Austenitic stainless steel and method for manufacturing strip products thereof

Abstract

Austenitic stainless steel having the following composition by weight % (wt%): C 0.06 or less, Si 0.1 - 1, Mn 3 - 5, Cr 20.5 - 23.5, Ni 11 - 15, Mo 1 - 4, Nb 0.50 - 0.70, N 0.40 - 0.60, P 0.050 or less, S 0.005 or less, optionally W 3 or less, optionally Co 0.50 or less, optionally V 0.30 or less, optionally Cu 0.30 or less, optionally B 0.005 or less, optionally Al 0.25 or less, optionally either Ca or Mg 0.05 or less, or REM 0.5 or less, the balance being Fe and normally occurring impurities, and the composition satisfies the criterion of [wt% of Mo] + 2 × [wt% of W] ≥ 3.

Description

Technical Field

[0001] The present disclosure generally relates to austenitic stainless steels. More specifically, the present disclosure generally relates to austenitic stainless steels suitable for use in hydrogen storage applications. The present disclosure also generally relates to methods of manufacturing products of austenitic stainless steels.

Background Art

[0002] Due to efforts to reduce harmful emissions in the automotive industry, there has been increasing interest in the development of vehicles that use at least partially hydrogen as fuel. An example of such a vehicle is a fuel cell vehicle. This also means that in addition to the need to store hydrogen in such vehicles, there is also a need to store hydrogen at hydrogen fuel stations and the like.

[0003] Hydrogen can be physically stored either as a gas or a liquid. To store hydrogen as a gas, a high-pressure tank is typically required, which is achievable at ambient temperature. In the liquid form, hydrogen can be stored either in a pure liquid form or in a cryo-compressed form, and in either option, it is necessary to store at a very low temperature. Hydrogen can alternatively be stored by using a solid, either by adsorption of hydrogen onto the surface of the solid or by absorption of hydrogen into such a solid. However, hydrogen storage using a solid can often be undesirable because it can increase the weight of the storage body and thus the weight of the vehicle, and / or increase the size required for the storage container.

[0004] Components used in combination with storage in the form of hydrogen gas, i.e., components configured to contain and / or be exposed to hydrogen gas, may be constructed of stainless steel. One of the most important properties when selecting which stainless steel to use for such components is the resistance of the stainless steel to hydrogen embrittlement. Hydrogen embrittlement is a phenomenon that can occur in combination with stress in steel when the steel is exposed to hydrogen and the hydrogen diffuses into the steel. Hydrogen embrittlement can lead to a loss of ductility and / or toughness, as well as a reduction in the load-bearing capacity of the steel, even at stress levels below the yield strength of the steel. Also, hydrogen embrittlement can cause cracks to occur suddenly without any warning, which can lead to catastrophic defects. Ferritic stainless steels and duplex stainless steels are well known to be prone to hydrogen embrittlement. Furthermore, martensitic stainless steels also have relatively low resistance to hydrogen embrittlement. Therefore, austenitic stainless steels seem to be the most promising candidates. In addition to having high resistance to hydrogen embrittlement, austenitic stainless steels are also desired to have a high tensile strength to withstand high pressures. Furthermore, the higher the tensile strength, the thinner the steel thickness can be, which also enables weight reduction of components that are important in the automotive industry. Additionally, austenitic stainless steels need to have sufficient properties to be used in the temperature range from -50°C to +85°C in order to be a suitable option.

[0005] Today, an example of an austenitic stainless steel used in these types of applications is SS 316L. This is because it has resistance to hydrogen embrittlement. However, SS 316L is not always a suitable option because it has a relatively low tensile strength.

[0006] European Patent Application Publication No. 1605073 (EP 1605073 A1) discloses another example of austenitic stainless steel intended for use in a high-pressure hydrogen gas environment. United States Patent Application Publication No. 2017 / 314092 (US 2017 / 314092 A1) discloses an austenitic stainless steel described as having high strength and good hydrogen embrittlement resistance.

[0007] SUMMARY OF THE INVENTION An object of the present invention is to provide an austenitic stainless steel having high tensile strength and good hydrogen embrittlement resistance so as to be suitable for hydrogen storage applications.

[0008] This object is achieved by the subject matter of the appended independent claims.

[0009] According to the present disclosure, an austenitic stainless steel is provided. The austenitic stainless steel has the following composition in weight % (wt%): C 0.06 or less, Si 0.1 - 1, Mn 3 - 5, Cr 20.5 - 23.5, Ni 11 - 15, Mo 1 - 4, Nb 0.50 - 0.70, N 0.40 - 0.60, P 0.050 or less, S 0.005 or less, Optionally W 3 or less, Optionally Co 0.50 or less, Optionally V 0.30 or less, Optionally Cu 0.30 or less, Optionally B 0.005 or less, Optionally Al 0.25 or less, Optionally either Ca or Mg 0.05 or less, or REM 0.5 or less, with the balance consisting of Fe and normally occurring impurities, and having the above composition, [% weight of Mo] + 2 × [% weight of W] ≥ 3 satisfies the following criterion.

[0010] The austenitic stainless steel according to the present disclosure has a high tensile strength in combination with excellent hydrogen embrittlement resistance. Further, very good toughness and weldability can be obtained. As a result, the austenitic stainless steel is an excellent candidate for use in components configured to contain hydrogen gas and / or liquid hydrogen and / or to be exposed to hydrogen gas and / or liquid hydrogen.

[0011] The composition of the austenitic stainless steel preferably [% weight of Mo] + [% weight of W] ≥ 3.0 satisfies the following criterion. This further contributes to the high tensile strength of the austenitic stainless steel.

[0012] The above composition of the austenitic stainless steel means that the sum of the niobium content and 2.5 times the nitrogen content (i.e., [% weight of Nb] + 2.5 × [% weight of N]) is at least 1.50. This is important for achieving high strength. The composition of the austenitic stainless steel is suitably [% weight of Nb] + 2.5 × [% weight of N] ≥ 1.55 such that the following criterion is satisfied and contains niobium and nitrogen in amounts that can further increase the tensile strength.

[0013] The austenitic stainless steel can suitably be in a solution annealed condition.

[0014] When in the solution annealed condition, the austenitic stainless steel according to the present disclosure can have a tensile strength of at least 930 MPa.

[0015] Furthermore, the austenitic stainless steel according to the present disclosure is a Z-phase strengthened stainless steel.

[0016] The present disclosure further provides a method for manufacturing the above-described austenitic stainless steel products. The method includes casting a melt having the above composition to obtain a casting material, hot working the casting material to obtain an intermediate product, cold working the intermediate product to obtain the intended final thickness or the intended final diameter of the product, and solution treating the cold-worked product at a temperature exceeding 1000°C. This includes

[0017] Suitably, the solution treatment of the cold-worked product is carried out at a temperature of 1050°C or higher, preferably 1080°C or higher.

[0018] The present disclosure also relates to the use of the above-described austenitic stainless steel for constructing components adapted to contain hydrogen gas and / or liquid hydrogen and / or exposed to an environment containing hydrogen gas and / or liquid hydrogen or consisting of hydrogen gas and / or liquid hydrogen. Such components can be, for example, a container configured to store pressurized hydrogen gas, a valve in an arrangement configured to store or transport pressurized hydrogen gas, or another component in an arrangement configured to store or transport pressurized hydrogen gas (such as a conduit, pipe, tube, or machined part). Alternatively, the component can be a container configured to store hydrogen in pure liquid or cryogenic compressed form, or a component in an arrangement configured to store or transport hydrogen in liquid or cryogenic compressed form.

[0019] The present disclosure also relates to components adapted to contain hydrogen gas and / or liquid hydrogen (especially pressurized hydrogen gas) and / or exposed to hydrogen gas and / or liquid hydrogen (especially pressurized hydrogen gas), said components being constructed of the austenitic stainless steel described above or below.

Brief Description of the Drawings

[0020]

Figure 1

Figure 2

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Figure 4

Mode for Carrying Out the Invention

[0021] Hereinafter, the present invention will be described in more detail with reference to exemplary embodiments. However, the present invention is not limited to the discussed exemplary embodiments and can vary within the scope of the appended claims.

[0022] In the present disclosure, when a range is disclosed, such a range includes the end values of the range, unless it is explicitly disclosed otherwise. Similarly, when an open range (only one of the upper limit or the lower limit) is disclosed, the open range includes one of the end values of the open range, unless it is explicitly disclosed otherwise.

[0023] The austenitic stainless steels according to the present disclosure are primarily intended for products in the form of strips or plates that can manufacture various components adapted to contain and / or be exposed to a high-pressure hydrogen environment. Examples of such components include, but are not limited to, storage containers for pressurized hydrogen gas, valves, or other machined parts. However, it should be noted that austenitic stainless steels can also be used in other forms (bars, tubes, forgings) if desired. Such bars can be extruded bars or rolled bars. Similarly, such tubes can be extruded tubes or rolled tubes.

[0024] For example, welding is required to manufacture a storage container from a stainless steel strip. Therefore, it is important for the austenitic stainless steels of the present application to have good weldability. This can be achieved when the austenitic stainless steel is in a solution-treated state. Thus, the austenitic stainless steels of the present application have been developed with the aim of obtaining high strength and good resistance to hydrogen embrittlement when in a solution-treated state. Also, it is desirable for the austenitic stainless steel to have high impact toughness to avoid cracking in the material when subjected to sudden impacts such as collisions. Furthermore, it is desirable for the austenitic stainless steel to maintain a good balance between the above properties within a temperature range from -50°C to at least +85°C, as it is a truly suitable candidate for components related to the storage of pressurized hydrogen gas.

[0025] Although the austenitic stainless steels according to the present disclosure have been primarily developed for use in products related to the storage of high-pressure hydrogen gas at ambient temperature (here meaning a temperature in the range of -50°C to at least +85°C), the austenitic stainless steels are also suitable for use in cryo-applications. In other words, the austenitic stainless steels described herein can also be used as components of an arrangement (such as tanks, pipes, tubes, or valves) for storing hydrogen in its pure liquid form or cryo-compressed form.

[0026] According to the present disclosure, there is provided an austenitic stainless steel capable of forming a thermodynamically stable Z phase. The Z phase can be described as a stoichiometric phase of niobium, chromium, and nitrogen. In some cases, vanadium may also be abundant in the Z phase. In order to obtain precipitation of the Z phase in combination with solution strengthening of the base material, it has been found that by appropriately selecting the composition of the austenitic stainless steel, high tensile strength can be achieved in the solution-treated state, and no special means / processes are required during the production of the austenitic stainless steel into the intended product form (e.g., strip or plate). In other words, the austenitic stainless steel can be cast, hot-worked, and cold-worked according to conventional methods and then solution-treated. The only requirement is that the solution treatment must be carried out at a temperature above the temperature at which potential carbides and nitrides dissolve.

[0027] The austenitic stainless steel described herein can provide an austenitic stainless steel that can be easily welded and has the following properties in the solution-treated state: · A tensile strength of at least 930 MPa, suitably at least 935 MPa, when tested at room temperature according to SS-EN ISO 6892-1, · An impact toughness (Charpy V) of more than at least 25 J, suitably 40 J, at -50 °C when tested according to SS-EN ISO 148-1, and · The hydrogen embrittlement resistance is represented by the reduction ratio of the area between the hydrogen-filled environment and the inert environment and is 0.90 or more (evaluated by slow strain rate testing (SSRT) at a temperature of 4 °C at a strain rate of 1×10 -5 s -1 while electrochemically charging), where the electrochemical charging is carried out by applying a cathodic current density of 5 mA / cm 2 in 0.5 M H2SO4 purged with N2, and the inert test is carried out in distilled water purged with air (without applying current).

[0028] The importance of various alloying elements in austenitic stainless steels will be briefly described below. All percentages regarding chemical composition are expressed in weight % (wt%) unless explicitly disclosed otherwise. The upper and lower limits of the individual elements of the composition can be freely combined within the broadest limits defined in the claims unless explicitly disclosed otherwise.

[0029] Carbon (C): 0.06% or less Since the solubility of carbon in austenite is limited, if the carbon content is too high, carbides may precipitate, which can reduce toughness. Therefore, it is desirable for the carbon content to be 0.06% or less. The carbon content can be maintained below 0.05%.

[0030] Carbon is not an element added purposefully, and there is no critical lower limit for the carbon content. However, if one tries to keep the carbon content very low, the processing cost will increase excessively. Therefore, in practice, carbon may be present in an amount of at least 0.01% or at least 0.02%.

[0031] Silicon (Si): 0.1 - 1% Silicon is used as a deoxidizer during steel manufacturing and can also improve fluidity during welding. Therefore, austenitic stainless steels contain at least 0.1% Si. Silicon preferably exists in an amount of at least 0.2%.

[0032] However, if the silicon content is excessive, it may lead to the precipitation of undesired intermetallic compound phases (such as sigma phase). The presence of the sigma phase can lead to, for example, a decrease in hot workability and should be avoided. Therefore, austenitic stainless steels contain at most 1% Si. Silicon preferably exists in an amount of 0.6% or less. Silicon more preferably exists in an amount of 0.45% or less.

[0033] Manganese (Mn): 3 - 5% Manganese is an element that stabilizes austenite and reduces the risk of forming ferrite that can form intermetallic compound phases after cooling. Manganese contributes to the improvement of strength, ductility, and toughness. Furthermore, manganese is an alloying element that is less expensive than nickel. The increase in manganese also contributes to the increase in the solubility of nitrogen in austenitic stainless steel, which can thereby contribute to the increase in strength. Therefore, the austenitic stainless steel according to the present disclosure contains 3% or more of Mn. Manganese is preferably present in an amount of 3.2% or more. In the austenitic stainless steel of the present application, manganese may be present in an amount of 3.5% or more.

[0034] If the manganese content is too high, in some cases, it may have a negative impact on the weldability and / or corrosion resistance of austenitic stainless steel. Furthermore, since a manganese content exceeding 5% is not required, the austenitic stainless steel of the present application contains 5% or less of manganese. In the austenitic stainless steel of the present application, manganese may be present in an amount of 4.6% or less, or 4.4% or less.

[0035] Chromium (Cr): 20.5 - 23.5% Chromium is an element added to provide sufficient corrosion resistance to austenitic stainless steel. Chromium further increases the solubility of nitrogen, thus contributing to the possibility of adding a desired amount of nitrogen to austenitic stainless steel. Chromium further contributes to the formation of the Z phase and is thus preferably present in a relatively large amount. Therefore, austenitic stainless steel contains at least 20.5% of Cr. The austenitic stainless steel of the present disclosure may contain at least 21% of Cr, for example, at least 21.5% of Cr.

[0036] However, if the chromium content is too high, the risk of forming undesired intermetallic compound phases increases, and the ductility and / or toughness may decrease. Therefore, chromium is present in an amount of up to 23.5%. In the austenitic stainless steel of the present application, the upper limit of the chromium content can be 23%. According to one option, chromium can be present in an amount of up to 22.8%.

[0037] Nickel (Ni): 11 - 15% Nickel improves the stability of austenite and reduces the ferrite content. Ferrite may form intermetallic compound phases during cooling, which is undesirable as it reduces impact toughness and hydrogen embrittlement resistance. Nickel also contributes to the improvement of ductility. Therefore, austenitic stainless steel contains at least 11%, or 11%, of nickel. Appropriately, nickel is present in an amount of 12% or more, or 12.5% or more.

[0038] However, nickel is a relatively expensive alloying element, and in the austenitic stainless steel of the present application, an addition exceeding 15% is not necessary. Also, if the nickel content is too high, the tensile strength may decrease. Therefore, nickel is present in an amount of 15% or less. Appropriately, nickel may be present in an amount of 14% or less, for example 13.5% or less.

[0039] Molybdenum (Mo): 1 - 4% Since molybdenum is a solid-solution strengthening element, it is important for the strength of austenitic stainless steel. Molybdenum can optionally be partially replaced by tungsten, which is also a solid-solution strengthening element. It is generally accepted in this field that replacing molybdenum with tungsten can be done in a ratio such that 1% of Mo is replaced by 2% of W. However, completely replacing molybdenum with tungsten may increase the risk of precipitation of the chi phase, which is undesirable. Therefore, the austenitic stainless steel of the present application contains 1% or more of Mo. Appropriately, molybdenum is present in an amount of 1.2% or more.

[0040] However, austenitic stainless steel needs to contain sufficient solution strengthening elements to obtain sufficient strength. Therefore, the composition should at least [% by weight of Mo] + 2 × [% by weight of W] ≥ 3 needs to meet this standard. In other words, when tungsten is not added, molybdenum is present in an amount of at least 3%.

[0041] However, if the content of molybdenum is too high, the ductility may decrease, so it should be avoided. Therefore, molybdenum is present in an amount of 4% or less. Molybdenum is preferably present in an amount of 3.2% or less. When tungsten is present in a sufficient amount to meet the above standard, molybdenum may be present in an amount of 2.0% or less.

[0042] Niobium (Nb): 0.50 - 0.70% Niobium contributes to the formation of the Z phase and thus to the strength, so it is an essential element for the austenitic stainless steel according to the present disclosure. To obtain the desired amount of the Z phase, niobium is present in an amount of at least 0.50%, preferably in an amount exceeding 0.50%. Such an amount of niobium is much higher than the amount added to conventional austenitic stainless steels for the purpose of forming carbides and / or nitrides (including carbonitrides). However, in the austenitic stainless steel of the present application, since niobium is added to form the Z phase, niobium is not added for the purpose of forming nitrides and / or carbides. Niobium is preferably present in an amount of 0.51% or more.

[0043] The presence of niobium and nitrogen in the austenitic stainless steel according to the present disclosure means that the austenitic stainless steel meets the standard that [% by weight of Nb] + 2.5 × [% by weight of N] is 1.5 or more. This standard is important for achieving high tensile strength as it reflects the contribution of the formed Z phase based on its stoichiometric composition.

[0044] However, if the amount of niobium is too large, the alloying cost increases and no further increase in strength is predicted. Also, if the amount of niobium is too large, the hot workability may decrease. Therefore, niobium is present in an amount of 0.70% or less. Appropriately, niobium is present in an amount of 0.65% or less.

[0045] Nitrogen (N): 0.40 - 0.60% Nitrogen is an essential alloying element in the austenitic stainless steel according to the present disclosure because it contributes to the formation of the Z phase and thus to the strength. Nitrogen can also contribute to solid-solution strengthening. Therefore, nitrogen is present in an amount of 0.40% or more. Appropriately, nitrogen is present in an amount of 0.42% or more.

[0046] As described above for niobium, the austenitic stainless steel preferably contains nitrogen in an amount such that the criterion of [wt% of Nb] + 2.5 × [wt% of N] ≥ 1.55 is satisfied.

[0047] However, if the amount of nitrogen is too large, it may lead to the formation of chromium nitride, which in turn reduces the ductility, increases the strain hardening, and thus increases the force required during cold working. In this case, an intermediate anneal may be required during cold working, which can increase the manufacturing cost. Therefore, nitrogen is present in an amount of 0.60% or less. The nitrogen content can appropriately be present in an amount of 0.55% or less, more preferably 0.50% or less.

[0048] Tungsten (W): Optionally 3% or less Tungsten is not strictly necessary for the austenitic stainless steel according to the present disclosure, but it is preferably added. Since tungsten is a solid solution strengthening element as described above, it can be added for the purpose of increasing the strength of the austenitic stainless steel of the present application. When tungsten is used as a substitute for molybdenum as described above, it contributes similarly to the strength of the steel, but tungsten has the advantage of a lower risk of forming sigma phase compared to molybdenum. Reducing the amount of sigma phase is advantageous because it increases the number of atoms available for solid solution strengthening of the austenitic stainless steel. Therefore, by adding tungsten, the process control during the production of the austenitic stainless steel of the present application can be made easier compared to the case where only molybdenum is present.

[0049] Furthermore, as described above regarding molybdenum, it has been found that austenitic stainless steel can achieve a very high tensile strength if the total of the molybdenum content and the tungsten content is 3.0 or more. Therefore, it is necessary to contain a sufficient amount of solid solution strengthening elements. Thus, according to a plurality of embodiments, the austenitic stainless steel [wt% of Mo]+[wt% of W]≧3.0 satisfies the criterion.

[0050] The austenitic stainless steel suitably contains 1% or more of W. The austenitic stainless steel preferably contains at least 1.4% of tungsten.

[0051] However, if the amount of tungsten is too large, it can have a negative impact on the properties of the steel of the present application, so it should be avoided. Thus, the austenitic stainless steel contains a maximum of 3% of W. Tungsten is suitably present in an amount of 2.5% or less.

[0052] Cobalt (Co): Optionally 0.50% or less The austenitic stainless steel of the present application does not necessarily need to contain cobalt, but cobalt can be added if desired. Cobalt is an element that stabilizes austenite and can contribute to an increase in strength. In order not to excessively increase the alloying cost, cobalt can be added in an amount of 0.50% or less. Appropriately, cobalt is present in an amount of 0.30% or less, or 0.20% or less.

[0053] Also, it should be noted that even when cobalt is not intentionally added, cobalt may still be present as an impurity resulting from the raw materials used. A cobalt content of less than 0.20% is considered an impurity in the present disclosure.

[0054] Vanadium (V): Optionally 0.30% or less The austenitic stainless steel does not need to contain vanadium. However, vanadium can be added in an amount of 0.30% or less if desired. Since vanadium can be rich in the Z phase, it can contribute to the improvement of strength. According to one option, vanadium is present in an amount of 0.10% or less.

[0055] Copper (Cu): Optionally 0.30% or less The austenitic stainless steel does not need to contain copper, but an addition of 0.30% or less can be tolerated without having a negative impact on the properties. Copper is an element that stabilizes austenite and can be added for the purpose of increasing strength in some cases. However, a large amount of copper may lead to the risk of forming an undesired intermetallic compound phase and should be avoided. Here too, it should be noted that even when not intentionally added, copper may still be present as an impurity as a result of the raw materials used to produce the austenitic stainless steel. Appropriately, the copper content is 0.25% or less.

[0056] Boron (B): Optionally 0.005% or less Austenitic stainless steel does not necessarily contain boron, but boron can be added to improve hot workability. Boron can also be added as a grain refiner, thereby increasing the strength of the austenitic stainless steel. However, an overly high content should be avoided so as not to adversely affect hot workability. When added, boron can be present in an amount of 50 ppm or less, preferably 30 ppm or less.

[0057] Aluminum (Al): Optionally 0.25% or less Austenitic stainless steel does not necessarily contain aluminum, but aluminum can be added as a deoxidizer during the production of the steel. When added, aluminum can be present in an amount of 0.25% or less.

[0058] Calcium (Ca), magnesium (Mg), or rare earth metals (REM) Austenitic stainless steel does not necessarily contain Ca, Mg, or REM. Nevertheless, any of Ca, Mg, or REM can be added to improve the hot workability of the materials during the manufacturing process. The calcium content is preferably at most 0.05%, suitably 0.01% or less. The Mg content can suitably be at most 0.05%. The REM content can suitably be at most 0.5%.

[0059] Normally occurring impurities In the present disclosure, normally occurring impurities are considered to be those arising from the manufacturing process and / or the raw materials used. In this specification, normally occurring impurities are intended to mean both impurities and trace amounts of elements. Generally, austenitic stainless steel does not contain normally occurring impurities in a total amount exceeding about 1.5 wt%, and typically contains a maximum of about 1 wt% in total.

[0060] An example of an impurity that usually occurs is phosphorus (P). If the phosphorus content is too high, it can, for example, have an adverse effect on hot workability and toughness. Therefore, the phosphorus content is preferably 0.050% or less, more preferably 0.030% or less.

[0061] Another example of an impurity that usually occurs is sulfur (S). If the sulfur content is too high, it can, for example, deteriorate hot workability and / or toughness, and also weldability. Therefore, the sulfur content is preferably 0.005% or less, more preferably 0.003% or less.

[0062] Another examples of impurities that usually occur are tin (Sn), arsenic (As), lead (Pb), bismuth (Bi), and titanium (Ti).

[0063] Manufacturing process Previously, or hereinafter described austenitic stainless steels can be cast, hot-worked, and cold-worked according to conventional methods. In the case of strip products, this means that austenitic stainless steels can be cast, hot-rolled, and then cold-rolled according to conventional methods.

[0064] However, in order to achieve a desired balance among properties such as tensile strength, hydrogen embrittlement, toughness, and weldability, it is desirable that the austenitic stainless steel be in a solution-treated state. Therefore, the austenitic stainless steel is preferably solution-treated after being cold-worked to the intended final dimensions.

[0065] The Z phase will already precipitate during the casting and hot working of austenitic stainless steels, which leads to the strengthening of austenitic stainless steels. Subsequent solution treatment of austenitic stainless steels leads to a fine structure substantially free of nitrides and carbides.

[0066] As will be apparent to those skilled in the art, the temperature at which potential nitrides and carbides (including carbonitrides) dissolve depends on the specific composition of the austenitic stainless steel. However, the solution treatment can typically be carried out by heat-treating the austenitic stainless steel at a temperature above 1000°C. The solution treatment can be carried out at a temperature of 1050°C or higher to ensure that nitrides or carbides (including carbonitrides) formed during casting dissolve and thereby nitrogen is available for the formation of the Z-phase in sufficient amounts. The solution treatment can be carried out at a temperature of 1080°C or higher. Increasing the temperature of the solution treatment step is considered to improve the impact toughness in addition to ensuring that there are no carbides and nitrides in the austenitic stainless steel. The solution treatment is suitably carried out at 1200°C or lower, for example 1150°C or lower, for the purpose of suppressing excessive grain growth and reduction of the tensile strength. However, higher temperatures are also possible if the duration of the solution treatment step is relatively short.

[0067] The duration of the solution treatment step varies depending on the dimensions of the austenitic stainless steel. The appropriate duration of the solution treatment is usually 1 to 5 minutes, for example 1 to 2 minutes, per millimeter of thickness / diameter of the cold-worked product, although the present disclosure is not limited thereto. Here, it should also be noted that the appropriate maximum temperature of the solution treatment step depends on the duration of the solution treatment. For example, when a strip product with a thickness of less than 10 mm is solution-treated at about 1200°C for a relatively long duration of about 1 hour, the tensile strength can decrease on the order of 50 to 100 MPa compared to the maximum tensile strength obtained by this method.

[0068] To avoid the formation of intermetallic compound phases, the product should be cooled sufficiently quickly from the solution treatment temperature. The product can be suitably cooled from the solution treatment temperature, for example by water quenching, or by forced gas or air cooling.

[0069] According to one exemplary embodiment for manufacturing a strip product of the austenitic stainless steel described herein, the method comprises Casting a melt having the above composition to obtain a casting material, Hot rolling the casting material to obtain a hot-rolled intermediate product, Cold rolling the hot-rolled intermediate product to the intended final thickness for the strip product, and Solution-treating the cold-rolled product at a temperature above 1000 °C, preferably at a temperature of 1050 °C or higher (more preferably at a temperature of 1080 °C or higher), are included.

[0070] Experimental tests Fourteen different alloys (chemical compositions defined in Table 1) were produced and tested. The balance of the melt was Fe and unavoidable impurities. The alloy melts were produced on a laboratory scale in an HF furnace and cast into ingots of approximately 15 kg each. These ingots were hot rolled at a temperature of approximately 1220 °C and then cold rolled to a thickness of approximately 8 mm or approximately 16 mm at approximately room temperature (the thicker material was used for the impact test and all other tests were performed on the material with a thickness of approximately 8 mm). Since there are limitations on the force of the laboratory-scale rolling mill used for cold rolling, an intermediate solution treatment at approximately 1100 °C was performed between two consecutive cold rolling steps to overcome the strain hardening caused by cold rolling. However, such an intermediate solution treatment during cold rolling is not expected to be necessary in full-scale production and is considered not to affect the resulting properties of the austenitic stainless steel.

[0071] Thereafter, the specimens numbered 1 to 9 were solution-treated at 1065 °C and then water quenched. For the specimens numbered 10 to 14, solution treatment at 1100 °C or solution treatment at 965 °C was performed and then water quenched.

[0072] Various tests were performed on the obtained strip specimens as described below.

[0073] Tensile test Tensile strength (R m) And elongation (A) were determined by a tensile test (room temperature) conforming to SS-EN ISO 6892-1. The samples were longitudinal with respect to the rolling direction. The test was carried out on a material with a thickness of approximately 8 mm.

[0074] Impact toughness Impact toughness (Charpy V-notch) was tested in the sample direction following the rolling direction in accordance with S-EN ISO 148-1. Test pieces of 10×10 mm obtained from a material with a thickness of approximately 16 mm were used.

[0075] Grain size number The grain size number was determined in accordance with ASTM E112.

[0076] Characterization of the microstructure The microstructure was evaluated by combining a scanning electron microscope (SEM) with electron backscatter diffraction (EBSD). The number of Z-phase particles per 1 μm 2 was determined by image analysis performed on an image obtained in the BSE mode at a magnification of 10,000 times with a SEM. For all the heated bodies, the number of particles in the size range of 0.05 - 1 μm was determined, and for some of the heated bodies, the number of particles larger than 1 μm was also determined.

[0077] The results obtained from the above tests are shown in Table 2. From these results, it can be seen that for the heated bodies No. 6, 10, 11, 12, and 13, a very high tensile strength has been achieved in the solution-treated state, and all of them have a tensile strength exceeding 935 MPa. All of these are [wt% of Mo] + [wt% of W] ≥ 3 and [wt% of Nb] + 2.5×[wt% of N] > 1.50 heated bodies that satisfy both criteria. Furthermore, all of the heated bodies No. 6, 10, 11, 12, and 13 [wt% of Nb] + 2.5×[wt% of N] ≥ 1.55 satisfy the criterion.

[0078] Figure 1 shows the tensile strength of the solution-treated samples as a function of [[Nb wt%]] + 2.5 × [[N wt%]]. Generally, it can be seen that the tensile strength increases as the amount of [[Nb wt%]] + 2.5 × [[N wt%]] increases. This indicates the contribution of the Z phase to the high tensile strength of the austenitic stainless steels described herein.

[0079] Figure 2 shows the combined effect of [[Mo wt%]] and [[W wt%]], and also [[Nb wt%]] + 2.5 × [[N wt%]] on the tensile strength of the solution-treated samples. From this figure, it can be clearly seen that very high tensile strength for the solution-treated samples is achieved when both the criteria of [[Mo wt%]] + [[W wt%]] ≥ 3 and [[Nb wt%]] + 2.5 × [[N wt%]] > 1.50 are satisfied. In this figure, the heated body numbers 6, 10, 11, 12, and 13 are circled.

[0080] Furthermore, from the test results shown in Table 2, it can be seen that treatment at 965 °C leads to a higher tensile strength compared to solution treatment at 1100 °C. However, when the samples are treated at 965 °C, the impact toughness becomes considerably lower. The decrease in impact toughness is considered to be the result of chromium nitride precipitation during treatment at 965 °C. In contrast, when solution treatment is carried out at 1100 °C, the temperature is too high for chromium nitride to precipitate.

[0081] Figure 3 shows an SEM image of the sample of the heated object No. 12 when processed at 965°C, and Figure 4 shows an SEM image of the sample of the heated object No. 12 when solution-treated at 1100°C. The images shown in Figures 3 and 4 were taken at the same magnification. The image shown in Figure 3 clearly shows the presence of chromium nitride (dark particles indicated by arrow A) and the presence of Z-phase particles (indicated by arrow B). The chi-phase exists at the grain boundaries and the chi-phase appears as small white particles indicated by arrow C. In contrast, the image shown in Figure 4 represents the solution-treated sample and the presence of nitrides or other intermetallic compound phases is not shown. However, the Z-phase (bright particles indicated by arrow D) is present.

[0082] Evaluation of the microstructure indicates that none of the samples subjected to the solution treatment substantially contain nitrides and intermetallic compound phases. Therefore, it can be concluded that the austenitic stainless steel according to the present disclosure has good weldability when solution-treated as described above. In contrast, intermetallic phases were present in each of the samples processed at 965°C. More specifically, after the treatment at 965°C, the heated objects No. 10 and 11 contained the sigma phase, and the heated objects No. 12 to 14 contained the chi phase.

[0083] From Table 2, it can be further seen that the specimens numbered 1 to 9 of the heated body have lower impact toughness than those numbered 10 to 14 of the heated body in the solution-treated state. This is considered to be due to the lower temperature during the solution treatment (1065 °C for the specimens numbered 1 to 9 of the heated body, while 1100 °C for the specimens numbered 10 to 14 of the heated body). Thus, it is predicted that solution treatment at a higher temperature will lead to higher impact toughness even for the specimens numbered 1 to 9 of the heated body. For the purpose of confirming this, the specimen numbered 6 of the heated body was also solution-treated at 1100 °C, and then the Charpy-V test was conducted, showing a result of 42.5 J. Here, it should be noted that the Charpy-V test was conducted as described above, except that the specimen direction was perpendicular to the rolling direction. In the specimen direction perpendicular to the rolling direction, the impact toughness can usually be lower compared to the case of the specimen direction following the rolling direction (see the results shown in Table 2).

[0084] For the solution-treated specimens numbered 6 and 10 to 13 of the heated body, the number of particles with a size of 50 to 1000 nm was larger compared to the reference specimens numbered 1 to 2 of the heated body. The larger the number of particles, the more the mechanical strength of the material is improved.

[0085] Hydrogen embrittlement resistance The hydrogen embrittlement resistance was tested according to two different methods as described below. The results are shown in Table 3.

[0086] To evaluate the hydrogen embrittlement resistance of the austenitic stainless steels described in this specification, two different 316L compositions (referred to as 316L-1 and 316L-2) were also tested with different Ni contents. 316L-2 is a composition known in this field to have excellent resistance to hydrogen embrittlement and is used as a component for hydrogen storage. The compositions of 316L-1 and 316L-2 are shown in Table 1. The specimens of 316L-1 and 316L-2 were obtained from commercially produced bar materials. Rolling billets were produced from the bars and made 7 mm thick by hot rolling and cold rolling. Then, the cold-rolled materials of 316L-1 and 316L-2 were solution-treated at 1100 °C and then water quenched.

[0087] The hydrogen embrittlement resistance of the heated body No. 6, as well as the heated bodies No. 2, 316L-1, and 316L-2, was tested with H2 at -40°C and 87.5 MPa during a slow strain rate test (SSRT). Using a strain rate of 5×10 -5 s -1 , the test was conducted by clamping the test piece in a container equipped with a cooling block and cooling it with liquid nitrogen. The test was carried out in accordance with ASMT G-129. The reduction in area at fracture was measured and compared with the reduction in area at fracture in an inert test at -40°C. From the results, it can be seen that the heated body No. 6 has higher hydrogen embrittlement resistance than either of the heated bodies No. 2 and 316L-1. Furthermore, the hydrogen embrittlement resistance of the heated body No. 6 is equivalent to that of 316L-2.

[0088] Furthermore, the hydrogen embrittlement resistance of the heated body No. 6, the heated body No. 12, 316L-1, and 316L-2 was evaluated by a slow strain rate test (SSRT) with electrochemical charging. The SSRT with electrochemical charging was carried out by applying a cathodic current density of 5 mA / cm 2 in 0.5 M H2SO4 purged with N2 at 4°C. The reduction in area at fracture was measured and compared with an inert test conducted in distilled water purged with air at 4°C. The strain rate was 1×10 -5 s -1 .

[0089] From these results, it can be seen that both the heated body No. 6 and the heated body No. 12 have resistance to hydrogen embrittlement, are much higher than the hydrogen embrittlement resistance of 316L-1, and are equivalent to the hydrogen embrittlement resistance of 316L-2. As described above, 316L-2 is an austenitic stainless steel known for its excellent hydrogen embrittlement resistance. However, neither 316L-1 nor 316L-2 has the mechanical properties of the austenitic stainless steel according to the present disclosure. For example, when manufactured as described above, the tensile strength of 316L-1 is 591 MPa, and the tensile strength of 316-2 is 566 MPa (tested at room temperature in accordance with S-EN ISO 6892-1).

[0090] Furthermore, according to the results of the hydrogen embrittlement test of the austenitic stainless steel described in this specification, it has been shown that the area reduction when hydrogen is filled is the same as in an inert environment. The ratio of the specimen filled with hydrogen to the specimen in an inert environment is close to 1. Usually, a ratio of 0.90 or more is considered to show no influence of hydrogen embrittlement. No brittle fracture or secondary cracks were observed in the tested samples.

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Claims

1. The following composition in weight percent (wt%): C 0.06 or less, Si 0.1 to 1, Mn 3-5, Cr 20.5-23.5, Ni 11-15, Mo 1-4, Nb 0.50-0.70, N 0.40-0.60, P 0.050 or less, S 0.005 or less, Selectively choose W 3 or less, Optionally, Co 0.50 or less, Optionally, V 0.30 or less, Optionally, Cu 0.30 or less, Optionally, B 0.005 or less, Selectively, Al 0.25 or less, Optionally, either Ca or Mg is 0.05 or less, or REM is 0.5 or less. The remainder consists of Fe and normally occurring impurities. an austenitic stainless steel having the composition, [Weight percentage of Mo] + 2 × [Weight percentage of W] ≥ 3 Austenitic stainless steel that meets the following criteria.

2. The aforementioned composition is [Weight percentage of Mo] + [Weight percentage of W] ≥ 3.0 It meets the following criteria: The austenitic stainless steel according to claim 1.

3. The aforementioned composition is [Weight % of Nb] + 2.5 × [Weight % of N] ≥ 1.55 An austenitic stainless steel according to claim 1 that satisfies the following criteria.

4. The austenitic stainless steel according to claim 1, wherein the composition contains more than 0.50% by weight of Nb, preferably 0.51 to 0.65% by weight of Nb.

5. The austenitic stainless steel according to claim 1, wherein the composition contains 0.40 to 0.55% by weight of N, preferably 0.42 to 0.50% by weight of N.

6. The austenitic stainless steel according to claim 1, wherein the composition contains 0.1 to 0.6% by weight of Si, preferably 0.20 to 0.45% by weight of Si.

7. The austenitic stainless steel according to claim 1, wherein the composition contains 3.2 to 4.6% by weight of Mn, preferably 3.5 to 4.4% by weight of Mn.

8. The austenitic stainless steel according to claim 1, wherein the composition contains 21 to 23% by weight of Cr, preferably 21.5 to 22.8% by weight of Cr.

9. The austenitic stainless steel according to claim 1, wherein the composition contains 12 to 14% by weight of Ni, preferably 12 to 13.5% by weight of Ni.

10. The austenitic stainless steel according to claim 1, wherein the composition contains 1.2 to 3.2% by weight of Mo, preferably 1.2 to 2.0% by weight of Mo.

11. The austenitic stainless steel according to claim 1, wherein the composition contains at least 1% by weight of W, preferably 1.4 to 2.5% by weight of W.

12. The austenitic stainless steel according to claim 1, wherein the composition contains 0.25% by weight or less of Cu.

13. The impurities P and S that normally occur are P up to 0.030% by weight, and S Maximum 0.003% by weight The austenitic stainless steel according to claim 1, limited to the following.

14. The austenitic stainless steel according to claim 1, wherein, in a solution-treated state, it has a tensile strength of at least 930 MPa when tested at room temperature according to SS-EN ISO 6892-1.

15. The austenitic stainless steel according to claim 1, wherein the steel is Z-phase strengthened.

16. A method for producing an austenitic stainless steel product according to any one of claims 1 to 15, To obtain a casting material by casting a molten material having the above composition, The aforementioned cast material is subjected to hot working to produce an intermediate product. The aforementioned intermediate product is cold-worked to achieve the intended final thickness or intended final diameter of the product, and The process of solution-treating cold-worked products at temperatures exceeding 1000°C. Methods that include...

17. The method according to claim 16, wherein the solution treatment is carried out at a temperature of 1050°C or higher, preferably 1080°C or higher.

18. Use of the austenitic stainless steel according to any one of claims 1 to 15 for constructing components adapted to contain hydrogen gas and / or liquid hydrogen, and / or to be exposed to an environment containing hydrogen gas and / or liquid hydrogen, or an environment consisting of hydrogen gas and / or liquid hydrogen.

19. A component adapted to contain hydrogen gas and / or liquid hydrogen, and / or to be exposed to hydrogen gas and / or liquid hydrogen, which is constructed from the austenitic stainless steel described in any one of claims 1 to 15.

20. The aforementioned components are as follows: - A container configured for storing pressurized hydrogen gas, hydrogen in pure liquid form, or hydrogen in cryogenically compressed form. Valves for configurations for storing or transporting pressurized hydrogen gas, pure liquid hydrogen, or cryogenically compressed hydrogen. - Conduits or pipes for configurations for storing or transporting pressurized hydrogen gas, pure liquid hydrogen, or cryogenically compressed hydrogen, and - Machined parts for configurations for storing or transporting pressurized hydrogen gas, pure liquid hydrogen, or cryogenically compressed hydrogen. A component according to claim 19, selected from the group including the following.

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