Process - microstructure - property relations of nanotwinned steels

Abstract

The modern advanced high strength steels (AHSS) that combine high strength and good formability/ductility are highly desired with the aim of improving passenger safety and weight-saving in the automotive industry. As the second generation of AHSS, twinning-induced plasticity (TWIP) steel is characterised by both high strength and large ductility. Recently, twin boundary strengthening is considered as an important strengthening mechanism that increases simultaneously strength and ductility. The nanoscale twins can be induced in the TWIP steel by a simple processing route (i.e. cold rolling followed by a recovery heat treatment) to produce the nanotwinned steels which possess extremely high strength and high ductility. However, the microstructure and related mechanical properties are highly dependent on the processing parameters of the metallurgical routes. Although the mechanical performance has been studied corresponding to different thermomechanical treatment, there are still questions that restrict the widely industrial application of nanotwinned steel such as the alternation of microstructure during the thermomechanical treatment, the deformation behaviours of the nanotwinned steel under different conditions and so on. In the present thesis, the mechanical properties of TWIP steel were first studied as a comparison to the nanotwinned steel. Then the effect of the recovery on the microstructure of the cold rolled TWIP steel was investigated. The recovery kinetics of cold rolled TWIP steel was studied based on the evolution of the dislocation density under different temperatures. An empirical model was used to reveal the development of dislocation density during the recovery, and the relatively mechanical properties can be predicted by a dislocation-based constitutive model. The deformation mechanisms of the nanotwinned steel were studied. Detailed transmission electron microscopy observations revealed that the twin volume fraction of the nanotwinned steel remained constant during tensile deformation. In contrast, the dislocation density of the nanotwinned steel increased with strain as measured by the synchrotron X-ray diffraction experiments. In other words, the plastic deformation of the nanotwinned steel was mainly accommodated by glide and multiplication of dislocations. Based on the experimental results, an analytical model was developed to capture the effects of dislocations and twins on the strength and ductility of the present nanotwinned steel. The nanotwinned steel could be considered as an armour material to resist projectiles because of its excellent combination of high yield strength and high ductility. The deformation mechanisms during ballistic impact penetration tests were investigated in both TWIP steel and nanotwinned steel. Similar to the quasi-static deformation, the twinning and dislocation glide were the main deformation mechanism in the TWIP steel during the ballistic impact penetration tests. However, three phenomena including deformation twinning, dislocation multiplication and recrystallization were observed in the nanotwinned steel during the ballistic impact penetration tests.