EP3967422A1

EP3967422A1 - Electromagnetic stirring and heating of an ingot

Info

Publication number: EP3967422A1
Application number: EP20195612.5A
Authority: EP
Inventors: Martin SEDÉN
Original assignee: ABB Schweiz AG
Current assignee: ABB Schweiz AG
Priority date: 2020-09-10
Filing date: 2020-09-10
Publication date: 2022-03-16

Abstract

An electromagnetic stirring and heating device (6) for an ingot casting device for ingot casting is presented. The ingot casting device comprises a mould (1-3) for casting of an ingot. The electromagnetic stirring and heating device (6) is arranged at a top of the mould. The electromagnetic stirring and heating device is configured to macroscopic stir the ingot with a varying frequency in the range 0-10 Hz, and to heat at a top of the ingot with a superimposed varying frequency in the range 20-200 Hz, on top of the electromagnetic stirring varying frequency. A method and an ingot casting device therefore are also presented.

Description

TECHNICAL FIELD

The present disclosure relates to electromagnetic stirring and heating of a steel ingot for ingot casting.

BACKGROUND

Steel ingot casting is an old and widely spread technique to manufacture steel where ingot moulds are filled with molten steel, followed by a solidification process of several hours depending on ingot size. Nowadays, several ingot moulds are generally filled simultaneously by the up-hill teeming technique giving a low and controlled rising speed of the steel in the mould. In progressive solidification, the steel solidifies starting from the side walls and the base of the mould. The solidification front then moves inwards towards the centre of the ingot. The resulting temperature gradients in the ingot initiate free convection which is the weak driving force of the flow in the liquid steel. The strongest flow speed appears along the mush-liquid interface where melt locally flows downwards with typical speeds of a few centimetres per second. Closer to the centre of the ingot, the flow recirculates upward again.
To induce forced and controllable convection in primarily the molten steel, electromagnetic stirrers to several other steel industry processes has been developed, such as e.g. continuous casting which is an efficient casting method for high productivity. For some special high-end steel grades however, the slower ingot casting method yields a superior quality of the final product. The application of controlled strong stirring in the ingot casting process has the potential of taking the manufacturing of clean steel to a higher level by increasing the quality of the end product by means of increasing molten steel flow speeds to 0.5 m/s. So far, electromagnetic stirring for ingot casters is a relatively unexplored field.
Recently, increased efficiency demands have surfaced for the ingot casting process. Due to market variations in the cost of raw materials and steel company acquisitions, the amount of production from steel scrap has increased the level contaminations in the incoming steel. Also, improved quality and cleanliness of the finished solidified product is requested for a broad span of new applications for steel materials. Together, this raises the requirements of refinement in the ingot casting process.
To handle the increased complexity and demands, more sophisticated casting methods, e.g. casting in vacuum may be approached in the future. In such a process, the conditions are different than in conventional ingot stirring. For instance, the slag layer at the top of the ingot will be removed as oxidation is no longer an issue, and the risk of slag inclusions can be eliminated in this way. Vacuum casting will also provide a different scenario when it comes to stirring of the melt; degassing, inclusion removal and segregation reduction will all benefit from a very strong stirring where the melt is regularly exposed to the ambient vacuum. As the slag layer is removed, there is no longer a limitation on the strength of the stirring imposed on the melt. However, the removal of the top slag layer will also remove the heat insulating properties of the top of the ingot, which may impose some difficulties for homogeneous solidification.
Uncontrolled traditional progressive solidification typically results in a number of defects in the cast ingot; e.g. segregation, cracks, inclusions and centre porosity. This leads to undesired low yields for high quality steels as defect areas or entire ingots must be re-melted and cast again. These metallurgical problems can all be related to low flow speeds and insufficient turnaround of the melt.
PTC (Plasma Treatment Casting) is a competing technology for stirring of ingots. PTC imposes an electric field over the height of the ingot by applying a rotating DC electrode above the ingot and another electrode at the base of the mould. A plasma arc strikes the top surface of the ingot in a rotating manner, feeding a varying current density to the steel in the mould. The varying current density induces a varying magnetic flux density, and together, the current and the magnetic field create a time varying stirring force on the ingot. The applied stirring is claimed to increase yield by about 10% for high grade steel ingots.
In vacuum casting without insulating top slag layer, an additional problem is the loss of heat vertically up and out which will lead to inhomogeneous solidification where the top ingot solidifies first.
Electromagnetic stirring of ingots provides melt flow speeds at least ten times higher than those of free convection. An electromagnetic stirrer can easily adapt its strength, thus controlling the different stages of solidification where different amounts of convection may be needed.
Compared to PTC, electromagnetic stirring is a contactless technology with minimum maintenance. The electromagnetic fields can penetrate mould, covers and mould powder and apply stirring without risking oxidation of the top surface of the ingot steel such as with the plasma arc technology. PTC needs to replace electrodes frequently, but there is no wear on the electromagnetic stirrer which has a very long lifetime.

SUMMARY

One objective of the present invention is to improve top ingot porosity deficiencies for electromagnetic casting of ingots.
According to a first aspect there is presented a method for electromagnetic stirring and heating for ingot casting. The method is performed during casting of an ingot in an ingot mould. The method comprises electromagnetic stirring of the ingot. The electromagnetic stirring is configured to macroscopic stir the ingot with a varying frequency in the range 0-10 Hz. The method further comprises electromagnetic heating of the ingot. The electromagnetic heating is configured to superimpose a varying frequency in the range 20-200 Hz, on top of the electromagnetic stirring varying frequency, to induce heat at a top of the ingot.
A same coil hardware may be used for inducing both the electromagnetic stirring as well as the electromagnetic heating.
The electromagnetic stirring may be configured to produce a horizontal, linear, straight stirring with a vertically circulating flow pattern of the ingot.
The electromagnetic stirring maybe configured to produce a circulating horizontal stirring at the top of the ingot.
The method may be performed during vacuum casting of the ingot.
According to a second aspect there is presented an ingot casting device for ingot casting. The ingot casting device comprises a mould for casting of an ingot. The electromagnetic stirring and heating device is arranged at a top of the mould. The electromagnetic stirring and heating device is configured to macroscopic stir the ingot with a varying frequency in the range 0-10 Hz, and to heat at a top of the ingot with a superimposed varying frequency in the range 20-200 Hz, on top of the electromagnetic stirring varying frequency.
The electromagnetic stirring and heating device may be configured to produce a horizontal, linear, straight stirring with a vertically circulating flow pattern of the ingot.
The electromagnetic stirring and heating device may alternatively be configured to produce a circulating horizontal stirring at the top of the ingot.
According to a third aspect there is presented an ingot casting device for ingot casting. The device comprises a mould for casting of an ingot and an electromagnetic stirring and heating device arranged at a top of the mould. The electromagnetic stirring and heating device is configured to macroscopic stir the ingot with a varying frequency in the range 0-10 Hz, and to heat at a top of the ingot with a superimposed varying frequency in the range 20-200 Hz, on top of the electromagnetic stirring varying frequency.
The mould may be arranged in a vacuum chamber.
The electromagnetic stirring and heating device may be configured to produce a horizontal, linear, straight stirring with a vertically circulating flow pattern of the ingot.
The electromagnetic stirring and heating device may be configured to produce a circulating horizontal stirring at the top of the ingot.
By superimposing a varying frequency in the range 0-10 Hz with a superimposed varying frequency in the range of 20-200 Hz a heat is induced in the top of the ingot to compensate for heat loss without the need to use of mould powder at the top of the melt. Frequencies below 20 Hz do not create inductive heating, and a frequency higher than 200 Hz is not practical due to the wall thickness for magnetic field penetration into the melt.
Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to "a/an/the element, apparatus, component, means, step, etc." are to be interpreted openly as referring to at least one instance of the element, apparatus, component, means, step, etc., unless explicitly stated otherwise. The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects and embodiments are now described, by way of example, with reference to the accompanying drawings, in which:

Fig. 1 is a schematic diagram of an electromagnetic ingot casting device;
Fig. 2a is a schematic diagram of a top view of an electromagnetic ingot casting device;
Fig. 2b is a schematic diagram of a side view of the electromagnetic ingot casting device illustrated in Fig. 2a;
Fig. 3 is a schematic diagram of a side view of an electromagnetic ingot casting device;
Fig. 4 is a schematic diagram of a top view of an electromagnetic ingot casting device; and
Fig. 5 is a schematic diagram of a side view of the electromagnetic ingot casting device.

DETAILED DESCRIPTION

The aspects of the present disclosure will now be described more fully hereinafter with reference to the accompanying drawings, in which certain embodiments of the invention are shown.
These aspects may, however, be embodied in many different forms and should not be construed as limiting; rather, these embodiments are provided by way of example so that this disclosure will be thorough and complete, and to fully convey the scope of all aspects of invention to those skilled in the art. Like numbers refer to like elements throughout the description.
For vacuum casting, electromagnetic stirring as presented herein offers the following features compared to non-vacuum casting:

Improving top ingot porosity deficiencies
Increase yield
Inclusion removal (Mg, Ca, C)
Reduction of segregation
Degassing (N, H, O)
Deoxidizing with C to CO
Enables larger ingots
Cheaper, dirtier scrap can be used and cleaned

An electromagnetic ingot casting device for ingot casting is presented herein. The ingot casting device is configured to superimpose varying frequencies to accommodate strong macroscopic stirring with a low frequency in the range 0-10 Hz together with a high frequency component in the range 20-200 Hz to induce heat at the top of the ingot to compensate for the heat loss without mould powder at the top of the melt. The macroscopic stirring achieved with the low frequency range 0-10 Hz provides a strong stirring in the whole volume of the ingot. The top of the ingot is the riser head of the ingot. By superimposing the heating and stirring frequencies both heating and stirring is achieved at the same time, saving production time and reducing ingot porosity in the riser head of the ingot.
Application of electromagnetic stirring to an ingot during solidification drastically strengthens and controls the flow in the fluid parts of the ingot. A strong flow homogenizes temperatures in the liquid and breaks off dendrite tips and hence increases the equiaxed ratio of a solidified product. Flow circulation also washes non-metallic inclusions off the solidification front and by the buoyancy effect due to the density difference between steel and non-metallic particles, helps float inclusions to the top of the ingot where they can be extracted from a solidified clean steel. A strong flow is a stirring speed bigger than 0.1 m/s. By the use of the term ingot in this document, steel ingot is meant.
The quality and yield of the cast products can be increased significantly thus increasing productivity and efficiency.
With vacuum casting and multi-frequency electromagnetic stirring, a large step is taken toward clean steel in reducing the ceramic materials and mould powder inclusions.
An embodiment of an ingot casting device for ingot casting is presented with reference to Fig. 1. The ingot casting device comprises a mould for casting the ingot and an electromagnetic stirring and heating device (EMS) 6. The mould comprises a head mould 2 and a bottom mould 1 separated by insulation 3. The insulation 3 is further arranged on the inside of the head mould 2, in order to reduce thermal loss into the atmosphere and the stirrer. The insulation 3 between the head mould 2 and the bottom mould 1 is used to support the insulation arranged on the inside of the head mould 2. The insulation 3 between the head mould 2 and the bottom mould 1 further reduces heat loss. The head mould 2 is configured to support the riser head of the ingot during casting. The ingot casting device also comprises a vacuum chamber 4, in which the mould 1-3 is arranged. The ingot casting device further comprises the EMS 6 arranged on top of the vacuum chamber 4.
The EMS 6 is configured to induce a horizontal, linear fluid flow at the top of the ingot. The induced flow is illustrated with an arrow. The induced horizontal, linear fluid flow is further configured to induce a circular fluid flow down into the bottom mould 1. The EMS 6 is further configured to induce heat in the top of the ingot, illustrated with undulating waves.
The EMS 6 is configured to macroscopic stir the ingot with a varying frequency in the range 0-10 Hz, and to heat at a top of the ingot with a superimposed varying frequency in the range 20-200 Hz, on top of the electromagnetic stirring varying frequency.
The ingot casting device is configured to cast a steel ingot in the mould 1-3 without use of mould powder at the top of the melt.
Vertical speeds in the range 0.3-0.5 m/s in the liquid transport molten steel in a vertical circulation loop. Primary focus on vertical transport of inclusions. Much stronger flow speeds can be used in vacuum process for a more efficient circulation and inclusion seclusion. In the presented embodiment the diameter of the top of the ingot can be about 0.5 m or larger, and the wall thickness of the mould can be about 0.3 m.
An embodiment of an ingot casting device for ingot casting is presented with reference to Figs. 2a and 2b. The ingot casting device comprises a mould for casting the ingot and an EMS 6. The mould comprises a head mould 2 and a bottom mould 1 separated by insulation 3. The insulation 3 is further arranged on the inside of the head mould 2 and on top of the ingot. The head mould 2 is configured to support the riser head of the ingot during casting. The ingot casting device also comprises the EMS 6 arranged at the top of the ingot. The EMS 6 is arranged around the head mould 2.
The EMS 6 is configured to induce a circular, horizontal fluid flow of the ingot at the top of the ingot. The induced flow is illustrated with arrows in Fig. 2b. The induced circular, horizontal fluid flow is further configured to induce a dual circular fluid flow down into the bottom mould 1, illustrated with arrows in Fig. 2a. The EMS 6 is further configured to induce heat in the top of the ingot, although not as important with the use of mould powder 5 arranged on top of the ingot.
The EMS 6 is configured to macroscopic stir the ingot with a varying frequency in the range 0-10 Hz, and to heat at a top of the ingot with a superimposed varying frequency in the range 20-200 Hz, on top of the electromagnetic stirring varying frequency.
The ingot casting device may be configured to cast a steel ingot in the mould 1-3 with or without use of mould powder 5 at the top of the melt.
Creation of a strong circulating flow covering the entire solidification front in a homogeneous fashion. This flow has velocities in the range 0.3-0.5 m/s. Primary focus on segregation problems. Vertical speeds in the range 0-0.15 m/s.
An embodiment of an ingot casting device for ingot casting is presented with reference to Figs. 3-4. The ingot casting device comprises a mould for casting the ingot and an EMS 6. The mould comprises a head mould 2 and a bottom mould 1 separated by insulation 3. The insulation 3 is further arranged on the inside of the head mould 2. The insulation 3 between the head mould 2 and the bottom mould 1 is used to support the insulation arranged on the inside of the head mould 2. The insulation 3 between the head mould 2 and the bottom mould 1 further reduces heat loss. The head mould 2 is configured to support the riser head of the ingot during casting. The ingot casting device also comprises a vacuum chamber 4, in which the mould 1-3 is arranged. The ingot casting device further comprises the EMS 6 arranged on top of the vacuum chamber 4.
The EMS 6 is configured to induce a horizontal, linear fluid flow at the top of the ingot. The induced flow is illustrated with an arrow. The induced horizontal, linear fluid flow is further configured to induce a circular fluid flow down into the bottom mould 1. The EMS 6 is further configured to induce heat in the top of the ingot, illustrated with undulating waves.
The EMS 6 is configured to macroscopic stir the ingot with a varying frequency in the range 0-10 Hz, and to heat at a top of the ingot with a superimposed varying frequency in the range 20-200 Hz, on top of the electromagnetic stirring varying frequency.
The ingot casting device is configured to cast a steel ingot in the mould 1-3 without use of mould powder at the top of the melt.
An embodiment of an ingot casting device for ingot casting is presented with reference to Figs. 4-5. The ingot casting device comprises a mould for casting the ingot and an EMS 6. The mould comprises a head mould 2 and a bottom mould 1 separated by insulation 3. The insulation 3 is further arranged on the inside of the head mould 2 and above the ingot. The head mould 2 is configured to support the riser head of the ingot during casting. The ingot casting device further comprises the EMS 6 arranged on top of the top part of the insulation 3.
The EMS 6 is configured to induce a horizontal, linear fluid flow at the top of the ingot. The induced flow is illustrated with an arrow. The induced horizontal, linear fluid flow is further configured to induce a circular fluid flow down into the bottom mould 1. The EMS 6 is further configured to induce heat in the top of the ingot, illustrated with undulating waves.
The EMS 6 is configured to macroscopic stir the ingot with a varying frequency in the range 0-10 Hz, and to heat at a top of the ingot with a superimposed varying frequency in the range 20-200 Hz, on top of the electromagnetic stirring varying frequency.
The ingot casting device may be configured to cast a steel ingot in the mould 1-3 with or without use of mould powder 5 at the top of the melt.
An embodiment of an EMS 6 for an ingot casting device for ingot casting is presented with reference to Figs. 1-5. The ingot casting device comprises a mould 1-3 for casting of an ingot. The EMS 6 is arranged at a top of the mould. The electromagnetic stirring and heating device is configured to macroscopic stir the ingot with a varying frequency in the range 0-10 Hz, and to heat at a top of the ingot with a superimposed varying frequency in the range 20-200 Hz, on top of the electromagnetic stirring varying frequency.
The EMS 6 may be configured to produce a horizontal, linear, straight stirring with a vertically circulating flow pattern of the ingot.
The EMS 6 may alternatively be configured to produce a circulating horizontal stirring at the top of the ingot.
Electromagnetic stirring in the presented embodiments is based on the common principle of a multiphase AC current being fed to an electromagnetic stirrer of the EMS 6 which generates a travelling magnetic field in the ingot. The direction of this travelling magnetic flux density wave is for the linear fluid flow along the circumference of the ingot. For the circular fluid flow the direction of the travelling magnetic flux density wave is along the stirrer length at the top of the ingot. These harmonic magnetic field waves induce eddy currents in an electrically conducting ingot, combining with the magnetic field to produce a force distribution in the ingot. The main force direction is aligned with the travelling magnetic wave, thus creating a stirring flow in the molten metal.
Electromagnetic heating, providing local heat at the top of the ingot, is created using the same coil hardware as for stirring, but with a higher frequency range. The current drives are setup to produce a linear superimposition of coil currents i_k : $i_{k} (t) = {\hat{ι}}_{stir} \cos (ω_{stir} t - φ_{k, stir}) + {\hat{ι}}_{heat} \cos (ω_{heat} t - φ_{k, heat})$
For phase k=i, 2, ..., N (N is typically 3 for 3-phase stirring) î_stir is the current amplitude for the stirring component, ω_stir is the angular frequency for the stirring component, ϕ_k,stir is the phase displacement for phase k of the stirring component, î_heat is the current amplitude for the heating component, ω_heat is the angular frequency for the heating component, ϕ_k,heat is the phase displacement for phase k of the heating component.
ϕ_k,stir is important for travelling wave stirring.
The aspects of the present disclosure have mainly been described above with reference to a few embodiments and examples thereof. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the invention, as defined by the appended patent claims.

Claims

A method for electromagnetic stirring and heating for ingot casting, the method being performed during casting of an ingot in an ingot mould, and comprising:
- electromagnetic stirring of the ingot, the electromagnetic stirring configured to macroscopic stir the ingot with a varying frequency in the range 0-10 Hz; and

- electromagnetic heating of the ingot, the electromagnetic heating configured to superimpose a varying frequency in the range 20-200 Hz, on top of the electromagnetic stirring varying frequency, to induce heat at a top of the ingot.
The method according to claim 1, wherein a same coil hardware is used for inducing both the electromagnetic stirring as well as the electromagnetic heating.
The method according to claim 1 or 2, wherein the electromagnetic stirring is configured to produce a horizontal, linear, straight stirring with a vertically circulating flow pattern of the ingot.
The method according to claim 1 or 2, wherein the electromagnetic stirring is configured to produce a circulating horizontal stirring at the top of the ingot.
The method according to any one of claims 1 to 4, wherein the method is performed during vacuum casting of the ingot.
An electromagnetic stirring and heating device (6) for an ingot casting device for ingot casting, wherein the ingot casting device comprises a mould (1-3) for casting of an ingot, wherein
- the electromagnetic stirring and heating device (6) is arranged at a top of the mould; and

- the electromagnetic stirring and heating device is configured to macroscopic stir the ingot with a varying frequency in the range 0-10 Hz, and to heat at a top of the ingot with a superimposed varying frequency in the range 20-200 Hz, on top of the electromagnetic stirring varying frequency.
The device according to claim 6, wherein the electromagnetic stirring and heating device is configured to produce a horizontal, linear, straight stirring with a vertically circulating flow pattern of the ingot.
The device according to claim 6, wherein the electromagnetic stirring and heating device is configured to produce a circulating horizontal stirring at the top of the ingot.
An ingot casting device for ingot casting, the device comprising:
- a mould (1-3) for casting of an ingot;

- an electromagnetic stirring and heating device (6) according to claims 6-8.
The device according to claim 9, wherein the mould is arranged in a vacuum chamber (4).