Manufacturing ultra-high strength load bearing parts using high strength/low initial yield steels through tubular hydroforming process

Abstract

Rather than using a conventional stamping forming process with steels having high ultimate tensile strength and relatively low initial yield, tubular hydroforming techniques are introduced to synergize with BIW part forming, or forming of other load bearing parts. Such steels can have ultimate tensile strengths of greater than 1000 MPa and initial yields of less than 360 MPa In some embodiments, the steels have elongation of at least 40%. Such steels can include retained austenite.

What is claimed is: 
     
         1 . A process for forming steels with high ultimate tensile strength and low initial yields comprising the steps of:
 a. Forming an initial tube blank by selecting a raw tube diameter sufficient to permit stretching of the steel in a later hydroforming step to reach a pre-determined yield strength while keeping induced material thinning within pre-determined failure limits and pre-determined design tolerance and forming an initial tube blank conforming to said diameter;   b. Pre-bending said initial tube blank to generate a smooth curvature in a bent tube;   c. Hydroforming the bent tube in a mold by filling the bent tube with a pressurized liquid until walls of the bent tube contact the mold to form a part;   d. Trimming the formed part.   
     
     
         2 . The process of  claim 1  wherein the steel contains retained austenite. 
     
     
         3 . The process of  claim 1  wherein before hydroforming the steel has an ultimate tensile strength of greater than 1000 MPa and an initial yield strength of less than 360 MPa. 
     
     
         4 . The process of  claim 3  wherein before hydroforming the steel has an ultimate tensile strength of greater than 1150 MPa. 
     
     
         5 . The process of  claim 1  wherein the tube blank is subject to at least one of preforming or intermediate hydroforming after pre-bending and before hydroforming.