EP0369184A2

EP0369184A2 - Casting machine having a travelling block mold assembly

Info

Publication number: EP0369184A2
Application number: EP89119431A
Authority: EP
Inventors: Masayuki Patent & License Dept. Nakada; Masami Patent & License Dept. Komatsu; Hideto Patent & License Dept. Takasugi; Yoshihiko Patent & License Dept. Kawai; Akichika Patent & License Dept. Ozeki; Shiro Osada
Original assignee: IHI Corp; NKK Corp; Nippon Kokan Ltd
Current assignee: IHI Corp; JFE Engineering Corp
Priority date: 1988-10-19
Filing date: 1989-10-19
Publication date: 1990-05-23
Also published as: CA2000618A1; JPH02108442A; EP0369184A3; BR8905304A; KR900006043A; JPH0470107B2

Abstract

A casting machine comprises a pair of travelling block mold assemblies (22) each having a plurality of block mold members (23) endlessly connected to one another to define a cavity (27), and a mechanism for driving the block mold members (23) such that the block mold members of one travelling block mold assembly (22) move in synchronism with those of the other in a casting direction. Latticed grooves (25, 34, 35, 36) are form on at least an area of the inner wall of each of the block mold members (23) which combine with one another to form the cavity (27). Delays in the solidification of molten steel in the cavity (27) are intentionally caused by these grooves, with the result that the concentration of contraction stress caused by the solidification delays at those seams (3) along which the adjacent block mold members (23) are connected to each other to form the cavity (27) can thus be reduced to prevent the creation of cracks on surfaces of cast products.

Description

The present invention relates to a casting machine having a pair of travelling block mold assembly capable of continuously casting a thin plate product at high speed.
The so-called block casting method of carrying out continuous casting while travelling in the casting direction an assembly which comprises block mold members endlessly connected to one another has appeared these days. This block casting method is intended to cast thin products continuously and at high speed and it attracts attention as a continuous casting method which will become prevailing in the future.
In the case of the conventional continuous casting method which uses the stationary mold, the product in the process of solidification, is forcedly pulled out of the stationary mold. Therefore, they are broken because of their shells bonded by walls of the stationary mold. This is the so-called "sticker type breakout". In the case of the block casting method, however, the sticker type breakout can be effectively avoided because molding blocks can be drived in response to the speed at which the product in the mold is solidified. According to the block casting method, therefore, casting speed can be made ten times or higher than that of the conventional continuous casting machine having the stationary mold.
A block type casting machine for casting thin products each having a thickness of several tens millimeters, for example, is provided with a pair of travelling block units (block mold assembly). Each of these paired travelling block units comprises a plurality of block mold members each having L-shaped section and endlessly connected to one another, and both of these units are travelled synchronously with each other. Casting cavity is defined by a plurality of the block mold members.
As shown in Fig. 1, however, the cooling deficiency of molten steel 4 takes place at seam 3, along which block mold members 2 are connected to each other, to thereby form solidification-delayed portion 6 along seam 3, when molten steel 4 comes into contact with casting walls of the block mold members 2 and shell 5 starts to develop at a certain speed. Contraction stress thus concentrates to solidification-delayed portion 6 along seam 3 of the block members and crack forms on the surface of the cast product along the dendrite axis of the product. This is the so-called "transversal crack". This transversal crack also forms in the case of the common stationary mold. It forms in the wide face, extending from one end to the other end of the width of the product in worst case.
When a flaw parallel to the casting direction exists on the inner wall of the block mold member 2, local cooling deficiency is caused at this flaw to form the solidification-delayed portion, which cooperates with the distortion of the product in the width direction thereof to result the so-called "longitudinal crack" on the surface of the product.
These transversal and longitudinal cracks can be avoided to some extent by controlling cooling speed to the product at the secondary cooling zone which follows the casting process. Because the casting speed is high, however, it is difficult to control the cooling speed to the product and it is impossible to completely eliminate these surface flaws from the product. This makes it necessary to apply a treatment such as the scarfing to the surface of the product before the following rolling process, thereby reduces the productivity and causes to a higher cost.
The object of the present invention is to provide a casting machine capable of preventing flaws such as the transversal and longitudinal cracks at the surfaces of the cast products. It is particularly to provide a casting machine capable of preventing from the surface flaws on the products in the case where the products for thin plates are continuously cast at high speed.
According to an aspect of the present invention, a casting machine comprising; at least a pair of travelling mold assemblies, each having a plurality of block mold members endlessly connected to one another to define a cavity and a means for driving the block mold members such that the block mold members of one travelling block mold assembly move in synchronism with those of the other in a casting direction, wherein grooves are formed in the shape of a latticed on at least inner walls of the block mold members.
When solidification delays of molten steel exist intentionally caused at the lattice of grooves on the surface of the cast product, they can be scattered over the whole surface of the product, including those portions thereof which correspond to seams of the block mold members connected and vertical flaws on the casting walls of the block mold members. This can reduce the concentration of contraction stress.
The cavity defined by the block mold members may have any sectional shape such as the slender rectangle, slender ellipse, triangle, hexagon, and octagon. Whatever sectional shape the cavity may have, however, it is preferable to form the lattice of grooves at the wide face of the mold wall.
It is preferable that each of the grooves formed on the mold wall is small in depth and width. It is more preferable that each of the grooves has a depth D of 0.5 mm and a width W of 0.5 - 1.0 mm.
It is preferable that the pitch interval L of the latticed grooves is 5 - 10 mm. When the groove pitch L is smaller than 5 mm, contact between the molten steel and the casting walls of the block mold members becomes incomplete. As the result, the molten steel is not cooled enough to thereby increase the frequency of so called breakout. When the groove pitch L is larger than 10 mm, however, the merit of preventing the transversal and longitudinal cracks cannot be found. These are the reasons why the groove pitch L is kept 5 - 10 mm.
This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which:

Fig. 1 is a vertically-sectioned partial view showing conventional block mold members which is under casting;
Fig. 2 is a schematic view of a block casting system in which an embodiment of casting machine according to the present invention is employed;
Fig. 3 is a transversally sectioned view showing block mold members;
Fig. 4 is a perspective view showing a part of the block mold members arranged to one another;
Fig. 5 is an enlarged perspective view showing grooves formed on the casting wall of a block mold member;
Figs. 6 through 8 are sectional views showing variations of the groove formed on the casting walls of the block mold members; and
Fig. 9 is a graph, on a horizontal axis of which groove pitches L are plotted and on a vertical axis of which the frequency of cracks caused in products and the index of breakout occurrence are plotted to explain how the quality of products cast is influenced by groove pitches L.

Some embodiments of the present invention will be described in detail with reference to the accompanying drawings.
As shown in Fig. 2, tundish 10 is located on the upstream side of block type continuous casting machine 20 and nozzle 14 of tundish 10 through which molten steel is fed is inserted into the upstream side of a cavity 27 of casting machine 20.
As shown in Fig. 3, cavity 27 is defined by block mold assembly 22 of paired upper and lower casting units 21.
Plural spray nozzles 32 and pinch rollers (not shown) are located on the downstream side of casting machine 20. Casting machine 20 is tilted by a certain angle.
Casting machine 20 includes the paired casting units 21 combined to face each other. Each of block mold assembly 22 comprises a plurality of block mold members 23, substantially the same in shape, endlessly connected to one another between a pair of gear wheels 28 and 29 and these members 23 of each of assembly 22 are endlessly driven from upstream to downstream, or from downstream to upstream by a drive mechanism (not shown). Speeds "a" and "b" at which upper and lower block mold assemblies 22 are driven and speed c at which product 31 is pulled out of cavity 27 are computer-controlled in response to the volume of the molten steel fed and the change of the molten surface in the cavity 27.
As shown in Fig. 4, each of block mold members 23 has an L-shaped section in a direction perpendicular to the casting direction and it is provided with a lattice of grooves 25 formed all over its wider inner wall 24a. In other words, grooves 25 extend along seam 3 of adjacent block mold members 23 connected and from one end to the other end of wider inner wall 24a. They also extend in a direction perpendicular to the wider inner wall 24a of member 23.
Grooves 25 may be formed on a narrower inner wall 24b of block mold member 23 in addition to wider inner wall 24a thereof. Or they may be formed on a part of wider inner wall 24a, that is, only that portion of wall 24a which is adjacent to seam 3 of members 23 connected.
As shown in Fig. 5, each of latticed grooves 25 has a V-shaped section whose depth D is smaller than its width W. For example, groove width W is 0.5 - 1.0 mm, groove depth smaller than 0.5 mm, and groove pitch (distance between adjacent grooves) L 5 - 10 mm.
As shown in Figs. 6 through 8, the sectional shape of the groove may be changed to saw-tooth-shaped groove 34, U-shaped groove 35 or rectangular shaped groove 36.
There will be described a case where low-carbon aluminum killed steel in which carbon of about 0.05 weight percents is contained is continuously cast through above-described machine 20.
Molten steel 12 adjusted to have certain components and a certain temperature is transferred from a ladle (not shown) to tundish 10. Molten steel 12 thus transferred into tundish 10 is fed into cavity 27, into which a dummy bar (not shown) is inserted, through nozzle 14. The cavity 27 is previously under the atmosphere of inert gas and casting is carried out, feeding the molten steel 12 into the cavity 27 while pulling the dummy bar out of the cavity 27 and keeping the front end of nozzle 14 separated from the molten surface in the cavity 27. This so-called "open casting method" may be carried out or the so-called "closed casting method" which is carried out keeping the front end of nozzle 14 immersed in the molten steel 12 in the cavity 27 may be employed. Further, the feeding volume of the molten steel is precisely adjusted by a sensor and a controller (not shown) located on the side of the tundish 10 to best control both of the speed "a" and "b" at which each of casting units 21 is endlessly driven and the speed "c" at which the product 31 is pulled out of the cavity 27.
When molten steel 12 is fed into cavity 27 through nozzle 14, it contacts the casting walls of water-cooled block members 23 which define cavity 27 and becomes shell 26, which is further cooled by spraying water through spray nozzles 32 and solidified completely. Casting speed is more than 20 meters per minute, for example.
Cooling deficiencies are caused at seams 3 and grooves 25 during solidifcation of molten steel in the mold. Solidification delays are thus caused and tensile stress (contraction stress) is created at these specific areas when the molten steel is solidified and shrunk. Because the contraction stress is scattered on all of the surface of the shell 26, however, the concentration of the contraction stress can be reduced to some extent.
Even if cracks should be caused, they can be stopped by grooves 25 not to develop to larger ones because some grooves 25 will be formed perpendicular to them.
Shell 26 is developed to have a sufficient thickness and a certain strength at the outlet of the mold. The solidification of product 31 is controlled by water jetted through spray nozzles 32, during pulling shell 26 out of the mold. A thin slab 50 mm thick and 1000 mm wide, is produced in this manner.
The quality of slabs producting by the block members which have various kinds of grooves thereon will be described referring to Fig. 9.
Fig. 9 is a graph, on the horizontal axis of which groove pitch L of the V-shaped grooves is plotted and on the vertical axis of which the frequency of cracks caused in products and the index of breakout occurrence are plotted to explain the quality of slab 31 changing the groove pitch L variously but keeping other conditions substantially the same. In the Fig. 9, black spots represent transversal cracks, white spots represent longitudinal cracks, and white triangles represent breakout occurrence. The index of breakout occurrence means in this case that frequencies of breakout caused in products are replaced by indexes. As apparent from Fig. 9, the frequency of transversal and longitudinal cracks caused in products is quickly increased when groove pitch L becomes longer than 10 mm. When groove pitch L is shorter than 5 mm, the index of breakout occurrence is increased because contact between the molten steel in the cavity and the inner walls of the block mold members becomes insufficient and the molten steel is not cooled enough, accordingly. When low-carbon aluminum killed steel is to be cast, therefore, it is preferable that groove pitch L is in a range of 5 - 10 mm.
Although each of the latticed grooves formed on the inner walls of the block mold members in the cavity has the V-shaped section in the above-described embodiment, it may be like saw-tooth-shaped groove 34, U-shaped groove 35 or rectangular-shaped groove 36, as shown in Figs. 6 through 8.
Although the latticed grooves have been formed only on the wider inner wall 24a of each of the block mold members 23 in the cavity in the case of the above-described embodiment, they may be formed on the narrower inner wall 24b of each of the block mold members 23, or on a part of the wider inner wall 24a, or adjacent to the seams along which the block mold members 23 are connected to one another.
According to the present invention, the occurrence of transversal and longitudinal cracks on surface of slab 31 can be effectively prevented. After-treatments (e.g. surface treatment etc.) which will be applied to surfaces of slab 31 at the following process can be thus reduced to a great extent. The productivity of products can be therefore enhanced and their manufacturing cost can be made lower.

Claims

1. A casting machine comprising:
a casting unit (21) having at least a pair of travelling block mold assemblies (22), each having a plurality of block mold members (23) endlessly connected to one another to define a cavity (27); and
a means for driving the block mold members (23) such that the block mold members of one travelling block mold assembly (22) move in synchronism with those of the other (22) in a casting direction;
wherein grooves (25, 34, 35, 36) are formed in the shape of a latticed on at least inner walls of the block mold members (23) which define the cavity (27).

2. The casting machine according to claim 1, characterized in that each of the block mold members (23) has two types of inner walls, one type being wider than the other.

3. The casting machine according to claim 1 or 2, characterized in that the grooves (25, 34, 35, 36) are formed only on the wider one of the inner walls (24a).

4. The casting machine according to any of claims 1 to 3, characterized in that the interval between the adjacent latticed grooves (25, 34, 35, 36) is in a range of 5 - 10 mm.

5. The casting machine according to any of claims 1 to 4, characterized in that the depth of each of the grooves (25, 34, 35, 36) is less than 0.5 mm.

6. The casting machine according to any of claims 1 to 5, characterized in that the width of each of the grooves (25, 34, 35, 36) is in a range of 0.5 - 1.0 mm.

7. The casting machine according to any of claims 1 to 6, characterized in that each of the grooves (25) is V-shaped in section.

8. The casting machine according to any of claims 1 to 6, characterized in that each of the grooves (35) is U-shaped in section.

9. The casting machine according to any of claims 1 to 6, characterized in that each of the grooves (34) has a saw-tooth-shaped section.

10. The casting machine according to any of claims 1 to 6, characterized in that each of the grooves (36) has a rectangular-shaped section.