Branagan, D., Frerichs, A., Meacham, B., Cheng, S. et al., "Overcoming the Paradox of Strength and Ductility in a New Generation of AHSS," SAE Technical Paper 2016-01-0357, 2016, doi:10.4271/2016-01-0357.
The historical development of autobody steels has demonstrated a paradoxical relationship between strength and ductility, with increasing strength necessary for lightweighting commensurate with reductions in ductility necessary for cold formability. This in turn creates geometric constraints in part design and manufacturing, ultimately limiting usage of these higher strength steel grades in automobiles. Quench and tempering including variants such as quench, partitioning, and tempering are known approaches to increase strength while attempts to overcome the paradox have focused on increasing ductility through three distinct deformation mechanisms including; 1) shear band induced plasticity (SIP), 2) transformation induced plasticity (TRIP), and 3) twinning induced plasticity (TWIP). In this presentation, a new and novel 4th approach is described to overcome this paradox and achieve 3rd Generation Advanced High Strength Steel (AHSS) properties with MPa% higher than 65,000 through the formation of a targeted microstructure consisting of two distinct microconstituent structures which, while having distinct roles in deformation and strengthening, act synergistically to develop novel combinations of strength and ductility. This Mixed Microconstituent structure will be described in detail including its deformation response through sequential stress activated processes resulting in a combination of dislocation dominated mechanisms with formation of characteristic high density dislocation networks, twin / defect annihilation, phase transformation, matrix phase refinement to the nanoscale, and nanoprecipitation. The microstructural pathway and resulting properties, enabled by novel structural formation mechanisms will be presented in detail through a simulation of each stage of the manufacturing process from cast slabs, to hot band coils, cold rolled coils, recrystallized coils, and final deformation during forming operations.