The invention relates to steel production by the electroslag remelting method, and more specifically to electroslag remelting furnaces with relative displacement of a mould and an ingot being cast.
The principle of operation of a electroslag remelting furnace with relative displacement of a mould and an ingot being cast and comprising an automatic control system based on the weight rate of the ingot casting, may be used in plasma-arc remelting furnaces, electron beam remelting furnaces and electroslag welding installations using this system.
Known in the art are electroslag remelting furnaces having systems for controlling production parameters in stationary moulds using indirect control parameters, such as electrode flashing current, voltage drop across a slag bath, furnace transformer voltage, feed rate of electrodes, resistance of an interlectrode gap and the like. Several parameters may also be used in combination.
Known in the art is a furnace comprising a stationary water chilled mould wherein the electroslag remelting and formation of an ingot from molten metal take place. Consumable electrodes remelted in the mould are connected to a furnace transformer by means of power supply cables. The electrodes are fixed with the top part thereof to an electrode holder mounted for movement along a furnace column.
At the beginning of the process, a carriage of the electrode holder with electrodes secured therein is at the top portion of the furnace column. During the melting of the electrodes, the carriage moves down along the column. The automatic control of the process is effected by controlling the feed of the electrodes. The electrodes are fed in accordance with the slag bath resistance, which is the resistance between the electrodes and the ingot being cast. For that purpose, the furnace is provided with a control system for controlling the displacement of the electrodes which is placed at a control board and consists of a series circuit including: a unit for measuring active resistance, a comparator unit, an amplifier and a drive for displacement having a reducing gear. Signals proportional to the flashing current of the electrodes (from a current transformer) and to the furnace transformer voltage are fed to the unit for measuring active resistance of the slag bath. The signal at the output of the unit for measuring active resistance, which is proportional to the actual resistance of the slag bath, is compared to a preset signal value. The difference in these signals is fed, via the amplifier, to the drive for displacing the electrodes which restores the preset resistance value.
It should be noted, however, that the measurements in the method of controlling the process in accordance with the resistance of the slag bath are effected with great errors. The consumable electrodes are heated during the remelting and are shortened as they are consumed. The active resistance of the electrodes varies during the entire casting period, and the control system takes no account of that. The resistance of the slag bath is an indirect parameter of the process which cannot insure an accurate maintenance of the rate of ingot remelting which represents the main parameter affecting the quality of metal.
Known in the art is also an electroslag remelting furnace for casting ingots of up to 14 tons. The furnace comprises a stationary water chilled mould which is mounted on a bottom and has a slag bath in which an electrode is melted under the action of current fed from a furnace transformer and in which molten metal is cast into an ingot. A consumable electrode of a round section has a standard head for locking in an electrode holder. The electrode holder is mounted on a carriage which is positioned at the top portion of the column at the beginning of the process. During the entire remelting period, the carriage of the electrode holder with the electrode secured therein moves down along the column as the electrode is consumed.
The primary control of the process for maintaining and controlling the melting speed is effected by a system for weighing the consumable electrode.
The standard electrode head is screwed into a weight cell which is mounted in the electrode holder cross piece. In this position, the weight cell senses the entire weight of the electrode holder, a standard head, electrode residue and a part of the weight of water for cooling the cables. Electric signals from the cell are fed to an amplifier and therefrom, to a measuring instrument. As the electrode is consumed, its weight changes, and the rate of remelting of an ingot may be computed.
The use of the weight cell in the control system for controlling the electroslag remelting process is very difficult because there are a number of factors influencing the readings of the cell which cannot be taken into account. The load on the cell from the power supply cables varies with the altitude of the electrode holder. When the electrode holder is in the upmost position, the cable weight is at its maximum. As the carriage is lowered, the cable weight decreases, and this variation is non-linear. The current value change during the melting results in a change in the magnetic field which negatively affects the accuracy of the readings of the weight cell.
Therefore, the prior art systems for controlling the electroslag remelting process have a number of substantial disadvantages. They cannot provide for maintaining a sufficiently accurate rate of ingot remelting since the process control is being conducted in accordance with indirect parameters, whereas the rate of ingot remelting is the main production parameter affecting the quality and structure of an ingot being cast.