Austenitic material with high strength and method for producing the same

Abstract

Austenitic material for use in the construction industry for pipes, tension wires or fasteners, in the paper industry, for chemical plant construction or for springs or valves or pumps or in the electronics industry, comprising or consisting of the following elements (all values ​​in wt.%): Carbon (C) 0.05 - 0.40 Silicon (Si) < 0.50 Manganese (Mn) 8.00 - 12.00 Phosphorus (P) < 0.05 Sulfur (S) < 0.005 Chromium (Cr) 17.50 - 20.50 Molybdenum (Mo) 0.50 - 3.50 Tungsten < 3.00 Nickel (Ni) 0.20 - 3.00 Vanadium (V) < 0.50 Copper (Cu) < 0.50 Cobalt (Co) < 0.50 Titanium (Ti) < 0.50 Aluminum (Al) < 0.50 Niobium (Nb) 0.01 - 0.10 Nitrogen (N) 0.30 - 0.60 Balance iron and unavoidable impurities, where: 10≤C+N / Nb≤34.

Description

[0001] The invention relates to an austenitic material, in particular for use in the construction industry for pipes, tension wires or fastening elements, in the paper industry, for chemical apparatus construction or for springs or valves or pumps, and a method for its production.

[0002] High-nitrogen alloyed austenitic materials are used in the construction industry as tension wire or fastening elements. These applications are characterized by a diameter of ≤ 125 mm.

[0003] From EP 1 069 202 A1, a paramagnetic, corrosion-resistant, austenitic steel with high yield strength, strength, and toughness is known, which is said to be particularly corrosion-resistant in media with high chloride concentrations, wherein this steel contains 0.6 to 1.4 wt.% nitrogen. It also contains 17 to 24 wt.% chromium and manganese.

[0004] From EP 4 279 628 A1, a non-magnetic austenitic stainless steel is known which has a purely austenitic structure. To ensure this structure, a nickel content of between 9 and 15 wt.% is added. This makes the material very expensive to produce and its use in the jewelry industry very difficult due to potential nickel allergies.

[0005] From EP 2 455 508 B1, an austenitic, corrosion-resistant steel with reduced manganese content (8 to 12 wt%) and a carbon-to-nitrogen ratio (C + N) between 0.6 and 1 wt% is known. It is further disclosed that the alloy must contain at least 0.3 wt% nitrogen. Due to the lower manganese content, a complex melting process in a vacuum furnace under a nitrogen atmosphere is necessary to achieve the required nitrogen content in the steel alloy. Furthermore, the examples provided disclose that the austenitic alloy contains more carbon than nitrogen.

[0006] From EP 0 875 591 B1, a corrosion-resistant, nickel-free steel alloy is known as a material for objects that come into at least partial or temporary contact with the skin or bodily fluids of living beings. However, in the examples cited, the alloy is produced from a powder or in expensive remelting processes, for example in a vacuum induction furnace or in the pressure electroslag remelting process.

[0007] From EP 2 924 514 B1, a watch or jewelry spring made of a stainless steel alloy with specified carbon-to-nitrogen ratios is known, wherein 0.40 wt.% ≤ (C + N) ≤ 1.5 wt.% and 0.125 ≤ (C / N) ≤ 0.55 wt.%. Additionally, the nitrogen content of the alloy is specified as 0.40 to 0.75 wt.%.

[0008] Key parameters for corrosion resistance include the so-called PREN OPT value, which is calculated as PREN OPT = Cr + 3.3 × (Mo + 0.5 × W) + 20 × (C + N) - 0.5 × Mn, and the so-called "pitting equivalent number", which is defined as MARC OPT = Cr + 3.3 × Mo + 20 × N + 20 × C - 0.5 × Mn (element contents in wt.%).

[0009] The object of the invention is to create an austenitic material which, with regard to its manganese and nickel content, has a significantly reduced alloy design while simultaneously exhibiting high corrosion resistance, high strength and good paramagnetic behavior.

[0010] The problem is solved using an austenitic material with the features of claim 1. Advantageous embodiments are characterized in dependent claims.

[0011] Furthermore, it is an object of the invention to provide a method for producing the austenitic material, which exhibits high strength and good paramagnetic behavior in addition to increased corrosion resistance.

[0012] The problem is solved by the features of claim 10. Advantageous further developments are characterized in the dependent subclaims.

[0013] Any percentages given below are always given in wt.% (weight percent).

[0014] The inventors recognized the particular advantages of increasing the chromium content while simultaneously reducing the manganese content. In the present material, the chromium content is increased, primarily to enhance corrosion resistance. Furthermore, the manganese content, which is necessary for nitrogen solubility, is reduced in the alloy. This has the positive effect of minimizing fretting corrosion and thus increasing the theoretical service life.

[0015] To ensure that the austenitic alloy or material is non-magnetic, it is necessary to use a non-ferritic steel alloy. By increasing the chromium content and simultaneously decreasing the manganese content, the austenitic alloy must be adjusted by the other austenite-forming elements, such as nickel, nitrogen, cobalt, and carbon. Empirical studies have shown that it is particularly advantageous for the alloy if 10 ≤ (C + N) / Nb ≤ 34 (element contents are given in wt.%), hereinafter referred to as Formula A, applies. This ensures that the microstructure is austenitic, in which niobium carbonitrides contribute to increased strength. Furthermore, the grain refinement effect of niobium carbonitrides allows yield strengths of more than 1900 MPa to be achieved after work hardening.

[0016] According to the invention, the austenitic alloy shall have a completely austenitic microstructure with niobium carbonitrides, wherein the magnetic permeability µ r < 1.01.

[0017] After the cast block has been subjected to a hot forming step, which can be either a rolling process or a forging process, the yield strength R p0,2 of the present alloy is R p0,2 > 450 MPa.

[0018] After cold working, the yield strength of the present alloy or austenitic material is reliably R p0,2 > 1900 MPa, with values ​​up to 2100 MPa being achieved in practice.

[0019] The yield strength R p0,2 is determined by the standard DIN EN 2002-001.

[0020] The corrosion resistance against pitting corrosion is specified by the standard ASTM G48 - Method E: Critical Pitting Temperature Test for stainless steel.

[0021] Until now, a good combination of strength and corrosion resistance could not be guaranteed in austenitic materials. This excellent combination of strength and corrosion resistance was previously neither achievable nor expected. However, it is achieved through the specific alloy composition used here. This is based on a synergistic effect, which is achieved through a targeted and closely coordinated selection of alloying elements.

[0022] The individual elements, possibly in combination with the other alloying components, are described in more detail below. All information regarding the alloy composition is given in weight percent (wt%). The upper and lower limits of the individual alloying elements can be freely combined within the limits of the invention.

[0023] Carbon is a strong austenite former and has a beneficial effect on high mechanical properties. In the presence of niobium and nitrogen, carbon forms very fine niobium carbonitrides, which also have a positive effect on strength properties. A lower limit of 0.05, 0.06, 0.07, 0.08, 0.09, 0.10, 0.12, or 0.15 wt.% is necessary for the formation of niobium carbonitrides. With increasing carbon content, the driving force for coarse (diameter > 5 µm) carbide precipitates, such as M₂₃C₆ or M₇C₃, also increases. In this context, coarse precipitates are defined as carbides with a mean diameter > 2 µm. These coarse carbides extract chromium and carbon from the steel matrix, thus reducing corrosion resistance. They also cause embrittlement of the material. For this reason, an upper limit of 0.40 or 0.38 or 0.35 wt.% should be chosen.Preferably, a carbon content between 0.05 and 0.40 wt.% is provided. A particularly preferred range is between 0.15 and 0.40 wt.%.

[0024] Silicon primarily serves to deoxidize steel. It also increases strength through the formation of solid solutions. To achieve these effects, a lower limit of 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, or 0.10 wt.% is required. However, if silicon is added in excessive amounts, there is a risk of intermetallic phase formation. Since silicon is also a ferrite former, the upper limit is defined by a safety margin. In particular, silicon can be added in amounts of < 0.50, 0.45, 0.40, 0.35, or 0.30 wt.%. Preferably, a silicon content of < 0.50 wt.% is used. A particularly preferred range lies between 0.10 and 0.30 wt.%.

[0025] Manganese increases nitrogen solubility. It was previously assumed that manganese contents of more than 19 wt.%, ideally more than 20 wt.%, were necessary for high nitrogen solubility. Surprisingly, with the present alloy, it has been found that high nitrogen solubility is achieved even with low manganese contents, without the need for expensive pressure nitriding. The lower limit for manganese is 8.0 wt.%. The upper limit for manganese can be 12.0, 11.5, or 11.0 wt.%. This is a remarkably low value compared to high-nitrogen materials according to the prior art. Preferably, a manganese content of 8.0 to 12.0 wt.% is used. A particularly preferred range lies between 8.0 and 11.0 wt.%.

[0026] According to the literature, the addition of copper proves advantageous for resistance in sulfuric acid. However, it has been shown that copper values ​​> 0.5 wt% increase the tendency for chromium nitride precipitation, which in turn negatively affects corrosion properties. Therefore, the upper limit is set at 0.5, 0.15, or 0.10 wt%. The copper content can also be chosen so that it is below the detection limit (i.e., no deliberate addition of copper).

[0027] Chromium increases the corrosion resistance of the alloy and is also essential for maintaining nitrogen solubility. Chromium contents of 17 wt.% or more are necessary for improved corrosion resistance. The present alloy contains at least 17.5 wt.% and at most 20.5 wt.% chromium. This optimally influences resistance to pitting and stress corrosion cracking. The lower limit for chromium is 17.5 wt.%. However, with an increase in chromium content, the tendency to form coarse chromium carbides (diameter > 5 µm) also increases, which reduces the corrosion resistance of the matrix. To avoid this adverse effect, the upper limit for the chromium content is 20.5, 20.0, 19.5, or 19.0 wt.%. Preferably, a chromium content of 17.5 to 20.5 wt.% is used. A particularly preferred range lies between 17.5 and 19.0 wt.%.

[0028] Molybdenum contributes significantly to corrosion resistance in general and pitting corrosion resistance in particular, with its effect being enhanced by tungsten. To utilize this positive effect of molybdenum, a lower limit of 0.5 wt.% is required. However, high molybdenum contents necessitate electroslag remelting (ESR) or pressure electroslag remelting (PESR) to prevent segregation. Such remelting processes are very complex and expensive. Therefore, according to the invention, PESR or PESR processes are to be avoided. Consequently, a maximum of 3.5, 3.2, or 3.1 wt.% molybdenum is added. Preferably, a molybdenum content of 0.5 to 3.5 wt.% is used. A particularly preferred range lies between 0.5 and 3.1 wt.%.

[0029] Tungsten exhibits the same properties as molybdenum, thus contributing to increased corrosion resistance and strength. In this alloy, the tungsten content can be up to 3.0 wt.%. If this upper limit is exceeded, significant segregation will occur, necessitating remelting of the alloy.

[0030] Nickel improves corrosion resistance. Furthermore, nickel is an austenite-stabilizing element. The lower limit for nickel is therefore 0.20 wt.%, 0.25 wt.%, 0.30 wt.%, 0.35 wt.%, 0.40 wt.%, 0.45 wt.%, or 0.50 wt.%. The upper limit for nickel can be 3.00 wt.%, 2.50 wt.%, or 2.00 wt.%. Preferably, a nickel content between 0.20 wt.% and 3.00 wt.% is provided. A range of 0.50–3.00 wt.% is particularly preferred. A further preferred range is 0.50–2.50 wt.%.

[0031] Cobalt may be used, in particular, to substitute for nickel. The upper limit for cobalt is 0.5, 0.4, 0.3, 0.2, or 0.1 wt.%. The cobalt content may also be chosen so that it is below the detection limit (i.e., no deliberate addition of cobalt).

[0032] Nitrogen is added to ensure high strength. Furthermore, nitrogen contributes to corrosion resistance and is a strong austenite former. Therefore, nitrogen contents higher than 0.30 wt.%, and especially higher than 0.35 wt.%, are particularly advantageous. However, to avoid nitrogenous precipitates, especially chromium nitride, the upper limit of the nitrogen content is restricted to 0.60, 0.55, or 0.50 wt.%. Surprisingly, it has been found that, despite the very low manganese content of 8.0 to 12.0 wt.% compared to known alloys, these high nitrogen contents in the alloy can be achieved without pressure nitrogenation or a nitrogen atmosphere. Preferably, a nitrogen content between 0.30 and 0.60 wt.% is used. A particularly preferred range is between 0.35 and 0.50 wt.%.

[0033] Niobium is an important element in the present alloy for grain refinement. For this reason, it is included at a lower limit of 0.01 wt.%, preferably 0.02 wt.%. In combination with carbon and nitrogen, niobium forms niobium carbonitrides at the grain boundaries. These not only refine the grain but also increase its strength. Niobium is included in the alloy at an upper limit of 0.10, 0.08, or 0.06 wt.%. Preferably, a niobium content of 0.01 to 0.10 wt.% is provided. A particularly preferred range lies between 0.02 and 0.06 wt.%.

[0034] It has proven particularly advantageous if the levels of carbon, nitrogen and niobium meet the ratio according to formula A: 10 ≤ C + N / Nb ≤ 34

[0035] In formula A, carbon (C), nitrogen (N) and niobium (Nb) are used in their respective wt.%.

[0036] The austenitic alloy exhibits very good strength properties and also good corrosion resistance, making it very suitable for use in the construction industry, the paper industry, chemical apparatus construction, springs, valves, pumps, or the electronics industry with a diameter of ≤ 125 mm.

[0037] The specified limit ranges for the elements carbon (0.05 to 0.40 wt.%), nitrogen (0.30 to 0.60 wt.%) and niobium (0.01 to 0.10 wt.%) yield important insights in connection with formula A.

[0038] Carbon and nitrogen are required to ensure a fully austenitic microstructure (≥ 99.5 vol% austenitic phase in the microstructure) and sufficient strength. In this alloy, the manganese content is reduced to improve pitting corrosion resistance. Reducing the manganese content also necessitates a reduction in austenite stabilizers and decreases the alloy's nitrogen solubility. Surprisingly, however, the desired nitrogen content in this alloy can be achieved without expensive pressure nitrogen treatment or additional remelting under a nitrogen atmosphere. Furthermore, the carbon content in the alloy is also increased, with the carbon, as an interstitially dissolved element, providing the required strength.

[0039] The ratio (C + N) / Nb serves as an indicator for the formation of niobium carbonitrides. This precipitation can be particularly well controlled when formula A (10 ≤ (C + N) / Nb ≤ 34) is fulfilled, as very fine niobium carbonitrides with an average diameter of < 0.5 µm then form. These increase strength and, through the pinning effect, refine the grain structure. Furthermore, another positive effect is that the driving force for the formation of coarse chromium carbides or chromium nitrides is no longer sufficient. While niobium carbonitrides do reduce corrosion resistance to some extent, they cannot be compared to coarse chromium nitrides or chromium carbides in this respect.For the application of this alloy with a diameter ≤ 125 mm in the construction industry for pipes, tension wires, or fasteners, in the paper industry, for chemical plant engineering, for springs, valves, or pumps, or in the electronics industry, practical experience has shown that the advantages of the increased strength provided by the niobium carbonitrides outweigh the negative effects of the reduced corrosion resistance. Values ​​for (C + N) / Nb below 10, indicating a high niobium content, always lead to the precipitation of primary niobium carbides. Due to their average size of > 0.5 µm, these carbides negatively affect corrosion resistance and impact strength. At values ​​above 34, the driving force for niobium carbonitrides decreases, thereby reducing the positive effect of grain refinement.

[0040] For these reasons, 10 ≤ (C + N) / Nb ≤ 34 applies, and 10 ≤ (C + N) / Nb ≤ 30 is particularly preferred.

[0041] Optionally, boron, aluminum and sulfur may be included as additional alloying elements.

[0042] The alloying elements vanadium and titanium are not necessarily present in the steel alloy in question. Although these elements contribute to the solubility of nitrogen, high nitrogen solubility can also be ensured in the alloy in question even in their absence.

[0043] It is advantageous if the following relationship holds true: PREN OPT : Cr + 3,3 × Mo + 0,5 × W + 20 × C + N − 0,5 × Mn ≥ 36 .

[0044] The PREN formula is optimized such that the limit of 36 is necessary.

[0045] The invention thus relates to an austenitic material which has a composition comprising or consisting of the following elements (all values ​​in wt.%): Carbon (C) 0,05 - 0,40 Silicon (Si) < 0,50 Manganese (Mn) 8,00 - 12,00 Phosphorus (P) < 0,05 Sulfur (S) < 0,005 Chromium (Cr) 17,5 - 20,5 Molybdenum (Mo) 0,50 - 3,50 Wolfram (W) < 3,00 Nickel (Ni) 0,20 - 3,00 Vanadium (V) < 0,50 Copper (Cu) < 0,50 Cobalt (Co) < 0,50 Titanium (Ti) < 0,50 Aluminum (Al) < 0,50 Niobium (Nb) 0,01 - 0,10 Nitrogen (N) 0,30 - 0,60

[0046] Residual iron and unavoidable impurities, whereby the following applies: 10 ≤ C + N / Nb ≤ 34 .

[0047] Further training stipulates that the austenitic material, after solution annealing, contains a microstructure of > 99.5 vol% austenite and < 0.5 vol% niobium carbonitrides.

[0048] Further training stipulates that the average diameter of the niobium carbonitrides is between 0.1 and 0.5 µm.

[0049] Further training requires that the condition 10 ≤ (C + N) / Nb ≤ 30 is met.

[0050] Further training stipulates that the nickel content should be 0.50 - 3.00 wt.%.

[0051] Further training stipulates that the nickel content should be 0.50 - 2.50 wt.%.

[0052] Further training stipulates that the yield strength R p0,2 according to the standard DIN EN 2002-001 in the solution annealed condition is > 500 MPa and the elongation A5 is 55 - 80%.

[0053] Further training stipulates that the yield strength R p0,2 according to the standard DIN EN 2002-001 in the cold-worked state is > 1900 MPa.

[0054] Further training stipulates that the optimized PREN value (PREN OPT ) Cr + 3.3 × (Mo + 0.5 × W) + 20 × (C + N) - 0.5 × Mn ranges from 36 to 51.

[0055] Further training stipulates that the critical pitting temperature according to ASTM Pitting G48 Method E is 30°C to 60°C.

[0056] The invention further relates to a method for producing the austenitic material, wherein an austenitic alloy comprising the following elements (all values ​​in wt.%): Carbon (C) 0,05 - 0,40 Silicon (Si) < 0,50 Manganese (Mn) 8,00 - 12,00 Phosphorus (P) < 0,05 Sulfur (S) < 0,005 Chromium (Cr) 17,50 - 20,50 Molybdenum (Mo) 0,50 - 3,50 Wolfram (W) < 3,00 Nickel (Ni) 0,20 - 3,00 Vanadium (V) < 0,50 Copper (Cu) < 0,50 Cobalt (Co) < 0,50 Titanium (Ti) < 0,50 Aluminum (Al) < 0,50 Niobium (Nb) 0,01 - 0,10 Nitrogen (N) 0,30 - 0,60

[0057] Residual iron and unavoidable impurities, whereby the following applies: 10 ≤ C + N / Nb ≤ 34 , The alloy is melted and treated using secondary metallurgy, the alloy is then cast into blocks and allowed to solidify, the blocks are then heated and immediately hot-formed into forgings and / or rolled pieces by forging and / or rolling, the forgings and / or rolled pieces are then subjected to further cold forming and finally mechanical processing.

[0058] Further training stipulates that hot forming is carried out in several partial steps.

[0059] Further training provides that the forging and / or rolled part is reheated between the hot forming steps, and after the last hot forming step, if necessary, solution annealing is carried out at 1000 to 1200°C for a duration of 1 to 48h.

[0060] Further training stipulates that water quenching takes place after the last hot forming step and / or solution annealing.

[0061] Further training stipulates that after water quenching, cold forming with a degree of cold forming of 10-50% takes place.

[0062] The invention also relates to the use of the austenitic material as a construction application, as a tension wire or as a fastening screw with a diameter of ≤ 125 mm.

[0063] All of the above-mentioned training courses can be combined with each other.

[0064] The invention is explained by way of example with the aid of a drawing. The drawing shows: Figure 1: a table showing the components of the alloy according to the invention; Figure 2: highly schematic representation of the manufacturing process; Figure 3: a table with four different alloys (A to C) within the specified alloy limits and four alloys (D to G) outside the specified limit ranges; Figure 4: mechanical properties and microstructure characterization of all alloys in the solution annealed condition.

[0065] The first column of the table in Figure 1 Figure 1 shows the elemental composition of the austenitic alloy according to the invention, which is characterized by increased corrosion resistance, high strength and toughness, and good paramagnetic properties. Preferred variants fall within the alloy ranges specified, whereby not all alloying elements necessarily have to be present in limited quantities.

[0066] In the present alloy, it is particularly surprising that nitrogen values ​​above 0.30 wt.% can be achieved despite the low contents of nitrogen solubility-promoting elements, such as manganese, without a special nitrogen enrichment process such as pressure electroslag remelting (DESU) or melting in a vacuum melting furnace under a nitrogen atmosphere.

[0067] All alloys are produced in the same way and are in Figure 2The process is shown in a highly schematic form. The components are melted under atmospheric conditions in an electric arc furnace and subsequently undergo secondary metallurgical treatment. Alternatively, it is also conceivable that the alloys are melted in a vacuum induction melting unit (VID) and subsequently treated with secondary metallurgical processes. Ingots are then cast and immediately hot-forged. "Immediately" in the context of the invention means that no additional remelting process, such as electroslag remelting (ESR) or pressure electroslag remelting (PESR), is performed.

[0068] The alloy according to the invention has the advantage that homogenization annealing or remelting is not necessary.

[0069] The solidified blocks are then hot-formed in several steps, for example, pre-forged on a forging press and brought to final dimensions on a rotary forging machine, or formed in a roughing mill and then brought to final dimensions on a finishing mill. Depending on the requirements, a solution annealing step at 1000 to 1200°C for 1 to 48 hours and / or water cooling may also be carried out.

[0070] After the final hot forming step, the intermediate products are cooled to room temperature with water. This special process step allows critical temperature ranges to be traversed more quickly and the formation of small (diameter < 0.5 µm) niobium carbonitrides to be controlled.

[0071] To determine the final properties, the necessary cold forming steps, which involve work hardening, are carried out on a long forging machine or by drawing on a drawbench. The degree of deformation during work hardening ranges between 10 and 50%, always referring to the initial area. The degree of deformation is calculated by subtracting the final area A1 from the initial area A0 and then dividing by the initial area A0: (A0 - A1) / A0. For example, if the initial area is 100 mm² and the final area is 30 mm², the degree of work hardening is (100 - 30) / 100 = 0.7 or 70%.

[0072] After cold forming, mechanical processing takes place, which can be turning, peeling or grinding in particular.

[0073] The present austenitic alloy with its advantageous properties can be achieved not only through the described (and especially in) Figure 2 The production route shown) can be used, but also via a powder metallurgical production route.

[0074] Figure 3 Figure 1 shows the chemical analysis of the alloy compositions and the calculated ratios of formula A for four alloys according to the invention (A to C) and four alloys not according to the invention (D to G).

[0075] In Figure 4 All mechanical properties, microstructure characterization and evaluated corrosion properties of all alloys are determined. Figure 3 The samples were characterized in the solution-annealed state, with the solution annealing carried out between 1000 and 1200°C for a duration of 1 to 48 hours. The alloys were solution-annealed at 1100°C for 8 hours and then quenched with water.

[0076] The determination of the mechanical properties tensile strength R m , yield strength at 0.2% elongation (R p0,2 ) and elongation A5 were carried out according to the standard DIN EN 2002-001.

[0077] The resistance to pitting corrosion is specified by the standard ASTM G48 - Method E: Critical Pitting Temperature Test for stainless steel. The sample is immersed in an iron(III) chloride solution for 24 hours at a constant temperature between 0 and 85°C. Afterward, the sample is cleaned and examined for pitting corrosion. Pitting is considered present if the localized attack has a depth of at least 0.025 mm. If no pitting is detected, the test temperature is increased, and the test is repeated in a fresh solution with an untested sample. The starting temperature for the test is determined using the empirical formula CPT(°C) = 3.2 × Cr + 7.6 × Mo + 10.5 × N - 81.0. Thus, the starting temperature for an alloy of Fe-18Cr5Mo0.5N corresponds to 19.85°C (rounded up to 20°C). The sample is immersed in a solution of iron(III) chloride for 24 hours at 20°C, then cleaned and finally tested.

[0078] In addition, the corrosion resistance was calculated according to the optimized PREN formula: PREN OPT : Cr + 3,3 × Mo + 0,5 × W + 20 × C + N − 0,5 × Mn ≥ 36

[0079] The formula was optimized so that tungsten also contributes positively to the increase in corrosion.

[0080] The microstructure, including the proportions of the individual phases, can be examined using a scanning electron microscope (SEM) at 20,000x magnification. Preferably, a sample is cut from a rod with a diameter ≤ 125 mm, followed by grinding and polishing of the sample cross-section. The surface can also be etched to better identify the phases.

[0081] The characterization of samples A to C, all within the specified ranges for chemical analysis, yielded very similar results for mechanical properties, microstructure, and corrosion characteristics in the solution-annealed condition. The evaluation of the mechanical properties consistently showed yield strength Rp0.2 values ​​> 500 MPa. Likewise, all measured strain values ​​A5 were > 60%, which is a very good value for this type of alloy.

[0082] Microstructural characterization of alloys A to C revealed the presence of niobium carbonitrides at the grain boundaries. These have an average diameter of 0.1 to 0.5 µm. The total inclusion content is < 0.5 vol.%. The remainder of the microstructure consists of an austenitic matrix. No other carbide, nitride, or carbonitride precipitates could be identified.

[0083] The corrosion results also yielded a uniform evaluation scheme. This allowed the optimized PREN value for alloys A to C to be set at ≥ 36. Furthermore, the ASTM pitting temperature G48 according to method E was at least 30°C in all samples.

[0084] The reference samples D to G are described in more detail below. These deviate from the alloy composition according to the invention with respect to at least one element or with respect to fulfilling the ratio according to formula A (10 ≤ (C + N) / Nb ≤ 34).

[0085] Sample D exhibits similar mechanical properties Rm and Rp0.2 to the alloys according to the invention. The optimized PREN value of 35.9 is also only slightly lower than specified according to the invention. However, it is evident that the sample is almost free of niobium, and thus no niobium carbonitrides are detectable. This results in a very high value of 330 for the ratio (C + N) / Nb. This is disadvantageous because there is no pinning effect of the austenite grains, and the grain size consequently increases. Surprisingly, sample E, with a PREN OPT value of 35.9, has a critical pitting temperature of only 27°C. This is due to the fact that the chromium content in alloy E is only 17 wt.%.

[0086] Among the reference samples, only sample E has a niobium content of > 0.10 wt.%. This results in a value of 6 for the ratio (C + N) / Nb. This example is intended to show that at high niobium content, primary melt carbides form in the alloy, which have a detrimental effect on corrosion resistance (ASTM pitting temperature G48 according to method E of 20°C). The negative effect is due to the size of the primary carbides, which are typically > 1 µm.

[0087] Sample *F also contains virtually no niobium. Furthermore, its carbon content is increased compared to the alloy according to the invention (0.48 wt.%). Here, even with small diameters of ≤ 125 mm, chromium carbides in the core are unavoidable, as the driving force for precipitation formation is so great that it cannot be suppressed. Sample *G also contains no nickel, with the Mo + W ratio being below the intended 2.5 wt.%, resulting in very poor resistance to pitting corrosion (ASTM pitting temperature G48 according to method E of 15°C).

[0088] Sample G also contains virtually no niobium. Compared to all samples, it also exhibits the lowest value for the sum of carbon and nitrogen contents, at 0.57. This results in a lower strength level in the parameters Rm and Rp0.2. However, with regard to elongation, it shows similar values ​​to the alloys according to the invention. Nevertheless, sample H exhibits significantly poorer corrosion resistance, given its low optimized PREN value of 24.7 and a low ASTM pitting temperature G48 according to Method E of 22 °C. This is due to the absence of the elements molybdenum and tungsten.

[0089] The most significant difference between samples A to C according to the invention lies in the microstructure characterization and, consequently, in the corrosion resistance of the austenitic alloy. The goal of specific construction industry applications with a diameter of ≤ 125 mm, for example, tension wires or fastening screws, is always a combination of high strength and improved corrosion resistance. This can be achieved with the present alloy concept by adhering to formula A (10 ≤ (C + N) / Nb ≤ 34) and the specified contents of chromium, nickel, molybdenum, and tungsten, whereby formula A specifically highlights the positive effect of niobium carbonitrides. If formula A is fulfilled, the advantage of the pinning effect of the niobium carbonitrides at the grain boundaries predominates. With the present alloy concept, an ASTM pitting temperature G48 according to method E of at least 30°C to 60°C can be achieved for all samples.This results in excellent corrosion resistance.

Claims

1. Austenitic material comprising or consisting of the following elements (all values ​​in wt.%): Carbon (C) 0,05 - 0,40 Silicon (Si) < 0,50 Manganese (Mn) 8,00 - 12,00 Phosphorus (P) < 0,05 Sulfur (S) < 0,005 Chromium (Cr) 17,50 - 20,50 Molybdenum (Mo) 0,50 - 3,50 tungsten < 3,00 Nickel (Ni) 0,20 - 3,00 Vanadium (V) < 0,50 Copper (Cu) < 0,50 Cobalt (Co) < 0,50 Titanium (Ti) < 0,50 Aluminum (Al) < 0,50 Niobium (Nb) 0,01 - 0,10 Nitrogen (N) 0,30 - 0,60 Residual iron and unavoidable impurities, whereby the following applies: 10 ≤ C + N / Nb ≤ 34 .

2. Austenitic material according to claim 1, characterized by the fact that The austenitic material, after solution annealing, contains a microstructure of > 99.5 vol% austenite and < 0.5 vol% niobium carbonitrides.

3. Austenitic material according to any one of the preceding claims, characterized by the fact that The average diameter of the niobium carbonitrides is between 0.1 and 0.5 µm.

4. Austenitic material according to any one of the preceding claims, characterized by the fact that The condition 10 ≤ (C + N) / Nb ≤ 30 is met.

5. Austenitic material according to any one of claims 1 to 4, characterized by the fact that The nickel content is 0.50 - 3.00 wt.%.

6. Austenitic material according to any one of claims 1 to 4, characterized by the fact that The nickel content is 0.50 - 2.50 wt.%.

7. Austenitic material according to any one of the preceding claims, characterized by the fact that the yield strength R p0,2 in the solution-annealed state > 500 MPa and the elongation A5 is 55 - 80%.

8. Austenitic material according to any one of the preceding claims, characterized by the fact that the yield strength R p0,2 in the cold-worked state > 1900 MPa.

9. Austenitic material according to any one of the preceding claims, characterized by the fact that for the optimized PREN value (PREN OPT ) Cr + 3.3 × (Mo + 0.5 × W) + 20 × (C + N) - 0.5 × Mn = 36 to 51 and that the critical pitting temperature according to ASTM Pitting G48 Method E is 30°C to 60°C.

10. Method for producing an austenitic material, in particular according to one of claims 1 to 9, characterized by the fact that an austenitic alloy comprising or consisting of the following elements (all values ​​in wt.%): Carbon (C) 0,05 - 0,40 Silicon (Si) < 0,50 Manganese (Mn) 8,00 - 12,00 Phosphorus (P) < 0,05 Sulfur (S) < 0,005 Chromium (Cr) 17,50 - 20,50 Molybdenum (Mo) 0,50 - 3,50 tungsten < 3,00 Nickel (Ni) 0,20 - 3,00 Vanadium (V) < 0,50 Copper (Cu) < 0,50 Cobalt (Co) < 0,50 Titanium (Ti) < 0,50 Aluminum (Al) < 0,50 Niobium (Nb) 0,01 - 0,10 Nitrogen (N) 0,30 - 0,60 Residual iron and unavoidable impurities, whereby the following applies: 10 ≤ C + N / Nb ≤ 34 , The alloy is melted and treated using secondary metallurgy, the alloy is then cast into blocks and allowed to solidify, the blocks are then heated and immediately hot-formed into a forging or rolled piece by forging and / or rolling, the forging or rolled piece is then subjected to further cold forming and finally mechanical processing.

11. Method according to claim 10, characterized by the fact that The hot forming process takes place in several stages.

12. Method according to one of claims 10 - 11, characterized by the fact that Between the hot forming steps, the forging or rolled piece is reheated, and after the last hot forming step, if necessary, solution annealing is carried out at 1000 to 1200°C for a duration of 1 to 48 hours.

13. Method according to any one of claims 10 to 12, characterized by the fact thatWater quenching takes place after the last hot forming step and / or solution annealing.

14. Method according to any one of claims 10 to 13, characterized by the fact that After water quenching, cold forming is carried out with a degree of cold forming of 10 - 50%.

15. Use of the austenitic material according to claims 1 to 14 as a construction application as a tension wire or as a fastening screw with a diameter of ≤ 125 mm.

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