Steel material for line pipe with high hydrogen embrittlement resistance and method for producing the same, and steel pipe for line pipe with high hydrogen embrittlement resistance and method for producing 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

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

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

[0002] Line pipes for natural gas transportation are an existing energy infrastructure. Steel materials for these pipes have been required to suppress the occurrence of hydrogen-induced cracking in sour environments. Meanwhile, hydrogen has recently attracted significant attention worldwide as a clean energy source for building a decarbonized society. Therefore, with the aim of transporting large quantities of hydrogen gas, the construction of hydrogen gas transportation networks that use natural gas mixed with a portion of hydrogen in natural gas line pipes or pressurized hydrogen gas as an alternative is being considered. The expected transport pressure during operation of these pipelines is high, ranging from 1 to 40 MPa, and the line pipes will be exposed to high-pressure hydrogen gas. Steel materials used in such environments are susceptible to "hydrogen embrittlement," in which hydrogen penetrates the steel and deteriorates its properties. Therefore, steel materials must combine the high toughness and sour resistance required of conventional line pipes with the resistance to hydrogen embrittlement required in hydrogen gas environments.

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

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

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

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

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

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

[0009] Furthermore, Patent Document 6 proposes a steel material in which the metal structure is a bainite-based structure with an area fraction of 90% or more, and 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] Non-Patent Document 1 describes the fatigue strength values ​​of low alloy steels.

[0011] JP 2005-2386 A JP 2009-46737 A JP 2009-275249 A JP 2009-74122 A JP 2010-37655 A JP 2012-107332 A

[0012] Matsunaga et al., Int J Hydrogen Energy, Vol. 40 (2015), pp. 5739-5748; Japan Heat Treatment Association (author), Introduction to the Structure and Properties of Metallic Materials - Heat Treatment and Structure Control to Make the Most of Materials, 2004

[0013] The pressure inside linepipes fluctuates during operation and undergoes periodic shutdowns, subjecting structures to repeated stresses. Therefore, fatigue fracture must be considered when designing steel structures such as linepipes. However, as shown in Non-Patent Document 1, it is known that the fatigue life of materials decreases in high-pressure hydrogen environments. This means that if linepipe materials are designed based on conventional natural gas linepipes, their service life will be shortened. However, while the above-mentioned conventional technologies can suppress the occurrence of hydrogen-induced cracking in sour environments, they are unable to sufficiently increase fatigue strength in hydrogen gas. In other words, it is difficult to achieve both the suppression of hydrogen-induced cracking in sour environments and high fatigue strength in hydrogen gas.

[0014] In view of the above-mentioned problems of the conventional technology, the present invention aims to provide a steel material for line pipes that has high strength and excellent resistance to hydrogen embrittlement in high-pressure hydrogen gas environments, suitable for steel structures to be used in high-pressure hydrogen gas environments, such as line pipes for 100% hydrogen gas or natural gas containing hydrogen at a hydrogen partial pressure of 1 MPa or more (natural gas is a gas whose main components are hydrocarbons such as methane and ethane), a manufacturing method thereof, and a steel pipe for line pipes and a manufacturing method thereof.

[0015] Here, "excellent resistance to hydrogen embrittlement in a high-pressure hydrogen gas environment" refers to a case in which a fatigue test is conducted in accordance with ASTM E466, Fatigue Testing, at a frequency of 1 Hz, a repetitive waveform of sine wave, a control method of load control, loading conditions of uniaxial tension and compression, and a stress ratio of R = -1.0 in both environments of room temperature (20±10°C), hydrogen gas at a pressure of 1 MPa or more, or a mixed atmosphere of natural gas (mainly hydrocarbons such as methane and ethane) containing hydrogen at a partial pressure of 1 MPa or more, and the fatigue limit stress in hydrogen, which is the stress at which the material does not fracture after 2 million cycles, is 200 MPa or more, and the ratio of the fatigue limit stress in hydrogen to the fatigue limit stress in an inert gas environment is 0.90 or more.

[0016] Furthermore, 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 / fatigue limit stress in an inert gas environment is 0.90 or more, it becomes possible to design long-life steel structures for hydrogen use, such as line pipes, within a thickness range that can be manufactured using processes for manufacturing seamless steel pipes, UOE steel pipes, and other steel pipes.

[0017] The term "steel material" as used herein includes thin steel plates, thick steel plates, seamless steel pipes, electric resistance welded steel pipes, steel sections, steel bars, and the like.

[0018] The present inventors have conducted extensive research into the conditions that steel materials must satisfy in order to obtain steel plates and steel pipes for line pipes that are excellent in hydrogen embrittlement resistance, and have invented new high-strength steel plates and steel pipes for line pipes. 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] A steel sheet containing, by 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: 0.0010% or less, or further containing 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 to 0.10%, A steel material for linepipe having excellent hydrogen embrittlement resistance, the steel material having a chemical composition containing one or more elements selected from 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%, and Sb: 0-0.3%, with the balance being Fe and unavoidable impurity elements, the steel material having an area fraction of retained austenite of 0-3%, an area fraction of bainite of 90% or more at a quarter-thickness position, a fatigue stress limit in hydrogen of 1 MPa or more of 200 MPa or more, and a fatigue stress limit in hydrogen of 1 MPa or more / fatigue stress limit in an inert gas environment of 0.90 or more. [2] Furthermore, the chemical composition, in mass%, is: Nb: 0.001 to 0.10%, Ca: 0.0001 to 0.005%, Ti: 0.005 to 0.1%, Ni: 0.01 to 2.0%, Cu: 0.01 to 1.0%, Cr: 0.01 to 1.0%, Mo: 0.01 to 0.60%, W: 0.01 to 1.0%, V: 0.01 to 0.10%, Zr: 0.0001 to 0.050%, REM: 0.0001 to 0.050%, Mg: 0.0001 to 0.050%, B: 0.0001 to 0.0020%, Hf: 0.0001 to 0.2%, [3] A steel material for line pipe having excellent hydrogen embrittlement resistance according to [1], wherein Ta: 0.0001 to 0.2%, Re: 0.0001 to 0.005%, Sn: 0.0001 to 0.3%, and Sb: 0.0001 to 0.3%. [4] A steel material for line pipe having excellent hydrogen embrittlement resistance according to [1] or [2], wherein the steel material has the chemical composition according to [1] or [2], and the steel material is subjected to a heating process at a rolling end temperature of Ar.３ a hot rolling step of rolling the hot rolled steel sheet under conditions of a temperature above 1000°C; and a cooling step of cooling the hot rolled steel sheet obtained in the hot rolling step under conditions of a temperature above 1000°C. ３a controlled cooling step of cooling the hot-rolled steel plate at a temperature of 750°C to 550°C under the conditions that the difference in cooling start time between the head end and tail end of the hot-rolled steel plate is 50 seconds or less, the average cooling rate from 750°C to 550°C is 15 to 50°C / s at the temperature at the center of the plate thickness, and the cooling stop temperature is 250 to 650°C; and a dehydrogenation treatment step of holding the steel plate obtained in the controlled cooling step at a temperature in the range of room temperature to 550°C. [4] A steel pipe for line pipe, comprising, by 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: 0.0010% or less, or further comprising 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 to 0.10%, A steel pipe for linepipe having excellent hydrogen embrittlement resistance, the steel pipe having a chemical composition containing one or more elements selected from 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%, and Sb: 0-0.3%, with the balance being Fe and unavoidable impurity elements, the steel pipe having an area fraction of retained austenite of 0-3%, and an area fraction of bainite of 90% or more at a wall thickness 1 / 4 position from the inner surface of the steel pipe, a fatigue stress limit in hydrogen of 1 MPa or more of 200 MPa or more, and a fatigue stress limit in hydrogen of 1 MPa or more / a fatigue stress limit in an inert gas environment of 0.90 or more.[5] Furthermore, the chemical composition, in mass%, is: Nb: 0.001 to 0.10%, Ca: 0.0001 to 0.005%, Ti: 0.005 to 0.1%, Ni: 0.01 to 2.0%, Cu: 0.01 to 1.0%, Cr: 0.01 to 1.0%, Mo: 0.01 to 0.60%, W: 0.01 to 1.0%, V: 0.01 to 0.10%, Zr: 0.0001 to 0.050%, REM: 0.0001 to 0.050%, Mg: 0.0001 to 0.050%, B: 0.0001 to 0.0020%, Hf: 0.0001 to 0.2%, A steel pipe for line pipe having excellent hydrogen embrittlement resistance according to [4], wherein Ta: 0.0001 to 0.2%, Re: 0.0001 to 0.005%, Sn: 0.0001 to 0.3%, and Sb: 0.0001 to 0.3%. [6] A heating step of heating a steel material having the chemical composition according to [4] or [5] at 1000 to 1250°C, and rolling the steel material heated in the heating step at a rolling end temperature of Ar. ３ a hot rolling step of rolling the hot rolled steel sheet under conditions of a temperature above 1000°C; and a cooling step of cooling the hot rolled steel sheet obtained in the hot rolling step under conditions of a temperature above 1000°C. ３ a controlled cooling process in which the hot-rolled steel plate is cooled under conditions where the difference in cooling start times between the head and tail of the hot-rolled steel plate is 50 seconds or less, the average cooling rate from 750°C to 550°C is 15 to 50°C / s at the temperature at the center of the plate thickness, and the cooling stop temperature is 250 to 650°C; either one of a pipe-making process in which, after the controlled cooling process, the hot-rolled steel plate is bent and both ends are butt-welded, or 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 and both circumferential ends of the cylindrical shape are butt-welded; and a dehydrogenation process in which the steel pipe obtained in the pipe-making process is kept at a temperature in the range of room temperature to 550°C.

[0020] According to the present invention, steel materials having extremely improved hydrogen embrittlement resistance in a high-pressure hydrogen gas environment can be easily and simply manufactured, which is of great industrial benefit. Furthermore, according to the present invention, the hydrogen embrittlement resistance of steel structures such as line pipes for high-pressure hydrogen gas can be significantly improved, and the fatigue resistance can be improved, which has the effect of significantly contributing to the extension of the life of the steel structures.

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

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

[0023] C: 0.02 to 0.15% C effectively contributes to improving strength, but if the content is less than 0.02%, sufficient strength and fatigue stress limit cannot be ensured. Therefore, the C content is set to 0.02% or more. Preferably, the C content is 0.03% or more. On the other hand, if the C content exceeds 0.15%, weldability deteriorates. Therefore, the C content is limited to 0.15% or less. Preferably, the C content is 0.13% or less. Furthermore, if the C content exceeds 0.08%, the hardness of the surface layer and central segregation increases during controlled cooling, deteriorating SSCC (sulfide stress corrosion cracking) resistance and HIC (hydrogen induced cracking) resistance. Furthermore, toughness also deteriorates. Therefore, the C content is more preferably 0.08% or less. Even more preferably, the C content is 0.05% or less.

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

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

[0026] P: 0.0001 to 0.015% P is an inevitable impurity element that deteriorates weldability and also deteriorates HIC resistance by increasing the hardness of the center segregation. This tendency becomes significant when the content exceeds 0.015%, so the upper limit of the P content is set at 0.015%. The P content is preferably 0.010% or less, and more preferably 0.008% or less. The lower the content, the better, but from the perspective of refining costs, the P content is set to 0.0001% or more.

[0027] S: 0.0002 to 0.0015% S is an unavoidable impurity element, and since it becomes MnS inclusions in steel and deteriorates HIC resistance, the lower the content the better, but up to 0.0015% is acceptable. For this reason, the S content is set to 0.0015% or less. The S content is preferably 0.0010% or less, and more preferably 0.0008% or less. The lower the content, the better, but from the perspective of refining costs, the S content is set to 0.0002% or more.

[0028] Al: 0.005 to 0.15% Al is added as a deoxidizer, but if it is less than 0.005%, the added effect is ineffective. For this reason, the Al content is set to 0.005% or more. The Al content is preferably 0.01% or more, and more preferably 0.03% or more. On the other hand, if it exceeds 0.15%, the cleanliness of the steel decreases and the toughness deteriorates, so the Al content is limited to 0.15% or less. The Al content is preferably 0.10% or less, more preferably 0.08% or less, and even more preferably 0.05% or less.

[0029] O: 0.01% or less O causes the formation of oxide-based inclusions, so the lower the content, the better. However, an O content of 0.01% or less does not pose a problem. Therefore, the O content is set to 0.01% or less. The O content is preferably 0.005% or less. More preferably, the O content is less than 0.003%. There is no particular lower limit, but since reducing the oxygen content to 0% increases costs, the O content is preferably 0.001% or more.

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

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

[0032] In order to further improve the strength and toughness of the steel plate, the component composition of the present disclosure may 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.

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

[0034] Ca: 0 to 0.005% Ca is an element effective in improving HIC resistance by controlling the morphology of sulfide-based inclusions, but not only does this effect saturate, but it also deteriorates HIC resistance by reducing the cleanliness of the steel, so if Ca is contained, the Ca content is limited to 0.005% or less. The Ca content is preferably 0.003% or less. The Ca content is more preferably 0.002% or less. The Ca content may be 0% or more, but if it is less than 0.0001%, the effect of adding it is difficult to obtain, so if it is contained, it is preferably 0.0001% or more. The Ca content is more preferably 0.001% or more.

[0035] Ti: 0 to 0.1% Ti is an element effective for increasing the strength and toughness of steel, but if it exceeds 0.1%, the toughness of the weld deteriorates. Therefore, if Ti is contained, the Ti content is set to 0.1% or less. The Ti content is preferably 0.05% or less. The Ti content is more preferably 0.03% or less, and even more preferably 0.02% or less. The Ti content may be 0% or more, but if the Ti content is less than 0.005%, the effect of containing Ti is difficult to obtain, so if Ti is contained, it is preferably 0.005% or more. The Ti content is more preferably 0.008% or more.

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

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

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

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

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

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

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

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

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

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

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

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

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

[0049] Metal structure: Retained austenite 0-3% Remaining austenite in steel increases the amount of hydrogen in the steel, which may increase susceptibility to hydrogen embrittlement. Furthermore, when austenite transforms into martensite due to stress load during use, martensite is very hard and prone to hydrogen cracking, which may cause cracks to initiate from the martensite portion. In the present invention, the fatigue crack growth rate is reduced by limiting the retained austenite to 3% or less. Reducing the retained γ suppresses the occurrence of fatigue cracks in a hydrogen environment, thereby suppressing a decrease in the fatigue stress limit in hydrogen. For this reason, the retained austenite is set to 3% or less. The retained austenite is preferably 2% or less. The retained austenite is more preferably 1% or less. The retained austenite may be 0%.

[0050] In order to achieve a high strength of 520 MPa or more, with bainite at an area fraction of 90% or more at the 1 / 4 position in the plate thickness, the steel structure must be a bainite structure. Here, the bainite structure includes bainitic ferrite or granular bainite that transforms during or after controlled cooling, which contributes to transformation strengthening, and also includes tempered bainite. If the bainite structure contains heterogeneous structures such as ferrite, martensite, pearlite, island martensite, or retained austenite, the strength and toughness will decrease, so the smaller the volume fraction of structures other than the bainite phase, the better.

[0051] Furthermore, when a steel material contains both soft and hard phases, fatigue damage accumulates preferentially in the soft phase, making crack initiation more likely and lowering the fatigue stress limit. In a hydrogen environment, localized deformation is promoted, further accelerating fatigue damage to the soft phase and further lowering the fatigue stress limit in hydrogen. As a result, the fatigue stress limit / fatigue stress limit 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 is set to 90% or more. The area fraction of bainite is preferably 92% or more. The area fraction of bainite is more preferably 95% or more, and even more preferably 98% or more. There is no particular upper limit, and the area fraction of bainite may be 100%. Furthermore, since fatigue cracks initiate from the inner surface of the steel pipe, uniformity of the inner surface structure of the steel pipe is important. Therefore, the metal structure at the 1 / 4 position in the wall thickness from the inner surface of the steel pipe was specified, and for steel materials, the metal structure at the 1 / 4 position in the wall thickness was specified so that the above effect could be obtained regardless of which surface is on the inner surface side of the steel pipe.

[0052] The fatigue limit stress in hydrogen at 1 MPa or more is 200 MPa or more, and the fatigue limit stress in hydrogen at 1 MPa or more / inert gas environment is 0.90 or more. If the fatigue limit stress in hydrogen at 1 MPa or more is less than 200 MPa, and the fatigue limit stress in hydrogen at 1 MPa or more / inert gas environment is less than 0.90, this significantly differs from the design conditions of conventional pipelines, and it becomes necessary to increase the steel thickness (steel pipe thickness in the case of steel pipes). For this reason, the fatigue limit stress in hydrogen at 1 MPa or more is set to 200 MPa or more, and the fatigue limit stress in hydrogen at 1 MPa or more / inert gas environment is set to 0.90 or more. The fatigue limit stress in hydrogen at 1 MPa or more is preferably 220 MPa or more. It is more preferable that the fatigue limit stress in hydrogen at 1 MPa or more is 250 MPa or more, and even more preferable that it is 270 MPa or more. Although there is no particular upper limit, it is preferable that the fatigue limit stress in hydrogen at 1 MPa or more is 500 MPa or less. Furthermore, the fatigue limit stress in hydrogen of 1 MPa or more / in an inert gas environment is preferably 0.92 or more. The fatigue limit stress in hydrogen of 1 MPa or more / in an inert gas environment is more preferably 0.94 or more, and even more preferably 0.96 or more. There is no particular upper limit, but the fatigue limit stress in hydrogen of 1 MPa or more / in an inert gas environment may be 1.1 or less. Note that the inert gas referred to here includes the six elements of Group 0 of the periodic table, helium, neon, argon, krypton, xenon, and radon, as well as air, and the inert gas environment refers to an environment containing any of the above.

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

[0054] The thickness of the steel material is not particularly limited, but is preferably 5 mm or more, and is preferably 30 mm or less.

[0055] Next, a method for manufacturing a steel material according to the present invention will be described. The steel material according to the present invention can be manufactured by sequentially carrying out a heating process, a hot rolling process, a controlled cooling process, and a dehydrogenation process for a steel material (slab). In the following description, temperatures refer to the temperature at the center of the thickness of the steel material or the steel pipe, unless otherwise specified. The average cooling rate refers to the temperature at 1 / 4 of the wall thickness from the inner surface of the steel pipe. The temperatures at the center of the thickness and the temperature at 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 with a radiation thermometer using heat transfer calculations that take into account the heat transfer coefficient of the steel material.

[0056] Heating process Heating temperature of steel material: 1000 to 1250°C If the heating temperature of steel material such as billets or slabs is less than 1000°C, the diffusion of micro-segregated impurity elements such as C, P, and S is insufficient, and a homogeneous material cannot be obtained. For this reason, the heating temperature of the steel material is set to 1000°C or higher. On the other hand, if the heating temperature exceeds 1250°C, the crystal grains become too coarse, resulting in a deterioration in toughness. Therefore, the heating temperature of the steel material is set to 1250°C or lower. The heating temperature is preferably set to 1200°C or lower. The heating temperature is more preferably set to 1180°C or lower.

[0057] Hot rolling process Hot rolling end temperature: Ar ３ After reheating the steel material, it is hot rolled to the desired pipe or plate thickness. The end temperature of hot rolling is the ferrite formation temperature, Ar ３ Points or more. ３ If the temperature is lower than this point, the strength will decrease due to the formation of a soft ferrite phase in the process of immediately cooling after hot rolling. ３ The finishing temperature of the hot rolling is preferably Ar +30°C or higher. ３ It is more preferable to set the temperature at +50°C or higher. Furthermore, if the temperature exceeds 1250°C, the crystal grains become too coarse and the toughness deteriorates, so the upper limit is preferably set at 1250°C or lower. The end temperature of the hot rolling is more preferably set at 1200°C or lower, and even more preferably at 1150°C or lower.

[0058] Ar ３Since the transformation temperature varies depending on the alloying composition of the steel, it can be determined by measuring the transformation temperature experimentally for each steel, but it can also be determined from the composition of the steel using the following formula: ３ (°C) = 910 - 310C (%) - 80Mn (%) - 20Cu (%) - 15Cr (%) - 55Ni (%) - 80Mo (%) The content of each alloy element is expressed as a percentage by mass.

[0059] Controlled cooling process Cooling start temperature: Steel plate surface temperature ３ The surface temperature of the steel plate at the start of cooling is Ar ３ If the temperature is below this point, ferrite will form before controlled cooling, resulting in a significant decrease in strength. ３ The surface temperature of the steel sheet at the start of cooling is Ar ３ It is preferable to set the temperature at +30°C or higher. ３ It is more preferable that the cooling start temperature is +50°C or higher. 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. It is more preferable that the surface temperature of the steel plate at the start of cooling is 1200°C or lower, and even more preferably 1150°C or lower. 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 is the lowest.

[0060] Controlled cooling time difference between the leading edge and the tail edge of the steel sheet: 50 seconds or less. If the time difference between the leading edge and the tail edge in the rolling direction of the steel sheet at the start of cooling exceeds 50 seconds, the temperature difference between the leading edge and the tail edge at the start of cooling will be large, resulting in large temperature variations when cooling is stopped. This will increase the variation in Vickers hardness 0.25 mm below the steel sheet surface and deteriorate HISC resistance. Therefore, the cooling time difference between the leading edge and the tail edge of the steel sheet should be 50 seconds or less, preferably 45 seconds or less. More preferably, it should be 40 seconds or less. While it is possible to shorten the cooling time difference by shortening the steel sheet length, this will reduce manufacturability, so it is preferable to shorten the cooling time difference by increasing the steel sheet transport speed. The lower limit is not particularly limited and may be greater than 0 seconds.

[0061] Average cooling rate at the center of thickness from 750°C to 550°C: 15 to 50°C / s If the average cooling rate at the center of thickness from 750°C to 550°C 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 at the center of thickness is set to 15°C / s or more. From the viewpoint of suppressing structure variation, the average cooling rate at the center of thickness is preferably set to 17°C / s or more. The average cooling rate at the center of thickness is more preferably set to 20°C / s or more, and even more preferably set to 25°C / s or more. On the other hand, in order to suppress variation in bainite grain size, the average cooling rate at the center of thickness is set to 50°C / s or less. The average cooling rate at the center of thickness is preferably set to 45°C / s or less. The average cooling rate at the center of thickness is more preferably set to 40°C / s or less. Although there are no particular limitations on the cooling to a steel sheet temperature of 550°C or less at the center of the sheet thickness, from the viewpoint of suppressing variations in the structure and grain size, the average cooling rate from 550°C to 300°C is preferably 15°C / s or more, for example. The average cooling rate from 550°C to 300°C is preferably 50°C / s or less.

[0062] Cooling stop temperature: 250 to 650°C. If the cooling stop temperature after hot rolling exceeds 650°C, the bainite transformation will be incomplete, resulting in a significant decrease in material strength. Therefore, the cooling stop temperature is set to 650°C or lower. The cooling stop temperature is preferably set to 625°C or lower. The cooling stop temperature is more preferably set to 600°C or lower. On the other hand, if the cooling stop temperature is lower than 250°C, quench cracking during cooling is likely to occur. Furthermore, to obtain a uniform bainite structure, the cooling stop temperature is set to 250°C or higher. The cooling stop temperature must also be set to a predetermined temperature or higher in order to suppress the amount of hydrogen in the steel. Specifically, hydrogen present in the steel gradually escapes during cooling, and the higher the temperature, the greater the effect. However, if the cooling stop temperature is too low, supercooling occurs, leaving hydrogen remaining in the steel. Furthermore, if the cooling stop temperature is too low, retained austenite, which rapidly releases a large amount of hydrogen compared to other phases, is likely to form. Therefore, the cooling stop temperature must be set to 250°C or higher in order to reduce the amount of hydrogen in the steel. The cooling stop temperature is preferably 270° C. or higher. After cooling is stopped, the material may be allowed to cool naturally, but in order to promote the formation of bainite, it is more preferable to slowly cool the material 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.

[0063] Dehydrogenation Treatment Step If hydrogen is present in the steel material, the acceleration of fatigue crack propagation increases, and the fatigue life and fatigue stress limit in hydrogen decrease. Therefore, dehydrogenation treatment may be performed to release the hydrogen remaining after manufacturing. Dehydrogenation treatment can reduce the amount of hydrogen in the steel by holding the steel at high temperature for a certain time before use, and it is possible to obtain a steel plate with excellent resistance to hydrogen embrittlement 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 ) and therefore, it is preferable to satisfy the following formula (A): R≧t ２ / D (A) The hydrogen diffusion coefficient varies depending on the contained components and metal structure. For example, the hydrogen diffusion coefficient is 1×10 －５ ~ 5 x 10 －３ mm ２ / s may be adopted. More preferably, 5 × 10 －４ mm２ / s or less.

[0064] The dehydrogenation process is performed before pipe making or welding to connect steel pipes. High temperatures are preferred for dehydrogenation because the hydrogen diffusion coefficient D decreases at high temperatures, allowing hydrogen to escape more quickly. At high temperatures, calculations can be performed using the diffusion coefficient D' (diffusion coefficient at each temperature) at which the value of D in equation (A) is maintained. On the other hand, if the temperature T in the dehydrogenation process is too high, the material strength decreases significantly, so the dehydrogenation temperature T is set to 550°C or less. The dehydrogenation temperature T is preferably set to 500°C or less. The dehydrogenation temperature T is more preferably set to 400°C or less, and even more preferably set to 300°C or less. Furthermore, because dehydrogenation at temperatures lower than room temperature increases the processing time and costs, the dehydrogenation temperature T is set to room temperature or higher. The dehydrogenation temperature T is preferably set to 50°C or higher. The dehydrogenation temperature T is more preferably set to 100°C or higher, and even more preferably set to 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 temperature of the atmosphere (dehydrogenation temperature T) in the dehydrogenation treatment process. Therefore, even if the above-mentioned holding time R (sec) is met at the atmosphere temperature, if the center of the thickness does not reach the dehydrogenation temperature T (atmospheric temperature), the dehydrogenation treatment may be insufficient. Therefore, it is preferable to hold the temperature Tc for R (sec) or more after it reaches the target dehydrogenation temperature T. Furthermore, in order to obtain a predetermined hydrogen fatigue limit stress, a hydrogen fatigue limit stress of 1 MPa or more / a 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 thickness. For this reason, it is preferable to hold the temperature Tc at the dehydrogenation temperature T for R (sec) or more as defined by formula (A), and it is also preferable to hold the temperature Tc for R (sec) or more after it reaches the target dehydrogenation temperature T. The temperature Tc may be measured using a thermocouple or the like, or it may be predicted using the finite element method or the like.

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

[0067] Second Embodiment Furthermore, a UOE steel pipe, which is 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 will be specifically explained. The chemical 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 UOE steel pipe are the same as those explained for the steel material of the first embodiment, and in the manufacturing method, the heating process, hot rolling process, controlled cooling process after hot rolling, and dehydrogenation treatment process are carried out in the same manner as those explained for the steel material. Below, the pipe-making process after rolling will be specifically explained.

[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 and forming them into a steel pipe shape using a C press, U press, or O press, followed by seam welding of the butt joints by internal and external welding, and, if necessary, by a pipe expansion process. Any welding method may be used as long as it provides sufficient joint strength and joint toughness, but submerged arc welding is preferred from the viewpoints of excellent weld quality and manufacturing efficiency. Pipe expansion can also be performed on steel pipes that have been press-bent into a tubular shape and then seam-welded at the butt joints.

[0069] Third Embodiment Furthermore, an example of a high-strength steel pipe for line pipe according to the present invention is an electric resistance welded steel pipe, which can be obtained by limiting the manufacturing conditions shown below, and the manufacturing method and conditions will be specifically described below. The steel material's chemical 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 are the same as those described for the steel material of the first embodiment, and as for the manufacturing method, the steps other than the cooling step after rolling and the pipe-making step (heating step, hot rolling step, dehydrogenation treatment step) are carried out in the same manner as those described for the steel material.

[0070] Cooling Step After Rolling (Controlled Cooling Step) The cooling start temperature and the average cooling rate of the controlled cooling are the same as those described in the first embodiment.

[0071] Cooling stop temperature: 250 to 650°C. If the cooling stop temperature after hot rolling exceeds 650°C, the bainite transformation will be incomplete, resulting in a significant decrease in material strength. Therefore, the cooling stop temperature is set to 650°C or lower. The cooling stop temperature is preferably set to 620°C or lower. The cooling stop temperature is more preferably set to 580°C or lower. On the other hand, if the cooling stop temperature is lower than 250°C, quench cracking during cooling is likely to occur. Furthermore, to obtain a uniform bainite structure, the cooling stop temperature is set to 250°C or higher. The cooling stop temperature must also be set to a predetermined temperature or higher in order to suppress the amount of hydrogen in the steel. Specifically, hydrogen present in the steel gradually escapes during cooling, and the higher the temperature, the greater the effect. However, if the cooling stop temperature is too low, overcooling occurs, leaving hydrogen remaining in the steel. Furthermore, if the cooling stop temperature is too low, retained austenite, which rapidly releases a large amount of hydrogen compared to other phases, is likely to form. Therefore, the cooling stop temperature must be set to 250°C or higher in order to reduce the amount of hydrogen in the steel. The cooling stop temperature is preferably 390°C or higher. More preferably, the cooling stop temperature is 450°C or higher. The cooling stop temperature is even more preferably 480°C or higher. After cooling is stopped, the material may be allowed to cool naturally, but in order to promote the formation of bainite, it is more preferable to slowly cool the material 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.

[0072] The hot-rolled steel sheet obtained as described above is then wound into a coil at a coiling temperature of preferably 650°C or less, and more preferably 250°C or more.

[0073] Pipe-making process: An electric resistance welded steel pipe, as an example of the present invention, is manufactured by forming a cylindrical shape by cold roll forming and then butting and welding both circumferential ends of the cylindrical shape together. Furthermore, the electric resistance welded steel pipe may be manufactured by forming the electric resistance welded steel pipe material (electric resistance welded steel pipe) using a sizing roll that satisfies the following formula (1) (sizing process), and then applying an internal pressure p (MPa) that satisfies the following formula (2) to the inner surface of the electric resistance welded steel pipe material (internal pressure applying process). The cylindrical shape refers to a pipe with a "C"-shaped circumferential cross section. Diameter (mm) of sizing roll ≥ Thickness of hot-rolled steel sheet (mm) / 0.020 (1) The thickness of the hot-rolled steel sheet refers to the thickness of the hot-rolled steel sheet before the sizing process. X<p≦X×1.5 (2) where 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 above-mentioned internal pressure application can be carried out, for example, by sealing the pipe ends with rubber packing and applying water pressure to the inside of the pipe. Furthermore, in order to stabilize the shape, a mold of the desired diameter can be used as an outer frame, if necessary.

[0074] The wall thickness of the electric resistance welded 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 for the radius of the electric resistance welded steel pipe material, but since a larger radius increases the load on the equipment, the radius of the electric resistance welded steel pipe material is preferably 400 mm or less. The radius of the electric resistance welded steel pipe material is preferably 200 mm or more. Furthermore, the yield strength of the electric resistance welded steel pipe material is preferably 480 MPa or more in order to withstand the gas pressure in pipeline operation, and more preferably 500 MPa or more. On the other hand, the yield strength is preferably 560 MPa or less in order to avoid increased susceptibility to hydrogen embrittlement.

[0075] In the sizing process, bending deformation occurs in the tube axial direction along the roll shape as the tube passes through the roll, generating residual stress in the tube axial direction. The greater the bending strain in the bending deformation, the greater the absolute value of the residual stress in the tube axial direction. 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 the present invention, from the viewpoint of reducing the shear residual stress, the diameter of the sizing roll satisfies the above formula (1) in order to reduce the absolute value of the residual stress in the tube axial direction. If the diameter of the sizing roll is less than the right-hand side of formula (1), the shear residual stress targeted in the present invention cannot be obtained. Note that there is no particular upper limit for the diameter of the sizing roll, but since a larger sizing roll increases the load on the equipment, it is preferable that the diameter of the sizing roll be 2000 mm or less.

[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 the residual stress in the circumferential direction. The greater the internal pressure p (MPa) in 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 becomes higher as the radius of the steel pipe increases and the wall thickness of the steel pipe decreases.

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

[0078] As partly explained above, for the steel pipe of the present invention, the steel material disclosed in the present invention is formed into a tubular shape by press bending, roll forming, UOE forming, etc., and then the butt joints are welded, thereby making it possible to produce high-strength steel pipes for sour-resistant line pipes (UOE steel pipes, electric resistance welded steel pipes, spiral steel pipes, etc.) that have excellent material uniformity within the steel plate and are suitable for transporting crude oil or natural gas. Furthermore, by using the steel plate disclosed herein for steel pipes, it is possible to produce steel pipes that have excellent HISC resistance, even if a high hardness region exists in the weld.

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

[0080] First, billets were prepared with the chemical compositions shown in Tables 1-1, 1-2, and 1-3. The casting speed was 0.05 to 0.2 m / min. The billets were heated to 1000 to 1100°C. Then, hot rolling was carried out at 1000°C ± 50°C. The time difference between the head and tail of the hot rolling was 30 to 45 seconds, and the target thickness of the steel plate was 20 mmt. The cooling start temperature was the surface temperature, and Ar ３ When the temperature reached +50°C, controlled cooling was started. Thereafter, steel products were produced under the conditions shown in Tables 2-1, 2-2, and 2-3. For some steel products (Steel Product Nos. 1 to 14, 16 to 30, and 92), after the controlled cooling process, the hot-rolled steel sheet was bent and then subjected to a pipe-making process in which both ends were butt-welded. For some steel products (Steel Product Nos. 15, 31 to 55, and 93 to 98), after the controlled cooling process, the hot-rolled steel sheet was formed into a cylindrical shape by cold roll forming and then subjected to a pipe-making process in which both circumferential ends of the cylindrical shape were butt-welded, thereby obtaining Steel Pipe Nos. 1 to 14, 16 to 30, and 92, and Steel Pipe Nos. 15, 31 to 55, and 93 to 98, respectively. Furthermore, in the dehydrogenation treatment of Example 1, the dehydrogenation treatment was carried out at a temperature ranging from room temperature to 550°C. The dehydrogenation temperature Y listed in Table 2 indicates that dehydrogenation was performed in the range of room temperature to 550°C, and N indicates that dehydrogenation was performed at a temperature above 550°C. After confirming that the center temperature Tc reached the target temperature, room temperature, the temperature was held for R (sec) so as to satisfy the above formula (A).

[0081] Furthermore, billets having the chemical compositions 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 to 1100°C. Thereafter, hot rolling was carried out at 1000±50°C. The time difference between the head and tail of the hot rolling was 30 to 45 seconds, and the target thickness of the steel plate was 20 mmt. The cooling start temperature was the surface temperature, and Ar ３ Controlled cooling was initiated when the temperature reached +50°C. Subsequently, steel products and steel pipes were obtained under the conditions shown in Table 3. Steel products Nos. 15-1 to 15-3 and 56-1 to 15-3 were used as they were. Steel pipes Nos. 15-11, 15-12, 56-11, and 56-12 were manufactured by a pipe-making process in which hot-rolled steel sheets were bent and then butt-welded at both ends. Steel pipes Nos. 15-13 and 56-13 were obtained by a controlled cooling process, followed by cold-rolling the hot-rolled steel sheets into a cylindrical shape, and then butt-welding both circumferential ends of the cylindrical shape. The metallographic structure and mechanical properties were evaluated. The evaluation methods were as follows. The tempering temperature was adjusted arbitrarily so that the tensile strength of the material was in the range of 520 MPa to 700 MPa. The metal structure and properties of the obtained steel materials and steel pipes were evaluated, and the results are shown in Tables 2-1, 2-2, 2-3, and 3. The evaluation methods were as follows.

[0082] Measurement of retained austenite Samples for metallographic observation were taken from the longitudinal center of the steel material and steel pipe obtained as described above, and the cross section parallel to the longitudinal direction was used as the observation surface, followed by buffing. The surface layer was then chemically polished by etching with picric acid to remove the surface layer, and measurement was performed using X-ray diffraction. Specifically, a Co-Kα radiation source was used for the incident X-rays, and the area fraction of retained austenite was calculated from the intensity ratio of the (200), (211), and (220) planes of ferrite to the (200), (220), and (311) planes of austenite.

[0083] Measurement of Bainite Area Fraction Test specimens taken from the longitudinal center of the steel plate at 1 / 4 of the plate thickness, and test specimens taken from the longitudinal center of the steel pipe at 1 / 4 of the wall thickness, were buffed and then etched with 3 vol% nital. Three fields of view were then observed using an optical microscope at 100x magnification, and scanning electron microscope photographs were taken at appropriate magnifications between 1000x and 5000x to observe bainite. Bainite was visually determined by comparison with the structural photographs in Non-Patent Document 2, and the structural fraction was determined by image analysis using binarized images of bainite and other regions in the optical microscope photographs or SEM photographs based on the above determination. The bainite area fraction was determined as the average of the values ​​determined from the optical microscope photographs or SEM photographs.

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

[0085] Temperature-programmed hydrogen analysis The amount of hydrogen remaining in the steel was measured using temperature-programmed desorption analysis, using a low-temperature temperature-programmed hydrogen analyzer (gas chromatograph type) (JTF-20AL). Temperature-programmed desorption analysis was performed in the temperature range from room temperature to 400°C at a temperature rise rate of 200°C / h, and the sum of the results was taken as the amount of hydrogen. The test specimens were cylindrical, 30 mm long in the longitudinal direction of the steel pipe, and 7Φ in diameter, located at a position 1 / 4 of the way through the plate thickness of the steel plate and a position 1 / 4 of the way from the inner surface of the steel pipe. Note that this amount of hydrogen was measured before the steel was subjected to high-pressure hydrogen fatigue testing, which will be explained later in the section on aging, and is the amount of H shown in Tables 1-1, 1-2, and 1-3.

[0086] Fatigue Test: A fatigue test was conducted at room temperature (20±10°C) in a high-pressure gas atmosphere in air in accordance with ASTM E466, Fatigue Testing, with a frequency of 1 to 15 Hz, a sine wave as the repetitive waveform, load control as the control method, load conditions as uniaxial tension and compression, and a stress ratio of R = -1.0. The stress at which the specimen did not break after 10 million cycles was defined as the fatigue limit in air.

[0087] High-Pressure Hydrogen Fatigue Testing: Fatigue tests were conducted at room temperature (20±10°C) in a hydrogen gas (100% gas) atmosphere at a pressure of 40 MPa, or in a hydrogen gas atmosphere at a pressure of 1 MPa or higher, or in a natural gas atmosphere (mainly composed of hydrocarbons such as methane and ethane) containing hydrogen at a partial pressure of 1 MPa or higher. The fatigue test was conducted in accordance with ASTM E466, Fatigue Testing, under the following conditions: frequency: 1 Hz, repetitive waveform: sinusoidal wave, load control method: load conditions: uniaxial tension and compression, and stress ratio: R = -1.0. The stress at which no fracture occurred after 2 million cycles was defined as the hydrogen fatigue limit. A specimen was deemed to have passed the test if its hydrogen fatigue limit was 200 MPa or higher and the ratio of the hydrogen fatigue limit to the inert gas fatigue limit (the ratio of the hydrogen fatigue limit to the inert gas fatigue limit) to the inert gas fatigue limit was 0.90 or higher.

[0088] All of the inventive examples of the present invention had a fatigue limit stress in hydrogen of 200 MPa or more, and the ratio of the fatigue limit strength in an inert gas atmosphere to the fatigue limit stress in hydrogen / fatigue limit stress in an inert gas environment was 0.90 or more, which indicated excellent hydrogen embrittlement resistance. Furthermore, the tensile strength was 520 MPa or more.

[0089]

[0090]

[0091]

[0092]

[0093]

[0094]

[0095]

[0096] Examples verifying the effects of the present invention will be described below. In the following examples, steel pipes were manufactured under the following manufacturing conditions, and their properties were evaluated. Using steel types No. 1, 15, and 56 shown in Tables 1-1 and 1-2, steel pipes were manufactured under the same conditions as steel materials No. 1, 15, 56, 15-12, and 56-12 shown in Tables 2-1, 2-2, and 3 up to the controlled cooling step, and their properties were evaluated when the dehydrogenation treatment conditions were changed. Steel pipe forming was carried out in the same manner as in Example 1. The results are shown in Table 4.

[0097] In this example, for steel pipe and steel material Nos. 1A, 15A, 56A, 15-12A, and 56-12A, the dehydrogenation treatment temperature T (atmospheric temperature) was set to 50°C, and the holding time tc after the plate thickness center temperature Tc reached 50°C was set to satisfy formula (A). For steel pipe and steel material Nos. 1B, 15B, 56B, 15-12B, and 56-12B, the dehydrogenation treatment temperature T (atmospheric temperature) was set to 50°C, and the holding time tc after the plate thickness center temperature Tc reached 50°C was set to satisfy formula (A). However, the holding time tc after the plate thickness center temperature Tc reached 50°C did not satisfy formula (A). In the cases of 1C, 15C, 56C, 15-12C, and 56-12C, the dehydrogenation treatment temperature T (atmospheric temperature) is 50 ° C., but the holding time t of the atmospheric temperature and the holding time tc after the central temperature Tc of the plate thickness reaches 50 ° C. do not satisfy the above-mentioned formula (A).

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

[0099] Various evaluations were carried out by the methods described in Example 1.

[0100] All of the inventive examples of the present invention had a fatigue stress limit in hydrogen of 200 MPa or more, and the ratio of the fatigue limit strength in an inert gas atmosphere to the fatigue stress limit in hydrogen / fatigue stress limit in an inert gas environment was 0.90 or more. Furthermore, the tensile strength was 520 MPa or more. Among these, the fatigue properties were superior when the dehydrogenation treatment was carried out under more favorable conditions.

[0101]

Claims

1. Containing, by 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: 0.0010% or less, or further containing 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 to 0.10%, A steel material for linepipe having excellent hydrogen embrittlement resistance, the steel material having a chemical composition containing one or more elements selected from 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%, and Sb: 0-0.3%, with the balance being Fe and unavoidable impurity elements, the steel material having an area fraction of retained austenite of 0-3%, an area fraction of bainite of 90% or more at a quarter-thickness position, a fatigue stress limit in hydrogen of 1 MPa or more of 200 MPa or more, and a fatigue stress limit in hydrogen of 1 MPa or more / fatigue stress limit in an inert gas environment of 0.90 or more.

2. Furthermore, the chemical composition, in mass%, is: Nb: 0.001 to 0.10%, Ca: 0.0001 to 0.005%, Ti: 0.005 to 0.1%, Ni: 0.01 to 2.0%, Cu: 0.01 to 1.0%, Cr: 0.01 to 1.0%, Mo: 0.01 to 0.60%, W: 0.01 to 1.0%, V: 0.01 to 0.10%, Zr: 0.0001 to 0.050%, REM: 0.0001 to 0.050%, Mg: 0.0001 to 0.050%, B: 0.0001 to 0.0020%, Hf: 0.0001 to 0.2%, 2. A steel material for line pipes having excellent hydrogen embrittlement resistance according to claim 1, which contains Ta: 0.0001 to 0.2%, Re: 0.0001 to 0.005%, Sn: 0.0001 to 0.3%, and Sb: 0.0001 to 0.3%.

3. A heating step of heating a steel material having the chemical composition according to claim 1 or 2 at 1000 to 1250°C, and rolling the steel material heated in the heating step at a rolling end temperature of Ar. ３ a hot rolling step of rolling the hot rolled steel sheet under conditions of a temperature above 1000°C; and a cooling step of cooling the hot rolled steel sheet obtained in the hot rolling step under conditions of a temperature above 1000°C. ３ a controlled cooling step of cooling the hot-rolled steel plate at a temperature of 750°C to 550°C under the conditions that the difference in cooling start time between the head end and tail end of the hot-rolled steel plate is 50 seconds or less, the average cooling rate from 750°C to 550°C is 15 to 50°C / s at the temperature at the center of the plate thickness, and the cooling stop temperature is 250 to 650°C; and a dehydrogenation treatment step of holding the steel plate obtained in the controlled cooling step at a temperature in the range of room temperature to 550°C.

4. A steel pipe for line pipe containing, by 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: 0.0010% or less, or further containing 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 to 0.10%, A steel pipe for linepipe having excellent hydrogen embrittlement resistance, the steel pipe having a chemical composition containing one or more elements selected from 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%, and Sb: 0-0.3%, with the balance being Fe and unavoidable impurity elements, the steel pipe having an area fraction of retained austenite of 0-3%, and an area fraction of bainite of 90% or more at a wall thickness 1 / 4 position from the inner surface of the steel pipe, a fatigue stress limit in hydrogen of 1 MPa or more of 200 MPa or more, and a fatigue stress limit in hydrogen of 1 MPa or more / a fatigue stress limit in an inert gas environment of 0.90 or more.

5. Furthermore, the chemical composition, in mass%, is: Nb: 0.001 to 0.10%, Ca: 0.0001 to 0.005%, Ti: 0.005 to 0.1%, Ni: 0.01 to 2.0%, Cu: 0.01 to 1.0%, Cr: 0.01 to 1.0%, Mo: 0.01 to 0.60%, W: 0.01 to 1.0%, V: 0.01 to 0.10%, Zr: 0.0001 to 0.050%, REM: 0.0001 to 0.050%, Mg: 0.0001 to 0.050%, B: 0.0001 to 0.0020%, Hf: 0.0001 to 0.2%, 5. A steel pipe for line pipe having excellent hydrogen embrittlement resistance according to claim 4, which contains Ta: 0.0001 to 0.2%, Re: 0.0001 to 0.005%, Sn: 0.0001 to 0.3%, and Sb: 0.0001 to 0.3%.

6. A heating step of heating a steel material having the chemical composition according to claim 4 or 5 at 1000 to 1250°C, and rolling the steel material heated in the heating step at a rolling end temperature of Ar. ３ a hot rolling step of rolling the hot rolled steel sheet under conditions of a temperature above 1000°C; and a cooling step of cooling the hot rolled steel sheet obtained in the hot rolling step under conditions of a temperature above 1000°C. ３ a controlled cooling process in which the hot-rolled steel plate is cooled under conditions where the difference in cooling start times between the head and tail of the hot-rolled steel plate is 50 seconds or less, the average cooling rate from 750°C to 550°C is 15 to 50°C / s at the temperature at the center of the plate thickness, and the cooling stop temperature is 250 to 650°C; either one of a pipe-making process in which, after the controlled cooling process, the hot-rolled steel plate is bent and both ends are butt-welded, or 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 and both circumferential ends of the cylindrical shape are butt-welded; and a dehydrogenation process in which the steel pipe obtained in the pipe-making process is kept at a temperature in the range of room temperature to 550°C.