High-strength austenitic material and method for the production thereof

Abstract

An austenitic material for use: in the construction industry, for pipes, tensioning wires or fastening elements; in the paper industry; for chemical apparatus construction; or for springs or valves or pumps; or in the electronics industry; comprising or consisting of the following elements (all specifications 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 (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, with the remainder being iron and unavoidable impurities, wherein: formula A: 10 ≤ (C + N) / Nb ≤ 34.

Description

[0001] International patent application

[0002] Voestalpine BÖHLER Edelstahl GmbH & Co KG 240551WO

[0003] Austenitic material with high strength and methods for its production

[0004] 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.

[0005] High-nitrogen alloyed austenitic materials are used in the construction industry as tension wire or fastening elements. These applications are characterized by their diameter of less than 125 mm.

[0006] From EP 1 069 202 Al, 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, and which contains 0.6 to 1.4 wt.% nitrogen. It also contains 17 to 24 wt.% chromium and manganese.

[0007] From EP 4 279 628 Al, 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.

[0008] From EP 2 455 508 Bl, 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 melting 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. International patent application

[0009] Voestalpine BÖHLER Edelstahl GmbH & Co KG

[0010] 240551WO

[0011] From EP 0 875 591 Bl, 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.

[0012] From EP 2 924 514 Bl, a watch or jewelry spring made of a stainless steel alloy with specified carbon-to-nitrogen ratios is known, where 0.40 wt.% < (C + N) < 1.5 wt.% and 0.125 < (C / N) < 0.55 wt.%. The nitrogen content of the alloy is also specified as 0.40 to 0.75 wt.%.

[0013] From JP H06 235048 A, a high-strength, non-magnetic stainless steel and its manufacturing process are known. The chemical composition of the steel includes, among other things, carbon, silicon, manganese, phosphorus, nickel, chromium, molybdenum, and nitrogen, whereby the ratio of the alloying elements to one another is specifically controlled to ensure the desired properties. The manufacturing process involves cold forming followed by an aging treatment.

[0014] Key parameters for corrosion resistance include the so-called PRENopT value, which is calculated as PRENOPT = Cr + 3.3 x (Mo + 0.5 x W) + 20 x (C + N) - 0.5 x Mn, and the so-called "pitting equivalent number", which is defined as MARCOPT = Cr + 3.3 x MO + 20 x N + 20 x C - 0.5 x Mn (element content in wt.%).

[0015] 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.

[0016] The problem is solved using an austenitic material having the features of claim 1. Advantageous embodiments are characterized in dependent claims. International patent application

[0017] Voestalpine BÖHLER Edelstahl GmbH & Co KG

[0018] 240551WO

[0019] 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.

[0020] The problem is solved by the features of claim 10. Advantageous further developments are characterized in the dependent subclaims.

[0021] Any percentages given below are always given in wt.% (weight percent).

[0022] 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 vibrational corrosion cracking and thus increasing the theoretical service life.

[0023] 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. 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.

[0024] According to the invention, the austenitic alloy is said to have a completely austenitic microstructure with niobium carbonitrides, wherein the magnetic permeability p r < 1.01. International patent application

[0025] Voestalpine BÖHLER Edelstahl GmbH & Co KG

[0026] 240551WO

[0027] 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 is determined. P 0.2 of the present alloy at P 0.2 > 450 MPa.

[0028] After cold working, the yield strength of the alloy or austenitic material in question is reliably at P 0.2 > 1900 MPa, although in practice values ​​up to 2100 MPa are achieved.

[0029] The yield strength P 0.2 Is determined by the standard DIN EN 2002-001.

[0030] The corrosion resistance against pitting corrosion is specified by the standard ASTM G48 - Method E: Critical Pitting Temperature Test for stainless steel.

[0031] 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.

[0032] 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.

[0033] 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. [International patent application.]

[0034] Voestalpine BÖHLER Edelstahl GmbH & Co KG 240551WO

[0035] A higher carbon content also increases the driving force for coarse (diameter > 5 pm) carbide precipitates such as M23C6 or M7C3. In this context, coarse precipitates are defined as carbides with a mean diameter of > 2 pm. 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, 0.38, or 0.35 wt.% should be selected. Preferably, a carbon content between 0.05 and 0.40 wt.% is used. A particularly preferred range is between 0.15 and 0.40 wt.%.

[0036] 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.%.

[0037] 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.%.

[0038] According to literature, the addition of copper proves advantageous for resistance in sulfuric acid. However, it has been shown that copper at values ​​> 0.5 wt.% increases the tendency to develop into an international patent application.

[0039] Voestalpine BÖHLER Edelstahl GmbH & Co KG 240551WO increases the precipitation of chromium nitrides, 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).

[0040] 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 the 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 pm) 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.%.

[0041] 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.%.

[0042] Tungsten exhibits the same properties as molybdenum, thus contributing to increased corrosion resistance and strength. In the present alloy, the tungsten content can be up to 3.0 wt.%. If this upper limit is exceeded, severe segregation occurs, necessitating remelting of the alloy. International patent application

[0043] Voestalpine BÖHLER Edelstahl GmbH & Co KG 240551WO

[0044] 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.%.

[0045] 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).

[0046] 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.%.

[0047] 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 grains but also increase their 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.%. International patent application

[0048] Voestalpine BÖHLER Edelstahl GmbH & Co KG

[0049] 240551WO

[0050] It has proven particularly advantageous if the levels of carbon, nitrogen and niobium meet the ratio according to formula A:

[0051] Formula A: 10 < (C + N) / Nb < 34

[0052] In formula A, carbon (C), nitrogen (N) and niobium (Nb) are used in their respective wt.%.

[0053] The austenitic alloy exhibits very good strength properties and 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.

[0054] 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.

[0055] 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.

[0056] The ratio (C + N) / Nb serves as an indicator for the formation of niobium carbonitrides. This precipitation can be controlled particularly well when the formula A (10 < (C + N) / Nb < 34) is fulfilled, since then very fine niobium carbonitrides with an average international patent application are formed.

[0057] Voestalpine BÖHLER Edelstahl GmbH & Co KG 240551WO

[0058] Niobium carbonitrides form diameters of < 0.5 pm. 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, this is not comparable to that of coarse chromium nitrides or chromium carbides. For applications of this alloy with a diameter < 125 mm in the construction industry (for pipes, tension wires, or fasteners), in the paper industry, in chemical plant engineering, for springs, valves, 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 pm, 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.

[0059] For these reasons, 10 < (C + N) / Nb < 34 applies, and 10 < (C + N) / Nb < 30 is particularly preferred.

[0060] Optionally, boron, aluminum and sulfur may be included as additional alloying elements.

[0061] 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.

[0062] It is advantageous if the following relationship holds true:

[0063] PRENOPT: Cr + 3.3 x (Mo + 0.5 x W) + 20 x (C + N) - 0.5 x Mn > 36th International Patent Application

[0064] Voestalpine BÖHLER Edelstahl GmbH & Co KG

[0065] 240551WO

[0066] The PREN formula is optimized such that the limit of 36 is necessary.

[0067] The invention thus relates to an austenitic material which has a composition comprising or consisting of the following elements (all values ​​in wt.-

[0068] %): 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 Tungsten (W) < 3.00 Nickel (Ni) 0.20 - 3.00 Vanadium (V) < 0.50

[0069] 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

[0070] Residual iron and unavoidable impurities, whereby the following applies:

[0071] Formula A: 10 < (C + N) / Nb < 34.

[0072] Further training stipulates that the austenitic material, after solution annealing, contains a microstructure of > 99.5 vol% austenite and < 0.5 vol% niobium carbonitrides.

[0073] Further research suggests that the average diameter of the niobium carbonitrides lies between 0.1 and 0.5 pm. International patent application.

[0074] Voestalpine BÖHLER Edelstahl GmbH & Co KG 240551WO

[0075] Further training requires that the condition 10 < (C + N) / Nb < 30 is met.

[0076] Further training stipulates that the nickel content should be 0.50 - 3.00 wt.%.

[0077] Further training stipulates that the nickel content should be 0.50 - 2.50 wt.%.

[0078] Further training stipulates that the yield strength po,2 according to the standard DIN EN 2002-001 in the solution annealed condition is > 500 MPa and the elongation A5 is 55 - 80%.

[0079] Further training stipulates that the yield strength R P 0.2 according to the standard DIN EN 2002-001 in the cold-worked state is > 1900 MPa.

[0080] Further training stipulates that the optimized PREN value (PRENOPT) is Cr + 3.3 x (Mo + 0.5 x W) + 20 x (C + N) - 0.5 x Mn from 36 to 51.

[0081] Further training stipulates that the critical pitting temperature according to ASTM Pitting G48 Method E is 30°C to 60°C.

[0082] The invention further relates to a method for producing the austenitic material, wherein an austenitic alloy comprising the following elements (all values ​​in wt.-

[0083] %):

[0084] Carbon (C) 0.05 - 0.40 Silicon (Si) < 0.50 Manganese (Mn) 8.00 - 12.00

[0085] Phosphorus (P) < 0.05 Sulfur (S) < 0.005 Chromium (Cr) 17.50 - 20.50

[0086] Molybdenum (Mo) 0.50 - 3.50 Tungsten (W) < 3.00 Nickel (Ni) 0.20 - 3.00 Vanadium (V) < 0.50 Copper (Cu) < 0.50 International patent application

[0087] Voestalpine BÖHLER Edelstahl GmbH & Co KG 240551WO

[0088] Cobalt (Co) < 0.50

[0089] Titanium (Ti) < 0.50

[0090] Aluminium (Al) < 0.50

[0091] Niobium (Nb) 0.01 - 0.10

[0092] Nitrogen (N) 0.30 - 0.60

[0093] Residual iron and unavoidable impurities, whereby the following applies:

[0094] Formula A: 10 < (C + N) / Nb < 34, is melted and treated by secondary metallurgy, the alloy being subsequently cast into blocks and allowed to solidify, the blocks then being heated and immediately afterwards forged and / or rolled into forgings and / or

[0095] Rolled pieces are hot-formed, with the forgings and / or rolled pieces subsequently being subjected to further cold forming and finally mechanical processing.

[0096] Further training stipulates that hot forming is carried out in several partial steps.

[0097] 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.

[0098] Further training stipulates that water quenching takes place after the last hot forming step and / or solution annealing.

[0099] Further training stipulates that after water quenching, cold forming with a degree of cold forming of 10-50% takes place.

[0100] 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.

[0101] All of the above-mentioned training courses can be combined. International patent application

[0102] Voestalpine BÖHLER Edelstahl GmbH & Co KG

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[0104] The invention is explained by way of example with the aid of a drawing. The drawing shows:

[0105] Figure 1: a table showing the components of the alloy according to the invention;

[0106] Figure 2: highly schematic representation of the manufacturing process;

[0107] 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;

[0108] Figure 4: Mechanical properties and microstructure characterization of all alloys in the solution-annealed condition.

[0109] The first column of the table in 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, although not all alloying elements necessarily have to be present in limited quantities.

[0110] 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 nitrogenization process such as pressure electroslag remelting (DESU) or melting in a vacuum melting furnace under a nitrogen atmosphere.

[0111] The production of all alloys is carried out in the same manner and is shown in a highly schematic form in Figure 2. 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 by secondary metallurgy. [The following appears to be unrelated and possibly a separate document:] International Patent Application

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[0114] Blocks are cast and then 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 (PER), takes place.

[0115] The alloy according to the invention has the advantage that homogenization annealing or remelting is not necessary.

[0116] 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.

[0117] 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 pm) niobium carbonitrides can be specifically controlled.

[0118] 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 surface area. The degree of deformation is calculated by subtracting the final surface area Ai from the initial surface area Ao and then dividing by the initial surface area Ao: (Ao - Ai) / Ao. For example, if the initial surface area is 100 mm² 2 and the end surface 30 mm 2 , corresponds to the degree of deformation due to cold working (100 - 30) / 100 = 0.7 or 70%.

[0119] Following cold forming, mechanical processing takes place, which can include turning, peeling, or grinding. International patent application

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[0121] The present austenitic alloy with its advantageous properties can be produced not only via the described (and in particular illustrated in Figure 2) manufacturing route, but also by a powder metallurgical production route.

[0122] Figure 3 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).

[0123] Figure 4 shows all mechanical properties, microstructure characterization, and evaluated corrosion properties of all alloys from Figure 3. The samples were characterized in the solution-annealed state, with the solution annealing process carried out at temperatures 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 subsequently quenched with water.

[0124] Determination of the mechanical properties tensile strength R m , Yield strength at 0.2% elongation ( P o,2) and elongation A5 were carried out according to the standard DIN EN 2002-001.

[0125] The resistance to pitting corrosion is specified by the standard ASTM G48 - Method E: Critical Pitting Temperature Test for stainless steel. In this test, 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 x Cr + 7.6 x Mo + 10.5 x N - 81.0. Thus, the starting temperature for an alloy of Fe-18Cr5MoO,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.

[0126] Furthermore, the corrosion resistance was calculated according to the optimized PREN formula: International patent application

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[0129] P ENOPT: Cr + 3.3 x (Mo + 0.5 x W) + 20 x (C + N) - 0.5 x Mn > 36

[0130] The formula was optimized so that tungsten also contributes positively to the increase in corrosion.

[0131] The microstructure, including the proportions of the individual phases, can be examined using a scanning electron microscope (SEM) at 20,000x magnification. This is preferably done by cutting a sample from a rod with a diameter of < 125 mm, followed by grinding and polishing the sample cross-section. The surface can also be etched to better identify the phases.

[0132] 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 parameters in the solution-annealed condition. The evaluation of the mechanical properties revealed the following for the yield strength R: P 0.2 consistently shows values ​​> 500 MPa. Likewise, all measured elongation values ​​A5 show a value of > 60%, which is a very good value for such an alloy.

[0133] 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 pm. 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.

[0134] The corrosion results also yielded a uniform evaluation scheme. This allowed the optimized PREN value for alloys A to C to be set above 36. Furthermore, the ASTM pitting temperature G48 according to method E was at least 30°C in all samples.

[0135] Reference samples D to G are described in more detail below. These differ 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). International patent application

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[0138] Sample D exhibits similar mechanical properties R m and PThe 0.2 value is similar to that of the alloys according to the invention. The optimized PREN value, at 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 PRENopT 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.%.

[0139] 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 pm.

[0140] 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).

[0141] Sample G also contains almost no niobium. Compared to all samples, it also has the lowest value for the sum of carbon and nitrogen contents at 0.57. This results in a lower strength level in the characteristic parameters R. m and R P0.2. However, it exhibits similar elongation values ​​to the alloys according to the invention. Nevertheless, for sample H, given the low optimized PREN value of 24.7 and a low ASTM pitting temperature G48 according to method E of 22 °C, a significantly poorer corrosion resistance can be observed. This is due to the missing elements molybdenum and tungsten. International patent application

[0142] Voestalpine BÖHLER Edelstahl GmbH & Co KG 240551WO

[0143] 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

International patent application Voestalpine BÖHLER Edelstahl GmbH & Co KG 240551WO 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 Aluminium (Al) < 0.50 Niobium (Nb) 0.01 - 0.10 Nitrogen (N) 0.30 - 0.60 Residual iron and unavoidable impurities, whereby the following applies: Formula A: 10 < (C + N) / Nb < 34.

2. Austenitic material according to claim 1, characterized in that the material of the austenitic material contains, after solution annealing at 1000 to 1200°C for a duration of 1 to 48h, a microstructure consisting of > 99.5 vol-% austenite and < 0.5 vol-% niobium carbonitrides.

3. Austenitic material according to claim 2, characterized in that the average diameter of the niobium carbonitrides is between 0.1 and 0.5 pm. International patent application Voestalpine BÖHLER Edelstahl GmbH & Co KG 240551WO 4. Austenitic material according to one of the preceding claims, characterized in that the condition 10 < (C + N) / Nb < 30 is met.

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

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

7. Austenitic material according to one of the preceding claims, characterized in that the yield strength Rpo,z in the solution annealed condition is > 500 MPa and the elongation A5 is 55 - 80%, measured according to the standard DIN EN 2002-001.

8. Austenitic material according to one of the preceding claims, characterized in that the yield strength Rpo,2 in the cold-worked condition is > 1900 MPa, measured according to the standard DIN EN 2002-001.

9. Austenitic material according to one of the preceding claims, characterized in that the optimized PREN value (PRENOPT) is Cr + 3.3 x (Mo + 0.5 x W) + 20 x (C + N) - 0.5 x 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 in 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 International patent application Voestalpine BÖHLER Edelstahl GmbH & Co KG 240551WO 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 Aluminium (Al) < 0.50 Niobium (Nb) 0.01 - 0.10 Nitrogen (N) 0.30 - 0.60 Residual iron and unavoidable impurities, whereby the following applies: Formula A: 10 < (C + N) / Nb < 34, is melted and treated by 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 in that the hot forming is carried out in several partial steps.

12. Method according to one of claims 10 to 11, characterized in that the forging 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.

13. Method according to one of claims 10 to 12, characterized in that water quenching takes place after the last hot forming step and / or solution annealing. International patent application Voestalpine BÖHLER Edelstahl GmbH & Co KG 240551WO 14. A method according to any one of claims 10 to 13, characterized in 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 9 as a construction application as a tension wire or as a fastening screw with a diameter of < 125 mm.

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