Steel material for high-strength line pipe having excellent fracture toughness in hydrogen, method for producing same, steel pipe for high-strength line pipe, and method for producing same

Abstract

The purpose of the present invention is to provide natural gas suitable as 100% hydrogen or hydrogen having a hydrogen partial pressure of 1 MPa or more (natural gas being gas mainly composed of hydrocarbons such as methane or ethane) Provided are: a steel material for high-strength line pipes, which has excellent fracture toughness in hydrogen in a high-pressure hydrogen environment, and which is used in a steel structure used in a high-pressure hydrogen environment, such as a line pipe; a method for producing the steel material; a steel pipe for high-strength line pipes; and a method for producing the steel pipe. A steel material for a high-strength line pipe having excellent fracture toughness in hydrogen, characterized by having a specific composition and a specific structure, the tensile strength of the steel material being 520 MPa or more, and the hydrogen-induced crack propagation threshold value KIH of the steel material being 80 MPa.m1 / 2 or more in a high-pressure hydrogen atmosphere of 1 MPa or more.

Description

Technical Field

[0001] The present invention relates to a high-strength line pipe steel material suitable for use in hydrogen transport line pipes and the like, which has excellent fracture toughness in hydrogen in a high-pressure hydrogen environment of 1 MPa or more, a method for producing the same, a high-strength line pipe steel pipe, and a method for producing the same. Background Art

[0002] As an existing energy infrastructure, there are pipelines for transporting crude oil, natural gas, etc. These steel structures are used in an atmosphere 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, and it has been required to suppress it. In order to prevent the occurrence of hydrogen embrittlement such as hydrogen-induced cracking and sulfide stress corrosion cracking, various countermeasures have been taken, such as reducing the amount of MnS in the steel that becomes the starting point of cracks, suppressing the accumulation of Ti and Nb carbonitrides and oxides, or suppressing the segregation of the hardened phase of center segregation. In addition, from the viewpoint of suppressing the occurrence of crack starting points by improving the corrosion resistance of steel, it has been proposed to add Sn and Sb to steel (for example, Patent Documents 1 and 2).

[0003] In recent years, the use of hydrogen has been promoted as a clean energy for the purpose of building a decarbonized society. Therefore, in order to transport a large amount of hydrogen, the construction of a hydrogen transportation network that pressurizes natural gas mixed with hydrogen in a certain proportion in natural gas pipelines and hydrogen as an alternative is studied. The transportation pressure of these pipelines during operation is assumed to be a high pressure of 1 to 40 MPa, and the pipelines are exposed to a high-pressure hydrogen environment. In addition to the characteristics required in the existing acidic environment to inhibit corrosion on the inner surface of the steel pipe and reduce the accumulation of hydrogen in the material, the steel used in such an environment also needs to have the hydrogen resistance required in the hydrogen environment.

[0004] Steel structures used in high-pressure hydrogen environments use austenitic stainless steels such as SUS316L that show fracture toughness in hydrogen. However, steel is expensive and low-strength, and when designed to withstand high hydrogen pressure, the wall thickness becomes thicker and the price of pipeline pipes becomes very expensive, making them unsuitable for pipeline laying. Therefore, as a steel material for hydrogen pipelines, a steel material with lower cost and that can withstand high-pressure hydrogen environments is required.

[0005] In order to solve the above problems, for example, an austenitic steel material with a high Mn content is proposed in Patent Document 3. According to the technology described in Patent Document 3, a steel material with a lower cost than austenitic stainless steel can be provided, but since it is austenitic, the cost is higher than that of low alloy steel. In addition, no consideration is given to suppressing pitting corrosion that becomes the starting point of hydrogen-induced cracking, such as HIC resistance and SSCC resistance.

[0006] In addition, in pipelines, since operations are started and shut down repeatedly, stress is repeatedly applied to the pipeline pipe. Therefore, fatigue fracture needs to be considered when designing steel structures such as pipelines. The limit point of fatigue fracture of steel structures used in high-pressure hydrogen environments corresponds to the limit crack length, which is determined by the operating conditions of the pipeline and the hydrogen-induced crack growth threshold K, which is equivalent to the fracture toughness value of the steel in hydrogen. IH From the perspective of extending the life of hydrogen-using structures and improving safety, it is considered that increasing the K IH is a valid pointer.

[0007] The hydrogen pipeline is assumed to use a line pipe having a welded portion, that is, a weld metal portion, and a heat-affected portion. Patent Document 4 proposes a K IH The method for manufacturing excellent steel materials does not mention the properties of welded parts. Generally speaking, welded parts are more susceptible to the deterioration of properties caused by hydrogen than base materials. Therefore, K including welded parts IH The improvement is important.

[0008] In order to increase the K IH , for example, the upper bainite containing coarse carbides can be reduced.

[0009] Prior art literature

[0010] Patent Literature

[0011] Patent Document 1: Japanese Patent Application Publication No. 2011-26695

[0012] Patent Document 2: Japanese Patent Application Publication No. 2010-209461

[0013] Patent Document 3: Japanese Patent Application No. 2019-505675

[0014] Patent Document 4: International Publication No. 2017 / 047099

[0015] Non-patent literature

[0016] Non-patent literature 1: Japan Heat Treatment Technology Association (author), Introduction: Structure and properties of metal materials - Heat treatment and structure control to make full use of materials, 2004 Summary of the invention

[0017] The present invention has been made to solve the above-mentioned conventional problems, and its purpose is to provide a high-strength pipeline steel material having excellent fracture toughness in hydrogen in a high-pressure hydrogen environment, a manufacturing method thereof, a high-strength pipeline steel pipe, and a manufacturing method thereof, which is suitable for use as a steel structure used in a high-pressure hydrogen environment such as a pipeline pipe for 100% hydrogen or natural gas (natural gas is a gas with hydrocarbons such as methane and ethane as a main component) containing hydrogen at a hydrogen partial pressure of 1 MPa or more. As the high-pressure hydrogen environment, a high-pressure hydrogen of 1 MPa or more or an environment containing 0.2% or more hydrogen is assumed.

[0018] It should be noted that the term "excellent fracture toughness in hydrogen under high-pressure hydrogen environment" here refers to the hydrogen-induced crack growth threshold K obtained by conducting a fracture toughness test under two environments: room temperature (20±10°C) and a hydrogen pressure of 1 MPa or more, or a natural gas (mainly composed of hydrocarbons such as methane and ethane) containing hydrogen at a hydrogen partial pressure of 1 MPa or more. IH 80MPa·m 1 / 2 The above situation. It should be noted that the fracture toughness value refers to the value obtained by implementing the fracture toughness test according to ASTM E399, ASTM E1820 and ASTM E1681. Natural gas containing hydrogen at a hydrogen partial pressure of 1 MPa or more refers to, for example, a gas having a hydrogen concentration of 30% or less in terms of volume fraction and a gas pressure of 30 MPa or less.

[0019] The so-called "steel" here includes thin steel plates, thick steel plates, seamless steel pipes, electric resistance welded steel pipes, steel sections, steel bars, etc.

[0020] The present inventors have conducted technical research on the conditions that steel materials should satisfy in order to obtain high-strength line pipe steel materials and high-strength line pipe steel pipes with excellent fracture toughness in hydrogen under high-pressure hydrogen environments in order to suppress hydrogen absorption into steel materials, which is the root cause of hydrogen embrittlement. As a result, it was found that when the number of inclusions with an aspect ratio of 2.0 or more and a length of 10 μm or more is 15 / 100 mm, the steel materials should be able to withstand the conditions that the steel materials should meet. 2 In the metal structure in which the maximum grain size of the 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 growth threshold K of the steel material and the steel pipe is IH In addition, it was found that if the area fraction of retained austenite is 0 to 3%, and the area fraction of bainite from the surface of the steel material and the steel pipe to the center of the plate thickness is 90% or more, the hydrogen-induced crack growth threshold K of the steel material IH Further improvement. 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 the conditions have been successfully found. The present invention is based on these insights. In addition, high strength in the present invention refers to a tensile strength of 520MPa or more.

[0021] That is, the gist of the present invention is as follows.

[0022] [1] A high-strength steel material for line pipes having excellent fracture toughness in hydrogen, having the following chemical composition: containing, by mass%, C: 0.02-0.15%, Si: 0.01-2.0%, Mn: 0.5-1.5%, P: 0.0001-0.015%, S: 0.0002-0.0015%, Al: 0.005-0.15%, O: 0.01% or less, N: 0.010% or less, Nb: 0.10% or less, H: 0.02ppm or less, or further containing 0-0.005% of Ca, N i: 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: 0-0.3%, and the remainder is Fe and unavoidable impurity elements;

[0023] 15 inclusions / 100 mm having bainite and an aspect ratio of 2.0 or more and a length of 10 μm or more 2 The following metal structures,

[0024] 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.

[0025] The tensile strength of the above steel is 520MPa or more.

[0026] The hydrogen-induced crack growth threshold K of the above steel in a high-pressure hydrogen environment above 1MPa is IH 80MPa·m 1 / 2 above.

[0027] [2] The high-strength line pipe steel material with excellent fracture toughness in hydrogen according to [1], wherein the chemical composition is further composed, in mass%, of 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%, T a: 0.0001 to 0.2%, Hf: 0.0001 to 0.2%, Re: 0.0001 to 0.005%, Sn: 0.0001 to 0.3%, Sb: 0.0001 to 0.3%.

[0028] [3] The high-strength line pipe steel having excellent fracture toughness in hydrogen according to [1] or [2], wherein the retained austenite has an area fraction of 0 to 3%, and the bainite in the range from the surface of the steel to the center of the plate thickness has an area fraction of 90% or more.

[0029] [4] A method for producing a high-strength line pipe steel material having excellent fracture toughness in hydrogen, comprising:

[0030] A heating step of heating the cast slab having the composition described in [1] or [2] at 1000 to 1250° C.

[0031] In the hot rolling step, the total reduction ratio of the cast piece heated in the heating step within the recrystallization temperature range is 35% to 55%, the reduction ratio of the final rolling pass within the recrystallization temperature range is 10% or more, and the reduction ratio of the final rolling pass above (recrystallization temperature - 80°C) is 15% or more, and the rolling end temperature is Ar based on the steel plate surface temperature. 3 Rolling is performed under conditions above the phase transformation point, and

[0032] In a controlled cooling step, the hot-rolled steel sheet obtained in the hot rolling step is cooled to a temperature where the surface temperature of the hot-rolled steel sheet is Ar. 3 The hot rolled steel sheet is cooled above the transformation point, with the cooling start time difference between the front end and the rear end being within 50 seconds, the average cooling rate from 750°C to 550°C being 15 to 50°C / s based on the center temperature of the sheet thickness, and the cooling stop temperature being 250 to 650°C.

[0033] [5] A high-strength steel pipe for line pipe with excellent fracture toughness in hydrogen, having the following chemical composition: containing, 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: 0.02 ppm or less, or further containing 0-0.005% of Ca, N i: 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: 0-0.3%, and the remainder is Fe and unavoidable impurity elements;

[0034] 15 inclusions / 100 mm having bainite and an aspect ratio of 2.0 or more and a length of 10 μm or more 2 The following metal structures,

[0035] The maximum grain 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.

[0036] The tensile strength of the steel pipe is 520 MPa or more.

[0037] The hydrogen-induced crack growth threshold K of the above steel pipe in a high-pressure hydrogen environment above 1MPa is IH 80MPa·m 1 / 2 above.

[0038] [6] The high-strength steel pipe for line pipes with excellent fracture toughness in hydrogen according to [5], wherein the chemical composition is further composed, in mass%, of 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%, T a: 0.0001 to 0.2%, Hf: 0.0001 to 0.2%, Re: 0.0001 to 0.005%, Sn: 0.0001 to 0.3%, Sb: 0.0001 to 0.3%.

[0039] [7] The high-strength steel pipe for line pipe having excellent fracture toughness in hydrogen according to [5] or [6], wherein in the high-strength steel pipe for line pipe,

[0040] The retained austenite has an area fraction of 0 to 3%, and the bainite in the range from the surface of the inner surface of the steel pipe to the center of the plate thickness has an area fraction of 90% or more.

[0041] [8] A method for producing a high-strength steel pipe for line pipe having excellent fracture toughness in hydrogen, comprising:

[0042] A heating step of heating the cast slab having the composition described in [5] or [6] at 1000 to 1250° C.

[0043] In the hot rolling step, the total reduction ratio of the cast piece heated in the above-mentioned heating step within the recrystallization temperature range is 35% to 55%, and the reduction ratio of the final rolling pass within the recrystallization temperature range is 10% or more, and the reduction ratio of the final rolling pass above (recrystallization temperature-80°C) is 15% or more, and the rolling end temperature is Ar based on the steel plate surface temperature. 3 Rolling is carried out under conditions above the phase transformation point.

[0044] In the controlled cooling step, the hot-rolled steel sheet obtained in the hot rolling step is cooled to a temperature where the surface temperature of the hot-rolled steel sheet is Ar. 3 The hot rolled steel sheet is cooled at a temperature above the transformation point, with a cooling start time difference of within 50 seconds between the front end and the rear end, an average cooling rate from 750°C to 550°C at a plate thickness center temperature of 15 to 50°C / s, and a cooling stop temperature of 250 to 650°C; and

[0045] Any of the following pipe making processes, a pipe making process in which, after the controlled cooling process, the hot rolled steel plate is bent, the two ends are butted and welded; a pipe making process in which, after the controlled cooling process, the hot rolled steel plate is formed into a cylindrical shape by cold roll forming, the two circumferential ends of the cylindrical shape are butted and resistance welded.

[0046] According to the present invention, it is possible to easily and simply manufacture steel materials with greatly improved hydrogen fracture toughness in a high-pressure hydrogen environment, which has a significant industrial effect. In addition, according to the present invention, it is also possible to significantly improve the hydrogen absorption resistance of steel structures such as high-pressure hydrogen pipelines, which has a great effect of contributing to improving the safety of steel structures. DETAILED DESCRIPTION

[0047] Next, a method for implementing the present invention will be described in detail.

[0048] As a first embodiment, a steel material is specifically described. Then, as a second embodiment, a UOE steel pipe as an example of the steel pipe of the present invention is specifically described. As a third embodiment, an electric resistance welded steel pipe as an example of the steel pipe of the present invention is specifically described.

[0049] First embodiment

[0050] [Ingredients]

[0051] The reasons for limiting the base material components in the steel material of the present invention are described below. In the following description, all units expressed in % are mass % unless otherwise specified.

[0052] C: 0.02~0.15%

[0053] C effectively contributes to the improvement of strength. When the content is less than 0.02%, sufficient strength cannot be ensured, so the C content is 0.02% or more. Preferably, the C content is 0.03% or more. More preferably, the C content is 0.035% or more. More preferably, the C content is 0.04% or more. On the other hand, if it exceeds 0.15%, the weldability decreases. Therefore, the C amount is limited to 0.15% or less. Preferably, the C content is 0.10% or less. In addition, if it exceeds 0.08%, the hardness of the surface layer and the center segregation part increases during controlled cooling, so sometimes the SSCC resistance and HIC resistance deteriorate. In addition, the toughness also deteriorates. Therefore, the C content is more preferably 0.08% or less. More preferably, the C content is 0.06% or less.

[0054] Si: 0.01~2.0%

[0055] Si is contained for deoxidation. When the content is less than 0.01%, the deoxidation effect is insufficient, so the Si content is 0.01% or more. The Si content is preferably 0.02% or more. More preferably, the Si content is 0.05% or more. It is further preferred that the Si content is 0.08% or more. Since the above effect can be confirmed up to 2.0%, the Si content is 2.0% or less. The Si content is preferably 1.8% or less, and more preferably 1.5% or less. The Si content is further preferably 1.0% or less. However, if it exceeds 0.5%, the toughness and weldability may deteriorate, so the most preferred Si content is 0.5% or less.

[0056] Mn: 0.5~1.5%

[0057] Mn effectively contributes to the improvement of strength and toughness. When the content is less than 0.5%, the effect of its inclusion is insufficient, so the Mn content is 0.5% or more. The Mn content is preferably 0.6% or more, and more preferably 0.8% or more. It is further preferred that the Mn content is 1.0% or more. On the other hand, if it exceeds 1.5%, the hardness of the surface layer and the center segregation part increases during controlled cooling, so the SSCC resistance and HIC resistance deteriorate. In addition, the weldability also deteriorates. Therefore, the Mn amount is limited to 1.5% or less. The Mn content is preferably 1.4% or less. The Mn content is more preferably 1.3% or less, and more preferably 1.2% or less.

[0058] P: 0.0001~0.015%

[0059] P is an inevitable impurity element that deteriorates weldability and increases the hardness of the center segregation portion, thereby deteriorating HIC resistance. If it exceeds 0.015%, this tendency becomes significant, so the P content is limited to 0.015% or less. The P content is preferably 0.012% or less, and more preferably 0.010% or less. It is further preferred that the P content is 0.008% or less. The lower the content, the better, but from the perspective of refining cost, the P content is 0.0001% or more.

[0060] S: 0.0002~0.0015%

[0061] S is an inevitable impurity element. It becomes MnS inclusions in steel and deteriorates HIC resistance. Therefore, it is preferably less, but 0.0015% is allowed. Therefore, the S content is 0.0015% or less. The S content is preferably 0.0010% or less, and more preferably 0.0008% or less. The lower the content, the better, but from the viewpoint of refining cost, it is 0.0002% or more.

[0062] Al: 0.005~0.15%

[0063] Al is added as a deoxidizer, but when it is less than 0.005%, there is no effect, so the Al content is 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 is 0.15% or less. The Al content is preferably 0.12% or less, and more preferably 0.10% or less. It is further preferred that the Al content is 0.08% or less.

[0064] O: 0.01% or less

[0065] O is the cause of the formation of oxide inclusions, so the less the better. If the O content is 0.01% or less, this effect will not be a problem, so the O content is 0.01% or less. The O content is preferably 0.0080% or less. More preferably, the O content is less than 0.0030%. The lower limit is not particularly limited, and can be 0.0005% or more.

[0066] N: 0.010% or less

[0067] N effectively contributes to the improvement of strength. If the content exceeds 0.010%, the hardness increases during controlled cooling, so the toughness deteriorates. Therefore, the N content is 0.010% or less. The N content is preferably 0.008% or less, the N content is more preferably 0.006% or less, and the N content is further preferably 0.004% or less. However, when it is less than 0.00001%, sufficient strength cannot be ensured, and excessive reduction will increase the cost of steelmaking. Therefore, the content is preferably 0.00001% or more. More preferably, the N content is 0.002% or more.

[0068] Nb: 0.10% or less

[0069] Nb is an element effective in improving the strength and toughness of steel. When the content is less than 0.001%, the effect of its inclusion is insufficient, so it is preferably 0.001% or more. On the other hand, if it exceeds 0.10%, the toughness of the weld deteriorates, so the Nb content is 0.10% or less. The Nb content is preferably 0.095% or less. The Nb content is more preferably 0.090% or less, and further preferably 0.085% or less. The Nb content is most preferably 0.080% or less.

[0070] H: 0.02ppm or less

[0071] H is sometimes introduced into steel materials in various steps during manufacturing. If the amount introduced is large, the risk of cracking after solidification may increase, and K IH Significantly reduced. If it is below 0.02ppm, these effects will not be a problem, so the H content is below 0.02ppm. The H content is preferably below 0.015ppm, more preferably below 0.008ppm. The H content is further preferably below 0.005ppm, most preferably less than 0.002ppm. The lower limit is not particularly limited, but from the perspective of manufacturing cost, it is preferably above 0.0008ppm. The H content is more preferably above 0.001ppm. It should be noted that the hydrogen content is the amount of residual hydrogen after forming steel, steel pipe, UOE, etc.

[0072] The chemical composition of the present disclosure may further contain one or more selected from Ca, Ni, Ti, Cu, Cr, Mo, W, V, Zr, Mg, REM, B, Ta, Hf, Re, Sn, and Sb arbitrarily within the following range.

[0073] Ca: 0~0.005%

[0074] Ca is an element effective in improving HIC resistance by controlling the morphology of sulfide inclusions. Therefore, when Ca is contained, the Ca content may be 0% or more, but when it is less than 0.0001%, the effect of addition is insufficient. Therefore, when Ca is contained, the Ca content is preferably 0.0001% or more. More preferably, it is 0.0005% or more. On the other hand, when it exceeds 0.005%, not only the effect is saturated, but also the HIC resistance is deteriorated due to the decrease in the cleanliness of the steel. Therefore, when Ca is contained, the Ca content is limited to 0.005% or less. The Ca content is preferably 0.004% or less. The Ca content is more preferably 0.002% or less, and further preferably 0.0008% or less.

[0075] Ni: 0-2.0%

[0076] Ni is an element effective for improving toughness and increasing strength. When Ni is contained, the Ni content may be 0% or more, but in order to obtain this effect, it is preferably contained at least 0.01%. The Ni content is more preferably 0.1% or more. On the other hand, in order to suppress costs, when Ni is contained, the Ni content is 2.0% or less. The Ni content is preferably 1.8% or less. The Ni content is more preferably 1.4% or less, and further preferably 0.8% or less.

[0077] Ti: 0~0.1%

[0078] Ti contributes to the strength of steel, so when Ti is contained, the Ti content can be 0% or more. In order to obtain the above-mentioned effect, when Ti is contained, it is preferably 0.005% or more. More preferably, it is 0.008% or more. On the other hand, if the content exceeds 0.1%, the effect is saturated, which becomes the main reason for the cost increase, so when Ti is contained, the Ti content is 0.1% or less. The Ti content is preferably 0.08% or less, and more preferably 0.06% or less. In order to suppress costs, the Ti content is further preferably 0.05% or less. The Ti content is most preferably 0.04% or less.

[0079] Cu: 0~1.0%

[0080] Cu is an element effective for improving toughness and increasing strength. When Cu is contained, the Cu content may be 0% or more, but in order to obtain this effect, it is preferably contained at 0.01% or more. More preferably, it is 0.05% or more. On the other hand, if the content is too much, weldability deteriorates, so when Cu is contained, the Cu content is 1.0% or less. The Cu content is preferably 0.95% or less, and the Cu content is more preferably 0.9% or less. It is further preferred that the Cu content is 0.85% or less. The most preferred Cu content is 0.5% or less.

[0081] Cr: 0~1.0%

[0082] Cr is an element that is effective for obtaining sufficient strength even at low C, similar to Mn. When Cr is contained, the Cr content may be 0% or more, but in order to obtain this effect, it is preferably contained at 0.01% or more. More preferably, it is 0.05% or more. On the other hand, if the content is too much, hardenability is excessive, so SSCC resistance deteriorates. In addition, weldability also deteriorates. Therefore, when Cr is contained, it is 1.0% or less. The Cr content is preferably 0.95% or less. The Cr content is more preferably 0.9% or less, and further preferably 0.85% or less.

[0083] Mo: 0~0.60%

[0084] Mo is an element effective in improving toughness and increasing strength, and is an element effective in improving SSCC resistance and HIC resistance. When Mo is contained, the Mo content may be 0% or more, but in order to obtain this effect, it is preferably contained at least 0.01%. It is more preferably contained at least 0.10%. On the other hand, if the content is too much, hardenability is excessive, so SSCC resistance deteriorates. In addition, weldability also deteriorates. Therefore, when Mo is contained, the Mo content is 0.60% or less. The Mo content is preferably 0.50% or less. It is more preferably 0.40% or less. It is further preferably 0.35% or less.

[0085] W: 0~1.0%

[0086] W contributes to the improvement of the strength of the steel. When W is contained, the W content may be 0% or more, but in order to obtain the above-mentioned effect, when W is contained, it is preferably 0.01% or more. On the other hand, if the W content exceeds 1.0%, the effect is saturated, which becomes a major factor in the increase in cost, so when W is contained, the W content is 1.0% or less. The W content is preferably 0.9% or less, and more preferably 0.8% or less. In order to suppress costs, it is further preferably 0.5% or less.

[0087] V: 0-0.10%, Zr: 0-0.050%, Mg: 0-0.01%, and REM: 0-0.01%

[0088] V is an element that can be arbitrarily contained in order to improve the strength and toughness of steel. When V is contained, the V content can be 0% or more, but when the content is less than 0.01%, the effect of its inclusion is insufficient, so the V content is preferably 0.01% or more. The V content is more preferably 0.03% or more. On the other hand, if it exceeds 0.10%, the toughness of the weld deteriorates, so when contained, it is preferably 0.10% or less. The V content is preferably 0.09% or less. The V content is more preferably 0.07% or less, and further preferably 0.06% or less.

[0089] Zr, Mg and REM are elements that can be added arbitrarily in order to improve toughness by grain refinement or to improve crack resistance by controlling the properties of inclusions. When these elements are contained, their content can be 0% or more, but when the content is less than 0.0001%, the effect of their inclusion is insufficient, so the content is preferably 0.0001% or more. More preferably, it is 0.0005% or more. That is, the Zr content is preferably 0.0001% or more. The Zr content is more preferably 0.0005% or more. In addition, the REM content is preferably 0.0001% or more. The REM content is more preferably 0.0005% or more. The Mg content is preferably 0.0001% or more. The Mg content is more preferably 0.0005% or more.

[0090] On the other hand, if the Zr content exceeds 0.050%, and the Mg and REM contents exceed 0.01%, the effects are saturated, so when contained, it is 0.050% or less, and the Mg and REM contents are 0.01% or less. That is, when contained, the Zr content is 0.050% or less. The Zr content is preferably 0.040% or less. The Zr content is more preferably 0.020% or less. In addition, when contained, the REM content is 0.01% or less. The REM content is preferably 0.009% or less. The REM content is more preferably 0.008% or less. In addition, when contained, the Mg content is 0.01% or less. The Mg content is preferably 0.009% or less. The Mg content is more preferably 0.008% or less.

[0091] B: 0~0.0020%

[0092] B is an element that improves hardenability, contributes to the strength of steel, and inhibits the coarsening of original austenite grains, improving various properties of the billet. When B is contained, the B content can be 0% or more, but in order to obtain the above-mentioned effect, it is preferably 0.0001% or more. More preferably, it is 0.0008% or more. On the other hand, if the B content exceeds 0.0020%, the effect is saturated, which becomes the main reason for the increase in cost. Therefore, when B is contained, the B content is 0.0020% or less. The B content is preferably 0.0014% or less. The B content is more preferably 0.0012% or less. In order to suppress costs, it is further preferably 0.0010% or less.

[0093] Ta: 0~0.2%

[0094] Ta is an element that forms carbides and nitrides and contributes to improving strength. When Ta is contained, the Ta content may be 0% or more, but in order to obtain the above-mentioned effect, the Ta content is preferably 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 sometimes leads to a decrease in toughness, so when Ta is contained, the Ta content is 0.2% or less. Ta is preferably 0.16% or less. Ta is more preferably 0.12% or less, and further preferably 0.10% or less.

[0095] Hf: 0~0.2%, Re: 0~0.005%

[0096] These elements contribute to the increase of the strength of the steel. In order to obtain the above-mentioned effect, when contained, it is preferred that the content of these elements is 0.0001% or more. Preferably, it is 0.0010% or more. That is, when contained, the Hf content is preferably 0.0001% or more. The Hf content is more preferably 0.0010% or more. When contained, the Re content is preferably 0.0001% or more. The Re content is preferably 0.001% or more. On the other hand, when these elements are contained, if the Hf content exceeds 0.2% and the Re content exceeds 0.005%, the oxide increases, and the hydrogen resistance is impaired during agglomeration, so the Hf content is 0.2% or less and the Re content is 0.005% or less. That is, when contained, the Hf content is 0.2% or less. The Hf content is preferably 0.18% or less, and more preferably 0.12% or less. When contained, the Re content is 0.005% or less. The Re content is preferably 0.004% or less, and more preferably 0.003% or less.

[0097] Sn: 0~0.3%, Sb: 0~0.3%,

[0098] These elements contribute to the increase of strength and hardenability of steel. When Sn and Sb are contained, the contents of Sn and Sb can be 0% or more, but in order to obtain the above-mentioned effects, it is preferred that the contents are 0.0001% or more, respectively. Preferably, it is 0.001% or more. That is, when contained, the Sn content can be 0% or more, but the Sn content is preferably 0.0001% or more. The Sn content is more preferably 0.001% or more. When contained, the Sb content can be 0% or more, but the Sb content is preferably 0.0001% or more. The Sb content is more preferably 0.001% or more.

[0099] On the other hand, if each content exceeds 0.3%, the effect is saturated, which becomes the main reason for the cost increase. Therefore, when Sn and Sb are contained, the Sn and Sb contents are 0.3% or less. In order to suppress costs, it is preferably 0.01% or less. That is, when Sn is contained, the Sn content is 0.3% or less. The Sn content is preferably 0.2% or less. The Sn content is more preferably 0.1% or less. The Sn content is further preferably 0.01% or less. When Sb is contained, the Sb content is 0.3% or less. The Sb content is preferably 0.2% or less. The Sb content is more preferably 0.1% or less. The Sb content is further preferably 0.01% or less.

[0100] In the component composition of the steel material, the remainder other than the above-mentioned components (elements) is composed of Fe and inevitable impurity elements.

[0101] The metal structure of the steel material of the present invention will be described below.

[0102] Metal structure

[0103] The number of inclusions with an aspect ratio of 2.0 or more and a length of 10 μm or more is 15 / 100 mm 2 the following

[0104] Examples of inclusions in the material include elongated MnS and cementite. These act as hydrogen accumulation sources, causing a significant decrease in HIC resistance and becoming the hydrogen-induced crack growth threshold K. IH Therefore, the number of inclusions with an aspect ratio of 2.0 or more and a length of 10 μm or more is 15 / 100 mm. 2 The number density of the above inclusions is preferably 10 / 100mm 2 The lower limit is not particularly limited and can be 0 pieces / 100 mm. 2 .

[0105] Retained austenite is 0-3% (preferably)

[0106] Since retained austenite remains in the steel structure, it sometimes acts as a hydrogen trapping site, increasing the amount of hydrogen in the steel and increasing the sensitivity to hydrogen embrittlement. Furthermore, when using steel materials and steel pipes as steel structures, when the retained austenite transforms into martensite due to the stress load during use, the martensite is very hard and becomes the source of HIC generation or propagation path, which may cause K IH In the present invention, by making the retained austenite 3% or less, K IH Therefore, the retained austenite is preferably 3% or less. The retained austenite is more preferably 2% or less. Further preferably, it is 1% or less. The retained austenite may be 0%.

[0107] The area fraction of bainite in the range from the surface of the steel material (the surface of 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 (preferably)

[0108] In order to achieve a high tensile strength of 520 MPa or more as a material suitable for line pipes, the steel structure of the steel material needs to be a bainite structure. Here, the bainite structure includes bainitic ferrite or granular bainite that transforms during or after accelerated cooling, which contributes to phase transformation strengthening, and includes tempered bainite. If different types of structures such as ferrite, martensite, pearlite, island martensite, and retained austenite are mixed in the bainite structure, the strength is reduced, and the toughness and K in normal (atmospheric environment) are reduced. IH degradation.

[0109] In addition, the presence of steel structures with different hardnesses causes stress distribution in the steel material when the stress is applied during use, and acts as a source of hydrogen accumulation caused by stress-induced diffusion, thereby reducing the HIC resistance. Therefore, it is preferred that the bainite area fraction is 90% or more. The bainite area fraction is more preferably 92% or more, and further preferably 95% or more. The upper limit is not particularly limited, and may be 100%.

[0110] The maximum particle size in the range from the steel material surface (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.

[0111] By making the average grain size finer, the toughness is improved, but in Ar 3 When cooling starts from above the K point, the average grain size is limited. In the present disclosure, it is important to suppress the formation of coarse grains. In the case of containing grains with a large maximum grain size, uneven strain is generated inside the material, which promotes the accumulation of hydrogen, and thus the fracture toughness in a hydrogen environment deteriorates. In particular, grains with a maximum grain size of more than 25 μm from the surface of the inner surface of the steel to the center of the plate thickness are prone to accumulate strain around the grains, and are likely to become the starting point and expansion path of hydrogen cracks, so K IH Significantly degraded. Therefore, it is necessary to make the maximum grain size from the inner surface of the steel to the center of the plate thickness less than 25μm. The maximum grain size from the inner surface of the steel to the center of the plate thickness is preferably less than 24μm, more preferably less than 22μm, and further preferably less than 20μm. The lower limit is not particularly limited, but the maximum grain size is preferably greater than 4μm. The measurement range of the crystal grain size is 1mm×1mm, and the crystal grain size is defined as the Area grain size (the weighted average value when the boundary with an orientation difference of more than 15° is defined as the grain boundary).

[0112] The hydrogen-induced crack growth threshold K in a high-pressure hydrogen environment above 1 MPa IH 80MPa·m 1 / 2 above

[0113] In order to ensure the safe use of steel structures in hydrogen-containing environments, the hydrogen-induced crack growth threshold K of the high-strength steel disclosed in the present invention is 1000MW in a high-pressure hydrogen environment of 1 MPa or more. IH 80MPa·m 1 / 2 The upper limit is not particularly limited, but the hydrogen-induced crack growth threshold K of the steel material is IH Preferably 120 MPa·m 1 / 2 Below, more preferably 100 Pa·m 1 / 2 It should be noted that the hydrogen-induced crack growth threshold K IH It refers to the plane strain fracture toughness K obtained according to ASTM E399 and ASTM E1820 in high pressure hydrogen above 1MPa. IC Or its provisional value, or the crack growth threshold value or its provisional value obtained according to ASTM E1681.

[0114] The plate thickness of the steel material is not particularly limited, but is preferably 5 mm or more and 30 mm or less.

[0115] The present invention can obtain an excellent hydrogen-induced cracking threshold K in high-pressure hydrogen by having the above-mentioned chemical composition and metal structure. IH , can be applied to hydrogen pipeline pipes.

[0116] Furthermore, the high-strength steel material for line pipe of the present invention can be obtained by limiting the production conditions shown below. The production method and conditions will be specifically described.

[0117] Molten steel process

[0118] [Average cooling rate of molten steel: 50°C / min or more (preferable condition)]

[0119] In order to reduce inclusions, it is also effective to reduce the content of S or O. In the cooling process of molten steel, the inclusions defined in the present invention condense, so it is also effective to accelerate the average cooling rate of molten steel. Therefore, it is preferred that the average cooling rate in the temperature range from 1500°C to 1000°C is 50°C / min or more. The average cooling rate is more preferably 60°C / min or more, and further preferably 70°C / min or more. The upper limit is not particularly limited, but is preferably 90°C / min or less.

[0120] Heating process

[0121] [Slab heating temperature: 1000-1250°C]

[0122] When the heating temperature of billets, slabs and other castings is less than 1000°C, the diffusion of impurity elements such as microsegregated C, P, and S is insufficient, and a homogeneous material cannot be obtained, which causes an increase in the number of inclusions and uneven precipitation, reducing toughness. Therefore, the heating temperature of the casting is above 1000°C. The heating temperature of the casting is preferably above 1050°C, and more preferably above 1100°C. On the other hand, if it exceeds 1250°C, the grains are too coarse and the toughness deteriorates. Therefore, the heating temperature of the casting is below 1250°C. The heating temperature of the casting is preferably below 1200°C, and more preferably below 1150°C.

[0123] Rolling process

[0124] [Total reduction ratio within the recrystallization temperature range after heating the cast piece: 35% to 55%]

[0125] In order to reduce the maximum grain size of bainite, it is necessary to promote the recrystallization of grains and suppress the formation of coarse grains during hot rolling within the recrystallization temperature range after the casting is heated. When the total reduction rate within the recrystallization temperature range is less than 35%, the recrystallization is insufficient, so coarse grains remain. Therefore, the total reduction rate within the recrystallization temperature range is 35% or more. Preferably, it is 38% or more. The total reduction rate within the recrystallization temperature range is more preferably 40% or more, and more preferably 43% or more. On the other hand, if the total reduction rate within the recrystallization temperature range exceeds 55%, the coarsening of the grains can be suppressed, but the reduction of the non-recrystallized area is insufficient, so the grains in the final product cannot be refined. Therefore, the total reduction rate within the recrystallization temperature range is 55% or less. Preferably, it is 52% or less. The total reduction rate within the recrystallization temperature range is more preferably 50% or less, and more preferably 48% or less. Here, the lower limit temperature Tnr of recrystallization can be obtained, for example, by the following formula based on the composition of the steel. It should be noted that the surface temperature of the steel plate can be measured using a radiation thermometer or the like. The total reduction ratio within the recrystallization temperature range refers to the total reduction ratio above the lower limit temperature Tnr of recrystallization obtained by the following formula.

[0126] Tnr(℃)=174×log[%Nb][%C+12 / 14%N]+1444

[0127] Here, [%X] represents the content of element X in steel (mass %).

[0128] [Reduction ratio of the final rolling pass within the recrystallization temperature range: 10% or more]

[0129] In addition to making the total reduction ratio within the above-mentioned recrystallization temperature range 35% to 55%, it is also necessary to fully ensure the reduction ratio of the final rolling pass within the recrystallization temperature range, fully promote recrystallization, and thus start the partial recrystallization region rolling in the state of uniform grains without coarse grains. When the reduction ratio of the final rolling pass within the recrystallization temperature range is less than 10%, the recrystallization is insufficient, so coarse grains grow during the holding time from rough rolling to the start of finish rolling. Therefore, the reduction ratio of the final rolling pass within the recrystallization temperature range is 10% or more. The reduction ratio of the final rolling pass within the recrystallization temperature range is preferably 11% or more. The reduction ratio of the final rolling pass within the recrystallization temperature range is more preferably 13% or more, and further preferably 15% or more. The upper limit of the reduction ratio of the final rolling pass within the recrystallization temperature range is not particularly limited, and the higher the better, but if it exceeds 70%, the productivity is significantly reduced, so it is preferably 70% or less.

[0130] [Reduction ratio of the final rolling pass above (recrystallization temperature - 80°C): 15% or more]

[0131] Since recrystallization is partially carried out after rolling in the recrystallization zone is completed, recrystallization can be promoted by further increasing the reduction rate, which is effective for the refinement of the first 20% of the grain size. Therefore, the reduction rate of the final rolling pass above (recrystallization temperature - 80°C) is 15% or more. The reduction rate of the final rolling pass above (recrystallization temperature - 80°C) is preferably 16% or more. The reduction rate of the final rolling pass above (recrystallization temperature - 80°C) is more preferably 18% or more, and further preferably 20% or more. There is no particular upper limit on the reduction rate of the final rolling pass above (recrystallization temperature - 80°C), and the higher the better, but if it exceeds 40%, the productivity is significantly reduced, so it is preferably 40% or less.

[0132] Since rolling at a low temperature introduces a large amount of strain, rolling at a temperature lower than (recrystallization temperature - 80°C) is effective for grain refinement. Therefore, rolling at a low temperature is preferably performed within a range that allows the cooling start temperature of controlled cooling to be met.

[0133] In the hot rolling process, in order to reduce the grain size, the lower the rolling end temperature, the better. However, from the perspective of ensuring HISC resistance in a high-pressure hydrogen environment, it is necessary to set the cooling start temperature to the hot-rolled steel sheet surface temperature as Ar, which can ensure controlled cooling. 3 The rolling end temperature is set in the manner above. 3 The point refers to the ferrite transformation start temperature during cooling, and can be obtained, for example, from the following formula based on the composition of the steel. The surface temperature of the hot-rolled steel sheet can be measured using a radiation thermometer or the like.

[0134] Ar3 (℃)=910-310[%C]-80[%Mn]-20[%Cu]-15[%Cr]-55[%Ni]

[0135] -80[%Mo]

[0136] Here, [%X] represents the content of element X in steel (mass %).

[0137] Cooling process after rolling (controlled cooling process)

[0138] [Controlled cooling cooling start temperature: The surface temperature of the hot-rolled steel sheet is Ar 3 above the phase transition point]

[0139] The surface temperature of the steel plate at the beginning of cooling is less than Ar 3 Phase transition point (Ar 3 Point), ferrite is generated before controlled cooling, and the strength reduction becomes larger. Therefore, the surface temperature of the hot-rolled steel sheet at the start of cooling is Ar 3 The surface temperature of the hot-rolled steel sheet at the start of cooling is preferably Ar 3 Phase transition point +20°C or above, more preferably Ar 3 The surface temperature of the hot-rolled steel sheet at the start of cooling is the temperature of the tail end of the hot-rolled steel sheet where the cooling start temperature is the lowest. The surface temperature of the hot-rolled steel sheet at the start of cooling is preferably Ar 3 Phase transition point +120°C or less, more preferably Ar 3 Phase change point is below +80℃.

[0140] [The difference in cooling start time between the front end and the rear end of the hot-rolled steel plate during controlled cooling: within 50 seconds]

[0141] When the time difference between the front end and the tail end of the hot-rolled steel sheet in the rolling direction at the start of cooling exceeds 50 seconds (s), the temperature difference between the front end and the tail end at the start of cooling becomes larger, so the temperature deviation when cooling stops becomes larger, and the deviation of the Vickers hardness at 0.25mm from the steel surface (the surface of the inner surface of the steel pipe in the case of a steel pipe) becomes larger, and the HISC resistance deteriorates. Therefore, the cooling start time difference between the front end of the hot-rolled steel sheet and the tail end of the hot-rolled steel sheet is within 50 seconds. The cooling start time difference is preferably within 45 seconds. The cooling start time difference is more preferably within 40 seconds, and further preferably within 32 seconds. The cooling start time difference can be shortened by shortening the length of the hot-rolled steel sheet, but the manufacturability is reduced, so it is preferred to shorten the cooling start time difference by speeding up the hot-rolled steel sheet conveying speed. The cooling start time difference can be 0 seconds, but from the viewpoint of manufacturability, it is preferably more than 20 seconds.

[0142] [Average cooling rate of controlled cooling]

[0143] Average cooling rate from 750°C to 550°C in the center of the plate thickness: 15 to 50°C / s

[0144] 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 prescribed bainite structure containing granular bainite cannot be obtained, resulting in a decrease in strength. Therefore, the average cooling rate in the center of the plate thickness is 15°C / s or more. From the viewpoint of suppressing the deviation of the structure, the average cooling rate in the center of the plate thickness is preferably 17°C / s or more. The average cooling rate in the center of the plate thickness is preferably 20°C / s or more, and more preferably 25°C / s or more. On the other hand, in order to suppress the deviation of the grain size of the bainite structure, the average cooling rate is 50°C / s or less. The average cooling rate is preferably 48°C / s or less, and more preferably 45°C / s or less. The average cooling rate is further preferably 42°C / s or less, and most preferably 38°C / s or less. It should be noted that there is no particular limitation on cooling to a temperature of 550°C or less at the hot-rolled steel plate center of the plate thickness, but from the viewpoint of suppressing the deviation of the structure and grain size, the average cooling rate is preferably 15°C / s to 50°C / s. That is, for cooling below 550°C, the average cooling rate is preferably 15°C / s or more. The average cooling rate is more preferably 30°C / s or more, and further preferably 35°C / s or more. For cooling below 550°C, the average cooling rate is preferably 50°C / s or less. The average cooling rate is more preferably 48°C / s or less, and further preferably 42°C / s or less. The average cooling rate below 550°C is the average of the cooling rates from 550°C to 250°C.

[0145] [Cooling stop temperature: 250~650℃]

[0146] When the cooling stop temperature at the center of the plate thickness after hot rolling exceeds 650°C, the material strength is greatly reduced, and from the viewpoint of obtaining a uniform bainite structure, the cooling stop temperature at the center of the plate thickness is also set to 650°C or less. The cooling stop temperature at the center of the plate thickness is preferably 620°C or less, more preferably 615°C or less, and further preferably 600°C or less. On the other hand, when the cooling stop temperature at the center of the plate thickness is less than 250°C, quenching cracks are easily generated during cooling. In addition, in order to obtain a uniform bainite structure, the cooling stop temperature is above 250°C. The cooling stop temperature at the center of the plate thickness is preferably 300°C or more, more preferably 350°C or more, and further preferably 380°C or more. From the viewpoint of suppressing the amount of hydrogen in the steel, the cooling stop temperature also needs to be above the specified temperature. Specifically, the hydrogen present in the steel gradually escapes during cooling, and the higher the temperature, the greater the effect, but when the cooling stop temperature is too low, it becomes overcooling, and hydrogen remains in the steel. Furthermore, if the cooling stop temperature is excessively reduced, it is easy to form retained austenite with a large increase in hydrogen compared to other phases. Therefore, in order to reduce the amount of hydrogen in the steel, the cooling stop temperature also needs to be 250° C. or higher. After cooling is stopped, the steel may be left to cool, but in order to promote the formation of bainite, it is more preferable to cool slowly until the temperature drops by about 50° C. from the cooling stop temperature.

[0147] [Dehydrogenation treatment (preferred conditions)]

[0148] When hydrogen is already present in the steel, the acceleration of fatigue crack growth increases and the fatigue life decreases. Therefore, in order to release the hydrogen remaining after manufacturing, it is preferred to use a dehydrogenation treatment. The dehydrogenation treatment can reduce the amount of hydrogen in the steel by keeping it at a high temperature for a certain period of time before the product is used.

[0149] Alternatively, the dehydrogenation treatment can be performed by holding the steel at room temperature for a long time. The holding time is longer when the steel is held at room temperature, so the holding time is preferably 96 hours or more. Furthermore, the oxide scale on the steel surface hinders the dehydrogenation, so it is preferred to remove the oxide scale before the dehydrogenation treatment. The holding time R (sec) is preferably determined based on the plate thickness and tube thickness t (mm) of the steel plate and steel tube and the hydrogen diffusion coefficient D (mm·sec) in the steel at room temperature. －1 ) and adopt the following formula (A).

[0150] R≥t 2 / D···(A)

[0151] The hydrogen diffusion coefficient also varies depending on the components and metal structure contained, but for example, the hydrogen diffusion coefficient can be 1×10 －5 ~5×10 －3 mm 2 / s. More preferably, 5×10 －4 mm 2 / s or less.

[0152] The dehydrogenation treatment process is carried out before the welding work of pipe making or connecting steel pipes. It should be noted that since the hydrogen diffusion coefficient D decreases at high temperatures, the dehydrogenation treatment is preferably carried out at a high temperature in order to allow hydrogen to escape quickly.

[0153] In the case of high temperature, the diffusion coefficient D' (diffusion coefficient at each temperature) of the temperature at which the value of D of the above formula (A) is maintained can be used for calculation. On the other hand, when the temperature T of the dehydrogenation process is too high, the material strength is significantly reduced, so the dehydrogenation treatment temperature is preferably below 550°C. The dehydrogenation treatment temperature T is more preferably below 500°C. The dehydrogenation treatment temperature T is further preferably below 400°C, and most preferably below 300°C. In addition, since the dehydrogenation treatment at a temperature lower than room temperature is the main reason for the increase in processing time and cost, for this reason, the dehydrogenation treatment temperature T is preferably above room temperature. The dehydrogenation treatment temperature T is more preferably above 50°C. The dehydrogenation treatment temperature T is further preferably above 100°C, and most preferably above 150°C. The so-called dehydrogenation treatment temperature T here is the temperature of the atmosphere in the dehydrogenation treatment process. Room temperature refers to 20±10°C.

[0154] In particular, when heating is performed, it takes time for the temperature Tc of the center of the plate thickness of the steel and steel pipe to reach the temperature of the atmosphere (dehydrogenation treatment temperature T) in the dehydrogenation treatment process. Therefore, even if the above-mentioned holding time R (sec) is met at the atmosphere temperature, the dehydrogenation treatment may not be sufficient if the dehydrogenation treatment temperature T (atmosphere temperature) is not reached at the center of the plate thickness. Therefore, it is preferred to maintain the R time (sec) or more after the temperature Tc of the center of the plate thickness reaches the target dehydrogenation treatment temperature T. Furthermore, in order to obtain the hydrogen fracture toughness in the specified hydrogen gas, it is necessary to appropriately adjust the hydrogen content of the steel in the surface part and the center of the plate thickness. For this purpose, it is preferred to maintain the R (sec) specified in formula (A) at the dehydrogenation treatment temperature T (atmosphere temperature), and it is further preferred to maintain the above-mentioned holding time R (sec) or more after the temperature Tc of the center of the plate thickness reaches the target dehydrogenation treatment temperature T. In other words, at least the former can properly control the hydrogen content of the steel in the surface part of the steel and the steel pipe, and when the latter is implemented, the hydrogen content of the steel from the surface part to the center of the plate thickness of the steel and the steel pipe can be properly controlled. The plate thickness temperature is the plate thickness center temperature Tc, which can be measured using a thermocouple or the like, or can be estimated using a finite element method or the like.

[0155] It should be noted that the time and temperature of the dehydrogenation treatment process may include the temperature and time applied during the heating process in the pipe making process of the electric resistance welded pipe, UOE, etc. as described later. Furthermore, the scale on the surface of the steel hinders dehydrogenation, so it is preferred to remove the scale and perform the dehydrogenation treatment. The removal method is not limited, for example, it can be physical cleaning using high pressure cleaning, or a chemical method using a scale remover can be used. If about 100 μm is removed in thickness, the effect of scale removal can be obtained.

[0156] Second embodiment

[0157] Furthermore, a UOE steel pipe, which is an example of a high-strength steel pipe for line pipe, can be obtained by limiting the manufacturing conditions shown below. The manufacturing method and conditions are specifically described. Component composition, metal structure, hydrogen-induced crack growth threshold K of UOE steel pipe IH As with the steel plate of the first embodiment, the manufacturing method includes the molten steel step, the heating step, the hot rolling step, the controlled cooling step after hot rolling, and the dehydrogenation treatment step. The following specifically describes the pipe making step after rolling.

[0158] Pipe making process

[0159] UOE steel pipe is a hot-rolled steel plate subjected to bending processing, specifically, the end of the hot-rolled steel plate is grooved, and the steel pipe shape is formed by a C-type press, a U-type press, and an O-type press, and then the butt joint is seam welded by inner surface welding and outer surface welding, and further manufactured by a pipe expansion process as needed. In addition, any welding method can be used as long as it can obtain sufficient joint strength and joint toughness, but from the perspective of excellent welding quality and manufacturing efficiency, submerged arc welding is preferably used. In addition, the steel pipe obtained by seam welding the butt joint after forming into a tubular shape by press bending can also be expanded. Furthermore, when the above-mentioned inclusions are present in the weld heat affected zone after pipe making, they act as a hydrogen accumulation source like the parent material portion, causing deterioration of HIC resistance and KIH. In order to reduce the inclusions in the weld, it is also effective to reduce the content of S or O. Therefore, it is preferred that the average cooling rate of the welded steel pipe in the temperature range from 1500°C to 1000°C is 50°C / min or more. The average cooling rate is more preferably 55° C. / min or more, and even more preferably 60° C. / min or more. The upper limit is not particularly limited, but the average cooling rate is preferably 100° C. / min or less.

[0160] Third embodiment

[0161] Furthermore, the high-strength steel pipe for line pipe of the present invention can be an electric resistance welded steel pipe as an example, and the electric resistance welded steel pipe can be obtained by limiting the manufacturing conditions shown below, and the manufacturing method and conditions are specifically described. Steel material composition, metal structure, hydrogen-induced crack growth threshold K IH As described in the steel material of the first embodiment, the manufacturing method includes steps other than the cooling step after rolling and the pipe making step (molten steel step, heating step, hot rolling step, dehydrogenation treatment step) in the same manner as described in the steel material.

[0162] Cooling process after rolling (controlled cooling process)

[0163] The cooling start temperature of the controlled cooling and the average cooling rate of the controlled cooling are the same as those described in the first embodiment.

[0164] [Cooling stop temperature: 250~650℃]

[0165] When the cooling stop temperature at the center of the plate thickness after hot rolling exceeds 650°C, the material strength is greatly reduced, and from the viewpoint of obtaining a uniform bainite structure, the cooling stop temperature at the center of the plate thickness is also set to 650°C or less. The cooling stop temperature at the center of the plate thickness is preferably 620°C or less, more preferably 615°C or less, and further preferably 600°C or less. On the other hand, when the cooling stop temperature at the center of the plate thickness is less than 250°C, quenching cracks are easily generated during cooling. Therefore, the cooling stop temperature at the center of the plate thickness is 250°C or more. The cooling stop temperature at the center of the plate thickness is preferably 300°C or more, more preferably 350°C or more, and further preferably 380°C or more. In order 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 is most preferably 450°C or more. After cooling is stopped, it is sufficient to cool, but in order to promote the formation of bainite, it is more preferable to cool slowly until the temperature drops by about 50°C from the cooling stop temperature.

[0166] Then, the hot-rolled steel sheet obtained as described above is coiled into a coil. The coiling temperature is preferably 650°C or less. The coiling temperature is more preferably 620°C or less, more preferably 615°C or less, and further preferably 600°C or less. As for the lower limit, the coiling temperature is preferably 250°C or more, more preferably 300°C or more. Further preferably 350°C or more, and most preferably 380°C or more.

[0167] Pipe making process

[0168] The electric resistance welded steel pipe cited as an example of the present invention is formed into a cylindrical shape by cold rolling, and the circumferential ends of the cylindrical shape are butted and welded. Furthermore, it is also possible to manufacture the electric resistance welded steel pipe material (electric resistance welded steel pipe) by forming it into an electric resistance welded steel pipe material (electric resistance welded steel pipe) using a sizing roll satisfying the following formula (1) (sizing step), and applying an internal pressure p (MPa) satisfying the following formula (2) to the inner surface of the electric resistance welded steel pipe material (internal pressure loading step).

[0169] It should be noted that the above-mentioned cylindrical shape means that the tube circumferential cross section is a "C" shape.

[0170] Diameter of sizing roller (mm) ≥ thickness of hot-rolled steel plate (mm) / 0.020···(1)

[0171] The plate thickness of the hot-rolled steel plate refers to the plate thickness of the hot-rolled steel plate before the sizing step.

[0172] X<p≤X×1.5···(2)

[0173] It should be noted that X = (wall thickness of electric resistance welded steel tube billet (mm) / radius of electric resistance welded steel tube billet (mm)) × yield strength of electric resistance welded steel tube billet (MPa)

[0174] The load of the above-mentioned internal pressure can be implemented by, for example, sealing the pipe end with a sealing pad of rubber material and loading the pipe internal water pressure. In addition, in order to stabilize the shape, a mold of a desired diameter can also be used as an outer frame as required.

[0175] It should be noted that the wall thickness of the electric resistance welded steel tube billet cited as an example of the steel tube of the present invention is preferably 5 mm or more. The wall thickness of the electric resistance welded steel tube billet is preferably 30 mm or less. There is no upper limit for the radius of the electric resistance welded steel tube billet, but if it becomes larger, the load on the equipment increases, so the radius of the electric resistance welded steel tube billet is preferably 400 mm or less. The radius of the electric resistance welded steel tube billet is preferably 200 mm or more. In addition, in order to withstand the pipeline operating gas pressure, the yield strength of the electric resistance welded steel tube billet is preferably 480 MPa or more. The yield strength is more preferably 500 MPa or more. On the other hand, in order to avoid an increase in hydrogen embrittlement sensitivity, the yield strength is preferably 560 MPa or less. The yield strength is more preferably 550 MPa or less.

[0176] In the sizing process, bending deformation occurs in the tube axis direction along the roller shape when the roller passes, and residual stress occurs in the tube axis direction. The greater the bending strain in the bending deformation, the greater the absolute value of the residual stress in the tube axis direction. The smaller the diameter of the sizing roller and the greater the thickness of the hot-rolled steel sheet, the greater the bending strain.

[0177] Therefore, in the present invention, from the viewpoint of reducing the shear residual stress, the diameter of the sizing roll is set to satisfy the above formula (1) in order to reduce the absolute value of the residual stress in the pipe axial direction.

[0178] When the diameter of the sizing roll is smaller than the right side of the above formula (1), the target shear residual stress in the present invention cannot be obtained. It should be noted that there is no particular upper limit on the diameter of the sizing roll, but if the sizing roll becomes larger, the load on the equipment increases, so the diameter of the sizing roll is preferably 2000 mm or less.

[0179] In the internal pressure loading process, the electric resistance welded steel pipe material is expanded to generate tensile stress in the pipe circumferential direction, thereby reducing the absolute value of the residual stress in the pipe circumferential direction.

[0180] The greater the internal pressure p (MPa) in the internal pressure loading process, the smaller the absolute value of the residual stress in the pipe circumferential direction. The greater the radius of the steel pipe and the smaller the wall thickness of the steel pipe, the higher the tensile stress generated in the pipe circumferential direction.

[0181] The left side (X) of the above formula (2) corresponds to the internal pressure p when the tensile stress generated in the circumferential direction of the tube is equal to the yield stress of the electric resistance welded steel tube material.

[0182] In the present invention, from the viewpoint of reducing the shear residual stress, in order to reduce the absolute value of the residual stress in the pipe axial direction, the internal pressure p is set to a value greater than the left side (X) of the formula (2), and the electric resistance welded steel pipe billet is expanded to the plastic region. On the other hand, if the internal pressure p exceeds the right side (X×1.5) of the formula (2), the absolute value of the residual stress in the pipe circumferential direction becomes smaller, but the amount of work hardening due to the expansion becomes too large, the dislocation density on the pipe surface increases, and the fracture toughness in hydrogen decreases.

[0183] As described in part above, for high-strength steel pipes, the high-strength steel material disclosed in the present invention is formed into a tubular shape by press bending, roll forming, UOE forming, etc., and then the butt joints are welded, thereby manufacturing a high-strength steel pipe for sour-resistant line pipe (UOE steel pipe, electric resistance welded steel pipe, spiral steel pipe, etc.) with excellent material uniformity in the steel plate suitable for the transportation of crude oil and natural gas. In addition, by using the high-strength steel plate disclosed in the present invention for the steel pipe, a steel pipe with excellent HISC resistance can be manufactured even if there is a high hardness region in the welded portion.

[0184] Example 1

[0185] The present invention will be described in more detail below based on examples. The following examples are preferred examples of the present invention, but the present invention is not limited to the following examples at all.

[0186] Slabs having the composition shown in Tables 1-1 and 1-2 were prepared, hot-rolled, controlled cooled, and dehydrogenated to obtain steel products. The steel products were formed to produce steel pipes. The production conditions are shown in Tables 2-1 and 2-2.

[0187] No. 2-6, 12-22, 35, 37 were subjected to the following pipe-making process: the obtained steel material (hot-rolled steel plate) was bent, the two ends were butted and welded; No. 7-11, 23-33, 36, 38 were subjected to the following pipe-making process: the obtained steel material (hot-rolled steel plate) was formed into a cylindrical shape by cold roll forming, the two circumferential ends of the cylindrical shape were butted and resistance welded; steel pipe forming was performed. No. 1, 34 kept the steel material as it was.

[0188] The results of evaluation of the metal structure and material quality of each of the obtained steel materials and steel pipes are shown in Tables 3-1 and 3-2. The evaluation method is as follows.

[0189] Retained austenite determination

[0190] A sample for metal structure observation was taken from the central part of the plate width in the central part of the long side direction of the steel material and steel pipe obtained according to the above operation, and the cross section parallel to the long side direction was polished as the observation target surface, and then the surface layer was removed by chemical polishing using picric acid etching, and the measurement was performed using X-ray diffraction measurement. Specifically, a Co-Kα ray source was used as the incident X-ray, and the area fraction of retained austenite was calculated based on the intensity ratio of the (200), (211), and (220) planes of ferrite to the (200), (220), and (311) planes of austenite.

[0191] Calculation of the maximum particle size and area fraction of bainite

[0192] A sample for metal structure observation is taken from the central part of the plate width of the steel and steel pipe obtained according to the above operation, and the cross section parallel to the long side direction of the sample is used as the observation object surface. After the observation object surface is mirror-polished, it is etched with colloidal silica, and crystal data is collected by EBSD (Electron Backscatter Diffraction) method with a field of view of 1mm×1mm at the center of the sample (measurement step: 0.8μm). The crystal grain size is defined as the Area grain size (the weighted average value when the boundary with an azimuth difference of more than 15° is defined as the grain boundary). The grain size of each crystal is calculated from the above crystal data, and the maximum grain size is calculated.

[0193] In addition, for the area fraction, the above-mentioned observation target surface was etched with a 3 vol% nitric acid alcohol solution, and a scanning electron microscope photograph was taken at an appropriate magnification of 1000 to 5000 times to observe bainite. For bainite, it was compared with the structure photograph of Non-Patent Document 1 and judged by visual observation. For the structure fraction, based on the above judgment, the bainite and other regions in the SEM photograph were binarized, and the fraction was calculated by image analysis as the area fraction of bainite.

[0194] Observation of inclusions and calculation of number density

[0195] A sample for metal structure observation was taken from the central part of the plate width in the central part of the long side direction of the steel material and steel pipe obtained according to the above operation. For this sample, the cross section parallel to the long side direction of rolling was used as the observation object surface. After the observation object surface was mirror polished, it was etched with colloidal silica, and the scanning electron microscope (SEM) was used to observe the center of the sample with a field of view of 10mm×10mm. The observation magnification was set to 2000-5000 times, and the average value of the three fields of view was used as the number density of inclusions.

[0196] Tensile Strength (TS)

[0197] According to JIS Z 2201, JIS No. 14 proportional test pieces (diameter of parallel part 7 mm, distance between points 35 mm) were taken from the steel materials and steel pipes obtained in the above-mentioned operation, and the tensile strength was measured.

[0198] Hydrogen heating analysis

[0199] The amount of hydrogen remaining in the steel was determined using a temperature rise desorption analysis method using a low temperature type temperature rise type hydrogen analyzer (gas chromatograph type) (JTF-20AL). The temperature rise desorption analysis was performed at a temperature rise rate of 200°C / h in the temperature range from room temperature to 400°C, and the sum was taken as the amount of hydrogen. The test body was a cylindrical shape with a length of 30mm and a diameter of 7Φ along the long side of the steel pipe at a position 1 / 4 of the plate thickness of the steel material and a position 1 / 4 from the inner surface of the steel pipe. It should be noted that the amount of hydrogen is the amount of hydrogen before the high pressure hydrogen fatigue test described in the following matters, and is the amount of H shown in Tables 1-1 and 1-2.

[0200] Fracture toughness test in high pressure hydrogen

[0201] The test was carried out in accordance with ASTM E1820 in a mixed atmosphere of hydrogen (containing 100% hydrogen) at room temperature (20±10°C) and a pressure of 25 MPa or a natural gas (mainly composed of hydrocarbons such as methane and ethane) containing hydrogen with a hydrogen partial pressure of 1 MPa or more at the above temperature and pressure. The test piece used a CT test piece (plate thickness 12.7 mm, plate width 25.4 mm), and was taken in the direction parallel to the rolling direction of the steel and the direction of mechanical notch introduction. The fatigue precrack was introduced in the atmosphere, and the conditions were set to frequency: 1 Hz, repetitive load waveform: sine wave, control method: K value control, and stress ratio R: 0.1. Then, it was set to a hydrogen or hydrogen + natural gas mixed atmosphere. The fracture toughness test was carried out by unloading-elastic compliance method using a single test piece. The crosshead displacement speed during load loading was 0.002 mm / sec.

[0202] The evaluation results are shown in Tables 3-1 and 3-2. All the steel materials and steel pipes satisfying the examples of the present invention showed a hydrogen-induced crack growth threshold value K IH 80MPa·m 1 / 2 The above excellent hydrogen fracture toughness was shown, and the tensile strength was 520 MPa or more. It should be noted that the same results as those of the steel materials were obtained for the steel pipes in Tables 3-1 and 3-2.

[0203] [Table 1-1]

[0204]

[0205] [Table 1-2]

[0206]

[0207] [Table 2-1]

[0208]

[0209] [Table 2-2]

[0210]

[0211] [Table 3-1]

[0212]

[0213] [Table 3-2]

[0214]

[0215] Example 2

[0216] The following is a description of examples that verify the effects of the present invention. It should be noted that in the following examples, steel materials and steel pipes were manufactured under the following manufacturing conditions, and their properties were evaluated. Steel grades No. 2, 4, 8, 14, 22, and 33 shown in Tables 1-1 and 1-2 used in Example 1 were used, and the steel pipes 2, 4, 8, 14, 22, and 33 shown in Example 1 (Tables 2-1 and 2-2) were manufactured under the same conditions until the controlled cooling process. The steel pipe forming was also carried out under the same conditions as in Example 1, and the properties were evaluated when the dehydrogenation treatment conditions were changed. The above results are shown in Table 4.

[0217] The dehydrogenation treatment of steel pipes No. 2, 4, 8, 14, 22, and 33 implemented in Example 1 was carried out at a dehydrogenation treatment temperature T (atmosphere temperature) of the temperature and time shown in Tables 2-1 and 2-2, which respectively corresponds to the dehydrogenation holding time t of Y and the holding time tc of the plate thickness center temperature Tc of N in Table 4.

[0218] Steel pipes No. 2A, 4A, 8A, 14A, 22A, and 33A were subjected to dehydrogenation treatment at temperatures T 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 satisfied the formula (A).

[0219] For steel pipes No. 2B, 4B, 8B, 14B, 22B, and 33B, although the dehydrogenation treatment temperatures T are the temperatures shown in Table 4, the holding time t of the atmosphere temperature and the holding time tc after the plate thickness center temperature Tc reaches the above dehydrogenation treatment temperature T do not satisfy the above formula (A).

[0220] In Table 4, "dehydrogenation holding time t is Y" means that the dehydrogenation treatment temperature T (atmosphere temperature) is a specified temperature and the holding time t satisfies the formula (A), and "dehydrogenation holding time t is N" means that the dehydrogenation treatment temperature T (atmosphere temperature) is a specified temperature, but the holding time t does not satisfy the formula (A). In addition, "holding time tc of the steel material center temperature Tc is Y" means that the holding time tc after the plate thickness center temperature Tc reaches the specified temperature satisfies the formula (A), and "holding time tc of the steel material center temperature Tc is N" means that although the plate thickness center temperature Tc reaches the specified temperature, the holding time tc after Tc reaches the specified temperature does not satisfy the formula (A).

[0221] The fracture toughness and tensile strength in hydrogen, and the evaluation of the structure and inclusions were carried out in the same manner as in Example 1.

[0222] All the examples of the present invention meet the hydrogen induced crack growth threshold K IH 80MPa·m 1 / 2 The above conditions are those under which the tensile strength is 520 MPa or more. Among them, when the dehydrogenation treatment is carried out under more preferred conditions, the fracture toughness in hydrogen is more excellent.

[0223] It should be noted that, regarding the steel pipes in Table 4, the same results as those of the steel materials were obtained.

[0224] [Table 4]

[0225]

Claims

1. A high-strength steel material for line pipes having excellent fracture toughness in hydrogen, having the following chemical composition: containing, by mass%, C: 0.02-0.15%, Si: 0.01-2.0%, Mn: 0.5-1.5%, P: 0.0001-0.015%, S: 0.0002-0.0015%, Al: 0.005-0.15%, O: 0.01% or less, N: 0.010% or less, Nb: 0.10% or less, H: 0.02ppm or less, or further containing 0-0.005% of Ca, N i: 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: 0-0.3%, and the remainder is Fe and unavoidable impurity elements; 15 inclusions / 100 mm having bainite and an aspect ratio of 2.0 or more and a length of 10 μm or more 2 The following metal structures, 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 of the steel is 520 MPa or more. The hydrogen-induced crack growth threshold K of the steel in a high-pressure hydrogen environment above 1 MPa is IH 80MPa·m 1 / 2 above.

2. The high-strength line pipe steel material having excellent fracture toughness in hydrogen according to claim 1, wherein The chemical composition is further as follows 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%, Sb: 0.0001~0.3%.

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

4. A method for producing a high-strength line pipe steel material having excellent fracture toughness in hydrogen, comprising: A heating step of heating the cast piece having the component composition according to claim 1 or 2 at 1000 to 1250° C. In a hot rolling step, the cast piece heated in the heating step is rolled under the conditions that the total reduction ratio in the recrystallization temperature range is 35% to 55%, the reduction ratio in the final rolling pass in the recrystallization temperature range is 10% or more, and the reduction ratio in the final rolling pass above (recrystallization temperature - 80°C) is 15% or more, and the rolling end temperature is above the Ar3 transformation point as measured by the steel plate surface thermometer, and The hot-rolled steel sheet obtained in the hot rolling process is cooled under the conditions that the cooling start temperature is above the Ar3 transformation point in terms of the surface temperature of the hot-rolled steel sheet, the cooling start time difference between the front end and the tail end of the hot-rolled steel sheet is within 50 seconds, the average cooling rate from 750°C to 550°C is 15 to 50°C / s in terms of the center temperature of the plate thickness, and the cooling stop temperature is 250 to 650°C.

5. A high-strength steel pipe for line pipes having excellent fracture toughness in hydrogen, having the following chemical composition: containing, by mass%, C: 0.02-0.15%, Si: 0.01-2.0%, Mn: 0.5-1.5%, P: 0.0001-0.015%, S: 0.0002-0.0015%, Al: 0.005-0.15%, O: 0.01% or less, N: 0.010% or less, Nb: 0.10% or less, H: 0.02ppm or less, or further containing Ca: 0-0.005%, N i: 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: 0-0.3%, and the remainder is Fe and unavoidable impurity elements; 15 inclusions / 100 mm having bainite and an aspect ratio of 2.0 or more and a length of 10 μm or more 2 The following metal structures, The maximum grain 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 of the steel pipe is 520 MPa or more. The hydrogen-induced crack growth threshold K of the steel pipe in a high-pressure hydrogen environment above 1MPa is IH 80MPa·m 1 / 2 above.

6. The high-strength steel pipe for line pipe having excellent fracture toughness in hydrogen according to claim 5, wherein: The chemical composition is further as follows 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%, Sb: 0.0001~0.3%.

7. The high-strength steel pipe for line pipes having excellent fracture toughness in hydrogen according to claim 5 or 6, wherein In high-strength pipeline steel pipes, The retained austenite has an area fraction of 0 to 3%, and the bainite in the range from the surface of the inner surface of the steel pipe to the center of the plate thickness has an area fraction of 90% or more.

8. A method for producing a high-strength steel pipe for line pipe having excellent fracture toughness in hydrogen, comprising: A heating step of heating the cast slab having the component composition according to claim 5 or 6 at 1000 to 1250° C. In the hot rolling step, the cast piece heated in the heating step is rolled under the conditions that the total reduction ratio in the recrystallization temperature range is 35% to 55%, the reduction ratio in the final rolling pass in the recrystallization temperature range is 10% or more, and the reduction ratio in the final rolling pass above (recrystallization temperature - 80°C) is 15% or more, and the rolling end temperature is above the Ar3 transformation point as measured by the steel plate surface thermometer. A controlled cooling step, wherein the hot-rolled steel sheet obtained in the hot rolling step is cooled under the conditions that the cooling start temperature is at least the Ar3 transformation point in terms of the surface temperature of the hot-rolled steel sheet, the cooling start time difference between the front end and the rear end of the hot-rolled steel sheet is within 50 seconds, the average cooling rate from 750° C. to 550° C. is 15 to 50° C. / s in terms of the center temperature of the sheet thickness, and the cooling stop temperature is 250 to 650° C.; and Any one of the following pipe making processes, a pipe making process in which, after the controlled cooling process, the hot rolled steel plate is bent, the two ends are butted and welded; a pipe making process in which, after the controlled cooling process, the hot rolled steel plate is formed into a cylindrical shape by cold roll forming, the two circumferential ends of the cylindrical shape are butted and resistance welded.

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