performed under the condition that a piercing speed specified according to the length of a hollow piece immediately after completion of the piercing is 3.3 m / second or less, the average ferrite filling factor can be stably 0.80 or less, as shown in Fig. 1. Although not clearly understood, the mechanism may be as follows. By performing the piercing at a certain speed or lower, the amount of shear strain in the circumferential direction of the hollow piece increases, and a microstructure including ferrite and martensite entangled in a complex manner is formed. This may be the reason that the ferrite filling factor in the plane including the longitudinal and wall thickness directions of the steel pipe tends to be 0.80 or less.

[0021] The present invention has been completed by conducting further studies on the basis of the above findings. The present invention is summarized as follows. [1] A stainless steel seamless pipe having a chemical composition containing, in mass %, C: 0.06% or less, Si: 1.0% or less, Mn: 0.01 to 1.0%, P: 0.05% or less, S: 0.005% or less, Cr: 15.2 to 18.0%, Mo: 1.5 to 4.3%, Cu: 0.5 to 3.5%, Ni: 3.5 to 5.2%, V: 0.5% or less, Al: 0.10% or less, N: 0.10% or less, and O: 0.010% or less, with the balance being Fe and incidental impurities, wherein the stainless steel seamless pipe has a microstructure including a martensite phase at a volume fraction of 40% or more, a ferrite phase at a volume fraction of 15 to 55%, and a retained austenite phase at a volume fraction of 40% or less, with an average of ferrite filling factors being 0.80 or less, wherein the stainless steel seamless pipe has a yield strength of 758 MPa or more, and wherein each ferrite filling factor is a value defined by the following formula for one ferrite grain: [2] The stainless steel seamless pipe according to [1], wherein the chemical composition further contains, in mass %, one or two or more selected from the group consisting of Nb: 0.3% or less, Ti: 0.3% or less, W: 2.0% or less, Co: 1.0% or less, B: 0.010% or less, Ta: 0.3% or less, Zr: 0.3% or less, Ca: 0.010% or less, REMs: 0.3% or less, Mg: 0.01% or less, Sn: 1.0% or less, and Sb: 1.0% or less. [3] The stainless steel seamless pipe according to [1] or [2], wherein the volume fraction of the martensite phase is 45% or more, wherein the volume fraction of the ferrite phase is 15 to 55%, wherein the volume fraction of the retained austenite phase is 30% or less, and wherein the yield strength is 862 MPa or more. [4] A method for manufacturing the stainless steel seamless pipe according to any one of [1] to [3], the method including: when a steel material having the chemical composition is subjected to pipe making to produce a seamless steel pipe, performing a piercing step at a piercing speed of 3.3 m / second or less; then performing quenching treatment including heating the seamless steel pipe to a quenching temperature of 850 to 1150°C and then cooling the seamless steel pipe at a cooling rate of 0.01°C / second or more to a cooling stop temperature at which a surface temperature of the steel pipe is 50°C or lower; and then performing tempering treatment including heating the seamless steel pipe to a tempering temperature of 500 to 650°C. Advantageous Effects of Invention

[0022] According to the present invention, a stainless steel seamless pipe having high strength in terms of a yield strength of 758 MPa or more and also having good stress corrosion cracking resistance can be obtained. With the manufacturing method of the invention, the stainless steel seamless pipe having the above properties can be manufactured by optimizing the piercing conditions in the piercing step.Brief Description of Drawing

[0023] [Fig. 1] Fig. 1 is a graph showing the relation between a manufacturing condition of a seamless steel pipe and an average ferrite filling factor.Description of Embodiments

[0024] The present invention will next be described in detail.[Chemical composition]

[0025] The stainless steel seamless pipe of the invention has the chemical composition described above. Frist, the reasons for the limitations on the chemical composition will be described. In the following description, "% by mass" is denoted simply as "%" unless otherwise specified.C: 0.06% or less

[0026] C is an element that is inevitably contained in the steel making process. If C is contained in an amount of 0.06% or more, corrosion resistance deteriorates. Therefore, the content of C is set to 0.06% or less. The content of C is preferably 0.05% or less, more preferably 0.04% or less, and still more preferably 0.03% or less. From the viewpoint of the corrosion resistance, the lower the content of C, the better. Therefore, no particular limitation is imposed on the lower limit of the content of C. However, from the viewpoint of the cost of decarbonization, the content of C is preferably 0.002% or more, more preferably 0.003% or more, and still more preferably 0.005% or more.

[0027] In the present invention, carbon dioxide gas corrosion resistance, sulfide stress cracking resistance (SSC resistance), and stress corrosion cracking resistance (SCC resistance) are correctively referred to as "corrosion resistance."Si: 1.0% or less

[0028] Si is an element that functions as a deoxidizing agent. However, if Si is contained in an amount of more than 1.0%, the hot workability and the corrosion resistance deteriorate. Therefore, the content of Si is set to 1.0% or less. The content of Si is preferably 0.7% or less, more preferably 0.5% or less, and still more preferably 0.4% or less. No particular limitation is imposed on the lower limit of the content of Si. However, from the viewpoint of increasing the deoxidization effect, the content of Si is preferably 0.03% or more, more preferably 0.05% or more, and still more preferably 0.10% or more.Mn: 0.01 to 1.0%

[0029] Mn is an element that functions as a deoxidizing agent and a desulfurizing agent and improves the hot workability. To obtain the effect of the deoxidizing agent and the effect of the desulfurizing agent and to improve the strength, the content of Mn is set to 0.01% or more. The content of Mn is preferably 0.03% or more, more preferably 0.05% or more, and still more preferably 0.10% or more. However, if Mn is contained in an amount of more than 1.0%, its effects are saturated. Therefore, the content of Mn is 1.0% or less. The content of Mn is preferably 0.8% or less, more preferably 0.6% or less, and still more preferably 0.4% or less.P: 0.05% or less

[0030] P is an element that reduces the carbon dioxide gas corrosion resistance and the SSC resistance. To obtain the desired corrosion resistance, the content of P is set to 0.05% or less. The content of P is preferably 0.04% or less and more preferably 0.03% or less. The lower the content of P, the better. Therefore, no particular limitation is imposed on the lower limit of the content of P, and the content of P may be 0%. From the viewpoint of the manufacturing cost, the content of P is more preferably 0.005% or more.S: 0.005% or less

[0031] S is an element that significantly reduces the hot workability and inhibits stable operation in a hot pipe making process. S is present in the steel as sulfide-based inclusions and reduces the corrosion resistance. Therefore, the content of S is set to 0.005% or less. The content of S is preferably 0.004% or less, more preferably 0.003% or less, and still more preferably 0.002% or less. The lower the content of S, the better. Therefore, no particular limitation is imposed on the lower limit of the content of S, and the content of S may be 0%. From the viewpoint of the manufacturing cost, the content of S is more preferably 0.0005% or more.Cr: 15.2 to 18.0%

[0032] Cr is an element that forms a protective coating on the surface of the steel pipe and contributes to an improvement in the corrosion resistance. If the content of Cr is less than 15.2%, the desired stress corrosion cracking resistance cannot be obtained. Moreover, the carbon dioxide gas corrosion resistance deteriorates. Although Cr is an element that stabilizes the ferrite phase, the fraction of the ferrite phase is small if the content of Cr is less than 15.2%, so that the steel obtained does not have the desired phase fractions. Therefore, the content of Cr is 15.2% or more. The content of Cr is preferably 15.5% or more, more preferably 16.0% or more, and still more preferably 16.3% or more. However, if the content of Cr exceeds 18.0%, the fraction of the ferrite phase and the fraction of the retained austenite phase become excessively high, so that the desired strength cannot be obtained. Therefore, the content of Cr is 18.0% or less. The content of Cr is preferably 17.5% or less, more preferably 17.2% or less, and still more preferably 17.0% or less.Mo: 1.5 to 4.3%

[0033] Mo stabilizes the protective coating on the surface of the steel pipe, increases resistance to pitting corrosion caused by Cl -< or low pH, and thereby increases the corrosion resistance. To obtain the desired stress corrosion cracking resistance, the content of Mo is set to 1.5% or more. Although Mo is an element that stabilizes the ferrite phase, the fraction of the ferrite phase becomes small if the content of Mo is less than 1.5%, so that the steel obtained cannot have the desired phase fractions. The content of Mo is preferably 1.8% or more, more preferably 2.0% or more, and still more preferably 2.3% or more. If the content of Mo exceeds 4.3%, the fraction of the ferrite phase and the fraction of the retained austenite phase become excessively high, so that the desired strength cannot be obtained. Therefore, the content of Mo is 4.3% or less. The content of Mo is preferably 4.0% or less, more preferably 3.5% or less, and still more preferably 3.0% or less.Cu: 0.5 to 3.5%

[0034] Cu has the effect of strengthening the protective coating on the surface of the steel pipe to thereby increase the corrosion resistance, particularly the carbon dioxide gas corrosion resistance. To obtain the desired strength and the desired corrosion resistance, particularly the desired carbon dioxide gas corrosion resistance, the content of Cu is set to 0.5% or more. The content of Cu is preferably 0.8% or more, more preferably 1.5% or more, and still more preferably 2.0% or more. If the content of Cu is excessively large, the hot workability of the steel deteriorates, and outer surface flaws are formed during pipe making, so that the desired stress corrosion cracking resistance cannot be obtained. Since Cu is an element that stabilizes the austenite phase, the fraction of the ferrite phase decreases if an excessively large amount of Cu is added, and the steel obtained cannot have the desired phase fractions. Therefore, the content of Cu is 3.5% or less. The content of Cu is preferably 3.2% or less, more preferably 3.0% or less, and still more preferably 2.7% or less.Ni: 3.5 to 5.2%

[0035] Ni maintains the fraction of the austenite phase at high temperature and allows the required amount of the martensite phase in the invention to be obtained, thereby contributing to the strength enhancement. To obtain the desired strength, the content of Ni is set to 3.5% or more. Although Ni is an element that stabilizes the austenite phase, the fraction of the austenite phase at high temperature becomes small if the content of Ni is less than 3.5%, so that the desired phase fraction of the martensite phase transformed from the austenite is not obtained. The content of Ni is preferably 3.8% or more, more preferably 4.0% or more, and still more preferably 4.3% or more. If Ni is contained in an amount of more than 5.2%, the fraction of the austenite phase becomes excessively large. In this case, the hot workability of the steel deteriorates, and flaws are likely to be caused during hot rolling, so that the desired stress corrosion cracking resistance may not always be obtained. Moreover, since the austenite forming ability becomes high, the fraction of the ferrite phase decreases accordingly, so that the steel obtained cannot have the desired phase fractions. Therefore, the content of Ni is 5.2% or less. The content of Ni is preferably 5.0% or less.V: 0.5% or less

[0036] V is an element that forms carbonitrides and thereby increases the strength without impairing the toughness. Moreover, V easily forms carbonitrides. Therefore, although corrosion resistant elements such as Cr may form carbonitrides, a reduction in their amounts effective for corrosion resistance is suppressed, and good corrosion resistance, particularly the desired stress corrosion cracking resistance, can thereby be obtained. However, even when V is contained in an amount of more than 0.5%, its effect is saturated. Therefore, the content of V is set to 0.5% or less. The content of V is preferably 0.2% or less and more preferably 0.1% or less. No particular limitation is imposed on the lower limit of the content of V, but the content of V is preferably 0.01% or more and more preferably 0.03% or more.Al: 0.10% or less

[0037] Al is an element that functions as a deoxidizing agent. However, if Al is contained in an amount of more than 0.10%, the corrosion resistance deteriorates. Therefore, the content of Al is set to 0.10% or less. The content of Al is preferably 0.07% or less and more preferably 0.05% or less. No particular limitation is imposed on the lower limit of the content of Al. However, from the viewpoint of increasing the deoxidization effect, the content of Al is preferably 0.005% or more, more preferably 0.010% or more, and still more preferably 0.015% or more.N: 0.10% or less

[0038] N is an element inevitably contained during the steel making process but is an element that increases the strength of the steel. However, if N is contained in an amount of more than 0.10%, the amount of nitrides formed is excessively large, and the corrosion resistance deteriorates. Therefore, the content of N is 0.10% or less. The content of N is preferably 0.07% or less, more preferably 0.05% or less, and still more preferably 0.03% or less. No particular limitation is imposed on the lower limit of the content of N. However, an excessive reduction in the content of N causes an increase in the cost of steel making. Therefore, the content of N is preferably 0.002% or more, more preferably 0.003% or more, and still more preferably 0.005% or more.O: 0.010% or less

[0039] O (oxygen) is present as oxides in the steel and adversely affects various properties. Therefore, in the present invention, the lower the content of O, the better. In particular, if the content of O exceeds 0.010%, the hot workability and the corrosion resistance deteriorate. Therefore, the content of O is set to 0.010% or less. No particular limitation is imposed on the lower limit of the content of O, and the content of O may be 0%. Since an excessive reduction in the content of O causes an increase in the cost of steel making, the content of O is more preferably 0.0005% or more.

[0040] The stainless steel seamless pipe of the invention has the chemical composition containing the above-described components with the balance being Fe and incidental impurities.

[0041] The components described above are basic components, and these basic components allow the stainless steel seamless pipe of the invention to have the intended properties. In the present invention, in addition to these basic components, one or two or more selected from the group consisting of Nb, Ti, W, Co, B, Ta, Zr, Ca, REMs, Mg, Sn, and Sb may be optionally contained. Since Nb, Ti, W, Co, B, Ta, Zr, Ca, REMs, Mg, Sn, and Sb are optional steel components, their contents may be 0%.Nb: 0.3% or less

[0042] Nb is an element that forms carbonitrides, improves the strength and corrosion resistance, and may be optionally contained. However, the Nb carbonitrides tend to cause a deterioration in low-temperature toughness. Therefore, when Nb is added, the content of Nb is 0.3% or less. The content of Nb is preferably 0.2% or less and more preferably 0.1% or less. The content of Nb is more preferably 0.01% or more.Ti: 0.3% or less

[0043] Ti is an element that increases the strength and corrosion resistance and may be optionally contained. However, if Ti is contained in an amount of more than 0.3%, the low-temperature toughness deteriorates. Therefore, when Ti is added, the content of Ti is 0.3% or less. The content of Ti is preferably 0.2% or less and more preferably 0.1% or less. The content of Ti is preferably 0.001% or more and more preferably 0.01% or more.W: 2.0% or less

[0044] W is an element that contributes to an increase in the strength of the steel, stabilizes the protective coating on the surface of the steel pipe to thereby increase the corrosion resistance, and may be optionally contained. However, if W is contained in an amount of more than 2.0%, the fraction of the ferrite phase becomes excessively high, and the desired strength cannot be obtained. Therefore, when W is added, the content of W is 2.0% or less. The content of W is preferably 1.5% or less and more preferably 1.2% or less. The content of W is more preferably 0.3% or more and still more preferably 0.5% or more.Co: 1.0% or less

[0045] Co is an element that improves the corrosion resistance and may be optionally contained. However, even if Co is contained in an amount of more than 1.0%, its effect is saturated. Therefore, when Co is added, the content of Co is 1.0% or less. The content of Co is preferably 0.5% or less, more preferably 0.3% or less, and still more preferably 0.1% or less. The content of Co is more preferably 0.01% or more.B: 0.010% or less

[0046] B is an element that contributes to an improvement in hot workability and has the effect of reducing the occurrence of cracking and splitting in the pipe making process and may be optionally contained. However, if B is contained in an amount of more than 0.010%, the low-temperature toughness deteriorates. Therefore, when B is added, the content of B is 0.010% or less. The content of B is preferably 0.007% or less and more preferably 0.005% or less. The content of B is more preferably 0.0005% or more and still more preferably 0.0010% or more.Ta: 0.3% or less

[0047] Ta is an element that has the effects of increasing the strength and improving the corrosion resistance and may be optionally contained. However, if Ta is contained in an amount of more than 0.3%, its effects are saturated. Therefore, when Ta is added, the content of Ta is 0.3% or less. The content of Ta is more preferably 0.001% or more.Zr: 0.3% or less

[0048] Zr is an element that increases the strength and may be optionally added. Zr also has the effect of improving the SSC resistance. However, even if Zr is contained in an amount of more than 0.3%, its effect is saturated. Therefore, when Zr is added, the content of Zr is 0.3% or less. The content of Zr is preferably 0.0005% or more.Ca: 0.010% or less

[0049] Ca is an element that improves the hot workability through shape control of its sulfide and has the effect of reducing the occurrence of cracking and chipping in the pipe making process and may be optionally contained. When Ca is added in order to obtain these effects, the content of Ca is set to 0.001% or more. The content of Ca is preferably 0.002% or more, more preferably 0.003% or more, and still more preferably 0.005% or more. However, even if Ca is contained in an amount of more than 0.010%, its effects are saturated and are not expected to be commensurate with the content. Therefore, the content of Ca is 0.010% or less. The content of Ca is preferably 0.008% or less and more preferably 0.007% or less.REMs: 0.3% or less

[0050] REMs (rare earth metals) are elements that contribute to an improvement in the stress corrosion cracking resistance through shape control of their sulfides and may be optionally contained. However, even if REMs are contained in an amount of more than 0.3%, their effects are saturated and are not expected to be commensurate with the content. Therefore, when REMs are added, the content of REMs is 0.3% or less. The content of REMs is preferably 0.0005% or more.

[0051] The REMs in the invention are elements including scandium (Sc) with an atomic number of 21, yttrium (Y) with an atomic number of 39, and lanthanoid elements from lanthanum (La) with an atomic number of 57 to lutetium (Lu) with an atomic number of 71. The chemical composition of the stainless steel seamless pipe of the invention may optionally contain at least one of the REMs. Therefore, the content of REM in the invention is the total content of the above elements.Mg: 0.01% or less

[0052] Mg is an element that improves the corrosion resistance and may be optionally contained. However, even if Mg is contained in an amount of more than 0.01%, its effect is saturated and is not expected to be commensurate with the content. Therefore, when Mg is added, the content of Mg is 0.01% or less. The content of Mg is preferably 0.0005% or more.Sn: 1.0% or less

[0053] Sn is an element that improves the corrosion resistance and may be optionally contained. However, even if Sn is contained in an amount of more than 1.0%, its effect is saturated and is not expected to be commensurate with the content. Therefore, when Sn is added, the content of Sn is 1.0% or less. The content of Sn is preferably 0.001% or more.Sb: 1.0% or less

[0054] Sb is an element that improves the corrosion resistance and may be optionally contained. However, even if Sb is contained in an amount of more than 1.0%, its effect is saturated and is not expected to be commensurate with the content. Therefore, when Sb is added, the content of Sb is 1.0% or less. The content of Sb is preferably 0.001% or more.[Microstructure]

[0055] Next, the reasons for the limitations on the microstructure of the stainless steel seamless pipe of the invention will be described.

[0056] The microstructure of a stainless steel seamless pipe in one embodiment of the invention includes a martensite phase at a volume fraction of 40% or more, a ferrite phase at a volume fraction of 15 to 55%, and a retained austenite phase at a volume fraction of 40% or less, and the average ferrite filling factor in a plane including the longitudinal and wall thickness directions of the steel pipe described later is 0.80 or less.Martensite phase: 40% or more in terms of volume fraction

[0057] If the volume fraction of the martensite phase is less than 40%, the desired strength cannot be obtained. Therefore, the volume fraction of the martensite phase is set to 40% or more. The volume fraction of the martensite phase is preferably 45% or more, more preferably 50% or more, and still more preferably 60% or more. No particular limitation is imposed on the upper limit of the volume fraction of the martensite phase, but the volume fraction of the martensite phase is preferably 90% or less and more preferably 85% or less.Ferrite phase: 15 to 55% or less in terms of volume fraction

[0058] When the ferrite phase is contained, propagation of stress corrosion cracks can be prevented, and good corrosion resistance is obtained. However, if the volume fraction of the ferrite phase exceeds 55%, the desired strength cannot be obtained. Therefore, the volume fraction of the ferrite phase is set to 55% or less. The volume fraction of the ferrite phase is preferably 50% or less, more preferably 45% or less, and still more preferably 40% or less. If the volume fraction of the ferrite phase is less than 15%, the desired stress corrosion cracking resistance is not obtained. Therefore, the volume fraction of the ferrite phase is 15% or more. The volume fraction of the ferrite phase is preferably 20% or more and more preferably 25% or more.Retained austenite phase: 40% or less in terms of volume fraction

[0059] The presence of the retained austenite phase allows the ductility and low-temperature toughness to be improved. However, if a large amount of the retained austenite phase is precipitated, i.e., if its volume fraction exceeds 40%, the desired strength cannot be obtained. Therefore, the volume fraction of the retained austenite phase is set to 40% or less. The volume fraction of the retained austenite phase is preferably 30% or less and more preferably 25% or less. No particular limitation is imposed on the lower limit of the volume fraction of the retained austenite phase, but the volume fraction of the retained austenite phase is preferably 3% or more and more preferably 5% or more.

[0060] The volume fractions of the above phases can be measured by the following method.

[0061] First, a steel piece is cut from a plane including the longitudinal and wall thickness directions of the stainless steel seamless pipe. The steel piece is embedded in a resin and mirror-polished to produce a specimen for microstructure observation. The observation surface is electrolytically etched with a KOH solution (i.e., a solution mixture of 35 g of KOH and 100 g of pure water) at a current density of 3 A / cm 2< for 35 seconds and then etched with Vilella's reagent (i.e., a reagent mixture of 2 g of picric acid, 5 mL of hydrochloric acid, and 50 mL of ethanol) for 30 seconds. Next, an image of the microstructure of the test specimen for microstructure observation is taken using an optical microscope at a magnification of 400X. In the optical micrograph obtained, an image is cut from one selected position within an area of 300 µm in the longitudinal direction of the steel pipe × 200 µm in the wall thickness direction of the steel pipe in actual size and then analyzed using image analysis software (ImageJ 1.52p, National Institute of Health) to compute the microstructure fraction (area fraction (%)) of the ferrite phase. In the above analysis, a Weka Trainable Segmentation function is used for the optical micrograph. Three bright ferrite portions and three dark martensite portions are used as training data. The Segmentation function is used for other portions to automatically classify these portions, and the ferrite phase can thereby be extracted. The area fraction of the extracted ferrite phase is defined as the volume fraction (%) of the ferrite phase.

[0062] Next, a test specimen for X-ray diffraction cut from the stainless steel seamless pipe is ground and polished such that a cross section perpendicular to the pipe axis direction (i.e., a C cross section) serves as a measurement surface, and the microstructure fraction of the retained austenite (γ) phase is measured by the X-ray diffraction method. Specifically, the volume fraction of the retained austenite phase is computed from the integrated intensity of the austenite (220) plane and the integrated intensity of the ferrite (211) plane using the following formula. Vγ % = 100 / 1 + IαRγ / IγRα Here, Vγ: the volume fraction of the retained austenite phase, Iα: the integrated intensity of the ferrite (211) plane, Iγ: the integrated intensity of the austenite (220) plane, Rα: the crystallographic theoretical computational value of α (34.15), and Rγ: the crystallographic theoretical computational value of γ (22.33).

[0063] The remaining portion other than the ferrite phase and the retained γ phase determined by the above measurement method is used as the fraction of the martensite phase. The method for observing the microstructure will also be described in detail later in Examples.

[0064] The microstructure of the stainless steel seamless pipe of the invention includes the martensite phase, the ferrite phase, and the retained austenite phase.Average ferrite filling factor: 0.80 or less

[0065] The stainless steel seamless pipe of the invention has a microstructural form in which the average ferrite filling factor is 0.80 or less.

[0066] The average ferrite filling factor is determined by the following method. First, in the image subjected to the Segmentation and used to determine the microstructure fraction of the ferrite phase (i.e., the image at the "one selected position"), an Analyze Particle function is used for the ferrite positions to thereby obtain the number of ferrite grains and the feature of the ferrite grains. The Solidity outputted from the ImageJ corresponds to the filling factor of each ferrite grain. The average of the Solidity values of the ferrite grains is defined as the average ferrite filling factor in the invention.

[0067] Specifically, the "ferrite filling factor" is a value defined by the following formula for each individual ferrite grain.

[0068] When no martensite phase is present in the convex hull surrounding the ferrite grain, the filling factor is close to 1. However, when the convex hull has a shape with the martensite phase present thereinside, the filling factor is small.

[0069] The ferrite phase, which is richer in corrosion resistant elements such as Cr and Mo, acts as a barrier to occurrence of pitting corrosion serving as the starting point of stress corrosion cracking. When the filling factor is large, i.e., when the ratio of the ferrite phase in the convex hulls surrounding the ferrite grains is high, pitting corrosion is unlikely to occur in the ferrite portions. However, in this microstructure, the martensite phase and the ferrite phase are not entangled in a complex manner. Specifically, in this microstructure, the martensite phase is densely present. Therefore, pitting corrosion is likely to occur and grow in the martensite portions, and these martensite portions are disadvantageous for stress corrosion cracking.

[0070] When the filling factor is small, i.e., when the ratio of the ferrite phase in the convex hulls surrounding the ferrite grains is small, the ferrite phase and the martensite phase are entangled in a complex manner. In this microstructural form, the martensite phase, which is disadvantageous for pitting corrosion, is surrounded by the ferrite phase. Therefore, even when slight pitting corrosion occurs, the pitting corrosion is unlikely to grow, and therefore good stress corrosion cracking resistance is obtained.

[0071] Because of the reasons described above, it is important for the microstructural form in the invention that the average ferrite filling factor be adjusted to 0.80 or less. When the average filling factor is small, the stress corrosion cracking resistance is expected to be improved. Therefore, the average ferrite filling factor is preferably 0.75 or less and more preferably 0.70 or less.

[0072] As described above, the smaller the average filling factor, the better. However, the lower limit of the average ferrite filling factor is preferably 0.2 or more. The average ferrite filling factor is more preferably 0.4 or more.[Yield strength]Yield strength: 758 MPa or more

[0073] The stainless steel seamless pipe of the invention has a yield strength of 758 MPa or more. No particular limitation is imposed on the upper limit of the yield strength, but the yield strength is preferably 1034 MPa or less. The yield strength can be measured by a method described later in Examples.

[0074] Preferably, the stainless steel seamless pipe of the invention has a yield strength of 862 MPa or more. At this yield strength, the volume fractions of the above-described phases are as follows. The volume fraction of the martensite phase is 45% or more; the volume fraction of the ferrite phase is 15 to 55%; and the volume fraction of the retained austenite phase is 30% or less.

[0075] The application of the stainless steel seamless pipe of the invention is not limited, and the stainless steel seamless pipe can be used for any application. In particular, the stainless steel seamless pipe of the invention is extremely suitable for oil well applications and can also be used preferably as a CO 2 injection pipe for the CCS described above.[Manufacturing method]

[0076] Next, an embodiment of a method for manufacturing the stainless steel seamless pipe of the invention will be described.

[0077] The stainless steel seamless pipe of the invention can be manufactured by subjecting a steel material to pipe making to thereby obtain a seamless steel pipe and subjecting the obtained seamless steel pipe to quenchingtempering treatment under specific conditions.

[0078] No particular limitation is imposed on the steel material, and any material can be used. For example, the steel material used may be a billet, and the steel material used has the chemical composition described above.

[0079] No particular limitation is imposed on the method for manufacturing the steel material, and any method may be used to manufacture the steel material. For example, a steelmaking method using, for example, a converter is used to prepare molten steel having the chemical composition described above, and then a round billet-shaped steel material is formed using, for example, a continuous casting method or an ingot making-blooming method. Alternatively, for example, after the steelmaking, the steel may be cast into a cylindrical shape to directly produce a round billet-shaped steel material.[Pipe making]

[0080] The above-obtained steel material is subjected to pipe making to obtain a seamless steel pipe. In the present invention, the pipe making is performed by hot working. Specifically, in the hot working, the steel material is heated, and the heated steel material is formed into a hollow piece (i.e., a hollow pipe) using a piercer. Then the hollow piece is subjected to rolling such as forming to thereby obtain a seamless steel pipe having the desired dimensions. No particular limitation is imposed on the method for subjecting the steel material to hot working to produce the seamless steel pipe, and any method may be used. For example, a Mannesmann-plug mill method or a Mannesmannmandrel mill method may be used to obtain the seamless steel pipe. No particular limitation is imposed on the heating temperature during heating in the hot working. However, from the viewpoint of obtaining both the hot workability during pipe making and the low-temperature toughness of the final product simultaneously at high levels, the heating temperature is preferably 1100 to 1350°C. After the heating, a piercing step of piercing the steel material is performed.

[0081] In the following description of the manufacturing method, the temperature (unit: °C) is the surface temperature of the steel pipe material or the steel pipe (i.e., the seamless steel pipe after the pipe making), unless otherwise specified. The surface temperature can be measured using, for example, a radiation thermometer.[Piercing step]

[0082] In the present invention, it is important that, when the pipe making is performed to produce the seamless steel pipe, the piercing speed in a piercing process be optimized. Specifically, the piercing step is controlled such that the piercing speed specified based on the length of the hollow piece immediately after completion of the piercing step, i.e., after the piercing step but before a subsequent rolling step, is 3.3 m / second or less. This is because, by appropriately controlling the piercing step such that the piercing speed satisfies the above range, the average ferrite filling factor can be adjusted to 0.80 or less.

[0083] The piercing speed is determined by dividing the length of the hollow piece immediately after the piercing by the piercing time. The "length of the hollow piece immediately after the piercing" is the total length (unit: m) of the hollow piece in the longitudinal direction immediately after completion of the piercing step. The "piercing time" is the time (unit: second) required from when a rolling load by the piercer is applied to the steel material until the rolling load is no longer applied to the pierced steel material. In the piercing mill, the rolling load varies with time. The state in which the rolling load is applied means a state in which the rolling load differs from that when the steel material is not in contact with the rolls. The state in which no rolling load is applied means a state in which the rolling load, which varies with time, returns to the level when the steel material is not in contact with the rolls. However, if the rolling load cannot be measured, an indicator such as the displacement of a roll or a torque value that varies when the steel material is in contact with the rolls may be used instead of the rolling load.

[0084] The piercing speed is preferably 2.0 m / second or less, more preferably 1.0 m / second or less, and still more preferably 0.5 m / second or less. It is considered that, as the piercing speed decreases, the average ferrite filling factor decreases. Therefore, the lower limit of the piercing speed is not specified. However, if the piercing speed is extremely small, the manufacturing efficiency deteriorates. Therefore, the piercing speed is preferably 0.05 m / second or more, more preferably 0.10 m / second or more, and still more preferably 0.2 m / second or more.

[0085] When hot working is used for the pipe making, cooling treatment may be performed after the pipe making. No particular limitation is imposed on the cooling treatment, and the cooling treatment may be performed under any conditions. For example, it is preferable that the cooling treatment includes, after the hot working, cooling the steel pipe until the surface temperature of the steel pipe reaches room temperature at an average cooling rate equal to or more than that of natural cooling. The average cooling rate equal to or more than that of natural cooling means 0.01°C / second or more.[Quenching treatment and tempering treatment]

[0086] Next, the seamless steel pipe obtained is subjected to heat treatment including quenching treatment and tempering treatment under specific conditions. The quenching treatment conditions and the tempering treatment conditions will next be described.[Quenching treatment]

[0087] First, the seamless steel pipe is heated to a quenching temperature of 850 to 1150°C, and the heated seamless steel pipe is cooled to a cooling stop temperature of 50°C or lower at an average cooling rate of 0.01°C / second or more.Quenching temperature: 850 to 1150°C

[0088] If the heating temperature (i.e., the quenching temperature) in the quenching treatment is lower 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 obtained. Therefore, the quenching temperature is set to 850°C or higher. The quenching temperature is preferably 900°C or higher. If the quenching temperature is higher than 1150°C, the crystal grains coarsen, and the low-temperature toughness thereby deteriorates. Therefore, the quenching temperature is 1150°C or lower. The quenching temperature is preferably 1100°C or lower.

[0089] In the quenching treatment, after the seamless steel pipe is heated to the quenching temperature described above, soaking treatment in which the seamless steel pipe is held at the quenching temperature may be performed. By performing the soaking treatment, the temperature of the seamless steel pipe can be made uniform in the wall thickness direction, and variations in quality can be reduced. No particular limitation is imposed on the time for which the seamless steel pipe is held at the quenching temperature (i.e., the soaking time), but the soaking time is preferably 5 to 30 minutes.Average cooling rate: 0.01°C / second or more

[0090] If the average cooling rate in the quenching treatment is less than 0.01°C / second, the desired microstructure cannot be obtained. Therefore, the average cooling rate is set to 0.01°C / second or more. The average cooling rate is preferably 1.0°C / second or more, more preferably 5.0°C / second or more, and still more preferably 10.0°C / second or more.

[0091] No particular limitation is imposed on the cooling, and any method can be used. For example, the cooling is performed preferably by at least one of natural cooling and water cooling and performed more preferably by water cooling.Cooling stop temperature: 50°C or lower

[0092] If the cooling stop temperature is higher than 50°C, the desired microstructure cannot be obtained. If the cooling stop temperature is high, the transformation from austenite to martensite is insufficient, so that the fraction of retained austenite becomes excessively large. Therefore, the cooling stop temperature in the quenching treatment is set to 50°C or lower. The cooling stop temperature is the surface temperature of the seamless steel pipe.[Tempering treatment]

[0093] Next, the tempering treatment is performed in which the seamless steel pipe subjected to the quenching treatment is heated to a tempering temperature of 500 to 650°C.Tempering temperature: 500 to 650°C

[0094] If the tempering temperature is lower than 500°C, a sufficient tempering effect cannot be obtained, and therefore the low-temperature toughness deteriorates. Therefore, the tempering temperature is set to 500°C or higher. The tempering temperature is preferably 520°C or higher. If the tempering temperature is higher than 650°C, large amounts of intermetallic compounds are precipitated, and good low-temperature toughness is not obtained. Therefore, the tempering temperature is set to 650°C or lower. The tempering temperature is preferably 630°C or lower.

[0095] In the tempering treatment, the seamless steel pipe is heated to the tempering temperature and then may be held at the tempering temperature. No particular limitation is imposed on the time for which the seamless steel pipe is held at the tempering temperature (i.e., the holding time). From the viewpoint of making the temperature uniform in the wall thickness direction to prevent variations in quality, the holding time is preferably 5 to 90 minutes.

[0096] After the tempering treatment, the seamless steel pipe may be left to cool.

[0097] By performing the quenching treatment and the tempering treatment, the stainless steel seamless pipe obtained can have the strength described above and also have good stress corrosion cracking resistance. Moreover, the stainless steel seamless pipe of the invention can have good low-temperature toughness.EXAMPLES

[0098] The present invention will be further described based on Examples. However, the invention is not limited to the following Examples.

[0099] First, seamless steel pipes were produced by the following procedure using steel materials having chemical compositions shown in Table 1.

[0100] Specifically, a steel material was produced by casting using molten steel having a chemical composition shown in Table 1. Then the steel material is heated and subjected to hot working using a model seamless rolling mill to produce a seamless steel pipe having an outer diameter of 177.8 mm × a wall thickness of 16.0 mm, and the seamless steel pipe was cooled by natural cooling. In this case, the heating temperature of the steel material before the hot working was set to 1250°C. A piercing speed shown in Table 2 was used.

[0101] Next, the seamless steel pipe obtained was subjected to the quenching treatment and the tempering treatment under the following conditions to obtain a stainless steel seamless pipe.[Quenching treatment]

[0102] The seamless steel pipe obtained was subjected to the quenching treatment under conditions shown in Table 2. Specifically, the seamless steel pipe was heated to a quenching temperature shown in Table 2 and held at the quenching temperature for a soaking time shown in Table 2. Next, the seamless steel pipe was cooled to a cooling stop temperature of 5°C. The cooling was performed by water cooling. In the water cooling, the average cooling rate from the time when the seamless steel pipe was put into water until the temperature of the seamless steel pipe reached 50°C or lower was 20°C / second.[Tempering treatment]

[0103] Then the cooled seamless steel pipe was heated to a tempering temperature shown in Table 2 and held at the tempering temperature for a holding time shown in Table 2. Then the seamless steel pipe was cooled by natural cooling (i.e., left to cool). The average cooling rate in the natural cooling was 0.04°C / second.

[0104] A test specimen was cut from the obtained stainless steel seamless pipe and subjected to (1) microstructure observation, (2) a tensile test, and (3) a stress corrosion cracking test by methods described below.(1) Microstructure observation

[0105] The stainless steel seamless pipe obtained was used to measure the volume fraction of each phase by the method described above. The amount of the martensite phase was determined as "100% - the volume fraction (%) of the ferrite phase - the volume fraction (%) of the retained austenite phase." In the "Microstructure (volume fraction)" column in Table 2, the ferrite phase is denoted as "F," and the martensite phase is denoted as "M." The retained austenite phase is denoted as "γ."

[0106] The average ferrite filling factor was computed using the method described above.(2) Tensile test

[0107] An arc-shaped test piece for a tensile test was cut from the obtained stainless steel seamless pipe according to the specifications of API (American Petroleum Institute)-5CT such that the pipe axis direction coincided with a tensile direction, and the tensile test was performed to determine the yield strength (YS). When the yield strength YS was 758 MPa or more, the stainless steel seamless pipe was considered to have high strength and rated pass. When the yield strength YS was less than 758 MPa, the stainless steel seamless pipe was rated fail.(3) Stress corrosion cracking test

[0108] To evaluate the stress corrosion cracking resistance, the following test was performed.

[0109] The stainless steel seamless pipe obtained was machined to produce a test specimen having a thickness of 5 mm × a width of 15 mm × and a length of 115 mm, and then a four-point bending test was performed. The four-point bending test was performed as follows. NaHCO 3 was added to a 20% by mass aqueous NaCl solution such that the pH was 4.5, and the prepared solution was held in an autoclave (solution temperature: 200°C, a 50 atm CO 2 -0.01 atm H 2 S gas atmosphere). The specimen was immersed in the solution, and the immersion time was set to 30 days (i.e., 720 hours). The load stress was set equal to the yield stress at 200°C. After the test, corrosion products were removed, and the surface of the test specimen was observed to check the presence of cracking. When no cracking was found, the stainless steel seamless pipe was rated pass. When cracking was found, the stainless steel seamless pipe was rated fail. In the "Presence of cracking" column in Table 2, the symbol "o" represents "pass," and the symbol "×" represents "fail."

[0110] The weight of the corrosion test specimen with the corrosion products removed was measured, and the weight of the test specimen measured prior to the corrosion test was subtracted to determine the amount of reduction in weight by the corrosion test. The amount of reduction in weight was divided by the surface area of the test specimen used and the immersion time to thereby obtain the amount of reduction in weight per unit time and unit area. Then the amount of reduction in weight per unit time and unit area was divided by the density of the steel to convert it to the depth of corrosion per unit time and unit area. The thus-obtained depth of corrosion per unit time and unit area (mm / year) was used as the rate of corrosion. When the corrosion rate was 0.10 mm / year or less, the stainless steel seamless pipe was rated pass. When the corrosion rate was more than 0.10 mm / year, the stainless steel seamless pipe was rated fail.

[0111] The results obtained are shown in Table 2. As can be seen from the results shown in Table 2, all the stainless steel seamless pipes that meet the conditions of the invention have a high strength, i.e., a yield strength of 758 MPa or more, and good stress corrosion cracking resistance. Therefore, the stainless steel seamless pipe of the invention can be very suitably used for various applications including oil well steel pipes. [Table 1]Steel typeChemical composition (% by mass) *RemarksCSiMnPSCrMoCuNiVAlNOOtherA0.0150.300.380.0160.000817.352.842.674.410.0410.0380.0640.002-Inventive steelB0.0190.350.370.0140.000917.042.272.904.290.0640.0390.0150.002-Inventive steelC0.0280.260.310.0150.001216.542.452.634.090.0690.0400.0370.003-Inventive steelD0.0170.380.360.0160.001017.392.012.344.520.0570.0390.0340.002Nb: 0.07Inventive steelE0.0110.210.390.0160.001116.652.492.164.460.0420.0440.0700.001W: 0.8, Co: 0.05Inventive steelF0.0100.290.360.0170.000817.402.782.604.580.0420.0430.0660.001Ca: 0.003, Sb: 0.008Inventive steelG0.0260.210.350.0140.001017.302.352.894.210.0580.0430.0160.003Ti: 0.002, Sn: 0.019Inventive steelH0.0250.380.270.0140.001217.432.272.294.480.0560.0440.0690.001Ta: 0.14, Zr: 0.22, Mg: 0.004, B: 0.008Inventive steelI0.0120.340.320.0130.001014.822.262.164.750.0620.0440.0630.002-Comparative steelJ0.0250.210.290.0170.001218.542.452.883.950.0360.0390.0530.001-Comparative steelK0.0220.210.380.0150.000816.021.212.614.160.0520.0390.0440.002-Comparative steelL0.0110.380.240.0140.000916.934.912.764.280.0640.0400.0220.001-Comparative steelM0.0230.300.360.0160.001116.862.810.314.050.0640.0440.0520.001-Comparative steelN0.0250.360.330.0170.001017.692.773.834.070.0560.0400.0240.003-Comparative steelO0.0220.360.260.0150.001115.952.822.883.140.0500.0410.0230.001-Comparative steelP0.0250.300.250.0170.001016.242.592.315.670.0460.0380.0680.001-Comparative steel* The balance is Fe and incidental impurities. [Table 2] No.Steel typeManufacturing conditionsMeasurement resultsRemarksPiercing stepQuenching treatmentTempering treatmentMicrostructure (volume fraction)Ferrite filling factorStrengthStress corrosion cracking resistancePiercing speed (m / second)Quenching temperature (°C)Soaking time (minute)Tempering temperature (°C)Holding time (minute)M (%)F (%)γ (%)Yield strength YS (MPa)Presence of crackingCorrosion rate (mm / year)1A0.329302057530732250.53904○0.06Inventive Example2A2.989302057530692560.77900○0.06Inventive Example3A3.479302057530722350.92923×0.04Comparative Example4A0.7593020625304425310.57798○0.06Inventive Example5B0.519302057530692560.55899○0.05Inventive Example6C0.519302057530722350.55906○0.06Inventive Example7D0.6293020575306720130.56902○0.05Inventive Example8E1.0193020575305927140.59925○0.04Inventive Example9F1.2093020575305031190.61907○0.04Inventive Example10G0.7993020575306718150.57890○0.04Inventive Example11H1.0793020575306316210.60898○0.04Inventive Example12I1.169302057530841240.60910×0.19Comparative Example13J0.609302057530356410.55738○0.05Comparative Example14K0.739302057530861130.57926×0.18Comparative Example15L0.959302057530356410.59713○0.06Comparative Example16M0.279302057530583840.52912×0.15Comparative Example17N0.8693020575306414220.58907×0.05Comparative Example18O0.939302057530375850.58725○0.05Comparative Example19P1.0593020575306512230.59908×0.06Comparative Example20A1.508602057520652870.64910○0.06Inventive Example21A1.7211402057520613270.65914○0.06Inventive Example22A0.879302051020692650.58931○0.05Inventive Example23A0.9593020640204629250.59868○0.05Inventive Example

Claims

1. A stainless steel seamless pipe having a chemical composition comprising, in mass %, C :0.06% or less, Si: 1.0% or less, Mn: 0.01 to 1.0%, P: 0.05% or less, S: 0.005% or less, Cr: 15.2 to 18.0%, Mo: 1.5 to 4.3%, Cu: 0.5 to 3.5%, Ni: 3.5 to 5.2%, V :0.5% or less, Al: 0.10% or less, N: 0.10% or less, and O: 0.010% or less, with the balance being Fe and incidental impurities, wherein the stainless steel seamless pipe has a microstructure including a martensite phase at a volume fraction of 40% or more, a ferrite phase at a volume fraction of 15 to 55%, and a retained austenite phase at a volume fraction of 40% or less, with an average of ferrite filling factors being 0.80 or less, wherein the stainless steel seamless pipe has a yield strength of 758 MPa or more, and wherein each ferrite filling factor is a value defined by the following formula for one ferrite grain:

2. The stainless steel seamless pipe according to claim 1, wherein the chemical composition further comprises, in mass %, one or two or more selected from the group consisting of Nb: 0.3% or less, Ti: 0.3% or less, W: 2.0% or less, Co: 1.0% or less, B: 0.010% or less, Ta: 0.3% or less, Zr: 0.3% or less, Ca: 0.010% or less, REMs: 0.3% or less, Mg: 0.01% or less, Sn: 1.0% or less, and Sb: 1.0% or less.

3. The stainless steel seamless pipe according to claim 1, wherein the volume fraction of the martensite phase is 45% or more, wherein the volume fraction of the ferrite phase is 15 to 55%, wherein the volume fraction of the retained austenite phase is 30% or less, and wherein the yield strength is 862 MPa or more.

4. The stainless steel seamless pipe according to claim 2, wherein the volume fraction of the martensite phase is 45% or more, wherein the volume fraction of the ferrite phase is 15 to 55%, wherein the volume fraction of the retained austenite phase is 30% or less, and wherein the yield strength is 862 MPa or more.

5. A method for manufacturing the stainless steel seamless pipe according to any one of claims 1 to 4, the method comprising: when a steel material having the chemical composition is subjected to pipe making to produce a seamless steel pipe, performing a piercing step at a piercing speed of 3.3 m / second or less; then performing quenching treatment including heating the seamless steel pipe to a quenching temperature of 850 to 1150°C and then cooling the seamless steel pipe at a cooling rate of 0.01°C / second or more to a cooling stop temperature at which a surface temperature of the steel pipe is 50°C or lower; and then performing tempering treatment including heating the seamless steel pipe to a tempering temperature of 500 to 650°C.

Images