Forming method and forming stand for welded pipes

A method for bending edges of a metallic strip in a roll stand having a strip entry side and a delivery side, said strip having a predetermined width and a longitudinal centerline, said method comprising:

• providing in said roll stand an upper pair of rolls with each roll having a center, a horizontal axis passing through its center, and a vertical axis passing through its center, the horizontal axis of each roll of said upper pair being in a common first horizontal plane, and each roll of said upper pair of rolls having a convex circumferential periphery;
• providing in said roll stand a lower pair of rolls with each roll having a center, a horizontal axis passing through its center, and a vertical axis passing through its center, the horizontal axis of each roll of said lower pair being in a common second horizontal plane spaced below said first horizontal plane, and each roll of said lower pair of rolls having a concave circumferential periphery;
• spacing a first roll of said upper pair from a second roll of said upper pair at a distance in the horizontal direction determined by said width of said metallic strip;
• spacing a first roll of said lower pair from a second roll of said lower pair at a distance in the horizontal direction determined by said width of said metallic strip;
• turning each roll of the upper pair of rolls having said convex circumferential periphery about its vertical axis in a first direction to a selected upper turning angle .alpha.u.ltoreq.45.degree., wherein .alpha.u is an angle between the horizontal axis of each upper roll and a horizontal line perpendicular to the longitudinal centerline of said metallic strip so that the upper turning angle coordinates with an inner curvature to be formed by bending edges of said metallic strip;
• said turning each roll of the upper pair of rolls having said convex circumferential periphery about its vertical axis in said first direction resulting in each roll of the upper pair of rolls being directed to a respective edge of said metallic strip at the entry side of the roll stand and being directed towards the longitudinal center line of said metallic strip at the delivery side of said roll stand;
• maintaining each roll of the lower pair of rolls having said concave circumferential periphery in a position wherein the horizontal axis of each lower roll is perpendicular to the longitudinal centerline of said metallic strip;
• bending the edges of said metallic strip by passing said metallic strip between said upper pair of rolls, each having said convex circumferential periphery, and said lower pair of rolls, each having said concave circumferential periphery.

A roll stand for bending edges of a metallic strip having a width comprising:

• an upper pair of rolls disposed in said roll stand wherein each roll has a center, a horizontal axis passing through its center, and a vertical axis passing through its center, with the horizontal axis of each roll of said upper pair located in a common first horizontal plane;
• a lower pair of rolls disposed in said roll stand wherein each roll has a center, a horizontal axis passing through its center, and a vertical axis passing though its center, with the horizontal axis of each roll of said lower pair located in a common second horizontal plane spaced below said first horizontal plane;
• means for spacing a first roll of said upper pair from a second roll of said upper pair at a distance in the horizontal direction determined by the width of the metallic strip to be bent;
• means for turning each roll of said upper pair about its vertical axis;
• means for spacing a first roll of said lower pair from a second roll of said lower pair at a distance in the horizontal direction determined by the width of the metallic strip to be bent;
• means for turning each roll of said lower pair about its vertical axis;
• the pair of upper and lower rolls disposed in the roll stand wherein the upper pair of rolls are located at a different position from the lower pair of rolls in each common horizontal plane.