Modulated electromagnetic stirring of metals at advanced stage of solidification

Abstract

A method and apparatus for electromagnetic stirring of molten metals at an advanced stage of solidification, as may be used in continuous casting of steel billets and blooms, are disclosed. At least first and second stirrers are provided for generating first and second rotating magnetic fields of a differing frequency about an axis of solidifying molten metal. The stirrers are arranged about the molten metal in sufficiently close proximity to each other so that their respective magnetic fields superpose to produce a modulated magnetic field. The magnetic fields of the respective stirrers may either have common or opposing rotational directions. The modulated stirring produced by the magnetic fields results in oscillating primary and secondary flows and hence turbulence within the melt bulk in the region wherein temperature of the melt on its central axis is below the liquidus level and at least 10% of substantially solidified material is formed. Turbulent flow created by this stirring method disrupts formation of crystalline structures in the melt bulk and mixes solute enriched melt of the central region with the bulk volume which subsequently results in improvements of the solidification structure and overall internal quality of the cast products.

1 . A method of electromagnetic stirring a molten metallic material comprising:
 providing at least two stirrers for generating independent rotating magnetic fields about an axis extending through said molten material;   wherein at least first and second ones of said at least two stirrers produce independent first and second rotating magnetic fields have differing angular frequencies;   and wherein said stirrers are located about said molten metallic material in sufficiently close proximity to each other so that said independent rotating magnetic fields superpose to produce a modulated magnetic field that creates a turbulent flow of said molten metallic material in a transition region of said molten metallic material having a temperature below the liquidus along a central axis of said molten metallic material, and in which said molten metallic material is mixed with at least about 10% of substantially solidified molten metallic material.   
   
   
       2 . The method of  claim 1 , wherein said first and second rotating magnetic fields counter-rotate. 
   
   
       3 . The method of  claim 1 , wherein said first and second rotating magnetic fields rotate in the same direction. 
   
   
       4 . The method of  claim 1 , wherein the longitudinal extent of said first of said at least two stirrers about said molten metallic material is different from the longitudinal extent of said second of said at least two stirrers about said molten metallic material. 
   
   
       5 . The method of  claim 2 , wherein the frequency of said first and second rotating magnetic fields differ by less than about 3 Hz. 
   
   
       6 . The method of  claim 2 , wherein the difference in the frequency of said first and second rotating magnetic fields varies in time. 
   
   
       7 . The method of  claim 1 , wherein each of said stirrers comprises at least two pole pairs, each excited by a current from at least one multi-phase current source. 
   
   
       8 . The method of  claim 2 , wherein said molten metallic material is within a cast strand, downstream of a casting mold. 
   
   
       9 . The method of  claim 1 , wherein each of said first and second ones of said at least two stirrers produce a different magnetic flux density in said molten metallic material. 
   
   
       10 . The method of  claim 1 , wherein the magnetic flux density produced in said molten metallic material produced by at least one of said first and second of said at least two stirrers varies in time. 
   
   
       11 . The method of  claim 1 , wherein said turbulent flow has a turbulent viscosity having peaks in excess of 2 Ns/m 2    
   
   
       12 . The method of  claim 1 , wherein said region comprises substantially liquid molten metallic material, and crystalline material surrounded by a solid shell. 
   
   
       13 . The method of  claim 1 , wherein said turbulent flow disrupts formation of a crystalline network in said region. 
   
   
       14 . The method of  claim 1 , further comprising transferring molten metallic material through a mold upstream of said region, and providing a further stirrer about said mold to generate a rotating magnetic field within said mold. 
   
   
       15 . A casting apparatus comprising:
 a mold for casting a molten metal;   a first stirrer for generating a first rotating magnetic field about an axis extending through said molten metal, located downstream of said mold;   a second stirrer for generating a second rotating magnetic field, located downstream of said first stirrer;   at least one power source for generating said first and second magnetic field, at frequencies of rotation differing from each other;   wherein said first and second stirrers are arranged in proximity to each other so that said first and second rotating magnetic fields produce a modulated magnetic field that creates a turbulent flow in a molten metallic material in a region between said first and second stirrers.   
   
   
       16 . The apparatus of  claim 15 , wherein said first and second rotating magnetic fields are generated to counter-rotate by said at least one power source. 
   
   
       17 . The apparatus of  claim 15 , wherein said first and second rotating magnetic fields are generated to rotate in the same direction by said at least one power source. 
   
   
       18 . The apparatus of  claim 15 , wherein the longitudinal extent of said first stirrer about said molten metal is different from the longitudinal extent of said second stirrer about said molten metal. 
   
   
       19 . The apparatus of  claim 16 , wherein the frequencies of said first and second rotating magnetic fields differ by less than about 3 Hz. 
   
   
       20 . The apparatus of  claim 16 , wherein the difference in the frequency of said first and second rotating magnetic fields varies in time. 
   
   
       21 . The apparatus of  claim 15 , wherein each of said first and second stirrer comprises at least two pole pairs, each excited by a current from said at least one source. 
   
   
       22 . The apparatus of  claim 15 , wherein each of said first and second stirrers produce a different magnetic flux density in said molten metal. 
   
   
       23 . The apparatus of  claim 15 , wherein the magnetic flux density produced in said molten metal produced by at least one of said first and second stirrers varies in time. 
   
   
       24 . The apparatus of  claim 15 , wherein said turbulent flow has a turbulent viscosity having peaks in excess of 2 Ns/m 2    
   
   
       25 . The apparatus of  claim 15 , wherein said region comprises substantially liquid molten metal, and crystalline material surrounded by a solid shell. 
   
   
       26 . The apparatus of  claim 15 , wherein said turbulent flow disrupts formation of a crystalline network in said region. 
   
   
       27 . The apparatus of  claim 15 , further comprising a further stirrer about said mold to generate a rotating magnetic field within said mold. 
   
   
       28 . A method of electromagnetic stirring a metallic melt comprising:
 providing a first stirrer for generating a first rotating magnetic field that rotates about an axis extending through said melt, at an angular frequency of ω 1 ;   providing a second stirrer for generating a second rotating magnetic field that rotates at an angular frequency of ω 2 ;   wherein said first and second stirrers are located in sufficiently close proximity to each other so that said first and second rotating magnetic field produce a magnetic force having a component with frequency (ω 1 −ω 2 ) in said metallic melt in a region between said first and second stirrer, wherein (ω 1 −ω 2 ) is sufficiently small to allow said magnetic force to overcome the inertia of said melt.   
   
   
       29 . A method of electromagnetic stirring a molten metallic material comprising:
 providing a first stirrer for generating a first rotating magnetic field about an axis extending through said molten material;   providing a second stirrer for generating a second rotating magnetic field having a frequency of rotation differing from said first rotating magnetic field;   wherein said first and second stirrers are located about said molten metallic material in sufficiently close proximity to each other so that said first and second rotating magnetic fields superpose between said first and second stirrers to produce a modulated magnetic field that creates a turbulent flow of said molten metallic material in a transition region of said molten metallic material having a temperature below the liquidus along a central of the molten metallic material, and in which said molten metallic material is mixed with at least about 10% of substantially solidified molten metallic material.