Control of Springback in Bending and Flanging Advanced High Strength Steels (AHSS)

Springback is known to be more pronounced in forming AHSS compared to mild or HSLA steels because of the unique hardening behavior and higher elastic stress of AHSS. Dimensional accuracy of automotive structure components is very critical for assembly and welding processes. AHSS automotive parts often encounter very complicated springback results such as angular change, sidewall curl, and twist. Reliable prediction and practical compensation of springback are important ways to accommodate springback effects in product design and manufacturing stages.

EWI is conducting a Cooperative Research Program with OSU-ERC/NSM on this subject. In this study, springback for bending and flanging AHSS was investigated by using various experimental methods (V-bend, rotary bend and S-rail flanging tests) and computational models. The elastic and plastic properties of AHSS were determined by using the tensile test and the biaxial bulge test. Dual-phase (DP) 600 steel showed significant decreases about 10% of elastic modulus as the effective strain increased during loading and unloading cyclic loading-unloading tensile tests (Fig.1), while a common mild steel, aluminum-killed drawing quality (AKDQ) gave almost constant modulus. Therefore, a general expectation of constant elastic modulus of the material will not always apply to AHSS.

A simple V-bend test was used to investigate the effects of material properties, texture orientations, and die openings on springback. V-bend test results showed that TRIP 780 has relatively smaller springback compared to DP 780 and 780 HY (high yield) materials at two different die opening lengths. Experimental results were compared to the predictions of the computational model.

Preliminary tests and FE simulations for an S-rail flanging were conducted with AKDQ material to investigate the feasibility of this test method for springback study. Preliminary FE simulations for S-rail stamping showed good feasibility to test AHSS without excessive thinning of the material.

The variable elastic modulus for DP780 were determined by the cyclic loading unloading tensile test and used in FE simulations of S-rail flanging. The difference of resultant displacement of S-rail part after springback is compared between the constant elastic modulus (207 GPa) and the variable modulus (Fig. 2). Experimental validation of this model is currently underway at OSU.

For more information, please contact Hyunok Kim at hkim@ewi.org or call 614.688.5239, or Menachem Kimchi at mkimchi@ewi.org or call 614.688.5153.