The stability of metastable austenite grains in advanced high strength steels

Abstract

The transformation of the austenite to the martensite provides the transformation-induced plasticity (TRIP) effect which can contribute to the high work hardening rate and the good mechanical properties of 3rd advanced high strength steels (AHSS). The high strength offered by AHSS can result in thinner components while providing the same or higher safety standard, leading to a lighter vehicle. The present thesis is aimed to study the stability of the individual metastable austenite grains in AHSSs. Firstly, it is studied by the nanoindentation technique. Several pop-ins are found in the corresponding load-displacement curves. The first pop-in is induced by the nucleation of dislocations. The second pop-in is due to the martensitic transformation. Therefore, the load at the second pop-in is the critical load to trigger the transformation of metastable austenite to martensite. The critical load increases with the increase of Mn content of the individual austenite grains. Different to the well-known factors affecting the stability of retained austenite grains, such as the austenite chemical concentration, grain size and morphology, the present thesis investigates the effect of free surface by using the focused ion beam (FIB) milling. It is found that the retained austenite grains transform to martensite automatically once a free surface is introduced in the retained austenite grains. This is ascribed to the reduction of the martensite nucleation energy barrier by introducing a new free surface. The present thesis demonstrates the importance of mechanical stability of austenite grains on the tensile properties of a medium Mn steel. It is found that the tailored mechanical stability of austenite grains is optimal for the tensile properties. The austenite grains with different grain size and C content are produced in a single steel. The large austenite grains with low C content have a lower mechanical stability, which transform to martensite and provide TRIP effect at small strain. The austenite grains with high C content have higher mechanical stability so that they do not transform to martensite but provide twinning-induced plasticity (TWIP) effect in the medium strain regime till fracture. Some of the twinned austenite grains provide the martensite nucleation site and offer TRIP effect in the large strain regime up to fracture. Since fine ferritic phases are the intrinsic components in the 3rd AHSS, the present thesis investigates their mechanical behavior by the nanoindentation technique. The lath martensite has the higher nanohardness than bainite and ferrite due to its high pre-existing dislocation density, high carbon content and carbide precipitation. The investigation on the as quenched lath martensite reveals that the small martensite block has a higher nanohardness than the large martensite block. Therefore, the as-quenched martensite may be considered as a composite material with the small but strong martensite blocks embedded in the large but soft martensite blocks, which provides an important information for the micromechanical modelling of martensitic steel.