High strength strip steels with a yield strength of ~700 MPa are used in the yellow goods industry, with recent trends requiring thicker strips with an improved toughness. Strip steel was produced to thicknesses of 10 – 17 mm, with thinner strips showing improved Charpy impact toughness compared to the thicker strip. It was found that all samples (different thicknesses and test orientations) showed splits on the fracture surfaces in the upper transition region. The hot rolled strip steels showed a predominantly fine-grained (~4 µm ECD) ferrite microstructure, with some coarse grain patches (grains ≥5 µm grouped together). No significant differences were noted for the grain size, yield stress, tensile strength, hardness and texture for the different compositions. Splits were characterised and produced successfully by low blow Charpy testing with acoustic emission (AE) sensors indicating the presence of splits which occurred in the upper transition region predominantly without main crack propagation. Analysis by x-ray tomography and sectioning was carried out showing splits propagate by transgranular cleavage, preferentially following coarse-grained regions, with the 10 mm strip producing deeper splits which was related to elongated grains (high aspect ratio). Therefore the presence of deep splits improved toughness through a reduction of the impact transition temperature, which was achieved by ensuring the presence of a bimodal grain structure, containing coarse elongated grain patches.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:607276 |
| Date | January 2014 |
| Creators | Punch, Rachel Bridget |
| Publisher | University of Birmingham |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://etheses.bham.ac.uk//id/eprint/5143/ |
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