In the drive towards higher strength alloys, a diverse range of alloying elements is employed to enhance their strength and ductility. Limited solid solubility of these elements in steel leads to segregation during casting which affects the entire down-stream processing and eventually the mechanical properties of the finished product. Although it is thought that the presence of continuous bands leads to premature failure, it has not been possible to verify this link. This poses as increasingly greater risk for higher alloyed, higher strength steels which are prone to centre-line segregation: it is thus vital to be able to predict the mechanical behaviour of multi-phase (MP) steels under loading.
The work presented in this PhD shows that segregation during casting of alloying elements required for obtaining the desired mechanical properties, particularly aluminium, lead directly to banding in the final product. It has been demonstrated that no significant homogenisation is possible in this alloy within practical time constraints of the industrial thermo-mechanical process. A through-process model was developed to design a thermo-mechanical treatment aimed at reducing the effects of segregation on the formation of banding. A new polynomial function for calculating the local phase transformation temperature (Ae3) between the austenite + ferrite and the fully austenitic phase fields during heating and cooling of steel is presented.
Material was produced both with and without banding and used to study the effect upon the mechanical properties. The results show a significant reduction in tensile strength in banded steels for a similar level of ductility which is the result of a reduced work-hardening capacity. In situ measurement under uniaxial loading using high-energy synchrotron diffraction allowed direct quantification of the impact of the mechanically induced transformation of metastable austenite on the work hardening. The results show that the mechanically induced transformation of austenite does not begin until the onset of matrix yielding and for the first time it was demonstrated that the austenite transformation increases the work-hardening contribution, σw thereby supporting a driving force approach to transformation induced plasticity. The transformation work required leads to an increase in the macroscopic work-hardening rate after matrix yielding and continues to offset the decrease in the work-hardening rate in the ferrite and martensite phases up to the UTS. Steels with a high degree of banding do not show this extra contribution due to the more dominant anisotropic effect of martensite bands on the work-hardening of ferrite.