The effect of processing parameters on the microstructure and mechanical properties of low-Si transformation-induced plasticity steels

A base low Si, high-Al transformation-induced plasticity (TRIP) steel and one with 0.03Nb and 0.02Ti (wt%) additions were subjected to thermo-mechanical processing (TMP) and galvanising simulations. The microstructure and mechanical properties were analysed using a combination of optical and electron microscopy, X-ray diffraction and tensile testing and the results compared with those from intercritically annealed-galvanised steels. The addition of Nb and Ti results in microstructure refinement and an increase in the amount of the retained austenite after TMP which in turn, leads to increases in the tensile strength (similar to 750 MPa) and the total elongation (TE) (similar to 29 %). A deterioration in the volume fraction of retained austenite and the mechanical properties was noted in both steels after the additional galvanising simulation. For the base steel, all TMP and galvanised samples presented with continuous yielding during tensile testing. The Nb-Ti steel exhibited discontinuous yielding and extended Luders banding when TMP was followed by a longer coiling time. Both steels returned discontinuous yielding after the intercritical annealing-galvanising treatment. The discontinuous yielding behaviour was associated with the much finer ferrite grain size in the intercritically annealed steels and the ageing processes that take place during galvanising.