Temperature and strain rate dependence of the martensitic transformation and mechanical properties in advanced high strength steels

Abstract

Third-generation advanced high strength steels (3GAHSS) use the deformation-induced phase transformation of austenite to martensite to enhance their properties during forming and in service. However, deformation conditions may vary during these processes, which is problematic because the martensitic phase transformation is intrinsically responsive to strain rate and temperature. This is known to influence the balance of strain accommodation and strengthening via transformation-induced plasticity and dislocation slip. This thesis explores the processing-property relationships that govern the performance of 3GAHSS created with the Quenching and Partitioning (Q&P) process, with the goal of better understanding the influence of deformation processing factors on the austenite stability and mechanical performance. Two grades of Q&P steel, having different fractions of ferrite and ultimate tensile strengths of 980 and 1180 MPa, were mechanically tested over a range of temperatures from 22 to 250 ◦C and strain rates from 10−4 to 103 s−1, while the martensitic transformation was tracked by a combination of X-ray diffraction (XRD) and electron microscopy. In some instances, XRD was performed ex-situ with a lab diffractometer, while in higher resolution experiments, XRD was performed in-situ using high-energy synchrotron light. After fracture, spatially-resolved electron backscatter diffraction kernel average misorientation (EBSD-KAM) analysis was performed to complement the bulk XRD data. Several studies used ex-situ XRD to elucidate the effect of the martensitic transformation on the mechanical properties, as a function of tensile specimen orientation and strain rate, respectively. The orientation of tensile specimens relative to the rolling direction of the sheet appeared to strongly influence the extent of the martensitic transformation, which may have been caused by crystallographic texture that developed during cold rolling and Q&P heat treating. Meanwhile, increasing the strain rate appeared to suppress the martensitic transformation, but it was unclear whether deformation-induced heating or the strain rate increase was responsible. An additional experiment was performed using a thermal-mechanical simulator, to decouple the influences of deformation-induced heating and strain rate on the martensitic phase transformation. By the combination of ex-situ XRD and EBSD-KAM analysis, it was observed that deformation-induced heating suppressed the martensitic transformation, while increasing the strain rate promoted the transformation. To resolve the martensitic transformation with greater spatial and temporal resolution, quasi-static elevated-temperature tensile tests and dynamic-rate room-temperature tensile tests were performed, while XRD data were collected in-situ using synchrotron light. Because the XRD was performed in-situ, the bulk mechanical response, as well as the amount of deformation-induced martensite, the stresses/strains on individual phases, and the peak broadening due to the elastic strain fields of dislocations, were measured concurrently. A strong temperature dependence of the martensitic transformation was observed, and the highest quality fit against existing empirical models for the transformation was reached when the strain partitioning between the phases was incorporated. The temperature dependence may have been linked to both the driving force for the transformation and the number of available martensite nucleation sites. Meanwhile, in the high-strain rate experiment, a clear dependence of the martensitic transformation on the steel grade and strain rate was less obvious, because the fidelity of the measurements was limited by a relatively low signal-to-noise ratio.