Seamless steel pipe and method for manufacturing the same

Abstract

The purpose of the present invention is to provide: a seamless steel pipe which has high strength and excellent hydrogen embrittlement resistance; and a method for producing the same.　The seamless steel pipe has a specific component composition, and has a structure in which tempered martensite occupies 95% or more of the area, wherein the structure has precipitates, the total amount of Fe contained in precipitates having a maximum length of 100 nm or less among the precipitates is 0.005 mass% or more, and the total amount of Fe contained in precipitates having a maximum length of more than 100 nm among the precipitates is 1.700 mass%.

Description

[Technical Field]

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

[0002] Currently, various initiatives are being studied worldwide to realize a decarbonized society. One such initiative is the use of hydrogen energy. Fuel cell vehicles use hydrogen as fuel, thereby emitting no carbon dioxide (CO₂) and being excellent in energy efficiency. Therefore, the popularization of fuel cell vehicles is important for realizing a decarbonized society. In order to popularize such fuel cell vehicles, there is a need for a storage container (pressure accumulator) excellent in strength and durability that can safely store high-pressure hydrogen of 35 MPa or more, particularly 70 MPa or more, for use in hydrogen stations for supplying hydrogen to fuel cell vehicles and for in-vehicle use in which hydrogen is loaded onto fuel cell vehicles, and developments thereof are underway.

[0003] Since weight reduction is required for in-vehicle pressure accumulators, pressure accumulators obtained by coating a liner made of a lightweight material such as aluminum (Al) with carbon fiber reinforced plastic (CFRP) have been proposed. For example, Patent Document 1 describes a liner made of an Al alloy excellent in fatigue properties.

[0004] On the other hand, since weight reduction is not required for pressure accumulators for hydrogen stations, liners manufactured from low-alloy steel have been proposed. For example, Patent Document 2 proposes a pressure accumulator obtained by coating a Cr-Mo steel liner with carbon fiber or glass fiber. However, low-alloy steels including Cr-Mo steel are known to undergo embrittlement caused by hydrogen (hydrogen embrittlement). Therefore, Al alloys and SUS316 with low susceptibility to hydrogen embrittlement are recommended as materials for high-pressure hydrogen pressure accumulators of 35 MPa or more. 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, which increases the weight of the accumulator. This results in high material costs and poor economic efficiency. In addition, in the case of vehicle-mounted 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 were sometimes insufficient 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 due to the accumulation of hydrogen at grain boundaries, leading to fracture along the grain boundaries (grain boundary fracture). Based on the above mechanism, to suppress the occurrence of grain boundary fracture, i.e., to improve hydrogen embrittlement resistance, hydrogen trapping sites should be formed within the steel material (steel pipe). Therefore, the inventors diligently investigated hydrogen trapping sites and found that Fe-based precipitates act as strong hydrogen trapping sites, however, the presence of a certain amount or more of Fe-based precipitates with a maximum length exceeding 100 nm reduces hydrogen embrittlement resistance. The inventors further investigated and discovered the following mechanism: Fe-based precipitates with a maximum length exceeding 100 nm tend to aggregate strongly in the steel pipe, causing a non-uniform distribution of precipitates. In other words, the distribution of hydrogen trapping sites becomes uneven, and hydrogen is trapped locally. As a result, when stress is applied to the steel pipe, the stress concentrates in precipitates with a large amount of trapped hydrogen, and these precipitates become fracture initiation points, reducing the hydrogen embrittlement resistance. Therefore, we considered it important to suppress the formation of Fe-based precipitates with a maximum length exceeding 100 nm.

[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: 0.05~0.80%, 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: 0.25 to 1.20%, Nb: 0.001 to 0.020%, B: 0.0003 to 0.0030%, O: 0.0030% or less, and Ti: 0.003 to 0.025%, the balance consisting of Fe and unavoidable impurities, and having a component composition in which Cr+Si, which is the sum of said Cr and said Si, is 0.75% or more, having a structure in which tempered martensite accounts for 95% or more by area fraction, said structure comprising precipitates, among said precipitates, the total mass% of Fe contained in precipitates having a maximum length of 100 nm or less is 0.005% or more by mass, and further, among said precipitates, the total mass% of Fe contained in precipitates having a maximum length exceeding 100 nm is 1.700% or less, a seamless steel pipe. [2] Further, in terms of mass%, said component composition: Mo: 1.0% or less, 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 comprising at least one selected from the group consisting of the seamless steel pipe according to [1].[ [3] A steel pipe material having the component composition according to [1] or [2] is heated at a heating temperature of 1050°C or more and 1350°C or less, then subjected to hot rolling to obtain a seamless steel pipe, cooling said seamless steel pipe to a first cooling stop temperature of 200°C or lower, reheating said cooled seamless steel pipe to a reheating temperature of not lower than the Ac3 transformation point and not higher than 1000°C, and performing reheating and quenching treatment for quenching one or more times, Tempering treatment is performed on the seamless steel pipe after the said reheating and quenching treatment under the conditions that the average temperature rising rate until the temperature reaches a tempering temperature of 500°C or higher and Ac₃ transformation point -50°C or lower is 0.5°C / min or more, and the holding time at the said tempering temperature is 10 minutes or more and less than 60 minutes, A method for producing a seamless steel pipe, comprising cooling the seamless steel pipe after the said tempering treatment to a second cooling stop temperature of 300°C or lower at an average cooling rate of 0.1°C / s or more.

Effect of the Invention

[0011] According to the present invention, a seamless steel pipe having high strength and excellent hydrogen embrittlement resistance and a method for producing the same can be obtained. The seamless steel pipe of the present invention can be extremely suitably used as a seamless steel pipe for high-pressure hydrogen accumulators.

Brief Description of Drawings

[0012] [Figure 1] It is a drawing showing a compact tension (CT) test piece used in a fatigue crack growth test.

Mode for Carrying Out the Invention

[0013] Hereinafter, embodiments of the present invention will be described. It should be noted that the following description shows a preferred embodiment of the present invention, and the present invention is not limited in any way by the following description.

[0014] [Component Composition] First, the reasons for limiting the component composition of the seamless steel pipe (sometimes referred to as steel pipe) of the present invention will be described. Hereinafter, mass% in the component composition is simply expressed as %.

[0015] C: 0.20 to 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. 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.

[0016] Si: 0.05~0.80% Si is included as a deoxidizing agent, but if the Si content is less than 0.05%, the deoxidizing effect is insufficient. Furthermore, Si has a softening resistance (tempering softening resistance) effect during the tempering treatment described later, so the growth of Fe-based precipitates is suppressed, and the amount of Fe-based precipitates exceeding 100 nm can be reduced. As a result, the desired hydrogen embrittlement resistance is obtained. To obtain the above effect, the Si content should be 0.05% or more. The Si content is preferably 0.10% or more, more preferably 0.25% or more, even more preferably 0.30% or more, and most preferably 0.35% or more. On the other hand, the higher the Si content, the more the above effect is observed, but if it exceeds 0.80%, the above effect saturates. Therefore, the Si content should be 0.80% or less. The Si content is preferably 0.75% or less, more preferably 0.70% or less, even more preferably 0.65% or less, and most preferably 0.60% or less.

[0017] 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 0.50% or more, even more preferably 0.60% or more, and most preferably 0.65% 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 0.75% or less.

[0018] 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 structure of the steel pipe, 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 0.012% or less, more preferably 0.010% or less, even more preferably 0.008% or less, and most preferably 0.007% 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.

[0019] 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 (Stress Corrosion Cracking) resistance, so it is preferable to reduce it as much as possible. However, an S content of up to 0.005% is acceptable. For this reason, 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. For this reason, the S content should preferably be 0.0002% or more, more preferably 0.001% or more.

[0020] 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 0.085% or less, and most preferably 0.080% 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.

[0021] 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.

[0022] Cr: 0.25~1.20% 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 Fe3C, M7C3, and M 23 It forms precipitates such as C6 (where M is a metallic element) and has the effect of resisting tempering softening, similar to Si. To obtain such an effect, the Cr content should be 0.25% or more. Preferably, the Cr content is 0.30% or more, more preferably 0.35% or more, even more preferably 0.40% or more, and most preferably 0.45% or more. On the other hand, if the Cr content exceeds 1.20%, the disadvantage of increased cost contributes more than the hydrogen embrittlement resistance obtained. Therefore, the Cr content should be 1.20% or less. Preferably, the Cr content is 1.15% or less, more preferably 1.10% or less, and even more preferably 1.05% 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 0.005% 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 0.015% or lower, 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 0.0004% or more, more preferably 0.0005% or more, and even more preferably 0.0006% or more. On the other hand, if the B content exceeds 0.0030%, it precipitates as carbonitrides, etc., reducing hardenability and toughness. Furthermore, adding too much B generates Fe2B, causing red-hot brittleness. For this reason, the B content should be 0.0030% or less. Preferably, the B content should be 0.0028% or less, more preferably 0.0025% or less, even more preferably 0.0023% or less, and most preferably 0.0020% 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 O (oxygen) content as much as possible. However, an O (oxygen) content of up to 0.0030% is acceptable. Therefore, the O (oxygen) content should be 0.0030% or less. Preferably, the O (oxygen) 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 O (oxygen) content is not particularly limited and may be 0%. However, excessive reduction of O (oxygen) leads to an increase in refining costs. Therefore, preferably, the O (oxygen) content should be 0.0001% or more, more preferably 0.0003% or more, even more preferably 0.0005% or more, and most preferably 0.0007% 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 0.008% or more, even more preferably 0.010% or more, and most preferably 0.012% or more. On the other hand, when 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 0.023% or less, more preferably 0.020% or less, even more preferably 0.018% 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. An Mg content of 0.0008% or less and a Co content of 0.0008% or less are permissible.

[0028] Cr + Si ≥ 0.75% The inventors have found that the sum of Cr (Cr content) and Si (Si content) in the steel pipe (Cr+Si) being 0.75% or more is important for suppressing the formation of Fe-based precipitates with a maximum length exceeding 100 nm. As described above, Cr and Si have the effect of resisting tempering softening, so the formation of Fe-based precipitates exceeding 100 nm can be suppressed, and the desired hydrogen embrittlement resistance can be obtained. If Cr+Si is less than 0.75%, the above effect cannot be obtained, and the desired hydrogen embrittlement resistance cannot be obtained. Therefore, Cr+Si should be 0.75% or more. Preferably, Cr+Si should be 1.00% or more, more preferably 1.25% or more, even more preferably 1.50% or more, and most preferably 1.55% or more. On the other hand, the more Cr and Si there are, the better the above effect can be obtained, so it is possible to include them up to the upper limit of each. That is, there is no particular upper limit to Cr+Si, but it is possible to include up to 2.00%, and it may be 1.90% or less.

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

[0030] Mo: 1.0% or less Mo forms precipitates (Mo-based precipitates), contributing to further strength increases of steel pipes through precipitation strengthening. Furthermore, the formed Mo-based precipitates function as hydrogen trapping sites, further improving hydrogen embrittlement resistance. Even when dissolved in the steel pipe, Mo segregates at prior austenite grain boundaries, contributing to further improvement of hydrogen embrittlement resistance. Therefore, it can be included in any amount depending on the desired properties. However, Mo is an expensive element, and if the Mo content exceeds 1.0%, the cost increase outweighs the benefits of improved hydrogen embrittlement resistance. For this reason, when Mo is included, the Mo content should be 1.0% or less. Preferably, the Mo content is 0.8% or less, more preferably 0.6% or less, and even more preferably 0.4% or less. On the other hand, the lower limit of the Mo content is not particularly limited and may be 0%. However, from the viewpoint of fully obtaining the effects of Mo, the Mo content is preferably 0.1% or more, more preferably 0.2% or more, and even more preferably 0.3% or more.

[0031] 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, V is an expensive element, and 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 0.25% or less, more preferably 0.20% or less, and even more preferably 0.15% 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 from 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 0.04% or more.

[0032] 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 0.90% or less, more preferably 0.70% or less, even more preferably 0.50% or less, and most preferably 0.20% 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 0.02% or more, and even more preferably 0.03% or more.

[0033] 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.

[0034] 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 0.30% 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.

[0035] W: 3.0% or less 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 0.2% or more, and even more preferably 0.3% or more.

[0036] 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 should be 0.0050% or less. Preferably, the Ca content is 0.0045% or less, more preferably 0.0040% or less, and even more preferably 0.0035% 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, preferably, the Ca content is 0.0005% or more, more preferably 0.0010% or more, and even more preferably 0.0015% or more.

[0037] Hydrogen (H) may be introduced into steel pipes during various manufacturing processes. If the amount introduced is high, the risk of cracking after solidification increases and the hydrogen embrittlement resistance deteriorates. Therefore, it is preferable to reduce the H content of steel pipes. These effects are not a problem if the H content of steel pipes is 0.00100% or less. For this reason, if steel pipes contain H, the H content of steel pipes should be 0.0010% or less. Preferably, the H content of steel pipes should be 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 steel pipes, the better the hydrogen embrittlement resistance. Therefore, the lower limit of the H content of steel pipes is not particularly limited and may be 0%, or 0.00001% or more. Note that the H content of steel pipes is the amount of residual hydrogen after steel pipe forming. 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.

[0038] [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 above structure has precipitates, and the total amount of Fe contained in the precipitates with a maximum length of 100 nm or less is 0.005% by mass or more, and furthermore, the total amount of Fe contained in the precipitates with a maximum length exceeding 100 nm is 1.700% by mass or less.

[0039] Tempered martensite accounts for 95% or more of the area: 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%.

[0040] 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%.

[0041] The area ratio of the above-mentioned tissue is determined based on the method described in the examples.

[0042] 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.

[0043] 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 phase 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 17.5 or lower, even more preferably 17.0 or lower, and most preferably 16.5 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.

[0044] The particle size number of 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.

[0045] The total amount of Fe contained in precipitates with a maximum length of 100 nm or less is 0.005% or more by mass: In order to obtain the desired hydrogen embrittlement resistance, the steel pipe of the present invention must contain Fe-based precipitates with a maximum length of 100 nm or less. That is, the precipitates with a maximum length of 100 nm or less must contain 0.005% or more by mass of Fe. Since the Fe-based precipitates with a maximum length of 100 nm or less act as hydrogen trapping sites, it is important that the above structure contains a predetermined amount or more of Fe-based precipitates with a maximum length of 100 nm or less. That is, the precipitates with a maximum length of 100 nm or less must contain 0.005% by mass or more of Fe. The total amount of Fe contained in precipitates with a maximum length of 100 nm or less is preferably 0.007% by mass or more, more preferably 0.010% by mass or more, even more preferably 0.013% by mass or more, and most preferably 0.015% by mass or more. On the other hand, there is no particular upper limit to the total amount of Fe contained in precipitates with a maximum length of 100 nm or less, but the total amount of Fe contained in precipitates with a maximum length of 100 nm or less may be 0.200% by mass or less.

[0046] The total amount of Fe contained in precipitates with a maximum length exceeding 100 nm is 1.700% by mass or less: The inventors have found that reducing the number of Fe-based precipitates with a maximum length exceeding 100 nm is important for obtaining the desired hydrogen embrittlement resistance. Fe-based precipitates with a maximum length exceeding 100 nm tend to aggregate strongly in steel pipes, resulting in a non-uniform distribution of precipitates. In other words, the distribution of hydrogen trapping sites becomes non-uniform, and hydrogen is trapped locally. As a result, when stress is applied to the steel pipe, these become points where stress concentrates, leading to fracture initiation points and a decrease in hydrogen embrittlement resistance. Therefore, it is necessary to reduce the number of Fe-based precipitates with a maximum length exceeding 100 nm, that is, to reduce the total amount of Fe contained in the precipitates to 1.700% by mass or less. Therefore, the total amount of Fe contained in precipitates with a maximum length exceeding 100 nm is set to 1.700% by mass or less. The total amount of Fe contained in precipitates with a maximum length exceeding 100 nm is preferably 1.600% or less by mass, more preferably 1.500% or less, even more preferably 1.450% or less, and most preferably 1.400% or less. On the other hand, since it is better to have as few Fe-based precipitates with a maximum length exceeding 100 nm as possible, the lower limit of the total amount of Fe contained in precipitates with a maximum length exceeding 100 nm is not limited and may be 0%.

[0047] Furthermore, the inventors have found that the hydrogen embrittlement resistance can be further improved by setting the ratio of the total amount of Fe contained in precipitates with a maximum length of 50 nm or less to the total amount of Fe contained in precipitates with a maximum length of 100 nm or less to 30% or more. For this reason, the above ratio is preferably 30% or more, more preferably 60% or more, and even more preferably 70% or more. On the other hand, the upper limit of the above ratio is not particularly limited and may be 100%. 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 larger. Furthermore, there is no particular lower limit to the size of precipitates 50 nm or less, but they may be 5 nm or larger. Here, Fe-based precipitates refer to intermetallic compounds such as cementite, ε-carbides, χ-carbides, and Fe7C3.

[0048] The total amount of Fe contained in the precipitate can be determined by the extraction method by filter filtration described in Patent Document 5 and Reference 1. Specifically, a test specimen with dimensions of total wall thickness x 10 mmC x 10 mmL (C: circumferential direction, L: axial direction) is taken from a steel pipe to be used as an electrolytic test specimen. The obtained electrolytic test specimen is electrolyzed with an electrolyte solution, and after electrolysis, 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. Subsequently, the filtered filtrate 1 is further filtered using a filter with a pore size of 50 nm, and precipitates between 100 nm and 50 nm are collected on the filter. The resulting filtered filtrate 2 is dried, and precipitates smaller than 50 nm are collected. In this way, the precipitates are separated by size (50 nm or less, between 50 nm and 100 nm, and over 100 nm), and each size-classified precipitate is subjected to acid decomposition. Using ICP emission spectrometry, the absolute amount of Fe contained in each size-classified precipitate (absolute amount of Fe in precipitates over 100 nm: W1, absolute amount of Fe in precipitates between 50 nm and 100 nm: W2, absolute amount of Fe in precipitates under 50 nm: W3) is calculated. Then, by dividing W1, W2, and W3 by the weight difference W0 (amount of electrolysis) of the electrolytic test piece before and after electrolysis, the total amount of Fe (mass%) contained in precipitates over 100 nm, the total amount of Fe (mass%) contained in precipitates between 50 nm and 100 nm, and the total amount of Fe (mass%) contained in precipitates under 50 nm can be calculated. Furthermore, using the total amount of Fe (mass%) contained in the precipitates between 50 nm and 100 nm and the total amount of Fe (mass%) contained in the precipitates below 50 nm, the ratio of the total amount of Fe contained in the precipitates below 50 nm to the total amount of Fe contained in the precipitates below 100 nm 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

[0049] 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.

[0050] 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.

[0051] In this invention, properties such as microstructure, grain size number of prior austenite grains, tensile strength TS, and hydrogen embrittlement resistance are evaluated near the t / 2 position (t: wall thickness). 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 is slowest at the t / 2 position of the steel pipe. As a result, it is difficult to obtain the desired microstructure, such as martensite (quenched structure), and 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.

[0052] 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.

[0053] 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.

[0054] [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 and quenching, tempering, and cooling. In the following description, unless otherwise specified, temperature refers to the temperature on the surface of the steel pipe material or steel pipe. The temperature on the surface is the value measured with a radiation thermometer.

[0055] 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.

[0056] 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.

[0057] [Hot rolling] The above steel pipe material is heated and then hot-rolled to produce a seamless steel pipe.

[0058] 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, most preferably 1100°C or higher, even more preferably 1120°C or higher, and even more preferably 1150°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 forms 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. The heating temperature is preferably 1330°C or lower, more preferably 1300°C or lower, even more preferably 1280°C or lower, and most preferably 1250°C or lower.

[0059] Next, the heated steel pipe material is subjected to hot rolling to form a seamless steel pipe. For the hot rolling, a standard Mannesmann-plug mill method or a Mannesmann-mandrel mill method, including perforation rolling, can be used.

[0060] 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.

[0061] [cooling] The resulting seamless steel pipes are cooled to a first cooling stop temperature of 200°C or lower.

[0062] 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 195°C or lower, more preferably 190°C or lower, even more preferably 185°C or lower, and most preferably 180°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.

[0063] 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 is preferably 0.8°C / s or lower. 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.

[0064] [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 subjected to a reheating and quenching treatment at least once.

[0065] 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, more preferably 925°C or lower.

[0066] The seamless steel pipes, after reheating as described above, are then quenched. The cooling stop temperature during quenching is not particularly limited. However, if the cooling stop temperature during quenching exceeds 200°C, the martensitic transformation may not be completely completed, so it is preferably 200°C or lower, more preferably 195°C or lower, and even more preferably 190°C or lower. Furthermore, the lower limit of the cooling stop temperature during quenching is not particularly limited, but it may be above room temperature and may be 50°C or higher. Room temperature refers to a temperature of 10 to 35°C. Quenching refers to rapid cooling, and rapid cooling means cooling at an average cooling rate of 1.0°C / s or more from the reheating temperature to the quenching cooling stop temperature. The average cooling rate is preferably 1.3°C / s or more, more preferably 1.5°C / s or more, even more preferably 2.0°C / s or more, and most preferably 2.5°C / s or more. The cooling method is not particularly limited and can be water cooling, accelerated cooling, etc. The upper limit of the average cooling rate is not particularly limited, but from the viewpoint of manufacturing cost, 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.

[0067] 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.

[0068] The Ac3 transformation point shall be the value 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%.

[0069] [Tempering treatment] The seamless steel pipes, after reheating and quenching, are subjected to tempering treatment under the conditions that the average heating rate until a tempering temperature of 500°C or higher and below the Ac3 transformation point -50°C is reached is 0.5°C / min or higher, and the holding time at the said tempering temperature is 10 minutes or more and less than 60 minutes.

[0070] Tempering temperature: 500°C or higher, below the Ac3 transformation point (-50°C). Tempering is performed to form desired precipitates (Fe-based precipitates) and obtain toughness and desired hydrogen embrittlement resistance. If the tempering temperature is below 500°C, the formation of Fe-based precipitates will be insufficient, and the desired hydrogen embrittlement resistance cannot be secured. For this reason, the tempering temperature should be 500°C or higher. Preferably, the tempering temperature should be 550°C or higher, more preferably 575°C or higher, even more preferably 600°C or higher, and most preferably 625°C or higher. On the other hand, if the tempering temperature exceeds the Ac3 transformation point - 50°C, the microstructure will be austonitized, causing quench cracks, or the desired area ratio of tempered martensite cannot be obtained due to an increase in retained austonite. For this reason, the tempering temperature should be below the Ac3 transformation point - 50°C, depending on the steel type. Preferably, the tempering temperature should be below the Ac3 transformation point - 80°C, more preferably below the Ac3 transformation point - 100°C.

[0071] The average heating rate to reach the tempering temperature is 0.5°C / min or more. Fe precipitates during the heating process from the cooling stop temperature during quenching to the tempering temperature. If the average heating rate from the cooling stop temperature during quenching to the tempering temperature is less than 0.5°C / min, the size of the Fe precipitates will increase, and the desired hydrogen embrittlement resistance will not be obtained. Therefore, the average heating rate should be 0.5°C / min or more. The average heating rate is preferably 1.0°C / min or more, more preferably 2.0°C / min or more, even more preferably 3.0°C / min or more, and most preferably 5.0°C / min or more. 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 microstructure. Therefore, the average heating rate is preferably 50.0°C / min or less, more preferably 45.0°C / min or less, and even more preferably 40.0°C / min or less. The average heating rate can be determined by dividing the temperature difference from the cooling stop temperature during quenching to the tempering temperature by the time required for this heating.

[0072] Holding time: 10 minutes or more, less than 60 minutes Fe-based precipitates are most abundant during the tempering period. If the tempering period is less than 10 minutes, sufficient Fe-based precipitates will not form, and the desired hydrogen embrittlement resistance will not be obtained. Therefore, the tempering period should be 10 minutes or longer. Preferably, the tempering period should be 15 minutes or longer, more preferably 20 minutes or longer, and even more preferably 30 minutes or longer. On the other hand, if the tempering period is 60 minutes or longer, the size of the Fe-based precipitates will increase, and the desired hydrogen embrittlement resistance will not be obtained. Therefore, the tempering period should be less than 60 minutes. Since the tempering period is an energy-intensive factor that increases costs, it is preferably 59 minutes or less, more preferably 58 minutes or less, and even more preferably 57 minutes or less.

[0073] [cooling] The seamless steel pipes, after the tempering treatment described above, are cooled to a second cooling stop temperature of 300°C or lower at an average cooling rate of 0.1°C / s or higher.

[0074] Second cooling stop temperature: 300℃ or less After the above holding is complete, the seamless steel pipe is cooled to a second cooling stop temperature of 300°C or less. If the second cooling stop temperature exceeds 300°C, aggregation of Fe-based precipitates progresses, causing coarsening of the Fe-based precipitates, and the desired hydrogen embrittlement resistance cannot be obtained. Therefore, the second cooling stop temperature should be 300°C or less. The second cooling stop temperature is preferably 280°C or less, more preferably 250°C or less, even more preferably 230°C or less, and most preferably 200°C or less. On the other hand, the lower limit of the second 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.

[0075] Average cooling rate: 0.1℃ / s or more After the holding process described above is complete, the seamless steel pipe is cooled from the tempering temperature to the second cooling stop temperature at an average cooling rate of 0.1°C / s or higher. If the average cooling rate is less than 0.1°C / s, the Fe-based precipitates become coarser, and the desired hydrogen embrittlement resistance cannot be obtained. Therefore, the average cooling rate should be 0.1°C / s or higher. Preferably, the average cooling rate should be 0.5°C / s or higher, more preferably 1.0°C / s or higher, even more preferably 1.3°C / s or higher, and most preferably 1.5°C / s or higher. On the other hand, there is no particular upper limit to the average cooling rate, but if the average cooling rate is too fast, the steel pipe may crack. Therefore, preferably, the average cooling rate should be 3.0°C / s or lower, more preferably 2.8°C / s or lower. The average cooling rate can be determined by dividing the temperature difference from the tempering temperature to the second cooling stop temperature by the time required for this cooling.

[0076] 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.

[0077] 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.

[0078] 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.

[0079] Furthermore, since the present invention can provide seamless steel pipes without using expensive elements such as Mo and V, or even with the use of only small amounts of them, it is possible to provide seamless steel pipes that are cheaper than conventional seamless steel pipes that use Mo and V. [Examples]

[0080] 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.

[0081] [Table 1]

[0082] [Table 2]

[0083] [Table 3]

[0084] 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 and obtain a seamless steel pipe. After that, the seamless steel pipe was cooled to the first cooling stop temperature shown in Table 2. Next, the cooled seamless steel pipe was reheated under the conditions shown in Table 2, then quenched (water-cooled), tempered under the conditions shown in Table 2, and cooled under the conditions 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.

[0085] Organizational evaluation methods Area ratio of tempered martensite A specimen for microstructural observation was taken from near the center of the wall thickness (t / 2 position) of the seamless steel pipe obtained above, with dimensions of 10mmt x 10mmC x 10mmL (where C: circumferential direction, L: axial direction). At this time, the specimen for microstructural observation was taken such 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, microstructural images of the t / 2 position were acquired for three fields of view at appropriate magnifications of 1000 to 5000x, and tempered martensite, ferrite, bainite, and pearlite were observed. In the above microstructural images, the area of ​​tempered martensite was visually determined by comparing it with the microstructural photograph in Reference 2, 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 2] 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.

[0086] Grain size number of old austenite grains The particle size number of the prior austenite grains was measured using the specimens used for microstructural observation after obtaining the microstructural images described above. First, the observation surface of the specimens used for microstructural observation after obtaining the microstructural images 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 microstructural images were obtained at the t / 2 position in at least three fields of view. For the obtained microstructural 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.

[0087] Total amount of Fe contained in the precipitate As described above, the total amount of Fe 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 a total wall thickness x 10 mm C x 10 mm L (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 solution, 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 precipitates were extracted was filtered using a 100 nm pore size filter, and precipitates larger than 100 nm were collected on the filter. Subsequently, filtrate 1 was further filtered using a 50 nm pore size filter, and precipitates between 100 nm and 50 nm were collected on the filter. The resulting filtered filtrate 2 was dried, and precipitates smaller than 50 nm were collected. In this way, the precipitates were separated by size (50 nm or less, between 50 nm and 100 nm, and over 100 nm), and each size-classified precipitate was subjected to acid decomposition. The absolute amount of Fe contained in each size-classified precipitate (absolute amount of Fe in precipitates larger than 100 nm: W1, absolute amount of Fe in precipitates between 50 nm and 100 nm: W2, absolute amount of Fe in precipitates smaller than 50 nm: W3) was calculated using ICP emission spectrometry. Subsequently, by dividing W1, W2, and W3 by the weight difference W0 (amount of electrolysis) of the electrolytic test specimens before and after electrolysis, the total amount of Fe (mass%) contained in precipitates exceeding 100 nm, the total amount of Fe (mass%) contained in precipitates between 50 nm and 100 nm, and the total amount of Fe (mass%) contained in precipitates of 50 nm or less were calculated. Furthermore, using the total amount of Fe (mass%) contained in the precipitates with a wavelength between 50 nm and 100 nm, and the total amount of Fe (mass%) contained in the precipitates with a wavelength of 50 nm or less, the ratio of the total amount of Fe contained in the precipitates with a wavelength of 50 nm or less to the total amount of Fe contained in the precipitates with a wavelength of 100 nm or less was calculated.

[0088] Tensile strength Bar-shaped test specimens, as specified in JIS Z 2201 "Tensile Test Specimens for Metallic Materials," were taken from the steel pipes as tensile test specimens. The tensile test specimens were prepared such that the longitudinal direction (tensile direction) of the specimen aligned with the circumferential direction (C direction) of the steel pipe, and the center of the cross-section perpendicular to the tensile direction was at the t / 2 position of the steel pipe. The tensile test was performed using the method specified in JIS Z2241, and the tensile strength (TS) of the steel pipe was calculated by dividing the maximum load by the initial cross-sectional area. Steel pipes with a TS of 850 MPa or higher were deemed acceptable.

[0089] Hydrogen embrittlement resistance Hydrogen embrittlement resistance was evaluated by conducting fatigue crack propagation tests in hydrogen gas and determining the fatigue crack propagation rate. First, a compact tension (CT) test specimen 2, used for the fatigue crack propagation test, was taken from the steel pipe in accordance with ASTM E 647, with the load application direction parallel to the circumferential direction 3 (direction C), as shown in Figure 1. Here, the thickness of specimen 2 was set to 10 mm in the wall thickness direction of the steel pipe, and the center of the thickness of specimen 2 was positioned at t / 2. The front and back surfaces of specimen 2 were mirror-polished to avoid variations in fatigue crack propagation rate in hydrogen gas due to surface finish. Furthermore, fatigue pre-cracks were introduced into the specimen under atmospheric conditions. In Figure 1, 1 is the rolling direction (pipe axis (L) direction) and 3 is the circumferential direction (direction C). A fatigue crack propagation test was performed on the obtained specimen 2 at room temperature (20±10℃) in hydrogen gas at 5 MPa under the following conditions: frequency: 1 Hz, cyclic load waveform: sine wave, control method: load control, load condition: short-axis tension, stress ratio R: 0.1. The fatigue crack propagation rate was determined by measuring the length of the fatigue crack using a clip gauge and the compliance method. As for fatigue crack propagation rate, the stable growth region where Paris's law holds is defined as the stress intensity factor range ΔK = 25 (MPa·m). 1 / 2 The fatigue crack propagation rate (m / cycle) in hydrogen gas was used.

[0090] The results obtained are shown in Table 3. A higher fatigue crack propagation rate indicates inferior hydrogen embrittlement resistance. (Fatigue crack propagation rate of 1.00 × 10⁻⁶) -6 Steel pipes that yielded a m / cycle or less were deemed acceptable.

[0091] Tables 1, 2, and 3 show that all of the invention examples exhibit high strength (tensile strength of 850 MPa or more) and excellent resistance to hydrogen embrittlement (fatigue crack propagation rate of 1.00 × 10⁻¹⁰). -6 The requirement of m / cycle or less was satisfied. Furthermore, the present invention can achieve high strength and excellent hydrogen embrittlement resistance even without using expensive elements such as Mo and V, or with the use of only small amounts. [Explanation of Symbols]

[0092] 1. Rolling direction (pipe axis (L) direction) 2 CT (Compact Tension) Test Specimens 3 Circumferential direction (C direction)

Claims

1. In mass percent, C: 0.20-0.50%, Si: 0.05-0.80%, 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: 0.25-1.20%, 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 is such that the sum of Cr and Si, Cr+Si, is 0.75% or more. The structure has tempered martensite accounting for 95% or more of the area. The aforementioned structure has precipitates, The total amount of Fe contained in the precipitates with a maximum length of 100 nm or less is 0.005% by mass or more. Furthermore, the seamless steel pipe is characterized in that the total amount of Fe contained in the precipitates with a maximum length exceeding 100 nm is 1.700% or less by mass.

2. Furthermore, the above component composition is expressed in mass percent as follows: Mo: 1.0% or less 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 higher and 1350°C or lower, and then hot-rolled to form 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 between the transformation point and 1000°C, and then subjected to a reheating and quenching treatment at least once. The seamless steel pipe after the aforementioned reheating and quenching treatment is subjected to a temperature of 500°C or higher. ３ The tempering process is performed under the conditions that the average heating rate until the tempering temperature of -50°C or lower is reached is 0.5°C / min or more, and the holding time at the tempering temperature is 10 minutes or more and less than 60 minutes. A method for manufacturing seamless steel pipes, comprising cooling the seamless steel pipe after the tempering treatment to a second cooling stop temperature of 300°C or lower at an average cooling rate of 0.1°C / s or higher.

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