Seamless steel pipe, seamless steel pipe manufacturing method, pressure vessel, and pressure vessel manufacturing method

Abstract

An object of the present invention is to provide a seamless steel pipe having a high martensite fraction, a thick wall, and excellent quench cracking resistance, 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, and a method for manufacturing a pressure vessel. A seamless steel pipe having specific components, and a composition in which P satisfies the following formula (1), and having a metal structure in which the Mn segregation degree on the inner surface of the steel pipe is less than 1.50 and the area fraction of martensite is 85% or more. [P]≦0.042―0.002[C]―0.024[Mn]―0.001[Mo] ―0.001[Cr]―0.002[Ni]···(1) (In formula (1), [element] represents the content (mass%) of the element written in [ ].)

Description

[Technical Field]

[0001] The present invention relates to a seamless steel pipe, and more particularly to a thick-walled seamless steel pipe with a high martensite fraction that has excellent resistance to quench cracking and is particularly suitable for use as a component of a pressure vessel, etc., and a method for manufacturing the seamless steel pipe. The present invention also relates to a pressure vessel and a method for manufacturing the pressure vessel. [Background technology]

[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 pressure inside the steel pipe, the higher the strength of the steel material 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 occurs 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 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. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 9-104925 [Patent Document 2] International Publication No. 2018 / 055937 Summary of the Invention [Problem to be solved by the invention]

[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 the test specimens are made by drilling a circular hole in a disk, so that the wall thickness at the cross section of the test specimen is extremely heterogeneous and the balance between thermal stress and transformation stress is intentionally deteriorated. 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, adding large amounts of alloying elements to ensure hardenability increases 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. 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.

[0010] The present invention has been made in view of the above circumstances, and has an object to provide a seamless steel pipe which has a high martensite fraction, is thick-walled, and has excellent quench cracking resistance, and 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, and a method for manufacturing a pressure vessel. [Means for solving the problem]

[0011] The present inventors conducted research to solve the above-mentioned problems and found that by optimizing the P concentration in the seamless steel pipe's chemical composition, as well as the rolling conditions in the seamless steel pipe manufacturing process and the heat treatment conditions after the pipe manufacturing process, they were able to control P segregation on the inner surface of the seamless steel pipe, thereby achieving excellent quench cracking resistance, even for thick-walled seamless steel pipes. However, because the P concentration in seamless steel pipes is designed to be low in order to suppress P segregation, there was also the problem of difficulty in assessing 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, it is more reliable to evaluate the degree of P segregation by evaluating the degree of Mn segregation. Therefore, the inventors found that quench cracking 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] In mass %, C: 0.20~0.60%, 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: Contains 0.005 to 3.0% P satisfies the following formula (1), Or even more so, Ti: 0.0001 to 0.1%, Cu: 0.0001 to 5.0% Co: 0.0001 to 5.0%, B: 0.0001~0.01%, V: 0.0001 to 1.0%, W: 0.0001 to 5.0%, Nb: 0.0001 to 0.1%, Zr: 0.0001~0.2%, 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%, REM: 0.0001 to 0.5%, and one or more selected from the following: The balance is Fe and unavoidable impurities, and The degree of Mn segregation from the inner surface of the steel pipe to the 1 / 6 position of the wall thickness is less than 1.50, A metal structure in which the area fraction of martensite is 85% or more. Seamless steel pipe. [P]≦0.042―0.002[C]―0.024[Mn]―0.001[Mo] ―0.001[Cr]―0.002[Ni]···(1) (In formula (1), [element] represents the content (mass%) of the element written in [ ].) [2] The seamless steel pipe according to [1], wherein the steel pipe wall thickness 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 having the following: [1] or [2]. [4] A seamless steel pipe according to any one of [1] to [3], which has a cross-sectional hardness of 480 HV or more. [5] A method for producing a seamless steel pipe according to any one of [1] to [4], The heated material is heated to a temperature range of 860°C or higher, held at that temperature range for 30 minutes or more, and then water-cooled at an average cooling rate of 3°C / sec or higher in the temperature range of 800-350°C. During the water-cooling process, the steel pipe is rotated in the circumferential direction of the pipe, and a 0.5m diameter is formed on the inner surface of the steel pipe. 3 and a quenching step in which the pipe is cooled while being in contact with a cooling medium at a flow rate of at least 1 / min. [6] A method for manufacturing a seamless steel pipe according to [5], in which the steel pipe is manufactured so that its shape satisfies the following formula (2): (EXP (coefficient K × 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) (In formula (3), [element] represents the content (mass%) of the element written in [ ].) [7] A pressure vessel using the seamless steel pipe according to any one of [1] to [4] above. [8] A method for manufacturing a pressure vessel, which comprises processing the seamless steel pipe according to any one of [1] to [4] above. [Effects of the Invention]

[0012] The seamless steel pipe of the present invention has a high martensite fraction and is excellent in quench crack resistance when used as a thick-walled steel pipe. DETAILED DESCRIPTION OF THE INVENTION

[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 is preferably 480 HV or more, and to obtain 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 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. Furthermore, S forms MnS inclusions, which reduce toughness. These effects are not a problem if 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 set 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 set to 0.0002% or more, even more preferably set to 0.0005% or more, and most preferably set to 0.001% or more.

[0018] O: 0.005% or less O exists in steel as an impurity, and 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 costs in 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 steel 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 an increase in the strength of steel pipes. By improving the hardenability of steel materials, Mo also increases the proportion of structures other than ferrite (particularly martensite) in the metallographic structure, making it an essential element for obtaining 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 obtain the above-mentioned metallographic structure and reduces the risk of quench cracking during heat treatment of steel pipes. To achieve this effect, the Mo content is set to 0.005% or more. The Mo content is preferably set to 0.008% or more, and more preferably set to 0.1% or more. On the other hand, if the Mo content exceeds 1.0%, the effect saturates and costs increase, so the Mo content is set to 1.0% or less. The Mo content is preferably set to 0.8% or less, more preferably set to 0.5% or less, even more preferably set to 0.3% or less, and most preferably set to 0.25% or less.

[0022] Cr: 0.005 to 3.0% Cr is an element that improves hardenability, contributing to an increase in the strength of steel pipes. By improving the hardenability of steel pipes, 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-mentioned metallographic structure and reduce the risk of quench cracking during heat treatment of steel pipes. 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 Mn segregation level 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 improving 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. However, 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] P satisfies the following equation (1): It is important that P satisfies the following equation (1). [P]≦0.042―0.002[C]―0.024[Mn]―0.001[Mo]―0.001[Cr]―0.002[Ni]···(1) (Note that the parentheses in formula (1) represent the content (mass%) of the element written within the parentheses.) P is an element that contributes to increasing the strength of steel pipes through solid solution strengthening, but also reduces the quench cracking resistance of steel pipes. It is known that the mechanism of quench cracking is that P trapped at grain boundaries reduces the bonding strength of the grain boundaries. Grain boundaries in steel are planar lattice defects that disrupt the 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 the steel, caused by the different melting points of each element when the steel solidifies from liquid to solid. The second is microsegregation, which is the concentration distribution within the steel structure, caused by the diffusion of contained elements from the bulk to the grain boundaries during heat treatment of the steel. That is, it is possible to reduce the risk of quench cracking in the steel pipe that is finally obtained by reducing both the macrosegregation of P and the microsegregation of P. Hereinafter, in the present invention, the term "segregation" is defined to include both macrosegregation and microsegregation, and the two are 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. Therefore, it is important to ensure the quench cracking 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 the movement of dislocations within grains. This effect increases intragranular strength, which in turn increases the load on grain boundaries. In other words, since the inclusion of C increases the risk of quench cracking, it is preferable to reduce the P content. It has long been known that Mn has a particularly high affinity with P, and reducing the microsegregation of Mn results in reducing the microsegregation of P. In other words, since the inclusion of Mn increases the risk of quench cracking, 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, just like Mn, and are less likely to inhibit the diffusion of Mn atoms in the Fe bulk than other elements. In other words, Mn atoms are more likely to diffuse. In other words, Cr and Ni are elements that promote the microsegregation of Mn. Since the inclusion of Cr and Ni also increases the risk of quench cracking, it is preferable to reduce the P content. In the case of seamless steel pipes in which macrosegregation exists on the inner surface of the steel pipe, satisfying formula (1) makes it possible to suppress cracks (quench cracks) that occur on the inner surface of the steel pipe during quenching. However, excessive reduction in the P content reduces production efficiency and increases refining costs, so the P content is preferably 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 permitted to be present to the extent that they do not impair the objectives of the present invention. Examples of raw materials include iron ore, reduced iron, scrap, etc. Examples of impurities include Zn, Pb, As, and Bi.

[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: 0.0001-0.1%, Cu: 0.0001-5.0%, Co: 0.0001-5.0%, B: 0.0001-0.01%, V: 0.0001-1.0%, W: 0.0001-5.0%, Nb: 0.0001-0.1%, Zr: 0.0001-0.2%, Hf: 0.0001-0.2%, Ta: 0.0001-0.2%, Sb: 0.0001-0.1%, Sn: 0.0001-0.1%, Ca: 0.0001-0.01%, Mg: 0.0001-0.01%, and REM: 0.0001-0.5%.

[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 this causes an increase in costs. Therefore, when Ti is contained, the Ti content is set to 0.1% or less. To suppress 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 the coarsening of prior austenite grains, thereby improving various properties of the steel pipe. When Cu is contained to obtain 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 causes an increase in costs. 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. In order to suppress 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 obtain 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 the 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. In order to suppress 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. 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, when the V content exceeds 1.0%, the effect saturates and this causes an increase in 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 this causes an increase in costs. 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. In order to obtain 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 this causes an increase in costs. 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 suppress 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 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, when 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 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 reduce 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, etc., 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. If the Sb content exceeds 0.1%, it 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 steel pipes. 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%, it may lead to embrittlement of the steel pipe. 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:0.0001~0.01%, Mg:0.0001~0.01% Ca and Mg contribute to improving the state of inclusions. To achieve this effect, when Ca and Mg are contained, the Ca content and Mg content 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, each set to 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, and in order to reduce costs, the Ca content and Mg content are each more preferably set to 0.005% or less, and even more preferably set to 0.004% or less.

[0041] REM: 0.0001 to 0.5% Rare Earth Metals (REM) 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. Note that 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. The REM content here refers to the total content of these elements.

[0042] Next, the degree of segregation in seamless steel pipes will be explained.

[0043] Mn segregation level less than 1.50 from the inner surface of the steel pipe to 1 / 6 of the wall thickness As mentioned above, P has a high affinity with Mn. Since P is a direct cause of the risk of quench cracking, the P content (P concentration) is controlled to be low in the present invention. While it is basically necessary to suppress P segregation, the Mn concentration, which has a high affinity with P, is higher than the P concentration, making the analysis results using EPMA more reliable. Furthermore, there is a high correlation between the P concentration and the Mn concentration. Therefore, suppressing Mn segregation can reliably suppress P segregation. Therefore, when evaluating the degree of P segregation in seamless steel pipes, the degree of Mn segregation is used as an evaluation index. Furthermore, in the case of steel plates (steel plates) manufactured by rolling continuously cast slabs, macrosegregation is present in the center of the plate thickness (wall thickness), and the effect of rolling on macrosegregation mitigation is therefore smaller than that of seamless steel pipes. This is due to 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 the pipe. As a result, the macrosegregation that was present in the billet center is relocated to the inner side of the seamless steel pipe in the pipe shape. In other words, the inner side 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 side of the steel pipe than on the outer side of the steel pipe. Therefore, rolling is expected to mitigate P macrosegregation on the inner side of the steel pipe. For this reason, it is important to control P segregation, i.e., Mn segregation, on the inner side of the seamless steel pipe. Therefore, when evaluating the degree of Mn segregation in seamless steel pipes, it is important to manufacture and evaluate seamless steel pipes. We found that the Mn segregation degree can be calculated by using the ratio of the Mn concentration in the segregation region on the inner surface of the steel pipe, i.e., from the inner surface to the 1 / 6th of the wall thickness position, to the local Mn concentration near the outer surface of the steel pipe as the reference. The reason for using the Mn concentration near the 1 / 4th of the wall thickness position as the reference local Mn concentration is that this region, from the outer surface to the 1 / 4th of the wall thickness position, 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 a pressing load, and can be further removed from the inner surface of the steel pipe. On the other hand, the Mn segregation concentration is calculated by using the Mn concentration in the segregation region on the inner surface of the steel pipe, i.e., from the inner surface to the 1 / 6th of the wall thickness position. To reduce the risk of quench cracking, the degree of Mn segregation must be less than 1.50, preferably 1.45 or less, more preferably 1.44 or less, and even more preferably 1.43 or less. There is no particular lower limit to the degree of Mn segregation, 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 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] To evaluate 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 taken so that the cross section of the steel pipe in the C direction (perpendicular to the rolling direction) served as the observation surface, and the observation surface was mirror-finished. Next, on the above observation surface, if the wall thickness of the steel pipe is t, a mapping analysis of the Mn concentration in region 1 was performed, which was 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, with the center at a position 1 / 4 of the way in the wall thickness direction from the outer surface of the steel pipe. The analysis described below was then performed and calculated. The Mn segregation concentration is calculated by performing a mapping analysis of the Mn concentration in region 2, which is defined 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. The resulting 40,000 mapping data points are then statistically analyzed to calculate the degree of Mn segregation. The reference local Mn concentration is the average value of all mapping data obtained in region 1, excluding outliers. Outliers are mapping data points 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 calculated by the average value of the top 1% of mapping data points with the highest Mn concentrations, excluding outliers from all mapping data obtained in region 2. In this case, mapping analysis of region 1 is performed in three fields of view, and the local Mn concentrations calculated in the three fields of view are added together and divided by 3 to obtain the local Mn concentration. Similarly, mapping analysis of region 2 is also performed in three fields of view, and the segregated Mn concentrations calculated in the three fields of view 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 obtained in this way (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 martensite content in the metal structure is less than 85%, the fatigue strength of the seamless steel pipe will decrease. Therefore, in the present invention, from the viewpoint of improving the fatigue strength of the seamless steel pipe, the total area fraction of martensite in the metal 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, there is no particular upper limit for the area fraction of martensite, and it is sufficient if it is 100% or less. The metal structure in one embodiment of the present invention may be composed of martensite. Furthermore, the metal structure in other embodiments of the present invention may contain other structures in addition to martensite. The other structures are not particularly limited and may be any structure. Examples of the other structures include bainite, ferrite, pearlite, and retained austenite. When the metal 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. When the metal 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 metal 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 metal 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. In the present invention, "high martensite fraction" refers to an area fraction of martensite of 85% or more. The area fraction of the metal structure can be measured by the method described in the examples.

[0049] A test piece was taken from each seamless steel pipe, 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 used 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 of 1000 to 5000 times 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 wall thickness of the seamless steel pipe of the present invention will be explained.

[0051] Steel pipe thickness is 40mm or more (optimal condition) When seamless steel pipes are used as pressure vessels, the thicker the steel pipe, the smaller the stress that is generated on the pressure vessel, making it possible to design pressure vessels that can withstand higher pressures. Therefore, the steel pipe of the present invention preferably has a wall thickness of 40 mm or more. The steel pipe wall thickness is more preferably 45 mm or more, and even more preferably 50 mm or more.

[0052] 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 on the inner and outer surfaces of the steel pipe can be considered to be the same, so there is no clear difference in the temperature gradient from the center of the wall thickness to the outer surface and the temperature gradient from the center of the wall thickness to the inner surface.However, when comparing the difference in circumferential length based on the center of the wall thickness, the value is positive on the outer surface but negative on the inner surface. During hardening, even after the transformation from a face-centered cubic structure to a body-centered cubic structure is complete, a temperature gradient remains, and the inner and outer surfaces thermally shrink first. In other words, the difference in perimeter from the center of the wall decreases. The outer surface has a positive perimeter difference, 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 perimeter difference, so the absolute value of the difference increases, and the generated thermal stress increases. In other words, as the wall thickness increases, the risk of cracking on the inner surface increases, so a smaller wall thickness is preferable. Therefore, the wall thickness is preferably 70 mm or less, more preferably 65 mm or less, and even more preferably 60 mm or less.

[0053] Next, inclusions in seamless steel pipes will be described.

[0054] Inclusions are harder and less likely to deform than bulk Fe, so they are known to become a source of stress concentration when thermal stress occurs, increasing the risk of quench cracking.

[0055] 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 1 / 6 position of the wall thickness is 10 per 100 mm 2 Below (optimal conditions) It is known that there are various types of inclusions, such as sulfide-based inclusions, oxide-based inclusions, and nitride-based inclusions. In the rolling process of the pipe-making process, low-hardness inclusions elongate during rolling, so they can be characterized by the lengths of their long and short sides when the shape of the inclusion is assumed to be elliptical. The value obtained by dividing the length of the long side by the length of the short side is defined as the aspect ratio. The higher the hardness of the inclusions, the smaller the elongation during rolling and the smaller the aspect ratio. On the other hand, the higher the hardness of the inclusions, the higher the risk of quench cracking. In particular, on the inner surface of the steel pipe where segregation exists, that is, from the inner surface of the steel pipe to the 1 / 6 position of the wall thickness, inclusions with an aspect ratio of 2 or less after rolling and a long side length of 10 μm or more act as starting points for quench cracking, increasing the risk of quench cracking. In other words, the number of inclusions with a small aspect ratio is important to reduce the risk of quench cracking. When 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 If the number of inclusions with an aspect ratio of 2 or less and a long side length of 10 μm or more is 10 per 100 mm from the inner surface of the steel pipe to the position at 1 / 6 of the wall thickness, irregular quench cracks will not occur. 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 It is more preferable that the number of pieces is 5 pieces / 100 mm or less. 2 It is even more preferable if it is less than 4.8 pieces / 100mm 2 The lower limit of the number of inclusions is not particularly limited, but from the viewpoint of cost reduction, it is preferable that the number of inclusions is 1 / 100 mm 2 The number of inclusions can be measured by the method described in the examples.

[0056] As with macrosegregation, the amount of inclusions is greater at the center of the billet, which is the final solidification area. Therefore, in seamless steel pipes obtained using billets, the number of inclusions is greatest on the inner surface of the seamless steel pipe. As mentioned above, the inner surface of seamless steel pipes has a high risk of quench cracking due to the effects 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 inclusions can be controlled by controlling the amount of reduction when manufacturing seamless steel pipes from the raw material (billet).

[0057] Next, the cross-sectional hardness of a seamless steel pipe will be explained.

[0058] Since the seamless steel pipe of the present invention is intended for use in pressure vessels, from the viewpoint of fatigue design, the cross-sectional hardness is preferably 480 HV or more. The cross-sectional hardness is more preferably 520 HV or more, even more preferably 530 HV or more, and most preferably 540 HV or more. On the other hand, the upper limit of the cross-sectional hardness is not particularly limited, but it is preferably 700 HV or less, more preferably 650 HV or less. The cross-sectional hardness can be measured by the method described in the Examples.

[0059] Next, the method for producing a seamless steel pipe of the present invention will be described.

[0060] Material (billet) From the viewpoint of productivity, it is preferable to use a material (billet) produced by continuous casting for the production of seamless steel pipes. In continuous casting, if the mold is large, it becomes difficult to control the solidification process. Therefore, for 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, there is no particular lower limit for the billet diameter, but it is preferably 320 mm or more, and more preferably 330 mm or more. Furthermore, since macrosegregation in a billet can be improved by hot forging, it is also effective to hot forge, for example, a rectangular material produced by continuous casting into a circular material (billet).

[0061] (Pipe making process) Pipe manufacturing is carried out so that the ratio of the steel pipe inner diameter divided by the wall thickness is 2.0 or more. Macrosegregation at the billet center can be dissipated by plastic deformation during the piercing process of producing a seamless steel pipe by rolling a solid billet into a hollow seamless steel pipe. When rolling a seamless steel pipe into a pipe by rolling a billet into a hollow seamless steel pipe, increasing the inner diameter can increase the amount of plastic deformation. The ratio of the inner diameter of the steel pipe divided by the wall thickness is preferably 2.0 times or more. Since the larger this ratio, the greater the amount of plastic deformation, it is preferably 3.0 times or more, more preferably 4.0 times or more, even more preferably 4.1 times or more, and most preferably 4.2 times or more. The upper limit of this ratio is not particularly limited, but from the viewpoint of ensuring sufficient rolling reduction, it is preferably 8.0 times or less, and more preferably 7.0 times or less.

[0062] 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.

[0063] In the pipe-making process, the shape of the steel pipe satisfies formula (2) (favorable condition) (EXP (coefficient K × 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) (In formula (3), [element] represents the content (mass%) of the element written in [ ].) Increasing the wall thickness of a steel pipe increases the area of ​​the circumferential cross section. In other words, increasing the volume of a seamless steel pipe increases the total heat capacity of the seamless steel pipe, which also has the effect of reducing the cooling rate during quenching and reducing the risk of quench cracking. As described above, an increase in wall thickness has both positive and negative effects on the risk of quench cracking, and it is preferable to optimize the balance between these effects and take productivity into consideration. Furthermore, the temperature difference between the inner and outer surfaces of seamless steel pipe and the center of the wall thickness is affected by thermal conductivity, and is therefore influenced by the chemical composition. Generally, the physical properties (thermal conductivity) of carbon steel, manganese steel, chromium steel, nickel steel, etc. are measured, 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 more preferably 208 or less, and even more preferably 205 or less. Furthermore, from the perspective of pipe-making efficiency, the left-hand side of formula (2) is preferably 40 or more. The coefficient K in formula (2) can be calculated using formula (3) above.

[0064] (Quenching process) Heat to a temperature range of 860°C or higher and maintain that temperature range for 30 minutes or more In the following, all temperatures mentioned refer to the 1 / 4 position from the inside unless otherwise specified. Just before quenching, the material is kept at a temperature in the austenite region. Because this is a temperature range in which the material exists as a solid, it is not expected that macrosegregation will dissipate, but since atomic diffusion occurs actively, it is expected that contained elements will be detrapped from the grain boundaries, which is effective in improving microsegregation. The frequency of diffusion depends on the temperature and time. If the temperature is below 860°C, the detrapping effect is weak, so the temperature is set to 860°C or higher. The temperature is preferably set to 865°C or higher, more preferably 870°C or higher, and even more preferably 875°C or higher. On the other hand, an increase in 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 set to 950°C or lower, more preferably 930°C or lower, even more preferably 900°C or lower, and most preferably 890°C or lower. For homogenization, the holding time is set to 30 minutes or higher. The holding time is preferably set to 35 minutes or higher, more preferably 40 minutes or higher, and even more preferably 45 minutes or higher. Furthermore, an increase in the holding time leads to a decrease in productivity, so the holding time is preferably set to 90 minutes or lower, more preferably 60 minutes or lower, and even more preferably 55 minutes or lower.

[0065] Water cooling under conditions where the average cooling rate in the temperature range of 800 to 350°C is 3°C / sec or more The above-mentioned heating and water cooling after holding are essential to ensure a martensite area fraction of 85% or more. Additionally, the average cooling rate in the temperature range of 800 to 350°C is 3°C / s or more. Since a faster average cooling rate is expected to increase the martensite area fraction, a rate of 4°C / s or more is preferred, 5°C / s or more is more preferred, 5.5°C / s or more is even more preferred, and 6°C / s or more is most preferred. While there is no particular upper limit to the average cooling rate, it is preferably 15°C / s or less, and more preferably 10°C / s or less. The average cooling rate can be calculated by dividing the temperature difference in the temperature range of 800 to 350°C by the time required to cool within this temperature range.

[0066] During the water cooling, the steel pipe was rotated in the circumferential direction and the inner surface of the steel pipe was coated with a 0.5m 3 / min or more flow rate of the cooling medium It is important to remove the steam film that occurs during water cooling. The outer surface of seamless steel pipes has a high degree of freedom in terms of equipment, and it is possible to remove the steam film even with equipment with a simple structure, but there are many restrictions on equipment when removing the steam film on the inner surface of steel pipes. Therefore, it is necessary to bring the cooling medium into contact with the inner surface of the steel pipe. For example, an appropriate method such as running a jet stream of cooling medium can be used, and the method is 0.5m 3 / min or more. The flow rate is 0.55 m 3 / min or more is preferable, and 0.58m 3 / min or more is more preferable, and 0.6m 3 / min or more is more preferable, and 0.65m 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 recommended to set it at 1.5 m 3 / min or less. In addition, in order to shorten the time during which thermal stress is generated, 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 in the early stage of injection, the flow rate should be reduced to 0.5 m / min or less after 90 seconds from injection. 3 / min and less than 0.05m 3 / min or more. In addition, it is preferable that the surface temperature of the steel pipe after the end of spraying is 50°C or less. Since the cooling medium needs to have a sufficient cooling function, water at 50°C or less can 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.

[0067] 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.

[0068] The quenching process refers to a series of processes including heating, holding, and water cooling after the pipe-making process.

[0069] Since seamless steel pipes are cooled while rotating at high speed, it is difficult to directly measure the temperature using a general method of measuring temperature by attaching a thermocouple and calculate the average cooling rate during quenching. Therefore, a CCT diagram can be prepared and used to determine the average cooling rate by correlating it with the hardness measured on the steel pipe after quenching.

[0070] 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.

[0071] In other embodiments of the present invention, further processing described below can be performed.

[0072] The decarburized layer that forms on the surface of steel pipes during quenching 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.

[0073] 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.

[0074] 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 point 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. While the outer surface does not have the same macrosegregation as the inner surface, it is still 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, a value of 0.10 or more is preferred.

[0075] Next, the pressure vessel and the method for manufacturing the pressure vessel will be described.

[0076] 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.

[0077] 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. [Example]

[0078] Next, an example will be described.

[0079] Heated material (billet) with the chemical composition shown in Table 1 was pierced, expanded, and rolled to form a pipe so that the ratio of the steel pipe inner diameter to the wall thickness was 2.0 or more, as shown in Table 2, and the pipe was quenched under the conditions shown in Table 2 to produce seamless steel pipes with the outer diameter and wall thickness shown in Table 2. In the cooling process during quenching, the steel pipe was immersed in water and rotated in the circumferential direction of the pipe, and a 0.5 mm diameter hole was formed on the inner surface of the steel pipe. 3 The test was carried out while the cooling medium was in contact with the pipe at a flow rate of 1 / min or more. Test specimens were taken from the center of the longitudinal direction of each seamless steel pipe to observe the structure, evaluate the degree of Mn segregation (EPMA observation), and investigate inclusions.

[0080] (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, and a thermocouple was attached to the center of the wall. During the water-cooling process of the steel pipe, 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 difference in cooling rate in the circumferential direction is reduced by the rotation of the steel pipe. 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.

[0081] (structural 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. The plane consisting of the wall thickness direction and the circumferential direction (the so-called "C cross section") was used as the observation surface (10 mm × 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 the 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, and that the prior γ grains were in a fine lath-like state, indicating a structure dominated by 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 of martensite was calculated by subtracting the area fractions of retained austenite, ferrite, and pearlite from the total (defined as 100%).

[0082] (Inclusion) The inclusions were investigated in the area from the inner surface of the steel pipe to 1 / 6 of the wall thickness, which is the segregation area on the inner surface 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 and wall thickness directions of the steel pipe (the so-called "L cross section") was the observation surface, and after mirror polishing, a 10 mm x 10 mm area (100 mm) was observed under an optical microscope. 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 steel pipe of each level, 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 determined in accordance with JIS G0555:2020 (Microscopic examination method for non-metallic inclusions in steel).

[0083] (Mn segregation) To evaluate the degree of Mn segregation, test specimens were used, with the cross section of the steel pipe in the C direction (perpendicular to the rolling direction) being 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 of the way in the wall thickness direction from the outer surface of the steel pipe. The analysis was then performed as described below. 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, centered at a position 1 / 4 of the way in the wall thickness direction from the outer surface of the steel pipe. The analysis was then performed as described below. The Mn concentration mapping analysis was performed with an analysis pitch determined so that 40,000 mapping data points were obtained for Region 1 and Region 2. Next, the obtained 40,000 mapping data points were statistically analyzed to calculate the degree of Mn segregation. The reference local Mn concentration was the average value of all mapping data obtained in region 1, excluding outliers. Outliers were mapping data that were less than 0.05 times and more than 5 times the Mn concentration (Mn content) obtained from the steel pipe check analysis. The segregated Mn concentration was the average value of the top 1% of mapping data with the highest Mn concentration, excluding outliers from all mapping data obtained in region 2. In this case, mapping analysis was performed in three fields of view for region 1, and the local Mn concentrations calculated in the three fields were summed and divided by 3 to obtain the local Mn concentration. Similarly, mapping analysis was performed in three fields of view for region 2, and the segregated Mn concentrations calculated in the three fields of view were summed and divided by 3 to obtain the segregated Mn concentration. The ratio of the segregated Mn concentration to the local Mn concentration obtained in this way was calculated as the degree of Mn segregation. The above mapping analysis was carried out by EPMA as described in the embodiment.

[0084] (cross section hardness) For cross-sectional hardness, a test piece was taken so that the evaluation position was 1 / 4 of the way through the wall from the outer surface of the steel pipe, and in accordance with JIS2244 "Vickers hardness test - Test method," the hardness was measured at five points at 1 / 4 of the way through the wall from the outer surface of the steel pipe using a load of 10 kgf, and the average value was taken as the hardness.

[0085] (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 met 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 crack resistance was evaluated. That is, the presence or absence of quench cracking in the obtained steel pipes was confirmed, and the percentage of the number of steel pipes that did not develop quench cracking out of the 20 steel pipes, i.e., (number of steel pipes without quench cracking / 20) × 100 (%), was taken as the quench crack pass rate. A quench crack pass rate of 90% or more was judged to be acceptable.

[0086] As can be seen from the results shown in Table 2, seamless steel pipes that meet the conditions of the present invention have a metal structure with a high martensite fraction, but also satisfy the specified Mn segregation level, and have a quench crack pass rate of 90% or more in heat treatment, demonstrating excellent quench crack resistance.

[0087] As described above, the seamless steel pipe of the present invention has sufficient quench cracking resistance despite the high martensite fraction. Furthermore, the high martensite fraction can be judged to have excellent fatigue strength. This makes it possible to supply thick-walled seamless steel pipes.

[0088] [Table 1]

[0089] [Table 2]

Claims

1. In mass%, C: 0.20-0.60%, Si: 0.01-2.0%, Mn: 0.5-1.2%, S: 0.010% or less, O: 0.005% or less, N: 0.010% or less, Al: 0.01-0.08%, Mo: 0.005-1.0%, Cr: 0.005-3.0%, Ni: 0.005 to 3.0%; P satisfies the following formula (1), The balance contains Fe and unavoidable impurities, and The degree of Mn segregation from the inner surface of the steel pipe to the 1 / 6 wall thickness position is less than 1.50, The metal structure has an area fraction of martensite of 85% or more. Seamless steel pipe. [P]≦0.042-0.002[C]-0.024[Mn]-0.001[Mo] -0.001[Cr]-0.002[Ni]...(1) (In formula (1), [element] represents the content (mass%) of the element written in [ ].)

2. The component composition is, in mass%, further comprising: Ti: 0.0001 to 0.1%, Cu: 0.0001 to 5.0%, Co: 0.0001 to 5.0%, B: 0.0001-0.01%, V: 0.0001-1.0%, W: 0.0001-5.0%, Nb: 0.0001-0.1%, Zr: 0.0001 to 0.2%, Hf: 0.0001-0.2%, Ta: 0.0001-0.2%, Sb: 0.0001 to 0.1%, Sn: 0.0001-0.1%, Ca: 0.0001-0.01%, Mg: 0.0001-0.01%, 2. The seamless steel pipe according to claim 1, further comprising one or more selected from the group consisting of REM: 0.0001 to 0.5%.

3. 2. The seamless steel pipe according to claim 1, wherein the wall thickness of the steel pipe is 40 mm or more.

4. A seamless steel pipe as described in claim 2, having a steel pipe wall thickness of 40 mm or more.

5. 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 having the following: The seamless steel pipe according to claim 1.

6. 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 having the following: The seamless steel pipe according to claim 2.

7. 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 or less. The seamless steel pipe according to claim 3.

8. 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 or less. The seamless steel pipe according to claim 4.

9. The seamless steel pipe according to any one of claims 1 to 8, having a cross-sectional hardness of 480 HV or more.

10. A method for producing a seamless steel pipe according to any one of claims 1 to 8, The heated material is heated to a temperature range of 860°C or higher, held at that temperature range for 30 minutes or more, and water-cooled under conditions where the average cooling rate in the temperature range of 800 to 350°C is 3°C / sec or higher. During the water-cooling process, the steel pipe is rotated in the circumferential direction of the pipe, and a 0.5 mm diameter is formed on the inner surface of the steel pipe. ３ and a quenching step of cooling the seamless steel pipe while contacting it with a cooling medium at a flow rate of at least 1 / min.

11. A method for manufacturing a seamless steel pipe according to claim 9, The method for manufacturing a seamless steel pipe comprises a pipe-making process in which the heated material is made 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, and a quenching process in which the material is heated to a temperature range of 860°C or more, held at that temperature range for 30 minutes or more, and water-cooled under conditions such that the average cooling rate in the temperature range of 800 to 350°C is 3°C / sec or more, and the steel pipe is rotated in the circumferential direction during the water-cooling process while a cooling medium is brought into contact with the inner surface of the steel pipe at a flow rate of 0.5 m3 / min or more.

12. 11. The method for producing a seamless steel pipe according to claim 10, wherein the steel pipe is produced so that the shape of the steel pipe satisfies the following formula (2): (EXP (coefficient K × 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 (2), the thickness is in mm and the area is in mm 2 . (In formula (3), [element] represents the content (mass%) of the element written in each bracket.)

13. A method for manufacturing a seamless steel pipe as described in claim 11, wherein the steel pipe is manufactured so that its shape satisfies the following equation (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 (2), the thickness is in mm and the area is in mm 2 . (In formula (3), [element] represents the content (mass%) of the element written in each bracket.)

14. A pressure vessel using the seamless steel pipe according to any one of claims 1 to 8.

15. A pressure vessel using the seamless steel pipe described in claim 9.

16. A method for manufacturing a pressure vessel, comprising processing the seamless steel pipe according to any one of claims 1 to 8.

17. A method for manufacturing a pressure vessel, which comprises processing the seamless steel pipe described in claim 9.

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