High-strength steel material for line pipes with excellent hydrogen fracture toughness, method for manufacturing the same, high-strength steel pipe for line pipes, and method for manufacturing the same

Abstract

The purpose of the present invention is to provide a high-strength line pipe steel material that has excellent fracture toughness in hydrogen in a high-pressure hydrogen gas environment, and that is suitable for use for a steel structure used in a high-pressure hydrogen gas environment, such as a line pipe for 100% hydrogen gas or for natural gas (natural gas is gas containing a hydrocarbon such as methane and ethane as the main component) containing hydrogen at a hydrogen partial pressure of 1 MPa or more; a method for manufacturing the steel material; a steel tube for high-strength line pipes; and a method for manufacturing the steel tube.　This high-strength line pipe steel material having excellent fracture toughness in hydrogen is characterized by having a specific compositional makeup and a specific structure, having a tensile strength of 520 MPa or more, and having a hydrogen induced crack propagation lower limit KIH of 80 MPa·m1 / 2 or more in a high-pressure hydrogen gas environment of 1 MPa or more.

Description

[Technical Field]

[0001] The present invention relates to a high-strength steel material for line pipes that exhibits excellent fracture toughness in a high-pressure hydrogen gas environment of 1 MPa or higher, suitable for applications such as line pipes for transporting hydrogen gas, a method for manufacturing the same, and a high-strength steel pipe for line pipes and a method for manufacturing the same. [Background technology]

[0002] Existing energy infrastructure includes pipelines for transporting crude oil, natural gas, and other materials. These steel structures are used in atmospheres containing hydrogen sulfide, and the occurrence of hydrogen embrittlement, such as hydrogen-induced cracking (HIC) and sulfide stress corrosion cracking (SSCC), has become a safety issue, requiring its suppression. Various measures have been taken to prevent hydrogen-induced cracking and hydrogen embrittlement, such as sulfide stress corrosion cracking, including reducing the amount of MnS in the steel material which acts as a crack initiation point, suppressing the accumulation of Ti and Nb carbonitrides and oxides, and suppressing the segregation of the hardened phase with central segregation. Furthermore, from the perspective of suppressing crack initiation by improving the corrosion resistance of the steel material, the addition of Sn and Sb to the steel material has been proposed (for example, Patent Documents 1 and 2).

[0003] In recent years, the use of hydrogen as a clean energy source aimed at building a decarbonized society has been promoted. Therefore, to facilitate the large-scale transport of hydrogen gas, the construction of hydrogen gas transport networks is being considered, including those that use natural gas mixed with hydrogen at a certain ratio in natural gas pipelines, or those that use hydrogen gas as a substitute for natural gas. The transport pressure during operation of these pipelines is expected to be high, ranging from 1 to 40 MPa, exposing the pipelines to a high-pressure hydrogen gas environment. Steel materials used in such environments need to possess not only the properties required in existing sour environments—such as suppression of corrosion on the inner surface of steel pipes and reduction of hydrogen accumulation within the material—but also the hydrogen resistance required in a hydrogen gas environment.

[0004] Austenitic stainless steels such as SUS316L, which exhibit hydrogen fracture toughness, are used in steel structures for applications in high-pressure hydrogen gas environments. However, in addition to the high cost of the steel, its low strength and the fact that designing it to withstand high hydrogen pressure requires thicker walls, resulting in extremely high costs for the line pipes, make it unsuitable for pipeline construction. Therefore, there has been a demand for a lower-cost steel material that can withstand high-pressure hydrogen gas environments for hydrogen line pipes.

[0005] To address the above problems, for example, Patent Document 3 proposes an austenitic steel material with a high Mn content. The technology described in Patent Document 3 makes it possible to provide a steel material that is less expensive than austenitic stainless steel, but because it is austenitic, it is more expensive than low-alloy steel. Furthermore, it does not take into account the suppression of pitting corrosion, which is the initiation point of hydrogen-induced cracks, such as HIC resistance and SSCC resistance.

[0006] Furthermore, pipelines are subjected to repeated stresses due to the repeated starting and shutting down of operations. Therefore, fatigue failure must be considered when designing steel structures such as pipelines. The fatigue failure limit of steel structures used in a high-pressure hydrogen gas environment is determined by the operating conditions of the pipeline and the lower limit of hydrogen-induced crack propagation, which corresponds to the fracture toughness value of the steel in hydrogen gas. IH This corresponds to the critical crack length calculated from. From the perspective of extending the lifespan and improving the safety of hydrogen-related structures, the K of steel materials IH Raising this level is considered one effective guideline.

[0007] Hydrogen pipelines are envisioned to use line pipes with welded sections, i.e., weld metal sections and heat-affected zones. Patent document 4 describes K IH While a superior steel manufacturing method is proposed, the characteristics of the welded joint are not mentioned. Generally, compared to the base material, the welded joint is more susceptible to the degradation of properties caused by hydrogen. Therefore, the K including the welded joint should be considered. IH Improving this is important.

[0008] K of steel materials IH In order to increase it, for example, it is better to reduce upper bainite containing coarse carbides.

Prior art documents

Patent documents

[0009]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Non-patent documents

[0010]

Non-patent Document 1

Summary of the invention

Problems to be solved by the invention

[0011] The present invention has been made to solve the above-mentioned conventional problems, and is suitable for steel structures used in a high-pressure hydrogen gas environment such as a line pipe for 100% hydrogen gas or natural gas containing hydrogen with a hydrogen partial pressure of 1 MPa or more (natural gas is a gas mainly composed of hydrocarbons such as methane and ethane). An object is to provide a high-strength line pipe steel material excellent in hydrogen cracking toughness in a high-pressure hydrogen gas environment, a method for producing the same, a high-strength line pipe steel pipe, and a method for producing the same. As the high-pressure hydrogen gas environment, a high-pressure hydrogen gas of 1 MPa or more or an environment containing 0.2% or more of hydrogen gas is assumed.

[0012] Note that the phrase "excellent hydrogen-induced fracture toughness in a high-pressure hydrogen gas environment" as used herein refers to the hydrogen-induced crack propagation lower limit K determined by conducting a fracture toughness test in both environments of a hydrogen gas at room temperature (20 ± 10°C) and a pressure of 1 MPa or more, or a natural gas (main components are hydrocarbons such as methane and ethane) mixed atmosphere containing hydrogen with a hydrogen partial pressure of 1 MPa or more. IH is 80 MPa·m 1 / 2 or more. Note that the fracture toughness value refers to the value obtained by conducting a fracture toughness test in accordance with ASTM E399, ASTM E1820, and ASTM E1681. The natural gas containing hydrogen with a hydrogen partial pressure of 1 MPa or more refers to, for example, a gas with a hydrogen concentration of 30% or less by volume fraction and a total gas pressure of 30 MPa or less.

[0013] The "steel materials" as used herein include thin steel plates, thick steel plates, seamless steel pipes, electric resistance welded steel pipes, shaped steel, steel bars, etc.

Means for Solving the Problems

[0014] The inventors of the present invention aimed to suppress the hydrogen absorption into steel materials, which is the fundamental cause of hydrogen embrittlement, and conducted a technical study on the conditions that the steel materials for high-strength line pipes and steel pipes for high-strength line pipes, which are excellent in hydrogen-induced fracture toughness in a high-pressure hydrogen gas environment, should satisfy. As a result, in a metallographic structure where the number of inclusions with an aspect ratio of 2.0 or more and a length of 10 μm or more is 15 pieces / 100 mm 2 or less, and the maximum grain size of the above bainite in the range from the surface of the steel material and the steel pipe to the center of the plate thickness is 25 μm or less, the hydrogen-induced crack propagation lower limit K of the steel material and the steel pipe IH is improved. In addition, if the area fraction of retained austenite is 0 to 3% and the area fraction of bainite is 90% or more in the range from the surface of the steel material and the steel pipe to the center of the plate thickness, the hydrogen-induced crack propagation lower limit K of the steel material IHWe found that this could be further improved. In order to achieve such a steel structure, it is necessary to strictly control the rolling conditions in the hot rolling process and the cooling conditions after rolling, and we succeeded in finding these conditions. This invention is based on these findings. Furthermore, in this invention, high strength refers to a tensile strength of 520 MPa or higher.

[0015] In other words, the gist of this invention is as follows: [1] In mass%, C: 0.02~0.15%, Si: 0.01~2.0%, Mn: 0.5~1.5%, P: 0.0001~0.015%, S: 0.0002~0.0015%, Al: 0.005~0.15%, O: 0.01% or less, N: 0.010% or less, Nb: 0.10% or less, H: Includes 0.02 ppm or less, Or, furthermore, Ca: 0~0.005%, Ni: 0~2.0%, Ti: 0~0.1%, Cu: 0~1.0%, Cr: 0-1.0%, Mo: 0~0.60%, W: 0~1.0%, V: 0~0.10%, Zr: 0~0.050%, Mg: 0~0.01%, REM: 0~0.01%, B: 0~0.0020%, Ta: 0~0.2%, Hf: 0~0.2%, Re: 0~0.005%, Sn: 0~0.3%, Sb: Contains one or more selected from 0-0.3%, It has a chemical composition in which the remainder is Fe and unavoidable impurity elements. Fifteen bainite inclusions with an aspect ratio of 2.0 or greater and a length of 10 μm or greater were found per 100 mm. 2 The metal structure is as follows: The maximum grain size of the bainite in the range from the surface of the steel material to the center of the plate thickness is 25 μm or less. The tensile strength is 520 MPa or more, In a high-pressure hydrogen gas environment of 1 MPa or higher, the lower limit K for hydrogen-induced crack propagation IH 80 MPa·m 1 / 2 The above describes a high-strength steel material for line pipes with excellent fracture toughness in hydrogen. [2] Furthermore, the chemical composition is, in mass%, Ca: 0.0001~0.005%, Ni: 0.01~2.0%, Ti: 0.005~0.1%, Cu: 0.01~1.0%, Cr: 0.01~1.0%, Mo: 0.01~0.60%, W: 0.01~1.0%, V: 0.01~0.10%, Zr: 0.0001~0.050%, Mg: 0.0001~0.01%, REM: 0.0001~0.01%, B: 0.0001~0.0020%, Ta: 0.0001~0.2%, Hf: 0.0001~0.2%, Re: 0.0001~0.005%, Sn: 0.0001~0.3%, A high-strength steel material for line pipes with excellent hydrogen fracture toughness, as described in [1], with an Sb content of 0.0001-0.3%. [3] A high-strength steel material for line pipes with excellent hydrogen fracture toughness as described in [1] or [2], wherein retained austenite is 0-3% in area fraction and bainite in the range from the steel surface to the center of the plate thickness is 90% or more in area fraction. [4] A heating step of heating a cast slab having the component composition described in [1] or [2] above at 1000 to 1250°C, A hot rolling process is performed on the cast slab heated in the heating step, under the conditions that the total reduction ratio in the recrystallization temperature range is 35% or more and 55% or less, the reduction ratio of the final rolling pass in the recrystallization temperature range is 10% or more, the reduction ratio of the final rolling pass at (recrystallization temperature -80°C) or higher is 15% or more, and the rolling completion temperature at the steel sheet surface temperature is above the Ar3 transformation point. A controlled cooling process is performed to cool the hot-rolled steel sheet obtained in the hot-rolling process under the following conditions: the cooling start temperature is above the Ar3 transformation point at the surface temperature of the hot-rolled steel sheet; the difference in cooling start time between the leading and trailing ends of the hot-rolled steel sheet is within 50 seconds; the average cooling rate from 750°C to 550°C is 15 to 50°C / s at the center temperature of the sheet thickness; and the cooling stop temperature is 250 to 650°C. A method for manufacturing high-strength steel material for line pipes that has excellent hydrogen fracture toughness. [5] In mass%, C: 0.02~0.15%, Si: 0.01~2.0%, Mn: 0.5~1.5%, P: 0.0001~0.015%, S: 0.0002~0.0015%, Al: 0.005~0.15%, O: 0.01% or less, N: 0.010% or less, Nb: 0.10% or less, H: Includes 0.02 ppm or less, Or, furthermore, Ca: 0~0.005%, Ni: 0~2.0%, Ti: 0~0.1%, Cu: 0~1.0%, Cr: 0-1.0%, Mo: 0~0.60%, W: 0~1.0%, V: 0~0.10%, Zr: 0~0.050%, Mg: 0~0.01%, REM: 0~0.01%, B: 0~0.0020%, Ta: 0~0.2%, Hf: 0~0.2%, Re: 0~0.005%, Sn: 0~0.3%, Sb: Contains one or more selected from 0-0.3%, It has a chemical composition in which the remainder is Fe and unavoidable impurity elements. Fifteen bainite inclusions with an aspect ratio of 2.0 or greater and a length of 10 μm or greater were found per 100 mm. 2 The metal structure is as follows: The maximum particle size of the bainite in the range from the surface of the inner surface of the steel pipe to the center of the plate thickness is 25 μm or less. The tensile strength is 520 MPa or more, In a high-pressure hydrogen gas environment of 1 MPa or higher, the lower limit K for hydrogen-induced crack propagation IH 80 MPa·m 1 / 2 The above describes a high-strength steel pipe for line pipes with excellent hydrogen fracture toughness. [6] Furthermore, the chemical composition is, in mass%, Ca: 0.0001~0.005%, Ni: 0.01~2.0%, Ti: 0.005~0.1%, Cu: 0.01~1.0%, Cr: 0.01~1.0%, Mo: 0.01~0.60%, W: 0.01~1.0%, V: 0.01~0.10%, Zr: 0.0001~0.050%, Mg: 0.0001~0.01%, REM: 0.0001~0.01%, B: 0.0001~0.0020%, Ta: 0.0001~0.2%, Hf: 0.0001~0.2%, Re: 0.0001~0.005%, Sn: 0.0001~0.3%, A high-strength steel pipe for line pipes with excellent hydrogen fracture toughness, as described in [5], having an Sb content of 0.0001-0.3%. [7] In steel pipes for high-strength line pipes, A high-strength steel pipe for line pipes with excellent hydrogen fracture toughness as described in [5] or [6], wherein retained austenite is 0-3% in area fraction, and bainite in the area fraction from the surface of the inner surface of the steel pipe to the center of the plate thickness is 90% or more. [8] A heating step of heating a cast slab having the component composition described in [5] or [6] above at 1000 to 1250°C, A hot rolling process is performed on the cast slab heated in the heating step, under the conditions that the total reduction ratio in the recrystallization temperature range is 35% or more and 55% or less, the reduction ratio of the final rolling pass in the recrystallization temperature range is 10% or more, the reduction ratio of the final rolling pass at (recrystallization temperature -80°C) or higher is 15% or more, and the rolling end temperature at the steel sheet surface temperature is above the Ar3 transformation point. A controlled cooling process is performed to cool the hot-rolled steel sheet obtained in the hot-rolling process under the following conditions: the cooling start temperature is above the Ar3 transformation point at the surface temperature of the hot-rolled steel sheet; the difference in cooling start time between the leading and trailing ends of the hot-rolled steel sheet is within 50 seconds; the average cooling rate from 750°C to 550°C is 15 to 50°C / s at the center temperature of the sheet thickness; and the cooling stop temperature is 250 to 650°C. After the controlled cooling process, a pipe-making process is performed in which the hot-rolled steel sheet is bent and both ends are butt-welded; or a pipe-making process is performed in which the hot-rolled steel sheet is formed into a cylindrical shape by cold roll forming and both ends of the cylindrical shape are butt-welded using electric resistance welding; A method for manufacturing high-strength steel pipes for line pipes that have excellent hydrogen fracture toughness. [Effects of the Invention]

[0016] According to the present invention, steel materials with extremely improved hydrogen fracture toughness under high-pressure hydrogen gas conditions can be easily and simply manufactured, yielding remarkable industrial benefits. Furthermore, the present invention significantly improves the hydrogen absorption resistance characteristics of steel structures such as high-pressure hydrogen gas line pipes, contributing greatly to the improvement of safety of steel structures. [Modes for carrying out the invention]

[0017] Next, a method for carrying out the present invention will be specifically described.

[0018] As the first embodiment, a steel material will be specifically described, followed by a second embodiment, a UOE steel pipe which is an example of a steel pipe of the present invention, and a third embodiment, an electric resistance welded steel pipe which is an example of a steel pipe of the present invention.

[0019] First Embodiment [Component composition] The following explains the reasons for limiting the base material composition in the steel material of the present invention. In the following description, all units expressed in % are mass % unless otherwise specified.

[0020] C: 0.02~0.15% While carbon (C) effectively contributes to improving strength, sufficient strength cannot be ensured if the C content is less than 0.02%, therefore the C content should be 0.02% or higher. Preferably, the C content is 0.03% or higher. More preferably, the C content is 0.035% or higher. Even more preferably, the C content is 0.04% or higher. On the other hand, if it exceeds 0.15%, weldability decreases. For this reason, the C content should be limited to 0.15% or less. Preferably, the C content is 0.10% or less. Furthermore, if it exceeds 0.08%, the hardness of the surface layer and central segregation increases during controlled cooling, which may lead to deterioration of SSCC resistance and HIC resistance. Toughness also deteriorates. For this reason, a C content of 0.08% or less is more preferable. Even more preferably, the C content is 0.06% or less.

[0021] Si: 0.01~2.0% Si is included for deoxidation, but since the deoxidation effect is insufficient if the Si content is less than 0.01%, the Si content should be 0.01% or more. Preferably, the Si content is 0.02% or more. More preferably, the Si content is 0.05% or more. Even more preferably, the Si content is 0.08% or more. Since the above effect is observed up to 2.0%, the Si content should be 2.0% or less. Preferably, the Si content is 1.8% or less, and more preferably 1.5% or less. Even more preferably, the Si content is 1.0% or less. However, since exceeding 0.5% may degrade toughness and weldability, most preferably, the Si content should be 0.5% or less.

[0022] Mn: 0.5~1.5% While Mn effectively contributes to improving strength and toughness, its effect is negligible at concentrations below 0.5%, therefore the Mn content should be 0.5% or higher. Preferably, the Mn content is 0.6% or higher, more preferably 0.8% or higher. Even more preferably, the Mn content is 1.0% or higher. On the other hand, if the Mn content exceeds 1.5%, the hardness of the surface layer and central segregation increases during controlled cooling, resulting in deterioration of SSCC resistance and HIC resistance. Weldability also deteriorates. For this reason, the Mn content should be limited to 1.5% or less. Preferably, the Mn content is 1.4% or less. More preferably, the Mn content is 1.3% or less, and even more preferably 1.2% or less.

[0023] P: 0.0001~0.015% P is an unavoidable impurity element that degrades weldability and reduces HIC resistance by increasing the hardness of the central segregation area. This tendency becomes significant above 0.015%, so the P content should be limited to 0.015% or less. Preferably, the P content is 0.012% or less, and more preferably 0.010% or less. Even more preferably, the P content is 0.008% or less. A lower content is better, but from the perspective of refining costs, the P content should be 0.0001% or more.

[0024] S: 0.0002~0.0015% S is an unavoidable impurity element, and in steel it forms MnS inclusions that degrade HIC resistance, so it is preferable to have a low amount, but up to 0.0015% is acceptable. Therefore, the S content should be 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, it should be 0.0002% or more.

[0025] Al: 0.005~0.15% Al is added as a deoxidizing agent, but since it has no effect if it is less than 0.005%, the Al content should be 0.005% or more. On the other hand, if it exceeds 0.15%, the cleanliness of the steel decreases and the toughness deteriorates, so the Al content should be 0.15% or less. The Al content is preferably 0.12% or less, and more preferably 0.10% or less. Even more preferably, the Al content is 0.08% or less.

[0026] O: 0.01% or less Since oxygen (O) causes the formation of oxide inclusions, a lower amount is preferable. This effect is not a problem if the O content is 0.01% or less, so the O content should be 0.01% or less. Preferably, the O content is 0.0080% or less. More preferably, the O content is less than 0.0030%. There is no particular lower limit, but it may be 0.0005% or more.

[0027] N: 0.010% or less While nitrogen (N) effectively contributes to improving strength, if its content exceeds 0.010%, hardness increases during controlled cooling, leading to a deterioration of toughness. Therefore, the N content should be 0.010% or less. Preferably, the N content is 0.008% or less, more preferably 0.006% or less, and even more preferably 0.004% or less. However, below 0.00001%, sufficient strength cannot be ensured, and excessive reduction increases steelmaking costs. Therefore, it is preferable that the content be 0.00001% or more. More preferably, the N content is 0.002% or more.

[0028] Nb: 0.10% or less Nb is an effective element for increasing the strength and toughness of steel. Since its effect is negligible below 0.001%, a content of 0.001% or more is preferable. On the other hand, since exceeding 0.10% deteriorates the toughness of the welded joint, the Nb content should be 0.10% or less. A Nb content of 0.095% or less is preferable. A Nb content of 0.090% or less is more preferable, and 0.085% or less is even more preferable. A Nb content of 0.080% or less is most preferable.

[0029] H:0.02ppm or less H may be introduced into the steel material during various processes in manufacturing, and if the amount introduced is large, the risk of cracking after solidification increases, and K IH This can significantly reduce the hydrogen content. Since these effects are not problematic if the hydrogen content is 0.02 ppm or less, the hydrogen content should be 0.02 ppm or less. Preferably, the hydrogen content is 0.015 ppm or less, and more preferably 0.008 ppm or less. Even more preferably, the hydrogen content is 0.005 ppm or less, and most preferably less than 0.002 ppm. The lower limit is not particularly limited, but for manufacturing cost reasons, it is preferable to be 0.0008 ppm or more. It is even more preferable that the hydrogen content be 0.001 ppm or more. Note that the hydrogen content is the residual hydrogen content after molding of steel materials, steel pipes, UOE, etc.

[0030] The chemical composition of this disclosure may further optionally contain one or more elements selected from Ca, Ni, Ti, Cu, Cr, Mo, W, V, Zr, Mg, REM, B, Ta, Hf, Re, Sn, and Sb within the following ranges.

[0031] Ca: 0~0.005% Ca is an effective element for improving HIC resistance by controlling the morphology of sulfide inclusions. Therefore, when Ca is included, the Ca content may be 0% or more, but if it is less than 0.0001%, the effect of its addition is insufficient. For this reason, when Ca is included, it is preferable that the Ca content be 0.0001% or more. More preferably, it is 0.0005% or more. On the other hand, if it exceeds 0.005%, not only does the effect saturate, but the HIC resistance deteriorates due to a decrease in the cleanliness of the steel. Therefore, when Ca is included, the Ca content should be limited to 0.005% or less. A Ca content of 0.004% or less is preferable. A Ca content of 0.002% or less is more preferable, and 0.0008% or less is even more preferable.

[0032] Ni: 0~2.0% Ni is an effective element for improving toughness and increasing strength. When Ni is included, the Ni content may be 0% or more, but to obtain this effect, it is preferable to have a Ni content of 0.01% or more. A Ni content of 0.1% or more is more preferable. On the other hand, in order to control costs, when Ni is included, the Ni content should be 2.0% or less. A Ni content of 1.8% or less is preferable. A Ni content of 1.4% or less is more preferable, and 0.8% or less is even more preferable.

[0033] Ti: 0~0.1% Since Ti contributes to increasing the strength of steel, if Ti is included, the Ti content may be 0% or more. To obtain the above effect, if Ti is included, it is preferable that the Ti content be 0.005% or more. More preferably, it is 0.008% or more. On the other hand, if the content exceeds 0.1%, the effect saturates and becomes a factor in increasing costs, so if Ti is included, the Ti content should be 0.1% or less. It is preferable that the Ti content be 0.08% or less, and more preferably 0.06% or less. To suppress costs, it is even more preferable that the Ti content be 0.05% or less. It is most preferable that the Ti content be 0.04% or less.

[0034] Cu: 0~1.0% Cu is an effective element for improving toughness and increasing strength. When Cu is included, the Cu content may be 0% or more, but it is preferable to include 0.01% or more to obtain this effect. It is more preferable to include 0.05% or more. On the other hand, if the content is too high, the weldability deteriorates, so when Cu is included, the Cu content should be 1.0% or less. A Cu content of 0.95% or less is preferable, and a Cu content of 0.9% or less is more preferable. Even more preferable is a Cu content of 0.85% or less. Most preferably, a Cu content of 0.5% or less is preferable.

[0035] Cr: 0~1.0% Like manganese, chromium (Cr) is an effective element for obtaining sufficient strength even at low carbon concentrations. When Cr is included, the Cr content may be 0% or more, but it is preferable to include 0.01% or more to obtain this effect. It is more preferable to include 0.05% or more. On the other hand, if the content is too high, the hardenability becomes excessive, which deteriorates the resistance to SSCC (Steel Carbon Deposition) and also deteriorates the weldability. For this reason, when Cr is included, it should be 1.0% or less. A Cr content of 0.95% or less is preferable, a Cr content of 0.9% or less is more preferable, and 0.85% or less is even more preferable.

[0036] Mo: 0~0.60% Mo is an element effective in improving toughness and increasing strength, and is effective in improving resistance to SSCC and HIC. When Mo is included, the Mo content may be 0% or more, but to obtain this effect, it is preferable to have a content of 0.01% or more. It is more preferable to have a content of 0.10% or more. On the other hand, if the content is too high, the hardenability becomes excessive, and the resistance to SSCC deteriorates. Weldability also deteriorates. For this reason, when Mo is included, the Mo content should be 0.60% or less. The Mo content should preferably be 0.50% or less. More preferably 0.40% or less. Even more preferably 0.35% or less.

[0037] W: 0~1.0% W contributes to increasing the strength of the steel material. When W is included, the W content may be 0% or more, but in order to obtain the above effect, it is preferable that the W content be 0.01% or more. On the other hand, if the W content exceeds 1.0%, the effect saturates and becomes a factor in cost increase, so when W is included, the W content should be 1.0% or less. It is preferable that the W content be 0.9% or less, and more preferably 0.8% or less. For cost reduction, it is even preferable that it be 0.5% or less.

[0038] V: 0-0.10%, Zr: 0-0.050%, Mg: 0-0.01%, and REM: 0-0.01% V is an element that can be optionally included to increase the strength and toughness of steel. When V is included, the V content may be 0% or more, but since the effect of including it is poor if the content is less than 0.01%, it is preferable that the V content be 0.01% or more. It is more preferable that the V content be 0.03% or more. On the other hand, if it exceeds 0.10%, the toughness of the weld deteriorates, so it is preferable that if it is included, it be 0.10% or less. It is preferable that the V content be 0.09% or less. It is more preferable that the V content be 0.07% or less, and even more preferable that it be 0.06% or less.

[0039] Zr, Mg, and REM are elements that can be optionally added to enhance toughness through grain refinement or to improve crack resistance through control of inclusion properties. When these elements are included, their content may be 0% or more, but since the effect of including any of them is poor if the content is less than 0.0001%, it is preferable that the content be 0.0001% or more. More preferably, it is 0.0005% or more. In other words, it is preferable that the Zr content be 0.0001% or more. It is even more preferable that the Zr content be 0.0005% or more. It is also preferable that the REM content be 0.0001% or more. It is even more preferable that the REM content be 0.0005% or more. It is preferable that the Mg content be 0.0001% or more. It is even more preferable that the Mg content be 0.0005% or more.

[0040] On the other hand, if the Zr content exceeds 0.050%, and the Mg and REM content exceeds 0.01%, the effect saturates. Therefore, if they are included, the Zr content should be 0.050% or less, and the Mg and REM content should be 0.01% or less. In other words, if they are included, the Zr content should be 0.050% or less. It is preferable that the Zr content be 0.040% or less. It is more preferable that the Zr content be 0.020% or less. Also, if they are included, the REM content should be 0.01% or less. It is preferable that the REM content be 0.009% or less. It is more preferable that the REM content be 0.008% or less. Also, if they are included, the Mg content should be 0.01% or less. It is preferable that the Mg content be 0.009% or less. It is more preferable that the Mg content be 0.008% or less.

[0041] B: 0~0.0020% B is an element that improves hardenability, contributing to increased strength of steel materials, suppressing coarsening of prior austenite grains, and improving various properties of the material. When B is included, the B content may be 0% or more, but to obtain the above effects, it is preferable to have a B content of 0.0001% or more. More preferably, it is 0.0008% or more. On the other hand, if the B content exceeds 0.0020%, the effect saturates and becomes a factor in cost increase, so when B is included, the B content should be 0.0020% or less. It is preferable that the B content be 0.0014% or less. It is even more preferable that the B content be 0.0012% or less. To suppress costs, it is even more preferable to have a B content of 0.0010% or less.

[0042] Ta: 0~0.2% Ta is an element that forms carbides and nitrides and contributes to improving strength. When Ta is included, the Ta content may be 0% or more, but to obtain the above effect, it is preferable that the Ta content be 0.0001% or more. More preferably, the Ta content is 0.0008% or more. On the other hand, if the content exceeds 0.2%, it may lead to a decrease in toughness, so when Ta is included, the Ta content should be 0.2% or less. It is preferable that the Ta content be 0.16% or less. It is more preferable that the Ta content be 0.12% or less, and even more preferable that it be 0.10% or less.

[0043] Hf: 0~0.2%, Re: 0~0.005% These elements contribute to increasing the strength of the steel. To obtain the above effect, if these elements are included, it is preferable that their content be 0.0001% or more. Preferably, it is 0.0010% or more. That is, if included, the Hf content is preferably 0.0001% or more. The Hf content is more preferably 0.0010% or more. If included, the Re content is preferably 0.0001% or more. The Re content is more preferably 0.001% or more. On the other hand, if these elements are included, if the content of Hf exceeds 0.2% or the content of Re exceeds 0.005%, the oxides increase and, when aggregated, impair the hydrogen resistance properties. Therefore, the Hf content should be 0.2% or less and the Re content should be 0.005% or less. That is, if included, the Hf content should be 0.2% or less. The Hf content is preferably 0.18% or less, and more preferably 0.12% or less. If included, the Re content should be 0.005% or less. The Re content is preferably 0.004% or less, and more preferably 0.003% or less.

[0044] Sn: 0-0.3%, Sb: 0-0.3% These elements contribute to increasing the strength and improving the hardenability of the steel. When Sn and Sb are present, the Sn and Sb content may be 0% or more, but to obtain the above effects, it is preferable that the content of each be 0.0001% or more. Preferably, it is 0.001% or more. That is, the Sn content may be 0% or more when present, but it is preferable that the Sn content be 0.0001% or more. It is more preferable that the Sn content be 0.001% or more. The Sb content may be 0% or more when present, but it is preferable that the Sb content be 0.0001% or more. It is more preferable that the Sb content be 0.001% or more. On the other hand, if the content of each exceeds 0.3%, the effect saturates, leading to increased costs. Therefore, when Sn and Sb are included, the Sn and Sb content should be 0.3% or less. To control costs, it is preferable to keep it at 0.01% or less. In other words, when Sn is included, the Sn content should be 0.3% or less. It is preferable that the Sn content be 0.2% or less. It is more preferable that the Sn content be 0.1% or less. It is even more preferable that the Sn content be 0.01% or less. When Sb is included, the Sb content should be 0.3% or less. It is preferable that the Sb content be 0.2% or less. It is more preferable that the Sb content be 0.1% or less. It is even more preferable that the Sb content be 0.01% or less.

[0045] In the composition of the steel material, the remainder other than the components (elements) mentioned above consists of Fe and unavoidable impurity elements.

[0046] The following describes the microstructure of the steel material of the present invention.

[0047] metallographic structure Fifteen inclusions per 100mm with an aspect ratio of 2.0 or greater and a length of 10 μm or greater. 2 below Examples of inclusions in the material include elongated MnS and cementite. These act as hydrogen accumulation sources, leading to a significant decrease in HIC resistance and the lower limit of hydrogen-induced crack propagation K. IHThis can cause a decrease in quality. Therefore, if there are 15 inclusions per 100 mm with an aspect ratio of 2.0 or higher and a length of 10 μm or more, 2 The following applies: The number density of the above inclusions is preferably 10 pieces / 100 mm. 2 The following applies. The lower limit is not particularly restricted, and is 0 pieces / 100mm. 2 That's fine.

[0048] Residual austenite content is 0-3% (preferred). The presence of retained austenite in the steel structure can act as a hydrogen trapping site, increasing the amount of hydrogen in the steel and potentially increasing its susceptibility to hydrogen embrittlement. Furthermore, when steel materials or pipes are used in steel structures, stress loading during use can cause retained austenite to transform into martensite. Martensite is extremely hard and can become a source or propagation pathway for HIC (hydrogen-induced cirrhosis). IH This may significantly reduce the K. In the present invention, by keeping the retained austenite content at 3% or less, IH This improved the properties. For this reason, it is preferable that the retained austenite content be 3% or less. It is more preferable that the retained austenite content be 2% or less. Even more preferable is 1% or less. The retained austenite content may be 0%.

[0049] The bainite content in the area fraction from the surface of the steel material (or the inner surface of the steel pipe in the case of a steel pipe) to the center of the plate thickness is 90% or more (preferred). To achieve high strength with a tensile strength of 520 MPa or more, suitable for line pipes, the steel structure of the steel material must be bainite. Here, the bainite structure includes bainitic ferrite or granular bainite that transforms during or after accelerated cooling, contributing to transformation strengthening, and also includes tempered bainite. If dissimilar structures such as ferrite, martensite, pearlite, island martensite, and retained austenite are mixed in the bainite structure, it will reduce strength and toughness (under normal atmospheric conditions), and K IH Deterioration occurs. Furthermore, the presence of steel structures with different hardness levels can cause stress distribution within the steel material when stress is applied during use, acting as a source of hydrogen accumulation due to stress-induced diffusion and reducing HIC resistance. For this reason, it is preferable that the area fraction of bainite be 90% or more. It is more preferable that the area fraction of bainite be 92% or more, and even more preferable that it be 95% or more. There is no particular upper limit, but it may be 100%.

[0050] The maximum particle size within the range from the surface of the steel material (or the inner surface of the steel pipe in the case of a steel pipe) to the center of the plate thickness is 25 μm or less. While toughness can be improved by refining the average grain size, there are limitations to refining the average grain size when cooling is initiated at Ar3 or higher. In this disclosure, it is crucial to suppress the formation of coarse grains. When grains with large maximum grain sizes are present, they induce the generation of non-uniform strain within the material, promoting hydrogen accumulation and thus degrading fracture toughness in a hydrogen gas environment. In particular, grains with a maximum grain size exceeding 25 μm from the surface of the inner surface of the steel to the center of the plate thickness tend to accumulate strain around the grain, easily becoming the initiation point and propagation path for hydrogen cracks. IH The grain deteriorates significantly. Therefore, the maximum grain size from the inner surface of the steel material to the center of the plate thickness must be 25 μm or less. Preferably, the maximum grain size from the inner surface of the steel material to the center of the plate thickness must be 24 μm or less, more preferably 22 μm or less, and even more preferably 20 μm or less. There is no particular lower limit, but it is preferable that the maximum grain size be 4 μm or more. The measurement range for grain size was 1 mm × 1 mm, and the definition of grain size was Area grain size (weighted average when the boundary with an orientation difference of 15° or more is defined as a grain boundary).

[0051] Lower limit K of hydrogen-induced crack propagation in a high-pressure hydrogen gas environment of 1 MPa or higher IH 80 MPa·m 1 / 2 That's all. The high-strength steel materials disclosed herein are intended for the safe operation of steel structures in hydrogen-containing environments, and meet the lower limit of hydrogen-induced crack propagation in a high-pressure hydrogen gas environment of 1 MPa or higher. IH 80 MPa·m1 / 2 The above conditions shall be met. The upper limit is not particularly limited, but the lower limit K of hydrogen-induced crack propagation in steel materials IH is 120 MPa·m 1 / 2 Preferably, it has the following characteristics: 100 Pa·m 1 / 2 The following is more preferable. Note that the lower limit K for hydrogen-induced crack propagation IH This refers to the plane strain fracture toughness K, determined in accordance with ASTM E399 and ASTM E1820 in high-pressure hydrogen gas at 1 MPa or higher. IC This refers to either a provisional value or the lower limit value of crack propagation determined in accordance with ASTM E1681, or its provisional value.

[0052] Furthermore, while the thickness of the steel plate is not particularly limited, it is preferable that the plate thickness be 5 mm or more. It is preferable that the plate thickness be 30 mm or less.

[0053] The present invention, by having the above-described chemical components and metallic structure, achieves an excellent hydrogen-induced cracking lower limit K in high-pressure hydrogen gas. IH This allows for the acquisition of a suitable material, making it applicable to hydrogen line pipes.

[0054] Furthermore, the high-strength steel material for line pipes according to the present invention can be obtained by limiting the manufacturing conditions as shown below, and the manufacturing method and conditions will be explained in detail.

[0055] Molten steel process [Average cooling rate of molten steel: 50°C / min or higher (preferred conditions)] Reducing the content of S or O is also effective in reducing inclusions. Since the inclusions specified in this invention aggregate during the cooling process of molten steel, increasing the average cooling rate of the molten steel is also effective. For this reason, it is preferable to set the average cooling rate in the temperature range of 1500°C to 1000°C to 50°C / min or higher. It is more preferable to set the average cooling rate to 60°C / min or higher, and even more preferable to set it to 70°C / min or higher. There is no particular upper limit, but it is preferable to set it to 90°C / min or lower.

[0056] heating process [Heating temperature of cast slab: 1000~1250℃] When heating billets or slabs to temperatures below 1000°C, the diffusion of microsegregated impurity elements such as C, P, and S is insufficient, resulting in an inhomogeneous material. This leads to an increase in the number of inclusions and non-uniform precipitation, reducing toughness. Therefore, the heating temperature of the casting should be 1000°C or higher. Preferably, the heating temperature is 1050°C or higher, and more preferably 1100°C or higher. On the other hand, if the temperature exceeds 1250°C, the crystal grains become too coarse, and toughness deteriorates. Therefore, the heating temperature of the casting should be 1250°C or lower. Preferably, the heating temperature is 1200°C or lower, and more preferably 1150°C or lower.

[0057] Rolling process [Total reduction ratio in the recrystallization temperature range after heating the cast slab: 35% to 55%] To reduce the maximum grain size of bainite, it is necessary to promote the recrystallization of crystal grains and suppress the formation of coarse grains by hot rolling in the recrystallization temperature range after heating the cast slab. If the total reduction ratio in the recrystallization temperature range is less than 35%, recrystallization is insufficient, and coarse grains remain. Therefore, the total reduction ratio in the recrystallization temperature range should be 35% or more. Preferably, it should be 38% or more. The total reduction ratio in the recrystallization temperature range should be more preferably 40% or more, and even more preferably 43% or more. On the other hand, if the total reduction ratio in the recrystallization temperature range exceeds 55%, grain coarsening can be suppressed, but the reduction in the non-recrystallized region is insufficient, and grain refinement in the final product cannot be achieved. Therefore, the total reduction ratio in the recrystallization temperature range should be 55% or less. Preferably, it should be 52% or less. The total reduction ratio in the recrystallization temperature range should be more preferably 50% or less, and even more preferably 48% or less. Here, the lower limit temperature Tnr of recrystallization can be determined, for example, from the composition of the steel using the following formula. The surface temperature of the steel plate can be measured using a radiation thermometer or similar device. The total reduction ratio in the recrystallization temperature range refers to the total reduction ratio above the lower limit temperature Tnr of recrystallization, which is calculated using the following formula. Tnr(℃)=174×log[%Nb][%C+12 / 14%N]+1444 However, [%X] indicates the mass %) content of element X in the steel.

[0058] [Reduction ratio of the final rolling pass in the recrystallization temperature range: 10% or more] In addition to setting the total reduction ratio in the recrystallization temperature range to 35% or more and 55% or less as described above, it is necessary to ensure a sufficient reduction ratio in the final rolling pass in the recrystallization temperature range and sufficiently promote recrystallization so that partial recrystallization rolling can be started with a uniform grain state free of coarse grains. If the reduction ratio of the final rolling pass in the recrystallization temperature range is less than 10%, recrystallization is insufficient, and coarse grains will grow during the holding time from rough rolling to the start of finish rolling. Therefore, the reduction ratio of the final rolling pass in the recrystallization temperature range should be 10% or more. Preferably, the reduction ratio of the final rolling pass in the recrystallization temperature range should be 11% or more. More preferably, the reduction ratio of the final rolling pass in the recrystallization temperature range should be 13% or more, and even more preferably 15% or more. There is no particular upper limit to the reduction ratio of the final rolling pass in the recrystallization temperature range, and a higher is preferable, but productivity decreases significantly if it exceeds 70%, so it is preferable to keep it at 70% or less.

[0059] [Reduction ratio of the final rolling pass at a recrystallization temperature of -80°C or higher: 15% or more] Since partial recrystallization occurs even after the completion of rolling in the recrystallization region, further increasing the reduction ratio can promote recrystallization and is effective in refining the top 20% particle size. Therefore, the reduction ratio of the final rolling pass at a recrystallization temperature of -80°C or higher should be 15% or higher. Preferably, the reduction ratio of the final rolling pass at a recrystallization temperature of -80°C or higher should be 16% or higher. More preferably, the reduction ratio of the final rolling pass at a recrystallization temperature of -80°C or higher should be 18% or higher, and even more preferably 20% or higher. There is no particular upper limit to the reduction ratio of the final rolling pass at a recrystallization temperature of -80°C or higher, and a higher is preferable, but productivity decreases significantly if it exceeds 40%, so it is preferable to keep it at 40% or lower.

[0060] Rolling below the recrystallization temperature (-80°C) is effective for grain refinement because lower temperatures introduce more strain. Therefore, it is preferable to roll at low temperatures within the range where the cooling start temperature for controlled cooling can be maintained.

[0061] In the hot rolling process, a lower rolling end temperature is preferable to reduce the size of the crystal grains. However, from the perspective of ensuring resistance to HISC under high-pressure hydrogen environments, the rolling end temperature must be set so that the cooling start temperature for controlled cooling is above the Ar3 point on the surface temperature of the hot-rolled steel sheet. Here, the Ar3 point refers to the ferrite transformation start temperature during cooling, which can be determined, for example, from the composition of the steel using the following formula. The surface temperature of the hot-rolled steel sheet can be measured using a radiation thermometer or the like. Ar3(℃)=910-310[%C]-80[%Mn]-20[%Cu]-15[%Cr]-55[%Ni]-80[%Mo] However, [%X] indicates the mass %) content of element X in the steel.

[0062] Cooling process after rolling (controlled cooling process) [Controlled cooling start temperature: Surface temperature of hot-rolled steel sheet above the Ar3 transformation point] If the surface temperature of the hot-rolled steel sheet at the start of cooling is below the Ar3 transformation point (Ar3 point), ferrite will form before controlled cooling, resulting in a significant decrease in strength. For this reason, the surface temperature of the hot-rolled steel sheet at the start of cooling should be at or above the Ar3 transformation point. Preferably, the surface temperature of the hot-rolled steel sheet at the start of cooling should be at or above the Ar3 transformation point + 20°C, and more preferably at or above the Ar3 transformation point + 50°C. Note that the surface temperature of the hot-rolled steel sheet at the start of cooling is the temperature at the tail end of the hot-rolled steel sheet where the cooling start temperature is lowest. Preferably, the surface temperature of the hot-rolled steel sheet at the start of cooling should be at or below the Ar3 transformation point + 120°C, and more preferably at or below the Ar3 transformation point + 80°C.

[0063] [Difference in cooling start time between the leading and trailing ends of controlled-cooling hot-rolled steel sheets: within 50 seconds] If the time difference between the leading and trailing ends of the hot-rolled steel sheet in the rolling direction at the start of cooling exceeds 50 seconds, the temperature difference between the leading and trailing ends at the start of cooling becomes large, resulting in large temperature variations at the end of cooling. This leads to large variations in Vickers hardness at 0.25 mm from the surface of the steel material (or the inner surface of the steel pipe in the case of a steel pipe) and deterioration of HISC resistance. Therefore, the time difference between the starting of cooling at the leading and trailing ends of the hot-rolled steel sheet should be 50 seconds or less. Preferably, the time difference between the starting of cooling should be 45 seconds or less. More preferably, the time difference between the starting of cooling should be 40 seconds or less, and even more preferably, 32 seconds or less. It is possible to shorten the time difference between the starting of cooling by shortening the length of the hot-rolled steel sheet, but this reduces manufacturability. Therefore, it is preferable to shorten the time difference between the starting of cooling by increasing the conveying speed of the hot-rolled steel sheet. The time difference between the starting of cooling may be 0 seconds, but from the viewpoint of manufacturability, it is preferable to have a time difference of 20 seconds or more.

[0064] [Average cooling rate of controlled cooling] Average cooling rate from 750°C to 550°C at the center of the plate thickness: 15-50°C / s If the average cooling rate from 750°C to 550°C in the center of the plate thickness is less than 15°C / s, the desired bainite structure containing granular bainite cannot be obtained, resulting in a decrease in strength. For this reason, the average cooling rate in the center of the plate thickness should be 15°C / s or higher. From the viewpoint of suppressing variations in the structure, it is preferable that the average cooling rate in the center of the plate thickness be 17°C / s or higher. It is preferable that the average cooling rate in the center of the plate thickness be 20°C / s or higher, and more preferably 25°C / s or higher. On the other hand, in order to suppress variations in the grain size of the bainite structure, the average cooling rate should be 50°C / s or lower. It is preferable that the average cooling rate be 48°C / s or lower, and more preferably 45°C / s or lower. It is even more preferable that the average cooling rate be 42°C / s or lower, and most preferably 38°C / s or lower. Note that there are no particular limitations on cooling to a hot-rolled steel plate temperature of 550°C or lower in the center of the plate thickness, but from the viewpoint of suppressing variations in structure and grain size, it is preferable that the average cooling rate be 15°C / s or higher and 50°C / s or lower. In other words, for cooling below 550°C, it is preferable that the average cooling rate be 15°C / s or higher. More preferably, the average cooling rate is 30°C / s or higher, and even more preferably, 35°C / s or higher. For cooling below 550°C, it is preferable that the average cooling rate be 50°C / s or lower. More preferably, the average cooling rate is 48°C / s or lower, and even more preferably, 42°C / s or lower. The average cooling rate below 550°C is the average value of the cooling rates from 550°C to 250°C.

[0065] [Cooling stop temperature: 250~650℃] If the cooling stop temperature at the center of the plate thickness after hot rolling exceeds 650°C, the material strength decreases significantly. Furthermore, from the viewpoint of obtaining a uniform bainite structure, the cooling stop temperature at the center of the plate thickness should be 650°C or lower. Preferably, the cooling stop temperature at the center of the plate thickness should be 620°C or lower, more preferably 615°C or lower, and even more preferably 600°C or lower. On the other hand, if the cooling stop temperature at the center of the plate thickness is below 250°C, quench cracks are more likely to occur during cooling. Also, in order to obtain a uniform bainite structure, the cooling stop temperature should be 250°C or higher. Preferably, the cooling stop temperature at the center of the plate thickness should be 300°C or higher, more preferably 350°C or higher, and even more preferably 380°C or higher. From the viewpoint of suppressing the amount of hydrogen in the steel, it is also necessary to set the cooling stop temperature above a predetermined temperature. Specifically, hydrogen present in the steel gradually escapes during cooling, and the effect is greater at higher temperatures, but if the cooling stop temperature is too low, supercooling occurs, and hydrogen remains in the steel. Furthermore, if the cooling stop temperature is set too low, retained austenite, which rapidly increases the amount of hydrogen compared to other phases, is more likely to form. Therefore, in order to reduce the amount of hydrogen in the steel, the cooling stop temperature needs to be 250°C or higher. After cooling stop, it is fine to let it cool naturally, but it is more preferable to cool it slowly until the temperature drops by about 50°C from the cooling stop temperature in order to promote the formation of bainite.

[0066] [Dehydrogenation treatment (preferred conditions)] The presence of hydrogen in steel accelerates fatigue crack propagation and reduces fatigue life. Therefore, it is preferable to use dehydrogenation treatment 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. Furthermore, hydrogen can be dehydrogenated by holding it at room temperature for an extended period. Since holding at room temperature requires a longer holding time, a holding time of 96 hours or more is preferable. Additionally, since scale on the steel surface inhibits dehydrogenation, it is preferable to remove the scale before performing the dehydrogenation treatment. The holding time R (sec) is determined by the plate thickness and pipe thickness t (mm) of the steel plate and 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 fabrication or welding to join steel pipes. It is preferable to use high temperatures for the dehydrogenation treatment because the hydrogen diffusion coefficient D decreases at high temperatures, allowing hydrogen to escape more quickly. In the case of high temperatures, the calculation may be performed using the diffusion coefficient D' (diffusion coefficient at each temperature) at the temperature that maintains the value of D in equation (A) above. 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 refers to 20±10°C.

[0067] In particular, when heating, it takes time for the temperature Tc at the center of the thickness of the steel material and steel pipe to reach the ambient temperature (dehydrogenation treatment temperature T) in the dehydrogenation treatment process. Therefore, even if the above holding time R (sec) is met at the ambient temperature, if the center of the thickness has not reached the dehydrogenation treatment temperature T (ambient temperature), the dehydrogenation treatment may be insufficient. For this reason, it is preferable to hold the center of the thickness Tc for R time (sec) or more after it reaches the target dehydrogenation treatment temperature T. Furthermore, in order to obtain hydrogen fracture toughness in a predetermined 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 dehydrogenation treatment temperature T (ambient temperature) for R (sec) or more as defined by equation (A), and it is even more preferable to hold the center of the thickness Tc for R (sec) or more after it 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 plate thickness temperature, or the temperature at the center of the plate thickness Tc, can be measured using thermocouples or other methods, or it can be predicted using methods such as the finite element method.

[0068] 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 by removing a thickness of approximately 100 μm.

[0069] Second Embodiment Furthermore, UOE steel pipes, which can be cited as an example of steel pipes for high-strength line pipes, can be obtained by limiting the manufacturing conditions as shown below, and the manufacturing method and conditions will be explained in detail. The component composition, microstructure, and hydrogen-induced crack propagation lower limit K of UOE steel pipes. IHThe process is the same as described for the steel plate in the first embodiment, and the manufacturing method, including the molten steel process, heating process, hot rolling process, controlled cooling process after hot rolling, and dehydrogenation treatment process, is carried out in the same manner as described for the steel material. The pipe-making process after rolling will be described in detail below.

[0070] Pipe making process UOE steel pipes are manufactured by bending hot-rolled steel sheets, specifically by beveling the ends of the hot-rolled steel sheets, forming them into a steel pipe shape using C-press, U-press, and O-press, then seam welding the butt joints using internal and external welding, and further expanding the pipe as needed. Any welding method is acceptable as long as sufficient joint strength and toughness can be obtained, but submerged arc welding is preferred from the viewpoint of excellent welding quality and manufacturing efficiency. Furthermore, pipe expansion can also be performed on steel pipes that have been formed into a tubular shape by press bending and then seam-welded at the butt joints. In addition, if the above-mentioned inclusions are present in the heat-affected zone after pipe manufacturing, they act as hydrogen accumulation sources similar to the base material, causing deterioration of HIC resistance and KIH. Reducing the content of S or O is also effective in reducing inclusions in the welded area. For this reason, it is preferable to set the average cooling rate of the steel pipe in the temperature range of 1500°C to 1000°C after welding to 50°C / min or more. The average cooling rate is more preferably 55°C / min or higher, and even more preferably 60°C / min or higher. There is no particular upper limit, but the average cooling rate is preferably 100°C / min or lower.

[0071] Third Embodiment Furthermore, an example of a high-strength steel pipe for line pipes according to the present invention is an electric resistance welded (ERW) steel pipe, which can be obtained by limiting the manufacturing conditions as shown below. The manufacturing method and conditions will be explained in detail. The composition of the steel material, the microstructure, and the lower limit K of hydrogen-induced crack propagation. IH The process is the same as described for the steel material in the first embodiment, and the manufacturing method is also carried out in the same manner as described for the steel material, except for the cooling process after rolling and the pipe-making process (molten steel process, heating process, hot rolling process, dehydrogenation treatment process).

[0072] Cooling process after rolling (controlled cooling process) The cooling start temperature and average cooling rate of the controlled cooling are the same as those described in the first embodiment.

[0073] [Cooling stop temperature: 250~650℃] If the cooling stop temperature at the center of the plate thickness after hot rolling exceeds 650°C, the material strength decreases significantly. Also, from the viewpoint of obtaining a uniform bainite structure, the cooling stop temperature at the center of the plate thickness should be 650°C or lower. Preferably, the cooling stop temperature at the center of the plate thickness should be 620°C or lower, more preferably 615°C or lower, and even more preferably 600°C or lower. On the other hand, if the cooling stop temperature at the center of the plate thickness is below 250°C, quench cracks are likely to occur during cooling. For this reason, the cooling stop temperature at the center of the plate thickness should be 250°C or higher. Preferably, the cooling stop temperature at the center of the plate thickness should be 300°C or higher, more preferably 350°C or higher, and even more preferably 380°C or higher. To reliably suppress the formation of a hard structure on the surface of the steel plate, the cooling stop temperature at the center of the plate thickness should be 450°C or higher. After cooling stops, it is sufficient to allow it to cool naturally, but to promote bainite formation, it is more preferable to cool it slowly until the temperature drops by about 50°C from the cooling stop temperature.

[0074] Subsequently, the hot-rolled steel sheet obtained as described above is wound into a coil. The winding temperature is preferably 650°C or lower. More preferably 620°C or lower, even more preferably 615°C or lower, and still more preferably 600°C or lower. The lower limit is preferably 250°C or higher, more preferably 300°C or higher, even more preferably 350°C or higher, and most preferably 380°C or higher.

[0075] Pipe making process An example of the present invention is an electric resistance welded (ERW) steel pipe, which is manufactured by forming it into a cylindrical shape by cold roll forming and then welding the circumferential ends of the cylindrical shape together. Furthermore, it may also be manufactured by forming an ERW steel pipe material (ERW steel pipe) using a sizing roll that satisfies the following equation (1) (sizing step), and then applying an internal pressure p (MPa) that satisfies the following equation (2) to the inner surface of the ERW steel pipe material (internal pressure loading step). Furthermore, the term "cylindrical" refers to a pipe whose circumferential cross-section is "C" shaped.

[0076] Diameter of sizing roll (mm) ≥ Thickness of hot-rolled steel sheet (mm) / 0.020 ... (1) The thickness of a hot-rolled steel sheet refers to the thickness of the hot-rolled steel sheet before the sizing process. X <p≦X×1.5 ···(2) Note that X = (wall thickness of electric resistance welded steel pipe material (mm) / radius of electric resistance welded steel pipe material (mm)) × yield strength of electric resistance welded steel pipe material (MPa) The aforementioned internal pressure load can be implemented, for example, by sealing the pipe end with a rubber gasket and applying water pressure inside the pipe. Furthermore, to stabilize the shape, a mold of the desired diameter can be used as an outer frame if necessary.

[0077] Furthermore, the wall thickness of the electric resistance welded (ERW) steel pipe material, which is listed as an example of the steel pipe of the present invention, is preferably 5 mm or more. The wall thickness of the ERW steel pipe material is preferably 30 mm or less. Although there is no upper limit specified for the radius of the ERW steel pipe material, since a larger radius increases the load on the equipment, the radius of the ERW steel pipe material is preferably 400 mm or less. The radius of the ERW steel pipe material is preferably 200 mm or more. In addition, the yield strength of the ERW steel pipe material is preferably 480 MPa or more in order to withstand the gas pressure of pipeline operation. A yield strength of 500 MPa or more is more preferable. On the other hand, in order to avoid increased susceptibility to hydrogen embrittlement, a yield strength of 560 MPa or less is preferable. A yield strength of 550 MPa or less is more preferable.

[0078] During the sizing process, bending deformation occurs in the axial direction of the pipe along the roll shape as the pipe passes through the roll, generating residual stress in the axial direction of the pipe. The greater the bending strain in the bending deformation, the greater the absolute value of the residual stress in the axial direction of the pipe. The bending strain increases as the diameter of the sizing roll decreases and as the thickness of the hot-rolled steel sheet increases. Therefore, in this invention, from the viewpoint of reducing shear residual stress, the diameter of the sizing roll is set to satisfy equation (1) above in order to reduce the absolute value of residual stress in the axial direction of the pipe. If the diameter of the sizing roll is less than the right-hand side of equation (1) above, the shear residual stress targeted by the present invention cannot be obtained. Although there is no upper limit specified for the diameter of the sizing roll, it is preferable that the diameter of the sizing roll be 2000 mm or less, as a larger sizing roll increases the load on the equipment.

[0079] 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.

[0080] The left-hand side (X) of equation (2) above corresponds to the internal pressure p when the tensile stress generated in the circumferential direction of the pipe is equal to the yield stress of the electric resistance welded steel pipe material. In this invention, from the viewpoint of reducing shear residual stress, the internal pressure p is set to a value greater than (X) on the left side of equation (2) in order to reduce the absolute value of residual stress in the axial direction of the pipe, and the electric resistance welded steel pipe material is expanded to the plastic region. On the other hand, if the internal pressure p exceeds (X × 1.5) on the right side of equation (2), the absolute value of residual stress in the circumferential direction of the pipe decreases, but the amount of work hardening due to pipe expansion becomes too large, the dislocation density on the pipe surface increases, and the fracture toughness in hydrogen decreases.

[0081] As explained above, for high-strength steel pipes, the high-strength steel material of this disclosure can be formed into a tubular shape by press bending, roll forming, UOE forming, etc., and then the butt joints can be welded to produce high-strength steel pipes for sour line pipes (UOE steel pipes, electric resistance welded steel pipes, spiral steel pipes, etc.) with excellent material uniformity within the steel plate, suitable for the transportation of crude oil and natural gas. Furthermore, by using the high-strength steel plate of this disclosure for steel pipes, it is possible to produce steel pipes with excellent HISC resistance even if a high-hardness region exists in the welded area. [Examples]

[0082] The present invention will be described in more detail below based on the examples. The following examples illustrate preferred examples of the present invention, and the present invention is not limited in any way by the following examples.

[0083] Slabs with the component compositions shown in Tables 1-1 and 1-2 were prepared, and these slabs were hot-rolled, controlled-cooled, and dehydrogenated to obtain steel material. Steel pipes were then manufactured by shaping this steel material. The manufacturing conditions are shown in Tables 2-1 and 2-2. For Nos. 2-6, 12-22, 35, and 37, the obtained steel material (hot-rolled steel sheet) was bent and the ends were butt-welded to form a pipe. For Nos. 7-11, 23-33, 36, and 38, 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. For Nos. 1 and 34, the steel material was left as is. The results of the evaluation of the metallographic structure and material properties of the obtained steel materials and steel pipes are shown in Tables 3-1 and 3-2. The evaluation method is as follows.

[0084] Measurement of retained austenite Samples for metallographic observation were taken from the center of the plate width in the longitudinal direction of the steel material and steel pipe obtained according to the above procedure. The cross section parallel to the longitudinal direction was used as the observation surface and buffed, and then the surface layer was removed by chemical polishing using picric acid etching, and measurements were taken using X-ray diffraction. Specifically, a Co-Kα source was used for the incident X-rays, and the area fraction of retained austenite was calculated from the intensity ratio of the (200), (211), (220) planes of ferrite and the (200), (220), (311) planes of austenite.

[0085] Calculation of the maximum grain size and area fraction of bainite Samples for microstructure observation were taken from the center of the plate width of the steel material and steel pipe obtained according to the above procedure, and the cross section parallel to the direction of compression extension of these samples was used as the observation surface. After mirror polishing the observation surface, etching was performed with colloidal silica, and crystal data was collected using the EBSD (Electron Backscatter Diffraction) method in a 1 mm × 1 mm field of view at the center of the sample (measurement step: 0.8 μm). The grain size was defined as the area grain size (weighted average when the boundary with an orientation difference of 15° or more is defined as a grain boundary). Each grain size was determined from the above crystal data, and the maximum grain size was determined. Furthermore, regarding the area fraction, the observation surface described above was etched using a 3 vol% nital solution, and scanning electron microscope (SEM) images were taken at an appropriate magnification between 1000 and 5000x to observe bainite. Bainite was identified visually by comparing it with the tissue image in Non-Patent Literature 1. Based on this assessment, the tissue fraction was determined by binarizing the bainite and other regions in the SEM image and performing image analysis to obtain the area fraction of bainite.

[0086] Observation of inclusions and calculation of their number density. Samples for metallographic observation were taken from the center of the plate width at the longitudinal center of the steel material and steel pipe obtained according to the above procedure, and the cross section parallel to the longitudinal direction of the sample was used as the observation surface. After mirror polishing the observation surface, etching was performed with colloidal silica, and observation was carried out using a scanning electron microscope (SEM) at the center of the sample with a field of view of 10 mm × 10 mm. The observation magnification was set to 2000 to 5000 times, and the average of the three fields of view was taken as the number density of inclusions.

[0087] Tensile strength (TS) From the steel materials and steel pipes obtained according to the above, JIS No. 14 proportional test specimens (parallel section diameter 7 mm, gauge length 35 mm) were taken in accordance with JIS Z 2201, and their tensile strength was measured.

[0088] Hydrogen temperature rise analysis The amount of hydrogen remaining in the steel was determined using a low-temperature heating-type hydrogen analyzer (gas chromatograph type) (JTF-20AL) with a heating-intensity desorption analysis method. The heating-intensity desorption analysis was performed at a heating rate of 200°C / h in the temperature range from room temperature to 400°C, and the sum of these results was taken as the hydrogen amount. The test specimens were cylindrical in shape with a length of 30 mm in the longitudinal direction of the steel pipe and a diameter of 7Φ, located at the 1 / 4 position of the steel plate thickness and the 1 / 4 position from the inner surface of the steel pipe. This hydrogen amount was measured before the high-pressure hydrogen fatigue test described later in the aging section, and corresponds to the H amount shown in Tables 1-1 and 1-2.

[0089] High-pressure hydrogen gas fracture toughness test The tests were conducted in accordance with ASTM E1820 at room temperature (20±10℃) and a pressure of 25 MPa using hydrogen gas (containing 100% hydrogen), or in a mixed atmosphere of hydrogen and natural gas (mainly composed of hydrocarbons such as methane and ethane) with a hydrogen partial pressure of 1 MPa or higher at the above temperature and pressure. CT specimens (plate thickness 12.7 mm, plate width 25.4 mm) were used, and samples were taken in an orientation parallel to the direction of machine notch introduction and the rolling direction of the steel. Fatigue pre-cracks were introduced in air under the following conditions: frequency: 1 Hz, repeated load waveform: sinusoidal, control method: K-value control, stress ratio R: 0.1. Subsequently, the samples were subjected to a hydrogen gas or hydrogen gas + natural gas mixed atmosphere. Fracture toughness tests were performed using the unloading-elastic compliance method with a single specimen. The crosshead displacement rate during load application was set to 0.002 mm / sec.

[0090] The evaluation results are shown in Tables 3-1 and 3-2. All steel materials and steel pipes that satisfy the examples of the present invention have a hydrogen-induced crack propagation lower limit K. IH 80 MPa·m 1 / 2 The material exhibited excellent resistance to fracture in hydrogen, with a tensile strength of 520 MPa or higher. Similar results were obtained for the steel pipes shown in Tables 3-1 and 3-2.

[0091] [Table 1-1]

[0092] [Table 1-2]

[0093] [Table 2-1]

[0094] [Table 2-2]

[0095] [Table 3-1]

[0096] [Table 3-2] [Examples]

[0097] The following describes examples that verify 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. Using steel grades No. 2, 4, 8, 14, 22, and 33 shown in Tables 1-1 and 1-2 used in Example 1, the steel materials were manufactured under the same conditions as steel pipes 2, 4, 8, 14, 22, and 33 shown in Example 1 (Tables 2-1 and 2-2) up to the controlled cooling process, and the steel pipe forming was also carried out under the same conditions as in Example 1. The characteristics were then evaluated when the dehydrogenation treatment conditions were changed. The results are shown in Table 4.

[0098] The dehydrogenation treatment of steel pipes No. 2, 4, 8, 14, 22, and 33 in Example 1 was carried out at the dehydrogenation treatment temperature T (ambient temperature) shown in Tables 2-1 and 2-2, respectively. This corresponds to the dehydrogenation holding time t in Table 4 as Y, and the holding time tc at the plate thickness center temperature Tc as N.

[0099] For steel pipes No. 2A, 4A, 8A, 14A, 22A, and 33A, the dehydrogenation treatment temperature T was set to the temperatures shown in Table 4, and the holding time tc after the plate thickness center temperature Tc reached the dehydrogenation treatment temperature T shown in Table 4 was carried out so as to satisfy equation (A).

[0100] For steel pipes No. 2B, 4B, 8B, 14B, 22B, and 33B, the dehydrogenation treatment temperature T is as shown in Table 4, but the holding time t of the ambient temperature and the holding time tc after the plate thickness center temperature Tc reaches the above-mentioned dehydrogenation treatment temperature T do not satisfy equation (A) described above.

[0101] In Table 4, "Dehydrogenation holding time t is Y" means that the dehydrogenation treatment temperature T (ambient temperature) is set to a predetermined temperature 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 set to a predetermined temperature, 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 core temperature Tc reaches a predetermined temperature satisfies equation (A), while "Holding time tc at steel core temperature Tc is N" means that the plate core temperature Tc reaches a predetermined temperature, but the holding time tc after Tc reaches a predetermined temperature does not satisfy equation (A).

[0102] The hydrogen fracture toughness, tensile strength, microstructure, and inclusions were investigated using the same methods as in Example 1.

[0103] All examples of the present invention relate to the lower limit K of hydrogen-induced crack propagation. IH 80 MPa·m 1 / 2 In summary, the condition of a tensile strength of 520 MPa or higher was satisfied. Among these, the hydrogen fracture resistance was superior when the dehydrogenation treatment was carried out under more favorable conditions.

[0104] Furthermore, the same results were obtained for steel pipes in Table 4 as for steel materials.

[0105] [Table 4]

Claims

1. In mass percent, C: 0.02-0.15%, Si: 0.01-2.0%, Mn: 0.5-1.5%, P: 0.0001-0.015%, S: 0.0002-0.0015%, Al: 0.005-0.15%, O: 0.01% or less, N: 0.010% or less, Nb: 0.10% or less, H: Includes 0.02 ppm or less, Or, furthermore, Ca: 0-0.005%, Ni: 0-2.0%, Ti: 0 to 0.1%, Cu: 0 to 1.0%, Cr: 0-1.0%, Mo: 0 to 0.60%, W: 0-1.0%, V: 0-0.10%, Zr: 0 to 0.050%, Mg: 0 to 0.01%, REM: 0-0.01%, B: 0 to 0.0020%, Ta: 0-0.2%, Hf: 0-0.2%, Re: 0 to 0.005%, Sn: 0-0.3%, Sb: Contains one or more selected from 0 to 0.3%, The composition has the remainder being Fe and unavoidable impurity elements. Fifteen bainite inclusions with an aspect ratio of 2.0 or greater and a length of 10 μm or greater were found per 100 mm. 2 The metal structure is as follows: The maximum particle size of the bainite in the range from the surface of the steel material to the center of the plate thickness is 25 μm or less. The tensile strength is 520 MPa or more, In a high-pressure hydrogen gas environment with a pressure of 25 MPa, the lower limit K for hydrogen-induced crack propagation is... IH 80 MPa·m 1 / 2 The above-mentioned steel material for high-strength line pipes is a thin or thick steel plate with excellent hydrogen fracture toughness.

2. The aforementioned component composition is expressed in mass%, and further, Ca: 0.0001-0.005%, Ni: 0.01-2.0%, Ti: 0.005-0.1%, Cu: 0.01-1.0%, Cr: 0.01-1.0%, Mo: 0.01 to 0.60%, W: 0.01-1.0%, V: 0.01-0.10%, Zr: 0.0001 to 0.050%, Mg: 0.0001-0.01%, REM: 0.0001-0.01%, B: 0.0001 to 0.0020%, Ta: 0.0001-0.2%, Hf: 0.0001-0.2%, Re: 0.0001-0.005%, Sn: 0.0001-0.3%, A high-strength steel material for line pipes with excellent hydrogen fracture toughness according to claim 1, comprising one or more Sb selected from 0.0001 to 0.3%.

3. A high-strength steel material for line pipes with excellent hydrogen fracture toughness according to claim 1 or 2, wherein the retained austenite is 0 to 3% in area fraction, and the bainite in the range from the steel surface to the center of the plate thickness is 90% or more in area fraction.

4. A method for producing a high-strength steel material for line pipes with excellent hydrogen fracture toughness according to claim 1 or 2, A heating step of heating a cast slab having the above-mentioned component composition at 1000 to 1250°C, The cast slab heated in the heating step is subjected to the following conditions: a total reduction ratio of 35% to 55% in the recrystallization temperature range, a reduction ratio of 10% or more in the final rolling pass in the recrystallization temperature range, and a reduction ratio of 15% or more in the final rolling pass at (recrystallization temperature - 80°C) or higher, and the rolling completion temperature is Ar 3 A hot rolling process in which rolling is performed under conditions above the transformation point, The hot-rolled steel sheet obtained in the hot-rolling process is cooled at a temperature where the surface temperature of the hot-rolled steel sheet is Ar 3 A controlled cooling process is performed under the conditions that the temperature is above the transformation point, the difference in cooling start time between the leading and trailing ends of the hot-rolled steel sheet is within 50 seconds, the average cooling rate from 750°C to 550°C is 15 to 50°C / s at the center temperature of the sheet thickness, and the cooling stop temperature is 250 to 650°C. A method for manufacturing high-strength steel material for line pipes that has excellent hydrogen fracture toughness.

5. A method for manufacturing a high-strength steel material for line pipes with excellent hydrogen fracture toughness as described in claim 3, A heating step of heating a cast slab having the above-mentioned component composition at 1000 to 1250°C, The cast slab heated in the heating step is subjected to the following conditions: a total reduction ratio of 35% to 55% in the recrystallization temperature range, a reduction ratio of 10% or more in the final rolling pass in the recrystallization temperature range, and a reduction ratio of 15% or more in the final rolling pass at (recrystallization temperature - 80°C) or higher, and the rolling completion temperature is Ar 3 A hot rolling process in which rolling is performed under conditions above the transformation point, The hot-rolled steel sheet obtained in the hot-rolling process is cooled at a temperature where the surface temperature of the hot-rolled steel sheet is Ar 3 A controlled cooling process is performed under the conditions that the temperature is above the transformation point, the difference in cooling start time between the leading and trailing ends of the hot-rolled steel sheet is within 50 seconds, the average cooling rate from 750°C to 550°C is 15 to 50°C / s at the center temperature of the sheet thickness, and the cooling stop temperature is 250 to 650°C. A method for manufacturing high-strength steel material for line pipes that has excellent hydrogen fracture toughness.

6. In mass percent, C: 0.02-0.15%, Si: 0.01-2.0%, Mn: 0.5-1.5%, P: 0.0001-0.015%, S: 0.0002-0.0015%, Al: 0.005-0.15%, O: 0.01% or less, N: 0.010% or less, Nb: 0.10% or less, H: Includes 0.02 ppm or less, Or, furthermore, Ca: 0-0.005%, Ni: 0-2.0%, Ti: 0 to 0.1%, Cu: 0 to 1.0%, Cr: 0-1.0%, Mo: 0 to 0.60%, W: 0-1.0%, V: 0-0.10%, Zr: 0 to 0.050%, Mg: 0 to 0.01%, REM: 0-0.01%, B: 0 to 0.0020%, Ta: 0-0.2%, Hf: 0-0.2%, Re: 0 to 0.005%, Sn: 0-0.3%, Sb: Contains one or more selected from 0 to 0.3%, The composition has the remainder being Fe and unavoidable impurity elements. Fifteen bainite inclusions with an aspect ratio of 2.0 or greater and a length of 10 μm or greater were found per 100 mm. 2 The metal structure is as follows: The maximum particle size of the bainite in the range from the surface of the inner surface of the steel pipe to the center of the plate thickness is 25 μm or less. The tensile strength is 520 MPa or more, In a high-pressure hydrogen gas environment where the hydrogen gas has a pressure of 25 MPa, a high-strength line pipe steel pipe with excellent hydrogen-induced cracking toughness, where the lower limit of hydrogen-induced crack propagation K IH is 80 MPa·m 1 / 2 or higher.

7. The aforementioned component composition is expressed in mass%, and further, Ca: 0.0001-0.005%, Ni: 0.01-2.0%, Ti: 0.005-0.1%, Cu: 0.01-1.0%, Cr: 0.01-1.0%, Mo: 0.01 to 0.60%, W: 0.01-1.0%, V: 0.01-0.10%, Zr: 0.0001 to 0.050%, Mg: 0.0001-0.01%, REM: 0.0001-0.01%, B: 0.0001 to 0.0020%, Ta: 0.0001-0.2%, Hf: 0.0001-0.2%, Re: 0.0001-0.005%, Sn: 0.0001-0.3%, A high-strength steel pipe for line pipes with excellent hydrogen fracture toughness according to claim 6, comprising one or more Sb selected from 0.0001 to 0.3%.

8. In steel pipes for high-strength line pipes, A high-strength steel pipe for line pipes with excellent hydrogen fracture toughness according to claim 6 or 7, wherein the retained austenite is 0 to 3% in area fraction, and the bainite in the range from the surface of the inner surface of the steel pipe to the center of the plate thickness is 90% or more in area fraction.

9. A method for manufacturing a high-strength steel pipe for line pipes with excellent hydrogen fracture toughness according to claim 6 or 7, A heating step of heating a cast slab having the above-mentioned component composition at 1000 to 1250°C, The cast slab heated in the heating step is subjected to the following conditions: a total reduction ratio of 35% to 55% in the recrystallization temperature range, a reduction ratio of 10% or more in the final rolling pass in the recrystallization temperature range, and a reduction ratio of 15% or more in the final rolling pass at (recrystallization temperature - 80°C) or higher, and the rolling completion temperature is Ar 3 A hot rolling process in which rolling is performed under conditions above the transformation point, The hot-rolled steel sheet obtained in the hot-rolling process is cooled at a temperature where the surface temperature of the hot-rolled steel sheet is Ar 3 A controlled cooling process is performed under the conditions that the temperature is above the transformation point, the difference in cooling start time between the leading and trailing ends of the hot-rolled steel sheet is within 50 seconds, the average cooling rate from 750°C to 550°C is 15 to 50°C / s at the center temperature of the sheet thickness, and the cooling stop temperature is 250 to 650°C. After the controlled cooling process, a pipe-making process is performed in which the hot-rolled steel sheet is bent and both ends are butt-welded; or a pipe-making process is performed in which the hot-rolled steel sheet is formed into a cylindrical shape by cold roll forming and both ends of the cylindrical shape are butt-welded using electric resistance welding; A method for manufacturing high-strength steel pipes for line pipes that have excellent hydrogen fracture toughness.

10. A method for manufacturing a high-strength steel pipe for line pipes that has excellent hydrogen fracture toughness according to claim 8, A heating step of heating a cast slab having the above-mentioned component composition at 1000 to 1250°C, The cast slab heated in the heating step is subjected to the following conditions: a total reduction ratio of 35% to 55% in the recrystallization temperature range, a reduction ratio of 10% or more in the final rolling pass in the recrystallization temperature range, and a reduction ratio of 15% or more in the final rolling pass at (recrystallization temperature - 80°C) or higher, and the rolling completion temperature is Ar 3 A hot rolling process in which rolling is performed under conditions above the transformation point, The hot-rolled steel sheet obtained in the hot-rolling process is cooled at a temperature where the surface temperature of the hot-rolled steel sheet is Ar 3 A controlled cooling process is performed under the conditions that the temperature is above the transformation point, the difference in cooling start time between the leading and trailing ends of the hot-rolled steel sheet is within 50 seconds, the average cooling rate from 750°C to 550°C is 15 to 50°C / s at the center temperature of the sheet thickness, and the cooling stop temperature is 250 to 650°C. After the controlled cooling process, a pipe-making process is performed in which the hot-rolled steel sheet is bent and both ends are butt-welded; or a pipe-making process is performed in which the hot-rolled steel sheet is formed into a cylindrical shape by cold roll forming and both ends of the cylindrical shape are butt-welded using electric resistance welding; A method for manufacturing high-strength steel pipes for line pipes that have excellent hydrogen fracture toughness.

Images