Seamless steel pipe and manufacturing method therefor

Abstract

Provided are: a seamless steel pipe having a yield stress of at least 450 MPa and having excellent CO2 corrosion resistance in a CCS corrosive environment; and a manufacturing method therefor.　This seamless steel pipe has: a component composition containing, in terms of mass%, 0.05-0.15% of C, 0.05-0.50% of Si, 1.0-2.0% of Mn, at most 0.030% of P, at most 0.005% of S, 0.010-0.3% of Cr, 0.01-0.2% of Mo, 0.001-0.01% of Ca, 0.010-0.06% of V, 0.005-0.05% of Ti, 0.0001-0.0050% of B, 0.001-1.000% of Sb, and at most 0.100% of Al, with the remainder consisting of Fe and inevitable impurities; a metal structure in which the average crystal grain diameter at the center of the wall thickness is at most 40.0 μm and the maximum crystal grain diameter at the center of the wall thickness is at most 90.0 μm; a yield strength of at least 450 MPa; and an absorption energy of at least 250 J at -30ºC.

Description

Seamless steel pipe and method for manufacturing the same

[0001] The present invention relates to a seamless steel pipe for transport line pipes used in CO2 recovery and storage (hereinafter simply referred to as CCS) and a method for manufacturing the same, and in particular, a yield strength (YS) of 450 MPa or more, and contains H2O and NO in CO2. x This invention relates to a seamless steel pipe and a method for manufacturing the same, which exhibits excellent resistance to CO2 corrosion in environments containing impurities such as the above.

[0002] In recent years, carbon capture and storage (CCS) technology has been actively developed as a means of realizing a decarbonized society. In CCS, many oil and gas development technologies are used for CCS injection wells, which inject the captured CO2 into the ground such as depleted oil fields and aquifers. However, in the pipelines of CCS injection wells, the transported CO2 contains NO x Ya SO x Because it contains impurities such as these, it requires high corrosion resistance in a corrosive environment different from that of oil well pipes.

[0003] Furthermore, in CCS injection wells, pipelines are sometimes buried underwater to inject CO2 into depleted oil fields located on the seabed. Line pipes used in such environments require high strength and toughness to withstand seawater pressure and low-temperature environments.

[0004] Patent Document 1 describes a seamless steel pipe for line pipes having a carbon equivalent Ceq of 0.38 or more and a total content (mass%) of Ti, V, and Nb of less than 0.06. According to the technology described in Patent Document 1, it is possible to obtain a yield strength of 450 MPa or more and toughness with a ductile-brittle fracture transition temperature of -70°C or lower, while also obtaining sour resistance in a 5% NaCl aqueous solution saturated with 1 atm of H2S gas.

[0005] Patent Document 2 states that the content (mass%) of Ca and O is Ca × O ≤ 0.324 × 10 -5The invention also describes a seamless steel pipe for line pipes having a chemical composition that satisfies the relationship Ca / O < 0.5. According to the technology described in Patent Document 2, the amount of oxide inclusions is reduced when Ca and O satisfy the above relationship. Furthermore, by keeping the number density of oxide inclusions within a predetermined range, excellent low-temperature toughness with a ductile fracture surface ratio of 75% or more in a Charpy impact test at -40°C can be obtained.

[0006] Patent Document 3 describes a seamless steel pipe for line pipes with a wall thickness of 12.5 mm or more, having a composition containing, by mass%, 0.7 to 2.5% Mn, 0.005% or less N, 0.005 to 0.05% Ti, and further containing appropriate amounts of Cr, Mo, Cu, V, and Nb. This seamless steel pipe is manufactured by heating a cast slab having the above composition at a low temperature, perforating and rolling it, and then subjecting it to low-temperature rolling and accelerated cooling that satisfy predetermined conditions. The technology described in Patent Document 3 achieves high strength exceeding 450 MPa and fracture surface transition temperature v T rs It is claimed that this material possesses high toughness below -70°C while also achieving excellent resistance to sulfide stress corrosion cracking in a sour environment with a 5% NaCl aqueous solution saturated with H2S gas.

[0007] International Publication No. 2011 / 152240, Japanese Patent Publication No. 2017-8362, Japanese Patent Publication No. 2015-193868

[0008] As mentioned above, the line pipes used in CCS injection wells have high strength and toughness, as well as NO in CO2. x Ya SO x High corrosion resistance is required in corrosive environments containing impurities such as these.

[0009] In contrast, the technologies described in Patent Documents 1 to 3 do not consider CO2 corrosion resistance in the corrosive environment of CCS injection wells (hereinafter also simply referred to as the CCS corrosive environment), and it is difficult to say that they possess the performance required for line pipes used in a CCS corrosive environment.

[0010] The present invention aims to provide a seamless steel pipe and a method for manufacturing the same, which has a yield strength of 450 MPa or more and excellent CO2 corrosion resistance in a CCS corrosion environment.

[0011] In this context, "excellent CO2 corrosion resistance in a CCS corrosive environment" refers to the performance of a test specimen that does not develop pitting corrosion exceeding 25 μm after 30 days when the specimen is immersed in an aqueous solution and the gas phase (supercritical phase) under a total pressure of 22.4 MPa at 50°C (10 ppm NO2, 10 ppm SO2, 10 ppm O2, 10 ppm H2S, 1.0 mol% H2, 0.2 mol% CO, 0.991 mol% CH4, bal. CO2).

[0012] To achieve the above-mentioned objectives, the inventors diligently investigated various effects on the strength, toughness, and CO2 corrosion resistance of seamless steel pipes in CCS corrosion environments. As a result, they found that by including Mn, Cr, Mo, Ca, V, Ti, B, Sb, and Al within predetermined ranges, and optimizing the conditions of the quenching and tempering treatments to achieve an average particle size of 40 μm or less in the L-section at the center of the wall thickness, it is possible to produce a seamless steel pipe with the desired strength and high toughness, as well as excellent CO2 corrosion resistance in CCS corrosion environments.

[0013] This invention was completed based on the above findings and further investigations. In other words, the gist of this invention is as follows:

[0014] [1] A seamless steel pipe having, in mass%, a component composition containing: C: 0.05 to 0.15%, Si: 0.05 to 0.50%, Mn: 1.0 to 2.0%, P: 0.030% or less, S: 0.005% or less, Cr: 0.010 to 0.3%, Mo: 0.01 to 0.2%, Ca: 0.001 to 0.01%, V: 0.010 to 0.06%, Ti: 0.005 to 0.05%, B: 0.0001 to 0.0050%, Sb: 0.001 to 1.000%, and Al: 0.100% or less, with the balance being Fe and unavoidable impurities; having a metal structure where the average crystal grain size at the center of the wall thickness is 40.0 μm or less and the maximum crystal grain size at the center of the wall thickness is 90.0 μm or less; having a yield strength of 450 MPa or more; and having an absorbed energy at -30°C of 250 J or more. Here, the statement that the average crystal grain size at the center of the wall thickness is 40.0 μm or less means that the average crystal grain size of the metal structure in the L-section at the central position of the wall thickness of the seamless steel pipe, that is, the cross-section parallel to the rolling direction (pipe axis direction) of the seamless steel pipe, is 40.0 μm or less. Similarly, the statement that the maximum crystal grain size at the center of the wall thickness is 90.0 μm or less means that the maximum crystal grain size of the metal structure in the L-section at the central position of the wall thickness of the seamless steel pipe is 90.0 μm or less. [2] The seamless steel pipe according to [1], wherein the component composition further contains one or more selected from the group consisting of, in mass%, Nb: 0.080% or less, Cu: 0.50% or less, Ni: 0.50% or less, and Sn: 0.100% or less. [3] The seamless steel pipe according to [1] or [2], wherein the wall thickness is 35 mm or less. [4] A method for producing a seamless steel pipe according to any one of [1] to [3], comprising: a pipe forming step of forming a steel pipe material having the above component composition into a steel pipe; a quenching step of heating the steel pipe to a temperature equal to or higher than the Ac3 transformation point, and subsequently cooling the steel pipe to a cooling stop temperature of 100°C or lower; and a tempering step of tempering the steel pipe that has undergone the quenching step at a temperature equal to or lower than the Ac1 transformation point.

[0015] According to the present invention, in CO₂ containing H₂O and NO x , a seamless steel pipe having excellent CO₂ corrosion resistance and high strength with a yield stress (YS) of 450 MPa or more can be obtained in a CCS corrosive environment containing impurities such as the above.

[0016] <Seamless Steel Pipe> A seamless steel pipe according to one embodiment of the present invention will be described below. [Component Composition] First, the appropriate range and reasons for limiting the component composition of the seamless steel pipe of this embodiment will be explained. In addition, the "%" expressed in the component composition below means "mass %" unless otherwise specified. ・C: 0.05 to 0.15% C is an important element related to strength and is effective in improving strength. In the present invention, in order to ensure the desired strength, the C content is set to 0.05% or more, preferably 0.07% or more. Also, if C is included in excess, the hardness will increase and the toughness will decrease. For this reason, the C content will be set to 0.15% or less, preferably 0.12% or less. ・Si: 0.05 to 0.50% Since Si acts as a deoxidizing agent, the Si content will be set to 0.05% or more. Also, if Si is included in more than 0.50%, the resistance to CO2 corrosion and hot workability will decrease. For this reason, the Si content will be set to 0.50% or less. Furthermore, from the viewpoint of ensuring stable strength, the Si content is preferably 0.20 to 0.35%. ・Mn: 1.0 to 2.0% Mn is an element that improves strength, and its inclusion ensures the necessary strength. It also improves hot workability. To obtain these effects, the Mn content should be 1.0% or more, preferably 1.2% or more. On the other hand, adding too much Mn causes Mn to segregate in the steel, increasing hardness and decreasing toughness. Therefore, the Mn content should be 2.0% or less, preferably 1.6% or less. ・P: 0.030% or less P is an element that reduces CO2 corrosion resistance and toughness, and in this invention, it is desirable to reduce the P content as much as possible. For this reason, the P content should be 0.030% or less, preferably 0.020% or less. However, an extreme reduction in the P content will increase manufacturing costs. Therefore, it is preferable that the P content be 0.001% or more, within a range that does not drastically degrade the properties of the steel material and is industrially feasible at low cost. • S: 0.005% or less. Since S is an element that significantly reduces hot workability and toughness, it is desirable to reduce the S content as much as possible.By reducing the S content to 0.005% or less, pipe manufacturing becomes possible using the normal process. Therefore, the S content in this invention is set to 0.005% or less, preferably 0.004% or less. On the other hand, excessive reduction of the S content leads to a surge in smelting costs, so it is preferable that the S content be 0.0001% or more. Cr: 0.010-0.3% Cr is an element that improves strength and tempering softening resistance, refines the microstructure, and forms a protective film to improve corrosion resistance. To obtain this effect, the Cr content is set to 0.010% or more, preferably 0.05% or more, and more preferably 0.10% or more. On the other hand, adding too much Cr reduces toughness. Therefore, the Cr content is set to 0.3% or less, preferably 0.28% or less, and more preferably 0.25% or less. Mo: 0.01-0.2% Mo is an element that improves resistance to pitting corrosion. To obtain this effect, the Mo content should be 0.01% or more, preferably 0.05% or more. On the other hand, adding too much Mo reduces toughness. Furthermore, since Mo is an expensive element, excessive addition of Mo leads to increased manufacturing costs. Therefore, the Mo content should be 0.2% or less, preferably 0.18% or less. Ca: 0.001 to 0.01% Ca is an element that improves toughness and corrosion resistance by controlling the morphology of inclusions. To obtain this effect, the Ca content should be 0.001% or more, preferably 0.002% or more. On the other hand, adding too much Ca increases nonmetallic inclusions, reducing toughness and corrosion resistance. Therefore, the Ca content should be 0.01% or less, preferably 0.006% or less. V: 0.010 to 0.06% V is an element that improves the strength of steel through precipitation strengthening. Furthermore, it also has the effect of refining the microstructure. To obtain these effects, the V content should be 0.010% or more, preferably 0.02% or more, and more preferably 0.025% or more. On the other hand, if the V content exceeds 0.06%, the toughness decreases. Therefore, the V content should be 0.06% or less, preferably 0.05% or less, and more preferably 0.045% or less. Ti: 0.005 to 0.05% Ti forms carbides, reducing the amount of dissolved carbon and thus reducing hardness. Furthermore, it also has the effect of refining the microstructure.To obtain these effects, the Ti content should be 0.005% or more, preferably 0.01% or more, and more preferably 0.015% or more. Furthermore, if the Ti content exceeds 0.05%, nitrides, sulfides, etc., will form as inclusions, reducing toughness. Therefore, the Ti content should be 0.05% or less, preferably 0.04% or less, and more preferably 0.03% or less. ・B: 0.0001 to 0.0050% B is an element that contributes to increased strength and improves corrosion resistance by controlling the morphology of inclusions. To obtain this effect, the B content should be 0.0001% or more. Furthermore, if the B content exceeds 0.005%, toughness and hot workability will decrease. Therefore, the B content should be 0.0050% or less, preferably 0.0020% or less.・Sb: 0.001 to 1.000% Sb is an element that improves hot workability and low-temperature toughness. Furthermore, it contributes to improved corrosion resistance, especially resistance to CO2 corrosion in CCS corrosion environments. In addition, Sb provides protection by concentrating on the steel surface and suppresses the occurrence of pitting corrosion. To obtain these effects, the Sb content should be 0.001% or more, preferably 0.005% or more. If the Sb content exceeds 1.000%, these effects become saturated. Therefore, the Sb content should be 1.000% or less, preferably 0.100% or less. ・Al: 0.100% or less If the Al content exceeds 0.100%, it adversely affects toughness. Therefore, the Al content should be 0.100% or less, preferably 0.050% or less. In addition, Al acts as a deoxidizing agent. To obtain this effect, the Al content should be 0.010% or more.

[0017] The seamless steel pipe of this embodiment has a composition in which the above-mentioned elemental components are essential, with the remainder being iron and unavoidable impurities. Unavoidable impurities are impurities that are inevitably mixed in from raw materials, manufacturing processes, or manufacturing equipment, and are permissible to be included in a range that does not hinder the objective of the present invention. Examples of unavoidable impurities in the remainder include As, Bi, Co, Pb, Zn, O, Ta, W, Te, Hf, Ge, Sr, and Cs. Examples of raw materials for steel include iron ore, reduced iron, or scrap.

[0018] By containing the essential elements listed above, the seamless steel pipe of this embodiment can acquire the desired properties. In addition to the essential elements listed above, the following elements may be included as needed: One or more elements selected from Nb: 0.080% or less, Cu: 0.50% or less, Ni: 0.50% or less, Sn: 0.100% or less. Nb, Cu, Ni, and Sn are all elements that contribute to improving strength. On the other hand, excessive content may reduce toughness and hot workability. Therefore, when Nb, Cu, Ni, and Sn are included, the Nb content should be 0.080% or less, the Cu content 0.50% or less, the Ni content 0.50% or less, and the Sn content 0.100% or less. While there are no particular lower limits on the content of Nb, Cu, Ni, and Sn, from the viewpoint of improving strength, it is preferable that the Nb content be 0.001% or more, the Cu content be 0.001% or more, the Ni content be 0.001% or more, and the Sn content be 0.001% or more. [Metal structure] Next, the appropriate range and reasons for limiting the metal structure of the seamless steel pipe of this embodiment will be explained. In the following, area ratio means the area ratio to the entire steel plate structure. ・Average grain size at the center of the wall thickness: 40.0 μm or less By refining the average grain size at the center of the wall thickness of the seamless steel pipe, the occurrence of pitting corrosion in a CCS corrosion environment is suppressed. If the average grain size is coarse, pitting corrosion is more likely to occur. In addition, if coarse grains are present, toughness is significantly reduced. Therefore, in the seamless steel pipe of this embodiment, the average crystal grain size at the center of the wall thickness is set to 40.0 μm or less, preferably 30.0 μm or less, and more preferably 25.0 μm or less.

[0019] Here, an average grain size of 40.0 μm or less at the center of the wall thickness means that the average grain size of the metal structure in the L-section at the center of the wall thickness of the seamless steel pipe, that is, in the section parallel to the rolling direction (pipe axis direction) of the seamless steel pipe, is 40.0 μm or less. When investigating the grain size of the metal structure of a steel pipe, it is common to observe the L-section and the C-section. The C-section is a section perpendicular to the rolling direction (pipe axis direction). Since the grain size in the L-section parallel to the rolling direction is larger than the grain size in the C-section perpendicular to the rolling direction, in this embodiment of the seamless steel pipe, the average grain size of the metal structure in the L-section is specified to be 40.0 μm or less.

[0020] The average grain size described above is measured using the SEM / EBSD method. Specifically, in a cross section parallel to the pipe axis, the measurement surface is set at the center of the wall thickness, and a measurement area of ​​350 μm × 350 μm is measured with a step size of 0.5 μm. Based on the EBSD data obtained in this way, the weighted average grain size is obtained from the grain map using TSL's crystal orientation analysis software OIM Analysis™. Boundaries with an orientation difference of 15° or more are defined as grain boundaries, and the weighted average grain size is calculated. Maximum grain size at the center of the wall thickness: 90.0 μm or less If coarse grains are present in seamless steel pipes, pitting corrosion is likely to occur. In addition, toughness is also significantly reduced, so in the seamless steel pipe of this embodiment, the maximum grain size at the center of the wall thickness is set to 90.0 μm or less, preferably 85.0 μm or less, and more preferably 71.0 μm or less.

[0021] Here, "maximum crystal grain size at the center of the wall thickness of 90.0 μm or less" means that the maximum crystal grain size of the metal structure in the L-shaped cross section at the center of the wall thickness of the seamless steel pipe is 90.0 μm or less. ・Yield strength: 450 MPa or more In this embodiment of seamless steel pipes, the yield strength is set to 450 MPa or more in order to ensure high strength that can withstand seawater pressure, etc. ・Hardness: 250 HV1 or less If the hardness of the seamless steel pipe is high, it will be more susceptible to cracking in a CCS corrosion environment, leading to pitting corrosion and cracking. The seamless steel pipe of this embodiment can have a hardness of 250 HV1 or less by satisfying the above requirements. The "hardness" referred to here is the hardness determined by the Vickers test in a cross section perpendicular to the pipe axis direction at the center of the pipe wall thickness. The Vickers test is performed with a load of 1 kgf according to the method described in Japanese Industrial Standard JIS Z 2244:2020. - Absorbed energy at -30°C: 250 J or more Seamless steel pipes may require excellent low-temperature toughness when used in CCS corrosive environments. Low-temperature toughness was evaluated by the absorbed energy in a Charpy impact test in accordance with Japanese Industrial Standard JIS Z 2242:2023. The seamless steel pipe of this embodiment has an absorbed energy of 250 J or more at -30°C, preferably 300 J or more, and more preferably 350 J or more. - Wall thickness: 35 mm or less (preferred conditions) The wall thickness of the seamless steel pipe of this embodiment is not particularly limited, but it is preferable to have a wall thickness of 35 mm or less. If the wall thickness of the steel pipe is thick, it is difficult to sufficiently heat-treat the center of the wall thickness in the quenching process described later, and differences in the structure between the surface of the steel pipe and the center of the wall thickness tend to occur, resulting in differences in strength and toughness. Furthermore, the cooling rate in the center of the wall thickness is slower, which tends to coarseen the structure in the center of the wall thickness. Therefore, the wall thickness of the seamless steel pipe in this embodiment is preferably 35 mm or less, more preferably 30 mm or less. <Method for manufacturing a seamless steel pipe> Next, a method for manufacturing a seamless steel pipe according to one embodiment of the present invention will be described below.

[0022] In the manufacturing method of seamless steel pipes according to this embodiment, a steel pipe material having the above-mentioned component composition is used, but the manufacturing method of the steel pipe material is not particularly limited, and any known method for manufacturing seamless pipes can be applied.

[0023] It is preferable to melt molten steel having the above-mentioned component composition using a melting method such as a converter, and to produce steel pipe material such as billets using methods such as continuous casting or ingot-rolling. Subsequently, these steel pipe materials are heated and subjected to hot working and pipe forming processes using known pipe-making methods, such as the Mannesmann-plug mill method or the Mannesmann-mandrel mill method, to produce seamless steel pipes having the above-mentioned component composition.

[0024] The processing after forming the steel pipe material into a steel pipe is not particularly limited, but preferably involves a quenching process in which the steel pipe is heated to above the Ac3 transformation point, followed by a cooling stop temperature of 100°C or lower, and then a tempering process in which it is tempered at a temperature below the Ac1 transformation point. [Quenching Process] In the manufacturing method of seamless steel pipes of this embodiment, a quenching process is performed in which the steel pipe is reheated to a temperature above the Ac3 transformation point, preferably held for 5 minutes or more, and then cooled to a cooling stop temperature of 100°C or lower. This results in the refinement and increased toughness of the martensite and bainite phases. If the heating temperature in the quenching process is below the Ac3 transformation point, the structure will not become the austenite single-phase region, and a sufficient martensite structure and bainite phase cannot be obtained during subsequent cooling, making it impossible to achieve the desired high strength. Therefore, the heating temperature in the quenching process is set to above the Ac3 transformation point.

[0025] Furthermore, the same quenching process may be repeated twice to refine the microstructure.

[0026] While there are no restrictions on the cooling method during the quenching process, cooling is generally performed by air cooling or water cooling, and increasing the cooling rate refines the microstructure.

[0027] The cooling rate at 400 to 800°C is preferably 15°C / s or more at the t position (1 / 2) in the wall thickness direction of the steel pipe with wall thickness t mm, and more preferably 20°C / s or more.

[0028] The cooling rate at 450 to 700°C is preferably 20°C / s or more at the (1 / 3) t position in the thickness direction and 70°C / s or more at the (1 / 6) t position in the thickness direction, and more preferably 30°C / s or more at the (1 / 3) t position in the thickness direction and 80°C / s or more at the (1 / 6) t position in the thickness direction.

[0029] In the quenching process, if the wall thickness of the steel pipe is thick, the heat treatment cannot be sufficiently applied to the center of the wall thickness, resulting in differences in the microstructure between the surface and the center of the wall thickness, which leads to differences in strength and toughness. Furthermore, the slower cooling rate in the center of the wall thickness causes the microstructure in the center of the wall thickness to coarseen. As mentioned above, these problems can be prevented by making the wall thickness of the seamless steel pipe 35 mm or less, more preferably 30 mm or less. [Tempering process] Next, the steel pipe that has undergone the quenching process described above is subjected to a tempering process. The tempering process is a process in which the steel pipe is heated to below the Ac1 transformation point, preferably held for 10 min or more, and then air-cooled. If the heating temperature in the tempering process is higher than the Ac1 transformation point, the martensite phase will precipitate after tempering, and the desired high toughness and excellent corrosion resistance cannot be ensured. Therefore, the heating temperature in the tempering process should be below the Ac1 transformation point. The above-mentioned Ac3 transformation point (°C) and Ac1 transformation point (°C) can be measured by the Formaster test, which involves subjecting a test specimen to a heating and cooling temperature history and detecting the transformation point from minute displacements of expansion and contraction.

[0030] Examples of the seamless steel pipe and its manufacturing method according to the present invention will be described below. However, the seamless steel pipe and its manufacturing method according to the present invention are not limited to the examples shown below.

[0031] Molten steel with the components shown in Table 1 was melted in a converter and then cast into billets (steel pipe material) using a continuous casting method. Furthermore, these billets were hot-worked using a model seamless rolling mill to form pipes, and then cooled by air or water to produce seamless steel pipes with an outer diameter of 406.4 mm and a wall thickness of 29 mm.

[0032] Test specimens were cut from the seamless steel pipes obtained in this manner, and these specimens were subjected to quenching and tempering processes under the conditions shown in Table 2. Test pieces for microstructure observation were taken from the test specimens after the quenching and tempering processes, polished, and then subjected to EBSD measurement and hardness measurement. Furthermore, arc-shaped tensile test specimens were taken from the test specimens after the quenching and tempering processes, and tensile tests were conducted in accordance with the provisions of Japanese Industrial Standard JIS Z 2241:2022 to determine the tensile properties (yield strength YS, tensile strength TS). In addition, Charpy impact test specimens were taken so that the direction perpendicular to the axial direction of the pipe of the test specimen was the longitudinal direction of the specimen, and Charpy impact tests were conducted in accordance with the provisions of JIS Z 2242:2023 to determine the absorbed energy (J) at -30°C.

[0033] In Table 2, the Ac3 transformation point (°C) and Ac1 transformation point (°C) were measured using a Formaster test on a 4 mmΦ × 10 mm specimen taken from a test material that had undergone the quenching process. Specifically, the specimen was heated to 500°C at 5°C / s, then further heated to 920°C at 0.25°C / s and held for 10 minutes, and then cooled to room temperature at 2°C / s. The Ac3 transformation point (°C) and Ac1 transformation point (°C) were obtained by detecting the expansion and contraction of the specimen during this temperature history.

[0034] Corrosion testing in the CCS corrosive environment was performed by immersion testing, which replicated a high-temperature, high-pressure environment in an autoclave. First, a 50 mm × 20 mm × 3 mm specimen was taken from the test material. The specimen was placed in an autoclave at 50°C with a total pressure of 22.4 MPa (10 ppm NO₂, 10 ppm SO₂, 10 ppm O₂, 10 ppm H₂S, 1.0 mol% H₂, 0.2 mol% CO₂, 0.991 mol% CH₄, bal. CO₂) and immersed in an aqueous solution (liquid phase) and a supercritical phase (gas phase) for 30 days. The pass / fail evaluation was as specified in ASTM G48, where the specimen was considered pass if no pitting corrosion exceeding 25 μm occurred.

[0035] The test results obtained in this manner are shown in Table 2.

[0036]

[0037]

[0038] The examples of the present invention (steel pipes No. 1-28, 44-55) all possessed a yield strength of 450 MPa or higher and were confirmed to be seamless steel pipes with excellent CO2 corrosion resistance, preventing pitting corrosion exceeding 25 μm even in CCS corrosion environments. In contrast, comparative examples (steel pipes No. 29-43) that fell outside the scope of the present invention did not achieve the desired strength or toughness and could not ensure excellent CO2 corrosion resistance in CCS corrosion environments.

Claims

1. The composition is as follows, by mass%, containing C: 0.05-0.15%, Si: 0.05-0.50%, Mn: 1.0-2.0%, P: 0.030% or less, S: 0.005% or less, Cr: 0.010-0.3%, Mo: 0.01-0.2%, Ca: 0.001-0.01%, V: 0.010-0.06%, Ti: 0.005-0.05%, B: 0.0001-0.0050%, Sb: 0.001-1.000%, Al: 0.100% or less, with the remainder being Fe and unavoidable impurities. A seamless steel pipe having a metall structure in which the average grain size at the center of the wall thickness is 40.0 μm or less and the maximum grain size at the center of the wall thickness is 90.0 μm or less, a yield strength of 450 MPa or more, and an absorbed energy of 250 J or more at -30°C.

2. The seamless steel pipe according to claim 1, wherein the component composition further contains one or more selected from Nb: 0.080% or less, Cu: 0.50% or less, Ni: 0.50% or less, and Sn: 0.100% or less, in mass percent.

3. A seamless steel pipe according to claim 1 or 2, wherein the wall thickness is 35 mm or less.

4. A method for manufacturing a seamless steel pipe according to any one of claims 1 to 3, comprising: a pipe-making step of forming a steel pipe from a steel pipe material having the above-mentioned component composition; a quenching step of heating the steel pipe to an Ac3 transformation point or higher, and then cooling it to a cooling stop temperature of 100°C or lower; and a tempering step of tempering the steel pipe that has undergone the quenching step at a temperature of Ac1 transformation point or lower.

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