PURPOSE: To assuredly provide an I butt welding groove line by eliminating plate edge warping, process uses step forming comprising initial plate edge forming, medium center forming providing linear edge, and last finishing forming.

CONSTITUTION: A plate is cut to dimensions that will provide the desired pipe size. The so cut plate S is introduced between the edge forming rolls 14a, 14b, subsequent rolls 15, 16 so that both of the plate edge in less than 25% width are bent into curvature to more than 70% of the pipe to be produced. The edge width in this case must exceed 12.5% to provide sufficiently good formation. Next, the restraining roll pairs 18a, 18b...18g and 17a...17d furnish external curve bending by plate center natural function. Finally, the down roll 19, and upper roll 21 with fin 20 are used to form an accurate, edge-to-edge groove line for welding.