Method for casting molten metal, apparatus for the same, and cast slab

Abstract

The present invention provides a continuous casting method in which vibration is given to molten metal by a shifting magnetic field so that the equi-axed crystal ratio can be enhanced and the equi-axed crystals can be made fine without generating surface defects caused by powder trapping. Further, the present invention provides an apparatus to which the continuous casting method is applied. Furthermore, the present invention provides a cast slab produced by the above method and apparatus. The method of casting molten metal comprises the steps of: pouring molten metal into a mold and solidifying it in the mold while applying an electromagnetic force, which is generated by an electromagnetic coil arranged in the proximity of a molten metal pool in the mold, upon the molten metal; and vibrating the molten metal, which has been solidified in the mold or is being drawn out downward from the mold while being cooled and solidified, by a shifting magnetic field generated by the electromagnetic coil so that the molten metal is accelerated by a high intensity and a low intensity of acceleration in a range not exceeding a predetermined flow velocity when the directional vectors of high acceleration and low acceleration in the same direction or in the opposite direction are combined with each other.

1 . A method for casting molten metal comprising the steps of: pouring molten metal into a mold and solidifying it in the mold while applying an electromagnetic force, which is generated by an electromagnetic coil arranged in the proximity of a molten metal pool in the mold, upon the molten metal; and vibrating the molten metal, which has been solidified in the mold or is being drawn out downward from the mold while being cooled and solidified, by a shifting magnetic field generated by the electromagnetic coil so that the molten metal is alternately given a high intensity and a low intensity of acceleration.  
     
     
         2 . A method for casting molten metal comprising the steps of: pouring molten metal into a mold and solidifying it in the mold while applying an electromagnetic force, which is generated by an electromagnetic coil arranged in the proximity of a molten metal pool in the mold, upon the molten metal; and vibrating the molten metal periodically, which has been solidified in the mold or is being drawn out downward from the mold while being cooled and solidified, by a shifting magnetic field generated by the electromagnetic coil so that the molten metal is alternately given a high intensity and a low intensity of acceleration.  
     
     
         3 . A method for casting molten metal comprising the steps of: pouring molten metal into a mold and solidifying it in the mold while applying an electromagnetic force, which is generated by an electromagnetic coil arranged in the proximity of a molten metal pool in the mold, upon the molten metal; and vibrating the molten metal, which has been solidified in the mold or is being drawn out downward from the mold while being cooled and solidified, by a shifting magnetic field generated by the electromagnetic coil so that the molten metal is accelerated by a high intensity and a low intensity of acceleration in a range not exceeding a predetermined flow velocity when the directional vectors of high acceleration and low acceleration in the same direction or in the opposite direction are combined with each other.  
     
     
         4 . A method for casting molten metal comprising the steps of: pouring molten metal into a mold and solidifying it in the mold while applying an electromagnetic force, which is generated by an electromagnetic coil arranged in the proximity of a molten metal pool in the mold, upon the molten metal; and vibrating the molten metal periodically in the one direction and the opposite direction, which has been solidified in the mold or is being drawn out downward from the mold while being cooled and solidified, by a shifting magnetic field generated by the electromagnetic coil.  
     
     
         5 . A method for casting molten metal according to any one of  claims 1  to  4 , wherein a process conducted in the mold is a cooling and solidifying process, and also the process conducted in the mold is a continuous casting process for continuously casting a slab, bloom, slab of medium thickness, or billet.  
     
     
         6 . A method for casting molten metal according to any one of  claims 1  to  5 , wherein a high intensity of acceleration of the vibrating waves in the one direction and the opposite direction is not lower than 10 cm/s 2  and a low intensity of acceleration of the vibrating waves in the one direction and the opposite direction is lower than 10 cm/s 2 .  
     
     
         7 . A method for casting molten metal according to  claim 6 , wherein an acceleration and an acceleration time of the vibrating waves in the one direction, or an acceleration and an acceleration time of the vibrating waves in the opposite direction, and a coefficient of acceleration time (acceleration×acceleration time) satisfy the following expression. 
       50 cm/s≦coefficient of acceleration time 
     
     
         8 . A method for casting molten metal according to  claim 6 , wherein an acceleration and an acceleration time of the vibrating waves in the one direction, or an acceleration and an acceleration time of the vibrating waves in the opposite direction, and a coefficient of acceleration time (acceleration×acceleration time) satisfy the following expressions. 
       10η≦coefficient of acceleration timeη: viscosity cp of molten metal 
     
     
         9 . A method for casting molten metal according to  claim 6 , wherein a relation between carbon content C and acceleration satisfies the following expressions. 
       [C]<0.1%: 30 cm/s 2 ≦acceleration0.1%≦[C]<0.35%: −80[C]+38 cm/s 2 ≦acceleration0.35%≦[C]<0.5%: 133.3[C]−36.7 cm/s 2 ≦acceleration0.5%≦[C]: 30 cm/s 2 ≦acceleration 
     
     
         10 . A method for casting molten metal according to any one of  claims 1  to  5 , wherein an acceleration stop time or an electric power stop time, the period of which is not more than 0.3 sec and not less than 0.03 sec, is provided in the process of acceleration in the one direction and in the process of acceleration in the opposite direction.  
     
     
         11 . A method for casting molten metal according to  claim 6 ,  7 ,  8  or  9 , wherein an acceleration stop time or an electric power stop time, the period of which is not more than 0.3 sec and not less than 0.03 sec, is provided in the process of acceleration in the one direction and also in the process of acceleration in the opposite direction.  
     
     
         12 . A method for casting molten metal according to  claim 6 ,  7 ,  8  or  9 , wherein acceleration is generated for t1, subsequently a constant flow velocity is kept for t2, next acceleration is generated in the opposite direction for t3 and thereafter a constant flow velocity is kept for t4 in one period, and molten metal in the mold is periodically vibrated by repeating this period, and a vibration time t1+t2+t3+t4 in one period is determined to be not less than 0.2 sec and less than 10 sec.  
     
     
         13 . A method for casting molten metal according to any one of  claims 1  to  8  or  claim 9 , wherein the molten metal is periodically vibrated, and a rotating flow in the one direction and the opposite direction is given to the molten metal.  
     
     
         14 . A method for casting molten metal according to  claim 13 , characterized in that: when integration is generated for a certain period of time, the expression of integrated value of (acceleration time×acceleration) in the one direction>integrated value of (acceleration time×acceleration) in the opposite direction is satisfied; and an average rotating flow velocity caused by the difference between the integrated values is not more than 1 m/s.  
     
     
         15 . A method for casting molten metal according to  claim 13 , wherein acceleration of the molten metal in the mold is generated for t1, subsequently a constant flow velocity is kept for t2, next acceleration is generated in the opposite direction for t3 and thereafter a constant flow velocity is kept for t4 in one period, molten metal in the mold is periodically vibrated by repeating the period, t1a is a time until the vibrating flow velocity becomes zero in time t1, t1b is a time after the vibrating flow velocity becomes zero in time t1, an expression of t1b+t2>t4+t1a is satisfied, and a rotating flow velocity in one direction caused by the difference in time is not more than 1 m/s.  
     
     
         16 . A method for casting molten metal according to  claim 13 , wherein vibration is periodically given in a period of n cycles, a rotating flow is generated by giving acceleration only in a predetermined direction for the rotating time ΔTv after the vibration, and an average rotating flow velocity, number n of cycles and rotating time ΔTv satisfy the following expressions. 
       Average rotating flow velocity≦1 m/s1≦number n of cycles≦200.1≦rotating time ΔTv≦5 sec 
     
     
         17 . A method for casting molten metal according to  claim 13 , wherein a rotating flow is generated by increasing an acceleration in the one direction to be larger than an acceleration in the opposite direction, and an average rotating flow rate is not more than 1 m/s.  
     
     
         18 . A method for casting molten metal according to  claim 13 , wherein an electric current for rotation generating a rotating flow in one direction is further superimposed an electric current during vibration by an electric current of the electromagnetic coil for generating a shifting magnetic field so that an average rotating flow velocity can be not more than 1 m/s.  
     
     
         19 . A method for casting molten metal according to any one of  claims 1  to  9 , wherein the molten metal is periodically vibrated, and vibration of a short period is further added, and the frequency of the vibration of this short period is not less than 100 Hz and not more than 30 KHz.  
     
     
         20 . A method for casting molten metal according to any one of  claims 6  to  9 , wherein an electromagnetic coil is arranged in the mold or in the proximity of the molten metal pool in the mold when molten metal is poured into and solidified in the mold, the molten metal in the mold is periodically vibrated in the one direction and the opposite direction by a shifting magnetic field generated by the electromagnetic coil, and an electromagnetic brake, which is arranged in a range from the meniscus to a position under the mold distant by 1 m, is applied.  
     
     
         21 . A method for casting molten metal according to  claim 11 , wherein an electromagnetic coil is arranged in the proximity to the molten metal pool in the mold when molten metal is poured into and solidified in the mold, the molten metal in the mold is periodically vibrated in the one direction and the opposite direction by a shifting magnetic field generated by the electromagnetic coil, and an electromagnetic brake, which is arranged at a position under the mold distant from the meniscus by 1 m, is applied being synchronized with time at which acceleration of the electromagnetic coil is stopped in the mold or being synchronized with time at which an electric power source is stopped.  
     
     
         22 . A method for casting molten metal according to any one of  claims 6  to  15 , wherein the electromagnetic coil arranged in proximity to the molten metal pool in the mold is arranged in a range under the mold from right below the mold to a position distant from the mold by 10 m.  
     
     
         23 . A method for casting molten metal according to  claim 22 , wherein an electromagnetic brake, which is arranged in a range from a position above the electromagnetic coil distant by 1 m to a position below the electromagnetic coil distant by 1 m, is applied.  
     
     
         24 . A method for casting molten metal according to  claim 11 , wherein the electromagnetic coil arranged in proximity to the molten metal pool in the mold is arranged in a range from a position right below the mold to a position under the mold distant by 10 m, and the electromagnetic brake arranged in a range from the meniscus to a position under the mold distant by 1 m is applied being synchronized with the time at which acceleration of the electromagnetic coil is stopped in the mold or being synchronized with the time at which the electric power source is stopped.  
     
     
         25 . An electromagnetic coil device used for any one of  claims 1  to  24 , comprising: an electromagnetic drive device for periodically vibrating in the one direction and the opposite direction; and a control unit for controlling the electromagnetic drive device.  
     
     
         26 . An electromagnetic coil device used for any one of  claims 1  to  24  comprising; an electromagnetic coil; and an electric power source for supplying an electric current to vibrate the electromagnetic coil periodically in the one direction and the opposite direction or a waveform generating device.  
     
     
         27 . An electromagnetic coil device used for any one of  claims 1  to  24 , comprising: an electromagnetic drive device for vibrating molten metal periodically in the one direction and the opposite direction, the electromagnetic drive device having a function of raising an electric current to a command value in the case of changing a vibrating direction; and an electric current control device for controlling the electric current.  
     
     
         28 . An electromagnetic coil device comprising an electromagnetic drive device, a control device for controlling an electric current, and an electromagnetic brake used in any one of  claims 1  to  24 .  
     
     
         29 . A cast slab having a negative segregation zone composed of a multilayer structure, the pitch of which is not more than 2 mm and the number of the layers of which is not less than three, a dendrite or a crystalline structure zone composed of a deflection structure of a multilayer.  
     
     
         30 . A cast slab having a negative segregation zone composed of a multilayer structure, the pitch of which is not more than 2 mm and the number of the layers of which is not less than three, a dendrite or a crystalline structure zone composed of a deflection structure of a multilayer, wherein the thickness of the negative segregation zone, dendrite or crystalline structure zone is not more than 30 mm.  
     
     
         31 . A cast slab characterized in that: a corner point (C) of a central negative segregation line (m) of a negative segregation zone of an average profile of the negative segregation zone of a multilayer structure is determined, or a virtual corner point (C′) extrapolated from two adjoining sides of a central segregation line (m) of an arcuate negative segregation zone is determined; and parallel lines are drawn from points (E) on two adjoining sides, which are distant from the corner point to the inside of the cast slab by 5 mm, to the two adjoining sides, and a difference between shell thickness D 1  at a point of intersection (F) with the central segregation line (m) and shell thickness D 2  at the center in the cast slab width direction is not more than 3 mm.  
     
     
         32 . A cast slab characterized in that: a corner point of a center line of dendrite or a crystalline structure zone of deflection structure of a multilayer, which has an average profile thereof, is determined, or a virtual corner point extrapolated from two adjoining sides of a center line of the arcuate dendrite or crystalline structure zone is determined; and parallel lines are drawn from points on the two adjoining sides, which are distant from the corner point to the inside of the cast slab by 5 mm, to two adjoining sides, and a difference between shell thickness D 1  at a point of intersection with the central line and shell thickness D 2  at the center in the cast slab width direction is not more than 3 mm.  
     
     
         33 . A cast slab characterized in that: a shape of the cast slab is circular; and fluctuation of shell thickness at a point on a central segregation line (m) of a negative segregation zone of an average profile of the negative segregation zone of a multilayer structure is not more than 3 mm.  
     
     
         34 . A cast slab characterized in that: a shape of the cast slab is circular; and fluctuation of shell thickness at a point of a center line of a dendrite or a crystalline structure of an average profile of a dendrite structure or a crystalline structure zone of a deflection structure of a multilayer is not more than 3 mm.  
     
     
         35 . A cast slab provided when molten metal is poured into a mold and solidified while an electromagnetic force is applied to the molten metal by an electromagnetic coil arranged in the proximity of the mold according to  claim 31  or  33 , the cast slab comprising a negative segregation zone composed of a multilayer structure formed in the inner circumferential direction of the mold having pitch P defined by the following expression (2) in a range of D 0 ±15 mm in the thickness direction with respect to solidified shell thickness D 0  (mm) at the core center in the casting direction determined by solidified shell thickness D (mm) defined by the following expression (1). 
         D=k ( L/V ) n   (1) D: Solidified shell thickness    L: Length from meniscus to core center of electromagnetic coil    V: Rate of casting    k: Coefficient of solidification    n: Constant     P=U×t/ 2  (2) U: Rate of solidification (dD/dt (mm/s))    t: Period of vibration      
     
     
         36 . A cast slab according to one of  claims 31  to  35 , the cast slab having an equi-axed crystal ratio of not less than 50% on the inside of a negative segregation zone composed of a multilayer structure, on the inside of a dendrite or a crystalline structure zone composed of a multilayer-shaped deflection structure.  
     
     
         37 . A cast slab provided when molten metal is poured into a mold and solidified while an electromagnetic force is given to the molten metal by an electromagnetic coil arranged in the proximity of the mold according to  claim 32  or  34 , the cast slab comprising a dendrite or a crystalline structure zone, the growing direction of which is regularly deflected, having pitch P defined by the following expression (2) in a range of D 0 ±15 mm in the thickness direction with respect to solidified shell thickness D 0  (mm) at the core center in the casting direction determined by solidified shell thickness D (mm) defined by the following expression (1). 
         D=k ( L/V ) n   (1) D: Solidified shell thickness    L: Length from meniscus to core center of electromagnetic coil    V: Rate of casting    k: Coefficient of solidification    n: Constant     P=U×t/ 2  (2) U: Rate of solidification (dD/dt (mm/s))    t: Period of vibration