Austenitic stainless steel for cryogenic tank

Abstract

This steel has a composition comprising, in weight percentage: C ≤ 0.030%, Si ≤ 1.0%, Mn ≤ 1.50%, P ≤ 0.045%, S ≤ 0.03%, 16.0% ≤ Cr ≤ 19.0%, 6.0% ≤ Ni ≤ 8.0%, 0.115% ≤ N ≤ 0.20%, B < 0.0020%, Al ≤ 0.02%, Ti ≤ 0.040%, Nb ≤ 0.05%, Cu ≤ 1.0%, Mo ≤ 1.0%, Pb ≤ 0.03%, Co ≤ 0.5%, Sn ≤ 0.05%, W ≤ 0.10%, the remainder being iron and impurities resulting from the production process. The composition further satisfies the condition PREN ≥ 18; preferably PREN ≥ 20, where PREN = %Cr + 3.3 × %Mo + 16 × %N.

Description

[0001] Austenitic stainless steel for cryogenic tank

[0002] The present invention relates to the field of austenitic stainless steels. More particularly, it relates to austenitic stainless steels for the manufacture of cryogenic tanks.

[0003] Liquefied hydrogen, liquefied natural gas, liquefied oxygen, liquefied nitrogen and other cryogenic fluids require storage at very low temperatures.

[0004] Therefore, to store these fluids, structures such as pressure vessels made from materials with sufficient mechanical resistance at extremely low temperatures are required.

[0005] To address these issues, it has been proposed to manufacture such tanks from 304L and 316L steel. However, these steels do not exhibit very high mechanical strength, particularly a low yield strength at room temperature. Therefore, it is necessary to construct the tanks with thick walls using these grades, which increases both the weight and the cost of the tank. This weight is an even greater drawback if the tanks are intended to be transported (by ship or truck, for example).

[0006] We are also familiar with 301 stainless steel.

[0007] Although the use of such steel guarantees high mechanical strength, particularly in the work-hardened state, it has major drawbacks.

[0008] Indeed, cryogenic tanks are made from steel plates welded together. However, it is well known that welding 301 steel, due to its high carbon content, leads to the precipitation of chromium carbides in the heat-affected zone (HAZ) of the welds, and therefore to intergranular corrosion. Furthermore, embrittlement of the HAZ can also be observed.

[0009] The heat-affected zone (HAZ) is the area of ​​steel that, during welding, did not melt but experienced a temperature spike sufficient to induce significant changes in its microstructure and potentially its properties. Unlike the weld zone, whose microstructure is highly dependent on the chemical composition of the filler metal and shielding gas used, the HAZ microstructure depends solely on the composition of the base metal.

[0010] We are also familiar with 301 LN steel. This steel significantly reduces the risk of intergranular corrosion of welds compared to 301 steel thanks to its low carbon content. It also offers high mechanical strength, particularly in the work-hardened condition, close to that of 301. However, it still does not provide complete satisfaction, especially regarding low-temperature toughness. This property is indeed crucial in the event of impacts, particularly in the intended transportation applications.

[0011] One aim of the invention is therefore to provide an austenitic stainless steel for a cryogenic tank, such that the tank's dimensions can be optimized without compromising its impact resistance. The goal is to achieve both high mechanical properties and high toughness, including in the heat-affected zone of the welds, under normal operating conditions, i.e., at cryogenic temperatures.

[0012] Cryogenic temperature is defined as a temperature less than or equal to -100°C, including the temperatures of liquefied natural gas (-162°C), liquid nitrogen (-196°C) and liquid hydrogen (-252°C).

[0013] In what follows, in addition to ambient temperature, the temperature at which the structure and properties of steel are measured is set at -196°C. This temperature falls between that of liquefied natural gas (-162°C) and liquid hydrogen (-252°C), thus in the middle of the cryogenic temperature range mentioned above. Furthermore, this temperature is that of liquid nitrogen, which is easier to handle than liquefied natural gas and liquid hydrogen.

[0014] The target uniaxial tensile yield strength is typically greater than or equal to 700 MPa at room temperature and 1100 MPa at -196°C, these values ​​being achieved in a cold-worked condition by cold rolling of the steel.

[0015] We also aim for good intrinsic toughness, measured by Charpy tests at room temperature and that of liquid nitrogen (-196°C).

[0016] By ambient temperature, we mean a temperature of 20°C, + / -5°C.

[0017] In particular, for work-hardened steel, a tensile strength measured according to ASTM A0370-21 and ISO 148-1 standards is required, on a sub-size specimen (mini-specimen) 2.5 mm thick, with the length of the specimen parallel to the width of the sheet and the crack propagating along the rolling direction (length of the sheet) at a minimum of 100 J / cm². 2 at room temperature and at least 50 J / cm² 2 at the temperature of liquid nitrogen (-196°C).

[0018] Furthermore, in the work-hardened and welded state of the steel, in the heat-affected zone (HAZ), which is the softened zone where geometric defects could form during welding, a resilience, measured as described above, of at least 180 J / cm² is required. 2 at room temperature and at the temperature of liquid nitrogen (-196°C).

[0019] Preferably, in the annealed state of the steel, a resilience of at least 180 J / cm²2 , measured as described above, on a sub-size test specimen (mini-test specimen) of 2.5mm is obtained.

[0020] As specified below, the "annealed" state refers to the state of the steel after recrystallization and solution annealing, and the "work-hardened" state refers to the state of the steel after cold rolling.

[0021] An additional objective of the invention is to produce such steel with controlled work hardening capacity, such that the structure after work hardening comprises in use at least 40% austenite and at least 20% martensite, preferably between 25% and 60%, and at most 2%, preferably at most 1% residual ferrite, at room temperature and at cryogenic temperature.

[0022] Furthermore, by "austenitic stainless steel", we mean, in accordance with the definition of a person skilled in the art, a completely austenitic or almost entirely austenitic steel (comprising, for example, between 90 and 100% austenite), as well as steels derived from such steels which, after forming treatments leading to work hardening, include, in addition to austenite, martensite induced by plastic deformation.

[0023] To this end, the invention relates to an austenitic stainless steel for a cryogenic tank, the steel having a composition comprising, by mass percentage:

[0024] C < 0.030%

[0025] If < 1.0%

[0026] Mn < 1.5%

[0027] P < 0.045%

[0028] S < 0.03%

[0029] 16.0% < Cr < 19.0%

[0030] 6.0% < Ni < 8.0%

[0031] 0.115% < N < 0.20%

[0032] B < 0.0020%

[0033] Al < 0.02%

[0034] Ti < 0.040%

[0035] Nb < 0.05% Cu < 1.0%

[0036] Mo < 1.0%

[0037] Pb < 0.03%

[0038] Co < 0.5%

[0039] Sn < 0.05%

[0040] W < 0.10%

[0041] the remainder being iron and impurities resulting from the manufacturing process, the composition further satisfying the condition:

[0042] PREN > 18; preferably PREN > 20;

[0043] with PREN = Cr + 3.3*Mo + 16*N.

[0044] Cr, Mo and N denote the mass percentages of these elements in the composition of steel.

[0045] This steel comes in the form of a metallurgical product such as a flat product, for example a sheet, a plate...

[0046] Preferably, the steel composition is such that the parameter Md30, defined according to the formula below, where C, N, Si, Mn, Cr, Ni, Cu, Mo and Nb denote the mass percentages of these elements in the steel composition:

[0047] Md30=551 - 462*(C + N) - 9.2*Si - 8.1*Mn - 13.7*Cr - 29*(Ni + Cu) - 18.5*Mo -68*Nb

[0048] is between -10°C and 30°C, preferably between 0°C and 20°C.

[0049] In one embodiment, the steel exhibits one or more of the following characteristics, expressed as mass percentages:

[0050] 1.0% < Mn < 1.50%

[0051] 0.30% < If < 0.80%

[0052] 6.5% < Ni < 7.5%

[0053] 17.0% < Cr < 18.5%,

[0054] Ti < 0.020%.

[0055] Preferably, the steel structure, at room temperature, and at a temperature less than or equal to -196°C, consists of at least 20% martensite, preferably between 25% and 60%, at least 40% austenite and at most 2%, preferably at most 1% residual ferrite.

[0056] Austenite is deformed at room temperature, i.e., elongated in shape as shown below.

[0057] The resilience of the steel, measured according to ASTM A0370-21 and ISO 148-1 standards on a 2.5 mm thick mini-specimen, with the length of the specimen parallel to the width of the sheet and the crack propagating in the rolling direction, is preferably at least 100 J / cm² 2 , at room temperature and at least 50 J / cm 2 at -196°C.

[0058] In addition, the uniaxial tensile yield strength at room temperature is preferably at least 700 MPa and at least 1100 MPa at -196°C.

[0059] The invention also relates to a welded structure comprising at least two stainless steel products according to the invention, the steel products being joined to each other by at least one weld (therefore welded to each other), in which, in the area of ​​the products affected by heat, the structure is made up of at least 90% austenite and at most 10% ferrite.

[0060] Preferably, in the heat-affected zone, the impact resistance, measured according to ASTM A0370-21 and ISO 148-1 standards on a 2.5 mm thick mini-test specimen, is at least 180 J / cm² 2 , at room temperature and at -196°C.

[0061] The invention also relates to a cryogenic tank, comprising at least one welded structure according to the invention.

[0062] The invention also relates to a method for manufacturing a stainless steel product, characterized in that:

[0063] - a steel is produced having a composition according to the invention;

[0064] - a semi-finished product is cast from this steel;

[0065] - hot forming and, possibly, cold forming are carried out, and at least one heat treatment of said steel is performed between 900°C and 1200°C, the heat treatment ending with over-quenching at a cooling rate of at least 2°C / s between 900°C and 500°C,

[0066] - The steel is cold rolled with a reduction ratio of between 5% and 60%, preferably between 15% and 45%.

[0067] Preferably, prior to cold rolling, the steel has a structure comprising at least 98% austenite at room temperature, the remainder being at most 2% residual ferrite and inclusions inherent to the manufacturing process.

[0068] Preferably, after cold rolling, the steel has a structure consisting of at least 20% martensite, preferably between 25% and 60%, and at least 40% austenite and at most 2%, preferably at most 1% residual ferrite.

[0069] This austenite is a cold-rolled deformed austenite. By deformed, we mean that the austenite grains are elongated in the rolling direction. These grains thus have a greater length in the rolling direction than their width in the direction transverse to the rolling direction. The invention relates in particular to a stainless steel product, characterized in that:

[0070] - its composition is that of the previous steel;

[0071] - its microstructure consists of at least 20%, preferably between 25% and 60% martensite including any residual ferrite, and at least 40% austenite.

[0072] Austenite is austenite deformed at room temperature, that is, austenite deformed by cold rolling.

[0073] Preferably the microstructure comprises at least 50% austenite.

[0074] The residual ferrite fraction is generally at most 2%, or even at most 1%.

[0075] This structure corresponds to the structure of steel in the work-hardened state.

[0076] The invention also relates to a welded structure of at least two stainless steel products according to the invention, in the work-hardened state, the steel products being joined to each other by at least one weld, in which, in the heat-affected zone of the weld (HAZ), the structure consists of at least 90% austenite and at most 10% ferrite.

[0077] Other features and advantages of the invention will become apparent during the description below, given by way of example.

[0078] After numerous trials, the inventors showed that the various requirements reported above were met by observing the following conditions.

[0079] With regard to the chemical composition of the steel, the carbon content must be less than or equal to 0.030% in order to avoid the formation of chromium carbides in the welds, which would lead to sensitization to intergranular corrosion and a decrease in the toughness of the heat-affected zone.

[0080] Manganese promotes the stability of the austenitic phase. However, given the nitrogen content of the composition, which also stabilizes austenite, the Mn content should be limited to a maximum of 1.50% to allow for sufficient work hardening, ensuring high mechanical strength at low temperatures. Preferably, the Mn content is greater than or equal to 1.0%.

[0081] Silicon is an element that can be added to deoxidize liquid steel. It also participates in solution hardening and therefore influences work hardening. Its content is limited to 1.0% by weight because it tends to increase hot hardness and thus limit the shapes achievable by hot rolling. Preferably, the silicon content is at least 0.30%, this minimum being desirable when silicon is added to deoxidize steel. Preferably, the silicon content is less than or equal to 0.80%.

[0082] Chromium is an element that increases resistance to oxidation and corrosion, and its content for this purpose should be at least 16.0%, preferably at least 17.0%. However, chromium stabilizes the ferrite, whereas a predominantly austenitic structure is desired. Therefore, the chromium oxide content should be less than or equal to 19.0%, preferably less than or equal to 18.5%.

[0083] Nickel is an essential element for stabilizing austenite. Its effects are sufficient when its content is 6.0% or higher. Therefore, the Ni content is 6.0% or higher. However, at excessively high levels, Ni negatively impacts mechanical strength by inhibiting martensitic transformation induced by plastic deformation, such that the Ni content is limited to a maximum of 8.0%. Preferably, the Ni content is between 6.5% and 7.5%.

[0084] Nitrogen is an important element in the steel according to the invention. In particular, nitrogen is a hardening element in the interstitial solid solution, which increases mechanical strength. Furthermore, in solid solution, nitrogen is a powerful stabilizer of the austenitic phase, making it possible to limit the content of Ni, which is an expensive alloying element. The nitrogen content is therefore adjusted to at least 0.115% for these effects. The N content is at most 0.20% to allow sufficient work hardening and to avoid the formation of nitrides such as C^N.

[0085] Sulfur is an element that particularly degrades hot forgeability and corrosion resistance; its content must be kept below or equal to 0.03%, preferably below or equal to 0.01%, and even better below or equal to 0.002%. This limitation also ensures suitable weldability and toughness of the final product.

[0086] Phosphorus also degrades ductility when hot; its content must be less than or equal to 0.045%, preferably less than or equal to 0.035% to obtain satisfactory results.

[0087] Boron causes the formation of chromium borides, iron borides, or boron nitrides, which weaken steel. Therefore, the boron content must be less than 0.0020%, and generally as low as possible.

[0088] Aluminum is a powerful deoxidizing agent for liquid metal. However, aluminum tends to precipitate as nitrides, thus reducing the amount of nitrogen available to stabilize the austenite. Its content is therefore limited to a maximum of 0.02%. The composition may include titanium as an impurity, in a content of no more than 0.040%, preferably no more than 0.020%. This is because titanium leads to the formation of titanium carbides or nitrides, which are detrimental to toughness.

[0089] Besides iron, the rest of the composition consists of unavoidable impurities resulting from the manufacturing process, such as Nb, Cu, Mo, Pb, Co, W or Sn.

[0090] In particular, niobium may be present in the composition as an impurity, in levels of up to 0.05%.

[0091] Copper may be present in the composition at a concentration of 1.0% or less. Copper promotes austenite formation and contributes to corrosion resistance. Generally, copper is present as an impurity resulting from the manufacturing process, in a concentration that depends on the specific manufacturing method. However, the copper content remains at most 1.0%. In any case, above a concentration of 1.0%, the austenite becomes too stable at room temperature, and martensitic deformation is inhibited.

[0092] The Mo content must be at most 1.0%, the Pb content at most 0.03%, the Co content less than or equal to 0.5% by weight, and the Sn content must be less than or equal to 0.05%. The W content must remain less than or equal to 0.10%.

[0093] The other elements not mentioned are present only in trace amounts resulting from the manufacturing process. This term "traces" should generally be understood as meaning that these elements are not added intentionally during manufacturing, or that (as may be the case with AI), they are subsequently removed, for example by decanting the non-metallic inclusions they have formed, and are found only in very marginal amounts in the final steel.

[0094] In addition to complying with the conditions seen previously regarding the individual contents of the various elements, the composition of the steel must also comply with the condition PREN > 18; preferably PREN > 20;

[0095] with PREN = Cr + 3.3*Mo + 16*N.

[0096] Where Cr, Mo and N are the mass percentages of the corresponding elements in the composition.

[0097] The PREN (Pitting Resistance Equivalent Number) is an indicator of the corrosion resistance of stainless steels. It reflects the cumulative effects of the three elements Cr, Mo and N, which improve corrosion resistance.

[0098] The PREN must be greater than or equal to 18 in the case of the invention, preferably greater than or equal to 20, so that the steel can sufficiently resist corrosion. Preferably, the steel composition is such that the Md30 parameter, as described below, is between -10°C and 30°C, preferably between 0°C and 20°C:

[0099] Md30 = 551 - 462*(C + N) - 9.2*Si - 8.1*Mn - 13.7*Cr - 29*(Ni + Cu) - 18.5*Mo -68*Nb

[0100] Where C, N, Si, Mn, Cr, Ni, Cu, Mo and Nb are the mass percentages of the corresponding elements in the composition.

[0101] This Md30 parameter is indeed an indicator of the work hardening potential, which estimates the temperature at which 30% deformation induces the formation of 50% martensite.

[0102] A value between -10°C and 30°C, and preferably between 0°C and 20°C, optimizes the compromise between mechanical resistance and toughness at cryogenic temperatures.

[0103] The steel manufacturing process according to the invention typically comprises the following steps:

[0104] - the composition is developed according to the invention;

[0105] - a semi-finished product is cast from this steel;

[0106] - We perform hot shaping and, possibly, cold shaping.

[0107] - at least one heat treatment of the steel is carried out between 900°C and 1200°C, the heat treatment ending with an ultraquench at a cooling rate of at least 2°C / s between 900°C and 500°C.

[0108] After these steps, the steel is in the annealed state. Generally, the annealed structure comprises at least 98% austenite, the remainder being mostly residual ferrite and inclusions inherent to the manufacturing process.

[0109] Then, the steel is cold-rolled with a reduction ratio of between 5% and 60%, preferably between 15% and 45%. This produces a work-hardened product.

[0110] Work-hardened steel has a microstructure consisting of at least 20%, preferably between 25% and 60% martensite, and at least 40% austenite deformed at room temperature, and at most 2% ferrite, preferably at most 1% ferrite.

[0111] This product exhibits high mechanical resistance, whether at room temperature or at cryogenic temperature, with a uniaxial tensile yield strength of at least 700 MPa at room temperature and at least 1100 MPa at -196°C.

[0112] This work-hardened steel also has a tensile strength, measured on a 2.5 mm sub-size specimen according to ASTM A0370-21 and ISO 148-1:2016 standards, with the specimen length parallel to the sheet width and the crack propagating in the rolling direction, of at least 100 J / cm². 2 at room temperature and at least 50 J / cm² 2 at -196°C.

[0113] When the steel according to the invention (in the form of a steel product such as a sheet or plate) is welded to another steel product made of steel according to the invention, the resulting structure is such that in the heat-affected zone, the impact strength, measured according to ASTM A0370-21 and ISO 148-1:2016 on a 2.5 mm thick mini-test specimen, as described above, is at least 180 J / cm² 2 at room temperature and at the temperature of liquid nitrogen (-196°C)

[0114] We will now describe a series of experiments demonstrating the advantages of the invention. Laboratory castings were studied, the chemical analyses of which, in mass percentages, are given in Table 1. In this table, values ​​not conforming to the invention are underlined.

[0115] C Mn PS Si Ni Cr Cu Mo N PREN Md30 0.025 1.12 0.040 0.012 0.33 6.69 17.22 0.27 0.64 0.13 21 18 0.022 1.39 0.040 0.012 0.72 7.15 18.25 0.13 0.36 0.12 21 0 0.021 1.41 0.040 0.012 0.37 6.61 18.32 0.08 0.24 0.18 22 -6 0.024 1.36 0.040 0.012 0.53 6.53 17.39 0.45 0.54 0.15 22 4 0.023 1.08 0.040 0.012 0.36 7.27 17.46 0.12 0.09 0.13 20 13 0.029 1.24 0.040 0.012 0.48 7.06 17.85 0.38 0.12 0.14 20 -4 0.022 1.61 0.042 0.018 0.51 6.42 17.02 0.51 0.12 0.106 19 38 0.027 1.75 0.046 0.016 0.49 8.03 18.53 0.55 0.51 0.053 21 -17 0.058 1.73 0.044 0.014 0.38 8.14 18.56 0.53 0.26 0.056 20 -30 0.106 1.50 0.043 0.015 0.43 6.76 16.89 0.39 0.47 0.043 19 19

[0116]

[0117] Table 1

[0118] These steels were cast in ingot form and then hot-formed.

[0119] Then, the steel underwent heat treatment between 900°C and 1200°C, as a result of which the steel structure has a structure comprising at least 98% austenite, the remainder being at most 2% residual ferrite and inclusions inherent to the manufacturing process.

[0120] The heat treatment was followed by over-quenching at a cooling rate of at least 2°C / s between 900°C and 500°C. The steels were then cold-rolled with a reduction ratio as indicated in Table 2 below.

[0121] Following these treatments, the structure of the work-hardened steels was determined. These structures, in particular the martensite volume fraction, are shown in Table 2.

[0122] The uniaxial tensile yield strength was measured according to the standard “ISO 6892-1:2016 Metallic materials - Tensile testing”, at ambient temperature and at -196°C. It was thus possible to verify that the uniaxial tensile yield strengths of the steels according to the invention are at least 700 MPa at ambient temperature, and at least 1100 MPa at -196°C.

[0123] Furthermore, the resilience of the steel in the work-hardened state was measured according to ASTM A0370-21 and ISO 148-1:2016 standards, on a mini specimen 2.5mm thick, the length of the specimen being parallel to the width of the sheet and the crack propagating along the rolling direction, at room temperature and at -196°C.

[0124] The metallurgical products thus obtained were then welded in pairs, each steel being welded to an identical steel, in this case sheets, and the characteristics of the structure thus formed were then measured, in particular the resilience at 20°C and at -196°C, as well as the volume fraction of ferrite in the area affected by the heat.

[0125] Table 2 shows the reduction rate during cold rolling (“Reduction Rate LAF”), the volume fraction of martensite in the structure after cold rolling (i.e. in the work-hardened state) (martensite (%)), the yield strength in this work-hardened state at 20°C and -196°C respectively, and the volume fraction of ferrite in the heat-affected zone.

[0126] The inventors have also verified that the structure of steels 1 to 6 according to the invention, in the work-hardened state, comprises at least 40% of austenite deformed at room temperature and at most 2%, preferably at most 1% of residual ferrite.

[0127] The resilience of the steel in the work-hardened state and in the heat-affected zone after welding was also reported, both at 20°C and at -196°C, measured as described above.

[0128] Again, the values ​​that do not conform to the invention are underlined.

[0129] It was thus observed that in the heat-affected zone of the products according to the invention, the structure consists of at least 90% austenite and at most 10% ferrite. Yield strength

[0130] Charpy Resilience Rate (J / cm2)

[0131] Martensite reduction (MPa) Ferrite in ZAT LAF (%)

[0132] Written state (%) Yes Welded state

[0133] 20°C -196°C

[0134] 20°C -196°C 20°C -196°C

[0135] 1 20 27 857 1396 113 62 203 184 8 2 20 21 786 1152 136 78 199 201 9 3 40 41 994 1135 146 94 196 207 7 4 25 28 872 1254 129 80 211 186 7 5 25 31 849 1223 108 59 194 202 5 6 35 31 901 1194 137 77 188 198 3 7 20 51 932 1298 83 46 125 119 6 8 20 15 634 1024 153 94 197 203 5 9 25 16 694 1068 125 90 64 4 14

[0136] 10 20 26 852 1436 54 26 9 2 12

[0137]

[0138] Table 2

[0139] Furthermore, it has been observed that only the steels according to the invention exhibit high cryogenic weld toughness of at least 180 J / cm² 2 measured at room temperature and at -196°C, while maintaining high mechanical strength, the uniaxial tensile yield strength being greater than or equal to 700 MPa at room temperature and 1100 MPa at -196°C

[0140] In addition, the steels according to the invention exhibit, in addition to the above properties, good intrinsic toughness, measured by Charpy tests at ambient temperature and that of liquid nitrogen (-196°C), whether in the work-hardened state or in the heat-affected zone.

[0141] The steel according to the invention makes it possible to solve the technical problem as presented above, in particular by exhibiting high mechanical strength both at ambient temperature and at cryogenic temperatures, and high resilience at cryogenic temperatures including in welds.

[0142] This steel is therefore particularly suitable for the manufacture of cryogenic tanks.

Claims

DEMANDS 1. Austenitic stainless steel for a cryogenic tank, the steel having a composition comprising, in mass percentages: C < 0.030% If < 1.0% Mn < 1.50% P < 0.045% S < 0.03% 16.0% < Cr < 19.0% 6.0% < Ni < 8.0% 0.115% < N < 0.20% B < 0.0020% Al < 0.02% Ti < 0.040% N b < 0.05% Cu < 1.0% Mo < 1.0% Pb < 0.03% Co < 0.5% Sn < 0.05% W < 0.10% the remainder being iron and impurities resulting from the manufacturing process, the composition further satisfying the condition: PREN > 18; preferably PREN > 20; with PREN = Cr + 3.3*Mo + 16*N. Where Cr, Mo and N are the mass percentages of Cr, Mo and N respectively in the composition.

2. Austenitic stainless steel according to claim 1, wherein the steel composition is such that the parameter Md30, defined by: Md30=551 - 462*(C + N) - 9.2*Si - 8.1*Mn - 13.7*Cr - 29*(Ni + Cu) - 18.5*Mo - 68*Nb, where C, N, Si, Mn, Cr, Ni, Cu, Mo and Nb are the mass percentages of C, N, Si, Mn, Cr, Ni, Cu, Mo and Nb in the composition, is between -10°C and 30°C, preferably between 0°C and 20°C.

3. Austenitic stainless steel according to any one of claims 1 or 2, wherein 17.0% < Cr < 18.5%.

4. Austenitic stainless steel according to any one of claims 1 to 3, wherein 1.0% < Mn < 1.50%.

5. Austenitic stainless steel according to any one of claims 1 to 4, wherein 0.30% < Si < 0.80%.

6. Austenitic stainless steel according to any one of claims 1 to 5, wherein 6.5% < Ni < 7.5%.

7. Austenitic stainless steel according to any one of claims 1 to 6, wherein Ti < 0.020%.

8. Austenitic stainless steel according to any one of claims 1 to 7, wherein the structure of the steel, at room temperature and at a temperature less than or equal to -196°C, consists of at least 20% martensite, preferably between 25% and 60%, at least 40% austenite and at most 2%, preferably at most 1% residual ferrite.

9. Austenitic stainless steel according to any one of claims 1 to 8, wherein the impact strength of the steel, measured according to ASTM A0370-21 and ISO 148-1:2016 on a 2.5 mm thick mini-specimen, the length of the specimen being parallel to the width of the sheet and the crack propagating in the rolling direction, is at least 100 J / cm² 2 at room temperature, and at least 50 J / cm² 2 at -196°C.

10. Austenitic stainless steel according to any one of claims 1 to 9, having a uniaxial tensile yield strength, measured according to ISO 6892-1:2016, of at least 700 MPa at room temperature, and of at least 1100 MPa at -196°C.

11. Welded structure comprising at least two stainless steel products according to any one of claims 1 to 10, the steel products being joined to each other by at least one weld, wherein, in the area of products affected by the heat of welding steel products, the structure consists of at least 90% austenite and at most 10% ferrite.

12. Welded structure according to claim 11, wherein, in the heat-affected zone, the impact strength, measured according to ASTM A0370-21 and ISO 148-1:2016 and measured on a 2.5 mm thick mini-test specimen, is at least 180 J / cm² 2, at room temperature and at -196°C.

13. Cryogenic tank, comprising at least one welded structure according to any one of claims 11 or 12.

14. A process for manufacturing a stainless steel product, characterized in that: - a steel is produced having a composition according to any one of claims 1 to 7, - a semi-finished product is cast from this steel, - hot forming and, possibly, cold forming are carried out, and at least one heat treatment of said steel is performed between 900°C and 1200°C, followed by over-quenching at a cooling rate of at least 2°C / s between 900°C and 500°C, - The steel is cold rolled with a reduction ratio of between 5% and 60%, preferably between 15% and 45%.

15. A manufacturing process according to claim 14, wherein, after cold rolling, the steel has a structure consisting of at least 20% martensite, preferably between 25% and 60%, and at least 40% austenite, and at most 2% ferrite, preferably at most 1% ferrite.

16. Method of manufacturing a welded structure according to any one of claims 11 or 12, comprising supplying at least two steel products according to any one of claims 1 to 10 or products according to any one of claims 14 or 15, and a step of welding the steel products to each other by at least one weld.

Images