Preform, hydroforming method, and hydroformed product

A hydroformed product formed by:

• disposing a preform inside forming dies having cavity surfaces that correspond to an outer shape of a hydroformed product, said preform having edges overlapped and jointed each other and including first and second outer members for forming outer surfaces of the hydroformed product, and reinforcement members that are jointed to said first and second outer members to form reinforcement ribs that divide a hollow cross section of said outer surfaces, and said reinforcement members having dimensions capable of suppressing elongation in a tensile direction due to a tensile force that develops during hydroforming;
• and
• applying a hydraulic pressure in an inside of said preform while suppressing elongations of said reinforcement members in a tensile direction due to a tensile force developed during inflating deformation of said preform to form said reinforcement ribs that divide the hollow cross section of said hydroformed product,
• wherein said reinforcement ribs that divide the hollow cross section of said outer surfaces are formed by means of suppressing elongations of said reinforcement members due to a tensile force developed during hydroforming.