Abstract: A pair of slit type horizontal fin rolls (18) and a pair of vertical side rolls (19) having such a shape as having the conventional vertical side roll (19) formed by contracting its center portion are utilized, and due to the construction in which one set of each horizontal fin roll (18) and vertical side roll (19) adjustable horizontally, the depressing force is not applied to the material, and since the edge is restrained and shifted widthwise only with the horizontal force of the vertical side rolls (19) rotating freely, threading can be conducted very smoothly irrespective of the sectional shape of the material after break down, and even if the material size is changed, the threading can be conducted very smoothly without adjusting the roll position as strictly as in the past.

Abstract: To promote the forming performance of band material into pipes and to decrease the number of forming stages and changes of the forming rolls in the pipe mill, considering that it is superior when forming the band material into pipes of required diameters if the sectional shape of the band material is provided with a curved surface which changes its curvature continuously into the required diameter, we provide a curved roll surface of numerous curvatures changing continuously. In a continuous roll forming in accordance with the present invention, a forming roll having an approximate involute curve based upon a polygon comprising a plurality of sides is used. By using the conventional rolls in various combinations therewith in each stage of forming as a horizontal roll, side roll and upper intermediate roll or a cage roll, the number of stages of the forming roll can be reduced.