Iron-nickel alloy, method for the production thereof, strand-shaped product obtainable thereby and use of the strand-shaped product for producing a carrier cable for high-temperature lines

Abstract

The invention relates to an iron-nickel alloy which, in addition to a low coefficient of thermal expansion, also has a high tensile strength and elongation at break, a method for the production thereof, a strand-like product obtainable by means of this method, and a use of the strand-like product for producing a carrier cable for high-temperature lines. This iron-nickel alloy contains the following alloying elements in mass fractions: 0.18%-0.30% C; optionally, less than or equal to 0.50% Si; optionally, less than or equal to 0.30% Mn; 0.01%-0.70% Cr; 36.00%-39.00% Ni; optionally, less than or equal to 0.10% Al; optionally, less than or equal to 0.50% Mo; optionally, less than or equal to 0.10% Ti; optionally, less than or equal to 0.01% N; optionally, less than or equal to 0.01% B; optionally, less than or equal to 5.0% Co; optionally, less than or equal to 0.015% P; optionally, less than or equal to 0.015% S; 0.01-0.80% Cu; 0.30-0.75% W; 0.50-0.90% V; optionally, less than or equal to 0.10% Nb; and the remainder iron and unavoidable impurities.

Description

[0001] Iron-nickel alloy, process for its production, extruded product obtainable thereby, and use of the extruded product for the manufacture of a support cable for high-temperature lines

[0002] Description

[0003] Technical field

[0004] The present disclosure relates to an iron-nickel alloy which, in addition to a low coefficient of thermal expansion, also has high tensile strength and elongation at break, a method for its production, a strand-shaped product obtainable by this method, and a use of the strand-shaped product for the production of a support cable for high-temperature lines.

[0005] Specifically, the present disclosure relates to an Invar alloy which, in addition to a low coefficient of thermal expansion, also has high tensile strength and high elongation at break and is therefore particularly suitable for the production of a support cable for high-temperature lines.

[0006] State of the art

[0007] Iron-nickel alloys with a nickel content of 35% or higher are called Invar alloys. These are characterized primarily by their very low coefficient of thermal expansion. Invar alloys specifically optimized for minimal thermal expansion achieve coefficients of thermal expansion ranging from less than 2.0 to 10⁻⁶, depending on the temperature range. -6 K' 1 .

[0008] Invar alloys are used in a wide variety of applications, such as the production of aperture masks for cathode ray tubes, diaphragm tanks for liquefied gas vessels, laser housings, and wires in precision leveling rods for geodesy. They can also be used as mold material in the production of CFRP components for the aerospace industry to prevent distortion and stress between the mold and the component due to differing thermal expansion, thus avoiding potential damage.

[0009] The present invention seeks an iron-nickel alloy that is particularly suitable for the production of a support cable for high-temperature pipelines. These pipelines must withstand temperatures of up to 200 °C continuously and, in addition to a low coefficient of thermal expansion, also exhibit very high tensile strength and sufficiently high elongation at break.

[0010] A manufacturing process for a wire-shaped iron-nickel alloy with special mechanical properties is disclosed in EP 2 279 274 B1. In the production of wire-shaped precursors from the iron-nickel alloy by cold drawing, an annealing treatment is carried out between the drawing stages.

[0011] EP 1 979 501 B1 discloses the use of a creep-resistant and low-expansion iron-nickel alloy with higher mechanical strength in CFRP mold making. High tensile strength values ​​of R can be achieved. m = 1200 MPa or more can be achieved, however, the strain values ​​are then only 2% to 3%.

[0012] EP 4 194 121 A1 discloses an iron alloy that can contain 25-42% nickel and can be used as a reinforcing steel core in an aluminum stranded cable for overhead power lines. The alloy is characterized by the presence of finely dispersed oxides in the matrix.

[0013] An Invar alloy and a wire produced from it are disclosed in JP 2022 138809 A2. In this alloy, (VCr)-based composite carbides are specifically used to improve strength. During solution annealing in the wire manufacturing process, the surface layer is decarburized, resulting in a core hardness to surface hardness ratio of the wire corresponding to the formula: core hardness + 10 > surface hardness of the wire. V00102P-WG-0019

[0014] 3 / 25

[0015] CN 1743490 A discloses Invar alloys in which W, V, and C are selectively added to ensure sufficient strength. A W / V ratio > 1.5 is established to precipitate fine carbides of the MC, M2C, or MsC type, preventing the carbides from becoming coarse particles and thus effectively increasing the strength of the low-expansion Invar alloy.

[0016] The known Invar alloys that exhibit sufficiently high tensile strength for a stranded product are generally characterized by low elongation at break. This is partly due to the fact that the size of the carbides causing the fracture exceeds 20 pm, and the grain size increases due to solution annealing during the production of the stranded product. These alloys are therefore unsuitable for applications requiring both high tensile strength and sufficiently high elongation at break.

[0017] Object of the invention

[0018] Against this background, the object of the present invention is to provide an iron-nickel alloy which has, on the one hand, the lowest possible thermal expansion but nevertheless a very high tensile strength and the highest possible elongation at break, a method for producing a strand-shaped product from the iron-nickel alloy and a strand-shaped product obtainable by this method, wherein the strand-shaped product according to the invention is usable for the production of a support cable for high-temperature lines.

[0019] Description of the invention

[0020] This task was solved by providing an iron-nickel alloy with the following alloying elements in mass proportions:

[0021] C from 0.18% to 0.30%, optional Si less than or equal to 0.50%, optional Mn less than or equal to 0.30%.

[0022] Cr from 0.01% - 0.70%

[0023] Ni from 36.00% - 39.00%, V00102P-WG-0019

[0024] 4 / 25 optional AI with less than or equal to 0.10%, optional Mo with less than or equal to 0.50%, optional Ti with less than or equal to 0.10%, optional N with less than or equal to 0.01%, optional B with less than or equal to 0.01%, optional Co with less than or equal to 5.0%, optional P with less than or equal to 0.015%, optional S with less than or equal to 0.015%

[0025] Cu from 0.01% - 0.80%

[0026] W from 0.30% - 0.75%

[0027] V of 0.50% - 0.90%, optionally Nb with less than or equal to 0.10% and remainder iron and unavoidable impurities.

[0028] The iron-nickel alloy according to the invention is characterized, firstly, by a low thermal expansion with a coefficient of thermal expansion of less than or equal to 3.5–10 -6 K' 1The properties of the inventive iron-nickel alloy are twofold: firstly, it exhibits very high tensile strength values ​​of over 1100 MPa with an elongation at break of more than 10%. Secondly, it was shown that the coefficient of thermal expansion of the inventive iron-nickel alloy remains almost constant up to well over 200 °C.

[0029] For this reason, the iron-nickel alloy according to the invention is ideally suited as a support cable for high-temperature lines. When transmitting large amounts of electricity via overhead power lines (transit lines), temperatures exceeding 200 °C can occur, which can lead to an increase in cable sag between the pylons of up to 20% compared to conventional conductors. With the iron-nickel alloy according to the invention, support cables can thus be manufactured which, due to their low thermal expansion, help to reduce sag in high-temperature lines while simultaneously exhibiting high tensile strength and elongation at break. V00102P-WC-0019

[0030] 5 / 25

[0031] This was made possible by increasing the carbon content to boost the maximum strength while simultaneously controlling carbide precipitation. According to the invention, the mass fraction of C is 0.18% to 0.30%, more preferably 0.20% to 0.28%.

[0032] By controlling carbide precipitation, the occurrence of coarse carbides (> 20 pm) was minimized without simultaneously reducing the absolute amount of carbides. This was achieved, firstly, by limiting the mass fraction of V to < 1.0%, which lowers the driving force for the precipitation / growth of carbide VC. Furthermore, a mass fraction of V greater than 1% would increase thermal expansion, which is undesirable. Another measure was the alloying of W to selectively introduce carbide WC, thereby increasing the tensile strength.

[0033] Another aspect was achieving a sufficiently high elongation at break to improve cold formability, which can be achieved by keeping the grain size small. For this purpose, it is advantageous to process the alloy directly from rolling heat, thus eliminating the need for additional heat treatment.

[0034] According to a preferred embodiment, the iron-nickel alloy according to the invention contains the following alloying elements in mass proportions:

[0035] C from 0.18% to 0.30%, optional Si with at least 0.001% and less than or equal to 0.50%, optional Mn with at least 0.001% and less than or equal to 0.30%, Cr from 0.01% to 0.70%.

[0036] Ni from 36.00% to 39.00%, optional AI with at least 0.001% and less than or equal to 0.10%, optional Mo with at least 0.001% and less than or equal to 0.50%, optional Ti with at least 0.001% and less than or equal to 0.10%, optional N with at least 0.001% and less than or equal to 0.01%, optional B with at least 0.001% and less than or equal to 0.01%, optional Co with at least 0.001% and less than or equal to 5.0%, V00102P-WG-0019

[0037] 6 / 25 optional P with at least 0.001% and less than or equal to 0.015%, optional S with at least 0.001% and less than or equal to 0.015%, Cu from 0.01% - 0.80%, W from 0.30% - 0.75%, V from 0.50% - 0.90%, optional Nb with at least 0.001% and less than or equal to 0.10% and remainder iron and unavoidable impurities.

[0038] If the present application specifies the proportion of an alloying element with, for example, the indication "less than or equal to 0.10%", this means that this alloying element may be present with a "maximum 0.10%" or with a proportion of "less than 0.10%".

[0039] Furthermore, it proved advantageous to alloy with niobium at a mass fraction of less than or equal to 0.10%, which has a grain-refining effect and also causes the carbide NbC to precipitate. Due to the smaller grain sizes, improved ductility can be achieved. This is already achieved with a mass fraction of niobium as low as 0.01%. Therefore, the mass fraction of niobium is preferably 0.01% or more and less than or equal to 0.10%, more preferably 0.01% to 0.08%, and particularly preferably 0.01% to 0.05%.

[0040] Furthermore, it was shown that the mass fractions of W and V exhibit a symbiotic effect that is not yet fully understood. A suitable W / V ratio helps to enhance carbide formation while simultaneously preventing carbid coarsening. By avoiding larger melt carbides, the occurrence of defects that could trigger premature fracture can be minimized.

[0041] Reducing the mass fraction of vitrification (V) is advantageous because otherwise, more primary carbides are formed from the melt, which cannot be dissolved by annealing. However, reducing the mass fraction of V also leads to a decrease in tensile strength, which can be compensated for by selectively adding carbon (C) and tungsten (W), resulting in the very high tensile strength values ​​of V00102P-WG-0019.

[0042] 7 / 25 can achieve strengths of over 1100 MPa, which is above the usual strength values ​​of Invar alloys.

[0043] Thus, according to a preferred embodiment, the mass ratio W / V is from 0.45 to 1.3. More preferably, the mass ratio W / V is from 0.45 to 1.2, and even more preferably from 0.48 to 1.1, and particularly from 0.48 to 1.0. Most preferably, the mass ratio W / V is from 0.50 to 0.95, and particularly from 0.50 to 0.90.

[0044] In the processing of the iron-nickel alloy according to the invention, solution annealing could optionally be omitted, making it possible to achieve an even more advantageous combination of tensile strength and toughness. Consequently, the material could be processed in the hot-rolled state.

[0045] According to a preferred embodiment of the iron-nickel alloy, the mass fraction of Mo is less than or equal to 0.40%, preferably less than or equal to 0.20%, and more preferably 0.01% - 0.08%.

[0046] According to another preferred embodiment of the iron-nickel alloy, the mass fractions of Al and Ti are each independently 0.01% - 0.08%.

[0047] According to a preferred embodiment of the iron-nickel alloy, the mass fraction of Cr is less than 0.60%, preferably less than 0.50%, more preferably less than 0.40%, even more preferably less than 0.30%, and particularly preferably less than 0.25%.

[0048] The present invention further relates to a method for producing a strand-shaped product from the iron-nickel alloy according to the invention, in which, after melting the alloy components, an optional secondary metallurgical treatment of the obtained melt and the casting of a semi-finished product, hot forming is carried out, the intermediate product thereby obtained is subjected, after an optional solution annealing, to cold forming in order to form a strand-shaped product, and the strand-shaped product is subjected to a final heat treatment.

[0049] Corresponding melting processes and secondary metallurgical treatments are described, for example, in EP 3 899 063 B1 and EP 3 899 065 A1 and can also be used in the present process.

[0050] It was found that it is not absolutely necessary to remelt the semi-finished product before hot forming. Thus, the inventive method, according to a preferred embodiment, is characterized by the fact that the semi-finished product is not remelted before hot forming.

[0051] It was further shown that the intermediate product obtained by hot forming does not necessarily have to undergo solution annealing before cold forming. Thus, according to a further preferred embodiment, the inventive process is characterized in that the intermediate product obtained by hot forming is not subjected to solution annealing before cold forming.

[0052] If the intermediate product obtained by hot forming is nevertheless subjected to solution annealing before cold forming, it was advantageous to carry this out at 900 °C to 1200 °C for 1 s to 10 h, followed by quenching to room temperature at a cooling rate of more than 20 K / s. Pickling can also be carried out after quenching to room temperature, if necessary.

[0053] It has also proven advantageous to carry out a surface treatment at least once after a heat treatment during the course of the process according to the invention. This can be pickling, grinding and / or peeling, in particular draw peeling. Such a surface treatment can be carried out once during the process after one of several heat treatments, or after each V00102P-WG-0019

[0054] 9 / 25

[0055] Heat treatment can be carried out multiple times. Surface treatment facilitates further processing of the product or intermediate product, so that, for example, subsequently applied coatings adhere better.

[0056] In the cold forming step, a degree of deformation of 20% to 80% is preferably used.

[0057] In the method according to the invention, according to a further preferred embodiment, the final heat treatment of the strand-shaped product can be carried out at 500 °C to 750 °C, preferably at 550 °C to 700 °C, for 1 to 30 h.

[0058] The final heat treatment can also be referred to as tempering or aging.

[0059] According to a preferred embodiment of the method according to the invention, a further treatment step can be the grinding of the wire obtained by cold forming. This takes place before the final heat treatment.

[0060] Another aspect of the present invention is a strand-like product made of an iron-nickel alloy, which is obtainable by the inventive method described above. The strand-like product according to the invention has a tensile strength of at least 1100 MPa in the tempered condition and a coefficient of thermal expansion at 15-230 °C of less than or equal to 3.5 x 10⁻⁶. 6 K' 1 The lower limit of the coefficient of thermal expansion is preferably 1.5 x 1 O' 6 K' 1 .

[0061] According to a preferred embodiment, the strand-shaped product according to the invention has an elongation at break of more than 10%.

[0062] It was found that in the strand-shaped product according to the invention, the coefficient of thermal expansion up to 200 °C has a value of less than or equal to V00102P-WG-0019

[0063] 10 / 25

[0064] 3.5x10' 6 K' 1 The bending temperature is maintained, thus remaining above 200 °C and preferably in the range of 200 °C to 350 °C. More preferably, the bending temperature is between 220 °C and 320 °C, particularly preferably between 230 °C and 300 °C, and even more preferably between 240 °C and 290 °C.

[0065] In principle, the bending temperature depends on the further processing of the product, and the values ​​and ranges given above preferably apply to the product obtained after the final heat treatment.

[0066] The buckling temperature of a material is defined as the temperature at which the approximately linear curve of a graph plotting the relative thermal expansion dL / Lo of a body made from that material as a function of temperature changes. Figure 2 shows an idealized representation of such a graph, plotting the relative thermal expansion dL / Lo against temperature. The graph, which is linear below the buckling temperature TK in this idealized representation, changes upon reaching the buckling temperature TK and, in the temperature range above the buckling temperature TK, becomes linear again, but with a steeper slope. This means that the coefficient of thermal expansion of the material is greater above the buckling temperature than below it.The kink temperature, also known as the "kink temperature", is also consistent with the Curie temperature of a material, which is the temperature at which a material loses its ferromagnetic properties and becomes paramagnetic.

[0067] With regard to the present invention, this means that the coefficient of thermal expansion of the strand-shaped product according to the invention is less than 3.5 x 1 O' below the buckling temperature. 6 K' 1 is and above the bending temperature values ​​of more than 3.5x10 -6 K -1 exhibits.

[0068] According to a further preferred embodiment, the strand-shaped product according to the invention has an austenitic microstructure with at least 95 vol.% austenite and a maximum of 5 vol.% precipitates and non-metallic inclusions. In particular, it may contain up to 5 vol.% of the carbides VC, WC, NbC, and CnCs. The carbide diameter is preferably < 10 pm. Preferably, the strand-shaped product according to the invention has an austenitic microstructure with at least 97 vol.% austenite and a maximum of 3 vol.% of the precipitates and non-metallic inclusions mentioned as examples.

[0069] The strand-shaped product described above according to the invention can be used as a starting material for a support cable for high-temperature lines.

[0070] Accordingly, a further aspect of the present invention is the use of the strand-shaped product according to the invention for the manufacture of a support cable for high-temperature lines. According to a preferred embodiment, said use is characterized in that the strand-shaped product is subjected to further cold forming.

[0071] Containers such as pressure vessels can also be manufactured from the strand-shaped product by forming and rolling, whereby it has been shown that the material can be welded very well and welded joints with high strength are obtained.

[0072] Brief description of the characters

[0073] Fig. 1 is a schematic representation of the individual process steps of the inventive process for producing a strand-shaped product.

[0074] Fig. 2 is a diagram to illustrate the buckling temperature TK, where the relative thermal expansion dL / Lo is plotted against the temperature in °C;

[0075] Definitions and measurement methods

[0076] In this disclosure, the term coefficient of thermal expansion refers to the coefficient of thermal linear expansion a, which can also be called the linear coefficient of thermal expansion, in the unit 10'. 6 K -1 V00102P-WG-0019

[0077] The value is specified as 12 / 25 and applies to a temperature range of 15–230 °C. This parameter is measured according to DIN 51045-1:2005-08.

[0078] Tensile strength indicates the maximum stress a material can withstand while being pulled before it breaks. It is expressed in force per unit area (e.g., megapascals, MPa).

[0079] Elongation at break is a measure of how much a material can be stretched before it breaks. It is expressed as a percentage and indicates the change in length relative to the material's original length.

[0080] The material properties of tensile strength and elongation at break mentioned in this disclosure were determined in accordance with ISO 6892-1 Rev. 11.19. The measurements were carried out on tensile specimens as defined in the standard and are therefore not directly equivalent to properties obtained from tensile tests on differently shaped end products, such as a wire of any dimension.

[0081] Tensile test specimens that meet the specifications defined in the aforementioned standard ISO 6892-1 are described in DIN 50125 (2016). For the present disclosure, the tensile test specimens of form B described therein were used, which are characterized by an initial gauge length Lo corresponding to five times the specimen diameter do, i.e., Lo = 5 do.

[0082] After hot forming, a so-called cold forming process is usually carried out. This refers to the plastic deformation of alloys below the recrystallization temperature, resulting in an increase in strength, known as work hardening. The degree of deformation during plastic forming (also called work hardening) is defined as follows:

[0083] (Starting area A0 - Ending area A1) / Starting area A0.

[0084] For example, if the starting area is 100 mm 2 is, and the end surface is 40 mm 2The degree of deformation is calculated using the formula: (100-40) / 100 = 0.6 or 60%. The solution annealing process step is carried out to remove residual precipitates and pronounced textures from production. For this purpose, the formed pre-product is immersed in solution at a high temperature (e.g., equal to or greater than 1000 °C) for a sufficiently long period of time (e.g., 3 hours or more).

[0085] During aging treatment, precipitates (carbides) can be selectively cultivated. These precipitates have a strength-enhancing effect. The aging treatment of the cooled, formed pre-product can be carried out at a temperature between 500 °C and 750 °C, particularly between 550 °C and 700 °C. Specifically, the aging treatment can be performed at temperatures of at least 560 °C, 570 °C, 580 °C, 590 °C, or 600 °C and not exceeding 680 °C, 660 °C, 650 °C, or 640 °C. The duration of the aging treatment can range from 1 to 30 hours, particularly between 2 to 20 hours, 3 to 15 hours, 5 to 10 hours, and 8 to 10 hours.

[0086] According to a preferred embodiment, the aging treatment is carried out in a temperature range of 600 °C to 650 °C over a period of 1 to 4 hours.

[0087] After the aging treatment, an optional additional processing step can be carried out. This can include turning, grinding, polishing, or peeling, among other things.

[0088] The term “unavoidable impurities,” as used in this disclosure in defining the alloy composition, refers to the proportions of elements in the alloy that are not intentionally added during the production of an alloy but may be present as impurities due to the use of common raw materials or are unintentionally introduced during the usual manufacturing steps. These may also be referred to as “melting-related impurities” or “unavoidable melting-related impurities.” V00102P-WG-0019

[0089] 14 / 25

[0090] Description of the exemplary implementations

[0091] Exemplary embodiments of the present invention are described below on the basis of the accompanying figures.

[0092] Specifically, two alloys according to the invention were produced and processed into the material samples of Examples 1 and 2 according to the method described below. The alloy composition of Examples 1 and 2 is listed in Table 1 below.

[0093] Material samples for comparison examples 1 to 3 were prepared using the same procedure. The alloy composition of comparison examples 1 to 3 is also listed in Table 1 below.

[0094] Table 1

[0095] Fe +*: Residual iron and unavoidable impurities. The individual samples were melted in a vacuum melting system, then cast into ingots and subsequently hot-formed. Forming was carried out within a temperature range of 1000–1200 °C from an initial cross-section with a diameter of 250 mm to a final diameter of 13.5 mm. After hot forming, the samples of Examples 1 and 2 and of Comparison Examples 1 to 3 underwent a solution annealing treatment at 1150 °C for 1 hour, with the heat treatment being terminated by rapid quenching at > 20 K / s to room temperature. The sample of Example 3 was processed further after hot forming without solution annealing.

[0096] The material was then pickled in a continuous pickling process to remove oxide adhesions. Following this, it underwent cold forming using a wire drawing machine, reducing the diameter from 13.5 mm to 8.5 mm. The final degree of cold forming for all samples was 60%.

[0097] Subsequently, a final heat treatment was carried out for 3 hours in the temperature range between 600 °C and 650 °C.

[0098] The sequential process steps are schematically illustrated in Fig. 1. Fig. 1 also shows an optional secondary metallurgical treatment, which is preferably carried out when the starting materials are melted in an electric arc furnace (EAF). Examples of such treatments include vacuum oxygen decarburization (VOD), argon oxygen decarburization (AOD), and ladder refining (LF). Reference is also made to the secondary metallurgical treatments described in EP 3 899 063 B1 and EP 3 899 065 A1, which can also be used in the present process.

[0099] The material properties tensile strength, elongation at break, coefficient of thermal expansion, and buckling temperature were determined from the material samples obtained at the end of the procedure for the examples and comparison examples. The values ​​obtained are listed in Table 2 below. V00102P-WQ-0019

[0100] 16 / 25

[0101] Table 2

[0102] The material samples of Examples 1 to 3 according to the invention showed high tensile strengths of over 1100 MPa with simultaneously sufficiently high elongation at break values ​​of over 10%. In addition, the coefficients of thermal expansion were less than 3.5–10 -6 K' 1 In the alloy composition of these two examples, the ratio of mass fractions W / V was ~0.6, within the preferred range of 0.45 to 1.3.

[0103] The sample from example 2 showed the best results with a high tensile strength of 1135 MPa, an elongation at break of 16% and a coefficient of thermal expansion of 2.7-10- 6 K' 1. This example is characterized, among other things, by the fact that the alloy contains relatively little Cr and Cu and, compared to example 1, provided the same tensile strength, a lower coefficient of thermal expansion and better elongation at break with a lower mass fraction of C.

[0104] Particularly noteworthy is example 3, in which the sample was further processed after hot forming without solution annealing. Since no solution annealing took place, no grain coarsening occurred, resulting in a finer grain, higher elongation at break, and increased resistance to torsional stress. The required strength properties are also met, with a tensile strength of 1,110 MPa.

[0105] The sample in comparison example 1 did not achieve a tensile strength of 1100 MPa, likely due to its very low w / (m·K) content. Consequently, its w / (m·K) ratio of 0.06 was also outside the preferred range of 0.45–1.3. The sample in comparison example 2, with its high w / (m·K) and chromium content, achieved a high tensile strength of 1250 MPa, but its elongation at break was insufficient at only 8%.

[0106] The sample of comparison example 3 achieved a comparable tensile strength to the samples of examples 1 and 2 according to the invention, however, the thermal expansion was higher with a coefficient of thermal expansion of 4 10'. 6 K -1Insufficient. This is likely due to the high mass fraction of Cr and V, which leads to the formation of more carbides and increases thermal expansion. Since no W was present, the W / V ratio was also outside the preferred range of 0.45–1.3.

[0107] The microstructure of the material samples obtained at the end of the process from Examples 1 to 3 and the comparison examples 1 to 3 was also characterized, including the determination of the individual phases. This was assessed using scanning electron microscope (SEM) images at 20,000x magnification. For this purpose, a sample was cut from a strand of the respective example, and the sample cross-section was then ground and polished. Etching the surface can also be used to better identify the phases.

[0108] The results are shown in Table 3 below.

[0109] Table 3 nn = not detectable V00102P-WG-0019

[0110] 18 / 25

[0111] Table 3 shows that the samples according to the invention had an austenitic microstructure with at least 95 vol% austenite and a maximum of 5 vol% precipitates and non-metallic inclusions. If the carbide volumes are very small and cannot be sufficiently resolved by SEM, the values ​​can be calculated using a thermodynamic simulation. A corresponding simulation was performed for the samples described above in Examples 1 to 3 and the comparative examples using the software Thermo-Calc 2024a and the database TCFeO7 (temperature 1150 °C, single equilibrium). For Examples 1 and 2 and the comparative examples, the simulation reflects solution annealing at a temperature of 1150 °C; for Example 3, the simulation shows the solution state at a hot forming temperature of 1150 °C. The results obtained are listed in Table 4.

[0112] V00102P-WC-0019

[0113] 19 / 25

[0114] Table 4

[0115] As a first step, calculations were performed for all samples under the scenario of solution annealing at a temperature of 1150°C. Example 3 shows the simulation of the solution state at a hot forming temperature of 1150°C. The resulting alloys exhibit only very low proportions of solution annealing carbides, ranging from 0.16 vol% to 0.74 vol%, with the remainder being an austenitic phase.

[0116] In Example 1, the proportion of 0.31 vol% MC-type carbides, in which tungsten and vanadium are incorporated into the carbide, is 0.31 vol%, and the austenitic phase comprises 99.69 vol%. This austenitic phase thus contains all other dissolved elements (such as nickel, chromium, vanadium, tungsten, and the like) and serves as the basis for the precipitation potential for the formation of secondary hardening carbides. V00102P-WC-0019

[0117] 20 / 25

[0118] In the next step, calculations were performed of the conditions after an aging treatment at 625 °C (an equilibrium state that is reached after a sufficiently long duration of the aging treatment).

[0119] For example 1, this results in a proportion of additionally formed secondary carbides of 1.46 vol.% in the alloy, which are MC carbides with 1.34 vol.% MC (with incorporated W, V, Cr) and with 0.12 vol.% WC (tungsten carbide).

[0120] For example 1, this results in a total carbide content (solution annealing carbides and secondary carbides) of 1.77 vol.% and a ratio of solution annealing carbides to secondary carbides of 4.7. The values ​​for example 2, example 3, and the comparative examples can be found in Table 4.

[0121] Example 3 yields very similar results to Example 2 and shows an even larger proportion of secondary carbides, increasing the secondary carbides / solution annealing carbides factor to 9.4.

[0122] A comparison of the results of Example 2, Example 3, and the comparative examples 1 and 3 surprisingly reveals that, despite a similar proportion of the total solution annealing carbides and secondary carbides, the proportion of solution annealing carbides in Examples 2 and 3 (0.16 vol.%) is significantly lower than in comparative examples 1 and 3 (0.26 vol.%) and 0.32 vol.%, respectively. Since the alloy of Example 2, as shown above, exhibits the best results with regard to tensile strength, elongation at break, and coefficient of thermal expansion, it can be concluded that the relatively low proportion of solution annealing carbides promotes the development of high elongation at break values, while a sufficiently high proportion of secondary carbides ensures sufficiently high tensile strength values.

[0123] Consequently, for the present invention, the ratio of the proportion of secondary carbides to the proportion of solution annealing carbides shown in Table 4 can be used as a parameter that characterizes the balanced and V00102P-WQ-0019

[0124] 21 / 25 good properties of the alloy according to the invention. Preferably, this parameter is above 4.5, more preferably above 5.5 and particularly preferably above 6.0.

[0125] If the strand-shaped product according to the invention is used as a starting material for the production of, for example, a support cable for high-temperature pipelines, it is subjected to a further and final cold forming process with a degree of deformation of up to 95%. This can be done, for example, by drawing or rolling. As a result of this further cold forming, the tensile strength will increase, and the elongation at break and the coefficient of thermal expansion will decrease. However, the buckling temperature is hardly affected.

Claims

V00102P-WQ-0019 22 / 25 Claims 1. Iron-nickel alloy with the following alloying elements in mass fractions: C from 0.18% to 0.30%, optional Si less than or equal to 0.50%, optional Mn less than or equal to 0.30%. Cr from 0.01% - 0.70% Ni from 36.00% to 39.00%, optional AI less than or equal to 0.10%, optional Mo less than or equal to 0.50%, optional Ti less than or equal to 0.10%, optional N less than or equal to 0.01%, optional B less than or equal to 0.01%, optional Co less than or equal to 5.0%, optional P less than or equal to 0.015%, optional S less than or equal to 0.015%. Cu from 0.01% - 0.80% W from 0.30% - 0.75% V of 0.50% - 0.90%, optionally Nb with less than or equal to 0.10% and Residual iron and unavoidable impurities.

2. Iron-nickel alloy according to claim 1, wherein the ratio of the mass fractions W / V is from 0.45 to 1.

3.

3. Iron-nickel alloy according to claim 1 or 2, wherein the mass fractions of Al and Ti are each independently 0.01% - 0.08%.

4. Iron-nickel alloy according to any one of claims 1 to 3, wherein the mass fraction of C is 0.20% - 0.28%. V00102P-WG-0019 23 / 25 5. A method for producing a strand-shaped product from the iron-nickel alloy according to any one of claims 1 to 4, in which, after melting the alloying elements, an optional secondary metallurgical treatment of the obtained melt and casting of a semi-finished product, hot forming is carried out, the intermediate product thereby obtained is subjected, after optional solution annealing, to cold forming in order to form a strand-shaped product, and the strand-shaped product is subjected to a final heat treatment.

6. Method according to claim 5, wherein the semi-finished product is not subjected to remelting before hot forming.

7. Method according to claim 5 or 6, wherein the intermediate product obtained by hot forming is not subjected to solution annealing prior to cold forming.

8. Method according to claim 5 or 6, wherein the intermediate product obtained by hot forming is subjected to solution annealing at 900 °C to 1200 °C for 1 s to 10 h and quenching to room temperature with a cooling rate of more than 20 K / s prior to cold forming, and optionally pickling or peeling is carried out after quenching to room temperature.

9. Method according to any one of claims 5 to 8, wherein the final heat treatment of the strand-shaped product is carried out at 500 °C to 750 °C, preferably at 550 °C to 700 °C, for 1 to 30 h.

10. Strand-shaped product of an iron-nickel alloy, obtainable by a method according to any one of claims 5 to 9, having a tensile strength of at least 1100 MPa after final heat treatment and a coefficient of thermal expansion at 15-230 °C of less than or equal to 3.5x10- 6 K' 1 exhibits.

11. Strand-shaped product according to claim 10, which has an elongation at break of more than 10%. V00102P-WG-0019 24 / 25 12. Strand-shaped product according to claim 10 or 11, wherein the bending temperature is above 200 °C and preferably in the range of 200 °C to 350 °C. 5 13. Strand-shaped product according to one of claims 10 to 12, which has an austenitic microstructure with at least 95 vol% austenite and a maximum of 5 vol% precipitates and non-metallic inclusions.

14. Strand-shaped product according to one of claims 10 to 13, wherein the strand-shaped product is a starting material for a support cable for high-temperature lines.

15. Use of the strand-shaped product according to one of claims 10 to 14 for the manufacture of a support cable for high-temperature lines, characterized in that the strand-shaped product is subjected to further cold forming.

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