EP3899065B1 - Component for drill rod with high corrosion resistance and manufacturing method thereof - Google Patents

Description

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

In deep drilling technology, particularly in oil field or gas field technology, it is necessary to determine the course of a borehole as precisely as possible. This particularly applies to boreholes where drilling is not exclusively vertical or perpendicular, but also to boreholes where changes in direction are planned during the borehole. To do this, it is necessary to determine the course of the borehole as precisely as possible in order to be able to control the course of the borehole accordingly. This is usually done by determining the position of the drill head with the help of magnetic field probes, which use 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 parts of drill strings that are usually located in the immediate vicinity of magnetic field probes must have a relative magnetic permeability µ _R <0.1.

Such parts are in particular the so-called drill collars or MWD (Measurement While Drilling) and LWD (Logging While Drilling) parts, which are arranged above the actual drill head and serve, among other things, to accommodate the corresponding measuring electronics.

To ensure that the alloys from which these drill collars are made are non-magnetic, it is necessary to use non-ferritic steel alloys. These are essentially fully or super-austenitic alloys.

A further requirement for drill string components is that they withstand corrosive attack, especially attack in media with high chloride concentrations.

This is also related to the fact that drill string components are subject to particularly high torsional load cycling and torsional stresses. A corrosive attack would cause weakening through fatigue corrosion cracking, which would reduce the theoretical service life of such a drill string component.

In addition, it is important that such drill string components are not only made of the appropriate alloys, but that appropriate post-treatments ensure that a homogeneous, high-strength and, in particular, highly impact-resistant structure is present, in which no cracks are caused, for example, by intermetallic phases, coarse carbides or similar.

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, can withstand the dynamically changing loads that occur.

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

The corrosion-resistant austenitic steel alloy chosen here is an alloy that contains particularly high contents of manganese, chromium and molybdenum as well as nickel.

In order to achieve high strength, nitrogen is present in amounts of 0.35 wt.% to 1.05 wt.%, whereby nitrogen is also said to contribute to corrosion resistance and is a strong austenite former. On the other hand, as the nitrogen content increases, the tendency to form nitrogen-containing precipitates, especially chromium nitride, increases.

In order to achieve this high nitrogen solubility, manganese in particular is used in amounts of more than 19 - 30 wt.%. This is to ensure that pore-free materials can be produced even when solidifying under atmospheric pressure. In addition, the manganese At high degrees of deformation, the austenite structure is stabilized against the formation of martensite.

From the EP 1 069 202 A1 is a paramagnetic, corrosion-resistant, austenitic steel with high yield strength, strength and toughness, which is said to be corrosion-resistant in particular in media with high chloride concentration, whereby this steel is said to contain 0.6 mass-% to 1.4 mass-% nitrogen, with 17 to 24 mass-% chromium and manganese.

From the WO 02/02837 A1 is a corrosion-resistant material for use in media with high chloride concentrations in oil field technology. This is a chromium-nickel-molybdenum superaustenite that is formed with comparatively low nitrogen contents, but very high chromium and very high nickel contents.

These chromium-nickel-molybdenum steels usually have improved corrosion behavior 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. The chromium-nickel-molybdenum steels mentioned achieve significantly higher corrosion resistance than chromium-manganese-nitrogen steels, but are associated with significantly higher costs due to the very high nickel content.

Superaustenites usually have molybdenum contents > 4% in order to achieve high corrosion resistance. However, molybdenum increases the tendency to segregation and thus an increased susceptibility to precipitation, especially sigma or chi phases. This means that these alloys require homogenization annealing or, with values above 4% molybdenum, remelting is necessary to reduce segregation.

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

Such steels typically have a yield strength R _{p 0.2} of 140 KSI = 965 MPa.

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 , whereby a superaustenite is characterized by a PREN ₁₆ of o>42, where PREN = % Cr + 3.3 x % Mo + 16 x % N.

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

Classic drill collar steels are the chromium manganese nitrogen steels, which have already been mentioned because they have excellent properties and are still relatively inexpensive. They are used without niobium, whereby manganese sulphides form due to the higher manganese content, which has a negative effect on the corrosion properties.

Similar steel grades are also known for use as shipbuilding steels for submarines. These are chromium-nickel-manganese-nitrogen steels that are also alloyed with niobium to stabilize the carbon, but this reduces the impact toughness. These steels generally have little manganese and therefore have relatively good corrosion resistance, but do not achieve the strength of drill collar grades and, above all, not their toughness.

The object of the invention is to provide a drill string component, in particular for use in oil field technology, in particular a drill collar, which exhibits high corrosion resistance, high strength and good paramagnetic behavior.

The object is achieved with a component having the features of claim 1. Advantageous further developments are characterized in subclaims.

It is a further object of the invention to provide a method for manufacturing the component, with which a drill string component is created which exhibits high strength and good paramagnetic behavior with increased corrosion resistance.

The problem is solved with the features of claim 15. Advantageous further developments are characterized in the dependent subclaims.

Whenever percentages are given below, these are always wt.% (percent by weight).

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

After the hot forming step to which the cast ingot has been subjected, the yield strength at R _p0.2 is >450 MPa and can easily reach values >500 MPa, whereby the impact energy at 20°C is greater than 350 J and values up to 440 J can also be reached.

After work hardening, the yield strength is certainly R _p0.2 >1000 MPa, with values of up to 1100 MPa being achieved in practice, while the work hardened impact energy at 20°C is certainly greater than 80 J, with values of 200 J being achieved in practice.

The impact energy was determined according to DIN EN ISO 148-1.

This excellent combination of strength and toughness was previously unattainable and also not expected and is achieved by the special alloy layer according to the invention, which produces this synergistic effect.

According to the invention, values for the product of tensile strength Rm with notched impact strength KV of more than 100,000 MPa J, preferably > 200,000 MPa J, particularly preferably > 300,000 MPa J can be achieved.

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 tungsten (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

The rest consists of 100% iron (as noted in the table) and unavoidable impurities.

The first column (far left) shows the composition with which a drill collar according to the invention with the respective positive properties can be realized. Preferred variants are shown in the following columns, although not all alloying elements necessarily have to be present in a restricted manner; combinations of, for example, 5.2% Mn with 23.1% chromium are also conceivable.

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

What is completely surprising about the alloy according to the invention is that very high nitrogen levels can be achieved, which is extremely good for strength, and these nitrogen levels are surprisingly higher than those stated as possible in the specialist literature. According to empirical methods, the high nitrogen contents of the alloy according to the invention are not possible at all.

The individual elements and, where applicable, their interaction with the other alloy components are described in more detail below. All information regarding the alloy composition are given in weight percent (wt.%). Upper and lower limits of the individual alloying elements can be freely combined within the limits of the claims.

Carbon is present in amounts of 0.01 to 0.1%. Carbon is an austenite former and has a positive effect on high mechanical properties. In order to avoid carbide precipitation, the carbon content is set between 0.01 and 0.1% by weight. Silicon is provided in amounts of up to 0.5% by weight and is mainly used to deoxidize the steel. The specified upper limit safely prevents the formation of intermetallic phases. Since silicon is also a ferrite former, the upper limit has also been chosen with a safety margin in this regard. In particular, silicon can be provided in amounts of 0.1 - 0.3% by weight.

Manganese is present in contents of 4.0 - 7.0 3 0 wt.%. This is an extremely low value compared to state-of-the-art materials. Until now it was assumed that manganese contents of more than 19 wt.%, preferably more than 20 wt.%, are necessary for high nitrogen solubility. Surprisingly, it has been found with the present alloy that even with the low manganese contents according to the invention, a nitrogen solubility is achieved which is above what is possible according to prevailing expert opinion. However, it has been found with the present alloy that this is apparently not necessary due to unexplained synergistic effects. 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%.

The upper limit for copper is < 0.5 wt.% or < 0.15 wt.% or < 0.10 wt.% or below the detection limit (ie without any deliberate addition of alloys). Although according to the literature the addition of copper is beneficial for resistance in sulphuric acid, it has been shown that copper at values > 0.5% increases the tendency to precipitate chromium nitrides, which has a negative effect on the corrosion properties. According to the invention, the upper limit is therefore set at 0.5%. Chromium in contents of 17 wt.% or more proves to be necessary for higher corrosion resistance. According to the invention, at least 23% and at most 28% chromium is required. It has previously been assumed that contents higher than 24% by weight have a detrimental effect on the magnetic permeability because chromium is one of the ferrite-stabilizing elements. In contrast, it was found that in the alloy according to the invention, even very high chromium contents above 23% do not have a negative effect on the magnetic permeability in the alloy in question, but are known to have an optimal effect on the resistance to pitting and stress corrosion cracking. The lower limit for chromium can be selected at 24 or 25 or 26%. The upper limit for chromium can be selected at 29 or 30%.

Molybdenum is an element that makes a significant contribution 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 ESR treatment absolutely necessary to prevent segregation. Remelting processes are very complex and expensive. Therefore, according to the invention, DESU or ESR routes should be avoided.

According to the invention, tungsten is present in amounts below 0.5% and contributes to increasing corrosion resistance. The upper limit for tungsten can be selected at 0.5 or 0.4 or 0.3 or 0.2 or 0.1% or below the detection limit (i.e. without any deliberate addition of alloys).

According to the invention, nickel is present in contents of 11 to 15.5%, which achieves a 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%.

Cobalt is intended in contents of up to 5 wt.%, particularly for the substitution of nickel. The upper limit for cobalt can be selected at 5 or 3 or 1 or 0.5 or 0.4 or 0.3 or 0.2 or 0.1% or below the detection limit (ie without any deliberate addition of alloying).

Nitrogen is contained in amounts 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 contents higher than 0.52 wt.%, and especially higher than 0.54 wt.%, are advantageous. In order to avoid nitrogen-containing precipitations, particularly chromium nitride, the upper limit of nitrogen is limited to 0.85 wt.%. It has been proven that, despite the very low manganese content in contrast to known alloys, these high nitrogen contents in the alloy can be achieved without any pressure nitrogen addition (DESU).

Due to the good nitrogen solubility on the one hand and the disadvantages that arise with higher nitrogen contents, especially above 0.9%, any pressure nitrogen addition in the context of a DESU route is actually prohibited. Due to the low molybdenum content according to the invention, which is compensated by chromium and nitrogen, this is also not necessary. 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 or 0.60 or 0.65%. The upper limit for nitrogen can be selected at 0.90%.

According to VG Gavriljuk, H. Berns; "High Nitrogen Steels", p. 264, 1999 CrNiMn(Mo) austenitic steels melted under atmospheric pressure, such as the steel 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 advantage of the alloy according to the invention is that it has obviously been possible to achieve much higher nitrogen contents than expected without the need for pressure nitriding.

Boron, aluminium and sulphur may also be included as additional alloy components, but only optionally.

The alloying components vanadium and titanium are not necessarily included in the present steel alloy. Although these elements contribute positively to the solubility of nitrogen, the high nitrogen solubility according to the invention can be achieved even in their absence.

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

Figur 1:

eine Tabelle, zeigend die Bestandteile der Legierung;

Figur 2:

stark schematisiert den Herstellungsweg;

Figur 3:

eine Tabelle mit drei unterschiedlichen Legierungen innerhalb des Konzepts und den daraus resultierenden Ist-Werten des Stickstoffgehaltes gegen die rechnerische Stickstofflöslichkeit einer derartigen Legierung laut geltender Lehrmeinung.

Figur 4:

mechanische Eigenschaften der drei Legierungen aus Figur 3 nach einem Herstellungsverfahren mit Kaltverfestigung hergestellt

The invention is explained by way of example using a drawing. In the drawing:

Figure 1:

a table showing the components of the alloy;

Figure 2:

highly schematized production process;

Figure 3:

a table with three different alloys within the concept and the resulting actual values of the nitrogen content against the calculated nitrogen solubility of such an alloy according to current theory.

Figure 4:

mechanical properties of the three alloys from Figure 3 manufactured using a cold-work hardening process

The components are melted under atmospheric conditions and then subjected to secondary metallurgical treatment. Blocks are then cast and then hot forged. Direct in the sense of the invention means that no additional remelting process such as electroslag remelting (ESR) or pressure electroslag remelting (DESR) takes place.

The advantage of the alloy according to the invention is that homogenization annealing or remelting is not necessary.

Figure 2 shows examples of the possible process routes for the production of the alloy composition according to the invention. One possible route is now described as an example. In the vacuum induction melting unit (VID), the melt is simultaneously melted and treated using secondary metallurgy. The melt is then poured into molds (ingots) and solidifies there to form blocks. These are then hot-formed in several steps. For example, pre-forged on the P52 forging press and brought to final dimensions on the rotary forging machine. Depending on requirements, a solution annealing step and/or water cooling can also be carried out.

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

After the last hot forming step, the material is cooled down quickly to room temperature. This special process step allows critical temperature ranges to be passed through more quickly and prevents the formation of grain boundary precipitation. The product according to the invention shows that, for example, chromium nitride precipitation occurs to a much lesser extent, which has an optimal effect on the corrosion properties. The necessary cold forming steps then follow, during which work hardening takes place. The degree of deformation here is between 10 and 50%.

According to the invention, it is advantageous if the following relationship applies:
MARC _opt : $$40<\mathrm{wt}\%\mathrm{Cr}+3.3\times \mathrm{wt}\%\mathrm{Mo}+20\times \mathrm{wt}\%\mathrm{C}+20\times \mathrm{wt}\%\mathrm{N}-0.5\times \mathrm{wt}\%\mathrm{Mn}$$

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

This is followed by the necessary cold forming steps, in which work hardening takes place, and then the mechanical processing, which can in particular be turning or peeling.

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

In Figure 3 Three different variants within the alloy compositions are shown, with the nitrogen values measured in each case, which resulted from the procedure according to the invention in connection with the alloys according to the invention. These very high nitrogen contents contradict the nitrogen solubility 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." Different temperatures are indicated for Medovar. However, it is clear that the high nitrogen values far exceed those theoretically expected.

This is all the more surprising since the alloy according to the invention follows a path which would not lead one to expect such a high nitrogen solubility, in particular because the manganese content, which has a strongly positive influence on nitrogen solubility, is greatly reduced compared to known corresponding alloys.

In Figure 4 The three alloys are made of Figure 3 manufactured using a process and subjected to work hardening.

After this work hardening, R _p0.2 was around 1000 MPa for all three materials and the tensile strength Rm was between 1100 MPa and 1250 MPa. In addition, the notched bar impact energy was an excellent 270 J to even over 300 J (alloy C - 329.5 J).

This enabled an excellent combination of strength and toughness to be achieved, with the product of Rm*KV being more than 300000 MPa J for all three examples.

The invention therefore has the advantage that a drill collar alloy with increased corrosion resistance and low nickel content has been created, which at the same time shows high strength and paramagnetic behavior. Even after cold forming, a completely austenitic structure with a magnetic permeability µ _r <1.005 is present, so that it has been 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 (18)

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 - 0.1 silicon (Si) < 0.5 manganese (Mn) 4.0 - 7.0 phosphorous (P) < 0.05 sulphur (S) < 0.005 iron (Fe) residue chromium (Cr) 23.0 - 28 molybdenum (Mo) 2.5 - 4.0 nickel (Ni) 11 - 15.5 vanadium (V) < 0.5 tungsten (W) < 0.5 copper (Cu) < 0.5 cobalt (Co) < 5 titanium (Ti) < 0.1 aluminium (Al) < 0.2 niobium (Nb) < 0.1 boron (B) < 0.01 nitrogen (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 - 0.1 silicon (Si) < 0.5 manganese (Mn) 4.0 - 7.0 phosphorous (P) < 0.05 sulphur (S) < 0.005 iron (Fe) residue chromium (Cr) 24.0 - 28.0 molybdenum (Mo) 2.5 - 3.5 nickel (Ni) 12.0 - 15.5 vanadium (V) < 0.3 tungsten (W) < 0.1 copper (Cu) < 0.15 cobalt (Co) < 0.5 titanium (Ti) < 0.05 aluminium (Al) < 0.1 niobium (Nb) < 0.025 boron (B) < 0.01 nitrogen (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 - 0.10 silicon (Si) < 0.5 manganese (Mn) 5.0 - 6.0 phosphorous (P) < 0.05 sulphur (S) < 0.005 iron (Fe) residue chromium (Cr) 26.0 - 28.0 molybdenum (Mo) 2.5 - 3.5 nickel (Ni) 13.0 - 15.0 vanadium (V) below detection limit tungsten (W) below detection limit copper (Cu) < 0.1 cobalt (Co) below detection limit titanium (Ti) below detection limit aluminium (Al) < 0.1 niobium (Nb) below detection limit boron (B) < 0.01 nitrogen (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 µ_r < 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 - 0.1 silicon (Si) < 0.5 manganese (Mn) 4.0 - 7.0 phosphorous (P) < 0.05 sulphur (S) < 0.005 iron (Fe) residue chromium (Cr) 23.0 - 28 molybdenum (Mo) 2.5 - 4.0 nickel (Ni) 11 - 15.5 vanadium (V) < 0.5 tungsten (W) < 0.5 copper (Cu) < 0.5 cobalt (Co) < 5.0 titanium (Ti) < 0.1 aluminium (Al) < 0.2 niobium (Nb) < 0.1 boron (B) < 0.01 nitrogen (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: elements carbon (C) 0.01 - 0.1 silicon (Si) < 0.5 manganese (Mn) 4.0 - 7.0 phosphorous (P) < 0.05 sulphur (S) < 0.005 iron (Fe) residue chromium (Cr) 24.0 - 28.0 molybdenum (Mo) 2.5 - 3.5 nickel (Ni) 12.0 - 15.5 vanadium (V) < 0.3 tungsten (W) < 0.1 copper (Cu) < 0.15 cobalt (Co) < 0.5 titanium (Ti) < 0.05 aluminium (Al) < 0.1 niobium (Nb) < 0.025 boron (B) < 0.01 nitrogen (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 - 0.10 silicon (Si) < 0.5 manganese (Mn) 5.0 - 6.0 phosphorous (P) < 0.05 sulphur (S) < 0.005 iron (Fe) residue chromium (Cr) 26.0 - 28.0 molybdenum (Mo) 2.5 - 3.5 nickel (Ni) 13.0 - 15.0 vanadium (V) below detection limit tungsten (W) below detection limit copper (Cu) < 0.1 cobalt (Co) below detection limit titanium (Ti) below detection limit aluminium (Al) < 0.1 niobium (Nb) below detection limit boron (B) < 0.005 nitrogen (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.

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