Method for producing a cast strip of molten metal and cast strip

Abstract

A method for producing a cast strip of molten metal, in which the molten metal passes through a casting gap defined by two oppositely rotating casting rollers and is shaped into the cast strip, as well as the cast strip that is produced. By providing the cast strip in the casting gap with a different thickness in a first length section extending in the longitudinal direction of the cast strip than in a second length section bordering thereon, a metal strip is produced that has length sections with different thicknesses.

Claims

1. A method for producing a cast strip from molten metal, in which the molten metal is poured into a casting gap defined by two oppositely rotating casting rollers and, upon contacting the casting rollers, is shaped into a cast strip, wherein the cast strip is in the casting gap provided with a first thickness in a first length section extending in a longitudinal direction of the cast strip and a second thickness in a second length section bordering thereon, wherein the first thickness is greater than the second thickness, wherein at least one of the casting rollers has a circumferential surface section associated with the first length section that is spaced apart from the rotational axis of the respective casting roller by a different distance than a closest adjacent circumferential surface section of the at least one casting roller associated with the second length section, and in addition to the difference between the distance that the circumferential surface section associated with the first length section is spaced apart from the rotational axis of the respective casting roller and the distance that the closest adjacent circumferential surface section associated with the second length section is spaced apart from the rotational axis of the respective casting roller, a surface structure of the casting roller in the circumferential surface section associated with the first length section is different from a surface structure of the casting roller in the circumferential surface section associated with the second length section such that the cooling imparted to the cast strip is uniform.

2. The method according to claim 1, wherein the first length section and the second length section extend over a fraction of a width of the cast strip.

3. The method according to claim 2, wherein at least one of the casting rollers features a circumferential surface section that is spaced apart from the rotational axis of the respective casting roller by a different distance than a closest adjacent circumferential surface section of this casting roller.

4. The method according to claim 1, wherein the circumferential surface sections with different distances from the rotational axis of the casting roller extend around the casting roller.

5. The method according to claim 1, wherein the surface of the circumferential surface section associated with the first length section has a first roughness, the surface of the circumferential surface section associated with the second length section has a second roughness, and the first roughness is different from the second roughness.

6. The method according to claim 1, wherein the difference between the surface structure of the circumferential surface section associated with the first length section and the circumferential surface section associated with the second length section results in a difference in cooling between the circumferential surface section associated with the first length section and the circumferential surface section associated with the second length section.

7. The method according to claim 1, wherein the surface of the circumferential surface section associated with the first length section is coated with a first coating, the surface of the circumferential surface section associated with the second length section is coated with a second coating, and the first coating is different from the second coating.

8. The method according to claim 7, wherein a coating thickness of the first coating applied to the circumferential surface section associated with the first length section is different from a coating thickness of the second coating applied to the circumferential surface section associated with the second length section.

9. The method according to claim 8, wherein a thermal conductivity of the first coating applied to the circumferential surface section associated with the first length section is different from a thermal conductivity of the second coating applied to the circumferential surface section associated with the second length section.

10. The method according to claim 7, wherein a thermal conductivity of the first coating applied to the circumferential surface section associated with the first length section is different from a thermal conductivity of the second coating applied to the circumferential surface section associated with the second length section.

11. The method according to claim 1, wherein the heat transfer referred to the molten metal in the first length section is greater than the heat transfer referred to the molten metal in the second length section.

12. A method for producing a cast strip from molten metal, in which the molten metal is poured into a casting gap defined by two oppositely rotating casting rollers and, upon contacting the casting rollers, is shaped into a cast strip, wherein the cast strip is in the casting gap provided with a different thickness in a first length section extending in a longitudinal direction of the cast strip than in a second length section bordering thereon, and circumferential surface sections with different distances from a rotational axis of the casting roller and associated with the first length section and the second length section have a different heat transfer referred to the molten metal, wherein the difference in heat transfer between the first length section and the second length section is achieved by providing partitions in the casting gap separating the casting gap into sections that cause a filling level of the melt pool in a section corresponding to the first length section to be different than a filling level of the melt pool in a section corresponding to the second length section bordering thereon.

13. A method for producing a cast strip from molten metal, in which the molten metal is poured into a casting gap defined by two oppositely rotating casting rollers and, upon contacting the casting rollers, is shaped into a cast strip, wherein the cast strip is in the casting gap provided with a different thickness in a first length section extending in a longitudinal direction of the cast strip than in a second length section bordering thereon, and that circumferential surface sections with different distances from a rotational axis of the casting roller and associated with the first length section and the second length section have a different heat transfer referred to the molten metal, wherein a thickness of the cast strip associated with the first length section is greater than a thickness of the cast strip associated with the second length section and a width of the cast strip associated with the first length section is less than a width of the cast strip associated with the second length section.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention is described in greater detail below with reference to the drawings that show an exemplary embodiment. In these schematic drawings that are not true-to-scale:

(2) FIG. 1 shows a side view of a twin-roller casting machine for casting molten metal into a cast strip;

(3) FIG. 2 shows a top view of a detail of the casting gap of the twin-roller casting machine according to a first embodiment;

(4) FIG. 3 shows a top view of a detail of the casting gap of the twin-roller casting machine according to a second embodiment;

(5) FIG. 4 shows a top view of an enlarged detail of the casting gap of the embodiment of the twin-roller casting machine illustrated in FIG. 2;

(6) FIG. 5 shows a top view of an enlarged detail of the casting gap of the embodiment of the twin-roller casting machine illustrated in FIG. 3;

(7) FIGS. 6-10 show different strips cast in accordance with the invention in the form of a section transverse to their longitudinal direction;

(8) FIGS. 11-12 show top views of different strips cast in accordance with the invention;

(9) FIG. 13 shows a side view of another embodiment of a twin-roller casting machine for casting molten steel into a cast strip;

(10) FIG. 14 shows a top view of a detail of the casting gap of the twin-roller casting machine according to FIG. 13;

(11) FIG. 15 shows a top view of a detail of a pair of casting rollers used in a practical casting test, and

(12) FIG. 16 shows the cast strip produced with the pair of casting rollers in the form of a sectional representation according to FIGS. 6-10.

DESCRIPTION OF THE INVENTION

(13) The twin-roller casting machine 1 illustrated in FIG. 1 serves for casting molten steel S into a cast steel strip B and has, in principle, a conventional design with two casting rollers 2, 3 that are arranged axially parallel to one another and rotate in opposite directions about their rotating axes A2, A3, wherein said casting rollers define the longitudinal sides of a casting gap 4 formed between the casting rollers, as well as of the melt pool 5 that is situated above the casting gap and into which the molten steel S to be cast is introduced. The two lateral narrow sides of the casting gap 4 and of the melt pool 5 are not defined by the casting rollers 2, 3 and respectively sealed by the plate-shaped lateral seals shown.

(14) The cast steel strip B exiting the casting gap 4 is also conventionally transported away along a transport path 6. Starting at the casting gap 4, the transport path 6 features a first section that essentially extends vertically and then leads to a roller table that is essentially aligned horizontally in the form of an arc. Not-shown cooling devices are conventionally arranged along the transport path 6 and used for purposefully cooling the cast strip B in an accelerated fashion. The casting rollers 2, 3 respectively feature a roller body, the outer surface of which is made of a copper alloy.

(15) In order to produce a cast strip B1 with three length sections L1-L3 of different thicknesses, the casting rollers 2, 3 feature three circumferential surface sections 10, 11, 12; 10, 11, 12 that respectively extend around their circumference and are spaced apart from the respective rotational axis A2, A3 of the casting rollers 2, 3 by a shorter distance G1, G2 than the circumferential sections 13, 14, 15, 16; 13, 14, 15, 16 that are arranged in between and laterally thereof and respectively spaced apart from the rotational axis A2, A3 of the respective casting roller 2, 3 by a greater distance G3.

(16) In the embodiment illustrated in FIG. 2, the circumferential surface sections 10, 11, 12; 10, 11, 12 of the casting rollers 2, 3 are respectively spaced apart from the assigned rotational axis A2, A3 by the same shorter distance G1.

(17) In the embodiment illustrated in FIG. 3, in contrast, the distance G1 of the respectively outer circumferential surface sections 10, 12; 10, 12 is identical, but the respective distance G2 of the central circumferential surface 11; 11 from the assigned rotational axis A2, A3 is even shorter.

(18) The distances G3 of the circumferential surface sections 13, 14, 15, 16; 13, 14, 15, 16 of the casting rollers 2, 3 that respectively border laterally on the circumferential surface sections 10, 11, 12; 10, 11, 12 are constantly uniform. Accordingly, the circumferential surface sections 10, 11, 12; 10, 11, 12 are realized in the casting rollers 2, 3 like circumferential grooves, wherein their respective depth depends on the difference between their respective distance G1, G2 and the distance G3, by which the circumferential surface sections 13, 14, 15, 16; 13, 14, 15, 16 are respectively spaced apart from the assigned rotational axis A2, A3.

(19) Between the respectively opposing circumferential surface sections 10, 10; 11, 11; 12, 12 of the casting rollers 2, 3, the casting gap 4 therefore respectively has a clear width W1, W2 that is greater than the clear width W3 of the casting gap 4 between the mutually assigned circumferential surface sections 13, 13; 14, 14; 15, 15; 16, 16. The circumferential surface sections 13, 14, 15, 16; 13, 14, 15, 16 may have a width b1 that is wider than the width b2 of circumferential surface sections 10, 11, 12; 10, 11, 12. In order to allow the shells being formed of the molten metal S to grow faster and therefore more substantially in the region of the circumferential surface sections 10, 11, 12; 10, 11, 12 than in the region of the circumferential surface sections 13, 14, 15, 16; 13, 14, 15, 16, cooling lines 20, 21 are respectively arranged in the casting rollers 2, 3 in the region of the circumferential surface sections 10, 11, 12; 10, 11, 12 and positioned closely adjacent to one another, wherein a cooling fluid flows through said cooling lines during the casting operation. Two cooling lines 20, 21 arranged closely adjacent to one another are respectively assigned to each of the circumferential surface sections 10, 11, 12; 10, 11, 12 in the embodiment illustrated in FIG. 2 and also to the outer circumferential surface sections 10, 12; 10, 12 in the example illustrated in FIG. 3, but three cooling lines 20, 21, 22 are provided in the deeper circumferential surface section 11 in the embodiment illustrated in FIG. 3 in order to ensure an even more intensive heat dissipation and therefore a faster growth of the shell.

(20) In the region of the circumferential surface sections 13, 14, 15, 16; 13, 14, 15, 16 that are spaced apart from the respective rotational axis A2, A3 by a greater distance G3 and are wider than the circumferential surface sections 10, 11, 12; 10, 11, 12, in contrast, three cooling lines 23, 24, 25 are respectively provided and spaced apart from one another by a greater distance. Accordingly, less heat is dissipated in these regions during the casting operation than in the circumferential surface sections 10, 11, 12; 10, 11, 12 and the growth of the shells that form of the solidifying molten metal on the casting rollers and are joined into the cast strip B1 in the casting gap 4 progresses slower.

(21) The different solidification speed of the molten metal S in the region of the respective circumferential surface sections 10-16; 10-16 can also be promoted by providing the circumferential surface sections 10-16; 10-16 with a coating 26 of different thickness D1, D2 as indicated in FIG. 4. In this case, the coating 26 consist, for example, of a FeCu alloy or CrNi alloy. The thickness D1 of the coating 26 in the region of the circumferential surface sections 10, 11, 12; 10, 11, 12 with a shorter distance G1, G2 is smaller than the thickness D2 of the coating in the region of the circumferential surface sections 13, 14, 15, 16; 13, 14, 15, 16 with a greater distance G3. The coating 26 serves as a wear protection layer for the casting roller surface, but its insulating effect also influences the heat transfer from the molten metal S to the respective casting roller 2, 3. Due to the smaller thickness D1 of the coating 26 in the region of the circumferential surface sections 10, 11, 12; 10, 11, 12, more heat is accordingly dissipated from the molten metal S at these locations than in the region of the circumferential surface sections 13, 14, 15, 16; 13, 14, 15, 16, in which the coating 26 is thicker.

(22) The different solidification speed is also promoted in that the coating 26 or the circumferential surfaces of the casting rollers 2, 3 that respectively come in contact with the molten metal S have a different roughness R1, R2 in the region of the circumferential surface sections 10-16; 10-16 as indicated in FIG. 5. In this case, the circumferential surface sections 10-16; 10-16 were shot-peened in order to adjust their roughness R1, R2. The peening treatment was carried out in such a way that the circumferential surface sections 13, 14, 15, 16; 13, 14, 15, 16 have a greater roughness R1 than the circumferential surface sections 10, 11, 12; 10, 11, 12 with their roughness R2.

(23) According to FIG. 5, the casting rollers 2, 3 may furthermore be realized in the form of hollow shafts, through which a cooling medium flows. In this case, the different cooling effect in the region of the circumferential surface sections 13, 14, 15, 16; 13, 14, 15, 16 and in the region of the circumferential surface sections 10, 11, 12; 10, 11, 12 is achieved in that the wall thickness of the casting rollers 2, 3 is greater in the region 60 of the circumferential surface sections 13, 14, 15, 16; 13, 14, 15, 16 than in the region 61 of the circumferential surface sections 10, 11, 12; 10, 11, 12.

(24) FIGS. 6 to 10 show different examples of cross-sectional shapes and thickness profiles of a strip B1, B2, B3, B4, B5 cast in accordance with the invention. The strip B1 illustrated in FIG. 6 can be produced with casting rollers 2, 3 realized in accordance with FIG. 2 and the strip B2 illustrated in FIG. 7 can be produced with casting rollers realized in accordance with FIG. 3. In these embodiments, the thicknesses of the length sections L1-L7 are respectively realized symmetrically referred to the center plane M of the cast strip B1, B2 and extend over the entire length of the cast strip in the longitudinal direction L as shown in FIG. 11. In the embodiment according to FIG. 6, the thicker length sections L1, L2, L3 produced by the circumferential surface sections 10, 11, 12; 10, 11, 12 have the same thickness D10 and the length sections L4, L5, L6, L7 that lie in between and are produced by the circumferential surface sections 13, 14, 15, 16; 13, 14, 15, 16 have the same smaller thickness D13.

(25) In the exemplary embodiment according to FIG. 7, in contrast, the central length section L2 has a greater thickness D11 than the two other thicker length sections L1, L3 with their thickness D10, as well as the length sections L4-L7 that lie in between with their thickness D13.

(26) In the strips B3-B5 illustrated in FIGS. 8-10, the length sections L1, L2, L3 with greater thickness are arranged asymmetrically referred to the center plane M or varied with respect to their shape.

(27) According to FIG. 12, it is also possible to produce a thicker length section L8, which extends over the entire width Y of the cast strip B6 and has a length Z that is limited to a fraction of the circumferential length of the casting rollers 2, 3, on the cast strip B6 in accordance with the invention within periodically repeating distances X.

(28) FIGS. 13 and 14 show another option for controlling the growth of the shells that are respectively formed of the solidifying molten metal S in the region of the circumferential surface sections 10-16 producing the length sections L1-L7. In this case, the sections 70-76 of the casting gap 4 assigned to the oppositely arranged circumferential surface sections 10, 10; 11, 11; 12, 12; 13, 13; 14, 14; 15, 15; 16, 16 are separated from one another by partitions 77-82. This makes it possible to realize a higher filling level F1 of the molten metal S in the sections 71, 73, 75 that are assigned to the circumferential surface sections 10, 10; 11, 11; 12, 12; 13, 13 with a shorter distance G1 from the respective rotational axis A2, A3 of the casting rollers 2, 3 than in the other sections 70, 72, 74, 76, in which a lower filling level F2 is maintained.

(29) In this way, prolonged contact between the molten metal S and the respective casting roller 2, 3 is realized in the region of the sections 71, 73, 75 of the casting gap such that a longer time period is available for the growth of the shells being formed of the solidified molten metal S on the respective casting rollers 2, 3. Consequently, thicker shells are formed in the region of the circumferential surface sections 10-13 with a shorter distance G1 from the respective rotational axis A2, A3 of the casting rollers 2, 3 than in the region of the other circumferential surface sections 14-16 of the casting rollers 2, 3 such that the joining of the shells in the narrowest point of the casting gap can once again take place in a uniform fashion and the thickness of the still molten steel present in the interior of the cast strip is evenly distributed over the width of the strip. Analogous to the other measures described herein, this also makes it possible to realize a homogenous microstructure in the cast strip despite the uneven thickness distribution.

(30) A practical test is described below with reference to FIGS. 15 and 16: In order to produce a steel strip, for example, with a thickness of 2 mm, it is known from conventional casting machines that a contact time t.sub.c of 0.29 seconds is required for the solidification on casting rollers with a shot-peened surface while a contact time of 0.4 s is required for casting rollers that are merely coated with a sprayed layer. In this case, the empirically determined relation t.sub.c=c*d.sup.2 can generally be formulated with c=0.0725 s/mm.sup.2 for shot-peened roller surfaces and c=0.1 s/mm.sup.2 for thermally sprayed roller surfaces.

(31) At the same melt pool filling level, this relation results in a thickness ratio of the solidified strip shells of 0.85 between roller surface regions with a sprayed layer coating and shot-peened surfaces (determined in the form of the root of the two constants c). Since this applies to both rollers, the ratio between the strip thicknesses doubles after the strip exits the gap between the rollers.

(32) A strip B7 with a width of 150 mm should be profiled symmetrically referred to its center plane M. In a length section 51 bordering on one longitudinal side 50, the thickness of the length section 51 should amount to 1.4 mm over a width T of 50 mm. In the length section 53 bordering on the length section 51 and the other longitudinal side 52 of the cast strip B7, in contrast, the strip B7 should have a thickness of 2 mm.

(33) In order to produce such a strip, a pair of casting rollers 2, 3 with a width K of 150 mm was utilized. In this case, the rollers 2, 3 contained two circumferential surface sections 56, 57; 56, 57, wherein the circumferential surface section 56, 56 assigned to the thinner length section 51 of the cast strip B7 was spaced apart from the rotational axis A2, A3 of the casting rollers 54, 55 by a distance G2 that was 0.3 mm greater than the distance G3 of the circumferential surface section 57 assigned to the thicker length section 53. The circumferential surface section 56 was coated with a sprayed thermal layer while the circumferential surface section 56 was abraded by means of shot-peening. This made it possible to realize the strip cast of molten steel S with a thickness jump from 1.4 mm to 2.0 mm transverse to the longitudinal direction.