EP1289691A1 - Method for continuously casting a metal strand - Google Patents

Method for continuously casting a metal strand

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Publication number
EP1289691A1
EP1289691A1 EP01942855A EP01942855A EP1289691A1 EP 1289691 A1 EP1289691 A1 EP 1289691A1 EP 01942855 A EP01942855 A EP 01942855A EP 01942855 A EP01942855 A EP 01942855A EP 1289691 A1 EP1289691 A1 EP 1289691A1
Authority
EP
European Patent Office
Prior art keywords
strand
metal
cooling
model
solving
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Granted
Application number
EP01942855A
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German (de)
French (fr)
Other versions
EP1289691B2 (en
EP1289691B1 (en
Inventor
Christian Chimani
Kurt Dittenberger
Andreas Flick
Karl Mörwald
Helmut Resch
Josef Watzinger
Manfred Thalhammer
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SIEMENS VAI METALS Technologies GmbH
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Voest Alpine Industrienlagenbau GmbH
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Application filed by Voest Alpine Industrienlagenbau GmbH filed Critical Voest Alpine Industrienlagenbau GmbH
Priority to AT01942855T priority Critical patent/ATE346706T1/en
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Classifications

    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
    • B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
    • B22D11/16—Controlling or regulating processes or operations
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
    • B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
    • B22D11/16—Controlling or regulating processes or operations
    • B22D11/22—Controlling or regulating processes or operations for cooling cast stock or mould
    • B22D11/225—Controlling or regulating processes or operations for cooling cast stock or mould for secondary cooling

Definitions

  • the invention relates to a method for the continuous casting of a metal strand, in particular a steel strand, a strand being pulled out of a cooled continuous mold, supported in a strand support device arranged downstream of the continuous mold and cooled with coolant and optionally reduced in thickness, and a system for carrying out the method.
  • DE-C - 25 42 290 it is known from DE-C - 25 42 290 to predefine a specific temperature profile according to an optimal casting speed for which the coolant quantities for cooling the strand are set and during casting the measured real casting speed with the optimum casting speed to compare and to make adjustments for the coolant quantities from deviations of the actual casting speed from the optimal casting speed.
  • thermodynamic changes in state of the strand into account with great accuracy, so that disadvantages caused by such thermodynamic changes in state, e.g. responsible for internal cracks or edge cracks can be reliably avoided.
  • thermodynamic changes in the state of the entire strand such as changes in the surface temperature, the mean temperature, the shell thickness, and also the mechanical state, such as the deformation behavior, etc., are constantly included in a mathematical simulation model by solving the heat conduction equation, and the cooling of the strand is taken into account Depending on the calculated value, at least one of the thermodynamic state variables is set, the strand thickness and the chemical analysis of the metal and the continuously measured casting speed being taken into account for the simulation.
  • Another cause of surface cracks is the segregation of trace elements such as S, Sn, Cu etc. at the grain boundaries. These segregations result in hot brittleness of the rolled product after rolling.
  • the crack intensity is directly related to the initial grain size, i.e. the larger the grain, the higher the crack intensity.
  • the starting grain size is generally larger than with cold-charged slabs, which undergo a complete conversion from ⁇ to ⁇ .
  • This effect can also be positively influenced by targeted temperature-time control, in particular rapid cooling to approximately 500 ° C. having a favorable effect on the excretion processes. That concentrated precipitation of nitrides at the austenite grain boundaries is suppressed and replaced by an even distribution over the volume. Depending on the steel analysis and time of the temperature treatment, a fine pearlitic or bainitic structure is created. In spite of a slight loss of global strength, the material toughness increases. Local softening at the primary austenite grain boundaries is avoided and cracking is consequently suppressed. The effect applies both to A1N excretions and to trace elements, which cause hot fragility.
  • temperature control is usually carried out in accordance with theoretical predictions and calculations.
  • the amounts of water are controlled so that at different casting speeds approximately the same surface temperatures are reached on the strand.
  • a temperature measuring device is used for this feedback, which measures the surface temperature of the cast product before and after the intensive exposure to water. These values are compared with the calculated ones and the optimum amount of water is determined from them after appropriate tests.
  • the water control is therefore linked purely to the casting speed. Changes that arise due to transient conditions (short changes in speed, start of casting when the machine is cold, end of casting, etc.) so that they are not influenced unless a permanent temperature measurement is used. Measuring instruments used for this purpose usually have only a low measuring accuracy and are strongly influenced in particular by scale, which is located on the surface of the cast product. The feedback is imprecise, so it is not possible to apply water evenly and intensively.
  • Another disadvantage relates to the fact that, in the case of greatly changed casting speeds, the optimum length of the section in which the strand is to be intensively cooled has to be changed in order to achieve a certain depth of the influence of the intensive cooling and it is not sufficient to change only the amount of water , If you have only inaccurate temperature signals to specify the optimal length or depth of the area of influence, you will never achieve the desired optimum.
  • DE 196 12 420 AI describes a method for achieving improved strand cooling at varying strand speeds, with model parameters such as mold length, strand geometry, strand speed, melting temperature, solidification enthalpy and cooling water volume being taken into account for various cooling models.
  • the thermal model is expanded with the functionality of a neural network for adapting modeling parameters. A thermal Modeling of the casting process coupled with metallurgical modeling in order to influence the material properties online is not addressed here.
  • the steel quality is not taken into account.
  • some (sensitive) steel grades are overcooled and subjected to unnecessary thermal stress.
  • the desired phase transition effect is not achieved in some other types of steel.
  • the invention aims to avoid these disadvantages and difficulties and has as its object the further development of a continuous casting process of the type described in such a way that it is possible to specify the formation of a desired structure of the metal as a target, etc. for metals, i.e. Different chemical composition in the continuous casting of steel for all steel qualities or steel grades to be cast.
  • a target i.e. Different chemical composition in the continuous casting of steel for all steel qualities or steel grades to be cast.
  • This object is achieved in a method of the type described at the outset in that, in order to form a specific structure in the cast strand, the continuous casting is carried out using on-line calculation on the basis of a computational model describing the formation of the specific structure of the metal, the structure-influencing variable of the continuous casting process, such as, for example, the specific quantity of coolant provided for cooling the strand, can be set dynamically online, ie during the ongoing casting.
  • the calculation model uses thermodynamic changes in the state of the entire strand, such as changes in the surface temperature, the mean temperature, the shell thickness, by solving the heat conduction equation and solving an equation describing the phase transition kinetics, and the cooling of the strand is dependent on the calculated value set at least one of the thermodynamic state variables, the strand thickness and the chemical analysis of the metal and the continuously measured casting speed being taken into account for the simulation.
  • the calculation model which includes the formation of a specific time and temperature-dependent structure of the metal, it is possible to determine the variables of the continuous casting process that influence the structure, e.g. adapt the amount of coolant to be applied to the strand surface, the chemical analysis of the metal, and the local temperature history of the strand. In this way, a desired microstructure in the broadest sense (grain size, phase formation, excretions) can be achieved in the region of the strand near the surface.
  • a continuous phase conversion model of the metal is preferably integrated into the computing model, in particular according to Avrami.
  • the Avrami equation describes all diffusion-controlled conversion processes for the respective temperature under isothermal conditions.
  • ferrite, pearlite and bainite fractions can be set in a targeted manner in steel continuous casting, etc. also taking into account a holding time at a certain temperature.
  • the method is preferably characterized in that thermal changes in the state of the entire strand, such as changes in surface temperature, mean temperature, shell thickness, are solved by solving the thermal conduction equation and solving an equation describing the excretion kinetics, in particular non-metallic and intermetallic precipitates, and continuously the cooling of the strand is set as a function of the calculated value of at least one of the thermodynamic state variables, the strand thickness and the chemical analysis of the metal and the constant being used for the simulation Measured pouring speed are taken into account, the excretion kinetics due to free phase energy and nucleation and use of thermodynamic basic variables, in particular Gibb's energy, and the germ growth according to Zener advantageously being integrated into the calculation model.
  • Structural quantity relationships in equilibrium states according to multi-substance system diagrams in particular according to the Fe-C diagram, are also expediently integrated into the calculation model.
  • Grain growth properties are preferably integrated into the computing model, in particular taking into account recrystallization of the metal.
  • Dynamic and / or delayed and / or post-recrystallization i.e. a recrystallization that later takes place in an oven must be taken into account in the calculation model.
  • thermodynamic rolling for example high-temperature rolling, which takes place during the continuous casting thermodynamic rolling with a surface temperature greater than A c3 can be taken into account.
  • the mechanical state such as the deformation behavior
  • the mechanical state is preferably also constantly included in the calculation model by solving further model equations, in particular by solving the thermal conductivity equation.
  • a preferred embodiment is characterized in that phase components defined in terms of quantity are set by applying specific strand coolant quantities calculated on-line before and / or after the strand has solidified.
  • a defined structure is expediently set by applying an on-line strand deformation calculated before and / or after the strand has solidified, which causes the structure to recrystallize.
  • An advantageous variant of the method according to the invention is characterized in that the phase transformation that concludes the continuous casting with setting of a phase component of the strand that is defined in terms of quantity, calculated specific strand The amount of coolant is set after solidification of the strand in the end region of a secondary cooling zone in a cooling zone causing increased cooling.
  • the calculation model to be used according to the invention can calculate all transformation temperatures and data which are necessary for predicting and describing the transformation processes for the phase fractions ferrite, pearlite, bainite and martensite.
  • X is the proportion of the converted phase and b and n are parameters which are dependent on the nucleation and the growth of the phase formed. These parameters b and n are dependent on the analysis and can be determined by dilatometer tests.
  • the Avrami equation can be used to calculate the start and end times as well as the temperature for the ferrite, pearlite and bainite transformation under isothermal conditions.
  • t s (T) means a virtual start time of the conversion at a temperature T in accordance with the amount actually converted.
  • the temperature is defined as a function of time. Since the calculated conversion or excretion percentage according to Avrami does not provide any information about the actual microstructure / quantity ratios, but only reveals whether and how the equilibrium state is reached, the conversion fractions on the equilibrium lines from the iron / carbon are used to determine the microstructure ratio Diagram related and also taken into account in the calculation model.
  • ⁇ G chem ⁇ G ° A1N - R ⁇ T • (In XAI + In X N )
  • G .0 A I N is the standard Gibb energy for the formation of AIN
  • X AI is the molar fraction of aluminum in the austenite volume
  • X N is the average nitrogen content.
  • S is the density of nucleation in austenite.
  • ⁇ is the austenite / AIN interface energy.
  • k ß is the Boltzmann constant and D A ⁇ is the spreading capacity of aluminum in austenite.
  • Zener for example, discussed in JS Kirkaldy, "Diffusion in the Condensed State", The Universities Press, Harbor, 1985).
  • the calculation process takes place in two main stages. In the first stage the number of currently formed germs is determined and in the second stage the growth of all previously formed excretions is calculated.
  • a steel strand 1 is formed from a molten steel 2 with a certain chemical composition by casting in a continuous mold 3.
  • the molten steel 2 is poured from a ladle 4 via an intermediate vessel 5 and one from the intermediate vessel 5 into the continuous mold 3 by means of a pouring tube 6 extending under the casting level formed in the continuous mold 3.
  • strand guide rollers 7 are provided for supporting the steel strand 1, which still has a liquid core 8 and initially only a very thin strand shell 9.
  • the steel strand 1 emerging from the continuous mold with a straight axis is deflected in a bending zone 10 into a circular arc path 11 and is also supported in this by strand guide rollers 7.
  • a straightening zone 12 following the circular arc path 11 the steel strand 1 is again straightened and conveyed out via an outfeed roller table or directly reduced in thickness on-line, e.g. by means of an on-line mill stand 13.
  • the steel strand 1 To cool the steel strand 1, it is cooled directly or indirectly - via strand guide rollers 7 provided with internal cooling - so that a certain temperature can be set on its surface to a certain depth range.
  • the steel strand 1 is supplied with the amount of coolant required for the desired structure of the steel strand 1 via a closed or open control circuit by means of a computer 14.
  • Machine data m the format f of the steel strand 1, material data, such as the chemical analysis St C h of the molten steel 2, the pouring state z, the pouring speed v, the molten steel temperature tn at which the molten steel 2 enters the continuous mold 3, as well as the desired structure ⁇ / ⁇ and possibly a deformation w of the steel strand 1, which is on the way of the strand guidance is entered.
  • This deformation can also be given, for example, by straightening the steel strand 1 in the straightening zone 12.
  • a set amount of water Q s is calculated on the basis of a metallurgical calculation model that takes into account the phase change kinetics and nucleation kinetics according to the calculation models specified above, and a thermal calculation model that enables the temperature analysis based on the solution of the heat conduction equation, etc. due to the current, already applied amount of water Q A , which is also entered into the computer.
  • a solution of the heat conduction equation using a process computer is state of the art and e.g. dealt with in detail in DE-C2 - 44 17 808 for continuous casting.
  • the finite difference method with Lagrangian description is given as one way of solving the heat conduction equation.
  • the metallurgical calculation model takes the current steel analysis St C into account in order to cope with different material behavior.
  • the current temperature T A calculated by the thermal calculation model is fed on-line to the metallurgical calculation model and this continuously calculates the desired target temperature T s , on the basis of which the thermal calculation model calculates and automatically sets the target water quantity Q s for the individual strand cooling sections.

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  • Engineering & Computer Science (AREA)
  • Mechanical Engineering (AREA)
  • Continuous Casting (AREA)
  • Manufacture And Refinement Of Metals (AREA)
  • Treatment Of Steel In Its Molten State (AREA)

Abstract

The invention relates to a method for continuously casting a metal strand, especially a steel strand (1). According to the method, a strand (1) is drawn out of a cooled open-ended mold (3), is supported in a strand supporting device (7, 11) located downstream from the open-ended mold (3), is cooled by a coolant, and optionally undergoes a thickness reduction. The aim of the invention is to stipulate the formation of a desired structure of the metal. To this end, the continuous casting is carried out with online calculation while using, as a basis, an arithmetic model which describes the formation of the defined structure of the metal, whereby the variables of the continuous casting method which influence the structuring, e.g. the specific amount of coolant provided for cooling the strand, are set in an online dynamic manner, i.e. during continuous casting.

Description

Verfahren zum Stranggießen eines MetallstrangesProcess for the continuous casting of a metal strand
Die Erfindung betrifft ein Verfahren zum Stranggießen eines Metallstranges, insbesondere eines Stahlstranges, wobei ein Strang aus einer gekühlten Durchlaufkokille ausgezogen, in einer der Durchlaufkokille nachgeordneten Strangstützeinrichtung gestützt und mit Kühlmittel gekühlt sowie gegebenenfalls dickenreduziert wird, sowie eine Anlage zur Durchführung des Verfahrens.The invention relates to a method for the continuous casting of a metal strand, in particular a steel strand, a strand being pulled out of a cooled continuous mold, supported in a strand support device arranged downstream of the continuous mold and cooled with coolant and optionally reduced in thickness, and a system for carrying out the method.
Es ist eine beim Stranggießen bekannte Anforderung, die Kühlung eines kontinuierlich gegossenen Stranges derart einzustellen, daß die Strangoberflächentemperatur vorgegebenen Werten, die gegebenenfalls vom Alter eines Querschnittselementes des Stranges abhängen, möglichst nahekommt. Dies ist insbesondere bei Strangverzögerungen und oder Strangbeschleunigungen von besonderer Bedeutung.It is a known requirement in continuous casting to adjust the cooling of a continuously cast strand in such a way that the strand surface temperature comes as close as possible to predetermined values, which may depend on the age of a cross-sectional element of the strand. This is particularly important in the case of line decelerations and or line accelerations.
Aus der AT-B - 300.238 ist ein Verfahren zum Kühlen eines aus einer Durchlaufkokille austretenden Stranges bekannt, wobei die Sollwerte der Kühlwassermenge in Abhängigkeit von der chemischen Zusammensetzung des Strangmaterials, der Erstarrungszeit und weiters in Abhängigkeit vom augenblicklichen Integralwert der Gießgeschwindigkeit während des Weges des Stranges bis zur jeweiligen Kühlzone eingestellt werden, so daß die Strangoberflächentemperatur vorbestimmbar bleibt.From AT-B - 300.238 a method for cooling a strand emerging from a continuous mold is known, the setpoints of the cooling water quantity depending on the chemical composition of the strand material, the solidification time and furthermore depending on the instantaneous integral value of the casting speed during the path of the strand be set up to the respective cooling zone so that the strand surface temperature remains predeterminable.
Weiters ist es aus der DE-C - 25 42 290 bekannt, vor dem Gießen einen bestimmten Temperaturverlauf entsprechend einer optimalen Gießgeschwindigkeit, für welche die Kühlmittelmengen für die Kühlung des Stranges eingestellt werden, vorzugeben und während des Gießens die gemessene wirkliche Gießgeschwindigkeit mit der optimalen Gießgeschwindigkeit zu vergleichen und aus Abweichungen der tatsächlichen Gießgeschwindigkeit von der optimalen Gießgeschwindigkeit eine Nachsteuerung für die Kühlmittelmengen vorzunehmen.Furthermore, it is known from DE-C - 25 42 290 to predefine a specific temperature profile according to an optimal casting speed for which the coolant quantities for cooling the strand are set and during casting the measured real casting speed with the optimum casting speed to compare and to make adjustments for the coolant quantities from deviations of the actual casting speed from the optimal casting speed.
Aus der DE-A - 2 344 438 ist es bekannt, während des Gießens durch Integrieren der Geschwindigkeit einzelner Strangabschnitte über die Laufzeit und durch gleichzeitiges Festhalten der von einem Strangabschnitt im Kühlbereich verbrachten Zeit die auf einen einzelnen Abschnitt aufgebrachte Kühlmittelmenge zu ermitteln und mit einer Sollmenge zu vergleichen, auf diese Weise sogenannte "Rest-Kühlmittelmengen" zu bestimmen und aus ieser Bestimmung heraus die Verweilzeit einzelner Strangabschnitte im gesamten Kühlbereich konstant zu halten. Diese bekannten Verfahren ermöglichen Korrekturen der Kühlmittelmengen, die in erster Linie von der Gießgeschwindigkeit abhängen, also gießgeschwindigkeitsabhängige Regelungen, wobei jedoch die tatsächlichen thermodynamischen Zustandsänderungen des Stranges unberücksichtigt bleiben. Der Stand der Technik berücksichtigt also nur - kommt es zu einem Abweichen der tatsächlichen Gießgeschwindigkeit von der Gießgeschwindigkeit, für die die Strangkühlung eingestellt ist - Tendenzen, ohne jedoch den tatsächlichen Verhältnissen gerecht zu werden.From DE-A-2 344 438 it is known to determine the amount of coolant applied to an individual section during the casting by integrating the speed of individual strand sections over the running time and by simultaneously recording the time spent by a strand section in the cooling area and with a target amount to compare, in this way to determine so-called "residual coolant quantities" and to use this determination to keep the dwell time of individual strand sections constant in the entire cooling area. These known methods make it possible to correct the coolant quantities, which primarily depend on the casting speed, that is to say controls dependent on the casting speed, but the actual thermodynamic changes in the state of the strand are not taken into account. The prior art therefore only takes into account trends - if the actual casting speed deviates from the casting speed for which the strand cooling is set - without however taking into account the actual conditions.
Gemäß der DE-A - 44 17 808 werden in Weiterentwicklung zu obigen Verfahren thermodynamische Zustandsänderungen des Stranges mit großer Genauigkeit berücksichtigt, so daß durch solche thermodynamische Zustandsänderungen verursachte Nachteile, die z.B. für Innenrisse oder Kantenrisse verantwortlich sind, zuverlässig vermieden werden.According to DE-A-44 17 808, further developments to the above methods take thermodynamic changes in state of the strand into account with great accuracy, so that disadvantages caused by such thermodynamic changes in state, e.g. responsible for internal cracks or edge cracks can be reliably avoided.
Hierzu werden thermodynamische Zustandsänderungen des gesamten Stranges, wie Änderungen der Oberflächentemperatur, der Mittentemperatur, der Schalenstärke, und auch der mechanische Zustand, wie das Verformungsverhalten, etc., in einem mathematischen Simulationsmodell durch Lösen der Wärmeleitungsgleichung ständig mitgerechnet und es wird die Kühlung des Stranges in Abhängigkeit des errechneten Wertes mindestens einer der thermodynamischen Zustandsgrößen eingestellt, wobei für die Simulation die Strangdicke und die chemische Analyse des Metalles sowie die ständig gemessene Gießgeschwindigkeit berücksichtigt werden.For this purpose, thermodynamic changes in the state of the entire strand, such as changes in the surface temperature, the mean temperature, the shell thickness, and also the mechanical state, such as the deformation behavior, etc., are constantly included in a mathematical simulation model by solving the heat conduction equation, and the cooling of the strand is taken into account Depending on the calculated value, at least one of the thermodynamic state variables is set, the strand thickness and the chemical analysis of the metal and the continuously measured casting speed being taken into account for the simulation.
Beim Direktverbund einer Stranggießanlage mit einem Walzwerk hängen Ausscheidungsbildung und Phasenumwandlungen im Gußprodukt von der Kühlrate, dem Temperaturniveau und von der Deformationskinetik ab. Es wurde beispielsweise beobachtet, daß im Falle eines zeitlich verzögerten Chargierens von Brammen in einen Ofen, z.B. infolge langer Transportzeit, Oberflächenrisse (Netzrisse) am Walzprodukt entstehen, welche auf eine Schädigung entlang der Korngrenzen zurückzuführen sind. Insbesondere trifft dies auf Aluminiumnitridausscheidungen zu, welche sich verstärkt an den Korngrenzen ausscheiden und dort die Mobilität der Körner zueinander behindern. Bei einer Warmumformung entstehen an den Korngrenzen hohe Spannungen, welche im Falle solcher Ausscheidungen in Rissen nach dem Walzen enden. Die Ausscheidung von A1N im stabilen γ-Bereich ist von der Temperatur-Zeitgeschichte abhängig. Durch die Phasenumwandlung von γ in α, bei Temperaturen zwischen 900°C und 720°C, kommt es zur annähernd spontanen Ausscheidung der sich nicht im Gleichgewicht befindlichen Aluminiumnitride. Zur Vermeidung der mit A1N- Ausscheidungen verbundenen Nachteile ist es bekannt (EP-A - 0 650 790), den durcherstarrten Strang (Bramme, Vorblock, Knüppel) in oder nach der Stranggießanlage in solcher Art mit einem Kühlmedium zu kühlen, daß die Oberflächentemperatur einen bestimmten Wert von ca. 500 bis 550°C erreicht. Anschließend wird die Kühlung gestoppt und der gekühlte Abschnitt erwärmt sich von innen auf einen sich ergebenden Wert.In the direct connection of a continuous caster with a rolling mill, precipitation and phase changes in the cast product depend on the cooling rate, the temperature level and the kinetics of deformation. It has been observed, for example, that in the case of a delayed loading of slabs into an oven, for example as a result of a long transport time, surface cracks (network cracks) occur on the rolled product, which can be attributed to damage along the grain boundaries. This applies in particular to aluminum nitride precipitates, which are increasingly precipitated at the grain boundaries and hinder the mobility of the grains towards one another. In hot forming, high stresses occur at the grain boundaries, which in the case of such precipitations result in cracks after rolling. The excretion of A1N in the stable γ range depends on the temperature-time history. Due to the phase change from γ to α, at temperatures between 900 ° C and 720 ° C, there is an almost spontaneous elimination of the aluminum nitrides which are not in equilibrium. To avoid the disadvantages associated with A1N excretions, it is known (EP-A - 0 650 790) to cool the solidified strand (slab, bloom, billet) in or after the continuous casting installation with a cooling medium in such a way that the surface temperature is unified reached a certain value of approx. 500 to 550 ° C. The cooling is then stopped and the cooled section heats up from the inside to a resulting value.
Eine andere Ursache von Oberflächenrissen sind Seigerungen von Spurenelementen, wie S, Sn, Cu etc., an den Korngrenzen. Diese Seigerungen resultieren in Heißbrüchigkeit des Walzproduktes nach dem Walzen. Die Rissintensität steht in einem direkten Zusammenhang mit der Ausgangskorngröße, d.h. je größer das Korn ist, umso höher wird die Rissintensität sein. In einem Direktverbundsystem ist die Ausgangskorngröße im allgemeinen größer als bei kalt chargierten Brammen, welche eine vollständige Umwandlung von γ in α erfahren.Another cause of surface cracks is the segregation of trace elements such as S, Sn, Cu etc. at the grain boundaries. These segregations result in hot brittleness of the rolled product after rolling. The crack intensity is directly related to the initial grain size, i.e. the larger the grain, the higher the crack intensity. In a direct bond system, the starting grain size is generally larger than with cold-charged slabs, which undergo a complete conversion from γ to α.
Auch dieser Effekt kann durch eine gezielte Temperatur-Zeitsteuerung positiv beeinflußt werden, wobei insbesondere eine rasche Abkühlung auf ca. 500°C die Ausscheidungsvorgänge günstig beeinflußt. D.h. eine konzentrierte Ausscheidung von Nitriden an den Austenitkorngrenzen wird unterdrückt und durch eine über das Volumen gleichmäßige Verteilung ersetzt. Je nach Stahlanalyse und Zeit der Temperaturbehandlung entsteht eine fein perlitische oder bainitische Gefügestruktur. Trotz einer geringen globalen Festigkeitseinbuße erhöht sich damit die Materialzähigkeit. Lokale Entfestigung an den primären Austenitkorngrenzen werden vermieden und folglich wird die Rißbildung unterdrückt. Der Effekt gilt sowohl für A1N- Ausscheidungen als auch für Spurenelemente, welche Heißbrüchigkeit hervorrufen.This effect can also be positively influenced by targeted temperature-time control, in particular rapid cooling to approximately 500 ° C. having a favorable effect on the excretion processes. That concentrated precipitation of nitrides at the austenite grain boundaries is suppressed and replaced by an even distribution over the volume. Depending on the steel analysis and time of the temperature treatment, a fine pearlitic or bainitic structure is created. In spite of a slight loss of global strength, the material toughness increases. Local softening at the primary austenite grain boundaries is avoided and cracking is consequently suppressed. The effect applies both to A1N excretions and to trace elements, which cause hot fragility.
Die Temperatursteuerung erfolgt gemäß dem Stand der Technik üblicherweise entsprechend theoretischer Vorhersagen und Berechnungen. Die Wassermengen werden so gesteuert, daß bei unterschiedlichen Gießgeschwindigkeiten in etwa gleiche Oberflächentemperaturen am Strang erreicht werden. Üblicherweise wird dazu als Rückkoppelung eine Temperaturmeßeinrichtung verwendet, welche die Oberflächentemperatur des Gußproduktes vor und nach der intensiven Wasserbeaufschlagung mißt. Diese Werte werden mit berechneten verglichen und daraus nach entsprechenden Versuchen die optimale Wassermenge bestimmt.According to the state of the art, temperature control is usually carried out in accordance with theoretical predictions and calculations. The amounts of water are controlled so that at different casting speeds approximately the same surface temperatures are reached on the strand. Usually a temperature measuring device is used for this feedback, which measures the surface temperature of the cast product before and after the intensive exposure to water. These values are compared with the calculated ones and the optimum amount of water is determined from them after appropriate tests.
Die Wassersteuerung ist also rein mit der Gießgeschwindigkeit gekoppelt. Veränderungen, welche aufgrund von instationären Zuständen entstehen (kurze Geschwindigkeitsänderungen, Gießbeginn bei kalter Maschine, Gießende etc.), können damit nicht beeinflußt werden, außer man bedient sich einer permanenten Temperaturmessung. Hierzu dienende Messinstrumente haben üblicherweise nur eine geringe Meßgenauigkeit und werden stark inbesondere durch Zunder, welcher sich auf der Oberfläche des Gußproduktes befindet, beeinflußt. Die Rückkoppelung ist ungenau, ein gleichmäßiges intensives Beaufschlagen mit Wasser ist daher nicht möglich.The water control is therefore linked purely to the casting speed. Changes that arise due to transient conditions (short changes in speed, start of casting when the machine is cold, end of casting, etc.) so that they are not influenced unless a permanent temperature measurement is used. Measuring instruments used for this purpose usually have only a low measuring accuracy and are strongly influenced in particular by scale, which is located on the surface of the cast product. The feedback is imprecise, so it is not possible to apply water evenly and intensively.
Ein weiterer Nachteil betrifft den Umstand, daß sich bei stark veränderten Gießgeschwindigkeiten die optimale Länge der Strecke, in der der Strang intensiv zu kühlen ist, zur Erzielung einer bestimmten Tiefe des Einflusses der intensiven Kühlung verändern muß und es nicht ausreicht, nur die Wassermenge zu verändern. Hat man zur Vorgabe der optimalen Länge bzw. Tiefe des Einflußbereiches nur ungenaue Temperatursignale, erreicht man nie ein angestrebtes Optimum.Another disadvantage relates to the fact that, in the case of greatly changed casting speeds, the optimum length of the section in which the strand is to be intensively cooled has to be changed in order to achieve a certain depth of the influence of the intensive cooling and it is not sufficient to change only the amount of water , If you have only inaccurate temperature signals to specify the optimal length or depth of the area of influence, you will never achieve the desired optimum.
Der Aufsatz H.P. Hougrady et al; Möglichkeiten und Grenzen einer Simulation des Werkstoffverhaltens, Stahl und Eisen; Bd 116, Nr. 4 April 1996, Seiten 109 bis 113, gibt einen grundlegenden Überblick in Modelle, insbesondere physikalisch basierte Modelle, die zur Beschreibung von werkstoffkundlichen Vorgängen beim Verarbeiten von Metallen, insbesondere bei Walzprozessen, benutzt werden können. In diesem Dokument wird die Anwendbarkeit dieser Modelle zur Nachbildung von metallurgischen Vorgängen und deren Verifikation mit experimenteller Laborarbeit beschrieben. Hierdurch ist es möglich, sich grundlegend über physikalische Modelle zur Beschreibung von Phasenumwandlungen und Rekristallisation beim Walzumformen zu informieren. Ein Bezug auf eine Onlinemodellierung bzw. Regelung von Phasenumwandlungen des zu vergießenden Metalls in Stranggießanlagen ist in diesem Dokument nicht gegeben.The H.P. Hougrady et al; Possibilities and limits of a simulation of material behavior, steel and iron; Bd 116, No. 4 April 1996, pages 109 to 113, gives a basic overview of models, in particular physically based models, which can be used to describe material science processes in the processing of metals, in particular in rolling processes. This document describes the applicability of these models for the simulation of metallurgical processes and their verification with experimental laboratory work. This makes it possible to obtain basic information about physical models for describing phase changes and recrystallization during roll forming. This document does not refer to online modeling or regulation of phase transformations of the metal to be cast in continuous casting plants.
Das Dokument C. Biegus et al.; Ermittlung von Werkstoffdaten zur Gefugesimulation, Stahl und Eisen, Bd 116 Nr. 5, 1996, Seiten 43 bis 49 zeigt Methoden auf, die es erlauben, Werkstoffeigenschaften experimentell zu ermitteln, die zur physikalisch basierten Modellierung von Phasenumwandlungen bzw. Rekristallisation notwendig sind.The document C. Biegus et al .; Determination of material data for microstructure simulation, steel and iron, vol. 116 no. 5, 1996, pages 43 to 49 shows methods that allow material properties to be determined experimentally, which are necessary for the physically based modeling of phase transformations or recrystallization.
Die DE 196 12 420 AI beschreibt ein Verfahren zur Erzielung einer verbesserten Strangkühlung bei variierender Stranggeschwindigkeit, wobei für verschiedene Kühlmodelle Modellparameter, wie Kokillenlänge, Stranggeometrie, Stranggeschwindigkeit, Schmelztemperatur, Erstarrungsenthalpie und Kühlwasservolumen berücksichtig werden. Nach bevorzugten Ausführungsformen wird das thermische Modell mit der Funktionalität eines neuronalen Netzes zur Anpassung von Modellierparametern erweitert. Eine thermische Modellierung des Gießprozesses gekoppelt mit einer metallurgischen Modellierung um damit online die Werkstoffeigenschaften zu beeinflussen, ist hier nicht angesprochen.DE 196 12 420 AI describes a method for achieving improved strand cooling at varying strand speeds, with model parameters such as mold length, strand geometry, strand speed, melting temperature, solidification enthalpy and cooling water volume being taken into account for various cooling models. According to preferred embodiments, the thermal model is expanded with the functionality of a neural network for adapting modeling parameters. A thermal Modeling of the casting process coupled with metallurgical modeling in order to influence the material properties online is not addressed here.
In der DE 197 17 615 AI wird ein Simulationsansatz zur Beschreibung der Temperaturverteilung während des Warmwalzens beschrieben; es handelt sich um die Anwendung eines rein thermischen Modells.DE 197 17 615 AI describes a simulation approach to describe the temperature distribution during hot rolling; it is the application of a purely thermal model.
Gemäß der DE 195 08 476 AI ist eine pauschale Prozessautomatisierung für Bandgießverfahren ohne nähere Angaben über die Art der Prozessregelung beinhaltet. In . einer pauschalen Auflistung von Teilmodellen wird der Ausdruck Kornstruktur angesprochen, jedoch sind Angaben zu Modellierungsansätzen sowie zur Verwendung von diesem Teilmodell nicht enthalten. Es gibt keine Hinweise auf die Benutzung von Simulationtools um Phasenumwandlungen gezielt nach Produktanforderungen zu steuern.According to DE 195 08 476 AI, a flat-rate process automation for strip casting processes without further details on the type of process control is included. In . A general listing of sub-models addresses the term grain structure, but does not include information on modeling approaches or the use of this sub-model. There is no evidence of the use of simulation tools to control phase conversions according to product requirements.
Gemäß dem Stand der Technik wird die Stahlqualität nicht berücksichtigt. Dies hat zur Folge, daß manche (empfindliche) Stahlgüten überkühlt und unnötig thermisch beansprucht werden. Andererseits wird bei manch anderer Stahlsorte der gewünschte Effekt der Phasenumwandlung nicht erreicht. Insbesondere ist es nicht möglich, Phasenanteile in einem gewünschten Ausmaß, wie z.B. für einen Stahlstrang Phasenanteile an Ferrit, Perlit, Baimit und Martensit, am Gußprodukt - vor oder nach einer Walzung - sicherzustellen.According to the state of the art, the steel quality is not taken into account. As a result, some (sensitive) steel grades are overcooled and subjected to unnecessary thermal stress. On the other hand, the desired phase transition effect is not achieved in some other types of steel. In particular, it is not possible to phase components to a desired extent, e.g. for a steel strand to ensure phase fractions of ferrite, pearlite, baimite and martensite, on the cast product - before or after rolling.
Die Erfindung bezweckt die Vermeidung dieser Nachteile und Schwierigkeiten und stellt sich die Aufgabe, ein Stranggießverfahren der eingangs beschriebenen Art dahingehend weiterzuentwi ekeln, daß es möglich ist, als Zielvorgabe die Ausbildung eines gewünschten- Gefuges des Metalls vorgeben zu- können, u.zw. für Metalle, d.h. unterschiedlicher chemischer Zusammensetzung beim Stahl-Stranggießen für sämtliche zu gießenden Stahlqualitäten bzw. Stahlgüten. Beim Stahl-Stranggießen soll es inbesondere möglich sein, eine bestimmte Ferrit-, Perlit-Struktur einzustellen und/oder Ausscheidungen, wie Aluminiumnitridausscheidungen, an den Korngrenzen zu vermeiden.The invention aims to avoid these disadvantages and difficulties and has as its object the further development of a continuous casting process of the type described in such a way that it is possible to specify the formation of a desired structure of the metal as a target, etc. for metals, i.e. Different chemical composition in the continuous casting of steel for all steel qualities or steel grades to be cast. In the case of continuous steel casting, it should in particular be possible to set a certain ferrite or pearlite structure and / or to avoid precipitates, such as aluminum nitride precipitates, at the grain boundaries.
Diese Aufgabe wird bei einem Verfahren der eingangs beschriebenen Art dadurch gelöst, daß zur Ausbildung eines bestimmten Gefuges im gegossenen Strang das Stranggießen unter on-line-Berechnung unter Zugrundelegung eines die Ausbildung des bestimmten Gefuges des Metalles beschreibenden Rechenmodells durchgeführt wird, wobei die Gefügeausbildung beinflussende Variable des Stranggießverfahrens, wie zum Beispiel die zur Kühlung des Stranges vorgesehene spezifische Kühlmittelmenge, on-line-dynamisch, d.h. während des laufenden Gießens eingestellt werden. Hierbei werden gemäß einer bevorzugten Ausfuhrungsform mit dem Rechenmodell thermodynamische Zustandsänderungen des gesamten Stranges, wie Änderungen der Oberflächentemperatur, der Mittentemperatur, der Schalenstärke durch Lösen der Wärmeleitungsgleichung und Lösen von einer die Phasen-Umwandlungskinetik beschreibenden Gleichung ständig mitgerechnet und wird die Kühlung des Stranges in Abhängigkeit des errechneten Wertes mindestens einer der thermodynamischen Zustandsgrößen eingestellt, wobei für die Simulation die Strangdicke und die chemische Analyse des Metalles sowie die ständig gemessene Gießgeschwindigkeit berücksichtigt werden.This object is achieved in a method of the type described at the outset in that, in order to form a specific structure in the cast strand, the continuous casting is carried out using on-line calculation on the basis of a computational model describing the formation of the specific structure of the metal, the structure-influencing variable of the continuous casting process, such as, for example, the specific quantity of coolant provided for cooling the strand, can be set dynamically online, ie during the ongoing casting. According to a preferred embodiment, the calculation model uses thermodynamic changes in the state of the entire strand, such as changes in the surface temperature, the mean temperature, the shell thickness, by solving the heat conduction equation and solving an equation describing the phase transition kinetics, and the cooling of the strand is dependent on the calculated value set at least one of the thermodynamic state variables, the strand thickness and the chemical analysis of the metal and the continuously measured casting speed being taken into account for the simulation.
Durch die erfϊndungsgemäße Koppelung der Berechnung der Temperatur des Stranges mit dem Rechenmodell, das die Ausbildung eines bestimmten zeit- und temperaturabhängigen Gefuges des Metalles beinhaltet, ist es möglich, die Variablen des Stranggießverfahrens, die die Gefügeausbildung beeinflussen, wie z.B. die auf die Strangoberfläche aufzubringende Kühlmittelmenge, der chemischen Analyse des Metalles, sowie der örtlichen Temperaturgeschichte des Stranges anzupassen. Hierdurch kann gezielt eine gewünschte Gefügestruktur im weitesten Sinn (Korngröße, Phasenausbildung, Ausscheidungen) im oberflächennahen Bereich des Stranges erreicht werden.By coupling the calculation of the temperature of the strand according to the invention with the calculation model, which includes the formation of a specific time and temperature-dependent structure of the metal, it is possible to determine the variables of the continuous casting process that influence the structure, e.g. adapt the amount of coolant to be applied to the strand surface, the chemical analysis of the metal, and the local temperature history of the strand. In this way, a desired microstructure in the broadest sense (grain size, phase formation, excretions) can be achieved in the region of the strand near the surface.
Vorzugsweise ist in das Rechenmodell ein kontinuierliches Phasen-Umwandlungsmodell des Metalles integriert, insbesondere nach Avrami.A continuous phase conversion model of the metal is preferably integrated into the computing model, in particular according to Avrami.
Die Avrami-Gleichung beschreibt in ihrer allgemeinen Form alle diffusionsgesteuerten Umwandlungsvorgänge für die jeweilige Temperatur unter isothermen Bedingungen. Durch Berücksichtigung dieser Gleichung im Rechenmodell können ganz gezielt beim Stahl- Stranggießen Ferrit-, Perlit- und Bainit-Anteile eingestellt werden, u.zw. auch unter Berücksichtigung einer Haltezeit bei bestimmter Temperatur.In its general form, the Avrami equation describes all diffusion-controlled conversion 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 targeted manner in steel continuous casting, etc. also taking into account a holding time at a certain temperature.
Vorzugsweise ist das Verfahren dadurch gekennzeichnet, daß mit dem Rechenmodell thermische Zustandsänderungen des gesamten Stranges, wie Änderungen der Oberflächentemperatur, der Mittentemperatur, der Schalenstärke, durch Lösen der Wärmeleitungsgleichung und Lösen einer die Ausscheidungskinetik, insbesondere nichtmetallischer und intermetallischer Ausscheidungen, beschreibenden Gleichung ständig mitgerechnet werden und die Kühlung des Stranges in Abhängigkeit des errechneten Wertes mindestens einer der thermodynamischen Zustandsgrößen eingestellt wird, wobei für die Simulation die Strangdicke und die chemische Analyse des Metalles sowie die ständig gemessene Gießgeschwindigkeit berücksichtigt werden, wobei vorteilhaft die Ausscheidungskinetik aufgrund freier Phasenenergie und Keimbildung und Verwendung thermodynamischer Grundgrößen, insbesondere der Gibb'schen Energie, und das Keimwachstum nach Zener in das Rechenmodell integriert ist.The method is preferably characterized in that thermal changes in the state of the entire strand, such as changes in surface temperature, mean temperature, shell thickness, are solved by solving the thermal conduction equation and solving an equation describing the excretion kinetics, in particular non-metallic and intermetallic precipitates, and continuously the cooling of the strand is set as a function of the calculated value of at least one of the thermodynamic state variables, the strand thickness and the chemical analysis of the metal and the constant being used for the simulation Measured pouring speed are taken into account, the excretion kinetics due to free phase energy and nucleation and use of thermodynamic basic variables, in particular Gibb's energy, and the germ growth according to Zener advantageously being integrated into the calculation model.
Zweckmäßig werden auch Gefugemengenverhältnisse in Gleichgewichtszuständen gemäß Mehrstoffsystem-Diagrammen, insbesondere gemäß Fe-C-Diagramm, in das Rechenmodell integriert.Structural quantity relationships in equilibrium states according to multi-substance system diagrams, in particular according to the Fe-C diagram, are also expediently integrated into the calculation model.
Vorzugsweise sind in das Rechenmodell Komwachstumseigenschaften, insbesondere unter Berücksichtigung von Rekristallisation des Metalles, integriert. Hierbei kann eine dynamische und/oder verzögerte und/oder eine post-Rekristallisation, d.h. eine Rekristallisation, die später in einem Ofen stattfindet, im Rechenmodell berücksichtigt werden.Grain growth properties are preferably integrated into the computing model, in particular taking into account recrystallization of the metal. Dynamic and / or delayed and / or post-recrystallization, i.e. a recrystallization that later takes place in an oven must be taken into account in the calculation model.
Vorzugsweise wird als die Gefugeausbildung beeinflussende Variable des Stranggießens eine während des Ausforderns des Stranges stattfindende Dickenreduktion vor und/oder nach Durcherstarrung des Stranges zusätzlich zur den Strang beaufschlagenden spezifischen Kühlmittelmenge on-line eingestellt, so daß auch während des Stranggießens stattfindende thermodynamische Walzungen, beispielsweise Hochtemperatur-thermodynamische Walzungen bei einer Oberflächentemperatur größer Ac3 berücksichtigt werden können.Preferably, as the variable of the continuous casting which influences the formation of the microstructure, a reduction in thickness which takes place during the loading of the strand before and / or after solidification of the strand is set on-line in addition to the specific amount of coolant acting on the strand, so that thermodynamic rolling, for example high-temperature rolling, which takes place during the continuous casting thermodynamic rolling with a surface temperature greater than A c3 can be taken into account.
Weiters wird vorzugsweise mit dem Rechenmodell auch der mechanische Zustand, wie das Verformungsverhalten, durch Lösen weiterer Modellgleichungen, insbesondere durch Lösen der Wärmeleitgleichung ständig mitgerechnet.Furthermore, the mechanical state, such as the deformation behavior, is preferably also constantly included in the calculation model by solving further model equations, in particular by solving the thermal conductivity equation.
Eine bevorzugte Ausführungsform ist dadurch gekennzeichnet, daß mengenmäßig definierte Phasenanteile durch Aufbringen on-line errechneter spezifischer Strang-Kühlmittelmengen vor und/oder nach der Durcherstarrung des Stranges eingestellt werden.A preferred embodiment is characterized in that phase components defined in terms of quantity are set by applying specific strand coolant quantities calculated on-line before and / or after the strand has solidified.
Weiters wird zweckmäßig ein definiertes Gefüge durch Aufbringen einer on-line errechneten Strangverformung vor und/oder nach der Durcherstarrung des Stranges, welche eine Rekristallisation des Gefuges bewirkt, eingestellt.Furthermore, a defined structure is expediently set by applying an on-line strand deformation calculated before and / or after the strand has solidified, which causes the structure to recrystallize.
Eine vorteilhafte Variante des erfindungsgemäßen Verfahrens ist dadurch gekennzeichnet, daß die zur das Stranggießen abschließende Phasenumwandlung mit Einstellung eines mengenmäßig definierten Phasenanteiles des Stranges errechnete spezifische Strang- Kühlmittelmenge nach Durcherstarrung des Stranges im Endbereich einer Sekundärkühlzone in einer eine verstärkte Kühlung bewirkenden Kühlzone eingestellt wird.An advantageous variant of the method according to the invention is characterized in that the phase transformation that concludes the continuous casting with setting of a phase component of the strand that is defined in terms of quantity, calculated specific strand The amount of coolant is set after solidification of the strand in the end region of a secondary cooling zone in a cooling zone causing increased cooling.
Die Erfindung ist nachfolgend für das Stahlstranggießen näher erläutert. Eine Anwendung des erfindungsgemäßen Verfahrens für andere Metalle kann analog zu den nachstehenden Ausführungen vorgenommen werden.The invention is explained in more detail below for continuous steel casting. The method according to the invention can be used for other metals analogously to the explanations below.
Das erfindungsgemäß zu verwendende Rechenmodell läßt aufgrund einer vorgegebenen chemischen Analyse des Stahls, der Austenitkorngröße und der Temperaturgeschichte des Stranges sämtliche Umwandlungstemperaturen und -daten, die zur Vorhersage und Beschreibung der Umwandlungsvorgänge für die Phasenanteile Ferrit, Perlit, Bainit und Martensit notwendig sind, berechnen.Based on a predetermined chemical analysis of the steel, the austenite grain size and the temperature history of the strand, the calculation model to be used according to the invention can calculate all transformation temperatures and data which are necessary for predicting and describing the transformation processes for the phase fractions ferrite, pearlite, bainite and martensite.
Hierfür wird zunächst ein Kohlenstoffäquivalent für die einzelnen Legierungsbestandteile errechnet. Daraus ergeben sich analysenabhängige Starttemperaturen für die Ferritumwandlung, für die Perlitumwandlung, die Bainitbildung und die Martensitbildung (aufgrund des Eisen/Kohlenstoff-Diagramms).For this, a carbon equivalent is first calculated for the individual alloy components. This results in analysis-dependent starting temperatures for ferrite conversion, pearlite conversion, bainite formation and martensite formation (based on the iron / carbon diagram).
Aufgrund der Avrami-Gleichung, die in ihrer allgemeinen Form alle diffusionsgesteuerten Umwandlungsvorgänge für die jeweilige Temperatur unter isothermen Bedingungen beschreibt, lassen sich Grundgleichungen für die Umwandlungskurven ermitteln.Based on the Avrami equation, which describes in its general form all diffusion-controlled conversion processes for the respective temperature under isothermal conditions, basic equations for the conversion curves can be determined.
X = 1 - exp(-b-tn)X = 1 - exp (-bt n )
worin X der Mengenanteil der umgewandelten Phase und b und n Parameter bedeuten, die abhängig sind von der Keimbildung und dem Wachstum der gebildeten Phase. Diese Parameter b und n sind analysenabhängig und können durch Dilatometer- Versuche bestimmt werden. Im Zusammenhang mit ZTU-Diagrammen lassen sich mit Hilfe der Avrami- Gleichung sowohl die Start- und die Endzeit als auch die Temperatur für die Ferrit-, Perlit- und Bainit-Umwandlung unter isothermischen Bedingungen berechnen.where X is the proportion of the converted phase and b and n are parameters which are dependent on the nucleation and the growth of the phase formed. These parameters b and n are dependent on the analysis and can be determined by dilatometer tests. In connection with ZTU diagrams, the Avrami equation can be used to calculate the start and end times as well as the temperature for the ferrite, pearlite and bainite transformation under isothermal conditions.
Um nicht-isothermische Umwandlungen zu berücksichtigen, also die in der Stranggießanlage stattfindende - gegebenenfalls auch ungleichmäßig stattfindende - Kühlung des Stranges voll berücksichtigen zu können, wird aufgrund der im Rechner gespeicherten ZTU-Schaubilder und der Abhängigkeit der Temperatur als eine Funktion der Zeit der Anteil an umgewandeltem Material berechnet, u.zw. durch eine Integration der Avrami-Gleichung über die Kühlzeit des Stranges (vgl. T.T. Pham, E.B. Hawbolt, J.K. Brimacombe: "Preciding the onset of transformation under non continuous cooling conditions. II Application to austenite - pearlite transformation", Met. Mat. Trans. A, 26A, pp. 1993-2000, 1995).In order to take into account non-isothermal conversions, i.e. to be able to fully take into account the cooling of the strand taking place in the continuous casting installation - possibly also taking place non-uniformly - the proportion is shown as a function of time on the basis of the ZTU diagrams stored in the computer and the dependence of the temperature converted material, etc. by integrating the Avrami equation over the cooling time of the strand (cf. TT Pham, EB Hawbolt, JK Brimacombe: "Preciding the onset of transformation under non continuous cooling conditions. II Application to austenite - pearlite transformation ", Met. Mat. Trans. A, 26A, pp. 1993-2000, 1995).
X(t) = l s(τ)[l-exp(-b-tπ)] -dtX (t) = l s ( τ ) [l-exp (-bt π )] -dt
wobei ts(T) eine virtuelle Beginnzeit der Umwandlung bei einer Temperatur T in Übereinstimmung zur tatsächlich umgewandelten Menge bedeutet.where t s (T) means a virtual start time of the conversion at a temperature T in accordance with the amount actually converted.
Für diesen Berechnungsalgorithmus wird die Temperatur als Funktion der Zeit definiert. Da der berechnete Umwandlungs- bzw. Ausscheidungsanteil nach Avrami keine Auskunft über die tatsächlichen Gefüge/Mengen- Verhältnisse gibt, sondern lediglich erkennen läßt, ob und wie der Gleichgewichtszustand erreicht wird, werden zur Bestimmung des Gefügeanteils die Umwandlungsanteile auf die Gleichgewichtslinien aus dem Eisen/Kohlenstoff-Diagramm bezogen und ebenfalls im Rechenmodell berücksichtigt.For this calculation algorithm, the temperature is defined as a function of time. Since the calculated conversion or excretion percentage according to Avrami does not provide any information about the actual microstructure / quantity ratios, but only reveals whether and how the equilibrium state is reached, the conversion fractions on the equilibrium lines from the iron / carbon are used to determine the microstructure ratio Diagram related and also taken into account in the calculation model.
Keimbildungsvorgänge werden aufgrund der chemischen Gibb'schen Energie bzw. Phasenenergie im Rechenmodell berücksichtigt (nachstehend für Aluminiumnitride gezeigt).Nucleation processes are taken into account in the calculation model based on the chemical Gibb's energy or phase energy (shown below for aluminum nitrides).
ΔGchem = Δ G°A1N - R ■ T • (In XAI + In XN)ΔG chem = Δ G ° A1N - R ■ T • (In XAI + In X N )
wobei G .0 AIN die Standard Gibb'sche Energie für die Bildung von AIN, XAI der Molanteil von Aluminium im Austenitvolumen und XN der Durchschnittsstickstoffgehalt bedeuten. Die Keimbildungsrate läßt sich wie folgt berechnen:where G .0 A I N is the standard Gibb energy for the formation of AIN, X AI is the molar fraction of aluminum in the austenite volume and X N is the average nitrogen content. The nucleation rate can be calculated as follows:
I = S • DAι • XAI • expI = S • D A ι • XAI • exp
worin S die Dichte der Keimbildung im Austenit bedeutet.where S is the density of nucleation in austenite.
ΔGcrit =ΔG crit =
gibt die Bedingung für die Keimbildung wieder. Hierin ist σ die Austenit/AIN- Grenzflächenenergie. kß ist die Boltzmannkonstante und DAι das Ausbreitungsvermögen von Aluminium in Austenit. Das Keimwachstum wird nach Zener berücksichtigt (z.B. abgehandelt in J.S. Kirkaldy, "Diffusion in the Condensed State", The Universities Press, Belfast, 1985).reflects the condition for nucleation. Herein σ is the austenite / AIN interface energy. k ß is the Boltzmann constant and D A ι is the spreading capacity of aluminum in austenite. The germ growth is taken into account according to Zener (for example, discussed in JS Kirkaldy, "Diffusion in the Condensed State", The Universities Press, Belfast, 1985).
Das Rechenverfahren geht in zwei Hauptstufen vor sich. In der ersten Stufe wird die Anzahl der aktuell gebildeten Keime bestimmt und in der zweiten Stufe wird das Wachstum aller vorhergehend gebildeten Ausscheidungen berechnet.The calculation process takes place in two main stages. In the first stage the number of currently formed germs is determined and in the second stage the growth of all previously formed excretions is calculated.
Zur weiteren Erläuterung der Erfindung dient die beiliegende Figur.The attached figure serves to further explain the invention.
Gemäß dieser wird ein Stahlstrang 1 aus einer Stahlschmelze 2 mit einer bestimmten chemischen Zusammensetzung durch Gießen in einer Durchlaufkokille 3 gebildet. Die Stahlschmelze 2 wird aus einer Gießpfanne 4 über ein Zwischengefäß 5 und ein vom Zwischengefäß 5 mittels eines unter den in der Durchlaufkokille 3 gebildeten Gießspiegel reichenden Gießrohres 6 in die Durchlaufkokille 3 gegossen. Unterhalb der Durchlaufkokille 3 sind Strangführungsrollen 7 zur Abstützung des Stahlstranges 1 vorgesehen, der noch einen flüssigen Kern 8 und zunächst eine nur sehr dünne Strangschale 9 aufweist.According to this, a steel strand 1 is formed from a molten steel 2 with a certain chemical composition by casting in a continuous mold 3. The molten steel 2 is poured from a ladle 4 via an intermediate vessel 5 and one from the intermediate vessel 5 into the continuous mold 3 by means of a pouring tube 6 extending under the casting level formed in the continuous mold 3. Below the continuous mold 3, strand guide rollers 7 are provided for supporting the steel strand 1, which still has a liquid core 8 and initially only a very thin strand shell 9.
Der aus der Durchlaufkokille mit gerader Achse austretende Stahlstrang 1 wird in einer Biegezone 10 in eine Kreisbogenbahn 11 umgelenkt und in dieser ebenfalls durch Strangführungsrollen 7 gestützt. In einer der Kreisbogenbahn 11 nachfolgenden Richtzone 12 wird der Stahlstrang 1 wiederum geradegerichtet und über einen Auslaufrollgang ausgefördert oder direkt on-line dickenreduziert, z.B. mittels eines on-line angeordneten Walzgerüstes 13.The steel strand 1 emerging from the continuous mold with a straight axis is deflected in a bending zone 10 into a circular arc path 11 and is also supported in this by strand guide rollers 7. In a straightening zone 12 following the circular arc path 11, the steel strand 1 is again straightened and conveyed out via an outfeed roller table or directly reduced in thickness on-line, e.g. by means of an on-line mill stand 13.
Zur Kühlung des Stahlstranges 1 wird dieser direkt oder indirekt - über mit einer Innenkühlung versehene Strangführungsrollen 7 - gekühlt, wodurch an seiner Oberfläche bis in einen gewissen Tiefenbereich eine bestimmte Temperatur eingestellt werden kann.To cool the steel strand 1, it is cooled directly or indirectly - via strand guide rollers 7 provided with internal cooling - so that a certain temperature can be set on its surface to a certain depth range.
Die Versorgung des Stahlstranges 1 mit der für das gewünschte Gefüge des Stahlstranges 1 notwendigen Kühlmittelmenge erfolgt über einen geschlossenen oder offenen Regelkreis mittels eines Rechners 14. In den Rechner 14 werden Maschinendaten m, das Format f des Stahlstranges 1, Materialdaten, wie die chemische Analyse StCh der Stahlschmelze 2, der Gießzustand z, die Gießgeschwindigkeit v, die Flüssigstahltemperatur tn, mit der die Stahlschmelze 2 in die Durchlaufkokille 3 eintritt, sowie das gewünschte Gefüge α/γ und gegebenenfalls eine Verformung w des Stahlstranges 1, die am Wege der Strangführung durchgeführt wird, eingegeben. Diese Verformung kann z.B. auch durch das Geraderichten des Stahlstranges 1 in der Richtzone 12 gegeben sein. In dem Rechner 14 wird anhand eines metallurgischen Rechenmodells, das die Phasenumwandlungskinetik und Keimbildungskinetik gemäß der oben angegebenen Rechenmodelle berücksichtigt, und eines thermischen Rechenmodells, das die Temperaturanalyse aufgrund der Lösung der Wärmeleitungsgleichung ermöglicht, eine Soll- Wassermenge Qs errechnet, u.zw. aufgrund der aktuellen, bereits aufgebrachten Wassermenge QA, die ebenfalls in den Rechner eingegeben wird.The steel strand 1 is supplied with the amount of coolant required for the desired structure of the steel strand 1 via a closed or open control circuit by means of a computer 14. Machine data m, the format f of the steel strand 1, material data, such as the chemical analysis St C h of the molten steel 2, the pouring state z, the pouring speed v, the molten steel temperature tn at which the molten steel 2 enters the continuous mold 3, as well as the desired structure α / γ and possibly a deformation w of the steel strand 1, which is on the way of the strand guidance is entered. This deformation can also be given, for example, by straightening the steel strand 1 in the straightening zone 12. In the computer 14, a set amount of water Q s is calculated on the basis of a metallurgical calculation model that takes into account the phase change kinetics and nucleation kinetics according to the calculation models specified above, and a thermal calculation model that enables the temperature analysis based on the solution of the heat conduction equation, etc. due to the current, already applied amount of water Q A , which is also entered into the computer.
Eine Lösung der Wärmeleitungsgleichung mittels eines Prozeßrechners ist Stand der Technik und z.B. in der DE-C2 - 44 17 808 für das Stranggießen ausführlich abgehandelt. Als eine Möglichkeit zur Lösung der Wärmeleitungsgleichung ist das Finite Differenzen Verfahren mit Lagrangescher Beschreibungsweise angegeben.A solution of the heat conduction equation using a process computer is state of the art and e.g. dealt with in detail in DE-C2 - 44 17 808 for continuous casting. The finite difference method with Lagrangian description is given as one way of solving the heat conduction equation.
Das metallurgische Rechenmodell berücksichtigt die aktuelle Stahlanalyse StC , um unterschiedlichen Werkstoffverhalten gerecht zu werden. Die durch das thermische Rechenmodell errechnete aktuelle Temperatur TA wird on-line dem metallurgischen Rechenmodell zugeführt und dieses errechnet laufend die gewünschte Soll-Temperatur Ts, aufgrund der das thermische Rechenmodell die Soll-Wassermenge Qs für die einzelnen Strangkühlungsabschnitte errechnet und automatisch einstellt. The metallurgical calculation model takes the current steel analysis St C into account in order to cope with different material behavior. The current temperature T A calculated by the thermal calculation model is fed on-line to the metallurgical calculation model and this continuously calculates the desired target temperature T s , on the basis of which the thermal calculation model calculates and automatically sets the target water quantity Q s for the individual strand cooling sections.

Claims

P atentansprüche : Patent claims:
1. Verfahren zum Stranggießen eines Metallstranges, insbesondere eines Stahlstranges (1), wobei ein Strang (1) aus einer gekühlten Durchlaufkokille (3) ausgezogen, in einer der Durchlaufkokille (3) nachgeordneten Strangstützeinrichtung (7, 11) gestützt und mit Kühlmittel gekühlt sowie gegebenenfalls dickenreduziert wird, dadurch gekennzeichnet, daß zur Ausbildung eines bestimmten Gefuges im gegossenen Strang das Stranggießen unter online-Berechnung unter Zugrundelegung eines die Ausbildung des bestimmten Gefuges des Metalles beschreibenden Rechenmodells durchgeführt wird, wobei die Gefügeausbildung beinflussende Variable des Stranggießverfahrens, wie zum Beispiel die zur Kühlung des Stranges vorgesehene spezifische Kühlmittelmenge, on-line-dynamisch, d.h. während des laufenden Gießens eingestellt werden.1. A method for continuous casting of a metal strand, in particular a steel strand (1), a strand (1) being pulled out of a cooled continuous mold (3), supported in a strand support device (7, 11) downstream of the continuous mold (3) and cooled with coolant and if necessary, reduced thickness, characterized in that for the formation of a certain structure in the cast strand, the continuous casting is carried out using online calculation on the basis of a calculation model describing the formation of the particular structure of the metal, the structure formation influencing variable of the continuous casting process, such as, for example, the Cooling of the strand provided specific amount of coolant, on-line dynamic, ie can be set during the casting process.
2. Verfahren nach Anspruch 1, dadurch gekennzeichnet, daß mit dem Rechenmodell thermodynamische Zustandsänderungen des gesamten Stranges, wie Änderungen der Oberflächentemperatur, der Mittentemperatur, der Schalenstärke durch Lösen der Wärmeleitungsgleichung und Lösen von einer die Phasen-Umwandlungskinetik beschreibenden Gleichung ständig mitgerechnet werden und die Kühlung des Stranges in Abhängigkeit des errechneten Wertes mindestens einer der thermodynamischen Zustandsgrößen eingestellt wird, wobei für die Simulation die Strangdicke und die chemische Analyse des Metalles sowie die ständig gemessene Gießgeschwindigkeit berücksichtigt werden.2. The method according to claim 1, characterized in that thermodynamic changes in state of the entire strand, such as changes in the surface temperature, the mean temperature, the shell thickness by solving the heat conduction equation and solving an equation describing the phase conversion kinetics are continuously included in the calculation model and the cooling of the strand is set as a function of the calculated value of at least one of the thermodynamic state variables, the strand thickness and the chemical analysis of the metal and the continuously measured casting speed being taken into account for the simulation.
3. Verfahren nach Anspruch 2, dadurch gekennzeichnet, daß in das Rechenmodell ein kontinuierliches Phasen-Umwandlungsmodell des Metalles integriert ist, insbesondere nach Avrami.3. The method according to claim 2, characterized in that a continuous phase conversion model of the metal is integrated in the computing model, in particular according to Avrami.
4. Verfahren nach Anspruch 1, 2 oder 3, dadurch gekennzeichnet, daß mit dem Rechenmodell thermische Zustandsänderungen des gesamten Stranges, wie Änderungen der Oberflächentemperatur, der Mittentemperatur, der Schalenstärke, durch Lösen der Wärmeleitungsgleichung und Lösen einer die Ausscheidungskinetik, insbesondere nichtmetallischer und intermetallischer Ausscheidungen, beschreibenden Gleichung ständig mitgerechnet werden und die Kühlung des Stranges in Abhängigkeit des errechneten Wertes mindestens einer der thermodynamischen Zustandsgrößen eingestellt wird, wobei für die Simulation die Strangdicke und die chemische Analyse des Metalles sowie die ständig gemessene Gießgeschwindigkeit berücksichtigt werden. 4. The method according to claim 1, 2 or 3, characterized in that with the computer model thermal changes in the state of the entire strand, such as changes in the surface temperature, the mean temperature, the shell thickness, by solving the heat conduction equation and solving the excretion kinetics, in particular non-metallic and intermetallic excretions , descriptive equation are constantly included in the calculation and the cooling of the strand is set depending on the calculated value of at least one of the thermodynamic state variables, the strand thickness and the chemical analysis of the metal and the continuously measured casting speed being taken into account for the simulation.
5. Verfahren nach Anspruch 4, dadurch gekennzeichnet, daß die Ausscheidungskinetik aufgrund freier Phasenenergie und Keimbildung und Verwendung thermodynamischer Grundgrößen, insbesondere der Gibb'schen Energie, und das Keimwachstum nach Zener in das Rechenmodell integriert ist.5. The method according to claim 4, characterized in that the excretion kinetics due to free phase energy and nucleation and use of thermodynamic basic variables, in particular Gibb's energy, and the germ growth according to Zener is integrated in the computing model.
6. Verfahren nach einem oder mehreren der Ansprüche 2 bis 5, dadurch gekennzeichnet, daß auch Gefugemengenverhältnisse in Gleichgewichtszuständen gemäß Mehrstoffsystem-Diagrammen, insbesondere gemäß Fe-C-Diagramm, in das Rechenmodell integriert sind.6. The method according to one or more of claims 2 to 5, characterized in that structural quantity relationships in equilibrium states according to multi-substance system diagrams, in particular according to the Fe-C diagram, are integrated into the computing model.
7. Verfahren nach einem oder mehreren der Ansprüche 2 bis 6, dadurch gekennzeichnet, daß in das Rechenmodell Kornwachstumseigenschaften, insbesondere unter Berücksichtigung von Rekristallisation des Metalles, integriert sind.7. The method according to one or more of claims 2 to 6, characterized in that grain growth properties are integrated in the computing model, in particular taking into account recrystallization of the metal.
8. Verfahren nach einem oder mehreren der Ansprüche 1 bis 7, dadurch gekennzeichnet, daß als die Gefugeausbildung beeinflussende Variable des Stranggießens eine während des Ausfördems des Stranges stattfindende Dickenreduktion vor und/oder nach Durcherstarrung des Stranges zusätzlich zur den Strang beaufschlagenden spezifischen Kühlmittelmenge on-line eingestellt wird.8. The method according to one or more of claims 1 to 7, characterized in that as the structure forming variable of the continuous casting a thickness reduction taking place during the conveying of the strand before and / or after solidification of the strand in addition to the specific amount of coolant acting on the strand on-line is set.
9. Verfahren nach einem oder mehreren der Ansprüche 1 bis 8, dadurch gekennzeichnet, daß mit dem Rechenmodell auch der mechanische Zustand, wie das Verformungsverhalten, durch Lösen weiterer Modellgleichungen, insbesondere durch Lösen der Wärmeleitgleichung, ständig mitgerechnet wird.9. The method according to one or more of claims 1 to 8, characterized in that the mechanical model, such as the deformation behavior, is constantly included in the calculation by solving further model equations, in particular by solving the thermal conductivity equation.
10. Verfahren nach einem oder mehreren der Ansprüche 1 bis 9, dadurch gekennzeichnet, daß mengenmäßig definierte Phasenanteile durch Aufbringen on-line errechneter spezifischer Strang-Kühlmittelmengen vor und/oder nach der Durcherstarrung des Stranges eingestellt werden.10. The method according to one or more of claims 1 to 9, characterized in that quantitatively defined phase fractions are set by applying on-line calculated specific strand coolant amounts before and / or after solidification of the strand.
11. Verfahren nach einem oder mehreren der Ansprüche 1 bis 10, dadurch gekennzeichnet, daß ein definiertes Gefüge durch Aufbringen einer on-line errechneten Strangverformung vor und/oder nach der Durcherstarrung des Stranges, welche eine Rekristallisation des Gef ges bewirkt, eingestellt wird.11. The method according to one or more of claims 1 to 10, characterized in that a defined structure is set by applying an on-line calculated strand deformation before and / or after the solidification of the strand, which causes re-crystallization of the structure.
12. Verfahren nach einem oder mehreren der Ansprüche 1 bis 10, dadurch gekennzeichnet, daß eine abschließende Phasenumwandlung, gegebenenfalls unter Berücksichtigung einer nachfolgenden Rückumwandlung, nach Durcherstarrung des Stranges in einer eine verstärkte Kühlung bewirkenden Kühlzone eingestellt wird. 12. The method according to one or more of claims 1 to 10, characterized in that a final phase change, optionally under Consideration of a subsequent reverse conversion, after solidification of the strand is set in a cooling zone causing increased cooling.
EP01942855A 2000-06-02 2001-06-01 Method for continuously casting a metal strand Expired - Lifetime EP1289691B2 (en)

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AT0097200A AT409352B (en) 2000-06-02 2000-06-02 METHOD FOR CONTINUOUSLY casting a METAL STRAND
PCT/AT2001/000183 WO2001091943A1 (en) 2000-06-02 2001-06-01 Method for continuously casting a metal strand

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