Seamless steel pipe and method for manufacturing the 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

[Technical Field]

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

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

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

[0004] On the other hand, since weight reduction is not necessary for accumulators used in hydrogen stations, liners made of low-alloy steel have been proposed. For example, Patent Document 2 proposes an accumulator in which a Cr-Mo steel liner is covered with carbon fiber or glass fiber. However, low-alloy steels, including Cr-Mo steel, are known to become brittle due to hydrogen (hydrogen embrittlement). Therefore, for high-pressure hydrogen accumulators of 35 MPa or higher, Al alloys and SUS316, which are less susceptible to hydrogen embrittlement, are recommended.

[0005] However, Cr-Ni austenitic stainless steel has low strength (below 800 MPa). Therefore, if Cr-Ni austenitic stainless steel is used as the material for an accumulator and, for example, the hydrogen pressure is increased to 70 MPa, the wall thickness of the accumulator needs to be made extremely thick, increasing the weight of the accumulator. This results in high material costs and poor economic efficiency. In addition, in the case of automotive accumulators, there are restrictions on the weight and / or size of the accumulator, so it becomes necessary to reduce the size of the accumulator, i.e., the volume of the accumulator, which reduces the amount of hydrogen that can be stored in the accumulator.

[0006] Therefore, much research is underway to apply low-alloy steel, which has lower material costs, as a material for high-pressure hydrogen accumulators. For example, Patent Documents 3 and 4 propose steel for high-pressure hydrogen gas environments that utilizes fine V-Mo carbides in the steel as hydrogen trapping sites to suppress hydrogen embrittlement caused by diffusible hydrogen. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2009-024225 [Patent Document 2] Japanese Patent Publication No. 2009-293799 [Patent Document 3] Japanese Patent Publication No. 2010-037655 [Patent Document 4] Japanese Patent Publication No. 2009-074122 [Overview of the project] [Problems that the invention aims to solve]

[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 producing the same. [Means for Solving the Problems]

[0010] A high-pressure hydrogen accumulator is required to achieve both high strength and hydrogen embrittlement resistance. Accordingly, various factors affecting the strength and hydrogen embrittlement resistance of steel materials were investigated. As a result, the present inventors found that hydrogen embrittlement in a steel material having high tensile strength (TS) is caused by fracture along grain boundaries (intergranular fracture) occurring due to accumulation of hydrogen at grain boundaries. Furthermore, the present inventors found that accumulation of hydrogen at grain boundaries occurs as follows: pressure deformation (such as expansion) occurs in the pressure accumulator due to pressure fluctuation caused by hydrogen pressure inside the pressure accumulator, and hydrogen trapped by dislocations moves together with dislocations to grain boundaries when dislocations move to grain boundaries. Based on the above mechanism, the present inventors found that reducing dislocations contained in a steel material (steel pipe), that is, reducing dislocation density, is effective for suppressing the occurrence of intergranular fracture, that is, improving hydrogen embrittlement resistance.

[0011] The present invention has been completed based on the above findings, and the gist of the present invention is as follows. [1] In mass%, C: 0.20 to 0.50%, Si: more than 0.75% and 3.00% or less, 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%, having a chemical composition where the balance is Fe and inevitable impurities, The structure has tempered martensite accounting for 95% or more of the area. Dislocation density is 8.00 × 10 14 / m 2 The following is a seamless steel pipe. [2] The above component composition is further expressed in mass%, 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, A seamless steel pipe as described in [1], comprising at least one selected from Ca:0.0050% or less. [3] The dislocation arrangement parameter M value of the seamless steel pipe is 0.45 or less. Seamless steel pipe as described in [1] or [2]. [4] A steel pipe material having the component composition described in [1] or [2] above is heated at 1050°C or higher and 1350°C or lower, and then hot-rolled to form a seamless steel pipe. The seamless steel pipe is cooled to a first cooling stop temperature of 200°C or less. The seamless steel pipe, after cooling, is reheated to a reheating temperature of 1000°C or higher than the Ac3 transformation point and subjected to a reheating and quenching treatment at least once. A method for manufacturing seamless steel pipes, comprising subjecting the seamless steel pipes, after the aforementioned reheating and quenching treatment, to a tempering treatment at a tempering temperature of 450°C to 740°C. [5] The hot rolling end temperature of the hot rolling is 600°C or higher, The tempering temperature of the tempering treatment is 600°C or higher and 740°C or lower. [4] A method for manufacturing seamless steel pipes. [Effects of the Invention]

[0012] According to the present invention, a seamless steel pipe with high strength and excellent hydrogen embrittlement resistance, and a method for manufacturing the same can be obtained. The seamless steel pipe of the present invention can be used very suitably as a seamless steel pipe for high-pressure hydrogen accumulators. [Modes for carrying out the invention]

[0013] The embodiments of the present invention will be described below. The following description illustrates a preferred embodiment of the present invention, and the present invention is not limited in any way by the following description.

[0014] [Component composition] First, the reason for limiting the component composition of the seamless steel pipe (sometimes abbreviated as steel pipe) of the present invention will be explained. Hereafter, mass percentages in the component composition will be simply expressed as %.

[0015] C: 0.20~0.50% Carbon (C) contributes to increasing the strength of the steel pipe through solid solution, improves the hardenability of the steel pipe, and contributes to the formation of a martensite-dominant microstructure during quenching. To obtain these effects, the C content should be 0.20% or more. Preferably, the C content should be 0.22% or more, and more preferably 0.25% or more. On the other hand, if the C content exceeds 0.50%, cracks will occur during quenching, and manufacturability will be significantly reduced. For this reason, the C content should be 0.50% or less. Preferably, the C content should be 0.45% or less, more preferably 0.40% or less, and even more preferably 0.38% or less.

[0016] Si: Over 0.75% and under 3.00% Si is included as a deoxidizing agent. Furthermore, as described later, Si has a high effect in suppressing softening during tempering, so the strength does not decrease easily even at high tempering temperatures. Therefore, compared to steel pipes tempered at low temperatures to adjust to the same strength level, it is possible to reduce the dislocation density while maintaining high strength. As a result, it is possible to reduce the amount of hydrogen trapped in dislocations, and the desired hydrogen embrittlement resistance can be ensured. In addition, Si contributes to increasing strength, so the desired high strength can be ensured. 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, if the Si content is high, it becomes hard, and the ductility and toughness in the atmosphere deteriorate. For this reason, 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: 0.30~1.50% Mn, like C, is an element that improves the hardenability of steel pipes and contributes to increasing their strength. To obtain such effects, the Mn content should be 0.30% or more. Preferably, the Mn content should be 0.40% or more, more preferably 0.50% or more, even more preferably 0.55% 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 present, it forms localized hardened regions and reduces the resistance to hydrogen embrittlement. For this reason, the Mn content should be 1.50% or less. Preferably, the Mn content should be 1.30% or less, more preferably 1.20% or less, even more preferably 1.00% or less, and most preferably 0.80% or less.

[0018] P:0.015% or less Although phosphorus (P) is present in steel pipes as an unavoidable impurity, it is an element that not only segregates at grain boundaries in the steel pipe structure, causing grain boundary embrittlement, but also segregates and locally hardens the steel pipe, so it is preferable to reduce it as much as possible. However, a P content of up to 0.015% is acceptable. For this reason, the P content should be 0.015% or less. Preferably, the P content should be 0.013% or less, more preferably 0.010% or less, even more preferably 0.008% or less, and most preferably 0.006% or less. On the other hand, since it is desirable to reduce P as much as possible, the lower limit of the P content is not particularly limited and may be 0%. However, excessive reduction of P leads to an increase in refining costs. For this reason, the P content should preferably be 0.0001% or more, more preferably 0.001% or more, and even more preferably 0.002% or more.

[0019] S: 0.005% or less S is present in steel pipes as an unavoidable impurity, but most of it exists as sulfide-based inclusions, which reduce ductility, toughness, and even SCC (Stress Corrosion Cracking) resistance, so it is preferable to reduce it as much as possible. However, an S content of up to 0.005% is acceptable. For this reason, the S content should be 0.005% or less. Preferably, the S content should be 0.004% or less, more preferably 0.003% or less, and even more preferably 0.002% or less. On the other hand, since it is desirable to reduce S as much as possible, the lower limit of the S content is not particularly limited and may be 0%. However, excessive reduction of S leads to an increase in refining costs. For this reason, the S content should preferably be 0.0002% or more, more preferably 0.001% or more.

[0020] Al: 0.150% or less Al is an element that acts as a deoxidizing agent and is commonly used in the molten steel deoxidation process. However, if the Al content exceeds 0.150%, the cleanliness and toughness of the steel pipe decrease. Therefore, the Al content should be 0.150% or less. Preferably, the Al content should be 0.110% or less, more preferably 0.090% or less, even more preferably 0.080% 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 Al content, preferably, the Al content should be 0.005% or more, more preferably 0.007% or more, even more preferably 0.009% or more, and most preferably 0.010% or more.

[0021] N: 0.006% or less Nitrogen (N) is present in steel pipes as an unavoidable impurity. It combines with Al in the steel pipes to form AlN, and with Ti in the steel pipes to form TiN, thereby refining the prior austenite grains and improving toughness. However, if the N content exceeds 0.006%, the formed nitrides such as AlN and TiN become coarser, significantly reducing toughness. For this reason, the N content should be 0.006% or less. Preferably, the N content should be 0.005% or less, more preferably 0.004% or less, and even more preferably 0.003% or less. On the other hand, the lower limit of the N content is not particularly limited and may be 0%. However, from the viewpoint of enhancing the effect of N content, the N content should preferably be 0.001% or more, more preferably 0.002% or more.

[0022] Nb: 0.001~0.020% Nb forms precipitates, such as carbonitrides, contributing to increased strength of the steel pipe through precipitation strengthening. Furthermore, Nb refines austenite grains, contributing to improved toughness. To achieve these effects, the Nb content should be 0.001% or higher. Preferably, the Nb content is 0.003% or higher, more preferably 0.004% or higher, even more preferably 0.005% or higher, and most preferably 0.006% or higher. On the other hand, if the Nb content exceeds 0.020%, a large amount of Nb-based precipitates are generated, reducing the hydrogen embrittlement resistance. Therefore, from the viewpoint of achieving both the desired strength and excellent hydrogen embrittlement resistance, the present invention sets the Nb content to 0.020% or lower. Preferably, the Nb content is 0.018% or lower, more preferably 0.015% or lower, even more preferably 0.013% or lower, and most preferably 0.010% or lower.

[0023] B: 0.0003~0.0030% B segregates at austenite grain boundaries and suppresses ferrite transformation from the grain boundaries, thereby improving the hardenability, toughness, and strength of steel pipes even in trace amounts. To obtain such effects, the B content should be 0.0003% or more. Preferably, the B content is 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., reducing hardenability and toughness. Furthermore, adding it in excess generates Fe2B, causing red-hot brittleness. For this reason, the B content should be 0.0030% or less. Preferably, the B content is 0.0025% or less, more preferably 0.0020% or less, and even more preferably 0.0015% or less.

[0024] Oxygen (O): 0.0030% or less Oxygen (O) is an unavoidable impurity and exists in steel pipes as oxide inclusions. These inclusions become the starting point for cracking in a hydrogen gas environment and reduce hydrogen embrittlement resistance, so it is preferable to reduce the amount of O (oxygen) as much as possible. However, an O (oxygen) content of up to 0.0030% is acceptable. Therefore, the O (oxygen) content should be 0.0030% or less. Preferably, the O (oxygen) content should be 0.0025% 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 an increase in refining costs. Therefore, preferably, the O (oxygen) content should be 0.0001% or more, more preferably 0.0003% or more, and even more preferably 0.0005% or more.

[0025] Ti: 0.003~0.025% Ti combines with N during the solidification of molten steel to precipitate as fine TiN, and its pinning effect contributes to the refinement of austenite grains, thereby improving toughness and strength. When the Ti content is less than 0.003%, this effect is small, and in order to obtain such an effect, the Ti content should be 0.003% or more. Preferably, the Ti content is 0.005% or more, more preferably 0.007% or more, even more preferably 0.009% or more, and most preferably 0.011% or more. On the other hand, when the Ti content exceeds 0.025%, the TiN becomes coarser, and the above-mentioned pinning effect cannot be exerted, and toughness actually decreases. For this reason, the Ti content should be 0.025% or less. Preferably, the Ti content is 0.022% or less, more preferably 0.020% or less, even more preferably 0.017% or less, and most preferably 0.015% or less.

[0026] The steel pipe in one embodiment of the present invention has a component composition that includes the above elements, with the remainder being Fe and unavoidable impurities. Hereinafter, the steel pipe according to one embodiment of the present invention may contain only the above components and the remainder being Fe and unavoidable impurities. Furthermore, unavoidable impurities are impurities that are inevitably introduced from raw materials, manufacturing processes, or manufacturing equipment, and are permissible to be included in a range that does not hinder the objectives of the present invention. Examples of the raw materials include iron ore, reduced iron, and scrap. Examples of the impurities include Zn, Pb, As, Bi, Mg, Co, and H. The Mg content is permissible if it is 0.0008% or less, and the Co content is permissible if it is 0.0008% or less.

[0027] In other embodiments of the present invention, the above component composition may optionally further include at least one selected from 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. Furthermore, Cr combines with C during tempering to form Fe3C, M7C3, and M 23 It forms precipitates such as C6 (where M is a metallic element), contributing to the increased strength of steel pipes. Therefore, it can be included as desired depending on the required properties. However, if the Cr content exceeds 1.7%, a large amount of Fe3C, M7C3, M will be added. 23 C6 is formed, acting as a hydrogen trapping site and reducing resistance to hydrogen erosion. Furthermore, if a large amount of Cr is included, and Mo is also included, the Mo-based precipitates will coarseen. This coarsening of the Mo-based precipitates is due to the aggregation and coalescence of fine Mo-based precipitates, which reduces the number of fine Mo-based precipitates that contribute to improved resistance to hydrogen embrittlement, thus reducing the resistance to hydrogen embrittlement. In addition, Cr is an expensive element, and its inclusion leads to increased costs. Therefore, if Cr is included, the Cr content should be 1.7% or less. Preferably, the Cr content should be 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 obtaining sufficient benefits from Cr inclusion, the Cr content should preferably be 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), contributing to further strength increases of steel pipes through precipitation strengthening. Furthermore, the formed Mo-based precipitates function as hydrogen trapping sites, further improving hydrogen embrittlement resistance. Even when dissolved in the steel pipe, Mo segregates at prior austenite grain boundaries, contributing to further improvement of hydrogen embrittlement resistance. Therefore, it can be included in any amount depending on the desired properties. However, Mo is an expensive element, and when the Mo content exceeds 1.0%, the cost increase outweighs the benefits of improved hydrogen embrittlement resistance. For this reason, when Mo is included, the Mo content should be 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 effects of Mo, the Mo content is preferably 0.1% or more, 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 steel pipes. V also has the effect of refining austenite grains and improving toughness. Therefore, it can be included as desired depending on the required properties. However, V is an expensive element, and if the V content exceeds 0.30%, the effect of V content saturates, and the disadvantage of increased cost becomes significant. For this reason, when V is included, the V content should be 0.30% or less. Preferably, the V content should be 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 obtaining sufficient effects of V content, the V content should preferably be 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 effective element for improving toughness and increasing strength. Therefore, it can be included in any amount depending on the desired properties. However, if the Cu content exceeds 1.00%, elongation deteriorates due to the formation of intermetallic compounds. For this reason, when Cu is included, the Cu content should be 1.00% or less. Preferably, the Cu content is 0.80% or less, more preferably 0.70% or less, even more preferably 0.60% or less, 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 obtaining sufficient effects from the inclusion of Cu, the Cu content is preferably 0.01% or more, more preferably 0.03% or more, and even more preferably 0.05% or more.

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

[0033] Sn: 0.50% or less Tin (Sn) may be present in steel pipes as an unavoidable impurity when steel pipe materials are manufactured in electric furnaces. Sn tends to segregate at grain boundaries, and when steel pipes are used in high-temperature environments, it can cause a decrease in strength. In addition, Sn can reduce the ductility of steel pipes and degrade their workability. Therefore, it is preferable to reduce Sn as much as possible. However, a Sn content of up to 0.50% is acceptable. Therefore, if Sn is present, the Sn content should be 0.50% or less. Preferably, the Sn content should be 0.40% or less, and more preferably 0.30% or less. On the other hand, there is no particular lower limit to the Sn content, and it may be 0%. However, excessive reduction of Sn leads to an increase in refining costs. Therefore, preferably, the Sn content should be 0.05% or more, more preferably 0.10% or more, and even more preferably 0.15% or more.

[0034] W:3.0% or less Water (W) is an element that forms precipitates, contributing to increased strength of steel pipes through precipitation strengthening, and also contributes to further improvement of hydrogen embrittlement resistance by solid solution and segregating at prior austenite grain boundaries. Therefore, it can be included in any amount depending on the desired properties. However, if the W content exceeds 3.0%, the effect of W content saturates, and the effect commensurate with the W content cannot be expected, making it economically disadvantageous. Therefore, when W is included, the W content should be 3.0% or less. Preferably, the W content is 2.5% or less, more preferably 2.0% or less, even more preferably 1.5% or less, and most preferably 1.0% or less. On the other hand, the lower limit of the W content is not particularly limited and may be 0%. However, from the viewpoint of obtaining sufficient effects of W content, the W content is preferably 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 inclusions, and contributes to improving toughness and hydrogen embrittlement resistance through the control of sulfide inclusion morphology. Therefore, it can be included as desired depending on the required properties. However, if the Ca content exceeds 0.0050%, the effect of Ca content saturates, and the effect commensurate with the Ca content can no longer be expected, making it economically disadvantageous. Therefore, when Ca is included, the Ca content should be 0.0050% or less. Preferably, the Ca content is 0.0045% or less, more preferably 0.0040% or less, 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 obtaining sufficient effects from Ca content, the Ca content is preferably 0.0001% or more, more preferably 0.0005% or more.

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

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

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

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

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

[0041] The above-described structure in the seamless steel pipe of the present invention can be adjusted by appropriately controlling the cooling conditions after hot rolling of the steel pipe and the reheating temperature during the reheating quenching treatment.

[0042] The dislocation density is 8.00×10 14 / m 2 or less: Furthermore, in the seamless steel pipe according to the present invention, in order to obtain desired hydrogen embrittlement resistance, the dislocation density is required to be 8.00×10 14 / m 2 or less. By controlling the dislocation density to 8.00×10 14 / m 2 or less, the amount of hydrogen trapped by dislocations can be reduced, and hydrogen that migrates to grain boundaries along with dislocations can be reduced. As a result, intergranular fracture can be suppressed, and hydrogen embrittlement resistance is improved. Therefore, the dislocation density is set to 8.00×10 14 / m 2 or less. The dislocation density is preferably 7.50×10 14 / m 2 or less, more preferably 7.00×10 14 / m 2 or less, still more preferably 6.50×10 14 / m 2 or less, and most preferably 5.00×10 14 / m 2 or less. On the other hand, the lower the dislocation density is, the better the hydrogen embrittlement resistance becomes, so the lower limit is not limited and may be 0 / m 2 . However, in practice, it is difficult to make the dislocation density 0 / m 2 , and it may be 2.00×10 14 / m 2 or more, or may be 3.00×10 14 / m 2 or more. The dislocation density can be obtained by the method shown below. The dislocation density in the seamless steel pipe of the present invention can be controlled by adjusting the Si content and tempering temperature of the steel pipe appropriately.

[0043] Dislocation density is analyzed using an XRD (X-ray diffraction) instrument. A specimen for dislocation density measurement is taken from near the center of the steel pipe wall thickness (t / 2 position, t: wall thickness) with dimensions of 10mmt × 10mmC × 10mmL (C: circumferential direction, L: axial direction). The specimen is taken such that the measurement surface is a cross section parallel to the wall thickness direction and the circumferential direction (a cross section perpendicular to the axial direction: C section), and the center of the measurement surface is near the t / 2 position. However, if t is less than 10mm, the specimen is taken with dimensions of total wall thickness × 10mmC × 10mmL. Next, the measurement surface of the collected specimen is mechanically polished. To prevent strain from mechanical polishing from affecting the dislocation density, approximately 50μm of the surface layer of the measurement surface is removed by electrolytic polishing after mechanical polishing. Electropolishing can be performed, for example, using a solution prepared with 478 ml of HClO, 700 ml of ethanol, and 120 ml of distilled water, at a temperature of 23°C and a voltage of 27 V, with the test specimen at the anode and a Pt wire at the cathode. After electropolishing, the measurement surface is subjected to the modified-Williamson-Hall method (mWH / WA method) described in Reference 1, and the dislocation density ρ(1 / m) is measured using the diffraction profile. 2 ) is calculated. Specifically, for example, XRD measurements are performed under the following conditions: The XRD measurement is performed using CuKα rays as the X-ray source, with a tube voltage of 45kV and a tube current of 200mA. In the obtained diffraction profile, the 2θ range is set to 35~154°, and the measurement diffraction planes are BCC-Fe(110), (200), (211), (220), (310), (222), and the dislocation density ρ(1 / m³) is calculated. 2 Calculate ). [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, it is preferable to stabilize the dislocations, that is, to set the dislocation arrangement parameter M value to 0.45 or less, in order to obtain even better hydrogen embrittlement resistance. In this invention, the dislocation arrangement parameter M value is used as an indicator of the degree of dislocation stabilization. Here, a smaller M value means that the dislocations are more stable, that is, the number of mobile dislocations is smaller relative to the total number of dislocations. When there are fewer mobile dislocations, trapped hydrogen is less likely to move to the grain boundaries, grain boundary cracking is suppressed, that is, hydrogen embrittlement resistance is improved. For this reason, 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, since the lower the M value, the more stable the dislocations are, 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 method shown below. First, the dislocation density (1 / m³ 2 Using the diffraction profile used to calculate the dislocation density (1 / m), the magnitude of the strain field Re(m) due to dislocations is calculated according to the mWH / WA method. Subsequently, the dislocation density (1 / m) calculated earlier is used to calculate the magnitude of the strain field Re(m) due to dislocations. 2 The M value is calculated by substituting the magnitude of the strain field Re(m) due to the dislocations into the following formula. M=Re / d Here, d(m): average distance between dislocations (d=1 / √ρ), ρ(1 / m) 2 ): This is the dislocation density.

[0045] The dislocation arrangement parameter M value in the seamless steel pipe of the present invention can be controlled by controlling the hot rolling completion temperature and appropriately adjusting 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 and oxide inclusions can be reduced by the following control measures during the molten steel refining process. Specifically, desulfurization and dephosphorization should be performed in the molten iron pretreatment, followed by decarburization and dephosphorization in the converter, and then heated stirring refining (LF) and RH vacuum degassing in the ladle. In the heated stirring refining (LF) process, sufficient processing time should be ensured, and in the RH vacuum degassing process, sufficient processing time should also be ensured, and the RH reflux rate should be controlled. Furthermore, when producing cast slabs (steel pipe material) by continuous casting, in order to reduce inclusions, an inert gas seal should be applied when pouring from the ladle to the tundish, and electromagnetic stirring should be performed in the mold to separate the inclusions by flotation. Note that the refining process is not limited to the above, but it is preferable to manufacture under the above conditions.

[0047] The seamless steel pipe of the present invention has high tensile strength (TS). Therefore, it can also be used for high-pressure hydrogen containers (high-pressure hydrogen accumulators). The specific value of TS is not particularly limited, but TS is preferably 850 MPa or higher, more preferably 860 MPa or higher, even more preferably 880 MPa or higher, and most preferably 900 MPa or higher. On the other hand, as hydrogen embrittlement resistance tends to decrease as TS increases, TS is preferably 1000 MPa or lower, more preferably 990 MPa or lower. The above TS can be determined based on the method described in the examples.

[0048] In this invention, properties such as microstructure, dislocation density, dislocation arrangement parameter M value, tensile strength TS, and hydrogen embrittlement resistance are evaluated near the t / 2 position. The reason for evaluating near the t / 2 position is as follows: Since steel pipes are cooled from both the outer and inner surfaces, the cooling rate at the t / 2 position of the steel pipe is the slowest. As a result, it is difficult to obtain the desired microstructure, such as martensite (quenched structure), and the desired properties. Therefore, if the desired microstructure and properties are obtained near the t / 2 position, it can be determined that the desired microstructure and properties are obtained at any position in the steel pipe.

[0049] Furthermore, the wall thickness of the seamless steel pipe of the present invention is not particularly limited, but is preferably 2.0 mm or more, more preferably 3.0 mm or more, even more preferably 4.0 mm or more, and most preferably 5.0 mm or more. Also, the wall thickness is preferably 35.0 mm or less, more preferably 33.0 mm or less, even more preferably 30.0 mm or less, and most preferably 25.0 mm or less.

[0050] Seamless steel pipes for high-pressure hydrogen accumulator applications are suitable for high-pressure hydrogen applications. Specifically, they are applicable to high-pressure hydrogen applications where the hydrogen pressure is preferably 1 MPa or higher, more preferably 10 MPa or higher, even more preferably 35 MPa or higher, and most preferably 70 MPa or higher. On the other hand, there is no particular upper limit to the hydrogen pressure, but in practice it may be 115 MPa or lower, 105 MPa or lower, or 95 MPa or lower. Note that the above-mentioned high-pressure hydrogen accumulators include not only accumulators containing 100% hydrogen gas, but also accumulators containing hydrogen-containing gas with less than 100% hydrogen, so the above-mentioned hydrogen pressure refers to either the partial pressure of hydrogen or the total pressure of hydrogen.

[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 component composition to form a seamless steel pipe, followed by sequential cooling, reheating, quenching, and tempering. In the following description, unless otherwise specified, temperature refers to the temperature at the surface of the steel pipe material or steel pipe. The temperature at the surface is the value measured with a radiation thermometer.

[0052] Steel pipe material: The steel pipe material used in the present invention (hereinafter also simply referred to as a cast slab) is preferably produced by melting molten steel having the above-described component composition using a conventional melting method such as a converter, and then forming a cast slab (round cast slab), such as a billet, using a conventional casting method such as continuous casting. The cast slab may be further hot-rolled to form a round steel billet of a predetermined shape, or it may be a round steel billet that has undergone ingot-block rolling.

[0053] When casting steel pipe material into cast slabs (round cast slabs) using a casting method, the slower the casting speed, the lower the hydrogen concentration and inclusions in the steel pipe. This effect is particularly noticeable at casting speeds of 1.0 m / min or less; therefore, the casting speed is preferably 1.0 m / min or less, more preferably 0.8 m / min or less, and even more preferably 0.7 m / min or less. On the other hand, the lower limit of the casting speed is not particularly limited; 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 above steel pipe material is heated and then hot-rolled to produce a seamless steel pipe.

[0055] Heating temperature: 1050℃ or higher and 1350℃ or lower The steel pipe material is heated prior to hot rolling. The heating may be performed after the steel pipe material obtained by methods such as casting has been cooled, or the obtained steel pipe material may be subjected to the heating directly without cooling. If the heating temperature of the steel pipe material is below 1050°C, the dissolution of precipitates in the steel pipe material will be insufficient. For this reason, the heating temperature should be 1050°C or higher. Preferably, the heating temperature should be 1060°C or higher, more preferably 1070°C or higher, even more preferably 1090°C or higher, and most preferably 1100°C or higher. On the other hand, if the heating temperature exceeds 1350°C, the prior austenite grains become coarser, precipitates such as TiN that precipitated during solidification become coarser, and cementite also becomes coarser, reducing the toughness of the steel pipe. In addition, a thick scale layer is formed on the surface of the steel pipe material, which can cause surface defects during hot rolling and increases energy loss, which is undesirable from an energy-saving standpoint. For these reasons, the heating temperature should be 1350°C or lower. Preferably, the heating temperature should be 1330°C or lower, more preferably 1300°C or lower, even more preferably 1280°C or lower, and most preferably 1250°C or lower.

[0056] Next, the heated steel pipe material is subjected to hot rolling to form a seamless steel pipe. For the hot rolling, a standard Mannesmann-plug mill method or a Mannesmann-mandrel mill method, including perforation rolling, can be used.

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

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

[0059] [cooling] The resulting seamless steel pipes are cooled to a first cooling stop temperature of 200°C or lower.

[0060] First cooling stop temperature: 200℃ or less If the first cooling stop temperature exceeds 200°C, the martensitic transformation will not be completely finished, and tempered martensite with the desired area ratio cannot be obtained after the tempering treatment. Therefore, the first cooling stop temperature should be 200°C or lower. Preferably, the first cooling stop temperature should be 180°C or lower, and more preferably 150°C or lower. On the other hand, the lower limit of the first cooling stop temperature is not particularly limited, but it may be above room temperature, and may be 50°C or higher. Room temperature refers to a temperature between 10 and 35°C.

[0061] Furthermore, it is preferable that the average cooling rate from the end of hot rolling (hot rolling end temperature) to the first cooling stop temperature be an average cooling rate equal to or greater than that of air cooling. By setting the average cooling rate equal to or greater than that of air cooling, it is easier to obtain tempered martensite with the desired area ratio after the tempering treatment. Here, in the present invention, "average cooling rate equal to or greater than that of air cooling" refers to 0.1°C / s or higher. If the average cooling rate is less than that of air cooling, i.e., less than 0.1°C / s, the structure after cooling will be non-uniform, and it will be difficult to obtain tempered martensite with the desired area ratio after the subsequent reheating quenching and tempering treatments. The average cooling rate is more preferably 0.2°C / s or higher, and even more preferably 0.3°C / s or higher. On the other hand, there is no particular upper limit to the average cooling rate, but it is preferably 1.0°C / s or lower, more preferably 0.8°C / s or lower, and even more preferably 0.5°C / s or lower. The average cooling rate can be determined by dividing the temperature difference from the end of hot rolling temperature to the first cooling stop temperature by the time required for this cooling.

[0062] [Reheating and hardening treatment] The seamless steel pipe, after cooling as described above, is reheated to a reheating temperature between the Ac3 transformation point and 1000°C, and subjected to a reheating and quenching treatment at least once.

[0063] Reheating temperature: Above Ac3 transformation point and below 1000℃ If the reheating temperature is below the Ac3 transformation point, the material will not be heated to the austenite single-phase region, and a structure with martensite as the main phase will not be obtained. As a result, tempered martensite with the desired area ratio will not be obtained after tempering. For this reason, the reheating temperature should be above the Ac3 transformation point. Preferably, the reheating temperature should be above the Ac3 transformation point + 20°C, more preferably above the Ac3 transformation point + 50°C, and even more preferably above the Ac3 transformation point + 100°C. On the other hand, if the reheating temperature exceeds 1000°C, the austenite grains will coarseen, reducing toughness, and the oxide scale on the surface will thicken, making it prone to peeling and causing defects on the steel pipe surface. Furthermore, the load on the heat treatment furnace will become excessive, which is also a problem from the standpoint of energy conservation. For these reasons, the reheating temperature should be 1000°C or lower. Preferably, the reheating temperature should be 950°C or lower, more preferably 925°C or lower.

[0064] The seamless steel pipes, after reheating as described above, are quenched. Quenching means rapid cooling, and rapid cooling means cooling at an average cooling rate of 1.0°C / s or more from the reheating temperature to the quenching stop temperature. 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 can be water cooling, accelerated cooling, etc. The upper limit of the average cooling rate is not particularly limited, but from the viewpoint of manufacturing cost, it is preferably 5.0°C / s or less, more preferably 4.8°C / s or less, and even more preferably 4.6°C / s or less. The average cooling rate can be determined by dividing the temperature difference from the reheating temperature to the quenching stop temperature by the time required for this cooling. Furthermore, the quenching stop temperature (second cooling stop temperature) is preferably 200°C or lower, more preferably 150°C or lower, in order to generate Mo-based precipitates and further improve hydrogen embrittlement resistance when Mo is present. On the other hand, the lower limit of the second cooling stop temperature is not particularly limited, but may be above room temperature, and may be 50°C or higher. Room temperature refers to a temperature of 10 to 35°C.

[0065] Number of reheating and quenching treatments: 1 or more times The reheating and quenching treatment may be performed once or two or more times. Therefore, the number of reheating and quenching treatments should be one or more. By performing the reheating and quenching treatment two or more times, the prior austenite grains are refined, improving toughness and strength. Therefore, the number of reheating and quenching treatments should preferably be two or more, more preferably three or more. On the other hand, there is no particular upper limit to the number of reheating and quenching treatments, but from the viewpoint of manufacturing costs, it is preferably five or less.

[0066] The Ac3 transformation point shall be calculated using the following formula. Ac3 transformation point (℃)=937.2-436.5C+56Si-19.7Mn-16.3Cu-4.9Cr-26.6Ni+38.1Mo+124.8V+136.3Ti-19.1Nb+198.4Al+3315B (Here, C, Si, Mn, Cu, Cr, Ni, Mo, V, Ti, Nb, Al, B: Content of each element (mass %)) When calculating the Ac3 transformation point, if the element listed in the above formula is not present, the content of that element shall be assumed to be 0%.

[0067] [Tempering treatment] The seamless steel pipes, after reheating and quenching, are subjected to tempering treatment at a tempering temperature of 450°C to 740°C.

[0068] Tempering temperature: 450℃ or higher and 740℃ or lower Tempering is performed to reduce dislocation density and improve toughness and hydrogen embrittlement resistance. If the tempering temperature is below 450°C, the reduction in dislocation density is insufficient, and the desired dislocation density cannot be obtained, making it impossible to secure the desired hydrogen embrittlement resistance. For this reason, the tempering temperature should be 450°C or higher. Preferably, the tempering temperature should be 500°C or higher, more preferably 550°C or higher. Furthermore, by setting the hot rolling end temperature to 600°C or higher and the tempering temperature to 600°C or higher, the desired M value can be obtained, and the hydrogen embrittlement resistance is further improved. For this reason, the tempering temperature should be even more preferably 600°C or higher. On the other hand, if the tempering temperature exceeds 740°C, and the Mo content is low, the microstructure softens significantly, making it impossible to secure the desired strength. For this reason, the tempering temperature should be 740°C or lower. Preferably, the tempering temperature should be 710°C or lower, more preferably 700°C or lower, and even more preferably 690°C or lower.

[0069] The smaller the precipitate, the more likely it is to function as a hydrogen trapping site. When precipitates are formed, they precipitate during the tempering heating process, and their size increases. Therefore, if the heating rate to reach the tempering temperature during the tempering process is slow, the size of the precipitates may become too large, and the desired hydrogen embrittlement resistance may not be obtained. For this reason, 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, there is no particular upper limit to the average heating rate, but if it is too fast, non-uniformity of the temperature distribution will occur, resulting in non-uniformity of the structure. For this reason, the average heating rate is preferably 50.0°C / min or less, more preferably 45.0°C / min or less, and even more preferably 40.0°C / min or less. The average heating rate can be determined by dividing the temperature difference from the second cooling stop temperature to the tempering temperature by the time required for this heating.

[0070] Furthermore, precipitates are most often precipitated when the material is held at the tempering temperature. By holding the material at the tempering temperature for 10 minutes or more, sufficient precipitates may be formed, and the hydrogen embrittlement resistance may be further improved. For this reason, the holding time is preferably 10 minutes or more, more preferably 15 minutes or more, even more preferably 20 minutes or more, and most preferably 25 minutes or more. However, if the holding time is too long, the size of the precipitates may increase, and the hydrogen embrittlement resistance may decrease. For this reason, the holding time is preferably 60 minutes or less. Since the holding time is an energy-related cost factor, the holding time is more preferably 55 minutes or less, even more preferably 50 minutes or less, and most preferably 45 minutes or less.

[0071] Furthermore, if necessary, after reheating and tempering, a straightening treatment may be performed at a warm or cold temperature to correct any shape defects in the seamless steel pipe. Note that the warm straightening treatment refers to performing the treatment at a temperature of 300°C or higher and below the tempering temperature, while the cold straightening treatment refers to performing the treatment at room temperature.

[0072] Furthermore, the seamless steel pipes obtained above can also be applied to cylinders. Examples of cylinder shapes include those with an outer diameter of 200 mm or more and 600 mm or less, and a cylinder length in the axial direction of the pipe of 500 mm or more and 12,000 mm or less. That is, the outer diameter of the cylinder to which it is applied is preferably 200 mm or more, more preferably 250 mm or more, and even more preferably 300 mm or more. Also, the outer diameter of the cylinder is preferably 600 mm or less, more preferably 550 mm or less, and even more preferably 500 mm or less. The seamless steel pipe of the present invention can be suitably used in high-pressure hydrogen accumulators that store high-pressure hydrogen in cylinders. Here, high pressure refers to, for example, a hydrogen pressure of 1 MPa or more and 115 MPa or less.

[0073] Furthermore, since the present invention can provide seamless steel pipes without using expensive elements such as Mo, V, and Cr, or even with only small amounts of such elements, it is possible to provide seamless steel pipes that are cheaper than conventional seamless steel pipes that use Mo, V, and Cr. [Examples]

[0074] The present invention will be described in more detail below based on examples. The following description illustrates a preferred example of the present invention, and the present invention is not limited in any way by these examples.

[0075] [Table 1]

[0076] [Table 2-1]

[0077] [Table 2-2]

[0078] First, a billet, which is a steel pipe material having the component composition shown in Table 1, was manufactured at a casting speed of 0.6 m / min. Next, the obtained billet was heated and hot-rolled under the conditions shown in Tables 2-1 and 2-2 to expand it into a seamless steel pipe. After that, the seamless steel pipe was cooled to the first cooling stop temperature at the average cooling rate shown in Tables 2-1 and 2-2. Next, the cooled seamless steel pipe was reheated to the reheating temperature shown in Tables 2-1 and 2-2, then quenched (water-cooled), and subjected to the tempering treatment shown in Tables 2-1 and 2-2. The resulting seamless steel pipe (wall thickness t: 14 mm, outer diameter of steel pipe: 245 mm, axial length L: 980 mm) was evaluated for its microstructure and mechanical properties. The results are shown in Tables 2-1 and 2-2. The evaluation method was as follows.

[0079] Organizational evaluation methods Area ratio of tempered martensite A specimen measuring 10mmt x 10mmC x 10mmL was taken from near the center of the wall thickness (t / 2 position) of the seamless steel pipe obtained above for microstructure observation. At this time, the specimen was taken so that the cross section (C section) parallel to the wall thickness direction and the circumferential direction was the observation surface, and the vicinity of the center of the observation surface was the t / 2 position. The specimen was then observed using the following method. The observation surface was etched with a 3 vol% nital solution, and using a scanning electron microscope, microstructure images of the t / 2 position were acquired at appropriate magnifications of 1000 to 5000x for three fields of view, and tempered martensite, ferrite, bainite, and pearlite were observed. In the above microstructure images, the area of ​​tempered martensite was visually determined by comparing it with the microstructure photograph in Reference 2, and the area ratio of tempered martensite was determined by binarizing the tempered martensite and other areas using image analysis. The above procedure was performed on three fields of view per test specimen, and the average of the area percentages of the three fields of view was used as the area percentage of tempered martensite. The area percentages of other tissues were calculated by subtracting the above-mentioned area percentage of tempered martensite from 100%. [Reference 2] Japan Heat Treatment Technology Association (author), Introduction to the Structure and Properties of Metallic Materials - Heat Treatment and Microstructure Control for Maximizing Material Potential, 2004.

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

[0081] Tensile strength Bar-shaped test specimens, as specified in JIS Z 2201 "Tensile Test Specimens for Metallic Materials," were taken from the steel pipes as tensile test specimens. The tensile test specimens were prepared so that the longitudinal direction (tensile direction) of the specimen aligned with the circumferential direction (C direction) of the steel pipe, and the center of the cross-section perpendicular to the 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 calculated by dividing the maximum load by the initial cross-sectional area. Steel pipes with a TS of 850 MPa or higher were deemed acceptable.

[0082] Hydrogen embrittlement resistance The hydrogen embrittlement resistance was evaluated from the relative elongation (REL) of the specimen after a low strain-rate tensile test in hydrogen gas. In air, the steel pipe (the steel material constituting the pipe) undergoes plastic deformation, and the gauge length increases, thus reducing the elongation EL. air The elongation length increases. On the other hand, in hydrogen, the ductility of the steel decreases, causing it to break before it can be compressed, and the elongation does not increase as much as in air. Therefore, the elongation length after testing in hydrogen is less than in air. H Unlike in the atmosphere, the elongation becomes shorter. The hydrogen embrittlement resistance was evaluated using this rate of elongation reduction. The relative elongation (REL) was calculated using the following formula. Relative growth (REL) = EL H / EL air ×100 The above-mentioned test specimens (SSRT specimens) were round bar-shaped specimens taken from steel pipes in accordance with the specifications of ASTM G 142. Here, the specimens had a parallel section diameter of 6 mm, the longitudinal direction of the specimen was aligned with the circumferential direction of the steel pipe, and the center of the cross-section of the specimen, perpendicular to the longitudinal direction, was at the t / 2 position of the steel pipe. They were evaluated in air and hydrogen using n2 and n3, respectively, and their average values ​​were used to determine EL. air , EL H This was the request. In the above test, the hydrogen pressure of the hydrogen gas was 105 MPa, the purity of the hydrogen gas was 99.97%, and the tensile speed for the low 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. The higher the REL (Resilience Level), the better the hydrogen embrittlement resistance, and steel pipes with an REL of 70% or higher were deemed acceptable.

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

Claims

1. In mass percent, C: 0.20-0.50%, Si: Over 0.75% and under 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: 0.001-0.020%, B: 0.0003 to 0.0030%, O: 0.0030% or less, Ti: Contains 0.003-0.025%, The composition consists of Fe and unavoidable impurities. The structure has tempered martensite accounting for 95% or more of the area. Dislocation density is 8.00 × 10 １４ / m ２ The following is a seamless steel pipe.

2. The aforementioned component composition is further expressed in mass%, 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, A seamless steel pipe according to claim 1, comprising at least one selected from Ca: 0.0050% or less.

3. The dislocation arrangement parameter M value of the aforementioned seamless steel pipe is 0.45 or less. A seamless steel pipe according to claim 1 or 2.

4. A method for manufacturing a seamless steel pipe according to claim 1 or 2, wherein a steel pipe material having the above-mentioned component composition is heated at 1050°C or higher and 1350°C or lower, and then hot-rolled to form a seamless steel pipe, The seamless steel pipe is cooled to a first cooling stop temperature of 200°C or less. The seamless steel pipe after cooling is Ac ３ The material is reheated to a reheating temperature between the transformation point and 1000°C, and then subjected to a reheating and quenching treatment at least once. A method for manufacturing seamless steel pipes, comprising subjecting the seamless steel pipes, after the aforementioned reheating and quenching treatment, to a tempering treatment at a tempering temperature of 450°C to 740°C.

5. The hot rolling completion temperature of the aforementioned hot rolling is 600°C or higher. The tempering temperature of the tempering treatment is 600°C or higher and 740°C or lower. A method for manufacturing a seamless steel pipe according to claim 4.

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