Steel pipe for line pipe having excellent hydrogen embrittlement resistance, method for producing same, steel material for line pipe, and method for producing same

Abstract

The purpose of the present invention is to provide natural gas suitable as 100% hydrogen or hydrogen having a hydrogen partial pressure of 1 MPa or more (natural gas being gas mainly composed of hydrocarbons such as methane or ethane) A steel pipe for a line pipe having excellent hydrogen embrittlement resistance in a high-pressure hydrogen environment and having high strength for a steel structure used in a high-pressure hydrogen environment, such as a line pipe; a method for manufacturing the steel pipe; a steel material for a line pipe; and a method for manufacturing the steel material for a line pipe. A steel pipe for a line pipe having excellent hydrogen embrittlement resistance, characterized by having a specific component composition and a specific structure, having a fatigue limit stress in hydrogen at 1 MPa or more of 200 MPa or more, and having a fatigue limit stress in hydrogen at 1 MPa or more / fatigue limit stress in an inert gas atmosphere of 0.90 or more.

Description

Technical Field

[0001] The present invention relates to a steel pipe for a line pipe having excellent hydrogen embrittlement resistance and suitable for use in a line pipe for transporting hydrogen gas, a method for producing the same, a steel material for a line pipe, and a method for producing the same. Background Art

[0002] As an existing energy infrastructure, there are pipelines for natural gas transmission. These steels are required to suppress the occurrence of hydrogen-induced cracking in acidic environments. On the other hand, in recent years, hydrogen has received great attention worldwide as a clean energy for building a decarbonized society. Therefore, in order to transport hydrogen in large quantities, the construction of a hydrogen transmission network in which natural gas partially mixed with hydrogen and hydrogen are pressure-transmitted in natural gas pipelines as an alternative is studied. The transmission pressure during operation of these pipelines is assumed to be a high pressure of 1 to 40 MPa, and the pipelines are placed in a high-pressure hydrogen exposure environment. There is a concern that steel used in such an environment will suffer from "hydrogen embrittlement" in which hydrogen intrudes into the steel and the properties deteriorate. Therefore, it is necessary not only to have the high toughness and acid resistance required for conventional pipelines, but also to have the resistance to hydrogen embrittlement required in a hydrogen environment.

[0003] Steel structures used in high-pressure hydrogen environments have always used austenitic stainless steels such as SUS316L, which are less susceptible to hydrogen embrittlement than low-alloy steels. However, austenitic stainless steels such as SUS316L have high steel costs and low strength, so if they are designed to withstand high hydrogen pressures, the wall thickness becomes thicker and the price of the hydrogen structure itself becomes expensive. Therefore, as a steel material for hydrogen steel structures, there is a strong demand for a low-alloy steel material that is lower in cost and can withstand high-pressure hydrogen environments.

[0004] In response to such a demand, for example, the steel for high-pressure hydrogen environment described in Patent Document 1 is steel used in a high-pressure hydrogen environment. By reducing the diffusible hydrogen concentration ratio by Ca / S to less than 1.5 or greater than 11, embrittlement due to diffusible hydrogen is suppressed.

[0005] Patent document 2 is a technology that finds that by using a low alloy high strength steel adjusted to a specific component composition, within the tensile strength range of 900 to 950 MPa in the atmosphere, the drawing and elongation values ​​in a 45 MPa hydrogen atmosphere are greater than those of JIS G3128SHY685NS, and the steel has excellent resistance to high pressure hydrogen environment embrittlement.

[0006] In addition, Patent Document 3 is a Cr-Mo based high-strength low alloy steel, which is tempered at a relatively high temperature of 560 to 580°C to adjust the grain size after tempering to a grain size of 8.4 or more and a tensile strength of 900 to 950 MPa in an extremely narrow range, thereby becoming a low-alloy high-strength steel with excellent resistance to high-pressure hydrogen environment embrittlement and excellent elongation and drawing properties even in a 45 MPa hydrogen atmosphere.

[0007] In addition, the low alloy steel for high pressure hydrogen environment proposed in Patent Document 4 further increases the Mo content compared with the existing steel by adding V, raising the tempering temperature, and utilizing V-Mo carbides, thereby improving the carbide morphology at the grain boundary and greatly improving the resistance to hydrogen environment embrittlement.

[0008] In addition, a steel for a high-pressure hydrogen storage container having excellent hydrogen resistance is proposed in Patent Document 5. According to the technology described in Patent Document 5, by performing a long-term stress relief annealing after normalizing treatment during steel plate manufacturing, MC carbides (Mo, V) C are dispersed and precipitated finely and densely, and hydrogen resistance such as hydrogen embrittlement resistance of the steel is improved.

[0009] Patent Document 6 proposes a steel material having a metal structure of a bainite main structure with an area fraction of 90% or more, in which cementite having 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.

[0010] Prior art literature

[0011] Patent Literature

[0012] Patent Document 1: Japanese Patent Application Publication No. 2005-2386

[0013] Patent Document 2: Japanese Patent Application Publication No. 2009-46737

[0014] Patent Document 3: Japanese Patent Application Publication No. 2009-275249

[0015] Patent Document 4: Japanese Patent Application Publication No. 2009-74122

[0016] Patent Document 5: Japanese Patent Application Publication No. 2010-37655

[0017] Patent Document 6: Japanese Patent Application Publication No. 2012-107332

[0018] Non-patent literature

[0019] Non-patent document 1: Matsunaga et al., Int J Hydrogen Energy, Vol. 40 (2015), p. 5739-5748

[0020] Non-patent document 2: Japan Heat Treatment Technology Association (author), Introduction: Structure and properties of metal materials - Heat treatment and structure control to make full use of materials, 2004 Summary of the invention

[0021] The pressure inside the pipeline pipe repeatedly applies stress to the structure due to fluctuations during operation and periodic closures. Therefore, fatigue fracture needs to be considered when designing steel structures such as pipeline pipes. However, as shown in Non-Patent Document 1, it is known that the fatigue life of materials decreases in a high-pressure hydrogen environment. That is, it means that when designing pipeline pipes based on conventional natural gas pipelines, the service life of the pipeline pipes decreases. However, although the above-mentioned prior art can suppress the occurrence of hydrogen-induced cracking in an acidic environment, it cannot fully improve the fatigue strength in hydrogen, that is, there is a problem that it is difficult to strike a balance between suppressing the occurrence of hydrogen-induced cracking in an acidic environment and high fatigue strength in hydrogen.

[0022] In view of the above-mentioned problems of the prior art, the present invention aims to provide a steel pipe for pipeline having high strength and excellent resistance to hydrogen embrittlement in a high-pressure hydrogen environment, and a method for manufacturing the same, and a steel material for pipeline having the same and a method for manufacturing the same, which is suitable for use as a steel structure for use in a high-pressure hydrogen environment, such as a pipeline pipe for 100% hydrogen or natural gas (natural gas is a gas containing hydrocarbons such as methane and ethane as a main component) containing hydrogen at a hydrogen partial pressure of 1 MPa or more.

[0023] It should be noted that the so-called "excellent hydrogen embrittlement resistance in a high-pressure hydrogen environment" here refers to the case where the fatigue limit stress in hydrogen, which is the stress without fracture after 2 million repetitions, is 200 MPa or more, and the fatigue limit stress in hydrogen / fatigue limit stress in an inert gas environment is 0.90 or more, and the natural gas containing hydrogen at a hydrogen partial pressure of 1 MPa or more is, for example, a gas having a hydrogen concentration of 30% or less in volume fraction and a gas pressure of 30 MPa or less.

[0024] It should be noted that if the fatigue limit stress in hydrogen under the above environment is 200 MPa or more and the fatigue limit stress in hydrogen of the steel under the above environment / the fatigue limit stress in the inert gas environment is 0.90 or more, it is possible to design hydrogen steel structures such as long-life pipelines within the range of plate thickness that can be manufactured by the process for manufacturing seamless steel pipes, UOE and other steel pipes.

[0025] In addition, the so-called "steel materials" here include thin steel plates, thick steel plates, seamless steel pipes, electric resistance welded steel pipes, steel sections, steel bars, etc.

[0026] The present inventors have conducted intensive research on the conditions that a steel material should satisfy in order to obtain a steel pipe for a line pipe and a steel material for a line pipe having excellent hydrogen embrittlement resistance, and have invented a new steel pipe for a line pipe and a steel material for a line pipe. In addition, the steel pipe and the steel material of the present invention have high strength, and high strength in the present invention refers to a tensile strength of 520 MPa or more.

[0027] The gist of the present invention is as follows.

[0028] [1] A steel pipe for a line pipe having excellent hydrogen embrittlement resistance, having the following chemical composition: containing, by 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 further containing Nb: 0-0.10%, Ti: 0-0.1%, Ca: 0-0.0001% or less, or further containing Nb: 0-0.10%, Ti: 0-0.1%, Ca: 0-0.0001% or less, or further containing Nb: 0-0.10%, Ti: 0-0.1%, Ca: 0-0.0001% or less, or further containing Nb: 0-0.10%, Ti: 0-0.1%, Ca: 0-0.0002% or less, or further containing Nb: 0-0.10%, Si: 0-0.1%, Mn: 0-0.0001 ... Mn: 0-0.0001% or less, or further containing Mn: 0-0.10%, Si: 0-0.1%, Mn: 0-0.0001% or less, or further containing Mn: 0-0.10%, Mn: 0-0.0001% or less, or further containing 0.005%, Ni: 0-2.0%, Cu: 0-1.0%, Cr: 0-1.0%, Mo: 0-0.60%, W: 0-1.0%, V: 0-0.10%, Zr: 0-0.050%, REM: 0-0.050%, Mg: 0-0.050%, B: 0-0.0020%, Hf: 0-0.2%, Ta: 0-0.2%, Re: 0-0.005%, Sn: 0-0.3%, Sb: 0-0.3%, and the balance is Fe and unavoidable impurity elements;

[0029] The retained austenite is 0 to 3% in terms of area fraction, bainite or martensite is present at a position 1 / 4 of the wall thickness from the inner surface of the steel pipe, the bainite is 90% or more in terms of area fraction or the martensite is 90% or more in terms of area fraction, 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 inactive gas environment is 0.90 or more.

[0030] [2] The steel pipe for line pipe having excellent hydrogen embrittlement resistance according to [1], wherein the chemical composition further comprises, in mass%, Nb: 0.001-0.10%, Ti: 0.005-0.1%, Ca: 0.0001-0.005%, Ni: 0.01-2.0%, Cu: 0.01-1.0%, Cr: 0.01-1.0%, Mo: 0.01-0.60%, W: 0.01-1.0%, V : 0.01~0.10%, Zr: 0.0001~0.050%, REM: 0.0001~0.050%, Mg: 0.0001~0.050%, B: 0.0001~0.0020 %, Hf: 0.0001~0.2%, Ta: 0.0001~0.2%, Re: 0.0001~0.005%, Sn: 0.0001~0.3%, Sb: 0.0001~0.3%.

[0031] [3] A method for manufacturing a steel pipe for a line pipe, comprising:

[0032] A casting step of casting a steel billet having the chemical composition described in [1] or [2] at a casting speed of 1.8 m / min or less,

[0033] The heating process is carried out at a temperature below 1350°C.

[0034] In the hot rolling step, the steel billet heated in the heating step is rolled at a rolling end temperature of 820°C or above to form a steel pipe.

[0035] In a cooling step, the steel pipe obtained in the hot rolling step is kept at a temperature of Ac3 point to 1000°C, and then cooled under the following conditions: Group A or Group B, and

[0036] In the tempering step, the steel pipe obtained in the above cooling step is tempered at 400°C to Ac1 point.

[0037] Group A:

[0038] The steel pipe is cooled to 50°C or less at an average cooling rate from 800°C to 550°C of 15°C / s or more at a position 1 / 4 of the wall thickness from the inner surface of the steel pipe, and at an average cooling rate from 550°C to 50°C of 15°C / s or less at a position 1 / 4 of the wall thickness from the inner surface of the steel pipe.

[0039] Group B:

[0040] The steel pipe is cooled to 50°C or less at an average cooling rate from 800°C to 300°C of 10°C / s or more at a position 1 / 4 of the wall thickness from the inner surface of the steel pipe, and at an average cooling rate from 300°C to 50°C of 5°C / s or less at a position 1 / 4 of the wall thickness from the inner surface of the steel pipe.

[0041] [4] The method for producing a steel pipe for a line pipe according to [3], wherein a quenching step is provided before the tempering step, wherein the steel pipe is reheated to a temperature of Ac3 point to 1000°C, and the cooling conditions are the following group A or group B.

[0042] Group A:

[0043] The steel pipe is cooled to 50°C or less at an average cooling rate from 800°C to 550°C of 15°C / s or more at a position 1 / 4 of the wall thickness from the inner surface of the steel pipe, and at an average cooling rate from 550°C to 50°C of 15°C / s or less at a position 1 / 4 of the wall thickness from the inner surface of the steel pipe.

[0044] Group B:

[0045] The steel pipe is cooled to 50°C or less at an average cooling rate from 800°C to 300°C of 10°C / s or more at a position 1 / 4 of the wall thickness from the inner surface of the steel pipe, and at an average cooling rate from 300°C to 50°C of 5°C / s or less at a position 1 / 4 of the wall thickness from the inner surface of the steel pipe.

[0046] [5] The method for producing a steel pipe for a line pipe according to [3] or [4], wherein the casting speed is 1.0 m / min or less.

[0047] [6] A steel material for line pipes having excellent hydrogen embrittlement resistance, having the following chemical composition: containing, by 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 further containing Nb: 0-0.10%, Ti: 0-0.1%, Ca: 0-0.0001% or less, or further containing Nb: 0-0.10%, Ti: 0-0.1%, Ca: 0-0.0001% or less, or further containing Nb: 0-0.10%, Ti: 0-0.1%, Ca: 0-0.0001% or less, or further containing Nb: 0-0.10%, Ti: 0-0.1%, Ca: 0-0.0002% or less, or further containing Nb: 0-0.10%, Si: 0-0.1%, Mn: 0-0.0001 ... Mn: 0-0.10%, Mn: 0-0.0001% or less, or further containing Mn: 0-0.10%, Mn: 0-0.0002% or less, or 0.005%, Ni: 0-2.0%, Cu: 0-1.0%, Cr: 0-1.0%, Mo: 0-0.60%, W: 0-1.0%, V: 0-0.10%, Zr: 0-0.050%, REM: 0-0.050%, Mg: 0-0.050%, B: 0-0.0020%, Hf: 0-0.2%, Ta: 0-0.2%, Re: 0-0.005%, Sn: 0-0.3%, Sb: 0-0.3%, and the balance is Fe and unavoidable impurity elements;

[0048] The retained austenite has an area fraction of 0 to 3%, and there is bainite or martensite at 1 / 4 of the plate thickness, the above-mentioned bainite has an area fraction of more than 90% or the above-mentioned martensite has an area fraction of more than 90%, the fatigue limit stress in hydrogen above 1 MPa is more than 200 MPa, and the fatigue limit stress in hydrogen above 1 MPa / fatigue limit stress in an inactive gas environment is more than 0.90.

[0049] [7] The line pipe steel material according to [6], wherein the chemical composition, in mass%, further comprises Nb: 0.001-0.10%, Ti: 0.005-0.1%, Ca: 0.0001-0.005%, Ni: 0.01-2.0%, Cu: 0.01-1.0%, Cr: 0.01-1.0%, Mo: 0.01-0.60%, W: 0.01-1.0%, V: 0.01 ~0.10%, Zr: 0.0001~0.050%, REM: 0.0001~0.050%, Mg: 0.0001~0.050%, B: 0.0001~0.0020%, Hf: 0.0001 to 0.2%, Ta: 0.0001 to 0.2%, Re: 0.0001 to 0.005%, Sn: 0.0001 to 0.3%, Sb: 0.0001 to 0.3%.

[0050] [8] A method for manufacturing a steel material, comprising:

[0051] A casting step of casting a steel billet having the chemical composition described in [6] or [7] at a casting speed of 1.8 m / min or less,

[0052] The heating process is carried out at a temperature below 1350°C.

[0053] In the hot rolling step, the steel billet heated in the heating step is rolled at a rolling end temperature of 820°C or above.

[0054] In a cooling step, the steel obtained in the hot rolling step is kept at a temperature of Ac3 point to 1000°C, and then cooled under the following conditions: Group A or Group B, and

[0055] In the tempering step, the steel material obtained in the cooling step is tempered at 400°C to Ac1 point.

[0056] Group A:

[0057] The steel is cooled to below 50°C with an average cooling rate from 800°C to 550°C being 15°C / s or more at a position 1 / 4 of the wall thickness from the steel surface, and an average cooling rate from 550°C to 50°C being 15°C / s or less at a position 1 / 4 of the wall thickness from the steel surface.

[0058] Group B:

[0059] The steel is cooled to below 50°C with an average cooling rate from 800°C to 300°C of 10°C / s or more at a position 1 / 4 of the wall thickness from the steel surface, and an average cooling rate from 300°C to 50°C of 5°C or less at a position 1 / 4 of the wall thickness from the steel surface.

[0060] [9] The method for producing a line pipe steel material according to [8], wherein a quenching step is provided before the tempering step, wherein the steel material is reheated to a temperature of Ac3 point to 1000°C, and the cooling conditions are the following group A or group B.

[0061] Group A:

[0062] The steel is cooled to below 50°C with an average cooling rate from 800°C to 550°C being 15°C / s or more at a position 1 / 4 of the wall thickness from the steel surface, and an average cooling rate from 550°C to 50°C being 15°C / s or less at a position 1 / 4 of the wall thickness from the steel surface.

[0063] Group B:

[0064] The steel is cooled to below 50°C with an average cooling rate from 800°C to 300°C of 10°C / s or more at a position 1 / 4 of the wall thickness from the steel surface, and an average cooling rate from 300°C to 50°C of 5°C or less at a position 1 / 4 of the wall thickness from the steel surface.

[0065]

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

[0066] According to the present invention, it is possible to easily and simply manufacture steel pipes and steel materials with greatly improved hydrogen embrittlement resistance in a high-pressure hydrogen environment, which has a significant industrial effect. In addition, according to the present invention, it is also possible to significantly improve the hydrogen embrittlement resistance of steel structures such as high-pressure hydrogen pipelines, improve fatigue resistance, and greatly contribute to extending the life of steel structures. DETAILED DESCRIPTION

[0067] Next, a method for implementing the present invention will be described in detail. It should be noted that the following description represents a preferred embodiment of the present invention, and the present invention is not limited to the following description at all.

[0068] As a first embodiment, a method for implementing a steel pipe will be specifically described. Next, as a second embodiment, a method for implementing a steel material will be specifically described.

[0069] First embodiment

[0070] [Ingredients]

[0071] The reasons for limiting the chemical composition of the steel pipe (including steel materials) of the present invention are described below. It should be noted that "%" in the following description means "mass %" unless otherwise specified.

[0072] C: 0.10~0.45%

[0073] C is an element necessary for increasing strength. Therefore, the C content is set to 0.10% or more. The C content is preferably set to 0.13% or more. On the other hand, if the C content exceeds 0.45%, quenching cracks may sometimes occur during quenching, so the C content is set to 0.45% or less. The C content is preferably set to 0.25% or less, more preferably set to 0.20% or less, and further preferably set to 0.17% or less.

[0074] Si: 0.01~2.0%

[0075] Si is added for deoxidation, and when 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, when it exceeds 2.0%, its effect is saturated, 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 it exceeds 0.5%, the toughness and weldability are deteriorated, so the Si content is more preferably 0.5% or less.

[0076] Mn: 0.5~1.5%

[0077] Mn effectively contributes to the improvement of strength and toughness, but when the content is less than 0.5%, the effect of its addition is insufficient. Therefore, the Mn content is 0.5% or more. The Mn content is preferably 0.6% or more, more preferably 0.7% or more, and further preferably 0.8% or more. On the other hand, if it exceeds 1.5%, the hardness of the surface layer and the center segregation portion increases during controlled cooling, so the resistance to SSCC (sulfide stress corrosion cracking) and HIC (hydrogen induced cracking) deteriorates. In addition, weldability also deteriorates. Therefore, the amount of Mn is limited to less than 1.5%. The Mn content is preferably 1.4% or less, and further preferably 1.3% or less.

[0078] P: 0.0001~0.015%

[0079] P is an inevitable impurity element that deteriorates weldability and increases the hardness of the center segregation portion, thereby deteriorating HIC resistance. If it exceeds 0.015%, this tendency becomes significant, so the upper limit of the P content is set to 0.015%. The P content is preferably 0.010% or less, and more preferably 0.008% or less. The lower the content, the better, but from the viewpoint of refining cost, the P content is 0.0001% or more.

[0080] S: 0.0002~0.0015%

[0081] S is an inevitable impurity element. It becomes MnS inclusions in steel and deteriorates HIC resistance. Therefore, it is preferably less, but 0.0015% is allowed. Therefore, the S content is 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 viewpoint of refining cost, the S content is 0.0002% or more.

[0082] Al: 0.005~0.15%

[0083] Al is added as a deoxidizer, but when it is less than 0.005%, there is no effect of addition. Therefore, the Al content is 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 further preferably 0.05% or less.

[0084] O: 0.01% or less

[0085] O is the cause of the formation of oxide inclusions, so the less the better, but if the O content is 0.01% or less, it will not be a problem. Therefore, the O content is 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 making oxygen 0% is the main reason for a large increase in cost, so the O content is preferably 0.001% or more.

[0086] N: 0.010% or less

[0087] N has little effect on the fatigue properties of the steel pipe. If the N content is 0.010% or less, the effect of the present invention will not be impaired from the viewpoint of toughness. Therefore, the N content is 0.010% or less. The N content is preferably 0.008% or less, and the N content is more preferably 0.006% or less. The N content is further preferably 0.004% or less. On the other hand, from the viewpoint of improving toughness, a small N content is preferred, but excessive reduction increases the cost of steelmaking, so the N content is preferably 0.00001% or more. The N content is preferably 0.001% or more.

[0088] H: 0.0010% or less

[0089] H is sometimes introduced into steel in various processes during manufacturing. If the amount introduced is large, the risk of cracking after solidification increases, and fatigue crack propagation is accelerated. In addition, when the amount introduced is large, the fatigue limit stress is reduced, so it is important to reduce the amount of hydrogen in the steel pipe. If the H content is less than 0.0010%, these effects will not be a problem, so the H content is less than 0.0010%. The H content is preferably less than 0.0005%, more preferably less than 0.0003%, and further preferably less than 0.0001%. On the other hand, less than 0.00001% becomes the main reason for the increase in cost, so the H content is preferably more than 0.00001%. It should be noted that the hydrogen content is the amount of residual hydrogen after the forming of steel, steel pipe, UOE, etc.

[0090] In order to further improve the strength and toughness of the steel pipe, the component composition disclosed in the present invention may arbitrarily contain one or more selected from Nb, Ti, Ca, Ni, Cu, Cr, Mo, W, V, Zr, REM, Mg, B, Hf, Ta, Re, Sn, and Sb within the following range.

[0091] Nb: 0-0.10% and Ti: 0-0.1%

[0092] Nb is an element effective in improving the strength and toughness of steel, but if it exceeds 0.10%, the toughness of the weld deteriorates, so when it is contained, the Nb content is 0.10% or less. The Nb content is preferably 0.08% or less. The Nb content is more preferably 0.06% or less. The Nb content can be 0% or more, but if the Nb content is less than 0.001%, it is difficult to obtain the effect of its inclusion, so when it is contained, it is preferably 0.001% or more. The Nb content is more preferably 0.01% or more.

[0093] Ti is an element effective in improving the strength and toughness of steel, but if it exceeds 0.1%, the toughness of the weld deteriorates, so when Ti is contained, the Ti content is 0.1% or less. The Ti content is preferably 0.05% or less. The Ti content is more preferably 0.03% or less, and further preferably 0.02% or less. The Ti content can be 0% or more, but if the Ti content is less than 0.005%, it is difficult to obtain the effect of its inclusion, so when it is contained, it is preferably 0.005% or more. The Ti content is more preferably 0.008% or more.

[0094] Ca: 0~0.005%

[0095] Ca is an element effective in improving HIC resistance by controlling the morphology of sulfide inclusions, but not only is the effect saturated, but HIC resistance is deteriorated due to a decrease in the cleanliness of the steel, so when Ca is contained, the amount 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%, it is difficult to obtain the effect of its addition, so when contained, it is preferably 0.0001% or more. The Ca content is more preferably 0.001% or more.

[0096] Ni: 0-2.0%

[0097] Ni is an element effective for improving toughness and increasing strength, but in order to suppress costs, it is contained in an amount of 2.0% or less. The Ni content is preferably 1.5% or less. The Ni content is more preferably 1.2% or less, and further preferably 1.0% or less. The Ni content may be 0% or more, but in order to obtain the above-mentioned effects, it is preferably 0.01% or more of Ni.

[0098] Cu: 0~1.0%

[0099] Cu is an element effective in improving toughness and increasing strength, but if the content is too high, weldability deteriorates, so when Cu is contained, it is 1.0% or less. The Cu content is preferably 0.5% or less. The Cu content is more preferably 0.3% or less, and further preferably 0.2% or less. The Cu content may be 0% or more, but in order to obtain the above-mentioned effects, it is preferably 0.01% or more.

[0100] Cr: 0~1.0%

[0101] Cr is an element that is effective for obtaining sufficient strength even at low C, similar to Mn, but if the content is too high, hardenability is excessive, so SSCC resistance deteriorates. In addition, weldability also deteriorates. Therefore, when Cr is contained, it is 1.0% or less. The Cr content is preferably 0.8% or less. The Cr content is more preferably 0.5% or less, and further preferably 0.1% or less. The Cr amount may be 0% or more, but in order to obtain this effect, it is preferably 0.01% or more of Cr. The Cr content is more preferably 0.02% or more.

[0102] Mo: 0~0.60%

[0103] Mo is an element effective in improving toughness and strength, and is an element effective in improving SSCC resistance independently of the hydrogen sulfide partial pressure. However, if the content is too high, the hardenability is excessive, so the SSCC resistance is deteriorated. In addition, weldability is also deteriorated. Therefore, when Mo is contained, the Mo content is 0.60% or less. It is more preferably 0.50% or less, and further preferably 0.40% or less. The most preferred Mo content is 0.03% or less. The Mo content may be 0% or more, but in order to obtain the above-mentioned effect, it is preferred to contain 0.005% or more of Mo. It is more preferred to contain 0.01% or more of Mo.

[0104] W: 0~1.0%

[0105] W contributes to the strength of the steel pipe, but if the W content exceeds 1.0%, the effect is saturated and becomes a major factor in the cost increase. Therefore, when W is contained, the W content is 1.0% or less. The W content is preferably 0.8% or less. In order to further suppress costs, the W content is more preferably 0.5% or less. The W content is further preferably 0.03% or less. The W content may be 0% or more, but in order to obtain the above-mentioned effect, it is preferably 0.01% or more.

[0106] V: 0~0.10%

[0107] V is an element that can be arbitrarily contained in order to improve the strength and toughness of the steel pipe, but if the V content exceeds 0.10%, the toughness of the weld deteriorates, so when contained, it is 0.10% or less. The V content is preferably 0.08% or less. The V content is more preferably 0.06% or less, and further preferably 0.03% or less. The V content can be 0% or more, but if the content is less than 0.01%, it is difficult to obtain the effect of its inclusion, so it is preferably 0.01% or more.

[0108] Zr: 0~0.050%, REM: 0~0.050%, Mg: 0~0.050%

[0109] Zr, REM, and Mg are elements that can be arbitrarily contained in order to improve toughness by grain refinement or to improve crack resistance by controlling the properties of inclusions. On the other hand, if it exceeds 0.050%, the effect is saturated, so when contained, it is 0.050% or less. That is, when contained, the Zr content is 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. In addition, when contained, the REM content is 0.050% or less. The REM content is preferably 0.040% or less. The REM content is more preferably 0.030% or less. In addition, when contained, the Mg content is 0.050% or less. The Mg content is preferably 0.040% or less. The Mg content is more preferably 0.030% or less. The content of these elements may be 0% or more, but when the content is less than 0.0001%, it is difficult to obtain the effect of their inclusion, so it is preferably 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. In addition, 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.

[0110] B: 0~0.0020%

[0111] B is an element that improves hardenability, contributes to the strength of the steel pipe, and inhibits the coarsening of the original austenite grains, improving various properties of the billet. On the other hand, if the B content exceeds 0.0020%, the effect is saturated, which becomes the main reason for the cost increase, so when it is contained, the B content is 0.0020% or less. The B content is preferably 0.0015% or less. The B content is more preferably 0.0012% or less. In order to suppress costs, it is further preferably 0.0010% or less. The B content can be 0% or more, but in order to obtain the above-mentioned effects, the preferred content is 0.0001% or more. More preferably, the B content is 0.0005% or more.

[0112] Hf: 0~0.2%, Ta: 0~0.2%

[0113] These elements contribute to the increase in the strength of the steel pipe, but if the content exceeds 0.2%, the effect is saturated and becomes the main reason for the cost increase, so when contained, it is 0.2% or less. That is, when contained, Hf is 0.2% or less. Hf is preferably 0.1% or less. Hf is more preferably 0.05% or less. In addition, when contained, Ta is 0.2% or less. Ta is preferably 0.1% or less. Ta is more preferably 0.05% or less. The Hf and Ta contents can be 0% or more, but in order to obtain the above-mentioned effects, it is preferred that the content is 0.0001% or more. That is, the Hf content is preferably 0.0001% or more. The Hf content is more preferably 0.0010% or more. In addition, the Ta content is preferably 0.0001% or more. The Ta content is more preferably 0.0010% or more.

[0114] Re: 0~0.005%

[0115] Re contributes to the strength of the steel pipe, but if the content exceeds 0.005%, the effect is saturated and becomes a major factor in the cost increase, so when contained, it is 0.005% or less. The Re content is preferably 0.003% or less. The Re content is more preferably 0.002% or less. The Re content may be 0% or more, but in order to obtain the above-mentioned effect, it is preferably 0.0001% or more. More preferably, it is 0.001% or more.

[0116] Sn: 0~0.3%, Sb: 0~0.3%

[0117] These elements contribute to the increase in strength and hardenability of the steel pipe, but if the content exceeds 0.3%, the effect is saturated and becomes the main reason for the cost increase, so it is contained in an amount of 0.3% or less. That is, the Sn content is 0.3% or less. The Sn content is preferably 0.2% or less. The Sn content is more preferably 0.1% or less. In order to suppress costs, the Sn content is further preferably 0.01% or less. In addition, the Sb content is 0.3% or less. The Sb content is preferably 0.2% or less. The Sb content is more preferably 0.1% or less. In order to suppress costs, the Sb content is further preferably 0.01% or less. The content of Sn and Sb can be 0% or more, but in order to obtain the above-mentioned effects, it is preferred that the content is 0.0001% or more. That is, the Sn content is preferably 0.0001% or more. More preferably, the Sn content is 0.0010% or more. In addition, the Sb content is preferably 0.0001% or more. More preferably, the Sb content is 0.0010% or more.

[0118] In the chemical composition of the steel pipe, the remainder other than the above-mentioned components (elements) is composed of Fe and inevitable impurity elements.

[0119] The metal structure of the steel pipe of the present invention will be described below.

[0120] Metal structure

[0121] The area fraction of retained austenite is 0-3%.

[0122] Since austenite remains in the steel pipe, the amount of hydrogen in the steel increases, which sometimes increases the sensitivity to hydrogen embrittlement. Furthermore, when austenite transforms into martensite due to stress load during use, hydrogen-induced cracking easily occurs because martensite is very hard, and cracks sometimes occur from the martensite part. In the present invention, the fatigue crack growth rate is reduced by making the retained austenite less than 3% in terms of area fraction. It is preferably less than 2%. More preferably, it is less than 1%. The retained austenite can be 0%.

[0123] Bainite or martensite is present at a position 1 / 4 of the wall thickness from the inner surface of the steel pipe (in the case of steel, at a position 1 / 4 of the plate thickness from the surface of the steel), and the bainite area fraction is 90% or more, and the martensite area fraction is 90% or more.

[0124] In order to achieve a high strength with a tensile strength of more than 520MPa, the steel structure needs to be a bainite or martensite structure. On the other hand, when a soft phase and a hard phase are mixed in a steel pipe, fatigue damage is accumulated in the soft phase to a limited extent, and cracks are easily generated, thereby reducing the fatigue limit stress. In a hydrogen environment, local deformation is promoted, so fatigue damage to the soft phase is further accelerated, and the fatigue limit stress in hydrogen is reduced. As a result, the fatigue limit stress in hydrogen / fatigue limit stress in an inactive gas environment is less than 0.90. In order to improve this, it is necessary to reduce the relative proportion of the soft phase. Therefore, the metal structure needs to be a single structure of bainite or martensite, with either bainite or martensite, so that the structure is more than 90% in area fraction. Preferably, the structure of either bainite or martensite is more than 92% in area fraction, more preferably more than 95%. More preferably, it is more than 98%. The upper limit is not particularly limited and can also be 100%. Furthermore, since fatigue cracks are generated from the inner surface of the steel pipe, the uniformity of the inner surface structure of the steel pipe is important. Therefore, the metal structure at a position of 1 / 4 of the wall thickness from the inner surface of the steel pipe is specified, and for steel materials, the metal structure at a position of 1 / 4 of the plate thickness is specified in order to obtain the above effect regardless of which surface becomes the inner surface side of the steel pipe.

[0125] Here, the bainite structure includes bainitic ferrite or granular bainite that transforms during cooling (accelerated cooling or quenching) or after cooling, which contributes to phase transformation strengthening, and includes tempered bainite. When different types of structures such as ferrite, martensite, pearlite, island martensite, and retained austenite are mixed in the bainite structure, the strength is reduced and the toughness is deteriorated, so the volume fraction of the structure other than the bainite phase is as small as possible. Here, the martensite structure includes tempered martensite.

[0126] Furthermore, bainite and martensite structures can precipitate carbides such as cementite through tempering. By precipitating fine carbides, the straightness of the fatigue crack propagation path in hydrogen can be hindered, further reducing the fatigue crack growth rate. Therefore, tempered bainite or tempered martensite structures are preferred. Furthermore, it is preferred to precipitate carbides in a fine dispersion. Therefore, the average size of the carbides is preferably less than 200nm, more preferably less than 50nm. It should be noted that the average size X of the carbides is X=√(a when the long side is set to a and the short side is set to b). 2 +b 2 ) / 2OK.

[0127] 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 inert gas environment is 0.90 or more

[0128] In order to design hydrogen steel structures such as line pipes with long life within the plate thickness range that can be manufactured by the process within the scope of the present invention, the fatigue limit stress in hydrogen of 1 MPa or more of the steel pipe needs to be 200 MPa or more. The fatigue limit stress in hydrogen of 1 MPa or more is preferably 220 MPa or more. The fatigue limit stress in hydrogen of 1 MPa or more is more preferably 250 MPa or more, and further preferably 270 MPa or more. The upper limit is not particularly limited, but the fatigue limit stress in hydrogen of 1 MPa or more is preferably 500 MPa or less.

[0129] Furthermore, the fatigue limit stress in hydrogen of 1 MPa or more / fatigue limit stress in an inert gas environment of the steel pipe needs to be 0.90 or more. In addition, the fatigue limit stress in hydrogen of 1 MPa or more / fatigue limit stress in an inert gas environment is preferably 0.92 or more. The fatigue limit stress in hydrogen of 1 MPa or more / fatigue limit stress in an inert gas environment is more preferably 0.94 or more, and further preferably 0.96 or more. The upper limit is not particularly limited, but the fatigue limit stress in hydrogen of 1 MPa or more / fatigue limit stress in an inert gas environment can be 1.10 or less.

[0130] It should be noted that the inert gas referred to here refers to the six elements of Group 0 of the periodic table, namely helium, neon, argon, krypton, xenon, and radon, and also includes the atmosphere, and the inert gas environment refers to an environment including any of the above.

[0131] The present invention can suppress the decrease of toughness in a high-pressure hydrogen atmosphere by having the above chemical composition and metal structure, and can obtain a tensile strength of 520 MPa or more, and can be applied to hydrogen line pipes. The upper limit of the tensile strength is not particularly limited, but is preferably 950 MPa or less.

[0132] In addition, the plate thickness is preferably 5 mm or more and preferably 30 mm or less.

[0133] [Manufacturing method]

[0134] Next, the method for manufacturing the steel pipe of the present invention will be described. In the following description, the manufacturing method is described by taking the case where the steel pipe is a seamless steel pipe as an example, but it is obvious that an electric resistance welded pipe or a UOE steel pipe can be manufactured by treating the steel pipe in a manner to obtain the same thermal history.

[0135] The steel pipe of the present invention can be produced by sequentially performing the following steps (1) to (3).

[0136] (1) Process of casting after adjusting the composition of the steel billet

[0137] (2) a hot rolling process of heating and rolling the cast material to obtain a steel pipe, and

[0138] (3) A step of cooling (accelerated cooling) and tempering the steel pipe obtained in the hot rolling step (including the case of reheating and quenching before the tempering step)

[0139] Each process is described below. It should be noted that the temperature in the following description is the temperature at the center of the plate thickness of the steel billet or steel pipe unless otherwise specified. The average cooling rate refers to the temperature at a position 1 / 4 of the wall thickness from the inner surface of the steel pipe. It should be noted that the temperature at the center of the plate thickness and the temperature at a position 1 / 4 of the wall thickness from the inner surface of the steel pipe are temperatures estimated from the surface temperature of the steel pipe measured by a radiation thermometer using heat transfer calculations that take into account the heat transfer coefficient of the steel material.

[0140] [Casting process]

[0141] Casting speed: 1.8m / min. or less

[0142] The slower the casting speed, the more the hydrogen concentration and inclusions in the steel can be reduced. The effect becomes significant when the casting speed is below 1.8 m / min., so the casting speed is 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. 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, and the casting speed can be any speed as long as it exceeds 0 m / min.

[0143] [Heating process]

[0144] In order to perform hot rolling, the steel billet having the above-mentioned component composition is heated. The steel billet is not particularly limited, and for example, a billet obtained by a common continuous casting method can be used.

[0145] Heating to a temperature below 1350°C

[0146] If the heating temperature in the heating process exceeds 1350°C, the average grain size of the original austenite grains becomes too large and the various characteristics deteriorate, so the heating temperature is 1350°C or less. The heating temperature is more preferably below 1300°C, further preferably below 1250°C, and most preferably below 1200°C. On the other hand, the lower the heating temperature, the more the amount of hydrogen in the steel can be reduced, so it is preferred, but if it is too low, the finishing temperature decreases and rolling becomes difficult. Therefore, the heating temperature is preferably above 950°C. The heating temperature is more preferably above 1000°C. The heating time is not particularly specified, but if it is too long, the risk of increased hydrogen introduced into the steel pipe increases, so it is preferably 180 minutes or less. The heating time is more preferably 150 minutes or less, and more preferably 120 minutes or less. The lower limit is not particularly limited, but the heating time is preferably 30 minutes or more, and more preferably 60 minutes or more.

[0147] [Rolling process]

[0148] Next, the steel billet heated in the heating step is rolled into a steel pipe shape. The rolling may be performed by hot rolling including piercing rolling using a conventional Mannesman plug mill method or a Mannesman mandrel mill method.

[0149] Rolling end temperature: above 820℃

[0150] If the rolling end temperature is less than 820°C, the rolling load becomes too large and the risk of rolling failure increases. Therefore, the rolling end temperature is 820°C or more. The rolling end temperature is preferably 850°C or more, and more preferably 900°C or more. On the other hand, the upper limit of the rolling end temperature is not particularly limited, but if the temperature is too high, the metal structure tends to become uneven, so the rolling end temperature is preferably 1200°C or less. The rolling end temperature is more preferably 1150°C or less, and further preferably 1100°C or less.

[0151] [Cooling process (accelerated cooling process)]

[0152] In the cooling process, the steel material having the above-mentioned composition is directly or processed into a steel pipe, heated to a temperature of Ac3 point to 1000°C and maintained, and cooled under the cooling conditions of the following group A or group B. It is preferably maintained at the above temperature for more than 10 minutes. It is more preferably maintained for more than 15 minutes, and further preferably maintained for more than 20 minutes. The upper limit is not particularly limited, but it is preferably maintained at the above temperature for less than 60 minutes, and more preferably maintained for less than 45 minutes.

[0153] Heating temperature after steel pipe processing: Ac3 point ~ 1000℃

[0154] If the heating temperature in the cooling process is less than the Ac3 point, ferrite will remain in the steel after cooling, and the strength of the steel pipe and the fatigue limit stress in hydrogen will be reduced. Therefore, the heating temperature is above the Ac3 point. The heating temperature is preferably above Ac3 point + 30 ° C, and more preferably above Ac3 point + 50 ° C. However, for the component system where Ac3 point + 30 ° C and Ac3 point + 50 ° C exceed 1000 ° C, the above-mentioned Ac3 point + 30 ° C and Ac3 point + 50 ° C are not applicable. However, on the other hand, if the above-mentioned heating temperature is higher than 1000 ° C, the austenite grains are sometimes coarsened, causing the impact absorption energy value and toughness of the material after heat treatment to decrease. Therefore, the above-mentioned heating temperature is set to 1000 ° C or less. The heating temperature is preferably below 950 ° C, and more preferably below 900 ° C. However, for the component system where 950 ° C and 900 ° C are less than the Ac3 point, the above-mentioned 950 ° C or below 900 ° C or below is not applicable.

[0155] In the cooling process here, when the temperature after rolling satisfies the present heating conditions, the steel plate may be directly cooled, or may be heated again and cooled after rolling. In addition, when the steel plate is temporarily cooled by air cooling, it may be heated again to a temperature of Ac3 point to 1000°C, and then cooled under the cooling conditions of the following group A or group B.

[0156] In addition, in this invention, Ac3 point (°C) is calculated by the following formula.

[0157] Ac3(℃)=910-203[C]1 / 2-30[Mn]+44.7[Si]+700[P]

[0158] +100[Al]+31.5[Mo]-11[Cr]-15.2[Ni]-20[Cu]+104[V]

[0159] In the formula, [M] represents the content of element M (mass %).

[0160] Average cooling rate

[0161] Group A: Cooled 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 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.

[0162] When the average cooling rate from 800°C to 550°C at a position of 1 / 4 of the wall thickness 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 at a position of 1 / 4 of the wall thickness from the inner surface of the steel pipe is 15°C / s or more. From the viewpoint of suppressing the deviation of the structure, the average cooling rate is preferably 17°C / s or more. The average cooling rate from 800°C to 550°C is further preferably 20°C / s or more, and most preferably 22°C / s or more. On the other hand, in order to suppress the deviation of the grain size, the average cooling rate is preferably 50°C / s or less, more preferably 45°C / s or less, and further preferably 40°C / s or less.

[0163] 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, the retained austenite can be reduced and the amount of hydrogen in the steel can be reduced. Therefore, the average cooling rate from 550°C to 50°C is 15°C / s or less. The average cooling rate from 550°C to 50°C is more preferably 12°C / s or less, and further preferably 10°C / s or less. The lower limit is not particularly limited, but the average cooling rate from 550°C to 50°C is preferably 1°C / s or more.

[0164] The cooling method is not particularly limited, and any method such as water cooling, oil cooling, air cooling, etc. may be used alone or in combination. However, water cooling or oil cooling is preferred from 800°C to 550°C, and air cooling is preferred from 550°C to 50°C.

[0165] Group B: Cooled to 50°C or less at a position 1 / 4 of the wall thickness from the inner surface of the steel pipe with an average cooling rate of 10°C / s or more from 800°C to 300°C and an average cooling rate of 5°C / s or less from 300°C to 50°C

[0166] When the average cooling rate from 800°C to 300°C at a position of 1 / 4 of the wall thickness from the inner surface of the steel pipe is less than 10°C / s, more than 90% of the martensite structure cannot be obtained, and a mixture with the bainite structure is generated, resulting in a reduction in fatigue limit stress in hydrogen. Therefore, the average cooling rate at a position of 1 / 4 of the wall thickness from the inner surface of the steel pipe is 10°C / s or more. From the viewpoint of suppressing the deviation of the structure, the average cooling rate from 800°C to 300°C is preferably 12°C / s or more, the average cooling rate is more preferably 15°C / s or more, and the average cooling rate is further preferably 17°C / s or more. The upper limit is not particularly limited, but the above average cooling rate is preferably 60°C / s or less.

[0167] Furthermore, 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, the amount of hydrogen in the steel can be reduced. Therefore, the average cooling rate from 300°C to 50°C is 5°C / s or less. The average cooling rate from 300°C to 50°C is preferably 1°C / s or less. The lower limit is not particularly limited, but is preferably 0.1°C / s or more.

[0168] The cooling method is not particularly limited, and any method such as water cooling, oil cooling, air cooling, etc. may be used alone or in combination. However, water cooling or oil cooling is preferred from 800°C to 300°C, and air cooling is preferred from 300°C to 50°C.

[0169] [Reheating and quenching process (preferable conditions)]

[0170] Reheating temperature before tempering: Ac3 point ~ 1000℃

[0171] When the temperature in the center of the plate thickness is less than the Ac3 point, a part of the untransformed austenite remains, so the desired steel structure cannot be obtained after hot rolling, quenching, and tempering described later. Therefore, the pre-quenching heating temperature during reheating is preferably above the Ac3 point. Preferably, it exceeds the Ac3 point. It should be noted that in order to suppress the excessive coarsening of the initial austenite grain size and improve production efficiency, the pre-quenching heating temperature is preferably below 1000°C. More preferably, it is below 980°C, and further preferably below 960°C. Most preferably, it is below 950°C. By making the reheating temperature before quenching a temperature on the low temperature side within the range above the Ac3 point, the initial austenite grain size can be refined, and the fatigue limit stress in hydrogen can be reduced.

[0172] Average cooling rate during quenching: Group A or Group B below

[0173] Group A: Cooled 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 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.

[0174] When the average cooling rate from 800°C to 550°C at a position of 1 / 4 of the wall thickness from the inner surface of the steel pipe is less than 15°C / s, a bainite structure of 90% or more in terms of area fraction cannot be obtained, resulting in a decrease in strength. Therefore, the average cooling rate at a position of 1 / 4 of the wall thickness from the inner surface of the steel pipe is 15°C / s or more. From the viewpoint of suppressing the deviation of the structure, the average cooling rate is preferably 17°C / s or more. More preferably, it is 20°C / s or more, and further preferably, it is 22°C / s or more. On the other hand, in order to suppress the deviation of the grain size, the average cooling rate is preferably 50°C / s or less, more preferably 47°C / s or less, and further preferably 45°C / s or less.

[0175] 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, the retained austenite can be reduced and the amount of hydrogen in the steel can be reduced. Therefore, the average cooling rate from 550°C to 50°C is 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. The lower limit is not particularly limited, but the average cooling rate from 550°C to 50°C is preferably 1°C / s or more.

[0176] The cooling method is not particularly limited, and any method such as water cooling, oil cooling, air cooling, etc. may be used alone or in combination. However, water cooling or oil cooling is preferred from 800°C to 550°C, and air cooling is preferred from 550°C to 50°C.

[0177] Group B: Cooled to 50°C or less at a position 1 / 4 of the wall thickness from the inner surface of the steel pipe with an average cooling rate of 10°C / s or more from 800°C to 300°C and an average cooling rate of 5°C / s or less from 300°C to 50°C

[0178] When the average cooling rate from 800°C to 300°C at a position of 1 / 4 of the wall thickness from the inner surface of the steel pipe is less than 10°C / s, more than 90% of the martensite structure cannot be obtained, and a mixture with the bainite structure is generated, resulting in a decrease in the fatigue limit stress in hydrogen. Therefore, the average cooling rate at a position of 1 / 4 of the wall thickness from the inner surface of the steel pipe is 10°C / s or more. From the viewpoint of suppressing the deviation of the structure, the average cooling rate is preferably 17°C / s or more, more preferably 20°C / s or more, and further preferably 25°C / s or more. On the other hand, the upper limit of the above average cooling rate is not particularly specified, but if it exceeds 60°C / s, a large amount of hard structure is generated on the surface of the steel plate, and the steel structure having the target structure in the present invention cannot be obtained, and the fatigue characteristics in hydrogen are reduced. Therefore, the above average cooling rate is preferably 60°C / s or less.

[0179] Furthermore, 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, the amount of hydrogen in the steel can be reduced. Therefore, the average cooling rate from 300°C to 50°C is 5°C / s or less. The average cooling rate is preferably 3°C / s or less, more preferably 1°C / s or less. The lower limit is not particularly limited, but is preferably 0.1°C / s or more.

[0180] The cooling method is not particularly limited, and any method such as water cooling, oil cooling, air cooling, etc. may be used alone or in combination. However, water cooling or oil cooling is preferred from 800°C to 300°C, and air cooling is preferred from 300°C to 50°C.

[0181] Cooling stop temperature during quenching: below 50°C

[0182] If the cooling stop temperature exceeds 50°C, the above-mentioned phase transformation is not completed, so the desired steel structure cannot be obtained after tempering. Therefore, quenching is performed 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. The lower limit is not particularly limited, but the cooling stop temperature is preferably 25°C or more.

[0183] [Tempering process]

[0184] Heating to 400℃~A C1 point

[0185] By setting the tempering temperature to 400°C or higher, it is possible to reduce the retained austenite and the hydrogen in the steel. The tempering temperature is preferably 450°C or higher, more preferably 500°C or higher. C1 When the tempering temperature is below Ac1 point, the retained austenite may increase and the hydrogen in the steel may increase. Therefore, the tempering temperature is below Ac1 point. C1 -30)°C or less. It should be noted that the upper limit of the above average temperature rise rate during tempering is not particularly limited, but is preferably 1°C / s or less. The tempering time is not particularly specified, but the longer the tempering time, the less the retained austenite and hydrogen in the steel pipe, so it is preferably 60 minutes or more. The tempering time is more preferably 80 minutes or more, and further preferably 100 minutes or more. When the tempering time is too long, the material strength is excessively reduced and the effect is saturated, so the tempering time is preferably 180 minutes or less.

[0186] It should be noted that in the present invention, A is calculated by the following formula: C1 Point (℃).

[0187] Ac1=723-14Mn+22Si-14.4Ni+23.3Cr

[0188] In the above formula, the symbol of each element represents the content (mass %) of each element in steel, and an element not contained is represented by 0.

[0189] [Dehydrogenation treatment process]

[0190] When hydrogen is already present in the steel, the acceleration of fatigue crack growth increases, and the fatigue life and fatigue limit stress in hydrogen decrease. Therefore, in order to release the hydrogen remaining after manufacturing, dehydrogenation treatment can be used. Dehydrogenation treatment can reduce the amount of hydrogen in the steel by keeping it at high temperature for a certain period of time before the product is used, and can obtain steel plates with excellent hydrogen embrittlement resistance in high-pressure hydrogen environments.

[0191] The holding time R (h) is preferably determined based on the plate thickness and tube thickness t (mm) of the steel tube and the hydrogen diffusion coefficient D (mm·sec －2 ) and adopt the following formula (A).

[0192] R≥t 2 / D···(A)

[0193] The hydrogen diffusion coefficient also varies depending on the components and metal structure contained, but for example, the hydrogen diffusion coefficient can be 1×10 －11 ~5×10 －9 m 2 / s. More preferably, 5×10 －10 m 2 / s or less.

[0194] The dehydrogenation process is implemented before the welding construction of pipe making or connecting steel pipes. It should be noted that since the hydrogen diffusion coefficient D at high temperature becomes smaller, in order to make hydrogen escape quickly, the dehydrogenation treatment is preferably at high temperature. In the case of high temperature, the diffusion coefficient D' (diffusion coefficient at each temperature) of the temperature at which the value of D of the above (A) formula is maintained can be used for calculation. On the other hand, when the temperature of the dehydrogenation process is too high, the material strength is significantly reduced, so the dehydrogenation temperature is preferably below 550°C. The dehydrogenation temperature T is more preferably below 500°C. The dehydrogenation temperature T is further preferably below 400°C, and most preferably below 300°C. In addition, since the dehydrogenation treatment at a temperature lower than room temperature is the main reason for the increase in processing time and cost, for this reason, the dehydrogenation temperature T is preferably above room temperature. The dehydrogenation temperature T is more preferably above 50°C. The dehydrogenation temperature T is further preferably above 100°C, and most preferably above 150°C. The so-called dehydrogenation temperature T here is the temperature of the atmosphere in the dehydrogenation process. Room temperature refers to 20±10°C.

[0195] In particular, when heating is performed, it takes time for the temperature Tc of the center of the plate thickness of the steel material and the steel pipe to reach the temperature of the atmosphere (dehydrogenation treatment temperature T) in the dehydrogenation treatment process. Therefore, even if the above-mentioned holding time R (sec) is satisfied at the atmosphere temperature, if the dehydrogenation treatment temperature T (atmosphere temperature) is not reached at the center of the plate thickness, the dehydrogenation treatment may not be sufficient. Therefore, it is preferred to maintain R (sec) or more after the temperature Tc of the center of the plate thickness reaches the target dehydrogenation treatment temperature T. Furthermore, in order to obtain a specified crack growth rate in hydrogen, it is necessary to appropriately adjust the amount of hydrogen in the steel material at the surface and the center of the plate thickness. For this purpose, it is preferred to maintain R (sec) or more specified in formula (A) at the dehydrogenation treatment temperature T, and it is further preferred to maintain the above-mentioned holding time R (sec) or more after the temperature Tc of the center of the plate thickness reaches the target dehydrogenation treatment temperature T. In other words, at least the former can properly control the amount of hydrogen in the steel material at the surface of the steel material and the steel pipe, and when the latter is implemented, the amount of hydrogen in the steel material from the surface to the center of the plate thickness of the steel material and the steel pipe can be properly controlled. The plate thickness temperature is the plate thickness center temperature Tc, which can be measured using a thermocouple or the like, or can be estimated using a finite element method or the like.

[0196] Furthermore, the scale on the steel surface hinders dehydrogenation, so it is preferred to remove the scale and perform dehydrogenation treatment. The scale removal method is not limited, and for example, physical cleaning using high-pressure cleaning or a chemical method using a scale removal agent can be used. The thickness of the scale removed is not limited, but if about 100 μm is removed, the scale removal effect can be obtained.

[0197] Second embodiment

[0198] The steel material of the present invention is described in detail below. The component composition, metal structure, and fatigue limit stress of the steel material are the same as those described in the steel pipe. Regarding the manufacturing method, the steps (casting step, heating step, reheating and quenching step, tempering step, and dehydrogenation treatment step) other than the rolling step and the cooling step are also implemented in the same manner as those described in the steel pipe. The rolling step and the cooling step are implemented by the following operations.

[0199] [Rolling process]

[0200] Rolling end temperature: above 820℃

[0201] If the rolling end temperature is less than 820°C, the rolling load becomes too large and the risk of rolling failure increases. Therefore, the rolling end temperature is 820°C or more. The rolling end temperature is preferably 850°C or more, and more preferably 900°C or more. On the other hand, the upper limit of the rolling end temperature is not particularly limited, but if the temperature is too high, the metal structure tends to become uneven, so it is preferred to set the rolling end temperature to 1200°C or less. The rolling end temperature is more preferably 1150°C or less, and further preferably 1100°C or less.

[0202] [Cooling process (accelerated cooling process)]

[0203] In the cooling process, the steel material having the above-mentioned composition is hot-rolled, heated to a temperature of Ac3 point to 1000°C and maintained, and cooled under the cooling conditions of the following group A or group B. It is preferably maintained at the above temperature for more than 10 minutes. It is more preferably maintained for more than 15 minutes, and further preferably maintained for more than 20 minutes. The upper limit is not particularly limited, but it is preferably maintained at the above temperature for less than 60 minutes, and more preferably maintained for less than 45 minutes.

[0204] Heating temperature after hot rolling: Ac3 point ~ 1000℃

[0205] If the heating temperature in the cooling process is less than the Ac3 point, ferrite will remain in the steel after cooling, and the strength of the steel and the fatigue limit stress in hydrogen will be reduced. Therefore, the heating temperature is above the Ac3 point. The heating temperature is preferably above Ac3 point + 30 ° C, and more preferably above Ac3 point + 50 ° C. However, for the component system where Ac3 point + 30 ° C and Ac3 point + 50 ° C exceed 1000 ° C, the above-mentioned Ac3 point + 30 ° C and Ac3 point + 50 ° C are not applicable. On the other hand, if the above-mentioned heating temperature is higher than 1000 ° C, the austenite grains are sometimes coarsened, causing the impact absorption energy value and toughness of the material after heat treatment to decrease. Therefore, the above-mentioned heating temperature is set to 1000 ° C or less. It is more preferably below 950 ° C, and further preferably below 900 ° C. However, for the component system where 950 ° C and 900 ° C are less than the Ac3 point, the above-mentioned 950 ° C or below 900 ° C or below is not applicable.

[0206] In the cooling process here, when the temperature after rolling satisfies the present heating conditions, the steel plate can be directly cooled, or it can be heated again and cooled after rolling. In addition, when the steel plate is temporarily cooled by air cooling, it can be heated again to a temperature of Ac3 point to 1000°C, and then cooled under the cooling conditions of the following group A or group B (this case refers to quenching).

[0207] In addition, in this invention, Ac3 point (°C) is calculated by the following formula.

[0208] Ac3(℃)=910-203[C]1 / 2-30[Mn]+44.7[Si]+700[P]

[0209] +100[Al]+31.5[Mo]-11[Cr]-15.2[Ni]-20[Cu]+104[V]

[0210] In the formula, [M] represents the content of element M (mass %).

[0211] Average cooling rate

[0212] Group A: 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 steel surface, and at an average cooling rate of 15°C / s or less from 550°C to 50°C

[0213] When the average cooling rate from 800°C to 550°C at a position of 1 / 4 of the plate thickness from the surface of the steel is less than 15°C / s, a bainite structure with an area fraction of more than 90% cannot be obtained, resulting in a decrease in strength. Therefore, the average cooling rate at a position of 1 / 4 of the plate thickness from the surface of the steel is 15°C / s or more. From the viewpoint of suppressing the deviation of the structure, the average cooling rate is preferably 17°C / s or more. More preferably, it is 20°C / s or more, and further preferably, it is 22°C / s or more. On the other hand, in order to suppress the deviation of the grain size, the average cooling rate is 50°C / s or less. Preferably, it is 47°C / s or less, and more preferably, it is 45°C / s or less.

[0214] 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, the retained austenite can be reduced and the amount of hydrogen in the steel can be reduced. Therefore, the average cooling rate from 550°C to 50°C is 15°C / s or less. The lower limit is not particularly limited, but the average cooling rate from 550°C to 50°C is preferably 1°C / s or more.

[0215] The cooling method is not particularly limited, and any method such as water cooling, oil cooling, air cooling, etc. may be used alone or in combination. However, water cooling or oil cooling is preferred from 800°C to 550°C, and air cooling is preferred from 550°C to 50°C.

[0216] Group B: Cooled to 50°C or less at a position 1 / 4 of the thickness from the steel surface with an average cooling rate of 10°C / s or more from 800°C to 300°C and an average cooling rate of 5°C / s or less from 300°C to 50°C

[0217] When the average cooling rate from 800°C to 300°C at a position of 1 / 4 of the plate thickness from the surface of the steel is less than 10°C / s, more than 90% of the martensitic structure cannot be obtained, a mixture with the bainitic structure is generated, and the fatigue limit stress in hydrogen is reduced. Therefore, the average cooling rate at a position of 1 / 4 of the plate thickness from the surface of the steel is 10°C / s or more. From the viewpoint of suppressing the deviation of the structure, it is more preferably 12°C / s or more. The average cooling rate is more preferably 15°C / s or more, and further preferably 17°C / s or more. On the other hand, the upper limit of the above-mentioned average cooling rate is not particularly specified, but if it exceeds 60°C / s, a large amount of hard structure is generated on the surface of the steel plate, and the steel structure having the target structure in the present invention cannot be obtained, and the fatigue properties in hydrogen are reduced. Therefore, the above-mentioned average cooling rate is preferably 60°C / s or less.

[0218] Furthermore, 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, the amount of hydrogen in the steel can be reduced. Therefore, the average cooling rate from 300°C to 50°C is 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. The lower limit is not particularly limited, but is preferably 0.1°C / s or more.

[0219] The cooling method is not particularly limited, and any method such as water cooling, oil cooling, air cooling, etc. may be used alone or in combination. However, water cooling or oil cooling is preferred from 800°C to 300°C, and air cooling is preferred from 300°C to 50°C.

[0220] In addition, after the above cooling, it is not necessary to perform cooling in the case of a thick plate, but in the case of a thin steel plate, it is preferable to wind the steel plate into a coil.

[0221] Example 1

[0222] Next, the present invention will be described in more detail based on examples. The following examples are preferred examples of the present invention, and the present invention is not limited to the examples described.

[0223] A steel pipe made of a steel material having the composition shown in Tables 1-1, 1-2, 1-3, 2-1, and 2-2 is manufactured. The manufacturing steps are as follows. First, billets having the composition shown in Tables 1-1, 1-2, 1-3, 2-1, and 2-2 are manufactured. The casting speed when manufacturing the billets shown in Tables 1-1, 1-2, 1-3, and 2-1 is 0.05 to 0.2 m / min. The casting speed of the billets shown in Table 2-2 is 1.1 to 1.5 m / min. The billets are heated to 1000°C to 1100°C and hot rolled. Then, the pipe is expanded by a Mannesmann-head tube rolling method or a Mannesmann-mandrel seamless tube rolling method to obtain a seamless steel pipe (Seamless Steel Pipe) in a manner such that the rolling end temperature is 850°C or above. Then, the seamless steel pipe is slowly cooled by air cooling.

[0224] For steel pipes having an Ac3 point of 950°C or less, the steel pipes obtained by the above method were heated and held at 950°C. For steel pipes having an Ac3 point exceeding 950°C, the steel pipes obtained by the above method were heated and held at 1000°C, and then cooled to 50°C or less at the average cooling rate shown in Tables 3-1, 3-2, 3-3, 4-1, and 4-2.

[0225] Then, tempering was performed, and a portion of the steel pipes No. 16, 29, 35, 37, and 39 were subjected to dehydrogenation treatment, and the metal 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 was in the range of 520 MPa to 700 MPa. In the dehydrogenation treatment of Example 1, after confirming that the plate thickness center temperature Tc reached the room temperature as the target temperature, R (sec) was maintained so as to satisfy the above-mentioned formula (A).

[0226] The evaluation results are shown in Tables 3-1, 3-2, 3-3, and Tables 4-1 and 4-2. The evaluation method is as follows: The steel material collected from the center of the longitudinal direction of the steel pipe was processed into the steel material of the present invention.

[0227] Retained austenite determination

[0228] A sample for metal structure observation was taken from the central part of the plate width in the central part of the long side direction of the steel material and steel pipe obtained according to the above operation, and the cross section parallel to the long side direction was polished as the observation target surface, and then the surface layer was removed by chemical polishing using picric acid etching, and the measurement was performed using X-ray diffraction measurement. Specifically, a Co-Kα ray source was used as the incident X-ray, and the area fraction of retained austenite was calculated based on the intensity ratio of the (200), (211), and (220) planes of ferrite to the (200), (220), and (311) planes of austenite.

[0229] Determination of the area fraction of bainite and martensite

[0230] The metal structure at the 1 / 4 position of the wall thickness on the inner side of the obtained steel pipe was evaluated as follows. Test pieces were taken from the center of the long side direction of the steel pipe so that the 1 / 4 position of the wall thickness on the inner side and the center position of the wall thickness became the observation positions, and the cross-section of the taken test piece was etched using a 3 vol% nitric acid alcohol solution. Scanning electron microscope photos were taken at an appropriate magnification between 1000 and 5000 times to observe tempered martensite, ferrite, bainite, and pearlite. For martensite, ferrite, bainite, and pearlite, they were compared with the structure photos of non-patent document 2 and judged by visual observation. For the structure fraction, the image obtained by dividing the SEM photo area based on the above judgment was used, and the image analysis was used to obtain it (for example, when calculating the bainite fraction, the bainite and other regions were binarized to obtain the bainite fraction), which was used as the area fraction of each phase.

[0231] Tensile Strength (TS)

[0232] According to JIS Z 2201, JIS No. 14 proportional test pieces (diameter of parallel part 7 mm, distance between points 35 mm) were taken from the steel pipes and steel materials obtained in the above-mentioned operation, and the tensile strength was measured.

[0233] Hydrogen heating analysis

[0234] The amount of hydrogen remaining in the steel was determined using a temperature rise desorption analysis method using a low temperature type temperature rise type hydrogen analyzer (gas chromatograph type) (JTF-20AL). The temperature rise desorption analysis was performed at a temperature rise rate of 200°C / h in the temperature range from room temperature to 400°C, and the sum was taken as the amount of hydrogen. The test body was a cylindrical shape with a length of 30 mm and a diameter of 7Φ along the long side of the steel pipe at a position 1 / 4 of the thickness of the steel plate and a position 1 / 4 from the inner surface of the steel pipe. It should be noted that the amount of hydrogen is the amount of hydrogen before the high pressure hydrogen fatigue test described in the matters described later, and is the amount of H shown in Table 1-1, Table 1-2, Table 1-3, Table 2-1, and Table 2-2.

[0235] Fatigue test

[0236] The fatigue strength is obtained by performing fatigue tests in a high-pressure gas mixture atmosphere in the atmosphere at room temperature (20±10℃) according to ASTM E466, Fatigue Testing, with a frequency of 1 to 15 Hz, a repetitive waveform of sine wave, a control method of load control, a load condition of uniaxial tension and compression, and a stress ratio of R=-1.0. The stress at which the material does not break after 10 million repetitions is defined as the fatigue limit strength in the atmosphere.

[0237] High pressure hydrogen fatigue test

[0238] The fatigue limit stress in hydrogen is determined by conducting a fatigue test at room temperature (20±10°C) at a pressure of 40 MPa of hydrogen (100% gas) or a mixed atmosphere of hydrogen at a pressure of 1 MPa or more or natural gas (mainly hydrocarbons such as methane and ethane) containing hydrogen at a hydrogen partial pressure of 1 MPa or more according to ASTM E466, Fatigue Testing, with a frequency of 1 Hz, a repetitive waveform of a sine wave, a control method of load control, a load condition of uniaxial tension and compression, and a stress ratio of R=-1.0. The stress at which no fracture occurs after 2 million repetitions is defined as the fatigue limit stress in hydrogen. It should be noted that the fatigue limit stress in hydrogen obtained in this test is 200 MPa or more, and the ratio of the fatigue limit stress in hydrogen to the fatigue limit stress in an inert gas atmosphere, i.e., the fatigue limit stress in hydrogen / the fatigue limit stress in an inert gas environment is 0.90 or more, which is considered acceptable.

[0239] In the inventive examples of the present invention, as shown in Tables 3-1, 3-2, 3-3, 4-1, and 4-2, all fatigue limit stresses in hydrogen are 200 MPa or more, and the ratio of the above fatigue limit strength in an inert gas atmosphere, i.e., fatigue limit stress in hydrogen / fatigue limit stress in an inert gas environment, is 0.90 or more, and the tensile strength is 520 MPa or more, satisfying excellent hydrogen embrittlement resistance characteristics.

[0240] [Table 1-1]

[0241]

[0242] [Table 1-2]

[0243]

[0244] [Table 1-3]

[0245]

[0246] [Table 2-1]

[0247]

[0248] [Table 2-2]

[0249]

[0250] [Table 3-1]

[0251]

[0252] [Table 3-2]

[0253]

[0254] [Table 3-3]

[0255]

[0256] [Table 4-1]

[0257]

[0258] [Table 4-2]

[0259]

[0260] Example 2 Hereinafter, an example for verifying the effect of the present invention will be described. It should be noted that in the following examples, steel pipes were manufactured under the following manufacturing conditions and their properties were evaluated. Steel grades No. 1, 14, 46, and 91 shown in Tables 1-1, 1-2, and 2-2 were used, and the steel pipes were manufactured under the same conditions as those of Steel Pipes No. 1, 14, and 46 shown in Tables 3-1 and 3-2 and Steel Pipe No. 91 shown in Table 4-1 until the tempering process, and the properties were evaluated when the dehydrogenation treatment conditions were changed. The above results are shown in Table 5.

[0261] In the present Example 2, the steel pipes and steel materials No. 1A, 14A, 46A, and 91A were implemented so that the dehydrogenation treatment temperature T (atmosphere temperature) was 50°C and the holding time tc after the plate thickness center temperature Tc reached 50°C satisfied the formula (A). The steel pipes and steel materials No. 14B, 46B, and 91B were implemented so that the dehydrogenation treatment temperature T (atmosphere temperature) was 50°C and the holding time tc when the dehydrogenation treatment temperature T was 50°C satisfied the above formula (A), but the holding time tc after the plate thickness center temperature Tc reached 50°C did not satisfy the above formula (A).

[0262] For steel pipes and steel materials No. 14C, 46C, and 91C, the dehydrogenation treatment temperature T (atmosphere temperature) is 50°C, but the holding time t of the atmosphere temperature and the holding time tc after the plate center temperature Tc reaches 50°C do not satisfy the above formula (A).

[0263] In Table 5, "dehydrogenation holding time t is Y" means that the dehydrogenation treatment temperature T (atmosphere temperature) is 50°C and the holding time t satisfies the formula (A), and "dehydrogenation holding time t is N" means that the dehydrogenation treatment temperature T (atmosphere temperature) is 50°C, but the holding time t does not satisfy the formula (A). In addition, "holding time tc of the center temperature Tc of the steel material is Y" means that the holding time tc after the center temperature Tc of the plate reaches 50°C satisfies the formula (A), and "holding time tc of the center temperature Tc of the steel material is N" means that although the center temperature Tc of the plate reaches 50°C, the holding time tc after Tc reaches 50°C does not satisfy the formula (A).

[0264] The fatigue test, structure, tensile strength and other measurement methods are the same as those in Example 1.

[0265] All the examples of the present invention satisfy the requirements that the fatigue limit stress in hydrogen is 200 MPa or more, the ratio of the fatigue limit stress in an inert gas atmosphere, i.e., the fatigue limit stress in hydrogen / the fatigue limit stress in an inert gas environment is 0.90 or more, and the tensile strength is 520 MPa or more. Among them, when the dehydrogenation treatment is carried out under more preferred conditions, the fatigue characteristics are more excellent. It should be noted that the same characteristics can be obtained for the steel pipe and steel material of the same No.

[0266] [Table 5]

[0267]

[0268] Example 3 The following is an example that verifies the effect of the present invention. It should be noted that in the following examples, steel materials and steel pipes were manufactured under the following manufacturing conditions, and the characteristics were evaluated. Steel pipes and steel materials with the same composition as No. 14, 46 shown in Tables 3-1 and 3-2, and No. 91 shown in Table 4-2 were used, and the cooling process was carried out under the prescribed conditions. After the cooling process (before the tempering process), the steel pipes and steel materials were reheated under the conditions of Tables 6-1 and 6-2, and the quenching process was carried out. The characteristics of the obtained steel pipes and steel materials were evaluated. The results are also shown in Tables 6-1 and 6-2. The steel pipes and steel materials No. 14E, 14F, 46E, and 46F shown in Table 6-1 were obtained by reheating the steel pipes and steel materials No. 14 and 46 shown in Tables 3-1 and 3-2. In addition, the steel pipes and steel materials No. 91E and 91F shown in Table 6-2 were obtained by reheating the steel pipes and steel materials No. 91 shown in Table 4-2.

[0269] The fatigue test, structure, tensile strength and other measurement methods are the same as those in Example 1.

[0270] All the examples of the present invention satisfy the conditions that the fatigue limit stress in hydrogen is 200 MPa or more, the ratio of the fatigue limit stress in hydrogen to the fatigue limit stress in an inert gas atmosphere is 0.90 or more, and the tensile strength is 520 MPa or more. It should be noted that the same characteristics can be obtained for the steel pipe and steel material of the same No.

[0271] [Table 6-1]

[0272]

[0273] [Table 6-2]

[0274]

Claims

1. A steel pipe for a line pipe having excellent hydrogen embrittlement resistance, having the following chemical composition: containing, by 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 further containing Nb: 0-0.10%, Ti: 0-0.1%, Ca: 0-0 0.005%, Ni: 0-2.0%, Cu: 0-1.0%, Cr: 0-1.0%, Mo: 0-0.60%, W: 0-1.0%, V: 0-0.10%, Zr: 0-0.050%, REM: 0-0.050%, Mg: 0-0.050%, B: 0-0.0020%, Hf: 0-0.2%, Ta: 0-0.2%, Re: 0-0.005%, Sn: 0-0.3%, Sb: 0-0.3%, and the balance is Fe and unavoidable impurity elements; The retained austenite is 0 to 3% in terms of area fraction, bainite or martensite is present at a position 1 / 4 of the wall thickness from the inner surface of the steel pipe, the bainite is 90% or more in terms of area fraction or the martensite is 90% or more in terms of area fraction, 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 inactive gas environment is 0.90 or more.

2. The steel pipe for line pipe having excellent hydrogen embrittlement resistance according to claim 1, wherein: The chemical composition is further as follows in mass %: Nb: 0.001-0.10%, Ti: 0.005-0.1%, Ca: 0.0001-0.005%, Ni: 0.01-2.0%, Cu: 0.01-1.0%, Cr: 0.01-1.0%, Mo: 0.01-0.60%, W: 0.01-1.0%, V: 0.01-0.10%, Zr: 0.0001~0.050%, REM: 0.0001~0.050%, Mg: 0.0001~0.050%, B: 0.0001~0.0020%, Hf:

0. 0001~0.2%, Ta: 0.0001~0.2%, Re: 0.0001~0.005%, Sn: 0.0001~0.3%, Sb: 0.0001~0.3%.

3. A method for manufacturing a steel pipe for a line pipe, comprising: A casting step of casting a steel billet having the chemical composition of claim 1 or 2 at a casting speed of 1.8 m / min or less, The heating process is carried out at a temperature below 1350°C. In the hot rolling step, the steel billet heated in the heating step is rolled at a rolling end temperature of 820° C. or higher to form a steel pipe. A cooling step, after the steel pipe obtained in the hot rolling step is maintained at a temperature of Ac3 point to 1000°C, the cooling conditions are the following group A or group B, and A tempering step, wherein the steel pipe obtained in the cooling step is tempered at 400°C to Ac1 point; Group A: The steel pipe is cooled to 50°C or below at an average cooling rate of 15°C / s or above at a position 1 / 4 of the wall thickness from the inner surface of the steel pipe from 800°C to 550°C, and at an average cooling rate of 15°C / s or below at a position 1 / 4 of the wall thickness from the inner surface of the steel pipe from 550°C to 50°C, Group B: The steel pipe is cooled to 50°C or less at an average cooling rate from 800°C to 300°C of 10°C / s or more at a position 1 / 4 of the wall thickness from the inner surface of the steel pipe, and at an average cooling rate from 300°C to 50°C of 5°C / s or less at a position 1 / 4 of the wall thickness from the inner surface of the steel pipe.

4. The method for manufacturing a steel pipe for a line pipe according to claim 3, wherein: There is a quenching process before the tempering process, which reheats the steel pipe to Ac3 point ~ 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 above at a position 1 / 4 of the wall thickness from the inner surface of the steel pipe from 800°C to 550°C, and at an average cooling rate of 15°C / s or below at a position 1 / 4 of the wall thickness from the inner surface of the steel pipe from 550°C to 50°C, Group B: The steel pipe is cooled to 50°C or less at an average cooling rate from 800°C to 300°C of 10°C / s or more at a position 1 / 4 of the wall thickness from the inner surface of the steel pipe, and at an average cooling rate from 300°C to 50°C of 5°C / s or less at a position 1 / 4 of the wall thickness from the inner surface of the steel pipe.

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

6. A steel material for a line pipe having excellent hydrogen embrittlement resistance, having the following chemical composition: containing, by 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 further containing Nb: 0-0.10%, Ti: 0-0.1%, Ca: 0-0 0.005%, Ni: 0-2.0%, Cu: 0-1.0%, Cr: 0-1.0%, Mo: 0-0.60%, W: 0-1.0%, V: 0-0.10%, Zr: 0-0.050%, REM: 0-0.050%, Mg: 0-0.050%, B: 0-0.0020%, Hf: 0-0.2%, Ta: 0-0.2%, Re: 0-0.005%, Sn: 0-0.3%, Sb: 0-0.3%, and the balance is Fe and unavoidable impurity elements; The retained austenite has an area fraction of 0 to 3%, and there is bainite or martensite at 1 / 4 of the plate thickness, the bainite has an area fraction of 90% or more or the martensite has an area fraction of 90% or more, the fatigue limit stress in hydrogen above 1 MPa is above 200 MPa, and the fatigue limit stress in hydrogen above 1 MPa / fatigue limit stress in an inactive gas environment is above 0.

90.

7. The steel material for line pipe having excellent hydrogen embrittlement resistance according to claim 6, wherein: The chemical composition is further as follows in mass %: Nb: 0.001-0.10%, Ti: 0.005-0.1%, Ca: 0.0001-0.005%, Ni: 0.01-2.0%, Cu: 0.01-1.0%, Cr: 0.01-1.0%, Mo: 0.01-0.60%, W: 0.01-1.0%, V: 0.01-0.10%, Zr: 0.0001~0.050%, REM: 0.0001~0.050%, Mg: 0.0001~0.050%, B: 0.0001~0.0020%, Hf:

0. 0001~0.2%, Ta: 0.0001~0.2%, Re: 0.0001~0.005%, Sn: 0.0001~0.3%, Sb: 0.0001~0.3%.

8. A method for manufacturing a steel material for a pipeline pipe, comprising: A casting step of casting a steel billet having the chemical composition of claim 6 or 7 at a casting speed of 1.8 m / min or less, The heating process is carried out at a temperature below 1350°C. The hot rolling step is to roll the steel billet heated in the heating step at a rolling end temperature of 820° C. or higher. A cooling step, after the steel material obtained in the hot rolling step is maintained at a temperature of Ac3 point to 1000°C, the cooling conditions are the following group A or group B, and A tempering step, wherein the steel obtained in the cooling step is tempered at 400°C to Ac1 point; Group A: The steel is cooled to 50°C or below at an average cooling rate of 15°C / s or above at a position 1 / 4 of the wall thickness from the surface of the steel material, and at an average cooling rate of 15°C / s or below at a position 1 / 4 of the wall thickness from the surface of the steel material, from 550°C to 50°C. Group B: The steel is cooled to below 50°C with an average cooling rate from 800°C to 300°C of 10°C / s or more at a position 1 / 4 of the wall thickness from the steel surface, and an average cooling rate from 300°C to 50°C of 5°C or less at a position 1 / 4 of the wall thickness from the steel surface.

9. The method for manufacturing a steel material for a line pipe according to claim 8, wherein: There is a quenching process before the tempering process, which reheats the steel material to Ac3 point ~ 1000°C, and the cooling conditions are the following group A or group B, Group A: The steel is cooled to 50°C or below at an average cooling rate of 15°C / s or above at a position 1 / 4 of the wall thickness from the surface of the steel material, and at an average cooling rate of 15°C / s or below at a position 1 / 4 of the wall thickness from the surface of the steel material, from 550°C to 50°C. Group B: The steel is cooled to below 50°C with an average cooling rate from 800°C to 300°C of 10°C / s or more at a position 1 / 4 of the wall thickness from the steel surface, and an average cooling rate from 300°C to 50°C of 5°C or less at a position 1 / 4 of the wall thickness from the steel surface.

10. The method for manufacturing a steel material for a line pipe according to claim 8 or 9, wherein: The casting speed is below 1.0 m / min.

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