WO2018050751A1

WO2018050751A1 - Method for purifying a metal melt in an induction furnace

Info

Publication number: WO2018050751A1
Application number: PCT/EP2017/073150
Authority: WO
Inventors: Rüdiger Schwarze; Amjad ASAD; Christos G. Aneziris; Steffen Dudczig
Original assignee: Technische Universität Bergakademie Freiberg
Priority date: 2016-09-16
Filing date: 2017-09-14
Publication date: 2018-03-22
Also published as: DE112017000047A5; DE112017000047B4

Abstract

The invention relates to a method for purifying a metal melt in the crucible of an induction crucible furnace before said metal melt is poured into a casting mold or into a distributor of the strand casting system, wherein at least one ceramic filter body or filter system is dipped into the flowing metal melt and nonmetallic inclusions (NMIs) moving in the melt are removed.

Description

Process for the purification of molten metal in an induction furnace

The invention relates to a method and a device for cleaning a molten metal in the crucible of an induction crucible furnace, in particular the cleaning of a molten steel, wherein endogenous, moving in the melt non-metallic inclusions are removed.

Safety components, thin-walled or thick-walled castings or forged components with high demands on strength, toughness and fatigue resistance can become unusable due to an impermissible portion of non-metallic inclusions or undergo a dramatic reduction of the safety-relevant properties, such as e.g. Notched impact strength up to 40%. Non-metallic inclusions are impurities in solid form. These can either be introduced into the melt from the outside (exogenous inclusions) or melt (endogenous inclusions).

Exogenous inclusions are, for example, slag or dissolved refractory materials, which precipitate on cooling of the melt. Endogenous inclusions, such as non-metallic inclusions, may arise during the deoxidation of molten steel.

The reduction of non-metallic inclusions can take place in different ways, a) avoiding inclusion formation (via a suitable metallurgical process) and b) deposition of the inclusions. Especially in the field of secondary metallurgy and in the foundry, the melting process is a central topic. Different molten metal treatments and different casting processes lead to different generation of inclusions or inclusion combinations. For example, oxides, carbides, silicates, etc. may be mentioned here, which, owing to the high temperature of the melts, can occur very rapidly during the casting process. Due to their non-wettability, the non-metallic inclusions are partly turbulent in real castings and are partly turbulent in their shape. This often leads to sharp edges. This leads to internal cracks and thus to the failure of the load, especially in dynamic or cyclic load cases component.

DE 60 2006 000 294 T2 discloses a method and apparatus for removing slag produced by liquefying the metal material by heating and floating on the surface of the molten metal in an induction furnace. For this purpose, the induction heating is stopped at a predetermined temperature, so that the molten metal settles and the slag begins to float on the surface. A filter made of a heat-resistant, porous ceramic is moved along a surface of the molten metal, such that the lower half part of the filter is immersed in the molten metal and the upper part of the filter above the surface of the filter molten metal is. The filter traps the slag that floats on the surface of the molten metal and removes the slag from the molten metal.

A disadvantage of this method is that only the slag which is deposited on a melt at rest, i. with stopped induction heating, forming, is removed. Impurities that move freely in the melt, such as endogenous inclusions can not be removed.

WO 2007/015956 A2 discloses a process for removing impurities before the molten metal penetrates into the mold. The at least one filter here is not arranged within the casting vessel in which the molten material is contained. The at least one filter is arranged inside a filter vessel, above which the casting vessel containing the molten material and below the filter vessel a transfer vessel or a mold are arranged. The method comprises the steps of preheating at least one filter to a filter temperature which prevents breakage of the filter due to thermal shock from contact with the molten metal, and flowing of the molten metal through at least one filter.

From KR 10 2001 010 0625 A a filter system for the deoxidation of a molten metal in a horizontal continuous casting process using a low frequency induction furnace is known. The filter system consists of graphite filters arranged inside the melting chamber and the heat insulation chamber. Additional gas lines supply gas for deoxidation and purification of the molten metal.

The invention has for its object to increase the filtration efficiency and consequently the quality of the molten metal and the metallic end product. In particular, freely moving, endogenous, non-metallic inclusions are to be removed in a flowing molten metal.

The object is achieved by a method for cleaning a molten metal in the crucible of an induction crucible furnace, wherein an AC-driven coil generates a melt flow and prior to casting in a casting mold or in a distributor for continuous casting, at least one ceramic filter of refractory oxides and / or non Oxide is dipped in combination with carbon in the preferably flowing molten metal, so that the moving in the melt non-metallic inclusions (NME) are removed by reactive means.

Advantageously, with the method, endogenous inclusions, such as non-metallic inclusions, are reactively removed from the flowing molten metal. Crucible furnaces allow inductive heating and melting of the metals. This heating is done by an AC driven coil. This creates a magnetic alternating field, which drives the melt flow in the crucible furnace.

A crucible furnace in the context of the invention is an inductively heated furnace, also known as an induction furnace, induction melting furnace, crucible induction furnace or induction crucible furnace, wherein at least one coil is arranged around the crucible.

In one embodiment, the molten metal in an induction furnace is pre-cleaned by means of at least one, preferably two, ceramic filters before the melt flows into the ladle, into the distributor, into the casting mold or into the casting mold.

Advantageously, immersing the filter in the molten metal in the crucible induction furnace results in improving the quality of the molten metal because the filter is capable of trapping the molten NME.

Reactive removal of the non-metallic inclusions in the sense of the invention means that as a result of a chemical reaction between the filter material and the molten metal, the non-metallic inclusions are removed directly and / or indirectly from the molten metal.

The direct removal of non-metallic inclusions occurs via the formation of in-situ layers on the filter surface.

The filter bodies can advantageously contribute directly to the removal of inclusions (generation of in situ layers on their filter surfaces where the inclusions deposit and lead via sintering processes to an attachment to the filter surface).

Indirect removal of the non-metallic inclusions occurs by transporting the non-metallic inclusions to the surface of the molten metal. Upon immersion of the ceramic filter in the flowing molten metal, a chemical reaction takes place between the ceramic filter material and the flowing molten metal. This gaseous reaction products are formed, which are able to transport the non-metallic inclusions to the surface of the melt.

Upon immersion of the filters in the hot molten steel, the carbon and alumina of the carbon-bonded Al 2 O 3 filter react with each other to form gaseous products such as aluminum suboxides and carbon monoxide, the iron of the molten steel catalytically acting. The gaseous products act as gas bubbles in the molten steel and are capable of transporting non-metallic inclusions to the surface of the melt. The indirect removal takes place in the sense of a kind of flotation over the gaseous products formed, for example CO bubbles from the carbothermic reduction between carbon from the carbon-bonded filter material and the oxide filler of the filter material, eg alumina. The molten bath movement preferably supports the transport of the gaseous products, eg CO gas bubbles, via the switched-in induction field with the attached endogenous inclusions in the direction of the surface of the molten metal.

In addition, the filter bodies may indirectly result in the deposition of the non-metallic inclusions into the surface of the molten metal by the design of the filter geometry, in combination with the filter body rotation and the flow of the molten metal in the induction furnace. There is also preferably the slag for receiving the particles. In addition, preference can be given to generating larger agglomerates / clusters via the targeted adjustment of the flow, which move in the direction of slag due to their size (additional contribution of the buoyancy force).

In a further embodiment, the filter bodies and / or the filter systems contribute directly to the removal of inclusions with the generation of in situ layers on their filter surfaces, at which the inclusions deposit and lead via sintering processes to an attachment to the filter surface and / or indirectly, by shortening the design of the filter geometry and in combination with the filter body rotation and the flow of molten metal in the crucible of the induction crucible furnace, the way to deposit the non-metallic inclusions of a slag located on the melt surface.

Immersion of the at least one ceramic filter in the flowing molten metal in the sense of the invention means that the ceramic filter is located in the flowing molten metal, flows around it and is in intimate contact with the flowing molten metal. The at least one ceramic filter dives punctually in the flowing molten metal, wherein the position of the ceramic filter is not changed relative to the arrangement within the induction crucible during the process for cleaning the molten metal.

Pouring in the sense of the invention means that the molten metal is in ordered movement and a melt flow prevails within the induction crucible furnace, so that the molten metal circulates on recirculating paths in the induction crucible furnace.

Before casting in the sense of the invention means that the AC driven coil of the induction crucible furnace is turned on and the molten metal is in ordered movement and not in an idle state. At least one ceramic filter is immersed in the molten metal in ordered motion and the molten metal in ordered motion is cleaned by removing the non-metallic inclusions from the molten metal. The cleaned molten metal is then poured into a mold or manifold for continuous casting, with further purification steps following. In the filtration of metallic melts, consider a) the transport of the inclusions in the molten metal, which is influenced by the flow guidance, the size and relative density of the inclusion particles, the relative pore diameter and the cross-linking of the filter pores, and b) the actual process of deposition the inclusions on the filter wall.

The efficiency of the filtration process depends on the transport of the non-metallic inclusion particles (NME) in the molten metal. The flow of molten metal inside and outside the filter can play a crucial role, e.g. For example, the flow may or may not transport the NME to the walls of the filter.

By a suitable choice of the frequency of the alternating current to drive the coil, the molten bath movement in the induction crucible furnace can be controlled in a targeted manner.

Typically, in the induction crucible furnace a melt flow in the form of two counter-rotating vortex toroids, wherein the molten metal circulates on recirculating paths in the crucible of the induction crucible furnace and thereby directed to the walls of the induction crucible furnace, the bottom of the induction crucible furnace and / or the surface of the molten metal and then recombined.

According to the invention, oxides and / or non-oxides in combination with carbon serve as filter materials, for example carbon-bonded Al 2 O 3.

Advantageously, a high filtration efficiency is achieved by a carbon-bonded filter material. Preferably, with 1 to 20 g of such a filter material, 20 to 50 kg of a molten steel are cleaned in 10 seconds.

Refractory oxides and / or non-oxides in combination with or without carbon are preferably used as filter materials.

Cylindrical, carbon-bonded filter bodies based on foam ceramics or spaghetti structures with macrochannels with a minimum diameter of 10 mm are preferably used. Filter bodies with a minimum diameter of 100 mm diameter and 100 mm height are preferred.

In a further preferred embodiment, the at least one ceramic filter is changed and / or rotated about its axis within the flowing molten metal.

In order to accommodate the high flow rates of the molten metal as well as the limitations of the filter capacities, exchangeable filter bodies are preferably used at eccentric positions in the induction furnace. Further advantageously, the filtration efficiency is increased when the at least one ceramic filter is rotated about its axis, since the melt is accelerated around the filter and under the filter.

As a ceramic filter according to the invention filter body and / or filter systems are understood.

Preferably serve as a stationary, interchangeable and / or rotatable eccentrically positioned filter in the crucible of the induction furnace filter body in the form of an open-cell foam ceramic, a honeycomb body or a spaghetti filter. Preferably serve as filter systems porous, changeable and / or rotatable containers in which mashed fiber or fiber fabric or balls or splintery grain are present.

In a preferred embodiment, the molten metal to be cleaned is a molten steel.

In a further preferred embodiment, the at least one ceramic filter is immersed eccentrically in the molten metal.

The eccentric position of the filter advantageously serves to increase the mass flow of the melt flowing through the filter. The reason for this is that the filter is preferably positioned in an area where higher metal melt speeds are to be expected.

For a high interaction with the melt and accordingly also for a high cleaning efficiency, the filters are preferably immersed eccentrically in the crucible.

Preference is given to molten metals before casting

a) into a mold, e.g. when pouring or molding in a foundry

or

b) by the continuous continuous casting with the help of a distribution vessel and a continuous casting machine

cleaned in an induction furnace with eccentrically installed filter bodies and / or filter systems.

In a further preferred embodiment, the purification of a molten metal is carried out by cleaning the molten metal in a crucible of the induction crucible furnace before casting into a casting mold or into a distributor for continuous casting in an induction crucible furnace with the aid of at least one eccentrically positioned filter body and / or filter system.

Eccentric position according to the invention means the arrangement of the at least one ceramic filter outside the center of the induction crucible furnace. The position in the induction furnace and the number of filters used as well as their permeability, open porosity, pore size distribution, specific surface area and roughness can advantageously influence the efficiency of the cleaning process.

In a further preferred embodiment, the at least one ceramic filter has an active surface coating of aluminum oxide (Al ₂ O 3), magnesium aluminate spinel (MgO-Al ₂ O ₃ ), hercynite (FeO-Al ₂ O ₃ ), Jakobsit (MnO-Fe ₂ O ₃ ), Galaxite (MnO-Al ₂ 0 ₃ ), mullite (3AI ₂ 0 ₃ -2SiO ₂ ), rodonite (MnO-SiO ₂ ) or fayalite (2FeO-SiO ₂ ) or mixtures of the aforementioned substances.

Advantageously, the filtration efficiency of the at least one ceramic filter is further increased by the active surface coating.

Carbon-bonded carrier materials may preferably be coated with oxides on their surface analogously to the patent DE 10 201 1 109 681 B4.

In a further preferred embodiment, the at least one ceramic filter has a coating based on carbonaceous and / or ceramic nanoparticles.

Carbon-containing nanoparticles can be present in various forms, such as fullerenes, carbon nanotubes or carbon black, preferably as carbon nanotubes, particularly preferably as multi-walled carbon nanotubes.

Ceramic nanoparticles can be nanoscale oxidic or non-oxidic ceramic particles, preferably oxidic ceramic nanoparticles, particularly preferably Al ₂ O ₃ nanosheets.

Advantageously, the filtration efficiency is further increased by the coating based on carbonaceous and / or ceramic nanoparticles.

In a further preferred embodiment, the at least one ceramic filter is rotated about its axis at a speed of 5 to 60 revolutions per minute.

This causes the melt to be accelerated around the filter and under the filter. This principle advantageously makes it possible to increase the filtration efficiency.

The immersed filter body (s) should preferably be rotated about its axis at different rotational speeds.

In a further preferred embodiment, the at least one ceramic filter is immersed in the molten metal for 5 seconds to 30 minutes and then changed.

Particularly preferably, the at least one ceramic filter is immersed in the molten metal for 5 seconds to 10 seconds and then changed.

Advantageously, this proposes a method for purifying a molten metal in the crucible of an induction crucible furnace, which prior to casting into a mold or a distributor for continuous casting allows an efficient and rapid cleaning of a flowing molten metal within the induction crucible furnace.

Preferably, the filter bodies are immersed in the crucible of the induction furnace depending on the metal alloy and the upstream secondary metallurgy for 10 seconds to 30 minutes and then replaced.

In a further embodiment, the filter bodies and / or the filter systems are immersed in the crucible of the induction crucible furnace for 5 seconds to 30 minutes and then changed.

The invention also includes a device for cleaning a molten metal in the crucible of an induction crucible furnace

comprising an AC-driven coil for producing a melt flow, comprising

at least one ceramic filter of refractory oxides and / or non-oxides in combination with carbon,

and a drive unit for immersing the at least one ceramic filter, wherein the at least one ceramic filter filter body in the form of an open-cell foam ceramic, a honeycomb body or a spaghetti filter comprises.

In a further embodiment, the filter bodies and / or the filter systems are used in the form of an open-cell foam ceramic or a honeycomb body or a spaghetti filter.

In a preferred embodiment, the at least one ceramic filter further comprises, as a filter system, porous containers in which mashed fibers or fibrous webs or spheres or splintered grains are present.

The filter systems used are porous containers in which mashed fibers or fiber fabrics or spheres or splintery grains are present.

In a further preferred embodiment, the ceramic filter is rotatably and / or exchangeably connected to the drive unit and the drive unit is configured such that the vertical movement of the at least one ceramic filter for immersion in the molten metal and the rotation of the at least one ceramic filter about its own axis is possible.

The exchangeable ceramic filter advantageously makes it possible to accommodate the high flow rates of the molten metal as well as the limitations of the filter capacities. Further advantageously, the filtration efficiency is increased when the at least one ceramic filter is rotated about its axis, since the melt is accelerated around the filter and under the filter.

In one embodiment, changeable and / or rotatable filter bodies and / or filter systems are used in the crucible of the induction crucible furnace in the induction crucible furnace.

In a further preferred embodiment, filter bodies or filter systems for increasing the filtration efficiency are additionally attached to the crucible wall of the induction crucible furnace.

To increase the filtration efficiency filter body or filter systems can be preferably attached to the crucible wall.

In one embodiment, filter bodies or filter systems are mounted on the crucible wall to increase filtration efficiency.

In a further preferred embodiment, coatings based on the materials of the filter bodies or the filter systems for increasing the filtration efficiency are attached to the crucible wall of the induction crucible furnace.

Coatings based on the materials of the filter bodies or the filter systems can preferably also be applied to the crucible wall.

In one embodiment, coatings based on the materials of the filter bodies or the filter systems for increasing the filtration efficiency are applied to the crucible wall.

The filters or filter systems or coatings, which are additionally attached to the crucible wall, advantageously permit the deposition of non-metallic inclusions on the crucible walls based on the materials of the filter bodies or the filter systems.

embodiments

The invention will be explained in more detail with reference to some embodiments and associated figures. The embodiments are intended to describe the invention without limiting it. Showing:

Fig. 1 shows a molten metal in an induction crucible furnace and two eccentrically submerged filters

2 shows the results of a numerical simulation of the filtration rate as a function of the number of filters and their position in the induction crucible furnace

3 shows a molten metal in an induction crucible furnace and a centrically submerged filter body In Fig. 1 shows the schematic diagram according to the invention. Shown is an induction crucible furnace with a molten metal therein. A coil is arranged around the induction crucible furnace. Within the melt, represented as black dots are the non-metallic inclusions moving and to be removed in the melt. Within the molten metal, two ceramic filters are positioned.

The arranged around the crucible coil is operated with alternating current and thus generates a magnetic alternating field, which drives the melt flow. Within the melt, a melt flow is formed according to the illustrated counter-rotating vortex toroids, wherein the molten metal circulates on recirculating paths in the crucible of the induction crucible furnace and thereby directed to the walls of the induction crucible furnace, the bottom of the induction crucible furnace and / or the surface of the molten metal and then brought together again , Immersed in the flowing molten metal are two ceramic filters. Here it can be seen that the filters are positioned eccentrically in the induction furnace. The ceramic filters can be additionally rotated around their axis.

Fig. 2 shows the results of a numerical simulation on the dependence of the filtration rate on the number of filters and their position in-an induction furnace, wherein a filter centric, a filter eccentric or two filters are arranged eccentrically.

The filtration rate or filtration efficiency is calculated as:

Number of non-metallic inclusions after cleaning the molten metal Number of non-metallic inclusions before cleaning the molten metal

Here, the advantage of the eccentric position of the filter can be seen. The filtration rate is increased when the filter is positioned eccentrically or several filters are immersed in the induction furnace. A centric filter placed in the center of the induction crucible furnace achieves a filtration rate of 39%. If a filter is positioned eccentrically outside the center of the induction crucible, the filtration rate increases to 49%. A further increase of the filtration rate to 73% can be achieved by the arrangement of two eccentrically positioned filters.

Fig. 3 shows a laboratory device for cleaning a molten metal in the crucible of an induction crucible furnace. The induction crucible furnace, consisting of a crucible 1 and an induction coil 2 surrounding the crucible 1. The induction coil is operated with a power supply 3 with alternating current in the kHz frequency range. The entire induction crucible furnace is located in a furnace chamber 8, which is evacuated to 2 mbar before the steel alloy is melted and then filled with argon. The induction crucible furnace produces 30 kg of 42CrMo4 steel melted. In the molten steel 4, the endogenous inclusions 5 move. The molten steel 4 is in ordered movement in accordance with the melt flows 6 that are formed. The filter 7 is connected to a drive unit 10 via a holder 9. The filter 7 is immersed centrally in the flowing molten steel 4 at a temperature of the melt of 1670 ° C, in the center of the induction crucible furnace. The filter 7 with the dimensions 130 mm x 50 mm x 50 mm is made of carbon-bonded Al2O3 according to the Schwartzwalder method as foam ceramic filter of the pore class 10 ppi. The filter 7 is immersed for 10 seconds in the molten steel 4 and thereby rotated at 30 revolutions per minute about its axis. The filter 7 achieves a filtration efficiency of 95%.

reference numeral

1 crucible of the induction crucible furnace

2 induction coil

3 power supply, alternating current

4 molten steel

5 endogenous inclusions

6 melt flow

7 Ceramic filter

8 oven came mer

9 bracket filter

10 drive unit filter

Claims

claims

1 . A process for purifying a molten metal in the crucible of an induction crucible furnace, wherein an AC driven coil generates a melt stream and cast prior to casting into a casting mold or manifold, at least one refractory oxide and / or non-oxide ceramic filter in combination with carbon is immersed in the flowing molten metal so that the non-metallic inclusions moving in the melt are reactively removed.

2. The method according to claim 1, characterized in that the molten metal is a molten steel.

3. The method according to claim 1 or 2, characterized in that the at least one ceramic filter is changed and / or rotated about its axis within the flowing molten metal.

4. The method according to any one of claims 1 to 3, characterized in that the at least one ceramic filter has an active surface coating of aluminum oxide (Al2O3), magnesium aluminate spinel (MgO-A C), hercynite (FeO-A C), Jakobsit (MnO -Fe ₂ 0 ₃ ), galaxite (MnO-Al ₂ 0 ₃ ), mullite (3AI ₂ 0 ₃ -2Si0 ₂ ), rodonite (MnO-Si0 ₂ ) or fayalite (2FeO-Si0 ₂ ) or mixtures of the aforementioned substances.

5. The method according to any one of claims 1 to 4, characterized in that the at least one ceramic filter has a coating based on carbonaceous and / or ceramic nanoparticles.

6. The method according to any one of claims 1 to 5, characterized in that the at least one ceramic filter is rotated at a speed of 5 to 60 revolutions per minute about its axis.

7. The method according to any one of claims 1 to 6, characterized in that the at least one ceramic filter is immersed for 5 seconds to 30 minutes in the molten metal and then changed.

8. The method according to any one of claims 1 to 7, characterized in that the at least one ceramic filter is immersed eccentrically in the molten metal.

9. A device for cleaning a molten metal in the crucible of an induction crucible furnace with an AC-driven coil for generating a melt flow having

a drive unit for moving the at least one ceramic filter, wherein the at least one ceramic filter filter body in the form of an open-cell foam ceramic, a honeycomb body or a spaghetti filter comprises.

10. The device according to claim 9, characterized in that the at least one ceramic filter further comprises as filter systems porous container in which mashed fibers or fiber fabric or spheres or splintery grains present.

1 1. Apparatus according to claim 9 or 10, characterized in that the ceramic filter is rotatably and / or changably connected to the drive unit and the drive unit, the vertical movement of the at least one ceramic filter for immersion in the molten metal and the rotation of the at least one ceramic filter to the own axis allows.

12. Device according to one of claims 9 to 1 1, characterized in that in addition to the crucible wall of the induction crucible furnace filter body or filter systems are mounted to increase the filtration efficiency.

13. Device according to one of claims 9 to 12, characterized in that on the crucible wall of the induction crucible furnace coatings based on the materials of the filter body or the filter systems are mounted to increase the filtration efficiency.