In a batch type hot rolling line in which a steel piece is individually heated and rolled by a roughing mill and a finishing mill to make a steel sheet of a desired thickness, a line stop tends to occur because the rolled material does not successfully bite into a gap defined by the upper and lower rolls and faulty shapes of the leading and the trailing ends of the rolled material will considerably lower the product yield.
Therefore, an improved rolling method (endless rolling method) has been adopted in recent years in which the trailing end of a steel piece to be rolled is joined to the leading end of the succeeding steel piece to be rolled before the finish rolling and the resulting steel piece is continuously supplied to a hot rolling line.
In this respect, reference may be had to Japanese Unexamined Patent Publication No. 58-122,109 which describes a method wherein the trailing end of a preceding steel piece is butt-joined to the leading end of a succeeding steel piece over the entire area of the end plane thereof, before they are rolled. Also, Japanese Unexamined Patent Publication No. 4-89,120 describes a method in which the trailing end of a preceding steel piece is arranged against the leading end of a succeeding steel piece with a gap on the entry side of a hot finishing mill equipment and in this area an alternate magnetic field is applied to the steel pieces in their thickness direction to heat them and raise the temperatures thereof by induction heating and then both steel pieces are joined by pressing.
In such continuous hot rolling of steel pieces, a cutting step is added as a preparatory step for the joining process, in which defective portions at the leading and the trailing ends of the steel piece are cut by a crop shear, drum shear or the like. In the cutting step, however, there may be instances wherein the leading end and/or the trailing end of the steel piece tend to warp upward or downward. As a result, the steel pieces may be joined with an upward or downward displacement (hereinafter referred to as "displacement"), as they are pressed against each other.
In this respect, the reason why the end of the steel piece warps in the cutting step is that a moment is produced in the cut plane when the end of the steel piece is cut by a crop shear. Usually, the preceding steel piece and the succeeding steel piece are cut by the same cutting blade so that they warp in opposite directions. If the steel pieces are heated to raise their temperatures and pressed in such a condition, they tend to further warp upward and downward to substantially decrease the effective joint area. This tends to cause the demerit that the steel pieces are broken in the following finish rolling process at the joined portion. In particular, when a transverse-type induction heating is used as heating means and an alternate magnetic field is applied to the steel pieces in their thickness direction for heating them by induction heating, due to the requirement for the installation space of the heating coil, a clamp for sandwiching the steel pieces must be used at a location which is remote from the ends of the steel pieces. As a result, it has been considered that the influence of the warp in the pressing process of the steel pieces would be more significant by using a clamp having a pair of upper and lower jaw elements which extend toward the ends of the steel pieces.
Furthermore, Japanese Unexamined Patent Publication No. 5-185,111 describes a cutting device having a structure wherein two pairs of upper and lower drums are arranged along the transfer direction of the rolled material and respectively rotated in opposite directions from each other, and cutting blades are mounted on each pair of drums and oriented in opposite directions. Also, Japanese Unexamined Patent Publication No. 56-27,719 and Japanese Unexamined Patent Publication No. 56-119,311 each describes a cutting technique in which the shear members of a drum type shear are driven at different circumferential speeds to cut the crop end of the steel piece. Moreover, Japanese Unexamined Patent Publication No. 7-251,203 describes a cutting technique in which a flying shear for cutting the end of the steel piece in a parallel surface relation is applied when a plurality of elongate steel pieces for hot rolling are joined by laser welding and are continuously hot rolled. However, even when such device and/or techniques are applied to the cutting of the steel pieces, it is still impossible to reduce the warping of the steel piece which causes the displacement at the joined portion.
It is an object of the present invention to provide a method for reducing the displacement at the joined portion of the steel pieces even when a traverse-type high-frequency induction heating is used as heating method, thereby allowing a stable continuous hot rolling to be carried out without defective joining or breakage at the joined portion.