A conventional example of a rolling system comprising a plurality of rolling mill stands arranged in a row will be described with reference to FIG. 13. FIG. 13 shows a schematic side view of a conventional rolling system.
On a hot rolling line, a plurality of rolling mill stands F.sub.1 to F.sub.n are provided, and each rolling mill stand F is composed of a four-high roll type rolling mill 1. The four-high roll type rolling mill 1 includes a pair of (i.e., upper and lower) work rolls 2, and a pair of (i.e., upper and lower) backup rolls 3, the work roll 2 being driven in a downwardly pressed manner (i.e., a screw down manner) by a hydraulic screw down cylinder 4 via the backup roll 3. The paired upper and lower work rolls 2 are rotationally driven in symmetric directions by a drive motor (not shown) at the same speed or different speeds.
The hydraulic screw down cylinder 4 is driven by a driving device 5 provided per rolling mill 1, to roll a material 6 to be rolled (plate material) which passes through the pair of work rolls 2. Each driving device 5 is connected to a hydraulic screw down control device 7, and is fed with a drive command individually by the hydraulic screw down control device 7. The positions of a front end and a rear end of the plate material 6 are detected by a tracking device 8 based on the rotational speed of the roll, rolling load, etc.
A method for rolling the plate material 6 by the rolling system of the foregoing constitution will be described with reference to FIG. 14. FIG. 14 shows a flow chart representing a conventional rolling method.
With a roll gap between the work rolls 2 in each of the rolling mill stands F.sub.1 to F.sub.n being kept "open" (S1), the plate material 6 on the rolling line is transported to the rolling mill stands F.sub.1 to F.sub.n from upstream of the hot rolling line (S2). Under monitoring by the tracking device 8, the front end of the plate material 6 immediately before being engaged between the work rolls 2 of the most upstream rolling mill stand F.sub.1 is detected. In synchronism with this detection, the hydraulic screw down cylinder 4 of each of the rolling mill stands F.sub.1 to F.sub.n is driven in an expanded manner by the hydraulic screw down control device 7. Thus, the roll gap is adjusted and controlled to a set roll gap (S3).
Then, the timing of release of the rear end of the plate material 6 from the most upstream rolling mill stand F.sub.1 is detected under monitoring by the tracking device 8. In synchronism with this detection, the hydraulic screw down cylinder 4 of the rolling mill stand F.sub.1 is driven in a contracted manner by the hydraulic screw down control device 7. Thus, the roll gap of the work rolls 2 in the rolling mill stand F.sub.1 is controlled to "open". The succeeding rolling mill stands F.sub.2 to F.sub.n , are sequentially controlled in the same manner, and a rolling operation is repeated until a command for termination of operation is issued (S4, S5).
With the foregoing conventional rolling system, as shown in FIG. 15 indicating a state as viewed along arrows B--B in FIG. 13, when a rear end 6b of the plate material 6 has escaped from the most upstream rolling mill stand F.sub.1 on a rolling line 10, the rolling mill stand F.sub.2 located immediately behind the rolling mill stand F.sub.1 is screwing down the plate material 6. In this state, screw down forces, which are different between a work side Ws and a drive side Ds, may have acted on the plate material 6 at the work rolls 2 of the rolling mill stand F.sub.1 releasing the rear end 6b, thereby having caused a difference in tension between the right and left parts of the plate material 6. In this case, when the rear end 6b has left the rolling mill stand F, the rear end 6b of the plate material 6 that has become free may follow a zigzag path, as indicated by 6c, because of the difference in tension.
Upon zigzag movement of the rear end 6b of the plate material 6 as in 6c, a rear end 6c touches a side guide or the like of the rolling line 10, and is thus folded back. This folded-back rear end 6c is engaged between the work rolls 2 of the succeeding rolling mill stand F as a double layer, so that a so-called pinch fold appears. When the plate material 6 is rolled with its folded rear end being engaged between the work rolls 2 of the rolling mill stand F, the surface of the work roll 2 may be flawed, whereby the work roll 2 may be cracked or ruptured. If the work roll 2 is flawed, it is necessary to reassemble the damaged work roll assembly. Consequently, the frequency of reassembling per unit time increases, and the downtime of the system increases.
The present invention has been accomplished in light of the above circumstances. Its object is to provide a rolling system and a rolling method capable of avoiding the occurrence of a pinch fold by effectively preventing the zigzag movement of a rear end portion of a material to be rolled.