Martensitic stainless steel for high-pressure hydrogen environments

Abstract

Provided is a martensitic stainless steel having high tensile strength in high-pressure hydrogen environments and having excellent ductility.　The martensitic stainless steel for high-pressure hydrogen environments has a composition comprising, in mass%, 0.18-0.40% C, up to 1.50% Si, 0.35-1.50% Mn, up to 0.040% P, up to 0.010% S, up to 0.50% Ni, 12.50-14.60% Cr, 1.00-3.00% Mo and / or W according to the relational formula Mo+1 / 2W, up to 3.00% Cu, up to 0.30% Nb, and 0.0005-0.100% N, with the remainder comprising Fe and unavoidable impurities. When a hollow test piece is subjected to a low-strain-rate tensile test in high-pressure hydrogen of 1-15 MPaG, then the tensile strength is a value which is 1,500 MPa or greater and is greater than the rupture stress.

Description

[Technical Field]

[0001] The present invention relates to a martensitic stainless 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 high-pressure hydrogen environments, such as those described above, are required to maintain high strength and ductility even in hydrogen environments. 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, necessitating the need for even higher strength and ductility. Patent Document 1 describes an effective method for increasing the strength of austenitic stainless steels while maintaining their excellent hydrogen embrittlement resistance. However, it does not address whether the strength required for use as sliding components is achieved, and there is a risk of shortening component life 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. However, as with Patent Document 1, they may result in insufficient strength when used as sliding components in hydrogen environments. Therefore, an object of the present invention is to provide a martensitic stainless steel for use in high-pressure hydrogen environments that has high tensile strength and excellent ductility even in high-pressure hydrogen environments by improving the hydrogen embrittlement resistance of steel having a martensitic structure. [Means for solving the problem]

[0008] The present inventors have investigated hydrogen embrittlement, a major issue with stainless steels having a martensitic structure, and discovered that optimizing the chemical composition of martensitic stainless steels can improve hydrogen embrittlement resistance and achieve high strength even in high-pressure hydrogen environments, leading to the present invention.

[0009] That is, the present invention provides a martensitic stainless steel for use in a high-pressure hydrogen environment, which has a chemical composition, in mass%, of C: 0.18 to 0.40%, Si: 1.50% or less, Mn: 0.35 to 1.50%, P: 0.040% or less, S: 0.010% or less, Ni: 0.50% or less, Cr: 12.50 to 14.60%, one or two of Mo and W according to the relationship (Mo + 1 / 2W): 1.00 to 3.00%, Cu: 3.00% or less, Nb: 0.30% or less, N: 0.0005 to 0.100%, and the balance being Fe and unavoidable impurities, and which, when subjected to a hollow specimen high-pressure hydrogen slow strain rate tensile test in high-pressure hydrogen of 1 to 15 MPaG, has a tensile strength of 1500 MPa or more and a value greater than the fracture stress. [Effects of the Invention]

[0010] According to the present invention, it is possible to obtain a martensitic stainless steel for use in a high-pressure hydrogen environment that has high tensile strength even in a high-pressure hydrogen environment and also has excellent ductility. DETAILED DESCRIPTION OF THE INVENTION

[0011] As described above, an important feature of the present invention is the discovery of a martensitic stainless steel for use in a high-pressure hydrogen environment that exhibits excellent high strength properties and good ductility even in a high-pressure hydrogen environment of 1 to 15 MPa (preferably 5 to 15 MPa). The reasons for the composition limitations of the present invention will be explained below. Note that martensitic stainless steel for use in a high-pressure hydrogen environment may also be referred to simply as "martensitic stainless steel."

[0012] C: 0.18 to 0.40 mass% (hereinafter simply referred to as "%") C is an element effective for increasing the hardness and hydrogen embrittlement resistance of the martensitic structure after quenching and tempering. However, if there is too much C, the C forms coarse carbides, which relatively reduces the amounts of Cr and Mo dissolved in the matrix, deteriorating the corrosion resistance of the product. Furthermore, the coarse carbides cause stress concentration when stress is applied, which becomes the starting point for fracture due to hydrogen embrittlement, reducing 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 reduced hardness and tensile strength. Therefore, the C content is set to 0.18 to 0.40%, preferably 0.20% or more, and more preferably 0.30% or less.

[0013] ·Si: 1.50% or less Si is used as a deoxidizer during the melting process, and is an element that can be unavoidably contained. If there is too much Si, the hardness of the annealed material (i.e., the martensitic stainless 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 1.50% or less, preferably 1.00% or less, more preferably 0.80% or less, and even more preferably 0.50% or less.

[0014] Mn: 0.35 to 1.50% Mn is an element that has the effect of promoting the solid solution of nitrogen into the structure when solid-phase nitrogen absorption treatment is performed for the purpose of improving strength. However, if there is too much Mn, the austenite structure is stabilized, making it difficult to obtain a martensite structure and high surface hardness. Therefore, the Mn content is set to 0.35 to 1.50%. In order to easily obtain the above-mentioned effects, the Mn content is preferably 0.50% or more and 1.00% or less.

[0015] ·P:0.040% or less P is an element that deteriorates the toughness of the product and therefore its content should be limited to 0.040% or less, preferably 0.030% or less.

[0016] ·S:0.010% or less Too much S deteriorates hot workability, so the S content is set to 0.010% or less, preferably 0.005% or less, and more preferably 0.003% or less.

[0017] ·Ni: 0.50% 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 Cu phase in the metal structure of martensitic stainless steel becomes fine (the area ratio of the coarse Cu phase becomes small), resulting in poor cold workability. Furthermore, in martensitic stainless steel products after quenching and tempering, the austenite structure is stabilized, making it difficult to obtain a martensite structure and high surface hardness. Therefore, the Ni content is set to 0.50% or less. Preferably, it is 0.30% or less, more preferably 0.20% or less, and even more preferably 0.15% or less. However, if the effect of improving corrosion resistance can be compensated for by other additive elements, no Ni (0%) may be added.

[0018] Cr: 12.50~14.60% Cr is an element that forms an amorphous passive film on the surface of martensitic stainless steel, imparting corrosion resistance to the product. However, if there is too much Cr, the ferrite structure is stabilized, making it difficult to obtain a martensite structure and high surface hardness. Therefore, the Cr content is set to 12.50 to 14.60%, preferably 14.00% or less, and more preferably 13.50% or less.

[0019] One or both of Mo and W according to the formula (Mo+1 / 2W): 1.00 to 3.00% Mo and W are similar elements and can be treated equally in the relationship (Mo + 1 / 2W). Mo and W can be contained alone or in combination. For example, Mo may be contained alone, which provides the effects of the present invention while being less expensive than W. Mo and W have the effect of stabilizing the passivation film of stainless steel in a solid solution state and also contribute to improving the corrosion resistance of the product surface. Furthermore, Mo and W have the function of strengthening the repair ability of the passivation film at the damaged area when the passivation film formed by Cr is scratched. However, if the Mo and W contents are too high, the ferrite structure will be stabilized, making it difficult to obtain a martensite structure, as with the above-mentioned Cr. Therefore, the Mo and W contents are set to 1.00 to 3.00% in accordance with the relationship (Mo + 1 / 2W). Preferably, they are 1.20% or more. Furthermore, they are preferably 2.50% or less.

[0020] ·Cu:3.00% or less Cu is an essential element for simultaneously achieving high hardness and excellent corrosion resistance according to the present invention. In particular, Cu is an effective element for improving corrosion resistance against non-oxidizing acids such as formic acid, sulfuric acid, and hydrochloric acid. However, too much Cu significantly deteriorates hot workability. Therefore, the Cu content is set to 3.00% or less, preferably 2.50% or less. If the effects of improving corrosion resistance and hardness can be compensated for by other additive elements, Cu may be omitted (0%). When the effects of Cu addition are to be obtained, the lower limit is preferably set to 1.80%, and more preferably 1.90%.

[0021] ·Nb: 0.30% 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. Therefore, Nb can be contained in an amount of 0.30% or less as needed, and preferably 0.20% or less. Note that if the above-mentioned effect of Nb can be compensated for by other added elements, Nb can be omitted (0%). When Nb is added, the lower limit is preferably 0.05%, and more preferably 0.08%.

[0022] ·N:0.0005~0.100% or less Nitrogen (N) is an element that dissolves in the martensite structure or precipitates as a compound in quenched and tempered martensitic stainless steel products, improving strength, ductility, and hydrogen embrittlement resistance. While the reason for the above-mentioned effects of N is unclear, the addition of N converts carbides formed 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. Nitrogen also inhibits the precipitation of delta ferrite, which is harmful to the microstructure, in martensitic stainless steel before quenching and tempering. However, excessive N not only generates bubbles during casting, significantly impairing manufacturability, but also may cause the crystallization of coarse nitrides after solidification. Therefore, the N content is limited to 0.0005 to 0.100%. The preferred upper limit is 0.0800% or less, and the more preferred upper limit is 0.0600% or less. In addition to the elements described above, the elements contain Fe and unavoidable impurities.

[0023] Next, the test method specified in the present invention and its characteristics will be explained. In the present 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 to simulate use in a high-pressure hydrogen environment. The main high-pressure hydrogen environment test methods include a method in which a tensile test is performed using a solid tensile test specimen in a chamber capable of applying high pressure and a predetermined hydrogen pressure is applied, and a method in which a tensile test is performed by hollowing out a tensile test specimen and applying high-pressure hydrogen gas inside it. Both methods are capable of evaluating hydrogen embrittlement behavior in a high-pressure hydrogen environment, so in the present invention, the characteristics are evaluated using a hollow test specimen high-pressure hollow test, which is the simpler method among high-pressure hydrogen environment tests. 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, a low strain rate (e.g., target strain rate: 5.0 × 10 ) is used to clarify the presence or absence of hydrogen resistance. -5 / s) and perform a tensile test.

[0024] The martensitic stainless steel of the present invention is specified to have a tensile strength of 1500 MPa or more obtained in the above-mentioned hollow specimen low strain rate tensile test in high pressure hydrogen. Generally, the stronger a metal material is, the more susceptible it is to hydrogen. Therefore, materials with a tensile strength exceeding 1000 MPa are prone to hydrogen embrittlement and experience a significant decrease in strength in a hydrogen environment. High wear resistance and fatigue strength are required for sliding components used in a hydrogen environment, and high strength, which correlates with these properties, is used as an evaluation index. Therefore, in this invention, a tensile strength of 1500 MPa or more is specified to achieve excellent sliding properties. The martensitic stainless steel of the present invention exhibits a tensile strength greater than the fracture stress. In other words, fracture does not occur in the plastic region, which is the region from the elastic region until the tensile strength is reached. Generally, martensitic stainless steels with a tensile strength exceeding 1000 MPa fracture in the elastic region or immediately after reaching the plastic region from the elastic region, resulting in small elongation and reduction of area, resulting in an unstable fracture mode. The martensitic stainless steel of the present invention exhibits fracture behavior due to localized contraction after reaching the tensile strength indicating the maximum stress in a tensile test (in other words, the tensile strength is greater than the fracture stress). This suggests that the martensitic stainless steel of the present invention has high strength and excellent ductility in a hydrogen environment. For example, the tensile strength of the martensitic stainless steel of the present invention can be greater than the fracture stress by 50 MPa or more. A tensile strength greater than the fracture stress refers to a tensile strength greater than the fracture stress. Furthermore, in the present invention, the elongation obtained by a hollow test specimen in a low strain rate tensile test in high pressure hydrogen can be 7% or more, and the reduction in area obtained by a hollow test specimen in the present invention in a low strain rate tensile test in high pressure hydrogen can be 20% or more. The slow strain rate tensile test specified in the present invention is carried out by simulating a high-pressure hydrogen environment so that the pressure applied to the hollow test specimen is 1 to 15 MPa (preferably 5 to 15 MPa).

[0025] The martensitic stainless steel of the present invention can be obtained by quenching and tempering a steel material (steel mainly composed of ferrite phase) having the above-mentioned composition to form a metal structure mainly composed of martensite phase. In this case, the quenching temperature can be set to 1000 to 1090°C, and the tempering temperature can be set to 150 to 500°C. Furthermore, subzero treatment can be performed after the quenching step, and the temperature during the subzero treatment can be set to -50°C or lower. Here, when even higher strength properties are desired, the martensitic stainless 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 a steel material having the composition of the present invention in a nitrogen atmosphere during quenching. A quenching temperature of 1000 to 1100°C is preferred because it allows nitrogen to be solid-dissolved near the surface. Subzero treatment at -50°C or below 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.

[0026] The martensitic stainless steel of the present invention has a high tensile strength of 1500 MPa or more even in a high-pressure hydrogen environment and excellent ductility, making it suitable for use in hydrogen environment components. In particular, it is preferable to use it in applications requiring high strength and corrosion resistance, such as various sliding members and high-strength products in hydrogen production equipment, hydrogen transportation and storage equipment, hydrogen utilization equipment or infrastructure facilities, and hydrogen-fueled internal combustion engines. [Example]

[0027] 10 kg of molten metal melted in a high-frequency induction melting furnace was cast to produce martensitic stainless steel ingots No. A to No. G having 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, and subsequent hot forging with a forging ratio (cross-sectional area before forging / cross-sectional area after forging) of approximately 10 was performed. After cooling, the ingots were annealed by holding them at a holding temperature of 780 to 860°C for four hours or more, yielding martensitic stainless steel materials with thicknesses of approximately 15 to 20 mm.

[0028] [Table 1]

[0029] Subsequently, materials No. A to No. G were quenched and tempered to obtain inventive examples and comparative examples, samples No. 1 to 9. The relationship between sample No. and material No. is shown in Table 2. Inventive samples No. 1 and No. 2 were prepared by subjecting martensitic stainless steel materials of the material numbers shown in Table 2 to quenching treatment, in which the materials were held at 1090°C for 30 minutes and then cooled, followed by deep-freezing treatment at -77°C, tempering treatment at 365°C for 2 hours, and air-cooling. Comparative samples Nos. 3 to 7 were prepared by subjecting martensitic stainless steel materials of the material numbers shown in Table 2 to quenching treatment, in which the materials were held at 1050°C for 30 minutes and then cooled, followed by deep-freezing treatment at -77°C, tempering treatment at 150°C for 3 hours, and air-cooling. Inventive sample No. 8 was prepared by subjecting material No. B to quenching treatment, in which the materials were held at 1090°C for 30 minutes and then cooled, followed by deep-freezing treatment at -77°C, tempering treatment at 500°C for 1 hour, and air-cooling. Sample No. 9, an example of the present invention, was prepared by subjecting Material No. B to quenching treatment, which involved holding the material at 1000°C for 30 minutes and then cooling, followed by deep cooling at -77°C, and then tempering treatment, which involved holding the material at 365°C for 2 hours, followed by air cooling. The metallographic structure of all of the samples after quenching and tempering was primarily martensite. These samples were used to fabricate hollow tensile test specimens with a parallel section diameter of 4 mm, a parallel section length of 20 mm, a total length of 70-90 mm, and an internal through-hole diameter of 1 mm. Hydrogen was then introduced into these test specimens at room temperature to a pressure of 10 MPa in the internal through-hole, and the strain rate was 5.0 × 10 -5 A low strain rate tensile test was carried out on the hollow specimen in high-pressure hydrogen to obtain a strain rate of 1 / s. The test results are shown in Table 2. As shown in Table 2, Samples No. 1, No. 2, No. 8, and No. 9 of the present invention did not fracture in the plastic region from the elastic region until the tensile strength was reached, and exhibited high tensile strengths of 1500 MPa or more. The tensile strengths were more than 100 MPa greater than the fracture stress. Samples No. 2 and No. 9 exhibited values ​​more than 150 MPa greater, and Sample No. 8 exhibited a value more than 200 MPa greater. Additionally, elongation was more than 8% and reduction of area was more than 23%, indicating ductile fracture behavior. On the other hand, the tensile strengths of Comparative Samples Nos. 3 to 7 ranged from 1069 to 1742 MPa, and although some exhibited high values, the fracture stress was the same as the tensile strength, and they exhibited almost no elongation or reduction of area, resulting in brittle fracture. This indicates that the martensitic stainless steel of the present invention exhibits high strength and ductility, as well as hydrogen embrittlement resistance, even under high-pressure environments.

[0030] [Table 2]

Claims

[Claim 1] The composition is, in mass%, C: 0.18 to 0.40%, Si: 1.50% or less, Mn: 0.35 to 1.50%, P: 0.040% or less, S: 0.010% or less, Ni: 0.50% or less, Cr: 12.50 to 14.60%, one or two of Mo and W according to the relationship formula (Mo + 1 / 2W): 1.00 to 3.00%, Cu: 3.00% or less, Nb: 0.30% or less, N: 0.0005 to 0.100%, with the balance being Fe and unavoidable impurities; A martensitic stainless steel for use in high-pressure hydrogen environments, in which hydrogen is introduced into an internal through-hole at room temperature to a pressure of 10 MPa, and a hollow test piece is subjected to a low strain rate tensile test in high-pressure hydrogen at a strain rate of 5.0 x 10-5 / s, and the tensile strength is 1500 MPa or more, a value greater than the fracture stress.

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