EP0786531B1 - Process and installation for remelting of metals to a strand - Google Patents

Description

The invention relates to a process for continuous remelting of metals - especially of steels as well as Ni and Co-base alloys - to a strand by melting at least a self-consuming electrode in an electrically conductive Slag bath, which is in a short, open at the bottom Mold is provided. The invention also relates to a device to carry out this procedure.

In the production of high-alloy tool steels, for example - such as high-speed steels, ledeburitic Chrome steels or other strongly segregating steels and alloys - The production of continuously cast strands is smaller to medium cross-sections associated with problems.

DE 1 932 763 A is a device for carrying out the Electro-slag refining process with a water-cooled, vertically arranged mold for raw blocks in particular high-alloy steel; this mold is double-walled and offers you the outer dimensions of the one to be created Raw block determining lower area, its inner cross section at most equal to the cross section of one in a warm one Slag bath is consumable electrode to be melted, wherein the lower mold area preferably in an upward direction merges conically widening intermediate area and this again in an expanded upper mold area. Here takes the stream made its way to the ingot through the slag bath.

GB 1 413 508 A discloses a mold in the mold wall installed a non-consumable current-conducting element as well connected to the power supply via a rectifier is that both the electrode and the casting cross section - - in relation to the non-consumable element - constantly electric are negatively polarized.

From DE 1 483 646 A and AT 320 884 B there are also variants of the Electroslag remelting process known. The ones described there Enable self-consuming electrodes the production of remelting blocks with a good surface slow block build speed. The small ones that occur Swamp depths lead to an even Solidification between edge and core and thus to one good internal quality of the remelted blocks. The application short molds with lowerable base plates and electrode change allows relatively long formation here, too Strands. When producing small dimensions, however the generation of the required consumable electrodes difficult, and the procedural costs due to the then low Remelting rates are high.

To the problem of the manufacturability of electrodes with small To avoid cross-sections, the use of so-called funnel or T-chill proposed; the mold takes on a funnel-shaped part that extends upwards Slag bath and thus enables melting of Electrodes whose cross-section is that of the remelting block to be produced is.

While in the continuous casting of formats between 100 and 200 mm - round or square - even with slow pouring Casting capacities of at least 5 to 10 t per hour and Strand are required, the melting rates at ESU procedure maximum 100 - 200 kg per hour for the same Formats. This results in sump depths during continuous casting between 4 m and 8 m. The swamp depths in the ESU procedure measure 100 to 300 mm.

Another procedure is according to AT-PS 399.463 proposed strands of high-alloy steels with essential lower casting speeds - than with Continuous casting are common - casting to improve To reach the core zone while covering the Pouring mirror through an electrically heated slag bath, no disadvantages with regard to the formation of the surface have to put up with due to excessive cooling. It is assumed that the liquid metal over a longer period Time with constant temperature from a heatable Pan can be exposed.

The problem often arises with this method keeping larger quantities of liquid metal warm a longer period. This is particularly important if only one strand is used. So surrendered for example when casting melts 25 t total weight to one strand with e.g. 150 mm diameter with a pouring rate of, for example, 2000 kg / h Casting times of 12.5 hours. During this time the Keep the melt warm in an intermediate vessel or pan be, which in turn corresponding energy losses and a consumption of refractory lining Has.

On the other hand, there is also the problem of controlling the Casting speed in the range of 2000 kg / h, since the here used spouts with about 8 mm spout opening at low pouring temperatures for freezing or smearing tend.

The inventor was aware of this state of the art set the goal of eliminating the identified shortcomings and an improved process for electro-slag strand melting of metals.

The teaching of the independent leads to the solution of this task Claims; the subclaims give favorable residences on.

According to the invention, the melting rate in kg / h should correspond to 1.5 to 30 times the strand diameter - above all the equivalent strand diameter calculated from the circumference (U) of the casting cross section in accordance with the relationship D _equ = U / π, the ratio the cross-sectional area of one or more melting electrodes for the cross-sectional area of the casting cross section is selected to be greater than 0.5.

Experiments have shown that the disadvantages of the individual methods disclosed at the outset can be avoided or circumvented in a surprisingly simple manner if the known electroslag remelting method is used with considerably higher melting rates than previously, if at the same time melting electrodes are compared with one large cross-section can be used for the casting cross-section. Good results are already achieved when the cross-sectional area of the melting 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 mentioned melting rates in kg / h for round cross sections should be at least 1.5 times - but not more than 30 times - the diameter in mm. 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 _equ .

Particularly good results with regard to energy consumption and surface quality with a good center structure are achieved if the melting rate in kg / h corresponds to 5-15 times the equivalent diameter D _eq in mm and the ratio of the cross-sectional area of the melting electrode / s to the cross-sectional area of the one to be produced Pouring cross section is equal to or greater than 1.0. In this case, it must be remelted in a funnel or T mold known per se, the newly formed strand being formed in the lower, narrower part of the mold and the slag bath located above the casting level extending into the funnel-shaped part, where the Dip the tip of the melting electrode into it.

This advantageous principle according to the invention described here in principle The procedure can be followed in many ways Operator requirements are adjusted.

For example, the mold can be fixed in a work platform be installed and the strand pulled down become. The strand also likes on a fixed one Base plate built up and the mold raised in the way become as the strand grows. Pulling the Strands or lifting the mold can continuously or step by step.

There is also the possibility of oscillating the mold let what especially with a continuous Strand withdrawal will be of interest.

In the event of a gradual strand withdrawal or mold lifting movement can also directly at each lifting step connect a counter stroke step, the step length of the Counter stroke step up to 60% of the step length of the trigger stroke step can be.

In the conventional electroslag remelting process the melt current flows through the slag between the electrode tip and melting sump or in biphilar or three-phase systems between the electrodes. Such a current flow is also in the invention Procedure possible.

When working with funnel-shaped molds, also good results with current routing between electrodes and mold wall achieved.

For particularly good results with regard to heat distribution in the slag bath there is an arrangement in which the electrode is connected to one pole of the transformer is while the second pole of the transformer at the same time both with the strand and with at least one current-conducting element built into the mold wall connected is; this current-conducting element is in one extended upper mold part provided, in which the Slag bath in and at the bottom of the The outlet section determining the strand cross section connects.

Further advantages, features and details of the invention emerge from the following description of a preferred Embodiment and with reference to the drawing; in its only figure, this shows a sketchy one Longitudinal section through a device for electro-slag strand melting of metals with a side electrode on hold.

One pole one - either AC or DC emitting - current source 10 is via a supply line 12 with a suspension device 14 of a melting electrode 16 connected. The electrode 16 is by a in the Drawing not shown in detail moves that the free electrode end 17 always in one Slag bath 18 immersed.

The slag bath 18 is provided in a mold 20 which in its funnel-like mold bottom 22 a tubular outlet part 24 for a resultant therein Remelting strand 26 has a diameter D. At the top The edge of its wall 28 has the mold 20 projecting radially Flange 30 on, as a support for a counter flange 32 of a gas-tight mountable electrode 16 surrounding hood 34 is used.

The current supply to the other pole of the current source 10 takes place either on the line 26 via drive rollers 36 designed as current collectors and a high current return line 40 containing a high current isolator 38 or via current collectors 42 built into the mold wall 28 and another high current return line 40 _{a connected} thereto with high current isolator 38 _a . It is also possible to carry current together via line 26 and current collector 42; the return line is selected by actuating the high-current isolators 38 or 38 _a mentioned.

The proportion of the currents flowing through the current collectors 42 and the drive rollers 36 as contacts - if both of the high-current isolators 38, 38 _a provided in the respective high-current return lines 40, 40 _a are switched so that current passage is possible - depends on the ratio of the resistances in Slag bath 18 from. These are determined by the height of the slag bath 18 in relation to the current collectors 42 or 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 remelting strand 26 is correspondingly driven by the drive rollers 36 the melting of the melting electrode 16 is lowered and the mirror 44 of the liquid metal in the narrower Outlet part 24 of the mold 20 through a control device, in particular a radioactive radiation source 46, supervised. At the same time - as already described - they serve Drive rollers 36 also as a contact for the current return 40 from strand 26 to power source 10.

A cutting of the desired product sections from the remelting strand 26 is indicated, for example, by one at 48 Flame cutting system possible.

If the first melting electrode 16 is consumed, it can be removed from the melting area by means (not shown) and replaced by a new electrode 16 _a , which reaches the melting position from a waiting position outlined on the right, so that the melting process can be continued ; Continuous operation is made possible by melting several electrodes 16 in succession.

The electrode 16 _, 16 _a and the slag bath 18 are protected against air access by the hood 34 _, 34 _{a, which is} sealed against the mold flange 30 by means of its counter flange 32.

In the described device, the remelting can controlled atmosphere and in the absence of atmospheric oxygen take place, with which also the generation ultra-pure remelting strands 26 allows and a burn oxygen-affine elements is prevented. In doing so Fuse electrodes 16 are used, their cross-sectional area in relation to the casting cross-section as large can be designated.

In the case of strand shapes which deviate from the round cross section, an equivalent diameter D _eq for the remelting strand 26, which can be derived from the circumference U, is assumed D eq = U / π.

Example:

To test the technology according to the invention, a test was carried out on an ESCU with a lifting mold. Mold: Funnel mold, diameter below 160 mm, diameter above 350 mm Consumable electrode: 220 mm diameter Steel: S 6-5-2

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

The electrical output was 750 kW at 10 KA and 75 Volts set in the slag bath 18, the energy being about the electrode 16 is introduced into the slag bath 18 and both over the strand 26 as well as over the mold wall 28 derived from the funnel-shaped upper part has been.

Under these conditions there was a melting rate between 820 and 900 kg / h. Accordingly, the mold was 20 with a medium speed of 87 to 95 mm / min. raised, the lifting gradually with about 10 mm stride length. The stroke rate was over a radioactive casting level measurement checked and controlled.

A strand 26 of approximately 3.0 m in length was produced. The Surface quality was good, so that before hot working no surface treatment was required. Strand 26 became a stick with 100 mm square without difficulty pre-forged on a forging hammer.

The metallographic testing showed a uniform fine-grain carbide distribution. Center increases were not found.

Claims (10)

1. Method for remelting metals, in particular steels as well as Ni-based and Co-based alloys, into a billet by melting off at least one consumable electrode in an electrically conductive slag bath which is provided in a short, open-bottomed chill mould, characterised in that a melt-off rate is set in kg/h, corresponding to 1.5 to 30 times the billet diameter (D, D_aq) in mm, wherein the ratio of the cross-sectional area of one or more consumable electrodes to the cross-sectional area of the casting cross-section or of the billet to be produced is selected at greater than 0.5.
2. Method according to claim 1, characterised by an equivalent billet diameter (D_aq) calculated from the circumference (U) of the casting cross-section and differing from the round cross-section, according to the expression D_aq - U/π.
3. Method for remelting metals in a funnel chill mould according to claim 1 or 2, characterised in that the melt-off race in kg/h corresponds to 5 to 15 times the equivalent billet diameter (D_aq) calculated from the circumference (U) of the casting cross-section, and the ratio of the cross-seccional area(s) of the consumable electrode(s) to the cross-sectional area of the casting cross-section is equal to or greater than 1.0, wherein 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.
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 co 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 counterlifting step is performed in the opposite direction, and the stroke length of the counterlifting step is selected at not more than 60% of the stroke length of the preceding lifting step.
8. Mechod according to one or more of claims 1 to 7, characterised by a fusing current flowing between electrode and billet or by a fusing current flowing between electrode and chill mould.
9. Method according to one or more of claims 1 to 7, characterised by a fusing current flowing between on the one hand electrode and on the other hand simultaneously both billet and chill mould.
10. 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 (10) for a slag bath (18) extending into a wider upper portion (22) of the chill mould, at least one electrically conductive element (42) in the wider chill mould portion (22) of the chill mould wall (28), and also an outlet 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 terminal of the current source (10) and its other terminal is connected both to the billet (24) and to the electrically conductive element(s) (42) in the chill mould wall (28).

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