Apparatus for regulating the depth of immersion of consumable electrodes in electroslag remelting furnaces

The invention relates to a system for regulating the depth of immersion of melting electrodes in electrical slag remelting furnaces, consisting of an apparatus for the detection of the reciprocal resistance correspondence to the conductance and for changing this conductance upon the spatial displacement of the end of the electrode within the slag layer, a signal corresponding to the conductance being relayed to a regulating system for the electrode drive.

BACKGROUND 
The invention relates to an apparatus for the regulation of the depth of 
the immersion of consumable electrodes into the slag of electroslag 
remelting furnaces, consisting of a system for detecting the actual 
resistance and the change of this resistance as the electrode is moved 
within the slag layer, and of a regulating means for the driving of the 
electrode, to which there is transmitted a signal corresponding to the 
actual resistance and, as a correction factor, a signal corresponding to 
the change of the actual resistance, measurement conductors for the 
melting current i and the melting voltage U being connected with a 
divider, in accordance with U.S. Pat. No. 4,075,414. 
In the subject matter of the principal patent application, the quotient, 
EQU Resistance R=U/i=(Melting voltage/Melting current) 
is formed in the divider and corresponds to the actual resistance in the 
slag blanket. This resistance, represented in relation to the depth of 
immersion of the electrode, results in hyperbolic curves which 
asymptotically approach the abscissas and the ordinates of the coordinate 
system. On the basis of the various apparatus and melting parameters, sets 
of curves are plotted which have a substantially similar course. These 
curves are also referred to as "immersion curves." Immersion curves, such 
as develop in the subject matter of the principal patent application, are 
also given for purposes of comparison in FIG. 1 appended hereto. It can be 
seen that the actual resistance R of the slag bath falls off more or less 
steeply, depending on the tip length h.sub.s of the electrode, as the 
depth of immersion s increases, dropping all the more steeply as the 
length of the electrode tip is reduced. The knowledge of this relationship 
is important in designing the power supply and of the electrode feed 
control, and also to the operation of the electroslag remelting furnace. 
On the basis of the curves, slight changes in the depth of immersion, 
especially in the case of short electrode tips, produce fluctuations of 
the resultant bath resistance. When controlled-current power supplies are 
used in production apparatus, allowance must be made for this fact by 
providing for appropriate voltage reserves in the power supply to assure 
the operation of the current regulator at all times. Furthermore, the 
dynamic performance of this current regulator must be set up in accordance 
with the variation of the electrical furnace circuit time constant 
resulting from the variations of the bath resistance. 
The above situation can be expressed by saying that, on the basis of the 
"immersion curves" or resistance curves, there is a considerable variation 
in the amplification factor of the regulating system. The calibration of 
the electrode feed control is therefore performed, for reasons of 
stability, for the steep portion of the curves. In this portion, 
outstanding regulation properties result, i.e., especially in the case of 
shallow slight immersion and a short tip length h.sub.s on the electrode. 
This has the disadvantage that, for the flatter portion of the 
characteristic curves there will be a lower control circuit amplification, 
which will result in inaccurate control in this portion of the curve. 
THE INVENTION 
The object of the invention, therefore, is to linearize the control circuit 
amplification and thereby to make the control operation more uniform over 
the entire range of immersion depths and tip lengths than it is in the 
subject of the principal patent application. 
The achievement of the object of the invention is accomplished in the 
above-described apparatus in accordance with the invention in that the 
divider is constructed such that a signal proportional to the reciprocal 
of the measured resistance is delivered to its output. This means that in 
the divider, the quotient 
EQU 1/R=G=i/U=(melting current/melting voltage) 
is formed, which corresponds to the conductance of the slag blanket. If the 
conductance is plotted against the immersion depth, a set of straight 
lines will result, i.e., the conductance at a given tip length is 
proportional to the immersion depth, so that a uniform regulating circuit 
amplification results. The linearity of the relationship between the 
conductance and the immersion depth will also obtain for different tip 
lengths, a particular linear characteristic corresponding to each tip 
length, so that here again a set of linear characteristics is obtained.

DETAILED DESCRIPTION 
In FIG. 1 there is represented a graph on whose abscissa is plotted the 
immersion depth s of the electrode end in millimeters, while on the 
ordinates, on the left is given the ohmic resistance between the electrode 
clamp and the crucible terminal in milli-ohms, and on the right the 
reciprocal thereof. These values contain not only the ohmic resistance of 
the slag blanket, but inevitably also the resistances in the electrical 
connections and parts of the apparatus. The resistance is therefore called 
the system resistance. The curves a, b and c represented in solid lines 
indicate the change of the system resistance as the immersion depth is 
changed between about 10 mm and 200 mm. The left curve a applies to a 
consumable electrode tip length h.sub.s of 50 mm, the middle curve b to a 
tip length of 100 mm, and the right curve c to a tip length of 200 mm. In 
all cases the depth of the slag blanket was 250 mm and the electrode 
radius 415 mm. 
Basically two different portions can be discerned in the immersion curves 
a, b and c, namely the portion that becomes flat after the immersion of 
the entire electrode tip, and the portion that rises steeply after the 
partial withdrawal of the electrode tip from the slag bath. 
If the bath resistance that is to be maintained is specified at the level 
of the flat portions of the immersion curves, a clear-cut relationship to 
a particular length h.sub.s of the electrode tip is hardly possible. This 
is because at this point on the immersion curves, the amplification in the 
control portions of the curve is very low, i.e., for a slight change in 
the bath resistance there is a very great change of the immersion depth h 
and vice versa. As a result, it is scarcely possible to have a definite 
establishment of the length h.sub.s of the electrode tip when operating 
within these flat portions of the immersion curves, and this has been 
confirmed in practice. The broken lines d, e and f represent the 
relationship of the reciprocal of the resistance, that is, the electrical 
conductance of the slag. If the reciprocal is used as the control signal, 
the control portion of the characteristic consequently has a simple 
proportionality with a control amplification factor that remains constant 
over the entire range, so that, with a lesser expense in connection with 
the control system and power supply, a high control accuracy is achievable 
and with it an extremely uniform melting process. The uniformity of the 
remelting or crystallization process is an important requirement for the 
obtaining of a homogeneous ingot. 
In FIG. 2, 1 designates a consumable electrode of any metal or alloy, which 
is fastened by means of a shank 2 to a boom 3 of an electrode holding 
means. The boom 3 can be raised and lowered on a vertical guiding column 4 
by means of a screw 5. For this purpose a nut 6 is provided on the boom 3. 
The screw 5 is held at its upper end in a bearing 7 which is fastened to 
the guide column by means of a crosspiece 8. The bottom bearing 9 of the 
screw is contained in a gear box 10 in which the rotatory speed of a drive 
motor 11 is reduced to an appropriate value. Parts 2 to 11 constitute the 
electrode feed means. 
The consumable electrode 1 is situated with at least a portion of its 
length within an ingot mold 12 which consists of a wall 13 in the form of 
a hollow cylindrical jacket having connections 14 for the inlet and outlet 
of a cooling fluid 15. During the remelting phase, in which the apparatus 
is represented, the consumable electrode is immersed to a specific, 
controlled depth in a slag blanket 16, a conical tip 1a having the length 
h.sub.s being formed on the bottom end of the electrode. By the dripping 
of molten metal from the electrode 1 a pool 17 is formed which solidifies 
as the remelting proceeds, to form an ingot. The bottom of the ingot mold 
is a water-cooled floor 19 which rests with the other parts of the 
installation on a base plate 20. 
The electrical power is delivered through a flexible conductor 22 and a 
terminal clamp 23 to the shank 2 and thence to the electrode 1, on the one 
hand, and on the other hand it passes through a conductor 21 to the floor 
19. Often the floor 19 is electrically insulated from the mold 12 (not 
shown in the drawing). The conductors 21 and 22 are connected by terminals 
24 and 25 to a power supply which is not represented. The melting current 
i flowing in the system is taken from line 21 by means of a current 
transformer 26 and delivered through a conductor 27 to a divider 28. 
Moreover, the melting voltage is taken from line 22 and likewise delivered 
to the divider 28 in which the quotient of the melting current and the 
melting voltage i/U is formed, which represents the conductance G. The 
output of the divider 28 is delivered through a conductor 30 to an input 
resistance 31 of a regulator 32 for regulating the depth of immersion. By 
means of a potentiometer 36, a reference voltage corresponding to the 
preselected conductance is delivered to another input resistance 37 of 
regulator 32. From the regulator 32, a conductor 33 leads to a control 
block 34 which is connected by a conductor 35 to the motor 11 operating 
the electrode feed. In this manner a purely conductance-related regulation 
of the depth of immersion of the electrode 1 into the slag blanket 16 is 
achieved. 
From the divider 28 another conductor 38 leads to a differentiating circuit 
39 for the formation of a derivative dG/dt, whose output is delivered 
through a conductor 40 to a divider 41. The divider 41 is also to receive 
from a conductor 42 a voltage corresponding to the rate of movement of the 
electrode or the differential quotient ds/dt. Since this magnitude in turn 
depends on the rotatory speed of motor 11, a tachometer generator 44 is 
coupled to the latter by a shaft 43 and puts out a voltage corresponding 
to the rotatory speed. The derivative dg/dt and the derivative ds/dt are 
processed in divider 41 to form the quotient dG/ds, that is, the change in 
conductance in relation to the displacement of the electrode. In a block 
45, which is connected to the divider 41 by a conductor 46, the absolute 
value of the differential quotient dG/ds is formed. A conductor 47 leads 
from block 45 to a block 48 in which the differential quotient average is 
formed. This average is carried by a conductor 49 to an input resistance 
50 of a regulator 51 whose output is connected by a conductor 52 and a 
switch 53 to an input resistance 54 of regulator 32. The switch 53 is 
closed when the regulator is operated in the fully automatic mode, but it 
can be opened when the apparatus is started up and when manual control 
operations are performed. A reference voltage is delivered through an 
input resistance 55 to the regulator 51, which corresponds to the optimum 
value of the differential quotient dG/ds. This reference voltage is 
adjusted at a potentiometer 56 which is a motorized potentiometer driven 
by a motor 57. This motorized potentiometer permits a smooth input of the 
correction. This is accomplished by the closing of a switch 58 in a 
conductor 59 leading to the output of the regulator 51. 
The changeover to regulation with correction is performed by then opening 
the switch 58 and simultaneously closing the switch 53. Thus a smooth 
changeover takes place, because the reference voltage at the output of the 
potentiometer 56 is equal to the voltage present in conductor 49 at the 
moment of the closing of switch 58.