Martensitic steel

Abstract

The present invention relates to a steel for structural components to be used at elevated temperatures. The steel comprises the following main components (in wt%): Cr 8.0–14.0, Ni 4.0–14.0, Al 2.5–5.0, C 0.003–0.3, N ≤ 0.06, Mo + W ≤ 4.0, at least one of the following: Nb 0.01–1.0, Ta 0.01–1.0, Ti 0.01–1.0, Zr 0.01–1.0, Hf 0.01–1.0, Y 0.05–1.0, balance optional elements, Fe and impurities; and the steel composition satisfies the following condition: Cr (equivalent) + Ni (equivalent) ≤ 30, where Cr (equivalent) = Cr + 2Al + 1.5(Si + Nb + Ti) + Mo + 0.5W; and Ni (equivalent) = Ni + 10(C + N) + 0.5(Mn + Cu + Co).

Description

Technical Field

[0001] The present invention relates to an oxidation-resistant steel suitable for structural components used at high temperatures.

[0002] The present invention also relates to a heat exchanger structural component including such steel. Background Art

[0003] Ferrite-type alumina-forming FeCrAl steels are well-known for their excellent high-temperature oxidation and corrosion resistance. However, ferritic steels do not possess the mechanical strength required for construction materials, especially for components exposed to high loads, such as heat exchangers and steam generators. In recent years, alumina-forming austenitic (AFA) steels with sufficient creep properties at high temperatures have been developed. However, due to the slow diffusion of oxide-forming substances (such as Cr and Al) in austenitic steel structures, ferritic materials are inherently superior in terms of oxidation and corrosion properties.

[0004] The low-carbon martensitic structure is very similar to the ferritic structure, resulting in comparable diffusion properties. Interestingly, it has been shown that aluminum diffuses faster in steels with a martensitic structure than in similar steels with a ferritic structure, thus indicating that the oxidation properties in alumina-forming martensitic steels (AFM) may even be better than those of FeCrAl steels. In addition, martensitic steels generally have higher creep strength than austenitic steels. However, currently available corrosion-resistant martensitic steels are at best chromium oxide formers and are generally not intended for high-temperature use, i.e., martensitic precipitation-hardening stainless steels (such as 17-4PH) have a maximum use temperature of approximately 350 °C. Therefore, there is a lack of suitable material candidates for, for example, the combination of high-temperature and high-load components required in future high-efficiency power generation technologies.

[0005] By manufacturing alumina-forming martensitic (AFM) steel, the present invention combines the excellent oxidation resistance and corrosion resistance provided by an alumina scale and the excellent high-temperature mechanical properties of the martensitic structure (at least up to 650 °C).

[0006] CN107587080 A discloses a precipitation-strengthened heat-resistant steel including: C: 0.03 - 0.06%, Ni: 6 - 10%, Cr: 8 - 13%, Al: 1.5 - 2.4%, Co: ≤3%, Nb: ≤0.1%, Zr: ≤0.1%, and the balance being Fe. It teaches that when the Al content in the alloy is low, the strength is significantly poor and the toughness is not significantly improved, while when the Al content is too high, the toughness of the alloy drops sharply. Summary of the Invention

[0007] The main object of the present invention is to provide aluminized steels which are suitable for load-bearing applications at high temperatures, such as energy generation and energy conversion.

[0008] Another object of the present invention is to provide steels for structural components in liquid metal cooled nuclear reactors or in concentrated solar power plants.

[0009] The above objects and other advantages can be significantly achieved by providing a steel having a composition and structure as described in the alloy claims.

[0010] The alloys of the present invention form protective aluminum-rich oxides in a manner similar to or better than that of FeCrAl alloys. It has been shown that in very corrosive environments (such as liquid lead with high dissolved oxygen content), AFM steels have even better oxidation resistance than the best FeCrAl steels.

[0011] The steel is suitable for heat exchanger structural components, particularly heat exchanger (HX) tubes, and more particularly steam generator (SG) tubes.

[0012] The present invention is defined in the claims. Detailed Description

[0013] The importance of the individual elements of the claimed alloy and their interaction with each other and the limitations of the chemical composition are briefly described below. Throughout the specification, all percentages of the chemical composition of the steel are given in weight % (wt. %). The upper and lower limits of each element can be freely combined within the limits listed in the claims.

[0014] Chromium is present in an amount of at least 8% to provide good oxidation resistance and corrosion resistance. Cr is a ferrite stabilizing element which reacts with carbon to form carbides. Cr also favors the formation of a protective alumina scale by the so-called "third element effect".

[0015] If the chromium content is too high, this can lead to the formation of an undesirable brittle phase at lower temperatures, such as 400 - 600 °C. Therefore, the chromium content is limited to 13%. The lower limit can be 8.5%, 9.0%, 9.5%, 10.0%, 10.5% or 11.0%. The upper limit can be 11%, 11.5%, 12.0% or 12.5%.

[0016] Nickel is an austenite stabilizer and its main purpose is to stabilize the austenite phase at high temperatures (i.e., at austenitizing temperatures) in order to achieve a sufficient amount of martensite upon cooling (quenching). The required amount of Ni depends on the amount of ferrite stabilizing elements and the amount of other austenite stabilizers. Ductility can be controlled by nickel in AFM steel, and increased Ni results in improved ductility until the structure is not favorable for the formation of any martensite upon cooling. Depending on the composition of the steel, the Ni content has an upper limit to avoid excessive stabilization of austenite at room temperature, i.e., too little martensite is formed. Thus, the lower limit can be 4.0%, 4.5%, 5.0%, 5.5%, 6.0%, 6.5%, 7.0%, 7.5%, 8.0%, 8.5% or 9.0%, and the upper limit can be 9.0%, 9.5%, 10.0%, 10.5%, 11.0%, 11.5%, 12.0%, 12.5%, 13.0%, 13.5% or 14.0%.

[0017] Aluminum is essential for the formation of aluminum-rich oxides and is therefore added in an amount of 2.5 - 5%. However, excessive Al can lead to the formation of undesirable brittle phases. Aluminum is beneficial in the case of ferrite precipitation at low temperatures, e.g., 400 - 500 °C, as it inhibits phase separation (α'-formation) and spinodal decomposition. Thus, the lower limit can be 2.5%, 2.6%, 2.7%, 2.8% and the upper limit can be 4.5%, 4.75% or 5%.

[0018] Carbon is always present in steel, it forms carbides and stabilizes austenite and is crucial for the formation of martensite upon cooling / quenching. C, i.e., carbides, is also important for minimizing grain growth upon cooling from the melting or recrystallization temperature in AFM steel. The upper limit of carbon can be set to 0.3%, 0.25%, 0.2%, 0.15%, 0.10%, 0.09% or 0.06%. Depending on the content of boron and nitrogen, the lower limit can be as low as 0.003% or 0.03%.

[0019] Since N reacts with Al, nitrogen can be present in the steel in an amount ≤ 0.06%. N can also form precipitates with V, Nb, Ti, Zr and Y and is beneficial for strength and creep resistance.

[0020] Molybdenum and tungsten improve high-temperature mechanical properties, and are strong carbide-forming elements, and also strong ferrite formers, and can lead to the formation of brittle Laves phases. The addition of W and Mo increases creep properties and can help maintain a lath martensite structure. The amount of molybdenum and tungsten should be limited to a maximum of 4%, preferably 3% or less. If the alloy composition is prone to Laves phase precipitation, the upper limit can be 3%, 2.5%, 2%, 1.5%, 1%, 0.5% or 0.1%.

[0021] Niobium and tantalum form carbides, nitrides and carbonitrides and are beneficial to strength and creep resistance. Additionally, Nb tends to improve oxidation resistance in the same manner as REM. Therefore, Nb and Ta are present in amounts of 0.01 - 1%, preferably 0.01 - 0.5%.

[0022] Ti, Zr & Hf

[0023] Reactive elements that promote the formation of a protective alumina scale. Strong carbide formers and strong oxide particle formers, which are beneficial to high-temperature mechanical properties when alloyed with oxygen (so-called ODS alloys).

[0024] The amounts of Ti, Zr and Hf can be 0.01 - 1% respectively. If alloyed with oxygen, the preferred amounts are 0.5 - 1% (ODS). If oxygen is not deliberately added, the amounts can be 0.1 - 0.7%.

[0025] Yttrium

[0026] Reactive elements that promote the formation of a protective alumina scale. Strong carbide formers and strong oxide particle formers, which are beneficial to high-temperature mechanical properties when alloyed with oxygen (so-called ODS alloys).

[0027] The amount of Y can be 0.01 - 1%. If alloyed with oxygen, the preferred amount is 0.5 - 1% (ODS). If oxygen is not deliberately added, the amount can be ≤0.5%.

[0028] Silicon is beneficial to high-temperature oxidation properties but forms brittle phases at higher contents and should therefore be limited. The upper limit can be 1.0%, 0.6%, 0.55%, 0.5%, 0.45%, 0.4% or 0.35%.

[0029] Manganese

[0030] A strong austenite stabilizer and can replace Ni to a certain extent. Mn also improves mechanical properties to a certain extent. Mn is incorporated in carbides as well as oxides. Mn tends to promote secondary phases that may cause embrittlement, such as the σ phase. For some alloy compositions, the Mn content should be limited to ≤4%, but for alloy compositions sensitive to the σ phase, it is preferably ≤3%. The upper limit can be 3%, 2.5%, 2.0%, 1.5%, 1% or 0.5%.

[0031] Copper is an optional element that has an austenite stabilizing effect, but it can form brittle phases, especially under irradiation. Once copper is added, it is impossible to extract copper from the steel. This greatly makes waste treatment more difficult. For this reason, copper is usually limited to 3%, preferably ≤0.3%. Most preferably, Cu is not deliberately added.

[0032] Cobalt

[0033] In nuclear applications, the cobalt content should be as low as possible, but for other applications, it is beneficial for stabilizing the austenitic structure and improving strength at all temperatures. In compositions for nuclear applications, the amount is preferably ≤0.1%. In compositions where Co is deliberately added, the amount can be ≤2%. The upper limit can be 2, 1.5, 1.0, 0.5, 0.3 or 0.1.

[0034] Vanadium forms carbonitrides and carbides of the M(C,N) type and the Z phase in the steel matrix. However, if there are stronger carbide formers present, the V amount should be ≤0.3%. In other cases, the V amount can be ≤1%.

[0035] Sulfur

[0036] Sulfur should not be deliberately added as it reduces oxidation performance.

[0037] Boron

[0038] Boron can act as a substitute for carbon but is also a strong neutron absorber. Boron can improve the creep strength in martensitic steels by reducing the coarsening (thickening) of carbides at higher temperatures. Boron inhibits the nucleation of ferrite at austenite grain boundaries. The amount of B can be ≤0.1%, but preferably ≤0.01%, depending on the carbon content.

[0039] Bi, Se, Ca, Mg

[0040] These elements can be added to the steel in amounts claimed to further improve machinability, hot workability, and / or weldability.

[0041] Oxygen

[0042] It is usually combined with oxygen-active elements such as Y and RE to form small oxide particles, which are beneficial for high-temperature mechanical properties, namely the so-called ODS alloys. In the case of ODS alloying, the O amount can be ≤0.5%, but preferably 0.05 - 0.15%. In non-ODS alloys, O should not be deliberately added.

[0043] RE improves the oxide scale properties and, in combination with oxygen, is beneficial for high-temperature mechanical properties, namely the so-called ODS alloys. Since yttrium is separately defined, the (rare earth metals) used in this application include elements with atomic numbers 21 and 57 - 71. The amount of RE can be ≤0.3%.

[0044] The steel composition should further meet the following conditions in terms of Cr-equivalent and Ni-equivalent (in wt%):

[0045] Cr(equivalent) + Ni(equivalent) ≤ 30

[0046] Where

[0047] Cr (equivalent) = Cr + 2Al + 1.5(Si + Nb + Ti) + Mo + 0.5W; and

[0048] Ni (equivalent) = Ni + 10(C + N) + 0.5(Mn + Cu + Co).

[0049] The steel preferably has a martensitic structure of at least 35% by volume. The composition region of the AFM steel as a function of Cr equivalent and Ni equivalent is shown in Figure 1 the figure.

[0050] Examples

[0051] In this example, two commercial steels were compared with the steel of the present invention. These two commercial steels are the alumina-forming FeCrAl steel and the stainless steel AISI 316L.

[0052] The steel of the present invention was cast in a high-frequency induction furnace, about 100 g per piece. Then the heating material (charge) was hot-rolled into a 200 mm x 10 mm bar in a total of 8 steps, followed by annealing at 1100 °C for 15 minutes and air-cooling to room temperature. The compositions of the alloys studied are shown in Table 1.

[0053] Table 1: Elemental compositions of examples of some test alloys. All values are given in wt%.

[0054]

[0055] All alloys were cut into samples with dimensions of approximately 30 x 10 mm, having a thickness that varied depending on the initial shape (different thicknesses depending on the initial shape). All samples were polished using Struers abrasive SiC paper (final step #500) to remove any initial oxides, and finally cleaned in ethanol and ionized H 2 O.

[0056] Experiments were conducted in a COSTA (Corrosion Test Bench for Stagnant Liquid Lead Alloys) facility built by the Karlsruhe Institute of Technology (KIT). Using an alumina support as a holder, the samples were loaded into an alumina crucible and then filled with lead. Subsequently, all crucibles were placed on a nickel tray and placed inside the sealed quartz tube of the furnace. More information about the COSTA facility is provided in J. Nucl. Mater. 278 (2000) 85 - 95.

[0057] Two environmental conditions were selected, one condition was using lead as the liquid metal, and the other was using steam exposure. The oxygen concentration in the liquid lead was adjusted by including Ar, H 2 and H 2controlled by a gas mixture of O. H 2 / H 2 O ratio is set to about 10 -3 , corresponding to 10 -4 wt% oxygen dissolved in lead. A ZIROX SGM5 oxygen analyzer is used to monitor the oxygen partial pressure at the system gas outlet. The steam exposure is also carried out in a COSTA facility where an inert Ar gas is added to stabilize the gas flow in the furnace. Both corrosion tests, for lead exposure for 820 hours and for steam exposure for 680 hours, are carried out at 550 °C.

[0058] Before exposure, cross-sections are prepared by polishing one side of each sample to a final grade #4000 at about 45°. The samples are then cleaned with ethanol and ionized H 2 O and subsequently dried with pressurized air. Representative examples of the results obtained from the liquid lead and steam exposures are visible in Figure 2 and 3 respectively.

[0059] Figure 2 is an example of a very severe corrosion test for 820 hours at 550 °C in liquid lead at a very high oxygen concentration (i.e., close to the formation of PbO). The SEM cross-section at 45° shows the top surface and inside the steel sample. This test induced severe oxidation of the following materials: steels forming chromium oxide such as AISI 316L (a), and FeCrAl alloys that are generally oxidation-resistant (both commercial), and a newly developed Fe10Cr4Al-RE (b) that suffered internal oxidation. The only steel forming a protective alumina scale is the newly developed AFM steel (c).

[0060] Figure 3 is a test in a less corrosive environment, i.e., steam + argon at 550 °C for 680 hours. The SEM cross-section at 45° shows the top surface and inside the steel sample. The steel AISI 316L forming chromium oxide suffered internal oxidation to a depth of 50 μm and was covered with a thick non-protective oxide layer on the surface. The FeCrAl steel (b) and the AFM steel (c) performed equally well due to a completely protective oxide scale over the entire surface.

[0061] When tested in a very corrosive environment (i.e., liquid lead with a high oxygen partial pressure), both steels that form chromium oxide such as 316L and FeCrAl alloys suffered from internal oxidation and were covered with a thick oxide layer on the top surface. For both samples, the depth of internal oxidation was about 50 μm. However, the 316L sample also suffered to some extent from liquid metal dissolution and lead penetration, most likely due to the selective dissolution of Ni. Since Ni is an austenite stabilizer, the dissolution of this element can lead to a phase transformation from austenite (FCC) to ferrite (BCC).

[0062] The steel of the present invention is an alumina former like FeCrAl steel. However, while the FeCrAl steel is covered with non-protective internal and external oxides, the steel of the present invention has formed a very thin protective skin. Although the oxide skin is very thin and cannot be distinguished by SEM and the sample preparation used, it is clear that it has formed because no lead has penetrated into the material. In addition, no detectable oxidation or corrosion products were found across the entire cross-section studied, which demonstrates the very strong protective nature of the oxide skin.

[0063] Both FeCrAl and the steel of the present invention achieved the formation of an alumina-rich oxide skin in a steam environment, where no observable corrosion or oxidation attack was found. However, the 316L sample that forms chromium oxide suffered from severe internal and external oxidation of about 50 μm and 40 μm, respectively.

[0064] The results obtained from both exposures are summarized in Table 2.

[0065] Table 2: Summary of the results obtained from liquid lead and steam exposures.

[0066]

[0067] P.O. - Protective oxide. D.A. - Dissolution attack. I.O. - Internal oxidation

[0068] It is clear from Table 2 that the only alloy that achieved the formation of a protective oxide cover in both experiments is the steel of the present invention, which also did not suffer any dissolution attack. Therefore, the alloy claimed is considered to have very attractive properties for use as a structural component in contact with liquid metals such as lead or steam or both. Similar corrosion tests in liquid lead have been carried out at up to 650 °C with different oxygen contents, with the same result, i.e., the AFM steel of the present invention has the same or better oxidation resistance than the best FeCrAl steel in this temperature range.

Claims

1. High-strength steel, comprising, by weight%: At least one of the following: Optionally The balance of iron except for impurities, wherein the content of RE does not include the amount of Y, but only includes the amount of elements having atomic number 21 and / or any one of atomic numbers 57 - 71; and The steel composition satisfies the following conditions: Cr (equivalent) + Ni (equivalent) ≤ 30 Wherein Cr (equivalent) = Cr + 2Al + 1.5(Si + Nb + Ti) + Mo + 0.5W; and Ni (equivalent) = Ni + 10(C + N) + 0.5(Mn + Cu + Co), wherein the steel has a martensitic structure of at least 35% by volume.

2. The steel according to claim 1, wherein Al is at least 2.6% by weight.

3. The steel according to any one of claims 1 - 2, wherein Co ≤ 0.1% by weight.

4. A heat exchanger structural component, comprising the steel according to any one of the preceding claims.

5. The heat exchanger component according to claim 4, wherein the heat exchanger structural component is a heat exchanger (HX) tube.

6. The heat exchanger component according to claim 4, wherein the heat exchanger structural component is a steam generator (SG) tube.

Images