WO2012107143A1

WO2012107143A1 - Method for regulating a temperature of a strand by positioning a movable cooling nozzle in a strand guide of a strand casting system

Info

Publication number: WO2012107143A1
Application number: PCT/EP2011/073939
Authority: WO
Inventors: Kurt Dittenberger; Paul Pennerstorfer; Franz Ramstorfer; Philipp Wieser
Original assignee: Siemens Vai Metals Technologies Gmbh
Priority date: 2011-02-07
Filing date: 2011-12-23
Publication date: 2012-08-16
Also published as: DE112011104849A5; DE112011104849B4; KR20140004742A; CN103347626A; EP2673099A1; EP2673099B1; KR101806819B1; CN103347626B

Abstract

The invention relates to a method for regulating a temperature (16), or a temperature profile (21), of a strand (1) by positioning a movable cooling nozzle (4) in a strand guide of a strand casting system. The aim of the invention is to optimize a regulating method by positioning a movable cooling nozzle (4) so that as precise a temperature control of the strand (1) as possible can be achieved, even under starkly different operational conditions in the strand casting system. Said aim is achieved by a method comprising the following method steps: determining at least one temperature value T (16) of the strand (1) in a direction that is transverse to the casting direction (9) of the strand (1); determining a control error e (14) by subtracting the temperature value T (16) from a target temperature T_Soll (17) of the strand (1), that is, e = T _Soll -T; calculating a control variable r (15) based on the control error e (14) and applying a control law; and positioning the cooling nozzle (4) in a direction that is transverse to the casting direction (9) of the strand (1) based on the control variable r (15) so that the control error e (14) is minimized.

Description

description

Method for controlling a temperature of a strand by positioning a movable cooling nozzle in one

Strand guide of a continuous casting plant

The present invention relates on the one hand to a method for controlling a temperature of a strand, preferably a steel slab, by positioning a movable cooling nozzle in a strand guide of a continuous casting plant, and on the other hand, a method for controlling a

Temperature profile of a strand, preferably one

Steel slab, by positioning a movable

Cooling nozzle in a strand guide of a continuous casting plant.

It has long been known, the cooling nozzles of a

Continuous casting plant depending on the strand width and the expected operating conditions, in particular the

Casting speed, the strand cooling and the steel grade, adjust. A disadvantage of a fixed setting of

Cooling nozzles is that changing the operating conditions of the continuous casting plant can result in overcooling (i.e., excessive cooling) or undercooling (i.e., overheating) of the strand, particularly the strand edges, thereby significantly degrading the quality of the strand. Travelable cooling nozzles can at least partially overcome these problems.

Movable cooling nozzles are used in continuous casting plants,

in particular in slab casters, used in particular the edge temperature or the temperature near the edges of the strand as a function of the strand width

(See Fig la and lb) and the operating conditions to influence targeted. So it is already known, a cooling nozzle a certain value xl or x2 from the edge of the strand (see FIG. 2) as a function of the machine position, the strand age or the shell thickness of the strand (see FIG. 3) in a direction transverse to the casting direction of the strand

to prevent in particular the overcooling of the edge or the edge near the edge of the strand.

Movable cooling nozzles are used in continuous casting plants

typically used in the casting arc or in the straightening zone; However, it is also known, especially in slow-casting plants, to use movable cooling nozzles in the bending zone or the Rieht- or reverse bending zone. Is the

Strand temperature (in particular the edge temperature) in an unfavorable temperature range - in particular in the region of the ductility low (for conventional steel grades between about 750 ° C and 600 ° C) of the cast steel grade, so the steel behaves very brittle and it can edge cracks when bending or Straightening the strand occur. FIG. 5 shows that cooling nozzles should not be moved by an arbitrary value x (positive values indicate a displacement of the cooling nozzle in the direction of the center of the strand), since otherwise there would be an increase in temperature in the vicinity of the edge of the strand

Surface temperature of the strand in the strand center - and this would be associated with thermal stresses in the near-edge region - would come. The adjustment of the position of the cooling nozzles in the course of the optimization of the edge temperatures of the strand by temperature measurements or by the evaluation of

Edge breaks through micrographs, is very time consuming and always allows only a compromise within a certain range of casting speed. The positioning of the cooling nozzles due to the shell thickness sets a

Improvement is, however, so far the actual value to be optimized - namely, the edge temperature or the temperature in the near-edge region of the strand - not

be managed. The object of the invention is to overcome the disadvantages of the prior art and to provide a method for controlling a temperature or a temperature profile of a strand by the optimized positioning of a movable cooling nozzle, so that even with very different

Operating conditions of the continuous casting the most accurate temperature control of the strand is achieved. Furthermore, overcooling and supercooling of the strand, in particular of the strand edges, should be prevented to the greatest extent possible.

This object is achieved by a method of the aforementioned type, comprising the following method steps:

Determining at least one temperature value T of

Strand in a direction transverse to the casting direction of the strand;

Determination of a control error e by the subtraction of the temperature value T from a setpoint temperature T _So ii of the strand, specifically e = T _setpoint T;

- Calculation of a controlled variable r as a function of

Rule error e with the help of a rule law; and

- Positioning of the cooling nozzle in the direction transverse to the casting direction of the strand as a function of the controlled variable r, so that the control error e is minimized.

In this case, the controlled variable r is supplied to an actuator, which moves the cooling nozzle as a function of the controlled variable r, so that the control error e is minimized. In slab plants, it is expedient to carry out the determination of a temperature value as well as the positioning of the cooling nozzle in the width direction transversely to the casting direction of the strand. This results from the fact that with a slab, the width is much larger than the thickness, potentially resulting in much larger temperature differences between strand center and strand edge can result.

According to an advantageous embodiment, the form takes place

Determination of the temperature value T at a strand edge of the strand, since the strand edges are most sensitive to under- or over-cooling. The temperature control of the strand edges is particularly important for the quality of the strand.

Furthermore, the above task is also by a

Method of the type mentioned above, comprising the following method steps:

Determining a temperature profile T of the strand in a direction transverse to the casting direction of the strand;

- Determination of a temperature deviation profile ΔΤ by the subtraction of the temperature profile T from a target temperature profile Tsoii of the strand, specifically ΔΤ = Τ _8ο11 -Τ;

Calculation of a control error e by applying a scalar cost function f to the temperature deviation profile ΔΤ, specifically e = f (ΔΤ);

- calculation of a controlled variable r as a function of the control error e with the aid of a control law; and

In this case, the controlled variable r is supplied to an actuator which moves the cooling nozzle as a function of the controlled variable r, so that the control error e is minimized. The regulation of a

Temperature profile is particularly advantageous because not only a single temperature of the strand can be controlled by the positioning of a movable cooling nozzle, but it can actually be on a temperature gradient across the

Casting of the strand are regulated out. Under a temperature profile is used in this application e.g. one

Temperature vector understood that at least two

Temperature values, each having different locations (typically in the width direction of the strand)

assigned. It is also in this process

Particularly useful in slab plants, the determination of a temperature profile and the positioning of the

Make cooling nozzle in the width direction transverse to the casting direction of the strand. Scalar cost functions that convert a vectorial input (here ΔΤ) into a scalar variable (here the control error e), i. e = / (ΔΤ) are known to those skilled in the art e.g. from the field of optimization, see e.g. the lecture script G. Greiner et al. "Optimization III, Linear Optimization", FAU Erlangen-Nuremberg, summer semester 2008.

Both in the method for controlling a temperature and in the method for controlling a temperature profile, the control law can be either a linear behavior (e.g.

classical controller that describes the input-output behavior as a transfer function, or a state controller that describes the input-output behavior in the state space; see also claims 9 and 10), for example that of a simple P, PI or PID controller or even a linear state controller; Of course, the law of control can also be a nonlinear behavior (see claim 11)

exhibit. Of course, one skilled in the art knows how to determine the law of regulation (e.g.

Frequency characteristic method, see Gausch et al. : Digital control loops, Institute of Control Engineering, Graz University of Technology, 1991), so that the control error e is minimized. According to an advantageous embodiment, the form takes place

Determining the temperature value T or the temperature profile by observing a state observer, having a process model with a thermodynamic

Heat equation for the strand. Further details of a possible process model may e.g. WO 01/91943 AI be removed. The state observer design allows a variety of different temperatures to be determined without even measuring one of them.

In addition, the already existing in a continuous casting process models in a simple way to

Temperature control of the strand can be used. In general, state observers are of course familiar to the person skilled in the art, see e.g. Lutz, Wendt: Taschenbuch der Regelungstechnik, 7.

Edition, publisher Harri Deutsch.

Alternatively, it is of course also possible that the determination of the temperature value T or the temperature profile is carried out by measuring at least one temperature of the strand. This variant may have a higher accuracy than the observation of a state observer, but this is offset by a higher cost of measuring means.

When measuring the temperature, it is advantageous that this can be done by evaluating the heat radiation, e.g. by means of a pyrometer, the strand is done.

Especially for the so-called subcritical straightening of a strand, it is expedient that the scalar cost function f calculates the maximum norm. This will be the maximum

occurring strand temperature regulated. According to an alternative embodiment, the calculates

Cost function f the two-norm (also known as Euclidean norm), whereby overcooling as well as subcooling of the strand are weighted equally.

According to a simple embodiment, the control law has a linear control behavior, preferably that of a P, PI, PID, H ₂ , H or a state controller. The behavior and merits of classical controllers, for example, the input-output behavior as a transfer function

As well as state controllers that describe the input-output behavior in the state domain, are well known to those skilled in the art, see, e.g. Paperback the

Control technology.

According to an alternative embodiment, the control law has a non-linear control behavior, e.g. the one

Two-point, three-point controller or higher

Controller. The two-position controller is special

in particular because of the switching on and off (possibly also by the pulsed switching on and off, for example by a PWM modulation)

Coolant flow can be done to the cooling nozzle.

It is advantageous to perform the method in real time.

For the computing time, it is advantageous that the

Condition observers a strand half on the one hand one

Symmetry axis of the strand observed. Preferably, the axis of symmetry extends through the width direction of the slab.

Further advantages and features of the present invention will not become apparent from the following description limiting embodiments, reference being made to the following figures, which show the following:

Fig la an arrangement of two cooling nozzles at a

Steel slab with a first width;

Fig lb an arrangement of two cooling nozzles at a

Steel slab with a second width;

2 shows the positioning of a cooling nozzle at a certain distance from the strand edge;

3 shows the positioning of a cooling nozzle at a certain distance from the strand shell;

4 shows a representation of the thickness of the strand shell of a

Slab of the casting direction;

5 is a diagram of surface temperatures over the

Distance of a cooling nozzle from the strand edge;

6 shows a representation of the maximum temperature, the

Temperature in the center and the edge temperature above the

Distance of a cooling nozzle from the strand edge;

7 shows a schematic representation of a first control loop for carrying out the method according to the invention;

8 is a schematic representation in the determination of a temperature value of the strand by measurement and the

Positioning a movable cooling nozzle;

9 shows a representation of a control error over the distance of a cooling nozzle from the strand edge;

10 is a schematic representation of a second

Control circuit for carrying out the inventive

Method;

11 shows a discretization of a temperature profile of a slab in the width direction; and

12 and 13 each show a schematic representation of a third and a fourth control loop for carrying out the

inventive method. 1 a shows a steel slab 1 having a first width 3 which is cooled in a direction transverse to the casting direction of the slab by two movable cooling nozzles 4. The method of the cooling nozzles 4 takes place in a direction of travel 5. Each cooling nozzle has an injection pattern 6, which is a function of the pressure of the cooling fluid and the distance of the cooling nozzle 4 of the

Surface of the slab has. FIG. 1b shows a steel slab 1 which is narrower than FIG. 1a and which in turn is cooled by two cooling nozzles 4. The directional arrows 5 indicate the direction of travel of the cooling nozzles at a

Width change of the steel slab. Both figures la and lb have in common that the cooling nozzles 5 are assigned to the edge region of the slab (so-called "margin"). From the figures it can be seen that at the same water pressure the maximum of the water volume distribution 7 is higher for a narrower slab than for a wider slab.

FIG. 2 likewise shows a steel slab 1, which is cooled by a total of three cooling nozzles 4. A cooling nozzle is assigned to the center and lies on the symmetry axis 2 of the slab 1.

Two further cooling nozzles are associated with the edge region, these cooling nozzles 4 are designed as movable cooling nozzles. The movable in the direction of travel 5 cooling nozzle 4 has in the position shown a distance x2 to the strand edge 10. The distance xl indicates the distance in the horizontal direction from the outer boundary of the spray pattern 6 to the strand edge 10. A positive value of x1 or x2 corresponds to a displacement of the cooling nozzle in the direction of the center of the strand. As stated in the introduction, it is known, a movable cooling nozzle 4 dynamically depending on the

Machine position or the strand age of the strand edge 10 to make. Figure 3 also shows a known dynamic employment of a movable cooling nozzle 4, wherein xl indicates a horizontal distance of the outer boundary of the spray pattern 6 of the cooling nozzle 4 to the strand shell 8 of the slab 1 and x2 a distance of the center axis of the cooling nozzle 4 to the strand shell 8 of the slab. A top view of a strand 1 incl

Formation of the strand shell 8 in dependence of

Machine position is shown in FIG.

FIG. 5 shows the surface temperatures T on the

Broad side of a strand in the width direction of the slab for different distances xl between the outer

Limiting the spray pattern of the cooling nozzle 4 and the strand edge 10. Herein it can be seen that a method of the cooling nozzle in the direction of the slab center admittedly increased

Edge temperature leads, but from a certain distance - in this case, from about 50 mm - the temperature of the edge remains constant even with larger xl, so that by another

Moving the cooling nozzle the edge temperature can not be increased. A further method of the cooling nozzle in the direction of the slab center merely leads to the formation of a so-called "hot strip" in the region near the edge

Distances xl of the cooling nozzle 4 from the strand edge 10th

FIG. 7 shows a schematic control diagram of a first control loop for carrying out the invention

Process. In this very simple case, that is

is particularly suitable for the so-called "subcritical straightening" of a strand is only a single

Surface temperature at the strand edge 10 of a

Pyrometer 11 determines and a control device 12th fed. The control device 12 calculates a control error 14, according to e = T _SoU - T, and calculates under

With the aid of a PID control set a control variable 15. Although in the schematic representation of FIG. 7, the

Calculation of the control error 14 is shown outside of the control device 12, the calculation of e can both

inside and outside (e.g., by an analog subtractor) of the controller 12; this has no influence on the method according to the invention. In the controlled system 13, the cooling nozzle 4 by a not

shown, typically electrical or hydraulic actuator as a function of the control variable 15 so moved that the control error 14 is minimized. If the cooling nozzle is e.g. initially at xl = 100mm and thus the control path initially has a control error e = -55 ° C, so the actuator moves the cooling nozzle with a constant travel speed of eg. 5mm / s in the negative direction, so in the next

Sampling step (for example, after ls) xl = 95mm. FIG. 9 shows the control error e (reference symbol 14 in FIG. 7) over the distance x1. According to FIG. 9, the control error e is minimal at the position x1 = -10 mm, with the cooling nozzle 4 reaching this position after approximately 22 seconds. At xl = -10mm, e «0, so that a controlled variable r« 0 is also set. Consequently

the actuator remains at the position xl = -10mm, which minimizes the control error e.

According to an alternative embodiment, the actuator moves in dependence on the controlled variable r with variable

Speed, i.A. it is expedient to limit the maximum travel speeds of the actuator in the positive and negative directions.

FIG. 10 shows a second embodiment of the control loop for carrying out the method according to the invention, which works without a measurement of the temperature of the strand. Specifically, the surface temperature of the strand edge is calculated by a so-called state observer 18, which implements the thermodynamic heat equation for the strand in a process model. Specifically, in the process model for the metal strand, a three-dimensional, nonlinear and transient heat equation in enthalpy formulation is solved taking into account temperature-dependent density changes; for details, reference is made to WO 2009/141205 AI. The resulting surface temperature of the

The strand edge is fed to the controller 12 via the state feedback. Although it is advantageous, a

to use three-dimensional formulation of the heat equation; In many cases, however, a two-dimensional formulation is sufficient. In a three-dimensional formulation (including a discretization in the longitudinal, width and thickness direction of the strand) of the heat equation, the regulation of the temperature can not only one

Surface temperature of the strand, but even temperatures in the interior (i.e., having a thicknesswise distance to the strand surface) of the strand are taken into account.

Figure 11 shows a local discretization of the strand 1 in the width direction, wherein the temperature profile of the

Surface temperature of the strand T (y) through 2N + 1

Support points, where N is an element of natural numbers, is discretized.

FIG. 12 shows how the method according to the invention can also be used for the regulation of temperature profiles. Specifically, the control loop is a temperature profile 19, eg in vector form Solll SollN supplied, whereupon the

Temperature deviation profile 20 by the element

Subtraction of ΔΤ = Τ ₅ , _{ο //} -Τ is calculated. The observed

State variables T for the temperature of the strand result from the evaluation of a state observer 18, which contains a process model with a two-dimensional formulation of the heat conduction equation. Based on the

Temperature deviation profile ΔΤ = (Δ7 [... ^) the scalar control deviation 14 is calculated by applying a scalar cost function 22 to the temperature deviation profile ΔΤ, the cost function being the Euclidean norm of

Evaluates ΔΤ,

10, the control deviation 14 is a digital controller 12, specifically a PI controller,

supplied, which calculates the controlled variable 15. The controlled variable is supplied on the one hand to an actuator of the cooling nozzle 4, which moves the cooling nozzle 4 in the width direction of the steel slab 1. On the other hand, the position 23 of the traversed cooling nozzle 4 is also fed back to the status observer 18, which underlies the temperature distribution on the strand 1

Recalculation of the lost cooling nozzle recalculated. The resulting temperatures T are in turn fed to the control loop, so that the total deviation

between the target temperature 19 and the observed

Temperature 21 of the strand 1 is minimized.

FIG. 13 shows an alternative embodiment of FIG. 12, which does not require a status observer 18. In this case, the temperature profile 21 of the strand 1 is determined by the measurement of the surface temperatures of the strand, for example by a pyrometer 11, which is designed to be movable in the width direction of the strand 1. A possible Discretization of the temperature profile 21 is shown in FIG.

Reference sign list

1 steel slab

2 symmetry axis

3 slab width

4 cooling nozzle

5 Travel direction of the cooling nozzle

6 spray pattern

7 Water volume distribution

8 strand shell

9 casting direction

10 strand edge

11 pyrometers

12 control device

13 controlled system

14 control error e

15 controlled variable r

16 temperature value T

17 target temperature T _So ii

18 state observers

19 Target temperature profile T _So n

20 temperature deviation profile ΔΤ

21 Temperature profile T

22 cost function

23 Travel path of the cooling nozzle

T temperature

xl Distance in the horizontal direction to the outer boundary of the spray pattern

x2 Distance in horizontal direction to the cooling nozzle

Claims

claims

1. A method for controlling a temperature of a strand (1), preferably a steel slab (1), by positioning a movable cooling nozzle (4) in a strand guide of a continuous casting plant, comprising the following process steps:

- determining at least one temperature value T (16) of the strand (1) in a direction transverse to the casting direction (9) of the strand (1);

- Determination of a control error e (14) by the

Subtracting the temperature value T (16) from a setpoint temperature s _o n (17) of the strand (1), specifically e = T _setpoint T;

- calculation of a controlled variable r (15) as a function of the control error e (14) with the aid of a control law; and

- Positioning of the cooling nozzle (4) in the direction transverse to the casting direction (9) of the strand (1) in dependence of

Controlled variable r (15), so that the control error e (14) is minimized.

2. The method according to claim 1, characterized in that the determination of the temperature value T (16) at a

Strand edge (10) of the strand (1) takes place.

3. Method for controlling a temperature profile of a

Strand (1), preferably a steel slab, through the

Positioning a movable cooling nozzle (4) in one

Strand guide of a continuous casting plant, with the following

Process steps:

- Determining a temperature profile T (21) of the strand

(1) in a direction transverse to the casting direction (9) of the strand

(l); - Determining a temperature deviation profile ΔΤ (20) by the subtraction of the temperature profile T (21) of a target temperature profile T _So n (19) of the strand (1), specifically

ΔΤ = T _M - T;

Calculation of a control error e (14) by applying a scalar cost function f (22) to the

Temperature deviation profile ΔΤ (20), specifically e = f (\ T);

- Positioning of the cooling nozzle (4) in the direction transverse to the casting direction (9) in dependence of the controlled variable r (15), so that the control error e (14) is minimized.

4. Method according to one of claims 1 to 3, characterized in that the determination of the temperature value T (16) or the temperature profile T (21) by the observation of a state observer (18), including a process model with a thermodynamic heat equation for the strand ( 1).

5. The method according to any one of claims 1 to 3, characterized in that the determination of the temperature value T (16) or the temperature profile T (21) by measurement

at least one temperature of the strand (1).

6. The method according to claim 5, characterized in that the measurement of the temperature by evaluation of

Heat radiation of the strand (1) takes place.

7. The method according to claim 3, characterized in that the cost function f (22) calculates the maximum norm

e = / (AT) = || ΔΤ ||

8. The method according to claim 3, characterized in that the cost function f (22) calculates the two nuclear standard

e = / (AT) = || ΔΤ || ₂

9. The method according to any one of the preceding claims, characterized in that the control law is a linear

Has control behavior.

10. The method according to claim 9, characterized in that the control law is a characteristic of a P, PI, PID, H2, H «. or a state controller.

11. The method according to any one of claims 1 to 8, characterized in that the control law is a nonlinear

Has control behavior.

12. The method according to any one of the preceding claims,

characterized in that the method is carried out in real time.

13. The method according to claim 4, characterized in that the state observer (18) observes a strand half on the one hand an axis of symmetry (2) of the strand (1).