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)
• Treatment Of Steel In Its Molten State (AREA)
• Manufacture And Refinement Of Metals (AREA)

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• 2000
  • 2000-06-02 AT AT0097200A patent/AT409352B/de not_active IP Right Cessation
• 2001
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  • 2001-06-01 WO PCT/AT2001/000183 patent/WO2001091943A1/fr active IP Right Grant
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