Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/26—Methods of annealing
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D6/00—Heat treatment of ferrous alloys
  • C21D6/004—Heat treatment of ferrous alloys containing Cr and Ni
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0226—Hot rolling
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0236—Cold rolling
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
  • C21D8/0273—Final recrystallisation annealing
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/52—Ferrous alloys, e.g. steel alloys containing chromium with nickel with cobalt

Definitions

• the present disclosure relates in general to an austenitic stainless steel. More specifically, the present disclosure relates in general to an austenitic stainless steel suitable for use in hydrogen storage applications. The present disclosure also relates in general to a method for producing a product of the austenitic stainless steel. BACKGROUND
• the efforts made within the automotive industry to reduce harmful emissions has led to an increased interest in development of vehicles which are, at least in part, powered by hydrogen as fuel.
• a fuel cell vehicle In addition to requiring storage of hydrogen onboard such vehicles, this also means that hydrogen needs to be stored at for example hydrogen fueling stations.
• Hydrogen may be stored physically as either a gas or a liquid. Storage of hydrogen as a gas typically requires high-pressure tanks, and may be achieved at ambient temperatures. In the case of liquid form, hydrogen may be stored in pure liquid form or in cryo-compressed form, wherein both alternatives require storage at very low temperatures. Hydrogen may alternatively be stored by usage of solids, either by adsorption of hydrogen on surfaces of the solid or by absorption of hydrogen in such a solid. However, storage of hydrogen by usage of solids increases the weight of the storage and thus the vehicle, and/or may increase the necessary size of the storage container, and is therefore often not desirable. Components to be used in conjunction 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 stainless steel’s resistance to hydrogen embrittlement.
• Hydrogen embrittlement is a phenomenon that may occur when a steel is exposed to hydrogen diffusing into the steel in combination with stresses in the steel. Hydrogen embrittlement may lead to loss of ductility and/or toughness and a reduction in load bearing capability of the steel, even at stress levels below the yield strength of the steel. Furthermore, hydrogen embrittlement may lead to sudden development of cracks without any warning, and may thus lead to catastrophic failure. It is well known that ferritic and duplex stainless steels are susceptible to hydrogen embrittlement.
• martensitic stainless steels also have a relatively low resistance to hydrogen embrittlement. Therefore, it appears that austenitic stainless steels are the most promising candidates.
• austenitic stainless steels In addition to high resistance to hydrogen embrittlement, it is also desired that the austenitic stainless steel has a high tensile strength in order to resist high pressures. Moreover, higher tensile strengths may enable reduced thickness of the steel and thereby also a reduction of weight of the components, which is important within the automotive industry.
• the austenitic stainless steel needs to have sufficient properties to be used within the temperature range of from -50 °C to + 85°C in order to be a suitable option.
• An austenitic stainless steel used in these types of applications today is SS 316L because of its resistance to hydrogen embrittlement.
• SS 316L has a relatively low tensile strength and may therefore not always be a suitable option.
• EP 1605073 A1 discloses another example of an austenitic stainless steel intended to be used in high- pressure hydrogen gas environments.
• US 2017/314092 A1 discloses an austenitic stainless steel described to have high strength as well as good hydrogen embrittlement resistance.
• SUMMARY The object of the present invention is to provide an austenitic stainless steel with high tensile strength and good hydrogen embrittlement resistance such that it is suitable for hydrogen storage applications. The object is achieved by the subject-matter of the appended independent claim(s).
• an austenitic stainless steel has the following composition, in percent by weight (wt.-%): C equal to or less than 0.06, 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 equal to or less than 0.050, S equal to or less than 0.005, optionally W equal to or less than 3, optionally Co equal to or less than 0.50, optionally V equal to or less than 0.30, optionally Cu equal to or less than 0.30, optionally B equal to or less than 0.005, optionally Al equal to or less than 0.25, optionally one of Ca or Mg equal to or less than 0.05 or REM equal to or less than 0.5 balance consisting of Fe and normally occurring impurities; wherein the composition fulfils the criterion [wt.-% Mo]+2*[wt.-% W] ⁇ 3.
• the austenitic stainless steel according to the present disclosure has a high tensile strength in combination with an excellent hydrogen embrittlement resistance. It is furthermore possible to obtain very good toughness and weldability. This makes this austenitic stainless steel an excellent candidate for use in components configured to contain and/or be exposed to hydrogen gas and/or liquid hydrogen.
• the composition of the austenitic stainless steel fulfils the criterion [wt.-% Mo] + [wt.-% W] ⁇ 3.0. This further contributes to a high tensile strength of the austenitic stainless steel.
• the composition of the austenitic stainless steel as described above means that the sum of the niobium content and 2.5 times the nitrogen content (i.e.
• the composition of the austenitic stainless steel may suitably comprise niobium and nitrogen in such amounts that the criterion [wt.-% Nb] + 2.5 *[wt.-% N] ⁇ 1.55 is fulfilled. Thereby, the tensile strength may be further increased.
• the austenitic stainless steel may suitably be in a solution annealed condition. When in a solution annealed condition, the austenitic stainless steel according to the present disclosure may have a tensile strength of at least 930 MPa.
• the austenitic stainless steel according to the present disclosure is a Z-phase strengthened stainless steel.
• the present disclosure further provides a method for producing a product of the austenitic stainless steel as described above.
• the method comprises: casting a melt having the composition to obtain a cast material, hot working the cast material to an intermediate product, cold working the intermediate product to intended final thickness, or intended final diameter, of the product, and solution annealing the cold worked product at a temperature above 1000 ⁇ C.
• the solution annealing of the cold worked product is performed at a temperature equal to or above 1050 ⁇ C, preferably equal to or above 1080 ⁇ C.
• the present disclosure also relates to the use of the above described austenitic stainless steel for constructing a component adapted to contain and/or be exposed to an environment comprising, or consisting of, hydrogen gas and/or liquid hydrogen.
• the component may for example be a container configured for storing pressurized hydrogen gas, a valve in an arrangement for storing or transporting pressurized hydrogen gas, or another component of an arrangement for storing or transporting pressurized hydrogen gas (for example a conduit, a pipe, a tube or a machined part).
• the component may be a container configured for storage of hydrogen in pure liquid or cryo-compressed form, or a component of an arrangement for storing or transporting of hydrogen in liquid or cryo-compressed form.
• the present disclosure also relates to a component adapted to contain and/or be exposed to hydrogen gas and/or liquid hydrogen, in particular pressurized hydrogen gas, said component being constructed of the austenitic stainless steel described hereinabove or hereinafter.
• Fig.1 illustrate tensile strength, as a function of [wt.-% Nb]+2.5*[wt.-% N], of solution annealed samples according to experimental results
• Fig.2 illustrates the effect of the sum of [wt.-% Mo] and [wt.-% W] as well as [wt.-% Nb]+2.5*[wt.-% N] on tensile strength of solution annealed samples according to experimental results
• Fig.3 shows a SEM image of a sample of an austenitic stainless steel according to the present disclosure when annealed at 965 °C
• Fig.4 shows a SEM image of a sample of the same austenitic stainless steel as shown in Figure
• the austenitic stainless steel may also be used in other forms, such as bars, tubes, or forgings, if desired. Such bars may be extruded bars or rolled bars. Similarly, such tubes may be extruded tubes or rolled tubes. Manufacturing of for example storage containers from stainless steel strips requires welding. It is therefore important that the present austenitic stainless steel has a good weldability. This may be achieved if the austenitic stainless steel is in solution annealed condition. Therefore, the present austenitic stainless steel has been developed with the aim to obtain a high strength and a good hydrogen embrittlement resistance when being in solution annealed condition.
• the austenitic stainless steel has a high impact toughness in order to avoid that the material cracks if subjected to sudden impact, such as in a collision. Furthermore, the austenitic stainless steel should be able to retain a good balance between the above-mentioned properties within the temperature range of -50 °C to at least +85 °C to make it a truly appropriate candidate for components associated with storage of pressurized hydrogen gas.
• the austenitic stainless steel according to the present disclosure is primarily developed for use in products associated with storage of high-pressure hydrogen gas at ambient temperatures (here meaning temperatures in the range of -50 °C to at least +85 °C), the austenitic stainless steel is also suitable for use in cryo-applications.
• the austenitic stainless steel as described herein may also be used in a component (such as a tank, a pipe, a tube or a valve) of an arrangement for storage of hydrogen in pure liquid form or in cryo-compressed form.
• a component such as a tank, a pipe, a tube or a valve
• an austenitic stainless steel capable of forming thermodynamically stable Z-phase is provided.
• Z-phase may be described as a stochiometric phase of niobium, chromium and nitrogen.
• vanadium may also be enriched in the Z-phase.
• an austenitic stainless steel which may easily be welded and which, in a solution annealed condition, has the following properties: - tensile strength of at least 930 MPa, suitably at least 935 MPa, when tested according to SS- EN ISO 6892-1 at room temperature, - impact toughness (Charpy-V) at -50 °C of at least 25 J, suitably above 40 J, when tested according to SS-EN ISO 148-1, and - a resistance to hydrogen embrittlement, represented by a ratio of reduction in area between hydrogen charged and inert environment, of equal to or above 0.90, evaluated through Slow Strain Rate Testing (SSRT), at a temperature of 4°C and with a strain rate of 1 x 10 -5 s -1 , with electrochemical charging, wherein electrochemical charging is performed in 0.5 M H 2 SO 4 purged with N 2 with a cathodic current density of 5 mA/cm 2 applied and in
• the austenitic stainless steel comprises at least 0.1% Si.
• silicon is present in an amount of at least 0.2 %.
• excessive contents of silicon may lead to precipitation of unwanted intermetallic phases, such as sigma phase. Presence of sigma phase may for example lead to a reduction of hot workability and should therefore be avoided.
• the austenitic stainless steel comprises at most 1 % Si.
• silicon is present in an amount of equal to or less than 0.6 %. More preferably, silicon is present in an amount of equal to or less than 0.45 %.
• Manganese is an austenite stabilizing element and reduces the risk of forming ferrite that can form intermetallic phases after cooling. Manganese contributes to increased strength as well as improved ductility and toughness. Moreover, manganese is a less expensive alloying element than nickel. An increase of manganese also contributes to an increase in solubility of nitrogen in austenitic stainless steels and may thereby contribute to an increased strength. Therefore, the austenitic stainless steel according to the present disclosure comprises equal to or more than 3 % Mn.
• manganese is present in an amount of equal to or more than 3.2 %. In the present austenitic stainless steel, manganese may be present in an amount of equal to or more than 3.5 %. Too high contents of manganese may in some cases have a negative impact on the weldability of the austenitic stainless steel and/or the corrosion resistance. Furthermore, a manganese content of above 5 % is not needed and the present austenitic stainless steel therefore comprises equal to or less than 5 % manganese. In the present austenitic stainless steel, manganese may be present in an amount of equal to or less than 4.6 %, or equal to or less than 4.4 %.
• Chromium is an element added to provide sufficient corrosion resistance in austenitic stainless steels. Furthermore, chromium contributes to the possibility of adding the desired amount of nitrogen to the austenitic stainless steel as it increases the solubility of nitrogen. Furthermore, chromium contributes to the formation of Z-phase and should therefore be present in a relatively high amount. Therefore, the austenitic stainless steel comprises at least 20.5 % Cr.
• the austenitic stainless steel of the present disclosure may comprise at least 21 % Cr, such as at least 21.5 % Cr.
• too high contents of chromium increase the risk for formation of unwanted intermetallic phases which may lead to a decrease of ductility and/or toughness.
• chromium is present in an amount of at most 23.5 %.
• the upper limit for the chromium content may be 23 %.
• Chromium may according to one alternative be present in an amount of at most 22.8 %.
• Nickel (Ni): 11 – 15 % Nickel improves the austenite stability and results in low contents of ferrite. Ferrite may form intermetallic phases during cooling which reduces impact toughness and hydrogen embrittlement resistance and is therefore not desirable. Nickel also contributes to increased ductility. Therefore, the austenitic stainless steel comprises at least or equal to 11 % nickel.
• nickel is present in an amount of equal to or more than 12 %, or in an amount of equal to or more than 12.5 %.
• nickel is a relatively expensive alloying element and additions above 15 % are not needed in the present austenitic stainless steel. Too high contents of nickel may also reduce the tensile strength. Therefore, nickel is present in an amount of equal to or less than 15 %. Suitably, nickel may be present in an amount of equal to or less than 14 %, such as in an amount of equal to or less than 13.5 %.
• the present austenitic stainless steel comprises equal to or more than 1 % Mo.
• molybdenum is present in an amount of equal to or more than 1.2 %.
• the austenitic stainless steel needs to comprise enough solution strengthening elements to obtain sufficient strength. Therefore, the composition at least needs to fulfil the criterion of [wt.-% Mo]+2*[wt.-% W] ⁇ 3.
• molybdenum is present in an amount of at least 3 %. Too high contents of molybdenum may however lead to a decrease in ductility and should therefore be avoided. Therefore, molybdenum is present in an amount of equal to or less than 4 %. Suitably, molybdenum is present in an amount of equal to or less than 3.2 %. In case tungsten is present in a sufficient amount to fulfil the above criterion, molybdenum may be present in an amount of equal to or less than 2.0 %.
• Niobium is an essential element to the austenitic stainless steel according to the present disclosure as it contributes to formation of Z-phase and thereby the strength.
• niobium is present in an amount of at least 0.50 %, preferably in an amount of above 0.50 %.
• Such amounts of niobium are considerably higher than what is added to conventional austenitic stainless steels for the purpose of forming carbides and/or nitrides (including carbonitrides).
• niobium is added for forming Z- phase, thus niobium is not added for the purpose of forming nitrides and/or carbides.
• niobium is present in an amount of equal to or above 0.51 %.
• the presence of niobium and nitrogen in the austenitic stainless steel according to the present disclosure means that the austenitic stainless steel fulfills the criterion of [wt.-% Nb]+2.5*[wt.-% N] being equal to or above 1.5. It has been found that this criterion is important for achieving the high tensile strength since it reflects the contribution of the formed Z-phase based on the stochiometric composition thereof.
• the niobium and nitrogen contents are selected such that [wt.-% Nb]+2.5*[wt.-% N] is equal to or higher than 1.55.
• niobium is present in an amount of equal to or less than 0.70 %.
• niobium is present in an amount of equal to or less than 0.65 %.
• Nitrogen may also contribute to solution strengthening. Therefore, nitrogen is present in an amount of equal to or more than 0.40 %. Suitably, nitrogen is present in an amount of equal to or more than 0.42 %. As mentioned above with regard to niobium, it is preferred that the austenitic stainless steel comprises nitrogen in an amount such that the criterion [wt.-% Nb]+2.5*[wt.-% N] ⁇ 1.55 is fulfilled. However, too high amounts of nitrogen may lead to formation of chromium nitrides, which reduces the ductility, which will cause an increase in the deformation hardening and thus an increase in needed forces during cold working. The latter may result in a need for intermediate annealing during cold working, which may increase the production costs.
• nitrogen is present in an amount of equal to or less than 0.60 %.
• the nitrogen content may be present in an amount of equal to or less than 0.55 %, more preferably equal to or less than 0.50 %.
• Tungsten is not a strictly necessary alloying element in the austenitic stainless steel according to the present disclosure but is preferably added. As described above, tungsten is a solution strengthening element and may therefore be added for the purpose of increasing the strength of the present austenitic stainless steel.
• tungsten has a similar contribution to the strength of the steel when used as a replacement of molybdenum as described above, tungsten has the advantage of resulting in a lower risk of formation of sigma phase compared to molybdenum.
• a reduced amount of sigma phase leads to more atoms available for solution strengthening of the austenitic stainless steel and is therefore advantageous. Therefore, additions of tungsten may lead to easier process control during manufacture of the present austenitic stainless steel compared to if only molybdenum is present.
• the austenitic stainless steel needs to comprise a sufficient amount of solution strengthening elements as it has been found that if the total sum of the molybdenum content and the tungsten content is equal to or above 3.0, very high tensile strength may be achieved. Therefore, according to embodiments, the austenitic stainless steel fulfills the criterion [wt.-% Mo] + [wt.-% W] ⁇ 3.0.
• the austenitic stainless steel comprises equal to or above 1 % of W.
• the austenitic stainless steel comprises at least 1.4 % tungsten. However, too high amounts of tungsten may negatively affect the properties of the present steel and should therefore be avoided.
• the austenitic stainless steel comprises at most 3 % W.
• tungsten is present in an amount of equal to or less than 2.5 %.
• Cobalt (Co) optionally equal to or less than 0.50 %
• the present austenitic stainless steel need not necessarily comprise cobalt, but cobalt may be added if desired.
• Cobalt is an austenite stabilizing element and may contribute to an increase in strength. In order to not unduly increase the alloying costs, cobalt may be added in amounts of equal to or less than 0.50 %.
• cobalt is present in an amount of equal to or less than 0.30 %, or equal to or less than 0.20 %.
• Vanadium (V) optionally equal to or less than 0.30 %
• the austenitic stainless steel need not to comprise vanadium.
• vanadium may be added in an amount of equal to or less than 0.30 %, if desired.
• Vanadium may be enriched in Z-phase and therefore contribute to increased strength. According to one option, vanadium is present in an amount of equal to or less than 0.10 %.
• Copper (Cu) optionally equal to or less than 0.30 %
• the austenitic stainless steel does not need to comprise copper, but additions of equal to or less than 0.30 % may be allowed without negatively affecting the properties. Copper is an austenite stabilizing element and may in some cases be added for the purpose of increasing strength. However, higher amounts of copper may lead to a risk for formation of unwanted intermetallic phases and should therefore be avoided. It should here also be noted that, even if not purposively added, copper may still be present as an impurity as a result of the raw material used for producing the austenitic stainless steel. Suitably, the copper content is equal to or lower than 0.25 %.
• the austenitic stainless steel does not need to comprise boron, but boron may be added to improve the hot ductility. It can also be added as a grain refiner and thereby increasing the strength of the austenitic stainless steel. However, too high contents should be avoided so as to not adversely affect hot workability. If added, boron may be present in amounts equal to or less than 50 ppm, preferably equal to or less than 30 ppm. Aluminium (Al): optionally equal to or less than 0.25 % The austenitic stainless steel does not need to comprise aluminum, but aluminium may be added as a deoxidation agent during steel production.
• calcium is at most 0.05%, suitably equal to or less than 0.01%.
• the content of Mg may suitably be at most 0.05%.
• the content of REM may suitably be at most 0.5 %.
• Normally occurring impurities is herein intended to mean both impurities and trace elements.
• austenitic stainless steels do not comprise more than about 1.5 wt.-% in total, usually at most about 1 wt.-% in total, of normally occurring impurities.
• a normally occurring impurity is phosphorus (P). Too high contents of phosphorus may adversely affect for example hot workability and toughness. Therefore, the phosphorus content should suitably be equal to or less than 0.050 %, preferably equal to or less than 0.030 %.
• S sulfur
• Too high contents of sulfur may for example deteriorate hot workability and/or toughness as well as weldability.
• the sulfur content should suitably be equal to or less than 0.005 %, preferably equal to or less than 0.003 %.
• Another examples of normally occurring impurities are Tin (Sn), Arsenic (As), Lead (Pb), Bismuth (Bi) and Titanium (Ti).
• the austenitic stainless steel should be in solution annealed condition. Therefore, the austenitic stainless steel is preferably solution annealed after having been cold worked to intended final dimension. Z-phase will be precipitated already during casting and hot working of the austenitic stainless steel, which will lead to strengthening of the austenitic stainless steel. The subsequent solution annealing of the austenitic stainless steel leads to a microstructure substantially free from nitrides and carbides.
• the temperature at which potential nitrides and carbides (including carbonitrides) are dissolved depends on the specific composition of the austenitic stainless steel.
• solution annealing may typically be performed by heat treating the austenitic stainless steel at a temperature above 1000 ⁇ C.
• the solution annealing may be performed at a temperature equal to or above 1050 ⁇ C to ensure that any nitrides or carbides (including carbonitrides) formed during casting are dissolved and that nitrogen thereby is available for sufficient formation of Z-phase.
• the solution annealing may be performed at a temperature equal to or above 1080 ⁇ C.
• solution annealing is performed at a temperature equal to or less than 1200 °C, such as equal to or less than 1150 °C, for the purpose of limiting excessive grain growth and reduced tensile strength. Higher temperatures are however also possible in case of relatively short duration of the solution annealing step. The duration of the solution annealing step depends on the dimension of the austenitic stainless steel.
• a suitable duration of solution annealing could typically be 1 to 5 minutes per mm of the thickness/diameter of the cold worked product, such as 1 to 2 minutes per mm of the thickness/diameter of the cold worked product, although the present disclosure is not limited thereto.
• the suitable maximum temperature of the solution annealing step is dependent of the duration of solution annealing. For example, solution annealing of a strip product having a thickness of less than 10 mm at about 1200 °C with a relatively long duration of about 1 hour may lead to a reduction of tensile strength in the order of 50-100 MPa compared to the maximum obtainable tensile strength according to the present method.
• the method comprises: - casting a melt, having the above described composition, to obtain a cast material; - hot rolling the cast material to an intermediate hot rolled product, - cold rolling the intermediate hot rolled product to intended final thickness of the strip product, and - solution annealing the cold rolled product at a temperature above 1000 °C, preferably at a temperature equal to or above 1050 °C (more preferably at a temperature equal to or above 1080 °C).
• Grain size number was determined according to ASTM E112. Microstructure characterization The microstructure was assessed by Scanning Electron Microscopy (SEM), combined with electron backscatter diffraction (EBSD). The number of Z-phase particles per ⁇ m 2 was determined by image analysis performed at pictures obtained in SEM at 10000X magnification in BSE mode. For all heats, the number of particles in the size range 0.05 to 1 ⁇ m was determined, for some heats also the number of particles larger than 1 ⁇ m were determined. The results obtained from the above described tests are presented in Table 2.
• Figure 1 illustrates the tensile strength of the solution annealed samples as a function of [wt.-% Nb]+2.5*[wt.-% N]. It can be seen that, in general, the tensile strength increases with increasing amount of [wt.-% Nb]+2.5*[wt.-% N]. This illustrates the contribution of the Z-phase to the high tensile strength of the herein described austenitic stainless steel.
• Figure 2 illustrates the effect of the sum of [wt.-% Mo] and [wt.-% W] as well as [wt.-% Nb]+2.5*[wt.- % N] on the tensile strength of the solution annealed samples.
• FIG. 3 shows a SEM image of a sample of Heat No.12 when annealed at 965 °C
• Figure 4 shows a SEM image of a sample of Heat No.12 when solution annealed at 1100 °C. The images shown in Figures 3 and 4 are taken at the same magnification.
• the image shown in Figure 3 clearly shows presence of chromium nitrides, which are the dark particles illustrated by arrow A, as well as Z-phase particles as illustrated by arrow B. Chi phase is present at the grain boundaries, said chi phase visible as small white particles illustrated by arrow C.
• the image shown in Figure 4 representing the solution annealed sample, does not show any presences of nitrides or other intermetallic phases. There is however a presence of Z-phase, which are the light particles, illustrated by arrow D.
• the assessment of the microstructure showed that all samples being subjected to solution annealing were essentially free from nitrides and intermetallic phases.
• the austenitic stainless steel according to the present disclosure has a good weldability when solution annealed as described above.
• each of the samples subjected to annealing at 965 °C had intermetallic phases present. More specifically, Heat Nos.10 and 11 contained sigma phase, and Heats Nos.12-14 contained chi phase, after annealing at 965 °C.
• Heat Nos.1-9 have a lower impact toughness than Heat Nos. 10-14 in the solution annealed condition. This is believed to be due to the lower temperature during solution annealing (1065 °C for Heats No.1-9, compared to 1100 °C for Heat Nos.10-14).
• Rolling billets were produced from the bar, and hot and cold rolled to 7 mm thickness. Thereafter, the cold rolled 316L-1 and 316L-2 materials were subjected to solution annealing at 1100 °C followed by water quenching. The resistance to hydrogen embrittlement of Heat No.6, as well as Heat No.2, 316L-1 and 316L-2, were tested during Slow Strain Rate Testing (SSRT) at -40 °C and 87.5 MPa H 2 . A strain rate of 5 x 10 -5 s -1 was used, and the testing was performed in a vessel with cooling blocks clamped on the specimens, which were cooled with liquid nitrogen. The testing was performed according to ASMT G- 129.
• Heat No.6 has a higher resistance to hydrogen embrittlement than both Heat No.2 and 316L-1. Moreover, the resistance to hydrogen embrittlement of Heat No.6 is comparable to the resistance to hydrogen embrittlement of 316L-2. Furthermore, resistance to hydrogen embrittlement of Heat No.6, Heat No.12, 316L-1 and 316L-2 were evaluated through Slow Strain Rate Testing (SSRT) with electrochemical charging. The SSRT with electrochemical charging was performed in 0.5 M H 2 SO 4 purged with N 2 at 4°C with a cathodic current density of 5 mA/cm 2 applied.
• SSRT Slow Strain Rate Testing
• 316L-1 has a tensile strength of 591 MPa and 316-2 has a tensile strength of 566 MPa (tested according to SS-EN ISO 6892-1 at room temperature) when produced as described above.
• the results of the hydrogen embrittlement tests of the herein described austenitic stainless steel show that the reduction of area when subjected to hydrogen charging is similar to in the inert environment. The ratio between the hydrogen charged specimens and the inert environment is close to 1. Normally a ratio of 0.90 or above is considered as no effect of hydrogen embrittlement being observed. No brittle failure or secondary cracks were observed in the tested samples.

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