Steel material for line pipes with excellent resistance to hydrogen embrittlement, method for manufacturing the same, steel pipe for line pipes with excellent resistance to hydrogen embrittlement, and method for manufacturing the same

Abstract

The purpose of the present invention is to provide a line pipe steel material that is suitable for use in a steel structure used in a high-pressure hydrogen gas environment, such as a line pipe for 100% hydrogen gas or a natural gas (natural gas being a gas having a hydrocarbon such as methane or ethane as a primary component) containing hydrogen having a hydrogen partial pressure of at least 1 MPa, and that has high strength and excellent hydrogen embrittlement resistance in a high-pressure hydrogen gas environment. The purpose of the present invention is also to provide a manufacturing method therefor, as well as a line pipe steel tube and a manufacturing method therefor.　Provided is a line pipe steel material having excellent hydrogen embrittlement resistance that has a specific chemical composition and a specific structure, wherein the fatigue limit stress in hydrogen that is at 1 MPa or higher is at least 200 MPa, and the ratio of the fatigue limit stress in hydrogen that is at 1 MPa or higher to the fatigue limit stress in an inert gas environment is at least 0.90.

Description

Technical Field

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

Background Art

[0002] As an existing energy infrastructure, there are line pipes for transporting natural gas. For these steel materials, suppression of the occurrence of hydrogen-induced cracking in a sour environment has been required. On the other hand, in recent years, hydrogen has been attracting great attention worldwide as a clean energy source for building a decarbonized society. Therefore, for the purpose of transporting a large amount of hydrogen gas, construction of a natural gas pipeline in which a part of hydrogen is mixed with natural gas or a hydrogen gas transport network for pumping hydrogen gas as a substitute is being studied. The transport pressure during operation of these pipelines is assumed to be a high pressure of 1 to 40 MPa, and the line pipe will be placed in a high-pressure hydrogen gas exposure environment. For steel materials used in such an environment, there is a concern about the occurrence of "hydrogen embrittlement" in which hydrogen penetrates into the steel and the properties deteriorate. Therefore, it is necessary to have not only high toughness and sour resistance required for conventional line pipes but also resistance to hydrogen embrittlement required in a hydrogen gas environment.

[0003] Conventionally, austenitic stainless steels such as SUS316L, which are less likely to be hydrogen embrittled than low alloy steels, have been used for steel structures used in a high-pressure hydrogen gas environment. However, austenitic stainless steels such as SUS316L have a high cost of steel materials and low strength. Therefore, if designed to withstand a high hydrogen pressure, the wall thickness becomes thick and the price of the hydrogen structure itself also becomes high. Therefore, there has been a strong demand for low alloy steel materials that are lower in cost and can withstand a high-pressure hydrogen gas environment for hydrogen steel structures.

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

[0005] Patent Document 2 describes a technology that uses low-alloy high-strength steel adjusted to a specific component composition, which has been found to have greater reduction of area and elongation values ​​in a 45 MPa hydrogen atmosphere than JIS G3128SHY685NS in the tensile strength range of 900 to 950 MPa in air, and to have superior resistance to embrittlement in a high-pressure hydrogen environment.

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

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

[0008] Furthermore, Patent Document 5 proposes a steel for high-pressure hydrogen gas storage containers with excellent hydrogen resistance. According to the technology described in Patent Document 5, by performing long-term stress-relieving annealing after normalizing during the manufacturing of the steel sheet, MC-type carbides (Mo,V)C are dispersed and precipitated in a fine and high-density manner, thereby improving the hydrogen resistance of the steel, such as its resistance to hydrogen embrittlement.

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

[0010] Non-patent document 1 contains the fatigue strength values ​​for low-alloy steel. [Prior art documents] [Patent Documents]

[0011] [Patent Document 1] Japanese Patent Publication No. 2005-2386 [Patent Document 2] Japanese Patent Publication No. 2009-46737 [Patent Document 3] Japanese Patent Publication No. 2009-275249 [Patent Document 4] Japanese Patent Publication No. 2009-74122 [Patent Document 5] Japanese Patent Publication No. 2010-37655 [Patent Document 6] Japanese Patent Publication No. 2012-107332 [Non-patent literature]

[0012] [Non-Patent Document 1] Matsunaga et al., Int J Hydrogen Energy, Vol.40(2015), p.5739-5748 [Non-Patent Document 2] Japan Heat Treatment Technology Association (author), Introduction to the Structure and Properties of Metallic Materials - Heat Treatment and Microstructure Control for Maximizing Material Potential, 2004. [Overview of the project] [Problems that the invention aims to solve]

[0013] The pressure inside the line pipe fluctuates during operation and undergoes periodic shutdowns, subjecting the structure to repeated stress. Therefore, when designing steel structures such as line pipes, it is essential to consider fatigue failure. However, as shown in Non-Patent Document 1, it is known that the fatigue life of materials decreases under high-pressure hydrogen environments. In other words, if line pipe materials are designed based on conventional natural gas line pipes, the service life of the line pipe materials will decrease. However, the conventional technology described above can suppress the occurrence of hydrogen-induced cracking in sour environments, but it cannot sufficiently increase the fatigue strength in hydrogen gas. That is, it is difficult to achieve both the suppression of hydrogen-induced cracking in sour environments and high fatigue strength in hydrogen gas.

[0014] In view of the problems of the prior art described above, the present invention aims to provide a steel material for line pipes that is suitable for use in steel structures used in high-pressure hydrogen gas environments, such as line pipes for 100% hydrogen gas or natural gas containing hydrogen with a hydrogen partial pressure of 1 MPa or more (natural gas is a gas whose main components are hydrocarbons such as methane and ethane), a method for manufacturing the same, a steel pipe for line pipes, and a method for manufacturing the same.

[0015] In this context, "excellent resistance to hydrogen embrittlement under high-pressure hydrogen gas conditions" means that the fatigue limit stress in hydrogen, which is the stress at which fracture occurs after 2 million cycles, is 200 MPa or higher, and the fatigue limit stress in the hydrogen environment / fatigue limit stress in the inert gas environment is 0.90 or higher, determined by fatigue testing conducted in accordance with ASTM E466, Fatigue Testing, under both environments of hydrogen gas at room temperature (20±10℃) and a pressure of 1 MPa or higher, or a mixed atmosphere of natural gas (mainly hydrocarbons such as methane and ethane) containing hydrogen as a partial pressure of 1 MPa or higher, with a frequency of 1 Hz, repetition waveform: sine wave, control method: load control, load conditions: uniaxial tensile compression, and stress ratio: R = -1.0.

[0016] In addition, if the fatigue limit stress in hydrogen under the above environment is 200 MPa or more and the fatigue limit stress of the steel material in hydrogen / fatigue limit stress in an inert gas environment under the above environment is 0.90 or more, it is possible to design hydrogen steel structures such as long-life line pipes within the plate thickness range that can be manufactured in the process of manufacturing steel pipes such as seamless steel pipes and UOE.

[0017] In addition, the "steel materials" mentioned here include thin steel plates, thick steel plates, seamless steel pipes, electric resistance welded steel pipes, shaped steel, steel bars, etc.

Means for Solving the Problems

[0018] The inventors of the present invention have intensively studied the conditions that the steel materials should satisfy in order to obtain steel plates for line pipes and steel pipes for line pipes with excellent hydrogen embrittlement resistance characteristics, and have invented new high-strength steel plates for line pipes and steel pipes for line pipes. In addition, the steel materials and steel pipes of the present invention have high strength, and in the present invention, high strength refers to a tensile strength of 520 MPa or more.

[0019] The gist of the present invention is as follows. [1] In mass%, C: 0.02 to 0.15%, Si: 0.01 to 2.0%, Mn: 0.5 to 1.5%, P: 0.0001 to 0.015%, S: 0.0002 to 0.0015%, Al: 0.005 to 0.15%, O: 0.01% or less, N: 0.010% or less, H: containing 0.0010% or less, Or further, Nb: 0 to 0.10%, Ca: 0 to 0.005%, Ti: 0 to 0.1%, Ni: 0 to 2.0%, Cu: 0 to 1.0%, Cr: 0 to 1.0%, Mo: 0 to 0.60%, W: 0 to 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: Contains one or more selected from 0-0.3%, It has a chemical composition in which the remainder is Fe and unavoidable impurity elements. A steel material for line pipes with excellent hydrogen embrittlement resistance, having retained austenite at an area fraction of 0-3%, bainite at an area fraction of 90% or more at the 1 / 4 plate thickness position, a fatigue limit stress in hydrogen at 1 MPa or more of 200 MPa or more, and a fatigue limit stress in hydrogen at 1 MPa or more / fatigue limit stress in an inert gas environment of 0.90 or more. [2] Furthermore, in mass%, the chemical composition is, 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%, A steel material for line pipes with excellent hydrogen embrittlement resistance, as described in [1], with an Sb content of 0.0001-0.3%. [3] A heating step of heating a steel material having the chemical composition described in [1] or [2] above to 1000-1250°C, A hot rolling process is performed in which the steel material heated in the aforementioned heating process is rolled under the condition that the rolling completion temperature is Ar3 or higher. A controlled cooling process is performed to cool the hot-rolled steel sheet obtained in the hot-rolling process under the following conditions: the cooling start temperature is above the Ar3 point on the steel sheet surface temperature, the difference in cooling start time between the leading and trailing ends 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 at the temperature in the center of the sheet thickness, and the cooling stop temperature is 250 to 650°C. A dehydrogenation treatment step is performed to maintain the steel sheet obtained in the controlled cooling step at a temperature in the range of room temperature to 550°C. A method for manufacturing steel materials for line pipes. [4] In steel pipes for line pipes, In 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: Includes 0.0010% or less, Or, furthermore, 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: Contains one or more selected from 0-0.3%, A steel pipe for line pipes having a chemical composition in which the remainder is Fe and unavoidable impurity elements, retained austenite is 0-3% by area fraction, 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 / fatigue limit stress in an inert gas environment is 0.90 or more, exhibiting excellent resistance to hydrogen embrittlement. [5] Furthermore, in mass%, the chemical composition is, 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%, A steel pipe for line pipes with excellent hydrogen embrittlement resistance, as described in [4], with an Sb content of 0.0001-0.3%. [6] A heating step of heating a steel material having the chemical composition described in [4] or [5] above to 1000-1250°C, A hot rolling process is performed in which the steel material heated in the aforementioned heating process is rolled under the condition that the rolling completion temperature is Ar3 or higher. A controlled cooling process is performed to cool the hot-rolled steel sheet obtained in the hot-rolling process under the following conditions: the cooling start temperature is above the Ar3 point on the steel sheet surface temperature, the difference in cooling start time between the leading and trailing ends 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 at the temperature in the center of the sheet thickness, and the cooling stop temperature is 250 to 650°C. After the controlled cooling process, a pipe-making process is performed in which the hot-rolled steel sheet is bent and both ends are butt-welded; or a pipe-making process is performed in which the hot-rolled steel sheet is formed into a cylindrical shape by cold roll forming and both ends of the cylindrical shape are butt-welded using electric resistance welding; A dehydrogenation treatment process in which the steel pipes obtained in the pipe manufacturing process are held in a range of room temperature to 550°C, A method for manufacturing steel pipes for line pipes. [Effects of the Invention]

[0020] According to the present invention, steel materials with significantly improved hydrogen embrittlement resistance under high-pressure hydrogen gas environments can be easily and simply manufactured, yielding remarkable industrial benefits. Furthermore, the present invention significantly improves the hydrogen embrittlement resistance of steel structures such as high-pressure hydrogen gas line pipes, improving fatigue resistance and greatly contributing to extending the lifespan of steel structures. [Modes for carrying out the invention]

[0021] Next, a method for carrying out the present invention will be specifically described. The following description illustrates preferred embodiments of the present invention, and the present invention is not limited in any way by this description. A steel material will be specifically described as the first embodiment, followed by a UOE steel pipe, an example of a steel pipe of the present invention, as the second embodiment, and an electric resistance welded steel pipe, an example of a steel pipe of the present invention, as the third embodiment.

[0022] First Embodiment [Component composition] The reasons for the limitations on the component composition (chemical composition) of the steel material of the present invention are explained below. In the following explanation, "%" refers to "mass%" unless otherwise specified.

[0023] C: 0.02~0.15% While carbon (C) effectively contributes to improving strength, sufficient strength and fatigue limit stress cannot be ensured if the C content is less than 0.02%. Therefore, the C content should be 0.02% or more. Preferably, the C content is 0.03% or more. On the other hand, if it exceeds 0.15%, weldability decreases. Therefore, the C content should be 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 central segregation increases during controlled cooling, resulting in deterioration of SSCC (sulfide stress corrosion cracking) resistance and HIC (hydrogen-induced cracking) resistance. Toughness also deteriorates. Therefore, more preferably, the C content is 0.08% or less. Even more preferably, the C content is 0.05% or less.

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

[0025] Mn: 0.5~1.5% While manganese (Mn) effectively contributes to improving strength and toughness, its additive effect is poor at concentrations below 0.5%. Therefore, the Mn content should be 0.5% or higher. Preferably, the Mn content is 0.6% or higher, more preferably 0.7% or higher, and even more preferably 0.8% or higher. On the other hand, if the Mn content exceeds 1.5%, the hardness of the surface layer and central segregation increases during controlled cooling, resulting in deterioration of SSCC (sulfide stress corrosion cracking) and HIC (hydrogen-induced cracking) resistance. Weldability also deteriorates. Therefore, the Mn content should be limited to 1.5% or less. Preferably, the Mn content is 1.4% or less, and even more preferably 1.3% or less.

[0026] P: 0.0001~0.015% P is an unavoidable impurity element that degrades weldability and reduces HIC resistance by increasing the hardness of the central segregation area. This tendency becomes significant above 0.015%, therefore the upper limit for P content is set at 0.015%. A P content of 0.010% or less is preferable, and more preferably 0.008% or less. A lower content is better, but from the perspective of refining costs, the P content should be 0.0001% or more.

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

[0028] Al: 0.005~0.15% Al is added as a deoxidizing agent, but below 0.005%, it has no effect. Therefore, the Al content should be 0.005% or more. Preferably, the Al content is 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 should be limited to 0.15% or less. Preferably, the Al content is 0.10% or less, more preferably 0.08% or less, and even more preferably 0.05% or less.

[0029] O: 0.01% or less While a low oxygen content is preferable because it causes the formation of oxide inclusions, an oxygen content of 0.01% or less is not problematic. Therefore, the oxygen content is set to 0.01% or less. Preferably, the oxygen content is 0.005% or less. More preferably, the oxygen content is less than 0.003%. There is no particular lower limit, but since reducing oxygen to 0% increases costs, an oxygen content of 0.001% or more is preferable.

[0030] N: 0.010% or less The effect of nitrogen (N) on the fatigue properties of steel is small, and if the N content is 0.010% or less, the effects of the present invention are not impaired from the viewpoint of toughness. Therefore, the N content should be 0.010% or less. Preferably, the N content should be 0.008% or less, and more preferably 0.006% or less. Even more preferably 0.004% or less. On the other hand, from the viewpoint of improving toughness, a low N content is desirable, but excessive reduction increases steelmaking costs, so it is preferable that the N content be 0.00001% or more. Preferably, the N content be 0.001% or more.

[0031] H:0.0010% or less Hydrogen (H) can be introduced into steel materials during various manufacturing processes. High hydrogen content increases the risk of crack formation after solidification and accelerates fatigue crack propagation. Furthermore, high hydrogen content reduces the fatigue limit stress, making it important to reduce the hydrogen content in the steel. These effects are not problematic if the hydrogen content is 0.0010% or less, so the hydrogen content should be 0.0010% or less. Preferably, the hydrogen content is 0.0005% or less, more preferably 0.0003% or less, and even more preferably 0.0001% or less. On the other hand, since a hydrogen content of less than 0.00001% would increase costs, it is preferable for the hydrogen content to be 0.00001% or more. Note that the hydrogen content refers to the residual hydrogen after forming of steel materials, steel pipes, UOE, etc.

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

[0033] Nb: 0~0.10% Nb is an effective element for increasing the strength and toughness of steel materials, but if it exceeds 0.10%, the toughness of the weld deteriorates, so if it is included, the Nb content should be 0.10% or less. Preferably, the Nb content should be 0.08% or less. More preferably, the Nb content should be 0.06% or less. The Nb content may be 0% or more, but if the Nb content is less than 0.001%, the effect of including it is difficult to obtain, so if it is included, it is preferable to have a content of 0.001% or more. More preferably, the Nb content should be 0.01% or more.

[0034] Ca: 0~0.005% Ca is an effective element for improving HIC resistance by controlling the morphology of sulfide inclusions. However, its effect not only saturates, but it also degrades HIC resistance by reducing the cleanliness of the steel. Therefore, if Ca is included, the amount should be limited to 0.005% or less. A Ca content of 0.003% or less is preferable. A Ca content of 0.002% or less is even more preferable. The amount of Ca may be 0% or more, but since the effect of its addition is difficult to obtain below 0.0001%, if it is included, it is preferable to have a Ca content of 0.0001% or more. A Ca content of 0.001% or more is even more preferable.

[0035] Ti: 0~0.1% Ti is an effective element for increasing the strength and toughness of steel materials, but if it exceeds 0.1%, the toughness of the weld deteriorates. Therefore, if Ti is included, the Ti content should be 0.1% or less. A Ti content of 0.05% or less is preferable. A Ti content of 0.03% or less is more preferable, and 0.02% or less is even more preferable. The Ti content may be 0% or more, but if the Ti content is less than 0.005%, the effect of including Ti is difficult to obtain, so if it is included, it is preferable to have a Ti content of 0.005% or more. A Ti content of 0.008% or more is more preferable.

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

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

[0038] Cr: 0~1.0% Like manganese, chromium (Cr) is an effective element for obtaining sufficient strength even at low carbon content. However, if the content is too high, the hardenability becomes excessive, degrading the resistance to SSCC (Steel Carbon Deposition). Weldability also deteriorates. Therefore, if Cr is included, it should be 1.0% or less. A Cr content of 0.8% or less is preferable. A Cr content of 0.5% or less is more preferable, and 0.1% or less is even more preferable. The Cr content may be 0% or more, but to obtain this effect, it is preferable to include 0.01% or more Cr. A Cr content of 0.02% or more is more preferable.

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

[0040] W: 0~1.0% While W contributes to increasing the strength of steel pipes, the effect saturates when the W content exceeds 1.0%, leading to increased costs. Therefore, if W is included, the W content should be 1.0% or less. Preferably, the W content should be 0.8% or less. For further cost reduction, it is even more preferable to have a W content of 0.5% or less. Even more preferable is a W content of 0.03% or less. The W content may be 0% or more, but to obtain the above-mentioned effects, it is preferable to have a content of 0.01% or more.

[0041] V: 0~0.10% V is an element that can be optionally included to increase the strength and toughness of steel materials. However, if the V content exceeds 0.10%, the toughness of the weld deteriorates, so if it is included, it should be 0.10% or less. Preferably, the V content should be 0.08% or less. More preferably, the V content should be 0.06% or less, and even more preferably 0.03% or less. The V content may be 0% or more, but if the content is less than 0.01%, the effect of including it is difficult to obtain, so it is preferable to have a content of 0.01% or more.

[0042] Zr:0~0.050%, REM:0~0.050%, Mg:0~0.050% Zr, REM, and Mg are elements that can be optionally included to enhance toughness through grain refinement or to improve crack resistance through control of inclusion properties. However, their effects saturate above 0.050%, so if included, the amount of each should be 0.050% or less. Specifically, if included, the Zr content should be 0.050% or less. Preferably, the Zr content should be 0.040% or less. More preferably, the Zr content should be 0.030% or less. Even more preferably, the Zr content should be 0.010% or less, and most preferably, 0.005% or less. Furthermore, if included, the REM content should be 0.050% or less. Preferably, the REM content should be 0.040% or less. More preferably, the REM content should be 0.030% or less. Furthermore, if included, the Mg content should be 0.050% or less. Preferably, the Mg content should be 0.040% or less. More preferably, the Mg content should be 0.030% or less. While the content of these elements may be 0% or more, it is preferable that the content be 0.0001% or more, as the beneficial effect of their inclusion is difficult to obtain if the content is less than 0.0001%. Specifically, it is preferable that the Zr content be 0.0001% or more, and more preferably 0.0005% or more. It is also preferable that the REM content be 0.0001% or more, and more preferably 0.0005% or more. It is preferable that the Mg content be 0.0001% or more, and more preferably 0.0005% or more.

[0043] B: 0~0.0020% B is an element that improves hardenability, contributing to increased strength of steel pipes, suppressing coarsening of prior austenite grains, and improving various properties of the material. On the other hand, if the B content exceeds 0.0020%, the effect saturates and becomes a factor in cost increase, so if it is included, the B content should be 0.0020% or less. Preferably, the B content should be 0.0015% or less. More preferably, the B content should be 0.0012% or less. For cost reduction, it is even more preferable to have a B content of 0.0010% or less. The B content may be 0% or more, but in order to obtain the above effects, it is preferable to have a content of 0.0001% or more. More preferably, the B content is 0.0005% or more.

[0044] Hf: 0-0.2%, Ta: 0-0.2% These elements contribute to increasing the strength of steel, but if their content exceeds 0.2%, the effect saturates, leading to increased costs. Therefore, if they are included, the content should be 0.2% or less. Specifically, if Hf is included, it should be 0.2% or less. Preferably, Hf should be 0.1% or less. More preferably, Hf should be 0.05% or less. Also, if Ta is included, it should be 0.2% or less. Preferably, Ta should be 0.1% or less. More preferably, Ta should be 0.05% or less. The Hf and Ta content may be 0% or more, but to obtain the above effects, it is preferable that the content be 0.0001% or more. Specifically, the Hf content is preferably 0.0001% or more. More preferably, the Hf content is 0.0010% or more. Also, the Ta content is preferably 0.0001% or more. More preferably, the Ta content is 0.0010% or more.

[0045] Re: 0~0.005% Re contributes to increasing the strength of steel, but if the content exceeds 0.005%, the effect saturates and becomes a factor in cost increase; therefore, if it is included, it should be 0.005% or less. Preferably, the Re content should be 0.003% or less. More preferably, the Re content should be 0.002% or less. The Re content may be 0% or more, but in order to obtain the above effect, it is preferable that the content be 0.0001% or more. More preferably, it should be 0.001% or more.

[0046] Sn: 0-0.3%, Sb: 0-0.3% These elements contribute to increasing the strength and hardenability of steel, but if the content exceeds 0.3%, the effect saturates and becomes a factor in increasing costs. Therefore, if they are included, the content should be 0.3% or less. In other words, the Sn content should be 0.3% or less. Preferably, the Sn content should be 0.2% or less. More preferably, the Sn content should be 0.1% or less. For cost reduction, it is even more preferable that the Sn content be 0.01% or less. Also, the Sb content should be 0.3% or less. Preferably, the Sb content should be 0.2% or less. More preferably, the Sb content should be 0.1% or less. For cost reduction, it is even more preferable that the Sb content be 0.01% or less. The content of Sn and Sb may be 0% or more, but in order to obtain the above effects, it is preferable that the content be 0.0001% or more. In other words, it is preferable that the Sn content be 0.0001% or more. More preferably, the Sn content is 0.0010% or more. Furthermore, the Sb content is preferably 0.0001% or more. More preferably, the Sb content is 0.0010% or more.

[0047] In the composition of steel plates and steel pipes, the remainder of the components (elements) other than those mentioned above consists of Fe and unavoidable impurity elements.

[0048] The following describes the microstructure of the steel material of the present invention.

[0049] metallographic structure Residual austenite content: 0-3% The retention of austenite in steel can increase the amount of hydrogen in the steel, potentially increasing its susceptibility to hydrogen embrittlement. Furthermore, if austenite transforms into martensite due to stress loading during use, martensite is very hard and therefore prone to hydrogen cracking, potentially causing cracks to originate from the martensite portion. In this invention, the fatigue crack propagation rate is reduced by limiting retained austenite to 3% or less. Reducing retained γ suppresses the occurrence of fatigue cracks in a hydrogen environment, thereby suppressing the decrease in the fatigue limit stress in hydrogen. For this reason, retained austenite is limited to 3% or less. Preferably, retained austenite is 2% or less. More preferably, retained austenite is 1% or less. Retained austenite may be 0%.

[0050] At the 1 / 4 thickness point, bainite accounts for 90% or more of the area fraction. To achieve high strength with a tensile strength of 520 MPa or more, the steel microstructure must be bainite. Here, the bainite microstructure includes bainitic ferrite or granular bainite that transforms during or after controlled cooling, which contributes to transformation strengthening, and also includes tempered bainite. If dissimilar microstructures such as ferrite, martensite, pearlite, island martensite, and retained austenite are mixed in the bainite microstructure, a decrease in strength and deterioration of toughness will occur, so it is better to have a small volume fraction of microstructures other than the bainite phase.

[0051] Furthermore, when soft and hard phases are mixed in the steel material, fatigue crack initiation occurs because fatigue damage preferentially accumulates in the soft phase, making crack initiation more likely and thus lowering the fatigue limit stress. In a hydrogen environment, local deformation is promoted, further accelerating fatigue damage to the soft phase and further lowering the fatigue limit stress in hydrogen. As a result, the fatigue limit stress / fatigue limit stress in an inert gas environment falls below 0.90. To improve this, it is necessary to reduce the relative proportion of the soft phase, and the area fraction of bainite was set to 90% or more. It is preferable that the area fraction of bainite be 92% or more. It is more preferable that the area fraction of bainite be 95% or more, and even more preferable that it be 98% or more. There is no particular upper limit, and the area fraction of bainite may be 100%. Furthermore, since fatigue cracks originate from the inner surface of the steel pipe, the uniformity of the internal structure of the steel pipe is important. Therefore, the microstructure at the 1 / 4 thickness position from the inner surface of the steel pipe is defined, and for steel materials, the microstructure at the 1 / 4 thickness position is defined so that the above effect can be obtained regardless of which surface faces the inner surface of the steel pipe.

[0052] Fatigue limit stress in hydrogen at 1 MPa or higher is 200 MPa or higher AND fatigue limit stress in hydrogen at 1 MPa or higher / fatigue limit stress in an inert gas environment is 0.90 or higher If the fatigue limit stress in hydrogen above 1 MPa is less than 200 MPa, and the fatigue limit stress in hydrogen above 1 MPa / inert gas environment is less than 0.90, the design conditions will differ significantly from those of conventional pipelines, requiring an increase in steel thickness (or pipe thickness in the case of steel pipes). Therefore, the fatigue limit stress in hydrogen above 1 MPa should be 200 MPa or higher, and the fatigue limit stress in hydrogen above 1 MPa / inert gas environment should be 0.90 or higher. Preferably, the fatigue limit stress in hydrogen above 1 MPa should be 220 MPa or higher. More preferably, the fatigue limit stress in hydrogen above 1 MPa should be 250 MPa or higher, and even more preferably, 270 MPa or higher. There is no particular upper limit, but it is preferable that the fatigue limit stress in hydrogen above 1 MPa is 500 MPa or lower. Furthermore, it is preferable that the fatigue limit stress in hydrogen above 1 MPa / inert gas environment is 0.92 or higher. It is more preferable that the fatigue limit stress in hydrogen at 1 MPa or higher / fatigue limit stress in an inert gas environment be 0.94 or higher, and even more preferable that it be 0.96 or higher. There is no particular upper limit, but the fatigue limit stress in hydrogen at 1 MPa or higher / fatigue limit stress in an inert gas environment may be 1.1 or lower. In this context, "inert gas" refers to the six elements of Group 0 of the periodic table: helium, neon, argon, krypton, xenon, and radon, in addition to the atmosphere. An "inert gas environment" refers to an environment containing any of the above.

[0053] The present invention, by having the above-described chemical components and metallic structure, suppresses the increase in fatigue limit stress under a high-pressure hydrogen atmosphere and the decrease in fatigue limit stress in hydrogen / inert gas, while achieving a tensile strength of 520 MPa or higher, making it applicable to hydrogen line pipes. The upper limit of the tensile strength is not particularly limited, but it is preferably 950 MPa or less.

[0054] Furthermore, while the thickness of the steel plate is not particularly limited, it is preferable to have a thickness of 5 mm or more. A plate thickness of 30 mm or less is also preferable.

[0055] Next, the method for manufacturing the steel material of the present invention will be described. The steel material of the present invention can be manufactured by sequentially performing a heating step, a hot rolling step, a controlled cooling step, and a dehydrogenation treatment step of a steel material (slab). In the following explanation, unless otherwise specified, the temperature refers to the temperature at the center of the thickness of the steel material or steel pipe. The average cooling rate refers to the temperature at a point 1 / 4 of the way through the wall thickness from the inner surface of the steel pipe. The temperature at the center of the thickness and the temperature at a point 1 / 4 of the way through the wall thickness from the inner surface of the steel pipe are estimated using heat transfer calculations that take into account the heat transfer coefficient of the steel material, based on the surface temperature of the steel pipe measured with a radiation thermometer.

[0056] heating process Heating temperature for steel material: 1000~1250℃ When heating steel materials such as billets and slabs to temperatures below 1000°C, the diffusion of micro-segregated impurity elements such as C, P, and S is insufficient, resulting in an unholy material. Therefore, the heating temperature of the steel material should be 1000°C or higher. On the other hand, if the temperature exceeds 1250°C, the crystal grains become too coarse, and the toughness deteriorates. Therefore, the heating temperature of the steel material should be 1250°C or lower. It is preferable that the heating temperature be 1200°C or lower. It is even more preferable that the heating temperature be 1180°C or lower.

[0057] Hot rolling process Hot rolling completion temperature: Ar3 point or higher After reheating the steel material, it is hot-rolled to the desired pipe or plate thickness, but the end temperature of the hot rolling should be above the Ar3 point, which is the ferrite formation temperature. Below the Ar3 point, if the process involves immediate cooling after hot rolling, the formation of a soft ferrite phase will lead to a decrease in strength. Preferably, the end temperature of the hot rolling should be above Ar3 + 30°C. More preferably, the end temperature of the hot rolling should be above Ar3 + 50°C. Furthermore, if the temperature exceeds 1250°C, the grains become too coarse and the toughness deteriorates, so it is preferable to keep the upper limit below 1250°C. More preferably, the end temperature of the hot rolling should be below 1200°C, and even more preferably below 1150°C.

[0058] Since the Ar3 point varies depending on the alloy composition of the steel, it can be determined by experimentally measuring the transformation temperature of each steel, but it can also be determined from the component composition using the following formula. Ar3(℃)=910-310C(%)-80Mn(%)-20Cu(%)-15Cr(%)-55Ni(%)-80Mo(%) Each alloying element is given as its content (mass %).

[0059] Controlled cooling process Cooling start temperature for controlled cooling: Steel plate surface temperature above 3 Ar points. If the steel sheet surface temperature at the start of cooling is below the Ar3 point, ferrite will form before controlled cooling, resulting in a significant decrease in strength. Therefore, the steel sheet surface temperature at the start of cooling should be at or above the Ar3 point. Preferably, the steel sheet surface temperature at the start of cooling should be at or above Ar3 + 30°C. More preferably, it should be at or above Ar3 + 50°C. If the cooling start temperature is too high, the grain size will become too large and the toughness will decrease, so preferably, the steel sheet surface temperature at the start of cooling should be below 1250°C. More preferably, the steel sheet surface temperature at the start of cooling should be 1200°C or lower, and even more preferably 1150°C or lower. The steel sheet surface temperature at the start of cooling is the temperature at the tail end of the steel sheet where the cooling start temperature is lowest.

[0060] Time difference between the start of cooling at the leading and trailing ends of the steel plate under controlled cooling: within 50 seconds If the time difference between the leading and trailing ends of the steel sheet in the rolling direction at the start of cooling exceeds 50 seconds, the temperature difference between the leading and trailing ends at the start of cooling becomes large, resulting in large temperature variations at the end of cooling. This leads to large variations in Vickers hardness at 0.25 mm below the surface of the steel sheet and deterioration of HISC resistance. Therefore, the time difference between the leading and trailing ends of the steel sheet at the start of cooling should be 50 seconds or less, preferably 45 seconds or less. More preferably 40 seconds or less. It is possible to shorten the time difference at the start of cooling by shortening the length of the steel sheet, but this reduces manufacturability. Therefore, it is preferable to shorten the time difference at the start of cooling by increasing the steel sheet conveying speed. The lower limit is not particularly limited and may exceed 0 seconds.

[0061] Average cooling rate from 750°C to 550°C at the center of the plate thickness: 15-50°C / s If the average cooling rate from 750°C to 550°C in the center of the plate thickness is less than 15°C / s, a bainite structure will not be obtained, resulting in a decrease in strength. For this reason, the average cooling rate in the center of the plate thickness should be 15°C / s or higher. From the viewpoint of suppressing variations in the structure, it is preferable that the average cooling rate in the center of the plate thickness be 17°C / s or higher. It is more preferable that the average cooling rate in the center of the plate thickness be 20°C / s or higher, and even more preferable that be 25°C / s or higher. On the other hand, in order to suppress variations in the grain size of bainite, the average cooling rate in the center of the plate thickness should be 50°C / s or lower. It is preferable that the average cooling rate in the center of the plate thickness be 45°C / s or lower. It is more preferable that the average cooling rate in the center of the plate thickness be 40°C / s or lower. Note that there are no particular limitations regarding the cooling of the steel plate temperature in the center of the plate thickness below 550°C, but from the viewpoint of suppressing variations in structure and grain size, it is preferable that, for example, the average cooling rate from 550°C to 300°C be 15°C / s or higher. The average cooling rate from 550°C to 300°C is preferably 50°C / s or less.

[0062] Cooling stop temperature: 250~650℃ If the cooling stop temperature after hot rolling exceeds 650°C, the bainite transformation becomes incomplete, and the material strength decreases significantly. For this reason, the cooling stop temperature should be 650°C or lower. Preferably, the cooling stop temperature should be 625°C or lower. More preferably, the cooling stop temperature should be 600°C or lower. On the other hand, if the cooling stop temperature is below 250°C, quench cracking during cooling is likely to occur. Also, in order to obtain a uniform bainite structure, the cooling stop temperature should be 250°C or higher. From the standpoint of suppressing the amount of hydrogen in the steel, it is also necessary to set the cooling stop temperature above a predetermined temperature. Specifically, hydrogen present in the steel gradually escapes during cooling, and the effect is greater at higher temperatures, but if the cooling stop temperature is too low, supercooling occurs, and hydrogen remains in the steel. Furthermore, if the cooling stop temperature is set too low, retained austenite, which rapidly increases in hydrogen compared to other phases, is more likely to form. For this reason, the cooling stop temperature needs to be 250°C or higher in order to reduce the amount of hydrogen in the steel. Preferably, the cooling stop temperature should be 270°C or higher. After cooling stops, the material can be allowed to cool naturally, but it is preferable to allow it to cool slowly until the temperature drops by about 50°C from the cooling stop temperature in order to promote bainite formation. Note that the cooling stop temperature referred to here is the temperature at the center of the plate thickness.

[0063] Dehydrogenation treatment process The presence of hydrogen in steel accelerates fatigue crack propagation, reducing fatigue life and hydrogen-induced fatigue limit stress. Therefore, dehydrogenation treatment may be used to release residual hydrogen after manufacturing. Dehydrogenation treatment reduces the amount of hydrogen in the steel by holding it at a high temperature for a certain period of time before product use, resulting in steel sheets with excellent hydrogen embrittlement resistance in a high-pressure hydrogen gas environment. The holding time R (sec) is determined by the thickness of the steel pipe, the pipe thickness t (mm), and the hydrogen diffusion coefficient D (mm·sec) in the steel at room temperature. -1 ) Therefore, it is preferable to use the following formula (A). R≧t 2 / D···(A) The hydrogen diffusion coefficient varies depending on the components and metal structure, but for example, the hydrogen diffusion coefficient is 1 × 10⁻⁶. -5 ~ 5×10 -3 mm 2You may use / s. More preferably 5×10 -4 mm 2 It is less than or equal to / s.

[0064] The dehydrogenation treatment process is carried out before pipe manufacturing or welding to connect steel pipes. It is preferable to perform the dehydrogenation treatment at a high temperature because the hydrogen diffusion coefficient D decreases at high temperatures, allowing hydrogen to escape more quickly. At high temperatures, the diffusion coefficient D' (diffusion coefficient at each temperature) at which the value of D in equation (A) above is maintained may be used for calculation. On the other hand, if the temperature T of the dehydrogenation process is too high, the material strength will decrease significantly, so the dehydrogenation treatment temperature T should be 550°C or lower. It is preferable that the dehydrogenation treatment temperature T be 500°C or lower. It is more preferable that the dehydrogenation treatment temperature T be 400°C or lower, and even more preferable that it be 300°C or lower. Furthermore, because dehydrogenation treatment at temperatures lower than room temperature increases processing time and cost, the dehydrogenation treatment temperature T should be above room temperature. It is preferable that the dehydrogenation treatment temperature T be 50°C or higher. It is more preferable that the dehydrogenation treatment temperature T be 100°C or higher, and even more preferable that it be 150°C or higher. Room temperature refers to 20±10°C.

[0065] In particular, when heating, it takes time for the temperature Tc at the center of the thickness of the steel material and steel pipe to reach the ambient temperature (dehydrogenation treatment temperature T) in the dehydrogenation treatment process. Therefore, even if the above holding time R (sec) is met at the ambient temperature, if the center of the thickness has not reached the dehydrogenation treatment temperature T (ambient temperature), the dehydrogenation treatment may be insufficient. For this reason, it is preferable to hold the temperature Tc at the center of the thickness for R (sec) or longer after it reaches the target dehydrogenation treatment temperature T. Furthermore, in order to obtain a predetermined fatigue limit stress in hydrogen, or a fatigue limit stress in hydrogen of 1 MPa or more / fatigue limit stress in an inert gas environment, it is necessary to appropriately adjust the amount of hydrogen in the steel material at the surface and the center of the thickness. For this purpose, it is preferable to hold the temperature T at the dehydrogenation treatment temperature for R (sec) or longer as defined by equation (A), and furthermore, it is preferable to hold the temperature Tc at the center of the thickness for R (sec) or longer after it reaches the target dehydrogenation treatment temperature T. The temperature Tc at the center of the thickness may be measured using a thermocouple or the like, or it may be predicted using the finite element method or the like.

[0066] Furthermore, the time and temperature of the dehydrogenation treatment process may include the temperature and time applied during the heating process in the pipe manufacturing process, such as for electric resistance welded pipes or UOE pipes, as described later. In addition, since scale on the steel surface inhibits dehydrogenation, it is preferable to remove the scale before performing the dehydrogenation treatment. The removal method is not limited, but may include physical cleaning by high-pressure washing, for example, or a chemical method using a scale remover. The effect of scale removal can be obtained when a thickness of about 100 μm is removed.

[0067] Second Embodiment Furthermore, UOE steel pipes, which can be cited as an example of high-strength steel pipes for line pipes, can be obtained by limiting the manufacturing conditions as shown below, and the manufacturing method and conditions will be explained in detail. The component composition, metallographic structure, fatigue limit stress in hydrogen of 1 MPa or higher, and fatigue limit stress in hydrogen of 1 MPa or higher / fatigue limit stress in an inert gas environment of UOE steel pipes are the same as those described for the steel material in the first embodiment, and the manufacturing method, including the heating process, hot rolling process, controlled cooling process after hot rolling, and dehydrogenation treatment process, is carried out in the same manner as described for the steel material. The pipe-making process after rolling will be explained in detail below.

[0068] Pipe making process UOE steel pipes are manufactured by bending hot-rolled steel sheets, specifically by beveling the ends of the hot-rolled steel sheets, forming them into a steel pipe shape using C-press, U-press, and O-press, then seam welding the butt joints using internal and external welding, and further expanding the pipe as needed. Any welding method is acceptable as long as sufficient joint strength and toughness can be obtained, but submerged arc welding is preferred from the viewpoint of excellent welding quality and manufacturing efficiency. Furthermore, pipe expansion can also be performed on steel pipes that have been formed into a tubular shape by press bending and then seam-welded at the butt joints.

[0069] Third Embodiment Furthermore, an example of a high-strength steel pipe for line pipes according to the present invention is an electric resistance welded (ERW) steel pipe, which can be obtained by limiting the manufacturing conditions as shown below. The manufacturing method and conditions will be explained in detail. The component composition of the steel material, the metal structure, the fatigue limit stress in hydrogen of 1 MPa or more, and the fatigue limit stress in hydrogen of 1 MPa or more / fatigue limit stress in an inert gas environment are the same as those described for the steel material in the first embodiment. The manufacturing method is also carried out in the same manner as described for the steel material, except for the cooling process after rolling and the pipe-making process (heating process, hot rolling process, dehydrogenation treatment process).

[0070] Cooling process after rolling (controlled cooling process) The cooling start temperature and average cooling rate of the controlled cooling are the same as those described in the first embodiment.

[0071] Cooling stop temperature: 250~650℃ If the cooling stop temperature after hot rolling exceeds 650°C, the bainite transformation becomes incomplete, and the material strength decreases significantly. For this reason, the cooling stop temperature should be 650°C or lower. Preferably, the cooling stop temperature should be 620°C or lower. More preferably, the cooling stop temperature should be 580°C or lower. On the other hand, if the cooling stop temperature is below 250°C, quench cracking during cooling is likely to occur. Also, in order to obtain a uniform bainite structure, the cooling stop temperature should be 250°C or higher. From the standpoint of suppressing the amount of hydrogen in the steel, it is also necessary to set the cooling stop temperature above a predetermined temperature. Specifically, hydrogen present in the steel gradually escapes during cooling, and the effect is greater at higher temperatures, but if the cooling stop temperature is too low, supercooling occurs, and hydrogen remains in the steel. Furthermore, if the cooling stop temperature is set too low, retained austenite, which rapidly increases in hydrogen compared to other phases, is more likely to form. For this reason, the cooling stop temperature needs to be 250°C or higher in order to reduce the amount of hydrogen in the steel. Preferably, the cooling stop temperature is 390°C or higher. More preferably, the cooling stop temperature is 450°C or higher. Even more preferably, the cooling stop temperature is 480°C or higher. After cooling stop, it is sufficient to allow it to cool naturally, but to promote the formation of bainite, it is more preferable to cool it slowly until the temperature drops by about 50°C from the cooling stop temperature. Note that the cooling stop temperature referred to here is the temperature at the center of the plate thickness.

[0072] Subsequently, the hot-rolled steel sheet obtained as described above is wound into a coil. The winding temperature is preferably 650°C or lower. Alternatively, the winding temperature is preferably 250°C or higher.

[0073] Pipe making process An example of the present invention is an electric resistance welded (ERW) steel pipe, which is manufactured by forming it into a cylindrical shape by cold roll forming and then welding the circumferential ends of the cylindrical shape together. Furthermore, it may also be manufactured by forming an ERW steel pipe material (ERW steel pipe) using a sizing roll that satisfies the following equation (1) (sizing step), and then applying an internal pressure p (MPa) that satisfies the following equation (2) to the inner surface of the ERW steel pipe material (internal pressure loading step). Furthermore, the term "cylindrical" refers to a pipe whose circumferential cross-section is "C" shaped. Diameter of sizing roll (mm) ≥ Thickness of hot-rolled steel sheet (mm) / 0.020 ... (1) The thickness of a hot-rolled steel sheet refers to the thickness of the hot-rolled steel sheet before the sizing process. X <p≦X×1.5 ···(2) Note that X = (wall thickness of electric resistance welded steel pipe material (mm) / radius of electric resistance welded steel pipe material (mm)) × yield strength of electric resistance welded steel pipe material (MPa) The aforementioned internal pressure load can be implemented, for example, by sealing the pipe end with a rubber gasket and applying water pressure inside the pipe. Furthermore, to stabilize the shape, a mold of the desired diameter can be used as an outer frame if necessary.

[0074] Furthermore, the wall thickness of the electric resistance welded (ERW) steel pipe material, which is given as an example of the steel pipe of the present invention, is preferably 5 mm or more, and preferably 30 mm or less. There is no particular upper limit specified for the radius of the ERW steel pipe material, but since a larger radius increases the load on the equipment, the radius of the ERW steel pipe material is preferably 400 mm or less. Also, the radius of the ERW steel pipe material is preferably 200 mm or more. In addition, the yield strength of the ERW steel pipe material is preferably 480 MPa or more, and more preferably 500 MPa or more, in order to withstand the gas pressure of pipeline operation. On the other hand, in order to avoid increased susceptibility to hydrogen embrittlement, the yield strength is preferably 560 MPa or less.

[0075] During the sizing process, bending deformation occurs in the axial direction of the pipe along the roll shape as the pipe passes through the roll, generating residual stress in the axial direction of the pipe. The greater the bending strain in the bending deformation, the greater the absolute value of the residual stress in the axial direction of the pipe. The bending strain increases as the diameter of the sizing roll decreases and as the thickness of the hot-rolled steel sheet increases. Therefore, in this invention, from the viewpoint of reducing shear residual stress, the diameter of the sizing roll is set to satisfy equation (1) above in order to reduce the absolute value of residual stress in the axial direction of the pipe. If the diameter of the sizing roll is less than the right-hand side of equation (1) above, the shear residual stress targeted by the present invention cannot be obtained. Although there is no upper limit specified for the diameter of the sizing roll, it is preferable that the diameter of the sizing roll be 2000 mm or less, as a larger sizing roll increases the load on the equipment.

[0076] In the internal pressure loading process, the electric resistance welded steel pipe material is expanded to generate tensile stress in the circumferential direction of the pipe, thereby reducing the absolute value of residual stress in the circumferential direction. The greater the internal pressure p (MPa) during the internal pressure loading process, the smaller the absolute value of the residual stress in the circumferential direction of the pipe. The tensile stress generated in the circumferential direction of the pipe increases as the radius of the steel pipe increases and as the wall thickness of the steel pipe decreases.

[0077] The left-hand side (X) of equation (2) above corresponds to the internal pressure p when the tensile stress generated in the circumferential direction of the pipe is equal to the yield stress of the electric resistance welded steel pipe material. In this invention, from the viewpoint of reducing shear residual stress, the internal pressure p is set to a value greater than (X) on the left side of equation (2) in order to reduce the absolute value of residual stress in the axial direction of the pipe, and the electric resistance welded steel pipe material is expanded to the plastic region. On the other hand, if the internal pressure p exceeds (X × 1.5) on the right side of equation (2), the absolute value of residual stress in the circumferential direction of the pipe decreases, but the amount of work hardening due to pipe expansion becomes too large, the dislocation density on the pipe surface increases, and the fatigue resistance characteristics in hydrogen decrease.

[0078] As partially explained above, the steel pipes of the present invention can be manufactured by forming the steel material disclosed in this invention into a tubular shape using press bending, roll forming, UOE forming, etc., and then welding the butt joints, thereby producing high-strength steel pipes for sour line pipes (UOE steel pipes, electric resistance welded steel pipes, spiral steel pipes, etc.) with excellent material uniformity within the steel plate, suitable for the transportation of crude oil and natural gas. Furthermore, by using the steel plate disclosed in this disclosure for steel pipes, it is possible to manufacture steel pipes with excellent HISC resistance even if a high-hardness region exists in the welded area. [Examples]

[0079] Next, the present invention will be described in more detail based on examples. The following examples illustrate preferred examples of the present invention, and the present invention is not limited in any way by the examples described.

[0080] First, billets with the component compositions shown in Tables 1-1, 1-2, and 1-3 were prepared. The casting speed was 0.05 to 0.2 m / min. The billets were heated to 1000 to 1100°C. Then, hot rolling was performed at 1000°C ± 50°C. The time difference between the leading and trailing ends of the hot rolling was 30 to 45 seconds, and the target thickness of the steel plate was 20 mmt. Controlled cooling was started when the surface temperature reached Ar3 + 50°C. Subsequently, steel materials were manufactured under the conditions shown in Tables 2-1, 2-2, and 2-3. For some steel materials (steel materials No. 1-14, 16-30, 92), after the controlled cooling process, a pipe-making process was performed in which the hot-rolled steel sheets were bent and both ends were butt-welded. For some steel materials (steel materials No. 15, 31-55, 93-98), after the controlled cooling process, the hot-rolled steel sheets were formed into cylindrical shapes by cold roll forming, and the circumferential ends of the cylindrical shapes were butt-welded using electric resistance welding. This yielded steel pipes No. 1-14, 16-30, 92 and No. 15, 31-55, 93-98, 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 Table 2, the dehydrogenation treatment temperature Y indicates that the dehydrogenation treatment was performed in the range of room temperature to 550°C, and N indicates that the dehydrogenation treatment was performed at a temperature exceeding 550°C. After confirming that the plate thickness center temperature Tc reached the target temperature of room temperature, the temperature was held for R (sec) to satisfy equation (A) above.

[0081] Furthermore, billets with the component compositions shown for steel grade No. 15 in Table 1-1 and steel grade No. 56 in Table 1-2 were produced at various casting rates shown in Table 3, and these billets were heated to 1000-1100°C. Then, hot rolling was performed at 1000±50°C. The time difference between the leading and trailing ends of the hot rolling was 30-45 seconds, and the target thickness of the steel plate was 20mmt. The cooling start temperature was determined by the surface temperature reaching Ar3+50°C, at which point controlled cooling was initiated. Subsequently, steel materials and steel pipes were obtained by manufacturing under the conditions shown in Table 3. Steel materials No. 15-1 to 3 and 56-1 to 3 are in their raw state. Steel pipes No. 15-11, 15-12, 56-11, and 56-12 are manufactured by bending hot-rolled steel sheets and welding the ends together. Steel pipes No. 15-13 and 56-13 are obtained by cold-rolling hot-rolled steel sheets into a cylindrical shape after a controlled cooling process, and then welding the ends of the cylindrical shape together using electric resistance welding. The microstructure and mechanical properties were evaluated. The evaluation method was as follows: The tempering temperature was arbitrarily adjusted so that the tensile strength of the material was in the range of 520 MPa to 700 MPa. The results of the evaluation of the metallographic structure and material properties of the obtained steel materials and steel pipes are shown in Tables 2-1, 2-2, 2-3, and 2-3. The evaluation method is as follows.

[0082] Measurement of retained austenite Samples for metallographic observation were taken from the center of the plate width in the longitudinal direction of the steel material and steel pipe obtained according to the above procedure. The cross section parallel to the longitudinal direction was used as the observation surface and buffed, and then the surface layer was removed by chemical polishing using picric acid etching, and measurements were taken using X-ray diffraction. Specifically, a Co-Kα source was used for the incident X-rays, and the area fraction of retained austenite was calculated from the intensity ratio of the (200), (211), (220) planes of ferrite and the (200), (220), (311) planes of austenite.

[0083] Measurement of the area fraction of bainite Test specimens taken from the longitudinal center of a steel plate at the 1 / 4 thickness point, and from the longitudinal center of a steel pipe at the 1 / 4 thickness point, were buffed and then etched with 3 vol% nital. Subsequently, three fields of view were observed using an optical microscope at 100x magnification, and scanning electron microscope (SEM) images were taken at an appropriate magnification between 1000 and 5000x to observe bainite. Bainite was visually identified by comparing it with the microstructure image in Non-Patent Literature 2. Based on the above assessment, the area fraction was determined by image analysis using binarized images of bainite and other regions from the optical microscope or SEM images. The average value obtained from the optical microscope or SEM images was defined as the bainite area fraction.

[0084] Tensile strength (TS) From the steel materials and steel pipes obtained according to the above, JIS No. 14 proportional test specimens (parallel section diameter 7 mm, gauge length 35 mm) were taken in accordance with JIS Z 2201, and their tensile strength was measured.

[0085] Hydrogen temperature rise analysis The amount of hydrogen remaining in the steel was determined using a low-temperature heating-type hydrogen analyzer (gas chromatograph type) (JTF-20AL) with a heating-intensity desorption analysis method. The heating-intensity desorption analysis was performed at a heating rate of 200°C / h in the temperature range from room temperature to 400°C, and the sum of these results was taken as the amount of hydrogen. The test specimens were cylindrical in shape with a length of 30 mm in the longitudinal direction of the steel pipe and a diameter of 7Φ, located at the 1 / 4 position of the thickness of the steel plate and the 1 / 4 position from the inner surface of the steel pipe. Note that this amount of hydrogen was measured before being subjected to the high-pressure hydrogen fatigue test described later in the aging section, and is the H amount shown in Tables 1-1, 1-2, and 1-3.

[0086] Fatigue test Fatigue testing was performed at room temperature (20±10℃) in a high-pressure gas mixture atmosphere in the air, in accordance with ASTM E466, Fatigue Testing, with a frequency of 1-15Hz, repetition waveform: sine wave, control method: load control, load conditions: uniaxial tensile compression, and stress ratio: R=-1.0. The fatigue limit strength in air was defined as the stress at which fracture did not occur after several tens of millions of cycles.

[0087] High-pressure hydrogen fatigue test Fatigue testing was conducted in a mixed atmosphere of hydrogen gas (100% gas) at room temperature (20±10℃) and pressure: 40 MPa, or hydrogen gas at a pressure of 1 MPa or higher, or natural gas containing hydrogen as a partial pressure of 1 MPa or higher (main components being hydrocarbons such as methane and ethane), in accordance with ASTM E466, Fatigue Testing, with a frequency of 1 Hz, repetition waveform: sine wave, control method: load control, load conditions: uniaxial tensile compression, and stress ratio: R = -1.0. The fatigue limit stress in hydrogen was defined as the stress at which fracture did not occur after 2 million cycles. In this test, the hydrogen fatigue limit stress obtained was 200 MPa or higher, and the ratio of the above-mentioned fatigue limit stress in hydrogen to the fatigue limit stress in an inert gas environment (the ratio of the fatigue limit stress in hydrogen to the fatigue limit stress in an inert gas environment) was 0.90 or higher, which was deemed to be a passing grade.

[0088] In all of the examples of this invention, the hydrogen fatigue limit stress was 200 MPa or higher, and the ratio of the hydrogen fatigue limit stress to the fatigue limit stress in an inert gas atmosphere (hydrogen fatigue limit stress / fatigue limit stress in an inert gas environment) was 0.90 or higher, satisfying excellent hydrogen embrittlement resistance. Furthermore, the tensile strength was 520 MPa or higher.

[0089] [Table 1-1]

[0090] [Table 1-2]

[0091] [Table 1-3]

[0092] [Table 2-1]

[0093] [Table 2-2]

[0094] [Table 2-3]

[0095] [Table 3] [Examples]

[0096] The following describes examples that verify the effects of the present invention. In the following examples, steel pipes were manufactured under the following manufacturing conditions and their characteristics were evaluated. Using steel grades No. 1, 15, and 56 shown in Tables 1-1 and 1-2, the steel pipes were manufactured under the same conditions as steel grades No. 1, 15, 56, 15-12, and 56-12 shown in Tables 2-1, 2-2, and 2-3 up to the controlled cooling process, and their characteristics were evaluated when the dehydrogenation treatment conditions were changed. Steel pipe forming was carried out in the same manner as in Example 1. The above results are shown in Table 4.

[0097] In this embodiment, the dehydrogenation treatment temperature T (ambient temperature) for steel pipes and steel materials No. 1A, 15A, 56A, 15-12A, and 56-12A was set to 50°C, and the holding time tc after the plate thickness center temperature Tc reached 50°C was carried out so as to satisfy equation (A). For steel pipes and steel materials No. 1B, 15B, 56B, 15-12B, and 56-12B, the dehydrogenation treatment temperature T (ambient temperature) was set to 50°C, and the holding time tc at the dehydrogenation treatment temperature T was set to satisfy the aforementioned equation (A). However, the holding time tc after the plate thickness center temperature Tc reached 50°C did not satisfy the aforementioned equation (A). For steel pipes and steel materials No. 1C, 15C, 56C, 15-12C, and 56-12C, the dehydrogenation treatment temperature T (ambient temperature) is 50°C, but neither the holding time t at ambient temperature nor the holding time tc after the central plate thickness temperature Tc reaches 50°C satisfies equation (A) described above.

[0098] In Table 4, "Dehydrogenation holding time t is Y" means that the dehydrogenation treatment temperature T (ambient temperature) is 50°C and the holding time t satisfies equation (A), while "Dehydrogenation holding time t is N" means that the dehydrogenation treatment temperature T (ambient temperature) is 50°C, but the holding time t does not satisfy equation (A). Furthermore, "Holding time tc at steel core temperature Tc is Y" means that the holding time tc after the plate thickness center temperature Tc reaches 50°C satisfies equation (A), while "Holding time tc at steel core temperature Tc is N" means that the plate thickness center temperature Tc reaches 50°C, but the holding time tc after Tc reaches 50°C does not satisfy equation (A).

[0099] Various evaluations were carried out using the method described in Example 1.

[0100] All of the examples of this invention satisfied the requirement that the fatigue limit stress in hydrogen be 200 MPa or higher, and that the ratio of the fatigue limit stress in hydrogen to the fatigue limit stress in an inert gas atmosphere be 0.90 or higher. Furthermore, the tensile strength was 520 MPa or higher. Among these, the fatigue characteristics were superior when the dehydrogenation treatment was carried out under more favorable conditions.

[0101] [Table 4]

Claims

1. In mass percent, 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: Includes 0.0010% or less, Or, furthermore, Nb: 0 to 0.10%, Ca: 0-0.005%, Ti: 0 to 0.1%, Ni: 0-2.0%, Cu: 0 to 1.0%, Cr: 0-1.0%, Mo: 0 to 0.60%, W: 0-1.0%, V: 0-0.10%, Zr: 0 to 0.050%, REM: 0-0.050%, Mg: 0 to 0.050%, B: 0 to 0.0020%, Hf: 0-0.2%, Ta: 0-0.2%, Re: 0 to 0.005%, Sn: 0-0.3%, Sb: Contains one or more selected from 0 to 0.3%, It has a chemical composition in which the remainder is Fe and unavoidable impurity elements. A thin or thick steel plate for line pipes, exhibiting excellent hydrogen embrittlement resistance, having retained austenite at an area fraction of 0-3%, bainite at an area fraction of 90% or more at the 1 / 4 thickness position, a fatigue limit stress in hydrogen gas at a pressure of 40 MPa of 200 MPa or more, and a fatigue limit stress in a hydrogen gas environment at a pressure of 40 MPa / fatigue limit stress in an inert gas environment of 0.90 or more.

2. In mass%, the above chemical composition is further, Nb: 0.001 to 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 to 0.60%, W: 0.01-1.0%, V: 0.01-0.10%, Zr: 0.0001 to 0.050%, REM: 0.0001-0.050%, Mg: 0.0001-0.050%, B: 0.0001 to 0.0020%, Hf: 0.0001-0.2%, Ta: 0.0001-0.2%, Re: 0.0001-0.005%, Sn: 0.0001-0.3%, A steel material for line pipes with excellent hydrogen embrittlement resistance, as described in claim 1, comprising one or more Sb selected from 0.0001 to 0.3%.

3. A method for manufacturing steel material for line pipes according to claim 1 or 2, comprising a heating step of heating a steel material having the chemical composition at 1000 to 1250°C, The steel material heated in the above heating step is rolled to a rolling completion temperature: Ar 3 A hot rolling process in which rolling is performed under conditions of 1.5 or higher, The hot-rolled steel sheet obtained in the hot-rolling process is subjected to a cooling start temperature at Ar, where Ar is the surface temperature of the steel sheet. 3 A controlled cooling process is performed under the conditions that, above a certain point, the difference in cooling start time between the leading and trailing ends 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 at the temperature in the center of the sheet thickness, and the cooling stop temperature is 250 to 650°C. A dehydrogenation treatment step is performed to maintain the steel sheet obtained in the controlled cooling step at a temperature in the range of room temperature to 550°C. A method for manufacturing steel materials for line pipes.

4. In steel pipes for line pipes, In mass percent, 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: Includes 0.0010% or less, Or, furthermore, Nb: 0 to 0.10%, Ca: 0-0.005%, Ti: 0 to 0.1%, Ni: 0-2.0%, Cu: 0 to 1.0%, Cr: 0-1.0%, Mo: 0 to 0.60%, W: 0-1.0%, V: 0-0.10%, Zr: 0 to 0.050%, REM: 0-0.050%, Mg: 0 to 0.050%, B: 0 to 0.0020%, Hf: 0-0.2%, Ta: 0-0.2%, Re: 0 to 0.005%, Sn: 0-0.3%, Sb: Contains one or more selected from 0 to 0.3%, A steel pipe for line pipes having a chemical composition in which the remainder is Fe and unavoidable impurity elements, retained austenite is 0-3% by area fraction, 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 gas at a pressure of 40 MPa is 200 MPa or more, and the fatigue limit stress in hydrogen gas at a pressure of 40 MPa / fatigue limit stress in an inert gas environment is 0.90 or more, exhibiting excellent resistance to hydrogen embrittlement.

5. In mass%, the above chemical composition is further, Nb: 0.001 to 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 to 0.60%, W: 0.01-1.0%, V: 0.01-0.10%, Zr: 0.0001 to 0.050%, REM: 0.0001-0.050%, Mg: 0.0001-0.050%, B: 0.0001 to 0.0020%, Hf: 0.0001-0.2%, Ta: 0.0001-0.2%, Re: 0.0001-0.005%, Sn: 0.0001-0.3%, A steel pipe for line pipes having excellent hydrogen embrittlement resistance, as described in claim 4, comprising one or more Sb selected from 0.0001 to 0.3%.

6. A method for manufacturing a steel pipe for line pipes according to claim 4 or 5, comprising a heating step of heating a steel material having the chemical composition at 1000 to 1250°C, The steel material heated in the above heating step is rolled to a rolling completion temperature: Ar 3 A hot rolling process in which rolling is performed under conditions of 1.5 or higher, The hot-rolled steel sheet obtained in the hot-rolling process is subjected to a cooling start temperature at Ar, where Ar is the surface temperature of the steel sheet. 3 A controlled cooling process is performed under the conditions that, above a certain point, the difference in cooling start time between the leading and trailing ends 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 at the temperature in the center of the sheet thickness, and the cooling stop temperature is 250 to 650°C. After the controlled cooling process, a pipe-making process is performed in which the hot-rolled steel sheet is bent and both ends are butt-welded; or a pipe-making process is performed in which the hot-rolled steel sheet is formed into a cylindrical shape by cold roll forming and both ends of the cylindrical shape are butt-welded using electric resistance welding; A dehydrogenation treatment process in which the steel pipes obtained in the pipe manufacturing process are held in a range of room temperature to 550°C, A method for manufacturing steel pipes for line pipes.

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