Patent Application
20100132911
The invention relates to a method of and an apparatus for casting near-net-shape, rectangular strands from metal and a subsequent processing thereof into metal strips according to a DSC-method (direct strip casting) in a horizontal strip casting installation, wherein the metal melt is cast with a melt feeder on a horizontally circulating metal conveyor belt with a cooled bottom, and a liquid cast product is solidified to a pre-strip on the metal conveyor belt during displacement thereof and which after leaving the metal conveyor belt is fed, mechanically tensioned, to a driver by, e.g., smooth/pinch rollers. An installation with smooth/pinch rollers is not absolutely necessary, the installation can be realized without these rollers.
Because of uneven heat dissipation during solidification process of a strip, cast according to DSC-method under inert gas atmosphere without use of a casting compound, due to the upper surface of the strip being cooled only by convection with the ambient atmosphere and by heat radiation, while the bottom is in a direct contact with a cooled metal conveyor belt, the strip deforms already during solidification, firstly, upward and then downward. At the start of cooling, the bottom of the material layer of the strip contracts most due to a very large temperature gradient. The entire strip bends upwardly in the middle, which results in very high stresses in the upper layer. Because these stresses are greater than the flow stress, they are reduced during the course of solidification again by subsequent elongation (flow), whereby opposite bending of the strip middle downwardly takes place. As a result, the low layer remains elongated, and the upper one shortened.
When the strip, which is usually not guided on its upper surface, leaves the metal conveyor belt with which it is displaced, the temperature of the strip over the strip thickness equalizes due to the reduced cooling of the strip bottom, the thermal tension also equalizes. The upper shortened and the lower elongated strip regions are subjected only to the backward bending, whereby the strip arches upwardly. The produced, as a result, stresses are below or close to the yield point, so that no or a very small backward formation of the arch resulting from the flow process, can be observed. The curve upward remains and results in arching of the strip narrow sides and also in a strip head like a ski.
During a further displacement, the degree of freedom of these arches in a longitudinal direction is reduced due to the gravity force of the strip horizontally displaceable on the adjoined roller table and/or by one or more pinch or smooth rollers which follow the metal conveyor belt, and firstly the strip tip and then the entire strip is mechanically tensioned and is forced to plane-parallel displacement downwardly.
This reduction of the degree of freedom leads to a need to reduce the stresses in the strip in the non-tensioned region, and that is why the strip narrow sides arch upwardly immediately after the strip leaves the metal conveyor belt. This behavior extends backwardly up to the region of the metal conveyor belt, so that the solidified strip has no contact anymore with the metal conveyor belt, and, thus, with the cooling medium and, as a result, has a non-homogenous temperature distribution over width of the strip that has a gutter profile.
In order to deal with this problem and to prevent the backward displacement of the pre-strip profile in the casting region and to insure passing into the upstream located machine, WO 2006/066552A1 suggests to arrange a guide element at the end of a primary cooling zone and in front of a conventional secondary cooling zone. As a rule, the guide element consists of several rollers arranged above and below the pre-strip in top-to-top or in offset-to-each other condition.
With a particular arrangement of rollers, the pre-strip is displaced in a plane located above a casting line in order to absorb the elongation of the bottom of the pre-strip by the carried-out upward movement. A roller arrangement, with which the pre-strip passes through the rollers as a wave, is also possible, however, it has not been used up to now.
The drawback of the method disclosed in WO 2006/066552 A1 consists in that the guide element that follows the metal conveyor belt can only partially influence the thermal processes on the metal conveyor belt.
Proceeding from this known state-of-the art, it is an object of the invention to provide a method and an apparatus with which in a simple manner, a maximum contact of the cast product with the metal conveyor belt and, thereby, optimization and equalization of heat transfer from the cast product to the metal conveyor belt over the entire casting width can be insured.
According to the method, the stated object is archived with features of claim 1 that in order to prevent a possible backward arching of strip edges that can begin in an outlet region of the caster and in order to average heat transmission to the casting product during solidification thereof on the metal conveyor belt, the following method steps are combined with each other:
establishing a maximum contact of the cast product with the metal conveyor belt, and to this end, a pressure device, which is arranged in a region of an end of the metal conveyor belt located downstream in a casting direction, applies pressure to the cast product solidifying into the pre-strip, preferably, to the strip edges thereof from above, and
compensating a suddenly reduced cooling of a bottom of the pre-strip upon the pre-strip leaving the conveyor belt, and to this end, in a predetermined region, immediately behind the metal conveyor belt, the bottom and selectively and simultaneously, an upper surface of the pre-strip selectively over an entire width is additionally cooled.
As a result of application of pressure, according to the invention, to the cast product from above in the region of the end of the metal conveyor belt and in particular, to its edges, which induces a complete contact of the cast product bottom with the metal conveyor belt, in association with additional cooling of the pre-strip bottom, optimization and equalization of heat transfer from the cast product to the metal conveyor belt over the entire casting width and heat equalization within the pre-strip after it leaves the metal conveyor belt, can be achieved.
The necessary pressure is produced by a pressure roller acting on the entire width of the cast product or by partial pressure-applying rollers acting only on the strip edges. The pressure rollers are preferably separately driven and inwardly cooled. According to the invention, the necessary pressure can be applied with an abutting circulating pressure strip which likewise can be separately driven and cooled.
In combination with application of pressure to the cast product, according to the invention, simultaneously, cooling of the pre-strip bottom in a predetermined region immediately behind the metal conveyor belt is carried out, wherein the predetermined region can extend over the entire width of the pre-strip and, upon availability of smooth/pinch rollers, up to those. The cooling is effected by an open spray cooling, e.g., with water, and/or by closed cooling with a circulating cooling belt that, like the metal conveyor belt, is in contact with the bottom of the pre-strip. According to the invention it is possible, simultaneously, to provide a circulating cooling belt on the upper side of the pre-strip for guiding the pre-strip and for cooling the same in a predetermined adapted different manner.
Further particularities and advantages of the invention will be explained based on an exemplary embodiment shown in schematic drawings.
The drawings show:
a a section of
b a section of
a a plan view of a section of
b cross-sections of a cast product/leadership according to the state-of-the art;
a plan view of
b a cross-section of a cast product/pre-strip with a pressure roller;
In this state-of-the art, corresponding strip casting installation 1, there are provided, according to the invention, in a region of the deflection roller 8, which are located at the end of the metal conveyor belt 7, above the cast product 4, a pressure roller 11. The pressure roller 11, which engages on the cast product 4, can insure, upon application of corresponding pressure, a maximal contact at least of the strip edges of the cast product 4 with the metal conveyor belt 7.
In a section of
An alternative cooling of the pre-strip 5 in form of a closed cooling is shown in
To better explain the inventive pressure application to the cast product 4, the strip casting installation shown in
a shows, e.g., a section of the strip casting installation, starting from the distribution spout 3/melt feeder 3′ to the end of the metal conveyor belt 7 according to the state-of-the art. In
In
a shows a change in the arching of the strip edges 6 due to the use of a pressure roller 11 in the region of the cross-section “B.” the upwardly arching region 18 of the strip edges 6 begins only at the cross-section “B” with a noticeably smaller amount. The pressure roller 11 acts so that it suppresses the arching of the strip edges 6, reversing it, until they occupy a position corresponding to that in the region of the cross-section “A.” A further, forwardly directed, arching of the strip edges 6 up to the cross-section “C” cannot be completely suppressed by the pressure roller 11, however, it is noticeably smaller than in
In
A further alternative to the use of the pressure roller 11 and the partial pressure-applying rollers 12 consists in use of a pressure belt 13 that applies pressure as shown in
The invention is not limited to the shown embodiments but can be carried out, with regard to the used pressure devices and devices for additional cooling, with devices that differ from the described above if the inventive method is possible with these devices.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2007 036 969.9 | Aug 2007 | DE | national |
| 10 2007 054 554.3 | Nov 2007 | DE | national |
| 10 2007 057 278.8 | Nov 2007 | DE | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2008/006362 | 8/1/2008 | WO | 00 | 1/27/2010 |
