Drilling strand component with high corrosion resistance and method for its manufacture

Description

[0001] The invention relates to a drill string component, in particular for use in media with high corrosive attack, and a method for its manufacture.

[0002] In deep drilling technology, particularly in oil and gas fields, it is essential to determine the borehole trajectory as precisely as possible. This applies especially to drilling operations that are not exclusively vertical or perpendicular, but also those that involve changes in direction. Accurate borehole trajectory determination is crucial for controlling the borehole's path accordingly. This is typically achieved by determining the drill head's position using magnetic field probes, which utilize the Earth's magnetic field for measurement. For this purpose, certain components of the drill string are made of non-magnetic alloys. This means that sections of the drill string located in close proximity to magnetic field probes must have a relative magnetic permeability µR < 0.1.

[0003] Such parts include in particular the so-called heavy bars or MWD (Measurement While Drilling) and LWD (Logging While Drilling) parts, which are located above the actual drill head and serve, among other things, to accommodate the corresponding measuring electronics.

[0004] To ensure that the alloys used to manufacture these heavy bars are non-magnetic, it is necessary to use non-ferritic steel alloys. These are essentially fully austenitic or superaustenitic alloys.

[0005] Another requirement for drill string components is that they must withstand corrosive attack, especially in media with high chloride concentrations.

[0006] This is also related to the fact that drill string components are subject to particularly high torsional load cycles and torsional stresses. Corrosive attack would lead to a weakening through vibration-induced cracking, reducing the theoretical service life of such a drill string component.

[0007] Furthermore, it is important that such drill string components are not only made from the appropriate alloys, but that appropriate post-treatments ensure a homogeneous, high-strength and, in particular, high-impact structure in which, for example, intermetallic phases, coarse carbides or similar substances do not cause crack propagation.

[0008] Therefore, in order to be particularly suitable for deep hole drilling, such drill string components are selected so that the minimum values ​​of the mechanical properties, in particular the 0.2% yield strength and tensile strength, are sufficient to withstand the dynamically changing loads that occur.

[0009] Such a drill string component is known, for example, from AT 412 727 B. DE 3837457 C1 discloses a drill rod component.

[0010] The corrosion-resistant austenitic steel alloy chosen here is an alloy that includes particularly high levels of manganese, chromium and molybdenum as well as nickel.

[0011] 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. On the other hand, with increasing nitrogen content, the tendency to form nitrogenous precipitates, especially chromium nitride, also increases.

[0012] To achieve this high nitrogen solubility, manganese is used in concentrations exceeding 19–30 wt.%. This ensures that pore-free materials can be produced even when solidifying under atmospheric pressure. Furthermore, the manganese stabilizes the austenitic structure against the formation of deformation martensite at high degrees of deformation.

[0013] 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 corrosion-resistant in particular in media with high chloride concentration, wherein this steel is said to contain 0.6 wt.% to 1.4 wt.% nitrogen, and 17 to 24 wt.% chromium, as well as manganese.

[0014] WO 02 / 02837 A1 discloses a corrosion-resistant material for use in media with high chloride concentrations in oilfield technology. This material is a chromium-nickel-molybdenum superaustenite characterized by comparatively low nitrogen content but very high chromium and nickel content.

[0015] 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 cost-effective alloy composition that nevertheless offers an excellent combination of strength, toughness, and corrosion resistance. 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.

[0016] Superaustenitic alloys typically have molybdenum contents above 4% to achieve their high corrosion resistance. However, molybdenum increases the tendency for segregation and thus the susceptibility to precipitates, particularly sigma or chi phases. Consequently, these alloys require homogenization annealing, or, at molybdenum contents above 4%, remelting is necessary to reduce segregation.

[0017] Basically, it is necessary that the materials still have a magnetic permeability of µ r < 1.01 even after cold forming.

[0018] Such steels typically have a yield strength R p 0.2 of 140 KSI = 965 MPa.

[0019] Characteristic values ​​for corrosion resistance include the so-called PREN 16 value, whereby it is also common to define the so-called pitting equivalent number using MARC OPT, where a superaustenite with a PREN 16 of o>42 is characterized, where PREN = % Cr + 3.3 x % Mo + 16 x % N.

[0020] The well-known MARC formula for describing the pitting corrosion resistance of such steels is as follows: MARC = Cr + 3.3 Mo + 20 N + 20 C - 0.25 Ni - 0.5 Mn.

[0021] Classic heavy-duty steels are the chromium-manganese-nitrogen steels already mentioned, because they offer excellent properties while remaining relatively inexpensive. They are used without niobium, as the higher manganese content leads to the formation of manganese sulfides, which negatively impacts corrosion resistance.

[0022] Comparable steel grades are also known for use as shipbuilding steels for submarines; these are chromium-nickel-manganese-nitrogen steels, which are further alloyed with niobium to stabilize the carbon, although this reduces their impact toughness. These steels generally contain little manganese and therefore have relatively good corrosion resistance, but they do not achieve the strength of heavy-duty steel grades, and especially not their toughness.

[0023] The object of the invention is to create a drill string component, in particular for use in oilfield technology, especially a heavy rod, which exhibits high corrosion resistance, high strength and good paramagnetic behavior.

[0024] The problem is solved with a component having the features of claim 1. Advantageous further developments are characterized in dependent claims.

[0025] Furthermore, it is an object of the invention to provide a method for manufacturing the component with which a drill string component is created that exhibits high strength and good paramagnetic behavior in addition to increased corrosion resistance.

[0026] The problem is solved by the features of claim 15. Advantageous further developments are characterized in the dependent subclaims.

[0027] Any percentages given below are always given in wt.% (weight percent).

[0028] According to the invention, the drill string component should have a completely austenitic microstructure, in particular without deformation martensite, even after cold forming, wherein the magnetic permeability µr < 1.01, preferably µr < 1.005. Since ferrite or deformation martensite exhibit magnetic behavior, they increase the permeability and are therefore to be avoided according to the invention.

[0029] After the hot forming step to which the cast block was subjected, the yield strength is R p0.2 >450 MPa and can easily reach values ​​>500 MPa, with the impact energy at 20°C being greater than 350 J and values ​​up to 440 J also being reached.

[0030] After cold working, the yield strength is reliably at R p0,2 >1000 MPa, with values ​​up to 1100 MPa being achieved in practice, and the impact energy after cold working at 20°C is reliably greater than 80J, with values ​​of 200 J being achieved in practice.

[0031] The impact strength was determined according to DIN EN ISO 148-1.

[0032] This excellent combination of strength and toughness was previously unattainable and unexpected, and is achieved through the special alloy layer according to the invention, which creates this synergistic effect.

[0033] According to the invention, values ​​for the product of tensile strength Rm with impact strength KV of more than 100000 MPa J preferably > 200000 MPa J particularly preferably > 300000 MPa J can be achieved.

[0034] The alloy according to the invention comprises the following elements (all values ​​in wt.%): elements preferred further preferred Silicon (Si) < 0,5 < 0,5 < 0,5 Manganese (Mn) 5,0 - 6,0 Phosphorus (P) < 0,05 < 0,05 < 0,05 Sulfur (S) < 0,005 < 0,005 < 0,005 Iron (Fe) rest rest rest Chromium (Cr) 24,0 - 28,0 26,0 - 28,0 Molybdenum (Mo) 2,5 - 3,5 2,5 - 3,5 Nickel (Ni) -10,0 12,0 - 15,5 13,0 - 15,0 Vanadium (V) < 0,5 < 0,3 Below detection limit Wolfram (W) < 0,5 < 0,1 Below detection limit Copper (Cu) < 0,5 < 0,15 < 0,1 Cobalt (Co) < 5,0 < 0,5 Below detection limit Titanium (Ti) < 0,1 < 0,05 Below detection limit Aluminum (Al) < 0,2 < 0,1 < 0,1 Niobium (Nb) < 0,1 < 0,025 Below detection limit Boron (B) < 0,01 < 0,005 < 0,005 Nitrogen (N) 0,52 - 0,85 0,54 - 0,80

[0035] The remainder consists of 100% iron (as noted in the table) and unavoidable impurities.

[0036] The first column (far left) shows the composition with which a heavy bar according to the invention with the respective positive properties can generally be realized. Preferred variants are shown in the following columns, whereby not all alloying elements necessarily have to be present in limited quantities; combinations of, for example, 5.2% Mn with 23.1% chromium are also conceivable.

[0037] Such an alloy combines the positive properties of different steel grades.

[0038] In the alloy according to the invention, it is quite surprising that very high nitrogen values ​​can be achieved, which is extremely beneficial for the strength, with these nitrogen values ​​surprisingly exceeding those reported as possible in the literature. According to empirical methods, the high nitrogen contents of the alloy according to the invention are not at all possible.

[0039] The individual elements and, where applicable, their interaction 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 claims.

[0040] Carbon is present in concentrations of 0.01 to 0.1%. Carbon is an austenite former and has a beneficial effect on high mechanical properties. To prevent carbide precipitates, the carbon content is set between 0.01 and 0.1 wt.%. Silicon is present in concentrations up to 0.5 wt.% and primarily serves to deoxidize the steel. The specified upper limit reliably prevents the formation of intermetallic phases. Since silicon is also a ferrite former, the upper limit is further defined by a safety margin. In particular, silicon may be present in concentrations of 0.1 to 0.3 wt.%.

[0041] Manganese is present in concentrations of 4.0–7.0 wt.%. This is a remarkably low value compared to prior art materials. It was previously assumed that manganese contents of more than 19 wt.%, preferably more than 20 wt.%, were necessary for high nitrogen solubility. Surprisingly, with the present alloy, it has been found that even with the low manganese contents according to the invention, a nitrogen solubility is achieved that exceeds what is considered possible according to prevailing expert opinion. However, it has been found according to the invention that, due to as yet unexplained synergistic effects, this is apparently not necessary with the present alloy. The lower limit for manganese can be selected at 4.5 or 5.0%. The upper limit for manganese can be selected at 7.5 or 8.0%.

[0042] The upper limit for copper is chosen to be < 0.5 wt%, < 0.15 wt%, < 0.10 wt%, or below the detection limit (i.e., without any deliberate alloying). Although the literature indicates that alloying with copper is advantageous for resistance in sulfuric acid, it has been shown that copper at values ​​> 0.5% increases the tendency for chromium nitride precipitation, which negatively affects corrosion properties. Therefore, according to the invention, the upper limit is set at 0.5%. Chromium is found to be necessary for higher corrosion resistance at contents of 17 wt% or more. According to the invention, the chromium content is at least 23% and at most 28%. It was previously assumed that contents higher than 24 wt% would adversely affect magnetic permeability because chromium is one of the ferrite-stabilizing elements.In contrast, it was found that with the alloy according to the invention, even very high chromium contents above 23% do not negatively affect the magnetic permeability of the alloy, but, as is known, the resistance to pitting and stress corrosion cracking is optimally influenced. The lower limit for chromium can be selected at 24, 25, or 26%. The upper limit for chromium can be selected at 29 or 30%.

[0043] Molybdenum is an element that contributes significantly to corrosion resistance in general and pitting corrosion resistance in particular, with the effect of molybdenum being enhanced by nickel. According to the invention, 2.5 to 4.0 wt.% molybdenum is added. Higher molybdenum contents make electrostatic precipitate (ESP) treatment essential to prevent segregation. Remelting processes are very complex and expensive. Therefore, according to the invention, electrostatic precipitate (ESP) or electrostatic precipitate (ESP) routes should be avoided.

[0044] According to the invention, tungsten is present in concentrations below 0.5% and contributes to increased corrosion resistance. The upper limit for tungsten can be chosen to be 0.5%, 0.4%, 0.3%, 0.2%, or 0.1%, or below the detection limit (i.e., without any deliberate alloying).

[0045] According to the invention, nickel is present in concentrations of 11 to 15.5%, thereby achieving high stress corrosion cracking resistance in chloride-containing media. The lower limit for nickel can be selected at 12 or 13%. The upper limit for nickel can be selected at 16%.

[0046] Cobalt is intended to be used in concentrations up to 5 wt%, particularly as a substitute for nickel. The upper limit for cobalt can be chosen to be 5, 3, 1, 0.5, 0.4, 0.3, 0.2, or 0.1%, or below the detection limit (i.e., without any deliberate alloying).

[0047] Nitrogen is included in concentrations of 0.52 to 0.85 wt.% to ensure high strength. Nitrogen also contributes to corrosion resistance and is a strong austenite former, which is why concentrations higher than 0.52 wt.%, and especially higher than 0.54 wt.%, are advantageous. To avoid nitrogenous precipitates, particularly chromium nitride, the nitrogen content is limited to 0.85 wt.%. It has been shown that, despite the very low manganese content, these high nitrogen concentrations in the alloy can be achieved without any pressure nitriding (DESU), unlike in known alloys.

[0048] Due to the good solubility of nitrogen on the one hand, and the disadvantages that arise with higher nitrogen contents, especially above 0.9%, pressure nitriding within the framework of a DESU route is even contraindicated. This is also unnecessary due to the low molybdenum content according to the invention, which is compensated by chromium and nitrogen. It is particularly advantageous if the nitrogen-to-carbon ratio is greater than 15. The lower limit for nitrogen can be selected at 0.54%, 0.60%, or 0.65%. The upper limit for nitrogen can be selected at 0.90%.

[0049] According to VG Gavriljuk, H. Berns; "High Nitrogen Steels", p. 264, 1999, CrNiMn(Mo) austenitic steels melted under atmospheric pressure, such as the one according to the invention, achieve nitrogen contents of 0.2% to 0.5%. In the prior art, only CrMn(Mo) austenites achieve Mn contents of 0.5% to 1%. However, the alloy according to the invention has the advantage that it has apparently been possible to achieve much higher nitrogen contents than expected without the need for pressure nitriding.

[0050] Furthermore, boron, aluminum and sulfur may be included as additional alloying elements, but only optionally.

[0051] The alloying elements vanadium and titanium are not necessarily present in the steel alloy in question. Although these elements contribute positively to the solubility of nitrogen, the high nitrogen solubility required by the invention can also be achieved in their absence.

[0052] Niobium should not be included in the alloy according to the invention, as it can lead to precipitates which reduce toughness. Historically, niobium was only used to bind carbon, which is not necessary in the alloy according to the invention. Niobium contents up to 0.1% are still tolerable, but should not exceed the content of unavoidable impurities.

[0053] 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; Figure 2: A highly schematic representation of the manufacturing process; Figure 3: A table showing three different alloys within the concept and the resulting actual nitrogen content values ​​compared to the calculated nitrogen solubility of such an alloy according to current scientific opinion. Figure 4: Mechanical properties of the three alloys. Figure 3manufactured using a cold working process

[0054] The components are melted under atmospheric conditions and then further processed using secondary metallurgy. 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 (PER), is carried out.

[0055] The alloy according to the invention has the advantage that homogenization annealing or remelting is not necessary.

[0056] Figure 2Figure 1 illustrates the possible process routes for manufacturing the alloy composition according to the invention. One possible route is now described as an example. In the vacuum induction melting unit (VID), the material is simultaneously melted and undergoes secondary metallurgical treatment. Subsequently, the molten metal is poured into molds (ingots) and solidifies there into ingots. These are then hot-formed in several steps. For example, they are pre-forged on the P52 forging press and brought to their final dimensions on the rotary forging machine. Depending on the requirements, a solution annealing step and / or water cooling can also be carried out.

[0057] To determine the final properties, cold forming is carried out on a long forging machine, and the parts produced in this way are then further machined.

[0058] After the final hot forming step, rapid cooling to room temperature takes place. This particular process step allows critical temperature ranges to be traversed more quickly and prevents the formation of grain boundary precipitates. The product according to the invention shows that, for example, chromium nitride precipitates occur to a significantly lesser extent, thus optimally influencing the corrosion properties. The necessary cold forming steps then follow, during which work hardening occurs. The degree of deformation here is between 10 and 50%.

[0059] According to the invention, it is advantageous if the following relationship holds: MARC opt : 40 < wt % Cr + 3,3 × wt % Mo + 20 × wt % C + 20 × wt % N − 0,5 × wt % Mn

[0060] The MARC formula is optimized in such a way that it was found that the otherwise usual deduction of nickel does not apply to the system according to the invention and that the limit of 40 is necessary.

[0061] The necessary cold forming steps then take place, during which cold hardening occurs, followed by mechanical processing, which may include turning or peeling.

[0062] A superaustenitic material according to the invention can be used not only via the described (and in particular in) Figure 2 The advantageous properties of the alloy according to the invention can also be achieved by a powder metallurgical production route, as shown in the production routes.

[0063] In Figure 3Three different variants within the alloy compositions are shown, with the respective measured nitrogen values ​​obtained using the inventive process in conjunction with the inventive alloys. These very high nitrogen contents contradict the nitrogen solubility values ​​given in the right-hand columns according to Stein, Satir, Kowandar, and Medovar from "On restricting aspects in the production of non-magnetic Cr-Mn-N-alloy steels," Saller, 2005. Medovar specifies different temperatures. However, it is evident that the high nitrogen values ​​far exceed the theoretically expected values.

[0064] This is all the more surprising since the alloy according to the invention follows a path that does not suggest such a high nitrogen solubility, especially because the manganese content, which strongly positively influences nitrogen solubility, is greatly reduced compared to known corresponding alloys.

[0065] In Figure 4 The three alloys are made from Figure 3 manufactured according to a process and subjected to cold working.

[0066] After this cold working, the Rp0.2 for all three materials was approximately 1000 MPa, and the tensile strength Rm was between 1100 MPa and 1250 MPa. Additionally, the impact strength was excellent, ranging from 270 J to even over 300 J (alloy C - 329.5 J).

[0067] This enabled an excellent combination of strength and toughness to be achieved, with the product of Rm*KV exceeding 300000 MPa J in all three examples.

[0068] Thus, the invention offers the advantage of creating a heavy-duty bar alloy with increased corrosion resistance and low nickel content, which simultaneously exhibits high strength and paramagnetic behavior. Even after cold forming, a perfectly austenitic microstructure with a magnetic permeability µr <1.005 is present, making it possible to combine the positive properties of a cost-effective chromium-manganese-nickel steel with the outstanding technical properties of a chromium-nickel-molybdenum steel.

Claims

1. Drill string component, in particular drill collar, MWD- or LWD component for use in oil field technology and in particular in deep drilling, made of an alloy with the following composition, all values in % by weight, as well as unavoidable impurities: elements carbon (C)0.01 - 0.1silicon (Si)< 0.5manganese (Mn)4.0 - 7.0phosphorous (P)< 0.05sulphur (S)< 0.005iron (Fe)residuechromium (Cr)23.0 - 28molybdenum (Mo)2.5 - 4.0nickel (Ni)11 - 15.5vanadium (V)< 0.5tungsten (W)< 0.5copper (Cu)< 0.5cobalt (Co)< 5titanium (Ti)< 0.1aluminium (Al)< 0.2niobium (Nb)< 0.1boron (B)< 0.01nitrogen (N)0.52 - 0.85 wherein the drill string component has a yield strength cold-worked with a degree of deformation of 10 to 50% of Rp0.2 > 1000 MPa.

2. Drill string component according to Claim 1, characterized in that the alloy consists of the following elements, all values in % by weight, as well as unavoidable impurities: elements carbon (C)0.01 - 0.1silicon (Si)< 0.5manganese (Mn)4.0 - 7.0phosphorous (P)< 0.05sulphur (S)< 0.005iron (Fe)residuechromium (Cr)24.0 - 28.0molybdenum (Mo)2.5 - 3.5nickel (Ni)12.0 - 15.5vanadium (V)< 0.3tungsten (W)< 0.1copper (Cu)< 0.15cobalt (Co)< 0.5titanium (Ti)< 0.05aluminium (Al)< 0.1niobium (Nb)< 0.025boron (B)< 0.01nitrogen (N)0.52 - 0.85.

3. Drill string component according to Claim 1 or 2, characterized in that the alloy consists of the following elements, all values in % by weight, as well as unavoidable impurities: elements carbon (C)0.01 - 0.10silicon (Si)< 0.5manganese (Mn)5.0 - 6.0phosphorous (P)< 0.05sulphur (S)< 0.005iron (Fe)residuechromium (Cr)26.0 - 28.0molybdenum (Mo)2.5 - 3.5nickel (Ni)13.0 - 15.0vanadium (V)below detection limittungsten (W)below detection limitcopper (Cu)< 0.1cobalt (Co)below detection limittitanium (Ti)below detection limitaluminium (Al)< 0.1niobium (Nb)below detection limitboron (B)< 0.01nitrogen (N)0.54 - 0.80.

4. Drill string component according to Claim 1, characterized in that in the alloy composition, the element cobalt is < 5 or < 1 or < 0.5% or < 0.4% or < 0.3% or < 0.2% or < 0.1% or below the detection limit.

5. Drill string component according to one of Claims 1 to 4, characterized in that in the alloy composition, the element copper is < 0.3 or < 0.2 or < 0.1 or below the detection limit.

6. Drill string component according to one of Claims 1 to 5, characterized in that in the alloy composition, the element tungsten is < 0.5 or < 0.3% or < 0.2% or < 0.1% or below the detection limit.

7. Drill string component according to one of Claims 1 to 6, characterized in that nickel has an upper limit value of 15% or 15.5% or 15.8% and a lower limit value of 10.2% or 11% or 12% or 13%.

8. Drill string component according to one of the preceding claims, characterized in that the drill string component is achieved by secondary metallurgical treatment of the melt, casting into ingots, directly followed by hot forging, cold forging and, if necessary, further mechanical processing.

9. Drill string component according to one of the preceding claims, characterized in that the magnetic permeability is µr < 1.01 after cold forming.

10. Drill string component according to one of the preceding claims, characterized in that, when cold-worked, the notch impact energy at 20°C is greater than 80J, preferably > 110J, particularly preferably > 130J and in particular the product of Rm * KV > 100000 MPaJ.

11. Drill string component according to one of the preceding claims, characterized in that the material is completely austenitic, i.e. free of deformation martensite, after cold forming.

12. Drill string component according to one of the preceding claims, characterized in that, sulphur as an impurity does not exceed 0.005% by weight.

13. Drill string component according to one of the preceding claims, characterized in that, phosphorus is present as an impurity with no more than 0.05% by weight.

14. Method for producing a drill string component in particular according to one of the preceding claims, characterized in that an alloy with the following components, all values in % by weight, as well as unavoidable impurities: elements carbon (C)0.01 - 0.1silicon (Si)< 0.5manganese (Mn)4.0 - 7.0phosphorous (P)< 0.05sulphur (S)< 0.005iron (Fe)residuechromium (Cr)23.0 - 28molybdenum (Mo)2.5 - 4.0nickel (Ni)11 - 15.5vanadium (V)< 0.5tungsten (W)< 0.5copper (Cu)< 0.5cobalt (Co)< 5.0titanium (Ti)< 0.1aluminium (Al)< 0.2niobium (Nb)< 0.1boron (B)< 0.01nitrogen (N)0.52 - 0.85 is melted and then undergoes secondary metallurgical treatment, the alloy thus obtained is then cast into ingots and left to solidify and then heated up and is straight after hot formed by forging, wherein the forged parts are subject to further cold forming and subsequent mechanical processing, wherein a yield strength cold-worked with a degree of deformation of 10 to 50% of Rp0.2 > 1000 MPa is achieved.

15. Method according to Claim 14, characterized in that an alloy with the following components, all values in % by weight, as well as unavoidable impurities is melted: elementscarbon (C)0.01 - 0.1silicon (Si)< 0.5manganese (Mn)4.0 - 7.0phosphorous (P)< 0.05sulphur (S)< 0.005iron (Fe)residuechromium (Cr)24.0 - 28.0molybdenum (Mo)2.5 - 3.5nickel (Ni)12.0 - 15.5vanadium (V)< 0.3tungsten (W)< 0.1copper (Cu)< 0.15cobalt (Co)< 0.5titanium (Ti)< 0.05aluminium (Al)< 0.1niobium (Nb)< 0.025boron (B)< 0.01nitrogen (N)0.52 - 0.85.

16. Method according to Claim 14, characterized in that an alloy with the following components, all values in % by weight, as well as unavoidable impurities is melted elements carbon (C)0.01 - 0.10silicon (Si)< 0.5manganese (Mn)5.0 - 6.0phosphorous (P)< 0.05sulphur (S)< 0.005iron (Fe)residuechromium (Cr)26.0 - 28.0molybdenum (Mo)2.5 - 3.5nickel (Ni)13.0 - 15.0vanadium (V)below detection limittungsten (W)below detection limitcopper (Cu)< 0.1cobalt (Co)below detection limittitanium (Ti)below detection limitaluminium (Al)< 0.1niobium (Nb)below detection limitboron (B)< 0.005nitrogen (N)0.54 - 0.80.

17. Method according to one of Claims 14 to 16, characterized in that hot forming takes place in several sub-steps.

18. Method according to one of Claims 14 to 17, characterized in that in between the hot forming sub-steps, the forged part is heated up again, and after the last hot forming step, solution annealing takes place if necessary.