JP2811842B2 - How to start operation of DC arc furnace - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial applications]

TECHNICAL FIELD The present invention relates to a method for starting operation which enables smooth firing, ie, generation of an arc, when charging a scrap into a DC arc furnace and melting it by energizing. This method is particularly useful when applied during a so-called cold start.

[Prior art]

DC arc furnaces have been known for a long time, and have the advantages of significantly lower electrode wear, lower power consumption, and lower noise compared to AC furnaces. In the past, not very large ones were built. In recent years, advances in thyristor technology have eliminated this limitation, and DC arc furnaces have become practical. A typical DC arc furnace has a furnace bottom electrode and a graphite electrode lowered from the furnace top. Although there are several types of furnace bottom electrodes, many use one or more metal pins, and a single pin type is advantageous from the viewpoint of furnace construction and repair. However, when a scrap is charged into a DC arc furnace having a single-pin type bottom electrode and energized, there is a problem that ignition failure often occurs, that is, an arc is not generated smoothly. If an arc does not occur, as a countermeasure, a small amount of molten metal can be prepared after the temperature rise due to resistance heating, thereby enabling ignition, but it takes a long time. As a means of positively producing molten metal, there is a method in which iron powder and a spark are introduced into scrap and oxygen gas is blown to increase the temperature. However, this is also cumbersome, in any case prolonging the melting cycle and poorly matching other processes, such as continuous casting. If there is molten metal, ignition failure will not occur, so if the furnace is hot, a small amount of molten metal should be left for each charge, so-called heel melting may be performed, but heel melting damages the furnace bottom refractory The situation is difficult to understand, and it is not suitable for small-scale production of many varieties due to the problem of residual hot water contamination.
Even if heel melting is performed, the problem of poor ignition cannot be avoided at the start when the furnace is in a cold state.

[Problems to be solved by the invention]

It is an object of the present invention to avoid ignition failure even at the cold start and when performing heelless melting without leaving molten metal by using a DC arc furnace, particularly an arc furnace having a single-pin type bottom electrode. To generate an arc reliably and to achieve the planned melting cycle.
An object of the present invention is to provide a method for starting operation of a DC arc furnace.

[Means for Solving the Problems]

As shown in FIG. 1, a method for starting operation of a DC arc furnace according to the present invention comprises charging a DC arc furnace (1) with a scrap (5), and connecting a scrap top electrode (2) and a furnace bottom electrode (3). As soon as electricity was supplied to start melting, at least the first scrap was charged using scrap (5A) made of fine pieces to cover the area near the bottom electrode, and the remaining scrap (5B) was charged. It is characterized by being carried out. A typical example of scrap (5A) consisting of fine pieces is so-called “Dalai powder”, that is, cutting chips, but in addition to drill cutting chips, the edge cutting of cold-rolled steel plates called “choppers” Several mm in size, such as chips
Any height of 20 to 30 mm can be used. The remaining scrap can be several tens of mm in size, such as billet scrap. It goes without saying that the scrap may be composed entirely of fine pieces.

[Operation]

In pursuit of the cause of the ignition failure, it was confirmed that there was a poor contact between the charged scrap and the bottom electrode. Even with a single pin type, the hearth electrode has a diameter of about 300 mm, and there should be enough opportunity to contact the electrode end face even with ordinary scrap, but in reality, the arrangement shown in Fig. 2 Or contact may be insufficient.
The furnace bottom electrode, which is a metal rod, melts at its upper part with the progress of scrap melting and is lost together with the molten metal. The amount reduced by melting is replaced by a small amount of the remaining molten metal, but the slag may cover the surface even partially, and the cause of poor contact when large scrap is charged is exhausted. Absent. On the other hand, if a scrap made of fine pieces is first charged, the scrap is always in contact with the bottom electrode, as shown in FIG. 3, and the contact is maintained for each fine piece of scrap. Since the electrical contact between them and the large scrap is also secure, there is no electrical contact failure as a whole,
Arc can be generated smoothly. In addition, when a scrap made of fine pieces and a large scrap are put into the furnace at the same time, there is no guarantee that the former comes into contact with the bottom electrode, and the latter usually falls first and falls near the bottom electrode. Because they are often blocked, they should be charged separately.

【Example】

Scrap melting was performed in a 25 ton DC arc furnace. The bottom electrode of this furnace is SUS304 with a diameter of 350mm
Is provided at the center of the furnace bottom. The scrap was initially charged 10 tons and the additional load was 15 tons. A. 1-2 tons of Dalai powder and the remaining 9 to 8 tons as billet chips (length 30 to 120 cm, average 50 cm) are prepared in separate buckets, and the latter is loaded after the former is loaded. B. Scrap configuration is the same as A, but both are thrown in simultaneously from the same bucket. C. All 10 tons shall be billet waste. In the hot start, heel melting, which leaves a small amount of molten metal, and heelless melting, in which the entire amount is melted, were compared. The firing failure rate (the rate at which an arc could not be generated simultaneously with energization) in each case is as follows.

【The invention's effect】

According to the operation start method of the present invention, in melting a scrap in a DC arc furnace, an arc is reliably generated and melting can be started smoothly. In the event of poor ignition, there is no need to take measures such as blowing an oxygen gas with a fire or waiting for a rise in temperature due to resistance heating, which is often performed in the past, and there is no fear that the melting cycle will be out of order. Therefore, the advantages that the DC arc furnace has over the AC arc furnace can be fully enjoyed.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional view of a DC arc furnace in a state immediately before energization, for explaining an operation start method of the present invention. 2 and 3 are detailed views of the vicinity of the bottom electrode in FIG. 1 for explaining the operation mechanism of the operation start method of the present invention. FIG. 2 shows a comparative example, and FIG. The figures each show a case according to the invention. 1 DC arc furnace 2 Furnace top electrode 3 Furnace bottom electrode 5 Scrap 5A Fine scrap

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int. Cl. ⁶ , DB name) C21C 5/52 C22B 9/20 F27D 11/08

Claims (2)

(57) [Claims] When a scrap is charged into a direct current arc furnace and melting is started by energizing, a scrap made of fine pieces is used as a scrap to be charged at least first, and the vicinity of the bottom electrode is covered. And starting the operation of the DC arc furnace, wherein the remaining scrap is charged. 2. The method according to claim 1, wherein cutting scraps from a lathe or a drill are used as the scraps made of fine pieces.