WO2001087518A1 - Method and system for continuously casting slabs, especially thin slabs, with comparatively high casting speeds - Google Patents

Abstract

The invention relates to a method and a system for continuously casting slabs, especially thin steel slabs, with comparatively high casting speeds, for example over 3m/min, in a chill (1) able to be driven in periodic oscillation. Said system comprises a strand guiding system (3) located underneath the chill (1) and having rollers (5,6) for guiding and extracting a strand (4). At least a few of these rollers (5,6) can be driven in a rotating motion either in pairs or individually. At least the driven roller (5) of at least one pair of rollers is driven by an overriding periodic oscillation (8) as well as by the rotating motion, whereby driving devices which can be electromechanically or hydraulically controlled are provided for said rollers (5,6).

Description

Process and plant for the continuous casting of slabs, in particular thin slabs with comparatively high casting speeds

The invention relates to a method and a plant for the continuous casting of slabs, in particular thin slabs, with comparatively high casting speeds of, for example, more than 3 m / min, using a mold that can be driven in periodic oscillation, with a roller arranged below it, for guiding and conveying out a Cast strand having strand guide, of which at least some rollers can be driven in pairs or individually in a rotational movement.

In order to convey a strand out of the mold of a continuous caster, several rollers arranged below the mold are usually driven by electric motors. The driving force of the motors must be large enough to ensure that a given, uniform strand withdrawal speed is taken into account, taking all forces into account.

The forces transmitted from the strand to the driven rollers essentially include the dead weight of the strand, the frictional force in the mold, which changes periodically in accordance with the mold oscillation, the frictional forces in the roller bearings, the frictional forces of a grid arranged below the mold - if any - that Forces for bending and bending back the strand from the vertical strand guide into the horizontal strand guide and the forces for overcoming the bulging of the strand shell between the rolls. The accuracy of the alignment of the rollers and the strand shell temperature, which varies over the length of the strand, which depends in particular on the casting speed and the intensity of the cooling of the strand in the cooling chamber below the mold, have an influence on the forces mentioned. In thin slab plants with a funnel-shaped pouring area of the mold, additional frictional forces and forces in the mold are used to deform the strand shell taking into account the up and down movement of the mold.

Comparatively large forces must also be overcome if the strand is to be reduced in thickness by adjusting the guide rollers.

The number and arrangement of drivable rollers depends on the system type and system concept. In conventional continuous casting plants with a circular arc shape and a strand thickness of about 150 to 250 mm, it has become established that several pairs of rollers with oppositely driven rollers are distributed over almost the entire arc length and the straightening area at the machine outlet, whereby a uniform distribution of the strand extraction forces over the strand length is achieved , The drives of the rollers are designed so that a specified strand withdrawal speed can be maintained with the required accuracy and without fluctuations over a range.

Continuous casting plants for producing thin strands with a thickness of up to 150 mm, in particular with a thickness between about 30 to 100 mm, are typically designed in such a way that the solidification of the strand takes place in a vertical section which is several meters long and adjoins the mold. In this section, the strand is often passed through non-driven rollers and, if necessary, over a partial length through a support grid. To further reduce the thickness, the rolls can be set up so that they squeeze the strand with the core still liquid. At the end of the vertical section, where the strand has already solidified, there are driven rollers which pull the strand out of the system and at the same time bend it. At the end of the arcuate section there are further driven rollers with which the strand is bent and conveyed out into the horizontal. Mold oscillation is an essential part of the continuous casting process of metals. It ensures the required lubricating effect of the lubricant, for example casting powder or oil, and thus prevents the strand from sticking to the mold walls. When continuously casting steel, slag has established itself as a lubricant in the mold. It is created by melting casting powder, which is applied to the bathroom mirror in such a way that it permanently covers it.

The simplest technical solution, which also characterizes the predominant state of the art, is to set a continuous casting mold into an oscillating movement with the aid of a motor-driven eccentric. This results in a sinusoidal oscillation form of the mold, the frequency and amplitude of which is predetermined by the speed of the motor and by the eccentricity of the eccentric.

If you replace the drive unit consisting of motor, gearbox and eccentric with one or more hydraulic cylinders, there is the possibility of being able to specifically influence the lubrication in the mold by non-sinusoidal mold vibrations and by changing the lifting height during the casting process.

So that the slag as a lubricant can continuously penetrate into the gap between the cast strand and the mold wall, and to prevent the lubricating film from tearing off, it is necessary to adjust the amplitude and frequency of the mold so that it periodically overtakes the strand as it moves downward.

The time component Ts of an oscillation period T, in which the overtaking process Vκokiιie> V _c takes place during the downward movement of the mold, is generally referred to as a negative strip and is: V _c π - 2 • aresine ()

2 - A - π - n Sn = 100 (%)

(D

2 - π

The meanings: Vκ _0k iiie - mold speed (m / sec),

V _c - casting speed (m / sec),

A - amplitude of the mold oscillation (m), n - oscillation frequency of the mold oscillation

(1 / sec).

The so-called healing time corresponds to this negative strip during each oscillation period,

sn

Theal

(2)

during which the lubricant is transported in the gap between the strand shell and the mold wall in the casting direction.

It is known that the negative strip, healing time, amplitude and frequency of the mold oscillation as well as their combination tailored to the present operating case are decisive for the quality of the cast product and must be adjusted to the properties of the melt to be cast and the casting powder used. The selection of the oscillation parameters is an essential component in the optimization of the continuous casting process and essentially consists in the selection of an optimal combination of amplitude and frequency. The negative strip should be within certain limits, usually between 15 and 40% of an oscillation period, so that the required values of the healing time in the range between 40 and 200 msec can be achieved. The interrelationships represented by formulas (1) and (2) indicate that, regardless of the oscillation profile of the mold, any combination of the oscillation parameters is not possible if a certain healing time is to be achieved.

State of the art for continuous casting plants with a low casting speed (0.8-1.5 m / min) are sinusoidal mold vibrations with a constant stroke and a frequency which changes proportionally with the casting speed. In this way, an equal distance between the oscillation marks is achieved regardless of the casting speed, thereby suppressing the occurrence of defects on the surface of the cast product. The disadvantage - the healing time that varies with the casting speed - can be accepted due to the small range of the casting speeds used. Lines 1 and 2 in Table 1 show typical parameters of sinusoidal oscillation curves in the specified speed range.

At high casting speeds between 3.0 and 6.0 m / min and higher, which are typical for the continuous casting of thin slabs, efforts are made to maintain the oscillation parameters known from conventional continuous casting, in particular the healing time and the distance L between the oscillation marks, This ensures constant and even lubrication with conventional continuous casting powders and prevents slag and powder particles from being drawn into the strand skin that forms. So that the distance between the oscillation marks does not exceed a critical value of approximately 20 mm, the oscillation frequency must be increased in proportion to the casting speed. With sinusoidal oscillation of the mold, the healing time is reduced considerably, as can be seen from row 3 in table 1. Line 4 of Table 1 shows that by using non-sinusoidal curves, sufficient healing time can be achieved despite the high frequencies. However, it must be taken into account that with curve courses, which deviate from the sinus shape, significantly higher accelerations of the mold have to be accepted.

If the casting speed is increased to values above 5 m / min, for example to 8 m / min as listed in line 5 of Table 1, the requirement for a maximum permissible distance between the oscillation marks L leads to frequencies which cause extremely large accelerations of the mold and with their mass of about 20 t lead to overstressing of plant parts and to unacceptable vibrations of the entire continuous casting plant. Experiments have shown that when the mold oscillates with accelerations that exceed the gravitational acceleration of 9.81 m / sec ² , normal casting operation is almost impossible due to strong system vibrations and noise.

By using mold vibrations with a lower frequency and a longer stroke, the acceleration of the mold can be reduced. The fact is used that the acceleration increases linearly with the amplitude and quadratically with the frequency. The comparison of different sinusoidal oscillation curves (Table 1, lines 5 and 6) clearly shows that acceptable values of the acceleration a (below 5 m / sec ² ) can only be achieved at frequencies n at which the distance L between the oscillation marks exceeds the critical value of approx. 20, as a result of which a uniform and error-free formation of the strand surface is no longer guaranteed.

The use of non-sinusoidal oscillation curves is automatically prohibited at these high casting speeds because they cause an increased acceleration of the mold compared to the sinus shape. Table 1

Comparison of oscillation parameters for different casting speeds

Starting from the known prior art, the object of the invention is to provide a method and a system with which it is possible to achieve comparatively high casting speeds with conventional values of the healing time and the distance between the oscillation marks, without a critical mold acceleration a of about 5 m / sec ^{2 is} exceeded.

To achieve the object is proposed in a method of the type mentioned in the preamble of claim 1 with the invention that at least the driven roller of at least one pair of rollers with two opposite rollers, of which at least one roller is driven, a superimposed oscillation in addition to the rotary drive movement performs.

Further refinements of the method according to the invention are provided in accordance with subclaims 1 to 7.

The invention is particularly advantageous if the superimposed periodic upward and downward movement of the rollers preferably takes place at the frequency of the mold oscillation. In this case, the additional up and down movement of the rollers makes the speed of the strand within the Time which corresponds to a period of oscillation of the mold, a time period greater and another time period less than the uniform strand speed. If one adjusts the phase of the additional upward and downward movement of the rollers so that the period with the highest downward speeds of the mold and the period with the lowest strand withdrawal speeds overlap, one obtains a period for the overtaking process (T _hea i + delta T), which is greater by the amount (delta T) than the healing time (T _hea i) with a uniform casting speed of the strand.

If one assumes a required value for the healing time (T _hea i), the original amplitude of the mold _vibrations can be noticeably reduced thanks to the up and down movement of the rollers. This lower amplitude corresponds to a lower mold speed. As a result, the acceleration of the mold can be reduced to uncritical values.

The method further provides that the superimposed periodic up and down movement of the rollers differs with their speed profile and / or their amplitude from the speed profile or from the amplitude of the periodic up and down movement of the mold at the same frequency. This opens up the possibility of achieving the maximum possible healing time (T _hea i) for a predetermined frequency of the mold oscillation and for predetermined limit values of the acceleration of the strand and the mold.

A further embodiment of the invention provides that for automatic bath level control in the mold, a signal which is proportional to the speed of the periodic up and down movement of the rollers is used to control the supply of the melt into the mold.

Furthermore, the method is advantageously supplemented in that the speed curve and the amplitude of the superimposed periodic upward and downward movement of the rollers are set in any assignment to the casting speed of the strand and / or the frequency of the mold oscillation. A system for carrying out the method according to the invention provides that at least the driven roller of at least one pair of rollers with two opposing rollers, of which at least one roller is driven, are assigned electromechanically or hydraulically controllable drive devices for exciting an oscillation which can be superimposed on the rotary drive movement.

It is particularly advantageous to control such a drive device with signals which are supplied by the electronic control of the oscillating device of the mold.

For a further technical development of the method according to the invention, it is provided that the rollers are connected to roller holders or roller bearings which are designed for a periodic upward and downward movement in particular. Alternatively, the invention provides that the roller brackets are attached to system parts which are designed for a periodic up and down movement.

The invention can also be used in continuous casting plants in which the thickness of the strand is reduced by employing one or more rollers of the roller guide.

The invention is to be explained below with reference to drawings and exemplary embodiments. Show it:

Fig. 1: the schematic representation of a continuous caster;

Fig. 2: the speeds and accelerations of the mold and

Strand for a mold oscillation and a periodic up and down movement of the rollers with sinusoidal speed curve for two different frequencies of the mold oscillation;

3: the course of movement for a mold oscillation and a periodic upward and downward movement of the rollers, each with a sinusoidal course of speed;

4: the movement profile for a mold oscillation with a sinusoidal speed profile and a periodic up and down movement of the rollers with a trapezoidal speed profile;

5: the movement of the strand for three successive mold vibrations for the strand speed shown in FIG. 3;

Fig. 6: The movement of the strand for three successive mold vibrations for the strand speed shown in Fig. 4.

The continuous caster shown in FIG. 1 is used in particular for casting thin slabs, for example 50 mm thick, from steel, with comparatively high casting speeds of 6 m / min or more in some cases, using a mold 1 which can be driven in periodic oscillation by a vibration drive 2 Below the mold 1 there is an initially vertical strand guide 3 with rollers 4 which do not drive the strand 4 in the area of a still liquid core in the strand 4 and at the lower end with rollers 5, 6 for conveying out the cast strand 4, which has already solidified in this area which the roller 5 is motor-driven.

This is followed by a curve guide 9 with a number of rollers 5 ', 5 ", 5'" and 6 'arranged individually and in pairs, which guide the flexible strand 4 divert from the vertical into an arcuate transport direction, after which it is bent by the straightening unit 10 in the straight direction and continued. If necessary, pairs of rollers, for example 5 '", 6', are provided with motor drives.

A typical operating case provides a casting speed of 4 m / min. According to FIG. 2, the mold oscillation has a sinusoidal speed profile 10, which results from an oscillation frequency of 260 strokes / min and an amplitude of 3.4 mm. This combination of the oscillation parameters and the casting speed results in a healing time (T _ea i) of 56.3 msec and a distance L between the oscillation marks of 15.4 mm.

If the rollers 5, 6 are set in a sinusoidal periodic upward and downward movement (8) with the frequency of the mold oscillation 2 and an amplitude of 1.7 mm, the speed profile of the strand marked 12 is obtained. This speed curve increases the healing time by 39% from (Theai) = 56.3 msec to (Theai + delta T) = 78.3 msec.

A major advantage of the invention is therefore to be seen in the fact that the periodic upward and downward movement of the rollers 5, 6 can achieve significantly longer healing times in time with the mold oscillation. As a result, the lubrication of the strand 4 by melted casting powder is noticeably improved, whereby the occurrence of defects on the strand surface is avoided. It should be particularly emphasized that a noticeable increase in the healing time, in this case by 39%, is already achieved by a comparatively small vibration amplitude of the rollers 5, 6.

Another application of the invention provides for oscillating rollers 5, 6 to reduce the amplitude of the mold oscillation to such an extent that the original healing time T _hΘa ι is maintained. For the operating case shown in FIG. 2, the oscillation of the rollers 5, 6 with an amplitude of 1.7 mm can The mold oscillation can be reduced by 48.2% from 3.4 mm to 1.76 mm without changing the healing time. The speeds 13 and the accelerations 16 of the mold 1 and their maximum values decrease in the same ratio. The maximum acceleration of the mold 1 is reduced from 2.52 m / sec ² to 1.30 m / sec ² . Such a reduction in the accelerations opens up the possibility of making the continuous casting installation lighter or using molds with a greater weight, which is advantageous, for example, when using an electromagnetic brake.

FIG. 3 shows that with a casting speed of 8 m / min and a mold oscillation with 448 strokes / min, a healing time of (T _he ai) = 28.3 msec and a distance between the oscillation marks of L = 17.8 mm are achieved can, the maximum acceleration of the mold 1 and the strand being 4.0 m / sec ² and 5.0 m / sec ^2, respectively. Without the oscillation of the driver rollers 5, 6, the mentioned values of the healing time and the distance L between the oscillation marks could only be achieved with mold speeds at which the maximum acceleration of the mold is 7.9 m / sec ² . Such an acceleration would lead to such high loads on the system parts due to the large mass of a continuous casting mold of approximately 20 t that such a casting operation could not be carried out in practice.

In the example shown, the amplitude of the oscillation of the rollers 5, 6 is 1, 8 mm. The maximum acceleration of 5.0 m / sec ² given to the strand is not critical if, for example, a strand weight of 5 t is assumed, which results from the strand thickness 50 mm, the strand width 1300 mm and the strand length 10,000 mm.

The example in FIG. 3 illustrates that the periodic upward and downward movement of the rolls according to the invention opens up the possibility of carrying out the continuous casting at very high casting speeds. FIG. 4 shows that by using non-sinusoidal waveforms of the periodic upward and downward movement of the rollers 5, 6, the advantages already explained with reference to FIG. 3 can be supplemented by increasing the healing time. If the sinusoidal speed profile of the rollers 5, 6 shown in FIG. 3 is replaced by the trapezoidal speed profile shown in FIG. 4, the healing time increases from 28.3 msec to 39.4 msec, ie by 39%.

Another advantage of the invention is therefore to be seen in the fact that sufficiently long healing times can be achieved by the periodic upward and downward movement of the rollers 5, 6.

A particular advantage of the invention is that the oscillation of the rollers 5, 6 does not lead to an up and down movement of the strand. This is evidenced by the movement profiles of the line shown in FIGS. 5 and 6, which can be attributed to the speed profiles of the line shown in FIGS. 3 and 4. In both cases, the strand executes an uninterrupted downward movement, which slows down and accelerates in time with the oscillation of the mold or roller. If one also takes into account the high oscillation frequency of the mold 1 or the rollers 5, 6, a negative influence of the oscillation of the rollers on the stability of the bath level is excluded.

Claims

claims 1. A process for the continuous casting of slabs, in particular thin slabs of steel, with comparatively high casting speeds of, for example, more than 3 m / min, using a mold (1) which can be driven in periodic oscillation, with a roller (5, 6 ) for guiding and conveying a cast strand (4) having strand guide (3), of which at least some rollers (5, 6) can be driven in pairs or individually in a rotary movement, characterized in that at least the driven roller (5) has at least one pair of rollers two opposite rollers (5, 6), of which at least one roller (5) is driven, in addition to the rotary drive movement, executes a superimposed oscillation (8). 2. The method according to claim 1, characterized in that the amplitude and / or frequency of the superimposed periodic oscillation of the individually or in pairs driven rollers (5, 6) match the amplitude and frequency of the mold oscillation (1). 3. The method according to claim 1 or 2, characterized in that ^" the speed profile of the mold oscillation sinusoidal, and the speed profile of the oscillating roller pair (5, 6) is in particular trapezoidal. 4. The method according to one or more of claims 1 to 4, characterized in that the oscillation parameters such as speed curve and / or amplitude of the rollers (5, 6) are set in any assignment to the casting speed of the strand (4) and / or to the frequency of the mold oscillation. 5. The method according to claim 1 or 2, characterized in that the amplitude of the oscillating driven rollers (5, 6), at the same frequency as the mold oscillation, differs from their amplitude. 6. The method according to one or more of claims 1 to 5, characterized in that the superimposed oscillation of the rollers (5, 6) is started only after exceeding a predetermined casting speed, or is ended after falling below. 7. The method according to any one of claims 1 to 6, characterized in that a signal is used for automatic bath level control for the supply of melt in the mold (1) which is proportional to the speed of the periodic oscillatory movement of the rollers (5, 6). 8. Plant for the continuous casting of slabs, in particular thin slabs of steel, with comparatively high casting speeds of, for example, more than 3 m / min, using a mold (1) which can be driven in periodic oscillation, with a roller (5, 6) arranged below it for guiding and conveying out a strand guide (3) having a casting strand (4), in particular for carrying out the method according to the invention, characterized in that that at least the driven roller (5) of at least one pair of rollers with two opposing rollers (5, 6), of which at least one roller (5) is driven, are assigned electromechanically or hydraulically controllable drive devices (8) for exciting an oscillation which can be superimposed on the drive rotary movement. 9. Plant according to claim 8, characterized in that the drive device (8) for controlling the rollers (5, 6; 5 ', 6') has a signal connection with the electronic control of the mold oscillation. 10. System according to claim 8 or 9, characterized in that the oscillatable rollers (5, 6) are connected to system parts such as brackets or bearings, which are designed in operative connection with their oscillation drives (8) for a periodic upward and downward movement in particular. 11. Plant according to one or more of claims 8 to 10, characterized in that driven or non-driven roller pairs (5, 6) of the strand guide (3), on the strand thickness reducing roller gaps are adjustable.