Seamless steel pipe and method for manufacturing the same

Abstract

The purpose of the present invention is to provide a seamless steel pipe having high strength and excellent hydrogen embrittlement resistance and a method for manufacturing said seamless steel pipe.　The seamless steel pipe has a specific component composition and has a structure in which tempered martensite accounts for 95% or more in terms of surface area ratio. The structure has precipitates, 50 mass% or more of Mo in the structure is contained in the precipitates, the precipitates include a precipitate having a maximum length of 100 nm or less, 10 mass% or more of the Mo is contained in the precipitates having a maximum length of 100 nm or less, and the value of a dislocation configuration parameter M is 0.45 or less.

Description

[Technical Field]

[0001] The present invention relates to a seamless steel pipe and a method for manufacturing the same. [Background technology]

[0002] Currently, a variety of initiatives are being considered worldwide to realize a decarbonized society. One of these is the use of hydrogen energy. Fuel cell vehicles use hydrogen as fuel, so they do not emit carbon dioxide (CO2) and are also energy efficient. Therefore, the widespread adoption of fuel cell vehicles is important for realizing a decarbonized society. In order to promote the adoption of these fuel cell vehicles, there is a need for storage containers (accumulators) with excellent strength and durability that can safely store hydrogen at high pressures of 35 MPa or more, especially 70 MPa or more, for use at hydrogen stations to supply hydrogen to fuel cell vehicles and for on-board use to carry hydrogen into fuel cell vehicles, and development of such containers is underway.

[0003] Because vehicle-mounted pressure accumulators require lightweight construction, pressure accumulators have been proposed in which a liner made of a lightweight material such as aluminum (Al) is coated with carbon fiber reinforced polymer (CFRP). For example, Patent Document 1 describes a liner made of an Al alloy with excellent fatigue properties.

[0004] On the other hand, since weight reduction is not necessary for accumulators used in hydrogen stations, liners made of low-alloy steel have been proposed. For example, Patent Document 2 proposes an accumulator in which a Cr-Mo steel liner is covered with carbon fiber or glass fiber. However, low-alloy steels, including Cr-Mo steel, are known to become brittle due to hydrogen (hydrogen embrittlement). Therefore, for high-pressure hydrogen accumulators of 35 MPa or higher, Al alloys and SUS316, which are less susceptible to hydrogen embrittlement, are recommended. However, Cr-Ni austenitic stainless steel has low strength (below 800 MPa). Therefore, if Cr-Ni austenitic stainless steel is used as the material for an accumulator and, for example, the hydrogen pressure is increased to 70 MPa, the wall thickness of the accumulator needs to be made extremely thick, increasing the weight of the accumulator. This results in high material costs and poor economic efficiency. In addition, in the case of automotive accumulators, there are restrictions on the weight and / or size of the accumulator, so it becomes necessary to reduce the size of the accumulator, i.e., the volume of the accumulator, which reduces the amount of hydrogen that can be stored in the accumulator.

[0005] Therefore, much research is underway to apply low-alloy steel, which has lower material costs, as a material for high-pressure hydrogen accumulators. For example, Patent Documents 3 and 4 propose steel for high-pressure hydrogen gas environments that utilizes fine V-Mo carbides in the steel as hydrogen trapping sites to suppress hydrogen embrittlement caused by diffusible hydrogen. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2009-024225 [Patent Document 2] Japanese Patent Publication No. 2009-293799 [Patent Document 3] Japanese Patent Publication No. 2010-037655 [Patent Document 4] Japanese Patent Publication No. 2009-074122 [Overview of the Initiative] [Problems that the invention aims to solve]

[0007] However, the steels for high-pressure hydrogen gas environments described in Patent Documents 3 and 4 had room for improvement in terms of hydrogen embrittlement resistance.

[0008] This invention has been made in view of the above circumstances, and aims to provide a seamless steel pipe with high strength and excellent hydrogen embrittlement resistance, and a method for manufacturing the same. [Means for solving the problem]

[0009] High-pressure hydrogen accumulators require both high strength and resistance to hydrogen embrittlement. Therefore, various factors affecting the strength and hydrogen embrittlement resistance of steel materials were investigated. As a result, the inventors found that hydrogen embrittlement in steel materials with high tensile strength (TS) occurs when hydrogen accumulates at the grain boundaries, leading to fracture along the grain boundaries (grain boundary fracture). Furthermore, it was found that the accumulation of hydrogen at grain boundaries occurs when pressure fluctuations due to hydrogen pressure inside the accumulator cause pressure deformation (expansion, etc.) within the accumulator, and hydrogen trapped in mobile dislocations moves along with the mobile dislocations when they move to the grain boundaries. From the above mechanism, it was found that stabilizing the dislocations contained in the steel material (steel pipe), that is, reducing mobile dislocations by converting mobile dislocations to immobile dislocations, is effective in suppressing the occurrence of grain boundary fracture, i.e., improving hydrogen embrittlement resistance. Further investigation revealed that it is important to control the dislocation arrangement parameter M value, which is an indicator of dislocation stabilization, to 0.45 or less. Furthermore, precipitates, particularly Mo-based precipitates, are known to have the effect of improving strength and acting as hydrogen trapping sites. As a result of our investigations, we have found that by increasing not only the amount of Mo-based precipitates in the steel material, but especially the amount of fine Mo-based precipitates (maximum length of 100 nm or less), diffusible hydrogen can be more strongly trapped in the Mo-based precipitates, making it more difficult for hydrogen to move to the grain boundaries, and further improving hydrogen embrittlement resistance. Furthermore, we discovered that the amount of Mo-based precipitates can be controlled by adjusting the ratio of C and Mo contained in the steel pipe to an appropriate level.

[0010] This invention was completed based on the above findings, and its gist is as follows. [1] In mass%, C: 0.20~0.50%, Si: 0.05~1.00%, Mn: 0.30~1.50%, P: 0.015% or less, S: 0.005% or less, Al: 0.150% or less, N: 0.006% or less, Cr: 0.10~1.70%, Mo: more than 1.0% and 3.0% or less, Nb: 0.001~0.020%, B: 0.0003~0.0030%, O: 0.0030% or less, and Ti: 0.003~0.025%, the balance consists of Fe and unavoidable impurities, and the steel has a component composition in which Mo / C, which is the ratio of said Mo to said C, is 2.0~12.0, the steel has a structure in which tempered martensite accounts for 95% or more in terms of area fraction, said structure includes precipitates, 50% by mass or more of said Mo is contained in said precipitates, further, said precipitates include precipitates having a maximum length of 100 nm or less, 10% by mass or more of said Mo is contained in the precipitates having a maximum length of 100 nm or less, the dislocation arrangement parameter M value is 0.45 or less, seamless steel pipe. [2] Further, in terms of mass%, said component composition: V: 0.30% or less, Cu: 1.00% or less, Ni: 2.0% or less, Sn: 0.50% or less, W: 3.0% or less, and Ca: 0.0050% or less contains at least one selected from The seamless steel pipe according to [1].[・ [3] A steel pipe material having the component composition described in [1] or [2] above is heated at a heating temperature of 1050°C to 1350°C, and then hot-rolled at a hot-rolling completion temperature of 600°C to 950°C to obtain a seamless steel pipe. The seamless steel pipe is cooled to a first cooling stop temperature of 200°C or less. The seamless steel pipe after cooling is reheated to a reheating temperature of 1000°C or higher than the Ac3 transformation point, and then quenched to a second cooling stop temperature of 200°C or lower, and subjected to a reheating and quenching treatment once or more times. A method for manufacturing seamless steel pipes, comprising tempering the seamless steel pipe after the reheating and quenching treatment under the conditions that the average heating rate from the second cooling stop temperature to a tempering temperature of 600°C to 740°C is 0.5°C / min or more, and the holding time at the tempering temperature is 10 minutes or more but less than 60 minutes. [Effects of the Invention]

[0011] According to the present invention, a seamless steel pipe with high strength and excellent hydrogen embrittlement resistance, and a method for manufacturing the same, can be obtained. The seamless steel pipe of the present invention can be used very suitably as a seamless steel pipe for high-pressure hydrogen accumulators. [Modes for carrying out the invention]

[0012] The embodiments of the present invention will be described below. The following description illustrates a preferred embodiment of the present invention, and the present invention is not limited in any way by the following description.

[0013] [Component composition] First, the reason for limiting the component composition of the seamless steel pipe (sometimes abbreviated as steel pipe) of the present invention will be explained. Hereafter, mass percentages in the component composition will be simply expressed as %.

[0014] C: 0.20~0.50% Carbon (C) contributes to increasing the strength of the steel pipe through solid solution, improves the hardenability of the steel pipe, and contributes to the formation of a martensite-dominant microstructure during quenching. Furthermore, it contributes to the formation of Mo-based precipitates. To obtain these effects, the C content should be 0.20% or more. Preferably, the C content should be 0.22% or more, more preferably 0.25% or more, even more preferably 0.28% or more, and most preferably 0.30% or more. On the other hand, if the C content exceeds 0.50%, cracks will occur during quenching, and manufacturability will be significantly reduced. For this reason, the C content should be 0.50% or less. Preferably, the C content should be 0.45% or less, more preferably 0.40% or less, even more preferably 0.38% or less, and most preferably 0.35% or less.

[0015] Si: 0.05~1.00% Si is included as a deoxidizing agent, but if the Si content is less than 0.05%, the deoxidizing effect is insufficient. For this reason, the Si content should be 0.05% or more. Preferably, the Si content should be 0.10% or more, more preferably 0.15% or more, even more preferably 0.20% or more, and most preferably 0.25% or more. On the other hand, if the Si content exceeds 1.00%, the hardness of the steel pipe becomes too high and the ductility decreases, so the Si content should be 1.00% or less. Preferably, the Si content should be 0.80% or less. Furthermore, if the Si content exceeds 0.70%, the toughness and weldability deteriorate, so preferably the Si content should be 0.60% or less. Even more preferably, the Si content should be 0.55% or less, and most preferably 0.50% or less.

[0016] Mn: 0.30~1.50% Like carbon (C), manganese (Mn) is an element that improves the hardenability of steel pipes and contributes to increasing their strength. To obtain such effects, the Mn content should be 0.30% or more. Preferably, the Mn content should be 0.40% or more, more preferably 0.45% or more, even more preferably 0.50% or more, and most preferably 0.60% or more. On the other hand, Mn is an element that segregates in steel pipes and locally hardens them. If a large amount of Mn is present, it forms localized hardened regions and reduces hydrogen embrittlement resistance. For this reason, the Mn content should be 1.50% or less. Preferably, the Mn content should be 1.30% or less, more preferably 1.00% or less, even more preferably 0.90% or less, and most preferably 0.80% or less.

[0017] P:0.015% or less Although phosphorus (P) is present in steel pipes as an unavoidable impurity, it is an element that not only segregates at grain boundaries in the steel pipe structure, causing grain boundary embrittlement, but also segregates and locally hardens the steel pipe, so it is preferable to reduce it as much as possible. However, a P content of up to 0.015% is acceptable. For this reason, the P content should be 0.015% or less. Preferably, the P content should be 0.013% or less, more preferably 0.010% or less, even more preferably 0.008% or less, and most preferably 0.006% or less. On the other hand, since it is desirable to reduce P as much as possible, the lower limit of the P content is not particularly limited and may be 0%. However, excessive reduction of P leads to an increase in refining costs. For this reason, the P content should preferably be 0.0001% or more, more preferably 0.001% or more, and even more preferably 0.002% or more.

[0018] S: 0.005% or less S is present in steel pipes as an unavoidable impurity, but most of it exists as sulfide-based inclusions, which reduce ductility, toughness, and even SCC resistance (stress corrosion cracking (SCC) resistance), so it is preferable to reduce it as much as possible. However, an S content of up to 0.005% is acceptable. Therefore, the S content should be 0.005% or less. Preferably, the S content should be 0.004% or less, more preferably 0.003% or less, and even more preferably 0.002% or less. On the other hand, since it is desirable to reduce S as much as possible, the lower limit of the S content is not particularly limited and may be 0%. However, excessive reduction of S leads to an increase in refining costs. Therefore, preferably, the S content should be 0.0002% or more, more preferably 0.001% or more.

[0019] Al: 0.150% or less Al is an element that acts as a deoxidizing agent and is commonly used in the molten steel deoxidation process. However, if the Al content exceeds 0.150%, the cleanliness and toughness of the steel pipe decrease. Therefore, the Al content should be 0.150% or less. Preferably, the Al content should be 0.110% or less, more preferably 0.090% or less, even more preferably 0.070% or less, and most preferably 0.050% or less. On the other hand, the lower limit of the Al content is not particularly limited and may be 0%. However, from the viewpoint of enhancing the effect of Al content, preferably, the Al content should be 0.005% or more, more preferably 0.007% or more, even more preferably 0.009% or more, and most preferably 0.010% or more.

[0020] N: 0.006% or less Nitrogen (N) is present in steel pipes as an unavoidable impurity. It combines with Al in the steel pipes to form AlN, and with Ti in the steel pipes to form TiN, thereby refining the prior austenite grains and improving toughness. However, if the N content exceeds 0.006%, the formed nitrides such as AlN and TiN become coarser, significantly reducing toughness. For this reason, the N content should be 0.006% or less. Preferably, the N content should be 0.005% or less, more preferably 0.004% or less, and even more preferably 0.003% or less. On the other hand, the lower limit of the N content is not particularly limited and may be 0%. However, from the viewpoint of enhancing the effect of N content, the N content should preferably be 0.001% or more, more preferably 0.002% or more.

[0021] Cr: 0.10~1.70% Cr is an element that increases the strength of steel pipes by improving hardenability and also improves corrosion resistance. Furthermore, Cr combines with C during tempering to form M3C, M7C3, and M 23 It forms precipitates such as C6 (where M is a metallic element), and is an essential element, especially for increasing the strength of steel pipes. To obtain these effects, the Cr content should be 0.10% or more. Preferably, the Cr content should be 0.15% or more, more preferably 0.17% or more, even more preferably 0.19% or more, and most preferably 0.20% or more. On the other hand, if the Cr content exceeds 1.70%, a large amount of Fe3C, M7C3, M will form. 23 It forms carbides such as C6, which act as hydrogen trapping sites, reducing resistance to hydrogen erosion. Furthermore, a high Cr content leads to coarsening of Mo-based precipitates. This coarsening of Mo-based precipitates is due to the aggregation and coalescence of fine Mo-based precipitates, which reduces the number of fine Mo-based precipitates and thus reduces hydrogen embrittlement resistance. For this reason, the Cr content should be 1.70% or less. Preferably, the Cr content should be 1.50% or less, more preferably 1.30% or less, even more preferably 1.00% or less, and most preferably 0.50% or less.

[0022] Mo: Over 1.0% and under 3.0% Mo is an element that forms precipitates (Mo-based precipitates) and contributes to increasing the strength of steel pipes through precipitation strengthening. Furthermore, the formed Mo-based precipitates function as hydrogen trapping sites, thus effectively improving hydrogen embrittlement resistance. In addition, the formed Mo-based precipitates stabilize dislocations through their tempering softening resistance, effectively contributing to achieving the desired strength by reducing the ratio of mobile dislocations to the total number of dislocations. Mo also contributes to improving hydrogen embrittlement resistance by dissolving in the steel pipe and segregating at prior austenite grain boundaries. To obtain these effects, the Mo content should exceed 1.0%. Preferably, the Mo content is 1.1% or more, more preferably 1.2% or more, even more preferably 1.3% or more, and most preferably 1.4% or more. On the other hand, if the Mo content exceeds 3.0%, the steel pipe becomes too hard, reducing its ductility in air. Therefore, the Mo content should be 3.0% or less. The Mo content is preferably 2.8% or less, more preferably 2.5% or less, and most preferably 1.8% or less.

[0023] Nb: 0.001~0.020% Nb forms precipitates, such as carbonitrides, contributing to increased strength of the steel pipe through precipitation strengthening. Furthermore, Nb refines austenite grains, contributing to improved toughness. To achieve these effects, the Nb content should be 0.001% or higher. Preferably, the Nb content is 0.002% or higher, more preferably 0.003% or higher, even more preferably 0.004% or higher, and most preferably 0.006% or higher. On the other hand, if the Nb content exceeds 0.020%, a large amount of Nb-based precipitates are generated, reducing the hydrogen embrittlement resistance. Therefore, from the viewpoint of achieving both the desired strength and excellent hydrogen embrittlement resistance, the present invention sets the Nb content to 0.020% or lower. Preferably, the Nb content is 0.018% or lower, more preferably less than 0.015%, even more preferably 0.013% or lower, and most preferably 0.010% or lower.

[0024] B: 0.0003~0.0030% B segregates at austenite grain boundaries and suppresses ferrite transformation from the grain boundaries, thereby improving the hardenability, toughness, and strength of steel pipes even in trace amounts. To obtain such effects, the B content should be 0.0003% or more. Preferably, the B content should be 0.0005% or more, more preferably 0.0007% or more, and even more preferably 0.0008% or more. On the other hand, if the B content exceeds 0.0030%, it precipitates as carbonitrides, etc., reducing hardenability and toughness. Furthermore, adding it in excess generates Fe2B, causing red-hot brittleness. For this reason, the B content should be 0.0030% or less. Preferably, the B content should be 0.0025% or less, more preferably 0.0020% or less, even more preferably 0.0018% or less, and most preferably 0.0015% or less.

[0025] Oxygen (O): 0.0030% or less Oxygen (O) is an unavoidable impurity and exists in steel pipes as oxide inclusions. These inclusions become the starting point for cracking in a hydrogen gas environment and reduce hydrogen embrittlement resistance, so it is preferable to reduce the oxygen (O) content as much as possible. However, an oxygen (O) content of up to 0.0030% is acceptable. Therefore, the oxygen (O) content should be 0.0030% or less. Preferably, the oxygen (O) content should be 0.0028% or less, more preferably 0.0025% or less, even more preferably 0.0023% or less, and most preferably 0.0020% or less. On the other hand, the lower limit of the oxygen (O) content is not particularly limited and may be 0%. However, excessive reduction of oxygen (O) leads to an increase in refining costs. Therefore, preferably, the oxygen (O) content should be 0.0001% or more, more preferably 0.0002% or more, even more preferably 0.0003% or more, and most preferably 0.0005% or more.

[0026] Ti: 0.003~0.025% Ti combines with N during the solidification of molten steel to precipitate as fine TiN, and its pinning effect contributes to the refinement of austenite grains, thereby improving toughness and strength. When the Ti content is less than 0.003%, this effect is small, and in order to obtain such an effect, the Ti content should be 0.003% or more. Preferably, the Ti content is 0.005% or more, more preferably 0.007% or more, even more preferably 0.008% or more, and most preferably 0.010% or more. On the other hand, if the Ti content exceeds 0.025%, the TiN becomes coarser, and the above-mentioned pinning effect cannot be exerted, and toughness actually decreases. For this reason, the Ti content should be 0.025% or less. Preferably, the Ti content is 0.020% or less, more preferably 0.019% or less, even more preferably 0.017% or less, and most preferably 0.015% or less.

[0027] The steel pipe in one embodiment of the present invention has a component composition that includes the above elements, with the remainder being Fe and unavoidable impurities. Hereinafter, the steel pipe according to one embodiment of the present invention may contain only the above components and the remainder being Fe and unavoidable impurities. Furthermore, unavoidable impurities are impurities that are inevitably introduced from raw materials, manufacturing processes, or manufacturing equipment, and are permissible to be included in a range that does not hinder the objectives of the present invention. Examples of the raw materials include iron ore, reduced iron, and scrap. Examples of the impurities include Zn, Pb, As, Bi, Mg, Co, and H. The Mg content is permissible if it is 0.0008% or less, and the Co content is permissible if it is 0.0008% or less.

[0028] Mo / C:2.0~12.0 The inventors have found that the ratio of Mo (Mo content) to C (C content) in the steel pipe is an important parameter for controlling the amount of Mo-based precipitates. When Mo / C is less than 2.0, the Mo content is insufficient relative to the C content, and C is consumed in the formation of Fe-based carbides, resulting in an inability to obtain the desired number of Mo-based precipitates. As a result, the amount of trapped hydrogen decreases, and the hydrogen embrittlement resistance deteriorates. In addition, the effect of tempering softening resistance becomes smaller. When the effect of tempering softening resistance is small, the tempering temperature does not shift to the higher side, and tempering treatment is performed at a low temperature in order to achieve the desired strength. As a result, dislocation stabilization does not progress, and the desired M value cannot be obtained. For this reason, Mo / C should be 2.0 or higher. Preferably, Mo / C should be 2.5 or higher, more preferably 3.5 or higher, even more preferably 4.0 or higher, and most preferably 4.5 or higher. On the other hand, when Mo / C exceeds 12.0, the tendency for Mo-based precipitates to coarseen becomes pronounced, and toughness and hydrogen embrittlement resistance deteriorate. Therefore, Mo / C should be 12.0 or less. Preferably, Mo / C should be 10.0 or less, more preferably 8.0 or less, even more preferably 6.0 or less, and most preferably 5.5 or less.

[0029] In other embodiments of the present invention, the above component composition may optionally further include at least one selected from V, Cu, Ni, Sn, W, and Ca.

[0030] V:0.30% or less V is an element that forms precipitates, such as carbonitrides, and contributes to strengthening steel pipes. V also has the effect of refining austenite grains and improving toughness. Therefore, it can be included in any amount depending on the desired properties. However, if the V content exceeds 0.30%, the effect of V content saturates, and the effect commensurate with the V content can no longer be expected, making it economically disadvantageous. For this reason, when V is included, the V content should be 0.30% or less. Preferably, the V content should be 0.28% or less, more preferably 0.25% or less, even more preferably 0.23% or less, and most preferably 0.20% or less. On the other hand, the lower limit of the V content is not particularly limited and may be 0%. However, from the viewpoint of obtaining sufficient effects of V content, preferably, the V content should be 0.01% or more, more preferably 0.02% or more, even more preferably 0.03% or more, and most preferably 0.05% or more.

[0031] Cu: 1.00% or less Cu is an effective element for improving toughness and increasing strength. Therefore, it can be included in any amount depending on the desired properties. However, if the Cu content exceeds 1.00%, elongation deteriorates due to the formation of intermetallic compounds. For this reason, when Cu is included, the Cu content should be 1.00% or less. Preferably, the Cu content is 0.80% or less, more preferably 0.70% or less, even more preferably 0.60% or less, and most preferably 0.50% or less. On the other hand, the lower limit of the Cu content is not particularly limited and may be 0%. However, from the viewpoint of obtaining sufficient effects from the inclusion of Cu, preferably the Cu content is 0.01% or more, more preferably 0.03% or more, and even more preferably 0.05% or more.

[0032] Ni: 2.0% or less Ni is an element that contributes to increasing the strength of steel pipes and improves their toughness and corrosion resistance. Therefore, it can be included in any amount depending on the desired properties. However, if the Ni content exceeds 2.0%, the effect of Ni content saturates, and the effect commensurate with the Ni content cannot be expected, making it economically disadvantageous. For this reason, when Ni is included, the Ni content should be 2.0% or less. Preferably, the Ni content should be 1.8% or less, more preferably 1.5% or less, even more preferably 1.3% or less, and most preferably 1.0% or less. On the other hand, the lower limit of the Ni content is not particularly limited and may be 0%. However, from the viewpoint of obtaining sufficient effects from Ni content, the Ni content should preferably be 0.1% or more, more preferably 0.2% or more.

[0033] Sn: 0.50% or less Tin (Sn) may be present in steel pipes as an unavoidable impurity during the manufacturing of steel pipe materials in electric furnaces. Sn tends to segregate at grain boundaries, and when steel pipes are used in high-temperature environments, it can cause a decrease in strength. In addition, Sn can reduce the ductility of steel pipes and degrade their workability. Therefore, it is preferable to reduce Sn as much as possible. However, a Sn content of up to 0.50% is acceptable. Therefore, if Sn is present, the Sn content should be 0.50% or less. Preferably, the Sn content should be 0.40% or less, and more preferably 0.30% or less. On the other hand, there is no particular lower limit to the Sn content, and it may be 0%. However, excessive reduction of Sn leads to an increase in refining costs. Therefore, the Sn content should preferably be 0.10% or more.

[0034] W:3.0% or less Water (W) is an element that forms precipitates, contributing to increased strength of steel pipes through precipitation strengthening, and also contributes to further improvement of hydrogen embrittlement resistance by solid solution and segregating at prior austenite grain boundaries. Therefore, it can be included in any amount depending on the desired properties. However, if the W content exceeds 3.0%, the effect of W content saturates, and the effect commensurate with the W content cannot be expected, making it economically disadvantageous. Therefore, when W is included, the W content should be 3.0% or less. Preferably, the W content should be 2.5% or less, more preferably 2.0% or less, even more preferably 1.5% or less, and most preferably 1.0% or less. On the other hand, the lower limit of the W content is not particularly limited and may be 0%. However, from the viewpoint of obtaining sufficient effects of W content, the W content should preferably be 0.1% or more, more preferably 0.3% or more, and even more preferably 0.5% or more.

[0035] Ca: 0.0050% or less Ca combines with S to form CaS, an element that effectively controls the morphology of sulfide inclusions, and contributes to improving toughness and hydrogen embrittlement resistance through the control of sulfide inclusion morphology. Therefore, it can be included as desired depending on the required properties. However, if the Ca content exceeds 0.0050%, the effect of Ca content saturates, and the effect commensurate with the Ca content can no longer be expected, making it economically disadvantageous. Therefore, when Ca is included, the Ca content is preferably 0.0050% or less, more preferably 0.0040% or less, even more preferably 0.0030% or less, and most preferably 0.0020% or less. On the other hand, the lower limit of the Ca content is not particularly limited and may be 0%. However, from the viewpoint of obtaining sufficient effects of Ca content, the Ca content is preferably 0.0005% or more, more preferably 0.0008% or more, and even more preferably 0.0010% or more.

[0036] Hydrogen (H) may be introduced into steel pipes during various manufacturing processes. A high amount of H increases the risk of cracking after solidification and degrades hydrogen embrittlement resistance; therefore, it is preferable to reduce the H content of steel pipes. These effects are not problematic if the H content of the steel pipe is 0.00100% or less. Therefore, if the steel pipe contains H, its H content should be preferably 0.00100% or less. The H content of the steel pipe is preferably 0.00080% or less, more preferably 0.00070% or less, and even more preferably 0.00050% or less. On the other hand, the lower the H content of the steel pipe, the better its hydrogen embrittlement resistance; therefore, the lower limit of the H content of the steel pipe is not particularly limited and may be 0% or 0.00001% or more. Note that the H content of the steel pipe refers to the residual hydrogen after the steel pipe is formed. Furthermore, as a steel pipe, it is sufficient to obtain an H content of 0.00100% or less, and the H content contained in the steel pipe material described later is not particularly limited. However, the H content contained in the steel pipe material is preferably 0.00100% or less, more preferably 0.00080% or less, even more preferably 0.00070% or less, and most preferably 0.00050% or less. The lower limit of the H content contained in the steel pipe material is also considered in the same way as for the steel pipe, and the lower limit of the H content contained in the steel pipe material is not particularly limited and may be 0% or 0.00001% or more.

[0037] [Organization] The seamless steel pipe of the present invention has a structure in which tempered martensite accounts for 95% or more by area, and the structure has precipitates, in which 50% or more of the Mo content by mass is contained in the precipitates, the precipitates include precipitates with a maximum length of 100 nm or less, and 10% or more of the Mo content by mass is contained in the precipitates with a maximum length of 100 nm or less.

[0038] Area ratio of tempered martensite is 95% or higher: The seamless steel pipe of the present invention has high tensile strength (TS) and possesses the ductility and toughness necessary for a steel pipe. To achieve this, tempered martensite, obtained by tempering martensite, accounts for 95% or more of the area ratio. Preferably, the area ratio of tempered martensite is 96% or more, more preferably 97% or more, and even more preferably 98% or more. On the other hand, there is no particular upper limit to the area ratio of tempered martensite, and it may be 100%.

[0039] Other organizations: The microstructure in one embodiment of the present invention may consist of tempered martensite. In other embodiments of the present invention, the microstructure may contain other microstructures in addition to tempered martensite at an area ratio of 5% or less. The other microstructures may be any microstructure without particular limitation. For example, the other microstructures may be at least one selected from bainite, retained austenite, and pearlite, and may be a mixture of two or more phases. The lower limit of the area ratio of the other microstructures may be 0%.

[0040] The area ratio of the above-mentioned tissue is determined based on the method described in the examples.

[0041] The microstructure of the seamless steel pipe of the present invention can be adjusted by appropriately controlling the cooling conditions after hot rolling of the steel pipe and the reheating temperature during the reheating and quenching process.

[0042] Old austenite grains: In one embodiment of the present invention, if the particle size number of the prior austenite grains, which serves as an indicator of the structure before martensitic transformation, is less than 8.5, the substructure of the resulting martensite becomes coarser, and its toughness decreases. For this reason, the particle size number of the prior austenite grains is preferably 8.5 or higher, more preferably 8.8 or higher, even more preferably 9.5 or higher, and most preferably 10.0 or higher. On the other hand, there is no particular upper limit to the particle size number of the prior austenite grains, but the particle size number of the prior austenite grains is preferably 18.5 or lower, more preferably 18.0 or lower, and even more preferably 17.5 or lower. The particle size number of the prior austenite grains shall be the value measured in accordance with the provisions of JIS G 0551. Specifically, the particle size number of the prior austenite grains is determined based on the method described in the examples.

[0043] The particle size number of the prior austenite grains in the seamless steel pipe of the present invention can be adjusted by changing the reheating temperature and the number of times the reheating treatment is performed during the reheating quenching process. In mass%, more than 50% of Mo (Mo content) is contained in the precipitate: In the seamless steel pipe of the present invention, it is important that the precipitates contain 50% or more of the Mo content by mass in order to ensure the desired hydrogen embrittlement resistance. If the above percentage is less than 50%, the number of Mo-based precipitates present in the steel pipe will be small, sufficient hydrogen will not be trapped, and the desired hydrogen embrittlement resistance will not be obtained. Therefore, the above percentage should be 50% or more. Furthermore, the above percentage should preferably be 53% or more, more preferably 55% or more, even more preferably 58% or more, and most preferably 60% or more. On the other hand, if the above percentage is too high, the number of Mo-based precipitates present in the steel pipe will be too large, the steel pipe will become hard, its ductility will decrease, and its hydrogen embrittlement resistance will tend to decrease. Therefore, the above percentage should preferably be 80% or less, more preferably 78% or less, even more preferably 77% or less, and most preferably 75% or less.

[0044] In mass%, 10% or more of Mo (Mo content) is contained in precipitates with a maximum length of 100 nm or less: The inventors have found that among Mo-based precipitates, those with a maximum length of 100 nm or less (fine Mo-based precipitates) exhibit particularly strong hydrogen trapping capabilities, and that trapped diffusible hydrogen is less likely to move to other lattice defects during deformation of the steel pipe. Therefore, analysis revealed that it is necessary for at least 10% of the mass percentage of Mo (Mo content) to be contained in precipitates with a maximum length of 100 nm or less. If the above percentage is less than 10%, the number of fine Mo-based precipitates is small, the amount of diffusible hydrogen trapped decreases, and the desired hydrogen embrittlement resistance cannot be obtained. Therefore, the above percentage is set to 10% or more. Preferably, the above percentage is 15% or more, more preferably 20% or more, even more preferably 27% or more, and most preferably 35% or more. On the other hand, the upper limit of the above percentage is not limited and may be 100% or 90% or less. There is no particular upper limit to the size of precipitates exceeding 100 nm, but they may be 600 nm or less. Similarly, there is no particular lower limit to the size of precipitates 100 nm or less, but they may be 5 nm or more.

[0045] The analysis of the precipitate can be performed using the following procedure. First, the amount of Mo contained in the precipitate is determined by the extraction method by filter filtration described in Patent Document 5 and Reference 1. Specifically, a test specimen measuring the total wall thickness × 10 mmC × 10 mmL (where C is the circumferential direction of the pipe and L is the axial direction of the pipe) is taken from the steel pipe to be used as an electrolytic test specimen. After electrolysis of the obtained electrolytic test specimen with an electrolyte solution, the electrolytic test specimen is immersed in a dispersible solution, and ultrasonic waves are applied to extract precipitates adhering to the surface of the electrolytic test specimen into the dispersible solution. Next, the dispersible solution from which the precipitates have been extracted is filtered using a filter with a pore size of 100 nm, and precipitates larger than 100 nm are collected on the filter. The filtrate after filtering is then dried, and precipitates smaller than 100 nm are collected. In this way, the precipitates are separated by size (less than 100 nm and more than 100 nm), and each size-classified precipitate is subjected to acid decomposition. Using ICP emission spectrometry, the absolute amount of Mo contained in each size-classified precipitate (absolute amount of Mo in precipitates more than 100 nm: W1, absolute amount of Mo in precipitates less than 100 nm: W2) is calculated. Then, by dividing W1 and W2 by the weight difference W0 (amount of electrolysis) of the electrolytic test piece before and after electrolysis, the total amount of Mo (mass%) contained in precipitates more than 100 nm and the total amount of Mo (mass%) contained in precipitates less than 100 nm are calculated. By dividing the total amount of Mo (mass%) contained in precipitates exceeding 100 nm and the total amount of Mo (mass%) contained in precipitates 100 nm or less by the amount of Mo contained in the steel pipe (Mo content), the proportion of the amount of Mo contained in the steel pipe that is contained in precipitates exceeding 100 nm and the proportion of Mo contained in precipitates 100 nm or less can be calculated. [Patent Document 5] Japanese Unexamined Patent Publication No. 2010-127791 [Reference 1] Ishida et al., Analysis of the Formation State of Fine Precipitates in Steel, Iron and Steel Vol. 107 No. 8

[0046] In the present invention, hydrogen embrittlement resistance can be further improved by reducing nitride-based inclusions and oxide-based inclusions, such as MnS, which can serve as fracture initiation points. Inclusions such as nitride and oxide inclusions can be reduced by the following control measures during the molten steel refining process. Specifically, desulfurization and dephosphorization should be performed in the molten iron pretreatment, followed by decarburization and dephosphorization in the converter, and then heated stirring refining (LF) and RH vacuum degassing in the ladle. In the heated stirring refining (LF) process, sufficient processing time should be ensured, and in the RH vacuum degassing process, sufficient processing time should also be ensured, and the RH reflux rate should be controlled. Furthermore, when producing cast slabs (steel pipe material) by continuous casting, in order to reduce inclusions, an inert gas seal should be applied when pouring from the ladle to the tundish, and electromagnetic stirring should be performed in the mold to separate the inclusions by flotation. Note that the refining process is not limited to the above, but it is preferable to manufacture under the above conditions.

[0047] Furthermore, in the seamless steel pipe of the present invention, in order to obtain the desired hydrogen embrittlement resistance, it is necessary to stabilize the dislocations, that is, to set the desired dislocation arrangement parameter M value. In the present invention, the dislocation arrangement parameter M value is used as an index representing the degree of dislocation stabilization. Here, the smaller the M value, the more stable the dislocations are, that is, the fewer the number of mobile dislocations relative to the total number of dislocations. When there are fewer mobile dislocations, trapped hydrogen is less likely to move to the grain boundaries, grain boundary cracking is suppressed, that is, the delayed fracture resistance is improved. For this reason, the M value is set to 0.45 or less. The M value is preferably 0.43 or less, more preferably 0.39 or less, even more preferably 0.37 or less, and most preferably 0.35 or less. On the other hand, the lower the M value, the more stable the dislocations are, so the lower limit of the M value is not particularly limited and may be 0, 0.20 or more, 0.25 or more, or 0.30 or more. The M value can be determined by the method shown below. The M value in the seamless steel pipe of the present invention can be adjusted by appropriately controlling the Mo content and tempering temperature of the steel pipe, as well as controlling the hot rolling completion temperature.

[0048] The dislocation arrangement parameter M value is analyzed using an XRD (X-ray diffraction) instrument. A specimen for M value measurement is taken from near the center of the steel pipe wall thickness (t / 2 position, t: wall thickness) with dimensions of 10mmt × 10mmC × 10mmL. At this time, the specimen is taken so that the measurement surface is a cross section parallel to the wall thickness direction and the circumferential direction of the pipe (a cross section perpendicular to the pipe axis direction: C section), and the center of the measurement surface is near the t / 2 position. However, if t is less than 10mm, a specimen is taken with dimensions of total wall thickness × 10mmC × 10mmL. Next, the measurement surface of the taken specimen is mechanically polished. At this time, to prevent strain from mechanical polishing from affecting the M value, approximately 50 μm of the surface layer of the measurement surface after mechanical polishing is removed by electrolytic polishing. Electropolishing can be performed, for example, using a solution prepared with 478 ml of HClO, 700 ml of ethanol, and 120 ml of distilled water, under conditions of 23°C and 27 V, with the test specimen at the anode and the Pt wire at the cathode. The dislocation arrangement parameter M value is analyzed for the measurement surface after electropolishing, according to the modified-Williamson-Hall method (mWH / WA method) described in Reference 2. Specifically, for example, XRD measurements are performed under the following conditions: The XRD measurement is performed using CuKα rays as the X-ray source, with a tube voltage of 45kV and a tube current of 200mA. In the obtained diffraction profile, the 2θ range is set to 35~154°, and the measured diffraction planes are BCC-Fe(110), (200), (211), (220), (310), and (222), with a dislocation density of (1 / m³). 2 Next, calculate the dislocation density (1 / m). 2 The M value is calculated by substituting the magnitude of the strain field Re(m) due to the dislocations into the following formula. M=Re / d Here, d(m): average distance between dislocations (d=1 / √ρ), ρ(1 / m) 2 ): This is the dislocation density. [Reference 2] T. Ungar et al., The effect of dislocation contrast on x-ray line broadening: A new approach to line profile analysis, Applied Physics Letters, Vol. 69 No. 21 p. 3173.

[0049] The seamless steel pipe of the present invention has high tensile strength (TS). Therefore, it can also be used for high-pressure hydrogen containers (high-pressure hydrogen accumulators). The specific value of TS is not particularly limited, but TS is preferably 850 MPa or higher, more preferably 860 MPa or higher, even more preferably 880 MPa or higher, and most preferably 900 MPa or higher. On the other hand, as hydrogen embrittlement resistance tends to decrease as TS increases, TS is preferably 1000 MPa or lower, more preferably 990 MPa or lower. The above TS can be determined based on the method described in the examples.

[0050] In this invention, properties such as microstructure, grain size number of prior austenite grains, dislocation arrangement parameter M value, tensile strength TS, and hydrogen embrittlement resistance are evaluated near the t / 2 position. The reason for evaluating near the t / 2 position is as follows: Since steel pipes are cooled from both the outer and inner surfaces, the cooling rate at the t / 2 position of the steel pipe is the slowest. As a result, it is difficult to obtain the desired microstructure, such as martensite (quenched structure), and it is also difficult to obtain the desired properties. Therefore, if the desired microstructure and properties are obtained near the t / 2 position, it can be determined that the desired microstructure and properties are obtained at any position in the steel pipe.

[0051] Furthermore, the wall thickness of the seamless steel pipe of the present invention is not particularly limited, but is preferably 2.0 mm or more, more preferably 3.0 mm or more, even more preferably 4.0 mm or more, and most preferably 5.0 mm or more. Also, the wall thickness is preferably 35.0 mm or less, more preferably 33.0 mm or less, even more preferably 30.0 mm or less, and most preferably 25.0 mm or less. Seamless steel pipes for high-pressure hydrogen accumulator applications are suitable for high-pressure hydrogen applications. Specifically, they are applicable to high-pressure hydrogen applications where the hydrogen pressure is preferably 1 MPa or higher, more preferably 10 MPa or higher, even more preferably 35 MPa or higher, and most preferably 70 MPa or higher. On the other hand, there is no particular upper limit to the hydrogen pressure, but in practice it may be 115 MPa or lower, 105 MPa or lower, or 95 MPa or lower. Note that the above-mentioned high-pressure hydrogen accumulators include not only accumulators containing 100% hydrogen gas, but also accumulators containing hydrogen-containing gas with less than 100% hydrogen, so the above-mentioned hydrogen pressure refers to either the partial pressure of hydrogen or the total pressure of hydrogen.

[0052] [Manufacturing method] Next, a method for manufacturing a seamless steel pipe according to one embodiment of the present invention will be described. The seamless steel pipe of the present invention can be manufactured by heating and hot-rolling a steel pipe material having the above-described component composition to form a seamless steel pipe, followed by sequential cooling, reheating, quenching, and tempering. In the following description, unless otherwise specified, temperature refers to the temperature at the surface of the steel pipe material or steel pipe. The temperature at the surface is the value measured with a radiation thermometer.

[0053] Steel pipe material: The steel pipe material used in the present invention (hereinafter also simply referred to as a cast slab) is preferably produced by melting molten steel having the above-described component composition using a conventional melting method such as a converter, and then forming a cast slab (round cast slab), such as a billet, using a conventional casting method such as continuous casting. The cast slab may be further hot-rolled to form a round steel billet of a predetermined shape, or it may be a round steel billet that has undergone ingot-block rolling.

[0054] When casting steel pipe material into cast slabs (round cast slabs) using a casting method, the slower the casting speed, the lower the hydrogen concentration and inclusions in the steel pipe. This effect is particularly noticeable at casting speeds of 1.0 m / min or less; therefore, the casting speed is preferably 1.0 m / min or less, more preferably 0.8 m / min or less, and even more preferably 0.7 m / min or less. On the other hand, there is no particular lower limit to the casting speed; the casting speed is preferably 0.01 m / min or more, more preferably 0.1 m / min or more, even more preferably 0.2 m / min or more, and most preferably 0.3 m / min or more.

[0055] [Hot rolling] After heating the above steel pipe material, hot rolling is performed under conditions where the hot rolling completion temperature is between 600°C and 950°C to produce a seamless steel pipe.

[0056] Heating temperature: 1050℃ or higher and 1350℃ or lower The steel pipe material is heated prior to hot rolling. The heating may be performed after the steel pipe material obtained by methods such as casting has been cooled, or the obtained steel pipe material may be subjected to the heating directly without cooling. If the heating temperature of the steel pipe material is below 1050°C, the dissolution of precipitates in the steel pipe material will be insufficient. For this reason, the heating temperature should be 1050°C or higher. Preferably, the heating temperature should be 1060°C or higher, more preferably 1070°C or higher, even more preferably 1090°C or higher, and most preferably 1100°C or higher. On the other hand, if the heating temperature exceeds 1350°C, the prior austenite grains become coarser, precipitates such as TiN that precipitated during solidification become coarser, and cementite also becomes coarser, reducing the toughness of the steel pipe. In addition, a thick scale layer is formed on the surface of the steel pipe material, which can cause surface defects during hot rolling and increases energy loss, which is undesirable from an energy-saving standpoint. For these reasons, the heating temperature should be 1350°C or lower. Preferably, the heating temperature should be 1330°C or lower, more preferably 1300°C or lower, even more preferably 1280°C or lower, and most preferably 1250°C or lower.

[0057] Hot rolling completion temperature: 600°C to 950°C Next, the heated steel pipe material is hot-rolled to form a seamless steel pipe under the condition that the hot-rolling completion temperature is between 600°C and 950°C. For the hot-rolling, a standard Mannesmann-plug mill method or a Mannesmann-mandrel mill method, including perforation rolling, can be used. Here, the hot-rolling completion temperature is between 600°C and 950°C. If the hot-rolling completion temperature is below 600°C, the steel pipe material will be heavily strained, the prior austenite grains will not be refined, and the desired strength and excellent toughness cannot be obtained. Furthermore, if there is a lot of strain, too many dislocations will occur, and even if the tempering treatment described later is applied, the dislocations cannot be controlled, and the desired M value cannot be obtained. For this reason, the hot-rolling completion temperature is set to 600°C or higher. Preferably, the hot-rolling completion temperature is 650°C or higher, more preferably 700°C or higher, and even more preferably 750°C or higher. On the other hand, if the hot rolling completion temperature exceeds 950°C, strain and dislocations are not introduced, prior austenite grains are not refined, and the desired strength and excellent toughness cannot be obtained. Therefore, the hot rolling completion temperature should be 950°C or lower. Preferably, the hot rolling completion temperature should be 930°C or lower, more preferably 900°C or lower, even more preferably 880°C or lower, and most preferably 850°C or lower.

[0058] Alternatively, the pipes may be expanded after hot rolling to produce seamless steel pipes. Expanding the pipes using an expanding rolling mill can improve the roundness of the seamless steel pipes. The expansion ratio can be set appropriately according to the target wall thickness (pipe thickness), outer diameter, strength, and target roundness of the seamless steel pipe.

[0059] [cooling] The resulting seamless steel pipes are cooled to a first cooling stop temperature of 200°C or lower. First cooling stop temperature: 200℃ or less If the first cooling stop temperature exceeds 200°C, the martensitic transformation will not be completely finished, and tempered martensite with the desired area ratio cannot be obtained after the tempering treatment. Therefore, the first cooling stop temperature should be 200°C or lower. Preferably, the first cooling stop temperature is 190°C or lower, more preferably 180°C or lower, even more preferably 160°C or lower, and most preferably 150°C or lower. On the other hand, the lower limit of the first cooling stop temperature is not particularly limited, but it may be above room temperature and may be 50°C or higher. Room temperature refers to 10 to 35°C.

[0060] Furthermore, it is preferable that the average cooling rate from the end of hot rolling (hot rolling end temperature) to the first cooling stop temperature be an average cooling rate equal to or greater than that of air cooling. By setting the average cooling rate equal to or greater than that of air cooling, it is easier to obtain tempered martensite with the desired area ratio after the tempering treatment. Here, in the present invention, "average cooling rate equal to or greater than that of air cooling" refers to 0.1°C / s or higher. If the average cooling rate is less than that of air cooling, i.e., less than 0.1°C / s, the structure after cooling will be non-uniform, and it will be difficult to obtain tempered martensite with the desired area ratio after the subsequent reheating quenching and tempering treatments. The average cooling rate is more preferably 0.2°C / s or higher, and even more preferably 0.3°C / s or higher. On the other hand, there is no particular upper limit to the average cooling rate, but it may be 1.0°C / s or less, 0.8°C / s or less, or 0.5°C / s or less. The average cooling rate can be determined by dividing the temperature difference from the end of hot rolling temperature to the first cooling stop temperature by the time required for this cooling.

[0061] [Reheating and hardening treatment] The seamless steel pipe, after cooling as described above, is reheated to a reheating temperature between the Ac3 transformation point and 1000°C, and then quenched to a second cooling stop temperature of 200°C or lower, and subjected to a reheating and quenching treatment at least once.

[0062] Reheating temperature: Above Ac3 transformation point and below 1000℃ If the reheating temperature is below the Ac3 transformation point, the material will not be heated to the austenite single-phase region, and a structure with martensite as the main phase will not be obtained. As a result, tempered martensite with the desired area ratio will not be obtained after tempering. For this reason, the reheating temperature should be above the Ac3 transformation point. Preferably, the reheating temperature should be above the Ac3 transformation point + 20°C, more preferably above the Ac3 transformation point + 50°C, and even more preferably above the Ac3 transformation point + 100°C. On the other hand, if the reheating temperature exceeds 1000°C, the austenite grains will coarseen, reducing toughness, and the oxide scale on the surface will thicken, making it prone to peeling and causing defects on the steel pipe surface. Furthermore, the load on the heat treatment furnace will become excessive, which is also a problem from the standpoint of energy conservation. For these reasons, the reheating temperature should be 1000°C or lower. Preferably, the reheating temperature should be 950°C or lower.

[0063] Second cooling stop temperature: 200℃ or less The seamless steel pipe, after reheating as described above, is quenched to a second cooling stop temperature of 200°C or lower. If the second cooling stop temperature exceeds 200°C, the desired Mo-based precipitate cannot be obtained. Therefore, the second cooling stop temperature is set to 200°C or lower. Preferably, the second cooling stop temperature is 100°C or lower. On the other hand, the lower limit of the second cooling stop temperature is not particularly limited, but it may be above room temperature. Room temperature refers to a temperature of 10 to 35°C. Quenching means rapid cooling, and rapid cooling means cooling at an average cooling rate of 1.0°C / s or higher from the reheating temperature to the second cooling stop temperature. Preferably, the average cooling rate is 1.5°C / s or higher, more preferably 2.0°C / s or higher, and even more preferably 2.5°C / s or higher. The cooling method is not particularly limited and can include water cooling, accelerated cooling, etc. The above average cooling rate is not particularly limited, but from the viewpoint of manufacturing costs, it is preferably 5.0°C / s or less, more preferably 4.8°C / s or less, and even more preferably 4.6°C / s or less. The average cooling rate can be determined by dividing the temperature difference from the reheating temperature to the quenching stop temperature by the time required for this cooling.

[0064] Number of reheating and quenching treatments: 1 or more times The reheating and quenching treatment may be performed once or two or more times. Therefore, the number of reheating and quenching treatments should be one or more. By performing the reheating and quenching treatment two or more times, the prior austenite grains are refined, improving toughness and strength. Therefore, the number of reheating and quenching treatments should preferably be two or more, more preferably three or more. On the other hand, there is no particular upper limit to the number of reheating and quenching treatments, but from the viewpoint of manufacturing costs, it is preferably five or less.

[0065] The Ac3 transformation point shall be calculated using the following formula. Ac3 transformation point (℃)=937.2-436.5C+56Si-19.7Mn-16.3Cu-4.9Cr-26.6Ni+38.1Mo+124.8V+136.3Ti-19.1Nb+198.4Al+3315B (Here, C, Si, Mn, Cu, Cr, Ni, Mo, V, Ti, Nb, Al, B: Content of each element (mass %)) When calculating the Ac3 transformation point, if the element listed in the above formula is not present, the content of that element shall be assumed to be 0%.

[0066] [Tempering treatment] The seamless steel pipe after reheating and quenching is subjected to tempering treatment under the conditions that the average heating rate from the second cooling stop temperature to the tempering temperature of 600°C to 740°C is 0.5°C / min or more, and the holding time at the tempering temperature is 10 minutes or more but less than 60 minutes.

[0067] Tempering temperature: 600℃ or higher and 740℃ or lower Tempering is performed to stabilize dislocations, i.e., to control the M value, to form Mo-based precipitates, and to improve toughness and hydrogen embrittlement resistance. If the tempering temperature is below 600°C, dislocation stabilization and Mo-based precipitate formation will be insufficient, and the desired M value and a sufficient number of Mo-based precipitates will not be obtained, making it impossible to secure the desired hydrogen embrittlement resistance. For this reason, the tempering temperature should be 600°C or higher. Preferably, the tempering temperature should be 615°C or higher, more preferably 625°C or higher, even more preferably 635°C or higher, and most preferably 645°C or higher. On the other hand, if the tempering temperature exceeds 740°C, the microstructure will soften significantly if the Mo content is low, and the desired strength cannot be secured. Also, as the tempering temperature increases, the Mo-based precipitates grow, making it impossible to secure the desired number of fine Mo-based precipitates. For this reason, the tempering temperature should be 740°C or lower. The tempering temperature is preferably 730°C or lower, more preferably 710°C or lower, even more preferably 700°C or lower, and most preferably 690°C or lower.

[0068] The average heating rate from the second cooling stop temperature to the tempering temperature is 0.5°C / min or more. Mo-based precipitates form during the heating process until the tempering temperature is reached. If the average heating rate from the second cooling stop temperature to the tempering temperature is less than 0.5°C / min, the size of the Mo-based precipitates will be large, and the desired number of fine Mo-based precipitates will not be obtained, resulting in a failure to obtain the desired hydrogen embrittlement resistance. Therefore, the average heating rate should be 0.5°C / min or higher. The average heating rate is preferably 2.0°C / min or higher, more preferably 3.0°C / min or higher, even more preferably 4.0°C / min or higher, and most preferably 5.0°C / min or higher. On the other hand, there is no particular upper limit to the average heating rate, but if it is too fast, non-uniform temperature distribution will occur inside the steel pipe, resulting in non-uniformity of the structure. Therefore, the average heating rate is preferably 50.0°C / min or lower, more preferably 45.0°C / min or lower, even more preferably 40.0°C / min or lower, and most preferably 35.0°C / min or lower. The average heating rate can be calculated by dividing the temperature difference from the second cooling stop temperature to the tempering temperature by the time required for this heating.

[0069] Holding time: 10 minutes or more, less than 60 minutes Mo-based precipitates are most abundant when held at the tempering temperature. If the holding time at the tempering temperature is less than 10 minutes, sufficient Mo-based precipitates will not form, and the desired hydrogen embrittlement resistance will not be obtained. Therefore, the holding time at the tempering temperature should be 10 minutes or more. Preferably, the holding time should be 15 minutes or more, more preferably 20 minutes or more, even more preferably 25 minutes or more, and most preferably 30 minutes or more. On the other hand, if the holding time at the tempering temperature is 60 minutes or more, the size of the Mo-based precipitates will increase, and the desired number of fine Mo-based precipitates will not be obtained. Therefore, the holding time at the tempering temperature should be less than 60 minutes. Since the holding time is an energy-related cost increase factor, preferably it should be 59 minutes or less, more preferably 58 minutes or less, even more preferably 57 minutes or less, and most preferably 56 minutes or less.

[0070] Furthermore, if necessary, after reheating and tempering, a straightening treatment may be performed at a warm or cold temperature to correct any shape defects in the seamless steel pipe. Note that the warm straightening treatment refers to performing the treatment at a temperature of 300°C or higher and below the tempering temperature, while the cold straightening treatment refers to performing the treatment at room temperature.

[0071] Furthermore, the seamless steel pipe obtained above can also be applied to cylinders. Examples of cylinder shapes include those with an outer diameter of 200 mm or more and 600 mm or less, and a cylinder length in the axial direction of the pipe of 500 mm or more and 12,000 mm or less. Specifically, the outer diameter of the cylinder to which it is applied is preferably 200 mm or more, more preferably 250 mm or more, and even more preferably 300 mm or more. Also, the outer diameter of the cylinder is preferably 600 mm or less, more preferably 550 mm or less, and even more preferably 500 mm or less.

[0072] The seamless steel pipe of the present invention can be suitably used in high-pressure hydrogen accumulators that store high-pressure hydrogen in cylinders. Here, high pressure refers to, for example, a hydrogen pressure of 1 MPa or more and 115 MPa or less. [Examples]

[0073] The present invention will be described in more detail below based on examples. The following description illustrates a preferred example of the present invention, and the present invention is not limited in any way by these examples.

[0074] [Table 1]

[0075] [Table 2]

[0076] First, a billet, which is a steel pipe material having the component composition shown in Table 1, was manufactured at a casting speed of 0.6 m / min. Next, the obtained billet was heated and hot-rolled under the conditions shown in Table 2 to expand it into a seamless steel pipe. After that, the seamless steel pipe was cooled to the first cooling stop temperature at the first cooling rate shown in Table 2. Next, the cooled seamless steel pipe was reheated to the reheating temperature shown in Table 2, then quenched (water-cooled) to the second cooling stop temperature, and then subjected to the tempering treatment shown in Table 2. The resulting seamless steel pipe (wall thickness t: 14 mm, outer diameter of steel pipe: 245 mm, axial length L: 980 mm) was evaluated for its microstructure and mechanical properties. The results are shown in Table 3. The evaluation method is as follows.

[0077] Organizational evaluation methods Area ratio of tempered martensite A specimen measuring 10mmt x 10mmC x 10mmL was taken from near the center of the wall thickness (t / 2 position) of the seamless steel pipe obtained above for microstructure observation. At this time, the specimen was taken so that the cross section (C section) parallel to the wall thickness direction and the circumferential direction was the observation surface, and the vicinity of the center of the observation surface was the t / 2 position. The specimen was then observed using the following method. The observation surface was etched with a 3 vol% nital solution, and using a scanning electron microscope, microstructure images of the t / 2 position were acquired at appropriate magnifications of 1000 to 5000x for three fields of view, and tempered martensite, ferrite, bainite, and pearlite were observed. In the above microstructure images, the area of ​​tempered martensite was visually determined by comparing it with the microstructure photograph in Reference 3, and the area ratio of tempered martensite was determined by binarizing the tempered martensite and other areas using image analysis. The above procedure was performed on three fields of view per test specimen, and the average of the area percentages of the three fields of view was used as the area percentage of tempered martensite. The area percentages of other tissues were calculated by subtracting the above-mentioned area percentage of tempered martensite from 100%. [Reference 3] Japan Heat Treatment Technology Association (author), Introduction to the Structure and Properties of Metallic Materials - Heat Treatment and Microstructure Control for Maximizing Material Potential, 2004.

[0078] Grain size number of old austenite grains Using the specimens for microstructure observation after obtaining the above microstructure images, the particle size number of the prior austenite grains was measured using the following method. First, the observation surface of the specimen was polished, and the prior austenite grain boundaries were exposed by etching with a saturated picric acid solution. The specimens were then observed using an optical microscope (magnification: 1000x), and microstructure images were obtained at the t / 2 position in at least three fields of view. For the obtained microstructure images, the particle size number of the prior austenite grains was determined using the sectioning method in accordance with the provisions of JIS G 0551. The above procedure was performed for three fields of view per specimen, and the average value of the particle size numbers of the prior austenite grains in the three fields of view was taken as the particle size number of the prior austenite grains.

[0079] Analysis of precipitates As described above, first, the amount of Mo contained in the precipitate was determined by the extraction method by filter filtration described in Patent Document 5 and Reference 1. First, an electrolytic test specimen with the dimensions of total wall thickness × 10 mmC × 10 mmL (where C: circumferential direction of the pipe, L: axial direction of the pipe) was taken from the steel pipe. Next, the electrolytic test specimen was electrolyzed with an electrolyte, and after electrolysis, the electrolytic test specimen was immersed in a dispersible solution, and ultrasonic waves were applied to extract the precipitate adhering to the surface of the electrolytic test specimen into the dispersible solution. The dispersible solution from which the precipitate was extracted was filtered using a filter with a pore size of 100 nm, and precipitates larger than 100 nm were collected on the filter. The filtrate after filtering was dried, and precipitates smaller than 100 nm were collected. In this way, the precipitates were separated by size (less than 100 nm and more than 100 nm), and each size-classified precipitate was subjected to acid decomposition. Using ICP emission spectrometry, the absolute amount of Mo contained in each size-classified precipitate (absolute amount of Mo in precipitates more than 100 nm: W1, absolute amount of Mo in precipitates less than 100 nm: W2) was calculated. Subsequently, by dividing W1 and W2 by the weight difference W0 (amount of electrolysis) of the electrolytic test specimens before and after electrolysis, the total amount of Mo (mass%) contained in precipitates more than 100 nm and the total amount of Mo (mass%) contained in precipitates less than 100 nm were calculated. By dividing the total amount of Mo contained in precipitates exceeding 100 nm (mass%) and the total amount of Mo contained in precipitates 100 nm or less (mass%) by the amount of Mo contained in the steel pipe (Mo content), the proportion of Mo contained in precipitates exceeding 100 nm and the proportion of Mo contained in precipitates 100 nm or less, out of the total amount of Mo contained in the steel pipe, were calculated.

[0080] Dislocation arrangement parameters As described above, the dislocation arrangement parameter M value was analyzed using an XRD (X-ray diffraction) apparatus. A specimen for measuring the M value was taken from near the center of the steel pipe wall thickness (t / 2 position) with dimensions of 10 mmt × 10 mmC × 10 mmL. At this time, the measurement surface was taken with a cross section (C section) parallel to the wall thickness direction and the circumferential direction of the pipe, and the t / 2 position was near the center of the measurement surface. Subsequently, the measurement surface of the taken specimen was mechanically polished. At this time, to prevent the strain caused by mechanical polishing from affecting the M value, a thickness of approximately 50 μm was removed from the surface of the measurement surface after mechanical polishing by electrolytic polishing. Electrolytic polishing was performed using a solution prepared with 478 ml of HClO, 700 ml of ethanol, and 120 ml of distilled water, at a temperature of 23°C and a voltage of 27 V, with the specimen as the anode and a Pt wire as the cathode. The dislocation arrangement parameter M value was analyzed for the measurement surface after electropolishing, according to the modified-Williamson-Hall method (mWH / WA method) described in Reference 2. Specifically, XRD measurements were performed under the following conditions. The XRD measurements were performed using CuKα radiation as the X-ray source, with a tube voltage of 45kV and a tube current of 200mA. In the obtained diffraction profile, the 2θ range was set to 35~154°, and the measured diffraction planes were BCC-Fe(110), (200), (211), (220), (310), and (222), with a dislocation density of (1 / m³). 2 The magnitude of the strain field due to dislocations, Re(m), was calculated. Subsequently, the dislocation density (1 / m) was calculated. 2 The M value was calculated by substituting the magnitude of the strain field Re(m) due to the dislocations into the following formula. M=Re / d Here, d(m): average distance between dislocations (d=1 / √ρ), ρ(1 / m) 2 ): This is the dislocation density.

[0081] Tensile strength A rod-shaped test specimen specified in JIS Z 2201 "Tensile Test Pieces for Metallic Materials" was sampled from a steel pipe as a tensile test specimen. Here, the tensile test specimen was produced such that the longitudinal direction (tensile direction) of the tensile test specimen corresponds to the circumferential direction (C direction) of the steel pipe, and the center of the cross-section of the tensile test specimen perpendicular to the tensile direction is located at the t / 2 position of the steel pipe. The tensile test was carried out in accordance with the method specified in JIS Z 2241, and the value obtained by dividing the maximum load by the initial cross-sectional area was defined as the tensile strength (TS) of the steel pipe. Steel pipes that obtained a TS of 850 MPa or higher were evaluated as acceptable.

[0082] Hydrogen embrittlement resistance Hydrogen embrittlement resistance was evaluated based on the relative elongation (REL) of a test specimen after a low strain rate tensile test in hydrogen gas. In the atmosphere, the steel material constituting the steel pipe undergoes plastic deformation, which increases the gauge distance, resulting in an increased elongation EL air . On the other hand, in hydrogen, the ductility of the steel material decreases, so the steel fractures before necking, and the elongation does not become larger than that in the atmosphere. Therefore, unlike in the atmosphere, the elongation after testing in hydrogen EL H becomes shorter. Hydrogen embrittlement resistance was evaluated using this reduction rate of elongation. The relative elongation (REL) was calculated by the following formula. Relative elongation (REL) = EL H / EL air ×100 The above-mentioned test specimen (SSRT test specimen) is a round bar-shaped test specimen, which was sampled from the steel pipe in accordance with the content specified in ASTM G 142. Here, the test specimen was prepared such that the diameter of the parallel portion is 6 mm, the longitudinal direction of the test specimen corresponds to the circumferential direction of the steel pipe, and the center of the cross-section of the test specimen perpendicular to the longitudinal direction of the test specimen is located at the t / 2 position of the steel pipe. Evaluations were carried out in the atmosphere and in hydrogen with n2 and n3 test pieces respectively, and the average values thereof were obtained as EL air and EL H respectively. In the above test, the hydrogen pressure of the hydrogen gas was set to 105 MPa, the purity of the hydrogen gas was 99.97%, and the tensile speed in the low strain rate tensile test at room temperature was set to 0.002 mm / s. Here, the hydrogen pressure of the hydrogen gas was calculated assuming high-pressure hydrogen applications with a maximum hydrogen pressure of 95 MPa, by multiplying by a safety factor of 1.1. The higher the REL (Resilience Level), the better the hydrogen embrittlement resistance, and steel pipes with an REL of 70% or higher were deemed acceptable.

[0083] Tables 1, 2, and 3 show that all of the inventive examples satisfied high strength (tensile strength of 850 MPa or more) and excellent hydrogen embrittlement resistance (condition of REL of 70% or more in low strain rate tensile tests in hydrogen gas).

[0084] [Table 3]

Claims

1. In mass percent, C: 0.20-0.50%, Si: 0.05-1.00%, Mn: 0.30 to 1.50%, P: 0.015% or less, S: 0.005% or less, Al: 0.150% or less, N: 0.006% or less, Cr: 0.10-1.70%, Mo: Over 1.0%, 3.0% or less, Nb: 0.001-0.020%, B: 0.0003 to 0.0030%, O: 0.0030% or less, Ti: Contains 0.003-0.025%, The remainder consists of Fe and unavoidable impurities, The component composition has a ratio of Mo to C, where Mo / C is 2.0 to 12.

0. The structure has tempered martensite accounting for 95% or more of the area. The aforementioned structure has precipitates, The precipitate contains 50% or more of the Mo by mass, Furthermore, the precipitate includes precipitates with a maximum length of 100 nm or less. At least 10% by mass of the Mo is contained in the precipitate with a maximum length of 100 nm or less. The dislocation arrangement parameter M value is 0.45 or less. Seamless steel pipe.

2. Furthermore, the above component composition is expressed in mass percent as follows: V: 0.30% or less, Cu: 1.00% or less, Ni: 2.0% or less, Sn: 0.50% or less, W: 3.0% or less, Ca: 0.0050% or less Includes at least one selected from, A seamless steel pipe as described in claim 1.

3. A method for manufacturing a seamless steel pipe according to claim 1 or 2, wherein a steel pipe material having the above-mentioned component composition is heated at a heating temperature of 1050°C or more and 1350°C or less, and then hot-rolled under conditions where the hot-rolling completion temperature is 600°C or more and 950°C or less to obtain a seamless steel pipe, The seamless steel pipe is cooled to a first cooling stop temperature of 200°C or less. The seamless steel pipe after cooling is Ac ３ The material is reheated to a reheating temperature of 1000°C or higher than the transformation point, and then quenched to a second cooling stop temperature of 200°C or lower, at least once. A method for manufacturing seamless steel pipes, comprising tempering the seamless steel pipe after the reheating and quenching treatment under the conditions that the average heating rate from the second cooling stop temperature to a tempering temperature of 600°C to 740°C is 0.5°C / min or more, and the holding time at the tempering temperature is 10 minutes or more but less than 60 minutes.

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