Steel material for line pipe having excellent hydrogen embrittlement resistance, method for producing same, steel pipe for line pipe having excellent hydrogen embrittlement resistance, 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) Provided are: a steel material for a line pipe, which has excellent hydrogen embrittlement resistance in a high-pressure hydrogen environment and has high strength for a steel structure used in a high-pressure hydrogen environment, such as a line pipe; a method for producing the steel material; a steel pipe for a line pipe; and a method for producing the steel pipe. A steel material for a line pipe having excellent hydrogen embrittlement resistance, the steel material having a specific chemical composition and a specific structure, the fatigue limit stress in hydrogen at 1 MPa or more being 200 MPa or more, and the fatigue limit stress in hydrogen at 1 MPa or more / fatigue limit stress in an inert gas atmosphere being 0.90 or more.

Description

Technical Field

[0001] The present invention relates to a steel material 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 pipe 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] It should be noted that Non-Patent Document 1 describes the value of fatigue strength of low alloy steel.

[0011] Prior art literature

[0012] Patent Literature

[0013] Patent Document 1: Japanese Patent Application Laid-Open No. 2005-2386

[0014] Patent Document 2: Japanese Patent Application Publication No. 2009-46737

[0015] Patent Document 3: Japanese Patent Application Publication No. 2009-275249

[0016] Patent Document 4: Japanese Patent Application Publication No. 2009-74122

[0017] Patent Document 5: Japanese Patent Application Publication No. 2010-37655

[0018] Patent Document 6: Japanese Patent Application Publication No. 2012-107332

[0019] Non-patent literature

[0020] Non-patent document 1: Matsunaga et al., Int J Hydrogen Energy, Vol. 40 (2015), p. 5739-5748

[0021] 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

[0022] 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.

[0023] In view of the above-mentioned problems of the prior art, the present invention aims to provide a steel material for pipeline pipes having high strength and excellent resistance to hydrogen embrittlement in a high-pressure hydrogen environment, which is suitable for use as a steel structure used in a high-pressure hydrogen environment, such as a pipeline pipe for 100% hydrogen or natural gas (natural gas is a gas with hydrocarbons such as methane and ethane as a main component) containing hydrogen with a hydrogen partial pressure of 1 MPa or more, and a manufacturing method thereof, a steel pipe for pipeline pipes and a manufacturing method thereof.

[0024] It should be noted that the so-called "excellent hydrogen embrittlement resistance in a high-pressure hydrogen environment" here means that the fatigue test is carried out in accordance with ASTM E466, Fatigue Testing, with a frequency of 1 Hz, repetitive waveform: sine wave, control method: load control, load conditions: uniaxial tension and compression, and stress ratio: R = -1.0 at room temperature (20 ± 10 ° C) and a mixed atmosphere of hydrogen gas with a pressure of 1 MPa or more or a natural gas (mainly composed of hydrocarbons such as methane and ethane) containing hydrogen with a hydrogen partial pressure of 1 MPa or more, and the stress at which no fracture occurs after 2 million repetitions, that is, the fatigue limit stress in hydrogen is 200 MPa or more, and the above-mentioned fatigue limit stress in hydrogen / fatigue limit stress in an inert gas environment is 0.90 or more.

[0025] 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.

[0026] 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.

[0027] The present inventors have conducted intensive research on the conditions that steel materials should satisfy in order to obtain steel plates and steel pipes for line pipes having excellent hydrogen embrittlement resistance, and have invented new high-strength steel plates and steel pipes for line pipes. In addition, the steel materials and steel pipes of the present invention have high strength, and high strength in the present invention refers to a tensile strength of 520 MPa or more.

[0028] The gist of the present invention is as follows.

[0029] [1] A steel material for a line pipe having excellent hydrogen embrittlement resistance, comprising, in terms of mass%, 0.02-0.15% C, 0.01-2.0% Si, 0.5-1.5% Mn, 0.0001-0.015% P, 0.0002-0.0015% S, 0.005-0.15% Al, 0.01% or less O, 0.010% or less N, and 0.0010% or less H, or further comprising 0-0.10% Nb, 0-0.005% Ca, and 0-0.005% Ti: At least one of Ni: 0-0.1%, 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 remainder is Fe and unavoidable impurity elements;

[0030] The area fraction of retained austenite is 0 to 3%, the area fraction of bainite is 90% or more at the 1 / 4 position of the plate thickness, the fatigue limit stress in hydrogen above 1 MPa is 200 MPa or more, and the fatigue limit stress in hydrogen above 1 MPa / fatigue limit stress in an inactive gas environment is 0.90 or more.

[0031] [2] The steel material for line pipes having excellent hydrogen embrittlement resistance according to [1], wherein the chemical composition further comprises, in mass%, Nb: 0.001-0.10%, Ca: 0.0001-0.005%, Ti: 0.005-0.1%, 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%.

[0032] [3] A method for manufacturing a steel material for a pipeline pipe, comprising:

[0033] The heating step is to heat the steel billet having the chemical composition described in [1] or [2] at 1000 to 1250° C.

[0034] Hot rolling process, rolling the steel billet heated in the above heating process under the condition of rolling end temperature: Ar3 point or above;

[0035] a controlled cooling step of cooling the hot rolled steel sheet obtained in the hot rolling step under the conditions that the cooling start temperature is Ar3 point or higher in terms of the steel sheet surface temperature, the cooling start time difference between the front end and the rear end of the hot rolled steel sheet is within 50 seconds, the average cooling rate from 750°C to 550°C is 15 to 50°C / s in terms of the temperature at the center of the sheet thickness, and the cooling stop temperature is 250 to 650°C; and

[0036] In the dehydrogenation treatment step, the steel sheet obtained in the above-mentioned controlled cooling step is kept in a range of room temperature to 550°C.

[0037] [4] A steel pipe for a line pipe having excellent resistance to hydrogen embrittlement, wherein the steel pipe for a line pipe has a chemical composition comprising, by mass%, 0.02 to 0.15% C, 0.01 to 2.0% Si, 0.5 to 1.5% Mn, 0.0001 to 0.015% P, 0.0002 to 0.0015% S, 0.005 to 0.15% Al, 0.01% or less O, 0.010% or less N, and 0.0010% or less H, or further comprising 0 to 0.10% Nb, 0 to 0.005% Ca, 、Ti: 0-0.1%, 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;

[0038] The retained austenite is 0 to 3% by area fraction, the bainite is 90% or more by area fraction at a position 1 / 4 of the wall thickness from the inner surface of the steel pipe, 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 / the fatigue limit stress in an inactive gas environment is 0.90 or more.

[0039] [5] The steel pipe for line pipe having excellent hydrogen embrittlement resistance according to [4], wherein the chemical composition is further, in mass%, Nb: 0.001-0.10%, Ca: 0.0001-0.005%, Ti: 0.005-0.1%, 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%.

[0040] [6] A method for manufacturing a steel pipe for a pipeline, comprising:

[0041] The heating step is to heat the steel billet having the chemical composition described in [4] or [5] at 1000 to 1250°C.

[0042] In the hot rolling step, the steel billet heated in the heating step is rolled at a rolling end temperature of Ar3 point or higher.

[0043] The hot rolled steel sheet obtained in the hot rolling step is cooled under the conditions that the cooling start temperature is Ar3 point or higher in terms of the steel sheet surface temperature, the cooling start time difference between the front end and the rear end of the hot rolled steel sheet is within 50 seconds, the average cooling rate from 750°C to 550°C is 15 to 50°C / s in terms of the temperature at the center of the sheet thickness, and the cooling stop temperature is 250 to 650°C.

[0044] Any of the following pipe making processes: a pipe making process of bending the hot rolled steel plate after the controlled cooling process, butting both ends and welding; a pipe making process of forming the hot rolled steel plate into a cylindrical shape by cold roll forming after the controlled cooling process, butting both ends of the cylindrical shape in the circumferential direction and performing electric resistance welding, and

[0045] In the dehydrogenation treatment step, the steel pipe obtained in the pipe making step is kept in a temperature range of room temperature to 550°C.

[0046] According to the present invention, it is possible to easily and simply manufacture a steel material having 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

[0047] Next, a method for implementing the present invention will be described in detail. It should be noted that the following description shows a preferred embodiment of the present invention, and the present invention is not limited by the following description. As a first embodiment, a steel material is described in detail, and then as a second embodiment, a UOE steel pipe as an example of a steel pipe of the present invention is described in detail, and as a third embodiment, an electric resistance welded steel pipe as an example of a steel pipe of the present invention is described in detail.

[0048] First embodiment

[0049] [Ingredients]

[0050] The reasons for limiting the chemical composition of the steel material of the present invention are described below. It should be noted that "%" in the following description means "mass %" unless otherwise specified.

[0051] C: 0.02~0.15%

[0052] C effectively contributes to the improvement of strength, but when the content is less than 0.02%, sufficient strength and fatigue limit stress cannot be ensured. Therefore, the C content is 0.02% or more. Preferably, the C content is 0.03% or more. On the other hand, if it exceeds 0.15%, the weldability decreases. Therefore, the C amount is limited to 0.15% or less. Preferably, the C content is 0.13% or less. Furthermore, if it exceeds 0.08%, the hardness of the surface layer and the center segregation part increases during controlled cooling, so the resistance to SSCC (sulfide stress corrosion cracking) and HIC (hydrogen induced cracking) deteriorates. In addition, toughness also deteriorates. Therefore, it is more preferred that the C content is 0.08% or less. It is further preferred that the C content is 0.05% or less.

[0053] Si: 0.01~2.0%

[0054] 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.

[0055] Mn: 0.5~1.5%

[0056] 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.

[0057] P: 0.0001~0.015%

[0058] 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.

[0059] S: 0.0002~0.0015%

[0060] 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 perspective of refining cost, the S content is 0.0002% or more.

[0061] Al: 0.005~0.15%

[0062] 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.

[0063] O: 0.01% or less

[0064] 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 cause of increased cost, so the O content is preferably 0.001% or more.

[0065] N: 0.010% or less

[0066] N has little effect on the fatigue properties of steel. If the N content is 0.010% or less, the effect of the present invention will not be impaired from the perspective 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 perspective of improving toughness, a low N content is preferred, but excessive reduction will increase the cost of steelmaking, so the N content is preferably 0.00001% or more. The N content is preferably 0.001% or more.

[0067] H: 0.0010% or less

[0068] 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. 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.

[0069] In order to further improve the strength and toughness of the steel plate, the component composition disclosed in the present invention can arbitrarily contain one or more selected from Nb, Ca, Ti, Ni, Cu, Cr, Mo, W, V, Zr, REM, Mg, B, Hf, Ta, Re, Sn, and Sb within the following range.

[0070] Nb: 0~0.10%

[0071] 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 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 when the Nb content is less than 0.001%, it is difficult to obtain the effect of its inclusion, so when contained, it is preferably 0.001% or more. The Nb content is more preferably 0.01% or more.

[0072] Ca: 0~0.005%

[0073] 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 deteriorates 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 when it is less than 0.0001%, it is difficult to obtain the effect of addition, so when contained, it is preferably 0.0001% or more. The Ca content is more preferably 0.001% or more.

[0074] Ti: 0~0.1%

[0075] 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 when 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.

[0076] Ni: 0-2.0%

[0077] 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.

[0078] Cu: 0~1.0%

[0079] 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.

[0080] Cr: 0~1.0%

[0081] 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.

[0082] Mo: 0~0.60%

[0083] 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.

[0084] W: 0~1.0%

[0085] 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.

[0086] V: 0~0.10%

[0087] V is an element that can be arbitrarily contained in order to improve the strength and toughness of steel materials. However, 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.

[0088] Zr: 0~0.050%, REM: 0~0.050%, Mg: 0~0.050%

[0089] 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.

[0090] B: 0~0.0020%

[0091] 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 increase in cost, 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, it is preferably 0.0001% or more. More preferably, the B content is 0.0005% or more.

[0092] Hf: 0~0.2%, Ta: 0~0.2%

[0093] These elements contribute to the increase of the strength of the steel, but if the content exceeds 0.2%, the effect is saturated and becomes the main reason for the increase in cost, 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.

[0094] Re: 0~0.005%

[0095] Re contributes to the strength of steel, but if the content exceeds 0.005%, the effect is saturated and becomes a major factor in the increase of cost, 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.

[0096] Sn: 0~0.3%, Sb: 0~0.3%

[0097] These elements contribute to the increase in strength and hardenability of steel, but if the content exceeds 0.3%, the effect is saturated and becomes the main reason for the increase in cost, 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.

[0098] In the chemical composition of the steel plate and the steel pipe, the remainder other than the above-mentioned components (elements) is composed of Fe and inevitable impurity elements.

[0099] The metal structure of the steel material of the present invention will be described below.

[0100] Metal structure

[0101] Retained austenite is 0-3%

[0102] Since austenite remains in the steel, 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 is likely to occur 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%. The reduction of residual γ suppresses the occurrence of fatigue cracks in a hydrogen environment, thereby being able to suppress the reduction of fatigue limit stress in hydrogen. Therefore, the retained austenite is less than 3%. The retained austenite is preferably less than 2%. The retained austenite is more preferably less than 1%. The retained austenite can be 0%.

[0103] Bainite at 1 / 4 of the plate thickness has an area fraction of 90% or more

[0104] In order to achieve a high tensile strength of 520 MPa or more, the steel structure needs to be a bainite structure. Here, the bainite structure includes bainitic ferrite or granular bainite that transforms during or after controlled cooling, which contributes to phase transformation strengthening, and includes tempered bainite. If 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.

[0105] In addition, for the generation of fatigue cracks, when soft phases and hard phases are mixed in the steel, fatigue damage is preferentially accumulated in the soft phase, and cracks are easily generated, thereby reducing the fatigue limit stress. In a hydrogen environment, local deformation is promoted, thereby further accelerating fatigue damage to the soft phase, and the fatigue limit stress in hydrogen is further reduced. As a result, the fatigue limit stress / 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 so that the area fraction of bainite is 90% or more. Bainite is preferably 92% or more in terms of area fraction. Bainite is more preferably 95% or more in terms of area fraction, and more preferably 98% or more. The upper limit is not particularly limited, and bainite can be 100% in terms of area fraction. 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 very important. Therefore, the metal structure at a position 1 / 4 of the wall thickness from the inner surface of the steel pipe is specified. For the steel material, in order to obtain the above-mentioned effect regardless of which surface becomes the inner surface side of the steel pipe, the metal structure at a position 1 / 4 of the plate thickness is specified.

[0106] 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

[0107] If the fatigue limit stress in hydrogen of more than 1MPa is less than 200MPa, and the fatigue limit stress in hydrogen of more than 1MPa / fatigue limit stress in an inactive gas environment is less than 0.90, it is greatly different from the design conditions of the previous pipeline, so it is necessary to increase the thickness of the steel (the thickness of the steel pipe in the case of a steel pipe). Therefore, the fatigue limit stress in hydrogen of more than 1MPa is more than 200MPa, and the fatigue limit stress in hydrogen of more than 1MPa / fatigue limit stress in an inactive gas environment is more than 0.90. The fatigue limit stress in hydrogen of more than 1MPa is preferably more than 220MPa. The fatigue limit stress in hydrogen of more than 1MPa is more preferably more than 250MPa, and more preferably more than 270MPa. The upper limit is not particularly limited, but the fatigue limit stress in hydrogen of more than 1MPa is preferably less than 500MPa. In addition, the fatigue limit stress in hydrogen of more than 1MPa / fatigue limit stress in an inactive gas environment is preferably more than 0.92. The fatigue limit stress in hydrogen of more than 1MPa / fatigue limit stress in an inactive gas environment is more preferably more than 0.94, and more preferably more than 0.96. The upper limit is not particularly limited, but the fatigue limit stress in hydrogen of 1 MPa or more / the fatigue limit stress in an inert gas environment may be 1.1 or less.

[0108] It should be noted that the inert gas referred to here includes the atmosphere in addition to the six elements of Group 0 of the periodic table, namely helium, neon, argon, krypton, xenon, and radon, and the inert gas environment refers to an environment including any of the above.

[0109] The present invention has the above chemical composition and metal structure, and can improve the fatigue limit stress in a high-pressure hydrogen atmosphere and suppress the reduction of the fatigue limit stress in hydrogen / fatigue limit stress in an inert gas, 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.

[0110] The plate thickness of the steel material is not particularly limited, but is preferably 5 mm or more and preferably 30 mm or less.

[0111] Next, the method for producing the steel material of the present invention is described. The steel material of the present invention can be produced by sequentially performing a heating step, a hot rolling step, a controlled cooling step, and a dehydrogenation treatment step of a steel billet (slab).

[0112] 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.

[0113] Heating process

[0114] Heating temperature of steel billet: 1000~1250℃

[0115] When the heating temperature of steel billets such as billets and slabs is less than 1000°C, the diffusion of impurity elements such as microsegregated C, P, and S is insufficient, and a homogeneous material cannot be obtained. Therefore, the heating temperature of the steel billet is 1000°C or higher. On the other hand, if it exceeds 1250°C, the grains become too coarse and the toughness deteriorates. Therefore, the heating temperature of the steel billet is 1250°C or lower. The heating temperature is preferably 1200°C or lower. The heating temperature is more preferably 1180°C or lower.

[0116] Hot rolling process

[0117] Hot rolling end temperature: above Ar3 point

[0118] After the steel billet is reheated, it is hot rolled to the desired tube thickness or plate thickness, and the end temperature of hot rolling is the ferrite formation temperature, that is, Ar3 point or above. This is because when it is less than Ar3 point, in the case of a process in which cooling is performed immediately after hot rolling, the strength is reduced due to the formation of a soft ferrite phase. The end temperature of hot rolling is preferably Ar3+30°C or above. The end temperature of hot rolling is more preferably Ar3+50°C or above. In addition, if it exceeds 1250°C, the grains are too coarse and the toughness deteriorates, so the upper limit is preferably below 1250°C. The end temperature of hot rolling is more preferably below 1200°C, and further preferably below 1150°C.

[0119] The Ar3 point varies depending on the alloy composition of the steel, so it can be obtained by experimentally measuring the phase transition temperature of each steel, but it can also be obtained from the following formula based on the component composition.

[0120] Ar3(℃)=910-310C(%)-80Mn(%)-20Cu(%)-15Cr(%)-55Ni(%)-80Mo(%)

[0121] The content of each alloy element is (mass %).

[0122] Controlled cooling process

[0123] Controlled cooling start temperature: Ar3 point or above based on the steel plate surface thermometer

[0124] When the surface temperature of the steel plate at the start of cooling is less than the Ar3 point, ferrite is generated before controlled cooling, and the strength reduction becomes greater. Therefore, the surface temperature of the steel plate at the start of cooling is above the Ar3 point. The surface temperature of the steel plate at the start of cooling is preferably above Ar3+30°C. More preferably, it is above Ar3+50°C. It should be noted that if the cooling start temperature is too high, the crystal grain size becomes too large and the toughness decreases, so the surface temperature of the steel plate at the start of cooling is preferably less than 1250°C. The surface temperature of the steel plate at the start of cooling is more preferably below 1200°C, and further preferably below 1150°C. It should be noted that the surface temperature of the steel plate at the start of cooling is the temperature of the tail end of the steel plate where the cooling start temperature becomes the lowest.

[0125] The cooling start time difference between the front end of the steel plate and the rear end of the steel plate for controlled cooling: within 50 seconds

[0126] When the time difference between the front end and the tail end of the steel plate rolling direction at the start of cooling exceeds 50 seconds, the temperature difference between the front end and the tail end at the start of cooling becomes larger, so the temperature deviation when cooling stops becomes larger, the deviation of the Vickers hardness at 0.25mm below the surface of the steel plate becomes larger, and the HISC resistance deteriorates. Therefore, the cooling start time difference between the front end of the steel plate and the tail end of the steel plate is within 50 seconds, preferably within 45 seconds. More preferably, within 40 seconds. The cooling start time difference can be shortened by shortening the steel plate length, but the manufacturability is reduced, so it is preferred to shorten the cooling start time difference by speeding up the steel plate conveying speed. The lower limit is not particularly limited and can exceed 0 seconds.

[0127] Average cooling rate from 750°C to 550°C in the center of the plate thickness: 15 to 50°C / s

[0128] If the average cooling rate from 750°C to 550°C in the center of the plate thickness is less than 15°C / s, no bainite structure can be obtained, resulting in reduced strength. Therefore, the average cooling rate in the center of the plate thickness is 15°C / s or more. From the viewpoint of suppressing the deviation of the structure, the average cooling rate in the center of the plate thickness is preferably 17°C / s or more. The average cooling rate in the center of the plate thickness is more preferably 20°C / s or more, and further preferably 25°C / s or more. On the other hand, in order to suppress the deviation of the grain size of bainite, the average cooling rate in the center of the plate thickness is 50°C / s or less. The average cooling rate in the center of the plate thickness is preferably 45°C / s or less. The average cooling rate in the center of the plate thickness is more preferably 40°C / s or less. It should be noted that there is no particular limitation on cooling to 550°C or less based on the steel plate temperature in the center of the plate thickness, but from the viewpoint of suppressing the deviation of the structure and grain size, for example, the average cooling rate from 550°C to 300°C is preferably 15°C / s or more. The average cooling rate from 550°C to 300°C is preferably 50°C / s or less.

[0129] Cooling stop temperature: 250~650℃

[0130] When the cooling stop temperature after hot rolling exceeds 650°C, the bainite phase transformation becomes incomplete and the material strength is greatly reduced. Therefore, the cooling stop temperature is below 650°C. The cooling stop temperature is preferably below 625°C. The cooling stop temperature is more preferably below 600°C. On the other hand, when the cooling stop temperature is less than 250°C, quenching cracks during cooling are easily generated. In addition, in order to obtain a uniform bainite structure, the cooling stop temperature is set to above 250°C. From the perspective of suppressing the amount of hydrogen in the steel, the cooling stop temperature also needs to be above the specified temperature. Specifically, the hydrogen present in the steel gradually escapes during cooling, and the higher the temperature, the greater the effect, but when the cooling stop temperature is too low, it becomes overcooling, and hydrogen remains in the steel. Furthermore, if the cooling stop temperature is excessively reduced, it is easy to form residual austenite with a large increase in hydrogen compared to other phases. Therefore, in order to reduce the amount of hydrogen in the steel, the cooling stop temperature also needs to be above 250°C. The cooling stop temperature is preferably above 270°C. After the cooling is stopped, the plate may be left to cool, but in order to promote the formation of bainite, it is more preferable to cool slowly until the temperature drops by about 50° C. from the cooling stop temperature. The cooling stop temperature here refers to the temperature at the center of the plate thickness.

[0131] Dehydrogenation process

[0132] 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.

[0133] The holding time R (sec) 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) in the steel at room temperature. －1 ) and adopt the following formula (A).

[0134] R≥t 2 / D···(A)

[0135] 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 －5 ~5×10 －3 mm 2 / s. More preferably, 5×10 －4 mm 2 / s or less.

[0136] The dehydrogenation treatment 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) at the temperature that maintains the value of D in the above (A) formula can be used for calculation. On the other hand, when the temperature T of the dehydrogenation process is too high, the material strength is significantly reduced, so the dehydrogenation treatment temperature T is below 550°C. The dehydrogenation treatment temperature T is preferably below 500°C. The dehydrogenation treatment temperature T is more preferably below 400°C, and further 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 treatment temperature T is above room temperature. The dehydrogenation treatment temperature T is preferably above 50°C. The dehydrogenation treatment temperature T is more preferably above 100°C, and further preferably above 150°C. It should be noted that room temperature refers to 20±10°C.

[0137] 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 in the dehydrogenation treatment process (dehydrogenation treatment temperature T). Therefore, even if the above-mentioned holding time R (sec) is satisfied at the atmosphere temperature, if the center of the plate thickness does not reach the dehydrogenation treatment temperature T (atmosphere temperature), the dehydrogenation treatment may not be sufficient. Therefore, it is preferred to maintain R (sec) or more after the center temperature Tc of the plate thickness reaches the target dehydrogenation treatment temperature T. Furthermore, in order to obtain the specified fatigue limit stress in hydrogen, the fatigue limit stress in hydrogen of more than 1 MPa / the fatigue limit stress in an inert gas environment, it is necessary to appropriately adjust the hydrogen content of the steel material in the surface layer 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 R (sec) or more after the center temperature Tc of the plate thickness reaches the target dehydrogenation treatment temperature T. The center temperature Tc of the plate thickness can be measured using a thermocouple or the like, or it can be predicted using the finite element method or the like.

[0138] It should be noted that the time and temperature of the dehydrogenation treatment process may include the temperature and time applied when heating in the pipe making process such as electric resistance welded pipe and UOE as described later. Furthermore, since the scale on the surface of the steel hinders dehydrogenation, it is preferred to remove the scale and perform the dehydrogenation treatment. The removal method is not limited, for example, it can be physical cleaning using high pressure cleaning, or a chemical method using a scale remover can be used. If about 100 μm is removed in thickness, the effect of scale removal can be obtained.

[0139] Second embodiment

[0140] Furthermore, the UOE steel pipe cited as an example of a high-strength steel pipe for line pipe can be obtained by limiting the manufacturing conditions shown below, and the manufacturing method and conditions are specifically described. The component composition, metal structure, fatigue limit stress in hydrogen of 1 MPa or more, and fatigue limit stress in hydrogen of 1 MPa or more / fatigue limit stress in an inactive gas environment of the UOE steel pipe are the same as those described in the steel material of the first embodiment. As for the manufacturing method, the heating process, hot rolling process, controlled cooling process after hot rolling, and dehydrogenation treatment process are also implemented in the same manner as those described in the steel material. The pipe making process after rolling is specifically described below.

[0141] Pipe making process

[0142] UOE steel pipe is a hot-rolled steel plate subjected to bending processing, specifically, groove processing of the end of the hot-rolled steel plate, and formed into a steel pipe shape by a C-type press, a U-type press, or an O-type press, and then seam-welded to the butt joint by inner surface welding and outer surface welding, and further manufactured by a pipe expansion process as required. In addition, any welding method may be used as long as it is a method that can obtain sufficient joint strength and joint toughness, but submerged arc welding is preferably used from the viewpoint of excellent welding quality and manufacturing efficiency. In addition, the steel pipe obtained by seam-welding the butt joint after being formed into a tubular shape by press bending can also be expanded.

[0143] Third embodiment

[0144] Furthermore, the high-strength steel pipe for line pipe of the present invention can be exemplified by an electric resistance welded steel pipe, which can be obtained by limiting the manufacturing conditions shown below. The manufacturing method and conditions are specifically described. The component composition, metal structure, fatigue limit stress in hydrogen of 1 MPa or more, and fatigue limit stress in hydrogen of 1 MPa or more / fatigue limit stress in an inert gas environment of the steel material are the same as those described in the steel material of the first embodiment. Regarding the manufacturing method, the steps (heating step, hot rolling step, dehydrogenation treatment step) other than the cooling step after rolling and the pipe making step are also implemented in the same manner as those described in the steel material.

[0145] Cooling process after rolling (controlled cooling process)

[0146] The cooling start temperature of the controlled cooling and the average cooling rate of the controlled cooling are the same as those described in the first embodiment.

[0147] Cooling stop temperature: 250~650℃

[0148] When the cooling stop temperature after hot rolling exceeds 650°C, the bainite phase transformation is incomplete and the material strength is greatly reduced. Therefore, the cooling stop temperature is below 650°C. The cooling stop temperature is preferably below 620°C. The cooling stop temperature is more preferably below 580°C. On the other hand, when the cooling stop temperature is less than 250°C, quenching cracks during cooling are easily generated. In addition, in order to obtain a uniform bainite structure, the cooling stop temperature is set to above 250°C. From the perspective of suppressing the amount of hydrogen in the steel, the cooling stop temperature also needs to be above the specified temperature. Specifically, the hydrogen present in the steel gradually escapes during cooling, and the higher the temperature, the greater the effect, but when the cooling stop temperature is too low, it becomes overcooling, and hydrogen remains in the steel. Furthermore, if the cooling stop temperature is excessively reduced, it is easy to form residual austenite with a large increase in hydrogen compared to other phases. Therefore, in order to reduce the amount of hydrogen in the steel, the cooling stop temperature also needs to be above 250°C. The cooling stop temperature is preferably above 390°C. The cooling stop temperature is more preferably above 450°C. The cooling stop temperature is further preferably above 480°C. After the cooling is stopped, the plate may be left to cool, but in order to promote the formation of bainite, it is more preferable to cool slowly until the temperature drops by about 50° C. from the cooling stop temperature. The cooling stop temperature here refers to the temperature at the center of the plate thickness.

[0149] Then, the hot-rolled steel sheet obtained as described above is coiled into a coil. The coiling temperature is preferably 650° C. or lower. In addition, the coiling temperature is preferably 250° C. or higher.

[0150] Pipe making process

[0151] The electric resistance welded steel pipe cited as an example of the present invention is formed into a cylindrical shape by cold rolling, and the circumferential ends of the cylindrical shape are butted and welded. Furthermore, it is also possible to manufacture the electric resistance welded steel pipe material (electric resistance welded steel pipe) by forming it into an electric resistance welded steel pipe material (electric resistance welded steel pipe) using a sizing roll satisfying the following formula (1) (sizing step), and applying an internal pressure p (MPa) satisfying the following formula (2) to the inner surface of the electric resistance welded steel pipe material (internal pressure loading step).

[0152] It should be noted that the above-mentioned cylindrical shape means that the tube circumferential cross section is a "C" shape.

[0153] Diameter of sizing roller (mm) ≥ thickness of hot-rolled steel plate (mm) / 0.020···(1)

[0154] The plate thickness of the hot-rolled steel plate refers to the plate thickness of the hot-rolled steel plate before the sizing step.

[0155] X<p≤X×1.5···(2)

[0156] It should be noted that X = (wall thickness of electric resistance welded steel tube billet (mm) / radius of electric resistance welded steel tube billet (mm)) × yield strength of electric resistance welded steel tube billet (MPa)

[0157] The load of the above-mentioned internal pressure can be implemented by, for example, sealing the pipe end with a sealing pad of rubber material and loading the pipe internal water pressure. In addition, in order to stabilize the shape, a mold of a desired diameter can also be used as an outer frame as required.

[0158] It should be noted that the wall thickness of the electric resistance welded steel tube billet cited as an example of the steel tube of the present invention is preferably 5 mm or more, preferably 30 mm or less. There is no particular upper limit for the radius of the electric resistance welded steel tube billet, but if it becomes larger, the load on the equipment increases, so the radius of the electric resistance welded steel tube billet is preferably 400 mm or less. In addition, the radius of the electric resistance welded steel tube billet is preferably 200 mm or more. In addition, in order to withstand the pipeline operating gas pressure, the yield strength of the electric resistance welded steel tube billet is preferably 480 MPa or more. More preferably, it is 500 MPa or more. On the other hand, in order to avoid an increase in hydrogen embrittlement sensitivity, the yield strength is preferably 560 MPa or less.

[0159] In the sizing process, bending deformation occurs in the tube axis direction along the roller shape when the roller passes, and residual stress occurs in the tube axis direction. The greater the bending strain in the bending deformation, the greater the absolute value of the residual stress in the tube axis direction. The smaller the diameter of the sizing roller and the greater the thickness of the hot-rolled steel sheet, the greater the bending strain.

[0160] Therefore, in the present invention, from the viewpoint of reducing the shear residual stress, the diameter of the sizing roll is set to satisfy the above formula (1) in order to reduce the absolute value of the residual stress in the pipe axial direction.

[0161] When the diameter of the sizing roll is smaller than the right side of the above formula (1), the target shear residual stress in the present invention cannot be obtained. It should be noted that there is no particular upper limit on the diameter of the sizing roll, but if the sizing roll becomes larger, the load on the equipment increases, so the diameter of the sizing roll is preferably 2000 mm or less.

[0162] In the internal pressure loading process, the electric resistance welded steel tube material is expanded to generate tensile stress in the tube circumferential direction, thereby reducing the absolute value of the residual stress in the tube circumferential direction.

[0163] The greater the internal pressure p (MPa) in the internal pressure loading process, the smaller the absolute value of the residual stress in the pipe circumferential direction. The greater the radius of the steel pipe and the smaller the wall thickness of the steel pipe, the higher the tensile stress generated in the pipe circumferential direction.

[0164] The left side (X) of the above formula (2) corresponds to the internal pressure p when the tensile stress generated in the circumferential direction of the tube is equal to the yield stress of the electric resistance welded steel tube material.

[0165] In the present invention, from the viewpoint of reducing the shear residual stress, in order to reduce the absolute value of the residual stress in the pipe axial direction, the internal pressure p is set to a value greater than the left side (X) of the formula (2), and the electric resistance welded steel pipe billet is expanded to the plastic region. On the other hand, if the internal pressure p exceeds the right side (X×1.5) of the formula (2), the absolute value of the residual stress in the pipe circumferential direction becomes smaller, but the amount of work hardening due to the expansion becomes too large, the dislocation density on the pipe surface increases, and the fatigue resistance in hydrogen decreases.

[0166] As described in part above, the steel pipe of the present invention can be formed into a tubular shape by press bending, roll forming, UOE forming, etc., and then the butt joints are welded, thereby manufacturing a high-strength steel pipe for sour-resistant line pipe (UOE steel pipe, electric resistance welded steel pipe, spiral steel pipe, etc.) with excellent material uniformity in the steel plate suitable for the transportation of crude oil and natural gas. In addition, by using the steel plate disclosed in the present invention for the steel pipe, a steel pipe with excellent HISC resistance can be manufactured even if there is a high hardness area in the welded part.

[0167] Example 1

[0168] Next, the present invention will be described in more detail based on examples. The following examples are preferred examples of the present invention, but the present invention is not limited to the examples described.

[0169] First, billets with the composition shown in Tables 1-1, 1-2, and 1-3 are produced. The casting speed at this time is 0.05 to 0.2 m / min. The billets are heated to 1000 to 1100°C. Then, hot rolling is performed at 1000°C ± 50°C. The time difference between the front and rear ends of the hot rolling is 30 to 45 seconds, and the target thickness of the steel plate is manufactured to 20 mmt. Controlled cooling is started after the cooling start temperature reaches Ar3 + 50°C with a surface thermometer. Then, steel materials were manufactured under the conditions shown in Tables 2-1, 2-2, and 2-3. For some of the steel materials (steel materials No. 1 to 14, 16 to 30, and 92), the following pipe making process was performed: after the controlled cooling process, the hot-rolled steel plate was bent, and the two ends were butted and welded; and for some of the steel materials (steel materials No. 15, 31 to 55, and 93 to 98), the following pipe making process was performed: after the controlled cooling process, the hot-rolled steel plate was formed into a cylindrical shape by cold roll forming, and the circumferential ends of the cylindrical shape were butted and resistance welded; and steel pipes No. 1 to 14, 16 to 30, and 92 and No. 15, 31 to 55, and 93 to 98 were obtained, respectively. In addition, in the dehydrogenation treatment of Example 1, the dehydrogenation treatment was performed in the range of room temperature to 550°C. In the dehydrogenation treatment temperature recorded in Table 2, Y means that the dehydrogenation treatment was performed in the range of room temperature to 550°C, and N means that the dehydrogenation treatment temperature was higher than 550°C. After confirming that the plate thickness center temperature Tc has reached the room temperature as the target temperature, R (sec) is maintained so as to satisfy the above-mentioned formula (A).

[0170] Furthermore, billets with the composition shown in Steel No. 15 in Table 1-1 and Steel No. 56 in Table 1-2 were produced at various casting speeds shown in Table 3, and the billets were heated to 1000-1100°C. Then, hot rolling was carried out at 1000±50°C. The time difference between the front and rear ends of the hot rolling was 30-45 seconds, and the target thickness of the steel plate was manufactured to 20 mmt. Controlled cooling was started after the surface thermometer reached Ar3+50°C. Then, the steel and steel pipe were manufactured under the conditions shown in Table 3. Steel materials No. 15-1 to 3 and 56-1 to 3 are kept as they are, and steel pipes No. 15-11, 15-12, 56-11 and 56-12 are manufactured by the following pipe making process: the hot-rolled steel plates are bent, and the two ends are butted and welded; steel pipes No. 15-13 and 56-13 are obtained by the following pipe making process: after the controlled cooling process, the hot-rolled steel plates are formed into a cylindrical shape by cold roll forming, and the circumferential ends of the cylindrical shape are butted and resistance welded.

[0171] The metal structure and mechanical properties were evaluated by the following method: The tempering temperature was arbitrarily adjusted so that the tensile strength of the material was within the range of 520 MPa to 700 MPa.

[0172] The results of evaluation of the metal structure and material quality of each of the obtained steel materials and steel pipes are shown in Tables 2-1, 2-2, 2-3, and 3. The evaluation method is as follows.

[0173] Retained austenite determination

[0174] 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.

[0175] Determination of the area fraction of bainite

[0176] The test piece taken from the center of the long side direction of the steel plate and the position of 1 / 4 of the plate thickness and the test piece taken from the center of the long side direction of the steel pipe and the position of 1 / 4 of the wall thickness were polished and etched with 3 vol% nitric acid alcohol. Then, three fields of view were observed using an optical microscope at a magnification of 100 times, and scanning electron microscope photos were taken at an appropriate magnification between 1000 and 5000 times to observe bainite. For bainite, it was compared with the tissue photos of non-patent document 2 and judged visually. For the tissue fraction, the image obtained by binarizing the bainite and other areas in the optical microscope photos or SEM photos based on the above judgment was used, and the area fraction was calculated by image analysis. The average value of the values ​​obtained from the optical microscope photos or SEM photos was taken as the bainite area ratio.

[0177] Tensile Strength (TS)

[0178] 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 materials and steel pipes obtained in the above-mentioned operation, and the tensile strength was measured.

[0179] Hydrogen heating analysis

[0180] 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 30mm 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 following matters, and is the amount of H shown in Tables 1-1, 1-2, and 1-3.

[0181] Fatigue test

[0182] 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.

[0183] High pressure hydrogen fatigue test

[0184] The fatigue limit stress in hydrogen is determined by performing fatigue testing 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 (hydrocarbons such as methane and ethane as the main component) 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 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 strength 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.

[0185] In the inventive examples of the present invention, the fatigue limit stress in hydrogen 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 is 0.90 or more, which is excellent hydrogen embrittlement resistance. Furthermore, the tensile strength is 520 MPa or more.

[0186] [Table 1-1]

[0187]

[0188] [Table 1-2]

[0189]

[0190] [Table 1-3]

[0191]

[0192] [Table 2-1]

[0193]

[0194] Underline: indicates outside the scope of the present invention. γ: austenite, B: bainite

[0195] Dehydrogenation treatment temperature Y is carried out at room temperature to 550°C, and N is carried out at more than 550°C.

[0196] [Table 2-2]

[0197]

[0198] Underline: indicates outside the scope of the present invention. γ: austenite, B: bainite

[0199] Dehydrogenation treatment temperature Y is carried out at room temperature to 550°C, and N is carried out at more than 550°C.

[0200] [Table 2-3]

[0201]

[0202] Underline: indicates outside the scope of the present invention. γ: austenite, B: bainite

[0203] The dehydrogenation treatment temperature Y is carried out at room temperature to 550°C, and N is carried out at a temperature exceeding 550°C.

[0204] [Table 3]

[0205]

[0206] Example 2 The following is an example that verifies the effect of the present invention. 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, 15, and 56 shown in Tables 1-1 and 1-2 were used, and the steels No. 1, 15, 56, 15-12, and 56-12 shown in Tables 2-1, 2-2, and 3 were manufactured under the same conditions until the controlled cooling process, and the properties were evaluated when the dehydrogenation treatment conditions were changed. The steel pipe was formed by the same method as in Example 1. The above results are shown in Table 4.

[0207] In this embodiment, the steel pipes and steel materials No. 1A, 15A, 56A, 15-12A, and 56-12A were processed 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. 1B, 15B, 56B, 15-12B, and 56-12B were processed 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 formula (A), but the holding time tc after the plate thickness center temperature Tc reached 50°C did not satisfy the formula (A).

[0208] For steel pipes and steel materials No. 1C, 15C, 56C, 15-12C, and 56-12C, 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).

[0209] In Table 4, "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 steel material center temperature Tc is Y" means that the holding time tc after the plate center temperature Tc reaches 50°C satisfies the formula (A), and "holding time tc of the steel material center temperature Tc is N" means that although the plate center temperature Tc reaches 50°C, the holding time tc after Tc reaches 50°C does not satisfy the formula (A).

[0210] Various evaluations were carried out by the methods described in Example 1.

[0211] All the examples of the present invention satisfy the requirement that the fatigue limit stress in hydrogen is 200 MPa or more, and 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 atmosphere, is 0.90 or more. Furthermore, the tensile strength satisfies 520 MPa or more. Among them, when the dehydrogenation treatment is carried out under more preferred conditions, the fatigue characteristics are more excellent.

[0212] [Table 4]

[0213]

Claims

1. A steel material for a line pipe having excellent hydrogen embrittlement resistance, having the following chemical composition: containing, by mass%, C: 0.02-0.15%, 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%, Ca: 0-0.005%, Ti: 0 ~0.1%, 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 area fraction of retained austenite is 0 to 3%, the area fraction of bainite is 90% or more at the 1 / 4 position of the plate thickness, the fatigue limit stress in hydrogen above 1 MPa is 200 MPa or more, and the fatigue limit stress in hydrogen above 1 MPa / fatigue limit stress in an inactive gas environment is 0.90 or more.

2. The steel material 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%, Ca: 0.0001-0.005%, Ti: 0.005-0.1%, 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 material for a pipeline pipe, comprising: A heating step of heating the steel billet having the chemical composition of claim 1 or 2 at 1000 to 1250° C. A hot rolling step, rolling the steel billet heated in the heating step at a rolling end temperature of Ar3 point or higher; a controlled cooling step of cooling the hot rolled steel sheet obtained in the hot rolling step under the conditions that the cooling start temperature is Ar3 point or higher in terms of the steel sheet surface temperature, the cooling start time difference between the front end and the rear end of the hot rolled steel sheet is within 50 seconds, the average cooling rate from 750° C. to 550° C. is 15 to 50° C. / s in terms of the temperature at the center of the sheet thickness, and the cooling stop temperature is 250 to 650° C.; and In the dehydrogenation treatment step, the steel sheet obtained in the controlled cooling step is maintained at a temperature ranging from room temperature to 550°C.

4. A steel pipe for a line pipe having excellent hydrogen embrittlement resistance, wherein the steel pipe for a line pipe has a chemical composition comprising, by mass%, 0.02 to 0.15% C, 0.01 to 2.0% Si, 0.5 to 1.5% Mn, 0.0001 to 0.015% P, 0.0002 to 0.0015% S, 0.005 to 0.15% Al, 0.01% or less O, 0.010% or less N, and 0.0010% or less H, or further comprising 0 to 0.10% Nb, 0 to 0.005% Ca, 、Ti: 0-0.1%, 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% by area fraction, the bainite is 90% or more by area fraction at a position 1 / 4 of the wall thickness from the inner surface of the steel pipe, 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 / the fatigue limit stress in an inactive gas environment is 0.90 or more.

5. The steel pipe for line pipe having excellent hydrogen embrittlement resistance according to claim 4, wherein: The chemical composition is further as follows in mass %: Nb: 0.001-0.10%, Ca: 0.0001-0.005%, Ti: 0.005-0.1%, 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%.

6. A method for manufacturing a steel pipe for a line pipe, comprising: A heating step of heating the steel billet having the chemical composition of claim 4 or 5 at 1000 to 1250° C. The hot rolling step is to roll the steel billet heated in the heating step at a rolling end temperature of Ar3 point or higher. A controlled cooling step is performed to cool the hot rolled steel sheet obtained in the hot rolling step under the conditions that the cooling start temperature is Ar3 point or higher in terms of the steel sheet surface temperature, the cooling start time difference between the front end and the rear end of the hot rolled steel sheet is within 50 seconds, the average cooling rate from 750° C. to 550° C. is 15 to 50° C. / s in terms of the temperature at the center of the sheet thickness, and the cooling stop temperature is 250 to 650° C., Any of the following pipe making processes: a pipe making process in which, after the controlled cooling process, the hot rolled steel plate is bent, both ends are butted and welded; a pipe making process in which, after the controlled cooling process, the hot rolled steel plate is formed into a cylindrical shape by cold roll forming, both circumferential ends of the cylindrical shape are butted and resistance welded, and In the dehydrogenation treatment step, the steel pipe obtained in the pipe making step is kept in a temperature range of room temperature to 550°C.

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