JP2007302954A - Method for electrically remelting slag in vacuum and apparatus for electrically remelting slag in vacuum - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for electrically remelting slag in a vacuum, which can conduct an operation by using such a slag as to have a low melting point, be easily melted and be able to decrease the amount of oxide inclusions while avoiding the boiling of the slag, and to provide an apparatus therefor. <P>SOLUTION: This slag-remelting method includes refining a metal in a degree of vacuum in a range of 100 to 600 Torr by using a slag containing 20 to 80 wt.% CaF<SB>2</SB>and the balance mainly CaO and (or) Al<SB>2</SB>O<SB>3</SB>, while maintaining a threshold pressure condition at which the slag does not substantially start boiling. The degree of the vacuum in a furnace is adjusted by concurrently exhausting air with a vacuum pump and injecting argon gas in the furnace. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

The present invention relates to improvements in the vacuum electroslag remelting process and also relates to the apparatus used to carry out the process.

As is well known, electroslag remelting is a remelting technique in which consumable electrodes are melted by the Joule heat of molten slag in the air or under an inert gas atmosphere, and laminated and solidified in a copper water-cooled mold. is there. Specifically, in a water-cooled mold, an electric current is passed between a consumable electrode made of a metal to be refined, mainly steel, located above the molten slag, and a steel ingot that is laminated and solidified at the lower side. This is a technique in which metal droplets dripping or dropping from a consumable electrode are refined while passing through a molten slag.

Electroslag remelting in the atmosphere is less demanding on equipment and easy to operate, but the refining capacity, particularly deoxidation performance, associated with slag deterioration is unavoidable, and it is difficult to avoid moisture and nitrogen in the atmosphere. There is a problem of generation of hydrogen defects and nitrides due to the pickup. The active component in the molten steel is lost as a result of refining, and variations in the alloy composition occur. This is a weak point of electroslag remelting in the atmosphere.

As a smelting technique that solves these problems, a vacuum electroslag remelting method is performed in which a smelting atmosphere is placed under reduced pressure. The basic technique is the method disclosed as “metal electroslag remelting method”, wherein a) the remelting step is carried out under reduced pressure (specifically 200 to 10 ⁻² mbar), b) slag Using at least 80% by weight of an oxide having a boiling point higher than 2000 ° C., and c) performing AC heating (Patent Document 1).

Regarding the vacuum electroslag remelting method, the applicant disclosed a method of energizing a direct current by immersing an electrode with variable polarity in the slag as a “component control vacuum ESR method” (Patent Document 2). This technique can control components such as S, Al, and P using polarity. Subsequently, the applicant developed and disclosed an ESR technique using slag mixed with iron sulfide powder for the “electroslag remelting method of sulfur-containing steel” (Patent Document 3). Regarding the apparatus, a “vacuum electroslag remelting furnace” in which the power loss was reduced by making the furnace body and its surrounding components nonmagnetic was proposed (Patent Document 4).
JP 63-259031 (Patent 2588895) JP-A-3-197624 JP-A-7-62461 JP 7-238327 A

Regarding the basic vacuum electroslag remelting technique described above, in order to smoothly operate under a vacuum of “200 to 10 ⁻² mbar” (0.01 to 150 Torr), “at least 80% by weight as slag is required. That is, an oxide having a boiling point higher than 2000 ° C. ”, that is, an oxide-based slag has to be used. However, when oxide-based slag is used, there is a problem that melting point is high and melting is not easy. If the problem of slag boiling is avoided, remelting is carried out using a slag with a composition that is advantageous for electroslag remelting and contains an economically advantageous composition that contains a larger amount of components such as CaF _2. It is desirable to do.

The object of the present invention is to meet such demands, use a slag that has a low melting point, is easy to melt, and can reduce the amount of inclusions, and avoids boiling of the slag. The object is to provide a method of vacuum electroslag remelting that can be operated. It is also within the scope of the present invention to provide a vacuum electroslag remelting apparatus suitable for use in carrying out the method.

The vacuum electroslag remelting method of the present invention is a method of refining metal using a vacuum electroslag remelting furnace, containing 20 to 80% by weight of CaF ₂ , with the remainder mainly CaO and / or Al. _This is a method characterized by using slag composed of ₂ O ₃ and performing refining while maintaining a pressure condition in which slag boiling does not substantially occur within a vacuum range of 100 to 600 Torr. The slag may contain MgO in addition to the above components.

Since the vacuum electroslag remelting method of the present invention uses slag containing 20 to 80% by weight of CaF ₂ and the remainder mainly composed of CaO and Al ₂ O ₃ , the melting point thereof is higher than that of slag mainly composed of oxide. Low and easy to melt. The ease of melting means that it is advantageous for process control, and means that stable quality products can be provided at low cost.

The degree of vacuum suitable for carrying out the vacuum electroslag remelting method of the present invention is determined by factors such as the slag composition, the alloy composition of the consumable electrode and the size thereof within the range of 100 to 600 Torr. Since the limit of the degree of vacuum at which slag boiling occurs is substantially determined by the above-described operating conditions, it is necessary to first understand the limit of the operable vacuum. Those skilled in the art can know the limitations by performing some tests as needed. If the slag composition slightly changes with the progress of refining, the limit of the degree of vacuum to be maintained is determined in consideration of this.

This adjustment of the degree of vacuum by injecting argon gas can also be used at the start of vacuum electroslag remelting operation. That is, prior to operation, a start-up method is recommended in which the inside of the furnace is depressurized to a high degree of vacuum and then argon gas is injected to lower the degree of vacuum to achieve a desired degree of vacuum and then energize to start refining.

As shown in FIG. 1, the vacuum electroslag remelting apparatus of the present invention suitable for carrying out the above-described method includes a water-cooled copper crucible (1), a furnace body (2) provided thereon, a vacuum pump (3 ), A main line (6) having a main valve (V1) in the middle for connecting the inside of the furnace and the vacuum pump, a bypass line (7) having a control valve (V2) for bypassing the main valve, An argon gas source (4) and an argon line (8) with an argon valve (V3) for connecting the inside of the furnace to the argon gas source are essential components, and a power source (5) and electrode lifting / lowering means ( (Not shown). It goes without saying that there are incidental facilities such as a pressure gauge and a thermometer for measuring the degree of vacuum in the furnace.

The operation of this apparatus is performed as follows. First, the electrode raising / lowering means is operated to place the consumable electrode at an appropriate position, and the vacuum pump is operated to depressurize the inside of the furnace. At this time, the air is rapidly extracted through the main line (6), and the pressure inside the furnace is set to be slightly lower than the expected vacuum level. However, fine control is performed using the control valve (V2) of the bypass line (7). Do. Next, the main valve (V1) of the main line (6) is closed, the argon valve (V3) of the argon line (8) is opened, and argon gas is injected into the furnace to achieve a desired degree of vacuum.

When a desired degree of vacuum is reached, an appropriate amount of slag composition is placed in a water-cooled copper crucible, and the slag is melted by energization, and metal droplets are dropped from the consumable electrode and refined while it passes through the slag. Perform vacuum electroslag remelting as described. The energization can be either alternating current or direct current. During refining, the vacuum pump is operated to the limit necessary to maintain the degree of vacuum in the furnace. The condition in the furnace is grasped by a pressure gauge and, if necessary, by observation through a viewing window.

A slag composed of CaF ₂ : 70% and Al ₂ O ₃ : 30% by weight was prepared. Stainless steel vacuum electroslag remelting was performed using an apparatus configured as shown in FIG. 1 and having a maximum capacity of 1750 KVA. The start was that the degree of vacuum was once about 0.1 Torr by operating the vacuum pump, and then argon gas was injected to return the degree of vacuum to 150 Torr, and an alternating current was applied to start refining. The pressure in the furnace is continuously observed, the vacuum pump is operated as necessary, the vacuum in the furnace is maintained within the range of 120 to 180 Torr, and an average of 150 Torr, and the set consumable electrode is dissolved. He continued driving for hours. During this time, no slag boiling was observed. The ingot solidified in the water-cooled copper crucible was clean with high purity of H: 2 ppm, O: 20 ppm and N: 350 ppm.

The layout of the principal part which shows the structure of the vacuum electroslag remelting apparatus of this invention.

Explanation of symbols

1 Water-cooled copper crucible 2 Furnace 3 Vacuum pump 4 Argon gas source 5 Power supply 6 Main line 7 Bypass line 8 Argon line V1 Main valve V2 Control valve V3 Argon valve

Claims (3)

A method of refining metal using a vacuum electroslag remelting furnace, comprising 20 to 80% by weight of CaF ₂ , and using a slag mainly composed of CaO and / or Al ₂ O ₃ A vacuum electroslag remelting method characterized in that refining is performed while maintaining a pressure condition in which boiling of slag does not substantially occur within a range of 100 to 600 Torr. 2. The vacuum electroslag re-starting according to claim 1, comprising depressurizing to a high degree of vacuum, injecting argon gas to lower the degree of vacuum, realizing a desired degree of vacuum, and energizing to start refining. Start-up method of dissolution method. A water-cooled copper crucible (1) and a furnace body (2) provided on the upper part thereof, a vacuum pump (3), and a main line (6) provided with a main valve (V1) in the middle for connecting the furnace and the vacuum pump. ), Bypass line (7) with control valve (V2) bypassing the main valve, argon gas source (4), and argon valve (V3) for connecting the inside of the furnace to the argon gas source A vacuum electroslag remelting apparatus comprising an argon line (8) as an essential component and a power source (5) and electrode lifting / lowering means (not shown).

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