Post-partitioning treatment increases both yield strength and local formability in QP steel

Abstract

The low cost and excellent strength-formability combination of quenched and partitioned steels have enabled improvements in automotive safety and lightweighting. Despite this, the presence of fresh martensite in their microstructures has been identified as a key factor limiting further formability improvements. Prior work has shown that the hardness contrast of fresh martensite with its surrounding matrix phases leads to decohesion and microcrack nucleation during deformation. In this study we evaluate the efficacy of a post-partitioning treatment in tempering the fresh martensite and reducing its tendency for microcrack nucleation. To this end, mechanical properties are studied across different scales (i.e., using hole expansion tests, uniaxial tensile tests, nanoindentation); and connected to characterization of microstructural constituents by scanning electron microscopy, synchrotron X-ray diffraction, dilatometry, and scanning transmission electron microscopy. The results reveal that post-partitioning treatment can indeed decrease the hardness of fresh martensite, increasing the local formability and yield strength. The increase in local formability is primarily achieved by fresh martensite tempering which reduces the hardness mismatch at phase boundaries, increasing the strain required for microcrack nucleation. On the other hand, an additional increase in the yield strength is observed which is due to increased austenite stability from additional carbon partitioning, and to a lesser degree, bainitic transformation. The potential for post-partitioning treatment to improve mechanical properties in steels beyond the one studied here is discussed in terms of microstructural and compositional factors.