USE OF AN AUSTENITIC STEEL ALLOY WITH IMPROVED CORROSION RESISTANCE UNDER HIGH TEMPERATURE EXPECTATIONS IN A SOLAR POWER PLANT, ABSORBER PIPE, METHOD FOR MANUFACTURING A PIPE BODY FROM THE STEEL ALLOY

Description

[0001] The invention relates to the use of an austenitic steel alloy for operating temperatures of at least 600 °C to 800 °C in a solar power plant. Furthermore, the invention relates to an absorber tube for a solar receiver of a solar power plant made of a tubular body made of this steel alloy, as well as to a solar receiver with such an absorber tube. The invention also relates to a method for producing a tubular body with a cover layer made of this steel alloy.

[0002] The development of solar thermal power plants and the decline in fossil-fired power plants driven by the energy transition are creating a need for new markets, some with highly specialized application portfolios, and the need to provide the appropriate materials. One of these scenarios is solar thermal power plants based on molten salts, which contain pipes that transport heat in a similar way to steam power plants. Heat is transported via molten salts heated by solar radiation, particularly in the design of solar thermal power plants known as tower receivers. Here, solar radiation is concentrated onto the top of a tower by an array of individually tracked mirrors (heliostats). With this concept, temperatures of over 1,000 °C can be reached at the top of the tower.At the top of the tower is a solar receiver that converts the radiation into heat and transfers it to a heat transfer medium, which supplies the heat to a conventional power plant process.

[0003] The solar receiver receives solar radiation using a bundle of tubes. This bundle of tubes is sometimes coated with a black coating to improve radiation absorption. Molten salt, such as a nitrate salt mixture, flows inside the tube, transferring and storing the heat. In a heat exchanger, the heat is transferred from the molten salt to a steam circuit, which then generates electricity via the Carnot cycle. The temperatures of the molten salt are typically up to about 620 °C, although certain parts of the absorber tubes can sometimes reach even higher temperatures.

[0004] Steels for this application must withstand high levels of corrosion and thermal stress. High mechanical stability against time- and temperature-dependent creep damage, as well as thermo-mechanical fatigue caused by the superposition of mechanical fatigue loading with cyclic thermal loading, is also required. This requires a correspondingly high degree of structural stability. The changes that occur as a function of temperature and time primarily affect the dislocation arrangement, the grain structure, and the precipitates.

[0005] In addition to nickel-based austenitic steels, which are very cost-intensive due to their high Ni content, austenitic Cr-Ni steels are also disclosed for this purpose, for example, in the published patent application WO 2016 / 116227 A1. The steel composition for absorber tubes of a solar thermal power plant includes, on a weight basis: 0% to 0.025% C; 0.05% to 0.16% N; 2.4% to 2.6% Mo; 0.4% to 0.7% Si; 0.5% to 1.63% Mn; 0% to 0.0375% P; 0% to 0.0024% S; 17.15% to 18.0% Cr; 12.0% to 12.74% Ni; 0.0025% to 0.0045% B, with the remainder being Fe and possibly common impurities. This steel is designed for a temperature range of up to 580 °C on the absorber tube.

[0006] The published patent application WO 2015 / 014592 A2 also describes an austenitic steel with the following alloy composition on a weight basis for a temperature range of up to 580 °C on the absorber tube: 0.08% or less C; 0% to 0.18% N; 0% to 3.0% Mo; 0% to 1.0% Si; 0% to 1.0% Mn; 0% to 0.035% P; 0% to 0.015% S; 16.0% to 19.0% Cr; 9.0% to 14.0% Ni; 0.0015% to 0.005% B; 0% to 0.23% Cu; 0% to 0.007% Al; 0% to 0.013% Nb; 0% to 0.12% V; 0% to 0.19% Co; and Ti, where at least one of the following two conditions is met: Ti / C at least 6; Ti 0.24% - 0.64% and contains the remainder Fe and possibly usual impurities.

[0007] A disadvantage of the known steel grades is that they are not designed for operating temperatures of at least 600 °C to approximately 800 °C on the absorber tube, which would lead to a significantly improved economic efficiency of solar power plants, especially molten salt-based solar power plants.

[0008] Furthermore, parts of a steam turbine are known from European Patent Application EP 1 502 966 A2, which are operated at temperatures of 650 °C or higher and are made of an austenitic, heat-resistant steel alloy with the following chemical composition, each in weight %: C: 0.45 or less; Si: 1.0 or less; Mn: 2.0 or less; Cr: 19.0 to 25.0; Ni: 18.0 to 45.0; Mo: 2.0 or less; Nb: 0.1 to 0.4; W: 8.0 or less; Ti: 0.6 or less; Al: 0.6 or less; B: 0.01 or less; N: 0.25 or less; the remainder being iron and unavoidable impurities.

[0009] An austenitic steel has also been described in Japanese laid-open specification JP 621 99 753 A, which has the following chemical composition in % by weight: C: < 0.03; Si: <0.6; Mn: <5.0; P: <0.04; S: <0.03; Cr: 23.0 to 30.0; Ni: 5.0 to 18.0; N: 0.25 to 0.45 or further comprising one or more of the following elements: Mo: 0.1 to 3.0; Nb: 0.05 to 2.0; Ti: 0.02 to 0.5; Cu: 0.2 to 5; B: <0.01; Ce: <0.05 and Ca: <0.1. This steel also meets the formula: Ni + 0.5 x Mn + 30 × ( C + N) >= 20. This steel is used for pipes in a heating apparatus for the recovery of soda in paper production and has improved load-bearing capacity and resistance to intergranular corrosion.

[0010] European Patent Application EP 2 581 464 A1 discloses pipes made of an austenitic steel alloy with the following chemical composition in weight %: C: 0.07; Si: 0.4; Mn: 1.2; Cr: 25.0; Ni: 20.0; N: 0.24; Nb: 0.5; remainder iron.

[0011] The Chinese laid-open specification CN 101 967 611 A describes a tubular body made of a steel alloy comprising in weight %: C: 0.068; Si: 0.44; Mn: 1.45; Cr: 24.5; Ni: 20.5; Nb: 0.40; N: 0.25; balance iron.

[0012] The Chinese laid-open specification CN 103 710 643 A also discloses a seamless pipe made of austenitic steel, which has the following chemical composition (in weight %): C: 0.04 to 0.10; Si: <0.75; Mn: <2.0; P: <0.030; S: <0.015; Cr: 24.0 to 26.0; Ni: 19.0 to 22.0; N: 0.150 to 0.350; Nb: 0.20 to 0.60; remainder iron and impurities.

[0013] European Patent Application EP 2 915 893 A1 describes a tubular body whose austenitic steel alloy has the following chemical composition (in weight %): C: 0.06; Si: 0.32; Mn: 1.21; P: 0.015; S: 0.0021; Ni: 19.5; Cr: 24.2; Nb: 0.46; N: 0.25; B: 0.0012; remainder iron and unavoidable impurities.

[0014] The invention is therefore based on the object of specifying the use of an austenitic steel alloy with excellent corrosion resistance under high-temperature stresses of over 600 °C up to 800 °C in a solar power plant, particularly in molten salt-based solar power plants. Furthermore, an absorber tube of a solar receiver of a solar power plant consisting of a tubular body and a solar receiver with such an absorber tube are to be specified.

[0015] For an austenitic steel alloy, this problem is solved by the features of patent claim 1. Advantageous further developments are the subject of subclaims.

[0016] According to the teaching of the invention, an austenitic steel alloy consisting of the following chemical composition in weight %: C: 0.01 to 0.10; Si: max. 0.75; Mn: max. 2.00; P: max. 0.03; S: max. 0.03; Cr: 23 to 27; Ni: 17 to 23; Nb: 0.20 to 0.6; N: 0.15 to 0.35; remainder iron with melting-related impurities, at operating temperatures of at least 600 °C to 800 °C, has high corrosion resistance and mechanical stability against thermo-mechanical fatigue and significantly improved economic efficiency in solar power plants, in particular in molten salt-based solar power plants.

[0017] The austenitic steel alloy has approximately the same or even improved corrosion properties compared to conventional steel alloys at high operating temperatures of at least 600 °C to 800 °C and sufficient creep rupture behavior compared to known alloys.

[0018] The combination of the alloying elements chromium - 23 to 27% by weight - and nickel - 17 to 23% by weight - advantageously achieves a corrosion-reducing coating layer formation under operating conditions.

[0019] Advantageously, appropriate preconditioning can achieve the formation of a covering layer, which significantly reduces the initial corrosion rate and increases the service life.

[0020] According to the invention, this is achieved by a method for producing a tubular body from the steel alloy, in which the tubular body is annealed at annealing temperatures between 800°C and 900°C for an annealing time of 0.1 h to 24 h, preferably 2 to 4 h, in an atmosphere containing oxygen and / or nitrogen in such a way that a covering layer with a thickness of at least 2 µm, advantageously at least 5 µm, and a maximum of 20 µm is produced on the tubular body during annealing. The corrosion-inhibiting covering layer formed by the alloy composition according to the invention during operational use or by appropriate preconditioning is permanent and self-healing in the event of mechanical damage.

[0021] The top layer can be conditioned during preconditioning either before operational use by annealing in a suitable atmosphere, for example with oxygen, nitrogen or other gases, so that layers, e.g. made of oxides or nitrides, or combinations thereof, are formed.

[0022] Annealing can also be performed on the precursor material for tube production, on the component itself (absorber tube), or directly in the final assembled state of the component. Advantageous annealing temperatures are between 800 °C and 900 °C, with an annealing time at 800 °C of approximately 3 hours. The resulting covering layer has a thickness of at least 2 µm, preferably at least 5 µm, in order to reduce an initially high corrosion rate. Higher annealing temperatures then correspond to shorter annealing times in order to create a corresponding minimum thickness of the covering layer. However, a maximum annealing temperature of 900 °C should not be exceeded. In the case of preconditioning by annealing, the covering layer should not exceed a thickness of 20 µm to prevent spalling of the covering layer.It is also possible to carry out preconditioning with an Al-containing coating, thus further improving corrosion resistance, but this is not claimed. Furthermore, the alloy also achieves high resistance to thermal fatigue, i.e., after a large number of temperature cycles.

[0023] The required creep resistance, i.e., low time- and temperature-dependent plastic deformation under constant load, is achieved by the precipitation of carbides of the type M23C6 or Nb(C,N). For this purpose, the required minimum Nb content is determined depending on the existing C and N content of the steel during melting according to the following formula (1): 0 ,3 < Nb / C + N < 3 ,8

[0024] The following alloy contents for the austenitic steel alloy in weight percent have proven to be advantageous for the corrosion and mechanical properties: C: 0.04 to 0.10, particularly advantageous 0.05 to 0.08; Si: min. 0.1; Mn: min. 0.6; Cr: 23 to 25; Ni: min. 20, particularly advantageous min. 21; N: 0.20 to 0.30.

[0025] Compliance with the following limit for Nb, C and N according to the following formula (2) has proven particularly favorable for the combination of the required properties: 0 ,4 < Nb / C + N < 2 ,5

[0026] The required resistance to creep damage is achieved by the precipitation of carbides of the M23C6 or Nb(C,N) type. This requires the alloying elements C and N, as well as Nb and Cr. Compared to other special carbide or nitride formers, such as Ti and V, the addition of Nb also has a grain-refining effect, an effect that also leads to increased corrosion resistance due to finer grain.

[0027] The steel alloy can be advantageously used to produce flat steel products, such as strips or sheets, or tubular bodies as seamless or welded tubes. Welded tubes are advantageously produced from appropriately formed strips or sheets. Seamless tubes can be manufactured using the conventional "Mannesmann" tube manufacturing process or, for example, by extrusion.

[0028] One application of tubular bodies made of the steel alloy is as an absorber tube of a solar receiver of a solar power plant for transporting a liquid heat carrier, in particular a molten salt.

[0029] To improve absorber performance, in an advantageous embodiment of the invention, the absorber tube has a heat-absorbing black coating applied to its outer surface. This coating can be subsequently applied, for example, as a paint, or produced from precursors using thermal processes. The latter can be, for example, sol-gel layers that cure under sunlight after being applied to the tube.

[0030] Accordingly, a tubular body made of an austenitic steel alloy is used as an absorber tube in a solar receiver for transporting a liquid heat transfer medium, in particular a molten salt. Accordingly, a solar receiver comprises an absorber tube made of a tubular body made of an austenitic steel alloy.

[0031] Tests have shown that at required operating temperatures of the absorber tubes of 600 °C and more, due to the higher corrosion stress, the chromium content must be increased to at least 23% by weight and the nickel content to at least 17% by weight.

[0032] Alloying elements are generally added to steel to specifically influence certain properties. A single alloying element can influence different properties in different steels. The effect and interaction generally depend considerably on the quantity, the presence of other alloying elements, and the solution state in the material. The interrelationships are diverse and complex. The effect of the alloying elements in the alloy according to the invention will be discussed in more detail below. The positive effects of the alloying elements used according to the invention are described below: Carbon C: Carbon is required for the formation of carbides, stabilizes the austenite, and increases strength. Higher C contents impair welding properties and lead to a deterioration in elongation and toughness properties, which is why a maximum content of less than 0.1% by weight is specified.To achieve fine carbides precipitation, a minimum addition of 0.01 wt.% is required. For an optimal combination of mechanical properties, especially in combination with nitrogen, and weldability, the C content is advantageously set at 0.04 to 0.1 wt.%, and particularly advantageously at 0.05 to 0.08 wt.%.

[0033] Nitrogen N: Nitrogen is usually an impurity element in steel production. Binding the nitrogen in the form of nitrides is advantageously possible by alloying with Nb. This, together with chromium carbides, leads to an additional increase in strength. Therefore, the alloy according to the invention has a minimum content of 0.15 wt.%, which, in combination with Nb and C, leads to the formation of strength-increasing Nb(C,N). Dissolved nitrogen stabilizes the austenite and, at high concentrations, leads to embrittlement of the grain boundaries. In the alloy according to the invention, the nitrogen content is therefore limited to a maximum of 0.35 wt.%. N contents of 0.20 to 0.30 wt.% have proven advantageous.

[0034] Chromium Cr: Chromium increases strength, reduces the corrosion rate, and forms carbides. However, in austenites, it can lead to the formation of the embrittling intermetallic sigma phase (Fe, Cr), which is why an upper limit of 27, preferably a maximum of 25, weight percent is defined. To maintain strength and provide corrosion protection for the alloy according to the invention in high-temperature applications, a minimum content of 23 weight percent is required.

[0035] Nickel Ni: Nickel stabilizes austenite at lower temperatures and is therefore necessary for the formation of the austenitic structure. This is even more so the more chromium is required, for example, for corrosion protection. The risk of the embrittling sigma phase, which precipitates with increasing chromium content, is reduced by nickel, which is why nickel contents of 17 to 23 weight percent are necessary in the alloy according to the invention for chromium contents between 23 and 27 weight percent. Higher nickel contents make the concept uneconomical for the application according to the invention. Against this background, Ni contents of at least 20, and advantageously at least 21 weight percent, have proven to be uneconomical.

[0036] Niobium Nb: Niobium acts as a carbide former in the alloy according to the invention, increasing strength through the precipitation of Nb(C,N). The niobium contents must be balanced with the C and N contents to prevent the occurrence of primary Nb(C,N) precipitates, i.e., those already formed during the melting process. Therefore, a maximum content of 0.6 wt.% is specified. To precipitate the desired levels of secondary Nb(C,N), a minimum content of 0.2 wt.% is specified in the alloy according to the invention.

[0037] Manganese Mn: Manganese stabilizes the austenite and can therefore be used as a substitute for nickel. A minimum content of 0.6 wt.% is required for this purpose. However, since Mn reduces corrosion resistance compared to nickel, the content is limited to a maximum of 2 wt.%.

[0038] Silicon Si: The addition of silicon generally increases corrosion resistance by accelerating the formation of a Cr 2 O 3 layer. For this purpose, a minimum Si content of 0.1 wt.% is optionally added. High silicon contents accelerate the precipitation kinetics of the intermetallic sigma phase and complicate the welding process. Therefore, the silicon content is limited to a maximum of 0.75 wt.%.

[0039] Phosphorus P: Phosphorus is a trace or accompanying element from iron ore and is dissolved in the iron lattice as a substitution atom. Generally, attempts are made to reduce the phosphorus content as much as possible because, among other things, its low diffusion rate makes it highly susceptible to segregation and significantly reduces toughness. The accumulation of phosphorus at grain boundaries can cause cracks to occur along them during hot rolling. For the reasons stated above, the phosphorus content is limited to values ​​below 0.03 wt.%.

[0040] Sulfur S: Like phosphorus, sulfur is bound as a trace or accompanying element in iron ore or is introduced via coke during production via the blast furnace route. It is generally undesirable in steel because it tends to segregate strongly and has a strong embrittling effect, which impairs elongation and toughness properties. Therefore, efforts are made to minimize the amount of sulfur in the melt (e.g., through deep desulfurization). For the reasons stated above, the sulfur content is limited to values ​​below 0.03% by weight.

[0041] The alloy composition of the steel alloy enables very economical application in solar thermal systems at high operating temperatures of 600 °C and above.

[0042] The following Table 1 shows the chemical composition of the materials examined in weight % (excerpt): Steel C Si Mn Al Cr Fe Ni Mon Ti Nb Co N 1 (Compare) 0,09 0,22 0,8 0,005 18,3 Bal. 9,07 0,3 0,009 0,3 0,092 0,05 1 (Exp.) 0,059 0,36 1,1 and 25,6 Bal. 21,2 and and 0,46 - 0,2 2 (Compare) 0,06 <0,3 <1 <0,025 27,1 Bal. 32,3 and and 0.8 - 0,02 3 (Compare) 0,05 0,08 0,06 1,3 22,1 1,9 53,1 8,7 0,40 <0,02 11,9 <0,01 4 (Compare) 0,18 <0,5 <0,5 2,1 24,9 9,3 Bal. - 0,13 - - - Comp.: comparison alloy Req.: alloy according to the invention nd: not determined Bal.: balance

[0043] The technical requirements placed on the austenitic steel alloy require a combination of alloying elements that, on the one hand, enables a low corrosion rate through the formation of a surface layer under operating conditions or through conditioning, and, on the other hand, provides the required mechanical properties, i.e., high resistance to thermal fatigue. From an economic perspective, the Ni content should be as low as possible.

[0044] These conditions are fully met with the alloy composition according to the invention. While comparative steels 2, 3 and 4 have a very high Ni content or a Ni-based alloy, comparative steel 1 and steel 1 according to the invention have significantly lower Ni contents. With regard to the required properties, steel 1 according to the invention, with its low Ni content, shows the best results from an economic point of view. Corresponding sample sheets of the aforementioned alloys were subjected to a 1000-hour corrosion test in a molten salt of KNO 3 -NaNO 3 at temperatures of 700 °C, 660 °C, 640 °C, 600 °C and 570 °C. The test results for the comparative alloys 1 to 4 and the alloy 1 according to the invention (in the Figure 1 as Ref. 1, Ref. 2, Ref. 3, Ref. 4 and Exp. 1) are presented in the form of a bar chart per alloy in the Figure 1Five columns are shown for each alloy, assigned from left to right to the temperatures of 700 °C, 660 °C, 640 °C, 600 °C, and 570 °C of the molten salt. The thickness of a corrosion layer formed during the corrosion test is plotted on a y-axis in µm, which can be interpreted as a measure of the corrosion attack. The corrosion attack for the steel 2 according to the invention is comparatively moderate and, given the lower alloy costs, optimal.

[0045] The thermal fatigue behavior of a sample made of the steel 1 according to the invention (Exp.1) is shown in a diagram as Figure 2shown. On the y-axis, a value Δε is plotted as the strain of the sample between the beginning and end of the test in percent between 0.2 and 2.0, and on the x-axis, a number of load cycles is plotted as the number of crack initiation cycles for a load drop criterion of 5% N A5 (LW) with values ​​between 10 2< and 10 5< . R ε =-1 indicates alternating loading during the fatigue test. The steel 1 according to the invention (Exp. 1) displays excellent stability under alternating loading, which is almost independent of temperature. This property is of particular importance under operating conditions due to the relatively frequent temperature cycles in a solar power plant.

Claims

1. Use of an austenitic steel alloy for operating temperatures of at least 600°C to 800°C in a solar power plant, in particular in a molten salt-based solar power plant, wherein this steel alloy consists of the following chemical composition in weight %: C: 0.01 to 0.10 Si: max. 0.75 Mn: max. 2.00 P: max. 0.03 S: max. 0.03 Cr: 23 to 27 Ni: 17 to 23 Nb: 0.2 to 0.6 N: 0.15 to 0.35 the remainder being iron with impurities resulting from smelting.

2. Use according to claim 1, wherein the steel alloy comprises, in weight %, the following: C: 0.04 to 0.10, advantageously 0.05 to 0.08 Si: min. 0.1 Mn: min. 0.6 Cr: 23 to 25 Ni: min. 20, advantageously min. 21 N: 0.20 to 0.

303. Use according to either claim 1 or claim 2, wherein the steel alloy comprises, in weight %, the following: 0 .3 < Nb / C + N < 3 .8 , in particular 0 .4 < Nb / C + N < 2 .

54. Absorber tube of a solar receiver of a solar power plant for transporting a liquid heat carrier, in particular a molten salt, made of a tubular body made of an austenitic steel alloy for operating temperatures of at least 600°C to 800°C, wherein the steel alloy consists of the following chemical composition in weight %: C: 0.01 to 0.10 Si: max. 0.75 Mn: max. 2.00 P: max. 0.03 S: max. 0.03 Cr: 23 to 27 Ni: 17 to 23 Nb: 0.2 to 0.6 N: 0.15 to 0.35 the remainder being iron with impurities resulting from smelting.

5. Absorber tube according to claim 4, characterised in that the tubular body is a seamless tube.

6. Absorber tube according to claim 4, characterised in that the tubular body is a welded tube.

7. Absorber tube according to any of claims 4 to 6, wherein the steel alloy of the tubular body comprises, in weight %, the following: C: 0.04 to 0.10, advantageously 0.05 to 0.08 Si: min. 0.1 Mn: min. 0.6 Cr: 23 to 25 Ni: min. 20, advantageously min. 21 N: 0.20 to 0.30.

8. Absorber tube according to any of claims 4 to 7, wherein the steel alloy of the tubular body comprises, in weight %, the following: 0 .3 < Nb / C + N < 3 .8 , in particular 0 .4 < Nb / C + N < 2 .5 .

9. Absorber tube according to any of claims 4 to 8, characterised by a heat-absorbing coating applied to the outer surface.

10. Absorber tube according to claim 9, characterised in that the coating is a lacquer coating or a sol-gel coating.

11. Solar receiver having an absorber tube according to at least one of claims 4 to 10.

12. Method for producing a tubular body from an austenitic steel alloy for service temperatures of at least 600°C to 800°C, wherein the steel alloy consists of the following chemical composition in weight %: C: 0.01 to 0.10 Si: max. 0.75 Mn: max. 2.00 P: max. 0.03 S: max. 0.03 Cr: 23 to 27 Ni: 17 to 23 Nb: 0.2 to 0.6 N: 0.15 to 0.35 the remainder being iron with impurities resulting from smelting, characterised in that the tubular body is annealed at annealing temperatures between 800°C and 900°C for an annealing time of 0.1 h to 24 h, preferably 2 to 4 h, in an atmosphere containing oxygen and / or nitrogen in such a way that during the annealing, a covering layer with a thickness of at least 2 µm, advantageously at least 5 µm, and at most 20 µm is produced on the tubular body.