Metal founding – Process – Shaping liquid metal against a forming surface
Reexamination Certificate
1998-01-08
2001-08-21
Nguyen, Nam (Department: 1722)
Metal founding
Process
Shaping liquid metal against a forming surface
C164S452000
Reexamination Certificate
active
06276436
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and an apparatus for continuous casting plants for producing strands whose cross-section is reduced during solidification.
2. Description of the Related Art
It is known in the art that strands are manufactured in such high-speed plants generally with a solidification thickness of between 18 and 450 mm and casting speeds of up to at most 15 m/min., for example, in plants for casting slabs, blooms and billets with quadratic or round profiles, wherein a reduction of the strand cross-section is preferably carried out during the solidification after the strand emerges from the mold.
This technology of casting and rolling of thin slabs or round billets is known from German patents 44 03 048, 44 03 049 and 41 39 242; in the case of thin slabs, this technology is used daily in production plants.
For example, a thin slab having a thickness of, for example, 65 mm is reduced to 40 mm in segment 0 which is arranged directly underneath the mold. This strand thickness reduction of 25 mm or 38.5% may be a disadvantage with respect to the quality of certain steels which are sensitive to internal ruptures. Thus, the internal deformation of the strand, due to the strand thickness reduction or also called casting and rolling, may trigger internal ruptures because the critical deformation of the material is exceeded at the inner strand shell liquid/solid, but also at the outer strand shell.
The above example is based on a circular arc segment 0 which has a length of 2 m and which does not introduce bending work or bending deformation into the strand shell. In this case, the deformation speed of the strand shell during casting and during solidification, which represents a measure for the strand deformation, is 1.25 mm/s at a casting speed of 6 m/min. When the casting speed is increased to, for example, 10 m/min., this value of the deformation speed increases to 2.08 mm/s and becomes very critical. Such internal deformations caused by casting and rolling are not only critical for deep drawing steel qualities which are relatively insensitive to internal deformations, but primarily for sensitive steels, such as microalloyed APX-80 qualities.
In addition, in vertical bending units in which usually bending of the strand occurs in the segment underneath the mold simultaneously with the deformation caused by casting and rolling, the bending deformation introduced into the strand is significantly increased, so that the danger of exceeding the critical deformation and, thus, the formation of cracks is even further increased.
SUMMARY OF THE INVENTION
Therefore, in view of the findings and relationships describes above, it is the primary object of the present invention to provide technical method measures and simple apparatus features for predetermining the deformation density of the strand cross-section reduction in such a way that the critical deformation of the strand is not exceeded while taking into consideration the casting speed and also the steel quality.
In accordance with the present invention, the continuous casting method for producing strands, wherein the cross-section of the strands is reduced during the solidification, includes casting into a mold, particularly an oscillating mold, and reducing the strand cross-section linearly over a minimum length of the strand guiding means immediately underneath the mold, i.e., casting and rolling, and subsequently carrying out a further strand cross-section reduction through the remaining strand guiding means, i.e., soft reduction, up to maximum reduction immediately in front of the final solidification or sump tip.
The continuous casting plant according to the present invention for carrying out the above-described method includes the following elements:
an oscillating mold;
a segment 0 which linearly reduces the strand in its cross-section at most by 40% over a length of at least 1 m;
a remaining strand guiding means which reduces the strand in its cross-section up to at most immediately following the sum tip, i.e., soft reduction; wherein
the total reduction of the strand cross-section in segment 0 and in the remaining strand guiding means is configured to be up to 60%.
The features of the present invention are applicable to all sizes cast in a strand and also for all types of continuous casting plants.
The following unexpected solution according to the present invention for achieving the above-described objects will be explained in more detail in connection with a thin slab, wherein the invention is particularly discussed with respect to casting of thin slabs having a thickness of between 60 and 120 mm after solidification, i.e., the thickness of the slab in the edge areas is, for example, a minimum of 70 mm and a maximum of 160 mm at the mold exit. In accordance with the prior art, the reduction of the strand thickness, which usually takes place between the upper and the lower side of a strand guiding means, is today under test conditions at most 60%, i.e., a slab having a thickness of 50 mm is reduced to about 20 mm over a roll gap length of about 200 mm, and is under production conditions at most 38.5%, i.e., the strand is reduced from 65 to 40 mm over the length of the segment 0 which is about 2 m, wherein segment 0 is arranged underneath the mold. In both cases, the maximum casting speed is 6 m/min.
The invention will be described on the basis of an example of a thin slab having a thickness of 100 mm at the mold exit and a solidification thickness of 80 mm. The invention proposes a type of distribution and the realization of the slab thickness reduction during the solidification of the thin slab in the strand guiding stand for, for example, casting speeds of 6 and 10 m/min.
In tables 1 and 1.1, the essential process and apparatus data of the invention are compared to those of the prior art. Table 1 shows the data for casting speeds of 6 m/min and table 1.1 shows the data for casting speeds of 10 m/min.
In both tables, the total reduction of the thickness of the strand of 20 mm during the solidification is varied in its distribution between the segment 0 and the remaining strand guiding means, i.e., the segments 1 through at most 13. In the tables, the prior art is illustrated by a total reduction of the strand thickness of 20 mm carried out solely in segment 0 (compare items 19 through 22 in column 1). This clearly shows that the reduction speed of the strand is increased in the segment 0 which has a length of 3 m from 0.67 to 1.11 mm/s, triggered by the strand thickness reduction or the casting and rolling process and, thus, functionally the strand shell deformation, wherein the casting speed increases from 6 m/min to 10 m/min.
Items 19-22 and 23-28, columns 2, 3 and 4 and items 29-34 represent the solution according to the present invention which results in a significant lowering of the deformation density of the strand shell by a redistribution of the total thickness reduction of 20 mm between the segment 0 and the segments 1-n, also called soft reduction. This redistribution will be explained in detail with the aid of the following examples:
15 mm in segment 0 and 5 mm in the segments 1-n, items 19-28, column 2;
10 mm in segment 0 and 10 mm in segments 1-n, items 19-28, column 3;
5 mm in segment 0 and 15 mm in segments 1-n, items 19-28, column 4;
20 mm in segments 0-n, items 29-34.
In this manner, the reduction speed, and, thus the functional deformation density of the strand shell with a 20 mm total thickness reduction and 10 m/min casting speed can be reduced from:
1.11 mm/s, 20 mm in segment 0, according to the prior art, item 21, column 1, to
0.114 mm/s, 20 mm in segments 0-13, item 33.
However, as a result of displacing a portion of the thickness reduction from segment 0 into the segments 1-13 or 1-2, depending on the casting speed, the work to be introduced into the strand increases with increasing strand shell thickness. Therefore, the present invention takes into account that an optimum distribution of the total thickness reduction in the t
Friedrich Kueffner
Lin I.-H.
Nguyen Nam
SMS Schloemann--Siemag Aktiengesellschaft
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