Furnace roller for an annealing furnace

Abstract

A furnace roller for an annealing furnace has a roller body and a roller coating. The roller coating consists of an alloy, indicated in weight percent, which is composed of Cr: 10% to 30%, Al: 4% to 30%, Ta: ≤ 8%, Ti: ≤ 4%, Si: ≤ 2.5%, with 14% ≤ Cr + Al + Ta + Ti + Si ≤ 60%, Ni: 30% to 60%, Co: 18% to 28%, one or more of the elements Fe: ≤ 30%, Mo: < 8%, W: ≤ 8%, with 48% ≤ Ni + Co + Fe + Mo + W ≤ 72%, Y: ≤ 2.5%, Ce: ≤ 1.5%, Zr: ≤ 1.5%, La: ≤ 1.5%, Hf: ≤ 1,5%, with Y + Ce + Zr + La + Hf ≤ 2,5%, and optional hard materials HM with HM: ≤ 0.5%, the remainder unavoidable impurities.

Description

[0001] Furnace roller for an annealing furnace

[0002] The invention relates to a furnace roller for an annealing furnace for producing metal sheets, as well as to a method for producing such a furnace roller. Furthermore, the invention relates to the use of such a furnace roller for transporting manganese-containing steel strips in an annealing furnace.

[0003] In plants for the production of metal sheets, particularly in the production of steel strip in a continuous process, rollers are used to transport the metal sheet (metal strip). Special requirements are placed on such transport or conveyor rollers if they are arranged in an annealing furnace, in particular a continuous annealing furnace. This is because the metal sheet is transported at a high temperature, which can easily lead to adhesions (growths or pimples) on the surface of the furnace roller. This impairs the quality of the metal sheet, as the adhesions can detach from the furnace roller and / or cause impressions in the metal sheet. Particular difficulties can arise with metal sheets containing manganese, as manganese from the metal sheet can form manganese oxide growths (pimples) on the furnace roller.This can also occur with metal sheets containing silicon, since silicon from the metal sheet can also cause growths (pimples) on the furnace roller.

[0004] Furnace roller technologies aim to achieve the highest possible productivity at the lowest possible cost. Carbon fiber rollers with SiC infiltration are well known and are used as disposable products. However, they are susceptible to pimple formation and can carburize the strip inside during a plant shutdown. Fused silica cladding tubes are also disposable products. A disadvantage is their short lifespan, caused by chipping of the surface of the fused silica cladding tubes.

[0005] Brush rollers with NiCr bristles are also known. The disadvantage is that the bristles stick together over time, increasing the risk of breakouts. Furthermore, the bristles break out after a while.

[0006] Metallic roller bodies coated with a ceramic or cermet coating are also known. Ceramic coatings are mechanically sensitive, expensive, and, depending on the ceramic used, susceptible to thermal shock. Cermet coatings are used for vertical systems, which typically operate at temperatures below 700 °C. They are unsuitable for horizontal systems, which operate at significantly higher temperatures. Furthermore, the cermet coating often cannot adequately suppress the formation of pimples, especially when transporting high-alloy metal sheets (e.g., electrical steel).

[0007] EP 2 213 755 B1 describes a furnace roller for a continuous annealing furnace with a cermet coating consisting of 50-90 vol.% ceramic and the remainder of a heat-resistant alloy. The ceramic contains Cr3C2 and Al2O3. The heat-resistant alloy has 5-20 mass% Cr, 5-20 mass% Al, 0.1-6 mass% Y and / or Si, and the remainder consisting of Co and / or Ni as well as unavoidable impurities.

[0008] One object underlying the invention can be seen in the creation of a furnace roller with a roller coating that significantly suppresses the occurrence of deposits (spot formation / growths) on its surface. Furthermore, the furnace roller should cause as little chemical and mechanical change as possible to the surface of the metal sheet during high-temperature processes. Further objectives may include a long-life (e.g., more than 2 years) for the roller coating and a reusable furnace roller after reconditioning of the coating. Furthermore, roller coatings are sought that are more cost-effective (in terms of service life) than ceramic coatings or brush-hair applications.

[0009] The problem underlying the invention is solved by the features of the independent claims. Examples and further developments are the subject of the dependent claims.

[0010] Accordingly, a furnace roller for an annealing furnace comprises a roller body and a roller coating. The roller coating consists of an alloy specified in mass% from Gr: 10% to 30%, Al: 4% to 30%, Ta: < 8%, Ti: < 4%, Si: < 2.5%, with 14% < Cr + Al + Ta + Ti + Si d 60%, Ni: 30% to 60%, Co: 18% to 28%, one or more of the elements Fe: < 30%, Mo: < 8%, W: < 8%, with 48% < Ni + Co + Fe + Mo + W < 72%, and optionally Y: < 2.5%, Ce : < 1.5%, Zr: < 1.5%, La: < 1.5%, Hf: < 1.5%, with Y + Ce + Zr + La + Hf < 2.5%, and optional hard materials HS with HS: d 0.5%, the rest unavoidable contamination.

[0011] Accordingly, the coating consists of a metallic alloy based on Ni and Co in addition to the optional hard materials. The alloy contains both Cr and Al, which

[0012] As explained in more detail below, oxide layers form during the operation of the furnace roller (or during a previous annealing process), which determine the thermo-mechanical properties of the roller surface. Hard materials (HS) are present in the roller coating only to a very small extent (HS < 0.5%) or not at all.

[0013] Hard materials increase the hardness and wear resistance of the coating, but they negatively impact the desired diffusion of Cr and Al through the alloy, which is why they are largely avoided. Since hard materials provide rough, surface-rough starting points for pimple formation, a low proportion of hard materials or the avoidance of hard materials (HS = 0%) counteracts pimple formation.

[0014] Furthermore, a low hard material content enables a pore-free and low-oxide alloy coating, which accelerates internal diffusion processes and reduces mechanical interactions within the alloy between the ceramic particles and the metal matrix. Therefore, a small amount of hard material (HS < 0.5%) can be tolerated, but in most cases, it does not provide any advantages.

[0015] The hard materials, HS, can, for example, comprise one or more of the compounds Cr3C2, CrN, Cr2O3, CrB2, WC, WB, Y2O3, BN, A12O3, ZrO2, ZrB2, Ce2O3, CeO2, AIN, TaC, TiN, TiC or consist of one or more of these hard materials.

[0016] In addition to unavoidable impurities and optional hard materials, the roller coating consists of an alloy with alloying elements that can be assigned to the groups "base main elements", "noble main elements", their substitute elements and "optional minor elements".

[0017] The precious main elements Ni and Co, which together constitute a mass fraction of less than or equal to 72% of the alloy, provide the roll coating with the required toughness and strength. They also hold together the oxides that form from the base main elements during annealing of the open roll.

[0018] The proportion of the noble main elements is equal to or greater than 48% to prevent the roller coating from becoming too brittle due to an excessive oxide content. The upper limit of 72% ensures that the roller coating does not become too soft, which would prevent a covering oxide layer from being sufficiently stable.

[0019] When the furnace roll is annealed at sufficiently high temperatures (e.g., during operation at temperatures above 900°C, especially above 1000°C), the base elements Gr and Al form a covering oxide surface of the coating and stabilize the coating through dispersion hardening. This effect of "in-situ" formation of a ceramic component in the coating during annealing (before or during use of the furnace roll in an annealing furnace) is essential for the effectiveness and properties of the roll coating.

[0020] The optional minor element Y may be present in small amounts (maximum 2.5%). It stabilizes the oxides formed from the base elements and hardens the roller coating as a dispersoid.

[0021] Replacement elements for the noble main elements Ni and Co are Fe, Mo, W, replacement elements for the base main elements Al and Gr are Ta, Ti, Si, and replacement elements for the minor element Y are Ce, Zr, La, Hf.

[0022] The alloy can be homogeneous before annealing the furnace roll. Homogeneous, in the sense of the invention, means that the alloy is present, for example, without a layered structure or a concentration gradient. If particles or other morphological structures are present, they are evenly (homogeneously) distributed. Furthermore, the coating does not, for example, exhibit any discrete phases with different compositions that are not attributable to unavoidable, minimal, superficial oxidation during the spraying process.

[0023] The (first) annealing of the furnace roll can take place either outside the annealing furnace or during the first use of the furnace roll in the annealing furnace. As explained in more detail below, diffusion processes occur, which, among other things, cause an inhomogeneous distribution of the alloying elements in the (annealed) alloy of the roll coating.

[0024] According to a further aspect of the disclosure, a furnace roller for an annealing furnace comprises a roller body and a layered structure produced from a roller coating as described above by annealing (which can be carried out before or during use of the furnace roller in the annealing furnace) at a temperature T > 900 ° C, in particular T > 1000 ° C. The layered structure comprises a second layer and a first layer arranged between the roller body and the second layer.

[0025] The second layer is a Cr-rich layer, and the first layer is a Cr-poor layer containing Al2O3 precipitates. The term "Cr-rich" layer means that the Cr content in mass% is higher than in the (homogeneous) alloy before annealing. The term "Cr-poor" layer means that the Cr content in mass% is lower than the Cr content in the (homogeneous) alloy before annealing.

[0026] The second layer may have a continuous Cr2O3 layer adjacent to its surface. The annealed furnace roll may further comprise a third layer above the second layer consisting of Al2O3. This layer is created by diffusion of Al in the alloy toward the surface and formation of the corresponding oxide. The third layer may also be a continuous (covering) oxide layer.

[0027] When oxides are formed from the base elements Cr and Al, comparatively thick oxide layers can be achieved. For example, the second layer can have a thickness equal to or greater than 20 pm. The third layer, for example, can have a thickness equal to or greater than 2 pm.

[0028] A method for producing a furnace roller for an annealing furnace comprises coating the roller body with an alloy of the above-mentioned composition.

[0029] For coating, a thermal spraying process, particularly high-velocity oxyfuel spraying (HVOF), can be used. The alloy can be applied directly to a metallic surface of the roller body. However, other coating methods, such as a welding process, e.g., laser cladding or plasma transferred arc (PTA), can also be used.

[0030] A further aspect of the disclosure concerns the use of a furnace roller according to the preceding description in an annealing furnace for transporting manganese-containing steel strips, in particular electrical steel strips, wherein, due to the release of Mn from the manganese-containing steel strip, a MnAl2O4 layer forms on the surface of the roller coating above the underlying Al2O3 layer. For example, such an electrical steel strip can have a manganese content of 0.20 to 2.0% manganese. Furthermore, such a strip can have, for example, 1.0 to 5.0% silicon.

[0031] In this application, after wear of the MnAl2O4 layer and the underlying Al2O3 layer, a MnCr2O4 layer may form on a surface of the Cr-rich second layer (which is formed by a continuous Cr2O3 layer layer).

[0032] The formation of manganese oxide layers is advantageous because these layers have a high thermo-mechanical stability and form new, smooth oxide-ceramic covering layers that suppress the formation of spots (which is usually particularly problematic in manganese-containing steel strips) to a high degree.

[0033] The effects of the individual alloying elements of the roller coating are described below:

[0034] Main elements (noble), total maximum 72%

[0035] Nickel (Ni) increases the high-temperature resistance and increases the strength of the alloy. Furthermore, Ni provides corrosion protection when combined with Cr and Al. The mass fraction of Ni is between 30% and 60%, in particular between 34% and 59%. In particular, the mass fraction of Ni can be < 59%, Ni < 58%, Ni < 57%, Ni < 56%, Ni < 55%, Ni < 54%, Ni < 53%, Ni < 52%, Ni < 51% or Ni < 50%. With regard to the lower limit, the mass fraction of Ni can in particular be Ni > 35%, Ni > 37%, Ni > 39%, Ni > 41%, Ni > 43%, Ni > 45% or Ni > 47%. Cobalt (Co) causes solid solution hardening for Ni and increases the creep strength and wear resistance of the alloy. Solid solution hardening for Ni by Co is especially relevant when Cr and Al have been removed from the alloy by oxidation and / or diffusion.

[0036] The Co content is at least 18% to sufficiently achieve the desired solid solution hardening for Ni. Above a Co mass fraction of 28%, solid solution hardening for Ni is no longer improved. In particular, the Co content can be between 20% and 26%. In particular, the Co mass fraction can be > 20%, 21%, 22%, 23%, 24%, or 25%. In particular, the Co mass fraction can be < 26%, 25%, 24%, 23%, 22%, or 21%.

[0037] Considering the ratio of the noble main element Ni to Co, it is preferable to use relatively high amounts of Ni and relatively low amounts of Co. For example, the alloy can contain more than 1.6, 1.7, 1.8, 1.9, 2.0, or 2.1 times as much Ni as Co in mass%.

[0038] The mass fraction of the noble main elements Ni and Co is at least Ni + Co > 48%. For example, the mass fraction of Ni + Co can be > 55%, in particular Ni + Co > 60%, and particularly preferably Ni + Co > 65.0%. This ensures that the desired properties of these elements are present in the alloy in sufficient quantities.

[0039] Replacement of the main elements (noble)

[0040] Ni and Co can be partially replaced by the following elements: Iron (Fe) results in lower strength and lower hardness of the alloy in the high-temperature range than corresponding amounts of Ni and / or Co. However, the use of Fe is more economical and results in a smaller amount of critical raw materials (Ni and / or Co) being contained in the alloy. The mass fraction of Fe is therefore between 0% and 30%. In particular, the mass fraction of Fe can be < 20%, preferably Fe < 10% or Fe < 5%, Fe < 2% or Fe < 0.8%.

[0041] Molybdenum (Mo) increases creep strength. Higher Mo contents lead to excessive embrittlement of the alloy, which also becomes susceptible to oxidation at low temperatures. This susceptibility to oxidation cannot be reduced by Al and / or Cr. Therefore, the Mo mass fraction is between 0 and 8%.

[0042] Tungsten (W) increases creep strength, but at higher concentrations, leads to embrittlement of the alloy. Similar to Mo, W increases the alloy's susceptibility to oxidation, although this effect cannot be prevented by Al and / or Cr. Furthermore, W has a high density and thus leads to an undesirable increase in the weight of the furnace roll. The W mass fraction is therefore between 0 and 8%.

[0043] Main elements (base), in total maximum 60%

[0044] Aluminum (Al), as the least noble element, forms the upper covering oxide layer of Al2O3. Al hardens the alloy as a metal through solid solution hardening. If Al is oxidized in the bulk of the alloy, the (annealed) alloy hardens through dispersion hardening. Furthermore, Al prevents the oxidation of Cr within the alloy or reduces already formed chromium oxide, thereby keeping Cr mobile within the alloy. If there is too little Al in the alloy, a sufficient initial Al2O3 covering layer does not form. If there is too much Al in the alloy, oxides (Al2O3 precipitates) can no longer be held together by the alloy, causing the coating to become brittle. The mass fraction of Al is therefore between 4 and 30%.In particular, Al > 6.0% or Al > 8.0% or Al > 10.0% or Al > 11.0% or Al > 12.0% or Al > 13.0% or Al > 14.0% and / or Al < 25% or Al < 20% or Al < 15% or Al < 14% or Al < 13% or Al < 12% or Al < 11%.

[0045] Chromium (Cr) forms a permanent Cr2O3 layer beneath the initial Al2O3 layer. Cr diffuses within the alloy toward the surface, forming a reservoir of Cr beneath the Cr2O3 layer, allowing it to be regenerated from the reservoir during wear.

[0046] If the Cr content is too low, the Cr2O3 layer will not be thick enough or the reservoir of Cr will be too small for wear regeneration.

[0047] If the Cr content is too high, the coating becomes brittle and there is a risk of layer flaking caused by expansion stresses during the oxidation of Cr. The mass fraction of Cr is therefore between 10% and 30%. In particular, Cr can be > 13.0%, Cr > 15.0%, Cr > 16.0%, Cr > 17.0%, Cr > 18.0%, Cr > 19.0%, Cr > 20.0%, Cr > 21.0%, Cr > 22.0% or Cr > 23.0% and / or Cr < 28% or Cr < 25% or Cr < 24% or Cr < 23% or Cr < 22% or Cr < 21% or Cr < 20%.

[0048] Replacement of the main elements (base) The base main elements Cr and Al can be partially substituted by the following elements:

[0049] Tantalum (Ta) strengthens the alloy through the formation of (Cr, Ni, Al)2Ta. However, Ta slows the formation of the Al2O3 layer, hinders the desired diffusion of Al and Cr, and forms linear Ta2Os, which embrittles the alloy. The mass fraction of Ta is therefore between 0 and 8%.

[0050] In particular, Ta < 4.0% or Ta < 3.0%, particularly preferably Ta < 1.0%.

[0051] At higher concentrations, titanium (Ti) forms brittle Ni3Ti. Ti improves the cyclic oxidation behavior of the alloy up to a Ti content of approximately 3.5%. Depending on the alloy composition, however, a deterioration in the cyclic oxidation behavior can occur even at a Ti mass fraction of 1%. Ti is not required for the coating, but can be advantageous at low concentrations (e.g., equal to or less than 3.5%, 2.5%, or 1.5%). Therefore, a Ti mass fraction is between 0 and 4%.

[0052] Silicon (Si) improves the cyclic oxidation behavior of the alloy in the presence of yttrium (Y) and promotes the formation of the Al2O3 layer over the formation of the Cr2O3 layer. This strengthens the initial capping layer, but is undesirable during later operation (after the initial capping layer has been lost). The mass fraction of Si can therefore be between 0 and 2.5%. In particular, Si can be < 2.0%, Si < 1.0%, or Si < 0.2%.

[0053] Minor element (base), maximum 2.5%

[0054] Yttrium (Y) is finely dispersed in the alloy and oxidizes, thus leading to dispersion hardening. Furthermore, Y improves the adhesion of the Cr / Al oxide layers (i.e., the Cr2O3 layer and the Al2O3 layer).

[0055] Too low a Y content increases the risk of spalling of the Cr / Al oxide layers and reduces the strength of the alloy. Nevertheless, a low Y content can be beneficial, as it allows for removal / installation of the furnace roller at low temperatures (e.g., below 1000°C).

[0056] A mass fraction of Y is between 0 and 2.5%. In particular, Y > 0.1% or Y > 0.5% and / or Y < 1.5% or Y < 1.2% or

[0057] Y < 1.0% or Y < 0.8%.

[0058] Replacement of the secondary element, maximum total of 2.5%

[0059] Cerium (Ce), zirconium (Zr) and / or lanthanum (La) act like Y and can be used individually or in combination in equal amounts as a complete or partial replacement for Y.

[0060] Hafnium (Hf) reduces the intergranular hot cracking of the alloy. Hf is not necessary for the coating, but is also not detrimental and (like Y) improves the oxidation resistance of the alloy. Hf is very reactive and can therefore cause pimple bonding upon oxidation at higher concentrations. The mass fraction of Hf is between 0 and 1.5%.

[0061] Figure 1 shows a furnace roller 100 for an annealing furnace in a schematic sectional view. The annealing furnace can, for example, be a continuous annealing furnace (continuous annealing furnace). The furnace rollers 100 are arranged in the annealing furnace during operation and serve to transport metal sheets in strip form (steel strips, in particular electrical steel strips) through the annealing furnace. In particular, the furnace roller 100 can be used in a continuous annealing furnace designed as a horizontal system at high temperatures. The rotation axis of the furnace roller 100 is represented by a dashed line.

[0062] The furnace rollers 100 can be operated in the annealing furnace at temperatures higher than 1000°C, e.g. up to about 1300°C.

[0063] The furnace roller 100 comprises a roller body 110 and a roller coating 120 applied to the roller body 110. Neither the roller body 110 nor the roller coating 120 must be destroyed by the thermo-mechanical conditions of the system.

[0064] Furthermore, the roller coating 120 is intended to cause as little chemical and mechanical change as possible to the surface of the metal sheet (e.g., steel strip). This means that the roller coating 120 serves to create a chemically and mechanically inert surface for high-temperature applications.

[0065] In particular, the roller coating 120 is intended to prevent the formation of local growths (pimples) on the furnace roller 100, whereby this should be ensured even under difficult conditions, e.g., during the transport of steel strips with high Si or Mn contents. A further requirement for the roller coating 120 may be that the coating 120 is suitable for the entire continuous annealing process, i.e., that no different furnace rollers 100 are required in the plant.

[0066] The roller body 110 can, for example, be metallic.

[0067] For example, the roller body 110 can be realized as a centrifugally cast roller. The roller coating 120 is applied to the roller body 110, for example, by means of a thermal spraying process, in particular an HVOF spraying process. During thermal spraying, a metallic spray powder, optionally mixed with hard materials, is sprayed onto the roller body 110. Other application methods are also possible. The production of furnace roller coatings by thermal spraying is known and is widely used in industry.

[0068] According to the invention, a coating material (usually in powder form) is used for the roller coating 120, which consists exclusively of metals except for a small optional proportion of hard materials.

[0069] Such a largely or completely metallic coating is more cost-effective than ceramic coatings or hairbrushes (used in brush rollers). Furthermore, they can be produced pore-free and with low oxide content. Within the scope of the invention, it was recognized that such metallic coatings form surface-adjacent oxide layers "in situ" (i.e., by means of an annealing process that can be performed before or during operation of the furnace roller 100) that bring about the desired properties of the roller coating 120. In particular, it is possible to generate at least two oxide layers (an upper Al2O3 layer and an underlying Cr2O3 layer), both of which effectively prevent the occurrence of growths (pimple formation) on the coating surface and form a durable surface coating with the desired properties.

[0070] Figure 2 shows an enlarged section (Detail D) of Figure 1. After its manufacture and before annealing, the roller coating 120 is a homogeneous metallic alloy (optionally with a small hard material content HS < 0.5%). Therefore, for the sake of simplicity, Figure 2 does not show any morphological structures of this alloy (which, in this example, is realized without a hard material content). In particular, the alloying elements are largely evenly distributed in the coating 120 in the specific alloy composition predetermined by the powder mixture.

[0071] In particular, the roller coating 120 does not have, for example, any further, different layer(s), e.g., no cermet layer(s).

[0072] Table 1 shows examples 1-15 of the chemical compositions of roller coating 120. As already mentioned, the alloy is based on the noble main elements Ni and / or Co, the base main elements Al and Cr, and the optional minor element Y. The noble main elements can be partially replaced by one or more of the elements Fe, Mo, and W. The base main elements can be partially replaced by one or more of the elements Ta, Ti, and Si. One or more of the elements Ce, Zr, La, or Hf can be used as a replacement for the minor element Y.

[0073] In examples 1-11 and 13-15 of Table 1, no hard materials (HS = 0%) were added. However, as already mentioned, a low content of hard materials HS < 0.5% (see example 12) is possible in each of these examples. Table 1: Examples of chemical compositions of the roller coating

[0074] (in wt.%) Table 1 (continued): Examples of chemical compositions of the roller coating (in wt.%)

[0075] The examples No. 1 - 8 for the roll coating were made from

[0076] Powder mixtures were produced in which the ranges of the elements contained were specified by the manufacturer as follows:

[0077] Alloy No. 1

[0078] Powder manufacturer ranges: Ni: 35.2-58.9%,

[0079] Co: 18.0-28.0%, Cr: 13.0-21.0%, Al: 10.0-15.0%, Y: 0.1-0, 8%, others: <5.0%.

[0080] Alloy No. 2

[0081] Powder manufacturer's range information: Ni: 33.1-53.7%, Co: 20.0-26.0%, Cr: 18.0-23.0%, Al: 4.0-11.0%, Ta: 2.0-6.0%, Y: 0.3-0.9%, others: < 5.0%.

[0082] Alloy No. 3

[0083] Range information powder manufacturer: Co: 58.5-66.95%, Cr: 27.0-31.0%, Al: 5.0-9.0%, Y: 0.05-0.5%, Si: 1.0-3.0%, others: <1.0%.

[0084] Alloy No. 4

[0085] Powder manufacturer's range: Ni: 67.5-70.7%, Cr: 23.5-25.0%, Al: 5.0-7.0%, Y: 0.3-0.7%, others: <1.0%.

[0086] Alloy No. 5

[0087] Range information powder manufacturer: NiCrAl (no specific information).

[0088] Alloy No. 6

[0089] Powder manufacturer's range information: Ni: 32%, Co: 38.5%, Cr: 21%, Al: 8%, Y: 0.5%.

[0090] Alloy No. 7

[0091] Powder manufacturer's specifications: Ni: 47.05%, Co: 23%, Cr: 17%, Al: 12.5%, Y: 0.45%. Alloy No. 8

[0092] Powder manufacturer's range information: Ni: 34.8-58.9%, Co: 18.0-28.0%, Cr: 13.0-21.0%, Al: 10.5-15.0%, Y: 0.1-0.8%, Si: < 0.2%, Fe: < 0.2%, Ta: 0%, others: <1.0%.

[0093] The values ​​given in Table 1 were determined by measurements of coatings made from the above-mentioned powder mixtures. A value of 0 indicates either that the corresponding element is not present or is present at a concentration below the detection threshold.

[0094] When used as furnace roller coatings, alloys No. 1, No. 8, and No. 9 showed the best results. The similar alloy No. 7 showed good results. Poorer results were obtained with alloy No. 2, where the Ta content counteracted the desired diffusion of O2. Alloy No. 3 was softer because, without Ni, there was no solid solution strengthening. Alloy No. 4 also showed poorer results because, without Co, there was no solid solution strengthening for Ni, and little Al was available for the initial Al2O3 overlay and dispersion strengthening (Al2O3 precipitates). In addition, the Ni content was too high. In alloy No. 5, there was also no solid solution strengthening for Ni due to the lack of Co, and here too, little Al was available for the formation of the initial Al2O3 overlay and dispersion strengthening (Al2O3 precipitates). The Ni content was also too high. Alloy No.Alloy No. 6 also had little Al for the initial Al2O3 top layer and the reinforcement by dispersion hardening (Al2O3 precipitates). Furthermore, it was not in accordance with the invention due to the excessive Ni + Co proportion > 72% and the excessive Co content. Alloy No. 9 had similar properties to alloy No. 8 but performed slightly lower as it was somewhat softer due to the Fe content. Alloys Nos. 10 and 11 had little Al for the top layer formation, and furthermore, these coatings were comparatively soft due to the high Fe content. Furthermore, the Co contents were outside the limit values. In alloy No. 11, only little solid solution hardening occurred due to the low Co content. The results for alloys Nos. 10 and 11 were roughly comparable to those for alloy No. 4. Alloy No. 9 had similar properties to alloy No. 8 but performed slightly lower as it was somewhat softer due to the Fe content. 12 showed a slightly lower performance than alloys No. 1, 7, 8, 9. Alloy No.Alloy No. 13 was alloyed with Ti and, for reasons still unclear, showed no advantages over alloys No. 1 and 8. Alloy No. 14 showed similar properties to alloy No. 2; no improvements were achieved by adding La. Alloy No. 15 showed no advantages over alloys Nos. 1, 7, and 8 by adding Fe / Zr / Hf, with Ni and Co being outside the limits.

[0095] In general, alloys with chemical compositions within the following ranges can be advantageous: Ni: 34.0-59.0%, Co: 18.0-28.0%, Cr: 13.0-21.0%, Al: 10.0-15.0%, Y: 0.1-0.8%, Si: <0.2%, Fe: <0.2%, where Ta, Ti, Mo, W, Ce, Zr, La, Hf, for example, can be equal to 0% or can each be present below the range limits specified above.

[0096] Figure 3 illustrates the processes in the alloy during annealing. Several diffusion processes occur:

[0097] 1. Diffusion of Al in the alloy

[0098] Diffusion of Al towards the surface (arrow P3) causes a covering Al2O3 layer to form there. This layer is referred to below as the third layer 330. The layer thickness of the third layer 330 produced by annealing is significantly greater than the layer thickness of a native Al2O3 layer, which would be formed without annealing and thus without the diffusion of Al (arrow P3) to the surface of the roller coating 120. The third layer 330 can, for example, have a layer thickness between 2 and 5 pm. At low Al contents, the layer thickness can be less. The third layer 330 can be the uppermost, exposed layer of the roller coating 120.

[0099] Furthermore, Al diffusion to interfaces 345 of particles in the alloy that were deposited during the manufacturing process (spraying process) occurs. In combination with oxygen diffusion (arrow PI) into the alloy, this causes the formation of Al2O3 precipitates 340 in the alloy. The precipitates 340 cause the alloy to harden into a cermet.

[0100] 2. Diffusion of Gr in the alloy

[0101] Gr also diffuses toward the surface of the alloy (arrow P2). This forms a second layer 320. The second layer 320 has a continuous (covering) Cr2O3 layer on its upper side adjacent to the third layer 330 and further comprises an underlying "diffusion reservoir" of (mobile) Gr dissolved in the alloy. The second layer 320 is therefore also referred to as a Cr / Cr2O3 layer.

[0102] 3. Diffusion of O2 into the alloy

[0103] Oxygen diffuses through the surface of the roller coating 120 into the alloy (arrow P1). As a result, in addition to the covering Cr2O3 layer, Cr2Os can also form within the alloy from Gr and diffused-in oxygen. However, this Cr2Os in the interior of the second layer 320 is reduced by the Al diffusing through the second layer 320 (arrow P3) (Cr2O2 + 2Al — 2Cr + Al2O3). As a result, Cr becomes (again) mobile, resulting in the formation of the comparatively thick second layer 320 with the upper continuous Cr2O3 layer and an underlying reservoir of mobile Cr. The Cr concentration in the second layer 320 has a gradient indicated by gray levels in Figure 3.

[0104] In other words, Al causes a getter effect, whereby Cr2O2 in the interior ("bulk") of the alloy (i.e., beneath the covering Cr2O3 layer) is at least partially reduced. Cr thus remains mobile, while the Al2O2 formed by the reduction is incorporated into the bulk, e.g., at particle interfaces 345 in the form of precipitates 340, which solidify the coating 120.

[0105] A Cr-depleted region of the alloy below the layer boundary SG is also referred to below as the first layer 310. As indicated in Figure 3, Al2O3 precipitates 340 may also be present in the first layer 310.

[0106] Since the second layer 320 exhibits a diffusion-induced enrichment of Cr with increasing Cr content towards the top, its layer thickness (lower layer boundary SG) is not sharply defined. If, for example, a line SG is defined as the lower layer boundary of the second layer 320, along which line the actual Cr content corresponds to the average (or nominal) Cr content of the alloy composition, the second layer 320 can be referred to as a Cr-rich (or Cr-enriched) layer, and the first layer 310 lying below the line SG can be referred to as a Cr-poor (or Cr-depleted) layer. For example, the Cr-rich second layer 320 may have an upper region adjacent to the third layer 330 with a proportion of Cr greater than 5.0% or 10.0% higher than the alloy before annealing, and / or the Cr-poor first layer 310 may have a lower proportion of Cr greater than 5.0% or 10.0% lower than the alloy before annealing.In other words, the Cr gradient extending across the first layer 310 and the second layer 320 may represent a Cr concentration difference of at least, for example, 5.0%, 10.0% or even more (for example, 13.0% or 17.0%).

[0107] The second layer 320 may have a thickness of, for example, 20 - 25 pm.

[0108] Figure 4 shows the roller coating 120 (detail D) after wear of the third layer 330 during operation. For example, complete wear of the third layer 330 may have occurred after just several months (e.g., 2 months or more) in large-scale use. The protective effect of the roller coating 120 is taken over by the second layer 320 after wear of the third layer 330. As already mentioned, this layer has a significantly greater layer thickness than the third layer 330 and has a covering Cr2O3 layer with an underlying Cr enrichment reservoir, which, with continued oxygen diffusion (arrow PI) and successive wear of the covering Cr2O3 layer, leads to the renewal of this Cr2O3 layer on its underside. This process can continue until the layer boundary SG is reached, below which there is no longer sufficient Cr available for layer renewal.

[0109] The furnace roll 100 with the roll coating 120 is particularly advantageous for processing high-alloy steel strips, such as electrical steel strips. High-alloy steel strips can contain comparatively high levels of Mn and / or Si, which typically increase the occurrence of local growths (pimples).

[0110] When using Mn-containing steel strips, Mn is deposited from the steel strip onto the surface of the roller coating 120. During initial operation (with little or no wear), a MnAl2O4 layer forms over the third layer 330. This layer is referred to as the fourth layer 540.

[0111] The fourth layer 540 initially protects (i.e., as long as it is still present) the underlying third layer 330. Furthermore, it forms a chemically and mechanically inert surface that effectively prevents the formation of local growths.

[0112] Since Mn is constantly available from the steel strip during operation, the fourth layer 540 is renewed until the third layer 330 is worn away. This means that the renewal of the fourth layer 540 is in contrast to the abrasion of the surface. As long as the third layer 330 is present, the fourth layer 540 is built up and then, if necessary, abraded again. The fourth layer 540 can, for example, have a layer thickness of approximately 1 pm.

[0113] Figure 6 shows the roller coating 120 (Detail D) after wear of the third layer 330 when using Mn-containing steel strips. A uniform, covering MnCr2O4 layer is formed over the second layer 320. The Mn originates from the steel strip (arrow P4). The MnCr2O4 layer is referred to below as the fifth layer 650.

[0114] Similar to the fourth layer 540, the fifth layer 650 has a chemically and mechanically inert surface suitable for high-temperature applications and prevents the formation of growths (pimples). The layer thickness of the fifth layer 650 can, for example, be greater than 5 μm and approximately 10 μm. The fifth layer 650 forms a new, smooth oxide-ceramic cover layer that continuously renews itself during operation (MnO + Cr2Os - MnCrA) and thus remains intact as the second layer 320 wears, until the layer boundary SG is reached.

[0115] Figure 7 illustrates that subsequent grinding of the roller coating 120 would result in the destruction of the roller coating 120, meaning that a new protective layer cannot be formed. This distinguishes the roller coating 120 from conventional ceramic coatings or cermet coatings, where grinding processes can be used to repeatedly create new, smooth coating surfaces.

[0116] During the grinding process (indicated by arrow S), the second layer 320 would be removed. This largely removes Cr (which has accumulated in the second layer 320) from the roller coating 120. What remains is the Cr-poor first layer 310 with the Al bound as oxide (precipitates 340). This means that Al is immobile and cannot diffuse to the new surface that would be created by the grinding process. The Ni / Co alloy with the Al2O3 precipitates 340 but without a protective layer does not form a chemically and mechanically inert surface and quickly leads to pimple formation. In other words, the roller coating 120 would no longer be suitable for use in an annealing furnace after a grinding process S. However, it is conceivable that a new thermal coating would be applied.In summary, it can be stated that the roll coating 120 for both non-Mn-containing steel strips and Mn-containing steel strips largely fulfills the desired properties for high-temperature applications through an "in-situ" formation of covering Al and / or Cr oxide layers. The formation of the "double oxide layer" increases the effective layer thickness of this protective layer 320, 330.

[0117] The desired longevity of the roller coating 120 is primarily achieved by the thicker second layer 320 (i.e., the Cr / Cr2O3 layer). When using Mn-containing steel strips, an additional positive effect occurs through the formation of Mn-0 ceramic cover layers 540 and 650, respectively.

Claims

Claims 1. Furnace roller for an annealing furnace, comprising a roller body and a roller coating on the roller body, wherein the roller coating consists of an alloy specified in mass% from Cr: 10% to 30%, Al: 4% to 30%, Ta: < 8%, Ti: < 4%, Si: <2.5%, with 14% <Cr + Al + Ta + Ti + Si <60%, Ni: 30% to 60%, Co: 18% to 28%, one or more of the elements Fe: < 30%, Mon: < 8%, W: <8%, with 48% <Ni + Co + Fe + Mo + W <72%, Y: < 2.5%, Ce: < 1.5%, Zr : < 1.5%, La : < 1.5%, Hf: < 1.5%, with Y + Ce + Zr + La + Hf < 2.5%, and optional hard materials HS with HS: < 0.5%, the rest unavoidable impurities.

2. Furnace roller for an annealing furnace according to claim 1, wherein the Hard materials, HS, one or more of the compounds Cr3C2, CrN, Cr2O3, CrB2, WC, WB, Y2O3, BN, A12O3, ZrO2, ZrB2, Ce2O3, CeO2, AIN, TaC, TiN, TiC. Furnace roller for an annealing furnace according to claim 1 or 2, wherein HS 0% .

4. Furnace roller for an annealing furnace according to one of the preceding Claims, whereby Al: 6.0% to 25%, in particular 8.0% to 20% or 10% to 15%.

5. Furnace roller for an annealing furnace according to one of the preceding claims, wherein Gr: 13.0% to 21.0%, in particular 15.0% to 20.0%, in particular Gr: < 19.0% or 18.0%.

6. Furnace roller for an annealing furnace according to one of the preceding claims, wherein Ni: 34% to 59% and / or Co: 20% to 26%.

7. Furnace roller for an annealing furnace according to one of the preceding claims, wherein the ratio of Ni to Co in mass% in the alloy is greater than 1.6 or 1.

8.

8. Furnace roller for an annealing furnace according to one of the preceding claims, wherein Ni + Co: > 55.0%, in particular Ni + Co: > 60.0% or Ni + Co: > 65.0%.

9. Furnace roller for an annealing furnace according to one of the preceding claims, wherein Ta: < 4.0%, in particular Ta: < 3.0% or Ta: < 1.0%.

10. Furnace roller for an annealing furnace according to one of the preceding claims, wherein the roller body is metallic at least on its surface.

11. Furnace roller for an annealing furnace according to one of the preceding claims, wherein the alloy is homogeneous before annealing.

12. Furnace roller for an annealing furnace, which has a roller body and a layered structure produced by annealing at a temperature T > 900 ° C, in particular T > 1000 ° C from a roller coating according to one of the preceding claims, wherein the layered structure has a second layer and a first layer arranged between the roller body and the second layer, wherein the second layer is a Cr-rich layer which has a higher Cr content than the Cr content of the roller coating before annealing, and the first layer is a Cr-poor layer which has a lower Cr content than the Cr content of the roller coating before annealing, and in which Al2O3 precipitates are present.

13. Furnace roller according to claim 12, wherein the second layer has a continuous Cr2O3 layer adjacent to its surface. 14 . Oven roll according to claim 12 or 13 , further comprising : a third layer above the second layer , which consists of A12O3consists of .

15. Oven roller according to one of claims 12 to 14, wherein the second layer has a thickness equal to or greater than 20 pm and / or the third layer has a thickness equal to or greater than 2 pm.

16. A method for producing a furnace roller for an annealing furnace, comprising: Coating of a roller body, wherein the roller coating consists of an alloy specified in mass% of Cr: 10% to 30%, Al: 4% to 30%, Ta: < 8%, Ti: < 4%, Si: < 2.5%, with 14% < Cr + Al + Ta + Ti + Si < 60%, Ni: 30% to 60%, Co: 18% to 28%, one or more of the elements Fe: < 30%, Mon: < 8%, W: <8%, with 48% <Ni + Co + Fe + Mo + W <72%, Y: < 2.5%, Ce: < 1.5%, Zr : < 1.5%, La : < 1.5%, Hf: < 1.5%, with Y + Ce + Zr + La + Hf < 2.5%, and optional hard materials HS with HS: < 0.5%, the rest unavoidable impurities.

17. The method according to claim 16, wherein a thermal spraying process or a welding process is used for the coating.

18. Use of a furnace roller according to one of claims 1 to 15 in an annealing furnace for the transport of manganese-containing steel bands, whereby a MnAl2O4 layer and an underlying Al2O3 layer form on the surface of the furnace roll.

19. Use according to claim 18, wherein after wear of the MnAl2O4 layer and the underlying Al2O3 layer, an MnCr2O4 layer forms on a surface of the second layer.

20. Use according to claim 19, wherein the surface of the second layer is formed by a Cr2O3 layer.

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