Seamless steel pipe, method for manufacturing seamless steel pipe, pressure vessel, pressure vessel for hydrogen gas, and method for manufacturing pressure vessel or pressure vessel for hydrogen gas

Abstract

The purpose of the present invention, configured in view of circumstances, is to provide a thick-walled seamless steel pipe that has a high martensite fraction and high strength, as well as has both excellent heat crack resistance and delayed fracture resistance, the seamless steel pipe being highly suitable for use as a member of pressure vessels etc. Another purpose of the present invention is to provide: a method for manufacturing the seamless steel pipe; a pressure vessel; a pressure vessel for hydrogen gas; and a method for manufacturing the pressure vessel or the pressure vessel for hydrogen gas.　The seamless steel pipe contains a specific composition that satisfies formula (1), where the manganese segregation ratio on the inner surface of the steel pipe is less than 1.50, the metal structure has a martensite area fraction of 85% or more, and the cross-sectional hardness is at least 250 HV but less than 480 HV.　Formula (1): [P] ≤ 0.042 − 0.002[C] − 0.024[Mn] − 0.001[Mo] − 0.001[Cr] − 0.002[Ni] (in formula (1), [element] represents the content (mass%) of the element indicated in the brackets).

Description

Seamless steel pipe, manufacturing method of seamless steel pipe, pressure vessel, pressure vessel for hydrogen gas, manufacturing method of pressure vessel or pressure vessel for hydrogen gas

[0001] The present invention relates to a seamless steel pipe, and more particularly to a seamless steel pipe with a high martensite fraction, thick wall thickness, and high strength, which has excellent resistance to quench cracking and delayed fracture and is particularly suitable for use as a component of a pressure vessel, etc. The present invention also relates to a pressure vessel, a pressure vessel for hydrogen gas, and a method for manufacturing the pressure vessel or the pressure vessel for hydrogen gas.

[0002] Seamless steel pipes (also simply referred to as steel pipes) are used in a variety of applications, such as piping and structural components, due to their excellent strength and relatively low cost. They are particularly useful as components for structures, such as pressure vessels, that require stress concentration relief. The higher the internal pressure of a steel pipe, the greater the strength of the steel pipe must be. In addition, a thicker wall thickness is required to reduce fatigue strength due to pressure fluctuations. Furthermore, it is preferable to use steel pipes with a high martensite fraction for pressure vessels, as this ensures high fatigue strength. However, thick, high-strength steel pipes with a high martensite fraction are prone to quench cracking, which is a significant problem in that cracks occur during quenching, a process essential for achieving high strength.

[0003] Therefore, various techniques have been proposed to reduce the risk of quench cracking in high-strength steel pipes.

[0004] For example, Patent Document 1 proposes a technology for reducing the risk of quench cracking by controlling heat treatment so as to optimize thermal stress and transformation stress by cooling only the inner surface of the steel pipe.

[0005] Furthermore, Patent Document 2 proposes a technique for reducing the risk of quench cracking by reducing the concentration of P, among other elements in the composition of a steel pipe.

[0006] Japanese Patent Application Laid-Open No. 9-104925 International Publication No. 2018 / 055937

[0007] In the above Patent Document 1, the quench cracking resistance is evaluated in an actual steel pipe shape, but no opinion is given regarding the P concentration, which is known to significantly contribute to quench cracking resistance.

[0008] On the other hand, in the above Patent Document 2, the effect of P concentration is evaluated, but the evaluation is performed using test specimens taken from a quarter position of the wall thickness of the steel pipe, and test specimens in which the wall thickness at the cross section is extremely heterogeneous by drilling a circular hole in a disk, thereby intentionally worsening the balance between thermal stress and transformation stress. In addition, the contribution of P segregation, which inevitably occurs in processes using continuous casting technology, is not taken into consideration.

[0009] Furthermore, when manufacturing thick-walled, high-strength steel pipes with a high martensite fraction, ensuring hardenability is necessary. However, ensuring hardenability by adding large amounts of alloying elements leads to increased costs. Therefore, ensuring hardenability through cooling rate is more important, and the fastest possible cooling rate during quenching is preferable. Additionally, seamless steel pipes are manufactured by rolling a cast or forged solid cylindrical steel material while forming a hole in the center of the cross section. This results in P segregation on the inner surface of the steel pipe, unlike the P segregation distribution in plate-shaped steel materials. In other words, it is important to consider the balance between the maximum temperature distribution in the wall thickness direction and the maximum P segregation point. Furthermore, this effect becomes more pronounced as the wall thickness of the steel pipe increases. Therefore, it is important to evaluate the presence or absence of quench cracking due to the balance between thermal stress and transformation stress in the actual shape, but the above-mentioned Patent Documents 1 and 2 do not evaluate the actual shape. Furthermore, there is no specific mention of delayed fracture resistance.

[0010] The present invention has been made in view of the above circumstances, and has an object to provide a seamless steel pipe having a high martensite fraction, a thick wall, high strength, and excellent resistance to quench cracking and delayed fracture, which can be suitably used as a component for a pressure vessel, etc. Another object of the present invention is to provide a method for manufacturing a seamless steel pipe, a pressure vessel, a pressure vessel for hydrogen gas, and a method for manufacturing a pressure vessel or a pressure vessel for hydrogen gas.

[0011] The present inventors conducted research to solve the above-mentioned problems and found that optimizing the P concentration in the chemical composition of seamless steel pipes, as well as the rolling conditions in the seamless steel pipe manufacturing process and the heat treatment conditions after the pipe manufacturing process, can control P segregation on the inner surface of the seamless steel pipe, thereby achieving excellent quench cracking resistance and delayed fracture resistance, even in thick-wall seamless steel pipes. However, because the P concentration in seamless steel pipes is designed to be low in order to suppress P segregation, there is also the problem of difficulty in determining the degree of P segregation. Focusing on Mn, which has a high affinity with P, the inventors found a high correlation between the P concentration and the Mn concentration. Furthermore, since the Mn concentration in the steel pipe is designed high in the present invention, evaluating the degree of P segregation based on the Mn segregation degree is more reliable. Therefore, the inventors found that quench cracking resistance and delayed fracture resistance can be improved by optimizing the rolling conditions in the seamless steel pipe manufacturing process and the heat treatment conditions after the pipe manufacturing process to control the Mn segregation on the inner surface of the seamless steel pipe. The gist of the present invention is described below.[1] Contains, by mass%, C: 0.20 to 0.60%, Si: 0.01 to 2.0%, Mn: 0.5 to 1.2%, S: 0.010% or less, O: 0.005% or less, N: 0.010% or less, Al: 0.01 to 0.08%, Mo: 0.005 to 1.0%, Cr: 0.005 to 3.0%, Ni: 0.005 to 3.0%, P satisfies the following formula (1), or further contains Ti: 0.0001 to 0.1%, Cu: 0.0001 to 5.0%, Co: 0.0001 to 5.0%, B: 0.0001 to 0.01%, V: 0.0001 to 1.0%, The steel pipe has a chemical composition comprising one or more elements selected from W: 0.0001 to 5.0%, Nb: 0.0001 to 0.1%, Zr: 0.0001 to 0.2%, Hf: 0.0001 to 0.2%, Ta: 0.0001 to 0.2%, Sb: 0.0001 to 0.1%, Sn: 0.0001 to 0.1%, Ca: 0.0001 to 0.01%, Mg: 0.0001 to 0.01%, and REM: 0.0001 to 0.5%, with the balance being Fe and unavoidable impurities; the steel pipe has a metallographic structure in which the segregation degree of Mn from the inner surface to the 1 / 6 wall thickness position is less than 1.50 and the area fraction of martensite is 85% or more; A seamless steel pipe having a cross-sectional hardness of 250 HV or more and less than 480 HV. [P]≦0.042−0.002[C]−0.024[Mn]−0.001[Mo]−0.001[Cr]−0.002[Ni]... (1) ([Element] in formula (1) represents the content (mass%) of the element written in each bracket.) [2] A seamless steel pipe according to [1], having a steel pipe wall thickness of 40 mm or more. [3] The number of inclusions having an aspect ratio of 2 or less and a long side length of 10 μm or more from the inner surface of the steel pipe to a position 1 / 6 of the wall thickness is 10 pieces / 100 mm. ２[4] A seamless steel pipe according to [1] or [2], comprising the following: [4] A method for producing a seamless steel pipe according to any one of [1] to [3], comprising: a pipe-making step of forming a heated material into a pipe such that the ratio of the inner diameter of the steel pipe divided by the wall thickness is 2.0 or more; and a step of heating the material to a temperature range of 860°C or more, holding the material in the temperature range for 30 minutes or more, and water-cooling the material under conditions in which the average cooling rate in the temperature range of 800 to 350°C is 3°C / sec or more, and rotating the material in the circumferential direction during the water-cooling step to form a 0.5 mm thick film on the inner surface of the steel pipe. ３ [5] A method for producing a seamless steel pipe, comprising: a quenching step in which the pipe is cooled while being in contact with a cooling medium at a flow rate of 1 / min or more; and a tempering step in which, after the quenching step, the pipe is held in a temperature range of 400 to 680°C for 30 minutes or more. [5] A method for producing a seamless steel pipe according to [4], in which the pipe is manufactured so that the shape of the steel pipe satisfies the following formula (2): (EXP (coefficient K × wall thickness) / area of ​​circumferential cross section) × 10 ６ <210... (2) K = {0.41([C] - 0.24) + 4.58[Mn] + 0.21[Cr] + 0.25[Ni]} / 100... (3) (In formula (3), [element] represents the content (mass%) of the element written in each bracket.) [6] A pressure vessel using a seamless steel pipe according to any one of [1] to [3] above. [7] A pressure vessel for hydrogen gas using a seamless steel pipe according to any one of [1] to [3] above. [8] A method for manufacturing a pressure vessel or a pressure vessel for hydrogen gas, which comprises processing the seamless steel pipe according to any one of [1] to [3] above.

[0012] The seamless steel pipe of the present invention has a high martensite fraction, is thick and has high strength, and is excellent in quench crack resistance and delayed fracture resistance.

[0013] The present invention will be described in detail below. Note that the following description shows examples of preferred embodiments of the present invention, and the present invention is not limited thereto. First, the chemical composition of a seamless steel pipe (also simply referred to as a steel pipe) will be described.

[0014] C: 0.20 to 0.60% C is an element necessary to increase the strength of steel pipes. Strength is an important required characteristic for structural materials. The cross-sectional hardness of steel pipes after quenching and tempering is preferably 250 HV or more. To achieve such a cross-sectional hardness, the C content is set to 0.20% or more. The C content is preferably 0.25% or more, more preferably 0.30% or more, even more preferably 0.33% or more, and most preferably 0.35% or more. On the other hand, if the C content exceeds 0.60%, quench cracking may occur when quenching is performed, so the C content is set to 0.60% or less. To reduce the risk of quench cracking, the C content is preferably 0.50% or less, more preferably 0.45% or less, and even more preferably 0.42% or less.

[0015] Si: 0.01 to 2.0% Si is an element that contributes to increasing the strength of steel pipes through solid solution strengthening. To achieve this effect, the Si content is set to 0.01% or more. The Si content is preferably 0.10% or more, more preferably 0.15% or more, even more preferably 0.18% or more, and most preferably 0.20% or more. On the other hand, if the Si content exceeds 2.0%, the effect saturates, and the surface quality of the steel pipe deteriorates, and the rolling ability in the pipe-making process also decreases. Therefore, the Si content is set to 2.0% or less. The Si content is preferably 1.5% or less, more preferably 1.0% or less, even more preferably 0.5% or less, and most preferably 0.4% or less.

[0016] Mn: 0.5 to 1.2% Mn is an element that contributes to increasing the strength of steel pipes by improving solid solution strengthening and hardenability. To achieve this effect, the Mn content is set to 0.5% or more. The Mn content is preferably 0.55% or more, more preferably 0.6% or more, even more preferably 0.65% or more, and most preferably 0.68% or more. On the other hand, excessive Mn not only makes rolling in the pipe-making process difficult, but also causes residual austenite, resulting in reduced fatigue properties. Furthermore, as described below, excessive Mn content increases the degree of Mn segregation and increases the risk of quench cracking. Therefore, the Mn content is set to 1.2% or less. The Mn content is preferably 1.0% or less, more preferably 0.9% or less, and even more preferably 0.88% or less.

[0017] S: 0.010% or less Excessive S can cause hot shortness and lead to manufacturing defects. S also forms MnS inclusions, reducing toughness. Furthermore, it also reduces delayed fracture resistance. These effects are not a problem when the S content is 0.010% or less. Therefore, the S content is set to 0.010% or less. The S content is preferably set to 0.005% or less, and more preferably to 0.003% or less. On the other hand, excessive reduction of the S content to less than 0.0001% increases the desulfurization costs in the steelmaking process. Therefore, the S content is preferably set to 0.0001% or more, more preferably to 0.0002% or more, even more preferably to 0.0005% or more, and most preferably to 0.001% or more.

[0018] O: 0.005% or less O is present in steel pipes as an impurity. If the O content exceeds 0.005%, it forms coarse and hard oxides, which not only reduces mechanical properties such as toughness but also increases the risk of quench cracking. For this reason, the O content is set to 0.005% or less. The O content is preferably as low as possible, preferably 0.004% or less, more preferably 0.003% or less, and even more preferably 0.002% or less. On the other hand, excessive reduction of the O content to less than 0.0001% increases the cost of the steelmaking process. For this reason, the O content is preferably 0.0001% or more, more preferably 0.0002% or more, even more preferably 0.0005% or more, and most preferably 0.001% or more.

[0019] N: 0.010% or less The effect of N on the strength of a steel pipe is small, and an N content of 0.010% or less does not impair the effects of the present invention. Therefore, the N content is set to 0.010% or less. The N content is preferably set to 0.008% or less, more preferably 0.005% or less, even more preferably 0.0045% or less, and most preferably 0.004% or less. On the other hand, from the viewpoint of improving toughness, a low N content is desirable, but an excessive reduction increases the cost in the steelmaking process, so the N content is preferably 0.0001% or more, more preferably 0.001% or more.

[0020] Al: 0.01 to 0.08% Al is an element effective as a deoxidizer in the steelmaking process. To obtain this effect, the Al content is set to 0.01% or more. The Al content is preferably set to 0.02% or more. On the other hand, if the Al content exceeds 0.08%, the effect saturates, so the Al content is set to 0.08% or less. The Al content is preferably set to 0.06% or less, more preferably 0.05% or less, and even more preferably 0.04% or less.

[0021] Mo: 0.005 to 1.0% Mo is an element that improves hardenability, contributing to increased strength of steel pipes. By improving the hardenability of steel pipes, Mo also increases the proportion of structures other than ferrite (particularly martensite) in the metallographic structure, making it an essential element for achieving a high martensite fraction. Mo also improves quench cracking resistance by suppressing ferrite transformation. Furthermore, the inclusion of Mo reduces the cooling rate required to achieve the above-described metallographic structure, reduces the risk of quench cracking during heat treatment of steel pipes, and is also highly effective in improving delayed fracture resistance. To achieve these effects, the Mo content is set to 0.005% or more. The Mo content is preferably set to 0.008% or more, and more preferably to 0.1% or more. On the other hand, if the Mo content exceeds 1.0%, the effect saturates and increases costs, so the Mo content is set to 1.0% or less. The Mo content is preferably 0.8% or less, more preferably 0.5% or less, further preferably 0.3% or less, and most preferably 0.25% or less.

[0022] Cr: 0.005 to 3.0% Cr is an element that improves hardenability, contributing to increased strength of the steel pipe. By improving the hardenability of the steel pipe, Cr also increases the proportion of structures other than ferrite (particularly martensite) in the metallographic structure, making it an essential element for achieving a high martensite fraction. Cr also improves quench cracking resistance by suppressing ferrite transformation. Furthermore, the inclusion of Cr can reduce the cooling rate required to achieve the above-described metallographic structure and reduce the risk of quench cracking during heat treatment of the steel pipe. To achieve this effect, the Cr content is set to 0.005% or more. The Cr content is preferably set to 0.1% or more, more preferably 0.5% or more, even more preferably 0.7% or more, and most preferably 0.9% or more. On the other hand, if the Cr content exceeds 3.0%, the effect saturates, and the desired degree of Mn segregation cannot be achieved, resulting in poor quench cracking resistance. Therefore, the Cr content is set to 3.0% or less. The Cr content is preferably 2.0% or less, more preferably 1.5% or less, even more preferably 1.3% or less, and most preferably 1.2% or less.

[0023] Ni: 0.005 to 3.0% Ni is an element that improves hardenability and contributes to increasing the strength of steel pipes. Furthermore, as a result of improved hardenability, it is possible to increase the strength of steel pipes, even in those with a wall thickness of 40 mm or more, where portions with a slow cooling rate are likely to occur. Furthermore, Ni has the effect of increasing the proportion of structures other than ferrite (particularly martensite) in the metal structure by improving the hardenability of the steel pipe, and is an element necessary for obtaining a high martensite fraction. As a result, Ni improves quench cracking resistance by suppressing ferrite transformation. To achieve this effect, the Ni content is set to 0.005% or more. The Ni content is preferably set to 0.01% or more, more preferably 0.5% or more, and even more preferably 0.8% or more. On the other hand, since excessive Ni content increases costs, the Ni content is set to 3.0% or less. Preferably, the Ni content is set to 2.5% or less, more preferably 2.0% or less, and even more preferably 1.8% or less.

[0024] It is important that P satisfies the following formula (1): [P]≦0.042−0.002[C]−0.024[Mn]−0.001[Mo]−0.001[Cr]−0.002[Ni]...(1) (Note: The parentheses in formula (1) represent the content (mass%) of the element listed within.) P contributes to increasing the strength of steel pipes through solid solution strengthening, and also reduces the steel pipe's resistance to quench cracking. It is known that the mechanism of quench cracking is that P trapped at grain boundaries reduces the grain boundary bond strength. Grain boundaries in steel are planar lattice defects with a disordered crystal structure, and the density of lattice defects is lower than the density in the bulk, where atoms are regularly and periodically arranged. In other words, the volume of the gaps between atoms is large, making it easier to trap contained elements. The trapping of P at grain boundaries can be broadly divided into two factors. The first is macrosegregation, which is the concentration distribution within the cross section of a steel material caused by the different melting points of each element when the steel material solidifies from a liquid to a solid. The second is microsegregation, which is the concentration distribution within the steel material structure caused by the diffusion of contained elements from the bulk to grain boundaries during heat treatment of the steel material. In other words, reducing both P macrosegregation and P microsegregation can reduce the risk of quench cracking in the final steel pipe. Hereinafter, the term "segregation" in this invention will be defined to include both macrosegregation and microsegregation, and will be distinguished from each other. When manufacturing thick-walled, high-strength seamless steel pipes, it is essential to include elements that improve hardenability to ensure hardenability, but this increases the risk of quench cracking. Furthermore, it also increases the risk of reduced delayed fracture resistance. P is an element that reduces the bonding strength of grain boundaries. Reducing P microsegregation can suppress intergranular cracking, which is the initiation point of delayed fracture, and is therefore expected to improve delayed fracture resistance. Therefore, it is important to ensure the quench crack resistance and delayed fracture resistance of seamless steel pipes by reducing the segregation of P.

[0025] In formula (1), which is a necessary condition for reducing P segregation, the P content is related to the C, Mn, Mo, Cr, and Ni contents for the following reasons: C is a solute atom that contributes to increasing strength and strongly inhibits intragranular dislocation movement. This effect increases intragranular strength, thereby relatively increasing the load on grain boundaries. In other words, since the risk of quench cracking increases when C is contained, it is preferable to reduce the P content. Mn has long been known to have a particularly high affinity with P, and reducing Mn microsegregation ultimately leads to a reduction in P microsegregation. In other words, since the risk of quench cracking increases when Mn is contained, it is preferable to reduce the P content. Furthermore, Mo, Cr, and Ni are elements that improve hardenability, and since the risk of quench cracking increases when these elements are contained, it is preferable to reduce the P content. In addition, Cr and Ni are transition metals in the fourth period of the periodic table, the same as Mn, and are less likely to inhibit the diffusion of Mn atoms in the Fe bulk than other contained elements. In other words, Mn atoms become more easily diffused. That is, Cr and Ni are elements that have the effect of promoting the microsegregation of Mn, and the inclusion of Cr and Ni also increases the risk of quench cracking, so it is preferable to reduce the P content. In the case of a seamless steel pipe in which macrosegregation exists on the inner surface of the steel pipe, satisfying formula (1) makes it possible to suppress cracking (quench cracking) that occurs on the inner surface of the steel pipe during quenching. However, excessive reduction of the P content reduces production efficiency and increases refining costs, so it is preferable to set the P content to 0.001% or more.

[0026] The components described above are essential elements in the present invention.

[0027] A steel pipe according to one embodiment of the present invention has a composition containing the above elements with the balance being Fe and unavoidable impurity elements.

[0028] Here, the term "unavoidable impurities" refers to impurities that are inevitably mixed in from raw materials, manufacturing processes, manufacturing facilities, etc., and are allowed to be present to the extent that they do not impair the object of the present invention. Examples of raw materials include iron ore, reduced iron, scrap, etc. Examples of impurities include Zn, Pb, As, Bi, etc.

[0029] Furthermore, the chemical composition of the steel pipe in another embodiment of the present invention may further contain one or more of the elements listed below. The elements contained as needed are one or more selected from Ti: 0.0001 to 0.1%, Cu: 0.0001 to 5.0%, Co: 0.0001 to 5.0%, B: 0.0001 to 0.01%, V: 0.0001 to 1.0%, W: 0.0001 to 5.0%, Nb: 0.0001 to 0.1%, Zr: 0.0001 to 0.2%, Hf: 0.0001 to 0.2%, Ta: 0.0001 to 0.2%, Sb: 0.0001 to 0.1%, Sn: 0.0001 to 0.1%, Ca: 0.0001 to 0.01%, Mg: 0.0001 to 0.01%, and REM: 0.0001 to 0.5%.

[0030] Ti: 0.0001 to 0.1% Ti contributes to increasing the strength of steel pipes. To achieve this effect, when Ti is contained, the Ti content is set to 0.0001% or more. The Ti content is preferably 0.0010% or more, more preferably 0.0015% or more, even more preferably 0.002% or more, and most preferably 0.01% or more. On the other hand, if the Ti content exceeds 0.1%, the effect saturates and costs increase. Therefore, when Ti is contained, the Ti content is set to 0.1% or less. To reduce costs, the Ti content is preferably 0.05% or less, more preferably 0.04% or less, and even more preferably 0.03% or less.

[0031] Cu: 0.0001 to 5.0% Cu is an element that improves hardenability, contributes to increasing the strength of the steel pipe, and suppresses coarsening of prior austenite grains, thereby improving various properties of the steel pipe. When Cu is contained to achieve the above effects, the Cu content is set to 0.0001% or more. The Cu content is preferably 0.001% or more, more preferably 0.1% or more, and even more preferably 0.2% or more. On the other hand, if the Cu content exceeds 5.0%, the effect saturates and costs increase. Therefore, when Cu is contained, the Cu content is set to 5.0% or less. The Cu content is preferably 4.3% or less, more preferably 4.0% or less, and even more preferably 3.0% or less. To reduce costs, the Cu content is most preferably 2.0% or less.

[0032] Co: 0.0001 to 5.0% Co is an element that improves hardenability, contributes to increasing the strength of the steel pipe, and suppresses coarsening of prior austenite grains, thereby improving various properties of the steel pipe. When Co is contained to achieve the above effects, the Co content is set to 0.0001% or more. The Co content is preferably 0.001% or more, more preferably 0.1% or more, and even more preferably 0.2% or more. On the other hand, if the Co content exceeds 5.0%, the effect saturates and becomes a factor in increasing costs. Therefore, when Co is contained, the Co content is set to 5.0% or less. The Co content is preferably 4.0% or less, more preferably 3.0% or less, and in order to reduce costs, the Co content is even more preferably 2.0% or less.

[0033] B: 0.0001 to 0.01% B is an element that improves hardenability, contributes to increasing the strength of the steel pipe, and suppresses coarsening of prior austenite grains, thereby improving various properties of the steel pipe. To achieve this effect, when B is contained, the B content is set to 0.0001% or more. The B content is preferably 0.0005% or more, more preferably 0.001% or more, and even more preferably 0.002% or more. On the other hand, if the B content exceeds 0.01%, the effect saturates and causes an increase in costs. Therefore, when B is contained, the B content is set to 0.01% or less. The B content is preferably 0.008% or less, more preferably 0.006% or less. To reduce costs, the B content is more preferably 0.005% or less, and most preferably 0.004% or less.

[0034] V: 0.0001 to 1.0% V contributes to increasing the strength of steel pipes. It can also improve delayed fracture resistance. To achieve this effect, when V is contained, the V content is set to 0.0001% or more. The V content is preferably 0.0005% or more, more preferably 0.0010% or more, even more preferably 0.0020% or more, and most preferably 0.01% or more. On the other hand, if the V content exceeds 1.0%, the effect saturates and becomes a factor in increasing costs. Therefore, when V is contained, the V content is set to 1.0% or less. The V content is preferably 0.8% or less, and in order to reduce costs, the V content is more preferably 0.5% or less, and even more preferably 0.4% or less.

[0035] W: 0.0001 to 5.0% W contributes to increasing the strength of steel pipes. To achieve this effect, when W is contained, the W content is set to 0.0001% or more. The W content is preferably 0.0005% or more, more preferably 0.0010% or more, even more preferably 0.0020% or more, and most preferably 0.1% or more. On the other hand, if the W content exceeds 5.0%, the effect saturates and costs increase. Therefore, when W is contained, the W content is set to 5.0% or less. The W content is preferably 4.0% or less, more preferably 3.0% or less, even more preferably 2.0% or less, and most preferably 1.7% or less.

[0036] Nb: 0.0001 to 0.1% Nb contributes to increasing the strength of steel pipes. To achieve this effect, when Nb is contained, the Nb content is set to 0.0001% or more. The Nb content is preferably 0.0005% or more, more preferably 0.0010% or more, and even more preferably more than 0.001%. On the other hand, when the Nb content exceeds 0.1%, the effect saturates and costs increase. Therefore, when Nb is contained, the Nb content is set to 0.1% or less. The Nb content is preferably 0.08% or less, and in order to reduce costs, the Nb content is more preferably 0.06% or less, even more preferably 0.05% or less, and most preferably 0.045% or less.

[0037] Zr: 0.0001-0.2%, Hf: 0.0001-0.2%, Ta: 0.0001-0.2% Zr, Hf, and Ta contribute to increasing the strength of steel pipes. To achieve this effect, when Zr, Hf, and Ta are each contained, the Zr content, Hf content, and Ta content are each set to 0.0001% or more. The Zr content, Hf content, and Ta content are each preferably 0.0002% or more, more preferably 0.0005% or more, even more preferably 0.0010% or more, and most preferably 0.01% or more. On the other hand, if the Zr content, Hf content, and Ta content each exceed 0.2%, the effect saturates and costs increase. Therefore, when Zr, Hf, and Ta are each contained, the Zr content, Hf content, and Ta content are each set to 0.2% or less. The Zr content, Hf content, and Ta content are each preferably 0.1% or less, and in order to suppress costs, the Zr content, Hf content, and Ta content are each more preferably 0.09% or less, each further preferably 0.08% or less, and each most preferably 0.07% or less.

[0038] Sb: 0.0001 to 0.1% Sb is an element that is effective in suppressing denitrification, deboronation, and the like, and thus suppressing a decrease in the strength of steel pipes. To achieve this effect, when Sb is contained, the Sb content is set to 0.0001% or more. The Sb content is preferably set to 0.002% or more, more preferably set to 0.005% or more, and even more preferably set to 0.01% or more. An Sb content exceeding 0.1% may lead to embrittlement of the steel pipe. Therefore, when Sb is contained, the Sb content is set to 0.1% or less. The Sb content is preferably set to 0.08% or less, more preferably set to 0.07% or less, and even more preferably set to 0.06% or less.

[0039] Sn: 0.0001 to 0.1% Sn is an element that suppresses pearlite formation and is effective in suppressing a decrease in the strength of a steel pipe. To achieve this effect, when Sn is contained, the Sn content is set to 0.0001% or more. The Sn content is preferably set to 0.0002% or more, more preferably set to 0.005% or more, and even more preferably set to 0.01% or more. If the Sn content exceeds 0.1%, the steel pipe may become embrittled. Therefore, when Sn is contained, the Sn content is set to 0.1% or less. The Sn content is preferably set to 0.08% or less, more preferably set to 0.05% or less, and even more preferably set to 0.04% or less.

[0040] Ca: 0.0001 to 0.01%, Mg: 0.0001 to 0.01% Ca and Mg contribute to improving the state of inclusions. To achieve this effect, when Ca and Mg are contained, the Ca and Mg contents are each set to 0.0001% or more. Preferably, the Ca content and Mg content are each set to 0.001% or more, and more preferably, 0.002% or more. On the other hand, if the Ca content and Mg content each exceed 0.01%, the effect saturates and costs increase. Therefore, when Ca and Mg are contained, the Ca content and Mg content are each set to 0.01% or less. The Ca content and Mg content are each preferably set to 0.008% or less. To reduce costs, the Ca content and Mg content are more preferably set to 0.005% or less, and even more preferably to 0.004% or less.

[0041] REM: 0.0001 to 0.5% REM (Rare Earth Metals) contribute to improving the state of inclusions. To achieve this effect, when REM is contained, the REM content is set to 0.0001% or more. The REM content is preferably 0.001% or more, more preferably 0.002% or more, and even more preferably 0.01% or more. On the other hand, if the REM content exceeds 0.5%, the effect saturates and costs increase. Therefore, when REM is contained, the REM content is set to 0.5% or less. The REM content is preferably 0.3% or less, and to reduce costs, the REM content is more preferably 0.28% or less, and even more preferably 0.25% or less. REM is a collective term for Sc, Y, and 15 elements ranging from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71, and the REM content here refers to the total content of these elements.

[0042] Next, the degree of Mn segregation in a seamless steel pipe will be described.

[0043] Mn segregation from the inner surface of the steel pipe to the 1 / 6 wall thickness position: less than 1.50. Like P, Mn is an element that reduces the bonding strength of grain boundaries. Reducing Mn microsegregation can suppress intergranular cracking, which is the initiation point of delayed fracture, and is expected to improve delayed fracture resistance. As mentioned above, P has a high affinity with Mn. Furthermore, because P is a direct cause of quench cracking risk, the P content (P concentration) is controlled to be low in this invention. While suppressing P segregation is fundamentally necessary, the concentration of Mn, which has a high affinity with P, is higher than the P concentration, making the EPMA analysis results more reliable. Furthermore, P concentration and Mn concentration are highly correlated. Therefore, suppressing Mn segregation can reliably suppress P segregation. Therefore, when evaluating the P segregation degree of seamless steel pipes, the Mn segregation degree is used as an evaluation index. Furthermore, in the case of plate-shaped steel materials (steel plates) produced by rolling continuously cast slabs, macrosegregation is present in the center of the plate thickness (wall thickness), and therefore the effect of rolling to alleviate macrosegregation is smaller than that of seamless steel pipes. This is for the following reasons. In the seamless steel pipe manufacturing process, the center of a cylindrical raw material (billet) is pierced with a piercing machine and then rolled to form a pipe. Therefore, the position of macrosegregation that existed in the billet center is shifted to the inner surface of the seamless steel pipe in the pipe shape. In other words, the inner surface of the seamless steel pipe, where P macrosegregation is high, is at the highest risk of quench cracking. On the other hand, because the billet center is pierced and rolled while pressing down on the outer surface, the amount of plastic deformation is higher on the inner surface of the steel pipe than on the outer surface of the steel pipe. Therefore, rolling is expected to alleviate P macrosegregation on the inner surface of the steel pipe. For this reason, it is important to control P segregation, i.e., Mn segregation, on the inner surface of the seamless steel pipe. Therefore, when evaluating the degree of Mn segregation in a seamless steel pipe, it is important to manufacture and evaluate the seamless steel pipe. In this case, it was found that the degree of Mn segregation can be determined by using the ratio of the Mn segregation concentration in the segregation region on the inner surface of the steel pipe, i.e., from the inner surface of the steel pipe to the position 1 / 6 of the wall thickness, based on the local Mn concentration in the vicinity of the position 1 / 4 of the wall thickness from the outer surface of the steel pipe.The reason for using the local Mn concentration near the 1 / 4 position from the outer surface of the steel pipe as the reference local Mn concentration is that this region is free of macrosegregation and has an average composition. It is also far from the outer surface of the steel pipe, where plastic deformation occurs due to the pressure applied, and can be further removed from the inner surface of the steel pipe. On the other hand, the Mn segregation concentration is the Mn segregation concentration in the segregation region on the inner surface of the steel pipe, i.e., from the inner surface of the steel pipe to the 1 / 6 position. To reduce the risk of quench cracking, the Mn segregation ratio must be less than 1.50, more preferably 1.45 or less, even more preferably 1.44 or less, and most preferably 1.43 or less. The lower limit of the Mn segregation ratio is not particularly limited, but from the viewpoint of cost reduction, it is preferably 1.00 or more, and more preferably 1.10 or more.

[0044] The method for calculating the degree of Mn segregation is as follows.

[0045] In the present invention, the Mn segregation degree is used to quantitatively evaluate the P segregation on the inner surface side of a seamless steel pipe. The Mn segregation degree is defined as the value obtained by dividing the Mn segregation concentration from the inner surface of the seamless steel pipe to the 1 / 6 position in the wall thickness by the local Mn concentration from the outer surface of the steel pipe to the vicinity of the 1 / 4 position in the wall thickness. The Mn segregation degree is measured as follows.

[0046] In evaluating the degree of Mn segregation, we focused on mapping analysis using an electron probe microanalyzer (EPMA), which can measure the local concentration of elements. First, a test piece for mapping analysis was collected so that the cross section of the steel pipe in the C direction (direction perpendicular to the rolling direction) served as the observation surface, and the observation surface was mirror-finished. Next, on the observation surface, when the wall thickness of the steel pipe is t, a mapping analysis of the Mn concentration in region 1 surrounded by a length of t / 6 (mm) in the wall thickness direction (±t / 12 (mm) in the wall thickness direction from the 1 / 4 position in the wall thickness direction from the outer surface of the steel pipe) and a length of 10 (mm) in the circumferential direction was performed, centered on a position 1 / 4 of the way in the wall thickness direction from the outer surface of the steel pipe, and the analysis described below was performed and calculated. The Mn segregation concentration is calculated by performing a mapping analysis of the Mn concentration in region 2, which is surrounded by a length of t / 6 (mm) in the wall thickness direction from the inner surface of the steel pipe (t / 6 (mm) in the wall thickness direction from the surface of the inner surface of the steel pipe) and a length of 10 (mm) in the circumferential direction, and then performing the analysis described below. In the Mn concentration mapping analysis, the analysis pitch is determined so that 40,000 mapping data points are obtained for each of region 1 and region 2, and the mapping data are acquired. The obtained 40,000 mapping data points are then statistically analyzed to calculate the Mn segregation degree. The reference local Mn concentration is the average value of the mapping data obtained in region 1, excluding outliers. Outliers are mapping data that are less than 0.05 times and more than 5 times the Mn concentration (Mn content) obtained from the check analysis of the steel pipe. The Mn segregation concentration is the average value of the top 1% of mapping data with the highest Mn concentrations, obtained in region 2, excluding outliers. In this case, mapping analysis of region 1 is performed in three visual fields, and the local Mn concentrations calculated in the three visual fields are added together and divided by 3 to obtain the local Mn concentration. Similarly, mapping analysis of region 2 is also performed in three visual fields, and the segregated Mn concentrations calculated in the three visual fields are added together and divided by 3 to obtain the segregated Mn concentration. The ratio of the segregated Mn concentration to the local Mn concentration thus obtained (the value obtained by dividing the segregated Mn concentration from the inner surface of the seamless steel pipe to the 1 / 6 position in the wall thickness by the local Mn concentration from the outer surface of the steel pipe in the vicinity of the 1 / 4 position in the wall thickness) is calculated as the degree of Mn segregation.

[0047] Next, the reasons for limiting the metal structure of the seamless steel pipe of the present invention will be explained.

[0048] Martensite Area Fraction: 85% or More If the area fraction of martensite in the metallographic structure is less than 85%, the cross-sectional hardness of the seamless steel pipe will be low and the fatigue strength will decrease. Therefore, in the present invention, from the viewpoint of ensuring the cross-sectional hardness and improving the fatigue strength of the seamless steel pipe, the total area fraction of martensite in the metallographic structure of the seamless steel pipe is set to 85% or more. The total area fraction of martensite is preferably 88% or more, more preferably 90% or more, even more preferably 92% or more, and most preferably 94% or more. On the other hand, the upper limit of the area fraction of martensite is not particularly limited and may be 100% or less. Note that the metallographic structure in one embodiment of the present invention may consist of martensite. Furthermore, the metallographic structure in other embodiments of the present invention may contain other structures in addition to martensite. The other structures may be any structure without any particular limitation. Examples of the other structures include bainite, ferrite, pearlite, and retained austenite. When the metallographic structure contains bainite, the area fraction of bainite is preferably 10% or less. The area fraction of bainite is more preferably 5% or less. The area fraction of bainite may be 0% or more. Furthermore, when the metallographic structure contains ferrite, the area fraction of ferrite is preferably 2% or less. The area fraction of ferrite is more preferably 1% or less. The area fraction of ferrite may be 0% or more. When the metallographic structure contains pearlite, the area fraction of pearlite is preferably 2% or less. The area fraction of pearlite is more preferably 1% or less. The area fraction of pearlite may be 0% or more. When the metallographic structure contains retained austenite, the retained austenite is preferably 1% or less. Note that, in the present invention, a high martensite fraction refers to an area fraction of martensite of 85% or more. Furthermore, the area fraction of the above metallographic structure can be measured by the method described in the examples.

[0049] A test specimen was taken from each seamless steel pipe obtained, with the observation position being the longitudinal center of the seamless steel pipe, 1 / 4 of the way down from the outer surface of the steel pipe. The plane consisting of the wall thickness direction and the circumferential direction (the so-called "C-section") was designated as the observation surface. The observation surface was etched using a 3 vol. % nital solution. The observation surface was then observed using a scanning electron microscope (SEM) at an appropriate magnification between 1000 and 5000x to obtain a microstructure image. The obtained microstructure image was analyzed to evaluate the type of metal structure and the area fraction of each metal structure. The area fraction of retained austenite was determined by chemically polishing the observation surface and performing X-ray diffraction. A Co-Kα source was used for incident X-rays, and the area fraction of retained austenite was calculated from the intensity ratio of the (200), (211), and (220) planes of ferrite to the (200), (220), and (311) planes of austenite.

[0050] Next, the cross-sectional hardness of a seamless steel pipe will be explained.

[0051] Steel pipes with a martensite area fraction of 85% or more obtained by quenching alone have high cross-sectional hardness, but are at high risk of delayed fracture during storage (i.e., cracking due to delayed fracture rather than cracking immediately after quenching), and therefore lack excellent delayed fracture resistance. The delayed fracture described in this invention is caused by the intrusion of trace amounts of hydrogen into the steel pipe due to corrosion. On the other hand, in an environment where hydrogen constantly penetrates the steel pipe, there is a risk of cracking due to hydrogen embrittlement, in which hydrogen constantly penetrates the steel pipe and reduces its material properties, and therefore the steel pipes lack excellent hydrogen embrittlement resistance. Furthermore, crack susceptibility varies depending on the tensile strength. Therefore, it is necessary to adjust the cross-sectional hardness as needed. Note that tensile strength and cross-sectional hardness are correlated, and tensile strength can be adjusted by controlling the cross-sectional hardness.

[0052] Cross-section hardness: 250 HV or more and less than 480 HV Since higher strength requires a smaller amount of hydrogen for delayed cracking to occur, it is effective to appropriately reduce the strength of the steel pipe. To obtain excellent delayed fracture resistance, the cross-section hardness must be less than 480 HV. On the other hand, for pressure vessels, it is necessary to increase the strength of the steel pipe and reduce the wall thickness in order to reduce the total weight of the vessel, so the cross-section hardness must be 250 HV or more.

[0053] The seamless steel pipe of the present invention must have the above-mentioned cross-sectional hardness, but it is preferable to select a cross-sectional hardness within a preferred range depending on various applications. For example, when used as a pressure vessel, the cross-sectional hardness is preferably more than 350 HV and less than 480 HV, and when used in an application where the steel pipe is intentionally used in an environment where hydrogen constantly penetrates the steel pipe, the cross-sectional hardness is preferably 250 HV or more and 350 HV or less.

[0054] Cross-section hardness: More than 350 HV and less than 480 HV (preferred condition) As described below, tempering reduces the cross-section hardness, improving delayed fracture resistance, toughness, and machinability. Therefore, the cross-section hardness of seamless steel pipes is adjusted according to the specifications of various pressure vessels. For pressure vessels requiring high strength in order to reduce the weight of the vessel or to secure a larger internal volume for the same outer diameter, the cross-section hardness is preferably more than 350 HV. The cross-section hardness is more preferably 360 HV or more, and even more preferably 370 HV or more. For the reasons described above, the preferred upper limit of the cross-section hardness is less than 480 HV. The cross-section hardness is preferably 460 HV or less, more preferably 445 HV or less, and even more preferably 440 HV or less.

[0055] Cross-section hardness: 250 HV or more and 350 HV or less (preferred condition) Furthermore, unlike unavoidable corrosion phenomena, when steel pipes are intentionally used in environments where hydrogen constantly penetrates the steel pipe, such as in pressure accumulators for hydrogen stations for fuel cell vehicles, cracks occur due to hydrogen embrittlement, in which hydrogen penetrates into the steel pipe and degrades its material properties. In addition, the load applied to the steel pipe promotes the amount of hydrogen penetration. To prevent this hydrogen embrittlement, as with delayed fracture, appropriately reducing the strength of the steel pipe is effective. However, since the amount of hydrogen penetration is large, to obtain excellent hydrogen embrittlement resistance, the cross-section hardness is preferably 350 HV or less, more preferably 345 HV or less, even more preferably 315 HV or less, and most preferably 310 HV or less. For the reasons mentioned above, the cross-section hardness is set to 250 HV or more. It is preferably 260 HV or more, more preferably 265 HV or more, and even more preferably 270 HV or more.

[0056] Next, the wall thickness of the seamless steel pipe of the present invention will be explained.

[0057] Steel pipe wall thickness of 40 mm or more (preferred condition) When a seamless steel pipe is used as a pressure vessel, the greater the steel pipe wall thickness, the smaller the stress that is generated on the pressure vessel, making it possible to design a pressure vessel that can withstand higher pressures. Therefore, the steel pipe wall thickness of the present invention is preferably 40 mm or more. The steel pipe wall thickness is more preferably 45 mm or more, and even more preferably 50 mm or more.

[0058] On the other hand, as the wall thickness of a seamless steel pipe increases, its volume also increases. This means that the total heat capacity of the seamless steel pipe itself increases. During the quenching process, a coolant removes heat from the inner and outer surfaces of the steel pipe, resulting in a higher temperature at the center of the pipe's wall than at the inner and outer surfaces. Meanwhile, the inner and outer surfaces of the steel pipe are always at the same temperature as the cooling medium, such as water, used for water cooling, creating a temperature gradient. This effect becomes more pronounced as the wall thickness increases. A large temperature gradient worsens the balance between thermal stress and transformation stress, increasing the risk of quench cracking. Additionally, as the wall thickness increases for a steel pipe with the same outer diameter, the inner diameter also decreases. Because of the thick-walled steel pipe, a temperature distribution exists during quenching, with the lowest temperature at the inner and outer surfaces of the steel pipe and the highest temperature at the center of the wall. When the inner and outer surfaces of a steel pipe are sufficiently cooled, the heat exchange efficiency at the inner and outer surfaces can be considered the same, so there is no clear difference between the temperature gradient from the center of the wall to the outer surface and the temperature gradient from the center of the wall to the inner surface. On the other hand, when comparing the difference in circumferential length based on the center of the wall, the difference is positive at the outer surface but negative at the inner surface. During quenching, a temperature gradient exists even after the transformation from a face-centered cubic structure to a body-centered cubic structure is complete, and the inner and outer surfaces thermally shrink first. In other words, the difference in circumferential length from the center of the wall decreases. The outer surface has a positive difference in circumferential length, and the absolute value of the difference decreases, so the generated thermal stress also decreases. On the other hand, the inner surface has a negative difference in circumferential length, so the absolute value of the difference increases, so the generated thermal stress increases. In other words, a smaller wall thickness is preferable because the risk of quench cracking on the inner surface increases with increasing wall thickness. Therefore, the wall thickness is preferably 70 mm or less, more preferably 65 mm or less, and even more preferably 60 mm or less.

[0059] Next, inclusions in seamless steel pipes will be described.

[0060] Inclusions are known to be a source of stress concentration when thermal stress occurs because they are harder and less deformable than bulk Fe, which increases the risk of quench cracking and reduces the delayed fracture resistance.

[0061] The number of inclusions with an aspect ratio of 2 or less and a long side length of 10 μm or more from the inner surface of the steel pipe to the position at 1 / 6 of the wall thickness is 10 pieces / 100 mm ２ The following (preferred conditions) are known: It is known that inclusions include sulfide-based inclusions, oxide-based inclusions, and nitride-based inclusions. Low-hardness inclusions elongate during rolling in the pipe-making process, and can be characterized by the lengths of their long and short sides when the inclusions are assumed to be elliptical. The aspect ratio is defined as the value obtained by dividing the long side by the short side. The higher the hardness of an inclusion, the smaller the elongation during rolling and the smaller the aspect ratio. On the other hand, with regard to quench crack resistance, the higher the hardness of the inclusion, the higher the risk of quench cracking. In particular, inclusions with an aspect ratio of 2 or less after rolling and a long side length of 10 μm or more on the inner surface of the steel pipe, where segregation exists, i.e., from the inner surface of the steel pipe to the 1 / 6 wall thickness position, act as starting points for quench cracking and increase the risk of quench cracking. In other words, the number of inclusions with a small aspect ratio is important for reducing the risk of quench cracking. The number of inclusions with an aspect ratio of 2 or less and a long side length of 10 μm or more is 10 / 100 mm ２ Therefore, the number of inclusions with an aspect ratio of 2 or less and a long side length of 10 μm or more from the inner surface of the steel pipe to the position of 1 / 6 of the wall thickness is 10 pieces / 100 mm. ２ The number of inclusions with an aspect ratio of 2 or less and a long side length of 10 μm or more is preferably 8 / 100 mm. ２ More preferably, it is 5 pieces / 100 mm or less. ２ It is more preferable that the number is 4.8 / 100 mm or less. ２ The lower limit of the number of the inclusions is not particularly limited, but from the viewpoint of cost reduction, it is preferably 1 inclusion / 100 mm ２ The number of inclusions can be measured by the method described in the examples.

[0062] As with macrosegregation, the amount of inclusions is greater at the center of the billet, which is the final solidification area. Therefore, in a seamless steel pipe obtained using a billet, the number of inclusions is greatest on the inner surface of the seamless steel pipe. As mentioned above, the inner surface of the seamless steel pipe has a high risk of quench cracking due to the influence of segregation, so it is preferable to have fewer inclusions on the inner surface of the seamless steel pipe. Here, the inner surface of the steel pipe refers to the region from the inner surface of the steel pipe where macrosegregation exists to the 1 / 6 wall thickness position. The size of the inclusions can be controlled by controlling the amount of reduction when manufacturing a seamless steel pipe from a raw material (billet).

[0063] Next, the method for producing a seamless steel pipe of the present invention will be described.

[0064] Raw Material (Billet) From the viewpoint of productivity, it is preferable to use a raw material (billet) produced by continuous casting as the raw material (billet) used in making seamless steel pipes. In continuous casting, if the mold becomes larger, it becomes difficult to control the solidification process. Therefore, in order to achieve stable mass production, the billet diameter is preferably 400 mm or less, more preferably 350 mm or less, and even more preferably 345 mm or less. On the other hand, the lower limit of the billet diameter is not particularly limited, but is preferably 320 mm or more, more preferably 330 mm or more. In addition, since macrosegregation in billets can be improved by hot forging, it is also effective to hot forge, for example, a rectangular raw material produced by continuous casting into a circular raw material (billet).

[0065] (Pipe-making process) Pipe-making is performed so that the ratio of the inner diameter of the steel pipe divided by the wall thickness is 2.0 or more. Macrosegregation at the billet center can be dissipated during the piercing process for making a seamless steel pipe, when the solid billet is rolled into a hollow seamless steel pipe and plastically deformed. When the wall thickness is increased during rolling and the risk of quench cracking is expected to increase, the inner diameter can be increased to increase the amount of plastic deformation when the seamless steel pipe is rolled from the billet to make the pipe. It is preferable that the ratio of the inner diameter of the steel pipe divided by the wall thickness be 2.0 or more. Since the amount of plastic deformation increases as this ratio increases, it is preferable that the ratio be 3.0 or more, more preferably 4.0 or more, even more preferably 4.1 or more, and most preferably 4.2 or more. The upper limit of the ratio is not particularly limited, but from the viewpoint of ensuring the amount of rolling, it is preferably 8.0 or less, and more preferably 7.0 or less.

[0066] In order to increase the amount of plastic deformation, it is also effective to expand the pipe to an outer diameter larger than the billet diameter. The outer diameter of the seamless steel pipe is preferably larger than 1.0 times the billet diameter, more preferably 1.1 times or more, and even more preferably 1.2 times or more. From the viewpoint of handling, it is preferable to make it 2.0 times or less. Note that piercing and rolling are essential requirements in the pipe-making process. It is also preferable to carry out pipe expansion as necessary.

[0067] In the pipe-making process, the shape of the steel pipe satisfies formula (2) (favorable condition): (EXP (coefficient K × wall thickness) / circumferential cross-sectional area) × 10 ６<210... (2) K = {0.41([C] - 0.24) + 4.58[Mn] + 0.21[Cr] + 0.25[Ni]} / 100... (3) (The [element] in formula (3) represents the content (mass%) of the element listed in [ ].) Increasing the wall thickness of a steel pipe increases the circumferential cross-sectional area. In other words, increasing the volume of a seamless steel pipe increases the total heat generated by the seamless steel pipe, which also reduces the cooling rate during quenching and reduces the risk of quench cracking. As described above, increasing the wall thickness has both positive and negative effects on the risk of quench cracking, and it is preferable to optimize the balance to consider productivity. In addition, the temperature difference between the inner and outer surfaces of a seamless steel pipe and the center of the wall thickness is affected by thermal conductivity and is therefore influenced by the chemical composition. Generally, physical properties (thermal conductivity) are measured for carbon steel, manganese steel, chromium steel, nickel steel, etc., with C, Mn, Cr, and Ni contributing significantly. Since satisfying formula (2) reduces the risk of quench cracking, it is preferable to satisfy formula (2). The right-hand side of formula (2) is preferably 208 or less, and more preferably 205 or less. Furthermore, from the perspective of pipe-making efficiency, it is preferable that the left-hand side of formula (2) be 40 or more. The coefficient K in formula (2) can be calculated using formula (3) above.

[0068] (Quenching Process) Heat to a temperature range of 860°C or higher and maintain this temperature range for 30 minutes or more. Unless otherwise specified, the temperatures described below refer to the quarter-way point from the inner surface. The temperature is maintained at the austenite region immediately before quenching. Because this is a temperature range in which the material exists as a solid, dissipation of macrosegregation is not expected, but active atomic diffusion occurs, which is expected to detrap contained elements from grain boundaries, thereby effectively improving microsegregation. The frequency of diffusion depends on temperature and time. At temperatures below 860°C, the detrapping effect is weak, so the temperature is set to 860°C or higher. The temperature is preferably 865°C or higher, more preferably 870°C or higher, and even more preferably 875°C or higher. On the other hand, increasing the temperature promotes coarsening of prior austenite grains, resulting in not only a decrease in fatigue strength but also an increase in heating costs. Therefore, the temperature is preferably 950°C or lower, more preferably 930°C or lower, even more preferably 900°C or lower, and most preferably 890°C or lower. The holding time is 30 minutes or more for homogenization, preferably 35 minutes or more, more preferably 40 minutes or more, and even more preferably 45 minutes or more. Furthermore, since an increase in the holding time leads to a decrease in productivity, the holding time is preferably 90 minutes or less, more preferably 60 minutes or less, and even more preferably 55 minutes or less.

[0069] Water cooling is performed under conditions where the average cooling rate in the temperature range of 800 to 350°C is 3°C / sec or more. The water cooling after the heating and holding is essential to ensure a martensite area fraction of 85% or more. In addition, the average cooling rate in the temperature range of 800 to 350°C is 3°C / sec or more. Since a faster average cooling rate is expected to increase the area fraction of martensite, a rate of 4°C / sec or more is preferred, 5°C / sec or more is more preferred, 5.5°C / sec or more is even more preferred, and 6°C / sec or more is most preferred. On the other hand, the upper limit of the average cooling rate is not particularly limited, but is preferably 15°C / sec or less, and more preferably 10°C / sec or less. The average cooling rate is calculated by dividing the temperature difference in the temperature range of 800 to 350°C by the time required to cool this temperature range.

[0070] During the water cooling, the steel pipe was rotated in the circumferential direction and the inner surface of the steel pipe was coated with water for 0.5 m. ３ It is important to remove the vapor film that occurs during water cooling by contacting the cooling medium at a flow rate of 0.5 m / min or more. The outer surface of seamless steel pipe has a high degree of freedom in equipment, and it is possible to remove the vapor film even with equipment with a simple structure, but there are many restrictions on equipment when removing the vapor film on the inner surface of steel pipe. Therefore, it is necessary to contact the inner surface of the steel pipe with the cooling medium, and an appropriate method such as running a jet stream of the cooling medium can be used. ３ The flow rate is 0.55 m / min or more. ３ / min or more, and ３ / min or more is more preferable, and ３ / min or more, and more preferably 0.65 m ３ The upper limit of the flow rate is not particularly limited, but from the viewpoint of reducing the load on the equipment, it is preferable that the flow rate is 1.5 m ３ / min or less. In addition, in order to shorten the time during which thermal stress occurs, it is important to remove the heat remaining in the steel pipe even after the martensitic transformation is completed, and the jet flow injection time is preferably 3 minutes or more, more preferably 5 minutes or more, even more preferably 5.5 minutes or more, and most preferably 6 minutes or more. Note that, since a steam film is generated at the initial stage of injection, the flow rate should be reduced to 0.5 m / min or less after 90 seconds from injection. ３ / min or less and 0.05m ３ / min or more. In addition, it is preferable that the surface temperature of the steel pipe after the end of injection is 50°C or less. Since the cooling medium needs to have a sufficient cooling function, water at 50°C or less may be used. The temperature of the water is more preferably 45°C or less, and even more preferably 40°C or less. The temperature of the water is preferably 2°C or more.

[0071] Furthermore, in order to alleviate the heat distribution caused by uneven wall thickness during rolling, it is preferable to rotate the seamless steel pipe in the pipe circumferential direction at a rotation speed of 10 rpm or more when the jet stream is sprayed. A rotation speed of 12 rpm or more is more preferable, and a rotation speed of 15 rpm or more is even more preferable. On the other hand, there is no particular upper limit to the rotation speed, but a rotation speed of 1200 rpm or less is preferable.

[0072] The quenching process refers to a series of processes including heating, holding, and water cooling after the pipe-making process.

[0073] Since the seamless steel pipe is cooled while rotating at high speed, it is difficult to directly measure the temperature using a general temperature measurement method that uses a thermocouple and calculate the average cooling rate during quenching. Therefore, a CCT diagram may be prepared, and the average cooling rate may be determined by correlating it with the hardness measured on the steel pipe after quenching.

[0074] As the cooling equipment, any suitable equipment may be used, such as immersion equipment, mist cooling equipment, or shower cooling equipment. However, it is preferable to use immersion equipment, since the most uniform cooling is achieved by basically immersing the entire steel pipe in water, rotating it, and running a jet stream over the inner surface of the steel pipe.

[0075] (Tempering Process) Tempering is performed for 30 minutes or more at a temperature range of 400 to 680°C. Tempering reduces cross-sectional hardness and improves delayed fracture resistance, toughness, and machinability. Therefore, the cross-sectional hardness of seamless steel pipes is adjusted according to the specifications of various pressure vessels. For pressure vessels requiring high steel pipe strength to reduce the weight of the vessel or to secure a larger internal volume for the same outer diameter, a cross-sectional hardness of less than 480 HV and more than 350 HV is desirable, and the temperature is 400°C or higher, preferably 405°C or higher, more preferably 410°C or higher, and even more preferably 415°C or higher. Furthermore, the temperature is preferably less than 620°C. For pressure vessels storing hydrogen gas (hydrogen gas pressure vessels), a cross-sectional hardness of less than 350 HV and more than 250 HV is desirable, and the temperature is 680°C or lower, preferably 675°C or lower, and more preferably 660°C or lower. Furthermore, the temperature is preferably 620°C or higher. Regarding the tempering time, in order to uniformly temper the material in the thickness direction, it is necessary to maintain the material in the above temperature range for 30 minutes or more. The maintenance time at the above temperature is preferably 35 minutes or more, more preferably 40 minutes or more, and even more preferably 45 minutes or more. The maintenance time at the above temperature is preferably 90 minutes or less, and more preferably 80 minutes or less.

[0076] In other embodiments of the present invention, further processing described below can be performed.

[0077] The decarburized layer that forms on the surface of steel pipes during quenching and tempering is more brittle than the steel pipe parent phase and is more susceptible to cracking. Furthermore, rolling marks can also be the starting point for cracks. Therefore, it is effective to remove the decarburized layer and rolling marks by machining such as cutting and polishing, shot blasting, or both.

[0078] In addition, in order to suppress corrosion of the inner surface of seamless steel pipes, it is also effective to apply anti-rust oil or paint.

[0079] When cooling a seamless steel pipe, heat transfer between the inner and outer surfaces is dominant, so the outer surface, which has a larger surface area, is cooled more efficiently. Therefore, the highest temperature in the temperature distribution along the wall thickness shifts from the center of the wall toward the inner surface. In this case, the load on the outer surface increases, increasing the risk of quench cracks occurring on the outer surface. Although the outer surface does not have macrosegregation like the inner surface, it is affected by microsegregation, so an extremely large wall thickness is undesirable. Therefore, regarding the relationship between the inner and outer diameters, the difference between the outer periphery length and the inner periphery length divided by the periphery length at the wall center is preferably 0.45 or less, more preferably 0.40 or less. While there is no particular lower limit, it is preferable that it be 0.10 or more.

[0080] Next, the pressure vessel and the method for manufacturing the pressure vessel will be described.

[0081] The seamless steel pipe of the present invention can be used as a pressure vessel for high pressure, such as a pressure vessel for compressed fluids or high-pressure gases, which has specifications that require higher roundness than, for example, electric resistance welded pipes.

[0082] For example, when the seamless steel pipe of the present invention is used for a pressure vessel, the pressure vessel can be manufactured by subjecting the seamless steel pipe to necessary processing. The processing is not particularly limited, but examples thereof include processing to render flaws on the inner surface of the steel pipe harmless and processing to attach a lid to the end of the steel pipe. Examples of processing to render flaws on the inner surface of the steel pipe harmless include cutting the inner surface of the steel pipe with a tool, grinding with a grinding wheel, and shot blasting. Examples of processing to attach a lid include providing threads for screwing a lid onto the end of the steel pipe and forming a flange for fastening the lid with a bolt.

[0083] Furthermore, the seamless steel pipe of the present invention can be used for any application in which it is used in a state of contact with hydrogen gas, such as a hydrogen gas pressure vessel or hydrogen gas piping, etc. In particular, it can be suitably used as a seamless steel pipe having excellent hydrogen embrittlement resistance and used in a high-pressure hydrogen gas environment.

[0084] For example, when the seamless steel pipe of the present invention is used as a pressure vessel for hydrogen gas, the pressure vessel can be manufactured by subjecting the seamless steel pipe to necessary processing. The processing described above is not particularly limited, but examples thereof include processing to render flaws on the inner surface of the steel pipe harmless and processing to attach a lid to the end of the steel pipe. Examples of processing to render flaws on the inner surface of the steel pipe harmless include cutting the inner surface of the steel pipe with a tool, grinding with a grinding wheel, and shot blasting. Examples of processing to attach a lid include providing a thread for screwing a lid onto the end of the steel pipe, and forming a flange for tightening the lid with a bolt.

[0085] The seamless steel pipe of the present invention can be used without additional heat treatment such as adjusting strength, even when used as a pressure vessel for a gas other than hydrogen, or when used as a pressure vessel for hydrogen gas.

[0086] The seamless steel pipe of the present invention has excellent resistance to quench cracking and delayed fracture, and therefore can be used as a pressure vessel. Furthermore, the seamless steel pipe, which has excellent resistance to hydrogen embrittlement in addition to the above properties, can be used as a hydrogen gas vessel.

[0087] Next, an example will be described.

[0088] Heated material (billet) having the chemical composition shown in Table 1 was pierced, expanded, and rolled to form a pipe so that the ratio of the inner diameter of the steel pipe divided by the wall thickness was 2.0 or more as shown in Table 2. The pipe was then subjected to a quenching process and a tempering process under the conditions shown in Table 2 to produce seamless steel pipes having the outer diameter and wall thickness shown in Table 2. The cooling in the quenching process was carried out by immersing the steel pipe in water and rotating it in the circumferential direction of the pipe, and a 0.5 mm thick film was applied to the inner surface of the steel pipe. ３ The seamless steel pipes were each subjected to a cooling medium contact at a flow rate of at least 1 / min. Test specimens were taken from the longitudinal center of each seamless steel pipe to observe the structure, evaluate the degree of Mn segregation (by EPMA observation), and examine inclusions.

[0089] Average Cooling Rate During the Quenching Process: Because it is difficult to directly measure the average cooling rate of a rotating steel pipe using a thermocouple or other device, a 500 mm-long steel pipe taken from the end of the steel pipe was divided circumferentially into four test pieces of the same wall thickness. Thermocouples were attached to the center of the wall thickness and measured. During the water-cooling process, the difference in cooling rate across the wall thickness of the steel pipe is reduced by continuously contacting the inner surface of the steel pipe with a cooling medium. Furthermore, the rotation of the steel pipe reduces the difference in cooling rate across the circumferential direction. Therefore, the above measurement method was based on the idea that the water-cooling process of a steel pipe can be simulated simply by immersing the test piece in room-temperature water. Four test pieces were prepared for each steel type listed in Table 2, and the average cooling rate in the temperature range of 800 to 350°C was used as the average cooling rate of the steel pipe. The average cooling rates are shown in Table 2.

[0090] Microstructural observation: Test specimens were taken from each seamless steel pipe, with the observation position being the longitudinal center of the seamless steel pipe, at a position 1 / 4 of the wall thickness from the outer surface of the steel pipe. Here, the plane consisting of the wall thickness direction and the circumferential direction (so-called "C cross section") was used as the observation surface (10 mm x 10 mm). First, the area fraction of retained austenite was calculated for the observation surface using the following method. The observation surface was chemically polished and determined by X-ray diffraction. A Co-Kα radiation source was used for incident X-rays, and the area fraction of retained austenite was calculated from the intensity ratio of the (200), (211), and (220) planes of ferrite to the (200), (220), and (311) planes of austenite. Next, the observation surface was mirror-polished and then etched using a picric acid solution to leave the prior γ grain boundaries. The size and morphology of the prior γ grain boundaries were determined using an optical microscope at an appropriate magnification of 100 to 500x. These are indicators of martensite, i.e., the size and morphology of martensite were determined. Furthermore, the observation surface was mirror-polished and then etched using a 3 vol. % nital solution. Comparison of the images obtained with nital and picric acid etching confirmed that the morphology of the prior γ grain boundaries was not extremely flattened, that the prior γ grains were in a fine lath-like state, and that the structure was predominantly martensite (85% or more). The nital-etched structure was then examined using an optical microscope at an appropriate magnification of 100 to 500x. Coarse grains were determined to be ferrite, and the area fraction of ferrite was calculated. The nital-etched cross section was then observed using a scanning electron microscope (SEM) at an appropriate magnification of 1,000 to 10,000 times, and the area where lamellar carbides were confirmed was determined to be pearlite, and the area fraction of pearlite was calculated. The area fraction obtained by subtracting the area fractions of retained austenite, ferrite, and pearlite from the total (defined as 100%) was defined as the area fraction of martensite.

[0091] Inclusions The investigation of inclusions was carried out in the region from the inner surface of the steel pipe to the 1 / 6 position in the wall thickness, which is the segregation region on the inner surface side of the steel pipe. Specifically, test pieces for inclusion measurement were taken from the inner surface of the obtained steel pipe, measuring 20 mm in the longitudinal direction of the steel pipe, 5 mm in the circumferential direction, and 15 mm in the wall thickness direction. The test pieces were embedded in resin so that the surface consisting of the longitudinal direction and the wall thickness direction of the steel pipe (the so-called "L cross section") was the observation surface, and after mirror polishing, the test pieces were observed under an optical microscope to determine the size of the inclusions in a 10 mm x 10 mm area (100 mm ２ ) was measured for the number density of inclusions with an aspect ratio of 2 or less and a long side length of 10 μm or more. Ten test pieces for inclusion investigation were taken from each level of steel pipe, and the total number of inclusions with an aspect ratio of 2 or less and a long side length of 10 μm or more measured as described above using these 10 test pieces was taken as the arithmetic average, which was used as the number of inclusions (inclusion number density) for the steel pipe of that level. The aspect ratio and long side length of the inclusions were values ​​determined in accordance with JIS G0555:2020 (Microscopic Testing Method for Non-Metallic Inclusions in Steel).

[0092] Mn Segregation Degree To evaluate the Mn segregation degree, test specimens were used, which were taken so that the cross section of the steel pipe in the C direction (direction perpendicular to the rolling direction) served as the observation surface, and the observation surface was mirror-finished. In the observation surface, where the wall thickness of the steel pipe is t, the reference local Mn concentration was calculated by mapping analysis of the Mn concentration in region 1, which was surrounded by a length of t / 6 (mm) in the wall thickness direction (±t / 12 (mm) in the wall thickness direction, based on the 1 / 4 position in the wall thickness direction from the outer surface of the steel pipe) and a length of 10 (mm) in the circumferential direction, centered at a position 1 / 4 in the wall thickness direction from the outer surface of the steel pipe, and the analysis described below was performed. Furthermore, the Mn segregation concentration was calculated by mapping analysis of the Mn concentration in region 2, which was surrounded by a length of t / 6 (mm) in the wall thickness direction from the inner surface of the steel pipe and a length of 10 (mm) in the circumferential direction, and the analysis described below was performed. Here, the mapping analysis of the Mn concentration was performed in both region 1 and region 2, with an analysis pitch determined so that mapping data of 40,000 points could be obtained. Subsequently, the obtained 40,000 points of mapping data were statistically analyzed to calculate the degree of Mn segregation. The reference local Mn concentration was determined as the average value of the mapping data obtained in region 1, excluding outliers. Outliers were defined as mapping data that were less than 0.05 times and more than 5 times the Mn concentration (Mn content) obtained from the check analysis of the steel pipe. Furthermore, the segregated Mn concentration was determined as the average value of the top 1% of mapping data with the highest Mn concentration in the mapping data obtained in region 2, excluding outliers. In this case, the mapping analysis of region 1 was performed in three fields of view, and the local Mn concentrations calculated in the three fields of view were summed and divided by 3 to obtain the local Mn concentration. Similarly, mapping analysis of region 2 was performed in three visual fields, and the segregated Mn concentrations calculated in the three visual fields were added together and divided by 3 to obtain the segregated Mn concentration. The ratio of the segregated Mn concentration to the local Mn concentration thus obtained was calculated as the degree of Mn segregation. The mapping analysis was performed using EPMA as described in the embodiment.

[0093] Cross-sectional hardness For the cross-sectional hardness, a test piece was taken so that the evaluation position was a quarter of the wall thickness from the outer surface of the steel pipe, and in accordance with JIS 2244 "Vickers hardness test - Test method", the hardness was measured at five points at a quarter of the wall thickness from the outer surface of the steel pipe using a load of 10 kgf, and the average value was taken as the hardness.

[0094] Quench crack resistance: The presence or absence of quench cracking was evaluated in accordance with JIS G 0582 "Automatic ultrasonic flaw detection method for steel pipes." The threshold used to determine the presence or absence of quench cracking was set to 5% of the wall thickness, and steel pipes that satisfied this criterion were judged to be free of quench cracking. First, 20 steel pipes for each of the levels shown in Table 2 were prepared, and quench cracking resistance was evaluated. That is, the presence or absence of quench cracking in the obtained steel pipes was confirmed, and the proportion of the number of steel pipes that did not experience quench cracking among the 20 steel pipes, i.e., (number of steel pipes without quench cracking / 20) x 100 (%), was taken as the quench cracking pass rate. A quench cracking pass rate of 90% or more was judged to be acceptable.

[0095] Delayed fracture resistance: The delayed fracture resistance was evaluated by conducting a thermal cracking resistance evaluation test. The thermal cracking resistance evaluation test was performed according to the following procedure. First, in the evaluation of quench cracking resistance described above, steel pipes that did not experience quench cracking were stored outdoors for 30 days without direct rain exposure, and then, similar to the evaluation of the presence or absence of quench cracking, the presence or absence of thermal cracking was evaluated in accordance with JIS G 0582 "Automatic Ultrasonic Inspection Method for Steel Pipes." The threshold used to determine the occurrence of thermal cracking was set to 5% of the wall thickness, and steel pipes that satisfied this criterion were judged to have no thermal cracking. The presence or absence of thermal cracking in the steel pipes after 30 days of storage was confirmed, and the proportion of steel pipes that did not experience thermal cracking, i.e., (number of steel pipes without thermal cracking / number of steel pipes without thermal cracking) × 100 (%), was taken as the thermal cracking pass rate. Note that the thermal cracking resistance evaluation test was only performed when the quench cracking pass rate for quench cracking resistance was 90% or higher. A thermal cracking pass rate of 80% or higher was judged to have excellent delayed fracture resistance.

[0096] Hydrogen embrittlement resistance: Hydrogen embrittlement resistance was evaluated in accordance with ASTM G142 by conducting a tensile test in high-pressure hydrogen gas (105 MPa) at room temperature and determining the RTS (tensile strength in hydrogen) / tensile strength in air. Test specimens for tensile tests were taken from a cross section perpendicular to the steel pipe axis (C direction: circumferential direction), with the center at a position 1 / 4 of the wall thickness from the outer surface of the steel pipe, so that the longitudinal direction of the test specimen was the circumferential direction. The taken test specimens were processed into bar-shaped test specimens specified in JIS Z 2201 "Tensile test specimens for metallic materials." A tensile test was conducted using the bar-shaped test specimens, and an RTS of 0.98 or more was determined to have excellent hydrogen embrittlement resistance.

[0097] As can be seen from the results shown in Table 2, seamless steel pipes satisfying the conditions of the present invention have a metal structure sufficient to achieve high strength, i.e., a high martensite fraction, and a predetermined Mn segregation degree. As a result, they have a quench crack pass rate of 90% or more, a thermal crack pass rate of 80% or more, and a cross-sectional hardness of 250 HV or more and less than 480 HV, and are excellent in quench crack resistance and delayed fracture resistance. Furthermore, seamless steel pipes with an RTS of 0.98 or more also have excellent hydrogen embrittlement resistance.

[0098] As described above, the seamless steel pipe of the present invention has sufficient resistance to quench cracking and delayed fracture despite the high martensite fraction, and also has a predetermined cross-sectional hardness. Furthermore, the high martensite fraction can be judged to have excellent fatigue strength. This makes it possible to supply thick-walled, high-strength seamless steel pipes.

[0099]

[0100]

Claims

1. Contains, by mass%, C: 0.20 to 0.60%, Si: 0.01 to 2.0%, Mn: 0.5 to 1.2%, S: 0.010% or less, O: 0.005% or less, N: 0.010% or less, Al: 0.01 to 0.08%, Mo: 0.005 to 1.0%, Cr: 0.005 to 3.0%, Ni: 0.005 to 3.0%, P satisfies the following formula (1), or further contains Ti: 0.0001 to 0.1%, Cu: 0.0001 to 5.0%, Co: 0.0001 to 5.0%, B: 0.0001 to 0.01%, V: 0.0001 to 1.0%, The steel pipe has a chemical composition comprising one or more elements selected from W: 0.0001 to 5.0%, Nb: 0.0001 to 0.1%, Zr: 0.0001 to 0.2%, Hf: 0.0001 to 0.2%, Ta: 0.0001 to 0.2%, Sb: 0.0001 to 0.1%, Sn: 0.0001 to 0.1%, Ca: 0.0001 to 0.01%, Mg: 0.0001 to 0.01%, and REM: 0.0001 to 0.5%, with the balance being Fe and unavoidable impurities; the steel pipe has a metallographic structure in which the segregation degree of Mn from the inner surface to the 1 / 6 wall thickness position is less than 1.50 and the area fraction of martensite is 85% or more; A seamless steel pipe having a cross-sectional hardness of 250 HV or more but less than 480 HV. [P]≦0.042−0.002[C]−0.024[Mn]−0.001[Mo]−0.001[Cr]−0.002[Ni]...(1) (The [element] in formula (1) represents the content (mass%) of the element written in each bracket.) 2. A seamless steel pipe according to claim 1, wherein the wall thickness of the steel pipe is 40 mm or more.

3. The number of inclusions with an aspect ratio of 2 or less and a long side length of 10 μm or more from the inner surface of the steel pipe to the position at 1 / 6 of the wall thickness is 10 pieces / 100 mm ２ 3. A seamless steel pipe according to claim 1 or 2, comprising:

4. A method for manufacturing a seamless steel pipe according to any one of claims 1 to 3, comprising the steps of: forming a heated material into a pipe so that the ratio of the inner diameter of the steel pipe divided by the wall thickness is 2.0 or more; heating the material to a temperature range of 860°C or more, holding the material in the temperature range for 30 minutes or more, and water-cooling the material under conditions in which the average cooling rate in the temperature range of 800 to 350°C is 3°C / sec or more; and forming a 0.5m thick film on the inner surface of the steel pipe while rotating the steel pipe circumferentially during the water-cooling process. ３ a quenching step in which the steel is cooled while being in contact with a cooling medium at a flow rate of 1000 kJ / min or more; and a tempering step in which, after the quenching step, the steel is maintained at a temperature range of 400 to 680°C for 30 minutes or more.

5. The method for producing a seamless steel pipe according to claim 4, wherein the steel pipe is manufactured so that its shape satisfies the following formula (2): (EXP (coefficient K × wall thickness) / area of ​​circumferential cross section) × 10 ６ <210 (2) K = {0.41 ([C] - 0.24) + 4.58 [Mn] + 0.21 [Cr] + 0.25 [Ni]} / 100 (3) (In formula (3), [element] represents the content (mass%) of the element written in each bracket.) 6. A pressure vessel using the seamless steel pipe according to any one of claims 1 to 3.

7. A pressure vessel for hydrogen gas using the seamless steel pipe according to any one of claims 1 to 3.

8. A method for manufacturing a pressure vessel or a pressure vessel for hydrogen gas, which comprises processing the seamless steel pipe according to any one of claims 1 to 3.

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