Method of controlling the flow of outstreaming liquid metal from an inductively heated ladle

Abstract: A method and an equipment for accurate weighing in a weighing unit (2) of a metal melt in a ladle (3), in which the melt can be heated respectively stirred by power supplied by means of an induction coil (4), at which the coil is also applied to a weighing unit (1) by which can be measured the reaction force of the magnetic force from the coil influencing the melt. In the weighing system (5) the net weight of melt is calculated by subtracting the signal of the coil weighing unit from that of the ladle weighing unit. The weight change per time unit which represents the mass flow rate of melt from the ladle is calculated in the control system (6) and is used to control the pressure in the ladle in such a way the metal flow is kept constant.

Description: The present invention relates to a procedure of accurate weighing of the metal content in a vessel in a casting plant provided with an equipment for heating/stirring the melt by inductive methods. The accuracy of weighing in such applications until now has been unsatisfactory due to influence from the magnetic forces arising from the induction current as well as to variations of metal bath level and lining profile of the vessel, i.e. the wear. Said variations in turn influence the size of the magnetic forces. The invention presents a simple and well functioning solution to these problems and is principally based upon a simultaneous weighing of the vessel (ladle) with metal content and the equipment for heating/stirring. DE-A1-2513923 relates to an automatic controlling equipment for preparation of metallic crystals, where continuous weighing of the inductively heated vessel containing the liquid metal is performed. A compensation for the disturbance on the weighing result caused by the forces of the magnetic field has been introduced via a correction signal generator in which the influence of the disturbing force is calculated as a constant times the coil current. The correction signal generated in this way is based on a simplified theory about the origination of the disturbing force and gives no true imitation of the disturbing force during the whole casting procedure and therefore does not permit an accurate enough control of the metal flow in applications where this is required. The simultaneous weighing of both vessel with metal content and heating equipment characterizing the present invention has not been made in the related case. AU-B-1723-66 relates to automatic level control for continuous casting. It discloses a weighing of molten metal in a ladle with induction coil by load cells below the ladle. This may be done during maintenance of the temperature by induction heating through the coil. Methods of making spray-cast billets or metal powder require accurate control of the temperature of the liquid metal and of the metal flow rate. Common to all methods used is the forming of a spray of atomized metal by blowing gas jets of high speed against a liquid metal stream which disintegrates to droplets. Properties such as powder size, billet structure, grain size, porous layer thickness etc of gas atomized metal powder or spray-cast billets are primarily determined by the temperature and mass flow of the liquid metal during the formation of the atomized metal flow. Methods for controlling or keeping the temperature or flow rate of the atomizing gas constant represent well-known technique. Controlling or keeping the temperature of the liquid metal constant during atomizing can be performed by using an inductively heated and sealed ladle. Control of the temperature of the liquid metal in such a ladle is obtained by measuring the temperature of the melt and varying the power to the induction coil. This is a known technique which has been described in SE-A-8006244-1 (EP-A1-47431). The known methods and techniques for controlling the liquid metal flow through the opening of the outlet (nozzle) of an inductively heated ladle do not permit sufficient precision, however. Accurate control of the metal flow rate at the tapping of metal from a ladle is a necessity. The reason for this is that the metal has to be deposited with great precision in order to give correct and near net (final) dimension of the manufactured object. Disturbances such as clogging of the nozzle opening, affecting the deposition procedure have to be compensated for. The methods according to known technique being used so far are based upon measuring the level of the metal surface in the ladle and do not consider the lining wear of the ladle, the flow rate of the liquid inside the ladle or the wear of the nozzle opening. These methods of controlling the metal flow are based upon the theoretical relation between static pressure and flow rate derived from Bernoulli's equation. Because of factors such as temperature and viscosity of the melt, type of ceramic material, chemical reactions between the melt and the ceramic lining, the real flow rate deviates from the theoretically calculated. Another method of controlling the metal flow rate is to weigh the ladle including its metal content and calculate the total weight decrease per time-unit. The weighing equipment has to meet high demands with respect to resolution and accuracy, since the metal flow rate (weight decrease per time-unit) is so small in relation to the weight of the total load and the requirement of keeping the flow constant is so great. The requirement in the present plant is that it has to be able to detect weight changes of 0.5 kg which should be compared with the total weight of ladle and melt which is about 4 metric tons, of which the melt represents about 1 metric ton. The weighing operation is exposed to disturbances from the magnetic force emanating from the current in the induction coil for stirring and heating, which directly influences the weighing result. This disturbing force varies in an interval between 0 kg (at cut-off power supply) and up to about 40 kg depending upon the conditions prevailing in the process as described below. In the manufacturing process in question a compensation was initially introduced based upon an empirical model to correct the weight signal. This model calculates the disturbing force at the actual steel bath level and coil current. Steel tubes manufactured by means of the model showed, however, that the correction factor was influenced also by the profile of the ladle lining and that the level of the steel bath could not be defined as function of the coil frequency (being used as measure of the bath level) with sufficient accuracy. The model was modified in order to consider the inner diameter of the ladle too, but was abandoned because of its complexity and the unsatisfactory results obtained. The method and the equipment according to the invention are based upon direct measurement of the disturbing force and compensation for its disturbing influence on the weighing result. Since the disturbing force acting on the liquid metal, in turn influencing the ladle weighing equipment, emanates from the induction coil, the reaction force acting on the coil has to be equal but opposite in direction to the disturbing force on the melt, because no other exchange of force is taking place between the coil and its non-magnetic surroundings. By placing the induction coil on load cells and measuring the change in force caused by the variation in power supplied, the disturbing force can be measured. The signal processing to calculate true weight change in the ladle and thus correct measure of the metal flow rate is carried out in the measuring and control units. The invention is described below with references made to the attached Fig. 1 where the numbers used distinguish different parts of the equipment. The metal, to be deposited is present in a ladle 3. The tapping of the metal is controlled by setting the ladle under a pressure below atmospheric pressure. The pressure is controlled by means of the gas system 7. By changing said pressure 70 the metal column can be raised or lowered, alternatively, and in this way the metal flow rate can be controlled. The desired tapping rate is controlled by a ramp device 67 which generates a signal representing pressure to be achieved as a function of time 69 in order to obtain correct metal flow. The ladle with its metal content stands on the weighing equipment 2 consisting of load cells placed in positions along the circumference of the ladle (in the actual plant in 3 points). The weighing equipment is exposed to various mechanical disturbances and must be able to detect small weight changes (in the actual plant about 0.5 kg). In particular this means that it has to meet strong requirements on linearity (small tolerances on non-linearity). In order to remove the influence of mechanical disturbances (vibrations) the output signal of the weighing system is strongly filtered. The ladle weighing equipment can be placed in different positions depending upon the vertical position of the ladle, which is adjustable. This is indicated by the text "Pos" on the attached drawing. By weighing the ladle before the metal is poured into it, the tare weight of the ladle is measured. This is deducted from the weight of the ladle with metal content by the weighing system in order to obtain the net metal weight, represented by the force "F metal " in the figure. For stirring and heating the metal in the ladle, the ladle is supplied with a coil 4 surrounding it. From the coil a force, "F ind ", is induced which influences the ladle weighing equipment not to detect the correct metal weight in the ladle but a resulting weight represented by the force "F metal "+"F ind ". The efficiency of the power supplied to the ladle decreases as the amount of metal in the ladle decreases. Over a long period of usage the ladle lining is worn. This wear is unequally distributed over the lining and the profile of the ladle lining therefore often looks like a pear having the greatest wear at the slag-line. The force "F ind " varies with the amount of power supplied and the level of metal in the ladle. Furthermore, "F ind " is influenced by the wear of the lining. In order to compensate the ladle weighing equipment for the disturbance caused by the magnetic force from the coil on the melt, the coil in turn has been placed on a weighing equipment 1 which in a corresponding way as the ladle weighing equipment has load cells placed along the circumference of the coil. The ladle- and the coil-weighing units, respectively, have separate mechanical foundations. The signal from the coil weighing equipment is filtered in a way corresponding to that of the ladle weighing equipment in order to remove the influence of mechanical disturbances on the weighing result. The reaction force, "F ind ", obtained from the coil weighing equipment is equal but opposite in direction to the magnetic force by which the coil influences the ladle weighing equipment. In the weighing system 5 the net weight 51 of the metal is calculated by subtraction 52 of "F ind " from "F metal " + "F ind ". In the system 5 the ladle- and the coil-weighing equipments are represented by the squares 53 and 54, respectively. Based upon the net weight of metal obtained the control system 6 calculates 60 the weight change per time unit, being the metal flow rate 61. Said flow rate 61 is compared with the desired flow rate 62 in the controller 63 and the deviation 64 is used to control the pressure and the tapping out of metal. The flow-rate controller 63 also takes into account the time delay existing in the weighing system because of the filtration. In the control system there is also a vacuum controller 65 which obtains information about pressure setpoint value 66 (via the pressure ramp device 67), measured pressure value 68 and flow rate deviation 64. This controller in its turn acts upon the gas system 7, in which the gas pressure can be lowered via a vacuum pump 71 or raised by supply of nitrogen gas via a valve 72.

Claims: Method of accurate weighing of the metal content in a vessel (3) in a casting plant, supplied with equipment (4) for heating/stirring by power supplied with inductive methods, a simultaneous weighing being performed of the vessel including the metal content and of the power supplying device (4) at which the magnetic disturbance force from the power supplying device (4) is eliminated and a correct net weight of the metal content of the vessel (3) is obtained, characterized in that the vessel including the metal content is weighed on a separate weighing equipment (2) and that the power supplying device (4) is weighed on another weighing equipment (1) and that the signal from the latter weighing equipment is subtracted from that of the former one so that the influence of the magnetic disturbance force on the weighing result is eliminated.

Inventors: JOSEFSSON KENNETH, JOSEFSSON, KENNETH
Applicants: SANDVIK AB, SANDVIK AKTIEBOLAG
Application year: 1991
Main classification: G01G17
IPC codes: G01G11, G01G17, B22D23, B22D2, G01G23, G01G13