Patent ID: 7377041

Claim:
A method for forming a torsion beam having a cross-sectional shape of a letter U, comprising the sequential steps of: placing a pipe having a circular cross-sectional shape and a certain wall thickness on a lower mold; operating an upper mold, an upper punch, and side molds, thereby forming the entire length of the circular pipe to have a cross-sectional shape of an oval, wherein the formation of both end portions of the torsion beam is completed without deformation hereinafter; sealing the pipe by attaching plugs to both ends of the pipe, and injecting fluid through center hole(s) formed in at least one of the plugs so as to fill an inner space of the pipe with the fluid, thereby applying internal pressure to an inner peripheral surface of the pipe; pressing down the upper punch and pressing up a lower punch to form a U-shaped pipe, wherein the upper punch has a U-shaped convex portion corresponding to the concave surface of the U-shaped pipe, the lower mold has a concave portion corresponding to central regions of the convex external surface of the U-shaped pipe, and the lower punch has side surfaces rounded to combine with the concave portion of the lower mold to form the external surfaces of the U-shaped pipe; and applying axial pressures from both ends of the pipe toward the center of the pipe after sealing the pipe, wherein the applied internal pressure of the pipe is in the range of 4 Mpa to 12 Mpa, wherein the internal pressure is constantly maintained by a servo valve during the forming process, and wherein the smallest diameter of the oval-shaped pipe is 0.5 to 0.7 times of the diameter of the initial circular pipe.