Austenitic material with high corrosion resistance and high strength, and method for the production thereof

Abstract

Austenitic material having a composition comprising or consisting of the following elements (all specifications in wt.%): Elements: carbon (C) 0.05–0.30, silicon (Si) < 0.50, manganese (Mn) 8.50–13.00, phosphorus (P) < 0.05, sulfur (S) < 0.005, chromium (Cr) 18.00–22.50, molybdenum (Mo) 0.50–5.00, nickel (Ni) 0.50–6.00, vanadium (V) < 0.50, tungsten (W) < 3.00, copper (Cu) < 0.50, cobalt (Co) < 0.50, titanium (Ti) < 0.50, aluminum (Al) < 0.50, niobium (Nb) < 0.03, boron (B) < 0.01, nitrogen (N) 0.40–0.80, with the remainder being iron and unavoidable impurities, wherein: formula A [wt.%]: 0.55 ≤ (C + N) ≤ 0.95 and formula B [-]: 0.07 ≤ (C / N) ≤ 0.50, and wherein formula A is in wt.% and formula B is dimensionless.

Description

[0001] International patent application

[0002] Voestalpine BÖHLER Edelstahl GmbH & Co KG 240550WÖ

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

[0004] The invention relates to an austenitic material with high strength, in particular for use in media with high corrosive attack, and a method for its production.

[0005] Such materials are used, for example, in chemical plant construction, oil or gas field technology, seawater treatment plants, the paper industry or shipbuilding.

[0006] Another requirement for austenitic materials is that they resist corrosive attack. Such materials are known, for example, from CN107876562 A, CN104195446 A, or DE 43 42 188 Al.

[0007] AT 412 727 B discloses a superaustenitic alloy as a drill string component.

[0008] The corrosion-resistant austenitic steel alloy chosen here is an alloy that includes particularly high levels of manganese, chromium, molybdenum, and nickel.

[0009] To achieve high strength, nitrogen is present in concentrations of 0.35 wt% to 1.05 wt%, where it also contributes to corrosion resistance and is a strong austenite former. However, with increasing nitrogen content, the tendency to form nitrogenous precipitates, especially chromium nitride, also increases.

[0010] To achieve this high nitrogen solubility, manganese is used in concentrations exceeding 19 to 30 wt.%. This is intended to ensure that pore-free materials can be produced even when solidifying under atmospheric pressure. Furthermore, the manganese is subject to an international patent application.

[0011] Voestalpine BÖHLER Edelstahl GmbH & Co KG 240550WÖ stabilizes the austenite structure against the formation of deformation martensite at high degrees of deformation.

[0012] 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.

[0013] 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.

[0014] From EP 2 455 508 Bl, an austenitic, corrosion-resistant steel with reduced manganese content (8 to 12 wt%) and a carbon-to-nitrogen (C-N) ratio 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 partial pressure of 1 bar nitrogen gas is necessary to achieve the required nitrogen content in the steel alloy. Furthermore, the examples given disclose that the austenitic alloy contains more carbon than nitrogen.

[0015] WO 02 / 02837 Al discloses a corrosion-resistant material for use in media with high chloride concentrations in oilfield technology. This is a chromium-nickel-molybdenum superaustenite characterized by low nitrogen content but very high chromium and nickel content.

[0016] These chromium-nickel-molybdenum steels typically exhibit improved corrosion resistance compared to the previously mentioned chromium-manganese-nitrogen steels. Overall, chromium-manganese-nitrogen steels are a relatively inexpensive alloy composition that nevertheless offers an excellent combination of strength, toughness, and corrosion resistance. International patent application

[0017] Voestalpine BÖHLER Edelstahl GmbH & Co KG 240550WÖ offers the following: The aforementioned chromium-nickel-molybdenum steels achieve significantly higher corrosion resistance than chromium-manganese-nitrogen steels, but are associated with considerably higher costs due to their very high nickel content.

[0018] From EP 2 455 508 Al, a high-strength and corrosion-resistant austenitic stainless steel and its manufacturing process are known. The nickel content is limited to a maximum of 2 wt.%. Through the controlled addition of the interstitial elements carbon and nitrogen (C+N = 0.6 to 1.0 wt.%) and the additional elements manganese (8 to 12 wt.%), chromium (15 to 20 wt.%), tungsten (< 4 wt.%), and molybdenum (< 2 wt.%), a tensile strength of over 850 MPa and excellent pitting resistance are achieved in an atmospheric induction melting process followed by hot forming and water quenching.

[0019] Superaustenitic alloys typically have molybdenum contents > 4 wt.% and a MA Copt value > 42 to achieve high corrosion resistance. However, molybdenum increases the tendency for segregation, thus increasing the susceptibility to precipitation, particularly of the brittle sigma or chi phases. Consequently, these alloys require an additional processing step, such as homogenization annealing. At molybdenum contents above 4 wt.%, remelting is even necessary to reduce segregation. Even then, the tendency for precipitation is so high that dimensions larger than 80 mm cannot be produced without precipitation due to the reduced cooling rate at the center.

[0020] Basically, it is necessary that the materials still have a magnetic permeability of p even after cold forming. r exhibit < 1.01.

[0021] Furthermore, such superaustenitic steels typically have a yield strength R P 0.2 of greater than 900 MPa after cold forming.

[0022] One characteristic value for corrosion resistance is the so-called PRENie value. Furthermore, it is common practice, as in EP 389 9064 Bl, to define corrosion resistance using MARCopt: MARCopt = wt.% Cr + 3.3* wt.% Mo + 20* wt.% N + 20* International patent application

[0023] Voestalpine BÖHLER Edelstahl GmbH & Co KG 240550WÖ

[0024] wt.% C - 0.5* wt.% Mn. At a MARCo P With a t-value of 20 - 42, it is a corrosion-resistant, austenitic alloy.

[0025] Another characteristic value for corrosion resistance against pitting corrosion (critical pitting temperature (CPT)) is given by the standard ASTM G150: Critical Pitting Temperature Test for stainless steel. For this test, samples of the austenitic material with a sample size of 25 x 30 x 5 mm are prepared, ground, and cleaned with deionized water and acetone. The sample is then placed in a test solution of 1 molar NaCl (corresponding to 58.45 g NaCl diluted to dilution II with deionized water). Electrodes and a thermometer are installed in the test cell for the measurement. Initially, the test solution is kept at a constant 10°C for 10 minutes. The temperature of the sample solution is then increased at 1°C / min. The time, temperature, and current density are continuously measured. The temperature is increased until the measured current density exceeds 100 pA / cm² for one minute.

[0026] According to ASTM G150, the CPT (temperature per minute) is defined as the temperature at which the current density exceeds 100 pA / cm² for one minute. If the measured current density exceeds 100 pA / cm² for one minute at the start of the experiment, the result is considered to be < 10°C.

[0027] Austenitic alloys are also known for their use as shipbuilding steels for submarines. These are chromium-nickel-manganese-nitrogen steels, further alloyed with niobium to stabilize the carbon. However, the latter reduces impact strength. These steels generally contain little manganese and therefore have relatively good corrosion resistance, but they do not achieve the strength of CrMnN alloys used for heavy bars. Furthermore, they contain nickel (Ni) in 10–16 wt%, which puts them at a cost disadvantage compared to CrMnN steels.

[0028] The object of the invention is to create an austenitic material that, with respect to its manganese and nickel content, exhibits a significantly reduced alloy design while simultaneously offering high corrosion resistance, high strength and toughness, as well as good paramagnetic properties. International patent application

[0029] Voestalpine BÖHLER Edelstahl GmbH & Co KG

[0030] 240550WÖ

[0031] The problem is solved using an austenitic material with the features of claim 4. Advantageous embodiments are characterized in dependent claims.

[0032] Furthermore, it is an object of the invention to provide a method for producing an austenitic material which exhibits high strength and toughness and good paramagnetic behavior in addition to increased corrosion resistance.

[0033] The problem is solved by a method having the features of claim 1. Advantageous further developments are characterized in the dependent dependent claims.

[0034] Any percentages given below are always given in wt.% (weight percent).

[0035] According to the invention, the austenitic material should have a completely austenitic microstructure, in particular without carbides and / or nitrides and / or carbonitrides and / or without deformation martensite even after cold forming, wherein the magnetic permeability p r < 1.01 preferred p r < 1.005. Since ferrite or deformation methylenesite exhibit magnetic behavior, they increase the permeability and should therefore be avoided in this embodiment.

[0036] After the hot forming process to which the cast block was subjected, the yield strength is at R P 0.2 > 450 MPa and the impact energy is greater than 250 at 20°C

[0037] After hot forming, the process is called cold forming. 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: (initial area A0 - final area Al) / initial area A0. If the initial area is 100 mm² 2 and the end surface 40 mm 2 The degree of deformation is calculated using the formula: (100 - 40) / 100 = 0.6 or 60%. After cold working with a degree of deformation of 10–90%, the yield strength R is... P o, i greater than 900 MPa, with values ​​greater than 1100 MPa achieved in practice. International patent application

[0038] Voestalpine BÖHLER Edelstahl GmbH & Co KG 240550WÖ. The impact energy at 20°C is greater than 80 J, although values ​​greater than 130 J are achieved in practice.

[0039] The impact strength is determined by the standard DIN EN ISO 148-1.

[0040] The yield strength P 0.2 is determined by the standard EN ISO 6892-1.

[0041] To increase corrosion resistance, the chromium content is increased, particularly to resist attack in media with high chloride concentrations. The manganese content in the alloy is also reduced. This has the additional benefit of minimizing the corrosive attack of fretting corrosion, thus increasing the theoretical service life. To ensure that the austenitic material is non-magnetic, non-ferritic steel alloys must be used. Since the chromium content is increased and the manganese content decreased, the austenitic alloy must be adjusted by the other austenite-forming elements: nickel, nitrogen, cobalt, and carbon. Empirical studies have shown that it is particularly advantageous if the formulas A = 0.55 < (C + N) < 0.95 and B = 0.07 < (C / N) < 0.50 apply.This ensures that the microstructure is a purely austenitic structure that meets the required strength and is also free of inclusions such as carbides and / or carbonitrides and / or nitrides and / or deformation martensite.

[0042] Furthermore, it is important that such austenitic materials are not only made from the appropriate alloys, but also that appropriate post-treatments ensure that a homogeneous, precipitation-free, high-strength and, in particular, high-impact alloy is present.

[0043] Therefore, such austenitic materials with a diameter of 80 mm to 450 mm, such as fasteners in the offshore industry, are selected to meet minimum mechanical property values, particularly the 0.2% yield strength. P0.2 and tensile strength are able to withstand the dynamically changing loads that occur. International patent application

[0044] Voestalpine BÖHLER Edelstahl GmbH & Co KG 240550WÖ

[0045] 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.

[0046] Carbon (C) is a strong austenite former and has a beneficial effect on high mechanical properties. For this reason, a lower limit of 0.05 wt.%, 0.06 wt.%, 0.07 wt.%, 0.08 wt.%, 0.09 wt.%, or 0.10 wt.% should be defined. To avoid carbide precipitates and embrittlement of the material, an upper limit of 0.30 wt.% should be selected. Preferably, a carbon content between 0.05 and 0.30 wt.% is intended.

[0047] Silicon (Si) serves to deoxidize steel. It also increases strength through the formation of solid solutions. To achieve these effects, a lower limit of 0.05 wt.%, 0.06 wt.%, or 0.07 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. Specifically, silicon can be added in amounts of < 0.50 wt.%, < 0.45 wt.%, < 0.40 wt.%, < 0.35 wt.%, or < 0.30 wt.%. A particularly preferred range lies between 0.05 and 0.30 wt.%.

[0048] Manganese (Mn) increases nitrogen solubility. It was previously assumed that manganese contents of more than 19.00 wt.%, preferably 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 8.50 wt.%. The upper limit for manganese is 13.00 wt.%. This is a remarkably low value compared to high-nitrogen materials according to the prior art. Preferably, a manganese content between 8.50 and 13.00 wt.% is provided. A particularly preferred range lies between 9.50 and 13.00 wt.%. International patent application

[0049] Voestalpine BÖHLER Edelstahl GmbH & Co KG 240550WÖ

[0050] According to literature, the addition of copper (Cu) proves advantageous for resistance in sulfuric acid. However, it has been shown that copper at values ​​> 0.50 wt.% increases the tendency for chromium nitride precipitation, which in turn negatively affects corrosion properties. Therefore, the upper limit is set at 0.50 wt.%, 0.15 wt.%, or 0.10 wt.%. The copper content can also be chosen so that it is below the detection limit. This means no deliberate addition of copper.

[0051] Chromium (Cr) increases the corrosion resistance of the alloy and is also essential for maintaining nitrogen solubility. For improved corrosion resistance, chromium contents of 17.00 wt.% or more are necessary. According to the invention, at least 18.00 wt.% and at most 22.50 wt.% chromium are present. This optimally influences the resistance to pitting and stress corrosion cracking. The lower limit for chromium is 18.00 wt.%, 18.50 wt.%, 19.00 wt.%, 19.50 wt.%, or 20.00 wt.%. The upper limit for chromium can be 22.50 wt.% or 22.00 wt.%. Preferably, a chromium content between 18.00 and 22.50 wt.% is provided. A particularly preferred range lies between 20.00 and 22.00 wt.%.

[0052] Molybdenum (Mo) contributes significantly to corrosion resistance in general and pitting corrosion resistance in particular. To utilize this positive effect of Mo, a lower limit of 0.50 wt.%, 0.60 wt.%, 0.70 wt.%, 0.80 wt.%, 0.90 wt.%, 1.00 wt.%, 1.10 wt.%, 1.20 wt.%, 1.30 wt.%, 1.40 wt.%, or 1.50 wt.% is required. However, molybdenum contents > 5.00 wt.% necessitate electroslag remelting (ESR) or pressure electroslag remelting (PESR) to prevent segregation. Such remelting processes are very complex and expensive. Therefore, a maximum of 5.00 wt.%, 4.50 wt.%, or 4.00 wt.% molybdenum is added. Preferably, a molybdenum content between 0.50 and 5.00 wt.% is provided. A particularly preferred range is between 1.50 and 4.00 wt.%.

[0053] Tungsten (W) has the same properties as molybdenum, thus contributing to increased corrosion resistance and strength. The upper limit for the tungsten content can be found in the international patent application.

[0054] Voestalpine BÖHLER Edelstahl GmbH & Co KG 240550WÖ

[0055] The tungsten content can be 3.00 wt%, 2.00 wt%, or 1.00 wt%. Due to the additional alloying of molybdenum, a lower limit for the tungsten content is not required.

[0056] Nickel (Ni) improves stress corrosion cracking resistance in chloride-containing media. Nickel is also an austenite-stabilizing element. To utilize the positive effects of nickel, a lower limit of 0.50 wt.%, 1.00 wt.%, 1.50 wt.%, 2.00 wt.%, or 2.10 wt.% is required. With increasing nickel content, the driving force for precipitation also increases. At the specified carbon and nitrogen levels, this would lead to undesirable carbide, nitride, and / or carbonitride precipitation. For this reason, the upper limit for nickel is chosen to be 6.00 wt.%, 5.50 wt.%, 5.00 wt.%, or 4.50 wt.%. Preferably, a nickel content between 0.50 and 5.50 wt.% or between 1.50 and 5.50 wt.% is used. A particularly preferred range is between 2.00 and 5.50 wt.%.

[0057] Cobalt (Co) can be added, particularly as a substitute for nickel. The upper limit for cobalt is 0.50 wt%, 0.40 wt%, 0.30 wt%, 0.20 wt%, or 0.10 wt%. The cobalt content can also be chosen so that it is below the detection limit (i.e., no deliberate addition of cobalt).

[0058] Nitrogen is added to ensure high strength. Furthermore, nitrogen contributes to corrosion resistance and is a strong austenite former. Therefore, nitrogen contents higher than 0.40 wt.%, and particularly higher than 0.45 wt.%, are advantageous. However, to avoid nitrogenous precipitates, especially chromium nitride, the upper limit of the nitrogen content is restricted to 0.80 wt.%, 0.75 wt.%, or 0.70 wt.%. It has been shown that, despite the very low manganese content compared to known alloys, these high nitrogen contents can be achieved in the alloy without pressure nitrogenation. Preferably, a nitrogen content between 0.40 and 0.80 wt.% is used. A particularly preferred range is between 0.45 and 0.70 wt.%.

[0059] If the levels of manganese, chromium, nickel and molybdenum are within the limits listed above, empirical evidence has shown that a particularly advantageous, synergistic international patent application

[0060] Voestalpine BÖHLER Edelstahl GmbH & Co KG

[0061] 240550WÖ

[0062] The effect is achieved when the ratio of carbon to nitrogen, (C / N) and (C + N), is chosen such that the austenitic alloy fulfills formulas A and B:

[0063] Formula A [wt%]: 0.55 < (C + N) < 0.95

[0064] Formula B [-]: 0.07 < (C / N) < 0.50, where formula A is given in wt.% and formula B is dimensionless.

[0065] This has surprisingly proven to be particularly advantageous. The ratio according to formula A ensures a completely austenitic microstructure and sufficient strength. In the present alloy, the manganese content is reduced to improve pitting corrosion resistance. However, reducing the manganese content also necessitates a reduction in austenite stabilizers. This is compensated for by increasing the carbon content. Furthermore, the carbon, as an interstitially dissolved element, ensures the required strength and also increases the solubility of nitrogen in the austenite. The ratio (C + N) in the range of 0.60 to 0.90 is particularly advantageous: 0.60 < (C + N) < 0.90.

[0066] The ratio according to formula B is particularly advantageous for this alloy, as it has a positive effect on the precipitation behavior.

[0067] It is theoretically conceivable to omit carbon from the alloy to meet the ratio specified in Formula A. However, this negatively impacts the strength and austenite stability. Therefore, the carbon / nitrogen ratio must be selected with a lower limit of 0.07, 0.08, 0.09, or 0.10. With increasing carbon content, however, the driving force for undesirable carbide formation also increases. By appropriately adjusting the nitrogen content according to Formula B, surprising effects occur regarding the mutual inhibition of precipitation. These effects are based on an interstitial expansion of the crystal lattice, resulting in more carbon and nitrogen remaining in solution.

[0068] If the manganese, chromium, nickel, and molybdenum concentrations are within the limits listed above, and the carbon-to-nitrogen ratio is chosen according to formula B, the thermokinetics of precipitation formation is surprisingly positively influenced. International patent application

[0069] Voestalpine BÖHLER Edelstahl GmbH & Co KG

[0070] 240550WÖ

[0071] For the given elements, precipitation can result in the formation of M₂₃C₆ and / or M₂N. In the case of the carbide M₂₃C₆, the metal-to-carbon ratio is approximately 4:1, while in the nitride M₂N, the metal-to-nitrogen ratio is 2:1. The carbide tends to have a higher precipitation potential, but because dissolved nitrogen atoms are present in abundance, the precipitation processes inhibit each other, and precipitation is delayed. After the final hot forming step and solution annealing, water quenching is required. This makes it possible to produce a precipitation-free alloy. Precipitation-free in this context means that no precipitates such as carbides, carbonitrides, or nitrides are present.

[0072] The ratio of (C / N) is particularly advantageous in the ranges 0.10 to 0.48: 0.10 < (C / N) < 0.48.

[0073] Optionally, boron, aluminum and sulfur may be included as additional alloying elements.

[0074] The alloying elements vanadium and titanium are not necessarily present in this steel alloy. Although these elements contribute positively to nitrogen solubility, high nitrogen solubility can also be ensured in their absence.

[0075] Niobium should not be included in the alloy, as concentrations above 0.015 wt.% can lead to the precipitation of niobium carbides or niobium carbonitrides. For austenitic alloys used in particularly corrosive environments, these precipitates have a detrimental effect; the preferential precipitation at grain boundaries reduces impact toughness. Niobium is typically used only for carbon bonding, which is undesirable in this alloy embodiment. Niobium contents up to 0.03 wt.% are still tolerable but should not exceed the content of unavoidable impurities. International patent application

[0076] Voestalpine BÖHLER Edelstahl GmbH & Co KG 240550WÖ

[0077] The invention thus relates to an austenitic material which has a composition (all values ​​in wt.%) comprising the following elements:

[0078] Carbon (C) 0.05 - 0.30 Silicon (Si) 0.05 - 0.50 Manganese (Mn) 8.50 - 13.00 Phosphorus (P) < 0.05 Sulfur (S) < 0.005 Chromium (Cr) 18.00 - 22.50 Molybdenum (Mo) 0.50 - 5.00 Nickel (Ni) 0.50 - 6.00 Vanadium (V) < 0.50

[0079] Tungsten (W) < 3.00 Copper (Cu) < 0.50 Cobalt (Co) < 0.50 Titanium (Ti) < 0.50 Aluminum (Al) < 0.50 Niobium (Nb) < 0.03 Boron (B) < 0.01 Nitrogen (N) 0.40 - 0.80

[0080] Residual iron and unavoidable impurities, whereby the following applies:

[0081] Formula A [wt%]: 0.55 < (C + N) < 0.95

[0082] Formula B [-]: 0.07 < (C / N) < 0.50 and where formula A is given in wt.% and formula B is dimensionless.

[0083] Further training stipulates that the material must be completely austenitic after cold forming, i.e., free of deformation martensite and / or carbides and / or carbonitrides and / or nitrides.

[0084] Further training requires that the following conditions are met:

[0085] Formula A [wt%]: 0.60 < (C + N) < 0.90

[0086] Formula B [-]: 0.10 < (C / N) < 0.48. International patent application

[0087] Voestalpine BÖHLER Edelstahl GmbH & Co KG 240550WÖ

[0088] Further development provides that the chromium content is 18.5 - 22.5 wt.%, particularly preferably 20 - 22.5 wt.%.

[0089] Further training stipulates that the Critical Pitting Temperature (CPT), measured according to the ASTM G150 standard, lies between 25 and 90°C.

[0090] Further training specifies that the nickel content should be 0.50 - 5.5 wt.%, preferably 1.5 - 5.5 wt.%.

[0091] Further training stipulates that the nickel content should be 2.0 - 5.5 wt.%.

[0092] Further training envisages that the austenitic alloy is obtained through secondary metallurgical treatment of the melt, casting into blocks, immediately followed by hot forging, cold forging and, if necessary, further mechanical processing.

[0093] Further training stipulates that the magnetic permeability pr < 1.01 after cold working.

[0094] Further training stipulates that the yield strength, measured according to the standard EN ISO 6892-1, is cold-worked. P 0.2 > 900 MPa.

[0095] Further training stipulates that the impact energy at 20°C, measured according to the standard DIN EN ISO 148-1, is greater than 130 J.

[0096] The invention further relates to a method for producing an austenitic material, wherein an alloy comprising the following components (all values ​​in wt.%):

[0097] Carbon (C) 0.05 - 0.30

[0098] Silicon (Si) 0.05 - 0.50

[0099] Manganese (Mn) 8.50 - 13.00

[0100] Phosphorus (P) < 0.05

[0101] Sulfur (S) < 0.005 International patent application

[0102] Voestalpine BÖHLER Edelstahl GmbH & Co KG 240550WÖ

[0103] Chromium (Cr) 18.00 - 22.50 Molybdenum (Mo) 0.50 - 5.00 Nickel (Ni) 0.50 - 6.00 Vanadium (V) < 0.50 Tungsten (W) < 3.00 Copper (Cu) < 0.50 Cobalt (Co) < 0.50 Titanium (Ti) < 0.50 Aluminum (AI) < 0.50 Niobium (Nb) < 0.03 Boron (B) < 0.01 Nitrogen (N) 0.40 - 0.80

[0104] Residual iron and unavoidable impurities, whereby the following applies:

[0105] Formula A [wt%]: 0.55 < (C + N) < 0.95

[0106] Formula B [-]: 0.07 < (C / N) < 0.50 and where formula A is given in wt.% and formula B is dimensionless, is melted and then treated by secondary metallurgy, the alloy thus obtained is cast into blocks and allowed to solidify, if necessary remelting, then heated and immediately hot-formed by forging or rolling, after the last hot-forming step being water-quenched, and the forgings being subjected to further cold forming and subsequent machining after cooling.

[0107] Further training involves hot forming in several stages, during which the block is reheated, and after the last hot forming stage, solution annealing at 1050 to 1200°C for a duration of 0.5 to 12 hours. Water quenching follows solution annealing.

[0108] Further training stipulates that cold forming is carried out after hot forming, and the degree of cold forming is between 10 and 90%.

[0109] Further development stipulates that the cold-formed, austenitic material has a final diameter of 80 to 450 mm. International patent application.

[0110] Voestalpine BÖHLER Edelstahl GmbH & Co KG 240550WÖ

[0111] The invention is explained by way of example with the aid of a drawing. The drawing shows:

[0112] Figure 1: a table showing the components of the alloy according to the invention;

[0113] Figure 2: highly schematic representation of the manufacturing process;

[0114] Figure 3: a table with three different alloys within the concept according to the invention (A, B, C) and three non-inventive alloys (D, E, F).

[0115] Figure 4: Isothermal phase diagram (calculated with ThermoCalc 2024a, database TCFE 12) for alloy B at 1100°C, with the stable phases: austenite (face centered cubic, FCC), ferrite (body centered cubic, BCC), M23C6, M7C3, and M2N and a gas phase (Gas) and the respective combinations of the stable phases therefrom;

[0116] Figure 5: Mechanical properties of the alloys from Figure 3 in the solution-annealed and cold-worked condition with a degree of deformation of 10 - 90 %

[0117] Figure 6: Corrosion characteristics and microstructure characterization of the alloys

[0118] The first column of the table in Figure 1 shows the elemental composition of the austenitic material according to the invention, which is characterized by increased corrosion resistance, high strength and toughness, and good paramagnetic behavior. These latter positive properties are achievable if the concentrations of the respective elements are within the limit ranges specified in the second column and satisfy formulas A (0.55 < (C + N) < 0.95) and B (0.07 < (C / N) < 0.50), although not all alloying elements are necessarily present.

[0119] In the present alloy, it is particularly surprising that nitrogen values ​​exceeding 0.40 wt.% are found despite the low content of elements that promote nitrogen solubility. International patent application

[0120] Voestalpine BÖHLER Edelstahl GmbH & Co KG

[0121] 240550WÖ, for example, manganese, without a special remelting process such as pressure electroslag remelting (DESU) or melting in a vacuum melting furnace under a nitrogen atmosphere.

[0122] Figure 2 shows a highly schematic representation of a possible process for manufacturing the alloy according to the invention. In the vacuum induction melting unit (VID), the material is simultaneously melted and undergoes secondary metallurgical treatment. Alternatively, the melt can be melted in an electric arc furnace (EAF) and subsequently treated with secondary metallurgy in an argon oxygen decarburization (AOD) converter. The melt is then poured into molds (ingots) and solidifies into ingots. The solidified ingots are then hot-formed. "Directly" in the context of the invention means that no additional remelting process, such as electroslag remelting (ESR) or pressure electroslag remelting (PESR), is necessary.

[0123] The alloy according to the invention has the advantage that homogenization annealing or remelting is not necessary.

[0124] The hot forming process then takes place in several stages. After the final stage, solution annealing is performed at 1050–1200°C for 0.5–12 hours. Following solution annealing, water quenching is carried out. This special process step allows critical temperature ranges to be traversed more quickly and prevents the formation of grain boundary precipitates and / or carbides and / or nitrides and / or carbonitrides. To determine the final properties, further cold forming steps are carried out, such as forming on a long forging machine or forming in a multi-line rolling mill, with the diameter of the final products ranging from 80 to 450 mm.

[0125] After cold forming, mechanical processing takes place, which can be, for example, turning or peeling.

[0126] The present austenitic material can be produced not only via the manufacturing route described and in particular schematically illustrated in Figure 2, but also according to an international patent application.

[0127] Voestalpine BÖHLER Edelstahl GmbH & Co KG 240550WÖ through a powder metallurgical production process, whereby the advantageous properties of the alloy can also be achieved.

[0128] Figure 3 shows three different variants of the alloy compositions according to the invention, which satisfy formulas A and B. In addition, two reference examples (D and E) are shown, which do not both satisfy formulas A and B and also deviate from the intended composition in other ways, and a further reference example F, which satisfies formulas A and B, but whose chromium and nickel contents fall outside the specified chemical analysis.

[0129] All six alloys tested were produced using the same process. All alloys were melted in a vacuum induction melting unit (VID) and subjected to secondary metallurgical treatment. The molten metal was then poured into molds (ingots) where it solidified into ingots. These ingots were subsequently hot-formed in several stages on the P52 forging press. All alloys were then subjected to solution annealing at 1100°C for 8 hours. This was followed by water quenching and an initial characterization of their properties. Cold forming then took place on the long forging press. All alloys were cold-formed to a final diameter of 200 mm, with a cold forming degree of 15% for all alloys. A final characterization of their properties was then performed.

[0130] Figure 4 shows an isothermal phase diagram for alloy B (calculated using the program ThermCalc 2024a, database TCFE 12 at 1100°C). The phase diagram consists of the phase components austenite (face-centered cubic, FCC), ferrite (body-centered cubic, BCC), M₂₃C₆, M₇C₃, and M₂N, and a gas phase (Gas). The ratio (C / N) is plotted on the secondary axis. The aim of the alloys according to the invention is to be in the stable austenite phase. The equilibrium line between austenite and (austenite + M₂₃C₆) lies just above the calculated ratio line of (C / N) = 0.50. For this reason, maintaining the upper limit of (C / N) = 0.50 is very important, as otherwise carbide precipitates will form. For the calculated alloy B, this shows that the alloy is in the stable austenite phase.It can also be seen that if the ratio of formula B were > 0.50, the calculated point would shift into the stable phase region of the International Patent Application.

[0131] Voestalpine BÖHLER Edelstahl GmbH & Co KG 240550WÖ

[0132] (FCC + M23C6), which leads to the formation of undesirable carbide precipitates. For comparison, alloys A and C to F are also shown in the diagram. However, these points are only of limited validity, as the alloys differ significantly in their chemical analysis. For example, alloys A and C are shown in the phase diagram for alloy B in the boundary region between FCC and (FCC+BCC). Compared to alloy B (2.14 wt% Ni), both alloys have a significantly higher nickel content (alloy A: 5.45 wt% Ni, alloy C: 4.09 wt% Ni). Due to the austenite-stabilizing effect of nickel, these alloys also exhibit pure austenite at 1100°C. In alloys D and E, the BCC phase and the undesirable carbide precipitates are present in the calculated phase diagram. A comparison between alloy F and alloy B is not valid due to the nickel content of 8.0 wt% in alloy F.The high nickel content increases the driving force for precipitation formation so strongly that carbide deposits occur.

[0133] Figures 5 and 6 characterize the six alloys from Figure 3. The tensile strength Rm, yield strength at 0.2% elongation (Rp0.2), and elongation A5 were determined according to EN ISO 6892-1. The impact strength was measured according to DIN EN ISO 148-1. Slight differences in the mechanical properties are observed in the solution-annealed condition of the alloys. All measured values ​​for Rp0.2 are > 450 MPa and Rm is > 850 MPa. These values ​​were lowest for alloy D, which is attributed to the lowest C + N ratio of 0.52. This can be compensated for with an increased C + N ratio. Significant differences in Charpy V impact strength are observed in the work-hardened condition. No values ​​> 74 J were measured for alloys E to F. This is due to the carbide excretions, which are characterized in Figure 6.

[0134] The most significant difference between alloys A to C according to the invention and alloys outside the specified chemical analysis lies in the corrosion resistance characteristics (standard ASTM G150: Critical Pitting Temperature Test for stainless steel) and in the microstructure characterization. International patent application

[0135] Voestalpine BÖHLER Edelstahl GmbH & Co KG

[0136] 240550WÖ

[0137] The microstructure, including individual phases, can be examined using a scanning electron microscope (SEM) at 20,000x magnification. For this purpose, a 10 x 10 mm sample is cut from the formed block with a diameter of 80–450 mm, preferably directly from the center, as the center of the formed block is of particular interest for characterization. The samples are then ground and polished. Additional etching allows for better identification of the phases. The phases to be determined are austenite, ferrite, deformation-induced martensite, carbides, nitrides, and carbonitrides.

[0138] The microstructure characterization of alloys A to C consistently shows an austenitic matrix – free of carbides, carbonitrides, nitrides, and / or deformation martensite. Alloy D deviates significantly from formulas A and B, with a (C + N) ratio of 0.52 wt.%, which is below the intended limit of 0.55 to 0.95 wt.%. Furthermore, the (C / N) ratio of 0.70 also deviates considerably from the intended limit of 0.07 < (C / N) < 0.50. These deviations, combined with the 200 mm diameter, lead to the formation of carbide inclusions in the alloy. This makes the alloy very poorly suited for corrosion resistance in aggressive media. In alloy E, the (C / N) ratio was increased even further to investigate its effects. This revealed even worse corrosion behavior and a significantly higher carbide content in the microstructure.

[0139] The negative effects of deviations from the intended alloy composition are most clearly seen in the results of the CPT temperature test. In reference examples D to F, this temperature is only 15°C. In contrast to the invention, sample F is already a superaustenitic alloy according to the MarcopT formula (MarcopT value = 42.1). Even though alloy F fulfills formulas A and B, 3 vol% carbides can still be detected in the microstructure during microstructural characterization. This is due to the increased nickel and chromium content. If the nickel content exceeds 6 wt%, carbide precipitation is unavoidable during water quenching in the presence of > 0.10 wt% carbon. Therefore, the invention is advantageous in that the austenitic material with good corrosion resistance has been created with very low manganese and nickel content.By adding carbon and nitrogen according to the international patent application.

[0140] Voestalpine BÖHLER Edelstahl GmbH & Co KG 240550WÖ Under the conditions described by formulas A and B, it has been possible to produce a completely austenitic alloy without inclusions, such as carbides, carbonitrides, nitrides or deformation martensites, which exhibits both high strength and good paramagnetic behavior; even after cold forming, a completely austenitic microstructure with a magnetic permeability of p is present. r < 1.005. In addition, corrosion resistance, in the form of the critical pitting temperature, could be improved.

Claims

International patent application Voestalpine BÖHLER Edelstahl GmbH & Co KG 240550WÖ Claims 1. A method for producing an austenitic material, in particular according to one of claims 1 to 12, characterized in that an alloy comprising the following components (all values ​​in wt.%): Elements Carbon (C) 0.05 - 0.30 Silicon (Si) 0.05 - 0.50 Manganese (Mn) 8.50 - 13.00 Phosphorus (P) < 0.05 Sulfur (S) < 0.005 Chromium (Cr) 18.00 - 22.50 Molybdenum (Mo) 0.50 - 5.00 Nickel (Ni) 0.50 - 6.00 Vanadium (V) < 0.50 Tungsten (W) < 3.00 Copper (Cu) < 0.50 Cobalt (Co) < 0.50 Titanium (Ti) < 0.50 Aluminum (AI) < 0.50 Niobium (Nb) < 0.03 Boron (B) < 0.01 Nitrogen (N) 0.40 - 0.80 Residual iron and unavoidable impurities, whereby the following applies: Formula A [wt%]: 0.55 < (C + N) < 0.95 Formula B [-]: 0.07 < (C / N) < 0.50 and where formula A is given in wt.% and formula B is dimensionless, is melted and subsequently treated by secondary metallurgy, the alloy thus obtained is cast into blocks and allowed to solidify, if necessary remelting, then heated and immediately hot-formed by forging or rolling, wherein after the last hot-forming step a International patent application Voestalpine BÖHLER Edelstahl GmbH & Co KG 240550WÖ Water quenching is carried out and the quenched hot-formed blocks are subjected to further cold forming and subsequent machining, wherein the hot forming is carried out in several partial steps, wherein the block is reheated between the hot forming steps, wherein after the last hot forming step a solution annealing at 1050 to 1200°C is carried out for a duration of 0.5 - 12h, and wherein water quenching is carried out after the solution annealing.

2. Method according to claim 1, characterized in that cold forming is carried out after hot forming, and the degree of cold forming is between 10 and 90%.

3. Method according to claim 1 or 2, characterized in that the cold-formed austenitic material has a final diameter of 80 - 450 mm.

4. Austenitic material produced by a process according to one of the preceding claims, comprising a composition comprising or consisting of the following elements (all values ​​in wt. %): Carbon (C) 0.05 - 0.30 Silicon (Si) 0.05 - 0.50 Manganese (Mn) 8.50 - 13.00 Phosphorus (P) < 0.05 Sulfur (S) < 0.005 Chromium (Cr) 18.00 - 22.50 Molybdenum (Mo) 0.50 - 5.00 Nickel (Ni) 0.50 - 6.00 Vanadium (V) < 0.50 Tungsten (W) < 3.00 Copper (Cu) < 0.50 Cobalt (Co) < 0.50 Titanium (Ti) < 0.50 Aluminum (AI) < 0.50 International patent application Voestalpine BÖHLER Edelstahl GmbH & Co KG 240550WÖ Niobium (Nb) < 0.03 Boron (B) < 0.01 Nitrogen (N) 0.40 - 0.80 Residual iron and unavoidable impurities, whereby the following applies: Formula A [wt%]: 0.55 < (C + N) < 0.95 Formula B [-]: 0.07 < (C / N) < 0.50 and where formula A is given in wt.% and formula B is dimensionless, where the material is completely austenitic after cold working, i.e. free of deformation martensite and / or carbides and / or carbonitrides and / or nitrides.

5. Austenitic material according to claim 4, characterized in that the following conditions are met: Formula A [wt%]: 0.60 < (C + N) < 0.90 Formula B [-]: 0.10 < (C / N) < 0.

48.

6. Austenitic material according to claim 4 or 5, characterized in that the chromium content is 18.5 - 22.5 wt.%, particularly preferably 20 - 22.5 wt.%.

7. Austenitic material according to one of claims 4 to 6, characterized in that the Critical Pitting Temperature (CPT) according to the ASTM G150 standard is between 25 and 90°C.

8. Austenitic material according to one of claims 4 to 7, characterized in that the nickel content is 0.50 - 5.50 wt.% and particularly preferably 1.50 - 5.50 wt.%.

9. Austenitic material according to one of claims 4 to 8, characterized in that the nickel content is 2.00 - 5.

50. International patent application Voestalpine BÖHLER Edelstahl GmbH & Co KG 240550WÖ 10. Austenitic material according to one of claims 4 to 9, characterized in that the magnetic permeability pr is <1.01 after cold working.

11. Austenitic material according to one of claims 4 to 10, characterized in that the yield strength according to standard EN ISO 6892-1 cold work-hardened po,2 > 900 MPa.

12. Austenitic material according to one of claims 4 to 11, characterized in that the impact energy at 20°C according to the standard DIN EN ISO 148-1 is greater than 130 J.

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