RU2058406C1 - Method for control of heating and remelting - Google Patents

Abstract

FIELD: electrometallurgy and power and heat engineering, devices having aggressive heating media (electrolytes, plasma, etc). SUBSTANCE: method for control of heating and remelting includes measurement of temperature of electrode part not coming in contact with the main source of heating, and calculation on the basis of the obtained data of effective temperature of heater and its regulation by devication from the preset one for the period between measurements or for a time unit. EFFECT: higher efficiency.

Description

 The invention relates to the field of electrometallurgy and is intended for use in technology and equipment for electroslag, vacuum-arc and plasma-arc remelting of ingots of high-quality and special steels and alloys, as well as for their electroslag welding. Common to all these processes is the heating and melting of the electrodes (in the form of rods) under the action of heat released in the electrolyte or plasma by a passing current. The method can also be used to control and control other thermal processes in which heat-conducting rods are in contact with an aggressive heating medium, the temperature of which must be controlled (plasma generators, high-temperature sources of electric current, etc.).

 Known methods of controlling these processes [1,2] are divided into two main types: the first are based on direct measurement of temperature in the heating and melting zone (working zone) of the furnace, the second use data on the electric power of the furnace, heater resistance, radiation from the working zone and others indirect parameters.

 Another sign of the difference in the considered methods is the way in which the measured parameters are used for control. There are also two main approaches: in the first, the measured parameter is directly a criterion for changing the control action (for example, switching the stage of the supply furnace of the transformer), in the second, mathematical processing of the measurement results is carried out with the calculation of indirect parameters of other parameters that are closer to determining the quality of the product (depth metal bath in the furnace, the crystallization rate of this metal, etc., on which the structure and properties of the ingot and products from it depend).

 The reason for measuring not direct, but indirect parameters is the high temperature and chemical aggressiveness of the medium in the working space of the furnace during remelting (usually this is slag-electrolyte or arc plasma). Therefore, greater reliability of control is provided by systems with measuring indirect parameters, which include the claimed method.

 Analogs differ in type, method of influence on the remelting process, or, in other words, in the form of controlled parameters. Most often, these parameters are the voltage at the heating source (slag bath, arc, winding of the furnace), as well as the feed rate of the metal (remelted electrode), which is used in these analogues. However, other methods are also known, for example, the introduction of additives into a slag bath, on the resistance of which the current generates heat (A.S. USSR N 1507834, class C 22 B 9/18, 1989).

 By the number of indirect parameters measured, analogues can be divided into methods with one parameter or with several parameters. In the latter case, all parameters are processed by the calculation method using a single thermophysical model, which allows you to evaluate any direct technological parameter, according to which control is carried out. These methods are based on modern automated control systems (Makhnenko V.I. et al. Automation of industrial furnaces with electronic computers using computers. In collection Electroslag technology. Kiev: Naukova Dumka, 1988, pp. 38-44).

 However, the accuracy of the currently constructed mathematical models with several input indirect parameters is limited, and the cost of improving accuracy by adjusting the coefficients requires a large number of expensive industrial melts, therefore such systems are used only as auxiliary, and the simpler systems described above carry out the main regulation .

Analogs provide for the measurement of parameters related mainly to the heater (resistance, power, voltage, current, radiation of the slag bath, arc) and to the ingot obtained (ingot temperature, melt depth in the forming ingot, heat removal from the ingot) and rarely use information taken with a remelted electrode. An exception is the method of controlling the output voltage of the power source of the ESR furnace [3] in which control is carried out according to the data on the change in the voltage drop on the electrode resistance. It is control that is carried out by an indirect parameter using mathematical data processing:
U _n U _o + n · Z · J, where Z is the constant of the secondary circuit of the furnace;
n the number of electrode sections determined during melting, by which its length and resistance have changed (maximum n is chosen equal to the number of voltage change stages);
U _{n the} voltage at this stage of regulation, depending on the resistance of the electrode;
U _{about the} maximum voltage of the furnace power source (transformer).

 Thus, the essence of this analogue, taken as a prototype, consists in measuring the electrode parameter (n) and in changing the supply voltage of the furnace according to the change in the electrode parameter. This relationship is expressed by the above mathematical relationship.

The disadvantage of the prototype is that the resistance of the electrode is associated with the quality of the ingot in a very indirect way, and therefore the determination of the numerical values of U _o and Z requires a series of experimental melts for each grade of steel, slag, size of the ingot, which requires significant costs and is applicable only in serial production .

 The purpose of the invention is the reduction of the cost of experimental swimming trunks, during which the coefficients in the formulas used in the calculation of control parameters are specified. This goal is achieved by the fact that monitoring and control are carried out only by directly interconnected thermal parameters of the process, the temperature of the melting electrode and the temperature of its heating medium (slag electrolyte or plasma gas). In this case, the mathematical expression connecting these quantities has a relatively simple form (1), it includes only one directly undetectable quantity, the heat-transfer medium-electrode heat transfer coefficient. It can be determined with sufficient accuracy using just one experimental heat. Thus, by measuring the temperature in the solid electrode, which is quite easily and with high accuracy, it becomes possible to reliably estimate the temperature of the molten electrolyte or plasma in the zone of contact with the melting metal, which (temperature) cannot be continuously measured by direct methods (thermocouples) in connection with high temperature and chemical aggressiveness of the environment. The presence of layers of slag or plasma with a lower temperature around this zone does not allow the remote measurement of the parameters of these media to be used for control, for example, by radiation intensity (pyrometers, spectrometers, etc.). The use of more complex plasma diagnostic methods is not technically feasible.

 The essence of the claimed invention is as follows.

 The temperature of the heated electrode is measured along the length of its part that is not in contact with the heating medium (electrolyte or plasma). Calculate the temperature of the electrode along its length, which are compared with the measured values. In the calculation process carried out by the microprocessor, the value of the effective temperature of the heating medium (electrolyte or plasma) is found. The deviation of the found effective temperature of the electrolyte or plasma from that set by the melting technology controls (proportional change) the voltage of the furnace power source. Since the determined “effective” temperature directly characterizes the interaction of the melting metal and the heating medium, its control provides control of the heating, melting, and crystallization process of the remelted metal occurring under close conditions.

Expression (1):
c · ρ · (∂T _calculation ) / ∂ t λ · (∂ ² · T _calculation ) / ∂ x ² +
+ α ₁ · (T _air T _calc ) + α · (T _load T _calc ) + f, where c, ρ, λ heat capacity, density and thermal conductivity of the electrode material;
T _calc electrode temperature;
x coordinate of the temperature determination point along the length of the electrode;
α _{1 is} the electrode-air heat transfer coefficient (along the length of the electrode surface);
α heat transfer coefficient "electrode-heating medium" on the immersed part of the electrode;
f is the power released in the electrode by the passing current (Joule heat).

[α(T_изм-T_расч)T_нагр+b•T $\genfrac{}{}{0ex}{}{\text{2}}{\text{нагр}}$ ] dx где l длина электрода в момент измерения температуры;
х координата точки измерения и расчета температуры по длине электрода;
Т_изм измеряемая температура электрода в точке х;
Т_расч температура электрода Т, рассчитываемая по выражению (1) в точке х;
Т_нагр искомая температура нагревателя (электролита или плазмы);
α α ₂ коэффициент теплоотдачи "электрод-нагреватель";
b вспомогательный коэффициент (подбирается при расчетах в зависимости от погрешности измерений температуры δ Т, например b=

.Expression (2):
-

[α (T _measurement -T _calculation ) T _load + b • T $\genfrac{}{}{0ex}{}{\text{2}}{\text{heat}}$ ] dx where l is the length of the electrode at the time of temperature measurement;
x coordinate of the point of measurement and calculation of temperature along the length of the electrode;
T _meas measured electrode temperature at point x;
T _{calc the} temperature of the electrode T, calculated by the expression (1) at point x;
T _LOAD desired heater temperature (electrolyte or plasma);
α α ₂ heat transfer coefficient "electrode-heater";
b auxiliary coefficient (selected in the calculations depending on the error of temperature measurements δ T, for example, b =

.

Thus, the advantages of the proposed method compared with the prototype are as follows:
in the calculation part of the method, the value of only one coefficient of heat transfer "electrode-heater" (slag) is necessary, and in the prototype it is necessary to choose two coefficients U _o and Z (selection of coefficients requires expensive pilot melts, and the less coefficients you need to find, the less melting you need );
in the inventive method, the relationship between the measured temperature and the desired temperature of the heater is given by the well-known heat equation, which is a mathematical model for describing thermal processes in rods of any nature and can be used to control the processes of remelting of different steels, electrodes of any diameter and length. Therefore, the found heat transfer coefficients are slightly variable values and are easily refined, while the parameters U _o and Z in the prototype can arbitrarily change when changing technology.

 The claimed method as a sequence of actions in detail is as follows.

Before the start of the process, it is assumed that some value of the heater temperature T _heats and values of thermophysical constants c, ρ λ, α α ₁ are taken according to the experience of similar processes. A time discretization step Δ t is selected, through which T _loads will be updated and the process will be adjusted. The values of T _min and T _{max are recorded for the} smallest and greatest possible values for T _loads , which are taken according to the experience of similar processes or from reasonable considerations.

Equation (1) is solved from which T = T _calculation is found at the temperature measurement points for a time t = t _beg + Δ t. To solve the equation, you can use finite-difference methods.

In a specific thermophysical heating process the rod is measured at time t = t + Δ t _nach temperature T _edited in a number of points along the length of the rod above the area of its contact with the aggressive medium. The result is a series of numbers related to known coordinates of length x.

T_max где T_* α /2b · (T_изм Т_расч).According to the obtained data T _ISM and T _calculation for the time t = t _beg + Δ t find the minimum of expression (2). The integral is not calculated, but the well-known formula for the minimum point is used
T _load =

T _max where T _* α / 2b · (T _rev T _calculation ).

The resulting _heating and T is the desired value of the heater temperature at the time t = t + Δ t _nach
The obtained value of the temperature of the heater is compared with the required technology and according to any of the known laws of regulation (for example, proportional), the voltage on the heater or the flow rate of combustible gas or the intensity of another heat source is changed in accordance with the result of this comparison.

The onset of _early time point t + Δ t is taken as the starting point to repeat steps 2, 3, 4, 5 to the end of the process.

 The described procedure contains actions that distinguish it from the prototype, and actions performed by known methods (taking measurements, finding a minimum, proportional control). The distinctive essence of the proposed method in comparison with the prototype is that, as an indirect parameter, depending on the value of which the amount of heat supplied to the heating and remelting (melting) of the rod electrode is changed using one of the known methods, the effective temperature of the heating source located in contact with the electrode. The value of this indirect parameter is determined on the basis of data on the change in the temperature of the electrode on the part that does not contact the heat source, which does not allow direct temperature measurements due to its thermochemical aggressiveness or for other reasons.

 PRI me R. According to the above, the method is implemented in three sequential operations. When remelting a 22K steel ingot weighing 60 tons on an OKB-1111 furnace, the electrode temperatures were measured using 8 thermocouples (minted in 1 m increments) after 15 minutes during melting (44 hours). The calculation of the auxiliary model corresponding to expression (1) was carried out on an IВМ RS / AT computer using an explicit difference scheme (Samarsky AA Theory of difference schemes. M. Nauka, 1977). The temperature of the heater (slag) was determined as a minimum of expression (2). Fixed during melting with control according to the claimed method, the mode turned out to be close to that worked during numerous melts of this steel. As a result, a high-quality ingot corresponding to the technical conditions was obtained. The results of measurement and calculation of temperatures according to formulas (1) and (2) are shown in figures 1 and 2.

The advantages of the proposed method are the possibility of reducing the number of pilot melts of new steel grades of various sizes of ingots from 3-5 to 1-2. The reduction in the number of heats in comparison with the prototype is due to a simpler and more direct dependence of the measured parameter of the electrode with the quality indicators of the ingot (in the claimed method, this physical connection is the basis of the method, and in the prototype it is indirectly specified through the values of U _o and Z by practicing the technology in experimental melts ) Annually, on large furnaces, up to 10 experimental swimming trunks per 1 furnace are carried out.

Claims (1)

 METHOD FOR MANAGING THE HEATING AND REMITTING PROCESS of a predominantly rod electrode, including changing the amount of heat supplied to the process, depending on the value of the indirect parameter, characterized in that the effective temperature of the heat source in contact with the electrode, which is determined by the temperature, is taken as an indirect parameter the electrode on its part not in contact with the heating source.

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