Vertical continuous casting method and casting apparatus

Disclosed are a method and an apparatus for vertical continuous casting. The vertical continuous casting includes drawing a strand formed in a watercooled mold vertically downward therefrom, positioning light reduction pinch rollers just prior to the solidification completing point of the strand, and applying a light reduction force to the strand sufficient to compensate for contraction caused by solidification of the strand. By this procedure the formation of central cavities is prevented and central segregation is lessened. The light reduction pinch rollers are arranged to be adjustably positioned up and down along the strand, in order to compensate for fluctuations in the location of the solidification completing point of the strand.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to improvements in vertical continuous casting or 
the casting technique of obtaining cast pieces by drawing a strand formed 
in a water-cooled mold downward without incurvating, and then cutting the 
strand. 
2. State of the Art 
Nowadays, the continuous casting process holds an important position in a 
steel casting. However, the horizontal continuous casting process is 
somewhat problematical from a technical point of view. For such reasons it 
has only been tried in part. Accordingly, the curved and vertical 
continuous casting processes are overwhelmingly used today. The vertical 
continuous casting is disadvantageous because it requires equipment which 
has a high cost. Accordingly, the curved continuous casting process is 
predominately used. In this process a strand is incurvated (a water-cooled 
mold is structured so as to provide the curved strand) to extend 
horizontally, thereby keeping a height of the casting equipment within a 
certain limit. The strand extending horizontally is rectified to be 
straight with pinch rolls and cut with a frame cutter into cast pieces. 
However, such a process comprising curved casting and horizontal 
rectification is not necessarily preferable for some special kinds of 
steel with respect to producing castings of sufficient quality. For 
example, the rectification operation often causes cracks on the strands of 
valve steel, high-speed steel and other steels. Accordingly, this kind of 
steel will have to be casted by the vertical continuous casting wherein a 
strand is formed without curvature in the water-cooled mold, drawn 
straightly downward and cut to cast pieces. 
In the vertical continuous casting process, light reduction should be 
applied to the strand coming out of the water-cooled mold at a position 
immediately before an unsolidified portion disappears in the interior of 
the strand, whereby contraction of the strand due to solidification is 
compensated, and thus, formation of cavities at the center of the strand 
may be prevented, and center segregation is lessened as well. In this way 
cast pieces of better quality can be obtained. 
However, in the case of some steels for which the application of light 
reduction is effective, different kinds of steels are usually produced in 
relatively small quantities. Therefore, the sectional size and the 
pertinent drawing speed of a strand varies. For such reasons, the optimal 
position at which the light reduction is to be applied, namely, the point 
at which the unsolidified portion of the strand disappears can vary from 
one strand to another. 
SUMMARY OF THE INVENTION 
The object of this invention is to provide a method and apparatus for 
vertical continuous casting wherein a light reduction is applied to a 
strand at an optimal position at all times while compensating for 
frequently fluctuating casting conditions, enabling the production of cast 
pieces having superior quality.

DETAILED EXPLANATION OF PREFERRED EMBODIMENTS 
In a continuous casting method for drawing a strand formed in a 
water-cooled mold vertically, the method of the present invention 
comprises, as shown in FIG. 1, positioning light reduction means (4), or 
pinch rolls representatively, just prior to a solidification completing 
point of a strand (5A) or a point at which an unsolidified portion (52) 
within a shell (51) disappears. The light reduction is applied to the 
strand in an amount sufficient to compensate for contraction caused by 
solidification of the strand. 
A vertical continuous casting apparatus comprising, as shown in FIG. 2, 
includes a water-cooled mold (1) and its ancillary equipment, drawing 
drive rolls (2) and a strand cutter (3) provided below the water-cooled 
mold. The apparatus is also provided with means (not illustrated) for 
feeding a molten metal to the water-cooled mold. The apparatus of the 
present invention additionally comprises providing light reduction means 
(4) which can be adjustably positioned upward or downward along the strand 
relative to the water-cooled mold so as to apply light reduction to the 
strand strong enough to compensate for contraction due to solidification 
of the strand just before the solidification completing point of the 
strand. A reference character (5B) denotes a cast piece obtained from 
cutting the strand, which will be carried away by a separate device one by 
one. A device (10) can be included for detecting the solidification 
completing point and the location of the light reduction means can be 
adjusted based thereon. 
The light reduction means and a technique for providing it variably in 
position along the strand are arbitrary, however, the devices shown in 
FIG. 3 to FIG. 5 will be exemplified as typical. The device can be 
constructed such that three pairs of pinch rolls (41a, 41b, 41c) are 
mounted on a frame 42, and the frame (42) is made movable vertically along 
four pieces of vertical guide columns (43). Each pinch roll is mounted 
slidably on the frame to advance and retreat by hydraulic cylinders (44a, 
44b, 44c) fixed on the frame to pinch the strand (5A) from opposite sides, 
driven by motors (45a, 45b, 45c), and is capable of applying a desired 
light reduction force on the strand metal. 
As shown in FIG. 5, the positions of the pinch rolls (41a to 41c) can be 
changed by worm-jack mechanisms (7) moving vertically from having four 
pieces of vertical bars (46) on a lower portion of the frame (42) rotated 
according to rotations of a motor (6) and male screws of the vertical bars 
engaged with fixed female screws. 
FIG. 6 indicates another mode of providing the light reduction means 
variably in position along the strand. In this apparatus, a construction 
of the light reduction means, namely, the frame and the pinch rolls fitted 
thereon and other parts are the same as the example described above, 
however, what is different is that a vertical movement of the frame is 
realized by winding and unwinding a wire (9) about pulleys (8a, 8b, 8c, 8d 
and 8e). 
Operation 
The position of the light reduction means can be moved up and down along 
the axis of the strand, and is adjusted to apply a light reduction at the 
solidification completing point of the strand based on conditions such as 
the kind of steel to be subjected to the continuous casting, sectional 
size of a strand, drawing speed and others, and by carrying out a light 
reduction at an optimal position, advantages of the present invention can 
be achieved. 
A determination of the solidification completing point can be made, for 
example, as follows. That is, since a thickness (D) of the shell (51) of a 
strand coming out of the water-cooled mold increases, as a time (t) 
passes, in the relation: 
##EQU1## 
a value of k is determined on a technique wherein a spike is driven, for 
example, in the strand and then a cast metal is cut to determine whether 
or not an unsolidified portion is present at the position, and thus the 
solidification completing point in the strand can be estimated according 
to the above equation. 
Recently, a technique has been established for detecting a solidification 
completing point by utilizing the phenomenon that the propagation speed of 
ultrasonic wave varies in solid and liquid. It is therefore preferable 
that a device for detecting a solid/liquid interface on an ultrasonic wave 
is added to the light reduction means (4). In this way a solidification 
completing point for the strand can be found for positioning the light 
reduction means when starting the operation, the light reduction can be 
performed precisely; and when the solidification completing point 
fluctuates according to changes in the kind steel to be subjected to the 
continuous casting and in casting conditions, the change in the 
solidification point is determined and the position of the light reduction 
means is modified. 
The strand is drawn by the aforementioned drawing drive rolls, therefore it 
is not required for the light reduction pinch rolls to have a driving 
force. However, the light reduction pinch rolls can be utilized 
advantageously for inserting dummy bars into the water-cooled mold when 
starting the casting operation. The object is attained by providing a 
driving function to the light reduction pinch rolls, therefore, the 
aforementioned apparatus is provided with a driving motor. 
According to this invention, a light reduction can be applied to a strand 
just prior to a solidification completing point in casting a special steel 
vertically and continuously by adjusting the location of the light 
reduction along the strand to an optimal position always corresponding to 
the strand sectional size and changes in casting conditions. Thus, a 
quality product can be obtained for the special steels by preventing the 
occurrence of center cavities, lessening of center segregation and so 
forth.