Understanding Forming Characteristics of 980 MPa and 1180 MPa strength level AHSS with various Microstructures

For electrical vehicle (EV) automotive body-in-white (BIW) structures, protection of passengers and battery in crash event becomes critically important. In addition to energy absorption, intrusion protection for battery and vehicle becomes extremely important and Advanced high strength steels (AHSS), like dual phase (DP)/multi-phase (MP)/complex phase (CP)/retained austenite bearing (GEN3) steels and Press hardened steels (PHS), have become material of choice for design for these components. Higher yield strength materials especially in 980/1180MPa MP and CP category are chosen for part design over conventional low yield strength DP to meet crash and performance targets. In this study, the forming characteristics including both global and local formability are evaluated and compared among 980 DP/MP/CP grades. Formability tests, such as forming limit curve (FLC), true fraction strain, V bend, half dome, and hole expansion tests are conducted. Microstructure analysis to understand the effect of different grain structure and phases of DP/MP/CP grades is performed. A T-shape laboratory die trials are carried out to compare and understand the global formability of these steels. Experimental results indicate MP and CP grades tend to have better local and equivalent global formability than DP steels. Some best practices are recommended, when AHSS (MP/CP) are recommended to form, in place of traditional 980 DP type parts in terms of both global and local formability performances.