JP7810312B2 - Martensitic steel for high-pressure hydrogen environments - Google Patents

Description

The present invention relates to martensitic steel for high-pressure hydrogen environments, which is used in high-pressure hydrogen environments.

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, with a tensile strength of around 600 MPa, will not have enough strength. For this reason, various steels have been proposed that aim to increase strength without reducing hydrogen embrittlement resistance.

For example, Patent Document 1 proposes an austenitic stainless steel for high-pressure hydrogen gas, which contains, 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, wherein the impurities contain 0.050% or less of P and 0.050% or less of S, has a tensile strength of 800 MPa or more, a grain size number (ASTM E 112) of 8 or more, and contains 0.4 or ^more alloy carbonitrides/μm2 with a maximum diameter of 50 to 1000 nm when observed in cross section. This proposal is excellent in that it achieves both hydrogen embrittlement resistance and high strength without adding more alloy than necessary.

Patent Document 2 also proposes a high-strength steel that has high strength and excellent hydrogen embrittlement resistance in a high-pressure hydrogen environment by containing, by mass%, C: 0.20-0.50%, Si: 0.01-0.40%, Mn: 0.10-1.0%, P: 0.02% or less, S: 0.02% or less, Ni: 1.0-5.0%, Cr: 0.5-2.5%, Mo: 0.1-1.5%, V: 0.005-0.3%, and optionally containing one or both of Nb: 0.1% or less and Cu: 0.5% or less, with the balance consisting of Fe and unavoidable impurities, and by having a composition such that Ceq (Ceq = C + Mn/6 + Si/24 + Ni/40 + Cr/5 + Mo/4 + V/14) as shown in the following formula is 0.75 or greater. 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.

Furthermore, Patent Document 3 proposes a high-pressure hydrogen container containing, by 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, containing 0.010 to 0.050 mass% solid solution 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, while achieving high strength of 1000 MPa or more in addition to hydrogen embrittlement resistance.

International Publication No. 2012/132992 JP 2014-227573 A Japanese Patent Application Laid-Open No. 2019-183218

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.

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 achieve high strength even in high-pressure hydrogen environments, leading to the invention.

That is, the present invention provides a steel sheet having a component 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 use in 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 test piece in high-pressure hydrogen of 10 MPaG or more, a carbide area ratio of 0.1 to 10.0% in a cross-sectional structure, and an average carbide circle equivalent diameter of 0.10 to 1.0 μm.

According to the present invention, it is possible to obtain martensitic steel for high-pressure hydrogen environments that has improved hydrogen embrittlement resistance and extremely high tensile strength even in high-pressure hydrogen environments.

As mentioned 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, we will explain the reasons for limiting the composition of the present invention.

C: 0.25 to 0.75% by 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, 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, and more preferably 0.50% or more. The preferred upper limit of C is 0.70%.

・Si: 0.1-1.5%
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 the Si content is too high, 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 the 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.

・Mn: 0.1-1.5%
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 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 preferred upper limit of Mn is 1.0%, and more preferably 0.9% or less.

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.

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.

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.

・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. Furthermore, when solid-phase nitrogen absorption treatment is performed, it also has the effect of increasing the amount of nitrogen that can be dissolved in martensitic steel. 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%. In order 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.

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.

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.

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%.

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 necessary, 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 may be set to 0.01%.
In addition to the elements described above, the elements include Fe and unavoidable impurities.

・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%.

・Average diameter of carbide particles equivalent to a circle: 0.10 to 1.0 μm
In the present invention, in addition to the above-mentioned carbide area ratio, the average equivalent circle diameter of the carbides is also controlled within an appropriate range. That is, since coarse carbides promote stress concentration when stress is applied and may lead to a significant decrease in strength due to hydrogen embrittlement, the upper limit of the average equivalent circle diameter (area equivalent circle diameter) of carbides in the cross-sectional structure is set to 1.0 μm. A preferred upper limit of the average equivalent circle diameter is 0.8 μm, a more preferred upper limit of the average equivalent circle diameter is 0.6 μm, and an even more preferred upper limit of the average equivalent circle diameter is 0.5 μm. Furthermore, a preferred lower limit of the average equivalent circle diameter is 0.13 μm, preferably 0.15 μm, and even more preferably 0.18 μm.
In this embodiment, the carbide area ratio and average equivalent circle diameter can be calculated by observing carbides in a field of view with an area of 500 ^μm2 or more taken with a scanning electron microscope (magnification 5000x) in a cross-sectional structure perpendicular to the processing direction (stretching direction of rolling) of the martensitic steel, and analyzing the image. Note that the carbides targeted in the image analysis are limited to those with an equivalent circle diameter of 0.1 μm or more, and those with a diameter less than this are not targeted. In addition, the identification of carbides can be confirmed by elemental mapping using an EPMA (electron probe microanalyzer) attached to the 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 the resulting cold-worked material, subjecting it to subzero treatment (optional), and tempering. 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.

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.

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 to further miniaturizing various sliding parts such as infrastructure equipment 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 piece in high-pressure hydrogen at 10 MPaG.Furthermore, when subjected to a slow strain rate tensile test on a hollow test piece in high-pressure hydrogen at 20 MPaG, it can achieve a high tensile strength of more than 1800 MPa.

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 involves hollowing out a fatigue test specimen and applying high-pressure hydrogen gas therein. Similar to the tensile test described above, this hollow test specimen high-pressure hollow test is a simpler method of high-pressure hydrogen environment testing, making it possible to evaluate fatigue properties. When fatigue testing is performed, in the present invention, a fatigue strength value is preferably at least 450 MPa, and even better, 500 MPa, when a tension-compression fatigue test (number of cycles: 1 × 10 ⁷ ) is performed in high-pressure hydrogen of 10 MPaG or more (preferably 20 MPaG) with a load control at a stress ratio of −1. 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.

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 using a nitrogen atmosphere during quenching when quenching and tempering martensitic steel having the alloy composition of the present invention. A quenching temperature of 1000 to 1100°C is preferred, as this allows sufficient nitrogen to be dissolved in solid solution. After the quenching process, sub-zero treatment at -50°C or below can also be carried out 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.

10 kg of molten metal was melted in a high-frequency induction melting furnace and cast to produce martensitic steel ingots No. 1 to No. 13 with the multiple component compositions shown in Table 1. These ingots were then 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. After cooling, the ingots were annealed by holding them at a holding temperature of 780 to 860°C for at least 4 hours, yielding 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 this invention.

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 average carbide equivalent circle diameter were also measured. The carbide area ratio and the average carbide equivalent circle diameter were calculated by observing carbides in a field of view with an area of 500 μm2 or ^more in a cross-sectional structure perpendicular to the processing direction (stretching direction of rolling) of the martensitic steel after quenching, deep-freezing, and tempering treatments, photographed with a scanning electron microscope (magnification 5000x), and analyzing the image. Note that the carbides targeted in the image analysis were limited to those with an equivalent circle diameter of 0.1 μm or more; those with a diameter less than this were not considered. The resulting carbide area ratios and average carbide equivalent circle diameters are shown in Table 2. Note that the No. 2 specimens in Table 2 were analyzed. The martensitic steels Nos. 1 to 7 satisfied the carbide morphology specified in the present invention.

Hydrogen was then introduced into the obtained hollow tensile test specimens at room temperature or 200°C to a pressure of 10 or 20 MPaG in the internal through holes, and tensile tests were carried out at a strain rate of 5.0 x 10 ^-5 /s. The test conditions and test results are shown in Table 3.

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 examples of the present invention, achieved tensile strengths of 2000 MPa or more when the hydrogen atmosphere pressure was 10 MPaG, and even showed 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 the low amount of C. Furthermore, Comparative Examples No. 9 to No. 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 range and, as a result, low tensile strength.

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 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. These hollow fatigue test specimens were subjected to a tension-compression fatigue test (number of cycles: 1 × 10 ⁷ ) at room temperature, with hydrogen introduced into the internal through-hole at a pressure of 10 MPa, and with a load controlled at a stress ratio of -1. The test temperature was room temperature, the waveform was a sine wave, and the frequency was 20 Hz. The test was conducted with the load controlled to 500 MPa. As a result, both No. 1 and No. 6 exhibited good fatigue properties, with fatigue strengths of at least 500 MPa at 1 × 10 ⁷ 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 (3)

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