Steel pipe for line pipe with excellent hydrogen embrittlement resistance, manufacturing method thereof, steel material for line pipe and manufacturing method thereof

Abstract

The purpose of the present invention is to provide a steel pipe for a line pipe and a method of manufacturing the same, and a steel material for a line pipe and a method of manufacturing the same, the steel pipe for a line pipe being suitable for steel structures that are used in a high-pressure hydrogen gas environment, such as a line pipe for 100% hydrogen gas or natural gas containing hydrogen with a hydrogen partial pressure of more than or equal to 1 MPa (natural gas is a gas having hydrocarbon, such as methane or ethane, as a major component), the steel pipe for a line pipe having high strength and excellent hydrogen embrittlement resistance characteristics in a high-pressure hydrogen gas environment.　The steel pipe for a line pipe is characterized by having excellent hydrogen embrittlement resistance characteristics and a specific component composition and a specific structure, wherein the fatigue limit stress in hydrogen of more than or equal to 1 MPa is more than or equal to 200 MPa, and the value of the fatigue limit stress in hydrogen of more than or equal to 1 MPa over the fatigue limit stress in an inert gas environment is more than or equal to 0.90.

Description

[Technical Field]

[0001] The present invention relates to a steel pipe for line pipe having excellent hydrogen embrittlement resistance suitable for applications such as line pipe for transporting hydrogen gas, a method for manufacturing the same, a steel material for line pipe and a method for manufacturing the same. [Background technology]

[0002] Existing energy infrastructure includes line pipes for natural gas transportation. Steel materials for these pipes have been required to suppress hydrogen-induced cracking in sour environments. Meanwhile, hydrogen has recently attracted significant attention worldwide as a clean energy source for building a decarbonized society. To transport large quantities of hydrogen, the construction of hydrogen gas transportation networks using natural gas line pipes partially mixed with hydrogen or pressurized hydrogen gas as an alternative is being considered. These pipelines are expected to operate at high pressures of 1–40 MPa, exposing the line pipes to high-pressure hydrogen gas. Steel materials used in such environments are susceptible to "hydrogen embrittlement," in which hydrogen penetrates the steel and degrades its properties. Therefore, steels must combine the high toughness and sour resistance required of conventional line pipes with the resistance to hydrogen embrittlement required in hydrogen gas environments.

[0003] Austenitic stainless steels such as SUS316L, which are less susceptible to hydrogen embrittlement than low-alloy steels, have traditionally been used for steel structures used in high-pressure hydrogen gas environments. However, austenitic stainless steels such as SUS316L are expensive and have low strength, so when designed to withstand high hydrogen pressures, the wall thickness becomes thick, which makes the hydrogen structure itself expensive. For this reason, there has been a strong demand for low-cost low-alloy steels for hydrogen structures that can withstand high-pressure hydrogen gas environments.

[0004] In response to such demands, for example, Patent Document 1 describes a steel for high-pressure hydrogen environments, which is used in high-pressure hydrogen environments, and claims that by making the Ca / S ratio less than 1.5 or 11 or more, the diffusible hydrogen concentration ratio is reduced and embrittlement due to diffusible hydrogen is suppressed.

[0005] Patent Document 2 describes a technology that discovered the finding that by using a low-alloy high-strength steel adjusted to a specific component composition, the reduction of area and elongation values ​​in a 45 MPa hydrogen atmosphere are greater than those of JIS G3128SHY685NS in the air tensile strength range of 900 to 950 MPa, and that the steel has excellent resistance to high-pressure hydrogen environment embrittlement properties.

[0006] Furthermore, Patent Document 3 describes a Cr-Mo high-strength low-alloy steel that is tempered at a relatively high temperature of 560 to 580°C, and that has a grain size of 8.4 or more after tempering, and a tensile strength adjusted to an extremely narrow range of 900 to 950 MPa, thereby producing a low-alloy high-strength steel that exhibits excellent elongation and drawing characteristics even in a 45 MPa hydrogen atmosphere and has excellent resistance to high-pressure hydrogen environment embrittlement.

[0007] Furthermore, the low-alloy steel for use in a high-pressure hydrogen gas environment proposed in Patent Document 4 is obtained by adding V, increasing the Mo content compared to existing steels, raising the tempering temperature, and utilizing V-Mo carbides, thereby improving the carbide morphology at the grain boundaries and significantly improving resistance to embrittlement in a hydrogen environment.

[0008] Furthermore, Patent Document 5 proposes a steel for high-pressure hydrogen gas storage containers that has excellent hydrogen resistance. According to the technology described in Patent Document 5, when manufacturing a steel plate, long-term stress relief annealing is performed after normalizing treatment, whereby MC-based carbides (Mo, V)C are dispersed and precipitated finely and densely, improving the hydrogen resistance of the steel, such as its resistance to hydrogen embrittlement.

[0009] Furthermore, Patent Document 6 proposes a steel material in which the metal structure is mainly composed of bainite with an area fraction of 90% or more, and cementite with an average grain size of 50 nm or less and an average aspect ratio of 3 or less is dispersed and precipitated in the bainite. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] Japanese Patent Application Laid-Open No. 2005-2386 [Patent Document 2] Japanese Patent Application Laid-Open No. 2009-46737 [Patent Document 3] Japanese Patent Application Laid-Open No. 2009-275249 [Patent Document 4] Japanese Patent Application Laid-Open No. 2009-74122 [Patent Document 5] Japanese Patent Application Laid-Open No. 2010-37655 [Patent Document 6] Japanese Patent Application Laid-Open No. 2012-107332 [Non-patent literature]

[0011] [Non-Patent Document 1] Matsunaga et al., Int J Hydrogen Energy, Vol.40(2015), p.5739-5748 [Non-patent document 2] Japan Heat Treatment Technology Association (author), Introduction to the Structure and Properties of Metallic Materials - Heat Treatment and Structure Control to Make the Most of Materials, 2004 Summary of the Invention [Problem to be solved by the invention]

[0012] The pressure inside linepipes fluctuates during operation and undergoes periodic shutdowns, subjecting structures to repeated stresses. Therefore, fatigue fracture must be considered when designing steel structures such as linepipes. However, as shown in Non-Patent Document 1, it is known that the fatigue life of materials decreases in high-pressure hydrogen environments. This means that if linepipe materials are designed based on conventional natural gas linepipes, their service life will be shortened. However, while the above-mentioned conventional technologies can suppress the occurrence of hydrogen-induced cracking in sour environments, they are unable to sufficiently increase fatigue strength in hydrogen gas. In other words, it is difficult to achieve both the suppression of hydrogen-induced cracking in sour environments and high fatigue strength in hydrogen gas.

[0013] In view of the above-mentioned problems of the conventional technology, the present invention aims to provide a steel pipe for line pipe that has high strength and excellent resistance to hydrogen embrittlement in a high-pressure hydrogen gas environment, suitable for steel structures to be used in a high-pressure hydrogen gas environment, such as line pipe for 100% hydrogen gas or natural gas containing hydrogen at a hydrogen partial pressure of 1 MPa or more (natural gas is a gas whose main components are hydrocarbons such as methane and ethane), a manufacturing method thereof, and a steel material for line pipe and a manufacturing method thereof.

[0014] Here, "excellent resistance to hydrogen embrittlement in a high-pressure hydrogen gas environment" refers to a material having a hydrogen fatigue limit of 200 MPa or greater, which is the stress at which a material fails after 2 million cycles without fracture, as determined by fatigue testing in accordance with ASTM E466, Fatigue Testing, under conditions of room temperature (20±10°C), hydrogen gas at a pressure of 1 MPa or greater, or a natural gas mixture (mainly composed of hydrocarbons such as methane and ethane) containing hydrogen at a partial pressure of 1 MPa or greater, with a frequency of 1 Hz, a sinusoidal waveform, load control, uniaxial tension and compression, and a stress ratio of R = -1.0. The hydrogen fatigue limit / inert gas fatigue limit is 0.90 or greater. Natural gas containing hydrogen at a partial pressure of 1 MPa or greater refers, for example, to a material with a hydrogen concentration of 30% or less by volume and a total gas pressure of 30 MPa or less.

[0015] Furthermore, if the fatigue limit stress in hydrogen under the above environment is 200 MPa or more and the ratio of the fatigue limit stress in hydrogen of the steel under the above environment to the fatigue limit stress in an inert gas environment is 0.90 or more, it will be possible to design long-life steel structures for hydrogen use, such as line pipes, within the thickness range that can be manufactured using processes for manufacturing seamless steel pipes and UOE steel pipes.

[0016] The term "steel material" as used herein includes thin steel plates, thick steel plates, seamless steel pipes, electric resistance welded steel pipes, steel sections, steel bars, and the like. [Means for solving the problem]

[0017] The present inventors have intensively studied the conditions that steel materials must satisfy in order to obtain steel pipes and steel materials for line pipes that are excellent in hydrogen embrittlement resistance, and have come to invent new steel pipes and steel materials for line pipes. The steel pipes and steel materials of the present invention also have high strength, and in the present invention, high strength refers to a tensile strength of 520 MPa or more.

[0018] The gist of the present invention is as follows. [1] Steel pipe for line pipe, In mass%, C: 0.10~0.45%, Si: 0.01 to 2.0% Mn: 0.5 to 1.5%, P: 0.0001 to 0.015%, S: 0.0002 to 0.0015%, Al: 0.005 to 0.15%, O: 0.01% or less, N: 0.010% or less, H: 0.0010% or less, including Or even more so, Nb: 0 to 0.10% Ti: 0 to 0.1%, Ca: 0 to 0.005%, Ni: 0 to 2.0% Cu: 0-1.0% Cr: 0 to 1.0%, Mo: 0-0.60% W: 0-1.0%, V: 0~0.10%, Zr: 0 to 0.050%, REM: 0~0.050%, Mg: 0 to 0.050% B: 0~0.0020%, Hf: 0 to 0.2% Ta: 0 to 0.2%, Re:0~0.005%, Sn: 0 to 0.3% Sb: 0 to 0.3%, The balance is Fe and unavoidable impurity elements, A steel pipe for line pipe with excellent hydrogen embrittlement resistance, having an area fraction of retained austenite of 0 to 3%, and having bainite or martensite at a position 1 / 4 of the way through the wall from the inner surface of the steel pipe, with the bainite having an area fraction of 90% or more or the martensite having an area fraction of 90% or more, a fatigue limit stress in hydrogen of 1 MPa or more of 200 MPa or more, and a fatigue limit stress in hydrogen of 1 MPa or more / fatigue limit stress in an inert gas environment of 0.90 or more. [2] The chemical composition further comprises, in mass %, Nb: 0.001 to 0.10%, Ti: 0.005 to 0.1%, Ca: 0.0001 to 0.005%, Ni: 0.01 to 2.0% Cu: 0.01 to 1.0%, Cr: 0.01 to 1.0%, Mo: 0.01 to 0.60%, W: 0.01 to 1.0%, V: 0.01 to 0.10%, Zr: 0.0001 to 0.050%, REM: 0.0001~0.050%, Mg: 0.0001 to 0.050%, B: 0.0001~0.0020%, Hf: 0.0001 to 0.2%, Ta: 0.0001 to 0.2%, Re: 0.0001~0.005%, Sn: 0.0001 to 0.3%, A steel pipe for line pipe excellent in hydrogen embrittlement resistance according to [1], wherein Sb is 0.0001 to 0.3%. [3] A casting process of casting a steel material having the chemical composition described in [1] or [2] at a casting speed of 1.8 m / min or less; a heating step of heating at 1350°C or less; a hot rolling step in which the steel material heated in the heating step is rolled under the condition of a rolling end temperature of 820°C or higher to form a steel pipe shape; a cooling step in which the steel pipe obtained in the hot rolling step is held at a temperature of not less than Ac3 point and not more than 1000°C, and then the cooling conditions are group A or group B below; a tempering step of tempering the steel pipe obtained in the cooling step at a temperature of 400°C or higher and Ac1 point or lower. Group A: The steel pipe is cooled to 50°C or below at an average cooling rate of 15°C / s or more from 800°C to 550°C at a position 1 / 4 of the way through the wall from the inner surface of the steel pipe, and at an average cooling rate of 15°C / s or less from 550°C to 50°C at a position 1 / 4 of the way through the wall from the inner surface of the steel pipe. Group B: The steel pipe is cooled to 50°C or below at an average cooling rate of 10°C / s or more from 800°C to 300°C at a position 1 / 4 of the wall thickness from the inner surface of the steel pipe, and at an average cooling rate of 5°C / s or less from 300°C to 50°C at a position 1 / 4 of the wall thickness from the inner surface of the steel pipe. [4] A method for producing a steel pipe for line pipe according to [3], which includes a quenching step in which, before the tempering step, the steel is reheated to a temperature not lower than the Ac3 point and not higher than 1000°C, and the cooling conditions are the following group A or group B: Group A: The steel pipe is cooled to 50°C or below at an average cooling rate of 15°C / s or more from 800°C to 550°C at a position 1 / 4 of the way through the wall from the inner surface of the steel pipe, and at an average cooling rate of 15°C / s or less from 550°C to 50°C at a position 1 / 4 of the way through the wall from the inner surface of the steel pipe. Group B: The steel pipe is cooled to 50°C or below at an average cooling rate of 10°C / s or more from 800°C to 300°C at a position 1 / 4 of the wall thickness from the inner surface of the steel pipe, and at an average cooling rate of 5°C / s or less from 300°C to 50°C at a position 1 / 4 of the wall thickness from the inner surface of the steel pipe. [5] The method for producing a steel pipe for line pipe according to [3] or [4], wherein the casting speed is 1.0 m / min or less. [6] In mass %, C: 0.10~0.45%, Si: 0.01 to 2.0% Mn: 0.5 to 1.5%, P: 0.0001 to 0.015%, S: 0.0002 to 0.0015%, Al: 0.005 to 0.15%, O: 0.01% or less, N: 0.010% or less, H: 0.0010% or less, including Or even more so, Nb: 0 to 0.10% Ti: 0 to 0.1%, Ca: 0 to 0.005%, Ni: 0 to 2.0% Cu: 0-1.0% Cr: 0 to 1.0%, Mo: 0-0.60% W: 0-1.0%, V: 0~0.10%, Zr: 0 to 0.050%, REM: 0~0.050%, Mg: 0 to 0.050% B: 0~0.0020%, Hf: 0 to 0.2% Ta: 0 to 0.2%, Re:0~0.005%, Sn: 0 to 0.3% Sb: one or more selected from 0 to 0.3%, The balance is Fe and unavoidable impurity elements, A steel material for linepipe having excellent hydrogen embrittlement resistance, in which the area fraction of retained austenite is 0 to 3%, and the steel has bainite or martensite at the 1 / 4 position of the plate thickness, the area fraction of the bainite being 90% or more, or the area fraction of the martensite being 90% or more, the fatigue limit stress in hydrogen of 1 MPa or more being 200 MPa or more, and the fatigue limit stress in hydrogen of 1 MPa or more / fatigue limit stress in an inert gas environment being 0.90 or more. [7] The chemical composition further comprises, in mass %, Nb: 0.001 to 0.10%, Ti: 0.005 to 0.1%, Ca: 0.0001 to 0.005%, Ni: 0.01 to 2.0% Cu: 0.01 to 1.0%, Cr: 0.01 to 1.0%, Mo: 0.01 to 0.60%, W: 0.01 to 1.0%, V: 0.01 to 0.10%, Zr: 0.0001 to 0.050%, REM: 0.0001~0.050%, Mg: 0.0001 to 0.050%, B: 0.0001~0.0020%, Hf: 0.0001 to 0.2%, Ta: 0.0001 to 0.2%, Re: 0.0001~0.005%, Sn: 0.0001 to 0.3%, The steel material for line pipe according to [6], wherein Sb is 0.0001 to 0.3%. [8] A casting process of casting a steel material having the chemical composition described in [6] or [7] at a casting speed of 1.8 m / min or less; a heating step of heating at 1350°C or less; a hot rolling process in which the steel material heated in the heating process is rolled under the condition of a rolling end temperature of 820°C or higher; A cooling step in which the steel material obtained in the hot rolling step is held at a temperature of not less than Ac3 point and not more than 1000°C, and then the cooling conditions are the following group A or group B: A tempering step of tempering the steel material obtained in the cooling step at a temperature of 400°C or higher and Ac1 point or lower. Group A: Cool the steel to 50°C or below at an average cooling rate of 15°C / s or more from 800°C to 550°C at a position 1 / 4 of the way through the wall from the surface, and at an average cooling rate of 15°C / s or less from 550°C to 50°C at a position 1 / 4 of the way through the wall from the surface. Group B: Cool the steel to 50°C or below at an average cooling rate of 10°C / s or more from 800°C to 300°C at a position 1 / 4 of the thickness from the steel surface, and at an average cooling rate of 5°C / s or less from 300°C to 50°C at a position 1 / 4 of the thickness from the steel surface. [9] The method for producing a steel material for line pipe according to [8], further comprising a quenching step of reheating the steel material to a temperature not lower than the Ac3 point and not higher than 1000°C before the tempering step, and cooling the steel material under the following conditions: Group A or Group B. Group A: Cool the steel to 50°C or below at an average cooling rate of 15°C / s or more from 800°C to 550°C at a position 1 / 4 of the way through the wall from the surface, and at an average cooling rate of 15°C / s or less from 550°C to 50°C at a position 1 / 4 of the way through the wall from the surface. Group B: Cool the steel to 50°C or below at an average cooling rate of 10°C / s or more from 800°C to 300°C at a position 1 / 4 of the thickness from the steel surface, and at an average cooling rate of 5°C / s or less from 300°C to 50°C at a position 1 / 4 of the thickness from the steel surface.

[10] The method for producing a steel material for line pipe according to [8] or [9], wherein the casting speed is 1.0 m / min or less. [Effects of the Invention]

[0019] According to the present invention, steel pipes and steel materials having extremely improved resistance to hydrogen embrittlement in a high-pressure hydrogen gas environment can be easily and simply manufactured, which is of great industrial benefit. Furthermore, according to the present invention, the hydrogen embrittlement resistance of steel structures such as line pipes for high-pressure hydrogen gas can be significantly improved, and the fatigue resistance can be improved, which has the effect of greatly contributing to the extension of the life of the steel structures. DETAILED DESCRIPTION OF THE INVENTION

[0020] Next, a method for carrying out the present invention will be specifically described. Note that the following description shows a preferred embodiment of the present invention, and the present invention is not limited to the following description in any way.

[0021] A method for carrying out the process for steel pipes will be specifically described as a first embodiment, and then a method for carrying out the process for steel materials will be specifically described as a second embodiment.

[0022] First embodiment [Component composition] The reasons for limiting the chemical composition of the steel pipe (including steel material) of the present invention will be explained below. In the following explanation, "%" represents "mass %" unless otherwise specified.

[0023] C: 0.10 to 0.45% C is an element necessary for increasing strength. For this reason, the C content is set to 0.10% or more. The C content is preferably 0.13% or more. On the other hand, if the C content exceeds 0.45%, quench cracks may occur during quenching, so the C content is set to 0.45% or less. The C content is preferably 0.25% or less, more preferably 0.20% or less, and even more preferably 0.17% or less.

[0024] Si: 0.01 to 2.0% Si is added for deoxidation, but if the content is less than 0.01%, the deoxidation effect is insufficient. Therefore, the Si content is 0.01% or more. The Si content is preferably 0.08% or more, and more preferably 0.1% or more. On the other hand, if the Si content exceeds 2.0%, the effect saturates, so the Si content is 2.0% or less. The Si content is preferably 1.8% or less, and more preferably 1.0% or less. Furthermore, if the Si content exceeds 0.5%, toughness and weldability deteriorate, so the Si content is even more preferably 0.5% or less.

[0025] Mn: 0.5 to 1.5% Mn effectively contributes to improving strength and toughness, but if the content is less than 0.5%, the effect of addition is poor. Therefore, the Mn content is set to 0.5% or more. The Mn content is preferably 0.6% or more, more preferably 0.7% or more, and even more preferably 0.8% or more. On the other hand, if the Mn content exceeds 1.5%, the hardness of the surface layer and central segregation increases during controlled cooling, resulting in a deterioration in SSCC (resistance to sulfide stress corrosion cracking) and HIC (hydrogen induced cracking) resistance. Weldability also deteriorates. For this reason, the Mn content is limited to 1.5% or less. The Mn content is preferably 1.4% or less, and more preferably 1.3% or less.

[0026] P: 0.0001 to 0.015% P is an inevitable impurity element that deteriorates weldability and increases the hardness of the center segregation, thereby deteriorating HIC resistance. This tendency becomes more pronounced when the P content exceeds 0.015%, so the upper limit of the P content is set at 0.015%. The P content is preferably 0.010% or less, and more preferably 0.008% or less. While a lower content is better, the P content is set at 0.0001% or more from the viewpoint of refining costs.

[0027] S: 0.0002 to 0.0015% S is an unavoidable impurity element that forms MnS inclusions in steel, which degrades HIC resistance, so a low S content is preferable, but up to 0.0015% is acceptable. Therefore, the S content is set to 0.0015% or less. The S content is preferably 0.0010% or less, and more preferably 0.0008% or less. The lower the content, the better, but from the perspective of refining costs, the S content is set to 0.0002% or more.

[0028] Al: 0.005 to 0.15% Al is added as a deoxidizer, but if it is less than 0.005%, it has no effect. Therefore, the Al content is set to 0.005% or more. The Al content is preferably 0.01% or more, and more preferably 0.03% or more. On the other hand, if it exceeds 0.15%, the cleanliness of the steel decreases and the toughness deteriorates, so the Al content is limited to 0.15% or less. The Al content is preferably 0.10% or less, more preferably 0.08% or less, and even more preferably 0.05% or less.

[0029] O: 0.01% or less Since O causes the formation of oxide-based inclusions, the lower the content, the better, but there is no problem if the O content is 0.01% or less. Therefore, the O content is set to 0.01% or less. The O content is preferably 0.005% or less. More preferably, the O content is less than 0.003%. There is no particular lower limit, but since reducing the oxygen content to 0% increases costs, the O content is preferably 0.001% or more.

[0030] N: 0.010% or less The effect of N on the fatigue properties of steel pipes is small, and if the N content is 0.010% or less, the effects of the present invention are not impaired from the perspective of toughness. Therefore, the N content is set to 0.010% or less. The N content is preferably set to 0.008% or less, and more preferably set to 0.006% or less. The N content is further preferably set to 0.004% or less. On the other hand, from the perspective of improving toughness, a low N content is desirable, but an excessive reduction increases the steelmaking cost, so the N content is preferably set to 0.00001% or more. The N content is preferably set to 0.001% or more.

[0031] H:0.0010% or less H may be introduced into steel during various manufacturing processes, and a high introduction amount increases the risk of cracking after solidification and accelerates fatigue crack propagation. Furthermore, a high introduction amount reduces the fatigue stress limit, so it is important to reduce the amount of hydrogen in the steel pipe. These effects are not a problem if the H content is 0.0010% or less, so the H content is set to 0.0010% or less. The H content is preferably 0.0005% or less, more preferably 0.0003% or less, and even more preferably 0.0001% or less. On the other hand, a H content of less than 0.00001% increases costs, so the H content is preferably 0.00001% or more. The hydrogen content is the amount of hydrogen remaining after forming into steel, steel pipe, UOE, etc.

[0032] In order to further improve the strength and toughness of the steel pipe, the chemical composition of the present disclosure may optionally contain one or more elements selected from Nb, Ti, Ca, Ni, Cu, Cr, Mo, W, V, Zr, REM, Mg, B, Hf, Ta, Re, Sn, and Sb within the following ranges.

[0033] Nb: 0-0.10% and Ti: 0-0.1% Nb is an element effective in increasing the strength and toughness of steel, but if it exceeds 0.10%, the toughness of the weld deteriorates, so if it is contained, the Nb content is set to 0.10% or less. The Nb content is preferably set to 0.08% or less. The Nb content is more preferably set to 0.06% or less. The Nb content may be 0% or more, but if the Nb content is less than 0.001%, the effect of containing Nb is difficult to obtain, so if it is contained, it is preferably set to 0.001% or more. The Nb content is more preferably set to 0.01% or more.

[0034] Ti is an element effective in increasing the strength and toughness of steel, but if it exceeds 0.1%, the toughness of the weld deteriorates, so if Ti is contained, the Ti content is set to 0.1% or less. The Ti content is preferably 0.05% or less. The Ti content is more preferably 0.03% or less, and even more preferably 0.02% or less. The Ti content may be 0% or more, but if the Ti content is less than 0.005%, the effect of containing it is difficult to obtain, so if it is contained, it is preferably 0.005% or more. The Ti content is more preferably 0.008% or more.

[0035] Ca: 0 to 0.005% Ca is an element effective in improving HIC resistance by controlling the morphology of sulfide inclusions, but not only does this effect saturate, but it also reduces HIC resistance by reducing the cleanliness of the steel, so if Ca is contained, the Ca content is limited to 0.005% or less. The Ca content is preferably 0.003% or less. The Ca content is more preferably 0.002% or less. The Ca content may be 0% or more, but if it is less than 0.0001%, the effect of adding it is difficult to obtain, so if it is contained, it is preferably 0.0001% or more. The Ca content is more preferably 0.001% or more.

[0036] Ni: 0 to 2.0% Ni is an element that is effective in improving toughness and increasing strength, but to reduce costs, its content should be limited to 2.0% or less. The Ni content is preferably 1.5% or less. The Ni content is more preferably 1.2% or less, and even more preferably 1.0% or less. The Ni content may be 0% or more, but to obtain the above effects, it is preferable to contain 0.01% or more of Ni.

[0037] Cu: 0 to 1.0% Cu is an element effective in improving toughness and increasing strength, but if the content is too high, weldability deteriorates, so if Cu is contained, it should be 1.0% or less. The Cu content is preferably 0.5% or less. The Cu content is more preferably 0.3% or less, and even more preferably 0.2% or less. The Cu content may be 0% or more, but to obtain the above effects, it is preferable to contain 0.01% or more.

[0038] Cr: 0 to 1.0% Like Mn, Cr is an effective element for obtaining sufficient strength even with low C content. However, if the content is too high, the hardenability becomes excessive, resulting in a deterioration of SSCC resistance. It also deteriorates weldability. Therefore, if Cr is contained, it should be 1.0% or less. The Cr content is preferably 0.8% or less. The Cr content is more preferably 0.5% or less, and even more preferably 0.1% or less. The Cr content may be 0% or more, but to obtain this effect, it is preferable to contain 0.01% or more of Cr. The Cr content is more preferably 0.02% or more.

[0039] Mo: 0 to 0.60% Mo is an element effective in improving toughness and increasing strength, and is effective in improving SSCC resistance regardless of hydrogen sulfide partial pressure. However, if the content is too high, hardenability becomes excessive, resulting in deterioration of SSCC resistance. Weldability also deteriorates. Therefore, when Mo is contained, the Mo content is set to 0.60% or less. More preferably, it is set to 0.50% or less, and even more preferably, it is set to 0.40% or less. Most preferably, the Mo content is set to 0.03% or less. The Mo content may be 0% or more, but to obtain the above effects, it is preferable to contain 0.005% or more of Mo. It is more preferable to contain 0.01% or more of Mo.

[0040] W: 0 to 1.0% W contributes to increasing the strength of steel pipes, but if the W content exceeds 1.0%, the effect saturates and becomes a factor in increasing costs; therefore, if W is contained, the W content is set to 1.0% or less. The W content is preferably set to 0.8% or less. To further reduce costs, the W content is more preferably set to 0.5% or less. The W content is even more preferably set to 0.03% or less. The W content may be 0% or more, but to obtain the above-mentioned effects, the content is preferably set to 0.01% or more.

[0041] V: 0 to 0.10% V is an element that can be optionally added to improve the strength and toughness of steel pipes, but if the V content exceeds 0.10%, the toughness of the weld deteriorates, so if V is added, it should be 0.10% or less. The V content is preferably 0.08% or less. The V content is more preferably 0.06% or less, and even more preferably 0.03% or less. The V content may be 0% or more, but if the V content is less than 0.01%, the effect of adding it is difficult to obtain, so it is preferably 0.01% or more.

[0042] Zr:0~0.050%, REM:0~0.050%, Mg:0~0.050% Zr, REM, and Mg are elements that can be optionally added to improve toughness through grain refinement or to improve crack resistance through control of inclusion properties. However, their effects saturate when their content exceeds 0.050%, so if they are contained, they should all be 0.050% or less. That is, if contained, the Zr content should be 0.050% or less. The Zr content is preferably 0.040% or less. The Zr content is more preferably 0.030% or less. The Zr content is further preferably 0.010% or less, and most preferably 0.005% or less. Furthermore, if contained, the REM content should be 0.050% or less. The REM content is preferably 0.040% or less. The REM content is more preferably 0.030% or less. Furthermore, if contained, the Mg content should be 0.050% or less. The Mg content is preferably 0.040% or less. The Mg content is more preferably 0.030% or less. The contents of these elements may be 0% or more, but if the contents are less than 0.0001%, the effects of the inclusion are difficult to obtain, so it is preferable that they be 0.0001% or more. That is, the Zr content is preferably 0.0001% or more. The Zr content is more preferably 0.0005% or more. The REM content is preferably 0.0001% or more. The REM content is more preferably 0.0005% or more. The Mg content is preferably 0.0001% or more. The Mg content is more preferably 0.0005% or more.

[0043] B: 0 to 0.0020% B is an element that improves hardenability, contributes to increasing the strength of steel pipes, and inhibits the coarsening of prior austenite grains, improving various material properties. On the other hand, if the B content exceeds 0.0020%, the effect saturates and causes costs to increase. Therefore, if B is contained, the B content is set to 0.0020% or less. The B content is preferably set to 0.0015% or less. The B content is more preferably set to 0.0012% or less. To suppress costs, the B content is even more preferably set to 0.0010% or less. The B content may be 0% or more, but to obtain the above effects, the content is preferably set to 0.0001% or more. More preferably, the B content is 0.0005% or more.

[0044] Hf: 0 to 0.2%, Ta: 0 to 0.2% These elements contribute to increasing the strength of steel pipes, but if their content exceeds 0.2%, the effect saturates and costs increase, so if they are contained, they are set to 0.2% or less. That is, if they are contained, Hf is set to 0.2% or less. Hf is preferably set to 0.1% or less. Hf is more preferably set to 0.05% or less. Furthermore, if they are contained, Ta is set to 0.2% or less. Ta is preferably set to 0.1% or less. Ta is more preferably set to 0.05% or less. The Hf and Ta contents may be 0% or more, but to obtain the above effects, the contents are preferably set to 0.0001% or more. That is, the Hf content is preferably 0.0001% or more. More preferably, the Hf content is 0.0010% or more. Furthermore, the Ta content is preferably 0.0001% or more. More preferably, the Ta content is 0.0010% or more.

[0045] Re:0~0.005% Re contributes to increasing the strength of steel pipes, but if the content exceeds 0.005%, the effect saturates and becomes a factor in increasing costs, so if Re is contained, it is set to 0.005% or less. The Re content is preferably set to 0.003% or less. The Re content is more preferably set to 0.002% or less. The Re content may be 0% or more, but in order to obtain the above-mentioned effects, the content is preferably set to 0.0001% or more. More preferably, it is set to 0.001% or more.

[0046] Sn: 0-0.3%, Sb: 0-0.3% These elements contribute to increasing the strength and hardenability of steel pipes, but if their content exceeds 0.3%, the effect saturates and they become a factor in increasing costs. Therefore, if they are contained, they are set to 0.3% or less. That is, the Sn content is set to 0.3% or less. The Sn content is preferably set to 0.2% or less. The Sn content is more preferably set to 0.1% or less. To reduce costs, the Sn content is even more preferably set to 0.01% or less. Furthermore, the Sb content is set to 0.3% or less. The Sb content is preferably set to 0.2% or less. The Sb content is more preferably set to 0.1% or less. To reduce costs, the Sb content is even more preferably set to 0.01% or less. The Sn and Sb contents may be 0% or more, but to obtain the above effects, the contents are preferably set to 0.0001% or more. That is, the Sn content is preferably set to 0.0001% or more. More preferably, the Sn content is 0.0010% or more. The Sb content is preferably 0.0001% or more, and more preferably 0.0010% or more.

[0047] In the chemical composition of the steel pipe, the balance other than the above-mentioned components (elements) consists of Fe and unavoidable impurity elements.

[0048] The metal structure of the steel pipe of the present invention will be described below.

[0049] Metal structure Retained austenite area fraction: 0 to 3% Residual austenite in a steel pipe increases the amount of hydrogen in the steel, which can increase susceptibility to hydrogen embrittlement. Furthermore, when austenite transforms into martensite due to stress load during use, the martensite is very hard and prone to hydrogen cracking, which can lead to cracks initiating from the martensite portion. In the present invention, the fatigue crack propagation rate is reduced by setting the area fraction of retained austenite to 3% or less. This is preferably 2% or less, and more preferably 1% or less. The retained austenite may be 0%.

[0050] The steel pipe has bainite or martensite at a position 1 / 4 of the wall thickness from the inner surface (in the case of steel, the position 1 / 4 of the plate thickness from the steel surface), with bainite at an area fraction of 90% or more and martensite at an area fraction of 90% or more To achieve a high tensile strength of 520 MPa or more, the steel structure must be bainite or martensite. On the other hand, if a steel pipe contains both soft and hard phases, fatigue damage preferentially accumulates in the soft phase, making crack initiation more likely and lowering the fatigue stress limit. In a hydrogen environment, localized deformation is promoted, further accelerating fatigue damage to the soft phase and further lowering the fatigue stress limit in hydrogen. As a result, the fatigue stress limit in hydrogen / inert gas environment falls below 0.90. To improve this, the relative proportion of the soft phase must be reduced. Therefore, the metal structure must be a single structure of bainite or martensite, and must contain either bainite or martensite, with the area fraction being 90% or more. Preferably, the area fraction of either bainite or martensite is 92% or more, more preferably 95% or more. It is even more preferable to set it at 98% or more. The upper limit is not particularly limited, but it may be 100%. Furthermore, because fatigue cracks initiate from the inner surface of the steel pipe, uniformity of the steel pipe's inner surface structure is important. Therefore, the metallographic structure at the 1 / 4 position from the inner surface of the steel pipe was specified, and for steel materials, the metallographic structure at the 1 / 4 position in the plate thickness was specified so that the above effect could be obtained regardless of which surface is on the inner surface of the steel pipe.

[0051] Here, the bainite structure includes bainitic ferrite or granular bainite that transforms during or after cooling (accelerated cooling or quenching), which contributes to transformation strengthening, and also includes tempered bainite. The presence of heterogeneous structures such as ferrite, martensite, pearlite, island martensite, and retained austenite in the bainite structure reduces strength and toughness, so the smaller the volume fraction of structures other than the bainite phase, the better. Here, the martensite structure includes tempered martensite.

[0052] Furthermore, tempering bainite and martensite structures allows precipitation of carbides such as cementite. Precipitating fine carbides can hinder the straightness of the fatigue crack propagation path in hydrogen, and further reduce the fatigue crack growth rate. For this reason, tempered bainite or tempered martensite structures are preferred. Furthermore, it is preferable to precipitate carbides in a finely dispersed manner. Therefore, the average size of the carbides is preferably 200 nm or less, and more preferably 50 nm or less. The average size X of the carbides is calculated by the formula X=√(a) where a is the long side and b is the short side. 2 +b 2 ) / 2.

[0053] The fatigue limit stress in hydrogen of 1 MPa or more is 200 MPa or more, and the fatigue limit stress in hydrogen of 1 MPa or more / fatigue limit stress in an inert gas environment is 0.90 or more In order to design long-life steel structures for hydrogen use, such as line pipes, within the thickness range that can be manufactured using the process within the scope of the present invention, the hydrogen fatigue limit stress of the steel pipe must be 200 MPa or more. It is preferable that the hydrogen fatigue limit stress of 1 MPa or more is 220 MPa or more. It is more preferable that the hydrogen fatigue limit stress of 1 MPa or more is 250 MPa or more, and even more preferable that it is 270 MPa or more. There is no particular upper limit, but it is preferable that the hydrogen fatigue limit stress of 1 MPa or more is 500 MPa or less. Furthermore, the fatigue limit stress in hydrogen of 1 MPa or more / inert gas environment of the steel pipe must be 0.90 or more. It is also preferable that the fatigue limit stress in hydrogen of 1 MPa or more / inert gas environment is 0.92 or more. It is more preferable that the fatigue limit stress in hydrogen of 1 MPa or more / inert gas environment is 0.94 or more, and even more preferable that the fatigue limit stress in hydrogen of 1 MPa or more / inert gas environment is 0.96 or more. There is no particular upper limit, but the fatigue limit stress in hydrogen of 1 MPa or more / inert gas environment may be 1.10 or less. In this context, the term "inert gas" refers to the six elements in Group 0 of the periodic table: helium, neon, argon, krypton, xenon, and radon, as well as air, and an "inert gas environment" refers to an environment that includes any of the above.

[0054] The present invention has the above-described chemical composition and metal structure, which suppresses the toughness loss under a high-pressure hydrogen atmosphere and provides a tensile strength of 520 MPa or more, making it applicable to hydrogen line pipes. The upper limit of the tensile strength is not particularly limited, but it is preferably 950 MPa or less.

[0055] The plate thickness is preferably 5 mm or more and 30 mm or less.

[0056] [Manufacturing method] Next, a method for producing a steel pipe according to the present invention will be described. In the following explanation, the production method will be described using an example in which the steel pipe is a seamless steel pipe, but it goes without saying that electric resistance welded pipes and UOE steel pipes can also be produced by performing treatments to achieve a similar thermal history.

[0057] The steel pipe of the present invention can be produced by sequentially carrying out the following steps (1) to (3). (1) The process of casting steel material after adjusting its composition (2) a hot rolling process in which the cast material is heated and rolled to obtain steel pipes; and (3) The process of cooling (accelerated cooling) and tempering the steel pipe obtained in the hot rolling process (including the case of reheating and quenching before the tempering process). Each step will be explained below. In the following explanation, temperatures refer to the temperature at the center of the steel material or steel pipe thickness unless otherwise specified. The average cooling rate refers to the temperature at 1 / 4 of the wall thickness from the inner surface of the steel pipe. The temperatures at the center of the wall thickness and the temperature at 1 / 4 of the wall thickness from the inner surface of the steel pipe are estimated from the steel pipe surface temperature measured with a radiation thermometer using heat transfer calculations that take into account the heat transfer coefficient of the steel material.

[0058] [Casting process] Casting speed: 1.8m / min or less The slower the casting speed, the more the hydrogen concentration and inclusions in the steel can be reduced, and this effect is most pronounced at 1.8 m / min or less, so the casting speed is set to 1.8 m / min or less. Preferably, it is 1.5 m / min or less. More preferably, it is 1.0 m / min or less. Still more preferably, it is 0.5 m / min or less. Most preferably, it is 0.1 m / min or less. There is no particular lower limit, but the casting speed should be above 0 m / min.

[0059] [Heating process] In order to perform hot rolling, a steel material having the above-mentioned composition is heated. The steel material is not particularly limited, but for example, a billet obtained by a conventional continuous casting method can be used.

[0060] Heat to a temperature below 1350℃ If the heating temperature in the heating process exceeds 1350°C, the average grain size of the prior austenite grains becomes excessively large, resulting in deterioration of various properties. Therefore, the heating temperature is set to 1350°C or less. The heating temperature is more preferably 1300°C or less, even more preferably 1250°C or less, and most preferably 1200°C or less. On the other hand, a lower heating temperature is preferable because it can reduce the amount of hydrogen in the steel, but if it is too low, the finish rolling temperature decreases, making rolling difficult. Therefore, the heating temperature is preferably 950°C or more. The heating temperature is more preferably 1000°C or more. The heating time is not particularly specified, but if it is too long, there is a high risk of increasing the amount of hydrogen introduced into the steel pipe, so it is preferably 180 minutes or less. The heating time is more preferably 150 minutes or less, and even more preferably 120 minutes or less. There is no particular lower limit, but the heating time is preferably 30 minutes or more, and more preferably 60 minutes or more.

[0061] [Rolling process] Next, the steel material heated in the heating step is rolled into a steel pipe shape. For the rolling, hot rolling including piercing rolling using a conventional Mannesmann plug mill or Mannesmann mandrel mill can be used.

[0062] Rolling end temperature: 820°C or higher If the rolling end temperature is less than 820°C, the rolling load becomes excessive, increasing the risk of rolling problems. Therefore, the rolling end temperature is set to 820°C or higher. The rolling end temperature is preferably set to 850°C or higher, and more preferably to 900°C or higher. On the other hand, although there is no particular upper limit for the rolling end temperature, if the temperature is too high, the metal structure is likely to become non-uniform, so the rolling end temperature is preferably set to 1200°C or lower. The rolling end temperature is more preferably set to 1150°C or lower, and even more preferably to 1100°C or lower.

[0063] [Cooling process (accelerated cooling process)] In the cooling step, the steel material having the above-described chemical composition is heated to a temperature of not less than the Ac3 point and not more than 1000°C, either directly or after being processed into a steel pipe, and then cooled under the following cooling conditions of Group A or Group B. It is preferable to hold the steel material at the temperature for not less than 10 minutes, more preferably for not less than 15 minutes, and even more preferably for not less than 20 minutes. There is no particular upper limit, but it is preferable to hold the steel material at the temperature for not more than 60 minutes, and more preferably for not more than 45 minutes.

[0064] Heating temperature after steel pipe processing: Ac 3 points or more and 1000℃ or less If the heating temperature in the cooling process is lower than the Ac3 point, ferrite will remain in the steel after cooling, reducing the steel pipe's strength and fatigue stress limit in hydrogen. Therefore, the heating temperature is set to the Ac3 point or higher. The heating temperature is preferably set to the Ac3 point + 30°C or higher, and more preferably set to the Ac3 point + 50°C or higher. However, for composition systems in which the Ac3 point + 30°C or Ac3 point + 50°C exceeds 1000°C, the above-mentioned Ac3 point + 30°C or Ac3 point + 50°C temperatures do not apply. However, on the other hand, if the heating temperature is higher than 1000°C, austenite grains will coarsen, which may cause a decrease in the impact absorption energy value and toughness of the material after heat treatment. Therefore, the heating temperature is set to 1000°C or lower. The heating temperature is preferably set to 950°C or lower, and more preferably set to 900°C or lower. However, for composition systems in which 950°C or 900°C is lower than the Ac3 point, the above-mentioned 950°C or lower and 900°C or lower temperatures do not apply.

[0065] In the cooling process here, if the temperature after rolling satisfies the heating conditions, the steel sheet may be cooled as is, or may be reheated after rolling and then cooled. Also, if the steel sheet has been once cooled by air cooling, it may be reheated to a temperature of not less than the Ac3 point and not more than 1000°C, and then cooled under the cooling conditions of Group A or Group B below. In the present invention, the Ac3 point (°C) is calculated by the following formula. Ac3(℃)=910-203[C]1 / 2-30[Mn]+44.7[Si]+700[P]+100[Al]+31.5[Mo]-11[Cr]-15.2[Ni]-20[Cu]+104[V] In the formula, [M] represents the content (mass %) of element M.

[0066] average cooling rate Group A: Cooling to 50°C or less under the conditions that the average cooling rate from 800°C to 550°C at the 1 / 4 position of the wall thickness from the inner surface of the steel pipe is 15°C / s or more, and the average cooling rate from 550°C to 50°C is 15°C / s or less. If the average cooling rate from 800°C to 550°C at the 1 / 4 wall thickness position from the inner surface of the steel pipe is less than 15°C / s, a bainite structure with an area fraction of 90% or more cannot be obtained, resulting in a decrease in strength. Therefore, the average cooling rate from the inner surface of the steel pipe to the 1 / 4 wall thickness position is set to 15°C / s or more. From the viewpoint of suppressing microstructural variation, the average cooling rate is preferably 17°C / s or more. The average cooling rate from 800°C to 550°C is more preferably 20°C / s or more, and most preferably 22°C / s or more. On the other hand, to suppress grain size variation, the average cooling rate is preferably 50°C / s or less, more preferably 45°C / s or less, and even more preferably 40°C / s or less. Furthermore, by cooling to 50°C or less under the condition that the average cooling rate from 550°C to 50°C is 15°C / s or less, it is possible to reduce the amount of retained austenite and reduce the amount of hydrogen in the steel. For this reason, the average cooling rate from 550°C to 50°C is set to 15°C / s or less. The average cooling rate from 550°C to 50°C is more preferably 12°C / s or less, and even more preferably 10°C / s or less. Although there is no particular lower limit, the average cooling rate from 550°C to 50°C is preferably 1°C / s or more. The cooling method is not particularly limited, and any method such as water cooling, oil cooling, or air cooling can be used alone or in combination, but water cooling or oil cooling is preferred from 800°C to 550°C, and air cooling from 550°C to 50°C.

[0067] Group B: Cooled to 50°C or less under the conditions that the average cooling rate from 800°C to 300°C at a position 1 / 4 of the wall thickness from the inner surface of the steel pipe is 10°C / s or more, and the average cooling rate from 300°C to 50°C is 5°C / s or less If the average cooling rate from 800°C to 300°C at the quarter-wall position from the inner surface of the steel pipe is less than 10°C / s, a martensite structure of 90% or more will not be obtained, resulting in mixing with a bainite structure and a decrease in the fatigue stress limit in hydrogen. Therefore, the average cooling rate from the inner surface of the steel pipe at the quarter-wall position is set to 10°C / s or more. From the viewpoint of suppressing microstructural variation, the average cooling rate from 800°C to 300°C is preferably 12°C / s or more, more preferably 15°C / s or more, and even more preferably 17°C / s or more. While there is no particular upper limit, the average cooling rate is preferably 60°C / s or less. Furthermore, the amount of hydrogen in the steel can be reduced by cooling to 50°C or less under the condition that the average cooling rate from 300°C to 50°C is 5°C / s or less. For this reason, the average cooling rate from 300°C to 50°C is set to 5°C / s or less. The average cooling rate from 300°C to 50°C is preferably set to 1°C / s or less. There is no particular lower limit, but it is preferably set to 0.1°C / s or more. The cooling method is not particularly limited, and any method such as water cooling, oil cooling, or air cooling can be used alone or in combination, but water cooling or oil cooling is preferred from 800°C to 300°C, and air cooling from 300°C to 50°C.

[0068] [Reheating and quenching process (optimal conditions)] Reheating temperature before tempering: Ac3 point or higher and 1000°C or lower If the temperature at the center of the plate thickness is below the Ac3 point, some untransformed austenite remains, making it impossible to obtain the desired steel structure after hot rolling, quenching, and tempering (described later). Therefore, the pre-quenching heating temperature during reheating is preferably set to the Ac3 point or higher. Preferably, it is set to a temperature above the Ac3 point. Note that, in order to prevent excessive coarsening of the initial austenite grain size and improve production efficiency, the pre-quenching heating temperature is preferably set to 1000°C or lower. It is more preferably set to 980°C or lower, and even more preferably set to 960°C or lower. It is most preferably set to 950°C or lower. By setting the reheating temperature before quenching to a lower temperature within the range of the Ac3 point or higher, the initial austenite grain size can be refined, and the fatigue stress limit in hydrogen can be reduced.

[0069] Average cooling rate during quenching: Group A or Group B below Group A: Cooling to 50°C or less under the conditions that the average cooling rate from 800°C to 550°C at the 1 / 4 position of the wall thickness from the inner surface of the steel pipe is 15°C / s or more, and the average cooling rate from 550°C to 50°C is 15°C / s or less. If the average cooling rate from 800°C to 550°C at the quarter-wall position from the inner surface of the steel pipe is less than 15°C / s, a bainite structure with an area fraction of 90% or more cannot be obtained, resulting in a decrease in strength. For this reason, the average cooling rate from the inner surface of the steel pipe to the quarter-wall position is set to 15°C / s or more. From the perspective of suppressing microstructural variation, the average cooling rate is preferably 17°C / s or more, more preferably 20°C / s or more, and even more preferably 22°C / s or more. On the other hand, to suppress grain size variation, the average cooling rate is preferably 50°C / s or less, more preferably 47°C / s or less, and even more preferably 45°C / s or less. Furthermore, by cooling to 50°C or less under the condition that the average cooling rate from 550°C to 50°C is 15°C / s or less, it is possible to reduce the amount of retained austenite and the amount of hydrogen in the steel. For this reason, the average cooling rate from 550°C to 50°C is set to 15°C / s or less. The average cooling rate from 550°C to 50°C is preferably 12°C / s or less, and more preferably 10°C / s or less. Although there is no particular lower limit, the average cooling rate from 550°C to 50°C is preferably 1°C / s or more. The cooling method is not particularly limited, and any method such as water cooling, oil cooling, or air cooling can be used alone or in combination, but water cooling or oil cooling is preferred from 800°C to 550°C, and air cooling from 550°C to 50°C.

[0070] Group B: Cooled to 50°C or less under the conditions that the average cooling rate from 800°C to 300°C at a position 1 / 4 of the wall thickness from the inner surface of the steel pipe is 10°C / s or more, and the average cooling rate from 300°C to 50°C is 5°C / s or less If the average cooling rate from 800°C to 300°C at the quarter-wall position from the inner surface of the steel pipe is less than 10°C / s, 90% or more of the martensite structure will not be obtained, and mixing with the bainite structure will occur, resulting in a decrease in the fatigue stress limit in hydrogen. Therefore, the average cooling rate from the inner surface of the steel pipe to the quarter-wall position will be 10°C / s or more. From the viewpoint of suppressing microstructural variation, the average cooling rate is preferably 17°C / s or more, more preferably 20°C / s or more, and even more preferably 25°C / s or more. On the other hand, although there is no particular upper limit for the average cooling rate, if it exceeds 60°C / s, a large amount of hard structure will form on the steel sheet surface, preventing the steel structure having the structure targeted in the present invention from being obtained, and resulting in a decrease in fatigue properties in hydrogen, so the average cooling rate is preferably 60°C / s or less. Furthermore, the amount of hydrogen in the steel can be reduced by cooling to 50°C or less under the condition that the average cooling rate from 300°C to 50°C is 5°C / s or less. For this reason, the average cooling rate from 300°C to 50°C is set to 5°C / s or less. The average cooling rate is preferably 3°C / s or less, and more preferably 1°C / s or less. There is no particular lower limit, but it is preferably 0.1°C / s or more. The cooling method is not particularly limited, and any method such as water cooling, oil cooling, or air cooling can be used alone or in combination, but water cooling or oil cooling is preferred from 800°C to 300°C, and air cooling from 300°C to 50°C.

[0071] Cooling stop temperature during quenching: 50°C or less If the cooling stop temperature exceeds 50°C, the transformation will not be completed, and the desired steel structure will not be obtained after tempering. For this reason, quenching is carried out to a temperature of 50°C or less. The cooling stop temperature is preferably 45°C or less, and more preferably 40°C or less. Although there is no particular lower limit, the cooling stop temperature is preferably 25°C or more.

[0072] [Tempering process] 400℃ or aboveA C1 Temperature rises below the point By setting the tempering temperature to 400°C or higher, it is possible to reduce the retained austenite and hydrogen in the steel. The tempering temperature is preferably 450°C or higher, and more preferably 500°C or higher. C1 If the temperature is raised above the Ac1 point, the amount of retained austenite and hydrogen in the steel may increase. Therefore, the tempering temperature is set to Ac1 point or lower. C1The average temperature rise rate during tempering is in the range of -30°C or less. There is no particular upper limit to the average temperature rise rate during tempering, but it is preferably 1°C / s or less. The tempering time is not particularly specified, but a longer time reduces the amount of retained austenite and hydrogen in the steel pipe, so a tempering time of 60 minutes or more is preferred. A tempering time of 80 minutes or more is more preferred, and a tempering time of 100 minutes or more is even more preferred. If the tempering time is too long, the material strength will decrease too much and the effect will saturate, so a tempering time of 180 minutes or less is preferred.

[0073] In the present invention, A C1 The temperature (°C) is calculated using the following formula. Ac1=723-14Mn+22Si-14.4Ni+23.3Cr In the above formula, each element symbol represents the content (mass%) of each element in the steel, and elements that are not contained are represented as 0.

[0074] [Dehydrogenation process] If hydrogen is present in steel, the acceleration of fatigue crack propagation increases, resulting in a decrease in fatigue life and a decrease in the fatigue stress limit in hydrogen. Therefore, dehydrogenation treatment may be performed to release the hydrogen remaining after manufacturing. Dehydrogenation treatment reduces the amount of hydrogen in the steel by holding it at high temperature for a certain period of time before use, resulting in a steel sheet with excellent resistance to hydrogen embrittlement in a high-pressure hydrogen gas environment. The holding time R (h) is determined by the thickness of the steel pipe, the pipe thickness t (mm), and the hydrogen diffusion coefficient D (mm sec -2 ) it is preferable to use the following formula (A). R ≥ t 2 / D···(A) The hydrogen diffusion coefficient varies depending on the contained components and metal structure. For example, the hydrogen diffusion coefficient is 1×10 -11 ~5×10 -9 m 2 / s may be used. More preferably, 5×10 -10 m 2 / s or less.

[0075] The dehydrogenation process is performed before pipe making or welding to connect steel pipes. High temperatures are preferred for dehydrogenation because the hydrogen diffusion coefficient D decreases at high temperatures, allowing hydrogen to escape more quickly. At high temperatures, calculations can be performed using the diffusion coefficient D' (diffusion coefficient at each temperature) at which the value of D in equation (A) is maintained. However, if the dehydrogenation process temperature is too high, the material strength decreases significantly, so the dehydrogenation temperature is preferably 550°C or lower. The dehydrogenation temperature T is more preferably 500°C or lower. The dehydrogenation temperature T is even more preferably 400°C or lower, and most preferably 300°C or lower. Furthermore, because dehydrogenation at temperatures lower than room temperature increases processing time and costs, the dehydrogenation temperature T is preferably above room temperature. The dehydrogenation temperature T is more preferably 50°C or higher. The dehydrogenation temperature T is more preferably 100°C or higher, and most preferably 150°C or higher. The dehydrogenation temperature T referred to here refers to the ambient temperature during the dehydrogenation process. Room temperature refers to 20±10°C.

[0076] In particular, when heating, it takes time for the temperature Tc at the center of the thickness of the steel material and steel pipe to reach the temperature of the atmosphere (dehydrogenation temperature T) in the dehydrogenation process. Therefore, even if the above-mentioned holding time R (sec) is met at the ambient temperature, if the center of the thickness does not reach the dehydrogenation temperature T (ambient temperature), the dehydrogenation may be insufficient. Therefore, it is preferable to hold the temperature Tc at the center of the thickness for at least R (sec) after it reaches the target dehydrogenation temperature T. Furthermore, to obtain a predetermined crack propagation rate in hydrogen gas, it is necessary to appropriately adjust the hydrogen content of the steel material in the surface layer and the center of the thickness. For this reason, it is preferable to hold the temperature Tc at the dehydrogenation temperature T for at least R (sec) as defined by Equation (A), and it is even more preferable to hold the temperature Tc at the center of the thickness for at least R (sec) after it reaches the target dehydrogenation temperature T. In other words, at least the former method can appropriately control the hydrogen content of the steel material in the surface layer of the steel material and steel pipe, and if the latter method is also implemented, it is possible to appropriately control the hydrogen content of the steel material from the surface layer to the center of the thickness of the steel material and steel pipe. The thickness temperature Tc may be measured using a thermocouple or the like, or may be predicted using the finite element method or the like.

[0077] Furthermore, since scale on the steel surface inhibits dehydrogenation, it is preferable to remove the scale and then perform dehydrogenation treatment. The method for removing the scale is not important, but physical cleaning using high-pressure washing or chemical methods using descaling agents can be used. The thickness of the scale removal is not important, but removing approximately 100 μm will be effective for descaling.

[0078] Second embodiment The steel material of the present invention will be specifically described below. The chemical composition, metal structure, and fatigue stress limit of the steel material are the same as those described for the steel pipe, and the manufacturing method is also the same as that described for the steel pipe except for the rolling and cooling processes (casting, heating, reheating and quenching, tempering, and dehydrogenation). The rolling and cooling processes are carried out as follows.

[0079] [Rolling process] Rolling end temperature: 820°C or higher If the rolling end temperature is less than 820°C, the rolling load becomes excessive, increasing the risk of rolling problems. Therefore, the rolling end temperature is set to 820°C or higher. The rolling end temperature is preferably set to 850°C or higher, and more preferably to 900°C or higher. On the other hand, although there is no particular upper limit for the rolling end temperature, if the temperature is too high, the metal structure is likely to become non-uniform, so the rolling end temperature is preferably set to 1200°C or lower. The rolling end temperature is more preferably set to 1150°C or lower, and even more preferably to 1100°C or lower.

[0080] [Cooling process (accelerated cooling process)] In the cooling step, after hot rolling a steel material having the above-mentioned chemical composition, it is heated to a temperature of not less than the Ac3 point and not more than 1000°C, and then cooled under the following cooling conditions of Group A or Group B. It is preferable to hold the steel at the temperature for not less than 10 minutes, more preferably for not less than 15 minutes, and even more preferably for not less than 20 minutes. There is no particular upper limit, but it is preferable to hold the steel at the temperature for not more than 60 minutes, and more preferably for not more than 45 minutes.

[0081] Heating temperature after hot rolling: Ac3 point or higher and 1000°C or lower If the heating temperature in the cooling process is lower than the Ac3 point, ferrite will remain in the steel after cooling, reducing the steel's strength and fatigue stress limit in hydrogen. Therefore, the heating temperature is set to the Ac3 point or higher. The heating temperature is preferably set to the Ac3 point +30°C or higher, and more preferably set to the Ac3 point +50°C or higher. However, for composition systems in which the Ac3 point +30°C or Ac3 point +50°C exceeds 1000°C, the above-mentioned Ac3 point +30°C or Ac3 point +50°C does not apply. On the other hand, if the heating temperature is higher than 1000°C, austenite grains will coarsen, which may cause a decrease in the impact absorption energy value and toughness of the material after heat treatment. Therefore, the heating temperature is set to 1000°C or lower. More preferably, it is set to 950°C or lower, and even more preferably, it is set to 900°C or lower. However, for composition systems in which 950°C or 900°C is lower than the Ac3 point, the above-mentioned 950°C or lower and 900°C or lower temperatures do not apply.

[0082] In the cooling process here, if the temperature after rolling satisfies the heating conditions, the steel sheet may be cooled as is, or may be reheated after rolling and then cooled. Also, if the steel sheet has been cooled once by air cooling, it may be reheated to a temperature of not less than the Ac3 point and not more than 1000°C, and then cooled under the cooling conditions of Group A or Group B below (this is called quenching). In the present invention, the Ac3 point (°C) is calculated by the following formula. Ac3(℃)=910-203[C]1 / 2-30[Mn]+44.7[Si]+700[P]+100[Al]+31.5[Mo]-11[Cr]-15.2[Ni]-20[Cu]+104[V] In the formula, [M] represents the content (mass %) of element M.

[0083] average cooling rate Group A: Cooling to 50°C or less under the conditions that the average cooling rate from 800°C to 550°C at a position 1 / 4 of the plate thickness from the steel surface is 15°C / s or more, and the average cooling rate from 550°C to 50°C is 15°C / s or less If the average cooling rate from 800°C to 550°C at the 1 / 4 thickness position from the steel surface is less than 15°C / s, a bainite structure with an area fraction of 90% or more cannot be obtained, resulting in a decrease in strength. For this reason, the average cooling rate at the 1 / 4 thickness position from the steel surface is set to 15°C / s or more. From the viewpoint of suppressing the variation in the structure, the average cooling rate is preferably set to 17°C / s or more, more preferably 20°C / s or more, and even more preferably 22°C / s or more. On the other hand, in order to suppress the variation in grain size, the average cooling rate is set to 50°C / s or less, preferably 47°C / s or less, and more preferably 45°C / s or less. Furthermore, by cooling to 50°C or less under the condition that the average cooling rate from 550°C to 50°C is 15°C / s or less, it is possible to reduce the amount of retained austenite and reduce the amount of hydrogen in the steel. For this reason, the average cooling rate from 550°C to 50°C is set to 15°C / s or less. Although there is no particular lower limit, it is preferable that the average cooling rate from 550°C to 50°C be 1°C / s or more. The cooling method is not particularly limited, and any method such as water cooling, oil cooling, or air cooling can be used alone or in combination, but water cooling or oil cooling is preferred from 800°C to 550°C, and air cooling from 550°C to 50°C.

[0084] Group B: Cooled to 50°C or less under the conditions that the average cooling rate from 800°C to 300°C at a position 1 / 4 of the plate thickness from the steel surface is 10°C / s or more, and the average cooling rate from 300°C to 50°C is 5°C / s or less If the average cooling rate from 800°C to 300°C at the 1 / 4 thickness position from the steel surface is less than 10°C / s, 90% or more of the martensite structure will not be obtained, and mixing with the bainite structure will occur, resulting in a decrease in the fatigue stress limit in hydrogen. Therefore, the average cooling rate at the 1 / 4 thickness position from the steel surface is set to 10°C / s or more. From the viewpoint of suppressing microstructural variation, a cooling rate of 12°C / s or more is more preferable. The average cooling rate is more preferably 15°C / s or more, and even more preferably 17°C / s or more. On the other hand, although there is no particular upper limit for the average cooling rate, if it exceeds 60°C / s, a large amount of hard structure will form on the steel sheet surface, preventing the steel structure having the structure targeted in the present invention from being obtained, and resulting in a decrease in fatigue properties in hydrogen, so the average cooling rate is preferably 60°C / s or less. Furthermore, the amount of hydrogen in the steel can be reduced by cooling to 50°C or less under the condition that the average cooling rate from 300°C to 50°C is 5°C / s or less. For this reason, the average cooling rate from 300°C to 50°C is set to 5°C / s or less. The average cooling rate is preferably 1°C / s or less, and more preferably 0.8°C / s or less. There is no particular lower limit, but it is preferably 0.1°C / s or more. The cooling method is not particularly limited, and any method such as water cooling, oil cooling, or air cooling can be used alone or in combination, but water cooling or oil cooling is preferred from 800°C to 300°C, and air cooling from 300°C to 50°C.

[0085] After the cooling, it is not necessary to carry out this step in the case of a thick steel plate, but it is preferable to wind up the thin steel plate in a coil. [Example]

[0086] Next, the present invention will be described in more detail based on examples. The following examples are intended to illustrate preferred examples of the present invention, and the present invention is not limited to the described examples in any way.

[0087] Steel pipes were manufactured from steel materials having the chemical compositions shown in Tables 1-1, 1-2, 1-3, 2-1, and 2-2. The manufacturing procedure was as follows. First, billets were prepared having the chemical compositions shown in Tables 1-1, 1-2, 1-3, 2-1, and 2-2. The billets shown in Tables 1-1, 1-2, 1-3, and 2-1 were manufactured at a casting speed of 0.05 to 0.2 m / min. The billets shown in Tables 2-2 were manufactured at a casting speed of 1.1 to 1.5 m / min. The billets were heated to 1000°C to 1100°C and hot-rolled. Subsequently, the billets were expanded using a Mannesmann plug mill or a Mannesmann mandrel mill to obtain seamless steel pipes with a rolling finish temperature of 850°C or higher. The seamless steel pipes were then slowly cooled by air cooling. The steel pipes obtained by the above method were heated and held at 950°C for steel pipes with an Ac3 point of 950°C or less, or at 1000°C for steel pipes with an Ac3 point of more than 950°C, and then cooled to 50°C or less at the average cooling rates shown in Tables 3-1, 3-2, 3-3, 4-1, and 4-2. Thereafter, tempering was performed, and some of the steel pipes, Nos. 16, 29, 35, 37, and 39, were subjected to dehydrogenation treatment, and the metallographic structure and mechanical properties were evaluated. The tempering temperature was adjusted in the range of 400°C to 680°C so that the tensile strength of the material would be in the range of 520 MPa to 700 MPa. In addition, in the dehydrogenation treatment of Example 1, after confirming that the temperature at the center of thickness Tc reached the target temperature, room temperature, the temperature was held for R (sec) so as to satisfy the above-mentioned formula (A). The evaluation results are shown in Tables 3-1, 3-2, 3-3, 4-1, and 4-2. The evaluation methods are as follows: The steel material sampled from the center of the steel pipe in the longitudinal direction was treated as the steel material of the present invention.

[0088] Retained austenite measurement Samples for metallographic observation were taken from the longitudinal center of the steel material and steel pipe obtained as described above, and the cross section parallel to the longitudinal direction was used as the observation surface, followed by buffing. The surface layer was then chemically polished by etching with picric acid to remove the surface layer, and X-ray diffraction measurements were performed. Specifically, a Co-Kα radiation source was used for the incident X-rays, and the area fraction of retained austenite was calculated from the intensity ratio of the (200), (211), and (220) planes of ferrite to the (200), (220), and (311) planes of austenite.

[0089] Area fraction measurement of bainite and martensite The metallographic structure of the obtained steel pipes at the inner 1 / 4 position in the wall thickness direction was evaluated as follows. Test specimens were taken from the longitudinal center of the steel pipe so that the observation positions were the inner 1 / 4 position in the wall thickness direction and the center position in the wall thickness direction. The cross sections of the taken test specimens were etched using a 3 vol% nital solution. Scanning electron microscope photographs were taken at an appropriate magnification of 1000 to 5000 times, and tempered martensite, ferrite, bainite, and pearlite were observed. Martensite, ferrite, bainite, and pearlite were visually determined by comparing with the microstructural photographs in Non-Patent Document 2. The microstructural fractions were determined by image analysis using images obtained by dividing the SEM photographs into regions based on the above determination (for example, when calculating the bainite fraction, the bainite and other regions are binarized to determine the bainite fraction), and these were used as the area fractions of each phase.

[0090] Tensile strength (TS) From the steel pipes and steel materials obtained as described above, JIS No. 14 proportional test pieces (parallel diameter 7 mm, gauge length 35 mm) were taken in accordance with JIS Z 2201, and the tensile strength was measured.

[0091] Hydrogen temperature program analysis The amount of hydrogen remaining in the steel was measured using thermal desorption analysis with a low-temperature temperature-programmed hydrogen analyzer (gas chromatograph type) (JTF-20AL). Thermal desorption analysis was performed at a temperature range from room temperature to 400°C at a heating rate of 200°C / h, and the sum of the results was taken as the amount of hydrogen. The test specimens were cylindrical, 30 mm long and 7 mm in diameter, located at 1 / 4 of the plate thickness of the steel plate and 1 / 4 of the way from the inner surface of the steel pipe. Note that this amount of hydrogen was measured before the high-pressure hydrogen fatigue test described later in the aging section, and is the amount of H shown in Tables 1-1, 1-2, 1-3, 2-1, and 2-2.

[0092] Fatigue testing Fatigue tests were conducted at room temperature (20±10°C) in a high-pressure gas atmosphere in air, in accordance with ASTM E466, Fatigue Testing, with a frequency of 1 to 15 Hz, a cyclic waveform of sine wave, load control, loading conditions of uniaxial tension and compression, and a stress ratio of R = -1.0. The stress at which no fracture occurs after 10 million cycles was defined as the fatigue limit in air.

[0093] High-pressure hydrogen fatigue testing Fatigue tests were conducted at room temperature (20±10°C) in an atmosphere of 40 MPa hydrogen gas (100% gas), 1 MPa or higher hydrogen gas, or a mixture of natural gas (mainly hydrocarbons such as methane and ethane) containing 1 MPa or higher hydrogen partial pressure. The tests were performed in accordance with ASTM E466, Fatigue Testing, with a frequency of 1 Hz, a sinusoidal waveform, load control, uniaxial tension and compression, and a stress ratio of R = -1.0. The fatigue limit in hydrogen was defined as the stress at which no fracture occurred after 2 million cycles. A specimen was deemed to have passed the test if its fatigue limit in hydrogen obtained in this test was 200 MPa or higher and the ratio of the fatigue limit in hydrogen to the fatigue limit strength in an inert gas environment (the fatigue limit in hydrogen / fatigue limit in an inert gas environment) was 0.90 or higher.

[0094] As shown in Tables 3-1, 3-2, 3-3, 4-1, and 4-2, all of the inventive examples of the present invention had a fatigue limit stress in hydrogen of 200 MPa or more, and the ratio of the fatigue limit strength in an inert gas atmosphere to the fatigue limit stress in hydrogen / fatigue limit stress in an inert gas environment was 0.90 or more, and the tensile strength was 520 MPa or more, thereby satisfying excellent hydrogen embrittlement resistance.

[0095] [Table 1-1]

[0096] [Table 1-2]

[0097] [Table 1-3]

[0098] [Table 2-1]

[0099] [Table 2-2]

[0100] [Table 3-1]

[0101] [Table 3-2]

[0102] [Table 3-3]

[0103] [Table 4-1]

[0104] [Table 4-2] [Example]

[0105] Examples verifying the effects of the present invention will be described below. In the following examples, steel pipes were manufactured under the following manufacturing conditions, and their properties were evaluated. Steel types Nos. 1, 14, 46, and 91 shown in Tables 1-1, 1-2, and 2-2 were used to manufacture steel pipes under the same conditions up to the tempering step as steel pipes Nos. 1, 14, and 46 shown in Tables 3-1 and 3-2, and steel pipe No. 91 shown in Table 4-1. The properties were evaluated when the dehydrogenation conditions were changed. The results are shown in Table 5. In this Example 2, for steel pipes and steel materials Nos. 1A, 14A, 46A, and 91A, the dehydrogenation treatment temperature T (atmospheric temperature) was set to 50°C, and the holding time tc after the plate thickness center temperature Tc reached 50°C was set to satisfy formula (A). For steel pipes and steel materials Nos. 14B, 46B, and 91B, the dehydrogenation treatment temperature T (atmospheric temperature) was set to 50°C, and the holding time tc after the plate thickness center temperature Tc reached 50°C was set to satisfy formula (A). However, the holding time tc after the plate thickness center temperature Tc reached 50°C did not satisfy formula (A). For steel pipes and steel materials No. 14C, 46C, and 91C, the dehydrogenation treatment temperature T (ambient temperature) is 50°C, but the holding time t at the ambient temperature and the holding time tc after the center temperature Tc reaches 50°C do not satisfy the above-mentioned formula (A).

[0106] In Table 5, "dehydrogenation holding time t is Y" means that the dehydrogenation treatment temperature T (ambient temperature) is 50°C and the holding time t satisfies formula (A), while "dehydrogenation holding time t is N" means that the dehydrogenation treatment temperature T (ambient temperature) is 50°C, but the holding time t does not satisfy formula (A). Also, "holding time tc at steel center temperature Tc is Y" means that the holding time tc after the plate thickness center temperature Tc reaches 50°C satisfies formula (A), while "holding time tc at steel center temperature Tc is N" means that the plate thickness center temperature Tc reaches 50°C, but the holding time tc after Tc reaches 50°C does not satisfy formula (A).

[0107] The fatigue test and the measurement methods for the structure, tensile strength, etc. were the same as in Example 1.

[0108] All of the inventive examples of the present invention had a fatigue limit stress in hydrogen of 200 MPa or more, and the ratio of the fatigue limit strength in an inert gas atmosphere to the fatigue limit stress in hydrogen / fatigue limit stress in an inert gas environment was 0.90 or more, and a tensile strength of 520 MPa or more. Among these, the fatigue properties were better when the dehydrogenation treatment was carried out under more favorable conditions. Note that the steel pipes and steel materials with the same No. above obtained the same properties.

[0109] [Table 5] [Example]

[0110] Examples verifying the effects of the present invention are described below. In the following examples, steel materials and steel pipes were manufactured under the following manufacturing conditions, and their properties were evaluated. Steel pipes and steel pipes with the same chemical compositions as Nos. 14 and 46 shown in Tables 3-1 and 3-2 and No. 91 shown in Table 4-2 were used, and the steel pipes and steel pipes were subjected to a cooling process under specified conditions. After the cooling process (before the tempering process), the steel pipes and steel pipes were reheated under the conditions shown in Tables 6-1 and 6-2, and then quenched. The properties of the steel pipes and steel pipes were evaluated. The results are also shown in Tables 6-1 and 6-2. Steel pipes and steel pipes Nos. 14E, 14F, 46E, and 46F shown in Table 6-1 were obtained by subjecting the steel pipes and steel pipes Nos. 14 and 46 shown in Tables 3-1 and 3-2 to a reheating process. Steel pipes and steel pipes Nos. 91E and 91F shown in Table 6-2 were obtained by subjecting the steel pipes and steel pipes No. 91 shown in Table 4-2 to a reheating process.

[0111] The fatigue test and the measurement methods for the structure, tensile strength, etc. were the same as in Example 1.

[0112] All of the inventive examples of the present invention had a fatigue limit stress in hydrogen of 200 MPa or more, a ratio of the fatigue limit strength in an inert gas atmosphere (fatigue limit stress in hydrogen / fatigue limit stress in an inert gas environment) of 0.90 or more, and a tensile strength of 520 MPa or more. Note that the steel pipes and steel materials with the same No. above had the same properties.

[0113] [Table 6-1]

[0114] [Table 6-2]

Claims

1. A steel pipe for line pipe, In mass%, C: 0.10-0.45%, Si: 0.01-2.0%, Mn: 0.5-1.5%, P: 0.0001-0.015%, S: 0.0002-0.0015%, Al: 0.005-0.15%, O: 0.01% or less, N: 0.010% or less, H: 0.0010% or less, Or even more so, Nb: 0 to 0.10%, Ti: 0 to 0.1%, Ca: 0-0.005%, Ni: 0-2.0%, Cu: 0 to 1.0%, Cr: 0-1.0%, Mo: 0 to 0.60%, W: 0 to 1.0%, V: 0-0.10%, Zr: 0 to 0.050%, REM: 0-0.050%, Mg: 0 to 0.050%, B: 0 to 0.0020%, Hf: 0-0.2%, Ta: 0-0.2%, Re: 0 to 0.005%, Sn: 0-0.3%, Sb: 0 to 0.3%; The balance is Fe and unavoidable impurity elements, A steel pipe for line pipe having excellent hydrogen embrittlement resistance, wherein the steel pipe has an area fraction of retained austenite of 0 to 3%, has bainite or martensite at a position 1 / 4 of the way through the wall thickness from the inner surface of the steel pipe, the area fraction of the bainite being 90% or more, or the area fraction of the martensite being 90% or more, has a hydrogen fatigue limit stress of 270 MPa or more at room temperature (20±10°C) and a hydrogen gas pressure of 40 MPa, and has a hydrogen fatigue limit stress of 40 MPa / fatigue limit stress in an inert gas environment of 0.90 or more, and has a tensile strength of 578 MPa or more and 697 MPa or less.

2. The chemical composition is in mass %, and further comprises: Nb: 0.001 to 0.10%, Ti: 0.005-0.1%, Ca: 0.0001-0.005%, Ni: 0.01-2.0%, Cu: 0.01 to 1.0%, Cr: 0.01-1.0%, Mo: 0.01 to 0.60%, W: 0.01-1.0%, V: 0.01-0.10%, Zr: 0.0001 to 0.050%, REM: 0.0001-0.050%, Mg: 0.0001-0.050%, B: 0.0001 to 0.0020%, Hf: 0.0001-0.2%, Ta: 0.0001-0.2%, Re: 0.0001-0.005%, Sn: 0.0001-0.3%, 2. A steel pipe for line pipe having excellent hydrogen embrittlement resistance according to claim 1, further comprising one or more elements selected from the group consisting of Sb: 0.0001 to 0.3%.

3. A method for manufacturing a steel pipe for line pipe according to claim 1 or 2, comprising: a casting step of casting a steel material having the chemical composition at a casting speed of 1.8 m / min or less; a heating step of heating at 1350°C or less; a hot rolling step in which the steel material heated in the heating step is rolled under the condition of a rolling end temperature of 820°C or higher to form a steel pipe shape; The steel pipe obtained in the hot rolling step is ３ a cooling step in which the temperature is maintained at a temperature of not less than 1000°C and not more than the above temperature, and then the cooling conditions are group A or group B below; The steel pipe obtained in the cooling step is cooled to 400°C or more. １ and a tempering step in which tempering is performed at a temperature below this point. Group A: The steel pipe is cooled to 50°C or less at an average cooling rate of 15°C / s or more from 800°C to 550°C at a position 1 / 4 of the wall thickness from the inner surface of the steel pipe, and at an average cooling rate of 15°C / s or less from 550°C to 50°C at a position 1 / 4 of the wall thickness from the inner surface of the steel pipe. Group B: The steel pipe is cooled to 50°C or less at an average cooling rate of 10°C / s or more from 800°C to 300°C at a position 1 / 4 of the wall thickness from the inner surface of the steel pipe, and at an average cooling rate of 5°C / s or less from 300°C to 50°C at a position 1 / 4 of the wall thickness from the inner surface of the steel pipe.

4. Before the tempering process, Ac ３ 4. The method for producing a steel pipe for line pipe according to claim 3, further comprising a quenching step of reheating the steel pipe to a temperature above 1000°C and a cooling condition of the following group A or B: Group A: The steel pipe is cooled to 50°C or less at an average cooling rate of 15°C / s or more from 800°C to 550°C at a position 1 / 4 of the wall thickness from the inner surface of the steel pipe, and at an average cooling rate of 15°C / s or less from 550°C to 50°C at a position 1 / 4 of the wall thickness from the inner surface of the steel pipe. Group B: The steel pipe is cooled to 50°C or less at an average cooling rate of 10°C / s or more from 800°C to 300°C at a position 1 / 4 of the wall thickness from the inner surface of the steel pipe, and at an average cooling rate of 5°C / s or less from 300°C to 50°C at a position 1 / 4 of the wall thickness from the inner surface of the steel pipe.

5. 4. The method for producing a steel pipe for line pipe according to claim 3, wherein the casting speed is 1.0 m / min or less.

6. 5. The method for producing a steel pipe for line pipe according to claim 4, wherein the casting speed is 1.0 m / min or less.

7. In mass%, C: 0.10-0.45%, Si: 0.01-2.0%, Mn: 0.5-1.5%, P: 0.0001-0.015%, S: 0.0002-0.0015%, Al: 0.005-0.15%, O: 0.01% or less, N: 0.010% or less, H: 0.0010% or less, Or even more so, Nb: 0 to 0.10%, Ti: 0 to 0.1%, Ca: 0-0.005%, Ni: 0-2.0%, Cu: 0 to 1.0%, Cr: 0-1.0%, Mo: 0 to 0.60%, W: 0 to 1.0%, V: 0-0.10%, Zr: 0 to 0.050%, REM: 0-0.050%, Mg: 0 to 0.050%, B: 0 to 0.0020%, Hf: 0-0.2%, Ta: 0-0.2%, Re: 0 to 0.005%, Sn: 0-0.3%, Sb: one or more selected from 0 to 0.3%, The balance is Fe and unavoidable impurity elements, A steel material for linepipe having excellent hydrogen embrittlement resistance, wherein the steel material has an area fraction of 0 to 3% of retained austenite, and has bainite or martensite at a 1 / 4 position of the plate thickness, the area fraction of the bainite being 90% or more, or the area fraction of the martensite being 90% or more, and wherein the fatigue stress limit in hydrogen at room temperature (20±10°C) and a hydrogen gas pressure of 40 MPa is 270 MPa or more, and the fatigue stress limit in hydrogen at a hydrogen gas pressure of 40 MPa / fatigue stress limit in an inert gas environment is 0.90 or more, and wherein the tensile strength is 578 MPa or more and 697 MPa or less.

8. The chemical composition, in mass %, further comprises: Nb: 0.001 to 0.10%, Ti: 0.005-0.1%, Ca: 0.0001-0.005%, Ni: 0.01-2.0%, Cu: 0.01 to 1.0%, Cr: 0.01-1.0%, Mo: 0.01 to 0.60%, W: 0.01-1.0%, V: 0.01-0.10%, Zr: 0.0001 to 0.050%, REM: 0.0001-0.050%, Mg: 0.0001-0.050%, B: 0.0001 to 0.0020%, Hf: 0.0001-0.2%, Ta: 0.0001-0.2%, Re: 0.0001-0.005%, Sn: 0.0001-0.3%, 8. A steel material for line pipes having excellent hydrogen embrittlement resistance according to claim 7, further comprising one or more selected from Sb: 0.0001 to 0.3%.

9. A method for manufacturing a steel material for line pipe according to claim 7 or 8, comprising: a casting step of casting a steel material having the chemical composition at a casting speed of 1.8 m / min or less; a heating step of heating at 1350°C or less; a hot rolling step in which the steel material heated in the heating step is rolled under the condition of a rolling finish temperature of 820°C or higher; The steel material obtained in the hot rolling process is ３ a cooling step in which the temperature is maintained at a temperature of not less than 1000°C and not more than the above temperature, and then the cooling conditions are group A or group B below; The steel material obtained in the cooling step is cooled to 400°C or more. １ and a tempering step of tempering at a temperature below this point. Group A: The steel is cooled to 50°C or less at an average cooling rate of 15°C / s or more from 800°C to 550°C at a position 1 / 4 of the thickness from the surface of the steel, and at an average cooling rate of 15°C / s or less from 550°C to 50°C at a position 1 / 4 of the thickness from the surface of the steel. Group B: The steel is cooled to 50°C or less at an average cooling rate of 10°C / s or more from 800°C to 300°C at a position 1 / 4 of the thickness from the surface of the steel, and at an average cooling rate of 5°C / s or less from 300°C to 50°C at a position 1 / 4 of the thickness from the surface of the steel.

10. Before the tempering process, Ac ３ 10. The method for producing a steel material for line pipe according to claim 9, further comprising a quenching step of reheating the steel material to a temperature above 1000°C and a cooling condition of the following group A or B: Group A: The steel is cooled to 50°C or less at an average cooling rate of 15°C / s or more from 800°C to 550°C at a position 1 / 4 of the thickness from the surface of the steel, and at an average cooling rate of 15°C / s or less from 550°C to 50°C at a position 1 / 4 of the thickness from the surface of the steel. Group B: The steel is cooled to 50°C or less at an average cooling rate of 10°C / s or more from 800°C to 300°C at a position 1 / 4 of the thickness from the surface of the steel, and at an average cooling rate of 5°C / s or less from 300°C to 50°C at a position 1 / 4 of the thickness from the surface of the steel.

11. 10. The method for producing a steel material for line pipe according to claim 9, wherein the casting speed is 1.0 m / min or less.

12. The method for producing a steel material for line pipe according to claim 10, wherein the casting speed is 1.0 m / min or less.

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