Year

2014

Degree Name

Doctor of Philosophy

Department

School of Mechanical, Materials and Mechatronic Engineering

Abstract

Transformation-­‐induced plasticity (TRIP) steel has received significant research effort because its high strength to ductility ratios and high energy absorption make it eminently suitable for automotive applications. This work investigates the microstructure, crystallography, and mechanical properties of thermo-­mechanically processed (TMP) low-Si high-Al TRIP steels with and without the addition of 0.03Nb and 0.02Ti (wt%) using a combination of optical and electron microscopy, X-ray diffraction, and tensile testing.

The two TRIP steels investigated were subjected to simulated TMP and galvanising and then the results were compared with intercritically annealed–galvanised steels. After undergoing different thermo-­‐mechanical processing schedules it was found that adding Nb and Ti increased the tensile strength (~ 750 MPa) and total elongation (TE) (~ 29 %), as well as improving the strength-­‐ductility balance compared to the base steel (without Nb and Ti) due to a higher volume fraction of RA and a combination of microstructure refinement and precipitation strengthening effects. Despite these gains, both steels suffered a deterioration in the volume fraction of retained austenite and mechanical properties after the additional galvanising simulation, indeed all the TMP and galvanised base steel samples recorded a continuous yielding behaviour during tensile testing. The Nb–Ti steel exhibited discontinuous yielding and extended Lüders banding when TMP was followed by a longer coiling time, while both the base and Nb–Ti steels returned discontinuous yielding with long Lüders elongation after undergoing intercritical annealing–galvanising treatment. This discontinuous yielding behaviour was associated with the much finer ferrite grain size in the intercritically annealed steels and the ageing processes that occur during galvanising. The modified Crussard–Jaoul (C–J) analysis showed three stages of work hardening where the second stage behaviour was ascribed to the different rates of RA transformation.

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