EA009108B1 - Duplex stainless steel alloy for use in seawater applications - Google Patents

Abstract

The present invention relates to a stainless steel alloy, more precisely a duplex stainless steel alloy having ferritic-austenitic matrix and having high corrosion resistance in combination with good structural stability and hot-workability, in particular a duplex stainless steel having a ferrite content of 40-65 % and a well-balanced composition, which gives the material corrosion properties making it more suitable for use in chloride-containing environments than what has been found possible previously. The material according to the present invention has, in view of the high alloy content thereof, extraordinarily good workability, in particular hot-workability, and should thereby be very suitable to be used for, for instance, the manufacture of bars, pipes, such as welded and weld less pipes, weld material, construction parts, such as, for instance, flanges and couplings. These objects are met according to the present invention with duplex stainless steel alloys, which contain (in % by weight): C more than 0 and up to max 0.03, Si up to max 0.5, Mn 0-3.0, Cr 24.0-30.0, Ni 4.9-10.0, Mo 3.0-5.0, N 0.28-0.5, B 0-0.0030, S up to max 0.010, Co 0-3.5, W 0-3.0, Cu 0-2.0, Ru 0-0.3, Al 0-0.03, Ca 0-0.010%, balance Fe together with inevitable contaminations.

Description

The proposed invention relates to a corrosion-resistant alloy, and more particularly to a two-phase corrosion-resistant alloy steel having a ferritic-austenitic structure and having good corrosion resistance in combination with high structural stability and hot workability, in particular, two-phase stainless steel with a ferrite content of 40-65 vol.% and balanced residual composition, which gives the material corrosion properties that provide better use in a chloride environment than that of hitherto known materials ialov.

Offshore oil production involves drilling wells on the seabed to access oil fields. On the seabed, funds are provided for regulating the flow of crude oil and its further transportation to oil storages and oil refining units for oil refining and separation of finished products or intermediates. Devices located on the seabed include, but are not limited to, valves for controlling extraction, flow pressure, speed, and so on, as well as couplings attached to pipes with the possibility of injecting chemical additives into petroleum feedstocks. Often methanol is used for injection, which prevents the coagulation of crude oil, causing unforeseen pipe clogging.

Valves and couplings included in the controls located on the seabed are hydraulically and electrically controlled from a platform, ship, or from another object located on the surface of the sea or on land.

The requirements for connecting pipes forming a composite pipe are corrosive and mechanical properties. The material from which the pipes are made must have high resistance to corrosion in sea water surrounding the pipe from the outside. This property is most important because seawater has a very strong corrosive effect on stainless steel. In addition, the material must have high corrosion resistance since when immersed in an oil reservoir it can be exposed to corrosive environments. In addition, the material must be adapted to the use of hydraulic fluids without contamination of the latter.

The occurrence of contaminants adversely affects the surface functions of the controls located on the seabed.

The mechanical properties of the material used for the manufacture of said pipes have a significant effect on the manufacture of a composite pipe. Since oil production is often carried out at great depths, the moving part of the composite pipeline has a large length and weight. This weight is held by a platform or floating craft. In practice, there are two possibilities for reducing the weight of a composite pipeline having a given configuration. It is possible to choose a lighter material or a material having the same density, but a higher yield strength and tensile strength. When choosing a material having greater strength, pipes with a smaller wall thickness can be manufactured and due to this, the mass of the composite pipe can be reduced. With an increase in the depth at which oil is produced, great demands are placed on the specific gravity per unit length of the material used to make the composite pipeline.

In recent years, due to the tightening of the operating conditions of corrosion-resistant metal materials, the requirements for the corrosion and mechanical properties of the materials used have also increased. Two-phase corrosion-resistant alloy steels have found their application as an alternative to previously used materials, such as, for example, ferritic steel used previously for the specified purpose, nickel-based alloys or other highly alloyed alloys.

In addition, recent developments in the market for connecting composite pipes have led to an increase in the operational characteristics of materials. The requirements previously imposed on strength and corrosion resistance are fully observed in existing alloys. New requirements that will be presented in the future for structural materials for composite pipelines and related equipment will include more stringent requirements for corrosion resistance caused by the design of enterprises located in hot water or when considering the flow of hot media in composite pipelines. The new requirements put forward imply the resistance of the alloy to crevice corrosion in sea water at a temperature of 70-90 ° C. Today's structural materials do not meet the requirement of the necessary resistance to corrosion. This is a challenge to solve. However, the alloys available so far have had a weak spot in this part. An alloy with high resistance to corrosion in a chloride medium, which also has increased strength and high structural stability, should, on the other hand, provide the ability to comply with the requirements for composite pipelines.

A comparative tool for changing the corrosion resistance in a chloride-containing medium is the so-called equivalent pitting resistance (PBE), defined as

PBE =% Cr + 3.3% Mo + 16% Ν, where percentages of elements refer to the mass concentration of elements in the alloy.

With a high value of a certain parameter, increased corrosion resistance is achieved, in particular with respect to pitting corrosion. In principle, alloying elements providing

- 1 009108 the occurrence of this property according to the above formula include chromium, molybdenum, nitrogen. An example of such steel is given in document EP0220141, which is incorporated herein by reference. Steel of this class has the trade designation 8LE 2507 (υΝ8 823750), and is alloyed with chromium, molybdenum and nitrogen. Thus, it is designed to provide, among other things, high resistance to chloride corrosion. Additionally, the content of elements such as copper and tungsten is regulated in steel to further optimize the corrosion properties of steel in a chloride environment. In this case, tungsten is used as a partial replacement for molybdenum, as for example in the alloy №3№ (υΝ8 8 39274) or 2 of its 100, which contain 2.0 and 0.7% of tungsten, respectively. The latter contains 0.7% copper, which provides an increase in the corrosion resistance of the alloy in an acidic environment.

Additionally, tungsten requires clarification of the equivalent corrosion resistance (ΡΚΕ) taking into account tungsten (ΡΚΕ№), while tungsten acts as an equivalent to molybdenum, as described for example in document EP0545753 relating to two-phase stainless steels having increased corrosion properties, while

ΡΒΕ No. =% Cr + 3.3 (% Mo + 0.5% No.) + 16% Ν.

The steel classes described above have a value of ΡΚΕ, regardless of the calculation method, which is about 40, but no more than 43, since the upper value of ΡΚΕ is determined by maintaining the structural stability of the alloy. Higher alloying levels increase the risk of precipitation of intermetallic compounds; therefore, the alloying level in two-phase steels is limited by the coefficient значением value of not more than 43, regardless of the calculation method.

As an alloy having high corrosion resistance in a chloride medium, 8LE 2906 should be mentioned, the composition of which is disclosed in EP 0708845. Such an alloy is characterized by a higher content of chromium and nitrogen compared to, for example, alloy 8LE 2507, and is intended for use in conditions in which resistance to intergranular corrosion and corrosion caused by ammonium carbamate is most important, but it also requires high resistance to chloride corrosion.

An alloy having high resistance to chloride corrosion corresponds to an alloy υΝ8 8 32750, but has a higher yield strength To _p0 , ₂ . This gives the alloy an advantage over υΝ8 8 32750, when used as a material for composite pipelines, since the weight of the pipeline can be reduced. Corrosion resistance, however, is not improved in comparison with υΝ8 8 32750, which imposes a restriction on the use of pipes at high temperatures in the designed plants.

Alloy 19Ό (υΝ8 832001) is a two-phase alloy and is characterized by a chromium content of 19.5-21.5%, nitrogen 0.05-0.17% and not more than 0.6% molybdenum. This alloy has a ΡΒΕ value of about 22, so this alloy is unsuitable as a material for composite pipes used in sea water. Thus, to ensure the necessary corrosion resistance in this alloy, cathodic protection should be applied, for example, in the form of a layer of zinc deposited on the outer surface of the composite pipe. If the zinc layer dissolves or the surface gets damaged, the corrosion protection will decrease and there will be a risk of rapid destruction, which leads to lengthy repair work.

The problem with the alloys having a high ΡΚΕ value described above is the appearance of hard and brittle intermetallic precipitates in steel, such as, for example, the sigma phase, especially after heat treatment, which can be caused, for example, by lengthy welding. This leads to hardening of the material, a decrease in workability and a deterioration in corrosion resistance.

Another group of alloys with high corrosion resistance includes austenitic steels with a значение value of more than 55 and is achieved by introducing a large amount of chromium, molybdenum and nitrogen, in combination with a high nickel content. Such alloys should work well under severe corrosive conditions as a material for composite pipes. The disadvantage of these alloys is a lower yield strength than that of two-phase steel, and a significantly higher manufacturing cost, due for example to the high content of nickel, which is an expensive alloying material. Examples of austenitic materials having high resistance in a chloride environment are υΝ8 832654, with a value of ΡΒΕ about 55, and υΝ8 8 34565, with a value of ΡΚΕ about 45. Such materials have too low strength and too high cost to serve as an alternative to materials for the manufacture of composite pipes.

In order to, among other things, increase the resistance to pitting corrosion in biphasic steels, it is necessary to increase the value of ΡΚΕ in both phases, that is, in ferrite and austenite, without the risk of reducing the stability of the structure or workability of the material. If the composition of the two phases differs in the content of active alloying elements, one of the phases becomes more sensitive to pitting and crevice corrosion. Thus, the corrosion-sensitive phase determines the corrosion resistance of the alloy, while structural stability determines the most alloyed phase.

- 2 009108

The requirements for alloys suitable for future use as a material for composite pipes are summarized in table. 1 along with examples of the best known alternative alloys currently on the market. It is clear that all existing alloys in at least one indicator do not meet the new, toughened requirements for the manufacture of composite pipes.

Table 1

Property Requirement alloy according to the invention ϋΝ3 3 32750 yiz 3 32906 ϋΝ3 3 32654 of 3 32001 RNE Not less than 46 42.5 42 55 22 Tensile yield strength Cr ₀ , 2 (N / mm ² ) 720 550 650 430 450 Critical temperature of pitting corrosion onset, ° С > 90 fifty fifty > 95 <20 The critical temperature of the onset of crevice corrosion, ° C 60 35 35 60 <20 Structural stability No more than 0.5% sigma phase OK OK OK OK Manufacture Weldability by conventional means OK OK OK OK

SUMMARY OF THE INVENTION

The object of the present invention is a two-phase alloyed corrosion-resistant steel having high corrosion resistance in combination with improved mechanical properties and at the same time having high structural stability, suitable for use in conditions that require high resistance to general corrosion and local corrosion, for example, in chloride wednesday

In addition, the invention relates to two-phase alloyed corrosion-resistant steel having a critical pitting onset temperature, then CPT, more than 90 ° C, preferably more than 95 ° C, a critical temperature for crevice corrosion onset, then CCT, more than 60 ° C under conditions 6 % solution of FeCl ₃ .

The invention concerns an alloy having impact strength of more than 100 J at room temperature and a tensile strength of K _{p0 2} of at least 720 N / mm ² and elongation at tensile test at least 25% at room temperature.

The material according to the present invention, with respect to high alloy alloys, has unexpectedly high machinability, in particular hot machinability, and therefore is suitable in particular for the manufacture of rods, pipes, such as welded or seamless pipes, welding material, structural elements, for example flanges and couplings .

The invention provides a two-phase alloyed corrosion-resistant alloy containing, in wt. %:

Carbon 0 to 0.03 Silicon Up to 0, 5 Manganese 0-3.0 Chromium 24.0-30.0 Nickel 4.9-10.0 Molybdenum 3.0-5.0 Nitrogen 0.28-0.5 Boron 0-0.0030 Sulfur Up to 0.010 Cobalt 0-3.5 Tungsten 0-3.0 Copper 0-2.0

- 3 009108

Ruthenium 0-0.3

Aluminum 0-0.03

Calcium 0-0.010

The remainder is iron and inevitable impurities.

Brief Description of the Drawings

In FIG. Figure 1 shows the values of the critical temperature of the onset of pitting corrosion (CPT) on test samples in the modified test A8TM O48C in a solution of "green death" in comparison with two-phase steels BAR 2507, 8AR 2906.

In FIG. Figure 2 shows the values of the critical temperature at the beginning of crevice corrosion (CPT) on test samples in the modified A8TM O 48C test in a green death solution in comparison with biphasic steels 8AR 2507, 8AR 2906.

In FIG. Figure 3 shows the value of the corrosion rate in mm / year at a temperature of 75 ° C in a 2% HC1 solution.

In FIG. Figure 4 shows the results of hot ductility tests for most samples.

DETAILED DESCRIPTION OF THE INVENTION

Systematic studies unexpectedly showed that with a favorable ratio of the following elements of chromium, molybdenum, nickel, nitrogen, manganese and cobalt, an optimal distribution of elements in ferrite and austenite can be achieved, due to which a high corrosion resistance of the material and a small amount of sigma phase in the material are achieved. The material also has good machinability, providing the necessary hood in the production of seamless pipes. To achieve a combination of good corrosion resistance and high structural stability, a very narrow spread in the ratio of alloying elements in the material is required. Therefore, the alloy according to the invention contains, wt.%:

Carbon 0 to 0.03 Silicon Up to 0, 5 Manganese 0-3.0 Chromium 24.0-30.0 Nickel 4.9-10.0 Molybdenum 3.0-5.0 Nitrogen 0.28-0.5 Boron 0-0.0030 Sulfur Up to 0.010 Cobalt 0-3.5 Tungsten 0-3.0 Copper 0-2.0 Ruthenium 0-0.3 Aluminum 0-0.3 Calcium 0-0.010

The remainder is iron and inevitable impurities and additives, while the proportion of ferrite is 40-65 vol.%.

The influence of alloying elements is explained below.

Carbon (C) has limited solubility in ferrite and austenite. Limited solubility increases the risk of precipitation of chromium carbides, so the carbon content should not exceed 0.03 wt.%, Preferably 0.02 wt.%.

Silicon (81) is used as a deoxidizer in the production of steel, and also increases the fluidity and weldability. A very high silicon content leads to the release of intermetallic compounds, and therefore the silicon content should not exceed 0.5 wt.%, Preferably 0.3 wt.%.

Manganese (Mn) is added to increase the solubility of nitrogen in the material. However, the effect on nitrogen solubility is not limited to the introduction of manganese. On the contrary, there are also other elements affecting solubility. In addition, manganese in combination with a high sulfur content leads to the release of manganese sulfides, which are the initial zones for pitting corrosion. Therefore, the manganese content should be 0-3.0 wt.%, Preferably 0.5-1.2 wt.%.

Chromium (Cr) is a very active element in terms of increasing resistance to most types of corrosion. In addition, a high chromium content leads to a good solubility of nitrogen in the material. Thus, it is desirable to keep the chromium content as high as possible to increase corrosion resistance. For very high values of corrosion resistance, the chromium content should be at least 24.0 wt.%, Preferably 27.0-29.0 wt.%. However, a high chromium content increases the risk of precipitation of intermetallic compounds, so the chromium content should not exceed 30 wt.%.

Nickel (N1) is used as an austenite stabilizing element and is added in the required amounts to achieve the required ferrite content. In order to achieve the necessary ratio between the austenitic and ferritic phase, in which 40-65% vol. ferri

- 4 009108, a nickel content of 4.9-10.0 wt.%, Preferably 4.9-9.0 wt.%, In particular 6.0-9.0 wt.%, Is required.

Molybdenum (Mo) is an active element that increases corrosion resistance in a chloride environment, as well as in dilute acids. A very high molybdenum content in combination with a high chromium content increases the risk of the release of intermetallic compounds. According to the present invention, the molybdenum content should be between 3.0 and 5.0 wt.%, Preferably 3.6-4.9 wt.%, In particular 4.4-4.9 wt.%.

Nitrogen (Ν) is a very active element that enhances corrosion resistance, structural stability, and material strength. In addition, the high nitrogen content facilitates the conversion of austenite after welding, which ensures good weld properties. To obtain the effect, the nitrogen content should be at least 0.28 wt.%. A high nitrogen content increases the risk of chromium nitride formation, especially if the chromium content is also high. In addition, a high nitrogen content causes a risk of an increase in porosity when the nitrogen content is above the solubility limit. Based on the above considerations, the nitrogen content should be set to not more than 0.5 wt.%, Preferably 0.35-0.45 wt.%.

Too high a content of chromium, as well as nitrogen, leads to the formation of compound & ₂ Ν, which is undesirable, since it affects the properties of the material, especially during heat treatment, such as welding.

Boron (B) is added to increase the hot workability of the material. With a very high boron content, the workability and corrosion resistance of the material are reduced. Therefore, boron should be contained in an amount of 0-0.0030 wt.%

Sulfur (8) negatively affects the corrosion resistance due to the formation of readily soluble sulfides. In addition, sulfur reduces hot workability, so the sulfur content should be set to not more than 0.010 wt.%.

Cobalt (Co) is added to further improve structural stability and corrosion resistance. Cobalt is also an austenite stabilizing element. To achieve the effect, the cobalt content should be at least 0.5 wt.%, Preferably 1.0 wt.%. Since cobalt is an expensive element, its amount is limited to 3.5 wt.%.

Tungsten increases resistance to pitting and crevice corrosion. But the addition of too much tungsten at a high content of chromium and molybdenum increases the risk of separation of intermetallic compounds. The tungsten content of the present invention should be in the range of 0-3.0 wt.%, Preferably 0-1.8 wt.%.

Copper is added to increase corrosion resistance in an acidic environment such as sulfuric acid. Copper also affects structural stability. However, a high copper content can lead to excess solubility in the solid state. Thus, the copper content does not exceed 2 wt.%, Preferably 0.1-1.5 wt.%.

Ruthenium (Vi) is added to increase corrosion resistance. Ruthenium is a very expensive element, and therefore its content does not exceed 0.3 wt.%, Preferably 0-0.1 wt.%.

Aluminum (A1) as well as calcium (Ca) are used as deoxidizing agents in the production of steel. The aluminum content should not exceed 0.03 wt.% To prevent the formation of nitrides. Calcium has a positive effect on hot workability, but the calcium content should not, however, exceed 0.010 wt.% To prevent a large carry-over.

Ferrite content is an essential feature for achieving good mechanical properties and good weldability. From the point of view of corrosion and weldability, it is desirable that the proportion of ferrite lies between 40 and 65% to obtain optimal properties. In addition, the high ferrite content provides impact resistance at low temperatures and also reduces the risk of hydrogen embrittlement. In addition, the proportion of ferrite should be 40-65 vol.%, Preferably 42-60 vol.%, In particular 45-55 vol.%.

Preferred Embodiments

The examples described below show the composition of the experimental samples, which show the effect of changes in the composition of alloying elements on the properties.

Sample 605182 is a comparative example and is not included in the scope of protection of the present invention. None of the compositions of the samples are limiting and show only preferred embodiments of the invention. The indicated PBE values refer to the values calculated by the formula PBE \ U. even if it is not specifically indicated.

Example 1

Experimental samples were made in laboratory conditions by casting 170 kg of ingots and hot forging to obtain round rods. The latter were subjected to high-temperature extraction into the rod (round rod or varietal rod), and then samples were taken from the rods. In addition, the rods were tempered before cold rolling, and then additional samples were taken. From a technological point of view, this process corresponds to a large

- 5,009108 to the number of processes, in particular the production of seamless pipes, in which the hood is drawn before cold rolling. Tab. 2 shows the composition of the samples of the first group.

table 2

Sample Mp SG Νί Mo AND With V Ba ΤΪ N 605193 1,03 27, 90 8.80 4.00 0.01 0.02 0.04 0.01 0.01 0.36 605195 0, 97 27.90 9, 80 4.00 0.01 0, 97 0, 55 0.01 0.35 0.48 605197 1,07 28.40 8.00 4.00 1.00 1.01 0.04 0.01 0, 01 0.44 605178 0, 91 27.94 7.26 4.01 0.99 0.10 0.07 0.01 0,03 0.44 605183 1,02 28.71 6.49 4.03 0.01 1.00 0.04 0.01 0.04 0.28 605184 0.99 28.09 7.83 4.01 0.01 0,03 0.54 0.01 0.01 0.44 605187 2.94 27.74 4.93 3, 98 0.01 0, 98 0.06 0.01 0.01 0.44 605153 2.78 27.85 6, 93 4.03 1.01 0.02 0.06 0.02 0.01 0.34 605182 0, 17 23.48 7.88 5.75 0.01 0.05 0.04 0.01 0.10 0.26

According to the requirements of structural stability tests, the samples were tempered at 900–1150 ° C and then cooled in air and water, respectively. At low temperatures, intermetallic phases formed. Thus, the lowest temperatures at which the number of intermetallic phases remained negligibly small were determined using a light optical microscope. New samples from the same preforms were then tempered at the indicated temperatures for 5 minutes, and then the samples were cooled at a constant cooling rate of 140 ° C / min to room temperature. The surface fraction of the sigma phase in the material was determined by digitally processing the image obtained in the stream of reflected electrons in a scanning electron microscope. The results are presented in table. 3.

The T _max sigma phase was determined by Tegto-Ca1c (TC version N thermodynamic database of steels TSTE99) based on the usual influence of known elements in various ways. Tmax sigma phase is the splitting temperature of the sigma phase, at a high splitting temperature there is a low stability of the structure.

Table 3

The task of the test was to rank the material according to structural stability, which shows the proportion of the sigma phase in the test samples that were subjected to heat treatment and cooling before the corrosion test. It is important that the T _max sigma phase was calculated by Tegto-Ca1c and is not directly related to the real share of the sigma phase, however, it is clear that experimental samples that have the lowest T _max sigma phase also have the smallest sigma phase fraction.

Resistance to pitting corrosion was measured in the so-called “green death” solution, which contains 1% EtCl ₃ , 1% Cu Cl ₃ , 11% H ₂ 8 O ₄ and 12% HC1. The test is in accordance with the test procedure for

- 6 009108 pitting corrosion according to L8TM C 48C, but using a more aggressive green death solution.

In addition, some samples were tested using the L8TM C 48C methodology (2 experiments per 1 blank). In addition, electrochemical studies were carried out in a 3% solution of No. C1 (6 experiments on the workpiece). The results in the form of the critical temperature of the beginning of pitting corrosion in all experiments are given in table. 4 together with the parameter PBE \ Y (С’г + 3.3 (Mo + 0.5 \ Y) + 16U) on the total composition of the sample, i.e., on austenite and ferrite. Index “α” refers to ferrite and “γ” to austenite.

Table 4

Sample RVE a RKE γ RKE y / RKE a RKE СРТ ° С in the modified test АЗТМ С48С “Green Death” СРТ ° С АЗТМ С48С 6% EgoС1 ₃ CPT ° C 3% IA1 (600 mV ZSE) 605193 51.3 49.0 0.9552 46, 9 90/90 64 605195 51.5 48.9 0.9495 48.7 90/90 95 605197 53.3 53.7 1,0075 50.3 90/90 > 95 > 95 605178 50.7 52,5 1,0355 49, 8 75/80 94 605183 48.9 48.9 1,0000 46.5 85/55 90 93 605184 48.9 51.7 1.0573 48.3 80/80 72 605187 48.0 54,4 1,1333 48.0 70/75 77 605153 49.6 51.9 1,0464 48.3 80/85 85 90 605182 54,4 46.2 0.8493 46.6 75/70 85 62 ZAG2507 39, 4 42,4 1,0761 41.1 70/70 80 95 ZAG2906 39, 6 46,4 1,1717 41.0 60/50 75 75

It was suggested that there is a linear relationship between the lowest PKE in austenite and in ferrite and CPT in biphasic steel, but the results in Table. 4 showed that the value of the PKE alone does not determine the value of CPT.

It is clear that all tested materials have a better CPT value in the modified L8TM C 48C test than 8LE 2507 and 8LE 2906. Sample 605183 doped with cobalt showed good structural stability upon cooling at a rate of 140 ° C / min, even with the negative effect of a high content chromium and molybdenum, a better result was achieved than with 8LE 2507 and 8LE 2906. This study showed that a high value of PKE does not directly determine the value of CPT, however, the ratio of PKE of austenite / PKE of ferrite is the most important for the properties of two high-alloy high-alloy corrosion-resistant steel, and very narrow limits and an exact ratio of alloying elements are required to obtain the optimum value of this ratio at the level of 0.9-1.15, preferably 0.9-1.05, and when combined with the value of the PKE of about 46. The ratio of PKE of austenite / PKE of ferrite depending on CPT in the modified L8TM C 48C test for experimental samples is presented in Table. 4.

Strength at room temperature (CT), 100 and 200 ° C and impact strength at room temperature (CT) was determined for all samples and was shown in these three experiments.

Tensile samples (OK-5C50) were taken from stretched rods with a diameter of 20 mm, which were heat treated at temperatures, according to table. 2 for 20 minutes, followed by cooling in air and water (605195, 605197, 605184). The results of these tests are given in table. 5 and 6. The results of tensile tests showed that the content of chromium, nitrogen and tungsten strongly affects the yield strength of the material. All samples except 605153 had 25% elongation in tensile tests at room temperature (CT).

- 7 009108

Table 5

Table 6

Sample Vacation [° C / min] Cooling Impact Resistance (J) Vacation [° C / min] Cooling Impact Resistance (J) 605193 1100/20 Air 35 1100/20 Water 242 605195 1150/20 Water 223 605197 1100/20 Water 254 1130/20 Water 259 605178 1100/20 Air 62 1100/20 Water 234 605183 1050/20 Air 79 1050/20 Water 244 605184 1100/20 Water 81 1100/20 Air 78 605187 1050/20 Air 51 1100/20 Water 95 605153 1100/20 Air fifty 1100/20 Water 246 605182 1100/20 Air 22 1100/20 Water 324

The experiments showed very clearly that cooling in water is preferable to obtain the best structure and, accordingly, high impact strength. The requirement is 100 J for the test at room temperature, and all samples satisfy this requirement with the exception of 605184 and 605187, however, the last sample was very close to these requirements.

Tab. 7 shows the results of a melting test in an arc welding with a tungsten electrode in an inert gas (TJ), on samples 605193, 605183, 605184, and also on sample 605253 having

- 8 009108 stable structure in the heat-affected zone (ΗΑΖ). Titanium containing steels in the heat affected zone included titanium nitrides зоне.

Table 7

Sample Precipitation, argon shielding gas (99.99%) 605193 ΗΑΖ: OK 605195 ΗΑΖ: A large number of ΤίΝ and σ phases 605197 ΗΑΖ: A small content of Cr ₂ C in grains 5, present in small numbers 605178 ΗΑΖ: Ογ ₂ Ν in grains δ, otherwise OK 605183 ΗΑΖ: OK 605184 ΗΑΖ: OK 605187 ΗΑΖ: Cr ₂ C near the melting zone, the absence of others precipitation 605153 ΗΑΖ: OK 605182 ΗΑΖ: ΤίΝ along the grain boundaries δ / δ

Example 2. In the example below, additional experimental samples were used to determine the optimal composition according to the invention. Samples were selected based on the properties of the samples of example 1 having high structural stability and corrosion resistance. All samples listed in the table. 8 have a composition according to the present invention, wherein samples 1-8 were selected from a statistical data array, while samples e-th were additional experiments carried out according to the invention.

Many experimental samples were produced from 270 kg of casting, hot forged to round rods. The latter were subjected to exhaustion, after which samples were taken. Then the rods were tempered before cold rolling or long rolling, and then additional samples were taken. In the table. 8 shows the compositions of some experimental samples.

Sample Mp SG ΝΪ Mo one 605258 1,1 29, 0 6.5 4.23 2 605249 1,0 28.8 7.0 4.23 3 605259 1,1 29.0 6, 8 4.23 4 605260 1,1 27.5 5, 9 4.22 5 605250 1,1 28.8 7.6 4.24 6 605251 1,0 28.1 6, 5 4.24 7 605261 1,0 27.8 6.1 4.22 8 605252 1,1 28,4 6.9 4.23 e 605254 1,1 26, 9 6.5 4.8 ί 605255 1,0 28.6 6.5 4.0 d 605262 2.7 27.6 6, 9 3.9 and 605263 1,0 28.7 6, 6 4.0 ί 605253 1,0 28.8 7.0 4.16 ό 605266 1,1 30,0 7.1 4.02 to 605269 1,0 28.5 7.0 3.97 one 605268 1,1 28,2 6, 6 4.0 w 605270 1,0 28.8 7.0 4.2 η 605267 1,1 29, 3 6.5 4.23

Table 8

AND With Si In and IN N 1,5 0.0018 0.46 1,5 0.0026 0.38 0.6 0.0019 0.45 1,5 0.0020 0.44 0, 6 0.0019 0, 40 1,5 0.0021 0.38 0, 6 0.0021 0.43 0.5 0.0018 0.37 1,0 0.0021 0.38 3.0 0.0020 0.31 1,0 1,0 0.0019 0.36 1,0 1,0 0.0020 0, 40 1,5 0.0019 0.37 0.0018 0.38 1,0 1,0 0.0020 0.45 1,0 1,0 1,0 0, 0021 0, 43 1,5 0.1 0.0021 0.41 1,5 0.0019 0.38

- 9 009108

The distribution of alloying elements between ferrite and austenite was determined using a microprobe, the measurement results are given in table. nine.

Table 9

Sample Phase SG Mp Νί Mo AND With Si N 605258 Ferrite 29, 8 1.3 4.8 fifty 1.4 0.11 Austenite 28.3 1.4 7.3 3.4 1,5 0, 60 605249 Ferrite 29, 8 1,1 5,4 5.1 1.3 0.10 Austenite 27.3 1,2 7.9 3, 3 sixteen 0.53 605259 Ferrite 29, 7 1.3 5.3 5.3 0.5 0, 10 Austenite 28.1 1.4 7.8 3, 3 0, 58 0.59 605260 Ferrite 28,4 1.3 4.4 5,0 1.4 0.08 Austenite 26, 5 1.4 6, 3 3, 6 1,5 0, 54 605250 Ferrite 30, 1 1.3 5, 6 5.1 0.46 0,07 Austenite 27.3 1.4 8.8 3.4 0, 53 0, 52 605251 Ferrite 29, 6 1,2 5,0 5.2 1.3 0.08 Austenite 26, 9 1.3 7.6 3,5 1,5 0.53 605261 Ferrite 28.0 1,2 4,5 4.9 0.45 0,07 Austenite 26, 5 1.4 6.9 3, 3 0.56 0.56 605252 Ferrite 29, 6 1.3 5.3 5.2 0, 42 0.09 Austenite 27.1 1.4 8.2 3.3 0.51 0.48 605254 Ferrite 28.1 1.3 4.9 5.8 0.89 0.08 Austenite 26, 0 1.4 7.6 3.8 1,0 0, 48 605255 Ferrite 30, 1 1.3 5,0 4.7 2.7 0.08 Austenite 27.0 1.3 7.7 3.0 3, 3 0.45 605262 Ferrite 28.8 thirty 5, 3 4.8 1.4 0.9 0.08 Austenite 26, 3 3, 2 8.1 3.0 0, 85 eleven 0.46 605263 Ferrite 29, 7 1 s 5.1 5.1 1.3 0, 91 0,07 Austenite 27.8 1.4 7.7 3, 2 0.79 1,1 0, 51 605253 Ferrite 30,2 1.3 5,4 5,0 1.3 0.09 Austenite 27.5 1.4 8.4 3,1 1,5 0.43 605266 Ferrite 31,0 1.4 5.7 4.8 0.09 Austenite 29, 0 1,5 8.4 3,1 0, 52 605269 Ferrite 28, 7 1.3 5.2 5.1 1.4 0, 9 0, 11 Austenite 26, 6 1.4 7.8 3, 2 0, 87 1,1 0, 52 605268 Ferrite 29, 1 1.3 5,0 4.7 1.3 0, 91 0.84 0, 12 Austenite 26.7 1.4 7.5 3, 2 0, 97 1,0 1,2 0, 51 605270 Ferrite 30,2 1,2 5, 3 5,0 1.3 0, 11 Austenite 27.7 1.3 8.0 3, 2 1.4 0, 47 605267 Ferrite 30, 1 1.3 5.1 4.9 1.3 0.08 Austenite 27.8 1.4 7.6 3,1 1.8 0.46

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All samples were ranked by pitting corrosion resistance in the “green death” solution (1% GeCl ₃ , 1% CuCl ₃ , 11% H ₂ 8O ₄ and 1.2% HC1). Testing in principle corresponds to checking corrosion resistance according to L8TM O 48C, but the test was carried out in a solution of "green death", more aggressive than 6% GeCl ₃ . The test for general corrosion resistance was carried out in a 2% HC1 solution (2 experiments per sample) before the corrosion test at the dew point temperature (c1е \\ ροΐηί 1е81т§). The results of all tests are given in table. 10, and also in FIG. 2 and 3. All test samples showed better properties than 8LG 2507 in the solution of "green death". For all samples, the ratio of PKE of austenite / PKE of ferrite was observed at a level of 0.9-1.15, preferably 0.9-1.05, and at the same time, the total value of PKE for austenite and ferrite exceeded 44, in many samples it was significantly higher than 44. On in some samples, a limit value of PKE of 50 was reached. Interestingly, sample 605251 doped with 1.5% cobalt in behavior in a green death solution showed equivalent properties to sample 605250 doped with just 0.6% cobalt, despite the lower chromium content in Sample 605251. In particular, be very surprised. tionary that the sample has a value of 605,251 ERC equal to 48, which is higher than in the majority of two-phase steels, and at the same time index T _max sigma phase equal to 1010 ° C shows a high structural stability, given the values in the Table. 2 from example 1.

In the table. Figure 10 shows the PKE value \ (% Cr + 3.3 (% Mo- 0.5% A) -16% \) for the alloy, as well as the PKE values for austenite and ferrite, calculated according to the composition determined by the microprobe. The ferrite fraction was determined after heat treatment at 1100 ° C and subsequent cooling.

Table 10

Sample Share a RKEM common RKE a RKE γ ΡΚΕγ / ΡΚΕα CPT ° C in "green death" 605258 48,2 50.3 48.1 49.1 1,021 65/70 605249 59, 8 48.9 48.3 46, 6 0, 967 75/80 605259 49.2 50,2 48.8 48,4 0, 991 75/75 605260 53,4 48.5 46.1 47.0 1.019 75/80 605250 53, 6 49.2 48.1 46.8 0.974 95/80 605251 54,2 48,2 48.1 46.9 0.976 90/80 605261 50.8 48.6 45,2 46.3 1,024 80/70 605252 56, 6 48,2 48,2 45.6 0.946 80/75 605254 53,2 48.8 48.5 46.2 0.953 90/75 605255 57.4 46, 9 46, 9 44.1 0.940 90/80 605262 57.2 47.9 48.3 45.0 0.931 70/85 605263 53, 6 49, 7 49, 8 47.8 0.959 80/75 605253 52.6 48,4 48,2 45.4 0.942 85/75 605266 62.6 49.4 48.3 47.6 0, 986 70/65 605269 52.8 50,5 49, 6 46, 9 0, 945 80/90 605268 52.0 49, 9 48.7 47.0 0, 965 85/75 605270 57.0 49.2 48.5 45.7 0, 944 80/85 605267 59, 8 49.3 47.6 45.4 0.953 60/65

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Sample CPT Medium FTA Average Cro 12 CT Ct ct ACT Ζ CT 605258 84 68 725 929 40 73 605249 74 78 706 922 38 74 605259 90 85 722 928 39 73 605260 93 70 709 917 40 73 605250 89 83 698 923 38 75 605251 95 65 700 909 37 74 605261 93 78 718 918 40 73 605252 87 70 704 909 38 74 605254 93 80 695 909 39 73 605255 84 65 698 896 37 74 605262 80 83 721 919 36 75 605263 83 75 731 924 37 73 605253 96 75 707 908 38 73 605266 63 78 742 916 34 71 605269 95 90 732 932 39 73 605268 75 85 708 926 38 73 605270 95 80 711 916 38 74 605267 58 73 759 943 34 71

For a more accurate analysis of structural stability, the samples were tempered for 20 min at 1080 ° C, 1100 ° C and 1150 ° C and then cooled in water.

The temperature at which the proportion of intermetallic inclusions remained negligible was determined using a light optical microscope. Verification of the structure of the samples after tempering at 1080 ° C and subsequent cooling in water showed which samples are most prone to the formation of an undesirable sigma phase in them. The results are shown in table. 11. The analysis of the structure showed that samples 605249, 605251, 605252, 605253, 605254, 605255, 605259, 605260, 605255, as well as 6054267 do not contain unwanted sigma phase. In addition, sample 605249 doped with 1.5 wt.% Cobalt did not contain a sigma phase, while sample 605250 doped with 0.6% cobalt had a small amount of sigma phase. Both samples were alloyed with a high percentage of chromium, about 29 wt.%, As well as molybdenum at the level of 4.25 wt.%. The composition of samples 605249, 605250, 605251, 605252 were checked for the presence of a sigma phase, and it was found that the interval of optimal ratios of elements in the sample from the point of view of obtaining structural stability is quite narrow. In addition, it is obvious that sample 605268 contains only minor amounts of sigma phase, while sample 605263 contains more sigma phase. The samples also differed in the copper content, for example, sample 605268. Sigma phase is absent in samples 605266 and 605267, despite the high chromium content, while the latter sample is doped with copper. In addition, samples 605262 and 605263 were additionally doped with 1.0 wt.% Tungsten and contained significant amounts of sigma phase, while sample 605269, also containing 1 wt.% Tungsten and more nitrogen than samples 6052 62 and 6052 63, had a significantly smaller amount of sigma phase. Thus, to obtain good structural properties, careful selection of the ratio between alloying elements, in particular chromium and molybdenum, is required.

Tab. 12 shows the results of studies using a light optical microscope of samples subjected to tempering at 1080 ° C for 20 minutes and subsequent cooling. The amount of sigma phase was shown on a 5-point scale, with 1 showing the absence of a sigma phase, and 5 the largest number of sigma phases.

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Table 12

Sample Sigma phase SG Mo AND With Si N In and 605249 one 28.8 4.23 1,5 0.36 605250 2 28.8 4.24 0, 6 0.40 605251 one 28.1 4.24 1,5 0.38 605252 one 28,4 4.23 0.5 0.37 605253 one 28.8 4.16 1,5 0.37 605254 one 26, 9 4.80 1, about 0.38 605255 one 28.6 4.04 3.0 0.31 605258 2 29, 0 4.23 1,5 0.46 606259 one 29, 0 4.23 0, 6 0.45 605260 one 27.5 4.22 1,5 0.44 605261 2 27.8 4.22 0.6 0.43 605262 4 27.6 3.93 1,0 1,0 0.36 605263 5 28.7 3, 96 1,0 1,0 0.40 605266 one 30,0 4.02 0.38 605267 one 29.3 4.23 1,5 0.38 605268 2 28,2 3.98 1,0 1,0 1,0 0.43 605269 3 28.5 3.97 1,0 1,0 0.45 605270 3 28.8 4.19 1,5 0.41 0, 1

In the table. 13 shows the results of impact strength tests of some of the samples. The results were very good, which shows the presence of a fine structure after tempering at 1100 ° C and subsequent cooling in water, taking into account the fact that the requirements of 100 J were achieved on most samples.

Table 13

Sample Vacation ° C / min Cooling Wednesday Impact Resistance [J] Impact Resistance [J] Impact Resistance [J] 605249 1100/20 Water > 300 > 300 > 300 605250 1100/20 Water > 300 > 300 > 300 605251 1100/20 Water > 300 > 300 > 300 605252 1100/20 Water > 300 > 300 > 300 605253 1100/20 Water 258 267 257 605254 1100/20 Water > 300 > 300 > 300 605255 1100/20 Water > 300 > 300 > 300

FIG. 4 shows the results of a hot ductility test. Good workability is certainly required in the manufacture of products from a particular material, for example rods, pipes, for example welded and seamless pipes, braces, welded material, structural elements, such as flanges or couplings. Samples 605249, 605250, 605251, 605252, 605255, 605266 and 605267 had a nitrogen content of 0.38 wt.%, And better indicators of hot ductility.

Strain fatigue tests have shown how many times and how much material can be lengthened before the fatigue cracks caused by deformation lead to fracture of the material. After the pipes are welded together and a long section is formed, they are screwed onto the drum before they are assembled into a composite pipe, so plastic deformation is not unusual before starting to be used as a composite pipe. Strain fatigue tests show that the risk of breaks in composite pipes due to strain fatigue is reduced to zero.

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Conclusion

The requirements that will be presented to composite pipes in the future are ensured by an optimized alloy composition in which the PBE value is not less than 46, while the value of PBE in austenite and ferrite should exceed 45 to achieve sufficient resistance to pitting and crevice corrosion. It is required that the CPT in 6% of FeCl ₃ was greater than 90 ° C, and the CCT in 6% of FeCl ₃ was greater than or equal to 60 ° C.

To sufficiently reduce the mass of the composite pipeline, a certain strength is required, while the yield strength In _p0 , _{2 is} not less than 720 N / mm ² .

For the manufacture of composite pipelines and to ensure resistance to pitting and crevice corrosion, as well as the required mechanical properties, the following properties must also be observed: the alloy must be welded by conventional welding methods, the structure should not contain more than 0.5% sigma phase, maximum temperature splitting of the sigma phase should be 1010 ° C.

The material according to the present invention has high machinability, in particular good hot machinability, and is therefore suitable for use as material in the manufacture of rods, pipes, such as welded and seamless pipes, structural elements, such as couplings and flanges.

Claims (6)

CLAIM 1. Ferritic-austenitic two-phase alloyed stainless steel, containing the following components, wt.%: 0 to 0.03 Carbon Silicon Manganese Chrome Nickel Molybdenum Nitrogen Boron Sulfur Cobalt Tungsten Copper Ruthenium Aluminum Calcium Up to 0,5 0-3.0 24.0-30.0 4.9-10.0 3.0-5.0 0.28-0.5 0-0.0030 Up to 0.010 0-3.5 0-3.0 0-2.0 0-0.3 0-0.03 0-0.010 residue - iron and inevitable impurities and additives, while the ferrite fraction is 40-65 vol.%, The value equivalent to pitting corrosion resistance PKE =% Cr + 3.3% Mo + 16% N exceeds 46 for the total chemical composition , and the calculated PBE values for the austenitic and ferritic phases exceed 45, while the yield strength В _p0 , ₂ exceeds 720 N / mm ² , while the critical temperature of the onset of pitting corrosion (CPT) is more than 90 ° C, and the critical temperature for the onset of crevice corrosion ( CCT) is greater than or equal to 60 ° C. 2. Steel according to claim 1, characterized in that it contains chromium from 26.5 to 29.0 wt.%. 3. Steel according to claim 1 or 2, characterized in that it contains manganese from 0.5 to 1.2 wt.%. 4. Steel according to any one of claims 1 to 3, characterized in that it contains nickel from 5.0 to 8.0 wt.%. 5. Steel according to any one of claims 1 to 4, characterized in that it contains molybdenum from 3.6 to 4.9 wt.%. 6. Steel according to any one of claims 1 to 5, characterized in that it contains nitrogen from 0.35 to 0.45 wt.%.