Method and system of electromagnetic stirring for continuous casting of medium and high carbon steels

Abstract

A method and an apparatus are provided for electromagnetic stirring during a continuous casting process, especially for casting a billet and bloom of medium and high carbon steels. The method and apparatus provide a higher surface quality of the ingot, reduce the entrapping of nonmetallic inclusions into the ingot, and suppress issues regarding central segregation and central porosity. The method provides an improvement in the stirring process from the meniscus to the crater end—and relates to in-mold stirring and stirring in a zone of secondary cooling and up to the crater end. The in-mold stirring is geared towards the suppression of meniscus disturbance, for submerged casting, in particular, the reducing of helical and axial velocity components of the molten steel, the lowering of the initial solidification point to avoid the touching of a shell edge with the slag ring, and a decrease of oscillation marks.

1 . A method of electromagnetic stirring of molten metal in an unsolidified portion of a continuously cast strand of an ingot, which comprises the steps of:
 applying an alternating poly-harmonic current to at least four (six) coils of an in-mold stirrer;   supplying the alternating poly-harmonic current with two frequency components, including a low frequency component between f 1 =3.0-6.5 Hz and a high frequency component between f 2 =13-20 Hz;   setting a ratio of current amplitudes by equation:
   (a high frequency current amplitude)/(a low frequency current amplitude)=0.5-0.75; 
   setting of ratio low frequency current component f 3 =0.5 f intr , f intr —intrinsic frequency of liquid part of billet 0.9-1.2 Hz, (about 1.0 Hz for 7×7 inch billet) with amplitude k times (0.2<k<2.0) higher than current amplitude of low frequency component f 1 =3-6.5 Hz—creates a pressure pulsation with frequency equal to intrinsic frequency of melt oscillation in the liquid portion of ingot, the oscillating pressure spreads along billet as acoustic waves that generate the pulse flow at the solidification front;   setting a ratio of current in dependence on a size of an ingot cross section for inducing a stirring torque below the mold for reducing a meniscus disturbance, for reducing particle entrapment into the ingot through a meniscus, for increasing mold powder flow into a gap formed between the mold and the ingot, and for decreasing oscillating marks on the ingot.   
   
   
       2 . The method according to  claim 1 , which further comprises:
 providing a unit for intermediate and final electromagnetic stirring of the molten metal downstream of the in-mold stirrer; and   generating two alternating magnetic fields, the alternating magnetic fields including:
 a second rotating magnetic field directed substantially perpendicular to the ingot axis and induces in the ingot rotating current loops disposed in a longitudinal section of the ingot. 
   
   
   
       3 . The method according to  claim 2 , which further comprises providing the unit with a rectangular ferromagnetic core surrounding the ingot for containing the first pulsed magnetic field around the ingot and preventing magnetic flux leakage avoiding the ingot. 
   
   
       4 . The method according to  claim 3 , which further comprises:
 providing two groups of electromagnetic coils to the rectangular ferromagnetic core, the two groups of electromagnetic coils include:
 a first coil group having two coils surrounding the rectangular ferromagnetic core and creating a first magnetic flux inside of the rectangular ferromagnetic core; and 
 a second coil group having coil groups of two coils each in a saddle-shape form, and disposed between the rectangular ferromagnetic core and the ingot and create a second magnetic flux penetrating into the ingot. 
   
   
   
       5 . The method according to  claim 4 , wherein the unit includes first and second units for stirring the molten metal and are disposed along the cast strand, the first coil group inducing a current along the ingot that flows between the first and second units for stirring and rotates around the ingot axis. 
   
   
       6 . The method according to  claim 5 , which further comprises using the current flowing along the ingot axis and rotating around the ingot axis for creating an electromagnetic force for stirring the molten metal along from an initial entry point up to a final point of solidification. 
   
   
       7 . The method according to  claim 1 , which further comprises:
 inducing helical and axial components to the molten metal flowing inside a liquid portion of the ingot for suppression of segregation.   
   
   
       8 . The method according to  claim 4 , which further comprises connecting each of the coils of the first and second coil groups to two of three phases of an AC voltage system operating. 
   
   
       9 . The method according to  claim 4 , which further comprises using the two groups of electromagnetic coils to create in the ingot a rotating vector field of electromagnetic forces, never having a zero in a geometrical center of the ingot. 
   
   
       10 . The method according to  claim 4 , which further comprises using the two groups of electromagnetic coils to create in the ingot a radial component of electromagnetic forces and a motion of the molten metal for preventing an occurrence of segregation. 
   
   
       11 . The method according to  claim 3 , which further comprises surrounding a channel of the rectangular ferromagnetic core for passing of the cast strand with a dielectric and non-magnetic baffles for forming a high velocity stream of cooling water. 
   
   
       12 . An electromagnetic stirrer system for stirring molten metal in an unsolidified portion of a continuously cast strand of an ingot, comprising:
 a power source supplying an alternating poly-harmonic current; and   an in-mold stirrer having a mold and at least four or six coils connected to said power source, said in-mold stirrer forming magnetic fields upon receiving the alternating poly-harmonic current to said at least four or six coils, the poly-harmonic current having two frequency components including a low pulse frequency component between 0.9-1.2 Hz, a middle rotate frequency component 3.0-6.5 Hz, and a high rotate frequency component between 13-20 Hz, a ratio of current amplitudes being defined by equation:
   −a first low frequency current amplitude)/(a high frequency current amplitude)=1.5-1.7 
   in dependence on a size of an ingot cross section for inducing a pulse motion of molten metal along ingot axis up to final point of solidification;   −a second low frequency current amplitude)/(a high frequency current amplitude)=0.6-1.0 for creation of revolving torque in for rotate motion of melt inside mold;   and high 17-20 Hz frequency current amplitude for reducing a meniscus disturbance, for reducing particle entrapping into the ingot through a meniscus, for increasing mold powder flow into a gap defined between said mold and the ingot, and for decreasing oscillating marks on the ingot.   
   
   
       13 . The electromagnetic stirrer system according to  claim 12 , further comprising:
 a line/final stirrer disposed downstream of said in-mold stirrer, said line/final stirrer generating two alternating magnetic fields, the alternating magnetic fields including:
 a first pulsed magnetic field having a substantially helical direction relative to the ingot and surrounding the ingot and induces a secondary single-directed current toward an ingot axis; and 
 a second rotating magnetic field directed substantially perpendicular to the ingot axis and induces in the ingot current loops located in a longitudinal section of the ingot. 
   
   
   
       14 . The electromagnetic stirrer system according to  claim 13 , wherein said line/final stirrer having a rectangular ferromagnetic core surrounding the ingot for containing the first pulsed magnetic field around the ingot and preventing magnetic flux leakage to the ingot. 
   
   
       15 . The electromagnetic stirrer system according to  claim 14 , wherein said line/final stirrer having two groups of electromagnetic coils attached to said rectangular ferromagnetic core, said two groups of electromagnetic coils include:
 a first coil group containing two coils surrounding two opposite rods of said rectangular ferromagnetic core and creating a first magnetic flux inside of said rectangular ferromagnetic core; and   a second coil group having coil groups of two coils each with a saddle-shape form, and disposed between said rectangular ferromagnetic core and the ingot and creating a second magnetic flux penetrating into the ingot.   
   
   
       16 . The electromagnetic stirrer system according to  claim 15 , wherein said line/final stirrer includes two line/final stirrers, said first coil group induces a current along the ingot that flows between said two line/final stirrer and rotates around the ingot axis. 
   
   
       17 . The electromagnetic stirrer system according to  claim 16 , wherein the current flowing along the ingot axis and rotating around the ingot axis creates an electromagnetic force for stirring the molten steel on the ingot axis from an entry point up to a final point of solidification. 
   
   
       18 . The electromagnetic stirrer system according to  claim 15 , wherein:
 at least one of the in-mold stirrer and said line/final stirrer inducing helical and axial components to a molten metal flow inside a liquid portion of the ingot for suppression of segregation; and   at least one of the in-mold stirrer and said line/final stirrer inducing longitudinal and radial components to the molten metal flow for increasing a concentration of defects in an ingot center when a forging process or extraction of a center portion of the ingot is provided.   
   
   
       19 . The electromagnetic stirrer system according to  claim 15 , wherein said two groups of electromagnetic coils create in the ingot a rotating vector field of electromagnetic forces, never having a zero in a geometrical center of the ingot. 
   
   
       20 . The electromagnetic stirrer system according to  claim 15 , wherein said two groups of electromagnetic coils create in the ingot a radial component of electromagnetic forces in the ingot center and a motion of the molten metal for preventing an occurrence of segregation. 
   
   
       21 . The electromagnetic stirrer system according to  claim 15 , wherein said rectangular ferromagnetic core defines a channel for passing of the cast strand and has a dielectric and non-magnetic baffles for forming of high velocity stream of cooling water.