Processing and application of austenitic high manganese steels for highly improved strength and energy absorption in automotive crash relevant parts

Austenitic High Manganese TWIP-Steels (HMnS) exhibit outstanding specific energy absorption due to their high strain hardening. The properties result from specific deformation-mechanisms - namely dislocation SLIP and TWinning-Induced Plasticity (TWIP). Despite their high energy absorption, the steels have a low yield strength in the recrystallized state, which is a major disadvantage in application, compared to classical 1st gen AHSS.

To adjust and improve the mechanical properties of HMnS we developed

• a deformation-annealing processing to adjust the properties application-oriented and
• a new crash-box design, showing how HMnS is used most efficiently.

At first cold-rolling is applied to hot-strip material. Rolling introduces a high density of deformation twins and dislocations to the microstructure, increasing the yield strength. A subsequent recovery annealing, reduces the dislocation density while deformation twins are preserved. This combination of cold-rolling and annealing is easily applicable within most production lines and allows the production of a fully austenitic ultra-high strength steel with sufficient residual deformation. The process route allows easy adjustment of the mechanical properties to any given design of application. To describe the materials response, we analyzed several rolling degrees,annealing temperatures and annealing durations. The yield strength as well as the deformability of the HMnS are adjustable. A range from ~1.6 GPa yield strength and 10 % uniform elongation, down to lower yield strengths with simultaneous increasing residual deformation is configureable.

To demonstrate the new possibilities, we designed, simulated and manufactured an innovative 2-stage crash-box design. The first stage exploits the high specific energy absorption of the recrystallized HMnS during straining. The second stage is a high-strength recovery-annealed variant of the same HMnS. The crash-box demonstrates the advantages of both the process chain and the adapted design. The results show that the specific energy absorption capacity of the crash-box is significantly increased compared to conventional concepts.