JP4622387B2 - Vacuum arc remelting furnace - Google Patents

Description

In the present invention, an arc to be smelted is used as a consumable electrode, and an arc is generated between the molten metal in a water-cooled copper crucible under vacuum, and the heat is melted and solidified while the consumable electrode is melted and solidified. A vacuum arc remelting furnace that realizes stable operation and a method of operating the melting technique are provided.

For example, tool steel for producing plastic molds is refined by a vacuum arc remelting (VAR) technique because extremely high cleanliness is required. In general, an apparatus for performing VAR includes a consumable electrode to be remelted, a pair of stinger rods and screws for raising and lowering the consumable electrode, a vacuum tank and a water-cooled copper crucible disposed therein, a power source and a cable as essential components. To do.

The consumable electrode is usually a cylindrical ingot, and a member called a “stub” is welded to the upper end of the ingot electrode. After the end of one batch, the stub is welded to a new consumable electrode and reused. The consumable electrode is held by clamping the protrusion at the center of the stub of the holding inner cylinder having a ball arranged in an annular shape, and the current is supplied to the consumable electrode on the peripheral edge of the stub upper surface. The conductive outer cylinder is brought into contact with each other through a contact surface between the conductive outer cylinder and the stub through the stinger rod.

Since the vacuum arc remelting is performed with a large current of several tens of KA, if the electrical contact between the contact surface of the outer cylinder for conduction and the stub is not sufficient, the contact portion may become red hot and cause melting. is there. If the conductive outer cylinder or stub is melted and damaged, it is necessary to repair it before the next batch, resulting in a delay in operation and an increase in cost. The redness of the contact portion can be checked by visual observation through the observation window of the vacuum tank in the current apparatus, but it cannot always be seen. Therefore, there has been a demand for the appearance of some kind of monitoring means that knows poor contact before reaching red heat.

The purpose of the present invention is to meet the above-mentioned demands, and in vacuum arc remelting, always check the quality of electrical contact between the stub welded to the consumable electrode and the conductive outer cylinder supplying current to it. It is intended to provide a vacuum arc remelting furnace capable of performing monitoring and issuing an alarm if necessary, and a method of operating the same.

The operation method of the vacuum arc remelting furnace of the present invention that achieves the above-mentioned object is achieved by welding a stub to the upper end of a consumable electrode made of a cylindrical alloy and clamping the protruding portion at the center of the stub with a holding inner cylinder. A vacuum arc remelting furnace including holding a consumable electrode, bringing a conductive outer cylinder into contact with a peripheral portion of the upper surface of the stub, and supplying a current to the consumable electrode through a contact surface between the stub and the conductive outer cylinder The electric resistance of the contact surface is calculated by electrically insulating the holding inner cylinder and the conductive outer cylinder and measuring the voltage drop between the two during operation, It consists of operating so that the values are in the appropriate range for each furnace.

According to the operation method of the vacuum arc remelting furnace of the present invention, the contact resistance between the stub welded to the consumable electrode and the conductive outer cylinder for supplying current to the consumable electrode can be obtained without viewing from the viewing window of the vacuum tank. Always know and monitor the temperature of the contact area. Accordingly, it is possible to prevent an accident of red heat and melting due to poor electrical contact of the contact portion.

The vacuum arc remelting furnace of the present invention for carrying out the above operation method has a structure as shown in FIG. 1 and shows a consumable electrode (1) having a stub (2) welded to its upper end, a stub. A holding inner cylinder (3) that holds the consumable electrode by clamping the central protrusion (21), and a conductive outer cylinder that contacts the peripheral edge of the stub upper surface and supplies current to the consumable electrode through the contact surface ( 4) An insulating ring (5) for electrically insulating and fixing the holding inner cylinder to the conductive outer cylinder, a set of a stinger rod (6) and a screw (7) for raising and lowering the conductive outer cylinder, a water-cooled copper crucible (Not shown), and in addition to the power supply and cable, a voltmeter (8) for measuring the voltage drop between the holding inner cylinder and the conductive outer cylinder is provided, and the conductive outer A hand that calculates and displays the contact resistance between the cylinder and the peripheral edge of the stub upper surface With a (9). The conductive outer cylinder and the holding inner cylinder are electrically insulated from each other by an insulating material in a painted portion in FIG.

At the start of the operation, the holding inner cylinder (3) having a ball (31) arranged in an annular shape is placed on the protrusion (21) of the stub (2) welded to the upper end of the consumable electrode (1). At this time, if the holding inner cylinder is protruded with respect to the conductive outer cylinder (4) so that the ball is relatively lower than the insulating ring (5), the flange passes through the protrusion. Can do. Subsequently, when the outer cylinder is lowered downward and the ball is restrained by the insulating ring, the protruding portion of the stub cannot be removed from the holding inner cylinder. At the same time, the lower end of the conductive outer cylinder comes into contact with the peripheral edge of the stub upper surface to form an electrical passage. In FIG. 1, a thick broken line across the center indicates that the top is outside the vacuum tank and the bottom is inside.

In this state, electricity is supplied to start remelting by arc heating under vacuum, and a voltage drop between the holding inner cylinder and the conductive outer cylinder is measured by a voltmeter (8). The actual measurement positions are point A (holding inner cylinder) and point B (conducting outer cylinder) shown in FIG. 1, and the voltage appearing there is the voltage drop of the current flowing through the conductive outer cylinder. Although the voltage drop at the contact portion is added, the former is very small, and the voltage drop between A and B is almost governed by the latter. Since no current flows through the holding inner cylinder, the potential at point A may be identified with the potential of the stub. If the amount of voltage drop is measured, the electrical resistance of the contact portion is calculated.

Needless to say, the means (9) for calculating and displaying the contact resistance is preferably provided with a device for recording the calculated data throughout the operation. In addition, it is desirable to provide means for issuing an alarm to the operator of the vacuum remelting furnace when the calculated contact resistance value exceeds a predetermined upper limit. Thereby, even if the operator is not always paying attention, the abnormality is notified. When the notified operator visually observes the contact portion from the viewing window, it is possible to determine whether to stop or continue the operation by knowing the presence or tendency of red heat.

A true arc remelting furnace having the structure shown in FIG. 1 and having a rated current set to 20 KA was used to refine a plastic mold steel “NAK80”. It has been found that the contact resistance between the conductive outer cylinder of this furnace and the stub is at least 10 μΩ, but is preferably 30 μΩ or less. If the voltage drop between the point A and the point B is 500 mV, if it is all subject to contact resistance, it is 25 μΩ. As a result of continuing measurement of contact resistance during the operation of each batch (inside and outside of 10 hours), it was observed that there was a change pattern as shown in FIGS.

The pattern of FIG. 2 has a certain amount of contact resistance at the beginning of melting, but is suitable for declining quickly and ending the operation stably at a low value. This pattern is observed in most operations. . FIG. 3 shows a pattern in which the contact resistance fluctuates in the early stage of dissolution, but then stabilizes. FIG. 4 shows a pattern in which contact resistance increases in the latter half of melting. In the example of the figure, the melting could be completed without exceeding the allowable limit of 30 μΩ. However, if the increase in the contact resistance becomes significant, there is a possibility that the contact portion will become red heat-melted. Be careful when you see this pattern. Thus, a certain degree of prediction is possible by monitoring the change in contact resistance over time.

The longitudinal cross-sectional view of the principal part which shows the measurement point of the furnace structure and voltage about the vacuum arc remelting furnace of this invention. FIG. 2 is a chart depicting a change in contact resistance with time obtained in vacuum arc remelting according to the present invention, showing a representative and preferred pattern. FIG. It is the same chart as FIG. 2, and shows another pattern. It is the same chart as FIG. 2, Comprising: Another pattern is shown.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Consumable electrode 2 Stub 21 Protruding part 3 of stub center 3 Inner cylinder 31 Ball 4 Outer cylinder 5 Conductive ring 6 Stinger rod 7 Screw 8 Voltmeter 9 Means for calculating and displaying contact resistance

Claims (2)

Consumable electrode with a stub welded to the upper end, holding cylinder that holds the consumable electrode by clamping the protrusion at the center of the stub, and electrical conductivity that contacts the peripheral edge of the top surface of the stub and supplies current to the consumable electrode through the contact surface The inner cylinder for holding in the vacuum arc remelting furnace consisting of the outer cylinder, stinger rod and screw set for raising and lowering the conductive outer cylinder, the vacuum tank and the water-cooled copper crucible, and the power source and cable with electrically insulating an insulating ring fixed to the outer tube, provided with a voltmeter for measuring the voltage drop between the holding inner tube and the conductive outer cylinder for the value of the measured voltage, the furnace A vacuum arc remelting furnace having means for calculating and displaying a contact resistance between a conductive outer cylinder and a stub upper surface periphery based on a value of a flowing current . When the value of the calculated contact resistance exceeds a predetermined upper limit, a vacuum arc remelting furnace according to claim 1 including means for issuing an alarm to the operator of the vacuum arc remelting furnace.

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