CONINUOUS CASTING INSTALLATIONS

Abstract: Continuous casting installation comprises a pair of endless tracks (10,11) which co-operate over a portion of their length to define a mould cavity (20) and a tundish (12) from which molten metal is allowed to flow through a tundish nozzle (13) into one end of the mould cavity (20). The co-operating portions of the endless tracks (10,11) are moved in the same direction towards the other end of the mould cavity at which the casting is withdrawn. The pressure within the mould cavity is sensed by means of load cells (15,17) and a throttling valve (19) is operated in response to the magnitude of the pressure to vary the rate of flow of molten metal through the tundish nozzle (13) and thus maintain the pressure within the mould cavity (20) at a substantially constant level which is low enough to ensure that substantially no leakage of molten metal occurs through the gap between the outlet end of the tundish nozzle (13) and the mould assemblies (10,11).

Description: The present invention relates to continuous casting installations and to a method of operating such installations and is concerned with such installations of the type which comprises a pair of endless tracks which co-operate over a portion of their length to define a mould cavity, which co-operating portions move, in use, in the same direction, and is concerned with a method of the type which includes introducing molten metal into one end of the mould cavity and withdrawing a casting from the other end of the cavity. Figure 1 is a diagrammatic side elevation of a known continuous casting installation of moving mould type which includes a plurality of mould blocks a interconnected to form two endless tracks b. The two tracks are disposed, one above the other, in spaced relationship with one run of each parallel to the other thereby defining a mould cavity c. A tundish nozzle d extends into one end of the mould cavity c through which molten metal e is delivered to the mould cavity c. The mould assemblies b, and the metal within it , are continuously moved toward the other end of the mould cavity c, from which the solidified casting f is withdrawn. If molten metal penetrates into the gap defined between the portion of the tundish nozzle d within the mould cavity c and the mould assemblies b and solidifies, not only the tundish nozzle d but also the casting f are damaged. Therefore, the gap should be as small as possible to prevent the molten metal from penetrating into it. However, when the gap is made too small, it causes the mould assemblies b to contact the tundish nozzle d, due to vibrations produced during the movement of the heavy mould assemblies b. The tundish nozzle d is thus easily damaged. Thus, in practice the gap cannot be particularly small. It has been found to be almost impossible to maintain the gap less than 0.15 mm wide, without the possibility of damage due to contact caused by the vibrations, because of the limited dimensional accuracy of the various mechanical component parts, including the tundish nozzle. It follows, therefore, that since the gap is in practice at least 0.15 mm wide,the vertical height between the leading end of the tundish nozzle d and the surface level of molten metal e within the tundish g,i.e. the molten metal static head H, has to be reduced to some extent to alleviate the pressure on the molten metal forcing it through the gap. In general the head of the molten metal in the tundish g is as much as 0.5 m or more. Conventionally, the tundish nozzle d is disposed in an inclined position to allow the casting f to be withdrawn from the mould cavity c. This results in an increase in length of the tundish nozzle d and thus the molten metal static pressure head H is further increased. As a result it is almost impossible to prevent molten metal from penetrating into the gap under the action of the pressure caused by the static head H. It is an object of the present invention to provide a continuous casting installation and a method of operating such an installation in which molten metal is effectively prevented from penetrating into the gap between the tundish nozzle and the mould assemblies, whilst nevertheless maintaining an optimum rate of casting. It has been found, by the inventors, that there is a relationship between the gap between the tundish nozzle and the sides of the mould cavity in a continuous casting installation and the pressure exerted by the head of molten metal at which no penetration of the molten metal into the gap occurs. This relationship is shown in the graph of Figure 2 which will be described in more detail below. As is clear from Figure 2, when the gap is 0.15 mm or less wide and if the molten metal static pressure head H is 0.3 m or less, substantially no molten metal will penetrate into the gap. The present invention is based on the fact that the penetration of the molten metal into the gap can be prevented by controlling the molten metal static pressure in the mould cavity c to a value corresponding to a molten metal static pressure height H of 0.3 m or less. According to the present invention a method of operating a continuous casting installation of the type referred to above is characterised by sensing the pressure in the mould cavity and controlling the flow of molten metal into the mould cavity in response to the sensed pressure, thereby maintaining the pressure in the mould cavity substantially at a predetermined level. The pressure in the mould cavity is preferably sensed by sensing the force exerted on supporting frames supporting the endless tracks by the molten metal in the cavity. The flow of molten metal is preferably controlled by means of a throttling valve which preferably co-operates with the inlet to the molten metal supply nozzle which extends into the mould cavity. The flow of molten metal is preferably controlled to maintain the pressure within the mould cavity substantially at a predetermined value at which substantially no leakage occurs through the gap between the nozzle and the endless tracks. Thus regardless of the height of the surface level of the molten metal in the tundish above the discharge end of the nozzle, the rate at which molten metal is allowed to pass through the nozzle is controlled by, for instance, variable throttling means so that the pressure within the mould cavity is always maintained to substantiate the predetermined level whereby the quantity of the molten metal penetrating into the gap between the nozzle and the opposing surfaces of the mould assemblies is minimised. The present invention can be put into practice in several ways and one embodiment will now be decribed by way of example with reference to Figures 2 and 3 of the accompanying drawings in which; Figure 2 is a graph illustrating the relationship between the gap between a tundish nozzle inserted in the mould cavity and mould assemblies, and the molten metal static pressure height at which molten metal just penetrates the gap. Figure 3 is a side view of a moving-mould type continuous casting machine according to the present invention. Referring to Figure 3, the continuous casting machine comprises upper and lower block assemblies 10 and 11 respectively which each comprises a series of mould blocks 9 connected to form an endless track. The tracks are arranged so that a run of the upper assembly 10 runs parallel to and above an adjacent run of the lower assembly 11. The space between the two runs consitutes an inclined mould cavity 20. Each track is driven by a respective sprocket 1 or 2 mounted on an associated drive shaft 3 or 4. The shafts 2 and 4 are supported by a respective bearing 5 or 6 mounted on a frame 7 or 8 respectively. A tundish 12 has an inclined tundish nozzle 13 depending from its lower face at the same angle of inclination as that of the mould cavity 20. The nozzle 13 extends into the upper end of the mould cavity 20 between the two tracks. In the casting process the height H of metal above the exit of the nozzle 13 exerts a static pressure on the mould cavity 20. The pressure tends to urge the molten metal out of the cavity 20 through the gap between the mould blocks 9 and nozzle 13. A load cell 15 is interposed between the shaft 4 of the sprocket 2 of the lower mould assembly 11 and the frame 8 upon which it is mounted thereby detecting increases or decreases in the pressure acting on the lower mould assembly 11 as a result of pressure in the mould cavity 20. In like manner, a load cell 17 is interposed between the shaft 3 of the sprocket 1 of the upper mould assembly 10 and the frame 7 upon which it is mounted, thereby detecting variations in pressure acting on the upper mould assembly 10. A throttling valve 19 is vertically movable above a pouring opening 18 formed through the bottom of the tundish 12 to control the flow of molten casting metal from the tundish 12. In order to cause the vertical movement of the throttling valve 19, a hydraulic cylinder, a screw rod, a rack mechanism or the like may be used although these are not shown in the drawing. The throttling valve 19 is controlled manually in response to signals delivered from the load cells 15 and 17 or more preferably automatically with the aid of a logic circuit and a valve actuator, which are not shown, such that the molten metal static pressure in the mould cavity 20 is maintained at a predetermined level. Referring back to Figure 2, in which Vc represents the casting velocity, Ps the molten metal static pressure and r the surface tension, when the gap 8 between the portion of the tundish nozzle 13 which is inserted in the mould cavity 20 and the surface of the mould cavity 20 is 0.15 mm, the allowable molten metal static pressure height which allows no penetration of the gap by the molten metal is about 0.3 m. Therefore, the control resistance HC acting on the throttling valve 19, i.e. the back pressure exerted by the valve 19, can be calculated from the following equation: H N = H - H N - H C where H N : molten metal static pressure in the mould cavity; H : molten metal static pressure height; and H N : tundish nozzle resistance. The tundish nozzle resistance 4H N can be measured and the molten metal static pressure height H, which varies in response to the variation in quantity of molten metal in the tundish, can be monitored in use. In the continuous casting operation, when the degree of opening of the throttling valve 19 is so adjusted to obtain the control resistance 4H C thus calculated, the molten metal in the tundish 12 is poured into the mould cavity 20 through the molten metal pouring opening 18 at the bottom of the tundish 12 and the tundish nozzle 13. The pressure of the molten metal poured into the mould cavity 20 acts on both the upper and lower mould assemblies 10 and 11 and is detected by the load cells 17 and 15. The static pressure H N in the mould cavity thus detected tends to differ from the maximum allowable static pressure height of 0.3 m due to variations in the actual molten metal static pressure height H and other factors, so that the degree of opening of the throttling valve 19 is increased or decreased, whereby the control resistance H C becomes equal to a value at which the molten metal static pressure in the mould remains at a value of 0.3 m. When the rate of pouring of molten metal through the tundish nozzle 13 into the mould cavity 20 is too high, the molten metal static pressure in the mould cavity 20 rises above a predetermined level and is detected by the load cells 15 and 17. In response to the increase in molten metal static pressure in the mould cavity 20 thus detected, the degree of opening of the throttling valve 19 is manually or automatically reduced so that the penetration of the gap by the molten metal is thus prevented. On the other hand, if the rate of pouring of molten metal through the tundish nozzle 13 into the mould cavity 20 is too low, no molten metal penetrates into the gap 8, but the casting velocity Vc is decreased. Thus, in response to the output signals from the load cells 15 and 17, the degree of opening of the throttling valve 19 is increased and consequently the quantity of the molten metal poured into the mould cavity 20 is increased. It is to be understood that the present invention is not limited to the preferred embodiment described above and that various modifications may be effected. The present invention may equally be applied to dual-­ belt type continuous casting machines as well as moving-mould type continuous casting machines. As described above, in the method and installation of the present invention, the flow rate of the molten metal is controlled in response to the detection of the molten metal static pressure in the mould cavity. Therefore, the molten metal static pressure in the mould cavity can be maintained at a predetermined maximum pressure range without causing penetration of the gap by the molten metal. As a result, the present invention allows a maximum continuous casting velocity Vc to be maintained while effectively preventing molten metal from penetrating the gap between the tundish nozzle and the mould assemblies.

Claims: 1. A method of operating a continuous casting installation, the installation comprising a pair of endless tracks (10,11) which co-operate over a portion of their length to define a mould cavity (20), which co-operating portions move, in use, in the same direction, the method including introducing molten metal into one end of the mould cavity (20) and withdrawing a casting from the other end of the cavity (20), characterised by sensing the pressure in the mould cavity (20) and controlling the flow of molten metal into the mould cavity (20) in response to the sensed pressure thereby maintaining the pressure in the mould cavity (20) substantially at a predetermined level. 2. A method as claimed in claim 1, characterised by sensing the force exerted on supporting frames (7,8), by the molten metal in the cavity (20) as a measure of the pressure in the mould cavity (20) each frame supporting a respective one of the endless tracks (10,11). 3. A method as claimed in claim 1 or 2, characterised by controlling the flow of molten metal by means of a throttling valve (19) which co-operates with the inlet to a molten metal supply nozzle (13) which extends into the mould cavity (20). 4. A method as claimed in any one of the preceding claims characterised by controlling the flow of molten metal to maintain the pressure within the mould cavity (20) substantially at a predetermined value at which substantially no leakage occurs through the gap between the nozzle (13) and the endless tracks (10,11). 5. A continuous casting installation comprising a pair of endless tracks (10,11) which co-operate over a portion of their length to define a mould cavity (20) which co-operating portions are movable in the same direction, a metallingical vessel (12) and a nozzle (13) which extends from the vessel (12) into the mould cavity (20), characterised by sensing means (15,17) arranged to sense the pressure in the mould cavity (20) and means (19) for varying the flow of molten metal through the nozzle (13) in response to signals from the sensing means (15, 17) to vary the flow of molten metal into the mould cavity (20) to maintain the pressure in the mould cavity (20) substantially at a predetermined level. 6. An installation as claimed in claim 5 characterised in that the sensing means comprise one or more load cells (15, 17) disposed between one or both endless tracks (10,11) and an associated support frame (7,8). 7. An installation as claimed in claim 6 characterised in that each track (10,11) includes drive means comprising a drive sprocket (1,2) mounted on a drive shaft (3,4) which is supported by a support frame (7,8), a load cell (15,17) being interposed between the shaft (3,4) and the frame (7,8). 8. An installation as claimed in any one of claims 5 to 7 characterised in that the means for varying the flow of molten metal comprises a throttling valve (19) co-operating with the inlet to the nozzle (13).

Inventors: FUKASE, HISAHIKO, MATSUI, KUNIO, TSUCHIDA, YUTAKA
Applicants: ISHIKAWAJIMA-HARIMA  JUKOGYO KABUSHIKI KAISHA, NIPPON KOKAN KABUSHIKI KAISHA
Application year: 1987
Main classification: B22D11
IPC codes: B22D11