Metal founding – Process – Shaping liquid metal against a forming surface
Reexamination Certificate
1999-12-21
2002-04-02
Elve, M. Alexandra (Department: 1722)
Metal founding
Process
Shaping liquid metal against a forming surface
C164S416000
Reexamination Certificate
active
06363998
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention pertains to a process for producing the oscillation of a mold in a continuous casting machine, especially with the use of a hydraulically driven lifting device.
2. Description of the Related Art
Mold oscillation is an essential component of the continuous casting process of metals. It ensures the necessary lubricating action of the lubricant such as casting flux or oil and thus prevents the strand from sticking to the walls of the mold. For the continuous casting of steel, slag has become the accepted lubricant in the mold. It is formed by the melting of a casting flux, which is added to the surface of the steel in the mold in such a way that it forms a permanent cover.
The simplest technical solution, which simultaneously characterizes the predominant state of the art, consists in using a motor-driven cam to cause the continuous casting mold to oscillate. As a result, the mold oscillates sinusoidally and the frequency and amplitude can be specified in each case by adjusting the rpm's of the motor and the eccentricity of the cam.
So that the slag, acting as a lubricant, can penetrate continuously into the gap between the cast strand and the mold wall, and in order to prevent the film of lubricant from being pulled off, it is necessary to adjust the amplitude and frequency of the mold oscillations in such a way that, in the downward phase of its movement, the mold periodically overtakes the strand.
The time component of the oscillation period T during which the overtaking process V
mold
>V
c
occurs during the downward movement of the mold is usually referred as “negative strip” and is described as:
S
n
=
π
-
2
*
arcsin
(
V
c
2
*
A
*
π
*
n
)
2
*
π
*
100
(
%
)
(
1
)
where:
V
mold
=the mold speed, m/s;
V
c
=the casting rate, m/s;
A=the amplitude of the mold oscillations, m; and
n=the frequency of the mold oscillations, 1/s. Corresponding to this negative strip is the so-called “heal time” T
heal
:
T
heal
=S
n
of each oscillation period, during which the lubricant is able to penetrate into the gap between the strand shell and the mold wall.
It is known that the negative strip, the heal time, the amplitude, and the frequency of the mold oscillations as well as the way in which the combination of the these variables is adjusted to suit the operating case in question determine the quality of the cast product and must be adapted to the properties of the heat to be cast and the selected casting flux. The selection of the oscillation parameters is an essential component of the optimization of the continuous casting process and consists essentially in the selection of an optimum combination of amplitude and frequency, such that the negative strip lies within certain limits, usually in the range of 15-40%.
The relationships described by Formulas (1) and (2) indicate that, when the mold oscillates sinusoidally, the range within which the combinations of oscillation parameters can be selected is limited. The idea of having the mold oscillate in a nonsinusoidal manner is based on the effort to separate the oscillation parameters from each other, so that the processes which occur in the continuous casting mold can be influenced more effectively.
DE 37 04 793 C2 describes a lifting device with two camshafts, which can be rotated by a drive and which are connected either to a lifting platform on which the continuous casting mold is mounted or directly to the mold itself. At least one cardan shaft is inserted into the connection between the rotary drive and the camshafts; this cardan shaft satisfies at least one of the two following conditions:
(a) the head of the joint facing away from the camshaft is mounted in such a way that its bearing can be changed; and
(b) the heads of the joints are mounted so that they can be rotated with respect to each other.
The mold is caused to oscillate in a nonsinusoidal manner by the intentional use of the cardan error which occurs when the cardan shaft is not properly aligned between the shafts. By changing the height and lateral displacement of the rotary drive, the mold can be made to perform various types of nonsinusoidal motion.
It can also be derived from the specification of DE 37 04 793 C2 that, first, in order to obtain a strand with a surface as free as possible of chatter marks, it is necessary to keep the negative strip as small as possible. Second, it is said to be favorable for the strand withdrawal rate curve to intersect the mold speed curve in the fast descent phase. This means that the time during which the continuous casting mold and the strand are both moving in the same direction is short. As a result, however, a long negative strip is created, which is in turn unfavorable because of the formation of chatter marks.
EP 0,121,622 B1 describes a process for continuous casting with the use of a mold supported in a frame, the mold being made to oscillate by an electrohydraulic servo device. The oscillation-producing device is operated according to a preselected oscillation amplitude signal from a function generator at a frequency which is higher than the natural frequency of the oscillation device.
EP 0,618,023 A1 discloses a process for continuous casting in which a mold with comparatively long side walls and comparatively narrow transverse walls is used. Simultaneously with the oscillation of the mold, the side walls of the mold are moved away from the cast strand by a short distance in the transverse direction during the time phase of each oscillation in which the difference between the mold speed and the strand withdrawal rate exceeds a predetermined value. During the rest of the time, i.e., the time during which the strand and the mold are moving at approximately the same speed, the side walls are moved back again toward the strand. As a result of the alternating expansion and contraction of the mold, it is said that the tensile and compressive forces acting on the strand shell are reduced. The depth of the oscillation marks is thus decreased, and there is less liquation in the groove.
EP 0,618,023 A1 also proposes mold oscillation without an overtaking phase. In this case, the slag is allowed to penetrate into the gap between the strand and the mold wall as a result of the alternating widening and constricting of the mold.
An essential feature of the known course of the sinusoidal and nonsinusoidal speed curves is that, at a given oscillation frequency and amplitude, the course of the mold speed and distance curves is always the same during the time between two successive overtakings of the strand by the downward-moving mold.
For the thin-slab process with slab thicknesses of less than 100 mm, casting rates of more than 4 m/min are normal. According to Formula (1), the lifting frequencies of the mold at which the conventional values for negative strip are obtained are correspondingly high, i.e., 400-450 strokes/min. Because of the short periods of 0.13-0.15 s per oscillation at these high frequencies, deviations in the oscillation curve from a sine wave have hardly any effect on lubrication behavior or on the formation of the casting shell in the mold. It is therefore impossible in this way to achieve any improvement in the surface quality of the cast product. Test castings have confirmed this.
The disadvantage of the known nonsinusoidal curves is therefore to be seen in the fact that they are able to affect the behavior of the lubricant and the casting shell formation process in such a way as to improve the quality of the cast product only in the range of low casting rates and mold oscillation frequencies. When continuous casting is carried out at high strand withdrawal rates and high mold oscillation frequencies, however, such as those used in the thin-slab process, for example, it is therefore necessary to adopt a new approach.
SUMMARY OF THE INVENTION
Proceeding from this state of the art, the invention is based on the task of creating an oscillatory motion of the continuous casting mold whi
Capotosti Romeo
Cristallini Alessandro
Sucker Jürgen
Weyer Axel
Elve M. Alexandra
Friedrich Kueffner
SMS Schloemann--Siemag Aktiengesellschaft
Tran Len
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