Stainless steel seamless pipe and method for producing same

Abstract

The purpose of the present invention is to provide a stainless steel seamless pipe having high strength to ensure a yield strength of 758 MPa (110 ksi) or more, and also having excellent stress corrosion cracking resistance and local corrosion resistance.　The stainless steel seamless pipe has a specific composition, and a microstructure in which, in volume percentage, a martensite phase is 40% to 85%, a ferrite phase is 15% to 55%, and a residual austenite phase is 40% or less. In a cross section perpendicular to the pipe axis direction of the pipe, the variation coefficient of Cr is 6.0 or more, and the yield strength is 758 MPa or more.

Description

Stainless steel seamless pipe and its manufacturing method

[0001] The present invention relates to a stainless steel seamless pipe suitable for use in oil and gas wells (hereinafter simply referred to as oil wells) and carbon dioxide storage wells. ２ ), chloride ions (Cl － ), and in severe corrosive environments containing hydrogen sulfide (H ２ S), sulfur oxide (SO Ｘ ), nitric oxide (NO Ｘ ), oxygen (O ２ ), hydrogen (H ２ This invention relates to a stainless steel seamless pipe having improved corrosion resistance in environments containing uranium dioxide.

[0002] In the prior art, Patent Documents 1 to 4 disclose methods for controlling the average composition values ​​and microstructure of the entire bulk of the steel as methods for producing seamless steel pipes with excellent corrosion resistance.

[0003] Furthermore, Patent Document 5 also discloses a method of varying the reduction rate during continuous casting depending on the solid fraction at the center of the thickness of the cast slab in order to suppress center segregation of steel.

[0004] International Publication No. 2010 / 058304 Japanese Patent Application Laid-Open No. 2013-249516 International Publication No. 2017 / 022374 International Publication No. 2020 / 013197 Japanese Patent Application Laid-Open No. 3-90263

[0005] The prior art proposed in Patent Documents 1 to 4 discloses techniques for obtaining 17Cr stainless steels with superior corrosion resistance compared to 13Cr steels. However, its corrosion resistance, particularly its localized corrosion resistance, is not necessarily sufficient, and it has become clear that there are environments in which it cannot be used. Furthermore, in the case of seamless steel pipes through which corrosive fluids pass, the inner surface of the pipe is exposed to a corrosive environment, and this inner surface corresponds to the center of the billet material. Therefore, there is a problem in that central segregation of the material causes variations in the composition of the inner surface of the pipe after rolling, and the uneven distribution of corrosion-resistant components makes it susceptible to localized corrosion.

[0006] Furthermore, according to the technology described in Patent Document 5, there is a method in which the reduction rate during continuous casting is changed depending on the solid fraction at the center of the thickness of the cast slab. However, even when the reduction rate is controlled within this range, there is a problem in that center segregation remains in the form of particles.

[0007] An object of the present invention is to solve the problems of the prior art and to provide a stainless steel seamless pipe that has high strength, with a yield strength of 758 MPa (110 ksi) or more, and excellent resistance to stress corrosion cracking and localized corrosion.

[0008] Here, "excellent stress corrosion cracking resistance" means that the resistance to stress corrosion cracking is high enough to withstand a 20 mass % NaCl aqueous solution (liquid temperature: 200°C, CO 2 at 50 atmospheres) held in an autoclave. ２ -0.01 atmospheres of H ２ S gas, 0.005 atmospheres of SO ２ NaHCO 3 was added to the atmosphere to adjust the pH to 4.5. ３ A test piece is subjected to a four-point bending test with a stress equivalent to the yield stress at 200°C and immersed in the solution containing the compound for 720 hours. The test is conducted in accordance with NACE TM0316 and the test piece shows no signs of stress corrosion cracking.

[0009] Here, the term "excellent local corrosion resistance" refers to a corrosion resistance obtained by subjecting a specimen to a 20 mass % NaCl aqueous solution (liquid temperature: 200°C, CO pressure: 50 atmospheres) held in an autoclave. ２ -0.01 atmospheres of H ２ S gas, 0.005 atmospheres of SO ２ NaHCO 3 was added to the atmosphere to adjust the pH to 4.5. ３ A test piece is immersed in a solution containing the compound for 720 hours, and after the test in accordance with ASTM G 48, no localized corrosion is observed on the test piece, and the corrosion rate (mm / year) on the test piece surface is 0.127 mm / year or less.

[0010] To achieve the above-mentioned objective, the present inventors conducted extensive research into various factors that affect the properties of stainless steel, particularly its localized corrosion resistance. As a result, they discovered that by optimizing the conditions for producing a slab in a continuous casting process, which is the raw material for producing seamless steel pipes, and controlling the microstructural morphology and homogeneity of the steel, excellent localized corrosion resistance can be achieved without the need for large amounts of corrosion-resistant elements. Specifically, the inventors discovered that localized corrosion occurs when the chemical composition of the material surface is nonuniform, with anodic reactions occurring in areas with low concentrations of corrosion-resistant elements such as Cr, Mo, Ni, and W, and cathodic reactions occurring separately in areas with high concentrations of corrosion-resistant elements. This phenomenon is particularly pronounced for Cr, and they discovered that localized corrosion resistance can be improved by reducing the variation in the Cr concentration on the material surface. Specifically, we found that the smaller the variation in Cr concentration on the steel pipe surface relative to the average Cr concentration throughout the steel pipe, the fewer regions with high and low Cr concentrations, making anodic and cathodic reactions less likely to occur and suppressing localized corrosion. We also found that there is a high correlation between the coefficient of variation, calculated by dividing the average Cr concentration of a steel pipe by the standard deviation of the Cr concentration, and the localized corrosion inhibition properties of the steel pipe. The specific definition of the coefficient of variation is described in the present specification. The homogeneity of a steel can be evaluated by analyzing Cr using an electron probe microanalyzer (EPMA) over a 60-μm square area of ​​the structure on a surface of the steel pipe, including the circumferential and thickness directions, and then calculating the coefficient of variation, which is the average of the X-ray counts at each measurement point over the entire measurement area divided by the standard deviation. We found that a Cr coefficient of variation of 6.0 or more can achieve the desired excellent localized corrosion resistance.

[0011] Furthermore, a stainless steel seamless pipe (seamless steel pipe) with such a homogeneous distribution of corrosion-resistant elements can be obtained by, for example, reducing segregation, particularly center segregation, in the billet, which is the steel material, thereby improving the homogeneity of the inner wall thickness of the slab (corresponding to the inner diameter side of the steel pipe after pipemaking). Specifically, this can be achieved by rolling down the slab during continuous casting using rolls installed on both sides to push out the central liquid portion. Generally, a faster rolling down speed of the slab increases the effectiveness of pushing out the liquid portion. However, the inventors have found that if the rolling down speed is too fast, the liquid portion remains in the center of the slab, resulting in significant center segregation. Furthermore, if the rolling down speed is too slow, the effect of pushing out the liquid portion is insufficient. Therefore, they have found that controlling the rolling down speed within a predetermined range can reduce the amount of liquid remaining inside the slab.

[0012] The present invention was completed based on these findings and further investigations. [1] A steel pipe having a composition, in mass%, of 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, O: 0.010% or less, with the balance being Fe and unavoidable impurities, and the microstructure has, in volume fraction, a martensite phase of 40% to 85% and a ferrite phase of 15% to 55% and a retained austenite phase of 40% or less, and in a cross section perpendicular to the axial direction of the steel pipe, % or less, Nb: 0.3% or less, Ti: 0.2% or less, W: 2.0% or less, Co: 1.0% or less, B: 0.01% or less, Ta: 0.3% or less, Zr: 0.3% or less, Ca: 0.01% or less, REM: 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 microstructure has, by volume, 45% to 85% of martensite phase, 15% to 55% of ferrite phase, and 30% or less of retained austenite phase. [4] A method for producing a stainless steel seamless pipe, comprising the steps of: continuously casting a slab having the chemical composition according to [1] or [2] above under conditions in which the reduction rate in the thickness direction of the slab before solidification of the interior of the slab is completed is 0.0005 mm / sec to 0.01 mm / sec to obtain a steel material; forming the steel material into a stainless steel seamless pipe; and subjecting the steel material to quenching and tempering treatments.

[0013] According to the present invention, it is possible to obtain a stainless steel seamless pipe that has high strength, such as a yield strength of 758 MPa (110 ksi) or more, excellent local corrosion resistance, and excellent stress corrosion cracking resistance.

[0014] 1A and 1B are a side view of a continuous casting facility for producing slabs, in which FIG. 1A shows a state in which a slab is rolled down before molten steel is completely solidified, and FIG. 1B shows a side view of a state in which a slab including a crater end is rolled down.

[0015] The present invention will be described in detail below.

[0016] [Component Composition] The stainless steel seamless pipe of the present invention has the above-mentioned component composition. First, the reasons for limiting the above-mentioned component composition will be explained. Hereinafter, unless otherwise specified, "mass %" will be simply written as "%".

[0017] C: 0.06% or less C is an element that is inevitably contained in the steelmaking process. If the C content exceeds 0.06%, corrosion resistance decreases. Therefore, the C content is set to 0.06% or less. The C content is preferably 0.05% or less, more preferably 0.04% or less, and even more preferably 0.03% or less. On the other hand, from the viewpoint of corrosion resistance, a lower C content is preferable, so the lower limit of the C content is not particularly limited. However, from the viewpoint of decarburization costs, the C content is preferably 0.002% or more, more preferably 0.003% or more, even more preferably 0.005% or more, and most preferably 0.01% or more.

[0018] Si: 1.0% or less Si is an element that acts as a deoxidizer. However, if the Si content exceeds 1.0%, hot workability and corrosion resistance decrease. Therefore, the Si content is set to 1.0% or less. It is preferably set to 0.7% or less, more preferably set to 0.6% or less, even more preferably set to 0.5% or less, and most preferably set to 0.4% or less. On the other hand, although the lower limit of the Si content is not particularly limited, from the viewpoint of enhancing the deoxidizing effect, the Si content is preferably set to 0.03% or more, more preferably set to 0.05% or more, even more preferably set to 0.1% or more, and most preferably set to 0.15% or more.

[0019] Mn: 0.01 to 1.0% Mn is an element that acts as a deoxidizer and desulfurizer and improves hot workability. To obtain the effect of deoxidizing and desulfurizing and to improve strength, the Mn content is set to 0.01% or more, preferably 0.03% or more, more preferably 0.05% or more, even more preferably 0.1% or more, and most preferably 0.15% or more. On the other hand, even if the Mn content exceeds 1.0%, the effect saturates. For this reason, the Mn content is set to 1.0% or less. Preferably, it is set to 0.8% or less, more preferably 0.7% or less, even more preferably 0.6% or less, and most preferably 0.5% or less.

[0020] P: 0.05% or less P is an element that reduces carbon dioxide corrosion resistance and SSC resistance. To obtain the desired corrosion resistance, the P content is set to 0.05% or less, preferably 0.04% or less, and more preferably 0.03% or less. On the other hand, since it is preferable to reduce the P content as much as possible, the lower limit of the P content is not particularly limited, and may be 0%, more preferably 0.0001% or more, and even more preferably 0.001% or more.

[0021] S: 0.005% or less S is an element that significantly reduces hot workability and hinders stable operation of the hot pipe-making process. Furthermore, S exists as sulfide-based inclusions in steel and reduces corrosion resistance. Therefore, the S content is set to 0.005% or less. It is preferably set to 0.004% or less, more preferably 0.003% or less, and even more preferably 0.002% or less. On the other hand, since it is preferable to reduce the S content as much as possible, the lower limit of the S content is not particularly limited, and may be 0%, more preferably 0.0001% or more, and even more preferably 0.0002% or more.

[0022] Cr: 15.2 to 18.0% Cr is an element that forms a protective film on the surface of the steel pipe, contributing to improved corrosion resistance. If the Cr content is less than 15.2%, the desired carbon dioxide corrosion resistance and stress corrosion cracking resistance cannot be ensured. Therefore, the Cr content is set to 15.2% or more. Preferably, it is set to 15.5% or more, more preferably 16.0% or more, and even more preferably 16.3% or more. On the other hand, if the Cr content exceeds 18.0%, the ferrite fraction becomes too high and the desired strength cannot be ensured. Therefore, the Cr content is set to 18.0% or less. Preferably, it is set to 17.5% or less, more preferably 17.2% or less, and even more preferably 17.0% or less.

[0023] Mo: 1.5 to 4.3% Mo stabilizes the protective film on the steel pipe surface and prevents Cl － It increases resistance to pitting corrosion due to low pH or low Cr content, thereby improving corrosion resistance. To obtain the desired corrosion resistance, the Mo content is set to 1.5% or more. Preferably, it is set to 1.8% or more, more preferably 2.0% or more, even more preferably 2.2% or more, and most preferably 2.3% or more. On the other hand, if the Mo content exceeds 4.3%, the ferrite fraction becomes too high and the desired strength cannot be ensured. Therefore, the Mo content is set to 4.3% or less. Preferably, it is set to 4.0% or less, more preferably 3.5% or less, even more preferably 3.0% or less, and most preferably 2.9% or less.

[0024] Cu: 0.5 to 3.5% Cu strengthens the protective coating on the steel pipe surface and has the effect of improving corrosion resistance, particularly carbon dioxide corrosion resistance. To obtain the desired strength and corrosion resistance, particularly carbon dioxide corrosion resistance, the Cu content is set to 0.5% or more. Preferably, it is set to 0.8% or more, more preferably 1.5% or more, and even more preferably 2.0% or more. On the other hand, if the Cu content is too high, the hot workability of the steel will deteriorate, causing external surface defects during pipe formation, and the desired stress corrosion cracking resistance will not be achieved. Therefore, the Cu content is set to 3.5% or less. Preferably, it is set to 3.2% or less, more preferably 3.0% or less, even more preferably 2.7% or less, and most preferably 2.5% or less.

[0025] Ni: 3.5 to 5.2% Ni contributes to ensuring strength by maintaining the austenite phase fraction at high temperatures and obtaining the necessary amount of martensite phase. To obtain the desired strength, the Ni content is set to 3.5% or more. Preferably, it is set to 3.8% or more, more preferably 4.0% or more, even more preferably 4.3% or more, and most preferably 4.4% or more. On the other hand, if the Ni content exceeds 5.2%, the austenite fraction becomes too large, reducing the hot workability of the steel and making it more susceptible to defects during hot rolling, and the desired stress corrosion cracking resistance may not necessarily be obtained. For this reason, the Ni content is set to 5.2% or less. Preferably, it is set to 5.0% or less, more preferably 4.9% or less, and even more preferably 4.8% or less.

[0026] V: 0.5% or less V is an element that increases strength without impairing toughness by forming carbonitrides. Furthermore, because V readily forms carbonitrides, it prevents corrosion-resistant elements such as Cr from forming carbonitrides, thereby reducing the effective amount of carbonitrides that contribute to corrosion resistance, thereby achieving the desired corrosion resistance. However, even if V is contained in an amount exceeding 0.5%, the effect saturates. For this reason, the V content is set to 0.5% or less. It is preferably set to 0.4% or less, more preferably 0.3% or less, even more preferably 0.2% or less, and most preferably 0.1% or less. On the other hand, the lower limit of the V content is not particularly limited, but is preferably set to 0.01% or more, and more preferably 0.03% or more.

[0027] Al: 0.10% or less Al is an element that acts as a deoxidizer. However, if the Al content exceeds 0.10%, corrosion resistance decreases. Therefore, the Al content is set to 0.10% or less, preferably 0.07% or less, and more preferably 0.05% or less. On the other hand, although there are no particular restrictions on the lower limit of the Al content, from the viewpoint of enhancing the deoxidizing effect, the Al content is preferably set to 0.005% or more, more preferably 0.01% or more, and even more preferably 0.015% or more.

[0028] N: 0.10% or less N is an element that is inevitably contained during the steelmaking process, but it also increases the strength of steel. However, if the N content exceeds 0.10%, excessive nitride formation occurs, resulting in reduced corrosion resistance. For this reason, the N content is set to 0.10% or less. Preferably, it is set to 0.07% or less, more preferably 0.05% or less, and even more preferably 0.03% or less. On the other hand, although there is no particular lower limit for the N content, excessive reduction in the N content leads to increased steelmaking costs. Therefore, the N content is preferably set to 0.002% or more, more preferably 0.003% or more, and even more preferably 0.005% or more.

[0029] O: 0.010% or less O (oxygen) exists as an oxide in steel and has an adverse effect on various properties. For this reason, in the present invention, it is desirable to reduce the content as much as possible. In particular, if the O content exceeds 0.010%, hot workability and corrosion resistance decrease. For this reason, the O content is set to 0.010% or less. There is no particular lower limit, but the O content is preferably 0.0001% or more, more preferably 0.0002% or more, and even more preferably 0.001% or more.

[0030] The above elements are essential components of the stainless steel seamless pipe of the present invention, and the stainless steel seamless pipe in another embodiment of the present invention may further optionally contain one or more elements selected from Nb, Ti, W, Co, B, Ta, Zr, Ca, REM, Mg, Sn, and Sb. Nb, Ti, W, Co, B, Ta, Zr, Ca, REM, Mg, Sn, and Sb are steel components that can be optionally contained, and the content of these elements may be 0%.

[0031] Nb: 0.3% or less Nb is an element that forms carbonitrides and improves strength and corrosion resistance, and can be contained as needed. However, since Nb carbonitrides tend to reduce low-temperature toughness, when Nb is contained, the Nb content is set to 0.3% or less, preferably 0.2% or less, and more preferably 0.1% or less. On the other hand, the lower limit is not particularly limited, but is preferably 0.01% or more, and more preferably 0.02% or more.

[0032] Ti: 0.2% or less Ti is an element that increases strength and corrosion resistance and can be contained as needed. However, if Ti is contained in an amount exceeding 0.2%, low-temperature toughness decreases. Therefore, when Ti is contained, the Ti content is set to 0.2% or less, preferably 0.18% or less, more preferably 0.15% or less, and even more preferably 0.1% or less. There is no particular lower limit, but it is preferably 0.00001% or more, more preferably 0.00002% or more, and even more preferably 0.00003% or more.

[0033] W: 2.0% or less W is an element that contributes to improving the strength of steel and stabilizes the protective coating on the steel pipe surface, thereby enhancing corrosion resistance. However, if the W content exceeds 2.0%, the ferrite fraction becomes too high, making it impossible to ensure the desired strength. Therefore, when W is contained, the W content is set to 2.0% or less. Preferably, it is set to 1.5% or less, more preferably 1.2% or less, even more preferably 1.15% or less, and most preferably 1.1% or less. On the other hand, the lower limit of the W content is not particularly limited, but is preferably 0.1% or more, more preferably 0.3% or more, even more preferably 0.5% or more, and most preferably 0.6% or more.

[0034] Co: 1.0% or less Co is an element that improves corrosion resistance. However, even if Co is contained in an amount exceeding 1.0%, the effect saturates. Therefore, when Co is contained, the Co content is set to 1.0% or less. It is preferably set to 0.5% or less, more preferably set to 0.4% or less, even more preferably set to 0.3% or less, and most preferably set to 0.1% or less. There is no particular lower limit, but it is preferably set to 0.00001% or more, more preferably set to 0.00002% or more, and even more preferably set to 0.00003% or more.

[0035] B: 0.01% or less B is an element that contributes to improving hot workability and has the effect of suppressing the occurrence of cracks and breakages during the pipe-making process. However, if the B content exceeds 0.01%, low-temperature toughness decreases. Therefore, when B is contained, the B content is set to 0.01% or less. Preferably, it is set to 0.008% or less, more preferably 0.007% or less, even more preferably 0.006% or less, and most preferably 0.005% or less. On the other hand, the lower limit of the B content is not particularly limited, but is preferably set to 0.0005% or more, more preferably 0.001% or more, and even more preferably 0.0015% or more.

[0036] Ta: 0.3% or less Ta is an element that increases strength and improves corrosion resistance, and can be contained as needed. However, even if it is contained in an amount exceeding 0.3%, the effect is saturated. Therefore, when Ta is contained, the Ta content is set to 0.3% or less. The Ta content is preferably set to 0.27% or less, more preferably 0.25% or less, even more preferably 0.22% or less, and most preferably 0.2% or less. On the other hand, the lower limit of the Ta content is not particularly limited, but is preferably set to 0.001% or more, more preferably 0.01% or more, and even more preferably 0.1% or more.

[0037] Zr: 0.3% or less Zr is an element that increases strength and can be contained as needed. Zr also has the effect of improving SSC resistance. However, if the Zr content exceeds 0.3%, the effect saturates. Therefore, when Zr is contained, the Zr content is set to 0.3% or less. The Zr content is preferably set to 0.27% or less, more preferably set to 0.25% or less, and even more preferably set to 0.22% or less. On the other hand, the lower limit of the Zr content is not particularly limited, but is preferably set to 0.0005% or more, more preferably set to 0.0008% or more, even more preferably set to 0.0010% or more, and most preferably set to 0.01% or more.

[0038] Ca: 0.01% or less Ca is an element that improves hot workability by controlling the morphology of sulfides and has the effect of suppressing the occurrence of cracks and fractures during the pipe-making process. In order to obtain this effect, when Ca is contained, it is preferably 0.001% or more. More preferably, it is 0.002% or more, and even more preferably, it is 0.003% or more. On the other hand, even if Ca is contained in an amount exceeding 0.01%, the effect saturates and an effect commensurate with the content cannot be expected. Therefore, when Ca is contained, the Ca content is 0.01% or less. The Ca content is preferably 0.008% or less, more preferably 0.007% or less, and even more preferably 0.005% or less.

[0039] REM: 0.3% or less. REM (rare earth metal) is an element that contributes to improving stress corrosion cracking resistance by controlling the morphology of sulfides and can be contained as needed. However, if the content exceeds 0.3%, the effect saturates and no effect commensurate with the content can be expected. Therefore, when REM is contained, the REM content is set to 0.3% or less. Furthermore, the REM content is preferably set to 0.2% or less. On the other hand, although there is no particular restriction on the lower limit of the REM content, it is preferably set to 0.0005% or more. The REM content is more preferably set to 0.1% or more. In the present invention, REM refers to scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanides from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. The chemical composition of the stainless steel seamless pipe of the present invention can optionally contain at least one of the above REMs. The REM content in the present invention refers to the total content of the above elements.

[0040] Mg: 0.01% or less Mg is an element that improves corrosion resistance and can be contained as needed. However, even if it is contained in an amount exceeding 0.01%, the effect saturates and no effect commensurate with the content can be expected. Therefore, when Mg is contained, the Mg content is set to 0.01% or less. The Mg content is preferably set to 0.008% or less, more preferably 0.007% or less, and even more preferably 0.006% or less. On the other hand, the lower limit of the Mg content is not particularly limited, but is preferably set to 0.0005% or more, and more preferably 0.001% or more.

[0041] Sn: 1.0% or less Sn is an element that improves corrosion resistance and can be contained as needed. However, even if it is contained in an amount exceeding 1.0%, the effect saturates and no effect commensurate with the content can be expected. Therefore, when Sn is contained, the Sn content is set to 1.0% or less. The Sn content is preferably set to 0.9% or less, more preferably set to 0.8% or less, and even more preferably set to 0.7% or less. On the other hand, the lower limit of the Sn content is not particularly limited, but is preferably set to 0.001% or more, more preferably set to 0.002% or more, and even more preferably set to 0.003% or more.

[0042] Sb: 1.0% or less Sb is an element that improves corrosion resistance and can be contained as needed. However, even if it is contained in an amount exceeding 1.0%, the effect saturates and an effect commensurate with the content cannot be expected. Therefore, when Sb is contained, the Sb content is set to 1.0% or less. The Sb content is preferably set to 0.9% or less, more preferably set to 0.8% or less, and even more preferably set to 0.7% or less. On the other hand, the lower limit of the Sb content is not particularly limited, but is preferably set to 0.001% or more, more preferably set to 0.002% or more, and even more preferably set to 0.003% or more.

[0043] A stainless steel seamless pipe according to one embodiment of the present invention has a composition containing the above components with the balance being Fe and unavoidable impurities.

[0044] [Microstructure] Next, the reasons for limiting the microstructure (structure) of the stainless steel seamless pipe of the present invention will be explained.

[0045] In one embodiment of the present invention, the stainless steel seamless pipe has a martensite phase of 40% to 85% by volume, a ferrite phase of 15% to 55% by volume, and a retained austenite phase of 40% or less.

[0046] Martensite phase: 40% or more and 85% or less in volume fraction If the martensite phase is less than 40% in volume fraction, it may not be possible to ensure the desired strength. Therefore, the martensite phase is set to a volume fraction of 40% or more. Preferably, it is set to 45% or more, more preferably 50% or more, even more preferably 60% or more, and most preferably 65% ​​or more. On the other hand, the upper limit of the volume fraction of the martensite phase is set to 85% or less. It is preferably set to 84% or less, more preferably 82% or less, even more preferably 80% or less, and most preferably 78% or less.

[0047] Ferrite phase: 15% or more and 55% or less by volume By containing the ferrite phase, the progression of stress corrosion cracking can be suppressed, and excellent corrosion resistance can be obtained. However, if the ferrite phase exceeds 55% by volume, the desired strength cannot be ensured. Therefore, the ferrite phase is set to 55% or less by volume. Preferably, it is set to 50% or less, more preferably 45% or less, even more preferably 40% or less, and most preferably 38% or less. On the other hand, if the ferrite phase is less than 15% by volume, the desired stress corrosion cracking resistance cannot be obtained. Therefore, the ferrite phase is set to 15% or more by volume. Preferably, it is set to 18% or more, more preferably 20% or more, even more preferably 23% or more, and most preferably 25% or more.

[0048] Retained austenite phase: 40% or less by volume The presence of the retained austenite phase improves ductility and low-temperature toughness. However, if a large amount of austenite phase, exceeding 40% by volume, precipitates, the desired strength cannot be ensured. For this reason, the volume fraction of the retained austenite phase is set to 40% or less. Preferably, it is set to 30% or less, more preferably 28% or less, and even more preferably 25% or less. On the other hand, although there are no particular restrictions on the lower limit of the volume fraction of the retained austenite phase, the volume fraction of the retained austenite phase is preferably 3% or more, more preferably 5% or more, even more preferably 8% or more, and most preferably 10% or more.

[0049] Here, the volume fraction of each of the phases can be measured by the following method. First, a steel piece is cut out from a stainless steel seamless pipe so that a cross section perpendicular to the pipe axis direction (a cross section parallel to the longitudinal direction and wall thickness direction of the steel pipe) serves as the target surface, and a sample for microstructure observation is prepared by embedding the steel piece in resin and mirror polishing the surface. This surface is then immersed in a KOH solution (a mixture of 35 g of KOH and 100 g of pure water) at a voltage of 3 A / cm. ２ The specimen was then subjected to electrolytic corrosion at a current density of 0.015 g for 35 seconds, followed by corrosion with Virrella's reagent (a mixture of 2 g picric acid, 5 ml hydrochloric acid, and 50 ml ethanol, respectively) for 30 seconds. The microstructure of the specimen for microstructure observation was then imaged at 400x magnification using an optical microscope. An image measuring 300 μm in the longitudinal direction of the steel pipe and 200 μm in the wall thickness direction of the steel pipe was cut out from the obtained optical microscope photograph and analyzed using image analysis software (ImageJ 1.52p, National Institute of Health) to calculate the ferrite phase fraction (area fraction (%)). In the analysis, the Weka Trainable Segmentation function is used on the optical microscope photograph to extract the ferrite phase by using three bright ferrite regions and three dark martensite regions as training data, and automatically classifying the other regions using the Segmentation function. The area fraction of the ferrite phase extracted in this way is defined as the volume fraction (%) of the ferrite phase.

[0050] Next, an X-ray diffraction test piece taken from the stainless steel seamless pipe is ground and polished so that a cross section (C cross section) perpendicular to the pipe axis direction becomes the measurement surface, and the structural fraction of the retained austenite (γ) phase is measured using X-ray diffraction. Specifically, the volume fraction of the retained austenite phase is calculated from the integrated intensities of the austenite (220) plane and the ferrite (211) plane using the following formula: Vγ (%) = 100 / (1 + (IαRγ / IγRα)) where Vγ: volume fraction of the retained austenite phase, Iα: integrated intensity of the ferrite (211) plane, Iγ: integrated intensity of the austenite (220) plane, Rα: the crystallographic theoretically calculated value of α (34.15), and Rγ: the crystallographic theoretically calculated value of γ (22.33).

[0051] The remainder other than the ferrite phase and the residual γ phase determined by the above measurement method is defined as the volume fraction of the martensite phase. Note that the method for observing each of the above microstructures will also be described in detail in the examples below.

[0052] The microstructure of the stainless steel seamless pipe of the present invention is substantially composed of a martensite phase, a ferrite phase, and a retained austenite phase. That is, the microstructure may contain other microstructures as long as the effects of the present invention are not impaired. Examples of the other microstructures include intermetallic compounds and inclusions.

[0053] Coefficient of variation of Cr: 6.0 or more The stainless steel seamless pipe of the present invention has a microstructure in which the coefficient of variation of Cr is 6.0 or more. The coefficient of variation is explained below. First, to determine the X-ray count in a predetermined region, EPMA is used to analyze Cr in at least 10 fields of view within a 60 μm square area, with a cross section perpendicular to the axial direction of the steel pipe (a cross section parallel to the circumferential direction and wall thickness direction of the steel pipe) as the target surface. Since the inner surface of the steel pipe is the primary contact point with the corrosive fluid, the measurement position is near the inner surface of the steel pipe, and at least 20% of the measurement sample should include a region extending from the inner diameter side to one-third of the wall thickness. Next, the X-ray count obtained by measuring Cr using EPMA on the steel material is taken as the average value for the steel pipe. The coefficient of variation of Cr is calculated by calculating the average X-ray count for all measurement points in the predetermined region described above, and then dividing this average by the standard deviation of all measurement points. A large coefficient of variation results in a uniform concentration of corrosion-resistant elements on the material surface, improving local corrosion resistance. The coefficient of variation of Cr is preferably 6.5 or more, more preferably 7.0 or more, even more preferably 7.3 or more, and most preferably 7.5 or more. There is no particular upper limit, but the coefficient of variation of Cr is preferably 15.0 or less, more preferably 14.5 or less, even more preferably 14.2 or less, and most preferably 14.0 or less.

[0054] Furthermore, although there are no particular limitations on the coefficients of variation for Mo, Ni, and W, it is preferable that each of the coefficients of variation for Mo, Ni, and W be 3.0 or greater. More preferably, each is 3.3 or greater, and even more preferably, each is 3.5 or greater. Although there are no particular limitations on the upper limits, the coefficients of variation for Mo, Ni, and W are preferably 15.0 or less, 15.0 or less, and 14.0 or less, respectively; more preferably, 14.5 or less, 14.5 or less, and 13.5 or less, respectively; even more preferably, 14.0 or less, 14.0 or less, and 13.0 or less, respectively; and most preferably, 13.5 or less, 13.5 or less, and 12.5 or less, respectively. The upper limits of the coefficients of variation are set to the above values ​​because it is unavoidable in the manufacturing process to completely eliminate the variation in the elements. The method for determining each coefficient of variation is the same as for Cr. The coefficients of variation described in this invention are different from conventionally used coefficients of variation, and the coefficients of variation in this invention can be determined based on the details described in the Examples below. The problem of the present invention can be solved if the coefficient of variation of Cr, an element that improves corrosion resistance, is satisfied. However, because Cr, Mo, Ni, and W, which are elements that improve corrosion resistance, form a mixed layer at a concentration above a certain level and suppress corrosion, it is desirable to simultaneously satisfy the lower limits of the coefficients of variation of Mo, Ni, and W in addition to Cr, in order to further improve the properties.

[0055] The coefficient of variation of elements is greatly affected by the manufacturing conditions during continuous casting, particularly the solidification rate (reduction rate) of the slab before solidification, and therefore requires control of the conditions as described below.

[0056] Yield strength: 758 MPa or more The stainless steel seamless pipe of the present invention has a yield strength of 758 MPa or more. The yield strength is preferably 763 MPa or more, more preferably 768 MPa or more, even more preferably 773 MPa or more, and most preferably 778 MPa or more. There are no particular restrictions on the upper limit of the yield strength, but it is preferably 1034 MPa or less, more preferably 1029 MPa or less, even more preferably 1024 MPa or less, and most preferably 1019 MPa or less. The yield strength can be measured by the method described in the examples.

[0057] The stainless steel seamless pipe of the present invention can be used for any purpose without any particular limitation, but is particularly suitable for use in oil wells. ２ It can also be suitably used as a press-fit pipe.

[0058] [Manufacturing Method] Next, a preferred manufacturing method for the stainless steel seamless pipe of the present invention will be described.

[0059] The stainless steel seamless pipe of the present invention can be manufactured by obtaining a steel material from a slab by continuous casting, and then forging the steel material into a stainless steel seamless pipe. The stainless steel seamless pipe is then subjected to a quenching-tempering treatment under specific conditions.

[0060] The steel material is not particularly limited and any material can be used. A billet is typically used as the steel material. The steel material can be a material having the above-mentioned component composition.

[0061] [Continuous Casting] The steel material can be produced by continuous casting. For example, molten steel having the above-described composition can be produced by a conventional melting method using a converter or the like, followed by casting the molten steel into a slab. The slab can then be further rolled by a method such as hot rolling to produce a round billet-shaped steel material. A slab refers to a slab in which molten steel is cast into a continuous casting facility, with the outer surface solidified but the interior not yet completely solid. A steel material refers to a slab in which continuous casting is completed, the interior of the slab being completely solid (all solid), and the slab has been cooled. Any type of continuous casting facility can be used, and the facility configuration is not particularly limited. However, the inventors have discovered that, to solve the above-mentioned problems, it is necessary to control the coefficients of variation of corrosion-resistant elements, particularly the coefficient of variation of Cr. To obtain a desired coefficient of variation of Cr, it is important to reduce the steel so as to minimize center segregation in the unsolidified portion at the final stage of solidification during casting. This led to the present invention.

[0062] For example, it is preferable to use a curved continuous casting machine equipped with a slab-type or round billet-type continuous casting mold, pour molten steel supplied from a ladle into a tundish into the mold, and oscillate at a frequency of 1 to 50 Hz. That is, the oscillation frequency is preferably 1 Hz or more, more preferably 2 Hz or more, even more preferably 3 Hz or more, and most preferably 4 Hz or more. The oscillation frequency is preferably 50 Hz or less, more preferably 48 Hz or less, even more preferably 46 Hz or less, and most preferably 44 Hz or less. It is preferable to use a mold having a taper of 1° or more with respect to the casting direction. While the upper limit is not particularly limited, it is preferably 3° or less, and more preferably 2° or less. The reason why the above is preferable is that by applying the above conditions when pouring molten steel into the mold, extrusion of molten steel in which non-ferrous elements in the unsolidified portion have been concentrated during continuous casting is promoted, thereby suppressing compositional variations due to the concentration of P, S, C, Mn, etc., and as a result, it is easier to obtain a Cr coefficient of variation of 6.0 or more. Electromagnetic stirring may be performed under any conditions, but may not be performed.

[0063] 1(a) and 1(b) show an example of continuous casting equipment for producing slabs. (a) is a side view of the molten steel being reduced before it completely solidifies, and (b) is a side view of the reduction of a slab including a crater end. Molten steel 1 poured from a tundish is injected into a mold through a nozzle 3 in a continuous casting machine 2 shown in FIG. 1(a). During continuous casting, the slab 4 is cooled from the wall and eventually solidifies. As the slab 4 advances downstream 6, the width of the solidified shell 5 (solidified region) increases (as the solidification reaction progresses) until it reaches a crater end 7, where solidification begins to complete. To address these concerns, reduction is performed to accommodate the solidification shrinkage of the molten steel 1. Because the solidification state at the final stage of solidification significantly affects the quality of the slab 4, it is important to consistently and reliably reduce the region including the crater end 7. For example, a continuous casting method using multiple reduction zones with a zone length of 2 m and 40 pairs of reduction rolls 9 is shown in FIG. 1( b ) on the upstream side 8 and downstream side 6 of the crater end 7 where solidification of the slab 4 is completed. The number of reduction roll pairs 9 can be in the range of 20 to 100, and the zone length can be changed within the range of 1 to 5 m. That is, the number of reduction roll pairs 9 is preferably 20 or more, more preferably 21 or more, and even more preferably 22 or more. The number of reduction roll pairs 9 is preferably 100 or less, more preferably 98 or less, and even more preferably 96 or less. The zone length is preferably 1 m or more, more preferably 1.1 m or more. The zone length is preferably 5 m or less, more preferably 4.5 m or less. As described above, the number of reduction rolls for the slab may be a single pair or multiple stages. The shape of the reduction roll is preferably a flat roll when the slab surface is flat, but if the surface is not flat, a caliber roll may be used.

[0064] It is also preferable to control the reduction amount and the pressure value applied to each segment in which the reduction roll pair 9 is arranged.

[0065] For example, when a 740 mm wide, 275 mm thick slab is cast at the following slab thickness reduction rate with a secondary cooling water specific water volume of 0.30 L / kg Steel, in order to obtain the desired coefficient of variation, it is preferable to carry out reduction in each of the reduction zones by a reduction amount of 0.8 to 1.2 mm / m so that the slab becomes tapered, and then to control the pressure acting on the segment of the reduction zone in the final solidification stage, which particularly affects the unsolidified reduction, to within ±4 tonnes of the set value. The reduction amounts are preferably 0.8 mm / m or more, and more preferably 0.85 mm / m or more. The reduction amounts are preferably 1.2 mm / m or less, and more preferably 1.15 mm / m or less.

[0066] In particular, to obtain a desired Cr variation coefficient, it is important to control the reduction rate in the thickness direction of the slab just before the center of the slab solidifies. During continuous casting, the inner and outer surfaces of the slab are cooled by water, and just before the center of the slab solidifies, the reduction rate in the thickness direction of the slab is set to 0.0005 mm / sec (0.03 mm / min) or more. The reduction rate in the thickness direction of the slab is preferably set to 0.0006 mm / sec (0.036 mm / min) or more, more preferably 0.0007 mm / sec (0.042 mm / min) or more, even more preferably 0.0008 mm / sec (0.048 mm / min) or more, and most preferably 0.0009 mm / sec (0.054 mm / min) or more. On the other hand, the upper limit of the reduction rate in the thickness direction of the slab is set to 0.01 mm / sec (0.6 mm / min) or less. The extrusion speed is preferably 0.008 mm / sec (0.48 mm / min) or less, more preferably 0.004 mm / sec (0.24 mm / min) or less, even more preferably 0.0035 mm / sec (0.21 mm / min) or less, and most preferably 0.0030 mm / sec (0.18 mm / min) or less. By controlling the extrusion speed to the above conditions, it becomes possible to appropriately extrude the central segregation portion where corrosion-resistant elements such as Cr are concentrated in the solidified state, and the alloy elements such as Cr can be homogenized in the steel, thereby obtaining excellent corrosion resistance after forming into a steel pipe.

[0067] Even if the width and thickness of the slab vary from the above-mentioned sizes, the set values ​​of the reduction rate in the thickness direction of the slab, the specific water flow rate of the secondary cooling water, the reduction amount, and the pressure value acting on the segment of the reduction zone in the final stage of solidification are preferably set to the above-mentioned conditions. The width of the slab is preferably 300 mm or more, more preferably 305 mm or more, and even more preferably 310 mm or more. The width of the slab is preferably 900 mm or less, more preferably 895 mm or less, and even more preferably 890 mm or less. The thickness is preferably 150 mm or more, more preferably 155 mm or more, and even more preferably 160 mm or more. The thickness is preferably 300 mm or less, more preferably 295 mm or less, and even more preferably 290 mm or less.

[0068] In the case of a slab, it is further rolled into a round billet by any rolling method, and the outer diameter of the round billet is preferably 6 times or less, more preferably 5.7 times or less, and even more preferably 5.5 times or less, the diameter of the steel pipe as the final product. There is no particular lower limit, but it is preferably 1.5 times or more.

[0069] [Pipemaking] The above steel material is made into a stainless steel seamless pipe. In the present invention, the pipemaking method is performed by hot working. The method for processing the steel material into a stainless steel seamless pipe is not particularly limited and any method can be used. For example, a stainless steel seamless pipe can be obtained by either the Mannesmann plug mill method or the Mannesmann mandrel mill method. The heating temperature in the heating is not particularly limited, but from the viewpoint of achieving high levels of both hot workability during pipemaking and low-temperature toughness of the final product, it is preferably 1100°C or higher, more preferably 1110°C or higher, and even more preferably 1120°C or higher. It is also preferably 1350°C or lower, more preferably 1340°C or lower, and even more preferably 1330°C or lower. The temperatures described below in the present invention, including the above heating temperatures, refer to temperatures measured on the outer surface of the steel pipe using a radiation thermometer.

[0070] In the present invention, in the piercing process for drilling holes in a steel material when producing this stainless steel seamless pipe, it is preferable that only piercing is performed and that the piercing speed is 3.3 m / s or less. This piercing speed is calculated by dividing the length of the mother pipe immediately after piercing by the piercing time. The piercing time can be calculated as the time from the point at which the load on the rolls in the piercing mill changes over time, from the point at which the load is increased from the level when the steel material is not in contact with the rolls to the point at which the load returns to the original level when the steel material is not in contact with the rolls, after piercing. However, if the roll load cannot be measured, an index that changes when the steel material is in contact with the rolls, such as roll displacement or torque value, can also be used. The piercing speed is more preferably 2.0 m / s or less, even more preferably 1.0 m / s or less, and most preferably 0.5 m / s or less. Since it is believed that the lower the piercing speed, the lower the degree of ferrite filling, no lower limit is set for the piercing speed. However, an extremely low piercing speed reduces production efficiency. Therefore, the drilling speed is preferably 0.05 m / sec or more, more preferably 0.1 m / sec or more, and even more preferably 0.2 m / sec or more.

[0071] When pipe-making is performed by hot working, a cooling treatment may be performed after the pipe-making. The cooling treatment can be performed under any conditions without particular limitations. For example, after the hot working, it is preferable to cool the pipe to room temperature at a cooling rate similar to that of air cooling.

[0072] [Quenching-Tempering Treatment] Next, the obtained stainless steel seamless pipe is subjected to a heat treatment consisting of quenching and tempering (quenching-tempering treatment) under specific conditions. The conditions for the quenching-tempering treatment will be described below.

[0073] Quenching Treatment First, it is preferable to heat the stainless steel seamless pipe to a quenching temperature of 850 to 1150°C, and then cool the heated stainless steel seamless pipe to a cooling stop temperature of 50°C or less at an average cooling rate of 0.01°C / s or more. Unless otherwise specified, the temperature is a value measured on the outer surface of the steel pipe. The average cooling rate is the average cooling rate from 900 to 100°C, and can be determined by dividing the temperature difference between them by the time required for cooling.

[0074] Quenching temperature: 850 to 1150°C. 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, making it impossible to ensure the desired strength. Therefore, the quenching temperature is preferably 850°C or higher. More preferably, it is 900°C or higher, even more preferably, it is 905°C or higher, and most preferably, it is 910°C or higher. On the other hand, if the quenching temperature is higher than 1150°C, the crystal grains become coarse, resulting in a deterioration in low-temperature toughness. Therefore, the quenching temperature is preferably 1150°C or lower. More preferably, it is 1100°C or lower, even more preferably, it is 1095°C or lower, and most preferably, it is 1090°C or lower.

[0075] In the above-mentioned quenching treatment, the stainless steel seamless pipe may be heated to the quenching temperature and then subjected to a soaking treatment in which the pipe is maintained at the heating temperature. By performing the soaking treatment, the temperature of the stainless steel seamless pipe can be made uniform in the wall thickness direction, thereby reducing variations in material properties. The time for maintaining the pipe at the heating temperature (soaking time) is not particularly limited, but is preferably 5 minutes or more, more preferably 8 minutes or more, and even more preferably 10 minutes or more. Furthermore, it is preferably 30 minutes or less, more preferably 28 minutes or less, and even more preferably 25 minutes or less.

[0076] Average cooling rate: 0.01°C / s or more. If the average cooling rate during cooling after heating in the quenching treatment is less than 0.01°C / s, the desired microstructure cannot be obtained. Therefore, the average cooling rate is preferably 0.01°C / s or more. It is more preferably 1.0°C / s or more, even more preferably 5.0°C / s or more, most preferably 10.0°C / s or more, and most preferably 12.5°C / s or more. There is no particular upper limit, but the average cooling rate is preferably 100.0°C / s or less, more preferably 95.0°C / s or less, even more preferably 90.0°C / s or less, and most preferably 85.5°C / s or less. The average cooling rate is the average value of the cooling rates from 900 to 100°C and can be calculated by dividing the temperature difference between them by the time required for cooling.

[0077] The cooling method is not particularly limited and can be performed by any method. For example, the cooling method is preferably at least one of air cooling and water cooling, and more preferably water cooling.

[0078] Cooling stop temperature: 50°C or less If the cooling stop temperature is higher than 50°C, it is difficult to obtain the desired microstructure. That is, if the cooling stop temperature is high, the transformation from austenite to martensite does not occur sufficiently, and the fraction of retained austenite may become excessive. Therefore, the cooling stop temperature in the above-mentioned quenching treatment is preferably 50°C or less, more preferably 48°C or less, and even more preferably 45°C or less. On the other hand, the lower limit is not particularly limited, but is preferably 4°C or more, more preferably 5°C or more, and even more preferably 7°C or more. Here, the cooling stop temperature is the surface temperature of the seamless steel pipe.

[0079] Tempering Treatment Next, the stainless steel seamless pipe after the quenching treatment is subjected to tempering treatment by heating to a tempering temperature of 500 to 650°C.

[0080] Tempering temperature: 500 to 650°C. If the tempering temperature is less than 500°C, a sufficient tempering effect cannot be obtained, resulting in a deterioration of low-temperature toughness. Therefore, the tempering temperature is preferably 500°C or higher. More preferably, it is 520°C or higher, even more preferably, it is 525°C or higher, and most preferably, it is 530°C or higher. On the other hand, if the tempering temperature is higher than 650°C, a large amount of intermetallic compounds precipitates, and excellent low-temperature toughness cannot be obtained. Therefore, the tempering temperature is preferably 650°C or lower. More preferably, it is 630°C or lower, even more preferably, it is 627°C or lower, and most preferably, it is 625°C or lower. Furthermore, the higher the tempering temperature, the more strain in martensite is recovered and the lower the yield strength. Therefore, the tempering temperature is appropriately selected according to the desired yield strength of the stainless steel seamless pipe.

[0081] In the tempering treatment, the stainless steel seamless pipe can be heated to the tempering temperature and then held at the tempering temperature. The time for holding at the tempering temperature (holding time) is not particularly limited, but from the viewpoint of uniforming the temperature in the wall thickness direction and preventing material variations, it is preferably 5 minutes or more, more preferably 7 minutes or more, even more preferably 8 minutes or more, and most preferably 10 minutes or more. The holding time is preferably 90 minutes or less, more preferably 85 minutes or less, even more preferably 82 minutes or less, and most preferably 80 minutes or less.

[0082] After the tempering treatment, the material may be allowed to cool or water-cooled, but water-cooling is preferred. The average cooling rate in the cooling after the tempering treatment is preferably 0.01°C / s or more, more preferably 0.03°C / s or more. Also, it is preferably 5.0°C / s or less, more preferably 1.0°C / s or less. The average cooling rate is the average cooling rate from the start of cooling to 50°C, and can be calculated by dividing the temperature difference during that period by the time required for cooling.

[0083] By carrying out the above quenching and tempering treatment, it is possible to obtain a stainless steel seamless pipe that has the desired strength, as well as superior low-temperature toughness and corrosion resistance.

[0084] The present invention will be further described below based on examples, but the present invention is not limited to the following examples.

[0085] Stainless steel seamless pipes were produced using cast slabs having the chemical compositions shown in Table 1 according to the following procedure. Specifically, steel blanks were cast using molten steel having the chemical compositions shown in Table 1 in a curved continuous casting machine equipped with a slab-type continuous casting mold, while varying the reduction rate conditions shown in Table 2. The oscillation frequency was 40 Hz, and the water cooling rate was 0.30 L / kg Steel. The dimensions of the steel blanks were 275 mm thick, 745 mm wide, and 10 m long. The steel blanks were then heated and hot-worked and pierced using a model seamless rolling mill to produce stainless steel seamless pipes with an outer diameter of 177.8 mm and a wall thickness of 16.0 mm, which were then air-cooled. The heating temperature of the steel blanks before hot working was 1250°C. The piercing speeds in the pipe-making process were 0.2 to 3.3 m / s for Nos. 1 to 16, and 0.5 to 3.3 m / s for Nos. 1 to 18. Castings Nos. 17 and 18 were carried out at a speed of more than 3.3 m / s and not more than 4.0 m / s. The casting was carried out using multiple reduction zones, each 2 m long, equipped with 40 pairs of reduction rolls 9, on the upstream side 8 and downstream side 6 of the crater end 7 where solidification of the inside of the slab 4 is completed, as shown in FIG. 1(b). The reduction rate was 0.1 to 30 mm / m.

[0086] The obtained stainless steel seamless pipe was then subjected to a quenching treatment under the conditions shown in Table 2. That is, the stainless steel seamless pipe was heated to the quenching temperature shown in Table 2 and held at the quenching temperature for the soaking time shown in Table 2. Next, it was cooled to a cooling stop temperature of 5 to 10°C. The cooling was performed by water cooling. The average cooling rate from 900 to 100°C during the water cooling was 20.0°C / s. Thereafter, the stainless steel seamless pipe after cooling in the quenching treatment was heated to the tempering temperature shown in Table 2 and held at the predetermined tempering temperature for the holding time shown in Table 2. Thereafter, the stainless steel seamless pipe was air-cooled (naturally cooled). The average cooling rate from the start of cooling to 50°C during air cooling during the tempering treatment was 0.04°C / s.

[0087] Test pieces were taken from the obtained stainless steel seamless pipes, and measurements of the coefficient of variation, microstructural observation, tensile tests, stress corrosion cracking tests, and localized corrosion resistance tests were carried out. The test methods were as follows.

[0088] (1) Measurement of coefficient of variation From the obtained stainless steel seamless pipe, a cross section perpendicular to the pipe axis direction of the steel pipe (a cross section parallel to the circumferential direction and wall thickness direction of the steel pipe) was mirror-polished as the target surface, and Cr, Mo, Ni, and W were analyzed using EPMA over a 60 μm square area. The acceleration voltage was 15 kV, the probe current was 10 nm, the measurement time was 150 ms, and the measurement pitch was 0.6 μm. The coefficient of variation can be determined by calculating the average X-ray count number of all measurement points in the specified region described above, and then dividing this average by the standard deviation of all measurement points.

[0089] (2) Microstructure Observation Here, the volume fraction of each of the above phases can be measured by the following method. First, a steel piece was cut out from a stainless steel seamless pipe in a range from the inner surface of the pipe to one-third of the wall thickness, so that the cross section perpendicular to the axial direction of the steel pipe (a cross section parallel to the longitudinal direction and wall thickness direction of the steel pipe) was the target surface. A sample for microstructure observation was prepared by embedding the steel piece in resin and mirror polishing. This surface was immersed in a KOH solution (a mixture of 35 g of KOH and 100 g of pure water) at 3 A / cm ２Electrolytic corrosion was performed for 35 seconds at a current density of 1000 kJ / cm, and then corrosion was performed for 30 seconds with Villela's reagent (a mixture of picric acid, hydrochloric acid, and ethanol in proportions of 2 g, 5 ml, and 50 ml, respectively). The microstructure of the test piece for microstructure observation was then imaged at 400x magnification using an optical microscope. An image of a full-scale area measuring 300 μm in the longitudinal direction of the steel pipe and 200 μm in the wall thickness direction of the steel pipe was cut out from the obtained optical microscope photograph and analyzed using image analysis software (ImageJ 1.52p, National Institute of Health), and the structure fraction (area fraction (%)) of the ferrite phase was calculated. In the analysis, the Weka Trainable Segmentation function was used on the optical microscope photograph to extract the ferrite phase by using three bright ferrite regions and three dark martensite regions as training data, and automatically classifying the other regions using the Segmentation function. The area fraction of the ferrite phase extracted in this way was defined as the volume fraction (%) of the ferrite phase. The structural fraction of the ferrite phase was calculated by the average value of three fields of view.

[0090] X-ray diffraction specimens were taken from the resulting stainless steel seamless pipes, ground and polished so that a cross section (C cross section) perpendicular to the pipe axis direction served as the measurement surface, and the fraction of the retained austenite (γ) phase was measured using X-ray diffraction. Specifically, the volume fraction of the retained austenite phase was calculated from the integrated intensities of the austenite (220) plane and the ferrite (211) plane using the following formula: Vγ (%) = 100 / (1 + (IαRγ / IγRα)) where Vγ is the volume fraction of the retained austenite phase, Iα is the integrated intensity of the ferrite (211) plane, Iγ is the integrated intensity of the austenite (220) plane, Rα is the theoretically calculated value of α (34.15), and Rγ is the theoretically calculated value of γ (22.33).

[0091] The remainder other than the ferrite phase and the residual γ phase determined by the above measurement method was taken as the fraction of the martensite phase.

[0092] (3) Tensile test An arc-shaped tensile test piece was taken from the obtained stainless steel seamless pipe in accordance with the provisions of API (American Petroleum Institute)-5CT 10th Edition (2018) so that the pipe axis direction was the tensile direction, and a tensile test was performed to determine the yield strength (YS). Here, a yield strength YS of 758 MPa or more was considered to be high strength and passed, and a yield strength YS of less than 758 MPa was considered to be failed.

[0093] (4) Stress Corrosion Cracking Resistance Test The following test was conducted to evaluate stress corrosion cracking resistance. Test pieces with a thickness of 5 mm, width of 15 mm, and length of 115 mm were prepared by machining from the obtained stainless steel seamless pipe, and a four-point bending test was conducted. The test was conducted in an autoclave using a 20% by mass NaCl aqueous solution (liquid temperature: 200°C, CO 2 at 50 atmospheres). ２ -0.01 atmospheres of H ２ S gas, 0.005 atmospheres of SO ２ NaHCO 3 was added to the atmosphere to adjust the pH to 4.5. ３ The test specimens were immersed in a solution containing 200°C of ammonium nitrate, to which a stress equivalent to the yield stress at 200°C had been applied by four-point bending for 30 days (720 hours). After the corrosion test, the corrosion products were removed, and the presence or absence of stress corrosion cracking was judged by visual inspection and observation with an optical microscope at 10x magnification in accordance with NACE TM0316. Tests without stress corrosion cracking were evaluated as passing, and those with cracking were evaluated as failing. In Table 2, passing was indicated by "◯" and failing by "X".

[0094] (5) Localized Corrosion Resistance Test The following test was conducted to evaluate the localized corrosion resistance of the obtained stainless steel seamless pipe. Test pieces with a thickness of 5 mm, width of 50 mm, and length of 50 mm were prepared from the obtained stainless steel seamless pipe by machining, and a corrosion weight loss test was conducted. The test was conducted by soaking the pipe in a 20% by mass NaCl aqueous solution (liquid temperature: 200°C, CO 2 at 50 atmospheres) held in an autoclave. ２ -0.01 atmospheres of H ２ S gas, 0.005 atmospheres of SO ２ NaHCO 3 was added to the atmosphere to adjust the pH to 4.5. ３The test specimens were immersed in a solution containing the above-mentioned ammonium hydroxide for 30 days (720 hours). After the test, the corrosion products were removed, and the corrosion rate (mm / year) of the test specimen surface was calculated. The presence or absence of localized corrosion was determined by observing the test specimen surface in accordance with ASTM G 48. A sample without localized corrosion was deemed a pass, and a sample with localized corrosion was deemed a fail. In Table 2, a pass was indicated by "○" and a fail by "×." The corrosion rate was determined as follows: First, the weight of the corrosion test specimen after the corrosion products were removed was measured, and the weight loss was calculated by subtracting the weight of the corrosion test specimen after the corrosion products were removed from the weight of the test specimen before the corrosion test, which had been measured in advance. Next, the weight loss was calculated by dividing the weight loss by the surface area of ​​the test specimen used and the immersion period to obtain the weight loss per unit time and unit area. Furthermore, the weight loss per unit time and unit area was converted to the corrosion thickness per unit time and unit area by dividing the weight loss per unit time and unit area by the density of the steel. The corrosion thickness per unit time and unit area (mm / year) thus obtained was taken as the corrosion rate. A corrosion rate of 0.127 mm / year or less was judged to be good.

[0095] The results are shown in Table 2. As can be seen from the results shown in Table 2, all of the stainless steel seamless pipes satisfying the conditions of the present invention had high strength, with a yield strength of 758 MPa (110 ksi) or more, excellent stress corrosion cracking resistance, and excellent localized corrosion resistance. The reason why No. 3 in Table 2 has a higher yield strength than No. 1, despite having a smaller martensite fraction and a larger ferrite fraction, is that the tempering temperature of No. 3 is lower than that of No. 1, thereby suppressing the recovery of strain in the martensite. Therefore, the stainless steel seamless pipe of the present invention can be used extremely suitably for a variety of applications, including oil well steel pipes.

[0096]

[0097]

[0098] REFERENCE SIGNS LIST 1 molten steel 2 continuous casting machine 3 nozzle 4 cast piece 5 solidified shell 6 downstream side 7 crater end 8 upstream side 9 reduction roll pair

Claims

1. A steel pipe having a chemical composition, by mass%, of 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, O: 0.010% or less, with the balance being Fe and unavoidable impurities, and the microstructure has, by volume fraction, 40% to 85% martensite phase, 15% to 55% ferrite phase, and 40% or less retained austenite phase, and in a cross section perpendicular to the axial direction of the steel pipe, A stainless steel seamless pipe having a Cr variation coefficient of 6.0 or more and a yield strength of 758 MPa or more.

2. A stainless steel seamless pipe as described in claim 1, wherein the chemical composition further contains, in mass %, one or more elements selected from Nb: 0.3% or less, Ti: 0.2% or less, W: 2.0% or less, Co: 1.0% or less, B: 0.01% or less, Ta: 0.3% or less, Zr: 0.3% or less, Ca: 0.01% or less, REM: 0.3% or less, Mg: 0.01% or less, Sn: 1.0% or less, and Sb: 1.0% or less.

3. A stainless steel seamless pipe according to claim 1 or 2, wherein the microstructure comprises, by volume fraction, 45% to 85% martensite phase, 15% to 55% ferrite phase, and 30% or less retained austenite phase.

4. A method for manufacturing a stainless steel seamless pipe, comprising the steps of: continuously casting a slab having the chemical composition set forth in claim 1 or 2 under conditions in which the reduction rate in the thickness direction of the slab before solidification inside the slab is completed is 0.0005 mm / sec or more and 0.01 mm / sec or less to obtain a steel material; forming the steel material into a stainless steel seamless pipe; and subjecting the pipe to quenching and tempering treatments.

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