Casting roll for a two-roll continuous casting installation

Abstract

A casting roll of a two-roll continuous casting installation is to be able to be exposed to a changing temperature stress and roll pressures when casting strip made of non-ferrous metals, especially of aluminum or an aluminum alloy. For this purpose, its sleeve is made of an age-hardening copper alloy made of—as expressed in each case as weight %—0.4% through 2% cobalt, which is partially exchangeable for nickel, 0.1% through 0.5% beryllium, optionally 0.03% through 0.5% zirconium, 0.005% through 0.1% magnesium and possibly a maximum of 0.15% of at least one element of the group including niobium, manganese, tantalum, vanadium, titanium, chromium, cerium and hafnium, the remainder being copper and inclusive of manufacturing-conditioned impurities and usual processing additives.

Description

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The invention relates to a casting roll for a two-roll continuous casting installation.

[0003] 2. Description of Related Art

[0004] The worldwide aim, especially of the steel industry, to pour semifinished product as close to final dimensions as possible, in order to save hot and/or cold working steps, has led since about 1980 to a series of developments, such as single roll and two-roll continuous casting methods.

[0005] In these casting methods, very high surface temperatures appear at the water-cooled cylinders or rolls during casting of steel alloys, nickel, copper, as well as alloys that are only rolled with difficulty in the pouring range of the melt. In the case of close to final dimension casting of a steel alloy, for example, the temperatures are about 350° C. to 450° C., the sleeves of the continuous casting rolls being made of a CuCrZr material having an electrical conductivity of 48 Sm/mm2 and a heat conductivity of about 320 W/mK. Materials based on CuCrZr were used up to now predominantly for continuous casting dies and casting wheels that were thermally highly stressed. In the case of these materials, the surface temperature drops cyclically to about 150° C. to 200° C., by the cooling of the casting rolls, with each revolution, shortly before the casting range. On the cooled rear side of the casting rolls, however, the temperature remains largely constant during the cycle, at about 30° C. to 40° C. The temperature gradient between the surface and the rear side in combination with the cyclical change in the surface temperature of the continuous casting rolls causes thermal stress in the surface region of the sleeve material.

[0006] According to investigations of the fatigue properties of the CuCrZr materials used up to now, at various temperatures, using an expansion amplitude of +/−0.3% and a frequency of 0.5 Hertz—these parameters approximately correspond to a rotational speed of the continuous casting rolls of 30 rpm—one may expect, for example, in the favorable case, a service life of 3000 cycles until cracks form, using a maximum surface temperature of 400° C., corresponding to a wall thickness of 25 mm above the water cooling. Therefore, the continuous casting rolls have to be reconditioned after as relatively early an operating time as about 100 minutes, for the purpose of removing surface cracks. In this context, the service life between reworking is, among other things, substantially dependent on the effectiveness of the lubrication/release agents at the casting surface, the constructive and process-conditioned cooling as well as the casting speed. For the purpose of exchanging the continuous casting rolls, the casting installation has to be stopped and the casting process has to be interrupted.

[0007] A further disadvantage of the proven mold material CuCrZr for this particular application is the relatively low hardness of approximately 110 HBW to 130 HBW. For, in the case of a single roll or two-roll continuous casting method, it is not to be avoided that, even before the casting range, splashes appear on the roll surfaces. The solidified steel particles are then pressed into the relatively soft surfaces of the continuous casting rolls, whereby the surface quality of the poured strips of about 1.5 mm to 4 mm thickness are considerably impaired.

[0008] Compared to a CuCrZr alloy, the lower electrical conductivity of a known CuNiBe alloy, having an addition of up to 1% niobium, also leads to a higher surface temperature. Since the electrical conductivity behaves approximately proportionally to the heat conductivity, the surface temperature in the sleeve, of a continuous casting roll, made of the CuNiBe alloy as compared to a continuous casting roll having a sleeve made of CuCrZr, at a maximum temperature of 400° C. at the surface and 30° C. on the rear side will be increased to about 540° C.

[0009] Ternary CuNiBe and CuCoBe alloys do indeed basically demonstrate a Brinell hardness of more than 200 HBW, however, the electrical conductivity of the standard semifinished products made of these materials, such as rod for manufacturing resistance welding electrodes or sheet or strip for manufacturing springs or leadframes, reach values of at most in the range of 26 Sm/mm2 to about 32 Sm/mm2. Under optimum conditions, with the use of these standard materials, a surface temperature of only about 585° C. could be reached at the sleeve of a continuous casting roll.

[0010] Even from the CuCoBeZr and CuNiBeZr alloys basically known from U.S. Pat. No. 4,179,314, no hints are seen that conductivity values of >38 Sm/mm2 in conjunction with a minimum hardness of 200 HBW could be achieved.

[0011] Within the scope of EP 0 548 636 B1, the use of an age-hardening copper alloy is also related art, which has 1.0% to 2.6% nickel that may be fully or partially replaced by cobalt, 0.1% to 0.45% beryllium, optionally 0.05% to 0.25% zirconium and possibly up to a maximum of 0.15% of at least one of the group of elements including niobium, tantalum, vanadium, titanium, chromium, cerium and hafnium, the rest being copper inclusive of production-conditioned contaminations and usual processing additives, having a Brinell hardness of at least 200 HBW and an electrical conductivity greater than 38 Sm/mm2 as the material for producing continuous casting rolls and wheels.

[0012] Alloys having these compositions, such as the alloys CuCo2Be0.5 or CuNi2Be0.5, have disadvantages in their hot forming capability, because of their relatively high alloying element content. However, high heat deformation strains are required to attain a fine grained product having a grain size <1.5 mm (as per ASTM E 112), starting from a coarse-grained cast structure having a grain size of several millimeters. In particular, for large format casting rolls, up to this point, sufficiently large continuous casting rolls have been producible only at very high expenditure; however, technical shaping devices are hardly available for realizing, at a justifiable cost, a sufficiently high hot kneading for recrystallization of the cast structure into a fine grain structure.

SUMMARY OF THE INVENTION

[0013] It is an object of the invention to create a continuous casting roll as a component of a two-roll continuous casting installation, which, during close to final dimension casting of strips made of non-ferrous metals, may be exposed without any problem to changing temperature stresses and high roll pressures, while having a long service life.

[0014] These and other objects of the invention are attained by a casting roll for a two-roll continuous casting installation, which has a sleeve made of an age-hardening copper alloy made of—as expressed in each case as weight %—0.4% through 2% cobalt, which is partially exchangeable for nickel, 0.1% through 0.5% beryllium, optionally 0.03% through 0.5% zirconium, 0.005% through 0.1% magnesium and possibly a maximum of 0.15% of at least one element of the group including niobium, manganese, tantalum, vanadium, titanium, chromium, cerium and hafnium, the remainder being copper inclusive of manufacturing-conditioned impurities and usual processing additives. During the casting of strips made of non-ferrous metals, the casting roll may undergo changing temperature stress and high roll pressures.

DETAILED DESCRIPTION OF THE INVENTION

[0015] By the use of a CuCoBeZr(Mg) alloy having an intentionally graded low content of Co and Be, on the one hand, one may ensure a still sufficient age-hardenability of the material for achieving high strength, hardness and conductivity; on the other hand, only low heat deformation strain is required for the complete recrystallization of the cast structure and the setting of a fine-grained structure having sufficient ductility.

[0016] Due to a continuous casting roll thus developed as a component of a two-roll continuous casting installation, it is possible to increase the casting speed of a strip made of a non-ferrous metal, particularly of aluminum or an aluminum alloy, by more than double, compared to a roll installation in which the rolls are fitted with steel sleeves. In addition, a clearly improved surface quality of the cast strip is achieved. Also, a considerably longer service life is ensured for the sleeve.

[0017] This continuous casting roll may be developed as a hollow cylinder, i.e. inherently rigid without a core. The surface coming into contact with the strips to be cast, however, may also be a component of a sleeve having a core, especially a steel core. The sleeve may then be shrink fitted onto such a core as the carrier, hot isostatically pressed on or slipped on and then locked mechanically.

[0018] It is also conceivable, when using a sleeve, that this could be developed as a single layer or multiple layers.

[0019] The enveloping surface of the surface of the casting roll may be designed cylindrically or having a camber, so as possibly to compensate for the sagging of a roll.

[0020] A further improvement in the sleeve's mechanical properties, particularly an increase in tensile strength, may be advantageously achieved, if the copper alloy contains 0.03% to 0.35% zirconium, and 0.005% to 0.05% magnesium.

[0021] According to a further specific embodiment, the copper alloy contains a proportion <1.0% of cobalt, 0.15% to 0.3% of beryllium and 0.15% to 0.3% of zirconium.

[0022] It is also of advantage if the ratio of cobalt to beryllium in the copper alloy of the sleeve is between 2 and 15. Most preferably, this ratio of cobalt to beryllium is 2.2 to 5.

[0023] The invention permits having the copper alloy contain, in addition to cobalt, up to 0.6% nickel.

[0024] Further improvements of the mechanical properties of the casting roll may be achieved if the copper alloy of the sleeve contains up to a maximum of 0.15% of at least one element of the group including niobium, manganese, tantalum, vanadium, titanium, chromium, cerium and hafnium.

[0025] The sleeve is advantageously produced by the processing steps casting, hot working, solution treatment at 850° C. to 980° C., cold working up to 30% as well as age-hardening at 400° C. to 550° C. within a period of 4 to 32 hours, the sleeve having a maximum average grain size of 1.5 mm as per ASTM E 112, a hardness of at least 170 HBW, and an electrical conductivity of at least 26 Sm/mm2.

[0026] It is of particular advantage if the sleeve in the age-hardened state, has an average grain size of 30 &mgr;m to 500 &mgr;m as per ASTM E 112, a hardness of at least 185 HBW, a conductivity between 30 and 36 Sm/mm2, a 0.2% yield strength of at least 450 MPa and an elongation at break of at least 12%.

[0027] If the sleeve is provided with a coating which reduces the permeability to heat, or evens out the flow of heat, the product quality of the cast strip made of a non-ferrous metal, but particularly of aluminum or an aluminum alloy, is further enhanced. Based on the operating condition of the sleeve, this coating, specifically made of a copper alloy, is made effective, especially in the case of an aluminum strip, due to the fact that, at the beginning of a casting or rolling process, an adhesion layer forms from the acting together of copper and aluminum on the surface of the sleeve, from which, then, during the further course of the casting process aluminum penetrates the copper surface and there forms a stable, resistive diffusion layer, whose thickness and properties are essentially determined by the casting speed and cooling conditions. That clearly improves the surface quality of the aluminum strip and consequently the product quality.

[0028] The service life of the sleeve can be prolonged even further by using a coating having a great surface hardness.

[0029] The surface of the casting roll may be made smooth. This design is achievable particularly by rolling. In this manner, pressure stresses are induced in the edge zone, and these make possible additional resistance to the formation of cracks and the progression of cracks, so as to raise the life duration of the casting roll.

[0030] The surface of the casting roll may be textured. Texturing can be applied, for example, by cutting, roller-burnishing, eroding or blasting. With the use of such measures, the heat transfer coefficient may specifically be influenced.

[0031] In the depressions formed by the texturing, a substance may be embedded having a low heat conductivity compared to the heat conductivity of copper.

[0032] Besides being a metallic material, such as particularly nickel or a nickel alloy, such a substance may also be a ceramic material. Such a filling up of the depressions formed by the texturing on the surface of the casting roll is used to create good surface quality and to ensure a lasting influence on the heat conductivity.

[0033] The invention is explained in greater detail with reference to the examples below. In the light of seven alloys for the sleeve of a casting roll (alloys A to G) and three comparison alloys (H to J), it is shown how critical the composition is to achieving the combinations of properties aimed for.

[0034] All the alloys were smelted in a crucible furnace and cast into round billets of equal format. The composition of the individual layers is given below in Table 1. The addition of magnesium is made for the pre-deoxidization of the melt, and the addition of zirconium acts positively on the hot ductility. 1 TABLE 1 Alloy Co (%) Ni (%) Be (%) Zr (%) Mg (%) Cu (%) A 0.68 — 0.20 0.20 0.03 Rest B 1.0 — 0.22 0.22 0.03 Rest C 1.4 — 0.20 0.18 0.02 Rest D 0.65 — 0.29 0.21 0.04 Rest E 1.0 — 0.31 0.24 0.01 Rest F 1.4 — 0.28 0.19 0.03 Rest G 1.0 0.1 0.22 0.16 0.03 Rest H — 1.7 0.27 0.16 — Rest 1 2.1 — 0.55 0.24 — Rest J — 1.4 0.54 0.20 — Rest

[0035] The alloys were subsequently pressed into flat bars using a low pressure ratio (=cross section of the cast block/cross section of the pressed bar) of 5.6:1 on an extrusion press at 950° C. Thereafter, the alloys were submitted to an at least 30-minute solution treatment above 850° C., using a subsequent water quenching, and after that, were age-hardened for 4 to 32 hours at a temperature range between 400° C. and 550° C. The combinations of properties attained are shown in Table 2 below. 2 TABLE 2 Rm Rp0.2 A HBW 2.5 El. Cond. Grain Size Alloy MPa MPa % 187.5 Sm/mm2 mm A 694 492 21 207 36.8 0.09-0.25 B 675 486 18 207 32.8 0.09-0.18 C 651 495 18 211 30.0 0.045-0.13  D 707 501 19 207 31.4 0.09-0.25 E 735 505 19 229 33.6 0.045-0.18  F 735 520 19 224 32.3 0.09-0.25 G 696 513 18 213 33.5 0.065-0.18  H 688 556 10 202 41.0 2-3 1 784 541 11 229 30.3 1.5-3   J 645 510  4 198 30.9 4-6 Rm = tensile strength Rp0.2 = 0.2% yield strength A = elongation at break HBW = Brinell hardness

[0036] As may be seen from the combinations of properties, the alloys according to the present invention, for producing a sleeve of a casting roll, attain the aimed-for recrystallized fine grained structure while having an appropriately good elongation at break. In the case of comparison alloys H to J, there is a grain size of more than 1.5 mm, which reduces the ductility of the material.

[0037] An additional increase in strength may be attained by cold forming before the age-hardening. Table 3 below gives the property combinations of alloys A to J, which are achieved by solution treatment of the pressed material for at least 30 minutes above 850° C. and subsequent water quenching, 10% to 15% cold rolling (reduction in cross section) and then age-hardening from 2 to 32 hours at a temperature range between 400° C. and 550° C. 3 TABLE 3 Rm Rp0 2 A HBW 2.5 El. Cond. Grain Size Alloy MPa MPa % 187.5 Sm/mm2 mm A 688 532 20 211 36.7  0.13-0.25 B 679 534 18 207 34.6 0.045-0.18 C 741 600 17 227 34.4 0.065-0.18 D 690 537 21 207 32.6 0.065-0.25 E 735 576 19 230 34.7 0.045-0.18 F 741 600 17 227 34.4  0.13-0.25 G 695 591 15 224 33.0  0.18-0.35 H 751 689  9 202 40.9  2-4 1 836 712 10 229 31.0  2-3 J 726 651  6 198 31.5  3-6

[0038] Alloys A to G according to the present invention, in turn, demonstrate good elongations at break and a grain size less than 0.5 mm, while comparison alloys H to J have a coarse grain, having a grain size greater than 1.5 mm and lower values of elongation at break. Thus, these copper alloys have clear processing advantages during the production of sleeves, particularly for larger continuous casting rolls of two-roll casting installations, whereby it is made possible to produce a fine grained end product having optimum basic properties for their field of application.

Claims

1. A casting roll for a two-roll continuous casting installation which, during the casting of strips made of non-ferrous metals, undergoes changing temperature stress and high roll pressures, comprising a sleeve made of an age-hardening copper alloy, which includes in weight %: 0.4% through 2% cobalt, which may be partially substituted with nickel, 0.1% through 0.5% beryllium, and a remainder of copper.

2. The casting roll according to claim 1, in which the copper alloy further includes 0.03% through 0.5% zirconium and 0.005% through 0.1% magnesium.

3. The casting roll according to claim 1, in which the copper alloy further includes a maximum of 0.15% of at least one element selected from the group consisting of niobium, manganese, tantalum, vanadium, titanium, chromium, cerium and hafnium.

4. The casting roll according to claim 2, in which the copper alloy contains 0.03% to 0.35% zirconium and 0.005% to 0.05% magnesium.

5. The casting roll according to claim 2, in which the copper alloy contains less than 1.0% cobalt, 0.15% through 0.3% beryllium and 0.15% through 0.3% of zirconium.

6. The casting roll according to claim 1, in which the copper alloy has a ratio of cobalt to beryllium of between 2 and 15.

7. The casting roll according to claim 2, in which the copper alloy has a ratio of cobalt to beryllium of between 2 and 15.

8. The casting roll according to claim 3, in which the copper alloy has a ratio of cobalt to beryllium of between 2 and 15.

9. The casting roll according to claim 6, in which the copper alloy has a ratio of cobalt to beryllium of between 2.2 and 5.

10. The casting roll according to claim 1, in which the copper alloy further includes up to 0.6% nickel in addition to cobalt.

11. The casting roll according to claim 2, in which the copper alloy further includes up to 0.6% nickel in addition to cobalt.

12. The casting roll according to claim 2, in which the copper alloy further includes a maximum of 0.15% of at least one element selected from the group consisting of niobium, manganese, tantalum, vanadium, titanium, chromium, cerium and hafnium.

13. The casting roll according to claim 1, wherein the sleeve is produced by casting, hot working, solution treatment at 850° C. to 980° C., cold working up to 30% as well as age-hardening at 400° C. to 550° C. within a time period of 4 to 32 hours, the sleeve having a maximum average grain size of 1.5 mm as per ASTM E 112, a hardness of at least 170 HBW, and an electrical conductivity of at least 26 Sm/mm2.

14. The casting roll according to claim 13, in which the sleeve, in the age-hardened state, has an average grain size of 30 &mgr;m to 500 &mgr;m as per ASTM E 112, a hardness of at least 185 HBW, a conductivity between 30 and 36 Sm/mm2, a 0.2% yield strength of at least 450 MPa and an elongation at break of at least 12%.

15. The casting roll according to claim 1, in which the sleeve is provided with a coating that reduces the permeability to heat.

16. The casting roll according to claim 15, in which the coating has a great surface hardness.

17. The casting roll according to claim 1, in which the surface is designed to be smooth.

18. The casting roll according to claim 1, in which the surface is textured.

19. The casting roll according to claim 18, in which a substance is embedded in the depressions, formed by the texturing, which has a low heat conductivity compared to the heat conductivity of copper.