Roller leveler and plate flattening method using the same

A plate flattening method of flattening a plate with the use of a roller leveler having a leveling roll unit inducing a plurality of leveling rolls arranged on upper and lower sides of a pass line in a staggered manner and configured to rotate so as to pass the plate while flattening the plate sandwiched therebetween, a pushing cylinder provided at each of an entrance side and a discharge side of the leveling roll unit, at which the plate enters and is discharged, respectively, and configured to press the plate via the leveling rolls, a driving mechanism configured to rotate the leveling rolls to pass the plate, with said plurality of leveling rolls including 1) two or more first rollers each having a stepped structure comprising a lateral center portion with a uniform diameter D1 extending along a longitudinal axis of the first roller corresponding to a center portion of the plate with respect to a plate width direction, and a lateral end portion with a uniform diameter D2 smaller than D1 extending from the lateral center portion along said longitudinal axis of the first roller to an end of the first roller corresponding to an edge portion of the plate with respect to the plate width direction;

• and 2 two or more second rollers each having a straight form comprising a uniform diameter D3 extending from a center thereof to an outside edge corresponding to said edge portion of the plate, the two or more first rollers each having the stepped structure are disposed sequentially on a first side of the leveling roll unit without any of the second rollers in between, and the two or more second rollers each having the straight form only are disposed on a second side of the leveling roll unit without any of the first rollers in between,
• the plate flattening method comprising:
• sandwiching the plate between the plurality of leveling rolls including said two or more first rollers having the stepped structure and said two or more second rollers each having a straight form;
• and
• rotating the leveling rolls while the pushing cylinder presses the plate via the leveling rolls to pass and flatten the plate;
• and
• flattening the plate by applying a pressing amount at the center portion of the plate pressed between central sections of both 1) said two or more first rollers each having the stepped structure and 2) said two or more second rollers which is greater than a pressing amount at the edge portion of the plate having only been pressed by said two or more second rollers each having, the straight form only.