High-strength seamless steel pipe for high-pressure hydrogen containers and method for manufacturing the same

Abstract

The purpose of the present invention is to provide: a high-strength seamless steel pipe which is for a high-pressure hydrogen vessel and has excellent hydrogen embrittlement resistance; and a manufacturing method therefor.　This high-strength seamless steel pipe for a high-pressure hydrogen vessel has a specific composition and has a structure in which the area ratio of tempered martensite is at least 95%, wherein at least 50 mass% of Mo contained in the steel is contained in precipitates, at least 50 mass% of Mo contained in the precipitates is contained in precipitates having a diameter of at most 50 nm, and the tensile strength TS is at least 850 MPa.

Description

Technical Field

[0001] The present invention relates to a high-strength seamless steel pipe for a high-pressure hydrogen container and a method for manufacturing the same.

Background Art

[0002] Fuel cell vehicles using hydrogen as fuel are expected to solve the CO2 emission problem and the energy problem because they do not emit carbon dioxide (CO2) and have excellent energy efficiency. In order to popularize these fuel cell vehicles, containers with excellent strength and durability capable of safely storing high-pressure hydrogen of 35 MPa or more, particularly about 70 MPa or more, are required for hydrogen stations for supplying hydrogen to fuel cell vehicles and for in-vehicle use for loading hydrogen into fuel cell vehicles, and the development thereof is underway.

[0003] As an in-vehicle accumulator that requires weight reduction, a liner made of a lightweight material such as aluminum or resin coated with carbon fiber reinforced resin (CFRP) has been proposed. For example, Patent Document 1 describes a liner made of an Al-Mg-Si alloy having excellent fatigue characteristics.

[0004] On the other hand, in the case of an accumulator used in a hydrogen station, since it is not necessary to reduce the weight as much as in the case of an in-vehicle accumulator, an accumulator entirely formed of steel (for example, Patent Document 2) or an accumulator in which a liner made of Cr-Mo steel is coated with carbon fiber or glass fiber (for example, Patent Document 3) has been proposed.

[0005] On the other hand, low alloy steels including Cr-Mo steel are known to be embrittled by hydrogen. Therefore, materials for high-pressure hydrogen accumulators of 35 MPa or more are limited to aluminum alloys and SUS316 with less material deterioration due to hydrogen.

[0006] However, since stainless steels such as SUS316 have low strength, for example, when increasing the hydrogen pressure up to 70 MPa, the wall thickness of the storage container becomes extremely thick, increasing the container weight. Therefore, there are problems that the size of the storage container is limited, not only the amount of hydrogen stored in the container is reduced, but also the material cost becomes too high, resulting in poor economic efficiency.

[0007] As a material for high-pressure hydrogen storage containers, many studies have been conducted to apply low-alloy steels with lower material costs instead of austenitic stainless steels. In Patent Document 4, a steel for high-pressure hydrogen environment that utilizes fine V-Mo-based precipitates as trap sites for hydrogen in steel to make non-diffusible hydrogen and suppress embrittlement by diffusible hydrogen has been proposed.

[0008] In Patent Document 5, a low-alloy high-strength steel with excellent high-pressure hydrogen environment embrittlement resistance has been proposed, in which the tempering treatment is performed at a relatively high temperature in the quenching and tempering treatment of Cr-Mo steel to control the tensile strength within an extremely narrow range of 900 to 950 MPa.

[0009] As shown by these prior arts, in order to achieve cost reduction, the use of low-alloy steels with enhanced hydrogen embrittlement resistance as infrastructure materials in high-pressure hydrogen gas environments has been widely studied. Here, the high-pressure hydrogen gas environment refers to an environment where the total pressure is 1 MPa or more and the partial pressure of hydrogen is 1 MPa or more.

Prior Art Documents

Patent Documents

[0010]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Summary of the Invention

Problems to be Solved by the Invention

[0011] When low alloy steel is used for pressure vessels for hydrogen, etc., it is known that hydrogen embrittlement can reduce the tensile strength of the material or significantly accelerate the fatigue crack propagation rate. Pressure vessels such as accumulators for high-pressure hydrogen can be designed with a thinner wall thickness by increasing the strength of the material, so the need for high-strength steel materials is increasing for the purpose of expanding the storage tank capacity and reducing the product weight. On the other hand, when the steel strength exceeds 850 MPa, the hydrogen embrittlement phenomenon becomes prominent, and fracture may occur at a value lower than the maximum tensile strength when tested in the atmosphere.

[0012] The present invention has been made in view of the above circumstances, and an object thereof is to provide a high-strength seamless steel pipe for high-pressure hydrogen containers having excellent hydrogen embrittlement resistance characteristics and a method for manufacturing the same.

Means for Solving the Problems

[0013] In order to obtain a high-strength seamless steel pipe for high-pressure hydrogen containers (hereinafter, also simply referred to as high-strength seamless steel pipe) that achieves the above object, the inventors considered it necessary to balance the desired high strength and hydrogen embrittlement resistance characteristics, and thus intensively studied various factors affecting strength and hydrogen embrittlement resistance characteristics. As a result, it was found that in high-strength steel pipes with a tensile strength TS of 850 MPa or more, the occurrence or non-occurrence of intergranular fracture has a great influence on hydrogen embrittlement resistance characteristics. Therefore, it was found that by dispersing and precipitating fine Mo precipitates in the steel structure, the Mo precipitates become powerful hydrogen trapping sites, reducing the hydrogen accumulation at the grain boundaries and suppressing the occurrence of intergranular fracture. Furthermore, it was found that the smaller the size of the Mo precipitates is and the larger the amount is, the more the hydrogen embrittlement resistance characteristics are improved, provided that the size is 50 nm or less.

[0014] Based on such findings, the inventors came to the conclusion that in order to further improve the hydrogen embrittlement resistance characteristics of high-strength seamless steel pipes, it is necessary to increase the amount of Mo contained in the precipitates, and in particular, to adjust the amount of Mo contained in precipitates of 50 nm or less to an appropriate amount or more. [1] By mass percentage, C: 0.20 - 0.50%, Si: 0.05 - 2.00%, Mn: 0.30 - 1.50%, P: 0.015% or less, S: 0.005% or less, Al: 0.005 - 0.150%, N: 0.006% or less, Cr: More than 0.2% and 1.7% or less, Mo: More than 1.0% and 3.0% or less, Nb: 0.001 - 0.020%, B: 0.0003 - 0.0030%, O: 0.0030% or less, Ti: Containing 0.003 - 0.025%, and containing such that the ratio of the content of Mo to the content of C, Mo / C, is in the range of more than 2.0 to 12.0, with the balance consisting of Fe and unavoidable impurities, having a structure with tempered martensite of 95% or more by area ratio, Among the Mo contained in the steel, 50% or more by mass percentage is contained in the precipitates, Furthermore, among the Mo contained in the precipitates, 50% or more by mass percentage is contained in the precipitates with a diameter of 50 nm or less, A high-strength seamless steel pipe for high-pressure hydrogen containers with a tensile strength TS of 850 MPa or more. [2] In addition to the above composition, further, by mass percentage, V: 0.30% or less, Cu: 1.00% or less, Ni: 2.0% or less, W: 3.0% or less The high-strength seamless steel pipe for high-pressure hydrogen containers according to [1], containing one or more selected from the above. [3] In addition to the above composition, further, by mass percentage, H: 0.0010% or less The high-strength seamless steel pipe for high-pressure hydrogen containers according to [1] or [2], containing the above. [4] In addition to the above composition, further, by mass percentage, Ca: 0.0005~0.005% The high-strength seamless steel pipe for high-pressure hydrogen containers according to any one of [1] to [3], which contains [5] A method for producing a high-strength seamless steel pipe for high-pressure hydrogen containers according to any one of [1] to [4], A steel pipe material having the above composition is cast into a slab, and the slab is heated at a temperature in the range of 1050 to 1350°C; The cast piece is hot-rolled to form a seamless steel pipe of a predetermined shape. After the hot rolling, the seamless steel pipe is cooled at an average cooling rate equal to or faster than air cooling until the surface temperature is 200°C or less; After the cooling, the mixture is reheated to a temperature in the range of from the Ac3 transformation point to 1000°C, The steel is quenched at least once to a surface temperature of 200°C or less. After the quenching treatment, a tempering treatment is performed by heating to a tempering temperature of 600 to 740 ° C. A method for producing a high-strength seamless steel pipe for high-pressure hydrogen containers, wherein the average heating rate until the tempering temperature is reached is 0.5°C / min or more, and the holding time at the tempering temperature is 10 minutes or more and less than 60 minutes. [6] The method for producing a high-strength seamless steel pipe for high-pressure hydrogen containers according to [5], wherein the casting speed is 1.8 m / min or less. Effect of the Invention

[0015] According to the present invention, a high-strength seamless steel pipe for high-pressure hydrogen containers having a high strength of tensile strength TS: 850 MPa or more and excellent hydrogen embrittlement resistance can be easily and inexpensively manufactured, which is of great industrial benefit. In addition, by using a manufacturing method that contains appropriate amounts of alloy elements and promotes the formation of Mo precipitates, a high-strength seamless steel pipe for pressure containers having both the desired high strength and excellent hydrogen embrittlement resistance can be stably manufactured. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] [Component composition] First, the reasons for limiting the composition of the high-strength seamless steel pipe for high-pressure hydrogen containers (hereinafter simply referred to as high-strength seamless steel pipe) of the present invention will be explained. Hereinafter, mass % in the composition will be simply indicated as %.

[0017] C: 0.20-0.50% C dissolves in the steel and contributes to increasing the strength of the steel, improves the hardenability of the steel, and contributes to the formation of a structure in which the martensite phase is the main phase during quenching. In order to obtain such effects, the C content must be 0.20% or more. The C content is preferably 0.22% or more, more preferably 0.25% or more, and even more preferably 0.28% 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 is set to 0.50% or less. The C content is preferably 0.45% or less, and more preferably 0.40% or less. The C content is even more preferably 0.35% or less.

[0018] Silicon: 0.05 to 2.00% Si is added for deoxidation, but if the content is less than 0.05%, the deoxidation effect is insufficient. For this reason, the Si content is set to 0.05% or more. The Si content is preferably 0.10% or more. It is more preferably 0.20% or more, and even more preferably 0.30% or more. On the other hand, if the Si content exceeds 2.00%, the effect is saturated, so the Si content is set to 2.00% or less. The Si content is preferably 1.00% or less, and more preferably 0.80% or less. Furthermore, if the Si content exceeds 0.50%, the toughness and weldability are deteriorated, so the Si content is further preferably 0.50% or less.

[0019] Mn: 0.30-1.50% Mn, like C, is an element that improves the hardenability of steel and contributes to increasing the strength of steel. In order to obtain such an effect, the Mn content is set to 0.30% or more. The Mn content is preferably 0.40% or more, more preferably 0.45% or more, and even more preferably 0.50% or more. On the other hand, Mn is an element that segregates in steel and locally hardens the steel, and when a large amount of Mn is contained, it forms a locally hardened region, which has an adverse effect of reducing hydrogen embrittlement resistance. For this reason, in the present invention, the Mn content is set to 1.50% or less. The Mn content is preferably 1.20% or less, more preferably 1.00% or less, and even more preferably 0.80% or less.

[0020] P:0.015% or less P is an element that not only segregates to grain boundaries in the steel structure to cause grain boundary embrittlement, but also segregates to locally harden the steel. In the present invention, P is an unavoidable impurity and is preferably reduced as much as possible, but up to 0.015% is acceptable. For this reason, the P content is set to 0.015% or less. The P content is preferably 0.008% or less. The P content is more preferably 0.005% or less, and further preferably 0.003% or less. The lower the content, the better, but from the viewpoint of refining costs, the P content is preferably 0.0001% or more, and more preferably 0.001% or more.

[0021] S: 0.005% or less S is an unavoidable impurity, and most of it exists as sulfide-based inclusions in steel, which reduces ductility, toughness, and even SSC resistance. Therefore, it is preferable to reduce the S content as much as possible, but up to 0.005% is acceptable. For this reason, the S content is set to 0.005% or less. The S content is preferably 0.003% or less. More preferably, it is 0.002% or less. The lower the content, the better, but from the viewpoint of refining costs, the S content is preferably 0.0002% or more. The S content is more preferably 0.001% or more.

[0022] Al: 0.005 to 0.150% Al is added as a deoxidizer, but if it is less than 0.005%, it has no effect. Therefore, the Al content is set to 0.005% or more. The Al content is preferably set to 0.010% or more, and more preferably set to 0.020% or more. On the other hand, if it exceeds 0.150%, the cleanliness of the steel decreases and the toughness deteriorates, so the Al content is set to 0.150% or less. The Al content is preferably set to 0.130% or less, more preferably set to 0.100% or less, and most preferably set to 0.080% or less.

[0023] N: 0.006% or less N is present in steel as an inevitable impurity, but it combines with Al to form AlN, and when Ti is contained, it forms TiN, which acts to refine crystal grains and improve toughness. For this reason, the N content is preferably 0.0005% or more. More preferably, it is 0.001% or more. However, if the N content exceeds 0.006%, the nitrides formed become coarse and the toughness is significantly reduced. For this reason, the N content is set to 0.006% or less. The N content is preferably 0.005% or less, more preferably 0.004% or less, and even more preferably 0.003% or less.

[0024] Cr: more than 0.2% but less than 1.7% Cr is an element that increases the strength of steel by improving hardenability and also improves corrosion resistance. In addition, Cr combines with C during tempering to form M3C, M7C3, and M 23 It is an element that forms precipitates such as M7C6 (M is a metallic element) and improves temper softening resistance, and is an element that is particularly necessary for increasing the strength of steel pipes. In particular, M3C type precipitates have a strong effect of improving temper softening resistance. To obtain this effect, the Cr content is made to exceed 0.2%. The Cr content is preferably 0.3% or more, and more preferably 0.5% or more. On the other hand, if the Cr content exceeds 1.7%, a large amount of M7C3, M 23C6 is formed, which acts as a hydrogen trapping site, lowering hydrogen corrosion resistance. Furthermore, a large Cr content causes coarsening of Mo precipitates. Fine Mo precipitates coarsen due to aggregation and coalescence, and the number density of fine Mo precipitates decreases, thereby deteriorating hydrogen embrittlement resistance. For these reasons, the Cr content is set to 1.7% or less. The Cr content is preferably 1.5% or less, more preferably 1.0% or less, and even more preferably 0.8% or less.

[0025] Mo: more than 1.0% and less than 3.0% Mo is an element that forms precipitates and contributes to strengthening steel by precipitation strengthening, and effectively contributes to securing the desired high strength after reducing dislocation density by tempering. Mo also dissolves in steel and segregates at prior austenite grain boundaries, contributing to improving hydrogen embrittlement resistance. Furthermore, Mo has the effect of densifying corrosion products and suppressing the formation and growth of pits that are the starting points of cracks. In order to obtain such effects, the Mo content is made to be more than 1.0%. The Mo content is preferably more than 1.1%, more preferably more than 1.2%, even more preferably 1.3% or more, and most preferably 1.4% or more. On the other hand, if the Mo content exceeds 3.0%, it promotes the formation of needle-shaped M2C precipitates and, in some cases, Laves phase (Fe2Mo), thereby deteriorating hydrogen embrittlement resistance. For this reason, the Mo content is made to be 3.0% or less. The Mo content is preferably 2.8% or less, more preferably 2.5% or less, and even more preferably 1.8% or less. Most preferably, it is 1.5% or less.

[0026] Nb: 0.001 to 0.020% Nb forms precipitates or carbonitrides, and contributes to increasing the strength of steel by precipitation strengthening, and also contributes to refinement of austenite grains. In order to obtain such effects, the Nb content is set to 0.001% or more. The Nb content is preferably 0.005% or more, more preferably 0.006% or more, and further preferably 0.007% or more. On the other hand, since coarse Nb precipitates are likely to become crack initiation points of hydrogen-induced cracking, the presence of a large amount of Nb precipitates based on a large amount of Nb content exceeding 0.020% leads to a significant decrease in hydrogen embrittlement resistance in high-strength steel materials. For this reason, from the viewpoint of achieving both the desired high 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 0.015% or less, and more preferably less than 0.010%.

[0027] B: 0.0003 to 0.0030% B segregates at the austenite grain boundaries and inhibits ferrite transformation from the grain boundaries, thereby enhancing the hardenability of steel even when contained in a small amount. To obtain such an effect, the B content is set to 0.0003% or more. The B content is preferably 0.0007% or more, and more preferably 0.0010% or more. On the other hand, if B is contained in an amount exceeding 0.0030%, it precipitates as carbonitrides, etc., and the hardenability decreases, and therefore the toughness decreases. For this reason, the B content is set to 0.0030% or less. The B content is preferably 0.0025% or less. The B content is more preferably 0.0020% or less, and further preferably 0.0015% or less.

[0028] O (oxygen): 0.0030% or less O (oxygen) is an inevitable impurity and exists as oxide-based inclusions in steel. These inclusions become the starting point of generation in a hydrogen gas environment and reduce hydrogen embrittlement resistance, so in the present invention, it is preferable to reduce O (oxygen) as much as possible. However, excessive reduction leads to high refining costs, so the O (oxygen) content is permissible up to 0.0030%. For this reason, the O (oxygen) content is limited to 0.0030% or less. The O content is preferably 0.0025% or less, and more preferably 0.0020% or less. The O content is further preferably 0.0015% or less. Although the lower limit is not particularly limited, the O content is preferably 0.0010% or more.

[0029] Ti: 0.003 to 0.025% Ti combines with N during solidification of molten steel and precipitates as fine TiN, and contributes to the refinement of austenite grains due to its pinning effect. In order to obtain such an effect, it is necessary to contain Ti at 0.003% or more. When the Ti content is less than 0.003%, the effect is small. For this reason, the Ti content is set to 0.003% or more. The Ti content is preferably set to 0.005% or more, and more preferably set to 0.010% or more. On the other hand, when the Ti content exceeds 0.025%, TiN becomes coarse, and the above-mentioned pinning effect cannot be exerted, and the toughness is rather reduced. Furthermore, the coarse TiN causes a decrease in hydrogen embrittlement resistance. For these reasons, the Ti content is set to 0.025% or less. The Ti content is preferably set to 0.020% or less, and more preferably set to 0.015% or less.

[0030] Mo / C: Over 2.0 to 12.0 When the ratio of Mo content to C content, Mo / C, is less than 2.0, Mo is insufficient and the amount of Mo precipitates formed is small, so that sufficient Mo precipitates are not formed to improve hydrogen embrittlement resistance. For this reason, Mo / C is made to exceed 2.0. Mo / C is preferably made to be 2.5 or more, more preferably 3.0 or more, and even more preferably 3.5 or more. On the other hand, when Mo / C is larger than 12.0, the tendency of Mo precipitates to become coarse becomes significant, and toughness and hydrogen embrittlement resistance properties are deteriorated. Furthermore, since the coarsening of Mo precipitates occurs due to the aggregation and coalescence of fine Mo precipitates, the number density of fine Mo precipitates is also reduced. For these reasons, Mo / C is made to be 12.0 or less. Note that Mo / C is preferably 10.0 or less, more preferably 8.0 or less, and even more preferably 6.0 or less. Most preferably 5.0 or less.

[0031] The above-mentioned components are the basic components, but in addition to the basic composition, the alloy may contain optional elements such as one or more selected from V: 0.30% or less, Cu: 1.00% or less, Ni: 2.0% or less, and W: 3.0% or less, H: 0.0010% or less, or Ca: 0.0005 to 0.005%, or any combination of these.

[0032] One or more selected from the following: V: 0.30% or less, Cu: 1.00% or less, Ni: 2.0% or less, W: 3.0% or less V, Cu, Ni, and W are all elements that contribute to increasing the strength of steel, and one or more of them may be selected and contained as necessary.

[0033] V: 0.30% or less V is an element that forms precipitates and carbonitrides and contributes to the strengthening of steel. The content of V may be 0% or more, but in order to obtain the above effects, the content of V is preferably 0.02% or more, and more preferably 0.03% or more. On the other hand, even if V is contained in an amount exceeding 0.30%, the effect saturates, and an effect commensurate with the content cannot be expected, which is economically disadvantageous. Therefore, when V is contained, the V content is set to 0.30% or less. The V content is preferably 0.20% or less, and more preferably 0.15% or less.

[0034] Cu: 1.00% or less Cu is an element effective for improving toughness and increasing strength, but if the content is too high, the weldability deteriorates. Therefore, when Cu is contained, the Cu content is limited to 1.00% or less. The Cu content is preferably 0.75% or less, more preferably 0.50% or less, and even more preferably 0.25% or less. The Cu content may be 0% or more, but in order to obtain the above effects, it is preferably contained in an amount of 0.01% or more.

[0035] Ni: 2.0% or less Ni is an element that contributes to an increase in the strength of steel and improves toughness and corrosion resistance. In order to obtain such effects, the Ni content is desirably 0.03% or more. The Ni content is more preferably 0.1% or more. On the other hand, even if Ni is contained in an amount exceeding 2.0%, the effect saturates, and an effect commensurate with the content cannot be expected, which is disadvantageous in terms of economy. Therefore, when Ni is contained, the Ni content is limited to 2.0% or less. The Ni content is preferably 1.5% or less, more preferably 1.0% or less, and even more preferably 0.5% or less.

[0036] W: 3.0% or less W is an element that forms precipitates and contributes to the increase in the strength of steel by precipitation strengthening, dissolves in solid solution, segregates at the prior austenite grain boundaries, and contributes to the improvement of hydrogen embrittlement resistance. In order to obtain such effects, it is desirable that the content of W be 0.03% or more. The W content is preferably 0.1% or more. On the other hand, when the content of W exceeds 3.0%, the effect saturates, and an effect commensurate with the content cannot be expected, which is disadvantageous in terms of economy. Therefore, when containing W, the content of W is limited to 3.0% or less. The W content is preferably 2.5% or less. The W content is more preferably 2.0% or less, still more preferably 1.5% or less, and most preferably 1.0% or less.

[0037] H: 0.0010% or less H may be introduced into the steel material in various processes during manufacturing. When the introduction amount is large, the risk of cracking after solidification increases, and the hydrogen embrittlement resistance deteriorates. Therefore, it is important to reduce the amount of hydrogen in the steel material. Since these effects do not pose a problem if the H content is 0.0010% or less, when containing H, the H content is set to 0.0010% or less. The H content is preferably 0.0008% or less, more preferably 0.0005% or less, and still more preferably 0.0001% or less. Since making the H content less than 0.00001% is a factor contributing to cost increase, the H content is preferably 0.00001% or more. The H content is more preferably 0.00005% or more. Note that the hydrogen amount is the residual hydrogen amount after forming plates, steel pipes, etc.

[0038] Ca: 0.0005 - 0.005% Ca combines with S to form CaS, which is an element that effectively acts on the morphology control of sulfide inclusions. Through the morphology control of sulfide inclusions, it contributes to the improvement of toughness and hydrogen embrittlement resistance. To obtain such an effect, when Ca is contained, the Ca content needs to be 0.0005% or more. Preferably, the Ca content is 0.001% or more. On the other hand, when the Ca content exceeds 0.005%, the effect saturates, and an effect commensurate with the content cannot be expected, which is disadvantageous in terms of economy. Therefore, when Ca is contained, the Ca content is limited to 0.005% or less. The Ca content is preferably 0.004% or less, more preferably 0.003% or less, and even more preferably 0.002% or less.

[0039] The balance other than the above-described components consists of Fe and inevitable impurities. As the inevitable impurities, for example, Mg: 0.0008% or less and Co: 0.0008% or less are acceptable.

[0040] [Steel structure] The high-strength seamless steel pipe of the present invention has the above-described composition, and further has a structure mainly composed of tempered martensite, precipitates exist in the structure, and precipitates with a diameter of 50 nm or less exist.

[0041] Main phase: Tempered martensite phase In the high-strength seamless steel pipe of the present invention, in order to ensure a high strength with a tensile strength TS of 850 MPa or more, a structure mainly composed of a martensite phase is used. However, in order to maintain the ductility and toughness required as a structure, a tempered martensite phase obtained by tempering the martensite phase is used as the main phase. The "main phase" as used herein refers to the case where the tempered martensite phase is a single phase with an area ratio of 100%, or the case where the tempered martensite phase is 95% or more, including 5% or less in an area ratio such that the second phase does not affect the properties. The tempered martensite phase is preferably 97% or more, and more preferably 98% or more. As described above, the tempered martensite phase may be 100%. In addition, examples of the second phase in the present invention include a bainite phase, a retained austenite phase, pearlite, or a mixed phase thereof.

[0042] Regarding the above-described structure of the seamless high-strength steel pipe for high-pressure hydrogen containers of the present invention, it can be adjusted by appropriately selecting the heating temperature during quenching treatment and the cooling rate during cooling according to the steel components.

[0043] If the grain size number of the prior austenite grains is less than 8.5, the substructure of the formed martensite phase becomes coarse and the toughness decreases. Therefore, it is preferable that the grain size number of the prior austenite grains be 8.5 or more. The grain size number of the prior austenite grains is preferably 9.0 or more, more preferably 9.6 or more, still more preferably 10.0 or more, and most preferably 12.0 or more. The upper limit is not particularly limited, and it is better to be smaller, but for reasons such as difficulty in observation and difficulty in appropriate evaluation, 18.0 or less is preferable. The grain size number shall be the value measured in accordance with the provisions of JIS G 0551.

[0044] In the present invention, the grain size number of the prior austenite grains can be adjusted by changing the heating rate, heating temperature, holding temperature during quenching treatment, and further the number of times of quenching treatment.

[0045] Furthermore, in the high-strength seamless steel pipe for high-pressure hydrogen containers according to the present invention, in order to improve the hydrogen embrittlement resistance characteristics, the concentration of Mo precipitates is adjusted within an appropriate range according to the size. The identification of Mo precipitates was carried out by the extraction method described in Patent Document 6 and Reference 1 by filter filtration. In order to measure the Mo concentration of the precipitates, a 10 mm square sample taken from a cross-section perpendicular to the rolling direction of the steel pipe (a cross-section perpendicular to the pipe axis direction: C cross-section) was electrolyzed with an electrolytic solution, and then the precipitates adhering to the surface of the steel piece were put into a liquid having dispersibility and irradiated with ultrasonic waves to extract the precipitates into an aqueous solution. The aqueous solution containing the precipitates was filter-filtered, the precipitates were separated by size, and the precipitates classified by size were each dissolved in a dissolution solution, and the Mo content in the precipitates of each size was calculated by analyzing the concentration of Mo by ICP. In the concentration analysis by ICP, by introducing the dissolution solution into the plasma, an element-specific spectrum is emitted, and since the concentration of the element in the dissolution solution can be obtained from the emission intensity of the light, the concentration (mass%) of Mo in the precipitates can be calculated. By this method, the Mo content in the entire precipitates can be calculated, and the ratio (mass%) of Mo contained in the precipitates to Mo contained in the steel can be obtained from this value and the Mo content in the steel. Also, according to Patent Document 7, the solid solution concentration of Mo in the steel was obtained by analyzing the concentration of the solution after electrolysis described above by ICP. Furthermore, the Mo content contained in the precipitates remaining on the filter was analyzed by ICP, and the Mo content contained in the precipitates exceeding 50 nm was analyzed. By subtracting the Mo content contained in the precipitates exceeding 50 nm from the Mo content contained in all the precipitates, the ratio (mass%) of Mo contained in the precipitates with a diameter of 50 nm or less to Mo contained in the precipitates can be obtained. [Patent Document 6] Japanese Patent Application Laid-Open No. 2010-127791 [Patent Document 7] Japanese Patent Application Laid-Open No. 2009-031269 [Reference 1] Ishida et al., Analysis of the formation state of fine precipitates in steel, Iron and Steel Vol. 107 No. 08

[0046] Of the Mo contained in the steel, 50% or more by mass is contained in the precipitates. The inclusion of Mo in the composition of steel as precipitates improves the characteristics of the hydrogen environment. If Mo is in solid solution even when the amount of Mo contained in the steel is increased, such an effect cannot be expected. On the other hand, the greater the amount of Mo precipitates, the better the hydrogen trapping ability, and it is significantly improved when 50% or more of the Mo contained in the steel is included in the precipitates. Therefore, it is necessary that 50% or more by mass of the Mo contained in the steel is included in the precipitates. Incidentally, it is preferable that 60% or more by mass of the Mo contained in the steel is present in the precipitates. More preferably, it is 65% or more by mass, and even more preferably, it is 70% or more by mass. The upper limit is not particularly limited, but it is preferable that 95% or less by mass of the Mo contained in the steel is present in the precipitates. More preferably, it is 90% or less by mass.

[0047] Of the Mo contained in the precipitates, 50% or more by mass is contained in the precipitates with a diameter of 50 nm or less Mo precipitates inhibit hydrogen accumulation at grain boundaries by trapping hydrogen in steel, improving the grain boundary strength in a hydrogen environment. However, when their size exceeds 50 nm, the hydrogen trapping ability decreases, and the effect on improving the grain boundary strength becomes smaller. Therefore, it is necessary for the precipitates with a size of 50 nm or less to contain a large amount of Mo. Here, the more Mo is contained in the precipitate, the better the hydrogen trapping ability. The Mo contained in fine precipitates with a diameter of 50 nm or less preferably occupies 50% or more by mass of the Mo in the precipitate, which is significantly improved. For this reason, in the present invention, it is limited to the case where the Mo contained in fine precipitates with a diameter of 50 nm or less is 50% or more by mass of the Mo contained in the precipitate. It is preferable that the Mo contained in fine precipitates with a diameter of 50 nm or less is 60% or more by mass of the Mo contained in the precipitate. More preferably, it is 65% or more by mass, and even more preferably, it is 70% or more by mass. The upper limit is not particularly limited, but it is preferable that the Mo contained in fine precipitates with a diameter of 50 nm or less is 95% or less by mass of the Mo contained in the precipitate, and more preferably, it is 90 or less by mass. Since smaller Mo precipitates have better hydrogen trapping ability, it is more preferable that the diameter of the precipitate is 20 nm or less. As described above, the coarsening of Mo precipitates is caused by the aggregation and coalescence of fine Mo precipitates, and coarsening leads to a decrease in the quantity of fine Mo precipitates. The lower limit of the diameter of the target precipitate is not particularly limited, but it is preferable to target 1 nm or more.

[0048] In the present invention, in addition to the precipitation of Mo precipitates, it is also important to reduce nitride-based inclusions and oxide-based inclusions that can serve as fracture initiation points in order to improve the hydrogen embrittlement resistance characteristics. In particular, management in the steel melting process is important. In the hot metal pretreatment, desulfurization and dephosphorization are performed. After decarburization and dephosphorization are performed in the converter, heating and stirring refining treatment (LF) and RH vacuum degassing treatment are performed in the ladle. Then, sufficient treatment time for the heating and stirring refining treatment (LF) is ensured, and the treatment time for the RH vacuum degassing treatment is ensured, and the RH reflux amount is controlled. Further, when making a slab (steel pipe material) by the continuous casting method, in order to reduce nitride-based inclusions and oxide-based inclusions, at the time of pouring from the ladle to the tundish, sealing with an inert gas is performed, and electromagnetic stirring is performed in the mold to promote the floating and separation of inclusions. Note that the refining process is not limited to the above, but even in that case, management of the refining process is important.

[0049] [Manufacturing method] Next, a manufacturing method of the high-strength seamless steel pipe for high-pressure hydrogen containers in the present invention will be described.

[0050] In the present invention, the steel pipe material having the above-described composition is heated and hot-rolled to obtain a seamless steel pipe having a predetermined shape. The seamless steel pipe for high-pressure hydrogen containers is preferably applied to hydrogen containers with a hydrogen pressure of 1 MPa or more, more preferably 20 MPa or more. The upper limit of the hydrogen pressure is not particularly limited, but is targeted up to 120 MPa or less.

[0051] The steel pipe material used in the present invention (hereinafter, also simply referred to as steel material) is preferably obtained by melting molten steel having the above-described composition by a common melting method such as a converter and making a slab (round slab) by a common casting method such as the continuous casting method. The slab may be further hot-rolled into a round steel piece having a predetermined shape, or may be a round steel piece that has undergone ingot-making - block-rolling.

[0052] In the following description, the manufacturing method will be described by taking the case where the steel pipe is a seamless steel pipe as an example. Needless to say, by performing the treatment so as to obtain the same heat history, it is possible to manufacture an electric resistance welded pipe or a UOE steel pipe. For example, in the case of an electric resistance welded pipe, the steel plate is hot-rolled under the condition of a temperature in the range of 1000 ° C or less above the Ac3 transformation point, and subsequently, quenching treatment for rapidly cooling to a temperature of 200 ° C or less at the surface temperature is performed one or more times. Tempering treatment for heating to a temperature in the range of 600 to 740 ° C after the quenching treatment is performed, and the average heating rate until reaching the tempering treatment temperature is 0.5 ° C / min or more, and the holding time at the tempering reach temperature is 10 minutes or more and less than 60 minutes. After tempering, welding is performed to manufacture an electric resistance welded pipe, and the same characteristics can be obtained.

[0053] The steel pipe of the present invention can be manufactured by sequentially performing the following steps (1) to (3). (1) A step of casting the steel pipe material after adjusting the components (2) A rolling step of heating and rolling the slab (casting material) to obtain a steel pipe, and (3) A step of cooling and tempering the steel pipe obtained in the rolling step Hereinafter, each step will be described. In the following description, the temperature means the temperature on the surface of the slab or the steel pipe unless otherwise specified.

[0054] [Casting step] Casting speed: 1.8 m / min or less If the casting speed is too fast, inclusions increase and the hydrogen embrittlement resistance deteriorates. Therefore, the casting speed is preferably 1.8 m / min or less. Note that the slower the casting speed, the lower the hydrogen concentration and inclusions in the steel can be reduced, and the effect becomes more remarkable at 1.0 m / min or less. Therefore, the casting speed is more preferably 1.0 m / min or less. More preferably, it is 0.5 m / min or less, and most preferably 0.1 m / min or less. The lower limit is not particularly limited, but it is preferably 0.01 m / min or more because of the difficulty of device control.

[0055] [Heating step] To perform hot rolling, the ingot having the above-described component composition is heated. The ingot is not particularly limited, and for example, billets obtained by a normal continuous casting method can be used.

[0056] Heating temperature: 1050 - 1350 °C When the heating temperature is less than 1050 °C, the dissolution of precipitates in the steel pipe material is insufficient. Therefore, the heating temperature should be 1050 °C or higher. The heating temperature is preferably 1100 °C or higher, and more preferably 1150 °C or higher. On the other hand, when heating exceeds 1350 °C, the crystal grains coarsen, the precipitates such as TiN precipitated during solidification coarsen, and the cementite coarsens, resulting in a decrease in the toughness of the steel pipe. Also, when heated to a high temperature exceeding 1350 °C, a thick scale layer is formed on the surface of the steel pipe material, which causes surface defects during rolling and increases the energy loss, which is not preferable from the perspective of energy conservation. For these reasons, the heating temperature is limited to a temperature of 1350 °C or lower. The heating temperature is preferably 1300 °C or lower. The heating temperature is more preferably 1250 °C or lower.

[0057] [Hot rolling process] Next, the ingot heated in the above heating process is rolled into a steel pipe of a predetermined shape. For the rolling, hot rolling including piercing rolling by a normal Mannesmann - plug mill method or Mannesmann - mandrel mill method can be used. The predetermined shape refers to, for example, a cylindrical shape such as a steel pipe, and examples include a steel pipe with the end diameter of the steel pipe smaller than the central diameter and the shape of a cylinder represented by a pressure vessel. For example, the steel pipe having the cylindrical shape preferably has an outer diameter of 200 - 600 mm and a steel pipe length in the pipe axis direction of 500 - 12000 mm. Also, for a steel pipe with the end diameter of the steel pipe smaller than the central diameter, the outer diameter of the central part is preferably 200 - 600 mm, the diameter of the end is 50 - 550 mm, and the steel pipe length in the pipe axis direction is 500 - 12000 mm. Also, for the cylinder, the outer diameter is preferably 200 - 600 mm and the cylinder length in the pipe axis direction is 500 - 12000 mm. In addition, the hot rolling also includes a method of simultaneously performing a step of rolling a billet into a steel pipe shape (hot working step) and an expanding step. Also, if necessary, a sizing step for adjusting the plate thickness may be carried out after the reheating step described later.

[0058] The obtained seamless steel pipe is subjected to a cooling treatment in which, after the hot rolling is completed, it is cooled at a cooling rate of air cooling or higher until the surface temperature becomes 200°C or lower.

[0059] Cooling treatment after hot rolling: Average cooling rate: air cooling or higher, Cooling stop temperature: 200°C or lower Within the range of the component composition of the present invention, if it is cooled at an average cooling rate of air cooling or higher after hot rolling, a structure having a martensite phase as the main phase can be obtained. If the air cooling (cooling) is stopped when the surface temperature exceeds 200°C, the transformation may not be completely completed. Therefore, the cooling treatment after hot rolling is to cool at an average cooling rate of air cooling or higher until the surface temperature becomes 200°C or lower. Also, in the present invention, the "cooling rate of air cooling or higher" means 0.1°C / s or higher. If the average cooling rate is less than 0.1°C / s, the metal structure after cooling becomes non-uniform, and the metal structure after the subsequent heat treatment also becomes non-uniform. The average cooling rate is preferably 1.0°C / s or higher, more preferably 10.0°C / s or higher. The upper limit is not particularly limited, but the average cooling rate is preferably 1000.0°C / s or lower. The above-mentioned average cooling rate is the average value of the cooling rate from the Ac3 transformation point to 200°C.

[0060] [Heat treatment process] Reheating temperature for quenching treatment: 1000°C or lower and above the Ac3 transformation point When performing quenching treatment, if the reheating temperature is below the Ac3 transformation point, the austenite single-phase region cannot be heated, so a structure with a martensite phase as the main phase cannot be obtained. Therefore, the reheating temperature should be set above the Ac3 transformation point. The reheating temperature is preferably Ac3 + 30°C or higher, more preferably Ac3 + 50°C or higher. However, for component systems where Ac3 point + 30°C and Ac3 point + 50°C exceed 1000°C, the above Ac3 point + 30°C or higher and Ac3 point + 50°C or higher are not applicable. On the other hand, when exceeding 1000°C, in addition to the coarsening of crystal grains and the decrease in toughness, the surface oxidation scale becomes thick and is likely to peel off, which causes defects on the surface of the steel plate. Moreover, the load on the heat treatment furnace becomes excessive, which also poses a problem from the perspective of energy conservation. For these reasons, and also from the perspective of energy conservation, the reheating temperature for quenching treatment is limited to 1000°C or lower. Preferably, it is 980°C or lower, and more preferably 950°C or lower.

[0061] Also, after reheating, quenching treatment is performed. During the cooling of the quenching treatment, the surface temperature is rapidly cooled until it reaches 200°C or lower by the cooling of the quenching treatment. The rapid cooling means cooling from the Ac3 transformation point to 200°C at an average cooling rate of 2.0°C / s or higher. The average cooling rate is preferably 5.0°C / s or higher, more preferably 10.0°C / s or higher. The upper limit is not particularly limited, but the average cooling rate is preferably 1000.0°C / s or lower. Also, after satisfying the above conditions, it is preferable to perform water cooling at an average cooling rate of 2.0°C / s or higher from the temperature at the center position of the plate thickness to a temperature of 400°C or lower below the Ac3 transformation point. The upper limit is not particularly limited, but the average cooling rate is preferably 1000.0°C / s or lower. Also, it is preferable to cool the surface temperature to a temperature of 100°C or lower. The lower the surface temperature after cooling, the better, and it is preferably cooled to room temperature. Note that the quenching treatment may be repeated two or more times.

[0062] Note that the value calculated by the following formula shall be used as the Ac3 transformation point.

[0063] Ac3 transformation point (°C) = 937 - 476.5C + 56Si - 19.7Mn - 16.3Cu - 4.9Cr - 26.6Ni + 38.1Mo + 124.8V + 136.3Ti + 198Al + 3315B (Here, C, Si, Mn, Cu, Cr, Ni, Mo, V, Ti, Al, B: Content of each element (mass%)) In calculating the Ac3 transformation point, when not containing the elements described in the above formula, the content of the relevant element shall be calculated as zero%.

[0064] After performing a cooling treatment with a cooling rate of air cooling or higher, a tempering treatment is carried out. The tempering treatment is a treatment of heating to a temperature in the range of 600 to 740 °C.

[0065] Tempering temperature: 600 - 740 °C The tempering treatment is carried out for the purpose of reducing the dislocation density, precipitating Mo precipitates, and improving toughness and hydrogen embrittlement resistance characteristics. When the tempering temperature is less than 600 °C, the reduction of dislocations and the precipitation of Mo precipitates are insufficient, so excellent hydrogen embrittlement resistance characteristics cannot be ensured. For this reason, the tempering temperature shall be 600 °C or higher. It is preferably 620 °C or higher, more preferably 640 °C or higher, and even more preferably 660 °C or higher. On the other hand, at a temperature exceeding 740 °C, the softening of the structure is remarkable and the desired high strength cannot be ensured. For this reason, the tempering temperature is limited to a temperature of 740 °C or lower. Note that the tempering temperature is preferably 710 °C or lower. It is more preferably 700 °C or lower, and even more preferably 680 °C or lower.

[0066] The average heating rate until reaching the tempering temperature is 0.5 °C / min or more Mo precipitates form during the heating process of tempering, and their size increases. Therefore, if the heating rate until reaching a predetermined temperature in the tempering process is slow, the size of the precipitates becomes too large, and the desired hydrogen embrittlement resistance characteristics cannot be obtained. Therefore, the average heating rate until reaching the tempering temperature is set to 0.5 °C / min or more, preferably 1.0 °C / min or more, more preferably 2.0 °C / min or more. Most preferably, it is 5.0 °C / min or more. Although the upper limit is not particularly defined, if it is too fast, non-uniform temperature distribution occurs, resulting in inhomogeneous material structure, so 50.0 °C / min or less is preferred.

[0067] The holding time at the tempering temperature is 10 minutes or more and less than 60 minutes Mo precipitates are most precipitated during the holding of tempering. If this time is short, the desired hydrogen embrittlement resistance characteristics cannot be obtained without sufficient precipitation. The holding time at the tempering temperature is set to 10 minutes or more. The holding time at the tempering temperature is preferably 15 minutes or more, and more preferably 20 minutes or more. Also, if the holding time at the tempering temperature is too long, the size of the precipitates becomes too large, so it is less than 60 minutes. Note that since the holding time is an energy cost-increasing factor, the tempering time is preferably less than 50 minutes, more preferably less than 40 minutes. Even more preferably, it is less than 30 minutes.

[0068] In addition, in order to stably ensure the desired characteristics, after hot rolling, a cooling treatment of cooling at a cooling rate of air cooling or higher is performed, then reheated, and a quenching treatment of rapid cooling such as water cooling is performed one or more times, and then the above-mentioned tempering treatment is performed. The upper limit of the number of times of the above quenching treatment is not particularly limited, but it is preferably 5 times or less.

[0069] In addition, after performing the quenching treatment and the tempering treatment, if necessary, a straightening treatment for correcting the shape defect of the steel pipe may be performed at warm or cold temperature.

Example

[0070] The following describes the examples for verifying the effects of the present invention. The following description shows a preferred example of the present invention, and the present invention is not limited by these examples. In the following examples, the manufacturing method and property evaluation of seamless steel pipes for actual steel structures were studied. Table 1 shows the component compositions of steels numbered 1 to 24. Table 2 shows the tempering conditions, area ratio of tempered martensite, prior austenite grain size number, ratio of Mo contained in precipitates among the Mo contained in the steel by mass%, ratio of Mo contained in precipitates with a diameter of 50 nm or less among the Mo contained in the precipitates, TS, and relative reduction of area (RRA) for each of Nos. 1 to 24.

[0071]

Table 1

[0072]

Table 2

[0073] Billets with the component compositions shown in Nos. 1 to 24 of Table 1 were produced at a casting speed of 0.6 m / min, heated to 1250°C, and hot-worked and pipe-expanded to obtain seamless steel pipes. The production of the seamless steel pipes was carried out under the condition that the pipe expansion was completed at 820°C or higher, and after hot working, cooling was performed at a cooling rate of air cooling or higher until the surface temperature reached 200°C or lower. For the obtained steel pipes with an Ac3 transformation point of 950°C or lower, they were heated and held at 950°C, and for the steel pipes with an Ac3 transformation point exceeding 950°C, they were heated and held at 1000°C, then water-cooled to 200°C or lower under the condition of 5.0°C / s, and then tempered. The tempering process was carried out at the heating rate, holding temperature, and holding time described in Table 2. The tempering temperature was adjusted so that the tensile strength was in the range of 850 to 950 MPa. The obtained steel pipes were evaluated for their metallographic structure and mechanical properties.

[0074] Furthermore, Steel Pipes Nos. 25 to 39 in Table 3 were produced from any one of Billet Nos. 5, 8, and 12 having the component compositions shown in Table 1 at various casting speeds. The billets were heated to 1250°C and then expanded to obtain seamless steel pipes. The production of the steel pipes was carried out under the condition that the expansion was terminated at 820°C or higher, and after hot working, cooling was performed at a cooling rate of air cooling or higher until the surface temperature reached 200°C or lower. For the obtained steel pipes, those with an Ac3 transformation point of 950°C or lower were heated and held at 950°C, and those with an Ac3 transformation point exceeding 950°C were heated and held at 1000°C, then water-cooled to 200°C or lower under the condition of 5.0°C / s, and then annealing was carried out under the conditions described in Table 3. For the obtained steel pipes, the metallographic structure and mechanical properties were evaluated.

[0075] The evaluation methods are as follows.

[0076] Measurement of the area fraction of martensite The metallographic structure at the position of 1 / 4 of the wall thickness on the inner surface side of the obtained steel pipe was evaluated as follows. In a cross-section parallel to the longitudinal direction and the wall thickness direction of the steel pipe, samples were taken such that the positions of 1 / 4 of the wall thickness on the inner surface side and the center of the wall thickness were the observation target surfaces, and the cross-sections of the taken samples were etched using a 3 vol% nital solution. Scanning electron microscope (SEM) photographs were taken at an appropriate magnification between 1000 and 5000 times, and tempered martensite, ferrite, bainite, and pearlite were observed. Tempered martensite was visually judged by comparing with the micrographs in Reference 2. The tissue fraction was determined by image analysis using an image obtained by region-dividing the SEM photograph based on the above judgment, binarizing martensite and other regions, and obtaining the fraction of tempered martensite, which was taken as the area fraction of tempered martensite. [Reference 2] Japan Heat Treatment Technology Association (author), Introduction - Structure and Properties of Metallic Materials - Heat Treatment and Structure Control to Utilize Materials, 2004

[0077] Evaluation of the prior austenite grain size The old austenite (γ) was observed by polishing the cross-section (C cross-section) perpendicular to the longitudinal direction of the tube of the test piece for microstructure observation, corroding (using picral (picric acid-ethanol mixture)) to reveal the old γ grain boundaries, and observing with an optical microscope (magnification: 1000 times), and imaging was performed at three or more fields of view. For the obtained microstructure photographs, the grain size number of the old γ grains was determined using the cutting method in accordance with the provisions of JIS G 0551. The average value obtained above was taken as the grain size number of the old γ grains of each steel pipe.

[0078] Method for measuring Mo precipitates in steel materials Also, the method for measuring Mo precipitates in steel materials sampled from the steel pipe is as described below. The identification of Mo precipitates was carried out by an extraction method in which the steel material was electrolyzed and the obtained precipitates were filtered through a filter. A 10 mm square sample taken from the cross-section perpendicular to the rolling direction of the steel pipe (cross-section perpendicular to the tube axis: C cross-section) was melted by a constant current electrolysis method using a 10% AA-based electrolyte, placed in an aqueous solution of sodium hexametaphosphate with a concentration of 0.05 wt%, irradiated with ultrasonic waves, and the precipitates were taken out. The dissolved solution was filtered through a filter with a filter diameter of 50 nm to obtain precipitates of 50 nm or less. The precipitates of 50 nm or less that passed through the filter and the precipitates of more than 50 nm on the filter were subjected to heat white smoke treatment with sulfuric acid, perchloric acid, and nitric acid, and hydrochloric acid dissolution was performed. Thereafter, the Mo concentration (mass%) and the solid solution Mo concentration (mass%) contained in the precipitates of each size were calculated by performing concentration analysis on the precipitate dissolution solution and the electrolyte containing the solid solution component by ICP respectively. The total Mo amount contained in the steel was obtained by summing up the Mo amount and the solid solution Mo amount contained in all the precipitates obtained as described above, and the Mo amount in all the precipitates / total Mo amount and the Mo amount in the precipitates of 50 nm or less / Mo amount in all the precipitates were obtained.

[0079] Mechanical property evaluation The test piece was taken from a cross-section perpendicular to the tube axis of the steel pipe (C direction) with the inner surface of the steel pipe as the center of the wall thickness at 1 / 4 position, and a tensile test piece was taken such that the longitudinal direction of the test piece was the C direction. A bar-shaped test piece specified in JIS Z 2201 "Tensile test pieces for metallic materials" was used. The test was carried out using the method specified in JIS Z2241, and the maximum load was taken as the TS of the steel pipe. Note that it is preferable to center around the 1 / 4 wall thickness position. However, for steel pipes with a small wall thickness (for example, a wall thickness of 45 mm or less), a method of sampling without centering around the 1 / 4 wall thickness position can also be mentioned.

[0080] The evaluation of the hydrogen embrittlement resistance property was evaluated from the relative reduction area (RRA) of the test piece after a slow strain rate tensile test in hydrogen gas in accordance with ASTM G 142. In the atmosphere, the steel material undergoes plastic deformation and the area of the fracture surface becomes smaller, so the reduction area φ air becomes larger. On the other hand, in hydrogen, since the elongation of the steel material decreases, the material fractures before it can be reduced, and the area of the fracture part remains large. Therefore, the reduction area φ H of the fracture part after the test in hydrogen becomes smaller compared to that in the atmosphere. The hydrogen embrittlement resistance property was evaluated from this decrease in the reduction area. Note that the relative reduction area (RRA) is Relative reduction area (RRA) = φ H / φ air × 100 and is obtained as such. The relative reduction areas obtained from a slow strain rate tensile test (tensile speed: 0.002 mm / s) in hydrogen gas at 105 MPa and room temperature are shown in Table 2. The larger the RRA, the better the hydrogen embrittlement resistance property. In this evaluation, 60% or more was judged to be good. Note that φ air is the cross-sectional area of the test piece after the test in the atmosphere / the cross-sectional area before the test, and φ H is the cross-sectional area of the test piece after the test in hydrogen / the cross-sectional area before the test.

[0081] All of the inventive examples satisfied the conditions of a TS of 850 MPa or more in the tensile test in the atmosphere and an RRA of 60% or more in the slow strain rate tensile test in hydrogen gas.

[0082]

Table 3

Claims

1. In mass percent, C: 0.20-0.50%, Si: 0.05-1.00%, Mn: 0.30-1.20%, P: 0.015% or less, S: 0.005% or less, Al: 0.005-0.150%, N: 0.006% or less, Cr: more than 0.2% and 1.5% or less, Mo: more than 1.0% and less than 3.0%, Nb: 0.001-0.020%, B: 0.0003 to 0.0030%, O: 0.0030% or less, Ti: 0.003 to 0.025%; The ratio of the Mo content to the C content, Mo / C, is in the range of 3.0 to 12.0, The balance is Fe and unavoidable impurities, The steel has a structure in which the area ratio of tempered martensite is 95% or more, Of the Mo contained in the steel, 50% or more by mass is contained in precipitates, Furthermore, of the Mo contained in the precipitates, 50% by mass or more is contained in precipitates having a diameter of 50 nm or less, A high-strength seamless steel pipe for high-pressure hydrogen containers having a tensile strength TS of 850 MPa or more.

2. In addition to the above composition, further, in mass%, V: 0.30% or less, Cu: 1.00% or less, Ni: 2.0% or less, W: 3.0% or less 2. The high-strength seamless steel pipe for high-pressure hydrogen containers according to claim 1, which contains one or more selected from the following:

3. In addition to the above composition, further, in mass%, H: 0.0010% or less 2. The high-strength seamless steel pipe for high-pressure hydrogen containers according to claim 1, which contains

4. In addition to the above composition, further, in mass%, H: 0.0010% or less 3. The high-strength seamless steel pipe for high-pressure hydrogen containers according to claim 2, which contains

5. In addition to the above composition, further, in mass%, Ca: 0.0005-0.005% 5. The high-strength seamless steel pipe for high-pressure hydrogen containers according to claim 1, further comprising:

6. A method for producing a high-strength seamless steel pipe for high-pressure hydrogen containers according to any one of claims 1 to 4, A steel pipe material having the above composition is cast into a slab, and the slab is heated at a temperature in the range of 1050 to 1350°C; The cast piece is hot-rolled to form a seamless steel pipe of a predetermined shape. After the hot rolling, the seamless steel pipe is cooled at an average cooling rate equal to or faster than air cooling until the surface temperature is 200° C. or less; After cooling, Ac ３ Reheating to a temperature in the range of from the transformation point to 1000°C, The steel is subjected to a quenching treatment at least once, in which the steel is rapidly cooled to a surface temperature of 200°C or less. After the quenching treatment, a tempering treatment is performed by heating to a tempering temperature of 600 to 740°C. A method for producing a high-strength seamless steel pipe for high-pressure hydrogen containers, wherein the average heating rate until the tempering temperature is reached is 0.5°C / min or more, and the holding time at the tempering temperature is 10 minutes or more and less than 60 minutes.

7. A method for producing a high-strength seamless steel pipe for a high-pressure hydrogen container according to claim 5, A steel pipe material having the above composition is cast into a slab, and the slab is heated at a temperature in the range of 1050 to 1350°C; The cast piece is hot-rolled to form a seamless steel pipe of a predetermined shape. After the hot rolling, the seamless steel pipe is cooled at an average cooling rate equal to or faster than air cooling until the surface temperature is 200° C. or less; After cooling, Ac ３ Reheating to a temperature in the range of from the transformation point to 1000°C, The steel is subjected to a quenching treatment at least once, in which the steel is rapidly cooled to a surface temperature of 200°C or less. After the quenching treatment, a tempering treatment is performed by heating to a tempering temperature of 600 to 740°C. A method for producing a high-strength seamless steel pipe for high-pressure hydrogen containers, wherein the average heating rate until the tempering temperature is reached is 0.5°C / min or more, and the holding time at the tempering temperature is 10 minutes or more and less than 60 minutes.

8. 7. The method for producing a high-strength seamless steel pipe for a high-pressure hydrogen container according to claim 6, wherein the casting is performed at a casting speed of 1.8 m / min or less.

9. 8. The method for producing a high-strength seamless steel pipe for a high-pressure hydrogen container according to claim 7, wherein the casting is performed at a casting speed of 1.8 m / min or less.

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