Seamless steel pipe and method for producing 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

Seamless steel pipe and its manufacturing method

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

[0002] Currently, various efforts are being considered around the world to realize a decarbonized society. One of these is the use of hydrogen energy. Fuel cell vehicles use hydrogen as fuel, which reduces carbon dioxide (CO ２ ) and are highly energy efficient. Therefore, the widespread use of fuel cell vehicles is important in realizing a decarbonized society. To popularize fuel cell vehicles, storage containers (pressure accumulators) with excellent strength and durability that can safely store high-pressure hydrogen of 35 MPa or more, especially 70 MPa or more, are needed for hydrogen stations that supply hydrogen to fuel cell vehicles and for on-board use that loads hydrogen onto fuel cell vehicles, and development of such containers is underway.

[0003] Since vehicle-mounted pressure accumulators are required to be lightweight, pressure accumulators have been proposed in which a liner made of a lightweight material such as aluminum (Al) is covered with carbon fiber reinforced plastic (CFRP). For example, Patent Document 1 describes a liner made of an Al alloy that has excellent fatigue properties.

[0004] On the other hand, since weight reduction is not required for pressure accumulators for hydrogen stations, liners made of low-alloy steel have been proposed. For example, Patent Document 2 proposes a pressure accumulator in which a liner made of Cr-Mo steel is coated with carbon fiber or glass fiber. However, low-alloy steels, including Cr-Mo steel, are known to be embrittled by hydrogen (hydrogen embrittlement). Therefore, Al alloys and SUS316, which are less susceptible to hydrogen embrittlement, are recommended as materials for high-pressure hydrogen accumulators of 35 MPa or higher. However, Cr-Ni austenitic stainless steels have low strength (800 MPa or less). Therefore, if a Cr-Ni austenitic stainless steel is used as the material for the pressure accumulator and the hydrogen pressure is increased to, for example, 70 MPa, the wall thickness of the pressure accumulator must be significantly increased, resulting in an increase in the weight of the pressure accumulator. This results in high material costs and poor economic efficiency. Furthermore, in the case of an on-board pressure accumulator, restrictions are placed on the weight and / or size of the pressure accumulator, which necessitates a reduction in the size of the pressure accumulator, i.e., the volume of the pressure accumulator, resulting in a problem of a reduction in the amount of hydrogen that can be stored in the pressure accumulator.

[0005] Therefore, much research is being conducted to apply low-alloy steels, which have lower material costs, as materials for high-pressure hydrogen pressure vessels. For example, Patent Documents 3 and 4 propose steels for use in high-pressure hydrogen gas environments that utilize fine V-Mo carbides in the steel as hydrogen trapping sites to suppress hydrogen embrittlement due to diffusible hydrogen.

[0006] JP 2009-024225 A JP 2009-293799 A JP 2010-037655 A JP 2009-074122 A

[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] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a seamless steel pipe having high strength and excellent hydrogen embrittlement resistance, and a method for manufacturing the same.

[0009] High-pressure hydrogen storage vessels must have both high strength and hydrogen embrittlement resistance. Therefore, various factors affecting the strength and hydrogen embrittlement resistance of steel materials were investigated. As a result, the inventors discovered that hydrogen embrittlement in steel materials with high tensile strength (TS) occurs when hydrogen accumulates at grain boundaries, causing fracture along the grain boundaries (intergranular fracture). Based on this mechanism, hydrogen trapping sites can be formed in steel materials (steel pipes) to suppress the occurrence of intergranular fracture, i.e., to improve hydrogen embrittlement resistance. The inventors conducted extensive research into hydrogen trapping sites and found that Fe-based precipitates act as strong hydrogen trapping sites. However, the presence of a certain amount of Fe-based precipitates with a maximum length exceeding 100 nm reduces hydrogen embrittlement resistance. Through extensive research, the inventors discovered the following mechanism: Fe-based precipitates with a maximum length exceeding 100 nm have a strong tendency to aggregate in steel pipes, causing the precipitate distribution to become non-uniform. 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, stress concentrates on the precipitates that trap a large amount of hydrogen, and these precipitates become fracture origins, resulting in a decrease in hydrogen embrittlement resistance. Therefore, it was considered important to suppress the formation of Fe-based precipitates with a maximum length of more than 100 nm.

[0010] The present invention has been completed based on the above findings, and the gist and configuration of the present invention are as follows. [1] A steel sheet having a chemical composition containing, by mass%, C: 0.20 to 0.50%, Si: 0.05 to 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 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%, with the balance being Fe and unavoidable impurities, and wherein the sum of the Cr and the Si, Cr+Si, is 0.75% or more; and wherein the steel sheet has a structure in which tempered martensite accounts for 95% or more in area ratio; and wherein the structure has precipitates. [2] A seamless steel pipe according to [1], wherein the composition further comprises, in mass%, at least one selected from 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. [3] A steel pipe material having the chemical composition according to [1] or [2] is heated at a heating temperature of 1050°C or more and 1350°C or less, and then hot-rolled to form a seamless steel pipe, and the seamless steel pipe is cooled to a first cooling stop temperature of 200°C or less, and the seamless steel pipe after cooling is subjected to Ac ３ The seamless steel pipe is reheated to a reheating temperature of not less than the transformation point and not more than 1000°C and quenched at least once, and the reheating and quenching treatment is carried out at a temperature of not less than 500°C and Ac ３ A method for manufacturing a seamless steel pipe, comprising: performing a tempering treatment under conditions in which an average heating rate is 0.5°C / min or more until a tempering temperature of −50°C or less is reached, and a holding time at the tempering temperature is 10 minutes or more and less than 60 minutes; and cooling the seamless steel pipe after the tempering treatment at an average cooling rate of 0.1°C / s or more to a second cooling stop temperature of 300°C or less.

[0011] According to the present invention, a seamless steel pipe having 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 extremely suitably used as a seamless steel pipe for a high-pressure hydrogen storage tank.

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

[0013] Hereinafter, an embodiment of the present invention will be described. Note that the following description shows a preferred embodiment of the present invention, and the present invention is not limited to the following description in any way.

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

[0015] C: 0.20 to 0.50% C contributes to increasing the strength of steel pipes through solid solution, improves the hardenability of steel pipes, and contributes to the formation of a structure in which martensite is the main phase during quenching. To achieve these effects, the C content is set to 0.20% or more. The C content is preferably set to 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, significantly reducing manufacturability. For this reason, the C content is set to 0.50% or less. The C content is preferably set to 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 to 0.80%. Si is included as a deoxidizer, but if the Si content is less than 0.05%, the deoxidizing effect is insufficient. Furthermore, Si has the effect of improving softening resistance (temper softening resistance) during the tempering treatment described below, thereby suppressing the growth of Fe-based precipitates and reducing the amount of Fe-based precipitates larger than 100 nm. As a result, desired hydrogen embrittlement resistance is obtained. To achieve the above-mentioned effect, the Si content is set to 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 greater the above-mentioned effect, but if the Si content exceeds 0.80%, the above-mentioned effect saturates. Therefore, the Si content is set to 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 to 1.50% Like C, Mn is an element that improves the hardenability of steel pipes and contributes to increasing their strength. To achieve this effect, the Mn content is set to 0.30% or more. The Mn content is preferably set to 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 contained, localized hardened regions will form, reducing hydrogen embrittlement resistance. For this reason, the Mn content is set to 1.50% or less. The Mn content is preferably set to 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. P is present in steel pipes as an unavoidable impurity. However, it segregates at grain boundaries in the steel pipe structure, causing grain boundary embrittlement, and also segregates to locally harden the steel pipe. Therefore, it is preferable to reduce its content as much as possible. However, a P content of up to 0.015% is acceptable. Therefore, the P content is set to 0.015% or less. The P content is preferably set to 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 increases refining costs. Therefore, the P content is preferably set to 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 the S is present as sulfide-based inclusions, which reduce ductility, toughness, and SCC (stress corrosion cracking) resistance. Therefore, it is preferable to reduce the S content as much as possible. However, an S content of up to 0.005% is tolerable. For this reason, the S content is set to 0.005% or less. The S content is preferably set to 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, there is no particular limitation on the lower limit of the S content, and it may be 0%. However, excessive reduction of S leads to an increase in refining costs. For this reason, the S content is preferably set to 0.0002% or more, and more preferably 0.001% or more.

[0020] Al: 0.150% or less Al is an element that acts as a deoxidizer 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 is set to 0.150% or less. The Al content is preferably set to 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 the Al content, the Al content is preferably set to 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 N is present in steel pipes as an unavoidable impurity. It combines with Al in the steel pipe to form AlN and with Ti in the steel pipe to form TiN, thereby refining prior austenite grains and improving toughness. However, if the N content exceeds 0.006%, the resulting nitrides, such as AlN and TiN, become coarse, significantly reducing toughness. Therefore, the N content is set to 0.006% or less. The N content is preferably set to 0.005% or less, more preferably 0.004% or less, and even more preferably 0.003% or less. 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 inclusion, the N content is preferably set to 0.001% or more, more preferably 0.002% or more.

[0022] Cr: 0.25 to 1.20% Cr is an element that increases the strength of steel pipes by improving hardenability and also improves corrosion resistance. Cr also combines with C during tempering treatment to form Fe. ３ C, M ７ C ３ , M ２３ C ６(M is a metallic element) and other precipitates, which, like Si, have the effect of improving temper softening resistance. To achieve this effect, the Cr content is set to 0.25% or more. The Cr content is preferably set to 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 becomes greater than the hydrogen embrittlement resistance obtained. Therefore, the Cr content is set to 1.20% or less. The Cr content is preferably set to 1.15% or less, more preferably 1.10% or less, and even more preferably 1.05% or less.

[0023] Nb: 0.001 to 0.020% Nb forms precipitates, such as carbonitrides, and contributes to increasing the strength of steel pipes through precipitation strengthening. Nb also refines austenite grains and contributes to improving toughness. To achieve these effects, the Nb content is set to 0.001% or more. The Nb content is preferably set to 0.002% or more, more preferably 0.003% or more, even more preferably 0.004% or more, and most preferably 0.005% or more. On the other hand, if the Nb content exceeds 0.020%, a large amount of Nb-based precipitates is formed, resulting in a decrease in hydrogen embrittlement resistance. Therefore, from the viewpoint of achieving both the desired strength and excellent hydrogen embrittlement resistance, the Nb content is set to 0.020% or less in the present invention. The Nb content is preferably set to 0.018% or less, more preferably 0.015% or less, even more preferably 0.013% or less, and most preferably 0.010% or less.

[0024] B: 0.0003 to 0.0030% B segregates at austenite grain boundaries and inhibits ferrite transformation from the grain boundaries, thereby improving the hardenability of steel pipes and improving toughness and strength, even with a small amount of B. To achieve this effect, the B content is set to 0.0003% or more. The B content is preferably set to 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., which reduces hardenability and toughness. Furthermore, adding excessive B increases the hardenability of Fe. ２B is generated, causing red shortness. Therefore, the B content is set to 0.0030% or less. The B content is preferably set to 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] O (oxygen): 0.0030% or less O (oxygen) is an unavoidable impurity and exists as oxide-based inclusions in steel pipes. These inclusions act as crack initiation sites in hydrogen gas environments 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 is set to 0.0030% or less. The O (oxygen) content is preferably set to 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 increased refining costs. Therefore, the O (oxygen) content is preferably 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 to 0.025%. Ti combines with N during solidification of molten steel and precipitates as fine TiN. This pinning effect contributes to the refinement of austenite grains and improves toughness and strength. If the Ti content is less than 0.003%, this effect is small. To obtain this effect, the Ti content is set to 0.003% or more. The Ti content is preferably 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, if the Ti content exceeds 0.025%, the TiN becomes coarse, the pinning effect is not achieved, and toughness is actually reduced. For these reasons, the Ti content is set to 0.025% or less. The Ti content is preferably 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] A steel pipe according to one embodiment of the present invention has a chemical composition containing the above elements, with the balance being Fe and unavoidable impurities. Here, a steel pipe according to one embodiment of the present invention may contain only the above elements and the balance being Fe and unavoidable impurities. Furthermore, unavoidable impurities are impurities that are inevitably mixed in from raw materials, manufacturing processes, manufacturing equipment, etc., and are allowed to be included to a degree that does not impair 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. A Mg content of 0.0008% or less and a Co content of 0.0008% or less are allowed.

[0028] Cr + Si ≧ 0.75% The inventors have discovered that it is important to set the sum (Cr + Si) of Cr (Cr content) and Si (Si content) contained in the steel pipe to 0.75% or more in order to suppress the formation of Fe-based precipitates with a maximum length exceeding 100 nm. As described above, Cr and Si have the effect of improving temper softening resistance, so they can suppress the formation of Fe-based precipitates with a maximum length exceeding 100 nm, thereby achieving the desired hydrogen embrittlement resistance. If Cr + Si is less than 0.75%, the above-mentioned effect cannot be obtained, and the desired hydrogen embrittlement resistance cannot be obtained. Therefore, Cr + Si is set to 0.75% or more. Cr + Si is preferably set to 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 higher the Cr and Si content, the greater the above-mentioned effects, so Cr and Si can be contained up to their respective upper limits. That is, although there is no particular upper limit for Cr+Si, it can be contained up to 2.00% at most, and may be 1.90% or less.

[0029] In another embodiment of the present invention, the above-mentioned composition may further contain at least one element selected from the group consisting of Mo, V, Cu, Ni, Sn, W, and Ca.

[0030] Mo: 1.0% or less Mo forms precipitates (Mo-based precipitates) and contributes to further increasing the strength of steel pipes through precipitation strengthening. Furthermore, the formed Mo-based precipitates function as hydrogen trapping sites, thereby further improving hydrogen embrittlement resistance. Even when dissolved in a steel pipe, Mo segregates at prior austenite grain boundaries, contributing to further improvement of hydrogen embrittlement resistance. Therefore, Mo can be added at any amount depending on the desired properties. However, Mo is an expensive element, and if the Mo content exceeds 1.0%, the disadvantage of increased cost outweighs the benefits of improved hydrogen embrittlement resistance. Therefore, when Mo is contained, the Mo content is set to 1.0% or less. The Mo content is preferably set to 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 effect of containing 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 the steel pipe. V also has the effect of refining austenite grains and improving toughness. Therefore, V can be added at 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 addition saturates, and the effect commensurate with the V content cannot be expected, resulting in economic disadvantage. Therefore, when V is contained, the V content is set to 0.30% or less. The V content is preferably set to 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 fully obtaining the effect of V addition, the V content is preferably set to 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 element effective in improving toughness and increasing strength. Therefore, it can be added at any amount depending on the desired properties. However, if the Cu content exceeds 1.00%, elongation deteriorates due to the formation of intermetallic compounds. Therefore, when Cu is added, the Cu content is set to 1.00% or less. The Cu content is preferably set to 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 fully obtaining the effect of Cu inclusion, the Cu content is preferably set to 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 added at any amount depending on the desired properties. However, if the Ni content exceeds 2.0%, the effect of Ni content saturates, and no effect commensurate with the Ni content can be expected, resulting in an economic disadvantage. Therefore, when Ni is contained, the Ni content is set to 2.0% or less. The Ni content is preferably set to 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 fully obtaining the effect of Ni content, the Ni content is preferably set to 0.1% or more, more preferably 0.2% or more.

[0034] Sn: 0.50% or less Sn (tin) may be contained in steel pipes as an unavoidable impurity when steel pipe materials are produced in electric furnaces. Sn tends to segregate at grain boundaries, which can cause a decrease in strength when the steel pipe is used in a high-temperature environment. Sn can also reduce the ductility of the steel pipe and deteriorate its 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 contained, the Sn content is set to 0.50% or less. The Sn content is preferably set to 0.30% or less. On the other hand, the lower limit of the Sn content is not particularly limited and may be 0%. However, excessive reduction of Sn leads to increased refining costs. Therefore, the Sn content is preferably set to 0.10% or more.

[0035] W: 3.0% or less W forms precipitates, contributing to increased strength of steel pipes through precipitation strengthening, and dissolves and segregates at prior austenite grain boundaries, further improving hydrogen embrittlement resistance. Therefore, W can be added at any amount depending on the desired properties. However, if the W content exceeds 3.0%, the effect of W saturates, and the effect commensurate with the W content cannot be expected, resulting in economical disadvantages. Therefore, if W is contained, the W content is set to 3.0% or less. The W content is preferably set to 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 fully obtaining the effect of W, the W content is preferably set to 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-based inclusions. Through this morphology control, Ca contributes to improving toughness and hydrogen embrittlement resistance. Therefore, it can be added at any amount depending on the desired properties. However, if the Ca content exceeds 0.0050%, the effect of Ca addition saturates, and the effect commensurate with the Ca content cannot be expected, resulting in economic disadvantage. Therefore, if Ca is contained, the Ca content is set to 0.0050% or less. The Ca content is preferably set to 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 fully obtaining the effect of Ca addition, the Ca content is preferably set to 0.0005% or more, more preferably 0.0010% or more, and even more preferably 0.0015% or more.

[0037] H may be introduced into a steel pipe during various manufacturing processes. A high H content increases the risk of cracking after solidification and degrades hydrogen embrittlement resistance. Therefore, it is preferable to reduce the H content of the steel pipe. These effects are not a problem if the H content of the steel pipe is 0.00100% or less. Therefore, if the steel pipe contains H, the H content of the steel pipe is set to 0.0010% 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 the 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. The H content of the steel pipe is the amount of hydrogen remaining after steel pipe forming. Furthermore, it is sufficient for the H content of the steel pipe to be 0.00100% or less, and the H content of the steel pipe material described below is not particularly limited. However, the H content 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 in the steel pipe material is also based on the same concept as for steel pipes, and the lower limit of the H content in the steel pipe material is not particularly limited and may be 0% or 0.00001% or more.

[0038] [Structure] The seamless steel pipe of the present invention has a structure in which tempered martensite accounts for 95% or more in terms of area ratio, and the structure has precipitates, and the total amount of Fe contained in the precipitates having a maximum length of 100 nm or less is 0.005% or more in terms of mass%, and further the total amount of Fe contained in the precipitates having a maximum length of more than 100 nm is 1.700% or less in terms of mass%.

[0039] Tempered martensite area fraction of 95% or more: In order to provide the seamless steel pipe of the present invention with high tensile strength (TS) and the ductility and toughness required for a steel pipe, the area fraction of tempered martensite obtained by tempering martensite is 95% or more. The area fraction of tempered martensite is preferably 96% or more, more preferably 97% or more, and even more preferably 98% or more. On the other hand, the upper limit of the area fraction of tempered martensite is not particularly limited and may be 100%.

[0040] Other structures: The structure in one embodiment of the present invention may be composed of tempered martensite. Furthermore, the structure in other embodiments of the present invention may contain, in addition to tempered martensite, other structures at an area ratio of 5% or less. The other structures are not particularly limited and may be any structure. For example, the other structures may be at least one selected from bainite, retained austenite, and pearlite, or may be a mixed phase of two or more phases. The lower limit of the area ratio of the other structures may be 0%.

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

[0042] The above-mentioned structure of the seamless steel pipe of the present invention can be adjusted by properly controlling the cooling conditions after hot rolling of the steel pipe and the reheating temperature during reheating and quenching treatment.

[0043] Prior austenite grains: In one embodiment of the present invention, if the grain size number of the prior austenite grains, which is an index of the structure before martensitic transformation, is less than 8.5, the substructure of the resulting martensite phase will become coarse, resulting in reduced toughness. Therefore, the grain size number of the prior austenite grains is preferably 8.5 or more, more preferably 8.8 or more, even more preferably 9.5 or more, and most preferably 10.0 or more. While there is no particular upper limit to the grain size number of the prior austenite grains, the grain size number of the prior austenite grains is preferably 18.5 or less, more preferably 17.5 or less, even more preferably 17.0 or less, and most preferably 16.5 or less. The grain size number of the prior austenite grains is a value measured in accordance with the provisions of JIS G 0551. Specifically, the grain size number of the prior austenite grains is determined based on the method described in the examples.

[0044] The grain size number of the prior austenite grains in the seamless steel pipe of the present invention can be adjusted by changing the reheating temperature during reheating and quenching treatment and the number of times the reheating and quenching treatment is carried out.

[0045] The total amount of Fe contained in the precipitates having a maximum length of 100 nm or less is 0.005% by mass or more: In order to obtain the desired hydrogen embrittlement resistance, the steel pipe of the present invention must contain Fe-based precipitates having a maximum length of 100 nm or less. That is, 0.005% by mass or more of Fe must be contained in the precipitates having a maximum length of 100 nm or less. Because Fe-based precipitates having a maximum length of 100 nm or less act as hydrogen trapping sites, it is important that the above-mentioned structure contain a predetermined amount or more of Fe-based precipitates having a maximum length of 100 nm or less. That is, 0.005% by mass or more of Fe must be contained in the precipitates having a maximum length of 100 nm or less. The total amount of Fe contained in the precipitates having 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, the upper limit of the total amount of Fe contained in precipitates having a maximum length of 100 nm or less is not particularly limited, but the total amount of Fe contained in precipitates having a maximum length of 100 nm or less may be 0.200 mass% or less.

[0046] The total amount of Fe contained in the precipitates having a maximum length exceeding 100 nm is 1.700% by mass or less: The inventors have discovered that reducing the number of Fe-based precipitates having a maximum length exceeding 100 nm is important for achieving desired hydrogen embrittlement resistance. Fe-based precipitates having a maximum length exceeding 100 nm tend to aggregate in the steel pipe, resulting in a non-uniform distribution of the precipitates. In other words, the distribution of hydrogen trapping sites becomes non-uniform, resulting in localized hydrogen trapping. As a result, when stress is applied to the steel pipe, these precipitates become locations where stress concentrates and fracture origins, resulting in reduced hydrogen embrittlement resistance. Therefore, it is sufficient to reduce the number of Fe-based precipitates having a maximum length exceeding 100 nm, i.e., to set the total amount of Fe contained in the precipitates to 1.700% by mass or less. Therefore, the total amount of Fe contained in the precipitates having a maximum length exceeding 100 nm is set to 1.700% by mass or less. The total amount of Fe contained in precipitates having a maximum length of more than 100 nm is preferably 1.600% or less, more preferably 1.500% or less, even more preferably 1.450% or less, and most preferably 1.400% or less, in mass %. On the other hand, since the number of Fe-based precipitates having a maximum length of more than 100 nm is preferably as small as possible, the lower limit of the total amount of Fe contained in precipitates having a maximum length of more than 100 nm is not limited and may be 0%.

[0047] Furthermore, the present inventors have found that hydrogen embrittlement resistance can be further improved by setting the ratio of the total amount of Fe contained in precipitates having a maximum length of 50 nm or less to the total amount of Fe contained in precipitates having a maximum length of 100 nm or less to 30% or more. Therefore, the 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 ratio is not particularly limited and may be 100%. The upper limit of the size of the precipitates exceeding 100 nm is not particularly limited and may be 600 nm or less. The lower limit of the size of the precipitates having a maximum length of 100 nm or less is not particularly limited and may be 5 nm or more. The lower limit of the size of the precipitates having a maximum length of 50 nm or less is not particularly limited and may be 5 nm or more. Here, the Fe-based precipitates include, for example, cementite, ε-carbide, χ-carbide, and Fe ７ C ３ This refers to intermetallic compounds such as:

[0048] The total amount of Fe contained in the precipitates can be determined by the filter filtration extraction method described in Patent Document 5 and Reference 1. Specifically, a test piece measuring 10 mm C x 10 mm L (C: pipe circumferential direction, L: pipe axial direction) is collected from a steel pipe as an electrolysis test piece. The obtained electrolysis test piece is electrolyzed in an electrolytic solution, and then the electrolytic test piece is immersed in a dispersible solution and ultrasonic waves are applied to extract the precipitates attached to the surface of the electrolysis test piece 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 exceeding 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 of 100 nm or less but exceeding 50 nm are collected on the filter. The obtained filtered filtrate 2 is dried, and precipitates of 50 nm or less are collected. In this way, the precipitates were separated by size (50 nm or less, more than 50 nm but not more than 100 nm, and more than 100 nm), and the precipitates separated by size were each subjected to acid decomposition. The absolute amounts of Fe contained in the precipitates separated by size were calculated using ICP atomic emission spectrometry (absolute amount of Fe in precipitates larger than 100 nm: W1, absolute amount of Fe in precipitates larger than 50 nm but not more than 100 nm: W2, absolute amount of Fe in precipitates smaller than 50 nm: W3). Then, by dividing W1, W2, and W3 by the weight difference W0 (amount of electrolysis) between the electrolysis test pieces before and after electrolysis, the total amount (mass%) of Fe contained in precipitates larger than 100 nm, the total amount (mass%) of Fe contained in precipitates larger than 50 nm but not more than 100 nm, and the total amount (mass%) of Fe contained in precipitates smaller than 50 nm could be calculated. Furthermore, the ratio of the total amount of Fe contained in precipitates of 50 nm or less to the total amount of Fe contained in precipitates of 100 nm or less can be calculated using the obtained total amount of Fe (mass%) contained in precipitates of more than 50 nm and 100 nm or less and the total amount of Fe contained in precipitates of 50 nm or less. [Patent Document 5] JP 2010-127791 A [Reference Document 1] Ishida et al., Analysis of the Formation State of Fine Precipitates in Steel, Tetsu-to-Hagané, 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-based inclusions and oxide-based inclusions can be reduced by performing the following management during the molten steel refining process. Specifically, desulfurization and dephosphorization are performed in the hot metal pretreatment, followed by decarburization and dephosphorization in a converter, followed by a hot-stirring refining process (LF) and a RH vacuum degassing process in the ladle. A sufficient processing time is ensured in the hot-stirring refining process (LF), and a sufficient processing time is also ensured in the RH vacuum degassing process, and the RH reflux rate is controlled. Furthermore, when producing a slab (steel pipe material) by continuous casting, in order to reduce inclusions, an inert gas seal is applied during pouring from the ladle into the tundish, and electromagnetic stirring is applied in the mold to float and separate the inclusions. While the refining process is not limited to the above, it is preferable to produce the steel under the above conditions.

[0050] The seamless steel pipe of the present invention has a high tensile strength (TS). Therefore, it can also be used for high-pressure hydrogen containers (high-pressure hydrogen pressure vessels). The specific TS value is not particularly limited, but TS is preferably 850 MPa or more, more preferably 860 MPa or more, even more preferably 880 MPa or more, and most preferably 900 Pa or more. On the other hand, since the hydrogen embrittlement resistance tends to decrease as TS increases, TS is preferably 1000 MPa or less, more preferably 990 MPa or less. The TS is determined based on the method described in the examples.

[0051] In the present invention, properties such as the structure, the grain size number of prior austenite grains, the 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: Because a steel pipe is cooled from the outer and inner surfaces of the steel pipe, the cooling rate at the t / 2 position of the steel pipe is the slowest. As a result, it is difficult to obtain a desired structure, such as martensite (quenched structure), and it is also difficult to obtain desired properties. Therefore, if the desired structure and desired properties are obtained near the t / 2 position, it can be determined that the desired structure and desired properties are obtained at any position of the steel pipe.

[0052] 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. 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 pressure vessels are suitable for use in high-pressure hydrogen applications. Specifically, they are applicable to high-pressure hydrogen applications where the hydrogen pressure is preferably 1 MPa or more, more preferably 10 MPa or more, even more preferably 35 MPa or more, and most preferably 70 MPa or more. On the other hand, the upper limit of the hydrogen pressure is not particularly limited, but may be 115 MPa or less, 105 MPa or less, or 95 MPa or less depending on the actual situation. Note that the high-pressure hydrogen pressure vessels mentioned above include not only pressure vessels containing 100% hydrogen gas but also pressure vessels containing hydrogen-containing gases with less than 100% hydrogen, and therefore the hydrogen pressure mentioned above refers to hydrogen partial pressure or total hydrogen pressure.

[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 chemical composition to form a seamless steel pipe, and then sequentially cooling, reheating and quenching, tempering, and cooling. In the following description, temperatures refer to the temperatures at the surface of the steel pipe material or steel pipe, unless otherwise specified. The surface temperatures are values ​​measured with a radiation thermometer.

[0055] Steel pipe material: The steel pipe material used in the present invention (hereinafter also simply referred to as "bill") is preferably produced by melting molten steel having the above-described chemical composition using a conventional melting method such as a converter, and then forming a billet (round billet) using a conventional casting method such as a continuous casting method. The billet may be further hot-rolled to form a round billet of a predetermined shape, or may be produced by undergoing ingot making and blooming.

[0056] When a steel pipe material is cast into a slab (round slab) by a casting method, the slower the casting speed, the more effectively the hydrogen concentration and inclusions in the steel pipe can be reduced. This effect is most pronounced at a casting speed of 1.0 m / min or less, so 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. Meanwhile, the lower limit of the casting speed is not particularly limited, and 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 steel pipe material is heated and then hot rolled to form a seamless steel pipe.

[0058] Heating temperature: 1050°C or higher and 1350°C or lower. The steel pipe material is heated prior to hot rolling. Heating may be performed after the steel pipe material obtained by a method such as casting has been cooled, or the obtained steel pipe material may be directly subjected to the heating without being cooled. If the heating temperature of the steel pipe material is lower than 1050°C, precipitates in the steel pipe material will not dissolve sufficiently. Therefore, the heating temperature is set to 1050°C or higher. The heating temperature is preferably set to 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, prior austenite grains will coarsen, and precipitates such as TiN precipitated during solidification will also coarsen, and cementite will also coarsen, resulting in a decrease in the toughness of the steel pipe. Furthermore, a thick scale layer forms on the surface of the steel pipe material, which causes surface defects during hot rolling and increases energy loss, which is undesirable from the viewpoint of energy conservation. For these reasons, the heating temperature is set to 1350°C or less. The heating temperature is preferably 1330°C or less, more preferably 1300°C or less, even more preferably 1280°C or less, and most preferably 1250°C or less.

[0059] Next, the heated steel pipe material is subjected to hot rolling to produce a seamless steel pipe. For the hot rolling, hot rolling including piercing rolling of a conventional Mannesmann plug mill method or Mannesmann mandrel mill method can be used.

[0060] It should be noted that a seamless steel pipe may be obtained by expanding the pipe after hot rolling. The roundness of the seamless steel pipe can be improved by expanding the pipe using a pipe expanding mill. The expansion rate may be appropriately set depending on the target wall thickness (pipe thickness), outer diameter, strength, and target roundness of the seamless steel pipe.

[0061] [Cooling] The obtained seamless steel pipe is cooled to a first cooling stop temperature of 200°C or lower.

[0062] First cooling stop temperature: 200°C or less If the first cooling stop temperature exceeds 200°C, martensitic transformation will not be completed completely, and the desired area ratio of tempered martensite will not be obtained after tempering. Therefore, the first cooling stop temperature is set to 200°C or less. The first cooling stop temperature is preferably set to 195°C or less, more preferably 190°C or less, even more preferably 185°C or less, and most preferably 180°C or less. On the other hand, the lower limit of the first cooling stop temperature is not particularly limited, but may be room temperature or higher, or may be 50°C or higher. Here, room temperature refers to 10 to 35°C.

[0063] Furthermore, the average cooling rate from the end of hot rolling (hot rolling end temperature) to the first cooling stop temperature is preferably 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 easy to obtain tempered martensite with a desired area ratio after tempering. Here, in the present invention, "average cooling rate equal to or greater than that of air cooling" refers to 0.1°C / s or greater. If the average cooling rate is less than that of air cooling, i.e., less than 0.1°C / s, the structure after cooling becomes nonuniform, making it difficult to obtain tempered martensite with a desired area ratio after the subsequent reheating, quenching, and tempering treatments. The average cooling rate is more preferably 0.2°C / s or greater, and even more preferably 0.3°C / s or greater. On the other hand, the upper limit of the average cooling rate is not particularly limited, but is preferably 0.8°C / s or less. The average cooling rate can be calculated by dividing the temperature difference from the end temperature of hot rolling to the first cooling stop temperature by the time required for this cooling.

[0064] [Reheating and Quenching Treatment] The seamless steel pipe after cooling is subjected to Ac ３ The steel is reheated to a reheating temperature of not less than the transformation point and not more than 1000°C and quenched, and this reheating and quenching treatment is carried out one or more times.

[0065] Reheating temperature: Ac ３ Reheating temperature is Ac ３ If the temperature is lower than the transformation point, the steel will not be heated to the austenite single phase region, and therefore a structure with martensite as the main phase will not be obtained, and the desired area ratio of tempered martensite will not be obtained after tempering. ３ The reheating temperature is preferably Ac ３ Transformation point +20°C or higher, more preferably Ac ３ Transformation point + 50°C or higher, more preferably Ac ３The reheating temperature is set to 100°C or higher than the transformation point. On the other hand, if the reheating temperature exceeds 1000°C, the austenite grains will become coarse, the toughness will decrease, and the oxide scale on the surface will become thicker and more likely to peel off, which will cause scratches on the steel pipe surface. Furthermore, the load on the heat treatment furnace will become excessive, which will be a problem from the viewpoint of energy conservation. For these reasons, the reheating temperature is set to 1000°C or lower. The reheating temperature is preferably 950°C or lower, more preferably 925°C or lower.

[0066] The seamless steel pipe after reheating is quenched. The cooling stop temperature during quenching is not particularly limited. However, if the cooling stop temperature during quenching exceeds 200°C, there is a possibility that the martensitic transformation may not be completely completed. Therefore, the cooling stop temperature during quenching is preferably 200°C or less, more preferably 195°C or less, and even more preferably 190°C or less. The lower limit of the cooling stop temperature during quenching is not particularly limited, but may be room temperature or higher, such as 50°C or higher. Room temperature refers to 10 to 35°C. Quenching refers to rapid cooling, and rapid cooling refers to cooling from the reheating temperature to the cooling stop temperature during quenching at an average cooling rate of 1.0°C / s or higher. The average cooling rate is preferably 1.3°C / s or higher, more preferably 1.5°C / s or higher, even more preferably 2.0°C / s or higher, and most preferably 2.5°C / s or higher. The cooling method is not particularly limited, and examples include water cooling and accelerated cooling. Although there is no particular upper limit to the average cooling rate, from the viewpoint of production 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.

[0067] Number of reheating and quenching treatments: one 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 is one or more. By performing the reheating and quenching treatment two or more times, prior austenite grains are refined and toughness and strength are improved. Therefore, the number of reheating and quenching treatments is preferably 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] In addition, Ac ３ The transformation point is calculated using the following formula: ３ 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%)) Ac ３ In calculating the transformation point, if an element described in the above formula is not contained, the content of the element is set to 0 (zero)%.

[0069] [Tempering Treatment] The seamless steel pipe after reheating and quenching treatment is annealed at 500°C or more. ３ The tempering treatment is carried out under the conditions that the average temperature rise rate is 0.5°C / min or more until the tempering temperature reaches the transformation point -50°C or less, and the holding time at the tempering temperature is 10 minutes or more and less than 60 minutes.

[0070] Tempering temperature: 500°C or higher Ac ３ Tempering at a temperature below transformation point -50°C is carried out for the purpose of forming desired precipitates (Fe-based precipitates) and obtaining toughness and desired hydrogen embrittlement resistance. If the tempering temperature is less than 500°C, the formation of Fe-based precipitates will be insufficient and the desired hydrogen embrittlement resistance will not be ensured. For this reason, the tempering temperature is set to 500°C or higher. The tempering temperature is preferably 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 is less than Ac ３If the tempering temperature exceeds the transformation point -50°C, the structure will be austenitized, causing quench cracks, or the amount of retained austenite will increase, making it impossible to obtain tempered martensite with the desired area ratio. ３ The transformation point is -50°C or less. The tempering temperature is preferably Ac ３ Transformation point -80 ° C or less, more preferably Ac ３ The transformation point is -100°C or less.

[0071] Average heating rate of 0.5°C / min or more until reaching the tempering temperature: Fe-based precipitates precipitate 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-based precipitates increases, and the desired hydrogen embrittlement resistance cannot be obtained. Therefore, the average heating rate is set to 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, while there is no particular upper limit for the average heating rate, if it is too fast, uneven temperature distribution will occur within the steel pipe, resulting in uneven structure. 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 temperature rise 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 temperature rise.

[0072] Holding time: 10 minutes or more but less than 60 minutes Fe-based precipitates are most abundantly precipitated during holding at the tempering temperature. If the holding time at the tempering temperature is less than 10 minutes, Fe-based precipitates are not sufficiently precipitated, and the desired hydrogen embrittlement resistance cannot be obtained. Therefore, the holding time at the tempering temperature is set to 10 minutes or more. The holding time is preferably 15 minutes or more, more preferably 20 minutes or more, and even more 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 Fe-based precipitates increases, and the desired hydrogen embrittlement resistance cannot be obtained. Therefore, the holding time at the tempering temperature is set to less than 60 minutes. Since the holding time increases energy costs, it is preferably set to 59 minutes or less, more preferably 58 minutes or less, and even more preferably 57 minutes or less.

[0073] [Cooling] The seamless steel pipe after the tempering treatment is cooled to a second cooling stop temperature of 300°C or less at an average cooling rate of 0.1°C / s or more.

[0074] Second cooling stop temperature: 300°C or less After completion of the above-mentioned holding, 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 the Fe-based precipitates to become coarse, and the desired hydrogen embrittlement resistance cannot be obtained. Therefore, the second cooling stop temperature is set to 300°C or less. The second cooling stop temperature is preferably set to 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 may be room temperature or higher, or may be 50°C or higher. Note that room temperature refers to 10 to 35°C.

[0075] Average cooling rate: 0.1°C / s or more After completion of the above-mentioned holding, 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 more. If the average cooling rate is less than 0.1°C / s, Fe-based precipitates become coarse, and the desired hydrogen embrittlement resistance cannot be obtained. Therefore, the average cooling rate is set to 0.1°C / s or more. The average cooling rate is preferably 0.5°C / s or more, more preferably 1.0°C / s or more, even more preferably 1.3°C / s or more, and most preferably 1.5°C / s or more. 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, the average cooling rate is preferably 3.0°C / s or less, more preferably 2.8°C / s or less. The average cooling rate can be calculated 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 quenching and tempering, the seamless steel pipe may be subjected to a warm or cold straightening treatment to correct any shape defects. The warm straightening treatment refers to straightening treatment performed at a temperature of 300°C or higher and equal to or lower than the tempering temperature, and the cold straightening treatment refers to straightening treatment performed at room temperature.

[0077] The seamless steel pipe obtained above can also be used for cylinders, and the cylinder shape can be, for example, one having 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. That is, the outer diameter of the applicable cylinder is preferably 200 mm or more, more preferably 250 mm or more, and even more preferably 300 mm or more. The outer diameter of the cylinder is also 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 for a high-pressure hydrogen storage tank for storing high-pressure hydrogen, especially in a cylinder. High pressure here means, for example, a hydrogen pressure of 1 MPa or more and 115 MPa or less.

[0079] Furthermore, the present invention can provide a seamless steel pipe of the present invention without using expensive elements such as Mo and V, or even with the use of small amounts of these elements, so that a seamless steel pipe that is cheaper than conventional seamless steel pipes that use Mo or V can be provided.

[0080] The present invention will be described in more detail below based on examples. The following description shows preferred examples of the present invention, and the present invention is not limited to these examples in any way.

[0081]

[0082]

[0083]

[0084] First, a billet, which was a steel pipe material having the chemical composition shown in Table 1, was produced 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 the pipe, thereby obtaining a seamless steel pipe. The seamless steel pipe was then 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, quenched (water-cooled), tempered under the conditions shown in Table 2, and cooled under the conditions shown in Table 2. The microstructure and mechanical properties of the obtained seamless steel pipe (wall thickness t: 14 mm, outer diameter of the steel pipe: 245 mm, length in the pipe axial direction L: 980 mm) were evaluated. The results are shown in Table 3. The evaluation methods are as follows.

[0085] Microstructural Evaluation Method: Area Fraction of Tempered Martensite A test specimen for microstructural observation measuring 10 mmt x 10 mmC x 10 mmL (where C: circumferential direction, L: axial direction) was taken from near the center of the wall thickness (t / 2 position) of the seamless steel pipe obtained above. The test specimen for microstructural observation was taken so that a cross section (C cross section) parallel to the wall thickness direction and circumferential direction was the observation surface, and the vicinity of the center of the observation surface was the t / 2 position. The test specimen was then observed using the following method. The observation surface was etched using a 3 vol% nital solution, and microstructural images at the t / 2 position were obtained using a scanning electron microscope at appropriate magnifications of 1000 to 5000 times in three fields of view, and tempered martensite, ferrite, bainite, and pearlite were observed. In the above structural image, the tempered martensite region was visually judged by comparing it with the structural photograph in Reference 2, and the tempered martensite and other regions were binarized by image analysis to determine the area ratio of the tempered martensite. The above procedure was carried out for three fields of view per test piece, and the average value of the area ratios of the three fields of view was taken as the area ratio of the tempered martensite. The area ratio of other structures was calculated by subtracting the area ratio of the tempered martensite from 100%. [Reference 2] Japan Heat Treatment Technology Association (author), Introduction to the Structure and Properties of Metallic Materials - Heat Treatment and Structure Control to Make the Most of the Material, 2004

[0086] Grain size number of prior austenite grains: Using the test specimen for structure observation after the above-mentioned structure image was obtained, the grain size number of the prior austenite grains was measured by the following method. First, the observation surface of the test specimen for structure observation after the above-mentioned structure image was obtained was polished and etched with a saturated aqueous solution of picric acid to reveal the prior austenite grain boundaries, and the specimen was observed using an optical microscope (magnification: 1000x), and structure images were obtained at the t / 2 position in three or more visual fields. For the obtained structure images, the grain size number of the prior austenite grains was determined using the intercept method in accordance with the provisions of JIS G 0551. The above operation was performed for three visual fields per test specimen, and the average value of the grain size numbers of the prior austenite grains in the three visual fields was taken as the grain size number of the prior austenite grains.

[0087] As described above, the total amount of Fe contained in the precipitates was determined by the filter filtration extraction method described in Patent Document 5 and Reference 1. First, an electrolytic test piece measuring 10 mm C x 10 mm L (where C: circumferential direction of the tube, L: axial direction of the tube) was collected from the steel pipe. Subsequently, the electrolytic test piece was electrolyzed in an electrolytic solution, and the electrolytic test piece was immersed in a dispersible solution and ultrasonic waves were applied to extract the precipitates attached to the surface of the electrolytic test piece into the dispersible solution. The dispersible solution from which the precipitates were extracted was filtered using a filter with a pore size of 100 nm, and precipitates exceeding 100 nm were collected on the filter. Subsequently, the filtered filtrate 1 was further filtered using a filter with a pore size of 50 nm, and precipitates of 100 nm or less but exceeding 50 nm were collected on the filter. The obtained filtered filtrate 2 was dried, and precipitates of 50 nm or less were collected. In this way, the precipitates were separated by size (50 nm or less, more than 50 nm and less than 100 nm, and more than 100 nm), and the precipitates separated by size were each subjected to acid decomposition. The absolute amounts of Fe contained in the precipitates separated by size were calculated using ICP atomic emission spectrometry (absolute amount of Fe in precipitates greater than 100 nm: W1, absolute amount of Fe in precipitates greater than 50 nm and less than 100 nm: W2, absolute amount of Fe in precipitates less than 50 nm: W3). Then, W1, W2, and W3 were each divided by the weight difference W0 (amount of electrolysis) of the electrolysis test piece before and after electrolysis to calculate the total amount of Fe (mass%) contained in precipitates greater than 100 nm, the total amount of Fe (mass%) contained in precipitates greater than 50 nm and less than 100 nm, and the total amount of Fe (mass%) contained in precipitates less than 50 nm. Furthermore, the ratio of the total amount of Fe contained in precipitates of 50 nm or less to the total amount of Fe contained in precipitates of 100 nm or less was calculated using the total amount of Fe (mass%) contained in the precipitates of more than 50 nm and 100 nm or less and the total amount of Fe contained in precipitates of 50 nm or less.

[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. The tensile test specimens were prepared so that the longitudinal direction (tensile direction) of the tensile test specimen was 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 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 determined by dividing the maximum load by the initial cross-sectional area. Steel pipes that obtained a TS of 850 MPa or more were considered to have passed the test.

[0089] Hydrogen embrittlement resistance: Hydrogen embrittlement resistance was evaluated by conducting a fatigue crack propagation test in hydrogen gas and determining the fatigue crack propagation rate. First, a CT (compact tension) test specimen 2 for the fatigue crack propagation test shown in Figure 1 was taken from a steel pipe in accordance with ASTM E 647, with the load direction parallel to the pipe circumferential direction 3 (C direction). The thickness of the test specimen 2 was 10 mm in the wall thickness direction of the steel pipe, and the center of the thickness of the test specimen 2 was located at the t / 2 position. The front and back surfaces of the test specimen 2 were mirror-polished to avoid variations in the fatigue crack propagation rate in hydrogen gas due to surface finishing. Furthermore, fatigue pre-cracks were introduced into the test specimens in an air environment. In Figure 1, 1 indicates the rolling direction (pipe axis (L) direction), and 3 indicates the pipe circumferential direction (C direction). A fatigue crack growth test was carried out on the obtained test piece 2 in 5 MPa hydrogen gas at room temperature (20±10°C) under the following conditions: frequency: 1 Hz, repeated load waveform: sine wave, control method: load control, loading conditions: uniaxial tension, stress ratio R: 0.1. The fatigue crack growth rate was determined by measuring the length of the fatigue crack by the compliance method using a clip gauge. The fatigue crack growth rate was determined by the stress intensity factor range ΔK = 25 (MPa m １／２ The fatigue crack growth rate (m / cycle) in hydrogen gas at 1000 kJ / s was used.

[0090] The results are shown in Table 3. The higher the fatigue crack growth rate, the worse the hydrogen embrittlement resistance is. －６ The steel pipes that obtained a value of 0.1 m / cycle or less were rated as passing.

[0091] As can be seen from Tables 1, 2 and 3, all of the invention examples have high strength (tensile strength of 850 MPa or more) and excellent hydrogen embrittlement resistance (fatigue crack growth rate of 1.00 × 10 －６ Furthermore, the present invention can achieve high strength and excellent hydrogen embrittlement resistance without using expensive elements such as Mo and V, or even with only small amounts of these elements.

[0092] 1. Rolling direction (pipe axis (L) direction) 2. CT (compact tension) test piece 3. Circumferential direction of pipe (C direction)

Claims

1. A steel sheet having a chemical composition containing, by mass%, C: 0.20 to 0.50%, Si: 0.05 to 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 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%, with the balance being Fe and unavoidable impurities, and wherein the sum of the Cr and the Si, Cr+Si, is 0.75% or more; the steel sheet has a structure in which tempered martensite accounts for 95% or more in area ratio; and the structure has precipitates. a seamless steel pipe, wherein the total amount of Fe contained in the precipitates having a maximum length of 100 nm or less is 0.005% by mass or more, and further, the total amount of Fe contained in the precipitates having a maximum length of more than 100 nm is 1.700% by mass or less.

2. A seamless steel pipe according to claim 1, wherein the composition further includes, in mass%, at least one selected from 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.

3. A steel pipe material having the chemical composition according to claim 1 or 2 is heated at a temperature of 1050°C or more and 1350°C or less, and then hot-rolled to form a seamless steel pipe, and the seamless steel pipe is cooled to a first cooling stop temperature of 200°C or less, and the seamless steel pipe after cooling is treated with Ac ３ The seamless steel pipe is reheated to a reheating temperature of not less than the transformation point and not more than 1000°C and quenched at least once, and the reheating and quenching treatment is carried out at a temperature of not less than 500°C and Ac ３ A method for manufacturing a seamless steel pipe, comprising: performing a tempering treatment under conditions in which an average heating rate is 0.5°C / min or more until a tempering temperature of −50°C or less is reached, and a holding time at the tempering temperature is 10 minutes or more and less than 60 minutes; and cooling the seamless steel pipe after the tempering treatment at an average cooling rate of 0.1°C / s or more to a second cooling stop temperature of 300°C or less.

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