Method for producing a pressure vessel for hydrogen, pressure vessel produced by the method and use thereof

Abstract

The invention relates to a method for producing a pressure vessel for hydrogen, comprising the following steps: (a) Providing a steel with the chemical composition, in wt.%: C: 0.15 to 0.30%, Si: 0.10 to 1.00%, Mn: 3.75 to 7.00%, Ni: 0 to 2.00%, Cr: 0 to 4.00%, Mo: 0 to 1.0%, W: 0 to 2.0%, V: 0 to 1.0%, the remainder being iron and unavoidable impurities, the sum of Ni% + Mn% being 4.00 to 8.00%, (b) Manufacturing the geometry of the pressure vessel with an inside and an outside, (c) Thermochemical treatment of the inside of the pressure vessel by carburizing or carbonitriding to a C content between 0.85 and 2.00 wt.%, (d) Quenching the pressure vessel at a cooling rate > 30°C / s, and (e) tempering the pressure vessel at a temperature between 200 °C and 500 °C for a duration tAnl > 15 min.The invention also relates to the use of the steel provided in step (a) of the process according to the invention for producing pressure vessels for hydrogen, as well as to the pressure vessel obtainable by the process according to the invention and to the use of the pressure vessel for motor vehicles and railways.

Description

[0001] The invention relates to a process for producing a pressure vessel for hydrogen, the use of the steel provided in the process according to the invention for the production of pressure vessels, the pressure vessel for hydrogen obtainable by the process according to the invention and its use in motor vehicles, trucks and railways.

[0002] Where "%" information is given below regarding alloys or steel compositions, these always refer to the weight, unless expressly stated otherwise.

[0003] Hydrogen pressure vessels currently used in automotive engineering are typically made of fiber-reinforced plastic (Type 4). The disadvantages of fiber-reinforced plastic pressure vessels are, on the one hand, they are difficult to recycle and, on the other hand, they are expensive to manufacture. Conventional pressure vessels, such as gas cylinders, for hydrogen are usually made of heat-treated steel 1.7220 / 34CrMo4 and are significantly heavier than fiber-reinforced plastic pressure vessels.

[0004] The particular challenge for using steel hydrogen pressure vessels is that hydrogen penetrates the steel and accumulates at the grain boundaries. This leads to a reduction in the steel's ductility, so that even a small crack can lead to catastrophic material failure.

[0005] This hydrogen-induced corrosion, or hydrogen-induced stress corrosion cracking, is often referred to as "hydrogen embrittlement."

[0006] Some austenitic steels are resistant to cracking, but are mechanically very soft, so that these steels are not suitable for the production of pressure vessels due to their low strength.

[0007] Quenched and tempered steels with inherently higher strengths are prone to hydrogen-induced stress corrosion cracking, so appropriate safety margins must be applied to their use in the manufacture of pressure vessels. This means that the steel's yield strength is not fully utilized, but rather these steels are used for safety reasons for use at loads significantly below their actual yield strength.

[0008] It is known that hydrogen embrittlement of tempered steels can be avoided by providing the pressure vessels with a plastic coating on their inside, which acts as a diffusion barrier for hydrogen.

[0009] For example, DE 10 2018 220 655 A1 discloses a method for manufacturing hydrogen pressure vessels. In this method, an oxide layer is first formed on the inside of the pressure vessel, and a diffusion barrier layer made of hexamethyldisiloxane and molecular oxygen is applied to it. The diffusion barrier layer is formed alternatingly from an expandable intermediate layer and a high-density barrier layer or from a gradient layer that exhibits a continuous gradient from an expandable material to a high-density material.

[0010] However, the production of such pressure vessels with a plastic coating on the inside and their recycling is complex.

[0011] Against the background of the prior art explained above, the object of the invention was to provide a simple method for producing a pressure vessel for hydrogen which, despite its low weight, has the required strength and can be easily recycled.

[0012] For the production of pressure vessels for hydrogen which have optimum strengths and can be easily recycled by melting, the invention proposes the method according to claim 1.

[0013] A steel suitable for use in the manufacture of pressure vessels for hydrogen is proposed in claim 13. A pressure vessel for hydrogen produced by the method according to the invention has the features specified in claim 14. Finally, the use of the pressure vessel produced by the method according to the invention for motor vehicles and railways is proposed in claim 15.

[0014] Advantageous embodiments of the invention are specified in the dependent claims and are explained in detail below, as is the general inventive concept.

[0015] The process according to the invention opens up a robust and cost-effective production route for the manufacture of steel pressure vessels for hydrogen. The process according to the invention involves significantly fewer manufacturing steps than is the case with processes commonly used for this purpose today.

[0016] The invention is based on the finding that hydrogen diffusion in austenite is slower than in ferrite or martensite. Thus, at approximately 50 °C, the permeability of hydrogen in ferrite is a factor of 1000 lower than in austenite.

[0017] By means of the method according to the invention, a diffusion barrier between the hydrogen and the high-strength hydrogen-sensitive steel is obtained on the inside of the pressure vessel by producing an austenitic structure from the base steel material by means of a thermochemical treatment (carburizing or carbonitriding).

[0018] It has been surprisingly shown that by creating an austenitic structure through a thermo-chemical treatment on the inside of the pressure vessel, the hydrogen concentration in the steel can be reduced to such an extent that it does not exceed the critical value of approximately 1 ppm in the outer shell of the pressure vessel, i.e. the remaining steel structure of the pressure vessel apart from the austenitic interior.

[0019] In this respect, it proves to be particularly advantageous that a pressure vessel produced in this way has a comparable strength and hydrogen resistance to a pressure vessel for hydrogen made of fiber-reinforced plastic, but at the same time can be manufactured and recycled easily and cost-effectively.

[0020] According to the method according to the invention for producing a pressure vessel for hydrogen, at least the following steps are carried out: (a) Providing a steel with the chemical composition, in wt.%: C: 0.15 to 0.30%, Si: 0.10 to 1.00%, Mn: 3.75 to 7.00%, Ni: 0 to 2.00%, Cr: 0 to 4.00%, Mo: 0 to 1.0%, W: 0 to 2.00%, V: 0 to 1.0%, the remainder being iron and unavoidable impurities, the sum of Ni% + Mn% being 4.00 to 8.00%, (b) Manufacturing the geometry of the pressure vessel with an inside and an outside, (c) Thermochemical treatment of the inside of the pressure vessel by carburizing or carbonitriding to a C content between 0.85 wt.% and 2.00 wt.% and (d) Quenching the pressure vessel at a cooling rate > 30 °C / s, (e) Tempering the pressure vessel at a temperature between 200 °C and 500 °C for a duration t Anl > 15 min.

[0021] In step (a) of the inventive method for producing a pressure vessel for hydrogen, a steel with the chemical composition, in wt.%: C: 0.15 to 0.30%, Si: 0.10 to 1.00%, Mn: 3.75 to 7.00%, Ni: 0 to 2.00%, Cr: 0 to 4.00%, Mo: 0 to 1.0%, W: 0 to 2.0%, V: 0 to 1.0%, and the remainder iron and unavoidable impurities is used, with the proviso that the nickel content in wt.% Ni% and the manganese content in wt.% Mn% meet the following condition: Ni % + Mn % = 4,00 bis 8,00 % .

[0022] The steel provided in step (a) of the process according to the invention is alloyed and can be processed in such a way that the microstructure of the outer shell of the pressure vessel, i.e. the remaining steel structure of the pressure vessel apart from the austenitic interior, has a residual austenite content of less than 10 vol.% based on the entire microstructure of the outer shell of the pressure vessel. The unavoidable impurities in the steel provided in step (a) of the process according to the invention due to production reasons include all elements that are present in amounts that are ineffective from an alloying perspective with regard to the properties of interest here. In particular, the unavoidable impurities also include contents of Cu up to 0.3 wt.%, Al up to 0.05 wt.%, Sn up to 0.3 wt.%, Zn up to 0.3 wt.%, Ti up to 0.1 wt.%, V up to 0.1 wt.%, Nb up to 0.1 wt.% and rare earths up to 0.1 wt.%.According to a preferred embodiment, the total impurity content in the steel produced in step (a) of the process according to the invention is less than 2 wt.%. The total impurity content is understood to be the sum of the individual alloying elements considered unavoidable impurities.

[0023] A pressure vessel produced from the steel provided in step (a) of the process according to the invention is thus characterized by the fact that the outer shell of the pressure vessel, i.e., the remaining steel structure of the pressure vessel apart from the austenitic interior, has a structure consisting of no more than 10 vol.% residual austenite. The remaining structure is occupied by martensite.

[0024] With the alloying concept underlying the steel provided in step (a) of the process according to the invention, it is possible to adjust the martensite start temperature M s such that the residual austenite content of at most 10 vol.% in the structure of the outer shell of the pressure vessel is reliably achieved, while the thermochemical treatment according to step (c) of the process according to the invention can achieve a sufficiently low martensite start temperature M s * after the thermochemical treatment of below -20°C. This is achieved by selecting the alloying elements and their contents in the steel provided in the process according to the invention as follows: Carbon ("C") is present in the steel to be used according to the invention in amounts of 0.15 - 0.30 wt.% in order to contribute to increasing the strength and wear resistance of the material through carbide formation and to lower the martensite start temperature.This effect is particularly evident at a C content of at least 0.16 wt.%, especially at least 0.17 wt.%. By limiting the C content to a maximum of 0.30 wt.%, especially a maximum of 0.28 wt.%, the steel possesses good elongation and toughness properties despite its maximized strength. An optimized effect of the presence of C in the steel produced by the process according to the invention can be achieved by adjusting the C content to 0.17–0.27 wt.%.

[0025] Silicon ("Si") increases the through-hardenability of the steel, but simultaneously reduces its toughness. The Si content of a steel produced by the process according to the invention is therefore limited to 1.00 wt.%, in particular 0.50 wt.%. At the same time, Si contents up to this upper limit contribute to increasing strength through solid solution strengthening. In order to particularly reliably utilize the advantageous effects of Si in the steel to be used according to the invention, the Si content is therefore preferably set to at least 0.10 wt.%, in particular at least 0.12 wt.%.

[0026] Manganese ("Mn") is present in the steel produced by the process according to the invention in amounts of 3.75–7.00 wt.% to increase through-hardenability and lower the martensite initiation temperature. A minimum Mn content of 3.75 wt.% is required to achieve these effects. If these effects are to be achieved with particular reliability, a Mn content of at least 3.80 wt.% can be provided. Excessively high Mn contents would lead to an excessive reduction in the martensite initiation temperature and thus to an undesirably high residual austenite content at room temperature, which would negatively affect the strength of the steel. Therefore, the Mn content is limited to a maximum of 7.00 wt.%, in particular a maximum of 6.70 wt.%.

[0027] Optional Ni contents of up to 2.00 wt.%, in particular up to 1.80 wt.%, improve the toughness of the steel to be used according to the invention by stabilizing the austenite. If this effect is to be utilized, it occurs from a Ni content of at least 0.05 wt.%, in particular at least 0.10 wt.%.

[0028] Likewise, optional chromium ("Cr") contents of up to 4.00 wt.% increase the through-hardenability of the steel and, through the formation of additional carbide precipitates in the austenitic barrier layer created by the thermochemical treatment, contribute to a further improved hydrogen barrier effect of this layer. For this purpose, at least 0.05 wt.% or at least 0.08 wt.% Cr can be provided. The optimal effect of the presence of Cr is achieved with a Cr content of at least 0.09 wt.%. Cr contents above 4.00 wt.% would increase alloying costs, although through-hardenability is ensured for a given tank wall thickness. To reliably avoid this, the Cr content can be limited to up to 3.00 wt.%.

[0029] Molybdenum ("Mo") is optionally present in the steel produced by the process according to the invention in amounts of up to 1.0 wt.% to increase the through-hardenability of the steel. This effect is particularly evident when at least 0.01 wt.% is present in the steel. At contents of more than 1.0 wt.%, no economically viable further increase in the positive effect of Mo occurs, based on the use of the steel to be used according to the invention, which is the focus here. By limiting the Mo content to 1.0 wt.%, the formation of a molybdenum-rich carbide phase, which would negatively affect the toughness properties, is limited. Optimal effects of Mo in the steel to be used according to the invention can be expected when the Mo content is at least 0.02 wt.%. Mo contents of at most 0.80 wt.% or at most 0.60 wt.% have proven particularly effective.

[0030] Tungsten ("W") is optionally present in the steel produced by the process according to the invention in amounts of up to 2.0 wt.%, in particular up to 1.5 wt.%, preferably up to 1.0 wt.%, to ensure through-hardenability. If these effects are to be achieved particularly reliably, a W content of at least 0.10 wt.%, preferably at least 0.50 wt.%, can be set in the steel.

[0031] Likewise, optional vanadium ("V") contents of up to 1.0 wt.% contribute to the strength of the outer shell through the formation of additional carbide precipitates. For this purpose, at least 0.08 wt.% V can be provided. The optimal effect of the presence of V is achieved with a V content of at least 0.10 wt.%. V contents above 1.0 wt.% would lead to coarse carbides, reduced toughness, and high alloying costs. To reliably avoid this, the V content can be limited to up to 0.50 wt.%.

[0032] A steel alloy particularly suitable for the purposes of the invention therefore consists, according to the above explanations, of (in wt. %) 0.16 - 0.28% C, 0.12 - 0.50% Si, 3.80 - 6.70% Mn, 0.05 - 3.00% Cr, 0.01 - 0.60% Mo, 0.10 - 1.0% W, 0.05 - 1.80% Ni, 0.08-0.50% V, the remainder being iron and unavoidable impurities, for which the explanations already given above also apply.

[0033] The method according to the invention provides, in step (b), the production of the geometry of the pressure vessel with an inside and an outside. Conventional manufacturing processes known to those skilled in the art, such as hot extrusion of slugs, can be used to produce the geometry of the pressure vessel. For example, the geometry of the pressure vessel can be obtained by sheet metal rolling and longitudinal seam welding; however, this is not the preferred method due to the welded joint. The inside of the pressure vessel represents the side that, after production of the geometry of the pressure vessel, is on the inside and is in contact with the hydrogen in the finished application. The outside of the pressure vessel is the side of the pressure vessel that, after production of the geometry of the pressure vessel, is on the outside and is in contact with the atmosphere (air) in the finished application.

[0034] According to step (c) of the process according to the invention, the inside of the pressure vessel is subjected to a thermochemical treatment by carburizing or carbonitriding to a C content of between 0.85 and 2.00 wt.%, preferably between 0.90 and 2.00 wt.%. For the thermochemical treatment, the pressure vessel is filled with a medium containing carbon and, optionally, nitrogen. The duration of the thermochemical treatment, during which the inside of the pressure vessel is kept submerged in the medium, is selected in a manner known per se, depending on the size of the surface to be treated, as well as taking into account the medium used and the temperature at which the thermochemical treatment is carried out, so that an austenitic barrier layer with a C content of between 0.85 and 2.00 wt.%, preferably 0.90 and 2.00 wt.%, within the specifications of the invention is achieved.

[0035] In practice, the temperature at which the steel component is maintained during the thermochemical treatment according to step (c) is therefore at temperatures sufficient for austenitization of the steel. These are typically temperatures T TB between 875 and 1050°C. By selecting higher temperatures, the thermochemical treatment can be accelerated and, accordingly, the time required for the required carburization to a C content between 0.85 and 2.00%, preferably 0.90 and 2.00% by weight, can be shortened.

[0036] The duration t TB for the thermochemical treatment according to step (c) of the process according to the invention is preferably less than 8 hours. Under the stated conditions (T TB ), this time not only achieves the desired C content, but also ensures the formation of an austenitic barrier layer up to 1 mm thick. It has proven particularly advantageous for the purposes of the invention if the thickness of the austenitic barrier layer corresponds to 0.05 to 0.25 times the wall thickness of the pressure vessel.

[0037] According to step (d) of the method according to the invention, after the thermochemical treatment, the pressure vessel is quenched at a cooling rate of > 30 °C / s. The cooling rate is set such that, on the one hand, hardness-increasing precipitates form in the austenitic barrier layer obtained by carburizing or carbonitriding, and on the other hand, in the structure of the outer shell of the pressure vessel, i.e. in the remaining steel structure of the pressure vessel apart from the austenitic barrier layer on the inside, a residual austenite content of less than 10 vol.%, based on the entire structure of the outer shell of the pressure vessel, is established. This is achieved particularly reliably if the cooling rate is > 30 °C / s, in particular > 90 °C / s.

[0038] In the process according to the invention, quenching is followed by tempering the pressure vessel at a temperature between 200 °C and 500 °C for a duration t Anl > 15 min. Such tempering increases the toughness of the outer shell and thus reduces the risk of crack growth.

[0039] The quenching and tempering according to steps (d) and (e) of the process according to the invention does not lead to a structural transformation of the austenitic barrier layer established in step (c) of the process according to the invention, so that it is retained even after tempering and cooling to room temperature.

[0040] It goes without saying that step (e) of the method according to the invention is optionally followed by the final production of the pressure vessel. During final production, threads for valves and the final geometry are created on the pressure vessel by forming or machining.

[0041] The process according to the invention surprisingly allows the use of the hydrogen-sensitive steel mentioned in step (a) for the production of pressure vessels for hydrogen. The preferred embodiments already described above in this context for the steel provided in step (a) of the process according to the invention apply equally to the inventive use of the steel for the production of pressure vessels for hydrogen.

[0042] By applying the above-described inventive method, a hydrogen pressure vessel according to the invention can be produced that is characterized by comparable resistance to hydrogen embrittlement and comparable strength to hydrogen pressure vessels made of fiber-reinforced plastic or plastic-coated steel. Unlike pressure vessels known from the prior art, the pressure vessels obtainable by the inventive method can be easily recycled. Due to the absence of other materials, such as plastic or fiber reinforcement, the inventive pressure vessels can be easily melted down and recycled.

[0043] The pressure vessels according to the invention are suitable for use in motor vehicles and railways due to their advantageous properties.

[0044] The invention is explained in more detail below with reference to figures and exemplary embodiments. Fig. 1 :Condition of the pressure vessel at the beginning of the carburizing process Fig. 2 :Condition of the pressure vessel after tempering at room temperature

[0045] Figure 1 This is a schematic representation of a section through a pressure vessel 1 with an inner side 2 and an outer side 3 at the beginning of the carburizing process during carburization above the austenitizing temperature of the base material. The structure of the pressure vessel is completely austenitic.

[0046] Figure 2is a schematic representation of a section through a pressure vessel 1 after undergoing the process according to the invention, i.e., after tempering, at room temperature. The structure of the pressure vessel has an austenitic barrier layer on the inner side 2, while the remaining structure of the pressure vessel is predominantly martensitic. Examples

[0047] Sixteen steel melts V1 to V16 were melted in an induction furnace, the composition of which is given in Table 1. Steel melts V13 and V14 are steel melts not according to the invention.

[0048] Sheets measuring 30 mm x 60 mm x 1 m were rolled from these steel melts. To determine the martensite initiation temperature in the original composition M s , i.e., before thermochemical treatment, samples measuring 9 mm in length and 4 mm in thickness were prepared from these sheets. The martensite initiation temperature was determined using a dilatometer by length change according to SEP 1681, 3rd edition (July 2019). The results of this determination are shown in Table 1.

[0049] The thermochemical carburization treatment was performed on 5 mm thick samples with an adjusted carbon content of the carburizing agent (carburizing granules) at 960 °C for 72 h. The samples were quenched to room temperature at a rate of 180 °C / s.

[0050] The samples were then processed into 4 mm thick, 9 mm long specimens to determine the martensite initiation temperature (M s * ) of the thermochemically treated samples using a dilatometer by length change according to SEP 1681, 3rd edition (July 2019). The carbon content of these samples after thermochemical treatment (C * ) was also determined and can be found in Table 1.

[0051] Table 1 shows that the Ms temperature of the samples from the non-inventive steel melts V13 and V14 had an excessively low temperature before the thermo-chemical treatment and would therefore lead to an excessively high residual austenite content in the finished pressure vessel, so that sufficient strength of the pressure vessel cannot be guaranteed. The sample from the non-inventive steel melt V02, on the other hand, has an excessively high Ms temperature even after the thermo-chemical treatment due to the carbon content of 0.79 wt.% after carburization, which is too low, i.e. the austenitic barrier layer formed in a pressure vessel made from it would not be sufficiently stable and would convert into martensite even at room temperature, so that an adequate diffusion barrier effect against hydrogen cannot be achieved.

[0052] In contrast, the samples from the steel melts according to the invention have the properties desired according to the invention, ie in the production of a pressure vessel, a sufficiently stable austenitic barrier layer is formed due to the low Ms temperature after the thermochemical treatment Ms*. Table 1 Leg. C [%] Si [%] Mn [%] Cr [%] Month [%] W [%] Ni [%] V [%] Ms [°C] C* [%] Ms* [°C] V01 0,21 0,16 6,55 0,10 0,02 Sp. 0,11 Sp. 243 1,00 -90 V02 0,25 0,25 3,92 0,10 0,02 Sp. 1,51 Sp. 231 0,79 35 V03 0,25 0,25 3,92 0,10 0,02 Sp. 1,52 Sp. 231 1,04 -33 V04 0,24 0,25 4,03 2,89 0,02 Sp. 1,31 Sp. 182 0,91 -48 V05 0,24 0,25 4,03 2,89 0,02 Sp. 1,31 Sp. 182 1,03 -65 V06 0,26 0,22 4,20 0,20 0,10 Sp. 1,55 Sp. 217 0.99 -42 V07 0,26 0,22 4,20 0,20 0,10 Sp. 1,55 Sp. 217 1,21 -110 V08 0,18 0,30 3,90 0,15 0,50 Sp. 1,47 Sp. 253 0,95 -19 V09 0,18 0,30 3,90 0,15 0,50 Sp. 1,47 Sp. 253 1,12 -76 V10 0,22 0,35 5,21 0,14 0,03 Sp. 1,03 Sp. 208 0,95 -51 V11 0,22 0,35 5,21 0,14 0,03 Sp. 1,03 Sp. 208 1,05 -86 V13 0,35 0,24 4,48 0,15 0,03 Sp. 1,51 Sp. 176 1,00 -51 V14 0,45 0,26 4,48 0,10 0,02 Sp. 1,62 Sp. 140 1,10 -85 V15 0,20 0,24 4,02 0,12 0,02 0,94 1,62 Sp. 240 1,07 -33 V16 0,25 0,26 4,29 0,13 0,02 Sp. 1,53 0,16 217 1,12 -83 Data in wt.%, balance Fe and unavoidable impurities; C* = C content after thermochemical treatment, Ms* = martensite start temperature after thermochemical treatment, Sp. = traces; Underlined and bold figures are not in accordance with the invention.

Claims

1. A method for producing a pressure vessel for hydrogen, comprising the following steps: (a) Providing a steel with the chemical composition, in wt.%: C: 0.15 to 0.30%, Si: 0.10 to 1.00%, Mn: 3.75 to 7.00%, Ni: 0 to 2.00%, Cr: 0 to 4.00%, Mo: 0 to 1.0%, W: 0 to 2.0%, V: 0 to 1.0%, the remainder being iron and unavoidable impurities, the sum of Ni% + Mn% being 4.00 to 8.00 wt.%, (b) Manufacturing the geometry of the pressure vessel with an inside and an outside, (c) Thermochemical treatment of the inside of the pressure vessel by carburizing or carbonitriding to a C content between 0.85 and 2.00 wt.% and (d) Quenching the pressure vessel at a cooling rate > 30 °C / s, (e) tempering the pressure vessel at a temperature between 200 °C and 500 °C for a duration t Anl > 15 min.

2. Method according to claim 1, characterized in that the steel contains 0.10 to 0.25% Si.

3. Method according to claim 1 or 2, characterized in that the pressure vessel is started up in accordance with step (e) for a period of not more than 2 hours.

4. Method according to one of the preceding claims, characterized in that the thermochemical treatment in step (c) is carried out at a temperature between 875 and 1050°C.

5. Method according to one of the preceding claims, characterized in that the thermochemical treatment for a duration t TB < 8 h.

6. Method according to one of the preceding claims, characterized in that the steel contains 0.16 to 0.30 wt% C.

7. Method according to one of the preceding claims, characterized in that the steel contains 3.80 to 6.70 wt% Mn.

8. Method according to one of the preceding claims, characterized in that the steel contains 0.05 to 1.80 wt% Ni.

9. Method according to one of the preceding claims, characterized in that the martensite start temperature M sof the steel provided in step (a) is at least 180°C.

10. Method according to one of the preceding claims, characterized in that the steel contains 0.05 to 3.00 wt% Cr.

11. Method according to one of the preceding claims, characterized in that the thermochemical treatment in step (c) is carried out in such a way that an austenitic edge zone is formed which corresponds to 0.05 to 0.25 times the nominal wall thickness of the pressure vessel.

12. Method according to one of the preceding claims, characterized in that the carburizing or carbonitriding in step c) is carried out to a C content between 0.90 and 2.00 wt.%.

13. Use of a steel with the chemical composition, in wt.%: C: 0.15 to 0.30%, Si: 0.10 to 1.00%, Mn: 3.75 to 7.00%, Ni: 0 to 2.00%, Cr: 0 to 4.00%, Mo: 0 to 1.0%, W: 0 to 2.0%, V: 0 to 1.0%, the balance being iron and unavoidable impurities, the sum of Ni% + Mn% being 4.00 to 8.00%, for the manufacture of pressure vessels for hydrogen.

14. A pressure vessel for hydrogen, manufactured by a process according to claims 1 to 12.

15. Use of a pressure vessel for hydrogen according to claim 14 for motor vehicles and railways.

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