EP1069202B1 - A paramagnetic, corrosion resistant austenitic steel with high elasticity, strength and toughness and a process for its manufacture - Google Patents

Abstract

Production of components made of an alloy containing (in wt.%) maximum 0.1 degrees C, 0.21-0.6 silicon, 17.0-24.0 chromium, more than 20 but less than 30 manganese, more than 0.6 but less than 1.4 nitrogen, up to 2.5 nickel, up to 1.9 molybdenum, maximum 0.3 copper, up to 0.002 boron, up to 0.8 group IV and V element, and a balance of iron comprises melting the alloy, casting under atmospheric pressure, deforming at more than 850 degrees C and rapidly cooling, followed by further deforming at less than 600 degrees C and allowing to cool.

Description

The invention relates to a method for producing a paramagnetic material, particularly corrosion-resistant in media with a high chloride concentration, with a high yield strength, strength and toughness consisting of in% by weight
Max. 0.1 carbon
0.21 to 0.6 silicon
17.0 to 24.0 chromium as well as manganese and nitrogen
up to 2.5 nickel
up to 1.9 molybdenum
Max. 0.3 copper
up to 0.002 boron
up to 0.8 elements of groups 4 and 5 of the periodic table
Remainder iron, melting-related accompanying elements and impurities.

The invention further relates to an austenitic, paramagnetic material good corrosion resistance, especially in media with high Chloride concentration, and high yield strength, strength and toughness made of carbon, manganese, silicon, chromium and nitrogen, and optionally Nickel, molybdenum, copper, boron, carbide-forming elements, remainder iron, Melting-related accompanying elements and impurities.

Finally, the invention includes the use of one at the beginning mentioned method manufactured material.

High-strength materials that are paramagnetic and corrosion-resistant economic reasons mainly from chromium-manganese-iron alloys exist for chemical apparatus engineering, for facilities for electrical power generation and especially for components in the Oilfield technology used. Both the corrosion chemical and the mechanical properties of such materials are described in increasingly higher demands.

For essentially all of the uses listed above, it is essential that the material is completely homogeneous, highly non-magnetic or behaves paramagnetically. For generator cap rings with a high yield strength and toughness, for example, must be a slight one ferromagnetic behavior even in parts of the material with the highest security be excluded. For measurements when drilling holes, especially exploration drilling in oil or gas fields Collars made of materials with magnetic permeability values below 1.02 or less than 1.018 required to track the location closely determine the hole and deviations from the intended course of the same and to be able to correct.

Furthermore, it is also necessary that facilities of oil field technology and Drill string components have a high mechanical strength, in particular a have a high 0.2% proof stress in order to technical advantages and high operational reliability. As well in many cases it is important to have a high fatigue strength, because with one Rotation of the part or the drill collars swelling or changing Stresses can exist.

Finally, the corrosion behavior of the Material in aqueous or oil-containing media, especially with high Chloride concentration.

The requirements of recent developments in plant and According to Tierfbohrtechnik, materials are subject to ever stricter standards regarding the combination of paramagnetic behavior, high yield strength and the like strength and resistance to chloride-induced Stress corrosion cracking as well as pitting corrosion (pitting corrosion) and crevice corrosion posed.

There are materials made of Cr-Mn-Fe alloys known with regard to mechanical properties and the corrosion behavior definitely match those the relevant requirements are met, only their magnetic Permeability values prevent use for parts related to magnetic measurements are used and include, for example Use for drill collars. On the other hand, required non-magnetic materials with good strength properties Do not withstand corrosive attack and largely use paramagnetic parts high corrosion resistance often do not have the necessary high resistance mechanical values.

It is known that the mechanical and nitrogen contents Corrosion-chemical properties of essentially Cr -Mn-Fe alloys to improve, however, using expensive metallurgical processes involving increased Press work are required.

Iron-manganese-chromium alloys were also used for economic reasons developed that without pressure melting and the like casting process, so at Atmospheric pressure can be produced (WO 98/48070), whereby alloying measures a desired property profile of the Material should be achieved. Alloys of this type point to Improvement in corrosion resistance to a molybdenum content of over 2%, which brings advantages in particular with hole and crevice corrosion behavior. However, like chromium, molybdenum is a ferrite former and can be found in segregation areas lead to unfavorable magnetic properties of the material. Increased Nickel contents stabilize the austenite, but may also contribute higher copper concentrations worsening the mechanical Properties and intensify the crack initiation.

DE-A-3 143 096 an alloy for use in cap rings and turbine parts. However, this document does not disclose cold forming at elevated temperature from 350 to 600 ° C.

A balanced concentration of the alloying elements results in PCT / US91 / 02490 tried an austenitic antimagnetic stainless To create steel alloy, but with hot machining without subsequent Remuneration has an excellent combination of properties.

In particular, the mechanical properties of non-magnetic A method has already been proposed to improve drill string parts (EP-0207068 B1), in which the material is hot and cold formed is subjected to the cold forming at a temperature between 100 ° C. and 700 ° C with an at least 5% degree of deformation.

The invention aims to provide a method which Exhaustion of alloying measures includes deformation and synergistically a manufacture of a highly magnetic paramagnetic, in Media with a high chloride concentration are corrosion-resistant, ferrite-free Material with high yield strength, strength and toughness indicates.

Another object of the invention is a fully austenitic paramagnetic Material with good corrosion resistance and high mechanical values create.

The aim of the invention is achieved in a method according to claims 1 to 19.

The advantages achieved by the invention are essentially to be seen in that with high efficiency regarding the material costs and Manufacturing process by an alloy-technical optimization largest Corrosion resistance and a desired paramagnetic behavior of the Material can be achieved, whereby its high mechanical characteristics, especially the yield strength, without adversely affecting the above properties mentioned by a targeted cold forming increased temperature experienced a further significant improvement.

The carbon content of the alloy is max. 0.1% by weight, because higher levels result in both pitting and stress corrosion cracking chloride-containing media as well as intercrystalline corrosion from it manufactured parts. Compliance with this upper limit, with levels of Max. 0.06 and 0.05% by weight is preferred, as mentioned above, from Corrosion-chemical reasons important, although carbon increases the yield strength and has a strong austenite-forming effect.

Silicon is said to be a deoxidation metal with a concentration of at least 0.21 % By weight is present in the metal, an upper limit of 0.6% by weight being provided. Higher silicon levels lead to nitride formation and deterioration of the Resistance to stress corrosion cracking of the material. Because silicon is also strong Magnetic permeability can also have a ferrite-forming effect due to higher contents be adversely affected. A maximum content of 0.48 is advantageous % By weight effective.

Corrosion behavior, especially resistance to Stress corrosion cracking and pitting corrosion is caused by the chromium content of the Alloy determined. It is important that a largely homogeneous Chromium distribution in the material is present; in other words, the so-called Weak spots in the passive layer due to segregation and inclusions avoided are. To be largely secured, a desired corrosion resistance too chromium contents of more than 17% by weight, preferably of more than 19% by weight required. Chromium increases the solubility of the alloy for Nitrogen, however, has a ferrite-forming effect and is therefore unfavorable with regard to the desired one non-magnetic or paramagnetic behavior of the material, so that the highest chromium concentration is 24.0% by weight, preferably 22.0% by weight.

Nickel can increase the mechanical properties of the alloy and the stability the austenitic structure can be improved for sufficiently good ones Corrosion properties of the material, especially the Regarding stress corrosion cracking, nickel contents are less than 2.5% by weight. better, but less than 0.96 wt .-% required. Due to low nickel contents From 0.21% by weight up to the above maximum values it is possible without disadvantages in the corrosion behavior of the desired alloy, an increase in To reach yield strength.

The alloy element molybdenum improves the durability of the material against corrosion, especially against chloride-induced crevice and pitting corrosion. However, because this element is a strong ferrite former and the like Carbide formers and formers of socialized phases are the Molybdenum caps at 1.9% by weight, but better at 1.5% by weight. low Levels from 0.28 wt .-% molybdenum up to the above limits can Precipitation-free austenite structure of the micro-chemical structure Advantages bring.

The element copper, which is often effective against corrosion attack, has been used in the However, the alloy according to the invention was found to have a disadvantageous effect, wherein the copper contents less than 0.3% by weight, but better less than 0.25% by weight to achieve resistance to corrosion.

Boron can be used to improve the hot forming behavior of the material an amount of up to 0.002% by weight, preferably up to 0.0012% by weight. Higher amounts of boron lead to grain boundary deposits, Signs of embrittlement and undesirable structures.

Are particularly important for preventing stress cracking and pitting corrosion low levels of elements of group 4 and group 5 of the periodic table. These elements (Ti, Zr, Hg, V, Nb, Ta) are extremely strong carbide and nitride or Carbonitride formers and have values of less than 0.8 in total % By weight, better of less than 0.48% by weight. Higher concentrations cause excretions and thereby weak points in the passive layer on the Workpiece surface, which affects the corrosion resistance.

In terms of alloying, the element nitrogen is a strong austenite former. In addition, the yield strength and the resistance of the material to hole and Crevice corrosion due to nitrogen increased. Nitrogen is in iron based alloys however, only soluble to a limited extent, with increasing chromium and manganese contents Solubility limit is increased. The chromium-manganese and Nitrogen concentrations of the alloy synergistically for the invention To see material or for its properties. A chromium content of 17.0 to 24.0% by weight, preferably from 19.0 to 22.0% by weight, as before explained the material mainly for reasons of corrosion resistance and the paramagnetic behavior. The manganese content of more than 20 % By weight, but less than 30% by weight, the preferred ones Concentration ranges between 20.5 or 21.5 and 29.5 or 25.0% by weight lie on the one hand to increase nitrogen solubility and on the other hand to Stabilization of the austenitic or ferrite-free structure is provided. Finally, the nitrogen content is greater than 0.6% by weight, but less than 1.4% by weight, essentially the accessibility of high yield strength values.

Preferred nitrogen concentration ranges are:
0.64 to 1.3 wt .-%, in particular 0.72 to 1.2 wt .-% N. Low manganese contents of 20 wt .-% and lower and high nitrogen concentrations of 1.4 wt .-% and larger due to an abrupt decrease in the nitrogen solubility of the alloy when it solidifies into porous or leaky castings. With manganese contents of 30% by weight and more and with nitrogen contents of 0.6% by weight and less, high yield strengths cannot be achieved and embrittlement of the material can occur.

If, as intended for reasons of material quality and economy is, the casting block or the casting has solidified under atmospheric pressure, can this or this diffusion annealing, which is a homogenization of the Serves microstructure or a compensation of micro segregations become. This annealing can be carried out, for example, at a temperature of around 1200 ° C up to 60 hours.

The hot forming of the casting, which represents the first deformation step, mostly done by forging, whereby the forming temperature is higher than 850 ° C, to ensure a correspondingly favorable recrystallization of the mixed structure. The forging shaped in this way is, as a rule, from the forging heat cooled down at increased speed. This cooling, the avoidance of Excretions, especially at the grain boundaries, can be in one Water basin or with a continuous cooling section. there can also be advantageous if after the first step the deformed block intermediate annealing at an annealing temperature of over 850 ° C and thereafter is subjected to cooling at an increased rate because it does so any precipitates formed are brought back into solution.

In the second step, the forging is made at a temperature below 600 ° C reshaped, a solidification of the material, in particular a desired increase in the proof stress occurs. Despite the high chrome and Manganese content, in particular, surprisingly remains the material fully austenitic or ferrite-free; there is therefore no expected partial folding to form a structure with deformation martensite. It turned out to be proved favorable when the deformation of the forged casting in the second Step at elevated temperature, but safely below 600 ° C and then the deformed shaped part is allowed to cool to room temperature. Manufacturing technology, but also in terms of improved homogeneity and Material quality can be favorable if the block uses an ESR process will be produced.

The material quality can be further increased if the block in the first step with a degree of deformation, which is defined: output cross section through Final cross-section is at least 4 times thermoformed. This makes a fine, recrystallized, uniform ferrite-free austenite structure achieved.

After cooling at an elevated rate from a temperature above 850 ° C, which serves to prevent the formation of excretions, it will Forging in the second step with a deformation in%, defined as Output cross-section minus final cross-section broken by output cross-section times 100 deformed by less than 35%, whereby the yield strength and the strength of the Material can be increased. In the sense of an even increase in mechanical A recrystallization-free deformation range of 5% to 20% exposed.

Both for performing cold forming as well as for an effective one profound and embrittlement-free improvement of the material properties and a safe avoidance of the occurrence of deformation martensite has proven to be particularly advantageous, the forging in the second step in a To transform temperature in the range of 400 to 500 ° C.

An austenitic, paramagnetic material with the composition mentioned with good Corrosion properties that are thermoformed at least 3.5 times and below the Elimination temperature of nitrides and associated phases, however is cold-formed above a temperature of 350 ° C, shows the slightest traces of ferrite, practically no ferrite content in the preferred ranges of the composition on and behaves essentially paramagnetically with a relative Permeability below 1.05, in particular below 1.016.

The yield strength R _{P0.2 of} the material at room temperature is higher than 700 N / mm ² . The values for the impact strength at room temperature are greater than 52J and the FATT (Fracture Appearance Transition Temperature) is lower than -25 ° C. The material according to the invention also has a fatigue strength of greater than + 400 N / mm ² at N = 10 ⁷ load changes and has a hole potential in neutral solutions (according to ASTM G5 / 87) at room temperature of greater than 700 mV _H / 1000ppm chlorides and / or 200mV _H / 80000ppm chlorides.

The invention is explained in more detail with the aid of examples.
Table 1 shows the chemical composition of all reference materials and, additionally, the deformation data for samples 1 to 3 and A to E. Samples 4 to 6 come from comparative material that was available on the market.
Table 2 summarizes the results regarding the magnetic property, the mechanical values and the corrosion behavior.

Samples 2 and A were made from steel that was melted in the induction furnace and cast into blocks under protective gas.
Samples 1, 3, B to E come from ESR material.

The materials of samples 1 and 3 have low magnetic data with good magnetic data Yield strengths and strength values. Good toughness and sufficient FATT and the corresponding oxalic acid test pattern show low hole potentials opposite, whereby the materials due to an insufficient Eliminate property profile for high loads. The causes of this are in the low chromium and manganese contents as well as in the consequence low nitrogen concentrations.

The material of sample 2 does have a sufficiently high chromium content, however, low manganese and the like cause nitrogen in particular poor corrosion resistance.

The samples A to E produced by means of the method according to the invention are significantly improved in leaps and bounds in the entirety of the usage properties. Synergistically, the respective, coordinated concentrations of the alloying elements and the solidifying cold forming of the material produced without precipitation provide superior corrosion resistance with a low relative magnetic permeability and a substantial increase in the strength values of the same. This is also shown by the test results or measured values of the freely obtained alloy samples 4 to 6.

Claims (19)

1. A process for producing a paramagnetic austenitic steel article which is corrosion-resistant in media with a high chloride concentration and which has a high yield stress, strength and toughness, consisting of an alloy in % by weight max. 0·10 carbon 0·21 to 0·6 silicon more than 20·0 to less than 30·0 manganese more than 0·6 to less than 1·4 nitrogen 17·0 to 24·0 chromium up to 2·5 nickel up to 1·9 molybdenum max. 0·3 copper up to 0·002 boron up to 0·8 elements of Groups 4 and 5 of the Periodic System, remainder iron, accompanying elements caused by the melting and impurities, which alloy is melted and allowed to solidify under atmospheric pressure to form an ingot or cast part, and the ingot or the cast part formed is subjected in a first step to a hot shaping at a shaping temperature of over 850°C and after that it is cooled at an increased rate, after which a further shaping of the forged part takes place in a second step at an elevated temperature of under 600°C, but over 350°C, and after that the shaped part is allowed to cool to room temperature and the article is produced therefrom.
2. A process according to Claim 1, characterized in that an alloy consisting, in % by weight, of max. 0·06 preferably max. 0·05 carbon 0·21 to 0·48 silicon 19·0 to 22·0 chromium 20·5 to 29·5 manganese 0·64 to 1·3 nitrogen 0·21 to 0·96 nickel 0·28 to 1·5 molybdenum max. 0·25 copper up to 0·0012 boron up to 0·48 carbide-forming elements remainder iron, accompanying elements caused by the melting and impurities, is melted and processed.
3. A process according to Claim 1 or 2, characterized in that a content, in % by weight, of 21·5 to 25·0 manganese 0·72 to 1·2 nitrogen is set.
4. A process according to one of Claims 1 to 3, characterized in that the ingot is produced in accordance with an electroslag re-melting process.
5. A process according to Claim 1, characterized in that after the first step the shaped ingot is subjected to an intermediate annealing at an annealing temperature of over 850°C and after that it is subjected to a cooling at an increased rate.
6. A process according to one of Claims 1 to 5, characterized in that the ingot is hot-shaped in a first step with a degree of deformation of at least 3·5 times.
7. A process according to one of Claims 1 to 6, characterized in that the forged part is shaped in a second step with a deformation of less than 35%, preferably in a range of from 5% to 20%.
8. A process according to one of Claims 1 to 7, characterized in that the forged part is shaped in a second step at a temperature in the range of from 400 to 500°C.
9. A process according to one of Claims 1 to 8, characterized in that the forged part is cooled intensely to below a temperature of 600°C after the hot shaping in the first step and is held there, and after a temperature equalization over the cross-section is supplied to the deformation in the second step.
10. An austenitic, paramagnetic steel article with a good corrosion-resistance in media with a high chloride concentration and a high yield stress, strength and toughness, consisting of an alloy in % by weight max. 0·10 carbon 0·21 to 0·6 silicon more than 20·0 to less than 30·0 manganese more than 0·6 to less than 1·4 nitrogen 17·0 to 24·0 chromium up to 2·5 nickel up to 1·9 molybdenum max. 0·3 copper up to 0·002 boron up to 0·8 carbide-forming elements of Groups 4 and 5 of the Periodic System,
    remainder substantially iron, which material is produced in accordance with the process according to Claim 1, is hot-shaped with a degree of deformation of at least 3·5 times and is cold-shaped below the precipitation temperature of nitrides and of commercially produced phases, but at an elevated temperature of under 600°C and over 350°C and the article has a yield stress R_P0·2 of more than 700 N/mm² at room temperature, a notched-bar impact toughness at the same temperature of more than 52 J and an FATT (fracture appearance transition temperature) of under -25°C.
11. A material according to Claim 10, characterized in that the alloy contains less than 0·06% by weight of carbon.
12. A material according to Claim 10 or 11, characterized in that the alloy contains less than 0·49% by weight of silicon.
13. A material according to one of Claims 10 to 12, characterized in that the alloy contains from 19·0 to 22·0% by weight of chromium.
14. A material according to one of Claims 10 to 13, characterized in that the alloy contains at least from 21·5 to 29·5, and in particular approximately 25·0, of manganese in % by weight.
15. A material according to one of Claims 10 to 14, characterized in that the alloy contains at least 0·64, preferably from 0·72 to 1·3, and in particular 1·2, of nitrogen in % by weight.
16. A material according to one of Claims 10 to 15, characterized in that the alloy contains from 0·21 to 0·96 of nickel in % by weight.
17. A material according to one of Claims 10 to 16, characterized in that the alloy contains from 0·28 to 1·5 of molybdenum in % by weight.
18. A material according to one of Claims 10 to 17, which has a relative magnetic permeability of under 1·05, and in particular of under 1·016.
19. A material according to one of Claims 10 to 18, which has a yield stress R_P0·2 of more than 700 N/mm² at room temperature, and a notched-bar impact toughness at the same temperature of more than 120 J.