EP0786531B2 - Process and installation for remelting of metals to a strand - Google Patents

Description

The invention relates to a method for remelting metals - in particular steels and Ni and Co base alloys - into one strand by melting at least one self-consumable electrode at a rate of melting in kg / h in an electrically conductive slag bath, which in a short, is provided downwardly open mold. In addition, the invention covers a device for carrying out this method.

For example, in the production of high alloy tool steels - such as high speed steels, ledeburitic chrome steels or other high segregating steels and alloys - producing continuously cast strands of small to medium cross sections presents problems.

DE 1 932 763 A discloses a device for carrying out the electroslag refining process with a water-cooled, vertically arranged mold for ingots, in particular of high-alloy steel; this mold is double-walled and offers a lower region determining the outer dimensions of the ingot to be produced, whose inner cross-section is at most equal to the cross-section of a consumable electrode to be melted in a hot Schläkkenbad, wherein the lower Kokillenbereich merges into an upwardly preferably conically widening intermediate region and this in turn into an extended upper mold area. Here the stream takes the way to the crude block over the slag bath.

GB 1 413 508 A discloses a mold, in the mold wall a non-consumable current-conducting element is installed and connected via a rectifier so with the power supply, that both the electrode and the casting cross-section - -. With respect to the non-consumable element - constantly are electrically negatively polarized.

DE 1 483 646 A and AT 320 884 B likewise disclose variants of the electroslag remelting process. The self-consumable electrode methods described therein enable the production of remelt blocks with a good surface at a slow block build-up speed. The resulting low sump depths lead to a uniform solidification between the edge and core and thus to a good internal quality of the remelted blocks The use of short molds with lowerable bottom plates and electrode replacement also allows here to form relatively long strands. However, in the production of small size, generation of the required consumable electrodes becomes difficult, and the process costs due to the then low reflow rates become high.

In order to avoid the problem of the manufacturability of electrodes with small cross-sections, the use of so-called funnel or T-molds has been proposed; the mold takes up the slag bath in a funnel-shaped widened part, thus enabling melting of electrodes whose cross-section is that of the remelt block to be produced.

While in the continuous casting of formats between 100 and 200 mm - round or square - even with slow casting casting loads of at least 5 to 10 t per hour and strand are required, the Abschmelzraten the ESU process maximum 100 - 200 kg per hour at the same formats. During continuous casting, this results in swamps between 4 m and 8 m. By contrast, the sump depths in the ESU method measure 100 to 300 mm.

In another procedure, according to AT-PS 399 463 proposes to cast strands of high-alloy steels with much lower casting speeds - as they are usual in continuous casting - to achieve an improved core zone while covering the casting mirror by an electrically heated slag bath in order to avoid any disadvantages with regard to the formation of the surface due to excessive cooling. It is assumed that the liquid metal can be made available for a long time at a constant temperature from a heatable pan.

In many cases, the problem of keeping large amounts of liquid metal over a longer period of time arises again with this method. This is especially important; when working with only one strand Thus, for example, when casting melts of 25 t total weight, a strand of e.g. 150 mm diameter with a casting rate of, for example, 2000 kg / h casting times of 12.5 hours. During this time, the melt must be kept warm in an intermediate dish or a pan, which in turn results in corresponding energy losses and a consumption of liberated brick lining.

On the other hand, there is also the problem of controlling the casting speed in the range of 2000 kg / h, since the pouring spouts used here with approximately 8 mm spout opening at low casting temperatures tend to settle or smudge.
A report to the "3rd International Symposium on Electroslag and other Special Melting Technology" of June 1971 discloses for a 600 mm ingot that, generally at a depth of the liquid metal pool of about 0.5 times the ingot diameter, good results of the ingot structure are achieved with respect to product properties wherein electrodes / ingot diameter d / D ratios of about 0.35 to 0.85 are shown with correspondingly minimum and maximum possible melt rates of about 200 to 1000 kg / hr, respectively. There is no execution made to the power supply; the current is probably supplied via the electrode, the mold or the ingot, so that the process can be realized.
DE 23 40 525 A1 discloses a device which contains at least one electrode with a mold having an expanded mold part; the slag bath extends into the widened part of the mold, to the outside of a current-conducting element leads. At the conical part, a cylindrical outlet part connects, which determines the strand cross-section of an ingot, wherein the one pole of the power supply via a terminal to the electrode or the other pole via the terminal to the mold in the region of the enlarged part is connected or via a pipe and a platform with that ingot.

With this prior art in mind, the inventor has set himself the goal of eliminating the identified deficiencies and offering an improved process for the electroslag strand melting of metals.

To solve this problem leads the teaching of the independent patent claims; the dependent claims indicate favorable Wieterbildungen.

According to the invention, the melting rate in kg / h should correspond to 1.5 to 30 times the strand diameter, in particular the equivalent strand diameter calculated from the circumference (U) of the casting cross section according to the relationship D _eq = U / π--, and the ratio of Cross sectional area of one or more Abschmelzelektroden the Cross-sectional area of Gießquerschnitts or Stranger be prepared equal to or greater than 1.0 are selected; the melt stream is passed between the electrode on the one hand and the strand with / or the mold designed as a funnel mold on the other hand monitored the phase boundary between slag bath and middle mirror by means of a control device wherein the newly formed strand is formed in the lower, narrower part of the mold and the above the mold level existing slag bath extends into the funnel-shaped widened part, where then immersed the tip of the Abschmelzelektrode in this.

Experiments have shown that the above-described disadvantages of the individual methods that have become known can be avoided or avoided in a surprisingly simple manner if the electroslag remelting process known per se uses considerably higher melting rates than hitherto when simultaneously melting electrodes have a melting point in comparison with Casting cross section large cross section can be used. Good results are already achieved if the cross-sectional area of the consumable electrode / s is at least 50% of the cross-sectional area of the strand to be produced. The values according to the invention of the abovementioned melting rates in kg / h should be at least 1.5 times - but not more than 30 times - the diameter in mm for round cross sections. In the case of strand shapes deviating from the round cross section, it is readily possible to work with that value for the equivalent diameter D _eq .

Particularly good results in terms of energy consumption and quality of the surface with a good center structure are achieved when the melting rate in kg / h corresponds to 5 to 15 times the equivalent diameter D _eq in mm.

This advantageous method according to the invention, described here in principle, can be adapted in many ways to the requirements of the operator.

For example, the mold can be installed in a working platform and the strand can be pulled down. However, the strand may also be built on a fixed bottom plate and the mold raised in the manner in which the strand grows. The removal of the strand or raising the mold can be continuous or stepwise.

It is also possible to oscillate the mold, which will be of particular interest in a continuous strand take-off.

In the case of a stepwise strand withdrawal or Kokillenhubbewegung may additionally connect directly to each Hübschritt a Gegenhubschritt, wherein the stride length of the Gegenhubschritts can be up to 60% of the stride length of the trigger stroke.

In the conventional electroslag remelting process, the melt stream flows through the slag between the tip of the Etekirode and the melt sump or, in the case of biphilic or three-phase fed plants, between the electrodes. Such current conduction is also possible in the method according to the invention.

With funnel-shaped molds also good results with a current between the electrode and mold wall are achieved.

For particularly good results in terms of heat distribution in the slag bath leads in a associated with at least one electrode and a power source mold for reaching into an upper extended mold part slag bath, with at least one current-conducting element in the expanded mold part of the mold wall and a subsequent to the mold part downwards , the strand cross-section defining outlet part provided device - in which the. Electrode is connected to a pole of the power source whose other pole is connected to both the strand and with the / the current-conducting element (s) in the mold wall, the inventive assignment of a control device to the narrower outlet part of the mold for the continuous monitoring of the position the phase boundary between the slag bath and the metal mirror, below, the control device drive rollers are provided for the strand withdrawal, which are also designed as contacts for the return current from the strand to the power source.

Further advantages, features and details of the invention will become apparent from the following description of a preferred embodiment and from the drawing; this shows in its single figure a sketch longitudinal section through a device for electroslag strand melting of metals with a lateral electrode in the waiting position.

The one pole of a - either AC or DC donating - power source 10 is connected via a feed line 12 with a suspension device 14 of a Abschmelzelektrode 16. The Elekrode 16 is moved by a not reproduced in detail in the drawing device so that the free end of the electrode 17 is always immersed in a slag 18.

The slag bath 18 is provided in a mold 20 which, in its cross-sectionally funnel-like mold bottom 22, has a tubular outlet part 24 for a remelt strand 26 of a diameter D formed therein. At the upper edge of its wall 28, the mold 20 has a radially projecting flange 30, which serves as a support for a counter-flange 32 of a gas-tight placed the electrode 16 surrounding hood 34.

The power supply to the other pole of the power source 10 is carried out either on strand 26 via trained as a current collector drive rollers 36 and - a Hochstromtrenner 38 containing - high current return line 40 or via the mold wall 28 built-in current collector 42 and another, subsequent high-current return line 40 _a with high-current separator 38 _a . Also possible is a power supply via strand 26 and pantograph 42 together while the return line is selected by pressing the aforementioned high-current separator 38 and 38 _a .

The proportion of current flowing through the current collectors 42 and the driving rollers 36 as contacts - when both in the respective high-current return lines 40, 40 _a provided high-current separator 38, 38 _a are connected so that a passage of current is made possible - depends on the ratio of the resistors in the slag bath 18. These are determined by the height of the slag bath 18 with respect to the current collectors 42 and the distance of the free end 17 of the electrode 16 from the metal mirror 44 in the mold 20 for the remelting strand 26 solidifying in its outlet part 24.

The Umschmelzstrang 26 is lowered by the drive rollers 36 according to the melting of the Abschmelzelektrode 16 and the mirror 44 of the liquid metal in the narrower outlet part 24 of the mold 20 by a control device, in particular a radioactive radiation source 46 monitored. At the same time - as already described - the driving rollers 36 also serve as a contact for the current return line 40 from the strand 26 to the source of stomata 10.

Cutting the desired product sections from the remelt strand 26 is possible, for example, by means of a flame cutting system indicated at 48.

If the first consumable electrode 16 is consumed, it can be removed from the melting region by means (not shown here) and replaced by a new electrode 16a _, which reaches a melting position from a waiting position sketched on the right, so that the melting process can continue can; By melting several electrodes 16 in succession, a continuous operation is made possible.

The electrode 16, 16 _a and the slag 18 are protected by this - as I said, sealed by means of its mating flange 32 against the Kokillenflansch 30 hood 34, 34 _a against air access.

In the described device, the remelting can take place under a controlled atmosphere and in the absence of atmospheric oxygen, which also enables the production of highly pure remelting strands 26 and prevents the combustion of oxygen-containing elements. This melting electrodes 16 are to be used, the cross-sectional area in relation to the Gießquerschnitt can be described as large.

In strand shapes, dio deviate from the circular cross section, an equivalent diameter D _eq for the Umschmelzstrang 26 is assumed, which can be derived from the circumference U with $${\mathrm{D}}_{\tilde{\mathrm{a}}\mathrm{q}}=\mathrm{U}/\mathrm{\pi },$$

Example:

To test the technology according to the invention, an experiment was carried out on an ESU system with lifting canoe. mold: Casserole, diameter below 160 mm, diameter above 350 mm consumable: 220 mm diameter Stole: P. 6-5-2

After melting 55 kg slag of the composition 30% CaO, 30% Al ₂ O ₃ 40% CaF ₂ , the Kokillenhub was adjusted so that the steel mirror was held about 20 to 30 mm below the funnel neck in the lower mold part with 160 mm diameter.

The electrical power was adjusted to 750 kW at 10 KA and 75 volts in the slag bath 18, the energy being introduced via the electrode 16 into the slag bath 18 and dissipated via both the string 26 and the mold wall 28 of the funnel-shaped upper portion.

With these conditions, a melting rate between 820 and 900 kg / h arose. Accordingly, the mold 20 was raised at a mean speed of 87 to 95 mm / min / with the lifting stepwise with about 10 mm stride. The stroke frequency was controlled and controlled via a radioactive pouring mirror measurement.

A strand 26 about 3.0 m in length was produced. The surface finish was good, so that no surface treatment was required before hot working. Strand 26 was easily forged to a billet of 100 mm square on a forging hammer.

The metallographic testing showed a uniformly fine-grained carbide distribution center segregations were not found.

Claims (8)

1. Method for remelting metals, in particular steels as well as Ni-based and Co-based alloys, into a billet (26) by melting off at least one consumable electrode (16) with a melt-off rate in kg/h in an electrically conductive slag bath (18) which is provided in a short, open-bottomed chill mould (20), characterised in that the melt-off rate corresponds to 1.5 to 30 times the billet diameter (D, D_eq) in mm and the ratio of the cross-sectional area of one or more consumable electrodes (16) to the cross-sectional area of the casting cross-section or of the billet (26) to be produced is selected at equal to or greater than 1.0, wherein the melting current is conducted between the electrode (16) on the one hand and the billet (26) and/or the chill mould (20) designed as a funnel chill mould on the other hand, and the phase boundary between slag bath and metal level (44) is monitored by means of a monitoring device (8), and in that the billet is formed in the lower narrow portion of the funnel chill mould, and the slag bath extends as far as the wider upper portion thereof.
2. Method according to claim 1, characterised by an equivalent billet diameter (D_eq) calculated from the circumference (U) of the casting cross-section and differing from the round cross-section, according to the expression D_eq = U/n.
3. Method for remelting metals in a funnel chill mould according to claim 1 or 2, characterised in that the melt-off rate in kg/h corresponds to 5 to 15 times the equivalent billet diameter (D_eq) calculated from the circumference (U) of the casting cross-section.
4. Method according to any of claims 1 to 3, characterised in that the formed billet is drawn off continuously from the chill mould, wherein if occasion arises the formed billet is stationary and the chill mould is lifted continuously.
5. Method according to any of claims 1 to 3, characterised in that the formed billet is drawn off stepwise from the chill mould, wherein if occasion arises the formed billet is stationary and the chill mould is lifted stepwise.
6. Method according to claim 4, characterised in that the chill mould is moved in oscillating fashion.
7. Method according to claim 5, characterised in that immediately after each lifting step a counter-lifting step is performed in the opposite direction, and the stroke length of the counter-lifting step is selected at not more than 60% of the stroke length of the preceding lifting step.
8. Apparatus for carrying out the method according to one or more of the preceding claims with at least one electrode (16) as well as a chill mould (20) associated with a current source for a slag bath (18) extending into a wider upper portion (22) of the chill mould, with at least one electrically conductive element (42) in the chill mould wall (28), and also a discharge portion (24) which adjoins the chill mould portion at the bottom and defines the billet cross-section, wherein the electrode (16) is connected to one pole of the current source (10) and its other pole is connected both to the billet (24) and to the electrically conductive element(s) (42) in the chill mould wall (28), characterised in that a monitoring device (46) is fitted on the narrower discharge portion (24) of the chill mould (20) for constant monitoring of the position of the phase boundary between the slag bath and the metal level (44) in the discharge portion (24), and beneath the monitoring device are provided driving rollers (36) for drawing off the billet, which are also designed as contacts for the current return (40) from the billet (26) to the current source (10).

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