Process and device for improving the quality of continuously cast metallic bodies

Abstract

In a continuous-casting plant which uses a tank made of refractory material set on top of the crystallizer for removing the meniscus and the supernatant layer of covering powders from the solidification area, around the said tank there are electromagnetic means designed to generate variable magnetic fields for slowing down and making uniform the disordered flow of liquid metal inside the said tank, whilst at the joint between the tank and the crystallizer, other electromagnetic means are set which are designed to generate a direct magnetic field so as to remove locally the liquid metal from the walls of the tank and of the crystallizer.

1 . Device for continuous casting of blooms, slabs, or billets, consisting of a cooled crystallizer in the form of an open hollow body and of a tank made of refractory material set on top of the crystallizer, into which liquid metal to be cast is poured, wherein the said tank made of refractory material is equipped with means designed to slow down and render uniform the disordered motion of the liquid metal, and in that means are present, set at the height of the boundary between the crystallizer and the tank, which are designed to cause and to maintain detachment of the molten metal from the walls of the tank-crystallizer ensemble.  
     
     
         2 . Device according to    claim 1   , wherein the said means for slowing down and rendering uniform the disordered motion of the liquid metal consist of a continuous core set around the tank and equipped with at least four poles and with windings set around each pole, and of means, adapted to interrupt and modify periodically the electrical supply of the said windings, thus creating electromagnetic fields which are periodically interrupted and modified in direction and intensity.  
     
     
         3 . Device according to    claim 1   , wherein the said means adapted to cause and maintain detachment of the molten metal from the walls of the tank-crystallizer ensemble are made up of an induction coil, supplied by a pulse current, consisting of a plurality of turns inserted in a magnetic core.  
     
     
         4 . Device according to    claim 3   , wherein the said induction coil is set between the tank and the crystallizer, and is provided with a wall facing towards the inside of the tank-crystallizer ensemble.  
     
     
         5 . Device according to    claim 3   , wherein the said induction coil is set outside the crystallizer, the contact surface between the tank and the crystallizer consisting of profiles of the end parts of the tank and of the crystallizer which are complementary and generically ogival or parabolic.  
     
     
         6 . Device according to    claim 5   , wherein the crystallizer has, in its part close to the tank, through slots.  
     
     
         7 . Device according to    claim 6   , wherein the said slots are at least partially closed by material transparent to electromagnetic fields, in which case one part of the said slots that has remained free serves for passage of a lubricant.  
     
     
         8 . Device according to    claim 5   , wherein the said induction coil is provided with a plurality of ducts for introducing, into the said compartment, lubricating material, fed in by means of manifolds.  
     
     
         9 . Device according to    claim 7   , wherein the said crystallizer is provided with ducts for the coolant, which are connected to similar ducts made in the induction coil, for cooling the latter.  
     
     
         10 . Device according to    claim 4   , wherein the crystallizer, at its initial part set facing towards the tank, has inserts which have high permeability and are provided with an electrically insulating coating on their surfaces that are in contact with the liquid metal.  
     
     
         11 . Device according to    claim 1   , wherein outside the crystallizer, mechanical exciters are set of an electrodynamic, pneumatic, magnetostrictive, or piezoelectric type, and the like, the frequency of which corresponds to the resonance frequency of the crystallizer-cast body system and is between 100 Hz and 25 000 Hz.  
     
     
         12 . Process for improving the quality of metallic bodies obtained from continuous casting, in which a tank made of refractory material is used, which is set on top of a crystallizer having the shape of an open hollow body, and into which, by means of a special discharging device, liquid metal is continuously poured which advances with a disordered motion towards the crystallizer and in the latter starts to solidify at a joining area (referred to as “triple point”) between the tank and the crystallizer, thus forming a so-called “skin” of the cast body extracted continuously from the crystallizer, the said tank having the purpose of removing, from the region of start of solidification, the free surface of the metal, the supernatant scale, and the area of molten metal with a flow that is perturbed as a result of the continuous addition of metal, wherein, in the said tank, a first action is carried out of slowing down and rendering uniform the disordered motion of liquid metal in the tank, which is combined with a second action of detachment of the liquid metal from the walls of the tank and of the crystallizer at the said joining point.  
     
     
         13 . Process according to    claim 12   , wherein the said first action of slowing down and rendering uniform the motion of the liquid metal is obtained by means of electromagnetic fields that are periodically interrupted and modified in direction and intensity.  
     
     
         14 . Process according to    claim 13   , wherein the said electromagnetic fields are generated by means of an induction system consisting of a first continuous core, set around the tank, provided with at least four poles and with windings set around each pole, the said poles being energized in a pre-set order for a given period of time Δt, and with a time interval, between two successive energizings, of 0.1-0.2 Δt.  
     
     
         15 . Process according to    claim 14   , wherein the said time period Δt is between 1 and 15 seconds.  
     
     
         16 . Process according to    claim 15   , wherein the said time period Δt is between 4 and 10 seconds.  
     
     
         17 . Process according to    claim 14   , wherein the current used for generating the said electromagnetic fields is between 1 kA and 200 kA.  
     
     
         18 . Process according to    claim 12   , wherein the second action of detachment of the liquid metal from the triple point is obtained by means of a pulsating magnetic field generated by a second induction coil consisting of a plurality of turns inserted into a second magnetic core which completely surrounds the crystallizer and is electrically insulated from the external environment.  
     
     
         19 . Process according to    claim 18   , wherein the said second induction coil is activated by means of a pulse current, having an intensity of between 5 kA and 200 kA, with a pulse duration of between 50 μs and 500 μs, and a frequency of between 2 Hz and 150 Hz.  
     
     
         20 . Process according to    claim 19   , wherein said current intensity is between 30 kA and 200 kA.  
     
     
         21 . Process according to    claim 19   , wherein the duration of said current pulses is between 100 μs and 200 μs.  
     
     
         22 . Process according to    claim 19   , wherein the frequency of the said pulse current is between 100 Hz and 200 Hz.  
     
     
         23 . Process according to    claim 12   , wherein, in an area corresponding to the said triple point, lubricant is injected in order to favour sliding of the forming skin against the walls of the crystallizer.  
     
     
         24 . Process according to    claim 12   , wherein, in addition to the detachment of the liquid metal, also detachment of the solidifying skin from the walls of the crystallizer is obtained, which is favoured by subjecting the crystallizer to vibrations both in the transverse and in the longitudinal directions with respect to the axis of the crystallizer, by means of mechanical exciters of an electrodynamic, pneumatic, magnetostrictive, or piezoelectric type, or the like, applied outside the crystallizer, with at least one per wall.  
     
     
         25 . Process according to    claim 23   , wherein the said lubricant has ferromagnetic properties.  
     
     
         26 . Process according to    claim 25   , wherein the said lubricant comprises a mixture of ferromagnetic particles having a size smaller than 100 μm, in a quantity of between 5 wt % and 25 wt % of the total lubricant.