Steel pipe manufacturing method

Abstract

The purpose of the present invention is to provide: a steel material which has excellent fatigue characteristics in a high-pressure hydrogen gas environment and is suitable for use in a steel structure, which is used in the high-pressure hydrogen gas environment, such as a line pipe for 100% hydrogen gas or natural gas (which is gas mainly composed of hydrocarbons such as methane and ethane) that contains hydrogen having a hydrogen partial pressure of 1 MPa or more; a method for producing the steel material; a steel pipe; and a method for manufacturing the steel pipe.　The steel material has a specific component composition and a specific structure, and has excellent fatigue characteristics in hydrogen, wherein the crack growth rate da / dN at a stress intensity factor (= 20 MPa√m) in hydrogen of at least 1 MPa is at most 1.0×10-6m·cycle-1.

Description

[Technical Field]

[0001] This invention relates to a steel material with excellent fatigue properties in hydrogen, a method for producing the same, a steel pipe, and a method for producing the same. [Background technology]

[0002] Existing energy infrastructure includes pipelines for transporting natural gas. These steel materials have been required to suppress hydrogen-induced cracking in sour environments. Meanwhile, in recent years, hydrogen has attracted significant global attention as a clean energy source for building a decarbonized society. Therefore, to transport large quantities of hydrogen gas, the construction of hydrogen gas transport networks is being considered, either by mixing natural gas with some hydrogen into the pipelines or by using hydrogen gas as a substitute for pressurized transport. The transport pressure during operation of these pipelines is expected to be high, ranging from 1 to 40 MPa, meaning the pipelines will be exposed to a high-pressure hydrogen gas environment. In such environments, there is a concern that steel materials will experience "hydrogen embrittlement," where hydrogen penetrates the steel, degrading its properties. Therefore, in addition to the high toughness and sour resistance required for conventional pipelines, steel materials need to possess the resistance to hydrogen embrittlement required in a hydrogen gas environment.

[0003] For steel structures used in high-pressure hydrogen gas environments, austenitic stainless steels such as SUS316L have traditionally been used because they are less susceptible to hydrogen embrittlement than low-alloy steels. However, austenitic stainless steels such as SUS316L are expensive and have low strength. Therefore, designing them to withstand high hydrogen pressure requires thicker walls, resulting in higher costs for the hydrogen structures themselves. For this reason, there has been a strong demand for low-alloy steels that are more cost-effective and can withstand high-pressure hydrogen gas environments for hydrogen steel structures.

[0004] In response to such demands, 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, it reduces the diffusible hydrogen concentration ratio and suppresses embrittlement due to diffusible hydrogen.

[0005] Patent Document 2 states that by using 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 tensile strength range of 900 to 950 MPa in air, and that it has excellent resistance to embrittlement in a high-pressure hydrogen environment.

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

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

[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 for high-pressure hydrogen storage. The steel material described in Patent Document 6 has a bainite-dominant structure with an area fraction of 90% or more, in which 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

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Patent Document 6

Non - Patent Documents

[0012]

Non - Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[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 material 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 suppress the occurrence of hydrogen-induced cracking in sour environments while also obtaining high fatigue strength in hydrogen gas, which has a greater impact on service life.

[0014] In view of the problems of the prior art described above, the present invention aims to provide a steel material with excellent fatigue characteristics in a high-pressure hydrogen gas environment, a method for producing the same, a steel pipe, and a method for producing the same, which are suitable for steel structures used in a high-pressure hydrogen gas environment, 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).

[0015] In this context, "excellent fatigue characteristics under high-pressure hydrogen gas conditions" means that the fatigue test was conducted in both environments: room temperature (20±10℃) with hydrogen gas at a pressure of 1 MPa or higher, or in a mixed atmosphere of natural gas (mainly composed of hydrocarbons such as methane and ethane) containing hydrogen at a partial pressure of 1 MPa or higher. The test was conducted in accordance with ASTM E647, with a frequency of 1 Hz, repetition waveform: sine wave, control method: load control, and stress ratio: R=0.1. The crack propagation rate da / dN in the stress intensity factor range = 20 MPa√m was 1.0 × 10⁻⁶. -6 m·cycle -1 The following applies: Natural gas containing hydrogen at a partial pressure of 1 MPa or higher means, for example, gas where the hydrogen concentration is 30% or less by volume fraction and the total gas pressure is 30 MPa or less.

[0016] Furthermore, the crack propagation rate da / dN in a hydrogen environment is 1.0 × 10⁻⁶. -6 m·cycle -1 If the following conditions are met, it is possible to design hydrogen-resistant structural steel within the plate thickness range that can be manufactured using the manufacturing process. [Means for solving the problem]

[0017] From the above perspective, the inventors have diligently researched the conditions that various steel materials must satisfy in hydrogen gas, and have discovered new steel materials and steel pipes with excellent fatigue properties in hydrogen.

[0018] This invention was developed based on these new findings and further considerations, and its gist is as follows. [1] In mass%, C: 0.02~0.15%, Si: 0.05~0.5%, Mn: 0.3~2.0%, Al: 0.01~0.15%, N: 0.0005~0.008%, P: 0.03% or less, S: 0.01% or less, O: 0.01% or less, Nb: more than 0% and less than 2.5%, H: 0.0010% or less, Cu: 0~2.5%, Ni: 0-2.5%, Cr: 0-2.5%, Mo: 0~2.0%, V: 0~0.5%, Ti: 0~0.5%, W: 0~2.5%, B: 0~0.005%, Sn: 0~0.03%, Sb: 0~0.3%, Ca: 0~0.01%, Mg: 0~0.01%, REM: 0~0.005% It contains, with the remainder being Fe and unavoidable impurities, and has a component composition. Nb precipitates with an equivalent circle diameter of 2 nm to 100 nm: 10 particles / μm 2 That's all. The stress intensity factor in hydrogen above 1 MPa = the crack propagation rate da / dN at 20 MPa√m is 1.0 × 10⁻⁶ -6 m·cycle -1 The following steel materials exhibit excellent fatigue properties in hydrogen. [2] A heating step of heating a slab having the component composition described in [1] above at 1000 to 1250°C, A hot rolling process is performed in which the slab heated in the aforementioned heating process is rolled at a finish rolling completion temperature of Ar3 or higher. A controlled cooling process is performed to cool the steel sheet obtained in the hot rolling process under the conditions that the average cooling rate from 1000 to 400°C is 10°C / s or more 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 dehydrogenate the steel sheet obtained in the controlled cooling step, A method for manufacturing steel materials having the following characteristics. [3] In mass%, C: 0.02~0.15%, Si: 0.05~0.5%, Mn: 0.3~2.0%, Al: 0.01~0.15%, N: 0.0005~0.008%, P: 0.03% or less, S: 0.01% or less, O: 0.01% or less, Nb: more than 0% and less than 2.5%, H: 0.0010% or less, Cu: 0~2.5%, Ni: 0-2.5%, Cr: 0-2.5%, Mo: 0~2.0%, V: 0~0.5%, Ti: 0~0.5%, W: 0~2.5%, B: 0~0.005%, Sn: 0~0.03%, Sb: 0~0.3%, Ca: 0~0.01%, Mg: 0 to 0.01%, REM: 0 to 0.005% containing the same, and having a component composition consisting of the balance of Fe and inevitable impurities, with 10 or more Nb precipitates having a circle equivalent diameter of 2 nm or more and 100 nm or less per μm 2 or more, the crack propagation rate da / dN at a stress intensity factor in hydrogen of 1 MPa or more = 20 MPa√m is 1.0×10 -6 m·cycle -1 or less, and a steel pipe excellent in fatigue characteristics in hydrogen. [4] A heating step of heating a slab having the component composition according to [3] at 1000 to 1250°C, a hot rolling step of rolling the slab heated in the heating step under the condition that the finish rolling end temperature is above the Ar3 point, a controlled cooling step of cooling the hot rolled steel sheet obtained in the hot rolling step under the conditions that the average cooling rate from 1000 to 400°C is 10°C / s or more at the temperature at the center of the plate thickness and the cooling stop temperature is 250 to 650°C, after the controlled cooling step, a pipe manufacturing step of bending the hot rolled steel sheet and butt-welding both ends, and after the controlled cooling step, either a pipe manufacturing step of forming the hot rolled steel sheet into a cylindrical shape by cold roll forming and butt-welding both circumferential ends of the cylindrical shape by electric seam welding, a dehydrogenation treatment step of dehydrogenating the steel pipe obtained in the pipe manufacturing step, and a method for manufacturing a steel pipe having the same.

Advantages of the Invention

[0019] According to the present invention, it is possible to obtain a steel material and a steel pipe having extremely excellent fatigue characteristics in hydrogen in a high-pressure hydrogen gas environment, which is extremely useful industrially.

Embodiments for Carrying Out the Invention

[0020] 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.

[0021] First Embodiment [Component composition] The reasons for the limitations on the component composition of the steel material of the present invention are explained below. In the following explanation, "%" refers to "mass%" unless otherwise specified.

[0022] C: 0.02~0.15% Carbon (C) is included to ensure adequate hardenability, but its effect is insufficient if it is less than 0.02%. Therefore, the C content should be 0.02% or more. A C content of 0.03% or more is preferable. On the other hand, if it exceeds 0.15%, the toughness of the base material and the heat-affected zone of the weld deteriorates, and the weldability deteriorates significantly. Therefore, the C content should be 0.15% or less. A C content of 0.12% or less is preferable. A C content of 0.10% or less is more preferable, and 0.08% or less is even more preferable.

[0023] Si: 0.05~0.5% Si is included as a deoxidizing agent and an element that ensures hardenability during the steelmaking process, but its effect is insufficient if it is less than 0.05%. For this reason, the Si content should be 0.05% or more. A Si content of 0.1% or more is preferable. A Si content of 0.15% or more is more preferable. On the other hand, if it exceeds 0.5%, the grain boundaries become brittle, degrading the low-temperature toughness and fatigue properties in hydrogen. Therefore, the Si content should be 0.5% or less. A Si content of 0.4% or less is preferable. A Si content of 0.3% or less is more preferable, and 0.25% or less is even more preferable.

[0024] Mn: 0.3~2.0% Mn is included as an element to ensure hardenability, but its effect is insufficient if it is less than 0.3%. For this reason, the Mn content should be 0.3% or more. A Mn content of 0.4% or more is preferable. A Mn content of 0.5% or more is more preferable. A Mn content of 0.6% or more is even more preferable. On the other hand, if the content exceeds 2.0%, the grain boundary strength decreases and the low-temperature toughness deteriorates. Also, the hardness of the surface layer and the central segregation increases during controlled cooling, which deteriorates the fatigue properties in hydrogen. For this reason, the Mn content should be 2.0% or less. A Mn content of 1.8% or less is preferable. A Mn content of 1.5% or less is more preferable, and 1.3% or less is even more preferable.

[0025] Al: 0.01~0.15% Al is included as a deoxidizing agent and, as fine precipitates of Al-based nitride, pinns austenite grains during heating, suppressing grain coarsening. However, this effect is insufficient if the Al content is less than 0.01%. Therefore, the Al content should be 0.01% or more. An Al content of 0.02% or more is preferable. An Al content of 0.03% or more is more preferable. On the other hand, if the Al content exceeds 0.15%, the cleanliness of the steel decreases, and the toughness and fatigue properties in hydrogen deteriorate. Therefore, the Al content should be 0.15% or less. An Al content of 0.12% or less is preferable. An Al content of 0.10% or less is more preferable, and 0.08% or less is even more preferable.

[0026] N: 0.0005~0.008% N is included because it forms fine precipitates by forming nitrides with Nb, Ti, Al, etc., and pinning the austenite grains during heating, thereby suppressing grain coarsening and improving low-temperature toughness. A content of less than 0.0005% does not provide sufficient microstructure refinement. Therefore, the N content should be 0.0005% or more. A N content of 0.001% or more is preferred. A N content of 0.0025% or more is more preferred. On the other hand, a content exceeding 0.008% increases the amount of dissolved N, impairing the toughness of the base material and the heat-affected zone of the weld, and degrading the fatigue properties in hydrogen. Therefore, the N content should be 0.008% or less. A N content of 0.007% or less is preferred. A N content of 0.006% or less is more preferred, and 0.005% or less is even more preferred.

[0027] P:0.03% or less The impurity element P tends to segregate at grain boundaries, and if its content exceeds 0.03%, it reduces the bonding strength between adjacent grains, degrading low-temperature toughness and fatigue properties in hydrogen. Therefore, the P content should be 0.03% or less. Preferably, the P content is 0.02% or less, and more preferably 0.01% or less. There is no particular lower limit, but it is preferable to set it at 0.001% or more to avoid increased costs.

[0028] S: 0.01% or less S, an impurity element, tends to segregate at grain boundaries and readily forms nonmetallic inclusions such as MnS. If the S content exceeds 0.01%, the bonding strength between adjacent grains decreases, the amount of inclusions increases, and the low-temperature toughness and fatigue properties in hydrogen deteriorate. Therefore, the S content should be 0.01% or less. Preferably, the S content should be 0.008% or less. More preferably, the S content should be 0.005% or less, and even more preferably 0.002% or less. There is no particular lower limit, but it is preferable to have a lower limit of 0.0001% or more because it leads to increased costs. More preferably, the S content should be 0.001% or more.

[0029] O: 0.01% or less Since oxygen (O) forms oxides with aluminum (Al) and other elements, affecting the processability of the material, a lower amount is preferable. An O content exceeding 0.01% increases inclusions and impairs processability. Furthermore, the fatigue properties in hydrogen deteriorate with increasing inclusions. Therefore, the O content should be 0.01% or less. A preferred O content is 0.008% or less, and more preferably 0.005% or less. While there is no particular lower limit, it is preferable to have an O content of 0.0001% or more to avoid increased costs. A preferred O content is 0.001% or more.

[0030] Nb: More than 0% and less than 2.5% Nb has the effect of improving hardenability and, as fine precipitates of Nb-based carbonitrides, pinns austenite grains during heating, suppressing grain coarsening. The smaller the grain size, the greater the grain boundary area and the better the fatigue properties in hydrogen. For this reason, the Nb content should be greater than 0%. Preferably, the Nb content should be 0.005% or more. More preferably, the Nb content should be 0.01% or more. On the other hand, a content exceeding 2.5% deteriorates the toughness of the heat-affected zone during welding. Therefore, the Nb content should be 2.5% or less. More preferably, the Nb content should be 2.2% or less. Even more preferably, the Nb content should be 2.0% or less, and most preferably 1.5% or less.

[0031] H:0.0010% or less Hydrogen (H) can be introduced into steel materials during various manufacturing processes. High levels of H increase the risk of crack formation after solidification and accelerate fatigue crack propagation. Furthermore, high levels of H increase the crack propagation rate, making it important to reduce the amount of hydrogen in the steel material. These effects are not problematic if the H content is 0.0010% or less, so the H content should be 0.0010% or less. Preferably, it should be 0.0005% or less. More preferably, it should be 0.0002% or less. On the other hand, since a H content of less than 0.00001% increases costs, it is preferable to have an H content of 0.00001% or more. A H content of 0.0001% or more is even more preferable. Note that the hydrogen content is the residual hydrogen after forming of steel materials, steel pipes, UOE, etc. In this invention, the above H content can be achieved by performing a dehydrogenation treatment process.

[0032] In the present invention, it is preferable that the steel composition consists of Fe and unavoidable impurities as the remainder of the above component composition. However, depending on the desired properties, it is also preferable to include one or more of the following, individually or simultaneously: Cu: 0-2.5%, Ni: 0-2.5%, Cr: 0-2.5%, Mo: 0-2.0%, V: 0-0.5%, Ti: 0-0.5%, W: 0-2.5%, B: 0-0.005%, Sn: 0-0.03%, Sb: 0-0.3%, Ca: 0-0.01%, Mg: 0-0.01%, and REM: 0-0.005%.

[0033] Cu: 0~2.5% Cu has the effect of improving hardenability. Therefore, when Cu is included, the Cu content may be 0% or more, but since the above effect is difficult to obtain if it is less than 0.05%, it is preferable that the Cu content be 0.05% or more. If the Cu content exceeds 2.5%, hot cracking is likely to occur when heating or welding the steel billet. Therefore, when Cu is included, it should be 2.5% or less. A Cu content of 2.3% or less is preferable. A Cu content of 2.0% or less is more preferable, and 1.8% or less is even more preferable.

[0034] Ni: 0~2.5% Ni, like Cu, has the effect of improving hardenability and also improves toughness. Therefore, when Ni is included, the Ni content may be 0% or more, but since the above effects are difficult to obtain if it is less than 0.05%, it is preferable that the Ni content be 0.05% or more. If it exceeds 2.5%, it becomes economically unfeasible. Therefore, when Ni is included, the Ni content should be 2.5% or less. A Ni content of 2.3% or less is preferable. A Ni content of 2.0% or less is more preferable, and 1.8% or less is preferable.

[0035] Cr: 0-2.5% Cr is an element that ensures hardenability. When Cr is included, the Cr content may be 0% or more, but since the above effects are difficult to obtain if it is less than 0.1%, it is preferable that the Cr content be 0.1% or more. On the other hand, if the content exceeds 2.5%, the weldability deteriorates. Therefore, when Cr is included, the Cr content should be 2.5% or less. A Cr content of 2.3% or less is preferable. A Cr content of 2.0% or less is more preferable, 1.5% or less is even more preferable, and 1.2% or less is most preferable.

[0036] Mo: 0~2.0% Mo has the effect of improving hardenability, so if Mo is included, the Mo content may be 0% or more, but if it is less than 0.05%, the above effect is difficult to obtain, so it is preferable that the Mo content be 0.05% or more. On the other hand, a content exceeding 2.0% is economically unfeasible. Therefore, if Mo is included, the Mo content should be 2.0% or less. A Mo content of 1.8% or less is preferable. A Mo content of 1.5% or less is more preferable, and 1.2% or less is even more preferable.

[0037] V: 0~0.5% V has the effect of improving hardenability and, as fine precipitates of V-based carbides, pinns austenite grains during heating, suppressing grain coarsening. Therefore, when V is included, the V content may be 0% or more, but since the above effect is difficult to obtain if it is less than 0.005%, it is preferable that the V content be 0.005% or more. It is preferable that the V content be 0.01% or more. On the other hand, adding more than 0.5% deteriorates the toughness of the heat-affected zone during welding. Therefore, when V is included, the V content should be 0.5% or less. It is preferable that the V content be 0.4% or less. It is more preferable that the V content be 0.3% or less, and even more preferable that it be 0.2% or less.

[0038] Ti: 0~0.5% Ti has the effect of improving hardenability and, as fine precipitates of Ti-based carbonitrides, pinns austenite grains during heating, thereby suppressing grain growth. Therefore, when Ti is included, the Ti content may be 0% or more, but since the above effect is difficult to obtain if it is less than 0.005%, it is preferable that the Ti content be 0.005% or more. On the other hand, adding more than 0.5% deteriorates the toughness of the heat-affected zone of the weld. Therefore, when Ti is included, the Ti content should be 0.5% or less. A Ti content of 0.4% or less is preferable. A Ti content of 0.3% or less is more preferable, and 0.2% or less is even more preferable.

[0039] W: 0~2.5% Since W has the effect of improving hardenability, if W is included, the W content may be 0% or more, but if it is less than 0.05%, the above effect is difficult to obtain, so it is preferable that the W content be 0.05% or more. On the other hand, if it exceeds 2.5%, the weldability deteriorates. Therefore, if W is included, the W content should be 2.5% or less. A W content of 2.3% or less is preferable. A W content of 2.0% or less is more preferable, and 1.8% or less is even more preferable.

[0040] B: 0~0.005% Since B is included as an element to ensure hardenability, if B is included, the B content may be 0% or more, but if it is less than 0.0005%, the above effect is difficult to obtain, so it is preferable that the B content be 0.0005% or more. On the other hand, if it exceeds 0.005%, the toughness deteriorates. Therefore, if B is included, the B content should be 0.005% or less. A B content of 0.004% or less is preferable. A B content of 0.003% or less is more preferable, and 0.002% or less is even more preferable.

[0041] Sn: 0~0.03% Sn has the effect of improving the corrosion resistance of steel. Therefore, when Sn is included, the Sn content may be 0% or more, but since the above effect is difficult to obtain if it is less than 0.005%, it is preferable that the Sn content be 0.005% or more. On the other hand, adding more than 0.03% reduces high-temperature ductility and increases the possibility of cracking during casting. Therefore, when Sn is included, the Sn content should be 0.03% or less. A Sn content of 0.025% or less is preferable. A Sn content of 0.02% or less is more preferable, and 0.015% or less is even more preferable.

[0042] Sb: 0~0.3% Sb has the effect of improving the corrosion resistance of steel. Therefore, when Sb is included, the Sb content may be 0% or more, but since the above effect is difficult to obtain if it is less than 0.005%, it is preferable that the Sb content be 0.005% or more. It is more preferable that the Sb content be 0.01% or more. On the other hand, adding more than 0.3% reduces high-temperature ductility and hot-rollability. Therefore, when Sb is included, the Sb content should be 0.3% or less. It is preferable that the Sb content be 0.25% or less. It is more preferable that the Sb content be 0.2% or less, and even more preferable that be 0.15% or less.

[0043] Ca: 0~0.01% Ca has the effect of controlling the morphology of sulfide inclusions, transforming them into spherical inclusions of CaS, which are difficult to stretch by rolling, instead of MnS, which are easily stretched by rolling. Therefore, when Ca is included, the Ca content may be 0% or more, but the above effect is difficult to obtain if it is less than 0.0005%, so it is preferable that the Ca content be 0.0005% or more. It is preferable that the Ca content be 0.001% or more. On the other hand, if the content exceeds 0.01%, the cleanliness decreases, and the material properties such as toughness deteriorate. Therefore, when Ca is included, the Ca content should be 0.01% or less. It is preferable that the Ca content be 0.005% or less. It is more preferable that the Ca content be 0.003% or less, and even more preferable that it be 0.002% or less.

[0044] Mg: 0~0.01% Mg is sometimes used as a desulfurizing agent for molten iron. Therefore, when Mg is included, the Mg content may be 0% or more, but since the above effects are difficult to obtain if it is less than 0.0005%, it is preferable that the Mg content be 0.0005% or more. It is preferable that the Mg content be 0.001% or more. On the other hand, adding more than 0.01% leads to a decrease in cleanliness. Therefore, when Mg is included, the Mg content should be 0.01% or less. It is preferable that the Mg content be 0.005% or less. It is more preferable that the Mg content be 0.004% or less, and even more preferable that it be 0.003% or less.

[0045] REM: 0~0.005% REM improves resistance to SR cracking by reducing the amount of dissolved sulfur at grain boundaries through the formation of sulfurides as REM(O,S) in steel. Therefore, when REM is included, the REM content may be 0% or more, but since the above effect is difficult to obtain below 0.0005%, it is preferable that the REM content be 0.0005% or more. On the other hand, adding more than 0.005% leads to a significant accumulation of REM sulfides in the precipitated crystal zone, causing deterioration of the material. Therefore, when REM is included, the REM content should be 0.005% or less. A REM content of 0.003% or less is preferable. A REM content of 0.001% or less is even more preferable. REM stands for Rare Earth Metal, and refers to rare earth metals.

[0046] 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.

[0047] The metallographic structure of the present invention will be described in detail.

[0048] Nb precipitates with an equivalent circular diameter of 2 nm to 100 nm: 10 particles / μm 2 That's all. The pinning effect of Nb can be confirmed by obtaining a predetermined amount of precipitate. In this invention, the Nb precipitate size is 10 particles / μm with an equivalent circular diameter of 2 nm to 100 nm. 2 The above is correct. Preferably, there are 15 Nb precipitates / μm with an equivalent diameter of 2 nm to 100 nm. 2 The above is more preferable: 20 particles / μm 2 The above is preferable to 25 particles / μm 2 That concludes the explanation. The upper limit is set at 100 Nb precipitates / μm with an equivalent circle diameter of 2 nm to 100 nm, because excessive precipitate formation worsens hydrogen embrittlement. 2 The following is preferable: 90 particles / μm 2 More preferably, the following is true: 80 particles / μm 2 The following is even more preferable. For example, Nb precipitates include NbC, NbN, and Nb carbonitride. The cooling stop temperature is important for the precipitation of fine Nb precipitates, as will be discussed later. The reason for focusing on Nb precipitates larger than 2 nm is that they are difficult to confirm, and the reason for focusing on Nb precipitates smaller than 100 nm is that if the Nb precipitates become too coarse, it will negatively affect hydrogen embrittlement.

[0049] The structure of the present invention is not particularly limited, but the main structure has bainite, preferably 60% or more. More preferably 80% or more bainite, and even more preferably 90% or more. The bainite may also be 100%.

[0050] The stress intensity factor in hydrogen above 1 MPa = the crack propagation rate da / dN at 20 MPa√m is 1.0 × 10⁻⁶ -6 m·cycle -1 below Fatigue crack propagation rate is an important parameter in the design of gas containers such as line pipes and gas cylinders, and is necessary to ensure the safety and service life of the fracture structural members. In fracture structural members, it is difficult to eliminate cracks or crack initiation sites entirely, and cracks inevitably occur and propagate when subjected to repeated stress. Therefore, controlling the crack propagation rate of the steel used in the above fracture structural members is important from the viewpoint of service life. The crack propagation rate is small when the stress state at the crack tip is small, and increases as the stress state at the crack tip increases. In a hydrogen environment, hydrogen penetrates the steel material, making it easier for cracks to propagate. The degree to which hydrogen accelerates the crack propagation rate is greatly influenced by the material's microstructure and precipitates. In a crack propagation test in hydrogen at 1 MPa or higher, a fatigue test was conducted in accordance with ASTM E647, with a frequency of 1 Hz, repetition waveform: sine wave, control method: load control, and stress ratio: R=0.1. The crack propagation rate da / dN at a stress intensity factor of 20 MPa√m was found to be 1.0 × 10⁻¹⁰. -6 m·cycle -1 The following conditions ensure the service life of steel structures in a high-pressure hydrogen environment: In a crack propagation test in hydrogen at 1 MPa or higher, the crack propagation rate da / dN at a stress intensity factor of 20 MPa√m is 1.0 × 10⁻⁶. -6 m·cycle -1 The following applies: The crack propagation rate da / dN at a stress intensity factor of 20 MPa√m is preferably 0.9 × 10 -6 m·cycle -1 The following, more preferably 0.8 × 10 -6 m·cycle -1 The following, and more preferably 0.7 × 10 -6 m·cycle -1 The following applies: Regarding the lower limit, the closer the result is to and exceeds the atmospheric result, the better it can be considered. The crack propagation rate da / dN at an atmospheric stress intensity factor of 20 MPa√m is 0.05 × 10 -6 m·cycle -1 Therefore, the crack propagation rate da / dN at a stress intensity factor of 20 MPa√m is 0.05 × 10 -6 m·cycle -1 That's all you need.

[0051] Furthermore, while the thickness of the steel plate is not particularly limited, a plate thickness of 4 mm or more is preferred. A plate thickness of 5 mm or more is more preferred. Also, a plate thickness of 70 mm or less is preferred. A plate thickness of 30 mm or less is more preferred.

[0052] The steel material exhibiting excellent fatigue properties in hydrogen gas according to the present invention is not particularly limited, as long as it has the above-mentioned component composition and metal structure and satisfies the crack propagation rate in hydrogen.

[0053] The method for producing the steel material with excellent fatigue properties in hydrogen, according to the present invention, will be described below, using a thick plate, which is a steel material with excellent fatigue properties in hydrogen, as an example.

[0054] The steel material of the present invention can be manufactured by sequentially performing a heating process, a hot rolling process, a controlled cooling process, and a dehydrogenation treatment process. Furthermore, the steel material exhibiting excellent fatigue properties in hydrogen gas according to the present invention includes various classifications such as thin plates, thick plates, and steel pipes having the above-mentioned component composition and exhibiting excellent fatigue crack propagation characteristics in hydrogen, or it may be a steel material for hydrogen pipelines formed into a predetermined shape.

[0055] In the following description of manufacturing methods, unless otherwise specified, the temperature refers to the temperature at the center of the plate thickness of steel materials, steel products, and steel pipes. The temperature at the center of the plate thickness of steel products, etc., can be determined by calculating the temperature distribution within the cross-section of the steel plate, etc., using heat transfer analysis, and correcting the result by the surface temperature of the steel plate, etc. Note that the term "hot-rolled steel sheet" includes hot-rolled plates and hot-rolled steel strips.

[0056] In this invention, the method for melting the steel material (steel slab) is not particularly limited. For example, any known melting method such as a converter, electric furnace, or vacuum melting furnace is suitable. The casting method is also not particularly limited. For example, the steel material can be manufactured to the desired dimensions by a known casting method such as continuous casting. There is no problem in applying the ingot-parting rolling method instead of continuous casting. The molten steel may be further refined, such as ladle refining.

[0057] heating process Heating temperature: 1000℃ or higher and 1250℃ or lower If the heating temperature is less than 1000°C, the deformation resistance of the material to be rolled increases, making rolling difficult. For this reason, the heating temperature should be 1000°C or higher. Preferably, the heating temperature is 1050°C or higher, more preferably 1100°C or higher, and even more preferably 1120°C or higher. On the other hand, if the heating temperature exceeds 1250°C, the austenite grains become coarser, and fine austenite grains cannot be obtained in subsequent rolling (rough rolling, finish rolling), and the fatigue resistance in hydrogen decreases. For this reason, the heating temperature in the hot rolling process should be 1250°C or lower. Preferably, the heating temperature is 1230°C or lower. More preferably, the heating temperature is 1210°C or lower, and even more preferably 1200°C or lower.

[0058] Furthermore, in addition to the conventional method of manufacturing steel slabs (slabs), cooling them to room temperature, and then reheating them, the present invention can also be applied without any problems to energy-saving direct rolling processes, such as loading the slabs into the heating furnace while still hot without cooling them to room temperature, or rolling them immediately after a short period of heat retention.

[0059] Hot rolling process After heating as described above, hot rolling is performed, including rough rolling and finish rolling. Finish rolling is carried out under the following conditions. Hot rolling is performed using a hot rolling mill.

[0060] Finishing rolling completion temperature: Ar3 point or higher If the finish rolling completion temperature is below the Ar3 point, the steel sheet surface temperature falls below the ferrite transformation initiation temperature during finish rolling, generating processed ferrite with high dislocation density and reducing fatigue properties in hydrogen. For this reason, the finish rolling completion temperature should be at or above the Ar3 point. If the Ar3 point is lower than 770°C, the finish rolling completion temperature is preferably 770°C or higher. If the Ar3 point is higher than 770°C, the finish rolling completion temperature is preferably at or above the Ar3 point + 30°C, and more preferably at or above the Ar3 point + 50°C. On the other hand, there is no particular upper limit specified for the finish rolling completion temperature, but if the Ar3 point is lower than 850°C, exceeding 850°C may result in insufficient reduction in the austenite non-recrystallization temperature range, preventing the acquisition of fine austenite grains and potentially reducing fatigue resistance in hydrogen. For this reason, if the Ar3 point is lower than 850°C, the finish rolling completion temperature is preferably 850°C or lower, more preferably 830°C or lower. If the Ar3 point is higher than 850°C, the finish rolling completion temperature is preferably Ar3 point - 30°C or lower, and more preferably Ar3 point - 50°C or lower.

[0061] In this invention, the finished plate thickness (the thickness of the steel plate after finish rolling) is preferably 4 mm or more. On the other hand, there is no particular upper limit specified for the finished plate thickness, but from the viewpoint of temperature control of the steel plate, it is preferably 70 mm or less.

[0062] 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 %).

[0063] Controlled cooling process Average cooling rate at the center of the plate thickness from 1000 to 400°C: 10°C / s or higher If the average cooling rate in the center of the plate thickness is less than 10°C / s, the nucleation frequency of ferrite and bainite decreases, and these coarseen, thus degrading the fatigue properties in hydrogen. For this reason, the average cooling rate in the center of the plate thickness from 1000 to 400°C should be 10°C / s or higher. Preferably, it should be 12°C / s or higher, more preferably 15°C / s or higher, and even more preferably 18°C / s or higher. On the other hand, although there is no particular upper limit specified for the average cooling rate, if it exceeds 60°C / s, a large amount of hard structure is formed on the surface of the steel plate, and a steel structure having the structure targeted in the present invention cannot be obtained, resulting in degraded fatigue properties in hydrogen. For this reason, it is preferable that the average cooling rate in the center of the plate thickness be 60°C / s or lower. More preferably, the average cooling rate in the center of the plate thickness is 55°C / s or lower, and even more preferably 50°C / s or lower.

[0064] Cooling stop temperature at the center of the plate thickness: 250°C to 650°C If the cooling stop temperature at the center of the plate thickness is less than 250°C, the cooling stop temperature on the steel plate surface becomes too low, resulting in the formation of a large amount of hard structure on the steel plate surface. This prevents the acquisition of a steel structure with the structure desired in this invention, and reduces the fatigue properties in hydrogen. For this reason, the cooling stop temperature at the center of the plate thickness should be 250°C or higher. Preferably, it should be 280°C or higher. More preferably, it should be 300°C or higher. Even more preferably, it should be 390°C or higher, and most preferably, 480°C or higher. On the other hand, if the cooling stop temperature at the center of the plate thickness exceeds 650°C, the nucleation frequency of ferrite or bainite decreases, and these coarseen, preventing the acquisition of a structure with the average grain size desired in this invention. This reduces strength, and residual heat further coarses Nb precipitates. For this reason, the cooling stop temperature at the center of the plate thickness should be 650°C or lower. The cooling stop temperature at the center of the plate thickness should preferably be 620°C or lower, more preferably 600°C or lower, and even more preferably 580°C or lower. Most preferably, it should be 550°C or lower. Furthermore, it is most preferably 500°C or lower.

[0065] Furthermore, when manufacturing thin steel sheets, it is necessary to wind them into a coil after cooling stops, and for the same reasons as above, the temperature at which they are wound into a coil after cooling stops is important. The winding temperature should be 650°C or lower. Preferably, it should be 620°C or lower, more preferably 600°C or lower, and even more preferably 580°C or lower. Most preferably, it should be 550°C or lower. And most preferably, it should be 500°C or lower. On the other hand, if the winding temperature is too low, winding becomes difficult and a steel structure with the desired structure cannot be obtained, so it should be 250°C or higher. Preferably, it should be 280°C or higher. More preferably, it should be 300°C or higher. Even more preferably, it should be 390°C or higher, and most preferably, it should be 480°C or higher.

[0066] Furthermore, while there are no particular restrictions on the cooling stop temperature of the steel plate surface, if it is below 250°C, a large amount of hard structure will be formed on the steel plate surface, and a steel structure with the target microstructure fraction in this invention cannot be obtained, resulting in a decrease in fatigue resistance in hydrogen. For this reason, it is preferable that the cooling stop temperature of the steel plate surface be 250°C or higher. More preferably, the cooling stop temperature of the steel plate surface is 280°C or higher. On the other hand, if such a cooling stop temperature exceeds 650°C, the cooling stop temperature in the center of the plate thickness becomes too high, reducing the frequency of ferrite or bainite nucleation in the center of the plate thickness, and causing them to coarseen, so a structure with the target average grain size in this invention cannot be obtained, resulting in a decrease in strength. For this reason, it is preferable that the cooling stop temperature of the plate surface be 650°C or lower. More preferably, the cooling stop temperature of the steel plate surface is 470°C or lower.

[0067] In this invention, unless otherwise specified, the average cooling rate is the value (cooling rate) obtained by ((center temperature of the steel sheet thickness before cooling - center temperature of the hot-rolled steel sheet thickness after cooling) / cooling time).

[0068] Furthermore, cooling methods include water cooling, such as spraying water from a nozzle, and cooling by spraying a cooling gas. In the present invention, it is preferable to perform a cooling operation (treatment) on both sides of the steel plate so that both sides of the steel plate are cooled under the same conditions.

[0069] Dehydrogenation treatment process The presence of hydrogen in steel accelerates fatigue crack propagation, reducing fatigue life and hydrogen-induced fatigue crack propagation rate. Therefore, dehydrogenation treatment is necessary to release any remaining 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-induced fatigue properties in a high-pressure hydrogen gas environment. The holding time R (sec) is determined by the plate thickness and pipe thickness t (mm) of the steel material and steel pipe, 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 2 You may use / s. More preferably 5×10 -4 mm 2 It is less than or equal to / s. 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 is preferably 550°C or lower. It is more preferable that the dehydrogenation treatment temperature T be 500°C or lower. It is even more preferable that the dehydrogenation treatment temperature T be 400°C or lower, and most preferably 300°C or lower. Furthermore, it is preferable that the dehydrogenation treatment temperature T be above room temperature because dehydrogenation treatment at temperatures lower than room temperature increases processing time and costs. It is more preferable that the dehydrogenation treatment temperature T be 50°C or higher. It is even more preferable that the dehydrogenation treatment temperature T be 100°C or higher, and most preferably 150°C or higher. The dehydrogenation treatment temperature T mentioned here refers to the temperature of the atmosphere during the dehydrogenation treatment process. Room temperature is defined as 20 ± 10°C.

[0070] 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 ambient temperature satisfies the holding time R (sec), 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 steel material for at least the holding time R (sec) after the temperature Tc at the center of the thickness reaches the target dehydrogenation treatment temperature T. Furthermore, in order to obtain a predetermined crack propagation rate in hydrogen gas, 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 steel material for at least the R (sec) specified by equation (A) at the dehydrogenation treatment temperature T, and it is even more preferable to hold the steel material for at least the holding time R (sec) after the temperature Tc at the center of the thickness reaches the target dehydrogenation treatment temperature T. In other words, at least the former allows for appropriate control of the amount of hydrogen in the surface of the steel material and steel pipe, and if the latter is also implemented, the amount of hydrogen in the steel material from the surface to the center of the thickness can be appropriately controlled. The central temperature Tc of the plate thickness can be measured using thermocouples or other methods, or it can be predicted using methods such as the finite element method.

[0071] 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 to this, 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.

[0072] Second Embodiment The following describes in detail the UOE steel pipe, which is an example of the steel pipe of this invention. The composition, microstructure, and crack propagation rate of the steel pipe are the same as those described for the steel material, and the manufacturing method, including the heating process, hot rolling process, controlled cooling process, and dehydrogenation treatment process, is carried out in the same manner as described for the steel material. The pipe manufacturing process is carried out as follows.

[0073] Pipe making process The UOE steel pipe of the present invention is manufactured by bending a hot-rolled steel sheet, specifically by beveling the ends of the hot-rolled steel sheet, forming it into a steel pipe shape using a C-press, U-press, or 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. The welding method is not particularly limited, but submerged arc welding is one example. Furthermore, the plate thickness of the UOE steel pipe, which is listed as an example of the steel pipe of the present invention, is preferably 10 mm or more and 50 mm or less.

[0074] Third Embodiment The following describes an electric resistance welded (ERW) steel pipe, which is an example of the steel pipe of the present invention. The component composition, metal structure, and crack propagation rate of the ERW steel pipe are the same as those described for steel materials, and the manufacturing method, including the heating process, hot rolling process, and dehydrogenation treatment process, is also carried out in the same manner as described for steel materials. The controlled cooling process after hot rolling and the pipe manufacturing process are carried out as follows.

[0075] Controlled cooling process Average cooling rate at the center of the plate thickness from 1000 to 400°C: 10°C / s or higher If the average cooling rate at the center of the plate thickness from 1000 to 400°C is less than 10°C / s, the nucleation frequency of ferrite and bainite decreases, and these coarseen, thus degrading the fatigue properties in hydrogen. For this reason, the average cooling rate at the center of the plate thickness from 1000 to 400°C should be 10°C / s or higher. Preferably, it should be 12°C / s or higher, more preferably 15°C / s or higher, and even more preferably 18°C / s or higher. On the other hand, although there is no particular upper limit specified for the average cooling rate, if it exceeds 60°C / s, a large amount of hard structure is formed on the surface of the steel plate, and a steel structure having the structure targeted in the present invention cannot be obtained, resulting in degraded fatigue properties in hydrogen. For this reason, it is preferable that the average cooling rate at the center of the plate thickness be 60°C / s or lower. More preferably, the average cooling rate at the center of the plate thickness is 55°C / s or lower, and even more preferably 50°C / s or lower.

[0076] Cooling stop temperature at the center of the plate thickness: 250°C to 650°C If the cooling stop temperature at the center of the plate thickness is less than 250°C, the cooling stop temperature at the surface of the steel plate becomes too low, resulting in the formation of a large amount of hard structure on the surface of the steel plate. This prevents the steel structure having the structure desired in this invention from being obtained, and the fatigue properties in hydrogen decrease. For this reason, the cooling stop temperature at the center of the plate thickness should be 250°C or higher. Preferably, it should be 280°C or higher. More preferably, it should be 300°C or higher. Even more preferably, the cooling stop temperature at the center of the plate thickness should be 390°C or higher. To reliably suppress the formation of hard structure on the surface of the steel plate, the cooling stop temperature at the center of the plate thickness should most preferably be 450°C or higher. Even more preferably, it should be 480°C or higher. On the other hand, if the cooling stop temperature exceeds 650°C, the nucleation frequency of ferrite or bainite decreases, and these coarseen, so the structure having the average grain size desired in this invention cannot be obtained, and the strength decreases. In addition, Nb precipitates coarseen due to residual heat. For this reason, the cooling stop temperature at the center of the plate thickness should be 650°C or lower. Preferably, the temperature is 620°C or lower, more preferably 600°C or lower, and even more preferably 580°C or lower. Most preferably, it is 550°C or lower. And most preferably, it is 500°C or lower.

[0077] Subsequently, the hot-rolled steel sheet is wound into a coil. The winding temperature is preferably 650°C or lower.

[0078] Furthermore, while there are no particular restrictions on the cooling stop temperature of the steel plate surface, if it is below 250°C, a large amount of hard structure will be formed on the steel plate surface, and the steel structure having the structure desired in this invention cannot be obtained, resulting in a decrease in fatigue properties in hydrogen. On the other hand, if the cooling stop temperature of the steel plate surface exceeds 650°C, the cooling stop temperature in the center of the plate thickness becomes too high, reducing the frequency of ferrite or bainite nucleation in the center of the plate thickness, and causing these to coarseen, resulting in a decrease in strength and preventing the formation of the structure desired in this invention. The cooling stop temperature of the steel plate surface is preferably 280°C or higher, and preferably 470°C or lower.

[0079] In this invention, unless otherwise specified, the average cooling rate is the value (cooling rate) obtained by ((temperature at the center of the thickness of the hot-rolled steel sheet before cooling - temperature at the center of the thickness of the hot-rolled steel sheet after cooling) / cooling time).

[0080] Furthermore, cooling methods include water cooling, such as spraying water from a nozzle, and cooling by spraying a cooling gas. In the present invention, it is preferable to perform a cooling operation (treatment) on both sides of the steel plate so that both sides of the steel plate are cooled under the same conditions.

[0081] Pipe making process An example of a steel pipe of the present invention is an electric resistance welded (ERW) steel pipe, which is manufactured by forming a hot-rolled steel sheet into a cylindrical shape by cold roll forming, and then butt welding the circumferential ends of the cylindrical shape (pipe making process). Furthermore, it may also be manufactured by forming the ERW steel pipe material using a sizing roll that satisfies the following equation (1) (sizing process), 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 process). Furthermore, the term "cylindrical" refers to a pipe whose circumferential cross-section is "C" shaped.

[0082] Diameter of sizing roll (mm) ≥ Thickness of hot-rolled steel sheet (mm) / 0.020 ... (1) 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.

[0083] Furthermore, the plate thickness of the electric resistance welded steel pipe, which is listed as an example of the steel pipe of the present invention, is preferably 5 mm or more, and preferably 30 mm or less.

[0084] Furthermore, 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.

[0085] 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.

[0086] 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.

[0087] 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.

[0088] 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 properties in hydrogen deteriorate.

[0089] Under the above conditions, steel materials and steel pipes with excellent fatigue properties in hydrogen that satisfy a predetermined crack propagation rate in hydrogen can be obtained. [Examples]

[0090] The following describes examples that verified the effects of the present invention. In the following examples, steel materials and steel pipes were manufactured under the following manufacturing conditions and their characteristics were evaluated. Steel with the chemical composition shown in Tables 1-1 and 1-2 was melted and cast into a slab. Steel materials No. 1 to 71 and 80 were all heated to 1200°C, then hot-rolled under conditions where the rolling completion temperature was 950°C or higher, and cooled by water cooling to the cooling stop temperature shown in Tables 1-1 and 1-2 (accelerated cooling) to produce the steel materials. Steel pipes No. 2 to 71 and 80 were also formed using the obtained steel materials. For samples No. 2-15, 33-44, 63-70, and 80, the obtained steel material (hot-rolled steel sheet) was bent and the ends were butt-welded to form a pipe. For samples No. 16-32, 45-62, and 71, the obtained steel material (hot-rolled steel sheet) was formed into a cylindrical shape by cold roll forming, and the ends of the cylindrical shape were butt-welded using electric resistance welding to form a steel pipe. The average cooling rate during cooling was within the range of 12°C / s ± 2°C / s. The temperature of the steel material was measured using a thermocouple inserted in the center of the plate thickness. Furthermore, steel materials No. 1-31, 33-80 and steel pipes No. 2-31, 33-80 underwent dehydrogenation treatment by being left at room temperature for 96 hours (h) or more before evaluation of the steel material and steel pipe. Room temperature refers to 20 ± 10°C. Steel material No. 32 did not undergo dehydrogenation treatment. Steel materials No. 70 and 71, and steel pipes No. 70 and 71, could not be welded and therefore could not be used as steel pipes, and thus could not be evaluated.

[0091] Furthermore, for steel materials No. 72 to 79, slabs with the same component composition as steel material No. 58 were used, and the manufacturing conditions were also investigated. In particular, for conditions not specifically mentioned, manufacturing was carried out under the same conditions as for steel materials No. 1 to 71. Steel material No. 72 had a heating temperature of 1300°C, which exceeds the upper limit of the invention. Steel material No. 73 had a rolling completion temperature of 700°C, which falls below the lower limit of the invention. Steel material No. 74 was manufactured with a rolling completion temperature of 900°C, which is within the scope of the invention. Also, for steel material No. 75, the average cooling rate after rolling was 9°C / s, which is outside the scope of the invention. For steel material No. 76, the average cooling rate after rolling was 65°C / s, which is within the scope of the invention. With steel material No. 77, heating at 950°C resulted in a slab that was too hard to roll, and therefore could not be evaluated. Furthermore, with steel material No. 78, the cooling stop temperature was 700°C, which is outside the scope of the present invention. With steel material No. 79, the cooling stop temperature was 240°C, which is also outside the scope of the present invention. For steel materials No. 72 to 79, the obtained steel materials were used to form cylindrical shapes by cold roll forming, and the circumferential ends of the cylindrical shapes were butted together and electro-welded to obtain steel pipes.

[0092] The fatigue crack propagation characteristics were evaluated by fatigue crack growth tests. From each steel material, a CT (compact tension) test specimen (a nearly square specimen with a notch at one end) conforming to ASTM E 647 was taken so that the load direction was parallel to the rolling direction. The fatigue crack length was measured using the compliance method with a clip gauge, and the fatigue crack propagation velocity in 5 MPa high-pressure hydrogen gas was determined. For plate thicknesses of 10 mm or less, the specimen was ground down by 0.5 mm from the surface to 2 mm, 5 mm, 8 mm, and 9 mm respectively. For plate thicknesses other than these, a 10 mm thick specimen was taken from the t / 2 (t: plate thickness) position, and both the front and back surfaces of the crack propagation area were mirror polished. In this case, the stable growth region where Paris's law holds was defined as the stress intensity factor range ΔK = 20 (MPa·m). 1 / 2 The fatigue crack propagation rate (m / cycle) at ) was evaluated as a representative value. The results are shown in Table 1. Crack propagation rate da / dN was 1.0 × 10⁻⁶ -6 m·cycle -1 The following standards were deemed acceptable.

[0093] Furthermore, Nb precipitates were evaluated using the following method: Square pieces of wood were randomly taken from three locations per sample, machined to a mirror finish, and then etched. There was no specific size requirement, but the surface area was 10 cm². 2 The following standardization was performed. Subsequently, pretreatment was carried out using the extraction replica method, in which precipitates were transferred to a replica film produced by carbon deposition and observed, and Nb precipitates were observed and analyzed by EPMA. Nb precipitates between 2 nm and 100 nm were found at a concentration of 10 particles / μm.2 If the above is observed, it should be recorded as Y; if it is less than the above, it should be recorded as N in Tables 1-1 and 1-2.

[0094] Furthermore, the same results were obtained for steel pipes in Tables 1-1, 1-2, and 2 as for steel materials.

[0095] All of the examples of this invention are based on a crack propagation rate da / dN of 1.0 × 10⁻¹⁰ in hydrogen gas. -6 The following conditions were met: m / cycle.

[0096] [Table 1-1]

[0097] [Table 1-2] [Examples]

[0098] The following describes examples that verified the effects of the present invention. In the following examples, steel materials were manufactured under the following manufacturing conditions and their characteristics were evaluated. Using slabs with the same component composition as steel materials No. 17, 26, 55, and 67 shown in Tables 1-1 and 1-2, the steel materials were manufactured under the same conditions as steel materials No. 17, 26, 55, and 67 shown in Example 1 up to the controlled cooling process, yielding steel materials No. 17A, 26A, 55A, and 67A. Steel materials No. 17A, 26A, and 55A were formed into cylindrical shapes by cold roll forming of the obtained steel materials (hot-rolled steel sheets), and the circumferential ends of the cylindrical shapes were butt-welded to form a pipe. Steel material 67A was formed into a steel pipe by bending the obtained steel material (hot-rolled steel sheet) and butt-welding the ends. As shown in Table 2, the characteristics were evaluated when the dehydrogenation treatment conditions were changed. The results are shown in Table 2.

[0099] In Example 1, the dehydrogenation treatment of steel materials No. 17, 26, 55, and 67 was carried out with a holding time of 96 hours while the dehydrogenation treatment temperature T (ambient temperature) remained at room temperature. However, in this example, the dehydrogenation treatment temperature T (ambient temperature) was set to 50°C for all cases. For steel pipes No. 81, 84, 86, and 88, the dehydrogenation treatment temperature T (ambient 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 equation (A). For steel pipes No. 82, 85, 87, and 89, the dehydrogenation treatment temperature T (ambient temperature) was set to 50°C, and the holding time 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 pipe No. 83, the dehydrogenation treatment temperature T (ambient temperature) is 50°C, but neither the holding time t at the ambient temperature nor the holding time tc after the central plate thickness temperature Tc reaches 50°C satisfies equation (A) described above.

[0100] In Table 2, "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). All of the examples of this invention are based on a crack propagation rate da / dN of 1.0 × 10⁻¹⁰ in hydrogen gas. -6 The conditions of m / cycle or less were satisfied. Among these, the crack propagation characteristics were superior when the dehydrogenation treatment was carried out under more favorable conditions.

[0101] [Table 2]

Claims

[Claim 1] In mass%, C: 0.02-0.15%, Si: 0.05-0.5%, Mn: 0.3-2.0%, Al: 0.01-0.15%, N: 0.0005-0.008%, P: 0.03% or less, S: 0.01% or less, O: 0.01% or less, Nb: more than 0% and less than 2.5%, H: 0.0010% or less, Cu: 0 to 2.5%, Ni: 0 to 2.5%, Cr: 0-2.5%, Mo: 0-2.0%, V: 0-0.5%, Ti: 0 to 0.5%, W: 0-2.5%, B: 0 to 0.005%, Sn: 0 to 0.03%, Sb: 0 to 0.3%, Ca: 0-0.01%, Mg: 0 to 0.01%, REM: 0~0.005% It contains, with the remainder being Fe and unavoidable impurities, and has a component composition. The Nb precipitates with an equivalent circular diameter of 2 nm to 100 nm are 10 or more per μm². A method for manufacturing a steel pipe with excellent fatigue properties in hydrogen, wherein the stress intensity factor in hydrogen at 5 MPa = 20 MPa√m is 0.706 × 10⁻⁶ m·cycle⁻¹ or less, comprising a heating step of heating a slab having the above-mentioned component composition at 1000 to 1250°C, The slab heated in the above heating step is subjected to the finishing rolling completion temperature: Ar 3 A hot rolling process in which rolling is performed under conditions of 1.5 or higher, A controlled cooling process is performed to cool the hot-rolled steel sheet obtained in the hot-rolling process under the conditions that the average cooling rate from 1000 to 400°C is 10°C / s or more 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, and 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, or one of these two pipe-making processes, A dehydrogenation treatment process in which steel pipes obtained in the pipe manufacturing process are subjected to dehydrogenation treatment, A method for manufacturing steel pipes having [a certain characteristic].

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