WO 2009/041777 PCT/KR2008/005697 STRIP EDGE SHAPE CONTROL APPARATUS AND METHOD IN STRIP CASTING PROCESS Technical Field 5 The present invention relates to the control of the edge shape of a strip in strip casting, and more particularly to an apparatus and method for controlling the edge shape of a strip in a twin-roll strip casting process, which improve the quality of the strip by using cameras to photograph the edge portions of the strip 10 being cast by strip casting, analyzing the photographs, and then controlling edge dams according to the edge shape of the strip. Background Art 15 FIG. 1 is a view showing the construction of a strip casting apparatus according to the prior art. As shown in FIG. 1, an important process for strip casting generally occurs in a sump between two rolls 1 and 2 that rotate in directions opposite to each other. When molten steel is supplied to the sump 9 through a submerged entry 20 nozzle 5 from a tundish 4, the molten steel is solidified between a leader strip and the two rolls within 0.2 seconds and rolled. Herein, the molten steel, the leader strip that is solidified and a strip that is solidified and follows the leader strip generate roll separating force (RSF). The roll force is sensed by a load cell 10 placed at the rear part of the rolls. 25 The sides of the two rolls are blocked with ceramic edge dams 8 in order to prevent molten steel from flowing out through the sides of the rolls. The solidification capability of molten steel is proportional to the cooling capability of the casting rolls. In addition, the solidification capability is influenced by the distance between the two rolls, that is, the roll gap, the casting 30 speed (rotating speed) of the rolls, and the height of molten steel in the sump. The height of molten steel is measured using a height detection sensor 7. In general, when molten steel solidifies, it generates roll force between the rolls. The rolling reduction rate is influenced by the roll gap and the casting speed. The roll gap is measured using a distance measuring unit 3. For example, if the 35 roll gap is excessively great or if the casting speed is excessively high, the WO 2009/041777 PCT/KR2008/005697 2 solidification point will decrease below the temperature of the center line of roll nips, such that the roll force gradually decreases, thus causing the non solidification of molten steel and strip breakdown. In the contrary case, the solidification point will increase, leading to high roll force. Therefore, the roll 5 gap, the casting speed and the roll force are typical casting parameters indicating the solidification properties of the strip. The control of the height of molten steel fundamentally requires a high degree of precision and stability. Thus, it is sought to maintain the height of molten steel at a target value starting from immediately after the start of casting. 10 Accordingly, the roll force may seem to result from the relationship between the roll gap and the casting speed. In order to prevent molten steel from flowing out through the sides of the two rolls during casting in a twin-roll strip casting apparatus, the sides of the two rolls are blocked with edge dams made of a ceramic material. In this case, even 15 though there is an interval of only a few tens of micrometers (gm) between the sides of the rolls and the edge dams, molten steel can flow into the gap. If molten steel flows into the gap, it will deteriorate the quality of the side of a cast strip, that is, the edge quality of the cast strip, thus reducing the value of the cast strip and abnormally wearing the ceramic material of the edge dam to make 20 normal casting impossible. Moreover, in many cases, a skull that almost inevitably occurs due to the temperature drop of molten steel and the surrounding devices during casting is incorporated between the sides of the rolls and the edge dams, and thus the seal plate of the edge dam is damaged. For this reason, an edge fin occurs at the edge 25 portion of a cast strip being produced, leading to deterioration in the quality of the cast strip. In order to prevent the production of this skull, a method is used in which the edge dams are oscillated to remove the skull from the edge dams in a very small skull state at the same time at which the skull is produced. However, the 30 oscillation of the edge dams additionally wears the seal plate of the edge dams to form an unnecessarily worn portion between the seal plate of the edge dams and the end face of the rolls, and if molten steel penetrates into and is solidified in that portion, burrs will be formed on a cast strip being produced, thus making the edge shape of the strip poor. 35 The efficient control of the edge dams in a twin-roll strip casting process 3 intended for long-term casting is important for producing a high-quality strip and also plays a key role in reducing the production cost by reducing the wear amount of the edge dams. At the same time, the edge quality of a cast strip being produced is one of the most important indices determining the superiority and inferiority of the cast strip product. 5 Such issues must be solved in order to improve quality and productivity, and thus a control method for solving them is required. According to studies on edge dam control conducted through casting experience in a twin-roll strip casting apparatus so far, a method which can be used by operators when the edge quality of a cast strip is bad is to control the wear rate and upward shift 10 rate. In addition, when the ceramic material of the edge dams is broken during casting, the only method for coping with this situation is to stop the casting process. Accordingly, it is very important for operators to closely examine all data related to edge dam control during casting so as to prevent reaching such an extreme situation. Thus, it is very important during casting to immediately observe and analyze the quality 15 of a cast strip being produced and to reflect the analysis results on the control of the edge dams. However, much time is required for an operator to visually analyze the strip coming out of a casting machine, to return to an edge dam controller in an operation room and to analyze the analysis results. Due to this time delay, molten steel and a skull are 20 incorporated between the sides of the rolls and the edge dams, damaging the seal plate of the edge dams, so that the quality of the cast strip becomes poor and the life span of the expensive edge dams is shortened. Object of the Invention 25 It is the object of the present invention to substantially overcome or ameliorate one or more of the disadvantages of the prior art. Summary of the Invention The present invention provides an apparatus for controlling the edge shape of a 30 strip in a twin-roll casting process, the apparatus comprising: an edge dam driving unit which is attached to edge dams attached to both sides of casting rolls so as to control the pressing force of the edge dams against the casting rolls and the upward shift of the edge dams according to the wear and upward shift rate of the edge dams, wherein the edge dam driving unit comprises an upward cylinder which is 35 disposed at the upper area of the center of the side opposite the side of the edge dams that 4 comes in contact with the casting rolls, so as to move the edge dams upward, and wear cylinders which are disposed at the left and right sides of the upper portion of the edge dams and at the lower portion of the center of the edge dams so as to press the edge dams against the casting rolls; 5 cameras which are disposed at the strip outlet side of the casting rolls so as to photograph the edge portions of the strip; and a control unit which analyzes the edge shape of the strip from images photographed by the cameras and outputs a signal controlling the wear and upward shift of the edge dams, wherein the control unit comprises a main control unit which analyzes to the strip edge images photographed by the cameras so as to calculate the wear and upward shift rate of the edge dams, an upward-shift control unit which outputs a signal driving the upward cylinder depending on the upward shift rate and target shift rate of the main control unit, an upward-cylinder servo-valve which drives the upward cylinder according to the driving signal of the upward-shift control unit, a wear control unit which outputs a is signal driving the wear cylinders according to the wear rate calculated in the main control unit, and a wear cylinder servo-valve which drives the wear cylinders according to the driving signal of the wear control unit. The present invention also provides a method for controlling the edge shape of a strip in a twin-roll casting process, the method comprising: 20 a photographing step of photographing the images of edge portions of the strip using cameras disposed at the strip outlet sides of casting rolls; a control signal-producing step of analyzing the strip edge images photographed in the photographing step and outputting a control signal for a wear and upward shift rate; and 25 an edge dam control step of controlling the wear and upward shift of edge dams according to the control signal output in the control signal-producing step; wherein the control signal-producing step comprises: an edge analysis step of calculating the burr thickness of the edge dams and the length of an edge fin; 30 a step of outputting a signal for maintaining the edge dams at a default wear and upward shift rate set in the normal state of the edge dams, if the edge analysis step shows a burr thickness of less than 1 mm and no existence of the edge fin; a step of outputting a signal for additionally applying a wear and upward shift rate of I pm per m of the strip to the default wear and upward shift rate, if the edge 5 analysis step shows a burr thickness of more than 1 mm but less than 5 mm and an edge fin length of less than 1 cm; and a step of outputting a signal for additionally applying a wear and upward shifn rate of 1.5 ptm per m of the strip to the default wear and upward shift rate, if the edge s analysis step shows a burr thickness of more than 5 mm and an edge fin length of more than 1 cm. Preferably, the edge dam control step comprises the steps of: re-analyzing the edge portions of the strip after performing the control of the edge dams according to the signal output in the control signal-producing step; and io controlling the edge dams so as to return to the default value of the edge dam wear and upward shift rate, if the edge portions are normal. Preferably, the default value of the wear and upward shift rate is set within the range of 0.5 to 5 gm per m of the strip. At least in preferred embodiments, the present invention provides the effect of 15 improving the quality and productivity of a strip in a twin-roll strip casting process. In addition, the present invention provides the effect of allowing an expensive edge dam to be used for a long period of time. Namely, the present invention provides the effects of suppressing the generation of strip scraps and extending the life span of an edge dam, thus WO 2009/041777 PCT/KR2008/005697 6 reducing production cost. Brief Description of Drawings 5 FIG. 1 is a view showing the construction of a strip casting apparatus according to the present invention. FIG. 2 is a block diagram of an apparatus for controlling the edge shape of a strip in a twin-roll strip casting process (hereinafter referred to as "edge shape control apparatus") according to an embodiment of the present invention. 10 FIG. 3 is a view showing the position of cylinders attached to an edge dam. FIG. 4 is a flowchart showing the processing procedure of the inventive method for controlling the edge shape of a strip in a twin-roll strip casting process. 15 Best Mode for Carrying Out the Invention Hereinafter, the present invention will be described in detail with reference to the accompanying drawings. FIG. 2 is a block diagram showing the construction of an apparatus for 20 controlling the edge shape of a strip in a twin-roll strip casting process (hereinafter referred to as "edge shape control apparatus") according to an embodiment of the present invention, and FIG. 3 is a view showing the position of cylinders attached to an edge dam. As shown in FIG. 2, the edge shape control apparatus comprises an edge 25 dam driving unit 100 which is attached to both sides of casting rolls so as to control the pressing force of the edge dams against the casting rolls and the upward shift of the edge dams depending on the wear and upward shift rate of the edge dams; cameras 200 which are disposed at both sides of the edge dams at the outlet side of the casting rolls 12 through which a strip 12 is discharged, the 30 cameras serving to photograph the edge portion of the strip; and a control unit 300 which analyzes the images photographed by the cameras 200 so as to analyze the burr thickness and edge fin length of the edge portion and outputs to the edge dam driving unit 100 a signal controlling the wear and upward shift rate of the edge dams according to the thickness of a burr and the length of an edge fin. 35 In the above-described construction, the edge dam driving unit 100 WO 2009/041777 PCT/KR2008/005697 7 comprises: an upward cylinder 101 which is disposed at the upper area of the center of the side opposite the side of the edge dams that comes in contact with the casting rolls, so as to move the edge dam upward; and a wear cylinder 102 which is disposed horizontally at the back side of the edge dams so as to control 5 the forward and backward movement of the edge dams 100 with respect to the casting rolls. Herein, the upward cylinder 101 which is disposed at the upper area is connected to the upper portion 111 of the back side of the edge dam 110 as shown in FIG. 3. Also, the wear cylinder 102 is disposed at the back side of the edge dam 110 in order to move the edge dam forward and backward with respect 10 to the casting rolls. As shown in FIG. 3, the wear cylinder 102 may also be disposed at the back side of the edge dams at each of both sides 112 and 113 of the upper portion of the edge dams and the lower portion 114 of the center of the edge dams. The cameras 200 are disposed at a strip outlet side through which a cast 15 strip is discharged. The cameras photograph the edge portion of the strip 12 with a resolution allowing the edge portion to be analyzed up to a millimeter scale, and output the photographed images to the control unit 300. Then, the control unit 300 comprises: a main control unit 310 which analyzes the strip edge images photographed by the cameras 200 so as to calculate 20 the wear and upward shift rate of the edge dams; an upward-shift control unit 320 which outputs a signal driving the upward cylinder 101 depending on the upward shift rate and target shift rate of the main control unit 310; a first DA converter 321 which converts the driving signal of the upward-shift control unit 320 to an analog signal; a servo-valve 322 which drives the upward cylinder 101 according 25 to the output signal of the first DA converter 321; a first distance measuring sensor 323 which is disposed at each of the casting rolls 1 and 2 so as to sense the upward moving distance of the edge dam 110; a first AD converter 324 which converts the output signal of the first distance measuring sensor 323 to a digital signal and then outputs the digital signal to the upward shift control unit 320; a 30 wear control unit 330 which outputs a signal driving the wear cylinder 102 according to the edge dam wear rate and target wear rate of the main control unit 310; a second DA converter 331 which converts the driving signal of the wear control unit 330 to an analog signal; a wear cylinder servo-valve 332 which drives the wear cylinder 102 according to the output signal of the second DA converter 35 331; a second distance measuring sensor 333 which is disposed at each of the WO 2009/041777 PCT/KR2008/005697 8 casting rolls 1 and 2 so as to sense the distance of the edge dam 110 moved with respect to the casting rolls; and a second AD converter 334 which converts the output signal of the second distance measuring sensor 333 to a digital signal and then outputs the digital signal to the wear control unit 330. 5 FIG. 4 is a flowchart showing the procedures of the inventive method for controlling the edge shape of a strip in a twin-roll strip casting process. Hereinafter, the inventive method for controlling the edge shape of a strip in a twin-roll strip casting process will be described with reference to FIGS. 2 to 4. The edge shape control apparatus of the present invention performs the control of 10 the pressing force of the edge dam against the casting rolls and the upward shift of the edge dam to the default set wear and upward shift rate of the edge dam while performing casting as in the prior art. Herein, the default set wear and upward shift rate of the edge dam can be set within the range of 0.5-5 ym per m of a strip. Herein, the cameras 200 continuously photograph images of the edge 15 portions of the strip being cast and discharged and output the images to the control unit 300 (S 1). Then, the control unit 300 analyzes the strip edge images photographed by the cameras 200 and determines whether the state of the strip edge is poor (S2 and S3). 20 If the state of the edge is determined to be poor, the control unit 300 produces and outputs a signal for controlling the wear and upward shift of the edge dams. An example of the wear and upward shift rate of the edge dams will now be described. If the burr thickness of the edge portion is less than 1 mm and no edge fin 25 exists in the strip images photographed by the cameras 200, the main control unit 310 outputs a signal maintaining the edge dams at the wear and upward shift rate set at a default value in the normal state of the edge dams. Herein, the wear and upward shift rate set at a default value in the normal state of the edge dams can be set within the range of 0.5 to 5 gtm per m of the strip as described above. 30 If the burr thickness of the strip edge is more than 1 mm but less than 5 mm and the length of the edge fin is less than 1 cm, the main control unit 310 outputs a signal for additionally applying a wear and upward shift rate of 1 tm per m of the strip to the default set value. Then, if the burr thickness of the strip edge is more than 5 mm and the length of the edge fin is more than 1 cm, the main 35 control unit 310 outputs a signal for additionally applying a wear and upward shift WO 2009/041777 PCT/KR2008/005697 9 rate of 1.5 tm per m of the strip to the default set value. Such output signals of the main control unit 310 are input into the shift control unit 320 and the wear control unit 330 so as to drive the upward cylinder 101 and wear cylinder 102 connected to the back side of the edge dam 110, thus controlling the pressing 5 force of the edge dam against the casting rolls and the upward shift of the edge dam (S4). Then, the edge portions of the strip 12 which is produced while controlling the wear and upward shift rate of the edge dam 110 are continuously photographed with the cameras 200 in order to determine whether the state of the 10 edge portions has been restored to the normal state (S5). If the state of the edge portions has not been restored to the normal state, the control unit 300 continually controls the wear and upward shift of the edge dam according to the edge dam wear and upward shift rate output in step S4. However, if the state of the edge portions has returned to the normal state, the 15 edge dam wear and upward shift rate of the edge dam returns to the initially set default value, and the pressing force of the edge dam against the casting rolls and the upward shift rate of the edge dam are controlled (S6). In addition, the above-described processing procedure is repeated during the casting process, and when the casting process has ended, the processing 20 procedure is ended (S7).