EP1448330B1 - Method for continuous casting - Google Patents

Abstract

In a method for the continuous casting of a thin metal strip according to the two-roll method, metal melt is cast into a casting gap formed by two casting rolls of the thickness of the metal strip to be cast, under formation of a melting bath. In order to form a particular texture within the cast metal strip and/or to influence the geometry of the metal strip, continuous casting is carried out by an on-line calculation based upon an arithmetic model describing the formation of the particular texture of the metal and/or the formation of the geometry of the metal strip, wherein variables of the continuous casting method affecting the formation of the texture and/or the geometry are adjusted in an on-line dynamic fashion, i.e. while casting takes place.

Description

The invention relates to a method for casting casting a thin metal strip in the two-roll process, in particular a steel strip, preferably having a thickness less than 10 mm, wherein molten metal is poured into one of two casting rolls in the thickness of the metal strip to be cast casting gap to form a molten bath.

Processes of this type are described in WO 95/15233 and EP-B1 0 813 700 and in AT-B 408.198. The first two documents relate to process models based control methods for the two-roll casting, but have the disadvantage that only when the control variables of required actual values deviate corrective action, so that initially more or less large deviations from the desired state of the metal strip, e.g. in thickness, microstructure, etc., must be accepted, even if subsequently a correction of the process model is made, as described in EP-B 1 0 813 700.

From JP-A-60 027 458 it is known to control the geometry of the tape by measuring the thickness at the center as well as at the edges during operation. In order to influence the geometry, the crowning of the rolls is changed.

EP-A-0 813 700 A relates to the use of a computer model for controlling and adjusting a strip caster. The calculation model taught here demonstrates learning ability, knowledge gained through integration into the ongoing process. Solidification and Seigerwagsverhalten are not addressed directly here. In contrast to the invention, according to which online data are taken and processed, largely off-line calculations are carried out according to EP-A-0 813 700 A.

The invention aims to avoid these disadvantages and difficulties and has as its object to provide a continuous casting of the type described above, which makes it possible for the metal strip compliance with predetermined quality characteristics such as in particular the formation of a desired microstructure of the metal or the assurance of a particular To allow geometry, u.zw. for metals of different chemical composition, i. for a large number of steel grades or steel grades to be cast.

In particular, the invention has the task of avoiding deviations in the quality of the metal strip from the outset, u.zw. by making it possible to intervene in production stages in which an actual value of the metal strip which determines the quality is not yet readily recognizable or can not be ascertained directly.

This object is achieved in that the formation of a certain microstructure in the cast metal strip, the continuous casting is carried out under on-line calculation under Zugnmdelegung a the formation of the particular microstructure of the metal descriptive computing model, the microstructure influencing variables influencing the continuous casting on-line dynamically, ie during ongoing casting, are set.

Preferably, the structuring of the surface of the casting rolls is detected, preferably detected on-line, and integrated into the calculation model, taking into account the consequent solidification and segregation conditions, in particular in the primary solidification.

In the strip casting process, the structure of the casting roll surfaces forms an important factor in solidification. This structure is only replicated to a certain extent by the liquid metal, i. Depending on the structure of the surface of the casting rolls in certain surface areas to a stronger and in other surface areas to a delayed solidification. According to the invention, the structuring of the surface of the casting rolls is preferably detected, preferably detected on-line, and integrated into the calculation model, taking into account the consequent solidification and segregation conditions, in particular in primary solidification.

For the solidification of the metal on the surfaces of the casting rolls, it is essential to condition these surfaces, such as by cleaning, spraying, coating, in particular by purging with gas or with gas mixtures. This gas or these gas mixtures determine the heat transfer from the melt or already solidified metal to the casting rolls, and there are therefore according to a preferred embodiment, the chemical composition of the gas or the gas mixture and the amount and optionally the distribution over the length of Gießwalzen detected, preferably detected on-line, and in the calculation model, taking into account the resulting solidification and Seigerungsbedingungen, especially in the primary solidification integiert.

In this case, according to a preferred embodiment with the computer model thermodynamic changes in state of the entire metal strip, such as changes in Temperature, by dissolving a heat equation and solving a phase conversion kinetics describing equation or equation systems constantly counted and the temperature setting of the metal strip and optionally the casting rolls is set depending on the calculated value of at least one of the thermodynamic state variables, wherein for the simulation, the thickness of the metal strip , the chemical analysis of the metal and the casting speed are taken into account, the values of which are preferably measured repeatedly during casting, in particular the thickness concerning constantly measured.

By coupling the calculation of the temperature of the strand with the mathematical model according to the invention, which involves the formation of a specific time- and temperature-dependent structure of the metal, it is possible to use the variables of the continuous casting process, which influence the continuous casting, the chemical analysis of the metal and the local Adjust the temperature history of the strand. As a result, it is possible to ensure in a targeted manner a desired structural structure in the broadest sense (particle size, phase formation, precipitations) in the metal strip.

It has been found that, according to the invention, a heat equation can be applied in a greatly simplified form and nevertheless a sufficiently high accuracy in the solution of the object according to the invention is ensured. As a simplified. Heat equation, the first law of thermodynamics is sufficient. Great importance is attached to the determination of the boundary conditions.

Preferably, a continuous phase transformation model of the metal is integrated into the calculation model, in particular according to Avrami.

The Avrami equation describes in its general form all diffusion-controlled transformation processes for the respective temperature under isothermal conditions. By taking this equation into account in the calculation model, ferrite, pearlite and bainite fractions can be set in a very targeted manner during steel casting, u.zw. also taking into account a holding time at a certain temperature.

Preferably, the method is characterized in that with the calculation model thermodynamically changes in state of the entire metal strip, such as changes in temperature, by solving a heat equation and solving a the Ausscheidungskinetik during and / or after solidification, in particular non-metallic and intermetallic precipitations, describing equation or equation systems The temperature setting of the metal strip and, if appropriate, of the casting rolls is set as a function of the calculated value of at least one of the thermodynamic state variables, whereby the thickness of the metal strip, the chemical analysis of the metal and the casting speed are taken into account for the simulation, preferably their values during the Gießens be repeatedly measured, in particular the thickness concerning constantly measured.

It is advantageous that the Ausscheidungskinetik integrated due to free phase energy and nucleation and use of thermodynamic parameters, in particular the Gibbs energy, and the growth of germs after Zener in the calculation model.

It is expedient to integrate microstructure ratios according to multi-component system diagrams, such as, for example, according to the Fe-C diagram, into the calculation model.

Advantageously, grain growth properties and / or grain formation properties, optionally taking into account recrystallization of the metal, are integrated into the calculation model. Here, dynamic and / or delayed and / or post-recrystallization, i. a recrystallization, which takes place later in an oven, are taken into account in the calculation model.

Preferably, as a variable of the continuous casting, which also influences a structural formation, takes place during the Ausördernden the metal strip single or multi-stage hot and / or cold rolling in the computer model, which also takes place during continuous casting thermomechanical rolling, for example, high-temperature thermomechanical rolling, can be taken into account for a line temperature greater than A _C3 . As rolling according to the invention thickness reductions even after reeling the tape and in low-temperature ranges (eg at 200 - 300 ° C), which can also be performed on-line, ie viewed without prior coiling.

Furthermore, the mechanical state, such as the deformation behavior, by resolving further model equations, in particular by solving the continuum technical basic equations for the visco-elasto-plastic material behavior, is also always included in the calculation model.

A preferred embodiment is characterized in that a quantitatively defined structure is adjusted by applying an on-line calculated strand deformation, which causes a recrystallization of the microstructure.

Furthermore, a thermal influence of the molten metal and already solidified metal is expediently integrated into the computer model by the casting rolls with on-line detection of the casting roll cooling.

It is additionally advantageous if a thermal influence of the metal strip, such as cooling and / or heating, is integrated into the calculation model. If necessary, differences between the edge and the middle area of the metal strip should be taken into account.

An advantageous variant of the method according to the invention is characterized in that a rolling process model, preferably a hot rolling process model, is integrated into the calculation model, wherein the rolling process model expediently comprises a rolling force calculation and / or a roll bending force calculation and / or for specially profiled rolls a rolling displacement calculation and / or a rolling deformation calculation and / or for thermally induced rolling geometry changes has integrated a deformation calculation.

According to the invention, mechanical properties of the metal strip, such as yield strength, tensile strength, elongation, etc., can be calculated in advance with the mathematical model, so that a corrective action can be taken in good time when a deviation of these predicted values from predetermined target values is ascertained. in the most suitable generation stages, i. during solidification and subsequent thermal influences or during subsequent rolling, recrystallization, etc.

The invention is explained in more detail with reference to an embodiment shown in the drawing, wherein the illustrated Fig illustrates a continuous casting of the type described above in a schematic representation.

For casting a thin strip 1, in particular a steel strip with a thickness between 1 and 10 mm, is one of two parallel to each other and juxtaposed casting rolls 2 formed casting mold. The casting rolls 2 form a casting gap 3, the so-called "kissing point", at which the strip 1 emerges from the continuous casting mold. Above the Gießspaltes 3 is a space 4, which is shielded by a cover forming a cover plate 5 upwards, formed and which serves to receive a molten bath 6. The molten metal 7 is supplied via an opening 8 of the cover, through which a dip tube projects into the molten bath 6 below the bath level 9. The casting rolls 2 are provided with an internal cooling, not shown. Side of the casting rolls 2 side plates are provided for sealing the space 6 receiving the molten bath 4.

On the surfaces 10 of the casting rolls 2, there is in each case to form a strand shell, said strand shells in the casting gap 3, i. at the kissing-point, to be united to a band 1. For optimum formation of a band 1 of approximately uniform thickness, preferably with a slight buckling commensurate with the standard, it is essential that in the casting nip 3 a specific rolling force distribution, e.g. in rectangular or barrel shape.

To keep the structure of the surfaces of the casting rolls constant, brush systems can be provided, the brushes of which can be set against the surfaces 10 of the casting rolls 2.

For quality assurance of the cast steel strip 1 is a computer 11, in the machine data, the desired format of the metal strip, material data, such as the chemical analysis of molten steel, the casting state, the casting speed, the liquid steel temperature at which the molten steel enters between the casting rolls, and the desired structure and optionally a deformation of the steel strip, which can take place on-line or outside the continuous casting, are entered. The calculator calculates various parameters influencing the quality of the hot strip, such as influencing the temperature of the molten steel and / or the steel strip, using a thermal calculation model that enables the temperature analysis based on the solution of a heat transfer equation using a metallurgical calculation model that includes the phase transformation kinetics and nucleation kinetics Furthermore, the internal cooling of the casting rolls, the gas loading of the casting rolls, the degree of deformation of the on-line in the example shown roll stand 12, and optionally reel conditions for the reel 13, etc.

The calculation model used according to the invention is essentially based on a strip casting model and a rolling model. The former includes a cast roll, solidification, segregation, primary microstructure, phase transformation and precipitation model. The rolling model includes a thermophysical model, a phase transformation, hot rolling, precipitation, recrystallization and grain size model, and a model for predicting mechanical characteristics.

For the first solidification on the casting rolls 2, the structuring of the casting roll surfaces 10 is decisive. The surface profile of the casting rolls 2 is simulated by the steel 7, but only to a certain extent. Due to the surface tension of the liquid steel 7, "valleys" are often straddled, in which media (for example gases) are stored. Since the gases reduce the heat dissipation from the liquid steel 7 to the casting rolls 2, the solidification is delayed.

The interaction between specially created casting roll surfaces 10 and various gas mixtures is used to set a suitable temperature for the casting process. For this it is necessary to know and describe the nature of the casting roll surfaces 10 exactly. This is done by measuring the Gießwalzenoberfläche after finished surface processing at several points (ideally several times in the axial direction, eg with a highly sensitive measuring pin). The surface profiles obtained in this way are now filtered and divided into classes.

For each of these classes, heat transitions are determined off-line by flow simulations and experiments, thus assigning each surface class a specific distribution of heat fluxes. These heat flow / temperature distributions are transferred to the downstream program parts.

A presetting of the (integral) heat flows can be made possible by the setting of the casting roll temperature. This in turn is determined by the Gießwalzenwerkstoffe, the cooling water temperature and the amount of cooling water.

The first step of this calculation model is thus to describe the condition of the casting surface and to calculate and classify the relevant heat transfers (surface "mountains", gas-filled "valleys", transition areas) into classes (fuzzyfication) as well as to transmit the respective temperatures.

In a second step the primary solidification to the different classes is calculated. For this purpose, the primary solidification (dendrite growth, alignments, lengths, arm distances) was previously determined in experiments using solidification experiments and at the same time recalculated with simulation calculations in combination with (or by using a statistical model = cellular automaton) the temperature model. The aim of this step is the calculation of the size distribution and growth direction of the dendrites.

In this step, (nearly) parallel growing dendrites are grouped into grains. The result of this step is the estimation of the particle size distribution and possibly of a shape factor (length / width).

Segregation and excretion models are used to determine segregation and excretion. The latter, in combination with the temperature model for the particular tape position, determines the degree of exudates that are fuzzified.

By means of a mechanical model which, together with the temperature model, determines and fuzzifies the resulting structure stresses, it is possible to predict cracking.

All parameters are passed to a rolling model whose goal it is to predict structure, mechanical parameters and cooling conditions in the outlet part and geometric parameters, such as. Flatness, predict.

All fuzzyfied parameters are passed on an on-line calculation model, which determines the current conditions for the steel strip 1 on the basis of the constantly running temperature model and optionally influences the control parameters by means of control circuits.

From already produced tapes quality features are redirected and stored and correlated with the manufacturing parameters. In a self-learning loop new process parameters are proposed.

Examples of calculation models, as they can be used for the invention, can be found in the Austrian patent application A 972/2000.

Claims (22)

1. A method for the continuous casting of a thin metal strip (1) according to the two-roll method, in particular of a steel strip, preferably of a thickness which is less than 10 mm, wherein, under formation of a melting bath (6), metal melt (7) is cast into a casting gap (3) formed by two casting rolls (2) of the thickness of the metal strip (1) to be cast, characterized in that, to form a particular texture within the cast metal strip, continuous casting is carried out by an on-line calculation based upon an arithmetic model describing the formation of the particular texture of the metal, wherein variables of the continuous casting method affecting the formation of the texture are adjusted in an on-line dynamic fashion, i.e. while casting takes place.
2. A method according to claim 1, characterized in that, to influence the geometry of the metal strip, continuous casting is carried out by an on-line calculation based upon an arithmetic model describing the formation of the geometry of the metal strip, wherein variables of the continuous casting method affecting the geometry are adjusted in an on-line dynamic fashion, i.e. while casting takes place.
3. A method according to claim 1 or 2, characterized in that the structuring of the surface of the casting rolls is recorded, preferably is recorded on-line, and is integrated in the arithmetic model, under consideration of the conditions of solidification and segregation resulting therefrom, in particular during primary solidification.
4. A method according to claim 1, 2 or 3, characterized in that the surfaces (11) of the casting rolls (2) above the melting bath (6) are flushed with a gas or a gas mixture and the chemical composition of the gas or the gas mixture, respectively, as well as its amount and optionally its distribution throughout the length of the casting rolls are recorded, preferably are recorded on-line, and are integrated in the arithmetic model, under consideration of the conditions of solidification and segregation resulting therefrom, in particular during primary solidification.
5. A method according to one or several of claims 1 to 4, characterized in that thermodynamic changes of state of the entire metal strip such as changes in temperature are permanently joined in the calculation of the arithmetic model by solving a heat conduction equation and solving an equation or equation systems, respectively, describing the phase transition kinetics, and in that the temperature adjustment of the metal strip as well as optionally of the casting rolls is adjusted in dependence of the calculated value of at least one of the thermodynamic state quantities, wherein, for simulation, the thickness of the metal strip, the chemical analysis of the metal as well as the casting rate are taken into account, the values thereof being measured repeatedly, preferably during casting, and constantly, in particular with regard to the thickness.
6. A method according to claim 5, characterized in that a continuous phase transition model of the metal is integrated in the arithmetic model, in particular in accordance with Avrami.
7. A method according to one or several of claims 1 to 6, characterized in that thermodynamic changes of state of the entire metal strip such as changes in temperature are permanently joined in the calculation of the arithmetic model by solving a heat conduction equation and solving an equation or equation systems, respectively, describing the precipitation kinetics during and/or after solifidication, in particular, of nonmetallic and intermetallic precipitations and in that the temperature adjustment of the metal strip as well as optionally of the casting rolls is adjusted in dependence of the calculated value of at least one of the thermodynamic state quantities, wherein, for simulation, the thickness of the metal strip, the chemical analysis of the metal as well as the casting rate are taken into account, the values thereof being measured repeatedly, preferably during casting, and constantly, in particular with regard to the thickness.
8. A method according to one or several of claims 1 to 7, characterized in that the precipitation kinetics due to free phase energy and nucleus formation and the use of thermodynamic primary quantities, in particular Gibbs' energy, and the germ growth according to Zener are integrated in the arithmetic model.
9. A method according to one or several of claims 1 to 8, characterized in that quantitative relations of texture according to diagrams of multicomponent systems such as, for example, according to the Fe-C diagram, are also integrated in the arithmetic model.
10. A method according to one or several of claims 1 to 9, characterized in that grain growth characteristics and/or grain formation characteristics are integrated in the arithmetic model, optionally under consideration of the recrystallization of the metal.
11. A method according to one or several of claims 1 to 10, characterized in that single- or multiple-stage hot- and/or cold-rolling taking place during extraction of the metal strip is integrated in the arithmetic model as a variable of continuous casting affecting a formation of texture.
12. A method according to one or several of claims 1 to 11, characterized in that also the mechanical state such as the forming behaviour is permanently joined in the calculation of the arithmetic model by solving further model equations, in particular by solving the continuum-mechanical fundamental equations for the visco-elastoplastic material behaviour.
13. A method according to one or several of claims 1 to 12, characterized in that a texture defined quantitatively is adjusted by imposing strand forming which has been computed on-line and leads to recrystallization of the texture.
14. A method according to one or several of claims 1 to 13, characterized in that a thermal influence on the metal melt and on the already solidified metal by the casting rolls is integrated in the arithmetic model under on-line acquisition of the cooling of the casting rolls.
15. A method according to one or several of claims 1 to 14, characterized in that a thermal influence on the metal strip, such as cooling and/or heating, is integrated in the arithmetic model.
16. A method according to one or several of claims 1 to 15, characterized in that a rolling process model, preferably a hot-rolling process model, is integrated in the arithmetic model.
17. A method according to claim 16, characterized in that the rolling process model comprises a calculation of rolling force.
18. A method according to claim 16 or 17, characterized in that the rolling process model comprises a calculation of lateral rolling power.
19. A method according to one or several of claims 16 to 18, characterized in that the rolling process model comprises a calculation of roll shifting for specially shaped rolls.
20. A method according to one or several of claims 16 to 19, characterized in that the rolling process model comprises a calculation of roll deformation.
21. A method according to one or several of claims 16 to 20, characterized in that the rolling process model comprises a forming calculation for thermally induced changes in rolling geometry.
22. A method according to one or several of claims 1 to 21, characterized in that mechanichal characteristics of the metal strip such as apparent yielding point, resistance to extension, stretching etc. are permanently joined in the calculation by means of the arithmetic model or are calculated at least for the end of the strip casting process.

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