Steel for surfacing material

Abstract

A steel suitable for structural components used in nuclear reactors in contact with liquid lead or liquid lead alloys, comprising, by weight percent (wt.%): C 0.02–0.09; Si 0.1–1.6; Mn 1.5–3.0; Cr 9.0–12.0; Ni 10.0–16.8; Al 2.0–3.4; Ti 0.1-1.0; Nb≤0.5; V≤0.5; Ta≤1.5; Y≤0.5; Mo≤1.5; W≤1.5; Cu≤1.7; N≤0.06; Co≤1.0; B≤0.1; Zr≤0.5; Hf≤0.5; RE≤0.2; Ca≤0.1; Mg≤0.1; Bi≤0.1; SE≤0.1, and the balance Fe excluding impurities, wherein the content of RE does not include the amount of Y, but only includes the amount of elements with atomic numbers 21 and 57-71, wherein the steel meets one or more of the following requirements: a) Cr Eq＝ 18.5‑21 and Ni Eq＝ 11-20, of which Cr Eq＝ Cr+3Al+2Si+1.5[(Ti+Nb+V+Ta+Zr)‑4.5(C+N)] and Ni Eq =Ni+0.5((Mn+Cu+Co),b)5-25 volume%δferrite.

Description

[0001] Steel used for welding materials is suitable for structural components in nuclear reactors that come into contact with liquid lead or liquid lead alloys.

[0002] In many high-temperature environments, alumina-formed steels such as FeCrAl are generally superior to chromium-formed stainless steels in terms of oxidation and corrosion. FeCrAl steels have weaker high-temperature mechanical properties but can be used as weld overlays on load-bearing steels. However, welded FeCrAl structures have been shown to be non-ductile and prone to fracture during welding or cooling. This is a serious and unavoidable problem when construction must comply with pressure vessel specifications and / or bending test specifications.

[0003] Theoretically, one solution could be to use high-aluminum alloy austenitic steel or nickel-based alloys as a weld overlay (or as a single material), but this would be extremely expensive and not feasible in certain environments that require relatively low nickel content, such as liquid lead or lead-bismuth eutectic (LBE) environments.

[0004] To address this problem, the present invention offers a cost-effective solution. By using a modified and lean AFA (alumina-formed austenitic) composition, a ductile and corrosion-resistant weld overlay can be produced. Severe high-temperature corrosion problems can be solved by applying the weld overlay of the present invention to standard stainless steel accepted as a base for pressure vessels.

[0005] For over a decade, AFA compositions have shown promise as corrosion-resistant and creep-resistant steels, but have yet to enter the market due to formability and aging issues in certain conditions. Both of these issues are related to the loss of ductility.

[0006] WO20167039679A1 discloses an AFA alloy suitable for use in contact with liquid lead.

[0007] Since early work at Oak Ridge National Laboratory (ORNL), the presence of Nb has been reported as a necessary condition for alumina formation in AFA steels, and high amounts of austenite stabilizing elements, preferably Ni, and in addition to Ni, Cu and Mn, are added to obtain a single-phase stable and highly uniform austenitic microstructure, thereby avoiding a ferrite-austenite dual-phase structure. KR: Larsen, Materials Performance, 54(9):30-34 provides a review of the early work on AFA alloys. Its contents are available at the following link:

[0008] https: / / www.researchgate.net / publication / 283690362_Alumina-forming_ austenitic_alloys_resist_high-temperature_corrosion

[0009] as well as

[0010] http: / / www.materialsperformance.com / articles / material-selection- design / 2015 / 12 / alumina-forming-austenitic-alloys-resist-high-temperature- corrosion .

[0011] The "Fine-Tuning the Alloy Composition" section lists three requirements for AFA alloys. First, there must be 12% to 15% Cr and 2.5% to 4% Al. Second, 0.6% to 3% Nb must be added. Third, the amounts of N, Ti, and V must generally be minimized.

[0012] The inventors have surprisingly discovered that AFA alloys with good properties for contact with liquid lead or lead-based alloys can be produced even without complying with all three requirements set forth above.

[0013] This invention uses a finely tuned lean AFA composition in which the Cr content can be less than 12%, and where the intentional addition of Nb is not required, provided that the alloy contains 0.1-1.0% Ti. It has also been found that specific amounts of ferrite are not only acceptable, but the two-phase structure can also have a positive impact on the properties. However, the amount of ferrite should be limited to 5-25% by volume. A preferred amount is 10-25% by volume ferrite in the welded structure. This provides significant benefits in improving weldability (avoiding hot cracking), improving corrosion resistance (increasing Al diffusion), and maintaining ductility during long-term aging.

[0014] "Conventional" AFA steels with 98-100% austenite and a composition close to that of this invention are more readily dissolved in liquid lead and have lower oxidation resistance in steam.

[0015] However, to avoid a continuous network of ferrite and thus reduce ductility and the second brittle phase, and to avoid liquid metal embrittlement (LME) in liquid lead and lead-bismuth eutectic (LBE) alloys, the ferrite content should not exceed 25% by volume.

[0016] This innovative welding consumable / weld overlay can be applied to several components in lead-cooled reactors, such as the inside of the vessel, pump components, steam generator components, core barrel, and other components in the core structure, where corrosion protection and ductility are required.

[0017] Especially in liquid lead / LBE and in vapor, its oxidation resistance far exceeds that of conventional stainless steel such as AISI 316L.

[0018] The innovative lean welding consumables / surface layers have a unique combination of excellent welding, casting, corrosion, erosion and high-temperature mechanical properties, as well as LME resistance, with sustained ductility and cost competitiveness.

[0019] Welding consumables can be wire or strip, and welding methods can be TIG, MIG, laser welding, or surfacing processes used in heavy industry, including different arc welding techniques, SAW (using wire or strip), SMAW (using wire), GMAW (using wire), FCAW (using wire), electroslag welding, and ESW (using strip).

[0020] The most important feature of this invention is that the weld overlay meets the requirements of the standard bending test for corrosion-resistant surface layers in the pressure vessel specification. Attached Figure Description

[0021] Figure 1 Schaeffer diagrams of AFA steels containing 5-25% ferrite are marked in gray.

[0022] The last item in the Cr equivalent is the sum of all strong carbide formers (SCFs) (such as Ti, Nb, Ta, V, and Zr) in their free non-carbide / nitride forms (i.e., minus the amount of carbon and nitrogen).

[0023] Figure 2 . Figure 1 Examples of welded structures of the steels of the present invention marked with an X. Typical welded structures of the alloys of the present invention having 17% ferrite and 87% austenite.

[0024] Figure 3 Electron microscopy (SEM) cross-section of the steel of the present invention, which serves as a weld overlay on an AISI 316L base metal (substrate) exposed to liquid lead at 700°C. The steel consumable of the present invention... Figure 1 Marked with an X. A protective oxide has formed on the top surface (welded material) with residual lead.

[0025] Figure 4 Cross section and Figure 3 Same, but with a higher magnification. The weld overlay of the present invention forms a fully protective aluminum-rich oxide layer beneath the initial weld oxide. The aluminum content (not shown here) measured by EDX indicates up to 10% by weight of aluminum at the inner protective oxide layer, i.e., the actual Al content is much higher, since this layer is only about one micrometer thick.

[0026] Figure 5 SEM micrographs show the steel of this invention in its as-cast state after being exposed to steam for 700 hours at 700°C. Polished cross-sections and exposed surfaces, which were ground with 600# SiC paper prior to exposure, are also shown. A thin protective layer of Al-rich oxide is present, with no visible corrosion.

[0027] Figure 6 SEM micrographs show commercial AISI 316L steel exposed to lead for 700 hours at 700°C. Polished cross-section and exposed surface. Severe corrosion with nickel dissolution, lead penetration (light gray contrast), and internal oxidation (dark contrast), especially at grain boundaries.

[0028] Figure 7 SEM micrographs show commercial AISI 316L steel exposed to steam for 700 hours at 700°C. Polished cross-section. Severe oxidation, up to 100 micrometers deep.

[0029] Figure 8 Example of a bending test of the weld overlay of the steel of the present invention on an AISI 316L substrate. Detailed Implementation

[0030] The following briefly explains the importance of individual elements and their interactions with each other, as well as the limitations on the chemical composition of the claimed alloy. Throughout this specification, all percentages of the chemical composition of the steel are given in weight % (wt.%). Upper and lower limits for individual elements can be freely combined within the limits set forth in the claims.

[0031] Chromium, ranging from 9.0% to 12.0%, is present at a minimum content of 9% to provide good oxidation resistance and corrosion resistance. Cr is a ferrite stabilizing element; it reacts with carbon to form carbides. Cr also promotes the formation of protective alumina nodules through the so-called "third element effect." However, the chromium content should not exceed 12% because the amount of ferrite in the weldment must be minimized to a maximum of 25%.

[0032] Excessive ferrite in weldments reduces their ductility and may cause them to fail standard bending tests. It also increases the formation of undesirable brittle phases during aging at lower temperatures (400-600°C). Therefore, the chromium content is limited to 12%. Lower limits are 9.0%, 9.5%, 10.0%, 10.5%, or 11.0%, and upper limits are 11%, 11.5%, or 12.0%.

[0033] 10-16.8% nickel acts as an austenite stabilizer, and its primary purpose is to stabilize the austenitic phase. To obtain a ductile weld structure, it is important to keep the ferrite content below 25%, therefore the Ni content should be above 10%. To achieve a balanced structure with optimized corrosion properties, the nickel content should not exceed 16.8%. In particular, in liquid lead / LBE, if the nickel content exceeds 16.8%, the risk of nickel dissolution corrosion in the weld increases dramatically.

[0034] Therefore, the lower limit can be 10.0%, 10.5%, or 11.0%, and the upper limit can be 12.0%, 12.5%, 13.0%, 13.5%, 14.0%, 14.5%, 15.0%, 15.5%, 16.0%, or 16.5%.

[0035] Aluminum, ranging from 2.0% to 3.4%, is essential for the formation of Al-rich oxides and is therefore added in amounts of 2.0% to 3.4%. However, excessive Al can lead to the formation of an undesirable brittle phase. Aluminum stabilizes ferrite and must be balanced with austenite stabilizers to avoid excessive ferrite. Lower limits can therefore be 2.1%, 2.2%, 2.3%, or 2.4%, and upper limits can be 3.1%, 3.2%, or 3.3%.

[0036] Carbon is always present in steel; it forms carbides and stabilizes austenite. C (i.e., carbides) is also important for minimizing grain growth in the weldment during cooling from the melting temperature. Upper limits for carbon can be set at 0.09%, 0.08%, 0.07%, 0.06%, or 0.05%. Lower limits can be as low as 0.02%, depending on the boron and nitrogen content.

[0037] Nitrogen can be present in steel in amounts ≤0.06% because N reacts with Al. N can also form precipitates with Nb, Ti, Zr, V and Y, which are beneficial to strength and creep resistance.

[0038] Molybdenum and tungsten enhance high-temperature mechanical properties and are carbide-forming elements, as well as strong ferrite-forming elements, leading to the formation of a brittle Laves phase. The addition of W and Mo increases creep properties. The amounts of molybdenum and tungsten should each be limited to a maximum of 1.5%, preferably 1% or less. Lower limits can be 0.001, 0.005, 0.01, 0.05, or 0.1% respectively. Upper limits can be 0.5% or 0.1% if the alloy composition readily facilitates Laves phase precipitation and ferrite formation.

[0039] Niobium is an element that forms carbides, nitrides, and carbonitrides, and is beneficial for strength and creep resistance. Furthermore, Nb tends to improve oxidation resistance in the same way as REs (reactive elements). Nb does not need to be added, but can be present in amounts up to 0.5%. Preferably, the upper limit can be set at 0.4%, 0.3%, 0.2%, 0.1%, or 0.05%. The lower limit can be 0.001%, 0.005%, 0.01%, 0.05%, or 0.1%.

[0040] Tantalum forms carbides, nitrides, and carbonitrides, and is beneficial for strength and creep resistance. Furthermore, Ta tends to improve oxidation resistance in the same way as RE (reactive elements). Therefore, Ta exists alone in amounts up to 1.5%. Lower limits may be 0.001, 0.005, 0.01, 0.05, or 0.1%.

[0041] titanium

[0042] Ti is intentionally added to the alloy in an amount of 0.1-1%, preferably 0.2-0.9%, more preferably 0.3-0.9%. Ti is beneficial for the formation of a stable alumina layer and acts as a grain refiner.

[0043] Zr

[0044] Zr is a reactive element that promotes the formation of protective alumina nodules. It is a strong carbide-forming element and a strong oxide particle-forming element, beneficial to high-temperature mechanical properties. The amount of Zr can be up to 0.5%. Higher concentrations may negatively impact thermal ductility. The upper limit may be further limited to 0.4%, 0.3%, 0.2%, or 0.1%. The lower limit may be 0.01%.

[0045] Hf

[0046] It is a reactive element that promotes the formation of protective alumina nodules. A strong carbide-forming element and a strong oxide particle-forming element, it is beneficial to high-temperature mechanical properties. The amount of Hf can be up to 0.5%. The upper limit can be further limited to 0.4%, 0.3%, 0.2%, or 0.1%. In nuclear applications, the amount of hafnium is preferably less than 0.01%.

[0047] yttrium

[0048] Reactive elements that promote the formation of protective alumina nodules. They can be found in carbides and nitrides. They are strong oxide particle-forming elements and beneficial for high-temperature mechanical properties.

[0049] The amount of Y can be up to 0.5%. The upper limit can be ≤0.3%, because higher amounts can induce hot cracking in austenite. The upper limit can be further changed to 0.2%, 0.1%, 0.055%, or 0.03%. If added, the lower limit can be 0.001%, 0.01%, 0.05%, or 0.1%.

[0050] Silicon is beneficial for high-temperature oxidation, but it stabilizes ferrite and forms a brittle phase, and its content is relatively high, therefore it should be limited. The upper limit is 1.6%, and can be set at 1.2%, 1.0%, 0.9%, 0.8%, 0.5%, or 0.3%. The lower limit can be set at 0.2%, 0.3%, or 0.4%.

[0051] manganese

[0052] Mn is an austenitic stabilizer and can replace Ni to some extent. It exists in amounts from 1.5% to 3.0%. Mn also improves mechanical properties to some extent. Mn is present in both carbides and oxides. Mn tends to promote second phases, such as σ(sigma) phases, which can lead to embrittlement. At higher concentrations, oxidizing properties may be negatively affected. Upper limits are 3%, 2.9%, 2.8%, 2.7%, or 2.6%. Lower limits are 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2.0%, 2.1%, 2.2%, 2.3%, or 2.4%. However, depending on conceivable alternatives, the intentional addition of Mn is not necessary, provided that the required Cr content is met. Eq＝ 18.5-21 and Ni Eq＝ 11-20.

[0053] Copper is an optional element that has an austenite-stabilizing effect, but it can form a brittle phase, especially under irradiation. Once copper is added, it becomes impossible to extract it from steel. This significantly makes waste disposal more difficult. For this reason, copper is typically limited to 1.7%, preferably ≤1%.

[0054] If added, the lower limit can be 0.1%, 0.2%, 0.3%, 0.4%, or 0.5%. The upper limit can be 1.7%, 1.5%, 1.3%, or 1%.

[0055] cobalt

[0056] The cobalt content should be as low as possible in nuclear applications, but for other applications, it is beneficial for stabilizing the austenitic structure and improving strength at all temperatures. In compositions intended for nuclear applications, this amount is preferably ≤0.1%. In compositions where Co is intentionally added, this amount may be ≤1%.

[0057] Vanadium forms M(C,N) type and Z-phase carbides and carbonitrides in the steel matrix. However, the V content should be ≤0.5%. The lower limit can be 0.01, 0.05, 0.1, or 0.15%.

[0058] sulfur

[0059] Sulfur should not be added intentionally, as it reduces the oxidizing properties.

[0060] boron

[0061] Boron can serve as a substitute for carbon, but it is also a strong neutron absorber. Boron can increase the creep strength of martensitic steel by reducing the coarsening of carbides at high temperatures. Boron inhibits the nucleation of ferrite at austenite grain boundaries. The amount of boron can be ≤0.1%, but preferably ≤0.01%, depending on the carbon content.

[0062] Bi, Se

[0063] These elements can be added to steel in the amounts claimed to improve machinability, hot workability, and / or weldability. The maximum amount of each element is preferably less than 0.1%, more preferably less than 0.02%. Preferably, neither is intentionally added, as they may impair corrosion resistance.

[0064] Ca, Mg

[0065] These elements can be added to steel in the claimed amounts to further improve machinability, hot workability, and / or weldability. The maximum amount of each element is preferably less than 0.1%, more preferably less than 0.02%.

[0066] oxygen

[0067] O is not intentionally added. The amount of O is preferably less than 0.05%.

[0068] Reactive elements (REs) improve oxide scale properties and are beneficial to high-temperature mechanical properties. As used in this application, REs cover elements with atomic numbers 21 and 57-71, since yttrium is defined separately. The amount of RE can be ≤0.2%. The lower limit can be 0.001, 0.005, 0.01, 0.05, or 0.1%. For example, lanthanum can be added in the range of 0.001-0.2% or 0.01-0.1%.

[0069] The steel preferably has a ferrite and austenite matrix structure of 5-25% by volume. Based on experimental data, specialized Schaeffer diagrams were constructed for AFA steel and the welding consumable composition of the present invention. Figure 1 The welding consumables of the present invention shown have a composition range in terms of Cr and Ni equivalents (Cr Eq Ni Eq )count.

[0070] Preferably, the Cr and Ni equivalents (Cr Eq Ni Eq )satisfy:

[0071] Cr Eq＝ 18.5-21 and Ni Eq＝ 11-20, of which Cr Eq=Cr+3Al+2Si+1.5[(Ti+Nb+V+Ta+Zr)-4.5(C+N)] and Ni Eq =Ni + 0.5(Mn + Cu + Co).

[0072] The steel of the present invention can be used as a weld overlay on preferred austenitic base materials (such as AISI 316L and Alloy 800HT).

[0073] Figure 2 An example of the weld overlay structure of the present invention on a 316L substrate is shown. The amount of δ-ferrite in this example is 17%, which improves weldability without compromising the inherent austenitic phase immunity to liquid metal embrittlement (LME).

[0074] As previously mentioned, the preferred amount of δ-ferrite in the final weld overlay structure is 5-25%. The upper limit for δ-ferrite can be further limited to 23%, 21%, 19%, 17%, or 15%. The lower limit can be further limited to 6%, 7%, 8%, 9%, or 10%. A preferred range is 10-20%.

[0075] The amount of intermetallic phase is preferably less than 5% by volume, preferably less than 1%, and most preferably the steel contains no intermetallic phase. Examples of intermetallic phases are σ phase, Lavres phase, and χ(chi) phase.

[0076] application

[0077] Steel can be used as a weld overlay material for nuclear reactors or concentrated solar power plants. Specifically, nuclear reactors or concentrated solar power plants are cooled by lead or lead-bismuth alloys. In such applications, molten lead or lead-bismuth alloys may have a temperature ≤600°C and / or at least 10... -7 Oxygen content (%) by weight.

[0078] Nuclear pressure vessels may contain steel.

[0079] Steel can be in the form of strip or wire and can be used as welding consumables and / or weld overlay materials.

[0080] The composite material can be formed from a stainless steel substrate, on which the steel of the present invention as defined above is provided by overlay welding. The substrate material can be a steel selected from AISI 316, AISI 316L, AISI 316LN, ALLOY 800, or ALLOY 800HT. Preferably, the overlay layer comprises 5 to 25 vol% ferrite, more preferably 10 to 20 vol% ferrite.

[0081] Example

[0082] In this embodiment, the present invention (both as welding consumables and weld overlay) is compared with commercial stainless steel AISI 316L.

[0083] The eleven steel compositions of the present invention disclosed in Table 1a were selected from 42 larger experimental steel matrices with different compositions. All steels were cast in a high-frequency induction furnace, approximately 100g per batch. Twelve selected steels were directly cast into 3.5mm rectangular sheets, approximately 200×20mm. Small samples (approximately 30×4×3.5mm) of the as-cast material were cut. 3 It is used for corrosion and abrasion tests.

[0084] The plate was cut into strips 200 mm long with a cross-section of 3.5 × 1.5 mm using a wire electrical discharge machining (WEDM) machine, to be used as welding consumables. A 4 mm thick plate made of AISI 316L stainless steel was used as the base for different weld overlay layers. A Manual TIGwelding 100ADC was used.

[0085] The composition of the selected alloy is shown in Table 1a. The corresponding chromium equivalent (Cr...) eq ) and nickel equivalent (Ni eq (This is disclosed in Table 1b.)

[0086] Table 1a. Elemental composition of 11 optimal alloys (1-11) for weldability, corrosion properties, and ductility. All values ​​are given in weight percent. AISI 316L was the base material for the weld overlay test. All weld samples contained approximately 5-25% δ-ferrite.

[0087]

[0088] Table 1b. Cr and Ni equivalents optimized for lean AFA with a small amount of δ-ferrite. eq =Cr + 3Al + 2Si + 1,5(SCF - 4,5(C + N)). SCF is the sum of strong carbide-forming elements (such as Ti, Nb, Ta, V, and Zr). Ni eq =Ni + 0.5(Mn + Cu + Co), all values ​​are in weight percent.

[0089] alloy Cr-eq Ni-eq 1 20,3 18,1 2 19,6 12,7 3 19,2 13,5 4 20,0 15,4 5 20,1 17,8 6 20,7 17,2 7 20,1 18,0 8 20,1 18,0 9 20,2 18,0 10 18,9 18,1 11 20,7 18,2

[0090] The cast samples were ground and polished using Struers abrasive SiC paper to remove any initial oxides (final step #500), and finally cleaned with ethanol and deionized H2O. The weld overlay samples were then exposed to weld with the remaining weld oxides.

[0091] Corrosion experiments were conducted in a COSTA (Corrosion Test Stand for Liquid Metal Alloys) apparatus constructed by the Karlsruhe Institute of Technology (KIT). Samples were placed in alumina crucibles using an alumina support, and then filled with lead. All crucibles were then placed on a nickel tray and placed within a sealed quartz tube of the furnace. More information about the COSTA apparatus is presented in J. Nucl. Mater. 278 (2000) 85-95.

[0092] Two environmental conditions were selected: one using lead as a liquid metal, and the other involving exposure to vapor. The oxygen concentration in the liquid lead was controlled using a gas mixture containing Ar, H2, and H2O. The H2 / H2O ratio was set to approximately 10. -3 This corresponds to dissolving 10 in lead at an exposure temperature of 700°C. -5 Oxygen by weight (%). Exposure time was 700 hours. The oxygen partial pressure at the system gas outlet was monitored using a ZIROXS GM5 oxygen analyzer. Vapor exposure was also performed for 700 hours at 700°C. Inert Ar gas was added to stabilize the gas flow within the furnace during vapor exposure. After exposure, for the final step #4000, cross-sections were prepared by polishing one side of each sample at approximately 45°. The samples were then cleaned with ethanol and deionized H2O, followed by drying with compressed air. Representative examples of liquid lead and vapor exposure results can be found in [reference needed]. Figure 3-7 The results are summarized in Table 2. All cast and polished samples performed as well as or better than the weld overlay of the same material, which is expected, as the weld overlay is slightly thinner or mixed with the AISI 316L base material, typically measuring 5-10% on top of the weld overlay.

[0093] Table 2. Summary of results from exposure to liquid lead and vapor at 700°C.

[0094]

[0095] PO - Protective oxide, IO - Internal oxidation / corrosion.

[0096] All weld overlay samples on the AISI 316L substrate passed the bending test without any signs of crack initiation. An example of such a test sample is shown below. Figure 8 In the three-point bending test, the radius is smaller than that required by the ASME bending test standard, and the bending test angle is greater than required, i.e., greater than 180°.

Claims

1. A steel suitable for use in structural components of nuclear reactors in contact with liquid lead or liquid lead alloys, said steel comprising, by weight percent (wt.%), the following: C0.02-0.09 Si 0.1-1.6 Mn1.5-3.0 Cr9.0-12.0 Ni 10.0-16.8 Al2.0-3.4 Ti0.1-1.0 Optional Nb≤0.5 V≤0.5 Ta≤1.5 Y≤0.5 Mo≤1.5 W≤1.5 Cu≤1.7 N≤0.06 Co≤1.0 B≤0.1 Zr≤0.5 Hf≤0.5 RE≤0.2 Ca≤0.1 Mg≤0.1 Bi≤0.1 Se≤0.1 The remaining Fe after removing impurities, wherein the RE content does not include the amount of Y, but only the amount of elements with atomic numbers 21 and 57-71. And the steel described therein meets one or more of the following requirements: a) Cr Eq =18.5-21 and Ni Eq =11-20, where Cr Eq =Cr+3Al+2Si+1.5[(Ti+Nb+V+Ta+Zr)-4.5(C+N)] and Ni Eq =Ni+0.5((Mn+Cu+Co), b) 5-25% δ-ferrite by volume.

2. The steel according to claim 1, wherein the composition is balanced such that it falls within a region defined by the following coordinates: Ni Eq Represented on the y-axis, and Cr Eq Represented on the x-axis.

3. The steel according to any one of the preceding claims, wherein the steel comprises at least one of the following: Ti0.2-0.9 Nb≤0.3 Mo≤1.0 Mn2.1-3.

0.

4. The steel according to claim 1 or 2, wherein the steel comprises at least one of the following: Cu 0.5-1.5 Nb0.001-0.1 Ti 0.3-0.9 Y0.005-0.2。 5. The steel according to claim 1 or 2, wherein the steel is in the form of strip or wire.

6. A composite material comprising a stainless steel substrate, wherein the stainless steel substrate is provided with steel as defined in any one of claims 1-5 by overlay welding.

7. The composite material according to claim 6, wherein the base material is steel selected from AISI 316, AISI 316L, AISI316 LN, ALLOY 800 or ALLOY 800HT.

8. The composite material according to claim 6 or 7, wherein the weld overlay comprises 5 to 25% by volume of δ-ferrite.

9. The composite material according to claim 8, wherein the weld overlay comprises 10 to 20 vol% δ-ferrite.

10. The use of steel according to any one of claims 1-5 as a welding consumable and / or surfacing material in nuclear reactors or concentrated solar power plants.

11. The use of the steel according to claim 10, wherein the nuclear reactor or the concentrated solar power plant is cooled by lead or a lead-bismuth alloy.

12. The use of the steel according to claim 11, wherein the molten lead or lead-bismuth alloy has a temperature of ≤600°C and / or at least 10 -7 Oxygen content (by weight %).

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