Year

2016

Degree Name

Doctor of Philosophy

Department

School of Mechanical, Materials and Mechatronic Engineering

Abstract

Dual phase (DP) and transformation-induced plasticity (TRIP) steels have received a significant attention in automotive industry due to their high strength and good ductility. At present, they are industrially manufactured using hot rolling and cold rolling followed by intercritical annealing. Alternatively, strip casting is another potential way to produce these steels, which will significantly reduce energy consumption. Thus, for the first time, an investigation into the possibility of production of DP and TRIP steels using strip casting was carried out in this work on the laboratory scale.

For DP and TRIP steels, the processing parameters for simulated strip casting technique were determined based on prior austenite microstructures and continuous cooling transformation diagrams. The effects of interrupted cooling temperature, the amount of deformation and deformation temperature on microstructure evolution and tensile properties were studied in depth. The influence of isothermal bainite transformation (IBT) temperature on microstructure and mechanical behaviours of the TRIP steel was also investigated. Typical microstructures of DP and TRIP steels were obtained in strip cast steels, which were characterised using X-ray diffraction, energy dispersive spectrometry, scanning and transmission electron microscopy (TEM), electron backscattering diffraction (EBSD) and atom probe tomography (APT). The strain hardening behaviour after tensile tests was analysed using modified Crussard – Jaoul model. The strengthening after deformation was predominantly ascribed to grain refinement and dislocation strengthening. The mechanical properties of strip cast DP and TRIP steels outperformed industrially produced DP 600 and TRIP 690 steels, respectively, indicating the feasibility to produce these two steels using strip casting.

For DP steel, the average ferrite grain size was significantly refined from ~ 25 to ~ 3 μm by the triggering of strain-induced ferrite (SIF) formation when the samples were deformed between 700 and 800 °C at a reduction of ~ 0.41. The EBSD maps were segmented into polygonal ferrite, SIF and second phase regions using mean angular deviation and grain size. A diffusional mode for SIF formation was suggested based on the similarity in dislocation substructure features, misorientation angle distribution and the deviation from theoretical orientation relationships between polygonal ferrite and SIF. The contribution to strain hardening from every microstructure constituent was analysed using modified iso-work modelling.

The effects of neighbouring phases, retained austenite (RA) size and morphology on its carbon content were studied using site-specific APT correlated with EBSD. The results highlighted the dominant role of neighbouring phases on the carbon content in the RA. The analysis of C and Mn partitioning across the interfaces between the RA and ferrite in bainite showed that it could be explained using either diffusional mechanism of bainite formation and/or diffusionless one followed by tempering.

FoR codes (2008)

0912 MATERIALS ENGINEERING

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.