Release Agent-Free Aluminium Strip Casting

Abstract

A casting roller or belt for a revolving chill mould of a strip casting system for the manufacture of an aluminium alloy strip and a strip casting system for manufacturing an aluminium alloy strip comprising at least one revolving chill mould with a casting gap. A method for manufacturing an aluminium alloy strip by means of a strip casting system. The object of providing a casting roller or belt or a strip casting system, by means of which, adhesion to the casting roller or belt is avoided during strip casting and a low-segregation and crack-free aluminium alloy strip can be produced, in particular under industrial conditions, is achieved by a specific surface structure, in that the surface of the casting roller or belt has a roughness value Sa of more than 5 μm and an average peak count RPc(0.5 μm) of less than 42 cm.sup.−1.

Claims

1. Use of a casting roller or belt for a revolving chill mould of a strip casting system for manufacturing an aluminium alloy strip, wherein the surface of the casting roller or belt has a roughness value Sa of more than 5 μm measured according to DIN-EN-ISO 25178-2:2012 and an average peak count RPc(0.5 μm) of less than 42 cm.sup.−1 measured according to DIN EN 10049:2005 (application group 1—without further removal of the ripple and fine roughness proportions).

2. Use of a casting roller or belt according to claim 1, wherein the surface of the casting roller or belt has a roughness value Sa of more than 15 μm measured according to DIN-EN-ISO 25178-2:2012 and/or an average peak count RPc(0.5 μm) of less than 35 cm.sup.−1 measured according to DIN EN 10049:2005 (application group 1—without further removal of the ripple and fine roughness proportions).

3. Use of a casting roller or belt according to claim 1, wherein the surface of the casting roller or belt has a surface structure which, in an Abbott-Firestone curve, at an area proportion S.sub.mr of 10% has a height value c of at least 20 μm above the zero level measured according to DIN-EN-ISO 25178-2:2012, wherein the zero level is defined as the height value at an area proportion of 50%.

4. Use of a casting roller or belt according to claim 1, wherein the surface of the casting roller or belt has a roughness value Sa of 5 to 40 μm, preferably 15 to 30 μm, further preferably 20 to 25 μm, measured according to DIN-EN-ISO 25178-2:2012.

5. Use of a casting roller or belt according to claim 1, wherein the surface of the casting roller or belt is substantially isotropic in terms of the peak count and the ratio RPc in the X direction to RPc in the Y direction, measured according to DIN EN 10049:2005 (application group 1—without further removal of the ripple and fine roughness proportions), has the value 1±5%, wherein the X direction and Y direction are perpendicular to one another.

6. Use of a casting roller or belt according to claim 1, wherein the surface of the casting roller or belt has been subjected to a grinding with a removal of up to 45 μm, preferably between 30 and 40 μm, after structuring.

7. Use of a casting roller or belt according to claim 1, wherein in that at least the surface of the casting roller or belt has a material with a thermal conductivity of more than 100 W/(m*K), preferably of more than 200 W/(m*K), particularly preferably of more than 300 W/(m*K).

8. Use of a strip casting system for manufacturing an aluminium alloy strip comprising at least one revolving chill mould with a casting gap, wherein the at least one revolving chill mould has at least one casting roller or belt according to claim 1.

9. Use of a strip casting system according to claim 8, wherein the strip casting system has means for setting the composition of an atmosphere on the surface of the revolving chill mould.

10. Use of a strip casting system according to claim 8, wherein the strip casting system is a vertical or horizontal strip casting system.

11. Use of a strip casting system according to claim 8, wherein the strip casting system comprises means for supplying an aluminium alloy melt into a melt pool formed in front of the casting gap, via which the aluminium alloy melt can be supplied to the melt pool below the surface of the melt pool.

12. Method for manufacturing an aluminium alloy strip using a strip casting system according to claim 8, which comprises the following steps: forming a melt pool of an aluminium alloy melt in a casting region in front of the revolving chill mould; stabilising an oxide layer on the surface of the melt pool by applying an oxygen-containing gas mixture, for example air, to the aluminium alloy melt; drawing the oxide layer into the casting gap.

13. Method according to claim 12, comprising: setting a specific area load, when joining the strip shells forming during the solidification of the aluminium alloy melt, from 10 to 800 kN/m, preferably from 20 to 400 kN/m, further preferably from 100 to 200 kN/m.

14. Method according to claim 12, comprising: supplying the aluminium alloy melt into the melt pool below the surface of the melt pool.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Further configurations and advantages of the invention can be inferred from the following detailed description of a number of exemplary embodiments of the present invention, in particular in combination with the drawing. The drawing shows in:

[0046] FIG. 1 is a schematic sectional view of an exemplary embodiment of a vertical strip casting system according to the invention;

[0047] FIG. 2a is a surface section of an exemplary embodiment of a casting roller according to the invention;

[0048] FIG. 2b is an Abbott-Firestone curve of the surface of an exemplary embodiment of a casting roller according to the invention;

[0049] FIG. 3a is surface section of an exemplary embodiment of a casting roller according to the invention;

[0050] FIG. 3b is an Abbott-Firestone curve of the surface of an exemplary embodiment of a casting roller according to the invention;

[0051] FIG. 4a is a surface section of an exemplary embodiment of a casting roller according to the invention;

[0052] FIG. 4b is an Abbott-Firestone curve of the surface of an exemplary embodiment of a casting roller according to the invention;

[0053] FIG. 5a is surface section of a comparative example of a casting roller not according to the invention;

[0054] FIG. 5b is an Abbott-Firestone curve of the surface of a comparative example of a casting roller not according to the invention.

DETAILED DESCRIPTION

[0055] FIG. 1 shows a strip casting system 1 for manufacturing an aluminium alloy strip 6 comprising a revolving chill mould 2 with two revolving barriers, between which a casting gap 21 is formed, wherein the revolving barriers are in each case provided by a casting roller 22, i. e. the strip casting system 1 comprises a revolving chill mould 2 with a casting gap 21, wherein the revolving chill mould 2 has two casting rollers 22. The surface 23 of the casting roller 22 has a roughness value Sa of more than 15 μm and a peak count RPc(0.5 μm) of less than 35 cm.sup.−1. In addition, the surface 23 of the casting roller 22 has a surface structure which, in an Abbott-Firestone curve (measured thereon), at an area proportion S.sub.mr of 10% has a height value c of at least 20 μm above the zero level, wherein the zero level is defined as the height value at an area proportion of 50%, i. e. c(50%):=0 μm and c(10%)>20 μm. The surface 23 of the casting roller 22 can also have a roughness value Sa of 5 to 40 μm, preferably 15 to 30 μm. The surface 23 of the casting roller 22 is substantially isotropic in terms of the peak count with a ratio of Rpc (in X direction) to Rpc (in Y direction)=1(±5%). The casting roller 22 consists of a copper alloy having a thermal conductivity of more than 300 W/(m*K), which is effective from the surface up to the inner cooling channels. After the corresponding structuring, the surface 23 of the casting roller 22 can be subjected to a grinding with 35 μm removal. The strip casting system 1 also has means 4 for setting the composition of an atmosphere on the surface of the revolving chill mould 2 and/or the surface 31 of the melt pool 3. The means 4 allow a controlled application of an oxygen-containing gas mixture, for example air, to the corresponding surfaces.

[0056] A casting furnace is connected to the casting gusset here by a pipe system which comprises heatable ceramic pipes 5. Furthermore, the casting gusset has two side dams. The aluminium alloy melt is guided from above into the casting gusset through a supply pipe 51. The supply pipe 51 can in this case be designed as means for supplying the aluminium alloy melt into the casting gusset, via which the aluminium alloy melt can be supplied to the casting region below the surface of the melt pool 3 formed in the casting region. For example, the outflow opening of the supply pipe 51 can lie below the surface of the melt pool.

[0057] As a result, the unbroken oxide layer formed on the surface 31 of the melt pool 3 can be drawn into the casting gap 21 in a controlled and continuous manner. The drawn-in oxide layer 32 then advantageously forms a separating layer between the chill mould wall and the melt or the removed aluminium alloy strip 6. Advantageously, this oxide layer can be drawn into the casting gap 21 undamaged and can thus serve as a separating layer between the melt and the casting roller or casting roll, whereby abrasion is avoided and a uniform and clean surface of the produced aluminium alloy strip 6 can be achieved after strip casting.

[0058] The mentioned parameters, as well as the Abbott-Firestone curve, are typically determined by optical measurement of the 3D surface structure. The optical capture of the surface takes place, for example, areally via interferometry or preferably confocal microscopy. The measuring area must be chosen large enough to ensure a statistically representative measurement of the surface. For example, in the present roughness range, a preferably square measuring area with a side length of 7 mm each can be used. The lateral measuring point distance must be selected such that sufficient resolution of the individual surface characteristics is given, e.g. 1.6 μm. The roller curvature contained in the raw data of the measurement is removed by means of an F-operator (2nd order polynomial). The determination of the roughness value Sa and of the areal material proportion Smr based on the Abbott-Firestone curve is carried out in accordance with DIN-EN-ISO 25178-2:2012. The peak count RPc can also be determined from the optical measurement of the 3D surface structure by evaluating in each case the profile along a line, for example along or parallel to one of the sides of the measuring area, and by determining, starting from these line profiles, the mean peak count RPc of the surface following DIN EN 10049:2005 (application group 1—but without further removal of the ripple and fine roughness proportions). The use of RPc as a characteristic value has proven to be advantageous in the topographies presented here. A ripple filter is not used, as it would require, on the one hand, impractically large measuring areas at the very high roughness. On the other hand, the long waves are insignificant for the contact conditions of the aluminium melt on a casting roll or belt.

[0059] Measurement and evaluation are usually carried out with corresponding standard-compliant software.

[0060] FIG. 2a shows a representation of a square measuring region of X=7 mm and Y=7 mm, which was determined on the surface of an exemplary embodiment of a casting roller according to the invention by means of optical 3D measurement. The casting roller thereby had a copper surface.

[0061] The associated Abbott-Firestone curve S.sub.mr(c) measured on the surface of this exemplary embodiment of a casting roller according to the invention is plotted in FIG. 2b. This curve is the cumulative probability density function of the surface height profile S(c). It provides, for a percentage value S.sub.mr (area proportion) between 0 and 100% (plotted on the abscissa), the profile height c (sectional area position) above which the corresponding percentage proportion of the surface is located. It thus describes the material proportion of the surface depending on the height c of a sectional area through the surface.

[0062] From the Abbott-Firestone curve of FIG. 2b, it can be clearly seen that the zero level is defined as the height value at an area proportion of 50% and that an area proportion S.sub.mr of 10% has a height value c of at least 20 μm above the zero level, wherein the zero level is defined as the height value at an area proportion of 50%.

[0063] From the optical 3D measurement of the surface on a square measuring region of X=7 mm and Y=7 mm carried out to determine the Abbott-Firestone curve, a 2D evaluation was also carried out in each case along the X and Y direction to determine the size of the average roughness Ra, the peak count RPc (0.5 μm), the square average roughness value Rq and the average roughness depth Rz. This was done automatically along a large number of lines, each parallel to the sides of the measuring region. An average roughness Ra of 26.4(±5.1) μm, a mean square average roughness value Rq of 32.1(±5.5) μm, an average roughness depth Rz of 104.1(±13.0) μm and a peak count RPc(0.5 μm) of 17.0(±5.1) per cm resulted along the X direction. An average roughness Ra of 26.4(±2.9) μm, a mean square average roughness value Rq of 32.4(±3.2) μm, an average roughness depth Rz 104.8(±9.8) μm and a peak count RPc(0.5 μm) of 17.4(±4.4) per cm resulted along the Y direction. In particular, Ra along the X direction is equal to Ra along the Y direction and, due to the isotropy, in particular is equal to the roughness value Sa of 26.4(±2.9) μm. The ratio RPc (in X direction) to RPc (in Y direction)=0.98. The surface is in particular isotropic in terms of RPc, Ra and Rz.

[0064] When strip casting an AA8111 alloy with a casting roller, which had the surface characteristics represented in FIGS. 2a and 2b, good strip formation properties could be achieved.

[0065] The copper surface of a further exemplary embodiment of a casting roller according to the invention is exemplarily reflected in FIG. 3a by a representation of a square measuring region of X=7 mm and Y=7 mm. The associated Abbott-Firestone curve S.sub.mr(c) of this further exemplary embodiment is plotted in FIG. 3b. The Abbott-Firestone curve of FIG. 3b also shows a height value c of at least 20 μm above the zero level at an area proportion S.sub.mr of 10%. The optical 3D measurement of the surface carried out to determine the Abbott-Firestone curve was also used to determine the variables calculated for the exemplary embodiment of FIG. 2a/b. An average roughness Ra of 23.5(±2.9) μm, a mean square average roughness value Rq of 28.6(±3.5) μm, an average roughness depth Rz of 92.6(±11.2) μm and a peak count RPc(0.5 μm) of 16.1(±5.1) per cm resulted along the X-direction. An average roughness Ra of 23.8(±3.5) μm, a mean square average roughness value Rq of 28.9(±4.2) μm, a mean roughness depth Rz of 92.7(±14.3) μm and a peak count RPc(0.5 μm) of 16.1(±4.0) per cm resulted along the Y direction. A roughness value Sa of 23.6(±2.3) μm resulted. Also when strip casting an AA8111 alloy with a casting roller, which had the surface characteristics represented in FIGS. 3a and 3b, good strip formation properties could be achieved.

[0066] FIG. 4a shows a square region with 7 mm edge length of the surface of a further exemplary embodiment of a casting roller according to the invention. The associated Abbott-Firestone curve S.sub.mr(c) is plotted in FIG. 4b. For this exemplary embodiment, the surface of the casting roller, whose Abbott-Firestone curve is represented in FIG. 2b, has been subjected to a grinding with a removal of 35 μm. Due to the grinding, the Abbott-Firestone curve exhibits a flatter course towards small S.sub.mr values. Despite the grinding, also the Abbott-Firestone curve of FIG. 4b at an area proportion S.sub.mr of 10% exhibits a height value c of at least 20 μm above the zero level. In addition, the variables calculated for the exemplary embodiment of FIG. 2a/b have been determined again. An average roughness Ra of 25.6(±4.8) μm, a mean square average roughness value Rq of 30.8(±5.1) μm, a average roughness depth Rz of 92.7(±11.0) μm and a peak count RPc(0.5 μm) of 16.8(±5.1) per cm resulted along the X direction. An average roughness Ra of 25.6(±2.8) μm, a mean square average roughness value Rq of 31.1(±3.1) μm, a mean roughness depth Rz of 93.6(±8.8) μm and a peak count RPc(0.5 μm) of 17.4(±4.5) per cm resulted along the Y direction. A roughness value Sa of 25.6(±2.8) μm resulted.

[0067] Due to the grinding, such surfaces are more wear resistant and form a plateau that supports the melt well. At the same time, the essential structural properties are retained such that the surface in particular has deep pockets, which reduce the contact area. The peak count RPc and the roughness value Sa remain substantially unchanged within the measurement uncertainties despite the grinding. The bearing proportions form an isotropic net-shaped structure, as resulting from the 3D surface measurements and indicated by the only slight deviations of RPc in the X and Y direction.

[0068] For a comparative test, a strip made of an AA8111 alloy was cast using a casting roller with copper surface not according to the invention. FIG. 5a again shows a representation of a square measuring region with 7 mm edge length of the surface of the casting roller not according to the invention. The associated Abbott-Firestone curve is plotted in FIG. 5b. The surface is non-isotropic with a roughness transversely to the grinding direction of only 0.21(±0.01) μm and longitudinally to the grinding direction of 0.16(±0.08) μm as well as a peak density RPc of 10.3(±3.3) per cm transversely to the grinding direction and 0.0(±0.2) per cm longitudinally to the grinding direction. The mean square average roughness value Rq was 0.2(±0.1) μm longitudinally and 0.3(±0.0) μm transversely to the grinding direction; the average roughness depth Rz was 0.2(±0.1) μm longitudinally and 1.4(±0.1) μm transversely to the grinding direction. As is discernible from FIG. 5b, there is also an area proportion S.sub.mr of 10% at a height value c of significantly below 20 μm. Poor strip formation properties were exhibited in the comparative test with this casting roller not according to the invention.

[0069] By means of the described exemplary embodiments of the casting rollers according to the invention, manufacture of an aluminium alloy strip without release agents can be implemented by means of strip casting. In particular, this eliminates a barrier of the heat flow from the melt or strip shell into the revolving chill mould. This therefore has a direct effect on the possible productivity of the casting system. Furthermore, the use of a release agent, usually as a graphite suspension, can lead to undesirable deposits on the produced strips. This is avoided according to the invention. Nevertheless, the disadvantages of adhesion can be effectively avoided using the means described. Thus, a high-quality aluminium alloy strip can be provided particularly productively.

[0070] All references, including publications, patent applications, and patents cited herein are hereby incorporated by reference to the same extent as if each reference were individually and specifically indicated to be incorporated by reference and were set forth in its entirety herein.

[0071] The use of the terms “a” and “an” and “the” and similar referents in the context of describing the invention (especially in the context of the following claims) is to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The terms “comprising,” “having,” “including,” and “containing” are to be construed as open-ended terms (i.e., meaning “including, but not limited to,”) unless otherwise noted. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention.

[0072] Preferred embodiments of this invention are described herein, including the best mode known to the inventors for carrying out the invention. Variations of those preferred embodiments may become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventors expect skilled artisans to employ such variations as appropriate, and the inventors intend for the invention to be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context.