Cast solidification structures of high aluminium steels

Low density steels (LDS) are of industrial interest as lightweight vehicle body panels to improve fuel efficiency. However, they are difficult to manufacture using conventional steel processing: for example they suffer from the formation of large grain sizes during solidification, which affects hot ductility causing cracking. Alternative processing, such as twin roll or belt casting, with higher cooling rates offer options for casting these grades. It is important that the effects of composition and cooling rate on the solidification structures (secondary dendrite arm spacing, and grain size development) are understood, as has been considered in this thesis, specifically for LDS, but also the more general applicability of the approach to other grades of steel. A new technique (using a confocal scanning laser microscope, CLSM) for SDAS measurements based on in situ observation of the metals surface during solidification is described and verified by comparison to bulk solidification measurements for both the LDS grades and 304 stainless steel, giving confidence it is suitable for wider application. The main benefit of this technique is that it can be used on grades where conventional techniques for ex-situ room temperature characterisation can not be applied, for example where phase transformations alter the microstructure removing evidence of solidification structures. SDAS values with respect to cooling rate were obtained for the LDS using the CLSM, however it was found that grain size measurements could not be made as the sample size / field of view was too small considering the grain sizes in the materials. EBSD grain analysis was used to characterise the development of grain sizes and misorientation ranges through the columnar region for laboratory ingot castings which gave a range of cooling rates that covers both conventional casting techniques and near net shape techniques. It was found that the bake hardenable LDS had a very narrow mushy zone and very large grain width, whist the nano-precipitate strengthened LDS had a wider mushy zone and relatively finer grain width on solidification. For both steels the grain width increased with depth into the cast ingot due to grain competition. Relationships between the cooling rate and rate of columnar grain competition, and the rate of competition and SDAS:mushy length ratio are proposed based on these results. The modelling section discusses the development of a model in MICRESS to simulate the solidification of the LDS with respect to cooling rate with the intent that it can be more broadly applied to other grades, this is then tested against a TWIP grade. There were some discrepancies between the simulations and experimental results, suggested to be due to 2d compared to 3d effects, however the trends of grain coarsening were broadly similar, giving the current model qualitative predictive capacity that appears to be generally applicable to different grades undergoing single phase (delta ferrite or austenite) solidification. xi The hypothesise developed during study of the LDS with respect to the effect of cooling rate on solidification structure were tested against the TWIP results, using laboratory and commercial cast samples. while relating coarsening rate to cooling rate directly did not carry over to this new grade, the TWIP results are consistent with the SDAS:mushy length based hypothesis, suggesting an avenue for further study and potentially advancing understanding of metallic solidification in general