Seamless steel pipe and method for manufacturing same

Abstract

In view of the above-described background, 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 manufacturing the same.　Provided is a seamless steel pipe which has a component composition that contains, by mass%, 0.20-0.50% of C, more than 0.75% but not more than 3.00% of Si, 0.30-1.50% of Mn, 0.015% or less of P, 0.005% or less of S, 0.150% or less of Al, 0.006% or less of N, 0.001-0.020% of Nb, 0.0003-0.0030% of B, 0.0030% or less of O, and 0.003-0.025% of Ti, with the balance being made up of Fe and unavoidable impurities. The seamless steel pipe has a structure in which the area ratio of tempered martensite is 95% or more, and has a dislocation density of 8.00×1014 / m2 or less.

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 such as 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.

[0005] However, Cr-Ni austenitic stainless steel has low strength (800 MPa or less). Therefore, if 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 needs to be made extremely thick, which increases 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.

[0006] 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.

[0007] JP 2009-024225 A JP 2009-293799 A JP 2010-037655 A JP 2009-074122 A

[0008] 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.

[0009] 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.

[0010] High-pressure hydrogen accumulators must have both high strength and hydrogen embrittlement resistance. Therefore, the inventors investigated various factors affecting the strength and hydrogen embrittlement resistance of steel materials. 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). Furthermore, they discovered that hydrogen accumulation at grain boundaries occurs when pressure fluctuations due to hydrogen pressure in the accumulator cause pressure deformation (expansion, etc.) inside the accumulator, and hydrogen trapped in dislocations migrates to the grain boundaries along with the dislocations. Based on the above mechanism, they discovered that reducing the dislocations contained in the steel material (steel pipe), i.e., lowering the dislocation density, is effective in suppressing the occurrence of intergranular fracture, i.e., improving hydrogen embrittlement resistance.

[0011] The present invention was completed based on the above findings, and the gist of the present invention is as follows: [1] A steel sheet having a chemical composition containing, in mass %, C: 0.20 to 0.50%, Si: more than 0.75% and not more than 3.00%, Mn: 0.30 to 1.50%, P: 0.015% or less, S: 0.005% or less, Al: 0.150% or less, N: 0.006% or less, 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 consisting of Fe and unavoidable impurities, having a structure in which tempered martensite accounts for 95% or more in area fraction, and having a dislocation density of 8.00 × 10 １４ / m ２[2] The seamless steel pipe according to [1], wherein the chemical composition further includes, in mass%, at least one selected from Cr: 1.7% or less, Mo: less than 1.0%, 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] The seamless steel pipe according to [1] or [2], wherein the seamless steel pipe has a dislocation configuration parameter M value of 0.45 or less. [4] A seamless steel pipe comprising a steel pipe material having the chemical composition according to [1] or [2], heated at 1050°C or more and 1350°C or less, and then hot-rolled to form a seamless steel pipe, cooling the seamless steel pipe to a first cooling stop temperature of 200°C or less, and treating the cooled seamless steel pipe with Ac ３ A method for producing a seamless steel pipe, comprising: performing a reheating and quenching treatment one or more times, in which the seamless steel pipe is reheated to a reheating temperature of from a transformation point to 1000°C and quenched; and tempering the seamless steel pipe after the reheating and quenching treatment at a tempering temperature of from 450°C to 740°C. [5] A method for producing a seamless steel pipe according to [4], in which the hot rolling finish temperature is 600°C or higher, and the tempering temperature of the tempering treatment is 600°C to 740°C.

[0012] 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.

[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 abbreviated 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, and more preferably set to 0.25% 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, and even more preferably 0.38% or less.

[0016] Si: More than 0.75% and not more than 3.00% Si is included as a deoxidizer. Furthermore, as described below, Si is highly effective in suppressing softening during tempering, so strength is less likely to decrease even at high tempering temperatures. Therefore, compared to steel pipes tempered at lower temperatures to achieve the same strength level, dislocation density can be reduced while maintaining high strength. As a result, it is possible to reduce hydrogen trapped in dislocations, ensuring the desired hydrogen embrittlement resistance. Furthermore, Si contributes to increasing strength, ensuring the desired high strength. For this reason, the Si content is set to more than 0.75%. The Si content is preferably 0.80% or more, more preferably 0.85% or more, even more preferably 0.90% or more, and most preferably 1.00% or more. On the other hand, a high Si content increases hardness and deteriorates ductility and toughness in air. Therefore, the Si content is set to 3.00% or less. The Si content is preferably 2.75% or less, more preferably 2.50% or less, even more preferably 2.25% or less, and most preferably 2.00% 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 0.55% or more, and most preferably 0.60% or more. On the other hand, Mn is an element that segregates in steel pipes and locally hardens them. If a large amount of Mn is 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 1.20% or less, even more preferably 1.00% or less, and most preferably 0.80% 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 0.013% or less, more preferably 0.010% or less, even more preferably 0.008% or less, and most preferably 0.006% or less. On the other hand, since it is desirable to reduce P as much as possible, the lower limit of the P content is not particularly limited and may be 0%. However, excessive reduction of P 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 permissible. 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 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 0.080% or less, and most preferably 0.070% 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] 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 0.003% or more, more preferably 0.004% or more, even more preferably 0.005% or more, and most preferably 0.006% 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.

[0023] 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 0.0004% or more, more preferably 0.0005% or more, even more preferably 0.0007% or more, and most preferably 0.0008% or more. On the other hand, if the B content exceeds 0.0030%, it precipitates as carbonitrides, etc., which reduces hardenability and toughness. Furthermore, if added in excess, Fe ２ B is generated, causing red shortness. Therefore, the B content is set to 0.0030% or less. The B content is preferably set to 0.0025% or less, more preferably 0.0020% or less, and even more preferably 0.0015% or less.

[0024] O (oxygen): 0.0030% or less O (oxygen) is an unavoidable impurity present in steel pipes as oxide-based inclusions. 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 0.0025% or less, more preferably 0.0020% or less, even more preferably 0.0015% or less, and most preferably 0.0010% 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 set to 0.0001% or more, more preferably 0.0003% or more, and even more preferably 0.0005% or more.

[0025] 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. When 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 0.007% or more, even more preferably 0.009% or more, and most preferably 0.011% or more. On the other hand, when the Ti content exceeds 0.025%, the TiN becomes coarse, the pinning effect is not exerted, and toughness is actually reduced. For these reasons, the Ti content is set to 0.025% or less. The Ti content is preferably 0.022% or less, more preferably 0.020% or less, even more preferably 0.017% or less, and most preferably 0.015% or less.

[0026] 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 permitted to be present 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 permitted.

[0027] In another embodiment of the present invention, the above-mentioned composition may further contain at least one element selected from the group consisting of Cr, Mo, V, Cu, Ni, Sn, W, and Ca.

[0028] Cr: 1.7% or less 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 , M２３ C ６ (M is a metal element) and contributes to increasing the strength of the steel pipe. Therefore, it can be added arbitrarily depending on the required properties. However, if the Cr content exceeds 1.7%, a large amount of Fe ３ C, M ７ C ３ , M ２３ C ６ These form hydrogen trap sites, which act as hydrogen trap sites and reduce hydrogen corrosion resistance. Furthermore, a high Cr content causes coarsening of Mo-based precipitates when Mo is also contained. The coarsening of Mo-based precipitates occurs due to the aggregation and coalescence of fine Mo-based precipitates, which reduces the number of fine Mo-based precipitates that contribute to improving hydrogen embrittlement resistance, thereby reducing hydrogen embrittlement resistance. Furthermore, Cr is an expensive element, and its inclusion leads to increased costs. Therefore, when Cr is contained, the Cr content is set to 1.7% or less. The Cr content is preferably set to 1.5% or less, more preferably 1.3% or less, even more preferably 1.0% or less, and most preferably 0.8% or less. On the other hand, the lower limit of the Cr content is not particularly limited and may be 0%. However, from the viewpoint of fully obtaining the effect of the Cr content, the Cr content is preferably set to 0.1% or more, more preferably 0.2% or more, and even more preferably 0.3% or more.

[0029] Mo: Less than 1.0% 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 is 1.0% or more, the disadvantage of increased cost outweighs the hydrogen embrittlement resistance obtained. Therefore, when Mo is contained, the Mo content is set to less than 1.0%. The Mo content is preferably 0.9% or less, more preferably 0.8% or less, and even more preferably 0.6% 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, and more preferably 0.2% or more.

[0030] 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, resulting in a significant disadvantage of increased costs. Therefore, when V is added, the V content is set to 0.30% or less. The V content is preferably set to 0.20% or less, 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, and even more preferably 0.03% or more.

[0031] 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 0.80% or less, more preferably 0.70% or less, even more preferably 0.60% or less, most 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 0.03% or more, and even more preferably 0.05% or more.

[0032] Ni: 2.0% or less Ni is an element that contributes to increasing the strength of steel pipes and improves their toughness and corrosion resistance. Therefore, it can be 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.

[0033] 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 the Sn content 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 0.40% or less, more preferably 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 0.05% or more, more preferably 0.10% or more, and even more preferably 0.15% or more.

[0034] 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 economic disadvantage. 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.

[0035] 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 content 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, even more preferably 0.0035% or less, and most preferably 0.0030% 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 content, the Ca content is preferably set to 0.0001% or more, more preferably 0.0005% or more.

[0036] 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 0.00100% or less. The H content of the steel pipe is preferably 0.00080% or less, more preferably 0.00070% or less, and even more preferably 0.00050% or less. On the other hand, the lower the H content of the steel pipe, the better 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 steel pipe to have an H content of 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.

[0037] [Structure] The seamless steel pipe of the present invention has a structure in which tempered martensite accounts for 95% or more in area ratio.

[0038] 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%.

[0039] 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%.

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

[0041] 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.

[0042] Dislocation density is 8.00 × 10 １４ / m ２ Furthermore, in the seamless steel pipe of the present invention, in order to obtain the desired hydrogen embrittlement resistance, the dislocation density is set to 8.00 × 10 １４ / m ２ The dislocation density must be 8.00 × 10 １４ / m ２ By controlling the dislocation density to the following value, it is possible to reduce the amount of hydrogen trapped in dislocations, and the amount of hydrogen that moves to grain boundaries due to dislocations can be reduced. As a result, grain boundary fracture can be suppressed, and hydrogen embrittlement resistance is improved. Therefore, the dislocation density is 8.00 × 10 １４ / m ２ The dislocation density is preferably 7.50×10 １４ / m ２ or less, more preferably 7.00 x 10 １４ / m ２ More preferably, 6.50 x 10 １４ / m ２ Below, most preferably 5.00 × 10 １４ / m ２ On the other hand, the lower the dislocation density, the better the hydrogen embrittlement resistance, so there is no lower limit and it is 0 / m or less. ２However, in reality, the dislocation density is set to 0 / m ２ It is difficult to make it 2.00 x 10 １４ / m ２ or more, 3.00 x 10 １４ / m ２ The dislocation density can be determined by the following method. The dislocation density in the seamless steel pipe of the present invention can be controlled by optimizing the Si content and tempering temperature of the steel pipe.

[0043] Dislocation density is analyzed using an XRD (X-ray diffraction) device. Test pieces for dislocation density measurement are taken from the vicinity of the center of the wall thickness (t / 2 position, t: wall thickness) of the steel pipe, measuring 10 mmt x 10 mmC x 10 mmL (C: pipe circumferential direction, L: pipe axial direction). The test pieces are taken so that the cross section parallel to the wall thickness direction and the pipe circumferential direction (cross section perpendicular to the pipe axial direction: C cross section) is the measurement surface, and the vicinity of the center of the measurement surface is at the t / 2 position. However, when t is less than 10 mm, test pieces are taken with a size of the total wall thickness x 10 mmC x 10 mmL. Subsequently, the measurement surface of the taken test piece is mechanically polished. After mechanical polishing, approximately 50 μm of the surface layer of the measurement surface is removed by electrolytic polishing to prevent strain due to mechanical polishing from affecting the dislocation density. Electrolytic polishing is performed using, for example, HClO ４ The test piece was placed as the anode and a Pt wire as the cathode under the conditions of a temperature of 23°C and a voltage of 27V using a solution prepared from 78 ml of HCl, 700 ml of ethanol, and 120 ml of distilled water. The test piece was placed as the anode and a Pt wire as the cathode. The measurement surface after electrolytic polishing was subjected to the modified Williamson-Hall method (mWH / WA method) described in Reference 1, and the dislocation density ρ (1 / m 2 ) is calculated. Specifically, for example, XRD measurement is performed under the following conditions. The XRD measurement is performed using CuKα radiation as the X-ray source under the conditions of a tube voltage of 45 kV and a tube current of 200 mA. In the obtained diffraction profile, the 2θ range is set to 35 to 154°, the measured diffraction planes are BCC-Fe (110), (200), (211), (220), (310), and (222), and the dislocation density ρ (1 / m 2[Reference 1] T. Ungar et al., "The effect of dislocation contrast on X-ray line broadening: A new approach to line profile analysis," Applied Physics Letters, Vol. 69, No. 21, p. 3173

[0044] Furthermore, in the seamless steel pipe of the present invention, in order to obtain better hydrogen embrittlement resistance, it is preferable to stabilize dislocations, i.e., to set the dislocation configuration parameter M value to 0.45 or less. In the present invention, the dislocation configuration parameter M value is used as an index representing the degree of dislocation stabilization. Here, the smaller the M value, the more stabilized the dislocations are, i.e., the fewer the number of mobile dislocations relative to the total number of dislocations. Fewer mobile dislocations makes it difficult for trapped hydrogen to move to grain boundaries, suppressing intergranular cracking, i.e., improving hydrogen embrittlement resistance. Therefore, the M value is preferably 0.45 or less, more preferably 0.44 or less, and even more preferably 0.43 or less. On the other hand, the lower the M value, the more stabilized the dislocations are, so the lower limit of the M value is not particularly limited and may be 0, 0.10 or more, 0.15 or more, or 0.20 or more. The M value can be determined by the following method. First, the dislocation density (1 / m 2 The diffraction profile used in calculating the dislocation density (1 / m) is used to calculate the magnitude of the strain field due to dislocations, Re (m), according to the mWH / WA method. 2 ) and the magnitude of the strain field due to dislocations, Re (m), are substituted into the following equation to calculate the M value: M = Re / d, where d (m): average distance between dislocations (d = 1 / √ρ), ρ (1 / m ２ ): is the dislocation density.

[0045] The dislocation configuration parameter M value in the seamless steel pipe of the present invention can be controlled by controlling the hot rolling finish temperature and optimizing the tempering temperature.

[0046] In the present invention, hydrogen embrittlement resistance can be further improved by reducing nitride-based inclusions and oxide-based inclusions, such as MnS, which can serve as fracture initiation points. Inclusions such as nitride-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.

[0047] 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 MPa 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.

[0048] In the present invention, properties such as the microstructure, dislocation density, dislocation configuration parameter M value, tensile strength TS, and hydrogen embrittlement resistance are evaluated near the t / 2 position. The reason for evaluating near the t / 2 position is as follows: Because the 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 microstructure, such as martensite (quenched structure), and it is also difficult to obtain desired properties. Therefore, if the desired microstructure and desired properties are obtained near the t / 2 position, it can be determined that the desired microstructure and desired properties are obtained at any position of the steel pipe.

[0049] 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.

[0050] 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.

[0051] [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 subjecting the pipe to treatments of cooling, reheating, quenching, and tempering. 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.

[0052] 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.

[0053] 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.

[0054] [Hot Rolling] The steel pipe material is heated and then hot rolled to form a seamless steel pipe.

[0055] 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. Alternatively, the obtained steel pipe material may be directly subjected to the heating without cooling. 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, and most preferably 1100°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 will form on the surface of the steel pipe material, which may cause surface defects during hot rolling and increase 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 set to 1330° C. or less, more preferably 1300° C. or less, even more preferably 1280° C. or less, and most preferably 1250° C. or less.

[0056] 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.

[0057] The hot rolling end temperature is not particularly limited. However, by setting the hot rolling end temperature to 600°C or higher, it is possible to reduce the strain in the seamless steel pipe, refine the prior austenite grains, and obtain higher strength and excellent toughness. Furthermore, by performing the tempering treatment described below, the M value can be controlled to 0.45 or less. The hot rolling end temperature is preferably 700°C or higher, more preferably 750°C or higher. On the other hand, by setting the hot rolling end temperature to 950°C or lower, strain and dislocations are introduced, refine the prior austenite grains, and obtain higher strength and excellent toughness. Therefore, the hot rolling end temperature is preferably 950°C or lower, more preferably 930°C or lower, and even more preferably 900°C or lower.

[0058] 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.

[0059] [Cooling] The obtained seamless steel pipe is cooled to a first cooling stop temperature of 200°C or lower.

[0060] 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 180°C or less, more preferably 150°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.

[0061] 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 easier to obtain tempered martensite with the desired area ratio after tempering. 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 non-uniform, making it 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 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 1.0°C / s or less, more preferably 0.8°C / s or less, and even more preferably 0.5°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.

[0062] [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.

[0063] 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 set to 950°C or lower, more preferably 925°C or lower.

[0064] The seamless steel pipe after reheating is quenched. Quenching refers to rapid cooling, and rapid cooling refers to cooling from the reheating temperature to the quenching stop temperature at an average cooling rate of 1.0°C / s or more. The average cooling rate is preferably 1.5°C / s or more, more preferably 2.0°C / s or more, and even more preferably 2.5°C / s or more. The cooling method is not particularly limited, and examples include water cooling and accelerated cooling. The upper limit of the average cooling rate is not particularly limited, but 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. When Mo is contained, the quenching stop temperature (second cooling stop temperature) is preferably 200°C or less, more preferably 150°C or less, in order to form Mo-based precipitates and further improve hydrogen embrittlement resistance. On the other hand, the lower limit of the second cooling stop temperature is not particularly limited, but may be room temperature or higher, and may be 50° C. or higher. Room temperature refers to 10 to 35° C.

[0065] 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.

[0066] 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)%.

[0067] [Tempering Treatment] The seamless steel pipe after the reheating and quenching treatment is subjected to tempering treatment at a tempering temperature of 450°C or higher and 740°C or lower.

[0068] Tempering temperature: 450°C or higher and 740°C or lower Tempering is performed to reduce dislocation density and improve toughness and hydrogen embrittlement resistance. If the tempering temperature is lower than 450°C, the dislocation density is not sufficiently reduced, the desired dislocation density is not obtained, and the desired hydrogen embrittlement resistance cannot be ensured. Therefore, the tempering temperature is set to 450°C or higher. The tempering temperature is preferably set to 500°C or higher, more preferably 550°C or higher. Furthermore, by setting the hot rolling finish temperature to 600°C or higher and the tempering temperature to 600°C or higher, the desired M value is obtained and hydrogen embrittlement resistance is further improved. Therefore, the tempering temperature is more preferably set to 600°C or higher. On the other hand, if the tempering temperature exceeds 740°C, when the Mo content is low, the structure is significantly softened and the desired strength cannot be ensured. Therefore, the tempering temperature is set to 740°C or lower. The tempering temperature is preferably set to 710°C or lower, more preferably 700°C or lower, and even more preferably 690°C or lower.

[0069] The smaller the precipitate size, the more likely it is to function as a hydrogen trapping site. If precipitates are formed, they precipitate and increase in size during the tempering heating process. Therefore, if the heating rate to the tempering temperature during the tempering process is slow, the precipitates may become too large, making it difficult to obtain the desired hydrogen embrittlement resistance. Therefore, the average heating rate from the second cooling stop temperature to the tempering temperature is preferably 0.5°C / min or more, more preferably 1.0°C / min or more, even more preferably 2.0°C / min or more, and most preferably 5.0°C / min or more. On the other hand, although there is no particular upper limit for the average heating rate, if it is too fast, uneven temperature distribution and therefore uneven structure will occur. 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 calculated by dividing the temperature difference from the second cooling stop temperature to the tempering temperature by the time required for this heating.

[0070] Furthermore, precipitates are often most likely to precipitate when the steel is held at the tempering temperature. Holding the steel at the tempering temperature for 10 minutes or longer may result in sufficient precipitate formation, further improving hydrogen embrittlement resistance. Therefore, the holding time is preferably 10 minutes or longer, more preferably 15 minutes or longer, even more preferably 20 minutes or longer, and most preferably 25 minutes or longer. If the holding time is too long, the size of the precipitates may increase, resulting in a deterioration in hydrogen embrittlement resistance. Therefore, the holding time is preferably 60 minutes or shorter. Since the holding time increases energy costs, the holding time is more preferably 55 minutes or shorter, even more preferably 50 minutes or shorter, and most preferably 45 minutes or shorter.

[0071] 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.

[0072] The seamless steel pipe obtained above can also be used for a cylinder. Examples of the cylinder shape include an outer diameter of 200 mm or more and 600 mm or less, and a cylinder length in the axial direction 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 preferably 600 mm or less, more preferably 550 mm or less, and even more preferably 500 mm or less. The seamless steel pipe of the present invention can be suitably used for a high-pressure hydrogen accumulator that stores high-pressure hydrogen, among other cylinders. High pressure here refers to a hydrogen pressure of 1 MPa or more and 115 MPa or less, for example.

[0073] Furthermore, the present invention can provide a seamless steel pipe of the present invention without using expensive elements such as Mo, V, and Cr, or even if only small amounts of these elements are used, so that a seamless steel pipe that is cheaper than conventional seamless steel pipes that use Mo, V, and Cr can be provided.

[0074] 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.

[0075]

[0076]

[0077]

[0078] First, a billet, a steel pipe material having the chemical composition shown in Table 1, was produced at a casting speed of 0.6 m / min. The resulting billet was then heated and hot-rolled under the conditions shown in Tables 2-1 and 2-2 to expand the pipe, resulting in a seamless steel pipe. The seamless steel pipe was then cooled to the first cooling stop temperature at the average cooling rate shown in Tables 2-1 and 2-2. The cooled seamless steel pipe was then reheated to the reheating temperature shown in Tables 2-1 and 2-2, followed by quenching (water cooling), and then subjected to the tempering treatment shown in Tables 2-1 and 2-2. The microstructure and mechanical properties of the resulting seamless steel pipe (wall thickness t: 14 mm, outer diameter of the steel pipe: 245 mm, axial length L: 980 mm) were evaluated. The results are also shown in Tables 2-1 and 2-2. The evaluation methods were as follows.

[0079] Microstructural Evaluation Method: Area Fraction of Tempered Martensite A test specimen for microstructural observation measuring 10 mmt x 10 mmC x 10 mmL 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 the circumferential direction of the pipe 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 were taken at the t / 2 position in three fields of view using a scanning electron microscope at appropriate magnifications of 1000 to 5000 times, 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

[0080] Calculation method of dislocation density and M value As described above, the dislocation density and M value were analyzed using an XRD (X-ray diffraction) device. Test pieces for analyzing dislocation density and M value were taken from the vicinity of the center of the wall thickness (t / 2 position) of the steel pipe, measuring 10 mmt x 10 mmC x 10 mmL. At this time, the test pieces were taken so that the cross section (C cross section) parallel to the wall thickness direction and the pipe circumferential direction was the measurement surface, and the vicinity of the center of the measurement surface was at the t / 2 position. Next, the measurement surface of the taken test piece was mechanically polished. At this time, after mechanical polishing, a thickness of about 50 μm was removed from the surface layer of the measurement surface by electrolytic polishing so that strain due to mechanical polishing would not affect the dislocation density and M value. Electrolytic polishing was performed using HClO ４ Electrolytic polishing was performed using a solution prepared from 78 ml of SiO2, 700 ml of ethanol, and 120 ml of distilled water at a temperature of 23°C and a voltage of 27 V, with the test piece positioned as the anode and a Pt wire positioned as the cathode. The dislocation density ρ and M value were calculated for the measurement surface after electrolytic polishing according to the modified Williamson-Hall method (mWH / WA method) described in Reference 1. Specifically, XRD measurement was first performed under the following conditions. The XRD measurement was performed using CuKα radiation as the X-ray source at a tube voltage of 45 kV and a tube current of 200 mA. In the obtained diffraction profile, the 2θ range was 35 to 154°, the measurement diffraction planes were BCC-Fe (110), (200), (211), (220), (310), and (222), and the dislocation density (1 / m 2 ) and the magnitude of the strain field due to dislocations, Re (m), were calculated. 2 The M value was calculated by substituting Re(m) and M(m) into the following equation: M = Re / d, where d(m) is the average distance between dislocations (d = 1 / √ρ), ρ(1 / m ２ ): is the dislocation density.

[0081] 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 longitudinal 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.

[0082] Hydrogen embrittlement resistance The hydrogen embrittlement resistance was evaluated based on the relative elongation (REL) of the test piece after a slow strain rate tensile test in hydrogen gas. In the atmosphere, the steel pipe (the steel material that makes up the steel pipe) undergoes plastic deformation, and the gauge length increases, so the elongation EL ａｉｒ On the other hand, in hydrogen, the ductility of the steel material decreases, so the steel material breaks before it is drawn, and the elongation is not longer than in air. Therefore, the elongation EL after the test in hydrogen Ｈ The relative elongation (REL) was calculated using the following formula: Relative elongation (REL) = EL Ｈ / EL ａｉｒ × 100 The above test specimen (SSRT test specimen) was a round bar-shaped test specimen, and was taken from a steel pipe in accordance with the specifications of ASTM G 142. Here, the test specimen had a diameter of 6 mm at the parallel part, the longitudinal direction of the test specimen was the circumferential direction of the steel pipe, and the center of the cross section of the test specimen perpendicular to the longitudinal direction of the test specimen was at the t / 2 position of the steel pipe. Evaluations were made at n2 and n3 in air and hydrogen, respectively, and the average values ​​were taken as EL ａｉｒ , E.L. Ｈ The hydrogen pressure of the hydrogen gas in the above test was 105 MPa, the purity of the hydrogen gas was 99.97%, and the tensile speed in the slow strain rate tensile test at room temperature was 0.002 mm / s. Here, the hydrogen pressure of the hydrogen gas was calculated by multiplying by a safety factor of 1.1, assuming a high-pressure hydrogen application with a maximum hydrogen pressure of 95 MPa. It was determined that the larger the REL, the better the hydrogen embrittlement resistance, and a steel pipe that obtained an REL of 70% or more was considered to have passed the test.

[0083] As can be seen from Table 1 and Tables 2-1 and 2-2, all of the inventive examples satisfied the requirements of high strength (tensile strength of 850 MPa or more) and excellent hydrogen embrittlement resistance (REL of 70% or more in a slow strain rate tensile test in hydrogen gas). Furthermore, the present invention can achieve high strength and excellent hydrogen embrittlement resistance without using, or even with the use of small amounts of, expensive elements such as Mo, V, and Cr.

Claims

1. A steel sheet having a chemical composition containing, by mass%, C: 0.20 to 0.50%, Si: over 0.75% and not more than 3.00%, Mn: 0.30 to 1.50%, P: 0.015% or less, S: 0.005% or less, Al: 0.150% or less, N: 0.006% or less, 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 consisting of Fe and unavoidable impurities, and having a structure in which tempered martensite accounts for 95% or more in area ratio, and a dislocation density of 8.00 x 10 １４ / m ２ Below is a seamless steel pipe.

2. A seamless steel pipe as described in claim 1, wherein the chemical composition further includes, in mass%, at least one selected from the following: Cr: 1.7% or less, Mo: less than 1.0%, 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 seamless steel pipe according to claim 1 or 2, wherein the dislocation configuration parameter M value of the seamless steel pipe is 0.45 or less.

4. A steel pipe material having the chemical composition described in claim 1 or 2 is heated to 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 A method for manufacturing a seamless steel pipe, comprising: performing a reheating and quenching treatment at least once, in which the seamless steel pipe is reheated to a reheating temperature of not lower than the transformation point but not higher than 1000°C and quenched; and tempering the seamless steel pipe after the reheating and quenching treatment at a tempering temperature of not lower than 450°C but not higher than 740°C.

5. A method for producing a seamless steel pipe according to claim 4, wherein the hot rolling finish temperature is 600°C or higher, and the tempering temperature is 600°C or higher and 740°C or lower.

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