Abstract:
Segregation of carbon or alloying elements in a solidifying liquid core during casting of a continuous metal strand of high carbon steel or alloy steel, are disbursed by vibrating hammers engaged with a solidified shell enclosing a liquid steel core. The hammers are located before the end of the liquid core. A vibrator operating at a frequency of between 1000 and 5000 cycles per minute is coupled to the hammers by a support structure forming a dead weight mass for maintaining a metal-to-metal contact with the solidified shell while vibrated by the vibrator. The support structure is guided for stabilizing the hammers and for displacement of the hammers between an operative position and inoperative position.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not applicable 
     BACKGROUND OF THE INVENTION 
     1. Field of the invention 
     The present invention relates to a method and an apparatus to improve the internal quality of a continuously cast steel section and, more particularly, to mechanically vibrate a solidified shell of such a steel section at a site upstream of the end of a contained liquid steel core consisting of high carbon steel or alloy steel to reduce segregation by dispersing carbon or alloying elements during final solidification of the liquid core. 
     2. Description of the Prior Art 
     Inherent internal conditions in the process of continuous casting of steel sections such as billets, blooms, rounds and slabs have a significant influence on the internal quality of the steel section especially when casting high carbon steel and alloy steel. The inherent conditions are center looseness, center segregation, and equiaxed grain ratio. While center looseness and center segregation are not desirable, obtaining an equiaxed grain ratio is very desirable. Several methods of combating or enhancing the above mentioned conditions are known to produce a varying degree of success. Two such known methods are electromagnetic stirring and soft reduction. Electromagnetic stirring is accomplished by applying a magnetic field to the cast section liquid core to agitate the steel causing the breakage of the dendrite tips and dispersion of inclusions. This action promotes recrystallization in the solidification process and minimizes center segregation. The soft reduction method involves progressively squeezing a mushy zone in the solidifying section to refine the grain size at the center of the section, which also influences center segregation and center looseness. Electromagnetic stirring and soft reduction methods are capital intensive, when initially installing the necessary equipment into a new facility or when retrofitting the necessary equipment into an existing facility. 
     It is an object of the present invention to provide a method and an apparatus to introduce vibration by physically impacting a continuously cast section at a location before final solidification. 
     It is a further object of the present invention to provide a method and an apparatus to apply mechanical vibrations to an outer shell of a continuously cast section to vibrate an internal mushy zone sufficiently to cause the breakage of dendrite tips and thereby promote recrystallization and to enhance refinement of the grain structure by dispersion of segregated carbon or alloying elements, to produce an equiaxed and dense structure, and to reduce porosity by facilitating the floatation of gas bubbles to the top of the mold. 
     SUMMARY OF THE INVENTION 
     According to the present invention there is provided an apparatus for reducing segregation in a solidifying section with a contained liquid core during casting of a continuous metal strand of high carbon steel or alloy steel, the apparatus including the combination of at least one hammer having a face surface for engaging a solidified shell enclosing a liquid steel core having concentrations of carbon or alloying elements, a vibrator having an operating frequency of between 1000 and 6000 cycles per minute coupled to the hammer for vibrating the liquid core to disperse concentrations of carbon or alloying elements during solidification of the liquid core, a dead weight mass mechanically coupled to the hammer for maintaining a desired contact force on the solidified shell by the hammer while vibrated by the vibrator, and guides for stabilizing the hammer. 
     The present invention further provides a method for reducing segregation in a solidifying liquid core during casting of a continuous metal strand of high carbon steel or alloy steel, the method including the steps of, selecting a site along a cast strand upstream of the end of a liquid core contained within a solidified shell of a continuous casting installation for high carbon steel or alloy steel, and vibrating the solidified shell at the site at a frequency selected to disperse concentrations of carbon or alloying elements during solidification of the liquid core to refine the grain structure during solidification of the liquid core. Preferably, the solidified shell is vibrated at a frequency of between 1000 and 6000 cycles per minute. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     The present invention will be more fully understood when the following description is read in light of the accompanying drawings in which: 
     FIG. 1 is an elevational view of a continuous casting installation embodying the present invention; 
     FIG. 2 is an enlarged elevational view of an apparatus to vibrate a newly formed continuously cast strand at a site along a secondary cooling section of a continuous casting installation as shown in FIG. 1; 
     FIG. 3 is a sectional view taken along lines III—III of FIG. 2; 
     FIGS. 4,  5  and  6  are schematic illustrations of the transition of the apparatus to vibrate the casting between an inoperative position and an operative position; 
     FIG. 7 is a photograph of a cross section of a high carbon continuous steel casting produced by an ordinary or standard casting method; 
     FIG. 8 is a photograph of a cross section of a high carbon continuous steel casting produced by a modified casting method incorporating the present invention; 
     FIG. 9 is a photograph of a cross section of a high carbon continuous steel casting produced by an ordinary casting method with electromagnetic stirring; and 
     FIG. 10 is a photograph of a cross section of a high carbon continuous steel casting produced by a modified casting method incorporating the present invention combined with electromagnetic molds stirring. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1 and 2 illustrate one form of a continuous casting installation  10  suitable to practice the method and incorporates one embodiment of the apparatus according to the present invention to produce a continuously steel casting comprised of high carbon steel or alloy steel. The term high carbon steel is defined to mean carbon steel with a carbon content of 0.45% or greater and the term alloy steel is defined to mean an alloyed steel having enhanced properties by the presence of one or more special alloying elements or due to the presence of larger portions of elements such as manganese and silicon than are ordinarily present in carbon steel. The continuous casting installation  10  includes a ladle turret  12  for delivering molten steel in ladles  13  and  14  into and from a position directly above a tundish  15 . The tundish delivers a stream of liquid steel into a water-cooled mold  16  and a continuous strand S made up of a solidified shell surrounding a liquid core passes from the mold along a curved secondary cooling section  17 . The continuous strand S has a well-known cross sectional configuration such as a billet, a bloom, a round or a slab. The secondary cooling section  17  contains spaced apart guide rollers  18  interleaved with water spray headers, not shown, to continue the cooling process. Preferably, though not necessary, mold  16  also includes an electromagnetic coil assembly  19  to provide electromagnetic stirring of the liquid core in the continuous strand S. The rollers and spray headers of the secondary cooling section  17  are supported by consecutively arranged frames  20 , each having anchor rods  21  supported on pedestals  22  mounted on an underlying foundation. The secondary cooling section extends to a straightener section  23  provided with motor driven straightenering rolls  24  for straightenering and delivering the continuous strand S to a runout table  25 . 
     In accordance with the present invention there is provided an apparatus  26  to vibrate the continuous strand S in the continuous casting installation  10  at selected site upstream of the end of the liquid core within the solidified shell. The end of the liquid core is generally within or close to the straightener section  23 . The selected site in the embodiment shown in FIGS. 1 and 2 is in a gap that exists between the last of the guide rollers  18  of the secondary cooling section  17  and the first pair of motor driven straightener rolls  24  of a straightener section  23 . The selected site can be located in a space between driven straightener rolls  24 , or past the last pair of straightener rolls and before the point of solidification of the liquid core. 
     As shown in FIGS. 2 and 3, the apparatus  26  essentially includes an oscillating dead weight mass, driven by a vibration actuator  27  which can be electromagnetic or eccentrically driven by an air, hydraulic, or electric motor. The vibration actuator  27  is mounted on a vibrating head  29 , which pivots on a frame  28 , and is engaged and disengaged by a linear actuator retained at the selected site through the use of supporting structure provided by the existing foundation of the casting machine. However, the selected site may be located in an area behind, i.e. downstream of, the straightener rolls in the event that the liquid core extends into this area. When engaged and activated, the mass undergoing oscillation is comprised of the frame  28  and the vibrating head  29  will undergo dynamic impacting with the continuous strand S at a preselected and relatively low frequency typically at a frequency in the range of 1000 to 6000 cycles per minute, preferably in the range of 3000 to 4000 cycles per minute. The magnitude of the dead-weight mass required for static contact with the continuous strand S, the oscillation stroke and force of the vibration actuator  27  are chosen relative to the physical dimensions of the continuous strand S. The oscillation cycle, stroke and force are controllable parameters of the vibration actuator  27 . The construction of the frame  28  and vibrating head  29  are specifically engineered to establish a predetermined dead weight required for exerting contact forces by hammer face surfaces on the casting. A plate P may be added to support a counterweight W to modify the dead weight and center of gravity of head  29 , such modification to the center of gravity to compensate a change to the metallurgical composition of the continuous strand or a change to the thickness of the continuously cast strand. However, it is to be understood that the hanging weight of the dead weight mass and the center of gravity can be modified by the addition of weight to the cross head  36  and/or by the counterweight W. The vibrations generated by the vibration actuator  27  are transmitted from the vibrating head  29  by a shaft  30  to the frame  28 . The shaft  30  pivotally interconnects head  29  with spaced apart rails  32  of frame  28  extending along opposite sides of the of the continuous strand S where the rails slidably engage with guides  33  extending generally vertically for stabilizing the frame  28 . The link arms  31  extend from the shaft  30  in a cantilevered fashion along opposite sides of the continuous strand S for rigidly mounting the opposite ends of two hammers  34  and  35  in a spaced apart relation to thereby mechanically couple the hammers to the vibration actuator  27 . The hammers  34  and  35  have face surfaces arranged for engaging the upwardly and downwardly directed face surfaces of the continuously moving steel casting. It is sufficient to provide at least one hammer although two hammers are preferred. The vibration imparted to the hammers serves the additional and essential function of reducing friction between the hammers and the continuously moving steel casting which allows unimpeded forward movement of the casting without damage to the hammer support structure including the guides  33 . 
     The lower end portions of the spaced apart rails  32  are secured to a cross head  36  provided with an arm  37  having a lateral projection overlying a linear actuator  38  which can be electrically, pneumatically or hydraulically powered to displace an actuator rod  39 . The actuator is supported by a bracket  40  extending from the underside of a frame  41 , which extends in the direction of the flow of the casting for support by adjacent pedestals  22  at the boundaries of the gap at the selected site. The frame  41  includes upstanding frame  42 , which includes the guides  33  for supporting the rails  32 . Channels, one of which is identified by reference numeral  43 , for coolant water are strategically placed at diverse locations to cool the apparatus  26  during the operation of the continuous casting installation  10 . 
     As shown in FIG. 4, the actuator rod  39  of the linear actuator  38  is extended to hold the upper hammer  34  in an inoperative position at a location above the casting. The cantilevered relation of the hammers relative to the shaft  30  allows the lower hammer  35  to rotate to an inoperative position below the casting by rotation of the link arms  31  about the shaft  30  and the upper hammer  34  to rotate to an inoperative position above the casting. The apparatus  26  is moved into an operative position by retracting the actuator rod  39  and, as shown in FIG. 5, this allows downward travel of the rails  32  along the guides  33  with the receding movement by the actuator rod. The upper hammer  34  rotate to an operative position contacting the upper surface of the casting S and the lower hammer rotates toward an operative position for contact with the lower surface of the casting S. When the actuator rod  39  is fully retracted, as shown in FIG. 6, the upper hammer  34  remains in contacts the upper surface of the casting and the lower hammer  35  pivots about shaft  30  into contact with the lower surface of the casting. The dead-weight mass of the assembly, which moved to allow contact with the casting by the two hammers, establishes a metal-to-metal contact with the casting under a dead-weight load. At the position shown in FIG. 6, the actuator rod  39  is disengaged with arm  37  and thereby the entire weight of the head  29  and the frame  28  is hanging on the strand S. 
     The vibration imparted to the steel shell propagates to the internal liquid core and in directions of toward the mold and oppositely to essentially the end of the liquid core to disperse concentrations of carbon or alloying elements occurring in the continuous casting of high carbon steel or alloy steel, respectively. FIG. 7 illustrates extensive center segregation of the grain structure in a continuous strand produced without practicing the method or use of the apparatus of the present invention. FIG. 8 illustrates a very favorable refined central grain structure in a continuous strand S produced by the same casting machine used to produce the steel casting of FIG. 7 but modified by practicing the method and the use of the apparatus of the present invention. The absence of voids in the central area of the casting shown in FIG. 8 is a note worthy advancement as compared with high concentrations of voids and segregated grain structure visible in the cross section of FIG.  7 . 
     Experimental use of the present invention further included a trial to examine the benefits of vibrating the casting in a continuous casting machine equipped with electromagnetic stirring of the steel residing in the mold which produced the equiaxed and densely refined grain structure as shown in FIG.  9 . The operation of the continuous casting installation was altered by placing the apparatus to vibrate the casting in the inoperative position but use of the electromagnetic stirring was continued to recover a casting and examine the grain structure, which is shown in FIG.  10 . The benefits of breaking dendrite tips during cooling of the central core of the high carbon steel or alloy steel are readily apparent which also was found to accelerate the solidification process by the seeding of the liquid core with the broken dendrite tips. Additionally, vibrating the continuously cast strand promoted the discharge of gas bubbles from the core during solidification. 
     While the present invention has been described in connection with the preferred embodiments of the various figures, it is to be understood that other similar embodiments may be used or modifications and additions may be made to the described embodiment for performing the same function of the present invention without deviating there from. Therefore, the present invention should not be limited to any single embodiment, but rather construed in breadth and scope in accordance with the recitation of the appended claims.