Martensitic steel for high-pressure hydrogen environments

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

[Technical Field]

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

[0002] Traditionally, SUS316L, an austenitic stainless steel with low susceptibility to hydrogen embrittlement, has been used for components used in high-pressure hydrogen environments (for example, 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 pressures are also increasing, and it is expected that SUS316L, which has a tensile strength of around 600 MPa, will not have enough strength. For this reason, 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] 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 being Fe and unavoidable impurities, and that has a composition such that Ceq, as shown in 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 being Fe and impurities, containing 0.010 to 0.050 mass% of solid-solubilized C, having 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, yet achieves high strength of 1000 MPa or more in addition to hydrogen embrittlement resistance. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] International Publication No. 2012 / 132992 [Patent Document 2] Japanese Patent Application Laid-Open No. 2014-227573 [Patent Document 3] Japanese Patent Application Publication No. 2019-183218 Summary of the Invention [Problem to be solved by the invention]

[0007] Components used in the above-mentioned 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 required. Patent Document 1 describes an effective method for increasing the strength of austenitic stainless steel while maintaining its excellent hydrogen embrittlement resistance, but does not address whether the strength characteristics required for use as a sliding component are present, 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 with martensite or bainite structures, but, like Patent Document 1, they may result in insufficient strength when used as a sliding component 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. [Means for solving the problem]

[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 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 formula (Mo + 1 / 2W): 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, This martensitic steel for high-pressure hydrogen environments has a tensile strength of 1800 MPa or more when subjected to a slow strain rate tensile test on a hollow specimen in high-pressure hydrogen of 10 MPaG or more, a carbide area ratio of 0.1 to 10.0% in the cross-sectional structure, and an average carbide circle equivalent diameter of 0.10 to 1.0 μm. [Effects of the Invention]

[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. DETAILED DESCRIPTION OF THE INVENTION

[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 referred to simply as "strength") of the martensitic structure after quenching and tempering. However, if there is too much C, the carbides become coarse and stress concentration occurs when stress is applied, which becomes the starting point of fracture due to hydrogen embrittlement, resulting in a significant decrease in tensile strength. On the other hand, if there is too little C, there is less C that dissolves in the matrix and exerts the solid solution strengthening effect, resulting in a decrease in hardness and strength. Therefore, the C content is set to 0.25 to 0.75%. The C content is preferably 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 also expected to have the effect of suppressing softening during tempering by dissolving in steel. 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 for ensuring the effect of Si is 0.2%. The upper limit of Si is preferably 1.0%, more preferably 0.8% or less. The even more preferable upper limit is 0.5%, and it is particularly preferable 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 can be unavoidably included. Furthermore, when solid-phase nitrogen absorption treatment is performed to improve strength, Mn has the effect of promoting 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%. To ensure the effects of Mn, the lower limit is 0.3%, and more preferably 0.4% or more. Furthermore, 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 its content 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 S content is too high, hot workability deteriorates. Therefore, in the present invention, the S content is limited to 0.01% or less, preferably 0.005% or less, and more preferably 0.003% or less.

[0017] ·Ni: 0.5% or less Ni is an element effective in 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 martensitic steel products after quenching and tempering is stabilized, making it difficult to obtain a martensite structure and high hardness and strength. Therefore, Ni can be added 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 desired, 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: 9.0~15.0% 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 there is too much Cr, the ferrite structure is stabilized, making it difficult to obtain a martensitic structure, and it becomes difficult to obtain high hardness and strength. Therefore, the Cr content is set to 9.0 to 15.0%. To reliably obtain the effects of Cr, the lower limit is preferably 10.0%, more preferably 10.5% or more. The upper limit of Cr is preferably 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 relationship (Mo + 1 / 2W). Mo and W have the effect of stabilizing the passivation film of martensitic steel in their solid solution state, contributing to improving the corrosion resistance of product surfaces. Furthermore, when a Cr-based passivation film is scratched, Mo and W increase the Cr content at the scratched location, strengthening the repair ability of the passivation film. These elements increase the amount of N absorbed during quenching. For example, quenching in a nitrogen atmosphere can form a nitrogen-based hardened layer on the surface of martensitic steel, increasing its strength. However, excessive Mo and W content, like the above-mentioned Cr, stabilizes the ferrite structure, making it difficult to obtain a martensitic structure. Therefore, the Mo and W contents are set to 3.0% or less according to the relationship (Mo + 1 / 2W). Preferably, they are 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, then no addition of Mo or W (0%) is acceptable, but 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, then 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 martensitic steel products after quenching and tempering is stabilized, making it difficult to obtain a martensite structure and high hardness and strength. Therefore, Cu can be added up to 0.5% as needed. The preferred upper limit of Cu is 0.3%, and more preferably 0.2%. If the aforementioned effects of Cu addition are desired, 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 effects of adding Nb are expected, the lower limit should be set to 0.05%.

[0022] ·N: 0.10% or less Nitrogen (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 aforementioned effects of N is unclear, adding N converts the carbides formed during tempering into carbonitrides, which are finely dispersed, enabling both strength and ductility. Furthermore, the finely dispersed carbonitrides are thought to trap hydrogen, suppressing hydrogen localization and improving hydrogen embrittlement resistance. Nitrogen also inhibits 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 reducing manufacturability, but also leads to the crystallization of coarse nitrides after solidification. Furthermore, when finishing unquenched materials into product shapes, they are prone to work hardening during cold working, necessitating repeated intermediate annealing, and further degrading machinability. Therefore, N can be added up to 0.10% if necessary. The content 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 should be set at 0.01%. In addition to the elements described above, the elements contain Fe and unavoidable impurities.

[0023] ·Carbide area ratio: 0.1~10.0% 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, it is set to 0.1% or more. A more preferable lower limit of the carbide area ratio is 0.15%.

[0024] Average diameter of carbide particles: 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 the carbides is also controlled within an appropriate range. Specifically, 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). 2 The calculation can be performed 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 a circular equivalent diameter of 0.1 μm or more; those with a diameter smaller than this are not considered. Furthermore, the identification of carbides can be confirmed by elemental mapping using an EPMA (electron probe microanalyzer) 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., a hot-rolled material) having the above-described chemical 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, and then performing annealing, quenching, subzero treatment (optional), and tempering.

[0025] Hollow specimen slow strain rate tensile test The test method and its characteristics specified in this invention are described below. In this invention, a slow strain rate tensile test using a hollow test specimen (a hollow test specimen slow strain rate tensile test in high-pressure hydrogen) is used as the tensile test in a high-pressure hydrogen environment. Other high-pressure hydrogen environment testing methods different from those of this invention include a method in which a solid tensile test 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 test 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 this invention, the characteristics are evaluated using a hollow test 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 this invention, a tensile test 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 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 susceptibility. Therefore, materials with a strength exceeding 1000 MPa are prone to hydrogen embrittlement and exhibit a significant decrease in strength in a hydrogen environment. High wear resistance and fatigue strength are important for sliding components used in hydrogen environments, and high strength, which correlates with these properties, is used as an evaluation index. In the present invention, a tensile strength of 1800 MPa or more is specified to achieve excellent sliding properties. Generally, the higher the hydrogen pressure in the environment, the higher the hydrogen susceptibility 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). Therefore, it is expected to contribute to further improving the performance of hydrogen production equipment, hydrogen transportation and storage equipment, and hydrogen utilization equipment, as well as to further miniaturizing 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 using a hollow test piece in high-pressure hydrogen of 10 MPaG.Furthermore, when subjected to a slow strain rate tensile test using a hollow test piece in high-pressure hydrogen of 20 MPaG, it can achieve a high tensile strength of more than 1800 MPa.

[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 out and high-pressure hydrogen gas is applied inside the hollow test specimen. 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 a load controlled at a stress ratio of -1. 7The fatigue strength value when subjected to tensile tests and fatigue tests is preferably at least 450 MPa, and even better, 500 MPa. There is no particular upper limit on the hydrogen pressure during tensile tests and fatigue tests, but it can be set to, for example, 25 MPaG, taking into account the combustion pressure of the internal combustion engine.

[0028] If further improvement in strength is desired, the martensitic steel of the present invention can be subjected to solid-phase nitrogen absorption treatment. This solid-phase nitrogen absorption treatment can be carried out by quenching the 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 -50°C or lower can also be carried out after the quenching process and before the tempering process. On the other hand, if further improvement in hydrogen embrittlement resistance is desired, the solid-phase nitrogen absorption treatment can be omitted. [Example]

[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 chemical 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, and subsequent hot forging was performed with a forging ratio (cross-sectional area before forging / cross-sectional area after forging) of approximately 10. 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 No. 7 and No. 12 in Table 1 satisfy the composition ranges specified in the present invention.

[0030] [Table 1]

[0031] The materials Nos. 1 to 13 of the present invention were subjected to the quenching, deep-freezing (subzero treatment), and tempering treatments shown in Table 2. These materials were processed into hollow tensile test specimens with a parallel section diameter of 4 mm, a parallel section 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 mean circle equivalent diameter were also measured. The carbide area ratio and the carbide mean circle equivalent diameter were measured using a scanning electron microscope (magnification 5000x) to measure the area of ​​a field of view of 500 μm in a cross-sectional structure perpendicular to the processing direction (stretching direction of rolling) of the martensitic steel after quenching, deep-freezing, and tempering. 2 The carbides in the above fields of view were observed and subjected to image analysis for calculation. 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 ratios and average equivalent circle diameters obtained are shown in Table 2. Note that the martensitic steels Nos. 1 to 7 in Table 2 satisfied the carbide morphology specified in the present invention.

[0032] [Table 2]

[0033] Hydrogen was then 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 the strain rate was 5.0 × 10 -5 The test conditions and results are shown in Table 3.

[0034] [Table 3]

[0035] As shown in Tables 2 and 3, Nos. 1 to 7 of the present invention had a carbide area ratio 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 in a high-temperature, high-pressure hydrogen environment of 10 to 20 MPaG. In particular, Nos. 1 to 5, which are examples of the present invention, achieved tensile strengths of 2000 MPa or more when the hydrogen atmosphere pressure was 10 MPaG, demonstrating excellent 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 the small amount of C. Furthermore, Comparative Examples Nos. 9 to 12 had carbide area ratios of 10.8 to 18.7%, and Comparative Example No. 13 had an average equivalent circle diameter of 1.16 μm, so hydrogen embrittlement occurred in the stress concentration area, resulting in brittle fracture in the elastic region and, as a result, 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, deep-freezing, and tempering treatments shown in Table 2, and then machined into hollow fatigue test specimens with a parallel section diameter of 7 mm, a parallel section 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 conducted 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 sine wave, and the frequency was 20Hz. The load was controlled to 500MPa. As a result, both No. 1 and No. 6 had a load of 1×10 7 It also showed good fatigue properties with a fatigue strength of at least 500 MPa in cycles. From the above results, it was confirmed 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. The composition is, 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 two of Mo and W according to the relationship formula (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; A hollow tensile test piece having 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 was subjected to a high-pressure hydrogen low strain rate tensile test at room temperature, where hydrogen was introduced into the internal through-hole at a pressure of 10 MPa and the strain rate was 5.0 × 10 -5 / s. The tensile strength was 1800 MPa or more. A martensitic steel for use in a high-pressure hydrogen environment, having a carbide area ratio of 0.1 to 10.0% in a cross-sectional structure and an average carbide equivalent circle diameter of 0.10 to 1.0 μm.

2. 2. The martensitic steel for use in a high-pressure hydrogen environment according to claim 1, wherein the martensitic steel for use in a high-pressure hydrogen environment has a tensile strength of 2000 MPa or more when subjected to a hollow test piece slow strain rate tensile test in high-pressure hydrogen at 10 MPaG.

3. 2. The martensitic steel for use in a high-pressure hydrogen environment according to claim 1, wherein the martensitic steel for use in a high-pressure hydrogen environment has a tensile strength of 1800 MPa or more when subjected to a hollow test piece slow strain rate tensile test in high-pressure hydrogen at 20 MPaG.

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