Martensitic steel for high-pressure hydrogen environment

Abstract

Provided is a martensitic steel capable of exhibiting very high strength even under a high-pressure hydrogen environment.　The martensitic steel for a high-pressure hydrogen environment has a component composition comprising, in mass%, C: 0.25 to 0.75%, Si: 0.1 to 1.5%, Mn: 0.1 to 1.5%, P: 0.04% or less, S: 0.01% or less, Ni: 0.5% or less, Cr: 9.0 to 15.0%, one or both of Mo and W according to the relational expression (Mo+1 / 2W): 3.0% or less, Cu: 0.5% or less, Nb: 0.3% or less, N: 0.10% or less, with the balance being Fe and inevitable impurities, wherein, when a low strain rate tensile test using a hollow test piece is conducted in high-pressure hydrogen of 10 MPaG or more, the tensile strength is 1800 MPa or higher, the carbide area ratio in a cross-sectional structure is 0.1 to 10.0%, and the average circle equivalent diameter of the carbide is 0.10 to 1.0 μm.

Description

Martensitic steel for high-pressure hydrogen environments

[0001] The present invention relates to a martensitic steel for use in a high-pressure hydrogen environment.

[0002] Conventionally, SUS316L, an austenitic stainless steel with low susceptibility to hydrogen embrittlement, has been used for components used in high-pressure hydrogen environments (e.g., various sliding components in hydrogen production equipment, hydrogen transportation and storage equipment, hydrogen utilization equipment or infrastructure facilities, and hydrogen-fueled internal combustion engines). However, in order to efficiently utilize gaseous hydrogen, which has a lower energy density than liquid hydrogen, hydrogen pressure is also being further increased, and it is expected that SUS316L, which has a tensile strength of approximately 600 MPa, will not have enough strength. Therefore, various steels have been proposed with the aim of increasing strength without reducing hydrogen embrittlement resistance.

[0003] For example, Patent Document 1 discloses a steel sheet containing, by mass%, C: 0.10% or less, Si: 1.0% or less, Mn: 3% or more but less than 7%, Cr: 15 to 30%, Ni: 10% or more but less than 17%, Al: 0.10% or less, N: 0.10 to 0.50%, and at least one of V: ​​0.01 to 1.0% and Nb: 0.01 to 0.50%, with the balance being Fe and impurities, of which P is 0.050% or less and S is 0.050% or less, with a tensile strength of 800 MPa or more, a grain size number (ASTM E 112) of 8 or more, and alloy carbonitrides with a maximum diameter of 50 to 1000 nm at a density of 0.4 particles / μm when observed in cross section. ２ Austenitic stainless steel for high-pressure hydrogen gas has been proposed, characterized by the above-mentioned content. This proposal is excellent in that it achieves both hydrogen embrittlement resistance and high strength without adding more alloy than necessary.

[0004] Furthermore, Patent Document 2 proposes a high-strength steel that contains, by mass%, 0.20 to 0.50% C, 0.01 to 0.40% Si, 0.10 to 1.0% Mn, 0.02% or less P, 0.02% or less S, 1.0 to 5.0% Ni, 0.5 to 2.5% Cr, 0.1 to 1.5% Mo, 0.005 to 0.3% V, and optionally one or two of 0.1% or less Nb and 0.5% or less Cu, with the balance consisting of Fe and unavoidable impurities, and that has a composition such that Ceq, represented by the following formula (Ceq = C + Mn / 6 + Si / 24 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14), is 0.75 or more, thereby exhibiting high strength and excellent hydrogen embrittlement resistance in a high-pressure hydrogen environment. This proposal is excellent in that it combines high strength with hydrogen embrittlement resistance by using martensite or bainite structures, which tend to have higher strength than austenite structures but are highly susceptible to hydrogen embrittlement.

[0005] Furthermore, Patent Document 3 proposes a high-pressure hydrogen container having a chemical composition containing, in mass%, C: 0.20 to 0.50%, Si: 0.05 to 0.50%, Mn: 0.05 to 1.00%, P: 0.025% or less, S: 0.0200% or less, Al: 0.005 to less than 0.050%, Cr: 0.30 to 1.50%, Mo: 0.15 to 1.50%, Ti: 0.002 to 0.050%, B: 0.0001 to 0.0050%, N: less than 0.0070%, and O: less than 0.0050%, with the balance consisting of Fe and impurities, and containing 0.010 to 0.050 mass% of dissolved C, a tensile strength of 900 to 1100 MPa, and a yield ratio of 85% or more. This proposal is superior in that it has the same martensite or bainite structure as Patent Document 2, while achieving high strength of 1000 MPa or more in addition to hydrogen embrittlement resistance.

[0006] International Publication No. 2012 / 132992 Japanese Patent Application Laid-Open No. 2014-227573 Japanese Patent Application Laid-Open No. 2019-183218

[0007] Components used in the above-described high-pressure hydrogen environment are required to maintain high strength (e.g., tensile strength) even in the hydrogen environment. In addition, sliding components such as parts for hydrogen-fueled internal combustion engines (bearings, valves) and sealing components for hydrogen gas boosting compressors are prone to shortening their lifespan due to friction and wear with other components, and therefore require further strengthening. Furthermore, to improve hydrogen storage capacity and promote the miniaturization of hydrogen-related components, further increases in hydrogen pressure are also required. Patent Document 1 describes an effective method for increasing the strength of austenitic stainless steels while maintaining their excellent hydrogen embrittlement resistance, but does not examine whether they possess the strength characteristics required for use as sliding components, and there is a risk of shortening the component lifespan due to insufficient strength. Patent Documents 2 and 3 also describe effective methods for improving the hydrogen embrittlement resistance of steel products having martensite or bainite structures, but, like Patent Document 1, they may result in insufficient strength when used as sliding components in a hydrogen environment. Therefore, an object of the present invention is to provide a martensitic steel for use in a high-pressure hydrogen environment, which has extremely high tensile strength even in a high-pressure hydrogen environment by improving the hydrogen embrittlement resistance of a steel having a martensitic structure.

[0008] The inventors investigated the major issue of martensitic steel, which is its susceptibility to hydrogen embrittlement, and discovered that by optimizing the chemical composition and metal structure of martensitic steel, it is possible to increase resistance to hydrogen embrittlement and obtain high strength even in a high-pressure hydrogen environment, thereby arriving at the present invention.

[0009] That is, the present invention provides a martensitic steel for use in a high-pressure hydrogen environment, which has a chemical composition, in mass%, of C: 0.25 to 0.75%, Si: 0.1 to 1.5%, Mn: 0.1 to 1.5%, P: 0.04% or less, S: 0.01% or less, Ni: 0.5% or less, Cr: 9.0 to 15.0%, one or two of Mo and W according to the relationship (Mo + ½W): 3.0% or less, Cu: 0.5% or less, Nb: 0.3% or less, N: 0.10% or less, the balance being Fe and unavoidable impurities, and which, when subjected to a hollow specimen slow strain rate tensile test in high-pressure hydrogen of 10 MPaG or more, has a tensile strength of 1800 MPa or more, a carbide area fraction in a cross-sectional structure of 0.1 to 10.0%, and a carbide average circle equivalent diameter of 0.10 to 1.0 μm.

[0010] According to the present invention, it is possible to obtain a martensitic steel for use in a high-pressure hydrogen environment which has improved hydrogen embrittlement resistance and also has very high tensile strength even in a high-pressure hydrogen environment.

[0011] As described above, an important feature of the present invention is the discovery of a martensitic steel that exhibits extremely high strength properties even in a high-pressure hydrogen environment of 10 MPaG or more (preferably 20 MPaG). First, the reasons for limiting the composition of the present invention will be explained.

[0012] C: 0.25 to 0.75 mass% (hereinafter simply referred to as "%"). C is an element effective in increasing the hardness and strength (tensile strength, hereinafter also simply referred to as "strength") of the martensitic structure after quenching and tempering. However, if there is too much C, carbides become coarse and stress concentration occurs when stress is applied, which becomes the starting point of fracture due to hydrogen embrittlement, significantly reducing tensile strength. On the other hand, if there is too little C, there is less C that dissolves in the matrix and exerts a solid solution strengthening effect, resulting in reduced hardness and strength. Therefore, the C content is set to 0.25 to 0.75%. The preferred C content is 0.30% or more, more preferably 0.50% or more. The preferred upper limit of C is 0.70%.

[0013] Si: 0.1 to 1.5% Si is an element that is used as a deoxidizer during the melting process, and is expected to dissolve in steel and suppress softening during tempering. On the other hand, if there is too much Si, the hardness of the annealed material (i.e., the martensitic steel of the present invention before quenching and tempering (before adjusting to the product hardness)) increases, and cold workability decreases. Therefore, the Si content is set to 0.1 to 1.5%. The lower limit to ensure the effect of Si is 0.2%. The preferred upper limit of Si is 1.0%, and more preferably 0.8% or less. The even more preferred upper limit is 0.5%, and it is particularly preferred to set it to 0.45% or less.

[0014] Mn: 0.1 to 1.5% Mn is used as a deoxidizer during the melting process, and is therefore an element that may be unavoidably included. Furthermore, when solid-phase nitrogen absorption treatment is performed to improve strength, Mn is an element that promotes the solid solution of nitrogen into the structure. However, if there is too much Mn, the austenite structure is stabilized, making it difficult to obtain a martensite structure, and therefore high hardness and strength are difficult to obtain. Therefore, the Mn content is set to 0.1 to 1.5%. The lower limit to ensure the effects of Mn is 0.3%, and more preferably 0.4% or more. The upper limit of Mn is preferably 1.0%, and more preferably 0.9% or less.

[0015] P: 0.04% or less P is an impurity element that deteriorates the toughness of the product, so it is limited to 0.04% or less, preferably 0.03% or less.

[0016] S: 0.01% or less S is an element that forms MnS in the presence of Mn, improving the machinability of the product. However, if the content is too high, hot workability deteriorates, so in the present invention, the S content is limited to 0.01% or less. It is preferably limited to 0.005% or less, and more preferably to 0.003% or less.

[0017] Ni: 0.5% or less. Ni is an effective element for improving corrosion resistance against non-oxidizing acids such as formic acid, sulfuric acid, and hydrochloric acid. However, if the Ni content is too high, the austenite structure in a martensitic steel product after quenching and tempering is stabilized, making it difficult to obtain a martensite structure and high hardness and strength. Therefore, Ni can be contained in an amount of 0.5% or less as needed. It is preferably 0.3% or less, and more preferably 0.2% or less. If the aforementioned effects of Ni addition are to be expected, the lower limit can be set to 0.05%. In the present invention, the Ni content can be appropriately determined within a range of 0.5% or less in relation to other added elements. For example, if high hardness and high strength are prioritized, no Ni (0%) may be added.

[0018] Cr: 9.0 to 15.0% Cr is an element that forms an amorphous passive film on the surface of martensitic steel, imparting corrosion resistance to the product. It also has the effect of increasing the amount of nitrogen that can be dissolved in martensitic steel when solid-phase nitrogen absorption treatment is performed. However, if the Cr content is too high, the ferrite structure is stabilized, making it difficult to obtain a martensitic structure and high hardness and strength. Therefore, the Cr content is set to 9.0 to 15.0%. To ensure the effects of Cr, the preferred lower limit is 10.0%, more preferably 10.5% or more. The preferred upper limit of Cr is 14.0%, more preferably 13.5% or less.

[0019] One or both of Mo and W according to the formula (Mo + 1 / 2W): 3.0% or less. Mo and W are similar elements and can be treated equally according to the formula (Mo + 1 / 2W). Mo and W have the effect of stabilizing the passivation film of martensitic steel in a solid solution state, contributing to improving the corrosion resistance of the product surface. Furthermore, when the Cr-based passivation film is scratched, Mo and W increase the amount of Cr at the scratched location, strengthening the repair ability of the passivation film. These elements increase the amount of N absorbed during quenching, so for example, quenching in a nitrogen atmosphere can form a nitrogen-based hardened layer on the surface of martensitic steel, increasing its strength. However, if the amount of Mo and W is too high, the ferrite structure will be stabilized, making it difficult to obtain a martensitic structure, as with Cr. Therefore, the content of Mo and W is set to 3.0% or less, based on the relationship (Mo + 1 / 2W). It is preferably 2.0% or less, more preferably 1.5% or less, and even more preferably 1.0% or less. If the effect of stabilizing the passivation film described above can be compensated for by other additive elements, it is acceptable to add no Mo or W (0%). However, if the effect of adding Mo and / or W is expected, the lower limit should be set to 0.3% or more. Furthermore, if one of Mo and W must be selected, Mo should be used.

[0020] Cu: 0.5% or less. Cu is an effective element for improving corrosion resistance against non-oxidizing acids such as formic acid, sulfuric acid, and hydrochloric acid. However, if the Cu content is too high, the austenite structure in the martensitic steel product after quenching and tempering is stabilized, making it difficult to obtain a martensite structure and high hardness and strength. Therefore, Cu can be contained in an amount of 0.5% or less as needed. The preferred upper limit of Cu is 0.3%, and more preferably 0.2% or less. If the aforementioned effects of Cu addition are to be expected, the lower limit can be set to 0.05%. In the present invention, the Cu content can be appropriately determined within a range of 0.5% or less in relation to other additive elements. For example, if high hardness and high strength are prioritized, no Cu (0%) may be added.

[0021] Nb: 0.3% or less Nb has the effect of suppressing the growth of prior austenite grain boundaries and refining the martensite structure. Refining the crystal grains contributes to improving strength and ductility and has the effect of reducing susceptibility to hydrogen embrittlement. However, if there is too much Nb, C and N form Nb compounds, which reduces the solid solubility of C and N and reduces the effect of improving hardness and strength. Therefore, Nb can be contained in an amount of 0.3% or less (including no addition) as necessary. Preferably, it is 0.2% or less. If the effect of adding Nb described above is expected, the lower limit should be set to 0.05%.

[0022] N: 0.10% or less. N is an element that dissolves in the martensite structure or precipitates as a compound in quenched and tempered martensitic steel products, improving strength, ductility, and hydrogen embrittlement resistance. While the reason for the above-mentioned effects of N is unclear, adding N converts carbides produced during tempering into carbonitrides, which are finely dispersed, enabling both strength and ductility to be achieved. Furthermore, the finely dispersed carbonitrides are thought to trap hydrogen, suppressing hydrogen localization and improving hydrogen embrittlement resistance. Furthermore, N is an element that can suppress the precipitation of delta ferrite, which is harmful to the microstructure, in martensitic steel before quenching and tempering. However, excessive N content not only generates bubbles during casting, significantly degrading manufacturability, but also may cause the crystallization of coarse nitrides after solidification. Furthermore, when finishing the unquenched material into a product shape, it is prone to work hardening during cold working, necessitating repeated intermediate annealing, and further degrading machinability. Therefore, N can be contained in an amount of 0.10% or less as needed. It is preferably 0.08% or less, and more preferably 0.06% or less. If the above-mentioned effect of adding N is expected, the lower limit can be set to 0.01%. In addition to the elements described above, the only elements included are Fe and unavoidable impurities.

[0023] Carbide area ratio: 0.1 to 10.0% Next, the microstructure will be described. The martensitic steel of the present invention has martensite as its base and carbides containing mainly C and Cr. Carbides are necessary to improve the wear resistance of martensitic steel, but if the carbide area ratio is large, the amount of Cr dissolved in martensite decreases relatively, which may lead to a decrease in corrosion resistance and hydrogen embrittlement fracture originating from the carbides. Therefore, the carbide area ratio in the cross-sectional structure of the martensitic steel of the present invention is set to 10.0% or less. A more preferable upper limit of the carbide area ratio is 9.5%, and an even more preferable upper limit is 9.3%. Furthermore, since a small carbide area ratio significantly reduces wear resistance, the carbide area ratio is set to 0.1% or more. A more preferable lower limit of the carbide area ratio is 0.15%.

[0024] Average equivalent circular diameter of carbides: 0.10 to 1.0 μm In the present invention, in addition to the above-mentioned carbide area ratio, the average equivalent circular diameter of carbides is also controlled within an appropriate range. That is, since coarse carbides promote stress concentration during stress application and may lead to a significant decrease in strength due to hydrogen embrittlement, the upper limit of the average equivalent circular diameter (area equivalent circular diameter) of carbides in the cross-sectional structure is set to 1.0 μm. A preferred upper limit of the average equivalent circular diameter is 0.8 μm, a more preferred upper limit of the average equivalent circular diameter is 0.6 μm, and an even more preferred upper limit of the average equivalent circular diameter is 0.5 μm. Furthermore, a preferred lower limit of the average equivalent circular diameter is 0.13 μm, preferably 0.15 μm, and even more preferably 0.18 μm. In this embodiment, the carbide area ratio and the average circle equivalent diameter are determined by taking a photograph of a cross-sectional structure perpendicular to the processing direction (the elongation direction of the rolling process) of the martensitic steel with a field area of ​​500 μm 2 taken with a scanning electron microscope (magnification: 5000 times). ２The carbides can be calculated by observing the carbides in the above fields of view and analyzing the images. Note that the carbides targeted in image analysis are limited to those with an equivalent circle diameter of 0.1 μm or more; those with a diameter less than this are not considered. Carbides can be identified by elemental mapping using an electron probe microanalyzer (EPMA) attached to a scanning electron microscope. The martensitic steel of the present invention can be obtained, for example, by subjecting a hot-worked material (e.g., hot-rolled material) having the above-described composition to one or more cold working processes (e.g., cold rolling) and annealing, and then quenching, subzero treatment (optional), and tempering the resulting cold-worked material. The carbide area ratio and average carbide diameter of the present invention can be obtained by adjusting the heating and cooling conditions to achieve the carbide area ratio and average carbide diameter of the present invention, followed by annealing, quenching, subzero treatment (optional), and tempering.

[0025] Hollow Specimen Slow Strain Rate Tensile Testing Next, the test method and its characteristics specified in the present invention will be described. In the present invention, a slow strain rate tensile test using a hollow specimen (hollow specimen slow strain rate tensile test in high-pressure hydrogen) is used as a tensile test in a high-pressure hydrogen environment. Other high-pressure hydrogen environment testing methods different from those of the present invention include a method in which a solid tensile specimen is used in a chamber capable of applying high pressure and a predetermined hydrogen pressure is applied, and a method in which a hollow tensile specimen is hollowed and high-pressure hydrogen gas is applied thereto. Both methods are capable of evaluating hydrogen embrittlement behavior in a high-pressure hydrogen environment. Therefore, in the present invention, the characteristics are evaluated using a hollow specimen high-pressure hollow test, which is the simpler method of high-pressure hydrogen environment testing. Furthermore, hydrogen embrittlement is dependent on the strain rate during tensile testing, with hydrogen susceptibility increasing as the strain rate decreases. Therefore, in the present invention, tensile testing is performed at a slow strain rate to clarify the presence or absence of hydrogen resistance.

[0026] As described above, the martensitic steel of the present invention has a tensile strength of 1800 MPa or more obtained in a slow strain rate tensile test of a hollow specimen in high-pressure hydrogen of 10 MPaG or more (preferably 20 MPaG). Generally, the higher the strength of a metallic material, the higher its hydrogen sensitivity becomes. Therefore, materials with a strength exceeding 1000 MPa are prone to hydrogen embrittlement and exhibit a significant decrease in strength in a hydrogen environment. For sliding components used in a hydrogen environment, high wear resistance and fatigue strength are important, and high strength, which correlates with these properties, is used as an evaluation index. In the present invention, the tensile strength is specified as 1800 MPa or more to achieve extremely excellent sliding properties. Furthermore, generally, the higher the hydrogen pressure in the environment, the higher the hydrogen sensitivity and the lower the tensile strength. The martensitic steel of the present invention has an extremely high tensile strength of 1800 MPa even in a high-pressure hydrogen environment of 10 MPaG or more (preferably 20 MPaG), and is therefore expected to contribute to further improving the performance of hydrogen production equipment, hydrogen transportation and storage equipment, and hydrogen utilization equipment, as well as further miniaturization of various sliding parts such as infrastructure facilities and hydrogen-fueled internal combustion engines. As will be shown in the examples below, the martensitic steel of the present invention can achieve a high tensile strength of 2000 MPa or more when subjected to a slow strain rate tensile test on a hollow test specimen in high-pressure hydrogen of 10 MPaG. Furthermore, when subjected to a slow strain rate tensile test on a hollow test specimen in high-pressure hydrogen of 20 MPaG, a high tensile strength of more than 1800 MPa can be achieved.

[0027] In the present invention, a fatigue test using a hollow test specimen may be performed as a fatigue test in a high-pressure hydrogen environment. This fatigue test uses a technique in which a fatigue test specimen is hollowed and high-pressure hydrogen gas is applied thereto. As with the tensile test described above, the fatigue properties can be evaluated by a hollow test specimen high-pressure hollow test, which is a simpler technique among high-pressure hydrogen environment tests. When the fatigue test is performed, in the present invention, a tension-compression fatigue test (number of cycles: 1×10) is performed in high-pressure hydrogen of 10 MPaG or more (preferably 20 MPaG) with load control at a stress ratio of -1. ７The fatigue strength value when subjected to the tensile test and fatigue test is preferably at least 450 MPa, and even better, 500 MPa. The upper limit of the hydrogen pressure during the tensile test and fatigue test is not particularly limited, but can be set to, for example, 25 MPaG, taking into account the combustion pressure of the internal combustion engine.

[0028] If further enhancement of high strength properties is desired, the martensitic steel of the present invention can also be subjected to solid-phase nitrogen absorption treatment. This solid-phase nitrogen absorption treatment can be carried out by quenching martensitic steel having the alloy composition of the present invention in a nitrogen atmosphere during quenching. A quenching temperature of 1000 to 1100°C is preferred because this allows sufficient nitrogen to be dissolved in solid solution. Subzero treatment at a temperature of −50°C or lower can also be carried out after the quenching process and before the tempering process. On the other hand, if further enhancement of hydrogen embrittlement resistance is desired, the solid-phase nitrogen absorption treatment can be omitted.

[0029] 10 kg of molten metal melted in a high-frequency induction melting furnace was cast to produce martensitic steel ingots No. 1 to No. 13 having the multiple component compositions shown in Table 1. Next, these ingots were heat treated by holding them at a holding temperature of 1080 to 1150°C for 60 minutes. This holding temperature was used as the forging start temperature, followed by hot forging with a forging ratio (cross-sectional area before forging / cross-sectional area after forging) of approximately 10, followed by cooling. After cooling, the ingots were annealed by holding them at a holding temperature of 780 to 860°C for 4 hours or more to obtain martensitic steel materials with thicknesses of approximately 15 to 20 mm. Note that martensitic steels No. 1 to 7 and No. 12 in Table 1 satisfy the composition ranges specified in the present invention.

[0030]

[0031] The materials Nos. 1 to 13 of the present invention were subjected to the quenching treatment, deep-freezing treatment (subzero treatment), and tempering treatment shown in Table 2. These materials were processed into hollow tensile test specimens with a parallel portion diameter of 4 mm, a parallel portion length of 20 mm, a total length of 70 to 90 mm, and an internal through-hole diameter of 1 mm. The carbide area ratio and the carbide average equivalent circle diameter were also measured. The carbide area ratio and the carbide average equivalent circle diameter were measured in a cross-sectional structure perpendicular to the processing direction (the elongation direction of the rolling process) of the martensitic steel after quenching treatment, deep-freezing treatment, and tempering treatment, with a field area of ​​500 μm photographed with a scanning electron microscope (magnification 5000x). ２ The carbides in the above fields of view were observed and subjected to image analysis to calculate the carbide area ratio. Note that the carbides targeted in the image analysis were limited to those with an equivalent circle diameter of 0.1 μm or more, and those with a diameter less than this were not considered. The carbide area ratio and average equivalent circle diameter obtained are shown in Table 2. Note that the martensitic steels No. 1 to 7 in Table 2 satisfied the carbide morphology specified in the present invention.

[0032]

[0033] Then, hydrogen was introduced into the obtained hollow tensile test specimen at room temperature or 200°C so that the pressure in the internal through-hole was 10 or 20 MPaG, and strain rate was 5.0 × 10 －５ The test conditions and results are shown in Table 3.

[0034]

[0035] As shown in Tables 2 and 3, Nos. 1 to 7 of the present invention had an area ratio of carbides of 0.2 to 9.3%. The average equivalent circle diameter of the carbides was 0.15 μm or more and 0.60 μm or less. They exhibited extremely high tensile strengths of 1800 MPa or more at room temperature to 200°C and in a high-temperature, high-pressure hydrogen environment of 10 to 20 MPaG. In particular, Nos. 1 to 5, which are inventive examples, achieved tensile strengths of 2000 MPa or more when the hydrogen atmosphere pressure was 10 MPaG, and even demonstrated very good values ​​of 2100 MPa or more. Among these, Nos. 1 to 3 and 5 achieved tensile strengths of 2200 MPa or more. No. 1 exhibited an extremely high tensile strength of 1820 MPa or more in a high-temperature, high-pressure hydrogen environment of 20 MPaG. On the other hand, Comparative Example No. 8 had low tensile strength due to its low carbon content. Comparative Examples Nos. 9 to 10 also exhibited low tensile strength. In No. 12, the area ratio of carbides was 10.8 to 18.7%, and in No. 13, the comparative example, the average equivalent circle diameter of carbides was 1.16 μm, so hydrogen embrittlement occurred in the stress concentration area, resulting in brittle fracture in the elastic range and resulting in low tensile strength.

[0036] Next, the fatigue strength of the inventive examples was confirmed. The martensitic steel materials of inventive examples 1 and 6 were subjected to the quenching treatment, deep-freezing treatment, and tempering treatment shown in Table 2, and then machined into hollow fatigue test specimens with a parallel portion diameter of 7 mm, a parallel portion length of 20 mm, a total length of 95 mm, and an internal through-hole diameter of 1 mm. Hydrogen was introduced into the internal through-hole at room temperature so that the pressure became 10 MPa, and a tension-compression fatigue test (number of cycles: 1 × 10) was performed with the hollow fatigue test specimens under load control at a stress ratio of -1. ７ The test temperature was room temperature, the waveform was a sine wave, and the frequency was 20 Hz. The test was carried out by controlling the load to 500 MPa. As a result, both No. 1 and No. 6 had a 1 × 10 ７ The fatigue strength in the 1000 cycle test was at least 500 MPa, demonstrating favorable fatigue properties. These results confirm that the martensitic steel of the present invention exhibits high strength even in a high-pressure hydrogen environment due to its improved resistance to hydrogen embrittlement.

Claims

1. In terms of mass percentage, it consists of a component composition of C: 0.25 to 0.75%, Si: 0.1 to 1.5%, Mn: 0.1 to 1.5%, P: 0.04% or less, S: 0.01% or less, Ni: 0.5% or less, Cr: 9.0 to 15.0%, one or two of Mo and W according to the relational expression of (Mo + 1 / 2W): 3.0% or less, Cu: 0.5% or less, Nb: 0.3% or less, N: 0.10% or less, and the balance Fe and inevitable impurities. When a hollow test piece low strain rate tensile test is carried out in high-pressure hydrogen of 10 MPaG or more, the tensile strength is 1800 MPa or more, the carbide area ratio in the cross-sectional structure is 0.1 to 10.0%, and the average equivalent circle diameter of the carbide is 0.10 to 1.0 μm. A martensitic steel for high-pressure hydrogen environment.

2. The martensitic steel for high-pressure hydrogen environment according to claim 1, when a hollow test piece low strain rate tensile test is carried out in high-pressure hydrogen of 10 MPaG, the tensile strength is 2000 MPa or more.

3. The martensitic steel for high-pressure hydrogen environment according to claim 1, when a hollow test piece low strain rate tensile test is carried out in high-pressure hydrogen of 20 MPaG, the tensile strength is 1800 MPa or more.

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