Abstract: A process for producing large-sized steel pipe of rectangular or square configuration from a single plate includes providing a predetermined length of steel plate. The plate is bent to form a longitudinally extending base, opposed sidewalls and a top wall. The top wall is constituted by inwardly directed extensions of the sidewalls which terminate in spaced apart longitudinally extending edges. The sidewalls are angularly disposed relative to the base. At least one inner roller having outwardly facing substantially planar surfaces is positioned within the bent plate upon the base thereof. Outer rollers are positioned externally of the bent plate so as to be cooperable with the underside of the base and with the sidewalls. Relative longitudinal movement between the rollers and the bent plate is effected, the outer sidewall rollers are pressed against the planar surfaces of the inner rollers, and the longitudinally extending edges are brought into substantially abutting relationship.

Abstract: A process for producing large-sized square steel pipe includes the steps of continuously paying off steel plate from a roll of the same, removing strain from the plate while conveying the plate, and cutting the plate to predetermined lengths. The lengths of plate are successively bent into a shape resembling a rectangular or square pipe having corner portions with angles slightly larger the 90.degree.. The successive plates are tack-welded and opposed outer rollers are pressed against the outer sides of the plate while the plate is conveyed longitudinally thereof. The opposed edges of the plate are thereafter welded together by high frequency welding.