Steel material for line pipe, method for producing same, steel pipe for 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 a line pipe, which has excellent hydrogen embrittlement resistance in a high-pressure hydrogen environment and has high strength for 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 a line pipe; and a method for producing the steel pipe. A steel material for a line pipe has a specific chemical composition, the area fraction of retained austenite is 0-3%, the hydrogen diffusion coefficient at room temperature is 1.5 * 10 <-10 > m2 / s or more, and the hydrogen solid solubility is 0.05 mass ppm / [square root of] P or less.

Description

Technical Field

[0001] The present invention relates to a steel material for a line pipe suitable for use in a line pipe for transporting hydrogen, and a method for producing the same, and a steel pipe for a line pipe and a method for producing the same. Background Art

[0002] As an existing energy infrastructure, there are pipelines for natural gas transmission. These steels are required to suppress the occurrence of hydrogen-induced cracking in acidic environments. On the other hand, in recent years, hydrogen has received great attention worldwide as a clean energy for building a decarbonized society. Therefore, in order to transport hydrogen in large quantities, the construction of a hydrogen transmission network in which natural gas partially mixed with hydrogen and hydrogen are pressure-transmitted in natural gas pipelines as an alternative is studied. The transmission pressure during operation of these pipelines is assumed to be a high pressure of 1 to 40 MPa, and the pipelines are placed in a high-pressure hydrogen exposure environment. There is a concern that steel used in such an environment will suffer from "hydrogen embrittlement" in which hydrogen intrudes into the steel and the properties deteriorate. Therefore, it is necessary not only to have the high toughness and acid resistance required for conventional pipelines, but also to have the resistance to hydrogen embrittlement required in a hydrogen environment.

[0003] Steel structures used in high-pressure hydrogen environments have always used austenitic stainless steels such as SUS316L, which are less susceptible to hydrogen embrittlement than low-alloy steels. However, austenitic stainless steels such as SUS316L have high steel costs and low strength, so if they are designed to withstand high hydrogen pressures, the wall thickness becomes thicker and the price of the hydrogen structure itself becomes expensive. Therefore, as a steel material for hydrogen steel structures, there is a strong demand for a low-alloy steel material that is lower in cost and can withstand high-pressure hydrogen environments.

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

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

[0006] In addition, Patent Document 3 is a Cr-Mo based high-strength low alloy steel, which is tempered at a relatively high temperature of 560 to 580°C to adjust the grain size after tempering to a grain size of 8.4 or more and a tensile strength of 900 to 950 MPa in an extremely narrow range, thereby becoming a low-alloy high-strength steel with excellent resistance to high-pressure hydrogen environment embrittlement and excellent elongation and drawing properties even in a 45 MPa hydrogen atmosphere.

[0007] In addition, the low alloy steel for high pressure hydrogen environment proposed in Patent Document 4 further increases the Mo content compared with the existing steel by adding V, raising the tempering temperature, and utilizing V-Mo carbides, thereby improving the carbide morphology at the grain boundary and greatly improving the resistance to hydrogen environment embrittlement.

[0008] In addition, a steel for a high-pressure hydrogen storage container having excellent hydrogen resistance is proposed in Patent Document 5. According to the technology described in Patent Document 5, by performing a long-term stress relief annealing after normalizing treatment during steel plate manufacturing, MC carbides (Mo, V) C are dispersed and precipitated finely and densely, and hydrogen resistance such as hydrogen embrittlement resistance of the steel is improved.

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

[0010] It should be noted that Non-Patent Document 1 describes the value of fatigue strength of low alloy steel.

[0011] Prior art literature

[0012] Patent Literature

[0013] Patent Document 1: Japanese Patent Application Laid-Open No. 2005-2386

[0014] Patent Document 2: Japanese Patent Application Publication No. 2009-46737

[0015] Patent Document 3: Japanese Patent Application Publication No. 2009-275249

[0016] Patent Document 4: Japanese Patent Application Publication No. 2009-74122

[0017] Patent Document 5: Japanese Patent Application Publication No. 2010-37655

[0018] Patent Document 6: Japanese Patent Application Publication No. 2012-107332

[0019] Non-patent literature

[0020] Non-patent document 1: Matsunaga et al., Int J Hydrogen Energy, Vol. 40 (2015), p. 5739-5748

[0021] Non-patent document 2: Omura, Hydrogen addition method for reproducing the amount of hydrogen intrusion in the actual environment, The Japan Institute of Metals, Vol. 84, No. 9, (2020) p. 285-294

[0022] Non-patent document 3: 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

[0023] Non-patent document 4: Michiya Okada et al., Electrochemical measurement of hydrogen permeation in iron, Journal of the Japan Society of Metals, Vol. 50, No. 2 (1986) p. 201-207 Summary of the invention

[0024] The pressure inside the pipeline pipe repeatedly applies stress to the structure due to fluctuations during operation and periodic closures. Therefore, fatigue fracture needs to be considered when designing steel structures such as pipeline pipes. However, as shown in Non-Patent Document 1, it is known that the fatigue life of materials decreases in a high-pressure hydrogen environment. That is, it means that when designing pipeline pipes based on conventional natural gas pipelines, the service life of the pipeline pipes decreases. However, although the above-mentioned prior art can suppress the occurrence of hydrogen-induced cracking in an acidic environment, it cannot fully improve the fatigue strength in hydrogen, that is, there is a problem that it is difficult to strike a balance between suppressing the occurrence of hydrogen-induced cracking in an acidic environment and high fatigue strength in hydrogen.

[0025] In view of the above-mentioned problems of the prior art, the present invention aims to provide a steel material for pipelines having high strength and excellent hydrogen embrittlement resistance in a high-pressure hydrogen environment, a method for manufacturing the same, and a steel pipe for pipelines and a method for manufacturing the same, which are suitable for steel structures used in a high-pressure hydrogen environment such as pipelines for 100% hydrogen or natural gas containing hydrogen with a hydrogen partial pressure of 1 MPa or more (natural gas is a gas with hydrocarbons such as methane and ethane as the main component). As the hydrogen environment, an environment of high-pressure hydrogen of 1 MPa or more or a natural gas containing hydrogen with a hydrogen partial pressure of 1 MPa or more (hydrocarbons such as methane and ethane as the main component) is assumed.

[0026] It should be noted that the so-called "excellent hydrogen embrittlement resistance in a high-pressure hydrogen environment" here means that the fatigue crack growth rate da / dN mm / cycle obtained by fatigue testing at room temperature (20±10°C) and a pressure of 1 MPa or more of hydrogen 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 is 2.0×10 －3 mm / cycle or less.

[0027] It should be noted that if the fatigue crack growth rate da / dN mm / cycle is 2.0×10 －3mm / cycle or less, it is possible to design hydrogen steel structures such as long-life pipelines within the range of plate thickness that can be manufactured by the process of manufacturing seamless steel pipes, UOE and other steel pipes.

[0028] In addition, the so-called "steel materials" here include thin steel plates, thick steel plates, seamless steel pipes, electric resistance welded steel pipes, steel sections, steel bars, etc.

[0029] The present inventors have conducted in-depth research on the conditions that steel materials should meet in order to obtain pipeline steel and pipeline steel pipes with excellent hydrogen embrittlement resistance. It was found that the fatigue crack growth rate is greatly affected by the accumulation of hydrogen at the crack front and the stress (stress magnification coefficient) at the crack front. By reducing the hydrogen solid solubility in steel to below 0.05ppm / √P, the fatigue crack growth rate in hydrogen is significantly reduced. Furthermore, the greater the accumulation of hydrogen at the crack front, the more accelerated the fatigue crack growth rate in hydrogen. Moreover, the smaller the hydrogen diffusion coefficient, the greater the accumulation of hydrogen at the crack front, and the greater the fatigue crack growth rate. The present inventors have conducted a detailed analysis of the relationship between the fatigue crack growth rate and the hydrogen diffusion coefficient, and found that the hydrogen diffusion coefficient at room temperature is less than 1.5×10 －10 m 2 / s, the fatigue crack growth rate in hydrogen increases significantly. Based on the above-described insights, a new high-strength steel material and steel pipe for pipelines were invented. In addition, the steel material and steel pipe of the present invention have high strength, and high strength in the present invention refers to a tensile strength of 520 MPa or more.

[0030] The gist of the present invention is as follows.

[0031] [1] A steel material for a line pipe having the following chemical composition: containing, in terms of mass%, C: 0.02-0.15%, Si: 0.01-2.0%, Mn: 0.5-1.8%, P: 0.0001-0.015%, S: 0.0002-0.0015%, Al: 0.005-0.15%, O: 0.01% or less, N: 0.010% or less, H: 0.02 ppm or less, or further containing Nb: 0-0.10%, 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%, REM: 0-0.01%, Mg: 0-0.01%, B: 0-0.0020%, Hf: 0-0.2%, Ta: 0-0.2%, Re: 0-0.005%, Sn: 0-0.3%, Sb: 0-0.3%, and the balance is Fe and unavoidable impurity elements;

[0032] The area fraction of retained austenite is 0-3%, and the hydrogen diffusion coefficient at room temperature is 1.5×10 －10 m 2 / s or more, and the hydrogen solid solubility is 0.05 mass ppm / √P or less.

[0033] [2] The line pipe steel material according to [1], wherein the chemical composition, in mass%, further comprises Nb: 0.001-0.10%, 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: 0.0 1~0.10%, Zr: 0.0001~0.050%, REM: 0.0001~0.01%, Mg: 0.0001~0.01%, B: 0.0001~0.0020%, H f: 0.0001 to 0.2%, Ta: 0.0001 to 0.2%, Re: 0.0001 to 0.005%, Sn: 0.0001 to 0.3%, Sb: 0.0001 to 0.3%.

[0034] [3] The line pipe steel material according to [1] or [2], wherein the steel material has bainite or martensite at a 1 / 4 thickness position, and the bainite accounts for 90% or more in terms of area fraction, or the martensite accounts for 90% or more in terms of area fraction.

[0035] [4] A method for manufacturing a steel material for a pipeline pipe, comprising:

[0036] The heating step is to heat the steel billet having the chemical composition described in [1] or [2] at 1000 to 1250° C.

[0037] In the hot rolling step, the steel billet heated in the heating step is rolled at a rolling end temperature of Ar3 point or higher.

[0038] a controlled cooling step of cooling the hot rolled steel sheet obtained in the hot rolling step under the conditions that the cooling start temperature is Ar3 point or higher as measured by the steel sheet surface temperature, 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 as measured by the temperature at 0.25 mm below the steel sheet surface and at the center of the sheet thickness, and the cooling stop temperature is 250 to 650°C; and

[0039] Any one of a stabilization treatment step of stabilizing the steel sheet obtained in the controlled cooling step and a dehydrogenation treatment step of dehydrogenating the steel sheet obtained in the controlled cooling step.

[0040] [5] A steel pipe for a line pipe, wherein the steel pipe for a line pipe has the following chemical composition: in terms of mass%, C: 0.02-0.15%, Si: 0.01-2.0%, Mn: 0.5-1.8%, P: 0.0001-0.015%, S: 0.0002-0.0015%, Al: 0.005-0.15%, O: 0.01% or less, N: 0.010% or less, H: 0.02 ppm or less, or further contains Nb: 0-0.10%, 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%、REM:0~0.01%、Mg:0~0.01%、B:0~0.0020%、Hf:0~0.2%、Ta:0~0.2%、Re:0~0.005%、Sn:0~0.3%、Sb:0~0.3%、the remainder is Fe and inevitable impurity elements;

[0041] The area fraction of retained austenite is 0-3%, and the hydrogen diffusion coefficient at room temperature is 1.5×10 －10 m 2 / s or more, and the hydrogen solid solubility is 0.05 mass ppm / √P or less.

[0042] [6] The line pipe steel pipe according to [5], wherein the chemical composition is further, in mass%, Nb: 0.001-0.10%, 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: 0.0 1~0.10%, Zr: 0.0001~0.050%, REM: 0.0001~0.01%, Mg: 0.0001~0.01%, B: 0.0001~0.0020%, H f: 0.0001 to 0.2%, Ta: 0.0001 to 0.2%, Re: 0.0001 to 0.005%, Sn: 0.0001 to 0.3%, Sb: 0.0001 to 0.3%.

[0043] [7] The steel pipe for line pipe according to [5] or [6], wherein bainite or martensite is present at a position 1 / 4 of the wall thickness from the inner surface of the steel pipe, and the bainite has an area fraction of 90% or more, or the martensite has an area fraction of 90% or more.

[0044] [8] A method for manufacturing a steel pipe for a pipeline, comprising:

[0045] The heating step is to heat the steel billet having the chemical composition described in [5] or [6] at 1000 to 1250°C.

[0046] In the hot rolling step, the steel billet heated in the heating step is rolled at a rolling end temperature of Ar3 point or higher.

[0047] The hot rolled steel sheet obtained in the hot rolling step is cooled under the conditions that the cooling start temperature is Ar3 point or higher as measured by the steel sheet surface temperature, 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 as measured by the temperature at 0.25 mm below the steel sheet surface and at the center of the sheet thickness, and the cooling stop temperature is 250 to 650°C.

[0048] Any of the following pipe making processes: a pipe making process of bending the hot rolled steel plate after the controlled cooling process, butting both ends and welding; a pipe making process of forming the hot rolled steel plate into a cylindrical shape by cold roll forming after the controlled cooling process, butting both circumferential ends of the cylindrical shape and performing electric resistance welding, and

[0049] Any one of a stabilization treatment step of stabilizing the steel pipe obtained in the pipe manufacturing process and a dehydrogenation treatment step of dehydrogenating the steel pipe obtained in the pipe manufacturing process.

[0050] According to the present invention, it is possible to easily and simply manufacture a steel material having greatly improved hydrogen embrittlement resistance 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 embrittlement resistance of steel structures such as high-pressure hydrogen pipelines, improve fatigue resistance, and greatly contribute to extending the life of steel structures. DETAILED DESCRIPTION

[0051] Next, a method for implementing the present invention will be described in detail. It should be noted that the following description shows a preferred embodiment of the present invention, and the present invention is not limited to the following description. As a first embodiment, a steel material is described in detail, and then as a second embodiment, a UOE steel pipe as an example of a steel pipe of the present invention is described in detail, and as a third embodiment, an electric resistance welded steel pipe as an example of a steel pipe of the present invention is described in detail.

[0052] First embodiment

[0053] [Ingredients]

[0054] The reasons for limiting the chemical composition of the steel material of the present invention are described below. It should be noted that "%" in the following description means "mass %" unless otherwise specified.

[0055] C: 0.02~0.15%

[0056] C effectively contributes to the improvement of strength, but when the content is less than 0.02%, sufficient strength cannot be ensured. Therefore, the C content is 0.02% or more. Preferably, the C content is 0.03% or more. On the other hand, if it exceeds 0.15%, the weldability decreases. Therefore, the C content is limited to 0.15% or less. Preferably, the C content is 0.13% or less. In addition, if it exceeds 0.08%, the hardness of the surface layer and the center segregation part increases during accelerated cooling, so the SSCC resistance and HIC resistance are sometimes deteriorated. In addition, the toughness is sometimes deteriorated. Therefore, it is more preferred that the C content is 0.08% or less. It is further preferred that the C content is 0.05% or less.

[0057] Si: 0.01~2.0%

[0058] Si is added for deoxidation, but when the content is less than 0.01%, the deoxidation effect is insufficient. Therefore, the Si content is 0.01% or more. The Si content is preferably 0.08% or more, and more preferably 0.10% or more. On the other hand, when it exceeds 2.0%, its effect is saturated, so the Si content is 2.0% or less. The Si content is preferably 1.8% or less, and more preferably 1.0% or less. Furthermore, if it exceeds 0.50%, the toughness and weldability are deteriorated, and the hydrogen solid solubility increases, so the Si content is further preferably 0.50% or less.

[0059] Mn: 0.5~1.8%

[0060] Mn effectively contributes to the improvement of strength and toughness, but when the content is less than 0.5%, the effect of its addition is insufficient. Therefore, the Mn content is 0.5% or more. The Mn content is preferably 0.6% or more, more preferably 0.7% or more, and further preferably 0.8% or more. On the other hand, if it exceeds 1.8%, the hardness of the surface layer and the center segregation portion increases during controlled cooling, so the resistance to SSCC (sulfide stress corrosion cracking), resistance to HIC (hydrogen induced cracking) and hydrogen embrittlement resistance deteriorate. In addition, weldability also deteriorates, and the hydrogen solid solubility increases. Therefore, the amount of Mn is limited to less than 1.8%. The Mn content is preferably less than 1.5%, more preferably less than 1.4%, and further preferably less than 1.3%.

[0061] P: 0.0001~0.015%

[0062] P is an inevitable impurity element that deteriorates weldability, deteriorates HIC resistance by increasing the hardness of the center segregation portion, and deteriorates hydrogen embrittlement resistance by increasing the hydrogen solid solubility. If it exceeds 0.015%, this tendency becomes significant, so the upper limit of the P content is limited to 0.015%. The P content is preferably 0.010% or less, and more preferably 0.008% or less. The lower the content, the better, but from the viewpoint of refining cost, the P content is 0.0001% or more.

[0063] S: 0.0002~0.0015%

[0064] S is an inevitable impurity element. It becomes MnS inclusions in steel, deteriorating HIC resistance and deteriorating hydrogen embrittlement resistance due to increased hydrogen solid solubility. 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 perspective of refining cost, it is 0.0002% or more.

[0065] Al: 0.005~0.15%

[0066] Al is added as a deoxidizer, but when it is less than 0.005%, there is no effect of addition, so the Al content is 0.005% or more. The Al content is preferably 0.010% or more, and more preferably 0.030% or more. On the other hand, if it exceeds 0.15%, the cleanliness of the steel decreases and the toughness deteriorates, so the Al content is limited to 0.15% or less. The Al content is preferably 0.10% or less, more preferably 0.08% or less, and further preferably 0.05% or less.

[0067] O: 0.01% or less

[0068] O is the cause of the formation of oxide inclusions, so it is preferably less, but if the O content is 0.01% or less, it will not be a problem. Therefore, the O content is 0.01% or less. The O content is preferably 0.005% or less. More preferably, the O content is less than 0.003%. There is no particular lower limit, but setting oxygen to 0% is the main cause of increased cost, so it is preferably 0.001% or more.

[0069] N: 0.010% or less

[0070] N has little effect on the fatigue properties of steel. If the N content is 0.010% or less, the effect of the present invention will not be impaired from the perspective of toughness. Therefore, the N content is 0.010% or less. The N content is preferably 0.008% or less, and the N content is more preferably 0.006% or less. The N content is further preferably 0.004% or less. On the other hand, from the perspective of improving toughness, a low N content is preferred, but excessive reduction will increase the cost of steelmaking, so the N content is preferably 0.001% or more.

[0071] H: 0.02ppm or less

[0072] H is sometimes introduced into steel in various processes during manufacturing. If the amount introduced is large, the risk of cracking after solidification is sometimes increased, and fatigue crack propagation is accelerated. If the H content is less than 0.02ppm, these effects will not be a problem, so the H content is less than 0.02ppm. Preferably, the H content is less than 0.01ppm. More preferably, it is less than 0.005ppm. It is further preferred that the H content is less than 0.003ppm. The lower limit is not particularly limited, but if it is less than 0.001ppm, it becomes the main reason for the increase in cost, so the H content is preferably more than 0.001ppm.

[0073] It should be noted that the hydrogen content refers to the amount of residual hydrogen after steel materials, steel pipes, UOE, etc. are formed.

[0074] In order to further improve the strength and toughness of the steel plate, the component composition disclosed in the present invention can arbitrarily contain one or more selected from Nb, Ca, Ni, Ti, Cu, Cr, Mo, W, V, Zr, REM, Mg, B, Hf, Ta, Re, Sn, and Sb within the following range.

[0075] Nb: 0~0.10%

[0076] Nb is an element that contributes to the strength of steel, but if the content exceeds 0.10%, the effect is saturated and becomes the main cause of cost increase. Therefore, when Nb is contained, the Nb content is 0.10% or less. The Nb content is preferably 0.08% or less. The Nb content is more preferably 0.06% or less. In order to suppress costs, the Nb content is further preferably 0.05% or less. When Nb is contained, the Nb content can be 0% or more, but in order to obtain the above-mentioned effect, it is preferably 0.001% or more. The Nb content is more preferably 0.01% or more.

[0077] Ca: 0~0.005%

[0078] Ca is an element effective in improving HIC resistance by controlling the morphology of sulfide inclusions, but when it exceeds 0.005%, not only is the effect 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 amount is 0.005% or less. The Ca content is preferably 0.003% or less. The Ca content is more preferably 0.002% or less. When Ca is contained, the Ca content may be 0% or more, but when it is less than 0.0001%, the effect of its inclusion is not sufficient, and the Ca content is preferably 0.0001% or more. The Ca content is more preferably 0.001% or more.

[0079] Ni: 0-2.0%

[0080] Ni is an element effective in improving toughness and increasing strength, but if it contains more than 2.0%, fine cracks called fissures are easily generated in an environment with a low hydrogen sulfide partial pressure of less than 1 bar. Therefore, when Ni is contained, the Ni content is 2.0% or less. The Ni content is preferably 1.5% or less, more preferably 1.2% or less, and further preferably 1.0% or less. The Ni content is preferably 0.1% or less. It is most preferably 0.02% or less. When Ni is contained, the Ni content can be 0% or more, but in order to obtain the above-mentioned effect, it is preferably 0.01% or more Ni.

[0081] Ti: 0~0.1%

[0082] Ti contributes to the strength of steel, but if the content exceeds 0.1%, the effect is saturated and becomes a major factor in the increase of cost. Therefore, when Ti is contained, the Ti content is 0.1% or less. In order to suppress the cost, the Ti content is more preferably 0.05% or less. When Ti is contained, the Ti content may be 0% or more, but in order to obtain the above-mentioned effect, when Ti is contained, the content is preferably 0.005% or more. The Ti content is more preferably 0.008% or more.

[0083] Cu: 0~1.0%

[0084] Cu is an element effective in improving toughness and increasing strength, but if the content is too high, weldability deteriorates, and hydrogen embrittlement resistance deteriorates due to increased hydrogen solid solubility. Therefore, when Cu is contained, the Cu content is 1.0% or less. The Cu content is preferably 0.5% or less. The Cu content is more preferably 0.3% or less, and further preferably 0.2% or less. When Cu is contained, the Cu content may be 0% or more, but in order to obtain this effect, it is preferably 0.01% or more.

[0085] Cr: 0~1.0%

[0086] Cr is an element that is effective for obtaining sufficient strength even with low C, similar to Mn, but if the content is too high, the hardenability is excessive, so the SSCC resistance is deteriorated, and the hydrogen embrittlement resistance is deteriorated due to the increase in hydrogen solid solubility. In addition, weldability is also deteriorated. Therefore, when Cr is contained, the Cr content is 1.0% or less. The Cr content is preferably 0.8% or less, and more preferably 0.5% or less. More preferably, it is 0.1% or less. When Cr is contained, the Cr content may be 0% or more, but in order to obtain this effect, it is preferably 0.01% or more. More preferably, it is 0.05% or more.

[0087] Mo: 0~0.60%

[0088] Mo is an element effective in improving toughness and strength, and is an element effective in improving SSCC resistance independently of the hydrogen sulfide partial pressure. However, if the content is too high, the hardenability is excessive, so the SSCC resistance is deteriorated, and the hydrogen embrittlement resistance is deteriorated due to the increase in hydrogen solid solubility. In addition, the weldability is also deteriorated. Therefore, the Mo content is 0.60% or less. The Mo content is preferably 0.50% or less, and more preferably 0.40% or less. The Mo content may be 0% or more, but in order to obtain the above-mentioned effect, it is preferably 0.01% or more of Mo, and more preferably 0.10% or more.

[0089] W: 0~1.0%

[0090] W contributes to the strength of steel, but if the W content exceeds 1.0%, the effect is saturated and becomes a major factor for increasing costs. Therefore, when W is contained, the W content is 1.0% or less. The W content is preferably 0.8% or less. In order to suppress costs, it is preferably 0.5% or less. The W content may be 0% or more, but in order to obtain the above-mentioned effects, when W is contained, it is preferably 0.01% or more.

[0091] V: 0~0.10%

[0092] V is an element that can be arbitrarily contained in order to improve the strength and toughness of steel. If it exceeds 0.10%, the toughness of the weld deteriorates, and the hydrogen embrittlement resistance deteriorates due to the increase in hydrogen solid solubility, so it is contained in an amount of 0.10% or less. The V content is preferably 0.08% or less. The V content is more preferably 0.06% or less, and further preferably 0.03% or less. 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 when V is contained, it is preferably 0.01% or more.

[0093] Zr: 0~0.050%, REM: 0~0.01%, Mg: 0~0.01%

[0094] Zr, REM and Mg are elements that can be arbitrarily contained in order to improve toughness by grain refinement or to improve crack resistance by controlling the properties of inclusions. If Zr exceeds 0.050%, REM and Mg exceed 0.01%, the effects of these elements are saturated, so when these elements are contained, Zr is 0.050% or less, and REM and Mg are 0.01% or less. That is, when contained, the Zr content is 0.050% or less. The Zr content is preferably 0.0040% or less. The Zr content is more preferably 0.0030% or less. In addition, when contained, the REM content is 0.01% or less. The REM content is preferably 0.0040% or less. The REM content is more preferably 0.0030% or less. In addition, when contained, the Mg content is 0.01% or less. The Mg content is preferably 0.0040% or less. The Mg content is more preferably 0.0030% or less. The content of these elements may be 0% or more, but when the content is less than 0.0001%, the effect of their inclusion is insufficient, so it is preferably 0.0001% 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.

[0095] B: 0~0.0020%

[0096] B is an element that improves hardenability, contributes to the strength of the steel pipe, and inhibits the coarsening of the original austenite grains, improving various properties of the billet. 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, so when it is contained, the B content is 0.0020% or less. The B content is preferably 0.0015% 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. The B content can be 0% or more, but in order to obtain the above-mentioned effect, when B is contained, it is preferably 0.0001% or more. More preferably, it is 0.0005% or more.

[0097] Hf: 0~0.2%, Ta: 0~0.2%

[0098] These elements contribute to the increase of the strength of the steel. If the content exceeds 0.2%, the effect is saturated, which becomes the main reason for the increase in cost. Therefore, when these elements are contained, the content is 0.2% or less. That is, when contained, Hf is 0.2% or less. Hf is preferably 0.1% or less. Hf is more preferably 0.05% or less. In addition, when contained, Ta is 0.2% or less. Ta is preferably 0.1% or less. Ta is more preferably 0.05% or less. In order to suppress costs, the content is preferably 0.01% or less. The content of these elements can be 0% or more, but in order to obtain the above-mentioned effects, when contained, the Hf content is preferably 0.0001% or more. The Hf content is more preferably 0.001% or more. In addition, the Ta content is preferably 0.0001% or more. The Ta content is more preferably 0.001% or more.

[0099] Re: 0~0.005%

[0100] Re contributes to the improvement of the strength of steel, but if the content exceeds 0.005%, the effect is saturated and becomes the main cause of the cost increase, so when contained, it is 0.005% or less. The Re content is preferably 0.003% or less. The Re content is more preferably 0.002% or less. The Re content can be 0% or more, but in order to obtain the above-mentioned effect, when contained, the content is made 0.0001% or more. It is preferably 0.001% or more.

[0101] Sn: 0~0.3%, Sb: 0~0.3%,

[0102] These elements contribute to the increase of strength and hardenability of steel, but if the content exceeds 0.3%, the effect is saturated and becomes the main reason for the increase in cost, so when contained, it is 0.3% or less. That is, 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. In order to suppress costs, it is further preferably 0.01% or less. In addition, 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. In order to suppress costs, the Sb content is further preferably 0.01% or less. The content of Sn and Sb can be 0% or more, but in order to obtain the above-mentioned effects, when contained, it is preferred that the Sn content is 0.0001% or more. The Sn content is more preferably 0.001% or more. In addition, the Sb content is preferably 0.0001% or more. The Sb content is more preferably 0.0010% or more.

[0103] In the chemical composition of the steel plate and the steel pipe, the remainder other than the above-mentioned components (elements) is composed of Fe and inevitable impurity elements.

[0104] The metal structure of the steel material of the present invention will be described below.

[0105] Metal structure

[0106] The area fraction of retained austenite is 0-3%.

[0107] Since retained austenite remains in the steel, the amount of hydrogen in the steel increases, which sometimes increases the sensitivity to hydrogen embrittlement. Furthermore, when the retained austenite is transformed into martensite due to the stress load in use, hydrogen-induced cracking is likely to occur because the martensite is very hard, and cracks sometimes occur from the martensite part. In the present invention, by making the retained austenite less than 3% in terms of area fraction, the fatigue crack growth rate can be reduced, thereby improving the hydrogen embrittlement resistance. Therefore, the retained austenite is less than 3%. The retained austenite is preferably less than 2%. The retained austenite is more preferably less than 1%. The retained austenite can be 0%.

[0108] The steel material has bainite or martensite at a position of 1 / 4 of the plate thickness (in the case of a steel pipe, a position of 1 / 4 of the wall thickness from the inner surface of the steel pipe), and the bainite has an area fraction of 90% or more or the martensite has an area fraction of 90% or more (preferably).

[0109] In order to achieve a high tensile strength of 520 MPa or more, the steel structure is preferably bainite or martensite. On the other hand, when soft phase and hard phase are mixed in the steel, fatigue damage is accumulated in the soft phase in advance, cracks are easily generated, and the fatigue limit stress is reduced. In a hydrogen environment, local deformation is promoted, thereby further accelerating fatigue damage to the soft phase, and the hydrogen embrittlement resistance in hydrogen is reduced. As a result, the fatigue crack growth rate da / dNmm / cycle in hydrogen is difficult to achieve 2.0×10 －3 mm / cycle or less. In order to improve this, it is necessary to reduce the relative proportion of the soft phase. Therefore, the metal structure is preferably a single structure of bainite or martensite, preferably having either bainite or martensite and the structure is 90% or more in terms of area fraction. The above-mentioned structure is more preferably 92% or more in terms of area fraction, and further preferably 95% or more. The upper limit is not particularly limited, but is preferably 98% or less. The upper limit is not particularly limited, and bainite can be 100% in terms of area fraction. Furthermore, since fatigue cracks are generated from the inner surface of the steel pipe, the uniformity of the inner surface structure of the steel pipe is very important. Therefore, in the case of a steel pipe, the metal structure at a position 1 / 4 of the wall thickness from the inner surface of the steel pipe is specified.

[0110] Here, the bainite structure includes bainitic ferrite or granular bainite that transforms during or after controlled cooling, which contributes to phase transformation strengthening, and includes tempered bainite. When 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 is deteriorated. Therefore, the volume fraction of the structure other than the bainite phase is as small as possible. Here, the above-mentioned martensite structure includes tempered martensite.

[0111] The diffusion coefficient of hydrogen at room temperature is 1.5×10 －10 m 2 / s or more

[0112] The greater the amount of hydrogen accumulated at the crack front, the faster the fatigue crack growth rate in hydrogen. The smaller the hydrogen diffusion coefficient, the greater the amount of hydrogen accumulated at the crack front, and the faster the fatigue crack growth rate. The hydrogen diffusion coefficient is less than 1.5×10 － 10 m 2 / s, the fatigue crack growth rate in hydrogen increases significantly, so the hydrogen diffusion coefficient is 1.5×10 －10 m 2 / s or more. The preferred hydrogen diffusion coefficient is 2.0×10 －10 m 2 / s or more, more preferably 3.0×10 －10 m 2 / s or more. More preferably, the hydrogen diffusion coefficient is 5.0×10 －10 m 2 / s or more. The most preferred hydrogen diffusion coefficient is 6.0×10 －10 m 2 / s or more. The upper limit is not particularly limited, but in order to reduce the hydrogen diffusion coefficient, the strength is reduced. Therefore, considering the material strength, it is preferably 5.0×10 －9 m 2 / s or less. Since the hydrogen diffusion coefficient of retained austenite at room temperature is small, in order to make the hydrogen diffusion coefficient at room temperature the above value, the above-mentioned retained austenite fraction is required. In addition, hydrogen intrusion into the steel occurs from the surface of the steel (the inner surface of the steel pipe in the case of a steel pipe). Therefore, for the hydrogen diffusion coefficient, the value of the wall thickness at which the fatigue crack propagates in the wall thickness and causes rapid fracture is very important. The wall thickness until rapid fracture can be determined based on the fracture toughness value of the material and the stress generated in the steel pipe. However, in reality, the fatigue crack growth life of the steel (the steel pipe in the case of a steel pipe) needs to expand mostly in 1 / 4t of the wall thickness, so the hydrogen diffusion coefficient only needs to be measured at a value 1 / 4t from the inner surface of the steel pipe. In addition, the temperature dependence of the hydrogen diffusion coefficient is large, so the hydrogen diffusion coefficient of the present invention is defined as the value at room temperature (20±10°C).

[0113] Hydrogen solid solubility is less than 0.05 mass ppm / √P

[0114] In the present invention, hydrogen solid solubility is the most important factor. The fatigue crack growth rate is greatly affected by the accumulation of hydrogen to the crack front end and the stress (stress amplification coefficient) at the crack front end. That is, in order to obtain the desired fatigue crack growth rate, it is important to reduce the accumulation of hydrogen to the crack front end. The smaller the hydrogen solid solubility, the greater the reduction in the fatigue crack growth rate in hydrogen. In order to obtain the desired fatigue crack growth rate, the hydrogen solid solubility in the steel is 0.05 mass ppm / √P or less. Preferably, it is 0.03 mass ppm / √P or less, and more preferably 0.02 mass ppm / √P or less. In order to reduce the hydrogen solid solubility, stabilization treatment or dehydrogenation treatment is implemented. On the other hand, the heat treatment that makes the hydrogen solid solubility less than 0.005 mass ppm / √P leads to a decrease in the strength of the material and a substantial increase in the manufacturing cost, so it is preferably 0.005 mass ppm / √P or more.

[0115] It should be noted that the hydrogen solubility s here is the value of the slope [mass ppm / √P] of the amount of hydrogen H [mass ppm] intruding under a hydrogen pressure P [MPa] environment and the square root of the hydrogen pressure P [MPa]. P in a mixed gas environment can be translated as the hydrogen partial pressure equivalent P'. Specifically, in the case of a gas environment containing 20% ​​hydrogen in 25MPa, 25MPa×0.2=5MPa is the hydrogen partial pressure P'.

[0116] There are many ways to calculate the solid solubility of hydrogen, and one example is shown below. For example, a test piece is exposed to any pressure environment of 0 to 40 MPa high-pressure hydrogen environment for a specified time. Then, the amount of hydrogen in the steel is measured using a hydrogen analyzer, the relationship between H and √P is obtained, and s is calculated from the slope. Alternatively, it can also be calculated by a cathode hydrogen charging test that simulates a high-pressure gas environment, such as in Non-Patent Document 2.

[0117] The tensile strength is preferably 520 MPa or more, more preferably 580 MPa or more. The upper limit is not particularly limited, but the tensile strength is preferably 950 MPa or less, more preferably 800 MPa or less.

[0118] In addition, although not particularly limited, the plate thickness is preferably 5 mm or more and preferably 30 mm or less.

[0119] The steel material of the present invention can be produced by sequentially performing a heating step, a hot rolling step, a controlled cooling step, a stabilization step, and a dehydrogenation step of a steel billet (slab).

[0120] It should be noted that the temperature in the following description is the temperature at the center of the plate thickness of the steel billet or steel pipe unless otherwise specified. The average cooling rate refers to the temperature at a position 1 / 4 of the wall thickness from the inner surface of the steel pipe. It should be noted that the temperature at the center of the plate thickness and the temperature at a position 1 / 4 of the wall thickness from the inner surface of the steel pipe are temperatures estimated from the surface temperature of the steel pipe measured by a radiation thermometer using heat transfer calculations that take into account the heat transfer coefficient of the steel material.

[0121] Heating temperature of steel billet: 1000~1250℃

[0122] When the heating temperature of steel billets such as billets and slabs 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. Therefore, the heating temperature of the steel billet is 1000°C or higher. The heating temperature of the steel billet is preferably 1180°C or higher, and more preferably 1200°C or higher. On the other hand, if it exceeds 1250°C, the grains become too coarse and the toughness deteriorates. Therefore, the heating temperature of the steel billet is 1250°C or lower. The heating temperature of the steel billet is preferably 1230°C or lower, and more preferably 1210°C or lower.

[0123] Hot rolling end temperature: above Ar3 point

[0124] After the steel billet is reheated, it is hot rolled to the desired tube thickness or plate thickness, and the end temperature of hot rolling is the ferrite formation temperature, that is, Ar3 point or above. This is because when it is less than Ar3 point, in the case of a process in which cooling is performed immediately after hot rolling, the strength is reduced due to the formation of a soft ferrite phase. The end temperature of hot rolling is preferably above 770°C. When the Ar3 point is higher than 770°C, the end temperature of finish rolling is preferably above Ar3 point + 30°C, and more preferably above Ar3 point + 50°C. In addition, if it exceeds 1250°C, the grains are too coarse and the toughness is deteriorated, so the upper limit is preferably below 1250°C.

[0125] The Ar3 point temperature varies depending on the alloy composition of the steel, so it can be obtained by experimentally measuring the phase transition temperature of each steel, but it can also be obtained from the following formula based on the component composition.

[0126] Ar3(℃)=910-310C(%)-80Mn(%)-20Cu(%)-15Cr(%)

[0127] -55Ni(%) -80Mo(%)

[0128] The content of each alloy element is (mass %).

[0129] Controlled cooling process

[0130] Cooling start temperature: Ar3 point or above based on the steel plate surface thermometer

[0131] When the surface temperature of the steel plate at the start of cooling is less than the Ar3 point, ferrite is generated before controlled cooling, and the strength reduction becomes greater. Therefore, the surface temperature of the steel plate at the start of cooling is above the Ar3 point. The surface temperature of the steel plate at the start of cooling is preferably above 770°C. When the Ar3 point is higher than 770°C, the finishing temperature is preferably above the Ar3 point + 30°C, and more preferably above the Ar3 point + 50°C. The upper limit is not particularly limited, but is preferably below 1250°C. It should be noted that the surface temperature of the steel plate at the start of cooling is the temperature of the tail end of the steel plate where the cooling start temperature is the lowest.

[0132] The cooling start time difference between the front end and the rear end of the steel plate: within 50 seconds

[0133] When the time difference between the front end and the tail end of the steel plate rolling direction at the start of cooling exceeds 50 seconds, 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, the deviation of the Vickers hardness at 0.25mm below the surface of the steel plate becomes larger, and the HISC resistance deteriorates. Therefore, the cooling start time difference between the front end of the steel plate and the tail end of the steel plate is within 50 seconds, preferably within 45 seconds. More preferably within 40 seconds. The cooling start time difference can be shortened by shortening the steel plate length, but the manufacturability is reduced, so it is preferred to shorten the cooling start time difference by speeding up the steel plate conveying speed. The lower limit is not particularly limited, and the cooling start time difference can exceed 0 seconds.

[0134] Controlling the cooling speed of the cooling process

[0135] In order to obtain excellent HISC resistance and achieve high strength, it is necessary to control the cooling rate at 0.25 mm below the surface of the steel plate and in the center of the plate thickness. It should be noted that the cooling rate in the plate thickness direction is a value obtained by simulation based on the surface temperature measured by a radiation thermometer through heat transfer calculation, etc.

[0136] Average cooling rate from 750°C to 550°C at 0.25mm below the steel plate surface: 15-50°C / s

[0137] It is important to slow down the average cooling rate from 750°C to 550°C as measured by the steel plate thermometer at 0.25 mm below the steel plate surface to form granular bainite. The temperature range from 750°C to 550°C is an important temperature range in the bainite transformation, so it is important to control the cooling rate within this temperature range. When the cooling rate exceeds 50°C / s, there is a concern that the hardness will deviate, and the HISC resistance after pipe making will deteriorate. Therefore, the average cooling rate is 50°C / s or less. Preferably it is 45°C / s or less. More preferably, it is 40°C / s or less. On the other hand, if the cooling rate becomes too small, ferrite and pearlite are generated and the strength is insufficient. Therefore, from the perspective of preventing this situation, it is 15°C / s or more, preferably 17°C / s or more. More preferably, it is 20°C / s or more, and more preferably, 25°C / s or more. It should be noted that, for cooling to 550°C or less as measured by the steel plate thermometer at 0.25 mm below the steel plate surface, if the cooling rate is slow, it will not be cooling in a stable nucleate boiling state, and there is a concern that the hardness of the extreme surface layer of the steel plate will vary. Therefore, the average cooling rate from 550°C to the cooling stop temperature as measured by the steel plate thermometer at 0.25 mm below the steel plate surface is preferably 150°C / s or more. Since there is a concern that the hardness will vary, the average cooling rate is preferably 250°C / s or less.

[0138] Average cooling rate from 750°C to 550°C in the center of the plate thickness: 15 to 50°C / s

[0139] When the average cooling rate from 750°C to 550°C in the center of the plate thickness is less than 15°C / s, no granular bainite structure can be obtained, resulting in reduced strength. In addition, excessive retained austenite is generated, and the hydrogen diffusion coefficient at room temperature becomes smaller. 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 more preferably 20°C / s or more, and further preferably 25°C / s or more. On the other hand, in order to suppress the deviation of the grain size, the average cooling rate is 50°C / s or less, preferably 45°C / s or less. The average cooling rate in the center of the plate thickness is more preferably 40°C / s or less. It should be noted that there is no particular limitation on cooling below 550°C with the steel plate temperature in the center of the plate thickness, but from the viewpoint of suppressing the deviation of the structure and grain size, the average cooling rate in the center of the plate thickness is preferably 15°C / s or more. In addition, the average cooling rate in the center of the plate thickness is preferably 50°C / s or less.

[0140] In addition, when the amount of C is high, the phase transformation morphology changes from bainite transformation to martensite transformation. However, when the average cooling rate from 750°C to 550°C in the center of the plate thickness is less than 15°C / s, a mixed structure of martensite and bainite is formed. Therefore, the average cooling rate is 15°C / 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 more preferably 20°C / s or more, and further preferably 25°C / s or more. On the other hand, in order to suppress the deviation of the grain size, the average cooling rate is 50°C / s or less, preferably 45°C / s or less. The average cooling rate in the center of the plate thickness is more preferably 40°C / s or less. It should be noted that there is no particular limitation on cooling below 550°C with the steel plate temperature in the center of the plate thickness, but from the viewpoint of suppressing the deviation of the structure and grain size, the average cooling rate in the center of the plate thickness is preferably 15°C / s or more. In addition, the average cooling rate in the center of the plate thickness is preferably 50°C / s or less.

[0141] It should be noted that the temperature of the steel plate at 0.25 mm below the surface of the steel plate and at the center of the plate thickness cannot be measured directly physically, but the temperature distribution in the plate thickness section can be calculated by, for example, using a program computer through differential calculation based on the surface temperature at the start of cooling and the target surface temperature at the stop of cooling measured by a radiation thermometer, and the result can be obtained in real time. The temperature at 0.25 mm below the surface of the steel plate in the temperature distribution is referred to as the "steel plate temperature at 0.25 mm below the surface of the steel plate" in this specification, and the temperature at the center of the plate thickness in the temperature distribution is referred to as the "steel plate temperature at the center of the plate thickness" in this specification.

[0142] Cooling stop temperature

[0143] Cooling stop temperature: 250~650℃ at the temperature of the steel plate 0.25mm below the surface and at the center of the plate thickness

[0144] If the cooling stop temperature exceeds 650°C, the bainite phase transformation is incomplete and sufficient strength cannot be obtained. Therefore, the cooling stop temperature is 650°C or less. The cooling stop temperature is preferably 625°C or less, more preferably 600°C or less, and further preferably 500°C or less. In addition, when the cooling stop temperature is less than 250°C, the hardness increases, so the resistance to HISC degradation. Therefore, the cooling stop temperature is 250°C or more. The cooling stop temperature is preferably 270°C or more. The cooling stop temperature is more preferably 300°C or more.

[0145] Tempering process

[0146] In order to improve toughness and adjust material strength, tempering treatment can be performed. If the temperature is below 200°C, the tempering effect cannot be obtained, so if it is performed, the tempering temperature is preferably above 200°C. On the other hand, tempering is also a major cause of strength reduction, and if it becomes too high, the structure will change again, so it is preferably below the Ar3 point. The holding time can be determined arbitrarily, but it is preferably 10 minutes or more at a specified temperature in the center of the plate thickness. It is preferably 180 minutes or less.

[0147] Stabilization process

[0148] The hydrogen that penetrates into the steel is mainly captured by various defects such as dislocations. Since hydrogen is captured in these various defects, the hydrogen diffusion coefficient becomes smaller and the hydrogen solid solubility also increases. As a result, the hydrogen embrittlement resistance deteriorates. Therefore, it is important to reduce these defects or reduce the combination of these defects with hydrogen. Therefore, in order to weaken the combination of hydrogen and dislocations after manufacturing, a dislocation stabilization treatment is implemented. If it is kept at a specified temperature for a certain period of time before the product is used, the dissolved carbon can be fixed to the dislocation, and the dislocation is stabilized, thereby reducing the combination of hydrogen and dislocations. As a result, the hydrogen diffusion coefficient can be increased, the hydrogen solid solubility can be reduced, and a steel with excellent hydrogen embrittlement resistance in a high-pressure hydrogen environment can be obtained.

[0149] The stabilization treatment process is implemented before the welding construction of pipe making and connecting steel pipes. When the temperature is lower than room temperature (25°C ± 10°C), the diffusion of carbon is significantly low, so it is above room temperature. In addition, the carbon diffusion coefficient Dc at high temperature is small, and carbon diffuses in a short time, so it is preferably above 100°C, and more preferably above 200°C. On the other hand, when the temperature of the stabilization treatment process is too high, the material strength is significantly reduced, so the stabilization treatment temperature is implemented below the Ar3 point (°C) or below 700°C. In addition, in the case of tempered materials, when the stabilization treatment is implemented, it is preferably set at a temperature that is 50°C or more lower than the tempering temperature as the upper limit. The holding time is 72 hours or more when the stabilization treatment temperature is less than 100°C, and is 10 minutes or more when it is above 100°C. The holding time is preferably 400 hours or less when the stabilization treatment temperature is less than 100°C, and is preferably 100 hours or less when it is above 100°C. The temperature is the center of the plate thickness.

[0150] It should be noted that the time and temperature of the stabilization treatment step may be used as a heating step in the pipe manufacturing process of electric resistance welded pipes, UOE steel pipes, etc. This step refers to a step of performing a heating treatment after pipe manufacturing such as tempering and stress relief annealing.

[0151] Dehydrogenation process

[0152] When hydrogen is present in steel, the acceleration of fatigue crack growth increases, and the fatigue life and fatigue limit stress in hydrogen decrease. Therefore, in order to release the hydrogen remaining after manufacturing, dehydrogenation treatment can be used. Dehydrogenation treatment can reduce the amount of hydrogen in steel by keeping it at high temperature for a certain period of time before the product is used, and can obtain steel with excellent resistance to hydrogen embrittlement in high-pressure hydrogen environment.

[0153] The holding time R (sec) is preferably determined based on the plate thickness and tube thickness t (mm) of the steel material and the steel tube and the hydrogen diffusion coefficient D (mm) in the steel at room temperature. 2 ·sec －1 ) and adopt the following formula (A).

[0154] R≥t 2 / D···(A)

[0155] In addition, the above-mentioned content can be used for the hydrogen diffusion coefficient.

[0156] The dehydrogenation process is implemented before the welding construction of pipe making and connecting steel pipes. It should be noted that since the hydrogen diffusion coefficient D at high temperature becomes smaller, in order to make hydrogen escape quickly, the dehydrogenation treatment is preferably at high temperature. 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 (A) formula is maintained can be used for calculation. In addition, when the temperature T of the dehydrogenation process is too high, the material strength is significantly reduced, so the dehydrogenation temperature is preferably below 550°C. The dehydrogenation temperature T is more preferably below 500°C. The dehydrogenation 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 temperature T is preferably above room temperature. The dehydrogenation temperature T is more preferably above 50°C. The dehydrogenation temperature T is further preferably above 100°C, and most preferably above 150°C. The dehydrogenation temperature T is the temperature of the atmosphere in the dehydrogenation process. Room temperature refers to 20±10°C.

[0157] In the case of tempered materials, when dehydrogenation treatment is performed, the upper limit is set to a temperature that is 50° C. or more lower than the tempering temperature.

[0158] In particular, when heating is performed, it takes time for the temperature Tc of the center of the plate thickness of the steel material and the steel pipe to reach the temperature of the atmosphere in the dehydrogenation treatment process (dehydrogenation treatment temperature T). 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 R (sec) or more after the temperature Tc of the center of the plate thickness reaches the target temperature T. Furthermore, in order to obtain a specified crack propagation rate in hydrogen, it is necessary to appropriately adjust the amount of hydrogen in the steel material at the surface and the center of the plate thickness. For this purpose, it is preferred to maintain R (sec) or more specified in formula (A) at the atmosphere temperature T, and it is further preferred to maintain R (sec) or more after the temperature Tc of the center of the plate thickness reaches the target temperature T. In other words, at least the former can properly control the amount of hydrogen in the steel material at the surface of the steel material and the steel pipe, and when the latter is implemented, the amount of hydrogen in the steel material from the surface to the center of the plate thickness of the steel material and the steel pipe can be properly controlled. The plate thickness center temperature Tc may be measured using a thermocouple or the like, or may be estimated using a finite element method or the like.

[0159] It should be noted that the time and temperature of the dehydrogenation treatment process may include the temperature and time applied when heating is performed in the pipe making process of electric resistance welded pipe, UOE, etc. as described later. Furthermore, the scale on the surface of the steel hinders dehydrogenation, so it is preferable 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.

[0160] Second embodiment

[0161] Furthermore, the UOE steel pipe cited as an example of a high-strength steel pipe for line pipe can be obtained by limiting the manufacturing conditions shown below, and the manufacturing method and conditions are specifically described. The component composition, metal structure, hydrogen solid solubility, and hydrogen diffusion coefficient of the UOE steel pipe are the same as those described in the steel material of the first embodiment. As for the manufacturing method, the heating process, hot rolling process, controlled cooling process after hot rolling, stabilization treatment process, and dehydrogenation treatment process are also implemented in the same content as those described in the steel material. The pipe making process after rolling is specifically described below.

[0162] Pipe making process

[0163] UOE steel pipe is a hot-rolled steel plate subjected to bending processing, specifically, groove processing of the end of the hot-rolled steel plate, and formed into a steel pipe shape by a C-type press, a U-type press, or an O-type press, and then seam-welded to the butt joint by inner surface welding and outer surface welding, and further manufactured by a pipe expansion process as required. In addition, any welding method may be used as long as it is a method that can obtain sufficient joint strength and joint toughness, but submerged arc welding is preferably used from the viewpoint of excellent welding quality and manufacturing efficiency. In addition, the steel pipe obtained by seam-welding the butt joint after being formed into a tubular shape by press bending can also be expanded.

[0164] Third embodiment

[0165] Furthermore, the high-strength steel pipe for line pipe of the present invention can be exemplified by an electric resistance welded steel pipe, which can be obtained by limiting the manufacturing conditions shown below, and the manufacturing method and conditions are specifically described. The component composition, metal structure, hydrogen solid solubility, and hydrogen diffusion coefficient of the steel material are the same as those described in the steel material of the first embodiment, and the manufacturing method is also implemented in the same content as that described in the steel material except for the controlled cooling step after rolling and the pipe making step (heating step, hot rolling step, stabilization treatment step, dehydrogenation treatment step) except for the controlled cooling step after rolling.

[0166] Cooling process after rolling (controlled cooling process)

[0167] 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.

[0168] Cooling stop temperature: 250~650℃

[0169] When the cooling stop temperature after hot rolling exceeds 650°C, the bainite phase transformation is incomplete and the material strength is significantly reduced. Therefore, the cooling stop temperature is below 650°C. The cooling stop temperature is preferably below 620°C. The cooling stop temperature is more preferably below 600°C, and further preferably below 580°C. On the other hand, when the cooling stop temperature is less than 250°C, quenching cracks during cooling are easily generated. In addition, in order to obtain a uniform bainite structure, the cooling stop temperature is set to above 250°C. From the perspective 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 residual 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 above 250°C. The cooling stop temperature is preferably above 300°C, and more preferably above 390°C. The cooling stop temperature is more preferably 450° C. or higher. After cooling is stopped, the plate may be left to cool, but in order to promote the formation of bainite, it is more preferably cooled slowly until the temperature drops by about 50° C. from the cooling stop temperature. The cooling stop temperature here refers to the temperature at the center of the plate thickness.

[0170] Then, the hot-rolled steel sheet obtained as described above is coiled into a coil. The coiling temperature is preferably 550° C. or less.

[0171] Pipe making process

[0172] 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).

[0173] It should be noted that the above-mentioned cylindrical shape means that the tube circumferential cross section is a "C" shape.

[0174] Diameter of sizing roller (mm) ≥ thickness of hot-rolled steel plate (mm) / 0.020···(1)

[0175] The plate thickness of the hot-rolled steel plate refers to the plate thickness of the hot-rolled steel plate before the sizing step.

[0176] X<p≤X×1.5···(2)

[0177] 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)

[0178] 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.

[0179] 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, preferably 30 mm or less. There is no particular 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. In addition, 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 600 MPa or less. The yield strength is more preferably 560 MPa or less.

[0180] 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.

[0181] 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.

[0182] 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.

[0183] In the internal pressure loading process, the electric resistance welded steel tube material is expanded to generate tensile stress in the tube circumferential direction, thereby reducing the absolute value of the residual stress in the tube circumferential direction.

[0184] 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.

[0185] 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.

[0186] 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 hydrogen embrittlement resistance decreases.

[0187] As described in part above, the steel pipe of the present invention can be 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 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 area in the welded part.

[0188] Example 1

[0189] Next, the present invention will be described in more detail based on examples. The following examples are preferred examples of the present invention, but the present invention is not limited to the examples described.

[0190] A steel pipe made of a steel material having the composition shown in Table 1 is manufactured. The manufacturing steps are as follows. First, a billet having the composition shown in Table 1 is prepared. The casting speed at this time is implemented at 0.05 to 0.2 m / min. The billet is heated to 1000°C to 1100°C, and hot rolling is performed at a temperature of 950±50°C. Controlled cooling is started after the surface thermometer reaches 900°C. In addition, the cooling start time difference at the front and rear ends of the hot rolling is 30 to 45 seconds, and the cooling stop temperature is 300±50°C, and the target thickness of the steel plate is manufactured to 20 mm. The average cooling rate in the controlled cooling process is implemented under the conditions shown in Table 2. The following pipe making process was performed on a portion of steel materials (steel materials No. 1 to 11): the hot-rolled steel plates were bent after a controlled cooling process, and the two ends were butted and welded; in addition, the following pipe making process was performed on a portion of steel materials (steel materials No. 12 to 22): the hot-rolled steel plates were formed into a cylindrical shape by cold rolling after a controlled cooling process, and the two circumferential ends of the cylindrical shape were butted and resistance welded; and steel pipes No. 1 to 22 were obtained respectively.

[0191] Then, the items subjected to stabilization treatment (or dehydrogenation treatment) were indicated as 0. The treatment conditions at this time were all 200° C. and 30 minutes. The steel materials and steel pipes produced as described above were evaluated as follows.

[0192] Furthermore, billets with the composition shown in Steel No. 8 and Steel No. 22 in Table 1 were produced at various casting speeds shown in Table 3. The billets were heated to 1000°C to 1100°C and hot rolled at 950±50°C. Controlled cooling was started after the surface thermometer reached 900°C. In addition, the hot rolling was carried out in such a way that the cooling start time difference at the front and rear ends was 30 to 45 seconds and the cooling stop temperature was 300±50°C, and the target thickness of the steel plate was manufactured to 20 mm. The average cooling rate in the controlled cooling process was carried out under the conditions shown in Table 3 to obtain steel materials and steel pipes. Steel No. 8-1, 8-2, 8-3, 22-1, 22-2, 22-3 were kept as they were, and steel pipe No. 8-11, 8-12, 8-13 were manufactured by the following pipe making process: the hot-rolled steel plate was bent, and the two ends were butted and welded; Steel pipe No. 22-11, 22-12, 22-13 were obtained by the following pipe making process: the hot-rolled steel plate was formed into a cylindrical shape by cold rolling after the controlled cooling process, and the two circumferential ends of the above cylindrical shape were butted and resistance welded. Then, the items that were subjected to stabilization treatment (or dehydrogenation treatment) were expressed as 0, and the treatment conditions at this time were all implemented at 200°C and 30 minutes. The steel and steel pipe manufactured as described above were evaluated by the following contents.

[0193] Determination of area fraction of retained austenite

[0194] 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.

[0195] Determination of the area fraction of bainite and martensite

[0196] The metal structure at the 1 / 4 position of the wall thickness on the inner side of the obtained steel pipe was evaluated as follows. Test pieces were taken from the center of the long side direction of the steel pipe with the 1 / 4 position of the wall thickness on the inner side and the center position of the wall thickness as observation positions, and the cross-section of the taken test pieces was etched using a 3 vol% nitric acid alcohol solution. Scanning electron microscope photos were taken at an appropriate magnification between 1000 and 5000 times to observe martensite (including tempered martensite), ferrite, bainite, and pearlite. For martensite and bainite, the comparison was made with the structure photos of non-patent document 3 and judged by visual observation. For the structure fraction, the image obtained by dividing the SEM photo area based on the above judgment was used, and the image analysis was used to obtain it (for example, when calculating the bainite fraction, the bainite and other regions were binarized to obtain the bainite fraction), which was used as the area fraction of each phase.

[0197] Hydrogen heating analysis

[0198] The amount of hydrogen remaining in the steel is determined by the 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 is performed at a temperature rise rate of 200°C / h in the temperature range from room temperature to 400°C, and the sum is taken as the amount of hydrogen. The test body is a prism shape with a length of 20mm and a thickness of 10mm × 10mm width along the long side of the steel pipe at a position of 1 / 4 of the thickness of the steel plate and a position of 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 crack growth test and high pressure hydrogen exposure test described in the following items.

[0199] High-pressure hydrogen exposure test (calculation of hydrogen solid solubility)

[0200] The calculation method of hydrogen solid solubility is explained. First, the test body uses a prism shape with a length of 20mm and a thickness of 10mm × 10mm width along the long side direction of the steel and the steel pipe at 1 / 4 of the thickness of the steel plate and 1 / 4 of the inner surface of the steel pipe. Since the amount of hydrogen intrusion depends on the surface state of the test piece, it is cut and ground with sandpaper from No. 160 to No. 1000 to make the surface state consistent in all samples. Then, in order to remove the oxide film that may hinder hydrogen intrusion, Pd is plated on the entire surface of the test piece. The Pd plating can be other methods such as vapor deposition, or it can be replaced by Ni plating.

[0201] The above test body is exposed to a high-pressure hydrogen environment (hydrogen 99.999% or more by volume fraction) at room temperature (20±10°C) and pressures of 0, 5, 25, and 40 MPa for 72 hours. After exposure, it is quickly taken out of the exposure environment and stored in liquid nitrogen to prevent hydrogen from being released from the test body. The hydrogen storage amount H is calculated by the same hydrogen temperature rise analysis method as above for the amount of hydrogen stored. The square root of the exposure pressure √P [MPa] is plotted on the horizontal axis, and the measured hydrogen storage amount H [mass ppm / √P] is plotted on the vertical axis. The amount of hydrogen in the test body at 0 MPa (before the exposure test) is taken as the initial hydrogen amount (intercept), and the hydrogen solid solubility s [mass ppm / √P] is calculated from the slope of √P-H.

[0202] Hydrogen diffusion coefficient

[0203] The hydrogen diffusion coefficient was evaluated using a 1×40×40 mm test piece taken from the 1 / 4 position of the steel plate thickness and the center of the plate thickness at the 1 / 4 position from the inner surface of the steel pipe. Ni was plated on one side of the test piece, and a Devanathan type battery was used. The surface that was not plated with Ni was immersed in a 0.2% NaCl solution for cathode hydrogen charging, and the surface that was plated with Ni was immersed in a 0.1 N NaOH aqueous solution with an extraction potential of 0 V. The second rise in permeation current, i.e., the start time of hydrogen permeation (2nd Build up), was fitted with the theoretical curve of non-patent document 4 to obtain the diffusion coefficient.

[0204] Evaluation of the Crack Growth Rate of High-Pressure Hydrogen Fatigue

[0205] The fatigue test was carried out at room temperature (20±10°C), pressure: 25MPa hydrogen, or pressure: 1MPa or more hydrogen, or natural gas (mainly methane, ethane and other hydrocarbons) containing hydrogen at a hydrogen partial pressure of 1MPa or more, in accordance with ASTM E647, Fatigue Testing, with frequency: 1Hz, repetitive waveform: sine wave, control method: load control, load condition: uniaxial tension, stress ratio: R=0.1. It should be noted that steel and steel pipes with excellent hydrogen embrittlement resistance are those obtained in this test, and the fatigue crack growth rate in hydrogen da / dNmm / cycle is 2.0×10 －3 mm / cycle or less.

[0206] Tensile Strength (TS)

[0207] 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.

[0208] The steel materials and steel pipes satisfying the present invention show excellent effects on fatigue crack growth characteristics in a hydrogen environment. Furthermore, when the hydrogen solid solubility s is less than 0.02 mass ppm / √P, the fatigue crack growth rate is further improved by more than 30% compared with the material with a hydrogen solid solubility s of about 0.05 mass ppm / √P, and is reduced to 1.5×10 －3 mm / cycle or less, showing excellent effects.

[0209] Example 2

[0210] 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 pipes were manufactured under the following manufacturing conditions and their properties were evaluated. Steel grades No. 1, 8, 10, 12, and 22 shown in Tables 1-1 and 1-2 were used, and the steels were manufactured under the same conditions as steel No. 1, 8-2, 10, 12, and 22-2 shown in Tables 2 and 3 until the controlled cooling process, and the properties were evaluated when the dehydrogenation treatment conditions were changed. The steel pipe was formed by the same method as in Example 1. The above results are shown in Table 4.

[0211] In this embodiment, the steel pipes and steel materials No. 1A, 10A, 12A, 8-2A, and 22-2A were implemented so that the dehydrogenation treatment temperature T (atmosphere temperature) was 50°C and the holding time tc after the plate thickness center temperature Tc reached 50°C satisfied the formula (A). The steel pipes and steel materials No. 10B, 12B, 8-2B, and 22-2B were implemented so that the dehydrogenation treatment temperature T (atmosphere temperature) was 50°C and the holding time tc when the dehydrogenation treatment temperature T was 50°C satisfied the above formula (A), but the holding time tc after the plate thickness center temperature Tc reached 50°C did not satisfy the above formula (A).

[0212] For steel pipes and steel materials No. 10C, 12C, 8-2C, and 22-2C, the dehydrogenation treatment temperature T (atmosphere temperature) is 50°C, but the holding time t of the atmosphere temperature and the holding time tc after the plate center temperature Tc reaches 50°C do not satisfy the above formula (A).

[0213] In Table 4, "dehydrogenation holding time t is Y" means that the dehydrogenation treatment temperature T (atmosphere temperature) is 50°C 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 50°C, 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 center temperature Tc reaches 50°C satisfies the formula (A), and "holding time tc of the steel material center temperature Tc is N" means that although the plate center temperature Tc reaches 50°C, the holding time tc after Tc reaches 50°C does not satisfy the formula (A).

[0214] Various evaluations were carried out by the methods described in Example 1.

[0215] All of the examples of the present invention satisfy the requirement of excellent fatigue crack growth rate. Among them, when the dehydrogenation treatment is carried out under more preferred conditions, the fatigue crack growth rate is further improved.

[0216] [Table 1]

[0217]

[0218] [Table 2]

[0219]

[0220] [Table 3]

[0221]

[0222] [Table 4]

[0223]

Claims

1. A steel material for a line pipe having the following chemical composition: containing, by mass%, C: 0.02-0.15%, Si: 0.01-2.0%, Mn: 0.5-1.8%, P: 0.0001-0.015%, S: 0.0002-0.0015%, Al: 0.005-0.15%, O: 0.01% or less, N: 0.010% or less, H: 0.02ppm or less, or further containing Nb: 0-0.10%, 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%, REM: 0-0.01%, Mg: 0-0.01%, B: 0-0.0020%, Hf: 0-0.2%, Ta: 0-0.2%, Re: 0-0.005%, Sn: 0-0.3%, Sb: 0-0.3%, and the balance is Fe and unavoidable impurity elements; The area fraction of retained austenite is 0-3%, and the hydrogen diffusion coefficient at room temperature is 1.5×10 －10 m 2 / s or more, and the hydrogen solid solubility is 0.05 mass ppm / √P or less.

2. The steel material for line pipe according to claim 1, wherein: The chemical composition is further as follows in mass %: Nb: 0.001-0.10%, 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%, REM: 0.0001~0.01%, Mg: 0.0001~0.01%, B: 0.0001~0.0020%, Hf: 0.0 001~0.2%, Ta: 0.0001~0.2%, Re: 0.0001~0.005%, Sn: 0.0001~0.3%, Sb: 0.0001~0.3%.

3. The steel material for line pipe according to claim 1 or 2, wherein: Bainite or martensite is present at a 1 / 4 thickness position, and the bainite accounts for 90% or more in terms of area fraction, or the martensite accounts for 90% or more in terms of area fraction.

4. A method for manufacturing a steel material for a pipeline pipe, comprising: A heating step of heating the steel billet having the chemical composition of claim 1 or 2 at 1000 to 1250° C. The hot rolling step is to roll the steel billet heated in the heating step at a rolling end temperature of Ar3 point or higher. 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 Ar3 point or higher as measured by the steel sheet surface temperature, 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 as measured by the temperature at 0.25 mm below the steel sheet surface and at the center of the sheet thickness, and the cooling stop temperature is 250 to 650° C.; and Any one of a stabilization treatment step of stabilizing the steel sheet obtained in the controlled cooling step and a dehydrogenation treatment step of dehydrogenating the steel sheet obtained in the controlled cooling step.

5. A steel pipe for a line pipe, wherein the steel pipe for a line pipe has the following chemical composition: in terms of mass%, C: 0.02-0.15%, Si: 0.01-2.0%, Mn: 0.5-1.8%, P: 0.0001-0.015%, S: 0.0002-0.0015%, Al: 0.005-0.15%, O: 0.01% or less, N: 0.010% or less, H: 0.02 ppm or less, or further contains Nb: 0-0.10%, Ca: 0-0.005%, Ni: At least one of the following: 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%, REM: 0-0.01%, Mg: 0-0.01%, B: 0-0.0020%, Hf: 0-0.2%, Ta: 0-0.2%, Re: 0-0.005%, Sn: 0-0.3%, Sb: 0-0.3%, and the remainder is Fe and unavoidable impurity elements; The area fraction of retained austenite is 0-3%, and the hydrogen diffusion coefficient at room temperature is 1.5×10 －10 m 2 / s or more, and the hydrogen solid solubility is 0.05 mass ppm / √P or less.

6. The steel pipe for line pipe according to claim 5, wherein: The chemical composition is further as follows in mass %: Nb: 0.001-0.10%, 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%, REM: 0.0001~0.01%, Mg: 0.0001~0.01%, B: 0.0001~0.0020%, Hf: 0.0 001~0.2%, Ta: 0.0001~0.2%, Re: 0.0001~0.005%, Sn: 0.0001~0.3%, Sb: 0.0001~0.3%.

7. The steel pipe for line pipe according to claim 5 or 6, wherein: Bainite or martensite is present at a 1 / 4 wall thickness position from the inner surface of the steel pipe, and the bainite accounts for 90% or more in terms of area fraction or the martensite accounts for 90% or more in terms of area fraction.

8. A method for manufacturing a steel pipe for a line pipe, comprising: A heating step of heating the steel billet having the chemical composition of claim 5 or 6 at 1000 to 1250° C. The hot rolling step is to roll the steel billet heated in the heating step at a rolling end temperature of Ar3 point or higher. The hot rolled steel sheet obtained in the hot rolling step is cooled under the conditions that the cooling start temperature is Ar3 point or higher as measured by the steel sheet surface temperature, 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 as measured by the temperature at 0.25 mm below the steel sheet surface and the center of the sheet thickness, and the cooling stop temperature is 250 to 650°C. 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, both 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, both circumferential ends of the cylindrical shape are butted and resistance welded, and Any one of a stabilization treatment step of stabilizing the steel pipe obtained in the pipe manufacturing process and a dehydrogenation treatment step of dehydrogenating the steel pipe obtained in the pipe manufacturing process.

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