A conventional type of installation for the continuous casting of metals includes a generally vertical, cooled, open-ended mold in which a metal strand is continuously formed. The mold is followed by a secondary cooling zone which is at least partially curved so as to turn the strand towards the horizontal. A straightener is arranged behind the secondary cooling zone and straightens the strand which subsequently follows a horizontal roller table to a cutting unit where it is cut into sections. The cut sections travel along another roller table, known as a runout table, to a cooling bed or to a mill.
One type of dummy bar for starting the withdrawal of the strand from the mold extends from the latter to the straightener when positioned in readiness for casting. The straightener draws the dummy bar away from the mold and thus also serves as a withdrawal device. Once the strand has reached the straightener, the dummy bar and the strand are disconnected and the dummy bar is stored.
An early dummy bar is made up essentially entirely of pivotally connected links which are short as compared to the distance between the mold and the straightener. Dummy bars of this type are flexible and can be stored flat yet have the ability to bend to the curvature of the secondary cooling zone. This early link design has several disadvantages.
To begin with, dummy bars of this design require a great deal of maintenance to insure that the links remain freely pivotable relative to one another. Furthermore, due to their flexibility, these link-type dummy bars must be supported along the entire span between the mold and the straightener when positioned in readiness for casting. This requires a large support structure which adds to the cost of the casting machine. In addition, dummy bars of this type cause non-uniform withdrawal of the strand from the mold since they tend to flex downwardly into the spaces between adjacent rollers of the roller tables following the straightener. Moreover, since the link-type dummy bars are normally stored in a flat condition, a long storage area is required thereby increasing the cost of the casting machine and the space requirements for the latter. This is particularly true for billet casting machines where the dummy bars are stored alongside the runout tables so that these tables must have lengths at least equal to those of the dummy bars.
In order to provide for a more uniform withdrawal of the strand from the mold, it has been proposed to construct the link-type dummy bars with limited flexibility. Here, the dummy bars are free to bend in one direction but can bend back only until they are flat. This prevents the dummy bars from flexing downwardly into the spaces between adjacent rollers of the roller tables and consequently promotes a more uniform withdrawal of the strand from the mold.
Another proposal eliminates the problem of non-uniform withdrawal as well as that of a long storage space. According to this proposal, the dummy bar is raised after leaving the straightener and is passed over a group of rollers located at the level of the casting platform. The rollers define an arc of radius smaller than the casting radius and a central portion of the dummy bar rests on the rollers while the remainder of the dummy bar is suspended on either side thereof. The dummy bar thus has an inverted U-shaped configuration while in storage. The dummy bar is here composed of curved links having radii equal to the casting radius so that the dummy bar conforms closely to the curvature of the casting machine when in the casting position.
The latter proposal does result in reduced storage space requirements for the dummy bar and also eliminates the problem of non-uniform withdrawal since the dummy bar does not travel onto the roller tables. However, the maintenance problems associated with the link-type dummy bars remain as does the need for a support structure along the entire span between the mold and the straightener.
A further proposal has been presented for curved-mold casting machines designed to cast strands such as billets of relatively small cross-sectional area. Here, a rigid, curved dummy bar is used which has a radius equal to the casting radius. The length of the dummy bar is somewhat greater than the distance between the mold and the straightener so that one end of the dummy bar can close the lower end of the mold while the other end is engaged by the straightener. This design eliminates the maintenance problems associated with the link-type dummy bars and has the further advantage that reduced support structure is required between the mold and the straightener.
The rigid dummy bar is stored behind the straightener in a storage unit which defines a curved path having a radius equal to the casting radius and hence to that of the dummy bar. This path forms a continuation of the casting path and the dummy bar enters the storage unit immediately after leaving the straightener. In its stored position, the dummy bar curves towards a location above the straightener.
Due to the relatively great length of the dummy bar, the latter projects upwardly to a level above that of the casting platform when in storage. This not only causes interference with the movement of the overhead cranes used in mills but also reduces the working space available on the casting platform. In addition, since the dummy bar curves towards a location above the straightener, access to the latter from the casting platform is made difficult. Aside from these disadvantages, the use of a rigid dummy bar leads to problems in the event of distortion of the dummy bar since it may no longer be possible to readily guide the dummy bar into the mold.
The latter disadvantage has been overcome by a dummy bar having limited flexibility. This dummy bar resembles the rigid dummy bar but, as opposed to the rigid dummy bar, is composed of several long links which are connected so as to permit limited, relative pivotal movement thereof. This makes it possible to compensate for minor distortion.
A proposal similar to the rigid dummy bar has been made for straight-mold casting machines in which the secondary cooling zone is not curved in its entirety but is made up of a straight section immediately following the mold and a curved section between the straight section and the straightener. In this case, the dummy bar is composed of a link-type flexible portion and a rigid portion. The flexible portion is designed to close the lower end of the mold prior to the start of a casting operation. It is flexible out of necessity since it must be able to conform to the curvature of the curved section of the secondary cooling zone and also be capable of assuming a linear configuration when in the straight section of this zone. The rigid portion of the dummy bar is curved and has a radius equal to the casting radius. This portion of the dummy bar is located in the curved section of the secondary cooling zone at the beginning of a casting operation and is engaged by the straightener.
The storage scheme set forth for the part-rigid, part-flexible dummy bar is the same as that for the rigid dummy bar and leads to the same disadvantages. The part-rigid, part-flexible dummy bar further eliminates one of the advantages of the rigid dummy bar, namely, a reduction in the support structure required between the mold and the straightener. Thus, support for the flexible portion of the part-rigid, part-flexible dummy bar must be provided along the entire span between the mold and the straightener.